studies in Surface Science and Catalysis 146 NANOTECHNOLOGY IN MESOSTRUCTURED MATERIALS
Studies in Surface Science and Catalysis 146
NANOTECHNOLOGY IN MESOSTRUCTURED MATERIALS
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studies in Surface Science and Catalysis Advisory Editors: B. Delmon and J.T. Yates Series Editor: G. Centi Vol. 146
NANOTECHNOLOGY IN MESOSTRUCTURED MATERIALS Proceedings of the 3*^^ International Mesostructured Materials Symposium, Jeju, Korea, July 8-11, 2002
Edited by Sang-Eon Park \ Ryong Ryoo ^, W h a - S e u n g Ahn ^ and Chul W e e Lee ^ and Jong-San Chang^ ^ Catalysis Center for Molecular Engineering, KRICT, Yusung, Taejon, 305-600, Korea ^ National Creative Research Initiative Center for Functional and Department of Chemistry, KAIST, Yusung, Taejon, 305-701, Korea
Nanomaterials
^ School of Chemical Science & Engineering, Inha University, Inchon, 402-751, Korea "^ Advanced Chemical Technology Division, KRICT, Yusung, Taejon 305-600, Korea
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CONTENTS
Preface Organizing committee
xxxi xxxiii
International advisory board
xxxiv
Local advisory board
xxxiv
Supporting organizations
xxxv
Financial support
xxxv
I. Synthesis and materials A new family of organic-bridged mesoporous materials
\
S. Inagaki Strategies to fabricate large-pore three-dimensional mesoporous materials with versatile
9
applications C Yu. B. Tian, J. Fan, X. Liu, H. Yang, L Wang. S. Shen, B. Tu and D. Zhao Mesostructured solid acids
15
S. Hamoudi, D. Trong on and S. Kaliaguine Template synthesis and catalysis of metal nanowires in mesoporous silicas
23
A. Fukuoka. Y Sakamoto, H. Araki, N. Sugimoto, S. Inagaki, Y. Fukushima and M. Ichikawa Mesostructured silica films with crystalline domains and structural features on multiple length
29
scales Y.-S. Lee, JR. Archer, andJ.F. Rathman Synthesis of mesoporous carbons with various pore diameters via control of pore wall thickness of
33
mesoporous silicas J.-S. Lee, S.H. Joo and R. Ryoo Ordered mesoporous carbon molecular sieves with functionalized surfaces
37
S. Jun, M.K. Choi. S. Ryu, H.-Y Lee and R. Ryoo Characterisation of ordered mesoporous carbons and their MCM-48 silica templates obtained by the replication technique using different carbon infiltration processes
41
J. Parmentier, C. Vix-Guterl, P. Gibot, M. Iliescu, J. Werckmann and J. Patarin Morphological control of highly ordered mesoporous carbon
45
C. Yu, J. Fan, B. Tian, F. Zhang, G.D. Stucky and D. Zhao Thermally induced structural changes in SBA-15 and MSU-H silicas and their implications for
49
synthesis of ordered mesoporous carbons S.H. Joo, R. Ryoo, M. Kruk and M. Jaroniec Regeneration of mesoporous inorganic materials using ordered mesoporous carbon as the template
53
J.M. Kim, M. Kang, S.H. Yi, J.E. Yie, S.H. Joo andR. Ryoo A novel preparation route for palladium-carbon composite materials - pore filling of SBA-15
57
mesoporous molecular sieve H.H.P Yiu, l.J. Bruce, F McGuinness and PA. Wright Structure of ultra-thin RbBr "Solution" in carbon nanospace
61
T. Ohkubo, H. Kanoh, Y. Hattori, T. Konishi and K. Kaneko Synthesis and characterization of mesoporous silica films by spin-coating on silicon:
65
photoionization of methylphenothiazine and photoluminescence of erbium 8-Hydroxyquinolinate in mesoporous silica films J. Y Bae, J.-I. Jung, O.-H. Park, B.-S. Bae, K.T. Ranjit andL
Kevan
Synthesis of 2D hexagonal mesoporous silica thin films via phase transition from lamellar structure
69
C.-W. Wu, K. Miyazawa and M. Kuwahara Nanostructured silicate film templated by discotic CT-complex column
73
A. Okahe, T. Fukushima, K. Ariga and T. Aida Mesoporous titania thin film with cubic mesostructure using photocalcination
77
U.-H. Lee. Y.K. Hwang and Y-U. Kwon Preparation of tin modified silica mesoporous
film
81
B. Yuliarto, H.-S. Zhou, T. Yamada. I. Honma and K. Asai Novel non-lithographic large area fabrication method to generate various polymeric nanostructures
85
W. Lee, M.-K. Jin, W.-C. Yoo and J.-K. Lee Mesoporous anodic alumina mbembrane with highly ordered arrays of uniform nanohole
89
CIV. Lee, C.I. Lee, Y Lee, H.S Kang YM. Hahm and YH. Chang Preparation and characterization of poly(ester)-silver and nylon-silver nanocomposites S.-H. Choi, K.-P LeeandS.-B.
93
Park
Synthesis of ordered three-dimensional large-pore mesoporous silica and its replication to ordered
97
nanoporous carbon J. Fan, C. Yu, L. Wang, Y Sakamoto, O. Terasaki, B. Tu and D. Zhao Morphology control of mesoporous SBA-16 using microwave irradiation Y.K. Hwang J.-S Chang Y-U. Kwon andS.-E. Park
101
One-Step synthesis of mesoporous silica SBA-15 with ultra-high microporosity
105
S.-C. Hung, H.-R Lin and C.-Y. Mou Controlling the pore sizes of SBA-15 mesoporous silica by the addition of poly(propylene oxide)
109
J.C. Park, J.H. Lee, P. Kim and J. Yi Synthesis of mesoporous silicas with different pore size by using EOmMAn diblock copolymers of
113
tunable block length as the templates Y.-T. Chan, H.-P Lin, C.-Y Mou and S.-T. Liu Polypropylene glycol as a swelling agent for the synthesis of mesoporous silica (SBA-15) by
117
amphiphilic block copolymer templating X. Cui, J.-H. Ahn, W.-C Zin, W.-J. Cho and C.-S. Ha Thermal decomposition-precipitation inside the nanoreactors high loading of W-oxide
121
nanoparticles into the nanotubes of SBA-15 L. Vradman, Y Peer, A. Mann-Kiperman and M. V. Landau Phase transition of SBA-1 induced by embedded heteropoly acids
125
S.H. Lim, H. Yoshitake and T. Tatsumi A further investigation on effect of basic media on the synthesis of MCM-41
129
C Yang, S. Ge and N. He Cationic templating with organic counterion for superstable mesoporous silica
133
P. Reinert, B. Garcia, C Morin, A. badiei, P. Perriat, O. Tdlement and L. Bonneviot. The synthesis of mesoporous materials with semicrystalline microporous walls
137
S.L Cho, YK. Kwon, S.-E. Park and G-J. Kim Synthesis of a mesoporous molecular sieve with hydrothermal stability
141
YK Kwon, G-J. Kim, J.H. Lim, D.H. Kim and B.D. Choi Diffusive characterization of large pore mesoporous materials with semi-crystalline zeolitic
145
framework H. V. Thang, A. Malekian, M. Eic, D. Trong On and S. Kaliaguine Synthesis of cubic mesoporous aluminosilicates with enhanced acidity
149
G Li, Q. Kan, T. Wu, C. Hou, F.-S. Xiao and J. Huang Synthesis and characterization of supersurface MCM-41 zeolite using additives
153
C.-M. Song Z.-F. Yan and H.-P Wang Preparation of large pore high quality MCM-48 silica by a simple post-synthesis hydrothermal
157
treatment J. Sun and M.-O. Coppens Synthesis and properties of aluminosilicate mesoporous material with adjustable pore structure
161
Y Zhang D. Wu, YH. Sun, S. Y Peng D. Y Zhao, Q. Luo and F Deng Variation of the pore properties of mesoporous silica after washing by water and ethanol-water
165
solutions L. Pasqua, F. Testa, R.Aiello, F. Di Renzo and F. Fajula Sythesis of ordered lamella mesophase from helix layered silicate (HLS)
169
M.-G. Song, J.-D. Kim and Y. Kiyozumi Sythesis of monolithic nanostructured silicate family materials through the lyotropic liquid
173
crystalline mesophases of non-ionic surfactant S.A. El-Safty and T. Hanaoka Synthesis and characterization of a new mesoporous molecular sieve
177
Q. Liu, C. Han, W. Sun, J. Yang and Y Zhou Direct- and post-hydrothermal treatments in ammoniated solution for the morphogenesis of
181
mesoporous silica nanotubes Z.-Y Yuan, B.-L. Su and W. Zhou Generalized homogeneous precipitation method for precisely controlled synthesis of mesoporous
185
silicas J. Rathousky and A. Zukal Sythesis of mesoporous silica particles preapared by using multiple emulsion
189
C Oh, J.-H. Park. S-i. Shin and S-G Oh Preparation and characterization of mesoporous silica spheres by polymerization induced colloid
193
aggregation method C.I. Lee, S. W. Lee, Y Lee, YH. Chang and YM. Hahm Preparation of mesoporous solids by agglomeration of silica nanospheres Y.K. Ferreira, M. Wallau and E.A.
197
Urquieta-Gonzdlez
Ordered mesostructured materials with composite walls of decavanadate and silica
201
Y-Y Chang, YK. Hwang, H. Choi and Y-U. Kwon Nanoporous alumina formation using mulit-step anodization and cathodic electrodeposition of
205
metal oxides on its structure / Oh, Y Jung. J. Lee and Y Tak Synthesis of mesoporous y-aluminas of controlled pore properties using alkyl carboxylate assisted
209
method Y Kim. C Kim. J. W. Choi. P Kim and J. Yi Synthesis and characterization of mesoporous alumina molecular sieves using cationic surfactants
213
HJ. Kim, H.C Lee, D.H Choo, H.C Lee. S.H. Chung. KH. Lee andJ.S. Lee Synthesis and characterization of mesoporous alumina molecular sieves with cationic surfactants in
217
the presence of formamide H.C. Lee. H.J. Kim. D.H Choo, H.C. Lee. S.H. Chung, KH. Lee andJ.S
Lee
Structure and properties of porous mesostructured zirconium oxo-phosphate with cubic(Ia-Id)
221
symmetry F. Kleitz, S.J. Thomson, Z. Liu, O. Terasaki and F. Schuth Synthesis and characterization of mesoporous titanium oxide
227
J.-L. Tsai, H.-W. Wang andS. Cheng Improvement of thermal stability of Ti-Zr mesoporous oxides using CTAB surfactant templates
231
mixed with auxiliary organic additives W. Li, X. Yang, Y. Zhang and W. Chu Synthesis and characterization of mesoporous zirconia
235
Y-W. Suh, J.-W. Lee and H.-K. Rhee A novel method to prepare mesoporous nano-zirconia
239
X.-M. Liu, M. G.Q. Lu, Z.-F Yan Control of ordered mesoporous molecular sieves synthesis using non-ionic surfactants by
243
incorporation of transition metal ions in the micellar solution A. Leonard, J.L. Blin, G. Merrier and B.-L. Su Texture of chromia aerogels and structure of their nanocrystals
247
M. Abecassis-Wolfovich, H. Rotter, M.V. Landau, E. Korin, A./. Erenhurg, D. Mogilyansky and E. Garshtein Preparation of ordered mesoporous NbTa mixed oxide with crystallized wall
251
T. Katou, B. Lee, D. Lu, J.N. Kondo, M. Hara and K. Domen Compositional effects of bimodal mesopore silica synthesized by a base-catalyzed ambient pressure
255
sol-gel processing X.-Z. Wang. W.-H. Li, T. Dou and B. Zhong A direct template synthesis of highly ordered mesostructured carbons using as-synthesized MCM-
259
48 as template S.B. Yoon, J. Y. Kim, Y.-S. Ahn, H.-S. Kim and J.-S. Yu
II. Characterization Gas adsorption: a valuable tool for the pore size analysis and pore structure elucidation of ordered
263
mesoporous materials M. Jaroniec and M. Kruk Three-dimensional transmission electron microscopy of disordered and ordered mesoporous
271
materials K.P. deJong, A.M. Janssen, P. van der Voort and A.J. Koster Structures of silica-mesoporous crystals and novel mesoporous carbon-networks synthesized within the pores
275
O. Terasaki, Z. Liu, T. Ohsuna, T. Kamiyama, D. Shindo, K. Hiraga, S.H.Joo, T.-W. Kim and R. Ryoo Phase transformations involved during silica, modified silica, and non-silica mesoporous
281
organized thin films deposition. The role of evaporation D. Grosso, E.L Crepaldi, GJ.de A. A. Soler Illia, F. Cagnol, N.Baccile, F. Babonneau, P.A. Albouy, H. Amenitsch and C. Sanchez Comparison of the mechanical stability of cubic and hexagonal mesoporous molecular sieves with
285
different pore sizes M. Hartmann and A. Vinu Fs-time-resolved diffuse reflectance and resonance Raman spectroscopic studies on MCM-41 as
289
microchemical reactor S. Y. Ryu and MJ. Yoon Detailed investigation of the microporous character of mesoporous silicas as revealed by small-
295
angle scattering techniques B. Smarsly, K. Yu and CJ. Brinker X-ray diffraction analysis of mesostructured materials by continuous density function technique
299
LA. Solovyov, O. V. Belousov, A.N. Shmakov, V.I. Zaikovskii, S.H. Joo, R. Ryoo, E. Haddad, A. Gedeon and S.D. Kirik Influence of aluminium, lanthanum and cerium on the thermal and hydrothermal stability of MCM-
303
41-type silicates M. Wallau, R.A.A. Melo and E.A.
Urquieta-Gonzalez
Enhanced acidity and hydrothermal stability of mesoporous aluminosilicate with secondary
307
building units characteristic of zeolite beta W. Guo, L. Kong, C.-S. Ha and Q. Li Chemical coating of the aluminum oxides onto mesoporous silicas by a one-pot grafting method
311
Y.-H. Liu, H.-P. Lin. C.-Y Mou. B.-W. Cheng and C.-F Cheng Acidity and temperature effect on the synthesis of SBA-1 M.-C
315
Liu, H.-S. Sheu and S. Cheng
HMS materials with high Al loading: a joint FT-IR and microcalorimetric study of their
319
acidic/basic properties B. Bonelli, B. Onida, B. Fubini, J.D. Chen, A. Galarneau, FD. Renzo and E. Garrone Effect of cations addition for the highly ordered mesoporous niobium oxide
323
B. Lee, D. Lu, J.N. Kondo and K. Domen Synthesis of zirconium-containing mesoporous silica Zr-MCM-48 membranes with high alkaline
327
resistance for nanofiltration D.-H. Park, H. Saputra, N. Nishiyama. Y. Egashira and K. Veyama Synthesis of siliceous MCM-41 grafted with transition metal carbonyls
331
R.-S. Raul, J.M. Dominguez, R. Feridand T.C. Alvarez Surface and pore structures of CMK-5 ordered mesoporous carbons studied by nitrogen adsorption
335
and surface spectroscopic methods H. Darmstadt, C. Roy, S. Kaliaguine, T.-W. Kim andR. Ryoo A comparison of the sorption properties of mesoporous molecular sieves MCM-41 and MCM-48
339
J.C. Vartuli, W.J. Roth, J.D. Lutner, S.A. Stevenson and S.B. McCullen Argon and nitrogen adsorption on ordered silicas with channel-like and cage-like mesopores:
343
implications for characterization of porous solids M Jaroniec and M. Kruk Mesopores developed in KL zeolite dealuminated with (NH4)2SiF6 solution
347
N. He, C. Yang, J. Tang and H. Chen Time-resolved in situ grazing incidence small angle x-ray scattering experiment of evaporation
351
induced self-assembly A. Gibaud, D. Doshi, B. Ocko, V. Goletto and C.J. Brinker Small angle neutron scattering study on the formation mechanism of mesostructures during sol-gel
355
processing Y.K. Kwon, D.H. Kim, G.-J. Kim, Y.-S. Han and B.-S. Seong Preparation of mesoporous silica anchored Mo catalysts and in-situ XAFS characterization under
359
propene photometathesis reaction N. Ichikuni, T. Eguchi, H. Murayama, K.K. Bando, S. Shimazu and T. Uematsu In-situ XAFS observation of formation of Pd-Pt bimetallic particles in a mesoporous USY zeolite
363
K.K. Bando, T. Matsui, L. Le Bihan, K. Sato, T. Tanaka, M. Imamura, N. Matsubayashi, and Y. Yoshimura Investigation of the internal pore structures of Beta/MCM-41 and ZSM-5/MCM-41 composites by
367
'^^Xe NMR W. Guo, L. Huang, C.-S. Ha and Q. Li Study of chromium species in the Cr-MCM-48 mesoporous materials by Raman spectroscopy
371
C Pak, H.S. Han and G L. Haller Covalent bonding of Disperse Red 1 in HMS silica: synthesis and characterization
375
B. Onida, L. Borello, S. Fiorilli, C Barolo, G Viscardi, D.J. Macquarrie and E. Garrone Accessibility of dye-molecules embedded in the micellar phase of hybrid mesostructured MCM41-
379
type materials B. Onida, B. Bonelli, L. Borello, S. Fiorilli, S. Bodoardo, N. Penazzi, C Otero Aredn, G Turnes Palomino and E. Garrone Influence of surface properties of MCM-48 on the formation of a nanocomposite structure based on MCM-48 and PVA J. He, J. Yang, S. Zhang, D. G Evans, X. Duan
383
A study on the structure of Si-O-C thin films with nano size pore by ICPCVD
387
T. Oh, K.-M. Lee and C.K. Choi Template effects on low k materials made from spin-on mesoporous silica
391
C.-Y. Ting, D.-F. Ouyan, W.-F. Wu and B.-Z. Wan Porosity tuning of single-wall carbon nanohoms with gaseous activation
395
E. Bekyarova, K. Murata, K. Kaneko, D. Kasuya, M. Yudasaka and S. lijima
III. Modification and composite Expanding horizons of mesoporous materials to non-siliceous systems
399
F Schiith, T. Czuryskiewicz, F. Kleitz, M. Linden, A. Lu, J. Rosenholm, W. Schmidt, A. Taguchi Structure and shape control in functional mesostructured materials from block copolymer
407
mesophases U. Wiesner Strategies for spatially separating photoactive molecules in mesostructured sol-gel silicate
films
413
R. Hernandez, P. Minoofar, M. Huang, A.-C. Franville, S. Chia, B. Dunn andJ.L Zink Design of supported catalysts by surface functionalization of micelle-templated silicas
419
D. Brunei, AC. Blanc, P.-H. Mutin, O. Lorret, V. Lafond, A. Galarneau, A. Vioux and F. Fajula Proteosilica-mcsopoTous silicates densely filling amino acid and peptide assemblies in their
427
nanoscale poresK. Ariga, Q. Zhang, M. Niki, A. Okahe and T. Aida Counteranion effect on the formation of mesoporous materials under acidic synthesis process
431
S. Che, M. Kaneda. O. Terasaki and T. Tatsumi Influence of alumination pathway on the steam stability of Al-grafted MCM-41
435
R. Mokaya Macroporous titanium oxides: from highly aggregated to isolated hollow spheres
439
P. Reinert. C Graillat, R. Spitz and L. Bonneviot Nanostructured mesoporous Ti02, Zr02 and Si02 synthesis by using the non-ionic Cm(EO)n -
443
inorganic alkoxyde system : toward a better understanding on the formation mechanism J. L. Blin, A. Leonard, L. Gigot, O. Provoost and B. L. Su Morphology control of hierarchically ordered ceramic materials prepared by surfactant-directed
447
sol-gel mineralization of wood cellular structures Y. Shin, L.-Q. Wang, J.H. Chang, WD. Samuels. GJ. Exarhos A NH3-responding material based on Reichardt's dye-impregnated mesoporous silica
453
B. Onida, S. Fiorilli, R. Gobetto, A. Russo, D.J. Macquarrie and E. Garrone Preparation and redox behavior of ordered porous zirconium oxide loaded with cerium
457
H.-R. Chen, J.-L Shi, J.-N. Yan, H.-G. Chen and D.-S. Yan Direct synthesis of bi-fimctionalized organo-MSU-X silicas
461
Y Gong, Z. Li, D. Wu, Y. Sun, B.H. Dong, F. Deng High-density modification of mesoporous silica inner walls with amino acid function by residue
465
transfer from template Q. Zhang, K. Ariga, A. Okabe and T. Aida The synthesis of optically active amino acid over Pd catalysts impregnated on mesoporous support
469
K.H. Chang, YK. Kwon and G.-J. Kim Sulfonic acid-functionalized periodic mesoprous organosilicas
473
S. Hamoudi and S. Kaliaguine Functionalized periodic mesoporous organosilicas with sulfonic acid group
477
X. Yuan, H.I. Lee, J. W. Kim, J.E. Yie andJ.M. Kim Exclusive incorporation of aluminum into tetrahedral site of the framework of periodic mesoporous
481
organosilica S.S. Park, J.H. Cheon and D.H. Park Functionalization of hexagonal mesoporous silica and their base-catalytic performance
485
C Yang X. Jia, Y Cao and N. He Microstructure of the organo-modified SBA-15 (Vinyl-SBA 15) prepared under different pH
489
B.-G Park, J. Park, W. Guo, W.-J. Cho and C-S Ha Surface coating of MCM-48 via a gas phase reaction with hexamethyldisilazane (HMDS)
493
A. Daehler, ML. Gee, F. Separovic, G W. Stevens and A.J. O'Connor Reacitvity of silica walls of mesoporous materials towards benzoyl chloride
497
L. Pasqua, F. Testa and R. Aiello Catalytic activitiy of chiral phosphinooxazolidine ligands immobilized on SBA-15 for the
501
asymmetric allylic substitution PH. Chong, YK. Kwon. C Y Lee and G.-J. Kim Preparation of guanidine bases immobilized on SBA-15 mesoporous material and their catalytic
505
activity in Knoevenagel condensation K.-S. Kim, J.H. Song, J.-H. Kim and G Seo MCM-41-supported norephedrine ligand for ruthenium-catalyzed asymmetric transfer
509
hydrogenation of ketones M.-J. Jin, S.-H Kim, S.-J Lee and W.-S Ahn Synthesis of silica support for biocatalyst immobilization
513
J.-K Kim, J.-K. Park and H.-K Kim Mesostructured materials for controlled macromolecular and supramolecular architectures M. Ikegame, K Tajima and T. Aida
517
Nanocomposites of MCM-41 and SBA-15 with polyaniline for electrorheological fluid
523
M.S. Cho, H.J. Choi, K. Y. Kim and W.S. Ahn Functionalized mesoporous adsorbents for Pt(II) and Pd(II) adsorption from dilute aqueous solution
527
T. Kang, Y. Park, J.C. Park, YS. Cho and J. Yi Environmentally benign removal of pollutant oxyanions by Fe adsorption center in functionalized
531
mesoporous silica T. Yokoi, T. Tatsumi and H. Yoshitake How can nanoparticles change the mechanical resistance of ordered mesoporous thin films?
535
E. Craven, S. Besson, M. Klotz, T. Gacoin, J.-P. Boilot and E. Barthel Nanoporous Si02 films prepared by surfactant templating method - a novel antireflective coating
539
technology H.-T. Hsu. C.-Y Ting, C.-Y Mou andB.-Z. Wan Textural and structural properties of Al-SBA-15 directly synthesized at 2.9 < pH < 3.3 region
543
M.S. Mel'gunov, E.A. Mel'gunova, A.N. Shmakov, V.I. Zaikovskii Fabrication of nanostructured SiC and BN from templated preceramic polymers
547
I.-K. Sung, T.-S. Kim. S.-B. Yoon, J.-S. Yu and D.-P Kim
IV. Application and catalysis Mesoporous solids for green chemistry
^^.
/ H. Clark Ultrastable acidic MCM-48-S assembled from zeolite seeds
^^-^
P.-C Shih. H.-P Lin and C.-Y Mou Acidic zeolite coated mesoporous aluminosilicates
^^^
D. Trong On and S. Kaliaguine Stable ordered mesoporous titanosilicates with active catalytic sites
c^r
F.-S. Xiao. Y Han. X. Meng. Y Yu. M. Yang, and S Wu W/Zr mixed oxide supported on mesoporous silica as catalyst for n-pentane isomerization
^^g
T. Li. S.-T. Wong. M.-C. Chao. H.-P Lin. C.-Y Mou andS. Cheng Au and Au-Pt bimetallic nanoparticles in MCM-41 materials: applications in CO preferential
^^^
oxidation S. Chilukuri. T. Joseph. S Malwadkar. C Damle. SB. Halligudi. B.S. Rao, M. Sastry and P. Ratnasamy Effective inclusion of chlorophyllous pigments into mesoporous silica for the energy transfer
^^^
between the chromophores H. Furukawa and K. Kuroda Biological applications of organically functionalised mesoporous molecular sieves and related
t:o^
materials H.H.R Yiu and IJ. Bruce One pot synthesis of mesoporous ternary V205-Ti02-Si02 catalysts
585
V. Pdrvulescu, V.I. Parvulescu, M. Alifanti, S. M. Jung and P. Grange Photocatalytic hydroxylation of benzene on Ti-modified MCM-41 with both framework and non
589
framework Ti- centers Z. Guo, J. He, S. Zhang, D. G. Evans, X. Duan The relationship between the local structures and photocatalytic reactivity of Ti-MCM-41 catalysts
593
Y. Hu, G Martra, S. Higashimoto, J. Zhang, M. Matsuoka, S. Coluccia and M. Anpo Photocatalytic epoxidation of propene with molecular oxygen under visible light irradiation on V
597
ion-implanted Ti-HMS and Cr-HMS mesoporous molecular sieves H. Yamashita, K. Kida, K. Ikeue, Y. Kanazawa, K. Yoshizawa, and M. Anpo Mesostructured Ti02 films as effective photocatalysts for the degradation of organic pollutants
601
J. Rathousky, M. Slabovd and A. Zukal Comparisons of the structural and catalytic properties of Ti-HMS synthesized using the
605
hydrothermal and molecular designed dispersion methods T. Williams and GQ.(Max) Lu Oxidation of methyl-propyl-thioether with hydrogen peroxide using Ti-SBA-15 as catalyst
609
DC. Radu, A. Ion, V.I. Pdrvulescu, V. Cdmpeanu, E. Bartha, D. Trong On and S. Kaliaguine Direct synthesis of hydrothermally stable mesoporous Ti-MSU-G and its catalytic properties in
613
liquid-phase epoxidation P. Wu, H. Sugiyama and T. Tatsumi Mesoporous V-containing MCM-41 molecular sieves: synthesis, characterization and catalytic
617
oxidation C.-W. Chen
andA.-N.Ko
Catalytic oxidation of H2S to elemental sulfur over mesoporous Nb/Fe mixed oxides
621
S.J. Jung, M.H. Kim, J.K. Chung, M.J. Moon, J.S. Chung, D. W. Park and H.C Woo Fe-MCM-41 catalyzed epoxidation of alkenes with hydrogen peroxide
625
Q. Zhang, Y. Wang, S. Itsuki, T. Shishido and K. Takehira Highly selective oxidation of styrene with hydrogen peroxide catalyzed by mono- and
629
bimetallic (Ni, Ni-Cr and Ni-Ru) incorporated MCM-41 silicas V. Parvulescu, C. Dascalescu and B.L. Su Mixed (Al-Cu) pillared clays as wet peroxide oxidation catalysts
633
S.-C. Kim, S.-S. Oh, G.-S. Lee, J.-KKang, D.-S. Kim and D.-K. Lee Finely-dispersed Ni/Cu catalysts supported on mesoporous silica for the hydrodechlorination of chlorinated hydrocarbons
637
Y.G Park, T.W. Kang, Y.-S. Cho, P. Kim. J.-C. Park and J. Yi New SO2 resistant mesoporous mixed oxide catalysts for methane oxidation
641
D. Trong On, S. V. Nguyen and S. Kaliaguine Decomposition of VOCs using mesoporous Ti02 in a silent plasma
645
W.-H. Hong. K.-S. Choi. G-J. Kim andD.-W. Park Preparation of mesoporous 12-tungstophosphoric acid HPW/Si02 and its catalytic performance
649
Z. Zhu. W. Lu and C. Rhodes Catalytic properties of heteropolyacids supported on MCM-41 mesoporous silica for hydrocarbon
653
cracking reactions J. N. Beltramini Preparation, characterization and catalytic activity of heteropolyacids supported on mesoporous
657
silica and carbon Z. Zhao. W. Ahn and R. Ryoo Novel SBA-15 supported heteropoly acid catalysts for benzene alkylation with 1-dodecene H.-O. Zhu. J. Wang. C.-Y Zeng andD.-Y
661
Zhao
Aluminum containing periodic mesoporous organosilicas: synthesis and etherification
665
J.-W. Kim, H.I. Lee. J.M. Kim, X.D. Yuan andJ.E. Yie Friedel-crafts alkylation over Al-incorporated mesoporous honeycomb
669
Y-S. Ahn. H.S. Kim, M.H. Han. S. Jun, S.H. Joo, R. Ryoo and S.J. Cho Heterogenization of AICI3 on mesoporous molecular sieves and its catalytic activity K.-K Kang and H-K
673
Rhee
Asymmetric dihydroxylation catalyzed by MCM-41 silica-supported bis-cinchona alkaloid
677
S.-H. Kim and M.-J. Jin Roles of pore size and Al content on the catalytic performance of Al-MCM-41 during
681
hydrocracking reaction W.-H. Chen. Q. Zhao. S.-J. Huang, C-Y Mou andS.-B Liu HDS of FCC gasoline: Mesoporous modified support catalyst and its effects on the hydrogenolysis
685
reaction selectivity G.H. Tapia, T. Cortez. R. Zarate. J. Herbert and J. L. Cano Synthesis of mesoporous carbon nanotubes and their application in gas phase benzene
689
hydrogenation DC Han. Z.Q. Zhu. A.M. Zhang. J.Z. Zhu andJ.L Dong Characteristics and reactivities of cobalt based mesoporous silica catalysts for Fischer-Tropsch
693
synthesis W.S. Yang. H. W. Xiang, YY Xu and Y.-W Li Physicochemical characteristics of Ti-PILC as a catalyst support for the reduction of NO by NH3
697
H.J. Chae. I.-S. Nam and S.B. Hong Performance of double wash-coated monolith catalyst in selective catalytic reduction of NOx with
701
propene H.-G. Ahn andJ.-D.
Lee
Copper loaded MCM-41. An alternative catalyst for NO reduction in exhaust gases ? - Study of its
705
acidic and redox properties M.S. Batista, M. Wallau, R.A.A. Melo and E.A.
Urquieta-Gonzdlez
Studies on synthesis and activity for selective catalytic reduction of NO over Pt supported MCM-48
709
J.-S.Yang, S.-C. Lee and S.-J. Choung Synthesis of titania-pillared clays and their application as catalyst supports for selective catalytic
713
reduction of NO with ammonia S.~C. Kim, J.-K. Kang, D.-S Kim and D.-K. Lee Hydrogenation of aromatics on Pt/Pd bimetallic catalyst supported by Al-containing mesoporous
717
silica S.-Y.Jeong High loading of short W(Mo)S2 slabs inside the nanotubes of SBA-15. Promotion with Ni(Co) and
721
performance in hydrodesulfurization and hydrogenation L. Vradman, M. V. Landau, M. Herskowitz, V. Ezersky, M. Talianker, S. Nikitenko, Y. Koltypin, A. Gedanken Cr-MCM-41 for selective dehydrogenation of lower alkanes with carbon dioxide
725
Y. Wang, Y Ohishi, T. Shishido, Q. Zhang and K Takehira Methane reforming on molybdenum carbide on Al-FSM-16
729
M Nagai, T. Nishivayashi and S. Omi Preparation of carbided WO3/FSM-16 and Al-FSM-16 and its catalytic activity
733
M Nagai, K. Kunieda, S. Izuhal and S. Omi AMI study on the catalytical isomerization of 1-hexene to 2-hexene on the surface of
737
aluminosilicate molecular sieves M. Pu, Z.~H. Li, S.-R. Zhai, D. Wu, Y.-H. Sun Isomerization and hydrocracking of n-decane over Pt/MCM-41//MgAPO-n composite catalysts
741
S.P. Elangovan and M. Hartmann Catalytic properties of mesoporous aluminosilicates and lanthanum containing mesoporous
745
aluminosilicates studied by m-xylene isomerisation M. Wallau, R.A.A. Melo and E.A.
Urquieta-Gonzdlez
Diels-Alder reaction catalyzed by ordered micro- and mesoporous silicates
749
Y. Kubota, H. Ishida, R. Nakamura and Y. Sugi Isospecific polymerization of propylene with metal-MCM-41
753
W.Z. Shen, J.T. Zheng, Y.L. Zhang, J.G Wang andZ.F. Qin A possibility of block-copolymer templated mesoporous silica films applied to surface photo
783
voltage (SPV) type NOx gas sensor T. Yamada, H.S. Zhou, H. Uchida, M. Tomita, Y. Ueno, Y. Katsube and I. Honma Proton conducting silica mesoporous/heteropolyacid-PVA/SSA nano-composite membrance for
787
polymer electrolyte membrane fuel cell Y.-H. Chu, J.-E. Lim, H.-J. Kim, C.-H. Lee, H.-S. Han and Y.-G Shut Hydrothermal synthesis of titania nanotube and its application for dye- sensitized solar cell
791
S. Uchida, R. Chiba, M. Tomiha, N. Masaki and M. Shirai Preparation of hydrophobic Ti-containing mesoporous silica by the F-modification and their
795
photocatalytic degradation of organic pollutant diluted in water H. Yamashita, H. Nakao, M. Okazaki and M. Anpo Synthesis of functionalised silicas for immobilisation of homogeneous catalysts
799
S.A. Riddel, W.R Hems, A. Chesney andS.R. Watson Author index Subject index Other volumes in the series
811 815
PREFACE The 3rd International Mesostructured Materials Symposium (IMMS 2002) was held successfully in Jeju Island, Korea from July 8 to July 11, 2002 under the auspices of the International Mesostructured Materials Association (IMMA). We would like to express sincere thank all the members of the IMMA council and the International Advisory Board for their active supports for the conference. The attendance in this conference was very encouraging with respect to the futuristic perspective of the scientific field in mesostructured materials and their applications. Four years ago, the first International Symposium on Mesoporous Molecular Sieves (ISMMS) was held in Baltimore. This symposium was followed by the second meeting two years later in year 2000 at Quebec, and the International Mesostructured Materials Association was organized following the success of the Quebec symposium. The title of the symposium was also changed to the International Mesostructured Materials Symposium (IMMS) in order to accommodate the rapidly expanding field of various types of mesostructured materials such as organic polymers, metals, organic-inorganic nanocomposite, and ordered mesoporous carbons. During the 4 day meeting of the IMMS 2002, 5 plenary lectures, 18 keynote lectures, and 25 papers were presented orally in 4 sessions and 174 papers as well as 19 recent research reports were presented as posters. Their topics of the IMMS 2002 covered the followings: synthesis and characterization of periodic mesoporous silicas and other metal oxides organic-inorganic hybrids with mesoscopic periodicity sol-gel approach for mesostructured materials synthesis and applications of mesoporous carbons synthesis of new nanostructured materials using mesoporous templates mesostructured and mesoporous organic polymers, pore size analysis and structure modeling host-guest interaction and molecular imprinting on mesoporous materials
-
catalytic applications of mesoporous materials adsorption and separation using mesoporous materials application of mesostructured materials for optical, electronic, electric, and magnetic
devices. We believe that the IMMS 2002 provided you with the most recent research results and stimulating scientific discussions with opening of novel and diverse mesostructured materials. This book was conceived as the proceedings of the IMMS 2002, which reflect various aspects of synthesis, characterization, and applications of mesostructured materials exhibiting
a mesoscopic periodicity. Recently, mesostructured materials including periodic mesoporous materials have been receiving much attention due to their potential uses in nanotechnology. Actually these materials are considered to be the promising candidates for designing nanoscopically-engineered materials due to their well-defined pore structures, tailor-made synthetic ability, and hosting ability for guest species exhibiting catalytic, optical, electronic and magnetic properties. In these proceedings the reader will indeed find regular papers from many groups worldwide, covering the most recent advances in mesostructured materials to give future perspective of nanotechnology. The organizers wish to express sincere appreciation to attendees of the IMMS 2002 and the authors for submitting their manuscripts to the proceedings. We are grateful to the outstanding scientists who accepted our invitation to overview vital research areas in plenary lectures and the keynote lectures that introduce the important topics of each session covered by the conference. We are also grateful to Prof Chang-Sik Ha, Prof Jong-Sung Yu, Dr. Jong-San Chang, Mr. Sang Hoon Joo, Prof Ji Man Kim, Dr. Sung June Cho and Prof Dong Ho Park who have spent so much time and efforts for the success of the symposium IMMS 2002. Furthermore, we wish to thank members of catalysis center for molecular cngineering(CCME), KRICT, especially Dr. Soo Min Oh, and members of center for functional nanomatcrials, KAIST, who very efficiently helped in the preparation of the proceedings. Finally we wish to acknowledge the help and generous financial support by co-operating organizations and sponsors from industry.
Jcju, July 2003 Sang-Eon Park Ryong Ryoo Wha-Seung Ahn Chul Wee Lee
Organization Organizing Committee Chairman Ryong Ryoo
KAIST, Korea
Co-Chairman Sang-Eon Park
KRICT, Korea
Secretary Wha-Seung Ahn
Inha Univ., Korea
Treasurer Chang-Sik Ha
Pusan National Univ., Korea
Scientific committee Jong-Sung Yu, Chair Dong Ho Park Ji Man Kim Jinwoo Cheon Jong-Ho Kim Kookheon Char Kyung Byung Yoon Seong-Geun Oh Seung-Kyu Park Sung June Cho Taeghwan Hyeon Yong Gun Shul Young-Uk Kwon
Hannam Univ., Korea Inje Univ., Korea Ajou Univ., Korea Yonsei Univ., Korea Chonnam National Univ., Korea Seoul National Univ., Korea Sogang Univ., Korea Hanyang Univ., Korea LG Co., Korea KIER, Korea Seoul National Univ., Korea Yonsei Univ., Korea Sungkyunkwan Univ., Korea
Program committee Chul Wee Lee, Chair Byung-Sung Kwak Chanho Pak Duk-Young Jung Geon-Joong Kim Wha Jung Kim Jung Hwan Park Jong-San Chang
KRICT, Korea SK Co., Korea SAIT, Korea Sungkyunkwan Univ., Korea Inha Univ., Korea Konkuk Univ., Korea Zeobuilder Co., Korea KRICT, Korea
Kwang Ho Park Myongsoo Lee Sang Sung Nam Soon-Yong Jeong Sun Keun Hwang
LG Co., Korea Yonsei Univ., Korea KRIC, Korea KRICT, Korea Aekyung PQ Adv. Mater Co., Korea
International committee Chung-Yuan Mou Dongyuan Zhao Osamu Terasaki Takashi Tatsumi
National Taiwan Univ., Taiwan Fudan Univ., China Tohoku Univ., Japan Yokohama National Univ., Japan
International Advisory Board Abdelhamid Sayari Avehno Corma C. N. R. Rao Charles T. Kresge Francois Fajula G. Q. Max Lu Galen D. Stucky George S. Attard Ilyun-Ku Rhee Jackie Y Ying James C. Vartuli Kazuyuki Kuroda Kenneth J. Balkus Klaus K. Linger Larry Kevan Laurent Bonneviot Masakazu Anpo Michael W. Anderson Mietek Jaroniec Mingyuan He Pierre A. Jacobs Serge Kaliaguine Shilun Qiu Shinji Inagaki Thomas J. Pinnavaia
Univ. Ottawa, Canada Univ. Politencnica de Valencia, Spain Jawaharlal Nehru Centre, India Dow Chemical Co., USA ENSCM, France Univ. Queensland, Australia Univ. California, Snata Barbara, USA Univ. Southampton, UK Seoul National Univ., Korea MIT USA Mobil, USA Waseda Univ., Japan Univ. Texas, Dallas, USA Johannes Gutenberg Univ., Germany Univ. Houston, USA Ecole Normale Superieure de Lyon, France Osaka Prefecture Univ., Japan UMIST UK Kent State Univ, USA Beijing Petro. Chem. Eng. Inst., China Katholieke Univ.. Leuven, Belgium Laval Univ., Canada Jilin Univ., China Toyota Central R&D Labs., Japan Michigan State Univ., USA
Local Advisory Board Baik-Hyon Ha Gon Seo
Hanyang Univ., Korea Chonnam National Univ., Korea
Hakze Chon Seong Ihl Woo Son-ki Ihm Yang Kim Young Sun Uh
KAIST, Korea KAIST, Korea KAIST, Korea Pusan National Univ., Korea Young Lin Instrument Co., Korea
Supporting Organizations Korean Ministry of Science and Technology Korea Advanced Institute of Science and Technology Financial Support The Organizing Committee gratefully acknowledges the receipt of financial support from R&D Center of SK Corp., Korea ATI Korea Co., Ltd. Korea I. T. S. Co., Ltd. Protech, Korea Young Lin Instrument Co., Ltd., Korea
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Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
A new family of organic-bridged mesoporous materials Shinji Inagaki Toyota Central R&D Labs., Inc., Nagakute, Aichi, 480-1192, Japan A new family of hybrid mesoporous materials containing a variety of bridging organic groups inside the pore walls is reported. A short review about the previous studies on the synthesis of organic-bridged mesoporous materials along with our recent research works on the formation of crystal-like ordered structure in the pore walls of organic-bridged mesoporous materials as well as the functionalization of ordered pore-walls of mesoporous benzene-silica with sulfuric acid groups is summarized. 1. INTRODUCTION The periodic mesoporous materials'"^^ have definite advantages of possessing uniform pores whose sizes are larger than those of zeolites, the high stability and the diversity in controlling framework composition and morphologies. The functionality of pore-wall surface was generally poor in previous mesoporous materials because the pore walls are composed of amorphous materials. The amorphous nature of the pore walls also narrows a range of application of the mesoporous materials. Various efforts have been made for the functionalization of pore walls of mesoporous materials by different approaches such as introduction of organic groups in the framework^"" ^ and crystallization of pore walls.'^''^^ The mentioned -Si(0R)3 approaches would be indeed {R0)3Sieffective to improve the functionality of pore walls. The
organic-bridged
mesoporous material has uniformly distributed orga
Fig.l Synthesis of organic-bridged mesoporous material and the pore-wall structure.
nic and inorganic moieties in the framework that are covalently bonded to each other and form stable framework (Fig.l). The materials have been synthesized from pure organosilane precursors (100%) having two or more silyl groups attached to the organic groups in the presence of surfactants. The organic-bridged mesoporous materials are quite distinct from conventional organic-grafted mesoporous materials having terminal organic groups in the pore space. In other reports, the attempts have been made to crystallize the pore walls of mesoporous materials include the synthesis of mesoporous transition metal oxides such as titania, zirconia with partially crystallized pore walls^^'^"^^ and the synthesis of mesoporous aluminosilicates composed of crystal seeds of zeolite in the framework.^'^'^''^ However, so far we have had no report on periodic mesoporous material possessing ordered structure in the whole region of pore walls. In this article, I, present a short review on the previous studies on the synthesis of organic-bridged mesoporous materials along with our recent works on the formation of crystal-like ordered structure in the pore walls of organic-bridged mesoporous materials as well as the functionalization of ordered pore-walls of mesoporous benzene-silica with sulfuric acid groups. 2. A SHORT REVIEW ON ORGANIC-BRIDGED MESOPOROUS MATERIALS Tabic 1 Hybrid mesoporous materials prepared from 100% of organic-bridged silane. Bridging organic8(-R-) -CHjCHj-
-CHjCHj(Block copolymer) -CH=CH-
-O^
-o^
Mesophases
Authors
2D & 3D-hex. Inagaki'^) Disrodered Stein 2") Cubic(film) Brinker^'^) Cubic Pm-3n Inagak'^' 2D-hex,Cubic Sayari'*** 2D-hex. Park' ) Brinker^^), Char^''), Roziere^^', Froba2«), Burleigh^'^), Jaroniec^"' Stein^') Disordered 2D-hex. Ozin^i) Ozin22) Disordered Inagaki'*2) 2D-hex. 2D-hex.
Inagaki^')
Disordered
Ozin22)
2D-hex.
Ozin2^)
2D-hex.
Ozin2^)
(PPO-PEO-PPO)
^Q<
-0^
x>^
The
synthesis
of
organic-bridged
mesoporous materials from the pure organic-bridging precursors (100%) is summarized in Table 1. We were the first to reported the synthesis of organic-bridged mesoporous materials from ethane-bridged precursor using alkyltrimethylammonium as a surfactant (Fig. 2).^^'*^^ Control of the synthesis temperature and alkyl-chain length of surfactant resulted in the formation of three mesophases of two- and three-dimensional hexagonal and cubic Pm-3n with highly ordered structures. The ethane-bridged mesoporous materials
showed
single
crystal-like
well-defined particle morphologies of hexagonal rod, spherical, and decaoctahedral shapes, whose shapes reflect the underlying the pore-arrangement symmetries.^^'^"^^ After our
(MeO)3Si-CH2CH2-Si(OMe)3
% NaOH/HgO Ci8H37N(CH3)3CI
Fig. 2 The first synthesis of organic-bridged mesoporous material.^^^ publication, Stein's and Ozin's groups reported the similar organic-bridged mesoporous materials including ethylene^°'^^\ benzene^^\ thiophene^^\ methane^"'^ toluene^"^^ etc. Drinker's group reported thin films of ethane-bridged mesoporous materials and spherical particles of benzene-bridged mesoporous materials by evaporation-induced self-assembly method.^^^ Recently, several groups described the synthesis of ethane- and benzene-bridged mesoporous materials with large pore size and wall thickness using nonionic triblock coplymers.^^-^^^ Table 2 lists the synthesis of bifunctionalized mesoporous materials by co-condensation of bridging and terminal (or TEOS) precursors. The co-condensation approach resulted in the synthesis of various mesoporous materials containing both of bridging organic moieties inside the walls and terminal groups protruding into the channel space.^^'"^''^ The bifunctional mesoporous materials have unique structure in which bridging organics play a structural and Table 2 Hybrid mesoporous materials mechanical role while the terminal groups are prepared from the mixtures of organicbridged and terminal(or TEOS) silanes. readily accessible for chemical transformation. Bridging organics(-R-)
Terminal organic(-R')
Alvaro et al. reported a mesoporous material containing viologen units in the framework by Ozin^2) -CH=CH- + -CH=CH2 co-condensation with TEOS."'^^ The pore walls -CHjCHj- + Burleigh^^ ^^) of the mesoporous material should show 'N/^NH^NH2 unique optoelectrical properties because viologenes are the most widely used electron acceptor units in a variety of charge transfer complexes and electron transfer processes. The bridged mesoporous materials containing Garcia"^^) + TEOS amine complexes in the framework have been Corriu^^) + TEOS also synthesized by co-condensation with » Mercier^**) + T E O S TEOS.^^'^^^ The reports also exist on the 'S/^N'^NA/^ organic-bridged mesoporous materials Inagaki'^'*) (-SO3H) incorporating Al and Ti in the framework.^'^ '^"^ By combining these previous synthesis approaches it is possible to design unique mesoporous catalyst containing hydrophobic and hydrophilic sites, acid sites and organic functional sites.
-O-O"
-o-
--0
Authors
2. FORMATION OF CRYSTAL-LIKE PORE WALL STRUCTURE The crystal-like periodic structure in the pore walls was first observed for benzene-bridged mesoporous material prepared under the controlled synthesis conditions."^^^ The benzene-bridged mesoporous material was synthesized by condensation of 100% of l,4-bis(triethoxysilyl)benzene [(C2H50)3Si-C6H4-Si(OC2H5)3, BTEB] in the presence of octadecyltrimethylammonium chloride surfactant under basic condition. The ^^Si and '^C NMR study revealed that the condensation 9000 reaction of the silylbenzene precursor proceeded ideally to form benzene-silica hybrid structure in the framework without any Si-C bond cleavage during the synthesis process. X-ray diffraction of the benzene-bridged 30 40 26 (degree) mesoporous material showed several Fig. 3 XRD pattern of benzene-bridged remarkable sharp reflections of d=7.6, 3.8, mesoporous material (surfactant free).^'* 2.5 and 1.9 A at medium-scattering angles in addition of low angle reflections of d= 45.5, 26.0 and 22.9 A due to the hexagonal mesostructure (Fig. 3). The medium angle reflections that never been observed for previously reported conventional mesoporous materials, were assigned as a lamellar structure with a basal spacing of 7.6 A. Transmission electron microscopy image showed that many lattice
^;..V
;<••
y^-'
KOMm Fig. 4 TEM image and periodic model of pore wall of benzenebridged mesoporous material."^^^
fringes with a basal spacing of 7.6 A exist on the pore walls of mesoporous materials (Fig. 4). The lattice fringes are stacked along the channels axes on the walls over the whole region. Electron diffraction spots corresponding to 7.6 A periodicity and its higher order are observed perpendicular to the diffraction spot owing to the mesopore arrangement with a=52.5 A. These results clearly show that the benzene-bridged mesoporous material has a lamellar structure with a basal spacing of 7.6 A inside the pore walls. Figure 5 shows a structural model of pore surface of benzene-bridged mesoporous material and the enlarged picture of pore
wall region. Benzene rings are aligned in a circle around the pore, fixed at
both sides by
silicate chains. The silicate is terminated by silanol (Si-OH) at the surface. Hydrophobic benzene layers and hydrophilic silicate layers array alternatively at the interval of 7.6 A along the channel direction. The periodic surface structure is a great advantage of orientation of guest molecules and clusters, which results in increasing selectivity and activity in catalysis and enhancing the physical properties of guest molecules and clusters. Moreover, the arrangement of organic molecules such as benzene with n electron in the walls has a potential to exhibit unique electrical and optical properties at the porous framework of mesoporous material. Fluorescence spectra indicated some interaction between benzene rings in the walls. In case, if the intermolecular distance of benzene rings 4.4 A in the benzene-bridged mesoporous material could be closer, 7t-7i conjugation of the rings would render the porous framework conducting. The benzene-bridged mesoporous materials also showed excellent thermal (500 °C) and hydrothermal stability.
Benzene layer Silicate layer
Fig. 5 a) CG images of ordered surface structure of benzenebridged mesoporous material and b) the enlarged image of pore wall."^^^
The periodic structure in the walls of benzene-bridged mesoporous material is generated by the self-organization of benzene-containing organosilane precursor due to the hydrophilic-hydrophobic interaction. Similar molecular-scale periodicity has been observed for the other organic-bridged mesoporous materials including biphenyl (-C6H4-C6H4-), 1,3-benzene and ethylene (-CH=CH-) bridging organics. The periodicities measured from XRD were 11.72, 7.64 and 5.6 A for biphenyl, 1,3-benzene and ethylene, respectively. The periodicity increased with increasing molecular length of organic groups, suggesting the validity of the lamellar structure with the organics as a pillar of the walls. While, ethane and methane-bridged mesoporous materials show no molecular-scale periodicity although they show highly ordered mesostructures. This indicates that strong hydrophobicity (or interaction) of organic groups is important to induce the organization of organosilane molecules to lamellar structure in the walls. The molecular-scale periodicity was not observed for the amorphous benzene-silica and other organic-inorganic hybrid materials produced by sol-gel methods without using surfactants."^^'"^^^ This suggests that the self-assembly of the surfactant and the resulting mesoctructure promote the formation of the
molecular-scale periodic structure. 4. FUNCTIONALIZAITION OF PERIODIC WALLS WITH SULFONIC GROUPS We have also attempted to design a highly functionalized acid-catalyst by introducing sulfonic groups (-SO3H) onto the periodic pore wall surface of mesoporous benzene-silica. Sulfuric acid (H2SO4) is one of the most frequently used acid-catalyst for chemical processes. However sulfuric acid has a drawback because it is a toxic liquid and as a liquid catalyst it is very difficult to be separated from products and reused. Therefore, the fixation of the sulfuric acid to a solid has been one of the most important subject to establishment of the eco-friendly chemical processes. We have successfully developed the two kinds of sulfuric-acid functionalized mesoporous benzene-silicas. The first is the mesoporous benzene-silica with sulfonic groups directly attached on phenylene groups in the walls."^^^ The sulfonation was carried out on the mesoporous benzene-silica using fuming sulfuric acid at 105-110 °C. In spite of the severe condition for the treatment, the mesoscopically ordered structure and the molecular-scale periodicity of the mesoporous material was preserved after the treatment. The content of sulfonic groups in the mesoporous material was 0.4 meq./g, which was estimated by the titration with sodium hydroxide solution. This indicates that approx. 10% of phenylene groups in the walls of mesoporous benzene-silica were attached with sulfonic groups (Fig. 6). The mesoporous material should show strong acid catalysis due to the molecular structure of sulfonic groups directly bonded to phenylene and the periodic structure of the walls. The sulfonic groups were kept with pore walls at the high temperature under 480 ' ^ ^ . "C. The excellent thermal stability allows us to apply this material as catalyst for both liquid and gas phase reactions. The other material we have developed is the mesoporous benzene-silica with propylsulfonic groups (-C3H6-SO3H) attached on the hydrophilic silicate layers (Fig. 7).'^'^^ The material was synthesized in two steps; first the synthesis of mesoporous
Fig. 6 CG image of pore surface of sulfuric acid-functionalized mesoporous benzene-silica . About 10% of phenylene groups in the walls are sulfonated."^^^
benzene-silica with mercaptopropyl groups (-C3H6-SH) at the silicate layers, and followed by the oxidative transformation of thiol groups (-SH) to sulfonic groups. The mercaptopropyl-functionalized mesoporous benzene-silica was synthesized by co-condensation of BTEB and 3-mercaptopropyltrimethoxysilane [(CH30)3Si-C3H6-SH, MPTMS] showed the similar molecular-scale periodicity (7.6 A) as observed for the mesoporous benzene-silica."^^^ Incorporation of mercaptopropyl groups in the framework could be confirmed by ^^Si and ^^C NMR
HH
Fig. 7 Structural i m a g e of bifunctional m e s o p o r o u s b e n z e n e - s i l i c a attached with p r o p y l s u l f o n i c acid g r o u p s on the silicate layers of ordered pore surface."^"^^
and IR measurements. The results clearly indicated that the mercaptopropyl groups are attached on the silicate layers of mesoporous benzene-silica. The maximum contents of mercaptopropyl groups in the mesoporous materials was 1.68 mmol/g. Subsequent oxidation treatment with HNO3 resulted in the formation of sulfonic groups. The conversion of oxidation of-SH to -SO3H was 41.7%, indicating the maximum acid amount of 0.70 mmol/g. Thus, in the material the catalytic acid sites and hydrophobic benzene sites are separately designed and apart from each other on the mesoporous surface. Indeed, such dimensionally designed surface structure is a better catalytic environment. 5. CONCLUSION The organic-bridged mesoporous material is a very interesting system because (i) chemical, electrical and optical functionalities could be integrated within the framework of mesoporous materials preserving highly ordered mesostructures, (ii) hierarchically ordered mesoporous material with crystal-like pore-walls can be synthesized by incorporating interactive bridging organic groups inside the pore walls, and (iii) organic groups within the pore walls can be modified by attaching with functional groups and further chemical transformation. REFERENCES 1. T. Yanagisawa, T. Shimizu, K. Kuroda and C. Kato, Bull. Chem. Soc. Jpn., 63 (1990) 988. 2. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck, Nature, 359, 710(1992); J. S. Beck et al., J. Am. Chem. Soc, 114 (1992) 10834. 3. S. Inagaki, Y. Fukushima, K. Kuroda, J. Chem. Soc, Chem. Commun., (1993) 680.; S. Inagaki, A. Koiwai, N. Suzuki, Y Fukushima, K. Kuroda, Bull. Chem. Soc Jpn., 69 (1996)1449. 4. R Yang, D. Zhao, D. I. Margolese, B. R Chmelka, G. D. Stucky, Nature, 396 (1998) 152. 5. M. J. MacLachlan, N. Coombs, G. A. Ozin, Nature, 397 (1999) 681.
6. U. Ciesla, S. Schacht, G. D. Stucky, K. K. Unger, F. Scuth, Angew. Chem. Int. Edn Engl., 35 (1996) 541. 7. G. S. Attard et al., Science, 278 (1997) 838. 8. R. Ryoo, S. H. Joo, M. Kruk, M. Jaroniec, Adv. Mater., 13 (2001) 677. 9. J. Y. Ying, C. P. Mehnet, M. S. Wong, Angew. Chem. Int. Ed., 38 (1999) 56. 10. A. Stein, B. J. Meldw, R. C. Schroden, Adv. Mater., 12 (2000) 1403. 11. A. Sayari, S. Hamoudi, Chem. Mater., 12 (2001) 3151. 12. R Yang, D. Zhao, D. I. margolese, B. F. Chmelka, G. D. Stucky, Nature, 396 (1998) 152. 13. B. Lee, D. Lu, J. N. Kondo, K. Domen, Chem. Commun., (2001) 2118. 14. Y. Liu, W. Zhang, T. J. Pinnavaia, J. Am. Chem. Soc, 122 (2000) 8791. 15. Z. Zhang, Y. Han, F-S. Xiao, S. Qiu, L. Zhu, R. Wang, Y. Yu, Z. Zhang, B. Zou, Y. Wang, H. Sun, D. Zhao, Y Wei, J. Am. Chem. Soc, 123 (2001) 5014. 16. S. Inagaki, S. Guan, Y Fukushima, T. Ohsuna, O. Terasaki, J. Am. Chem. Soc, 121 (1999) 9611. 17. S. Guan, S. Inagaki, T. Ohsuna, O. Terasaki, J. Am. Chem. Soc, 122 (2000) 5660. 18. S. Hamoudi, Y Yang, I. L. moudrakovski, S. Lang, A. Sayari, J. Phys. Chem. B, 105 (2001) 9118. 19. S. S. Park, C. H. Lee, J. H. Cheon, D. H. Park, J. Mater. Chem., 11 (2001) 3397. 20. B. J. Melde, B. T. Holland, C. F Blanford, A. Stein, Chem. Mater., 11 (1999) 3302. 21. T. Aefa, M. J. MacLachlan, N. Coombs, G. A. Ozin, Nature, 402 (1999) 867. 22. C. Y Ishii, T. Asefa, N. Coombs, M. J. MacLachlan, G A. Ozin, Chem. Commun., (1999) 2539. 23. T. Asefa, M. J. MacLachlan, H. Grondey, N. Coombs, G A. Ozin, Angew. Chem. Int. Ed., 39 (2000)1808. 24. G Temtsin, T. Asefa, S. Bittner, G A. Ozin, J. Mater. Chem., 11 (2001) 3202. 25. Y Lu, H. Fan, N. Doke, D. A. Loy, R. A. Assink, D. A. La Van, C. J. Brinkcr, J. Am. Chem. Soc, 122 (2000) 5258. 26. E.-B. Cho, K.-W. Kwon, K. Char, Chem. Mater., 13 (2001) 3837. 27. H. Zhu, D. J. Jones, J. Zajac, J. Roziere, R. Dutartre, Chem. Commun., (2001) 2568. 28. O. Muth, C. Schcllbach, M. Froba, Chem. Commun., (2001) 2032. 29. M. C. Burleigh, M. A. Markowita, E. M. Wong, J-S. Lin, B. P Gaber, Chem. Mater., 13 (2001) 4411. 30. J. R. Matos, M. Kruk, L. O. Mercuri, M. Jaroniec, T. Asefa, N. Coombs, G A, Ozin, T. Kamiyama, (). Terasaki, Chem. Mater., 14 (2002) 1903. 31. Y Goto, S. Inagaki, Chem. Commun., (2002) in press. 32. T. Asefa, M. Kruk, M. J. MacLachlan, N. Coombs, H. Grondey, M. jaroniec, G A. Ozin, J. Am. Chem. Soc, 123 (2(X) 1)8520. 33. M. C. Burleigh, M. A. Markowitz, M. S. Spector, B. P Gaber, J. Phys. Chem. B, 105 (2001) 9935. 34. M. C. Burleigh, M. A. Markowitz, M. S. Spector, B. P Gaber, Chem. Mater., 13 (2001) 4760. 35. M. C. Burleigh, M. A. Markowitz, M. S. Spector, B. P Gaber, Langmuir, 17 (2001) 7923. 36. M. Alvaro, B. Ferrer, V. Fornes, H. Garcia, Chem. Commun., (2001) 2546. 37. R. J. P Corriu, A. Mehdi, C. Reye, C. Thieuleux, Chem. Commun., (2002) 1382. 38. K. Z. Hossain, L. Mercier, Adv. Mater., 14 (2002) 1053. 39. K. Yamamoto, Y Nohara, T. Tatsumi, Chem. Lett., (2001) 648. 40. M.PKapoor, A. Bhaumik, S. Inagaki, K. Kuraoka, T Yazawa, J. Mater. Chem., 12 (2002) 3078. 41. S. Inagaki, S. Guan, T. Ohsuan, O. Terasaki, Nature, 416 (2002) 304. 42. D. A. Loy, K. J. Shea, Chem. Rev., 95 (1995) 1431. 43. R. J. P Corriu, Angew. Chem. Int. Edn. Engl., 39 (2000) 1376. 44. Q. Yang, M. Kapoor, S. Inagaki, J. Am. Chem. Soc, 124 (2002) 9694.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. Allrightsreserved
Strategies to Fabricate Large-Pore Three-Dimensional
Mesoporous
Materials with Versatile Applications C. Yu, B. Tian, J. Fan, X. Liu, H. Yang, L. Wang, S. Shen, B. Tu and D. Zhao,* Molecular Catalysis and Innovative Materials Laboratory, Department of Chemistry, Fudan University, Shanghai 200433, P. R. China, E-mail: dvzhao@fudan.edu.cn, Tel: +86-21-656.42036, Fax: +86-21-65641740 Five syntheses strategies have been developed to fabricate large-pore three-dimensional mesoporous materials (3DMMs) with variable compositions, controlled morphologies and high qualities. These strategies include the rational design and carefully choice of 1) surfactant templates; 2) co-solvents; 3) inorganic salts; 4) "acid-base pair" of inorganic precursors; and 5) "hard" template approach. L Introduction Since the pionnering work of the Mobil scientists, [IJ periodic MMs synthesized from supramolecular self-assembly have attracted much attention because of their potential uses in separation, catalysis, and sensors. [2] Recent advance in this research area has extended previous work from silica materials to other compostions, [3] framework with crystalline walls, [4] and the discovery of new applications. [5] It is generally accepted that 3DMMs with linked pore systems exhibit more advantages in mass diffusion and transportation compared with MCM-41 materials pocessing ID channels. However, the preparation of 3DMMs is somehow difficult and the resulted 3DMMs are limited in pore sizes, compositions and structure types. Here we demonstrate five approaches to synthesize highly ordered 3DMMs with large pores (2-22 nm) and extra-large mesotunnels or entrance sizes (up to 10.8 nm); rich structure types {Pm3n, Ia3d, ImSm, FdSm mesostructures); controlled morphologies (single crystal, rod, monolith, and thin film); and designable compositions (silica, carbon, TiP04, AIPO4, NbP04). 2. Syntheses Strategies
• This work was supported by the NSF of China (Grant No. 29925309 and 20173012), Shanghai Nanotech Pormote Center (0152nm029), and State Key Basic Research Program of P. R. China (G2000048001).
10
1) Surfactant templates The amphiphilic surfactants with high charge and large hydrophilic/hydrophobic volume ratio can be used to prepare cubic 3DMMs. We have used tri-head group surfactant amphiphiles [CniH2m+iN^(CH3)2CnH2nN^(CH3)2CsH2sN^(CH3)3-3Br]. (Cm-n-s-1, m=14, 16, 18) as the structure directing agents to synthesize a cubic silica mesostructure (possible space group of Fd3m Q^^^) under a basic condition at low temperature (~17°C). [6] The high charge of the surfactant headgroups greatly enhances the interactions between organic-inorganic species and results in the aggregation of organic-inorganic composites into face centered cubic mesophase. We have also used ternary mixed surfactants to significantly increase the micellar curvature, which led to high quality 3D (PmSn or P63/mmc) mesoporous Si02. [7] The hydrophobic part of surfactants with large molecular weight and low polar structure favors large pores of templated 3DMMs. Highly ordered, hydrothermally stable, caged cubic mesoporous silica structures {Im3m) FDU-1 with unusual large pore size (12 nm) have been synthesized by using commercial block copolymers with poly(butylene oxide) (PBO) as hydrophobic moiety (B50-6600, F.O39BO47HO39). | 8 | 2) Co-solvents A method involved in the introduction of organic co-solvent and high temperature treatment can be used to fabricate highly ordered 3DMMs. 3D mesoporous silica SBA-15 with extra-large mcsotunncls (2 ~ 8 nm) and the average mesostructure of hexagonal symmetry {p6m) has been reported. |9| Large pore (15.4 nm) mesoporous silica SBA-16L (lm3m) with ultra-large window sizes (up to 10.8 nm) have successfully been synthesized by using 1,3,5-trimethylbcnzene (TMB) as co-solvents (Figure la). We show a successful synthesis of large pore (up to 9.5 nm) mesoporous silica MClVl-48 at room temperature under acidic media by using triblock copolymer as a template (Pluronic PI23, F^02oP07()H02()) and the orgcinosiloxanc or organic molecules as additives (Figure 2). [ lOJ
[7-
100 nm
:.^--m[^
100 nm
Figure 1 TFM images of (a) calcined silica SBA-16L and (b) carbon C-SBA-161>.
A fast way of liquid paraffin medium protected solvent evaporation process to prepare large-sized, crack-free silica monoliths with highly ordered mesostructures is also developed. The use of inert liquid paraffin is critically important to protect macro-morphology of the monolith without cracking, and the processing time are greatly reduced (less than 8 hours). The transmission rate of the resulting silica monolith is higher than the normal glass, which may be potentially usefully in optic devices. lOOnm
3) Inorganic Salts Figure 2 TEM image of large pore mesoporous silica
The effect of inorganic salts has been MCM-48 synthesized at room temperature under acidic media by using triblock copolymer PI23 as a template and carefully studied in the synthesis of the organosiloxane or organic molecules as additives. 3DMMs with block copolymers as the structure-directing agents. The influence of ionic strength upon nonionic block copolymer systems is much stronger than that for ionic surfactants, therefore, compared to ionic surfactant templating systems, inorganic salts may play more prominent role in the structure adjustment and morphology control when block copolymers are used as the templates. A "salt effect" in the synthesis of MMs with block copolymers has been proposed. [11] The critical micelle concentration (CMC) and critical micelle tempreture (CMT) can be decreased in the presence of "salting-out" inorganic salts such as NaCl and KCl, thus the self-assembling ability of block copolymer templating systems can be greatly improved. The use of "salting-out" inorganic salts can dramatically widen the syntheses domain (in temperature, surfactant concentration, etc.) and broaden the range of surfactants that can be utilized to produce high quality 3D mesostructures. For example, highly ordered SBA-15 materials can be obtained at temperature as low as 10°C depending on the ionic strength used in the synthesis. Moreover, at relatively high temperatures such as 38°C, the concentration of templates in starting reactants can be lowered in the presence of inorganic salts. This may be quite useful when the price of the templates is expensive. This "salt effect" is much important in the fabrication of 3DMMs, since block copolymers usually have large hydrophilic/ hydrophobic volume ratio and large CMC values, which in turn decreases their ability to obtain composite phase separation even at relatively high temperature. By employing "salting-out" inorganic salts, ordered 3DMMs have been obtained by using 840-2500, F98 and F108 block copolymers as the templates, while only disordered MMs were obtained without inorganic salts in some cases.
12
Inorganic salts are also important in the morphology control of 3DMMs. We have synthesized a large pore (7.4 nm), cubic mesoporous silica single crystals with exclusively uniform rhombdodecahedron shapes (~ 1 jim) and -100% crystal yield by using triblock coploymer as a template. These 12 faces can be indexed to {110} planes. The unit cell is propagated throughout the faceted particles without twinning or apparent dislocations and fault planes, unambiguously confirming that the particles are perfect single crystals. The underlying role of inorganic salts in the morphology control of 3DMMs may be related to the competition between self-assembly free energy and surface tension of colloidal particles grown from solutions. [12] 4) "Acid-Base Pair" The "Acid-Base Pair" presents a new conception to choose the inorganic precursors especially for the synthesis of new MMs with binary-component inorganic frameworks such as metal phosphates. Metal phosphates are one important family of microporous materials that can be used as catalysts, adsorbents and ion exchangers. To date, only a few metal phosphates MMs have been reported and most of them are thermally unstable. The difficulty of such syntheses comes from the competition between cooperative self-assembly of organic species, metal precursors, phosphor precursors (designated as OI1I2) and other processes such as the condensation of inorganic precursors (I1I2, l|li, hh)- I he introduction of binary-component inorganic framework makes the synthesis more difficult and different than that for mesoporous silica systems. Obviously, in the former case, if the condensation rates arc different for two inorganic precursors, i.e., Iili>lil2, or bh^lib, then it can be inferred that the cooperative self-assembly of OI1I2 composites may not be the dominant process in such systems, since the condensation of inorganic species is necessary for the formation of the final mesostructures. Additionally, different nature of metal sources and phosphor sources makes it even more difficult to the homogeneous condensation of multi- components. The "Acid-base Pair" conception is developed in order to rationally design the synthesis of metal phosphates MMs (Scheme 1). As discussed above, by comparing and matching the relative reactivity of metal precursors and phosphor sources as ,.,, viewed as either acid or bases (Iil2>lili or I2I2), ' ••'" ^ the homogeneous condensation of inorganic ' ,'-"*! precursors and further cooperative self-assembly ^ . -^ \ .--'^^''^ of OI1I2 can be established. Furthermore, the , ''' nonaqueous solvents employed during synthesis 100 n m reduce the tendency of inorganic hydrolysis and polymerization (OIil2>lll2>IlIl. h h ) - By using this c o n c e p t , w e h a v e s u c c e s s f u l l y s y n t h e s i z e d
Figure 3 I H M image of large pore mesoporous AIPO, via ealcinations at SOOT.
13 high quality 3D mesoporous AIPO4 (Pds/mmc), NbP04and TiP04 (Ia3d), hexagonal (p6mm) AIPO4, NbP04, TiP04, and Zr04 by using copolymers as the templates. (Figure 3) These 3DMMs with various composition and mesophase are thermal stable up to 700 °C and have uniform large pores (above 9, 5 and 6 nm for mesoporous AIPO4, NbP04 and TiP04, respectively) [7]
Metal Source Metal Aikoxide
0P(0R)3
Metal chloride
(R - CI I3, 03115)
Phosphoric Ester
Phosphoric Acid
Phosphorus Chloride
Phosphor Source Scheme 1. Relative acidity and alkalinity of metal and phosphor precusors and "Acid-base pair" matching in the design of metal phosphate MMs systems. 5) Hard Templates Approach Ryoo and his co-workers firstly reported the synthesis of periodic mesoporous carbon materials. [13] By utilizing mesoporous silica structures as the hard templates, highly ordered large pore 3D cubic mesoporous carbons (ImSm, laSd) with very large surface area (up to 1900 m^g'') and pore volume (up to 2.23 cm^g'') have been synthesized. Highly ordered mesoporous carbon C-SBA-16L (lm3m) has successfully been synthesized from SBA-16L templates (Im3m) with caged structure and enlarged window sizes, indicating that the carbon
14
products are faithful replication of their silica templates without any structural transformation (Figure lb). It is noted that this is a new symmetry in ordered mesoporous carbon materials. By carefully controlling the morphology of SBA-15 templates, we have successfully synthesized highly orderd mesoporous carbon with fibers, rods, plates, donuts, and monoliths morphologies. Such ordered mesoporous carbon materials with uniform morphology, various pore parameter and extra large porosity may have potential use in catalysis, gas separation, chromatography, energy storage and as nano-reactors for hydrophobic precursors. 3. Applications of 3DMMs 3DMMs have been used to produce ordered low-dimesional nano-scale materials such as carbon nanotubes, [14] ordered semiconductor (CdS) nanorod arrays, [15] or uniform Au nanocrystals. [16] They can also be used as the substrates of HPLC to separate large proteins. [17]
REFERENCES [1] C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck, Nature 359 (1992) 710. [2] M. E. Davis, Nature 417 (2002) 813. [3] J. Y. Ying, C. P. Mehncrt, M. S. Wong, Angcw. Chem., Int. Ed. 38 (1999) 56. [41 S. Inagaki, S. Guan, T. Ohsuna, O. Terasaki, Nature 416 (2002) 304. [5] M. Vettraino, X. He, M. Irudeau, J. 1^. Drake, D. M. Antonelli, Adv. Funct. Mater. 12 (2002)174. [61 S. Shen, Y. Li, F. Wu, J. Fan, B. Tu, !•. Tao, D. Zhao, Chem. J. Chin. Univ. 23 (2002) 358. [7] B. lian, D. Zhao, manuscript in preparation. [81 C. Yu, Y. Yu, D. Zhao, Chem. Commun. (2000) 575. [9] J. Fan, C. Yu, L. Wang, B. lu, D.Y. Zhao, Y. Sakamoto, O. Terasaki, J. Am. Chem. Soc. 123(2001)12113. |10]X. Liu, D. Zhao, submitted. [11]C. Yu, B. Tian, J. Fan, G. D. Stucky, D. Zhao, Chem. Commun. (2001) 2726. [12]C. Yu, B. Tian, J. Fan, G. D. Stucky, D.Y. Zhao, J. Am. Chem. Soc. 124 (2002) 4556. [13] R. Ryoo, S. II. Joo, S. Jun, J. Phys. Chem. B, 103(1999), 7743-7746. |14]G. Zheng, H. Zhu, Q. Luo, Y. Zhou, D. Zhao, Chem. Mater. 13 (2001) 2240. [15]F. Gao, Q. Lu, X. Liu, Y. Yan, D. Zhao, Nano Lett. 1 (2001) 743. [16] J. Fan, D. Zhao, manuscript in preparation. [17] J. W. Zhao, F. Gao, Y.L. Fu, W. Jin, P. Yang, D. Zhao, Chem. Commun. (2002) 752.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
15
Mesostructured Solid Acids S. Hamoudi, D. Trong On and S. Kaliaguine* Department of Chemical Engineering, Laval University Ste-Foy, Quebec, GIK 7P4, Canada Fax: 418-656-3810. E-mail: kaliagui@gch.ulaval.ca A review of the main types of mesostructured solid acids synthesized to date is presented, mostly illustrated by developments made in our laboratory. These materials encompass the templated mesostructured silicas (TMS) substituted with trivalent elements, the TMS impregnated or grafted with inorganic acids, the mesostructured acidic phosphates and sulfates, the TMS/zeolite composite materials and the acid functionalized organosilicas. INTRODUCTION The preparation of acidic solids has consistently been a major theme throughout the history of mesostructured materials. Indeed the M41S family of solids was initially discovered in trying to enlarge the pores of zeolites which are mostly used as acidic catalysts. Mesoporous molecular sieves (MMS) by lifting the pore size constraints of zeolites have indeed broadened the scope of zeolites not only as acid catalysts but in other applications of acidic solids such as the ones which require proton conduction. In this presentation we will survey the various approaches to the synthesis of mesostructured solid acids describing in each case the pitfalls and assets of the various products. Substitution of trivalent elements in TMS It was early recognized that substitution of Ap^ in tetrahedral co-ordination in M41S led to weaker Bronsted acid sites than in zeolites and to less hydrothermally stable solids. We contributed to this early development by comparing B- and Al- ZSM-5 with B- and AlMCM-41 [1,2]. We showed that the MCM-41 solids were much less stable than the pentasils and that the order of acid strengths was B-MCM-41 < Al-MCM-41 < B-ZSM-5 « Al-ZSM-5. It was however also shown that the possibility to prepare very weak Bronsted acid sites is
16
of interest in several catalytic applications. For example the Prins condensation of formaldehyde with iso-butylene yielding isoprene is performed with 100% selectivity over B-MCM-41 compared to 6.6% over Al-ZSM-5 in similar conditions [3]. Mesostructured acidic phosphates (and sulfates) There are numerous examples in the literature of mesostructured phosphated oxides including zirconium oxo-phosphate, titanium phosphate as well as mixed TiZr oxophosphate, substituted ALPO's, SnP04, InP04, etc. We have focused our work on BPO4, which is a very strongly acidic solid prepared by co-condensation of boric acid and phosphoric acid. This solid is initially formed as a gel at room temperature. Increasing its calcination temperature from 400 to 900 "C allows to control its acid site density and resistance to dissolution in water. These materials have been extensively studied as catalysts but our interest is in proton conduction properties [4]. We report here the synthesis of mesostructured cubic BPO4 using phosphoric and boric acids as P and B sources respectively and monoalkylamines as templates. These materials collapse upon calcination but it was possible to perform partial template extraction with ethanol or ether. Figure 1 shows the XRD diagrams of a mesostructured BPO4 sample (a) prepared using monooctylamine as the template and of the same sample after partial ethanol extraction
1.-02 I--03
(a)
^'^
l-()4 I-05
(d)
l.-()7 -
water/solid (wt. % )
Fig.l. XRD pattern of (a) assynthesized BPO4 (template C8NH2), (b) partially solvent extracted BPO4.
Fig. 2. Room temperature proton conductivity of (a) cubic as synthesized BPO4, (b) partially solvent extracted BPO4, (c) Zirconium oxophosphate, (d) H-ZSM-5 (Si/Al = 31).
17
(b). In this second sample, TGA analysis showed that only about 25% of the template was extracted. It is seen from Fig.l that both samples display a cubic pore arrangement. Figure 2 compares the room temperature proton conductivity versus water content curves for these two samples with the ones obtained for a ZSM-5 layer and a sample of zirconium oxophosphate. The latter sample was kindly provided by Professor F. Schiith. It was synthesized by post synthesis phosphatation of a zirconium sulfate mesophase followed by calcination. It contains about 10 wt% P and 0.2 wt% of S. It is seen that the proton conductivity of the two BPO4 samples is significantly higher than the ones of ZSM-5 and ZrOP04. It is interesting to note that the BPO4 samples conductivity is less dependent on water content than the other solid acids. This behaviour could be related to a proton hoping mechanism which would not involve electroosmotic drag of water. Such a process would be of real significance in the management of water in the membrane of a PEM fuel cell.
B >
QOH
-. «™ •^
QOBD
5
(KUK
1
n
(KIM
I
00^
J"
Jl before
IT
• ^
1'""" \~ J V i 1"
A
1 C
d n
K.""
'" nil
dp (A)
dp (A)
.•3
befon
ID
Jjlj
beibi
•ft«r
ai
02 0.1 (M 05
07
08
09
LO
Fig. 3. N2 sorption isotherms before and after steaming for 24h at 800°C with 20% water vapor in N2: A) precursor MMS (worm-hole type structure, Si/Al=50), B) ULZSM-5(2) {24% crystallinity) and C) UL-ZSM-5(4) {60% crystallinity) (Inset: BJH pore diameter distribution).
4. TMS/ZEOLITE COMPOSITES The various strategies reported in the literature for the synthesis of materials that are simultaneously mesoporous and zeolitic may be summarized as follows: - Dual templating [5] - Secondary synthesis of MMS from precursor zeolites [6]
18
+ in presence of a liquid phase [7] + by solid phase crystallization [8] - Auto-assembly of zeolite nanocrystals [9, 10] - Coating of MMS with zeolite nanoslabs [11] Our lab has contributed to two of these syntheses namely the solid phase secondary crystallization of zeolites (UL-zeolites) and the coating with zeolite nanoslabs. The second of these two processes will be the object of another presentation in this meeting [II, 12]. Therefore only some new results about UL-zeolites will be discussed here. In Figure 3 the hydrothermal stability of a series of UL-ZSM-5 is presented. These materials were obtained using as a precursor, a mesostructured aluminosilicate (Si/Al=50) prepared by hydrolysis of chlorides in ethanol as described by Stucky et al. [13]. The solidstate crystallization was performed at 130°C in the presence of TPAOH. As shown in Table 1, the mesopore diameter and micropore volume increase regularly with crystallization time as crystallinity raises. After 6 days of crystallization as crystallinity exceeds (79%), the mesopore lattice disappears completely. Figure 4 reports the IR spectra of the materials showing a regular increase in the 550 cm' lines corresponding to the vibration of the pentasil ring. The IR spectra of adsorbed pyridine also indicated the build-up of both strong Lewis and Bronsted acidic sites. Figure 3 shows that the hydrothermal stability increases with 121)0 KMN) K(H) crystallinity to such an extent that the Wavenuniber (cm') 60% crystalline solid shows very little Fig. 4. FT-IR spectra: a) precursor MMS degradation of its pore structure after (Si/Al-50), b) UL_ZSM-5(2) (24% cryst.); c) 24 hours at 800 T under 20% water UL-ZSM-5(4) (60%, cryst.) vapor. In another paper presented at this meeting, we report a systematic study of gas phase diffusivities of probe molecules (nhcptane, toluene etc.) in these solids, measured by the zero length column technique (ZLC). These results indicate a large increase in Den/R^ in the composite materials compared to
19
pure zeolite or silicalite. This is coherent with the picture of the composite materials constituted of nanocrystals of zeolite accessible through large mesopores [14]. Table 1 Physico-chemical characterizations before and after steaming for 24h at 800°C with 20% water vapor in N2 of precursor MMS (Si/Al = 50), UL-ZSM-5 BJH pore Mesopore SBET SBJH Crystallinity Sample (mVg) volume diameter (A) (m'/g) (%) (cmVg) Al-MMS before 840 650 0.62 40 Al-MMS after
645
450
0.53
42
UL-ZSM-5(2) before
790
400
1.98
220
UL-ZSM-5(2) after
640
330
1.40
220
UL-ZSM-5(4) before
690
300
1.43
280
UL-ZSM-5(4) after
550
245
1.24
320
110
0.49
-
UL-ZSM-5(6)
490
24
60
79
Acidic organo-silica hybrids We have already reported the high proton conductivity of the hybrid materials obtained by a procedure initially proposed by Stucky et al [15]. This involves the co-condensation of tetraethylorthosilicate (TEOS) and mercaptopropyltrimethoxysilane (MPTMS) in the presence of Pluronic 123 in acidic conditions, followed by oxidation of the thiol group to a sulfonic acid group, by 33% H2O2 solution. At 10% and 20% nominal content of functionalized Si, the room temperature conductivity exceeded 10'^ Scm' [16]. In the present contribution we compare the conductivity of these materials with the ones of 1/ HMS materials prepared by the above functionalization technique but showing higher acid loading (up to 40% FS) [17] 2/ Entirely new materials produced by co-condensation of 1,2 bis(trimethoxysilyl) ethane (BTME) and MPTMS followed by H2O2 oxidation of the thiol moiety. These sulfonic acid functionalized mesoporous ethane silica materials (SAF-MES) were prepared either in acidic conditions using P-123 (SAF-MES-Al) or Brij-56 (SAF-MES-A2) surfactants or in basic conditions using CTAC (SAF-MES-B).
20
The new materials properties will be described in a separate contribution in this meeting [18]. Table 2 Synthesis conditions for the reported samples. Gel molar composition Material Surfactant SAF-SBA-15
PI 23
SAF-HMS
CA
SAF-MES-Al
PI23
Ageing time (hrs)
100
24
65
72
90
24
60
144
95
24
TEOS: 0.25 MPTMS: 0.02 P123:7.31 HCl:203H2O TEOS: 0.4 MPTMS: 0.31 CA: 7.14 ETOH: 230 H2O 0.75 BTME : 0.25 MPTMS : 0.05 P123 :36HC1: 1000 H2O 0.75 BTME : 0.25 MPTMS : 0.24 Brij : 83 HCl : 9260 H2O 0.75 BTME: 0.25 MPTMS: 0.57 CTAC: 2.36 NaOH : 353 H2O
SAF-MES-A2 Brij-56 SAF-MES-B
Ageing temperature (°C)
CTAC
The synthesis conditions for the reported samples including nature of the surfactant, gel composition, ageing temperature and time, are given in Table 2.
Table 3 summarizes the
structural properties of the SAF materials. Tabic 3 Structural properties of the SAF materials. Material
Surface area ^ (nr/g)
Pore size (nm)
Total pore volume
-SOiH contenl^'^
(cm /g)
(nieq/g)
SAF-SBA-15
521
6.0
0.56
0.92
SAF-IIMS
1124
2.8
0.65
0.74
SAF-MES-Al
522
5.4
0.69
0.62
SAF-MES-A2
1184
3.3
0.64
0.77
882 3.5 SAF-MES-B (IT Acid capacity determined by titration.
0.64
0.53
Figure 5 shows the TGA (under dry nitrogen) and DTG curves for sample MES-A2. Fig. 5a is for the as-synthesized (still containing the template) sample.
It shows that the Brij-
56 template is desorbed simultaneously with the decomposition of the mercaptopropyl
21
moiety. This is confirmed by the TGA curve of the extracted sample shown in Fig. 5b. Fig. 5c which corresponds to the extracted and oxidized sample (SAF-MES-A2) shows a peak between 400 and 520 °C. A peak observed in the same temperature range by stucky et al. [15] during the decomposition of SAF-SBA-15 was ascribed by these authors to the thermal desorption of the -C3H6-SO3H moiety. It is therefore apparent that in the SAFMES materials the thiol moieties which yield the peak at 350 °C are only partially oxidized. The organic fraction of the ethane silica walls is progressively decomposed between 520 and 750 °C. Figure 6 represents a comparison of the room temperature proton conductivity for the SAF materials described in Table 3. The high conductivity of these materials is obviously 100
90
1 8.. *
70
60
=::,^
100
y
\\ /^^"^^^
1 (\ V
1 1 i.
X ^
Temperature ("C)
a. As-synthesized MP-MES-A2
?
90
80
..^
/^^x\
\x \ 1
v
.^"^ ^ v
^--^ Temperature ("C)
Temperature ("C)
b. Solvent-extracted MP-MES-A2
c. SAF-MES-A2
Fig. 5. TGA and DTG profiles of sample MES-A2. related to their high acidity. The three MES samples display the lowest conductivity likely associated with the less hydrophilic character of their surface. The remarkably high conductivity of the SAF-HMS materials is associated with the high acidity and high hydrophilicity of this material. This solid is indeed as acidic as NAFION and almost as conductive as the 85% solution of phosphoric acid, the conductivity of which is 8x10'^ Scm'^ at Water content ( w / w % ) room temperature. Fig. 6. Room temperature proton conductivity as a function of water content.
22
CONCLUSION The mesostructured solid acids encompass a very rich variety of materials with highly diverse properties. In addition to controlling the mesostructured type and mesopore size they also allow to design catalysts with a very large range of acid strength, acid site density, acid site surface environment, acid site accessibility as well as thermal and hydrothermal stability. The hydrothermal stability which was considered a problem up to very recently was shown to be quite significantly improved by some of the new TMS/zeolite composite materials. REFERENCES 1. 1 .D. Trong On, P.N. Joshi, G. Lemay and S. Kaliaguine, Stud. Surf Sci. Catal., 97 (1995) 543. 2. D. Trong On, P.N. Joshi and S. Kaliaguine, J. Phys. Chem., 100 (1996) 6743. 3. E. Dumitriu, D. Trong On and S. Kaliaguine, J. Catal., 170 (1997) 150. 4. S. Mikhailenko, J. Zaidi and S. Kaliaguine, Catal. Today, 67 (2001) 25. 5. A. Karlsson, M. Stocker, R. Schmidt, Micropor. Mesopor. Mater., 27 (1999) 181. 6. Y. Goto, Y. Fukushima, R Ratu, Y. Imada, Y Kubota, Y Sugi, M. Ogura, M. Matsukala, J. Porous Mater., 9(2002)43. 7. K. R. Kloctslra, 11. Van Bckkum, J. C. Janscn, Chem. Commun., 1997, 2281. 8. D. Trong On, S. Kaliaguine, Angew. Chem. Int. Ed., 40 (2001) 3248. 9. Y Liu, W. Zhang, T J. Pinnavaia, J. Am. Chem. Soe. 2000, 722, 8791; Y Liu, W. Zhang, T J. Pinnavaia, Angew. Chem. Int. Ed. 40 (2001) 1255. 10. Z. Zhang, Y Han, L. Zhu, R. Wang, Y Yu, S. Qiu, D. Zhao, F.-S. Xiao, Angew. Chem. Int. Ed. 40 {200\) 1258. 11. D. Trong On, S. Kaliaguine, Angew. Chem. Int. Ed., .,41 (2002) 1036. 12. D. Trong On, S. Kaliaguine, this meeting. 13. P Yang, D. Zhao, D. 1. Margolese, B. F. Chmelka, G. D. Stucky, Nature 396 (1998) 152. 14. V. T Uoang, A. Malekian, M. Hie, D. Trong On, S. Kaliaguine, this meeting. 15. D. Margolese, J.A. Melero, S.C. Christiansen, B.F. Chmelka and G.D. Stucky, Chem. Mater., 12(2000)2448. 16. S. Mikhailenko, D. Desplantier-Giscard, C. Danumah and S. Kaliaguine, Micropor. Mesopor. Materials, 52 (2002) 29. 17. Y Mori and T.J. Pinnavaia, Chem. Mater., 13 (2001) 2173. 18. S. Hamoudi, S. Kaliaguine, this meeting.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
23
Template synthesis and catalysis of metal nanowires in mesoporous silicas Atsushi Fukuoka/'* Yuzuru Sakamoto,^ Hidenobu Araki,^ Noriaki Sugimoto,*' Shinji Inagaki,^ Yoshiaki Fukushima,^ andMasaru Ichikawa^* ^Catalysis Research Center and Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0811, Japan Toyota Central R&D Labs., Inc., Nagakute, Aichi 480-1192, Japan Several monometallic and bimetallic nanowires of transition metals were synthesized by a photoreduction method using mesoporous silicas FSM-16 and HMM-1 as templates. The metal nanowires were characterized by physicochemical methods such as TEM, XRD, XAFS, XPS, IR, and gas adsorption. Mechanism for formation of Pt nanowires is discussed in terms of relative rates of migration and reduction of Pt ions in the one-dimensional channels. Dissolving silica matrix with HF gave unsupported nanowires, and in the presence of surfactants or ligands the nanovsdres were stabilized in solutions. The catalytic performances of Pt nanowires and nanoparticles in FSM-16 were studied in water-gas-shift reaction and hydrogenolysis of butane. It is noteworthy that Pt nanowire/FSM-16 shows higher activities than Pt nanoparticle/FSM-16 in both the reactions. 1. INTRODUCTION Mesoporous silica materials such as FSM-16, MCM-41, SBA-15, and organic-inorganic HMM-l have large pores (2-10 nm) and high surface area (up to 1000 m^ g"'), thus providing great opportunities in separation, catalysis, and sensors. They are also attractive hosts to confine metal complexes, nanoparticles, and nanowires. In particular, metal nanowires have attracted recent interest in terms of nanotechnology, because they are expected to show unique physical and chemical properties based on the quantum-size effect and the low-dimensionality. The well-defined nanocomposites are also expected to show improved catalytic activity and selectivity different from those of conventional supported metal catalysts. Among various preparation methods of the nanowires, one of the most promising methods is the template synthesis using porous materials as hosts. Our interest has been in the ship-in-bottle synthesis of uniform metal clusters and particles in microporous zeolites and mesoporous silicas. In the course of the study, we found that Pt nanowires were formed by the photoreduction of H2PtCl6 in FSM-16 [1,2]. Here we have investigated preparation, characterization, and formation mechanism of metal nanowires in FSM-16, HMM-1, and mesoporous thin film. As an application of the nanowires in heterogeneous catalysis, performances of metal nanowires and nanoparticles have been studied using water-gas-shift reaction and hydrogenolysis of butane as test reactions.
24
2. EXPERIMENTAL FSM-16 [3] and HMM-1 [4] were prepared according to the published methods: FSM-16, BET surface area 950 m^ g"' and pore diameter 2.7 nm; HMM-1 812 m^g"' and 3.1 nm. Mesoporous silica film (pore diameter 2.7 nm) was prepared on a Si wafer under acidic conditions [5]. The channels run parallel to the Si surface. HMM-1 with CH2CH2 groups in the framework is thermally stable up to 523 K; thus the thermal treatment was performed below 473 K. In a typical preparation of Pt nanowires, dry FSM-16 was impregnated with H2PtCl6*6H20 (metal loading 5 wt%), and vapors of water (ca. 20 Torr) and methanol (ca. 100 Torr) were adsorbed in the impregnated sample. Then UV light was irradiated to the sample for 48-72 h with a high-pressure mercury lamp (100 W, 250-600 nm). Hydrogen reduction of H2PtCl6/FSM-16 was done in flowing H2 at 673 K for 2 h. Structural characterization was performed by TEM, XRD, XPS, XAFS, IR, and gas adsorption. Water-gas-shift reaction was performed in a closed circulation system, and hydrogenolysis of butane in a fixed bed flow system. 3. RESULTS AND DISCUSSION I H20/MeOH r UV, room temp.
i
H2, 673 K (or 473 K)
3.L Synthesis and characterization of metal nanowires in FSM-16 and HMM-1 Pt nanowires were formed by the photoreduction of H2PtCl6 in FSM-16 in the presence of vapors of water and methanol, while H2-reduction at 673 K gave nanoparticle-arrays of Pt (Fig. 1) [6-8]. Fig. 2 is a TEM image of Pt nanowire/FSM-16, in which the nanowires are Fig. 1. Template synthesis observed as dark stripes. The diameter of the nanowires is of Pt nanowires and 2.5 nm in accord with the pore diameter of FSM-16 (2.7 nanoparticles. nm), showing that the nanowires are formed inside the 1D channels. The length of the nanowires ranges from 10 to 800 nm. The HRTEM observation of Pt nanowires gave a clear image of (111) planes o^fcc Pt, indicating that the nanowires are single crystals. On the other hand, H2-redcution of H2PtCl6/FSM-16 yielded spherical Pt nanoparticles of 2.5 nm in FSM-16 with a small amount of short nanowires. Therefore, the photoreduction is an effective method to prepare the Pt nanowires. Similar results were obtained when MCM-41 was used as a support. In the XRD study of Pt/FSM-16, no significant change was observed for FSM-16 after incorporation of Pt, confirming the retention of the pore structure. Typical peaks 10 nm f\ of ycc Pt crystalline were observed in the high angle region. In the EXAFS analysis of Pt/FSM-16, the first shell of Pt-Pt was analyzed, and as is expected from the morphology the Fig. 2. TEM image of Pt coordination number of Pt-Pt for Pt nanowire/FSM-16 is nanowires in FSM-16.
25
larger than that for Pt nanoparticle/FSM-16: 10.1 and 5.8. Similar results were obtained in the uptake of H2 and CO: Pt nanowire/FSM-16, H/Pt =0.058 and CO/Pt = 0.080; Pt nanoparticle/FSM-16, H/Pt =0.17 and CO/Pt = 0.19. In the XPS study of Pt nanowire/FSM-16, 4f5/2 and 4f7/2 peaks were detected at 75.0 and 71.7 eV, while Pt nanoparticle/FSM-16 gave the peaks at 74.1 and 71.0 eV similarly as Pt foil (Fig. 3). In the IR of CO adsorption for Pt nanowire/FSM-16 (Fig. 4), a small peak of linear CO was observed at 2080 cm'\ which was shifted to high frequency fi-om the peak for Pt nanoparticle/FSM-16 (2060 cm'^). These results suggest that the surface of the Pt nanowire is slightly Fig. 3. XPS of (a) Pt electron-deficient compared to the Pt nanoparticle and Pt nanowire/FSM-16, (b) Pt nanoparticle/FSM-16, and foil. (c) Pt foil. HMM-1 has a highly ordered 2D hexagonal structure with long one-dimensional channels [4], and the ordered channels are attractive hosts to synthesize nanowires. In fact, long Pt nanowires were formed in HMM-1 by the same photoreduction method (Fig. 5), which is further confirmed by the TEM observation of unsupported nanowires by dissolving silica with HF. The aspect ratio I reaches ca. several hundred to 1000 (diameter 3 nm, length "| 2-3 jLim). Rh and bimetallic Pt-Rh and Pt-Pd were also < rb^ prepared in HMM-1 [9]. The HRTEM and EDX analyses show that the bimetallic nanowires are alloy with high (a) crystallinity. The surface of the metal nanowires in HMM-1 is not straight but curved like an array of Wavenumber / cm' necklaces. In contrast, rod-like nanowires with smooth surfaces are formed in FSM-16, thus implying that the Fig. 4. IR of CO adsorption for necklace-structure is due to weak interaction of the Pt (a) Pt nanowire/FSM-16 and surface with the organic (b) Pt nanoparticle/FSM-16. fragments. When mesoporous thin film was used as templates, uniform nanoparticles of Au and Pt with a diameter of 2.5 nm were synthesized (Fig. 6) [10]. The Au nanoparticles are closely packed in the mesopores, forming an ordered structure in the mesoporous Fig. 6. TEM image of Au Pt networks. Short Au Fig. 5. TEM image nanoparticle arrays in nanowires are also formed nanowires in HMM-1. mesoporous thin film. as a minor species.
26
3.2. Mechanism for formation of Pt nanowires in mesoporous materials The mechanism for formation of Pt nanowires in the photoreduction was studied using TEM and XAFS by varying the irradiation time. The results suggest that Pt chloride ions are reduced on the surface of Pt nanoparticles initially formed in the mesopores. It seems plausible that water and methanol adsorbed in the mesopores work as solvents in the "nanoflask". The Pt ions can migrate in the solution phase in the one-dimensional channels to reach the surface of tiny nanoparticles initially formed in the photoreduction. Then the Pt ions are reduced by hydrated electrons or radicals generated under UV irradiation, leading to the growth of nanowires. In contrast, H2-reduction is faster than the migration of Pt ions, resulting in the formation of Fig. 7 Formation mechanism of Pt nanowires in HMM-1 nanoparticles or short nanowires. by photoreduction. 3.3. Separation of metal nanowires The metal nanowires and nanoparticles were separated from FSM-16 and HMM-1 by dissolving the silica network with diluted aqueous HF solutions and subsequent centrifugation. In aqueous or organic solution, unsupported Pt nanowires are initially stable but aggregated to form big particles in ca. 2 day. However, the nanowires can be stored as solid. In the presence of surfactants or phosphine ligands such as [NR4]C1 and PR3 in the solutions, the unsupported nanowires are well dispersed (Fig. 8). Presumably, the nanowires is stabilized like organosols by the interaction with the ligand. The separation experiments show the formation of longer nanowires in HMM-1 than in FSM-16 (vide supra). Nanoparticles were also isolated from FSM-16 or HMM-1 by the same method. 3.4. Applications of metal nanowires in mesoporous silicas 25 nm Magnetic susceptibility was measured on the samples of Pt nanowires and nanoparticles in FSM-16 at 5-300 K [1]. Pt Fig. 8. Unsupported Pt nanowire/FSM-16 obeyed the Curie-Weiss law below 70 K, but nanowires from HMM-1 there was a deviation from the law above that temperature. This stabilized with fNR4lCl. behavior is different from that of Pt particle/FSM-16, which may be due to the anisotropic orientation of the Pt nanowires and their interaction with the internal surface of mesopores. Magnetization data were also obtained for the Pt-containing nanowires in HMM-1 [9]. The magnetic susceptibilities of the samples seem to obey Curie's law, although some deviation is observed for Pt-Pd/HMM-1 at ca. 60 K. It is interesting to note that Pt-Pd/HMM-1 shows an increase in the susceptibility below 90 K, which is two or three times higher than expected from the simple sum of the values of bulk Pt and Pd. This enhancement is attributable to the low-dimensionality of the metal morphology.
27
Water-gas-shift reaction was used as a test reaction to study the catalytic performances of Pt nanowire/FSM-16 and Pt nanoparticle/FSM-16 (Eq. 1) [6]. The reaction was performed in a closed circulation system, and the conditions were as follows: catalyst 300 mg (5 wt% Pt), initial/7(CO) 200 Torr and;7(H20) 20 Torr, reaction temperature 373 K. CO + H2O
CO2
(1)
H2
The initial rate based on total Pt atoms for Pt nanowire/FSM-16 is three times larger than that for Pt nanoparticle/FSM-16: 3.4 x 10'^ versus 1.2 x 10"^ niolco2 gcat^ h ^ As shown above, the dispersion of Pt nanowire/FSM-16 is smaller than that of Pt nanoparticle/FSM-16. Therefore, these data suggest that the surface of Pt nanowires is more active than that of Pt nanoparticles. We propose that the nucleophilic attack of water (:0H2) to slightly positive carbon of CO adsorbed on Pt (Pt-C^^O^) is enhanced on the electron-deficient surface of Pt nanowire/FSM-16 where the back-bonding fi-om Pt to CO is weak compared to Pt nanoparticle/FSM-16. This is in agreement with the IR data of CO adsorption (Fig. 4).
4
6
Reaction time (h)
Fig. 9 Formation of CO2 m water-gas-shift reaction by Pt/FSM-16.
Hydrogenolysis of butane was also performed using Pt nanowire/FSM-16 and Pt nanoparticle/FSM-16 as catalysts (Eq. 2) [7]. This reaction is known as a structure-sensitive reaction, and methane and propane are formed from the cleavage of terminal C-C bond and ethane from the central C-C bond cleavage. Table 1 summarizes the catalytic results at 606 K. The TOF based on surface Pt atoms for Pt nanowire/FSM-16 is 70.7 h'^ that is 35 times higher than for Pt nanoparticle/FSM-16 in spite of the smaller dispersion. The selectivity of ethane is also enhanced over Pt nanowire/FSM-16 catalyst. This result implies that the morphology of nanowire produces active surface sites. CH3-CH2-CH2-CH3
+
H2
->
CH4
+
CH3-CH3
+
CH3-CH2^H3
(2)
Table 1. Hydrogenolysis of butane by Pt and/or Rh/FSM-16 catalysts.* Catalyst
TOF /h"'' ^ Selectivity /%' A^H M CH4 C2H6 C3H8 i-C4Hio Ptnanowire/FSM-16 4.10(70.7) 0 40 29 31 0 Ptnanoparticle/FSM-16 0.330(1.94) 0 55 0 45 0 a) Conditions: catalyst 190 mg (5 wt%), temperature 606 K, pressure 1 atm, flow rate 100 ml min' , C4H,o:H2 = 1:9, SV = 20000-30000 h'V b) A^H = TOF for hydrogenolysis, A^i = TOF for isomerization. TOFs are calculated based on the total metal atoms, and values in parentheses are based on the surface Pt atoms. c) Product distribution in mol%.
28
4. CONCLUSIONS In this work, we have demonstrated that the photoreduction is a good preparative method to yield long metal nanowires in mesoporous silica templates FSM-16 and HMM-1. Metal nanoparticles are obtained in the thermal H2-reduction of the impregnated samples. HMM-1 gives long nanowires owing to the highly ordered channel structure. In the formation mechanism, the nanowires grow by reducing metal ions on the surface of nanoparticles that are initially formed in the mesoporous channels. The results of catalytic test reactions indicate that nanowires can provide more active sites than nanoparticles. Separation of nanowires and nanoparticles would find opportunities of their utilization as building blocks for nanodevices and catalysts. We are currently studying the large-scale synthesis of nanowires and the catalytic applications of separated nanowires. ACKNOWLEDGMENT This work was financially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan (no. 13650836).
REFERENCES [1]
M. Sasaki, M. Osada, N. Sugimoto, S. Inagaki, Y. Fukushima, A. Fukuoka, and M. Ichikawa, Microporous Mesoporous Mater., 21 (1998) 597 [2] M. Sasaki, M. Osada, N. Higashimoto, T. Yamamoto, A. Fukuoka, and M. Ichikawa, J. Mol. Catal. A, 141(1999)223. [3] S. Inagaki, Y. Fukushima, and K. Kuroda, J. Chem. Soc, Chem. Commun., (1993) 680. [4] S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna, O. Terasaki, J. Am. Chem. Soc, 121 (1999)9611. [5] M. Ogawa, Chem. Commun., (1996) 1149. [6] A. Fukuoka, N. Higashimoto, Y. Sakamoto, M. Sasaki, N. Sugimoto, S. Inagaki, Y. Fukushima, and M. Ichikawa, Catal. Today, 66 (2001) 23. [7] A. Fukuoka, N. Higashimoto, Y. Sakamoto, S. Inagaki, Y. Fukushima, and M. Ichikawa, Microporous Mesoporous Mater., 48 (2001) 171. [8] A. Fukuoka, N, Higashimoto, Y. Sakamoto, S. Inagaki, Y. Fukushima, and M. Ichikawa, Topics in Catal., 18 (2002) 73. [9] A. Fukuoka, Y. Sakamoto, S. Guan, S. Inagaki, N. Sugimoto, Y. Fukushima, K. Hirahara, S. lijima, and M. Ichikawa, J. Am. Chem. Soc, 123 (2001) 3373. [10] A. Fukuoka, H. Araki, Y. Sakamoto, N. Sugimoto, H. Tsukada, Y. Kumai, Y. Akimoto, and M. Ichikawa, Nano Lett., in press.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
29
Mesostructured silica films with crystalline domains and structural features on multiple length scales Yoon-Seob Lee, Jared R. Archer, and James F. Rathman Chemical Engineering Department, The Ohio State University, 140 W. 19^*^ Ave., Columbus, OH 43210-1180, USA. FAX: (614) 292-3769. rathman.l@osu.edu The cooperative self-organization of surfactant molecules with reactive silicate species is a key factor in the synthesis of mesoporous materials. Mesostructured films can be produced by exploiting similar self-assembly phenomena at the surface of a solid substrate in contact with a liquid solution; however, in this approach, the properties of the resulting film are strongly influenced by chemical and physical properties of the solid. Alternately, films can be synthesized at vapor/liquid or liquid/liquid interfaces and then transferred to solid substrates. Confinement of the reaction environment to a fluid/ fluid interface provides an additional level of control over the structural evolution that occurs during the reaction, while avoiding undesired influences from a solid phase. This paper presents two examples of mesostructured silica films synthesized at fluid/liquid interfaces: 1) ultrathin films, produced at a gas/liquid interface, having highly regular stripes on two discrete length scales; 2) relatively thick mesoporous silica/collagen composite films, synthesized at a liquid/liquid interface, that are partially crystalline. 1. INTRODUCTION Langmuir films can be used to prepare ultrathin mesostructured films. In the approach used here, the self-assembly of insoluble amphiphiles at the interface between two fluid phases directs the reaction of silicate species in the subphase near the interface. The resulting solid film is then transferred to a solid substrate by conventional Langmuir-Blodgett deposition. The varied types of patterns that can be attained in Langmuir monolayers can be exploited to produce solid films with a wide variety of structures.' The primary challenges are to preserve the patterned structure when depositing onto the solid, and to minimize the defects in the film over large length scales. Well-defined two-dimensional patterns of organic materials are tunable by controlling surface pressure, temperature, subphase composition, and the external shear flows applied to the subphases."^' ^'"^ Films having regular stripe patterns can be produced by Langmuir-Blodgett deposition of monolayers under very specific conditions. The wavelength of the repeating stripe pattern can be varied from 50 to several hundred nm. Transferring rate and pulling direction strongly affect the stripe geometry. Dynamic instabilities caused by autooscillation of the meniscus during transfer is an important factor. Although the list of interesting and useful applications of mesostructured materials continues to grow, the poor thermal and mechanical stability of many of these amorphous materials limits their usefulness in processes in which the structure would be exposed to high
30
temperature or significant mechanical stress. Efforts to synthesize crystalhne mesostructured sihca, which would be expected to have the same excellent thermal stability as zeolites, have met with limited success. With regard to mechanical stability, one promising strategy is to follow the example presented by naturally-occurring materials, which often have nanosized features on multiple length scales. This multiscale architecture provides better mechanical properties than materials having features of a single, uniform size. 2. EXPERIMENTAL Materials used were l,2-dipalmitoyl-5«-glycero-3-phosphocholine (DPPC, Sigma), tetraethylorthosilicate (TEOS, Aldrich), dodecyl-, tetradecyl-, and hexadecyl-trimethyl ammonium chloride (DTAC, TTAC, CTAC, from Aldrich), «-decyl alcohol (Aldrich), and Type IV acid-soluble collagen from calf skin (Sigma). Films were transferred to borosilicate glass slides or freshly cleaved mica. Ultrathin silica films were synthesized by spreading a monolayer of the lipid DPPC on an aqueous TEOS solution in a Langmuir trough. The zwitterionic phosphocholine groups in the lipid headgroups template the reaction of silicate species in solution directly below the monolayer, resulting in the formation of an ultrathin silica film that is then transferred to the glass substrate by Langmuir-Blodgett deposition. In situ imaging of monolayers was done using Brewster angle microscopy. The monolayer was first compressed to a target surface pressure, stabilized for 10 minutes, then transferred at a constant pulling rate (e.g., 1 mm/min). The LB film was dried at room temperature in air before taking AFM images, acquired using a Nanoscope Ilia controller with a Multimode microscope (Digital Instruments). A different approach was used to synthesize thicker films. A precursor gel was first spread at a liquid/liquid interface and allowed to react for a prescribed period of time, after which the partially-formed wet film was transferred to a solid substrate. This method eliminates structural reorganization within the film caused by contact with a solid surface, and so provides a way to selectively synthesize films of a desired mesostructure (e.g., lamellar, hexagonal, or cubic) on a wide variety of different solids. Formulation of the precursor gel is the key step in this process; films described here were synthesized from a prescursor gel containing a cationic surfactant (DTAC, TTAC, or CTAC), «-decyl alcohol, a biopolymer such as collagen, TEOS, water, benzene, and sodium hydroxide for pH control. Advantages of this approach and specific details of the method are described elsewhere.^ Precursor surfactant-silicate-collagen gels were also prepared for all three surfactant systems studied. Films were characterized using powder X-ray diffraction (XRD) and ^"^Si magic angle spinning (MAS) solid-state nuclear magnetic resonance (NMR) spectroscopy. 3. RESULTS AND DISCUSSION 3.L Ultrathin films: nanoscale double-stripe patterns Surface pressure-area isotherms were measured to compare the behavior of DPPC monolayers on water and on silicate solution (10'"^ M) at room temperature. The surface phase transitions from liquid-compressed (LC) to solid (S) was observed near 35 mN/m for both systems. The surface area per DPPC molecule was significantly higher on silicate solution subphase than on water at the same surface pressure in the LC region, but these values were approximately the same in the S region. Above 35 mN/m the two isotherms become nearly
31
identical and have the same slope in the S region. Increased electrostatic repulsion between head groups of DPPC due to binding of silicate anions is responsible for the increased area per molecule in the LC region. Upon deposition onto a solid substrate, double stripe patterns were observed only for films transferred at 35 mN/m. AFM images for an ultrathin silica film synthesized beneath a DPPC monolayer at surface pressure 35 mN/m and then transferred to borosilicate substrate are shown in Figure 1. This film exhibits an extremely regular and long-range stripe patterning with two "wavelengths": a fine-scale spacing of 3.4 nm super-imposed on a larger-scale spacing of 31 nm. These dimensions can be interpreted by considering the lattice geometry of the polymerized silica network. The role of the DPPC monolayer is important in obtaining films with structural features on multiple length scales; I 12 similar films are not observed when 'Sl^'i'^i^^i^^lR3^^^^S^r^^^!^^i^7i'^'^^ synthesized in the presence of monolayers composed of conventional cationic surfactants. The stripes are oriented perpendicular to the transfer direction, indicating that the formations of these stripes are closely related to the meniscus of the three phase contact line during the 1 \im 50 nm transfer step. Preliminary results show that the width of the larger Fig. 1. AFM height images of ultrathin silica film having stripes were affected by the film regular stripe patterns on multiple length scales. transfer rate, but none of the features of small stripes were affected by varying the transfer rate. 3.2. Composite silica/collagen films with crystalline domains Figure 2 shows XRD and NMR results for the "thick" mesostructured silica film synthesized with cetyltrimethyl ammonium chloride in the presence of collagen. The first five peaks in the low angle region of the XRD clearly identify this as a cubic mesostructure with Pm3n symmetry. The 11 peaks in the higher angle region suggest that the silica walls have crystalline domains. "^^Si CP/MAS NMR spectra of nanostructured silicas that have amorphous silica walls typically show broad peaks in the range -90 to -120 ppm since the resonances from different SiOx(OH)4-x groups are overlapped; however, three well-resolved peaks at -95.0, -101.1, and -112.0 are observed in this spectrum, providing additional evidence of crystalline silica. Crystalline walls should provide much better thermal and mechanical stability. This bio-mimetic synthesis was conducted at room temperature, another advantage compared to the high temperatures often required for the preparation of mesoporous silicas. 26 Silica/collagen films were also synthesized using Fig. 2. XRD pattern for cubic (PmSn) dodecyltrimethyl ammonium chloride and mesostructured silica/collagen composite film. Inset shows ^^Si NMR spectra.
32
tetradecyltrimethyl ammonium chloride surfactants at the same conditions. Despite the changes in the hydrocarbon chain length, all the films synthesized with collagen exhibited the cubic mesostructure with the same PmSn symmetry. The unit cell sizes of the cubic structures varied with increasing length of the surfactant hydrocarbon chain length. This work was supported by the U.S. National Science Foundation and the Petroleum Research Fund of the American Chemical Society.
REFERENCES 1. Lehn, J-M., Ball, P. 'Supramolecular Chemsitry' in ''The New Chemistry Hall, N., Ed., Cambridge University Press, 2000, 300. 2. Viswanathan, R., Madsen, L.L., Zasadzinski, J.A., Schwartz, D.K. Science, 1995, 269, 51. 3. Takamoto, D.Y.; Aydil, E.; Zasadzinski, J.A.; Ivanova, A.T.; Schwartz, D.K.; Yang, T; Cremer, RS. Science, 2001, 293, 1292. 4. Ignes-Mullol, J.; Schwartz, D.K. Nature, 2001, 410, 348. 5. Lee, Y.S.; Rathman, J.F. in Reactions and Synthesis in Surfactant Systems, Surfactant Science Series, Vol. 100, Texter, J., Ed.; Marcel Dekker, 2001, p. 779.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved
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Synthesis of mesoporous carbons with various pore diameters via control of pore wall thickness of mesoporous silicas Jae-Seung Lee, Sang Hoon Joo and Ryong Ryoo* National Creative Research Initiative Center for Functional Nanomaterials and Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon, 305-701, Republic of Korea A synthesis strategy for the systematic control of the pore wall thickness has been developed for the mesoporous silicas with 2-D hexagonal order using ionic and nonionic surfactant mixtures. The mesoporous silicas have been used as templates for the synthesis of 2-D hexagonally ordered mesoporous carbons with controlled pore diameters. The synthesis strategy and results are useful not only for tailoring the properties of the mesoporous materials but also for extending our insights into the synthesis mechanism. 1. INTRODUCTION Mesoporous carbons are important in many areas of modem science and technology.' Recently, ordered mesoporous carbons have been synthesized by casting carbon frameworks with mesoporous silica templates.^'^ The mesoporous carbons, which are constructed with a regular array of uniform nanopores, exhibit high specific surface areas (typically 1300 ~ 2500 m^g'), uniform pore diameters ( 2 - 6 nm), large adsorption capacities (1 - 2 . 5 cm^g"'), and high thermal, acid-base, and mechanical stabilities. They have received great technological attention for the development of advanced separation systems, catalysts, hydrogen-storage systems, fuel-cell electrodes^, and double-layer capacitors.^ The mesoporous carbons exhibit wide varieties of pore shapes, connectivity, and pore wall thickness, depending on silica templates that are synthesized with various structures and pore diameters. In addition to these structural variations, it is important to control the pore diameters of mesoporous carbons for various applications. However, until now, this has remained as a challenge since there were no effective methods for the control of pore wall thickness of silica templates. Although there were several ways of tailoring the pore wall thickness of 2-D hexagonal silica, most of the reported methods were insufficient to provide a continuum of the pore wall thicknesses, or they suffered from poor reproducibility.^'^ Here we report a synthesis strategy for the systematic control of the pore wall thickness of hexagonal mesoporous silicas, using surfactant mixtures. We demonstrate that the mesoporous silicas are suitable as templates for the synthesis of mesoporous carbons with various pore diameters.'^
34
2. EXPERIMENTAL The synthesis of the hexagonal mesoporous sihcas was performed under acidic condition, using sodium siHcate as a siHca source. The mixture of the hexadecyltrimethylammonium bromide (HTAB) and two polyoxyethylene hexadecyl ethers, Ci6H33(C2H50)20H (C16EO2) and Ci6H33(C2H50)ioOH (CieEOio), was used as surfactant. The nonionic surfactants were mixed in HCl solution to give nominally CieEOg. After the silicate and surfactant solutions were mixed at 308 K, the mixture was maintained at 308 K for 12 h under static conditions and then heated to 373 K for 12 h. The products were filtered and calcined at 823 K. The obtained silicas are designated as SiO2-3:0, Si02-2:1, Si02-1:2, and SiO2-0:3, according to the HTAB:Ci6E08 ratios. The synthesis of carbon was carried out using mesoporous silicas as the templates and s sucrose as a carbon source. The silica-to-sucrose ratios were optimized depending on the pore volume of the silica templates. The details of the procedures were performed in the same way as for the synthesis of synthesis of CMK-3 carbon using SBA-15 silica.^ The CMK-3-type carbons thus obtained are denoted as CMK-3{x:y), where x:y refers to the same HTABiCioEOs ratio as for the silica templates. X-ray powder diffraction (XRD) patterns were taken with a Rigaku Miniflex instrument. Pore size distribution was analyzed with N2 adsorption, following the Barrett-Joyncr-Halenda algorithm and the KJS calibration." 3. RESULTS AND DISCUSSION The XRD patterns in Figure 1 show that all the carbon products have hexagonal structures corresponding to the faithful replication of the silica templates. The hexagonal structures suggest that the CK^EOH surfactant-assembled mesoporous silica (SiO2-0:3) should contain a sufficient number of complementary pores that are enough to afford freestanding, highly ordered carbon structure as in the case of SBA-15-templated CMK-3 synthesis. The amount of the complementary pores contained in the silica pore walls seems to decrease with the increase of cationic surfactant, as revealed by less resolved XRD peaks in carbons as the nTAB:Ci6E08 ratio in surfactants increases. The pore size distribution curves of ordered carbons are shown in Figure 2. The pore diameters are very narrow in distribution, and the pore size at the maximum of the distribution is systematically shifted from 2.2 to 3.3 nm against the HTAB:Ci6E08 ratio used for the synthesis of templates. The pore size variation of the carbons indicates that the pore-wall thickness of the silica templates increased with the MTABICKJEOS ratio, which is reasonable since the silica species interacting with the head group corona'^ through hydrogen bonding'"^ would increase in number with increasing the EO segments per surfactant, while the cationic HTAB surfactant would have the electrostatic interaction with silica species in an almost one-to-one ratio, independent of the various ratios used in the starting mixtures.
35
(b) Carbon
(a) Silica
2
4
6
8
2
4
6
8
26 (degrees)
Fig. 1. XRD patterns for calcined mesoporous silicas and templated carbon samples
2
3
4
5
Pore Size (nm)
Fig. 2. Pore size distribution of the carbon samples from nitrogen adsorption It is interesting to note that, even in the case of the mesoporous silica template synthesized using only the cationic HTAB surfactant-the same surfactant as for the synthesis of MCM-41, the templated carbon CMK-3(3:0) still retains the ordered structure as shown in Figure 1. This result is comparable to the carbon synthesis using MCM-41, where entangled nanofiberlike carbons are obtained.'"* Both MCM-41 and the SiO2-3:0 sample are synthesized with HTAB and sodium silicate, but their difference in synthesis conditions (acidic condition for SiO2-3:0 as compared with basic condition for MCM-41) seems to cause a remarkable difference in the
36
pore connectivity. The formation of the fiberUke carbon is evidence for the one-dimensional channel structure of MCM-41 without connectivity. On the other hand, the synthesis of the hexagonally structured carbon from SiO2-3:0 indicates that the mesoporous sihca from acidic synthesis conditions (designated as SBA-3 following Stucky*^) has the mesoporous channels somewhat interconnected by complementary pores in the silica pore wall. 4. CONCLUSIONS In this work, the thickness of mesoporous silica pore walls can be controlled systematically by the number of the functional groups that can attract silica species such as the ethylene oxide portion of the surfactant. The silica products with various pore wall thicknesses are suitable as templates for mesoporous carbons with controlled pore diameters, which would be of great interest for applications requiring fme-tuning of pore diameters. ACKNOWLEDGEMENTS This work was supported in part by the Creative Research Initiative Program of the Korean Ministry of Science and Technology, and by the School of Molecular Science through the Brain Korea 21 project. REFERENCES 1. C. R. Bansal, J.-B. Donnct and F. Stoeckli, Active Carbon, Marcel Dekker, New York, 1988. 2. R. Ryoo, S. 11. Joo and S. Jun, J. Phys. Chcm. B, 103 (1999) 7743. 3. S. Jun ct al., J. Am. Chcm. Soc, 122 (2000) 10712. 4. R. Ryoo, S. 11. Joo, M. Kruk and M. Jaroniec, Adv. Mater., 13 (2001) 677. 5. S. 11. Joo ct al., Nature, 412 (2001) 169. 6. J. Lcc et al., Chcm. Commun., (1999) 2177. 7. F. Di Rcnzo ct al.. Stud. Surf. Sci. Catal., 105 (1997) 69. 8. A. Sayari, P. Liu, M. Kruk and M. Jaroniec, Chcm. Mater., 9 (1997) 2499. 9. M. Kruk, M. Jaroniec, C. II. Ko and R. Ryoo, Chcm. Mater., 12 (2000) 1961. 10. J.-S. Lcc, S. H. Joo and R. Ryoo, J. Am. Chem. Soc, 124 (2002) 1156. 11. M. Kruk, M. Jaroniec and A. Sayari, Langmuir, 13 (1997) 6267. 12. M. Impcror-Clcrc, P. Davidson and A. Davidson, J. Am. Chem. Soc, 122 (2000) 11925. 13. Q. Huo et al., Nature, 368 (1994) 317. 14. R. Ryoo, unpublished result.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Ordered mesoporous carbon molecular sieves with functionalized surfaces Shinae Jun^, Minkee Choi^, Suyoung Ryu^, Hee-Yoon Lee^ and Ryong Ryoo^* ^National Creative Research Initiative Center for Functional Nanomaterials and Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon, 305-701, Republic of Korea ^Center of Molecular Design and Synthesis, Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon, 305-701, Republic of Korea Ordered mesoporous carbon molecular sieves designated as CMK-1 and CMK-5 were synthesized by using nanocasting technique with the synthetic procedures reported previously. After carboxylic acid groups were generated on the carbon frameworks through the partial oxidation with nitric acid, pore walls of the mesoporous carbons were functionalized with various organic and organometallic functional groups using esterification and amidization reactions. Mn-Schiff base complex grafted on the mesoporous carbon surface by using ester bond formation showed remarkable catalytic activity for partial oxidation of cyclohexene. The mesoporous carbons were also functionalizable with tetraethylenepentamine (TEPA), which could be used for the effective removal of heavy metal ions. 1. INTRODUCTION Recently, various types of ordered mesoporous molecular sieves composed of carbon frameworks have been synthesized by exploiting the "nanocasting" technique. '"^ The mesoporous carbons have high specific surface areas, excellent mechanical and chemical stabilities, good electrical conductivity and capability of supporting platinum nanoparticles with excellent dispersions. With these remarkable properties, mesoporous carbons attract much scientific attention for the development of advanced separation systems, highly selective catalysts, hydrogen-storage systems, and catalysts for electrochemical energy conversion. Here we show that the pore walls of the mesoporous carbons can be functionalized with various kinds of organic and organometallic groups applying a general method that is commonly adopted for the surface modification of carbons. We believe that functionalized carbon materials could be used for various application even under hydrothermal condition. 2. EXPERIMENTAL Two kinds of mesoporous carbons designated as CMK-1 and CMK-5 were prepared by synthetic procedures reported previously.*"^ These carbons were treated with cone. HN03for 15 min at 383 K to generte -COOH groups on the surface of carbon frameworks. This is the
38
Scheme 1. schematic representation of synthesis and grafitization of manganese Schiff-base on carbon surface. optimized condition for introducing acid groups as much as possible while the carbon framework remains intact. Before grafting various functional groups, -COOII groups were first converted to more reactive acyl chloride (-C0C1) through the reaction with thionyl chloride (SOCh) in dichloromethane at room temperature. A Schiff base ligand, 3-[N,N'-bis-3-(3,5-di-tert-butyl salicylidcnamino) propyl] hydroxypropylamine [C in Scheme I] was grafted onto the carbons through the esterification between -COCl and compound C. In the synthesis, 3-[N,N'-bis-3(3,5-di-tert-butyl salicylidcnamino) propyl] amine [B] was prepared via the condensation reaction between 3,3'-diaminopropylamine (Aldrich) and 3,5-di-tert-butyl salicyladchyde (Aldrich)."*"^ Compound B was reacted in excess under reflux condition in toluene solution with A, which was prepared by reacting 3-chloropropanol with tetrahydropyran to protect the alcohol group. The product was purified through usual workup and chromatographic separation and then protection group in the product was removed by cleaving ether linkage with methanol. After grafting C onto carbons, the product (D) was complexed with a Mn^^ ion using Mn(acac)2 (Aldrich) in methanol at 355 K under argon. The Mn^* complex was subsequently oxidized to Mn ^ in saturated NaCl solution in air. Products were thoroughly washed with methanol and water in order to remove species physically adsorbed. The catalytic activity of the carbon-supported Mn Schiff base was measured for the partial oxidation of cyclohexene with tetrabutylhydroperoxide. The reaction was started with 0.1 g catalyst, 20 mL CH2CI2, 0.5 niL cyclohexene (freshly distilled after purchase, Acros 99%) and 0.4 mL tetrabutylhydroperoxide (5.5 M solution in decane, Aldrich) under reflux conditions at 333 K in argon. Gas chromatographic analysis (HP 5890, Carbowax lOM column) of the reaction mixture was performed to check the conversion of the reaction.
39 The functionalization of the mesoporous carbons for metal chelation has been performed with amines, using the same amidization procedure reported in a previous work on carbon fibers.^'^ CMK-1 was reacted with tetraethylenepentamine [HN(CH2CH2NHCH2CH2NH2)2, Aldrich, TEPA for brievity] under reflux condition at 463 K for 15 h after the partial oxidation with HNO3. This carbon was immersed in a sufficiently large amount of 0.01 M Cu(N03)2 aqueous solution for 24 h. After filtered, thoroughly washed with distilled water and dried, the copper content was analysed by ICP. 3. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns and FT-IR spectra of CMK-1 before and after surface oxidation. From XRD patterns IR spectra XRD patterns, we could recognize that, mesoscopic order was not impaired significantly even after oxidation and the FT-IR peaks appearing around 1750 and 1650 cm"' confirmed the presence of COOH group. By using the general titration method with standard NaOH solution, the amount of acid groups introduced was calculated as 1 x 10'^ mol per g CMK-1. Through the ester bond formation with the acid group produced, we grafted Mn Schiff-base onto mesoporous carbon surface. In the case of the CMK-5 carbon, the grafting experiment could be also performed onto the carbon/silica composite during the carbon synthesis. ' — I — " ^ T n 2(XX) 15(X) 1000 Subsequent removal of the silica framework with \\F 4 gave a CMK-5 sample containing the Schiff-base 2 tlida / (ic^vc Wi\aTuiitxj(aii') complex selectively grafted inside the nanopipes. From the inductively coupled plasma emission (ICP) Fig. 1. XRD and FT-IR spectra of CMK-1 before and after carbon analysis data, Mn contents for different catalysts are as surface. follows; 20 miiol g"' for CMK-1, 42 ^imol g ' for CMK-5 (20 i^mol g'' for inside-selective grafting) compared to 48 |amol g ' of MCM-48 catalyst synthesized by the same method reported previously ' \ The catalytic activities of Mn Schiff-bases grafted on different supports arc represented in Figure 2. Interestingly, the catalytic activities of the carbon catalysts reduced by Mn content were at least twice as much as that of an MCM-48 silica catalyst. The higher catalytic activity of the mesoporous carbons may be related to the increased concentration effect of the reactants in the case of the more hydrophobic carbon catalysts. All the carbon catalysts exhibited highly reproducible catalytic activities even after the reaction was repeated 5 times. The carbon catalysts retained the structural order and catalytic activities even after boiling in water for 10 days while the MCM-48'based catalyst lost structural order and catalytic
40
5
10
15
20
Time / hour Fig. 2. Conversion of cyclohexene for partial oxidation of cyclohexene plotted with reaction time: (A) CMK-5, (o) ClVIK-5 functionalized selectively inside the nano-pipes, (V) CMK-1 carbon and (fl) MCM-48 silica.
activities almost completely. Finally, TEPA-functionalized CMK-1 adsorbed 0.4 mmol Cu^^ per g carbon, whereas the original CMK-1 exhibited undetectable copper adsorption. It is noteworthy that this adsorption capacity for Cu^^ was much higher than the value of 0.1 mmol Cu^^ per g, which was reported with molecularimprinting functionaliza-tion of MCM-41 silicas with ethylene-diamine.^The metal chelating effect may be useful for water treatments, since the carbons functionalized in this way can remove heavy metals and organic pollutants simultaneously. While the pore-wall functionalization is a wellknown strategy used for MCM-41 silicas, low hydrothermal stability has been a serious problem with the silica-based mesoporous materials. The functionaliz-ation of the mesoporous carbons exhibiting much higher hydrothermal and acidbase stabilities could offer a remarkable adventage especially for the applications under hydrothermal conditions.
ACKNOWLEDGEMENTS This work was supported in part by the Creative Research Initiative Program of the Korean Ministry of Science and Technology, and by the School of Molecular Science through the Brain Korea 21 project. REFERENCES 1. R. Ryoo, S. H. Joo, S. Jun, J. Phys. Chem. B 103 (1999) 7743. 2. S. Jun, S. H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna and O. Terasaki, J. Am. Chem. Soc. 122(2000) 10712. 3. S. H. Joo, S. J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Nature 412 (2001) 169. 4. P Sutra, D. Brunei, Chem. Commun. 21 (1996) 2485. 5. D. Brunei, N. Bellocq, P. Sutra, A. Cauvel, M. Lasperas, P. Moreau, F. Di Renzo, A. Galarneau, F. Fajula, Coord. Chem. Rev. 178-180 (1998) 1185. 6. C. U. Pittman, Jr., G.-R. He, B. Wu, S. D. Gardner, Carbon 35 (1997) 317. 7. C. U. Pittman, Jr., Z. Wu, W. Jiang, G.-R. He, B. Wu, W. Li, S. D. Gardner, Carbon 35 (1997)929. 8. S. Dai, M. C. Burleigh, Y. Shin, C. C. Morrow, C. E. Barnes, Z. Xue, Angew. Chem. Int. Ed. 38(1999) 1235.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Characterisation of ordered mesoporous carbons and their MCM-48 silica templates obtained by the replication technique using different carbon infiltration processes J. Parmentier*^ C. Vix-Guterl^ P. Gibot^ M. Iliescu', J. Werckmann', J. Patarin^ ^Laboratoire des Materiaux Mineraux, Ecole Nationale Superieure de Chimie de Mulhouse, UMR CNRS 7016, Universite de Haute Alsace, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France. E-mail: J.Pamientier@univ-mulhouse.fr ^'Institut de Chimie des Surfaces et Interfaces, UPR CNRS 9069, 15 rue Jean Starcky - B.P 2488 - 68057 Mulhouse Cedex, France '^Institut de Physique et de Chimie des Materiaux de Strasbourg, UMR CNRS 7504, 23 rue du Loess, 67037 Strasbourg Cedex, France The influence of carbon infiltration processes onto the formation of ordered mesoporous carbons have been investigated by taking into account the physico-chemical properties of the obtained carbon materials and their silica templates after carbon impregnation and subsequent oxidation. It has been shown, that when sucrose is used as carbon precursor (liquid infiltration process), the high temperature of carbonisation and the water vapour release greatly affect the structural characteristic of the silica template contrary to what is observed when a gaseous hydrocarbon precursor is used (CVD process). 1. INTRODUCTION Mesoporous ordered carbons have attracted recently much scientific interest for their specific applications in areas such as shape-selective catalysts and adsorbents, sensors, hydrogen containers and electrode materials. They have been synthesised by the pyrolysis of a carbon-precursor impregnating an ordered mesoporous silica template; silica being removed selectively afterwards by chemical etching (HF). Different routes for the carbon impregnation have already been investigated; using either a liquid carbon precursor (e.g. a sucrose solution [1], phenol [2]), monomers (e.g. divinylbenzene [3]) or a gaseous hydrocarbon precursor (e.g. propylene [4] or acetylene) associated with the Chemical Vapour Deposition technique (CVD). Depending on the process used, the carbonisation takes place inside the silica mesopores at different temperatures and with a release of different gaseous species. To our knowledge, comparisons of the mechanisms of the carbon formation inside the pore according to the infiltration process have not been really investigated. In this frame, the structural characterisations of mesoporous ordered carbons (obtained by two different processes) and their corresponding silica templates after carbonisation and subsequent oxidation can bring new insights in the understanding of the formation of ordered mesoporous carbons. 2. EXPERIMENTAL The starting MCM-48 silica template was synthesised according to the procedure described by Schumacher et al [5]. The infiltration of carbon by the CVD process uses the pyrolytic decomposition of propylene at 750°C [4]. The second impregnation procedure is the one
42
described by Ryoo [1, 6]. A sucrose solution, in the presence of a sulfuric acid solution, was infiltrated twice in the silica matrix and then carbonised under vacuum at 900°C. The final carbon content was gravimetrically determined. The resulting Si02/C materials were then chemically etched by HF or by oxygen to recover the carbon or the silica matrix, respectively The materials were characterised by chemical analysis,N2 adsorption/desorption, X-Ray Diffraction (XRD) and High Resolution Transmission Electron Microscopy (HRTEM). 3. RESULTS AND DISCUSSION As evidenced by the TEM images, both processes were suitable for the formation of a carbon replica of the MCM-48 silica (Figure 1). Nevertheless, some differences can be observed for both carbons and their corresponding silica matrices recovered after oxidation of the Si02/C materials.
Fig. 1. TBM pictures of ordered mcsoporous carbons obtained by sucrose (A) and CVD process (B). The X-ray diffraction patterns of both carbon replicas (Figure 2) show the presence of the (110) diffraction peak which do not appears for the MCM-48 silica material. One can notice that its intensity is lower in the case of the carbon obtained by CVD than in the case of carbon prepared by the sucrose process. This can be attributed to stronger deformation of the cubic structure in the latter case and a more faithful replication in the former case, it is well known that the carbon replica prepared by the sucrose route exhibits a lower symmetry than the starting MCM-48 materiel (cubic Ia-3d for MCM-48). This could be attributed to the formation of tetragonal domains of I4i/a symmetry for the replica [6] and was evidence by the appearance of the peak (110) on the diffraction pattern. However, the authors have shown that a more faithful replica (using a CVl process) preserve the template symmetry and its original diffractogram (absence of (110) peak) [6]. Characterisations of the corresponding silica matrices (after carbon impregnations and subsequent oxidation) were performed by XRD technique and N2 adsorption/desorption. For the liquid infiltration, the corresponding XRD pattern displays the disappearance of peaks in the 4 - 6° 2 0 range, characteristic of a loss of long-range organization (Figure 3). It is worth noting that the same sucrose process leads to considerable shrinkage of the silica matrix (15%) whereas for a similar heat-treatment without carbon impregnation, the unit cell decreased of only 4% (Table 1). At the opposite, with the CVD process, only a small contraction was observed (5 %) close to the one of the silica matrix having followed the same heat-treatment (2%) but without carbon impregnation. With the sucrose process, the silica matrix has undergone major transformations. Indeed, as
43
confirmed by its Nitrogen adsorption isotherm, the porous volume has drop considerably (50%) and the mesopores narrow distribution, characteristic of MCM-48 silica, are replaced by broadly distributed secondary micropores and primary mesopores (Figure 4 and Table 2). A different trend was observed for the silica resulting of the CVD process, with only minor
MCM-48 silica after oxidation of (SiCyCJbvD material
MCM-48 silica after oxidation of (SiQ/C)t,^„^^ nnaterial
MCM-48 silica (starting material)
2(-) (degree) 1Fig. i g . 2. z,. X-ray A - r a y diffraction umiav^^iivjii patterns paLiv^iii;^ KJI of
the carbon replicas: (a) sucrose process, (b) CVl process
20 (degree)
Fig. 3. X-ray diffraction patterns of the different silica matrices
transformations of the starting silica template. These facts can be related to the carbonisation of the sucrose which takes „-^ 400 place at high temperature (900°C) with a E release of gaseous species such as H2O (but also CO and CO2 ). Water vapour and the strong gas pressure present in the - MCM-48 silica (starting material) confmed media of the Si02/C sample MCM-48 silica after oxidation of (SiOj/C)(-v, material - MCM-48 silica after oxidation of (SiO^/C),^ ^ material may then promote drastic reduction in the structural ordering and the creation of micropores in the silica matrix. Such a Fig. 4. N2 adsorption isotherms of MCM-48 phenomenon is well known since MCMsilica and the recovered silica matrix after carbon 48 displays a low hydrothermal stability. This behavior leads to a carbon replica infiltration processes and oxidation. with high surface-area and porous volume (Table 2) At the opposite, for the one step CVD process, the pyrolysis takes place at a lower temperature (750°C) with removal of mainly H2 and hydrocarbon as gaseous species that do not affect strongly the mesoporous characteristics of the starting silica material. Moreover, it allows a higher carbon content in the silica matrix and lower oxygen content compare to the sucrose process (Table 2).
44
Table 1 Variation (%) of the unit cell parameter a^ (cubic systeim) of the silica matrices according to the carbon infiltration processes and the heat-treatments . Infiltration process Heat-treatment 750°C 20h Argon 900°C 3h vacuum 1200°C 1 h Argon None -2 -4 0 (collapsing) Sucrose / -15 -20 CVI ^5 / -1_ The Si02/C materials were previously oxidised in air before characterisations Table 2 Characteristics of the materials
Process Sucrose CVD (90Q°C) (75Q°C) 37 50 Carbon content in the SiOz/C material (wt %)* C loH i .7O0.52 C loH 1.5O0. i Composition of the carbon material Total Porous volume of the carbon material (cm^/g) 0.9 0.6 BET surface area (m^/g) 1400 800 0.45 Pore volume of the silica matrix (cm"^/g) after the oxidation of the 0.8 Si02/C material** *: the maximum theoretical carbon content is 60 wt %.; **: the total porous volume of the MCM-48 silica is 0.9 cm"^/g. 4. CONCLUSION This study has shown that according to the carbon infiltration process, there are similarities in the structural characteristics of carbon replica and its corresponding silica matrix. For the sucrose process, the high temperature of carbonisation and the big amount of gas released (mainly water) lead after oxidation for carbon removal, to a high shrinkage of the silica template associated to a loss of long-range ordering, and a disappearance of the initial mcsoporosity. The corresponding carbon displays a lower symmetry than the starting MCM48 silica (appearance of the (110) peak), a high surface area and a high porous volume induced likely by the released of gaseous species. At the opposite, the CVD is a more "soft" process which preserves the silica template integrity and allows a more faithfull carbon replica with low oxygen content. Therefore, carbon infiltration processes have a great influence on the porosity, symmetry, long-range organisation and composition of the ordered mesoporous carbons but also on the physico-chemical characteristics of the silica template. REFERENCES R. Ryoo, S. H. Joo, S. Jun; J. Phvs. Chem B 1999, 103, 7743. J. Lee, S. Yoon, T Hyeon, S. M. Oh, K. B. Kim, Chem Comm, 1999, 2177. J.Y. Kim, S.B. Yoon, J.-S. Yu, Chem Commun., 2001, 559. J. Parmentier, J. Patarin, J. Dentzer, C. Vix-Guterl, Ceramics International 28(1), 2002, 1. K. Schumacher, C. Von Hohenesche, K.K. Unger, R. Ulrich, A. Du Chesne, U. Wiesner and H.W. Spiess, Adv. Mater., 1999, 11, 1194. 6. M Kaneda, T. Tsubakiyama, A. Carlsson, Y. Sakamoto, T. Ohsuna, O. Terasaki, S.H. Joo, R. Ryoo,y. Phys. Chem. B, 2002, 106, 1256.
1. 2. 3. 4. 5.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
45
Morphological control of highly ordered mesoporous carbon C. Yu \ J. Fan \ B. Tian \ F. Zhang \ G. D. Stucky ^* and D. Zhao '* ^Department of Chemistry, Fudan University, Shanghai, 200433, P. R. China. Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106, USA. Highly ordered mesoporous carbon materials with fiber-like, plate-like, rod-like and donutlike morphologies have been synthesized by using mesoporous silica as the templates. 1. INTRODUCTION Recently, periodic mesoporous carbon materials CMK-X synthesized from ordered mesoporous silica templates [1,2] have attracted much attention because of their versatile uses in gas separation, catalysis, chromatography, and energy storage. [3,4]Such new mesostructured carbon materials were synthesized by using mesoporous silica with threedimensional (3-D) pore networks (MCM-48, SBA-1, and SBA-15) as hard templates and sucrose, furfuryl alcohol as suitable carbon sources. In the case of mesoporous carbon CMK-3, [2] the structure is exactly an inverse replica of its silica templates SBA-15 [5] without any structural transformation. For practical applications, the fabrication of desired morphologies is important as well as the control in composition, structure, porosity, etc. Mesostructured fibers, donuts [6] and crystal morphologies [7] have been obtained for silica powders synthesized by using commercial nonionic block copolymers as supramolecular templates. Until now, however, no successful morphology control in mesoporous carbon has been reported. Here we show a successful morphological control of hexagonally ordered mesoporous carbon materials with fiber-like, rod-like, plate-like and donut-like morphologies by using SBA-15 silica as the hard templates. 2. EXPERIMENTAL Highly ordered mesoporous SBA-15 silica with different morphologies were synthesized by using poly(ethylene oxidcs)-/?-poly(propylene oxides)-/?-poly(ethylene oxides) (PEO-PPOPEO) triblock copolymer EO20PO70EO20 (BASF) as the structure directing agent. Rod-like SBA-15 was synthesized using TMOS as a silica source at 313 K under static conditions. This work was supported by the NSF of China (Grant No. 29925309 and 29873012), Shanghai Sci. Tech. Committee (0152nm029), State Key Basic Res. Prog. (G2000048001), and the National Science Foundation of the Unites States under Grant DMR-9634396.
46
Plate-like SBA-15 was synthesized using TMOS as a silica source at 303 K under static conditions. The fiber-like and donut-like SBA-15 was synthesized as reported in literature. [6] In a typical synthesis of SBA-15 rods, 2.0 g (0.34 mmol) of EO20PO70EO20 were dissolved in 60 g of 2.0 M HCl at 38 °C. To this solution, 3.3 g (22 mmol) tetramethyl orthosilicate (TMOS) were added under vigorous stirring. The final reactants molar composition is 0.015 EO20PO70EO20/5.5 HCl/150 H2O/I TEOS. After stirring for 6 min, the mixture was kept in static conditions at the same temperature for one day, then the mixture was transferred into an autoclave and heated at 100 °C for another 24 h. The solid products were collected by filtration and dried at room temperature in air. The resulting powders were calcined at 550 °C for 4 h in order to obtain mesoporous silica materials. The synthesis of mesoporous carbon materials from SBA-15 silica hard templates was performed with sucrose as described in literature. [2] The dissolution of silica with 10% HP resulted in mesoporous carbon materials. 3. RESULTS AND DISCUSSION Scanning electron microscopy (SEM) images of mesoporous SBA-15 with different morphologies are shown in Figure la, b, c, and d. The well-defined fiber-like, rod-like, platelike and donut-like morphologies confirm that desired morphologies of SBA-15 have been obtained in high yield. It is interesting to note that the stirring condition is essential to the morphology of SBA-15, which leads to fiber like (with stirring) and rod-like SBA-15 materials (without stirring). Furthermore, the synthesis temperature can be used in the aspect ratio control of highly ordered SBA-15 materials, as can seen in the cases of rod-like and plate-like SBA-15 synthesized with the same reactant compositions while the synthesis temperature is different (313 K for rods and 303 K for plates). X-ray diffraction (XRD), N2 sorption analysis results (Table 1) and transmission electron microscopy (TEM) images (data not shown) confirm that these silica templates are highly ordered hexagonal mesostructures. Table 1 Physical Data of Mesoporous Silica/Carbon with Sample Morphology J(IOO) /nm 9.06 Mesoporous Fibers Silica 8.74 Rods SBA-15 Plates 8.72 9.11 Donuts 7.82 Mesoporous Fibers Carbon 7.96 Rods C-SBA-15 6.98 Plates 8.03 Donuts
Different Wa /nm 6.8 6.6 6.3 7.8 3.8 3.9 3.9 3.4
Morphologies wj S /nm /m^g"' 7.0 916 6.9 811 910 6.5 7.7 983 1746 4.6 1899 4.5 1738 4.6 1558 4.3
V /cm"^g'' 1.21 0.99 1.09 1.44 1.48 1.65 1.45 1.22
Notes: d{\00) is d-spacing of mesoporous silica/carbon calculated from XRD data; Wa and wj are pore size of mesoporous silica/carbon calculated using BdB model from adsorption and dcsorption branch, respectively. S is BET surface area, and V is pore volume.
47
Fig. 1. SEM images (a), (b), (c) and (d) of hexagonal mesoporous silica SBA-15 materials with fiberlike, rod-like, plate-like and donut-likc morphology, respectively. SEM images (e), (f), (g) and (h) and TEM images (i), (j), (k) and (1) of hexagonal mesoporous carbon with fiber-like, rod-like, plate-like and donut-like morphology, respectively. SEM images were obtained with a JEOL 6300-F microscope using 3.0 KV acceleration voltages. TEM micrographs were obtained with a JOEL 2000 transmission electron microscope operating at 200KV. XRD patterns and N2 sorption isotherms of mesoporous carbon materials synthesized from SBA-15 fibers (C-SBA-15) are shown in Figure 2a and 2b, respectively. The well-resolved three diffraction peaks can be indexed to 2D hexagonal structure (p6mm), indicating that the carbon products are faithful replication of their silica templates. Other C-SBA-15 synthesized from rod-like, plate-like and donut-like SBA-15 templates show similar XRD patterns and N2 sorption curves, and the physical data can be found in Table 1. Moreover, SEM (Figure le, f, g, h) and TEM images (Figure li, j , k, 1) confirm that the replication of these mesoporous
48
carbon materials is strict in both mesostructure and morphology compared to their silica templates. It is noted that the length and curvature of mesopore channel within these C-SBA15 materials are different; and these mesoporous carbon materials with controlled morphologies have large surface area (up to 1900 m^g"') and pore volumes (up to 1.65 cm"^g').
^
(100)
Fig. 2. (a) XRD patterns; (b) N2 adsorption-dcsorption isotherm plots and pore size distribution curve (inset) of mesoporous carbon materials C-SBA-15 with fiber-like morphology. The X-ray data were collected on a Scintag PADX diffractometer using Cu Ka radiation. The isotherms were measured using a Micromcritics ASAP 2000 system. The samples were degassed at 200 °C overnight on a vacuum line.
4. CONCLUSIONS In conclusion, highly ordered hexagonal mesoporous carbon materials with fiber-like, rodlike, plate-like and donut-like morphologies have been synthesized. Such ordered mesoporous carbon materials with uniform morphology, various pore parameter and extra large porosity may have potential use in catalysis and as nano-reactors for hydrophobic precursors. REFERENCES 1. R. Ryoo, S.H. Joo, S. Jun, J. Phys. Chem. B 103 (1999) 7743. 2. S. Jun, S.H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna, O. Tcrasaki, J. Am. Chem. Soc. 122(2000) 10712. 3. R. Ryoo, S.H. Joo, M. Kruk, M. Jaroniec, Adv. Mater. 13 (2001) 677. 4. S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Nature 412 (2001) 169. 5. D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Frederickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548. 6. D. Zhao, J.Y. Sun, Q.Z. Li, G.D. Stucky, Chem. Mater. 12 (2000) 275. 7. C. Yu, B. Tian, J. Fan, G.D. Stucky, D. Zhao, J. Am. Chem. Soc. 124 (2002) 4556.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved
49
Thermally induced structural changes in SBA-15 and MSU-H silicas and their implications for synthesis of ordered mesoporous carbons Sang Hoon Joo,^ Ryong Ryoo,^ Michal Kruk^ and Mietek Jaroniec^ ^National Creative Research Initiative Center for Functional Nanomaterials and Department of Chemistry, Korea Advanced Institute of Science and Technology, Taejon, 305-701, Korea ^Department of Chemistry, Kent State University, Kent, Ohio 44242, USA SBA-15 and MSU-H silicas were synthesized and subjected to calcination at temperatures in the range from 823 to 1273 K. The porous structures of the resulting samples were studied using X-ray diffraction and nitrogen adsorption. The pore connectivity was elucidated using the inverse carbon replication procedure. The structures of carbon inverse replicas were studied using XRD and nitrogen adsorption. 1. INTRODUCTION The recent development of ordered mesoporous carbons using ordered mesoporous silicas as templates [1,2] opened remarkable new opportunities in the synthesis of ordered mesoporous materials suitable for numerous advanced applications [1-5]. The necessary condition for the formation of ordered mesoporous carbon replica is a three-dimensional (3D) connectivity of pores in the structure of the template [3]. The first reported ordered mesoporous carbon, CMK-1, was synthesized using MCM-48 silica template [1,2] that exhibits two interwoven, but disconnected 3-D porous systems. Consequently, CMK-1 exhibited an ordered structure, but the latter was different from that of the template because of the mutual displacement of the two interwoven carbon frameworks after the silica template dissolution [1]. The first ordered mesoporous carbon that faithfully replicated the structure of the silica template was synthesized using SBA-15 silica as a template [4]. SBA-15 exhibits a two-dimensional (2-D) hexagonally ordered structure with large mesoporous channels that are connected through complementary pores (micropores and mesopores) in the pore walls, as was demonstrated earlier using platinum inverse replication [6]. Consequently, SBA-15templated carbon, referred to as CMK-3, is a 2-D hexagonal array of carbon rods. The formation of connecting pores in the SBA-15 structure was explained as an effect of occlusion of poly(ethylene oxide) chains of the triblock copolymer structure-directing agent in the silica framework [6]. It was further hypothesized that this behavior is common for silicas templated by polymers and oligomers with poly(ethylene) oxide blocks [6]. This contention was confirmed by a successful synthesis of CMK-3 carbons not only using SBA-15 templates assembled from TEGS as a silica source under acidic conditions in the presence of triblock copolymer structure-directing agent, but also using silica templates prepared in the presence of triblock copolymers or oligomers under different conditions. These templates include MSU-H silica synthesized from sodium silicate under nearly neutral conditions in the presence of triblock copolymer [7,8] and SBA-15 silica synthesized from sodium silicate
50
rather than from TEOS [8]. Moreover, CMK-3 carbons were successfully synthesized using 2-D hexagonal silica templates synthesized in the presence of oligomers [9]. Despite the fact that CMK-3 carbons can be formed using silica templates synthesized under various conditions in the presence of polymeric or oligomeric structure-directing agents, some differences in the structure of these templates are expected that may influence the structure of the resulting carbon replicas. In particular, it was reported that SBA-15 silica synthesized at 403 K exhibits relatively large tunnels or holes in the pore walls, which provide connectivity between the ordered, large mesopores [10]. These connecting pores are observable by transmission electron microscopy (TEM) as ruptures in the pore walls, whereas connecting pores present in SBA-15 synthesized at 373 K are not readily observable by TEM. It was also suggested on the basis of nitrogen adsorption data that the heat treatment of SBA15 synthesized at 373 K leads to depletion and perhaps even to closure of the connecting pores [6]. These findings highlighted the need for a systematic study of pore connectivity in 2-D hexagonally ordered mesoporous silicas synthesized under various conditions in the presence of polymeric and oligomeric structure-directing agents. In particular, an effect of calcination temperature on the pore connectivity in these silicas deserves much attention, as the successful elimination of connecting pores without the destruction of the periodic structure of the material would lead to extra-large-pore MCM-41 analogues [11] that are desirable as templates for nanowires and as model adsorbents. It is also interesting to investigate the effect of the calcination of the silica template on the structure of its carbon inverse replica. This contribution reflects our ongoing research effort in these important directions. 2. EXPERIMENTAL SBA-15 and MSU-H silicas were synthesized as described elsewhere using Pluronic PI23 poly(cthylcne oxide)-poly(propylcne oxidc)-poly(ethylene oxide) triblock copolymer as a structure-directing agent [8,12,13]. After the removal of the copolymer via calcination, inverse carbon replicas were synthesized as reported earlier [4]. 3. RESULTS AND DISCUSSION It is convenient to elucidate the pore connectivity in ordered silicas on the basis of the structural properties of their carbon inverse replicas [3,8,11,14]. We have already employed this methodology in studies of structural changes in SBA-15 brought about by calcination at temperatures above 823 K [11] (823 K is a standard calcination temperature for silicas synthesized using surfactant, oligomeric and polymeric structure-directing agents). SBA-15 silica selected for this study was synthesized initially at 308 K and later aged at 373 K, which are typical conditions for the SBA-15 synthesis [12]. It was found that thus synthesized SBA15 sample retained its ordered structure after calcination at 1153 and 1243 K, although its unit-cell size and pore diameter were reduced and its adsorption capacity was diminished to a half and to one forth, respectively, when compared to the sample calcined at 823 K. The SBA15 samples calcined at 823 and 1153 K were successfully used as templates for the synthesis of ordered CMK-3 carbons, whereas the inverse replication of the SBA-15 silica calcined at 1243 K afforded a disordered high-surface-area carbon with a broad distribution of pores in the micropore and mesopore ranges. We interpreted this result as evidence that connecting pores in pore walls of large mesoporous channels of SBA-15 synthesized at 373 K are
51
eliminated at 1243 K, thus leading to the formation of large-pore MCM-41 analogue. Unfortunately, this MCM-41 analogue exhibited a low adsorption capacity and relatively small pore diameter (about 5 nm). It is interesting to note that in the case of the SBA-15 sample synthesized under conditions discussed above, the increase in the calcination temperature did not lead to any appreciable change in the pore diameter of the inverse carbon replica. This is rather unexpected because the higher calcination temperature results in structural shrinkage, which is expected to increase the pore wall thickness and thus to lead to larger pore diameter of the inverse carbon replica. However, it needs to be kept in mind that the pore wall of SBA-15 synthesized under conditions considered is highly porous and that the increase in the calcination temperature appears to lead to its consolidation. Therefore, the effect of structural shrinkage, which is expected to increase the pore wall thickness, is likely to be counterbalanced by the wall consolidation with reduction of the volume of the complementary pores in the walls. These two opposite effects may lead to a relatively constant pore wall thickness for SBA-15 samples synthesized at 373 K and calcined at different temperatures. In addition to SBA-15 silicas synthesized at 373 K, MSU-H silicas synthesized at the same temperature were studied. As reported elsewhere [7,8], MSU-H synthesized at 333 K as well as at 373 K is suitable as a template for the synthesis of CMK-3 carbons. The resultant inverse carbon replica exhibits properties similar to those for CMK-3 carbon prepared using SBA-15 template [7,8]. MSU-H silica calcined at 973 and 1073 K was also suitable for the synthesis of CMK-3 carbons, whose properties were similar to those of typical CMK-3 carbons (see Figure 1). This indicates that the connectivity of pores in MSU-H silica synthesized at 373 K was retained at 973 and 1073 K. In contrast to the results of calcination at temperatures up to 1073 K, higher calcination temperatures for the MSU-H sample considered allowed us to obtain ordered mesoporous carbons with very interesting, novel properties. These properties can be explained as results of structural changes of the MSU-H template upon heating. We intend to continue this research to gain a better understanding of these fascinating phenomena and their implications in the synthesis of ordered mesoporous carbons, and we plan to report these results later.
H
H
CO
l - c
o
<
o B < Relative Pressure
Relative Pressure
Fig. 1. Nitrogen adsorption isotherms for MSU-H silicas calcined at 973 and 1073 K (left graph) and for their inverse carbon replicas of CMK-3 type (right graph).
52
4. CONCLUSIONS An inverse carbon replication is a convenient tool for the characterization of pore connectivity in ordered mesoporous silicas. In particular, this method allowed us to demonstrate that 2-D hexagonally ordered silicas synthesized in the presence of triblock copolymer structure-directing agent under various conditions and from different silica sources exhibit 3-D pore connectivity. This connectivity is often retained even after extensive thermal treatments at high temperatures, although in the case of SBA-15 silica synthesized from TEGS at 373 K, it appears to be possible to apply the calcination at 1243 K to eliminate the connectivity of large, uniform mesopore channels with retention of 2-D hexagonal ordering. We found that MSU-H synthesized at 373 K can be calcined at temperatures above 1073 K in order to obtain ordered carbons with interesting, novel properties, which we intend to further explore. ACKNOWLEDGMENTS This work was supported in part by Creative Research Initiative Program of the Korean Ministry of Science and Technology, and by School of Molecular Science through Brain Korea 21 project. The donors of the Petroleum Research Fund administered by the American Chemical Society are gratefully acknowledged for the support of this project. REFERENCES 1. 2. 3. 4.
R. Ryoo, S. H. Joo and S. Jun, J. Phys. Chem. B, 103 (1999) 7743. J. Lee, S. Yoon, T. Hyeon, S. M. Oh and K. B. Kim, Chem. Commun., (1999) 2177. R. Ryoo, S. H. Joo, M. Kruk and M. Jaroniec, Adv. Mater., 13 (2001) 677. S. Jun, S. M. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna and O. Terasaki, J. Am. Chem. Soc, 122 (2000) 10712. 5. S. H. Joo, S. J. Choi, 1. Oh, J. Kwak, Z. Liu, O. Terasaki and R. Ryoo, Nature, 412 (2001) 169. 6. R. Ryoo, C. H. Ko, M. Kruk, V. Antochshuk and M. Jaroniec, J. Phys. Chem. B, 104 (2000)11465. 7. S.-S. Kim and T. J. Pinnavaia, Chem. Commun., (2001) 2418. 8. S. H. Joo, R. Ryoo, M. Kruk and M. Jaroniec, J. Phys. Chem. B, 106 (2002) 4640. 9. J.-S. Lee, S. H. Joo and R. Ryoo, J. Am. Chem. Soc, 124 (2002) 1156. 10. J. Fan, C. Yu, L. Wang, B. Tu, D. Zhao, Y. Sakamoto and O. Terasaki, J. Am. Chem. Soc, 123(2001) 12113. 11. H. J. Shin, R. Ryoo, M. Kruk and M. Jaroniec, Chem. Commun., (2001) 349. 12. D. Zhao, Q. Uuo, J. Feng, B. F. Chmelka and G. D. Stucky, J. Am. Chem. Soc, 120 (1998)6024. 13. S.-S. Kim, T. R. PaulyandT. J. Pinnavaia, Chem. Commun., (2000) 1661. 14. J. Lee, S. Yoon, S.M. Oh, C.-H. Shin and T. Hyeon, Adv. Mater., 12 (2000) 359.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
53
Regeneration of mesoporous inorganic materials using ordered mesoporous carbon as the template Ji Man Kim^*, Min Kang^*^, Seung Hwan Yi^, Jae Eui Yie^, Sang Hoon Joo*^ and Ryong Ryoo^ ^Functional Materials Laboratory, Department of Molecular Science & Technology, Ajou University, Suwon, 442-749, Korea ^Catalyst and Surface Laboratory, School of Chemical Engineering and Biotechnology, Ajou University, Suwon, 442-749, Korea ^National Creative Research Initiative Center for Functional Nanomaterials and Department of Chemistry, Korea Advanced Institute of Science and Technology, Taejon, 305-701, Korea Regeneration of mesoporous inorganic materials from ordered mesoporous carbons has been successfully performed in the present work. A mesoporous silica SBA-15 are used as a template for the synthesis of the mesoporous carbon CMK-3. Subsequently, mesoporous inorganic replica materials can be obtained from the mesoporous carbon. This synthetic principle is very useful for the preparation of various kinds of the mesoporous inorganic materials. 1. INTRODUCTION After discovery of ordered mesoporous materials such as MCM-41, MCM-48, SBA-15, etc. [1,2], their syntheses, characterization and applications have attracted much attention due to the large pore diameters, compared with those of conventional microporous zeolites [3]. It is well known that mesoporous materials are synthesized by the synergistic self-assembly between surfactant micelles and inorganic species to form mesoscopically ordered organicinorganic composites [1,2], Some reports have demonstrated the formation of porous materials by using other templates such as polymer beads, emulsions, etc. instead of surfactant micelles [4,5]. It is reasonable to divide the templating materials as two types: one is soft such as surfactant micelles and emulsions and the other rigid such as polymer beads. In case of the soft type of templates, it is necessary to optimize the synthetic conditions such as reaction temperature, pH, template concentration, molar ratios between the reactants, etc. for preparation of highly ordered mesoporous materials. The rigid one is supposed to have excellent structure-directing abilities, because it is not necessary to control the synthetic conditions for the formation of mesostructures. Moreover, this synthetic strategy may be useful for the preparation of various kinds of mesoporous inorganic materials, especially transition metal oxides. However, it is difficult to find a good rigid template to obtain the mesoporous materials with well-ordered structure and high surface area. Recently, a new series of ordered mesoporous carbon materials, which are obtained by using mesoporous silica materials as templates, has been attracted much attention as adsorbents, catalyst supports, and materials for advanced electronics applications [6-8]. It is
54
interesting to investigate a possibility to use the mesoporous carbon materials as the templates for the mesoporous inorganic materials because the carbons have well-ordered mesoporosity and rigidity. Here, we present regeneration of mesoporous inorganic materials using the ordered mesoporous carbon as a template [9]. 2. EXPERIMENTAL Mesoporous silica SBA-15 and mesoporous carbon CMK-3 were obtained following the procedures described elsewhere [6,10], using a triblock polymer Pluronic PI23 as the structure-directing agent and sodium metasilicate as the silica source for the silica material, and using a calcined SBA-15 as the template and sucrose as the carbon source for carbon material. The CMK-3 material thus obtained was impregnated with aqueous solution of sodium metasilicate by wetness method using a rotary evaporator, and subsequently the sample was hydrothermally treated under acidic condition to obtain the fully condensed inorganic framework [9]. Finally, the samples were calcined at 823 K in air under static condition to remove the carbon frameworks. In case of transition metal oxides, the samples were heated at 1073 K in nitrogen flow before the carbon removal. 3. RESULTS AND DISCUSSION
Silica Replica
SBA-15
1
2
3 4 2 ^/degree
5
Figure 1. XRD patterns for the mesoporous materials.
Figure 1 shows powder X-ray diffraction (XRD) patterns for the SBA-15, CMK-3 and silica replica from the CMK-3. All exhibit XRD patterns with a very intense diffraction peak and two or more weak peaks, which are characteristic of 2-d hexagonal {P6mm) mesostructures [1,2,6]. The XRD pattern for the silica replica shows (210), (300) and (310) peaks, which indicates excellent textural uniformity of the material. Hexagonal unit cell parameters (a = 2d\()()N'i) for the SBA-15, CMK-3 and silica replica are 11.99, 10.69, 10.20 nm, respectively, which arc calculated from ^loo spacings in Figure 1. There arc no significant differences between XRD patterns for the SBA-15 and silica replica materials, except for the expected lattice contraction (~ 15 %) upon the thermal treatment to remove the carbon template. The result shows that the mesoporous silica can be regenerated by faithful negative replication from the mesoporous carbon. Further evidence for the 2-d hexagonal mesostructures of the materials is provided by transmission electron microscopic (TEM) images as shown in Figure 2. The TEM images also indicate that all the materials have a highly ordered 2-d hexagonal structure. TEM images for SBA-15 and CMK-3 in Figure 2a and 2b are very similar to those in the literatures [2,6,10]. TEM image for the silica replica material in Figure 2c shows a series of parallel and straight fringes, which is viewed along the c axis for the 2-d hexagonal mesostructures.
55
All the materials in Figure 1 give type IV nitrogen adsorption-desorption isotherms with hysteresis loops [9]. The isotherms for the SBA-15 and CMK-3 materials coincide with the data reported elsewhere [2,6]. The CMK-3 with narrow size distribution has well ordered hexagonal structures corresponding to the replication of the SBA-15 as can be shown in TEM images (Figure 2). The silica replica obtained from the CMK-3 also exhibits a very narrow pore size distribution centered at 6.47 nm. The peak width of 0.40 nm, measured on the basis of the width at half-maximum for the pore size 50 nm distribution, indicates that the material has well defined uniform pore dimensions. The pore size of the silica replica material is less than 8.11 nm of ^' '' SBA-15, which coincides with lattice contraction in XRD results. _ .^ J(j|r./ * N'*.'• " Figure 3 represent the schematic diagram for ^ * ' • f ^ f i - - ' • : . - • • * Vl":J»^ffi the replication between mesoporous silica and mesoporous carbon materials. Framework thickness and pore sizes in Figure 3 can be 50 nm obtained from the XRD data and nitrogen adsorption-desorption isotherms for the materials. As shown in Figure 3, the pore size of the CMK-3 is very similar with the wall thickness of the SBA15 and the silica replica, respectively. These results indicate the reversible replications from silica to carbon and from carbon to silica materials. In other words, carbon sources can be placed within the void mesoporous channels of the SBA15, and subsequent removal of silica framework results in the mesopores of the CMK-3. In an Figure 2. TEM images for mesoporous opposite way, the void space of the CMK-3 is materials: (a) SBA-15, (b) CMK-3 and filled with silica species and the carbon rods give the mesopores in the silica replica by calcination. (c) silica replica. In the field of the mesoporous materials, most of the researches have focused on the silica as the framework constituent. The synthesis of mesoporous silica materials containing transition metals or pure transition metal oxides has been less successful. One difficulty may be a facile crystallization of most transition metal oxides during the mesostructure formations and the removal of the organic templates. In the
mk
Mesoporous Silica
Mesoporous Carbon
Carbon Filling & Silica Removal
I
Replica
Silica Filling & Carbon Removal
Figure 3. Schematic diagram for the replication between mesoporous silica and carbon.
56
present work, the CMK-3 material can act as a rigid template for the formation of mesoporous inorganic oxides. Various kinds of mesoporous materials constructed with alumina, titania, zirconia, manganese oxide, etc. can be successfully obtained by using this synthetic strategy. It is reasonable that the rigid nature and well-defined mesoporosity of the mesoporous carbon framework help to form and stabilize the mesostructured inorganic frameworks. The pore sizes (5.5 - 8.8 nm) of the non-silica materials are similar with that of the silica replica material, indicating that the CMK-3 material can be act as the good template for the mesoporous inorganic materials. BET surface areas and total pore volumes of the inorganic replica materials are listed in Table 1. However, XRD patterns and nitrogen adsorptiondesorption isotherms for the non-silica replica materials indicate that the mesoscopic orders of the materials are not good qualities, compared with that of the silica replica material. It is necessary to develop the optimum conditions for highly ordered mesoporous inorganic materials. This work was supported by Korea Research Foundation Grant (KRF-2002-003-D00079). Table 1 BET surface areas and total pore volumes of the inorganic repl ica materials. Materials
s,3ET (m^/g)
Vp(cm^/g)
Materials
SBET (m %)
Vp(cmVg)
SBA-15
776
1.27
Ti02 replica
92
0.31
Silica replica
685
0.90
ZrOz replica
60
0.21
AI2O3 replica
336
0.66
Mn02 replica
63
0.30
REFERENCES 1. C. T. Kresgc, M. E. Leonowicz, W. J. Roth, J. C. Vatuli and J. S. Beck, Nature, 359 (1992) 710. 2. D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka and G. D. Stucky, Science, 279 (1998) 548. 3. A. Corma, Chem. Rev., 97 (1997) 2373 and references therein. 4. A. Imhof and D. J. Pine, Nature, 368 (1997) 317. 5. B. T. Holland, C. F. Blanford and A. Stein, Science, 281 (1998) 538. 6. S. Jun, S. H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna and O. Terasaki, J. Am. Chem. Soc, 122 (2000) 10712. 7. S. H. Joo, S. J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki and R. Ryoo, Nature, 412 (2001) 169. 8. R. Ryoo, S. H. Joo, S. Jun, T. Tsubakiyama and O. Terasaki, Stud. Surf Sci. Catal., 135 (2001) 150. 9. M. Kang, S. H. Yi, H. I. Lee, J. E. Yie and J. M. Kim, Chem. Commun., revised (2002). 10. J. M. Kim and G. D. Stucky, Chem. Commun., (2000) 1159.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
57
A novel preparation route for palladium-carbon composite materials - pore filling of SBA-15 mesoporous molecular sieve Humphrey H.P. Yiu^*, Ian J. Bruce^ Fiona McGuinness^ Paul A. Wright'' ^School of Chemical and Life Sciences, University of Greenwich, Wellington Street, Woolwich, London, SE18 6PF, U.K. ^School of Chemistry, University of St. Andrews, The Purdie Building, North Haugh, St. Andrews, Fife, Scotland, KY16 9ST, U.K. A new, simple preparation route for mesoporous carbon-palladium materials is reported. In this method, a direct decomposition of palladium acetate into palladium metal was applied during carbonisation. The size of the palladium metal particles ranged from 5 to 10 nm in diameter. 1. INTRODUCTION Supported palladium nanoparticles have been used as a heterogeneous catalyst in many well-known reactions e.g. as selective hydrogenations [1], hydrodechlorination [2] and Heck reactions [3]. The choice of specific support involved varies from inorganic oxides (amorphous silica and alumina) to carbon-based materials (carbon nanotubes and carbon fibres). Similarly to common supported metal catalysts, the catalytic activity of these supported palladium materials increases with decreasing metal particle size. The palladium nanoparticles have been shown to be active in Heck reactions when supported on siliceous mesoporous molecular sieves MCM-41 [4]. A chemical vapour deposition (CVD) method was used to prepare this catalyst but is a rather complicated route to prepare supported such nanoparticles. Carbon-based materials are preferential choices as supports in many cases over other materials for metal catalysts because of the ease with which the catalyst can be recovered e.g. via simple combustion. However, in these catalysts, metal nanoparticles are mainly supported on the surface of the carbon support or inside the micropores. Because of this, the catalysts become less active in the majority of liquid phase reactions due to the poor diffusion of reactants and products into and out of the pores. A new synthetic route for preparation of ordered mesoporous carbon, named CKM, templated by SBA-15 mesoporous molecular sieves was reported by Ryoo et. al. [5]. This provided a new family of materials for supporting catalysts such as palladium nanoparticles, as well as other metal catalysts such as platinum, copper and nickel. As supports for metal catalysts, the CKM materials possess the advantages of both mesoporous molecular sieves (enhancing diffusions of reactants and products in liquid phase reactions) and carbon-based materials (known to be good catalyst supports). *Corresponding author: Tel: +44 20 83318340. E-mail address: H.H.P.Yiu@gre.ac.uk.
58
In this report, as an alternative to CVD, we have introduced a novel one-step preparation route of palladium nanoparticles/carbon/mesoporous silica composite material, which can be used in various catalytic reactions. 2. EXPERIMENTAL The preparation procedure for SBA-15 has been reported previously [6]. The PEO-PPOPEO template was removed by calcination at 540*'C in air. Palladium acetate (Aldrich) and sucrose (Aldrich) were used as the source of palladium and carbon respectively. Mesoporous silica template SBA-15 (1.0 g) was suspended in an aqueous solution (6 cm^) of sucrose (0.625g). A palladium acetate solution was prepared by dissolving palladium acetate (0.025 g) in ethanol (6 cm"^) and added into the silica suspension. The solvents were then removed by heating in an oven at 100°C and the resultant solid was heated in flowing nitrogen at 900°C in a tube furnace for 16 hours. The sample is denoted as Pd-C/SBA-15. The material was studied using transmission electron microscopy (TEM) and powder x-ray diffraction. 3. RESULTS AND DISCUSSIONS 3.1. Characterisation of siliceous SBA-15 support The results from XRD (figure 1) and TEM (figure 2) showed that the SBA-15 template possessed a highly ordered 2-D hexagonal array of straight pores. The BET surface, the pore diameter and pore volume were found to be 918 m^ g'\ 70 A and 1.0 cm"^ g"^ respectively.
20/°
Fig. 1. The x-ray diffraction pattern of the SBA-15 template shows a typical 2-D hexagonal structure.
Fig. 2. The TEM microgram of the SBA15 template shows a highly ordered 2-D hexagonal array of mesopores .
59
3.2. XRD and TEM studies of the Pd-C/SBA-15 composite The XRD pattern for the Pd-C/SBA-15 is depicted in figure 3. The peak at 40.5° was assigned to the Pd (111) diffraction and the broad peak at 22° was due to the silica wall of the SBA-15 template. The low intensity of the Pd (111) peak indicates the small size of the Pd particles. In fact this is in agreement with the TEM study of the sample.
Fig. 3. The x-ray diffraction pattern (26 = 5° - 50°) of Pd-C/SBA-15. The broad peak at 22° was assigned to the silica wall of SBA-15 while the peaks at 40.5° and 47° were assigned to Pd (111) and Pd (200) respectively. (a)
_,,^ii(M^A^^''' • ^ " ^ - - 1 , ^
(M
100 nm . ...,.^ '^^' Fig. 4. TEM micrographs of Pd-C/SBA-15. Figure 4(a) shows the palladium nanoparticles were uniform in dimension and evenly distributed. Figure 4(b) is the image of the same sample at a higher magnification. The palladium nanoparticles are clearly located inside the mesopores of the SBA-15 template. The particle size and the locations of the palladium particles in the Pd-C/SBA-15 composite material were studied with TEM imaging. Figure 4a and 4b are the electron micrographs of Pd-C/SBA-15 composite. In figure 4a, the palladium particles were shown to
60
have fairly uniform size and the diameter was ranging from 5-10 nm. Figure 4b shows the same Pd-C/SBA-15 sample with a higher magnification and the palladium nanoparticles were encapsulated inside the mesopores of the SBA-15 template are clearly visible. The TEM study also show that the mesostructure of Pd-C/SBA-15 composite materials was retained after the heating process at 900 °C. Usually, palladium nanoparticles are prepared by reducing palladaium oxide nanoparticles formed by thermal decomposition of a palladium salt [7]. In this case, since the carbonisation process takes place at 900°C, the palladium oxide particles decompose to form palladium metal particles. Normally, the high temperature decomposition of metal oxide leads to the formation of large metal particles due to sintering. Here, the sintering of metal particles was suppressed by the dimensions of SBA-15 pores and the filled carbon materials. The particle size of the palladium particles ranges from 5-10 nm. Without the carbon fillings, the palladium particle size increases. Therefore it seems likely that the carbon fillings not only act as the metal particle support but also the "sintering barrier" for the metal particles. 4. CONCLUSION Palladium nanoparticles have been successfully prepared via a simple one-step synthetic route. Due to the mesoporous structure of the SBA-15 template and its carbon fillings, the particle size of the palladium nanoparticles was controlled to between 5 to 10 nm in diameter and the SBA-15 silica template can be removed using NaOH/EtOH solution to "restore" the mesoporous structure. This new material has the potential to be used in numerous palladiumcatalysed reactions. ACKNOWLEDGEMENT We would like to thank BASF for kindly supplying the Pluronic P-123 surfactant. Dr. Sanming Yang and Mr. Marc Mamak (both Toronto) for the low angle powder XRD, Dr. George Fern (Greenwich) for TEM imaging, and EU for funding HHPY (project no. G5RDCT-2001-00534).
REFERENCES 1. A. Stanislaus, B.H. Cooper, Catal. Rev. - Sci. Eng. 36 (1994) 75. 2. L.M. Gomez-Sainero, A. Cortes, X.L. Seoane, A. Arcoya, Ind. Eng. Chem. Res. 39 (2()()()) 2849. 3. A. Biffis, M. Zecca, m. Basato, J. Mol. Catal. A: General 173 (2001) 249. 4. C.P. Mehnert, D.W. Weaver, J.Y. Ying, J. Am. Chem. Soc. 120 (1998) 12289. 5. R. Ryoo, S.H. Joo, M. Kruk, M. Jaroniec, Adv. Mater. 13 (2001) 677. 6. H.H.P. Yiu, P.A.Wright, N.P. Botting, J. Mol. Catal. B: Enzym. 15 (2001) 81. 7. S. Polizzi, P. Riello, A. Balerna, A. Benedetti, Phys. Chem. Chem. Phys. 3 (2001) 4614.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
61
Structure of ultra-thin RbBr "Solution" in carbon nanospace T. Ohkubo', H. Kanoh^'^ Y. Hattori^ T. Konishi'^ and K. Kaneko*''^ 'Graduate School of Science and Technology and ^Center for Frontier Electronics and Photonics, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, JAPAN The hydration structure of RbBr electrolytic solution confined in carbon nanospaces was determined with EXAFS technique and related analyses. The analytical results indicate that the electrolytes confined in hydrophobic nanospaces have an incomplete dehydration structure, because of an intensive restriction and cluster formation of adsorbed water molecules in the nanospaces. 1. INTRODUCTION The hydration structures of ions have gathered much attention for a long time. The Frank-Wen hydration model takes into account only the orientation of nearest hydration water molecules.' However, understanding of the electrolytes adsorbed in nanospaces is indispensable to develop the energy capacitor systems and to provide carrier mechanism of ions in biochemical systems. An intensive confinement of molecules in nanospace is expected to provide new physics and/or chemistry. Recently, it was reported that solvent molecules such as water and alcohol form highly ordered structure in hydrophobic nanospaces even at 303 K.^'^ These results stem from the hydrogen bonds of adsorbed molecules and strong potential well of pores.^' In this paper, we will show unusual local structure of both hydrated cations and anions confined in carbon nanospace by using EXAFS technique. 2. EXPERIMENTAL We used pitch-based activated carbon fiber (ACF; w=l.l nm) as an adsorbent. RbBr was introduced into nanospaces of ACF using 0.1 and 1 M aqueous solution. After filtrating and drying ACF sample containing RbBr, the samples were installed in the glass cell with the window of Mylar film (350 }im: Toray Ind. Inc.) and evacuated under 0.1 mPa and 383 K for 2 hours. Then, the samples were exposed to water vapor of the saturated vapor pressure at 303 K; those were named RbBr-xM-nanosolution (x: concentration of RbBr solution). We measured the EXAFS spectra of K-edges of Rb and Br at BL-lOB Station, Photon Factory (PF) at National Laboratory for High Energy Accelerator Research Organization (KEK) in Tsukuba. The radial structure functions (RSFs) of confined RbBr solutions for Rb and Br were calculated and analyzed by FEFF.^
62
3. RESULTS AND DISCUSSION 3.1. Determination of concentration of electrolytes confined in nanospaces The adsorption isotherms of nitrogen on ACF and RbBr-adsorbed ACFs at 77 K were of lUPAC type I, indicating the uniform microporosity. Fig. 1 shows the nitrogen adsorption isotherms of ACF and RbBr-adsorbed ACFs at 77 K. The adsorption isotherm in terms of log(P/Po) supports no serious micropore blocking. The saturated adsorbed amount of nitrogen in carbon nanospaces decreases with elevating the concentration of initial RbBr aqueous solution, indicating that the additional RbBr molecules reduced the effective surface area of carbon nanospaces. Table 1 summarizes the pore parameters obtained from the as plots and subtracting pore effect (SPE) analysis.^' ^ The concentrations of RbBr in nanospace were calculated from the reduction ratio of total surface area and micropore volume of ACF. Therefore, the values of volume fraction of RbBr in nanospace of ACF were determined by 0.25 and 0.09 for RbBr-1 M-nanosolutin and RbBr-0.1 M-nanosolution, respectively.
_0 0
ouu
•
/OO
•
600
•
500
•
• •
.
-
* *
RbBr-1 M-nanosolution RbBr-0.1 M-nanosolution ACF
700 600
i
•
800
•
400
1
500
c
400
, , • >
O
•
300 ]
RbBr-1 M-nanosoIution RbBr-0.1 M-nanosoIution ACF
•
200 <
300 C D O
E
100 1
<
01 1 . • • 1 0.2
,
•
•
.
.
.
.
•
1
.
.
'%
O
200 100 0
•
0.6
Lg^
r.^ -3 log(IVPo)
p/I^.
Fig. 1. Adsorption isotherms of nitrogen on RbBr-adsorbed ACF at 77 K. Table 1 Total surface area aa, external surface area a^xu micropore volume Wo, and the effective concentration of RbBr confined in ACF YRbBrYKbBr/M
a„/m^g''
a ex / m" g '
W «/nilg-'
RbBr-1 M-nanosolution
1290
33
0.72
1.4
RbBr-0.1 M-nanosolution
1580
34
0.83
0.7
ACF
1730
40
0.92
0
3.2. Local structure of RbBr "solution" in nanospace Fig. 2 shows RSFs of RbBr nanosolutions of 0.1 and 1 M for a Rb ion. In the figure, RSF of 1 M RbBr bulk solution is shown for comparison. The highest peak stems from the Rb-0 coordination of the nearest hydration shell. The reliable parameters by the curve fitting with FEFF calculation were also summarized in Table 2. The Rb-0 distance of the RbBr nanosolution well agrees with that of the bulk RbBr solution. However, the relative
63
coordination numbers of water molecules around a Rb ion in the nanosolution are 2.3 and 5.1 for 0.1 M and 1 M nanosolutions, respectively. Here, the relative coordination number was obtained from the assumption that a Rb ion of the bulk 1 M RbBr solution is 6. Hence the hydration number around a Rb ion in the nanospace should be less than that in the bulk aqueous solution. The Debye-Waller (DW) factor around a Rb ion for nanosolution is much smaller than that in the bulk aqueous solution. Therefore, there should be an ordered structure in the nanosolution compared with the bulk aqueous solution. Fig. 3 shows the Br-centered RSFs of RbBr nanosolution and the bulk aqueous solution of 1 M. The analytical coordination distances with FEFF, as shown in Table 2, well agree with the literature value of Br-O distance in bulk aqueous solution (0.29-0.34 nm).'^' '' The hydration number around a Br ion in the nanosolution is quite small compared with that of the bulk aqueous solution. Also, the DW factor of the nanosolution is smaller than that of the bulk solution. Accordingly, the hydration structure around a Br ion is completely unique compared with that in the bulk solution. Results of concentration of RbBr in nanospace and parameters from EXAFS measurements
RbBr aqueous solution (IM
RbBr aqueous solution (1 M)
RbBr-0.1 M-nanosolution
0.2
0.3
0.4
0.2
r / nm
Fig. 2. RSFs around a Rb ion for RbBr nanosolutions and aqueous bulk solution of 1 M.
0.3
0.4
r / nm
Fig. 3. RSFs around a Br ion for RbBr nanosolutions and aqueous bulk solution of 1 M.
Table 2 Structural parameters obtained by least-squares fit of Fourier-filtered EXAFS spectra; distances between two atoms r, relative coordination number N, and DW factor a for each sample by using FEFF. The first and second elements in the subscripts denote the central and scattering atoms, respectively. Sample rRb-o (nm) rer-o (nm) 0.323 RbBr-1 M-nanosolution 0.281 0.329 RbBr-0.1 M-nanosolution 0.282 0.325 RbBr solution (IM) 0.283 0.31-0.34 Literature value 0.288
NRb-o
Nflr-O
5.1 2.3 6
2.4 1.3 6
aRb-o(nm) CTBr-o(nm) 0.0070 0.0182 0.0069 0.0121 0.0111 0.0197
64
indicate the hydration anomaly of nanosolution confined in hydrophobic nanospace. This hydration anomaly is associated with the stability of water cluster in the nanosolution. The stable water cluster deprives the ion of the hydrated molecules, giving the observed dehydration. As a more stable water cluster should be formed in RbBr-O.lM-nanosolution compared with RbBr-1 M-nanosolution, the remarkable dehydration must occur in RbBr-O.lM-nanosolution. However, the competitive partition of water molecules for the ion and ordered water cluster in the nanosolution should be studied more. ACKNOWLEGEMENT This work was supported by Research Fellowships of the Japan Society for the Promotion of Science (JSPS) for Young Scientists. REFERENCES 1. H.S. Frank and W.Y. Wen, Discuss. Faraday Soc, 24 (1957) 133. 2. T. liyama, M. Ruike, and K. Kaneko, Chem. Phys. Lett., 331 (2001) 359. 3. T. Ohkubo, T. liyama, K. Nishikawa, T. Suzuki, and K. Kaneko, J. Phys. Chem. B, 103 (1999) 1859. 4. T. Ohkubo, T. liyama, and K. Kaneko, Chem. Phys. Lett., 312 (1999) 191. 5. T. Ohkubo and K. Kaneko, Colloid Surf A, 187-188 (2001) 177. 6. F. Rouquerol, J. Rouquerol, and K.S.W. Sing, Adsorption by Powders & Porous Solids, Academic Press, London, 1999. 7. A.L. Ankudinov, B. Ravel, J.J. Rchr, and S.D. Conradson, Phys. Rev. B, 58 (1998) 7565. 8. K. Kaneko and C. Ishii, Colloid Surf, 67 (1992) 203. 9. N. Setoyama, T. Suzuki, and K. Kaneko, Carbon, 36 (1998) 1459. 10. Y. Marcus, Chem. Rev., 88 (1988) 1475. 11.11. Ohtaki and T. Radnai, Chem. Rev., 93 (1993) 1157.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
65
Synthesis and characterization of mesoporous silica films by spin-coating on silicon: photoionization of methylphenothiazine and photoluminescence of erbium 8-hydroxyquinolinate in mesoporous silica films J.Y. Bae,' J.-I. Jung," O.-H. Park," B.-S. Bae," K.T. Ranjit,^ and L. Kevan^ "Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea* ^Department of Chemistry, University of Houston, Texas 77204-5641, USA** Transparent mesoporous silica films were successfully prepared by spin-coating on silicon wafers at room temperature. The x-ray diffraction patterns of the films indicate that both hexagonal and cubic mesoporous films can be formed by varying the surfactant to silicon mole ratio. These films have reasonable thermal stability and are calcinable up to 670 °C. Methylphenothiazine is incorporated into the silica films and after photoionization by ultraviolet light, the radical cation was characterized by electron spin resonance spectroscopy. Incorporation of erbium 8-hydroxyquinolinate into mesoporous silica films was characterized by photoluminescence. 1. INTRODUCTION Scientists at Mobil Corporation synthesized silica-based mesoporous molecular sieves with hexagonal, cubic, and lamellar structure called M41S materials in 1992. Zhao et al. [1] and Kimura et al. [2] reported the successful formation of mesoporous hexagonal aluminophosphate and silicoaluminophosphate by a modified ion-pair process. Recently, hexagonal mesoporous silicoaluminophosphate possessing improved thermal stability (calcinable up to 670 °C) was synthesized by utilizing charge density matching with optimal composition and stirring conditions at room temperature [3,4]. For possible applications of mesoporous oxide materials as sensors and optical materials, it is important to develop thin film materials. Zhao, Yang, and Stucky [5] reported dispersion of a bulk silica phase into a liquid which was dip-coated onto silicon wafers and glass slides and resulted in a continuous, uniform coating of colloidal particles. In the present study, optically transparent and crack-free mesoporous silica films have been synthesized by an evaporation-induced self-assembly process and spin-coated on silicon. Methylphenothiazine is incorporated into these films and photoionization yields stable radical cations. Incorporation of erbium 8-hydroxyquinolinate (ErQ) into mesoporous silica films is 'This research was supported by grant No.RO 1-2000-00224 from the Korea Science & Engineering Foundation and the Brain Korea 21 project. **This research was supported by the Robert A. Welch Foundation and the U.S. Department of Energy.
66
characterized by PL and isothermal nitrogen physisorption studies [6]. These materials are promising for applications such as sensor, optical devices, optical amplifiers, nanoreactors, and hosts for large organic molecules. 2. EXPERIMENTAL Tetramethylorthosilicate (TMOS, Aldrich, 98%) was hydrolyzed under acidic conditions (HCl, J.T. Baker, 36.5-38%), and then methanol (CH3OH, Merck, 99.8%) was added into the hydrolyzed TMOS at room temperature. Finally, cetyltrimethylammonium chloride (CTACl, Aldrich, 25%) was added so that the final reactant mole ratios were 1-3 TMOS : 8-16 H2O : 0.09-0.11 HCl : 18-30 CH3OH : 0.2-0.8 CTACl. The mesoporous thin films were calcined in flowing air at 550 "C for 12 h at rate of 1 "C/min. Methylphenothiazine is incorporated into the silica films and after photoionization by ultraviolet light, the radical cations are characterized by electron spin resonance spectroscopy. Incorporation of erbium 8-hydroxyquinolinate (ErQ) into the silica films was characterized by photoluminescence and isothermal nitrogen sorption studies [6]. 3. RESULTS AND DISCUSSION XRD patterns of as-synthesized and calcined mesoporous silica films in Figure 1 indicate that a mesoporous structure is formed on the silica substrate. XRD patterns show a prominent peak at 20 = 2.0-4.0° and some broad peaks at 26 = 4.0°-7.0° characteristic of hexagonal structure, which is similar to the XRD of hexagonal MCM-41 materials. In addition to preparation of hexagonal mesoporous silica film, materials with cubic phase were also synthesized by varying the surfactant to silicon mole ratio. At a surfactant/Si ratio of less than 0.25, the predominant phase is hexagonal as shown in Figure 1. As the surfactant/Si ratio increases beyond 0.25, a cubic phase is produced as shown in Figure 1. These films have reasonable thermal stability and arc calcinablc up to 670 "C. The ordered structure of the calcined mesoporous silica film was further confirmed by TFM as in Figure 2, which clearly shows a hexagonal periodicity. The calcined mesoporous sihca film thickness is 433 ± 2 nm as measured by cross-sectional SFM. The surface roughness of the calcined film was studied by AFM, and the average roughness is estimated to be less than 2 nm over a length span of 1000 nm. Nitrogen sorption shows an lUPAC type IV nitrogen adsorption and desorption isotherm. Calcined films have a BLT surface area of 920 m^/g and an average pore diameter of 2.1 nm.
^ XI0(d)
(b)
X 1 0 (^)
29/°
29/°
>-
*4!^!!^>{- -
(c)
>f%^^S*i^f^^-^^-.
':^" "A
''•«fcij^^^/-5r
Fig. 1. XRD patterns of as-synthesized Fig. 2. TEM (a), SEM (b) and AFM (c) of (a,c) and calcined (b,d) mesoporous silica calcined mesoporous silica films films
67
(a) Thin Film
1500
1550
1600
Wavelength (nm)
Fig. 3. ESR spectra of methylphenothiazine Fig. 4. PL spectrum of mesoporous silica in mesoporous silica film (a) and powder (b). film with incorporated ErQ at 300 K. Mesoporous silica films with impregnated methylphenothiazine show a weak ESR signal before ultraviolet irradiation [3,7]. After being irradiated by 320 nm light at 100 K for 20 min, the samples showed a large ESR signal as shown in Figure 3. Figure 3 shows the ESR spectra of impregnated methylphenothiazine in mesoporous silica film (a) and powder (b) after being irradiated by 320 nm light at 100 K for 20 min. These ESR spectra are an asymmetric partially resolved sextet at g = 2.0055. The photoyield of methylphenothiazine cation radical is about 35 % higher in the film compared to the powder. The relative high efficiency of the formation and stabilization of methylphenothiazine cation radical in mesoporous silica films suggest that such films are promising materials for various applications. Figure 4 shows the photoluminescence (PL) spectrum of mesoporous silica film with incorporated ErQ. It is expected that the peak at 1475nm is due to the gratings in monochromator. The main luminescence peak is at 1545nm. The bandwidth at half-maximum is 72nm. This is much wider than for any other Er-doped materials [8]. The wide bandwidth is obtained by emission from Er atoms in different local environments. Such a broad spectrum enables a wide gain bandwidth for optical amplification. Therefore it is considered that the mesoporous silica film is a good matrix to be doped by a rare-earth complex homogeneously. 4. CONCLUSIONS Transparent mesoporous silica films with hexagonal and cubic phases are formed by control of the surfactant concentration. Transparent mesoporous silica films are fairly homogeneous and relatively easy to produce. The mesoporous silica films with hexagonal and cubic phases show possibilities of application as advanced materials. The incorporation of methylphenothiazine into mesoporous silica films shows successful photoionization by ESR and the incorporation of erbium 8-hydroxyquinolinate (ErQ) into mesoporous silica films is characterized by PL and isothermal nitrogen physisorption studies. Such mesoporous films with impregnated photo-functional materials may find application as sensor, optical devices, nanoreactors, and hosts for large organic molecules. REFERENCES 1. D. Zhao, Z. Luan and L. Kevan, Chem. Commun., (1997) 1009. 2. T. Kimura, Y. Sugahara, K. Kuroda, Chem. Commun., (1998) 559.
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3. J.Y. Bae, K.T. Ranjit, Z. Luan, R.M. Krishna, L. Kevan, J. Phys. Chem. B, 104 (2000) 9661. 4. J.Y. Bae, L. Kevan, Microporous Mesoporous Mater., 50 (2001) 1. 5. D. Zhao, R Yang and G.D. Stucky, Adv. Mater., 10 (1998) 1380. 6. O.-H. Park, J.Y. Bae, J.-I. Jung, B.-S. Bae, Submitted for publication. 7. Z. Luan, J.Y. Bae and L. Kevan, Chem. Mater., 12 (2000) 3202. 8. W. J. Miniscalco, Lightwave J. Techn., 9 (1991) 234.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved
69
Synthesis of 2D hexagonal mesoporous silica thin films via phase transition from lamellar structure Chia-Wen Wu, Kunichi Miyazawa and Makoto Kuwabara* Department of Materials Engineering, University of Tokyo, 7-3-1 Hongo, Tokyo, Japan. A phase transition from lamellar to 2-dimensional (2D) hexagonal structure has been observed to occur in silica thin films during heat treatment up to 80 °C in a humid atmosphere. By way of this phase transition, transparent highly ordered 2D hexagonal mesoporous silica thin films have been successfully synthesized from a silicon alkoxide (tetraethoxysilane: TEOS) precursor solution using self-assembly triblock copolymer PI23 (E02oP07()E02()) as a template. The heat treatment conditions that allow the phase transition were investigated in detail with respect to heating rate and the hold temperature and time. Characterization of the obtained mesoporous films was made using SA-XRD, FE-SEM and TEM. 1. INTRODUCTION There have been many reports [1-3] dealing with the synthesis of silica powders and thin films with various mesostructures, such as lamellar, hexagonal and cubic ones, by controlling synthetic or aging conditions. It has been shown that the mesostructures formed in powders or thin films can be estimated by some phase determining factors such as the surfactantpacking factor and the charge density matching between the surfactant and silica species [4-5]. Moreover, the mesostructurc of the as-made silica materials can be transformed (for example from lamellar to hexagonal) by controlling these phase determining factors. The phase transformation in mesoporous materials has been widely studied mainly in silica/surfactant composites in a powder form, by putting as-made samples into hot water [4-7], but there have been little reports on thin films. In this paper, we report the phase transformation from lamellar to highly ordered 2-D hexagonal in silica thin films by heating lamellar structured films under water vapor hydrothermal conditions. 2. EXPERIMENTAL Silica/surfactant mesostructured thin films were prepared as follows: tetraethyl orthosilicate (TEOS, 98%, Wako) was partially hydrolyzed under a strong acidic condition (pll~2) [8] at 60 °C for 2 h, and then mixed with a triblock copolymer (E02()P07()E02(), Mav = 5800, Aldrich) cthanol (EtOH) solution. In this study, the final precursor solution with the
70
compositions of ITEOS: 0.01 EO20PO70EO20: 30EtOH: 0.12HC1: II.5H2O (in molar ratio) was prepared and spin cast coated on glass substrates to form thin films. Lamellar structured silica thin films were obtained by aging the as-made films from room temperature (R.T.) to 120 °C for 24 h in a dry atmosphere. Hexagonal structured silica thin films were obtained by either aging the as-made films or the lamellar structured films under water vapor hydrothermal conditions. The water vapor hydrothermal conditions were carried out by placing the as-made or lamellar structured thin film samples in a glass container with an ample of water, and heating from R.T. to 150 °C with varied ramp rates (1-3 °C/min) in an electric furnace (Miwa, MT-1100). Inside the glass container, the film sample was exposed to water vapor atmosphere, which was produced as the temperature increased. Mesostructures of the synthesized silica thin films were characterized with small angle X-ray diffractometry (SA-XRD, Rigaku-Rint2000, Cu Ka ), field-emission scanning electron microscopy (FESEM, Hitachi S5000), and transmission electron microscopy (TEM, HitachiSOO, 200kV). 3. RESULTS AND DISCUSSION Transparent mcsoporous silica thin films with a lamellar structure can be synthesized when the deposited films were aged at a temperature from R.T. to 120 °C in a dry atmosphere for 24 hours. The presence of strong (001) and (003) diffraction peaks and a weak (002) peak of the as-synthesized film indicate the formation of highly ordered alternating silica/P123 layers (Fig. la) [9]. The (001) d spacing and intensity of lamellar structured silica films is about 8.5 nm while aging at R.T. and decreases to 4.7 nm while aging at higher temperatures (Fig.2). The 2D hexagonal structured silica thin films can be synthesized when the deposited films were aged at a temperature from R.T. to 80 °C under water vapor hydrothermal conditions for 6 hours. The formation of a highly ordered 2D hexagonal mesoporous structure in the films was clearly demonstrated by five well resolved (100), (200), (300), (400) and (500) diffraction peaks in Fig. lb: the silica film was calcined at 450°C to remove the template.
5
6
Fig. 1. The mcsoporous silica films with (a) a lamellar and (b) a 2D hexagonal structure.
100
120
H*at tr«a(in«nt temp (*C)
Fig. 2. The shift of the d spacing and intensity of lamellar structured silica f ilms as function of aging temperature for 24 hours
71 The (100) d spacing and intensity of the hexagonal structured silica films is about 9 nm. While heating lamellar structured silica films at the ramp rate of l~3°C/min from R.T. under water hydrothermal conditions, the lamellar-to-hexagonal phase transformation was observed (Fig.3). The phase transformed, 2D hexagonal mesoporous silica films show the highly ordered structures that pore channels in the hexagonal mesoporous structure are highly oriented to be parallel to the surface of the substrate [10,11]. This can indeed be seen from a TEM image recorded on the silica thin film, as show in Fig. 4a,b. The FE-SEM images (Fig. 4c, d) also show the smooth surfaces and continuous mesostructures. In summary, we have synthesized lamellar and hexagonal structured silica films by controlling the aging conditions, and observed the phase transformation from lamellar to 2D highly ordered hexagonal in mesostructured silica thin films by heating as-made film samples under water vapor hydrothermal conditions. XRD patterns show the gradual transition process, and the SEM and TEM images prove the transformed films are transparent with smooth
(D) (C)
^^^^/rv.^.|^,^^^;^
20(deg.)
Fig. 3. Small-angle XRD patterns corresponding to a lamellar-to-hexagonal phase transformation in mesostructured silica thin films synthesized using EO20PO70EO20 surlaetant. The sample was heated under a water vapor hydrothermal condition, where the temperature was ramped at a rate of 1 °C/min from 25 °C (trace A), to 80 °C (trace B), 110 °C (trace C), and 150 °C (trace D), respectively.
Fig. 4. a,b) TEM images of hexagonal structured films after phase transformation alon g a) [110] zone axes and b) along [001] zone axes. c,d) SEM micrographs of c) surface and d) cross section of phase transformed hexagonal silica films after calcination.
72
surface and highly ordered mesostructures. The exact mechanism of this phase transition has not been clarified yet, but one may recognize that water vapor played the most important role in the phase transition. Water vapor strongly affects the hydrolysis and condensation reactions of alkyl silica groups, resulting in the reduction of the interfacial charge density leading to the phase transformation. This phase transformation process not only benefits us to understand the mesophase formation mechanism during the synthesis of mesoporous silica thin films with block copolymer as template, but also is expected to supply a novel pathway to prepare highly ordered mesoporous thin films via precise control of the phase transformation process.
REFERENCES 1. M. Ogawa and N. Masukawa, Micro. Meso. Mater., 38 (2000) 35. 2. D. Zhao, Q. Iluo, J. Feng, B. F. Chmelka, and G D. Stucky, J. Am. Chem. Soc, 120 (1998)6024. 3. S. Besson, C. Ricollcau, T Gacoin, C. Jacquiod, and J. P. Boilot, J. Phys. Chem. B., 104 (2000) 12095. 4. Q. Huo, D. I. Margolcsc, and G D. Stucky, Chem. Mater., 8 (1996) 1147. 5. S. H. Tolbert, C. C. Landry, G D. SUicky, B. F. Chmelka, P. Norby, J. C. Hanson, A. Monnier, Chem. Mater, 13 (2001) 2247. 6. C. C. Landry, S. H. lolbcrt, K. W. Gallis, A. Monnier, G D. Stucky, P. Norby, and J. C. Hanson, Chem. Malcr., 13 (2001) 1600. 7. K. W. Gallis and C. C. Landry, Chem. Mater., 9 (1997) 2035. 8. S. Yun, K. Miyazawa, 11. Zhou, I. Honma, and M. Kuwabara, Adv. Mater., 18 (2001) 1377. 9. Y. Lu, Y. Yang. A. Sellingcr M. Lu, J. Huang, H. Fan, R. Haddad, G Lopez, A. R. Bums, D. Y Sasaki, J. Shclnutt, and C. J. Brinker, Nautre, 410 (2001) 913. 10. I. A. Aksay, M. Trau, I. Honma, Y L. Zhou, P. Fenter, P. M. Eisenberger, and S. M. Gruner, Science, 273 (1996) 892. 11. D. Zhao, R Yang, N. Mclosh, J. Feng, B. R Chmelka, and G D. Stucky, Adv. Mater., 16 (1998) 1380.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
73
Nanostructured silicate film templated by discotic CT-complex column A. Okabe, T. Fukushima, K. Ariga and T. Aida ERATO Nanospace Project, JST, National Museum of Emerging Science and Innovation Bldg., 2-41 Aomi, Koto-Ku, Tokyo 135-0045, Japan Nanostructured
silicate
films
were prepared
composed of a triphenylene-based
using charge-transfer
disk-like molecule as templates.
(CT)
complexes
A triphenylene
derivative having hydrophilic triethyleneglycol groups was newly synthesized as an electron donor that can form CT complexes with various kinds of electron acceptors, and transparent nanostructured silicate films were obtained with these CT complexes by various coating techniques, i.e., spin-coating, dip-coating and casting.
The CT columns immobilized in the
mesopores are stable against external perturbation, and possibly have long-range structural ordering. 1. INTRODUCTION Various macroscopic morphologies of mesoporous silica have been so far reported [1-4]. Among them, mesoporous silica films containing various functional molecules have been especially paid much attention as materials for a wide-range of application including electro-optical devices [2].
Although mesoporous silicate films can be easily obtained
through solvent evaporation by dip-coating [3] or spin-coating [4], lack in methodologies to immobilize functional
molecules limits preparation of functionally-attractive
materials.
Here we propose use of CT complexes of a disk-like molecule for mesoporous silica syntheses, because columnar structures of the CT complexes [5] are appropriate for the mesoporous silica templates.
In the obtained nanocomposites, the CT columns are isolated
in the mesopores and would express various novel and attractive properties and functions [6]. In this report, fabrication of novel nanostructured silicate films using triphenylene-based CT complexes is demonstrated (Figure I) [7]. 2. EXPERIMENTAL An amphiphilic triphenylene derivative TP was newly synthesized (Figure 1). complexes
were
prepared
by
mixing
with
electron
acceptor
molecules
such
CT as
74 R = 2,4,7-trinitro-9-fluorenone (TNF), Donor -{CH2)io—(OCH2CH2)30CH3 2, 3, 6, 7, 10, 11- hexacyanohexaazatriphenylene (HAT), 7, 7, 8, TP 8tetracyanoquinodimethane (TCNQ) chloranil (CA), and 1, 2, 4, Acceptors 5- tetracyanobenzene (TCNB) in TNF, HAT, TCNQ, benzene solution. Tetrabutoxy CA, TCNB silane (TBOS) was partially hydrolysed and polymerized in the presence of the CT complexes with small amount of H2O (H20/Si = 5) in HCl/ethanol solution at room temperature for 2-48 h. The solution was then coated on substrates by spin-coating, dip-coating, or casting, followed by drying at room temperature for CT Column 12 h. Glass, mica and graphite sheets were used as the substrates. Fig. 1. Schematic representation of mesoporous silicate The obtained films were calcined films templated by CT column. at 723 K for 3 h after dried at 373 K for 12 h. The structure of the films were characterized by XRD and TEM. The CT columns immobilized in nanostructured silicate films were characterized by electronic spectroscopy.
3. RESULTS AND DISCUSSION The silicate films prepared on glass substrates by spin-coating, dip-coating, or casting from an equimolar mixture of triphenylene derivative TP and TNF (TP/TNF/TBOS/ethanol = 1/1/20/4600 in molar ratio), showed XRD peaks that were apparently different from those of non-silicate triphenylene assemblies (Figure 2a). However, these peaks disappeared after calcination, indicating that the formed phase was lamellar. This structural characteristic was also comfirmed by TEM observations (Figure 2b). Modification of the preparative conditions by increasing TBOS/TP ratio and decreasing ethanol content (TP/TNF/TBOS/ethanol = 1/1/60/1540 in molar ratio) induced the structure featured by hexagonal XRD patterns. Unlike the former case, the (100) peak remained even after the template removal by calcination (Figure 3a). TEM observations also showed hexagonally aligned pore arrays (Figure 3b). Hexagonal structures were similarly obtained
75 a)
1 25000 cps (100)
I 2500 cps
(110)
(200) uncalcined
(200)
JI
.
ft
11
uncalcined calcined
1
,
,
,
I
,
calcined
. ', „..J
4 6 2 theta / degree
4 6 2 theta / degree
50 nm Fig. 2. a ) XRD pattern and b) TEM image of lamellar cast film on glass.
Fig. 3. a ) XRD pattern and b) TEM image of hexagonal cast film on glass.
from the CT complexes of the other acceptors. The obtained siHcate films were all highly transparent, and stained in blue to red colors depending on used acceptors. Table 1 summarizes absorption maxima that are characteristics of CT interaction, and the maxima showed red-shifted features compared with those of the corresponding CT complexes in nonstructured media. The latter fact may indicate long-range structural ordering of the CT columns in mesopores. In addition, the colors of the CT complexes in the hexagonal mesoporous silicate films stably remained even when the films were immersed in acetnitrile solution containing the other acceptor molecules. In contrast, the lamellar composite film was immediately decolored upon exposure to acetnitrile. These Table 1 results indicate that the hexagonally Absorption maxima (nm) of mesoporous silicate films -arranged silica framework is an containing CT colomns. appropriate medium for stable TNF TCNB HAT CA TCNQ accommodation of the one -dimensional CT columns in 490 548 615 700 890,410 well-ordered pore structures.
76
4. CONCLUSION We successfully demonstrated the first example of immobilization of one-dimensional columnar CT complexes into the pores of transparent mesoporous silicate film. The CT columns segregated by the hexagonally-arranged silica framework are highly stable and possibly have long-range structural ordering. The films obtained in this research would be highly useful for nano-fabricated devices based on various electro-optical properties. REFERENCES 1. Q. Huo, D. Zhao, J. Feng, K. Weston, S. K. Buratto, G. D. Stucky, S. Schacht and F. Schuth, Adv. Mater., 9(1997)974. 2. Y. Lu, Y. Yang, A. Sellinger, M. Lu, J. Huang, H. Fan, R. Haddad, G. Lopez, A. R. Bums, D. Y. Sasaki, J. Shelnutt and C. J. Brinker, Nature, 410 (2001) 9131. 3. Y. Lu, R. Ganguli, C. A. Drewien, M. T. Anderson, C. J. Brinker, W. Gong, Y Guo, H. Soyez, B. Dunn, M. H. Huang and J. I. Zink, Nature, 389 (1997) 364. 4. M. Ogawa, Chem. Comm., (1996) 1149. 5. H. Bengs, M. Ebert, O. Karthaus. B. Kohne, K. Praefcke, H. Ringsdorf, J. H. Wendorff and R. Wustefeld, Adv. Mater., 2 (1990) 141. 6. N. Boden, R. J. Bushby, J. Clements, B. Movaghar, J. Mater. Chem., 9 (1999) 2081. 7. Okabe, T. Fukushima, K. Ariga and T. Aida, Angew. Chem. Int. Ed., in press.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
Mesoporous Titania Photocalcination
Thin
Film with
77
Cubic
Mesostructure
using
U-Hwang Lee, Young Kyu Hwang and Young-Uk Kwon Department of Chemistry and BK-21 School of Molecular Science, Sungkyunkvy^an University, Suwon 440-740, Korea. FAX: +82-31-290-7070. Well ordered mesoporous titania thin films with cubic structures were prepared by evaporation-induced self-assembly of a mixture composed of titania nanoparticles and diblock copolymers followed by UV-irradiation treatment to remove the surfactant molecules. This new strategy of using nanoparticles shows enhanced reproducibility over the others reported for mesoporous materials of non-silica compounds. The photocalcination provides further improvement by having less structural distortion upon the organic template removal process. 1. Introduction Ti02 thin films are used in dye-sensitized photoelectrochemical cells, as antireflection material in solar cells, as gas sensors, in photoelectrocatalysis, in photocatalysis, and in luminescence.''^ However, attempted syntheses of mesoporous titania have achieved only limited success in terms of the control of the mesostructure and reproducibility. Especially, the low reproducibility of reported synthesis methods for non-silica mesoporous materials is a serious problem for the future development of this class of materials. It is partly because the condensation of the inorganic precursors does not occur in harmony with the self-assembly process of the template surfactant molecules. Previously, we have reported a new approach of using titania nanoparticles to solve these problems.'^ Because the condensation step and self-assembly step were separately controlled, this approach gives reproducible results and easy control of the mcsostructures. However, the thermal calcination to remove the organic molecules accompanies structural distortions, which is an important drawback of this process. In order to solve this problem, we have employed the photocalcination technique in the place of thermal calcination and have synthesized mesoporous titania thin films with little structural distortions from the ideal cubic or hexagonal mcsostructures.'^^ 2. Experimental Section 2.1 Synthesis The synthesis of mesoporous titania is achieved as described in our previous paper except that
78
the removal of the surfactant molecules is performed by photocalcination instead of thermal calcination at 350-450°C. The synthesis of mesoporous titania is achieved in four steps of 1) synthesis of nanoparticles, 2) blending nanoparticles with template molecules into thin films, 3) aging the blended mixtures into mesostructures under appropriate conditions, and 4) calcination to remove the organic templates by UV-irradiation treatment. Stock solutions of Ti02 nanoparticles were prepared according to the literature procedure with slight modifications.'^ TiCUwas dissolved in absolute ethanol to make the final concentration 20 wt. %. A mixed solution of cone. HCl and 35% H2O2 was added and the solution was refluxed at 80°C for 2h. A Brij-type block copolymer, CnH2n+i(OCH2CH2)yOH, with n/y= 16/20, was dissolved into the Ti02 stock solution. The molar composition of the final solution was TiCl4/CnEOy/HCl/H202/EtOH/H20 = 1/0.083/3.8/0.97/6.1/15. The solution was dip-coated on silicon substrates, and the resultant thin films were dried and aged at 18°C under a controlled huminity of 80%. Finally, the as-made thin films whose mesostructures were confirmed with powder X-ray diffraction were photocalcined with a UV-ozone lamp (253.Inm + 184.9nm, 5W) at room temperature. 2.2 Characterization Characterization of the thin film by X-ray diffraction (XRD) was carried out with a Rigaku D/max-RC. FT-IR spectra were measured by using a Nicolet 1700 FT-IR spectrometer. TEM images were obtain by a HRTEM (JEOL-3011, 300kV). 3. Result and disscusion The selective removal of organic surfactant using the UV-irradiation treatment was verified by FT-IR measurements. The as-made (before calcine) thin films show pattern for the C-H stretching frequencies at about 2800cm ', methylene (-CH2) and methyl (-CH3) groups
( d )
""^
-''
( c )
^,/ /
\p H 3 0 0 0
w
( b )
\/' '' ( a )
- H 2 5 0 0
a v c n u m
b c r
( c m
')
Fig 1. FT-IR spectra of mesoporous Ti02 thin films after UVirradiation a) Omin, b) 15min, c) 30min, d) 45min
79
bending frequencies at about 1300 - 1600 cm ~' and bending frequencies of many CH2 groups in a open chain (long chain) at about 650 - 750 cm ' from the surfactant molecules. (Figure la) Figure lb ~ d show the gradual decrease of absorption bands as a function of UVirradiation time. In the sample treated with UV-irradiation over 45 minute, these peaks are absent, indicating complete removal of the organic molecules.
( b ) 5 .5 ni
( a ) 7 .1 n 1
20/degree
Fig 2. XRD patterns of mesoporous Ti02 thin films prepared using C16EO20 templates aged at 18 °C (a) as-made (b) after UV-irradiation for 45min. The as synthesized film prepared with C16EO20 block copolymer formed a mesostructure as evidenced by the XRD peak with d = 7.1 nm. (Figure 2a) The XRD peak was shifted to d = 5.5 nm upon UV-irradiation indicating a lattice contraction. (Figure 2b) Although the XRD patterns do not reveal the details of the structure with only one peak, the observed TEM images in Figure 3 show a cubic mesostructure view in the [110] direction with duo = 3.9 nm in direct correspondence with the XRD result which give dioo == 5.5 nm in an excellent agreement. The TEM images displayed a regular pore structure with the mean pore size of about 3nm. The pore walls are continuous with a thickness of about 2.5 ~ 3nm. Unfortunately,
Fig 3. TEM images of the calcined thin film that show a cubic structure Ti02thin film after UV-irradiation
80
our TEM and XRD data of UV-irradiated Ti02 thin films did not provide any evidence of crystalline Ti02 in the walls, probably because the particles were not well crystallized or were too small in size. The UV-irradiated Ti02 thin film shows less contraction of the dioo-space as well as decrease in the intensity of the (100) peak than thermally calcined Ti02 film. These results show that photocalcination of mesostructured Ti02 thin film has less structural distortion than thermal calcinations. 4. Conclusion Compared with our previous results with thermal calcination of the same material that produced a distorted cubic mesostructure, the advantage of the photocalcination for producing well-ordered mesoporous titania is evident. The effectiveness of the photocalcination can be assessed with the infrared spectra of the film materials before and after the UV-irradiation. The characteristic absorption peaks for the organic molecules disappeared completely. Probably, the photocatalytic effect of titania for decomposing organic molecules also has contributed to the almost complete removal of the surfactant molecules. This work was supported by KOSEF(SRC, CNNC) and School of Molecular Science through BK21 project. References (1) Y. Matsumoto, Y. Ishikawa, M. Nishida and S. li, J. Phys. Chem. B 20(X), 104,4204. (2) A. Hagfcldt and M. Griitzcl, Ace. Chem. Res., 2000, 33, 269. (3) W. Lin, W. Pang, J. Suh and J. Shen, J. Mater. Chem. 1999, 9, 641. (4) Thompson, D. W. and Meyer, G. Langmuir 1999, 15, 650 (5) Doeswijk, L. M. and Rogalla, H. M. Appl. Phys. A 1999, 69, S409 (6) Lin, H. M. and Tung, C. Y. Nanostruct. Mater. 1997, 9, 747. (7) Ichikawa, S. and Doi, R. Thin Solid Film 1997, 292, 130. (8) Xagas, A. P. and Falaras, P. Thin Solid Film 357, 173 (9) Y. K. Hwang, K. C. Lee and Y. U. Kwon, Chem. Commun., 2001, 1738. (10) Keene, M. T. J. and Llewellyn, P.L. Chcm.Commun.1998, 20, 2203 (11) H. K. Park, D. K. Kim and C.H. Kim, J. Am. Ceram., Soc, 1997, 80, 743
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights resei*ved
Preparation of Tin Modified Silica Mesoporous Film Brian Yuliarto'', Hao-Shen Zhou***, Takeo Yamada'', Itaru Honma'', and Keisuke Asai'* ^ Department of Quantum Engineering and System Science, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo,Bunkyo, Tokyo 113-8656, Japan. ^ Energy Material Group, Energy Electronic Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba-shi, Ibaraki 305-8568 Japan.
Tin modified silica mesoporous films have been prepared by direct synthesis for the first time. The films were synthesized using cetyltrimethylammonium chloride as structure-directing agents and deposited on glass substrates by spin coating and finally calcined at 400^^ C. Characterization including small axis x-ray diffraction patterns, nitrogen absorption desorption and atomic force microscope were observed. The resulting films with the Sn/Si ratios of 0.005-0.03 have shown to maintain mesoporous structure after calcinations. The pore size uniformity was confirmed by the nitrogen absorption desorption isotherm although the hexagonal pore arrangements were disordered as the increasing amount of tin loaded into the solution. This novel tin modified silica mesoporous film has promising application in catalyst as well as sensor devices. 1. INTRODUCTION The unique and prospective of mesoporous silica properties, which contain large surface area and porosity, uniform pore size distribution, order pore arrangement, and possible surface engineering, has stimulated the development of application in various fields [1-3]. Recently, the research related to mesoporous silica has been conducted to improve the performance by modifying the structure of it. The modification by incorporating transition metal ions into the mesoporous material is of great interest for catalytic application as well as sensing devices. In oxide condition tin would be very interesting phenomena, because tin oxide has semi conductive properties and has been widely used as a catalyst for oxidation of organic compounds, optical electronics and especially as a gas sensors [4]. To date, the previous result of tin containing silica mesoporous material concerned only in powder forms [5,6]. Further more, it is well known that film is an ideal morphology for the applications of mesoporous materials to optical, electronic and sensing devices. As a consequence, it is necessary to synthesize tin modified mesoporous silica materials in thin film form. In this paper, we report the first synthesis (to our knowledge) of tin modified mesoporous silica film using sol gel methods. The molar ratio of tin and silica is varied between 0.005 and 0.05 molar. Moreover, the effect of amount of tin added into silica mesoporous thin film was investigated. Corresponding author. Tel.: •81-298-61-5795; fax: < 81-298-61-5829. E-mail: hs.zhour«)-aist.go.jp
82
2. EXPERIMENTAL SECTION Tetradecyltrymethylammonium chloride (abbreviated as C16TAC) was obtained from Tokyo Kasei Industries Co. Tetraetyl orthosilicate (abbreviated as TEOS), tin (IV) chloride, 1-propanol, 2-butanol, and hydrochloric acid were purchased form Wako Pure Chemicals Industries, Ltd, All materials are used without further purification. The tin modified mesoporous silica film was synthesized according to our group published procedure with quite alteration to introduce tin material [7,8]. The samples were prepared in the following way. The certain amount of TEOS was mixed with 1 -propanol and stirred for several minutes. The TEOS was hydrolyzed via the addition of a solution of previously mixed HCl and water, after that 2-butanol was added and stirred. In addition, the SnCU with variation of concentration was added while the solution was stirred. The C16TMA-CI surfactant solution was slowly added under stirring into the previously prepared sol. Finally the solution was the spin coated on a glass substrate. The final gel composition is 1 TEOS, 6.4981 1-Propanol, 2.6505 2-Butanol, 6.8472 H2O, 0.2632 IN HCl, and xSnCU, with x = 0.005, 0.01, 0.02, 0.03, and 0.05. Calcinations for the resulted films were performed at 400" C for 60 minutes. Atomic force microscopy (AFM) analysis was carried out to investigate surface morphology of the films on AFM SPA300HC from Seiko Instrument Inc. The obtained sample films were also characterized by X-ray diffraction (XRD), and Nitrogen absorption desorption isotherm. The small angle X-ray diffraction (SAXRD) pattern were observed for both after synthesis and after calcinations sample on a Mac Science M03XHF22 using CuK a radiation operated at 40 kV and 50 mA. Nitrogen absorption desorption of the calcined films were measured using Bell Sorp 18+ (Bell Japan Inc.) at 77 K. The sample for absorption desorption measurement was prepared on cover glass with several micrometer thickness. The Brunauer-Emmet-Teller (BET) calculation and Dollimore-Heal (DH) method were applied to calculate the specific surface area and the pore size distribution, respectively.
CI)
(b)
Ui
Fig. 1. AFM surface morphology of tin-modified mesoporous film with Sn/Si - 0 (a), 0.01 (b), 0.03 (c).
3. RESULTS AND DISCUSSION The film produced from the sol gel method was transparent film even after addition for all percentage of loading SnCU- The thickness of film can be adjusted by controlling the spinning rate of coating process. According to the AFM record, the film surface is rougher as tin content increases, as shown in Figure 1. The x-ray diffraction record for as synthesis samples with different Sn content are shown in Figure 2-1. From the picture, it can be observed that all samples with ratio between 0.005
83
and 0.05 reveal peak as synthesized, indicates that mesostructure was formed. However the peak intensity became weak as the increasing of Sn/Si ratio, except it was quite strong when the amount of Sn/Si was 0.03. [2-1]
[2-2]
(0 (e)
M JSl (b) (a) 3
4
26 L"J
3
4
20 ["]
Fig. 2. X-ray difiraction pattern of the as-synthesizcd [2-1] and as calcined [2-2] tin-modified silica mesoporous film with Sn/Si - 0 (a), 0.005 (b), 0.01 (c), 0.02 (d), 0.03 (c), and 0.05 (0-
The x-ray diffraction pattern for Sn-containing silica thin film of the as calcinations in the different Sn contents are shown in Figure 2-2. Nevertheless, when the percentages of SnCU reached to 0.5% of TEOS, the solution was no longer homogenous, and the resulting film tend to slight turbid. This phenomenon indicates that the films have a homogeneous structure until 3% of Sn/Si ratio, however when the ratio reach 5 %, the tin oxide was being isolated particle so that the heterogeneous solution was formed. Therefore, although the diffraction peak was observed in samples up to 0.003 of Sn/Si ratio, the peak became broad with the increase in the loaded Sn amount. Finally the peak did not disappeared at all for 0.05 of Sn/Si ratio, showing that mesoporous structure was broken. TTie values of d spacing increased as the increasing of Sn/Si ratio. Furthermore, at the same ratio condition, the d values decreased upon calcinations. This fact indicates that wall structure of film sample shriveled after removal of surfactants. Nitrogen absorption desorption isotherms of as-calcined films in all ratio of Sn/Si are shown in Figure 3-1. From the absorption desorption records, it is clear that the isotherm are type IV as identified by TUPAC for all loading Sn amount. This indicates that mesoporous structure was formed within the film samples. TTie adsorption isotherm reveals a large inflection in the partial pressure (P/Po) range around 0.2, which is the typical graph of capillary condensation within uniform mesoporous. The surface areas according to the BET calculation method are 1150 m'^g, 661 m"'^g, 620 m~^g, 514 m^ g, and 312 m~^g for pure silica, 0.5%, 1%, 2%, and 3% respectively. The calculations according to DH plots of the derivative of the pore volume per unit weight with respect to the pore radius are performed as shown in Figure 3-2. A narrow pore size distribution is observed in all samples ratio. Nevertheless, the pores size distribution after tin incorporating is shifted towards lower values as the increasing
84 [3-1]
1.107
[3-2]r - • - 0% Sn / Si -•-0.5%Sn/Si - A - 1 % Sn/Si - • - 2% Sn/Si - • - 3% Sn/Si
mrli-M I i 1.0
0.2
0.4
0.6
0.8
10
R e l a t i v e pressure [P/P^I
0
1
2
3
Pore Radius [nm]
Fig. 3. Nitrogen adsorplion/desorption isotherm [3-1J and DII pore distribution [3-2J of the calcined tin-modified silica mcsoporous film in all condition
of Sn/Si ratio, indicates that the pore structure is being disturbed as the increasing of tin, which is consistent with x-ray diffraction pattern. Additionally, these four samples posses a similarity of peak location at 2.2 A of diameter. This fact indicates that the addition of tin into silica mesoporous does not change the pore size. In addition, the phenomenon also explain that tin disperse within the framework of the film sample in spite of in the pore. 4. CONCLUSION It is concluded that the synthesis modification of silica mesoporous film allows to introduce tin into the mesoporous silica film. The film properties of tin-modified silica mesoporous samples depend on tin loading. According to the evidences, the mesoporous silica structure suffer degradation their structure after modification of 3% ratio of Sn/Si upon direct synthesis. Moreover, the direct synthesis to prepare the samples gives convenience method for metal transition incorporating into mesoporous silica film. REFERENCES 1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. 2. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmidtt, C.T-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B.Higgins, J.L. Schlenker, J.Am. Chem. Soc. 114(1992) 10834. 3. S. Inagaki, Y. Fukushima, K. Kuroda, J. Chem. Soc, Chem. Commun. (1993) 680. 4. F. Chen, M. Liu, Chem. Commun. (1999) 1H29. 5. G. Li, S. Kawi, Sensors and Actuator B 59 (1999) 1. 6. Y. Teraoka, S. Ishida, A. Yamasaki, N. Tomonaga, A. Yasutake, J. Izumi, I. Moriguchi, S. Kagawa, Microporous and Mesoporous 48 (2001) 151. 7. H.S. Zhou, D. Kundu, I. Honma, J. of European Cer. Soc. 19(1999) 1361. 8. Honma, H.S. Zhou, D. Kundu, A. Endo, Adv. Mater. 12 (2000) 1529.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved
85
Novel non-lithographic large area fabrication method to generate various polymeric nanostructures Woo Lee, Mi-Kyoung Jin, Won-Cheol Yoo, and Jin-Kyu Lee School of Chemistry and Molecular Engineering, Seoul National University, Seoul 151 -747, Korea. FAX: +82-2-882-1080. E-mail: jinklee@snu.ac.kr A simple and completely non-lithographic route has been developed to fabricate freestanding nanostructured polymeric films or polymer/nanoparticle composite films with a close-packed hexagonal array of nanolenses or nanoposts by using electrochemically prepared textured aluminum sheets or mesoporous anodic aluminum oxides (AAOs) as a replication master. TEM, FE-SEM, and AFM analyses revealed that our nanofabrication procedure could provide a convenient route to produce multiple copies of polymeric nanostructures over several square centimeters, even in an ordinary laboratory where one could not make routine access to state-of-the-art lithography facilities. 1. INTRODUCTION Fabrication of the materials with nanometer-scale periodic array is of utmost importance due to their potential technological applications in high-density magnetic memories, singleelectron devices, and optical media.''^ The most common techniques used for generating periodic array in nanometer scales so far are lithographies (i.e., ion- and electron-beam lithography, x-ray lithography, probe-tip based lithography, and etc.). However, they have some fundamental drawbacks, such as low throughput and high cost requiring state-of-the-art facilities. Herein we report facile and completely non-lithographic routes for fabricating large area nanostructured polymeric films with a two-dimensional vast array of nanolenses or spatially well-separated nanoposts using electrochemically prepared textured aluminum sheets or mesoporous AAO. We also demonstrated that the textured aluminum sheets could be used as a replication master for the fabrication of the polymer/nanoparticle composite films, whose structural features are characterized as a close-packed hexagonal array of polymeric nanoembosses containing nanoparticles. The financial support from the Interdisciplinary Research Program (Grant No. 1999-2-121-004-5) of the KOSEF is greatly acknowledged.
86
2. EXPERIMENTAL Textured aluminum masters with 2-D hexagonal array of approximately hemispherical concaves on their surfaces were prepared by the two-step electrochemical oxidation of Al using 0.3 M H2SO4 (10 ^'C), 0.3 M H2C2O4 (17 °C), and 10 wt. % H3PO4 (-3 °C) to give different sizes of concaves, followed by the complete removal of porous AI2O3 films using an aqueous acid mixture of 1.8 wt. % chromic acid and 6 % H3PO4. On the other hand, mesoporous AAO replication master was prepared by briefly anodizing textured aluminum for 100 s. Fabrication of free-standing thin films of polystyrene (PS) replicas of respective replication master has been realized by spin-on assisted replica molding or nanoimprint pattern transfer, followed by stripping of nanostructured polymeric films. The structures of the replication masters and the replicated polymeric films have been investigated by using AFM, FE-SEM, and TEM. 3. RESULTS AND DISCUSSION FE-SEM investigation revealed that the surface of the textured aluminum consists of closepacked hexagonal arrays of approximately hemispherical concaves (i.e., a honey-comb structure). The radius (r) of each concave varies as a function of the anodization voltage with r = 2.6 nm/V; the average radii of concaves in the textured aluminums prepared from H2SO4 (25 V), H2C2O4 (40 V), and H3PO4 (160 V) are 61 nm, 111 nm, and 420 nm, respectively (Fig. 1).
Fig. 1. FE-SEM images of (a ~ c) anodic aluminum oxide (AAO) and (d ~ e) textured aluminum master produced from (a and d) 25 V H2SO4, (b and e) 40 V H2C2O4, and (c and f) I6OVH3PO4.
87
On the other hand, further anodization of the textured aluminum master produces highly ordered mesoporous AI2O3 film with cylindrical channels at the precise center of hemispherical concaves, generating another replication master. In this case, the interpore distance of the mesoporous AI2O3 film is predefined by the center-to-center distance of concaves on a textured aluminum and the length of channels depends on the anodization time. Fabrication of a free-standing thin films of polymer replica of the present masters has been realized by spin-on assisted replica molding or nanoimprint pattern transfer technique. In spin-on assisted replica molding method, the replication master was supported on the chuck of a conventional spin-coater and then polymer solution was placed on the surface of the master. Typically we used commercial polystyrene (PS) (M.W. = 1 x 10^) dissolved in methylene chloride (10 wt. %). By subjecting the master to a specified spinning rate (typically, 3000 rpm for 30 s) the solvent of the polymer solution was allowed to evaporate. The free-standing polymer thin film with vast arrays of nanostructure was easily separated from the replication master by simply immersing the sample into distilled water. In nanoimprint pattern transfer (see Schemel 1.), the replication master was placed directly on the polymer substrate. A pressure was applied to hold the master against the polymer substrate. The whole assembly was heated uniformly to a temperature slightly above the glass transition temperature (Tg) of polymer, and then cooled down to room temperature. The master was easily removed from the polymer substrate to give the large area nanostructured polymer surface. (a)
textured . aluminum'
,„......„.^^^^
substrate (\)\
^
p^^^^^^^
heating (T >'l\ of polymer)
^
(rrr^
cooling and master removal
Schemel 1. Schematic illustration of nanoimprint pattern transfer using (a) textured aluminum sheet and (b) mesoporous AAO as replication master According to our AFM and FE-SEM studies, the surface structures of polymeric films replicated from the textured aluminums or mesoporous AAO masters are mainly characterized as a 2-D hexagonal arrangement of nanolenses and nanoposts, respectively (Fig. 2); the surface of the replicated polymeric films have exactly complementary structures of the replication masters, manifesting the high fidelity of the spin-on assisted replica molding or nanoimprint pattern transfer technique for producing a periodic nanostructures over large area.
Fig. 2. FE-SEM micrographs of freestanding polystyrene (PS) films with 2-D hexagonal array of (a and b) nanolenses and (c and d) nanoposts.
Fig. 3. FE-SEM image of polystyrene/ferrite nanoparticle composite film; TEM image of the sample is also presented as an inset.
Replication masters could also be used to produce various nanocomposite films, whose structural features are characterized as a close-packed hexagonal array of nanoembosses containing inorganic nanoparticles. This has been simply achieved by stepwise spin-on assisted loading of nanoparticles and polymer solution, followed by stripping of nanocomposite films. In Figure 3, we presented a typical FE-SEM image of polystyrene/ferrite nanoparticle composite film produced by this procedure, together with TEM micrograph as an inset. The present electron micrographs reveal clearly that each nanoemboss contains several ferrite nanomagncts (ca. 10 nm), forming close-packed hexagonal arrangement. It is worthy to note here that this novel process could provide ample variations of the constituents of the nanocomposite. The nanoparticles could be nanometersized polymer beads, quantum-sized metals, catalytically important semiconductor nanoparticles, or oxide nanomagncts. The polymeric matrices could also varied from elastomers, thermoplastics, to conducting polymers. Considering a broader perspective, the utilization of textured aluminum sheets or mesoporous AAOs as replication master presents a novel and exciting methodology for preparing various surface nanostructures. In addition, it is expected that our process will open up the new possibility of generating various functional materials for many interesting applications (for instances, anti-reflection, anti-fogging, electromagnetic wave shielding, and etc.) and that it will attract much attention from the academic and industrial communities. REFERENCES 1. Black, C. T. et al., Science, 290 (2000) 1131. 2. Postma, H. W. Ch. et al.. Science, 293 (2001) 76.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
89
Mesoporous anodic alumina membrane with highly ordered arrays of uniform nanohole C.W. Lee', C.I. Lee^ Y. Lee^ H.S. Kang^ Y.M. Hahm' and Y.H. Chang^* 'Department of Chemical Engineering, Dankook Univ., Seoul, 140-714, Korea. ^Department of Chemical Engineering, Inha Univ., Inchon, 402-751, Korea. The mesoporous anodic alumina(AA) membrane with highly ordered arrays of uniform nanohole was prepared by anodic oxidation process in an aqueous solution of sulfuric acid at 20 °C. Morphology, pore size, pore size distribution and thickness for mesoporous AA membrane was examined with several anodizing conditions; reaction temperature, current density, electrolyte concentration, amount of additive, etc. The pore density in the array was approximately 3.5-5.8 X lO'"* m"^ with pore diameter and membrane thickness of approximately 25-35 nm and 50 j^m. 1. INTRODUCTION The properties of materials or devices can be tailored by controlling their microstructure on atomic level, which has become an emerging interdisciplinary field based on solid state physics, chemical, biology and material science[l]. Recently, the fabrication of quantum dot or nanodot arrays has attracted considerable attention, because these nanostructures show not only the novel physical properties, but also the potential application in the electronic devices, catalysts, gas absorption/separation membranes and efficient sensors[2-4]. In general, porous AA membranes are used in a number of diverse applications, such as filtration, bioreactors, analytical device including sensor and as supports for active materials[5-7]. However, most fabrication methods are not satisfactory due to some drawbacks, such as low uniformity of the shapes and sizes of pores, etc. Moreover, preparation of mesoporous A A membrane with highly ordered arrays of uniform nanohole was very difficult. We reported herein the mesoporous AA membrane with highly ordered arrays of uniform nanohole was prepared by anodic oxidation using DC power supply in an aqueous solution of sulfuric acid at 20°C. 2. EXPERIMENTAL The aluminum plate used in this study has 99.8% purity (size: 30X70X0.6mm). Prior to anodic oxidation, sample was washed several times using distilled water and acetone to eliminate the impurities on the surface. After washing, thermal oxidation was executed for 15 min at 580 °C to make better formation of pores. Subsequently, a chemical polish was made with a solution of H3P04(3.5 vol%)-Cr03(45 g/L) for 10 min at 80°C. Electrochemical polish
90
was made at a constant current of 2.87A with a solution of H3P04(85wt%)-H2S04(98wt%)H20(7:2:lby volume), which contained 35g/L CrOs, for 10 min at 40°C and washed once more in distilled water. In order to prepare PAA membrane by anodic oxidation, the sample was sealed with silicon rubber except reaction side. In order to mesoporous AA membrane with highly ordered arrays of uniform nanohole was examined with several anodizing conditions; reaction temperature, current density, electrolyte concentration, amount of additive, etc. The anodic oxidation was carried out at constant current density. To prevent the chemical dissolution of mesoporous AA membrane during anodic oxidation, aluminum sulfate and aluminum nitrate as additive were added to electrolytic solution of sulfuric acid at a reaction temperature of 20°C. The remaining Al substrate of the PAA membranes was removed by 0.1 M CuCb solution containing 32wt% HCl at a room temperature. Scanning electron microscope (SEM) photographs were obtained with a Jeol JSM-5800, N2 adsorption measurements were performed at 77K using a Micromeritics ASAP 2010 analyzer utilizing Barrett-Joyner-Halenda (BJH) calculations of pore volume and pore size distributions. 3. RESULTS AND DISCUSSION Figure 1 shows the scanning electron microscopy (SEM) photographs of the surface of mesoporous AA membrane prepared by anodic oxidation at various reaction temperature in an aqueous solution of sulfuric acid. As can be seen from this figure, we get the mesoporous A A membrane with smooth surface and uniform pore diameter at low temperature.
Fig. 1. SEM photographs of mesoporous AA membrane prepared at various temperature in sulfuric acid. [cone. : 15wt%, current density : 20 mA/cm^](a) 20°C, (b) 10°C, (c) 0°C In case of high reaction temperature, degree of chemical dissolution increased because of high ion activity. Forward reaction rate for second ionization reaction of sulfuric acid is reduced at lower temperature (see Van't Hoff Eq.[8]). Hydrogen sulfate (HSO4) as weak acid with lower dissolution activity was formed since second ionization constant of sulfuric acid was decreased. It plays an important role in electrode reaction of anode and it reduces degree of chemical dissolution because of space charge formation by proton. Figure 2 shows the SEM photographs of the surface of mesoporous AA membrane prepared by anodic oxidation in an aqueous solution of sulfuric acid containing Al2(S04)3 as an additive at 20 °C reaction temperature. The mesoporous AA membranes, with highly ordered arrays of uniform nanohole, was obtained.
91
Fig. 2. SEM photographs of mesoporous AA membrane prepared according to current density. [Cone. : 5wt%, Al2(S04)3: 10 g/£] (a) 10 mA/cm^', (b) 30 mA/cm^ (c) 50 mA/cm^ Figure 3 and 4 shows effect of additive quantity on pore diameter of mesoporous AA membrane prepared at various current density and electrolyte concentration. The amount of aluminum sulfate was increased with concentration of electrolyte increasing. Namely, 15g additive per 1 L electrolyte was proper. Pore diameter scarcely varied by current density and electrolyte concentration with the use of additives during anodic oxidation.
l -
1
•
• A -'-^ ^
1
# • •
1
1
-
•
•
.A •
•
30 mA/cm 40 mA/cm' 50 mA/cm
Fig. 3. Effect of additive quantity on pore diameter of mesoporous AA membrane prepared at various current density.[Conc. : 15wt%]
1
-
-L-^
•
1
-•5wt% - • - 10wt% A 15wl% 1
1
1
1
10
15
20
AI,(SO,)3(g/l|
Fig. 4. Effect of additive quantity on pore diameter of mesoporous AA membrane prepared at various electrolyte concentration, [current density : 30 mA/cm^]
92
Mesoporous AA membranes were prepared with pore sizes in the range of approximately 25 to 35nm, and corresponding pore density in the range of approximately from 3.5X10'"* to 5.8X io'"* m"^ and membrane thickness of 50 ^m. 4. CONCLUSION It was made an attempt to get mesoporous anodic alumina membrane by adding an additive in sulfiiric acid during anodic oxidation at 20 °C and aluminum sulfate as an additive suppressed the degree of chemical dissolution. Moreover, the amount of aluminum sulfate was increased with concentration of electrolyte increasing. Namely, 15g additive per 1 L electrolyte was proper. Pore diameter scarcely varied by current density and electrolyte concentration with the use of additives during anodic oxidation. It was possible preparing of anodic alumina membrane with highly ordered arrays of uniform nanohole. The pore density in the array was approximately from 3.5 xio'"* to 5.8Xl0''*m'^ with pore diameter and membrane thickness of approximately 25 ~35 nm and 50 Mm. REFERENCES 1. H. Gleiter, Acta Mater., 48, (2000), 1. 2. I. Amlani, et al.. Science, 282, (1988), 1473. 3. S. Fafard, et al., Phys. Rev., B 59, (1999), 15368. 4. S. Drecker, et al., J. Am. Chem. Soc, 118, (1996), 12465. 5. G. Graff, Science, 253, (1991), 1097. 6. W. R. Bowen, and D. T. Hughes, J. Membrane Sci., 51, (1990), 189. 7. S. K. Dalvie, and R. E. Baltus, J. Membrane Sci., 71, (1992), 247. 8. P. W. Atkins, Physical Chemistry, 4th Ed., 219, 1989.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
93
Preparation and characterization of poly(ester)-silver and nylon-silver nanocomposites Seong-Ho Choi^, Kwang-Pill Lee^* and Sang-Bong Park'' ^Department of Chemistry Graduate School, Kyungpook National University, Daegu 702-701, South Korea. Polymerization R&D Team, R&D Center, Kolon Industries, Inc., Kumi, 730-030, South Korea. The carboxylic acid-modified Ag nanoparticles were prepared by reaction of Ag colloidal nanoparticles and mercaptosuccinic acid.
The carboxylic acid-modified Ag nanoparticle was
precipitated
bonding
by
means
mercaptosuccinic acid.
of
hydrogen
of
an
carboxylic
acid
group
onto
The Ag nanocomposites were also prepared by polymerization of
poly(ester) and nylon in the presence of the carboxylic acid-modified Ag nanoparticle.
In a
poly(ester)-Ag nanocomposite, the Ag nannoparticle was aggregated in the poly(ester) matrix, whereas the Ag nanoparticle was dramatically dispersed in a nylon matrix. 1. INTRODUCTION The nano metal particle-organic polymer composites have attracted considerable interest in recent years.
These composites not only combine the advantageous properties of metals and
polymers but also exhibit many new characteristics that single-phase materials do not have. They have a wide range of applications including electromagnetic inference shielding, heat conduction, discharging static electricity, conversion of mechanical to electrical signal, and the like.'-^ The nano metal particle-polymer composites can be simply prepared by homogenizing polymer and nano powder.
In order to homogenize nano metal and polymer, combination
between the hydrophilic properties of nano metal and the hydrophobic properties of the polymer matrix can be considered. In this study, the Ag colloidal nano particles were synthesized by y-irradiation using silver salt in H2O in the presence of radical scavenger and colloidal stabilizers.
The surface of the
Ag nanoparticle was simply immobilized by the self-assembling of the thiol group in •Corresponding author. Tel.: +82-53-950-5901; fax: +82-53-952-8104. E-mail address: kplee@knu.ac.kr
94
mercaptosuccinic acid. The mercaptosuccinic acid-modified Ag nanoparticle powder was analyzed by XRD and TEM. The poly(ester)-Ag nanocomposite and nylon-Ag nanocomposite were prepared by a homogenizing method. The characteristics of the poly(ester)-Ag nano composite and nylon-Ag nanocomposite are discussed. 2. EXPERIMENTAL 2.1. Preparation of silver colloidal nano particle."^'^ The preparation procedure of the Ag nanoparticle and the carboxylic acid-modified Ag nanoparticle. The solution (500 mL) containg the AgNOs (25.3g), 2-propanol (3.3 mL) as radical scavenger, and PVP (0.5g) as stabilizer was prepared, oxygen was removed by bubbling with pure nitrogen for 30 min. and then irradiated by Co-60 y-ray source. 2.2. Immobilization of carboxylic acid onto surface of silver colloidal nanoparticle. In order to obtain Ag nanoparticles powder in Ag colloidal solution, the carboxylic acid group was introduced onto the surface of an Ag colloidal nanoparticle. A typical preparation procedure was described below. The mercaptosuccinic acid solution (I.OXIO"^ M) was prepared and then added to the Ag colloidal solution by sonicating. The Ag nanoparticle was precipitated and separated by centrifuge. 2.3. Synthesis of poly(ester)-Ag nanocomposite and nylon-Ag nanocomposite. Poly(ester)-Ag nanocomposite. After the condensation reaction of the dimethylterephthalate and ethylene glycohol at 140 ~ 230 °C using magnesium acetate as catalyst, the ethylene glycohol dispersed Ag nano powder was added to the reaction solution. The reaction solution was reacted in the presence of arsenic(lll) oxide (AS2O3) as catalyst at 280 °C for 3 hrs in a vacuum state. Nylon-Ag nanocomposite. The e-caprolactam dispersed with an Ag nanoparticle was maintained on 15 kgf/mm^ at 260 °C for 1 hr and repeatedly reacted in previous condition after decompressed to normal pressure for 1 hr. 3. RESULTS AND DISCUSSION Figure 1 shows the UV spectra of the silver colloidal solution prepared by y-irradiation. The band at about 400 nm, which is due to colloidal nano silver, which is due to silver cluster plasmons. Fujita et al.^ began the synthesis of metal aggregates by the radiolytic reduction of metal cations in solution. In order to obtain Ag nanoparticles powder, the author selected the compound containing the carboxylic acid group (-COOH) and thiol group (-SH) because
95
carboxylic acid groups have hydrogen bonding sites in solution and the thiol group bonded the surface of the metal particle. Figure 2 shows XRD spectra of the Ag nanoparticle (a) and carboxylic acid-modified Ag nanoparticle (b). XRD patterns show the products are metallic silver. The average size crystallite sizes calculated from peak broadening of XRD patterns by the Scherrer equation. The average size of the Ag nanoparticle and the carboxylic acid-modified silver was to be 18.8 and 5.4 nm, respectively. The size of the silver nanoparticle precipitated by centrifuge higher than that of carboxylic acid-modified Ag silver. It may be considered that the large size Ag particle was precipitated by centrifuge of 45000rpm/min. Figure 3 shows TEM image of the Ag nanoparticle (a) and the carboxylic acid-modified Ag nanoparticle. The shape of Ag nanoparticle and the carboxylic acid-modified Ag nanoparticle was spherical-type powder. Figure 4 shows the SEM image of the poly(ester)-Ag and nylon-Ag nanocomposite: surface of poly(ester)-Ag nanocomposite (a), surface of poly(ester)-Ag nanocomposite etched by plasma (b), surface of nylon-Ag nanocomposite (c), and surface of nylon-Ag nanocomposite (d). The Ag nanoparticle were aggregated onto a poly(ester) matrix, whereas the Ag nanoparticle was dramatically dispersed onto a nylon matrix. For these reasons, the carboxylic acid-modified Ag nanoparticle were dispersed onto nylon with a hydrophilic backbone chain, whereas the carboxylic acid-modified Ag nanoparticle was aggregated on poly(ester) with a hydrophobic backbone chain.
10
20
30
40
50
60
20 (cleg.)
Fig. 1. UV spectra of Ag nanoparticle prepared by y-irradiation.
Fig. 2. XRD spectra of Ag nanoparticle (a) and carboxylic acid-modified Ag nanoparticle (b).
96 ;.5i-::>;..'^vrv,i:.-
J\ •
(a)
(b)
Fig. 3. TEM image of Ag nanoparticlc (a) and COOH-modificd Ag nanoparticlc (b).
Fig. 4. SEM image of surface of the poly(estcr)-Ag (a), surface of the poly(estcr)-Ag etched by plasma (b) surface of the nylon-Ag (c), and surface of nylon-Ag nanocomposite etched by plasma (d).
ACKNOWLEDGEMENT This work was supported (in part) by the Ministry of Science & Technology (MOST) and the Korea Science and Engineering Foundation(KOSEF) through the Center for Automotive Parts Technology(CAPT) at Keimyung University.
REFERENCES 1. S.T. Selvan, T. Hayakawa, and M. Nagami, J. Phys. Chem. B, 103 (1999) 7441. 2. X. Xu, Y. Yin, X. Ge, H. Wu, and Z. Zhang, Materials Letter, 37 (1998) 354. 3. J.-C. Huang, X.-F. Qian, J. Yin, Z.-Kang Zhu, and Il.-J. Xu, Mater. Chem. Phys., 69 (2001) 172. 4. H. S. Nalwa, Handbook of Nanostructured Materials and Nanotechnology, ACADEMIC PRESS, INC., New York, 2000, chapter 9. 5. H. Fujita, M. Izawa, and H. Yamazaki, Nature, 196 (1962) 666.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
97
Synthesis of Ordered Three-Dimensional Large-pore Mesoporous Silica and Its Replication to Ordered Nanoporous Carbon Jie Fan^, Chengzhong Yu^, Limin Wang^, Yasuhiro SakaInoto^ Osamu Terasaki*"^, Bo Tu^, Dongyuan Zhao^* ^ Department of Chemistry, Fudan University, Shanghai 200433, P. R. China, ^ Department of Physics, Tohoku University Sendai 980-8578, Japan "^CREST, Japan Science and Technology Corporation, Tohoku University Sendai 980-8578, Japan Ordered three-dimensional (3D) large-pore mesoporous channels have been fabricated based on mesoporous silica SBA-15 and SBA-16 by a new synthesis strategy, which involves the introduction of organic co-solvents followed by a high temperature hydrothermal process. The previous small entrances (2.3 nm) for caged cubic mesoporous SBA-16 can be enlarged (up to 10.8 nm) by using this approach. These highly ordered mesoporous silica materials with large entrances have been verified to be suitable templates for the synthesis of ordered cubic carbon replicas with a novel ball-type pore structure. 1. Introduction Highly ordered large pore mesoporous silica shows great importance for many applications, such as catalysis, separation, adsorption and fabrication of nanostructured matcrials.|l-5| Compared with ID channel MCM-41, 3D mesoporous materials have the advantage in the mass diffusion and transport because of their interconnecting networks. Except for bicontinuous cubic MCM-48 {Ia3d), 3D mesoporous materials, including cubic SBA-1 (/^w3^), 3D hexagonal SBA-2 and SBA-12 {P63/mmc\ and cubic SBA-16 and FDU-1 {Im3m) structures, have caged mesostructures, in which small entrances block the large pore channels. Erom a standpoint of applications, it is oi^ great importance to break the channel dimensional limitation for original ordered mesoporous silica SBA-15, as well as tailor the cavity and entrance dimension for SBA-16 (specially enlarge their original small entrances). |6| Recently, many efforts have been taken to the synthesis of ordered mesoporous carbons templated from mesoporous silica templates for their potential applications in advanced electronic devices, shape-selective catalysts, and hydrogen-storage systems. [7-10] Various ordered mesoporous silicas have been chosen as the templates for the variation of the
98
mesostructure of mesoporous carbon. Up to date, mesoporous carbon with 3D cubic {ImSm) structure has not been reported. Scheme 1
•:?,!;"\"™:r''
Scheme 2 window size < 4 nm
window size 10 8 nm
MX
lanocrystal in caged mesoporous silica
Schematic illustration for the pore structure of 1) 3D mesoporous SBA-15 and 2) entrances expanding of 3D cubic caged mesoporous silica SBA-16. 2. Experimental Section The modified 3D mesoporous SBA-15 was synthesized from Pluronic P123 {MOK^O-IOMOIO) under an acidic condition. Organic co-solvent (1,3,5-trimethylbenzene, fMB) was introduced into embryo mesostructured material (if desired), and the as-synthesized products were under a high temperature hydrothermal treatment (up to 150°C). The caged cubic mesoporous SBA-16 with large entrances (denoted as SBA-16L) was synthesized by using triblock copolymer with long V\0 segments (such as HOio^POvoP^Oio^, 1'127) as a template. Different from that for 3D mesoporous SBA-15, TMB acts as co-surfactant in the presence ol' inorganic salts. Dispersed Au nanocrystals were prepared by using the large window mesoporous silica as the hard templates according to an easily loading approach reported previously. 111 j The synthesis of carbon using SBA-16L as a hard template was similar to that reported by Ryoo and co-workers | 7 | except for difference of silica-to-sucrose ratios. 3. Results and Discussion 3.1 3D SBA-15 3D large-pore mesoporous SIiA-15 can be prepared by a high temperature hydrothermal process, which involves the introduction of '1MB as an organic co-solvent into embryo mesostructured SBA-15, illustrated in Scheme 1. XRD patterns show that 3D modified mesoporous silica SBA-15 has an average mesostructure of hexagonal space group symmetry p6m. [12] HREM imgaes show that 3D SBA-15 has many nanosized ( 2 - 8 nm) connections /tunnels that are randomly distributed between the ID-channels (Figure 1). The presence of the interconnected tunnels results in the formation of 3D large pore (average pore size up to 22.3 nm) networks.
99
Figure 1. TEM images of calcined 3D mesoporous SBA-15 viewed along a) [100] and b) [110] direction. 3.2 SBA-16 with Large Entrances (SBA-16L) Highly ordered large pore (15.4 nm) mesoporous silica SBA-16 with large entrances (up to 10.8 nm) has successfully been synthesized at high temperature (130°C) according to Scheme 2. XRD patterns and TEM images show that the SBA-16L with large entrances has excellent structural ordering for a cubic space group {Jm3m) with a cell parameter (a) of 21.8 nm (Figure 2a, b). The nitrogen sorption isotherms show the entrance size of SBA-16L can be large up to 10.8 nm, suggesting that the window of SBA-16 can be enlarged after high temperature hydrothemal process in the presence of inorganic salt such as NaCl or KCl. The
_caicined
\ 0
V a 1
2
as-synthesized 3
4
2 Theta value
100 nm
5
60nhi 2 Theta value
Figure 2. XRD patterns (a, c) and TEM images (b, d) for silica SBA-16L (a, b) and its carbon replica (c, d).
100
negative diluted Au nanocrystals prepared from SBA-16L products as a hard templates can be used to image the opened connectivity of the neighbouring spherical cavities, as well as entrance dimension. 3.3 Cubic Mesoporous Carbon Using SBA-16L as Hard Template Such caged cubic mesoporous silica SBA-16L with large entrances has unblocked 3D pore networks and facilitates to prepare stable mesoporous carbon. Here we have successfully synthesized a highly ordered cubic mesoporous carbon with a novel ball-type pore structure by using above large entrance sized SBA-16L as a template. XRD pattern of the mesoporous carbon (Figure 3a) shows characteristic of body center cubic mesostructure, similar to the silica template. TEM images for the carbon sample reveal that it has the same structural symmetry with the silica SBA-16L template (Figure 3b). Such fully ordered carbon products can not be obtained by using conventional SBA-16 with small entrances as a template. We think that the interconnected channels (less than 2.3 nm) of mesoporous carbon prepared from conventional SBA-16 is quite small, resulting in that the mesostructure of the carbon is unstable during the preparation. 4. Conclusions Ordered 3D large-pore (22.3 nm) mesoporous SBA-15 and cubic caged SBA-16 with large entrances (up to 10.8 nm) have been successfully synthesized. Such large entrances SBA-16L facilitates to prepare stable mesoporous carbon with a novel ball-type pore structure. This work was supported by NSF of China (Grant No. 29925309 and 20173012), Shanghai Promote Center (0/52nm029). State Key Basic Research Program ofPRC
Nanotech.
(G2000048001).
References I . e . T. Krcsge, M. \\. Lconowicz, W. J. Roth, J. C. VartuH, J. S. Beck, Nature 1992, 359. 710. 2. J. Wu, A. F Gross, S. H. lolbert,./ Phys. Chem. B 1999, 103. 2374. 3. S. S. Kim, W. / . Zhang, X. J. Pinnavaia, Science 1998, 2H2. 1302 4. Z. Zhang, S. Dai,./. Am. Chem. Soc. 2001, 123. 9204. 5. D. Zhao, Q. Muo, J. Feng, B. \\ Chmelka, G. D. Stucky,./. Am. Chem.Soc. 1998, 120, 6024. 6. a) Y. Sakamoto, M. Kancda, O. Terasaki, D. Zhao, J. M. Kim, G. D. Stucky, H. J. Shin, R. Ryoo, Nature 2000, 40S, 449. b) M. Kruk, V. Antochshuk, J. R. Matos, L. R Mercuri, M. JaroniccJ. Am. Chem. Soc. 2002, 124. 768. 7. R. Ryoo, S. H. Joo, S. Jun,./. Phys. Chem. B 1999, J03. 7743. 8. S. H. Joo, S. J. Choi, 1. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Nature 2001, 412. 169 9. J. Lee, S. Yoon, T. Hyeon, S. M. Oh, K. B. Kim, Chem Commun 1999, 2177. 10. S. Kim, T. J. Pinnavaia, Chem. Commun. 2001, 2418. 11. Y. J. Han, J. M. Kim, G. D. Stucky, Chem. Mater 2000, 12. 2068. 12. J. Fan, C. Yu, L. Wang, i^. Tu, D. Zhao, Y. Sakamoto, O. lerasaki, ./ Am. Chem. Soc. 2000,/2J, 12113.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
101
Morphology control of mesoporous SBA-16 using microwave irradiation Young Kyu Hw^ang^'^, Jong-San Chang^, Young-Uk Kwon^, and Sang-Eon Park^* ^Catalysis Center for Molecular Engineering, KRICT, PO Box 107, Yusong, Taejon, 305-600, Korea ''Department of Chemistry and BK-21 School of Molecular Science, Sungkyunkwan University, Suwon 440-740, Korea. FAX: +82-42-860-7676. Well ordered mesoporous SBA-16 was successfully synthesized within an one hour by employing microwave-irradiation (MI). Morphologies of these materials prepared by controlling the aging time of silica sol, can be addressed with decaoctahedron and spherical shape. Particle size of mesoporous mateials is also controlled by an effective heat transfer agent as a microwave active material, such as ethylene glycol. 1. INTRODUCTION Well ordered large pore mesoporous materials which have the pore-size distribution from 2 to 30nm have been researched for their applications such as catalysis, separation and nanoscience.''^ Moreover, three dimensional cubic mesoporous silicas have advantage compared to hexagonal mesoporous material with the one-dimensional channels. In addition to the pore size tuning, morphology control of mesoporous silica have been reported such as fibers, spheres, hollow tubulars, and monoliths. Compared with conventional hydrothermal method, microwave-synthesis of nanoporous material have the advantages of the rapid crystallization time and homogeneous nucleation. In this regards, microwave irradiation technique has been widely introduced to the synthesis of nanoporous materials such as zeolite A, Y, ZSM-5 and MCM-41'^"'^ Recently, ordered hexagonal mesostructured SBA-15 under acidic condition by employing microwave has been reported by Kormaneni and co-workers'^^ In our previous work, we reported synthesis of MCM-41 using microwave heating, and mechanism study for mesoporous MCM-41.'' Herein, we report the microwave-synthesis of mesoporous SBA-16 with the morphology of dodecahedron and spherical shapes obtained by controlling the aging time and amount of the ethylene glycol.
2. EXPERIMENTAL 2.1. Synthesis Mesostructures of silica-polymer were obtained following the synthesis procedure reported elsewhere ^ except for the use of the microwave synthesis. In a typical synthesis, 1.6g of EOi()6P07oEOi()6 polymer mixture was dissolved in 41.2g of distilled water and then 4.7 Ig of the gel was stirred for x Min.(x=0, 30, 120). The gel obtained was loaded into microwave
102
oven to increase degree of silanol group condensation under microwave irradiation condition at 373K for 1 hour. The molar composition of the final gel mixture was 1.0 Si02 : 3.17x10'"^ F127; 6.68 HCl : 137.9 H2O : 1.4-8.4 EG. Microwave synthesis was performed using MAR5(CEM Corp., Matthews, NC) microwave digestion system. We denote samples depending on stirring tiems and microwave irradiation times, such as stirring for 30min and microwave irradiation for 60min(S30/M60). 2.2. characerization Mesostructures were monitored by X-ray diffraction (XRD, Rigaku D/max-RC). Transmission electron microscopic (TEM) images were taken using a JEM-3011 instrument (JEOL) equipped with slow-scan CCD camera operating at 300 keV. Scanning electron microscopic (SEM) images were collected with a JEOL 630-F microscope operating at 5 kV. N2 adsorption-desorption isotherms were obtained using a Micromeritics ASAP 2040 apparatus at liquid N2 temperature. 3. RESULTS AND DISCUSSION 3.1. Structure and morphology of SBA-16 XRD pattern of the as-made and calcined samples obtained by stirring for 30min and MI 120min (S30/M60), can be indexed as a cubic mesophase with (110), (200), and (310) diffractions (Im3m space group, a=155A and 133 A for assynthesized and calcined samples, respectively) which is consistent with the reported for SBA-16.^ Although 3 the XRD patterns do not reveal the < details of the structure with these diffraction patterns, the TEM image of the calcined material, in Figure Ic, 50nm C can be explained with Im3m cubic o structure with the [11 l]-direction, a = 130A. On the contrary, sample which is not seen grown without stirring (S0/M60) has a single intense peak at 1 2 3 4 5 6 7 a d spacing of 112 A, typical of the 2 6 I degree peak corresponding to disordered Fig. 1. XRD patterns of (a) as-made and (b) calcined and mesostructured silica prepared by nonionic surfactant templating.'^ The TEM image of (c) calcined SBA-16 N2 adsorption and dcsorption isotherms of calcined sample give a BET(Brunauer-Emmett-Teller) surface area of 822.9 m^g" ' and a pore volume of 0.73 cm^g'. The morphology of well ordered mesoporous SBA16(S30/M60) has a decaoctahcdron shape, with 6 squares and 12 hexagon, which has a relatively uniform size of ~2 //m.(Figure 2). The morphology of the observed crystals of this material is also consisted with a cubic structure.'^ The formation of a well-defined external morphology of the SBA-16 suggests that the mesostructured material has a highly ordered structure and a low level of imperfections or defects in the lattice. When samples prepared without stirring (S0/M60) have spherical shape.
103
• ^
*
3.2. Controlling particle of disordered materials M e s o s t r u c t u r e d material having disordered spherical shape prepared without stirring before MI has an irregular size of 0.5-5 (im in Figure 3a was obtained. When the EG as a heat transfer agent under MI is added into the synthetic solution, particle size distribution of spherical shape of disordered mesostructured materials having smaller one can be controlled depending on an amount of ethylene glycol Fig. 2. SEM image of as-made SBA-16(S30/M60) as can be s e e n from the SEM images(Figure 3). In Figure 3a, the size of the mesoporous materials have wide ranges of distribution from 1 fim to 10 fim. In a EG/H2O ratio of 0.04, particle size has been almost homogeneously distributed with a ~ 2 //ni. As M *^Jf
^jL^-y-.J: ^\ -f^ w^ i >*^-«
I: LK'
X^^d^
».'-•• v,"^:/?". /j^?»' <'*-r-Ti
Fig. 3. SEM images showing the external morphology of disordered matcrials(S0/M60) for the different EG contents (a)EG/H2O=0.0 (b) EG/H2O=0.02 (c) EG/H2O=0.04, and (d) EG/H2O=0.06 increasing the amount of EG/H2O more than 0.06, size distribution has widely dispersed again. With the view of amount of EG, even though water {z= 71.9 at 25 °C) has higher dielectric constant than ethylene glycol (8= 41.4 at 25 °C), the effect of microwave energy on improving monodispersity of particles was more significant in the addition of ethylene glycol into water compared to water medium itself. These results are in good agreement with those reported under the microwave irradiation for the MCM-41 .^ and demonstrates that upon the microwave preparation of mesostructured silica the use of small amount of ethylene glycol contributes to improving the isolation of aggregated gels or nuclei with resulting in more homogeneous nucleation.
104
4.CONCLUSION Microwave irradiation technique is an effective method for the synthesis of well ordered cubic SBA-16 within an Ihour and large pore disordered mesoporous silicas. Morphology and structure of mesostructued silicas have been controlled by initial stirring times and MI times. Effective heat transfer agent such as ethylene glycol, has a good candidate for a controlling the particle size distribution. We appreciate the Korean Energy Management Corporation (Institutional Research Program, TS-0110) for supporting this work. Y. K. Hwang also appreciates School of Molecular Science through BK21 project. REFERENCES 1. Kresge, C. T.; Leonowiez, M. E.; Roth W. J.; Vartuli, J. C ; Beck, J. S. Nature, 1992, 359, 710. 2. Zhao, D.; Huo, Q.; Feng, J.; Chemlka, B. F; Stucky , G. D. / Am. Chem. Soc, 1998, 120, 6024. 3. Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science, 1995, 269, 1242 4. C.S. Cundy, Collect. Czech. Chem. Commun. 63 (1998) 1699. 5. S. A. Galema, Chem. Soc. Rev., 26 (1997) 233. 6. C. Gabriel, S. Gabriel, E. H. Grant, B. S. J. Halstead, D. M. P. Mingos, Chemical Society Reviews, 27 (1998) 213. 7. A. Arafat, J.C. Jansen, A.R. Ebaid, and H. van Bekkum, Zeolites, 13 (1993) 162. 8. S.-E. Park, D. S. Kim, J.-S. Chang, and W. Y. Kim, Catal. Today, 44 (1998) 301. 9. J.C. Jansen, A. Arafat, A.K. Barakat and H. van Bekkum, in M.L. Occelli and H.E. Robson (Eds.), Synthesis of Microporous Materials, Van Nostrand Reinhold, Vol. 2, New York, 1992, p. 507. 10. Newalkar, B. L.; Komarncni, S.; Katsuki, U. Chem. Commun., 2000, 2389 11. Sung-Suh, H. M.; Kim, D. S.; Park, S. -E. J. Ind. Eng. Chem. 1999, 5, 191. 12. Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242 13. Yu, C ; Tian, B.; Fan, J.; Stucky, G. D.; Zhao, D.. J. Am. Chem. Soc, 2002, 124, 4556
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
One-step synthesis microporosity
of
mesoporous
105
silica
SBA-15
with
ultra-high
Sen-Chi Hung^ Hong-Pin Lin^ and Chung-Yuan Mou^' ^ * ^Institute of Chemistry, National Taiwan University, Taipei, 106, Taiwan. ''Institute of Chemistry, Academia Sinica, Taipei, 115, Taiwan. ^Center of Condensed Material Research, National Taiwan University, Taipei, 106, Taiwan. Mesoporous silica materials SBA-15 with tunable microporosity were synthesized starting from sodium silicate solutions and triblock copolymer surfactants Pluronic 123 (E02oP07oE02(), Mav = 5800). Microporosity within the ordered mesoporous silica SBA-15, confirmed by physisorption, could be controlled by varying HVSi molar ratios (pH value) of synthetic gel. The fraction of the micropore volume in the total volume can be adjusted up to 0.22. 1. INTRODUCTION Hexagonal ordered mesoporous silica materials SBA-15 were synthesized using triblock copolymers and tetraethoxysilane by Zhao et al \ but the high cost of silicon alkoxides was an undesirable features of the preparative chemistry. Sodium silicate is a cheaper candidate to replace the silicon alkoxides as the silica source.^""^ The nonionic surfactants and sodium silicate result in the formation of nanocomposites through hydrogen bond or coulomb interaction. Recently, several detailed structural studies " on SBA-15 have been reported, it has been confirmed that the presence of micropores within the pore walls of SBA-15. A synthesis for the systematic control of micropore volume has been developed for the mesoporous silicas SBA-15 under acidic condition using nonionic surfactant mixture, and the microporosity was dependent on the temperature of syntheses and the TEGS-to-surfactants ratios . The origin of these micropores is due to the hydrophilic character of PEG chains of the surfactants '^. For this reason, the EG groups of the surfactants are proposed to chelated with the Si-GH groups of the walls and are responsible for the generation of microporosity. The hydration of the EG groups and the condensation of Si-GH groups would be key roles for microporosity. We explore the role of pH in controlling microporosity.
2. EXPERIMENTAL 2.1. Sample preparation Gur approach used sodium silicate as the silica source and N^ surfactant as the structure director. The surfactant, amphiphillic triblock copolymer (ethylene oxide)2o(propylene
106
oxide)7o(ethylene oxide)2o (EO20PO70EO20, PI23; MW 5800, BASF), and a desired amount of acid, sulfuric acid (H2SO4) which is used to control the degree of acidity of reactive mixture, were first mixed at ambiemt temperature and then the mixture was added to the sodium silicate solution (-27% Si02, -14% NaOH, Aldrich) to form reactive silica. Immediately the polymerizing silica was aging under stirring at a temperature in the range 30-50°C for a period of 12-20 h. This process allowed the assembly of framework structure at different pH and avoided the need for readjusting the pH once after the reactive silica has been formed. The composition to synthesize mesoporous silica with tunable micropore-mesopore ratio was carried out at a molar ratio N^/Si in the range 0.008-0.012 and H20/Si in the range 240-350. The resulting materials were recovered by filtration, washing with water and drying at 60°C for 24 h. The nonionic surfactant were removed by calcination in air at 560°C for 6 h. 2.2. Characterization X-ray powder diffraction (XRD) patterns were performed with a 0.02° step size and 1.5-s step time over the range 0.3° < 26 < 3.0° on the Wiggler-A beamline {X = 0.1326 nm) of the Synchrotron Radiation Research Center, Hsinchu, Taiwan. The N2 adsorption-desorption isotherms were collected at 77 K on a Micromeritics ASAP 2010 apparatus. The calcined samples were degassed at 200 °C for 6 h in 10'^ torr before analysis. The pore size distribution was derived from the analysis of the adsorption branch of the isotherms by the BJH (Barrett-Joyner-Halenda) method. 3. RESULTS AND DISCUSSION 3.1. X-ray powder diffraction (XRD) analysis. Calcined SBA-15 samples prepared with different M VSi molar ratio displayed patterns with
A .A M M
t
\
(h) (K)
\
(0
\
»
(c)
\
(c)
(d)
\
(d)
(c) (b)
(c)
^
(b)
(")
0.5 1.0 1.5 2.0 2.5 3.0 0.5 1.0 1.5 2.0 2.5 3.0 20/dt'Krcc
20/deKrec
Fig. 1. X-ray powder patterns of A) SBA-15 samples prepared in the H^/Si molar ratio of (a) 0.96, (b) 0.99, (c) 1.03, (d) 1.04, (e) 1.06, (f) 1.08, (g) I.IO, (h) 1.39 at 3 0 ^ . B) SBA-15 samples prepared in the H"*7Si molar ratio of (a) 0.99, (b) 1.04, (e) 1.05, (d) 1.06, (e) 1.08, (0 1.14, (g) 1 . 3 7 a t 5 0 r .
Fig. 2. N2 adsorption-desorption isotherm of A) SBA-15 samples prepared in the H*/Si molar ratio of (a) 0.96, (b) 0.99, (e) 1.03, (d) 1.04, (e) 1.06, (0 1.08, (g) 1.10, (h) 1.39 at 30 C- B) SBA-15 samples prepared in the H^/Si molar ratio of (a) 0.99, (b) 1.04, (e) 1.05, (d) 1.06, (e) 1.08,(0 114,(g) 1 . 3 7 a t 5 0 r .
107
a sharp peak in the range of 0.5°-1.0° and two long order weak peaks in the range of 1.2°-1.5°. X-ray diffraction results confirmed the hexagonal structure for SBA-15 materials synthesized at 30-50°C (Figure 1 A, B). The unit cell parameters (ao) corresponding to the djoo spaces for the calcined samples are listed in Table 1. Overall, the increase in HVSi molar ratio decreased the unit cell parameter and the increase in temperature increased the unit cell parameter. However, there were no ordering arrays for samples synthesized in HVSi molar ratio below 0.99. Furthermore, the relative intensity of the dwo peaks (Figure lA) for samples prepared at 30°C was found to be much higher than those prepared at 50°C. This could be due to the decreased uniformity in the SBA-15 framework with the increased temperature of reaction. 3.2. Nitrogen adsorption-desorption measurements. Porosity data (Table 1) determined on nitrogen adsorption/desorption for the calcined SBA-15 samples were shown in Figure 2. In the case of SBA-15 prepared at 30°C, larger unit cell parameter and pore diameter, reported in Figure 2A and 2B, were firmed in higher HVSi molar ratio of synthesis, as witnessed by the shift to higher relative pressure of the pore-filling step of the isotherms in Figure 2. Table 1 Porous Parameters for SBA-15^ Reaction temperature ("O 3()'C
40"C
50C
IT/Si ratio
pH value
0.96
9.30
0.99
8.68
«»(nm) 12.54
DBJH (nxn)
/ (nm)
F, ( m i g ' )
9.55
-
0.67
K^y, ( m l g ' ) 0.02
K;^,, ( m l g ' ) 0.62
0.02
7.75
4.79
0.82
0.01
0.75
0.00 0.13
Vf,,,/VTratio
1.03
7.52
10.97
5.94
5.03
0.51
0.12
0.38
1.04
4.66
10.45
5.26
5.19
0.51
0.18
0.32
0.19
1.06
3.27
10.45
5.14
5.31
0.52
0.16
0.35
0.17
1.08
2.96
10.45
5.13
5.32
0.53
0.16
0.36
0.16
1.10
2.69
10.45
5.23
5.22
0.59
0.17
0.42
0.16
9.98
5.11
4.87
1.39
1.90
0.96
9.45
0.51
0.10
0.40
0.11
0.74
0.01
0.70
0.01
1.01
8.37
12.19
7.63
1.03
7.59
11.55
6.87
4.56
0.81
0.00
0.78
0.00
4.68
0.68
0.10
0.56
1.04
6.67
11.55
0.09
7.14
4.41
0.68
0.21
0.48
1.05
3.64
0.18
10.97
5.45
5.52
0.48
0.19
0.28
1.08
0.21
3.02
10.70
5,24
5.46
0.50
0.10
0.38
0.11
1.55
1.71
10.45
5.09
5.36
0.48
0.07
0.39
0.08
0.99
9.19
1.02
0.00
0.94
0.00
1.04
7.82
12.19
8.74
3.45
0.86
0.04
0.79
0.03
1.05
7.41
12.19
8.33
3.86
0.80
0.10
0.68
0.08
1.06
4.03
12.19
7.21
4.98
0.64
0.24
0.38
0.22
1.08
3.05
12.19
5.96
6.23
0.41
0.16
0.23
0.19
1.14
2.53
11.86
5.74
6.12
0.42
0.17
0.24
0.19
1.37
1.83
9.75
6.07
3.68
0.53
0.18
0.33
0.18
9.05
10.58
^Key: tt/Si molar ratio; ao, lattice constant determined by XRD; DBJH, pore diameter calculated from the adsorption branch of the isotherm by the BJH method; t, wall thickness calculated on lattice constant-pore diameter; V(, total pore volume calculated at P/Po = 0.98; V jup and Vmp, microporous and mesoporous volumes detected on t-plot respectively; Vj, total volume of silica, Vj = V/,p + V,np + V^.aii •> VwaU, volume of silica wall, V^aii = l/psiO:-
108
The unit cell parameter, reported in Figure 2A as a function of HVSi molar ratio of syntheses, decreased with the HVSi molar ratio of syntheses from 12.54 nm (for HVSi molar ratio = 0.99) to 9.98 nm (for HVSi molar ratio = 1.39). And the pore diameter, got the same trend, decreased from 7.75 nm to 5.11 nm. The increase of unit cell parameter and pore diameter corresponded to a property of nonionic surfactants, which the micell size inceased with a fall of HVSi molar ratio. A fall of HVSi molar ratio to brought about a partial dehydration of the PEO units and decreased the volume of the hydrophilic corona. At low pH value (< 3.0), the high concentration of S04^" anions promotes the micellization capability of the tri-blockcopolymer and reduces the microporostiy of the final product. 3.3. ^^Si MAS NMR spectra. The ^^Si MAS NMR Spectra of the SBA-15 samples prepared in the different HVSi molar ratios were shown in Figure 3. The positions of resonances of SBA-15 samples and the assignments for these resonances were listed below: the ca. -110 (Q4), -102 (Q3), and -94 (Q2) ppm resonances were attributable to the Si atoms of Si-04-Si, OsSi-OH, and 02Si-(OH)2 groups. The samples possessed low and middle V^p/Vj ratio (0.03 and 0.08) presented the Q4/Q3 ratios of about 2 (Figure 3A and 38), and those possessed high V^p/Vj ratio (0.18) presented the Q4/Q3 ratios of about 1 (Figure 3C). Simultaneously, the Qj peak of the samples possessed high V^p/Vj ratio was also showed in Figure 3C. The samples possessed high V^p/Vj ratio was found to be much higher ratio of Si-OIl groups uncondcnscd and bound to II^O^ and EO groups. 4. CONCLUSIONS
Fig. 3. '''Si MAS NMR spectra of SBA-15 samples synthesized in different H+/Si molar ratio of A) 1.04 (V,p/V^-0.03); B) 1.05 (V,p/V^0.08); C) 1.14 (Vj.p/V^ - 0.18) at 50'C.
In the present investigation, we have demonstrated a convenient approach to vary the content of connecting micropores by controlling the miccllar behavior. The tcxtural properties of the SBA-15 samples, such as pore size and pore volume, were found to decrease with an increase in the H VSi molar ratio. REFERENCES 1. D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F Chmelka, and G. D. Stucky, Science, 279(1998)548. 2. J. M. Kim, and G D. Stucky, Chem. Commun., (2000) 1159. 3. S. -S Kim, T. R. Pauly and T. J. Pinnavaia, Chem. Commun., (2000) 1661. 4. M. Impcror-Clerc, P. Davidson, and A. Davidson, J.Am. Chem. Soc, 122(2000) 11925. 5. M. Kruk, M. Jaronicc, C. H. Ko, and Ryong Ryoo, Chem. Mater., 12 (2000) 1961. 6. R. Ryoo, C. II. Ko, M. Kruk, V. Antochshuk, and M. Jaronicc, J. Phys. Chem. B, 104 (2000)114. 7. B. L. Newwalkar and S. Komameni, Chem. Mater. 13 (2001) 4573.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
109
Controlling the Pore Sizes of SBA-15 Mesoporous Silica by the Addition of Poly(propylene oxide)** Jong Chul Park, Jae Ho Lee, Pil Kim and Jongheop Yi* School of Chemical Engineering, Seoul National University, San 56-1, Shillim, Kwanak, Seoul 151-742, Korea Poly(propylene oxide) (PPO) was added during the synthesis of SBA-15 type mesoporous silica in order to increase the pore diameter. Synthesized mesoporous materials showed different morphologies and pore properties compared to the SBA-15 mesoporous silica without PPO addition due to the presence of PPO. The pore properties were affected by the amount of added PPO in the synthetic mixtures and those values were increased compared to the SBA-15 without PPO addition. The hexagonal pore structure of the SBA-15 mesoporous silica was not altered by the PPO addition, which was confirmed by the TEM and SAXS analysis. However, size of the primary particles was increased in the axial direction resulted in a longer particles. 1. INTRODUCTION Among the large variety of mesoporous silicas, SBA-15 type one was known to have more hydrothermal stability due to its thicker pore walls, which is an important property for the applications such as adsorbents or catalyst supports.'"^ A lot of researches were performed on the synthetic methods and its applications to enhance the properties of this type mesoporous silica. However, only a little cflbrt has been dedicated to enhance the pore diameter of the SBA-15 type mesoporous silica. For the HMS type mesoporous silica, trimethylbenzene (TMB) was used in order to increase the pore diameter."* For the synthesis of HMS, hcxadecylamine (HDA) was used as a structure-directing agent, however, for the synthesis of SBA-15 type mesoporous silica, non-ionic pluronic surfactants were used. Pluronic surfactant is a tri-block copolymer in which contains two poly(ethylene oxide) group as a hydrophilic part and a poly(propylene oxide) group as a hydrophobic part. Considering that the pluronic is a polymer, HDA is not, it was thought that the poly(propylene oxide), a polymer, would be the better reagent for the synthesis of SBA-15 type mesoporous silica with enhanced pore properties. In this work, different amount of poly(propylene oxide), which have a similar molecular weight with a poly(propylene oxide) group in the pluronic surfactant PI 23, was used for the synthesis of SBA-15 type mesoporous silica. Physical properties of the synthesized materials were analyzed and compared with the SBA-15 mesoporous silica synthesized without poly(propylene oxide) addition.
* Corresponding author: ivi@snu.ac.kr Financial support by the National Research Laboratory (NRL) of the Korean Science and Engineering Foundation (KOSEF) is gratefully acknowledged.
110
2. EXPERIMENTAL Synthesis of the SBA-15 type mesoporous silica was performed under acidic conditions using TEOS (tetraethoxyorthosilicate, Aldrich Co.) and pluronic surfactant (PI23, EO20-PO70EO20, BASF Co.). Synthetic medium was prepared by mixing 10.4 ml of 35 wt.% HCl and 64.6 ml of deionized water. PI23, 2.0 g, was added to this mixture and followed by 4.57 ml of TEOS addition. Different amount of poly(propylene oxide), with PPO:P123 molar ratio of 1:1, 2:1 and 4:1 were added before or after TEOS addition. The molecular weight of poly(propylene oxide) (Aldrich Co.) used was ca. 3,500. The resulting mixture was stirred for 24 hr at 35°C and aged at lOO^C with stirring for an additional 24 hr. As-synthesized materials were filtered, dried and followed calcinations at 450"C for 3 hr to remove organic templates. Nitrogen adsorption/desorption isotherms were measured by sorptometer (ASAP 2010, Micromeritics Co.) and pore size distribution was calculated from the isotherm data using BJH (Barret, Joyner and Halenda) method with desorption branch. In addition. X-ray scattering patterns were measured using Samll-Angle X-Ray Scattering (GADDS, Brucker Co.) and TEM images were obtained using transmission electron microscope (JEM-200CX, JEOL Co.). 3. RESULTS AND DISCUSSION Poly(propylene oxide) is a hydrophobic liquid and thus, it was expected that poly(propylene oxide) might act as a swelling agent for the pluronic surfactant. The pore properties of the SBA-15 mesoporous silica were affected by the poly(propylene oxide) addition and those results measured by nitrogen adsorption were summarized in Table 1 below. Table 1 Pore characteristics of SBA-15 and PPO added Sample Molar ratio 'Pore ^Wall of PPO to size/ thick PI 23 nm ness/ nm SBA-15 5.25 5.16 1:1 5.40 5.45 PPO'1-SB A 2:1 6.12 5.21 PP02-SBA 4:1 6.14 5.19 PP04-SBA
samples Surface area/ mV'
Micropore area/ mg
Pore volume/ cm3g-1
Micropore volume/ cmV'
686 831 784 773
47 82 96 118
0.796 1.007 1.050 1.004
0.012 0.021 0.026 0.035
Peak pore sizes obtained from desorption branch of nitrogen isotherm by BJH method ^ Wall thickness was calculated using 29 of the primai primary peak in the SAXS pattern ^' "x" means the molar ratio of the added PPO to PI 23 All of the pore properties of the synthesized materials were increased by the PPO addition compared to the SBA-15 mesoporous silica. Especially, micropore areas and volumes were greatly increased in the PP04-SBA samples. Pore diameter of the PPO added samples were increased with the amount of added PPO, however, wall thickness and surface area showed an opposite trend. The increase in the pore diameter was only 0.15, 0.87 and 0.89 nm. It was thought that this relatively smaller increase in the pore diameter was due to the characteristics of the pluronic surfactant PI 23, which is composed of three polymer blocks connected with each other. The extension of the centered PPO block in this surfactant might be restricted by
Ill
the sterical hindrance caused by the interactions between the surfactants and added PPO molecules.
E o^ 0)
E _2 o
> o
CL
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/PJ
Pore Diameter (nm)
Fig. 1. Nitrogen adsorption/desorption isotherms (left) and pore size distributions (right) of (a) SBA-15; (b) PPOl-SBA; (c) PP02-SBA; and (d) PP04-SBA. The nitrogen adsorption/desorption isotherms and pore size distributions calculated by BJH method using desorption branch of the isotherm are shown in Fig. 1. The hysteresis loop of the PPO added samples showed almost the same behavior with the typical SBA-15 mesoporous silica with a little shift in the position to a higher relative pressure. The pore size distributions showed that all the materials have micropores less than 2 nm and the peak position in the mesopore region was shifted to a larger diameter with the amount of added PPO in the synthetic mixtures. The SAXS patterns of the samples are shown in Fig. 2. In this figure, SBA-15 and PPO added samples showed an intense primary peak around 26 = 0.9 and the ratios between the 1000A three peaks are close to 1:1.73:2, which / Id 800supports that these samples have the highly ordered hexagonal pore 600structures. In addition, TEM images of i: the SBA-15 and PP04-SBA sample in 400 H ' b Fig. 3 shows the particle morphologies 200^ and the highly ordered hexagonal pore structures of these samples. In this 0^ figure, it was shown that the polymer added PP04-SBA samples have the 20 same morphologies with the SBA-15 mesoporous silica. However, PP04Fig. 2. SAXS patterns of (a) SBA-15; (b) PPOl- SBA sample showed the larger particle sizes in the axial direction. SBA: (c) PP02-SBA: and (d) PP04-SBA.
1\ lie
L
—
'
—
1
—
'
?..
—
1
112
(b)
(a) 4009C'3fe
(c) (d) Fig. 2. T¥M images of 2()(),()0() limes (left) and 5,000 times (right) magnification, (a) and (b) for the SBA-15; and (C) and (d) for the PP04-SBA 4. CONCLUSIONS Poly(propylene oxide) was added to the synthetic mixtures of the SBA-15 type mesoporous silica. The added polymer alTected the pore properties of the synthesized mesoporous silicas and the results were an increase in the pore diameter, surface area and pore volume, especially, the micropores were remarkably increased. Also, the polymer addition has afTected the morphologies of the synthesized particles and thus, the particle size was increased in the axial direction. However, the highly ordered hexagonal pore structures of the SBA-15 type mesoporous silica were conserved with the amount of added polymer.
REFERENCES 1. A. Galameau, D. Desplantier-Ciiscard, V. D. Renzo and F. I-ajuIa, Catalysis Today, 68 (2001) 191. 2. Y. S. Cho, J. C. Park, W. Y. Lee and J. Yi, Catalysis Letters, in press (2001) 3. Y. S. Cho, J. C. Park, B. S. Choi, J. Moon and J. Yi, Stud. Surf. Sci. Catal., 133 (2001) 559 4. H. G. Karge and J. Wcitkamp, Molecular Sieves: Synthesis, Springer-Verlag, New York, 1998 5. Y. Wang, M. Noguchi, Y. lakahashi and Y Ohtsuka, Catalysis Today, 68 (2001) 3
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved
113
Synthesis of mesoporous silicas with different pore-size by using EOmMAn diblock copolymers of tunable block length as the templates Yi-Tsu Chan^, Hong-Ping Lin^, Chung-Yuan Mou^'^ and Shiuh-Tzung Liu^* ^ Department of Chemistry, National Taiwan University, Taipei 106, Taiwan. ^ Institute of Chemistry, Academia Sinica, Nankang, Taipei 115, Taiwan. '^ Center of Condensed Matter, National Taiwan University, Taipei 106, Taiwan. The EOmMAn diblock copolymers with various unit number of MA segment can be synthesized via a typical ATRP method and used as the templates for preparation of mesoporous silicas with well-ordered hexagonal mesostructures. With the feasible adjustment of hydrophobic MA parts, the pore size of mesoporous silicas can be tuned in a wide range from 4.0 to 20.0 nm. 1. INTRODUCTION Since the discovery of M41S mesoporous silicas by Mobil Oil Corp. in 1992, this area of research has received much attention due to the high surface area(~ 1000 m^/g), tunable pore size (1.0-10.0 nm) and uniform as well as stable pore structures of these materials.[l] Besides of quaternary ammonium surfactants, the low-cost, biodegradable, and natural friendly neutral polyethylene oxides surfactants recently have been extensively performed to synthesize mesoporous silicas with various mesostructures and morphologies. It is well known that mesostructure, wall thickness, morphology and porosity of the mesoporous silicas rely on surfactant micelles and liquid-crystal arrays of micelles as structure-directing agents.[2] However, the amphiphilic surfactant used for this function is not well understood because few polyethylene oxide surfactant sources are available. [3] Recently, the synthetic methods leading to well-controlled copolymers have been developed, which raises the interest in design of diblock amphiphilic polymers for surfactant to build the mesoporous materials. Herein, we reported a synthetic approach via atom transfer radical polymerization (ATRP) method to obtain various poly(ethylene oxide)-/7-poly(methyl acrylate) diblock copolymers (denoted as EOmMAn) with various polymerization degree of MA segment, [4-6] which were used for construction of mesoporous silica with different mesostructures and pore sizes in a wide dimension of 4.0-20.0 nm. 2. EXPERIMENTAL 2.1. Synthesis of EOmMAn diblock copolymers The CuBr/MceTREN (tris[2-(dimethylamino)ethyl]amine) and the poly(ethylene oxide)-2-bromoisobutyrate were used as the catalyst and the macroinitiator for the polymerization of MA, respectively. The detail synthetic procedures, chemical composites and reaction condition have been reported in elsewhere.[5-7]
114
2.2. Preparation of mesoporous silicas The EOmMAn-silica mesostructural composites were synthesized in acidic media as that for SBA-15 siHcas."^ The as-synthesized mesoporous siHca products were obtained after 1 day agitation at 25-50 °C. The final gel composition (in gram) is: (0.3~0.5)g EOmMAn: (20.0~25.0)g H2O : (4.0~6.0)g 37%HC1 : (l~2.5)g TEOS. In order to increasing the mesostructural ordemess and stability, 1.0 g dried acid-made mesoporous silicas was combined with 50.0 g water (pH «7.0) and then hydrothermally treated at 100 °C for 24 hr. [8] 2.3. Measurement The powder x-ray diffraction patterns (XRD) were taken on Wiggler-A beamline (k = 0.1326 nm) of Taiwan Synchrotron Radiation Research Center. The mesostructures of mesoporous silicas were recorded on Hitachi S 7100 transmission electron microscope (TEM) with an operating voltage of 100 keV. N2 adsorption-desorption isotherms were obtained at 77 K on a Micromeritics ASAP 2010 apparatus, and the pore size distribution was calculated from the adsorption isotherms using the Barrett-Joyner-Halenda (BJH) method. 3. RESULTS AND DISCUSSION Fig. lA shows the XRD patterns of the calcined mesoporous silicas synthesized from the EO17MA12, EO45MA72, and EO45MA94-TEOS-HCI-H2O compositions. One can clearly see that all these calcined mesoporous silicas exhibit 3 XRD peaks at low angle range of 0.3-2.0°. All these calcined mesoporous samples have the representative (100), (110) and (200) peaks indicating of hexagonal mesostructure. Moreover, every mesoporous silica sample possesses a sharp capillary condensation in each N2 adsorption-desorption isotherm (Figure IB). 1500 -
T
1400 -
A
B
r^J^
1200 1 1 00 -
900 -
(110) (100)
700 600 -
II
500 -
II
.^ Cl'j^O) 00)
300-
•i-J
I
KOO -
1
\lOO)
III
1 '• t
/
200 -
(••0)
(2,„
100-
ill
0 2 0.5
1.0
1.5
2.0
2.5
0 4
3.0
2e/degree
Fig. 1. (A) The XRD patterns and N2 adsorption-desorption isotherms of the mesoporous silica synthesized from E0mMAn-TE0S-HCl-H20 composition. I. EO45MA94; 11. EO45MA72; III. EO17MA12.
115
•S'S
Fig. 2. The TEM micrographs of the calcined mesoporous silica using different EOmMAp diblock copolymers. A. EO17MA12; B. EO45MA72; C. EO45MA94. However, the capillary condensation occurs at different relative pressure (P/Po). Using the Barrett-Joyner-Halenda calculation method, the pore size of the mesoporous silica from EO17MA12, EO45MA72, and E045MA94are 3.8, 12.0 and 19.1 nm, respectively. In Figure 2, the TEM micrographs demonstrate the well-ordered hexagonal array of the nanochannels of the mesoporous silicas in Figure 1. This TEM result is parallel to that of XRD. The existence of the mesostructure pattern of the calcined sample suggests that this mesostructure is thermal stable as those of SBA-15 mesoporous silicas prepared by using Pluronic 123 triblock copolymer."* With an approximate comparison and measurement on the pore dimension, we found the measured pore size is close to that of N2 adsorption-desorption isotherms and increase in the order of EO17MA12 < EO45MA72 < EO45MA94. According to the prediction of the core-shell model, [9] it was supposed that the H pore size and d-spacing increase with the increase of the hydrophobic units in block Number oflMA units copolymers. In EOmMAn diblock copolymers, Fig. 3. The plot of the pore size vs. the the hydrophobic part is MA segment. To confirm this ideal, we performed various number of the MA units in E045MAn E045MAn diblock copolymers with varying diblock copolymers. MA units as the templates to synthesize the mesoporous silicas. After the analysis the pore size from the N2-adsorption branches, a plot of pore size vs. MA units was illustrated in Figure 3. The pore size linearly increases with the increase of the number of MA units and the slope is about 0.25/MA unit. While doing an extra-plot, the intercept is about 1.92 nm, ascribed to the contribution of 45 EO units. Consequently, the hydrophobic MA plays the major role on controlling the pore size rather that hydrophilic EO segments. These results
t *
+
116
almost match the core-shell model. Thus, changing the hydrophobic MA units upon the synthesis of EOmMAn diblock copolymers, the pore size can be fme-tuned. In addition, the pore size of the mesoporous silicas can be feasibly swollen to 20.0 nm at MA unit =110 without the addition of hydrocarbon expanders, which have been used to expand the pore size of SBA-15 orMCM-41 mesoporous silicas. From further analysis of the N2 adsorption-desorption isotherms, it shows that all calcined mesoporous silica samples aforementioned have the advantages of high surface area (400-800 m^/g), tunable pore size (4.0-20.0 nm) and thick wall thickness (2.5-4.0 nm). In summary, the EOmMAn diblock copolymers are a new family of organic templates to generate the high surface -area mesoporous materials with the desired pore size and porosity. 4. CONCLUSION In brief, the ATRP synthetic method provides a convenient way to control the composition and combination of the neutral diblock or triblock copolymers. With a well tuning in the surfactant micellar properties and liquid-crystal phases, one can design the porosity, pore size, morphologies and mesostructures for extending the applications of mesoporous materials. [10] ACKNOWLEDGEMENT We thank Mr. Chung-Yuan Tang and Ching-Yuan Lin for helping the TEM micrographs preparation. This research was financially supported by National Science Council of Taiwan (NSC-90-2113-M-002-038). REFERENCES 1. C. T. Krcsge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. 2. D. Zhao, R Yang, Q. Hou, B. R Chmelka and G. D. Stucky, Current Opinion in Solid State and Materials Science, 3 (1998) 111. 3. (a). D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F Chmelka, and G. D. Stucky, Science, 279 (1998) 548.; (b). S. A. Bagshaw, E. Prouzet and T. J. Pinnavaia, Science, 269(1995) 1242. 4. Y T. Chan, H. R Lin, C. Y Mou and S. T. Liu, unpublished result. 5. R. N. Keller and H. D. Wycoff, Inorg. Synth., 2 (1946) 1. 6. M. Ciampolini and N. Nardi, Inorg. Chem., 5 (1966) 41. 7. K. Jankova, X. Y Chen, J. Kops and W. Batsberg, Macromolecules, 31 (1998) 538. 8. D. Zhao, Q. Huo, J. Feng, B. F Chmelka and G. D.Stucky, J. Am. Chem. Soc, 120 (1998) 6024. 9. J. H. Chang, L. Q. Wang, Y Shin, B. Jeong, J. C. Bimbaum and G. J. Exarhos, Adv. Mater., 14(2002)378. 10. F S. Bates and G. H. Fredrickson, Phys. Today, 52 (1999) 32.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved
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Polypropylene glycol as a swelling agent for the synthesis of mesoporous silica (SBA-15) by amphiphilic block copolymer templating Xiuguo Cui^, Joong-Hyun Ahn^, Wang-Cheol Zin^, Won-Jei Cho ^, and Chang-Sik Ha^* ^Department of Chemical Engineering, Yanbian University, Yanji 133002, P. R. China ^Department of Materials Engineering, Pohang University of Science and Technology, Pohang 790-390, Korea. ''"Department of Polymer Science & Engineering, Pusan National University, Pusan 609-735, Korea. The role of poly(propylene glycol) (PPG) as a polymeric swelling agent in the synthesis of mesoporous silica, SBA-15, by amphiphilic triblock copolymer templating was investigated. Two different molecular weights of PPG were compared. It was found that even a low concentration of PPG expands effectively the pore size of the SBA-15 in the presence of the triblock copolymer, poly(ethylene oxide)-poly(propylene oxide)-poly(propylene oxide) (PEOPPO-PEO) without sacrifying the wall thickness of the framework and the original morphology of pores. 1. INTRODUCTION The traditional methods to control the pore size of mesoporous materials are the postsynthesis hydrothermal treatments[l,2], the addition of an organic swelling agent (i.e. 1, 3, 5trimethylbenzene, TMB)[3] as well as the use of templates with different lengths of hydrophobic chains. In order to obtain mesoporous silica with large-sized pores, amphiphilic triblock copolymers have been also utilized as a template[4]. However, unlike low molecular weight surfactants whose hydrophobic chain length could be finely adjusted, the length of the block segment in the amphiphilic triblock copolymer, PEG-PPG-PEG, is not easily controlled in a small-scale region without the use of delicate and thorough synthetic skills. Furthermore, it is not easy to obtain commercially available block copolymers that have various defined block lengths. Here we report the role of a polymeric swelling agent, poly(propylene glycol) (PPG) with two different molecular weights in the synthesis of SBA-15 by the triblock copolymer templating. 2. EXPERIMENTAL The template solutions were prepared by dissolving PEG2o-PPG7o-PE02o(average molecular weight of 5800, abbreviated as EPE5800), and a swelling agent, PPG(average molecular weight of 2000 and 2700, abbreviated as PPG2000 and PPG2700, respectively), in de-ionized water under moderate stirring at 308K for 6h, then a 2M HCl solution was added into the template solution. Gnce the PPG suspension was adsorbed and a transparent solution was obtained, tetraethyl orthosilicate (TEGS) was dropped slowly into the acidic template
solution while stirring. In a typical synthesis, a mixture of 2.03g of EPE5800 and 0.4g of PPG2000 dissolved in 30g of de-ionized water was added to 60g of a 2M HCl solution. Then, 11.1ml of TEOS were dropped into the template solution that has a pH of 0.7, while being stirred at 308K. Other procedure is similar as that for the SBA-15. The mesoporous silicas were characterized using small angle X-ray scattering (SAXS) and N2 sorption experiments. 3. RESULTS AND DISCUSSION Figure 1 illustrates the N2 adsorption/desorption isotherms for mesoporous silica prepared with or without hydrophobic PPG. The isotherms exhibit a typical type IV curve, which characterize properties of mesoporous materials exhibiting a capillary condensation step and a hysteretic loop. The BJH pore size distributions (inset in Figure 1) exhibit a single narrow peak. The mesoporous silica synthesized in the absence of PPG has a pore size of 39 A , which is smaller than that of SBA-15 prepared at 408K for 20h[4], because this sample was prepared at 408K for 8h without any post-treatment procedure. Table 1 Physicochemical Properties of Mesoporous Silica Prepared Using Hydrophobic PPG as a Swelling Agent and Amphiphilic Copolymer as a Structure-directing agent Sample Content of PPG: Pore Wall ABJH ABI;T ^Langmuir dioo ao Size Thickness /g(io"Wir') (mV) (mV") (mV) (A) (A)
0
1044
382
522
(A) 39
(A)
80.8
93.3
54.3
PPG2000/0.4g 501 681 1434 42 83.2 96.1 54.1 (2.0) PPG2000/1.0g 58.9 1266 108.9 50 443 601 94.3 (5.0) PPG2700/0.54g 517 711 60 1367 94.4 109.0 49.0 (2.0) PPG27001.08g 62 540 744 1438 96.1 111.0 49.0 ^ _ . . (4.0) ABJH, ABI:T, ALangmuir arc surface areas obtained from adsorption branch results calculated by software; the pore sizes were determined from the BJH pore size distribution; a()=2di()()/V3; and the wall thickness^ao-porc size A condensation step was displayed at P/Po=0.45 (Figure la). In contrast, adding 2.0x10'^ mol.r' of PPG2000 resulted in pore sizes of 42 A (see inset in Figures lb), and shifts of mesoporous adsorption steps from P/Po=0.45 to near P/P()=0.5 in the isotherm. Hysteretic loops in the sorption isotherm for sample 1 is type H2, which is different from type H I , whose adsorption and dcsorption branches should be almost vertical[4]. Similarly, a clear H2 - type and a modified H2 - type hysteretic loop, respectively corresponding to mesoporous silica prepared with PPG2000 and PPG2700, were observed in the isotherms. The BET surface area and the other characteristic parameters obtained from the N2 adsorption/desorption experiment are summarized in Table 1. These results indicate that the mean pore size of mesoporous silica was enlarged by increasing the amount of PPG, and for expanding the pore size of mesoporous silica, PPG2700 with a higher molecular weight is more effective than PPG2000 with a lower molecular weight.
119 400
3.9nm
5,2.5
350
^ 1.5
Q.
feaoo •tlaso
2 0.5 o ^0.0
T3 200
0
250
T
80 160 240 320 400
Pore Diameter, (Angstroms)
50 0.2
0.4
in 0 100 200 300 400 50r
Pore Diameter, (angstrom*?)
150
0.6
0.8
' Adsorption Desorption
100 50
(a)
0
s
200
Adsorption Desorption
0.0
EPE580(>+PPG200Q (0.4g)
300
(A
3
4.2nm
350
I 1.0
•s O 150
E
400
EPE5800+PPG (Og)
E 2.0
(b)
0.0
1.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/PQ) Relative Pressure (P/PQ) Fig. 1. N2 adsorption and desorption isotherms of mesoporous silicas prepared with(a) or without(b) polymer swelling agent. Insets are the BJH pore size distributions. (100)
E P E 5 8 0 0
( 2 0 0 ) E P E 5 8 0 0
+ 0.54g
P P G 2 7 0 0
+
P P G 2 0 0 0
0.4g
>|^ ( 2 1 0 )
h k I 10 0 2 0 0 2 10 E P E 5 8 0 0
d (n m ) 8.0 8 4.0 5 3.0 5 + Og P P G
S (n m -1 )
Fig. 2. Small angle X-ray scattering patterns of mesoporous silicas prepared using a polymer swelling agent with different molecular weights. Small angle X-ray scattering patterns of mesoporous silicas are shown in Figure 2. The results demonstrate that mesoporous silicas synthesized with EPE5800 as a structure directing agent and PPG as a swelling agent exhibit a well-ordered hexagonal pore shape. In the absence of PPG, the SAXS pattern of mesoporous silica shows three well resolved peaks with d spacings of 80.8 A, 40.5 A, and 30.5 A (Figure 2A). These three peaks display a d value ratio of Vl: V4: V?, which are indexable as (100), (200) and (210) reflections, respectively, in the hexagonal space group. Thus, the unit cell distance ao between pore centers can be
120
calculated by the formula ao=2^ioo/V3, and the thickness of the framework wall determined by subtracting the mean pore size from ao is 54.3 A. Similarly, three peaks were shown in the SAXS pattern when adding 2.0x10'^ mol.l'' of PPG2000 in the synthesis of the mesoporous silica. These peaks were also assigned to (100), (200), and (210) reflections, respectively (Figure 2B). The mesoporous silica prepared by using EPE 5800 as a template and 2.0x10'^ mol.r' of PPG2700 as a swelling agent showed a well-resolved hexagonal SAXS pattern (Figure 2C). Three peaks were observed with d spacings of 94.4A, 54.2 A, and 47.3 A {dvalue ratio of Vl: V3: V4), indexing as (100), (110), and (200) reflections, respectively. It is clear that the d spacing increases with the increase in the molecular weight of the hydrophobic polymer used for a swelling agent. These results are identical with those of the N2 adsorption/desorption experiments. In summary, a new polymer swelling agent, hydrophobic poly(propylene glycol) (PPG), was used to prepare the mesoporous silicas possessing fme-controllable pore size with a hexagonal porous structure in the presence of a triblock copolymer, PEO-PPO-PEO as a structure-directing agent. The synthetic procedure did not involve a post-synthesis treatment at high temperature. With the addition of PPG of two different molecular weights in different amounts, mesoporous silicas maintained the original porous structure and a thick framework wall. The pore size of mesoporous silicas increased when the amount of the swelling agent was increased. The PPG with the molecular weight of 2700g.mor^ is more effective for expanding the pore size of mesoporous silica, than one with 2000g.mor'. The engineering of pore size control can be completed at a low concentration of the swelling agent (below 2wt%), and without a treatment procedure for the swelling. The advantages of a polymer-swelling agent reported here have allowed us to approach other mesoporous materials to fmely control their pore size. ACKNOWLEDGEMENTS The supports of the Center for Integrated Molecular Systems, POSTECH, Korea and the Brain Korea 21 Project are gratefully acknowledged. Prof. Cui thanks to the Program of Young Scientist Exchange of Korea-China in 1999.
REFERENCES 1. Khushalani, D. M.; Kupperman, A.; Ozin, G; Tanaka, A. K.; Garces, J.; Olken, M. M.;Coombs, N. Adv. Mater. 1995, 7, 842. 2. Huo, Q.; Margolese, D. I.; Stuck, G D. Chem. Mater. 1996, 8, 11477. 3. Kim, M. J.; Ryoo, R. Chem. Mater. 1999, 11, 487-491. 4. Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G H.; Chmelka, B. F.; Stucky, G D. to'cwe 1998, 279, 548.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
121
Thermal decomposition-precipitation inside the nanoreactors. High loading of W-oxide nanoparticles into the nanotubes of SBA-15. L. Vradman, Y. Peer, A. Mann-Kiperman and M. V. Landau. Blechner Center for Industrial Catalysis and Process Development, Chemical Engineering Department, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel. The solution thermal decomposition-precipitation (ThDP) of oxide precursor inside the nanotubes (nanoreactors) of mesoporous silica support under atmosphere saturated with solvent at the oxide precursor decomposition temperature was explored for loading the Woxide nanoparticles into SBA-15. ThDP of W-Ethoxide solution in decalin yielded WO3/SBA-I5 composites with W-phase located exclusively inside the pores in form of nanocrystals strongly blocking the pores. ThDP of W(C0)6 yielded W-phase up to 32 wt% spread as an amorphous monolayer on the pore walls with minimal pore blocking. 1. INTRODUCTION Since the discovery of MCM-41 and related materials [1], many attempts were done to employ them as supports for catalytic phases dispersions [2]. However, the conventional impregnation methods yielded poor dispersion of the active phase and significant pore blocking even at relatively low loading of active components [3]. Moreover, at least part of the active phase was formed outside the mesopores. In the present study we tried to overcome this limitation by thermal treatment of the support impregnated with W-oxide precursor solution in a closed reactor with a gas phase saturated with a solvent. In this way the solution was forced to remain inside the nanotubes during decomposition of the precursor. Hence the precipitation of an active phase occurred exclusively inside the mesopores. 2. EXPERIMENTAL The wide-pore pure silica SBA-15 material with surface area of 800 m^/g, uniform mesopore diameter of 6.5 nm and pore volume of 1 cc/g was prepared according to published procedure [4] modified by increasing the duration of the hydrothermal treatment at 100 ^C to 3-7 days to decrease the micropore contribution to less than 10%. I g of SBA-15 was impregnated with decalin solution of W-Ethoxide or W(C0)6 at 80°C to increase the solubility. After solid separation by filtration, the sample together with 5 ml of pure decalin were introduced into the different areas of the 50 ml stainless steel vessel and pressurized to 20 atm with air. ThDP was performed in two steps. First, the temperature was increased up to 350 °C and kept for 3 h. Next, the temperature was increased up to 450 °C and the vessel pressure was released. Due to the limited solubility of W(C0)6 in decalin, the concentration of WO3 in the composite was about 7 wt% after one ThDP step and subsequent ThDP steps were necessary to increase the loading. Reference sample was prepared by conventional wet
122
impregnation of SBA-15 with water solution of ammonium tungstate, followed by drying at 120 *^C and calcination at 550 °C. Unsupported WOx was prepared by similar procedures in absence of SBA-15. The chemical composition of the solid catalysts was measured by EDS analysis with a JEOL JEM 5600 microscope (SEM-EDS). Surface areas, pore volumes and pore size distributions were obtained from N2-adsorption-desorption isotherms using conventional BET and BJH methods. XRD patterns were recorded on a Phillips diffractometer PW 1050/70 (CuKa radiation) equipped with a graphite monochromator. HRTEM micrographs were obtained on a JEOL FasTEM-2010 electron microscope operating at 200 kV and equipped with an analytical EDS-system for composition analysis. 3. RESULTS AND DISCUSSION The XRD patterns showed that conventional impregnation led to formation of crystalline WO3 phase (Figure 1, a). The average crystals size of 15 nm was much higher than SBA-15 pore diameter suggesting that at least part of the WO3 phase was located outside the pores of SBA-15. It was also confirmed by TEM and TEM-EDS measurements. Furthermore, the pore blocking extent, calculated from normalized surface area [3], was 57% as a result of partial pore blocking. ThDP of W-Ethoxide yielded WO3 phase with average crystals size of 5.5 nm (XRD) (Figure 1, b). This suggests that entire WO3 phase was located only inside the nanotubes of SBA-15 since the WO3 phase obtained by ThDP of W-Ethoxide at the same conditions in absence of SBA-15 has much higher average crystal size (>20 nm). This was also confirmed by TEM-EDS. The blocking extent, however, was also high (73%) as a result of blocking the SBA-15 mesopores with WO3 nanocrystals. This correlates well with HRTEM investigations (Figure 2, a) where large nanoparticles could be recognized at the openings of the hexagonally arranged nanotubes of SBA-15 support. Some of the pores seems to be completely blocked in agreement with strong reduction of the measured surface area of the 32 wt% WOx/SBA-15 sample prepared by ThDP of W-Ethoxide (145 m^/g) compared with parent SBA-15 (800 m'/g). ThDP of W(C0)6 yielded highly dispersed almost XRD amorphous WOx phase (Figure 1, c). WOx phase obtained by the same method in absence of SBA-15 had an average crystal size of 7.1 nm. Therefore, ThDP after impregnation of SBA-15 yielded WOx phase located exclusively inside the SBA-15 nanotubes that was also confirmed by TEM-EDS. Furthermore, the pore blocking extent was minimal (14%) which correlates well with XRD data. HRTEM micrograph of 32 wt% WOx/SBA-15 sample obtained by ThDP of W(C0)6 (Figure 2, b) clearly demonstrates the openings of SBA-15 nanotubes that are not blocked with any particles as opposed to the sample obtained by ThDP of W-Ethoxide (Figure 2, a). At the same time, several EDS analysis, taken from this area with 15-25 nm probe size, yielded an average WOx concentration similar to that measured be SEM-EDS. This means that WOx phase is spread on the SB A-15 pore walls in form of amorphous layer in agreement with XRD and N2-sorption measurements. This demonstrates high efficiency of thermal decomposition-precipitation method for loading the transition metal oxide into the mesopores without blocking them by using the optimal precursor.
123
(a)
rt 32 wt% W0,/SBA-15, CS=15 nm, SA=235 m^/g
3
Unsupported WO^, CS>20 nm, SA=10 m^/g
c
0)
\L^ j (b)
(1 32wt%WO,/SBA-15,CS=5.5nm,SA=145m2/g
Unsupported WO^, CS>20 nm, SA=7 m^/g c c
32 wt% W0,/SBA-15, CS<1.5 nm, SA=468 m^/g
^it^'^f^^'M'tiW^^il^^^
c
S c
Unsupported WO^, CS=7.1 nm, SA=60 m^/g
10
20
30
40
50
60
70
80
Angle (2e), O Fig. 1. XRD patterns of unsupported WO^ and WO^/SBA-IS samples prepared by impregnation from water solution (a), ThDP of W-Ethoxide (b) and W(CO)^ (c). CS - crystal domains size calculated from the XRD patterns. SA - BET specific surface area calculated from N2 adsorption.
124
^c-^.. • » > l t # . , • . • . •••.
-^i^i•;--4^ci
^ 0) l i s :li
:;W Fig. 2. HRTBM micrographs of 32 wt% WOx/SBA-15 samples prepared by ThDP of WEthoxide (a) and W(C0)6 (b). REFERENCES 1. U. Ciesla, F. Schuth, Micropor. Mesopor. Mater., 27 (1999) 131. 2. F. Schuth, A. Wingen, J. Sauer, Micropor. Mesopor. Mater., 44-45 (2001) 465. 3. M. V. Landau, L. Vradman, M. Herskowitz, Y. Koltypin, A. Gedanken, J. Catal., 201 (2001)22. 4. D. Zhao, J. Sun, Q. Li, G. D. Stucky, Chem. Mater., 12 (2000) 275.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
125
Phase transition of SBA-1 induced by embedded heteropoly acids SungHyun Lim,^ Hideaki Yoshitake^, and Takashi Tatsumi^ ^Graduate School of Engineering, Yokohama National University, 79-5 Hodogaya - k u Tokiwadai, Yokohama 240-8501, Japan ^Graduate School of Environment & Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan Molybdenum
incorporated
SBA-1
was
synthesized
under
acidic
conditions
using
hexaammonium heptamolybdate (NH4)6Mo7024 * 6H2O, phosphomolybdic acid (H3PM012O40), and silicomolybdic acid (H4SiMo 12040) as Mo precursors.
Treatment of as-synthesized
Mo-SBA-1 obtained from ll4SiMoi204o in 1-octanol resulted in the phase transition from P m 3 n cubic to a lamellar structure.
The AHM- and H3PMi204o-derived materials gave
mixtures of these two phases while complete phase transition occurred for the H4SiMoi204o -derived one. 1.
INTRODUCTION Keggin-type heteropoly acids (HPA), H8.XXM12O40, where X is the central atom ( Si, P,
etc.), X is the oxidation state of X and M is the metal ion (Mo^^, W^\etc.), are widely used as acid catalysts due to their strong Bronsted acidity and unique structural properties.
They are
known to possess stronger acid properties than the mineral and synthetic solid acids such as amorphous
Si02-Al203
The
acidity
of
heteropoly
acids
f0ll0WS:l l3PMOi204(), n3PWi704(), H4SiMOi2O40 > H4SiW,2O40 »
in
acetic
acid
are
as
H2SO4, HNO3.
The catalytic applications of hctcropolyanions have attracted increasing interest in the past few decades.
Catalysts based on polyoxometalates with the Keggin structure have been
extensively studied: these compounds with both acid-basic and redox properties have been shown to be efficient catalysts for a wide variety of chemical processes.
Silica supported
heteropoly acid''^ has been studied but direct incorporation of heteropoly acids into mesoporous silica has been scarcely studied.
In this report, we studied the incorporation of
heteropoly acid into the SBA-1 silica mesostructure and the effect of heteropoly acid on the framework structure.
126
2. EXPERIMENTAL The incorporation of Mo precursors into SBA-1 were conducted in a manner similar to the procedures described elsewhere\ SBA-1 containing Mo was treated in 1-octanol at 90°C for 3 d, washed with methanol and then dried overnight at 100°C. After the treatment in 1-octanol, the SBA-1 containing Mo acid were characterized by XRD, CHN, FT-IR and XANES. H4SiMoi204o, H3PM012O40, and AHM are abbreviated as SiMo, PMo and Mo hereafter. 3. RESULTS AND DISCUSSION Typical patterns of the SBA-1 cubic structure before heat treatment in 1-octanol are shown in Fig. 1(a). The SBA-1 structure was slightly decomposed upon the treatment of SBA-1 and Mo-SB A-1 (Figs.l (b), (d)). For the PMo-SBA-1 sample, the original pattern of SBA-1 still remained, and the pattern attributable to a layered structure also appeared (Fig.l (f)). SiMo-SBA-1 samples showed a lamellar structure accompanied by the disappearance of the Pm 3 n structure (Fig.l (h)). This phase transition was observed over the range of Si/Mo= 10-100 in the synthesis gel. The CHN elemental analysis and ICP revealed that the surfactant remaining after the treatment decreased with increasing loading of Mo as shown in Table 1. The remaining surfactant was nearly proportional to the concentration of [SiMoi204o]'*', suggesting that the surfactant molecules are anchored by silicomolybdic acid binding to the 2theta/degree layers. The structure of heteropoly Fig. 1. XRD patterns of samples after and before treated in acid in samples after 1-octanol treatment 1-octanol. (a) Pure SBA-1 as-synthesized, (b) Pure SBA-1 was investigated by XANES and IR. treated in 1-octanol, (c) Mo-SB A-1 (Si/Mo-^12) assynthesizcd, (d) Mo-SBA-1 (Si/Mo-12) treated in 1The spectra of samples containing octanol, (c) PMo-SBA-1 (Si/Mo-12) as-synthesized, (f) heteropoly acids are similar to those of PMo-SBA-1 (Si/Mo-12) treated in 1-octanol, (g) SiMopure heteropoly acids. SBA-1 (Si/Mo= 12) as-synthesized (h) SiMo-SBA-1 (Si/Mo^l2) treated in 1-octanol
127
Table 1. CHN elemental analysis of samples results.
SBA-l Mo-SBA-1 Mo-SBA-1 PMo-SBA-1 PMo-SBA-1 SiMo-SBA-1 SiMo-SBA-1 SIMo-SBA-l
After treatment wt %)
Before treatmenl Cwt %) H C N 6.7 1.6 31.8 33.4 7.0 1.7 6.9 1.7 33.0 1.5 6.5 30.3 1.6 6.7 31.1 30.6 6.7 1.6 1.6 30.1 6.7 1.8 33.5 7.5
Si/Mo 10 12 10 12 10 12 90
C 1.1(0.6) 7.4(5.3) 4.6(3.4) 18.3(6.6) 8.8(7.2) 21.5(15.5) 14.2(12.5) '•^^'')
H 1.4 2.5 1.6 4.1 2.6 4.4 3.0 1.5
Surfactant removal (wt %)
N 0.03 0.3 0.2 0.4 0.4 0.8 0.7 0.1
98 83 89 77 76 48 59 94
( )* : The C wt % of remaining surfactant was calculated by using the observed value of N wt %. Figure 2 shows the infrared spectra of SiMo-SBA-1 and PMo-SBA-1 samples after 1-octanol treatment. The Keggin structure of XM12040"*" consists of one XO4 tetrahedron surrounded by four M3O13, formed by three edge-sharing octahedrons. All of M3O13 are linked together through oxygen atoms. Here are four kinds of oxygen atoms in XM12040"^'; four X-Oa in which oxygen atom connects with heteroatom, 12 M-Ob-M oxygen bridges in comer-sharing between different M3O13, 12 M-Oc-M oxygen-bridges in edge-sharing within M3O13, and 12 M-Od terminal oxygen atoms. Fig. 2 shows that the bands in the spectrum of sample shift toward the lower wavenumbers compared to those in the spectrum of pure HPAs. This is probably due to chemical interaction between the HPA anion and the cation of surfactant remaining after treatment in 1-octanol. The shifts of bands are similar to the outer-sphere Si-0„ Mo-O,,
Mo-Ot,-Mo
1
•
j Mo-0,-Mo
952 902 i ^ ^
773
850^.^
(a)
< * U % H . . — _ ——^
y
(c)
789
r. \db8f\
(b)
'
896 942
»^^^'*^^^
944 895
\
^gg
853^
noo 900 V\/&vemumber(cm-1)
\ \
,
\
A\
N
500
19940
20000 E/eV
20060
Fig. 2 IR spectra of SiMo samples, (a) SiMo/KBr (5%), (b) Fig. 3 XANESspecta of pure heteropoly acid and samples SiMo»CTi:ABr/KBr(l()%), (c) Si/Mo=i() SiMo-SBA-l/KBr treated with 1-octanol. (a) Si/Mo=12 PMo-SBA-1 treated (10%) sample, (b) pure PMo, (c) pure SiMo, (d) Si/Mo=12 SiMoSBA-1 treated sample.
128
cation effects on the vibrational spectra of HPAs reported by C.Rocchiccioli-Deltcheff et al."^ IR spectra demonstrate that the Keggin structure of both heteropoly acids was preserved in samples treated with 1-octanol. The results of IR are in agreement with the results of XANES of Fig.3. Obtained lamellar samples show a dioo-spacing of 3.1 nm, comparable with the original pore size of SBA-1, (2.9 nm). It is inferred that once formed pore structure changed into layered structure by 1-octanol treatment. From the above results, we suggest the layered structure as shown in Fig. 4.
lamellar Fig. 4. Schematic representation of phase transition from cubic to lamellar structure REFERKNCES 1. Y. 1/umi, M. Ono, M. Kitagawa, M. Yoshida, K. Urabe, Microporous Mater., 5, 255 (1995) 2. W. Chu, X. Yang, Y. Shan, X. Ye, Y. Wu, Catal Lett., 42, 20 (1996) 3. S.Chc, Y. Sakamoto, W. Yoshitake, O.Terasaki, T. Tatsumi, J.Phys. Chem. B, 105, 10565 (2001) ; H.Yoshitake, S-H.Lim, S.Che, T. Tatsumi, Stud. Surf. Sci. Catal., 135, 2333 (2001) 4. C. Rocchiccioli-Deltchcff, M. Fournier, R. Frank and R. Thouvenot, Inorg. Chem. 22, 207 (19S3)
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
129
A further investigation on effect of basic media on the synthesis of MCM-41 Chun Yang^' ^ Shuxun Ge^ and Nongyue He"'' ^Key Laboratory of New Materials and Technology of China National Packaging Corporation, Zhuzhou Engineering College, Zhuzhou 412008, P. R. China ^College of Chemistry and Environmental Science, Nanjing Normal University, Nanjing 210097, P R . China ^Key Laboratory for Molecular and Bio-molecular Electronics of Ministry of Education, Southeast University, Nanjing 210096, P. R. China Pure siliceous MCM-41 samples were synthesized in strong and weak alkaline media (sodium hydroxide and ethylenediamine). Different pH ranges for synthesis were obtained for different alkaline media. Ethylenediamine gave the samples higher structural order and stability, and allowed the use of a broader pH range that makes it easier to synthesize MCM-41 of good quality. The advantage of ethylenediamine may results from the increase of charge density on the surface of surfactant micelles owing to the penetration of conjugated acid cation of the organic base.
1. INTRODUCTION Hexagonal MCM-41-type mesoporous materials showing a long-range order are potentially useful in some applications requiring regular nano-metric porous structure owing to their adjustable pore size and hexagonally patterned one-dimensional channel structure [1]. Therefore, many studies have been focused on the synthesis of MCM-41 with regular mesostructure [2-4]. These materials usually are hydrothermally synthesized in an alkaline medium through S^I" mechanism, and strong bases, such as NaOH and (CH3)4NOH, are usually employed as alkaline media for the synthesis. However, thermal stability of the samples prepared from strong alkaline media is so poor that the regularity in structure can't be maintained during the calcinations [3, 5]. In recent reports, weak bases (NH3, methylamine, ethylamine, dimethylamine and diethylamine) were used in the synthesis of MCM-41 [6-8]. It was found that the weak bases could endow the samples with high structural order and hydrothermal stability [8]. Therefore, it is important to investigate the mechanism of effect of alkaline media in the synthesis. Here we compare the synthesis of MCM-41 in weak organic base (ethylenediamine (EDA)) with that in strong base (NaOH), and give an explanation for the advantage of weak base exhibited in the synthesis.
130
2. EXPERIMENTAL Given amount of base (NaOH and EDA) was mixed into the aqueous solution of cetyltrimethylammonium bromide (CTAB) under stirring at room temperature. Then tetraethyl orthosihcate (TEOS) was introduced dropwise until the molar composition was: ITEOS : 0.12CTAB : x Base : I3OH2O, where x was varied from 0.16 to 18.40 as listed in Table 1. After further stirring for 24 h, the solid products were separated by filtration, fully washed with distilled water and dried for 10 h at 323 K. The obtained samples were calcined at 823 K for 6 h in air to remove templates. The samples synthesized in NaOH and EDA were designated as MCM-OH and MCM-EDA, respectively. XRD patterns were recorded on Rigaku D/max-yA X-ray diffractometer and N2 sorption measurements were conducted on Micromeritics ASAP 2000 instrument after evacuation at 573 K and 5x10'^ mmHg for 2 h. 3. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of samples synthesized in NaOH medium. It can be found that, as NaOH/TEOS ratio increases from 0.16 to 0.70, the half peak width gradually becomes narrow and the peaks (110), (200) and (210) appear (Fig.lA), meaning that the structural regularity of as-synthesized samples is improved under the higher alkalinity. However, after calcination to remove template (Figure 1B), no sample exhibits high structural order even though it possesses good XRD pattern before the calcination. This reveals that it is difficult to obtain a sample with both high structural order and stability under our experimental conditions of NaOH medium. Table 1 Samples and their structural parameters Sample
diool[nm)
X **
A.C
Ad 100''
(nm)
SBI;T
MCM-OH-1
0.70
13.4
3.84
2.79
1.05
inm)_ 3.22
MCM-OH-2
0.50
13.3
3.84
3.40
0.44
3.93
V
(cmVg)
(nm)
(nrn)_
2.14
1.08
1020
0.46
MCM-OH-3
0.23
13.0
4.55
4.09
0.46
4.72
2.47
2.25
756
0.61
MCM-OH-4
0.16
12.8
4.75
4.70
0.05
5.43
2.53
2.53
834
0.81
MCM-EDA-1
18.40
13.4
4.13
3.59
0.54
4.15
2.79
1.36
759
0.51
MCM-EDA-2
9.70
12.8
3.98
3.49
0.49
4.03
2.33
1.70
946
0.66
MCM-EDA-3
4.70
12.4
4.01
3.68
0.33
4.25
2.72
1.53
821
0.58
2.59 771 0.54 2.04 MCM-EDA-4 11.8 4.55 4.01 4.63 0.62 0.40 a ^^ NaOH/EDAl(molar ratio); ^ Initial pi I in synth esis system; ^ Before calcinations and after calcinations;
TEOS '' Adio()=aio() (B.C.) - d|oo (A.C.); *^ ao = 2 d|oo (A.C.) /V3; ^ Thickness of pore wall, L= ao-D.
BJH desorption pore diameter;
131
From the data for thickness of pore wall and Adioo shown in Table 1, it can be deduced that the polymerization between silica species decreases as the amount of NaOH increases, thus, more silica species with negative charges and low polymerization degree assemble around the micelles of surfactant to form a regular but low-polymerized mesostructure, which is destructible during the calcinations. Therefore, it is clear that the polymerization degree of silica species and the interaction between organic and inorganic species intensively depend on the alkalinity of system in NaOH medium. A suitable pH range in which MCM-41 sample of high quality can be obtained is very limited. The synthesis in EDA, a weak organic base, is different from that in NaOH as shown in Figure 2. When the EDA/TEOS ratio increased from 0.40 to 18.40, the samples with both good structural regularity and stability can always be obtained even though initial pH changes in a larger range of 13.4-11.8 as listed in Table 1. The data for wall thickness and Adioo, which are neither too high nor too low with the alkalinity unlike those in the case of NaOH medium, suggest also that the polymerization is at a moderate level and no over-polymerization and over-unpolymerization occur under lower and higher alkalinities, respectively. That is to say, the silica species perform well in both polymerization and assembly along the template micelles. These indicate that the quality of sample is not sensitive to the alkalinity in this system. EDA shows a "buffer effect" and allow the use of a broader pH range in synthesis, which make it easier to synthesize MCM-41 of high quality. The above experimental phenomena indicate that the alkaline media significantly influence the polymerization of the silica species and the interaction between the silica and the surfactant micelles. It was reported that the pH was more constant in the system of weak organic base than that in NaOH system because the weak base could continuously supply OH' ions by hydrolysis, thus, the charge density and polymerization degree of silica species were more uniform during the reaction, leading to a regular and stable structure in the weak base system [8]. Of course, this is a reason why the quality of sample is improved for the system located within the pH range
100
100
A
100
B
110200 210
110200 210 a
-
-
5
a 110 200
d 29 / (°)
B
b c
3
100
A
7 1
3
5
c
c
d
d
29 / (°)
Fig. 1. XRD patterns of samples synthesized in NaOH medium. (A) before calcination; (B) after calcination (a) MCM-OH-1, (b) MCM-OH-2, (c) MCM-OH-3, (d) MCM-OH-4.
3
5
29 / (°)
7
3
5
a b c d 7
29 / (°)
Fig. 2. XRD patterns of samples synthesized in EDA medium. (A)before calcination; (B) after calcination (a) MCM-EDA-1, (b) MCM-EDA-2, (c) MCM-EDA-3, (d) MCM-EDA-4.
132
suitable to synthesis. But it can't explain why the different pH ranges are required in the different alkaline media. Huo et al. found that the surfactant with high charged head group is favorable to the formation of MCM-41 of high quality [2]. This may be due to high charge density on the surface of surfactant micelles, which can promote the interaction between organic and inorganic species, hi the system of weak organic base, accompanied by hydrolysis of weak base, a number of conjugated acid cations form. We consider that these cations with organic hydrophobic tail can probably insert into the palisade region of surfactant micelles and leave their hydrophilic head groups on the surface, resulting in the increase of charge density on micelle surface. Therefore, in EDA medium, there is a stronger interaction between organic and inorganic species and thus the over-polymerization is suppressed when the alkalinity of system is at a lower level. On the other hand, the weak basic property of EDA allows considerable polymerization to occur at higher alkalinity. Consequendy, a broader pH range can be used for the synthesis of MCM-41 of high quality in EDA medium. REFERENCES 1. C.T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992)710. 2. Q. S. Huo, D. I. Margolese and G D. Stucky, Chem. Mater., 8 (1996) 1147. 3. R. RyooandJ. M. Kim, J. Chem. Soc, Chem. Commun., 711 (1995). 4. W. J. Kim, J. C. Yoo and D. T. Hayhurst, Microporous and Mesoporous Mater., 39 (2000) 177. 5. J. M. Kim, J. H. Kwak, S. Jun and R. Ryoo, J. Phys. Chem., 99 (1995) 16742. 6. M. Grun, K. K. Unger, A. Matsumoto and K. Tsutsumi, Microporous and Mesoporous Mater, 27 (1999) 207. 7. W.-Y. Lin, W.-Q. Pang, C.-P Wei, D.-M. Li and K.-J. Zhcn, Chem. J. Chin. Univ., 20 (1999)1495. 8. W-Y Lin, Q. Cai, W-Q. Pang, Y Yue and B.-S. Zou, Microporous and Mcsoporoas Mater, 33 (1999) 187.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
133
Cationic templating with organic counterion for superstable mesoporous silica P. Reinert", B. Garcia"'^', C. Morin^ A. Badiei^ P. Perriat^ O. Tillement', L. Bonneviot"'^* ^Laboratoire de Chimie Theorique et Materiaux Hybrides, Ecole Normale Superieure de Lyon, 46 allee d'ltalie, 69364 Lyon et Institut de Recherches sur la Catalyse, 2 av A. Einstein, 69626 Villeurbanne. ''UMR CNRS-GEMPPM 5510, INSA de Lyon, 20 avenue A. Einstein, 69621 Villeurbanne ^Laboratoire de Physico-Chimie des Materiaux Luminescents, 43 bd 11 novembre 1918, 69622 Villeurbanne '^Department of Chemistry, Universite Laval, Ste-Foy (QC), Quebec, Canada, G1K7P4 Superstable mesoporous silicas LUS-1 templated using cationic surfactants of various chain lengths were synthesized in the presence of tosylate anions in basic media with low surfactant to silicium ratios. The highly structured hexagonal silica mesophases are stable in boiling water for several days and, after calcination up to 1093 K. A stabilizing hydrothermal treatment before calcination further improves the stability up to 1273 K. It is believed that the outstanding stability of the LUS-1 is related to the hydrophobic control obtained using an organic surfactant counterion in the surfactant-silicate interface during polycondensation. 1. INTRODUCTION Mesostructured porous materials are very attracting for industrial applications as, for instance, specific absorbents, molecular sieves, catalyst supports and, for metal remediation. They are indeed characterized by very narrow pore size distribution in the range 2-30 nm and high surface area of about 1000 m^/g. Nonetheless, these materials are not very stable [1]. This is particularly true for the {S\ X', V} or {S', T} synthesis routes implying interfaces with either cationic or anionic surfactant S^ or S", respectively and, a positively charged inorganic phases r . The inorganic anions X" or the sulfonates or phosphonates S' surfactants generate more hydrophilic interfaces than quaternary ammonium surfactants in the {S^, I'} route. The latter route is known to generate more stable materials in which the surfactant counterion plays an important role on the long-range order, the nature of the phase and on the hydrophobicity of the organic-inorganic interface [2]. Besides, non-ionic route using amines in HMS materials or block copolymers such as polyethers leads to more hydrophobic interface, larger pore wall and better-condensed inorganic matter. The latter are indeed very stable materials however their structures are quite disordered [3]. To go further with the idea that counterion may control the interface, TMS were prepared according to the {S^, 1} route in the presence of hydrophobic tosylate (Tos) and tested for their both thermal stability and hydrothermal stability [4].
to whom correspondence should be sent, present address in a) ENS-Lyon and IRC, France
134
2. EXPERIMENTAL The mesoporous templated silicas were prepared from clear sodium silicate solutions, alkyltrimethylammonium (RTMA) and tosylate (Tos", paratoluenesulfonate) as counterion for LUS-1. To compare counterions, MCM-41 were prepared in the same conditions than that of the LUS-1 using various halogenides, X" = CI", Br" and I". However, for stability tests, the comparison was made with a MCM-41 made according the optimized recipe of Song et al. [5]. Typical gel compositions for a LUS-1 were: 1 Si02; 0.266 Na20; m RTMAX, 79.9 H2O where R = CIO to C22 n-alkyl and, the surfactant to silica ration m = 0,01 0,1 [4]. The silica source may be commercial water silica or Ludox solution. The hydrothermal treatment was performed at 403 K for 24 h followed by calcination at 823 K. Further stabilization consisted in a second hydrothermal treatment of the solid immerged in pure water for 24 h at 403 K. The XRD data were measured on the stabilized and calcined sample exposed to ambient air using a Siemens D5005 diffractometer. The SEM pictures were taken using a Hitachi S800 FEG. For ^^Si Solid State NMR, the samples were loaded under pure oxygen into 4 mm rotor. The rotation frequency was about 8 KHz and the data were collected using a 400 MHz Bruker solid state NMR spectrometer. The hydrothermal stability was performed in boiling water for several days using the appropriate vessel equipped with a cooling column. The samples were calcined in ambient air ramping the temperature at 1 K/min up to the desired temperature, the latter maintained for 1 hour before cooling. 3. RESULTS AND DISCUSSION Four well-distinguishable XRD peaks (100, 110, 200 and 210) typical of the hexagonal mesophase, were obtained for C12 to C18 surfactants (the narrowest peaks for C12 and CI4). A single broad XRD peak was observed for CIO and C22. For the materials prepared using the C16-surfactant, 20% up to 70% structure losses were measured after exposure to boiling water for 48 and 72 hours respectively, according the XRD peak intensity. MCM-41 prepared with bromide as counterion reaches these levels of degradation in less than 6 and 12 hours, respectively. Calcination in air shows clearly that the stability is improved when the anion is changed from CI" to I', with a structure collapse at 1093 up to 1273 K, respectively. The mesophases obtained with tosylate, which leads to slightly less stable structure upon calcination than iodine, are further improved, as shown on Figure 1, by postsynthesis hydrothermal stabilization according to the procedure describe in the literature [5]. Up to 1173 K, there is a progressive decrease of the silanol concentration measured from FT-IR and H-NMR with minor morphology changes (dioo and pore diameter kept constant at about 4 nm and 2.9 nm, respectively) and loss of surface area (from 1100 to 935 m^/g) (see table 1). At 1273K, the structure shrinks by 15 %, the surface drops down to 635 mVg accompanied with wall thickening bringing the solid to the limit of the microporosity. For higher temperature, the structure begins to collapse as illustrated by the decrease of the intensity on the diffractograms (figure Id, e). However, at 1373K a well ordered hexagonal structure (illustrated by a narrow half-width) is kept and according to the dioo equal to 2.9 nm corresponds to pores, which are probably microporous. The surface area of 33 m^/g measured for non-calcined stabilized LUS-1 is consistent with the external surface of fibers of 40 nm diameter. On figure 2, the fibers are much larger than 40 nm. However, on Transmission Electron Microscopy, the well-ordered domains are indeed of 30 to 60 nm wide. A detailed observation of the SEM pictures reveals that the fibrous material
135
8 29
Fig.
Fig. 1. XRD of stabilized LUS-1 calcined at a) 873, b) 1173 c) 1273, d) 1373, e) 1473 K
2. SEM micrograph of stabilized LUS-1 showing the typical fiber-like morphology
Table 1 Morphology characteristicsft"omN2 adsorption-desorption and "^^Si MAS NMR data Pore Wall diameter thickness (nm) (nm) (d) (c) / /
LUS-1 Treatment (a)
Surface area m'/g(b)
Porous volume cmVg
Stabilized
33
/
873 K
1100
0.84
3.1
1173K
935
0.67
1273 K
635
0.34
Q4 Ciiirr
%
Q3
%
Q2
%
(Q2+Q3)/
%(e)
32
59
24
17
41
1.5
137
59
37
4
41
2.9
1.7
70
83
14
3
17
2.1
1.8
17
/
/
/
/
(a) as-made followed by stabilizing hydrothermal treatment and calcined at various temperature between 873 and 1273 K (b) calculated using the BET equation for P/Po < 0.15 (c) calculated using the Kelvin-Thomson equation (d) calculated from XRD and pore diameter (e) chemical shift for Q2, Q3, Q4 respectively at 92, 100, 109 ppm (ref TMS)
136
is made of primary fiber of about 50 nm diameter merged together into secondary more or less bulky (some times hollow) fibers (figure 2) of about 1 pim diameter. These secondary fibers are themselves associated one with another in much larger fibers of rather heterogeneous diameter sizes centered at about 10 ^m. The ^^Si MAS NMR experiments performed on the different stabilized silicas. The stabilized state was of particular interest since it has been found to increase further the thermal stability of the material by about 100 K. For a calcination at 873 K the surface silanol groups undergo a strong rearrangement leading to an increase of the concentration of Q3 species at the expense of the Q2 species while the concentration of the Q4 species remains constant. The OH density calculated from the surface area and the weight loss at 1273 K from TGA is 4.2 OH/nm^. However, after a calcination temperature of 1173 K, the number of Q4 species is increased by about 50% mainly due to the condensation of the Q3 species. The silanol density drops down to 0,25 OH/nm^, which confer to the material a rather hydrophobic nature characterized by a BET constant of 70 (instead of 137 after calcination at 873 K, table 1). For a higher calcination temperatures (1273 K), the silanol density drops even further and, concomitantly, the BET constant value decreases down to 17. Further questions remains to be answered concerning the stability in boiling water since the OH density is almost the same as in MCM-41. It is believed that the difference lies on the localization of the silanol groups that might be rather internal to the wall in the LUS-1. A post hydrothermal treatment that decreases the resistance toward boiling water without changing the OH density according to ^^Si MAS-NMR would be interpreted as a rearrangement of the OH groups "moving" from internal to external position by successive OH condensations and siloxane bridge openings. 3. CONCLUSION Super stable templated mesoporous silicas arc obtained in a one-pot synthesis procedure using hydrophobic counterion believed to increase the hydrophobicity of the organic-inorganic interface and generate highly ordered silica mcsophases. Eamclar and cubic mesophases obtained according to a similar recipe are currently under investigation. REFERENCES [1] J. M. Kim, R. Ryoo, Bull. Korean Chem. Soc, 17 (1996) 66; G. Gu, P. P. Ong, C. Chu, J. Phys. Chem. Solids, 60 (1999) 943. [2] A.-R. Badiei, S. Cantournet, M. Morin, L. Bonneviot, Langmuir, 14 (1998) 7087; B. Echchahed, M. Morin, S. Blais, A.-R. Badiei, G. Berhault, L. Bonneviot, Micropor. Mesopor. Mater., 44-45 (2001) 53. [3] S. S. Kim, W. Zhang, T. J. Pinnavaia, Science, 282 (1998) 1302. [4] L. Bonneviot, M. Morin, A. Badiei, PCT/CAO1/00062 extension WO 01/55031 Al (2001). [5J K. M. Reddy, C. Song, Catal. Lett., 36 (1996) 103. [6] Q. Huo, D. I. Margolese, G. D. Stucky, Chem. Mater., 8 (1996) 1147.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
137
The synthesis of mesoporous materials with semicrystalline microporous walls Sung II Cho ^, Yong Ku Kwon^, Sang-Eon Park'^ and Geon-Joong Kim^ ^Department of Chemical Engineering, Inha University, Incheon 402-751, Korea. ^Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Korea. ^Korea Research Institute of Chemical Technology, Taejon 305-600, Korea. The route to synthesize the mesoporous materials having microporous semicrystalline frameworks has been developed in this study; the successful incorporation of the microporous ZSM-5 within the framework walls of the ordered mesoporous structure significantly improves the hydrothermal stability and acidity of MCM-41 and related mesoporous materials. 1. INTRODUCTION Recent research efforts have been paid to the synthesis of microporous[l] or mesoporous materials[2-4] with large pore sizes and uniform pore distribution[5]. However, typical mesoporous materials have the amorphous frameworks that are insufficient to withstand the severe process conditions, e.g. steam regeneration of deactivated catalysis[l]. Because of these limitations in use, future applications in the conversion of large molecules would depend on improving their characteristics. Obviously, significant advances of the physicochemical properties of these mesoporous materials can be expected when the crystalline zeolites such as ZSM-5 or Y-zeolite are incorporated into the silica frameworks. In this paper, we report the synthesis of highly ordered mesoporous silica and aluminosilicate having semicrystalline, microporous zeolite frameworks. Our approach is mainly based on the sequential synthesis of MCM-41 and MFI-type zeolite by two step treatments using tetrapropylammonium bromide (TPABr) and cetyltrimethylammonium bromide, Ci6Hi3(CH3)3NBr (C16TMAE), surfactant as a structure-directing agent. In addition, the catalytic properties of these materials were examined in the alkylation of diisopropylbenzene with isopropanol as a test reaction. 2. EXPERIMENTAL For the MFI synthesis, silicon oxide (SiOz), aluminum oxide (AI2O3), sodium oxide (NazO), TPABr and H2O was used at 50 °C and then the aqueous solution of CieTMAB surfactant was added to the resulting precursor MFI nuclei solution with stirring at the same temperature and the mixed solution was aged at 100°C for 1 day. And the pH was adjusted to approximately 11 by the addition of hydrochloric acid (HCl) with vigorous stirring. Then, the mixture was heated again at 100 °C for 1 day. 1,3,5-Trimethylbenzene (TMB) was added to the mixed solution that was heated to reflux for additional 1 day. And, the pH value of the final mixture
138
was adjusted to 12 by addition of tetrapropyl ammonium hydroxide (TPAOH) solution. Finally, methanol with the same volume as TMB was added to the reactant mixture. The final mixture was transferred to a Teflon-coated autoclave for further reaction at 100 - 170°C. XRD patterns were recorded on a Rigaku Rotaflex diffractometer using CuKa radiation. The morphology of the samples was examined by TEM (Phillips CM-220) and SEM (Hitachi S-4200). N2 adsorption and desorption isotherms were determined on a Micrometrics ASAP 2000 sorptometer at -196 °C. NH3-TPD curves were performed in the range of 150 - 500 °C 3. RESULTS AND DISCUSSION Typical X-ray diffractograms of the silica-surfactant assembly with the addition of TMB display a series of intense and sharp peak intensities in the small and wide angle region, indicating the formation (IHU-1) of both MCM-41 mesophase and crystalline ZSM-5 microphase as shown in Figure 1. The intense (100) peak for the silica-surfactant assembly before the heat treatment at 170 °C (Figure lA) slightly moves to the lower angle region during crystallization (Figure IB, C and D), indicating the expansion of the hexagonal packing distance between the mesopores during the second crystallization to induce the microporosity of ZSM-5. The fact that higher order peaks near 26 = 3 - 7° are absent after the second crystallization (Figure ID) indicates that the structural perfection of the hexagonal mesophase is reduced due to the crystallization of the MFI-zeolite nuclei, leading to the disordered channel formation of IHU-1.The XRD pattern in Figure IE shows simultaneous formation of the disordered mesoporous and ZSM-5 structure. As a result, the physical mixture of ZSM-5 crystal and collapsed MCM-41 was obtained as a product without addition of TMB and methanol. All of the mesoporous structures were collapsed and pure ZSM-5 crystals were formed through the transformation from meso-phase with the prolonged reaction time (Figure IF). Transmission electron microscopy (TEM) micrograph of IHU-1 shows a partially disordered pore structure with thick framework walls (Figure 2). The repeated addition of hydrochloric acid leads to MCM-41 with thicker framework (IHU1) walls, within which a high density of nanocrystallites can be nucleated and grown.
2th*ta(d«ar**t)
Fig. 1. XRD data of the IHU-1, followed by additional heat treatment at 170 °C for: (A) 0 hours: (B) 3 hours; (C) 7 hours; (D) 24 hours
Fig. 2. Tem image of the IHU-1 at lOOt for 3days with repeated pH adjustment to 11 and additional heat treatment at 170°C for 3 hours
139
These data are coincident with the XRD results of MCM-41 containing ZSM-5 nuclei and IHU-1 with semicrystalline walls (Figure l).These data also confirm the conversion of the zeolite precursors into the ZSM-5 nano-crystals in the frameworks The calcined IHU-1 had a N2 BET surface of 920 m^/g and a pore volume of l.Ocm^/g. Aluminum containing IHU-1 (IHU-2) was prepared to test its catalytic activity and selectivity for the alkylation of diisopropylbenzene(DIPB) with isopropanol. For comparison, H-type Al MCM-41 was also prepared and tested as a catalyst. The amount of aluminium in catalysts was almost the same. Catalytic activities determined in the alkylation of DIPB with isopropanol over various catalysts are given in Figure 3. In the alkylation of DIPB, the conversion of DIPB over the H-type Al MCM-41 was lower than that over the H-type IHU-2 because of the abundance of acidic sites on the surface of IHU-2. The conversions of DIPB at 400 °C were 18% over IHU-2 and 10% over H-type Al MCM-41, respectively. Triisopropylbenzene (TIPB) was found mainly in the product mixture at 200 - 300 °C, whereas the cracking of DIPB was proceed at 400 - 600°C, resulting in the production of benzene and mono-isopropylbenzene(MIPB). The main products obtained over IHU-2 were benzene and MIPB over 300 °C. The high selectivity to benzene and MIPB over IHU-2 suggests that IHU-2 has stronger acid sites than Al containing H-type MCM-41 catalyst (Figure 4 and Figure 5). HZSM-5 was inactive for this reaction. Microporous ZSM-5 cannot adsorb TIPB in the pore because of its small size, whereas MCM-41 with the larger pore size converts it into various hydrocarbons. As a result, the incorporation of aluminum into the mesoporous walls (IHU-2) greatly enhances the acidity and reactivity of IHU-1 to be utilized as one of the most promising catalysts. The catalytic activity of the separate agglomerates of H-type ZSM-5 and H-type Al MCM-41 was similar to that of Al MCM-41, confirming that the reaction of TMB molecules occurred only within the mesopores of MCM-41/MFI composite. The acidic nature of H-type AlMCM-41 MOVWI -a/AH3oanu) (mole ratio of Si02/Al203=35) and H-type IHU-2 (mole ratio of Si02/Al203=39) was measured by temperature-programmed desorption (TPD) of ammonia. They have very strong acidic site distribution, similar to that present in the H-type ZSM-5 zeolite. The acidity of MCM-41 was found to be comparable to that of amorphous silicaalumina, but weaker than that of ZSM-5. ZSM-5 offers unique catalytic activities 400 500 and selectivity alteration for certain Teniperature('C) reactions compared to amorphous silicaFig. 3. The catalytic activities of IHU-2 and alumina because of porosity, crystallinity HMCM-41 in the alkylation of DIPB with and strong acidity. It is evident that Hisopropanol type IHU-2 has strong acidic site distribution, similar to that present in the H-type ZSM-5 zeolite.
140 100-
80-
geo- A 1 ^ 40-
% \
(/5 —0—bUHHK
\
20-
300
3BD
400
490
5D0
5QD
eOO
'foiperatm(T)
Fig. 4. The selectivity versus reaction temperature over IHU-2
—
*
0— I — I — I — I — I — 1 — I — I — ^ — I — I — I — | —
—•—dinne
•
^_____::zz=—•
— 1 — ' — 1 — ' — 1 — • — 1 — 1 — I — ' — 1 — 1 — r
3D0
3BD
400
493
aD
5BD
eOO
liii|xraim(t)
Fig. 5. The selectivity versus reaction temperature over MCM-41
When the MFI microporous structure is incorporated into the walls of MCM-41, a significant increase in peak area around 350 *^C is observed compared to H-type Al MCM-41, indicating that the strong acid sites are introduced within the micropores of ZSM-5 walls. Consequently, the H-type IHU-2 has strong acid sites within the framework walls that catalyze large aromatic molecules to produce various hydrocarbons in the catalytic reaction mentioned above. 4. CONCLUSION The successful incorporation of the microporous ZSM-5 within the framework walls of the ordered mesoporous structure significantly improves the hydrothermal stability and acidity of MCM-41 and related mesoporous materials. The excellent catalytic properties of a series of the IHU expand the area for the application of porous materials and can be used in a number of commercial processes in the future. REFERENCES 1. A. Corma, Chem. Rev. 97 (1997) 2373. 2. J. S. Beck, M. C. Vartuli, Current Opinion in Solid State and Mater. Sci. 1 (1996) 76. 3. D. Zhao, Q. Huo, J. Feng, B. F Chmelka, G. D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. 4. D. M. Antonelli, Y. Ying, Angew. Chem. Int. Ed. 35 (1996) 426. 5. Q. Huo, D. I. Margolese, G. D. Stucky, Chem. Mater. 8 (1996) 1147.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
Synthesis of a Mesoporous Hydrothermal Stability
141
Molecular
Sieve
with
Y. K. Kwon^ G.-J. Kimh, J. H. Llm^ D. H. Kim^ and B. D. Choi^> 'Department of Polymer Science and Engineering, Inha University, 253 Yonghyun-Dong, Nam-Gu, Incheon 402-751, Korea ''Department of Chemical Engineering, Inha University, 253 Yonghyun-Dong, Nam-Gu, Incheon 402-751, Korea Ordered mesoporous materials with a hydrothermally stahle microporous framework have been synthesized using a mixture of a non-ionic block copolymer and cationic surfactant under the basic conditions. Varying the relative molar ratio between the polymer and surfactant leads to the change in morphology of the final calcined mesostructures, indicating that the ack^quate amount of polymer for the surfactant forms a hard-gel type micellar structure that persists in the solid rearrangement process of th(^ inorganic species. 1. I N T R O D U C T I O N To enhance hydrothermal stability of mesoporous materials, much scientific effort has been focused on modifying their pore walls into somewhat hydrothermally stable materials. Recent approaches to build hydrothermally stable mesostructures have been carried out by either the direct assembly of transition metal oxides or the sequential synthesis of the frameworks with zeolitic precursors. We recently reported the sequential synthetic method to build the ordered mesoporous material with the semicrystalline zeolite framework (IHU-1) using cetyltrimethylammonium bromide (CTAB) and trimethylbenzene (TMB) as swelling agent.' It was shown that the addition of TMB improves the stability of the organic-inorganic supramolecular micellar structures during the solid rearrangement process of zeolite precursors at high temperature, converting the amorphous precursors into zeolitic semicrystals. IHU-1 displayed both characteristics of zeolitic and mesoporous material with highly enhanced thermal, hydrothermal stability and acidity. Here we report another synthetic route to produce ordered mesoporous mat(M*ials with zeolite seed frameworks with excellent hvdrothermal stabilitv.
142
Our approach is based on the sequential synthesis of MCM-41 mesoporous frame and ZSM-5 seed frameworks using a mixture of amphiphilic block copolymer, poly(ethylene oxide)-6-poly(propylene oxide)-6-poly(ethylene oxide) [E02()P07()E02(), where 20 and 70 denote the numbers of ethylene oxide (EO) and propylene oxide (PO) monomers per block] and cetyltrimethylammonium bromide, Ci6H33(CH3)3NBr (CTAB) as structure du'ecting agent. The relative amount of E02()P07oE02() was changed to determine the adequate molar ratios between E02()P07()E02() and CTAB in the mixtures which form a hard-gel-type intramicellar structure with the inorganic species during the reaction in base media. 2. EXPERIMENTAL SECTION To make ZSM-5 precursor solution, 3.15 g of tetrapropylammonium bromide (TPABr) was initially dissolved in aqueous sodium hydroxide (NaOH) solution (1.25 g NaOH / 12.5 g H2O) with stirring at 50 "C for 1 hour and then 5 g of Ludox HS-40 was added and stirred for additional 6 hours at the same temperature. The organic templating phases were prepared by dissolving 4.5 g mixture of CTAB and EO^OFOTOEOLW in 10 g C2Hr,0H with stirring at 50^>C for 1 hour. The molar ratios of CTAB to EOLWFOTOEOLW in the mixtures were 16, 40, 56, 72 and 100. Then they were added into the ZSM-5 precursor solution with stirring at the same temperature and aged at 100 "(^ for 1 day. The resultant inorganic-organic solutions were cooled to room temperature and the pH was adjusted to approximately 11 by dropwise addition of aqueous HCl with vigorous stirring. Then the solutions were heated again at 100 "C for additional 2 days. The pH adjustment was repeated several tim(\s during additional aging, due to NaOH, produced during the reaction that shifted the solution pH toward a strong base. After aging at 100 "(\ th(^ half of the precipitates was filtered, washed and dried for analyti(!al measurements and the remains were transferred to a Teflon-coated autoclave for further heat treatment for the formation of zeolite seeds at 175 "C for 12 hours. The as-made samples were calcined at 500 "C for 4 h in air. For convenience, the calcined samples synthesized with the CTAB/ K02()P07oE02() molar ratios of 16, 40, 56, 72, 100 were denoted as C16, C4(), (^56, C72 and ClOO. 3. RESULTS AND DISCUSSION X-ray diffractometer (XRD) scan data (Fig. 1 a) of C56 shows an intense Bragg peak near 29 = 2" and a series of weak high order peaks in the range of 2.6 < 20(degrees) < 7.5, indicating the existence of a mesoscopic ordering of the pore structure. To measure the hydrothermal stability of the C56, it was placed in boiling water for 24 hrs and 1 N NaOH solution for 3 hrs. Fig. 1 b shows a typical XRD data of C16 after heat-treating in boiling watei- for
143
24 hrs. The data show t h a t the intense small angle peak was still observed near the same 20 as t h a t seen in Fig. 1 a. The peak intensity was decreased by approximately 35 % which may be ruptured in boiling water. After treating in I N NaOH, the main peak decreased and slightly shifted to the wide angle region. These data indicated that the part of the mesostructure was dissolved under the basic condition and the distance between neighboring pores became smaller.
c
03
> 15
1.5
^H
25
35
45
55
65
20 (degrees)
8.5
15.5
29
22.5
29.5
(degrees)
Fig. 1. XRD data of C56: (a) after calcination; (b) after dipping in boiling water; (c) after dipping in IN NaOH solution. The main intense peak of the XRD data for the specimens prepared without heat treatment at 175"C was almost disappeared due to the collapse of the amorphous framework. By addition of EOL^OPOTOEO^O in CTAB micellar solutions, the electrostatic molecular interactions between EOLWFOTOEOLU) and CTAB enhance the molecular packing density especially in the hydrophobic core, leading to a hard-gel type micellar formation. With the tightly-coiled molecular conformation, the organic-inorganic templates may not be ruptured even during the solid phase conversion of the MFI-type zeohte precursors into the ZSM-5 seeds at high temperatures and pH ^ 12. The successful incorporation of non-ionic EO^OFOTOEOL'O in the cationic CTAB solution depends on the molar ratio of two components with given molecular weights which changes the molecular packing density or interfacial curvature of the mixed micelles which determines the hydrothermal stability of the organic-inorganic templates during treatment of ZSM-5 precursors. To analyze the pore geometry and structure, N2 adsorption-desorption isotherms were measured. In Fig. 2, the adsorption and desorption branches of C72 gradually increases and decreases with pressure, indicative of the
144
absence of mesoporosity of these compounds. The desorption branch of C5() was steeper than the adsorption branch, typical of mesoporous structures with
1(f) Ui
E o •a
/ OU
600
•
500
.
400
o
^ ^ o (A
^ M"^^.-"'
''
/^
.-l-^^j^feafeM
_^^^^I^C^MHHH
i^^n^pTHnf^BggiiMHI 't^^nSsSmKK^^K^^A
;'^&^KB^^^^HI
200
o
1 no
245
"E O 210
" a
300
•a
<
P 280 CO
adsorption — • — desorption
^s^^H^^^BHI "'•-^^^^HHSr**^
'
- '
1 f*LL_--w.,!^.,
.T'^-'WiW
« 175
o (/)
<
140
L ^
adsorption —•—desorption «
^^^^^^^^mmtmmt^^^^^^^
02
04
1
06
•
1
t
1
08
Relative Pressure, (P/P )
Relative Pressure, (P/P^)
Fig. 2. BET data of (a) C56 and (b) C72.
t>
Insets are their TEM mage
distribution in framework wall thickness. ACKNOWLEDGEMENT This work was supported by Grant No. 2()()l-2-3()8()()-()()l-3 from the Basic R(\sear(*h Program of the Korea Science and Engineering Foundation and b\' In ha University through project.
REFERENCES 1. (J.-J. Kim, S. D. Choi, Y. K. Kwon, S. E. Park, Chem. Mater, submitted in 2002.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
145
Diffusive characterization of large pore mesoporous materials with semicrystalline zeolitic framework H. V. Thang^ A. Malekian^ M. Eic^*, D. Trong On' and S. Kaliaguine' 'Department of Chemical Engineering, Laval University, Sainte-Foy, Quebec, Canada, G I K 7P4 ^Department of Chemical Engineering, University of New Brunswick, P.O. Box 4400, Fredericton, N.B., Canada E3B 5A3. Fax:+1-506-453-3591, e-mail:meic@unb.ca. Diffusion (kinetics) characterization of composite mesoporous materials that contain nanozeolite particles in the mesoporous walls (UL-zeolite) is considered very important to further assess their potential as unique materials for separation and catalysis. Two different bimodal materials were investigated using two probe molecules, i.e., n-heptane and toluene of different kinetic diameter. Results showed varied behavior with respect to diffusion rates, as well as mass transport mechanisms. Differences in the kinetic results were related to structural properties of the UL-zeolite samples. 1. INTRODUCTION One of the most important advantages of a new class of mesoporous molecular sieves that contain inter-grown zeolitic nano-particles in their walls (UL-zeolite) are their great potential for applications in separation and catalysis involving large molecules, which are not possible with conventional zeolites ''^. In contrast to microporous materials like zeolites, which have been extensively investigated with respect to their diffusive properties, so far mesoporous molecular sieves (MMS) of MCM-41 or SBA-15 type have been the subject of a very limited number of diffusion studies '^'^. The present study is based on Zero Length Column (ZLC) technique to measure effective diffusivities of n-heptane and toluene as probe molecules on two types of UL-zeolite materials, i.e., UL-ZSM-5 and UL-silicalite. In addition the measurements were also carried out on single ZSM-5 and silicalite crystals, as well as Al-SBA-15 precursor, and obtained results compared with UL-zeolites. 2. THEORY Diffusion measurements were carried out employing the ZLC gas chromatographic method. The analysis of the ZLC experimental desorption curves involves a fitting procedure based on a solution of the Fickian diffusion equation with appropriate initial and boundary conditions. The solution for spherical geometry and linear adsorption isotherm conditions is given as: corresponding author
146
00 ^""^[-Pl^eff^'^^ /3^+L{L-\) n=\
(1)
where C and Co are concentration in the effluent gas at time t and initial (feed) concentration respectively, and Deff/R^ is effective diffusion time constant. The long time (asymptotic) solution of Eqn. (1) is approximated by a straight line on a semi-log plot (C/Co vs.t). Definitions of "„ and L, as well as details of the method are given elsewhere^. The corresponding expression for a system in which desorption rate is controlled by a surface resistance to mass transfer is: (2)
— = exp{-3kt/R) where k is a solid surface mass transfer coefficient. 3. SAMPLE PREPARATIONS
UL-ZSM-5 and UL-silicalite samples were synthesized using SBA-15 as a precursor with thick walls (>4 nm), and impregnated with TPAOH as a template for zeolite crystallization. The second stage involved solid state crystallization at 130^C and different times of aging (08 days) ''^. Some alumina was also added in the precursor solution for the UL-ZSM-5 synthesis (Si/Al=100). Structural data of the samples are shown in Table 1. Table 1 Structural properties of Al-SBA-15 (precursor) and UL-Zeolite samples Micropore Mesopore Sample* SBI:T SBJH ^micropore volume volume (m'/g) (m'/g) (mVg) (cmVg) (cmVg) 0.65 0.058 144 Al-SBA-15(100/0) 914 770 0.089 1.25 188 UL-ZSM5(lOO/2) 974 786 0.145 371 UL-ZSM5( 100/6) 904 533 2.19 0.151 0.48 373 UL-ZSM5( 100/8) 479 106 309 0.133 UL-Silicalite(oo/6) 421 112 0.18 *The numbers in parenthesis indicate Si / Al ratio, e.g., 100 and crystallization e.g., 0,2,6 etc.
Mesopore diameter(A) 38 110 195
-
40 time in days ,
4. RESULTS AND DISCUSSION Diffusion measurements, according to the standard ZLC method, were conducted at low concentrations (partial pressures), e.g., 0.1-0.2 Torr, which are considered to be within the linear range of adsorption isotherms. In one of the earlier studies'* it was reported that the diffusion measurements under these conditions revealed parallel microporous structure of MMS. The ZLC desorption curves shown in Figure 1 for toluene in different UL-ZSM-5 samples, as well as the reference (precursor) Al-SBA-15 sample indicate interesting and
147
distinctive features regarding mass transfer processes occurring in these samples at low concentration levels. Two UL-ZSM-5 samples with different times of crystallization, e.g., 2 and 6 days, as well as the Al-SBA-15 sample show typical ZLC curves of the Fickian diffusion. The original micropore structure from Al-SBA-15 precursor is retained in the ULzeolite structure, and is exposed to diffusing sorbate molecules in a fashion similar to micropore diffusion in zeolites. Taking an average diameter lOfim for a single particle, as revealed by SEM images, one can obtain diffusivities in the range of 10''^ to 10"'^ m^/s range from the data presented in Tables 2 and 3, which are typical of micropore diffusion in zeolites^. However, the ZLC curve for Ul-ZSM-5 ((100/8), top curve), which has the highest crystallinity or the longest aging time (eight days) showed a linear behavior on the semi-log scale for the entire desorption time range indicating a response typical of the surface barrier controlled process, as described by Eqn. (4). A plausible explanation for this type of the transport mechanism could be related to the collapse of meso-structure, as is evident from the structural data presented in Table 1, i.e., a drastic reduction of the mesopore area (SBJH) and mesopore volume when inter-grown zeolite crystallinity becomes close to 100 % (8 days of aging). This collapse of the mesopore structure is likely to cause obstruction of micropores within the original SBA-15 precursor's structure. The exactly same pattern of behavior was also observed for n-heptane diffusion in the UL-ZSM-5 samples. Figure 2 illustrates toluene diffusion in composite UL-silicalite particles in comparison with single silicalite crystals. Effective time constants (Dco/R^) determined from these curves confirm faster diffusion process in the composite UL-silicalite particles (3-4 times). Similar results were obtained for other comparative systems involving n-heptane and toluene in ULZSM-5 particles and ZSM-5 crystals. Generally all these results confirmed facilitated mass transport involving UL-zeolite composite particles. Summary of the results presented in Tables 2 and 3 shows that toluene effective time constants are 2-3 times higher in the UL-ZSM- 5 (2 and 6 days) compared to the UL-silicalite sample. This is in contrast to diffusivites involving zeolite single crystals, where diffusion of the same or similar sorbates is always larger in silicalite than ZSM-5 due to the absence of active acid centers in the former. Property data shown in Table 1 indicate very small mesopore volume of UL-silicalite (0.18 cmVg) in comparison with 2.19 cmVg associated with the UL-ZSM-5 (100/6) which could be indicative of a partially collapsed mesoporous structure in the UL-silicalite, and possibly some obstruction of the micropores. In addition these results show significantly higher effective time constants for n-heptane in UL-ZSM-5 than toluene as expected, due to interactions between the acid sites of the adsorbent with aromatic ring of toluene. 5. CONCLUSIONS AND RECOMMENDATIONS At the low concentration levels diffusion processes in UL-zeolite samples are entirely governed by micropores that are associated with the precursor, SBA-15 structure. Effective diffusion time constants for the samples with developed meso-stucture are generally much higher in comparison to the corresponding zeolite single crystals. If a mesoporous structure is destroyed due to a shrinkage of mesopore volume, then the transport process is entirely controlled by a surface barrier mechanism
148
Fig. 1. ZLC curves of toluene in different samples of UL-ZSM-5 and SBA-15 at 80°C l^*'.^
^ • ^ ^ ^ ^ ^^>toLr*^Cfc»
i
'"'-^Mx>^. "^''**'^.'-v'v.'„
,..-'^^**^J'^^*'-S8A15(100) UL-Z8M5(100/8)
y
^ ^ « ^ , „ ^ ^ Silicalite "" ^ ^ l ^ ^ ' ^ H i t ^ ^
C/Co ' •"^,,
^ ^ ^ ^ ^ f e j B - j .
J
I
^^ ^
UL-ZSM5(100/t)
xsT"^'^--^ ^ v '"^^'->-1
Fig. 2. ZLC curves of toluene in silicalite and UL-silicalite samples at 80"C
':
"-K
UL-silicaiite
^ " " ^
^^^^^
^^^^S^^^Sikaft
UL-ZSM5(100/2)
0
too
30
200
50
0
Table 2 Summary of the ZLC results for toluene in different adsorbents Sample AI-SBA-15 (100) UL-ZSM-5 (6) UL-Silicalitc (6) Silicalite
ZSM-5 (100)
60 80 100 80 100 120 60 80 100 60 80 100 120 60 80 100
(xlO^) 5.9 10.7 22.3 13.0 22.6 49.1 2.5 5.4 10 5 0.7 1.5 3.1 6.2 1.8 2.9 4.5
150
200
250
300
35
Table 3 Summary of ZLC results for n-heptane in different adsorbents Sample
Temp.
CO
100
T i m e (sec)
Time(sec)
Temp.
(kJ/mol) 32.2
Deo/R'
H. (kJ/mol)
("C) Ai-SBA-15
60
4.6
(100)
80
11.4
44.53
100 38.16
UL-ZSM-5 (6)
37.60
UL-Silicalite (6)
29.36 Silicalite 23.05
ZSM-5 (100)
60
15.1
80
24.9
100
51.4
60
2.2
80
4.3
100
8.1
120
19.6
60
1.23
100
6.9
80
1.9
100
2.9
120
4.1
21.20
39.23
21.20 21.91
The ZLC measurements involving higher (non-linear) concentration levels of sorbates arc recommended in a future work to assess a role of mesoporcs and inter-grown nano-crystals of zeolites in the overall diffusion process. REFERENCES 1. D. Trong On, D. Lutic and S. Kaliaguine, Microp. Mesop. Mat., 44-45 (2001) 435. 2. D. Trong On and S. Kaliaguine, Angew. Chem.lnd. Ed., 40 (2001) 3248. 3. F. Stallmach, A. Graser, J. Karger, C. Krause, M. Jeschke, U. Oberhagenmann and S. Spange, Microp. Mesop. Mat., 44-45 (2001) 745. 4. D.S. Campos, M. Eic and M.L. Occelli, Stu.Surf.Sci.Cat., 129, A. Sayari et al. eds., Elsevier Science (2000) 639. 5. M. Jiang, M. Eic and D.M. Ruthven in Fundamentals of Adsorption 7, K. Kaneko et al. eds.. International Adsorption Society (2002) 732 6. J. Karger and D. M. Ruthven, Diffiision in Zeolites and Other Microporous Materials, J. Wiley and Sons, New York (1992).
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
149
Synthesis of cubic mesoporous aluminosilicates with enhanced acidity Gong Li, Qiubin Kan*, Tonghao Wu, Changmin Hou, Feng-Shou Xiao, Jiahui Huang College of Chemistry, Jilin university, Changchun 130023,RR.China. Cubic mesoporous aluminosilicate(AlMB48) with enhanced acidity was synthesized by two-step crystallization and characterized by XRD, N2 physical adsorption-desorption, ^^Al MAS NMR , IR and NH3-TPD methods. A1MB48 possessing stronger acid centers showed higher activity for the cumene cracking and the catalytic alkylation of 2,4-ditert-butylphenol with tert-butanol than conventional mesoporous materials. 1. INTRODUCTION M41S molecular sieves have only weaker acid strength owing to the amorphous character of the pore wall, which limited their applications for the catalytic conversion of large hydrocarbons or other organic molecules. Much effort has been undertaken to synthesize new types of materials which combined the advantages of mesoporous and microporous molecular sieves^''^'. Kloetstra et al'"^^ have tried to synthesize the mesosoporous materials by recrystallization of MCM-41 and IIMS in the presence of tetrapropylammonium cations and found that the acidity and catalytic activity of both materials were improved after recrystallization of the pore wall. Karlsson '^' and Huang et al '^'' have ever tried other ways in which microporous and mesoporous composite matetials with hexagonal ordered structure could be prepared in the presence of a mixed template and using a dual templating method through a process of two-step crystallization. Zhang et a r ' a n d Liu '^'synthesized exceptionally acidic and steam-stable hexagonal aluminosilicate mesostructures from protozeolitic nanoclusters. However, the mesoporous materials mentioned above are all hexagonal mesostructures and the cubic mesoporous materials with stronger acidic strength have never been published up to date. Here we make use of the precursor containing the structure units of zeolite Beta at low concentration of cetyltrimethylammonium bromide to synthesize a cubic mesoporous aluminosilicate designated AIMB48 with enhanced acid centers in the mesoporous wall and improved catalytic activity for the reaction of larger molecules. 2. EXPERIMENTAL General procedure for preparing AIMB48 is as follows: 0.1906g sodium aluminate was dissolved in 2.9 mL water and then 14.3 mL25% aqueous solution of tetraethylammonium hydroxide(TEAOH) and 2.25 mL of 3.70 moll'* HCl were respectively added, followed by addition of 2.5g fumed silica under vigorous agitation. The whole mixture was stirred for 1 h at room temperature to form a homogeneous gel with the composition of Si02: O.O2AI2O3: 0.028Na2O: 0.6TEAOH: 0.2HCI: 2OH2O. The gel mixture was loaded into a teflon-lined •Corresponding author, E-mail address: qkanr^f-mail.ilu.edu.cn. Fax: 86-431-8949334
150
Stainless steel autoclave and heated at 140 °C for 22 h. The products which were verified by XRD and IR spectra not to contain the zeolite phases but to embody the secondary structure units of zeolite were cooled to room temperature, stirred for 25 min and kept on for second step of crystallization. The above precursor was combined with 9 ml of 19.28 % aqueous solution of cetyltrimethylammonium bromide (CTMAB) and 1.12 ml of 3.70 M HCl and stirred for 1 h to form homogeneous composition of SiOz: O.O2AI2O3: 0.028Na2O: 0.6TEAOH: 0.14CTMAB: 0.3HC1: 33H2O. This mixture was heated in autoclave under static condition at 140 °C for 24 h. After cooling to room temperature,the solid product was recovered by filtration, washed with deioned water, dried in air at ambient temperature and calcined at 540 °C (1 °C /min) for Ih in flowing nitrogen followed by calcination in flowing air for 8h. The obtained mesoporous sample was designated as A1MB48(25), where 25 in parentheses represented the Si/Al ratio of the reactant mixture. As-synthesized sample was exchanged with 2 molL"' NH4NO3 solution (pH=3) for three times at 80 ""C for 5 h and calcined at 540 ""C to generate H-form, HMB48(25). For comparison purposes, AlMCM-48(25) was prepared according to proceures reported in literature^ \ Samples were measured by X-ray diffraction, N2 adsorption and desorption, ^^Al MAS NMR, IR and NH3-TPD. Cumene cracking reactions were performed in a pulsed microreactor with 50mg catalytst at 350 °C. Hydrogen was used as carrier gas at a flow rate of 50 ml /min, the amount of cumene injected for each test was l|iL. The catalytic alkylation of 2,4-ditert-butylphenol with tert-butanol was investigated by a continuous flow fixed bed reactor. The reaction was carried out with 500 mg of catalyst and 2,4-ditert-butylphenol / tert-butanol molar ratio of 1:2 at 120 "C. The WHSV was 2.20 h-^ and average conversion was reported in 10 h. 3. RESULTS AND DISCUSSION The design of A1MB48 is based on two-step crystallization procedure. The zeolite precursor was firstly prepared. This step does not allow to form complete structure of zeolite phase but should generate secondary structure units of zeolite which possess stronger acidic strength than amorphous aluminosilicatc. The second crystallization in the presence of organic surfactant (CTMAB) makes the precursor prepared in the first step to construct the framework of mesoporous materials. We have investigated the crystallization kinetics of zeolite Beta and found that the Beta precursor with Si/Al=25 containing structure units could be formed within 20-24hat 140"C. The XRD patterns of as-synthesized and calcined A1MB48(25) in Fig. 1 show the characteristic cubic (la3d) structure of mesoporous materials''"' without observing any diffraction peaks of the zeolite in 20 region 10-50^\ Calcination of the sample leads to the contraction of the unit cell from 101.0 to 94.0 A. The TMAB/ Si02 ratio of reactant mixture in the second crystallization of A1MB48 is much lower than that in the synthesis of conventional mesoporous MCM-48'^'""'^'. The two-step crystallization procedure can be used to synthesize cubic mesoporou A1MB48 with various Si/Al ratios from 15-100. ^^Al MAS NMR spectroscopy of as-synthesized and calcined A1MB48 in Fig.2(a,b) show a single resonanse at ca. 54ppm which not only indicates the tetrahedral aluminum environment but implies the absence of zeolite Beta crystal which will present a peak at ca. 60-63 ppm. Although NH4^ exchange treatment causes migration of aluminum from the framework to outside of A1MB48, most aluminum is present in the framework of HA1MB48 as tetrahedral coordination(see Fig. 2c). IR spectra in Fig.3 show a vibrational bond at 550-600 cm'' region for A1MB48(25), which is characteristic of five-membered rings'*^^, indicating the presence of
151
the secondary structure units of zeolite. On the other hand, AlMCM-48(25) which has amorphous pore wall only shows a very weak absorption at this range.
At 4 6 20/degrees
8
Fig. 1. XRD patterns of (a) assynthesized and (b) calcinedd A1MB48(25).
50 6/ppm
0
Fig. 2. ^^Al MAS NMR spectra of (a) as-synthesized and (b) calcined A1MB48(25), (c) H A1MB48(25).
The nitrogen adsorption-desorption isotherms of calcined A1MB48(25) are typical Type IV curves of mesoporous materials (Figure not shown here). The sharp step between p/po=0.3 and 0.4 indicate a narrow distribution of mesopore. The pore diameter (DBJH), cumulative pore volume (VBJH) and BET surface area (ABHT) of A1MB48(25) are 28.48A, 0.75cm^g'' and 881m^g'', respectively. There are two peaks at ca.360 "C and 185 ''C in the NH3-TPD profiles of HA1MB48(25) as shown in Fig.4, which imply the presence of stronger acid centers besides weaker acid sites on HA1MB48(25). This is not the case for HAlMCM-48(25) on which only weaker acid sites exist. The acidic amounts of HA1MB48(25) and HAlMCM-48(25) is 0.80 and 0.72 mmolg"', respectively.
(b)AIMCM-48(25)
561cm
400
^ (a)AIMB48(25)
600 800 1000 J 200 Wavenumber/cnn'
Fig. 3. FTIR absorption spectra of (a) A1MB48(25) and (b) A1MCM-41(25).
100 200 300 400 500 600 Temperature/"C Fig.4. NH3-TPD profiles of (a) HAlMCM-48(25) and (b) HA1MB48(25).
The catalytic tests also announce the difference between HA1MB48(25) and HAlMCM-48(25). For the standard reaction of cumene cracking, the conversion of cumene
152
over HA1MB48(25) (89.7%) is much higher than that over HAlMCM-48(25) (52.9%) under the same conditions. For alkylation of 2,4-ditert-butylphenol with tert-butanol producing 2,4,6-tritert-butylphenol with larger molecules size, the results are shown in table 1 and the orders of activity and selectivity are as HAlMB48(25)>HAlMCM-48(25)>HBeta(25), indicating the advantage of HA1MB48(25) for this reaction. Table 1 Alkylation of 2,4-ditert-butylphenol with tert-butanol ^ . Conversion of "^P 2,4-ditert-butylphenol / % HA1MB48(25) 22.7 HAlMCM-48(25) 15.7 HBeta(25) 14^2
selectivity of 2,4,6-tritert-butylphenol / % 45.6 35.9 14J
ACKNOWLEDGMENT We thank financial support by the Natural Science Foundation of China (29973001). REFERENCES 1. C. J. H. Jacobsen, C. Madsen, J. Houzvicka, I. Schmidt, J. Am. Chem. Soc, 122 (2000) 7116. 2. M. J. Verhoef, P. J. Kooyman, J. C. van der Waal, M. S. Rigutto, J. A. Peters, H. van Bekkum, Chem. Mater, 13 (2001) 683. 3. Y. Liu, W.Zhang, T. J. Pinnavaia, J. Am. Chem. Soc, 122 (2000) 8791. 4. K. R. Kloetstra, W. van Bekkum, J. C. Jansen, J. Chem. Soc, Chem. Commun., (1997) 2281. 5. A. Karlsson, M. Stocker, R. Schmidt, Microporous Mesoporous Mater., 27 (1999) 181. 6. L. M. Huang, W. R Guo, R Deng, Z. Y. Xue, and Q. Z. Li, J. Rhys. Chem. B, 104 (2000) 2817. 7. Z.-Z. Zhang, Y. Han, L. Zhu, R.-W. Wang, Y. Yu, S.-L. Qiu, D.-Y. Zhao, and R-S. Xiao, Angew. Chem. Int. Ed., 40 (2001) 1258. 8. Y. Liu. W. Zhang, T. J. Pinnavaia, Angew. Chem. Int. Ed., 40 (2001) 1255. 9. A. A. Romero; M. D. Alba; J. Klinowski, J. Phys. Chem. B, 102 (1998) 123. 10. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck, Nature, 359 (1992) 710. 11. J. Xu, J. Luan, H. He, W. Zhou, L. Kevan, Chem. Matter., 10 (1998) 3690. 12. A. Sayari, J. Am. Chem. Soc, 122 (2000) 6504. 13. J. C. Jansen, R J. van der Gaag, H. van Bekkum, Zeolite, 4, (1984), 369. 14. C. E. A. Kirschhock, R. Ravishankar, F. Verspeurt, P. J. Grobet, P. A. Jacobs, J. A. Martens, J. Phys. Chem. B, 103 (1999) 4965.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
153
Synthesis and characterization of supersurface MCM-41 zeolite using additives Chun-Min Song
Zi-Feng YAN*
Huai-Ping Wang
State Key Laboratory for Heavy Oil Processing, the Key Laboratory of Catalysis, CNPC, University of Petroleum, Dongying, Shandong , China, 257061 1. INTRODUCTION MCM-41 is one member of a new family of mesoporous materials, designated as M41S and discovered by Mobil^''^^ The two most investigated materials, MCM-41 with a 2-D hexagonal structure and MCM-48 with a 3-D cubic structure, are synthesised using rt-alkylammonium salts as templates. It has attracted considerable attention for potential application as catalyst supports or adsorbents because of its high surface area and large pore volume^'"^l At present, the synthesized MCM-41 materials usually have the specific surface area of about lOOOm^.g''. For the MCM-41-supported catalyst, it is rather important that the active sites in such mesoporous zeolite could be well dispersed, which requires that the supported MCM-41 zeolite possess large surface area. Furthermore, super-high surface MCM-41 with larger meso-porosity might be potentially used as the multi-way catalyst carriers as well as shape-selective adsorbents. However, studies for the MCM-41 zeolites with super high surface area have not yet been reported in the literature. The aim of the current study was to develop the novel method to obtain MCM-41 zeolites with super-high surface area and uniform mesoporosity by means of XRD and nitrogen adsorption techniques. 2. MATERIAL AND EXPERIMENTS 2.1. Materials and synthesis process The chemicals used in the experiment were hexadecyltrimethyl ammonium bromide (CieTMABr, A.R.), sodium silicate (Na2Si03.9H20, A.R.), sodium aluminate (C.R), sulfuric acid (A.R.), acetic acid (A.R.), ammonium citrate (A.R.), ammonium nitrate (A.R.), etc. The experimental procedure has been described in our previous work^'^l For the sake of completeness, it is briefly restated here. The mesoporous MCM-41 molecular sieves were prepared with starting gel compositions of 1.0SiO2: O.O5AI2O3: (0.25 ~ 0.125) CieTMABr: 6OH2O: (0 ~ 0.3) additives at 373K for different times, where additives are some compounds such as ammonium citrate, ammonium nitrate etc. The solid products were recovered by filtration, washed with deionized water, and dried at 373K for 24 h. The samples were calcined at 823K for 6 hours to remove the template. These samples were denoted as MCM(t), where t stands for the crystallization time in hours. The other two samples obtained from the same composition, with temperatures of the treatment of 373 and 413K, respectively, are denoted as MCM-41(373) and MCM-41(413). * Corresponding author.
Email: zfyancat(a^hdpu.cdu.cn
154
2.2. Characterization Powdered X-ray diffraction (XRD) were performed on a Rigaku D/MAX-IIIA X-ray powder diffractometer using Cu Ka radiation, and operated at 40kV and 40mA. The X-ray diffraction pattern was recorded over the range from 1 to 10° 26. Nitrogen adsorption measurements were performed at 77K on an ASAP-2010 volumetric adsorption analyzer manufactured by Micromeritics. Before the adsorption analysis, the calcined samples were outgassed for 4 h at 673K in the degas port of the adsorption analyzer. The BET specific surface area was calculated using nitrogen adsorption data in the relative adsorption range from 0.04 to 0.2. The mesopore diameter was evaluated using the BJH method^^^.
u 1
ij
^ 11
1
^
^1
c
ft as-synlhesized
1
3
4
5
fi
7
2
3
4
5
6
7
2
:)
4
5
6
7
2
3
4
5
fi
7
2
3
4
5
6
Fig. 1. XRD patterns of MCM-41 samples with different crystallization time (a) 28 h, (b) 32 h, (c) 48 h, (d) 96 h, (e) 120 h Table 1 Sample MCM(28) MCM(32) MCM(48) MCM(96) MCM(120)
Unit cell parameter (nm) 4.40 4.64 4.31 4.60 4.60
BET specific surface area 903 1005 1282 1245 1104
Pore volume (cm\g-') 0.92 1.02 1.31 1.23 1.23
Pore diameter (nm) 3.46 3.22 3.28 3.12 3.46
3. RESULTS AND DISCUSSION 3.1. X-ray diffraction analysis XRD spectra of the synthesized samples are shown in Figure 1. It can be seen that the XRD spectra of the samples bear four well-resolved reflection peaks indexed as (100), (110), (200), and (210) at 26 range of 1-7°, based on a hexagonal lattice for high quality MCM-41 mesoporous materials. MCM-41 synthesized gives one smart and strong (100) peak, which is indicative of a highly ordered material while the disordered amorphous silica gel showed a broad peak in the X-ray diffraction pattern. The subsequent calcination resulted in the peak intensity to became stronger while the peak location shifted toward higher 2lvalues and the d spacings became smaller. It indicated that further partial cross-linking and reconstruction of aluminosilicate species has been occurred to give better-organization of the mesostructures while calcination. Simultaneously, as the crystallization times increases, the XRD peak intensity and resolution initially increases and subsequently decreases, but the unit cell parameter varies only slightly. Figure 1 displayed that the higher crystallinity is obtained
155
when crystallization times are in the 32 ~ 100 h range. It revealed that the speed of crystallization is relatively rapid and considerably constricted by thermal dynamics of this process. The crystallizing products are thermodynamically metastable and will further turn into a more stable amorphous phase if crystallization time is excessively extended.
700-
<^)J
+ Adsorption o DesorpBon
600-
j
24 -
'<« 500-
20 • i ^
400300-
|,,; :>" 08 -
200-
04 -
J 2
0-
3 4 lb Df/nm
1
p/p.
Fig. 2. Nitrogen adsorption isotherms and mesopore size distributions of calcined MCM-41 samples (a) 48h; (b) 96h 3.2. Nitrogen adsorption results analysis The nitrogen adsorption isotherms and mesopore size distributions of the calcined MCM-41 samples are shown in Figure 2. It can be seen that adsorption isotherms are similar to type IV which possess triangular hysteresis loops, and exhibit remarkably sharp capillary condensation steps. This is in accord with the feature of hysteresis loops reported in the literature^^l Besides, it is interesting that the isotherm has two hysteresis loops. The hysteresis loop close to the saturation pressure is a common phenomenon of porous adsorbents, which is the capillary condensation in the pores formed between the crystal grains. The other hysteresis loop is related to the capillary condensation occuring within the MCM-41 mesopores. It is closely related to the degree of order and smoothness of the mesoporous channels. The decrease in the degree of order causes diminishing or even disappearance of the hysteresis loop. Then this indicates these samples contain the presence of certain amounts of secondary mesopores. Simultaneously, it can be seen in Figure 2 that the degree of order of the synthesized samples initially increases and then is reduced while crystallization time is being extended. The samples with in the 30 ~ 100 h range have a higher degree of structural ordering, and have a lower degree of structural ordering when crystallization times are under 30 h or over 100 h (not shown). This is in accord with the results of the XRD patterns. Figure 2 also indicated that pore size distributions of the synthesized samples exhibit sharp peaks with maxima, which showed the uniformity of the mesopores. Table 1 shows structural properties of the calcined MCM-41 samples. Of interest is that the surface area and total pore volume of the sample MCM (48) is the largest among MCM (t) samples, while the crystallinity of this sample is also the highest. The sample MCM (48) synthesized can reach values up to 1282 m /g and the pore structure is highly uniform and exhibits a unidirectional pore system composed of a regular hexagonal array of tubes. It may be supposed that the surface area and pore volume are related to crystallization time, and it has influences on crystal granularity and the growth of the number of mesopores. It can be seen in Figure 1, the MCM (32), MCM (48) and MCM (96) samples have almost the same width of the main peak, which indicates that the crystal granularity of the molecular sieves is almost identical. The result indicates that the improvement of the specific surface area does not come from the reduction in crystal granularity. For the sample MCM (48), surface area
156
and pore volume is maximal while this sample appears to have the highest degree of structural ordering. This reveals that the crystal structure of the MCM (48) sample is the best, and its mesoporous structure is the most perfect in the MCM (t) samples. Furthermore, crystal structure of the samples with short crystallization times is not perfect enough, and the crystal may locally dissolve with longer crystallization times. We supposed the mesopore loss phenomenon increases with increasing crystallization times, and resultes in specific surface area reduction. This also can be validated by the change of the hysteresis loop in Figure 2. The result can be explained by the synthesis mechanism of MCM-41, which is a transformation mechanism from a lamellar to a hexagonal phase^^^. Table 2 showed the structural properties of the some samples. It can be seen that the specific surface area and pore volume increase while synthesis temperature increases. For the synthesized sample at 413K, the result shows the specific surface area can reach 1636 m^.g"'. The sample synthesized without additives has relatively small surface area. The function of the additives might improve the electronic density and coordinative regioselectivity of the electrostatic layer and pH value of gel solution. Table 2 Structural properties of the calcined MCM-41 samples Sample MCM-41 ( 3 7 3 ^ ^ MCM-41(413)'^ MCM-41 (N)^^
Unit cell parameter (nm) 4.40 4.66 4^6^6
BET specific surface area(m^.g') ' 903 1636 752
Pore volume (cm^.g"') 0.92 1.12 0^92
Pore size (nm) 3.46 3.28 3^99
l)The initial gel composition was 1.0 SiOj: O.O5AI2O3: 0.25Ci6TMABr : 6OH2O : 0.2 additive, pH = 10.3, crystallization time 28h. 2)The initial gel composition of MCM-41(N) was 1.0 SiOj: O.O5AI2O3: 0.25C,6TMABr : 6OH2O. 4. CONCLUSIONS The current study shows that high-quality MCM-41 materials could be synthesized by the hydrothermal procedure using certain additives such as ammonium citrate and ammonium nitrate etc. The surface area and pore volume of the synthesized samples in the 30-lOOh range is larger. The surface area is higher than 1200 m^.g', and the pore diameter is about 3.2 nm, and the pore volume is over 1.2 cm^.g"'. For the synthesized sample MCM-41 (413), the surface area is maximal, 1636 m^.g"*. The results indicate that increasing temperature of the hydrothermal treatment has the advantage of increasing the surface area and pore volume. REFERENCES 1. C.T Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature, 359(1992) 710 2. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T-W. Chu, D.H. Olson, E.W Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc, 114(1992)10834 3. X.S. Zhao, GQ. Lu, GJ. Millar, Ind. Eng. Chem. Res., 35(1996) 2075 4. CM. Song, Z.R Yan, H.R Wang, M. Lu, J. Nat. Gas Chem., 9(2000) 237 5. S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press: London, 1982 6. M. Kruk, M. Jaroniec, Y. Sakamoto, O. Terasaki, R. Ryoo, C.H. Ko, J. Phys. Chem. B, 104(2000) 292
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
157
Preparation of Large Pore High Quality MCM-48 Silica by a Simple PostSynthesis Hydrothermal Treatment Jihong Sun and Marc-Olivier Coppens* Reactor & Catalysis Engineering, DelftChemTech, Julianalaan 136, 2628BL Delft, The Netherlands
Delft
University of
Technology,
Large pore, high quality MCM-48 silica was synthesized using CTAB as the only template. The pore size could be tailored from 2.6 nm to 4.2 nm by a post-synthesis hydrothermal treatment. Nitrogen adsorption, XRD, TEM and SEM were used to characterize the samples, showing an improved long-range structural order and expanded pore size, which depends on the post-synthesis treatment conditions, such as the aging temperature during the second synthesis step.
1. INTRODUCTION MCM-48 is a nano-structured porous material of the M41S family, which contains a cubic, bi-continuous, three-dimensional array of pore channels {laSd). Guest molecules can access the active sites more easily than in the one-dimensional MCM-41, and pore network blockage is reduced in three dimensions, making this material potentially interesting for applications in catalysis and separations. However, high quality MCM-48 is particularly difficult to synthesize due to the very sensitive synthesis conditions [IJ. Conventional methods to expand the pore size of MCM-48 proceed via a hydrothermal route, mainly changing the pH of the reaction medium, the catalyst, the reaction time and the temperature. Recent reports show that post-synthesis treatment and tailoring the alkyl chain length of the cationic surfactant by using mixtures of surfactants lead to a considerably increased pore size as well [2, 3]. In all these cases, the pore size of the good quality MCM-48 silica was found to be in the range 2-3.2 nm. More recently, high quality MCM-48 with up to 3.8 nm pore diameter was synthesized with no organic additive [4]. Here, we report on a simple, reproducible, post-synthesis hydrothermal restructuring route to prepare MCM-48 with improved long-range structural order and increased pore size.
* To whom correspondence should be addressed. E-mail: M.0.Coppens@tnw.tudelft.nl Tel: +31-15-2784399 Fax: +31-15-2788267
158
2. SYNTHESIS AND CHARACTERIZATION CTAB was added to a mixture of deionized water and ethanol [5]. Adding aqueous ammonia and TEOS at room temperature (final molar composition TE0S:CTAB:H20: C2H50H:NH40H = 1:0.4:174:54:12.5) and stirring led to a gel, which was filtered and repeatedly washed with distilled water. This white gel was divided into two parts. One part was dried and then calcined in air at 550°C for 6 hours, leading to "primary" MCM-48 (sample 1). The other part was immersed into an aqueous solution of 2 wt% CTAB, aged for 2 days in an autoclave at lOO^C (values for "base case"), filtered, washed, dried and calcined ("secondary" MCM-48, sample 2). Powder X-ray diffraction (XRD) studies were performed using a Philips XRD spectrometer (PW1840) with CuKai radiation, operating at 40 kV and 50 mA. Nitrogen adsorption/desorption isotherms were measured with a Micromeritics ASAP2000 sorption analyzer, utilizing the BdB method to evaluate the pore volume and pore size distributions for the desorption portion of the isotherm. Transmission electron micrographs (TEM) were made with a JEOL JEM 4000-EX electron microscope operating at 400 kV. Scanning electron microscope (SEM) images were recorded using a Philips XL20. 1. RESULTS AND DISCUSSION
2:900 28000
H
21000 C'
"^ 600 14000 7000
o
>
X)
<
300 0 0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/Po) Figure 1 XRD patterns of MCM-48. The pattern for sample 2 is shifted by 5000
Figure 2 Nitrogen adsorption isotherms and corresponding pore size distribution of MCM-48
The XRD patterns for the calcined samples show the characteristic cubic order of MCM48 (Figure 1). Compared to the primary material, the XRD pattern of the secondary material shows a significantly increased intensity of the (211) peak, as well as other peaks. The reflections are also more intense, indicating improved order. Meanwhile, the ^2// spacing of sample 2 has increased from 3.1 to 3.9 nm, suggesting increased unit cell parameters (as can be seen in Table 1).
159
The nitrogen adsorption isotherms and the corresponding pore size distributions (Figure 2) indicate that the mesopore size increases from 2.6 nm to 3.7 nm, most Hkely by swelHng of the organic structure-directing entities in the primary material. The sharpness of the nitrogen condensation step (P/Po = 0.3-0.4) and the well-defmed XRD peaks provide strong evidence of the very high quahty of this material. The hysteresis loop of sample 2 at a relative pressure P/Po = 0.45-0.9 is due to inter-particle porosity.
m p5 -
,. ir.
• 4 •;..%••
AccV
S()i)l M.iqn
?()()kv;!()
Figure 3 TEM image of sample 2
;iO()()x
\h'\
;;i
WD
isni
hxfi
ii
I
(ii.r, - sv,', MI l o o m
Figure 4 SEM image of sample 2
TEM (Figure 3) confirms that the structure of the MCM-48 material (sample 2) is cubic. Along the [100] direction, a very regular pattern is clearly observed [6]. The SEM image in Figure 4 reveals that the resulting particles (sample 2) are practically of the same size and close to spherical in shape. No aggregation is visible, and the particle size is in the range of 1.0-1.3 |im. Table 1 Pore structure parameters of synthesized MCM-48 materials Aging a" S^ V D'^ ^^"^P'^ temperature ("C) (nm) (m^/g) (cmVg) (nm) 1 7.75 1510 0.74 2.6 2
100
9.53
1170
1.24
3.7
//' (nm) 1.2 1.2
3 60 8.54 1470 0.94 3.2 1.2 4 150 12.3 740 0.78 4.2 1.9 ^ a = dhki {h^+k^+l^y^ is a cubic lattice parameter calculated from XRD. ^ S is the specific surface area obtained from nitrogen adsorption and desorption. ^' K is the specific pore volume. ^ D is the mean pore diameter. '^ H= a/3.092 - D/2 is the mean pore wall thickness. Table 1 summarizes the textural characteristics of the primary and secondary MCM-48 materials. The highly ordered secondary MCM-48 (sample 2) has a pore wall thickness of approximately 1.2 nm, hardly changed from that in the primary MCM-48 (sample 1). This indicates that the unit cell enlargement can be almost entirely attributed to an increase in pore size.
160
The post-synthesis hydrothermal treatment (second synthesis step) rearranges the structure of the primary MCM-48 gel in a way that depends on the synthesis conditions. Figure 5, e.g., shows the XRD pattern of secondary MCM-48 as a function of the aging < temperature, keeping the other parameters constant. The degree of order of the MCM-48 goes through an optimum around 100"C (base case, Hne b in Figure 5). The increase in peak intensity is accompanied by a shift of the (211) cubic reflection toward lower diffraction angles, i.e., toward a larger cubic unit cell size, from 8.5 nm at 60"C (line a in Figure 5, 10 0 corresponding to sample 3 in Table 1) to 12.3 2 (9/" nm at 150°C (line c in Figure 5, corresponding to sample 4 in Table 1), while the pore size Figure 5 XRD patterns of MCM-48 expands up to 4.2 nm. Interestingly, the pore a:60"C;b: 100 "C;c: 150"C walls of MCM-48 aged at 150 "C (sample 4) are thicker (around 1.9 nm) than those of the primary MCM-48 (sample 1), suggesting that rearrangement of the silica framework takes place when the temperature is high during the second synthesis step. 4. CONCLUSION It was shown that restructuring of a primary MCM-48 gel using CTAB via a hydrothermal post-synthesis method yields to an expansion of the pores from 2.6 nm to 4.2 nm and a higher degree of order (eight well-rcsolvcd diffraction peaks in XRD pattern), while maintaining thick pore walls around 1.2 nm, and a very high surface area. The texture properties depend on the synthesis conditions, in particular, the aging temperature during the hydrothermal post synthesis procedure. More studies have been conducted to investigate other synthesis parameters such as the pll, the synthesis period and the amount of CTAB during the second synthesis step, and their effects will be discussed in a forthcoming paper. ACKNOWLEDGEMENT We would like to thank Dr. Chia-Min Yang for the TEM and Mr. Johan C. Groen for measuring the nitrogen adsorption isotherms. REFERENCES 1. 2. 3. 4. 5. 6.
M. L. Pena, Q. Kan, A. Corma, F. Rey, Micro. Meso. Mater., 44-45 (2001) 9. R. Ryoo, S. H. Joo, J. M. Kim, J. Phys. Chem. B, 103 (1999) 7435. M. Kruk, M. Jaroniec, Chem. Mater., 12 (2000) 1414. A. Sayari, J. Am. Chem. Soc, 122 (2000) 6504. K. Schumacher, M. Grun, K. K. Unger, Micro. Meso. Mater., 27 (1999) 201. J. Xu, Z. H. Luan, H. He, W. Zhou, L. Kevan, Chem. Mater., 10 (1998) 3690.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
161
Synthesis and properties of aluminosilicate mesoporous material adjustable pore structure Y.Zhang" D. Wu'
Y. H. Sun"
S. Y Peng" D. Y Zhao^ Q. Luo'
with
F.Deng'
"State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, P. R. China. (Fax: +86-0351-4041153 E-mail: yhsunra).sxicc.ac.cn) ^Chemistry department of Fudan University, shanghai, 200433, P. R. China '^State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics & Mathematics, Chinese academy of Sciences, Wuhan, 430071, P. R. China Mesoporous aluminosilicate material with different Si/Al ratios was synthesized from tetraethylorthosilicate (TEOS) and aluminum nitrate nonahydrate (ANN) via sol-gel route. CTAB was used as the structure-directing agent to create mesopore within the colloidal particles. The pore structure of the finally obtained samples changed regularly with the altering of Si/Al ratio by carefully controlling of the sol-gel process. Bimodal pore structure was then obtained when the Si/Al ratio reached a suitable range. As a result of the sol-gel synthesis method and the tcmplating function of CTAB, nanometer mesoporous molecular sieves were obtained. Structural properties of the samples were characterized by means of XRD, BET, NMR and HRTEM. 1. INTRODUCTION Ordering mesoporous material MCM-41 was firstly prepared in 1992 ''^. It was considered to have great potential use in fields of catalytic, adsorption and separation, etc. The incorporation of hetcroatom such as aluminum into the inorganic framework generated active sites in the framework and made the material being applicable in petroleum processing ''''*. Generally, this kind of material was synthesized under hydrothermal conditions with the particle size located at hundreds of nanometers or larger. As a result, the material usually possessed long one-dimensional pores that was not in favor of the molecular's exiting. Catalysts with hierarchical structure are proposed for potential applications since they combined the fast mass transport of a large pore size system with high specific surface of a small pore size network^. In the present work, mesoporous material with different Si/Al ratios was synthesized via sol-gel route. Nanometer bimodal mesoporous material was obtained by adjusting the Si/Al ratio and by carefully controlling of the sol-gel process. The structural properties of the material were characterized by means of XRD, HRTEM, NMR and BET etc. 2. EXPERIMENTAL TEOS and ANN were used as the silica and alumina source, respectively. The crystallization solution was prepared as the following molar ratio: l.OTEOS: (0~0.2)ANN: *: The authors would like to thank the National Key Basie Rescareh Spceial Foundation (G2000048001) and the National Nature Scicnee Foundation (Grant No. 29973057 and 29623057) for the finaneial support.
162
40H2O:8C2H5OH:0.25CTAB:10NH4OH. Crystallization was performed at 50°Cfor 3 days. The gel was dried and then calcined at 923K to remove the template. In comparison, amorphous silica-alumina and MCM-41 with the Si/Al ratio at 7.5 were also prepared. XRD patterns were obtained on a D/max-rA diffractometer with Cu-K a radiation. Nitrogen adsorption-desorption isotherms were measured on an ASAP2000 apparatus at 77K and the surface area and pore size distribution data were analyzed by BET method and BJH model respectively. The high-resolution transmission electron micrograph (HRTEM) was carried out on H9000 instrument. Solid-state ^''AI MAS NMR experiments were conducted on an Infmityplus-400 spectrometer with resonance frequencies of 79.48MHz. 3. RESULTS AND DISCUSSION 3.1. XRD analysis The packing order of the mesopores of samples was characterized by XRD spectra. The samples only exhibited one diffraction peak in the 29=1-6° region by using the sol-gel preparing method. Pinnavaia^^^ and Quanzhi Li^^^ have demonstrated that similar single diffraction type material still exhibit crystallographic symmetry analogous to MCM-41 phase. With the decreasing of Si/Al ratio, the intensity of the 100 peak decreased because of the lowering long-range order by the incorporation of Al atoms into the framework. The diffraction peaks completely disappeared when the Si/Al ratio reached 5.0 which was similar to the patterns of amorphous silica-alumina materials. 3.2. Pore structure of the samples The nitrogen adsorption-desorption isotherms of the samples that prepared with and without CTAB were compared in Figure 1. For the highly ordered pure siliceous MCM-41, the isotherms exhibited an obvious increasement at P/Po=0.25-0.35, which was indicative of mesoporous structure of the material'^^. The increasement became smaller and smaller with the increasing of Al content in the sample, indicating the decreasing of ordering-packed mesopores. For amorphous Si02-Al203, no such an increasement at low relative pressure was 2.5
1.5
0.5
10 0
0.2 0.4 0.6 0.8 1 Relative Pressure(P/Po) - ^ 5 - * - 7 . 5 - ^ 2 5 - ^ & - - ^ 7 . 5 , w t h o u t CTAB
Fig. 1. The N2 adsorption-desorption curves of the calcined samples prepared with different Si/Al ratio or without templates
100
1000
Pore Diameter(nm) Fig. 2. The BJH pore size distribution curve of calcined bimodal sample
163
observed but one hysteresis loop appeared at P/Po^O.8, which was typical of amorphous material with large and disordering-packed pores^^l However, both kinds of the sudden increasement simultaneously appeared at P/Po=0.25~0.35 and P/Po^O.8 in the isotherm of the sample with the Si/Al ratio at 7.5, indicating that there might exist two kinds of pores in the sample which were ordering and disordering-packed, respectively. The BJH pore size distribution in Figure 2 further showed that the diameter of these two kinds of pores was located at 2.42nm and 65.6nm, respectively. The BJH pore size and the BET surface area as well as the pore volume of samples with different Si/Al ratios were listed in Table 1. Those indicated that the amorphous Si02-Al203 possessed appreciably low surface area. The pores concentrated at 46nm was considered to be formed by the aggregation of the colloidal particles during the sol-gel process ^^\ With the addition of CTAB, another mesopore appeared at about 2.4nm due to the templating function of surfactant micelles and the surface area also increased greatly as a result of the creation of mesopore within the colloidal particles. The mesopore was proved to be packing in long-range order but not as high as that of MCM-41^^^ by HRTEM (see Fig.3) in correspondence with the XRD results. The HRTEM image of the bimodal sample simultaneously showed that the particle size of the material was about lOOnm which would have relative short one-dimensional mesopore and in turn made the reactant and product molecules easier to access or exit^^l Table 1 The structural properties of the samples Sample ~ Si/Al(mol) No. Si/Al(mol) ^^^,^.^^^ (^^^^tant) p^^^^^^^
g^(^2/g)
I 7^ ~ Pore Diameter r.(nm) r^Cnm)
~ ~ Pore Volume ^"'^^^
1 (without CTAB)
7.5
8.51
149.2
--
75.76
0.93
2
5.0
5.56
424.9
--
40.2
1.25
3
7.5
9.04
513
2.42
65.6
0.85
4
15
-
498.3
2.32
70.3
0.68
5
25
32.6
927.6
2.51
--
0.82
1235.66
2.78
6
CO
1.0
The relative larger pore located at above 60nm was formed by the aggregation of the colloidal particles by carefully controlling of the sol-gel process. For the pure siliceous ordering material, the particle size was too large (0.7 u m) and the inter-particle pore could be omitted. The particle size of the obtained material increased gradually with the increasing of Si/Al ratio in the sample and as a result, the amount of the relative larger inter-particle pore decreased with the increasing of Si/Al ratio. In general, the ordering-packed mesopore increased and the inter-particle pore decreased with the increasing of the Si/Al ratio in the present sol-gel synthesis. Furthermore, NMR measurements indicated that Al atoms were present in a tetrahedral state in the as-synthesized sample. Thus, nanometer bimodal silica-alumina mesoporous molecular sieves could be obtained when the Si/Al ratio reached the suitable range.
164
v^^4V;r^l80nn [
1
Fig. 3. The HRTEM images of the bimodal material 4. CONCLUSIONS Mesoporous silica-alumina materials with adjustable pore structure and particle size were synthesized via sol-gel route. Nanometer bimodal mesoporous aluminosilicate molecular sieves with most Al atoms in a tetrahedral state were obtained by carefully controlling of the sol-gel process. The two kinds of pores were respectively formed by the templating function and the aggregation of colloidal particles which were proved by XRD and HRTEM analysis. REFERENCES 1. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck. Nature, 1992, J59.710. 2. J. S. Beck, J.C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt et.al. J. Am. Chem. Soc. 1992, IN, 10834. 3. K. M. Reddy, C. Song. Catal. Today 1996, 31, 137. 4. A. Corma. A. Martinez, V. Martinez-Soria. / Catal. 1997, 7(59,480. 5. Peidong Yang, Tao Deng, Dongyuan Zhao, et al., ScL, 1998, 282, 2244. 6. Tanev R T.; Chibwe M. and PinnavaiaT. J., Nature, 1994, 368, 321. 7. Chen X.; huang L.; Ding G; Li Q. Catal. Lett. 1997, 44, 123. 8. Drinker C. J. and Soberer G. W. SOL-GEL SCIENCE. The Phy.sics and Chemistry of Sol-Gel Proce.ssing, Acdemic Press, inc. 1990.
9. Robert Mokaya, / Catal. 1999, 186, 470.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved
165
Variation of the pore properties of mesoporous silica after washing by water and ethanol-water solutions L. Pasqua^, F. Testa'', R. Aiello^, F. Di Renzo^ and F. Fajula'^ ^Universita di Napoli Federico II, DIMP Chimica Applicata, 80125 Napoli, Italy ^Dipartimento di Ingegneria Chimica e dei Materiali, Universita degli Studi della Calabria, Via Pietro Bucci, 87030 Rende, Italy. FAX +39-0984492058. E-mail: r.aiello@unical.it '^Laboratoire de Materiaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS, ENSCM, 8 rue de I'Ecole Normale, 34296 Montpellier Cedex 5, France. Pure-silica and iron-containing mesoporous materials are obtained by different synthesis procedures in the presence of cetyltrimethylammonium cation at neutral or slightly acidic pH. Several salts or hydrochloric acid are used as condensing agents. The property of the samples are characterized as a function of the washing procedure. Washing by water alone does not extract the template and does not affect the stability of the solid. Washing by ethanolic solution is an effective way to extract the template, and severely affect the stability and the porosity of the samples. 1. INTRODUCTION The acid synthesis pathway of mesoporous materials is based on the interaction between neutral or positively charged silica species and micelles of cationic surfactant whose positive charge is compensated by inorganic anions [1]. The polarisation of silica and the nature of the charge-compensating anion influence the properties of the materials [2-4]. Samples obtained from syntheses near the isoelectric point of silica usually undergo an important shrinking upon extraction of the template, due to their thin pore walls [4]. In this way, microporous materials can be obtained from precursor mesophases with 4 nm micelles [5]. Any method of extraction of the template, washing or calcination, brings about a pore shrinking and a corresponding increase of wall thickness. These effects proceed until a threshold of wall thickness is reached, at which the silica surface area is decreased enough to minimize the excess energy of the system [6]. Secondary treatment of the synthesis systems by water has been shown to positively affect the properties of several samples [7, 8]. Nevertheless, degradation or rearrangement of pore system can also occur through thermally induced hydrolysis of silicate upon hydrothermal treatment. Bagshaw et al submitted some assynthesised mesoporous materials to hydrothermal treatment at 373 K and materials with thicker pore walls resulted [9]. In this communication, the effect of washing on mesoporous silicas and iron silicates prepared in the presence of different salts is dealt with. 2. EXPERIMENTAL Molar compositions of the synthesis batches were nFe(NO3)3/MX/0.21 CTMABr/TEOS/146 H2O, with n in the range 0-0.05 and MX a salt or acid among NH4NO3,
166
NaNOs, NH4CI, NaCl, NH4F, HCl. The detailed preparation of samples and preliminary investigations on their stability were presented in a previous report [6]. For each composition, the products of five parallel preparations were washed according to one of five different procedures: (a) filtration without washing, filtration and washing with (b) 300 cm^ water, (c) 900 cm^ water, (d) 50 cm^ ethanol in 300 cm^ of water, (e) 150 cm ethanol in 300 cm of water. The amount of TEOS used in each synthesis batch corresponded to 4 g Si02. The washed samples were characterized by X-ray diffraction, thermogravimetry and N2 sorption on the solid calcined at 823 K in air flow. 3. RESULTS AND DISCUSSION In Figure 1, the evolution of the a parameter of the hexagonal cell is reported as a function of the washing procedure. Washing with water alone does not seem to affect the cell parameter in a significant way, while washing by ethanolic solution brings about a decrease of the cell parameter for all samples.
50
100
150
200
0
washing H2O / cm3 g-i
10
20
30
40
50
washing EtOH / cm3 g-i
Fig. 1. Hexagonal cell parameter of non calcined samples as a function of washing procedure. Amounts of solvent normalised on the total silica present in the system. Samples prepared with NaCl (round dots), HCl (lozenges), Fe and HCl (squares), Fe and NH4F (triangles). I.5O1
0.00
50
100
150
200
washing H2O / cm3 g-i
3
10
20
30
40
50
washing EtOH / cm3 g-i
Fig. 2. Cetyltrimethylammonium content of non calcined samples as a function of washing procedure. Samples as per Figure 1.
167
The variation of the cell parameter can be related to the change of composition of the samples, reported in Figure 2. Washing with water alone is not an effective method to extract the surfactant, while ethanolic solution is able to extract most of the surfactant from the solid. It can be observed that the cell shrinking presents a not linear dependence on the amount of surfactant extracted [10]. The (3c parameter of the hexagonal cell for the calcined materials is reported in Figure 3. Its dependence on the washing procedure closely parallels the effects observed for the cell parameter of the non-calcined materials.
50
100
150
200
0
washing H2O / cm3 g-i
10
20
30
40
50
washing EtOH / cm3
Fig. 3. Hexagonal cell parameter of calcined samples as a function of washing procedure. Samples as per Figure 1. 1.25[ 1.001 0.7^ 0.50 0.25 0.00
50
100
150
200
washing H2O / cm3 g-i
0
10
20
30
40
50
washing EtOH / cm3
Fig. 4. Structural mesopore volume of calcined samples as a function of washing procedure. Samples as per Figure 1. The structural pore volume of the calcined materials are reported in Figure 4. When water alone is used for washing, the pore volume of non-washed solids is somewhat lower than the pore volume of washed solids for all samples except the silica sample prepared in the presence of HCl. It looks like some retention of salts for samples prepared at pH between 2 and 7 can negatively affect the calcination. The evolution of pore volume with ethanolic washing follows the trends of the surfactant content reported in Figure 2, albeit in a nonquantitative way. The decrease of pore volume after washing is lower than the decrease of
168
template content. Very likely, the surfactant left in the solid after ethanolic washing does not occupy the whole available volume. In Figure 5, typical isotherms for a ferrisilicate sample prepared in the presence of HCl are presented. The positive effect of water washing is evident, as well the negative effect of washing by ethanolic solution.
0.4 0.6 Relative pressure P/P^
0.8
1.0
Fig. 5. Nitrogen adsorption-dcsorption isotherms of a ferrisilicate sample prepared in the presence of HCl (a) non-washed, (b) washed with water, (c) washed with ethanolic solution. 4. CONCLUSIONS It can be concluded that, for samples synthesized at neutral or moderately acidic pH, the extraction of template by solvent at room temperature implies a degradation of the structure far exceeding the usual shrinking observed during calcination. REFERENCES 1. Q. Huo, D.I. Margolese, U. Ciesla, P. Feng, T.E. Gier, P. Sieger, R. Leon, P.M. Petroff, F. Schuth and G.D. Stucky, Nature 368 1994, 317 2. H.P. Lin, S. Cheng and C.Y. Mou, Microporous Mater. 10 1997, 111. 3. H. Yang, G. Vovk, N. Coombs, I. Sokolov and G.A. Ozin, J. Mater. Chem. 8 1998, 743. 4. F. Di Renzo, F. Testa, J.D. Chen, H. Cambon, A. Galarneau, D. Plee and F. Fajula, Microporous Mesoporous Mater. 28 1999, 437. 5. A.C. Voegtlin, A. Matijasic, J. Patarin, C. Sauerland, Y. Grillet and L. Huve, Microporous Mater. 10 1997, 137. 6. L.Pasqua, F. Testa, R. Aiello, F. Di Renzo and F. Fajula, Stud. Surf. Sci. Catal. 135 2001, 06-P-28. 1. Q. Huo, D.I. Margolese, G.D. Stucky, Chem. Mater. 8 1996, 1147. 8. L. Chen, T. Horiuchi, T. Mori, K. Maeda, J. Phys. Chem. B 103 1999, 1216. 9. S. A. Bagshaw. Stud. Surf. Sci. Catal., 117 1998, 381. 10. F. Di Renzo, D. Desplantier, A. Galarneau and F. Fajula, Catal. Today 66 2001, 75.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
169
Synthesis of ordered lamella mesophase from helix layered silicate (HLS) Myung-Geun Song^, Jong-Duk Kim^ and Y. Kiyozumi^ ^Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusung-dong, Yusung-gu, Daejon 305-701, Republic of Korea ''National Institute of Materials and Chemical Research, 1-1-1 Higashi, Tsukuba, Ibaraki 3058565,Japan The lamellar mesostructure was synthesized from helix layered silicate (HLS) using cationic surfactant of alkyltrimethylammonium salt. The structural ordering was affected by the several factors such as the amount of surfactant, pH of the suspension and hydrothermal condition. The hexagonal or cubic structure was not obtained, since the frameworks of HLS were composed with a significant amount of Q"* species. The interplanar spacing can be tuned by the change of alkyl chain length of surfactant, and the intercalated surfactant molecules were easily removed by the repeated washing with EtOH. 1. INTRODUCTION Mesoporous silicate derived from a layered polysilicate kanemite named FSM-16 has an ordered one-dimensional pores system similar to MCM-41. This method consists of interlayer cross-linking of a layered silicate in the ion exchange reaction with organic cations. During the ion exchange with organic surfactant such as alkyltrimethyl ammonium ions, the silicate layers of the single-layered polysilicate are condensed to form three-dimensional silicate networks'. Recently, novel layered silicate with helical morphology named helix layered silica (HLS) was synthesized by Akiyama, et al? The frameworks of HLS are different with those of traditional layered silicates such as kanemite and magadiite
(NaSi70i3(OH)3-4(H20)). In
this paper, organic modification of HLS was carried out in various synthetic conditions under the presence of cationic surfactants, alkyltrimethylammonium salts with different alkyl chain length.
170
2. EXPERIMENTAL A typical synthesis of HLS was carried out as follows. lOg of Cab-o-Sil(M5) was mixed with 61.8g of Dl-water, and then 13.2g of tetramethylammonium hydroxide (TMAOH) and 60.Og of IN-NaOH solution were successively added into the gel mixture. Finally, 1,3dioxane were added and stirred vigorously. After 1 hour later, the gel was heated in a 300ml volume of teflon-lined static autoclave, to 150°C for at least 5 days. The precipitated product was filtered, washed with acetone, and dried at 80°C in an oven for several hours. For the transformation of HLS into porous material, cationic surfactants with different alkyl chain length, CnTACl or CpTABr were used as structure-directing agent. The 15ml of aqueous surfactant solutions with known amounts were prepared. After adding of 1 g of HLS into the surfactant solution, the suspension was stirring for one at least 24hrs at room temperature. Several experimental variables such as pH of the suspension, the amount of surfactant and additives were changed to find out the optimum condition. After hydrothermal treatment, the modified HLS particles were recovered by filtration and washed with water, and then were dried at 60"C for several hours. The solid products were characterized by using powder XRD. 3. RESULTS AND DISCUSSION Fig. 1 shows the XRD patterns of (a) as-synthcsizcd HLS powder prepared from the above experimental condition and (b) calcined HLS at 700"C for 6 hours. The XRD pattern of (a) shows the characteristic of a layered structure. It is revealed that HLS is converted to quartz phase if it was calcined at high temperature. Fig 2 shows the XRD patterns of C16TACI-HLS composite prepared from various amount of surfactant. The ratio of C16TACI to HLS varied from 0.445 to 4.450 (w/w). As shown in this figure, the HLS is easily transformed to lamellar phase by intercalation of surfactant molecule, since the intensities of two peaks at 2.62 and 5.220 of 26 (d,Q()=3.37nm), which characterize the lamella phase, increase with the amount of surfactant. However, the intensities decreases slightly from the ratio reach to about 2.226. The structural ordering of lamellar phase also can be affected seriously by the pH of suspension. Fig 3 shows the XRD patterns of C16TACI-HLS powders, which the pHs of suspensions were controlled as 4.3, 8.0, 10.0 and 12.8 before the addifion of pure HLS. The ordering of lamella formed in C16TACI-HLS complex becomes very weak as the pH of suspension decreases. The best pH condition for the lamella phase was revealed as around 10.0, since the peak intensity at 12.8 of pH was weaker than at 10.0. At low pH condition, the negafive charge of silica surface may be weak, since the isoelectric point (lEP) of silica is around 2.0. Therefore the
171
interaction between cationic surfactant molecules and silica layer may be weakened and the lamella structure is poorly ordered. At high pH condition, the excess Na^ ions may destabilize the layer structure of HLS, and the regularity of structure becomes weak.
^^^ILOJUlMLAjunj 20
25
30
35
40
45
2 e (degrees)
Fig. 1. XRD patterns of (a) assynthesized HLS and (b) calcined HLSat700''Cfor6h.
50
2 e (degrees)
Fig 2. XRD patterns of CjJACl-HLS prepared with different amount of C,(,TAC1. Ci^TACl/HLS (w/w) was varied as (a) 0.445, (b) 0.779, (c) 1.113, (d) 2.226, (e) 3.338, and (f) 4.450.
During the experiment of transformation of HLS into mesoporous materials, another mesophase such as hexagonal or cubic structure could not be obtained in spite of various synthetic conditions. The change of temperature or additives such as short chain alcohol and hydrogen peroxide, etc, has no effect for the transformation HLS into hexagonal or cubic phases. This may be caused by the structural difference between HLS and kanemite, that is, there is a significant amount of Q"* silicate species (completely polymerized) in HLS materials, and the mesoporous materials cannot be synthesized from it. Presumably, Q"* species indicate some "cross-linking" of silica layers that is not reversible. In the report of Chen et al."^, Nakanemite which has no Q'* species can be converted into ordered mesoporous materials by hydrothermal treatment. The interplanar spacing of surfactant-HLS composite can be tuned by changing the alkyl chain length of cationic surfactant species. The dioo spacings of C12TACIHLS and C16TACI-HLS hybrid materials are 2.94nm and 3.37nm, respectively. Instead of calcination process for the detemplating of surfactant, repeated EtOH washing is available to eliminate the surfactant molecules intercalated into layers. Fig 4 shows the XRD patterns of
172
CpTA-HLS mesophases detemplated by repeated EtOH washing. After elimination of surfactant intercalated between layers, the interplanar spacing is serious decreased as 1.87nm and 2.07nm for C12TACI-HLS and CieTACl-HLS materials, respectively.
'1
•
f-. •
' ' ' . . • ' ' "
••.,-,•
••••
^
M C
s 1
5
10
15
i^'M
20
25
30
35
40
4J
10
15
20
25
30
35
40
2 e (degrees)
2 e (degrees)
Fig. 3. XRD patterns of C, JACl-HLS prepared at different initial pH conditions; (a) 4.3, (b) 8.0, (c) 10.0, and (d) 12.8
Fig. 4. XRD patterns of CJAX-HLS after detemplating of surfactant by repeated EtOH washing; (a)C,6Cl, (b)C,6Br (c)C,4Cl and (d) C,2Cl.(a) 4.3, (b) 8.0, (c) 10.0, and (d) 12.8
ACKNOWLEDGMENT This work was supported in part by the winter institute program of the Japan International Science & Technology Exchange Center. REFERENCES 1. C. T Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, and J. S. Beck, Nature, 359, 710 (1992). 2. S. Inagaki, Y. Fukushima, and K. Kuroda, J. Chem. Soc, Chem. Commun., 680(1993). 3. Y. Akiyama, F. Mizukami, K. Maeda, H. Izutsu, K. Sakaguchi, Angew. Chem. Int. Ed, 38, 1420(1999). 4. C. Y. Chen, S. Q. Xiao, and M.E. Davis, Microporous Materials, 4, 1 (1995).
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
173
Synthesis of monolithic nanostructured silicate family materials through the lyotropic liquid crystalline mesophases of non-ionic surfactant S. A. El-Safty*, T. Hanaoka Tohoku Center, National Institute of Advanced Industrial Science and Technology 4-2-1, Nigatake, Miyagino-ku, Sendi, 983-8551, JAPAN, Fax:+81-022-2375226, E-mail: SherJf.El-Saftvr(^)aist.gQ.ip, and E.mail: Hanaoka-takaaki@aist.go.ip
(AIST),
A family of highly ordered mesoporous silica materials designated as (HOM) has been synthesized by using high concentration of non-ionic surfactant of Brij 56 as a template. The monolithic silicate molecular sieves with regular arrays and extend periodicity have been produced in acidic condition and at different temperatures 25-45 °C range. The liquid crystal mesophase properties affect in the monolithic mesostructured morphology. The three dimensional (3D) accessible mesoporous silicates of cubic Im3m (HOM-1), cubic Ia3d (HOM-5), primitive-centered cubic Pn3m (HOM-7), and 3-d hexagonal P63/mmc (HOM-3) have been generated at 35, 70, 69, and 85 wt% of the mass ratio of Brij 56/TMOS, respectively. In addition, a stable well-defmed ordered two dimensional hexagonal p6mm space group (HOM-2), lamellar Loo (HOM-6), and solid phase (S) with a cubic Ia3d space group (HOM-4) have been prepared with mass ratio ca. 50, 75, and 70 wt % of Brij 56/TMOS at 35, 40, and 25 °C, respectively. 1. INTRODUCTION Lyotropic liquid crystalline mesophases formed at high concentration of surfactant have been employed for the synthesis of large uniform nanoporous silicates. The long-range ordering mesoporous materials are obtained in this regime, which is independent of the structure and charge of the amphiphiles as shown by Attard [1,2]. Recently, we have reported that a well-defmed long-range ordered mesoporous silicate family including hexagonal (Hp Si02), lamellar (Loo-Si02), lamellar solid phase (S-Si02), cubic (Ia3d-Si02), cubic (Im3mSi02), and 3-d hexagonal (P63/mmc-Si02) materials have been synthesized adopting lyotropic liquid crystal mesophases of Brij 76 [3]. These materials have a great of special interest in the separation treatment of water from the organic contaminated materials [4], catalysis [5,6], semiconductors [7], storage batteries [8]. 2. EXPERIMENTAL In a typical synthesis [3], as-synthesized silica materials were prepared by mixing tetramethoxysilane (TMOS) with Brij 56. The exothermic hydrolysis of TMOS was carried out quickly by addition of HCl (pH= 1.3). * Permanent address; Chemistry department. Faculty of Science, Tanta University, TantaEgypt. E-mail: saes682001 (?^yahoo.co.uk
172
CpTA-HLS mesophases detemplated by repeated EtOH washing. After elimination of surfactant intercalated between layers, the interplanar spacing is serious decreased as 1.87nm and 2.07nm for C12TACI-HLS and CieTACl-HLS materials, respectively.
'1
•
f-. •
' ' ' . . • ' ' "
••.,-,•
••••
^
M C
s 1
5
10
15
i^'M
20
25
30
35
40
4J
10
15
20
25
30
35
40
2 e (degrees)
2 e (degrees)
Fig. 3. XRD patterns of C, JACl-HLS prepared at different initial pH conditions; (a) 4.3, (b) 8.0, (c) 10.0, and (d) 12.8
Fig. 4. XRD patterns of CJAX-HLS after detemplating of surfactant by repeated EtOH washing; (a)C,6Cl, (b)C,6Br (c)C,4Cl and (d) C,2Cl.(a) 4.3, (b) 8.0, (c) 10.0, and (d) 12.8
ACKNOWLEDGMENT This work was supported in part by the winter institute program of the Japan International Science & Technology Exchange Center. REFERENCES 1. C. T Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, and J. S. Beck, Nature, 359, 710 (1992). 2. S. Inagaki, Y. Fukushima, and K. Kuroda, J. Chem. Soc, Chem. Commun., 680(1993). 3. Y. Akiyama, F. Mizukami, K. Maeda, H. Izutsu, K. Sakaguchi, Angew. Chem. Int. Ed, 38, 1420(1999). 4. C. Y. Chen, S. Q. Xiao, and M.E. Davis, Microporous Materials, 4, 1 (1995).
175
(a) HOM-1
a,-10.6nm
HOM-4
\f^5jM\jl_ 2
3
4
5
6
7
2
4
3
4
c" 8.81 nm c/a« 1.628
5
6
7
(d) HOWW
Fig. 2. XRD of the three dimensional mesoporourus material synthesized at different lotropic concentration (a) at 35 wt%, (b) Cubic solid phase at 70wt%, (c) at 85 wt%, and (d) at 69 wt% of Brij 56, respectively. In addition, the four well-known diffraction peaks that are corresponding to (100), (200), (300), and (400) planes of highly ordered lamellar structure (IIOM-6) are also investigated within the Brij 56 mesophases. The X-ray diffraction pattern shows five well-ordered peaks that are indicative of (100), (110), (200), (210), and (300) reflections for hexagonal P6mm (HOM-2). It reveals that the observable X-ray reflections indicate that the liquid crystal phases have well-defmed high ordered nanoporous monoliths [2-4]. SEM micrographs (Figure 3) reveal that the particle morphologies of the mesophase samples are consisted of crystal structures with variable sizes and shapes.
w^ w
(a)
' ^
(b)
(c)
j--i'":-*r
Fig. 3. SEM micrographs of (HOM-2), (HOM-6), and (HOM-7) particle morphologies.
176
The high quaHty nanoporous structures are also investigated by TEM micrographs, Fig. 4. TEM images of the mesophases along the observable orientations show that the structures are well-defmed and regular. The regularity of the spots in the images corresponds to the channels running along these directions [3]. (c)
f..- i i I'% 3 . 1 V t -^ •;
(f)
Fig. 4. TEM images of the silica monoliths with different mesophases of (a) HOM-6, (b) HOM-1, (c) HOM-2, (d)H0M-3, (e) HOM-5, and (f) HOM-7 In summary, the synthesis of such bulk mesophases from only one non-ionic surfactant established that the direct templating method is considered a reliable synthetic methodology for producing a high crystalline with high quality mcsoscopic morphology. In addition, the synthetic conditions keep the easily and simply producing of mesophases without loss of texture order. We believe that this periodic mesophase family with large cage, uniform mesopore arrays over the long-range of all lattice symmetries, stabilizing wall architectures, and high crystallinity open a great valuable opportunity in application fields of nanoporous materials. ACKNOWLEDGMENT Authors thank the Japan Society for Promotion Science (JSPS) for financial support. REFERENCES 1. G. S. Attard et.al. Nature, 378 (1995) 366. 2. G. S. Attard et.al. Science, 278 (1997) 838. 3. S. A. El-Safty, J.Evans, J. J. Mater. Chem., 12 (2002)117. 3. S. A. El-Safty, J. Mater. Chem., under published, 2002. 5. J. Evans, M. Y. El-Sheikh, A. B. Zaki, S. A. El-Safty, Phys.Chem.B, 104 (2000) 10271. 6. S. A. El-Safty, J.Evans, M. Y. El-Sheikh, A. B. Zaki, Colloid Surf. A, 203 (2002) 217. 7. N. Iris et. al., Chem. Mater., 13 (2001) 3840. 8. G. S. Attard, Macromol.Symp., 156 (2000) 179.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
177
Synthesis and characterization of a new mesoporous molecular sieve Quanjie Liu, Chongren Han, Wanfii Sun, Jun Yang and Yong Zhou FuShun Research Institute of Petroleum and Petrochemicals, FuShun China 113001. FAX: +86-0413-6429551. E-mail:liuquanjie@fripp.com.cn In this presentation, a new mesoporous molecular sieve (MPL-1) was successfully synthesized from the feedstock of pseudobeohmite (72.6wt% AI2O3), orthophosphoric acid (85wt% H3PO4) and silica sol (30wt% SiOz) under the hydrothermal process with an organic compound as template reagent. The structures and properties of as-synthesized molecular sieve were investigated by XRD and liquid nitrogen physical absorption characterization methods. The experimental results show that the molecular sieve exhibits larger pore diameters, surface area and pore volume. The hetero-atoms such as Ti, Fe and Zn may be incorporated in the framework of as-synthesized molecular sieve to produce derivatives of the mesoporous materials such as Al-Si-Ti-P, Al-Si-Fe-P, and Al-Si-Zn-P molecular sieves which have higher thermal and hydrothermal stability. 1. INTRODUCTION Porous inorganic materials especially crystalline zeolites have been widely applied in the fields of catalysis and adsorption separation because of their abundant microporous structure and larger surface areas. Although the study on zeolites is quite mature, the pore diameters of most prepared zeolites are below 1.0 nm, and the maximum pore diameter reported is only 1.3 nm^''. According to the definition of lUPAC, the material with pore diameter below 2 nm belongs to the microporous materials, and the material with pore diameter in the range of 2 nm to 50 nm belongs to the mesoporous material, and the material with pore diameter exceed 50 nm belongs to the macroporous material. Based on this definition, most of the prior zeolites belong to the microporous zeolites. Beck et al^^^ disclosed a process for synthesizing a mesoporous MCM-41 zeolite and its properties in 1992. This sort of zeolite has a structure of hexagonal symmetry. It could provide favorable space and effective acidic active sites for the large molecules because of its higher surface area, uniform pore distribution, adjustable pore diameter and acidity, accessible active sites, and small diffusion resistance. However, the zeolite has so poor thermal stability (especially hydrothermal stability), its crystal lattice can be retained in boiling water for only several hours or even shorter, so it would be hard for them to have any value for practical applications. To overcome the above mentioned disadvantages and problems, a new molecular sieve (hereinafter names it MPL-1) was synthesized in this presentation. MPL-1 exhibits its mesoporous structure, large pore diameter and specific surface area, strong adsorption capacity, high thermal and hydrothermal stability. 2. EXPERIMENTAL 2.L Synthesis of template (phenethoxy-2-hydroxypropyItriinethylammoniuni chloride) a. Synthesis of phenethyl glycidic ester 122 g (1 mole) of phenethyl alcohol, 240 g of 50 wt% sodium hydroxide solution and 1000 ml of toluene were sequentially added into a 2000 ml three-necked flask. The mixmre was
178
Stirred magnetically at room temperature for 1 h, and then 184 g (2 moles) of epoxy chloropropane was added into the mixture. The resulting solution was heated to 75°C for 6 h under intensive stirring, and then the stirring was stopped after the solution was cooled to room temperature. The lower layer solution was separated with a separating funnel. 171 g (0.96 moles) of the product phenethyl glycidic ester with a yield of 96 wt% was obtained after toluene and unreacted epoxy chloropropane being removed from the upper layer solution in a rotatory evaporator. b. Synthesis of phenethoxy-2-hydroxypropyltrimethylammonium chloride 142 g (0.8 moles) of phenethyl glycidic ester of step (a), 100 g of a solution containing 48 g (0.8 moles) of trimethyl amine and 500 ml of anhydrous ethanol were sequentially added into a 1000 ml of three-necked flask. The solution was neutralized with 37 wt% of hydrochloric acid and refluxed for 2 h to conduct the reaction, and then ethyl alcohol and water were evaporated. The residue was re-crystallized twice with acetone and petroleum ether (2:1 volume ratio of acetone to petroleum ether), and then dried in vacuum at room temperature to obtain 191 g (0.70 mole) of product phenethoxy-2-hydroxypropyltrimethylammonium chloride(PTMAC) with a yield of 87 wt%. The phenethoxy-2-hydroxypropyl trimethylammonium bromide (PTMAB) was synthesized with the same process of above, and yield was 74 wt%. 2.2. Synthesis of molecular sieves All molecular sieves were synthesized under the hydrothermal process. The reactants were pseudobeohmite(72.6wt%Al203/technical grade) as the aluminum source, orthophosphoric acid(85wt%/analytical grade) as the phosphorous source, silica sol (30wt%SiO2 ) as silicon source, TiCl4(analytical grade) as titanium source, FeCb or ZnCb as metal source, phenethoxy-2-hydroxypropyltrimethylammonium chloride(PTMAC) or phenethoxy-2-hydroxypropyl trimethylammonium bromide (PTMAB) as template reagent. Tetrabutylammonium hydroxide was used as an alkali to adjust the pH value of the mixture. The MPL-1 and its derivatives were prepared by the process of: (a) A template, an aluminum source, a silicon source, a metal source and a phosphorus source were mixed with water, the mixture was stirred to homogeneous solution and the pH value was adjusted using tetrabutylammonium hydroxide to obtain the gel; (b) Crystallizing the resulting mixture of step (a), a crystal is formed, recovering and washing and drying the solid product to obtain the as-synthesized molecular sieve; (c) Calcining the as-synthesized molecular sieve of step (b) to remove the template reagent to obtain the mesoporous molecular sieve of the present information. During the synthesize process, if necessary, metal compounds may be added into synthesize derivatives of containing the corresponding hetero-atoms. The synthesis conditions of several samples were shown in table 1. The crystallinity of MPL-1 treated in different conditions (synthesized by No. 1, calcined at 700°C for 2 h, and being treated in boiling water for 10 h) was determined by XRD, respectively.
179
Table 1 the synthesized conditions of samples Ex. Nos.
Gel composition
a
b
c
1 2 3 4
0.2SiO2:1 .OAI2O3:1.0P2O5:0.4Rl: 1 SOUjO 0.ITiOz: 0.1 Si02:1 .OAI2O3:1. 1P205:0.5R1:200H2O 0.05Fe2O3:0.1 Si02:1 .OAI2O3:1.0P2O5:0.25R2:200H2O 0. lZnO:0.15Si02:1.0Al2O3:0.9P2O5:0.20R2:200H2O
150 160 170 170
6.0 6.5 8.0 7.5
24 24 36 36
a. crystallization temperature(°C); b. pH of the mixture; c. crystallization time (h)
Rl: (PTMAC) ; R2: (PTMAB)
2.3. Characterization The structure type and crystalinity of all samples were checked by powder X-ray diffraction (XRD) on a D/max-rb automated diffraction system using a Cu Ka radiation, operating at 40kV and 40mA. The spectra were recorded in the angular range from 1 to 10 degrees with a scanning speed of 0.2 degrees (2-theta)/min. N2 adsoption/desorption data were obtained at 77K with an ASAP2400 equipment, the specific surface area (SBET) was determined from the linear part of the BET plot (0.06
180
mesoporous molecular sieves have higher thermal and hydrothermal stabilities. Its crystal lattice is not damaged after being calcined at 700°C for 2 h and its crystallinity is not substantively decreased after being treated in boiling water for 10 h. It is also observed from figure 2 that the position of the peaks (100) slightly shift towards higher 2theta angle, showing that after thermal and hydrothermal treatment, the crystal lattice shrinks as usual. The sorption of N2 has been widely used to determine the surface area and to characterize the pore distribution of mesoporous materials^'^l The N2 adsorption/desorption isotherms of various samples belong to the type IV in the lUPAC classification and exhibit two distinct features: a sharp capillary condensation step at a relative pressure p/po of around 0.3, and all the isotherm exhibits a smaller hysteresis loops at relative p/po of about 0.9. The above two distinct features of mesoporous materials can be used as an criterion to indicate the materials have mesoporous structure and their pore sizes distribute in concentration ^^\ The values or specific surface area(SBET), diameter(dBjH) of the mesopores and total pore volume(Vt) for several molecular sieves synthesized are listed in table 2, and it shows that these materials have larger pore diameters, surface area and pore volume and confirm these materials are mesoporous molecular sieves. From this table, we could also observe that the specific surface area (SBHT), diameter (dejH) of the mesoporous and total pore volume (Vt) were increased after introducing metals into products, and this may be caused by the lacuna or distortion of crystal lattice after the metal substitution. In these processes, we find that the ratio of silicon to aluminum of products as almost the same as the reactant gel, but the ratio of phosphorous to aluminum is decreased. It shows that the silicon may substitute the hypothetical phosphorous position of framework Table 2 Properties of the synthesis materials Ex. Nos.
Samples
Product composition
V, (ml/g)
dBJH
(m'/g)
(nm)
(%)
1
MPL-1
0.18SiO2:1.0Al2O.v0.92P2O5
576
0.48
3.4
100
2 3 4
Ti-MPL-1 Fe-MPL-1 in-MPL-1
0.lTi02: 0.1 Si02:1.0Al2O.,:0.9P2O5 0.04Fc2O3:0.1 Si02:1.0Al2O3:0.9P2O.s 0.1 ZnO:0.13Si02:1.0Al2O.v0.9P2O.s
633 843 865
0.53 0.60 0.62
3.4 3.5 4.1
92 84 90
SBI;T
Crystalinity
4. CONCLUSIONS The samples, synthesized in this paper, were proved to be mesoporous molecular sieves by their XRD pattern and the results of N2 adsorption/desorption of them. The hetero-atoms such as Ti, Fe and Zn may be incorporated in the framework to produce derivatives of the mesoporous materials. These mesoporous materials have higher thermal and hydrothermal stabilities. REFERENCES 1. 2. 3. 4. 5.
Davis M E, Saldarriaga C, et al. Nature, 1991, 352: 320 Beck J S, Chu Cynthia; Johnson I D, etal., US 5,108,725, 1992. BeckJ.S, US 5057296.1991. On D.T, Zaidi M.S, Kaliaguine S, Microporous and Mesoporous Materials, 1998, 22:211 Ciesla U, Microporous and Mesoporous Materials, 1999, 27:131
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
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Direct- and post- hydrothermal treatments in ammoniated solution for the morphogenesis of mesoporous silica nanotubes Zhong-Yong Yuan^, Bao-Lian Su^'* and Wuzong Zhou^ ^Laboratory of Inorganic Materials Chemistry, The University of Namur (FUNDP), 61 rue de Bruxelles, B-5000 Namur, Belgium. E-mail: bao-lian.su@fundp.ac.be ^School of Chemistry, University of St. Andrews, St. Andrews, Fife KYI 6 9ST, UK The surface patterning of mesoporous silica nanotubes was observed in the mesoporous materials hydrothermally synthesized under mild-alkaline conditions in the presence of cetylpyridinium surfactants. Direct hydrothermal synthesis in an ammoniated solution or posthydrothermal treatment of room-temperature-synthesized materials were performed alternatively. The formation mechanisms were investigated. It is revealed that the roles of ammonia during direct- or post-hydrothermal treatment are different. 1. INTRODUCTION The disclosure of M41S periodic mesoporous silicates had an immediate strong impact on the research area of mesostructured inorganic materials [1]. The ability to tailor the nanostructure and morphology of ordered mesostructured inorganic solids is a key factor in their applications in the fields such as catalysis, molecular separation, chemical sensors, and optoelectronic devices. Many synthesis approaches based on surfactant templating have been developed, and various mesoporous silicas in the forms of films, fibers, spheres, monoliths, curved shapes and designed patterns have been achieved. Recently we have reported a novel morphology of mesoporous silicas with nanotube surface patterning which was formed during direct hydrothermal synthesis [2], and Lin et al. also presented mesoporous silica tubes after a post-synthesis hydrothermal treatment [3]. However, the formation mechanisms are still not clear. A further detailed study is necessary to make some questions on the mechanisms clear. We thus investigated the syntheses of mesoporous silicas either by direct hydrothermal synthesis or by post-hydrothermal treatment of the primary room-temperature-synthesized materials, and the results are shown herein. 2. EXPERIMENTAL The mesoporous silica materials were prepared with cetylpyridinium chloride (CPCl) under a mild alkaline condition either at room temperature or by low-temperature (for example, 80 °C) hydrothermal treatment. Post-synthesis hydrothermal treatment of room-temperaturesynthesized mesoporous silicas was also performed with or without ammonia at lowtemperature (lower than 100 °C). The initial synthetic mixture composition was 1 TEGS : 9.2 NH4OH : 0.5 CPCl : 130 H2O. Calcination in order to remove the organic species in the mesochannels was taken at 540 °C for 5 h in air. The texture properties and the morphology of the synthesized materials were characterized by X-ray diffraction (XRD), N2 adsorption analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
182 3. RESULTS AND DISCUSSION The solid materials collected from the mixture of CPCI-H2O-NH4OH-TEOS system at room temperature, without further hydrothermal treatment, have shown the well-defined hexagonal mesostructure. 4 diffraction peaks can be seen at low 2 ^ region of their powder XRD pattern (Fig. la). TEM images confirm the hexagonal mesoporous structure, and the particle surface is smooth. N2 adsorption analysis data reveal that the calcined sample has a narrow pore size distribution 2 ^(degree) centered at 2.3 nm with BET surface area of 1320 m^/g. Its wall thickness is about 1.4 Fig. 1. XRD patterns: (a) room-temperaturcnm accordingly. However, if the mixed gel synthesized sample; (b) after ammonia posthydrotreatment at 80 °C of (a); (c) after were subjected to hydrothermal treatment calcination at 540 °C of (b); (d) dircctat low-temperature, the structural features hydrothermal synthesized sample (80 °C). would be improved greatly. After direct hydrothermal synthesis in the ammoniated solution at 80 °C for 3 days, the products exhibit at least 5 sharp XRD peaks (Fig.Id), indicating the higher-ordered hexagonal array of mesopores. The hexagonal unit cell parameter is 5.0 nm. Calcination to remove the organic species in the channels results in only a very small decrease of unit cell parameter of 0.1 nm, suggesting the high thermal stability of the samples. The uniform pore size is about 3.1 nm in diameter, and the surface area is 1002 mVg, based on N2 adsorption-desorption isotherms. The wall thickness is therefore about 1.9 nm. The enlargement of pore size by hydrothermal treatment has been well documented [1], and the pore size could be still increased with the hydrotreatment time. TEM images of these direct-hydrothermal-synthesized mesoporous silicas show a paintbrush-like morphology of the nanotubc surface patterning (Fig. 2). Many single silica nanotubcs with the diameter of about 5 nm and bundles of single nanotubes with average diameter of 20 - 40 nm appear on the (001) surfaces of the particles. All the silica nanotubes are parallel to the mesopore axis of the MCM-41 structure, and the length of the nanotubes is almost uniform. It is found that the length of the nanotubes could be varied with the synthesis time, up to above 250 nm. Such nanotube surface patterning, even single Fig. 2. TEM image of direct-hydrothermal-synthesized mesoporous nanotubes, are stable, and silicas showing paintbrush-hkc surface. Insert is an image at the could be kept unchanged initial period of nanotube growing, showing one nanotube of about after calcination or under electron beam irradiation. 5 nm diameter is growing. Diffusion process of chemical
183
Species in the solution and in the surfactant-sihca particles is quite important [4,5]. The initial nuclei of mesoporous silica formed before the hydrothermal treatment, though their mesostructure was less condensed with lower ^loo spacing. There are still silicate species and surfactant micelles in the precursor solution as detected by ^H, '^C and ^^Si MAS NMR. Those partially hydrolyzed silicate species would continue to aggregate with the surfactant micelles to generate condensed rods during the mild hydrothermal treatment, and the condensation stabilizes the rods. The spherical end cap of the cylindrical rod is relatively unstable due to the lack of geometric matching, which would proceed on the surface of initial mesostructured silica nucleus particles and connect together by the geometric matching between the occupied area of surfactant molecules and silicate molecules respectively. Thus the mesochannels would elongate from the initial silica particle surface during crystallization. Meanwhile, mesostructured silicate-surfactant composite particles could be restructured during hydrocrystallization by the additional silicate species, forming better ordered array of mesochannels with larger d\oo spacing. The proposed formation mechanism of surface nanotube patterning is supported by the TEM image at the initial stage of nanotube growing from the particle surface (Fig. 2). Hydrothermal treatment is necessary for the formation of this novel surface patterning since it is not found the same morphology in the initial mesostructured silica particles before hydrothermal treatment. Post-hydrotreatment of the room-temperature-synthesized silicas in an ammonia solution at the temperature lower than 100 °C can also result in the pore size enlargement, accompanying with the change of the morphology. As shown in Fig. 1, after post-hydrothermal treatment at 80°C for 5 days, the XRD pattern shows a significant shift of the reflections to lower 2 Wangle, though the diffraction peaks of (110), (200) and (210) broadened too much to be identified clearly. Further calcination to remove the organic species results in an increase in intensity of the diffraction peaks and the clear visualization of the (110), (200) and (210) peaks (Fig. Ic). The surface area is 1068 m^/g. The pore size enlarged to be 3.6 nm after post-treatment, but the wall thickness decreased to be l.O nm, which is opposite of the case of direct-hydrothemal synthesis. It has been known that post-synthesis hydrothermal treatment could offer significant restructuring for a good thermal stability and extending pore size [3,6]. Mesopore size could be enlarged markedly at high temperature (130-150 °C) of post-synthesis treatment, but almost unchanged when the temperature was lower than 100 °C in the previous reports. Lin et al. [3] performed post-hydrothermal treatment on their acidmade mesoporous silicas in an ammonia solution at higher temperature (150 °C). This treatment resulted in the expansion of pore size and extrusion of silica nanotubes from the mesochannels, but no evident changes in the pore size and morphology at lower temperature (< 100 °C). However, different phenomena are observed in our present work, i.e. not only the pore size increased but also Fig.3. TEM images of the post-hydrotreated particle morphology changed after lowsample. Some zones of the studded vaults temperature post-hydrotreatment of mildextruded from the silica nanotubes arc circled. alkaline-made mesoporous silicas at only The direction of the particle elongation is also 80 °C. Fig. 3 shows TEM images of the indicated by a long white arrow.
184
products after the post-hydrotreatment. Some short nanotubes were extruded from the bulk particle through the direction of mesoporous axis. Many small vaults with nano-scale present on the particle surface, forming the novel surface patterning of studded vaults, hidividual vaults on the tips of nanochannels with the width of about 5 nm can be seen. Simultaneously the particles were elongated in the direction perpendicular to the pore direction, due to the enlargement of mesochannels by ammonia post-hydrotreatment. Such subtle morphology changes could be preserved after calcination, which suggests that the lattice expansion and the morphology change should occur during the post-hydrotreatment. In order to better understand the effect of ammonia in the hydrothermal treatment, post-hydrotreatment in pure water system gSi-0'®«^vv« j was also carried out. Only an increase of 0.4 BSi-0'®«'vs/v> t post-synthesis nm in pore size was noted after the posthydrotreatment eSi-0'©^vN/v> \ treatment in water at 80 °C for 5 days, and no evident changes in morphology was ... ,, crNii^oTT) -Si-O ( thicker wall observed. The particle surface is as smooth thinner wall Fig. 4. A schematic model for postas that of initial mesoporous materials. This hydrothcrmal-synthesis of mesoporous silicas indicates that ammonia may take an in ammonia solution. important role in the changes of morphology and structural property of mesoporous silicas during post-synthesis hydrothermal process. Khushalani et al. [ 6] believed that pore expansion is mainly due to the penetration of water into the pores during the posthydrotreatment. Our present study is carried out at low-temperature (80 °C). Due to the volatility of ammonia, ammonia molecules should be easier to penetrate inside the nanochannels than water, and then the channels were swelled up, resulting in the pore expansion. The possible diffusion of volatile ammonia species in the nanochannels upon mild hydrothermal treatment would drive the movement of surfactant-silica species, inducing the extrusion of some silica nanotubes on the particle surface, accompanied with the decrease of wall thickness (Fig. 4). Therefore, the formation mechanisms of mesoporous silica nanotubes during direct- and post-hydrotreatments are quite different. The growing surfactant/silicate aggregates are invoked to explain the larger pore diameter, thicker and highly condensed pore walls and the paintbrush-like morphology observed in direct-hydrothermal synthesis. Whereas the penetration and diffusion of ammonia may be the cause of the pore expansion, pore wall reduction and the extrusion of silica nanotubes in post-synthesis treatment. The findings may be significant for the design and synthesis of mesostructured materials with controllable pore system and morphology. REFERENCES 1. A. Corma, Chem. Rev., 97 (1997) 2373. 2. Z. Yuan and W. Zhou, Chem. Phys. Lett., 333 (2001) 427. 3. H.P. Lin, C.Y. Mou and S.B. Liu, Adv. Mater., 12 (2000) 103. 4. W. Zhou and J. Klinowski, Chem. Phys. Lett., 292 (1998) 207. 5. W. Zhou, R. Mokaya, Z. Shan and T. Maschmeyer, Prog. Nat. Sci., 11 (2001) 33. 6. D. Khushalani, A. Kuperman, G.A. Ozin, K. Tanaka, J. Garces, M.M. Olken and N. Coombs, Adv. Mater., 7 (1995) 842.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved
185
Generalized homogeneous precipitation method for precisely controlled synthesis of mesoporous silicas Jifi Rathousky and Amost Zukal* J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3, CZ-182 23 Prague 8, Czech Republic A simple synthesis pathway has been developed which affords organized mesoporous silicas using inexpensive sodium metasilicate as the silica source and nonionic surfactants as the structure-directing agents. Due to their precipitation from an alkaline medium, these materials exhibit markedly different structure features from those prepared in an acid medium, being non-microporous. The precipitation should be carried out at strictly quiescent conditions, as stirring leads to an inevitable decrease in the pore ordering. 1. INTRODUCTION Recently, we have developed a new procedure for the synthesis of organized mesoporous silica (OMS), which is based on the homogeneous precipitation in an alkaline medium of a water solution of sodium metasilicate and quaternary ammonium surfactant [1,2]. The decrease in pH, which causes the formation of a solid product, is achieved by the hydrolysis of a suitable ester of acetic acid. The acidification of the reaction mixture occurs under quiescent conditions without local variations. Due to the possibility to control both the rate of the decrease in pH and its final value, this procedure enables to prepare siliceous mesoporous materials with different structure features as well as the long length scale control of the particle formation. Nonionic alkyl poly(ethylene oxide) surfactants and poly(alkylene oxide) triblock copolymers are important families of surfactants, being low-cost, nontoxic and biodegradable. The synthetic procedures based on the use of nonionic surfactant and tetraalkyl orthosilicate are not as commercially viable as they might be due to the high cost of this silica source. Therefore, syntheses starting from sodium silicate solutions as an inexpensive inorganic silica source were recently reported [3-7]. In all these procedures an amount of acid is added to the reaction mixture to lower its pH; in some cases pH is then adjusted to the desired value. This study aims at the development of a new procedure for the synthesis of OMS, which combines the advantages of both the precipitation under quiescent conditions and the application of nonionic surfactants. Unlike above cited procedures, the formation of the silica mesophase occurs at gradually decreasing pH from highly alkaline to neutral region. Typical results obtained with three different types of nonionic surfactants are reported in this contribution.
•Corresponding author; Telephone: +4202-66053865; Fax: +4202-86582307; E-mail: amost.zukal@jh-inst.cas.cz.
186
2. EXPERIMENTAL 1 g of nonionic surfactant and 2.5 g of Na2Si03 were dissolved in 900 mL of H2O. The decrease in pH, which caused the precipitation of OMS, was due to the addition and ensuing hydrolysis of 5 mL of ethyl acetate at 35 - 80 °C. The resulting solid was recovered by filtration, washed with water, dried and calcined at 600 °C in air. As structure directing agents, alkyl poly(ethylene oxide) surfactant Brij 56, poly(alkylene oxide) triblock copolymer Pluronic P 123 and poly(ethylene oxide) sorbitan monostearate Tween 60 were used. (The designation of samples and synthesis temperatures are given in Table 1.) Adsorption isotherms of nitrogen at 77 K were taken with a Micromeritics ASAP 2010 instrument. Powder X-ray diffraction (XRD) patterns were collected on a Siemens D 5005 diffractometer. Scanning electron microscopy (SEM) was performed using a JEOL JSM5500LV microscope. 3. RESULTS AND DISCUSSION It was found that stirring should be only applied to achieve homogenization of the reaction mixture after adding ethyl acetate, while the synthesis proper should be carried out at strictly quiescent conditions. If this requirement is not observed, either less well-organized or even disordered material is obtained depending on the concentration of reaction mixture. The diffractograms of all the samples (not shown) exhibited only single reflection in the low 20 range. These diffractograms are typical for the mesopore structure with wormhole motifs; the reflection can be interpreted as an indication of the distance between nearest neighbours, rather than as distance between lattice planes. Adsorption isotherms on samples prepared at the temperature 80 "C are shown in Fig. 1 A. It is obvious that the structure of OMSs, which were prepared with Brij 56 or Pluronic P 123, is not substantially influenced by the type of surfactant. On the other hand, the structure of OMS is influenced by the specific shape of the Tween 60 surfactant molecule with a short hydrophobic chain compared with a large hydrophilic head made of three free poly(ethylene Table 1 Synthesis conditions and material parameters Sample
856/35 856/50 856/65 856/80 P123/80 Tw60/80
Surfactant
Brij 56 Brij 56 Brij 56 Brij 56 Pluronic P 123 Tween 60
Synthesis temperature (^C)
Sm-.i
^Ml-SO
^Mi':sc)
(m'g')
(cm^g-')
(nm)
35 50 65 80 80 80
546.4 485.4 477.4 439.6 559.7 415.4
0.584 0.777 0.957 1.062 1.099 1.351
4.1 6.5 9.5 12.2 6.4 7.1; 24
The mesopore volume ^^MESO and mean mesopore diameter DMESO were obtained from the desorption branch of the nitrogen isotherm using the BJH method.
187
-|
0.0
0,2
1
1
T"
0,4
0,6
0,8
1,0
PlPo
Fig. 1. (A) Adsorption isotherms of nitrogen at 77 K on samples PI 23/80 (a), B56/80 (b) and Tw60/80 (c). (B) BJH pore size distribution of samples B56/35 (a), B56/50 (b), B56/65 (c) and B56/80 (d). oxide) chains and one linking the ring to the hydrophobic tail leads to a distinctly differing product. The size of mesopores can be tailored by the choice of the synthesis temperature. Increasing step-by-step the synthesis temperature over the range of 35 - 80 "C leads to a substantial increase in the mesopore size and volume as illustrated by the samples prepared withBrij 56 (Figure IB). The texture parameters (Table 1) were evaluated from adsorption data using the BET and BJH methods (Table 1). They confirm the influence of the surfactant nature and synthesis temperature on the mesopore size and volume. The maximum in the pore size distribution of the sample Tw60/80 at 7.1 nm and the shoulder at 24 nm illustrate the specific role of the Tween 60 surfactant as a structure directing agent. The analysis of adsorption isotherms performed by means of the comparison plot method has shown that neither of the materials prepared contains micropores. The shape of adsorption isotherms and the absence of micropores represent a marked difference from analogous materials prepared in strong acid media [8,9], where the ethylene oxide (EO)n moieties of the surfactant associate with hydronium ions forming units, which can be designated as S^H^. Below the aqueous isoelectric point of silica, the assembly of OMS proceeds through an intermediate in the form (S"H^)(X 1^) where X" is an anion such as CI' in the HCI medium and r is a protonated Si-OH moiety [8]. However, the assembly of OMS in the alkaline media is based on another type of interaction between the nonionic surfactant and silicate species. OMSs are formed from silicate anions I" in a reaction pathway, which can be denoted as (S'^M^)r, wherein electrostatic forces are introduced into the assembly process through (EO)n complexation of small metal cations M"^ (such as Na^ cations for Na2Si03 used as the silica source) [10].
188
•
g
' (^^'; " ^ V '» ,•*-
Sjiiii
^
•
Fig. 2. Scanning electron micrographs of samples B56/50 (A), B56/80(B) and Tw60/80(C). Scanning electron micrographs of samples 356/50, 856/80 and Tw60/80 reveal that the nature of the surfactant strongly influences the assembly pathway. All the samples prepared with Brij 56 are characterized by spherical particles (Fig. 2A and 2B), which proves that the particles are liquid-like after their assembly and solidify only due to their subsequent aging. At lower synthesis temperatures the coalescing of particles can be observed (Fig. 2A). On the contrary, the sample Tw60/80 is characterized by large irregular particles, from which it follows that they grow as a solid phase from the very beginning. 4. SUMMARY The process described in this contribution provides a new insight into the synthesis of OMS in an alkaline medium using nonionic surfactants as structure-directing agents. The materials prepared exhibit structure features, which differ from silicas tcmplated by nonionic surfactants in an acid medium. ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft (WA 1116/7-1). REFERENCES 1. G. Schulz-Ekloff, J. Rathousky and A. Zukal, Microporous Mcsoporous Mater., 27 (1999) 273. 2. G. Schulz-Ekloff, J. Rathousky and A. Zukal, J. Inorg. Mater., 1 (1999) 97. 3. L. Sierra, B. Lopez, H. Gil and J.-L. Guth, Adv. Mater., 11 (1999) 307. 4. L. Sierra and J.-L. Guth, Microporous Mcsoporous Mater., 27 (1999) 243. 5. J.M. Kim and G.D. Stucky, Chem. Commun., 2000, 1159. 6. S.-S. Kim, T.R. Pauly and T.J. Pinnavaia, Chem. Commun., 2000, 1661. 7. C. Boissiere, A. Larbot and E. Prouzet, Chem. Mater., 12 (2000) 1937. 8. D. Zhao, Q. Huo, J. Feng, B.F. Chmelka and G.D. Stucky, J. Am. Chem. Soc, 120 (1998)6024. 9. M. Kruk, M. Jaroniec, C.H. Ko and R. Ryoo, Chem. Mater., 12 (2000) 1961. 10. S.A. Bagshaw, T. Kemmitt and N.B. Milestone, Microporous Mcsoporous Mater., 22 (1998)419.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
189
Synthesis of mesoporous silica particles prepared by using multiple emulsion Chul Oh, Jae-Hyung Park, Seung-il Shin and Seung-Geun Oh* Division of Chemical Engineering and Center for Ultramicrochemical Process System(CUPS), Hanyang University, 17 Haengdang-Dong, Seongdong-gu, Seoul, 133-791, Korea The spherical silica particles with meso- and macropores at the surface and inside of particles were prepared
in the hexane/water/n-decyl
alcohol
multiple-emulsion.
Also
micrometer-sized hollow silica particles could be prepared by controlling the viscosity of the aqueous phase in W / 0 emulsion with polyethylene glycol (PEG). The morphology of silica particles was influenced by the concentration of PEG, HPC polymer and the external oil phase (O2). 1. INTRODUCTION The preparations of mono-dispersed silica particles have attracted more and more attention in the recent years because of their wide technological applications [1]. Specially, hollow silica particles are widely applied to drug delivery system (DDS), catalysis, composite materials, and protecting sensitive agents, etc., owing to their low density [2]. Generally, for the preparation of mono-dispersed silica particles from aqueous solution of silicon alkoxides, the silica particles are formed by two-step of hydrolysis and condensation [3]. Emulsions are dispersions of two immiscible fluids such as water and oil. Multipleemulsion may have either of the water-in-oil-in water type (W/O/W) or of the oil-in-water-inoil type (0/W/O) [4]. 0 / W / O multiple-emulsion is a good reaction medium for the preparation of particles which have meso- and macropores. In this study, we prepared the silica particles with meso- and macropores at the surface or inside of the particles and the hollow particles like pin-pong ball shape. As changing the reaction environments, the variation of size and morphology of particles were investigated by field emission scanning electron microscopy (FE-SEM).
* Corresponding author : e-mail (seongoh@hanyang.ac.kr)
190
2. EXPERIMENTAL 2.1. Materials Tetraethyl orthosilicate (TEOS, 98%) as a silica source, hydroxylpropyl cellulose (HPC) as a stabilizer, n-hexane (99%+) as an oil phase (Oi) and Tween 20 as high HLB surfactant were purchased from Aldrich Chemical Company. N-decyl alcohol (minimum 98%) as an external oil phase (O2), Span 80 as low HLB surfactant and polyvinyl-pyrrolidone (PVP) were purchased from Sigma Chemical Company. 1-Octanol as an oil phase (O2) and polyethylene glycol (PEG) as a water stabilizer were obtained from Junsei Chemical Company. Also, ammonium hydroxide as a catalyst and ethanol as a washing reagent were purchased from Acros and Teamin Chemical Company. All chemicals were used as received without further purification. The water was deionized by Milli-Q Plus system (Millipore, France). 2.2. Preparation methods and characterization A multiple-emulsion preparation procedure can be described as followings: In the first step, Oi/W emulsion was prepared by dispersing n-hexane in an aqueous solution containing Tween 20. After stirring with magnetic stirrer, NH4OH and PEG were added to the water phase of Oi/W emulsion. To make an external oil phase (O2), HPC and Span 80 was solubilized in n-decyl alcohol at 50°C. In the second step, Oi/W emulsion solution of 10 wt% was added to an external oil phase. And then we mixed the multiple emulsion using the magnetic stirrer for Ih at 40°C. In order to prepare spherical silica particles in O1/W/O2 multiple emulsion, TEOS was added into the O2 phase. After reaction for an appropriate time, the samples were centrifuged at 2,500 rpm for 10 minutes to obtain the silica particles. The obtained particles were washed with ethanol 2 times. In the case of preparation of hollow silica particles, n-hexane in the water phase was excepted. So, the multiple-emulsion was changed to W/0 emulsion. FE-SEM was used to investigate the morphology of silica particles. 3. RESULTS AND DISCUSSION 3.L Preparation of silica particles with dimple structure by using multiple emulsion Though general emulsion techniques such as microemulsion method were utilized to obtain spherical silica particles, it was difficult to prepare the particles which had various-sized pores at the surface and inside of particles. For this reason, the multiple-emulsion technique was applied to this system. In the previous work, spherical silica particles with meso- and macropores inside were prepared by using the o/w/o multiple-emulsion as reaction media [5]. In order to increase the stability of multiple-emulsion and control the distribution of macropores in silica particles, HPC and PEG were employed. HPC played the key role in
191
growing the primary particles into the spherical silica particles in the range of l-3|im through the aggregation, while PEG affects the morphology of surface of spherical silica particles. Without HPC, the primary particles ranging 30-40nm didn't have the spherical shape and resulted in the flat form with irregular pores. When HPC concentration increase from 0.5wt% to 0.7wt%, the spherical particles were obtained and the size were tailored from 5|im to Ijim. When both HPC and PEG were added into the multiple-emulsion, the spherical silica particles with meso- and macropores at the surface and inside of particles were formed. As shown in figure 1, under the condition of Rw=4, 2wt% PEG, and 0.7wt% HPC, the surface structure like the dimpled surface of golf ball was observed very well and the particle size was more or less larger than the other samples. As PEG concentration increased to 6wt%, dimple structure was exchanged to the structure with pores inside of particles. (Figure 2)
^Sr •1,1
.1 I
r
.',^^'•
U\.UC
-.rn
..VV,
. rr
Fig. 1. SEM micrograph of silica particles with dimpled surface
If
^
i
Fig. 2. SEM micrograph of silica particles with many pores inside
3.2. Synthesis of hollow silica particles in W/O emulsion with PEG and HPC To prepare hollow silica particles, water/1-octanol (w/o) emulsion with PEG and HPC was used as reaction matrix. The preparation of hollow silica particles is based on the hydrogen bonding of water-PEG interaction and viscosity effect in the aquous phase. According to the reaction conditions, hollow and micrometer-sized spherical silica particles were obtained. As shown in figure 3, the sample prepared under conditions of 2wt% PEG didn't have the hollow structure and the size distribution was more or less broad. Though hollow silica particles didn't exist in the samples, the density of the aggregation among primary particles in a particle is lower than other samples prepared by emulsion method without PEG polymer. When PEG concentration was changed from 2wt% to up to 6wt%, the hollow silica particles as shown in figure 4 were obtained. The size of hollow silica particles was nearly same as that of the particles in figure 3. The shell thickness ranging from 200 to 500nm could be observed through the magnifying SEM image of the hollow silica particles. This variation of shell
192
width resulted from the diversity of concentration of PEG polymer which exist in each water droplets. Because of influence of HPC and PEG polymers, many mesopores exist at the surface of hollow silica particles.
H
1,
Fig. 3. SEM micrograph of silica particles with dense structures
..^.^.^
^ir,
iir
Fig. 4. SEM micrgraph of silica particles with hollow structures
ACKNOWLEDGEMENT This work was supported in part by Center for Ultramicrochemical Process System (CUPS). REFERENCES 1. F. Garbassi, L. Balducci, and R. Undrarelli, J. Non-Cryst. Solids, 223 (1998) 190. 2. F. Caruso, R.A. Caruso, and H. Mohwald, Science, 282 (1998) 1111. 3. W. Stober, A. Fink, and E. Bohn, J. Colloid Interface Sci., 26 (1968) 62. 4. J. Sjoblom (eds.). Encyclopedic Handbook of Emulsion Technology, Marcel Dekker, New York (2001) 5. M.H Lee, S.G. Oh, S.K. Moon, and S.Y. Bae, J. Colloid Interface Sci., 240 (2001) 83.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
193
Preparation and Characterization of Mesoporous Silica Spheres by Polymerization Induced Colloid Aggregation Method C. I. Lee'\ S. W. Lee', Y. Lee', Y. H. Chang'and Y M. Hahm'* 'Department of Chemical Engineering, Dankook Univ., Seoul, 140-714, Korea. ^Department of Chemical Engineering, Inha Univ., Inchon, 402-751, Korea. Mesoporous silica spheres having a diameter in the 3 to 15 micron range were produced by polymerization induced colloid aggregation method. Uniform sized silica spheres with a narrow pore size distribution were controlled by reaction conditions such as composition of polymeric materials, concentration of colloidal silica, temperature of heat treatment and amount of acidic catalysts. 1. INTRODUCTION Mesoporous silica spheres have been used as versatile catalysts, catalyst supports, packing materials for normal phase chromatography, etc 11-2]. It is now apparent that the morpiiological control as well as handling and texture oi^ mesoporous silica is extremely important for these applications. Mesoporous silica spheres with a narrow pore size distribution are expected to use as a packing material in chromatography or an easy-to-handle from orMCM-41 for catalytic purpose. Various techniques for synthesis of spherical silica developed such as oil emulsion, spray drying, sol-gel, etc. Ihese methods have produced a polydisperse collection of spheres ranging from 0.5 to 500j.mi in diameter. Another method for preparation of spherical porous particles was referred to polymerization induced colloid aggregation (PICA) because polymer growth occurs along with colloid aggregation. In this method, acid-catalyzed polymerization take place and the oligomer so formed adsorbs onto the surface of the colloid particles causing them to aggregates. It may proceed by the formation of polymer linkage between colloids | 3 | . Both inorganic and organic acid as acidic catalysts used for control of surface characteristics of silica spheres |4]. Here, we report the variations of particle size and shape, structural characteristics of pores according to reaction conditions such as composition of polymeric materials, concentration of colloidal silica, temperature of heat treatment and amount of acidic catalysts.
194
2. EXPERIMENTAL SECTION The preparation of the mesoporous siHca spheres was performed under acidic condition using polymerization induced colloid aggregation. Colloidal silica containing 40 wt.% of Si02 was diluted with ultra pure water and the pH adjusted to 1-3 by adding concentrated hydrochloric acid (HCl) or various organic acid while stirring rapidly. Urea and 35 wt.% formaldehyde were added and stirred until dissolved. pH of mixture was again adjusted to 1-3. Within a few minute, the mixture had turned white and opaque, due to the formation of spherical particles of a complex of silica and polymer. After the reaction, the clear aqueous supernatant liquid was discarded and remained white cake washed with water. The washed product, in the form of wet settled cake, was dried in vacuum at 60 "C for 24 hours. The vacuum dried material was then heated in tube furnace at 400-1000 °C in air atmosphere, rising the temperature slowly, to burn off organic material. Scanning electron microscope (SHM) photographs were obtained with a Jeol JSM5800, FT-IR spectra were obtained with a shimadzu DR-8011, N2 adsorption measurements were performed at 77K using a Micromeritics ASAP 2000 analyzer utilizing Brunauer-Hmmett-Teller (BHT) calculations of surface area and Barrett-Joynerllalenda (BJII) calculations of pore volume and pore size distributions. Ihermogravimetric analysis (TCiA) was performed on a lA instruments 2100 analyzer with temperature rate of lOTVmin in air. 3. RESULTS AND DISCUSSION Micromcler-sized mesoporous silica spheres can be synthesized in present of a mixture oi^ polymer and colloidal silica by PICA method, figure 1 shows the PT-IR spectra of urea-formaldehyde resin (UP resin) and synthesized spherical silica. UI* Resin has three strong absorption peak at 1670, 3300 and 1200 cm" , which are assigned to C=(), N-Il and C-N bond. After heat treatment of synthesized silica sphere, shows three inherent ab.sorption peak of amorphous silica at 480. 800, 1 100 cm"'.
\ CD 0 C 03
E
CO
c
2 h=
\
\y
/•~"'
-—--. ""^-v
V
\ f'
.
\j\.J
\j
-^
V
v^_^.^"""
" ' •
1
3000
~-
/
^\
'
'J
V
• - > _
••'
i
\y
I
,
1
figure 1. I'l-IR spectra of UP resin and synthesized silica sphere. (a) UP resin, (b) UP resin & silica composite before heat treatment, (c) pure silica after heat treatment
195
Thermogravimetric analysis (TGA) of UF^ resin, silica composited with polymer and heat treated silica provides information about the weight loss steps corresponding to physically adsorbed water, amounts of polymeric material, weight percentage of silica (see Figure 2). As can be seen from this figure the UF resin removed at 600 °C completely. Hence, heat treatment temperature fixed 600 °C, or higher.
1
UI' Resin
j
Ul- R c s i n - S i l i c a j Silica
Figure 2. TGA curves of UF silica, and their composite. 0
200
400
600
800
resm.
1000
rcnipciaturc | V \
SEM images (Figure 3, 4) of the silica spheres show their impacts to sphere shape with relatively uniformed micrometer-sized. The effect of molar ratio (silica/polymeric materials) was insignificant (see Figure 3). The size of silica spheres is depended on both primary particle size of colloidal silica and pi I of solution (see I^igure 4). With increase of the pi I of solution from 0.7 to 2.8, the average particle size of mesoporous silica spheres increases from 4 to 10 |.un.
Figure 3. SEM photographs of mesoporous spherical silica with molar ratio, polymeric material/silica, (a) 0.5 (b) 1.0 (c) 2.0 (d) 3.0
Figure 4. SEM photographs of mesoporous spherical silica with pH of solution, (a) 0.7(b) 1.5(c) 2.0(d) 2.8
196
Figure 5 shows nitrogen adsorption/desorption isotherm and pore size distribution in surface of spherical silica prepared with inorganic acid (HCl) or organic acid (L-tartaric acid) as acidic catalyst. In case of using organic acid, specific surface area, pore size and pore volume increased. Pore diameter and pore volume could be adjusted by addition of organic acid and heat treatment conditions.
—•— —I'— —T— —V—
Inorganic Acid (ads ) Inorganic Acid (dcs.) Organic Acid (ads ) Organic Acid (dcs.)
,
(a) E
Ji^p^
200
ij\ iji^^
jj;3[^.^ ^ ^>->^-^-^ ^'-^ 0.2
0.3
0.4
0.5
0.6
0.7
Relative Pressure |l*/Po|
0.8
0.9
1.0
KM) Pore D i a m e t e r j A |
Figure 5. BET analysis for silica sphere prepared with inorganic/organic acid (a) nitrogen adsorption/desorption isotherm, (b) pore size distribution. 4. CONCLUSION Polymerization induced colloid aggregation method has been introduced to control the morphology and the porosity of micrometer size silica spheres. Urea and formaldehyde were used as polymeric materials, colloidal silica as a silica source, organic and inorganic acid as a catalyst. The particle size could be adjusted in range of 3-15 \xm by pH of solution, amount of colloidal silica and stirring conditions. Pore diameter and pore volume could be adjusted by addition of organic acid and heat treatment conditions. These spherical particles are used as packing materials for various separation techniques such as High Performance Liquid Chromatography (IIPLC) or as supports for catalysts.
REFERENCES 1. R. K. Her, The Chemistry of Silica, John Wiley & Sons, New York, 1979. 2. Qian Luo, et al.. Studies in Surface Sci. Catalysis, 129 (2000) 37. 3. U. Trudinger, et al., Chromatogr., 535 (1990) 111. 4. H. Izutsu, et al., J. Mater. Chcm., 7 (1997) 1519.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
197
Preparation of mesoporous solids by agglomeration of silica nanospheres Yuri K. Ferreira, Martin Wallau and Ernesto A. Urquieta-Gonzalez*' ^ Departamento de Engenharia Quimica, Universidade Federal de Sao Carlos, Caixa Postal 676, 13565-905 Sao Carlos - SP, Brasil Silica nanospheres were synthesized with diameters in the range from 40 to 170 nm. The size and shape of the spheres were determined by Photon Correlation Spectroscopy (PCS) and Scanning Electron Microscopy (SEM). The nanospheres were agglomerated by centrifugation and solvent evaporation and characterised by SEM and nitrogen adsorption. Partly regular mesostructures could be observed by SEM. However, these structures easily crumbles when they are handle and therefore no surface area related to the formation of mesopores was detected by nitrogen sorption. 1. INTRODUCTION Ordered mesoporous materials have great potential for application as catalysts, adsorbents and as host material for the preparation of electronic and optical devices. Besides mesoporous materials derived from the M41S family, recently porous materials using monodispersed nanospheres as cast have attracted much interest [1]. Silica nanoshperes firstly described by Stober et al. [2] are obtained by ammonia catalysed hydrolysis of tetraethyl orthosilicate (TEOS) in ethanol and can be modified by lining with functionalised organic molecules [3]. Silica nanospheres form after agglomeration mesoporous solids with pore diameters varying approximately between 0.15 to 0.5 times of the particle diameter, depending of the structural arrangement of the nanospheres. Transformation of the sphere surface into zeolitic structures permits the preparation of hierarchical porous materials [4]. Here we describe the synthesis of silica nanospheres and their agglomeration into mesoporous structures. 2. EXPERIMENTAL A 3^ factorial assay of the experiments were used to study the factors influencing the particle diameter and its standard deviation. While the TEOS concentration (0.14 mol/L) and the reaction time (24 h) were kept constant, the reaction temperature and the concentration of ammonia and water were varied as it is indicated in Fig. 1. The samples were denominated in the form E([NH3]/[H20])[T], with the ammonia concentration [NH3] and the water concentration [H2O] given in (mol/L) and the reaction temperature [T] in (°C). The sphere diameter and its standard deviation were determined by Photon Correlation Spectroscopy (PCS). Selected samples were further characterised by Scanning Electron Microscopy (SEM). Agglomerates were prepared by evaporation of the supernatant alcoholic solution at room temperature or by centrifugation (1000 - 3000 rpm). The obtained solids were subsequently dried in an exsiccator and characterised by SEM and nitrogen sorption (BET) corresponding author: FAX: +55-16-260-8266. E-mail: urquieta@power.ufscar.br ^ Acknowledgements are given to CNPq Brazil for the financial support (proc. 461444/00-3 and 300373/01-5) and to Professor Fernando Galembeck, IQ/Unicamp-Brazil for the opportunity to realise the PCS measurement.
198
3. RESULTS AND DISCUSSION The dependency of the nanosphere diameter (0sph.) from the reaction conditions is demonstrated in Fig. 1. It can be seen that, in general, the diameter increases with increasing water [H2O] or ammonia concentration [NH3] but decreases with increasing temperature [T]. The influence of the parameters can be described by the equation 1.
• 20**c D 40 *C H 60**C
[NH3] (mol/L)
IH201 (mol/L)
Fig. 1. Influence of the reaction parameters on the sphere diameter determined by PCS. 0,ph.(nm) = 17.31 + 235.17 [NH3] + 16.63 [H2O] -0.78 [T] (1) The influence of the reaction parameters on the sphere diameter might be explained considering that the nanospheres formation comprises hydrolysis of TEOS, nucleation and particle growth. It was observed that the TEOS hydrolysis is the rate limiting step in the formation of the nanospheres and that particle growth proceeds mainly via addition of monomers and small oligomers on particle nuclei [5]. However, as suggested by van Blaaderen et al., in the early stage of the particle formation, aggregation of nanometer sized sub-particles also occurs [5]. At low NH3 concentration, small particles are stabilised and the aggregation of sub-particles decreases. Therefore, a larger number of particle nuclei leading to smaller sized spheres is present at low NH3 concentration. It was found by Weres et al. [6], that the critical nuclei size is inverse proportional to the reaction temperature Therefore increasing temperature further stabilises small nuclei, resulting also in small particles. ->' In general it could be observed that larger spheres are more uniform. The uniformity of the nanospheres is r\ demonstrated in the SEM micrograph of a typical sample shown in Fig. 2. Using the standard deviation (00) of the mean diameter, as a measure of the i uniformity, the influence of the reaction parameters may be described by equation 2.
if
It can be seen from Fig. 3, that evaporation of the solvent leads to compact but irregular agglomerates of the spheres.
^1
Fig. 2. SEM micrograph of E(0.2/6.())2() (scale bar = 300 nm; 0 = 160.9 nm).
00 (%) = 17.13 - 38.18 [NH3] - 6.95 [H2O] +0.23 [T] + 2.24 [NH3]^ + 0.77 [HzO]^ + + 0.0008 [T]^ + 7.13 [NH3][H20] - 0 . 3 3 [NH3][H20] - 0.33 [NH3KT] - 0 . 0 3 [H20][T]
(2)
199
On the other hand, as it can be seen from the micrographs shown in Fig. 4, the agglomerates obtained by centrifugation are more ordered. These micrographs reveal further that the • uniformity of the agglomerates increases with increasing the rotation frequency. The agglomeration of the nanospheres is influenced by directed gravitational sedimentation and undirected dislocation due to Brownian motion. It can be calculated for silica spheres with diameter in the range of pig. 3. E(0.3/4.0)40 ( 0 = 131.8 nm) 170 to 130 nm in ethanol at 20 °C, that agglomerated by evaporation (bar = 1 pm). the displacement of the spheres due to the Brownian motion is around 110 to 220 times higher than the displacement caused by the gravitational sedimentation. For nanospheres of sample E(0.2/6.0)20, with diameters around 160 nm, the ratio Brownian dislocation to sedimentation dislocation (B/S) is decreased to 21 and 3 when they are centrifuged at 1000 and 3000 rpm, respectively. This explains why the agglomerate shown in Fig. 4b is more ordered. The predominance of the Brownian motion for small particles ( 0 « 40 nm) explains why these particles could not agglomerated, even at 4500 rpm where B/S is still around 50. (a)
^^^
(b)
Fig. 4. E(0.2/6.0)20 ( 0 = 160.9 nm) agglomerated (a) 1000 rpm, (b) 3000 rpm (bar = 300 nm). Although the agglomerates prepared by centrifugation show a higher uniformity than that prepared by evaporation of the solvent, they are brittle and therefore difficult to handle. This fragility might be the reason for the unexpected adsorption behaviour. Although that one would expect from the dense arrangement of the spheres shown in Fig. 4b, that the agglomerate could possess mesopores in the range of 24 to 66 nm (0.15 - 0.41 x 0sphcrc)^ it shows an adsorption isotherm, given in Fig. 5, classified as type II, which is typical for nonor macroporous materials. This could have been caused by disintegration of the agglomerate during the sample preparation. As for different packing structures of spheres with a density of 2.17 g/cm"^, pore volumes in the range of 0.2 to 0.9 cmVg are expected, the disintegration of the agglomerate structure (shown in Fig. 4) is strengthened by the low observed pore volume of 0.039 and 0.088 cm^g for the samples agglomerated at 1000 and 3000 rpm, respectively.
200
The observed BET surface area of 20.4 and 21.2 m^/g for these agglomerates is higher than the expected specific surface area of 17.2 m^/g, calculated for spheres with diameters of 160.9 nm and density of 2.17 g/cm"'. The observed specific surface area would correspond to a density of 1.87 g/cm"'. Giesche [7] observed by helium pycnometry also densities lower than 2.0 g/cm^ for silica nanospheres prepared by the Stober method [2] and found that the density of the particles increases to values typical for amorphous silica (~ 2.2 g/cm"') after calcination at temperatures above 800 °C. Therefore he concluded that these nanospheres, prior to calcinations, still contain difficulty accessible micropores with pore diameters around 0.3 nm [7] which are not accessible for nitrogen molecules (kinetic diameter = 0.36 nm).
k
loose 96
g94t
8 92+ n
S
90+
esj eel 0,4
0,6
1,0
p/R
Fig. 5. Isotherm of N2 sorption on sample E(0.2/6.0)20 agglomerated at 3000 rpm.
200
400
600
800
Temperature [°C]
Fig. 6. Thermogravimetry of sample E(0.2/6.0)20 agglomerated at 30(X) rpm.
A typical example of thermoanalysis of the agglomerates is shown in Fig. 6. It can be observed that the weight loss of the nanospheres occurs in two steps. Desorption of physically adsorbed water (9.2 %) until approximately 190 **C and desorption of ammonia accompanied by dehydroxylation of the surface hydroxyl groups (4.0 %) between 250 and 700 °C, as it is schematised in equation 3. -0-Si
—^-* 2NH, + 2HO - Si -
(3) -^INH.+H^O + Si- O-Si The number of surface (Si-O')-groups (77 jxmol/m^), estimated from equation 3 and the observed weight loss, is much higher than that reported [8] for commercial silica (7.0 - 9.5 Hmol/m^). This do not only indicate the presence of hydroxyl groups in micropores, which cannot be detected by nitrogen adsorption, but also that no or only a small number of covalent Si-O-Si bonds exist between the nanospheres agglomerated by centrifugation. Then they must be connected only by H-bridging bonds, thus explaining their low mechanical stability. 2NH;
REFERENCES 1. A. Stein, Microporous Mesoporous Mater., 44-45 (2001) 227. 2. W.Stober, A. Fink and E. Bohn, J. Colloid Interface Sci., 26 (1968) 62. 3. C. Beck, W. Hartl and R. Hempelmann, Angew. Chem., I l l (1999) 1380. 4. M.W. Anderson, S.M. Holmes, N. Hanif and C.S. Cundy, Angew. Chem., 112 (2000) 2819. 5. A. van Blaaderen, J. Van Geest and A. Vrij, J. Colloid Interface Sci., 154 (1992) 481. 6. O. Werres, A. Yee and L. Tsao, J. Colloid Interface Sci., 84 (1981) 379. 7. H. Giesche, J. Eur. Ceram. Soc, 14 (1994) 189. 8. K. Unger, Angew. Chem. Int. Ed. Engl., 11 (1972) 267.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved
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Ordered mesostnictured materials with composite walls of decavanadate and silica Yoon-Young Chang, Young Kyu Hwang, Hyuk Choi, and Young-Uk Kwon* Department of Chemistry and BK-21 School of Molecular Science, Sungkyunkwan University, Suwon 440-740, Korea. FAX: +82-31-290-7075 E-mail: ywkwon@chem.skku.ac.kr Ordered mesostructured materials composed of decavanadate-silica composite wall materials were synthesized from the reactions of decavanadate-silica composite sol solution with a structure-directing agent MTAB (CH3(CH2)i3N(CH3)3Br). The sol solution was prepared by inducing hydrolysis of silica in the presence of decavanadate ions. Controlled reactions of the process parameters such as composition, aging time of the sol solution, and pH produced mesostructured materials with the ID hexagonal symmetry of a = 4.32nm. While thermal calcination even at low temperatures destroyed the mesostructure, photocalcination of these materials provides a viable means to generate mesoporous materials with vanadia-silica composite wall. 1. INTRODUCTION There have been many attempts to modify the walls of mesoporous silica materials with metal ions for catalytic purposes. In most of the cases, the metal ions are impregnated onto the walls of preformed mesoporous silica. We have recently synthesized a composite material in which polyoxometalate ions are encapsulated by silica layers to form nanoscopic sol particles,
-h
Scheml. Reaction scheme of the formation of a mesostructure with POM-silica composite wall materials.
202
which may be a useful precursor to synthesize mesostructured materials. In this study, we have explored this possibility with decavanadate ions stabilized by silica layers. 2. EXPERIMENTAL The vanadia-silica mesostructured materials were synthesized by using MTAB, sodium metavanadate and sodium silicate solution. NaVOs was dissolved in distilled-water and 2M HCl was added until pH 4.5 to prepare a decavanadate solution. A sodium silicate solution was added to the decavanadate solution. After the vanadia-silicate solution was stirred for Ih at room temperature, a 6wt% MTAB solution was added. A yellow precipitates formed immediately. 2M HCl solution was added to adjust the pH to some designated values shown in Table 1. The resultant mixture was rapidly stirred at room temperature for 12hr. The yellow precipitates were aged for 2-3 days at 80°C in an oven. The precipitate was filtered, washed, and vacuum dried. The surfactant molecules of assynthesized vanadia-silica materials were removed either by UV irradiation or thermal calcination. The mesophases are characterized by powder X-ray diffraction (RIGAKU D/max-RC) and transmission electron microscopy (JEOL-3011, 300kV). 3. RESULTS AND DLSCUSSION For the successful synthesis of pure mesostructured materials with vanadia-silica composite wall, the important parameters were aging time, stirring time, p\\ and surfactant/inorganic precursor ratio. Fig. 1 shows the progression of the mesostructure formation from a pH = 4.5 reaction as a function of aging time. Just before the aging, there arc unreactcd crystalline MTAB and a small amount of what appears to have the desired hexagonal mesostructure. After 1
2
3
4
2G(degree)
5
Fig. 1. XRD patterns of as-synthcsizcd materials with various aging time, (a) MTAB only (b) before aging (c) aging 8hr (d) aging 48hr
aging for 8hr, there grew a sharp peak at 20 ^ 3° that may be assigned to a mesostructure composed of decavanadate and MTA ions in addition to the hexagonal mesostructure and the MTAB. It takes over 48 hr of aging to form a pure hexagonal mesostructure in this system.
203
Tablel Peak indices, peak positions for as-synthesized materials with various pH conditions pH
4.5
5.5
6
8
(100)
38.37
36.78
36.48
37.40 23.35
(110)
22.18
21.25
20.96
(200)
19.11
18.39
18.32
21.43
2e(degree) Fig. 2. XRD patterns of as-synthesized materials with various pH conditions
As shown in Figure 2, characteristic peaks for a hexagonal mesostructure were observed in materials synthesized below pH 7, but a mixed phase was observed at the higher pH. We have tried to remove the surfactant to obtain mesoporous materials. However, unfortunately the structure collapsed under thermal calcination at 200°C, probably because of the low thermal stability. On the other hand, the hexagonal mesostructure appears to be intact under photocalcination condition as evidenced by the TEM image (Fig.3) and XRD pattern (Fig.4). However, the IR spectrum shows that there is some residual organic materials C-H stretching peak (2850~3000cm"^) remaining even after photocalcination.
(100)
d CO
(110) (200) 'yv___vN...^___,_^
• • - *
CO
cCD
(a)
c
(b) 1
.
1 ^
^$§:
^
2 e ( d e g r e e ) Fig. 3. XRD patterns of a) as-synthesized hexagonal materials and b) after photocalcination
Fig. 4. TEM image photocalcination
after
204
4. CONCLUSIONS We have synthesized mesostnictured materials with vanadate-silica composite walls by using decavanadate ions. The decavanadate ions are encapsulated by silica layers and are kinetically stabilized at all pH range we have studied. Although we have failed in obtaining pure mesoporous materials from this approach primarily because of the instability of the composite wall material, this approach may be utilized in synthesizing mesostructured materials with composite walls of various polyoxometallate ions. REFERENCES 1. Hyuk Choi, Young-Uk Kwon, and Oc Hee Han, Chem. Mater., 1999, 11, 1641. 2. Matthew T. J. Keene, Renaud Denoyel, Philip L. Llewellyn, Chem. Commun., 1998,11, 2203. 3. Theotis Clark, Jr., Julia D. Ruiz, Hongyou Fan, C. Jeffrey Brinker, Basil I. Swanson, and Atul N. Parikh, Chem. Mater., 2000,12, 3879. 4. Abdelhamid Sayari, Ping Liu, Microporous Materials 1997, 12, 147. 5. Victor Luca and James M. Hook, Chem. Mater., 1997, 9, 2731.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
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Nanoporous alumina formation using multi-step anodization and cathodic electrodeposition of metal oxides on its structure Jaeho Oh, Youngwoo Jung, Jaeyoung Lee^, and Yongsug Tak Department of Chemical Engineering, Inha University, Incheon 402-751, Korea. ^Water Protection Research Team, Research Institute of Industrial Science & Technology, Pohang 790-330, Korea Highly ordered nanoporous alumina structure was fabricated with a solution pretreatment method and a time-efficient anodization process and, this structure was tested as a template for the electrochemical synthesis of metal oxides, CU2O and ZnO. Cathodic electrodeposition of oxides inside pores was retarded because of the variation of pore structure caused by the pH increase, but nano-thin oxide films was formed at pore mouth. 1. INTRODUCTION Template synthesis method has been widely used for the fabrication of nano-size materials and a highly ordered nano-porous alumina has been actively applied as a template structure*. Partly hydrated alumiunm oxide, which consists of thin barrier layer and porous overlayer, is prepared by anodization in an acid electrolyte. Thickness of two-layer depends on the cell voltage applied and the regularity of pore distribution depends on the anodization time. Masuda et. al. developed a repetitive formation and dissolution of oxide layers to obtain the honeycomb structures of anodic alumina. Longer first anodization time at 40 V is requisite for the ideally arranged structure^. This nanopore structure has been used as a template for the fabrication of ceramic nanowires, LiCo02^ and LiCoo.5Mno.5O2'*, using sol-gel process. Electrochemical deposition of metal oxides has advantages over conventional methods due to higher deposition rate, precise morphology control, and low synthesis temperature^. ZnO and CU2O are potential materials for application in solar cells due to semiconducting behaviors and can be electrodeposited by the control of surface pH. In this work, different pretreatment and anodization steps were utilized to control the nanopore array structure of aluminum oxide. This structure was utilized for the electrochemical preparation of nanostructural metal oxides. This work was supported by the Korea Science & Engineering Foundation under a grant from the Engineering Research Center for Energy Conversion and Storage.
206
2. EXPERIMENTAL A high purity aluminum plate (Aldrich 99.999%) was employed in this experiment. Prior to anodization, aluminum metal was electropolished and pretreated with IM NaOH solution. Specimen was embedded in a Teflon holder with exposed surface area of 1.18 cm^ and electrolyte was rigorously stirred and maintained at S'C during anodization. Basically, two-step electrochemical anodization process is used for obtaining a porous alumina structure. Copper and zinc nitrate solutions were used for the formation of metal oxides by applying a direct cathodic current. Surface morphologies of the anodic alumina and metal oxides were investigated with a scanning electron microscope (SEM, Hitachi s-4300). 3. RESULTS AND DISCUSSION Prior to anodization, aluminum metal was electropolished to remove an air-formed oxide and smooth the surface. Its surface roughness was estimated to be 0.7335 nm over 3 |im^. Figure 1(a) shows the irregularly ordered pore arrangement after anodization, however, the ordered region exists at the bottom of pores.^ When the irregular porous alumina was removed, the concave pore bottom remails and this textured structure works as pore initiation sites in the following anodization steps. Similar concave structure is found during uniform aluminum dissolution in an alkaline solution because of the hydrogen gas evolution. In this work, electropolished surface was chemically pretreated with NaOH solution and it results in the regularity of pore distribution after the first anodization. Figure 1(b) show the regularly spaced hexagonal pore structure after 12 hrs of second anodization. Cross-section views of specimen indicate the straight and parallel pores of which has a high aspect ratio over 1,000, as shown in Fig. 1(c). It has been known that pore structures, interpore distance and barrier layer thickness, are dependent on the applied cell potential and electrolyte composition. In oxalic acid solution, long-range ordering takes place at 40 V of anodization,^ which requires a time-consuming process. At less anodization voltage, the bottom of pores is less ordered and it results in less ordered pore distribution. Three-step anodization process including the second removal of oxides gives a better pore distribution and a reduced anodization time, and it provides the control method of pore length by simply adjusting anodization time without sacrificing ordered pore distribution. Figure 2 shows the cross-section views of 200 nm-long pores, formed after 30 sec of the third anodization. Ordered pore structure was used as a template for the formation of semiconducting oxides, CU2O and ZnO. Figure 3 shows a linear sweep voltammogram in zinc nitrate solution after the removal of barrier film at the pore bottom.
207
Fig. 1. Porous structure of alumina prepared by anodization; (a) first anodization, (b) second anodization, and (c) cross section view of straight proes. 0
E
-5 •10 -15
O 0 55158
i0.0kv
X60 . ek'•'ae'erirn
Fig. 2. Porous alumina prepared 30 sec of anodization time in three-step process.
-20
:
- J
:
r^
^
-J 1
-2.5
.
1
-2.0
.
1
-1.5
.
1
-1.0
.
1
-0.5
.
1
1
0.0
Potential ( V vs. SCE ) Fig. 3. Linear sweep voltammogram in obtained in zinc nitrate solution with porous alumina electrode.
Obtained potential-current curve shape is similar to a reported polarization curve measured on a flat electrode but the potential is shifted to cathodic direction.^ Its behavior can be ascribed to the existence of thick porous alumina. Current plateau around -1.6 V indicates the formation of passivating layers or mass transfer limited current and electrodeposition of ZnO was performed at this plateau potential. Figure 4(a) shows a tortuous pore structure, compared to Figure 2. Cathodic reaction takes place at the pore bottom and it is considered to be in a neutral solution. 2H20 + 2e' -> H2 + 2OH" As the result, pH of the electrode surface increases and it makes straight pore into tortuous by dissolving alumina. On the other hand, Figure 4(b) shows that pore mouth was covered with a transparent nano-thin ZnO film. It suggests that the dissolution of alumina occurs ahead of the precipitation of ZnO. Similar phenomena were observed during cathodic deposition of CU2O and Figure 5 shows the twisted pore structure. When a buffering agent, diammonium hydrogen citrate, is added during electrodeposition, destruction of pore wall is diminished. Crystalline CU2O is only nucleated around pore mouth.
208
nucleated around pore mouth. (a) ,vy'>^>r- .
.<
(b)
Tr. f-^^"*ij,iS\ '^^ai tU
Fig. 4. (a) Surface and (b) cross-section view of cathodically electrodeposited ZnO.
Fig. 5. Cross-section view of cathodically electrodeposited CU2O. 4. CONCLUSIONS This work includes the preparation of porous alumina and cathodic deposition of metal oxide on its surface. Ordered nanoporous alumina was manufactured with a controlled three-step anodization of which provides a time-efficient process. Electrochemical deposition of metal oxide was tested with porous alumina electrode. However, pH rise inside pores favors the dissolution of pore wall rather than the precipitation of oxides inside pores. A transparent nano-thin ZnO film is prepared on the surface and the nucleation of CU2O is restricted to pore mouth. REFERENCES 1. Z. Wang, Y.-K. Su, H.-L. Li, Applied Physics A Material Science and Processing, 74 (2002) 563. 2. Hideki Masuda, Kenji Fukuda, Science, 268 (1995) 1466. 3. Y. Zhou, C. Shen, H. Li, Solid State Ionics, 146 (2002) 81. 4. Y. Zhou, H. Li, J. of Solid State Chemistry, 165 (2002) 247. 5. J. Lee, Y. Tak, Electrochemistry Communication, 2 (2000) 765. 6. Hideki Masuda, Masahiro Satoh, Jpn J. Appl Phys., 35 (1996) LI26. 7. K. Nielsch, F. Muller, A-P Li, U Gosele, Adv. Mater., 12 (2000) 582.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
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Synthesis of mesoporous y^luminas of controlled pore properties using alkyl carboxylate assisted method** Younghun Kim, Changmook Kim, Jang Wook Choi, Pil Kim and Jongheop Yi* School of Chemical Engineering, Seoul National University, San 56-1, Shillim, Kwanak, Seoul 151-742, Korea Mesoporous y-alumina (MA) was prepared by alkyl carboxylate assisted method. Pore properties of MAs could be controlled by carbon tail length of template, the molar ratio of sec-butanol to isooctane or water to aluminum precursor, and calcination conditions. The crystalline phase of MA after calcinations was y-A^Os. The pore size of MAs decreased from 7.7 nm to 3.5 nm with the decrease in the ratio of water to aluminium ion, while the pore uniformity was enhanced. Isooctane as a co-solvent acted as an expander of the pores. In addition, as increasing the molar ratio of sec-butanol to isooctane, pore size increased from 2.3 nm to 3.5 nm and both pore uniformity and framework porosity was improved. 1. INTRODUCTION The synthesis and characterization of mesoporous silica of the M41S type has been widely documented.' Mesoporous materials of this type arc not restricted to silica, and have also been reported for transition metal oxides (AI2O3, Ti02, Zr02, etc.). However, in the case of mesoporous aluminium oxide, the strategics typically used in the synthesis of mesoporous silica have not always yielded satisfactory results. Moreover, relatively few studies on the synthesis of pure alumina have been reported to date.^"^ Davis and coworkers^ prepared mesoporous aluminas by the hydrolysis of aluminium alkoxides in the presence of a carboxylate surfactant. The resulting materials, however, had an approximately constant pore size (ca. 20 A) that could not be tailored by changing the length of surfactant chains. Yada et al."^ reported the preparation of aluminium-based dodecyl sulfate mesophase using an electrostatic S'V assembly, and Pinnavaia et al.'* obtained mesoporous aluminas from electrically neutral assemblies of polyethylene oxide and aluminium alkoxides. One limitation of the above synthetic strategies for mesoporous aluminas is that the rates of hydrolysis (and condensation) of aluminium alkoxides are much faster than that of the silicon alkoxides. Therefore, the hydrolysis reactions in aqueous media also lead to the formation of lamellar hydrated hydroxides, even in the presence of surfactant.^ Amoros and coworkers^ solved this problem by controlling the hydrolysis rate of the aluminum suspension by adding hydrolysisretarding agents. Here we report the synthesis strategy for mesoporous y-alumina via posthydrolysis method.^'^ * Corresponding author: Jxi({{:-^liy.-^.-^-ikr Financial support by the National Research Laboratory (NRL) of the Korean Science and Engineering Foundation (KOSEF) is gratefully acknowledged.
210
2. EXPERIMENTAL Mesoporous y-alumina was prepared by adjusting the molar ratio of sec-butanol as parent alcohol to isooctane as co-solvents. Aluminum sec-butoxide as a precursor and alkyl carboxylates [CH3(CH2)nCOOH; n=4, 10, 16] as chemical templates were separately dissolved in parent alcohol, and the two solutions then mixed. Small amount of water were dropped into the mixture as a rate of 1 ml/min at ambient condition. The co-solvent ratio was optimized depending on the pore uniformity and porosity. The carbon length of template, the molar concentraton of water or co-solvent and calcination condition controlled the pore properties of MA, and those were analyzed with N2 sorptometer. The crystalline phase was obtained from TGA, DSC and XRD analysis. 3. RESULTS AND DISCUSSION FT-IR analysis illustrated the removal of template from as-made MAs was dependent on the calcination time and temperature. As the calcination time increased from 1 to 5 h at 360"C, tail groups (2857 cm'') of alkyl carboxylate disappeared at relatively low temperature compared with head groups (1461 and 1573 cm'), and templates were completely removed after 5 h at 360"C. However, after the samples were calcined at 5 h, 360"C, the pores channel shrank, while the pores of calcined MA at 3 h, 420"C did not shrink, even after the template was completely removed. The crystalline phase of as-made MA shows boehmite and bayerite. TGA curve shows the occurrence of two group of mass change, which is physically adsorbed water and carboxylate. This result agrees well with DSC analysis. However, another peak appeared around 220"C in DSC curve. It was believed to be the phase transformation aluminium hydroxide to y-A^O.^ that was retained up to 550"C. Calcined MA was thermally stable up to 550"C . Table 1 Pore characteristics of mesoporous aluminas Sample Molar ratio of ^Poresize/nm FWHM of Surface area/ water to PSD/nm m^g' aluminum 2.9 48 7.7 386 MA-1 32 7.7 3.0 377 MA-2 16 369 2.9 7.1 MA-3 MA-4 8 485 1.0 3.5 4 1.4 420 3.5 MA-5 ^ Pore sizes obtained from desorption branch of nitrogen isotherm.
Pore volume/ cm^g"' 1.0 1.0 0.9 0.5 0.6
The pore properties of the aluminas were dependent on the ratio of water to aluminium precursor as listed in Table 1. Pore size decreased with the decrease in this ratio. However, the pore uniformity, based on the FWHM of the pore size distribution, was enhanced due to the complete condensation of aluminium hydroxide and surfactant micelle. In the case of MA-1 to MA-3, a larger quantity of water in the reaction increased the pore size, presumably the
211
results of a swelling effect of solvent inner micelles. Nevertheless, pore uniformity was similar. When a very small amount of water was used a catalyst, as in the case of MA-4 and MA-5, the pore size distribution was very narrow, and the surface area increased. The N2 isotherm of the MA-5 showed a different shape compared with the others, as shown in Fig. 1.
04
06
08
10
P/PQ
pore size (nm)
Fig. 1. Nitrogen isotherms (left) and pore size distributions (right) of (a) MA-1, (b) MA-4, and (c) MA-5. The porosity of the mesoporous silica varied according to the synthesis conditions, as shown in Fig. I. The N2 adsorption/desorption isotherm of MA-5 was a typical form of type IV like MCM-41, with hysteresis curves near the relative pressure of 0.5 and/or 0.9. Tanev et al.'^ defmed and classified the terms as framework porosity and textural porosity. The framework porosity represents the porosity contained within the uniform channels of the templated framework, while the textural porosity represents the porosity arising from the noncrystalline intra-aggregate voids and spaces formed by the interparticle contacts. In this Figure, MA-5 showed both framework and textural porosity, while MA-1 and MA-4 showed only - •••^^•f^'.^Ji-^ —=^—^ textural porosity and framework porosity,
I
I
,' ' ^-'^l^r^^.. '
? 0 0 8 '^ J1
Fig. 2. TEM image of mesoporous alumina, MA4, calcined at 420 ""C (1 bar:20 nm).
respectively. This is probably because the presence of a small quantity of water resulted in the formation of small precipitated aggregates, and that upon further aggregation. These gave rise to textural and framework porosity of the MA-5. From MA-1 to MA-3, only textural porosity showed poorly organized and incomplete condensed structure. The TEM image of MA-4 in Fig. 2 shows the surface morphology of mesoporous alumina prepared after calcinations at 420^C. The pores were
212
wormhole or sponge-like in appearance, which impHes the advantage of having a highly interconnected pore system. Similar pore morphology was found for disordered mesoporous silicas' and alumina"*'^ when cationic or neutral surfactants were used. Pore properties of MA are able to control using isooctane, which is very hydrophobic and could be used as micelle structure reinforcement agent and swelling agent. BET results show the FWHM of pore size distribution was reduced with increasing of moral ratio (Fig. 3). When the isoocatne is not used as co-solvent, MA-0.0 has a hyesteresis loop of HI type at above 0.9 P/PQ. That loop was corresponding to the textural porosity. This porosity, however, disappeared with using isooctane and pore uniformity was Fig. 3. Pore size distributions and enhanced. The pore size increased with nitrogen isotherms of MAs. Numbers increasing the molar ratio. When the ratio is 0.2, of notation denote the molar ratio of sec- this material shows bimodal pore structure at 2.3 butanol to isooctane. and 3.5 nm due to the small amount of isooctane. Results imply that one part of micelle (hydrophobic portion) is containing the isooctane. Therefore MA-0.2 shows medium step of the pore growth. The optimal molar ratio value is 1.0 for enhanced pore uniformity and framework porosity. 4. CONCLUSIONS We report here the synthesis of mesoporous aluminas with pore sizes that can be tailored at ambient temperature. XRD result confirmed that the thermal stability of prepared alumina was maintained up to 550"C. Hydrophobic reagent, isooctane acts like a swelling agent that can control the pore properties of mesoporous alumina.
REFERENCES 1. Stein, B. J. Melde and R. C. Schroden, Adv. Mater.,12 (2000) 1403. 2. F. Vaudry, S. Khodabandeh and M. E. Davis, Chem. Mater., 8 (1996) 1451. 3. M. Yada, M. Machida and T. Kijima, Chem. Commun., (1996) 769. 4. W. Zhang and T. J. Pinnavaia, Chem. Commun., (1998) 1185. 5. S. Valange, J.-L. Cuth, F. Kolenda, S. Lacombe, and Z. Gabelica., Micropor. Mesopor. Mat., 35-36 (2000) 597. 6. S. Cabrera, J. El Haskouri, J. Alamo, A. Beltran, S. Mendioroz, M. D. Marcos, and P. Amoros, Adv. Mater., 11 (1998)379. 7. Y. Kim, B. Lee and J. Yi, The Korean J. of Chem. Eng. in press, (2002). 8. P. Kim, Y. Kim, C. Kim, I.-K. Song and J. Yi, The Korean J. of Chem. Eng. in press, (2002). 9. P. T. Tanev and T. J. Pinnavaia, Chem. Mater., 8 (1996) 2068.
Studies in Surface Science and Catalysis 146 Park et aJ (Editors) © 2003 Elsevier Science B.V. All rights reserved
213
Synthesis and characterization of mesoporous alumina molecular sieves using cationic surfactants Hae Jin Kim,^' ^ Hyun Chul Lee,'^ Dae Hyun Choo,^ Hee Cheon Lee,^ Soo Hyun Chung/ Kyung Hee Lee,^ and Jae Sung Lee"^'* ^Department of Chemistry and Division of Molecular and Life Science, Pohang University of Science and Technology, Pohang, 790-784, Korea ''Korea Basic Science Institute, Daejeon, 350-333, Republic of Korea '^Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea* ^Korea Institute of Energy Research, Daejeon, 305-343, Republic of Korea We report a new synthetic route of thermally stable alumina mesoporous molecular sieves with a cationic surfactant, CH3(CH2)n-iN(CH3)3Br, n=12, 14 and 16. We were able to synthesize the mesoporous alumina from aluminum alkoxide precursor in the presence of cationic surfactant under hydrothermal condition without any additives. As a result, the stable alumina mesoporous molecular sieves could be synthesized in a much simpler manner. The synthesized mesoporous alumina molecular sieve (P-AMS) showed the 'wormhole-like' pore structure, high surface area, thermal stability, and aluminum sites of different coordinations. 1. INTRODUCTION Alumina is most commonly used for catalysis and catalyst supports due to their thermal, chemical and mechanical stability. Recently, efforts have been devoted to the synthesis of mesoporous alumina with an ordered pore structure.''^ Unfortunately, the well-established procedures used for the synthesis of siliceous mesoporous materials have often failed. Nevertheless, there are a few successful examples of mesoporous alumina molecular sieves *This work has been supported by BK-21 program of Korea Ministry of Education and ERC and NRL program of Korea Ministry of Science and Technology.
214
having thermal stabihty. Mesoporous alumina molecular sieves prepared with the nonionic polyethylene oxide surfactant was known to show a regular, wormhole-like channel motif with high surface areas between 420 and 535 m^/g.'"^ Although materials with similar structure were also prepared with carboxylic acids or other anionic surfactants,^'"^ synthesis with a cationic surfactant was most troublesome. Nevertheless, it was first achieved by Cabrera et al.^ by adding triethanolamine as a "hydrolysis-retarding agent" of the aggregates present in the mother solution. In this study, we report a new synthetic route of thermally stable alumina mesoporous molecular sieve (hereafter denoted as P-AMS) with only a cationic surfactant, CH3(CH2)niN(CH3)3Br, n=12, 14 or 16. In order to control the synthesis process without adding any extra agent, the amount of water was limited to the minimum that was required for the hydrolysis of the aluminum precursor in an alcohol solution. 2. EXPERIMENTAL The synthetic procedure for the mesoporous alumina molecular sieves (denoted as P-AMS) is as following: Distilled water was added very slowly to a homogeneous mixture of cationic surfactants and aluminum tri-sec-butoxide dissolved in 1-butanol solvent while stirring. The molar composition (surfactant: Al: water) of the resulting gel was 0.5:1:2 After stirring until the homogeneity was obtained, the resulting well-mixed gel was put into Teflon-lined autoclave vessel. Then, hydrothermal reaction was followed at a desired temperature for 24 h under autogenous pressure and static condition. The product was washed with absolute ethanol, dried and then calcined at 773 K for 4 h in the flow of air. Characteristics of the mesoporous materials prepared through this new method were investigated with XRD (MAC Science Co, M18XHF diffractometer), HRTEM (JEOL JEM 2010F, Field Emission Electron Microscope), nitrogen adsorption (ASAP 2010, Micromeritics) and solid state NMR (Varian Unity Inova 300 MHz spectrometer equipped with a 7mm Chemagnetics MAS probe head using a sample rotation rate of 6 kHz). 3. RESULTS AND DISCUSSION Table 1 shows the average pore size and the calculated BET specific surface areas for synthesized alumina molecular sieves (P-AMS). The pore size distributions were determined by the BJH model.^ The pore size increased with the chain length of the cationic surfactants from 4.5 to 6.7 nm. These results suggest that the pore size might be controlled to some extent by the employed chain length of the cationic surfactants. Figure 2 shows the representative
215
Table 1 Characteristics of mesoporous alumina molecular sieves synthesized with cationic surfactants under hydrothermal conditions at 373 K for 24 h. BET Surface Area
BJH Pore Size
SgCm^/g)
(nm)
[CH3(CH2)„N(CH3)3]Br
429
4.5
P-AMS-2
[CH3(CH2),3N(CH3)3]Br
241
6.5
P-AMS-3
[CH3(CH2),5N(CH3)3]Br
337
6.7
Materials
Surfactants
P-AMS-1
small and wide angle range powder X-ray diffraction patterns for the mesoporous alumina molecular sieves sample after removal of surfactant by calcinations at 773 K. For small angle XRD, only one XRD peak was obtained, which has been directly related to the disordered mesoporous structure.^ Thus, our samples prepared with cationic surfactants were also considered to have disordered pore architectures. Dependence of N2 adsorption/desorption isotherms and pore size distribution on surfactant chain lengths is displayed for P-AMS calcined at 773 K in Figure 2. A typical Type IV adsorption isotherm with a hysteresis loop is observed for all samples. The isotherm of PAMS alumina exhibits a broad curvature, but well-defmed step in the adsorption isotherm curve in p/po range from 0.4 to 0.8, which is characteristic of capillary condensation within uniform pores.
700 A
vy\\ ?n d
i
^tf) c
sc.
\ ^1 \ c
\^
I
V^A _,, „ - , — , — . . . . . ,
\^_^
10 20 30 40 50 60 70 80 90
26 (Degrees)
500 - U « * * ^
Fig. 1. Small angle XRD P-AMS-3 after calcination 773 K for 4 h
rX>-qCPt*^
[>^ 00
26 (Degrees)
rTTT
0.2
0.4
0.6
0.8
Relative Pressure (P/Po)
1.0
r^
0
50
100
150
200
Pore diameter (A)
Fig. 2. Dependence of N2 adsorption/desorption isotherms (A) and pore size distribution (B) on surfactant chain length, (a) P-AMS-1, (b) P-AMS2, (c) P-AMS-3.
216
tOOnm 10
20
30
40
50
60
70
80
90
29
Fig. 3. HRTEM image of P-AMS-3 after calcination 773 K for 4 h
Fig. 4. XRD patterns for P-AMS-3. (a) before calcinations, (b) after calcination at 773 K, (c) 873 K, (d) 973 K for 4 h.
Figure 3 shows HRTEM image for P-AMS-3. In the HREM image, there seems to be no discemable long range order of the pore structure, although it shows pores with a quite regular diameter. Thus, the packing of pore in P-AMS-3 seems to be rather random with "wormholelike" or "sponge-like" morphology, which is typically observed for disordered mesoporous silicas and aluminas.^'^ The XRD patterns in Figure 1 and 4 imply that the wall of P-AMS materials after calcination consists of bulk y-alumina with a low crystallinity. We could conclude that the thermally stable mesoporous alumina molecular sieves could be synthesized in a much simple manner with only cationic surfactant under hydrothermal condition without any additives.
REFERENCES 1. S. A. Bagshaw, E. Prouzet, T. J. Pinnavaia, Science 1995, 2(59, 1242-1244. 2. S. A. Bagshaw, T. J. Pinnavaia, Angew. Chem. Int. Ed. Engl. 1996, J5, 1102-1105. 3. F. Vaudry, S. Khodabandeh, M. E. Davis, Chem. Mater. 1996, 8, 1451-1464. 4. M. Yada, M. Machida, T. Kijima, Chem. Commun. 1996, 769-770. 5. S. Cabrera, J. E. Haskouri, J. Alamo, A. Beltran, D. Beltran, S. Mendioroz, M. D. Marcos, P. Amor6s,^Jv. Mater. 1999, //, 379-381. 6. S. Valange, J. -L. Guth, F. Kolenda, S. Lacombe, Z. gabelica, Micropor Mesopor Mater 2000, 35-36, 597-607. 7. V. Gonzalez-Pefia, I. Dias, C. Marques-Alvarez, E. Sastre, J. Perez-Pariente, Micropor Mesopor Mater 2001, 44-45, 303-310. 8. E. R Barett, L. G. Joyner, R R Halender, J. Am. Chem. Soc. 1951, 73, 373-380.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
217
Synthesis and characterization of mesoporous alumina molecular sieves with cationic surfactants in the presence of formamide Hyun Chul Lee,^ Hae Jin Kim,^ Dae Hyun Choo,^ Hee Cheon Lee,^ Soo Hyun Chung,^ Kyung Hee Lee^ and Jae Sung Lee^ ^Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea* ''Department of Chemistry and Division of Molecular and Life Science, Pohang University of Science and Technology, Pohang, 790-784, Korea '^Korea Institute of Energy Research, Daejeon, 305-343, Republic of Korea We have synthesized alumina mesoporous molecular sieves with a cationic surfactant (CTAB) exhibiting thermal stability and high surface area and were also able to synthesize the mesoporous alumina from aluminum alkoxide precursor with cationic surfactant under hydrothermal condition in the presence of formamide as an external additive. The pore sizes and surface area could be controlled by changing of the amount of formamide. The HRTEM image of P-AMS-F showed the disordered interwoven network of the pore architecture. The ^^Al NMR results clearly demonstrate the existence of the different electronic environment around resonant aluminum nuclei. 1. INTRODUCTION Considerable interests have been devoted to developing new synthetic methods, pathways, and also to characterizing nano-structured molecular sieves since the discovery by Mobil researchers of the ordered mesoporous M41S family by means of the templating of surfactant micelle structures.' The synthetic strategy used for the silica-based materials has been extended to the preparation of non-siliceous mesoporous oxides. In spite of extensive efforts recently devoted to the synthesis of mesoporous alumina with an ordered pore structure^"^, the well-established procedures used for the synthesis of siliceous mesoporous * This work has been supported by BK-21 program of Korea Ministry of Education and ERC
218
and NRL program of Korea Ministry of Science and Technology. materials have often failed. Nevertheless, there are a few successful examples of mesoporous alumina molecular sieves having thermal stability. It was first achieved by Cabrera et al^ for the mesoporous alumina molecular sieves with a cationic surfactant by adding triethanolamine as a 'hydrolysis retarding agent' of the aggregates in the mother solution. All of these mesoporous alumina molecular sieves showed 'wormhole-like' or 'sponge-like' morphologies. Here we report synthesis of mesoporous alumina molecular sieves with cationic surfactants in the presence of formamide. 2. EXPERIMENTAL The method of preparation for the mesoporous alumina molecular sieves in the presence of formamide (denoted as P-AMS-F) is as following: formamide and distilled water was added very slowly to a homogeneous mixture of cationic surfactants ([CH3(CH2) i5N(CH3)3]Br, CTAB) and aluminum tri-sec-butoxide dissolved in 1-butanol solvent while stirring. The molar composition, (surfactant: Al: formamide: water) of the resulting gel, was 0.5: 1: x: 2 (x=0-l). After stirring until the homogeneity was obtained, the resulting well-mixed gel was put into Teflon-lined autoclave vessel. Then, hydrothermal reaction was followed at a desired temperature for 24 h under autogenous pressure and static condition. The product was washed with absolute ethanol, dried and then calcined at 773 K for 4 h in the flow of air. Characterizations of the mesoporous materials prepared through this method were carried out with XRD (MAC Science Co, M18XHF diffractometer), HRTEM (JEOL JEM 201 OF, Field Emission Electron Microscope), nitrogen adsorption (ASAP 2010, Micromeritics) and solid state NMR (Varian Unity Inova 300 MHz spectrometer equipped with a 7mm Chemagnetics MAS probe head using a sample rotation rate of 6 KHz). 3. RESULTS AND DISCUSSION The XRD experiments showed that P-AMS-F consisted of aluminum oxide such as gamma alumina and oxyhydroxide species with a low crystallinity. Table 1 shows the characteristics of mesoporous alumina molecular sieves synthesized with cationic surfactant (CTAB) in the presence of formamide (P-AMS-F) after removal of surfactant by calcination at 773 K. As the amount of formamide increased from 0 to 0.45 (mol ratio to Al), the average pore size in mesoporous alumina decreased from 6.7nm to 3.0 nm and its distribution became sharper while calculated BET specific surface area increased. These results suggest that the pore size might be controlled to some extent by adjusting the amount of the formamide.
219
Table 1 Effect of the amount of formamide on the synthesis of mesoporous alumina molecular sieves with cationic surfactant under hydrothermal conditions at 373 K for 24 h. Amount of formamide (mol/ Al mol)
Materials
BET Surface Area
BJH Pore Size
Sg(m'/g)
(nm)
P-AMS-F (0)
0
337
6.7
P-AMS-F(O.l)
0.1
370
3.8
P-AMS-F (0.2)
0.2
388
3.7
P-AMS-F (0.45)
0.45
404
3.0
Figure 1 shows the dependence of N2 adsorption/desorption isotherms and pore size distribution on the amount of formamide added to synthesize P-AMS-F materials. The narrower pore size distribution was observed as the adding amount of formamide raised. The packing of channel systems in P-AMS-F (Fig. 2) appears to be an interwoven network of pore architecture, rather than so called 'wormhole-like' or 'sponge-like'^'"^ morphology often observed for disordered mesoporous silicas and aluminas in general, and P-AMS without formamide addition. The HRTEM images for the P-AMS-F showed no discemable long range order in the pore structure. The representative ^^Al solid-state NMR spectra of calcined P-AMS-F are depicted in Figure 3. ^^Al MAS NMR spectra (Figure 3a) show two well-resolved ^^Al NMR peaks in all samples, which can be assigned to Al centers coordinated to a donor atom with tetrahedral and octahedral geometries, respectively. In addition to these two peaks, P-AMS-F show an additional very weak NMR signal at 33 ppm which is assigned to a penta-coordinated aluminum site.^ This result implies the existence of amorphous domains with a poor crystallinity as defects arising from distorted octahedrally coordinated Al in P-AMS-F.
O.025 0.020 "
1. ^
Q
0.O15
0.010 0.005 0.000
0.2
OA
0.6
0.8
R e l a t i v e Rnessure (P/Po)
f\\ /y-^ "^
(d)
\ ^
_
(c)
(b)
^^-~—(£>. 50 100 150 F=tane clamE*er (/>^
Fig. 1. N2 adsorption/desorption isotherms and pore size distribution of the samples. Amount of formamide (mol/Al mol)= FA, (a) FA=0, (b) FA=0.1, (c) FA=0.2, (d) FA=0.45.
220
(a) 3
^, z''' (b) •^':v=-'"--''
^fe^,.
r\''.; ^ i,-••-*-'
i^lS-
•J.?..
-"^..jigte ^ ..g^
1 50nin
if^
200
100
0
-100
-200200
100
0
-100
-200
Chemical Shift from AliU^O)^^"^
Fig. 2. HRTEM images after calcination 773 K
Fig. 3. ^^Al NMR spectra. (A)
for 4 h.
AMS-F(0.45), (B) P-AMS-F(O).
P-
(a) P-AMS-F (0), (b) P-AMS-F (0.45)
(a) ^^Al MAS NMR, (b) ^^Al CPMAS NMR.
Also, ^^Al CPMAS (Cross Polarization Magic Angle Spinning) NMR spectrum (Figure 3b) of the calcined P-AMS-F exhibits three well-resolved NMR peaks at 72, 33, and -1 ppm. The cross polarization effect will increase the relative intensity of penta-coordinated aluminum center, largely due to the magnetization transfer from proton to the aluminum center. Nevertheless, the peak from penta-coordinated aluminum site at 33 ppm nearly did not change for P-AMS-F(0.45) compared with that of P-AMS-F(O). Thus, from the resuh of ^^Al NMR experiments, the amorphous domains in P-AMS-F are relatively small and not directly involved with proton sites. In conclusion, we could successfully control the pore size in mesoporous alumina molecular sieves with a variation of the amount of formamide. They showed a morphology of an interwoven network of pore architecture. In addition, we confirmed the different electronic environments in aluminum sites via ^^Al NMR experimental techniques.
REFERENCES 1. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck, Nature 1992, 359, 710. 2. S. A. Bagshaw, T. J. Pinnavaia, Angew. Chem. Int. Ed. Engl. 1996, 35, 1102. 3. F Vaudry, S. Khodabandeh, M. E. Davis, Chem. Mater. 1996, 5, 1451. 4. S. Cabrera, J. E. Haskouri, J. Alamo, A. Beltran, D. Beltran, S. Mendioroz, M. D. Marcos, P Amoros, Adv. Mater. 1999, 7/, 379. 5. S. Valange, J. -L. Guth, F Kolenda, S. Lacombe, Z. gabelica, Micropor Mesopor Mater 2000, 35-36, 597.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
221
Structure and properties of porous mesostructured zirconium oxophosphate witli cubic (Ia3d) symmetry Freddy Kleitz^^ Stuart J. Thomson^^, Zheng Liu*', Osamu Terasaki^ and Ferdi Schiith^* ^Max-Planck-Institut fiir Kohlenforschung 45470 Miilheim an der Ruhr, Germany. ^Japan Science and Technology Corporation (CREST) and Department of Physics, Tohoku University, Sendai 980-8578, Japan The synthesis and characterization of the first porous zirconium oxo-phosphate material structured on the nanoscale with a cubic Ia3d symmetry is described. The new ordered porous material was obtained in aqueous solution by the self-assembly of a simple cationic surfactant combined with the inorganic zirconium sulfate precursor. The cubic zirconium oxo-phosphate was characterized by X-ray diffraction (XRD), high resolution electron microscopy (HREM), N2 sorption and FTIR spectroscopy. 1. INTRODUCTION The developments in the field of non-siliceous mesostructured and mesoporous materials have recently been reviewed.''^ In particular, transition metal-based ordered mesoporous materials have been synthesized on the basis of titanium, zirconium, niobium or tantalum, most of them being either hexagonally ordered or rather disordered.''^ However, considerably less attention has been given to non-hexagonal structures,^ mainly due to the higher difficulty in achieving stable well-ordered porous solids."*'^ We previously reported the synthesis of mesoporous zirconium 0x0phosphates with 2-D hexagonal phase.^'^ These well-ordered and thermally stable zirconium oxo-phosphate materials, show relatively large adsorption capacity, high surface area, and Lewis and Bronsted acidity. The desire to create porous materials combining acid-base properties and the advantages of a well-defined 3-D structure led us to develop the synthesis of a cubic Ia3d mesoporous zirconium-based analogue.^ However, in the initial study we were not able to remove the template without structural collapse. By carefully examining the synthesis conditions and the method used for the template removal, we have now succeeded in removing the template without destroying the structure.^ The present report focuses on the characterization of this newly synthesized material. 'Author for correspondence. E-mail: schueth@mpi-muelheim.mpg.de ^Present address: Center for Functional Nanomaterials, Korea Advanced Institute of Science and Technology, 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea. ^Present address: Materials Division, Australian Nuclear Science and Technology Organisation (ANSTO) PMB 1, Menai, NSW, Australia, 2234. The European Community (project HPRN-CT-99-00025) and the Japan Science and Technology Corporation are gratefully acknowledged forfinancialsupports.
222
2. EXPERIMENTAL SECTION The addition of an aqueous solution of Zr(S04)2«4H20 to N-benzyl-N,Ndimethyloctadecylammonium chloride in water leads to rapid formation of a zirconium sulfate-surfactant composite mesophase. The reactants molar ratio of the reagents used was Zr(S04)2: C18BDAC : H2O =\ I r I All. Two surfactant to zirconium sulfate molar ratios (r) were studied, r = 0.40 and r = 0.54. The mesostructured material obtained under acidic conditions was then hydrothermally aged (3 days) and subsequently posttreated with an aqueous solution of phosphoric acid (0.5 M), following a method described previously.^'' As a comparison, hexagonal analogous materials were synthesized, with r - 0.40 and r = 0.54, according to a method described previously.'' Template-free products were obtained after air calcination in a box furnace with a plateau at 300°C for 3 hours followed by 3 hours at 500°C. Slow heating rates (0.5°C/min) were used as this has been shown to have a critical effect on the mesostructure.*^'^ Full details of the syntheses and characterization procedures have been recently reported.^ 3. RESULTS AND DISCUSSION The XRD patterns of the assynthesized samples show reflections suggesting a cubic Ia3d symmetry (Fig. la and lb). Only reflections within 2-8 ° (20), which are due to the ordering of the pores, are observed. This indicates that no condensed crystalline phases arc present. The unit cell parameter of the cubic lattice, calculated from d(211), is generally about ao = 9.9 nm for assynthcsizcd materials. It has been shown previously that the surfactant to zirconium ratio range, where the cubic Ia3d mesophase with a well-resolved diffraction pattern is obtained, is rather 2 theta (•] 2 theta ['] narrow, between r = 0.40 and r = 0.67.^ No significant variations in Fig. 1: XRD patterns, a) and b) As-synthesized cubic zirconium oxo-phosphates. c) and d) As-synthesized the (211) d-spacing (d = ca. 4 nm) hexagonal zirconium oxo-phosphates. e) and f) Calcined is observed. However, even within cubic materials, g) and h) Calcined hexagonal materials. this r range only the two first The dashed lines materialized low angle scattering reflections, assigned to the cubic intensity cut by beam block. space group Ia3d, arc well defined. The higher order reflections appear with low signal-to-noise ratios. In contrast, materials obtained under the same synthetic conditions using CI STAB as a template, exhibit well-resolved XRD patterns of 2-D hexagonal p6mm phase (Fig. Ic and Id).^'^ This highlights the unique role of the surfactant molecular geometry to direct the final
223
mesophase structure. The unit cell parameter of the hexagonal phase is usually around 5.3 nm.^The HREM images of an as-synthesized sample synthesized in presence of Nbenzyl-N,N dimethyloctadecylammonium ions with r = 0.54 reveal domains of highlyordered mesostructure (Fig. 2). Images of the [111] and [100] zone axis are presented in Fig. 2a and 2d, respectively. In the electron diffraction (ED) pattern (Fig. 2b), only diffuse rings are observed indicating that the wall structure of the as-prepared samples is amorphous. Fig. 2c, which is the Fourier diffractogram obtained from the HREM image in Fig. 2a, suggests that the material is commensurate with Ia3d symmetry. As-prepared samples synthesized with r = 0.40 show similar features. In agreement with the XRD, the HREM investigations confirm that the architecture of the zirconium oxo-phosphate surfactant mesophase is characteristic of the cubic Ia3d phase.
®D
202
' o6o 121 ]
•fl
' 2nm'^
0.2nm"^
Fig. 2: Typical HREM image and electron diffraction (ED) pattern of an as-prepared sample with r = 0.54. Fig. 2a) HREM image taken along the [111] zone axis. Fig. 2b) Electron diffraction pattern. Fig. 2c) Fourier diffractograms obtained the area labeled by 1. Fig. 2d) HREM image taken along the [100] zone axis.
The samples were carefully calcined as described. Fig. le and If show the XRD patterns recorded for calcined samples synthesized with r = 0.40 and r = 0.54, respectively. Generally, the structure shrinks drastically and the (220) reflection appears only as a shoulder. No higher order reflections can be detected. The sample synthesized with r = 0.54 undergoes a larger shrinkage (about 30%, acaicined-o.54 = 7 nm) than that with r = 0.40 (about 25%, acaicined-o.4o = 7.5 nm). On the other hand, the shrinkage is slightly less pronounced for hexagonal phase materials (21% for r = 0.40, 25% for r = 0.54) and the reflections at higher 2 theta angles are retained. But a lower ordering is evidenced in all cases. The samples synthesized with less surfactant (r = 0.40) are more stable and undergo less contraction upon calcination. Although the X-ray diffraction patterns recorded for the calcined cubic materials are poorly resolved (Fig. le and If), the HREM image reveals large domains of highlyordered mesostructure (Fig. 3a and 3d). The HREM images presented in Fig. 3a and Fig. 3d are consistent with the Ia3d symmetry and show the uninterrupted channels along the observation direction. In the electron diffraction pattern (Fig.3b), one can observe diffuse electron diffraction rings, indicating that the walls remain amorphous after calcination. This is also supported by the absence of wide-angle reflections in the XRD pattern. The Fourier diffractogram (Fig. 3c) indicates that the zirconium oxophosphate material is also commensurate with the Ia3d symmetry after calcination. Therefore, Fig. 3 gives the clear evidence that the cubic Ia3d mesostructure is retained after the removal of the template by thermal treatment. The sample with r = 0.40, investigated by EM shows similar well-resolved cubic domains.
224
[ ^
202 .
2nm"'
0^0
I 0.2nrn"^
-121
^^
Fig. 3: Typical HREM image and electron diffraction (ED) pattern of a sample with r = 0.54 after calcination at 500°C. Fig. 3a) HREM image taken along the [111] zone axis. Fig. 3b) Electron diffraction pattern. Fig. 3c) Fourier diffractograms obtained from the HREM image in Fig. 3a. Fig. 3d) HREM image taken along the [100] zone axis (inset is the ED pattern).
The N2 sorption isotherms are similar to Type I isotherms characteristic for microporous materials, and likely correspond to pore sizes in the upper micropore range or lower mesopore range.^ In general, the total nitrogen adsorption capacity decreases rapidly with increasing surfactant-to-zirconium sulfate ratio. The highest adsorption capacity is measured for r = 0.40. This cubic zirconium oxo-phosphate sample exhibits total nitrogen adsorption capacity of up to 130 cm^/g and has a pore volume of up to 0.20 cmVg. In addition, the physisorption data indicate a smaller pore size for the cubic zirconium oxo-phosphate compared to Wavenumber [cm^] the corresponding hexagonal phase Fig. 4: Typical FTIR spectra recorded on a material.^ zirconium-based cubic mesophase. a) Zirconium The FTIR spectra recorded on a cubic sulfate mesophase. b) As-synthesized zirconium zirconium-based mesophase (r = 0.40), oxo-phosphate. c) Calcined zirconium oxoprior to and after the phosphation step, phosphate. Samples in KBr. Offset is for clarity. and after removal of the template by calcination, are detailed in Fig. 4a, 4b and 4c, respectively. The broad unresolved peak observed for all synthesis stages at about 3200-3600cm'' is characteristic of hydrogenbonding from 0-H groups. The peak observed at 1630-1640 cm'^ is due to the bending mode of water adsorbed on the sample surface, which also contributes to the broad 0-H stretching band above 3200 cm"'.'''^ The absorption bands observed around 1470 cm"' and 2800-3000 cm', in Fig. 4a and 4b, originate from the surfactant species and are due the C-H hydrocarbon deformation and stretching modes, respectively. In addition, the weak absorption bands observed around 3065 cm' originate from the aromatic ring of the surfactant head group. All these bands disappear after calcination (Fig. 4c).'^Several absorption bands attributed to the sulfate groups in the zirconium sulfate-surfactant mesophase are observed between 900-1300 cm' (Fig. 4a). After phosphation of the sample, an intense broad
225
band centered at 1040-1060 cm"^ assigned to the stretching region of phosphates^ ^''^ is observed at the same frequency range (Fig. 4b). Furthermore, the spectrum in Fig. 4a exhibits medium intensity peaks around 610-650 cm"'. After phosphation, these signals are reduced (611 cm'^ Fig. 4b), and a new absorption band appears at ca. 515 cm'. After thermal treatment at 500°C, the bands at 610-650 cm' seem to vanish, while the band at ca. 515 cm'' is retained. The appearance of all peaks in the phosphated sample in Fig. 4b likely suggests therefore the presence of both sulfate and phosphate species, which may act to increase the disorder in the zirconium-based fi-amework.^ In addition, a weak absorption peak is observed around 742 cm"' for the calcined zirconium oxophosphate (Fig. 4c), and might be due to the presence of pyrophosphate groups"''^ (P0-P bending) suggesting phosphate condensation during calcination. The intensity around 2440 cm"' is probably due to overtone and combination bands. Pyridine sorption followed by IR spectroscopy shows that the samples contain both Bronsted and Lewis acid sites, the concentrations of which depend on the synthesis parameters.^ In terms of relative peak intensities, the largest Bronsted : Lewis (B : L) peak ratio, determined using the ratio of the 1540 cm"' (B) and the 1446 cm' (L) peak, was observed in the 0.54 sample. The sample with r = 0.40 has more Bronsted acidic bridging OH groups. This sample has the highest pore volume (0.20 cmVg), and the highest thermal stability. 4. CONCLUSIONS The cubic structure inferred from XRD is confirmed for the template free materials by HREM, which enables precise structure assignment. The porous zirconium oxophosphate described is therefore one of the first transition metal-based analogues of MCM-48-type materials. The zirconium oxo-phosphate exhibits total nitrogen adsorption capacity of up to 130 cm^/g and has a pore volume of up to 0.20 cmVg, with pore sizes reaching the upper micropore range. Pyridine sorption followed by IR spectroscopy shows that the samples contain both Bronsted and Lewis acid sites. As prospects, it could be expected that such high surface area ordered porous zirconium oxo-phosphates could find interest as metal or metal sulfide catalyst supports for hydrotreatment processes'^ or low-temperature methanol decomposition reactions.'^ 5. REFERENCES 1. A. Sayari, Chem Mater., 8 (1996) 1840. 2. F. Schuth, Chem. Mater., 13 (2001) 3184. 3. P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka and G.D. Stucky, Chem. Mater., 11 (1999) 2813. 4. D.M. Antonelli, A. Nakahira and J.Y. Ying, Inorg. Chem., 35 (1996) 3126. 5. H. Hatamaya, M. Misono, A. Tagushi and N. Mizuno, Chem. Lett., (2000) 884. 6. U. Ciesla, S. Schacht, G.D. Stucky, K.K. Unger and F. Schuth, Angew. Chem. Int. Ed. Engl., 35 (1996)541. 7. U. Ciesla, M. Fr6ba, G.D. Stucky and F. Schuth, Chem. Mater., 11 (1999) 227. 8. F. Schuth, U. Ciesla, S. Schacht, M. Thieme, Q. Huo and G. Stucky, Mater. Res. Bull., 34 (1999) 483. 9. F. Kleitz, S.J. Thomson, Z. Liu, O. Terasaki and F. Schuth, Chem. Mater., in press. 10. F. Kleitz, W. Schmidt and F. Schuth, Microporous Mesoporous Mater., 44-45 (2001) 95. 11. D.E.C. Corbridge and E.J. Lowe, J. Chem. Soc, (1954) 493. 12. K. Segawa, Y. Kurusu, Y. Nakajima and M. Kinoshita, J. Catal., 94 (1985) 491. 13. M.S. Wong and J.Y. Ying, Chem. Mater., 10 (1998) 2067. 14. Y. Sun, P. Afanasiev, M. Vrinat and G. Coudurier, J. Mater. Chem., 10 (2000) 2320. 15. M. Ziyad, M. Rouimi and J.L. Portefaix, Appl. Catal. A, 183 (1999) 93 16. M.P. Kapoor, Y. Ichihashi, W.-J. Shen and Y. Matsumura, Cata. Lett., 76 (2001) 139.
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Synthesis and characterization of mesoporous titanium oxide Jia-Long Tsai, Hsiao-Wan Wang and Soofin Cheng* Department of Chemistry, National Taiwan University, Taipei 106, Taiwan *Fax No: +886-2-2363-6359; Email: cheml031@ccms.ntu.edu.tw Mesoporous Ti02 powders were synthesized in the presence of surfactant-type pore-directing agents. Monolaureth phosphate was used as the pore-directing agent because of its low cost and industrial convenience. Mesoporous Ti02 was successfully obtained by stabilizing the titanium source with acetylacetone and by hydrolyzing it in strong acidic condition. The crystallinity of hexagonal arranged meso-structure was improved by adding butanol as a co-surfactant and post-treatment with ammonia solution. Several methods were used to remove the organic templates in the structure, and their effect on the porous structure and surface area was compared. The photo-catalytic activity of the resultant porous TiOz in degradation of phenol was also studied. 1. INTRODUCTION Titanium oxide has been extensively studied because of its attractive properties and numerous applications. It has high refractive index and commonly used in pigments. It is a semiconductor and used as catalyst supports or photo-catalysts. Mesoporous Ti02 is attractive as a result of its additional potential applications in chemical sensor, photonic crystal and solar cell. Mesoporous materials are usually synthesized by self-assembling of surfactants or block copolymers as templates. Mesoporous Ti02 was first prepared by Antonelli et al. [ 1] by using titanium isopropoxide as starting material and potassium tetradecylphosphate as pore-directing agent, but phosphate was still present in the structure after calcination at 350°C. Block copolymer templating syntheses were also used to prepare mesoporous TiO: [2,3]. Aging time of seven days was required while employing titanium chloride as titanium source. Besides, a variety of surfactants were also applied to prepare mesoporous TiO: [4,5]. However, the materials were suffered from low thermal stability. Here, we report an easy synthesis method using a convenient and low-cost surfactant. We also compare the influence of different types of surfactants on the structure of resultant Ti02 products. 2. EXPERIMENTAL Anionic monolaureth phosphate (MLP) was used as pore directing-agent. An aqueous solution of titanium /so-propoxide and acetyl acetone was added into a solution containing MLP, «-butanol and HCl. After stirring at room temperature for 16 h, the precipitate was separated by filtration. The resultant powders were heated with 0.5M NH3 solution at 80 C for 2 days to obtain mesoporous Ti02. Several methods, including ion-exchange with 0.5 M NaCl(aq), calcination, and irradiation with UV light, were applied to remove the phosphate templates. This work was supported by the Ministry of Education and the National Science Council of Taiwan.
228
XRD patterns were recorded on a Scintag XI instruments. BET surface area was obtained using a Micrometric ASAP 2000 physisorption system. IR data were taken using a Bomem MBIOO spectrometer. The elemental analysis was obtained using an Allied Analytical System (Jarrell-Ash), Model IC AP9000 ICP-AES. The morphology and pore structure were examined with a Hitachi S-2400 SEM and a Hitachi H-7100 TEM, respectively. 3. RESULTS AND DISCUSSION Fig. 1. compares the structures of Ti02 obtained by using surfactants of different charged natures: cationic CTMABr (cetyltrimethylammonium bromide), neutral hexadecylamine, and anionic MLP. The XRD patterns show that highly crystalline meso-structure was obtained when using MLP as pore-directing agent. On the other hand, a meso-structure with a very broad XRD peak was obtained when applying neutral surfactant, hexadecylamine as template, and no precipitation could be seen when using cationic surfactant, CTMABr. Moreover, anatase phase Ti02 was formed if the solution of CTMABr was neutralized with base. These results imply that the titanium precursor formed in the synthesis solution containing titanium /50-propoxide and acetyl acetone is likely a cationic complex, and an anionic surfactant would be a proper pore-directing agent to synthesize mesoporous Ti02 materials.
I \^y*4^^^f^^ -^->-w^___
K^.
(b)
(c) 10
2 0
Fig. I. XRD patterns of Ti02 samples prepared with different surfactants as templates: (a) monolaureth phosphate, (b) sample (a) after NH^-treatment, (c) C16H33NH2 amine, (d) sample (c) after NH.vtreatment, and (e) CTMABr
Fig. 2. XRD patterns of Ti02 samples synthesized (a) with w-butanol, (b) with /i-butanol but without HCl, and (c) without «-butanol.
Addition of/i-butanol and HCl into the synthesis gel was found to improve the crystallinity of the mesoporous Ti02 (Fig. 2). «-Butanol was considered to play the role of co-surfactant, which probably interacts with the hydrophilic end of MLP and helps the formation of rod-shaped micelles. On the other hand, HCl can slow down the hydrolysis of titanium complexes and prevent the formation of dense Ti02 structure. It can be seen that a material of poor crystallinity was obtained when n-butanol and HCl were not added to the synthesis gel.
229
(a) (b)
r^ Fig. 3. XRD patterns of Ti02 products synthesized with (a) MLP, and (b) treated with 0.5M NH3 solution at 80°C for 2 days.
5
(c) (d)
*
-JJJL^ —//
20
- — 1
30
* 40
(e) X, 50
2 9
Fig. 4. XRD patterns of Ti02 products (a) synthesized with MLP, (b) ion-exchange with 0.5M NaCl(aq) for 16 h, (c) irradiated with 300 nm UV for 65 h, (d) calcined at 500°C for 6 h, and (e) calcined at 800°C for 6 h. * anatase TiOz, ^ TiP207.
The XRD patterns in Fig. 3 show that before ammonia-treatment, the structure of the as-synthesized TiOz product is more like lamellar compound. After ammonia-treatment, the lamellar structure seems to reorganize and transform to hexagonal arranged mesoporous structure. Several methods were used to remove the organic template, including ion-exchange, calcination and irradiation with UV light. The extent of template removal was examined by the C-H stretching intensity in the IR spectra. The organic phosphate template cannot be completely removed by ion-exchange with NaCl, probably due to the strong interaction between Ti and phosphate. A nearly complete removal of the template was achieved by calcination at 500°C. UV light irradiation could also decompose the organic template, depending on the irradiation period. However, the hexagonal arranged structure collapsed when the template was removed, as shown in Fig. 4(c) and 4(d). When the as-synthesized sample was calcined at 800°C for 6 h, a cubic phase TiPzOv [6] and anatase TiOz formed, as shown in Fig. 4(e). These results imply that the hexagonal arranged meso-structure TiO: has strong interaction with the phosphate template. In other words, the meso-structure material is Table 1 Photodegradation activity of TiOz compounds. Conversion (%) Catalyst Eg(eV) CO2 yield (%) 68 anatase 3.09 83 26 89 rutile 2.85 18 27 3.38 K2Ti409 48 67 Meso-Ti02 after UV radiation* 3.22 28 31 Meso-Ti02 after UV radiation 3.36 50 mLof 0.5 mM phenol solution over 0.01 g catalyst, radiated with 300nm UV for 6 h. * without NH3 treatment.
230
likely a composite of titanium oxide and titanium phosphate. The elemental analysis of meso-Ti02 samples with ICP-AES showed the presence of P. The P/Ti atomic ratio in the as-synthesized sample is ca. 0.58. That value decreased to ca. 0.29 after 300 nm UV irradiation for 65 h, indicating that a large portion of phosphorus was also removed by UV irradiation. Although the structure loses its crystallinity, irradiation with UV light is a promising method to remove phosphate template from meso-Ti02. Fig. 5 shows the N2 adsorption-desorption isotherms of the meso-TiOz samples after different post-treatments. A sample with surface area of 68 m^/g was obtained with UV irradiation and 125 m^/g for the sample calcined at 500°C. In contrast, relatively high surface areas (150-360 m^/g) were obtained for the samples ion-exchanged with NaCl. The hexagonal arranged pore structure was detected on the ion-exchanged samples but hardly seen on the calcined sample due to the collapse of ordered-structure during heat treatment. Fig. 6 shows the TEM images of TiOa products after ion-exchange. Table 1 shows that the meso-Ti02 materials demonstrated photo-catalytic activities in degradation of phenol. The meso-Ti02 samples were irradiated with 300 nm UV light to decompose the organic templates before they were used as photo-catalysts. As shown in Table 1, porous Ti02 demonstrated higher photo-catalytic activities than commercial rutile or K2Ti409 but lower than that of anatase. Besides, the meso-Ti02 has larger energy gap than that of anatase and rutile. The blue-shift in energy gap comparing with pure Ti02 is because the meso-Ti02 is a composite of titanium oxide and titanium phosphate. 100 nm
*
—
•
.
.
_
_
B ^
Fig. 5. N2 adsorption-desorption isotherms of meso-Ti02 products (a) calcined at 500^^0 for 6 h, and (b) irradiated with 300 nm UV light.
Fig. 6. TEM image of meso-TiOz products after ion-exchange with NaCl.
REFERENCE 1. 2. 3. 4. 5. 6.
Antonelli, D. M. et al., Angew. Chem. Int. Ed. Engl. 34 (1995) 2014. Yang, P D. et al.. Nature 396 (1998) 152. Yang, P D. et al., Chem. Mater. 11 (1999) 2813. Antonelli, D. M. et al.. Micro. Meso. Mater. 30 (1999) 315. Khushalani, D. J. Mater. Chem. 9 (1999) 2491. Joint Committee for Powder Diffraction Standard, Powder Diffraction File No. 38-1468. (JCPDS International Center for Diffraction Data, 1987)
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Improvement of thermal stability of Ti-Zr mesoporous oxides using CTAB surfactant templates mixed with auxiliary organic additives Weibin Li , Xufei Yang, Yu Zhang, Wenbo Chu The Environmental Catalysis Group, Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, China. Several auxiliary organic additives such as dodecylamine, Triton-X 100, triethanolamine and hexamethylenetetramine (HMTA) were all found to be able to improve thermal stability of the 1:1 molar ratio Zr/Ti mesoporous oxides prepared from inorganic salt precursors using cetyltrimethylammonium bromide (CTAB) under hydrothermal conditions. Particularly, a high surface area, i. e., 386 m^/g was available on the Ti-Zr mesoporous oxides prepared from CTAB and HMTA after calcinations at 450 °C, meanwhile the mesoporous structures still retained on the calcined sample. 1. INTRODUCTION Mesoporous transition metal oxides arc very attractive in the fields of catalysis and gas adsorption because of their unique pore structures and redox properties [1]. However, as compared to siliceous MS41 mesoporous materials, it is much difficult to keep their mesoporcs after removing organic templates due to their easily collapsed structures. Several attempts has been made to stabilize their mesoporous structures including using amphiphilic poly (alkylcne oxide) block copolymers [2], addition of sulfate and phosphoric acid during the gel reaction by Ciesla et al [3] and Ying et al [1], respectively. Post-synthesis treatment with phosphoric acid was also employed on mesoporous Ti-Zr oxides by Chen et al [4]. But it is difficult to remove phosphor or sulfur species on the final samples, and hence limiting their applications on some catalytic reactions or adsorption process because of the poisoning [5-6]. In this presentation, several auxiliary organic components were chosen to be mixed with cetyltrimethylammonium bromide (CTAB) template solution in an attempt to improve thermal stability of the mesoporous Zr-Ti oxides during the synthesis process. 2. EXPERIMENTAL The Ti-Zr mesoporous mixed oxides were synthesized from titanium sulfate and zirconium nitrate through templating by CTAB and other auxiliary organic components, i.e. dodecylamine (DDA), triethanolamine, TritonX-100, and hexamethylenetetramine (HMTA) in aqueous To whom correspondence should be addressed. E-mail: wbli(a'mail.tsi^^hua.cdii.cn
232
solution at 100-110°C for 2 days; the Ti/Zr molar ratio in the starting gel mixture was varied from 20-80 mol% of the titanium; the molar ratio of DDA/CTAB, triethanolamine /CTAB, TritonX-lOO/CTAB, and HMTA/CTAB, and CTAB/Ti-Zr in the gel mixture with 1:1 Ti/Zr molar ratio was 0.2, 0.55, 0.12, 5.2, and 0.5, respectively. After stirring for 2 hours, the Ti-Zr containing gel mixtures were transferred into Teflon autoclave, and subsequently heated at 100°C for 2 days. After filtration and washing, the powders were dried at 100°C in air, a part of dried sample was chosen for small angle XRD test, the remaining part of the sample were further calcined at 350 or 450°C in nitrogen followed by in oxygen. Small angle X-ray diffraction (XRD) patterns were obtained on a Rigaku D/max RB X-ray diffractometer using Cu K a radiation. TEM images were obtained on a JEM-200C transmission microscope. Nitrogen adsorption/desorption isotherm was determined at 77K by means of Micromeritics ASAP 2010 surface area analyzer. Elemental analysis was done with X-ray fluorescence (XRF) analyzer on Shimadzu XRF-1700 spectroscopy. 3. RESULTS AND DISCUSSION 3.1. Effect of the Ti/Zr ratio
Figure 1. XRD patterns of various Ti-Zr samples
Figure 1 shows XRD patterns of the various Ti-Zr samples prepared from CTAB dried at 100°C, indicating that the crystallinity depends strongly on the Ti/Zr molar ratio in the gel mixtures. The results show that the intensity of XRD peak first increased and then decreased with an increase in the Ti/Zr molar ratios. The most intense and sharp peak at 2.3° 2 0 , with dioo=3.84 nm was observed on the sample with the 1:1 Ti/Zr molar ratio. It is likely that the better ordered mesostructure could be obtained on the Ti-Zr mixed oxides as compared to that on the pure Ti or Zr oxide., Similiarly, Chen et al recently reported that doping with 10 mol % titania could significantly increase the thermal stability of Ti02-Zr02 samples after post-synthesis treatment with phosphoric acid solution [4].
3.2. Effect of auxiliary organic components Figure 2 shows that addition of DDA could improve the mesopore structure of the 1:1 Ti/Zr mixed oxides at 350 "C remarkably, and the effect was also observed slightly for the addition of Triton-X 100 and triethanolamine. Additionally, the peak position of XRD pattern was shifted to a lower 2 9 value after addition of triethanolamine during the synthesis process, which indicated a wider pore diameter was obtained on the sample. Further heating these samples up to 450 "C unfortunately led to the disappearance of XRD peak at a small 2 0 angle, which indicated the collapse of the mesoporous structure. As for the sample prepared with the CTAB template mixed with HMTA, the XRD peak at a
233
small angle still retained after the sample was calcined at 450 °C for 2h as shown in Figure 3. It is clear that HMTA could improve the thermal stability of Ti-Zr mesoporous oxides more pronouncedly than other auxiliary organic components such as dodecylamine (DDA) did.
3 4 5 6 7 8 9
2 Theta (° )
10
Fig. 2. XRD patterns of the Ti-Zr samples prepared with and without (A) auxiliary organic components after calcination at 350 °C : DDA (B), triethanolamine (C), TritonX-100 (D).
Fig. 4. TEM images of the Ti-Zr sample prepared from CTAB and DDA after calcination at 350°C.
1 2
3
4
5
6
2 Theta (" )
7
Fig. 3. XRD patterns of Ti-Zr samples calcined at 450 "C (A), 350 "C (B) and 100 ''C (C) with the molar ratio of HMTA/CTAB/Ti+Zr as 5.2:1:0.5.
Fig. 5. TEM images of the Ti-Zr sample prepared from CTAB and HMTA after calcination at 450°C.
Figure 4 shows a TEM image of the Ti-Zr sample prepared from the mixture of CTAB and DDA after calcination at 350°C, indicating a lamellar ordered channels with continuous walls were clearly found on the sample; While Figure 5 shows a TEM image of the Ti-Zr sample prepared from the mixture of CTAB and HMTA after calcination at 450°C, illustrating a "worm-like" mesoporous structure was obtained. The two different mesopore structures imply that HMTA was possibly playing different role in the synthesis of Ti-Zr mesoporous oxides with DDA did. Apparently, the partial pyrolysis of HMTA and the resulting change in pH value of the gel acidity during the hydrothermal synthesis process may be one of the reasons for the difference. Figure 6 shows the Nitrogen adsorption/desorption isotherm and the BJH pore size distribution of the Ti-Zr sample prepared from the mixture of CTAB and HMTA after calcination at 450 °C. It revealed that the adsorption/desorption loop at relative pressure (p/po)
234
of 0.4-0.7 and 0.75-1.00, indicating the sample structures are of bimodal mesoporous, its BET specific surface area is 386 m^/g. The pore size distribution calculated from BJH equation is centered at 2.5 nm and 55 nm was also shown in Figure 6. The first peak was associated with the primary mesopores and the second peak was due to the secondary mesopores formed by secondary ^O) 3 0 0 particle aggregates [7], which was consistent with the TEM image in Figure 5. The similar worm-like Pore Diameter (nmV..'^.-' pore was observed on mesoporous alumina as reported by Bagshav et al [8] Elemental Analysis by XRF shows that only desorption minor amount of sulfur species, i. e., 0.026% of Relative Pressure (p/Po) sulfur by weight was detected on the calcined sample as compared to the higher amount of sulfur Fig. 6. Nitrogen adsorption/ desorption isotherm and pore size distribution from BJH species, i. e., 5-8 wt% sulfate for the sample (inset) for the Ti-Zr oxide sample. reported by Ciesla et al [9]. 4. CONCLUSIONS A well-ordered mesoporous Zr-Ti mixed oxide sample with the 1:1 Zr/Ti molar ratio was synthesized from inorganic salt precursors using either cetyltrimethylammonium bromide (CTAB) or a mixture of CTAB and an auxiliary organic component. It was also found that thermal stability of the mesoporous Zr-Ti oxides could be improved by the presence of dodecylamine, Triton-X 100, tricthanolamine, or hcxamethylenctctraminc (HMTA) under hydrothcrmal conditions. It is noteworthy that a high surface area, i. c., 386 mVg after calcinations at 450 °C was available on the Ti-Zr mesoporous oxides prepared from CTAB and IIMTA, moreover the mesoporous structures could remain on the calcined sample with only minor amount of sulfur species, i. e., 0.026% of sulfur by weight. ACKNOWLEDGEMENT National Natural Science Foundation of China (#29907003) is gratefully acknowledged. REFERENCES 1. D. M. Antonelli and J. Y. Ying, Angew. Chem. Int. Ed. Egnl., 34 (1995) 2014. 2. P. D. Yang, D. Y. Zhao, D. I. Margolese et al. Nature, 396 (1998) 152. 3. U. Ciesla, S. Schacht, G. D. Stucky, K. linger and F. Schuth , Angew. Chem., Int. Ed. Engl., 35(1996)541. 4. H. R. Chen, J. L. Shi, Z. L. Hua et al. Mater. Lett., 51 (2001) 187. 5. D. Trong On, Langmuir, 15(1999) 8561. 6. H. Fujii, M. Ohtaki and K. Eguchi, J. Am. Chem. Soc, 120 (1998) 6832. 7. M. L. Occelli, S. Biz, A. Auroux, G. J. Ray, Mesopor. Mesopor. Mater., 26 (1998) 193. 8. S. A. Bagshav, T. J. Pinnavaia, Angew. Chem. Int. Ed., 35 (1996) 1102. 9. U. Ciesla, M. Froba, G. D. Stucky and F Schuth , Chem. Mater 11 (1999) 227.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Synthesis and characterization of mesoporous zirconia Young-Woong Suh, Jung-Woo Lee and Hyun-Ku Rhee School of Chemical Engineering and Institute of Chemical Processes, Seoul National University, Seoul 151-742, Korea* Mesoporous zirconia has been synthesized using zirconium chloride and PEO nonionic surfactant, Triton X-100, as a zirconium source and a structure directing agent, respectively, in aqueous medium. From XRD, BET, SEM and TEM analyses, it can be known that the material treated with UV and ozone has a wormhole structure and a spherical morphology with uniform size. 1. INTRODUCTION Since the discovery of mesoporous silicates based on amphiphilic supramolecular templates [1], a number of studies have been reported concerning the preparation conditions, synthesis mechanism, characterization and use of these materials as catalysts and catalyst supports for various reactions [2]. This surfactant templating procedure was extended to the formation of non-silica mesoporous oxides [3], e.g., titania, niobia, tantala, alumina, manganese oxide, ceria, hafnia and zirconia. Among these non-silica oxides, zirconium oxide is of particular interest for acid catalysis [4]. Hence, much effort has been directed to the preparation of mesoporous zirconia using cationic quaternary ammonium [5-7], anionic surfactants [3,8] and primary amines [9] as the structure directing agents. More recently, Stucky and co-workers [10] prepared mesoporous Zr02 using PEO-PPO-PEO block copolymers and zirconium chloride in a nonaqueous medium. This material was reported to have a two-dimensional hexagonal structure with a semicrystallinc wall. They utilized inorganic salts as metal precursors and carried out the synthesis of mcsostructure in a nonaqueous medium such as ethanol solution, because the presence of excess water makes the hydrolysis and condensation of the reactive metal alkoxides as well as the subsequent mcsostructure assembly process difficult to control. In this study PEO nonionic surfactant with alkyl and aryl groups is used as the structure directing agent. Mesoporous zirconia is prepared in an aqueous medium in contrast to the work of Stucky and co-workers [10]. Finally, the material obtained in this work is compared to the one synthesized in a nonaqueous medium. 2. EXPERIMENTAL 2.1. Synthesis of mesoporous zirconia Triton X surfactants have structures given as (CH3)3CCH2CH(CH3)C6H40(CH2CH20);,H, where x = 8 (TX-114) or 10 (TX-lOO). The latter surfactant was utilized as the structure directing agent in the synthesis of mesoporous zirconia. In a typical preparation, 0.002 mol of 'Address for correspondence: E-mail, hkrhccq/'snu.ac.kr
Fax. +82-2-888-7295
Tel. +82-2-880-7405
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Triton X-100 was dissolved in 100 g of water. To this solution, zirconium chloride precursor in anhydrous ethanol (10 mL) was added very slowly with vigorous stirring. The molar ratio of Zr/surfactant was made equal to 8. The mixture was stirred in a thermostatic oil bath maintained at 100 °C for 48 h. Then, it was aged at 120 °C for 48 h to favor the mesostructure stabilization. After aging, the powder was obtained by the centrifiigation at 12,000 rpm for 30 min, washed with ethanol and dried at room temperature overnight. Instead of calcination, the material was finally treated with ultraviolet (UV) light and concomitantly generating ozone at room temperature to remove the occluded surfactants. It has been suggested in recent studies that UV/ozone treatment is an effective method for the removal of the template surfactants from bulk three-dimensional (3D) MCM-41 materials [11] or two-dimensional mesoporous silica thin films [12]. TX-lOO templated materials will be designated as TX-lOO—Zr02. 2.2. Characterization Powder X-ray diffraction patterns in the 26 range of 1—10° were collected at ambient temperature using Cu-Ka radiation, X = 1.54056 A, on a Philips X'Pert MPD diffractometer operating at 40 kV and 30 mA. Nitrogen adsorption and desorption isotherms were measured at 77 K on a Micromeritics ASAP 2010 system after the samples were vacuum-dried at 100 °C overnight. Surface areas were determined by the BET method in the 0.05-0.2 relative pressure range. The pore size distribution was determined by the Barrett-Joyner-Halenda (BJH) method. Transmission electron microscopy (TEM) studies were carried out on a JEOL JSM2000EXII electron microscope operating at 200 keV. The samples for TEM were mounted on a microgrid carbon polymer supported on a copper grid by placing a few droplets of a suspension of ground sample in ethanol on the grid, followed by drying at ambient conditions. Field emission scanning electron microscopy (FE-SEM) was performed on a JEOL JSM6700F microscope. 3. RESULTS AND DISCUSSION SEM image showing the particle texture of TX-lOO—Zr02 is shown in Figure 1. This surfactant provides the zirconia with well-defined elementary spherical morphology with a mean size of 200 nm. This particle texture was also observed in mesoporous Zr02 synthesized with Tween-20 surfactant [13]. These particles are much smaller than those usually obtained with MCM-41-type materials (mean size ~2 jim) [1]. Therefore, it is expected to obtain a textural porosity within the partial pressure range of 0.8 to 1. Figure 2 presents the XRD pattern for TX-100-ZrO2 treated with UV light and in-situ generating ozone at W[) 3.0mm ?.OkV X30,000 room temperature. The pattern resembles those obtained with MSUX materials with a single correlation Fig. 1. SEM image of UV/ozonc-treated TXpeak due to the 3D wormhole porous 100-ZrO2.
237
o-o o^_o^:0;*^-at«**^
:
y^ J
1..
. 1
0.0 2 theta /degrees
Fig. 2. XRD pattern of UV/ozone-treated TX-100-ZrO2.
0.2
0.4
0.6
'^w-^.-A-
1
Pora dlarTMl^r (nm)
0.8
1.0
Relative pressure (P/PQ)
Fig. 3. N2 physisorption isotherm of UV/ozone-treated TX-100-ZrO2. Inset: BJH pore size distribution.
framework structure [14]. This single peak pattern is typical of materials possessing uniform diameter pores in the mesoporous range, indicating that either the pore architectures of the materials are non-symmetrical or the particle sizes are small [14]. Since TX-lOO—Zr02 particles are relatively large [14], the single peak XRD pattern indicates that the particles have non-symmetrical worm-like pores. The peak is observed in the d spacing of 29.1 A, similar to the one (29.8 A) obtained from mesoporous zirconia synthesized with Tween-20 surfactant [13]. Mesoporosity of TX-100—Zr02 is illustrated by the N2 adsorption/desorption isotherms and pore size distribution as shown in Figure 3. The material exhibits a broad, but well-defined step in the adsorption isotherm and a clear hysteresis in the desorption isotherm over the relative pressure range of 0.4 to 0.8, which is indicative of the filling of the frameworkconfined mesopores. The existence of textural mesoporosity is evidenced by the presence of a hysteresis loop above PfP{) = 0.8. Some necking of the pore structure is suggested by the sharp curvature in the desorption leg of the hysteresis loop. Surface area determined by the BET method is 290m^/g, very high when compared to that of the conventional zirconia. The BJH model applied to the %. desorption branch of the isotherms verifies the expected bimodal framework (3.88 nm) and textural (21.7 nm) pore size distribution (see the inset of Figure 3). TEM image showing the ordered character of UV/ozone-treated TX100—Zr02 is presented in Figure 4. The spherical particles are observed in accordance with SEM analysis. It is Fig. 4. TEM image of UV/ozone-treated TXnoticed that no apparent order in the 100-ZrO2. pore arrangement exists, which is in
IIHL'''
238
good agreement with the absence of extra peaks in the X-ray diffraction patterns. In fact, the pore packing can be well described as wormhole-like or possibly sponge-like. Similar pore distributions have been observed for disordered mesoporous silicas and also aluminas when nonionic surfactants were used [14]. 4. CONCLUSIONS A mesoporous zirconia, UV/ozone-treated TX-lOO—Zr02, is synthesized using zirconium chloride and Triton X-100 in an aqueous medium. Apparently, the material is composed of elementary spherical particles with a mean size of 200 nm. It has both the framework and textural mesoporosities and a wormhole structure. In contrast to the work of Stucky and coworkers [10], the mesoporous zirconia of this study is synthesized in an aqueous medium using different kind of PEG nonionic surfactant. ACKNOWLEDGEMENT The authors gratefully acknowledge the financial support of LG-Caltex Oil Corporation and the partial aid from the Brain Korea 21 Program sponsored by the Ministry of Education.
REFERENCES 1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. 2. P.L. Llewellyn, Y. Ciesla, H. Decher, R. Stadler, F. Schuth and K.K. Unger, Stud. Surf. Sci. Catal., 84 (1994) 2013; A. Corma, M. Iglesia and F. Sanchez, Catal. Lett., 39 (1996) 153; P.T. Tancv, M. Chibwe and T.J. Pinnavaia, Nature, 368 (1994) 321. 3. M.S. Wong and J.Y. Ying, Chem. Mater., 10 (1998) 2067 and references therein. 4. T. Yamaguchi, Catal. Today, 20 (1994) 199. 5. U. Ciesla, S. Schacht, G.D. Stucky, K.K. Unger and F. Schuth, Angew. Chem., Int. Ed. Engl., 35 (1996) 541; U. Ciesla, F. Froba, G.D. Stucky and F. Schuth, Chem. Mater., 11 (1999) 227. 6. P. Liu, J.S. Reddy, A. Adnot and A. Sayari, Mat. Res. Soc. Symp. Proc, 431 (1996) 101. 7. J.A. Knowles and H.J. Hudson, J. Chem. Soc, Chem. Commun., 2083 (1995). 8. G. Pacheco, E. Zhao, A. Garcia, A. Sklyarov and J.J. Fripiat, Chem. Commun., 491 (1997). 9.N. Ulagappan, Neeraj, B.V.N. Raju and C.N.R. Rao, Chem. Commun., 2243 (1996); Y.-Y. Huang, T.J. McCarthy, W.M.H. Sachtler, Appl. Catal. A, 148 (1996) 135. 10. P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka and G.D. Stucky, Nature, 396 (1998) 152; P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka and G.D. Stucky, Chem. Mater., 11(1999) 2813. 11. M.T.J. Keene, R. Denoyel and P.L. Llewellyn, Chem. Commun., 2203 (1998). 12. T. Clark Jr., J.D. Ruiz, H. Fan, C.J. Brinker, B.I. Swanson and A.N. Parikh, Chem. Mater., 12 (2000) 3879. 13. Y.-W. Suh and H.-K. Rhee, Stud. Surf. Sci. Catal., 141 (2002) 289. 14. S.A. Bagshaw and T.J. Pinnavaia, Science, 269 (1995) 1242; P.T. Tanev and T.J. Pinnavaia, Science, 267 (1995) 865.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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A novel method to prepare mesoporous nano-zirconia Xin-Mei Liu''\ Max G. Q. Lu", Zi-Feng Yan"'^ ""Department of Giemical Engineering, University of Queensland, Brisbane 4072, AUSTRALIA ^State Key Laboratory for Heavy Oil Processing, Key Laboratory of Catalysis, CNPQ University of Petroleum, Dongying 257062, CHINA A novel method to prepare mesoporous zirconia was developed. The synthesis was carried out in the presence of PEO surfactants via solid-state reaction. The materials exhibit strong diffraction peak at low 2-theta angle and their nitrogen adsorption/desorption isotherms are typical of IV type with H3 hysteresis loops. The pore structure examined by TEM can be described as wormhole domains. The tetragonal zirconia nanocrystals are uniform in size (around 1.5nm) and their pores center at around 4.6nm. The zirconia nanocrystal growth is mainly via an aggregation mechanism. This study also reveals that the PEO surfactants can interact with the Zr-O-Zr framework to reinforce the thermal stabiHty of zirconia. The ratio of NaOH to ZrOCb, crystalhzation and calcination temperature play an important role in the synthesis of mesoporous zirconia. 1. INTRODUCTION Mesoporous nano-Zirconia is of particular interest recently because of its potential applications in chemical sensors of oxygen, solid oxide electrolyte of fuel cell, oxide electrode materials, and catalysis. The uniform mesoporosity of nano-zirconia is necessary to control the transport of the reactant molecules to active sites and determine the length of the triplephase boundary where charge transfer occurs for an electronically conducting electrode and is expedient to the percolation of electrons throughout the electrode microstructure '''"*'. The fme particle zirconia bears the better wear resistance '^' and the weaker diffusion resistance, which could be feasible to use the inner active sites in the catalyst and obtain the higher reaction conversion ratio. Simultaneously, the nanosize zirconia has higher adsorptive capacity, which exhibits the potential application in adsorption or separation. Thus very recently, nanosize zirconia with mesoporous texture has attracted considerable interest because of its large surface areas, unusual adsorptive properties, surface defects and fast diffusivities. Most zirconium oxides were generally synthesized via the sol-gel or precipitation processing using surfactant as template or scaffold agent in previous research. In this paper, a novel method combining solid-state reaction and in-situ crystallizing with polyethylene oxide surfactant to prepare the nanosize zirconia with mesoporous structure is tentatively presented. 2. EXPERIMENTAL 2.L Preparation of nano-zirconia The nanosize zirconia was prepared via solid-state reaction using zirconyl chloride (ZrOCl2-8H20) as precursors. Several procedures were investigated to elucidate the influence
240
of Zr/NaOH ratios, the calcinations and crystallization temperature, and the role of surfactants. Firstly, the ZrOCl2-8H20 and NaOH were milled to fine and mixed them at ambient temperature. Then the mixture were transferred to the autoclave and kept it at desired temperature for certain time. After that the mixture were washed with deionized water until free of CI' ions, and then washed with ethanol for two times to remove the water involved in the solid. Finally, the samples were dried at 383K for overnight and calcined at temperatures of 523K~ 773K in the fiimace for 20h using heating rate of 2^C/min. 2.2. Characterization of the samples The synthesized samples were characterized by nitrogen adsorption analyzer. X-ray diffraction (XRD), transmission electron microscopy (TEM) and thermal analysis (TG-DTA). 3. RESULTS AND DISCUSSION 3.1. XRD investigation
Fig. 1. XRD patterns of zirconia with the different of calcination temperature. Insert: High-angle peaks
Fig. 2. The XRD pattems of zirconia with the different of crystallizing temperature
Of interest is a rather broad low-angle peak occurred in XRD pattems of as-synthesized zirconia shown in Figure 1 and the other four peaks appeared at high 20 degrees, which shows that the synthesized samples are single-phase zirconia with tetragonal structure. It also means that such as-synthesized zirconia actually bears mesoporous skeleton. The broad shape of the XRD peaks means that the as-synthesized samples may possess the less ordered mesoporous structure and the particle may be ultrafine. The particle size estimated by the Sherrer equation is about 1.5nm as perceived from extensive comparison of TEM images. Figure 1 also illustrated that the XRD peaks tended to be sharp and strong with the increase of the calcinatiion temperature. It indicated that the agglomeration and surface reconstruction of as-synthesized nano-zirconia samples occurred in the process of calcinations. Such agglomeration and surface reconstruction might result in the growth of mesoporous nanozirconia particle sizes. The growth of the particle size can be attributed to condensation of the abounding surface hydroxyls groups, which causes the nucleation of new oxide crystals and the growth of the existing one at higher temperature ^^l It is noteworthy that the crystallizing temperature plays an important role on the crystal phase of the zirconia. Figure 2 illustrates that the sample crystallized at ambient temperature exhibits broader diffraction peaks with rather weak intensity, which shows that the sample is the amorphous pattern and particle size is ultrafine. As indicated by the position of the main diffraction peak and the ticks corresponding to tetragnol zirconia, the amorphous structure had
241
the tetragonal zirconia local order. The weak XRD peak means that mesoporous nano-zirconia samples bear tetraganol skeleton but enrich many defects and/or lattice vacancies. Such large number of lattice vacancies and local lattice disorder result in weak in diffraction intensity and even to disappear of crystal planes ^'^\ Upon heating at elevated temperatures, the zirconyl clusters can agglomerate each other and generate many small nuclei ^^\ This results in the larger particle size and more ordered nanocrystalline at higher temperature. However, the monoclinic phase can be formed when the temperature up to 200°C although the signal is not apparent. This means the target crystal phase can be obtained by controlling the crystallizing temperature.
100 200 300 400 500 600 700 800 900 100 Temperature / °C
Fig. 3. Nitrogen adsorption/desorption isotherm of zirconia with different calcinating temperature
Fig. 4. Profile of the TGA spectrum of as- prepared Zr02
3.2. Nitrogen adsorption isotherm The isotherms of the samples all are typical IV isotherms with type H3 hysteresis loops just as shown in Figure 3. It means that the zirconia prepared with this novel method is comprised of the aggregate of plate-like particles forming slit-like pores. It also exhibits that the calcinating temperature plays an important role in the pore structure formation. At the elevated temperature the thermal lattice contraction might occur and the particle size can grow, which results in the larger mesopore generation. NaOH/Zr ratio is another key factor to synthesize mesoporous nano-zirconia. An increase of ratio from 2 ~ 4.0 resulted in an enormous increase of the adsorption capacity of synthesized zirconia. However, the adsorption capacity will be slightly decreased when the NaOH/Zr ratio is above 4.0. Consequently, the specific surface area changed from 182.3 m^/g to 363.9 m^/g, and then decreased to 314.8 m^/g when the ratio is up to 5.0. Of interest is that the inception point of the hysteresis loop shifts to the lower pressure region with the increase of the NaOH/Zr ratio. It indicates that the mesopore diameter of synthesized zirconia obviously shrink with the increase of the NaOH/Zr ratio. It shows that the pore sizes of zirconia prepared with solid-state reaction can be tuned by choosing various ratio of NaOH to ZrOCb. 3.3. Thermal analyses Two weight loss stages were observed in TGA profile of nano-zirconia sample illustrated in Figure 4. The first one that located at low 373K corresponded to the evaporation of the water adsorbed in the sample. The weight loss presented between 523K and 773K is the removal of the terminal hydroxyl groups bonded on the surface of zirconia. Such great weight loss between 523 and 723K means that many hydroxyl groups enriched on the surface of synthesized nano-zirconia. When the samples were annealed above 773K, no further weight
242
loss was observed. This means that the thermal stability of the zirconia prepared with solidstate reaction is well. 3.4. TEM images
^dTZ"
Fig. 5. TEM images of zircoinia.
''l^...i
TEM imagines of nano-zirconia sample depicted in figure 5 positively supported the acquired XRD and nitrogen adsorption/desorpotion results. It confirmed synthesized zirconia samples actually have uniform mesopore and nano-crystalline particles. The mesopore architecture of these zeolite-like is best described as the worm hole. These pore structure have been noted in catalysis and adsorption owing to its greater accessibility to surface sites for gaseous species ^^\ The lattice images exhibit the necking between crystallites while a void region representing the pores winds extensively throughout the structure. 4. CONCLUSION The mesoporous nano-zirconia can be initiatively synthesized by solid-state reaction. 1) The pore size can be tuned by changing the NaOH/ZrOCb ratio. 2) The different crystal phase can be formed at different crystallizing temperature. 3) The particle growth is mainly via an aggregation mechanism 4) The nanostructure is strongly influenced by the NaOH/ZrOCb ratio, calcinating and crystallizing temperature. REFERENCES 1. M. Mamak, N. Coombs, G. Ozin, J. Am. Chem. Soc. 122 (2000) 8932. 2. H. Verveij, Adv. Mater 10 (1998) 1483. 3. A. Ziehfreund, U. Simon, W. F. Maier, Adv. Mater. 8 (1996) 424. 4. F.P.F. van Berkel, F.H. van Heuveln, J.P.P. Huijsmans, Solid state Ionics, 72 (1994) 240. 5. Y.J. He, A.J.A. Winnubst, A. J. Burggraaf, H. Verweij, PG. van der Varst, and B.G. De With, J. Am. Ceram. Soc. 9 (1996) 3090. 6. J.A.Wang, M.A. Valenzuela, J. Salmones, etc., Catal. Today, 68 (2001) 21. 7. G.G. Siu, M.J. Stokes, Y.L. Liu, Phy Rev. B, 59 (1999) 3173. 8. Michael Z.-C. Hu, Michael T. Harris, Charles H. Byers, J. Colloid and Interface Science, 198(1998)87. 9. M. Yoshimura, Am. Ceram. Soc. Bull. 67 (1998) 1950.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Control of ordered mesoporous molecular sieves synthesis using non-ionic surfactants by incorporation of transition metal ions in the micellar solution A. Leonard ^ J.L. Blin, G. Herrier ^ and B.-L. Su* Laboratoire de Chimie des Materiaux Inorganiques, ISIS, The University of Namur (FUNDP), 61, rue de Bruxelles, B-5000 Namur, Belgium Phone : +32-81-72-45-31, Fax : +32-81-72-54-14, e-mail: bao-lian.su@fundp.ac.be Ordered mesoporous silica molecular sieves were obtained using a series of non-ionic Cm(EO)n surfactants. The control of the hexagonal structure was achieved by adding transition metallic cations to the micellar solutions. It has been shown that highly organised CMI-2 and 4 materials could be obtained with Ci6(EO)io and Ci8(EO)io whereas disordered wormhole-like mesostructures were reached with Ci3(EO)6 and Ci3(EO)i2. 1. INTRODUCTION The control of the internal structure and texture as well as external morphology is essential in the design of new materials such as nanobiosensors and opto-electronic devices and their application in industrial processes. Large-pore mesoporous materials were recently prepared with use of polyoxyethylene alkyl ether surfactants [1-5]. This is a more environmentalfriendly way for synthesis as these surfactants are less toxic and more biodegradable than their ionic analogues generally used in the preparation of MCM-41. Besides, it appears that the recovery of the template is easier and so a further re-utilisation could be envisaged. Previous studies using this kind of surfactants have shown that the textural, structural and morphological features of the final mesoporous compounds were strongly affected by physico-chemical variables such as the surfactant / silica molar ratio, pH of the synthesis gel, stirring duration and hydrothermal treatment conditions [6, 7]. Especially the control of the structure of the materials is important, since the 3-dimensional structure of MSU could be more appropriate for catalysis. Whereas the fabrication of semi-conducting wires [8] would require a regular array of long straight channels. One way allowing the combination of the advantages of PEO-type surfactants with the yield of highly ordered materials was proposed by Pinnavaia et al. These authors induced an electrostatic control of the surfactant - silica assembly process by complexing small transition metallic cations by the hydrophilic oxyethylene heads of the template [9]. In this work, we have investigated if this method is effective in the obtention of highly ordered mesoporous molecular sieves by using a series of non-ionic Cm(EO)n surfactants.
^ : FRIA fellow : Corresponding author
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2. EXPERIMENTAL 2.1. Synthesis Micellar solutions with defined weight percentages of Cm(EO)n were prepared by dissolving the surfactant at a temperature below its cloud point value in an aqueous solution containing cobalt chloride. The cation / surfactant molar ratio was varied from 0.25 to 4.00. Sulfuric acid was then added to decrease pH to a value of 2. After homogenization, TMOS was added dropwise in order to reach a surfactant / silica molar ratio of 1.50. After stirring during 1 hour, the synthesis gel was poured in teflon-lined cartridges sealed in stainless steel autoclaves and submitted to hydrothermal treatment. The recovered gel was then extracted using a Soxhlet apparatus, dried and calcined at 550°C under nitrogen and oxygen. 2.2. Characterization XRD measurements and Transmission Electron Microscopy using a Siemens D-5000 diffractometer and a Philips Technai lOOkV microscope respectively assessed structural features. For TEM observations, the powders were embedded in an epoxy resin and sectioned with an ultramicrotome. The final morphologies were observed using a Philips XL-20 Scanning Electron Microscope. The textural characteristics of our compounds were evaluated by nitrogen adsorption-desorption measurements with a volumetric adsorption analyzer ASAP 2010 or Tristar 3000, both manufactured by Micromeritics. The pore size distributions were calculated by the BJH method applied to the adsorption branch. 3. RESULTS AND DISCUSSION 3.L C,6(EO),oand Ci8(EO^,o surfactants. The introduction of Co^ cations leads to the formation of organized materials obtained with 50 wt.% Ci6(EO)io micellar solutions as can be seen from TEM micrographs. Working in the same concentration domain without cations usually leads to wormhole-like structures [6]. The inserted FFT's (Fig. 1) show that ordering gets better if the concentration in cations ,„^ ..,,....._ ...-, ... is raised. The X-ray diffraction ^i':<^.:''^^yi .• ,^ • •:' "^^f / patterns however show poorly •:^; V >'. :>;. _ • ^::j- ; ~*L*J^"~ resolved secondary reflections, - V ' • / \ indicating that organisation is not perfect. The morphology (not - i'.: ' shown here) is also affected. Indeed, in this case, the particles .•. ^(),ljn ^^^^ ^h^ appearance of splitted .I . . "™ '"' *^'^' " " ' -'•—~ leaves compared with the smooth Fig. 1. TEM micrographs of compounds prepared with blocks that were obtained in the Co^^/Ci6(EO)io molar ratios of a: 0.25 and b : 1.25. absence of transition metallic cations. Using Ci8(EO)io, well-ordered materials can be prepared if Co^^ cations are added to the micellar solution. In the absence of these cations, hexagonal materials were obtained at concentrations below 30wt.% [10]. In this case, the compounds become organised below 40wt.% micellar solutions (Fig. 2b-f). Indeed, the diffractograms show secondary reflections, which can be indexed in a hexagonal system. The TEM pictures showing the honeycomb-like channel array also confirm this arrangement. This suggests that the domain of existence of isolated cylindrical micelles allowing the obtention of ordered materials through a cooperative mechanism is enlarged in the presence of the cations. The presence of the cation
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probably induces some changes in the packing parameter of the surfactant. Its value decreases if the relative surface of the hydrophilic head becomes larger by complexation of the cobalt ion, leading to the transition from a hexagonal phase to isolated cylindrical or spherical micelles. Regular hexagonal materials are then formed by a cooperative mechanism involving the assembly of silicate covered cylindrical micelles, as observed for CMI-1 and 3. The formers are also stabilised by the rigidification induced by complexation of the cation by the oxyethylene part of the template.
4
^
'
•
29 n
6
^ n pit^i Fig. 2. XRD patterns of compounds prepared with solutions containing Fig. 3. TEM micrograph of a compound 0.705M Co^' as a function of C,8(EO),o prepared with a Ci8(EO)io concentration of weight percentage : a : 50, b : 40, c : 30, 5 wt.% in the presence of Co^^ ions. d : 20, ein: the 10 and : 5 (d-spacings in Concerning texture, for both surfactants, there is a decrease poref size as the content in cobalt is raised (Table 1). The specific surface area of the materials however remains very high. When oxyethylene heads surround the cations, their conformation is frozen. This rigidification could be accompanied with a retraction of the hydrophilic head when increasing the cobalt chloride concentration. This contracted conformation could then account for the smaller pore sizes of the obtained materials, especially for the higher amounts of added transition metal.
3.2. Ci3(EO)i2and Ci3(EO)6 surfactants. Using Ci3(EO)i2, variable amounts of cobalt chloride were added to 15wt.% micellar solutions. Indeed, the materials obtained with 50wt.% were supermicroporous with pore sizes below 2.0 nm. From XRD measurements, it can be seen that secondary reflections appear and become more intense as the amount of added Co^"^ cations increases (Fig. 4). However, these Table 1 Evolution of pore sizes (in nm) as a function of the Co^^ / surfactant molar ratio for different Cm(EO)n surfactants C0^VCn.(E0)n 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 3.00 4.00 molar ratio 4.0 Ci6(EO),o5^w/.% 3.8 2.5 5.5 2.7 3.7 Cis{EO)io 50 wt.% 3.2 4.3 Ci8(EO),o 15 wt.% 2.9 <2.0 2.6 C,3(EO)6 50wt.% 3.7 3.4 3.1 3.5 Ci3(EO),2 15 wt.% : no data
246
peaks are not well resolved indicating that the materials obtained with this surfactant are not as ordered as those obtained with Ci8(EO)io. Without addition of cations, we already observed that it was impossible to obtain highly ordered phases by lowering the surfactant concentration in solution. The co-operative mechanism proposed in the cases of Ci6(EO)io and Ci8(EO)io for the low surfactant weight percentages could not apply in the case of Ci3(EO)i2 [5]. This was explained by their different hydrophilic to hydrophobic volume ratio (VHA^L) compared to the templates with the longer alkyl tails [11]. The much higher VHA'^L value leads to less homogeneity in the micellar system. The addition of cobalt cations to the micellar solution induces some rigidity of the hydrophilic head and confers a global positive charge to it. This could then account for the formation of materials, which, though not perfectly ordered, exhibit more regularity than their analogues prepared without addition of cations. It is also observed from table 1 that the pore size remains constant, showing again that there is a very different behaviour for this templating agent. Cobalt cations favour the obtention of a more homogeneous system of isolated micelles, leading to materials with well-defined pore sizes. 4. CONCLUSIONS
26 n Fig. 4. XRD patterns of compounds prepared at 15 wt.% Ci3(EO)i2 with Co^^ / surfactant molar ratios : a : 0, b: 1.0,c:2.5.
The addition of transition metallic cations to the micellar solution leads to a better control of the structure of the final mesoporous materials. Compounds obtained with Ci6(EO)io and Ci8(EO)io have an ordered channel array whereas more homogeneous disordered structures are prepared with Ci3(EO)i2. ACKNOWLEDGEMENTS This work has been performed within the framework of PAI/IUAP 4-10. Alexandre Leonard and Gontran Herrier thank FNRS (Fonds National de la Recherche Scientifique) for a FRIA scholarship. REFERENCES 1. G.S. Attard, J.C. Clyde and CO. Goltner, Nature, 378 (1995) 366. 2. D. Zhao, Q. Huo, J. Feng, B.F. Chmelka and G.D. Stucky, J. Am. Chem. Soc, 120 (1998) 6024. 3. E. Prouzet, F. Cot, G. Nabias, A. Larbot, P. Kooyman and T.J. Pinnavaia, Chem., Mater., 11 (1999) 1498-1503. 4. P.T. Tanev and T.J. Pinnavaia, Science, 267 (1995) 865. 5. J.L. Blin, A. Leonard and B.L. Su, Chem. Mater. 13(10) (2001) 3542. 6. J.L. Blin, A. Leonard and B.L. Su, J. Phys. Chem. B. 105 (2001) 6070. 7. A. Leonard, J.L. Blin and B.L. Su, Stud. Surf. Sci. Catal. 141 (2002) 109. 8. C.G. Wu and T. Bein, Chem. Mater., 6 (1994) 1109. 9. W. Zhang, B. Glomski, T.R. Pauly and T.J. Pinnavaia, Chem.Commun.,(\999) 1803. 10. G. Herrier and B.L. Su, Stud. Surf. Sci. Catal. 135 (2001) 08-O-02. 11. J.L. Blin, A. Leonard, G. Herrier, G. Philippin and B.L. Su, Stud. Surf Sci. Catal. 141 (2002) 117.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
247
Texture of chromia aerogels and structure of their nanocrystals M. Abecassis-Wolfovich''^ H. Rotter''^ M.V. Landau'^ E.Korin^ A.I. Erenburg", D. Mogilyansky^ and E.Garshtein^ ' The Blechner Center for Industrial Catalysis, ^ Chemical Engineering Department, BenGurion University of the Negev, Beer-Sheva 84105, Israel '^The Institutes for Applied Research, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel Mesoporous chromia aerogels with a surface area of 484-735 m V ^ a pore volume of 0.40.9 cm^ g'^ and a pore diameter of 3-9 nm were prepared by urea-assisted homogeneous precipitation from an aqueous Cr(N03)3 solution, followed by continuous supercritical CO: extraction. The aerogels were characterized by means of N2-adsorption isotherms, AA, HRTEM, FTIR, thermoanalytical methods TPD, TPO, DSC and X-ray diffraction in combination with structure modeling. For production of mesoporous aerogels with surface areas > 700 mV^was required Pco^ of ~ 400 bar. The chromia aerogels consisted of 3 to 5 nm nanoparticles representing a monoclinic analogue of a-CrOOH in which half the O atoms and OH groups were replaced with coordinately bonded water molecules. After dehydration at 550-600 K the materials retained their texture, being converted to two-dimensional fragments (clusters) of a -CrOOH crystals built on [Cr(OH)303] octahedra without bonding along the Zaxis (N2, vacuum) or to amorphous CrOi (air). At temperatures of >650 K in air and of > 773 K in an inert atmosphere, the material was converted to a-Cr203 with 50-nm particles . 1. INTRODUCTION Ultrafine nanostructured chromium(III) oxides with a high surface area find application in a wide variety of uses, particularly as pigments, coating materials and heterogeneous catalysts. The most efficient synthetic route for producing porous chromium (III) oxides is urea-assisted sol-gel processing giving a surface area in the range of tens to hundreds of square meters per gram ^^'^\ The gel that is formed can be dried in two different methods: oven drying "* or supercritical solvent release to eliminate collapse of the gels structure ^'^\ The structural characterization of urea-assisted chromia aerogels has never been systematically undertaken. In the present work, we focus on the two problems that have so far been overlooked in preparation of nanostructured chromium oxides: 1) the effect of low-temperature CO: supercritical extraction conditions on the texture of urea-assisted chromia aerogels, and 2) structure of primary nanoparticles forming aerogels and their texture/structure thermo transformations. 2. EXPERIMENTAL A wet gel of chromium(III) hydroxide was prepared by mixing aqueous solutions of urea and Cr(N03)3'9H20 in ratio of 1.5:1 (v/v), agitating the mixture at 368 K for 6 h, and final
248
aging at room temperature for 16 h. The wet gel was separated by filtration, washed and loaded into a flask for replacement of water for hexane-butanol by distillation. After solvent replacement, the wet gel was converted to an aerogel by supercritical drying (extraction) with CO2. The effects of supercritical drying conditions at different pressures (116 - 456 bars) and drying times (0.25-2 h) were studied at a temperature of 313 K and a flow rate of 1 ml min"^ The discharged aerogel was further dehydrated under vacuum (85 mbar) at 373-593 K to yield a high-surface area nanostructured chromium oxide material. The information about the texture of fresh and thermally treated Cr-aerogels was obtained from N2-adsorptiiondesorption isotherms and by HRTEM. The chemical composition and the structure of nanoparticles was evaluated from their elemental analysis, thermoanalytical behavior (TPD, DSC, TGA, TPO), FTIR and XRD data combined with the modeling of XRD patterns of the material by means of a Rietveld-based software program DBWS-9807. 3. RESULTS AND DISCUSSION The effect of supecritical drying conditions on the texture parameters of Cr-aerogels evacuated at 373 K is shown in Table 1. Increasing the pressure of CO2 extraction from 116 to 456 bars gradually increased the surface area of the chromia aerogels from ~ 530 to ~ 730 m V ' and more than doubled the pore volume from -0.42 to 0.93 cm^g\ while the mean mesopore diameter remained almost constant in the CO2 extraction pressure range of 184-456 bars. All of the aerogels displayed a type IV isotherm, with desorption hysteresis and the mesoporosity of a relatively narrow PSD. The PSD was shifted to higher pore diameters by increasing the CO2 pressure from 116 to 184 bars, while the further pressure increase caused the PSD narrowing (Fig. 1). Untreated Cr-aerogel consisted of disordered closely packed 3- to 5-nm, almost globular, primary particles (Fig.2a). Evacuation at 373 resulted in formation of faceted friable packed nanoparticles of the same size (Fig.2b). Their size and arrangement did not change up to 593 K (Fig.2c). By varying the calcination temperature up to 593 K it is possible to achieve the partial dehydration of nanoparticles removing up to 2 water molecules per I Cr-atom without substantial changes of materials texture. Table 1 Textural properties of chromia aerogel after evacuation at 373 K Pore diameter, nm Extraction Sample Surface area , m2**g-1 PCO:, conditions # kg*m"^ '
P,bar
T,h
2M
116
0.25
0.643
484
334
4M 3M lb 2b 3b 4b 5b
116 116 184 252 320 387 456
1 2
0.643 0.643 0.740 0.840 0.915 0.958 0.976
539 540 525 651 599 735 712
363 364 86 108 171 247 152
Total
Micropore Average
Pore volume.
Mean
cm^'g '
3.1
3.6
0.37
3.1 3.5 4.9 5.5 5.0 4.8 5.2
3.5 3.6 4.6 4.7 4.7 4.7 4.5
0.42 0.45 0.65 0.89 0.74 0.87 0.93
249
The XRD data (Fig.3) combined with a structure modeling approach taken together with the results of the AA, TPD, TGA, TPO, DSC and FTIR studies provide evidence for the crystallohydrate nature of the parent chromia aerogel extracted with supercritical CO2 (CrOOH 2H2O). Its structure (Fig.4) is similar to that of monoclinic analogue of a-CrOOH in which half the O atoms and OH groups were replaced with coordinately bonded water molecules
2b, 252 bar 3b, 320 bar 4b, 387 bar 4M, 116 bar
2
4
6
8
10
12
Pore diameter (nm)
Fig. 1. Pore size distributions of chromia aerogels extracted at different conditions
t
hO.O nm 2t_200000
O.OOnm 200000
. >.£:'
Fig. 2. HRTEM micrographs of a typical chromia aerogel (4b): (a) after CO2 extraction; (b) after CO2 extraction and evacuation at 373 K;(c)after CO2 extraction and evacuation at 593K
o-cn
•
-
•
'
<
• '-•L.
%i!^^
:i:«,«.T?'
i > 0 0 —I
Fig. 3. X-ray diffractograms of chromia aerogel (4b) after CO2 extraction at 313K (1), followed by further evacuation at 593K (2) aerogel after : a. calcination at 593 K (3) and 723K (5) in air b. calcination at 723 K in inert atmosphere (4)
Fig. 4. Model structure of a hydrated chromia aerogel CrOOH'2H20
250
This material undergoes gradual dehydration at elevated temperatures forming anhydrous CrOOH in inert atmosphere (vacuum) at - 593 K (Fig.3-2) or CrOz in air at the same temperature (Fig. 3-3). It yields nanostructured materials with a texture similar to the starting aerogel and surface area > 500 m^^- After endothermic dehydration in N2 or vacuum the nanoparticles contained two-dimensional fragments (clusters) of a -CrOOH crystals built on [Cr(OH)303] octahedra without bonding along the Z-axis (Fig.5). The dimensions of the octahedtra corresponded to the d-spacings calculated from the X-ray patterns (Fig3-2,4) of dehydrated aerogel are shown in Fig 5. After endothermic dehydration in air followed by exothermic Cr(III) ^Cr(IV) oxidation at 450-593 K the material consisted on almost amorphous Cr02 nanoparticles (fig 3-3) with texture similar to that of aerogels dehydrated in oxygen-free atmosphere. At higher temperatures depending on the treatment atmosphere, exothermic dehydration-recrystallization (CrOOH, N2, vacuum, > 773 K) or decompositioncrystallization (Cr02, air, >650 K) resulted in an exothermic glow transition into large (- 50 nm) crystals of well-defined a-Cr203 (Fig.3-5) with a low surface area of <200 m~g'' (Fig 6).
0.20 nm 0.15nm 0.24 run Fig. 5. Two dimensional clusters - fragments of a-CrOOH lattice dehydrated Cr - aerogel nanoparticles:
building blocks of
800 700-:^
600 500 400 300
D • Pore^iameter Pore volume Surface area
310
375
440
505
570
635
<^ ^ £ g
200 I
700
765
100 5 0 830
T(K)
Fig. 6. Effect of the calcination temperature in oxidative (open figures) or inert (N2 solid figures) atmosphere, on the texture parameters of chromia aerogel (4b).
REFERENCES 1. R.L. Burwell, K.C. Taylor, G.L. Haller, J. Phys. Chem., 71(13) (1967) 4580. 2. S. Music, M. Maljkovic, S. Popovic, R. Trojko, Croat. Chem. Acta, 72(4) (1999) 789. 3. J.N. Armor, E.J. Carlson, W.C. Conner, Reactivity of Solids, 3 (1987)155.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
251
Preparation of ordered mesoporous NbTa mixed oxide with crystallized wall T. Katou ^ B. Lee \ D. Lu ^ J. N. Kondo \ M. H a r a ' and K. Domen''^ ^Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, 226-8503, Japan ''Core Research for Evolutional Science and Technology, Japan Science and Technology
An ordered 2D-hexagonal mesoporous Nb and Ta (Nb:Ta=l:l) mixed oxide was successfully crystallized.
A new strategy for preserving mesoporous structure after the
crystallization of the walls was performed by re-filling a templating material to the mesopores before crystallization for suppressing the destruction of mesoporous structure. Transmission electron microscopy and electron diffraction analyses revealed the formation of crystallized mesoporous NbTa mixed oxide with single crystal phases in periodical mesoporous structure (denoted NbTa-TIT-2) at ca. 100-nm range. 1. INTRODUCTION Compared with silica-based mesoporous materials, less work has been directed to non-silica mesoporous materials although they have high potential in wide range of applications.' oxides
For example, in terms of photocatalysis, mesoporous pure and mixed Ta
showed
considerable
photocatalytic
activity
decomposition, in spite of the amorphous wall structure."^'^
for
the
stoichiometric
water
The crystallization of the walls
of mesoporous materials would be one of the important subjects for advancing various applications, although at present, the wall structure of almost all the mesoporous materials is amorphous and the preservation of the original mesoporous structure after crystallization is difficult.
We have attempted to solve this problem."^'^
One of the strategies is to re-fill the
This research was supported by Core Research for Evolutional Science and Technology (CREST) program of the Japan Science and Technology Corporation.
252
mesopores and cover the whole particles by thermally stable materials.
If this re-templating
material could be removed after crystallization of mesoporous substance, mesoporous structure would be preserved.
In this report, among various re-templating sources (sucrose,
ftirfuryl alcohol, glucose, silica, BaCh, etc), the results obtained by using amorphous carbon using furftiryl alcohol are shown. 2. EXPERIMENTAL The 2D-hexagonally ordered mesoporous NbTa oxide (amorphous precursor) was synthesized by the method introduced in previous report.^
The crystallized ordered
mesoporous NbTa oxide, NbTa-TIT-2, was prepared as follows.
The ftirfuryl alcohol vapor
(0.023 mol/min) with nitrogen gas (30 ml/min) was passed on the amorphous precursor fixed in reactor at 473 K.
After the furftiryl alcohol vapor treatment, polymerized fiirftiryl alcohol
is accumulated in pores of amorphous precursor.
Then, the carbonization of polymerized
furfuryl alcohol was performed at 823 K for 3 h in vacuo.
Then, the crystallization of the
carbon-filled and coated mesoporous NbTa oxide was conducted by the calcination at 923 K for 2 h in helium to avoid the removal of re-templated carbon during the high-temperature treatment.
Finally, the carbon included in the crystallized sample was eliminated by the
calcination at 773 K in air. X-ray diffraction (XRD) patterns were obtained using a Rigaku RINT 2100 diffractometer with CuKa radiation.
N2 sorption isotherms were recorded using a Coulter SA3100 system.
The sorption data were analyzed by Barrett-Joyner-Hallenda (BJH) method.
Transmission
electron microcopy (TBM) images and electron diffraction (ED) patterns were obtained by JEOLJEM2010F(200kV). 3. RESULTS AND DISCUSSION The physicochemical properties of mesoporous NbTa mixed oxide were considerably affected by the amount of metal sources, as well as the amount of added water.^
The results
of XRD, N2 sorption isotherm and a TEM image together with the ED pattern of mesoporous NbTa oxide prepared under the optimized condition are shown in Fig.l.
As shown in Fig.l,
the low-angle (1-6 degree) XRD pattern and the N2 adsorption isotherm are consistent with hexagonally ordered mesoporous structure.
The TEM image and ED pattern ftirther support
the proposed 2D-hexagonal mesoporous structure of NbTa oxide.
253 200 150
ra
100
"c
e
13
^
'o
50
>
CD
^ €
10.0 nm
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0 •
0
02
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oe P/Pu
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_
0)
^
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2
•
1
^- — 1
r
1
3 4 5 26 (degree)
'Mi^
1
Fig. 1. Low-angle XRD pattern (left), N2 sorption isotherm and representative TEM image and diffraction pattern (right) for NbTa oxide sample prepared under the optimized conditions of water and metal sources. It is noted that the wall thickness estimated by assuming a hexagonal structure was ca. 2.6 nm. Crystallization of the sample after calcination of 923 K was confirmed by wide-angle XRD pattern (not shown).
Fig.2(a) displays a typical TEM image of NbTa-TIT-2, crystallized
NbTa oxide with periodical mesoporous structure.
The mixed spot ED pattern (inset of
Fig.2(a)) obtained from a whole particle reveals the presence of multi crystal phases.
In
order to estimate the size of a single crystal domain, ED patterns were collected from places of different sizes.
The ED pattern (Fig.2(b)) taken from a 100-nm range (shown image)
shows a spot ED pattern, which indicates a single crystal phase in the range.
From high
resolution TEM (HRTEM) image of walls displayed in Fig.2(c), lattice fringes in a limited place, where ordered pores are directly observed, run in the same direction.
From these
results, it is considered that the ED pattern of mixed spots (inset of Fig.2(a)) taken from a whole particle is resulted from the fact that a crystallized particle with the ordered mesoporous structure consists of phases by ca. 100-nm ranged single crystal domains.
This
means that the ordered crystallized mesoporous NbTa oxide, NbTa-TIT-2, is different from single crystal particles of worm-hole mesoporous NbTa oxide, NbTa-TIT-1, in the size of a single crystal domain.
In the case of worm-hole structure, single crystal domain spreads to
several hundreds nanometer size, whereas ordered mesoporous structure
254
m
:5^" ii--#
-•K-
SO.Onm
(^\
Fig. 2. Typical TEM images and ED patterns of NbTa-TIT-2.
(a) A particle with periodical
mesoporous structure (inset : ED pattern collected from whole particle), (b) periodical mesoporous structure in 100-nm ranges single crystal domain (inset : ED pattern collected from the image) and (c) HRTEM image of walls. suppresses the size to ca. 100 nm.
It is also mentioned that in the case of the
2D-hexagonally ordered mesoporous NbTa oxide calcined under the same condition without re-filling template, the crystallized sample as NbTa-TIT-1 was obtained.
Therefore, the
presence of re-filling template appeared to be effective for preserving mesoporous structure during the crystallization. In conclusion, it was found that NbTa-TIT-2 prepared by the use of furfuryl alcohol as re-templating source possessed ca. 100-nm ranged single crystal domains in a particle with the original 2D-hexagonally ordered mesoporous structure.
We expect that this strategy
would be improved and become one of the general methods applicable to various materials. REFERENCES 1. U. Ciesla and F. Schiith, Microporous and Mesoporous Materials, 1999, 27, 131. 2. Y. Takahara, J. N. Kondo, T Takata, D. Lu and K. Domen, Chem. Mater., 13, 1200. 3. M. Uchida, J. N. Kondo, D. Lu and K. Domen, Chem. Lett., 498 (2002). 4. B. Lee, D. Lu, J. N. Kondo and K. Domen, Chem. Commun., 2001, 2118. 5. B. Lee, T. Yamashita, D. Lu, J. N. Kondo and K. Domen., Chem. Mater, 2002, 14, 867. 6. T. Katou, D. Lu, J. N. Kondo and K. Domen, J. Mater. Chem., 2002, 12 1480.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. Allrightsreserved
255
Compositional effects of bimodal mesopore silica synthesized by a basecatalyzed ambient pressure sol-gel processing X.- Z. Wang," ^ * W.- H. Li,^ T. Dou' and B. Zhong^ ^Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, 030024 China E-mail: \vangxiaozhong@tvut.edu.cn ''State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, the Chinese Academy of Sciences, Taiyuan, 030001 China The effects of tetraethylorthosilicate (TEOS) concentration and TEOS/surfactant molar ratio on the synthesis of bimodal mesopore silica (BMS) were studied. It was found that the BMS silica can be synthesized in a wide range of component concentration and its secondary mesopore size is more sensitive to the change of precursor concentration than its primary mesopore size and the secondary mesopore volumes can be up to 2.0 or more times as large as the primary mesopore volumes. The controllability of the bimodal mesopore size distributions, in particular the secondary mesopore size of BMS silica is of great interest to catalysis because they greatly facilitate mass transport to the primary mesopore. 1. INTRODUCTION The synthesis of inorganic frameworks with hierarchically structured pores, and an accurately controlled pore texture at different length scales is of potential importance in catalysis[l], separation technology[2] and biomaterials enginecring[3]. Since the first synthesis of mesoporous MCM-41 materials[4.5], there has been an unparalleled activity in the design and synthesis of a variety of mesoporous solids with different structural characteristic. In earlier investigations[6.7], we showed that careful controlling alkalinity affords a novel porous materials with well-defined bimodal mesopore size distribution (designed as BMS) in a cationic surfactant-contained synthesis system at ambient conditions, which used usually to prepare MCM-41 mesoporous materials. Further investigation showed that the first key factor for the formation of BMS silica was to control the relative rate of the hydrolysis and polycondensation of TEOS and then gelation[8]. Thus, any variation of the reaction components and its concentration may influence the reaction kinetics of sol-gel alkoxides and then influences the mesostructure of the resultant silica gel. Unquestionably, the unique bimodal mesopore structure and fairly thermal stability, more specifically the fine controllability of bimodal mesopore structure can be of great value in designing BMS materials as catalyst supporter, catalysts, adsorbents and sensor materials. Accordingly, in the present study we examine systematically the influence of TEOS concentration and TEOS/ surfactant molar ratio on the bimodal mesopore structure of BMS silica. At the same time we give a full account of the trend of pore size adjustment and an anew insight into the formation mechanism of BMS silica.
256
2. EXPERIMENTAL The synthesis procedure for BMS sihca was described elsewhere[6.7] and the standard molar ratio of the reaction gel mixtures was 1.0 Si02: 0.185 Ci6H33N(CH3)3Br : 0.6 NH3 : 115H2O. For the purposes of probing the effect of TEOS concentration and TEOS/surfactant molar ratio on the bimodal mesostructure porosity, BMS silicas were prepared over a wide range of TEOS concentration from 9.9wt% to 24.8wt% while holding the TEOS/CTAB molar ratio constant at 5.4, or holding other component constant to make the TEOS/CTAB molar ratio increasing from 5.4 to 16.3, and the pH values of the reaction mixture were adjusted with aqueous ammonia. All of the BMS reaction products were washed repeatedly with distilled water in a centrifliger, dried in air at 353K and finally calcined in air at 2K min'' to 823K for 6h to remove the template. The powder X-ray diffraction patterns (XRD) were recorded using a D/max-2500 powder diffractometer with Cu-Ka radiation (40kV, 100mA), 0.02"step size and 1 s step time over the range 1°< 2 9 < 8°. N2 adsorption isotherms were measures at -196°C using a ASAP2000 analyser. The volume of adsorbed N2 was normalized to standard temperature and pressure. Prior to the experiments, samples were dehydrated at 350°C for 12h.The pore-size distribution was calculated using the desorption branches of the N2 adsorption isotherm and the Barrett-Joyner-Halenda (BJH) formula. 3. RESULTS AND DISSCUSSION XRD patterns of all BMS silica samples prepared under different component 1000 concen- tration conditions exhibit qualitatively equivalent diffraction features. Figure 1 provides the representative X-ray B 500 powder diffraction patterns for the calcined BMS silicas prepared under different TEOS/ CTAB molar ratios. The patterns all contain a single strong, relatively broad 0 2 4 6 8 reflection at low 2 0 angle. However, the ZTheta positions of the intense reflection are dependent by the TEOS/CTAB molar ratios of the reaction medium. As the Fig. 1. Powder X-ray diffraction patterns of TEOS/CTAB molar ratios increased, the calcined BMS silicas prepared from dioo values gradually increased from different TEOS/CTAB molar ratio: (a) 5.4; 4.49nm to 5.74nm. An analogous increase {b)7.6;(c) 13.3; (d) 16.3. in the dioo value with increasing the TEOS concentration at constant TEOS/CTAB molar ratios was also observed. The basal spacings represented by the strong diffraction line are correlated with the BJH pore sizes, even though the framework lacks regular long-range order. Figure 2 shows the corresponding N2 adsorption isotherms and the BJH pore size distributions for calcined BMS samples mentioned above. As can be seen from the adsorption plots, the samples all exhibit type IV isotherms as expected for mesopore silica but with a characteristic hysteresis loop lifted up
257
0
0.2
0.4
0.6
0.8
10
1
100
1000
10000
Pore Size (A)
Relative Pressure (p/po)
Fig. 2. N2 adsorption-desorption isotherms and BJH pore size distributions of calcined BMS silicas prepared with different TEOS/CTAB molar ratio: (a) 5.4;(b) 7.6;(c) 13.3;(4) 16.3. sharply in the p/po region of 0.8-1.0, corresponding a bimodal mesopore size distribution was observed in the BJH plots. Following the increase of TEOS/CTAB molar ratio, the adsorption step at the position of p/po=0.8-1.0 is shifted gradually to higher relative pressure, but the adsorption step at the position of p/po=0.25-0.45 is not shifted obviously. Corresponding, the secondary mesopore size of BMS silicas increased systematically with increasing TEOS/CTAB molar ratio from I8.9nm at 5.4 to 45.5nm at 16.3 and the primary mesopore size was not changed obviously. An analogous shift in the bimodal mesopore size distributions with increasing the TEOS concentration at constant TEOS/CTAB molar ratio can be also observed in Figure 3, however, the increasing extent of the secondary mesopore size (such as from 18.9nm to 37.7nm) was far lower than that of the former and Table 1 provided the relevant structure parameters. It is clear that the secondary mosopore size of BMS silica is more sensitive to the change of precursor concentration than that of its primary mesopore size. Since the primary framework mesopore Table 1 Physical parameters for calcined BMS silicas prepared under different TEOS concentration and TEOS/CTAB molar ratio. CTEOS
(wt %) 9.9 13.3 19.4 24.8 13.3 19.4 24.8
TEOS/CTAB (molar ratio) 5.4 7.6 13.3 16.3 5.4 5.4 5.4
dioo (nm) 4.49 4.85 5.29 5.74 4.62 4.81 5.15
Primary mesopore
Secondary mesopore
(m'/g)
(cmVg)
Dp (nm)
(m'/g)
ABET
Vs (cm'/g)
Ds (nm)
1064.6 806.5 592.1 608.8 908.5 843.5 886.2
0.66 0.50 0.37 0.40 0.60 0.58 0.63
2.80 2.70 2.70 2.65 2.71 2.70 2.94
243.1 230.7 184.1 137.5 255.2 273.2 220.8
1.18 1.50 1.74 1.14 1.12 1.41 1.94
18.9 26.0 43.0 45.5 20.0 22.7 37.7
ABET
258
007
^5
_
a
n
lie
h^ij4
1 1 ™B»P*^i ttAAMoUr1 1 1 1 mill
0
0.2 0.4 0.6 0.8 Relative Pressure (p/po)
1
10
100
1000
10000
PbreSEE(/^
Fig. 3. N2 adsorption-desorption isotherms and BJH pore size distributions of BMS silicas prepared with different TEOS concentration at constant TEOS/CTAB molar ratio: (a) 9.9wt%; (b) 13.3wt%; (c) 19.4wt%; (d) 24.8wt%. of BMS silica results from the removal of surfactant template and the secondary textural mesopore results from the interparticle porosity[8], the above results indicate that the change of compositional concentration used here main affects the relative rate of the hydrolysis and condensation of TEOS and then affects the particle sizes of resultant silica gel, but has a little effect on the micelle size, which decides the primary mesopore size. On the contrary, the element that can alter the micelle size, such as by altering the surfactant alkyl chain length or adding an auxiliary organic solvent such as 1,3,5-trimethylbenzene (TMB) into the reaction systems, can alter also simultaneously the relative rate of the hydrolysis and condensation of TEOS, and the fmal result is that the bimodal size distributions of DMS silicas can be well-matched adjusted in certain range at the same time[9]. ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Grant No.20073029) and the Province Youth Science Foundation of Shanxi (Grant No.981007).
REFERENCES 1. P.T. Tanev, M.Chibeve and T.J.Pinnavaia, Nature., 368(1994)321. 2. R.Burch, N.Cruise, D.Gleeson and S.C.Tsang, J.Chem.Soc.,Chem.Commun., 1996, 951. 3. R.M. Barren Hydrothermal Chemistry of Zeolites. Academic, London, 1982. 4. J.S.Beck, J.C.Vartuli, W.J.Roth, M.E.Leonowicz, C.T.Kresge, K.D.Schmitt, C-T.W.Chu, D. H.Olson, E.W.Sheppard, S.B.McCullen, J.B.Higgins and J.L.Schlenkere, J.Am.Chem.Soc, 114(1992)10834. 5. C.T.Kresge, M.E.Leonowicz, W.J.Roth, J.C.Vartuli, J.S.Beck, Nature.,359( 1992)710. 6. X.Z. Wang, T.Dou and Y.Z.Xiao, Chem.Commun., 1998,1035. 7. X.Z. Wang, T.Dou, Y.Z.Xiao and B.Zhong, Stud.Surf Sci.Catal., 135 (2001) 199. 8. X.Z. Wang, T.Dou, D.Y.Zhao and B.Zhong, submitted to Chem.Mater., 9. X.Z. Wang, T.Dou, D.Wu and B.Zhong, Stud.Surf Sci.Catal., 2002, Nanoporous Mater-III.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
259
A direct template synthesis of highly ordered mesostructured carbons using as-synthesized MCM-48 as template S. B. Yoon,' J. Y. Kim,' Y.-S. Ahn,'' H.-S. Kim^ and J.-S. Yu'* ''Department of Chemistry and Institute of Infor-Bio-Nano Materials, Hannam University, Taejon, 306-791, Korea Functional Materials Research Team, KIER, Taejon, 305-343, Korea A direct template carbonization using as-synthesized MCM-48 as template provides a simple and efficient synthetic method for highly ordered mesostructured carbons with great mechanical. 1. INTRODUCTION Porous carbons have been greatly studied as adsorbents and electrode materials [1]. Various porous carbon materials have been fabricated using inorganic templates including zeolites [2J, opals [3J and silica gels [4]. Recently, a new class of mesoporous carbons was reported using mesoporous materials as templates [5]. In these previous works, before the carbonization, the surfactant molecules in as-synthesized templates were completely removed by calcination process. Such process may often cause some partial lattice collapse or shrinkage of mesoframework as observed by line broadening or signal shift in their powder x-ray diffraction. The process also wasted the expensive surfactants, usually organic hydrocarbons or block copolymers, which can be a good carbon source. To help this end, we report here a simple synthetic method called "a direct template synthesis" of porous carbons using as-synthesized mesostructures as templates. The surfactant in the as-synthesized host was also used as a carbon source. This work can save extra labor, time and energy required for the calcinations process, and yet is found to be an efficient way of synthesizing high quality nanoporous carbons with great mechanical stability. 2. EXPERIMENTAL Mesoporous silica MCM-48 was prepared using hexadccyltrimcthylammonium bromide (Ci6H33N(CH3)3Br) and Brij 30 (polyoxyethylene (4) lauryl ether, Ci2(EO)4) as surfactants and colloidal silica Ludox HS40 as a silica source [5]. As-synthesized silica MCM-48 template is
260
denoted as AM48T in this work. For comparison purposes, some of the as-synthesized MCM-48 was calcined in air at 823 K to remove surfactant molecules. The calcined silica MCM-48 template is named as CM48T. Each of AM48T and CM48T was transferred to a reaction flask in a dry box and dried under vacuum at 373 K for 3 h prior to introduction of carbon precursor. Divinylbenzene (DVB) with a free radical initiator, azobisisobutyronitrile (AIBN) (DVB/AIBN mole ratio D 24) was used as a carbon precursor. The carbon precursor was incorporated into the mesopore of the dried MCM-48 templates. Although the composites in the as-synthesized form are relatively dense, DVB molecules still can enter the pores. The large inner inorganic/organic surface can provide an area for the filling of the carbon precursor solution. The resulting template/polymer composites were then carbonized under argon gas flow by heating at ca.I273Kfor7h. 3. RESULTS AND DISCUSSION Fig. 1 shows powder X-ray diffraction (XRD) patterns of the silica hosts and the resulting carbons, respectively. The AM48T shows the first intense (211) XRD signal at 26^ = 2.1. Calcination process used in this work caused framework shrinkage as indicated in a slight shift of the first signal io 20 = 2.2 as shown for CM48T. Two intense signals ai 269 = 1.4 and 2.4, and 2(-) = 1.5 and 2.5 were observed for AM48T-C (carbon) and for CM48T-C, respectively. The overall XRD intensity of the AM48T-C (formed from both surfactant and DVB as carbon precursors) was usually better than that of the CM48T-C.
20
20
Fig. 1. Powder X-ray diffraction patterns using Cu Ka radiation of (a) as-synthesized MCM-48 (AM48T) and (b) calcined MCM-48 (CM48T) and the resulting nanoporous carbons prepared from (a) AM48T and (b) CM48T
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The first new (110) intense signal not seen in the MCM-48 host was the result of the phase transition of a cubic MCM-48 to a new cubic phase upon removal of the silica framework [5]. The same XRD signal was also observed for mesostructured polymers templated in MCM-48 [6]. Interestingly, the two intense signals of the CM48T-C as compared with those of the AM48T-C were found to shift to higher 2 theta values by about the same 10 = 0.1 as the shift of the first (211) signal of the CM48T in comparison with that of the AM48T. Transmission electron microscope (TEM) images show highly regular arrays of holes separated by walls, indicating equally great structural integrity and order for the both carbons. The values of unit cell parameter, BET surface area, total pore volume and pore diameter are listed in Table 1. Interesting pore size changes were observed from morphological alterations during the replication process, in which the pores and walls of the silica host were transformed to the walls and pores in the resulting carbon network, respectively. The AM48T-C has a greater unit cell dimension and slightly smaller pore size distribution as compared with those of the CM48T-C. The greater unit cell (or d interplanar spacing) of the former stems from direct template use of the intact AM48T. The framework shrinkage observed in the CM48T is considered to occur mainly in the pore, which will be filled by carbon precursor, rather than in silica wall, thus resulting in thin wall in the corresponding CM48T-C. At least 7 % thicker cross-sectional wall diameter was observed for the AM48T-C as compared with that of the CM48T-C, thus allowing one way of a fine-tuning for carbon wall thickness control. Mechanical strength was measured by monitoring XRD intensity changes after pressurizing Table 1 Structural properties of the AM48T and CM48T silica hosts and the corresponding nanoporous AM48T-C and CM48T-C unit cell , total pore ^ sample d spacing' BET surface pore size parameter ^ volume name (nm) ^ area (nr/g) (nm) ao (nm) (ml/g) AM48T 4.2 10.3 63 0.15 CM48T 3.9 9.6 1130 1.15 3.3 AM48T-C 6.3 8.9 1116 0.94 2.3 CM48T-C 5.9 8.3 1147 0.88 2.4 ''The d spacings were determined from (211) and (110) reflections for the MCM-48 templates and corresponding carbon replicas, respectively. ^XRD unit cell parameter equal to 6'''xd(211) for AM48T and CM48T and equal to 2'''xd(l 10) for AM48T-C and CM48T-C, respectively. ^Maximum value of the BJH pore size distribution peak calculated from the adsorption branch of the N2 isotherm.
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pelletized carbons at each of different pressures. The relative intensity decreases mainly at low pressure range less than 120 MPa and slowly decreases at higher pressure range. The intensity of the CM48T-C decreased more rapidly than that of the AM48T-C against pressure with ca. 72 % and ca. 85 % of their corresponding initial intensity after 470 MPa, respectively, indicating the latter showed much better mechanical stability. This may be mainly due to the difference in wall thickness. With an assumption of cylindrical shape for the wall, simple calculations indicates at least 14 % larger cross-sectional wall area for the AM48T-C as compared with that of the CM48T-C. In contrast to the carbon replicas, the CM48T silica with high structural order maintained only 38 % of the initial intensity after 470 MPa. 4. CONCLUSIONS It has been demonstrated that the direct synthesis method using as-synthesized MCM-48 as templates and divinylbcnzene as a carbon precursor is simple and energy-saving, and yet also an efficient way of synthesizing ordered nanoporous carbons. The composite carbon formed from both surfactant and DVB showed no structural instability and defects from the heterogeneity, and together with direct use of the intact as-synthesized hosts, rather greatly increased its structural integrity and mechanical stability as compared with the carbon templated in the calcined hosts. ACKNOWLEDGEMENT Authors thank KOSBF for support (Project No. RO1-2001-00424) and Korea Basic Science Institute (in Taejon) for TliM pictures. REFERENCES 1. F. Rodriguez-Reinoso, in Introduction to Carbon Technology, cd. 11. Marsh, \i. A. Ileintz and P. Rodrigucz-Reinoso, Univcrsidad dc Alicante, Secretariade de Pub. Alicante, (1997) p35. 2. Z. Ma, T. Kyotani and A. Tomita, Chcm. Commun., (2000) 2365. 3. A. A. Zakhidov, R. H. Boughman, Z. Iqbal, C. X. Cui, I. Khayrullin, S. O. Danta, L. Marti and V. G. Ralchcnko, Science, 282 (1998) 897. 4. (a) J.-S. Yu, S. B. Yoon and G. S. Chae, Carbon, 39 (2001) 1442. (b) S. B. Yoon, K. Sohn, J. Y. Kim, C. H, Shin, J.-S. Yu and T Hycon, Adv. Mater., 14 (2002) 19. (c) S. Kang, J.-S. Yu, M. Kruk and M. Jaronicc, Chcm. Commun. (2002) 1670. 5. (a) S. B. Yoon, J. Y Kim and J.-S. Yu, Chem. Commun., (2001) 559. (b) R. Ryoo, S. H. Joo and S. Jun, J. Phys. Chem. B, 103 (1999) 7743. 6. J. Y Kim, S. B. Yoon, F. Kooli and J. -S. Yu, J. Mater. Chem., 11 (2001) 2912.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Gas adsorption: a valuable tool for the pore size analysis and pore structure elucidation of ordered mesoporous materials Mietek Jaroniec and Michal Kruk Department of Chemistry, Kent State University, Kent, Ohio 44242, USA A overview and outlook is presented for the application of ordered mesoporous materials (OMMs) in the development of accurate and reliable methods for the determination of pore size distributions (PSDs) and the elucidation of the pore connectivity. Current status of the use of MCM-41 as a model adsorbent, including the evidence of suitability of adsorption branches of isotherms for the PSD calculations, and the methods for the consistent evaluation of PSDs from nitrogen adsorption at 77 K and argon adsorption at 77 and 87 K are discussed. Opportunities in the use of OMMs with various porous structures to develop PSD calculation methods are outlined. Recently proposed methods for the pore entrance size determination are overviewed and emerging opportunities in the pore connectivity elucidation based on gas adsorption isotherms are discussed. 1. INTRODUCTION The determination of the pore size distribution (PSD) and pore connectivity is one of crucial aspects of characterization of adsorbents, and heterogeneous catalysts [1-3]. Gas adsorption has been an important tool for the elucidation of these important structural properties [1-4]. Many methods to calculate PSD from gas adsorption data have been developed and some of them have been extensively applied [1-4]. Unfortunately, different methods proposed to determine PSDs from gas adsorption data often produce inconsistent results [1,4-6], so much so that in some relatively common cases, completely different estimates of the number, shape and position of peaks on PSDs are indicated by different PSD calculation methods. There were also numerous attempts to elucidate the pore connectivity from gas adsorption data [7], although so far, none of the elaborated methods has gained much practical importance. Experimental verification of the methods to determine PSDs and pore connectivity from gas adsorption data was hampered by the lack of mesoporous solids with well-defined pore shape, size and connectivity. This situation changed dramatically during the last decade thanks to the discovery of ordered mesoporous materials (OMMs) [8-10] that are now available in a wide range of structure types and pore sizes. These structural features can be determined using methods based primarily on X-ray diffraction (XRD), and transmission electron microscopy (TEM) [11-13], which are independent from adsorption methods of the PSD calculation and pore connectivity characterization. Therefore, it is now possible to use OMMs to experimentally test the methods for PSD calculations based on adsorption data and to elaborate new ones [14-18] that would provide the accuracy and reliability required in the emerging nanotechnology
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research. The challenge remains in realizing all the opportunities in the elaboration of methods for PSD evaluation that arose from the discovery of OMMs. Moreover, adsorption methods to elucidate the pore connectivity can now be put to the test [19], and a better understanding of the opportunities and limitations in the use of gas adsorption data for the characterization of pore connectivity is expected to emerge from studies of OMMs with well-defined cage-like pores [11,20]. 2. DISCUSSION In 1997, we first demonstrated how MCM-41 silicas with a wide range of pore sizes can be used as model adsorbents to develop a practical method to calculate PSDs for silicas with cylindrical pores [14]. The MCM-41 pore sizes were determined on the basis of a geometrical relation that involves the XRD interplanar spacing and volume of ordered pores [12,13]. Subsequently, experimental relations between the capillary condensation pressure and the pore size as well as between the capillary evaporation pressure and the pore size were determined for nitrogen adsorption at 77 K. It was found that the capillary condensation pressure tended to gradually and systematically increase as the pore diameter increases. On the other hand, the relation between the capillary evaporation pressure and pore size was more complicated in the adsorption-desorption hysteresis region. In particular, a relatively narrow range of capillary evaporation pressures close to the lower limit of hysteresis corresponded to an appreciable range of pore diameters. It was also apparent that the capillary evaporation tended to be somewhat delayed in MCM-41 samples of lower degree of structural ordering, which we attributed to "single-pore" pore blocking effects similar to those observed in porous networks with constrictions [7], but related to the variations in diameter along single channel-like pores [14,21]. Subsequent studies of argon adsorption at 87 and 77 K [16,18] as well as nitrogen adsorption at 77 K on an extensive set of MCM-41 silicas [4] confirmed that our initial observations were representative for adsorption behavior of different gases at different temperatures on MCM-41 silicas. It has become clear that adsorption branches of isotherms are suitable for PSD calculations because of a well-defined relationship between the capillary condensation pressure and pore size, whereas PSD calculations from desorption branches of isotherms are inherently difficult and may be highly unreliable. Based on this work, we have developed a practical method to calculate PSDs [14] using the well-known Barrett-Joyner-Halenda (BJH) algorithm [22], which we implemented in a rigorous way without approximations originally proposed. We have found that consistent PSD assessment can be made from adsorption branches of nitrogen adsorption isotherms at 77 K, and argon adsorption isotherms at 87 and 77 K [16,18], although argon at 77 K does not allow one to evaluate PSDs for pores of diameter above about 15 nm [18]. Our work mostly involved the use of MCM-41 as a model adsorbent, therefore the developed PSD calculation method was primarily suited for silica-based materials with cylindrical pores. However, this method can be readily extended on materials with different surface properties, for instance on silicas with chemically bonded organic groups [15,17], The pore diameter of MCM-41 samples used in the above studies was restricted to 6.5 nm and we intended to extend the pore size range for the model adsorbents used. SBA-15 silica appeared to be very promising from this point of view, as it can be synthesized with
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pores much larger than those of MCM-41 [23], but SBA-15 was found to exhibit connections between 2-D hexagonally ordered pores [24], thus making it less suitable as a model adsorbent. Recently, much progress has been made in the synthesis of other OMM structures with tailored pore size, including cubic Ia3d structure of MCM-48 with channel-like branched pores [8], cubic Pm3n structure of SB A-1 and SBA-6 with cage-like pores [11], and cubic Im3m structure of SB A-16 with cage-like pores [11]. These structures have recently been elucidated in detail using electron crystallography [11,25] and their pore size can be estimated from XRD and pore volume data using simple geometrical equations [19,26,27]. The use of these OMMs as model adsorbents for the development of methods for the PSD calculation is anticipated. During the last two years, methods for the elucidation of the pore entrance size in OMMs with cage-like pores were developed [11,20]. One of them was based on the electron crystallography, which solves the 3-D structure of OMM, providing a wealth of information about pore diameter, pore entrance size and pore connectivity [11]. The other method was based on the modification of the OMM surface with ligands of different size to determine the smallest size of ligand that renders the pores inaccessible to gas molecules [20]. Specifically, the OMM with cage-like pore structure is modified with several ligands of gradually increasing size and adsorption isotherms for the resultant modified materials are measured. These ligands can be selected among organosilanes that are commercially available in a wide range of structures and sizes of organic groups. However, one needs to ensure that the modifier forms a monolayer of a predictable thickness on the surface rather than an ill-defined multilayer with thickness that is difficult to predict or is spatially inhomogeneous. This restricts the choice of organosilanes to monofunctional ones, such as organomonochlorosilanes. In addition, the modification conditions need to be chosen in such a way that sufficiently high coverage of surface groups is introduced. From our experience, a high surface coverage results when template-free siliceous OMM is modified with organochlorosilane in the presence of pyridine under reflux conditions [15,20,24], Typically, cage-like pores of OMMs are accessible after the surface modification with smaller organosilanes (such as trimethylchlorosilane), but become inaccessible after the surface modification with larger silanes. For instance, an FDU-1 sample synthesized at room temperature, the pores were accessible after the introduction of trimethylsilyl ligands, but the introduction of triethylsilyl groups made most of the cage-like pores inaccessible [20]. Another sample that was synthesized in the same manner and additionally subjected to hydrothermal treatment for 6 hours at 100°C exhibited accessible porosity after the modification with much larger butyldimethylsilyl ligands, but the pores were inaccessible after the modification with octyldimethylsilyl ligands. In this case, the modification with hexyldimethylsilyl groups, whose size was between the sizes of the two aforementioned ligands, resulted in a partial pore blockage [20]. In judging the degree of pore accessibility after the modification, it is important to keep in mind that in the case of any rigid pore system, the successftil surface modification reduces adsorption capacity and pore diameter of the material. The degree of such a reduction for a material with cage-like pores that are accessible after modification can be estimated from the results for modification of channel-like pores of MCM-41 [28] or SBA-
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15 [24]. When such a comparison is made, it is suggested to select the channel-like material with pore diameter similar to the cage diameter of the material with cage-like pores. It is important to be able to relate the size of the surface ligand that causes the pore blockage to the pore entrance size. The pores are expected to become inaccessible when the thickness of the layer of bonded groups on the surface is larger than the pore radius minus the radius of adsorbed gas atom or molecule. In the case of the modification with organochlorosilane groups, the maximum possible thickness of the bonded layer can be readily related to the structure of the silane on the basis of the bond angles and bond lengths. However, many organosilanes useful for the considered modifications exhibit much structural flexibility related to the fact that they feature long hydrocarbon chains without branching. Therefore, the maximum possible thickness of the bonded layer is likely to be larger than the actual thickness. To investigate this effect, MCM-41 silicas with cylindrical pores were used and it was found that the pore blocking is observed in cases where the maximum possible ligand size is about the same as the pore radius minus of the radius of the adsorbate atom or molecule [20]. So, in the case of pore entrances that have a geometry close to circular, one can readily relate the size of the surface modifier that causes the pore blocking to the pore entrance size. It should be noted that in general, flexible surfacebonded ligands are not expected to adopt fully extended geometry, because they can coil. The pore size reduction observed in the case of modifications with ligands much smaller than the pore radius provides the confirmation of this contention [28]. However, in the case of high surface coverage of ligands, whose maximum extension is close to the pore radius, bonded on the surface of cylindrical or spherical pore, the geometrical constraints (related to the fact that there is more space close to the surface than in the center of the pore) may force at least some of the ligands to adopt fully extended configurations, which would explain the experimental findings for pore-blocked MCM-41. Therefore, the assumption that the pore blocking takes place for ligands whose maximum extension is equal to the pore radius minus the adsorbate molecule radius appears to have both experimental and theoretical basis lor cylindrical pores. Using the methodology discussed above, it was concluded that the pore entrance diameter of FDU-1 synthesized at room temperature is larger than 1.2 nm, but most of the entrances have diameters smaller than 1.4 nm. On the other hand, FDU-1 that was additionally subjected to heating at 100°C for 6 hours had pore entrances larger than 1.9 nm and smaller than 2.9 nm in diameter [20]. This methodology for the assessment of size of entrances to cage-like pores is expected to be particularly useful for silicas and organosilicas with hybrid organic-inorganic frameworks, which are two important types of materials with cage-like pores. The maximum pore entrance diameter that can be assessed using this method is likely to be about 5 nm on the basis of the size of commercially available organosilanes [20]. It is also expected that the accuracy of the pore entrance size evaluation will be lower for larger entrance sizes because of higher uncertainty in estimation of the size of the surface groups. The fact that the above method of the pore entrance size evaluation is not likely to be suitable for entrances larger than about 5 nm in diameter does not appear to be a major limitation, as the sizes above this limit can be characterized simply on the basis of the shape of desorption branches of isotherms in the adsorption-desorption hysteresis region. This opportunity arises from the fact that the capillary evaporation in pores with constrictions
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takes place either (i) at pressure where the constrictions themselves exhibit capillary evaporation or (ii) at lower pressure limit of adsorption-desorption hysteresis. The first scenario is a widely adopted hypothetical mechanism of delayed capillary evaporation in ink-bottle pores [3]. However, it needs to be kept in mind that the second scenario is often prevalent. The unambiguous evidence of the validity of the second scenario was obtained for FDU-l silicas, whose sizes were assessed via surface modification to be below 3 nm, or even in the micropore range (below 2 nm). These FDU-l samples exhibited capillary evaporation at a relative pressure of about 0.48, whereas the capillary evaporation from the constrictions of size below 2-3 nm is expected to take place at much lower relative pressures. So, the phenomenon of delayed capillary evaporation does not provide any specific information about the constrictions in which capillary evaporation takes place below the lower limit of adsorption-desorption hysteresis. However, this phenomenon is expected to provide information about constrictions that exhibit capillary evaporation above the lower limit of hysteresis. For instance, it was reported [19] that FDU-l silicas subjected to extended hydrothermal treatments at 100°C exhibit nitrogen capillary evaporation at 77 K that starts to take place above the lower limit of adsorption-desorption hysteresis (relative pressure of 0.48). We attributed this behavior to the formation of defects in the pore entrance structure as a result of overly extended hydrothermal treatments [19]. Therefore, the examination of the shape of the adsorption-desorption hysteresis loop for large-pore OMMs with cage-like pores promises to be useful in the investigation of defects in the pore opening structure and in the pore entrance size elucidation in general. However, when nitrogen adsorption at 77 K is used, this method appears to allow one to study openings of sizes down to only about 5 nm (the lower limit of adsorption-desorption hysteresis at a relative pressure of 0.48 corresponds to the capillary evaporation from uniform cylindrical pores about 5 nm in size), which is beyond the typical range of sizes of entrances to mesoporous cages. There is a strong incentive to find gases and experimental conditions that would allow one to obtain information about practically important pore entrance sizes from the shape of hysteresis loops of adsorption-desorption isotherms. To this end, argon adsorption at 77 K was identified as promising, because in this case, the adsorption-desorption hysteresis extends to somewhat smaller pore sizes than in the case of nitrogen at 77 K. We expect that the use of argon at 77 K allows one to probe pore entrance sizes down to about 4 nm on the basis of the shape of the desorption branch of the hysteresis loop. We currently investigate this possibility. 3. CONCLUSIONS Gas adsorption is an important tool in the characterization of ordered mesoporous materials. The determination of pore size distributions can be accomplished by analyzing adsorption branches of isotherms. Well-ordered OMMs with simple pore geometry and a wide range of pore sizes assessed using independent methods can conveniently be used as model adsorbents suitable for the testing and development of methods to calculate PSDs. We have used MCM-41 silicas for this purpose and achieved consistent PSD estimates from nitrogen adsorption data at 77 K and argon adsorption data at both 77 and 87 K. However, we found that the use of argon at 77 K in PSD calculations is restricted to pores of diameter below about 15 nm. In favorable cases, the examination of desorption branches
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of hysteresis loops allows one to gain insight about the size of entrances to cage-like pores. In the case of nitrogen adsorption at 77 K, we expect that the desorption branch may provide information about entrances above 5 nm in diameter, whereas the use of argon adsorption at 77 K is suggested to allow one to obtain information about entrances of diameter above 4 nm, thus providing additional information about entrances in the 4-5 nm interval. The pore entrance size below 5 nm in silicas and organosilicas with cage-like pores can be assessed in another way. Namely, the surface modification of the sample with cage-like mesopores with monolayer of ligands of gradually increasing sizes allows one to find the smallest ligand size that causes a complete pore blocking, which results from the reduction of the pore entrance size to that below the size of the adsorbate molecule. The pore accessibility is monitored by gas adsorption and the size of smallest ligand that caused the pore blocking is used to assess the pore entrance size. Consequently, gas adsorption can conveniently be use to determine pore size distributions of OMMs and may provide information about the pore entrance size, which can be accomplished on the basis of either desorption branches of hysteresis loops or changes in adsorption properties of the material after modification with surface groups of gradually increasing size. 4. ACKNOWLEDGMENTS The donors of the Petroleum Research Fund administered by the American Chemical Society are gratefully acknowledged for support of this research. This work was also supported in part by NSF Grant CHE-0093707. REFERENCES I. S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982 2. M. Jaroniec and R. Madey, Physical Adsorption on Heterogeneous Solids, Elsevier, Amsterdam, 1988. 3. F. Rouquerol, J. Rouquerol and K. Sing, Adsorption by Powders and Porous Solids, Academic Press, San Diego, 1999. 4. M. Kruk and M. Jaroniec, Chem. Mater. 13 (2001) 3169. 5. P. Selvam, S. K. Bhatia and C. G. Sonwane, Ind. Eng. Chem. Res., 40 (2001) 3237. 6. P. I. Ravikovitch, D. Wei, W. T. Chueh, G. L. Haller and A. V. Neimark, J. Phys. Chem. B, 101 (1997)3671. 7. H. Liu, L. Zhang and N. A. Seaton, J. Colloid Interface Sci., 156 (1993) 285. 8. J. S. Beck, J. C. Varluli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, C. T. W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, J. B. Higgins and J. L. Schlenker, J. Am. Chem. Soc, 114 (1992) 10834. 9. T. Yanagisawa, T. Shimizu, K. Kuroda and C. Kato, Bull. Chem. Soc. Jpn., 63 (1990) 988. 10. S. Inagaki, Y. Fukushima and K. Kuroda, J. Chem. Soc, Chem. Commun., (1993) 680. II. Y. Sakamoto, M. Kaneda, O. Terasaki, D. Y. Zhao, J. M. Kim, G. Stucky, H. J. Shin and R. Ryoo, Nature, 408 (2000) 449. 12. T. Dabadie, A. Ayral, C. Guizard, L. Cot and P. Lacan, J. Mater. Chem., 6 (1996) 1789. 13. M. Kruk, M. Jaroniec and A. Sayari, J. Phys. Chem. B, 101 (1997) 583.
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14. M. Kruk, M. Jaroniec and A. Sayari, Langmuir, 13 (1997) 6267. 15. M. Kruk, V. Antochshuk, M. Jaroniec and A. Sayari, J. Phys. Chem. B, 103 (1999) 10670. 16. M. Kruk and M. Jaroniec, Chem. Mater., 12 (2000) 222. 17. M. Kruk and M. Jaroniec, Microporous Mesoporous Mater., 44-45 (2001) 725. 18. M. Kruk and M. Jaroniec, J. Phys. Chem. B, 106 (2002) 4732. 19. J. R. Matos, L. P. Mercuri, M. Kruk and M. Jaroniec, Langmuir, 18 (2002) 884. 20. M. Kruk, V. Antochshuk, J. R. Matos, L. P. Mercuri and M. Jaroniec, J. Am. Chem. Soc. 124(2002)768. 21. M. Kruk, M. Jaroniec and A. Sayari, Adsorption, 6 (2000) 47. 22. E. P. Barrett, L. G. Joyner and P. P. Halenda, J. Am. Chem. Soc, 73 (1951) 373. 23. D. Zhao, Q. Huo, J. Feng, B. F. Chmelka and G. D. Stucky, J. Am. Chem. Soc, 120 (1998)6024. 24. R. Ryoo, C. H. Ko, M. Kruk, V. Antochshuk and M. Jaroniec, J. Phys. Chem. B, 104 (2000)11465. 25.M. Kaneda, T. Tsubakiyama, A. Carlsson, Y. Sakamoto, S. H. Joo and R. Ryoo, J. Phys. Chem. B, 106(2002) 1256. 26. P. I. Ravikovitch and A. V. Neimark, Langmuir, 16 (2000) 2419. 27. P. I. Ravikovitch and A. V. Neimark, Langmuir, 18 (2002) 1550. 28. C. P. Jaroniec, M. Kruk, M. Jaroniec and A. Sayari, J. Phys. Chem. B., 102 (1998) 5503.
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Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Three-Dimensional Transmission Electron Microscopy of Disordered and Ordered Mesoporous Materials
K. P. de Jong^, A.H. Janssen^, P. van der Voort^ and A.J. Koster*^ ^Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands ^Laboratory of Adsorption and Catalysis, University of Antwerp, Universiteitsplein 1, B-2610 Wilrijk, Belgium '^Molecular Cell Biology, Utrecht University, Padualaan 8, 2584 CH Netherlands
Utrecht, The
The use of 3D-TEM, in particular electron tomography, for the characterisation of mesoporous materials is introduced. In 3D-TEM a tilt series of the specimen collected in bright field mode comprises typically 150 images over tilt angles ranging from - 7 0 ° to +70°. The tilt series are used to calculate a full 3D- image reconstruction of the specimen in question. The first example delt with comprises the study of zeolite Y crystals that contain mesopores. The pore shape, size and connectivity of the zeolite Y crystal is obtained with great clarity and detail. The second example involves SBA-15 materials. The curved nature of the pores in the particles of SBA-15 is clearly demonstrated from tilt series. 1.
INTRODUCTION
Transmission Electron Microscopy (TEM) is one of the most powerful techniques to characterize mesoporous materials. Typical images from MCM-41 obtained by TEM are displayed in Figure 1. The side-on view of the mesopores (left) combined with a view into the pores (middle) has led to the general belief that MCM-41 can be considered as to consist of hexagonally-packcd straight channels (right). It should be realized though that in a transition electron microscope the image obtained essentially is a 2D projection of the 3D object. Recently, in materials science great strides have been made to obtain a 3D reconstruction from 2D images obtained by TEM. For an overview of the several modes of 3D-TEM we refer to a recent paper [1]. Here we focus on a particular mode of 3D-TEM, viz. electron tomography. First wc will describe briefly the essential features of electron tomography. Second, we present a study on mesopores in zeolite Y and in SBA-15 using 3D-TEM.
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* ^
'
TEM images of MCM-4 with the electron beam perpendicular to the pores (letl) and parallel to the pores (middle). Pore architecture derived from these images (right). 2. ELECTRON TOMOGRAPHY - PRINCIPLES AND PRACTICE Figure 2 provides an important feature of image formation in TEM in bright-field mode. High-energy electrons transverse through the 3D object and give rise to a 2D projection that displays density contrast variations. By tilting the object typically by 1° increments a series of 2D projections is obtained. By using an automated electron microscope and a CCD camera one can obtain 150 images in 20-30 minutes. The automated mode of TEM operation guarantees minimal exposure of the sample thus limiting damage due to the beam. In our work discussed here cither an FEG-CM200 or a Technai-20 microscope has been used. Both microscopes combine good resolution with the option to apply high tilt angles. Having acquired the tilt series alignment of the data stack using fudicial markers is carried out. Subsequently, image reconstruction using Fourier Transformation and back transformation (cf. Figure 2) is done. Finally, image presentation is pursued using several modes ranging from movies to contour surfaces to virtual slices [2]. 3D Object
3D FT of Object
2D Projection
20 FT of Projection
Fig. 2. Principle of 3D-TEM techniques; a series of 2D projections is used to allow Fourier transformation (FT) to obtain a reconstruction of the 3D object.
273
Fig. 3. 2D-TEM image of a steamed and leached zeolite Y crystal (left) and a thin slice through the 3D reconstruction of this crystal (right). Scale bars 200 nm. 3. MESOPORES IN ZEOLITE Y CRYSTALS Here we summarize results that have been or will be published in more detail elsewhere [3,4]. Zeolite Y samples that have previously been steamed and acid leached were studied with conventional techniques such as nitrogen physisorption and mercury intrusion. From earlier work it is known that next to dealumination, steaming and leaching bring about mesopore development in zeolite Y. In Figure 3 we compare a conventional 2D-TEM image (left) with a virtual slice (thickness 2 nm, right) from a 3D reconstruction obtained using 3DTEM as described above. The contrast variation displayed by the 2D image reveals the occurrence of mesoporcs. However, no details on these pores can be obtained from this image. The virtual slice from 3D-TEM, on the other hand, shows the mesopores with great clarity and detail. By studying subsequent slices (not shown) we could conclude that the pores are either cylindrical or ink bottle-type. Whereas the cylindrical pores appear to run through the crystal and are connected to the crystal surface, the inkbottle pores are cavities that are only connected to the outside world via the micropore network of the zeolite. In ref. 4 we present a detailed study comparing the results from 3D-TEM with texture analysis data. In particular the pore dimensions fit very well with the nitrogen data. The quantitative establishment of the fraction of inkbottle versus cylindrical pores up till now could not be done with 3D-TEM but can be derived from a combination of nitrogen and mercury data [4]. 4. ON THE SHAPE AND LENGTH OF PORES IN SBA-15 SBA-15 materials display similar pore ordering as indicated in Figure 1 for MCM-41. In many cases it has been suggested that the simplified picture of straight channels that are hexagonally ordered holds also for these materials. In order to study the pore architecture, we have acquired tilt series of SBA-15. In Figure 4 we show two images of an identical part of the specimen at different tilt angles. Each picture shows three different parts. On the right hand side of the picture, one can see a spherical particle of about 75 nm. This particle does not display ordered mesopores. Left of this spherical particle there is a particle of about 300 nm that displays more or less straight channels that run 'once through' through the particle as can
274
\..
:
200nm
"
Fig. 4. TEM images of the same SBA-15 particles at different tilt angles.
200nm
be indicated from Figure 3 and be proven from the full tilt series of 150 images. The largest particle displays more complex phenomena. The left image suggests also straight channels with slight curvature at top and bottom. The right-hand image, however, reveals that the majority of the pores bends backwards and run both forth and back the particle. Therefrom one can conclude that the lengths of the pores in this particle are twice the size of the particle. For details on the studies on (modified) SBA-15 we refer to the literature [5,6]. 5. CONCLUSIONS 3D-TE1V1 or more specifically electron tomography is a powerful technique to study the details of pore size, shape, length and connectivity in mesoporous materials. For steamed and leached samples of/oolite Y inkbottle and cylindrical pores could be visualized. For SBA-15 we have shown that noodle-like pores give rise to pore lengths that can be twice the particle size. REFERENCES 1. K.P. de Jong and A.J. Koster, ChemPhysChem 3 (2002) 776. 2. A.J. Koster, R. Grimm, D. Typke, R. Hegerl, A. Stoschek, J. Walz and W. Baumeister, J. Struct. Biology 120 (1997) 276. 3. A.H. Janssen, A.J. Koster and K.P. de Jong, Angew. Chem. Int. Ed. 40 (2001) 1102. 4. A.H. Janssen, A.J. Koster, K.P. de Jong, J. Phys. Chem. B, accepted. 5. P. Van Der Voort, IM. Ravikovitch, K.P. de Jong, M. Benjelloun, E. Van Bavel, A.H. Janssen, A.V. Neimark, B.M. Weckhuysen and E.F. Vansant, J. Phys. Chem. B 106 (2002) 5873. 6. A.H. Janssen, P. Van Der Voort, A.J. Koster and K.P. de Jong, Chem. Commun. (2002) 1632.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Structures of Silica-Mesoporous Crystals and Novel Mesoporous Carbon-Networks Synthesized within the Pores O. Terasaki''*, Z. Liu^, T. Ohsuna^, T. Kamiyama^, D. Shindo^, K. Hiraga^, S. H. Joo^ T.-W. Kim^ and R. Ryoo^ '*Department of Physics andCIR, Tohoku University, Sendai 980-8578, Japan. FAX: +81-22-217-6472. E-mail: terasaki@msp.phys.tohoku.ac.jp ^ Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan ^ Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan "^ National Creative Research Initiative Center for Functional Nanomaterials and Department of Chemistry, KAIST, Daejeon, 305-701, Korea. Summary: After three-dimensional structures of mesoporous crystals have been solved, these crystals can be used as molds to design and synthesize nano-structured materials utilizing the available spaces within the microporous crystals. The designed nanostructured materials can be further utilized as molds, to create new meso- or nanostructured materials. Thus the nano-network structures are basically controlled by the geometries of spaces available within the mesoporous materials used, and this route opens new field for synthesis of novel nano-structured materials, which, otherwise, can not be synthesized directly on their own. To date, several ordered carbon and Pt nanonetworks have been synthesized from silica-mesoporous crystals. During our studies, by close structural investigation employing electron microscopy (EM), we observed new structural features in these materials. Here, we discuss the structural features from both atomic and mesoscopic scales and possibilities to control their fine structures for further usage. Introduction Mesoporous crystals, using surfactant molecules synthesized either from layered silicate or water soluble silica source[l,2], have attracted considerable attentions in various fields. Recently, we have developed a method to solve threedimensional (3d-) structure of mesoporous crystals uniquely from Fourier analysis of a set of HREM images, which is one of the electron crystallography (EC) methods[3,4]. We have solved several 3d-structures of mesoporous-silica crystals which include MCM-48, SBA-1, SBA-II, SBA-12, SBA-15 and SBA-16 [3-5]. From these structural studies it is evident that these materials possess uniform nanoporous spaces which are ideal to employ as molds for the synthesis of nanostructured materials. The greatest advantage of these mesoporous crystals is that their pore dimensions can be finely controlled by a choice of amphiphilic molecule and its concentration. In addition, subsequent to the preparation of nan-structured materials, the molds can be easily removed. Based on our structural studies, several nano-structured materials such as carbon and Pt networks have been successfully synthesized within silica mesoporous crystals of MCM-48, SBA-1 and SBA-15 [6-9].
276
In this report, we will begin with importance of phase in crystal structure factor, principles of diffraction and imaging (EC) before discussing the structure determination and characterization procedures by taking examples of silica-mesoporous crystal MCM48 and carbon networks CMK-1 and CMK-4. Structural details of SB A-15 and carbon networks, CMK-3 are also discussed. Phase of crystal structure factor, Babine's principle and CMK-4 The crystal structure factor for ^-reflection, F(hkl) where g is given by index of hkl in reciprocal space, is g-th component of Fourier transformation of the structure. F(hkl) is generally complex and described by F(hkl) = Amp[F(hkl)]exp[ia(hkl)], where Amp[F(hkl)] and a(hkl) are amplitude and phase terms for ^ -reflection, respectively. If the crystal has a center of inversion, then a(hkl) is either 0 or 7C by taking the origin of coordinates at the inversion center. HREM image carries both the amplitude and the phase information, while Silica Wall and Surfactant Rods diffraction intensity gives only the amplitude term. Once we have a complete 3d-data set of the factors from analysis of HREM images, the structure can be determined uniquely and straightforwardly by Channel Network an inverse Fourier transformation. Silica Wall Network of Surfactant-Rod The structure of the silica wall of MCM-48, which F'ig.l Structure solution obtained from \iC for IV1CIVI-48, silica was filled with surfactant wall and channel structure. rods, obtained froin EC is shown in Fig. 1. After removing the surfactant by calcination, carbon networks (prepared from different carbon sources, see ref. for details of the preparation) were formed uniformly within the mesopores of MCM-48. These nano-structured carbons obtained by complete dissolution of silica MCM-48 arc designated as CMK-1 and CMK-4. HREM images of silica MCM-48 and CMK4 together with Fourier diffractograms obtained from the images are shown in Fig. 2 for [-11IJ
Fig.2. HREM images of [-III] incidence and their Fourier diffractograms. MCM-48 (left) and CMK-4 (right).
277
incidence. It is apparent from the contrast in the HREM images of MCM-48 and CMK4(Fig.2) they are opposite to each other. Although the structures of MCM-48 and CMK4 are different and complement to each other, as shown in Fig. 1, both have the same symmetry, Ia-3d, and give same Fourier diffractograms shown in Fig. 2. The structures shown in Fig. 1 (as silica wall and network of surfactant-rod) are two solutions out of many possible structures, which give the same diffraction pattern as shown in Fig. 2. This comes from the differences in the phases of crystal structure factors, which clearly indicates the importance of phases to give structure solutions. The crystal structure factors of MCM-48 and CMK-4, obtained from EC, are shown in the Table below. If we assume that the silica wall and the carbon rods have the same scattering density, it is obvious from the table that, the amplitudes of crystal structure factors for hkl reflection are same but the phases are anti-phase, which is known as Babine's principle in optics. Therefore if the mesopores are occupied completely by the material with uniform and equal scattering density as the silica wall, then we cannot observe any density modulation except the uniform scatterer of crystal shape, which gives only shapefunction information at very close to the origin, 000, in reciprocal space. This is the reason why intensity of Bragg peaks representing mesostructures will be diminished when the mesopores are filled, which is typically observed in many powder XRD patterns(see Fig. 3b). MCM-48 CMK-4 Amp. Phas hkl d/nm Amp. Phase e 211 0 100. 100 3.52 n 71 220 3.04 0 43.6 41.7 0 321 2.30 K 4.1 5.3 0 71 400 2.15 14.5 9.7 0 K 420 10.7 4.6 1.92 71 0 332 13.5 6.5 1.83 n 0 5.3 2.2 422 1.75 Fig.3. Powder XRD pattern with Cu K„. MCM-48 (a), MCM-48/CMK-4 K 0 3.4 0.6 1.69 431 (b), CMK-4 (c) and CMK-1 (e).
Symmetry change in CMK-1 It is interesting to note that a new reflection {110} is observed for CMK-1 (see Fig3(e)), which was not observed for the material and are not allowed in la-^d, is observed for CMK-1. The HREM image of CMK-1 shows domain character. In addition the image taken with the [111] incidence shows deviation from three-fold symmetry, therefore it is clear that CMK-1 is not cubic. The Fourier diffractogram of the HREM image of CMK-1 taken with the [110] incidence indicates that CMK-1 keeps 4, symmetry along the c direction, which is a part of the symmetry element of la-M, and thus CMK-1 is tetragonal. The unique axis of tetragonal can be either [100] or [010] or [001] of cubic and therefore CMK-1 consists of small domains. This also explains an appearance of a cubic system in unit cell size as an average structure within our experimental resolution. Tetragonal l4/a is the most probable space group. To explain the powder XRD and ED patterns, crystal structure factors for several reflections were calculated as a function of magnitude for mutual displacement along the [100] based on
278
the space group I4/a as shown in Fig. 4. For example, all {110}-type reflections are degenerate and zero if the crystal system is cubic. However, the crystal system can be tetragonal by a displacement ,. , which
.,, will
, result
•
• 101 O i l
/.^J0b0^^^\ ^^S^
j ^
KjisL ^ laJd
«»BrraBim.i
yT^]^iy^j^*~
Fig- 4. Crystal structure factors observed [111] Incidence as a function ^f ^^^ relative displacement (right). ED pattern and projected structure
in 1-10 reflection for CMK-l (left), being nondegenerate with structure factor zero and the other 10-1 and 01-1 reflections, which are still degenerate contributing to the intensity. This relative shift model explains the powder XRD pattern of CMK-l shown in Fig. 3(e). Domain character and relative shift of {110} fringes among the domains observed in 5SS^ HREM image taken with [(X)1J (not shown here) are well explained by the relative shift along [1(X)], [010] and [0-10]. SBA-15 and CMK-3
Fig. 5. 2d hex. p6mm structure is schematically drawn as Id-pipe convoluted by 2d-hex plane lattice.
The SBA-15 has the same 2dhexagonal symmetry, p6mm, as MCM41. Structures of both SBA-15 and MCM-41 are assumed to be honeycombtype structures, that is. Id-pipes are arranged on 2d-heagonal lattice (Fig. 5, left). Mathematically they are described as a pipe (along the z-axis) convoluted by 2d-heagonal lattice with p6mm{x-y plane), as schematically shown in Fig. 5(right). Hence the crystal structure factors for SBA-15 and MCM-41 are multiples of Fourier transforms of the pipe and that of the 2d-heagonal lattice, i.e., hexagonally arrayed lines. In addition, it should be noted that the Fourier transform of the pipe can produce not a systematic extinction but
Fig. 6. Channel structure of CMK-3 obtained through HREM image of Pt nanowire-cast.
279
an accidental one, which is currently being investigated in detail. The difference in the structure factor between SBA-15 and MCM-41, therefore, comes only from the difference in shape of pipes for SBA-15 and MCM-41, and it will be very difficult to determine the difference from diffraction experiments alone. However, recent studies pointed out that the two mesoporous materials could have different channel connectivity. The Pt cast obtained from the MCM-41 and SBA-15 molds (after complete removal of the silica frameworks with HF) are single crystal straight nanowires [10] and rigidly
w^ \
Fig. 7. High resolution SEM image of CMK-3, Field Emission Gun.
30 nm Fig.8. TKM image and KD pattern of CMK-3, taken along the channel direction.
Fig.9. HREM image of CMK-3 and Fourier difTractogram taken along the channel direction.
connected 2d- hexagonal arrays[l 1], respectively. The HREM images of Pt-nanowires, which were formed within the mesopores of SBA-15, clearly revealed the presence of 3d-channel connectivity (see Fig. 6)[11]. By choosing appropriate carbon source, we can synthesize 2d-hexagonal array of either carbon-rods or pipes, CMK-3. It is to be noted that the modern FE-SEM instrument can produce a high-resolution image, as shown in Fig. 7, which gives finer details of carbon structures. Fig. 8 shows transmission EM image of CMK-3 and corresponding ED pattern. This image indicates CMK-3 carbon is exactly an inverse replica of SBA-15. The spatial hexagonal ordering of carbon pipes is very high and gives higher order reflections in the ED pattern compared to SBA-15. HREM-image of CMK-3 (Fig. 9) suggests that we can tune both outer and inner pore diameters. The partially ordered graphitic wall structures are also clearly observed in the image as graphitic lattice fringes. The diffuse ring in the inserted Fourier diffractogram of the image corresponds to the spacing of the fringes. Platinum nano-particles with narrow particle-size distribution can be prepared on CMK-3 with Pt loading as high as 50 wt.%. Low magnification TEM image of Ptloaded CMK-3(Fig. 10, left) suggests that the nano-particles are located both within and
280
between the carbon pipes. The HREM image (Fig. 10, right) clearly shows a uniform distribution of Pt nano-particles, which are nano-single crystal particles. Conclusions Using well-tuned silica mesoporous crystals as molds, we can synthesize novel nanostructured materials such as carbon rod or pipe nano-networks, which cannot be synthesized otherwise. The new nano-networks also can be used as molds to obtain mesoporous Fig. 10. TEM image (left) and HREM image (right) of Pt/CMK-3. silica similar to the host material[I2]. More importantly, this strategy can be adopted to produce other nanostructured materials with different elements and compositions. It is important that the structures must be characterized in atomic detail at every step. Although electron microscopy offers the best approach, at present, it is important to develop new methods for crystallographic study of materials with orders in multi-length scales. One of the approach will be lens-less imaging based on over-sampling mcthod[13]. Acknowledgement This work was supported in part by CREST project by JST (OT) and CR! Program of the Korean Ministry of Science and Technology, and by School of Molecular Science through Brain Korea 21 project (RR). References (1) C.T. kresge, M.E. Leonowic/,, W.J. Roth, J.C. Vartuli & J.S. Beck, Nature, 1992, 359, 710. (2) S. Inagaki, Y. Fukushima and K. Kurcxla,./. Chem. Soc. Chem. Commun. 1993, 680. (3) A. Carlsson, M. Kaneda, Y. Sakamoto, (). Terasaki, R. Ryoo & H. Joo / Electron Microscopy 1999 4H, 795. (4) Sakamoto, M. Kaneda, O. Terasaki, D.Y. Zhao, J.M. Kim, G. Stucky, H.J. Shin & R. Ryoo, Nature 20()0 4()H, 449. (5) Y. Sakamoto, I. Dfa/, O. Terasaki, D. Zhao, J. Perez-Pariente, J. M. Kim & G. D. Stucky, J. Fhys. Chem. 8 2002 I()6,?,\\S. (6) M. Kaneda, T. Tsubakiyama, A. Carlsson, Y. Sakamoto, T. Ohsuna, O. Terasaki, S.H. Joo and R. Ryoo., / Phys. Chem. 8 2002 l()6, 1256. (7) Ryoo, R. et al.,./ Phys. Chem. B 1999, 103,11A7>. (8) S. Jun, S. H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna & O. Terasaki, J. Am. Chem. Soc. 2000, 122, 10712. (9) S. H. Joo, S. J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki and R. Ryoo, Nature, 2001, 412, 169. (10) Z. Liu, Y. Sakamoto, T. Ohsuna, K.Hiraga, O. Terasaki, C.H. Ko, H.J. Shin and R. Ryoo, Angew. Chem. Int. Ed. 2000, 39, 3110. (11) Z. Liu, O. Terasaki, T. Ohsuna, K. Hiraga, H. J. Shin & R. Ryoo, ChemPhysChem 2001, 2 29. (12) M. Kang, S.H. Yi, H.I. Lee, J.E. Yie and J.M. Kim, To be published in Chem. Commun. 2002. (13) J. Miao, T. Ohsuna, O. Terasaki, K.O. Hodgson & M.A. O'Keefe, To be published in Phys. Rev. Letter. 2002.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Phase transformations involved during silica, modified silica, and non-silica mesoporous-organized thin films deposition. The role of evaporation. D. Grosso", E. L. Crepaldi^ G. J. de A. A. Soler Illia\ F. CagnoP, N. Baccile^ F. Babonneau', P. A. Albouy^, H. Amenitsch'^ and C. Sanchez^. "^Chimie de la Matiere Condensee, UPMC - CNRS, 4 place Jussieu, 75005 Paris, France. Fax: 33 (0)1 4427 4769; E-mail: grosso@ccr.jussieu.fr. ^Lab. de Physique des Solides, Universite Paris-Sud, 91405 Orsay, France. ''Institute of Biophysics and X-ray Structure Research, Austrian Academy of Sciences, Steyrergasse 17/VI, 8010 Graz, Austria. A high flux X-ray source was used to follow in-situ and in real time the formation of mesoorganised Si02, CeHs modified Si02, Ti02, Zr02, VO2 and AI2O3 thin films by dip-coating. These investigations have shown that disordered, bi and tri-dimensional meso-organisations can be obtained by varying chemical and processing conditions. This study allowed (i) to understand the mechanism of micelle segregation and self-assembly during fast concentration increasing, (ii) to determine the critical parameters and to fix the optimal conditions for each system, and (iii) to adapt the conditions to the process of meso-organised particle preparation. 1. INTRODUCTION Recently, the preparation of thin SiOs [l-3],Ti02 [4], ZrOs [5], VO2.X [6] and AI2O3 [7] meso-structured films by combining the sol-gel chemistry and the surfactant tcmplating approach were reported. The self-assembly requires the spontaneous organisation via interactions at surfactant/inorganic phase interfaces, induced by the rapid evaporation of solvent associated with dip-coating. By carefully adjusting the composition of the initial solutions, the coating parameters, and the treatment conditions, several structures can be obtained. The mesoporous film is thereafter obtained by thermal degradation of the surfactant. Even if the chemistry of the inorganic precursors and the mesophase behaviour of surfactants in various media are well known, the self-assembly process taking place during thin film formation remains still poorly understood because of the very low quantity of matter and the rapid kinetic of organisation. We recently succeeded to follow the structural evolution of the system by SAXS (Small Angle X-ray Scattering) during the first minutes that followed the deposition. This was made possible by using synchrotron radiation (ELETTRA, Trieste, Italy). For silica based systems, we have found that the final mesostructure obtained is strongly dependent on the surfactant type, the initial sol composition, the sol ageing time and the deposition processing conditions, while for the non-silica systems, the critical parameters are mainly related to the film processing and to the post treatment. 2. EXPERIMENTAL Initial solutions were prepared by hydrolyzing TEGS or 8 5 % molar TEGS + 15% molar of (EtG)3Si(C6H5) or MCln (M = Al, Ti, Zr, V) in an ethanol and water mixture followed by the addition of a certain amount of CTAB or Brij 58 or Pluronic F127 surfactants. The solution pH was fixed as to minimize the polycondensation of inorganic entities : HCl/Si = 0.004[3], HCl/M"^ « n[4]. Precise compositions of initial sols are listed in Table 1. Films were prepared by dip-coating in a sealed cabinet equipped with gas flow controller allowing to adjust the relative humidity and eliminate the extra ethanol vapour coming from the sol. The evolutions
282
of the film structures were deduced from the diffracted patterns collected every Is with a 2DCCD detector. Details of such an experiment has been detailed somewhere else [8-10] a simplify scheme is given in Figure 1. 3. RESULTS AND DISCUSSION A selection of in-situ 2D-SAXS patterns recorded at various times after deposition of a typical Pm3n silica film from system C in Table 1 are shown in Fig 2. In the present case, the drying line was observed at 22 s by interferometry, suggesting that the whole transformation process takes place in a non-totally dry system. One can clearly see that the formation of the cubic Pm3n structure involves the formation of various intermediate phases for which recorded d-spacing enter in the characteristic dimensions of CTAB micelles. 10 s after deposition, no diffraction is observed, suggesting that the film is not organised. After 13 s, a single well-defined peak starts to appear at q = 0.0183 A ' (55 A). This peak, labelled L(OOl) corresponds to planes that are parallel to the surface and since no other diffraction are present one can assume the phase to be lamellar. A feint diffusion ring is also present, suggesting that 1 ^^ I S
^K''--
13s
L(OOI)
;- j : - .
15s
H(002)
S1 " H(IOO)
19s
17s
Sanple H
(XD detector
Fig. 1. Scheme of the insitu SAXS experiment.
¥
\
,
c(2n)
32s
4k
i ' 1 « f
.<
M ii
Fig. 2. 2D-SAXS diffraction patterns collected during the deposition of a Pm3n cubic silica films templatcd with CTAB.
randomly located and oriented micelles (average distance % Increasing CTAB 60A) start to form at this stage. concentration : di>crea3>in|> From 14s, together with the micelle curvature diffusion ring and the lamellar phase peak, characteristic diffraction peaks corresponding to the H(002) (d()02 = 53A), H( 101) and H(IOO) reflections of the 3D-hexagonal P63/mmc are recorded, confirming that spherical micelles are present and organise in the latter Fig. 3. Model of Pm3n thin film formation from compact structure. After 16s, the TEOS/CTAB initial sol (see Table 1 sample C). characteristic diffraction pattern of the Pm3n cubic structure begins to overlay the 3D-hexagonal and the lamellar ones, while the diffusion ring is not visible any longer. The characteristic C(211) diffraction is located at d2ii = 50 A and on the in-plane profile line, suggesting that the domains have their (211) planes parallel to the film surface. At this stage one may assume that the whole film is
283
organised in three different mono-oriented mesostructures. At 20 s and at 21 s the lamellar and the 3D-hexagonal phases respectively disappear, while the cubic structure remains the only phase present in the dry film. It is lost if the film is allowed to stay in an ethanol saturated atmosphere (within the sealed dip-coater) for more than 25 min. During this process, the location of the phases with respect to both interfaces concords with the general behaviour of surfactant in composition phase diagrams : isotropic -^ arrangement of spherical micelles -> arrangement of cylindrical micelles -^ lamellar, with increasing concentrations. A model of such film formation is draw in Figure 3. Similar experiments performed on the other systems showed that (i) the organization occurs via a disordered to ordered transition and is also governed by the presence of both substrate/film and film/air interfaces for non ionic surfactants[8], (ii) the same mesostructure as pure silica is obtained for organically modified silica despite a different phase transition sequence, (iii) a similar mechanism applies for silica and non-silica systems leading to 2D-hexagonal structure despite their difference in chemical properties. Results concerning all the studied systems are given in Table 1. Table 1 Final structures for each system with related chemical and processing parameters. LP. : Inorganic precursor; S/M, EtOH/M, H2O/M, and HVM: molar ratios of surfactant, ethanol, water and proton to metal respectively; HR: relative humidity during dip-coating; F.S.: final film structure (D: disordered, H3: P63/mmc, CI: Pm3n, H2: P6m, C2: Im3m; + for phase coexistence).* similar results for Brij or Pluronic surfactants with different S/M.** 85% TEOS + 15% (C6H5)Si(OEt)3. Sample LP. Surf S/M EtOH/M H2O/M H V M Sol age HR F.S. A TEOS CTAB <0.I0 25 5 0.004 0-15 days 50% D B TEOS CTAB O.IO 25 5 0.004 4-10 days 50% H3 C TEOS CTAB 0.12-0.14 25 5 0.004 4-15 days 50% CI D 50% CI CTAB 0.14 25 5 0.004 4 ** E TEOS CTAB O.I 6-0.18 25 5 0.004 2-10 days 50% H2 F TEOS CTAB 0.18 50% H2 25 5 0.07 3 days G 20%) D TEOS CTAB 0.18 25 5 0.07 3 days 80%) CI TEOS CTAB 0.18 25 5 0.07 3 days \\ I 50%) CI+H2 TEOS CTAB 0.18 25 5 0.07 15 days 0.004J* 40 4-20 «4 O-I year <50% D TiCU FI27 0.02 0.004K* 40 4-20 «4 0-1 year >50% H2+C2 TiCU FI27 0.02 L* <30%o H2+C2 AlCh Brij 58 0.03-0.1 150 40 «2 70%) H2+C2 ZrCU FI27 In view of these results one observes that silica and modified silica films exhibit a final mesostructure that depend on the following critical parameters: - The surfactant content: higher CTAB concentration promotes low curvatures (see A to E). - The sol ageing time: longer periods of ageing promote silica condensation, changing silica species/micelles charge matching and preventing micelle rearrangement due to higher viscosity (see F and I)[9] - The relative humidity within the dip-coater: diffusion of water outside the film (low RH) favours cylindrical micelles while diffusion of water inside the film (high RH) favours hydrophilic matching and formation of spherical micelles (see F, G H) [I I]. Therefore, the mesostructure obtained for silica-based films prepared by E.I.S.A. [12] is controlled by a thermodynamic quasi-equilibrium (surfactant quantity), by the kinetic of diffusion of volatile species (water and ethanol), and by the kinetic of silica condensation (pH, ageing time, ...etc). For non-silica based films templated by neutral surfactants, the critical
284
parameter is mainly the RH as the condensation is quenched by the very low pH. The formation of micelles is mainly attributed to the quantity of water inside the film that should be optimised with the type of inorganic precursor and its related chemistry [4-7]. Also, once these optimised conditions where found and understood, it is just a matter of process to use them to form aerosols mesostructured particles via spray-drying for which the E.I.S.A. is just transposed on spherical droplets [13]. The following TEM pictures in Figures 4 show thermally treated SiOz, ZrOz and Ti02 particles prepared by spray-drying from solutions F, K, N from Table 1 respectively. The preparation and characterisation of such particles will be published in forecoming papers. (d)
1 10
30 20/o 50
70
Fig. 4. TEM of microtomed mesoporous particles of (a) Si02, (b) Zr02 and (c) Ti02 prepared from solutions F, K, and N respectively. The Ti02 particles contained a macropore in its centre generated by a latex sphere template dispersed in the N sol, previous to deposition, (d) XRD diagram of Ti02 particles of picture (c) caracteristic of anatase (101, 004, 200, 105, 211, and 204 with increasing 20) nanoparticles composing the inorganic meso-framework REFERENCES 1. H. Yang, A. Kuperman, N. Coombs, S. Mamicheafara & G. Ozin, Nature 379 (1996) 703. 2. D. Zhao, P. Yang, D. I. Margolese, B. F. Chmelka & G. D. Stucky, Chem. Comm. (1998) 2499. 3. D. Grosso, A. R. Balkenende, P.A. Albouy, M. Lavcrgne, and F. Babonneau, J. Mater. Chem. 10(2000)2085. 4. D. Grosso, G. A. A. Soler lllia, F. Babonneau, C. Sanchez, P. A. Albouy, A. BrunctBruneau & R. Balkenende, Adv. Mater. 13 (2001) 1085. 5 E. Crepaldi, G. A. A. Soler lllia, D. Grosso, P. A. Albouy, & C. Sanchez, Chem. Com. (2001)1582. E.L. Crepaldi, D. Grosso, G. J. de A. A. Soler-Illia, P.-A. Albouy & C. Sanchez, Chem. Mater. (2002) in press. L. Pidol, D. Grosso,G. A. A. Soler lllia, E. Crepaldi,, P. A. Albouy, H. Amenitsche, P. Euzcn & C. Sanchez, J. Mater. Chem. 3 (2002) 557. 8 D. Grosso, A.R. Balkenende, P.A. Albouy, A. Ayral 11. Amenitsh, F. Babonneau, Chem. Mater. 13(2001) 1848. 9. D. Grosso, F. Babonneau, P.A. Albouy, H. Amenitsch, A.R. Balkenende, A. BrunetBruneau & J. Rivory. Chem. Mater. 2 (2002) 931. 10. D. Grosso, F. Babonneau, G. J. de A. A. Soler-Illia, P.-A. Albouy, H. Amenitsch, Chem. Comm. (2002) 748. 11. K. Fontell, A. Khan, B. Lindstrom, D. Macicjwcska, & S. P. Puang-Ngem, Colloid & Polymer Science 7 (1991) 269. 12. C. J. Brinker, Y. Lu, A. Sellinger, & H. Fan, Adv. Mater. 11 (1999) 579. 13. Y. Lu, H. Fan, A. Stump, T. L. Ward, T. Rieker & C. J. Brinker, Nature 398 (1999) 223.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
285
Comparison of the mechanical stability of cubic and hexagonal mesoporous molecular sieves with different pore sizes Martin Hartmann and Ajayan Vinu University of Kaiserslautem, Department of Chemistry, Chemical Technology, P.O. Box 3049, D-67653 Kaiserslautem, Germany The mechanical stability of mesoporous molecular sieves, viz. MCM-41, MCM-48, SBA-1 and SBA-15, is compared employing X-ray powder diffraction, nitrogen and organics adsorption. The adsorption capacity is considerably decreased by mechanical compression between 20 and 400 MPa. It has been found that SBA-1 exhibits the highest mechanical stability, while SBA-15 is mechanically unstable despite its thick walls (ca. 2 nm). 1. INTRODUCTION Mechanical stability is an important characteristic of an adsorbent or catalyst for most practical applications. Prior to adsorption and catalytic studies, IR or electric conductivity measurements, the fme powders obtained by hydrothermal synthesis are routinely compacted at high pressure into pellets. Therefore, crushing strength is a critical requirement to conserve the pore volume, surface area and pore diameter of the starting material. Data on the mechanical strength of mesoporous materials such as MCM-41 and MCM-48 have already been published [1-4], but no unifying approach also including less well studied materials such as SBA-1 and SBA-15 has been presented. In the present contribution, the mechanical stability of different mesoporous materials is investigated by adsorption of nitrogen and organic vapors such as benzene, cyclohexane and ^-heptane. Emphasis has been placed on the comparison of cubic and hexagonal materials and the comparison of materials with different pore size and wall thickness. 2. EXPERIMENTAL The syntheses of the mesoporous silicas was performed employing standard procedures as described elsewhere [4,5]. To test the mechanical stability, the samples were compressed in a steel die of 24 mm diameter, using a hand-operated press, for 30 min. The different external pressures applied were calculated from the external force and the diameter of the die. Subsequently, the obtained disc was crushed and sieved to obtain pellets with a diameter of 0.22 to 0.35 nm, which were used for all further experiments. Powder X-ray diffraction patterns were recorded on a Siemens-axs D5005 diffractometer using CuKa radiation. Nitrogen adsorption isotherms were measured in a Quantachrome Autosorp-1 instruments at 77 K. The adsorption isotherms of the organic vapors were collected in a home-built volumetric adsorption instrument at 25 °C. Prior to the adsorption measurements, the samples were outgassed at 200 °C under vacuum for 4 h. To whom correspondence should be addressed. Fax : +49 631 205 4193 E-mail: hailmann(«^rhrk.uni-kl.de
286
3. RESULTS AND DISCUSSION The mechanical stability of hexagonal MCM-41 materials with different pore sizes has been studied employing X-ray powder diffraction, nitrogen and /t-heptane adsorption and mercury porosimetry. In Figure 1, the XRD patterns of compressed MCM-41 materials synthesized with Ci6TMABr and CnTMABr are displayed. With increasing pressure, the intensities of the reflections decrease for both materials. Analogous XRD patterns are obtained for MCM-41/12 (MCM-41 synthesized with CizTMABr as template) and MCM-41/10 (not shown). With increasing pressure, the steep increase in the N2 adsorption isotherms (not shown) is less pronounced and the amount of adsorbed N2 is reduced. The height of the pore size distribution (which is a measure of the fraction of pores with a certain pore diameter) decreases with increasing pressure and broadens to the low diameter side. This gives further evidence for our model that a fraction of the mesopores is completely destroyed upon compression. Similar models are also discussed in the literature [6].
OMPa 52MPa 130 MPa 260 MPa
4 6 Angle 20 /'
OMPa 52 MPa 130 MPa 260 MPa
yj 30000 h
10
4 6 Angle 20 / '
10
Fig. 1. XRD patterns of MCM-41/14 (left) and MCM-41/16 (right) after compression. The tcxtural properties of MCM-41/16 and MCM-41/14 were also characterized by mercury porosimetry (Figure 2). Mercury porosimetry is largely used to evaluate the porosity of industrial catalysts and other porous materials. Due to the high pressure needed to fill small pores, this method is at present limited to pores with dp > 3 nm. The mean pore diameters for MCM-41/16 and MCM-41/14 are determined to 5.0 nm and 4.2 nm, respectively, which is about 2 nm larger than the results from BJH analysis of the adsorption branch of the respective nitrogen isotherm. It is, however, widely accepted that BJH analysis underestimates the pore diameter by about 1 nm in this mesopore range [7,8]. Furthermore, the inaccuracy of mercury porosimetry in the range of such narrow mesopores is probably also ca. 1 nm, so that the pore diameters calculated from both methods are comparable within experimental error. The pore diameter decreases slightly with increasing pelletizing pressure and the pore size distribution broadens. The second maximum around 0.2 ^m and 1 fim for MCM-41/14 and MCM-41/16, respectively, (Figure 4.36) is ascribed to the filling of the void spaces (macropores) between the primary particles. The study of the intergranular porosity shows that the particles are getting closer under pressure. Therefore, the size and the number of these pores decrease with increasing pelletizing pressure. For the pellets, a new maximum at ca. 100 |im is observed which represents the toroidal void space of a collection of solid particles [9]. The size of these macropores mainly depends on the pellet size and, hence, is independent of the pelletizing pressure and the catalyst under investigation.
287 I I I
iMiiii 1 1
; ,'1il
imiiT I 1
MCM-41-52 MCM-41-130 MCM-41-260
'o) 3 -
1
Q ' D) O •o
>
-
-1
/11 1 II
/
iiim 1 T
11 1 1
-
:
a
fi
:
11
/
0
imiTTi 1
llllll 1 1 1
:
n
f ^
/
' 'V^ /' ^
•o
III! 1 1 1 I
11
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/
i f\
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MCM-41-52 MCM-41-130 MCM-41-260-
-^0.4 :
/
H
llllll 1 ( 1
\
\
]
100
10 1 0.1 0.01 0.00 100 10 1 0.1 0.01 0.00 Pore diameter / |jm Pore diameter / fjm Fig. 2. Mercury porosimetry analysis of MCM-41/14 (left) and MCM-41/16 (right). In Figure 3, the relative pore volumes of the compressed cubic phases MCM-48 and SBA-1 are compared. The mechanical stability of these materials differs only slightly with MCM-48 being more stable. The difference might be attributed to the different structure of the two cubic phases. For the hexagonal phases MCM-41 and SBA-1, the mechanical stability decreases with decreasing wall thickness / unit cell size (w / (w + dp) (Figure 4). Consequently, SBA-15 is mechanical less stable compared to MCM-41/16 despite its significantly thicker walls (2 nm vs. 1.1 nm). 1 .z
I
1
I
1
I
I
I
I
I
I
^o ^ 10^ ^ ' ^ ^ ^ ^ E D #^^^^^^ o 0.8 > * \ ^ ^ 0
o
-
0.6
§ ^ ^ \,^^
0)
> '% 0.4
-
-
ff
0 1
0
1
100
1
1
200
1
1
300
400
,
500
, 600
Pressure / MPa
Fig. 3. Comparison of the relative pore volumes V/Vo of pelletized (in the calcined form) SBA-1 (closed symbols) and MCM-48 (open symbols) molecular sieves calculated from the adsorption of nitrogen (o), benzene (V), cyclohexane (n) and A7-heptane (A). In comparison to MCM-41/14, which exhibits a pore volume of 0.58 cm'^ g"' (68 % of the parent material) and a BET surface of 960 m^ g"' after application of 260 MPa , the MCM-48 materials tested in our study are more stable. A significant lower stability is reported for other MCM-41 materials [1], which were probably of inferior quality compared to the materials prepared in our study. The higher stability of MCM-48 is most likely due to the threedimensional arrangement of the pore system, which also profits from a higher wall thickness as compared to MCM-41. Nevertheless, the mesoporous materials investigated in this work exhibit inferior mechanical stability compared to other adsorbents and catalysts such as
288
zeolites, silica and alumina [10,11], which may result from their large porosity and the absence of a stabilizing crystal structure.
50
100
150
200
300
Pressure / MPa Fig. 4. Comparison of the relative pore volumes V/Vo of pelletized (in the calcined form) SBA-15 and MCM-41 molecular sieves with different pore sizes calculated from the adsorption of nitrogen. 4. CONCLUSIONS Data on the mechanical strength of mesoporous materials have been obtained from nitrogen and organic vapor adsorption as well as from mercury porosimetry. For these materials, it has been shown that the loss of pore volume takes place without a significant decrease of the pore diameter. It is, therefore, surmised that a fraction of the pores is completely destroyed, while the rest of the material is unaffected by the compression. . The mechanical stability increases with decreasing pore diameter and with increasing wall thickness. Mesoporous molecular sieves were found to possess rather low mechanical stability as compared to other materials, viz. zeolites and aluminophosphates which are crystalline
REFERENCES 1. V. Y. Gusev, X. Feng, Z. Bu, G.L. Mailer and J.A. O'Brien, J. Phys. Chem., 100 (1996) 1985. 2. T. Tatsumi, K.A. Koyano, Y. Tanaka and S. Nakata, J. Porous Mater. 13 (1999) 6. 3. Desplanticr-Giscard, O. Collart, A. Galarneau, P. Van dcr Voort, F. Di Renzo and F. Fajula, Studies in Surface Science and Catalysis 129 (2000) 665. 4. M. Hartmann, and C. Bischof, J. Phys. Chem. B 103 (1999) 6230. 5. M. Hartmann, S. Racouchot, and C. Bischof, Microp. Mesop. Mater., 27 (1999) 309. 6. A. Galarneau, D. Desplantier-Giscard, F. Di Renzo and F. Fajula, Catal. Today, 68 (2001) 191. 7. A. Sayari, M. Kruk and M. Jaroniec, Catal. Lett., 49 (1997) 147. 8. P.I. Ravikovitch and A.V. Neimark, Langmuir, 16 (2000) 2419. 9. R. Mayer, and R.A. Stowe, J. Phys. Chem., 70 (1966) 3867. 10. V. Bosacek, M.M. Dubinin, O. Kadlets, K.O. Murdmaa and V. Navratil, Dokl. Chem., 174(1967)305. 11. S.J. Gregg, and J.F. Langford, J. Chem. Soc, Faraday Trans., I 73 (1977) 747.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
289
Fs-time-resolved diffuse reflectance and resonance Raman spectroscopic studies on MCM-41 as microchemical reactor Su Young Ryu and Minjoong Yoon* Department of Chemistry, Chungnam National University, Daejon, 305-764, Korea We have encapsulated porphyrin derivatives into MCM-41, TiMCM-41, and Cu°-A1MCM41, and its photoinduced electron transfer were investigated by steady state and ultrafast timeresolved spectroscopy. All spectroscopic results measured for tetraphenyl Porphine Manganese (III) Chloride (Mn(III)TPPCl) encapsulated in MCM-41 and TiMCM-41 reveal that framework modification by incorporating the Ti02 into the MCM-41 enhances the electron accepting ability of the MCM-41 framework. And also, Raman and UV-Vis spectroscopic investigations for Zn" tetraphenylporphyrin (Zn^TPP) encapsulated into MCM41 and CU"A1MCM-41 allow us to conclude that Zn"TPP in MCM-41 is oxidized to a considerable extent. Furthermore, central metal ion exchange of Zn"TPP encapsulated into CU"A1MCM-4 1 gives Cu" tetraphenylporphyrin (CU"TPP) with almost unit transformation efficiency. In conclusion, metal ion exchanged MCM-41, Cu°AlMCM-41, might be used for microchemical reactor metal-ion exchange reactions of the porphyrin derivatives. 1. INTRODUCTION Among the heterogeneous hosts, a mesoporous silica, MCM-41, whose pore size is between 2 and 10 nm, has been known to have promising usage for catalysis due to its regular hexagonal array of uniform silica tube with a narrowly distributed diameters.' These unique properties of MCM-41 might allow one to selectively ionize porphyrin macrocycles and reduce the back electron transfer rate by separating the donor and acceptor, which will eventually increase the ionization efficiency."^ If the tetrahedral Si^"* of the mesoporous silica is replaced with transition metal, it is known to be more acidic as well as more reactive with adsorbates than that of MCM-41.^ Due to the catalytic potential of transition metal ions adsorbed, the synthesis and characterization of MCM-41 modified by exchanging the metal ions is one of active research fields."* In this work, we have attempted to investigate the fast electron or charge-transfer processes between metallo-porphyrin and mesoporous silica by using the ultrafast time-resolved diffuse reflectance spectroscopy. And Zn^TPP encapsulated into CU^'A1MCM-41, was also investigated with respect to the utility of mesoporous silica as a michrochemical reactor to control the product selectivity of the central metal exchange reaction in porphyrin macrocycles. 2. EXPERIMENTAL 1.
The synthesis procedure and characterization of the mesoporous silica used in this work are already reported elsewhere.^ X-ray diffractograms were recorded by M03X-HF diffractometer. The diffuse reflectance UV-VlS spectra were recorded by using a Shimadzu
To whom correspondence should be addressed. This work was financially supported by the Korea Research Foundation and the Korea science and Engineering Foundation.
290
UV-3101PC spectrometer. The details of femtosecond diffuse reflectance spectroscopic system have been reported elsewhere.^' ^ Briefly, a light source of self-mode-rocked Ti:sapphire laser pumped by an Ar^ laser and a Tiisapphire regenerative amplifier system with Q-switched Nd:YAG laser. Resonance Raman signal upon photoexcitation at 442 and 458 nm from HeCd and Ar ion laser, respectively, was dispersed by a Raman UIOOO double monochromator and detected with cooled photomultiplier in a photon counting configuration. To determine which metals were altered by the encapsulation of Zn°TPP into Cu AIMCM41, the metal concentration in the toluene solution extracted from the used mesoporous silica was measured by inductively coupled plasma mass spectroscopy(ICP-MS). 3. RESULTS AND DISCUSSION 3.1. The photoinduced electron transfer of Mn (III) chloride tetraphenylporphine incorporated TiMCM-41 and MCM-41 The efficiency of photoinduced electron transfer is generally limited by the occurrence of deactivating back electron transfer, which completes with other reactive pathway of the generated radical ion pairs. The large pore molecular sieves like MCM-41 is used to provide an appropriate microenvironment for retarding dramatically the back electron transfer and increasing enormously the lifetime of the photogenerated ion pairs. The ground-state absorption spectra of Mn°^TPP(Cl) encapsulated into MCM-41 and TiMCM-41 show a dramatically change compare to that in benzene, i.e. i) the Q-band was blue-shifted, ii) the ratio of Soret bands to Q-bands was reduced, and iii) the Soret bands became broader. These absorption spectral changes indicate that Mn"' TPP(Cl) molecules are adsorbed well onto MCM-41 and TiMCM-41, and that the porphyrin Ji-electrons interact with the surface hydroxyl group of MCM-41 and TiMCM-41.^'' ^^^ To understand the microenvironmental effects on the photophysical dynamics of Mn TPP(Cl), we have performed the femtosecond diffuse reflectance photolysis. Figure la shows the transient absorption spectra of Mn"' TPP(Cl) in benzene at delay times 1 ps with an excitation of the soret band at 390 nm. Since Mn'" TPP(Cl) has a d"^ ground-state electron configuration, the (ji, ji*) transition states of complex are consist of a quintet singlet state ( S i ) and a "tripmultiplet" manifold (^Ti, '^Ti, ^Ti). The transient species at 450-500 nm with a decay time of approximately 6 ps in benzene is assigned that the tripquintet, Ti (;i, JT ), which is apparently formed rapidly via intersystem crossing from the lowest singquintet, Si(jr, JT ) decaying very rapidly (2-3 ps).'^''^ The transient spectra in MCM-41 and TiMCM-41 were observed greatly different with that in benzene as shown fig. 1(b) and 1(c) : In MCM-41 and TiMCM-41, its spectral feature shows the broad transient absorption not only around 450-500 nm but also in the low energy region around 550-800 nm. According to suggestion by Holtern et al.,^^' ^^ we ascribe the transient absorption around 550-800 nm to a
400
500
600
700
wavetengji/nm
450
500
550
600
650
Wavetength/nm
700
750
450
500
550
600
650
Wav^engih/nm
Fig 1. Transient absorption spectra Mn"'TPP(Cl) in benzene (a), in MCM-41(b), and in TiMCM-41(c).
^CO 750
291
quintet CT state. The temporal decay profile of the transient absorption indicates that two different transient species (c.a. 10 ps and c.a. 80 ps) were observed in MCM-41 and TiMCM41. The short-lived component should be originated from relaxation of a "tripmultiplet" state and the longer-lived component should be attributed to the spin-forbidden relaxation via a quintet CT state. And also, we found that the longer-lived component in TiMCM-41 is even more enhanced than in MCM-41, indicating that the framework of TiMCM-41 could be more easily interacted with Mn^TPP(Cl) compared with one of MCM-41, and the spin-forbidden relaxation via a quintet CT state were favorable in the order TiMCM-41> MCM-41. After irradiation, MnTPPCl^* radicals are detected in MCM-41 or TiMCM-41, indicating that the mesoporous silicate framework plays good electron acceptor. Furthermore, it has been found that the formation MnTPPCr* is easier in TiMCM-41 than in MCM-41, indicating that framework modification by incorporating the Ti"^^ into the MCM-41 enhances the electronaccepting ability of the MCM-41 framework. Therefore, those photogenerated electrons and Mn TPP(Cl) cation radical in this system could be applied to the photocatalytic reaction. 3.2. Resonance raman studies on Zn" Tetraphenylporphyrin encapsulated into MCM-41 and Cu^AlMCM-41: catalytic ionization of Zn^TPPand its central metal ion exchange We have encapsulated Zn°TPP into MCM-41 and Cu°AlMCM-41. Fig. 2(a) shows the resonance Raman spectrum of Zn"TPP encapsulated into MCM-41.The resonance Raman spectra of Zn"TPP in crystal and toluene were also displayed in Fig. 2(b) and 2(c) for comparison. The frequency and half-width of the Raman band in crystal are identical to those in toluene solution. The most evident change due to encapsulation of Zn"TPP into MCM-41 is the enhancement of u 4 mode intensity as well as the broadening of u 2 mode to the lower frequency region. It is interesting to compare Raman spectrum of Zn"TPP-MCM-41 with that of Zn"TPP radical cation electrochemically generated in solution because mesoporous silica is well known to have an oxidative catalytic properties, and the radical cation is rather stabilized to be long-lived.^' ^ The metallo-porphyrin radical cation like Zn^TPP^ and Cu"TPP^exhibit a rather strong enhancement in the intensity of the Raman modes related to phenyl substituent such as u 4 and u 1 modes as well as appreciable down Shift of u 2 modes.''' These observations were consistent well with the results of the observed Zn"TPP-MCM-41. All the
1300 1400 1500 R a m a n S h ift ( c m ' ' !
350 400 450 500 550 600 650 700 750 800 W a v e l e n g t h (nm )
Fig. 2. Resonance Raman Spectra of Zn'^P-MCM-
Fig. 3. UV-Vis absorption spectra of Zn"TPP-MCM-41
41(a), Zn"TPP crystal (b), Zn°TPP-Cu"AlMCM-
(a), ZnVPP-Cu°AlMCM^l (b), sample in toluene
41(c), Zn"TPP (d), and Cu°TPP in solution (e).
extracted &om Zn°TPP-MCM41 (c), and Zn'^PCU"A1MCM-41 (d), neat Cu°TPP in toluene (e).
292
results led us safely to propose that MCM-41 can efficiently oxidize Zn"TPP encapsulated. Fig. 3a shows absorption spectra of Zn"TPP-MCM-41, the Soret absorption band maximum of Zn"TPP was observed blue shifts accompanied with an appreciable broadening compare to that in toluene solution. In addition to this, the red shifts of Q-band with a new band at 650 nm could be also observed. The characteristic features in the electronic spectra of porphyrin cation radical compared to those of neutral porphyrins are a diminished intensity, the broadening of the Soret band and the appearances of new visible bands at 600-700nm.^^ Therefore, The broadening and blue shifts observed in the Soret band and the additional band at 650 run in Q-band can be explained in terms of the catalytic oxidation of Zn°TPP into MCM-41. Raman and UV-Vis spectroscopic investigations allows us to conclude that Zn TPP in MCM-41 is oxidized to a considerable extent. Tabel 1 Resonance Raman Frequencies (cm"^) for Zn"TPP(crystal), Zn"TPP in solution, Zn"TPPMCM-41, Zn"TPP-Cu'^AlMCM-41, and CU"TPP in solution. Zn"TPPMClVI-41
Zn"TPP-
Zn"TPP in solution
Cu"TPP in solution
CU"AIMCM-41
(a)
Zn"TPP (crystal) (b)
(c)
(d)
(e)
1234
1234
1236
1234
1238
1292
-
-
-
-
1351
1352
1368
1348
1366
1414 1467
-
-
1492
1490
-
1490
-
Vll
1545
1545
1561
1548
1563
V2
1593
1592
1601
1597
1601
4
assignment
Vi
" V4
V2«
The encapsulation of Zn"TPP into CU"A1MCM-41 exhibits a broad Soret band at 410 nm (Figure 3 (b)), which is lower wavelength compared to that of Zn"TPP-MCM-41. The Q-band of Zn"TPP-Cu"AlMCM-41 also exhibits the spectral features quite different from that of Zn"TPP-MCM-41. The resonance Raman spectrum in CU"A1MCM-41 is shown in Figure 2 (c). Of very interest it is to note that o 2 and u 4 modes is up-shifted more than 10 cm ' compared to those for Zn"TPP in crystal and toluene. It should be also noticed that the spectrum is quite different from that of Zn"TPP-MCM-41 (See Table 1). Surprisingly, both UV-Vis absorption and Raman spectral feature of Zn"TPP-Cu"AlMCM-41 were almost identical to the previous reported spectra of Cu" TPP in solution.^^ To obtain further spectroscopic data on this peculiar system, UV-Vis spectrum of the extracts with toluene from the solid Zn"TPP-Cu"AlMCM-41 is shown in Figure 3 (d). For comparison. Figure 3 (c) shows the electronic absorption spectrum observed from the toluene extracts from Zn"TPPMCM-41. Of quite interest is to note that the two spectra are different from each other even though the same porphyrin, Zn"TPP is initially encapsulated. To gain better understandings of the interesting central metal ion exchange occurred in mesoporous silica, we have measured UV-Vis as well as Raman spectra on the extracted solutions from both Zn"TPP-Cu"AlMCM41 and Zn"TPP-MCM-41. Zn"TPP is recovered without any noticeable changes in both UV-
293
Vis and Raman spectrum after encapsulation into MCM-41. However, the extracted solutions from Zn"TPP-Cu ' A I M C M - 4 1 shows that the spectroscopic observations are identical to those from C U " T P P dissolved in toluene. From the ICP-MS studies the relative ratio between Cu" to Zn" quantity in mole fraction is found to be (32.5±1.0) (not shown). Central metal ion exchange of Zn^TPP encapsulated into Cu°AlMCM-41 gives Cu ° tetraphenylporphyrin with almost unit transformation efficiency. All the experimental results in addition to the above considerations led us to safely suppose that the mobile Cu" ions in CU"A1MCM-41 might replace Zn" ions from the porphyrin macrocycles, if the resultant C U " T P P is stable in the mesoporous silica (see Scheme 1).
ClPTPP Scheme 1. Microreactor controlled metal-ion exchanged reactions of porphyrin derivatives 4. CONCLUSION Mesoporous MCM-41 and TiMCM-41 molecular sieves are found to be promising hosts for photoinduced charge separation of adsorbed Mn"'TPP(Cl). In MCM-41 or TiMCM-41, Mn'" TPPCl *' radicals are generated by irradiation, indicating that the mesoporous silicate framework plays good electron acceptor. The Mn'" TPPCl *' generation increases in the order MCM-41 < TiMCM-41, indicating that framework modification by incorporating the Ti^^ into the MCM-41 enhances the electron-accepting ability of the MCM-41 frame work. Also the metal ion exchanged MCM-41 and Cu°AlMCM-41 might be used for microchemical reactor controlled metal-ion exchange reactions of the porphyrin derivatives.
REFERENCES P Selvam, S.K. Bhatia, C.G. Sonwane, Ind. Eng. Chem. Res. 40 (2001) 3237. H. M. Sung-Suh, Z. Luan, L. Kevan, J. Phys. Chem. B 101 (1997) 10455 J. Xu, J.-S. Yu, S.J. Lee, B.Y. Kim, L. Kevan, J. Phys. Chem. B 104 (2000) 1307. S. Sinlapadech, R.M. Krishna, Z. Luan, L. Kevna, J. Phys. Chem. B 105 (2001) 4350. Y. Kim, J.R. Choi, M. Yoon, A. Furube, T. Asahi, H. Masuhara, J. Phys. Chem. B 105 (2001)8513. 6. A. Furube, T. Asahi, H. Masuhara, H. Yamashita, M. Anpo, J. Phys. Chem B 103 (1999) 3120-3127. T. Asahi, A. Furube, H. Fukimura, M. Ichikawa, H. Masuhara, Rev. Sci. Instrum. 69 (1998)361. H. M. Sung-Suh, Z. Luan, L. Kevan, J. Phys. Cheln. B 101 (1997) 10455-10463. R A. Leennker, H. T. Thomas, L. D. Weis, R C. James, J. Am. Chem Soc. 88 (1966) 5075.
1. 2. 3. 4. 5.
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10. A. Ron, M. Folman, 0. Schnepp, J. Phys. Chem 36 (1962) 2449. 11. L. Mochida, K. Tsuji, H. Fujitsu, K. Takeshida, J. Am Chem.Soc. 84 (1980) 3159. 12. M. Goutennan, In The Polphyrins, Dolphin. D., Ed., Academic: New York. Vol.3 (1978) 1-165. 13. M. P. Irvine, R. J. Harrison, M. A. Stahand, G. S. Beddard, Bunsen-Ges. Ber, Phys. Chem 89 (1985) 226-232. 14. X. Yan, C. Kinnaier, D. Holten, Inorg. Chem. 25 (1986) 4774-4777. 15. D. Holten, M. Gouterman, In Optical Properties and Strucfure of Tetrapyrroles, Blauer, G. Sund, H., Eds., de Gruyter: Berlin. (1985) 64. 16. L. Pekkarinen, H. Linschitz, J. Am. Chem Soc. 82 (1960) 2407-241150 17. R.S. Czernuszewicz, K.A. Macor, X.-Y. Li, J.R. Kincaid, T.G. Spiro, J. Am. Chem. Soc. I l l (1989) 3860. 18. R.H. Felton, D. Dolphin, The Porphyrins, Academic Press, New York, 5 (1978) Chapter 3.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Detailed investigation of the microporous character of mesoporous silicas as revealed by small-angle scattering techniques B. Smarsly"', K. Yu^ and C. J. Brinker''^ ^University of New Mexico, Center for Microengineered Materials, Advanced Materials Laboratory, 1001 University Blvd., SE, Albuquerque, NM 87106, USA ^Sandia National Laboratories, MS 1349, Albuquerque, NM, 87185, USA 1. INTRODUCTION Mesoporous
silica bulk materials, prepared
by templating with
amphiphilic
block
copolymers, have recently been shown to contain a substantial degree of additional micropores, located in the silica walls. Significant experimental evidence from sorption and SAXS/SANS (Small-Angle X-ray/Neutron Scattering) suggest that these micropores originate from the hydrophilic poly(ethylene oxide) (PEO) chains being tightly embedded in the matrix and creating voids of similar dimension after template removal [1-2]. This microporosity was found in bulk silicas obtained from several polymer templates such as PEO-PEO-PEO (SBA15) and poly(styrene)-/?-poly(ethylene oxides) (PS-PEO). While the degree of microporosity seems to be tunable by a variation of the preparation conditions, the determination of micropores in the presence of mesopores still is an experimental challenge. In particular, standard evaluations of microporosity based on nitrogen sorption measurements have to be regarded as inappropriate for these materials. T-plot and as-plot analyses do not have the required accuracy because of the lack of suitable reference materials. It was recently demonstrated that suitable SAXS/SANS techniques provide a practical and
accurate
determination of microporosity, and even the micropore size could be determined. Thus, this analytical tool allows studying the influence of different preparation methods on the microporosity. In this study, mesoporous silica films were prepared by using PS-PEO polymers as structure directing agents and applying the pathway of evaporation-induced self'correspondence author: bsmarslv@unm.edu This work was supported by Sandia National Laboratories, a Lockheed Martin Company, under Department of Energy Contract DE-AC04-94AL85000, the Air Force Office of Scientific Research Award Number F49620-01-1-0168, the DOE Office of Basic Energy Sciences, the DOD MURI Program Contract 318651, and Sandia's Laboratory Directed Research and Development Program.
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assembly. Starting from a diluted solution of the polymers in THF/H2O, containing TEOS as precursor, this preparation method is characterized by a very low polymer concentration at the beginning of the evaporation, which is different from the procedures for powder materials. The objective of this study is two-fold. Firstly, it was attempted to produce well-defined mesostructured porous silica films, which were characterized by TEM and SAXS in grazing incidence geometry (GISAXS). Secondly, detailed GISAXS and sorption studies were carried out in order to check for microporosity due to PEO chains. The GISAXS data will be compared with corresponding analyses on powder materials obtained also from PS-PEO templates. 2. EXPERIMENTAL In a typical synthesis, PS(22)-Z7-PEO(70) diblock copolymer, which has 22 styrene units and 70 ethylene oxide units, was dissolved in tetrahydrofiiran (THF) at 1 wt.%. Afterwards, a certain amount of tetraethoxysilane (TEOS), hydrogen chloride (HCl), as well as water (Milli Q) were added to the dilute copolymer solution in THF [3-4]. The quantity of the silica precursor, namely MTES added was such as to achieve a weight ratio of ca. 1 copolymer : 7 precursor. The total amount of HCl and water added was such as to achieve molar ratios of 1 TEOS : 0.004 HCl : 5 H2O. After 30 minutes of sonication, one drop of the solution was cast to obtain a silica/diblock thin film on a silicon wafer. Calcination in Argon (with a heating rate of l°C/min to 400 °C for 3 hours) removed the diblock, as confirmed by thermogravimetric analysis, and produced mesostructured porous silica films. The GISAXS measurements were performed directly on cast thin films, using the 5-meter pinhole instrument in the SAXS laboratory at the Center of Micro-Engineered Materials at the University of New Mexico, with an available range of s values between 0.08 - 1.4 nm', where .v = 2sin(0)/A. and 20 is the scattering angle and X the CuKa wavelength (0.154 nm). 3. RESULTS AND DISCUSSION Fig. 1 shows a TEM (Transmission Electron Mikrograph) picture of a mesoporous silica film, obtained by using a PS(22)-Z7-PEO(70) block polymer. Tilting experiments indicate a cubic lattice. The structure corresponds to a bcc or fee structure, but a differentation between them was not possible based on TEM. It is seen that the pore size is about 5nm assuming spherical mesopores, but the determination of mesopore sizes from TEM involves certain inaccuracies. It could be shown by thermogravimetric analysis that by the calcinations step almost all of the polymer is removed at 400 degrees Celsius. Interestingly, these materials showed almost no absorption in nitrogen sorption measurements, indicating that the mesopores are isolated voids.
297
Fig. 1. TEM of a mesoporous silica film obtained from PS(22)-Z7-PEO(70) polymer. The scale bar corresponds to 50nm. A and B correspond to different tilting angles, the difference is 15 degrees.
/^. 1000 J
y
[200]
100 J CO
10
[110]
[110]
14 0.07 0.10
0.33
0.67 1.00 -ii
scattering vector s [nm" ] Fig. 2. A. GISAXS data of a mesoporous silica film, templated with PS(22)-6-PEO(70) (curve 1, crosses), mesoporous silica film without any template (curve 2, open circles), SAXS data of mesoporous SEIOIO silica powder according to ref [1] (curve 3, crosses), and Porod's law (s"^). B. 2D GISAXS pattern corresponding to A, curve 1. The peak indexing is based on a bcc cubic structure. The two non-indexed peaks are due to the primary reflected beam.
298
Fig. 2 shows SAXS and GISAXS data from silica films and powder materials. Curve 1 corresponds to a mesoporous silica film prepared by using a PS(22)-Z?-PEO(70) block copolymer template described by the procedure above. The curve was obtained by averaging the 2D GISAXS data in Fig. 2B in the region of the [110] reflections, if the GISAXS pattern is interpreted in terms of a cubic bcc structure. Curve 2 represents the GISAXS data of a silica film prepared under identical conditions as the sample belonging to curve 1, without using a template. The non-templated film was studied to study the microstructure of the silica matrix: if micropores were present, this should be apparent at larger scattering vectors. Since the two GISAXS data sets were obtained under comparable conditions (angle of incidence, etc), the relative scattering intensities can be directly compared. It is seen that curves 1 and 2 are almost identical in shape and intensity at scattering vectors beyond ca. s = 0.3 nm'. This identity at larger s already suggests the absence of additional micropores due to PEO. This assumption is further support by comparing the GISAXS data of the thin films with SAXS data obtained on mesoporous silica powders, which were also obtained from PS-PEO templates (Fig. 2A, curve 3). This material was obtained from a PS(9)-b-PS-23) polymer and shows several reflections in SAXS patterns, resulting from ordered mesopores. These materials were shown to contain a significant degree of microporosity. It was concluded that SAXS curves at larger scattering vectors s significantly differ from a theoretical mesostructure without any microporosity. The microporosity gets apparent by a significant deviation from Porod's law (I(s) a s'^) at larger s. A comparison with the GISAXS patterns in Fig. 2A obtained from the silica films therefore furthermore indicates the absence of additional microporores, which were shown to have sizes of about 0.5-1.5nm. Based on the microporosity in mesoporous silica porous, determined in our previous publications [1-2], the upper limit for the micropore volume can be estimated to be about 0.01 ml/g. The reason for the absence for microporosity in the mesoporous silica films may be related to the significantly different preparation conditions, compared to mesoporous bulk silicas: the self-assembly is carried out over a much longer period of time (about 1 day) and in THF, which is a good solvent for PS. Our results suggest that during the solvent evaporation the polymer continuously gets more insoluble, while the silica framework is still fragile enough to rearrange. Hence, the PEO chains may retract from the matrix as a result of the solvent removal. REFERENCES 1. C. G. Goltner, B. Smarsly, B. Berton, M. Antonietti, Chem. Mater., 13 (2001) 1617. 2. B. Smarsly, C. Goltner, M. Antonietti, W. Ruland, E. Hoinkis, J. Phys. Chem. B, 10 (2001)831. 3. K. Yu, A. J. Hurd, C. J. Brinker, A. Eisenberg, Langmuir, 17 (2001) 7961. 4. B. Smarsly, K. Yu, C. J. Brinker, Mat. Res. Soc. Symp. Proc, SI.9 (2002) 728.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved
299
X-ray diffraction analysis of mesostructured materials by continuous density function technique L.A. Solovyov^ O.V. Belousov^ A.N. Shmakov\ V.I. Zaikovskii^ S.H. Joo", R. Ryoo', E. Haddad^ A. Gedeon'^ and S.D. Kirik' ^Institute of Chemistry and Chemical Engineering, K.Marx av., 42, Krasnoyarsk 660049, Russia. E-mail: leosol@icct.ru ^Boreskov Institute of Catalysis, Novosibirsk 630090, Russia "^Department of Chemistry (School of Molecular Science-BK21) Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea "^Pierre and Marie Curie University, Paris 75252, France A continuous density function technique has been developed for X-ray diffraction (XRD) structural investigations of mesostructured materials. The technique is designed for the analysis of the density distribution in materials exhibiting nanoscale (2-50 nm) ordering of structural elements without atomic long-range order. The results of structure investigations of a series of silicate, metallosilicate and carbon mesostructured materials with hexagonal and cubic symmetry are presented. 1. INTRODUCTION The X-ray powder diffraction is one of the main methods applied to the characterization of mesostructured materials. However, apart form a number studies published, this technique is mostly used for the qualitative ascription of materials to known structural types and for determining the lattice dimensions. The detailed analysis of mesostructures basing on the diffraction peak intensities still presents a challenge due to the severe disordering of these substances at atomic level. Recently, a novel XRD structure analysis method has been proposed and applied in the structural investigations of a series of two-dimensionally ordered mesoporous materials [1, 2]. Here we report further development of this technique for threedimensionally ordered mesostructures and present the results of its application to the structure analysis of different materials, including MCM-41, SBA-15 (2-D hexagonal p6mm), MCM48 (3-D cubic Ia3d) and carbon molecular sieves. 2. EXPERIMENTAL X-ray powder diffraction data were collected on a laboratory diffractometer DRON-4 (CuKa radiation) and on a high-resolution diffractometer located in Siberian Center of Synchrotron Radiation. The scheme of the continuous density function technique used for structural investigations is described as follows. The averaged density distribution in the material is modeled by
300
flexible analytical function p with adjustable parameters. The intensities of the diffraction reflections are calculated by the Fourier-transform of the model function. The adjustable parameters of yoare refined by minimizing the difference between calculated and experimental powder diffraction profiles using the Rietveld technique [3]. The structure characteristics are determined from the refined model parameters. Additional unknown structure details can be revealed from the density distribution maps calculated by the inverse Fourier-transform based on the reflection intensities extracted from the powder diffraction profile and the initial phases derived from the refined model function. The two-dimensional hexagonal mesostructures were modeled by the density distribution function proposed in [1]. To simulate the density distribution in cubic MCM-48 material and its carbon derivative CMK-1 [4] we designed a density function [5] based on the nodal approximation of the "Gyroid" periodic minimal surface [6]. The structural disorder was allowed for by the Debye-Waller factor. 3. RESULTS AND DISCUSSION The application of the developed approach to the structure investigations of a series of mesoporous silicate, alumosilicate, titanosilicate and carbon mesostructured materials allowed their comprehensive structural characterization. In Fig. 1 some results of structure modeling of three samples of MCM-41 silicate mesoporous material are presented. Sample 1 was synthesized at room temperature from an aqueous mixture of cetyltrimethylammonium bromide (CTAB), Ethanol, NH3 and Tetraethoxysilane (TEOS). Sample 2 was obtained by heating sample 1 in the mother liquor for 2 hours at 383 K, and sample 3 was a calcined form of sample 2. The structure characteristics obtained for samples 1, 2, 3 are: unit cell - 4.37(1), 4.36(1), 4.26(1) nm; pore diameter - 3.67(4), 3.32(4), 3.32(4) nm; wall thickness - 0.70(4), 1.04(4), 0.94(4) nm; pore hexagonality - 67, 54, 37 %. As seen, the material formed at room temperature has the least wall thickness and the most hexagonal pores. After the thermal
3 5 20 (degree)
Fig. 1. Experimental (circles) and calculated (solid line) XRD powder patterns of three MCM-41 materials studied. Respective density distribution maps are shown.
1 2 20 (degree)
Fig. 2. Experimental and calculated XRD powder patterns of SBA-15 aluminosilicate at two stages of hydrothermal treatment and respective density distribution maps.
301
treatment in the mother Uquor the wall thickened without unit cell expansion. The room temperature sample 1 was found to be unstable after calcination and boiling in water, but sample 3 was stable in boiling water and strong hydrochloric acid. We believe that this improvement of the material stability was due to the silica polycondensation and wall thickening without unit cell expansion. The surfactant distribution in the mesopores was found to be not uniform with distinct minimum in the pore centers. This feature was also observed in our previous studies [1, 2] for as-made silicate and metal-silicate MCM-41 materials obtained from different media. In Fig. 2 the results of XRD structural studies of SBA-15 aluminosilicate mesoporous materials obtained at two stages of hydrothermal treatment are presented. The materials were synthesized in the presence of Pluronic 123 surfactant via hydrothermal treatment at 373 K for 16 (sample 1) and 48 (sample 2) hours. Both materials exhibited cylindrical mesopores of nearly the same diameter 9.85(5) nm and their unit cells were determined to be 13.39(3) and 13.66(2) nm respectively. The density distribution maps demonstrate the elution of surfactant from mesopores during the hydrothermal treatment. Our structural studies of different silicate and metal-silicate SBA-15 materials show that their pores are, basically, of less hexagonality than those of MCM-41 and the surfactant within the pores of SBA-15 is distributed more uniformly. The structural studies of MCM-48 mesoporous material were carried out on samples synthesized by the well known procedure [7] from an aqueous mixture of CTAB, NaOH and TEOS at 383 K. The samples were highly ordered, exhibiting XRD reflections with d-spacing up to 1 nm. The results for the as-made form of the material are illustrated by Fig. 3. The refined structure model provided nearly perfect XRD profile fitting. The as-made and calcined materials were found to have the same wall thickness of 0.83(5) nm and unit cells of 9.57(1) and 9.00(1) nm respectively. The analysis of the Fourier density maps revealed that the distribution of surfactant in the pores of MCM-48 was not uniform (Fig. 3) with a central
1 3
5 7 20 (degree)
Fig. 3. Experimental (circles) and calculated (solid line) XRD powder patterns of as-made MCM-48. Sections (100) and (211) of the 3-D density distribution map are shown.
2 3 20 (degree)
1 2 20 (degree)
Fig. 4. XRD powder patterns and density distributions for CMK-1 (left) and CMK-3 (right) carbon molecular sieves.
302
minimum similar to that observed for MCM-41. This feature may be ascribed to the strong interaction between the cationic surfactant and inorganic wall in MCM-type materials. The technique was also applied in structural investigations of mesostructured carbon molecular sieves CMK-1 and CMK-3 [4] obtained by replication of the mesoporous silicas MCM-48 and SBA-15 with carbon. The results are illustrated by Fig. 4. The structure of CMK-1 was characterized as an ordered interwoven assembly of two enantiomeric carbon sub-frameworks reproducing the shape of the MCM-48 mesopores. It was shown that the subframeworks are displaced with respect to one another to form contacts without significant distortions. The structure model was explicitly confirmed by the transmission electron microscopy. The effective thickness of the frameworks (2.9 nm) and their mutual displacement distance (1.4 nm) were determined by the XRD structure model refinement [5]. The structure of CMK-3 material was described as fairly long-range ordered hexagonal arrangement of cylindrical carbon nanorods with average diameter of 7.1 nm [8]. The results were consistent with both TEM and adsorption data. The density distribution Fourier-map for this material displayed bridges of non-zero density between the nanorods (Fig. 4), which could be attributed to the carbon matter providing the mesostructure connectivity derived from former complementary porosity of the SBA-15 template. 4. CONCLUSIONS The developed continuous density function technique is demonstrated to be a powerful tool for comprehensive and precise XRD structure analysis of mesostructured materials, providing important information on the mesostructure characteristics. The technique is flexible and can be applied to substances with different anatomy and composition. ACKNOWLEDGEMENTS This work is supported by the INTAS Fellowship grant for Young Scientists YSF 2001/2-3, INTAS grant 01-2283 and joint grant KRSF-RFBR 02-03-97704. REFERENCES 1. L.A. Solovyov, S.D. Kirik, A.N. Shmakov, and V.N. Romannikov, Microporous and Mesoporous Mater., 44-45 (2001) 17. 2. L.A. Solovyov, V.B. Fenelonov, A.Y. Derevyankin, A.N. Shmakov, E. Haddad, A. Gedcon, S.D Kirik and V.N. Romannikov, Stud. Surf Sci. Catal., 135 (2001) 287. 3. H.M. Rietveld, J. Appl. Cryst., 2 (1969) 65. 4. R. Ryoo, S.H. Joo, M. Kruk and M. Jaroniec, Adv. Mater., 13 (2001) 677. 5. L.A. Solovyov, V.I. Zaikovskii, A.N. Shmakov, O.V. Belousov and R. Ryoo, J. Phys. Chem. B, submitted. 6. P.J.F. Gandy, S. Bardham, A.M. Mackay, J. Klinowski, Chem. Phys. Lett., 333 (2001) 427. 7. A. Monnier, F. Schuth, Q. Huo, D. Kumar, D. Margolese, R.S. Maxwell, G.D. Stucky, M. Krishnamurty, P. Petroff, A. Firouzi, M. Janicke, B.F. Chmelka, Science, 261 (1993) 1299. 8. L.A. Solovyov, A.N. Shmakov, V.I. Zaikovskii, S.H. Joo and R. Ryoo, Carbon, in press.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved
303
Influence of aluminium, lanthanum and cerium on the thermal and hydrothermal stability of MCM-41-Type silicates Martin Wallau, Rogerio A. A. Melo* and Ernesto A. Urquieta-Gonzalez^' * Departamento de Engenharia Quimica, Universidade Federal de Sao Carlos, Caixa Postal 676, 13565-905, Sao Carlos - SP, Brasil MCM-41 were prepared in the presence of aluminium (Al-MCM-41), lanthanum (LaMCM-41) or cerium (Ce-MCM-41) and lanthanum and aluminium (La/Al-MCM-41). Aluminium in Al- and La/Al-MCM-41 and lanthanum in La-MCM-41 were incorporated. Otherwise cerium in Ce- and lanthanum in Al/La-MCM-41 was found as extra-framework species. With respect to Si-MCM-41, cerium decreases the thermal and increases the hydrothermal stability, while Al-MCM-41 show similar stability. Extraframework lanthanum below 5 % (w/w) in Al/La-MCM-41 enhances its stability but decreases it above this content. 1. INTRODUCTION MCM-41 is widely studied as adsorbent, catalyst and catalyst support [ 1 ]. In these applications thermal and hydrothermal stability are crucial. However, the reported data arc relatively lacking for MCM-41 synthesised with hetero-elements other than aluminium. Although some works report that lanthanum enhances the MCM-41 stability [2], little work was published concerning the effect of cerium on MCM-41 stability. Here we describe the preparation of MCM-41 with aluminium and/or lanthanum or cerium and discuss the influence of these metals on the thermal and hydrothermal stability of MCM-41. 2. EXPERIMENTAL MCM-41 samples were prepared at 373 K using the gel compositions given in Table 1. The solids, calcined at 823 K, were characterised by elemental analysis before and after ion exchange, which was developed with NH4CI (1 mol/L) for Al-MCM-41 or with HCl (0.1 mol/L) for Ce-, La-, and La/Al-MCM-41. To verify their stability, the calcined samples before ion exchange, were treated thermally and hydrothermally. In the first case in dry air at 1153 K for 2 hours and in the second in a water saturated nitrogen flow at 933 K for 1 hour. All samples were characterised before and after specified treatments by XRD and nitrogen sorption (BET). One Al-MCM-41 (sample III) was additionally characterised by "'^Si and " Al MAS NMR, while Ce-MCM-41 was supplementary examined by ESR spectroscopy.
* Present adress: Centro Universitario do Sul de Minas, Faculdade de Engenharia Quimica, Av. Gel. Jose Alvcs. 37010-540 Varginha - MG, Brasil ^ Corresponding author: FAX: +55-16-260-8266. E-mail: urquieta@power.ufscar.br * This work was financially supported by CNPq (461444/00-3; 300373/01-5), FAPEMIG (TEEC - 1241/01).
304
Table 1 Elemental composition of the synthesis gels and of the calcined MCM-41 samples before and after ion exchange. Gel* Calcined Solid XAI2O3 yMzOs before ion exchange after ion exchange 0 (I) 0 0 (II) 0.017 Nao.027 [ S io.960AI0.040O2] Nao.023[Sio.958Alo.04202] 0 (III) 0.05 Nao.035[Sio.923Alo.07702] Nao.o79[Sio.9ooAlo.ioo02] 0.017^ (IV) 0 Naz[Sio.986Lao.oi402] Naz[Sio.985Lao.oi502] 0.017* NazCeo.oo9[Si02] (V) 0 NazCeo.oo7[Si02] 0.005^ (VI) 0.05 Nao.o6Lao.ooi5[Sio.9Alo.i02] Nao.o3Lao.ooo7[Sio.9Alo. 102] 0.018^ (VII) 0.05 Nao.o5Lao.oi3[Sio.92Alo.o802] Nao.02Lao.001 [Sio.93 AI0.07O2] 0.035^ (VIII) 0.05 Nao.o4Lao.o2o[Sio.94Alo.o602l Nao.03Lao.004iSio.95Alo.05O2] *Si02 : 0.07 NazO : 0.03 CTMA^ 0.14 TMA^ : X AI2O3: y M2O3: 100 H2O •••M = L a ; * M = Ce.; CTMA^ = cetyltrimethylammounium ; TMA^ = tetramethylammonium. 3. RESULTS AND DISCUSSION The idealised sum formulae of the calcined solids before and after ion exchange are given in Table 1 in the form NawM'x[SiyM"z02] where M ' means extraframework cations (/. e. Ld^^ and Ce"^^) and M " framework incorporated cations (/. e. A\^^ and La^^). It can be seen that the aluminium content of the samples II, III and VI do not differ significantly from that of the synthesis gel, while the lanthanum and cerium content in the samples IV-VIIl are much lower. This indicates that aluminium is easier incorporated into the wall structure than lanthanum and cerium. However, the presence of lanthanum seems to hamper the aluminium incorporation in sample VII and VIII. However, this cannot be taken as an indication for competitive framework incorporation of lanthanum, because ion exchange decreases the lanthanum content dramatically (see Table 1), and therefore lanthanum is probably present in extra-framework sites in the La/Al-silicates (VI-VIII). On the other hand, in these samples (VI-VIII), as well as in the Al-MCM-41 (II, III), the aluminium content is not affected by ionexchange, this indicating its framework incorporation. In sample III aluminium in framework position is confirmed by the ^^Si - and ^^Al MAS NMR spectra shown in Fig. 1. -106 -102 -
\A
-110
b
,
S3
1
-1
-91
1
1
^ 1
1
-100
-120
ppm (TMS)
1^**^""^^ 11
100 90
80
70
\-*L
i
1
60
50
I—k—k
1
40
30
20
10
"T'^^T"^
-10 -20 -30 -40
ppm (AI(H20)J
Fig. 1. MAS NMR spectra of calcined sample (III): (a) ""Si; (b) '^Al. While the peaks at -110, -102 and the shoulder at -91 ppm are usually attributed to (-0-)4Si (Q4), (-0-)3SiOH (Q3) and (-0-)2Si(OH)2 (Q2) units [3], respectively, the peak at -106 ppm can be attributed to (-0-)3Si-0-Al units [3]. The presence of Al in tetrahedral coordinated framework positions is confirmed by the peak at 53 ppm in the ^^Al MAS NMR. As it was shown by Janicke et al. [ 4 ] , aluminium atoms incorporated into the MCM-41
305
framework can be co-ordinated to two other water molecules resulting in a peak at around 0 ppm. Therefore, the peak at - 1 ppm due to octahedral co-ordinated aluminium, can not be taken as a conclusive proof for the presence of extra-framework aluminium. The lanthanum content in La-MCM-41 (sample IV) remains unchanged after ion exchange, which might indicate the incorporation of lanthanum into the framework. On the other hand in La/Al-MCM-41, where the lanthanum content decreases after ion exchange, the lanthanum is probably be present as extra-framework species. It can be calculated from Table 1, that for the calcined La/Al-MCM-41 before ion exchange, the ratio (Na^ + 3 La^^)/Al is 0.65, 1.11, and 1.67 for sample VI, VII and VIII, respectively. This show, that in sample VI and VII, the positive charges of the sodium and lanthanum cations did not exceed significantly the negative framework charge, expected for the complete incorporation of all aluminium atoms. Otherwise, for sample VIII, the number of cations is much higher than the number of the expected negative framework charges. This suggest the presence of lanthanum cations located on ion exchange sites in sample VI and VII, while sample VIII will also contain neutral lanthanum species, probably as La203. From the absence of any signal in the ESR spectrum (not shown) of Ce-MCM-41, we conclude that Ce"^^ was oxidised during the synthesis to Ce"^^, which might form insoluble Ce02. Therefore, the nearly unchanged cerium content after acid treatment, do not allow to conclude that cerium was incorporated into the MCM-41 framework. Only around 20 % of the cerium used in the synthesis gel is present in the obtained solid, indicating that its incorporation is hampered or impossible. 1200 The XRD patterns reveal the presence of before thermal a hexagonal mesostructure by a broad peak ^ 1000 treatment around 2 °(20) (dioo) and additional, sometimes overlapped peaks (duo, d2oo), ^ 800i between 4 and 5 °(20). As a typical after thermal 600 example, the XRD patterns of Si-MCM-41 treatment (Sample I) before and after thermal ^ 400 treatment are shown in Figure 2. After the thermal treatment at 1153 K the intensity 200H of the dioo peak is decreased to 73 %. Decreased intensity of the X-ray 0 reflections is often taken as an indication 0 8 10 2e' of structural degradation [2]. However, it Fig. 2. XRD patterns of Sample 1. was pointed out, that for mesostructured materials an intensity loss of the XRD peaks cannot necessarily be interpreted as a loss of "crystallinity" [5]. Therefore we used the relative change of the specific surface area as a measure of the thermal and hydrothermal stability of studied MCM-41 type materials. The results shown in Figure 3 indicate that the thermal and hydrothermal stability of Al-MCM-41 (Sample II, III) is in the same order or slightly better than that of Si-MCM-41 (Sample 1). A higher stabilisation of the mesoporous structure is observed for La-MCM-41 (Sample IV) and for La/Al-MCM-41 with lanthanum contents below 5 % (w/w) /. e. Sample VI and VII. On the other hand, while the mesostructure of Ce-MCM-41 (sample V) is completely destroyed after thermal treatment at 1153 K, it shows a slightly better hydrothermal stability than SiMCM-41. The presence of lanthanum amounts higher than 5 % (w/w) strongly decreases the thermal and hydrothermal stability of La/Al-MCM-41 (Sample VIII). While our observation confirms the higher stability of La-MCM-41 claimed by He et al. [2], the observed thermal
306
and hydrothermal behaviour of Al-MCM-41 (sample II and III) is in contrast with the results of these authors, who observed lower stability for Al-MCM-41. As it can be seen from Table 2, the Al-MCM-41 studied here possess thicker walls (13 to 30%) than the sample studied by He et al. [2], which might have caused the enhance in the stability. The wall thickness of LaMCM-41 is only slightly enhanced, and its higher stability might be due to lanthanum incorporation. As it was discussed above, Sample V and Sample Fig. 3. Relative specific surface area (BET) of the calcined VIII might contain the Me-MCM-41 ( • ) and after thermal ( • ) and hydrothermal strong Ce02 and La203 (CZD treatment (sample identification as in Table 1). bases, respectively, and as known, even weak bases Table 2 in the mesopores of MCM Wall thickness (s) of MCM-41 samples -41 leads to structure MCM Si Al* Al^ Al^ La^ La** degradation [6] by hydroThis work 1.50 1.75 2.04 1.62 lysis of the T-0- T bonds. 1.41 Ref[2] 1.45 1.49 T bonds. The elemental Si/M = *29; '^24; ^9; §66; **42. analysis of a La/Al-MCM41 studied by He et al. [2] with ca. 4 % (w/w) of lanthanum also suggest the presence of La203, which would explain the low observed stability. Then, the enhanced stability of La/AlMCM-41 containing less than 5 weight % (Sample VI and VII), might be attributed to the presence of lanthanum cations located on ion exchange sites, which can react with the framework structure: (a) La^^ + Si-(0- )-Al + H2O -^ La(OH)^^ + Si-(OH)-Al; (b) La^^ + 2 SiOH -> La(H20)^^ + Si-O-Si, thus preventing hydrolysis of the T-O-T bonds. The stabilising effect of cerium in the hydrothermal treatment of Ce-MCM-41 (Sample V) is still unclear. A possible explanation might be an enhancement of the hydrophobicity as it was observed for Ce-X [7]. REFERENCES 1. F. Schuth, Stud. Surf Sci. Catal., 135 (2001) 1. 2. N.-Y. He, S.-L. Bao, Q.H. Xu, Stud. Surf Sci. Catal., 105 (1997) 85. 3. Z. Luan, C.-F. Cheng, W. Zhou, J. Klinowski, J. Phys. Chem., 99 (1995) 1018. 4. M.T. Janicke, C.C. Landry, S.C. Christiansen, D. Kumar, G.D. Stucky, B.F. Chmelka, J. Am. Chem. Soc, 120 (1998) 694. 5. B. Marler, U. Oberhagemann, S. Vortmann, H. Gies, Microporous Mater., 6 (1996) 375. 6. C.N. Perez, E. Moreno, CA. Henriques, S. Valange, Z. Gabelica, J.L.F. Monteiro, Microporous Mesoporous Mater., 41 (2000) 137. 7. Q.J. Chen, T. Ito, J. Fraissard, Zeolites 11 (1991) 239.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Enhanced acidity and hydrothermal stability of mesoporous aluminosilicate with secondary building units characteristic of zeolite Beta Wanping Guo^'^, Lingdong Kong^, Chang-Sik Ha^ and Quanzhi Li^* ^Department of Chemistry, Fudan University, Shanghai 200433, P. R. China ''Department of Polymer Science and Engineering, Pusan National University, Pusan 609-735, Korea Ordered hexagonal mesoporous aluminosilicate (SBU-MCM-41) with secondary building units characteristic of zeolite Beta has been characterized and compared with the corresponding MCM-41 by means of XRD, SEM, N2 adsorption, IR spectra of pyridine, catalytic cracking activity and hydrothermal treatment. The results indicate that the introduction of secondary building units characteristic of zeolite Beta into the mesopore wall is an effective route to improve the acidity and hydrothermal stability of mesoporous material. 1. INTRODUCTION The development of mesoporous material MCM-41, which possesses highly regular arrays of uniform-sized pore channels ranging from 1.5 to 10 nm in size, large surface area and good thermal stability, has inspired a great deal of interest in processing large molecules [1,2]. The acid strength of mesoporous aluminosilicate is, however, weaker than that of traditional zeolites owing to the amorphous character of mesopore wall, which limits potential applications in petroleum industry [1]. In order to improve the framework property of mesoporous material, it is practicable to introduce zeolite-like structural order into the mesopore wall. Up to now, there have been only a few articles involving this issue [3-7]. Our recent study [8] reported that a series of ordered hexagonal mesoporous molecular sieves (SBU-MCM-41) with various Si02/Al203 ratios, of which the mesopore wall was constructed by a lot of secondary building units characteristic of zeolite Beta, were prepared through pretreating the aluminosilicate gel in the absence of alkali metal cations. Here we present the characterization of the SBU-MCM-41 mesoporous aluminosilicate compared with the corresponding mesoporous material in terms of structure, acidity, catalytic performance and hydrothermal stability.
308
2. EXPERIMENTAL The preparation of the SBU-MCM-41 mesoporous aluminosilicate was carried out under hydrothermal condition avoiding the presence of alkali metal cations according to the procedure described elsewhere [8]. As a reference, a corresponding MCM-41 mesoporous material was prepared using the same procedure as the SBU-MCM-41 except pretreatment process. 3. RESULTS AND DISCUSSION The XRD patterns of calcined SBU-MCM-41 and the corresponding MCM-41 are shown in Fig. 1. The typical hexagonal lattice for the SBU-MCM-41 and the corresponding MCM-41 can be verified by the observation of a strong peak (100) at very low angle although the (110) and (200) peaks for both samples are ill-defined and overlap to give a single broad peak. After calcination, there is shrinkage of 7.0 % (from 3.98 nm to 3.70 nm) in dioo spacing of the SBU-MCM-41 while the dioo spacing of the corresponding MCM-41 shrinks by 11 % (from 3.77 nm to 3.35 nm). Fig. 2 displays scanning electron micrographs of the SBU-MCM-41 and the corresponding MCM-41. It is found that the SBU-MCM-41 appears in spindle "crystal-like" morphology and the corresponding MCM-41 shows typical loose aggregates. From the N2 adsorption-desorption isotherms and the BJH pore size distribution plots for the SBU-MCM-41 and the corresponding MCM-41, it can be seen that both samples exhibit a well-expressed hysteresis loop at relative pressure P/PQ of 0.2-0.4, characteristic of framework-confined mcsoporcs [7]. The pore structure data of the SBU-MCM-41 and the corresponding MCM-41 are presented in Table 1. It can be seen that the BET surface area and Table 1 Pore structure data of the SBU-MCM-41 and the corresponding MCM-41 Sample BET surface Pore volume Pore diameter ao (nm) area (m^ g"') (cm^ g'') D (nm) SBU-MCM-41 942 1.54 2.75 4.27 Corresponding MCM-41 1178 1.48 2.72 3.87
t(nm) 1.52 1.15
pore volume of the SBU-MCM-41 are as high as 942 m^ g'^ and 1.54 cm^ g\ respectively. It should be noted that the pore wall thickness (1.52 nm) of the SBU-MCM-41 is much higher than that (1.15 nm) of the corresponding MCM-41. The IR spectra of pyridine adsorbed on protonated samples in the region 1600-1400 cm' show that both Bronsted and Lewis acid sites of the SBU-MCM-41 are much higher than those of the corresponding MCM-41 at different desorption temperatures. A good correlation can be observed between the cumene cracking activity and the number of acid sites (see Table 2). Therefore, the SBU-MCM-41 with much more Bronsted and Lewis acid sites contributed by many secondary building units characteristic of zeolite Beta in the mesopore wall exhibits much higher catalytic activity for cumene cracking than the corresponding MCM-41. The test of hydrothermal stability of calcined samples refluxed in water at 373 K shows That
309
1.5 2
4
_
,
6
10
2 tneta
Fig. 1. Powder XRD patterns of calcined samples (a) the SBU-MCM-41 and (b) the corresponding MCM-41.
a
•' "' •
'
-
^
' •"•'' " ^
l i — ' — '
•'""
^
' • ' ' ^ ' • -
'
Fig. 2. SEM images of (a) the SBU-MCM-41 and (b) the corresponding MCM-41.
Table 2 Acidity and catalytic activity of the SBU-MCM-41 and the corresponding MCM-41 Sample SBU-MCM-41 Corresponding MCM-41
Acidity (x lO'^g') Bronsted 393K 453K 513K 3.11 2.59 1.76 2.13 1.65 0.89
Cumene conversion (%) Lewis 393K 453K 7.29 5.05 5.88 3.56
513K 2.94 2.13
523K 15.4 7.3
573K 36.1 20.4
623K 60.4 30.8
673K 89.9 53.4
310
the hexagonal pattern of the corresponding MCM-41 almost disappears after hydrothermal treatment for 16 h. By contrast, hydrothermal treatment for the same time has little effect on the structural integrity of the SBU-MCM-41. Moreover, the SBU-MCM-41 refluxed for 48 h is still well ordered. 4. CONCLUSIONS The ordered SBU-MCM-41 mesoporous aluminosilicate has been investigated and compared with the corresponding MCM-41 for their structure, acidity, catalytic performance and hydrothermal stability. The much enhanced acidity and hydrothermal stability of the SBU-MCM-41 can be attributed to the presence of zeolite-like structural order (secondary building units characteristic of zeolite Beta) in the framework wall. ACKNOWLEDGEMENT This work was supported by the National Natural Science Foundation of China (Grant No. 29733070) and financial support from the Center of Integrated Molecular Systems, POSTECH, Korea and the Brain Korea 21 Project is gratefully acknowledged. REFERENCES 1. A. Corma, Chem. Rev., 97 (1997) 2373. 2. J.Y. Ying, C.P Mehnert and M.S. Wong, Angew. Chem. Int. Ed., 38 (1999) 56. 3. Y. Liu, W. Zhang and T.J. Pinnavaia, J. Am. Chem. Soc, 122 (2000) 8791. 4. Y Liu, W. Zhang and T.J. Pinnavaia, Angew. Chem. Int. Ed., 40 (2001) 1255. 5. Z. Zhang, Y Han, F.S. Xiao, S. Qiu, L. Zhu, R. Wang, Y Yu, Z. Zhang, B. Zou, Y Wang, H. Sun, D. Zhao and Y Wei, J. Am. Chem. Soc, 123 (2001) 5014. 6. Z. Zhang, Y Han, L. Zhu, R. Wang, Y Yu, S. Qiu, D. Zhao and F.S. Xiao, Angew. Chem. Int. Ed., 40 (2001) 1258. 7. W. Guo, L. Huang, P. Deng, Z. Xue and Q. Li, Microporous Mesoporous Mater., 44-45 (2001)427. 8. W. Guo, L. Kong and Q. Li, Stud. Surf Sci. Catal., in press.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Chemical coating of the aluminum oxides onto mesoporous silicas by a one-pot grafting method Yi-Hsin Liu^ Hong-Ping Lin^* Chung-Yuan Mou^'', Bo-Wen Cheng"^ and Chi-Feng Cheng"^ ^Department of Chemistry National Taiwan University, Taipei 106, Taiwan. ^Department of Chemistry, National Cheng Kung University, Tainan 701, Taiwan. '^Center of Condensed Matter Science, National Taiwan University, Taipei 106, Taiwan. ^Department of Chemistry, Chung-Yuan Christian University, Chung- Li, Taiwan. Using a suitable solvent (such as, 1-propanol) that prevents the self-condensation of aluminum /^o-propoxide and extract the quaternary ammonium surfactant out of the nanochannels, a high-coverage aluminum oxide (Al/Si = 0.3-0.5) can be chemically coated onto acid-made mesoporous silica. The aluminum oxide coated mesoporous silica has the both advantages of aluminum oxide (highly hydrothermal stability) and mesoporous materials (large pore size of 2.7 nm, high surface area of around 800 mVg, and well-ordered mesostructure). With the well-dispersity, high accessibility and la»*ge pore size, the aluminum oxide coated mesoporous silica was supposed to be an acid-catalyst for large molecules. 1. INTRODUCTION Mesoporous silica with the advantages of high surface area (1000 m^/g), tuneable pore size (1.0-30.0 nm) have reasonably been considered as a superior support that can promote well-dispersed and high accessibility of active sites for catalytic applications [1]. Typically, the metal oxides-grafting on calcined mesoporous silica must be done in an extremely dry condition to avoid the fast hydrolysis and self-condensation of metal alkoxides [2]. Basically, one should think the metal alkoxides grafting as a simple chemical surface modification of mesoporous silica via covalent bonding of metal alkoxide and surface silanol groups [3]. By using this idea, it is possible that a convenient method for high grafting of metal oxides onto the mesoporous silica could be achieved by choosing a proper solvent, and precursor of metal oxide via judicious synthesis processes. Here, we provided a convenient one-pot strategy for aluminum /^o-propoxide grafting onto the acid-made mesoporous silica in a low-toxicity 1-propanol solution at low reaction temperature of 80 °C [4]. The high-coverage of aluminum oxide makes the aluminum oxide-coated mesoporous silica highly hydrothermal stable, and demonstrates a high catalytic activity of cumene cracking reaction. 2. SYNTHESIS AND METHOD 2.1. One-Pot grafting of aluminum oxide on acid-made mesoporous silica The acid-made quaternary ammonium surfactant micelle-templated silicas were prepared by typical method reported in the previous literatures,[5] and the detailed procedures and
312
chemical compositions were demonstrated elsewhere. The one-pot process of aluminum /i-o-propoxide grafting is described as followed: 1.0 g dried acid-made mesoporous silica was added into 40-100 g 1-propanol (99.5%, Acros) solution containing the suitable amount of aluminum /5o-propoxide (Al(/-OC3H7)3, Acros, 98%). Then, that solution was re fluxed at 80 ^C for 24-60 hr. Filtration, washing, and drying recovered the aluminum wo-propoxide grafted mesoporous materials. Finally, the aluminum oxide coated mesoporous silicas products were obtained after calcined at 560 °C to remove the unreacted /5o-propoxide. 2.2. Measurement The powder x-ray diffraction patterns (XRD) were collected on Scintag XI diffractometer using Cu Kct radiation (X = 0.154 nm). The mesostructural observations of mesoporous silica were recorded on Hitachi S 7100 or Philips CM 200 transmission electron microscope (TEM) with an operating voltage of 100 and 200 keV, respectively. N2 adsorption-desorption isotherms were obtained at 77 K on a Micromeritics ASAP 2010 apparatus, and the pore size distribution was calculated from the adsorption isotherms using the Barrett-Joyner-Halenda (BJH) method. 2.3. Cumene cracking reaction The cumene cracking reaction was performed in a continuous-flow fixed bed system at different temperature from 250 to 400 °C. The reactant, cumene (saturated vapor pressure at 0 ^C), was continuously mixed into the nitrogen carrier gas stream of flow rate = 20 ml/min, and the catalyst weight is 0.05 g. The products were analyzed on-line by a Shimadzu GC-7A gas-chromatograph. 3. RESULTS AND DISCUSSION Fig. lA shows the XRD patterns of the Al(/-OC3H7)3 grafted mesoporous silica. The Al(/-OC3H7)3 grafted mesoporous sample has the well-ordered hexagonal mesostructure as the original acid-made mesoporous silica. Combining the analysis of N2 adsorption-desorption isotherm (Figure 2), the uncalcined aluminum oxide grafted mesoporous silica possesses a sharp capillary condensation at P/Po « 0.38 and high porosity (~ 0.9 ml/g) as that of the surfactant-free mesoporous materials. The result indicates that the quaternary surfactants (S^X) in nanochannels of acid-made mesoporous silicas are almost completely taken out during the chemical grafting of Al(/-OC3H7)3. To confirm the successful grafting of Al(/-OC3H7)3, we performed a Induced Couple Plasma-Atomic Emission Spectrometer to analyze the Al content in the Al(/-OC3H7)3 grafted mesoporous silica, and the Si/Al ratio is around 2.3. Consequently, this grafting reaction could be regarded as a energy-favored interaction transformation from weak hydrogen-bonding of silica and surfactant to chemical bond of silanol and Al(/-OC3H7)3. Moreover, the avoidance of high-temperature calcination, the high silanol group density can be remained to promote the high loading of metal-oxides. Therefore, this one-pot grafting method provides a new and convenient way to chemically grafting the well-dispersed aluminum oxide onto the acid-made mesoporous silica. For converting the Al(/-OC3H7)3 grafted mesoporous silica to the aluminum oxide coated mesoporous silica, a high-temperature was directly used. One can see that the XRD pattern, pore size and porosity are preserved as the uncalcined one (Figure 1).
313
P/P„
Fig. 1. (A) XRD patterns and (B) N2-adsoption-desorption isotherms of aluminum wo-propoxide grafted and aluminum oxide coated mesoporous silica samples. (I). Aluminum /50-propoxide grafted mesoporous silica. (II). Aluminum oxide coated mesoporous silica. (Ill) Aluminum oxide coated mesoporous silica after a hydrothermal reaction at 100 °C for 20 hr. With the examination of the high-angle XRD pattern and TEM micrographs, no diffraction peaks and images characteristic of nano-sized AI2O3 clusters as byproduct were found in the aluminum oxide coated mesoporous silica. Owing to high content of aluminum oxides (Si/Al ratio = 1.8-2.5) and uniform pore size, it was reasonably supposed that the aluminum oxide was homogeneously coated on the silica nanochannels. It should be mentioned that the aluminum oxide coated mesoporous silica demonstrate the thicker wall thickness (~ 2.5 nm) than the un-grafted one(~ 2.0 nm), and the difference between these values is about 0.5 nm. Because the aluminum oxide layer behaves like a protecting film, the aluminum oxide coated mesoporous silica shows highly hydrothermal stability,[6] which can stand for at least 20 hr in boiling water without the apparent mesostructural damage (Fig. lA). In contrast, the mesostructures of the calcined acid-made mesoporous completely collapsed after 6 hr. To investigate the effects of Al(iOPr)3 concentration and reflux time on the physical properties and Al content, we adjusted both factors to get a proper reaction condition with relatively low Al(iOPr)3 concentration and short reflux time. In Table 1, one can find that the high aluminum content can be almost achieved by using a low Al(iOPr)3 concentration of around 50 mM and refluxing for 24 h. High Al(iOPr)3 concentration can not increase the Al Table 1 Physical properties of aluminum oxide coated mesoporous silicas samples prepared from different Al(iOPr)3 concentration and reflux time. Reflux time Si/Al" [Al(iOPr)3] dioo Dwall' ^porc /m'/g Ik /mM /hr Ik Ik 48.4 43.3 157.0 417.0
19 60 19 19
898 844 877 854
42.4 43.3 42.8 42.9
24.8 24.2 23.8 23.8
24.2 25.8 25.6 25.7
2.38 2.10 2.07 1.97
a. Dwall = 2d 100/ 3 - Rporc. b. The Si/Al ratio was obtained from Induced Couple Plasma-Atomic Emission Spectrometer.
314
content, but may induce the formation of AI2O3 nano-clusters out of the nanochannels. The aluminum oxide coated mesoporous siHca was recommended as one kind of acid-catalyst. [7] Thus, the catalytic activity of aluminum oxide coated mesoporous silica toward cumene cracking has been investigated at different temperature. The high conversion (78 %) at 400 °C showed the high accessibility and well-dispersity of the active sites. In contrast, the cumene conversion of Al Time (/lO min) incorporated MCM-41 (Si/Al = 20) is only 20 Fig. 3. A plot of cumene conversion vs. %. As the decreasing reaction temperature, the time at 250-400 °C. catalytic activity decreases. However, the activity completely recovered upon another temperature-raising process. The high activity can maintain for longer than 24 h. 4. CONCLUSION The one-pot grafting method is convenient and efficient to coat aluminum oxide to mesoporous silica. To extend this concept, other metal oxides also can be coated to mesoporous silicas by choosing suitable precursors and solvent. Combining with the acid-treatment process,[8] coating of metal oxides on the alkaline-made MCM-41 and MCM-48 mesoporous silicas could be achieved. In brief, the desired surface property and activity of the mesoporous silica based catalysts possibly will be obtained to fit the applications by using well-designed post-treated processes. ACKNOWLEDGEMENT This research was financially supported by National Science Council of Taiwan (NSC-90-2113-M-002-038). REFERENCES 1. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. 2. A. Sayari and S. Hamoudi, Chem. Mater., 13 (2001) 3151. 3. R. Ryoo, S. Jun, J. M. Kim and M. J. Kim, Chem. Comm., (1997) 2225. 4. H. P. Lin, L. Y. Yang, C. Y. Mou, S. B. Liu and H. K. Lee, New, J. Chem., 24 (2000) 253. 5. H. R Lin, C. R Kao, C. Y. Mou and S. B. Liu, J. Phys. Chem. B, 104 (2000) 7885. 6. R. Mokaya, Chem. Comm., (2001) 633. 7. Y Han, R S. Xiao, S. Wu, Y Sun, X. Meng, D. Li and S. Lin, J. Phys. Chem. B, 105 (2001) 7963. 8. H. P. Lin, P. C. Shih, Y. H. Liu and C. Y. Mou, Chem. Lett., (2002) 566.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Acidity and temperature effect on the synthesis of SBA-1 Ming-Chang Liu," Hwo-Shuenn Sheu'' and Soofin Cheng*" ""Department of Chemistry, National Taiwan University, Taipei 106, Taiwan. FAX: +886-2-2363-6359. '^Research Division, Synchrotron Radiation Research Center, Hsinchu 300, Taiwan The formation of cubic phase SBA-1 is strongly influenced by the synthesis condition. In-situ XRD experiments showed that SBA-1 crystallized more quickly in higher acidic conditions, and no phase transformation was observed in the synthesis gel when varying either the reaction period or the acidity (HCl: TEOS molar ratio = 2~5). Moreover, the cubic phase was less stable when the solid was separated from the mother liquid. A phase transformation from cubic to hexagonal was observed on the precipitates formed in short crystallization period when they were dried at room temperature. To the best of our knowledge, it was the first time in the literature to report that the phase transformation was correlated to the removing of the solvent from the precipitate. The stability of SBA-1 crystal structure was greatly improved by post-treatment of the wet products with ammonia solution. 1. INTRODUCTION Since the discovery of the M41S family of mesoporous molecular sieves, a great effort has been focused on their possible applications as catalysts and adsorbents. Cubic phase SBA-1 (Pm3n) possesses uniformly sized pore structure and open windows [1-2]. It is considered to be suitable for catalytic reactions because of its high surface area and three-dimensional inter-connected pores. However, the material is suffered from its relatively low stability. Recently, Che et al. [3] reported that a phase transformation from hexagonal to cubic occurred during the synthesis ofSlBA-l and a co-solvent 1,3,5-trimethylbenzene could slow down the transformation. Phase transformation was also reported on other meso-structurc materials [4-7]. These transformations were all occurred in hydrothermal condition at reaction temperatures greater than 100"C. This subject is of great interest and importance for the purpose of synthesizing the desired meso-structure material. However, the mechanism of surfactant-templated phase transformation reaction was not well understood yet. In the present study, the synthesis condition of SBA-1 and the factors affecting the phase transformation were examined by ex- and in-situ x-ray diffraction techniques. The stability of SBA-1 was also improved by a post-treatment. 2. EXPERIMENTAL SBA-1 was synthesized following the procedures in ref 2. The molar composition of the synthesis gel was TEGS/ CTEABr/ HCl/ H20= 1/ 0.13/ x/ 125, where x= 2-5. The reaction temperature was maintained at ca. 0-2°C, if not specified. In ex-situ study, powder XRD
316
patterns were recorded using a Scintag XI instrument with a Cu Ka radiation. In-situ XRD studies of the synthesis gels were conducted at the Synchrotron Radiation Research Center, Hsinchu, Taiwan. The patterns were recorded in the transmission mode with X= 1.32633nm radiation (1.85 GeV and 200 mA). 3. RESULTS AND DISCUSSION SB A-1 was synthesized under strong acidic condition. The effect of acidity was examined by varying the amount of hydrochloric acid used. Ex-situ powder XRD was utilized to determine the crystalline structure of the precipitates dried at 100°C overnight. For precipitates synthesized in higher acidic conditions (x^ 5), only cubic phase was observed. However, for those formed in lower acidic conditions (x= 2- 4), the hexagonal phase was
LA t
-(k)
(I) (k) (J)
ML
(i)
-(h)
(h)
-(g) -(0 -(e) -(d)
(g)
(0 (c) (d)
-(c)
(c)
-(b)
(b)
-(a)
2TH
Fig. 1. In-situ XRD patterns of SBA-1 gel (x-3 at 0"C) (a) O.lh, (b) 0.3h, (c) 0.6h, (d) 0.9h, (e) 1.4h, (f) 1.6h, (g) 1.9h, (h) 2.2h, (i) 2.4h, Q) 2.6h, (k) 4.8h
(a)
2TH
Fig. 2. In-situ XRD patterns of SBA-1 (x=3, crystallized for 2.8h) precipitate exposed to air at room temperature for (a) 5m, (b) 30m, (c) l.lh, (d) 1.5h, (e) 1.6h, (f) 1.68h, (g) 1.77h, (h) 1.85h, (i) 1.92h, (j)2.0h, (k)3.5h,(I) I2h.
observed for the samples crystallized for short period time. As the reaction prolonged, samples of cubic phase were obtained. These results deduced that increasing the acidity of the synthesis gel would accelerate the appearance of cubic phase and the resultant crystal structure would be more stable. In other words, strong acidity was favorable for the formation of cubic crystalline phase. These results are consistent with those reported in the literature [3,8]. The in-situ XRD experiments of the synthesis gel showed that only cubic phase was formed in the gel when the HCl concentration was varied from x= 2-5. The results were contradictory
317
to those observed on the dried samples. The crystallinity of the gels increased with the aging period, as shown in Fig. 1. No hexagonal phase was observed even the synthesis gel was in low acidic conditions. On the other hand, the HCl concentration would affect the rate of crystallization. For synthesis gel with HCl content x^ 2, 3, 4, and 5, it took 7 h, 1.4h, 0.7h, and 0.6h, respectively, for the resolved cubic phase to appear. The reaction temperature was also examined with in-situ experiment. For the gel with HCl content x= 3 condition, cubic phase was the only phase observed when the synthesis gel was heated from 0 to 80"C, and the crystallinity increased with the gel temperature. In another experiment, the wet precipitate crystallized for short period of time was sealed in between two tapes. The cubic phase of SBA-1 was retained when heating the precipitate from room temperature to lOO^C. All these results demonstrate that acidity and temperature are not the factors for phase transformation. Fig. 2 shows that a phase transformation from cubic to hexagonal occurred when the wet precipitate crystallized for short period of time was left in open-air environment. The phase transformation apparently has to do with the solvent evaporation. In other words, the cubic SBA-1 phase, which is stable in the presence of solvent, transforms to hexagonal phase when it is dried up. However, for the precipitate crystallized for long period of time, the cubic phase was retained even the precipitate is dry. The phenomenon of phase transformation at room temperature has never been reported on mcsoporous silica. The solvent evaporation would lead to the surfactant and the acid being concentrated. The spherical micelles probably transform to rod-shape micelles when the concentration of surfactant increases. That accounts for the phase transformation observed in this system. ^\Si MAS NMR studies confirmed that
(b)
20
AH
60
80
100
IM
1«
Iffl
-180
-140
-160
-180
Q3/"i
(a)
I ', Q4
2TH
(ppm)
Fig. 3. Ex-situ XRD patterns of SBA-1 (x=5) samples: (a) as-synthesis, (b) post-treatment with 0.1 M NH3 for 2 days at lOO^'C, (c) hydrothermal treatment of sample (b) with water at lOOTfor 1 day.
Fig. 4. ''^Si MAS solid state NMR spectra of SBA-1 (x=5) samples: (a) as-synthesis, (b) post-treatment with IM NH3 at lOOT for 2days.
318 Q4 peak (-llOppm) increased significantly after solvent evaporated. This implied that silica condensation continued when the precipitate left the mother solution. The thermal stability of the SBA-1 products could be improved by hydrothermal treatment of the wet samples with O.IM NH3 at lOO^'C for 2 days (Fig. 3). After NH3 treatment, the XRD intensity of the sample was retained while the diffraction peaks shifted slightly toward lower angles. This result implies that the crystal lattice is expanded slightly during the ammonia treatment. In the hydrothermal stability test, O.lg of the samples in powder form were suspended in lOg of water and boiled at 100"C for 1 day. The crystalline structure of SBA-1 collapsed no matter the experiment was started with wet as-synthesized samples or the calcined ones. In contrast, the crystalline structure retained for the NH3 post-treated samples. The effect of post-treatment was examined by taking the ^'^Si MAS NMR spectra on the SBA-1 samples before and after ammonia treatment. Fig. 4 shows that the Q2peak at -91 ppm disappeared and the intensity of the Q4peak at -110 ppm increased after NH3 post-treatment. This implies that NH3 post-treatment should make the silica condensation more complete. The resultant silica framework of relatively low defects was more tolerable to hydrothermal treatment. REFERENCES 1. Q. Huo,D.l. Margolcsc and G.D. Stucky, Chcm. Mater. 8 (1996) 1147. 2. S. Che, Y. Sakamoto, H. Yoshitakc,0. Tcrasaki and T. Tatsumi, J. Phys. Chcm. B 105 (2001) 10565. 3. S. Che, S. Kamiya, O. Tcrasaki and T. Tatsumi, J. Am. Chcm. Soc. 123 (2001) 12089. 4. J. Xu, Z. Luan, H. He, W. Zhou and L. Kcvan, Chem. Mater. 10 (1998) 3690. 5. C.F. Cheng, D.H. Park and J. J. Klinowski, Chem. Soc, Faraday Trans. 93 (1997) 193. 6. M.T. Anderson, J.E. Martin, J. G Odinck, and P.P. Newcomer, Chcm.Matcr. 10 (1998) 311. 7. A.F. Adam, E.J. Ruiz and S.H. Tolbcrt, J. Phys. Chem. B 104 (2000) 5448. 8. M.J. Kim and R. Ryoo, Chcm. Mater. 11 (1999) 487.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
319
HMS materials with high Al loading: a joint FT-IR and microcalorimetric study of their acidic/basic properties B. Bonelli,^ B. Onida,^ B. Fubini,^ J.D. Chen,' A. Galameau,' F. Di Renzo' and E. Garrone^ ^Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi 24, Torino, 1-10129, Italy ^Dipartimento di Chimica Inorganica, Fisica e dei Materiali Universita degli Studi di Torino, V. Pietro Giuria 7,1-10125, Torino, Italy '^Laboratoire de Materiaux Catalytiques et Catalyse en Chimie Organique, ENSCM, 104, rue de la Galera - 34097 Montpellier, France HMS materials with Si/Al ratio = 2.5 have been synthesised and characterized. "^^Al NMR spectra showed minimal occurrence of octahedral phase after template removal. FT-IR and microcalorimetric results showed that, though having the same chemical composition of Y zeolites, such materials exhibit drastically different acidic/basic properties. 1. INTRODUCTION Since their discovery in 1992, ordered mesoporous materials have attracted much interest because of their high surface area and uniform distribution of mesopores diameters.' HMS (hexagonal mesoporous silicate) materials are usually synthesised by means of a "neutral route", in which the template is a primary amine. They may be prepared with different Al contents: as the Si/Al ratio decreases, however, the fraction of Al not incorporated in the lattice increases and only few papers deal with micelle-templated silicates with a low Si/Al ratio, with Al sitting stably in tetrahedral coordination.^ This work reports on the synthesis and the characterization of three HMS samples with Si/Al ratio = 2.5, either in the H form or after partial exchange with Li^ or Na^ cations. 2. EXPERIMENTAL The parent H sample was synthesised in ethanol (95%, Carlo Erba) at neutral pH with dodecylamine, TEGS and aluminium isopropoxide in molar ratios 1.0/2.5/1.0, respectively. Li- and Na-exchanged samples were obtained by contacting H sample with LiCl and NaCl alcoholic solutions, and finally washing with alcohol/water 95% mixture, in order to remove most of the template. Remaining template was removed by successive thermal treatments. Exchange was not complete, as the final ratios were Li/Al = 0.43 and Na/Al = 0.82. The parent H sample, containing a large template amount, was calcined for 8 hours in dry-air fiow at 723 K (temperature ramp = 5°/min). After calcination, ^^Al NMR only featured the peak of tetrahedral Al, with minimal contribution of the octahedral phase and ^Si NMR indicated a
320
thorough dispersion of Al. To allow FT-IR measurements, powders were pressed into thin, self-supported wafers and out-gassed at 573 and 873 K in an IR cell equipped with KBr windows. The acidic/basic properties of the specimens have been characterized by means adsorption of i) CO at 77 K; ii) propene and CO2 at RT. On samples Li and Na outgassed at 873 K, adsorption of CO2 was followed also by means of a Tian-Calvet microcalorimeter (Setaram) operated at 303 K. 3. RESULTS AND DISCUSSION 3.1. Hydroxyls population of samples outgassed at 573 and 873 K Figure 1 compares normalised spectra in the region of 0-H stretch of the three samples outgassed at 573 K (section a) and 873 K (section b). The intensity of the absorptions shows that, after thermal treatment and exposure to air, re-hydration takes place to a large extent, especially on H sample (section a). The prominent peak at 3742 cm' is due to free silanols and the broad absorption at lower frequencies to hydroxyls interacting via H-bonding. On Li and Na samples, such absorption is markedly less intense: exchange with alkali-cations brought about a change in surface properties, in that Li and Na samples show a different pattern of re-hydration. After treatment at 873 K (Figure lb), a sharp peak, due to OH stretch of isolated silanols, is seen at 3748 - 46 H 0.6 J cm'. On H sample, such peak is tailed on the low frequencies side (arrow), showing that several types of hydroxyls 8 with different acidic strength are C Li expected, whereas after ionic exchange the more acidic species have probably exchanged a proton for Li^ or Na' cations. The disappearance of the tailing when passing from H sample to ^"<^ Li and Na samples corresponds to the 0.0-i 3600 3200 exchange of more acidic hydroxyls. 3800 3700 The peak on Na sample is less intense Wavenumbers (cm'^) than on Li sample: compositional data (Experimental Section) show that Fig. 1. hydroxyls spectra of samples outgassed at exchange with Na^ ions was more 573 K (section a) and 873 K (section b). extensive with respect to Li' ions.
0
1
1 ^^
1
3.2. Acidic properties: adsorption of CO at nominal 77 K and propene at RT. Figures 2a and 2b report difference spectra recorded upon CO adsorption on H sample outgassed at 573 K. In the OH stretching range (section a), at least three types of hydroxyls showing different acidity are seen at 3660, 3720 and 3747 - 3742 cm'', giving rise upon interaction with CO to broad absorptions centred at 3440, 3570 and 3650 cm'', respectively. As the band at 3660 cm'' is typical of Al-linked hydroxyls formed in de-aluminated zeolites,^ it is assigned to Al-OH species, with sizeable acidity (Av = 3660 - 3440 = 180 cm"') and still anchored to the mesoporous structure, since ^^Al-NMR did not show sizeable phase segregation of aluminium oxide. Hydroxyls absorbing at 3720 cm'' are less acidic Al-OH
321
species (Av = 3720 - 3570 = 150 cm''), similar to those found on transition aluminas."* The smaller shift suffered by silanols (Av = 3747 - 3650 = 97 cm"') is due to their moderate acidity, confirmed by the fact that component at 3650 cm'' is formed at higher CO pressures, i.e. when other components are saturated. Figure 2b shows the formation of -OH—CO adducts at 2172 cm'' (interaction with more acidic hydroxyls) and 2161 cm"' (interaction with silanols), besides the adsorption of CO on small amount of extra-framework Ap^ species (band at 2226 cm''). No band due to Bronsted sites has been detected around 3620 cm''. Adsorption of propene at RT has been carried out on the sample pre-treated in the same way: no bands due to the formation of oligomeric species have been observed, confirming the absence of strong Bronsted sites capable of proton transfer on this system, in spite of its chemical composition, close to that of H-Y zeolite. On Li and Na samples hydroxyls spectra showed that bands of most acidic hydroxyls disappeared (Figure 1) as protons were exchanged for the cations. As it concerns CO adsorption on Lewis sites, bands at 2185 cm"' and 2173 cm'' have been observed, due to CO adsorbed on Li^ and Na^ ions, respectively. These frequencies are close to those found on Liand Na-ZSM-5 zeolites.^ 0.4 2161
0.5-
CD
O
H
0.0
<
2172 2226
-0.4
0.0-
3600
3200
2200
2100
Wavenumbers (cm'^)
Fig. 2. CO adsorption on H sample out-gassed at 573 K; hydroxyls (section a) and CO stretching (section b) ranges. 3.3. Study of the basic properties: adsorption of CO2 as followed by FT-IR and microcalorimetry. Figure 3 reports spectra recorded after CO2 adsorption on Li sample outgassed at 873 K. As far as basic properties are concerned, CO2 adsorption did not lead to the formation of any carbonate species on Li and Na samples, and only Hnear adducts on cations have been detected with spectroscopic features analogous to those of ZSM-5 zeolites.^ CO2 molecules coordinated on Li^ and Na^ ions absorb respectively at 2366 and 2356 cm"' (V3 mode). The inset in Figure 3 shows the vi streching mode (IR inactive in the free moiety) of CO2 molecules linearly adsorbed on Li^ cations (1382 cm''). Differential heats of adsorption (low coverage) are 32 and 27 kJ mol"' on Li and Na samples, respectively, and decrease with pco2 (Figure 4). Volumetric isotherms (not reported) show that adsorption of CO2 is reversible at 303 K and that a large amount of Li^ and Na^ cations is not accessible to CO2 molecules.
322
^1 1
Li
0.025
1400
1350
40 2400
2300
60
P (Torr)
100
Wavenumbers (cm'^)
Fig. 3. CO2 adsorption on Li sample out-gassed at 873 K.
Fig. 4. differential heats of adsorption for CO2 on Li (squares) and Na (circles) samples outgassed at 873 K.
As a whole, the cations-exchanged samples show a lack of basicity, though the composition is close to that of a basic zeolite, e.g. M-Y (M = Li or Na). This means that the presence of cations is not sufficient to impart basic properties to the HMS framework, in sharp contrast with what predicted by Sanderson electronegativity theory,^ only based on the chemical composition of the material. 4. CONCLUSIONS HMS samples with Si/Al ratio = 2.5, either in the H form or after partial exchange with Li^ or Na' cations show much more less marked acidic/basic properties, though their chemical composition is close to that of Y zeolites, whose protonic form (H-Y) is markedly acidic, and those alkali-exchanged (Na-Y, for example) are markedly basic. This feature is ascribed to the amorphous nature of the walls to be contrasted with the crystalline structure of zeolites. REFERENCES 1. J.S. Beck et al., J. Am. Chem. Soc. 114 (1992) 10834. 2. R.B. Borade et al., Catal. Lett. 31 (1995) 267 ; K.R. Kloetstra et al., Catal. Lett. 33 (1995) 57. 3. E. Garrone et al.. Proceedings of the 9th IZC 2 (1993) 267, Butterworth, Boston. 4. A. Zecchina et al., Appl. Spectrosc. Rev. 21 (1985) 259. 5. A. Zecchina et al., J. Phys. Chem. 98 (1994) 9577. 6. B. Bonelli et al., J. Phys. Chem. B 104(47) (2000) 10978. 7. R.T. Sanderson, Chemical Bonds and Bond Energy, Academic Press, 1976.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
323
Effect of cations addition for the highly ordered mesoporous niobium oxide B. Lee", D. L u \ J. N. Kondo' and K. Domen"'^ ^ Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan ^ CREST, JST Wormhole-like mesoporous niobium oxide was prepared by neutral templating route. Addition of trace amount of cations (Li"^, Na^, K^, Mg^^, Ca^^, or Ba^^) promoted structural regularity to 3-dimensionally (3D) ordered mesoporous niobium oxide. The 3D ordered mesoporous niobium oxide show^ed 5 and 3 nm of pore diameter and wall thickness, respectively. X-ray diffraction (XRD) and electron diffraction (ED) patterns verified 3D hexagonal structure of the mesoporous niobium oxide similarly to the mesoporous silica, SBA-2 and SBA-12, with Pds/mmc mesoporous structure. 1. INTRODUCTION Non-siliceous mesoporous materials have been investigated by neutral templating ' or ligand-assisted templating methods.^ Various ordered mesoporous metal oxides have been reported on titanium, zirconium, tungsten, niobium and tantalum oxides,"^ while typical mesoporous metal oxides show non-ordered porous structure. Differently from the structural variety of mesoporous silica, which have 2D or 3D hexagonal, ^ cubic, ^ lamellar, "^ and wormhole-like structures,'* little structural selectivity have been known in non-siliceous mesoporous materials. Even in the case of ordered mesostructures, ambiguous XRD patterns indicate the poor structural regularity of whole mesoporous particles.' In neutral templating route, ion addition promotes the structural regularity of mesoporous silica."*'^ The results are applicable in the preparation of mesoporous metal oxides, where the electrostatic interaction between template and ions promote the structural regularity. We have explored the systematic change of mesoporous structures in metal oxides framework by cation addition in neutral templating route. In this study, effect of ion addition is reported in the preparation of a mesoporous niobium oxide with highly ordered 3D mesoporous structure. This work was supported by Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology (JST).
324
2. EXPERIMENTAL Mesoporous niobium oxide was prepared by neutral templating route. Triblock copolymer, (HO(CH2CH20)26(CH2CH(CH3)0)39(CH2CH20)26H) (P-85, Adeka), and niobium pentachloride were employed as the template and inorganic source, respectively. Li^, Na"^' K^ Mg^^ Ca^\ Ba^^ were selected as the cations. Typical synthesis was proceeded in the molar ratio, Nb:P-85:propanol:cation:H20 = 35:1:835:0.25:280. After the reaction of niobium pentachloride in template solution, the sol was aged at 40 °C for one week to obtain a gel product. The gel was calcined at 450 °C for 5 hours in air to remove the template. X-ray diffraction (XRD) patterns were measured with a Rigaku RINT 2100 diffractometer using Cu Ka radiation. The TEM images were obtained with a 200kV JEOL JEM2010F. Nitrogen gas adsorption-desorption isotherm and BJH (Barrett-Joyner-Halenda) pore-size distribution were measured by a SA-3100 systems. 3. RESULTS AND DISCUSSION While wormhole-like mesoporous structure was found by neutral templating route, the addition of cations in niobium precursor/template system resulted highly ordered mesostructurcs. Although a sharp peak and a small second peak are found in mesoporous niobium oxide without any additives (Fig. la), the TEM observation revealed wormholc-Iikc mesoporous structure. On the other hand, by addition of Ca^^ ion, the XRD pattern shows the 3D hexagonal structure (Fig. lb). The c/a ratio was evaluated as 1.63, which indicates hep packing. The XRD pattern well agreed with 3D hexagonal mesoporous silica, SBA-2 ^ andSBA-12.^ N2 gas adsorption-desorption isotherms indicate change of mesoporous structure by cation addition. Type IV isotherm curve is found in the wormhole-like mesoporous niobium oxide (Fig. 2a). On the other hand, two-step raisings of adsorption and desorption curves at 0.5-0.7 P/Po range are observed in the 3D hexagonal mesoporous niobium oxide, which implies the existence of channel interconnection (Fig. 2b).^ Pore size distribution was estimated as 5 nm both adsorption and desorption curves, which is indicative of uniform mcsopore size. To dcfme the reproducible synthetic route, optimization of various factors was investigated: amount of metal source, cation concentrations, timing of cation addition, and the type of cations. The optimized condition gave the hexagonally arrayed 3D mesoporous structure that 1 ml of 0.05 M of cation solution was added in the niobium chloride/templatc/propanol system at 5*^ minute of reaction time. Smaller or larger amount of cation or niobium
325
chloride, or inappropriate addition time resulted in wormhole-like mesoporous structure that the hexagonally ordered phase was obtained under extremely controlled condition. The TEM and electron diffraction verifies the 3D hexagonal mesostructured niobium oxide prepared by Na^ addition (Fig. 3). Electron diffractions collected from different zone axis indicate hexagonally ordered mesoporous structure while long-range ordered mesoporous structure is observed from the TEM images. Consequently, it is considered that the wormhole-like porous structure is changeable by addition of cations, which might control the electrostatic interaction among the neutral template. Only a small amount of cations is applicable for the improvement of structural regularity, while large amount of alkaline addition resulted in structural degradation. Moreover, in the case of niobium oxide, the mesoporous structure is changed during the calcination step. As-synthesized mesoporous niobium oxide showed no specific XRD patterns of 3D hexagonal structure, while 3D hexagonal structure is found after template removal by calcination. Therefore, the structure control process is different in the case of mesoporous transition metal oxides from that for mesoporous silicas. 3D hexagonal mesoporous structure is obtained by optimized cation addition and calcination.
200
a. I(/) 8,
150 100
0)
E
_3 O
>
2
4 6 2 e (deg.)
Fig. 1. XRD pattern of mesoporous niobium oxide with (a) and without Ca^"^ addition (b).
Fig. 2. N2 gas adsorption-desorption isotherm of mesoporous niobium oxide with (a) and without Ca^^ addition (b).
326
a lOnm?,'/;
.i10nml^'
V ^ rf y
*
I
'>:
t #
10nm
Fig. 3. TEM image of 3D hexagonal mesoporous niobium oxide collected from different zone axes. The electron diffractions are shown in inset of each image: [0001] zone axis (a), [1213] zone axis (b), and [0111] zone axis (c).
REFERENCES 1. P. Yang, D. Zhao, D. I. Margolese, B. F. Chmelka and G. D. Stucky, Chem. Mater., 11 (1999)2813. 2. T. Sun and J. Y. Ying, Nature, 389 (1997) 704. 3. D. Zhao, Q. Huo, J. Feng, B. F. Chmelka and G. D. Stucky, J. Am. Chem. Soc, 120 (1998)6024. 4. P. T. Tanev and T. J. Pinnavaia, Science, 267 (1995) 865. 5. S. A. Bagshaw, Chem. Commun., (1999) 1785. 6. W. Zhang, B. Glomski, T. R. Pauly and T. J. Pinnavaia, Chem. Commun., (1999) 1803. 7. Q. Huo, R. Leon, P. M. Petroff and G. D. Stucky, Science, 268 (1995) 1324.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
Synthesis of zirconium-containing mesoporous silica membranes with high alkaline resistance for nanofiltration
327
Zr-MCM-48
Dong-Huy Park, Hens Saputra, Norikazu Nishiyama, Yasuyuki Egashira and Korekazu Ueyama Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka, 560-8531, Japan Mesoporous silica containing zirconium (Zr-MCM-48) membranes were synthesized on a porous alumina support. The Zr-MCM-48 membranes showed high stability in the alkaline solution of pH 12, indicating that only 3 % of Zr effectively enhanced the alkaline resistance. The permeation of gases through the calcined Zr-MCM-48 membrane was governed by the Knudsen diffusion mechanism. There was no contribution of viscous flow, which occurs in large pinholes. The result of permporometry measurements suggested the narrow pore size distribution of the membrane. 1. INTRODUCTION Mesoporous silica MCM-48 is an attractive material for many possible applications such as catalysis, sensors and membrane separations because MCM-48 has a three-dimensionally accessible pore structure. We have synthesized mesoporous silica MCM-48 membranes for pervaporation and nanofiltration [1-3]. However, silicate materials dissolve in water and alkaline solutions, which decreases the possibility of practical use. Some researchers [4,5] have prepared porous glass films [4,5] and membranes [6,7] containing zirconium with high resistance against water and alkaline solutions. We have reported that the introduction of zirconium into MCM-41 and MCM-48 powders effectively enhances their stability in alkaline solutions [8]. In the present study, we synthesized zirconium-containing MCM-48 (Zr-MCM-48) membranes on a porous alumina support. Structural stability in alkaline solutions and gas permeation characteristics of Zr-MCM-48 membranes were studied. Pore size distributions of Zr-MCM-48 powders and membranes were measured using N2 adsorption and permporometry, respectively. 2. EXPERIMENTAL A Zr-MCM-48 membrane was prepared as follows. A porous a-alumina support (NGK Insulators, Ltd.) with an average pore diameter of 0.1 im was placed in a tetraethyl orthosilicate (TEOS) and zirconium propoxide (ZrPr) mixture. A solution which consists of the quaternary ammonium surfactant, Ci6H33(CH3)3NBr (C16TAB), NaOH, and deionized
328
water was added to TEOS and ZrPr containing the a-alumina support. The molar ratio of the mixtures was 0.97 TEOS: 0.03 ZrPr: 0.4 CieTAB: 0.5 NaOH: 61 H2O. After the mixture was stirred for 2 h, the mixture and support were transferred to an autoclave. The reaction was carried out at 423 K for 24 h. The product was calcined at 773 K for 7 h. The product was identified by X-ray diffraction (XRD). The alkaline resistance of the Zr-MCM-48 membranes was evaluated by the XRD measurements before and after treatments in alkaline solutions with pH 10 ~ 12 at 303 K for 3 h. Gas permeation measurements using an as-synthesized and a calcined Zr-MCM-48 membranes were carried out with N2, He and H2 gases. The pore size distributions of MCM-48 and Zr-MCM-48 powders were calculated from N2 adsorption isotherms using the BJH method. The permporometry of Zr-MCM-48 membranes was carried out by monitoring N2 flux in the presence of capillary condensation of water vapor. 3. RESULTS AND DISCUSSION Fig. 1 shows the XRD patterns of MCM-48 and Zr-MCM-48 membranes before and after the treatment in the alkaline solution of pH 10-12. The peaks of the MCM-48 membrane disappeared after the alkaline treatment (Fig. 1(a)). This means that the structure of MCM-48 membrane cannot be maintained in the alkaline solution of pH 10. Fig. 1(b) shows that the structure of the Zr-MCM-48 membrane was maintained even after the alkaline treatment. The improvement of alkaline resistance seems to be caused by the strong Si-O-Zr network near the surface of the pore wall.
1 2
3 4 5 6 2 Theta [degree]
2
3 4 5 6 7 2 Theta [degree]
8
Fig. 1. X R D patterns of (a) M C M - 4 8 and (b) Z r - M C M - 4 8 m e m b r a n e s before and after treatment in the alkaline solutions
In the N2 gas permeation measurement using the as-synthesized Zr-MCM-48 membrane, no gas permeation was observed. This suggests that Zr-MCM-48 membrane has no pinholes or cracks before calcination. Fig. 2 shows the permeance of gases as a function of pressure drop through the a-alumina support and the Zr-MCM-48 membrane at 295 K. The permeance of N2 through the a-alumina support was proportional to the pressure drop, showing the characteristic of viscous flow. On the other hand, the permeance of gases through the calcined
329
Zr-MCM-48 membrane was constant with pressure drop, indicating that the gas permeation is governed by the Knudsen flow. There was no contribution of viscous flow to the permeation of gases, which occurs in large pinholes.
M_
c S 0.5
ZJp [kPa]
zip [kPa]
Fig. 2. The permeance of gasses as a function of the pressure drop through (a) a-alumina support and (b) Zr-MCM-48 membrane (295 K). 0:N2, DiHj, A: He.
0.16
<
~aO
0.14 0.12
"E 0.10 o
4>
E 3
O
0.08 0.06
> 0.04
i.
0.02 0.00
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Relative pressure, pip,, [-]
Pore diameter [nm]
Fig. 3. (a) N2 adsorption-desorption isotherms and (b) pore size distributions; (Ads), OMCM-48 (des), • Zr-MCM-48 (ads), D Zr-MCM-48 (des).
lMCM-48
Fig. 3(a) shows the adsorption and desorption isotherms of N2 at 77 K on MCM-48 and Zr-MCM-48 powders. Fig. 3(b) shows the pore size distributions calculated by the BJH method using the adsorption isotherm. Both the MCM-48 and Zr-MCM-48 showed narrow pore size distributions, implying that the introduction of Zr into MCM-48 did not destroy the ordered mesostructure. The average pore sizes of MCM-48 and Zr-MCM-48 were calculated to 2.4 and 2.8 nm, respectively. The Zr-MCM-48 showed larger pore size, pore volume and BET surface area than MCM-48. The permporometry is a useful technique for measuring open pores passing across the membranes. Fig. 4(a) shows the N2 flux as a function of relative vapor pressure of water. The
330
(a)
(b)
• • 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Relative vapor pressure of water, p/p^ [-]
3
4
5
6
7
Kelvin diameter [nm]
Fig. 4. (a) Nj flux as a function of relative vapor pressure of water and (b) Nj flux as a function of Kelvin diameter. relative vapor pressure was converted to Kelvin diameter using the Kelvin equation as shown in Fig. 4(b). A steep decrease in N2 flux at relative vapor pressure of 0.55'^0.6 indicated narrow pore size distribution of the Zr-MCM-48 membrane. The corresponding Kelvin diameter of the Zr-MCM-48 membrane was calculated to be about 2.5 nm. This steep decrease in N2 flux is attributed to the capillary condensation of water into the intrinsic mesopores of Zr-MCM-48. The contribution of N2 flux through large pinholes was small (less than 5 %) to the total flux. ACKNOWLEDGMENTS We thank NGK Insulators, Ltd. for supporting a-alumina supports. We also thank GHAS at the Department of Chemical Engineering at Osaka University for XRD measurements and K. Suzuki (NGK Insulators, Ltd.) for N2 adsorption measurements. REFERENCES 1. N. Nishiyama, A. Koide, Y. Egashira and K. Ueyama, Chem. Commun., (1998) 2147. 2. N. Nishiyama, D.H. Park, Y. Egashira and K. Ueyama, J. Membr. Sci., 182 (2001) 235. 3. D.-H. Park. N. Nishiyama, Y Egashira and K. Ueyama, Ind. Eng. Chem. Res., 40 (2001) 6105. 4. M. Nogami and Y Morita, Yogyo-Kyokai-Shi, 85 (1977) 449. 5. N. Tohge, A. Matsuda and T. Minami, Nihon-Kagakukai-shi, 11 (1987) 1952. 6. T. Yazawa, H. Tanaka, H. Nakamichi and T. yokoyama, J. Membr. Sci., 60 (1991) 307 7. T. Tsuru, H. Takezoe and M. Asaeda, AICHE J. 44 (1991) 765 8. D.-H. Park, M. Matsuda, N. Nishiyama, Y Egashira and K. Ueyama, J. Chem. Eng. Japan, 34(2001) 1321.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Synthesis of Siliceous MCM-41 grafted with transition metal carbonyls Rosas-Salas Raul ^''^, Dominguez J.M ^, Rachdi Ferid ^ and Alvarez T.C '^. ^ Institute Mexicano del Petroleo, Programa de Ingenieria Molecular/Competencia de Catalisis, Eje Central L. Cardenas 152, C.P. 07730, Mexico D.F,* jmdomingfalimp.mx. ^ G.D.P.CUniversite Montpellier 2,CC026. Place E. Bataillon,34095 Montpellier Cedex 05, France. '• Institute de Quimica UNAM. Circuito exterior C.U. Mexico D.F. C. P. 04310 ABSTRACT Grafted transition metal carbonyls over siliceous MCM-41 have been synthesized and further characterized by IR spectroscopy, showing bands in the range between 1800 c m ' and 2100 cm'', which are characteristic of metal carbonyls. ^'^Si MAS NMR spectra indicate that the interaction of the metal complexes with the silica wall of MCM-41 provokes a modification of the relaxation time of the silanol surface groups. The XRD study confirmed the stability of the hexagonal structure of the parent MCM-41 after the grafting procedure. The Thcrmo-analytical characterization of the silica-grafted complexes indicates the complete removal of the carbonyl bonds in a single cndothermic step, leaving an oxide residue. 1.-INTRODUCTION Transition metal carbonyls grafted on porous materials are of special interest in catalysis, separation technology and materials science^'\ for example the periodic mesoporous structure of MCM-41 ^^\ The use of metal carbonyls as active metal precursor allows the possible control of metal location and specific clustering into the mesoporous channels of MCM-41, thus offering a better control of activity and selectivity for potential catalytic reactions ^^\ In turn, these materials could be used as heterogeneous catalysts for decarbonylation, bond substitution and other catalytic reactions that could be sensitive to active species attached on the surface groups of the porous materials. The study of these novel materials should provide also a route for obtaining highly dispersed low valence metals ^^\ At present. Chemical Vapor Deposition (CVD) is used for anchoring metal carbonyls on oxide type surfaces, but in most of these cases a weak interaction between carbonyl complexes and surface groups occurs ^""'^^
332
Then, in this work transition metal carbonyls were grafted over MCM-41 siliceous mesoporous materials: Cr(C0)6, Mn2(CO)io, and Co2(CO)8. These species were grafted using UV radiation for generating coordinated non saturated species; ultrasound was applied in order to provoke further interaction between Fe2(CO)9 and MCM-41. The surface species were characterized by FTIR, MAS NMR and nitrogen adsorption at 78 K, TEM and thermal analysis. 2. EXPERIMENTAL The synthesis of the hexagonal mesoporous silica (HMS) was performed at room temperature, under basic conditions, using tetraethylortosilicate (TEOS) as silica source and hexadecyltrimethylammonium bromide (CTAB) as the organic surfactant agent. The intercalation of the transition metal carbonyls into the porous MCM-41 structure was performed using a proper amount of MCM-41 suspended in anhydrous diethyl-ether, then the metal carbonyls were added, and the mixture was irradiated using UV at room temperature, with the purpose of grafting the Cr(CO)6, Mn2(CO)io and Co2(CO)8. Ultrasound radiation at 30 °C was used instead of UV for the species Fe2(CO)9. 3. RESULTS AND DISCUSSION The IR spectrum of the synthesized materials shows characteristic bands corresponding to metal carbonyls between 1800 cm"' and 2200 cm"'. 3.L Thermal analysis The thermal decomposition, i.e., loss weight, was determined from TGA profiles while the temperature changes were recorded by means of DTA, The GTA curves corresponding to the grafted carbonyl complexes (fig.l) indicate that the CO ligand is removed between 180 and 300 °C, the DTA profiles showing an exothermic signal from 240 to 310 "C (fig 2), which can be assigned to the oxidation process occurring after removing the CO ligands.
S0.0 170.0 260.0 3!M).0 «40.0 930.0 620.0 710.0 800.0 TetnperaAure ^ C
Fig. 1. Typical T(iA profile of MCM-41 Grafted with Metal carbonyls
Fig 2. Typical DTA Profile of MCM-41 Grafted with Metal carbonyls
333
3.2. HRTEM The HRTEM images obtained from the parent siliceous MCM-41 and the modified materials are shown in Figs 3a and 3b, respectively. The hexagonal symmetry of the pore arrays is conserved after grafting, which is confirmed by XRD. In addition, there are not metallic particles present in the MCM-41 materials grafted with the transition metal carbonyls, thus suggesting that the interaction between the metal carbonyls and surface groups is strong enough to form chemical bonds.
^^i«lk4t
4 • • • • • • • *jj'
*
Figure 3a. TEM Photomicrograph of as - synthesized MCM-41.
Figure 3b. . TEM Photomicrograph of MCM-41 after anchoring of Chromium Carbonyl species.
3.3. XRD The X ray diffraction patterns of the parent siliceous MCM-41 and the materials modified after anchoring the metal carbonyl species, i.e. the as-synthesized MCM-41 and those containing Cr(C0)6, Mn2(C0)i(), and Co2(CO)8, show an intense XRD peak, but no d-displacements occur. Apparently, there is no change of the lattice parameters upon the anchoring process, but the Fe2(CO)9-MCM-41 show a XRD peak less intense and wider with respect to the other solids (Fig.4). These observations are interpreted as a loss of order in the hexagonal array of the porous system.
Figure 4:XRD pattern profiles.
334
3.4. ^'Si MAS NMR ^^Si MAS NMR spectra obtained using Is of repetition time for either parent and modified samples (Fig.5) show that the presence of transition metal complexes provokes modification of the relaxation time for the surface groups on MCM-41 i.e. Q'*, Q^ and Q^ species. The signal at -110 ppm corresponding to Q"* species can be observed for all the samples additionally a shoulder at 6 = -101 ppm is observed for the solids containing Cr and Fe, but those containing Mn and Co show a signal that can be assigned to Q^ (=Si-OH) species. The strongest effect is observed in the solid containing Mn, where a small signal at 5 = -91 ppm, assigned to Q^ (=Si(0H)2) species, is observed. These results indicate a chemical interaction between MCM-41 surface groups and the metallic center of the carbonyl complex.
-50 ,
. -150
Figure 5: '^Si MAS NMR.
4. CONCLUSION Molecular species of transition metal carbonyls have been anchored on the silica walls of MCM-41 materials. The hexagonal arrays that are characteristic of these materials are unchanged after the grafting procedure, as observed by using DRX and HRTEM. The metallic center of the carbonyl complexes and the surface groups of Siliceous MCM-41 seems to interact strongly, as evidenced by ^'^Si MAS NMR.
REFERENCES.
1.2.3.-
4.5.-
Myllyoja, S. y Pakkanen, T. T, / Mol. Cat. A: Chemical 156(2000) 195. C.Liu, Y.Y.Fan, M.Liu, H.T.Cong, H.M. Cheng and M.S. Dresselhaus, Science, 286 (1999)1127. H. Yoshitake, L.Sung Hun, S.Che, T. Tatsumi, in Zeolites and Mesomorphous Materials 14-P-26 (2001), Studies in Surface Science and Catalysis, Eds. A.Galameau, F.Di-Renzo, F.Fajula, J.Vedrine, Montpellier, april 12, 2001. Psaro R., Recchia S. Catalisis Today A\ (1998) 139. Suvanto, S. Hirva, P. y Pakkanen T. A. Surface Science 465: (3) (2000) 277-285.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
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Surface and pore structures of CMK-5 ordered mesoporous carbons studied by nitrogen adsorption and surface spectroscopic methods Hans Darmstadt ^, Christian Roy ^, Serge Kaliaguine ^, Tae-Wan Kim ^ and Ryong Ryoo ^ ^Departement de genie chimique,Universite Laval, Quebec, Qc, GIK 7P4, Canada ^National Creative Research Initiative Center for Functional Nanomaterials and Department of Chemistry (School of Molecular Science-BK21), Korea Advanced Institute of Science and Technology, Taejon, 305-701, Korea 1. INTRODUCTION Porous carbons are widely used as absorbents and catalyst supports. In many applications, carbons with mesopores of defined dimensions are desirable. By synthesis in a suitable matrix, ordered mesoporous carbons (OMCs) can be produced in a convenient way [1-4]. In the present work, OMCs were synthesised by polymerisation of furfuryl alcohol in SBA-15 aluminosilicates with different Si/Al ratios. The polymerisation of furfuryl alcohol is normally acid catalysed. The addition of an acid catalyst is required when the synthesis is performed in non-acidic silica [5]. However, this is unnecessary if performed in an acidic aluminosilicate as in the present work. The polymerisation reaction is catalysed by the Bronsted acid sites of the matrix. The introduction of aluminium not only influences the acidity of the matrix, it may also affect its pore structure. Therefore, in the present work, the matrices used for the OMC synthesis were characterised by nitrogen adsorption. The OMCs were studied by X-ray photoelectron spectroscopy (XPS) and by nitrogen adsorption. By XPS, only information on the external surface is obtained. However, it was shown in previous studies on similar OMCs that the surface spectroscopic results are representative for the entire sample [6]. 2. EXPERIMENTAL Aluminum-free SBA-15 silica was synthesised as reported elsewhere [7]. Different amounts of aluminum were introduced by slurrying the silica with an aqueous solution of AICI3 for approximately 30 min [8]. The Si/Al molar ratio ranged from 5 to 80. For the synthesis of the OMCs the pores of the aluminosilicates were filled at room temperature with furfuryl alcohol by an incipient wetness method. The amount of furfuryl alcohol corresponded to the pore volume of the aluminosilicate. The loaded aluminosilicate was heated to 95 °C in order to polymerise the furfuryl alcohol. Then, the SBA-15 template containing the carbon source was heated with increasing temperature to 900 °C under vacuum. Finally, the OMCs were liberated by treatment with hydrofluoric acid [2]. Elemental analysis of the aluminosilicates for Si/Al ratios was performed with inductively coupled plasma emission spectroscopy (Shimadzu, ICPS-IOOOIII). The powder X-ray diffraction (XRD) spectra were measured for calcined aluminosilicate samples at room temperature using a Rigaku Multiplex instrument (Cu Ka source, 3 kW). The details of the XPS and nitrogen adsorption experiments have already been described elsewhere [6]. The mesopore size distribution of the aluminosilicates was calculated with a modified BJH
336
method [9] using desorption data, whereas in spite of its shortcomings, for the OMCs the "traditional" BJH method [10] was used. 3. RESULTS AND DISCUSSION 3.1. Structure of the AI-SBA-15 aluminosilicates The X-ray diffractograms of the SBA-15 aluminosilicates showed intense narrow diffraction lines, indicating highly ordered structures. The Si/Al ratio only had a minor influence (not shown). It should be considered, however, that extra-framework aluminium species, possibly present in the pores, would not cause new X-ray diffractions to appear. Extra-framework species may narrow the pore diameter or block sections of the pore system. The mesopore volume and surface area of the aluminosilicates decreased indeed significantly with decreasing Si/Al ratio (Table 1). The pore size distribution, however, depended much less on the Si/Al ratio. For the sample with a Si/Al ratio of 80 a narrow pore size distribution with a maximum at widths of 75 to 80 A was observed. With decreasing Si/Al ratio, the widths of the mesopores changed very little (Table 1). Only for the most aluminium-rich sample (Si/Al ratio of 5), in addition to the above-mentioned pores, a small population of narrower pores with widths of 60 to 75 A was found. The differences between the pore widths are too small to explain the differences in the pore volumes. Therefore, the observations discussed above may be attributed mainly to the presence of extra-framework species that occupy the entire cross section of some parts of the mesopores. The presence of extra-framework aluminium species on the external surface could be ruled out. The concentration of aluminium on the surface, as determined by XPS, was smaller (higher Si/Al ratio) or only slightly higher as compared to the bulk (Table 1). 3.2. Chemical nature of the OMC surface It was discussed in the previous section that the Si/Al ratio of the aluminosilicale had an important influence on the structure of its pore system and its catalytic activity. This in turn should influence the properties of the OMCs formed there. The influence of the aluminosilicate Si/Al ratio on the OMC properties is first presented for the OMC chemistry. The XPS carbon spectra were dominated by an intense asymmetrical so-called graphite peak and by a smaller 7t-^ TI* peak (not shown). Such spectra are typical for polyaromatic, "graphite-like" carbonaceous solids as carbon blacks [11] and carbon fibres [12]. The full Table 1 Properties of the SBA-15 alumino-silicates Pore volume Surface Pore [cmVg] width area MicroMesoBulk Surface' [mVg] [A] pores pores 619 5 9.8 74 0.01 0.70 730 0.03 0.82 10 74 23.8 779 0.05 0.86 20 75 866 0.07 0.93 40 76 889 70.6 0.07 0.94 80 76 ^ Determined by XPS Si/Al
Table 2 XPS parameter for the graphitic character of the OMC surface Sample, Si/Al Relative area FWHM ratio of matrix o f t h e 71—>>7r* [eV] in parenthesises peak [%] 6.3 CMK-5 (5) 1.20 6.8 CMK-5(10) 1.19 7.2 CMK-5 (20) 1.18 6.1 CMK-5 (40) 1.21 6.0 CMK-5 (80) 1.22 Graphitised 0.82 8.9 carbon black
337
width at half maximum (FWHM) of the graphite peak depends on the graphitic character of the surface. It becomes narrower with increasing graphitic character [13]. For the OMCs, the FWHM of the graphite peak first decreased with increasing Si/Al ratio of the matrix and then increased again. A minimum value was found for a Si/Al ratio of 20 (Table 2), suggesting that the OMC synthesised in a matrix with a "medium" Si/Al ratio had the highest graphitic character. This finding is supported by the dependence of the JT;-> JI* peak area on the Si/Al ratio. A large Jt-» ji* peak indicates a carbon surface with a high graphitic character [14]. The largest jr-* Ji* peak was found for the OMC synthesised in the matrix with a Si/Al ratio of 20 (Table 2). The observation that the OMC with the highest graphitic character was formed in the matrix with a medium Si/Al ratio is explained as follows: upon heating, the OMC underwent several reactions (e.g. aromatisation and condensation) that increase its graphitic character. At least a portion of these reactions was catalysed by the acid sites of the matrix. Usually, the strength of acid Br0nsted sites increases with increasing Si/Al ratio. However, when the Si/Al ratio increases the increasing strength of individual sites is accompanied by a decreased concentration of acid sites. Therefore, the catalytic activity of aluminosilicates can often be described by a volcano curve where the highest activity is observed at medium Si/Al ratios. This is exactly the case for the dependence of the OMC graphitic character on the Si/Al ratio. 3.3. Structure of the ordered mesoporous carbons If during the OMC synthesis the entire pore system of the matrix is filled with the carbon product, the OMC can be described as a network of carbon rods (e.g. CMK-3). However, as in the present work, it is also possible to form the carbon product only on the pore walls of the matrix, without filling the entire pore. The produced CMK-5 OMCs consist of a network of nanopipes. The pore size distribution (PSD) of the CMK-5 OMCs showed the presence of two types of mesopores (Fig. 1). Pore widths and volumes were determined by fitting as shown for one sample (CMK5 (80)). The pores with widths between 34 and 37 A were assigned to the voids inside the nanopipes, whereas the pores with width 25 to 30 A were assigned to the voids in-between the nanopipes [2]. In an earlier study on CMK-3 OMCs it was observed that with increasing graphitic character of the OMCs the carbon rods are shrinking [6]. A similar shrinkage > seems to have occurred in the case of the CMK-5 OMCs. The nanopipes have the same initial external diameter (pore diameter of the matrix). However, when the nanopipes shrink the distance between them becomes larger. The largest pore width between the nanopipes was found for the sample with the highest graphitic character (CMK-5 (20)). This sample also had the narrowest pores inside the nanopipes (Table 3). The extra-framework species present in the pores of 10 20 30 40 50 60 the matrix also influenced the OMC structure. As mentioned above, in pore sections where the extraPore Width, w [A] framework species were present, they blocked the entire cross section of the pores. It is very likely that Fig. 1. OMC pore size distribution
338
Table 3 Characteristics of the CMK-5 OMCs Sample CMK-5 (5) CMK-5 (10) CMK-5 (20) CMK-5 (40) CMK-5 (80)
Pore width [A] Specific pore volume [cmVg] Surface SBA-15 Mesopores area Between the Inside the Si/Al ratio Total Between the Inside the [mVg] Nanopipes nanopipes Nanopipes nanopipes 26.2 5 2040 36.3 1.85 1.37 0.49 27.4 10 1.85 35.3 1.38 2010 0.45 1.82 1.21 20 1840 29.5 35.1 0.54 26.2 1.58 40 2450 36.6 2.26 0.67 25.0 36.4 2.03 1.38 0.62 80 2280
in these sections no carbon was formed. The corresponding OMCs consisted therefore of nanopipes with "missing" sections. This structure may have advantages when the OMCs are used in adsorption applications. As compared to a structure with long intact nanopipes, diffusion to adsorption sites inside the nanopipes should be much faster when "missing" sections provide additional entrances into the nanopipes. The very high pore volumes and surface areas indicate that the OMC are attractive adsorbents. Mesopore volumes of up to 2 g/cm^ were found. For most samples, the BET surface areas were well above 2000 mVg (Table 3). REFERENCES 1. R Ryoo, S.H. Joo, S. Jun, J. Phys. Chem. B 1999, 103, 7743. 2. S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Nature 2001, 412, 169. 3. S. Jun, S.H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna, O. Terasaki, J. Am. Chem. Soc. 2000, 122, 10712. 4. R. Ryoo, S.H. Joo, M. Kruk, M. Jaroniec, Advanced Materials 2001, 13, 677. 5. M. Kruk, M. Jaroniec, R. Ryoo, S.H. Joo, J. Phys. Chem. B 2000, 104, 7960. 6. H. Darmstadt, C. Roy, S. Kaliaguine, S.J. Choi, R. Ryoo, Carbon 2001, International Conference on Carbon, July 14-19, 2001, Lexington, Kentucky, USA. 7. M. Kruk, M. Jaroniec, C.H. Ko, R. Ryoo, Chem. Mater. 2000, 12, 1961. 8. S. Jun, R. Ryoo, J. Catal. 2000, 195, 237. 9. W.W. Lukens, P. Schmidt-Winkel, D. Zhao, J. Feng, G.D. Stucky, Langmuir 1999, 15, 5403. 10. E.P. Barrett, L.G. Joyner, P.P. Halenda, J. Am. Chem. Soc. 1951, 73, 373. 11. H. Darmstadt, N-Z. Cao, D. Pantea , C. Roy, L. Summchen, U. Roland, J.-B. Donnet, T.K. Wang, C.H. Peng, P.J. Donnelly, Rubber Chem. Technol. 2000, 73, 293. 12. E. Desimoni, G.I. Casella, A.M. Salvi, T.R.I. Cataldi, A. Morone, Carbon 1992, 30, 527. 13. K. Morita, A. Murata, A. Ishitani, K. Muragana, T. Ono, A. Nakajima, Pure Appl. Chem. 1986, 58, 456. 14. S.R. Kelemen, K.D. Rose, P.J. Kwiatek, Appl. Surf Sci. 1992, 64, 167.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved
339
A comparison of the sorption properties of mesoporous molecular sieves MCM-41 and MCM-48 J.C. Vartuli, W. J. Roth, J.D. Lutner, S.A. Stevenson, and S.B. McCullen ExxonMobil Research and Engineering, Corporate Strategic Research 1545 Route 22 East, Annandale, NJ 08801-3059. TEL: 1-908-730-3341 FAX: 1-908-730-3031 Email: Jim.C.Vartuli@ExxonMobil.com The benzene and triisopropylbenzene sorption capacity and rates of adsorption for several samples of MCM-41 and MCM-48 were compared. The results indicate that for samples with similar pore diameters, MCM-41 has approximately 30% more total capacity for the two hydrocarbons than MCM-48. Pore sizes estimated from the benzene capacity measurements agree well with those calculated from argon adsorption. Estimates made from benzene capacity measurements indicate that for the MCM-41, samples the thickness of the pore walls is approximately 8A and is independent of pore diameter; the pore wall thickness of the MCM-48 sample is estimated to be slightly larger, approximately lOA. Sorption rates for all samples were found to be indicative of macropore diffusion limitations rather than mesopore diffusion, hence, no conclusions can be drawn concerning the relative rates of diffusion of benzene through the mesoporous channels characteristic of these samples. 1. INTRODUCTION One of the unique features of the M41S molecular sieve family is the sorption of molecules within a uniform mesopore channel of dimensions from 15 to -lOOA [1]. The sorptive properties of MCM-41, the initial member of this family of mesoporous molecular sieves, have been extensively characterized using a variety of molecules [2-4]. These data indicate that MCM-41 has a narrow pore size distribution and that hydrocarbon sorption capacity is extraordinary compared to that of a classical microporous molecular sieve. The sorption properties of MCM-48, the cubic member of this mesoporous molecular sieve family, have not been as extensively examined, however the sorption characteristics appear to be comparable to that of the MCM-41 [5,6]. This paper compares both the sorption capacity and rates of adsorption of benzene and triisopropylbenzene in MCM-41 and MCM-48. 2. EXPERIMENTAL The syntheses of the MCM-41, prepared using dodecyltrimethylammonium (CI2), myristyltrimethylammonium (CI4), and cetyltrimethylammonium (CI6) bromide and MCM48 sample, prepared by using cetyltrimethylammonium bromide have been described previously [7]. X-ray powder diffraction data were obtained on a Scintag XDS 2000 diffractometer using CuKa radiation. Argon physisorption was used to determine pore diameters. Sample crystallite sizes were measured using TEM. The benzene and triisopropylbenzene (TIPB) sorption data were obtained on a DuPont Model 951
340
Thermogravimetric Analyzer at 5 and 15 torr benzene partial pressures and <1 torr for TIPB. The data are shown in Figure 1 and Table I. Table I Sorption Capacities and Physical Data Pore Sample
Pore
Total sorption^, g/g
diameteri (A) diameter2 (A) Benzene
MCM-41 C12 Ci4
void
wall
TIPB^
fraction
thickness (A) 8.2
26
25
0.53
0.52
0.58
28
28
0.62
0.58
0.61
7.8
Cl6 MCM-48
36
32
0.77
0.78
0.67
8.0
Cl6
28
28
0.45
0.45
0.54
10.1
1. from argon physisorption 2. from benzene sorption 3. g-sorbed/g-sample at p/po = 0.5 and 21°C 4. TIPB - tri-isopropylbenzene
100
100 MCM-41 (16)
c o O
80
C
MCM-41 (14)
»-r « - c . . : ^ - -
60
o s S
— - - - - "
*""'^'"
(/I
MCM-41 (12)
/ 1 /
^
20
0
10
60
5-?
1'/ 11/ / lA
*o
9
80
£ 40 n>
2 1
J
1
1
1
20
30
40
50
60
70
Benzene Vapor Pressure (torr)
20
10
20
30
40
50
60
Benzene Vapor Pressure (torr)
Fig. 1. MCM-41/48 benzene sorption isotherms 3. RESULTS AND DISCUSSION The benzene isotherms of MCM-41 and MCM-48, exhibit three unique characteristics. The first is the exceptionally high hydrocarbon sorption capacity (>50 wt.% benzene at 50 torr). The second characteristic is the sharp inflection of the isotherm indicative of capillary
70
341
condensation within uniform pores. The third feature is the position of the inflection point at relatively high partial pressure (p/po) suggesting large diameter pores. The total benzene sorption capacity and the partial pressure associated with the inflection point increases with increasing pore size. For the MCM-41 samples the total benzene capacity was 51% for the MCM-41 (CI2) sample, 62% for the MCM-41 (CI4) sample and 78% for the MCM-41 (CI6) sample. The total sorption capacity of the MCM-48 (CI6) sample (45%)) is about 30% less than that of the MCM-41 sample with a similar pore diameter. The benzene sorption rates are also 50% greater for the MCM-41 materials compared to those of the MCM-48 (see Figure 1 for data). The wall thicknesses were calculated to be approximately 8A for all three MCM-41 samples, suggesting that the thickness of the pore walls is independent of alkyl chain length. The pore wall thickness of the MCM-48 sample is estimated to be approximately lOA, or 25% larger than that observed for the MCM-41 samples. These values are in agreement with those previously reported by Chen et al. [8]. The triisopropylbenzene (TIPB) adsorption profiles are consistent with the benzene sorption characteristics. For the MCM-41 samples, the TIPB uptakes increase with increasing pore size (52% for the MCM-41 (CI2) sample, 58% for the MCM-41 (CI4) sample and 78% for the MCM-41 (CI6) sample). The MCM-48 (CI6) sample sorbed 45%, again approximately 30% less than the corresponding pore sized MCM-41 sample. The TIPB sorption rates are also about 50% greater for the MCM-41 materials compared to those of the MCM-48. The fact that the uptakes of benzene and TIPB are essentially the same suggests that the pore channels of both MCM-41 and MCM-48 are readily accessible for the molecules up to ~9 A. The kinetic diameter of benzene is 6 A and that of TIPB is 8.5 A. MCM-41 and MCM-48 demonstrate extraordinary sorption capacity for both benzene and triisopropylbenzene. For similar pore diameters, the MCM-41 samples have approximately 30% greater capacity for both hydrocarbons than MCM-48. The sorption rates are also 50% greater for the MCM-41 materials compared to those of the MCM-48. However, these rates and capacities differences are due to differences in macropore diffusion rates rather than differences in diffusion rates in the mesoporous channels.
REFERENCES 1. 2. 3. 4. 5. 6. 7.
Beck, J.S., et. al., J ACS, 114(27) (1992), 10834. Beck, J.S., et. al., Chem. Mater., 6(10) (1994), 1816. Branton, P. J., et. al., J. Chem. Soc. Faraday Trans., 90(19) (1994), 2965. Rathousky, J., et. al., J. Chem. Soc. Faraday Trans., 90(18) (1994), 2821. Thommes, M., et. Al., Studies in Surface Science and Catalysis, 135 (2001), 2893. Vartuli, J.C, et. Al., Microporous and Mesoporous Materials, 44-45 (2001), 691. Vartuli, J.C, et. al.. Zeolites and Related Microporous Materials: State of the Art 1994 (Proceedings of the 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, 7/17-22/94), J. Weitkamp, H.G. Karge, H. Pfeifer, and W. Holderich, eds., Elsevier Science, 53(1994). 8. Chen, C.Y., et. al., Microporous Mater., 2 (1993), 17.
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Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Argon and nitrogen adsorption on ordered silicas with channel-like and cage-like mesopores: implications for characterization of porous solids Mietek Jaroniec and Michal Kruk Department of Chemistry, Kent State University, Kent, Ohio 44242, USA Argon adsorption isotherms at 77 and 87 K and nitrogen adsorption isotherms at 77 K were measured for ordered silicas, including MCM-41 with channel-like mesopores. Nitrogen adsorption isotherms at 77 K were also measured for FDU-1 silica with cage-like mesopores. The studies of MCM-41 silicas with a wide range of pore sizes strongly suggest that adsorption branches of isotherms are suitable for the calculation of pore size distributions (PSDs). Moreover, for a given porous solid, similar PSD estimates can be obtained from adsorption branches of isotherms of different gases (nitrogen and argon at 77 K), or the same gas at different temperatures (argon at 77 and 87 K). On the other hand, in cases where adsorption-desorption hysteresis is observed, desorption branches of the isotherms are usually less suitable or even not suitable at all for the PSD determination. However, the desorption branches may provide important information about size of constrictions in the porous structure, as inferred from nitrogen adsorption data for some FDU-1 silicas. 1. INTRODUCTION The discovery of ordered mesoporous materials opened a new era in the field of gas adsorption in porous media. This discovery has made it possible to experimentally verify theories and reexamine empirical knowledge about gas adsorption in porous solids. In particular, it is now possible to experimentally study gas adsorption behavior in uniform channel-like and cage-like pores of different sizes and to provide a definite answer as to how adsorption data can be used to reliably assess pore size distributions (PSDs) [1,2]. Our earlier studies [1,3] demonstrated that in the cases of nitrogen adsorption at 77 K and argon adsorption at 87 K in channel-like pores of MCM-41, the capillary condensation pressure gradually increases as the pore diameter increases, whereas the relation between the capillary evaporation pressure is much more complicated in the pressure range of adsorption-desorption hysteresis. We have proposed to use the experimental relation between the capillary condensation pressure and the pore diameter for MCM-41 in PSD calculations [1,3], and demonstrated that when this approach is adopted, very similar PSDs are assessed from nitrogen adsorption data at 77 K and argon adsorption data at 87 K. We also observed that in the case of FDU-1 silicas with large cage-like pores, the capillary evaporation relative pressure is about 0.48 and varies very little for samples synthesized under different conditions, which suggests that this pressure does not reflect the pore diameter [4]. This relative pressure value would correspond to a pore diameter of about 4 nm when one assumes the validity of the Kelvin equation for hemispherical meniscus. We have demonstrated [5] that it is possible to synthesize FDU-1 samples that exhibit pore entrance size on the borderline between micropore (width below 2 nm) and mesopore (width between 2 and 50 nm) ranges. Therefore, the relative pressure of 0.48 is related to neither the pore diameter nor the pore entrance
344
diameter. Herein, we further examine the suitability of adsorption and desorption branches of gas isotherms in the pore size analysis, with particular emphasis on argon adsorption at 77 K [6]. 2. EXPERIMENTAL MCM-41 silicas with pore diameters below 5 nm were synthesized using alkylammonium surfactants of different structures and alkyl chain length from 8 to 22 carbon atoms, as reported in [7-9], where their properties were described. MCM-41 silicas with pores above 5 nm were synthesized using the postsynthesis hydrothermal restructuring method, and their properties are described elsewhere [10]. The synthesis of FDU-1 silica was carried out as originally proposed by Zhao et al. [11] using a poly(ethylene oxide)-poly(butylene oxide)poly(ethylene oxide) triblock copolymer and the details are described in [4]. Argon adsorption isotherms at 77 and 87 K, and nitrogen adsorption isotherms at 77 K were measured on a Micromeritics ASAP 2010 volumetric adsorption analyzer [1,3,4,6]. Before the measurements, the samples were outgassed for 2 hours at 473 K in the port of the adsorption analyzer. 3. RESULTS AND DISCUSSION Argon adsorption isotherms were measured at 77 K for MCM-41 silicas with pore diameters from 2 to 6.5 nm in order to establish the experimental relation between the pore diameter and the capillary condensation/evaporation pressure for cylindrical pores [6|. fhe capillary condensation pressure tended to increase gradually as the pore diameter was increased, which allowed us to establish a relation useful in PSD calculations. To extend this relation on the pore diameters larger than 6.5 nm, argon adsorption isotherms for SBA-15 and disordered large-pore silicas were also acquired and examined [6J. It was shown that PSDs calculated for MCM-41 from adsorption branches of argon isotherms at 77 and 87 K. and nitrogen isotherms at 77 K were very similar [6]. However, argon adsorption at 77 K was found suitable to study pores of diameter below 15 nm, because there is no capillary condensation in larger pores. The desorption branches of argon isotherms at 77 K were found to provide much less information about the pore size, because the relation between the pore diameter and capillary evaporation pressure exhibited much scatter. Moreover, in many cases, the steepness of the desorption branches of isotherms did not reflect the degree of structural ordering of the materials. These observations based on argon adsorption data at 77 K are the same as those made earlier in cases of nitrogen adsorption at 77 K [1] and argon adsorption at 87 K [3], but in the case of argon adsorption at 77 K, the unsuitability of desorption branches of isotherms was particularly striking. Therefore, we recommend that PSDs be calculated from adsorption branches of gas isotherms and we discourage the use of desorption data in the pore size analysis even for materials with channel-like pores [12]. It will be discussed below that in cases where adsorption-desorption hysteresis is observed, desorption data are certainly not suitable for the determination of pore size distributions for materials with cage-like pores. Nitrogen adsorption at 77 K on FDU-1 silicas was also studied. It was found earlier that in the case of nitrogen adsorption, the analysis of the desorption branch of the isotherm may provide important insight into the pore connectivity in FDU-1 and related materials [4]. Namely, good-quality FDU-1 (pore diameter of at least 8.5 nm) exhibits capillary evaporation delayed down to the lower limit of adsorption-desorption hysteresis, which in this case
345
corresponds to a relative pressure of about 0.48. It is clear that this relative pressure does not correspond to the actual pore entrance size, because FDU-1 samples with pore entrances in the micropore range, or in the lower end of the mesopore range (below 3 nm) all exhibit very similar capillary condensation pressure, as inferred from the results reported elsewhere [5]. This result suggests that a classical picture of delayed capillary evaporation related to constrictions in the porous structure [13,14] is largely valid, but needs to be modified to reflect the behavior at the lower limit of adsorption-desorption hysteresis [13]. One can conclude that capillary evaporation from a large cage-like pore with narrow entrance is delayed either to the pressure at which the capillary evaporation in the entrance takes place or to the lower pressure limit of adsorption-desorption hysteresis, whichever pressure is higher [4]. Therefore, the examination of the delayed capillary evaporation can provide information only about pore entrances that exhibit capillary evaporation above the lower limit of adsorption-desorption hysteresis in the wider pore parts. In the case of nitrogen at 77 K. this corresponds to pore entrances of diameter above about 5 nm. FDU-1 samples subjected to extensive hydrothermal treatment at 373 K or higher temperatures can serve as examples. In their cases, the onset of capillary evaporation of nitrogen at 77 K takes place relative pressures higher than 0.48, which suggests the presence of pore connections of size larger than 5 nm between some of the large, uniform mesopores. The occurrence of such large pore entrances, which is not observed for FDU-1 samples synthesized at lower temperatures or at short heating times, can serve as an indication of structural degradation of uniform cage-like structure during extended hydrothermal treatments [4]. The information about the loss of uniformity of the pore entrance size is not easy to extract from adsorption branches of nitrogen isotherms, so the analysis of desorption branches is much more informative in this case. However, in the case of nitrogen adsorption at 77 K, this methodology is expected to be suitable for studies of pore openings of diameter above about 5 nm. It is expected that argon adsorption at 77 K may offer important advantages over nitrogen in studies of pore connectivity in cage-like structures of large pore diameter. This is because it is known for silicas with channel-like pores that the adsorption-desorption hysteresis extends to appreciably lower pore sizes in the case of argon at 77 K, when compared nitrogen adsorption at 77 K [6]. But it needs also to be kept in mind that argon adsorption at 77 K is not expected to be suitable for studies of entrances to pores of diameters much larger than 15 nm, because argon at 77 K does not exhibit capillary condensation in such large pores [6]. 4. CONCLUSIONS The studies of ordered mesoporous materials with channel-like and cage-like porous structures allowed us to conclude that adsorption branches of isotherms are suitable for the calculation of pore size distributions, whereas desorption branches are often not particularly good, or even are completely unsuitable for PSD calculations. This is to large extent because of the lack of a clear relation between the capillary evaporation pressure and the pore diameter in the adsorption-desorption hysteresis region, which is particularly striking for cage-like pores, but observed also to some extent in channel-like pores. On the other hand, the position of the capillary evaporation step of the hysteresis loop can be related to the pore entrance size, unless the step happens to be at the lower limit of adsorption-desorption hysteresis. It is suggested that pore entrance sizes above 5 nm can be elucidated from desorption data in the hysteresis region in the case of nitrogen at 77 K. The use of argon adsorption at 77 K is
346
promising for the extension of the characterization capabilities to somewhat lower pore entrance diameters. ACKNOWLEDGMENTS Professors Ryong Ryoo and Abdelhamid Sayari are gratefully acknowledged for providing MCM-41 silicas. The authors also thank Professor Jivaldo R. Matos for the synthesis of FDU1 silicas. The donors of the Petroleum Research Fund administered by the American Chemical Society are gratefully acknowledged for support of this research. This work was also supported by NSF Grant CHE-0093707. The authors also thank Dr. Rene Geiger from Dow Chemicals for providing the triblock copolymer suitable for the synthesis of FDU-1 silica. REFERENCES 1. 2. 3. 4. 5.
M. Kruk, M. Jaroniec and A. Sayari, Langmuir, 13 (1997) 6267. P. I. Ravikovitch and A. V. Neimark, Langmuir 18 (2002) 1550. M. Kruk and M. Jaroniec, Chem. Mater., 12 (2000) 222. J. R. Matos, L. P. Mercuri, M. Kruk and M. Jaroniec, Langmuir, 18 (2002) 884. M. Kruk, V. Antochshuk, J. R. Matos, L. P. Mercuri and M. Jaroniec, J. Am. Chem. Soc. 124(2002)768. 6. M. Kruk and M. Jaroniec, J. Phys. Chem. B 106 (2002) 4732. 7. R. Ryoo, I.-S. Park, S. Jun, C. W. Lee, M. Kruk and M. Jaroniec, J. Am. Chem. Soc. 123 (2001)1650. 8. M. Kruk, M. Jaroniec, Y. Sakamoto, O. Terasaki, R. Ryoo and C. H. Ko, J. Phys. Chem. B, 104(2000)292. 9. M. Kruk, M. Jaroniec, H. J. Shin, R. Ryoo, Y. Sakamoto and O. Terasaki, Microporous Mesoporous Mater., 48 (2001) 127. 10. A. Sayari, P. Liu, M. Kruk and M. Jaroniec, Chem. Mater., 9 (1997) 2499. 11. C. Yu, Y. Yu and D. Zhao, Chem. Commun., (2000) 575. 12. M. Kruk and M. Jaroniec, Chem. Mater. 13 (2001) 3169. 13. M. Kruk, M. Jaroniec and A. Sayari, Adsorption, 6 (2000) 47. 14. S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area and Porosity, Academic Press. London, 1982.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
347
Mesopores developed in KL zeolite dealuminated with (NH4)2SiF6 solution Nongyue He^ ^, Chun Yang^, Jianxin Tang^ Hong Chen^ ^Key Laboratory of New Materials and Technology of China National Packaging Corporation, Zhuzhou Engineering College, Zhuzhou 412008, P. R. China ^Key Laboratory for Molecular and Biomolecular Electronics of Ministry of Education, Southeast University, Nanjing 210096, P. R. China. '^Department of Chemistry, Nanjing Normal University, Nanjing 210024, P. R.. China KL zeolite was dealuminated with a solution of (NH4)2SiF6 for secondary synthesis of mesoporous KL zeolite materials. In the absence of NH4AC as a buffer agent, some mesopores were generated but the distribution in pore size was broad. In the presence of NH4AC, or elongation of corrosion time, not only a higher crystallinity of samples was obtained, but also the mesopore size distribution was narrowed. Moreover, an empiric linear relationship between the 1)770 frequency and XAI was found. 1. INTRODUCTION Since a new family of mesoporous materials designated as M41S was first introduced to the scientific community by the scientists from Mobil Corporation [1,2], much attention has been paid to the synthesis, modification, characterization and application of the materials, and a great deal of work has been reviewed [3-6]. We have found that this kind of material showing very high alkylation activity and very strong photoluminescent effect [7-8]. In deed, this kind of novel material shows many very interesting new phenomena and promising application in many related fields. However, if the pore size distribution is not strictly required, the approach to develop nano-metric mesopores from conventionally used microporous zeolite molecular sieve materials by "secondary synthesis" is still a very useful tool owing to the low cost and the stability of the obtained products. We have treated KL of a one-dimensional channel structure with (NH4)2SiF6 solution and investigated the effect of corrosion conditions on the pore distribution in detail. 2. EXPERIMENTAL The treatment of KL with (NH4)2SiF6 solution was referred to that described previously [9]. X-ray powder diffraction (XRD) patterns were taken with a Rigaku D/max-yA instrument.
348
Pore size distribution was analyzed on a Micromeritics ASAP 2000 instrument at 77 K with N2 adsorption, following the Barrett-Joyner-Halenda algorithm [10]. The framework vibration infrared (IR) spectra of samples were reported for the wafers of mixture of 1% sample in KBr. The mixtures were ground by hand in pestle and mortar for 5 min and were then pressed at 4 tons to give a pellet (15 mm in diameter). Spectra were ao/c ) recorded on a Nicolet 51 OP FT-IR spectrometer Fig. 1. XRD patterns of parent KL(a) with a resolution of 2 cm'. Compositions were and modified KL sample (b). Si/Al: determined by conventional chemical analysis. a-2.90, b-3.94. 3. RESULTS AND DISCUSSIONS Figure 1 shows the XRD patterns of the KL samples before and after treatment. We can find that the obtained KL sample remained a good crystallinity. The good retention of the crystallinity after the modification is also demonstrated by the relative crystallinities of the samples listed in Table 1. A shift of the XRD peaks toward higher 20 degree direction was ascribed to the substitution of the shorter Si-0 bond for the longer Al-0 bond [11-12], but no linearity between the shift and XAI was gained after a systematic study. Interestingly, from the framework vibration FT-IR spectra (not shown here) and Table 1 wc find that the frequency for the internal tetrahedron symmetric band at -770 cm' increased significantly with the raise of Si/Al ratio and the frequency maximum position can always be determined. We plotted the frequency maximum of this band against the atom fraction of Al (XAI) in the framework tetrahedral sites, an empiric linear relationship between the frequency and XAI is obviously shown in Figure 2:
Table 1 Molar compositions and frequencies of framework IR spectra for samples. Sample
Si/Al (mole)
(mole)
1 2 3 4 5 6 7 8
2.75 2.90 2.95 3.41 3.94 4.98 6.14 7.37
0.267 0.257 0.254 0.227 0.203 0.167 0.140 0.119
XAI
Relative Frequency (cm' ) Crystallinity Asymmetric Symmetric T-0 bend (%) 1024.33 767.77 725.33 476.48 101 100 1028.19 768.71 727.26 476.48 769.70 726.29 478.41 105 1027.23 1030.21 97 773.01 727.45 478.41 1034.46 776.45 729.67 477.44 105 99 1035.40 782.23 726.11 477.44 indiscernible 785.12 730.19 478.83 88 99 indiscernible 788.02 729.19 476.46
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XAI = -7.309x10-^ (1)770-760) + 0.3242 This linear relationship was verified by a series KL samples whose compositions were carefully determined by conventional chemical analysis in our investigation. Moreover, upon investigating the relationship between the XAI and D770, we found the dealumination was limited and approximately 50% of Al in framework can be removed (not shown here), in accordance with the estimate by Vansant et al [13]. If one hopes to dealuminate more than 50% of Al atoms in the framework of KL, a partial breakdown of the framework of KL is inevitable. That partial breakdown together with the above dealumination makes some micropores evolve into nano-metric mesopores. However, the pore size distribution of the developed mesopores is usually very broad as shown in Figure 3 (a) without the introduction of NH4AC.
790 \ \
780 770 760
0
0.1
0.2
0.3
XAI /(Al/(Si+Al)) Fig. 2. Frequency of the internal tetrahedron symmetric band at -770 cm' vs. the atom fraction of Al (XAI) in the framework for KL zeolite molecular sieves samples.
Upon the introduction of buffer agent NH4AC, the pore size distribution was narrowed obviously as shown in Figure 3 (b). That is due to that the release of F' ions was controlled and, therefore, the severe collapse of framework resulting from the serious depletion of Al atoms was suppressed and the insertion of Si was enhanced [9, 14]. When the aging time was elongated (Figure 3 (c), NH4AC/KL =0.6, aging for 9 h), the mesoporc size distribution was further narrowed owing to the more complete replenishment of Si species into framework. After the NH4AC/KL ratio further increased to 1.3 (Figure 3 (d)), the mesopore size
0.20
<
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>
0.20
Ij
0.15
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>
0.15
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0.10
j
j
0.10
(-1
o
'i j / d i • ' 1 y I ^ ^ ^^"'•-,
0.05
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0.05
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Fig. 3. Pore size distributions of siliceous MCM-41 and the dealuminated KL samples at different NH4AC/KL molar ratios and corrosion time (a-0, 3 h; b-0.6, 3 h; c-0.6, 9 h; d-1.2, 3 h; e-MCM-41).
350
distribution was further narrowed and the volume of mesopore was increased, even if the sample underwent a relatively short aging period (3 h). However, upon the further increase of NH4AC/KL ratio (2.2, not shown here), the pore size distribution was not further improved and the volume of mesopore decreased. In the end, the mesopore size distribution of KL obtained by secondary synthesis described above is generally much broader than the directly synthesized mesoporous material MCM-41 and the pore size (~20 nm) of the former is much bigger than that (~3 nm) of the later, as clearly shown in Figure 3 (d) and (e). ACKNOWLEDGMENT This research is supported by the Natural Science Foundation of China, the Excellent Teacher's Foundation of Ministry of Education, P. R. China, Natural Science Foundation of Hunan Province, China, and Education Committee of Hunan Province, China. REFERENCES 1. C.T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992)710. 2. J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, C. T.-W. Chu, D. H. Olson, E. D. Sheppard, S. B. McCullen, J. B. Higgins and J. L Schlenker, J. Am. Chem. Soc, 114(1992) 10834. 3. A. Sayari, Stud. Surf Sci. Catal., 102 (1996) 1. 4. U. Ciesla, F. Sch
th, Microporous and Mesoporous Materials, 27 (1999) 131.
5. Y. Ma, W. Tong, H. Zhou and S. L. Suib, Microporous and Mesoporous Materials, 37 (2000), 243. 6. G. D. Stucky, Q. Huo, A. Firouzi, B. F. Chmclka, S. Schacht, I. G. Voigt-Martin and F. Schuth, Stud. Surf Sci. Catal., 105 (1997) 3. 7. N. He, S. Bao and Q. Xu, Appl. Catal. A: General., 169 (1998) 29. 8. N. He, C. Yang, L. Liao, C. Yuan, Z. Lu, S. Bao and Q. Xu, Supramolecular Science, 5 (1998)523. 9. N. He, Q. Shi, X. Qiu, X. Zhang and K. Zhu, Petrochemical Technology (Chinese), 22 (1993) 19. 10. Barrett, E. P, Joyner and L. G., Halenda, P P, J. Am. Chem. Soc. 73 (1951) 373. 11. D. W. Breck and E. M. Flanigen, Molecular Sieves, 1968, Society of the Chemical Industry, London. 12. Edith. M. Flanigen, Khatami, H., Szymanski, Herman A., Molecular Sieves-1, American Chemical Society, Washington D. C , 1971, pp 201-227. 13. E. F. Vansant and G. Peeters, J. Chem. Soci. Faraday Trans. L, 73 (1977) 1574. 14. W. Breck, H. Blass, USP 4503023 (1985).
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Time-resolved in situ grazing incidence small angle x-ray scattering experiment of evaporation induced self-assembly A.Gibaud', D. Doshi^, B. O c k o \ V. Goletto^ and C.J. Brinker^ 'Universite du Maine, UMR 6087 CNRS, 72085 Le Mans,Cedex 9, FRANCE ^ University of New Mexico, Albuquerque, NM 87106, USA P h y s i c s Department, BNL, Upton, Long Island New-York USA "^Chimie de la Matiere Condense, UMPC, 75005 Paris, FRANCE ^Sandia National Laboratories, Advanced Materials Laboratory, 1001 University Blvd SE, Albuquerque, New Mexico 87106, USA A time-resolved in-situ grazing incidence small angle x-ray scattering (GISAXS) experiment combined to gravimetric measurements of the slow evaporation of a liquid film of CTAB surfactant molecules dissolved in a mixture of TEOS and ethanol is presented. The complete investigation of the film formation, starting from the very beginning when the film is still in a liquid state to its final dried state is reported. 1. INTRODUCTION The synthesis of sihca-bascd mesostructured materials by using supramolccular selfassembly of surfactant molecules to template the condensation of inorganic species has attracted considerable interest in the past decade'. Geometrical considerations show that above the CMC (Critical Micelle Concentration), surfactant molecules can self-assemble into spherical, cylindrical or lamellar shapes micelles^. As shown in 1992, by researchers of the Mobil Corporation micelles can further self assemble into well-organized 2D or 3D mesostructural phases^'"^ and template the condensation of inorganic materials. Mesoporous phases with tailored porosity^'^' can be obtained by thermally removing the surfactant. Recently, there have been some attempts of in-situ characterization, using resolved fluorescence-depolarization experiments or in-situ luminescence of probe molecules but those techniques do not provide any information on the organization of the film^. For a system prepared using a block-copolymer as the templating agent, some insight into the film formation were obtained using in situ time-resolved SAXS experiments^. But so far, no complete study was done to follow the whole structural process of self-assembly. We present here a time-resolved in-situ grazing incidence small angle x-ray scattering (GISAXS) experiment combined to gravimetric measurements of the slow evaporation of a liquid film of CTAB surfactant molecules dissolved in a mixture of TEOS and ethanol. The complete investigation of the film formation, starting from the very beginning when the film is still in a liquid state to its final dried state is reported. The time evolution of the GISAXS patterns is exploited to come out with a self-explanatory mechanism of the self-assembly process.
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2. EXPERIMENTAL The sol containing surfactant, silica precursor, ethanol, water and hydrochloric acid (initial molar composition ITEOS: 20 C2H5OH: 5.4 H2O:0.004 HCl: 0.10 CTAB) is dispensed on to a silicon (100) substrate that is sitting on a plateau of a weighing balance confined in an evaporation cell. Evaporation of ethanol concentrates the system in silica and surfactant finally resulting in a mesostructured film. Typical experiment time is 20 min, which allows us to follow the different periods of the self-assembly and correlate it to the mass of the film. The measurements were preformed on the liquid spectrometer of the X22B beam line of the NSLS (National synchrotron Light Source, BNL, USA). The sample was kept horizontal during the course of the measurements and the incident beam was deflected by a Ge monochromator so that it could impinge at a controlled incident angle on the surface of the liquid film. The incident wavelength was fixed to 1.582A and the scattering was monitored with a MAR CCD 2D detector (see figure 1).
Incident beam
Scattered ^^^^
140120100^ h
Liquid film 21) detector
\
35
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Fig. 1. Schematic representation of the xray set-up
Fig. 2. Evolution of the mass of the liquid film and of the weight percent of CTA B as a function of time
3. RESULTS AND DISCUSSION Figure 2 shows how the mass of the film changes as a function of time upon controlled evaporation of ethanol. The self-assembly occurs in different steps which can be divided in 5 consecutive periods. The first step corresponds to the rapid evaporation of ethanol (t < 400s). It is characterized by the absence of any definite off-specular features in the scattering image (Fig 3a). At t~400s, w = 19 %, the first premises of an organized structure suddenly appear. As seen in Figure 3b a Bragg reflection appears along the specular direction at qz=0.178 A ' (d = 35.4 A). The presence of a Bragg spot is the clear signature of the layering of the constituent species parallel to the substrate. From the relatively high intensity of the Bragg reflection one can clearly state that the layering consists of several layers and not only of a single monolayer. The observation of only one Bragg peak indicates that the smectic ordering is short ranged. This second step can be labeled "the smectic ordering of surfactant molecules mediated by silica". The organization of the smectic phase does not persist for long. The continuous evaporation of ethanol forces the surfactant molecules to interact more strongly. The next step can be well identified by the concomitant disappearance of the Bragg spot arising with the development of a broad arch at a somewhat larger wave vector transfer than
353
the one of the disappearing Bragg spot (figure 3c). The broad arch strongly evolves during this fourth step, which can be considered as the "cylindrical rod-like micelle ripening ". As a consequence of better defined distances between the formed rods the arch becomes sharper. The radius of the arch gives the average distance between neighboring organized species. As the ring is continuous, one can infer that these species are on the average separated by this distance but that no clear 3 D periodicity has yet been achieved. The radius of the ring continuously shrinks and sharpens during this period indicating that the organized species which are forming are moving further apart or growing in size. The radius goes from 0.205 to 0.153 A ' which gives a d-spacing going from 30.6 to 41 A.. In less than 2 minutes after the starting of the silica induced cylindrical rod-like micelle ripening, one assists to the selfassembly of the ripe rods into a 2D hexagonal phase. This transformation is clearly a first order transition. This is beautifully demonstrated by the very sudden emergence of well defined Bragg spots in the specular and off-specular directions : this is called "the 2D hexagonal self-assembly of ripe cylindrical rods" (figure 3d).
(b)
(a)
0-"
-0 15
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-0 05
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0 1
0 15
%{/<-')
(d)
(c)
"5^ -0 15
^(A-^)
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Fig. 3. In-situ 2D x-ray scattering patterns measured during the phase transformation of the diluted liquid film to the 2D hexagonal mesophase; (a) liquid phase, (b) smectic phase (the arrow indicates a Bragg reflection), (c) micellar phase, (d) 2D hexagonal phase. During the phase transformation the intensity of the Bragg peak suddenly diverges whereas its width drastically decreases. The simultaneous observation of the ring together with the Bragg spots shows the coexistence of ordered and disordered phases and is in favor with a first order transformation. On further drying the intensity of these spots grows as a function of time showing that this process further develop until almost no more ethanol is present in
354
the film. At that time the final state is almost reached. One can however observe upon further drying that the Bragg spots slowly change in position. In particular, in this last step the specular Bragg spots move away from the origin as a consequence of "the shrinkage of the silica network". Similar experiments were carried out under different conditions of evaporation and of film compositions to test how the sequence of phase transformation was affected. In particular the evaporation rate was slowed down by adjusting the leak rate. Slowing down the rate of evaporation of ethanol indeed increases the degree of condensation of the silica network. Consequently the fast condensafion of the silica network impedes the development of the hexagonal phase. The phase transformation starts identically to the previous one but after the development of the arch, the ultimate arrangement of the rods into a hexagonal lattice is made impossible. The fast condensation of the silica network traps the rods into a disordered configuration and the diffraction pattern of the final dried film exhibits the intermediate ring. A liquid film which did not contain any TEOS was also prepared to further probe the influence of the silica condensation on the mesophase transformation. From the above experiments it is extremely clear that the development of the silica network plays a crucial role on the development of the mesophase. Without the silica we observed that upon evaporation of ethanol, the surfactant orders into a lamellar phase which is conserved all along the evaporation process. ACKNOWLEDGEMENTS This work was supported by the UNM/NSF Center for Micro-Engineered Materials, the French ACI "Nanostructure" under project N° 03-01, the AFOSR, the DOE Basic Energy Sciences Program and SNL's Laboratory Directed R&D program. Research carried out in part at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. REFERENCES 1. For a recent review see Ying, J. Y., Mchnert, C.P. and Wong, M.S. Angew. Chem. Int. Ed., 38,56-77, 1999 2. Israclachvili, J., "Intcrmolccular and Surface forces" 2nd Ed., Academic Press, London 1992. 3. Kresgc, C.T., Leonowicz, M.E., Roth, W.J., Vartuli, J.C, and Beck, J.S., Nature, 359, 710-712,(1992) 4. Beck, J.S., Vartuli, J.C, Roth, Leonowicz, M.E., W.J., Kresgc, C.T., Schmitt, K.D., Chu C.T.W., Olson, D.H., Shcppard, E.W., McCullen, S.B., Higgins, J.B. and Schlcnker J.L. J. Am. Chem. Soc, 114, 10834-10843, (1992) 5. Lu, Y., Cangull, R., Drewlen, C.A., Anderson, M.T., Brinkcr, C.J., Gong, W., Guo, Y., Soyez, H., Dunn, B., Huang and Zing, J.l. Nature, 389, 364,(1997). 6. Zhao,D., Yang, P., Melosh, N., Feng, J., Bradley, B.F., Cmelka, F. and Stucky, G.D., Adv.Mater., 10, 1380,(1998). 7. Chen, C.Y., Burkett, S.L., Li, H.X. and Davis, M.E., Microporous Mater., 2, 27-34, (1993) 8. Grosso, D., Balkenende, A.R., Albouy, P.A., Ayral, A., Amenitsch, H. and Babonneau, F. Chem.Matcr., 13, 1848-1856, (2001); Grosso,D, Babonneau, F., Soller-IUia, G., Albouy, P.A, Amenitsch, H., Chem. Com., 748-749, 2002.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Small Angle Neutron Scattering Study on the Formation Mechanism of Mesostructures During Sol-Gel Processing Y. K. Kwon^ D. H. Klm^ G.-J. Kimh, Y.-S. Han^ and B.-S. Seong^ "Department of Polymer Science and Engineering, Inha University, 253 Yonghyun-Dong, Nam-Gu, Incheon 402-751, Korea ''Department of Chemical Engineering, Inha University, 253 Yonghyun-Dong, Nam-Gu, Incheon 402-751, Korea 'Korea Atomic Energy Research Institute, P. O. Box 105, Yusong, Daejon 3()(;-G()(), Korea The formation mechanism of the mesoporous structures in the silica solgel solutions with a structure-directing amphiphilic triblock copolymer was monitored by SANS measurement at various stages during synthesis. Th(^ SANS data showed that a long-range liquid crystal-like mesoscopic ordering of" the polymer was almost collapsed shortly after being mixed with thc^ ])rehydr()lyzed inorganic silica solution, but it was gradually recovered into th(^ m(\sostructure with a long range order during evaporating the solv(Mits in the gelation process. 1. INTRODUCTION Mesoporous materials with pore sizes in a nanometer range have been prepared with various synthetic methods. Typical examples in this category ar(^ M41S silicate materials, transition metal-doped and transition metal oxide mesoporous molecular sieves. In general, the synthesis of thesc^ materials has been carried out using templating surfactants, inorganic prcH'ursors and reaction media usually with addition of a catalyst.' The hydr()})hilic i)art of surfactants is associated with the surrounding inorganic ])recurs()rs by intermolecular interactions, including van der Waals, hydrogen bonding, covalent bonding and electrostatic interactions.^ Several formation mechanisms of a supramolecular organic-inorganic selfassembly have been proposed by adjusting the charge densities of the surfactant species and the inorganic species under the acidic or basic reaction conditions. Mobil researchers proposed the liquid crystal templating mechanism that described a supramolecular self-assembly of
356
organic molecules or those associated with inorganic species. Since the concentration of surfactants in the synthesis solution is less t h a n what is needed to form lyotropic micellar phases, the cooperative, dynamic mechanism has been suggested instead. In this model, the organic surfactants are associated with the inorganic species in the reaction solutions. The self-organization of the organic-inorganic assembly in two-dimensional lattices is gradually acquired during the condensation of the inorganic species.^^ However, any experimental evidence has not been reported precisely on the details of the formation of supramolecular organic-inorganic assembly. In this study, we investigate the formation mechanism of ordered mesoporous structures from the homogeneous sol-gel solutions with a nonionic block copolymer in a D2O/H2O mixed solvent. In the sol-gel process, a long-range mesoscopic ordering is formed during evaporating mediating constituents. In the present study, small angle neutron scattering (SANS) technique is employed at various reaction stages to provide the detailed information on the self-organization of the organic and inorganic species into a supramolecular organic-inorganic assembly during the reaction. 2. EXPERIMENTAL SECTION The inorganic precursor solution of tetraethoxyorthosilane (TEOS) in ethanol (EtOH) was prehydrolyzed in aqueous HCl solution. The molar ratio of TEOS : EtOH : H2O : HCl was 1 : 3 : 4 : 0.04. This solution was r(^fluxed at G()*'C for 90 min and then stirred at 25"C for 15 min. The final solution was aged at 55 '>C for additional 15 min without stirring. The solution of organic phase was prepared by dissolving the 10 g of KO^OPOTOEOI^O in a mixture of 23.33 g of D2O/H2O (GO/40 in vol. %) mixed solvent and aqueous hydrochloric acid solution (0.145 g HCl / 2.()58 g of D^O/H^O (60/40 in vol %) mixed solvent). The molar ratio of TEOS to EO^OPOTOEO^O was set to be 40. The interaction between the poly (ethylene oxide) (PEO) part of E02()P07()E02() and hydrolyzed TEOS through PEO-SiO. complexation was mediated by HCl in 2.8 g 0.5 M aqueous HCl solution. Th(^n both organic and inorganic precursor solutions were mixed for further relictions at 55 "C for 5 hrs. The resultant mixture was then poured into a ])etri dish and dried in air at room temperature for several weeks. SANS measurement was carried out on the facility at the HANARO ccMitcnin KAERl with two-dimensional position sensitive 05 x ()5 cm- area dc^tcn-tor. Th(^ neutron scattering intensity vt^as measured using a ncnitron wavelc^ngth of 5.08 A and the sample-to-detector distance of 3 m. The equipment employs circular pinhole collimation with Bi/Be filter. 3. RESULTS AND DISCUSSION P'lgure 1 shows the SANS intensity profiles of the solution of organic-
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inorganic complexes as a function of reaction time during the evaporation of the solvent. In Fig. la, the solution of the block copolymer E02oP07()E02() with 30 wt % concentration gave an intense peak at Q » 0.037A-^ which corresponded to an intermicellar spacing of about 170A (Fig. la). The SANS intensities in the range of 0.06 < Q (A-^ < 0.15 occurred due to two-dimensional micellar ordering of E02oP07oE02(), but their weakness revealed that it appeared only in a short-range order. It has been known that the E02()P07oE02() micelles are spherical in this concentration regime and arranged in a cubic lattice.
Absolute Scattering ^ Cross Section (cm ')
Reaction Time Fig. 1. SANS data measured in the sol-gel process: (a) E02()P07()E02() solution i'M) wt %) at room temperature and the E02()P07()E02()/TEOS mixed solution: (b) just after mixing; (c) after stirring for 5 hrs at 60 "C; after evaporation of (d) K; vol. %; (e) 46 vol. % and (f) 66 vol. % of the solvent. When both the organic and inorganic precursor solution were mixed at room temperature, the E02()P07()E02()concentration was decreased to about 15 wt % wh(M'e the polymers are almost in an isotropic state. Therefore, the SANS })(»ak intensity was almost disappeared (Fig. 1 b). However, the data showed an extremely weak peak near Q ^ 0.072 ± 0.003 A ' which may occur due to a short-range ordered, molecular association between small numbers of F():i()P()7()E02() in the isotropic solution. Stirring the mixed solution for 5 hrs at 60 "C causes the intermolecular association between the hydrophilic PEO and the prehydrolyzed TEOS to form PE0-Si02 complexes that also associate^ into a micellar structure in the solution. In Fig. 1 c, the peak intensity was slightly increased by a better molecular association of these complexes. Its position was also moved to the small angle region, due to the slightly larger sizes of these complex micelles as compared to those in the reaction stage b . Increasing the concentration of the mixed solution by evaporating thc^ solvent results in the continuous growth of this weak peak intensity and the peak shift to the lower angle region. The increase in peak intensity at higher*
358
concentration regime in Fig. 1 d and e reveals the increased packing order of the EO20PO70EO20 -Si02 complex micelles. At higher concentrations, more E02()P07()E02()-Si02 complexes are associated into the micellar structure and the domain spacing becomes larger to show the continuous shift of the observed q value to the small angle region. Up to this stage of reaction, the self assembly process of E02()P07()E02() and prehydrolyzed TEOS molecules into a long-range ordered, supramolecular organic-inorganic structure is not completed. In the final stage of the reaction, the molecular rearrangement and relocation was almost completed. During the evaporation in this concentration regime (Fig. If), the silanol groups in the framework phase are gradually condensed into an interconnected, rigid network structure. In this stage of reaction, water and ethanol were additionally produced during the condensation of the silica species and the pre-organized self-assembled structure of the organic-inorganic complexes became contracted to exhibit smaller domain spacings as compared to those measured in Fig. 1 a through e. lOU
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.
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Fig. 2. Variation in the observed domain spacings and the reaction stages
ACKNOWLEDGEMENT This work was supported by Grant No. 2001-2-3()8()()-()()l-3 from the Basic R(\search Program of the Korea Science and Engineering Foundation and by Inh[i University through project.
REFERENCES 1. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuh, J. S. Beck, Nature 359(1992) 710. 2. A. Firou/i, F. Atef, A. G. Oertli, G. D. Stucky, B. F. Chmelka, J. Am. Chem. Soc. 119(1997) 3596. 3. G. S. Attard, J. C. Glyde, C. G. Goltner, Nature 378 (1995) 366.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Preparation of mesoporous silica anchored mo catalysts and in-situ XAFS characterization under propene photometathesis reaction Nobuyuki Ichikuni *, Taku Eguchi, Haruno Murayama, Kyoko K. Bando \ Shogo Shimazu and Takayoshi Uematsu Department of Materials Technology, Faculty of Engineering, Chiba University, Inage-ku, Chiba 263-8522, JAPAN ^ National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8565, JAPAN FAX: +81-43-290-3401, E-mail: ichikuni@tc.chiba-u.ac.jp Mo catalysts anchored on MCM-41 and incorporated into SBA-3 framework were prepared. XRD and TEM analysis revealed that both catalysts maintained their mesoporous structure. However, Mo/MCM-41 catalyst prepared by conventional impregnation lost the ordered mesoporous structure. Structural differences were analyzed by in-situ XAFS measurement under propene photometathesis condition. Structure and the catalysis were also discussed based on the differences between irradiated and unirradiated structures. 1. INTRODUCTION The specific surface area of the highly ordered hexagonal mesoporous silica was typically more than 1000 m^g'*, and seemed to be good support for catalyst. However, the mesoporous structure was easily dcstructcd by some treatments, especially immersing to solvents. We tried to prepare the MCM-41 supported Mo catalysts from M0CI5. Moreover, Mo species incorporated into silica walls during hydrothermal synthesis was also prepared. Isolated molybdenum ions were known to have the higher activity toward propene photometathesis reaction [1]. The catalysis depends on the local structure around Mo atom and support. We have already prepared an in-situ XAFS (X-ray absorption fine structure) cell that could be used under irradiation and reaction gas passage [2]. However, the efficiency of the irradiation was not enough. In this study, new type in-situ XAFS cell was developed and applied to the Mo catalysts under propene photometathesis reaction. We elucidate the differences in the catalyst structures in the course of photo-catalytic reaction using in-situ XAFS technique. 2. EXPERIMENTAL The hexagonal mesoporous silica MCM-41 was hydrothermally synthesized using sodium silicate and [CH3(CH2)i3N(CH3)3]Br at 373 K for 144 h. Mo/MCM-41 was prepared from M0CI5 and MCM-41 in cyclohexane under N2 atmosphere. The solution was stirred for overnight to complete the reaction of M0CI5 with surface OH groups, consequently evacuated
360
to remove the solvent and calcined in air [Mo/MCM-A]. Conventional impregnation catalyst was also prepared from ammoniumheptamolybdate (AHM) aqueous solution [Mo/MCM-B]. The incorporated Mo catalyst in the mesoporous silica wall was synthesized under acidic condition using TEOS, AHM and [CH3(CH2)i5N(CH3)3]Br at 293 K for 24 h [Mo-SBA-3]. TEM (transmission electron microscopy) images were observed by JEM-4000FX1I (JEOL) operated at 400 kV. XAFS measurements were performed at BL-lOB of the Photon Factory of KEK IMSS (proposal no. 2000G286). XRD (x-ray diffraction) measurement was performed by MXP3 (MAC Science). The samples were pressed into self-supported disks and placed in the in-situ XAFS cell designed for photocatalyst characterization. Mo K-edge in-situ XAFS spectra were collected in a transmission mode with C3H6 flow under 75 W highpressure Hg lamp irradiation. 3. RESULTS AND DISCUSSION 3.1. eX'Situ characterization Mo loading of the Mo-SBA-3 was determined from ICP-MS analysis. The loading of Mo/MCM-41 catalysts was 5 wt%. XRD patterns in low degree range showed that Mo/MCM-A and 1.2 wt% Mo-SBA-3 catalysts maintained their hexagonal mesoporous structure, while, Mo/MCM-B catalyst lost the hexagonal mesoporous feature and only showed amorphous phase (Fig. 1). 2 3 4 5 The d\Qo value was calculated to 3.7 and 3.5 nm 26^/degree for Mo/MCM-A and Mo-SBA-3, respectively. BET surface area of Mo/MCM-A, Mo/MCM-B Fig. 1. XRD patterns for Mo catalysts and Mo-SBA-3 were 1070, 650 and 1270 m^-g" in low degree region: (a) Mo/MCM-B, \ respectively. The impregnation of MCM-41 (b) Mo-SBA-3 and (c) Mo/MCM-A. with AHM aqueous solution caused the destruction of hexagonal mesoporous structure and lead to the diminishment of the surface area. XRD patterns for Mo oxide range were shown in Fig. 2. M0O3 diffraction peaks were clearly observed on Mo/MCM-B. To prepare highly dispersed supported Mo/MCM-41 catalysts and to maintain the highly ordered mesoporosity, using AHM aqueous solution 30 35 4() should be avoided. On the other hand, XRD 26^/degree patterns for Mo/MCM-A and Mo-SBA-3 shows Fig. 2. XRD patterns for Mo catalysts only amorphous Si02 peaks. It is supposed in Mo oxide range: (a) Mo/MCM-B, (b) that three dimensional bulk growth of M0O3 Mo-SBA-3 and (c) Mo/MCM-A. had not been occurred and highly dispersed Mo
361
oxide phase might be expected on these catalysts. Conventional ex-situ EXAFS spectra were collected by transmission mode and analyzed by curve-fitting (CF) method. FT for Mo/MCM-B catalyst (not shown) exhibited a clear Mo(O)-Mo peak suggesting the aggregation of Mo oxide. From the CF analysis, Mo/MCM-A and Mo-SBA-3 have a dimeric Mo unit and an isolated Mo unit, respectively. So the Mo-0 bond in the Mo-SBA-3 seemed not to change so much during the photometathesis reaction. The photocatalytic activity of propene photometathesis reaction on 5 wt% Mo/MCM-A and 1.2 wt% Mo-SBA-3 were 0.84 and 2.5 mmol-min'^-gMo', respectively. The higher photoactivity of Mo-SBA-3 suggests the higher dispersion of Mo species inside the mesopore. 3.2. in-situ characterization The schematic of the in-situ XAFS cell designed for photo-catalyst characterization was illustrated in Fig. 3. The cell was constructed from SUS and had a quartz window to penetrate the UV light to the sample disks. water jacket The cell has two acrylic windows at the gas in both end of the x-ray path. The absorption acrylic window^ of x-rays by the windows seemed to be sufficiently small enough at Mo K-edge [3]. X-ray Self-supporting 20 mm diameter disks are placed in the cell at 45 degree to x-ray path. water jacket 3 to 5 disks were placed in the cell to reach the sufficient edge jump at Mo K-edge. Figure 4 showed the in-situ XAFS spectra around Mo K-edge region for 1.8 wt% MoIIV irradiation SBA-3 catalyst. Catalysts were treated Fig. 3. Top view of in-situ XAFS cell for with following conditions during the photocatalysis. measurements; (1) pre-treatment with O2, (2) UV irradiation under N2 flow, (3) unirradiation under N2 flow, (4) UV reirradiation under N2 flow, (5) UV irradiation under propene flow, (6) reoxidation after the reaction. The ls-4d transition peaks were observed at around 20000 eV for all the spectra. This peak became stronger as the structure around Mo was in Tj symmetry rather than in Oh symmetry. The peak height became 20()4() 19960 2(K)0C) weaker under UV-irradiation which reflects photon energy / cV the photo-excitation process, from Mo^^=0^" to Mo^'^-O". As propene was Fig. 4. In-situ XANES spectra for Mointroduced under UV-irradiation, the SBA-3 catalyst in the course of propene diminishment of the pre-edge peak became photometathesis reaction. clear, suggesting the structural change around Mo atom.
362
3.5
1 (2) (3)j (4) ' (5)1 1 1 CF analysis for Mo-0 coordination was carried out by using K2M0O4 as a 3.0TO reference with a program REX2000 T 6 (Rigaku Co.). Coordination number ^JQ^ 52.5 (CN) of Mo-0 (about 0.20 nm) for Moo SB A-3 was shown in Fig. 5. The CN U 2.0 Lh was diminished as photometathesis reaction proceeded. Double bonded I I 1 1 1.5 1 h ; Mo=0 (0.17 nm) site is thought to be the 4(X) 2(X) 300 100 0 active site [1,4]. Thus, the CN of Mo-0 / / min would not change in the reaction state. Fig. 5. Change in CN of Mo-0 in Mo-SBA-3 This tendency could be explained as beat during propene photometathesis reaction: (2) phenomena, which means that Mo-O irradiated, (3) unirradiated, (4) re-irradiated, (5) bond length changed during the reaction. irradiated under propene flow. Many kind of Mo-0 coordination length existed in the catalyst. These distributions of Mo-0 length lead to the interference of waves and diminished the EXAFS oscillation. In other words, incorporated Mo species in the SBA3 walls was not rigid but change their surroundings during the reaction. The structural change of around anchored Mo was clearly observed in this in-situ XAFS technique.
'tM^t •
M
i
-]
4. CONCLUSION Mo catalysts anchored on MCM-41 maintaining the highly ordered mesoporous structure was prepared from M0CI5 precurosor. Mo incorporated into SBA-3 framework was prepared and characterized by XAFS technique. In-situ XAFS measurement of Mo-SBA-3 catalysts during propene photometathesis reaction revealed that SBA-3 walls incorporated Mo species were not rigid during the reaction. This work was supported in part by the Sasakawa Scientific Research Grant from The Japan Science Society.
REFERENCES 1. M. Anpo, M. Kondo, Y. Kubokawa, C. Louis and M. Che, J. Chem. Soc, Faraday Trans. /, 84(1988)2771. 2. N. Ichikuni, H. Murayama, K. K. Bando, S. Shimazu and T. Uematsu, Anal. Sci., 17s (2001) ill93. 3. K. K. Bando, T. Saito, K. Sato, T. Tanaka, F. Dumeignil, M. Imamura, N. Matsubayashi and H. Shimada, J. Synchrotron Rad, 8 (2000) 581. 4. J. M. Aigler, V. B. Kazansky, M. Houalla, A. Proctor and D. M. Hercules, J. Phys. Chem., 99(1995) 11489.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
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In-situ XAFS observation of formation of Pd-Pt bimetallic particles in a mesoporous USY zeolite Kyoko K. Bando, Takashi Matsui, Lionel Le Bihan, Koichi Sato, Tomoaki Tanaka, Motoyasu Imamura, Nobuyuki Matsubayashi, and Yuji Yoshimura National Institute of Advanced Industrial Science and Technology, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan In-situ XAFS observation of reduction process of USY zeolite supported Pd and Pd-Pt catalysts was performed to elucidate the structure of active Pd and Pd-Pt metal particles in connection with catalytic activity. Some of the metal particles were supposed to be located in mesopors ranging from 10 to 50 nm in diameter. Pt was found to promote reduction of Pd. The average size of Pd-Pt metal particles was similar to that of Pd. Pt atoms formed fine particles and were dissolved in Pd particles. 1. INTRODUCTION USY (Ultra Stable Y-type) zeolite-supported Pd catalysts (Pd/USY) are highly active for hydrogenation of aromatic compounds. These catalysts deactivate rapidly in hydrodearomatization (HDA) of diesel fuel containing sulfur compounds due to formation of inactive sulfided species. Addition of Pt to Pd/USY improves the sulfur tolerance of the catalyst [1]. In order to clarify the factor which determines sulfur tolerance of a catalyst, it is necessary to know the structure of active sites under reaction conditions. In this work, we carried out an in-situ X-ray absorption fine structure (XAFS) spectroscopy analysis of the Pd-Pt catalysts during the reduction process and observed how metal particles were formed on the mesoporous USY support. The same analysis was applied to the monometallic Pd catalyst and difference in structure of reduced Pd and Pd-Pt catalysts was discussed in connection with catalytic activity. 2. EXPERIMENTAL Pd and Pd-Pt (molar ratio of Pd/Pt was 4) catalysts were prepared using USY (SiOz/ AI2O3 = 13.9) by impregnation method (Pd/USY and Pd-Pt/USY for short, respectively). The
364
precursors were Pd(NH3)4Cl2xH20 (41.21 wt% Pd) and Pt(NH3)4Cl2 XH2O (55.6 wt% Pt). The total metal content was 1.2 wt%. After impregnation, the samples were calcined at 573 K for 3 h. Pd K-edge and Pt Lm-edge XAFS measurements were made in a transmission mode at BLIOB and BL9A of the Photon Factory in the Institute of Materials Structure Science, High Energy Accelerator Research Organization in Japan. The sample was set in an in-situ cell designed for measurements under a flow of a reactant gas [2]. XAFS analysis of the obtained data was conducted with commercially available software (REX, Rigaku Co.). Parameters for backscattering amplitudes and phase shift functions were derived from the oscillations of Pd, Pt, and Pd-Pt (Pd/Pt = 4) foil observed at the same temperature with the in-situ experiments. The pore size distribution were determined by measurement of the N2 isotherm at 77 K with ASAP 2010 (Micrometrics). The BJH method with the N2 desorption isotherm was applied to the pore size distribution analysis.
3. RESULTS AND DISCUSSION Figure 1 shows the results of pore size distribution analysis. The USY used in this study has a bimodal mesopore structure, that is, there is a sharp peak at 3 nm and a broad band at 20 nm. The mesopore structure of the calcined catalysts was almost the §0.3 (b) 2 0.3 same as that of an untreated USY for -Sf A both Pd/USY and Pd-Pt/USY The J 0.2 pore distribition of larger mesopores ^ 0.1 b£ changed by reduction of the catalysts bo under 20 % H2 (diluted by He or Ar) I 0.0 §0.0 at 573 K for 1 h. Therefore it is 1 10 100 1 10 100 Pore Diameter (R) / nm Pore Diameter (R) / nm supposed that some of the metal Fig. 1. Pore size distribution of (a) Pd/USY particles formed in the mesopores and (b) Pd-Pt/USY. (The solid line ranging from 1 0 - 5 0 nm in diameter represents USY, the dotted line calcined catalysts, and dash-dotted line reduced or larger mesopores were deformed catalysts. by the treatment.
}\\
V
VV
Table 1 Coordination numbers obtained by curve-fitting analysis for Pd/USY and Pd-Pt/SUY at 573 K under 20 % H2. Coordination Number Sample Pt-Pt Pd-Pd Pd-Pt Pt-Pd Pd/USY 7.0 2.1 Pd-Pt/USY 6.7 0.6 4.2
^
365
Figure 3 shows change in Fourier transform of Pd K-edge EXAFS (extended x-ray absorption fine structure) spectra (k"^ (k), where "k" represents the photoelectron wave number, and " " is the normalized EXAFS oscillation) observed for Pd/USY and Pd-Pt/USY in the reduction process. In the spectra observed in air before reduction, there are peaks at 0.19 nm and it is assigned to Pd-Cl and Pd-0 scattering. When hydrogen was introduced onto the catalysts at r.t., a new peak appeared at 0.27 nm for both catalyst. This new peak is assigned to Pd-Pd scattering of metallic particles. Intensity of the Pd-Pd peak in Pd-Pt/USY was stronger than that in Pd/USY. As the temperature increased, the intensity of the Pd-Pd peak increased. The Pd-Pd peak became dominant at 323 K for Pd-Pt/USY and at 373 K for Pd/USY. Therefore, reduction of Pd in Pd-Pt/USY was promoted by Pt, compared with Pd in Pd/USY. Curve-fitting analysis of the main peak observed at 573 K was carried out. The results are shown in Table 1. The average coordination number (CN) of metal-metal scattering was almost the same, that is, 7.0 for Pd/USY and 7.1 for Pd-Pt/USY, which implies that the average size of the particles were the same for the two catalysts. Figure 3 shows change in Fourier transform of Pt Lm-edge EXAFS spectra observed for Pd-Pt/USY in the reduction process. Before reduction, a peak due to Pt-Cl and Pt-0 was observed at 0.19 nm. Influence of this peak remained until 323 K under a flow of 20% H2/He. A band ranging from 2 to 3 nm was due to Pt-Pd or Pt-Pt scattering of metal particles. At 373 K, CN's for Pt-Pd and Pt-Pt were 1.6 and 2.5, respectively, which changed to 4.2 and 2.1 at 573 K (Table 1). These facts imply that initially Pt atoms formed rather monometallic fine particles apart from Pd particles and as the temperature increased Pt particles dissolved into Pd particles. Therefore, it is suggested that concentration of Pt atoms on the surface of Pd-Pt particles became high. Pt particles located on the surface of Pd is supposed to play an important role in the catalytic reactions.
573 K .523 K '473 K '423 K
20 % H/Ar r.t. in air 4 6 0 2 Distance / (0.1 nm)
'373 K 323 K r.t. t/vyv Vv—'v^. in 20% H/He j-.t. in air Distance / (0.1 nm)
Fig. 2. Change in Fourier transform of Pd K-edge EXAFS spectra (k'^x(k)) during reduction of 1.2 wt% (a) Pd/USY and (b) Pd-Pt/USY under a flow of 20 % H2.
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Judging from the average coordination number, the size of metal particles is supposed to be less than 1.3 nm, which is smaller than the size of the supercage. Most of the particles were supposed to be located in micropores. But, the analysis of the porosity showed that during reduction of metal species the pore distribution changed. Some of the metal atoms might have become larger particles and been located in mesopores. Pt atoms in Pd-Pt/USY formed fine particles and were dissolved in Pd particles and a cluster-in-cluster like structure was generated in Pd-Pt/USY [3]. Pt atoms near the surface is supposed to be directly involved in the reaction and promoted the catalytic activity.
•^573 K 523 K 473 K
.10
423 K H fe
•373 K 323 K 20%H/He r.t. in air 0
1 Distance / (0.1 nm)
Fig. 3. Change in Fourier transform of Pt Liiredge EXAFS spectra (k^xCk)) during reduction of 1.2 wt% Pd-Pt/USY under a flow of 20 % H2/He at a flow rate of 120 ml/min.
REFERENCES 1. Y. Yasuda, and Y. Yoshimura, Catal. Lett., 46 (1997) 43. 2. K. K. Bando, T. Saito, K. Sato, T. Tanaka, F. Dumeignil, M. Imamura, N. Matubayashi, and H. Shimada, J. Sync. Rad., 8 (2001) 581. 3. K. Asakura, Y. Yamazaki, H. Kuroda, M. Harada, N. Toshima, Jpn. J. Appl. Phys., 32 (1993) 448.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
367
Investigation of the internal pore structures of Beta/MCM-41 and ZSM-5/MCM-41 composites by ^^^Xe NMR Wanping Guo^'^, Limin Huang^, Chang-Sik Ha^ and Quanzhi Li^* ""Department of Chemistry, Fudan University, Shanghai 200433, P. R. China ^Department of Polymer Science and Engineering, Pusan National University, Pusan 609-735, Korea The application of '^'^Xe NMR to investigating the internal pore structures of Beta/MCM-41 and ZSM-5/MCM-41 composites shows that the mesopore wall of the Beta/MCM-41 composite is constructed by a lot of secondary building units characteristic of zeolite Beta, while the ZSM-5/MCM-41 composite contains interconnected mesopore and micropore. This has demonstrated that '^^Xe NMR spectroscopy is a powerful tool to study the internal structures of porous materials. 1. INTRODUCTION '^'^Xe NMR has been used as a sensitive probe for the investigation of the pore structures of zeolites and other porous materials in the past few years [1-4]. The '^'^Xc atom, with a spin of 1/2 and relative high natural abundance (26.4 %), is particularly sensitive to its neighboring environment because of the very large and extremely polarizable electron cloud of xenon. Small variations in the physical interactions with xenon atoms may give rise to remarkable perturbations of the electron cloud, which are transmitted directly to the xenon nucleus and greatly affect the NMR chemical shift. The advantage of xenon in comparison with other adsorbates is due to its chemical inertness and excellent sensitivity [5]. It is very interesting to apply '^'^Xe NMR technique to determining certain properties of zeolites, which are much difficult or even impossible to detect by conventional physicochemical techniques [6]. Recently, we have synthesized two types of composites of Beta/MCM-41 and ZSM-5/MCM-41 [7,8]. The Beta/MCM-41 composite was prepared through a two-step crystallization process of combining low crystallized zeolite Beta synthesis gel with surfactant cetyltrimethylammonium bromide (CTAB) solution. A synthesis gel of zeolite Beta with low crystallinity was first obtained by controlling the crystallization time. There were a great number of secondary building units (repeating structural sub-units in zeolite frameworks) characteristic of zeolite Beta besides the small Beta crystals in the aluminosilicate gel. With the addition of surfactant CTAB, the gel would condense around the self-assembling aggregate of CTAB and many secondary building units in the initially crystallized
368
aluminosilicate could be introduced into the mesopore wall of the Beta/MCM-41 composite. The ZSM-5/MCM-41 composite was prepared using a dual templating method through a process of two-step crystallization. Mesoporous MCM-41 was first synthesized using the self-assembling of surfactant CTAB and subsequently the amorphous wall of MCM-41 was recrystallized with a structure-directing agent tetrapropylammonium bromide, which was introduced into the MCM-41 wall through a pre-treatment process. Because the synthesis approach of the Beta/MCM-41 is different from that of the ZSM-5/MCM-41, these two kinds of composites may possess respective specific pore structures. In this paper, we employ '^^Xe NMR technique to investigate the internal pore structures of Beta/MCM-41 and ZSM-5/MCM-41 composites. 2. EXPERIMENTAL Beta/MCM-41 and ZSM-5/MCM-41 composites were prepared according to the procedures described by Guo et al. [7] and Huang et al. [8], respectively. The '^^Xe NMR spectra were recorded at 293 K with a Bruker MSL-300 instrument operating at 83.0 MHz. The samples studied were packed in an NMR tube equipped with a re-sealable valve suited for attachment to a vacuum line, then dehydrated by gradual heating to 573 K in vacuum of 0.027 Pa and maintained at this temperature for 5 h before xenon adsorption. Finally, the samples were equilibrated under the same xenon adsorption capacity (1.0 x 10^' atoms/g) and the same xenon equilibrium pressure at room temperature before the NMR tube was sealed. The relaxation delay of 0.5 s was used to get the spectrum. The chemical shifts were expressed relative to that of xenon gas extrapolated to zero pressure [1,3]3. RESULTS AND DISCUSSION As a reference, a mechanical mixture of Beta and MCM-41 was prepared with the same weight percent of Beta and Si/Al ratio as the Beta/MCM-41 composite [7]. The '^'^Xc NMR spectra for the Beta/MCM-41 composite and the corresponding mechanical mixture of Beta and MCM-41 are shown in Fig. 1. Free xenon gas line at about 0 ppm can be observed in the '^*^Xe NMR spectra for the Beta/MCM-41 composite and the mechanical mixture. The chemical shift at 119.2 ppm for the Beta/MCM-41 composite is close to that at 120.8 ppm for the mechanical mixture, which can be attributed to the strong adsorption of xenon inside the micropore of zeolite Beta [9]. The chemical shift at 96.3 ppm for the mechanical mixture is characteristic of the xenon adsorption in the mesopore of MCM-41 and this value has little variation with the adsorption pressure of xenon [10]. However, the chemical shift at 109.1 ppm for the Beta/MCM-41 composite is 12.8 ppm higher than that at 96.3 ppm for the mechanical mixture under the same xenon adsorption pressure. This phenomenon indicates that there is stronger adsorption of xenon inside the mesopore of the Beta/MCM-41 composite. The specific two-step crystallization process has suggested that a lot of secondary building units characteristic of zeolite Beta in the initially crystallized aluminosilicate may be introduced into the mesopore wall during the formation of the Beta/MCM-41 composite. Therefore, the xenon adsorption in the mesopore becomes stronger to produce one '^'^Xe
369
NMR line with higher average chemical shift at 109.1 ppm, a value between 120 ppm and 96 ppm. From the results discussed above, it can be concluded that the Beta/MCM-41 composite possesses specific mesopore wall containing many secondary building units characteristic of zeolite Beta.
120.8
300
200
100
S4.6
0
Chemical shift (ppm) Fig. 1. •^'^Xe NMR spectra for (a) the Bcta/MCM-41 composite and (b) the mechanical mixture of Beta and MCM-41.
300 5oo Too 0 Chemical shift (ppm) Fig. 2. "'Xc NMR spectra for (a) the ZSM-5/MCM-41 composite and (b) the mechanical mixture of ZSM-5 and MCM-41.
A mechanical mixture of ZSM-5 and MCM-41 phases, which contained the same weight percent of ZSM-5 and Si/Al ratio as the ZSM-5/MCM-41 composite, was used as a reference [8]. The ^^'^Xe NMR spectra for the ZSM-5/MCM-41 composite and the corresponding mechanical mixture of ZSM-5 and MCM-41 are presented in Fig. 2. Fig. 2b shows that there arc three chemical shifts at 174.0, 84.6 and 0 ppm, corresponding to the adsorption of xenon in the micropore of ZSM-5, the mesopore of MCM-41, and free xenon in the gas phase, respectively. However, the '^^Xe NMR spectrum for the ZSM-5/MCM-41 composite is quite different. In Fig. 2a, we can see that the chemical shift of the line at 90.1 ppm is higher than that of the line at 84.6 ppm and this line shape becomes broadened and unsymmetrical, whereas the chemical shift of the 159.4 ppm line is lower than that of the 174.0 ppm line and this line shape becomes sharpened. Considering the specific two-step crystallization process for synthesizing the ZSM-5/MCM-41 composite, this phenomenon probably results from the presence of interconnected pores between the micropore and mesopore in the
370
ZSM-5/MCM-41 composite. The adsorbed xenon atoms can exchange rapidly between the micropore and mesopore, resuhing in one higher average chemical shift at 90.1 ppm and one lower chemical shift at 159.4 ppm. Therefore, the results of ^^^Xe NMR spectra show that the ZSM-5/MCM-41 composite possesses novel porous structures characteristic of the interconnection between the micropore and mesopore. 4. CONCLUSIONS Our results show that two types of composites, Beta/MCM-41 and ZSM-5/MCM-41, which have been prepared through different synthetic approach, are in possession of distinctive internal pore system. This information can't be obtained by any single conventional technique such as the nitrogen adsorption and demonstrates the usefulness of '^^Xe as a probe for studying internal pore structures with NMR spectroscopy. ACKNOWLEDGEMENT This work was supported by the National Natural Science Foundation of China (Grant No. 29733070) and financial support from the Center of Integrated Molecular Systems, POSTECH, Korea and the Brain Korea 21 Project is gratefully acknowledged. REFERENCES 1. J. Fraissard and T. Ito, Zeolites, 8 (1988) 350. 2. T. Ito and J. Fraissard, Stud. Surf Sci. Catal., 49 (1989) 579. 3. S.B. Liu, B.M. Fung, T.C. Yang, E.C. Hong, C.T. Chang, P.C. Shih, F.H. Tong and T.L. Chen, J. Phys. Chem., 98 (1994) 4393. 4. J.L. Bonardet, J. Fraissard, A. Gedeon and M.A. Springuel-Huet, Catal. Rev., 41 (1999) 115. 5. Q.J. Chen and J. Fraissard, J. Phys. Chem., 96 (1992) 1814. 6. S.B. Liu, J.F. Wu, L.J. Ma, T.C. Tsai and I. Wang, J. Catal., 132 (1991) 432. 7. W. Guo, L. Huang, H. Chen and Q. Li, Chem. J. Chinese Universities, 20 (1999) 356. 8. L. Huang, W. Guo, R Deng, Z. Xue and Q. Li, J. Phys. Chem. B, 104 (2000) 2817. 9. R. Benslama, J. Fraissard, A. Albizane, F. Fajula and F. Figueras, Zeolites, 8 (1988) 196. 10. J.M. Kim, J.H. Kwak, S. Jun and R. Ryoo, J. Phys. Chem., 99 (1995) 16742.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
371
Study of chromium species in the Cr-MCM-48 mesoporous materials by Raman spectroscopy Chanho Pak*', Hyouk Soo Han^ and Gary L. Haller" ^Materials and Devices laboratory and ^Analytical Engineering center, Samsung Advanced Institute of Technology, P.O. Box 111, Suwon, 440-600, Korea. FAX:+82-31-280-9308. E-mail: netgem(a)sait.samsuna.co.kr ^' Department of Chemical Engineering, Yale University, P.O. Box 208286, New Haven, CT 06520-8286, USA FAX: +1-203-432-4387. E-mail: gary.haller@yale.edu
Highly ordered Cr containing MCM-48 materials with different average pores size were prepared by a hydrothermal procedure using mixtures of cationic and neutral surfactants. The mesoporous structure of MCM-48 was proved by eight well resolved X-ray diffraction peaks and a sharp capillary condensation in the nitrogen adsorption isotherm. The main species of Cr was identified as a dichromate, Cr207'", in the MCM-48 materials, as revealed by Raman spectroscopy. 1. INTRODUCTION Mesoporous MCM-48 materials have attracted increasing attention as potential catalysts, adsorbents, supports and host materials because they have a high surface area and unique pore system, which is indexed in the space group laSd 'dnd is a bicontinuous, three-dimensional array of separate channels [1,2]. Incorporation of various transition metals into the framework or channels of MCM-48 has also been studied to develop catalytic activity for fme chemical applications [3-5]. Chromium-based catalysts are very important for the production of several chemicals and also used for selective oxidation of hydrocarbons [6,7]. Kawi and Te reported that MCM-48 supported chromium catalyst showed excellent activity for the oxidative destruction of trichloroethylene [7]. Characterization of the local environment of Cr in the catalysts is essential and requires various physicochemical methods because the activity and selectivity vary with the oxidation states and local structures. The direct incorporation and stability of Cr in the MCM-41, MCM-48 and KlT-1 mesoporous structures were investigated recently by X-ray diffraction (XRD), N2 adsorption and X-ray absorption by several authors [8-10]. It was suggested that the coordination of Cr in the mesoporous materials is tetrahedral and is revcrsibly transformed to octahedral structure by reduction and vice versa. In this paper, the further identification of Cr species in the MCM-48 materials, prepared by direct incorporation of Cr with difTerent mixtures of cationic and neutral surfactants, was studied by Raman spectroscopy.
372
2. EXPERIMENTAL Chromium-substituted MCM-48 (Cr-MCM-48) was synthesized by a modified method used for pure MCM-48 [1] with Cr(N03)2- 9H2O. The sodium silicate solution containing 9 wt% Si02 was prepared with colloidal silica Ludox HS 40 (Aldrich), doubly deionized water and NaOH. The cationic surfactants were n-alkyltrimethylammonium bromides (Aldrich), where the carbon number of alkyl chain was varied from 14 to 18. The neutral surfactant used was Brij-30 (Aldrich) for C14 and C16 cationic surfactants and Triton-X 100 (Aldrich,) for the CI8 cationic surfactant, respectively. Prior to mixing the surfactant solution with sodium silicate solution, drop by drop, Antifoam A, (Sigma, 0.0075 g) [11] was added to the surfactant solution for all samples in order to decrease excess foam from the surfactants. The pH control and heating procedure followed the literature specification [1]. An aqueous solution with dissolved amount of Cr(N03)3 • 9H2O corresponding to Si/Cr = 50, was added to the solution following the first pH adjustment. The resulting solid product was recovered by filtration, washed with doubly deionized water, and dried at ambient. Calcination of product was performed by heating to 823 K at IK/min and holding for 10 h in static air. Samples were named Cr-MCM-48C/7, where n is the alkyl chain carbon number of cationic surfactants. The experimental details of XRD and nitrogen adsorption have been described in an earlier report [8]. Raman spectra was obtained by a Renishaw Raman system 3000 spectrometer equipped with a holographic notch filter and an integral microscope. The 633 nm radiation from a 25mW air-cooled He-Ne laser was used as an excitation source. Raman scattering was detected with 180° geometry using a thermoelectric-cooled CCD detector. The Raman band of a silicon wafer at 520 cm"' was used to calibrate the spectrometer.
3. RESULTS AND DISCUSSION The XRD patterns of Cr-MCM-48 samples display eight well-resolved diffraction peaks corresponding to the cubic /^i^i structure [8]. The lattice constant, as listed in Table 1, increased with increasing carbon number of the alkyl chain of cationic surfactants from 9.20 to 9.83 and then to 10.6 nm for CI4, CI6, and CI8, respectively. The nitrogen adsorption isotherms of the samples showed a sharp capillary condensation (without hysteresis) in the range of 0.2 to 0.4 Table 1 The structure parameters for Cr-MCM-48 samples ^^ \ Sample Cr-MCM-48Cy'^
Surface area' ,^2^A 119
Pore size^ /^^ 3.2
Lattice Constant^ ,^ 9.20
Wall Thickness"* /^^ 1.4
Cr-MCM-48C76
1229
3.6
9.83
1.4
Cr-MCM-48C/(^
1046
3.9
10.6
1.5
' Calculated by BET method in the range of 0.05-0.15 relative pressures (p/po). ^ Evaluated by corrected BJH method [1]. ^ Obtained from d2i 1 spacing by ao = V6 • d2i 1. ^ Estimated according to the literature [2,8].
373
J\ HA
j\ „^^
400
^
1
J \
^^-—
b
J
600 800 1000 1200 Raman shift (cm') Fig. 1. Raman spectra of (a) CrOs, (b) K2Cr04 and (c) KzCrzOy
400
600 800 1000 1200 Raman shift (cm-1) Fig. 2. Raman spectra of Cr-MCM48C« samples, where n is (a) 14, (b) 16 and (c) 18.
relative pressure, which is a characteristic of Type IV isotherm and mesoporous structures [8-10]. The surface areas determined by the BET method for Cr-MCM-48C« samples ranged from 779 to 1229 m^g'' as listed in Table 1. These values are smaller than those of the respective pure MCM-48 samples [8]. Based on the XRD, sharp capillary condensation and high surface area, it was concluded that the samples have a highly ordered mesoporous MCM-48 structure [8]. The main features in the Raman spectra (Fig. 1) of CrOs, K2Cr04 and K2Cr207 were recorded in the range of 200 to 1000 cm'' as reported earlier [12]. The most intense peak of dichromate in K2Cr207 and polychromate in Cr03, which have Cr-O-Cr linkage is observed at 910 or 980 cm"', respectively. Polychromate species has another strong peak at 500 cm''. For Cr04^'in K2Cr04, the main peak is composed of four peaks at 852, 866, 880 and 905 cm''. Two weak peaks at 570 and 754 cm"', which can distinguish between Cr04^' and Cr207^', are attributed to the symmetric and asymmetric vibration of Cr-O-Cr in Cr207^' [12]. Below the 400 cm'', Cr04^' showed two peaks at 387 and 397 cm'', however Cr207^' displayed broad one peak at 384 cm''. All Cr-MCM-48 samples showed very similar Raman features after calcination. The main peaks at 902 and 955 cm'' are assigned to the vibration of CrOs fragment in a dichromate. Three peaks at 610, 505 and 371 cm'' suggest that the dichromate, Cr207^' is the main species in the samples. The peak at 505 cm'' is in a similar position as one of the strong peaks of polychromate. However, CrOs did not display the other two peaks around 610 and 370 cm'. Thus, it was suggested that polychromate could be excluded. A shoulder at 860 cm'' of Cr-MCM-48 samples indicates that the monochromate is present as a minor species. These two species contain tetrahedral Cr coordination. This agrees well with the XANES results of Cr-MCM-48 samples [8]. All samples showed the pre-edge peak at 5993 eV, which is a signature for tetrahedral coordination of Cr in the MCM-48 samples. After treatment of the samples in a H2 atmosphere at 823 K, the Raman signal, measured with He-Ne laser (633 nm) as a excitation source, of Cr-MCM-48 samples disappeared. This can be attributed the change of coordination by reduction of Cr as proved by XANES [8]. To detect the Raman signal from different structures of Cr, the excitation source was changed to 514.5 nm (Ar laser). The main peak of the Cr-MCM-48C7(5, after reduction was observed at 555 cm' by changing the excitation source. The other samples showed a weak signal around at 555 cm''. This peak corresponds to the main peak of Cr203 that have a Cr^^ ion with octahedral coordination, which indicates that the Cr in
374
the samples was reduced and changed its coordination simultaneously from tetragonal to octahedral. This interpretation confirms the previous results [10]. However, it is suggested that Cr has a distorted octahedral structure by the weak intensity of signal in the Raman spectra. The short bond length of octahedral Cr^"^ species in the channels of MCM-48 could allow the tetrahedral coordination of Cr^"^ in the dichromate to form easily by condensation of water in the oxidation atmosphere. Thus, the main species, dichromate in the samples could explain the facile redox process of Cr in the MCM-48 samples [8]. 4. CONCLUSION Chromium was incorporated into the highly ordered MCM-48 structure having different pore size by a hydrothermal procedure using mixtures of cationic and neutral surfactants. Specification of Cr coordination in the Cr-incorporated MCM-48 samples was deduced from Raman spectroscopy. All Cr-MCM-48 samples displayed the main features of dichromate, Cr207^" with a small contribution of chromate, Cr04^". The facile redox process of Cr in the MCM-48 samples may be attributed to this Cr structure. ACKNOWLEDGEMENT We are grateful to the Office of Basic Energy Science, DOE, for financial support and to the NSLS, Brookhaven National Laboratory for beam time. One of us (C. Pak) acknowledges the Korea Science and Engineering Foundation (KOSEF) for partial financial support for the post-doctoral fellowship. REFERENCES 1. M. Kruk, M. Jaroniec, R. Ryoo, S. H. Joo, Chem. Mater. 12 (2000) 1414. 2. K. Schumacher, R I. Ravikovitch, A. D. Chesne, A. V. Neimark, K. K. linger, Langmuir 16 (2000) 4648. 3. Ralf Kohn, Michael Froba, Catal. Today 68 (2001) 227. 4. M. S. Morey, S. O'Brien, S. Schwarz, G. D. Stucky, Chem. Mater. 12 (2000) 898. 5. K. Schumacher, M. Grun, K. K. Unger, Micropo. Mesopo. Mater. 27 (1999) 201. 6. B. M. Weckhuysen, I. E. Wachs, R. A. Schoonheydt, Chem. Rev., 96 (1996) 3327. 7. S. Kawi, M. Te, Catal. Today 44 (1998) 101. 8. C. Pak, G. L. Haller, Stud. Surf Sci. Catal., 135 (2001), 200. 9. C. Pak, G. L. Haller, Micropo. Mesopo. Mater. 44-45 (2001) 321. 10. C. Pak, G. L. Haller, Micropo. Mesopo. Mater. 48 (2001) 165. U . S . Lim, G. L. Haller, Appl. Catal. A: General 188 (1999) 277. 12. J. Ramsey, L. Xia, M. W. Kendig, R. L. McCreery, Corrosion Sci. 43 (2001) 1557.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
375
Covalent bonding of Disperse Red 1 in HMS silica: synthesis and characterization. B. Onida', L. Borello\ S. Fiorilli\ C. Barolo^ G. Visca^di^ D. J. Macquarrie^ and E. Garrone*. 'Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Torino, Italy. FAX: +39-011-564-4699. E-mail:garrone@athena.polito.it ^Dipartimento di Chimica Generale ed Organica Applicata, Universita di Torino, Torino, Italy. ^Department of Chemistry, University of York, Heslington, York, UK. Optical properties of the dye Disperse Red 1 covalently bonded to HMS surface via a onestep procedure are strongly affected by the polarity at the silica surface, which is modulated by the amount of residual template molecules and ethoxy groups formed during extraction in refluxing hot ethanol. 1. INTRODUCTION Ordered mesoporus silica can host organic molecules and polymers with conjugated chain structure which are known to be favorable for photonic and optical applications because of their large optical nonlinearity and rapid optical response'. Disperse Red 1 (2-[ethyl[4-[(4-nitrophenyl)azo]phenyl]amino]-ethanol, here referred to as DRl) and its derivatives have long been investigated as optical nonlinear dopants in polymeric matrix and in glasses obtained by sol-gel synthesis,^ as well as in self- assembled thin films produced by physical vapour deposition.^ As to crystalline hosts, Wark et al. have reported the encapsulation of DRl in HY via an in-situ (ship-in-the-bottle) synthesis procedure."^ In the present contribution the covalent bonding of DRl to the internal surface of HMS silica is reported. HMS silicas are prepared using an amine as a template, which can be removed by solvent extraction.^ This ^ allows the preparation of organicallyf modified mesoporous silica in a one-step ^ 0 \ ^ ^ synthesis.^ The recovery of the template I is also environmentally benign. For the N^^^^ *^^ ^ synthesis of the mesoporous hybrid 1 ^ 0 ^ material, a properly modified DRl is ^^' used, containing a triethoxy-group O^ --^^-^ /^^O (Scheme 1) which allows i I copolymerisation with tetraethyl O orthosilicate (TEOS). Scheme 1
376
A similar procedure has been used in the recent past to prepare an ordered organo-siHcasurfactant mesophase with a MCM-41 structure containing a covalently-Hnked chromophore, both as powder and a thin film.^ 2. EXPERIMENTAL DRl has been prepared and modified introducing the triethoxy functionahty according the procedure described in hterature."^ For the synthesis of the DRl-HMS hybrid material TEOS and modified DRl were added simultaneously to a stirred mixture of w-dodecylamine, ethanol and distilled water. The coloured mixture was stirred for 18 hours, yielding a thick suspension. This was filtered and dried at 80 °C for 1 hour. Filtrate was only slightly red coloured. The amine was removed by heating the solid (ca 10 g) at reflux in absolute ethanol (100 ml) for 6 hours. This extraction was repeated two times. After extraction the powder was coloured whereas the extracting solvent was colourless. This indicates that the dye is stably anchored to the silica matrix. The sample was characterised by XRD and BET analysis confirming the ordered structure, the high surface area (ca 1100 m^/g) and the mesoporosity of the silica (average pore diameter of 2.5 nm). TG-DTA, UV-visible and FTIR characterization were also carried out. The solvatochromic behaviour of the dye in various solutions was studied by UV-visible spectroscopy. 3. RESULTS AND DISCUSSION Figure 1 reports FTIR spectra of DRl-HMS after the first (curve 1) and the second (curve 2) extraction, outgassed at room temperature. In both spectra the absorptions due to silanols, both free (peak at 3747 cm"') and engaged in H-bonding Abroad band centred at 3500 cm" ) are observed. Also the typical bands of DRl^ are seen and J N^ labelled with asterisks: their presence confirms that the ]) functional groups are intact. /\ * *AN? In curve 1 absorptions //^ labelled with arrows are due to the presence of residual amine K\^ molecules: NH2 stretching modes are present in the range 3500-3250 cm"', together with y CH2 stretching modes at 2927 1 .^Ji___yV V ^ cm"' and 2855 cm''. The peak at 3500 3000 2500 2000 1500 3700 cm'' is probably due to ... ^ ^^^^e^^u silanols interacting with the
V
—i—/-
Wavenumbers (cm )
Fig. 1. FTIR spectra of DR1 -HMS second (curve 2) extraction
after the first (curve 1) and the
weight loss related to these species appears at about 290 °C.
n r
u •
r *u
•
T^U
alkylic chain of the amine. The presence confirmed
of by
surfactant is DTG analysis
reported in Figure 2, where the
377
The second extraction yields the complete removal of surfactant. In the IR spectrum (curve 2, Figure 1) absorptions due to amine molecules disappear, and the same occurs for the weight loss at 290 °C in the DTG pattern (curve 2, Figure 2). Together with removal of template molecules, extraction in hot ethanol causes the ethoxilation of the surface. C-H stretching modes of ethoxy groups are visible in curve 2 of Figure 1 (peaks in the range 29902900 cm"'). The weight loss of ethoxy groups is responsible of the peak at 550 °C in the DTG curve, which increases passing from curve 1 to curve 2 of Figure 2. Figure 3 reports UV-Visible spectra of the sample as-synthesized (curve 1) and after the first (curve 2) and the second (curve 3) extraction. A blu-shift of the absorption maximum is observed (from 510 nm to 484 nm), passing from curve 1 to curve 3.
,
3.5-
I'l \ *
3.0-
T r
2.5-
§ 2.0nj
100
200
300
400
500
600
700
i
1.5H
3
I
^
1 i,oJ
800
/' /l V;
\J/\\\J /
'\
0.5 J
Temperature cc)
Fig. 2. DTG analysis of DRl-HMS after the first (curve 1) and the second (curve 2) extraction.
V
0,0 J 200
300
400
500
600
700
800
Data concerning DRl solutions are reported in the Table 1, which suggests a solvatochromic Fig. 3. UV-Visible spectra of DRl-HMS asbehaviour involving a blue-shift with decreasing synthcsizcd (curve 1), and after the first the solvent polarity. The polarity of each solvent (curve 2) and the second (curve 3) extraction is expressed by the normalized molar electronic transition energy ET^ of dissolved Reichardt's dye, which ranges from 0.000 for TMS, the least polar solvent, to 1 for water, the most polar solvent.^ Table 1 solvent
toluene
CH2CI2
EtOH
2,2,2
(95%)
trifluoroethanol
X(nm)
472
484
488
508
ET^
0.096
0.321
0.654
0.889
On the basis of the blue shift observed in Figure 3, extraction seems to cause a decrease of the polarity of DRl environment in DRl-HMS. The more marked polarity of the system
378
extracted at a lesser extent is in agreement with the larger amount of water present, as shown by the weight loss at about 100 °C in Figure 2. This may be ascribed to the substitution of amine polar heads at the silica-surfactant interface in the as-synthesized sample with apolar aliphatic ends of ethoxy groups on the silica surface in the extracted material. Scheme 2 gives a pictorial description of these two different situations. as-synthesized
- ^ ^ y ^
C
NH^
^
^
OH
^ ^ ^ y ^
yn^ 0
Scheme 2 4. CONCLUSIONS DRl has been successfully anchored to the internal surface of a ordered silica mesoporous host of HMS type via a one-pot synthesis. Extraction of surfactant by hot ethanol yields ethoxylation of the silica surface. The polarity at the silica surface affects the chromophore and increases upon extraction. This may be a crucial factor when the system is considered for optical purposes. Preparation and characterization of the hybrid material in thin films, which are required for NLO applications, are in progress. REFERENCES 1. G. Schulz-Ekloff, D. Wohrle, B. van Duffel and R. A. Schoonheydt, Microporous and Mesoporous Materials, 2002, 57, 91. 2. D. Riehl, F. Chaput, Y. Levy, J.-P. Boilot, F. Kajzar, P.-A. Chollet, Chem. Phys. Lett., 1995, 245, 36 3. H. Taunaumang, Herman, M. O. Tjia, Optical Mater., 2001, 18, 343. 4. M. Wark, M. Ganschow, Y. Rohlfmg, G. Schuz-Ekloff and D. Wohrle, Stud. Surf. Sci. Catal., 2001, 135, 160. 5. P. T. Tanev and T. J. Pinnavaia., Science., 1995, 269, 865. 6. D. J. Macquarrie., Chem. Commun., 1996, 1961. 7. C. E. Fowler, B. Lebeau and S. Mann, Chem. Commun., 1998, 1825. 8. B. Lebeau, C. E. Fowler, S. R. Hall and S. Mann, J. Mater. Chem., 1999, 9, 2279. 9. C. Reichardt, Chem. Rev. 1994, 94, 2319.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
379
Accessibility of dye-molecules embedded in the micellar phase of hybrid mesostructured MCM41-type materials. B. Onida^ B. Bonelli\ L. Borello', S. Fiorilli^ S. Bodoardo', N. Penazzi^ C. Otero Arean^, G. Tumes Palomino^ and E. Garrone^ ' 'Dipartimento di Scienza dei Material] e Ingegneria Chimica, Politecnico di Torino, Torino, Italy. FAX: +39-011-564-4699. E-mail:garrone@athena.polito.it ^ Departamento de Quimica, Universidad de las Islas Baleares, 07071 Palma de Mallorca, Spain. Silica-based hybrid mesostructured materials (MCM-41 type) containing surfactantembedded Methylene Blue have been prepared and characterized by several techniques, including UV-Vis spectroscopy and cyclic voltammetry, and compared to other systems containing the same dye molecule. The micellar phase of the hybrid material was found to be permeable to protons, which may be of interest for potential applications in chemical and biological sensing devices. Introduction As recently reviewed by Scott and coworkers,' mesoporous materials are excellent hosts for sensing molecules, and dye-containing surfactant-silica mesostructured systems can also find application for NLO and lasers as well as photochromic materials. For both optical and sensing applications, permeability of the micellar phase and accessibility of guest molecules from the outer environment are critical factors. Very recently it has been observed that pH indicators embedded in the micellar phase of hybrid MCM-41-type systems are accessible to both molecules from the gas phase and ions from solution, and they can be reversibly protonated and deprotonated.^ In the present contribution the assessment of permeability is extended to the redox indicator methylene blue (MB) encapsulated in a mesostructured hybrid system. Activity and accessibility of the encapsulated molecules to electrons and protons have been investigated by means of cyclic voltammetry. Corresponding results were also obtained for the same molecule encapsulated in the zeolite X, and compared to those obtained for a MB solution. Redox mechanisms of MB, which differ at low and high pH, are described in scheme 1. The blue form, used for the synthesis, is the oxidized one. Experimental section The MB-doped hybrid hexagonal mesostructured material, denoted hereafter as MBMCM-41, was synthesized in basic conditions according to the procedure described in reference 2. The dye-free material (MCM-41) has been prepared following the same procedure, without adding MB to the synthesis solution. MB-containing X zeolite (MBX) was obtained by adding MB to the zeolite synthesis gel and following standard procedures. Samples have been characterized by XRD and UV-visible spectroscopy.
380
For cyclic voltammetry (CV), modified carbon paste electrodes have been prepared,^ by mixing a proper amount of the sample with graphite and a drop of dodecane. Measurements have been carried out using electrolytes at different pH values, ranging from 1 to 6. pH<5
(CH3),N
Results and Discussion Figure 1 reports the XRD pattern of MB-MCM-41 compared to that of MCM-41: typical peaks of the hexagonal phase are observed in both cases. A small difference in the dioo parameter is seen, suggesting that the cell parameter of MB-MCM41 (0,420 nm) is slightly smaller than that of the dye-free sample (0,444). This probably indicates that MB molecules are not at the core of the micelle, but act as cosurfactants, by intercalating their MB-MCM-41 amine heads between those of surfactant molecules during the MCM-41 micelle assembly. This leads to an increase of the average distance between polar heads and 8 2 theta consequently to a decrease in micelle diameter."* Figure 1. XRD patterns of MB-containing and MB-tree The UV-Vis spectrum (Figure mesosotructured materials. 2, curve 1) revealed the presence of monomers (component at 600 nm ) and dimers (component at 560 nm) as expected on the basis of the concentration of the synthesis solution (10~^ mol/1).^ Both absorptions are some 50 nm blue-shifted with respect of those known for MB in aqueous solution. This shift is too large to be ascribed exclusively to the encapsulation of the dye in the solid matrix.^
381
A blue-shift of MB absorptions may also be due to progressive demethylation of the molecule, yielding NH(CH3) and NH2 species/ The progressive demethylation is promoted in basic medium, like ammonia and amines solutions, and indeed the loss of methyl groups has been observed for MB encapsulated in zeolite X, as a consequence of the synthesis conditions/ The spectrum of MB-X (Figure 2, curve 2) shows broad absorptions, indicative of strong 1 host-guest interactions,^ v^ith a maximum at 560 nm and a shoulder around 650 nm. The former is ascribed to dimers of demethylated 2molecules and the latter to monomers of / ^ MB.^ The presence, however, of c / '' ^» 1 demethylated monomers (absorption around (0 600) cannot be ruled out. jQ In conclusion, in both MB-MCM-41 and MB-X, because of the basic synthesis condition, most of incorporated molecules '' contain at least one -NH(CH3) group. MB-X is stable to light, exposure instead of MB-MCM-41 to visible light causes the complete fading of the material. This is 0ascribed to the photochemical reduction of 400 MB by surfactant amines, yielding the leuco wavenumber (nm) form similarly to what was observed for MB in amine solutions.* This confirms the Figure 2. UV-Vis spectra of MB-MCM-41 embedding of MB molecules inside the (curve 1) and MB-X (curve 2) surfactant-silica mesostructure and their location in the proximity of polar heads. The CV curve of MB-MCM-41 is reported in Figure 3 (upper section). The composite material is electrochemically active, whereas the dye-free mesostructured system is not, as expected (Figure not reported). The electrochemical activity of MBMCM-41 is stable, even after 24 hours of cycling. Cathodic and anodic peaks are broader than those observed in CV curve of MB solution, indicating that the kinetics of the redox process is affected by the immobilization of MB. The midpoint potential, defined as the average of cathodic (Epc) and anodic (Epa) potentials, is -0.23 V, very close to that observed for MB in solution (-0.22 V). This indicates that the electrochemical behaviour is not significantly affected by the presence of the micelles.'^ The CV of MB-X (Figure 3, lower section) is different from that of MB-MCM-41. Epc and Epa values and the midpoint potential (-0.38 V) are shifted to more negative values, indicating that the reduction of MB molecules is more difficult when they are included within zeolite X with respect to MCM-41. This may be ascribed to the high local positive charge density, due to presence of sodium ions, experienced by the MB molecules inside the zeolites supercages,^ which destabilizes the double positive charged species (reduced form, scheme 1). Indeed CV experiments conducted at pH = 6 yield curves for MB-MCM-41 and MB-X (not shown) showing no significant differences. At pH > 5 the reduced form is uncharged and the MB reduction is less affected by the local charge of the matrix. ^Vs.^^\
382
Besides the two main redox peaks, two couples of less intense peaks are seen, at higher and lower potentials. The former (labelled with asterisks) is ascribed to external MB molecules at MB-MCM-41 Epc the matrix/graphite interface, since it was observed also with a similar paste electrode obtained by mixing graphite with a few drops of MB solution, instead of the sample powder. A y * couple of peaks with the same midpoint potential, though less F defined, is observed for MB-MCM-41 curve. MB-X The couple of peaks at lower potentials (labelled with dots), absent * in the MB-MCM-41 curve, is ascribed to MB molecules which experience a pH gradient. Indeed, Epc and Epa are pH-dependent and shift to more * negative values with increasing pH. • The gradient of pH inside the zeolite matrix is probably due to diffusion -0.2 0.1 limitations. Voltage (V)
' 1
Figure 3. CV curves for MB-MCM-41 (upper abscissa) and MB-X (lower abscissa)
Conclusions Methylene blue molecules embedded both in surfactant-silica MCM-41 mesostructures and in zeolite X are electrochemically active, so confirming their accessibility to protons involved in the redox mechanism. The presence of surfactant does not affect significantly the electrochemical activity of MB. Immobilization affects instead the kinetics of the redox process with respect to solution. With acidic electrolytes (pH < 5) the reduction is more difficuh for MB-X than for MB-MCM-41. References (1) B. J. Scott et al., Chem. Mater, 2001, / i , 3140. (2) B. Onida, B. Bonelli, L. Flora, F. Geobaldo, C. Otero Arean and E. Garrone Chem. Com/www., 2001,2216. (3) S. Bodoardo et al., Electrochem. Commun., 2000, 2, 349. (4) F. Di Renzo, F. Testa, J.D. Chen, H. Cambon, A. Galameau, D. Plee, F. Fajula, Micr. Mesop. Mater, 1999, 28, 437. (5) K. Patil, R. Pawar and P. Talap Phys. Chem. Chem. Phys., 2000, 2, 4313 (6) S. Wohlrab, R. Hoppe, G. Schulz-Ekloff; D. WOhrle, Zeolites, 1992, 12, 862. (7) R. Hoppe, G. Schulz-Ekloff; D. W5hrle, C. Kirschhock, H. Fuess, L. Uytterhoeven and R. Schoonheydt, Adv. Mater, 1995, 7, 61. (8) F. C. Schaefer and W. D. Zimmermann, Nature, 1968, 220, 66. (9) G.C. Zhao, J.J. Zhu, J.J. Zhang, H.Y. Chen, Anal. Chim. Acta, 1999, 394, 337
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Influence of surface properties of MCM-48 on the formation of a nanocomposite structure based on MCM-48 and PVA Jing HE" Jia YANG'
Shichao ZhANG^
D.G.EVANS'
Xue DUAN*'
'The Key Laboratory of Science and Technology of Controllable Chemical Reactions, Beijing University of Chemical Technology, Ministry of Education, Beijing 100029, China ^Beijing University of Aeronautics and Astronautics, Xueyuan Lu, Beijing, China The influence of the surface properties of MCM-48 on the formation of a nanocomposite structure based on MCM-48 and PVA has been investigated. The structure of the PVA/MCM-48 nanocomposite has been characterized by FT-IR, XRD and N2 adsorptiondesorption techniques. A hydrophobic surface favors the introduction of PVA inside the mesopores of MCM-48, producing a nanocomposite structure in which the organic phase inside the nano-sized pores is isolated by the inorganic phase. INTRODUCTION The conventional way to prepare nanocomposite materials is to mix nano-dimensional inorganic particles with a polymer matrix. Aggregation of the inorganic particles as well as the distinct differences in surface character between the discontinuous inorganic phase and the continuous organic matrix however, has an adverse effect on the dispersion of the former and the interphase compatibility. The use of intercalation reactions of preformed polymers and in-situ polymerization of intercalated monomers only ameliorates these problems to some extent^^'^l The development of an additive which exhibits good interphase compatibility with a polymer matrix is an important goal. The class of mesoporous M41S materials, including MCM-41, MCM-48 and MCM-50^^^, possesses tailorable nano-sized pores together with nano-thickness pore walls. The large diameters of the channels and their tailorability provide new opportunities for construction of nano-sized architecture. It can be expected that a polymer can either be introduced directly or produced through in-situ polymerization of organic monomers inside the mesopores to form nanocomposites based on the MCM-41 and MCM-48 structures. The organic phase extends along the channels to the openings in the nanocomposite structure, optimizing the interaction between the nanocomposite and a bulk polymer matrix. A nanocomposite structure based on MCM-41 and polyethylene has been formed through in-situ polymerization of ethylene in our previous work^'^l This paper reports a nanocomposite structure based on MCM-48 and polyvinyl acetate (PVA). The influence of the surface properties of MCM-48 on the formation and structural characteristics of the resulting nanocomposite structure is investigated. ^ This work was supported by the National Science Foundation of China (Grant no. 59973004).
384
2. EXPERIMENTAL MCM-48 was synthesized under hydrothermal conditions using TEOS as Si source and cetyltrimethylammonium bromide (CTABr) as template. TEOS was first pre-hydrolysed with NaOH solution and then added dropwise to the 25 wt% aqueous solution of CTABr. The final gel composition was lSi02 • 0.21Na2O : 0.65CTABr : 62H2O. The mixture was heated at 373 K for 7 days, and then filtered and dried at 373 K overnight to give the as-synthesized MCM-48 samples. The inorganic-organic nanocomposite structure based on MCM-48 and poly (vinyl acetate) was produced by two procedures. In one procedure (method 1), as-synthesized MCM-48 was first silylated with (CH3)3SiCl and then used as the inorganic host. In the other procedure (method 2), as-synthesized MCM-48 was first calcined at 773 K for 10 h and then used as the inorganic host. PVA was introduced by reaction of the inorganic host with a solution of PVA in ethanol. Powder XRD patterns were obtained using an XRD-6000 instrument with Cu Ka source, step size of 0.02° and scan rate of l°/min. The low-temperature N2 adsorption-desorption and vapor adsorption-desorption experiments were carried out using an Autosorb-1 system. The FT-IR spectra were recorded on Bruker Vector 22 spectrometer (resolution 4 cm''), the samples being pressed into disks with KBr. TEM micrographs were taken on a JlM-2010 transmission electron microscope. 3. RESULTS AND DISCUSSION 3.1. Structure and surface properties of the IVICIVl-48 inorganic host The structural characteristics of the MCM-48 inorganic host were investigated by XRD, TEM and N2 adsorption experiments. XRD patterns show that both calcined and silylated MCM-48 exhibit the reflections characteristic of a well-ordered cubic structure'^l The well-ordered arrays of channels can be clearly observed by TEM as shown in Figure 1. The pore structural parameters given in Table 1 indicate that both calcined and silylated MCM-48 samples possess a narrow distribution of nano-sized pores, large specific surface areas and high mesopore volumes. The above structural characteristics suggest that both calcined and silylated MCM-48 possess the properties required for an inorganic host in a nanocomposite with an organic phase. Table 1 Pore structural parameters of calcined and silylated MCM-48 Samples
Pore volume (cm^/g) Total Mesopore
Pore diameter (nm)
Surface area (m /g)
Calcined MCM-48
2.5
1203
1.28
0.62
Silylated MCM-48
Z5
[112
096
0.54
The surface properties of calcined and silylated MCM-48 were evaluated by water and isopropanol vapor adsorption experiments. The water and isopropanol vapor adsorption-
385
desorption isotherms of both calcined and silylated MCM-48 are typical of type IV. The water vapor adsorption-desorption isotherms of silylated MCM-48 exhibit a closed hysteresis loop characteristic of a hydrophobic surface, whereas the hysteresis loop for calcined MCM-48 is open which is characteristic of a hydrophilic surface. The data given in Table 2 reveal that the maximum adsorption volume of water on calcined MCM-48 is more than 4 times higher than that on silylated MCM-48 based on either weight (g) or surface area (mVg). Moreover, calcined MCM-48 has a higher adsorption volume for water than for isopropanol, whilst silylated MCM-48 has much a higher adsorption volume for isopropanol than for water. These results indicate that silylated MCM-48 possesses more a hydrophobic surface than calcined MCM-48. Table 2 Adsorbed volumes of water and isopropanol vapor on MCM-48 Maximum volume of Maximum volume of Surface area « . j . j i. j • i Adsorbents ^ adsorbed water vapor adsorbed isopropanol vapor ^^'^^ calcined MCM-48
1203
(cmVg)
(cmVm^)
(cmVg)
(cmVm^)
1301
1.08
321
0.267
silylated MCM-48 1172 25.7 2.19x10'^ 251 0.214 3.2. Structure of the nanocomposite based on PVA/MCM-48 The FT-IR spectra of nanocomposites based on MCM-48 and PVA show vibrational bands characteristic of-(CH2)n groups at 2930cm"' and 2860cm', -CH3 groups at 2950cm'', C-0 groups in -COOH moieties at 1370cm' and 1240cm"', and C=0 groups in -COOH moieties at 1740cm"'. The FT-IR results indicate that PVA as organic phase has been integrated with MCM-48 as inorganic host. The XRD patterns of the nanocomposites based on MCM-48 and PVA exhibit the reflections characteristic of a well-ordered cubic structure similar to MCM-48 itself, as shown in Figure 2. Figure 3 shows the N2 adsorption-desorption isotherms of the nanocomposite produced by method 1. For comparison, the isotherms of silylated MCM-48 are also shown. In contrast to silylated MCM-48, the PVA/MCM-48 nanocomposite shows an adsorption isotherm typical of type III, indicating that the mesopores have been fully occupied by PVA. The specific surface area decreased dramatically from 1173m^/g for silylated MCM-48 to 8 m^/g for the PVA/MCM-48 nanocomposite, a percentage loss of greater than 99%, further suggesting the complete occupation of mesopores by PVA. The N2 adsorption-desorption isotherms of the PVA/MCM-41 nanocomposite produced by method 2 (not shown here) are similar to those of calcined MCM-48, and the specific surface area decreased from 1203 m^/g to 522 m^/g, indicating that method 1 is more favorable for the formation of the hybrid between MCM-48 and PVA than method 2. The mesopore surface of the calcined material is hydrophilic due to the presence of hydroxyl groups, whilst the mesopore surface of silylated MCM-48 is hydrophobic resulting from the transformation of hydroxyl groups to -0-Si(CH3)3 groups. The surface of silylated MCM-48 exhibits a stronger affinity for PVA than that of calcined MCM-48, and as a result the former favors the formation of the inorganic-organic nanocomposite structure.
386
L '_ ' : ) A
a
^-^---v__ 4
b
1
,"
1
5
26/° Fig. 2. XRD patterns of (a) siliceous MCM-41 and nanocomposites based on MCM-41 and PVA formed by (b) method 2 and (c) method
Fig. 1. TEM micrograph of silylated MCM-48
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
P/Po P/Po Fig. 3. N2 adsorption-desorption isotherms of (a) silylated MCM-48 and (b) the PVA/MCM-48 nanocomposite formed by method 1 REFEFENCES 1. R.Schollhom, Chem.Mater., 8 (1996), 1747 2. Y.Chen, X.Y.Wang, Z.M.Gao, X.G.Zhu and Z.N.Qi, Acta Polymerica Sinica, 1 (1997), 73 3. J.S.Beck, C.Vartuli, W.J.Roth, M.E.Leonowicz, C.T.Kresge, K.D.Achmitt, C.T-W.Chu, D.H.Olson, E.W.Sheppard, S.B.McCuUen, J.B.Higgins and J.L.Schlenker, J.Am.Chem.Soc, 114(1992), 10834 4. J.He, D.G.Evans, X.Duan and R.F.Howe, J.Porous Mater., 9 (2002), 49
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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A study on the structure of Si-O-C thin films with nano size pore by ICPCVD Teresa Oh', Kwang-Man Lee^ and Chi Kyu Choi Research Institute of Advanced Technology \ Faculty of Electrical and Electronic Engineering, Research Institute of Advanced Technology^, Dept. of Physics, Cheju National University, Jeju 690-756, Korea, FAX: +82-64-756-3506. E-mail: cckvu@cheiu.ac.kr Si-O-C thin films with a low dielectric constant were deposited on a p-type Si(lOO) substrate by an inductively coupled plasma chemical vapor deposition (ICPCVD). Bistrimethylsilylmethane (BTMSM, H9C3-Si-CH2-Si-C3H9) and oxygen gas were used as precursor. Hybrid type Si-O-C thin films with organic properties have been generated many pores by annealing. Therefore, the low dielectric behavior of the Si-O-C films can be explained by mean of attractive (electrostatic) and repulsive (steric) forces from inductive effect. The characteristic analysis of Si-O-C thin films was performed by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The dependence of the bonding structure on thermal annealing was studied. 1. INTRODUCTION A low dielectric material has been required for the multilevel interconnection of the ULSI (ultra large scaled integrated circuits) devices for the device performance. The low dielectric (low-/:) material is used to reduce propagation delay times, cross-talk noise between metal layers, and power dissipation from RC coupling[l,2]. Since the development and integration of those new low dielectric materials have been examined recently, the hybrid-type film mixed with organic and inorganic materials is being examined as possible candidates suitable to low-^ material in ultra high density device integration. One of the hybrid-type low-/: material is Si-O-C thin films, which is a promising candidate for low-/: material with stable thermal properties[3~7]. The bonding structure of organic materials is induced by the electronegativity, resonance and inductive effect[8]. It was reported that the compound with hydrogen bonding and high electronegative atom had a chemical shift[9,10]. In this work, SiO-C thin films were investigated by a chemical analysis and it was discussed about the correlation between the chemical shift and variation of the bonding structure by annealing. 2. EXPERIMENTAL The Si-O-C thin-films were deposited on the p-type Si(lOO) substrate by ICPCVD with rf power of about 300W atl3.56MHz. Precursor mixed with BTMSM and O2 was used. The flow rate ratio of 02:BTMSM was 10 seem : 10 seem. Working pressure was 450 mTorr and the substrate was not heated. To improve thermal stability of the films, these were annealed at 400 °C and 500 °C for 30 minutes in vacuum and then analyzed. The dielectric constant of the film was measured by C-V measurement using a MIS (metal-insulator-semiconductor) structure. The structural study of Si-O-C thin films mixed with organic-inorganic properties
388
was performed by FTIR and XPS. The relationship between relative dielectric constant and the chemical structure was investigated from these analyses. 3. RESULTS AND DISCUSSION Figure 1 shows the FTIR survey scan spectra of as deposited film, annealed film at 400 °C and annealed film at 500°C, respectively. The FTIR spectra have strong peak near 1000cm"', and the OH related bonds show broad peak about 3300 ~ 4000 c m ' owing to hydrogen bond.
^
0 40 0 35
Oj:BTMSM-10:10
0 30 0 25 0 20 0 15
UiD
• M
-
l.
005
annealed at 400*C ^
c-o-a
0-H(Hydrogen bond)
000
as-deposite film 500
-
annealed at 500*C -
/I
0 10
"
1000 1500 2000
2500
3000
3500
4000
-j 1 4500
Wav* numbertcm')
Fig. 1. FTIR spectra of Si-O-C thin films with various annealing temperature. Figure 2 shows the FTIR spectra of the films from lOOOcm' to 1350 cm"'. The band due to the Si-O-R is found in the frequency range, 1000cm"'-1100cm"', and the band due to CH3M(methyl-metal) is found in the frequency range, 1100~ 1350cm"'. The CH3-M bond can be formed by attaching alkyl group to a metal atom(or P, S, Si)[7,8]. Compounds containing methyl-metal bond give rise to the normal CH3 vibrations, and its structure is described by inductive effect due to the electron density. The effect is divided into positive inductive effect by electron-releasing due to the electron affluence by a bonding (0 or 71 bond), and negative inductive effect by electron-withdrawing such as the electron deficiency by an antibonding (o* or, 71* bond)[8]. The FTIR spectrum of as-deposited film of Fig. 2(a) showed broad C-O-Si annealecl at 400*C
1300
1400
1300
1400
900
(a) As deposited film, (b) Annealed film at 400 °C, (c) Annealed film at 500 °C. Fig. 2. The FTIR spectra of Si-O-C thin films in the range of 1000~1350cm'' with various annealing temperature.
389
bond between 1000cm*' and 1350cm'', which was overlapped the Si-O-R bond and CH3-M bond. Because the as deposited film was mainly occurred the electron-withdrawing owing to C-H 0 bond. However, the FTIR spectra of the annealed film shown in Fig 2(b) and (c) were shifted by the electron-releasing due to the C-H o bond and n* bond. One H2O molecule and an oxygen atom were generated by two 0-H bond coupling in the annealing process. Then the oxygen attached to silicon and the H2O evaporated. The films decreased the electron density due to H2O evaporation, and the CH3-M bond was attacked by the repulsive force of 71 antibonding (71 bond) owing to the electron deficiency. Therefore the CH3-M bond weakened. The ji* bond formed the Si-0 bond instead of CH3-M bond by this repulsive force[8]. The SiO bond surrounded a void by the steric effect of the repulsive force. The electron-releasing of the carbon-hydrogen bond in annealed films shown in Fig. 2(b) and (c) showed that the FTIR spectra of annealed films between 1000cm"' and 1250cm'' shifted to high frequency. The peak due to the C-O-Si of the annealed films shifted to higher frequency than that of as-deposited film by 32.4cm'' and the intensity of the peaks increased by increasing the annealing temperature as shown in Fig. I. The spectra due to CH3-M of annealed films were decreased. Because the peak of CH3-Si bond divided into the CH3 related bond and Si related bond, and the CH3 related bond combined with the Si-O-R bond, and the Si related bond made the Si-0 bond. Therefore, the carbon related bond between 1000cm' and 1250cm'' increased by thermal annealing and the annealed film might have pores between repulsive force by the CH3-M bond and attractive force by alkyl group. The dielectric constant of as-deposited film was 3.8 and those of annealed films were 3.1 and 2.8 at 400°C and 500°C, respectively. Decrease of dielectric constant of the annealed films can be explained by both H2O evaporation and increasing the relative carbon concentration by annealing. Figure 3 shows the Si 2p electron orbital spectra by XPS in Si-OC thin films. In order to investigate the analysis of content, XPS spectrum was deconvoluted by fitting the data with a number of Gaussian peaks. By comparison as deposited film with annealed films, the peak position of as deposited film was 102.0 eV, while that of annealed film at 400 °C was 102.2 eV, and that of annealed film at 5 0 0 t was 103.2 eV.
Binding EnergyfeV]
Binding Energy(eVl
(a) As deposited
film,
(b) Annealed film at 400 °C,
Binding EnergyfeV]
(c) Annealed film at 500 °C
Fig. 3. The Si2p electron orbital spectra in Si-O-C thin films by XPS.
390
The bonding structure of annealed films became harder than that of as-deposited film because of shifting to the high bonding energy. As previously mentioned, it was verified that the bonding structure of annealed film involved the recombined Si-0 bond, such as the silicate-wall by oxygen attaching to silicon. Consequently, it is said that the annealing process is essential to decrease the dielectric constant of Si-O-C thin films, and to improve the hardness of the film owing to the surrounding pores by the silicate wall. The relationship between Si-O-C thin film with a pore and annealing process was explained by the attractive (electrostatic) force due to the C-H o bond with repulsive (steric) force due to the TC* bond, therefore the dielectric constant of the films decreased. 4. CONCLUSION Si-O-C thin films with the O2:BTMSM=10sccm:10sccm flow rate rado showed that the bonding structure of the films was changed by variation of the electron density due to H2O evaporation by thermal annealing. The peak of the C-O-Si bond of annealed film shifted to higher frequency than that of as-deposited film by 32.4 eV. The as-deposited film was only formed by the electron-withdrawing due to the C-H 0* bond, but the annealed film was formed by a balance between attractive force due to the C-H 0 bond with repulsive force of the CH3-M which was attacked by the ir bonding. These bonding structures of annealed film made the Si-O-C thin films with pores and the film had low dielectric constant by increasing the relative carbon concentration. This work was supported by grant No. Ml-0104-00-0071 from the National Research Laboratory Program of the Ministry of Science and Technology, and by grant No. ROl-1999000-00092-0 from Korea Science and Engineering Foundation. REFERENCES 1. M. A. Tamor, C. H. Wu, J. Appl. Phys. 67(2) (1990) 1007. 2. Yoshiaki Ohkawara and Hidetoshi Saitoh, Jpn. J. Appl. Phys. 40(12) (2001) 7007. 3. A. Grill, V. Patel, J. Appl. Phys. 85 (1999) 3314. 4. Y. H. Kim, K. Lee, and H. J. Kim, J. Vac. Sci. Technol. A18 (2000) 1216. 5. Shou-Young Jing and Chi Kyu Choi, J. Korean Phys. Soc. 39 (2001) 302. 6. Y-H Kim and H-J Kim, J. Korean Phys. Soc. 30 (2002) 94. 7. George Socrates, Infrared and Raman characteristic group frequencies, John Wiley & Sons, Ltd. (2001)243. 8. Robert V. Hoffman, Organic chemistry, Oxford University Press. (1997) 289. 9. X. Li, L. Liu, and H. B. Schlegel, J. Am. Chem. Soc. 124 (2002) 9639. 10. Sofie N. Delanoye, Wouter A. Herrebout and Benjamin J. Van der Veken, Am. Chem. Soc. 124(2002)7490.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
391
Template effects on low k materials made from spin-on mesoporous silica Chih-Yuan T i n g \ De-Fa Ouyan^ Wen-Fa Wu*' and Ben-Zu Wan^'* ^Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 106, R.O.C. National Nano Device Laboratories, Hsinchu, Taiwan 300, R.O.C. Mesoporous silica films prepared through surfactant-templating process is one of promising candidates as low-dielectric-constant (k) materials, which is important in the future IC industry. In this paper, the difference of surface morphology and that of electrical properties of mesoporous silica films prepared with different templates are reported. It is suggested that tween is a superior surfactant and a template for making porous silica low k films. 1. INTRODUCTION As the packing density of metal lines on semiconductors continues to increase, accompanied problems such as cross-talk noise and propagation delay become more serious. Using interlevel dielectric films with k<2.5 is one of the possible ways to solve these problem. Recently it has been reported that porous silica is one of the pormising materials to meet this requirement.' For the porous silica, the dielectric constant is a combination of that of air (about 1) and that of solid Si02 (3.9-4.5); therefore, the k values of silica films can be reduced by making pores in the material. Mesoporous films can be prepared through surfactant-templating process, and show great potential to be applied in the future IC industry. In this research, TPABr, C16TMAB, P I 2 3 , and Tween series compounds were chosen to prepare low-k films. Each represents a particular type of template. TPABr was the smallest template used in this research and is generally used for making microporous zeolite (Le. ZSM-5). C16TMAB is an ionic surfactant. It is one of the alkylammonium salts, CnH2n+i(CH3)3-NBr (n = 8, 10, 12,14,16,18), acting as templates for making mesoporous silica. PI23 (a tri-block-copolymer) and Tween series surfactants (polyoxyethylene(20) sorbitan compounds) are two kinds of non-ionic surfactants. They were also used to synthesize mesoporous materials in the past.^ However, the template effect on mesoporous films prepared by spin-coating process have not been investigated yet. In order to prepare and improve the quality of mesoporous silica low-k films, it is worth a detailed examination. 2. EXPERIMENTAL The coating solution was prepared by mixing tetraethyl orthosilicate (TEOS), deionized water, ethanol, hydrochloric acid and templates at room temperature for 3 h. After the coating solution was spin-coated on a silicon wafer (4 inch or 6 inch), the wafer was baked at
392
106 °C and then was calcined in air. Finally, the film surface was modified to be hydrophobic by immersing it in a HMDS/tolune solution. The Capacitance, the leakage current density and the elastic modulus were measured in National Nano Device Laboratory in Taiwan. Capacitance measurements were performed with a Keithley Model 82 CV meter. The frequency and the oscillation level were IM Hz and 100 mV, respectively. The dielectric constant was calculated from the capacitance, the film thickness and the area of electrode. The elastic modulus was obtained by a Nano indenter XP system (MTS). The film surface morphology and thickness were obtained by scanning electron microscopy (SEM) taken on an S-800 (Hitachi). The low angle XRD experiments were conducted in Synchrotron Radiation Research Center in Taiwan. 3. RESULTS AND DISCUSSION Table 1 shows the dielectric constants of films made from different templates, when the weight ratio of template to TEOS in the coating solution is 0.41. It is shown that the dielectric constant of the film using TPABr as templates was much higher than those of the others. This is because TPABr is not a surfactant and cannot form micelles in the solution. Thus only a few micropores may be formed in the film. As a result, silane molecules were not able to diffuse into these pores for modifying silanol surface, which causes water adsorption and high film dielectric constant. For films using CiaTMAB, PI23 and Tween 80 as templates, the dielectric constants were lower than that of a dense silica film. It suggests that mesoporous films were successfully prepared by these three surfactants. Table I K values (dielectric constants) of films prepared by different templates. Templating agents k value TPABr 8.5 C16TMAB 2.5 P123 2.5 Tween 80 L8 Although the dielectric constant of the film using CieTMAB as templates can be below 3.0, charged ions in ionic surfactant (CIGTMAB) can hardly be removed during the preparation process, and may interfere the electrical performance. From this point, ionic surfactants are not suitable for templating nanoporous dielectric films. Therefore, in the later part of this research, nonionic surfactants such as Tween series surfactants and copolymer (PI23 in this research) were chosen for detailed study. In order to prepare films with lower k values, the weight ratio of surfactant over TEOS was raised from 0.41 to 0.81. It was found that films templated with PI23 cracked severly, as shown in Figure 1. (a). On the other hand, films templated with Tween 20 and Tween 80 remained crack-free all the time, as shown in Figure I. (b), (c). The crack on the film was probably resulted from the solubility problem of PI23 polymers in the coating solution, and from less significant difference of the hydrophilic and the hydrophobic groups in PI23. The crack was more pronounced, when more PI23 were in the coating solution for making films with higher porosity. Therefore, the better surfactant property of Tween can cause a higher solubility and a better interaction with silanes in the coating solution, which resulted crack-free films.
393
Fig. 1. Top view of films, (a). Templated with P123 ; (b). Templated with Tween 20 ; (c). Templated with Tween 80. The weight ratio of surfactant over TEOS is 0.81. The cracks in (a) are apparent. Table 2 K values and elastic modulus of films prepared by different nonionic surfactants. Templating agents k value Elastic Modulus (GPa) P123 1.87 6 Tween 20 1.47 10 Tween 80 : Severe cracks were observed. Table 2 shows the dielectric constants and the elastic modulus of films when the weight ratio of template to TEOS is 0.81. It was found that films templated with Tween 80 possessed an ultra low k value of 1.47 and a modulus value of 10 GPa. Films templated with Tween 20 exhibited a low k value of 1.87 and a modulus value of 6 GPa. It is noticed that for a dense chemical vapor deposition silica film used in the current industry, the modulus is about 70 GPa. And a modulus of 6GPa is usually considered to a threshold value for industrially viable low-k dielectrics^. Therefore, for films prepared with Tween 20 and Tween 80, the elastic moduli are acceptable. 5 0.12 Furthermore, films with k values lower than 2.0 can be obtained. On the other hand, the weight ^ 0.08 After baking ratio of PI23 to TEOS in the coating solution is limited because of film cracks. As a result, 0.00 films with lower k values prepared with PI23 were difficult to acquire through the process 1.5 2.0 presented in this research. Thus it is suggested 29 / degrees that Tween series surfactants are superior templates for preparing spin-on mesoporous films. Fig. 2. X-ray diffraction patterns from films templated by Tween 80. Since mesopores were formed in the film, it The shoulder near 1 degree is the is interesting to examine that whether these diffraction signal of the film. pores were arrayed orderly. To examine the pore arrangement, low angle XRD
394
characterizations were adopted, and the grazing incidence X-ray diffraction technique was applied. Fig.2 shows the results from the samples using Tween 80 as the template. There are two spectra in the figure, one is from that while the film was just coated on the silicon wafer, the other is from that after the coated wafer was baked at 106°C. It can be found that both spectra possess strong X-ray bands when the 29 angles are smaller that 0.6°. From the calculation, they represent the reflection of X-ray from the surface of silicon ; therefore, they cannot be included for the diffraction analysis. Moreover, there are peaks ranged in 26 values between 0.7 and 1.2. Although the intensities are rather low, these peaks may represent the X-ray diffraction from the film templated by Tween 80. And the broad X-ray diffraction indicates that the d-spacing of the detected porous structure are ranged between 6 to 10 nm. Furthermore, the results in Fig.2 show that the X-ray diffraction was less pronounced after baking, which suggests that the porous structure became more disordered during baking process. Thus it can be concluded that pores in the film are more likely to be randomly distributed, because the low intensity of the X-ray diffraction patterns. The lack of periodic structure in the film is reasonable since spin-coating process has been applied. During spin coating, the rapid solidification of the solution and the intense shear force in it prevent the formation of ordered mesostructures. 4. CONCLUSION In this research, four kinds of templates were attempted for the preparation of spin-on mesoporous silica films. When the weight ratio of template to TEOS was 0.41, films with low-k values were obtained except for films using TPABr as a template. This is because TPABr can not form micelles in the solution. To acquire films with ultra low dielectric constants, the weight ratio of template to TEOS needs to be increased. However, films templated with PI23 cracked severely when the ratio is increased, while films templated with Tween series surfactants remained crack-free and possessed superior properties (k = 1.47, elastic modulus = 10 GPa). The cracks may be resulted from the solubility of PI23 in the coating solution. From this research, Tween series surfactants are suggested to be the better templates for the preparation of spin-on mesoporous low-k films.
REFERENCES 1. S. J. Martin, J. P. Godschalx, M. E. Mills, E. O. Shaffer II, and P. H. Townsend, Adv. Mater., 23(2000) 1769. 2. D. Zhao, Q. Huo, J. Feng, B. F. Chmelka, and G. D. Stucky, J. Am. Chem. Soc, 120(1998) 6024. 3. Z. Wang, A. Mitra, H. Wang, L. Huang, and Y. Yan, Adv. Mater., 13(2001) 1463.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
395
Porosity tuning of single-wall carbon nanohorns with gaseous activation E. Bekyarova", K. Murata", K. Kaneko^'', D. Kasuya^ M. Yudasaka' and S. lijima"'^'" ^JCORP-JST, c/o NEC Corporation, 34 Miyukigaoka, Tsukuba, Japan ^Department of Chemistry, Faculty of Science, Chiba University, Chiba, Japan *^Center for Frontier Electronics and Photonics, Chiba University, Chiba, Japan ^Meijo University, 1-501 Shiogamaguchi, Tempaku, Nagoya, Japan ^ E C Corporation, 34 Miyukigaoka, Tsukuba, Japan In this paper we discuss the effect of the heat treatment in oxidizing atmosphere, O2 and CO2, on the pore structure of single-wall carbon nanohorns (SWNHs). The pore structure of SWNHs could be controlled by varying the oxidizing gas, temperature and time of treatment. Both treatments, heating in O2 and CO2, enhance the microporosity. We have found that treatment in CO2 effectively develops the mesoporosity in SWNHs. 1. INTRODUCTION The discovery of carbon nanotubes (CNTs) in 1991 [1] opened new opportunities for the scientists interested in quasi one-dimensional physics and chemistry. The great advance in understanding the growth mechanism [2] led to a wide availability of nanotubes and consequently to progress in the research concerning their properties and potential applications. SWNHs is a carbon nanotube-related material synthesized in large quantities by laser ablation of graphite at room temperature in absence of a catalyst [3]. In addition to the high production rate (50 g/h), the high purity and the possibility to control the structure by the synthesis conditions [4] make this nanostructured material especially attractive for adsorption application and fundamental studies of nanoconfmement phenomena. SWNHs consist of a single graphene sheet folded into a cylinder with a typical diameter of 2 to 3 nm [3, 4]. SWNHs are produced closed at their ends with conical tips. To insert species inside the nanotubular space the nanohorns must be open. Gas-phase oxidation is an effective approach for opening of carbon nanotubular structures. Gaining control over the nanotube opening is important for development of advanced materials, which might fmd application in catalysis, gas storage and
396
separation. Here we report on the opening of SWNHs by heating in O2 and CO2. We discuss the possibility to control the pore structure of SWNHs by varying the heat treatment conditions. 2. EXPERIMENTAL Bud-type SWNHs were produced by CO2 laser ablation of graphite in He (760 Torr) [4]. To open the nanohorns SWNHs were heat treated in flowing CO2(50 ml/min) at 1173 K and 1273 K. The samples are designated as h-NH-x/y, where x is the temperature in K and>^ is the time in hours at which the treatment was conducted. The treatment in oxygen (760 Torr) was carried out for 10 min at 623 K {b-NH-623) and 693 K {b-NH-693). 3. RESULTS AND DISCUSSION ^W(i-SWNHs, used in this study, consists of individual nanohorn-likc units with a diameter of the tubular part 2 to 3 nm. The nanohorns assemble in spherical bundles of about 70 nm, which arc attached to each other by van dcr Waals forces. THM observations showed that heating in O2 and CO2 docs not change significantly the SWNHs
"•'•.'i^^:^^:^?^'^-)^'''^ ' •^' ;•..'•'•.••••'S>'*;;v:"^>"^ .
morphology. Figure 1 illustrates the preserved SWNH structure after Irealmcnt in CX)2 at 1273 K for 1 hour. In this study physical adsorption analysis was applied to understand the elTect oftiic
•
•, • ' ^ • .
; • > . * * ' V
•
'••\'\ . — ^'S' ' • " - ^ ''"^S''' "'" h-SWNH-l 273/1.
gaseous treatment on the nanohorn opening. The nitrogen adsorption isotherms of SWNHs treated in O2 and CO2 are given in Figure 2 {a and b). The isotherms clearly indicate an enhanced adsorption capacity after the gaseous treatment, suggesting that both gases, O2 and CO2, open the nanohorns at the applied treatment conditions. Additionally, we have found that treatment in CO2 for 10 h changes the slope of the isotherm at moderate and high pressures. The steeper uptake at relative pressures between 0.4 and 0.9 suggests the presence of larger amount of mesoporcs. The mesopore size distributions (PSD) given in Figure 2 (c and d) confirm that treatment in different oxidizing atmospheres has a different effect on the mesopore structure of SWNHs. Thus, there are no changes in the PSD curves of the nanohorns treated in O2 independently on the temperature of treatment (Figure 2c). On the contrary, the PSD curve becomes broader after treatment in CO2 and the higher the temperature of treatment the broader the distribution (Figure 2d). Whereas the development of
397
micropores and pores of about 3 nm is associated with the nanohom opening, mesopores larger than 3 nm could be formed only between adjacent spherical assemblies. Respectively, development of such pores suggests rearrangement of the spherical assemblies during the treatment due to detachment of the assemblies and increasing the space between them. Table 1 Pore structure parameters estimated by as-method. Sample
at
Vme
Vmi
d
(m'/g)
(cm'/g)
(cmVg)
(nm)
b-NH
320
021
on
-
b'NH-623
600
0.29
0.23
1.8
b-NH-693
830
0.32
0.34
1.9
b-NH-1173/1
566
0.23
0.22
1.5
b-NH-1173/5
558
0.22
0.22
1.7
b-NH-1173/10
560
0.38
0.22
2.1
b-NH-1273/1
668
0.34
0.26
2.0
b-NH-1273/5
670
0.48
0.27
2.4
b-NH-1273/10
820
0.62
0.35
2.7
The pore structure parameters estimated by the subtracting pore effect method for as-plot [5] are given in Table 1. The specific surface area (a,) and the micropore volume (Vmi) increase after the oxidation due to opening of the nanohoms. Consequently, both parameters, a, and I 'mi, increase with the temperature as a result of the higher reaction rate and the opening of larger number of nanohoms. The enhancement of the microporosity is more pronounced for SWNHs treated in O2. The data reveal that a significant development of the mesoporosity (Vme) is achieved after a long treatment in CO2. Additionally, the diameter (d) of the internal pores of the open nanohoms could be controlled by the temperature and time of treatment in CO2. The treatment in O2 has little effect on the average diameter of the nanopores. This is associated with the different reactivity of O2 and CO2 towards carbon. The oxidizing agent etches away carbon atoms from the nanohom walls creating nanowindows. The number and the size of the nanowindows depend on the reactivity of the graphitic walls towards the oxidizing agent. The slow reaction between CO2 and carbon allows formation of nanowindows of different size depending on the time and temperature of treatment. In summary, treatment of SWNHs in oxidizing gaseous atmosphere allows tuning of the
398
pore structure by control of the temperature and time of treatment.
800 •
600
b-SWNH-623 jfl
• b-SWNH-693 400 200
1600
(a)
is
:^4
^
^1200
b-SWNH-1273/10
I
b-SWNH-1173/10 pristine J
800
™ 400
pristine
0 0.0
0.2
0.4 0.6 P/Po
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
40
50
PlPo
•o
0
10 20 pore size, x (nm)
30
0
10
20
30
pore size, x (nm)
Fig. 2. Nitrogen adsorption isotherms at 77 K of SWNMs treated in O2 (a) and COiih). The PSD curves estimated by Barrett-Joyner-Halenda method are plotted in (<:•) and (d) for SWNF-Js treated in O2 and in CO2, respectively.
REFERENCES 1. S. lijima, Nature (London), 56 (1991) 354. 2. S. Ijima, P. M. Ajayan and T. ichihashi, Phys. Rev. Let., 69 (1992) 3100. 3. S. lijima, M. Yudasaka, R. Yamada, S. Bandow, K. Suenaga, F. Kokai and K. Takahashi, Chem. Phys. Lett., 165 (1999) 309. 4. D. Kasuya, M. Yudasaka, K. Takahashi, F. Kokai and S. lijima, J. Phys. Chem. B, 106 (2002) 4947. 5. N. Setoyama, T. Suzuki and K. Kaneko, Carbon, 36 (1998) 1459.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
399
Expanding horizons of mesoporous materials to non-siliceous systems Ferdi Schiith^*, Teresa Czuryskiewicz'', Freddy Kleitz^, Mika Linden'', Anhui Lu'' Jessica Rosenheim'', Wolfgang Schmidt^, Akira Taguchi^ ^Max-Planck-Institut fiir Kohlenforschung, 45470 Miilheim an der Ruhr, Germany. ''Department of Physical Chemistry, Abo Akademi University, 20500 Turku, Finland. Non-siliceous mesoporous materials are becoming more readily accessible w^ith novel methods for their preparation. Non-silica compositions are now available w^ith wide variety in the cation framework, but also first non-oxide materials are emerging. The highest degree of versatility seems to be provided by the nanocasting pathway introduced by Ryoo's group in 1999 for carbons, which - as repeated nanocasting introduced here - can be expanded also to other framework compositions. This survey will try to cover recent developments in the synthesis of mesoporous non-silica materials with a strong bias towards work from the groups of the authors, but also try to cover selected contributions of other groups which are thought to be very important. 1. INTRODUCTION The developments in the field of non-siliceous mesostructured and mesoporous materials have recently been reviewed.^ The state of the art until about the beginning of 2001 has been covered there. However, the development in this field is very rapid, and exciting novel discoveries have been published in the meantime. While most novel discoveries rely on known pathways for the synthesis of mesostructured and mesoporous materials, repeated nanocasting was only forecasted in previous publications''^, first successful syntheses have only been achieved over the past year^^. This pathway seems to allow the synthesis of an even wider range of possible framework compositions, since it does not require matching solution chemistries between inorganic species and surfactant, but rather compatibility with the processing conditions of carbon removal. However, also following the more conventional synthesis procedures exciting new discoveries have been reported. In the next sections it will be attempted to highlight some more recent developments together with work from the groups of the authors. When looking at the great advances made over the last years, one should not forget other materials which are mesoporous with pores sized in the range discussed here, and ordered to a similar degree or even fully crystalline. Partly overlapping with the pore size range opened by the synthesis of MCM-4T^ and^SM-16^ is anodic alumina which can have pores with sizes down to below 10 nm in a hexagonal array if prepared under ' Author for correspondence. Email: schueth@mpi-muelheim.mpg.de
400
suitable conditions with sulfuric acid as electrolyte^. Anodic alumina can be considered complementary to ordered mesoporous oxides formed by surfactant templating, since the first are membranes while the latter are obtained as powders. Another class of related materials are metal organic framework materials with sufficiently long linkers between the vortices of the net. Such materials, which are fully crystalline, have extensively been investigated by the group of Yaghi^. Window sizes of such materials reach now 2 nm, cavity sizes are in the range of 3 nm, while porosities can be as high as 90% for the MOF-16 material. 2. STATE OF THE ART Most of the syntheses of non-silica ordered mesoporous materials still rely on one of the cooperative formation mechanisms, where either charged surfactants react with charged inorganic solution species or neutral surfactants, such as amines or blockpolymers, interact with the inorganic solution species by hydrogen bonding. The range of compositions accessible covers many transition and base metals oxides, including TiOz (also phosphate stabilized), ZrOz (also phosphate stabilized), Hf02 (phosphate stabilized), Nb205, TazOs, Y2O3, different rare earth metal oxides, AIPO4, AI2O3, and others (for a more complete list with references, see ref. 1). Over the last two years, several highly interesting compositions have been added to the list, which shall be discussed in some more detail in the following. When trying to synthesize non-silica compositions, one faces several additional obstacles: (i) Many metal species are cationic and thus would need for the most simple pathway an anionic surfactant. However, the metal ion solutions are strongly acidic, and precipitation often occurs upon basification. In strongly acidic solutions, the normally anionic surfactants are typically protonated and can thus not act as supramolecular templates, (ii) For some metal ion species very stable units exist which are difficult to condense further, such as for instance the group-5 Keggin ions. The resulting compound has thus more salt like character than that of an extended framework material, (iii) The metal species often are redox active, and redox reactions will occur during calcination, leading normally to structural collapse, (iv) Hydrolysis and condensation reactions are often much faster than for silica, which does not leave sufficient time for a well ordered mesostructure to develop, (v) Most oxides crystallize much easier and at much lower temperatures than silica. However, for a crystalline material it will be much more difficult to accommodate the curvatures typically associated with ordered mesoporous materials, and upon crystallization of the inorganic framework, again collapse of the mesostructure has to be expected. Most of these problems can be solved by one or several different means. With the choice of good precursor species charge matching is possible, alternatively, one can use the ligand assisted pathway^ or use non-ionic surfactants to induce self-assembly. Insufficient condensation can be improved by different post treatment methods, such as phosphatation for zirconias and titanias. Rates of hydrolysis and condensation reactions can be slowed down by using stabilizing ligands, low amounts of water or nonhydrolytic pathways, surfactant can be removed by extraction instead of calcination, and thus many different mesoporous materials have been synthesized as detailed above. The synthesis of mesoporous zirconium oxophosphate is an example, where the choice of precursors and a post treatment step have been used to achieve a stable material. In addition, in this system the first synthesis of a cubic Ia3d material was
401
achieved with a non-silica composition. A good starting point for the search of suitably charged solution precurors is a review paper of Livage et al.^^. When one looks at the different zirconium species which have been identified in aqueous solutions with different anions, sulfate solutions seem to provide good precursors for a cooperative assembly, since in sulfate solutions oligomeric polycharged zirconium oxo-hydroxosulfato anions are present. Consequently, the addition of alkyltrimethylammonium surfactants to zirconium sulfate solutions under acidic conditions leads to the formation of ordered mesostructures^^'^^. However, upon calcination the structure collapses and only dense zirconia phases can be obtained. As a reason for this collapse we identified the insufficient crosslinking of the inorganic framework and the removal of sulfate upon calcination. Treatment of the as-made zirconium-surfactant composite with phosphoric acid, however, leads to substitution of sulfate by phosphate which results in a more strongly condensed framework. The material thus produced can then be calcined without structural collapse, and an accessible pore system results^^'^^. This synthesis approach has subsequently been used to synthesize and stabilize a variety of other transition metal frameworks, reviewed in ref. 1. However, attempts to also produce a non-silica framework analogous to MCM-48 are very scarce in the literature. This is probably due to the more difficult synthesis of the Ia3d structure, together with the problems listed above in the synthesis of non-silica frameworks in general. It has already early been reported that alkylbenzyldimethylammonium surfactants can favor the formation of the cubic Ia3d phase due to the different packing requirements of this surfactant compared to alkyltrimethylammonium. This approach is also possible to synthesize the cubic zirconium oxophosphate with the Ia3d structure* \ However in this early study attempts to stabilize the structure so that it could be calcined to result in an accessible pore system failed. Only after careful evaluation of the processes occurring during template removal and optimization of the surfactanl-tozirconium ratio used in the synthesis was it possible to develop a calcination protocol involving holding points to remove the template and create a porous material which still retained the Ia3d mesostructure '"^ , as evidenced by XRD, TEM and sorption experiments. It is thus possible to tune a synthesis so that also structures other than the P6 structure can be generated. Another challenge still open was the swelling of non-silica 1 mesostructured materials. The framework of many of the reported mesophases synthesized with 2^ molecular surfactants strongly shrinks (0 during calcination, and often the pores c IL.--.^ 0=2 are only in the upper micropore range. 1 Swelling, however, is strongly J V _ 0=1 0 = 0 hindered by the fact that the hydrolysis and condensation of the 1 3 4 5 8 6 non-silica materials is rather fast. 2e(deg) Mesophase assembly can occur on the Fig. 1: XRD-patterns of titanium oxophosphate time scale of less than one second'\ prepared in the presence of 1-octanol as cosurfactant and this might often not be sufficient and mesitylene as swelling agent. O = for oil incorporation in the mesophase. mesitylene/zirconium (mol/mol). Swelling with organic additives, such
J
402
as trimethylbenzene, was therefore found to be not as efficient for the synthesis of titanium oxophosphate^^ as in the case of silica, although some swelling by about 3 nm was possible. Optimizing the stirring conditions and adding 1-octanol, which enhances the solubilization capacity of the micelles, allowed to achieve a total swelling from a lattice parameter of 4.8 nm to about 9.1 nm for the cetyl-trimethylammonium surfactant (Fig. 1)^^. This also brought about a marked improvement of the sorption capacity of this material. The unswollen material had a nitrogen sorption capacity of about 140 ml/g after calcination at 350°C, which was reduced to about 40 ml/g after 400°C and to less than 10 ml/g after calcination at 450*'C. In contrast, the capacity of a swollen material decreased from about 180 ml/g (350°C) to about 150 ml/g (400°C) to still approximately 120 ml/g after calcination at 450°C. Although it is possible to prepare a wide range of different non-silica materials by pathways making use of small molecular surfactants, the most versatile method for the synthesis of various different mesoporous materials seems to be the blockcopolymer route, introduced initially for the synthesis of SBA-15^^. This pathway allows to synthesize mesostructured and mesoporous transition and base metal oxides by hydrolysis of, typically, chloride precursors in essentially alcoholic solution in the presence of triblockcopolymers. Thus also assemblies of crystalline nanoparticles, for instance of Ti02 in the anatase phase, into a hexagonal mesostructure can be prepared. This pathway also seems to be very useful to control the morphology of the resulting solid. While work to control the morphology of siliceous mesophases has already begun in the middle of the 1990s (reviewed in ref. 19), attempts to produce, for instance, thin films of non-silica materials only emerged in the beginning of the new millennium"""'. The films can be prepared with blockcopolymers as described above and show a relatively high degree of order in the TEM. Interestingly, a slight structural change was observed by Grosso et al.^" in their study, where the hexagonal unit cell was distorted to a centered rectangular one. This was attributed to the forces acting during drying and condensation of the film on the substrate. Great progress has been made over the last years in the synthesis of semiconducting and conducting ordered porous materials via a cooperative assembly pathway. Hyodo el al.^^ reported the successful synthesis of ordered mesoporous Sn02 which was stable at a thermal treatment to about 700°C. Higher temperatures lead to collapse of the structure to result in small Sn02 particles. Sn02 is used as a sensor material to detect reducing gases, such as H2 or CO, via changes of its conductivity. The mesoporous material was thus tested in this application. While the sensitivity towards CO was comparable to a conventional sensor, performance in the detection of hydrogen exceeded conventional Sn02 based sensors, which was tentatively attributed to a better diffusivity of hydrogen. The successful synthesis of mesostructured indium tin oxide (ITO) could open up novel opportunities, in fundamental studies as well as in applications. The synthesis used cetyltrimethylammonium as surfactant and triethanolamine atrane complexes of indium and tin as inorganic precursors'^. The calcined materials had surface areas around 300 m'/g and showed a disordered pore system. More importantly, however, the ITO in the mesoporous form has a conductivity of 10'^ S/cm which is about three orders of magnitude lower than ITO thin films, but still significant and useful for applications. One could, for instance, adsorb electrochemically switchable dye molecules in the pores to construct displays, or use doped films as sensors, the advantage of them being that the readout would be electrical.
403
In comparison to attempts to alter the cationic framework constituents from silicon to other elements, few attempts were published to synthesize non-oxidic frameworks using the cooperative pathway. An ordered titanium fluorophosphate framework has been synthesized by the group of Ferey, which, however, could not be calcined without loss of order. High surface area, however, was retained^"^. For the synthesis of the higher chalkogenide frameworks, the Ge4Sio unit was found to be a useful building block in the assembly of mesophases and has been employed primarily by the groups of Froba, Kanatzidis and Ozin (see ref. 1 with further references). Attempts to create nitridic frameworks have been published. Nitridation of silica frameworks with NH3 at high temperatures is known to convert part of the 0x0 groups to nitride groups, and this was also possible for ordered porous silica " Alternatively, the condensation of 0 0,1 0,2 0,3 silicon amine precursors in the P/Po presence of ammonia, surfactants Fig. 2: Nitrogen sorption isothermes of siliconimidonitrides and solvents can lead to the synthesized with different chain length alkylamincs formation of supermicroporous (normalized for comparison). From left to right the chain silicon imido nitrides, which can length increases from C12, to C14, Cu, and Cis, indicating the increase of pore size with increasing chain length (after^^) act as shape selective superbases after loading with alkali metals~'\ However, although the pore sizes are dependent on the size of the templating amines (Fig. 2), the mechanism is probably not a supramolecular assembly but rather molecular templating, since the pore sizes are not compatible with supramolecular aggregates of the alkylamines. The cooperative assembly mechanism does thus not seem to be well suited for the synthesis of non-oxidic frameworks. This is very much different for other mechanisms: The true liquid crystal templating (TLCT) pathway originally developed for silica was soon after used for the creation of mesoporous platinum particles. To do this, a preformed liquid crystal with C16EO8 was infiltrated with a H2PtCl6 or (NH4)2PtCl4 precursor solution. The platinum was then reduced by less noble metals or N2H4. After removal of the surfactant, ordered mesoporous platinum particles could be recovered^^. Following the same strategy of TLTC, films of such materials could be produced by electroreduction of solution species in the presence of the liquid crystal. Recently, Attard et al. succeeded in synthesizing a mesoporous semiconducting selenium film^^. The most exciting opportunities, however, were opened by the nanocasting route to create ordered mesoporous carbons pioneered by Ryoo and coworkers^^. The major steps in the process are (i) the impregnation of a mesoporous silica with a carbon precursor, such as a sugar solution, furfuryl alcohol, or a resorcin-formaldehyde resin which is formed in the pores, (ii) carburization of this precursor by pyrolysis and (iii) removal of the silica mold by leaching with HF or NaOH. Depending on the structure of the parent silica, different structures of the resulting carbon can be achieved. The first material produced was the so called CMK-1, which was formed from MCM-48.
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However, other than MCM-48, the symmetry is not Ia3d, but I4i/a^^. This reduction of the symmetry is brought about by the fact that in MCM-48 a silica minimal surface separates two independent pore systems, which are filled with the carbon in the nanocasting process. After removal of the silica, there is nothing to keep the two carbon systems separated so that they can move with respect to each other, lowering the overall symmetry. However, the group of Ryoo also succeeded in producing the exact replica of MCM-48 which they called CMK-4, by a CVD process in which a MCM-48, surface enriched with aluminum, was filled with carbon from the thermal decomposition of acetylene^\ Most interesting are also the CMK-3^^ (replica of SBA-15) and the CMK5^"^, a replica of SBA-15, in which only the pore walls of the precursor were coated and thus an ordered array of carbon nanotubes results. Especially the latter has highly interesting properties since it can accommodate high loadings of platinum in the range of several ten percent while maintaining very small particles with sizes of only a few nanometers (2.5 nm at a loading of 50 wt.%)'' . It should be mentioned here that independently related materials were also produced by the group of Hyeon, which are called SNU (Seoul National University)-n. The first material such made was templated with a HMS-type silica and thus had the wormhole type pore structure resulting from the more disordered framework'^"^. Last, but not least, the organic matter in the pores of the silica does not necessarily have to be carburized, but can be recovered as an ordered mesoporous polymer, if the crosslinking of the polymer is high enough to maintain sufficient rigidity, as shown for the example of divinylbenzene polymer . It seems that the ordered silica templated carbon materials will have great application potential in several fields, including their use as an ordered template for other materials, which will be addressed in the last section. 3. REPEATED NANOCASTING As already stated in the introduction, repeated nanocasting had been suggested some time ago as a possible means to achieve an even wider variety of framework compositions than accessible at present. The first realizations are now at hand. Following this approach, an ordered mesoporous carbon is used as a mold for another material which fills the pores and is condensed in the pores. After sufficient condensation in the pore system of the carbon has been achieved, the carbon template is removed, easiest by simple calcination. In order to first obtain a proof of concept, we choose to nanocast silica in a CMK-3 carbon mold (a more extensive description has been submitted for publication elsewhere"). Silica was chosen because there are several different precursors available which are easy to handle and do not hydrolyze too quickly: CMK-3 was chosen as the mold, because it can be prepared with relatively large pores which should be easy to infiltrate. A typical preparation resulting in high quality material is described in the following: As the silica source we chose tetraethoxysilane (TEOS), which was introduced in the pores of a CMK-3 mold. The parent materials SBA-15 and CMK-3 were synthesized following published procedures. 0.15 g of CMK-3 were infiltrated with 0.15 g TEOS dropwise under vigorous agitation and then one drop of HCl (pH=l) was added to induce hydrolysis. After 10 min the sample was heated for three hours in a box oven at 40°C, then for an additional three hours at 80°C. After cooling down, the procedure was repeated until the desired loading with TEOS was achieved. Then the sample was
405
heated to 700°C in nitrogen before it was calcined in air at 550°C to remove the CMK-3. Following this pathway, a hexagonal ordered mesoporous silica was obtained which we call NCS-1 (nanocasted silica No. 1). The thermal treatment conditions are crucial in determining the quality of the resulting silica. The yield was typically 1.5 - 2 g of silica per gram of carbon, depending on the pore volume of the starting material. -— —Fig. 3 shows a TEM picture of a ; typical material. It should be emphasized • that the whole sample looks like the sections shown in the TEM, all over the ' sample the typical hexagonal and parallel line structures were visible. Often the morphology of particles corresponded to the noodle shaped morphology often :>; L: observed in SBA-15 which had been the starting point of the synthesis. The XRD " pattern reveals the typical low angle reflections with the (11) and (20) ^" reflection discernible, the sorption isotherm shows the typical step in the . . . ' " mesopore range (BET surface areas ._...„_ 200 nm around 500 m^/g). The quality of the resulting material was found to depend Fig. 3: TEM of a typical NCS-l material critically on the quality of the starting obtained as a SiOo nanocast from CMK-3.
_
.
• ,
^ j „ ^^i^^
^
^n
^ ^^ •u .
materials used as molds as well as on the processing conditions, especially on the thermal treatment. After having been successful with silica, the process was extended to alumina. So far, a material with the same high degree of order as the silica was not obtained, but the results already show, that the process is transferable also to other oxides. As aluminum source we used different aluminumalcoholates. Fig. 4a shows the isotherm for one example. Fig. 4b a TEM, which demonstrates that the pore system is still rather disordered, in agreement with the single, relatively broad low angle reflection in the XRD. The pore size as seen in the TEM, however, corresponds to the pore size calculated from the isotherm.
^nm 0.2 0.4 0.6 0.8 Relative pressure (P/Po)
Fig. 4: (a) Sorption isotherm of a nanocasted alumina NCA-1 obtained from CMK-3 and aluminun tributylate (b) TEM of the same sample.
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4. CONCLUSIONS Great advances in the synthesis of non-siliceous mesoporous materials have been made over the last years, and the field is still expanding. It is probably no great risk to state that in the years to come repeated nanocasting will be one of the main pathways by which even more unusual framework compositions will become accessible. 5. REFERENCES 1. F. Schuth, Chem. Mater. 13 (2001) 3184. 2. F. Schuth, Stud.Surf.Sci.Catal. 135 (2001) 101. 3. A.H. Lu, W. Schmidt, A. Taguchi, B. Spliethoff, B. Tesche, F. Schuth, submitted. 4 . J.M. Kim, M. Kang, S.H. Yi, J.E. Yie, S.H. Yoo, R. Ryoo, 3rd IMMS, July 8-11, 2002, Jeju, Korea, PA-7. 5 . C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. 6. T. Yanagisawa, T. Shimizu, K. Kuroda, C Kato, Bull.Chem.Soc.Jpn. 63 (1990) 988. 7. D. Honicke, E. Ditsch, Anodic Alumina, in: F. Schuth, K.S.W. Sing, J. Weitkamp (eds.). Handbook ol Porous Solids, Wiley-VCH, Weinheim, 2002. 8. M. Eddouadi, J. Kim, N. Rosi, D. Wodak, J. Wachter, M. O'Keeffe, O.M. Yaghi, Science 295 (2002) 469, with further references. 9. D.M. Antonelli, J.Y. Ying, Angew.Chem.Int.Ed.Engl. 35 (1996) 426. 10. J. Livage, M. Henry, C. Sanchez, Progr.Sol.State Chem. 18 (1989) 259. 11. U. Ciesla, S. Schacht, G.D. Stucky, K.K. Unger, F. Schuth, Angew.Chem.Int.Ed.Engl. 35 (1996) 541. 12. U. Ciesla, M. Froba, G.D. Stucky, F. Schuth, Chem.Mater. 11 (1999) 227. 13. F. Schuth, U. Ciesla, S. Schacht, M. Thieme, Q. Huo, G.D. Stucky, Mater.Res. Bull. 34 (1999) 483. 14. F. Kleitz, S.J. Thomson, Z. Liu, O. Terasaki, F. Schuth, Chem.Mater., in print. 15. M. Linden, J. Blanchard, S. Schacht, S.A. Schunk, F. Schuth, Chem.Mater. 11 (1999) 3002. 16. J. Blanchard, F. Schuth, P. Trens, M. Hudson, Microporous and Mesoporous Mater. 39 (2000) 163. 17. T. Czuryskicwicz, J. Rosenholm, F. Kleitz, F. Schiith, M. Linden, in preparation 18. D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, J.Am.Chem.Soc. 120 (1998) 6024. 19. U. Ciesla, F. Schuth, Microporous and Mesoporous Mater. 27 (1999) 131. 20. D. Grosso, G.J. de A.A. Soler-Illia, F. Babonneau, C. Sanchez, P.A. Albouy, A. Brunet-Bruncau. A.R. Belkcnende, Adv.Mater. 13 (2001) 1085. 21. H.S. Yun, K. Miyazawa, H. Zhou, I. Honma, M. Kuwabara, Adv.Mater. 13 (2001) 1377. 22. T. Hyodo, T. Nishida, Y. Shimizu, M. Egahira, Sensors and Actuators B 83 (2002) 209. 23. T.T. Emons, J. Li, L.F. Nazar, J.Am.Chem.Soc, in print. 24. C. Serre, M. Hervieu, C. Magnier, F. Taulelle, G. Ferey, Chem.Mater. 14 (2002) 180. 25. J. El Haskouri, S. Carbrera, F. Sapina, J. Latorre, C. Guillem, A. Beltran-Porter, D. Bellran-Porlcr, M.D. Marcos, P. Amoros, Adv.Mater. 13 (2001) 192. 26. D. Farrusseng, K. Schlichte, B. Spliethoff, A. Wingen, S. Kaskel, J. Bradley, F. Schuth, Angew.Chem. 113 (2001) 4336. 27. G.S. Attard, C.G. Goltner, J.M. Corker, S. Henke, R.H. Templer, Angew.Chem.Int.Ed.Engl. 36 (1997)1315. 28. I. Nandhakumar, J.M. Elliott, G.S. Attard, Chem.Mater. 13 (2001) 3840. 29. R. Ryoo, S.H. Joo, S. Jun,.J.Phys.Chem.B 103 (1999) 7743. 30. M. Kaneda, T. Tsubakiyama, A. Karlsson, Y. Sakamoto, T. Oshuna, O. Terasaki, S.H. Joo, R. Ryoo, J.Phys.Chem.B 106 (2002) 1256. 31. R. Ryoo, S.H. Joo, S. Jun, T. Tsubakiyama, O. Terasaki, Stud.Surf.Sci.Catal. 135, Elsevier, Amsterdam 2001. 32. S. Jun, S.H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Oshuna, O. Terasaki, J.Am.Chem.Soc. 122(2000)10712. 33. S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu., O. Terasaki, R. Ryoo, Nature 412 (2001) 169. 34. J. Lee, S. Yoo, T. Hyeon, S.M. Oh, K.B. Kim, Chem.Commun. (1999) 2177. 35. J.Y. Kim, S.B. Yoon, F. Kooli, J.S. Yu, J.Mater.Chem. 11 (2001) 2912.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Structure and shape control in functional mesostructured materials from block copolymer mesophases Ulrich Wiesner Department for Materials Science & Engineering, Cornell University, 329 Bard Hall, Ithaca, NY 14850, USA 1. INTRODUCTION The synthesis of functional mesostructured organic-inorganic hybrid materials using organic molecules as structure-directing agents or templates is an area of rapid growth w^ith diverse applications, such as separation technology and catalysis. Block copolymers can be regarded as macromolecular analogues of low molecular weight surfactants.^'^ Use of block copolymers has recently been shown to extend the structural feature size of mesostructured hybrid materials as well as the pore sizes of the resulting ordered porous silica to hundreds of Angstroms.^^'^^ Combined principles of polymer, colloidal and inorganic chemistries have been used to synthesize materials with uniform and adjustable pore sizes, with thick, hydrothermally stable walls and with both, 2-dimensional (2D) hexagonal structures as well as 3-dimensional (3D) cubic morphologies with more accessible pores.^^"''^ Unprecedented morphology control is obtained for mesostructured materials by changing from conventional silicon precursors to organically modified ceramic (ormocer) precursors in the block copolymer directed synthesis.^"^^ Most of the mesophase morphologies observed in block copolymers or copolymer-homopolymer mixtures have been obtained for such organicinorganic hybrid materials.^'''^^ The basis for this morphological control is a unique polymerceramic interface which can be characterized by solid state NMR techniques.^'^^ The hydrophilic blocks of the amphiphilic copolymers are completely integrated into the ceramic phase analogous to what is found in biological hybrid materials. This leads to a "quasi two phase system" allowing for a more rational hybrid morphology design based on the current understanding of the phase behavior of block copolymers and copolymer-homopolymer mixtures.^'"^^ Through the unique interface the ceramic phase is plasticized by the polymers thus generating an approach to a novel class of materials referred to as 'flexible ceramics' in the following. In the present paper, after elucidating the structural control obtained through the block copolymer-ormocer approach, several examples illustrate areas for potential applications of these hybrid materials. They include applications as solid hybrid polymer (SHyP) electrolytes, polymer-hybrid nanocomposites, and mesoporous materials.
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2. PHASE SPACE OF ORMOCER DERIVED MESOSTRUCTURED HYBRIDS (H3CO)3SP
In ormocer derived meso-structured '4J^>======^NL^^ AI(0*Bu)j hybrids the morphology of the final material is mainly a function of the weight of the added inorganic fraction components and essentially independent of the microstructure of the block copolymer.^''^ This allows access to a wide variety of morphologies starting from a single block copolymer by simply mixing in the inorganic components. This is Fig.l. Schematic drawing of approach for demonstrated schematically in Figure 1 for synthesizing mesostructured hybrid materials: the ormocer precursors, (3-glycidyloxyp- Left: the morphology of the precursor polymers; Right: the resulting morphologies ropyl)-trimethoxy si lane (GLYMO) and after addition of various amounts of metal aluminum ^ec-butoxide (Al(0^Bu)3) in a alkoxides. molar ratio of 80:20, with the diblock copolymer poly(isoprene-block-ethyleneo-xide), (PI-b-PEO), as structure directing agent (for details of the procedure see reference 11 and references therein). Representative transmission electron microscopy (TEM) micrographs of selected hybrid morphologies are depicted in Figure 2. Starting from a PI-^-PEO block copolymer with bcc structure (1.410'* g/mol, /PEG ~ 0.13), increasing the content of GLYMO and Al(0^Bu)3 leads to spheres (WINORG ^ 0.23, Fig. 2A), hexagonally packed cylinders (WINORG = 0.32, Fig.2B), lamellae (WJNORG = 0.45, Fig. 2C),
the inverse cylinder morphology (WINORG = 0.65,
Fig.2D)
and
randomly packed wormlike micelles of PI in an inorganic-rich matrix (vviNORG = 0.82). Employing a block copolymer exhibiting a hexagonal array of cylinders (M= 16400, /PEG = 0.38) as structure directing agent, lamellae (WINGRG = 0.53), a bicontinuous cubic Plumber's Nightmare structure^'^^ Fig.2. TEM micrographs of some of the mesostructured (wiNGRG = 0.56, Fig.2E), an hybrid materials. If not otherwise shown, magnifications inverse cylinder morphology are as depicted by the bar in (A). Except for (E) all and inverse images were taken under bright-field conditions (bright (wiNGRG = 0.73) (WINORG = 0.79, Fig.2F) spheres inorganic and dark polymer phase). In (E) the contrast is are found. inverted (dark-field conditions). When comparing the sequence of hybrid morphologies in Figure 1 with that observed for the pure PI-^-PEO and other block copolymers^'"^'^^^ it is evident that the overall structural control is similar and the sequence of morphologies indeed closely follows what is expected ft*om block copolymer phase diagrams. Nevertheless, subtle differences are observed. First,
409
only for the block copolymer with larger PEO content a bicontinuous cubic structure could be obtained. As described in an earlier publication,^^^^ SAXS and TEM data on this bicontinuous structure does not agree with the double gyroid morphology expected from block copolymer phase diagrams^'"^^ but rather is consistent with a so called "Plumber's nightmare" morphology. Second, also the inverse spherical morphology could only be reached through addition of inorganic material to the copolymer with larger PEO content. In contrast, addition of large amounts of inorganic material (WINORG > 0.8) to the block copolymer with bcc morphology leads to wormlike rather than spherical micelles. This morphology has been reported for A2Bmictoarm star polymers^'^^ and diblock copolymer/homopolymer (AB/A) mixtures.^'^'^^^ The occurrence of worm-like micelles instead of spheres arranged on a cubic lattice has been ascribed to interface-curvature constraints.^'^^ 3. SOLID HYBRID POLYMER ELECTROLYTES Solid polymer electrolytes (SPEs) are potential materials for application as electrolytes and separators in secondary lithium and lithium-ion batteries. The prototypical SPE is polyethylene oxide (PEO) blended with the lithium salt of a large soft anion. Cross-linking can improve the mechanical strength of such SPEs. The addition of nanoscale ceramic materials inhibits the recrystallization of PEO, increases the cationic conductivity and stabilizes the Li electrolyte interface. The present PEO/Al-GLYMO composites when combined with a lithium salt provide a novel type of lithium ion conductor with an unprecedented combination of properties: a Solid Hybrid Polymer (SHyP) electrolyte.^''^^ Because of the molecular-scale mixing of the components, crystallization of PEO is completely suppressed, while strength, conductivity, and lithium transference numbers all are high compared to prototype Fig. 3. Energy Filtering (EF) TEM SPEs. Cyclic voltammetry shows that lithium micrograph (right) of isolated nano can be plated and stripped from these cylinders of a PI-b-PEO/Al-GLYMO electrolytes and suggests a reasonable cycling composite and its molecular interpretation efficiency. Finally, as demonstrated in Figure (^'g^^; ^.PP^^ P^^' ^ark-field image - , . -^ .-' , ,^ • J • reveahng silicon; lower part: carbon map. 3, this composite can be self-organized using o r r diblock copolymer technology into nanometer scale plates and rods, paving the way to making lithium-conducting cables, for example, and hence solid-state electrochemical devices of sizes down to 10 nm. 4. MODEL BLOCK COPOLYMER NANOCOMPOSITES Using the block copolymer assisted sol-gel synthesis a series of silica based fillers with well-defined shapes, i.e. spheres, rods and plates, and surface potentials ("hairy objects") can be synthesized, see scheme in Figure 4 (see also Figure 3).^'^^ These filler particles can subsequently be incorporated into, e.g., a lamellar poly(styrene-b-isoprene) block copolymer matrix to generate model nanocomposites. The influence of filler dimensionality on orderdisorder phase transition of the block copolymer matrix can then be studied.^^^^ The addition
410
of as little as 0.5wt. % fillers drastically alters the thermodynamic properties of the nanocomposites. The order-disorder transition temperature is lowered by 1523°C and is accompanied by a significant broadening of the transition temperature window. The dimensionality of the fillers plays a significant and non-trivial role in the process of the order-disorder phase transition. Rod-like fillers induce the largest depression and broadening of the phase transition. The findings can be rationalized based on varying defect energy density arguments as also supported by recent computer simulations. Experimental work to further elucidate the origins of the observed behavior is now in progress in our laboratory.
^Mfs^
- c r ^ AI(OBu\
0.5%
99.5%
5. MESOPOROUS MATERIALS Fig. 4. Schematics of how to generate model nanocomposites. Mesostructured hybrid materials with an inverse cylindrical (cf Figs. 1 & 2D) and a Plumber's Nightmare morphology (cf Figs. 1 & 2E) can be converted successfully to the corresponding mesoporous materials after heat treatment in several stages up to 600 ^c.^'^'"^ A schematic representation of the resulting structures as well as the corresponding TEM micrographs of the calcined materials are shown in Figure 5. In case of the bicontinuous cubic Plumber's Nightmare, after calcinations the bulk material consists of a particularly fascinating morphology with an aluminosilicate matrix interwoven with two discrete, continuous nano-channel systems that do not touch each other (see Scheme in Fig.5C). Because of its interwoven and regular, branched cubic bulk structure, the resulting mesoporous material is expected to provide excellent mass transfer kinetics Fig. 5. Schematic representations (left) in catalytic and separation technologies. From NMR and TEM micrographs (right) of studies about half of the aluminum is incorporated in mesoporous materials with hexagonal and bicontinuous cubic the silicon network as fourfold coordinated (top) aluminum. Calcinations of the as-made materials Plumber's Nightmare morphologies leads to an even larger amount of such in-frame (bottom). aluminum with respect to the precursor material thus providing a pathway to materials with acid catalytic activity (data not shown). It is striking that the structures are well preserved after calcinations even though the unit cell volumes sometimes decrease by as much as 75%! The large mass loss and shrinkage is due to the large fraction of organic moieties even in the inorganic phase of the hybrids. Preservation of the structures indicates that the bonding network formed by the inorganic precursors is extremely robust. This may be a general feature of the present block copolymer-ormocer derived hybrid materials. Both mesoporous materials exhibit a nitrogen sorption isotherm of
411
type IV according to BDDT classification with specific surface areas typically around 300 m^/g (according to the Brunnauer-Emmett-Teller (BET) method) for materials with pore diameters of about 9 nm. It is interesting to note that, e.g., in the hexagonal mesoporous materials with about 12 nm the wall thickness is about doubled with respect to that of materials described in the literature.^^'^'^ This should lead to significantly improved stability. Furthermore, the pore sizes (as well as the wall thicknesses) of the present materials can be varied through a simple variation of the molecular weight of the precursor PI-Z?-PEO obtained through anionic polymerization. As an example, from a PI-^-PEO sample with A/n = 8.4-10'^,/pEo ^ 0.08 and a polydispersity, Mw /Mn =1.05, a hybrid material with inverse hexagonal morphology was prepared at (WINORG =0.28) and then calcined. Analysis of the corresponding nitrogen sorption isotherm revealed a pores size of about 50 nm.
REFERENCES 1. B. M. Discher, Y. Y. Won, D. S. Ege, J. C. M. Lee, F. S. Bates, D. E. Discher, D. A. HamrnQT, Science 1999,25^,1143. 2. S. A. Bagshaw, E. Prouzet, T. J. Pinnavaia, Science 1995, 269, 1242. 3. C. G. Goltner, M. Antonietti, Adv. Mater. 1997, P, 431; C. G. Goltner, S. Henke, M. C. Weissenberger, M. Antonietti, Angew. Chem. 1998, 110, 633-636; Angew. Chem. Int. Ed. ^•wg/. 1998, J7, 613. 4. M. Templin, A. Franck, A. Du Chesne, H. Leist, Y. Zhang, R. Ulrich, V. Schadler, U. Wiesner, Science 1997, 278, 1795. 5. D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka, G. D. Stucky, Science 1998, 279, 548. 6. D. Zhao, Q. Huo, J. Feng, B. F. Chmelka, G. D. Stucky, J. Am. Chem. Soc. 1998,120, 6024. 7. D. Zhao, P. Yang, N. Melosh, J. Feng. B. F. Chmelka, G. D. Stucky, Adv. Mater. 1998,10, 1380. 8. P. Yang, D. Zhao, D. I. Margolese, B. F. Chmelka, G. D. Stucky, Nature 1998, 396, 152. 9. P. Yang, T. Deng, D. Zhao, P. Feng, D. Pine, B. F. Chmelka, G. M. Whitesides, G. D. Stucky, Nature 1998, 282, 2244. 10. A. C. Finnefrock, R. Ulrich, A. Du Chesne, C. C. Honeker, K. Schumacher, K. K. Unger, S. M. Gruner, U. Wiesner, Angew. Chem. Int. Ed. 2001, 40, 1208 11. P. F. W. Simon, R. Ulrich, H. W. Spiess, U. Wiesner, Chem. Mater. 2001, 13, 3464. 12. R. Ulrich, A. Du Chesne, M. Templin, U. Wiesner, Adv. Mater. 1999, //, 141. 13. S. M. De Paul, J. W. Zwanziger, R. Ulrich, U. Wiesner, H. W. Spiess, J. Am. Chem. Soc. 1999, 121, 5727. 14. The Physics of Block Copolymers, I. W. Hamley, Oxford University Press, Oxford 1998. 15. G. Floudas, B. Vazaiou, F. Schipper, R. Ulrich, U. Wiesner, H. latrou, N. Hadjichristidis, Macromolecules 2001, 34, 2947. 16. D. J. Pochan, S. P. Gido, S. Pispas, J. W. Mays, A. J. Ryan, J. P. A. Fairclough, I. W. Hamley, N. J. Terrill, Macromolecules 1996, 29, 5091. 17. T. Hashimoto, H. Tanaka, H. Hasegawa, Macromolecules 1990, 23, 4378. 18. D. J. Kinning, K. I. Winey, E. L. Thomas, Macromolecules 1988, 21, 3502. 19. R. Ulrich, J. W. Zwanziger, S. M. De Paul, A. Reiche, H. Leuninger, H. W. Spiess, U. Wiesner, Adv.Mater. 2002, in press. 20. A. Jain, J. S. Gutmann, C. Garcia, Y. Zhang, M. Tate, S. Gruner, U. Wiesner, Macromolecules 2002, 35, 4862. 21. P. Yang, D. Zhao, D. I. Margolese, B. F. Chmelka, G. D. Stucky, Chem. Mater. 1999, 7/, 2813.
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Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
Strategies for spatially separating mesostructured sol-gel silicate films
413
photoactive
molecules
in
Raquel Hernandez, Payam Minoofar, Michael Huang, Anne-Christine Franville, Shinye Chia, Bruce Dunn and Jeffrey I. Zink Department of Chemistry and Biochemistry, University of California, Los Angeles, 405 Hilgard Ave., Los Angeles, California 90095 USA Three strategies for placing molecules in designated regions of mesostructured thin films made by the sol-gel dip-coating technique are demonstrated. These strategies all involve one-step syntheses where all of the components are present in the sol from which the substrate is pulled. Silicate films templated by ionic surfactants contain three spatiallyseparated regions: a silicate framework, an organic region formed by the hydrocarbon tails of the surfactants, and an intervening ionic interface formed by the charged surfactant head groups. Luminescent molecules are placed in each of these regions, and the formation is monitored spectroscopically. 1. MESOSTRUCTURED FILMS The sol-gel process is a technique used for preparing transparent inorganic glasses at room temperature. The process used in this paper begins by reacting a silicon alkoxide such as tetraethoxysilane with water. The alkoxide undergoes hydrolysis reactions Si(0R)4 + H2O ^ H0-Si(0R)3 + ROH. The hydrolyzed molecules then undergo condensation polymerization reactions to produce a three dimensional network: (0R)3Si-0H + H0-Si(0R)3 -^ (0R)3Si -0-Si(0R)3 + H2O -> -> Si02. We discovered methods of making mesostructured sol-gel thin films formed by a rapid dip-coating method with structures that possess a high degree of long-range order [1]. Macroscopic materials having nanostructured long range order are fabricated on a support or substrate. The desired structure is built into films by using molecular templates such as surfactant molecules. Nanostructured sol-gel films are formed by the co-assembly of the inorganic silica and the organic surfactant molecules. The self assembly of the surfactants forms the desired structure which is locked in place by the silica [1,2]. The specific structure that is obtained depends on the type and concentration of surfactant used. For example, a lamellar layered structure can be produced by SDS (sodium dodecyl sulfate) surfactant and a hexagonal array of rods is formed when CTAB (cetyltrimethylammonium bromide) is used. These structures are shown in Figure 1. 2. STRUCTURAL REGIONS On the molecular level, the structures that are formed have three distinct regions as sketched in Figure 1. The solid transparent sol-gel silicate structure that holds the material together is called the framework. In most of the materials in this paper the framework is on the order of 20 A thick. The region containing the hydrophobic long-chain hydrocarbon tails of the surfactant is called the organic region. It is a planar sheet in the lamellar materials and a rod in the hexagonal materials. The dimensions of the organic region are controlled by the length of the hydrocarbon tail of the surfactant used in the preparation.
414
When dodecyl sulfate surfactant (twelve carbon chain length) is used, the organic region in the lamellar structure is about 20 A thick and the measured lattice spacing is 39 A. The third region that contains the ionic head group of the surfactant and the counterions is called the ionic region. It is a few Angstroms thick and forms the interface between the framework and the organic region. In the as-pulled films, it also Fig. 1. Schematic diagram of the processes that occur contains residual water during film pulling. The left side shows a film with as well as ions from the lamellar mesostructure templated by SDS, the right side a acid catalyst and buffer 2-d hexagonal structure templated by CTAB. The three (if they were used in the spatially separated regions of the final mesostructured preparation.) The films—the silicate framework, the organic interior of the thickness of the ionic micelles, and the ionic interfaces between the framework layer cannot be readily and the surfactant head group—are shown in the center. controlled, but its composition can be modified by the choice of surfactant and the salts added to the initial sol used to pull the film. The total thickness of the film is measured by profilometry and is usually 100-200 nm thick. 3. STRATEGIES OF DELIBERATE PLACEMENT IN SPATIALLY SEPARATED REGIONS Three strategies for placing molecules preferentially in any one of the three distinct regions [3] depicted in Figure 1 are demonstrated here. The first is iermcd philicity, and is summarized by "like dissolves like." Organic compounds are dissolved by the hydrophobic interior of micelles and ionic compounds will accumulate at the ionic interface. The second strategy is termed bonding. Bonding entails the use of molecules derivatized with trialkoxysilyl groups that can co-condense with the silica precursors in the sol to become fully integrated into the silicate framework of the final film. In other cases, the bonding strategy may apply to covalent attachment of functional molecules to surfactants, thereby placing the dopants in the hydrophobic core of the films. The third strategy is termed bifunctionality, and it applies to molecules that simultaneously incorporate both of the above strategies. These molecules possess both a physical affinity for a particular region and the capability of bonding to another. 4. EXAMPLES OF FILMS SYNTHESIZED BY USING THE STRATEGIES The focus of this paper is on the placement of luminescent molecules in structured films. In all cases, the structure is verified by x-ray diffraction. The formation of long-range
415
ordered structures is a delicate balance involving formation of micelles and the ordered phase of the template and also hydrolysis and condensation of the silicate. 4.1. Philicity The philicity strategy is a useful approach to the deliberate placement of molecules into certain regions of the mesostructured sol-gel materials [4-8]. The most common application involves non-polar molecules that reside preferentially in the hydrophobic interiors of micelles. The first example of the use of this strategy involved pyrene, a non-polar luminescent molecule, as a probe of micelle formation during film pulling. In this study, a TEOS sol is prepared and an anionic surfactant sodium dodecylsulfate (SDS) is used as the templating agent. The final mesostructure is lamellar with alternating surfactant micelle layers and silicate layers (Figure 1). As the film is pulled, the non-polar hydrophobic pyrene molecule becomes incorporated into the micellar interior when the micelle first forms, and the change in luminescence is used to monitor the formation process of the mesostructured sol-gel films [9,10]. The intent of these studies is to monitor the dynamics of film formation in real time, and the final film contains pyrene deliberately placed in the organic region. To further characterize the dynamic properties, interferometry is used to monitor the film thickness at the same time that in-situ photoluminescence spectroscopy of the pyrene probe monitors micelle formation. Monochromatic light is used to illuminate the film. Because of the continuous decrease in the film thickness during the dip-coating process, constructive pyrene band III to band I ratios
percentage of H2O in the sol solvent
1-2-hour-oldfilm R = 1.45 ±0.11 0th fringe R = 1.21 ±0.03 10 R = 1.03 ±0.07
R= 1.39 ±0.29 5
-\R = 1.54 ±0.22
R = 1.12 ±0.12
Fig. 2. Dynamic changes occurring during film pulling. The light and dark interference fringes on the film are used to measure the thickness. The time of the process and the distance above the sol are shown on the left. The pyrene molecule is incorporated in the surfactant by the philicity strategy. The ratios R of the vibronic bands in the luminescence spectra monitor micelle formation (R=1.54), reorganization (R = 1.03) and final lamellar mesostructure formation (R > 1.21).
416
and destructive interference leads to the appearance of light and dark fringes on the developing film that can be used a convenient scale to monitor the thickness. The positions of the fringes on the moving film do not change, A schematic diagram of the results of pyrene luminescence spectroscopy and interferometry of SDS sol-gel films is shown in Figure 2. Changes in the relative vibronic band intensities (band III to band I ratios) in the luminescence spectra are related to changes in the polarity of the probe environment. Low band III to band I ratios correspond to a polar environment, whereas a high III/I ratio is indicative of a non-polar environment. The results show that micelles are formed early in the film formation process (III/I ratio increases from 1.12 to 1.54 within 5 sec.) and that pyrene is incorporated into the micellar interior and experiences a non-polar environment. Then the ratios gradually decrease (III/I ratio decreases to 1.03) and finally increase again at the end of the process (III/I ratios increase to 1.21 and eventually to 1.45). During this period, the initially-formed micelles undergo a phase transformation as the surfactant concentration continues to increase. Micelles break up and pyrene becomes re-exposed to the polar solvents. Finally when the micellar reorganization is complete and the final lamellar phase is formed, pyrene becomes incorporated into the surfactant layer and again reports a non-polar environment. This example shows that pyrene's philicity can be used to incorporate it selectively into the micellar region of the mesostructured sol-gel films, and in the process it can be used to probe the micelle formation and its transformation into the mesostructured sol-gel films. 4.2. Bonding The bonding strategy for incorporating luminescent molecules in the framework requires molecules that contain at least two alkoxysilane substituents on opposites sides [11-13]. Trialkoxysilyl groups undergo condensation with TEOS, the silica precursor in the starting sol, to become integrated into the silicate framework. In all of the examples discussed in this paper, six trialkoxysilyl substituents that surround the molecule in three dimensions are used. An example of a ligand that binds luminescent lanthanide ions is shown in Figure 3 [13]. The incorporated complexes exhibit both the characteristic lanthanide emission spectra and excitation spectra consistent with the absorption-transfer-emission (ATE) mechanism of luminescence typical of these complexes. The Eu complex depicted in Figure 3 is incorporated into the framework of hexagonally structured thin films [13]. This localization is evidenced by the relative intensities of the 616 nm and 592 nm Eu"^^ emission peaks that are the same in both silicate (no surfactant) and nanostructured films. In addition, the Eu"'^ emission lifetimes are the same in both types of films. The silylated ligand enables other 500 550 600 650 700 lanthanides to be deliberately placed in Emission Wavelength (ran) the framework. Terbium and cerium Fig. 3. Placement of a silylated europium have also been studied; Tb^^ emission complex in the framework by using the lifetimes were used to demonstrate the bonding strategy. The molecular structure and placement of the terbium [14]. If this the placement are sketched on the left; the ligand is synthesized without the luminescence spectra of both the silicate groups, its lanthanide mesostructured and silicate films are shown. complexes are hydrophobic and the Films were excited at 330 nm.
417
SDS Templated Film
CTAB Templated Film
li-:iiiU'U(irk
Fig. 4. Placement of a silylated ruthenium complex by using the bifunctional strategy. The molecular structure is sketched at the top. When the surfactant is anionic, the positively charged ruthenium extends into the ionic interface, but when a cationic surfactant is used the ruthenium is repelled and located in the framework.
philicity strategy can be used to place them in the organic region.In another example of the bonding strategy, a silylated Ru(bpy)3(PF6)2 complex that has six condensable trialkoxysilane groups is incorporated into the silicate framework [14]. The luminescence spectra is the same in both CTAB templated films and silicate films indicating that the ruthenium complex is located in the same region, the silicate matrix, in both films. This bidentate ligand strongly binds many transition metals and can be used to place other metals in the framework.
4.3. Bifunctionality of The strategy bifunctionality requires a molecule that has characteristics of two of the regions within the mesostructured film. An example of a bifunctional molecule is the singly silylated Ru(bpy)2ATT (Figure 4) that has the ability to bond one end to the silicate matrix and also has an ionic end with an affinity for the ionic region of the film [3]. The position of the luminescence band maximum is sensitive to the metal's environment and is used to characterize its location. In the first studies of the bifunctional strategy with silylated Ru(bpy)2ATT, the molecule was incorporated in films templated with the anionic surfactant SDS (Figure 4). The emission band maximum is at 650 nm. In a control experiment, the molecule is incorporated in a silica film (no surfactant). In this film, where the molecule is located in the silicate region, the band maximum is at 665 nm. The shift of the emission band maximum to shorter wavelengths shows that the metal-containing end of the 320 360 400 440 molecule is not in the framework but Wavelength (nm) instead extends into the ionic interface Fig. 5. Simultaneous placement of region. In subsequent studies, silylated paraterphenyl in the organic region by using Ru(bpy)2ATT was incorporated in the philicity strategy and the silylated CTAB templated films. The emission europium complex in the framework by the bonding strategy. The molecular structures, band maximum, 665 nm, is the same in and luminescence spectra, collected with both the pure silicate and the 266 nm excitation, are shown. mesostructured film, and shows that the
418
ruthenium resides in the same type of environment, the silica region, in both films. The above studies suggest that the charge on the surfactant's head group plays a major role in the final location of the cationic metal end of the bifunctional molecule. When anionic SDS is used, the metal is attracted into the interface region, but when cationic CTAB is used, the metal is repelled from the interface and becomes incorporated into the framework. 4.4. Dual placement: philicity and bonding Simultaneous incorporation and separation of two different luminescent molecules in structured thin films is possible when the molecules take advantage of two different strategies. An example is the simultaneous incorporation of the silylated Eu complex in the framework by the bonding strategy, and of paraterphenyl (PTP), a hydrophobic laser dye, in the organic region by the philicity strategy [13]. Figure 5 shows emission spectra obtained from a mesostructured thin film containing both the Eu complex and PTP prepared in a one-step, one-pot synthesis from a sol containing both luminescent molecules. This experiment demonstrates that two dopants can be incorporated into nanostructured thin films and simultaneously be directed to specific regions of the same films. The dual placement can produce functional films that undergo intermolecular energy or electron transfer. 5. SUMMARY Luminescent molecules are deliberately placed in one of the three spatially separated regions of mesostructured films by using the strategies of philicity, bonding or biftinctionality. All of the components are present in the sol from which the film is pulled. The structure is confirmed by x-ray diffraction and the location of the molecule is determined by luminescence spectroscopy. Simultaneous placement of two different molecules in two different regions is also demonstrated. Functional films that undergo intermolecular electron transfer and energy transfer can be synthesized in a one-pot, onestep procedure.
REFERENCES 1. Lu, Y.; Gangull, R.; Drewlen, C. A.; Anderson, M. T.; Brinker, C. J.; Gong, W.; Quo, Y.; Soyez, H.; Dunn, B.; Huang, M. H.; Zink, J. I. Nature 1997, 389, 364-368. 2. Sellinger, A.; Weiss, P. M.; Nguyen, A.; Lu, Y.; Assink, R. A.; Gong, W.; Brinker, C. J. Nature 1998, 394, 256. 3. Hernandez, R.; Franville, A.-C; Minoofar, P.; Dunn, B.; Zink, J. I. J. Am. Chem. Soc. 2001,723, 1248-1249. 4. Franville, A.; Dunn, B.; Zink, J. I. J. Phys. Chem. B 2001, 105, 10335-10339. 5. Ogawa, M. Langmuir 1995, 11, 4639. 6. Ogawa, M. Chem. Mater. 1998, 10, 1382. 7. Wimsberger, G.; Scott, B. J.; Chmelka, B. F.; Stucky, G. D. Adv. Mater. 2000,12, 1450. 8. Wu, J.; Abu-Omar, M. M.; Tolbert, S. Nano Letters 2001, /, 27-31. 9. Huang, M. H.; Dunn, B. S.; Soyez, H.; Zink, J. I. Langmuir 1998, 14, 7331. 10. Huang, M. H.; Dunn, B. S.; Zink, J. I. J. Am. Chem. Soc. 2000, 122, 3739-3745. 11. Lebeau, B.; Fowler, C. E.; Hall, S. R.; Mann, S. Journal of Materials Chemistry 1999, 9, 2279-2281. 12. Li, H.; Fu, L.; Wang, S.; Zhang, H. New J. Chem. 2002, 26, 674. 13. Minoofar, P.; Hernandez, R.; Franville, A.; Dunn, B.; Zink, J. Journal of Sol-Gel Science and Technology 2002, in press. 14. Minoofar, P.; Hernandez, R.; Chia, S.; Dunn, B.; Zink, J. I.; Franville, A. submitted.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
419
Design of supported catalysts by surface functionalization of micelletemplated silicas D. Brunei'*, A.C. Blanc", P-H. Mutin^ O. Lorret", V. Lafond^ A. Galameau', A.Vioux'' and F. Fajula"
'Laboratoire des Materiaux Catalytiques et Catalyse en Chimie Organique - UMR-5618CNRS-ENSCM, 8, rue de I'Ecole Normale, F-34296 -MONTPELLIER Cedex 5, France. FAX: +33-4-67-14-4349. e-mail: brunel@cit.enscm.fr. ^Laboratoire de Chimie Moleculaire et Organisation du Solide - UMR-5637-CNRS-UM II, Place Eugene Bataillon, F-34095 -MONTPELLIER Cedex 5, France. The functionalization of the surface of micelle-templated silicas with catalytic sites opened up a wide range of opportunities for advanced applications in supported catalysis. In this field, the control of the different modification steps is critical in the catalyst design. This presentation deals with the best control of the surface coverage of the mineral surface with organic silane bearing chiral ligand or guanidine, by surface sol-gel polymerization process. This strategy provides more efficient enantioselective catalyst and/or catalyst stability. On the other hand, the enhancement of the chemical stability of base supported catalysts by other coupling strategies will allow their application in fine organic chemistry. 1. INTRODUCTION Increasing interest is renewed on the use of immobilized homogeneous catalysts on mineral supports owing to the possibility of the easy recovery and reuse of the catalyst. Moreover, the discovery of the highly structured silica such as M41-S family and others has opened up new possibilities for their use as nanostructured supports '. In particular, these materials offer larger accessible surface than zeolites, and therefore present considerable advantages for their applications as catalyst supporting solids in fine organic chemistry. Hence, the surface of these materials have been covalently grafted with organic moieties bearing catalytic sites. Depending on the nature of the tethered catalyst, these hybrid materials are potential heterogeneous catalysts for either acid or base or hydrogenation or oxidation reaction. Besides the advantages provided by the particular texture of the micelle-templated silicas (MTS), which often avoid difflisional limitation, the homogeneous chemical nature of their surface could conveniently allow dispersion of the catalytic sites, hence their isolation towards mutual site-site interaction. Nevertheless, the activity and selectivity of such supported catalysts could be strongly changed compared to that of their homogeneous couterparts due to sitesurface interactions which depends on the type of the used anchoring methodogy. This aspect is developed here. Morover, one of the drawbacks of such solids which could hamper their
420
widespread use as catalysts, is their chemical instability towards hydrolysis or solvolysis particularly during their use under basic conditions.^ However, up to now, although some materials obtained recently by post-synthesis treatments exhibited a considerable improvement of their chemical stability,^ this has remained as a challenge since there was no effective methods for the preservation of the texture of MCM-41 containing strong bases. In this paper, we present two possible routes to increase the stability of the hybrid organic-silicic mesoporous materials and some applications in catalysis. 2. EXPERIMENTAL 2.1. Materials MCM-41 materials were synthesized from alkaline silicate solution in the presence of hexadecyltrimethylammonium bromide"*. Different pore sizes were achieved by adding various amounts of swelling agent: 1,3,5-trimethylbenzene. 2.2. Surface modifications of MCM-41 Three different methods were investigated: Type 1 Silylation: The organosilane chains were grafted on dehydrated surface of MCM-41 samples by reaction of organotrialkoxysilane, mainly 3-chloropropyltrimethoxysilane, in anhydrous refluxing toluene. Type 2. Coating: MCM-41 samples were functionalized by surface sol-gel polymerization of chloropropyltrimethoxysilane molecules which were firstly adsorbed on MCM-41 surface (5 molecules/nm^), according to a procedure leading to an optimal surface coverage on silica^. Type 3. Two-step modification'. The MCM-41 surface was first covered by TiOx overlayers according to a method reported in ref 6, then functionalized by grafting with 3-bromopropanphosphonic acid. 2.3. Anchorage of catalytic sites Ephedrine or guanidine was anchored by nucleophilic substitution of chlorine atom born by the differently grafted organosilane chains, or (in type 3 modification) by substitution of bromine atom born by the grafted alkylchain phosphonate groups. 2.4. Characterization Hybrid mesoporous materials were characterized by a battery of techniques including XRD, sorption measurements, ^'^Si and *^C NMR, IR, UV-Vis, TGA and elemental analyses. 2.5. Catalytic tests 2.5.1. Enantioselective additions of diethylzinc to benzaldehyde were conducted in presence of stirred dispersions of various ephedrine-grafted MCM-41 samples in anhydrous toluene at 273 K. 2.5.1. Transesterification reactions were performed by addition of ethylpropionate to stirred dispersions of various guanidine-grafted MCM-41 samples in butanol at 373 K. 3. RESULTS AND DISCUSSION MCM-41 possessing different mesopore sizes have been functionalized with 3aminopropyl- and 3-halogenopropylsilane by surface grafting according to silylation- or coating-type methods. The amine-containing MCM-41 revealed base-catalysis efficiency in Knoevenagel condensation and fatty acid addition on glycidol and the chloropropane -
421
containing MCM-41 was an useful precursor for the preparation of various supported homogeneous catalysts. For example, other organic bases such as piperidine or guanidine and different ligands or metal transition complexes have been covalently anchored on MCM-41 by halogen substitution reaction.
^
<
ZnEto
Chiral Auxiliary ^
OZnEt \
'Et y^
^ Hydrolysis
H
When chiral P-aminoalcohols such (-)-ephedrine were anchored on 3-chloropropylsilane chains previously grafted on mineral oxides by silylation, these supported catalysts displayed moderate enantioselectivity (33% ee) in alkylation of prochiral aldehyde with dialkylzinc. We have shown that these lower ee's compared to those obtained by the same ligands anchored on polymers or used in homogeneous conditions (65% ee) result from the competitive achiral activation of dialkylzinc by the residual silanols provided by the uncovered mineral surface.
^ Me 2jn '0-;ZrrR R
33 ee%
-55>
Achiral catalytic site
-^cxH^V-ci
•o-'^,^^^' "Coating"
\\ '-N
n
q
Me
CI ee% up to 65% -homogeneous performance
Taking into account that grafting performed by silylation under anhydrous conditions proceeds mainly on the hydrophobic portion of the surface, the uncovered surface features hydrophilic properties able to activate dialkyl zinc as measured using naked MCM-41 surface. Hence this achiral activity depends on the coverage degree of the surface so that a total coverage was aimed at. In this respect, MCM-41 was functionalized with 3chloropropylsilane chains by a new surface sol-gel polymerization method, which afforded high surface loadings. Hence, the accessible surface after ephedrine substitution featured mainly either grafted ephedrine as efficient chiral auxiliairies or unreacted and inactive 3halogenopropyl chains. This new generation of catalysts demonstrated remarkable enantioselectivity at the level of the homogeneous catalysis''. On the other hand, kinetic studies of the alkylation reaction were carried out with ephedrine anchored on different MCM-41 possessing larger mesopore sizes
422
and/or poorer structure qualities. Using MCM-41 supports of 5.2 nm pore diameter in place of 3.5 nm led to increased rate without any effect on the enantioselectivity ^. Moreover an increase in the chain loadings in the case of the MCM-41 having 3.5 nm or around 5.2 nm pore diameter in the case of poorly ordered materials led to a decrease in the turn-over frequency (TOF) which is observed in the case of well ordered MCM-41 possessing 5.2 and 10 nm. Moreover with these last supported catalysts possessing variable ephedrine-grafted chain/ unfunctional chain ratios, the observation of constant TOF value (3.4 h'') is consistent with an absence of diffusional limitation. However, the activity remains lower than in homogeneous catalysis for which the TOF is 8 h ' using (-)-A^-propyl-ephedrine which can be considered as analogue of grafted (-)-ephedrine. These supported catalysts featuring higher hydrophobic properties showed higher chemical and textural stabilities because they can be reused several times without notable loss in enantioselectivity and activity. Other organic bases different from primary amine such as piperidine and guanidines have been anchored on MCM-41 according to the same methodologies than previously. These supported bases have been tested in different catalyzed reactions, either Knoevenagel condensation reaction and epoxide-ring opening or in more demanding reaction such as transesterification. Primary amine- and tertiary amine- containing MCM-41 have been used to catalyze the Knoevenagel condensation of benzaldehyde and ethylcyanoacetate in the presence of DMSO as solvent.
:c^o +
CN
n.c
""cOzEt
Basic Catalyst
/CN
c =- c\
COzEt
The activities of the primary amine grafted solid were significantly higher than that of the corresponding tertiary one. This result was explained by a transient formation of imine group resulting from rcation of benzaldehyde with anchored primary amine, which cannot occur with the tertiary one. The anchoring of 3-aminopropylsilane on MCM-41 surface was carried out by silylation procedure, hence the environment of the grafted chains is also constituted by uncovered surface although the loadings in 3-aminopropylsilane chains are higher than that of 3chloropyl or unfunctional alkyl chains, probably due to the nucleophilic assistance induced by
COzEt
EtOzC
423
amine groups. It is noteworthy that the TOF of the catalyzed reaction (4.7 mn') was lowered when the residual silanols of the functionalized solid were passivated by trimethylsilylation (3.9 mn-'). Considering the imine formation in the catalytic cycle, the concentration in this transition intermediate could be kinetically favoured by an electrophilic activation by surface silanol at the carbonyl function of benzaldehyde. Moreover, the hydrophilic properties of the uncovered surface could also help to displace the equilibrium reaction leading to water in addition with the grafted imine. The difference in activities between passivated and not passivated primary amine-containing silica support is more pronounced with the use of silica gel in place of MCM-41. Silica gel having higher surface silanol density (5-6 silanols per nm^) than MCM41 silica (2-3 silanols per nm^), this result argues in favour of the role played by the uncovered surface. The catalytic activity of the amine-grafted MCM-41 has been investigated in the formation of monoglycerides by direct addition of fatty acids on glycidol. With MCM-41 containing aminopropyl groups, 15% yield of monoglyceride were observed after 6h of reaction. A higher yield of 59% was achieved in the presence of grafted piperidine which could not be converted into p-aminoalcohol. The selectivity has been improved by treating the grafted catalysts with trimethylsilylating agent such as hexamethyldisilazane, thereby masking residual silanol groups of MCM-41 material which undergo side reactions with glycidol such as polymerization. Attention has been focussed recently on the developpement of strong solid bases to perform more demanding reaction such as Michael reaction or trans esterification reaction. CH3CH2C02Et
n-Bulanol ^-^
CH3CH2C02n-Bu + EtOH
Catalyst
Strong organic bases such as guanidinc have been also anchored on MCM-41.""'^ Among the different guanidine anchored to MCM-41, l,5,7-triazabicyclo[4.4.0]dec-5-cne (TBD) revealed an activity in transcsterification of cthylpropionatc with /7-butanol. The catalytic activity of TBD-supported MCM-41 prepared v/a silylation method, was almost totally lost after the first run due to the structure collapse upon silica restructuration. In order to enhance the chemical stability of the support, the chloropropyl-grafted MCM was prepared by the coating method, and then functionalized by TBD substitution.
Although the chemical stability of these materials was significantly improved, the activity demonstrated by the recovered catalyst after the first run was notably reduced (1^ run : TON 7.5). : 22; 2"' run: TON :
424
This result prompted us to investigate another way to enhance the texture preservation of the siHca framework. In the hterature, the surface of MCM-41 has been already overlayered by aluminium or tin oxides and that of MCM-48 by titanium oxide . Though titanium oxide is more resistant toward hydrolysis under basic conditions, we have investigated the MCM-41 overlayering by TiOx followed by the functionalization of the asmodified surface by halogenopropyl phosphonic acid. The aim of such modification was to test both surface coverage by Ti02 and the chemical resistance of the titanium-phosphonate linker under -p-^Ti. basic conditions. This solid revealed three time SiOJ lower catalytic activity than the TBD-containing MCM-41 prepared by coating method, but higher improvement of both texture stability of the silicic O framework and preservation of the catalytic activity during the 2""^ run (>80%) were acquired by this new mode of functionalization. 4. CONCLUSION While organofunctionalized MCM-41 are truly fascinating for their potential application in fine organic chemistry, they possess a major drawback ie the chemical instability of silica framework, which could hamper their use as catalysts. At this stage, supported catalyst bound on MTS materials prove very efficient under experimental conditions avoiding the presence of water or strong bases, which promote Si-O-Si solvolysis. Advances in the use of MTS based materials under alkaline conditions can be expected provided the silica surface be protected by organic lining/coating or overlaying by base resistant mineral oxides. REFERENCES 1. C.T.Kresge, M.H. Lconowicz, W.J. Roth, J.C. Vartuli,.J.S. Beck, Nature. 359 (1992) 710. 2. D. Brunei, A.C. Blanc, A. Galarneau, F. Fajula, Catal. Today, 73 (2002) 139. 3. D. Trong On, S. Kaliaguine, Angew.Chem. Int. Ed., 41 (2002) 1036. 4. D. Desplantier-Giscard, A. Galarneau, F. Di Renzo, F. Fajula , Stud. Surf Sci.Catal., 135 (2001)205(06-0-27). 5. T. Martin, A. Galarneau, D. Brunei, V. Izard, V. Hulea, AC. Blanc, S. Abramson, F. Di Renzo, F. Fajula, Stud. Surf Sci. Catal. , 135 (2001) 205 (29-O-02). 6. X. Zhao, G.Q. Lu, X. IIu, Microporous Mesoporous Mater., 41 (2000) 37; K. Kchrijnemakers, E.F. Vansant, J. Porous Mater., 8 (2001) 83 ; M. Wildenmeyer, S. Grasser, K. Kohler,R. Anwander, Microporous. Mesop. Mater., 44 (2001) 327. 7. S. Abramson, M. Lasperas, D. Brunei, Tetrahedron: Asymmetry, 13 (2002) 357. 8. N. Bellocq, S. Abramson, M. Lasperas, D. Brunei, Tetrahedron Asymmetry, 10 (1999) 3229. 9. M. Lasperas, T. Llorett, L. Chaves, I. Rodriguez, A. Cauvel, D. Brunei, Stud. Surf Sci. Catal., 108(1997)75. 10. A. Cauvel, G. Renard, D. Brunei, J. Org. Chem., 62 (1997) 749. 11. Y.V. Subba Rao, D.E. De Vos, P.A. Jacobs, Angew. Chem. Int Ed.Eng., 36 (1997) 2661. 12. A. Derrien, G. Renard, D. Brunei, Stud. Surf Sci. Catal., 117 (1998) 445.
425
13. R. Sercheli, R.M. Vargas, R. Sheldon, U. Schuchardt, J. Mol. Catal.,148 (1999) 173. 14. S. Jaenicke, G.K. Chuah, X.H. Lin, X.C. Hu, Microporous Mesoporous Mater., 35/36 (2000) 143. 15. M. Lakshmi Kantam, P. Screekanth, Catal. Lett., 77 (2001) 241.
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Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
427
Proteosilica - mesoporous silicates densely filling amino acid and peptide assemblies in their nanoscale pores K. Ariga,* Q. Zhang, M. Niki, A. Okabe and T. Aida* ERATO Nanospace Project, National Museum of Emerging Science and Innovation Bldg., 2-41 Aomi, Koto-Ku, Tokyo 135-0045, Japan In order to prepare protein-like structure in inorganic mesostructured framework, methodologies to densely fill assemblies of amino acids and peptides in mesoporous silica (Proteosilica) are proposed in this study.
Surfactants having a peptide residue with
hydrophobic chain at C-terminal and polar group at N-terminal were newly synthesized, and mesoporous silicates in both powder and film forms were successfully prepared with these templates.
Especially a spin-coating method effectively provided transparent
mesoporous films, and formation of regular structures significantly depends on hydrogen bonding formed between the peptide moieties. 1. INTRODUCTION Highly-sophisticated functions are performed in many biological systems where proteins play a central role.
Controlled arrangement of peptide segments in protein
nanospaces leads to biological functions with incredible precision and efficiency. Protein architecture can be regarded as ultimate specimen of nanosizcd engineering and technology. However, mechanical and thermal weakness of protein structures does not fit practical engineering conditions.
Therefore, protein-like fine organization has to be hybridized
with nanostructured strong frameworks.
Recent development of mesoporous silica
technologies [1-4] would provide a chance to prepare nanofabricated inorganic framework suitable for peptide mimic immobilization.
We have proposed novel kinds of
peptide/silica nanocomposites with regular structure that arc named as "Proteosilica". They can be obtained by densely filling amino acids and peptides in mesoporous silica by
428
two methodologies (Figure 1). The first one is preparation of mesoporous composites between sihca and peptide-carrying surfactants (type 1).
The second method is based on
covalent-immobiHzation of surfactant to silica backbone and subsequent removal of alkyl tails through hydrolysis (type 2).
In this report, preparation of highly transparent
mesoporous film by type 1 method is briefly introduced.
I I I I I N*
>=o,,
''H-N
N*
\=o,
''H-N
N'
\=o.,
''H-N
N*
N*
>=o.-,
>=o.,
N-H*
N-H*
'H-N
'H-N
' ' S i " ^ S i ' - ' S " S i - 0 - S i " ° - S . - 0 Si°'Si-0-Si"°'Si'« N-H''
N-H''
N-H'"
/v )=o
N
°=\
Type 1 o
R
/x >=o
/s >=o
/s )=o
/x y=o
H-N
H-N
H-N
°=\
°=\
o=\ ^=\
H-N
Type 2
''^'^^N"^}^-(CH2)i5CH3 Br H o
i]
(CH2)l5CH3 Br
H
Ala, 1 ; (Ala)2, 2: (Ala)4, 3; Gly, 4; Phe 5
Fig. 1 Illustration of two types of proteosilica that densely fill amino acid and peptides in their pores. 2. EXPERIMENTAL SECTION Tetramethoxysilane (TMOS) in aqueous methanol was lightly gelated the for 10 minutes in presence of appropriate amount of HCl, and then further reacted for 20 minutes upon addition of the peptide carrying surfactants.
The obtained transparent mixed solution
429
was spin-coated on cover glass.
The obtained films were analyzed by XRD, FT-IR, and
TEM techniques. 3. RESULTS AND DISCUSSION Condition for preparation of transparent mesoporous silica films was first optimized using alanylalanine-containing surfactant (2).
As shown in Figure 2, sharp XRD patterns
are observed in appropriate range of methanol contents.
This pattern remained even after
calcination treatment with some shifts in ^loo value, indicating formation of hexagonallyarranged mesoporous silica films. (A) 1100000 cps
LA
g
VA
J J
e
d
^
b
a
s
2
1
^ ^ ^ 3
4
5
_ ^ , 1
20/deg.
Fig. 2 XRD patterns (A, uncalcined; B, calcined) of mesoporous silica films obtained from 2 at [2]/[H20]/[HCl]/[TMOS] = 0.1/1.73/0.0076/1 in molar ratio with various amount of MeOH. fMeOHl/[TMOSl in preparative condition:a, 4; b , 5 ; c , 6 ; d , 7 ; e , 8;f,9;g, 10. The optimized condition at [MeOH]/[TMOS] of 8 (mol/mol) was applied to the other surfactants.
Hexagonal structure is similarly obtained except surfactant 3 (Table 1).
430
Transparent appearance of the film from 1 is demonstrated in Figure 3, and hexagonallyaligned mesopore structures in this film were also confirmed by TEM observation.
FT-
IR measurement on amide I region of these films revealed that unstructured film from 3 forms too strong hydrogen bonding probably in parallel p-sheet motif and that are not unfavorable for rod-like micellar template formation.
How^ever, mixing other components
such as 1 or 2 effectively moderated hydrogen bonding strength resulting in successful mesoporous silica formation.
Similarly, various mesoporous silica films densely
containing amino acids and peptides can be obtainable under appropriate control of hydrogen bonding formed in mesopores. Table 1 XRD data for various silica films Surfactant
dmJ k
1 2 3 4 5
Uncalcined 40.5 42.4 No Peak 42.0 41.6
d\ml k Calcined 30.0 29.8 No Peak 34.0 31.0
Fig. 3 Transparent appearance of mesoporous silica film obtained from 1.
4. CONCLUSION We have demonstrated successful preparation of Proteosilica that is mesoporous silicates densely filling various amino acids and peptides in its pores.
The amino acid
residues immobilized in these Proteosilica films are not in strongly-bonded state, and it would be open to binding of external molecules at activated hydrogen bonding sites. Therefore, the inner pores of the Proteosilica can provide environment similar to hydrophobic binding sites or reaction pockets of actual proteins. REFERENCES 1. F. Schuth and W. Schmidt, Adv. Mater., 14 (2002) 629. 2. M. E. Davis, Nature, 417 (2002) 813. 3. A. Okabe, T. Fukushima, K. Ariga and T. Aida, Andcw. Chcm. Int. Ed., in press. 4. K. Kageyama, J. Tamazawa and T. Aida, Science, 285 (1999) 2113.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
431
Counteranion effect on the formation of mesoporous materials under acidic synthesis process Shunai Che,^'* Mizue Kaneda,^ Osamu Terasaki^ and Takashi Tatsumi ^ ^Division of Materials Science & Chemical Engineering, Faculty of Engineering, Yokohama National University, 79-5 Tokiwadai, Yokohama 240-8501, Japan ^Department of Physics, Graduate School of Science, and Center for Interdisciplinary Research Tohoku University, Sendai, 980-8578, Japan Well ordered mesophases 3d-hexagonal Pdj/mmc, cubic Pm3n, 2d-hexagonal p6mm, and cubic Ia?d can be synthesized with the same surfactant cetyltriethylammonium bromide (CTEABr), in the presence of various acids. The 3d-hexagonaI Pdj/mmc mesophases have been rationally explained by EM observations, scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM). The cubic 7^3^ phase has been first synthesized under acidic conditions. 1. INTRODUCTION In the energetic self-organization process it is thought that the packing of the organic surfactant and the charge density matching between the surfactant and the inorganic precursor are essential for the formation of the ordered mesostructure [1]. The surfactant packing depends on the molecular geometry of the surfactant species, such as the number of carbon atoms in the hydrophobic chain, the degree of chain saturation, and the size or charge of the polar head group [1, 2]. In addition, it has been reported that the formation of mesostructures were affected by the solution conditions, including the surfactant concentration, pH, the presence of co-surfactant and temperature [3, 5]. However, although it has been shown that counteranions affect the formation of mesophase structure [2, 6], and the kinetics [7], the effect has still remained poorly understood and elusive, largely because of the complicated nature of the multicomponent mixtures, which often requires elaborate control of the synthesis conditions and examination of the results.
* Present address: "Nanotubulites", ICORP (International Cooperative Research Project), JST (Japan Science Technology Corporation) c/o NEC Corporation, 34, Miyukigaoka Tsukuba, Ibaraki, Japan 305-0841
432
2. EXPERIMENTAL Mesoporous materials were synthesized by using CTEABr as the surfactant and tetraethyl orthosiHcate (TEOS) as a siHca source in the presence of various acids: H2SO4, HCl, HBr, HNO3. Typically, the molar composition of the reaction mixture was CTEABr:TEOS:Acid:H2O=0.13:l:x:125, where x were 3.6, 7.2, 5.0, and 1.0 for H2SO4, HCl, HBr, and HNO3, respectively. The mixture was allowed to react at 0°C under static conditions for 1 day after homogenization. The resultant white precipitates were filtered, and dried at 100 "C overnight. 3. RESULTS AND DISCUSSION Fig. la shows the X-ray diffraction (XRD) patterns of the sample synthesized in the presence of H2SO4. The three well-resolved peaks in the range of 26 = 1.5-3'' and additional three weak peaks in the range of 3.5 '^ to 6 ° are characteristic of the 3d-hexagonal P63/mmc mesophase [8] with the unit cell parameters, a = 49.473 A and c = 80.978 A, for as-synthesized sample. This gives a c/a ratio of 1.637, which is close to the ideal c/a ratio of 1.633 for the hexagonal close-packed (hep) structure of hard spheres. This sample demonstrates distinct 20 crystal faces with one 6-fold axis (Fig. 2a). The surfaces of the particles are indexed as shown in Fig. 2b, which is consistent with 6/mmm point group symmetry. The electron transmission micrograph of the Pds/mmc crystallite taken with the (100) incidence is presented in Fig. 2c. Obviously, cages are stacked along the c-axis solely in the "ABAB... "sequence characteristic of the hep structure. The corresponding electron diffractogram (inset) supports extinction conditions of the reflections, for the 3d-hexagonal symmetry. The powder from synthesis gej^s with HCl shows XRD pattern (Fig. lb) of the cubic P/W3A2 mesostructure [9]. The morphology, surface indices and HRTEM images of this material have already been reported [10]. When the HBr acid was used in the synthesis, the material shows the 2dhexagonal p6mm system (Fig. Ic). This sample has spiral or gyroid morphology (not shown), the typical morphologies of the 2d-hexagonal p6mm mesophases synthesized under acidic conditions [11]. From the XRD patterns presented in Fig. Id, it can be seen that highly ordered _ mesoporous materials consistent with the cubic/(33c/ symmetry are obtained with HNO3. We cannot observe a crystal with morphology characteristic of the symmetry.
(c)
Fig. 1. XRD patterns of assynthcsized materials synthesized with various acids at 0 °C for different times, (a) H2SO4, (b) HCl, (c) HBr, and (d) HNO3.
433
f
(ii)
Fig. 2. SEM images (a), surface index (b) and HREM image (c) of the samples synthesized with H2SO4 The diagrams of the products for the silica-surfactant mesophases synthesized in the presence of various acids at 0 °C for 1 day have been drawn. The 3d-hexagonal Pds/mmc mesostructures are obtained with H2SO4 or HCl as acid; H2SO4 gives this mesophase in the wider composition range than HCl. The cubic Pm3n mesostructures are obtained with three acids of H2SO4, HCl, and HBr; HCl gives this mesophase in the widest composition range. The 2d-hexagonal p6mm mesostructure is synthesized in the wide range of reactants compositions when HBr or HNO3 is used. Only HNO3 produces cubic/a3J mesophase among the tested acids. It is noteworthy that the diagrams are dependent on the H2O/TEOS molar ratio and temperature as well as the synthesis time. The effect of counteranions on the formation of mesostructures can be explained in terms of the adsorption strength on the head groups of the surfactant micelle. It is useful to introduce surfactant packing parameter g, g=v/al where v is the chain volume, a is hydrophobic/hydrophilic interfacial area and / is chain length. The X ions are more or less hydrated in the surfactant solution. Less strongly hydratcd ions have in general smaller ionic radii and bind more closely and strongly on the head-group of the surfactant. The small anions contribute to the partial reduction in the electrostatic repulsion between the charged surfactants head-groups and the decrease in the effective area of surfactant a, therefore resulting in a significant increase in the g value. The aggregation number or the ionic radii is reported to decrease in the following order: 1/2S04^ > CI > Br > NO3 [12]. Thus, it is reasonable that H2SO4 leads to facile formation of 3d-hcxagonal^ Pdj/mmc mesophase with a smaller g parameter, and that HNO3 favors the formation of /a3n , 3d-hexagonal P6i/mmc < 2d-hexagonal p6mm < cubic/a3^ [13]. From our results, it can be concluded that the 3d-hexagonal P6i/mmc lyotropic phase has a smaller g parameter than the cubic Pm'in .
434
ACKNOWLEGEMENT The authors are grateful to Y. Shimada (Instrumental Analysis Center, Yokohama National University,) for taking scanning electron micrographs. For financial support, O.T. thanks CREST, JST and T.T. thanks JCII, NEDO, respectively. REFERENCES 1. Huo, Q., Margolese, D. I., Ciesla, U., Demuth, D. G, Feng, P., Gier, T. E., Sieger, P., Firouzi, A., Chmelka, B. F., Schuth, F., Stucky, G. D., Chem. Mater, 6 (1994 ) 1176. 2. Israelachvili, J. N., Mitchell, D. J., Ninham, B. W., J. Chem. Soc, Faraday Trans. 2, 72 (1976) 1525. 3. Vartuli, J. C , Schmitt, K. D., Kresge, C. T, Roth, W. J., Leonowicz, M. E., McCullen, S. B., Hellring, S. D., Beck, J. S., Schlenker, J. L., Olson, D. H., Sheppard, E. W., Chem. Mater. 6(1994)2317. 4. Tolbert, S. H., Landry, C. C , Stucky, G D., Chmelka, B. F., Norby, P, Handon, J. C , Monnier, A., Chem. Mater. 13 (2001) 2247. 5. Che, S. A., Kamiya, S., Terasaki, O., Tatsumi, T, J. Am. Chem. Soc. 123 (2001) 12089. 6. Kim, J., Ryoo, R., Chem. Mater. 11 (1999) 487. 7. Lin, H. P, Kao, C. P, Mou, C. Y, Liu, S. B., J. Phys. Chem. B, 104 (2000 ) 7885. 8. Huo, Q., Leon, R., Petroff, P M., Stucky, G D., Science, 268 (1995) 1324. 9. Sakamoto, Y, Kaneda, M., Terasaki, O., Zhao, D., Kim, J. M., Stucky, G D., Shin, H. J., Ryoo, R., Nature, 408 (2000) 449. 10. Che, S. A., Sakamoto, Y, Terasaki, O., Tatsumi, T, Chem. Mater. 13 (2001) 2237. 11. Yang, H., Coombs, N., Ozin, G A., Nature, 386 (1997) 692. 12. Ray, A., Nemethy, G, J. Am. Chem. Soc. 93 (1971) 6787. (b) Gaillon, L., Lelievere, J., Gaboriaud, R., J. Colloid Interface. Sci. 213 (1999) 287. 13. Huo, Q., Margolese, D. I., Stucky, G D., Chem. Mater, 8 (1996) 1147.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
435
Influence of alumination pathway on the steam stability of Al-grafted MCM-41 Robert Mokaya School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, UK. The method used to prepare Al-grafted MCM-41 materials is an important factor in determining steam stability. Al-grafted MCM-41 materials prepared via supercritical fluid mediated alumination of pure silica MCM-41 or via so-called 'dry' grafting (i.e., pure silica MCM-41 grafted with Al in hexane) exhibit remarkably higher steam stability compared to materials grafted in aqueous media. The order of steam stability is, supercritical grafted > dry grafted > wet grafted. We propose that the difference in steam stability between supercritical, dry and wet grafted Al-MCM-41 materials is due to the way in which the Al interacts with the host silica framework and in particular the extent to which the Al is sorbed onto rather than into the framework. 1. INTRODUCTION Mcsoporous aluminosilicates are currently attracting considerable research effort due to their potential use as heterogeneous solid acid catalyst, especially for large molecule transformations.'"^ Although, in general, mesoporous aluminosilicates are only moderately acidic compared to zeolites, they are however potentially useful catalysts for large molecule transformations that do not require very strong acid sites. A key requirement for their successful use as solid acids (or ion exchangers), is good hydrothermal stability in hot aqueous solutions and under steaming (high temperature hydrothermal) conditions."^ Recent work has shown that mesoporous aluminosilicates with improved hydrothermal stability may be prepared via post-synthesis grafting routes or from zeolite seeds as inorganic framework precursors."''^ Mesoporous aluminosilicates prepared via post-synthesis alumination routes offer certain advantages over similar but directly synthesised materials with respect to accessibility to active (Al) sites and structural ordering. Post-synthesis alumination is therefore fast becoming an attractive alternative route for the preparation of mesoporous aluminosilicates derived from various forms of mesoporous silicas. Previous studies on the hydrothermal stability of Al-grafted MCM-41 have shown that the post-synthesis alumination pathway (i.e., grafting in either aqueous or non-aqueous media) does not have any significant effect on the level of stability in boiling water.^ We have now investigated the high temperature hydrothermal (steam) stability of Al-grafted mesoporous aluminosilicates and show here that the post-synthesis alumination pathway is a critical factor in determining the steam stability of Al-grafted MCM-41. In particular we show that the solvent used during post-synthesis alumination of pure silica MCM-41 has a significant effect on the steam stability of the resulting Al-containing MCM-41 materials.
436
2. EXPERIMENTAL The Al-grafted materials were prepared, at a target Si/Al ratio of 10, via an aqueous, nonaqueous or supercritical fluid (SCF) mediated alumination method; in the aqueous (or wet) method 1.0 g of calcined purely siliceous MCM-41 was added to a 50 ml solution of aluminium chlorhydrate (ACH) and stirred for 2 hours. The solid was obtained by filtration and thoroughly washed with distilled water (until free of CI ions), dried at room temperature and calcined in air at 550^0 for 4 hours to obtain the (wet) Al-grafted material which was designated HIO. In the non-aqueous (or dry) route 2.0 g of Si-MCM-41 was dispersed in 50 ml dry hexane and added to 150 ml dry hexane containing the appropriate amount of aluminium isopropoxide. The resulting mixture was stirred for 10 minutes and allowed to react at room temperature for 24 hours. The obtained powder was filtered, washed with dry hexane, dried at room temperature and calcined at 550"C for 4 hours to yield a sample designated PIO. For supercritical fluid mediated alumination, the required amounts of calcined mesoporous silica and aluminium isopropoxide were placed in a 60 ml magnetically stirred, high pressure autoclave and while under vigorous stirring, the temperature was raised to llO^C before pressurization with supercritical propane (150 bar). Vigorous stirring was continued for 19 hours after which the autoclave was depressurized slowly over 15 min. The autoclave was allowed to cool to room temperature and the (dry) sample recovered. The dry sample was then calcined at 600"C for 4 hours to obtain the Al-grafted material designated SIO. The high temperature hydrothermal (steam) stability of the Al-graftcd materials was evaluated by subjecting them to heat treatment at 900"C for 4 hours in a flow of nitrogen saturated with water vapour at room temperature. 3. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of the wet, dry and supercritically grafted materials (all with a Si/Al ratio of ca. 10) before and after steaming at 900"C for 4 hours. The patterns of the Al-graftcd materials are typical of well ordered MCM-41; all three samples exhibit an intense (100) diffraction peak and some well resolved higher order (110), (200) and (210) peaks. The influence of the alumination pathway on steam stability is clearly illustrated in Figure 1. The XRD patterns of the steamed samples clearly show that despite a similar Al content, the supercritically grafted sample is much more steam stable compared to the other two samples. It is also clear that the dry grafted sample is more stable compared to the wet grafted sample. The wet grafted sample (HIO) is severely degraded after the hydrothermal treatment. The supercritically grafted sample (SIO) on the other hand, still retains excellent structural ordering after steaming. The dry grafted sample (PIO) also exhibits considerable stability, which is intermediate between that of the wet-grafted and supercritically grafted samples. Table 1 shows the textural properties of the three samples before and after steaming (the steamed samples are designated as SHIO, SPIO and SSIO). After steaming, the wetgrafted sample retained only 10% of its original surface area and 20% of pore volume. The dry grafted sample, on the other hand, retained 78% of its original surface area and 53% of pore volume. The supercritically grafted sample retained much of its surface area (88%) and
437
pore volume (79%) after steaming. Another indicator of stability is the extent to which the pore size reduces after steaming. As shown in Table 1, steaming induced reduction in pore size is lowest for the supercritically grafted sample.
J 0
2
(a)
4
2^/°
6
8
0
2
4
6
2 91°
8
0
2
4
6
2er
Fig. 1. Powder XRD patterns of (a) wet-grafted, (b) dry-grafted and (c) supercritically grafted Al-MCM-41 before (top) and after (bottom) steaming at 900°C for 4 hours. Table 1 Textural properties and acidity of Al-grafted materials before and after steaming Pore APD[a] Sample Surface Volume Area
(A) (cm'/g) (m'/g) HIO 0.73 35.0 835 0.15 SHIO 86 36.8 0.83 PIO 902 0.44 25.0 SPIO 705 0.92 36.0 SIO 833 729 0.73 33.1 SSIO [^IAPD = Average Pore Diameter estimated using the relation APD = 4V/S, where V is the mesopore volume. The pore size estimates are given here only as an indication of the trend and extent of reduction after steaming. Figure 2 shows the nitrogen sorption isotherms of the three samples before and after steaming at 900'^C. All three samples exhibit sorption isotherms characteristic of well ordered MCM-41. This is consistent with the XRD traces in Figure 1 and confirms that the alumination pathway does not have any significant influence on the structural ordering. This is a key observation in a comparative study - and we can rule out structural ordering as a factor in determining steam stability. The sorption isotherms indicate that the wet grafted sample completely losses its mesoporous structure - the sorption isotherm of the steamed wet grafted sample (SHIO) does not exhibit any mesopore filling step. The isotherm of the steamed dry grafted sample (SPIO) has a much-reduced mesopore-filling, which indicates considerable degradation but still retains some mesoporous character. The supercritically grafted sample, on the other hand, presents very high steam stability; the isotherms of both the parent and steamed sample exhibit a sharp mesopore filling step characteristic of wellordered MCM-41 materials.
438
n
1iji
E (0 03
600
(a)
600
(b)
600 (c)
400
400
400
200
200
200
"o >
0 00
0.5
1 0
0 0.0
0.5
1 0
0.0
0.5
1.
Partial pressure (P/Po) Fig. 2. Nitrogen sorption isotherms of (a) wet-grafted, (b) dry-grafted and (c) supercritically grafted Al-MCM-41 before (top) and after (bottom) steaming at 900°C for 4 hours. The results show that Al-grafted MCM-41 materials prepared via dry grafting exhibit considerably higher steam stability compared to materials grafted in aqueous media. The difference in stability between dry grafted and wet grafted Al-MCM-41 materials is most likely due to the way in which the grafted Al interacts with the host silica framework and in particular the extent to which the Al is sorbed onto rather than into the framework. Under dry grafting conditions it is likely that the Al is sorbed mainly on the outermost surface of the host Si-MCM-41 while under wet (aqueous) grafting conditions the Al may penetrate the framework (due to greater hydrolysis of the host silica framework) and occupy both surface and near surface sites. The extent to which Al is sorbed into (penetrates) the host silica framework is expected to be lower when the grafting is performed under dry conditions. This is because the host silica framework does not undergo any significant hydrolysis during the 'dry' grafting procedure. For supercritically grafted samples, it is likely that the low solvating power of SCFs ensures even more efficient deposition of Al onto rather than into the silica framework. No hydrolysis of the host silica framework occurs during the SCF mediated alumination. Furthermore, better dispersion of Al achieved under SCF conditions can be expected to coat efficiently the surface of the host Si-MCM-41 with a protective aluminosilicate layer. Removal of Al (i.e., dealumination) which occurs during steaming is therefore more detrimental to the structural integrity of wet grafted samples due to extraction of Al sited deeper within the framework. Steam stable Al-grafted MCM-41 materials are therefore best prepared via alumination pathways that efficiently coat the outermost parts (i.e., pore wall surfaces) of the host pure silica material with Al without introducing Al deep into the silica (pore wall) framework. REFERENCES 1. J.Y. Ying, C.P. Mehnert and M.S. Wong, Angew. Chem. Int. Ed., 38 (1999) 56. 2. D.T. On, D. D. Giscard, C. Danumah and S. Kaliaguine, Appl. Catal. A, (2001) 299. 3. R. Mokaya, Angew. Chem. Int. Ed., 38, (1999) 2930. 4. R. Mokaya, Chem. Commun. (2001) 633; S.C. Shen and S. Kawi, Chem. Lett. (1999) 1293 5. Y. Liu and T.J Pinnavaia, J. Am. Chem. Soc, 122, (2000) 8791. 6. Z. Zhang, et al., Angew. Chem. Int. Ed., 40, (2001) 1258. 7. D.T. On and S. Kaliaguine, Angew. Chem. Int. Ed., 41, (2002) 1036. 8. R. Mokaya, ChemPhysChem, 3, (2002) 360.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Macroporous titanium oxides: from highly aggregated to isolated hollow spheres P. Reinert^ C. G^aiIlat^ R. Spitz^ and L.Bonneviot^ ^Laboratoire de Chimie Theorique et Materiaux Hybrides, Ecole Normale Superieure de Lyon, 46 allee d'ltalie, 69364 Lyon et Institut de Recherches sur la Catalyse, 2 av A. Einstein, 69626 Villeurbanne. ^Laboratoire de Chimie et Procedes de Polymerisation, Ecole Superieure de Chimie Physique Electronique de Lyon, 43 boulevard du 11 novembre, 69616 Villeurbanne The control of both poly condensation of the titanium alkoxide, [Ti(OR)4], and alkoxide coverage of the templating monodispersed polystyrene (PS) beads in suspension lead to various morphologies of macroporous solids after calcination. By increasing the ratio H20/[ri(OR)4], the solids progressively evolve from a dense interconnected macroporous network to almost isolated hollow spheres characterized using SEM. 1. INTRODUCTION In the process to further improve the diffusion of liquid or solid within a porous solid (absorbent or catalyst), the search for hierarchical porosity is a very challenging topic [1]. In this newborn field, the control at the macroscopic scale among that of the meso- and microscopic scales is the newest [2-5]. A better knowledge on the conditions required to generate connected macropores is necessary. The sedimentation-aggregation method may lead to v£irious stages of aggregation of the templating PS beads and, therefore, to solids of v£irious density. The preparation of hollow spheres aggregates of low connectivity generate a superporosity at a higher scale while isolated spheres may find interesting applications. There are a variety of methods used to fabricate hollow spheres and one of the newest is based on polystyrene bead templating. One of these method developped by Caruso et al [6] consists of coating nanoparticles of oxides on the beads by using layers of charged polymers, that allows the adherence of the successive inorganic layers and control their thickness. Encapsulation has also been envisaged by Zhong et al. who obtained hollow spheres using a gelification process in the presence of polystyrene beads entrapped between two glass plates [7]. Other authors [8] performed the poly condensation of the alkoxide precursor using a carbon template and are able to move from macroporous to hollow spheres of oxides by changing the number of infiltration. The present method consists of an impregnation of the PS beads by titanium alkoxide in solution followed by a sedimentation-aggregation process [9] that allows also a control of the morphology of the solid. The packing of the impregnated beads depends on the hydrolysis rate of the alkoxide controlled here varying both water/alkoxide ratio and stirring rate during the addition of the alkoxide in the suspension of the PS beads.
440
2. EXPERIMENTAL Non-crosslinked PS beads were synthesized by emulsifier-free emulsion polymerization. The polymerization was carried out in a 1 -L glass reactor equipped with an anchor stirrer and filled with water heated to 70 °C and degassed under N2. Ammonium persulfate was used as initiator. The beads obtained have an average size of 630 nm and their monodispersity was determined by light-scattering measurement and also estimated using SEM. The beads were kept in water and were transferred in absolute ethanol just before the synthesis and their size increases up to 710 nm. Titanium ethoxide (diluted in absolute ethanol) was slowly added under stirring to the suspension of polystyrene beads in absolute ethanol. Water may be added at this stage in order to control the condensation of the oxide precursor. After 20 min of stirring, the mixture was kept at room temperature to allow the sedimentation of the beads to proceed. After 5 days, the supernatant solution was eliminated and the solid dried at 80°C overnight. Polystyrene was removed from the solid by calcination in air at 500°C for 8 hours. 3. RESULTS AND DISCUSSION Various synthesis parameters such as water content in the mixture, alkoxide content or stirring time were studied (Table 1). For all the samples, the beads diameter measured after the drying step is larger (730 nm) than the initial one (710 nm). This suggests that the titanium alkoxide has reacted with water giving a layer of titanium oxide precursor on the beads (amorphous according to XRD measurement). Moreover this diameter increases from 730 to 770 nm when H20/Ti molar ratio goes from 0.2 to 1.9 for the same amount of alkoxide (compare samples A to E). This shows that thicker walls are obtained at higher degree of polycondensation of titanium oxide (higher H20Ari ratio). For ratios H20Ari ^ 1, the titanium oxide obtained after calcination at 500°C (anatase phase) is a macroporous solid (samples A and B). Table Synthesis conditions and morphological characteristics of the titanium oxides Coated beads Calcined solid (500°C) H20/Ti x^ Sample diameter ^ (nm) molar ratio Pore diaWall thickMorphology -meter ^ (nm) -ness ^ (nm) 02 730 500 macroporous solid A 2x50 1 750 macroporous solid 500 2x50 B 1.4 765 500 55 C hollow spheres 765 hollow spheres 1.6 560 D 65 1.9 770 hollow spheres 545 E 55 0.2 1.25 765 macroporous solid 520 F 2x50 780 550 hollow spheres 0.15 1.5 C} 55 0.1 2 H dense gel / / / 1 I^ 0.2 770 hollow spheres 490 60 ^ weight composi tion of he s tarting mixture: 1 PS : x TET : 18 Ethanol ^ estimated from SEM observations ^ stirring time : 16 hours
441
This material is characterized by cavities (average size of 500 nm) which are connected through windows of about 110 nm diameter (Figure 1 b). These windows resuh from the numerous contact points existing between beads at low hydrolysis rate. This can be observed on the beads reported in Figure la, which surfaces present imprints resulting from the contact with other spheres. These cavities in the solid are delimited by double-wall of about 2x50 nm large. It should be noted that the complete merging of these walls can be achieved during the synthesis procedure with a centrifiigation step just after decantation. The framework contraction calculated for this morphology is about 20%. For higher ratios, 1< H20/Ti <2.5 hydrolysis is favored and hollow spheres with various degree of connectivity are formed. The latter (Figure 1 d) have an external diameter varying from 610 to 690 nm depending on the hydrolysis rate (Samples C to E). These spheres are not completely isolated since contact points exist between PS beads coated with
6^' .^•r-
(b)
(a)
'^i^^-'^
(c)
v.. V.
(d)
Fig. 1. Scanning electron micrographies of (a,c) polystyrene beads coating with titanium precursor (samples A and D respectively) and (b,d) anatase phase obtained after calcination at 500°C (samples A and D respectively)
442
amorphous titanium oxide precursor (see Figure Ic). These contact points are however in a lower proportion than for samples obtained with low hydrolysis rate (compare Figure la and Ic). It seems that for a given rate of polycondensation of the alkoxide on the beads, the interconnection between beads cannot proceed further and a wall thickness of about 55 nm is large enough to stabilize hollow spheres. This is confirmed by a framework contraction of only 10 % for H20/Ti ^ 1.6 indicating a more rigid structure. A similar evolution towards the formation of isolated spheres was observed when the coverage was increased using 50 % more of Ti(OEt)4 precursor in the starting mixture (sample G). Only 25 % more of Ti(0Et)4 still leads to a macroporous solid (sample F). Adding more than 50 % of Ti precursor produces a dense gel (sample H). The morphology of hollow spheres is also obtained when the stirring time was increased from 20 min to 16 hours (sample I). In this case the coating is increased as confirmed by the diameter of the beads which varies from 730 to 770 nm (compare samples A and I). The compacity of the solid depends clearly on the degree of hydrolysis of the titanium ethoxide covering the PS beads. The rational might resides in a low degree of polycondensation favoring the merge of the deposited layers of titanium precursor between different spheres. This is consistent with the surface of the decorated spheres shown on figures la and c. The former are softer than the latter exhibiting rougher surface and cracks for a higher hydrolysis rate. Anyhow, the hydrolysis needs to be incomplete to avoid hydrolysis in the solution and formation of bulky Ti02 particles. According to the IR spectra the bead coatings are indeed made of ill-defined polycondensates of general formula [TiOxOHy(OEt)z]n. Calcination up to 500°C leads to pure Ti02 anatase. The anatase to rutile transition has been found retarded arising at 800°C for non-macroporous Ti02 and above for macroporous solid. This peculiarity that is presently under investigation is related to the weak dimensionality of the pore walls. 4. CONCLUSION Dense macroporous, low density hierarchically structured and almost isolated hollow sphere of Ti02 were obtained using a sedimentation-aggregation of PS beads in presence of titanium ethoxide. The morphology of the oxides is governed by the rate of hydrolysis of the alkoxide and the degree of coverage of the beads. This method is likely to be applicable for other macroporous oxides with such a variety of morphologies. REFERENCES [1] Ch. Danumah, S. Vaudreuil, L. Bonneviot, M. Bousmina, S. Giasson, S. Kaliaguine, Microporous and Mesoporous Materials, 44-45 (2001) 241. [2] B.T. Holland, C.F. Blandford, A. Stein, Science, 281 (1998) 538. [3]. J.E.G.J. Wijnhoven, W.L. Vos, Science, 281 (1998) 802. [4] A. Imhof and D. J. Pine, Adv. Mater., 10 (1998) 697. [5] M. Antonietti, B. Berton, C. Goltner, H.-P. Hentze, Adv. Mater., 10 (1998) 154. [6] D. Wang, R. A. Caruso, F. Caruso, Chem. Mater., 13 (2001) 364. [7] Z. Zhong, Y. Yin, B. Gates, Y. Xia, Adv. Mater., 12 (2000) 206. [8] Z. Lei, J. Li, Y. Ke, Y. Zhang, H. Zhang, F. Li, J. Xing, J. Mater. Chem., 11 (2001) 2930. [9] S. Vaudreuil, M. Bousmina, S. Kaliaguine, L. Bonneviot, Adv. Mater, 13 (2001) 1310.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
443
Nanostructured mesoporous TiOi, ZrOi and SiOi synthesis by using the non-ionic Cm(EO)n - inorganic alkoxyde system : toward a better understanding on tlie formation mechanism J.L. Blin, A. Leonard ^, L. Gigot, O. Provoost and B.L.Su* Laboratoire de Chimie des Materiaux Inorganiques, ISIS, The University of Namur (FUNDP), 61, rue de Bruxelles, B-5000 Namur, Belgium. Phone : 00-32-81-72-45-31 The present work deals with a mechanistic study of mesoporous materials (SiOz, Zr02 and Ti02) formation. A series of polyoxyethylene alkyl ether surfactants such as Ci3(E0)n (n = 6, 12, 18), Ci6(EO)io and Ci8(EO)io have been used to prepare nanostructured mesoporous Si02, Ti02 and Zr02. In the case of silica mesoporous materials, we have correlated the structural characteristics of the recovered mesoporous molecular sieves with the VH/VL ratio of the template. The present research is expected to enlarge the amount of available informations on the possible synthesis mechanisms of mesoporous materials. 1. INTRODUCTION Until now, a large series of cationic, anionic, gemini and neutral surfactants have been used in the synthesis of ordered mesoporous silicas or non-silicas and materials labeled MCM [1,2], HMS [3], TUD [4], SBA [5], MSU [6] and CMI [7] have been obtained. The synthesis of the last ones is achieved through a neutral N^l^ pathway, in which hydrogen bondings are responsible for the cohesiveness between the non-ionic recoverable and biodegradable surfactant (N") and the inorganic precursor (1*^). Various oxide-based mesostructured materials have been explored over the past few years since these materials are very promising in applications involving electron transport, magnetic interaction, photocatalysis, semiconductors and etc. [8-10]. However, behind this important and encouraging progress and the strong attraction of new materials synthesis, less knowledge has been acquired on the synthesis mechanisms, despite the great amount of results published and available in literature and due to the important divergences in scientific findings from different authors. These disparities mainly result from the different, even though slightly, synthesis conditions used and have been object of a dispute that is far from being settled. In the present study we have investigated the effect of a series of surfactants and inorganic precursors on the synthesis of mesoporous molecular sieves in order to shed some light on the synthesis mechanism of these compounds. 2. EXPERIMENTAL Silica mesoporous materials : A micellar solution with a weight percentage of Cni(EO)n varying from 5 to 60 was prepared by dissolving the surfactant at a temperature below its cloud point value in an aqueous solution during 3 hours. The pH value of the micellar solution was then adjusted with H2SO4 to 2.0. The obtained medium was stirred for three hours at this temperature before adding drop by drop the silica source : tetramethoxysilane (TMOS). The # : FRIA Fellow * : Corresponding author
444
surfactant / silicium molar ratio is fixed to 1.5. The obtained gel was sealed in Teflon cartridges enclosed in stainless steel autoclaves and heated. The final products were recovered after ethanol extraction with a soxhlet apparatus during 30 hours and calcination under nitrogen and then air atmosphere at 550°C for 18 hours in order to remove all the surfactant and impurities. Non - silica mesoporous materials : A 50 wt.% micellar solution of Ci3(EO)6 was prepared by dissolving the surfactant at room temperature in an aqueous solution during 3 hours. The obtained medium was further stirred for three hours at room temperature before adding drop by drop the inorganic source : zirconium propoxide [Zr(OC3H7)4] or titanium propoxide [Ti(OC3H7)4]. The surfactant / zirconia or titania molar ratio was varied from 0.5 to 10. The obtained gel was sealed in Teflon autoclaves. Hydrothermal treatment was performed during 2 days at 60°C. The surfactant was removed by ethanol extraction. The final products were characterized by X-ray diffraction, electron microscopies (SEM and TEM) and nitrogen adsorption - desorption analysis. 3. RESULTS AND DISCUSSION 3.1. Silica mesoporous moleclar sieves Whatever the template is, if the weight percentage is located in the range where a hexagonal array is detected on the phase diagram, the addition of TMOS disturbs the arrangement of the micelles in solution. Indeed, the TMOS molecules interact with the hcxagonally ordered surfactant micelles and induce a displacement of the channels. This lead to recovered materials with DWM (Disordered Wormhole Mesostructure) characteristics. When decreasing the weight percentage of dccaoxyethylcnc cetyl or oleyl ether, well ordered hexagonal CMl-n materials, analogous to MCM-41 but with larger pore sizes, can be prepared [7]. In this case, isolated micelles are formed in solution and, after the addition of silica, to reach a complete polymerization of tctramethoxysilane, the supramolccular template-silica assemblies have to pack together. This cooperative effect of silica source - rod micelles interaction and polymerization of these assemblies afford highly ordered mesoporous channel arrays. Polyoxyethylene tridecyl ether, however, only affords DWM-1 type materials for Ci3(EO)i2 and Ci3(EO)i8 or DWM-2 for Ci3(EO)6. This phenomenon is reflected in the VH/VL values (Table 1), which are much different for these templates with shorter alkyl chain length compared to Ci6(E0)i() and Ci8(E0)i{) that posses a similar ratio and the same final structures. Taking into account the ratio of hydrophilic to hydrophobic volumes, it appears thus that there would be an optimal ratio that could lead to ordered structures. Disordered but homogeneous frameworks would likely to formed beyond this value whereas smaller hydrophilic portions of the template would not afford any structured materials. The use of Cm(EO)n with a hydrophilic portion that could lead to a VH/VL ratio similar to that of Ci6(E0)i() and Ci8(E0)i() will be useful in confirming this proposition. Of course our interpretations will not last universally as so many synthesis variables have to be taken into account in such preparations. In the case of ionic templates and in particular for quaternary ammonium salts QTMAX (X = CI, Br, OH...) it was reported that if n is located between 6 and 18, the hexagonal MCM-41 structure is formed whereas for n >18 only an amorphous phase is obtained [11]. It was however reported that Cji alkyl chains could lead to MCM-41 if the methyls of the headgroup were replaced by ethyl entities [12]. No micelles are formed if the value of n is less than 6. In the present study we propose that the VM/VL ratio can play a similar role as n.
445
Table 1 Contribution of the hydrophilic (VH) and lipophilic (VL) part of surfactant, obtained structure and weight percentage below which this phase appears (wt.%). C,3(EO)6
Ci8(EO),o
Ci6(EO),o
C,3(EO),2
C,3(EO),8
VH/VL
1.04
1.23
1.39
2.04
3.04
Obtained structure
a
Hexagonal
Hexagonal
Wormhole
Wormhole
25
15
*
30 wt.% a : amorphous, * value not yet determined
3.2. Non - silica mesoporous molecular sieves The Cm(EO)n-inorganic alkoxydes system was also employed to prepare pure mesoporous titania or zirconia without adding stabilizing agents such as phosphate or sulfate anions. DWM-1 type materials are recovered with titanium propoxide whereas only DWM-2 are formed with zirconium propoxide. For porous zirconia, our results obtained by XRD, TEM and nitrogen adsorption-desorption analysis, not reported here in detail, evidenced the role played by the template and the inorganic source during the synthesis. These results also led us to propose the mechanism of formation represented in Figure 1. When zirconium propoxide is added to the micellar solution, its rapid polymerization will disturb the micellar array. Furthermore, the large amount of propanol produced during the hydrolysis of Zr(0Pr)4 will provoke a change of organization of surfactant micelles in solution. This perturbation of the surfactant molecules associated with the high reactivity of Zr(0Pr)4 will lead to the destruction of the 111 micellar arrangement phase. Isolated spherical and cylindrical micelles can coexist in solution. Hydrogen bonding interactions will take place between the zirconium propoxide and the ill-aggregated entities of template. But, as the reactivity of zirconium is predominant on these interactions, no cooperative mechanism leading to the formation of highly ordered mesoporous materials can occur and only supermicroporous molecular sieves are formed after surfactant removal. Concerning mesoporous titania, complementary studies arc needed to propose a formation mechanism. 4. CONCLUSIONS Ordered mesoporous silica CMl-1 and CMl-3 can be prepared by decreasing the weight percentage of Ci6(E0)i() and Ci8(EO)io in solution. This is a cooperative effect of interface interactions of hydroxyl groups of silica source-rod micelles and polymerization of silica source covering the outer layer of rod micelles. Whereas for polyoxyethylene tridecyl ether, only DWM-1 type materials [for Ci3(EO),2 and C,3(E0)IK] or DWM-2 [for C,3(EO)6] are obtained. The different behavior is explained by considering the VH/VL ratio. In the case of non silica molecular sieves, the present work reveals that the control of the balance between the precipitation rate of zirconia and the interaction of zirconium source with micellar system will be fatal in the formation of nanostructured porous zirconia. The same experimental conditions with the surfactant micelles lead to DWM-1 materials when using titanium propoxide as inorganic precursor instead of zirconium propoxide.
446
Destruction of the hexagonal array l
Spherical Micelles
Addition of Zr(OPr)4 Hexagonal array of micelles in solution
Rod Micelles Isolated molecules of Precipitation of Zr(OPr)4 surfactant around the ill-aggregated micelles of Ci3(EO)6
Fig. 1. Proposed mechanism for the synthesis of porous zirconia. ACKNOWLEDGEMENTS This work has been performed within the framework of PAI/IUAP 4-10. Alexandre Leonard thanks FNRS (Fond National de la Recherche Scientifique, Belgium) for a FRIA scholarship. REFERENCES 1. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresgc, K.D. Schmitt, C.T-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schenker, J. Am. Chem. Soc, 114(1992) 10834. 2. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. 3. P.T. Tanev and T.J. Pinnavaia, Chem. Mater., 8 (1996) 2068. 4. TUD-1, Shen, Ziping; Jansen, and al., Mesoporous amorphous silicate materials and process for the seperation. Patent application, EU4741, (1998). 5. Q. Huo, D.I. Margolese and C D . Stucky, Chem. Mater., 8 (1996) 8 1147. 6. S.A. Bagshaw, E. Prouzet and T.J. Pinnavaia, Science, 269 (1995) 1242. 7. J.L. Blin, A. Leonard and B.L. Su, Chem. Mater., 13 (10) (2001) 3542. 8. D.M. Antonelli and J.Y. Ying, Angew. Chem. Int. Edn. Engl., 35 (1996) 426. 9. P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka and G.D. Stucky, Nature, 396 (1998) 152. 10. J.L. Blin, R. Flamant. and B.L. Su, International J. Inorg.Mater., 3(7) (2001) 959. 11. M. Kruk and M. Jaroniec, Langmuir, 15 (1999) 5279. 12. R. Ryoo, C. H. Ko and I.S. Park, Chem. Commun., (1999) 1413.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Morphology control of hierarchically ordered ceramic materials prepared by surfactant-directed sol-gel mineralization of wood cellular structures Yongsoon Shin,* Li-Qiong Wang, Jeong Ho Chang, William D. Samuels, Gregory J. Exarhos Pacific Northwest National Laboratory, P.O. Box 999, MSIN: K2-44, Richland, WA 99352 We here report the synthesis of ordered ceramic materials with hierarchy produced by an in-situ mineralization of ordered wood cellular structures with surfactant-templated sol-gel at different pHs. At low pH, a silicic acid is coated onto inner surface of wood cellular structure and it penetrates into pores left, where degraded lignin and hemicellulose are leached out, to form a positive replica, while at high pH the precipitating silica particles due to fast condensation clog the cells and pit structures to form a negative replica of wood. The calcined monoliths produced in different pHs contain ordered wood cellular structures, multi-layered cell walls, pits, vessels well-preserved with positive or negative contrasts, respectively. The surfactant-templated mineralization produces ordered hexagonal mesopores with 2nm in the cell walls after calcination. 1. INTRODUCTION Self-assembly approaches to the synthesis of ordered inorganic oxide architectures are of great interest since such materials show marked improvement in separation efficiency,^'^ catalytic behavior,^^'^' and bioengineering applications'"*'. A cadre of biological and nonbiological based approaches have been used to both self-assemble nanoparticle building blocks and then assemble these fundamental units into larger structures'^'. However, postassembly removal of the templating agents used to form resulting inorganic architectures restricts the size of the ordered pore channels to a few hundred angstroms'^l Synthetic materials with ordered structures on micro, meso, and macroporous dimensions have not been successfully prepared even though considerable efforts have been expended that invoke that either temperature-programmed hydrothermal processing '^' or dual templating approaches'^^ Ordered porous materials with larger pore sizes are of considerable interest since they could provide for more rapid mass transport to the active sites thereby improving reaction kinetics. Previous efforts to silicify,'*^'"' or to carbonize''^^ wood tissues produced small segments of the negative replica of the wood structure in a porous ceramic form, but did not reproduce the structural nuances intrinsic of the natural wood material. Rather than simple deposition of a ceramic phase within the wood interstices or infiltration of the ceramic into the unoccupied void space as seen in the earlier publications, work reported here demonstrates chemical control of the deposition process through solution pH and the presence of a templating agent. This approach allows formation of either positive or negative replicas of the wood structure to a greater degree than has been achieved in the earlier reported studies. Our approach to synthesize silica-based systems with multidimensional pore diameters is to mineralize wood cellular structures using a surfactant-templated sol-gel solution method shown in Scheme 1. In both routes, several pieces of wood are soaked in a surfactanttemplated sol-gel solution. Heating at low temperature (40-60°C) enhances lignin leaching and penetration of silicate solution into the inner surfaces of the wood cellular networks.
448
JEOS/CIAC OHU
Sodium silicate/
!_^
l.Dry
CTAC
Scheme 1. Two routes for the formation of ordered mineralized silicate networks using wood cellular templates: (up) in acidic solutions, infiltration and mineralization of lignin-washed swollen spaces with mesostructured silicate sol (60"C), (down) in basic solutions, precipitation of silica-surfactant mesophases in wood cellular spaces by rapid condensation (40"C); A parallelgram indicates the wood cellular structure and a dot indicates silicate sol.
2. EXPERIMENTAL The molar ratios of the components in the surfactant sol-gel solution, TEOS/CTAC/HCl/EtOH/HsO, were 1.0:0.24:0.05:4.0:2.0. In the basic system, 27.0% sodium silicate solution was used as a silicate precursor: Si/CTAC/NaOH/F^O = 1.0:0.5:2.0:120. The surfactant-templated sol-gel solution was changed and renewed before precipitation started. Removal of the organic content by calcination at 550^^C gave silica monoliths with wood cellular structures. Two different types of wood samples were chosen: Poplar {Populus spp.), which is a hard wood, contains close-packed tubular cells of different sizes, ranging from a few micrometers to about one hundred micrometers, while pine (Pinus), belonging to the soft wood family, contains close-packed rectangular cells of roughly the same size. 3. RESULTS AND DISCUSSION Figure 1 shows photographs of the mineralized wood samples prepared in different pi I solutions using the surfactant-templated sol-gel solutions before and after the organic content were removed. The samples remained almost completely intact and their original shapes for both solutions with the exception that shrinkage occurred toward the middle of the samples during calcination. This problem was avoided by multiple replacements of the precursor solution. The samples prepared without the surfactant were crushed to a powder and appeared discolored after calcination due to the shortage of ordered mesopores.
.,mm
m
(a)
(b)
Fig. 1. Photographs of surfactant-templated wood samples prepared in (a) acidic (b) basic solutions before and after calcination.
449
Figure 2 shows SEM images of silica samples mineralized in acid and in base after calcination. These ordered samples retain their original cellular structures with positive or negative contrasts at both low and high magnification. At low magnification samples prepared at low pH show intact cells, cell walls, and pit structures in the cell walls. In addition, under high magnification, they show ray pitting of poplar (Fig.2a; x3000), rectangular-type cells with pine fibrous arrays (Fig.2b; xlOOO), and bordered pits (Fig.2c; x500). However, calcination of silica samples prepared at high pH leads to negative replicas. At low magnification, filled tube-type cells with negative images of pits were observed, and at high magnification negative vessel pittings in the cell walls (Fig.2d; xlOOO) and negative donuttypes of bordered pits were observed (Fig.2e; xlOOO). These negative replicas are formed at high pH due to fast condensation kinetics of silica precursors, while at low pH, the silica precursor (TEOS) is hydrolyzed quickly and condensation proceeds slowly^'^l Contrasting mechanisms drive positive and negative replica formation under different conditions. We observed positive forms of silica-wood composites with soft surfaces after 2 days at low pH, while at high pH all the cells and pits were filled or half-filled with silica after 2 days. Under high magnification SEM images show gradual filling of pits with silica as a function of time.
(a)
(b)
(c)
(d)
(c)
Fig. 2. SEM images of poplar (a, d) and pine (b, c, c) samples prepared in acidic (a, b, c) and basic (d, e) solutions after calcination at 550"C for 6 hours: scale bar; lOjum
We also investigated lignin leaching and formation of different types of the poplar-silica during the mineralization process. Significant leaching of lignin during the process may form a positive replica because lignin is an integral main cell wall constituent in all vascular plants of arborescent gymnosperms and angiosperms^'^l At low pH, EtOH keeps silicate from precipitating and promotes leaching of noncrystalline lignin and hemicellulose. At 60°C lignin was removed more quickly than in basic solution; more than 95% lignin was removed after 3 cycles. CP-MAS '"^C NMR spectra of silica-wood composites confirm the above results. The poplar tissue comprised of crystalline cellulose^'"^^ and lignin''^^ showed dominant cellulose peaks at 67.5, 72.5, 84.0, 88.8, and 105.2ppm, and lignin associated resonances were observed to be broad at 136.9, 154.0, 172.3, 194.7ppm. After a two day treatment in acid/EtOH, lignin was leached out significantly, but the crystalline cellulose peaks and the CTAC surfactant in the low chemical shift region (14-35ppm) were constant. However, basic treatment of the poplar-silica composite for 2 days showed unleached lignin. XRD patterns of as-synthesized and calcined materials in both conditions showed that the crystallinity of wood cellular structures was nearly totally preserved even after acid or base treatment (Fig. 3) and a surfactant-templated liquid-crystalline solution produced hexagonally ordered nanoporous silicate networks. In Figure 4, TEM images show that calcined poplar
450
samples prepared under both conditions are hexagonally ordered with 20A pores. Type IN2 sorption isotherms were observed, and the Brunauer-Emmett-Teller (BET) measurement of calcined samples prepared under both conditions showed higher surface area (up to 700 m^/g) than silica materials prepared without the surfactant (< 50 m^/g). Single pulse ^^Si NMR spectra of poplar-silicate composite indicate high ratios of Q^ due to the use of prehydrolyzed silicate precursors in both solutions.
after calcination 3 cycles 2 cycles ^^.^____
1 cycle
poplar untreated 20
30
40 50 2theta
eO
70
after calcination 3 cycles 2 cycles
poplar untreated
Fig. 3. Timc-dcpcndcnt X-ray diffractions of poplar silica composites prepared in (a) acidic and (a) basic solutions.
451
(a) (b) Fig. 4. TEM images of poplar samples prepared in (a) acidic and (b) basic solutions after calcination: scale bar; 20nm, 5nm (inset).
In conclusion, pH-controlled biomineralization of wood cellular structures using a templated sol-gel technique, successfully controlled the positive or negative replica form of wood cellular structures. These hierarchical silica materials contain hexagonally ordered nanopores formed by surfactant-templating at both low and high pH. Future studies address transport properties and chemical reactivity of these surfaces following chemical modification of the internal pore surfaces. ACKNOWLEDGEMENTS Pacific Northwest National Laboratory is operated by Battelle Memorial Institute for the U.S. Department of Energy under Contract DE-AC06-76RL0 1830. This work is supported by the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, of U.S. Department of Energy. REFERENCES 1. X. Feng, G. E. Fryxell, L.-Q. Wang, A. Y. Kim, J. Liu, K. M. Kemner, Science 216, 923 (1997). 2. K. Kageyama, J. Tamajawa, T. Aida, Science 285, 2113 (1999). 3. P. T Tanev, M. Chibwe, T. J. Pinnavaia, Science 368, 321 (1994). 4. Y. Shin, J. 11. Chang, J. Liu, R. Williford, Y.-K. Shin, G. J. Exarhos, J. Contr. Rel. 73, 1 (2001). 5. S. A. Davis, S. L. Burkett, N. H. Mendelson, S. Mann, Nature 385, 420 (1997). 6. C. T. Kresgc, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck, Nature 359, 710 (1992). 7. A. Karlsson, M. Stocker, R. Schmidt, Microporous Mesoporous Mater 11, 181 (1999). 8. B. T. Holland, L. Abrams, A. Stein, A. J. Am. Chem. Soc. 121. 4308 (1999). 9. G Scurfield, E. R. Segnit, Sedment. GeoL 39, 149 (1984). 10. R. W. Drum, Science 161, 175 (1968). 11. T. Ota, M. Takahashi, T. Hibi, M. Ozawa, S. Suzuki, Y. Hikichi, J. Am. Ceram. Soc. 78, 3409(1995). 12. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, ed. C. J. Brinker and G. W. Scherer (Academic Press: San Diego, CA, 1990). 13. Lignin: Properties and Materials, ed. W. G. Glasser and S. Sarkanen, S. (ACS Symposium Series 397; American Chemical Society: Washington, DC, 1989). 14. R. H. Atalla, J. C. Cast, D. W. Sindorf, V. J. Bartuska, G E. Maciel, J. Am. Chem. Soc. 102,3249(1980). 15. N. G Lewis, R. A. Razal, K. R Dhara, E. Tamamoto, G H. Bokelman, J. B. Wooten, J. Chem. Soc. Chem. Commun. 1626 (1988).
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Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
A NH3-responding mesoporous silica.
453
material based on reichardt's
dye-impregnated
B. Onida', S. Fiorilli', R. Gobetto^ A. Russo^ D. J. Macquarrie^ and E. Garrone' 'Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Torino, Italy. FAX: +39-011-564-4699. E-mail:garrone@athena.polito.it ^Dipartimento di Chimica Inorganica, Fisica e Materiali, Universita di Torino, Torino, Italy. ^Departement of Chemistry, University of York, Heslington, York, UK. Reichardt's dye impregnated HMS gives a selective, reversible and fast optical response to NH3 in air. When adsorbed on a poorly dehydrated HMS surface, Reichardt's betaine dye does not display its typical charge-transfer transition, which appears upon contact with ammonia. 1. INTRODUCTION Ordered mesoporous silicas are excellent hosts for sensing molecules because of the highly uniform porosity which allows facile diffusion and the high surface area which creates the potential to dope these materials at high concentrations.' HMS silicas are prepared using an amine as a template, which allows an environmentally benign recovery of the template by solvent extraction.^ Reichardt's betaine dye [2,6-diphenyl-4-(2,4,6-triphenyl-N-pyridinio)-phenolate, hereafter RD] is an excellent solvatochromic compound"^ with a Charge Transfer (CT) transition in the visible spectra suffering a shift of- 328 nm when passing from a polar to a non polar solvent. It has been immobilized in polymeric substrates'*, silica^ and glasses^ in order to obtain optical chemical sensors for humidity, alcohols, and other polar and non-polar vapors. The structure of the fundamental and excited state is reported in Scheme 1.
-a^;yo Scheme I
454
In this contribution the impregnation of HMS silica with RD is reported. The system was characterized by XRD, BET, TG-DTA, UV-Visible and FTIR spectroscopy. The optical response of the material to ammonia in gas phase has been investigated. 2. EXPERIMENTAL HMS silica was synthesized at room temperature using tetraethylorthosilicate, ndodecylamine, ethanol and distilled water. The solution was stirred for 18 hours, yielding a thick white suspension. This was filtered and dried at 80 °C for 1 hour. The amine was removed by heating the solid (ca 10 g) at reflux in absolute ethanol (100 ml) for 6 hours. The treatment was repeated two times. Thermal analysis showed that the template was completely removed. XRD and BET analysis confirmed the ordered structure (dioo = 0,34 nm) and the high surface area and porosity (ca 1100 m^/g, pore diamater = 0,25 nm) of the silica. Before impregnation, two amounts of silica were calcined at 300°C (HMS-300) and 600°C (HMS-600), respectively. After calcination, both were put in contact with a dichloromethane solution of RD (green) and kept under stirring overnight. Powders were recovered by filtration: solutions were colourless in both cases, i. e. the dye was totally adsorbed onto the mesoporous silica. The two systems obtained from HMS-300 and HMS-600 are designated hereafter as R-HMS-300 and R-HMS-600, respectively. Response to NH3 was investigated both at atmospheric pressure and by dosing ammonia in a UV-visible cell (quartz, home made) in controlled atmosphere. 3. RESULTS AND DISCUSSION Though impregnated with RD, the sample RHMS-300 was white. Indeed, the UV-visible spectrum (curve 1, Figure 1) does not reveal any CT absorption in the visible region, whereas the UV absorptions due to the conjugated aromatics rings are seen below 400 nm. In the FTIR spectrum (figure not reported) typical vibrational modes of RD are observed. The reason for the absence of CT transition in RD molecules is most likely protonation, which leads to a phenolic species with no transition in the visible region.^ The only acidic Br0nstcd sites present arc silanols. the pKa of which has been evaluated to be close to 7, i. e. silanols are probably more acidic than Reichardt's dye (pKa = ). Thus, it is reasonable to hypothesise a proton transfer where silanols act as donor and RD as acceptor.
Fig. 1. UV-visible spectra of R-HMS-300 (curve 1) and HMS-300 (curve 2).
455
3800 440
480
520
560
600
3600 3400 3200 wavenumber (cm'^)
3000
wavenumber (nm)
Fig. 2. Visible spectra of R-HMS-600 (curve 1), R-HMS-300 (curve 2) and HMS600 (curve 3).
Fig. 3. FTIR spectra of HMS-300 (curve 1) and HMS-600 (curve 2)
Figure 2 reports the UV spectrum of R-HMS-600 (pale pink, curve 1) as compared to those of R-HMS-300 (curve 2) and HMS-600 (curve 3). The CT absorption is visible at 490 nm: a certain amount of dye molecules are not protonated, because calcination at 600 °C decreases the number of Bronsted sites able to protonate RD. As it is known (and confirmed by FTIR spectra in Figure 3), calcination at temperature higher than 300°C causes dehydroxilation of the silica surface, with a decrease of H-bonded silanols (broad absorption at 3500 cm') and an increase of isolated silanols (peak at 3747 cm'). Terminal silanols in Hbonded SiOH chains are known to be more acidic than isolated SiOH/ Therefore it appears likely that H-bonded SiOH patches are responsible for protonation of RD. The protonation is perturbed by NH3 (a stronger base with respect to the phenolate). In presence of gaseous NH3 R-HMS-300 changes from white to pink and R-HMS-600 from pale to strong pink. 800 400 500 600 700 The change is immediate and completely reversible. CT intensity is NH3 pressure dependent, wavenumber (nm) as shown by spectra in Figure 4 recorded at Fig. 4. Visible spectra of R-HMS-300 in incresing NH3 pressure. contact with increasing amount of A small shift of the CT maximum is observed NH3 (broken line: highest NH3 with increasing NH3 pressure, indicating a change
456
in the polarity of the system. Figure 5a represents the FTIR spectrum related to R-HMS-300 (curve 1) in contact with 10 torr of NH3, as compared to HMS-300 in the same condition (curve 2). Besides the band at 1627 cm"', observed also for HMS-300, due to the bending mode of adsorbed ammonia, a band is observed at 1465 cm'', assigned to the bending mode of ammonium ions. The intensity of this band is negligible in curve 2. Ammonium species are formed reversibly, in that outgassing leads to the vanishing of related bending mode (Figure 5b). Ammonium species are formed on R-HMS-300 by reversible deprotonation of RD: the phenolate formed by deprotonation probably stabilizes the ammonium ion, which is not formed on the naked silica surface (HMS-300).
1600 1500 1400 wavenumber (cm^)
1700
1600 1500 1400 wavenumber (cm'^)
1300
Fig. 5. Section a: FTIR spectra related to R-HMS-300 (curve 1) and HMS-300 (curve 2) in contact with 10 torr of NH3. Section b: FTIR spectra related to outgassing after adsorption of NH3 on R-HMS-300. 4. CONCLUSIONS Reichardt's dye adsorbed on poorly dehydrated HMS surface is protonated, most probably by H-bonded silanols, and thus colourless. In these conditions it can reversibly protonate ammonia with re-appearance of the typical CT transition in the visible spectrum occurs. In conclusion, R-HMS-300 gives a fast, reversible, and pressure dependant optical response to NH3. REFERENCE 1. 2. 3. 4. 5. 6.
B. J. Scon etal., Chem. Mater. 2001, 13, 3140. P. T. Tanev et ai, Chem. Mater., 1996, 8, 2068. C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, VCH, Weinheim, 1988. Y. Sadaoka, M. Matsuguchi, Y. Sakai and Y. Murata, Chem. Lett., 1992, 53; Spichiger D. Crowther and X. Liu, J. Chem. Soc, Chem. Commun., 1995, 2445. F. L. Dickert, U. Geiger, P. Lieberzeit and U. Reutner, Sensors and Actuators B, 2000, 70, 263. 7. B. Fubini, V. Bolis, A. Cavenago, E. Garrone and P. Ugliengo, Langmuir, 1993, 9, 2712.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Preparation and redox behavior of ordered porous zirconium oxide loaded with cerium Hang-Rong Chen, Jian-Lin Shi Ji-Na Yan, Hong-Guang Chen, Dong-Sheng Yan State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050 Cerium incorporated ordered porous zirconia materials have been prepared by the surfactant-assisted route and post-grafting method. The XRD patterns, TEM, nitrogen adsorption, and UV-Vis spectroscopy were adopted for the characterization of the synthesized materials. It was shown that small amount of cerium can be incorporated into the framework of Zr02 at the cerium loading not higher than 4wt%, excess amount of cerium could exist as nano-crystalline ceria onto the pore outer surfaces. The introduction of cerium induced more shrinkage of the zirconium framework but both the specific surface area and the ordering of pore structure were retained. The H2-TPR measurement shows much more increased redox activity of the prepared mesoporous zirconia sample containing 10wt% ceria than that of the reference sample of Ceo.35Zro.65O2 prepared by the co-precipitation process. 1. INTRODUCTION The synthesis of MCM-41 silicate mesoporous molecular sieves and its application in catalysis has stimulated a great interest in the synthesis of all kinds of mesoporous materials[l]. A wide range of these new porous materials, including titania, alumina, zirconia, and manganese oxide[2-4], for use as selective ion exchangers, catalysts, etc., were prepared. Much of the early work was concentrated on the synthesis of porous zirconia with highsurface-area and well ordered pore structure even after calcinations[5]. Recently, a few researchers have tried to introduce some elements into the framework of mesoporous zirconia for structural and/or property modification[6,7]. Ceria is a crucial component of modem catalysts, since Ce02 can easily absorb and desorb oxygen owing to its nonstoichiometric behavior. It plays an indispensable role in the so called three-way catalysts[8]. Much work has been focused on the study of Ce02-Zr02 solid solution, because the reducibility and the thermal stability of Ce02 can be greatly enhanced by mixing with zirconium oxide[9]. In this paper, the ordered mesoporous Zr02 material loaded with cerium has been prepared by post-grafting method, both high-surface-area and longrange order of pore arrangement were retained. The redox behavior of such prepared sample was also reported. 2. EXPERIMENTAL The mesoporous Zr02 was synthesized by the surfactant assisted route, an aqueous solution of surfactant cetyltri-methylammonium bromide Ci6TMABr, and zirconium sulfate Zr(S04)2 • 4H2O were well mixed with a molar composition of 0.32Ci6TMABr:lZr(SO4)2 • 4H2O:120H2O. After treatment with phosphoric acid solution Corresponding author. E-Mail: jlshiCa'jsunm.shcnc.ac.cn Fax: 86-21-52413122.
458
(0.25M, 200ml) for at least 2h, the as-synthesized sample was calcined in flowing air at 500°C for 6h to remove the surfactants, named as PZr. The loading of cerium was then carried out by using a grafting method: 20 milliliters of 0.25M ammonia solution (25wt%) was dropped into a cerium(III) nitrate solution of a certain concentration to yield the transparent filemot cerium sol under a controlled pH value at 4-5 . Total of 1.5g PZr material was degassed at 140°C in a vacuum for 6h and then cooled to room temperature. This well treated sample was then mixed into the cerium sol under continuous stirring for at least 6h. The following filtering and drying (100°C, 2h) then gave the cerium-loaded Zr02 powders. Finally, the loaded sample was heat treated at 673K for 4h. In this series of experiments, mesoprous cerium-zirconium oxides were prepared with cerium contents ranging from 4-10wt%, named as Ce4, Ce6 and CelO, respectively. The X-ray powder diffraction results of samples were recorded on a Rigaku D/Max-RB Xray diffractometer with Cu target (40KV, 60mA). N2 adsorption-desorption isotherms were measured at 77.35K using a Micromeritics Tristar 3000 analyzer. Diffuse reflectance UV-Vis spectra were taken on a SHIMADZU UV-3101PC, UV-Vis-NIR scanning spectrophotometer. Hydrogen consumption was evaluated by temperature-programmed reduction (TPR) experiments using a thermal conductivity detector (TCD) of a gas chromatograph (HP 6890 series GC system). To minimize the effects of adsorbed species on TPR, all of the samples were pretreated in O2 (50ml/min) at 673K for 2h before the initial TPR experiment. The TPR characterization was performed on 0.15g catalyst under a 1%H2+99%N2 flow with a flow rate of 50ml/min from room temperature to 1173K. 3. RESULTS AND DISCUSSION The XRD patterns of the cerium-loaded samples after treatment at 673K are shown in Figure 1. The peak (100) at low 20 range (2° -^10° ) shows clearly the existence of ordered pore structure in the cerium-zirconium materials. The d (100) values distinctively decrease at the higher contents of cerium loading, which means more shrinkage of the inorganic wall of zirconium oxide. Furthermore, it should be noted that the intensity of the peak (100) gradually decreases at the higher cerium content. This indicates that the surplus amount of doped cerium may react with zirconia, inducing more condensation of the inorganic wall and the simultaneous decrease of the ordering of the pore structure. The XRD patterns for these samples within the range of 10° -80° shown in the figure 2. It can be seen that the inorganic wall of the small cerium-loading sample Ce4 has a similar noncrystalline framework structure as the undopcd sample PZr. However, some weak and broadening peaks occur in the cerium doped samples from the non-crystalline framework structure when the cerium doping was higher than 4wt%. This can be attributed to the possible interaction between the doped cerium and the zirconium framework, and the surplus cerium might form as nano-crystalline particles of Ce02 dispersed onto the pore outer surface. The N2 adsorption-desorption isotherms of both the PZr sample and the cerium-doped samples can be classified as type I, which is typical for the pore sizes between micro- and mesoporous. The values of the specific surface area for these cerium-loaded samples were between 100 to 200m^/g. Figure 3 shows the UV-Vis spectra of these cerium-loading samples, and some reference samples are shown in the small inset. The porous zirconia (sample PZr) gives an absorption peak close to ca. 210nm (trace a), and the reference sample of pure Ce02 (trace c), presents two absorption peaks at c«.360nm and 250nm. For comparison, the spectrum of the reference sample, MixZC, which was made by mixing 4wt% amount of Ce02 with sample PZr treated at 673K, is also shown in trace b. It can be seen that the sample of MixZC presents both the
459
hkl d^nm 100 2.96
hkl d/nm 100 3.044
c
b
hkl cVnm 100 3.154
2"meta
Fig. 1.The XRD patterns of the ceriumloaded samples (a)Ce4, (b) Ce6, (c) CelO.
Fig. 2. The XRD patterns within the range of 10° - 8 0 ° ofthe samples: (a) Ce4,(b) Ce6,(c)CelO.
absorption peak of Zr-0 bonding at ca.2\0nm and the Ce02 absorption band. This suggests that there is no interaction between ceria and zirconia in such a mixture. However, for the cerium-loaded porous samples, the intensity of the absorption peaks is greatly enhanced than that of sample PZr. Moreover, all of these Ce-loaded samples present the similar absorption bands close to ca300nm (trace e, f, g), which belongs neither to the Zr02 absorption nor to the Ce02 absorption. This suggests that cerium may have reacted with the zirconia and formed Ce-O-Zr linkages. In addition, the absorption bands of the cerium-loaded samples clearly redshifted at increased cerium contents. For comparison, another reference sample CCQ35Zro6502 solid solution was prepared from the chemical co-precipitation method. It is worthy noting that the UV spectrum of this sample (trace h) shows the similar absorption band to those cerium-loaded samples. This gives another support for the conclusion that the cerium has been successfully incorporated into zirconia framework. The dispersion of the ceria-zirconia sample on the surface of pore structure can be directly observed by the HRTEM. It seems that the dispersion of the nanocrystalline Ce02 on the surface of pore structure is somewhat homogeneous, which is very helpful to the application of redox catalysis. The H2-TPR profiles of these cerium loaded samples as well as the reference sample of Ce().35Zr().6502 solid solution are reported in figure4. There are altogether three peaks for the reference sample of Ce().35Zr().6502 solid solution demonstrated at around 500 °C, 600°C and 800 °C respectively, which are attributed to the reduction processes in the bulk. It can be seen from this figure that all the cerium loaded samples show the peak of the largest H2 consumption at around 600 °C similar to the reference sample Ceo.35Zro.65O2, which confirms that there are actually the same Zr-O-Ce bonds in the prepared cerium loaded samples as in the Ceo.35Zro.65O2 solid solution. In addition, the peak at the highest temperature gradually disappears and the amounts of H2 consumption greatly increase with the increase of cerium loading. Furthermore, a small peak at the lowest temperature, which is due to reduction of the surface, shifts to the lower temperature. This suggests that the cerium-loaded mesoporous zirconia possess much higher redox activity. This better redox behavior is believed to be due to smaller particle size and much higher surface area of the Ce-loaded mesoporous zirconia samples than that of Ceo.35Zro.65O2 solid solution.
460 isle1412-
""
300-
- -^
~h\
z
g
\
c 260-
o
. 10-
|-240-
^0.80.60.40.20.0-
• •
-•-a • b c • d
-j 280-
200
300
i 220o
\\ \\
o
^^^200— ' — 1 — ' — 1 — ' — 1 — • — T " -1—1
T ,
•
400
i-T—* 1 ' 1 '"'l "'
' 1
200 250 300350400450500550600650 700 750 800
Wavelength/nm Fig. 3. UV-Vis spectra for the samples: (a) PZr, (b) reference sample MixZC, (c) reference sample CeOz, (e) Ce4, (f) Ce6, (g) CelO, (h) reference sample Ceo.35Zro.65O2 solid solution.
180-
*
l\
•
^^' 1
1—'—r
100
200300400500600
700
800900
Temperature/(°C) Fig. 4. The H2-TPR profiles for the samples: (a) Ce4, (b) reference sample Ceo.35Zro.65O2 solid solution, (c) Ce6, (d) CelO.
In conclusion, the post-grafting method for the loading of cerium into mesoporous zirconium oxide materials has been demonstrated. Both the structural characteristics and spectroscopy results reveal that small amount (<4wt%) of cerium can be incorporated into the zirconium framework, whereas, nano-crystalline of Ce02 may form and separate onto the outer surface of the materials at the higher cerium loading. The prepared cerium incorporating mesoporous zirconia sample shows much high redox activity at the lower cerium consumption according to the H2-TPR measurement, and this new material may have potential as a novel catalyst. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China with contract No. 50172057. REFERENCES 1. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature 1992, 359, 710. 2. D. M. Antonelli and J. Y. Ying, Angew. Chem. Int. Ed. Engl., 1995, 34, 2014. 3. J. A. Knowles and M. J. Hudson, J. Chem. Soc, Chem. Commun.,\995, 2083. 4. Z. R. Tian, W. Tong, J. Y. Wang, N.G.Duan, V. V. Krishnan and S. L. Suib, Science. 1997,276,926. 5. U. Ciesla, S. Schacht, G. D. Stucky, K. K. linger, F. Schuth, Angew. Chem. Int. Ed. Engl. 1996,35,541. 6. E. Zhao, S. E. Hardcastle, G. Pacheco, A. Garcia, A. L. Blumenfeld and J. J. Fripiat, Microporous and mesopourous Materials. 1999, 31,9. 7. M. Mamak, N. Coombs and G. Ozin, Adv. Mater. 2000, 12, 198. 8. T. Masui, Y. Peng, K. Machida and G. Adachi, Chem. Mater., 1998, 10, 4005. 9. T. Ozaki, T. Masui, K.Machida, G. Adachi, T. Sakata and H.Mori, Chem. Mater., 2000, 12, 643.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
461
Direct synthesis of bi-functionalized organo-MSU-X silicas YanJunGong', ZhiHong Li", Dong WU", YuHan Sun'*, Baozhong Dong^ Feng Deng' 'state Key Laboratory of Coal Conversion, Shanxi Institute of Coal Chemistry, CAS, Taiyuan 030001, China; ^Synchrotron Radiation Laboratory, Institute of High Energy Physics, CAS ,Beijing 100039 "^State Key Laboratory of Magnetic Resonance and Atomic Molecular Physics, Wuhan Institute of Physics and Mathematics, CAS, Wuhan 430071, China. Organically bi-functionalized MSU-X silicates were prepared by one-pot synthesis methodology. The solids were characterized by XRD, FT-IR, ^^Si MASNMR, TGA, N2 physisorption techniques and Small Angle X-ray Scattering (SAXS). The surface morphology and textural properties of such materials varied with synthesis mechanism and various kinds of groups in the channels. SAXS results showed that the binary organic groups uniformly distributed in MSU-X and so-produced materials displayed the fractal characteristics. 1. INTRODUCTION Recently, the synthesis of hybrid inorganic-organic mesoporous silicates enjoyed much interest and their recent applications are highlighted in catalysis, sorption, and host/guest chemistry ^''^'. It is very important to have precise control over the surface properties and pore sizes of the mesoporous sieves for specific applications, while at the same time stabilizing the materials towards hydrolysis. Many kinds of organic molecules were incorporated into silica frameworks by using post-synthesis or one-pot co-condensation strategy to form mesoporous structures with one kind of covalent bonding of organic moiety, especially for organically functional-MCM-41, HMS and MSU-X. We previously studied the synthesis of organically modified MSU-X with active or passive groups, in which MSU-X containing phenyl sulfuric acid has a good performance in catalysis^^l Here we report the synthesis of the bi-functionalized MSU-X and the investigation of the interface between the pore and silica framework due to the presence of the two organic moieties in the framework. 2. EXPERIMENTAL 2.1. Synthesis The bi-functionalized mesoporous silicates were prepared under neutral condition using the commercially available non-ionic alkylpolyethyleneoxide surfactants Cii-,i5H23-3i(CH2CH20)9H (AEO9) or C8Hi7(C6H4)(EO)i5H (Tx-15) , and tetraethoxysilane (TEOS) and other two organosiloxane(RTES) silicon sources to form R1-R2-MSU-I and R1-R2-MSU-2 respectively. The molar ratio of R1/R2/TEOS is 0.05/0.05/0.9. The organosiloxsanes (Arcol chemicals) are methyltriethoxysilane (MTES), vinyltriethoxysilane (VTES), phenyltriethoxysilane (PhTES) National Natural Science Foundation (No.29973057); National Key Basic Research Special Foundation (NO.G20000480). Fax +86-351-4041153. E-mail: vhsunralsxicc.ac.cn
462
and ureidopropyltrimethoxysilane (UPTMS). Bi-flinctionalized mesoporous silicates containing phenyl along with methyl, ureidopropyl or vinyl group were denoted PhM-MSU, PhU-MSU, PhV-MSU respectively. The ones containing methyl along with vinyl or ureidopropyl moieties were denoted MV-MSU and MU-MSU respectively. A typical synthesis of MV-MSU-2 was as following: A mixture of TEOS and MTES and VTES was added into aqueous Tx-15 solution at ambient temperature. After stirring for Ih, KF was added and then the final molar gel composition was 0.9 TEOS: 0.05MTES: 0.05VTES: 0.34 Tx-15: 0.12KF: 49OH2O. Finally the mixture aged under stirring at SOV for 72h. The resultant solids were filtered, water-rinsed and dried. The surfactant was removed by extraction over ethanol for 48h. The others containing phenyl group and other organic moieties were synthesized by the same procedures as that for MV-MSU-X, except the starting materials were TEOS with both selected RTES and AEO9 instead of Tx-15. 2.2. Physical measurements X-ray diffraction (XRD) patterns were collected on a D/MAX-rA diffractometer with CuKa at 30KV/30mA. Nitrogen physisorption were obtained at -196''C on an ASAP 2000 Micromeritics (USA). Sample was degassed at 150''C for 6h prior to the analysis. "^^Si and •^CCPMAS NMR spectra were obtained on a Bruker MSL-400 spectrometer at 79.46MHz. FT-IR spectra were recorded on FTS-25PC IR analyzer. Scanning electron micrograph (Hitachi H-600) was operated at an accelerating voltage of 50 keV. Thermo gravimetric analysis (TGA) was carried out on a SETARAM TGA-92 thermal analyzer. Sample was heated from 25-700 °C at 10°C/min under air. SAXS experiment was performed using Synchrotron Radiation as X-ray source with a long-slit collimation system (A.=0.154nm). Data analysis was directly based on slit-smeared intensity J (^). 3. RESULTS AND DISCUSSION 3.1. Mesostructure analysis Figure 1 showed the XRD patterns of MV-MSU-2 containing methyl and vinyl two moieties, which suggested that the bi-functionalized mesoporous silicas had single peak similar to that in organo-MSU-X and their parent MSU-X^'^'^l The MV-MSU-2 sample remained well-structured and just the d\oo spacing decreased as the calcination temperature changed from 350 to SSO'C. All other bi-functionalized mesophases showed normal XRD patterns and their ^100 values and N2 adsorption data were summarized in Table 1. Except for binary PhU-MSU-X with phenyl and ureidopropyl, the pore size of solids was more than 3nm, while that was about 2nm corresponding to v-MCM-41 or R-HMS mesostructures^^l The possible reason was that the assembly was controlled by N^I^ mechanism via hydrogen bonding interaction between surfactant head group and silicate species, in which the electronic density could match well in the co-condensation process of organosiloxane Figl. XRD patterns of M5V5-MSU-2 processors with TEOS and then the organic groups might be a, extracted , b, c, d, is calcined at 350, 550, and 850°C respectively
463
auxiliary agents to influence the porosity parameters. The F" could enlarge the pore size^''^ The order of BET surface area and pore volume of so-produced samples was MV-MSU> PhV-MSU> PhM-MSU >MU-MSU> PhU-MSU (see Table 1). The wall thickness for the binary organo-MSU was in the range of 3.1-6.6nm larger than that of organically modified MCM and SBA^^l The effects of organic groups for the modified mesoporous silica through nonionic assembly mechanism was less well understood, but the results indicated that the texture of the solids was adjusted by mixing the binary organic moieties in the mesostructures due to the various organic groups with different properties in chain length, polarity and spacer ligands etc. Table 1 Physico-chemical property of some binary organo-MSU-X mesoporous silicates Organo Pore Pore ^100 Organic Sample Template Substitution volume Size (nm) (mV) , xa , 3 -K group (nm) (cm g ) (mol%) PhV-MSU-2 MV-MSU-2 MV-MSU-2-350* MV-MSU-2-550* MV-MSU-2-850* PhU-MSU-1 MU-MSU-1 PhM-MSU-1
7.1 7.6 7.0 6.8 6.4 8.8 7.2 9.8
757 789 787 683 443 360 419 521
3.2 3.5 3.5 3.5 3.3 3.1 2.8 3.2
0.98 0.72 0.70 0.63 0.45 0.41 0.65 0.68
Ph+V M+V
..__ _... ..._
Tx-15 Tx-15 Tx-15 Tx-15 Tx-15
Ph+U M+U Ph+M
AEO9 AEO9 AEO9
5+5 5+5 5+5 5+5 5+5 5+5 5+5 5+5
•calcinations at 350,550,850°C respectively" BJH pore diameter 3.2. Composition analysis For PhM-MSU and PhU-MSU samples, FT-IR spectra showed the following adsorption at 1431, 3000-3100, 797,740,700 cm"' attributable to the aromatic ring C=C, C-H, and it's finger region characteristic bands, respectively. Additionally, both the characteristic bands of Si-C [1280 cm" (for Si-CHj)] and H2NCONH- group [1646,1603,1565 cm"'] could be found in their corresponding spectra. '^CMAS NMR spectrum confirmed the co-presence of vinyl and methyl groups (MV-MSU), which showed three resonances for surfactant-extracted MV-MSU-2, corresponding to unsaturated carbons at S 128.7, 136.0 ppm and saturated one at 58.2 ppm. UV-Raman spectrum gave it further support by showing the C=C vinyl stretching mode at 1618cm"'. ^^Si MAS NMR data of all samples showed resonances that assigned to T^ [R(SiO)2Si-OH], T^ [R(SiO)3Si], Q^ [(SiO)2Si-(OH)2], Q^ [(SiO)3Si-OH] and Q'^[(SiO)4Si] silicon atoms, respectively. Comparison with single organic group incorporation, there was a little S change of T^ and T^ due to the effect of binary moieties in MSU framework^'^l The surface properties of the binary organo-MSU could be tailored via either choice of organosilane or change of their ratios with TEOS. The thermal stability of organic functional groups in the silica framework could be determined by TGA. For extracted bi-functionalized PhU-MSU sample, the loss at temperature lower than 150°C was due to adsorbed water. A large weight loss in the range of 200-400°C should be attributed to the incorporated ureidopropyl groups in the framework, which divided into two weight loss stage similar to that in the ureidopropyl-MSU-X sample^^l
464
The large weight loss at temperature over 500"C was attributable to covalently linked phenyl groups in the silica framework and to the loss of water resulted from condensation of Si-OH on the surface, but the later mass was typically quite small. For extracted bi-functionalized mesophase containing methyl along with phenyl (PhM-MSU), TGA analysis gave three exothermic weight losses due to the decomposition of two kind organic groups. Except the loss of adsorbed water and phenyl group, an additional methyl groups loss in the range of 200 to BOO^C could be found, as confirmed from methyl-modified MSU sample. Among the selected organic groups, the Si-Phenyl bond performed highest thermally stable. 3.3. Interface and fractal analysis SAXS is sensitive to the electron density contrast. For pure mesoporous MSU-X silicas (without attached organic group), the scattering of SAXS agreed with both Porod's and Bebye's law^'^'''\ indicating that the respective electron density in the silicate skeletons and the pores was homogeneous. SAXS results, for binary organo-MSU-X, showed obviously a negative deviation from Porod's and Bebye's theories (Fig.2 Debye plots), suggesting that an interfacial layer formed between pore and silica matrix due to the incorporated organic groups in the silica framework. According to SAXS data plotted as ln[J(q)]~lnq for MV-MSU, the results ^ of the fractal dimension (Ds) of so-produced ^ binary mesophases were obtained, indicative of ^ the presence of rough surface with 2.49 of Ds ^^"''I ^ : After the sample was calcined at 350, 550 and | 850 "C respectively, the fractal dimension Ds changed from 2.47 to 2.27 due to the removal of ' the organic groups gradually. The interface layer 0 1 2 3 4 5 thickness was different with the organic group and q^Cnm"^) it's amount in the mesoporous framework. The further investigation for the distribution of the two Fig-2. A typical negative deviation from organic moieties on the mesostructure is Debye's theory (I) and the deviation-corrected undergoing. Debye plot (11) of organo-MSU silica
REFERENCES 1. L.Mercier, T.J. Pinnavaia. Chcm. Mater., 12(2000), 188. 2. A. Sayari, S. Hamoudi. Chem. Mater., 13(2001 ),3151. 3. Y. J. Gong, Y.Li, D. Wu. Y. H. Sun. Catal. Lett., 74(3/4) (2001), 213. 4. S. A. Bagshaw, E. Prouzet and T. J. Pinnavaia. Science, 269 (1995)1242. 5. R. Richer, L. Mercier, Chem. Commun.( 1998) 1775. 6. M. H. Lim, C. F.Blandford, A. Stein. Chem. Mater., 10 (1998), 467. 7. C. Boissiere, A. Larbot, C. Bourgaux, E.Prouzet,et al. Chem. Mater., 13(2001), 3580. 8. F Babonneau, L. Leite, S. Fontlup. J. Mater. Chem. 9 (1999) 175. 9.Y. J. Gong, Z. Li, D. Wu, Y. H. Sun. J. Mater. Res., 17(2)( 2002),431. lO.Z.H. Li, Y.J. Gong, D.Wu, Y.H. Sun. J. Phys.D: Appl. Phys., 34(14) (2001), 2085. 1 l.Y J. Gong, Z.H. Li, D. Wu, Y. H. Sun. Microporous.Mesoporous Mater., 49(2001), 95.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
465
High-density modification of mesoporous silica inner walls with amino acid function by residue transfer from template Q. Zhang, K. Ariga,* A. Okabe and T. Aida* ERATO Nanospace Project, National Museum of Emerging Science and Innovation Bldg., 2-41 Aomi, Koto-Ku, Tokyo 135-0045, Japan Novel surfactants containing an alkoxysilane group and an amino acid residue have been synthesized.
Using these surfactants as templates, mesoporous silica/surfactant
covalent-composites were prepared.
Selective removal of the alkyl chain of the template
provided mesoporous silica with inner surface densely covered by the amino acid residue. Successful modification process was confirmed by XRD, FT-IR, TEM and TGA analyses. 1. INTRODUCTION Mesoporous silica provides unique nanospaces with precisely-defined dimension, and modification of its inner walls would lead to novel kinds of functional materials.
Co-
condensation and grafting methods were so far reported as major modification techniques, but they do not provide completely modified surfaces with maintaining structural stability of the silica [1-3].
Especially, lack of methodologies in effective modification of
mesopores with biological functionalities restricts potential possibilities in biological applications.
In this report, we propose a novel technique, residue transfer method, to
densely modify the silica inner walls with amino acid residues. The outline of this method is summarized in Figure 1. In the first step, mesoporous silica was synthesized using template of newly synthesized surfactant carrying an amino acid residue and a alkoxysilane group.
The surfactant template was covalently immobilized to the silica inner wall
through Si-O-Si linkage.
Next, the alkyl tail was selectively removed through hydrolysis
of ester linkage between the amino acid moiety and the alkyl tail. amino acid residue was transferred from the template to the silica wall.
As the results, the
466 (A) Cross-Sectional View of Mesoporous Silica
(B)
~->c - - 0 < / " - ^X^O(CH2),5CH3 y
Br
H
0
R = CH3 (L-isomer) CH3 (D-isomer) H CHj>Ph (L-lsomer) H
H
0
0
1
BrL-isomer
0
1 2 3 4
H
6
5
1
Aj^A,0(CHj),5CH3 .W H 0
^
(^
.^ N
f Br
0
N ..^.-.J: N C A 0
^f' L-isomer
1 ^> . H 6
.0(CHz),5CH3 6
Fig. 1 (A) Schematic illustration of mesoporous silica modification by the residue transfer method from template. (B) Surfactants used in this research. 2. EXPERIMENTAL SECTION Mesoporous
silica
was
prepared
using
tetramethoxysilicate
(TMOS)
or
tetraethoxysilane (TEOS) as silica sources under conditions described later. The obtained silica/surfactant composites (50 mg) were hydrolyzed by conc-HCl (0.5 g) in rcfluxcd THF (30 mL) for 8 hours.
Parts of the obtained materials were calcined at 500 °C for 6 hours.
3. RESULTS AND DISCUSSION The XRD patterns of the obtained materials from 1 are summarized in Table 1.
The
patterns assigned to thermally-stable mesoporous silica in hexagonal and cubic phases can be prepared under TEGS-containing acidic condition and TMOS-containing basic conditions, respectively.
As summarized in Table 2, mesoporous silica can be obtained
from surfactants containing various amino acid residues under the optimized conditions. Especially, in the cases of 1, 2, and 4, both hexagonal- and cubic-structured materials were obtained upon the selection of the catalyst and the silica source. structures obtained from 1 were confirmed by TEM observation.
The cubic and hexagonal
467
Table 1 XRD patterns of mesoporous silica templated by 1 under various conditions Silica
Catalyst
Source
XRD Peaks
XRD Peaks
d
(Uncalcined)
(Calcined)
1 nm
Assigned Structure
TMOS
HCl"
(100)
(100)
A.T
Hexagonal
TEOS
Hcr
(100), (110), (200)
No Peaks
A.y
Hexagonal (Unstable)
TEOS'^
HCl"
(100), (110), (200)
(100), (110)
A.y
Hexagonal
TEOS'
HCl"
(100)
(100)
4.5''
Hexagonal
TMOS
NaOH''
(211), (220), (332)
(211), (220), (332)
3.y
Cubic
TEOS
NaOH''
(211)
No Peak
3.6"
Cubic (Unstable )
4.5'' TEOS'' NaOH'' (100) Hexagonal (100) "Reaction Condition: 1/H20/Silica Source/HCl/EtOH = 0.13/131/1 /8.8/3 in molar ratio, 4h reaction at room temperature. ''TEOS was prehydrolyzed in aqueous ethanol with HCl at 70 °C for 2h. ' TEOS was prehydrolyzed in aqueous ethanol with HCl at 70 °C for 7h. ''Reaction Condition: 1/H20/Silica Source/NaOH = 0.12/141/1/0.27 in molar ratio, 4h reaction at room temperature. '' Value of (100) peak for uncalcined sample. ^Value of (211) peak for uncalcined sample.
Table 2 XRD patterns of mesoporous silica templated by various surfactants Surfactant
Catalyst
XRD Peaks
XRD Peaks
d
(Uncalcined)
(Calcined)
1 nm
Structure
(100), (110)
4.3'
Hexagonal Hexagonal
(100), (110), (200)
Assigned
2
HCl"
3
HCl"
(100), (110)
(100)
4.4'
4
HCl"
(100), (110)
(100)
4.5'
Hexagonal
5
HCl"
(100), (110)
(100)
4.7'
Hexagonal Disordered
6
HCl"
(100)
No Peaks
4.8'
2
NaOH''
(211), (220), (332)
(211), (220), (332)
3.5''
Cubic
3
NaOH''
(100)
No Peaks
4.0'
Disordered
4
NaOH''
(211), (220), (332)
(211), (220), (332)
3.6''
Cubic
5
NaOH''
(100), (200)
(100)
4.0'
Hexagonal
(100) NaOH'' (100), (200) 6 4.4' Hexagonal "Reaction Condition: Surfactant/H.O/TEOS/HCl/EtOH = 0.13/131/1/8.8/3 in molar ratio, 4h reaction at room temperature. ''Reaction Condition: Surfactant/HjO/TMOS/NaOH = 0.12/141/1/0.27 in molar ratio, 4h reaction at room temperature. TEOS was prehydrolyzed in aqueous ethanol with HCl at 70 °C for 2h. 'Value of (100) peak for uncalcined sample. ''Value of (221) peak for uncalcined sample.
468
Maintenance of regular structures of hexagonal and cubic silica from 1 during the hydrolysis process was also revealed by XRD measurement.
Transfer of the amino acid
residue from the template to silica backbone was next investigated by FT-IR spectroscopy. In IR spectrum of the cubic mesoporous silica composite from 1, characteristic peaks for V3XCH2), v(CO, ester), and v(CO, amide) were detected at 2925, 1742, and 1685 cm', respectively.
Selective removal of the alkyl tails by hydrolysis with HCl under refluxed
condition was confirmed by disappearance of v^,(CH2) peak and preservation of v(CO, amide) peak.
The peak originally observed at 1742 cm"' was shifted to 1734 cm"' that can
be assigned to CO stretching vibration for COOH group.
The similar spectral features
were observed for hexagonal silica prepared from 1. The composition of the silica/1 composites were analyzed by TGA, and 44% and 46% of organic components were detected for the cubic and hexagonal samples, respectively. The organic components in hydrolyzed silica decreased to 26% and 27% for the cubic and hexagonal ones, respectively.
The latter values are in good agreement with theoretical
values (22% for cubic and 23% for hexagonal) calculated with molecular weight change upon hydrolysis of the ester linkage.
These thermal analytical data confirm that the alkyl
chains were selectively removed only with ignorablc decomposition of the amino acid moiety. 4. CONCLUSION In conclusion, mesoporous hybrid preparation using the novel kinds of the surfactants and subsequent selective removal of alkyl chains provide mesoporous silica with inner surface densely covered by amino acid residues.
This residue transfer method
will be also applied to effective immobilization of the other biological functions to inorganic silica mesopore structure and be highly useful for preparation of bio-inorganic nanocomposites. REFERENCES 1. A. Stein, B. J. Melde and R. C. Schroden, Adv. Mater., 12 (2000) 1403. 2. A. Sayari and S. Hamoudi, Chem. Mater., 13 (2001) 3151. 3. K. Mollcr and T. Bcin, Chcm. Mater., 10 (1998) 2950.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
469
The synthesis of optically active amino acid over Pd catalysts impregnated on mesoporous support Kyung Hye Chang^, Yong Ku Kwon'' and Geon-Joong Kim^ ^ Department of Chemical Engineering, Inha university, Incheon 402-751, Korea ^ Department of Polymer Sciene and Engineering, Inha university, Incheon 402-751, Korea Pd metals immobilized on SBA-15 and NaY were applied as catalysts in the synthesis of amino acid. These catalysts afford a high level of enantioselectivity in the asymmetric hydrogenation of a-keto acids to corresponding amino acids. Indeed, this reaction has been investigated and reported only using Pd metals immobilized on active carbons. 1. INTRODUCTION Almost enantioselective catalysts are soluble metal complexes containing some type of chiral ligands. It appears that such catalysts are effective because of the chiral environment created around the active metal center by the chiral ligands. The heterogeneous cataysts are usually produced by attaching ligands to an insoluble matrix and then using these insoluble ligands to complex with the active metal species. The other type of chiral heterogeneous catalyst can be either a supported metal which has been treated with a chiral modifier or an active metal on a chiral support. Asymmetric syntheses of a-amino acids from their corresponding a-keto acids have been reported[l]. Hiskey and Northrop[2] have demonstrated the synthesis of optically pure a -amino acids by catalytic hydrogenation and subsequent hydrogenolysis of the Schiff bases of a-keto acids with chiral a-methylbenzylamine. Harada[3] reported the syntheses of optically active amino acids in a way principally similar to those done by Hiskey but by the use of a-phenylglycine in alkaline aqueous solution (optial purity 40-65%). These reactions are interesting because they are essentially a kind of asymmetric transamination reaction performed by catalytic hydrogenation and hydrogenolysis. To date, however, very few publications have dealt with the use of zeolites as catalysts for asymmetric reactions. The aim of this study is to demonstrate the aptitude of Pd-containing mesoporous materials for enantioselective catalytic hydrogenation. In this study, Pd metals immobilized on SBA-15 and NaY were applied as efficient catalysts in the synthesis of amino acid. These catalysts afford a high level of enantioselectivity in the asymmetric hydrogenation of a-keto acid to corresponding amino acids. Indeed, this reaction has been investigated and reported only using Pd metals immobilized on active carbons.
2. EXPERIMENTAL The (S)-(-)-a-Methylbenzylamine(2.42g, 0.02 mole) in ethanol(30 iii^) was added to pyruvic acid(0.88g, 0.01 mole) in cold ethanol (40 ni^). The mixture was allowed to stand for 30 min at room temperature. To the solution was added palladium on supports, and then it was
470
CHj
Pd / Support Hydrogenation COOH
'NH
Pd(0H)2 Hydrogenoloysis coo-
U H3C'
Scheme 1. Reation pathway to synthesize chiral amino acid hydrogenated for 8 hr at room temperature. The catalyst was removed by fikration and washed with hot water. The combined solution was evaporated to 20 mL To the concentrated solution was added 30 % aqueous ethanol (50 iii^ and palladium hydroxide on charcoal. The hydrogenolysis was carried out at room temperature for 12 h using Pd(0H)2 as shown in Scheme 1. The filtrate was concentrated to 5 111^ in vacuo. (S)(+)-Alanine was obtained (0.07g), and ee% was determined by instrumental analysis. 3. RESULTS AND DISCUSSION In this work, Hiskey-type reaction was carried out in order to screen the effect of support. Initially comparative investigations were carried out under the given reaction conditions to establish the suitability of the prepared Pd-containing catalyst for hydrogenation. The optical purities of the resulting amino acids were dependant on the kinds of supports and the enantiomeric excess values vary according to the composition of zeolitic materials. Figure 1 shows the relation between the initial Pd wt% and enantioselectivity. In this case, the reaction was conducted with a hydrogen pressure of 3.5 atm. The ee(%) increased with the increase in the loading amount of palladium on the support. An optical purity of about 81% was obtained on the 10%Pd/SBA-15, and the highest optical yield of 88% was obtained
100
100
80
80
60
60
40
—•—Pd/C —0—Pd/N«Y —A— Pd / AljOj —A—Pd/SBA-15
201-
4
6 Wt
8
10
:^^^^^\
3 ^
40 20 0
•
—•—Pd/( —0—Pd/NaN —A—Pd/AljO, -A—Pd/SBA-15
2
3
4
%
Fig. 1. Relation beteen the initial Pd Wt % and ee %
Fig. 2. The effect of hydrogen pressure on the product ee%
471 COOH HjN——H
1?K
NHT^O
icture I.Major structure
PhR"
R (S)
II. Minor structure
Scheme 2. Conformation of substrates when NaY was used as the support. 10%Pd/Active Carbon and Na-Mordenite gave a relatively low enantioselectivity of around 65 ee% for the synthesis of alanine from Pyruvic Acid. The unsupported Pd black itself also gave a low enantioselectivity, showing 52 ee(%). When using acidic support such as HY, as compared to zeolite Y in sodium form, a decrease in the optical yield was investigated. No improvement in enantioselectivity was achieved by using acidic supports in the hydrogen form. In Figure 2, the plots show the effect of hydrogen pressure on the product ee%. Optical yields mainly depended on hydrogen pressure. As shown in Figure 2, the maximum ee% of the product was found at the hydrogen pressure of 3.5atm. The effect of Pd metal size was also investigated in this reaction. As mentioned above, the enantioselectivity was influenced by the loading amount of Pd on supports. This result indicates that larger crystallite size of Pd would provide the suitable surfaces for the effective enantio-differentiation in the hydrogenation. Figure 3 shows the TEM images of Pd metal supported on the mesoporous materials. The Pd metals were observed to be apparently aggregated, and the mean size of metal particle became larger with the increased amount of Pd on the supports. Nitta el al.[4] have reported that the catalyst with the larger crystallite size gave the higher optical yield in the enantio-differentiating hydrogenation of methyl acetoacetate. They predicted
1%. (a) 10 % Pd loading on MCM-41
(b) Enlargement of photograph(a)
Fie. 3. TEM imaee of Pd-loaded MCM-41
472
that the catalyst with a larger crystallite size had regularlyH R arranged metal atoms on the _ / catalyst surface providing sites for ••S^'//, / \ a strong and regular adsorption of 0 ^N Ph the modifier, propitious to obtain \ / a high optical yield. (Pd)n When this fact was taken into account, the results in Fig. 1 indicate that larger metal would Scheme 3. Major coformation adsorpted on Pd provide the appropriate surface metals for the enantio-differentiating hydrogenation of Pyruvic Acid to (S)-alanine. The enantioselective mechanism proposed in the literature stated that the structure I might be the most predominant structure and structure II might be a minor structure. Structure I resulted in (S)-amino acid when (S)-amine was used. On the other hand, structure II resulted in (R)-amino acid when (S)-amine was used. When the alkyl group of keto acid is methyl(pyruvic acid), conformation of reactant might be composed mainly of structure 1, therefore resulting in highly optically active alanine as indicated in Scheme 2. However, according to the experimental results, structure 1 seems to be a major conformation in this reaction. The structure I might form a five-membered cyclic structure on Pd metal and then the structure would be adsorbed at the less bulky side of the molecule. On the other hand, structure II might not form such a cyclic structure because of the steric hindrance. Ihe difference in the ease of formation of the cyclic complex between structure 1 and II might be an important factor why structure I is a major conformation in the reaction. It is assumed that the adsorpted state of reactants as structure I or II may be influenced by the reaction conditions such as the Pd metal size, resulting in the different enantioselectivity. ACKNOWLEDGMENT This work was supported by grant No. 2000-1-30700-002-3 from the Basic Reseach Program of the Korea Science & Engineering Foundation. REFERENCES 1. K. Harada and K. Matsumoto, J. Org. Chem. 32 (1967) 1794. 2. R.G.Hiskey and R.C.Northrop, J. Am. Chem. Soc. 83 (1961) 4798. 3. K. Harada, Nature, 212 (1966) 1571. 4. Y.Nitta, F.Sekine, T.Imanaka, and S. Teranishi, Bull.Chem.Soc.Jpn. 54 (1981) 980.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
473
Sulfonic acid-functionalized periodic mesoporous organosilicas S. Hamoudi and S. Kaliaguine Department of Chemical Engineering, Laval University, Quebec, GIK 7P4, Canada Mesoporous ethane-silica materials containing sulfonic acid bearing groups were synthesized using bis(trimethoxysilyl)ethane (BTME) and mercaptopropyltrimethoxysilane (MPTMS) as framework precursors under acidic or basic conditions. Pluronic PI23 or polyoxyethylene (Brij-56) were used as surfactants for the acidic synthesis, while cetyltrimethylammonium chloride (CTAC) was used for the basic synthesis. Conversion of the mercaptopropyl groups into sulfonic acid moieties was achieved via oxidation using hydrogen peroxide. Ordered hexagonal mesostructures with high surface areas (up to 1180 mVg) and narrow pore size distributions (up to 5.4 nm) were obtained. 1. INTRODUCTION At the end of the last decade, a new class of mesostructured silica cumulating organic and inorganic moieties within the framework was discovered [1-3]. These novel periodic mesoporous organosilica (PMO) materials exhibited a homogeneous distribution of organic fragments and inorganic oxide within the framework and demonstrated highly ordered structures and uniform pores. In order to confer to PMO materials additional functionalities, a combination of both bridging organic functional moieties in the framework and terminal organic functional groups protruding into the pores stands for a judicious strategy. The main objective of the present work is to take advantage of both surface functionalization and framework modification to design sulfonic acid bearing hybrid mesoporous ethane-silica materials. In fact, there has been considerable interest in the development of mesostructured solid acid materials for their potential use as advanced materials or as acid catalysts. This opens a new route for the engineering of acidic mesoporous materials with hydrophobic properties, wherein the amount of hydrophobic groups is not limited as is the case for conventional mesoporous materials. Indeed, the combination of both functionalities (hydrophobic and acidic) may result in interesting surface properties facilitating for instance diffusion and/or adsorption of reactants and products in acid catalyzed reactions. 2. EXPERIMENTAL Sulfonic acid functionalizcd mesoporous ethane-silica materials were prepared under acidic conditions using either PI23 copolymer or polyoxyethylene Brij-56 as surfactants. The resulting materials are denoted here as SAF-MES-Al and SAF-MES-A2, respectively. When PI23 copolymer was used as surfactant, the synthesis method used herein was recently described by Burleigh et al. [4], except that in our work a supplementary step for the incorporation of MPTMS was added. Therefore, the synthesis gel molar composition was: BTME, 0.75; MPTMS, 0.25; PI23, 0.05; HCl, 36; H2O, 1000. In the presence of Brij-56, the synthesis procedure was adapted from the method reported by Coleman and Attard for all-
474
silica materials [5]. The synthesis gel molar composition was: BTME, 0.75; MPTMS, 0.25; Brij-56, 0.24; HCl, 83; H2O, 9260. For the material prepared under basic conditions (SAFMES-B), a synthesis procedure modified from method II described by Inagaki et al. was adopted [1]. Hence, the synthesis gel molar composition was: BTME, 0.75; MPTMS, 0.25; CTAC, 0.57; NaOH, 2.36; H2O, 353. For all the materials reported herein, conversion of the thiol groups into sulfonic acid moieties was carried out on the solvent-extracted samples by oxidation with hydrogen peroxide [6]. Adsorption measurements were performed on a Coulter Omnisorp 100 gas analysis apparatus. Pore size distributions were calculated using the Barrett-Joyner-Halenda (BJH) method. XRD spectra were obtained on a Philips X-ray diffractometer. Thermogravimetric analysis was carried out on a Perkin Elmer 7 scries thermal analyzer from ambient temperature to 800 °C at a heating rate of 5 °C/min under nitrogen atmosphere. The acid capacity of the SAF-materials was determined by NaOH titration. The proton conductivity was assessed using impedance spectroscopy. 3. RESULTS AND DISCUSSION Nitrogen adsorption analysis for extracted materials displayed type IV isotherms with marked hysteresis loop for SAF-MES-Al material and sharp adsorption step at relative pressures ranging between 0.5 and 0.8 (Figure 1). Furthermore, the SAF-MES-A2 and SAFMES-B exhibited quite similar adsorption isotherms without hysteresis loop. The corresponding BJH pore distributions were reasonably narrow, and centered around 3.5-5.4 nm (Figure 1, inset). As reported in Table 1, the BET surface areas ranged from [520 to 1180 mVg], whereas the pore volumes reached ca. 0.64 to 0.69 cm7g. Moreover, the SAF-MES-A2 and SAF-MES-B materials exhibited prominent sharp peaks in the diffraction patterns at approximately 20 = 1.6° and 2°, respectively (Figure 2), characteristic of hcxagonally ordered mcsoporous materials. Table 1 Pore Pore S03H^'^ (mcq/g) size volume (nm) (cmVg) 0.69 5.4 522 SAF-MES-Al 0.83 1184 0.64 SAF-MES-A2 3.3 0.93 0.64 882 3.5 SAF-MES-B 0.62 ^'^Propyl-sulfonic acid loading determined from TGA peak
Sample
SBET
(m'/g)
SOiH^^^ (mcq/g) 0.62 0.77 0.53 at 450 °C.
Proton conductivity^"^ (S/cm) 1.31 X 10-^ 1.38 X 10-' 4.78 X 10-^
^^^ Determined by titration and defined as mmol HVg Si02. ^"'^ Proton conductivity at ambient temperature and relative water content of 60 %. As depicted in Figure 3, thermogravimetric analysis showed that all the sulfonic acid modified materials displayed a peak centered at 100 °C attributed to the desorption of water. Above this event, slightly different thermal behaviors depending on the surfactant used were observed. Indeed, both the samples synthesized under acidic conditions were thermally stable until ca. 300 °C, whereas the SAF-MES-B exhibited moderate weight loss (3 %) with a maximum at 240 °C assigned to the partial thermal decomposition of the alkyl-sulfonic acid groups leading to SO2 release [6]. Subsequently, all the three materials exhibited comparable
475
thermal profiles. A more or less marked weight loss (ca. 7 %) taking place between 280 and 380 °C was assigned to the pyrolysis of the pendant unreacted mercaptopropyl groups in the pore systems. A subsequent event occurring between 380 and 520 °C was attributed to the thermal decomposition of the whole alkyl sulfonic acid groups [7]. The last event occurring above 520 °C was ascribed to the partial decomposition of the ethane bridging groups in the framework [3, 7]. Impedance spectroscopy analysis was performed at ambient temperature on the SAF-MES materials at different water contents. As depicted in Figure 4, an increase in the samples water content yielded a continuous rise in the proton conductivity exceeding 10'^ S/cm at water to solid ratio above 100 %. Furthermore, the registered profiles clearly indicated that the materials synthesized under acidic conditions exhibited the highest proton conductivities if compared to their homologue synthesized under basic conditions. Moreover, the acid capacity of the different samples in agreement with their propyl-sulfonic acid amount determined by TGA also followed the same tendency (see Table 1). Such behavior was attributed to a better conversion of the thiol groups into sulfonic acid moieties reached for the materials synthesized under acidic conditions, as previously reported for all-silica materials [8].
SAF-MHS-A2
3 5 2-Thcta (Degree)
Fig. 1. Nitrogen adsorption/desorption isotherms and BJH pore size distributions (inset).
7
Fig. 2. X-ray diffraction patterns.
4. CONCLUSION Sulfonic acid bearing ethane-silica mesostructurcd materials were synthesized for the first time. The procedure involved the synthesis of mercaptopropyl-attached materials, followed by conversion of the mercaptopropyl groups into sulfonic acid moieties using hydrogen peroxide. Different synthesis procedures under acidic and basic conditions led to ordered mesostructurcs. The sulfonic acid modified materials synthesized under acidic conditions were shown to be thermally stable up to 300 °C. These materials exhibited appreciable sulfonic group concentrations neighboring 1 meq HVg as well as high proton conductivities beyond 10'^ S/cm at ambient temperature.
476 A 100
:^.^-^
'? ie-44.
B
SAF-MES-Al SAF-MES-A2 SAF-MES-B
^
Water content (w/w %)
Fig. 4. Room temperature proton conductivity as function of water content. 200
400
600
Temperature (°C)
Fig. 3. (A) Thermogravimetric weight loss curves and (B) derivative plots for (a) SAF-MES-Al; (b) SAF-MES-A2 and (c) SAF-MES-B.
REFERENCES 1. S. Guan, Y. Fukushima, T. Ohsuna, O. Terasaki and S. Inagaki, J. Am. Chem. Soc, 121 (1999)9611. 2. B. J. Melde, B. T. Holland, C. F. Blanford and A. Stein, Chem. Mater., 11 (1999) 3302. 3. T. Asefa, M. J. MacLachlan, N. Coombs and G. A. Ozin, Nature, 402 (1999) 867. 4. M. C. Burleigh, M. A. Markowitz, E. M. Wong, J. Lin and B. P. Gaber, Chem. Mater., 13 (2001)4411. 5. N. R. B. Coleman and G. S. Attard, Micropor. Mesopor. Mater., 44-45 (2001) 73. 6. S. Mikhailenko, D. Desplantier-Giscard, C. Danumah and S. Kaliaguine, Micropor. Mesopor. Mater., 52 (2002) 29. 7. D. Margolese, J. A. Melero, S. C. Christiansen, B. F. Chmelka and G. D. Stucky, Chem. Mater., 12 (2000) 2448. 8. D. Trong On, D. Desplantier-Giscard, C. Danumah and S. Kaliaguine, Appl. Catal. A. Gen., 222 (2001) 299.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
477
Functionalized periodic mesoporous organosilicas with sulfonic acid group Xingdong Yuan^'', Hyung Ik Lee^, Jin Won Kim*^, Jae Eui Yie'^ and Ji Man Kim^* ^ Functional Materials Laboratory, Department of Molecular Science & Technology, Ajou University, Suwon, 442-749, Korea ^Department of Petrochemical Technology, Fushun Petroleum University, Fushun, 113001, China '^Catalyst and Surface Laboratory, School of Chemical Engineering and Biotechnology, Ajou University, Suwon, 442-749, Korea Periodic mesoporous organosilicas (PMO) functionalized with sulfonic acid group have been successfully synthesized by co-condensation of bis(triethoxysily)ethane and 3mercaptopropyltrimethoxysilane in the presence of octadecyltrimethylammonium chloride as the structure-directing agent under basic conditions. The PMO materials have been characterized by nitrogen adsorption, powder X-ray diffraction, IR, and thermogravimetric analysis. The results indicate that the materials exhibit well-ordered mesostructures, high surface areas and solid acid properties. 1. INTRODUCTION Mesoporous silicas functionalized with organic groups, which can be designed for uniform and inorganic framework, have attracted much attention because of new catalytic and adsorption functions [1,2]. Both post-synthetic grafting and co-condensation methods have been used for preparation of various kinds of organically functionalized mesoporous materials. The organic functional groups not only give a new function to mesoporous materials but also enhance their hydrophobicity. However, the materials may exhibit weak hydrothermal stability for catalytic applications in the presence of water because the organic groups exist only on the surface of pore wall so that the framework structures (-Si-O-Si-) may be disintegrated during the application under hydrothermal conditions [3]. Recently, the development of PMO has led to great interest in the field of mesoporous materials [4,5]. The synthesis of PMO materials combines the structural properties of ordered mesoporous materials with the chemical properties of both silica and the organic bridging groups. The presence of organic groups within the frameworks is expected to give these materials a lot of favorable properties: structural rigidity and a degree of hydrophilic character that are useful for applications in aqueous systems. However, there are few reports on the modification and applications of PMO materials [6]. In the present work, we describe the direct synthesis of thiol-modified PMO materials (PMO-SH) and preparation of PMO containing sulfonic acid moieties (PMO-SO3H) through subsequent oxidation. The synthetic conditions for the functionalized PMO materials will also be discussed.
478
2. EXPERIMENTAL Bis(triethoxysily)ethane (BTSE), 3-mercaptopropyltrimethoxysilane (MPTMS) and octadecyltrimethylammonium chloride (ODTMACl) were used as received from Aldrich and Kogyo, Co. LTD. PMO-SH was synthesized using BTSE as the main framework source and MPTMS as functional group and ODTMACl as the structure-directing agent[7] , The synthesis procedure was as follows: 1.0 g of ODTMACl was added to 31.7 g of doubly distilled water under stirring in a polypropylene bottle to give a clear solution, and subsequently 0.47 g of NaOH was added to the surfactant solution at room temperature. 1.55 g of BTSE and 0.15 g of MPTMS were mixed in a separate vial. The framework source mixture was then added to the surfactant solution, and the resulting mixture was stirred at room temperature for 20 h. The gel compositions were (\-x) BTSE : x MPTMS : 0.57 ODTMACl : 2.4 NaOH : 350 H2O (;c = 0 - 0.25). The reaction mixture was heated at 95 °C in an oven for 21 h under static condition. The white precipitate solid was filtered off, washed with doubly distilled water and dried at 60°C overnight.. In order to remove the surfactant, 1.0 g of solid product was treated with 150 ml of a mixture of ethanol and HCl at 70 °C for 12 h. The product was filtered, washed with ethanol and dried at 60 °C for 12 h. This extraction procedure was repeated one more time to remove the surfactant completely. The PMO-SH was oxidized with H2O2 at room temperature for 24h. Finally the solid product was acidified by H2SO4 to produce sulfonic moieties (PMO-SO3H). Ion-exchange capacities of the PMOSO3H materials were determined using aqueous solution of NaCl (2.0 M) as exchange agent. In a typical experiment, 0.05g of PMO material was added to 10 g of NaCl solution. After reaching to equilibrium, the suspension was titrated using an aqueous solution of NaOH (0.01 M). A pure silica PMO material (Si-PMO) was synthesized under the similar conditions without MPTMS to compare with the PMO-SO3H 3. RESULTS AND DISCUSSION
2
3 4 5 2-Theta(degree)
Fig. 1. Powder X-ray diffraction patterns of (a) Si- PMO, (b) PMO-SH (0.1), (c) PMOSH(a 15), (d) PMO-SH (0.2), (e) PMO-SH (0.25) and (f) PMO-SO3H (0.25).
Figure 1 shows XRD patterns for the PMO materials after the extraction of surfactant. All the materials exhibit a very intense Bragg peak at low-angle and two or more weak peaks, which are characteristic of 2-d hexagonal {P6mm) mesostructures. There are no significant changes upon the oxidation of -SH group to -SO3H group, as shown in Figure 1(f). The t/100 intensities in XRD patterns decrease as the amount of thiol precursor increase. This may be related to the fact that the MPTMS contains fewer hydrolysable groups, so that when its amount increases, the degree of cross-linking within the framework decreases [8]. When x is above 0.2, the PMO-SH materials exhibit somewhat broad XRD patterns and a shoulder at low angle, which indicates that the materials are mixture with disordered material or different from perfect 2-D hexagonal structure.
479
1600 I
1
These results may be due to alkylthiol group of MPTMS that results in the disturbance 1400 for interaction between the surface of surfactant micelles and framework sources. 1200 IR spectra indicate that all the functionalized PMO materials exhibit strong 1000 bands at 2920 and 2890 cm'^ assigned to CH stretching and deformation vibrations, 800 1410 and 1270 c m ' corresponding to C-H deformation vibrations of the framework i 600 organic group. The peaks at 780 and 690 cm"' assigned to Si-CH2 stretching 400 vibrations. A weak peak at 2580 cm' for the PMO-SH materials, corresponding to S200 h H stretching vibration is disappeared after oxidation to the sulfonic acid group. N2 adsorption-desorption isotherms for P/P' the PMO-SH materials after surfactant Fig. 2. N2 sorption isotherms for extracted PMO extraction are shown in Figure 2. The materials: (a) Si-PMO, (b) PMO-SH (0.1), (c) isotherms for the PM0-S03H materials PMO-SH (0.15), (e) PMO-SH (0.2) and (f) PMO- coincide with data in Figure 2. When x = 0 SH (0.25). 0.2, the materials exhibit type IV isotherms without hysteresis loops, which are the wellknown characteristics of 2-d hexagonal mesoporous materials. A well defined step of the adsorption and desorption appears between partial pressures pIpQ of 0.3 ~ 0.4. The materials exhibit a very narrow pore size distribution, which means well-defined uniform pore dimensions. However, the PMO-SH {x = 0.25) gives somewhat flat and broad step in the mesoporous range, indicating that a disordered materials is formed as expected from XRD results. Table 1 summarizes BET surface areas, total pore volumes and pore sizes for the materials. The decrease in the surface areas and pore sizes after x = 0.2 also means disordered nature of the materials. Table 1 Structural properties of PMO materials Sample Si-PMO PMO-SH (0.1) PMO-SH (0.15) PMO-SH (0.2) PMO-SH (0.25)
SBET (mVg)
1050 1043 1116 873 783
Vp(cmVg) 0.838 1.133 0.852 0.759 0.646
Pore size (nm) 2.87 2.98 2.95 2.81 2.61
Figure 3 shows thermogravimetric analysis (TGA) results under nitrogen atmosphere for as-synthesized PMO materials. A weight loss of 2 ~ 5 wt% below 120°C is attributed to the loss of small amounts of residual water adsorbed to the materials. This is followed by weight loss of 30 - 35 wt% from 120 to 250°C due to surfactant decomposition. The PMO-SH material exhibits a weight loss around 350°C, which is the decomposifion of thiol group and is not observed from Si-PMO material. The PMO-SH materials with higher x values result in more weight loss in this range. An additional weight loss of 5 - 7 wt% above 500°C indicates
480
the decomposition of organic bridging group within the framework. The TGA results mean that the PMO materials containing functional group can be used below than 350°C. The PMOSO3H material can be used over 24 hr without any loss of catalytic activity for alkylation of phenol at 150°C, which means the material has also excellent hydrothermal stability. Table 2 shows the results obtained 1 • I 100 200 300 400 500 600 700 from ion exchange with NaCl and titration with NaOH to investigate the Temperature (°C) amount of acid sites for the PMOSO3H materials. All the acid capacities Fig 3. Thermogravimetric weight loss curves defined as mmol of H^ per g catalysts. Table 2 indicates that the acid capacities of the PMO-SO3H materials are good agreement with x value in the synthesis gel mixtures. The MPTMS in the initial mixture can be grafted on the surface when the x is lower than 0.15, whereas some thiol precursor may be remained as soluble species or incorporated within the framework. The authors are grateful for support by the Research Initiation Program at Ajou University (20012010) and Department of Molecular Science & Technology through Brain Korea 21 Project. 100
PMO-SH(O.l) Si-PMO
Table 2 Acid capacities of PMO-SO3H materials Sample
Calculated (mmol H^/g)
Titrated (mmol HVg)
Incorporation degree (%)
PMO-SOjHCO.l) PMO-SO3H(0.15) PMO-S O3H (0.2) PMO-S O3H (0.25)
0.57 0.85 1.15 1.45
0.57 0.84 0.93 1.07
100 98.8 80.9 73.8
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
I. Diaz, F. Mohino, P P Joaquin and E. Sastre, Appl. Catal. A, 205 (2001) 19. X. H. Lin, G. K. Chuah and S. Jaenicke, J. Mol. Catal. A, 150 (1999) 287. S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna, et al, J. Am. Chem. Soc, 121 (1999) 9611. S. Hamoudl, Y. Yang, I. L. Moudrakovskl, S. Lang, et al., J. Phys. Chem., B, 105 (2001) 9118. S. Y Guan, S. Inagaki, T. Ohsuna, O. Terasaki, Micro. Meso. Mater, 44 (2001) 165. T. Asefa, M. J. Maclachlan, H. Grondey, N. Coombs, et al., Angew. Chem. Int. Ed., 39 (2000) 10. M. C. Burieigh, M. A. Markowitz, M. S. Spector, and B. P Gaber, J. Phys. Chem. B., 105 (2001)9935. M. C. Burleigh, M. A. Markowitz, M. S. Spector, and B. P Gaber, Chem. Mater, 13 (2001) 4760.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
481
Exclusive incorporation of aluminum into tetrahedral site of the framework of periodic mesoporous organosilica Sung Soo Park^, Jong Hyeon Cheon'' and Dong Ho Park^* ^National Science Research Institute, Korea Advanced Institute of Science and Technology, Daejeon, 305-701, Korea ^Department of Chemistry, Inje University, Kimhae, Kyongnam, 621-749, Korea.
Al-containing periodic mesoporous organosilicas (Al-PMO) with hexagonal symmetry have been synthesized with varying the source and concentration of aluminum. Unlike MCM41, all Al species used in this study, except of aluminum hydroxide, were exclusively incorporated into the tetrahedral site of framework of PMO synthesized under the optimized reaction condition. The incorporation of aluminum into framework was identified by ^^Al and ^^Si - MAS NMR. The strength of Bronsted acid site of Al-PMO was weaker than that of Al-MCM-41, which is calculated by using HF/6-31G(d) basis set in Gaussian 98 program. 1. INTRODUCTION Surfactant-mediated synthesis method for mesoporous materials, which was initiated by Mobil group[l], made it possible to produce periodic mesoporous organosilicas[2]. These organic-inorganic hybrid materials containing of covalently-linked organic groups to Si inside the channel open up the possibility for many applications such as catalysis, sensing, enantioselective separation, asymmetric syntheses, chromatographic supports and so on[3,4|. With potential catalytic applications in mind, much attention has been given to isomorphous incorporation of heteroatoms into the silicate framework of MCM-41. Unlike MCM-41, the literatures relevant to heteroatom-incorporated PMO are limited yet. Since the Bronsted acidity of hydroxyl groups associated with 4-coordinate Al incorporated into framework is essential for acid catalytic application of PMO, the effort for synthesis of framework Al containing PMO is necessitated. We have found that all Al species except of aluminum hydroxide are exclusively incorporated into the tetrahedral site of framework of PMO under our synthetic condition, while the ratio of framework vs. nonframework Al were changed depending on Al source and concentration, in case of MCM-41. Here we report the exclusive incorporation of Al into tetrahedral site of the framework of PMO. The acidic property of Al-PMO was theoretically compared with that of Al-MCM-41 by Gaussian 98 program.
Corresponding author. Fax. +082-55-321-9718; e-mail: chempdhfglijnc.lnie.ac.kr; research grant: No. R052002-00922-0 from the Basic Program of the Korea Science & Engineering Foundation.
482
2. EXPERIMENTAL The synthesis of the Al-PMO was performed under basic condition with sodium hydroxide(Aldrich) using l,2-bis(trimethoxysilyl)ethane (BTME, Aldrich), cetyltrimethylammonium bromide (CTABr, Aldrich), and various aluminum source such as aluminum isopropoxide(Fluka), aluminum sulfate(Aldrich), aluminum nitrate(Aldrich), aluminum phosphate(Aldrich), aluminum acetylacetonate(Aldrich), aluminum hydroxide(Fluka), and sodium aluminate(Kokusan) at 95 °C for 21 h with varying the concentration of Al sources. X-ray powder diffraction (XRD) patterns were obtained by a Rigaku Miniflex 2200 diffractometer using Cu Ku radiation. Nitrogen adsorption-desorption isotherms were measured at 77 K on a Micromeritics ASAP 2010 instrument. Surface areas were determined by the Brunauer-Emmett-Teller (BET) method and the pore size distribution curve was obtained using Barrett-Joyner-Halenda (BJH) method from the adsorption branch isotherms. The chemical environment of aluminum and silicon was characterized by ^^Si and ^^Al MAS NMR spectra using a Bruker DSX400 spectrometer in air at 6 kHz and 12 kHz, respectively. In order to compare the acidic properties of Al-PMO with that of Al-MCM-41, the electron density in modeling compound [Ai(OSi(OH)3)4]"' for Al-MCM-41 and [Al(OSi(OH)2CH3)4]'' for Al-PMO, respectively, is calculated by HF/6-31G(d) in Gaussian 98 program. 3. RESULTS AND DISCUSSION Al-PMO's were synthesized under the optimized reaction condition[5] from reaction mixture with the molar composition of 1.0 BTME - (0.0-0.1) Al source - 0.57 CTABr - 2.73 NaOH - 380 H2O. XRD patterns of as-prepared Al-PMO from reaction mixtures with Si/Al 53 2 (tEtrahedrai) molar ratio on the range of 10-300 reflect p6mm I hexagonal symmetry lattice (not shown). As compared with the XRD of aluminum-free PMO, n si/Ai even low level of aluminum incorporation \\ molar ratio affects the quality of XRD pattern, which is
483
of Al-PMO with 80 of Si/Al ratio show a type-IV isotherm, indicating that this material had uniform 0.8 mesopores with average pore diameter of 28 P • >^ calculated by BJH method. BET surface area was ^•6 753 mV'(riot shown). E ^O" • y Figure 1 show the ^^Al MAS NMR spectra of
y^4
I
1
«
>
-66.9 53.2 (tetrahedral)
II
7.5 (octahedral)
^
_J\
/v_^jjminu m hydroxide
"^
> CO
Aluminum sulfate
I
CO
>
Aluminum acetylacetonate
i
Sodium aluminate
CO
Aluminum nitrate Aluminum phosphate 200
100
0
-100
p p m from AI(H20)63*
Fig. 3. ^^Al-MAS NMR of Al-PMO from different sources, of aluminum
-30
-40
-50
-60
-70
-80
-90
ppm from TMS CSKOSDj : - 6 6 . 9 ppm CSi(0AI)(0Si)2: - 6 2 . 7 ppm CSi(OH)(OSi), : - 5 6 . 5 ppm
Fig. 4. '^'Si MAS NMR of AlPMO and PMO.
484
framework of hexagonal mesostructure, independent of the amount and source of Al. Figure 3 shows ^^Al MAS NMR spectra of as-prepared Al-PMO from different aluminum sources with Si/Al=80. All samples except of synthesized using aluminum hydroxide as an aluminum source show a single peak at 53.4 ppm. As shown in Figure 4(a), ^^Si MAS NMR spectrum of aluminum-free PMO shows two bands at -56.5 and -66.9 ppm, which are assigned to partially condenced silicon. SiC(OH)(OSi)2, and fully condenced silicon, SiC(0Si)3, respectively[7]. As shown in Figure 4(b), ^^Si MAS NMR spectrum of Al-PMO with Si/Al=20 shows three bands at -56.5, -62.7 and -66.9 ppm. The additional band at -62.7 ppm may be attributable to silicon. SiC(OAl)(OSi)2, neighboring to incorporated aluminum. We confirmed by the acid strength of Al-PMO compared with Al-MCM-41, using HF/631G(d) in Gaussian 98 program. The modeling compound for Al-MCM-41 and Al-PMO is [Al(OSi(OH)3)4]"' and [Al(OSi(OH)2CH3)4]'\ respectively. The electron density, which is represented by the atomic charge, of Al, O bridged between Al and Si, Si, and O of hydoxyl group in [Al(OSi(OH)3)4]'' is 1.16, -0.98, 1.60, and -0.39, respectively. The electron density of Al, O bridged between Al and Si, Si, O of hydoxyl group, and C of methyl group in [Al(OSi(OH)2CH3)4]'' is 1.30, -0.95, 1.50, -0.40, and -0.31, respectively. From the results of theoretical calculation, the electron density of oxygen bridged between Al and Si of Al-PMO is lower than that of Al-MCM-41. That is, the difference of electron density between oxygen and hydrogen in Bronsted acid site of Al-PMO is smaller than that of Al-MCM-41. It means that the strength of site of Al-PMO was weaker than that of Al-MCM-41. The preference of tetrahedral site to octahedral one in PMO would be higher than that in MCM-41, which may be due to the difference of framework polarity and structural flexibility between PMO and MCM-41. It is anticipated that Al-PMO has weaker acidic site than AlMCM-41 due to organic moiety in the framework and therefore, could be applied to mild acid catalysis of bulky molecules.
REFERENCES 1. C. T Kresge, M. E. Leonowitz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature 359 (1992) 710. 2. C. Yoshina-Ishii, T. Asefa, N. Coombs, M. J. MacLachlan and G. A. Ozin, Chem. Commun. (1999)2539. 3. I. A. Aksay, M. Trau, S. Manne, I. Honma, N. Yao, L. Zhou, P. Fenter, P. M. Eisenbcrger and S.M. Gruner, Science 273 (1996) 892. 4. D. Zao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka and G. D. Stucky. Science 279 (1998) 548. 5. S. S. Park, C. H. Lee, J. H. Cheon and D. H. Park, J. Mater. Chem, 11 (2001) 3397. 6. Z. Luan, C.-F. Cheng, W. Zhou and J. Klinowski, J. Phys. Chem.. 99 (1995) 1018. 7. A. Sayari, S. Hamoudi, Y. Yang, I. L. Moudrakovski and J. R. Ripmeester, Chem. Mater. 12 (2000)3857
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Functionalization of hexagonal mesoporous silica and their base-catalytic performance Chun Yang^ ^, Xueping Jia^, Yudan Cao^ and Nongyue He^ ^ ^Key Laboratory of New Materials and Technology of China National Packaging Corporation, Zhuzhou Engineering College, Zhuzhou 412008, P. R. China ^ College of Chemistry and Environmental Science, Nanjing Normal University, Nanjing 210097,R R.China ^ Key Laboratory for Molecular and Bio-molecular Electronics of Ministry of Education. Southeast University, Nanjing 210096, P. R. China
3-aminopropyltriethoxysilane (AM), 3-ethyldiaminopropyltrimethoxysilane (ED) and 3-piperazinylpropyltriethoxysilane (PZ) were used to react with calcined hexagonal and hexagonal-like mesoporous silica SBA-3 and HMS (DDA-HMS and ODA-HMS) to produce functionalized mesoporous molecular sieves. Grafting level of functional groups on the surface increased with the raise in dosage of organosilanes. The general order of loadings for different mesoporous materials and organosilanes are 0DA-HMS>SBA-3>DDA-HMS and ED>AM>P/, respectively, depending on the pore volume, molecular size and reactivity of organosilane. These functionalised samples are effective catalysts with high activity for the Knoevenagcl condensation reaction of benzaldehyde with ethyl cyanoacetate. However, the over-loading is unfavorable to the initial activity. 1. INTRODUCTION Grafting organic functional groups onto the internal surface of mesoporous materials has become an effective method to modify these materials for catalysis and other applications in recent years [1-5]. Based on this approach, one can obtain novel solid base catalysts by functionalizing the surface with basic groups. Here we modify the hexagonal and hexagonal-like mesoporous silica materials SBA-3 and HMS with three N-containing organosilanes, 3-aminopropyltriethoxysiIane (AM), 3-ethyldiaminopropyltrimethoxysilane (ED) and 3-piperazinylpropyltriethoxysilane (PZ), to prepare solid base catalysts. An investigation on the effects of the sizes of organosilanes and the pore volumes of mesoporous materials on the loadings of functional groups is made. Then the base-catalytic properties of the functionalised samples were studied using the model Knoevenagel condensation reaction.
486
2. EXPERIMENTAL The SBA-3 was synthesized as previously described [6]. HMS were prepared using dodecyl amine (DDA) and octadecyl amine (ODA) as templates, respectively, at a composition ratio: lTEOS:0.27DDA(ODA):6.5EtOH:36H2O. The resulted samples are designated as DDA-HMS and ODA-HMS, respectively. These mesoporous materials were used as mother samples for further functionalization after calcinations to remove the templates. The samples grafted organosilane were prepared following the procedures: the mother samples were mixed with the given organosilane in toluene, followed by stirring for 3 h at given temperature. The resulted samples were filtered and the extra organosilane were extracted with CH2CI2 in a Soxhlet apparatus twice. The obtained samples are designated as AM (ED, PZ)-SBA-3 (DDA-HMS, ODA-HMS). The XRD patterns were recorded on Rigaku D/max-yC X-ray diffractometer, N2 sorption measurements were performed on Micromeritics ASAP 2000 instrument after evacuation at 573 K and 5x10"^ mmHg. The element analyses for C and N were conducted on Perkin-Elmer 2000 instrument to determine loading levels. The catalytic activities of samples for Knoevenagel condensation were investigated at 353 K and in toluene solvent. Dosage of catalyst was 4.5% of the total weight of reactants, and benzaldehyde and ethyl cyanoacetate were adopted 8 mmol each. The reaction mixtures were analyzed by gas chromatography (Varian 3400). 3. RESULTS AND DISCUSSION Listed in Table 1 are some structural parameters of the mother samples. ODA-HMS possesses the largest pore size and pore volume. As expected, the amount of used organosilane influence the loading level (see Figure 1). When the dosage of AM is increased from 0 to 1 mmol/g mother sample, the loading (mmol/g mother sample) increases rapidly and all of AM is grafted onto the surface. No significant difference in AM loading is observed for the three different mother samples at this stage because the influence of pore size on diffusion o( organosilane is inconsiderable at low loading level. When the AM dosage further increases, the raise in loading slower and the sequence of loading level on three mother samples is ODA-HMS>SBA-3>DDA-HMS, consistent with that of their pore volume. Table 1 Some parameters of mothei' samples Sample
dioo/nm
ao/nm ^
D/nm^
L/nm'
SBHi/m-g-''^
V/mLg''
SBA-3 DDA-HMS ODA-HMS
3.24 3.56 4.80
3.74 4.11 5.54
2.12 2.35 3.04
1.62 1.76 2.50
1276 943 718
0.66 0.53 0.86
^ ao=2 dioo/V3 , ^BJH desorption pore diameter, '^Thickness of pore wall, L = ao - D, ^ Bl: surface area and ^ Total pore volume.
487
The loading level of organosilanes is less 00 influenced by reaction temperature and time. Figure 2 shows the functionalization of SBA-3 with different organosilanes. It is found that the sequence of loading level is ED>AM>PZ, especially at high loadings. Two reasons are responsible for the sequence. o (1) The bulky PZ molecules diffuse into the channel more difficultly than the smaller ED < and AM molecules do. On the other hand, larger size makes the number of PZ 0 12 molecules anchored in the same area of Dosage of organosilane / mmol g surface less than that of ED and AM Fig. 1. Effect of dosage of organosilane on molecules. (2) The reactivity of RO- group of loading of AM. (Reaction temp.: 120°C) organosilane influences the loading level. The CH3O- group of ED molecule is more active than the C2H5O- group of AM molecule [7] and, therefore, the loading 0 4 h amount of ED is greater. The same sequence 0 /^ for loading level is observed on DDA-HMS d m "^ < (not shown here). OQ c C/3 0 On ODA-HMS, however, the loading of 00 0 ^0 2 h .c 3 AM is similar to that of ED (not shown here). 6 This is attributed to the larger pore size of 0 b 1 h C/) G/) ODA-HMS, which allows the entrance of C more AM molecules even though the Ti cd 0 reactivity of C2H5O- group is lower. It should J I II III IV also be mentioned that the loading levels of Reaction conditions all organosilanes arc higher on ODA-HMS than on SBA-3 and DDA-HMS, indicating Fig. 2. Loadings of different organosilanes that the larger pore size favors the grafting of on SBA-3. functional groups on the surface. Reaction temp, and dosage of organosilanes: Knocvenagel condensation reaction of (1) r.t., 1 mmol/g SBA-3, (Il)80°C,5mmol/gSBA-3, benzaldchyde with ethyl cyanoacetate was (1I1)120°C, 5 mmol/g SBA-3 used to investigate the base-catalytic (1V)120"C, 12 mmol/g SBA-3 properties of the functionalizcd samples. The results on HMS samples are shown here as an example. Since the selectivity for condensation product, a a, P-unsaturated ester, is 100%, the yield of this product can be considered as the activity of reaction. Figure 3 exhibits the change of activity as a function of reaction time on DDA-HMS grafted with AM. No notable activity is detected on the mother sample (Figure 3(1)); but very high yields are observed after grafting AM molecules onto the surface. For most of the samples, the yields approximate 100% after 2.5-3 h of reaction time and the difference in activity appears only at initial stage ^-1
488
of reaction. In order to compare the catalytic performance of various samples, we investigated the activities at 0.5 h of reaction time and show those of ED-grafted samples in Figure 4 as a representative. It can be seen from Figure 4 that the change of yield with loadings is not monotone and a maximum exists for both DDA-HMS and ODA-HMS, suggesting that over-loading is unfavorable to the initial activity, especially for DDA-HMS with a smaller pore size. This is because the pore volume decreases (not shown here), i.e., the channels are partly blocked by organic molecules at the higher loading levels so that the diffusion of reactants and products is hindered and accessible active sites decrease. This effect of diffusion occurs only under higher loadings and similar activities are observed for DDA-HMS and ODA-HMS at lower loading levels as shown by the yields before reaching the maximum in Figure 4.
lUU
80
S2
a
;(3)
"
~^V^
60
-o 40
(2)
/(5)
-53 ^
r.
20 n { U '
(1)
1 2 Reaction time / h
3
Fig. 3. Activities change with reaction time on AM-DDA-HMS at loading of (1) 0 mmol/g, (2) 0.49 mmol/g, (3) 0.95 mmol/g, (4) 1.80 mmol/g, (5) 2.60 mmol/g.
Loadings/mmol g" Fig. 4. Effect of loading on initial activity for (1) ED-DDA-HMS, (2) ED-ODA-HMS
REFERENCES 1. X. Feng, G.. E. Fryxell, L.-Q. Wang, A. Y. Kim, J. Liu and K. M. Kemner, Science, 276 (1997)923. 2. I. Diaz, C. Marquez-Alvarez and F. Mohino et al, J. Catal., 193 (2000) 283. 3. D. Brunei, Micro. Meso. Mater., 27 (1999) 329. 4. W. A. Carvalho, M. Wallau and U. Schuchardt, J. Mole. Catal. A, 144 (1999) 91. 5. S. Jaenicke, G. K. Chuah, X. H. Lin and X.C. Hu, Micro. Meso. Mater., 35-36 (2000) 143. 6. X.-P. Jia, C.Yang, N.-Y He and Z.-H. Lu, Chinese J. Inorg. Chem., 17 (2001) 256. 7. Z.-D. Du, J.-H. Chen, X.-L. Bei and C.-G. Zhou, Chemistry of Organosilicon Compounds, Beijing, Higher Education Press, 1990:195
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
489
Microstructure of the organo-modified SBA-15 (Vinyl-SBA 15) prepared under different pH Byimg-Gyu Park, Jiyong Park, Wanping Guo, Won-Jei Cho, and Chang-Sik Ha* Department of Polymer Science & Engineering, Pusan National University, Pusan 609-735, Korea. Microstructure of the organo-modified SBA-15(vinyl SBA-15) prepared under different pH was investigated using a scanning electron microscopy(SEM). It was found that the morphology of the organo-modified mesoporous materials depended on pH conditions during sol-gel reaction. The periodic mesoporous structure was formed when the materials are obtained in rod-like shapes. 1. INTRODUCTION Much progress has been made in the last years in the development of organo-modified periodic nanoporous materials[l-3]. Chemical functionalization of the inorganic framework of porous materials through the covalent coupling of an organic moiety is a promising approach to specific pore surface properties such as hydrophobicity, polarity, and catalytic, optical, and electronic activity. Silsesquioxanes or bridged silsesquioxanes are used as coprecursors with tetraalkoxy silane for the surface modification of organo-modified periodic nanoporous silica materials. A few attempts have been made to use porous inorganic materials with their superior thermal and mechanical properties as the carrier for the preparation of organo-modified silica gels. Hybrid catalysts with organic groups attached to the support by standard silica functionalization techniques have been proposed. It is apparent that materials with wide pores are required in order to accommodate the functional groups and to allow easy access of reactants to the active sites. We prepared organo-modified SBA-15 by introducing silsesquioxane, triethoxy vinyl silane(TEVS), to the silica framework by the direct synthesis method. In this paper, we report on the microstructure of the organo-modified mesoporous SBA-15 like materials prepared by using silsesquioxanes under different pH conditions. 2. EXPERIMENTAL For a typical synthesis, a triblock copolymer, poly(ethylene oxide)-poly(propylene oxide)poly(propylene oxide) (PE02o-PP07o-PE02o;EPE) was dissolved in water and stirred at 40 °C
490
for 3 hours. A catalyst, such as HCl or NaOH, was added to this solution, then the mixture of TEOS and TEVS of a given mole ratio was put into the solution under different pH conditions and stirred for 40 hours. After reaction, the precipitated powder products were filtered and dried in air at ambient temperature for 1 day, then put into an oven at 60 °C for 4 days. The products prepared under acidic conditions were washed with distilled water before drying. As-synthesized samples were extracted by acidic solution containing hydrochloric acid and methanol at 80 °C for 48 hours. Table 1 summarizes samples prepared in this work. Small angle X-ray scattering(SAXS) patterns were obtained on 4C2 beam lines with a Co Ka radiation operated at 2.5 GeV and 140m(wavelength, A<=1.608A) in the Pohang Accelerator Laboratory, POSTECH, Korea. The morphology of final compounds was studied using a Scanning electron microscopy(SEM) using conventional sample preparation and imaging techniques. Nitrogen adsorption and desorption isotherms were measured at 77K. Surface area was determined by the BET method. Table 1 Textural Properties of Organo-modified Mesoporous Materials Extracted by Acidic Solution , ^ ^ r, 1 Average por b Wall c' u la IT "100 Surface are Pore volume ^. ^ ao ^i,:^i,„^oo^ Symbol pH , , , , 2, . , 3, . size „ thickness ^ ^1 (A) (m/g) (cm/g) ^^^ (A) ^^^ 80 E4P1H 440 0.196 92 69 E4P2H 2 0.275 21 90 78 480 79 E4P3H 99 751 0.392 20 86 3 E4P5H 44 5 539 0.606 E4P7N 70 0.710 7 565 E4P9N 9 95 683 1.626 a. The mole ratio of TEOS to TEVS is 0.75:0.25. The amount of EPE is fixed as 0.004mol. b. ao is the lattice parameter from the SAXS data using the formula ao= 2 d]oo/^3. c. Wall thickness = the lattice parameter - average pore size. 3. RESULTS AND DISCUSSION The hydrolysis includes reacfions of alkoxide groups(-OR) with hydroxyl groups(-OH). Subsequent condensation reactions involving the silanol groups produce siloxane bonds(Si0-Si) plus alcohol(ROH) or water as by-products. Under general conditions, condensation commences before hydrolysis is complete because water and alkoxysilanes are immiscible. When large amount of water was used, as for our experiments, hydrolysis reaction dominantly took place, followed by low rate of condensafion reaction. Fig. 1 shows SEM images of vinyl-SBA15 series prepared under different pH conditions. Bulk phases of over 50 j^ni size at pH 1, rod-like powder of 10-50 ^m size at pH 2-3, and aggregation of ca. 100 nni particles above pH 5 were formed. Various morphology of the vinyl-SBA15 serise can be explained by the reaction pathway during the sol-gel reaction.
491
PF^^C
*1
Fig. 1. Scanning electron microscopy(SEM) images of as-synthesized organo-modified mesoporous materials ; (a)E4PlH, (b)E4P2H, (c)E4P3H, (d)E4P5H, (e)E4P7N, and (f)E4P9N Under acid conditions, it is likely that an alkoxide group is rapidly protonated in first step. Electron density is withdrawn from silicon, making it electrophilic and thus more susceptible to be attacked by water. Thus, hydrolysis rate is very fast, then condensation reaction is followed. Below pH 2, a vinyl-SBA15 appears to bulk phase because the polymerization rale is proportional to [H^] and is affected by the hydrolysis effect of TEVS. Near the isoelecuic point(IEP) of silica, the growth and aggregation processes occur together and may be indistinguishable. Despite low solubility of silica near pH 2, rod-like powders were made at pH 2 and 3 because high reactivity of TEVS in comparison to TEOS attributed to the grown particles of the vinyl-SBA15 due to hydrolysis effect. Under basic conditions, it is likely that water rapidly dissociates to produce nucleophilic hydroxyl anions in the first step. When the hydroxyl anion attacks the silicon atom, oxygen of alkoxide push out nucleophilic hydroxyl anions so that it can induce steric hindrance. In this way, the hydrolysis reaction is much slower in comparison to the case when same concentration of acid catalyst is used. Condensation reaction is nucleophilic substitution of deprotonated silanol. Therefore, it is considered that steric hindrance limits the growth of particles and then they aggregate one another with the size of ca.lOOnm. Generally speaking, the condensation ratio is proportional to the concentration of hydroxyl anions[-OH] between pH 2 and pH 6 [4]. For E4P5H sample, which was prepared under pH 5, therefore showed similar morphology with samples prepared under basic conditions as well. The SAXS studies showed that vinyl-modified mesoporous materials didn't show any structural regularity above pH5, while the vinylSBA15 materials showed ordered hexagonal mesopore structures below pH3. N2 adsorpliondesorption isotherms for E4P2H(a) and E4P7N(b) samples are shown in Fig. 2. The E4P2H sample exhibits substantial framework-confined and textural mesoporosity, which is ca. 20-
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23 A of average pore size distribution. A sharp increase in N2 adsorbed volume is observed for the sample at P/Po value(N2 relative pressure) of ca. 0.2W 0.4, owing to capillary condensation of N2 in mesopores. In the case of E4P7N sample, the capillary condensation was observed at high P/Po 0.0 0.2 0.4 0.6 0.8 1.0 value of ca. 0.6-0.9 indicating large pore size of Relative Pressure (P/P.) these materials. The E4P7N sample showed typical type n isotherm in comparison to the ordered mesoporous material showing typical type IV isotherm for the E4P2H sample, even though the former has large mesopore of ca. 70-95A. In spite of possessing a sharp increase of capillary 0.0 0.2 0.4 0.6 0.8 1.0 condensation, organo-modified mesoporous Relative P r e s s u r e (P/P^) materials showed the broad pore size distribution. The SAXS analysis data are given in Table 1. It Fig. 2. N2 adsorption-desorption was found that periodic mesoporous structure was isotherms for E4P2H(a) and E4P7N(b) samples. formed when the materials are obtained in rodlike shapes. The textural properties taken from SAXS and the N2 adsorption-desorption isotherms are also summarized in Table 1. It was concluded that the morphology of organomodified mesoporous materials was significantly affected by pH conditions.
ACKNOWLEDGEMENTS This work was supported by the Center for Integrated Molecular System, POSTI^CIl, Korea and the Brain Korea 21 Project in 2001. The Pohang Accelerator Laboratory. POSTECH, Korea is also acknowledged for SAXS experiments. REFERENCES (1) B. G. Park, N. J. Jo, W. J. Cho, and C. S. Ha, Polym. IntT, in press. (2) V. Goletto, M. Imperor, and F. Babonneau, Stud. Surf. Sci. Catal., 129(2000), 287. (3) P Mukherjee, S. Laha, D. Mandal, and R. Kumar, Stud. Surf Sci. Catal., 129(2000), 283. (4) J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Loonowicz, C. T. Kresge, K. D. Schmitt, C. T. Chu, D. H. Olsen, E. W. Sheppard, S. B. Mccullen, J.B. Higgins, and J. L. Schlenker, J. Am. Chem. Soc, 114(1992), 10834.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
493
Surface coating of MCM-48 via a gas phase reaction with hexamethyldisilazane (HMDS) A. Daehler,^ M.L. Gee,^ F. Separovic,^ G.W. Stevens^ and A.J. O'Connor^ Particulate Fluids Processing Centre, ^Department of Chemical Engineering and ^School of Chemistry, The University of Melbourne, 3010 VIC, Australia. Hydrophobic coatings on silica M41S materials may have the dual benefits of improving their stability in aqueous environments and their adsorption selectivity for solutes such as proteins and vitamins. In this work, the surface of MCM-48 was successfully coated by a gas phase reaction with hexamethyldisilazane (HMDS). Characterisation by gas adsorption showed that the method is highly reproducible. Solid-state NMR results demonstrated that the coating was chemically attached to the surface. The material was much more stable than untreated MCM-48 in aqueous buffer solution for up to 40 days. 1. INTRODUCTION MCM-48, the cubic mesoporous molecular sieve of the M41S family,' is expected to provide better access to its pores than MCM-41, its better known hexagonal counterpart, due to its interconnected pore structure. This is of particular interest for the use of M41S materials as adsorbents. The application of appropriate robust coatings on M41S materials may improve their selectivity of adsorption and stability in aqueous environments.^"'* Here we report data on hydrophobically coated MCM-48. 2. EXPERIMENTAL MCM-48 was synthesized using the method of Schmidt et ai^ A hydrophobic coating was applied to MCM-48 by gas phase reaction with HMDS (Merck, 98%) as follows. MCM-48 was heated under vacuum to 350°C in the reaction chamber to remove any adsorbed water, and then reacted with the HMDS for 4 h or 16-17 h as the reaction chamber was allowed to cool. Gas adsorption and desorption isotherms were determined by nitrogen adsorption at 77 K, using a Micromeritics ASAP 2000. The MCM-48 was degassed before gas adsorption measurements for a minimum of 6 h at 250°C for the untreated material and at 150°C for the coated material. Single point pore volumes were estimated based on the amount of nitrogen adsorbed at a relative pressure of about 0.99. Pore size distributions were estimated using the BJH model and specific surface areas were estimated using the BET model.^' Powder x-ray diffraction (XRD) spectra were recorded using a Philips PW 1800 x-ray diffractometer (40 kV, 30 mA, step size 0.02°, count time 5 s, Cu K a-radiation). Nuclear magnetic resonance (NMR) analysis was performed on an Inova 300 Varian (Palo Alto, USA) spectrometer, operating at 75.452 MHz for '"^C and 59.606 MHz for ^^Si. Acquisition and processing of data were performed using Varian software. The '^C and ^^Si solid-state spectra were recorded at the magic angle (54.7'') by spinning the sample at a frequency of ~6 kHz with
494
proton decoupling to enhance resolution. The ^^C spectra were recorded with crosspolarisation for H-MCM-48. 3. RESULTS AND DISCUSSION Several samples from the same synthesis batch of MCM-48 were coated by the procedure described above, with reaction times as shown in Table 1. The general shape of the nitrogen adsorption isotherms (Fig. 1(a)) did not change upon coating, which indicates that the mesoporous structure of MCM-48 remained intact. This was also confirmed by powder X-ray diffraction which showed no significant change in the spectra before and after coating. The amount of nitrogen adsorbed decreased for the coated material, as did the single point pore volume (Table 1), with good reproducibility for 16-17 h reaction time. A reaction time of 4 h resulted in a smaller change in adsorbed volume and total pore volume but repeating the coating reaction on the same sample for a further 16 h yielded results comparable to the longer reactions. The BJH pore size distributions (Fig. 1(b)) confirmed these results: after 4 hours reaction with HMDS, the maximum in the PSD was shifted to smaller values than for the original MCM-48, and this effect was stronger for the longer reaction times (16-17 h), with little variation between samples S2-S4. Repeating the coating reaction for 16 h on sample SI yielded similar results to those for samples S2-S4. The four samples with reaction times of 16-20 h were combined after characterization to provide a larger sample which maintained the narrow pore size distribution and characteristic XRD spectrum of the constituent samples. Table 1 Properties of MCM-48 before and after gas phase reaction with HMDS Sample Reaction Pore volume Peak position in BJH pore Ccm'/g) size distribution (nm) time (h) 0.97 2.18 MCM-48 6 4 0.81 2.00 H-MCM-48-S1 4 + 16 0.65 1.69 H-MCM-48-Slr 16 0.66 1.83 H-MCM-48-S2 17 0.66 1.69 H-MCM-48-S3 17 0.67 1.69 H-MCM-48-S4 16-20 0.67 1.72 H-MCM-48* H-MCM-48 is a combination of samples H-MCM-48-Slr, S2, S3 and S4.
BET surface area (m~/g) 1181 1121 1318 1194 1139 1248 1225
Chemical binding and the quality of the coating were determined by solid-state magic angle spinning (MAS) ^^Si and '^C NMR, demonstrating successful covalent binding of the coating to the silica surface. Fig. 2 shows the ^^Si spectra with peaks for the MCM-48 at - 93 ppm. 103 ppm and -112 ppm, assigned to geminal silanol groups (Q^), silanol groups (Q^) and framework silica (Q'*), respectively. The H-MCM-48 spectrum showed significant changes in comparison to MCM-48. An additional peak at 13 ppm, not present in the MCM-48 spectrum, was attributed to the groups attached by the coating reaction. The position of this additional peak, labelled T, was in good agreement with the peak position reported for attaching trimethylsilane moieties on other mesoporous materials.^"'^ No Q^ peak was detected in HMCM-48. The relative height of the Q^ peak compared to the Q"^ peak decreased significantly after coating, as expected. "'^ This drop in the Q /Q^ ratio, together with the position of the additional peak in the H-MCM-48 spectrum, are strong evidence for the covalent attachment of trimethylsilane moieties to the surface of the MCM-48.
495
600
1
(a)
/••
H 500
-
i^^**
^ 400
-
* i****
300 200 100
4
r •» 1 f>
r
MCM-48 H-MCM-48-S1
. • '
H-MCM-48-Slr H-MCM-48-S2 H-MCM-48-S3 H-MCM-48-S4
> 1-5
• • o o
MCM-48 H-MCM-48-S1 H-MCM-48-Slr H-MCM-48-S2 H-MCM-48-S3 n H-MCM-48-S4
•Q
0.5 > ^
0.2
0.4 0.6 Relative Pressure
0.8
g>
Pore Size (nm)
Fig. 1 .(a) Nitrogen adsorption isotherms and (b) BJH pore size distributions for MCM-48 before and after reaction with HMDS To obtain quantitative information about the composition of the samples, the ^^Si MAS NMR spectra were deconvoluted with least squares fits, using Gaussian normal distributions, shown in Fig.2. The concentrations of the silanol (SiOH) and trimethylsilane groups (Si(CH3)3), shown in Table 2, were thereby determined relative to the mass of the original MCM-48.'' The surface densities of these groups were calculated using the BET surface area. The silanol group density decreased by about 50 % upon coating with a corresponding increase in the density of Si(CH3)3 groups, consistent with the 1:1 stoichiometry expected for reaction of silica with HMDS. The sum of the concentrations of the silanol and trimethylsilane groups on H-MCM-48 was within 3 % of the concentration of silanol groups on the original MCM-48, further evidence for covalent attachment of trimethylsilane moieties.
yW-vVV^.jArA/V^'^^^^ 50
-50
-100
-150
29c Fig.2. '^Si solid-state MAS NMR spectra of MCM-48 and H-MCM-48 showing deconvolution of the H-MCM-48 spectrum. Typically 920 transients were recorded using a 2 i^s (Tr/4) pulse, 60 s recycle time and a 5 mm MAS rotor.
496
Table 2 Surface characteristics of MCM-48 before and after coating as determined by NMR MCM-48 H-MCM-48 [SiOH] (mmol/g) SiOH group density densil (number/nm^) [Si(CH3)3] (mmol/g) Si(CH3)3 group density densit (number/nm^) [SiOH] + [Si(CH3)3] (mmol/g)
6.9 3.5 6.9
3.5 1.8 3.6 1.8 7.1
The H-MCM-48 sample was immersed in aqueous buffer solution (0.5 M 2-[Nmorpholino] ethane-sulfonic acid, pH 6) for up to 40 days to test its structural stability. Without the hydrophobic coating, MCM-48 degrades significantly over this time period." In contrast, the H-MCM-48 sample showed no significant change in either its XRD spectrum or nitrogen adsorption isotherm between 1 and 40 days of solution contact. A small drop (< 10%) in the pore volume was observed after 1 day's immersion but thereafter the pore volume remained constant at 0.62 cm"^/g and the BET surface area stayed high at 1030 m^/g. These observations demonstrate that H-MCM-48 has greatly improved stability relative to MCM-48 in the presence of water. 4. CONCLUSIONS A robust, covalently attached, hydrophobic coating was applied to MCM-48 with a high surface coverage of trimethylsilane moieties. The coating procedure did not affect the ordered structure of the underlying silica matrix. Narrow pore size distributions were maintained with a decrease in pore diameter compared to the original MCM-48 due to the attached moieties. The hydrophobic H-MCM-48 was stable in aqueous buffer solutions, under the conditions tested, a major improvement compared to the original, uncoated material, which would make it useful for applications like the separation of biological molecules. The authors gratefully acknowledge Dr M.I. Burgar (CSIRO) for his help with Si NMR. The support of a HSPIII postgraduate scholarship from the German Academic Exchange Service (DAAD) for AD and the Selby Research Award for AO are gratefully acknowledged.
REFERENCES
1. Kresge, C.T., et al, Nature, 1992, 359, 710. 2. Daehler, A., et al., Proc. 6th World Congress Chem. Eng., 2001, Melbourne, Australia. 3. Kisler, J., et al.. Mater Phys. Mech., 2001, 4, 89. 4. Yang, ].,etal.. Stud Surf. Sci. CataL, 2002, 141, 221. 5. Schmidt, R., et al., Microporous Materials 1995, 5, 1. 6. Branton, RJ., et al., Chem. Soc, Faraday Trans., 1997, 93, 2337. 7. Kruk, M., et al., Langmuir, 1997, 13, 6267. 8. Kruk, M., et al., Langmuir, 1999, 15, 5279. 9. Zhao, X. S. and G. Q. Lu, J. Phys. Chem. B, 1998, 102, 1556. 10. Sutra, R, et al.. Colloids & Surfaces A, 1999, 158(1-2), 21. 11. Wouters, B. H., et al, Micropor Mesopor Mater, 2001, 44, 453.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
497
Reactivity of silica walls of mesoporous materials towards benzoyl chloride L. Pasqua^ F. Testa^ R. Aiello^* ^Dipartimento di Ingegneria dei Materiali e della Produzione, Universita di Napoli Federico II, 80125 Napoli, Italy ^Dipartimento di Ingegneria Chimica e dei Materiali, Universita degli Studi della Calabria, Via Pietro Bucci, 87030 Rende, Italy. FAX +39-0984492058. E-mail: r.aiello@unical.it Mesoporous materials synthesised using sodium silicate and an industrial neutral surfactant activated by calcination or solvent extraction are modified by reaction with benzoyl chloride to compare surface properties of both samples. FT-IR spectra of modified samples show a peak centred at 1702 cm"' assigned to the stretching vibration of carbonyl group of benzoic ester. Modified samples also show lower specific surface areas and pore volume compared to the corresponding values for starting materials. Amount of chemical modification in hydrolysis solutions is quantitatively evaluated by UV-spectrofotometry and TG analysis showing that the mesoporous sample activated by solvent extraction possess a much higher reactivity because of high hydroxyl groups population not affected by thermalinduced condensation. 1. INTRODUCTION Mesoporous ordered silicate can have important uses in catalysis, metal ion extraction and optical applications because they are characterized by an hexagonal array of mesopore whose diameters range, depending on synthesis parameters, between 15 and 100 A. On the other hand pore walls can be easily modified so that active sites can be designed to host as better as possible the substrate. Catalytically active materials have been prepared through introduction of inorganic heteroatoms by grafting metallocene complexes on mesoporous silica creating well-defined active sites [1] Silica sources containing a non-hydrolysable Si-C bond can be used to produce hybrid inorganic-organic materials by post-synthetic grafting or by simultaneous condensation [2-4]. In a recent work a functionalised hexagonal mesoporous SBA-15-type molecular sieves have been prepared using non-ionic block copolymers, was used for the immobilization of the enzyme trypsin [5]. MCM-41 was also studied as a drug delivery system. In particular inclusion and delivery mechanism, of Ibuprofen, an antiinflammatory drug, were investigated [6]. In this work mesoporous materials are synthesised using a simple synthesis procedure which employs sodium silicate and an industrial neutral surfactant. Activation of as-synthesised samples is performed according two alternative methods: calcination and solvent extraction by using ethanol-water mixtures. Pore surfaces of activated samples are modified by benzoyl chloride and the different reactivities compared. 2. EXPERIMENTAL The neutral surfactant polyoxyethilene(10)isononylphenylether (Nonfix 10, Condea) was used in the synthesis of sample A. The molar composition of the gel was: SiO2-0.6NaOH-0.064 Nonfix 10-0.8 HCl-58.1 H2O
498
Silica source was sodium silicate. The sample was calcined at 550°C in air with a heating rate of l°C/min. The surfactant was extracted from the as-made materials by three extraction cycles with water/ethanol 2/1 VV at 85 °C. Chemical modification was induced in tetrahydrofuran with benzoyl chloride (Sigma) at room temperature. UV spectra were acquired at 240 nm in corrispondence of the main absorption in the benzoic acid spectrum. A Shimadzu UV 160-A instrument was used. UV quantitative analysis was carried out on solutions (Buffer solution pH 1, Merck) used for hydrolysis of the modified sample. Hydrolysis was carried out at room temperature for 36 h. A 5 standards calibration curve was used whit the buffer solution as blank. Tg and DSC analyses were acquired on a Netzsch STA 409 instrument. 3. RESULTS AND DISCUSSION Table 1 shows SBET and pore volume of active (Acaic and Aextr) and modified samples (AcaicBz and AcxtrBz). Extracted sample shows higher values of specific surface area and pore volume. Table 1 SBET and pore volume of activated and modified samples SBET(niVg) Sample •^calc 791 Aextr 984 Acalc B Z 797 AcxtrBz 638
PV (cm7g) 0.48 0.75 0.46 0.41
Esterification of free silanols on pore walls was evidenced by FT-IR spectroscopy and evaluated by UV-spcctrophotomctry and thcrmogravimctrical analyses. Porosity of modified samples was evaluated by nitrogen adsorption-dcsorption at 77 K. FT-IR spectra of modified samples show a peak at 1702 cm" assigned to the stretching vibration of carbonyl group of benzoic ester. This peak, not present in the spectra of starting materials, disappears after hydrolysis of ester function.
4000
3500
3000 2500 2000 1500 Wavenumbcr (cm" )
1000
Fig. 1. FT-IR spectrum of sample Aextr after chemical modification.
500
499
360 H320 too
280
j1
B 240
^'•^^^X^^'^'—X-"^—
1
/
^ 200
—
^3
120 •] 0.0
DCS A^_^|^
— • — Ads A^^i^ — ^ — Des A^,^|^ Bz — • — Ads A . . Bz
160
<
=sst^^^^
1
0.2
0.8 0.4 0.6 Relative Pressure P/P,,
11
l.O
Fig. 2a. Nitrogen adsorption-desorption isotherms of sample Acaic and sample Acaic after chemical modification (Acaic Bz) 560 H 480 H OH
00 toD
400
j^ 320
> 240
-e o
160^
C/5
<
0.4 0.6 0.8 Relative pressure P/P„
1.0
Fig. 2b. Nitrogen adsorption-desorption isotherms of sample Aextr and sample Aexi after chemical modification (Aextr Bz) Nitrogen adsorption-desorption isotherms of sample Acaic before and after chemical modification are shown in Fig. 2a. The two couple of curves are almost not distinguishable being just slightly in upper position the unmodified material curves. Presence of benzoyl groups does not affect pore structures of calcined samples. Fig. 2b shows nitrogen adsorptiondesorption isotherms of sample Aextr and sample Aextr after chemical modification (AextrBz). The presence of benzoic ester is able to shift to lower relative pressures the main nitrogen adsorption, indicating a reduction in average pore diameter and, moreover drastically reduces pore volume. UV quantitative measurements of benzoic acid concentration in hydrolysis solution show that, as expected, chemical modification on sample activated by solvent extraction is much larger in comparison with calcined sample. Benzoic acid/silica mass ratios derived from benzoic acid concentrations detected in hydrolysis solutions are 4.30»10''^ and 6.95»10'^ for
500
solutions coming from hydrolysis on calcined and extracted sample, respectively. Nevertheless benzoic acid is only partially released in hydrolysis solution. TG and DSC analysis were used to evaluate surface properties of both calcined and extracted materials and their modified derivatives. DSC analysis shows the exothermal peak assigned to the combustion of benzoyl group around 400°C. A blank thermogravimetric analysis was runned on template-free samples in order to compare the weight loss due to the high temperature condensation of vicinal and geminal hydroxyls in calcined and extracted samples. An OH group concentration of 0.23 and 0.56 mol in lOOg of dry Si02 in was calculated for calcined and extracted samples respectively. Benzoic group/dry composite sample mass ratio from thermogravimetric analysis of modified samples arc 4.12-10'^ and 2.30-10"' for materials obtained from calcined and extracted samples, respectively. Table2 Quantitative data on benzoyl and hydroxyl modification Sample Bz/(Si02 + Bz OH cone. Bz/(Si02 + Bz) Bz/Si02 + Bz (mass ratio)** (mol/lOOg) + ads. H2O) mol/lOOg (mass ratio) Dry Si02 0.042 4.30-10-'^* 0.23 4.12-10-^ Acalc BZ 0.23 6.95-10-^* 0.56 2.30-10-' AcxtrBz *Data obtained from UV quantitative measurements of hydrolysis solution **Data obtained from TG analyses
%ofOH reacted with Bz 18 41
0.042 and 0.23 moles of benzoyl group arc contained in 100 g of dry composite samples indicating that 18% and 41% of detected silanols, respectively, for calcined and extracted samples react with the carbonil group. Isolated silanols, not detectable, arc not considered in the total OH group concentration. Nevertheless they can react with benzoyl chloride. 4. CONCLUSIONS UV quantitative measurements show that the release of benzoic acid in hydrolysis solutions is not complete. Mcsoporous silica activated by solvent extraction exhibits a much higher surface reactivity compared to calcined samples. Hydroxyl group population remain quasi intact after solvent extraction of surfactant allowing massive reaction with benzoyl chloride. Calcination drastically reduces OH groups so decreasing the reactivity of pore walls.
REFERENCES 1. 2. 3. 4. 5. 6.
T. Maschmeier, F. Rey, G. Sankar and J.M. Thomas. Nature, 378 1995, 159. S.L. Burkett, S.D. Sims and S. Mann. Chem. Commun, 1996. 1367. M.H. Lim and A. Stein. Chem. Mater. 11 .1999, 3285. Brunei D. . Microp. and Mesop. Mater., 27.1999, 329. H.H.P. Yiu, P.A. Wright, N. P. Botting, J. Molee. Catal. B: Enzymatic 15, 2001, 81 M. Vallet-Regi, A Ramila, R:P. Del Real and J. Perez Pariente. Chem. Mater., 13 2001, 308.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
501
Catalytic activitiy of chiral phosphinooxazolidine ligands immobilized on SBA-15 for the asymmetric allylic substitution Pong Hyun Chong^, Yong Ku Kwon^, Chung Young Lee^ and Geon-Joong Kim^ ^Department of Chemical Engineering, Inha University Incheon 402-751, Korea. ^Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Korea. ^Department of Industrial Chemistry, Inha Technical College, Incheon 402-752, Korea. The asymmetric Pd-catalyzed allylic substitution using chiral phosphinooxazolidines immobilized on SBA-15 mesoporous silica could be applied with success & high enantioselectivities were attainable in the allylic substitution. 1. INTRODUCTION Asymmetric catalysis is becoming the preferred approach because of its low environmental impact and high potential productivity. In particular, asymmetric allylic alkylation has become one of the more useful reactions. During the last decade, various enantioselective catalysts have been developed for palladium-catalyzed allylic substitution reactions[l]. In particular, chiral phosphinooxazolidine ligands are one of the effective ligands in this reaction[2,3]. In this study, we developed new chiral oxazolidines and immobilized these ligands on the supports. Chiral phosphinooxazolidine ligands could be readily synthesized by condensation of alkyl aminoalcohols and (2-diphenylphosphino) benzaldehyde. So, we have synthesized new oxazolidines using aminoalcochols containing sufur atom(methionol, s-methyl cysteinol, sbenzyl cysteinol) and various aldehydes(2-thiophenecarbxaldehyde, 2-pyridinealdehyde). The sequent anchoring method of reacting a functionalized support with reactive groups of aminoalcohols, step by step, make it possible to synthesize various unsymmetrical chiral oxazolidines of different structure and to immobilize them onto solid supports. The heterogeneous catalysts offer practical advantages of the facile separation from reactants and products, as well as recovery and reuse[4-7]. But some disadvantages can be expected in heterogeneous catalysis in terms of reaction rates and enantioselectivity. Our attention was directed to the development of a method anchoring the optically selective chiral ligands on the solid supports[5]. A few studies concerning the immobilization of amino alcohol derivatives onto MCM-41 mesoporous materials and their catalytic properties in the asymmetric allylic alkylation have been published in the open literature[5-7]. The use of these mesoporous silicas with regular pore diameters has expanded the range of applications for the catalysis of large substrates and incorporating bulky complexes for enantioselective catalytic reactions.
502
Ri
Qupporp
CI
OH
*^2C03
Scheme. 1 Preparation of phosphinooxazolidines immobilized on supports 2. EXPERIMENTAL 2.1. Preparation of phosphinooxazolidines immobilized on the mesoporous material Scheme. 1 shows the method to immobiUze phosphinooxazolidines on the mesoporous supports. Heterogenized phosphinooxazolidines ligands were prepared by the reaction of chloro functionalized supports with an amino alcohol in boiling toluene. 2.2. Allylic substitution reaction Similarly to the Trost's procedure[8], which is very often employed as a test allylic substitution reaction, the reactions of l,3-diphenyl-2-propenyl acetate with nucleophile produced by reacting dimethylmalonate, M(9-bis(trimethylsilyl)acetamide(BSA), and potassium acetate as a catalyst were performed at room temperature in the presence of nallylpalladium chloride dimmer and the chiral ligands. 3. RESULTS AND DISCUSSION The catalytic activities of homogeneous phosphooxazolidines were investigated in the asymmetric allylic substitution of l,3-diphenyl-2-propenyl acetate with dimethyl malonate and the results are summarized in Table 1. The high enantioselectivity was obtained when 2.5 mol% Pd-allyl dimmer and 10 mol% ligand to the substrate were employed. As can be seen in this Table, the enantioselectivity was strongly dependent on the structure of amino alcohols. The homogeneous phosphinooxazolidine ligand Al and A3 show the very high enantioselectivities. The structure of ligands used as catalysts in this work is shown in Scheme 2.
Scheme. 2
503
The increase of reaction temperature led to a slight decrease of enantioselectivity. These results show that low reaction temperature is efficient for the increase of enantioselectivity. THF solvent is usually used in the Palladium-catalyzed allylic substitution reaction. The high enantioselectivity was also observed by using dichloromethane as a solvent. We have prepared new phosphinooxazolidine ligands starting with the sulfiir-containing amino alcohols and used them as catalysts in the same reaction. Chiral phosphooxazolidine (Al) prepared from methioninol showed a high ability to induce chirality in the target molecules with 88% ee. The greater enantioselectivity provided by ligand prepared from methioninol in comparison with s-methinonol may be rationalized by the longer tether between the nitrogen and sulfur as a donor atoms. The chiral phosphooxazolidine ligands immobilized on the mesoporous material were applied for the pd-catalyzed reaction to evaluate their catalytic activity against the parent homogeneous catalysts. The results obtained on a supported chiral catalyst were compared with those obtained by using the analogous homogeneous catalyst. A higher ee% was obtained under the same reaction conditions employing Merrifield's resin as a support. When the chiral phosphooxazolidine ligands anchored on polystyrene were used in the reaction, B1 and B3 gave products of 80% ee and 89% ee, respectively. In the case of SBA-15, capping of silanols with TMS improved the enantioselectivity, but this inorganic support was less efficient in the Pd-catalysed allylic substitution reaction. Heterogenized catalyst was simply separated by filtration from the product and it could be reused again. Table 1 Asymmetric Pd-catalyzed allylic substitution of l,3-diphenyl-2-propenyl acetate Q^^
3eqCH2(C02Me)2
^
^^
3eqBSA 3mol%KOAc 2.5mol% [Pd(allyl)CIJ2 10mol% ligand
CH(C0iMe)2
^
Entry
Catalyst
Benzaldehyde'^
TempCt:)
Yield(%)
ee%'
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Al Al Bl (Polymer) B1(SBA-15) Bl (Silica gel) A2 B2(Polymer) B2(SBA.15) A3 B3(Polymer) B3(SBA-15) B3(Silica gel) A4 B4(SBA-15) CI C2
PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 thiophene pyridyl
10 50 10 10 10 10 10 10 10 10 10 10 10 10 10 10
98 100 88 74 62 88 66 40 98 75 70 60 99 55 60 58
88 61 80 65 43 55 44 35 98 89 72 58 84 52 3 20
>h2=2--diphenylposphinobenzaldehyd e, THF solvent
•etermi nded by HPLC with chiralcel OD column(25cm x 0.46cm) : 1% 2-propanol in hexane, flow rate=0.5mL/min, tR(min)=25.6(R), 27.5(S)
504
As summarized in Table 1, the enantioselective catalytic activities of the phosphinooxazolidines immobilized on solid supports are slightly lower than those of the corresponding homogeneous phosphino-oxazolidines. SBA-15-supported catalysts gave much higher reacon rates and higher asymmetric induction than silica gel-supported ones. Highly ordered mesoporous silica supports were found to be better inorganic support than amorphous silica gel. The ligands prepared from 2-pyridinealdehyde and 2-thiophenecarboxaldehyde afford 320% ee. On the basis of asymmetric allylic substitution reaction, the chiral complexes immobilized on mesoporous material by the present procedure can be applied as an effective heterogenized homogeneous catalyst for the asymetric reactions. 4. CONCLSIONS New heterogeneous catalysts employing various amino alcohols immobilized on SBA-15 have been synthesized and they were applied to the asymmetric allylic substitution. The enantioselectivity was strongly dependent on the structure of amino alcohol and the enantiomeric excess varied substantially from one amino alcohol to another. SBA-15 has served as a potential support for the heterogenized chiral catalysts in the asymmetric reduction of aromatic ketones to alcohols. ACKNOWLEDGMENT This work was supported by grant No. 2000-1-30700-002-3 from the Basic Research Program of the Korea Science & Engineering Foundation and partially by Inha Technical College. REFERENCES 1. A. K. Ghosh, P. Mathivanan and J. Cappiello, Tetrahedron Asymmetry, 9 (1998) 1. 2. H. Steinhagen, M. Reggelin and G. Helmchen, Angew. Chem. Int. Ed. Engl. 36 (1997) 2108. 3. B. J. Nagy, P. Sutra, F. Fajula, D. Brunei, P. Lentz, G. Daelen, Colloids and Surfaces. 158 (1999)21. 4. G. Giffels, J. Beliczey, M. Felder and U. Kragel, Tetrahedron; Asymmetry, 9 (1998) 691. 5. G.-J. Kim and J.-H. Shin, Tetrahedron Lett., 40 (1999) 6827. 6. S. W. Kim, S. J. Bae, T. Hyeon and B. M. Kim, Microporous and Mesoporous Materials, 44(2001)523. 7. N. Bellocq, D. Brunei, M. Lasperas, P. Moreau, Stud. Surf Sci. Catal., 108 (1997) 485. 8. B. M. Trost and D. L. van Vranken, Chem. Rev. 96 (1996) 395.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
505
Preparation of guanidine bases immobilized on SBA-15 mesoporous material and their catalytic activity in knoevenagel condensation Keun-Sik Kim, Jong Hun Song, Jong-Ho Kim and Gon Seo. Department of Chemical Technology & The Research Institute for Catalysis, Chonnam National University, Gwangju, 500-757, Korea. Guanidine was immobilized on SBA-15 mesoporous material by a consecutive addition reaction of precursors and a condensation reaction between presynthesized guanidinecontaining silane and hydroxyl groups of supports. Immobilized guanidine was thermally stable and showed the high activity in the Knoevenagel condensation between cyclohexanone and benzylcyanide. 1. INTRODUCTION Guanidine, non-ionic organic base, is widely employed as active base catalysts in various organic synthesis because of its strong basicity and high miscibility with organic reactants [1 ]. The difficulty in the separation of guanidine from products, however, reduces its economic feasibility by increasing separation expense. In addition, heating for distillation accelerates the formation of by-products, lowering the purity of desired products. Organic bases can be immobilized by the reactions between bases and chlorinated polystyrene supports [2]. Although immobilized bases show reasonable activity in basecatalyzed reactions, their low thermal stability and easy breaking of benzylic groups of polymer inhibit to achieve high activity and multiple use. Reaction of alkoxysilane with hydroxyl groups of solid silica supports provide an effective way to immobilize organic bases on them. Exceptional thermal stability of silica and strong Si-C chemical bond promise better performance of silica as catalyst support. In this study, three different kinds of immobilized guanidine base catalysts were prepared following the procedures shown in Scheme 1: through the stepwise reaction of 3-amino propyltriethoxysiliane (APTS) and N, N'-dicyclocarbodiimide (DCC) consecutively, and the reaction of presynthesized guanidine-containing silanes with hydroxyl groups of SBA-15 mesoporous material. The physico-chemical property and catalytic activity of guanidineimmobilized catalysts in Knoevenagel condensation were discussed relating to the basic character of immobilized guanidine. 2. EXPERIMENTAL SBA-15 mesoporous material was synthesized using an acidic reactant composing of tetraethoxysilane, polyalkylene oxide copolymer (Pluronic-123), trimethylbenzene (TMB), and hydrochloric acid [3]. Calcinated SBA-15 mesoporous material was used as a catalyst support in this study and guanidine was immobilized through the procedure described in Scheme 1.
506
O-NCNHQ -OH
-(gSi
r-BuOH, reflux, 24 h
toluene, reflux, 12 h
Nil
•6
[guan(step)/SBA]
OEt r, ^ . . ^ ^.. /-BuOH, reflux, 24 h
HN-(3
Art <-'Et
NH |
0
OEt EtO-Si-'^^^-^^N'
m
^IIP-OH" NH
6
[guan(mono)/SBA
Q-N:C:NHQ>
Q
OEt Nil EtO-Si ^ ' ^ ^ - ^ N ^ ^ ^ N=< N=< y - ^
0
r-OH ^—011 '—OH
toluene
"-O"
toluene, reflux, 12 h
^ • ^ ^?2S'
iN=<
y
_,.HN-Q
0 [guanCdiVSBAI
0" N'
N < )
Scheme 1. Preparation of guanidine base catalysts immobilized on SBA-15 mesoporous material. Two types of Knoevenagel condensation were carried out over guanidine-immobilized catalysts: one was the condensation between benzaldehyde (BA) and ethylcyanoacetate (ECA. p/Ca=9), and the other was that between cyclohexanone (CH) and benzylcyanide (BC\ pKa=2\.9). Equal moles (10 mmol) of reactants were added to a nitrogen-purged 100 iiiC three neck flask accompanying with 0.1 g of base catalysts. Products were analyzed using HPLC equipped with a Capcell Pak CI8 column and an UV detector. Conversion was determined from the consumed amount of BA or CH as percentage. 3. RESULTS AND DISCUSSION TG curves of guanidine-immobilized catalysts show two weight decreasing regions: the first decrease below 100 "C is due to water desorption and the second one at 300-600 V is due to the combustion of immobilized guanidine. Immobilized amounts of guanidine on the SBA-15 support were estimated from weight loss in the TG curves and elemental analysis results. Those were 0.78, 0.33, 0.75 meq of guanidine per gram support on the guan(di)/SBA.
507
guan(step)/SBA and guan(mono)/SBA catalysts, respectively. Fig. 1 shows nitrogen adsorption isotherms of guanidine-immobilized catalysts. The SBA15 support synthesized using a mixed template of P-123 copolymer and TMB had large pores ranged from 130 to 150 A. A significant decrease in pore volume with guanidine immo bilization indicated that guanidine was mainly immobilized in mesopores of the support. IR spectra of guanidine-immobilized catalysts provide the immobilized state of guanidine (Fig. 2). Only a sharp absorption band at 3740 cm' attributed to external hydroxyl groups was observed on the evacuated support. By reacting APTS with hydroxyl groups lost methoxy groups as methanol and led to propylamine immobilization, disappearing 0-H band (3740 cm') and appearing N-H band (3300 cm') and C-H bands (2800-3000 cm' and 1610 cm'). Further reaction of DCC brought about a clear guanidine band at 1650 cm' on the guan(step)/SBA catalyst. Similar guanidine band was observed on the guan(di)/SBA catalyst, indicating that guanidine groups were immobilized on the SBA-15 support, regardless of preparation procedure. 1500 guan(step)/SBA
'SD
E
(J
a
2
guan(di)/SBA
1000
500 1500
'B
1000
o s
500
3
o
£
1500 1000
O
•o
500
3800
P/Po
Fig. 1. Adsorption-desorption isotherms of nitrogen on support and propylamine and guanidine-immobilized catalysts.
3300
2800
2300
1800
1300
Wavenumber (cm')
Fig. 2. IR spectra of the support and propylamine- and guanidine-immobilized catalysts after evacuation at 100 °C.
All prepared base catalysts were active in the Knoevenagel condensation of BA and HCA because of easy deprotonation of ECA. However, the deprotonation of methylene group of BC is more difficult than that of ECA, requiring strong base catalysts for the Knoevenagel condensation between CH and BC. Table 1 exhibits conversion and product composition over prepared base catalysts. The PA/SBA catalyst was not active in this Knoevenalgel condensation while both the guan(mono)/SBA and the guan(di)/SBA catalysts were active. Therefore, it was evident that guanidine-immobilized catalysts were more basic than propylamine-immobilized catalyst did. The yield of the dehydrated condensation product from CH and BC approached to 60% over the guan(di)/SBA catalyst at 150 °C. Although this value was low compared to that obtained on homogeneous l,5,7-triazabicyclo[4,4,0]dec-5-ene
508
(TBD) at 110 °C, the guan(di)/SBA was more feasible than TBD in the aspects of easy separation, repeated use, energy saving operation without using solvent, and the workability at elevated reaction temperature. Table 1. Knoevenagel condensation of CH and EC over immobilized base catalysts. CN
6 • "^a,CH
BC
C/ti. Dehydrated
Adduct
Catalyst PA/SBA guan(mono)/SBA guan(di)/SBA // PA/SBA guan(mono)/SBA guan(di)/SBA PA/SBA guan(mono)/SBA guan(di)/SBA TBD
CN
OH
-OCH3
Temp. (°C) 25
Solvent neat
Time (h) 48 12 48
80
t-BuOH
6
// II II
150
neat
2 II II
110
toluene
6
Conv. (%) 0 5 20 30 0 62 59 0 65 61 83
Product Composition (%) Adduct Dehydrated
-
5 20 30
2
40 49
-
46
5
74
60
4. CONCLUSION Guanidine could be stably immobilized on SBA-I5 mesoporous material through the reaction between guanidine-containing silane and hydroxy 1 groups of catalyst support. Guanidine-immobilized catalysts showed high activity in the Knoevenagel condensation between CH and BC which was not activated by primary amine-immobilized catalysts. ACKNOWLEDGEMENT This work was supported by Korean Science and Engineering Foundation (2000-1-30700002-3). REFERENCES 1. G. Barcelo, D. Grenouillat, J.P. Senet and G. Sennyey, Tetrahedron, 46 (1990) 1839. 2. U. Schuchardt, R.M. Vargas and G. Gelhard, J. Mol. Catal. A: Chemical, 109 (1996) 37. 3. D. Zhao, Q. Huo, J. Feng, B.F. Chmelka and G.D. Stucky, J. Am. Chem. Soc, 120 (1998) 6024.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
509
MCM-41-supported norephedrine ligand for ruthenium-catalyzed asymmetric transfer hydrogenation of ketones Myung-Jong Jin, Sang-Han Kim, Sang-Joon Lee, and Wha-Seung Ahn School of Chemical Science & Engineering, Inha University, Inchon, 402-751, Korea Optically active norephedrine was anchored on MCM-41 silica material. The insoluble system was utilized as an enantioselective ligand in the asymmetric transfer hydrogenation of ketones. This reaction provided (/?)-secondary alcohols with satisfactory enantioselectivities (up to 95% ee) in good conversion. The results are comparable to those of the analogous homogeneous counterpart. 1. INTRODUCTION Transition metal complex-catalyzed asymmetric transfer hydrogenation of ketones is an attractive method for the synthesis of optically active secondary alcohols.'"^ Efficient chiral ligands have been developed for the homogeneous catalysis. Successful development of homogeneous ligands has been sometimes followed by attempts to attach the ligands on an insoluble polymeric support. This strategy offers practical advantages such as simplified separation, easy recovery of catalyst, and potential reuse.^'^ Polystyrene resin, silica gel and alumina have been most commonly used as supports for the immobilization of the ligands. Recently, mesoporous molecular sieves MCM materials with large uniform pore diameters and high specific surface areas have become of high interest as inorganic supports.'^ Our interest in the field led to prepare a new MCM-41 silica-supported norephedrine ligand 3. Herein, we describe the application of the supported ligand for the asymmetric transfer hydrogenation of ketones. Scheme I Mq,
(—^" MCM-41 —OH 1
i •
|—^\ ^ w x r~^7^' C' 2
ii
?h
|^ 3
i) (McO)3Si(CH2)3Cl, toluene, reflux ii) (+)-norcphcdrinc - EtjN, toluene, reflux 2. EXPERIMENTAL 2.1. Preparation of MCM-41 silica 1. MCM-41 silica 1 was prepared according to the literature method using surfactant Ci6H33N(CH3)3Br as the template.'^ 2.2. Preparation of 3-chIoropropyl MCM 2. To a solution of 3-(chloropropyl)trimethoxysilane (0.72 g, 3.6 mmol) in toluene (10 mL)
510
was added MCM-41 silica (1.0 g). The mixture was gradually heated to 110 °C and allowed to react for 12 h. The MCM powder was collected by filtration and washed repeatedly with CH2CI2. After drying in vacuo at 50 °C, 1.3 g of 3-chloropropyl MCM 2 was obtained. Elemental analysis and weight gain showed that 3.0 mmol of 3-(chloropropyl) trimethoxysilane was immobilized on 1.0 g of MCM-41 silica 1. 2.3. Preparation of MCM-41-supported norephedrine 3. To a solution of (+)-norephedrine (0.68 mg , 4.5 mmol) and triethylamine (0.33 g, 3.3 mmol) in toluene (10 mL) was added 3-chloropropyl MCM 2 (1.0 g). The mixture was gradually heated to 110 °C and allowed to react for 48 h. The powder was collected by filtration and washed successively with H2O, ethanol, and CH2CI2. After drying in vacuo at 50 "C, 1.2 g of MCM-41-supported norephedrine 3 was obtained. Elemental analysis and weight gain showed that 1.83 mmol of norephedrine was anchored on 1.0 g of 3-chloropropyl MCM 2. 2.4. Typical procedure for the Ru-catalyzed asymmetric transfer hydrogenation using MCM-41 silica-supported norephedrine 3. A preparative experiment using 1.67 mmol of acetophenone (S/C=100) was performed as follows: A suspension formed by mixing [{RuCl2(p-cymene)}2] (5.1 mg 0.008 mmol) and MCM-41 silica-supported norephedrine 3 (10 mg, 0.07 mmol) in 2-propanol (5 mL) was heated at 80 °C for 30 min under an nitrogen atmosphere. To the resulting solution, a degassed solution of ketone (0.83 mmol) in 2-propanol (10 mL) and a solution of KOH (2.2 mg, 0.04 mmol) in propane-2-ol (1.0 ml) were added. The mixture was stirred at RT for 12-20 h, neutralized with dilute hydrochloric acid and concentrated in vacuo. The residue was diluted with ethyl acetate and the organic solution was washed with brine. The organic layer was dried over MgS04, concentrated under reduced pressure. The residue was purified by column chromatography (hexane-ethyl acetate,95:5, as eluent) and vacuum distillation to yield the pure alcohol. The enantiomeric excess was determined HPLC analysis using Chiralcel OB-H column (5% 2-propanol in hexane, 0.5ml/min). 3. RESULTS AND DISCUSSION The immobilization of norephedrine onto MCM-41 silica material was performed in two steps (Scheme 1). Reaction of MCM 1 with an excess of (3-chloropropyl) trimethoxysilane in refluxing toluene gave chloropropylsilanized MCM 2 (3.0 mequiv /g). Subsequent treatment of 2 with 1.5 equiv of (-i-)-norephedrine in refluxing toluene in the presence of 1.1 equiv of triethylamine afforded MCM-supported norephedrine 3 (1.83 mequiv/g). The MCMsupported chiral Ru(II) complexes were prepared in situ by heating a mixture of 3 and [Ru(arene)Cl2]2'^ in 2-propanol. Asymmetric transfer hydrogenation of several ketones with isopropanol as a hydrogen source was examined in the presence of the Ru(II) catalyst. As indicated in Table 1, all the ketones were reduced to (i?)-secondary alcohols with moderate to high enantioselectivities in reasonable conversions. The e.e. and conversion seem to depend on the structure of the substrate. The highest enantioselectivity was observed for the hydrogen transfer of atetralone. An even better e.e. can be obtained when the reaction is carried out in the presence of [Ru(HMB)Cl2]2". As the bulkiness of the alkyl substituent increases, the extend of e.e. (enantiomeric excess) is lowered. The conversion of the product decreases with increasing the e.e. It is noteworthy that the MCM-supported ligand 3 is as effective as comparable free (+)-ephedrine ligand in terms of enantioselectivity."^
511
Table 1. Asymmetric transfer hydrogenation of ketones in 2-propanol using MCM-supported ligand 3^
c/-
MCM-41-supported ligand 3 [Ru(arene)Cl2]2 /PrOH, KOH
OH
C
Entry
Substrate
Arene
Time (h)
Conv. (%)
% e.e."
1
R=Me
/?-cymene
12
96
81
2b
R=Me
/?-cymene
12
74
81
3
R=Et
/?-cymene
12
90
71
4^
R=Et
/?-cymene
15
70
70
5
R=Me
HMB'*
14
73
85
7
R=Et
HMB
16
60
78
8
a-tetralone
/?-cymene
18
70
80
9
a-tetralone
p-cymene
20
72
89
10
a-tetralone
HMB
16
50
95
"The reaction was carried out at RT using 0.1 M solution of ketone (5.0 mmol) in 2-propanol; ketone: Ru: ligand: KOH = 100: 1 : 1 : 5 . ^Ketone: Ru: ligand: KOH = 200: 1: 2: 5. "^Determined by HPLC analysis using Chiralcel OB-H column (5% 2-propanol in hexane, 0.5ml/min). ''HMB = hexamethylbenzene.
In conclusion, we have shown that the MCM-41 can be served as a potential support for the heterogeneous chiral ligand. Our studies strongly support the possibility of achieving high reactivity and enantioselectivity in heterogeneous systems. Further synthesis of MCMsupported chiral ligands and their use to asymmetric catalysis are underway in our laboratory. This work was supported by the Center for Advanced Bioseparation Technology, Inha University. REFERENCES 1. Zassinovich, G.; Mestronu, G.; Gladiali, S. Chem. Rev. 1992, 92, 1051. 2. Noyori, R.; Hashiguchi, S. Ace. Chem. Res., 1997, 30, 97.
512
3. Takehara, J.; Hashiguchi, S.; Fujii, A.; Inoue, S.-I.; Ikariya, T.; Noyori, R. J. Chem. Soc, Chem. Commun., 1996, 233. 4. Murata, K.; Ikariya, T. J. Org. Chem., 1999, 64, 2186. 5. Yamakawa, M.; Ito, H.; Noyori, R. J. Am. Chem. Joe, 2000, 122, 1466. 6. Alonso, D. A.; Guijarro, D.; Pinho, P.; Temme, O.; Andersson, P. G. . J. Org. Chem., 1998, 63, 2749. 7. Hodge, P and Sherrington, D. C. in Polymer-Supported Reactions in Organic Synthesis, Wiley, New York, 1980 8. Smith, K. Solid Supports and Catalysts in Organic Synthesis, Ellis Horwood and Prentice Hall, New York, 1992. 9. Liu, C.-H.; Yu, W.-Y. J. Org Chem., 1998, 63, 7364. 10. Bennete, M. A.; Smith, A. K. J. Chem. Soc, Dalton Trans., 1974, 233. 11. Bennete, M. A.; Matheson, T. W.; Robertson, G. B.; Smith, A. K.; Tucker, P. A. Inorg. Chem., 1980, 19, 1014 12. Ryoo, R.: Jun, S. J. Phys. Chem. B, 1997, 101, 317.
Studies in Surface Science and Catalysis 146 Park et al (Editors) y 2UU3 Hisevier Science B. V. Allrightsreserved
513
Synthesis of silica support for biocatalyst immobilization Jong-Kil Kim, Jin-Koo Park and Ho-Kun Kim* Department of Applied Chemistry, Hanyang University Ansan, 425-791, Korea Bioconversion processes utilizing an immobilized biocatalyst (enzyme) are widely used to synthesize various kinds of medicines. The support for biocatalyst, such as silica, plays an important role in the overall yield of the bioconversion reaction because they determine the loading capacity of the active biocatalyst. In this study, nanoporous silicas suitable for the support, were synthesized via a salt route and the pore morphology in silicas were characterized. Then, D-amino acid oxydase, the enzyme for the conversion reaction to synthesize an antibiotic, was immobilized in the above silicas and the activity of immobilized enzyme was investigated in relation to the pore morphology. 1. INTRODUCTION Recently, the conversion processes that can replace chemical synthetic processes in the field of medicine production has been widely studied according to the progress of biotechnology^'"^l Studies on the bioconversion processes utilizing an immobilized biocatalyst (enzyme), especially, to produce antibiotics are most active. When an enzyme, which is a kind of protein, is used in the bioconversion reaction, it is necessary to protect the enzyme during the reaction because it has low stability to chemical and physical attacks. Immobilization, the process of fixing an enzyme into the specific spaces of inert support, is generally carried out. Thus, the immobilized enzyme can be kept active during the reaction and re-use of the enzyme becomes possible as well'^^l Porous materials of organic, inorganic or organic-inorganic hybrid compounds are usually used as the support for the enzyme immobilization. Inorganic supports were lately developed by H. H. Weetall^'^ in 1969, then extensive studies on the immobilization yield and enzyme activity under various reaction conditions have been performed with a given inorganic support^^. However, the studies on how the physicochemical characteristics of the inorganic support influence the immobilization yield or enzyme activity are very few. In this study, silicas with nano sized pores were synthesized via a salt route and characterizations of silicas including pore morphology were carried out. The relationships between the pore morphology of the silicas and the activity of immobilized enzyme were investigated. 2. EXPERIMENTAL Silicas with 30~80 nm pore diameters that could be used as supports of biocatalyst were synthesized by acid decomposition reaction using silicate salts as starting materials. 5 N-HCl was added to the starting materials Na20-3.4Si02 in 5 L reactor with a quantitative pump and the overall reactions were controlled by a continuous reaction regulating system. During the decomposition reaction, acids were supplied excessively to prevent rapid gelation. Reactions proceeded at room temperature, however, the temperature in the reactor was raised up to 60 °C by the heat released during the reactions. After the reaction was completed, the reaction products were washed with distilled water and aged to prepare silica hydrogel. Tel : +82-31-400-5493, Fax : +82-31-407-3863, e-mail : hkkim@hanyang.ac.kr
514
Washing and aging were performed using an auto controlling system under the condition of pH 3.5 to 10, temperature 25 °C to 75 °C in range. The hydrogel was converted to xerogel by drying at 140 °C for 4 hours and the characteristics of xerogel were measured. The hydrothermal process was adopted to control the pore size in the xerogel silicas. After the hydrothermal treatment, the surface area and pore volume were measured by the BET method that exploited N2 gas adsorption. The calculation of pore size in silicas was done using the following Wheeler's formula. APD
= 40,000 V/S
(1)
Here, APD is the average pore diameter in A unit, V and S are pore volume(ml/g) and surface area(m^/g), respectively. Aqueous silanization on the pore surfaces was allowed using silane linked to amine group(3aminopropyltriethoxysilane), then glutaraldehyde was attached to the amine group of the silane and D-amino acid oxydaze, an enzyme for bioconversion process to produce GL-7ACA which is an intermediate of antibiotic, was immobilized. Figure 1 schematically shows the immobilization procedures described above.
OHC-HC-CH-(CH2)?CH-(CH2)4CHO
9 T'
^+C•(CH2)JC•H
htji
SI—O-Si—(CH3)2
-Si-O-Si—(CH3)2
Glutaraldehyde
O
Polymerized plutaraldchyde OHC-(CH2)2 OHC-HC (D-amino acid oxidase)
^
'
'
[-CH-(CH2)2-CH-]—CH-(CH2)3CHO
I
Protein
L
CHO " NH protein
Fig.l. Covalent immobilization of D-amino acid oxidase to nano-porous silica. 3APTES : 3-aminopropyltriethoxysilane The immobilized enzyme catalyzes the following bioconversion reaction effectively as shown in Figure 2.
^^v^-s^.-^Y'^^v^^
COOH'
COOH Cephalosporin C
Y
D-amino acid oxidase
6
Glutaryl-7-Aminoccphalosporanic acd (Gl.-7-AC'/\)
Fig. 2. 1^^ step enzymatic conversion of cephalosporin C to GL-7-ACA by immobilized enzyme (D-amino acid oxidase).
515
The activity of immobilized enzyme during the above reaction was measured by the well defined spectroscopic method. One unit of enzyme activity was defined as the amount of enzyme that produced 1 pmol of GL-7-ACA per min at 37 °C, pH 8. 3. RESULT AND DISCUSSION Pore size and surface area varied with PH and temperature at which time the silicas undertook polymerization. Under the constant pH and temperature condition, the pore size changed in relation to the polymerization time. The variations of average pore size with polymerization time are shown in Figure 3. The pore size increases gradually with the increasing polymerization time in an acidic solution as shown in Figure 3; the pore diameter becomes 4.4 nm from 1.4 nm as the time changes to 48 hours from 3 hours. However, in Figure 3, the pore diameter increases rapidly to 23 nm from 12 nm in an alkaline solution at the same time interval as above. These results agree with the silica polymerization model which is suggested by R. K. fir in "The Chemistry of Silica". It is considered that the alkaline condition accelerates the condensation polymerization of silanol groups more effectively than the acidic condition during the gelation reaction.
Time(hr8)
Fig. 3. Variations of average pore size with polymerization time. • :pH3.5,Temp. 35°C, • : pH 9.5, Temp. 65°C Silicas synthesized via a salt route described in the experimental section, were heated hydrothermally at 130 °C for 24 hours to modify the pore morphology. Figure 4 shows the relation of average pore size to the hydrothermal heating time. Pore diameter increases to 53 nm from 20 nm with increasing heating time of hydrothermal process as shown in Figure 4. The enlarging of the pore diameter is considered to be due to the condensation polymerization of silanol groups existing in the silica pores. In general, enlarging of the pore volume is preferable to increasing the immobilization yield for the enzyme, however, the surface area that can provide active sites for the enzyme decreases with the enlarged pore volume. In addition, an increase in pore diameter could result in the reduction of the mechanical strength of the support. Optimizing the volume, surface area and diameter of pores in support, therefore, is very important in improving support efficiency. It is noticeable that the optimum pore conditions vary with the kinds of support and optimizing suited for a given enzyme should be carried out. Activity of D-amino acid oxidaze as a function of pore diameter is shown in Figure 5. It can be seen from Figure 5 that the activity is influenced to a great extent by the pore diameter. The activity is markedly low (nearly 0) when the pore diameter distributes below 15 nm as shown in Figure 5. It means that the immobilization cannot be performed in the diameter
516
range of less than 15 nm. From Figure 5, activity increases with increasing pore diameter and it shows 71 % of the maximum value when the diameter reaches 33 nm.
50
.
F
c 0)
1
40
.
0)
o
^ 20 0) C3)
^
<
^^
^•^^% •^
M^
/•
^
10
)
12
15
18
21
24
27
Time(hr8)
Fig. 4. Relation of average pore size to the hydrothermal heating time(pH 7.6, Temp. 130°C).
Pore diameter(nm)
Fig. 5. Eftect of pore size on the activity of immobilized enzyme. Activity : enzyme units/support mass(gram) The maximum value of the activity, 48units/g, is found in the pore diameter range of 45 ~ 46 nm in Figure 5. In the case of a diameter larger than 46 nm, also in Figure 5, the activity decreases with increasing diameter contrary to the above situation. Particularly, the immobilized enzyme was separated from the support during the bioconversion reaction if the diameter exceeded 60 nm. When the pore diameter is larger than 60 nm, the numbers of active sites in the support become extremely few and it could be one reason of the enzyme separation from support. Entrance of an enzyme with an average length of 2 ~ 10 nm into pores with diameter less than 15 nm is dilTicult, consequently, the activity decreases and fall to nearly 0 in the case of a pore diameter less than 15 nm. REFERENCES 1. Sung, M. H., et. al., Bioindustry, 1998, 11, 23. 2. S. W. Kim, et. al, Reactive& Functional Polymers, 2002, 51, 79-92. 3. K. H. Chung, et. al. Biochemical Eng., 2002, 3598, 1-7. 4. Park, Y. H., et. al., Bioindustry, 1996, 13, 6. 5. Weetall, H. H., et. al.. Science, 1969, 166, 615. 6. A Pierre, et. al., J. Mol Catalysis B:Enzymatic, 2001, 11, 639.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
Mesostructured Materials for Controlled Supramolecular Architectures
517
Macromolecular and
Midori Ikegame, Keisuke Tajima and Takuzo Aida* Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. This paper focuses attention on our recent work on applications of mesostructured materials to the control of macromolecular and supramolecular architectures, with a view to develop bottom-up approaches to nanoscale materials design [1-5]. INTRODUCTION Formation of macromolecules in biological systems occurs in constrained or organized media, resulting in rigorous control of primary and even higher-orderd structures.
On the other hand, chemical synthesis of macromolecules, usually
conducted in homogeneous, non-constrained media, just follows statistics of numerous elementary reaction steps, leading to the formation of macromolecules with poorly controlled architectures.
In this respect, utilization of constrained media for artificial
macromolecular synthesis and supramoleclar architectonics is an interesting subject, which possibly allows spatial control of primary structures of polymeric and self-assembling structures, and even their two and three-dimensional multi-level structuring [1].
Here we report that mesoporous silicate materials with aligned
mesoscopic channels are powerful candidates for ordering macromolecular and supramolecular architectures.
We would also like to propose a general and
conceptually new methodology for controlled macromolecular synthesis, which makes use of polymerizable surfactants as templates for the sol-gel synthesis of mesoporous
518
silicates. 1.
Polymerization of Etliylene Within Mesoporous Silicates It has been reported that titanocenes are able to be grafted onto the surface of
MCM-41 by a post-treatment of a siliceous mesoporous zeolite with a grafting reagent, and the resulting titanium mounted MCM-41 (Ti-MCM-41) is effective for the oxidation of bulky cyclic alkenes.
We have found that Ti-MCM-41 in the presence of
methylaluminoxane (MAO) as co-catalyst initiates polymerization of ethylene to give a linear polyethylene (PE) with an ultrahigh molecular weight. For the preparation of the titanocene-grafted MCM-41, powdery MCM-41 was suspended in a CH2CI2 solution of titanocene dichloride, containing triethylamine as
MCM-41 (27A diameter)
Scheme 1. Schematic representation for the preparation of Ti-MCM-41. proton-scavenger, and the mixture was stirred for 1 h. MCM-41 was isolated by filtration
The titanocene-grafted
and washed with fresh
CH2CI2.
The
cyclopentadienyl ligands of this material were removed by calcination at 540 °C under air, leaving Ti- MCM-41 (Scheme 1). Typically, the polymerization was carried out with 6 mL of toluene as solvent containing MAO at a ratio Al/Ti of 100 in a glass-lined autoclave (100 mL) containing dried Ti-MCM-41 powder (0.05 g).
To this system was
introduced ethylene gas at an initial pressure of 10 atm at room temperature with stirring magnetically.
The ethylene pressure was reduced with time, and a complete
consumption of ethylene was achieved in 70 h.
After the polymerization, small
amount of methanolic HCl was added to the mixture, and the polymer was isolated by
519
filtration and washed with MeOH.
A PE fraction, isolated by extraction with
1,2-dichlorobenzene at 180 °C, was subjected to GPC to determine the molecular weight.
Interestingly, the molecular weight of PE (A/w = 1,500,000) was found to be
much higher than that produced by homogeneous titanocene catalyst under otherwise identical conditions to the above (A/w = 100,000).
By decreasing the ratio Al/Ti to 25,
for example, a much higher molecular weight PE (Afw = 2,700,000) was obtained. further interest, triethylaluminium and trimethylaluminium were also
Of
effective
co-catalysts for the polymerization of ethylene to produce high molecular weight PEs. The structure of PE was linear, as confirmed by the absence of any '^C NMR signals due to branched units.
From these observations, it is clear that Ti-MCM-41 has a high
potential utility for the synthesis of linear, ultra high molecular weight PE. We have also found that a supported titanocene by a particular type of mesoporous silica called "mesoporous silica fiber", in conjunction with methylaluminoxane (MAO), initiates "extrusion polymerization" of ethylene to form a cocoon-like polymeric mass consisting of 30-50 nm ultra-thin fibers of polyethylene [2].
'^C NMR spectroscopy of
the polyethylene again indicated a linear sequence of the repeating ethylene units without any branch structures.
Furthermore, the polymer also had an ultrahigh
viscometric molecular weight (A/v = 6,200,000).
Small-angle x-ray scattering analysis
and DSC measurement indicated that the polyethylene fibers consist predominantly of extended-chain crystals.
This is a rare example of the successful control of the ternary
structures of synthetic polymers by the reaction loci [3].
Fig. 2.
Conceptual scheme for the growth of crystalline polyethylene fibers.
520
2.
Synthesis of Novel Polymer/Silica Composite Materials Unidirectional arrangement of conjugated polymer nanodomains is important for the
development of novel electroconductive and optoelectronic devices.
From this point
of view, template-assisted polymerizations in organized media have attracted great SIO2
1
Si02
(R0)4Si
Heating
Condensation Step
Polymerization
Me(CH2)ioC=C-CEC-CH2N*Me3 B r
Poly(diacetylen L ^ L A ^
1a
Me(CH2)7C=C-C=C-(CH2)4N*Me3 Br 1b
Fig. 3.
Schematic representation of the polymerization of a surfactant monomer 1
within mesostructured silica 2. attention.
Recently, we succeeded in the fabrication of a novel micrometer-scale
photoluminescent silicate stick with segregated nanodomains of a conjugated polymer by a "sol-gel based template polymerization" of diacetylenic surfactant monomers 1 used as templates for the formation of mesostructured silicates (Fig. 3) [4]. This method is expected to allow complete filling of the silicate channels with ordered diacetylenic monomers.
Such a predetermined approach for the inclusion of guest
molecules is also quite essential for the polymerization of 1, since diacetylenic monomers can be polymerized only topochemically in condensed, crystalline and semi-crystalline phases. Thus, 2,4- and 5,7-hexadecadiynyltrimethylammonium bromides (la, lb) as template surfactants were synthesized.
Mesostructured silicates 2a and 2b were
successfully synthesized according to a method reported by Stucky et al. for the preparation of mesoporous silica fiber.
Typically, to 6 wt-% aqueous hydrochloric acid,
upon stirring magnetically, were successively added 0.5 g of 1 and 0.98 g of tetrabutyl
521
orthosilicate, and the resulting phase-separated system was allowed to stand for 5 days at 20 °C.
When la and l b were used as templates, white fibrous precipitates were
formed, which were collected by filtration and dried in vacuo.
X-ray diffraction
(XRD) of 2a showed a peak with a d spacing of 37.7 A, which is characteristic of a [100]-diffraction originating from the hexagonal structure of the silicate.
On the other
hand, fibrous silicate 2b, prepared from surfactant monomer lb, showed a smaller d spacing of 34.9 A. Exposure of fibrous mesostructured silicates 2a and 2b to a UV light (/Imax = 254 nm) from a high-pressure mercury lamp for 2 h did not result in any color change associated with the polymerization of the included monomers.
In contrast, upon
heating at 170 °C for 2 h under N2, 2a and 2b both showed a marked color change from white to orange.
Infrared spectroscopy of the colored material from 2a, for example,
showed a complete disappearance of the C^C vibrational band at 2260 cm'\ indicating that the diacetylenic moiety of 2a is polymerized
to
form
an
-C=C-C=C-C=C- sequence.
IR-silent Electronic
absorption spectroscopy of 3a in the solid state showed a broad band centered at 350 nm with an upper threshold around 650 nm, assignable to a 71-71* electronic transition
of the conjugated
polymer
Fig. 4.
Fluorescence micrograph of 3a.
backbone. Fluorescence microscopy of 3a (Fig. 4) and 3b showed that the stick-like fibers entirely emit a yellowish green fluorescence.
The emission spectrum
of 3a showed a broad fluorescence band
(B)
1
v
\^ ^•'^
centered at 550 nm, which is red shifted from
those
of
ordinary 300
poly(phenyleneethynylene)s.
On the
(A)
1
1
1
1
400
500
600
700
800
wavienBth(nm)
other hand, the excitation spectrum,
pjg 5
Electronic absorption spectra of
monitored at 550 nm, displayed a band
/ ^ \ 2a and (B) 3a.
522
centered at 450 nm, which corresponds to the long-wavelength part of the absorption band of 3a (Fig. 5). Absorption and emission profiles of poly(diacetyIene) derivatives are known to be susceptible to the ordering of the conjugated polymer chains. Polymerized diacetylenes in highly ordered, crystalline states are stained blue and not emissive, while those in bilayer films at a lower level of molecular ordering are red-colored and luminescent.
Thus, it is likely that the conjugated polymer chains
within the nanoscopic silicate channels are arranged with a certain freedom of molecular ordering. As an extension of this method, we have quite recently succeeded in the fabrication of novel silica composites with other conjugated polymers [5], which are expected to possess
interesting
electrochemical
properties
associated
with
a
"nanoscopic
confinement effect" by the silica wall. CONCLUSION In synthetic polymer chemistry, attention has mostly been focused on control of primary structures of polymers, while control of higher-order structures under polymerization conditions has been much less explored to date.
In the present paper,
we demonstrated that mesoporous silicates are promising candidates for long-range molecular ordering of synthetic polymers, which is quite important for nanoscale materials design of functional polymers. REFERENCES 1. K. Tajima and T. Aida, Chem. Commun., (2000) 2399. 2. K. Kageyama, J. Tamazawa and T. Aida, Science, 285 (1999) 2113. 3. K. Tajima, G. Ogawa and T. Aida, J. Polym. Sci, Part A: Polym. Chem., 38S (2000) 4821. 4. K. Tajima and T. Aida, Angew. Chem,, Int. Ed., 40 (2001) 3803. 5. M. Ikegame, K. Tajima, and T. Aida, submitted.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. Allrightsreserved
Nanocomposites of MCM-41 electrorheological fluid
and
523
SBA-15
with
polyaniline
for
M. S. Cho", H. J. Choi*', K. Y. Kim^ and W. S. Ahn*^ 'Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Korea ^Department of Chemical Engineering, Inha University, Incheon 402-751, Korea Nanocomposites of mesoporous silica MCM-41 and SBA-15 with conducting polyaniline inside the channels were prepared and tested as a dispersed phase in electrorheological (ER) fluids. Their ER characteristics were demonstrated using a rotational rheometer with a high voltage generator. 1. INTRODUCTION Mesoporous silicas, due to extremely large surface areas (> 1000 m^/g) and the close tuning of pore sizes, are considered to be useful for various fields such as catalytic applications and adsorbents. In addition, they can be utilized in preparing organic-inorganic composites having well-defined nanosized structure. Composites between a mesoporous host and other guest materials such as carbon, polymer and metal, have been investigated after establishing the nano-scale alignment of the encapsulated molecules in the host channel [1|. l:leclrical properties of nanostructured conducting polymers and carbon, in particular, have been actively investigated by means of implementing ''molecular wires" in electronic devices |2|. As one of the most spectacular smart materials, ER fluids, which can be transformed into a solid-like state by an applied electric field, are composed of dispersions of polarizable or semiconducting particles in insulating oils and represent a unique class of electroaclive intelligent materials that exhibit drastic change in rheological and electrical properties |3. 4|. Various semiconducting polymers and inorganic materials have been used as particulates in the ER fluids [5-7] and recently, MCM-41 suspension in silicone oil was reported to show ER properties [8]. In this study, we prepared an organic-inorganic nanocomposite in which the conducting polyaniline (PANI) is located insides the mesoporous silicas, MCM-41 and SBA-15. and its potential use as an ER fluid system was investigated.
524
2. EXPERIMENTAL MCM-41 was prepared following the procedure of Ryoo et al. [9]. Ludox AS-40 (Si02 40wt% colloidal silica in water, Dupont) was added under vigorous stirring to a 40wt% TEAOH (tetraethylammonium hydroxide) solution. This solution mixture was then combined with a 25wt% CTMACl (cetyltrimethylammonium chloride) solution, and the gel obtained was stirred at room temperature for an hour. Subsequently, the mixture was placed in an autoclave and kept at 373 K for 24 h in a conventional oven for drying. The reaction mixture was cooled to room temperature, and acetic acid was added dropwise under vigorous stirring until the pH reaches 10.2. This mixture was then heated again to 373K at 24h. The pH adjustment and subsequent heating were repeated twice more. The solid product obtained was filtered and surfactant was extracted from MCM-41 in EtOH-HCl solution at room temperature (Ig solid / EtOH 20ml + HCl 2ml). After drying, it was calcined in air at 823K. Concurrently, SBA-15 was synthesized using nonionic triblock copolymer, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (Pluronic PI23, BASF) as a template according to the method reported[10]. In a typical synthesis, lOg of Pluronic PI23 was added to 380ml of 1.6M HCl. After stirring for Ih, a clear solution was obtained. 21.3g of tetraethylorthosilicate (TEOS, 98%, Aldrich) was then added to the solution with vigorous stirring for lOmin. Resulting mixture was left for 24h at 308K, and subsequently heated for 24h at 373K. The solid product obtained was filtered without washing, and dried overnight al 373K. After drying, product was calcined at 823K for 4h to remove the surfactant. SBA-15 was further dried before use at 473K under vacuum for 2 h. To synthesize PANI/MCM-41 and PANI/SBA-15 nanocomposite, the hosts were contacted with aniline gas at 313K for 24 h [11]. Either MCM-41 or SBA-15 containing aniline was then immersed in 0.2M HCl aqueous solution and the same mole of oxidant initiator ammonium peroxysulfate as absorbed aniline was added to the reaction system with stirring at room temperature [12]. The polymerization was conducted for 24 h. The PANI/MCM-41 was washed several times with aqueous HCl solution and methanol, and it was dried at room temperature under reduced pressure. To prepare as a dispersing phase in ER fluid system, the nanocomposites were further dried at 383K, and then mixed with electrically insulating silicone oil to the concentration of 10%(w/w).
ER property of PANI/MCM-41 suspension in silicone oil was obtained b> a
rotational rheometer (Physica, MCI20) equipped with a DC high voltage generator.
The
measuring geometry was a concentric cylinder and all measurements were conducted at 25''C. Dielectric spectrum of ER fluids were also measured by an impedence analyzer using a measuring fixture for liquids, in order to investigate their interfacial polarization.
525
3. RESULTS AND DISCUSSION In the PANI/MCM-41 nanocomposite, the PANI content was ca. 10% (w/w) as confirmed by a TGA thermogram. Its conductivity was 10'^ S cm'' measured by 2-probe method using a pressed disk of PANI/MCM-41. The conductivity of doped PANI is generally known to be about 1 S cm"'. Based on the observation of much lower conductivity of PANI/MCM-41 compared with PANI, we can indirectly come to the conclusion that all the synthesized PANI is located inside the MCM-41 channel. In the case of the PANI/SBA-15 nanocomposite, the PANI content was ca. 35%(w/w) with its conductivity of 10"^ S cm"'. This indicates that under the same condition, SEA-15 contains more polyaniline than MCM-41, because pore size and pore volume of the SBA-15 is larger than that of the MCM-41. Meanwhile, since its conductivity is too high to use it as dispersed phase for ER fluids, causing electrical short under the applied electric field, the conductivity of PANI/SBA-15 particles was lowered about 10'^ S cm"' through a dedoping process. The polymerization confined within channel was also confirmed by a nitrogen sorption experiment. The residual pore volume of PANI/MCM-41 is reduced by 0.63 ml/g from 0.96 ml/g for the empty MCM-41 (Fig. 1(a)) and that reduced to 0.53 ml/g from 0.98 ml/g for PANI/SBA-15 (Fig. 1(b)), respectively.
600
•
„u..*—*
(b) SBA-15
Q
fe 500-
V)
™ 200
0.2
0.4
0.6
Relative pressure ( P / P J
Fig.
0.8
'
/ /
E^ 400 -2 300 o
/
m*^ _,^jmr
/
/ PANI/SBA-15
J ^ 0.2
0.4
0.6
08
10
Relative pressure ( P / P Q )
N2-adsorption isotherm curves before (square) and after (circle) aniline polymerization. Solid symbols represent adsorption process and open symbols desorption process.
Figure 2 shows flow curves of PANI/MCM-41 and MCM-41 ER fluids under an applied electric field of 3kV/mm. Flow curves of empty MCM-41 host materials are also shown for comparison. Polarizing species inducing ER characteristics under electric fields are conducting polymer PANI for PANI/MCM-41 ER fluid and absorbed moisture for MCM-41 ER fluid [8]. The developed stress for PANI/MCM-41 by an applied electric field was found to be larger in whole shear rate regime than that of MCM-41 alone.
526
nD°
PANI/MCM-41
10^
Uooooo o
Ooooo ooooo
•4
oo^
MCM-41
Shear rate (1/sec)
Fig. 2. Shear stress as a function of shear rate under 3kV/mm, with a particle concentration of 10%(w/w).
ACKNOWLEDGEMENT This study was supported by research grants from the KOSEF through the Applied Rheology Center at Korea University, Korea. REFERENCES 1. K. Moller and T. Bein, Chem. Mater. 10 (1998) 2950. 2. M. J. MacLachlan, P. Aroca, N. Coombs, I. Manners and G. A. Ozin, Adv. Mater. 10 (1998) 144. 3. I. S. Sim, J. W. Kim, H. J. Choi, C. A. Kim and M. S. Jhon, Chem. Mater. 13 (2001) 1243. 4. H. J. Choi, M. S. Cho, J. W. Kim, C. A. Kim and M. S. Jhon, Appl. Phys. Lett. 78 (2001) 380. 5. W. H. Jang, J. W. Kim, H. J. Choi and M. S. Jhon, Colloid Polym. Sci. 279 (2001) 823. 6. J. W. Kim, H. J. Choi, S. H. Yoon and M. S. Jhon, Int. J. Mod. Phys. B 15 (2001) 634. 7. M. S. Cho, H. J. Choi, I. J. Chin and W. S. Ahn, Micropor. Mesopor. Mater. 32 (1999) 233. 8. H. J. Choi, M. S. Cho, K. K. Kang, W. S. Ahn, Micropor. Mesopor. Mater. 39 (2000) 19. 9. R. Ryoo, C.H. Ko, and R.F. Howe, Chem. Mater. 9 (1997) 1607. 10. D. Y. Zhao, J. T. Feng, Q. S. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka. (}. D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. 11. C. G. Wu and T. Bein, Science 264 (1994) 1757. 12. J. H. Lee, M. S. Cho, H. J. Choi, M. S. Jhon, Colloid Polym. Sci. 277 (1999) 73.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
527
Functionalized mesoporous adsorbents for Pt(II) and Pd(II) adsorption from dilute aqueous solution Taewook Kang, Younggeun Park, Jong Chul Park, Young Sang Cho* and Jongheop Yi** **School of Chemical Engineering, Seoul National University, Seoul, 151-742, Korea *Korea Institute of Science and Technology, Seoul, 136-791, Korea The surface of the SBA-15 was functionalized with imidazole or thiol functional group via grafting method. Binding behaviors of the adsorbents toward Pt(II) and Pd(II) were examined. The properties of the adsorbents such as pore structure and pore uniformity were also investigated. The pore structure of as-synthesized adsorbents was conserved throughout the preparing steps. The results showed that imidazole- or thiol-functionalized adsorbents showed a high affinity for Pt(II) and Pd(II) metals in aqueous solution. 1. INTRODUCTION For the metal extractions from dilute aqueous solution, solid-phase adsorbents have greater applicability than traditional solvent extraction. The recent discovery of mesoporous molecular sieves have stimulated a renewed interest in developing a novel adsorbcnt.^'^ However, little researches have been reported for the adsorption of noble metal ions using mesoporous silica."* It was reported that polymeric extractants with heterocyclic amine units exhibited efficient adsorption of Pt(II) and Pd(II) from aqueous solutions, and also reported that polymers containing functional groups with donor N and S atoms were the promising reagents toward noble metal ions.^'^' In this study, mesoporous adsorbents functionalized with chelating ligands (imidazole group or thiol group) via grafting method were synthesized and we investigated their binding capability for noble metal ions such as Pt(II) and Pd(II). 2. EXPERIMENTAL The synthesis of hexagonally ordered SBA-15 was performed as described in the literature.^ In order to graft metal adsorptivc functional group containing silanc on the mesopore wall, SBA-15 was silanized with N-(3-triethoxysilylpropyl)-4,5-dihydroimidazole (Imidazole, Gclest Inc.) or 3-mcrcaptopropyltricthoxysilane (MPTES, Gelcst Inc.). In the specific synthesis, 2.0 g of SBA-15 was refiuxed for 20 hr with 60 mL of dry toluene and 3 mL of silanes containing imidazole or thiol functional group. Solid products were filtered off and washed with solvents in the order of toluene, acetone and ethanol. The materials were labeled as Imi-SBA-15 and Thio-SBA-15, where Imi represents imidazole functional group and Thiol Corresponding author: ivid^/^snu.ac.kr Financial support by the National Research Laboratory (NRL) of the Korean Science and Engineering Foundation (KOSEF) is gratefully acknowledged.
528
4000
2thcta
3500
3000
2500 2000
1500
1000
500
Wavenumber, [1/cm]
Fig. 1. SAXS patterns (right figure) of (a) SBA-15, (b) Thio-SBA-15 and (c) ImiSBA-15
Fig. 2. FT-IR spectra of (a) SBA-15, (b) Thio-SBA-15 and (c) Imi-SBA-15
groups of SBA-15 was decreased and a very weak S-H stretching peak was seen at 2572-2589 cm"' for Thio-SBA-15. For Imi-SBA-15, the intensity of-OH stretching band of silanol groups was also reduced, while 1544-1734 cm"' absorption peak (C=N stretching), which resulted from the grafting of imidazole functional group, appeared.'^ The amounts of functional groups, based on the elemental analysis, were determined to be 2.54 mmol/g SBA15 (Imi-SBA-15) and 0.58 mmol/g SBA-15 (Thio-SBA-15), respectively. Metal adsorption experiments were carried out using SBA-15, Imi-SBA-15 and Thio-SBA15 in buffer solution at pH 4. The amounts of adsorbed metal ions are shown in Table 2. The extent of metal adsorption capability can be represented by distribution coefficient, K^, which is defined as the ratio of the amount of metal ions in solid matrix to those in liquid matrix as listed in Table 2. The Kd value of Imi-SBA-15 was 16000 for Pt(II), 4300 for Pd(II) and the Kd value of Thio-SBA-15 was 38000 for Pt(II), 990000 for Pd(II) in single solution. A Kj value of 990000 for Thio-SBA-15, to our knowledge, the highest value reported for metal ions adsorption in similar conditions although target metal ions were different.'°'" Table 2 Physicochemical properties of surface functionalized mesoporous silicas Pd(II)
Pt(II) Adsorbent
Uptake
/ %
mlg"
Capacity/ mmolg"'
SBA-15
20.9
26
0.019
Imi-SBA-15
99.4
16000
Thio-SBA-15
99.7
38000
Kd/
Uptake
mlg-'
Capacity/ mmolg'
13.5
16
0.012
0.091
97.7
4300
0.093
0.095
99.9
990000
0.098
/ %
Kd/
The results (Table 2) showed that SBA-15 had binding affinity for Pt(II)and Pd(II) metals. Probably oxygen atom in the silanol group of the surface interacted with Pd(II) and Pt(II) by
529
denotes thiol functional group. A variety of properties of as-synthesized adsorbents were characterized with SAXS (BRUKER), FT-IR (Jasco), elemental analysis (MT-2, Yanaco) and N2 sorptometry (ASAP 2010, Micromeritics). A batch technique was applied to determine the metal binding ability of as-synthesized adsorbents. Typically, 0.1 g of adsorbent was equilibrated with 10 mL of ca. 1 mM K2PdCl4 or K2PtCl4 (pH 4.01 buffer) in vials, and these mixture was shaken for 12 hr and the metal ion uptake was determined by analyzing supernatant solution using Inductively Coupled PlasmaAtomic Emission Spectrometer (ICP-AES). 3. RESULTS AND DISCUSSION N2 adsorption/desorption isotherms of SB A-15, imidazole functional group grafted SBA-15 (Imi-SBA-15) and thiol functional group grafted SBA-15 (Thio-SBA-15) showed irreversible type IV adsorption isotherms with a HI hysteresis loop as defined by lUPAC. The physical properties of SBA-15 and the functionalized silicas were listed in Table 1. Surface area, pore diameter of the SBA-15 decreased due to the grafting of organic functional group. Moreover, the decrease in surface area and pore diameter was observed in the order of size of functional group. Surface area and pore diameter of imidazole-functionalized SBA-15 was more sharply decreased than thiolated analogue. Pore size distributions ofSBA-15, Imi-SBA15 and Thio-SBA-15 were similar except for decreasing pore diameter approximately 1-2 nm throughout the preparing steps. No change occurred in hexagonal mesoporous structure of the Tabic 1 Physical properties of the samples Functional group/ BET surface area/ *Pore diameter/ Sample mmolg rn g nm SBA-15 721 8.3 Imi-SBA-15 2.54 161 5.8 Thio-SBA-15 0.58 437 7.3 *Pore size calculated from the desorption branch using BJH formula SBA-15 through the preparing steps. This conservation of the mesoporous structure is confirmed precisely by the SAXS data (Fig. 1). The SAXS pattern of SBA-15, Imi-SBA-15 and Thio-SBA-15 showed three reflections, respectively. The X-ray diffraction pattern of ImiSBA-15 and Thio-SBA-15 showed a very intense peak (100) and two additional high order peaks (110, 200) with lower intensities. This result was characteristic of a hexagonal pore structure. The functional groups contained by three samples were identified using FT-IR (Fig. 2). Silanol groups on the silica surface exists as several types, such as isolated, hydrogenbonded, and geminal types of silanol.^ The IR absorption bands of these silanol groups are corresponding to the peaks at 3738 cm"', 3200-3600 cm' and 3738 cm'. The results showed that the surface silanol group was mainly of the hydrogen-bonded type, IR absorption bands observed at 3200-3600 cm"'. The siloxane, -(SiO)n-, peak appeared at 1000-1100 cm"'. Si-0 bond stretching was detected at 960 cm"'. The intensity of-OH stretching band of silanol
530
ion-pairing mechanism with K^ ion as balancing counter-ion.^ The maximum loading capacities of Imi-SBA-15 were 1.1 mmol/g for Pt(II), 1.0 mmol/g for Pd(II) and the maximum loading capacities of Thio-SBA-15 were 0.60 mmol/g for Pt(II) and 0.88 mmol/g for Pd(II). Metal/functional group ratios were approximately 0.5 (both Pt(II) and Pd(II)) for Imi-SBA-15 and 1 (Pt(II)), 0.7 (Pd(II)) for Thio-SBA-15. In summary, we have shown that the introduction of chemical functional groups such as thiol and imidazole, to the mesoporous silica support leads to remarkable increase of the binding capacity for Pt(II) and Pd(II). REFERENCES 1. J. S. Kim and J. Yi, Separ. Sci. Technol., 34 (1999) 2957. 2. H. Lee and J. Yi, Separ. Sci. Technol., 36 (2001) 2433. 3. B. Lee, Y. Kim, H. Lee and J. Yi, Micropor. Mesopor. Mat., 50 (2001) 77. 4. T. Kang, Y. Park, J. C. Park, Y. S. Cho and J. Yi, The Korean J. of Chem. Eng., 19 (2002) 5. R. Liu, Y. Li, H. Tang, J. Appl. Polym. Sci., 83 (2002) 1608. 6. G. G. Talanova, L. Zhong, O. V. Kravchenko, K. B. Yatsimirskii, R. A. Bartsch, J. Appl. Polym. Sci., 80(2001)207. 7. D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka, G. D. Stucky. Science, 279(1998)548. 8. X. S. Zhao, G. Q. Lu, J. Phys. Chem. B, 102 (1998) 1556. 9. G. Socrates (2"^^), Infrared Characteristic Group Frequencies: Tables and Charts. John Wiley & Sons Ltd., Chichester, 1994. 10. S. Dai, M. C. Burleigh, Y. H. Ju, H. J. Gao, J. S. Lin, S. J. Pennycook, C. E. Barnes, and / . L. Xue, J. Am. Chem. Soc, 122 (2000) 992. 11. M. C. Burleigh, S. Dai, E. W. Hagaman, and L. S. Lin, Chem. Mater., 13 (2001) 2537
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
531
Environmentally benign removal of pollutant oxyanions by Fe adsorption center in functionalized mesoporous silica Toshiyuki Yokoi^, Takashi Tatsumi and Hideaki Yoshitake^* ^Graduate School of Environment and Information Sciences, Yokohama National University. 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan. ''Division of Materials Science and Chemical Engineering, Graduate school of Engineering, Yokohama National University. 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan corresponding author, yos@ynu.ac.jp
The specific molecular adsorption sites were built on the pore wall of MCM-41 by means of fixation of ethylenediamine group followed by cationization with Fe ^. The iron-anchored surface sites lead to selective adsorptions of arsenate, chromate, selenate and molybdate. The used adsorbent can be regenerated by an environmentally benign process.
1. INTRODUCTION Recently, the pollution of groundwater by arsenic has attracted a considerable attention and a trace amount of pollutants in groundwater need to be removed for human health. Precipitation methods have been utilized as a removal technique of the pollutant in water environment. However, those methods are not satisfactory and have several disadvantages, such as large amount of secondary waste products. Although the adsorption on solid surfaces can be an efficient method for removing pollutants from water, the specific adsorption of the oxyanions is generally difficult because As is shelled by four oxygen atoms (arsenate HAs04^' or H2ASO4') and several kinds of anions, such as sulfate and chloride often compete. Mesoporous silica with a well-ordered structure and a high specific surface area has been expected to be applicable to catalyses and adsorptions [1-3]. In addition the high density of silanol groups on the surface is beneficial to the introduction of functional groups with high coverage by silylations [4]. We report here the synthesis and the utilization of Fe(in)-chelated ligands immobilized on the surface for a high performance and environmentally benign adsorbent for pollutant oxyanions.
532
2. EXPERIMENTAL 2.1. Synthesis of adsorbent The surface of MCM-41, which was synthesized by a conventional method [5], was modified with an organosilane containing amino groups, l-(2-aminoethyl)-3-amino propyltrimethoxysilane (NN-MCM-41) [6]. Fe(III) ion was coordinated with amino Hgands to form a stable complex on the surface in the pores (Fe/NN-MCM-41). The composition and structure of MCM-41, NN-MCM-41 and Fe/NN-MCM-41 were characterized by CHN and ICP elemental analyses, XRD, nitrogen adsorption, ^'^Si-NMR and FT-IR spectroscopy. 2.2. Adsorption experiments Fe/NN-MCM-41 was utilized as an adsorbent for arsenate, chromate, selenate and molybdate. Typical adsorption experiments were carried out by using 50 mg of Fe/NN-MCM-41 stirred in 10 ml of aqueous solutions containing the oxyanion, KH2ASO4, K2Cr04, K2Se04 and K2M0O4 for 10 h at 298 K. The concentration of initial and residual oxyanion in the solution was analyzed by ICR
3. RESULTS AND DISCUSSION 3.L Structural properties of adsorbent XRD patterns demonstrated that NN-MCM-41 and Fc/NN-MCM-41 retained the original ordered mcso structure of MCM-41, though the surface area, pore volume and pore size decreased (Table 1). Thus the high accessibility of ions from the outside of the pore to Fe(lll) cations center in the pore is likely to be maintained. *^'^Si-NMR and FT-IR spectra of NN-MCM-41 showed the presence of Si-C bonds and amino groups of the organosilane. According to the elemental analyses, the molar ratio of N / Fe ^ was 4, suggesting that one Fe * was tethered to four N atoms and also coordinated with Cf or H2O ligands. Table 1 Characteristics of Functinalized MCM-41. A HI./
MCM-41 NN-MCM-41 Fc/NN-MCM-41
(m'g') 1283 586 310
v,.^ (cnr^g"') 1.05 0.50 0.25
2R,.^ (nm) 2.9 2.6 2.2
Fc^' content (mmol g ' )
-
0.55
C/N N content'' (molar ratio) ( m m o l g ' )
-
2.65 2.73
2.76 2.09
"AHI;T: B E T specific surface area. V|.: primary mesopore volume. '^Rp: pore radius (by the BJH method). ''Assuming that -NH groups content is equal to nitrogen atoms content.
533
3.2. Adsorption behavior for Oxyanions The adsorption isotherms of various oxyanions are shown in Figure 1. The maximum adsorption amount reached as large as 1.56, 0.99, 0.81 and 1.29 mmol g'' for arsenate, chromate, selenate and molybdate, respectively. The maximum leaching of Fe(in) cations during the adsorption of oxyanions was less than 7 wt%. The molar ratio of the cations
-As(V) -Se(VI)
-Cr(VI) -Mo(VI)
to
-r-l S
0 2 4 6 8 10 (Fe-^^) to the anion (HAs04^', Cr04 Equilibrium concentration of oxyanions Se04^" or Mo04^') at the adsorption /mmol r' saturations was almost in agreement with Fig. 1. Adsorption isotherms of oxyanions, # the electric charge balance. This nearly A s ( V ) , B C r C V l X ^ S c C V I ) , A M o ( V I ) , by Fe/NN-MCM-41. Reaction conditions; 50 mg stoichiometric adsorption demonstrated adsorbent, 10 ml H2O as a solvent, reaction time the molecular nature of adsorption site. 10 h and reaction temperature 298 K. When the density of the organic groups was increased, the surface area and the structural order of the silica framework decreased,
resulting in lower adsorption capacity. Purely inorganic silica (MCM-41) showed negligible adsorption capacity compared to Fc/NN-lVICM-41, indicating that the complexation was indispensable to adsorptions of anions. 3.3. Inhibition by competing anions Several kinds of anions in the hydrosphere can be adsorbed competitively during removal of the pollutant oxyanions. We carried out coadsorption experiments in the presence of sulfate and chloride anions at low initial concentration of oxyanions (0.5 mmol l') in order to clarify the inhibition effect on the adsorption capacities. The results showed that in the presence of sulfate or chloride with the concentration of 10 molar times as high as all four kinds of oxyanions, over 80 and 90 % of the adsorption capacities were maintained, respectively, which implies that Fc/NN-MCM-41 will work as an effective adsorbent of oxyanions effectively in a real environment. The strong resistance against competing anions is attributed to the strong specific affinity between Fe(lII) cation center and the target oxyanions. 3.4. Regeneration of used adsorbent We also explored the reusability of our adsorbent. Table 2 shows the result of the
534
regeneration of arsenate-saturated Fe/NN-MCM-41, where the complete desorption of arsenates from the adsorbent was achieved. However, this process did also simultaneously lead to the leaching of almost all the Fe(in) cations from lSrN-MCM-41 probably due to the strong acidity of the treatment solution. Because most of the organic groups were retained after the HCI treatment, we re-introduced Fe(III) cation into the fiinctiolized silica to restore the adsorption sites (Table 2). The adsorption of arsenates on the regenerated Fe/NN-MCM-41 showed 84 % (1.31 mmol g'') of the initial adsorption capacity. The decrease in the adsorption amount was nearly in accordance with the decrease in the adsorption site. Table 2 Regeneration of Fe/NN-MCM-41 by Treatment with 1 M HCI aq.. Residual oxyanion" (mmol g')
Fe^^ content N content N / Fe^^ (mmol g"') (mmol g') (molar ratio) Before adsorption 0.55 2.09 3.8 1.50 1.98 3.9 After adsorption of arsenate 0.51 0.001 1.53 0.03 After HCI treatment 0.001 0.42 1.51 After re-incorporation of Fe^^ 3.6 "Calculated from solid analysis. Treatment conditions: 1 M HCI aq. 200 ml g' , treatment time 10 h, treatment temperature 298 K.
4. CONCLUSIONS Fe(in)-cation coordinated with the amino ligands on the surface of mesoporous silica work as excellent adsorbing sites for pollutant oxyanions, arsenate, chromate, selenate and molybdate. The selectivity to the target oxyanion was not interfered by abundant sulfate and chloride anions, suggesting the strong specific affinity between Fe(III) cations and the target oxyanions. Adsorbed oxyanions were successfully desorbed by a simple acid treatment and the degree of regeneration was found satisfactory to a recycled use of the adsorbent. REFERENCES 1. L. Mercier, T. J. Pinnavaia, Environ. Sci. Technol., 32 (1998) 2749. 2. H. Yoshitake, T. Yokoi, T. Tatstumi, Chem. Lett., 6 (2002) 586. 3. G. E. Fryxell, J. Liu, T. A. Hauser, Z. Nie, K. F. Ferris, S. Mattigod, M. Gong, R. T. Hallen, Chem. Mater., 11 (1999) 2148. 4. Zhao, X. S.; Lu, G. Q.; Whittaker, A. J.; Millar, G. J.; Zhu, H. Y. J. Phys. Chem. B, 101(1997)6525. 5. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J.S.Beck, Nature, 359 (1992) 710. 6. K. Moller; T. Bein, Chem. Mater. 10 (1998) 2950.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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How can nanoparticles change the mechanical resistance of ordered mesoporous thin films ? Edward Craven', Sophie Besson'' ^, Michaela Klotz', Thierry Gacoin^, Jean-Pierre Boilot^ and Etienne Barthel' ^Laboratoire CNRS / Saint-Gobain "Surface du Verre et Interface" UMR 125 ;39 Quai Lucien Lefranc ; F- 93303 Aubervilliers, France ^Laboratoire de Physique de la Matiere Condensee, UMR CNRS 7643 ;Ecole Polytechnique ; F-91128 Palaiseau, France Nanoindentation was performed on mesoporous thin films in order to investigate their mechanical behaviour. It has been found that the empty mesoporous films behave plastically while films filled with nanoparticles exhibit more elastic deformations. This contrasfing behaviour is consistent with the different indent morphologies observed by optical and electronic microscopy. KEYWORDS : mesoporous thin films, nanoindentation, elastic modulus and hardness 1. INTRODUCTION Mesoslructured thin films arc studied for their potential applications in a variety of fields including separation technology, sensors and catalysis. These films are formed by the association of sol-gel chemistry and a templating mesophase. Combining the advantages of each component, one can form a crack free thin film that exhibits a beautifully ordered mesostructure. Most studies aim at the understanding of the formation mechanism and fiinctionalisation of these films. However, the mechanical behavior - most prominently the resistance to wear - has to be considered for industrial applications. In this contribution, we present the results of indentation tests performed on two different types of mesostructured films: a mesoporous film presenting an isotropic hexagonal micellar structure and the same structure uniformly filled with CdS particles. The results will be correlated to the structural properties of the thin film. 2. EXPERIMENTAL Mesoporous thin films are prepared under acidic conditions using cetyltrimethylammonium bromide as a template. The synthesis procedure has previously been reported'. CdS particles are subsequently grown in situ by impregnation of the mesoporous film with a cadmium containing solution of pH 9.5. The film is then treated with H2S leading to the precipitation of CdS particles. These steps were repeated until film saturation. The complete preparation procedure and characterization is reported in ^. An additional sample is prepared following the same procedure except that the pH 9.5 solution used for impregnation is prepared using ammonium hydroxide. This chemical treatment of the film is thus considered equivalent but will not lead to the precipitation of nanoparticles. In the following, we will refer to this sample as "base treated".
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Indentation tests are performed with an XP Nano Indenter (MTS). The experiments are carried out using Berkovich type tip (three sided pyramid). In order to study the mechanical properties of the film and suppress the influence of the substrate, only indents of 1/10^^ of the film thickness are considered. 3. RESULTS AND DISCUSSION The mesoporous thin films are highly textured with single domains occupying the entire film thickness. They exhibit a 3D hexagonal structure (P63/mmc) described in more detail in '. The total pore volume was determined by ellipsometry measurements^. The approximative part of mesoporosity and microporosity of the walls was deduced by simple calculation knowing the pore diameter and the structural parameters of the structure. The results are summarized in table 1. The detailed characterization of the nanocomposite formed by in situ growth of CdS particles^ show that the 3D hexagonal structure is maintained during the filling process. The nano-crystals were found to always be located within the pores and 100% of the pores were filled. It was also found that the particle diameters, determined from UVvisible absorbance spectra, were equal to the initial pore size of 3.5 nm. Table 1 Structural and porous properties of the empty mesoporous thin film Structural parameters a
c
5.6 nm 6.1 nm
Porosity
Total Pore Mesopore Microporosity Film Pore thickness diameter volume volume within the walls 300 nm
3.6 nm
55%
25%
40%
The Young's modulus and hardness (table 2) for the films can be derived from the nanoindcntation force curves (Figure 2): The empty 3D hexagonal film was found to behave as an almost perfectly plastic material, with a low yield stress, and negligible elastic recovery. The SEM image (Figure la) shows no other surface effect than the trace of the plastic deformation of the material. Comparatively, the CdS filled structure exhibits a sizably larger modulus (+80%) and hardness (+100%), resulting in a more brittle layer than the empty one. Identical penetrations in the CdS filled layer lead to delamination along with the formafion of flakes (Figure Ic). Finally, the results on the base treated sample shows that the influence of the chemical treatment during the growth process is limited in terms of mechanical behavior.
^
a) Empty mesoporous film b) base treatment Fig. 1. SEM images of the residual trace of l)im indents
-
_
-
^
c) CdS filled
537
.4x10 1.2 1.0
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' I ' ' ' ' I ' ' ' ' I '
silica Pyrex CdS filled empty base treatment
£0.20
0.8 0.6
0.25 f-'
t0.15 E ra ^0.10 o
.,jr
-^•^
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.
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silica Pyrex CdS filled empty base treatment
T3
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o 0.05
0.0 Vyy'^^ffr: ^^v^»,'^-,-r-T 0 5 10 15 20 25 Displacement into surface (nm)
tJ»iyyfy'(i.f>.yJT •
30
5
0.00
a) Harmonic contact stiffness
10 15 20 25 Displacement into surface (nm)
b) Load on sample
Fig. 2. Nanoindentation curves for 300 nm thick layers on a glass substrate. Table 2 Elastic modulus and hardness of the different films Elastic Modulus Hardness Sample (GPa) (GPa) Silica
71.6
8.35
Empty
11.0
0.75
Base treatment
10.9
0.49
CdS filled
19.7
1.45
The results can be correlated to the structure of the thin film by the use of simple models. First, wc used Voigt's model'^ to determine the elastic modulus of the walls from the elastic modulus of the empty film. This model supposes equal strain in the 2 phases composing the material. Then
£
tilm
={\-(l) V
)E ,,+(/)
' ni.vo/xir.' /
wall
T mifd/uirf
E
mesupore
Knowing that the film is composed of 25% of mesopores and that the elastic modulus of the void mesopores equals to zero, we determined that the elastic modulus of the walls is 14.5 GPa. This value is much lower than for dense silica. This is attributed to the important part - 40% - of microporosity in the walls. The previous model is not applicable in the case of the CdS filled thin film. We suppose in this case that the walls and the CdS particles get equal stress^ during the nanoindentation experiment. Then
1
E ..
E
The elastic modulus of CdS is 45GPa. Using the elastic modulus of the walls previously determined, one can calculate the theoretical value of the nano-particle filled sample: 17.3 GPa. This result is in good correlation with the experimental value.
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4.CONCLUSION The mechanical behavior of mesostructured thin film have been studied. Empty mesoporous thin films were found to behave like a very plastic material. The elastic modulus of the walls have been deduced. The calculated values are much lower than those of dense silica, which can be correlated to the microporosity within the walls. The filled layer behaves like a more brittle material. The film's capacity to store stresses causes delamination instead of a distributed plastic deformation as observed for the empty film. The chemical treatment which leads to the formation of the CdS nano-cristals was shown to have a weak impact on the mechanical behavior of the empty film, especially for the Young's modulus. The film's elastic modulus correlates well with a simple additive model, thus, the modification of the behavior is mainly due to the presence of the nano-particles. The mechanical properties of the mesoporous thin film could be improved by the in-situ growth of nano-cristals. However, the filled film is more sensitive to delamination which causes visible optical defects.
4. REFERENCES l.S. Besson et al., J. Mater. Chem., vol. 10, p. 1331,2000 2. S. Besson et al., Nano Lett., vol. 2, p. 409, 2002. 3. S. Besson, PhD thesis, Ecole Polytechnique, Palaiseau (France), 2002 4. W. Voigt, Lehrbuch der Kristallphysik, Teubner, Leipzig, 1910 5. A. Rcuss, Z. Angcw. Math. Mech., p9-49, 1929
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
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Nanoporous SiOi films prepared by surfactant templating method - a novel antireflective coating technology Heui-Ting Hsu^, Chih-Yuan Ting^, Chung-Yuan Mou^ and Ben-Zu Wan^* ^Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 106, R.O.C. ^Department of Chemistry, National Taiwan University, Taipei, Taiwan 106, R.O.C. A sol-gel derived antireflection (AR) coating by surfactant templating method is presented. The spin coating process was used to deposit Si02 thin films on glass. The pore size and the pore volume of the film were controlled by the size and the volume of the template (i.e. Tween 80). It was found from this research that the transmittance was increased from 91.7% to above 99.0% either by a single-layer or by a double-layer porous Si02 antireflection coating, on both sides of the glass pane at a specific wavelength. 1. INTRODUCTION Antireflection coatings can reduce the intensity of light-reflection (or increase the intensity of transmittance) and increase the quality of optical lens system. Porous Si02 antireflection films arc commonly prepared by sol-gel deposition.' Reflectance can be minimized at a particular wavelength /l,, at normal incidence, when refractive index and thickness satisfy the following two conditions': (1).Light amplitudes reflected at air/film and film/substrate interfaces must be equal. That is
where nc, no, and ns arc refractive indices of film, air and substrate, respectively. (2). Film thickness (tc) must be 1/4 of a reference wavelength in the film, for the reflected light to interfere destructively. That is t,=Z,/(4n^.)
(2)
Therefore, when /l,)~510nm is chosen and a glass (ns=1.52) is used as a substrate, the optimum refractive index and thickness of coated antireflection film can be calculated as nc~1.23 and tc~100nm, respectively. Because the refractive index of a dense Si02 film is also 1.52, the desired film refractive index must be reduced by adjusting the porosity of the Si02 film. Therefore in this paper, a sol-gel derived antireflection (AR) coating by surfactant templating method is introduced. The results of antireflection (or enhanced-transmittance) is demonstrated.
540
2. EXPERIMENTAL SECTION Silica sols were made from Tween 80, ethanol, H2O, TEOS, and HCl. Tween 80 is a non-ionic surfactant and act as a template in the sols. The spin coating was used for the deposition of sols on silicon wafers and glass substrates^ The coated substrates were baked on a hot plate at 106°C, then were calcined at 400°C for 3 h. Later, the Si02 surface was grafted with silane by immersing the samples in a HMDS/toluene solution at 80°C, in order to increase the surface hydrophobicity. On the other hand, extraction of template molecules and simultaneous grafting of silane were also studied by treating uncalcined samples in HMDS/ethanol solution at 50 °C (direct surface modification). For the transmittance measurement, a spectrophotometer (Hitachi, UV-visible 3410) was employed to record transmittance of the AR coated glass in a 200-800 nm wavelength range. Refractive index and thickness of the thin film was measured with a n&k analyzer 1200. Surface morphologies were characterized by a scanning electron microscope (SEM, Hitachi S-2400). 3. RESULTS AND DISCUSSION 3.1. Controlling refractive index and thickness of the film For an antireflection coating, it is necessary to control the refractive index and the thickness of the films. In the case of nanoporous Si02 films, both can easily be achieved. The refractive index of the film can be tuned by varying the film porosity, which can be achieved by varying the Table 1 Refractive indices and thicknesses of weight ratio of Tween 80 to TEOS in the coating solution. Table 1 shows the refractive indices and the films prepared by different ratios of the thicknesses of the films made from different Tween 80/TEOS Tween 80 Refractive Thickness weight ratios of Tween 80/TEOS, when the molar Index /TEGS ratios of TEOS/E1OH/M2O/HCN 1/60/4.2/0.24 arc 0.13 1.43 74 fixed. It was found that the refractive indices 0.41 93 1.25 decreased from 1.43 to 1.19, when the weight 1.21 0.62 103 ratio was increased from 0.13 to 0.83. Fig. 1 118 0.83 1.19 shows the empirical dependence of the refractive index on Tween 80 mixing ratio, which allows precise adjustment of the film refractive index in the later experiments. The thickness of the film can be controlled by varying the concentrations of TEOS, Tween 80 and ethanol in the coating solution. Changing the spin-coating speed is another way for varying thickness. It is concluded from this research that when the molar ratio of TEOS /EtOH/H20/HCl is 1/48-72/8/4.2/0.24 and the weight ratio of Tween 80/TEOS is 0.13-0.83 in the coating solution, the Tween 80/TEOS desired film thickness for this research can be obtained. Fig.I. Refractive indices as a function of the ratios of Tween 80/TEOS
541
3.2. Single-layer AR coating Substrate Pretreatment
Prepare coating solutiai
^ i n coating Baking(106-'120°C)
Calcinations(400°C)
400
500 600 Wavelength(nm)
800
Surface i m3dification(80°C)
Treating uncaldned sanple in HMDS/BOH solution at 50°C
Fig.3. Experimental procedure
Fig. 2. Transmittance of porous Si02 AR coatings prepared by (a) calcinations or by (b) direct surface modification in HMDS/EtOH solution and (c) uncoated ^lass.
The transmittance spectrum in Fig.2.(a) shows a broad-band AR from a single-layer coated glass, calcined at 400°C. It can be found that the transmittance of more than Table 2 Refractive indices and thicknesses of films 96% is over the whole visible spectrum. The prepared by different ways for removing transmittance at 475 nm even reaches 99.3%. The refractive index and the thickness of the template film are 1.22 and 90 nm, which are listed in Refractive Thickness Table 2. The high transmittance Index (nm) demonstrates that the surfactant-templated (a)Calcinations 1.22 90 nanoporous film is a potential candidate for (b)HMDS/EtOH 1.29 120 AR coating. However, it should be noted that the high temperature (400"C) may damage the glass substrate and cause the coated film to shrink. In order to prevent these, the direct surface silyl modification'^ to remove the templates in the solution at low temperature was developed. Experimental procedure are shown in Fig. 3. And the transmittance spectrum from the coated glass is shown in Fig.2.(b). It can be found that the transmittance spectrum shows a broad-band AR in the range of 600 to 800 nm. The refractive index and thickness listed in Table 2 are 1.29 and 120nm, respectively. The highest transmittance (98.5%)) appears at 710 nm. It is apparent that the different wavelength at the highest transmittance in Fig.2. (a) and (b) results from the different film thicknesses. And the difference of their maximum transmittance in Fig.2 (a) and (b) are caused by the different refraction indices in the films. 3.3 Double-layer AR coating Single-layer AR coatings normally cover narrow bandwidth. In order to achieve broadband optical performance, a multi-layer stack is usually necessary. Multi-layers coating can further
542
reduce the reflected light but require layers of refractive index below 1.2. In this research, the optimal optical parameters for the coatings were determined from a theoretical calculation^. Fig. 4(a) shows the measured transmittance of a glass substrate coated with double-layer AR coating. The glass was calcined at 400°C. It consists of two porous Si02 layers with refractive indices 1.19 and 1.44 and with thickness 141 and 86 nm, respectively. It can be found that the transmittance spectum in 570~800nm wavelength range increased from 91.7% to >99%, and the transmittance is 99.4% at ~713nm, which are better than those from a single-layer coating. Moreover, Fig. 4. (b) is the result form computer simulation according to the experimental parameters (refractive index and the thickness). The similar trend between the experimental and the simulation results suggests that the surfactant tcmplating method is one of the effective ways for the preparation of antireflectivc films.
400
500
600
700
800
Wavelength(nm) Fig. 4. Transmittance of (a) glass substrate coated with two porous Si02 layers, (b) transmittance from theoretical calculation and (c) uncoated glass.
4. CONCLUSION Surfactanl-templatcd nanoporous silica films have been applied as AR coatings for the first time. The refractive index and the thickness of the films are easily adjusted via coating solution composition and spin coating speed. The transmittance of a single-layer AR coated glass can be enhanced from 91.7% to 99.3% measured at a single wavelength. The transmittance of the double-layer AR coated glass is increased from 91.7% to an excellent value of >99% in the 570~800nm wavelength range and approaches to 99.4% at ~713nm. The film preparation process reported in this research is simple and cheap. Both the high performance of the film and the simplicity of the process make the reported technology a potential candidate for AR coatings application in the future. REFERENCES 1. C. J. Brinker, G.W. Scherer, Sol-Gel Science, Academic Press, Boston, 1990. 2. H. A. Macleod, Thin-Film Optical Filters, Mcgraw-Hill, New York, 1986. 3. S. Walheim, E. Schaffer and J. Mlynek, U. Steiner, Science, 283 (1999) 520. 4. H. P. Lin, L.Y. Yang, C.Y. Mou, S. B. Liu and H.K. Lee, New J. Chem., 24 (2000 ) 253.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
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Textural and structural properties of Al-SBA-15 directly synthesized at 2.9 < pH < 3.3 region Dedicated to memory of Dr. V.N. Romannikov Maxim S. Mel'gunov, Elena A. Mergunova, Alexander N. Shmakov, Vladimir I. Zaikovskii Boreskov Institute of Catalysis, SB RAS, Novosibirsk, 630090, Russia. FAX: +7-3832-343056. E-mail: 2max(a)bk.ru Implantation of Al in a carcass of SBA-15 silicate directly during its synthesis at pH in a range of 2.9-3.3 using sodium silicate and aluminum sulfate as precursors results in formation of a well organized hexagonally structured mesophase. The effect of mesophase perfection improvement due to Al implantation is reported. The obtained materials demonstrate high thermohydrostability and mechanical strength. 1. INTRODUCTION Various synthetic procedures for the preparation of Al-SBA-XX silicates have already been reported. There are two methods of Al incorporation into a SBA mesophase, including directand post-synthesis. The latter is generally based on impregnation of a pre-prepared SBA mesophase with an Al-containing precursor followed by conversion of the precursor to the surface grafted AlOx. However, this method results in formation of relatively big AlOx clusters weakly bounded to the mesophase surface, thus the final material has low catalytic activity. The direct synthesis allows implantation of Al in a mesophase carcass resulting in higher dispersion and stability of AlOx clusters, increasing catalytic activity. Catalytic and hydrothermal properties of Al-SBA mesophase can also be improved when synthesis proceeds in weak acid conditions. For example, recently Yue et al [\] have reported the direct synthesis of Al-SBA-15 using tetraethyl orthosilicate and aluminum tri-/er/-buthoxide as precursors at ambient temperature and pH of 1.5 followed by hydrothermal treatment at 373K and calcination at 823K. The main disadvantage of this method is usage of metal-organic precursors that results in low commercial viability. To bypass this, Kim and Stucky [2] have offered sodium silicate as Si02 precursor for the synthesis of pure siliceous mesophases with various structure at pH of-0.6. Following these trends, in this paper we report the synthesis and textural and structural study of Al-SBA-15 materials prepared at 2.9
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poly(propylene glycol)-/7/oc/:-poly(ethylene glycol) composition of (EO)2o(PO)7o(EO)2o (Pluronic P123) at a molar composition of [P123]:[Al((SO)4)3]:[Si02] 1:X:60 (0.1<X<10). Our distinctive step includes co-precipitation of silica and alumina precursors at final pH around 3.0. Amount of Al in a synthetic mixture was varied in a range of 0.01 - 0.18 Al:Si mol. The mixes were aged for 50h after precipitation at ambient conditions followed by 50h of autoclave hydrothermal treatment at 383K. The final precipitates were filtered and calcined at BOOK and are refereed hereafter to as as-prepared. After cooling down the samples were washed in IM NH4CI aqueous solution in order to remove residual Na ions and nonframework alumina. Then the samples were repeatedly calcined at BOOK. These samples will be refereed to as ammonia chloride washed. The prepared samples were characterized by N2 adsorption at 77K. The N2 adsorption isotherms were compared with a reference adsorption isotherm (RI) measured for a set of non-porous samples of different chemical nature (the method of comparison is analogous to a more widely used as method [3]). As a result total surface area, external surface area of aggregates, volume of mesoporcs in these aggregates were measured. The isotherm reported in [4,5] was used as RI. The X-ray diffraction of synchrotron radiation (>L=0.154nm) and high resolution electron microscopy were also used. Mechanical strength of a sample with 0.07 Al:Si mol. composition was examined by static prcssurization of sample powder. 3. RESULTS AND DISCUSSION As it follows from data summarized in Table 1 the chosen synthetic conditions do not allow implanting of Al in ratio of Al:Si more than -0.07. The most plausible explanation of this is the decrease of pll of a final mixture from 3.3 for sample 1 to 3.0 2.9 for samples 5 11, while an affordable region for A1(()I1)3 precipitation is in between 3
545
diameter. The unit cell parameter is about 11 nm and the wall thickness is about 3 nm. Sample 11 consists of particles more uniformed in size in a range of 0.3 - 1.5 jam.
Fig. 2. High resolution electron microscopy images of samples 1 and 11. The pores are almost straight and have a length comparable to the size of particles and aspire 1.5 jam. The unit cell parameter, pore diameter and wall thickness are close to that for sample 1. The more accurate estimation of pore diameter and wall thickness can be done on the basis of analysis of both synchrotron radiation diffraction and N2 adsorption data. Typical for SBA-15 materials N2 adsorption - desorption isotherm (77K) over sample 11 is shown in Figure 3 in regular and P/PQ comparative coordinates. Comparative plot b) in Figure 3 allows quantitative description material's texture. Two linear parts are observed on this plot. The tangent of slope angle of the interpolation line at low P/Po region gives the value of the total surface area, A, the intercept on ordinate axis gives the volume of micropores, V^. In the case of materials under study the values of V^ are equal to 0.0 indicating absence of measurable amount of micropores. Fig. 3. N2 adsorption isotherm (77K) for sample 11 in After capillary condensation filling of mesopores observed is another linear region on the comparative plot. The tangent of slope angle of the line gives the value of the external surface area, Aext, the intercept on ordinate axis
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gives the volume of mesopores, Vmes- The values of, A, Aexh Vmes for all the samples are collected in Table 1. The values of mesopores average diameter are shown in Table 1. One can observe insignificant decrease in average diameter of mesopores with Al loading. Taking into account a slight increase of the values of unit cell parameter it is possible to consider a slight increase of pore wall thickness around 3 nm that correlates well with HREM data. Table 1 Textural and structural characteristics of ammonia chloride washed samples Specific surface area, m /g B o c 8 Total External h E 8 < < > 00
E T3
1 3 5 7 9 11
7.58 7.43 7.15 6.79 7.07 7.10
0.009 0.036 0.072 0.108 0.144 0.180
0.009 0.031 0.050 0.074 0.067 0.072
12.0 12.1 11.9 12.1 12.3 12.3
0.15 0.16 0.21 0.20 0.19 0.19
0.75 0.71 0.71 0.70 0.94 1.00
687 666 677 726 734 741
380 391 413 446 388 303
0.90 0.83 0.69 0.65 0.74 0.87
6
to that for sample 11 To estimate the mechanical strength the static pressure of 25, 50, 75 and 100 atm was applied to the sample 5. The X-ray diffraction patterns demonstrate (Fig.4) that this impact does not cause a dramatic destruction of the mesophase. The reflections become slightly broader and their intensity decreases. That possibly means that particle size decrease, but lattice constant of the mesophase has not been changed. 2C"), degs
Fig. 4. XRD patterns of the Sample 6 after impact of various pressures.
This work was supported by RFBR (Grants 01-03-32711, 01-03-32391) and INTAS (Project 2283), CRDF (Rec-008).
REFERENCES 1. Yue, Y.-H. et al, Chem. Comm., 1999, 1967. 2. Kim, J.M.; Stuchy, G.D., Chem. Comm., 2000, 1159. 3. Gregg,S.J.; Sing, K.S.W. Adsorption, Surface Area and Porosity,2nd ed.; Academic Press: London, 1982. 4. Kamaukhov, A.P.; Fenelonov, V.B.; Gavrilov, V.Yu. Pure Appl Chem. 1989, (57, 1913.5. Fenelonov, V.B.; Romannikov, V.N.; Derevyankin, A. Yu. Microporous Mesoporous Mater. 1999, 28, 57. 5. Fenelonov, V.B.; Romannikov, V.N.; Derevyankin, A. Yu. Microporous Mesoporous Mater. 1999, 28, 57.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Fabrication of nanostructured SiC and BN from templated preceramic polymers ' In-Kyung Sung, ^Taek-Su Kim,^Suk-Bon Yoon, %ng-Sung Yu and ^Dong-Pyo Kim* ^ Department of Fine Chemical Engineering & Chemistry, Chungnam National University, Taejon305-764, Korea. FAX:+82-42-823-6665 E-mail: dpkim(a)cnu.ac.kr ^Department of Chemistry and Institute of Infor-Bio-Nano Materials, Hannam University, Taejon, 306-791, Korea. E-mail: jsyu@mail.hannam.ac.kr Macroporous SiC and BN with a different degree of ordered pore arrays were prepared by infiltrating liquid precursors into sacrificial crystalline array of monodisperse colloidal silica spheres in range 70~500nm. The polymethylsilane and polycarbosilane were used as the SiC precursors, and borazine was used as the BN precursor The silica template was subsequently etched off after pyrolysis in an argon atmosphere at 1200-1300 °C. An alternative nanotublar morphology of stoichiometric SiC was produced using alumina membranes as templates. 1. INTRODUCTION The polymeric route to non-oxide ceramics via wet fabrication and subsequent pyrolysis offers a unique opportunity for manufacturing ceramic components such as fibers, coatings, and composites[l]. The SiC compounds derived from various polysilanes have been investigated for a variety of high temperature applications, while boron nitride (BN) prepared from the pyrolysis of polyborazine has useful properties such as low density, high thermal conductivity, exceptional strength and high chemical inertness at very high temperatures[2]. On the other hand, only a few reports devoted to the synthesis of porous non-oxide ceramic frameworks are available[3]. It has been recently demonstrated that silicon oxycarbidc ceramic foams can be produced from mixtures of fillers and polyorganosiloxanes[4]. However, there are no reports on non-oxide ceramics with a regular pore array prepared from liquid preceramic polymers. Our recent discovery, which is a simple, straightforward method for generating well-aligned, monodispersed macroporous SiC based on the use of a colloidal silica templating technique in combination with preceramic polymers, appears quite promising[5]. In this paper, the developed process is shown to offer a great versatility in terms of the possible shapes and morphologies, and allows the synthesis of nanostructured materials with a tailored composition and microstructure, which depend primarily on the chemical natures of their polymeric precursors. 2. EXPERIMENTAL As shown in Table 1, monodisperse silica spheres, 70nm, lOOnm and 220nm in diameter, were synthesized using a previously reported procedure[6], and commercial 500nm silica sphere powder (Lancaster 16985) were used to prepare the templates. A commercial alumina membrane (Whatman Anopore filters) with a 60|im thickness and a lOOnm or 200nm pore size was used as a template for the nanotublar stoichiometric SiC (ns-SiC). In order to modify the silica sphere surface to hydrophobic, a mixture of pure hexamethyldisilazane
548
{HMDS([Si(CH3)2]2NH)} and the silica was stirred for 5 hours at 150°C[7]. The closedpacked silica templates were fabricated using a simple sedimentation method as previously reported[8], which was performed in absolute ethanol for the non-modified silica. However, purified normal hexane was used for the surface modified silica. As summarized at Table 1, Table 1 Materials used as templates and precursors Template
Sphere & Pore size
Colloid Si09 AI2O3 membrane
70, 100,220, 500nm lOOnm. 200nm
Polymer (Molecular Weight: Mn) PMS(700), PCSf990), PBN(2019) stoichiometric precursor (PMS : PCS = 1 : 0.23)
polymethylsilane (PMS); -[Si(H)(CH3)]n-, was used as a SiC precursor and polyborazine was used as a BN precursor. These were synthesized using methods reported elsewhere[9, 10]. In addition, polycarbosilane (PCS) ; -[Si(H)(CH3)CH2]n-, which was synthesized as previously reported, was used as an alternative SiC precursor[9]. Each 30 wt% solution of PMS or PCS for porous SiC, and a 50% solution of a PMS and PCS mixture (1:0.23 in weight ratio) for nsSiC, was dissolved in anhydrous tetrahydrofuran. The dried silica templates and the alumina membranes were dipped individually in the solutions to fill the void spaces of the templates under a nitrogen atmosphere at room temperature for 24 hr. However, the silica template was infiltrated by the borazine monomer, which was subsequently polymerized at 70''C for 50hr. Prior to pyrolysis, the infiltrated polymer-silica and polymer-alumina composites were cured at 160"C for 6 hr in a mechanical vacuum after evaporating the volatile solvents and unreacted monomer[9]. The SiC preceramic polymer-silica composite was finally pyrolyzed at 1200"C for 1 hr in an argon atmosphere at a heating rate of 10*^C/min (in particular, 5''C/min in the temperature range of 2 0 0 - 600"C due to the characteristic weight loss), while the polyborazine-silica composite was pyrolyzed at 1300"C at a heating rate of 4"C/min in the same atmosphere. The pyrolyzed composites were dissolved in 48% aqueous HF to remove either the silica or the alumina templates. The resulting porous SiC, BN and ns-SiC were washed with distilled water and dried at 100"C for 6 hr. The properties of the porous ceramics were analyzed using Scanning Electron Microscopy (SEM, LE01455VP), Thermal Gravimetric Analysis (TGA2050, TA Instrument), N2 adsorption (BET ASAP 2400, Micromeritics, and Gel Permeation Chromatography (Waters 2690). 3. RESULTS In order to control the pore size in porous materials, silica spheres with various diameters 500nm, 220nm, lOOnm and 70nm were used. Fig 1 shows SEM images of the porous ceramics formed using the templates. The pore sizes were approximately proportional to the size of the silica spheres used. However, the extent of the ordered macropore array became lower with as the silica sphere diameter decreased. Macroporous SiC from the PMS precursor and 500nm silica template exhibited the highest homogeneous regularity even by a single infiltration. The image definitely shows a highly ordered pore structure with a pores size of approximately 350nm, indicating a 30% shrinkage during the pyrolysis process when compared to the 500nm size of the original silica sphere. Moreover, small windows were observed in the macroporous SiC from the 500nm and 220nm silica template and PMS, which corresponded to the contact points between the initial neighboring silica spheres. In contrast, the SiC product obtained from the lOOnm and 70nm silica template displayed a worm-like
549
morphology without an ordered pore array. On the other hand, the pore morphology depended on the types of precursors used. The PMS-derived porous SiC produced a better pore array than the PCS-derived SiC. This suggests that the degree of infiltration is related to the molecular weight of the precursor. Besides, the BN products did not develop the fine replica with regular porosity, presumably due to the high sensitivity of the borazine monomer against moisture adsorbed on the inner wall of the hydrophilic silica. When the surface was modified to a hydrophobic surface with HMDS prior to infiltration, the pore morphology was barely Table 2 Comparative thermal stabilities of porous products (in dry air) Templates Product Precursor Weight change at 600°C at 1000°C Porous SiC ns-SiC
500nm SiOz Sm-500nm Si02 500nm SiOz alumina membrane (200nm pore)
PMS PMS PCS PMS/PCS = 1/0.23
+7%, +0.5%, -11%, -0.7%
+8% +4% -7% -2%
Sm; surface modified + ; weight gain, -; weight loss
Fig. 1. SEM images of nanostructured ceramics prepared from a) PMS and 500nm silica spheres, b) PMS and 220nm silica spheres, c) PMS and 70nm silica spheres, d) stoichiometric SiC precursor and alumina membrane, e) borazine and 500nm surface modified silica spheres. observable, as shown in Fig. 1. This suggests that surface modified hydrophobic silica prevented the decomposition of the borazine monomer due to less adsorption of moisture than the non-modified silica. Therefore, in order to avoid the initial decomposition, alternative methods are currently under investigation. In particular, an alumina membrane template was infiltrated with a stoichiometric SiC precursor of PMS and PCS mixture, dissolved at a weight ratio of 1 : 0.23[11]. As shown in Fig. 1, the morphology appeared to be mixture of nanofibers and hollow nanotubes, with a 200nm diameter and 60|am length. Presently, the conditions used to prepare homogeneous nanofibers or nanotubes are under investigation. It appears that the high stability of the non-oxide nanostructured ceramics is greatly advantageous for practical applications under high temperature and chemically harsh conditions, compared to the developed oxide and carbonaceous porous materials. Table. 2 summarizes the thermal stability of the products. The PMS-derived porous SiC showed a gradual increase in weight from 400°C due to oxidation of excess Si and SiC, reaching 8 wt.%
550
gain at 1000°C, while the PCS-derived SiC showed initial 11 wt.% loss by excess carbon below 600°C, and a subsequent gain of 4 wt.% at 1000°C. The ns-SiC exhibited a better thermal stability with a loss of ca. 2 wt.% at 1000°C. This is consistent with other report results showing that stoichiometric SiC had a higher oxidation resistance than nonstoichiometric SiC[l 1]. I should be pointed out that porous carbon was completely burned off at 500°C in air. In addition, it is interesting that the use of the organically modified templates had a positive influenced on the thermal stability of the final products. Macroporous SiC from the surface modified silica templates showed a lower weight gain at high temperature oxidation conditions than the non-modified templates. This suggests that the exterior carbon layer may protect the inner SiC from the chemical attack of HF, reducing the formation of active sites for oxidation. Table 3 shows the pore characteristics of the macroporous SiC from the 500nm, lOOnm and 70nm silica template measured by the nitrogen adsorption isotherms. The BET surface areas of the products became smaller from 150-172 m^/g with a total pore volume of 0.24-0.26 Table 3 Pore characteristics of macroporous SiC Surface area Silica sphere size m'/g 70nm lOOnm 500nm
25.53 40.80 150-172
Pore diameter nm ca. 50, 1-3 ca. 80. ca. 4 ca. 350, ca 5
Pore volume cm^/g 0.05 0.18 0.24-0.26
cm^/g to 25 m'^/g with a total pore volume of 0.05 cm'^/g as the diameter of silica spheres decreased from 500nm to 70nm. This is a comparable order of porosity to the 100 m^/g of polysiloxane-derived SiOC foams annealed at 1000°C[4]. Furthermore, bimodal pore structures were commonly observed with macroporous SiC with 350nm, 90nm and 50nm pores surrounded by microporous walls with narrow size distributions of 5nm, 4nm and l~3nm, respectively. This might have been caused by pyrolysis of the polymer framework. ACKNOWLEDGEMENT D.-P. Kim and J.-S. Yu acknowledge the Korea Research Foundation Grant ( KRF-2001-005-E00033) and the Korea Research Foundation (KRF-2001-041-D00177) for financial support, respectively.
REFERENCES l.C.K. Narula, Ceramic Precursor Technology and Its Applications, Marcel Dekker, New York, 1995. 2. A. W. Weimer, Carbide, Nitride and Boride Materials Synthesis and Processing, Chapman & Hall, London, UK, 1997. 3. (a) W. Schnick, J. Lucke, Angew. Chem. Int. Ed., 31 (1992) 213. (b) H. Huppertz, W. Schnick, Angew. Chem. Int. Ed., 36 (1997) 2651. 4. H. Schmidt, D. Koch, G. Grathwohl, J. Am. Ceram. Soc, 84 (2001) 2252. 5. I. K. Sung, S. B. Yoon, J. S. Yu, D. P. Kim, ChemCommun., in press. 6. E. Bohn, J. Colloid Inter. Sci., 26 (1968) 62. 7. N. L. Wu, S. Y. Wang, I. A. Rusakova, Science, 285 (1999)1375 8. J. S. Yu, S. B. Yoon, G S. Chai, Carbon, 39(2001) 1421 9. D. P Kim, J. Mat. Sci. Let., 19 (2000) 303 10. P J. Fazen, J. S. Beck, A. T. Lynch, E. Remsen, L. G Sneddon, Chem. Mater. 2 (1990) 96 11. F. Cao, D.-P Kim, X. Li., J. Mat. Chem., 12 (2002) 606
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
551
Mesoporous solids for green chemistry James H Clark Clean Technology Centre, University of York, YOlO 5DD, UK 1. INTRODUCTION Mesoporous solids have widespread potential for green chemistry including catalysis, reagents, trapping and transport. We report a number of chemically modified porous materials including those based on silica, acid-treated clays, zirconia and expanded starches along with illustrative examples of their application. Economic, societal and legislative pressures are forcing the chemical and allied industries to reconsider the way they have practised chemistry in the last century'. Chemical processes must be more efficient and less wasteful and chemical products must be less harmful to the environment as well as to life. One key area of clean technology is the replacement of hazardous acids, bases, metal and other hazardous traditional reagents especially for use in liquid phase organic reactions widely used in the speciality chemical and pharmaceutical industries. We have developed a series of mesoporous solids with surfaces designed to provide the activity appropriate for many reactions and structures designed to enable fast throughput of many organic molecules. Successful applications include acid-catalysed rearrangements and oligomerisations, partial oxidations including those of steroids, immobilised palladium and solid basecatalysed C-C bond forming reactions." 2. SOLID ACIDS rhc widespread importance of acid catalysis makes it the primary target for developing more environmentally acceptable processes. Better control of the type and strength of the acid sites as well as facilitating the separation of the acid at the end of the process are necessary improvements. Solid acids offer solutions in both these areas as well as being advantageous in terms of storage and handling. We have developed mesoporous solid Lewis Bronsted and mixed Lewis-Bronsted acid processes for important areas including alkylations, acylations, rearrangements, oligomerisations and polymerisations. Most recently we have shown how supported aluminium chloride can be used to give both better activity and control in the polymerisation of aliphatic and aromatic alkenes. ' (Figure 1)
Monomers ^•3-
^"^q_n^^ ^
^
^ ^ ^^
O ; ' .OAfCL ^g.
2
*"
Controlled Molecular Weight Polymers
Fig. 1. Use of supported aluminium chloride for the controlled polymerisation of unsaturated hydrocarbons.
552
Applications for this chemistry include the manufacture of hydrocarbon resins. These are used in areas such as adhesives, printing inks, paints and coatings. They are produced via the cationic polymerisation of monomer feedstreams which are often refinery mixtures of various unsaturated as well as saturated compounds. Current manufacturing processes use soluble Lewis acids notably AICI3 and BF3 with all the associated handling difficulties and waste problems resulting from the aqueous quench production stage. Clean technology in chemicals manufacturing will often require a combination of clever chemistry and innovative engineering. The successful uptake of solid catalysts by the speciality chemical manufacturing industries may well require more than traditional batch reactors. The greater fiexibility of more continuous reactors with 'just-in-time' manufacturing that reduces the need for the storage of large volumes of chemicals is another feature of greener chemical cz IM ^^^ manufacturing. In an exciting extension of our research on solid acids we have successfully developed the first exampl Fig. 2. SDR Reactor By using a mesoporous silica-supported zinc trifiate^, for example, fixed to a metal spinning disc we have been able to carry out the ultrafast rearrangement of a-pincnc oxide to campholenic aldehyde with a selectivity not greatly less than that obtained in the commercial process using homogeneous zinc halide, but with vastly improved reaction times and catalyst lifetimes (figure 3).^
silica-Zn2* CSDR Fig. 3. The rearrangement of a-pinene oxide to campholenic aldehyde using a catalytic spinning disc reactor. Conversion levels are unaffected over at least 15 runs demonstrating the excellent durability of the CSDR. Conversion notes and reaction selectivities are affected by fiow rates and disc speed so that the reactor conditions can be adjusted to suit the application. We can compare the best CSDR results to typical batch results: Table 1 Comparison of CSDR to typical Approximate conversion rate kg/h CSDR 170 Batch 1.2
batch results Selectivity % 53 64
553
Other mesoporous support materials that we have successfully chemically modified and used as catalysts in liquid phase organic reactions include acid-treated clays (e.g. for immobilisation of aluminium chloride^) and zirconia. Sulfated mesoporous zirconia is a powerful solid acid catalysis for some liquid phase Friedel-Crafts reactions notably alkylations using alkenes^, benzoylations using benzyl chloride^, and most recently some Fries rearrangements (Figure 4).
mesoporous sulfated zirconia
Fig. 4. Fries rearrangement of benzoates to hydroxybenzophenones using mesoporous sulfated zirconia Using conventional Lewis acids such as AICI3 the destructive separation of the inorganics leads to large volumes of hazardous waste and results in 'catalyst' turnover numbers of <1. In our heterogeneous reaction, we can achieve turnover numbers of >1 and the catalyst is easily recoverable and reusable with little loss in activity. 3. IMMOBILISED METAL COMPLEXES Metal-catalysed reactions are also diverse and widely used in all areas of chemical manufacturing. The prohibitive costs of waste remediation coupled with the high value of many metallic catalysts will increasingly make the loss of metal in any significant amount, unacceptable. The ability to chemically modify the surfaces of many high surface area support materials means that quite complex surface architectures with, for example, metal complexing abilities, can be built up. The low dimensionality of most of these surfaces means that it is relatively easy to combine strong surface interactions with weakly interacting non-surface bound ligands (e.g. solvent molecules). In this way, we can aim to combine catalyst stability with the coordinative unsaturation necessary for catalytic activity. Two examples of this which we have recently demonstrated are supported palladium for Heck and Suzuki reactions'"'" and supported cobalt for oxidation reactions including the selective allylic oxidation of steroids '^' '^ (Figure 5). The steroidal oxidations are a good illustration of the value of this approach. Traditional methods commonly involve highly toxic reagents or catalysts and almost inevitably lead to hazardous waste. Organic-inorganic hybrids based on mesoporous silica can be designed to have metal complexing surfaces of controllable binding ability and with sufficient coordinative unsaturation to allow substrates to bind to and be activated by the immobilised metal centres. Quite simple materials such as shown in Figure 6 will bind Cor\ Mn^^ and V^' sufficiently well to prevent leaching in quite polar solvents such as MeCN.
554
Fig. 5. Some organic reactions catalysed by metal complexes immobilised on mesoporous solids
I \
^O—Si-
-(CH,
O
O Me
h-0 Fig. 6. Complexation of metal ions using chemically modified mesoporous silica rhese are good catalysts for the allylic oxidation of steroids ^BuOOH. After reaction, the materials can be simply filtered off (when used in stirred tank reactors) and then used again with no significant loss in activity . 4. EXPANDED STARCHES There is a growing momentum in the scientific and technological communities to make more use of renewable biomass. In certain sectors, such as lubricants, the preparation of products derived from renewable feedstocks is becoming significant (5% in much of Western Europe). Apart from destruction of the biomass to generate useful
555
chemicals, such as ethanol and lactic acid, it is also possible to use the bulk materials as substitutes for more conventional materials. We have found that a quite simple waterbased treatment of cornstarch leads to a massive expansion of the starch structure making it highly suitable for use as a porous adsorbent, base material for polymer composites or catalyst support (figures 7 and 8).''^ Heating in Water Swelling
Dry in"
Collapse & Dispersion
Aggregation
O-'Or-i Starch granule
Retrogradation
Fig. 7. Expansion of Starch Granules
expansion
surface area 2m*'/g
surface area > 1 OOm'^g
Fig. 8. Expansion of Corn Starch We are now working on the chemical modification and subsequent application of these very promising materials. We are also expanding our research in the area of natural mesoporous materials to vegetable-derived carbons and cellulose. ACKNOWLEDGEMENTS The work described here is due to the excellent efforts of many researchers whom 1 thank. 1 am particularly grateful to my colleagues in the Green Chemistry Group at York and to the various organisations who have financially supported the research, particularly the UK Engineering and Physical Sciences Research Council. REFERENCE Green Chemistry and Technology, J.H. Clark and D.J. Macquarrie eds, Blackwcll Science, Abingdon, 2002 See for example P.M. Price, J.H. Clark and D.J. Macquarrie, J. Chem Soc. Dalton Trans, 2000, 101 (perspective) J.H Clark, Ace. Chem. Res., 2002, in the press J.H. Clark, K. Shorrock, V. Budarin and K. Wilson, J.Chem Soc. Dalton Trans, 2002,423 J.H. Clark, D.J. Macquarrie and V. Sage, unpublished results
556
6. K. Wilson, A. Renson and J. H. Clark, Catal. Lett., 1999, 61, 51 7. M. Vicevic, K. Wilson, Chem. Eng., 2002, in press 8. J. H. Clark, G. L. Monks, D. J. Nightingale, P. M. Price, and J. F. White, Catalysis of Organic Reactions, Marcel Dekker, New York, 2000, 135 9. J. H. Clark, G. L. Monks, D. J. Nightingale, P. M. Price, and J. F. White, J. Catal., 2000,193, 348 10. E.M. Mubofu, J.H. Clark and D. J. Macquarrie, Green Chem., 2001, 3, 23 11. J.H. Clark, D.J. Macquarrie and E.B Mubofu, Green Chem., 2000, 2, 53 12. J. Salvador and J. H. Clark, Chem. Commun., 2001, 33 13. J. Salvador and J. H. Clark, Green Chem., 2002, in the press 14. J. H. Clark, S. Doi, D. J. Macquarrie and K. Milkowski, unpublished results and UK Patent Application (2002)
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
557
Ultrastable acidic MCM-48-S assembled from zeolite seeds Pei-Chun Shih^'", Hong-Ping Lin^and Chung-Yuan Mou'* ^Deh Yu College of Nursing and Management, Keelung, Taiwan 203 ^Institute of Chemistry, Academia Sinica, Taipei, Taiwan 115 "^Department of Chemistry and Center of Condensed Matter Science, National Taiwan University, Taipei, Taiwan 106 Highly ordered Ultrastable Acidic Mesoporous aluminosilicates (MCM-48-S) with cubic structure was synthesized from assembly of cetyltrimethylammonium bromide (CTAB) and pre-formed zeolite Beta seed. The materials retain its structure after in boiling water for 10 days. 1. INTRODUCTION Mesoporous molecular sieves such as hexagonally ordered MCM-41 and cubic structure MCM-48 have attracted much attention because of their potential use as versatile catalysts and catalyst supports, especially large molecules [1]. Compared with conventional zeolites, MCM-41 and MCM-48 have weaker acidity and much less hydrothermal stability. This limits their catalytic applications. The relatively low acidity of mesoporous materials such as MCM-41, as compared to zeolites, can be attributed to the amorphous nature of the pore walls. Recently, two research groups succeeded in synthesizing mesoporous aluminosilicates with high acidity and hydrothermal stability [2-4]. They used quaternary ammonium such as TEACH to separately develop zeolitic nanocluster as silica precursor to make the mesoporous materials with cationic surfactant. The success relies on formation of better-structured wall with pre-formed zeolitic nanocluster. The mesotructures reported for this type of stable acidic mesoporous materials are hexagonal [2-4], cellular foam [5], and large-pore hexagonal [6]. However, the corresponding three-dimensional bicontinuous cubic (laScI) structure of MCM-48, has not been reported yet. By using two-steps procedure we have successfully synthesized a new strongly acidic and highly hydrothermal stable mesoporous material with laSd cubic structure. The first step was the preparation of zeolite seeds that were produced by using tetraethylammonium hydroxide (TEACH) as the template and mixing sodium hydroxide, sodium aluminate, fumed silica and water at 100°C for 5-18 h. The second step was to synthesize cubic aluminosilicate mesporous product by the reaction of zeolite seeds and cetyltrimethylammonium bromide (CTAB) solution at 150°C for 6-48 h. We successfully synthesize the final product with molar ratio of Si/Al from 37 to 60. The XRD and HRTEM showed that the mesoporous material possesses cubic (laSd)
558
mesostructures. The products showed high hydrothermal stabiHty (in boihng water over 10 days) and high acidity (NH3-TPD-MASS over 500°C). The properties of products present that are emerging as a new usefully acidic catalyst to catalyze petroleum refme reactions. 2. EXPERIMENATL 2.1. Synthesis Zeolite seeds were prepared by mixing NaAI02(Riedel-de Haen), NaOH (Shimakyu's Pure Chemicals, Japan), fumed silica (Sigma), TEAOH aqueous solution (20%) (Acros), and water under stirring at 50°C for 2-5 h, then transferring the gel-solution into the autoclave at 100 °C oven for 18 h, and then will get a clear solution. MCM-48-S mesoporous were synthesized by the reaction of zeolite seeds with cetyltrimethylammonium bromide solution (CTAB, Acros) at 150°C for 6-48 h. Samples were then recoveryed by filtration, washed with water, dried at 100°C, and calcined at 580°C for 6 h. The molar ratios of reactants NaAlOz: SiOz: NaOH: TEAOH: C16TMAB: H2O were 1:37-67:1.5-9:11-22:18.3:3000-3500. The proton form were prepared by ion exchange of 0.1 M NH4CI, followed by calcination at 500°C for2h. 2.2. Characterization The powdered X-ray diffraction patterns were recorded on a Scintag XI diffractometer using Cu Ka (>t=0.154 nm) radiation. N2 adsorption-desorption isotherms were obtained on a Micromeritics ASAP 2010 sorptometer at 77 K. The infrared spectra were measured on MAGNA-IR 500 spectrometer in the range of 400-1000 cm' with a resolution of 2cm'. The NH3-TPD-MASS curves were determined in the range 110-800"C at a temperature-increasing rate of 10"C/min on a AutoChem 2910 and Thermo ONIX ProLab system. HRTEM micrographs were taken with Philips CM200 Microscope. 3. RESULT AND DISCUSSION 3.1. X-ray diffraction patterns By controlling the right pH value range (-11.2) and reaction temperature, we can successfully synthesize the MCM-48-S with Si/Al ratio between 40 and 60(Fig. 1.). They cannot get good structures when the aluminate's content is too much. The XRD patterns (Fig. 2.) show the MCM-48-S has well-ordered cubic {laSd) mesostructures, even treated in boiling water for 10 days or after calcination at 900"Cfor 6 h they show only limited decay of structure. 3.2. N2 adsorption-desorption isotherm BET surface area and pore volume (Table 1) shows a 30-40% reduction after 5-days hydrothermal reaction and keeps nearly the same to 10 days. According to Schumacher's method [7], for calculating the wall thickness of MCM-48-S we found the wall thickness of MCM-48-S is about 10 A close to typical MCM-48. It indicates that the gained stability is due to a stronger rather than thicker wall. Table 1 lists the physical properties of the calcined products (Si/Al ratio is 60) that treated in boiling water for different time.
559
800
29/degree
29/degree
Fig. 1. The XRD patterns of samples with different Si/Al ratio.
700
600
500
400
Wavenumber (cm')
Fig. 2. The XRD patterms of calcined MCM-48-S with different times in boiling water (Si/Al=60).
Fig. 3. Infra-Red spectrum of calcined MCM-48-S (Si/Al=50).
Table 1 Comparison of physical characteristics of calcined products (Si/Al ratio is 60) with different rehydrothermal time (A) non rehydrothermal, (B) 24 h, (C) 48 h, (D) 5 days, (E) 10 days, (F) MCM-48 Sample
Surface area (mVg)
A
1312
B
Pore volume (cm^/g)
d-spacing
(A)
Ao(A)
h(A)
22
0.97
34.76
85.14
8.25
1213
19
0.81
33.70
82.54
8.60
C
1206
18.5
0.80
34.22
83.82
8.95
D
959
15
0.57
34.22
83.82
9.8
E
931
15
0.58
33.96
83.18
9.7
F
1322
23.5
0.87
34.22
83.82
8.70
Pore size
(A)
3.3. IR spectroscopy We further took an IR spectrum (Fig. 3) of MCM-48-S. An absorption shoulder at 550 cm"' was observed indicating the presence of five-member ring structure for the siloxanc connections. 3.4. Transmission electron microscopy The HRTEM image (Fig. 4) of the calcined samples shows excellent periodic structure. The distance between the pores is in good agreement with that determined from XRD pattern.
560
3.5. Acidity Measuring from the NH3-TPD-MASS curves of the H-form MCM-48-S (Fig. 5.), NHs-desorption lasts until a fairly high temperature of 500 °C which is much higher than that of typical MCM-48 at 320 °C. Similar to the desorption temperature of the acidic HZSM-5, we thus have a fairly acidic mesoporous aluminosilicates in MCM-48-S. The combination of 3-D interconnecting channel system, strong acidity and highly hydrothermal stability will be useful in many catalytic applications.
200
Fig. 4. High Resolution Transmission Electron Microscopy (HRTEM) image of calcined MCM-48-S. (Si/Al=60).
300 400 500 600 700
Temperature/ C
Fig. 5. The NH3-TPD-MASS curves of HMCM-48-S (Si/AH37).
4. CONCLUSIONS The MCM-48-S has been synthesized by the reaction of CTAB solution and zeolite seed. By the characterization of XRD and NH3-TPD-MASS, we prove the material possesses highly hydrothermal stability and strong acidity. In future, it would be very useful for catalytic reaction. REFERENCES Kresge, C.T., Leonowicz, M.E., Roth, W. J., Vartuli, J.C, Beck, J. S., Nature 359 (1992) 710. Y. Liu, W. Zhang, and T J. Pinnavaia, T. J., J. Am. Chem. Soc. 122 (2000) 8791. Z. Zhang, Y. Han, L. Zhu, R. Wang, Y. Yu, S. Qiu, D. Zhao, and F. S. Xiao, Angew. Chem. Int. Ed. 40(2001) 1258. Z. Zhang, Y. Han, F. S. Xiao, S. Qiu, L. Zhu, R. Wang, Y Yu, Z. Zhang, B. Zou, Y Wang, H. Sun, D. Zhao, and Y. Wei, J. Am. Chem. Soc. 123 (2001) 5014. Y Liu, and T. J. Pinnavaia, Chem. Mater. 14 (2002) 3. Y Han, F. S. Xiao, S. Wu, Y Sun, X. Meng, D. Li, and S. Lin, Phys. Chem. B 105 (2001) 7963 K. Schumacher, P. I. Ravikovitch, A. Du Chesne, A. V. Neimark and K. K. linger, Langmuir, 16(2000)4648.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Acidic zeolite coated mesoporous aluminosilicates D. Trong On and S. Kaliaguine Department of Chemical Engineering, Laval University, Ste-Foy, Quebec G I K 7P4, Canada, A new approach to the synthesis of unusual zeolite coated mesoporous aluminosilicate (ZCMAS) using a diluted clear gel solution containing primary zeolite units is reported. Hydrothermally ultra-stable and highly acidic ZCMAS are achieved due to the nanocrystalline zeolitic nature of their pore wall surface, which opens up new opportunities for the use of these materials as high temperature acid catalysts. 1. INTRODUCTION Because of their amorphous wall character, the acidity and hydrothermal stability of mesoporous aluminosilicates (MAS) are relatively low compared to those of zeolites, which limits their potential applications as catalysts.' One might expect to improve both the stability and acidity of these materials if zeolite-like order could be introduced into the mesopore walls. The use of zeolite seeds as precursors for the assembly of mesoporous aluminosilicates was reported.^ Furthermore, recent results from our group showed the preparation of a new type of materials (UL-zeolites) with semi-crystalline zeolitic mesopore walls. The results indicated that nanocrystals were embedded in the continuous amorphous inorganic matrix to form semicrystalline wall structures while preserving the mesoporous structure. Herein, we describe another approach namely the production of zeolite coated mesoporous aluminosilicates (ZCMAS) using diluted clear solutions containing primary zeolite units."* 2. EXPERIMENTAL The synthesis of zeolite coated mesoporous materials involves two steps: The first step consists of the preparation of the MAS precursors such as SBA-15 ^ or a mesocellular foam (MCF) ^ and the desired clear zeolite gel containing primary nanocrystal units. The second step is the coating of zeolite nanocrystals on the MAS surface using the diluted clear zeolite gel. The calcined mesoporous precursors after contacting the diluted ZSM-5 gel under vigorous stirring at room temperature for 1 h were filtered, washed with distilled water and dried at 80°C. The resulting materials were subsequently suspended in glycerol and then transferred into a Teflon lined autoclave and heated at 130°C for 24 hours. Finally the solid product was filtered, washed with distilled water, dried at 80°C and calcined at 550°C. The materials were characterized using BET, FTIR, and TEM. Solid-state ^^Al and ^^Si MAS NMR spectra were recorded at room temperature using a Bruker ASX 300 spectrometer.
562
3. RESULTS AND DISCUSSION It is of special concern that due to the size of primary ZSM-5 units templated by tetrapropylammonium ions (28 A in diameter), the pore diameter of the mesoporous precursor molecular sieves should be higher than 30 A. Mesoporous aluminosilicate precursors, such as SBA15 ^ and meso-cellular foam (MCF) ^ were used in this context. The N2 adsorption/desorption isotherms of the SBA-15 aluminosilicate presursor and ZSM-5 coated SBA15 exhibit the typical behavior of a mesoporous molecular sieve with a mesopore volume of-1.56 and 0.78 cmVg (BJH surface area: 800 and 465 m^/g), respectively (Fig. 1). Further, a significant decrease in pore diameter (from 70 to 54 A) and a narrower pore diameter distribution of the coated sample compared to that of the parent sample could conceivably be ascribed to the ZSM-5 nanocrystals coated inside the mesopore channels of the host. Similar results were also obtained for a MCF aluminosilicate precursor before and after coating (Fig. 2). The mesopore volume and the pore diameter decrease from 2.4 to 0.7 cm^/g and 315 to 175 A, (surface area from 875 to 435 m^/g), respectively.
k
.—7^
I
XV
s
a
Fig. 1. N2 sorption isotherms and BJH pore diameter distributions from the desorption branch a) SBA-15 and b) ZSM-5 coated SBA15.
^
Fig. 2. N2 sorption isotherms and BdBFHH pore diameter distributions from the adsorption branch a) MCF and b) ZSM-5 coated MCF.
The ZSM-5 coated samples of both SBA-15 and MCF show a FTIR absorption band at 550 cm'\ which is essentially not present in the parent samples. The band around 550 cm"' is characteristic of five-membered ring units in pentasils indicating that ZSM-5 nanocrystals are present within the mesopore walls. The acidity of the materials after coating is enhanced. The FTIR spectra of adsorbed pyridine on the parents, ZSM-5 coated samples in H-form and on H-ZSM-5 with almost the same Al content indicate that strong Bronsted and Lewis acid sites are created in the ZSM-5 coated samples. These sites are only slightly weaker than in ZSM-5 itself showing however more Lewis sites (not shown). The order of the acid strength is as follows: the parent samples « ZSM-5 coated MCF < ZSM-5 coated SBA-15 < ZSM-5.
563
0
25
50
75
100
125
150
175 200
Pore diameter (A)
Fig. 3. Si^^ MAS NMR spectra a) parent and b) ZSM-5 coated SBA-15 samples.
Fig. 4. BJH pore size distributions of ZSM-5 coated SBA-15a) before and b) after steaming.
Table 1 Physico-chemical properties of the parent mesoporous aluminosilicates (SBA15) and ZSM-5 coated SBA15 (ZC-SBA15) samples before and after hydrothermal treatments. N" Materials Treatment SBIT SBJH Mesop. Vol. BJH pore time (days) (m^/g) (m^/g) (cm^/g) diameter (A) Boiling water at 100"C SBA15-0-W* 1080 1 0 70 800 1.56 SBA15-2-W 415 375 1.72 2 120 2 465 0.78 ZC-SBA15-0-W 495 52 0 3 485 0.85 ZC-SBA15-2-W 55 475 2 4 58 495 1.35 ZC-SBA15-5-W 485 5 5 Steaming of 20% vapor water in N2 at 800"C ZC-SBA15-1-S 1 445 400
0.70
53
* SBA15-x-y where: x treatment time in days, y: boiling water (W) or steaming (S) treatment
The ^^Si MAS NMR spectrum of the parent SBA-15 sample (atomic Si/Al= 65/1) shows a ^'^Si MAS NMR spectrum typical of mesoporous alumosilicates, which contains four main features (Fig. 3). Two main resonances at -112 and -100 ppm and a weak peak at -92 ppm correspond to Si(0Si)4 (Q'*), (H0)Si(0Si)3 (Q^), (HO)2Si(OSi)2 (Q^) silicate species, respectively; a shoulder at -105 ppm has been assigned to (A10)iSi(0Si)3 species due to the tetrahedral aluminum structure. However, the ^^Si MAS NMR spectrum of the ZSM-5 coated SBA-15 sample (atomic Si/Al= 50/1) shows a main resonance centered at -112 ppm, which is attributed to Q'* silicon of the silicalite framework and a shoulder (A10)iSi(0Si)3 band at -105 ppm (Fig. 3). Only a weak resonance attributable to Q^ silicon from surface hydroxyl groups is observed at ~100 ppm. The increase in intensity of the Q"* resonance and
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concomitant decrease in intensity of the Q"^ and Q^ resonances reflect the transformation of the hydrophiHc surface of the precursor into a more hydrophobic one upon coating. The same trend was also observed for the MCF aluminosilicate sample before and coating (not shown). The hydrothermal stability in boiling water at 100°C and steam stability with water vapor of the parent SBA-15 and ZSM-5 coated SBA-15 samples were also studied. In boiling water, the mesopore structure of the parent sample was collapsed after 2 days. However, no significant collapse of the mesopore structure was observed for the coated sample after 2 days in the same treatment conditions. Even after 5 days, the mesopore structure was still uniform. The coated sample was also steamed with 20% water vapor in N2 at 800°C (Fig. 4) and showed no essential change in the pore size distribution after one day of steaming indicating that the coated sample is hydrothermal I y ultra-stable. The remarkable hydrothermal stability of the coated sample observed here involves the zeolite seeds coated on the mesopore surface, which act to heal defect sites in the mesopore surface, and should be associated with the lowered silanol surface concentration. 4. CONCLUSION We demonstrate a general approach to coating zeolite nano-units within the mesopore structure. This synthesis methodology can be extended beyond the coating by ZSM-5 seeds, since primary units with diameters of 28 A for MFI/MEL structures, 26 A for zeolite beta, 15 A for ZSM-12 and 16 A for zeolite sodalite have been reported. They can potentially be used to prepare a large variety of zeolite coated mesoporous molecular sieves. This new type of materials has potential application as high temperature acid catalysts. REFERENCES 1. D. Trong On, D. Desplanticr-Giscard, C. Danumah, S. Kaliaguinc, Applied Catalysis A: General, im\, 222,299. 2. Y. Liu, W. Zhang, T. J. Pinnavaia, Angew. Chem. Int. Ed. 2001, 40, 1255. 3. D. Trong On and S. Kaliaguine, Angew. Chem. Int. Ed. 2001, 40, 3248. 4. D. Trong On and S. Kaliaguinc, Angew. Chem. Int. Ed. 2002, 41, 1036. 5. D. Zhao, Q. Huo, J. Feng, B. F. Chmelka, G. D. Stucky, J. Am. Chem. Soc. 1998, 120, 6024. 6. P. S. Winker, W. W. Lukens, P. Yang, D. I. Margolese, J. S. Lettow, J. Y. Ying, G. D. Stucky, Chem. Mater. 2000, 12, 686.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Stable ordered mesoporous titanosilicates with active catalytic sites Feng-Shou Xiao*, Yu Han, Xiangju Meng, Yi Yu, Miao Yang, and Shuo Wu Department of Chemistry & State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130023, China Stable ordered mesoporous titanosilicates with highly active catalytic sites (MTS-9) have been successfully synthesized from assembly of pre-formed titanosilicate precursors with polymer surfactant (PI23) in strongly acidic media. Mesoporous MTS-9 shows good hydrothermal stability in boiling water (over 120 hours). Catalytic data show that MTS-9 is very active catalyst in catalytic conversion of both small molecule of phenol and bulky molecule of 2,3,6-trimethylphenol. The MTS-9 samples were characterized with infrared, UV-Raman, and TEM techniques. The results suggest that the good hydrothermal stability of MTS-9 is attributed to thicker mesoporous wall and zeolite-like connectivity of TO4 (T=Si, Ti) in the mesostructure. The high catalytic activity of MTS-9 are due to the TS-1-like enviromcnt of the Ti species in MTS-9. 1. INTRODUCTION Since the discovery of microporous TS-1 [1], a series of microporous titanosilicates have been reported, which exhibit remarkable catalytic properties in oxidation of alkanes, epoxidation of alkenes, and hydroxylation of phenol [2,3]. However, one disadvantage of these microporous titanosilicate catalysts is that their pores are too small for access by bulky reactants. Recent progress in solving this has been the substitution of titanium ions into the silicon sites of mesoporous materials (MCM-41) [4-6]. These mesoporous titanosilicates have pore diameters of 30-200 A and exhibit catalytic properties for the oxidation of bulky reactants under mild conditions, but unfortunately, when compared with TS-1, the oxidation ability and hydrothermal stability are relatively low [7]. The low oxidiation ability and hydrothermal stability can be attributed to the amorphous nature of the mesoporous wall [7]. On the other hand, microporous crystals of zeolites are very stable, and are widely used commercial catalysts [8]. Recently, it has been reported the successful synthesis of titanosilicate nanoclusters with zeolite primary and secondary structural building units [9]. More recently, there had been great progress in the preparation of mesostructured materials assembled from nanoclusters such as mesoporous aluminosilicate nanoclusters [10,11]. In our preliminary work [12,13], we have briefly reported the synthesis of an ordered mesoporous titanosilicate (MTS-9) via self-assembly of pre-formed titanosilicate precursors with triblock copolymers in a strong acidic media. We demonstrate here that MTS-9 shows excellent
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hydrothermal stability and very high activity for the oxidation of the smaller molecules of phenol and styrene and also of the bulky molecule of 2,3,6-trimethylphenol (TMP). 2. EXPERIMENTAL The preparation of MTS-9 have been described in elsewhere [12,13]. X-ray diffraction (XRD) patterns were obtained with a Siemens D5005 diffractometer. Transmission electron microscopy (TEM) images and electron diffraction (ED) patterns were recorded on a JEOL2010FEG. The nitrogen isotherms at -196 °C were measured using a Micromeritics ASAP 2010 system. Infrared (IR) spectra of the samples were recorded on a Perkin-Elmer FT-IR spectrometer (PE 430). UV-Raman spectra were recorded on an UV-Raman spectrometer built by the State Key Laboratory for Catalysis, Dalian, China. Catalytic experiments were run in a 50 ml glass reactor and stirred with a magnetic stirrer. Phenol hydroxylation, styrene epoxidation, and hydroxylation of 2,3,6-trimethylphenol were performed at 80 °C for 4h, 45 °C for 3h, and 80 °C for 2h with molar ratio of reactant/H202 at 3/1, respectively. The products were analyzed by gas chromatography (GC-17A, Shimadzu). 3. RESULTS AND DISCUSSION 3.L X-Ray diffraction The small-angle X-ray diffraction pattern for a typical as-synthesized MTS-9 sample shows well-resolved peaks that can be indexed as (100), (110), and (200) reflections associated with the hexagonal symmetry [4,6]. The (100) peak reflects a d spacing of 112 A (a()=130A). No diffraction peak was observed in the region of higher angles 10-40°, which indicates the absence of large microporous crystals in the sample, suggesting that MTS-9 sample is a pure phase. Interestingly, after treatment of the sample in boiling water for more than 120 h, the XRD patterns still show those peaks assigned to the hexagonal symmetry, suggesting that MTS-9 is extremely hydrothermally stable, as compared to Ti-MCM-41 and SBA-15. 3.2. Transmission electron microscopy TEM image of MTS-5 exhibits ordered hexagonal arrays of mesopores with uniform pore size [4,6]. From high-dark contrast in the TEM image of the sample, the distance between mesopores is estimated to be 120 A. Furthermore, we observed that the wall thickness of MTS-9 is greater than that of SBA-15 reported in the literature. 3.3. Adsorption isotherms The N2 adsorption-desorption parameters of various samples are presented in Table 1. Notably, MTS-9 shows that the BET surface is 980 m^/g and 8.0 nm. Ti-MCM-41 and SBA-15 give the values at 1080 mVg, 2.7 nm, 870 m^/g and 7.6 nm, respectively. After treatment in boiling water, only MTS-9 keeps its structure
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Table 1 MTS-9, Ti-MCM-41, & SBA-15 samples before and after treatment in boiling water for 120 h Pores:size (nm) Wall thickness (nm) Surface area (m^/g) Samples After Before After Before After Before 8.9 4.8 4.3 980 720 8.0 MTS-9 ___ _-_ 1080 55 Ti-MCM-41 1.5 2.7 3.6 870 187 7.2 SBA-15 — — 3.4. IR spectroscopy. IR spectrum of SBA-15 shows a borad band at 460 cm', which is similar to those of amorphous materials. However, MTS-9 exhibits 550 cm'' band, which are similar to those of 5-membered rings of T-O-T (T=Si or Al) in microporous zeolites [8]. These results suggest that MTS-9 has zeolite primary and secondary building units. 3.5. UV-vis spectroscopy UV-vis spectrum of Ti-MCM-41 exhibits the relatively broad band centered at 230 nm, which is assigned to the distorted 4-coordinated Ti specie. However, MTS-9 gives at 215 nm, suggesting that both Ti species in TS-1 and MTS-9 are the same (4-coordinated) [1]. 3.6. UV-Raman spectroscopy UV-Raman spectroscopy is very sensitive to the coordination environment of titanium species. The framework titanium ions of TS-1 have been identified with UV-Raman spectroscopy by the asymmetric stretching of Ti-O-Si species, and the asymmetric stretching vibration mode of the tetrahedral framework titanium of TS-I is found to be at 1125 cm"' [14]. UV-Raman spectrum of MTS-9 shows the peak at 1122 cm', indicating that the coordinated environment of Ti in MTS-9 is very similar to that of TS-1 [14]. On the contrary, Ti-MCM-41 exhibits the peak at 1110 cm"' [15], suggesting that the coordinated environment of Ti in MTS-9 and Ti-MCM-41 is distinguishable. 3.7. Catalytic tests Catalytic activities for the oxidation of aromatics by H2O2 over MTS-9, Ti-MCM-41, and TS-1 catalysts are summarized in Table 2. In phenol hydroxylation, Ti-MCM-41 shows very low catalytic activity, but MTS-9 exhibits very high catalytic activity, with a phenol conversion of 26% which is comparable with TS-1 [1]. In styrene epoxidation, MTS-9 shows activity and selectivity similar to those of TS-1. In 2,3,6-trimethylphenol hydroxylation, Ti-MCM-41 is inactive due to the relatively low oxidation ability of Ti species in the amorphous wall of Ti-MCM-41, and TS-1 is also inactive due to the inaccessibility of the small micropores of TS-l to the large diameter of a bulky molecule like 2,3,6-trimethylphenol. However, MTS-9 is very active for this reaction with a conversion of 18.8%, indicating that MTS-9 is an effective catalyst for the oxidation of bulky molecules.
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Table 2 Catalytic activities in oxidation reactions by H2O2 over MTS-9, Ti-MCM-41, and TS-1 samples Sample Reaction TOF Conv. H2O2 Product Selectivity"^, % (%) Eff.^ PI P2 __P3__ MTS-9 Phenol Hydroxylation*^ 6.8 26.3 79.5 0.7 59.5 39.8 Ti-MCM-41 Phenol Hydroxylation® 60.1 0.5 2.5 7.6 1.9 38.0 84.8 48.6 5.5 28.0 50.4 Phenol Hydroxylation® 1.0 TS-1 Styrene Epoxidation 9.4 56.4 56.4 29.3 MTS-9 42.7 28.0 Styrene Epoxidation^ 5.2 54.6 54.6 58.3 29.0 TS-1 13.3 Styrene Epoxidation^ 48.3 Ti-MCM-41 48.3 6.1 100 — — 18.8 68.3 MTS-9 TMP Hydroxylation*^ 7.4 66.7 21.1 12.2 4.1 20.9 TMP Hydroxylation*^ 1.4 Ti-MCM-41 4.6 25.5 69.8 TMP Hydroxylation*^ 1.2 4.2 71.1 0.3 11.3 TS-1 17.6 #: The efficiency conversion of H2O2 was calculated as follows: H2O2 eff. conv. = IOOXH2O2 (mols) consumed in formation of products/total H2O2 (mols) added. +: The product selectivity: PI (or P2 or P3)/(P1 + P2 + P3). @: The products are catechol (PI), hydroquinone (P2), and benzoquinone (P3). $: The product are styrene epoxide (PI), phenylacetaldehyde (P2), and benzaldehyde (P3). &: The product are trimethylhydroquinone (PI), trimethylbenzoquinone (P2), others (P3).
REFERENCES 1. M. Taramasso, G. Perego and B. Notari, U.S. Patent 4,410,501 (1983). 2.T. Blasco, A. Corma, and J. Perez-Pariente, J. Am. Chem. Soc. 115 (1993) 11806. 3. B. Notari, Catal. Today 18 (1993) 163. 4. C. T. Krcsge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck, Nature 352 (1992) 710. 5. P T. Tanev, M. Chibwe & T. J. Pinnavaia, Nature, 368 (1994) 321. 6. D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F Chmelka, G. D. Stucky, Science 279 (1998) 548; Z. Luan, J. Y. Bae, and C. Kcvan, Chem. Mater., 12 (2000) 3202. 7. A. Corma, Chem. Rev. 97 (1997) 2373. 8. H. Van Bekkum, E. M. Flanigen, P. A. Jacobs, J. C. Jansen (Eds.), Introduction to Zeolite Science and Practice, Elsevier, Amsterdam, 2002. 9. X. Xu, M.S. Thesis, Jilin University, China, 1999. 10. Y. Liu, W. Z. Zhang, T. J. Pinnavaia, J. Am. Chem. Soc. 122 (2000) 8791; Angew. Chem. Int. Ed. Engl. 40(2001) 1255. 11 .Z. Zhang, Y Han, L. Zhu, R. Wang, Y Yu, S. Qiu, D. Zhao, and F.-S. Xiao, Angew. Chem. Int. Ed. Engl. 40 (2001)1258; J. Am. Chem. Soc, 123 (2001) 5014. 12. Y Han, F-S. Xiao; S. Wu; Y Sun; X. Meng; D. Li; S. Lin, F Deng, X. Ai, J. Phys.Chem. B 105(2001)7963. 13.F-S. Xiao, Y. Han, Y. Yu, X. Meng, M. Yang, S. Wu, J. Am. Chem. Soc, 124 (2002) 888. 14.C. Li, G. Xiong, Q. Xin, J. Liu, P Ying, Z. Feng, J. Li, W. Yang, Y Wang, G. Wang, X. Liu, M. Lin, X. Wang, E. Min, Angew. Chem. Int. Ed. 38 (1999) 2220. 15.G. Xiong, C. Li, H. Li, Q. Xin, Z. Feng, Chem. Commun. (2000) 677.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
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W/Zr mixed oxide supported on mesoporous silica as catalyst for «-pentane isomerization Tao Li, She-Tin Wong, Man-Chien Chao, Hong-Ping Lin, Chung-Yuan Mou and Soofin Cheng Department of Chemistry, National Taiwan University, Taipei 106, Taiwan. FAX: +886-22363-6359. E-mail: cheml031@ccms.ntu.edu.tw W03/Zr02 mixed oxides supported on various porous silica were prepared. For the mesoporous silica, SBA-15 was found to retain the crystalline structure better than MCM-41 after loading W/Zr mixed oxides. Tungstated zirconia is mainly dispersed into the mesoporous channels of SBA-15, which causes the dramatical decrease in the surface area and pore volume. All the mesoporous siliceous materials supported solid acid samples showed strong acidity. The SBA-15 supported W03/Zr02 materials promoted with Pt were highly efficient in catalyzing the isomerization of n-pentane with a high selectivity of isopentane. SBA-15 with 1 %Pt/20%WO3/40%ZrO2 gave the highest catalytic activity. 1. INTRODUCTION Due to the hazardous properties of liquid acids such as HF and H2SO4 commonly employed in petrochemical industry, a great of effort has been focused on the development of more environmentally friendly strong solid acids [1]. Mesoporous silica, such as MCM-41 and SBA-15 have uniformed hexagonal arrays of mesopores and very high surface area [2,3]. However, the material itself has no acidity and low catalytic activities. The objective of this work is to introduce acid function on the mesoporous materials by supporting W/Zr mixed oxide on them and to examine their catalytic activities in isomerization of n-pentane. 2. EXPERIMENTAL Pure siliceous MCM-41 and SBA-15 were synthesized according to the literatures [4,5], The W/Zr mixed oxides were supported on the mesoporous silicas by co-impregnation of zirconium(IV) acetylacetonate and ammonium metatungstate hydrate. The resultant calcined materials were impregnated by the incipient wetness method with aqueous platinum tetrachloride and calcined at 773K in air. The resultant catalyst was characterized with various techniques such as XRD, N2 physisorption, HRTEM and DRIFTS of pyridine. The temperature-programmed desorption of ammonia (NH3-TPD) was carried out on a Micromcritics AutoChcm 2910 instrument. The desorption process was monitored by a Quadruple Mass Spectrometer and the mass number 16 was followed to obtain the TPD profiles of NH3. The catalytic activities in the isomerization of ^-pentane were carried out in a fixed-bed micro-reactor at atmospheric pressure. On-line product analysis was done using a GC equipped with a FID detector. This work was supported by China Petroleum Corporation and National Science Council of Taiwan.
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3. RESULTS AND DISCUSSION 3.1. Characterization It was found that SBA-15 could keep its mesoporous structure better than MCM-41 after supporting W/Zr mixed oxide. The XRD patterns showed that the structures corresponding to hexagonal SBA-15 were still retained for the samples calcined at 800°C with the WO3 and Zr02 contents as high as 20wt% and 40wt%, respectively, while the MCM-41 lost its hexagonal structure with the same amount of WO3 and Zr02 loadings. For the SBA-15 with WO3 and Zr02 loadings up to 20wt% and 40wt%, respectively, only pure tetragonal Zr02 phase was observed and nearly no peaks due to WO3 crystallites could be seen. When the WO3 and Zr02 loadings were further increased, the crystalline WO3 phase and a little monoclinic Zr02 phase would appear. The physical properties measured from nitrogen adsorption-desorption isotherms of some solid acid catalysts are depicted in Table 1. Since all the SBA-15 supported mixed oxide samples were calcined at 800°C, the parent SBA-15 sample calcined at such a high temperature was compared. It has surface area, pore volume and pore diameter of 452 m^/g, 0.62 cmVg and 7.5nm, respectively. After supporting tungstated zirconia on SBA-15, the BET surface area and pore volume decreased gradually with the metal oxide loading. These results imply that the supported oxides should be dispersed onto the internal surfaces of the mesopores of SBA-15. Moreover, the supported catalysts have much larger surface areas (>92mVg) than the unsupported W03/Zr02 (24m^/g). The HRTEM photographs confirmed that the hexagonal arranged mesopores of SBA-15 were still retained and tungstated zirconia was mainly dispersed inside the pores. In comparison, the decreases in surface area and pore volume are more drastic for MCM-41 loaded with W03/Zr02. The temperature-programmed desorption of ammonia (NH3-TPD) was performed to determine the amount and strength of acid sites on the catalysts. Figure 1 compares the Nn3Table 1 Physical properties of some solid acid catalysts Sample
W03/Zr02 (wt/wt)
Zr02/support (wt/wt)
-
-
0.50 0.50 0.50 0.50 0.50 0.20 0.35 0.50 0.65
0.10 0.25 0.54 1.0 1.86 1.0 1.0
Si02 gel W03/Zr02/Si02 W03/Zr02 MCM-41 W03/Zr02/MCM-41
-
-
0.50 0.17
1.0
SO/-Zr02/MCM-41
-
SBA-15 WO^/ZrOj/SBA-lS WO^/ZrO./SBA-lS W03/Zr02/SBA-15 W03/Zr02/SBA-15 WOj/ZrOz/SBA-lS W03/Zr02/SBA-15 WOj/ZrO./SBA-lS W03/Zr02/SBA-15 W03/Zr02/SBA-15
0.50
1.0 1.0
1.0 1.0
Calc. Temp. ("C) 800 800 800 800 800 800 800 800 800 800 560 800 800 560 800 680
BHT area (m'/g) 452 411 301 201 137 92 216 166 137 125 660 141 24 1085 162 476
BJH pore volume Pore diameter (nm) (cm-Vg) 0.62 0.60 0.42 0.30 0.21 0.14 0.25 0.22 0.21 0.20 0.67 0.17 0.03 1.0 0.11 0.41
7.5 8.0 7.0 6.8 6.2 6.2 6.3 5.8 6.2 6.9
26.8 21.3 25.0
571
200
3(K)
400
500
6(X)
7(K)
800 Calcination temperature (°C)
Temperature ("C)
Fig. I.NH3-TPD profiles of (a) l%Pt/50%SZr02/MCM-41, and 1 %Pt/20%WO3/40%ZrO2 onvarious supports (b) SBA-15, (c) MCM-41, (d)silica gel, and (e) l%Pt/17%W03/Zr02.
Fig. 2.Catalytic activities of l%Pt/20% WO3/40%ZrO2/SBA-15 catalyst as a function of calcination temperature of the supported mixed oxides.
TPD profiles of W03/Zr02 supported on various silica supports, SBA-15, MCM-41, and silica gel, the unsupported V^IO^IZrOi and sulfated Zr02 supported on MCM-41. The WO3 and Zr02 contents in the three supported samples were similar, 20%WO3 and 40%ZrO2. The results demonstrate that W03/Zr02 supported on either SBA-15, MCM-41 or silica gel contains much more acid sites than the unsupported sample, while the acid strength does not have much change. In comparison to the MCM-41 supported sulfated zirconia with the same Zr02 loading, the acid strength of the supported mixed oxides is slightly weaker. These results accompanying with the BET results in Table 1 suggest that supporting W03/Zr02 on various silica can fonn mixed oxide of high dispersion and generate large amount of acid sites with medium acid strength. 3.2. Catalytic studies The catalytic properties of SBA-15 supported W03/Zr02 were investigated in the isomerization of /7-pentane to /.vo-pentane. The Pt-promoted W/Zr mixed oxide supported on SBA-15 was found to be efficient catalysts in the A7-pentane isomerization. For the Pt-free catalysts, nearly no activity was observed. Introducing a small amount of Pt (0.5wt%) onto the catalyst can cause a great increase in both the conversion of ^-pentane and selectivity to /.vo-pentane. The catalytic activity increases with the increase in Pt loading, while the selectivity of/.sY;-pentane keeps constant around 97%. For the catalyst with Pt loading higher than lwt%, no significant increase in promotion effect of Pt was observed. Under our reaction conditions, the major product was /.vr;-pentane. The main by-products were cracking products such as methane, ethane, propane, A2-butane and /.vc^-butane. Figure 2 shows that the «-pentane conversion over l%Pt/20%WO3/40%ZrO2/SBA-15 was dependent on the calcination temperature of the W03/Zr02/SBA-15 sample before impregnation of Pt. The optimal activity was observed on the catalyst calcined at SOO'^C. Figure 3 compares the catalytic activities in «-pentane isomerization of Pt-promoted W/Zr mixed oxide without support and that supported on various silica materials. It shows that SBA-15 support gives the highest A2-pentane conversion. In contrast, MCM-41 and silica gel supports give relatively low conversions. The BET surface areas of the three supported catalysts were in a close range of 140-160 m^/g, but the BJH pore diameter of SBA-15
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TOS(h)
Fig. 3. Conversion of A?-pcntane versus time on stream over different catalysts: (a) l%Pt/ 20%WO3/40%ZrO2/SB A-15, (b) unsupported l%Pt/17%W03/Zr02, (c) l%Pt/50%SZrO2/ MCM41, (d) l%Pt/20%WO3/40%ZrO2/MCM-41 and (e) l%Pt/20%WO3/ 40%ZrO2/SiO2.
TOS (h)
Fig. 4. Conversion of n-pentane versus time on stream over l%PtAV03/Zr02/SBA-15 with W03/Zr02 (wt/wt) ratio of 0.50 and different Zr02/SBA-15 (wt/wt) ratio (a) 1.0, (b) 1.86, (c)0.54, (d) 0.25 and (e) 0.10.
catalyst was ca. 6 nm and much larger than 2 nm of MCM-41. Therefore, the easier diffusion of gaseous reactants and products in SBA-15 probably accounts for its higher activity. The unsupported W/Zr mixed oxide gives similar conversion as that over SBA-15 supported catalyst. However, the unsupported catalyst decays with time-on-strcam more easily than the supported ones. These results imply that the supports play a role in stabilizing the catalytic active centers. The MCM-41 supported sulfated zirconia shows very high initial activity, but it decreases abruptly with time-on stream. These results are elucidated by that the acidity of supported sulfated zirconia is too strong and leads to severe coking and decay of the catalyst. Figure 4 compares the catalytic performance of l%Pt/W03/Zr02/SBA-15 samples with W03/Zr02 (wt/wt) ratio of 0.50 and different Zr02/SBA-15 (wt/wt) ratio. For catalysts with low Zr02 loadings (profiles d and e), the conversions of A7-pentanc are lower than 2%, and isopentane selectivities are lower than 50%. The catalytic activity increases drastically with Zr02 content. The optimal catalytic activity was observed on the catalyst with Zr02/SBA-15 wt/wt ratio of 1.0 (20wt% WO3 and 40wt% ZrO:). Further increase in Zr02 content would cause the decrease in /7-pentane conversion. This maybe due to the block of the mesoporous pores by too large amount of mixed oxide. These results also imply that the crystallites of tungstated zirconia must meet the required size to become the active sites for /7-pentane isomerization.
REFERENCES 1. A. Corma and A. Martinez, Catal. Rev.-Sci. Eng. 35 (1993) 483. 2. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature 359 (1992) 710. 3. D.-Y. Zhao, J.-L. Feng, Q.-S. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka and G.D. . Stucky, Science 279 (1998) 548. 4. D. Das, C.-M. Tsai and S. Cheng, Chem. Commun. (1999) 473. 5. C.-P. Kao, H.P.Lin and C.Y. Mou, J. Phys. Chem. Solid, 62 (2001) 1555.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Au and Au-Pt Bimetallic Nanoparticles in MCM-41 materials: Applications in CO Preferential Oxidation Satyanarayana Chilukuri', Trissa Joseph', Sachin Malwadkar', Chinmay Damle^, S.B. Halligudi\ B.S. Rao', Murali Sastry^and Paul Ratnasamy Catalysis' and Materials^ Chemistry Divisions, National Chemical Laboratory, Pune-411 008, India, Abstract Nanosized Au and Au-Pt bimetallic particles of different atomic ratios were synthesized from HAuCU and HPtCU inside the channels of amine functionalized MCM-41. These were characterized through chemical analysis, XRD and TEM. The size of the bi-metallic particles was found to be in the range of 2-4 nm. Their catalytic activities were evaluated in simulated gas mixtures that typically contain 0.5 and 0.96% CO in presence of large proportions (-74%) of H2. The catalysts that contain higher concentrations of Pt were found to be active and offer good CO preferential oxidation activity. Catalysts that contain an optimum amount of Au along with Pt have shown highest activities at lower temperatures. 1. INTRODUCTION Polymer electrolyte membrane fuel cells (PEMFC's) require hydrogen, free from CO impurities (<5 ppm). Supported noble metals such as Au and Pt exhibit good activity for CO oxidation. However, Pt based catalysts operating at higher reaction temperatures (130-200"C) are not that selective for CO oxidation in hydrogen rich streams'. As a result, valuable H2 is oxidized to water. Gold catalysts, on the other hand are active at very low temperatures (at or below room temperature) and hence selectively oxidise CO without consuming much H2 in the process . However, the Au loadings (-5%) in these catalysts are very high, so any reduction in the amounts brought about by better dispersion is welcome. There are three factors that govern the performance of the gold catalysts, viz., (i) type of support, (ii) size of the gold particles and (iii) the strength of interaction between gold particle and the support'*'*. It is reported that gold catalysts are active at lower temperatures only if they are supported on reducible supports such as oxides of Mn, Ti and Fe etc'. Gold catalysts, supported on metal oxides show good activity and selectivity towards CO oxidation, but majority of these deactivate after few hours, in gas streams that contain CO2 This deactivation is attributed to the build up of surface carbonate species on the support. For applications in PEM fuel cells, a catalyst has to be active atleasl for 5000 hours. Since, it is known that metal particle (gold or platinum) size plays a pivotal role, it is appropriate to load gold and/or platinum on to high surface areas materials such MCM-41 to yield nano sized particles. It is also expected that weakly acidic supports such as, MCM-41 may prevent the formation of carbonate species thus preventing the deactivation of gold catalysts. In this paper, we demonstrate the use of amine functionalized MCM-41 materials in the entrapment of PtClc' and AuCV anions within the silicate framework via electrostatic interactions with the pendant amine groups. Reduction of the above anions, in-situ at room temperature resulted in the formation of mono as well as bimetallic particles of well defined dimensions. These catalysts show excellent catalytic activity in the preferential oxidation
574
(PROX) of CO, demonstrating the utility of these materials for fuel cell applications in the complete removal of CO from fuel reformates. This study encompasses preparation of nanosized Au, Pt and Au-Pt bimetallic particles of various compositions and concentrations, their characterization using TEM and their applications as PROX catalysts 2. EXPERIMENTAL Synthesis of MCM-41 materials was carried out as per reports. For preparing Au, Pt and Au-Pt nano-clusters, freshly calcined MCM-41 (surface area llOOm^/g) was re fluxed with 3aminopropyl triethoxy silane (APTES) in toluene for 5h. It was filtered and washed with toluene and diethyl ether to remove any adsorbed APTES and was vacuum dried. This amine functionalized MCM-41 was suspended in solutions of (10"^ M) HAuCU and H2PtCl6 of required amounts, for 12h, to allow the migration of metal containing anions inside the pores. The solid was then filtered, washed and vacuum dried. These samples were reduced, to form nanoparticles, using 10"^ M NaBH4 solution. As synthesized as well as modified materials were characterized using powder XRD (Rigaku Geiger flex) and TEM (JEOL Model 1200 EX operating at 80 KV). Bulk chemical analysis of the catalyst samples was carried out by dissolving them in HF-aquaregia mixture and estimating the concentration by ICP-AES (Perkin Elmer 1000). CO oxidation experiments were conducted in a down flow glass reactor using two simulated gas streams (that mimick real output of fuel reformer), consisting of 0.50% and 0.96% CO, rest being carbon dioxide (24.3 and 23.75%), methane (2.02 and 1.93%) and hydrogen (73.12 and 73.32%). Product gases were analysed on-line with TCD and FID detectors in series, through methanation of trace amounts of CO, which is estimated by FID. Gas chromatographic (Shimadzu 15A) analysis was performed using Spherocarb column. 3. RESULTS & DISCUSSION The description of various samples along with their metal contents and composition is given in Table 1. As may be seen, the total metal contents were in the range 0.06 to 0.17 wt% which were close to the values that were attempted. Catalysts are described in terms of the metal atomic ratios that are taken up for the exchange. Tablc-1: Chemical composition of various Au-Pt catalysts
Catalyst
Au-MCM-41 Pt-MCM-41 Au5Pt,o(rMCM-41 Au,5PWMCM-41 Au5oPt5o-MCM-41 Au,o()Pt5-MCM-41
Metal loading (wt%)
Attempted 0.09 0.09 0.18 0.18 0.09 0.09
Actual 0.081 0.098 0.150 0.170 0.060 0.090
Au:Pt ratio
Attempted
Actual
5: 100 15:90 50:50 100:5
5:95 17:83 54 : 46 96:4
-
—
3.1. Physico-chemical characterization Figure-1 shows UV spectra of Au-Pt solutions before and after exchange. After exchanging the metals, the solid was separated by filtration and the filtrate was analyzed by UVVIS Spectroscopy. As evident from the figure that most of the Au and Pt is taken up by MCM41-NH2, which on further reduction resulted in the formation of bimetallic nanoparticles. Figure 2 shows the XRD patterns of MCM-41, MCM-41-NH2 and MCM-41-NHz-Au-Pt. XRD pattern of MCM-41 shows a very intense peak assigned to reflections at (100) and two additional peaks with low intensities at (110) and (200) reflections respectively, which can be indexed to hexagonal lattice. Crystallinities of Au and Au-Pt bi-metal incorporated MCM-41's were close
575 to the parent materials, though some loss in the intensities of the peaks was observed upon modificaion with 3-amino propyl triethoxy silane (Fig. lb) as evidenced by XRD.
fL Au/Pt 10 M 5: 100
/
\
1
MCM -41
"^"i
- M C M - 41
MCM-41 -NH^-A u/Pt
Filtrate 300
400
SCO
2 theta
W avelngth (nm) Fig.l:UV-VIS Solution spectra before & after exchange
Fig.2: XRD pattern; MCM-41, NH2MCM-4I andAu-Pt-MCM-41
These results are in line with those reported in the literature^. Attempts to monitor the particle size of gold, platinum and gold-platinum nanoparticles through XRD were not successful probably because of very low metal content and small particle size in the samples.
39f!|i •'^'
The particle size of the nanoparticles was investigated using a high resolution transmission electron microscope operating at 80KV and photographs were obtained upto magnifications of 100,000. Pore structure of MCM-41 can clearly be seen in the Fig.3. The TEM investigations show that Au and Pt particles are in nano (2-4 nm) meter range as envisaged.
Fig 3: TEM Photograph of Au5Pt,oo-MCM-41 3.2. Oxidation of CO Preferential oxidation of carbon monoxide in the presence of excess amounts of hydrogen was carried out on various catalysts at O2/CO ratio of one, instead of the stoichiometrically required ratio of O2/CO=0.5. It was found that stoichiometric concentrations of oxygen does not offer good conversion of CO. The results on various catalysts are compared in Table-2. The catalysts that contain Pt in excess were found to be active. The catalyst with Au:Pt in the ratio of 5:95 (Au5Ptioo-MCM-41) that has a total metal content of 0.15% was found to be active even at lower temperatures (80°C). Considering that Pt catalysts offer good performance
576 Table-2: Comparision of various catalysts in preferential oxidation of CO Reaction Gas com ponent (Vol%) Catalyst Temp, "C CO* CH4 H2 CO2 Reactant gas mix. 9600 73.36 1.93 23.75 _-_ Proudct eas mix. Au-MCM-41 150 4200 73.07 24.53 1.98 Pt-MCM-41 71.30 2.01 150 0.9 26.69 Au5Pt,oo-MCM-41 80 0.3 71.76 2.05 26.18 Au,5Pt9o-MCM-41 157 71.55 150 2.02 26.43 Au5oPt5o-MCM-41 200 169 70.90 2.09 27.00 Au,ooPt5-MCM-41 175 3940 72.26 2.10 25.25 * in ppm, Oxygen/Carbon monoxide=l only in 130-200 °C range, wherein H2 is also oxidized in considerable quantities, the present result is remarkable. This performance may be attributed to the small nanoparticles that are obtained through the methods adopted in the present study. This approach also helped in large savings of metal. The interesting part is that the performance of this Au-Pt bimetallic system is better than that of pure Pt-MCM-41. Results show that the bimetallic systems should contain Au as minor quantity (5-20%), while the major component being Pt.
j
' '.
I
2.50j 2.25 J 2.00 H
-
~1 _
1.75]
*-
125QC
•
1500C
—•— awe
2
4
6
1
•
I
•
1
•
A30
8 10 12 14 16 18 20 22 24 TIME IN HOURS RQ-4 CO Oxidation on AUjR^MCM41 Catalyst
2
1.00H
•
/ y
<
I
— ^ GHSV-5000 (0.96% CO)
1.25 j
-.— - - -' 0
•
I.50I
—800C -»
,
^ GHSV-2S00(a5%CO) » - GHSV-2500(0.9e%CO)
/ '
•
'
•
• : -i
0.75] 0.50]
O25J 0.00 4-
10 12 14 16 18 TIME IN HOURS Fig.-5 CO Oxidation on Au Pt MCM-41 at various GHSV
Performance of Au5Ptioo-MCM-41 at various reaction temperatures is given in Fig.4. The catalyst is highly active at 80"C, while higher temperatures led to increased concentations of CO in the product. At higher temperatures, the CO oxidation activity falls with a simultaneous increase in the oxidation of H2. This behaviour is typical of any PROX catalyst that operates at higher tempertures. The performance of this catalyst at different gas hourly space velocities (GHSV's) for the gases containing 0.5 and 0.96% CO is compared in Fig.5. It may be seen that at both CO inputs, the catalyst did not show any deactivation at least up to 16 hours, while at higher GHSV (5000), it deactivated after few hours on stream. Effect of higher loading of metal is being investigated on the performance of these catalysts at higher GHSV.
REFERENCES 1. 2. 3. 4. 5.
C D . Dudfield, R.Chen and P.L. Adcock, Intl. J. Hydrogen Energy 26 (2001) 76 M. Haruta and M. Date, Appl. Catal. A 222 (2001) 427. G.C. Bond and D.T. Thompson, Catal. Rev.-Sci.Eng. 41 (1999) 319. M. Haruta, Catalysis Today, 36(1997) 153 M.M. Schubert, S. Hackenberg, A.C. van Veen, M. Muhler, V. Plzak and R.J. Behm, J. Catal. 197 (2001) 113. 6. D. Brunei, N. Bellocq, P. Sutra, A. Cauvel, M. Lasperas, P. Moreau, F. Di Renzo, A. Galameau and F. Fajula, Coord. Chem. Rev. 180 (II) (1998) 1085.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
577
Effective inclusion of chlorophyllous pigments into mesoporous silica for the energy transfer between the chromophores Hiroyasu Furukawa and Kazuyuki Kuroda* Department of Applied Chemistry, Waseda University, Okubo-3, Shinjuku-ku, Tokyo 169-8555, Japan Silica/chlorophyll nanocomposites were synthesized for mimicking partly a light-harvesting apparatus of higher plants. The energy transfer between chlorophyll a and b in alkanediol-modified mesoporous silica was observed by fluorescence measurements. To develop an effective immobilization method for the highly efficient energy transfer, chlorophyll derivatives possessing triethoxysilyl groups were also synthesized and grafted onto FSM-type mesoporous silica. In this case, the energy transfer efficiency was higher than that of the former adsorption way. 1. INTRODUCTION The capture of low-density solar energy and the following energy transfer (ET) play a key role for the natural photosynthesis, and such a series of physicochemical processes is performed by chlorophyllous pigments themselves. To mimic the highly efficient ET systems, many different donor/acceptor (D/A) systems which immobilized in various media have been reported [1-2]. On the other hand, we have recently proposed the utilization of FSM-type mesoporous silica (FSM) as an adsorbent of photosynthetic pigments, because of silica/pigments nanocomplexes without solvents are suitable for simple handling [3-4]. However, it is difficult to immobilize chlorophyllous pigments in the mesopores without pigments denaturation, because these h\) pigments are prone to denature by heat or solvent treatments. This has ^^\-^W>s^ prompted us to improve an efficient adsorption method of Chls on mesoporous silica for the highly efficient ET Energy transfer systems. In this study, to suppress pigment M Entry Symbol Re RA RB denaturation, alkanediol modified FSM Mg COOMe Me 1 Chi a O-C20H39 materials were synthesized and the ET 2 Chi 6 Mg COOMe CHO O-C20H39 3 Zn-APTES-ChI Zn H Me NH(CH2)3Si(OEt)3 behavior between Chls in mesopores 4 Cu-APTES-ChI Cu H Me NH(CH2)3Si(OEt)3 was investigated by fluorescence Fig. 1. Molecular structures of chlorophyllous ' Corresponding author. pigments. (Inset) Schematic representation of ET E-mail address: kuroda@waseda.jp in the mesopores.
578
measurements. Moreover, to improve the ET efficiency in the mesopores, chlorophyll derivatives possessing triethoxysilyl groups were also synthesized and grafted on the silica surface. 2. EXPERIMENTAL Synthetic procedures of FSM were described in the literature [3, 5]. Dried FSM was refluxed in 1,6-hexanediol (HD) for 24 h under an N2 atmosphere. The esterified product (HD-FSM) was washed to remove unreacted diol and dried [3, 5]. Extraction procedure of Chls was reported previously [6]. The prescribed amount of Chi (1, 2, or the mixture of 1 and 2) was dissolved in toluene, and then HD-FSM was dispersed in the Chi solution. The mixture was stirred at room temperature for 3 h in the dark, and centrifuged to remove supernatant and dried in vacuo [5]. The amidation of Chi is as follows [7]: zinc C13^-demethoxycarbonyl-chlorophyllide a was added to a mixture of 3-aminopropyltriethoxysilane (APTES), dimethylaminopyridinium tosylate, and l-(3-dimethylaminopropyl)-3-ethylcarbodiimide in dry CH2CI2. The mixture was stirred for 12 h under N2 at room temperature. A crude compound was purified by a preparative high-performance liquid chromatography (HPLC). FSM powders grafted with Chls were prepared by the following method. An amidated Chi (3, 4, or the mixture of 3 and 4) and APTES, a modifier to suppress denaturation of Chls, were dissolved in dry CH2CI2. Then FSM powders were dispersed in this solution. The mixtures were stirred at room temperature for 24 h in the dark, centrifuged, washed with acetone, and dried. 3. RESULTS AND DISCUSSION The XRD pattern of HD-FSM had four peaks which are characteristic of a typical hexagonal array of mesopores, indicating that mesostructure did not change after the modification. Based on the data by solid-state NMR, CHN elemental and N2 adsorption analyses, the pore structure parameters of FSM materials were obtained (Table 1). This clearly shows that a large part of silanol groups were esterified by diols. The adsorption of Chi a (1) in the FSM pore system was performed by liquid-phase adsorption. The absorption spectral shape of the FSM/1 compounds was similar to that of Chi a in acetone. The suppression of the pigment degradation in the mesopores was confirmed by the analysis of the extracts from FSM/1 by HPLC. These findings indicate that Chls were adsorbed into FSM without denaturation. Next, to mimic some functions of photosynthesis, such as energy transfer from Chi h to Chi Table 1 Pore characterization of FSM and HD-FSM. BET Pore Pore size ^^ Number of grafted Number of silanol surface area volume silanol groups groups "^ ( m^ • g"') ( mL • g"') ( "tn ) ( groups • nm"^) ( groups • nm ") FSM 1020 0.82 3.2 2.8 HD-FSM 610 042 TT lA 1) Calculated by the BJH method 2) Based on the ^IQ^ ratio of ^"^Si MAS NMR measurements
579
a (2 to 1), both Chi a and Chi b (1 and 2) were co-adsorbed into the HD-FSM pore system. When the ratio of 2/1 was 1/10, the peak area of the emission from Chi b (2) was about 0.3 times smaller than that from the single 2 system (Fig. 2a). With the decreasing of emission at around 655 nm, new emission in the wavelength region longer than the 655-nm band was 650 700 750 also observed in the difference spectrum 800 Wavelength / nm (Fig. 2b), though the fluorescence Fig. 2. (a) Fluorescence emission spectra of quantum yield of Chi a (1, ca. 0.2 [8]) is FSM/Chls. (b) Difference spectrum of (a). relatively small. This indicates that the Forster type D/A ET should occur. In the case of 2/1 ratios were 1/4 and 1/1, however, the peak areas of the emission from 2 were about 0.65 and 0.9 times smaller than that from the single 2 system. The results suggest that the energy transfer becomes more difficult as the 2/1 (D/A) ratio increases. This leads that it is necessary to optimize the adsorption states of the Chls in the mesopores. The arrangement between neighboring chromophores in mesopores was hardly controlled by simple liquid-phase adsorption, which is not suited for the efficient ET. To overcome this problem, the compounds 3 and 4 (Fig. 1) were synthesized and grafted on the silica surface. The advantages of grafting are as follows: (i) the interaction between adsorption site on silica surface and Chls is decreased, and (ii) the grafted chromophores may be centered in the mesopores. All grafted FSM powders were scarcely bleached by repeated washing with acetone. This is in sharp contrast with the fact that Chls incorporated into HD-FSM by simple liquid-phase adsorption were easily desorbed from mesopores by washing with acetone. This finding strongly supports that the amidated Chls (3 and 4) were grafted onto the surface of mesopores as well as APTES (modifier) molecules. Fig. 3 depicts the visible absorption spectra of the FSM powders grafted with the Chi derivatives. The top and middle curves illustrate 3 and 4 onto FSM, respectively. In the co-adsorbed state, the spectral shape was roughly interpreted as a summation of those of 3 and 4. For investigation of the ET behavior, the fluorescence emission spectra were recorded. The emission peaks of Zn-APTES-Chl (3, donor) and Cu-APTES-Chl (4, acceptor) on FSM were located at 666 and 670 nm (data not shown). The emission intensity of 4 was much weaker than that of 3 in FSM due to the small quantum yield of Cu-Chl. In the co-adsorption systems the emission intensity of 3 was decreased with an increase in the amount of 4. The ET efficiency from 3 to 4 is approximately estimated by the decrease in the emission area in comparison with that of FSM/3 complex (90% for 3/4 = 1/2; 50% for 1/1). Based on the pore volume and pore size of grafted FSM, the distance between neighboring chromophores (RQ) is calculated to be ca. 12 nm. It is of interest that this efficiency is much higher than that presented above (ET efficiency was ca. 70% for 2/1 = 1/10), though the pigment concentration and RQ in mesopores are basically same. Taking
580
into account the trend that the highly efficient ET is obtained in this grafted system in comparison with the conventional adsorption method, the planes of chromophores are probably perpendicular to the silica walls in the mesopores (Fig. 1 inset). 4. CONCLUSION
400
500
600
700
Wavelength / nm
Fig. 3. Visible absorption spectra of Chls grafted We have demonstrated that the use of on FSM powders. FSM has enabled us to fabricate new inorganic/organic nanocomposite containing no solutions in the system. The modification on silica surface is also effective techniques for inclusion of Chls without denaturation. The ET efficiency of the system that Chls were incorporated by liquid-phase adsorption was not so high in comparison with that of the grafted system. We believe that the grafting of Chi on FSM surface has allowed us to optimize the pigment orientation for efficient ET. ACKNOWLEDGEMENTS The authors are grateful to Prof. T. Watanabe (Univ. of Tokyo) for HPLC analyses and Messrs. T. Shigeno and S. Murata (Waseda Univ.) for their experimental assistance. This work was partially supported by a Grant-in-Aid for COE Research, Japan.
REFERENCES 1. A.G. Tweet, W.D. Bellamy, and G.L. Gaines, J. Chem. Phys., 41 (1964) 2068. 2. K. Colbow, Biochim. Biophys. Acta, 314 (1973) 320. 3. S. Murata, H. Hata, T. Kimura, Y. Sugahara, and K. Kuroda, Langmuir, 16 (2000) 7106. 4. H. Furukawa, K. Kuroda, and T. Watanabe, Chem. Lett., (2000) 1256. 5. S. Murata, H. Furukawa, and K. Kuroda, Chem. Mater., 13 (2001) 2722. 6. H. Furukawa, T Oba, H. Tamiaki, and T. Watanabe, Bull. Chem. Soc. Jpn., 73 (2000) 1341 7. H. Furukawa, T. Watanabe, and K. Kuroda, Chem. Commun., (2001) 2002. 8. H. Scheer (ed.). Chlorophylls, CRC press, Boca Raton, 1991.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Biological applications of organically functionalised mesoporous molecular sieves and related materials Humphrey H.P. Yiu and Ian J. Bruce School of Chemical and Life Sciences, University of Greenwich, Wellington Street, Woolwich, London, SE18 6PF, U.K. The adsorption behaviour of mesoporous molecular sieves SBA-15 and MCF was studied using a family of polysaccharide molecules, dextrans. The results provide some insight into the accessibility of the mesopores to biological molecules. The effect of an amine functionalised surface of the solid was also examined. 1. INTRODUCTION The discovery of the mesoporous molecular sieves M41S in the early 1990s had opened up a new research direction for materials sciences. The breakthrough regarding the limitation of microporous dimensions of zeolites permits the potential use of molecular sieves with larger molecules, such as biomolecules. Early candidates of mesoporous molecular sieves such as MCM-41 and MCM-48 (pore diameter 25 to 35 A) were considered to be unsuitable for biological applications but with the discovery of SBA-15 which possess relatively large pore dimensions (pore diameter ca. 60 A), biological applications of mesoporous molecular sieves became possible and the biomolecular adsorption behaviour of SBA-15 had been studied previously [1]. Research into the biological application of mesoporous molecular sieves is still limited. Enzyme immobilisation [2] and, more recently, protein separations [3] are the two major areas which have been explored. We are reporting a new route for analysis the accessibility of mesoporous molecular sieves SBA-15 and MCF by biomolecules. Dextrans, a family of polysacharride molecules, have been used to develop model systems to study adsorption which are relatively simple molecules compared with other biomolecules such as proteins. 2. EXPERIMENTAL 2.L Siliceous and amine functionalised SBA-15 The method for the preparation of pure siliceous SBA-15 has been reported previously [4]. The PEO-PPO-PEO template was removed by calcination. Propylamine functionalised SBA15 (PrNH2-SBA-15 with 1 mol % propylamine) was prepared following the in situ route already reported in the literature [2]. The PEO-PPO-PEO template was removed by extraction with ethanol at 78°C for 5 hours three times and the solid product was filtered, washed with ether and air-dried. * Corresponding author: Tel. +44 20 83318215. E-mail address: I.J.Bruce@gre.ac.uk
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2.2. Siliceous and functionalised MCF materials The method for the preparation of siliceous and amine functionaUsed MCF materials has been reported previously [3]. Two oil to surfactant ratios (0.3 and 0.6) were used and the products were denoted as MCF-3 and MCF-6 respectively. 2.3. Dextran adsorption to solid supports Dextran standard solutions ranging from 0 to 2 mg cm"^ were prepared by dissolving dextrans (Sigma and Fluka) in potassium phosphate buffer solutions (pH = 5 to 9). Dextrans with 4 different molecular weights were used (see table 1). The solid supports (20 mg) were suspended in the dextran solution in a microcentrifuge tube and incubated in a rotating disc for 16 hours at 25°C to ensure an equilibrium in adsorption of dextran molecules. The dextran content of the supernatant was determined colorimetrically (see below). Table 1 The notations for dextrans used in the adsorption experiment. Dextran Average molecular weight Dextran A 10,400 15,000--20,000 (or 17,500) Dextran B 40,000 Dextran C 68,800 Dextran D
Supplier Sigma Fluka Fluka Sigma
2.4. Colorimetric assay for dextrans Dextran content of the supernatants was determined using the anthronc assay for sugars [6]. A standard anthronc solution was prepared by dissolving 20 mg of anthronc (Sigma) in 100 cm"^ 80% w/w sulphuric acid. 0.3 cm"^ of supernatant was added to a test tube followed by 0.3 cm"^ cone. HCl and 30 |.il 90% formic acid. After the solution was thoroughly mixed, 2.4 cm^ anthronc standard solution was added slowly. The solution was then heated at 90°C for 5 minutes and cooled in a water bath immediately. The absorbance of the solution at 630 nm was measured. 3. RESULTS AND DISCUSSIONS 3.1. Molecular weight and adsorption to solid support The dextran adsorption isotherms of SBA-15 are shown in Figure 1. The amount of dextran molecule adsorbed to the solid support appeared to decrease with increasing dextran molecular weight in the range of 10 kD to 40 kD. Increase in molecular weight above this showed no significant difference in the adsorption isotherm. This could be because dextran molecules with molecular weights higher than 40 kD are too large to fit inside the mesopores of SBA-15. As a result, larger dextran molecules (C and D) can only be adsorbed to the outer surface of SBA-15. The result is in agreement with the protein adsorption isotherms from literature [I].
583
0.01
0.02
0.03
0.04
0.05
equilibrium concentration (mM)
Fig. 1. Dextran adsorption isotherms of SB A-15. (Key: • dextran A (av. mw = 10.4 kD), • dextran B (av. mw = 17.5 kD), x dextran C (av. M.w. = 40 kD) and A dextran D (av. M.w. = 68.8 kD) 3.2. Effect of pore size on dextran adsorption Since the majority of biomolecules have dimensions larger than the pore diameter of SBA15, supports with pore diameters larger than those are needed for more general biological applications. Recently MCF materials formed by microemulsion have been used in the immobilisation of chloroperoxidase [7] and, as a result, were selected to study the effect of pore size on dextran adsorption. The dextran B and C adsorption isotherms are depicted in figure 2. From the dextran B adsorption isotherms, MCF-3 adsorbed a larger amount than molecules than SBA-15 (figure 2a). This could be because the pore dimension of MCF-3 is larger and so more molecules could be packed inside its cages. However, on increasing the pore dimension, MCF-6 appeared to adsorb a lower amount than MCF-3. A possible reason for this was that the pore size of MCF-6 was too large dextran B molecule packing to occur and only a monolayer of molecules was adsorbed. On increasing the molecular weight of dextran to 40 kD, the amount adsorbed by SBA-15 decreased significantly possibly because the molecules were too large to enter its pores. MCF-3 still adsorbed a considerable amount but less than the amount adsorbed MCF-6. It seemed that when the size increased to a certain extend, molecules could be packed inside the pores of MCF-6. 3.3. Effect of the surface chemistry on dextran adsorption Propylamine functionalised mesoporous materials have been used in protein separation [3] because the amine group on the surface can possess anion exchange property. However, in our experiment, no effect on the adsorption and desorption of dextran was observed.
584
0
0.02
0.04
0.06
equilibrium concentration (mM)
0.01 0.02 0.03 0.04 equilibrium concentration (mM)
Fig. 2. Dextran adsorption isotherms of SBA-15, MCF-3 and MCF-6. Figure 2a is the comparison for the adsorption behaviour of the three supports using dextran B (av. M.w. = 17.5 kD) and dextran C (av. Mw. = 40 kD) was used in figure 2b. (Key: • SBA-15, D MCF3 and x MCF-6) 4. CONCLUSIONS Dextran adsorption isotherms have been used as a model to study the accessibility of the porous structure of mesoporous molecular sieves by biomolecules. The pore size of the supports and the molecular weight of dextrans have significant effect on the adsorption isotherms. Because of their larger pore size, MCF materials seem to be more useful for biological applications than SBA-15. ACKNOWLEDGEMENT We would like to thank BASF for kindly supplying the Pluronic P-123 surfactant, and EU for funding HIIPY (project no. G5RD-CT-2001-00534). REFERENCES 1. H.H.P. Yiu, C.H. Botting, N.P. Bottign, P.A.Wright, Phys. Chem. Chem. Phys. 3 (2001). 2. H.H.P. Yiu, P.A.Wright, N.P. Botting, J. Mol. Catal. B: Enzym. 15 (2001) 81. 3. Y.J. Han, G.D. Stucky, A. Butler, J. Am. Chem. Soc, 121 (1999) 9897. 4. H.H.P. Yiu, P.A.Wright, N.P. Botting, Microporous Mesoporous Mater. 44 (2001) 763. 5. P. Schmidt-Winkel, C.J. Glinka, G.D. Stucky, Langmuir, 16 (2000) 356. 6. Lisa M. Higgins, M.Ph. Thesis, University of Greenwich, 1996. 7. Y.J. Han, J.T. Watson, G.D. Stucky, A. Butler, J. Mol. Catal. B: Enzym. 17 (2002) 1.
Studies in Surface Science and Catalysis 146 Park et al (Editors) ©2003 Elsevier Science B.V. All rights reserved
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One pot synthesis of mesoporous ternary V205-Ti02-Si02 catalysts V. Parvulescu^ V. I. Parvulescu^*, M. Alifanti^'", S. M. Jung^ and P. Grange'^ ^Institute of Physical Chemistry of the Romanian Academy of Sciences, Splaiul Independentei 202, Bucharest, Romania. ^University of Bucharest, Faculty of Chemistry, Department of Chemical Technology and Catalysis, B-dul Regina Elisabeta 4-12, Bucharest 70346, Romania, E-mail: V_PARVULESCU@chim.upb.ro. c
Universite Catholique de Louvain, CATA, Croix du Sud 2/17, 1348 Louvain-la-Neuve, Belgium. Mixed vanadia-titania-silica catalysts (3 or 6w^t% V2O5, and 16-34wt% Ti02) were one pot prepared by sol-gel and hydrothermal methods in the presence of different surfactants. Tetraethylortosilicate (TEOS) was used as precursor for silica, tetraisopropylorthotitanate (TIPOT) for titania, and vanadyl sulfate for vanadia. As surfactants were used cetyltrimethylammonium bromide (CTMABr) or octadecyl-trimethylammonium bromide (ODCABr). The catalysts were characterized by adsorption and desorption curves of N2 at 77 K, NH3-DRIFTS, H2-TPR, XRD, in-situ Raman spectroscopy, XPS, and TEM. 1. INTRODUCTION Almost all the preparations concerning ternary vanadia-titania-silica considered the deposition of vanadium on mixed Ti02-Si02 oxides prepared either by co-precipitation or by sol-gel method. Shikada et al. [1] and Odenbrand et al. [2] prepared mixed Ti02-Si02 oxides by co-precipitation and vanadium was subsequently introduced by impregnation with NH4VO3. Shikada et al. [1] and Vogt et al. [3] proposed a procedure in which both titanium and vanadium were introduced by impregnation from several precursors. Rajadhyaksha et al. [4] prepared silica-supported titania samples and then V2O5 was impregnated on these supports. Handy et al. [5] also prepared V205-Ti02-Si02 catalysts by reacting vanadyl triisopropoxide with sol-gel prepared Ti02-Si02 mixed oxide. Reiche et al. reinvestigated the structural properties of these catalysts [6] by preparing vanadia grafted on Ti02-Si02. Very recently, Sorrentino et al. [7] attributed the performances of vanadia grafted on Ti02-Si02 catalysts to the size of V2O5 clusters. Here we report for the first time on the one pot synthesis of mesoporous ternary vanadiatitania-silica catalysts for reduction of nitrogen oxides with ammonia. Several questions like the location and the state of vanadia, thermal and hydrothermal stability of the catalysts were adressed.
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2. EXPERIMENTAL VTS1-VTS4 catalysts were obtained by the polymeric sol-gel method when the molar ratio of Si02/Ti02A^205 was 1.00/0.14/0.022. The molar ratio of surfactant/silica was 0.2/1. For the VTSl sample, the sol-gels of silica (A) and titania (B) were obtained by two different modalities. As for (A), a mixture of TEOS, ethanol and water (Si02/EtOH/H20 for a molar ratio of 1/5.2/5.2) was refluxed (pH=l) at 353 K for 2h. Separately for (B), a mixture of TIPOT, 2-propanol and acetic acid (TIPOT/C3H7OH/CH3COOH molar ratio: 1/9.5/1.5.2) was stirred for 3h. CTMABr was added as mixture (C). This resulted by mixing CTMABr with water for Ih. Vanadyl sulfate was added, under stirring, to the mixture (B). Then the (A+B+C) mixture was mixed and the gelation was carried out at room temperature. Acetylacetone was added to mixture (B) after the addition of vanadyl sulfate (the amount of acetylacetone was calculated as function of Ti02; the molar ratio Ti02/acetylacetone was 1/0.15). VTS2-VTS4 samples were prepared by mixing firstly the silica sol-gel (A) with the titania sol (B). For these catalysts, the mixture (B) was obtained by mixing a solution of TIPOT in 2-propanol with acetylacetone and vanadyl sulfate. The solution (C) was added to the mixture (A+B) and the gelation was carried out at room temperature. The composition of the gel was 1.00 Si02: 0.14 Ti02: 0.022 V2O5 for VTS3 and VTS4, and 1.00 Si02: 0.14 Ti02: 0.06 V2O5, for VTS2. For VTS4 the surfactant was ODCABr. The catalysts were characterized by adsorption and desorption curves of N2 at 77 K, NH3DRIFTS, H2-TPR, XRD, in-situ Raman spectroscopy, XPS, and TEM. Activity measurements were performed in a continuous flow fixed bed reactor operating at atmospheric pressure on 0.08g of the sample. The total flow rate was lOOml/min and feed composition was: nitric oxide 0.1vol%; ammonia 0.1vol%; 3vol% oxygen, in helium. The inlet and outlet gas compositions were measured using a quadrupole mass spectrometer QMC 311 Balzers coupled to the reactor. 3. RESULTS AND DISCUSSIONS The use of the sol-gel method in the presence of surfactants led to very high surface area materials (Table 1). The structures resulted in these preparations were better organized, resulting in MCM-41 textures as for VTS2, VTS3 and in a small extent for VTSl and VTS4. TEM analysis confirmed such a behavior (Fig. 1). Monomodal pore size distribution was determined for these samples with a BJH diameter between 2 and 6 nm (Fig. 2). However, differences exist between these procedures, and that used for the VTS2-VTS4 catalysts led to more organized textures. Actually, this procedure seems to provide a more intimate interaction of titanium and vanadium in the silica matrix. The agglomeration of vanadium is supposed to determine ruptures in the MCM channels, and as a direct consequence, smaller surface areas. The differences between VTS2-VTS3 and VTS4 are related to a direct effect of the surfactant. These above textures seemed to be fairly stable because no change was observed after 6 h reaction, neither from the adsorption-desorption curves of N2 at 77K nor from the TEM analysis. Vanadium is better dispersed in these structures as result both from the values of the XPS binding energies and from the Raman evidence of mono-oxo tetrahedral monomeric vanadyl species. The same XPS investigation showed that titanium is tetrahedrally coordinated in these samples. H2-TPR profiles indicated a very small hydrogen consumption with peaks centered at high temperatures, namely over 773 K. These results come in the same
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Table 1 Chemical composition and textural characteristics of the investigated catalysts Catalyst Chemical composition, wt.% Surface area Pore size V2O5 Ti02 Si02 m^ g'^ , nm 16 81 709 VTSl 5.5 16 81 1206 VTS2 3.7 32 62 994 VTS3 2.1 34 63 873 VTS4 3.3 Table 2 XPS binding energies, relative XPS and chemical ratios Catalyst Binding energy, eV Atomic XPS ratios V/(Ti+Si+V) Si2D Ti2p3/2 V2p3/2 0.17 459.5 517.5 103.3 VTSl 0.20 459.4 517.4 VTS2 103.8 0.26 459.5 517.8 103.8 VTS3 517.4 0.17 459.3 103.6 VTS4
Atomic chemical ratios V/(Ti+Si+V) 0.023 0.023 0.047 0.023
line with XPS and Raman spectroscopy data indicating that for these catalysts both titanium and vanadium are more rigidified in a defined surrounding. NH3-DRIFT spectra indicated for all the investigated catalysts the existence of both Bronsted (1430 cm"') and Lewis (1610 cm"') acid sites. The source of the acidity might be either the linkage between Ti and Si or V and Si or V and Ti. The increase of the temperature leads to a decrease of the intensity of the band located at 1430 cm"', which almost disappeared at 523 K indicating that in such conditions the Bronsted acid contribution is very low. A
Pore diameter, Fig. l.TEM picture of VTS3
Fig. 2. Pore size distribution for VTS3
588
100 T
Fig. 3. NO conversion over the investigated catalysts comparatively to the commercial catalyst (total flow rate lOOml/min, 80 mg catalyst, 0.1vol% nitric oxide, 0.1 vol% ammonia, 3vol% oxygen, in helium, 350 °C).
Fig. 4. Selectivity of the investigated catalysts (total flow rate lOOml/min, 80 mg catalyst,0.1vol% nitric oxide, 0.1vol% ammonia, 3vol% oxygen, in helium, 350
decrease of the intensity also occured for the bands due to ammonia adsorbed on Lewis acid sites, but these are still present after the temperature was raised to 523 K. Figs. 3-4 indicate the catalytic performances of the above catalysts. Except for the commercial V205/Ti02 catalyst, a direct relation between the conversion and the surface area of these materials has been determined. This means that, indeed, in terms of turnover the commercial catalyst is still more active, but in terms of productivity the new catalysts are more effective. These data may also confirm the dispersion of vanadia inside the mesoporous structure. The selectivity of these catalysts was found to be superior to that exhibited by the V205/Ti02 catalyst (Fig. 4). In conclusion, one pot synthesis of mesoporous ternary V205-Ti02-Si02 systems led to catalysts which exhibit a good productivity in selective catalytic reduction of NO with ammonia. In addition, these systems exhibit a good stability under the investigated catalytic conditions and a good selectivity. REFERENCES 1. T. Shikada, K. Fujimoto, T. Kunugi, H. Tominaga, J. Chem. Technol. Biotechnol. A33 (1983)446. 2. C. U. I. Odenbrand, S. T. Lundin, L. A. H. Anderson, Appl. Catal., 18 (1985) 335. 3. E. T. C. Vogt, A. Boot, A. J. Van Dillen, J. W. Geus, F. J. J. Janssen, F. M. G. van den Kerkhof, J. Catal., 114(1988)313. 4. R. A. Rajadhyaksha, G. Hausinger, H. Zeilinger, A. Ramstetter, H. Schmelz, H. Knozinger, Appl. Catal., 51 (1989) 67. 5. B. E. Handy, A. Baiker, M. S. Marth, A. Wokaun J. Catal., 133 (1992) 1. 6. M. A. Reiche, E. Orteli, A. Baiker, Appl. Catal B., 23 (1999) 187. 7. A. Sorrentino, S. Rega, D. Sannino, A. Magliano, P. Ciambelli, E. Santacesaria, Appl. Catal. A: General, 209 (2001) 45.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Photocatalytic hydroxylation of benzene on Ti-modified MCM-41 with both framework and Non-framework Ti centers Zongying GUO' Jing HE"* Shichao ZHANG*' D.G EVANS" Xue DUAN"* "The Key Laboratory of Science and Technoloy of Controllable Chemical Reactions, Beijing University of Chemical Technology, Ministry of Education, Beijing 100029. ^Beijing University of Aeronautics and Astronautics, Xuanyuan Lu, Beijing. Ti-modified MCM-41 possessing well ordered long-range and pore structures has been prepared and characterized by XRD, low temperature N2 adsorption-desorption measurements, AAS, UV-visible diffuse reflectance spectroscopy and XPS technique. The hydroxylation of benzene was carried out using Ti-MCM-41 containing both framework and non-framework Ti centers as photocatalyst with molecular oxygen as oxidant in the absence of any co-oxidant species. 1. INTRODUCTION There is currently great interest in catalytic oxidation technologies for the production of bulk and fine chemicals that are cleaner, safer and more environmentally friendly than traditional methods''"^'. Using redox molecular sieves as catalysts and H2O2 in aqueous solution as the oxidant, organic compounds can be transformed without the production of environmentally unfriendly side products. The high cost and low efficiency of H2O2 however, limits its applications on an industrial scale. In previous research^"'' we found that Ti-substituted MCM-41 exhibited high initial conversion of benzene and selectivity to phenol in the hydroxylation process. The benzene conversion decreased with the number times the catalyst was reused however, resulting most probably from the changes in the long-range and pore structures of Ti-substituted MCM-41 and, especially, the local structure of the Ti sites. According to the possible mechanism reported for TS-l'^', tetrahedral framework Ti centers are the active sites for selective oxidation reactions. The transformation of some framework Ti to non-framework Ti centers will therefore result in the decrease in the selective oxidation activity of Ti-substituted MCM-41. In this paper we investigate whether the hydroxylation in the liquid phase will result in the occurrence of non-framework Ti centers and whether these Ti centers can be used as photocatalytic sites on which molecular oxygen is transformed to the active oxidant species either for the hydroxylation of benzene on framework Ti centers or to interact directly with benzene to produce phenol. A previous report ^^^ on the photocatalytic oxidation of hydrocarbons in the liquid phase to give oxygenates indicates that the photocatalysis may be a potential low-temperature alternative for selective oxidation processes with molecular oxygen as oxidant. 'This work is supported by the State Major Basic Research Project of China (2000048009) and NSFC (20173003) *Co-corresponding authors E-mail: iinghe(g',263.net.cn or duanx(g),mail.buct.edu.cn
590
2. EXPERIMENTAL Ti-substituted MCM-41 samples (also denoted as fresh Ti-MCM-41) were synthesized as described in our previous paper^^l Fresh Ti-MCM-41 was used as the catalyst for the hydroxylation of benzene at 65 °C with 30% H2O2 in aqueous solution as oxidant and acetone as solvent, to produce the spent catalyst (also denoted as used Ti-MCM-41). MCM-41 supported Ti02 was prepared by physically mixing of Ti02 with MCM-41. The total Ti contents determined by AAS are close to 4wt.% in all three samples. The atomic absorption spectra (AAS) measurements were carried out on a HITACHI Z-8000 atom absorption spectrometer. Powder XRD patterns were obtained using an XRD-6000 diffractometer with Cu Ka radiation, step size of 0.02° and scan rate of l°/min. The low-temperature N2 adsorption-desorption experiments were carried out using a Quantachrome Autosorb-1 system. The XPS spectra were obtained using a VG ESCA LAB 5 spectrograph with Al K^ radiation (9 kV, 18.5 mA). Diffuse reflectance UV-visible spectra were recorded by means of a TU-1221 spectrometer equipped with an integrating sphere attachment using BaS04 as background. Photocatalytic reaction was performed in a quartz reactor using a high-pressure mercury lamp (100 W, >t=200-700 nm) as UV-light source. Air was bubbled into the reactor during the reaction time with a flow rate of 80 ml/min. H2O2 yield was determined by titration with KMn04 standard solution after deionized water(100 g) and catalysts(0.1 g) were irradiated by UV light at 40°C for 3h. OH yield was determined as described in literature'^'. Photocatalytic hydroxylation of benzene was performed on the same photocatalytic reactor using acetic acid as solvent. Benzene (6 g, 77 mmol), acetic acid (15 g, 250 mmol) and deionized water (5 g, 278 mmol) were added to a specified quantity (0.1 g if not indicated) of catalyst. The mixture was vigorously stirred with irradiation at 40°C for 3h with an air flow of 80ml/min. The products were analyzed using a TU-1221 UV-visible spectrometer. 3. RESULTS AND DISCUSSION 3.1. Structural characteristics of catalysts The structural characteristics of catalysts were characterized by XRD, N2 adsorption experiment, UV-Visible diffuse reflectance spectroscopy and XPS technique. The XRD patterns (shown in Fig. 1) indicate that the used Ti-MCM-41 retains the well-ordered long-range structure of the fresh material. The results obtained from N2 adsorption-desorption isotherms given in Table 1 show that, the used Ti-MCM-41 sample possesses pore structure characteristic typical of mesoporous materials including a narrow distribution of pore diameters, high mesopore volume and specific surface area. The properties of the used catalyst are similar to those of the fresh catalyst and the Ti02/MCM-41 mixture. The XPS results (see Table 1) show that the Ti 2p3/2 B.E of used Ti-MCM-41 lies between the B.E values of fresh Ti-MCM-41 in which the Ti centers are located largely in the framework and Ti02/MCM-41 in which the Ti centers can be completely attributed to non-framework Ti sites. The XPS results indicate that the Ti centers in the used Ti-MCM-41 probably comprise both framework and non-framework Ti as predicted. The UV-Visible diffuse reflectance spectrum of fresh Ti-MCM-41 (Fig.2 (a)) shows a maximum absorption at about 240 nm which can be assigned to tetrahedral framework Ti (IV) centers'^''^'. For the used Ti-MCM-41 (Fig.2 (b)), the maximum shifts to about 270 nm with a
591
Table 1 Structural characteristics of Ti-modified MCM-41 Catalyst UsedTi-MCM-41 Fresh Ti-MCM-41 Ti02/MCM-41
Pore diameter at the maximum
Pore volume
Specific surface area
Surface Ti/Si molar ratio
2.7 2.7 2.7
0774 0.78 0.93
1042 1055 1258
0.12 0.07 0.07
An^l
Ti 2p3/2
B.E (eV) 458.6 459.9 458.1
shoulder at 240 nm. The origin of UV-visible absorption maxima at about 260-280 nm has been the subject of some dispute in the literature ^^'^l Calcination of used catalyst did not give a material with an absorption maximum at the same position as that of the fresh catalyst however (Fig.2(c)), indicating that the absorption maximum at 260-280 nm most probably results from non-framework Ti (IV) centers. Comparison of the spectrum of used Ti-MCM-41 (Fig. 2(b)) with that of Ti02/MCM-41 (Fig. 2(d)) indicates that bulk Ti02 is not present in the used catalyst. The diffuse reflectance UV-Visible spectra indicate that the used Ti-MCM-41 contains both framework and non-framework Ti centers, consistent with the XPS results discussed above. c di
u
c en
X)
b a
O
C/3
X3
<
^A—\d V
\ \ \
\
\\ \\ \\ \ ^\ "~^-.
200
Fig. 1. XRD patterns of (a) fresh and (b) used Ti-MCM-41
^
300
400
500
600
Wavclcnlh/nm Fig. 2. UV-Visible diffuse reflectance spectra of (a) fresh Ti-MCM-41, (b) used Ti-MCM-41, (c) used Ti-MCM-41 regenerated by calcination and (d) Ti02/MCM-41
3.2. Photocatalytic performance of used Ti-MCM-41 catalyst Table 2 shows the yields of OH radicals and H2O2 as well as the TON for benzene and phenol selectivity for Ti-MCM-41 with both framework and non-framework Ti certers (used Ti-MCM-41), fresh Ti-MCM-41 and Ti02/MCM-41. The H2O2 yield on used Ti-MCM-41 is lower than that on Ti02/MCM-41, and the yield of OH radicals on the former is similar to that on the latter, but all higher than those on fresh Ti-MCM-41, indicating that the capacity to activate molecular oxygen decreases in the order Ti02/MCM-41>used Ti-MCM-41 >fresh Ti-MCM-41. In the hydroxylation of benzene however, used Ti-MCM-41 gives higher TON for benzene than Ti02/MCM-41 probably because the used Ti-MCM-41 contains active sites for both photocatalysis and hydroxylation reactions. Blank experiments in the absence of UV light runs gave no measurable amount of oxidation products, thereby demonstrating that the
592
hydroxylation reaction was proceeding via a photocatalyzed mechanism. Unlike a previous report^^^ in which the selective hydroxylation of benzene to phenol under UV irradiation was carried out with H2O2 as additional oxidant, in this work we have shown that hydroxylation of benzene can be carried out in the absence of any oxidant apart from the oxidant species produced by photocatalytic activation of molecular oxygen. Table 2 The photocatalytic performance of different catalysts Fresh Ti-MCM-41 Catalyst >0.12 H2O2 yield per mol Ti (mol) •OH yield per mol Ti (mol) 0.37 TON for benzene 0.9 Selectivity to phenol 96%
Ti02/MCM-41 >0.72 3.05 0.4 92%
Used Ti-MCM-41 >0.26 3.51 1.8 63%
REFERENCES 1. R.A.Sheldon, J. Mol. Catal. A: Chemical, 107 (1996) 75 2. I.W.C.E.Arends, R.A.Sheldon, M.Wallau and U.Schuchardt, Angew.Chem.Int.Ed.Engl., 36(1997) 1144 3. J.He, W.Xu, D.GEvans and X.Duan, Microporous Mesoporous Mater., 44-45(2001) 581 4. M.A.Gonzalez, S.GHowell and S.K.Sikdar, J.Catal., 183(1999) 159 5. S.Bordiga, S.Coluccia, C.Lamberti, L.Marchese, A.Zecchina, F.Boscherini, F.Buffa, F.Genoni, GLeofanti, GPetrini and GVlak, J.Phys.Chem., 98 (1994) 4125 6. RZhang, X.Guo, X.Wang, GLi, J.Zhou, J.Yu and C.Li, Catal.Lett., 72 (2001) 235 7. M.H.Zahedi-Niaki, M.RKapoor, and S.Kaliaguine. J. Catal., 177(1998) 21 8. K.Zama, A.Fukuoka, Y.Sasaki, S.Inagaki, Y.Fukushima and M.Ichikawa, Catal.Lett., 66(2000)251 9. K.Ishibashi, A.Fujishima, T.Watanabe, K.Hashimoto, J. Photochem. Photobiol. A: Chemistry, 134(2000) 139
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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The relationship between the local structures and photocatalytic reactivity of Ti-MCM-41 catalysts Yun Hu^, Gianmario Martra'', Shinya Higashimoto^, Jinlong Zhang*^, Masaya Matsuoka^, Salvatore Coluccia'^ and Masakazu Anpo^* ^Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan. E-mail: anpo@ok.chem.osakafu-u.ac.jp. ^Dipartimento di Chimica IFM, Universita di Torino, Via P. Giuria 7, 10125 Torino, Italy. '^Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, P.R. China. A series of Ti-MCM-41 materials with Ti loading in the 0.15-2.00 wt% range was synthesized as photocatalysts for the decomposition of NO into N2 and O2, and characterized by XRD, XAFS (XANES and FT-EXAFS), diffuse reflectance UV-Vis, photoluminescence spectroscopies, and FTIR spectroscopy of the adsorbed ammonia as a probe molecule. 1. INTRODUCTION Nitric oxide, a highly toxic air pollutant, is emitted largely from the reaction between N2 and O2 in high temperature combustion processes, and its effective abatement is currently an urgent and challenging issue. Meanwhile, microporous and mesoporous materials functionalized with transition metal ions (TMI) have attracted much attention as effective photocatalysts for the decomposition of NOx [1-3]. Such systems exhibit the advantages of having a high dispersion of TMI due to the high internal surface area, and of a nano-scaled pore reaction field. However, their photocatalyitc activity appears to be mainly dependent on the local structure of the TMI centers, although the relationship between the local structure and photocatalytic reactivity has yet to be clarified. In this work, we report on the structural, photophysical and photocatalytic features of Ti(lV) centers, and their interconnections with the mesoporous MCM-41 type materials by various investigation techniques. 2. EXPERIMENTAL MCM-41 and Ti-MCM-41(x) (content of Ti as wt%, x= 0.15, 0.60, 0.85, 2.00) were prepared by an ambient temperature method [4], using tetraethylorthosilicate, tetraisopropylorthotitanate and cetyltrimethylammonium bromide as the sources of the silica, titanium and template, respectively. The XRD patterns (Shimadzu XD-Dl, Cu Ka radiation) of the samples calcined at 823 K (in air, 6 h) were recorded. The calcined samples were outgassed at 723 K (2 h) and characterized by XAFS (XANES and FT-EXAFS, Photon Factory, Tsukuba, Japan; Ti K-edge absorption spectra This work was supported in part by the Research Institute of Innovative Technology for the Earth, Japan.
594 recorded in the transmission or fluorescence mode), diffuse reflectance UV-Vis absorption (Shimadzu UV-2200A), photoluminescence (Spex 1943D3) and IR (adsorbed NH3; Jasco FT/IR 660) spectroscopies. UV irradiation of the catalysts under NO was carried out using a 100 W Hg lamp (k > 240 nm) at 295 K. The reaction products were analysed by gas chromatography. 3. RESULTS AND DISCUSSION The XRD patterns showed that the Ti-MCM-41 catalysts have a MCM-41 mesoporous structure. The XAFS experiments indicated that in Ti-MCM-4 catalysts, the Ti(IV) centers are tetrahedrally coordinated to the oxygen anions even for a Ti content as high as 2.00 wt%. Nevertheless, as can be seen in Fig. 1, their diffuse reflectance UV-Vis spectra exhibited different degrees of dependence on the Ti loading. In fact, besides a progressive increase in intensity, the increase in the Ti content resulted in a progressive shift of the absorption maximum from 205-208 nm [Ti-MCM-41(0.15) and Ti-MCM-4 l(0.60)](Fig. 1, a, b) to 215 nm [Ti-MCM-41(2.00)] (Fig.l,d), while a shoulder at ca. 230 nm became evident in the spectrum of Ti-MCM-41(0.85) (Fig.l, c), contributing to a larger extent to the spectral profile of the Ti-MCM-41(2.00) sample. All the components cited above are in the absorption ranges where the ligand-to-metal charge transfer involving an electron transfer from O^' to Ti"^^ of the tetrahedrally coordinated Ti-oxide species to form its charge transfer excited state, (Ti^^-O)* [5]. The bands at 205-215 nm may be attributed to the isolated tetrahedrally coordinated Ti-oxide species, while we propose that the band at 230-250 nm is due to the dimeric or oligomeric tetrahedrally coordinated Ti-oxide species, the formation of which may be favoured at higher Ti loadings. Moreover, the tail at >t > 250 nm observed for Ti-MCM-41(2.00) can be attributed to a minor moiety of the Ti(IV) centers in pentaand/or octahedral coordination [6]. In order to monitor the relative amount of the Ti-oxide species exposed at the surface of the wall of the various Ti-MCM-41 materials, the IR spectra of ammonia irreversibly adsorbed at room temperature were recorded. No bands due to the adsorbed species were observed for pure MCM-41. Conversely, the spectra of the Ti-MCM-41 samples in contact with ammonia exhibit, in
215 1
iq08
c
l\ 1 \
D CD
Q)
n r 205\ D
0.02 A.u
230
d
o
\c
A'
1 1
200
MCM-41
^
250 300 Wavelength / nm
350
Fig. 1. Diffuse reflectance UV-vis spectra of the Ti-MCM-41 with different Ti contents (a) 0.15, (b) 0.60, (c) 0.85, (d) 2.00 wt%.
1800
1700 1600 1500 1400 Wavenumber (cm"') Fig. 2. FT-IR spectra of NH3 molecules adsorbed on MCM-41 and Ti-MCM-41 (a, 0.15; b, 0.60; c, 0.85; d, 2.00 Ti wt%) observed after admission of 10 Torr NH3 and subsequent outgassing at room temperature for 1 minute.
595
the 1800-1350 cm" range, a characteristic band at 1608 cm' (5asym NHsads) due to the NH3 Lifetime (ms) molecules irreversibly adsorbed on the Ti(IV) 0.110 r "^ V b centers of the tetrahedrally coordinated 0.098 Ti-oxide species [7], as shown in Fig. 2. The Sy 0.027 increase in the Ti content leads to an increase in \ 0.024 yd the intensity of this band, indicating that the amount of tetracoordinated Ti(IV) sites exposed at the surface walls of the channels of the Ti-MCM-41 materials increased with the Ti / ^ loading. The bands at 1708, 1660, 1453 cm' can be assigned to the vibrational bending / / ^ modes of NR^"^, formed by a protonation of the ammonia molecules by some acidic surface dir''^^ 1 1 1 r "^ hydroxyl groups. The band at 1553 cm' is 350 400 450 500 550 600 650 700 attributed to the Ti-NH2 or Si-NH2 bending Wavelength /nm mode formed by the irreversible reaction of Fig. 3. The photolumincsccncc spectra of Ti-MCM-41 with different Ti contents (a) NH3 with the Si-O-Ti bridges or distorted 0.15, (b) 0.60, (c) 0.85, (d) 2.00 wt% measured surface Si-O-Si bridges formed due to the at 295 K. incorporation of Ti into the Si-O-Si networks [7]. As shown in Fig. 3, the Ti-MCM-41 catalysts exhibit a photoluminescence spectrum at around 470 nm upon excitation at around 240 nm at 295 K. The observed photoluminescence spectra were attributed to the radiative decay process from the charge transfer excited state to the ground state of the isolated Ti-oxides in tetrahedral coordination [8-10]. As also shown in Fig. 3, the overall intensity of the photoluminescence increases with an increase in the Ti content up to 0.60 wt% and then decreases sharply for the higher Ti content. Furthermore, it was found that an increase in the Ti content from 0.60 wt% to 0.85 wt% leads to a decrease in the phosphorescence lifetime from 0.1 ms to 0.025 ms. On the basis of these data, it can be proposed that, when photoexcited, only the isolated tetrahedrally coordinated Ti-oxide species, which are more abundant for Ti contents up to 0.60 wt%, can stay in the excited state long enough to allow the occurrence of some radiative decay to the ground state. The dimeric and/or oligomeric tetrahedrally coordinated Ti-oxide species, which are likely to be the overwhelming species present at higher Ti loadings, decay quickly to the ground state through not-radiative, vibrational relaxation processes, which apparently are favoured by their clustered structure. We have found that UV irradiation of 0.60 0.85 Tiwt% Ti-MCM-41 in the presence of NO leads to Fig. 4. Relationship between the yields of N2, the the formation of N2 and O2, their yields intensities of the IR band of NH3 molecules increasing linearly against irradiation time. adsorbed on tetrahedral Ti(IV)-oxides (a) and These results clearly showed that the photoluminescence spectra (b) of Ti-MCM-41 photocatalytic decomposition reaction of NO with various Ti contents.
596
proceeds on Ti-MCM-41 at 295 K. As shown in Fig. 4, the efficiency of the photocatalytic decomposition of NO on these catalysts under UV irradiation at 295 K, which did not occur under dark conditions or by UV irradiation on the pure MCM-41, appears to be correlated to the intensities of the photoluminescence spectra. Thus, the amount of the isolated tetrahedrally coordinated Ti-oxide species, not the total amount of the Ti-oxide species exposed at the surface of the walls of the Ti-MCM-41 channels as monitored by the intensity of the 6asymNH3 band, appeared to play an important role. These results suggest that only the highly dispersed isolated tetrahedrally coordinated Ti-oxide species act as active sites in the photocatalytic decomposition ofNO into N2 and O2. 4. CONCLUSIONS The characterization of mesoporous Ti-MCM-41 prepared at ambient temperature with Ti contents in the 0.15-2.00 wt% range, using various spectroscopic methods such as XAFS, UV-vis, FT-IR, photoluminescence, showed that for Ti contents up to 0.60 wt%, isolated tetrahedrally coordinated Ti-oxide species are formed, while at higher Ti contents, dimeric or oligomeric Ti-oxide species, still with Ti(IV) in tetrahedral coordination, became overwhelming. The comparison between the intensity of the photoluminescence spectra, which are due only to the isolated tetrahedrally coordinated Ti-oxide species, and the yield in the photocatalytic decomposition of NO into N2 and O2 allowed us to conclude that only these species are responsible for such photocatalytic reactivity. REFERENCES 1. M. Anpo, Stud. Surf Sci. Catal., 130, 12th Int. Congr. Catal., Part A, A. Corma, F.V. Melo, S. Mendioroz, J.L.G. Fierro (Eds.), Elsevier, Amsterdam, (2000) 157. 2. M. Anpo, S. Higashimoto, Y. Shioya, K. Ikeue, M. Harada and M. Watanabe, Stud. Surf. Sci. Catal., A. Gamba, C. Colella, S. Coluccia (Eds.), Elsevier, Amsterdam, 140 (2001) 27. 3. J.-L. Zhang, M. Minagawa, T. Ayusawa, S. Natarajan, H. Yamashita, M. Matsuoka and M. Anpo, J. Phys. Chem. B., 104 (2000) 11501. 4. W. Zhang, M. Froba, J. Wang, P. T. Tanev, J. Wong and T. J. Pinnavaia, J. Am. Chem. Soc, 118(1996)9164. 5. L. Le Noc, D. Trong On, S. Solomykina, B. Echchahed, F. Beland, C. Cartier dit Moulin and L. Bonneviot, Stud. Surf. Sci. Catal., 101, 11th Int. Congr. Catal., Part A, J.W. Hightower, W.N. Delgass, E. Iglesia, A.T. Bell (Eds.), Elsevier, Amsterdam, (1996) 611. 6. L. Marchese, E. Gianotti, V. Dellarocca, T. Maschmeyer, F. Rey, S. Coluccia and J.M. Thomas, Phys. Chem. Chem. Phys., 1 (1999) 585. 7. L. Marchese, E. Gianotti, T. Maschmeyer, G. Martra, S. Coluccia and J.M. Thomas, II Nuovo Cimento, 19D(1997) 1707. 8. M. Anpo, N. Aikawa, Y. Kubokawa, M. Che, C. Louis and E. Ciamello, J. Phys. Chem., 89 (1985)5017. 9. J.-L. Zhang, Y. Hu, M. Matsuoka, H. Yamashita, M. Minagawa, H. Hidaka and M. Anpo, J. Phys. Chem. B., 105 (2001) 8395. 10. M. Anpo and M. Che, Adv. Catal., 44 (1999) 119.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
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Photocatalytic epoxidation of propene with molecular oxygen under visible light irradiation on V ion-implanted Ti-HMS and Cr-HMS mesoporous molecular sieves Hiromi Yamashita*, Keiko Kida, Keita Ikeue, Yukiya Kanazawa, Katsuhiro Yoshizawa, and Masakazu Anpo* Department of Applied Chemistry, Graduate School of Engineering Osaka Prefecture University, Gakuen-cho 1-1, Sakai, Osaka 599-8531, Japan FAX: +81-72-254-9287. E-mail: yamashita@chem.osakafu-u.ac.jp The metal ion-implantation with V ions was effective to shift the absorption band of Ticontaining mesoporous molecular sieves (V/Ti-HMS, V/Ti-MCM-41) to the longer wavelengths. V/Ti-HMS and V/Ti-MCM-41 performed the photoepoxidation of propene with molecular oxygen under UV irradiation with the longer wavelength (K> 340 nm), while no reaction proceeded on the original Ti-HMS. Cr-containing mesoporous molecular sieves (CrHMS) could absorb visible light and showed the photoepoxidation under visible light irradiation (\> 450 nm). The charge transfer excited state of the tetrahedral chromium oxide moieties dispersed on mesoporous silica played a significant role in the photocatalytic reaction. 1. INTRODUCTION Ti-containing mesoporous molecular sieves exhibit high photocatalytic reactivity for the epoxidation of propene with molecular oxygen under UV irradiation (220-260 nm). It is vital to develop a photocatalyst that can operate efficiently under visible light irradiation [1,2]. In the present study, we have investigated on the photocatalytic reactivities of Ti-containing mesoporous molecular sieves implanted with V ion by an ion-implantation method (V/TiHMS, V/Ti-MCM-41) and chemically produced Cr-containing mesoporous molecular sieves (Cr-HMS) for the photoepoxidation under UV irradiation with the longer wavelengths (K > 340 nm) and visible light irradiation {\> 450 nm). 2. EXPERIMENTAL Ti-HMS, Ti-MCM-41 (Si/Ti=100) and Cr-HMS (Si/Cr=50) were synthesized using tetraethylorthosilicate, titaniumisopropoxide and Cr(N03)-9H20 as the starting materials, respectively, and templates: dodecylamine (HMS), cetyltrimethylammonium bromide (MCM41). The metal ion-implantation with V ions (0.66 p mol/g-cat) to the parent Ti-HMS and TiMCM-41 was carried out using an ion-implanter consisting of a metal ion source, mass analyzer, and high voltage ion accelerator (150 keV). Prior to the photocatalytic reactions, the
598
catalysts were degassed and calcined in 02 at 723 K for 2 h, then degassed at 473 K for 2 h. The photoluminescence were measured at 77 K and XAFS spectra was measured in the fluorescence mode at BLOIBI of SPring-8 (project No.: 2002A0613). The photocatalytic oxidation of propene (16 u mol) with 02 (32 y mol) was carried out with the catalysts (50 mg) using a high-pressure Hg lamp through a UV cut filter (A > 250 nm, A, > 340 nm, X > 450 nm) at 295 K. The products collected in the gas phase and after heating of catalysts at 573 K were analyzed by g.c. 3. RESULTS AND DISCUSSION 3.1. V ion-implanted Ti-HMS molecular sieves The results on the XRD patterns and the BET surface area of the Ti-HMS and Ti-MCM-41 indicated that these catalysts have a hexagonal lattice having mesopores larger than 20 A with a high BET surface area. The results obtained using XAFS analysis indicated that the local structure of the titanium oxide species of the Ti-HMS and Ti-MCM-41 is highly dispersed and exists in tetrahedral coordination (tetra-Ti-oxide), while Ti02 powder have the octahedral coordination. This tetra-Ti-oxide can exhibit the unique photocatalytic reactivity. Although the tetra-Ti-oxide included within mesoporous silica can exhibit the high selectivity for epoxide formation in alkene oxidation, they can only absorb and utilize UV light at around 220-260 nm to form the charge transfer exited state as active species. Fig. 1 shows the effect of metal ion implantation on the diffuse reflectance UV-Vis absorption spectra of Ti-HMS having the tetra-Tioxide. As shown in Fig. 1, the V ion-implanted TiHMS can absorb the light at the longer wavelengths (-450 nm) while the original un-implanted Ti-HMS absorbs the UV light at around 220-260 nm. These results indicate that the metal ion-implantation is effective to modify the Ti-HMS to absorb the light 200 250 300 350 400 450 500 with the longer wavelengths and exhibit the Wavelength/nm photocatalytic reaction under irradiation at the longer Fig. 1. UV-Vis absorption spectra of wavelengths. the V ion implanted Ti-HMS. Table 1 shows the results of the photocatalytic oxidation of propene with 02 under Table 1 The products in the photocatalytic oxidtion of propene irradiation of light with the various with O2 on the various catalysts under the light irrdiation with wavelengths ( \ > 250 nm, \> 340 the various wavelengths. Selectivity / % Light Conv. PO-yield nm, A,> 450 nm). Under UV Catalysts /nm /% /% PO HC COx HO irradiation with the longer wavelengths (K> 340 nm), the photo-epoxidation of propene with 02 to form propylene oxide (PO) proceeded on the ion-implanted V/Ti-HMS and V/Ti-MCM-41, while no reaction occurred on the original un-implanted Ti-HMS.
V/Ti-HMS >340 V/Ti-MCM-41 >340 >340 TI-HMS Ti-HMS >250
0.8 0.9 0 7
0.2 0.2 0 1.5
26 24 0 22
66 17 0 57
8 60 0 11
0 0 0 10
Cr-HMS
>450
10
1.2
12
63
4
20
CrS-1
>450
3.3
0.1
4
67
21
25
HO: propanal+acetone+acrolein+ethanal+alcohols, HC: hydrocarbons, CO,: CO2+CO
599
3.2. Chemically produced Cr-HMS mesoporous molecular sieves The results of XRD analysis indicated that ACF-O Cr-O-Cr the Cr-HMS have the structure of HMS R/A N 1.98 5.5 mesoporous molecular sieves and the Croxide moieties are highly dispersed in the framework of HMS and the CrS-1 have the structure of MFI zeolite. As shown in XAFS spectra (Fig. 2), Cr-HMS and CrS-1 exhibit CO n a sharp and intense preedge peak in the Ow XANES regions which is characteristic of < Cr-oxide moieties in tetrahedral coordination (tetra-Cr-oxide). In the FTEXAFS spectrum, only a single peak due to 5990 6010 6030 6050 Energy / eV Distance / A the neighboring oxygen atoms (Cr-0) can be observed and the curve fitting analysis Fig. 2. XANES (A-C ) and FT-EXAFS ( a-c ) spectra. indicated that tetra-Cr-oxide existed as in an (a) imo-Cr/HMS. (b) Cr-HMS. (c) CrS-1. isolated state with two terminal bonds {Cr=0) in the shorter distance of 1.57 A and two single bonds (Cr-0) of 1.82 A, while CrS-1 has tetra-Cr-oxide with the high Td symmetry (four oxygen atoms at 1.78 A). As shown in Fig. 3, the UV-Vis spectra of Cr-HMS exhibits three distinct absorption bands at around 280, 370 and 490 nm which can be assigned to charge transfer from O^" to C/'^ of the tetra-Cr-oxide. The tetra-Cr-oxide in Cr-HMS with the low Td symmetry exhibits absorption band at 490 nm (forbidden Ai-Ti transition), while it was not observed with CrS-1 having tetra-Cr-oxide in the high Td symmetry. Cr-HMS exhibited a photoluminescence spectrum at around 550-750 nm upon excitation of the absorption (excitation) bands at around 250-550 nm. Fig. 4 shows the 0.8 \l photoluminescence spectra of Cr-HMS observed at Ky V a 77 K upon the excitation at 280, 370, 500 nm, 1 ^"^ respectively. These three spectra were observed 0 1 h ^ ^^^*'«T ^ 1 1.2 at the same position. In the excitation spectrum 1 A of Cr-HMS monitored at 640 nm (Fig. 4), three excitation bands are observed in the same wavelengths to those observed in the UV-Vis 0 I 1 1 ^^^~"'-'— absorption spectra (Fig. 3). These results suggest 200 300 400 500 600 that the photoluminescence occurs as the radiation Wavelength / nm decay process from the same excited state Fig. 3. UV-Vis absorption spectra of CrHMS(a), Cr-HMS with 25 Torr H2O (b), independently to the excitation wavelength. These CrS-1 (c),and Cr04^" in K2Cr04 solution (d).
Am
600
0^-
V
a
Scheme. 1 Charge transfer process of the tetrahedrally coordinated Cr-oxide moieties.
absorption and photoluminescence spectra can be attributed to the charge transfer processes on the tetra-Cr-oxide involving an electron transfer from O^' to Cr^^ and a reverse radiative decay [3,4], respectively, as shown in scheme. 1. After the addition of reactants onto the Cr-HMS the efficient quenching of the photoluminescence was found, their intensity depending on the amount of added gases accompanied by the shortening of the emission lifetime of the excited triplet state. These results indicated not only that the charge transfer excited state of the tetra-Cr-oxide easily interact with the added gases and plays a significant role in the photocatalytic reaction but also that the large mesoporous cavities are significant in inducing for efficient photoreactions. 550
800 650 700 750 Wavelength / nm Fig. 4. The photoluminescence spectra of Cr-HMS at 77K. Excitation wavelength : 370 nm (a), 500 nm (b), 280 nm (c).
600
Under visible light irradiation (A> 450 nm), the photoepoxidation of propene with molecular oxygen to form propylene oxide proceeded on the both Cr-HMS and CrS-1 and CrHMS exhibits the highest PO yields among the present catalysts. 4. CONCLUSIONS The metal ion-implantation with V ions was effective to shift the absorption band of Ticontaining mesoporous molecular sieves to the longer wavelengths. The ion-implanted V/Ti-HMS and V/Ti-MCM-41 were found to exhibit the photoepoxidation of propene with molecular oxygen even under UV irradiation with the longer wavelength ( \ > 340 nm ). Chemically produced Cr-HMS can absorb visible light and act as an efficient and selective photocatalysts under visible light irradiation. The charge transfer excited state of the tetrahedral chromium oxide moieties dispersed on mesoporous silica are responsible for the efficient photocatalytic reactivities. REFERENCES 1. 2. 3. 4.
Yamashita, H., Yoshizawa, K., and Anpo, M., Chem. Commun., 2001, 435. Murata, C , Yoshida, and H., Hattori, T., Chem. Commun., 2001, 2412. M. Anpo, I. Takahashi, and Y Kubokawa, J. Phys. Chem., 86 (1982) 1. M. F. Hazenkamp and G Blasse, J. Phys. Chem., 96 (1992) 3442.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Mesostructured TiOi films as effective photocatalysts for the degradation of organic pollutants Jifi Rathousky*, Marketa Slabova and Amost Zukal J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3, CZ-182 23 Prague 8, Czech Republic The porous structure of titania films prepared using a poly(alkylene) block copolymer in an ethanolic medium depends on their thickness because the completeness of the hydrolysis of the titania precursor is decided by its accessibility for air humidity. A well-organized film, whose photocatalytic activity is comparable with that of the most active commercial anatase powders, is formed only from a completely hydrolyzed precursor. 1.
INTRODUCTION
In the field of organized mesoporous materials most of the published experimental research has focused on silica as the inorganic framework constituent. The application of the reaction schemes originally developed for siliceous materials has been found much less successful for the synthesis of mesoporous transition metal oxides, especially titanium dioxide. Recently, a novel approach has been developed, enabling to obtain mesoporous titanium dioxide with highly interesting structural properties based on the usage of amphiphilic poly(alkylene oxide) block copolymers as structure directing agents in non-aqueous solution for organizing the network-forming titanium dioxide species [1]. Only very recently first successful preparations of mesoporous titania films have been reported [2-5]. In this communication the preparation of mesoporous titania films based on the mentioned approach will be analyzed with respect to the effects of decisive processing parameters and the obtained films will be tested in the photocatalytic destruction of an important organic water pollutant, viz. 4-chlorophenol. 2.
EXPERIMENTAL
2.1. Preparation of mesoporous titanium dioxide using block copolymers First, 0.9 g of Pluronic P-123 (BASF) were dissolved in 11 mL of ethanol. To this solution, 1 mL of titanium tetrachloride was added under vigorous stirring. The mixture was maintained in an open beaker at 40"C for 5 days, the evaporated ethanol being filled up every 12 h. Thus prepared clear yellowish solution could be stored at room temperature for several weeks without apparent changes. Films of different thickness were prepared by spreading various amounts of the stock solution on the glass support. The liquid layer was subsequently •Corresponding author; Telephone: +4202-66053865; Fax: +4202-86582307. E-mail: jiri.rathousky@jhinst.cas.cz.
602
gelled in air at 40°C for 7 days and calcined at 400°C for 5 h in air. Finally, the titania films were peeled off the support (samples I-A, I-B, I-C, I-D). 2.2. Measurements Scanning electron micrographs were obtained by a Hitachi S-900 apparatus. Powder X-ray diffraction data were collected with a Siemens D 5005 diffractometer in the Bragg-Brentano geometry using CuKa radiation. Adsorption isotherms of nitrogen and krypton were measured at -196°C with an ASAP 2010 instrument (Micromeritics). UV/Vis were measured with an Perkin Elmer Lambda 19 spectrometer. ESCA analysis was performed with a Scienta 310 instrument (Gammadata AB). Photocatalytic activity of the Ti02 samples was studied using 4-chlorophenol as model pollutant. Photodegradation of this compound was examined employing a tube photoreactor where Ti02 was dispersed in water. After illumination by a medium pressure arc mercury lamp with the dominating 366 nm line, 4-chlorophenol follows three separate reaction pathways: hydroxylation, substitution and direct charge-transfer oxidation forming 4-chlorocatechol, hydroquinone and non-aromatic compounds as primary intermediates, respectively. The reaction rate was calculated according to the first-order kinetics. 3. RESULTS AND DISCUSSION The stock solution for the film preparation contains an ethoxide-modified titanium chloride, formed by the reaction: TiCU + X EtOH -^ TiCl4-x(0Et)x + x HCl, where x « 2. The formed TiClx(OC2H5)4-x species, which are rather stable against hydrolysis, associate preferentially with poly(ethylene oxide) moieties to produce a self-assembling complex. The necessary prerequisite for the formation of ordered material is the hydrolysis of titanium-containing species. Due to their stability, this process is strongly dependent on such parameters as the sufficient supply of water vapor and the length of the hydrolysis. Finally, calcination in air removes quantitatively the organic template. Chemical analysis by XPS has confirmed that the product does not contain any detectable amounts of elements other than titanium and oxygen, i.e. the removal of the organic component was complete. Because of the intended application in the continuous effluent decontamination and the aimed study into the effect of the completeness of the hydrolysis on the structure properties of mesoporous titania, the samples were prepared in the form of films of variable thickness. With thin films (samples I-A and I-B, density of 2 mg/cm^ and 4 mg/cm^, respectively), the full hydrolysis occurs due to a good accessibility for the air humidity during the aging. This ensures the creation of a highly uniform and regularly arranged porous structure with a narrow pore size distribution as has been proved by SEM and N2 adsorption (Fig. 1). With medium (sample I-C, density of 6 mg/cm^) and thick films (sample I-D, density of 8 mg/cm^) the hydrolysis is far from being complete. Consequently, larger pores are formed in addition to smaller ones during the calcination of the non-hydrolyzed fraction. This leads to the formation of a bimodal porous structure (Fig. 1, right). The structure parameters of all the films studied are given in Table 1. X-ray diffractograms and the Raman spectra (not shown here) evidence that all the samples contain a pure anatase phase. The presence of an amorphous titania component is probable because X-ray diffractograms exhibit decreased intensity of reflections due to anatase in comparison with
603
Table 1 Structure parameters of films prepared using block copolymers d^ Sample SBET (mg/cm^) (mVg) (nm) I-A 105 4.4 I-B 94 4.6 I-C 127 4.0, 5.2 I-D 104 6.2,16.0 ^density of the film, BET surface area, ^ mean pore size (two values correspond to a bimodal porous structure).
Fig. 1. SEM image of the thinnest film I-A (left) and adsorption isotherms of N2 at -196°C (right). The start point of each isotherm is shifted by p/po = 0.4. The solid symbols denote desorption. a pure reference material (Bayer). This component does not seem, however, to exhibit any characteristic Raman signal, which would distinguished it from anatase. As the absorption spectra in the UV range (not given here) show that there is no shift in the position of the absorption edge due the size-quantization effect, the obtained films are suitable for photocatalyzing the mineralization of 4-chlorophenol due to illumination with given light source. It was recently demonstrated that mesoporous titania prepared using ligand assisted templating methods has low photocatalytic activity compared to the crystalline phase despite its high surface area [6]. This low activity is due to the incomplete extraction of the surfactant and the amorphous titania channel walls. The authors conclude that partially crystallized titania is essential for obtaining high photocatalytic activity. It this study we have found that by optimizing the synthesis condition a highly active photocatalysist can be synthesized using block copolymers, whose activity compares well even with the best commercial materials (such as PKP 09040, Bayer). There are, however, severe requirements, which should be met. The preparation of a highly active photocatalyst requires the complete hydrolysis of the precursor, as that is the case with samples I-A and I-B. Consequently such a photocatalyst is characterized by a regularly arranged porous structure
604
with a narrow pore size distribution. Rate constants of the decomposition of 4-chlorophenol calculated according to the first-order kinetics are given in Table 2. Table 2 Decomposition of 4-chlorophenol Rate constant of the decomposition of 4-chlorophenol Sample I-A I-B Non-optimum films Bayer
(loS-') 3.49 3.42 1.4-2.4 3.37
4. CONCLUSIONS The porous structure of titania films prepared using a poly(alkylene) block copolymer in an ethanolic medium depends on their thickness because the completeness of the hydrolysis of the titania precursor is decided by its accessibility for air humidity. Well-organized film is formed only from a completely hydrolyzed precursor. The photocatalytic activity of such organized film is comparable with that of the most active commercial anatase powder. ACKNOWLEDGMENT This work was supported by the Grant Agency of the Academy of Sciences of the Czech Republic (contract No. A4040804) and the Deutsche Forschungsgcmeinschaft (WA 1116/7-1). REFERENCES 1. P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka and G.D. Stucky, Nature, 396 (1998) 152. 2. L. Kavan, J. Rathousky, M. Gratzel, V. Shklover and A. Zukal, J. Phys. Chem. B 104 (2000)12012. 3. D. Grosso, G.J. de A.A. Soler-Illia, F. Babonneau, C. Sanchez, P.-A. Albouy, A.Brunet-Bruneau and A.R. Balkenende, Adv. Mater. 13 (2001) 1085. 4. H.-S. Yun, K. Miyazawa, H. Zhou, I. Honma and M. Kuwabara, Adv. Mat. 13 (2001) 1377. 5. J. Yu, J.C. Yu, W. Ho and Z. Jiang, New. J. Chem. (2002) 607. 6. V.F. Stone and R.J. Davis, Chem. Mater. 10 (1998) 1468.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Comparisons of the structural and catalytic properties of Ti-HMS synthesized using the hydrothermal and molecular designed dispersion methods Tristan Williams and G. Q. (Max) Lu ^The NanoMaterials Centre, School of Engineering, The University of Queensland., St Lucia, Queensland, 4072, Australia. Fax: +64-7-3365-6074. E-mail: tristanw@cheque.uq.edu.au Titanium containing wormhole-like mesoporous silicas, denoted Ti-HMS, synthesized both via the hydrothermal synthesis route and the post synthesis grafting technique, known as molecular designed dispersion, have been successfully applied in the gas phase oxidation of Toluene to CO and CO2. Selectivity towards CO2 for all catalysts, at temperatures between 400-600"C, was above 80%. Benzene and benzaldehyde were observed at temperatures above 450°C, but in very low concentrations. The conversion of toluene was shown to increase significantly when the VIHX/VMHSO ratios were increased from 0.07 to 0.84. No significant difference in catalytic activity was observed for catalysts prepared via the different synthesis techniques. The catalytic activity also depends on the concentration of tetrahedrally coordinated titanium atoms and not on the total concentration of titanium in the catalyst. 1. INTRODUCTION Ti-HMS materials exhibit greater catalytic activity than their corresponding Ti-MCM-41 counterparts. Some researchers [1, 2] have attributed this to the presence of textural mcsoporosity which facilitates access of reactant molecules to the active Ti sites. Others [3] hoverer, have observed very little difference in textural mcsoporosity between Ti-HMS and Ti-MCM-41 and have sought other factors to explain this difference. Since spectroscopic techniques cannot discriminate between the two materials, one such factor could be the location of Ti active sites on the silica wall. It has been observed that samples prepared by grafting Ti onto the surface of MCM-41 mesopore walls are just as active as Ti-HMS. Thus, they suggested that in contrast to Ti-MCM-41 the Ti species in Ti-HMS are concentrated on the surface of the mesopores and not randomly distributed in the silica framework. In this work we have compared the catalytic activity of the Ti-HMS synthesized via these two different routes. The effect of textural mcsoporosity on catalytic activity was also investigated. 2. EXPERIMENTAL The hydrothermal synthesis of Ti-HMS was performed as described by Zhang et al. [2] and the molecular designed dispersion method was performed as described by Schrijnemakers et
606
al. [4]. The molecular designed dispersion technique involved first making HMS samples with the desired structural properties and then grafting them with the desired amount of Titanium. N2 sorption measurements were performed on a Quantachrome Autosorb IC; UV-vis diffiise reflectance measurements and fixed wavelength absorbance measurements were performed on a Jasco UV-vis spectrometer. The catalj^ic tests were carried out at atmospheric pressure in a continuous flow, fixed-bed quartz microreactor. The reactor was loaded with 0.05 g of calcined catalyst and had a fixed bed volume of 0.25 cm' . Toluene was introduced into the reactor by flowing air through a saturator maintained at 95°C. The volumetric flow through the catalyst was held constant at 100 ml min"' and the concentration of toluene was 1000 ppm. The reactions were carried out at 400, 450, 500, 550 and 600°C. Before each reaction the catalyst was activated in air flowing at 400 °C for 30 min. The reactants and the reaction products were analyzed using two Shimadzu GC-17A gas chromatographs. Toluene and its incomplete oxidation products, benzene and benzaldehyde were measured online, using a 30 m DB-5 capillary column connected to a flame ionization detector. The O2, N2, CO and CO2 were analyzed on the second gas chromatograph fitted with a thermal conductivity detector and were separated using a 30 ft Porapak Q packed column. 3. RESULTS AND DISCUSSION The degree of textural mesoporosity of the Ti-HMS catalysts can be quantified using the VTI.:X/VMHSO ratio. This ratio is simply the textural mesopore volume, VTHX, divided by the mesopore volume VMI:SO. The textural pore volume is the difference between the total pore volume measured at a partial pressure of 0.98 and the mesopore volume. In Table 1 the properties of the catalysts investigated in this study are reported. Figure 1 compares two isotherms of Ti-HMS, one with low textural mesoporosity and the other with high textural mesoporosity. The high textural mesoporosity is indicated by the large increase in volume of nitrogen adsorbed above partial pressures of 0.9. Table 1 Physical and Chemical Properties of Catalysts Tested. Ti BJH Mesopore Total Textural VTEXA/MESO Synthesis BET Volume Volume Method ;Surface Pore Pore Loading Volume Area Radius cc/g cc/g Wt% A m'/g cc/g 1.07 0.42 17.0 MDD 0.65 6.99 768 0.65 1.30 0.69 0.77 0.53 5.93 909 15.5 MDD 1.01 HydroThermal 1152 0.12 0.14 13.7 3.73 0.89 0.47 15.4 1.30 HydroThermal 973 0.56 0.83 3.69 0.07 0.83 0.05 HydroThermal 939 13.8 0.78 6.69 1.71 0.84 17.2 0.78 HydroThermal 873 6.22 0.93 All catalysts are mesoporous and have high surface areas varying between 768 to 1159 m^/g. The titanium content varies between 3.69 Ti Wt 5 and 6.99 Ti Wt %.
607
1800
80 70
1500
-^^-3.7TlHMS-0.13 -<>-3.7TiHMS-0.56 -X - 6.7TiHMS-0.06
- High Textural Porosity - Low textural Porosity
60 ^ -^>-6.2TIHMS-0.84
1200
50
o
c .2
I 900
12
- A - MDD5.93TiHMS-0.5:^ -•.-.MDD6.99TiHMS-0.69
40
0)
g 30 o o
> 600
20
300
0.0
10 i
0.2
0.4
0.6
P/Po
0.8
1.0
Fig. 1. N2 sorption Isotherms of Ti-HMS.
200
300
400
500
Temperature (C)
Fig. 2. Conversion of Toluene.
Figure 2 shows the behavior of the catalysts in the oxidation of toluene to CO and CO2. Samples prepared via the molecular designed dispersion are indicated with the prefix MDD. The numerical prefix indicates the titanium content as a weight percent while the numerical suffix indicates the VTHx/VMiiso ratio. For all catalysts this oxidation gives rise to greater than 80% conversion to CO2 at all temperatures between 400 and 600"C. Benzene and benzaldehyde were observed at temperatures above 450°C, but below the calculated minimum detectable limit of the FID. Comparison of the catalysts 3.7Ti-HMS-0.13, 3.7Ti-HMS-0.56, 6.7Ti-HMS-0.06 and 6.2Ti-HMS-0.84, all prepared hydrothermally, show that under these flow conditions an increase in textural porosity from around 0.1 to above 0.5 results in a near doubling of toluene conversion at SOO^C. To investigate the effect the synthesis method has on toluene conversion, we compare catalyst 6.2Ti-HMS-0.84 with catalysts MDD5.93Ti-HMS-0.53 and MDD6.99Ti-HMS-0.69. Catalyst 6.2-Ti-HMS-0.84, synthesized hydrothermally, is less than 5% more active at 500°C than MDD5.93Ti-HMS-0.53, but this minor increase may be possibly attributed to its increased textural mesoporosity. Catalyst MDD6.99Ti-HMS-69, synthesized via molecular designed dispersion, however, is almost 10% more active than 6.2-Ti-HMS-0.84 at 500°C. This slight increase may possibly be attributed to its greater titanium content. In either case, the differences in toluene conversion are very small so it appears that the type of synthesis method has negligible impact on the activity of the catalysts. Another trend observed for all catalysts was that the titanium content has very little effect on the over all activity of the catalyst. To fiirther investigate the effect of titanium loading, it was necessary to perform UV-vis diffuse reflectance measurements on the catalysts. Figure 3 contains the diffuse reflectance UV-vis spectra of all 6 catalysts. Figures 3a, b and c clearly show 2 distinct peaks. The first is a sharp peak at 210 nm, which corresponds to Ti
608
species in the tetrahedral site-isolated form. The second much broader peak at 260-270 nm corresponds to site isolated Ti atoms in a penta- or octahedral coordination [2, 5]. r\ A
3.7TiHMS-0.14 3.7TiHMS-0.56
0.3 -
K
0.6 -
250 300 350 Wavelength (nm)
0.7 -1
0.6
0.5 -
0.5
80.4 -
80.4
\
5°^ 1
0.1 -
200
6.7TiHMS-0.07 1 6.2TiHMS-0.84 |
400
0.2
0.2 -
0.1 -
0.1 -
0 2C)0
250
300
350
W a v e l e n g t h (nm)
400
0 200
\
MDD5.971HMS-0.69 MDD6.9TiHMS-0.65
250
300
350
400
W a v e l e n g t h (nm)
Fig. 3a. UV-vis spectra of Fig. 3b. UV-vis spectra of Fig. 3c. UV-vis spectra of TiTi-HMS synthesized by the Ti-HMS synthesized by the HMS synthesized by the hydrothermal method. hydrothermal method. MDD method. It can be seen in Figures 3a, b and c that as the titanium content increases, the maximum absorbance measured at 210 nm varies only slightly, while the absorbance at 260-270 nm of the much broader peak increases significantly. This suggests that the catalytic activity of the catalysts is dependent on these tetrahedrally coordinated titanium atoms, and not on the total concentration of titanium in the catalyst. 4. CONCLUSIONS The catalytic oxidation of toluene has been investigated for a series of Ti-HMS catalysts. Toluene is oxidized to CO and CO2. Selectivity towards CO2 is approximately 80% and is not affected by changes in temperature. Benzene and benzaldehyde were the only other incomplete oxidation products observed, but in very low concentrations. Greater textural mesoporosity results in higher catalytic activity, while the type of synthesis method employed has negligible impact. UV-vis diffuse reflectance measurements show that catalytic activity depends on the concentration of tetrahedrally coordinated titanium atoms, and not on the total titanium concentration.
REFERENCES 1. Tanev, P.T., Chibwe, M., Pinnavaia, T.J., Nature, 1994. 368: p. 321-323. 2. Zhang, W., Froba, M., Wang. J., Tanev, P.T., Wong, J., Pinnavaia, T.J., J. Am. Cham. Soc, 1996. 118: p. 9164-9171. 3. Tuel, A., Microporous and Mesoporous Materials, 1999. 27: p. 151-169. 4. Schrijnemakers, k. and E.F. Vansant, J. Porous Mater, 2001. 8: p. 83-90. 5. Blasco, T., Corma, A., Navarro, M.T., Pariente, J.P., J. Catal., 1995. 156: p. 65.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
609
Oxidation of methyl-propyl-thioether with hydrogen peroxide using Ti-SBA-15 as catalyst D. C. Radu^ A. Ion\ V. I. Parvulescu^*, V. Campeanu^ E. Bartha^ D. Trong On' and S. Kaliaguine" ^University of Bucharest, Faculty of Chemistry, Department of Chemical Technology and Catalysis, B-dul Regina Elisabeta 4-12, Bucharest 70346, Romania, E-mail: V PARVULESCU@chim.upb.ro. ^Institute of Organic Chemistry of the Romanian Academy of Sciences, Splaiul Independentei 202, Bucharest, Romania. Department of Chemical Engineering, Laval University, Ste-Foy, Quebec, GIK 7P4 Canada. A series of Ti-SBA-15 catalysts (atomic ratio Ti/Si = 1.5%) with various average pore diameters was prepared. These catalysts were tested in chemoselective oxidation of methylpropyl-thioether with hydrogen peroxide. The modification of these catalysts with tartaric acid was found to improve the chemoselectivity to sulfoxide. No leaching of Ti was detected under the investigated conditions. 1. INTRODUCTION Heterogeneous oxidation of organic thioethers and sulfoxides received a special attention in recent years. Chemo- but mainly stereoselective oxidation of these substrates may provide very useful intermediates and products. Both kinds of reactions were carried out in the presence of titanium catalysts. Chemoselective oxidation of thioethers and sulfoxides over several zeolite-type catalysts was reported.[1-2] Choudary et al.[3] indicated titanium-pillared montmorillonite modified with tartrates as a very efficient heterogeneous Sharpless catalyst. Very recently, Iwamoto et al.[4] reported the asymmetric oxidation of sulfides to sulfoxides using tartaric acid modified Ti-containing mesoporous silica catalysts. Our group reported the oxidation of various pyrimidine-derivatives on titania-silica mixed oxides prepared by sol-gel [5] Although the chemoselectivity was excellent, the modification with tartaric acid led to very poor stereoselectivity. The present study reports on the chemoselective oxidation of methyl-propyl thioether on Ti-SBA-15 catalysts. The oxidized substrate is a very effective intermediate in synthesis of flmgicides, acaricides, etc. Several questions like the titanium leaching and the stability of the catalysts under the investigated conditions were addressed.
610
2. EXPERIMENTAL A series of Ti-SBA-15 catalysts (atomic ratio Ti/Si of 1.5%) with various pore diameters were prepared in a strong acidic medium (2M HCl solution) using HO(CH2CH20)2o(CH2CH(CH3)0)7o(CH2CH20)2oH (Pluronic P-123, BASF) as the surfactant and tetraethyl orthosilicate and tetrapropyl orthotitanate as silicon and titanium sources. The modification of the above catalysts was made in situ by treating 10 mg catalyst with 3 mg tartaric acid in hydrogen peroxide solution 12wt% in dioxane at 298 K for 2.5h. The catalysts were characterized using adsorption-desorption isotherms of N2 at 77K, XRD, and XPS. The sulfides were prepared following a reported procedure [5]. The catalytic tests have been carried out in a glass-flask. Standard experiments used 10 mg catalyst and 100 mg sulfide under inert atmosphere. The reactions were carried out for a sulfide: H2O2 ratio between 1:1.2 and 1:12, temperatures between 293 and 323 K, and reaction times between lOmin and 5h. Dioxane was used as solvent. The analysis of the products was done by ' H and '^C NMR operating at 300 MHz for ^H and 75 MHz for '^C. The optical purity of produced sulfoxide was assessed by ' H NMR using the Pirkle's reagent. This was expressed as enantiomeric excess. 3. RESULTS AND DISCUSSIONS 3.1. Catalysts characterization Table 1 compiles the synthesized Ti-SBA-15 catalysts and the characteristics of these materials. All the catalysts contain the same amount of titanium. The catalysts pore diameter and surface area were adjusted from 3.6 to 5.6 nm and 610 to 920 m^/g by varying the heating temperature and time of the gel mixture during the synthesis process The SB A structure was well evidenced from XRD patterns. The binding energies corresponding to Ti2p.? 2 species were very close to those of Ti(!V) [6]. In addition, these data may give some information about the coordination of titanium ions. According to several authors [7-9] the binding energy of 459.5 eV to 460 eV is typical for tetrahedrally coordinated titanium. Table 1. Chemical composition, textural properties and Ti2p.v2 XPS binding energies of Ti-SBA-15 catalysts Ti loading. Surface area. Pore size. Ti2p3 2 XPS binding Catalyst DOl D02 D03 D04
mol % 1.5 1.5 1.5 1.5
m^g' 610 715 830 920
nm 3'6 4.2 5.1 5.6
energy, eV 459.4 459.5 459.4 459.5
3.2. Catalytic activity in oxidation of methyl-propyl-thioether The oxidation of methyl-propyl-thioether may occur selectively to sulfoxide or nonselectively to sulfone (Scheme 1). The stereocontrol of the first step to one of the two stereoisomers makes this reaction even more interesting leading to valuable products.
611
610
715
830
920
BET surface area (m g' )
BET surface area (m^g'^) Fig. 1: Selectivity to sulfoxide as a function of the surface area of the Ti-SBA-15 catalysts (298 K, 100% conversion)
Fig. 2: Selectivity to sulfoxide as a function of the surface area of the tartaric acid modified Ti-SBA-15 catalysts (298 K, 100% conversion)
On the investigated catalysts oxidation was complete after 2.5 h. The selectivity to the corresponding sulfoxide is presented in Fig. 1. Except for the sample with 830 m^ g\ the selectivity was higher than 84% for a conversion of 100%. These data indicated almost no dependence of selectivity on the texture characteristics. The hydrogen peroxide efficiency was over 60% in all the cases. O
/ 101
,c-^N,c,-c„,^
"3C
,
CH^CH^-CH-,
H3C
- i ^ U H3C
CH^- CH^- CH3
CH^- CH^- CH3
Scheme 1. Oxidation of methyl-propyl-thioether Similar tests in the absence of the catalysts showed that the conversion reached 100% after 5h reaction with a 100% selectivity to sulfone. These data clearly evidenced the contribution of the catalysts, indicating that the chemoselective oxidation of the thioether is a heterogeneous catalytic mediated reaction. Robinson et al. [2] also investigated the oxidation of small thioethers using a TS-1 catalyst. These authors reported that the selective sulfoxidation was achieved merely as a homogeneous catalytic step. Actually, under the conditions of our investigation no leaching
612
was observed. Further oxidation of the liquor separated after the hot filtration of the catalysts indicated only an enhancement of conversion identical with that observed for oxidation of the same substrate without any catalyst. The reuse of the catalysts for five times indicated the same performances. Such a behavior might be well correlated with the tetrahedrally coordinated titanium state found by XPS. Previous data [5] using sol-gel prepared mixed titania-silica catalysts in which part of titanium existed as octahedrally coordinated species showed that under these conditions part of titanium may leach to the solution. No e.e. was observed under these conditions. The modification of the catalysts with tartaric acid caused no changes in the catalysts activity, the conversion being also completed after 2.5 h. But, as it can be observed from Fig. 2 the modification with tartaric acid determined important changes in the chemoselectivity. Except for the sample with surface area of 920 m^ g\ the selectivity was increased. However, the e.e. remained very small being less than 3% in all cases. These results might again be correlated with the coordination state of titanium. For tetrahedrally coordinated species, titanium belonging to the solid network has not enough free coordinating valences to bond both the ligand and the substrate. This is completely different from the case of the sol-gel prepared mixed titania-silica catalysts where e.e. of maximum 30% were obtained because these catalysts contained a part of titanium as octahedrally coordinated species. The stability of titanium in the network of Ti-SBA-15 under the investigated conditions might also be appreciated from the fact that the presence of tartaric acid, which is a corrosive reactant, caused no leaching of titanium, as was determined both from the chemical analysis of the used catalysts and the reproducibility of catalytic results. 4. CONCLUSIONS In conclusion, heterogeneous oxidation of thioethers on Ti-SBA-15 catalysts indicated these catalysts as selective systems, leading with a good chemoselectivity to the synthesis of sulfoxides. No Ti leaching has been detected under the investigated conditions. The modification of the catalysts with optically active tartaric acid did not yield a stereoselective oxidation Both the characterization and catalytic data indicated that Ti is well rigidified in these catalysts as tetrahedrally coordinated species. REFERENCES 1. V Hulea, P. Moreau, F. DiRenzo, J. Mol. Catal. A \\\ (1996) 325 2 D J Robinson, L Davies, N.McMorn, D.J Willock, GW Watson, P C B Page, D.Bethell, G.J.Hutchings, Phys. Chem. Chem. Phys., 2 (2000) 1523. 3 B. M. Choudary, V.L.K. Valli, A. Durga Prasad, Chem. Commun, (1990) I 186. 4 M. Iwamoto, Y.Tanaka, J.Hirosumi, N.Kita, Chem Lett., (2001) 226. 5 D C Radu, V.Parvulescu, V. Campeanu, E. Bartha, A. Jonas, P. Grange, VI Parvulescu, Appl. Catal. A., in press. 6. M. A. Stranick, M. Houalla, D. M. Hercules, J. Catal., 106 (1987) 362. 7 M. A. Reiche, E. Orteli, A. Baiker, Appl. Catal B., 23 (1999) 187. 8. A. Y. Stakheev, E. S. Shpiro, J. Apijok, J. Phys. Chem , 97 (1993) 5668. 9 S. Kaliaguine, Stud. Surf Sci. Catal., 102 (1996) 191.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
613
Direct synthesis of hydrothermally stable mesoporous Ti-MSU-G and its catalytic properties in liquid-phase epoxidation Peng Wu, Hiroyuki Sugiyama and Takashi Tatsumi Division of Materials Science & Chemical Engineering, Faculty of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan Ti-MSU-G has been directly synthesized to investigate its hydrothermal stability and catalytic properties by comparing to pure silica MSU-G and Ti-MCM-41. Ti-MSU-G with ordered mesostructure and containing mainly tetrahedral Ti species in the silica framework is successfully synthesized in the Si/Ti ratio range of 30-70. Showing superior hydrothermal stability to MCM-41, Ti-MSU-G withstands the treatment in boiling water up to 75 h. No Ti leaching occurs when Ti-MSU-G catalyst is used repeatedly in the epoxidation of cyclohexene with either hydrogen peroxide or tert-huty\ hydroperoxide (TBHP). Nevertheless, the states of tetrahedral Ti species of Ti-MSU-G prove to be more stable in TBHP than in aqueous H2O2. 1. INTRODUCTION Titanium-containing mesoporous molecular sieves such as Ti-MCM-41 [1], having ordered mesopores and extremely high specific surface area, has received researchers' much attention because of their potential apphcations to the oxidation of bulkier substrates in comparison to microporous titanosilicates of TS-1, Ti-MWW and Ti-Beta. Nevertheless, Ti-MCM-41 suffers a distinct disadvantage of low hydrothermal stability due to its thin wall thickness and high hydrophilicity derived from abundant surface silanol groups. Although trimethylsilylation modification significantly improves both the hydrophobicity and catalytic activity of Ti-MCM-41 by removing the surface silanols [2], it is desirable to prepare mesoporous titanium molecular sieves hydrothermally stable by themselves. Based on this consideration, we have prepared Ti-SBA-15 of thick silica walls by a postsynthesis method, and verified that Ti-SBA-15 exhibits not only superior hydrothermal stability but also Ti stability against leaching in actual liquid-phase epoxidation reactions [3]. Recently, MSU-G mesoporous silica, synthesized with electrically neutral gemini surfactant, is reported to have superior hydrothermal stability even much higher than SBA-15 because of its high degree of silica framework cross-linking [4]. Moreover, its vesicular morphology promises that MSU-G may serve as an excellent support of catalytic species. With the purpose of applying MSU-G to the catalysis, the incorporation of Al has been carried out by a post-synthesis method to prepare acid catalyst [5], but still no researches have been reported on the incorporation of transition metals. We report here for the first time the hydrothermal synthesis of Ti-MSU-G and its catalytic properties in comparison to Ti-MCM-41.
614
2. EXPERIMENTAL Ti-MSU-G was hydrothemally synthesized using tetraethyl orthosilicate (TEOS), tetrabuthyl orthotitanate (TBOT) and neutral gemini surfactant of CioH2iNH(CH2)2NH2by modifying the procedures reported on pure sihca MSU-G [4]. In a typical run, deionized water and surfactant were mixed and stirred at room temperature for 1 h. The solution of TEOS and TBOT was then added under vigorous stirring to obtain a gel with a molar composition of I.O Si02 : x Ti02 • 0.3 surfactant : 78 H2O where x was 0-0.033. This gel was then heated statically under autogenous pressure at 373 K for 48 h. The solid product was gathered by filtration and calcined in air at 873 K for 10 h to remove the surfactant. The physico-chemical properties of Ti-MSU-G were characterized by XRD, ICP, UV-visible spectroscopy, and N2 or H2O adsorption measurements. Its catalytic properties were tested for the epoxidation of cyclohexene with H2O2 or TBHP in liquid-phase at 333 K. 3. RESULTS AND DISCUSSION Fig. 1 shows the XRD patterns of Ti-MSU-G samples with various Ti content after the calcination. The incorporation of Ti into the structure showed no obvious influence on the sharpness of the 001 diffraction peak, verifying the presence of highly ordered mesostructure in the samples. The N2 adsorption-desorption isotherms of pure silica MSU-G and Ti-MSU-G were characteristic of mesoporous materials (Fig. 2 a). The hysteresis loops at higher relative pressure is probably attributable to the vesicular particle morphology of this type of mesoporous material [4]. The D-H pore size distribution curves showed that the samples contained highly ordered mesopores of ca. 2.5 nm in diameter (Fig. 2b). All the Ti-MSU-G samples exhibited in 800 600
C/5 (DO
1 ^
M S U ^ c
400
JO O
200 o
en
T3
4 6 10 2theta Fig. 1. XRD patterns of calcined pure silica MSU-G (a), and Ti-MSU-G synthesized from the gel with Si/Ti ratio of 70 (b), 50 (c) and 30 (d) 0
<
•H^^^^TTMSU-G
0 \
J
1
1
1
0 0.2 0.4 0.6 0.8
12
3 4 5 6 7
Pore diameter /nm P/Po Fig. 2. N2 adsorption-desorption isotherms (a) and pore size distribution (b) of pure silica MSU-G and Ti-MSU-G
200 250 300 350 400 Wavelength /nm Fig. 3. UV-visible spectra ofTi-MSU-G
615
0
10 2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 2theta 2theta 2theta Fig. 4. XRD patterns of MCM-41 (a), MSU-G (b), and Ti-MSU-G (c) after the treatment in boiling water for a different period of time
the UV-visible spectra the Table 1 Physico-chemical properties of pure silica MSU-G main band at 220 nm and Ti-MSU-G treated in boiling water attributed to the tetrahedral Ti Boiling time ABET Pore size Water adsorbed species (Fig. 3). ICP analyses Sample /h /UyO nm"^ Wg-' /nm showed that the Si/Ti ratios of 2.5 0 510 1.1 Ti-MSU-G samples were 69, MSU-G 2 2.7 490 46 and 31, respectively, indicating that nearly all the Ti 2.5 460 25 added in the gel was 410 2.5 75 effectively introduced into the Ti-MSU-G 2.5 530 0 2.7 silica framework. 2.4 510 2 The hydrothermal stability 400 2.5 75 of Ti-MSU-G has been compared to that of pure silica MCM-41 and MSU-G (Fig. 4 and Table 1). When MCM-41 was suspended in boilingwater, its mesostructure was severely destroyed within 2 h (Fig. 4a), while that of MSU-G was almost maintained up to 75 h (Fig. 4b). The structure Ti-MSU-G also withstood the treatment in boiling water although the 001 diffraction peak somewhat decreased in intensity than MSU-G at 75 h (Fig. 4c). This slight difference is presumably due to a higher hydrophilicity originating from the framework Ti species in Ti-MSU-G. In fact, Ti-MSU-G showed a higher water adsorption amount than MSU-G (Table 4). The hydrophi lie Ti species would make the silica walls attacked by water adsorption more easily. In spite of this difference, BET and surface measurements were consistent with the above XRD results, indicating that Ti-MSU-G was a highly hydrothermally sable material. The catalytic properties of Ti-MSU-G have been investigated in the liquid-phase epoxidation of cyclohexene with H2O2 or TBHP. Ti-MSU-G showed catalytic activity comparable to Ti-MCM-41 (not shown). Used Ti-MSU-G was regenerated by calcination in air and then repeatedly applied to the reaction to investigate the stability of Ti species. No obvious Ti leaching was observed for both H2O2 and TBHP oxidants (Table 2). The cyclohexene conversion was almost constant in the case of TBHP, while it decreased gradually following the reaction-regeneration cycle in H2O2. XRD and N2 adsorption confirmed that the mesostructure was totally maintained during the repeated reactions. However, UV-visible spectra showed that a part of tetrahedral Ti species of Ti- MSU-G was tetrahedral Ti species of Ti- MSU-G was
616
Table 2 Epoxidation of cyclohexene over regenerated Ti-MSU-G^ Si/Ti Conv. Oxidant Repeated Sel. /% time epo. /% diol l-ol 1-one 17 8.2 31 15 11 56 1 H2O2 7.4 4.8 8.4 2 11 28 76 4.0 2.6 27 74 9.9 3 13 4 3.6 2.8 14 29 12 71 96 13 31 3.3 TBHP 1 0.1 0.3 97 2 0.2 15 32 0.3 2.3 97 2.2 0.2 16 29 0.3 3 0.2 96 0.4 15 4 3.0 30 ^Conditions: cat., 0.1 g; 5 mmol cyclohexene : 2.5 mmol H2O2 (31 wt%) or TBHP (70 wt%): 5 ml MeCN; temp., 333 K; time, 2 h. irreversibly b changed into octahedral (U 0 coordination in the case of H2O2 as evidenced by the gt-l ^ \ \ ^ X) appearance of the band at \Vv^ 2 0C/5 260 nm (Fig. 5a). TBHP, on X) \^^4 the other hand, little < affected the state of 1 1^^*-^ Tispecies (Fig. 5b). These 200 250 300 350 400 200 250 300 350 400 results indicate that although Ti-MSU-G has Wavelength /nm Wavelength /nm hydrothermally stable silica Fig. 5. UV-visible spectra of Ti-MSU-G after repeatedly walls, the stability of Ti used in the epoxidation with H2O2 (a) and TBHP (b) greatly depends on the nature of oxidant. In aqueous H2O2, some Ti-O-Si bonds could be cleaved due to the synergistic attack by H2O2 and H2O molecules. Therefore, Ti-MSU-G prefers the organic oxidant to H2O2. 4. CONCLUSIONS Hydrothermally stable Ti-MSU -G with tetrahedral Ti species has been directly synthesized in a wide Si/Ti ratio range. Ti-MSU-G proves to be an active liquid -phase epoxidation catalyst. It is stable against Ti leaching in the reaction, but its Ti states changes more easily in aqueous H2O2 than in TBHP. REFERENCES 1. Corma, Chem. Rev., 97 (1997) 2373. 2. T. Tatsumi, K. A. Koyano and N. Igarashi, Chem. Commun., (1998) 325. 3. P. Wu, T. Tatsumi, T. Komatsu and T. Yashima, Chem. Mater., 14 (2002) 1657. 4. S.-S. Kim, W. Zhang and T. J. Pinnavaia, Science, 282 (1998) 1302. 5. S.-S. Kim, Y. Liu and T. J. Pinnavaia, Micropor. Mesopor. Mater., 44 (2001) 489.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
Mesoporous V-containing MCM-41 characterization and catalytic oxidation
617
molecular
sieves:
synthesis,
Chih-Wei Chen and An-Nan Ko* Department of chemistry, Tunghai University, Taichung, Taiwan. Fax: + 886-4-23590426. E-mail: anko^mail.thu.edu.tvv Two types of vanadium-containing MCM-41 were synthesized via direct hydrothermal (V-MCM-41) and impregnation (V/MCM-41) methods. The catalysts were characterized by ICP-AES, XRD, N2-sorption, ^^Si and ^V NMR, TPR of H2, and chemisorption of O2. Based on ^V NMR results, calcination followed by hydration in the air results in the transformation of tetrahedral V^^ into octahedral V^"*^ coordination due to addition of water molecules. For all samples, an increase of V content diminishes the surface area but enhances both the amount and the reduction temperature of H2 gas in the TPR experiments. The catalysts were applied to the oxidation reaction of diphenylmethane with tert-butyl hydroperoxide to produce benzophenone. The catalytic results correlate well with the properties of V-containing MCM-41 molecular sieves. 1. INTRODUCTION Side-chain oxidation of alkly aromatic compounds is an important transformation in the synthetic organic chemistry. Traditionally, these reactions were carried out with a large excess of chromium and manganese reagents under homogeneous condition [1]. As these materials are environmentally undesirable, recently, heterogeneous catalysts such as V205-Fe203 [2], Mn-MCM-41 [3] and Cr-MCM-41 [4] were utilized for catalyzing benzylic oxidations. Vanadium-modified molecular sieves have been investigated for selective oxidation of hydrocarbons. However, most of them are microporous which restrict the oxidation of large molecules. V-containing MCM-41 was utilized as a catalyst for the oxidation of benzene in the liquid phase [5]. Benzophenone is a starting material for flavoring, soap fragrance and pharmaceuticals. In this study, V-containing MCM-41 catalysts were prepared and characterized. These materials were used in the catalytic synthesis of benzophenone (BP) from diphenylmethane (DPM) and tert-buty\ hydroperoxide (TBHP). The catalytic results were correlated to the catalyst properties. 2. EXPERIMENTAL Two types of V-containing MCM-41 were synthesized via the direct hydrothermal (V-MCM-41) and impregnation (V/MCM-41) method. The synthesis of V-MCM-41 was performed at pH 9-10 by using vanadyl sulfate hydrate, sodium silicate, hexadecyltrimethylammonium bromide and sulfuric acid according to the literature [6], whereas V/MCM-41 was obtained by impregnating solution of vanadyl sulfate hydrate on Si-MCM-41.
618
These catalysts were characterized with various techniques, viz. ICP-AES (Jobin Yvon JY38 Plus), XRD (Shimadz XRD-6000), sorption analysis (Quantasorb), ^^Si & ^'V NMR (Braker DSX 400WB), H2-TPR, and Oi-chemisorption (Micromeritics Pulse Chemisorb 2705). The catalytic reactions were carried out at 1 atm and 60°C in a stirred batch reactor (250 ml) connected with a condenser. 50 mmol of DPM, 100 mmol of TBHP and 41.8 ml of acetonitrile (solvent) were added into the glass reactor. After the reaction temperature was reached, appropriate amount of catalyst (0.3 g) was added into the reactor to start the reaction. The products were identified with a GC-MS (Micromass Trio 2000) and were periodically analyzed with a GC (China Chromatography 9800), fitted with a FID and a HP-5 column (30 x0.32mm). 3. RESULTS AND DISCUSSION Both calcined V-MCM-41 and V/MCM-41 samples exhibit characteristic MCM-41 structures according to the powder XRD patterns. The Si/V ratio of the samples has no profound influence on the peak intensities, indicating the similarities among the crystal structures. For the ^^Si NMR spectra of calcined V-MCM-41 samples, three resonances were observed at -90, -100 and -110 ppm. As various samples of V-MCM-41 with different Si/V ratios reveal similar patterns, the Si/V ratio causes little effect on the environment of silicon. Fig. 1 shows the ^'V NMR spectra of V-MCM-41 samples. For as-synthesized samples of V-MCM-41 (50 & 25), only one peak appears at -500 ppm. Calcination followed by hydration in the air results in two peaks at -300 and -500 ppm. The new peak at -300 ppm is assigned to octahedral V=^0(H20)2(OSi)3 species at the surface, whereas the peak at -500 ppm is
11
/N (C) \
' • • •
(b)
M -^^^\j\/^-^j^^r'^
500
O
w-^^^A—x
(a)
-500 -1000 -1500 ppm Fig. l.-''V NMR of V-MCM-41. Si/V= 10 0(a); 50(b); 25(c); 12.5(d); as synthesized 50(c); 25 (f).
\
•A/V
500
0
,
(a)
-500 -1000-1500 ppm
Fig. 2. ^'V NMR of x%V/MCM-41. x=2.4: (a); 4.8(b); 9.1(c).
619
attributed to vanadium in tetrahedral V=0(0Si)3 symmetry. These results imply that the tetrahedral V^"^ species are converted to octahedral V^"^ by coordination with water molecules as was reported elsewhere [7]. The small shoulder at -500 ppm infers that a small amount of vanadium species are unable to coordinate with water molecules probably due to their incorporation into the pore walls. Similar results were found for the V/MCM-41 samples (Fig. 2).Fig. 3 shows the H2-TPR profiles. Both V-MCM-41 and V/MCM-41 samples exhibit a single peak at ~550°C due to the reduction of surface vanadium species which is different from those of V2O5 [8]. For all samples, the amount and the reduction temperature of H2 gas increase with increasing the V content in the sample. Table 1 indicates the catalyst properties and the catalytic results. The Si/V ratios in the framework are apparently larger than those in the starting mixture, hi addition, an increase of V content diminishes the surface area. In the reaction of DPM with TBHP over (V)MCM-41, the products include BP and benzhydryl-/er/-butyl peroxide. Fig. 4 shows the relation of catalyst V content with the relative amount of H2 in the TPR experiment and the DPM conversion. The relative amount of H2 gas is proportional to the V content. Consequently, the capacity of catalyst reducibility enhances with the catalyst V content. Furthermore, the catalytic activity is parallel to the V content except at the highest V loading (9.1% V/MCM-41), probably due to its remarkably lower surface area. As shown in Table 1, V-MCM-41 (12.5) and 4.8%V/MCM-41 have similar V contents and surface areas. However, the former catalyst exhibits the higher catalytic activity due to its larger V dispersion, i.e., 71.4% as compared to 55.2% on 4.8%V/MCM-41. 120 100 CL
(g)
JTL JeL
0.6
80
0.4
h 60
0.2
40 CL O
(d)
0.0
A9L JbL (a)
100
300 500 Temperature ( C )
700
Fig. 3. TPR of H2 from (V)MCM-41. V-MC M-41, Si/V= 100(a); 50(b); 25(c); 12. 5(d); x %V/ MCM-41, X- 2.4(e); 4.8(f); 9.1(g).
20
2
4
6
]
10
V content (wt%)
Fig. 4. Relation of catalyst V content with relative amount of H2 in TPR and DPM conversion.
620
Table 1 Catalyst properties and catalytic results. Reaction condition: 1 atm; 60°C; 200 rpm; 3 h Catalyst V-MCM-41(100) V-MCM-41(50) V-MCM-41(25) V-MCM-41(12.5) 2.4%V/MCM-41 4.8%V/MCM-41 9.1%V/MCM-41
SiA^ ratio Si/V ratio V content Surface area DPM conv. in gel (wt%) by ICP-AES (mol%) (mVg) 100 294 0.23 895 5.6 50 154 0.45 877 14.6 25 37 799 33.2 1.8 4.0 12.5 16 785 58.0 27.1 — — 2.4 876 — — 793 52.6 4.8 — — 9.1 625 55.3
BP select. (mol%) 61.1 45.4 60.6 80.7 41.3 55.6 71.0
REFERENCES 1. 2. 3. 4. 5. 6.
F.A. Luzzio and W.J. Moore, J. Org. Chem. 58 (1993) 512. A. Bruckner and M.Baerns,Appl.Catal. A: Gen. 157(1997)311. V. Caps and S.C. Tsang, Catal. Today 61 (2000) 19. N. Srinivas, V. Radha, S.J. Kulkami and K.V. Raghavan, J. Mol. Catal. 179 (2002) 221. Y.-W. Chen and Y.-H. Lu, Ind. Eng. Chem. Res. 38 (1999) 1893. K.M. Reddy, I. Moudrakovski and A. Sayari, J. Chem. Soc, Chem. Commun. (1994) 1059. 7. L. Zhaohua, X. Jie, H. Heyong, K. Jacek and K. Larry, J. Phys. Chem. 100 (1996) 19595. 8. F. Arena, N. Giordano and A. Parmaliana, J. Catal. 167 (1997) 66.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
621
Catalytic oxidation of H2S to elemental sulfur over mesoporous Nb/Fe mixed oxides S.J. Jung", M.H. Kim", J. K. Chung", M. J. Moon", J.S. Chung^ D.W. Park' and H.C. Woo'^ "Department of Chemical Engineering, Pukyong National University, Pusan 608-739, Korea. ^Department of Chemical Engineering, POSTECH, Pohang 790-784, Korea. *^Department of Chemical Engineering, Pusan National University, Pusan 609-735, Korea. The mesoporous Nb/Fe mixed oxides with different Nb/Fe ratios were prepared by shortchain amine (hexylamine) templating method via co-precipitation process, and found to be an efficient catalyst for the selective oxidation of H2S to elemental sulfur. Compared with the conventional co-precipitated Nb/Fe mixed oxide, the mesoporous oxide shows higher yields (ca. 91 % at 493 K) of elemental sulfur. The high catalytic activity and good stability is believed to be due to the high surface area and the relatively thick pore wall of mesoporous Nb/Fe mixed oxide. 1. INTRODUCTION Most hydrogen sulfide is removed from industrial waste gases via the Claus process [1]. However, the catalytic reaction SO2 + 2H2S = 3/n Sn + 2 H2O being an equilibrium, the best sulfur recovery efficiency of Claus plants units dose not exceed 97-98 %. The remaining H2S has to be eliminated and one of the most satisfactory processes is the direct H2S oxidation to elemental sulfur. Recently, various metal oxides have been applied to selective oxidation of H2S as efficient catalysts [2,3]. However, metal oxides were not selective toward sulfur, the most selective being V2O5, M0O3, and Fe203. Some authors used Fe203 and Cr203 supported on alumina to improve selectivity and avoid alumina sulfation and its consequent deactivation. In the present study, we prepared the mesoporous Nb/Fe mixed oxide by short-chain amine templating method via co-precipitation process, and report its catalytic properties for the seletive oxidation of H2S to elemental sulfur. The perfomance is also compared to that of conventional co-precipitated oxides. 2. EXPERIMENTAL The synthesis of mesoporous Nb/Fe mixed oxides with four different atomic ratios (Nb/Fe = 1/0, 5/1, 2/1, and 1/1) were carried out using the short-chain amine as a template. In general, iron nitrate and niobium penta chloride were dissolved in HCl aqueous solution and the pH of the solution was adjusted to pH 3.05 by addition of aqueous ammonia. This led to a colloidal solid precipitate. Hexylamine was then introduced as a templating agent to form an organicinorganic composite. The mixture was aged at 393 K for 24 h in a closed Teflon autoclave. The precipitate was filtered, washed, and dried at 373 K. The as-made powder sample was then calcined at temperature of 623, 723 or 873 K during 4 hours with air. For comparison purposes the Nb/Fe mixed oxide was also prepared by the conventional co-precipitation method. The specific surface area and pore size distribution were analyzed with N2 adsorption-desorption isotherms at 77 K. The phase analysis was performed by small-angle XRD (Rigaku D/max-RB) using a filtered Cu K a radiation. The selective oxidation of H2S to
622
elemental sulfur was carried out at the temperature range from 453 to 613 K and atmospheric pressure (H2S/O2 ratio = 2/1) with a space velocity, GHSV=30,000 l/kg-catJh. Reaction products were analyzed by an on-line gas chromatograph(Shimadzu 17A) equipped with a thermal conductivity detector and a 2 m Porapak T column (80-100 mesh) at 120 °C. 3. RESULTS AND DISCUSSION 3.1. Characterization The XRD patterns of the samples calcined at 623 K showed the small angle reflections (Fig. 1), typical of a mesoporous sample, indicating the presence of a long-range order with regular pore system. The patterns can be indexed to a hexagonal unit cell as (100) reflections correspond to ^-spacings of -73-88 A (Table 1). Fig. 2 shows N2 adsorption-desorption isotherms that are representative of mesoporous Nb/Fe mixed oxides. All isotherms of the samples are of type IV. From the adsorption-desorption isotherm results, the pore size distribution calculated from the BJH method is very narrow, around 12.9 A, except for that of the single niobium oxide (abbreviated as NbFelO). The introduction of iron into the mesoporous molecular sieves during the synthesis (abbreviated as NbFeSl, NbFe21 and NbFe 11) dose not move the N2 adsorption-desorption isotherms towards type IV. The pore walls have thicknesses of-64-79 A.
2Theta(0 )
Fig. 1. XRD patterns of calcined samples with different Nb/Fe ratios.
Relative pressure(P/Po)
Fig. 2. N2 adsorption-desorption isotherm of calcined samples with different Nb/Fe ratios.
The results above mentioned were similar to those reported by Yang et al [4]. Finally, our results indicate that the samples are somewhat disordered mesoporous Nb/Fe mixed oxide with relatively thick amorphous walls. The stability of the mesoporosity of sample NbFe 11 was tested by calcining the material for 4h at various temperatures, followed by XRD analysis. Table 1 Physico-chemical propert ies of mesoporous Nb/Fe mixed oxides Pore radius dwall Nb/Fe Sample SBHT VBJH (A) (cmVg) (m^/g) Nominal Actual (A) 264 17.4 1/0 1.0/0 74.7 0.11 NbFelO 224 12.9 7.3/1 71.6 0.11 5/1 NbFe51 206 12.9 4.2/1 79.3 0.10 2/1 NbFe21 249 12.9 1.3/1 64.8 0.12 1/1 NbFe 11
d(ioo)
(A)
85.0 83.3 88.7 73.3
623
NbFell-823K
NbFell-723K
i
air-dried
It was also found that up to a calcination temperature of 623 K the characteristic small angle XRD peak is retained (Fig. 3). With the calcination temperature the ^-spacing for the main peak (100) was quite lower and shifted along with lower angle. This indicates a decrease or collapsing in the long-range order of the hexagonal pore arrangement upon calcinations
2Theta(e )
Fig. 3. Change of XRD patterns with different calcination temperatures 3.2. Catalytic activity Mesoporous Nb/Fe oxide was compared with co-precipitated Nb/Fe oxide under the same reaction conditions of GHSV=30,000 l/kg-cat./h and H2S/02=2. As shown Table 2, mesoporous sample shows better activity than co-precipitated sample. Table 2 Catalytic oxidation of H2S to elemental sulfur over Nb/Fe oxides NTcmp.(K)
453
Sample^^ Mesoporous Coprccipitated
(mVg)
XH2S
Ss
XH2S
(%)
(%)
(%)
275
52.6
99.2
83.4
72
26.2
99.6
53.5
90 0
ao S^ 70 •>, 60 tH •5
^
1 f-
//
/
^
1 1/ 1/
-H^ 0 -T--V -
40 30
440
460
-—-^*^-~^^^
520
SeO
NbFelO NbFeSl NbFe21 NbFell
600
640
Temperdture(K)
Fig. 4. Sulfur yield as a function of reaction temperature
513
493
473
SBET
Ss
XH2S
Ss
XH2S
Ss
(%)
(%)
(%)
(%)
98.8
99.1
92.2
98.3
91.8
99.1
82.1
98.2
89.6
96.6
(%)
At the temperature of 493 K, both mesoporous and co-precipitated oxides showed good activity and stability with time-on-stream. Although selectivity was somewhat lower, conversion was much higher than that of co-precipitated oxide. Consequently, mesoporous Nb/Fe oxide showed higher yields by ca. 11% compared with coprecipitated oxide, mainly due to higher conversions. Fig. 4 shows the sulfur yield as a function of reaction temperature. The maximum sulfur yield obtained for the mesoporous mixed oxide catalysts with Nb/Fe = 5/1, 2/1 and 1/1 was 85.9 %, 86.9 % and 87.7 %, respectively The maximum sulfur yield obtained with mesoporous single niobium oxide was only 78.0 % at 573 K. Therefore, the sulfur yield of the mesoporous
624
Nb/Fe mixed oxide catalysts was significantly oxide with different calcination temperatures better than that of corresponding mesoporous single Temp. SBET XH2S Ss Sample niobium oxide catalysts, temperatures (K) (%) (%) better than that of corresponding 250 97.6 89.8 NbFel 1-623 623 mesoporous single niobium oxide 91.8 93.3 75.7 NbFe 11-723 723 catalysts. 41.9 91.3 56.9 NbFel 1-823 823 The results suggest that the mesoporous Nb/Fe binary oxide exhibits strong synergistic behavior in the performances for hydrogen sulfide selective oxidation. On the other hand, the H2S oxidation activity was founded to decrease progressively by increasing the calcination temperature from 623 to 723K ,then drastically decreased by increasing the temperature from 723 to 823 K. It is believed that this is related to the decrese of active suface area due to the collapsing of order pore structure. Our final results obtained indicate that the high catalytic activity and good stability of mesoporous Nb/Fe oxide are originated from its high surface area and relatively thick pore wall. Table 3 Catalytic activities ofNb/Femesoporous mixed
4. CONCLUSION Synthesis of mesoporous Nb/Fe mixed oxide was successfully achieved by short-chain amine (hexylamine) templating method. The catalytic activity mainly depends on its surface area and the calcination temperature. The mesoporous catalysts calcined at mild temperature had a higher surface area and were the most efficient in H2S oxidation. Synergistic phenomena in catalytic performances were observed for the catalysts with different Nb/Fe ratios. ACKNOWLEDGEMENT This work was supported by Korea Research Foundation Grant (KRF-2000-042-E00076). REFERENCES 1. J.A. Lagos, J. Borsboom and RH. Bezben, Oil and Gas J., 10 (1988) 68. 2. S.W. Chun, J.Y. Jang, D.W. Park, H.C. Woo and J.S. Chung, Appl. Catal. B, 16 (1998) 235. 3. K-T. Li and N-S. Shyu, Ind. Eng. Chem. Res., 36 (1997) 1480. 4. P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka and G.D. Stucky, Nature, 396 (1998) 152..
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. Allrightsreserved
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Fe-MCM-41 catalyzed epoxidation of alkenes with hydrogen peroxide Qinghong Zhang^, Ye Wang^, Satoko Itsuki^, Tetsuya Shishido^ and Katsuomi Takehira^ ^Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-hiroshima 739-8527, Japan ^State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen 361005, China Fe-MCM-41 has been characterized in detail and studied for epoxidation of alkenes with H2O2. UV-vis, ESR, Fe K-edge XAFS and UV-Raman spectroscopic studies suggest that iron in Fe-MCM-41 is highly dispersed and mainly incorporated into the framework of MCM-41. Fe-MCM-41 is effective for epoxidation of alkenes including styrene, cyclohexene and many other cyclic alkenes with H2O2. The tetrahedrally coordinated iron sites in the framework of MCM-41 are probably responsible for the epoxidation reactions. 1. INTRODUCTION Metal ion-containing MCM-41, which possesses uniform nano-order mesopores and high concentration of isolated active sites, has attracted much attention as a new type of oxidation catalyst, especially for liquid phase oxidation reactions since facile diffusion of relatively large molecules can be expected. Many studies contributed to the syntheses and characterizations of metal ion-containing MCM-41, and some of these materials, e.^., Ti-MCM-41 and V-MCM-41 showed unique catalytic properties for the reactions of larger molecules. Since iron functions as catalytic center in monooxygenase enzymes and many selective oxidation systems such as cytochrome P-450 and Gif system, we attempt to design selective oxidation catalysts by introducing iron into MCM-41, which possesses uniform mesopore and large surface area [1]. A few studies have been reported on the syntheses and characterizations of Fe-MCM-41 with different methods [2-4], but the details about the location and the coordination environment of iron and the amount of iron cations incorporated inside the framework of MCM-41 are still not clear. In this work, the coordination environment of iron introduced to MCM-41 by direct hydrothermal synthesis (DHT) and template-ion exchange (TIE) methods is characterized and the catalytic performances in epoxidation of alkenes are investigated.
626
2. EXPERIMENTAL Fe-MCM-41 was prepared by hydrothermal synthesis (DHT) and template ion exchanging (TIE) methods. After hydrothermal synthesis or ion exchange, the resultant solid was recovered by filtration, thoroughly washed with deionized water, dried at 313 K in vacuum and finally calcined in flow air at 823 K for 6h. Ferrisilicate with MFI structure and Fe203/Cab-0-Sil were also prepared as a reference. XRD and N2 adsorption at 77 K were measured to obtain information about the mesoporous structure. The coordination environment of Fe was studied by the diffuse reflectance UV-vis, ESR, Fe K-edge XAFS and UV-Raman. The epoxidation reactions were performed at 343 K using a batch type reactor. Typically, to suppress the side reactions added 10 mmol H2O2 (30 % aqueous solution) separately by 4 times every 30 min to the reaction mixture during the reaction. A standard reaction mixture contained 10 mmol of alkene, 10 ml of DMF (solvent) and 0.2 g of the catalyst. Products were analyzed by GC-MS and GC. 3. RESULTS AND DISCUSSION 3.1. Properties of catalysts XRD measurements showed that the diffraction lines of (100), (110), (200) and (210) at 2B degrees of ca. 2.2"", 3.6", 4.3"" and 5.7"" indexed to the hexagonal regularity of MCM-41 were observed for the Fe-MCM-41 samples by both the DHT and TIE methods, suggesting that the hexagonal array of mesopores of MCM-41 was sustained after the introduction of Fe with both methods. The porous properties obtained from N2 adsorption measurements at 77 K are shown in Table 1. All the Fe-MCM-41 samples exhibited large surface area of ca. 1000-1200 m^ g'^ and pore volume of ca. 0.75-1.0 ml g"^ Narrow pore size distribution around 2.5-3.0 nm was observed for all the samples. Figure 1 shows the diffuse reflectance UV-vis spectra. UV-vis measurements of DHT samples with iron content of 0.9, 1.1 and 1.8 wt% (Si/Fe= 105-50) showed a single band at ca. 260 nm, which is assigned to charge transfer between Fe and O in the framework of molecular sieves. On the other hand, in addition to the peak at 265 nm, bands at ca. 385 and 510 nm, which were mainly observed for Fe/Cab-0-Sil also appeared for the TIE samples, and both bands became stronger with increasing Fe content. Three bands at ca. 575, 1075 and 1125 c m \ which are probably related to framework Fe-O-Si structure, were observed in UV-Raman spectrum of Fe-MCM-41 by using 325 nm laser as the source, while these bands were absent in that of a-Fe203 or Fe/Cab-0-Sil. The incorporation of the majority of Fe"^^ into the framework of MCM-41 with a tetrahedral coordination structure is
627
Table 1 Properties of Various Iron-Containing Samples SBET
sample^
pore vol.
/m^
MCM-41
1025
Fe-MCM-41-TIE (102)
1220
Fe-MCM-41-TIE (70)
1212
0.89 0.82 1.03 0.89 0.89 0.83 0.75 0.38
pore dia.
ao
/nm
/ nni
as synthesized
calcined
2.7 2.7 2.5 2.7 3.0 3.0 3.0 0.55
4.37 4.41 4.38 4.61 4.70 4.71 4.65
white brown brown white white white white white brown
white brown brown white white off white off white white brown
1043 1173 1078 Fe-MCM-41-DHT(86) 1016 Fe-MCM-41-DHT(50) 350 Ferrisilicate (MFI, 48) — — 150 FezOs/Cab-O-Sil ^The number in the parenthesis is the Si/Fe atomic ratio. ^Unit cell parameter. Fe-MCM-41-DHT(163)
Fe-MCM-41-DHT (105)
—
color of sample
further inferred from Fe K-edge XAFS studies.
Fe K-edge XAFS
results
showed that both the Fe-0 distance (1.85 A)
and
the
coordination
number
(4.2-4.5) of DHT samples were similar to those of the ferrisilicate with MFI structure, whereas the iron in a-Fe203 possessed two kinds of Fe-0 bonds with the lengths of 1.91 and 2.04 A and the coordination numbers of 3.0 and 2.9, respectively.
All these results strongly
suggest that the iron in DHT sample is highly dispersed and isolated in the framework of MCM-41, whereas most of the iron species exist in the form of oxide clusters in the TIE sample and Fe/Cab-0-Sil.
300
400 500 600 700 Wavelength /nm
800
Fig. 1. Diffuse reflectance UV-vis spectra of Fe-MCM-41 along with references, (a) MCM-41, (b)Fe-MCM-41-DHT(Si/Fe=105),(c)Fe-MCM-41DHT (Si/Fe=86),(d) Fe-MCM-41-DHT(Si/Fe=5()), (e)ferrisilicate(MFI,Si/Fe=48),(f) Fe-MCM-41-TIE (Si/Fe= 102), (g) Fe-MCM-41-TIE (Si/Fe= 70), (h) Fe/Cab-0-Sil.
628
Table 2 Comparison of Catalysts for Epoxidation of Styrene with Hydrogen Peroxide^ Catalyst Conversion / % Selectivity / % MCM-41 Fe203/Cab-0-SiI Ferrisilicate Fe-MCM-4I (TIE)^ Fe-MCM-41 (DHT)^
Styrene
H2O2
Epoxide
Aldehyde
Others
TOF' /h-'
2.1 6.6 1.9 3.5 13.8
45 82 95 86
36.8 33.8 58.2 45.7 41.8
52.2 43.0 41.7 45.9 37.3
0 10.4 0 0 12.2
1.7 0.8 2.5 8.9
Reaction conditions: catalyst 0.2 g, T= 343 K, styrene 10 mmol, H2O2 9.8 mmol, DMF 10 ml, reaction '^TOF: moles of styrene oxide produced per mole of Fe or Ti in eaction time 2 h., ^ 0.9wt%, ( the catalyst per hour. 3.2. Catalytic properties of Fe-MCM-41 The Fe-MCM-41 catalyzed efficiently the epoxidation of alkenes including styrene, stilbene, cyclohexene and cyclooctene with H2O2. Table 2 shows the results of epoxidation of styrene with H2O2. Although the conversion of H2O2 reached 45 %, MCM-41 without iron exhibited very low conversion of styrene, suggesting that iron was mainly responsible for the conversion of styrene with H2O2. For the Fe-MCM-41-DHT, the conversion of styrene was notably higher as compared with the Fe-MCM-41-TIE although the consumption of H2O2 was lower. Fe203/Cab-0-Sil showed not only lower conversion of styrene but also worse selectivity to styrene oxide. Ferrisilicate with MFl structure showed higher selectivity to styrene oxide but lower styrene conversion probably due to the diffusion limitation. Thus, the coordination environment of iron was crucial for the epoxidation with H2O2. We suggest that the iron incorporated inside the framework of MCM-41 accounts for the effective activation of H2O2 for the epoxidation reaction, while iron oxide cluster catalyzes the decomposition of H2O2.
REFERENCES 1. Q. Zhang, Y. Wang, S.Itsuki, T. Shishido, and K. Takehira, Chem. Lett., 2001, 946. 2. Z. Y. Yuan, S. Q. Liu, T. H. Chen, J. Z. Wang, and H. X. Li, J. Chem. Soc. Chem. Commun., 973 (1995). 3. B. Bourlinos, M. A.Karakassides, and D.Petridis, J. Phys. Chem. B 104, 4375 (2000). 4. M. Stockenhuber, M. J. Hudson, and R. W. Joyner, J. Phys. Chem. B 104, 3370 (2000).
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
629
Highly selective oxidation of styrene with hydrogen peroxide catalyzed by monoand bimetallic (Ni, Ni-Cr and Ni-Ru) incorporated MCM-41 silicas V. Parvulescu', C. Dascalescu' and B. L. Su* Laboratoire de Chimie des Materiaux Inorganiques, ISIS, The University of Namur (FUNDP), 61 rue de Bruxelles, B-50 Namur, Belgium Mesoporous nickel silicate molecular sieves having hexagonal structure (Ni/MCM-41, NiCr/MCM-41, Ni-Ru/MCM-41) were prepared by direct synthesis, characterized and tested for the liquid-phase oxidation of styrene w^ith H2O2. NiCr-MCM-41 catalysts have the highest conversion and selectivity to benzaldehyde. Leaching was evidenced for chromium and ruthenium at reaction temperature of 343K. 1. INTRODUCTION Transition metal ion-containing microporous molecular sieves have extensively been investigated because of their remarkable activity and selectivity in oxidative transformation of organic molecules [1-6]. The newly discovered hexagonal mesoporous molecular sieves MCM41 offer new opportunities for transition metal incorporation into silica framework in order to generate potential catalytic activity [7-10]. MCM-41 type materials have an regular pore system which consists of an hexagonal array of unidimensional hexagonally shaped pores and display interesting textural properties, combining both high surface area and high porosity [6, 11]. Transition metals containing MCM-41 mesoporous redox molecular sieves have shown a high activity and selectivity in the oxidative transformation of the organic molecules [8-15]. The properties of these high selective catalysts named as "mineral enzyme" can be tailored over a broad range, allowing the easy preparation. Transition metal ions can be introduced into molecular sieves in three different ways: ion exchange, impregnation and direct introduction by hydrothermal synthesis [9, 10, 15-18]. In this paper, we present the preparation, characterization and catalytic behavior in the oxidation of styrene using H2O2 (30%) of mono- and bimetallic (Ni, Ni-Cr and Ni-Ru) incorporated MCM-41 silica molecular sieves. 2.
EXPERIMENTAL
The reagents used were sodium silicate (25.5-28.5% silica), cethyltrimethylammonium bromid (CTMABr), tetramethylammonium hydroxide (25 wt% TMAOH in water), Ni(CH3COO)2-4H20, RuCl3-nH20, Cr(N03)2-9H20, NaOH and H2SO4. The ordered mono- and bimetallic substituted MCM-41 catalysts were synthesized from a mixture with following composition: 1 SiOj: (xM, + yM2): 0.96 Na20: 0.48 (CTMABr): 3.70 TMAOH: 222 H2O where ' On leavefromInstitute of Physical Chemistry " I.G. Murgulescu", Spl. Independentei 202, Bucharest, Romania *: Corresponding author (bao-lian.su@fundp.ac.be)
630
Ml = Ni and M2= Cr or Ru. The gel obtained was sealed into Teflon-lined steel autoclaves and heated 5 days at 373K. The as-synthesized samples were calcined at 773K. Ni-Ru/MCM-41 sample was calcined at 723K. The fresh and used catalysts were characterized by XRD, N2 adsorption/desorption, SEM, TEM, FTIR and TG-DSC analysis. Oxidation of styrene was carried out in the thermostated glass reactor or Teflon lined autoclave with magnetic stirring in the presence or absence of the solvent (acetonitrile). The reaction temperature and time varied from 293 to 343K and from 12 to 48h, respectively. The molar ratio of styrene /acetonitrile/ hydrogen peroxide was 1/3.6/3, 1/1.8/3 and 1/0/3, corresponding to 47.3, 31.0 and 0.0, molar percent of the solvent in reaction solution, respectively. After reaction, the catalyst was separated by centrifugation and the oxidation products were analyzed using a GC coupled with a FID detector (Carlo Erba) with a column containing OV-101. The catalysts were reutilized in the oxidation reactions and characterized after each utilization. Hydrogen peroxide consummation was determined by iodometric titration and Metal ions leaching was also verified. 3. RESULTS AND DISSCUTION Ni, Ni-Cr and Ni-Ru incorporated samples show diffraction lines (Fig. 1), characteristic of mesoporous materials with very regular hexagonal arrangement of their cylindrical channels. All the prepared samples have therefore MCM-41 structure. These properties were confirmed by TEM (Fig. 2A) and samples obtained have spherical morphology (Fig. 2B). The texture properties of all the catalysts are listed in Table 1. All catalysts obtained have a very high surface area. After reactions, the TEM micro-graph (Fig. 3a) shows that the MCM-41 structure remains. The IR spectra (Fig. 3b) of Ni/MCM-41 catalyst after first cycle reaction (spectrum 1) and after desorption at 293K (spectrum 2), 373K (spectrum 3) and 623K (spectrum 4) show a strong adsorption of benzaldehyde. Complete desorption of this specie after 623K was confirmed by thermal analysis, indicating the strong adsorption of benzaldehyde. The activity of the catalysts was found to increase with temperature, time and decreasing the solvent molar percent (Figs. 4 and 5). Under all experimental conditions investigated, the principal reaction product is benzaldehyde. In autoclaves, the conversion and efficiency of the
I,a.d
Ni-MCM-41 NiCr-MCM-41 NiRu-MCM-41 2
4
6
8
10 12 14 16 18 20 20
Fig.l. XRD patterns of calcined catalysts
Fig.2. TEM (A) and SEM (B) images of Ni-MCM-41 sample
631
Table 1. Characteristics of the mesoporous catalysts and results of styrene oxidation on fresh (I.Cst.) or reused (Il.Cst.) catalysts (a: in glass reacotr and b: in autoclave reactor) Catalyst
SBET
0BJH
I'Cgt*
Said,?
CelT.H202»
n.c,,,
% % % (mVg) (nm) % 16.7 12.8 5.5 62.8 2.85 945 Ni/MCM-4r 15.2 40.0 2.85 Ni/MCM-41' 24.6 945 92.7 79.5 2.74 84.2 31.2 914 NiCr/MCM-4r NiCr/MCM-41' 80.1 2.74 42.6 80.1 914 NiRu/MCM-4r 12.2 8.1 2.49 2.7 84.5 805 10.2 38.4 3.4 805 NiRu/MCM-4r 2.49 Reaction conditions: mcai:70g, temperature: 343K; time: 48h and molar ratio: 1/1.8/3;
CcfT.H202»
Sald.1
% 48.8
% 12.4
-
-
38.4
90.2
5.8
86.4
-
-
-
-
1^
r«'
i
A\ A •
f^
b » /•' '*
*^.
^-v--/
\
1 2 3
!\
' y\
•
^
'\
4 1
1300
.
1400
1
1500 1600 , 1700 Wave number, cm
Fig.3. TEM image (a) and FTIR spectra (b) of the used Ni-Ru/MCM-41 catalyst H)
/
7)
IS295K
«)
• 3SK
5[)
D1343K
4L)
30 2) 10 0
Ji 1 41
!Nfi-IVfIV1tl
Fig.4. Variation of the conversion with time and molar percent of the solvent
I
l^MhlVflVttl
Fig.5. Styrene conversion at three reaction temperatures
1800
632 H2O2 (H2O2 quantity used for oxidation/ H2O2 quantity transformed) are insignificantly higher than those obtained in the glass reactor. When catalysts were reused in second reaction, the conversion and efficiency of the H2O2 increase and selectivity to benzaldehyde decreases compared with that of the first reaction. Association of the nickel with another trivalent cations modifies activity. Introduction of Cr in Ni-MCM-41 gives a benefit effect in styrene conversion and efficiency of the H2O2. However the incorporation of Ru in Ni-MCM-41 reduces the styrene conversion and the efficiency of H2O2 (Table 1). Leaching of the ruthenium and chromium was also evidenced. 4. CONCLUSIONS Nickel, chromium and ruthenium species are able to incorporate into the framework of MCM-41 and act as active sites for the oxidation of styrene to benzaldehyde. ACKNOWLEDGMENTS This work was performed within the framework of PAJ-IUAP 4/10. VP thanks the SSTC (Federal scientific, technological and cultural office of Premier Minister, Belgium) for a scholarship and a research grant from The University of Namur. The grants from The Direction Generale des Relations Exterieures du Gouvemement de la Region Wallonne, Belgique for CC and CA, respectively are also acknowledged. REFERENCES 1. 2. 3. 4. 5. 6.
A. Corma, Chem. Rev., 97 (1997) 2373. D.C.M.Dutoit, M. Schneider, P. Fabrizioli and A. Baiker, J. Mater. Chem. 7 (1997) 271. M. Taramasso, G. Perego and B. Notari, U.S. Patent, 4410 501, 1983. V. Parvulescu, C. Dascalescu and B.L. Su, Stud. Surf Sci. Catal., 135 (2001) 4772 V. Parvulescu and B.L. Su, Catal. Today, Catal. Today, 69 (2001) 315 J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, S.B. McCuIlen, J.B. Higgins, J.L. Schlenker, J.Am.Chem.Soc. 114 (1992) 10834 7. Dusi, M., Mallat, T.,A. Baiker, A., Catal. Rev.Sci. Eng., 42, 213 (2000) 8. Biz, S., Occelli, M.L., Catal. Rev.Sci. Eng., 40, 329 (1998) 9. Wei, D., Chueh, W.-T., Haller, G., Catal. Today, 5, 501 (1999) 10. Pradier, CM., Rodrigues, F., Marcus, P., Landau, M.V., Kaliya, M.L., Gutman, A., Herskowitz, M., Appl. Catal. B., 27,73 (2000) 11. Carvalho, W.A., Wallau, M., Schuchadt, U., J. Mol. Catal. A., 144, 91 (1999) 12. M. Dusi, T. Mallat and A. Baiker, Catal. Rev.Sci. Eng., 42 (2000) 213. 13. S. Biz and M.L. Occelli, Catal. Rev.Sci. Eng., 40 (1998) 329. 14. V. Parvulescu, C. Dascalescu and B.L. Su, Stud. Surf Sci. Catal. 135 (2001). 15. V. Parvulescu and B.L. Su, Catal. Today, 69 (2001) 315. 16. C.W. Lee, D. H. Ahn, B. Wang, J.S. Hwang and S-E. Park, Microporous Mesoporous Mat. 44-45(2001)587. 17. M. Stockenhuber, R.W. Joyner, J.M. Dixon, M.J. Hudson and G. Grubert,, Microporous Mesoporous Mat. 44-45 (2001) 367. 18. G. Grubert, J. Rathousky, G. Schulz-Ekloff, M. Wark and A. Zukal, Microporous Mesoporous Mater., 22 (1998) 225.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
633
Mixed (Al-Cu) pillared clays as wet peroxide oxidation catalysts Sung-Chul Kim, Sang-Sin Oh, Geun-Seon Lee, Ju-Ki Kang, Dul-Sun Kim, Dong-Keun Lee Department of Chemical Engineering/Environmental Protection, Environment and Regional Development Institute, Gyeongsang National University, Kajwa-dong 900, Chinju, Kyungnam 660-701, Korea Al-Cu pillared clays were prepared by direct introduction of Al-Cu pillaring solution into the dilute bentonite suspension. Al-Cu pillared clays had dooi spacing of about 18A and had surface area of about 140mVg or higher. Al-Cu pillared clays showed excellent activity toward the catalytic wet peroxide oxidation of reactive black 5. Complete removal of reactive black 5 could be achieved within 20min at atmospheric pressure and 80 °C which are extremely milder operation condition when compared with the conventional catalytic wet oxidation process. The pillared clays were also stable against the leaching out of Cu. 1. INTRODUCTION Recently catalytic wet oxidation method has been the subject of numerous investigations to reduce the amount of organic pollutants in wastewaters[l,2]. The reaction is carried out under different conditions depending on the type of oxidant(02, O3, H2O2). Catalytic wet oxidation with H202(generally called wet peroxide oxidation) is believed a more efficient process due to strong oxidizing properties of H2O2, and therefore the reaction is performed in simple equipment under mild conditions. Copper could act as a catalyst to accelerate the decomposition of H2O2 into the hydroxy! radical[3]. For the production of hydroxy! radical both the supported Cu such as CU/AI2O3 and homogeneous Cu ^cation were used as catalysts. The heterogeneous CU/AI2O3 catalyst and homogeneous Cu^^ cation, however, had a serious shortcoming that Cu was leached out from the catalyst through the formation of copper hydroxides, and thus additional separation procedure was required. Pillared clays are thermally stable microporous solids which are promising catalysts in numerous areas[4]. Copper-containing pillared clay might be a promising catalyst for the production of hydroxyl radical which oxidize organic pollutants in wastewaters. In this work mixed (AJ-Cu)-pillared clays were prepared, and their catalytic properties toward the wet peroxide oxidation of reactive dye were investigated. 2. EXPERIMENTAL Al-Cu pillared clays were prepared by direct introduction of Al-Cu pillaring solution into the dilute bentonite suspension. The pillaring solutions were prepared by dissolving 0.1M Al and Cu nitrates in 0.2M NaOH solution. The hydrolysis molar ratio 0H/(A1+Cu) was kept to be 2. Solution volumes were adjusted to have a Cu/(A1+Cu) ratio between 0% and 20%. The This research was supported by Gyongnam Regional Environmental Technology Development Center.
634
dilute bentonite suspension(l%w/w) was prepared by adding the purified bentonite powder(DongYang Bentonite Co.) into the corresponding distilled deionized water. The pillaring reaction was carried out under continuous vigorous stirring at 40 °C by adding the pillaring solution drop by drop into the bentonite suspension. The prepared samples were filtered and washed with deionized water until Cl-free samples were obtained, and then dried at 120°C for 24h. The dried samples were finally calcined at 300 °C for 6h. Reactive black 5, which is a highly refractory material, was used as the model compound for catalytic wet peroxide oxidation with the mixed (Al-Cu) pillared clay catalysts. The oxidation of the reactive dye aqueous solution was performed in a glass reactor of IL capacity operated at atmospheric pressure and 80 °C. Liquid sample were immediately filtered and analyzed for total organic carbon(TOC), hydroxyl radical(HO-) and residual materials in water. TOC was measured with a Shimadzu 5000A TOC analyzer. Electron paramagnetic resonance(EPR) spin trapping of the HO • formed during the catalytic wet oxidation was performed using a Varian E-4 spectrometer. 5,5-Dimethyl-l-pyrroline N-oxide(DMPO, purchased from Aldrich Co.) was used as a trapping agent because it efficiently scavenges HO- through the following reaction to produce the DMPO/HO- adduct, which has a characteristic EPR spectrum[3]. MeJ LH Me. HOMe'
> ^ H
Me^N^OH
O"
O-
H2O2 concentration was measured by a colorimetric method using a UV/Visible DMS 90 Varian spectrophotometer[5]. 3. RESULTS AND DISCUSSION 3.1. Characterization of the catalyst X-ray diffraction patterns of the starting bentonite and Al-Cu pillared clays are shown in Figure 1. The prepared pillared clays will be abbreviated by the symbols of Al-PILC and Al(mole% in the pillaring solution)-Cu(molc% in the pillaring solution)-PlLC. Al-PILC denotes the pillared clay with alumina. Al(9())-Cu(l())PILC is, for example, the Al-Cu pillared clay prepared by the initial pillaring solution having the 90mole% Al and 10mole% Cu, respectively. The 26 angle of the (001) reflection of the pure bentonite was 7.58° which corresponded to a d-spacing of 11.77A. The corresponding 29 angles of the (hk) two-dimensional peaks were at 19.68° and 35.38°. The diffraction at 2G of 19.68° was the summation of hk indices of (02) and (01), and the diffraction of 35.38° was the summation of hk indices of (13) and (20). The peak at 26 of 27.98° was a reflection of the 26 Figure 2. XRD patterns of bentonite(a), Al- quartz impurity. Upon pillaring with Al and Cu the d^n peak PILC(b), Al(95)-Cu(5)-PILC(c), Al(90)shifted lower 26 values of about 4.98° corresponding Cu(10)-PILC(d),Al(80)-Cu(20)-PILC(e).
635
Table 1. Summarized properties of Al-Cu pillared clays Clay Bentonite Al-PILC Al(95)-Cu(5)-PILC Al(90)-Cu(10)-PILC Al(85)-Cu(15)-PILC Al(80)-Cu(20)-PILC
Cu(%)
surface area(m^/g) 33.2 142.3 164.5 149.4 146.8 142.9
dooi(A) 11.77 17.02 18.1 18.0 18.0 18.0
-
0.80 1.12 1.71 2.50
to the increase in the dooi spacing, while the rest of the structure was not clearly affected. The dooi spacing of the pillared clays was about 18.0A. In Table 1 are listed the summarized properties of the Al-Cu pillared clays. BET surface area increased significantly after intercalation. The state of copper was also investigated by using electron paramagnetic resonance(EPR) spectroscopy. When the copper was post ion-exchanged into Al-PILC sample, most of the copper existed mainly in the form of [Cu(H20)4]^"^. Cu° was coordinated to four water molecules in the x-y plane and two surface oxygen of the silicate lattice along the z-axis. When the sample was fully hydrated, copper was present as [Cu(H20)]^"^ which could be easily leached out from the sample into the solution under reaction condition. In the case of the mixed Al-Cu-PILCs, however, copper was generally grafted on the alumina pillars in the form of [Cu°(A10)n(H20)6-n]'^ 3.2. Catalytic wet peroxide oxidation of reactive black 5 Catalytic wet oxidation of reactive black 5 was carried out in a batch reactor. 2()mL of 0.5 N H2O2 solution was added, and the initial concentration of reactive black 5 solution was 1,0(X)mg/L. In Figure 2 are shown the removal of TOC together with the concentration of H2O2 consumed and HO- produced during the reaction with reactive black 5 in the presence of lOg Al(9())Cu(10)-PILC. The removal of TOC was shown to be strongly related to the consumption of H2O2 which will be decomposed into H0-. A separate experiment of H2O2 decomposition in the absence of any reactive black 5 was carried out at the same reaction condition. The concentration of H2O2 was the same as that in
or ^s.
5
Time(min)
Figure 2. Correlation between TOC removal(#), H2O2 consumption(A) and HO- formation(B) during the catalytic wet peroxide oxidation of reactive black 5 with 10gAl(90)-Cu(10)-PILC.
10
15
20
Time(min)
Figure 3. Time dependence of H2O2 conversion and HO- formation during H2O2 decomposition in the absence of the catalyst(#) and in the presence of lOg Al(90)-Cu(10)-PILC(A).
636
the experiment of Figure 2. The measured changes in the concentration of H2O2 and HO- are plotted in Figure 3. As seen, in accordance with the consumption of H2O2 the formation of HO- occurs during the reaction. The rates of both the H2O2 consumption and HO- production increased greatly by the action of Al(90)-Cu(l())-P1LC which must have played an important role on the activation of H2O2 decomposition and the subsequent HO* formation. The subtracted amount of H0-, corresponding to the difference between HO- formed in Figure 3 and HO- remained in Figure 2 must have participated in the oxidation of reactive black 5 in water. Figure 4. Effects of Cu content in Figure 4 shows a comparison between the results of the Al-Cu pillared clays on the wet peroxide oxidation with Al-Cu pillared clays removal of T0C(#:A1-PILC, having different amount of Cu. There was a A:A1(95)-Cu(5)-PILC, m:Al(90)- considerable increase in the reaction rate by using Al-Cu Cu(10)-PILC, TiAlCSSVCuClS)- pillared clays instead of Al-PILC. About 14% removal of TOC was achieved in 30min with the Al-PILC, while PILC, •:A1(80)-Cu(20)-PILC). in the presence of the Al-Cu pillared clays it took only about 20min for the complete removal of TOC. In addition the removal efficiency of TOC increased with increasing amount of Cu in the Al-Cu PILCs. 3.3. Stability of the catalyst During the catalytic wet peroxide oxidation the active component Cu might be leached out from the Al-Cu pillared clays. To investigate the stability of the Al-Cu pillared clays with respect to metal leaching, the concentrations of dissolved Cu and Al in the solution were analyzed using ICP. NO detectable amount of dissolved Cu and Al could be measured. 4. CONCLUSIONS Al-Cu pillared clays were prepared by direct introduction of Al-Cu pillaring solution into the dilute bentonite suspension, and upon intercalation the dooi spacing increased from W.ll A to 18.0A. In addition the surface area of the pillared samples also increased from 33.2m"/g of the pure bentonite upto 164.5m'^/g. Al-Cu pillared clays were proved to act as excellent catalysts for the wet peroxide oxidation of reactive black 5. Reactive black 5 could be completely removed in just 2()min with lOg Al(9())-Cu(10)-PILC at atmospheric pressure and 80°C. The catalysts were also extremely stable against the leaching out of active Cu component into the aqueous solution. REFERENCES 1. D.-K. Lee and D.-S. Kim, Catal. Today, 63 (2000) 249. 2. P. Gallezot, N. Laurin and P. Isnard, Appl. Catal. B, 9 (1996) L l l . 3. D.-K. Lee, D.-S. Kim and S.-C. Kim, Stud. Surf. Sci. Catal., 133 (2001) 297. 4. R.T. Yang, N. Tharappiwattananon and R.Q. Long, Appl. Catal. B, 19 (1998) 289. 5. GM. Eisenberg, Ind. Eng. Chem., 15 (1943) 327.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
637
Finely-dispersed Ni/Cu catalysts supported on mesoporous silica for the hydrodechlorination of chlorinated hydrocarbons Younggeun Park, Taewook Kang, Young-sung Cho, Pil Kim, Jong-chul Park and Jongheop Yi* School of Chemical Engineering, Seoul National University, Seoul, 151-742, Korea. FAX: +82-2-885-6670. E-mail: jyi^snu.ac.kr Ni/Cu and Ni catalysts supported on the mesoporous silica were prepared via grafting, metal adsorption and calcination steps. The prepared catalyst was characterized using XRD, TEM, N2 sorption, and SAXS. Pore structure of the support was maintained throughout the preparation procedures. In addition, no specific metal particle image was appeared in XRD, because the metal particles are highly dispersed in the support as a very small size. Hydrodechlorination of TCEa (Trichloroethane) was selected as a model system to test the catalytic activity and selectivity to VCM. The activity of Ni/Cu-E-SBA is higher than that of Ni-E-SBA. The decrease in activity of Ni/Cu-E-SBA was much slow, because copper was added in the Ni catalyst. Cu may plays an important role to retard the deactivation by, such as coking. Further research is on progress. 97% selectivity to VCM was achieved after about 600min on Ni/Cu-E-SBA catalyst. 1. INTRODUCTION Chlorinated hydrocarbons are widely used as solvents or raw materials. These chlorinated hydrocarbons cause serious environmental problems. A variety of technologies have been developed for the safe treatment or destruction of these hazadous materials produced as wastes or by-products. Catalytic hydrodechlorination of chlorinated hydrocarbons is a promising technology for the treatment of these pollutants and hazards a possibility to recover useful products [1-3]. Numerous studies have been performed to develop a hydrodechlorination catalyst using noble or transition metal supported on Si02, AI2O3 or mesoporous materials [4]. Recently, mesoporous molecular sieves have been attracted as a catalyst support due to its desirable properties such as large surface area, well arranged pore array and narrow pore size distribution. In addition, mesoporous silicas can be functionalized with organic or inorganic chemicals by grafting method. In this study, SBA-15 was synthesized using sol-gel method and used as a catalyst support. Ni/Cu supported on the SBA15 (Ni/Cu-E-SBA) was prepared by EDTA (N(trimethixysilylproply) ethylenediaminetriacetic acid salt) grafting, followed by metal adsorption and calcination. Hydrodechlorination of TCEa was carried out in a continuousflow fixed bed reactor using the prepared catalysts. Ni catalyst also was prepared by the same procedure. Conversion and selectivity of VCM among others were compared. * Corresponding author: ivi@snu.ac.kr ** Financial support by the National Research Laboratory (NRL) of the Korean Science and Engineering Foundation (KOSEF) is gratefully acknowledged.
638
2. EXPERIMENTAL 2.1. Catalyst preparation As a first step, mesoporous support was prepared and attached the metals onto the support by grafting method using a bridging chemical. In brief, SBA-15 mesoporous silica was synthesized using a non-ionic surfactant, Pluronic PI23 (EO20PO70EO20, BASF Co.) as a template and tetraethylorthosilicate (TEOS, Aldrich Chemical Co.) as a silica precursor [5-6]. In a typical experiment 10.0 g of Pluronic PI23 was dissolved in an aqueous HCl solution (1.6M, 358 ml). This solution was added to a mixture of 14.3 g of TEOS. The mixture was stirred for 20 h to form a microemulsion. After heating at 100°C for 24hr, the transparent solution was obtained. The resulting solid was filtered off and washed. The surfactant was then removed by hot ethanol extraction in a Soxhlet apparatus. In order to load a metal on the supports, EDTA was grafted onto surfaces of the SBA-15 by reflux of dry toluene. Ni and Cu were loaded by contacting with IM nickel nitrate solution and copper nitrate solution for 1 day. The catalysts were calcined at 450 °C for 5h in an atmosphere environment (Ni/Cu-ESBA). For the preparation of nickel catalyst (Ni-E-SBA), only nickel nitrate was used, but followed the same procedure describe above. 2.2. Catalyst characterization N2 adsorption-desorption isotherms were measured using Brunauer-Emmett-Teller equipment (BET; ASAP 2010, Micromeritics). Small-angle X-ray scattering (SAXS) patterns were collected on a Bruker GADDS diffractometer. Metal composition analysis was conducted with ICP-AES (SHIMADZU, ICPS-1000). XRD pattern were collected on a Philips XRD. 2.3. Hydrodechlorination (HDC) The HDC of TCEa was carried out in a continuous-flow fixed bed reactor at atmospheric pressure. The Ni-E-SBA and Ni/Cu-E-SBA catalyst were charged in a tubular quartz reactor and activated in a stream of hydrogen (20ml/min) and helium carrier (20ml/min) at 400 °C for 2h. The reaction temperature maintained at 300°C. GC-MS and GC analysis were carried out to determine the product distribution of the HDC reaction. 3. RESULTS AND DISCUSSION It is important to maintain the mesoporous structure during the steps of functionalization and calcination. SAXS patterns of the SBA-15,
0
,
2
3
pjg 1 sAXS patterns.
639
y
\ L
/
\
Ni/Cu-F-
Ni-E-SBAI
SBA15
0
Relative pressure
Fig. 2. BET analysis.
20
40 60 20 / degrees
80
Fig.3.. XRD patterns.
silanized, grafted and calcined catalysts were shown in Figure 1. All the samples studied exhibit a single diffraction peak corresponding to a ^spacing of 9.82nm. N2 adsorptiondesorption isotherm was a typical form of SB A-15 (Figure 2). Surface area, pore diameter and pore volume were decreased with EDTA grafting and Ni/Cu loading (Table 1). Metal contents of the catalyst were measured by the ICP-AES as of 6.5% of Ni and 4.5% of Cu after calcination. It is interesting that no specific metal particle image was appeared in XRD patterns of the prepared catalysts, as shown in Figure 3, because the metal particles are highly dispersed in the support as a very small size. TEM images of calcined Ni/Cu- and Ni-E-SBA showed the highly ordered hexagonal phase, as shown in Figure 4. SAXS, BET, and TEM results showed that mesopore structure was maintained during the preparation of steps. Hydrodechlorination (HDC) of trichloroethane (TCEa) was conducted in a fixed bed flow reactor using the prepared catalysts. The reaction temperature was 300 °C. The products consisted of VCM, methane, ethylene and TCEa. It is well known that any unsaturated chemical compound, such as ethylene, acts as a precursor. Figure 5 is the plot of conversion through the reaction time. The activity of Ni/Cu-E-SBA is higher than that of Ni-E-SBA. In addition, the decrease in activity of Ni/Cu-E-SBA was much slow, when copper was added in the catalyst. Cu may plays an important role to retard the deactivation by, such as coking. Further research is on progress. Figure 6 is the plot of VCM selectivity versus reaction time. VCM (VCM is the desired product.) was achieved after about 600min on Ni/Cu-E-SBA catalyst for 97% selectivity.
,^^f>^r'
% Fig.4. TEM image; (a) SBA15, (b) EDTA-SBA15, and (c) Ni/Cu-E-SBA 15.
640
Table 1 Pore structure properties Ave. Pore diameter (nm)
Surface area (m'/g)
Pore volume (cmVg)
SBA15
6.5
605.00
0.83
Ni/Cu-E-SBA
4.8
229.64
0.20
90
^ o LH
60
c o
V
LL
40 20
-Ni/Cu-E-SBA Ni-E-SBA
Conversion
j^.ie-is
m m m -»- • e- »
se
70 50
«
Ni/Cu-E-SBA
30
A
Ni-E-SBA
Selectivity
Selectivity
10
Conversion
0 200 Time
400 [min.]
Fig. 5. Conversion data.
200
400
600
T i m e [ m m .]
Fig. 6. TCEa selectivity.
4. CONCLUSIONS Ni- and Ni/Cu-E-SBA catalysts were prepared and characterized using an N2 sorption, and SAX. Pore structure of the support was maintained throughout the preparation procedures. From the XRD pattern and TEM image, Ni and Cu metal particle was finely dispersed onto the mesoporous silica support. The HDC of TCEa to VCM was carried out over the prepared catalysts in a continuous flow fixed-bed reactor. Ni/Cu-E-SBA showed stronger resistance to deactivation than catalyst prepared by Ni-E-SBA. Cu may plays an important role to retard the deactivation by, such as coking. Further research is on progress. REFERENCES Keane, M. H. et al., NJ. Mol. Catal. A: Chem., 142 (1999) 187. Jo, J. H. et al., J. ^m. Chem. Soc. in press. Allen, T., Ind. Eng. Chem. Res., 36 (1997) 3019. Kim, Y. H., Lee, B. H., and Yi J., accepted in The Korean J. of Chem. Eng. (2002). Stucky, D. et al., J. Am. Cheme. Soc, 120 (1998) 6024. Stucky, D. et al., Chem. Mater., 12 (2000) 275. Cho, Y. S., Park, J. C , Lee, W. Y., and Yi, J., Catalysis letters, in press. Cho, Y. S., Park, J. C , Lee, W. Y., and Yi, J., Stud. Surf. Sci. Catal. 133 (2001) 559.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
641
New SO2 resistant mesoporous mixed oxide catalysts for methane oxidation D. Trong On, S. V. Nguyen and S. Kaliaguine Department of Chemical Engineering, Laval University, Ste-Foy, Quebec GIK 7P4, Canada, An approach to the synthesis of a new type of mesoporous La-Co-Zr oxides v^ith various atomic (La+Co/La+Co+Zr) ratios is reported. These materials have uniform pore size with a wormhole-type structure, high surface area, high component dispersion and nano-crystalline channel walls after calcination at >450°C. They exhibit high catalytic activity for methane oxidation and good resistance to SO2 poisoning. 1. INTRODUCTION Metal mixed oxide catalysts have been the object of many investigations and have been recognized as active catalysts in a variety of catalytic processes such as the hydrocarbon oxidation. Among them, perovskite type oxides containing transition metals (e. g., Co, Cr, Mn etc.) are considered of great interest for the combustion of hydrocarbons and NOx selective reduction.^ However, these materials usually possess low specific surface areas and are severely poisoned by a few ppm SO2. Their potential applications as catalysts are therefore limited. The discovery of surfactant-templated mesoporous molecular sieves with high surface area and uniform pore size provided new opportunities for the synthesis of original catalysts.^ The potential applications of these materials as catalysts were reported in a recent review.^ Herein, we report the synthesis of a new type of ternary mesoporous mixed oxide materials (e.g., La-Co-Zr oxides). The catalytic activity in methane oxidation and resistance to SO2 poisoning of this type of materials are also studied, compared to those of conventional bulk LaCoOs perovskite and mesoporous silica supported LaCoOa perovskite. 2. EXPERIMENTAL Mesoporous mixed oxide materials with various atomic (La+Co/La+Co+Zr) ratios (designated as Meso-LCZ[x]) were prepared from amorphous La-Co citrate complexes, zirconium sulfate as La, Co and Zr sources, and cetyltrimethylammonium bromide as a surfactant. The synthesis method combines the preparation of a clear solution of soluble homogeneous mixed metal oxides containing cationic surfactant in acidic medium and the precipitation of this homogeneous mixture in basic medium at the pH of ~1L5, followed by hydrothermal treatment at 130°C for 48 h. The materials were characterized using BET, XRD, TEM. Methane oxidation was studied as catalytic test reaction. It was conducted in a tubular fixed bed micro-reactor with a quartz reactor tube at atmospheric pressure. The feed gas contained methane (0.25%), Ne (L0%) and O2 (balance). The feed mixture passed through a catalyst charge of 100 mg, installed in the reactor. The feed and product gases were analyzed using a gas chromatograph (GC). The effect of SO2 in catalytic activity was also studied using a feed gas containing 26 ppm SO2.
642
3. RESULTS AND DISCUSSION The nitrogen adsorption/desorption isotherm obtained from Meso-LCZ[0.5] after calcination at 500°C for 8 h exhibits typical type IV adsorption/desorption isotherms. The BJH pore size distribution is narrow with a 34.5 A average pore diameter indicating the textural uniformity of this sample. A TEM image of this sample shows a uniform pore size with a wormhole-type structure. With increasing calcination temperature (>450°C), a broader low-angle XRD reflection indicates less-uniform mesopores, but maintains its intensity; further, broad higher-angle XRD peaks that correspond to nano-crystalline La-Co-Zr oxides, an increase in the pore diameter and a broader pore size distribution suggest the formation of nano-crystals within the mesopore walls. Table 1 Textural properties of the mesoporous La-Co-Zr mixed oxide materials with various atomic (La+Co/La+Co+Zr) ratios designated as Meso-LCZ[x1* after calcination at 50CPC for 8h. SBET Pore volume Pore diameter Sample (LaCo)/(LaCoZr) (atomic %) (m^/g) (cmVg) (A) Mesoporous ZrOz 0.0 255 0.095 18.0 148 0.140 34.0 Meso-LCZ[0.2] 0.2 130 0.140 34.5 Meso-LCZ[0.4] 0.4 120 0.140 34.5 Meso-LCZ[0.5] 0.5 60 0.100 35.0 Meso-LCZ[0.6] 0.6 30 0.100 65.0 Meso-LCZ[0.8] 0.8 8 0.040 1.0 LaCoOs** perovskite * ) Meso-LCZ[x] where: LCZ = La-Co-Zr oxides, x = atomic (La+Co/La+Co+Zr) ratio. **) this sample prepared from the La-Co citrate complex precursor and calcined at 60iTC for 8 h.
A series of Meso-LCZ [x] with various atomic x = (La+Co)/(La+Co+Zr) ratios calcined at 500°C was also studied (Table 1). A decrease in the specific surface area (SBKT) and pore volume, an increase in the pore diameter and a broader pore size distribution were observed at increasing La-Co oxide content. No mesopore structure was obtained for the La-Co mixed oxide sample indicating that zirconium oxide is responsible for the stabilization of Meso-LCZ (Fig. 1). Meso LCZ[0.5] after calcinations at 400°C was tested as a catalyst in the methane oxidation reaction and compared with reference samples, such as a bulk LaCo03 perovskite having a specific surface area of 8 m^/g and a hexagonal mesoporous silica supported LaCoO^ perovskite containing 38.5 wt% LaCoOs with a specific surface area of 425rn^/g.'^The lightoff temperature (defined as 10% conversion of methane) are at 310, 335 and 370^C; and the half-conversion temperatures (T50) are at 360, 390 and 420°C for Meso LCZ[0.5], supported catalyst and bulk LaCoOs perovskite, respectively (Fig. 2). This indicates a higher catalytic activity for Meso LCZ[0.5] compared to the reference samples. This can be explained by active lattice oxygens, which contribute to deep oxidation reaction, in Meso LCZ[0.5] being much more abundant than in bulk LaCoOs perovskite, as presented by the O2-TPD spectra of these samples (not shown). Therefore, the presence of
643
i "
Pore diameter (A)
Fig. 1. BJH pore diameter distributions from the desorption branch of N2 isotherms of Meso-LCZ[x] with a) 0.2, b) 0.4, c) 0.6, d) 0.8, e) 1.0 (La+Co/La+Co+Zr) ratios.
TemperaturefC)
Fig. 2. Methane conversion in complete oxidation reaction over: a) Meso-LCZ[0.5], b) mesoporous silica supported perovskite c) bulk LaCoOs perovskite.
more active lattice oxygen and higher specific surface area in Meso LCZ[0.5] is the main reason for its markedly enhanced CH4 oxidation activity. Note that a mass transfer limitation was observed in the supported catalyst. This is associated with an internal diffusion effect in the highly ordered mesopore structure with one-dimensional channels of the support; however, this limitation suppressed is on Meso LCZ[0.5] likely due to the worm-hole-type mesopore structure. The SO2 poisoning resistance of Meso-LCZ[0.5] was also examined compared with those of the same reference samples. No significant influence on the catalytic activity in the presence of 26 ppm SO2 for Meso-LCZ[0.5] was observed over 3 days of reaction. By contrast, the rapid decline in methane oxidation activity for the mesoporous silica supported LaCoOs perovskite and the bulk LaCoOs perovskite indicates that Meso- LCZ[0.5] is highly resistant to SO2 poisoning during methane oxidation (Fig. 3). The resistance to SO2 poisoning could be associated with the high La-Co-Zr oxides dispersion and the presence of sulfate groups in the calcined Meso LCZ[0.5] sample. Fig 4 shows the FT-IR spectra of the MesoLCZ[0.5] sample after calcination for 8 h at 400, 500 and 600°C. Four sharp bands at ~1190, 1125, 1060 and 990 cm"^ can be assigned to sulfate species. The intensity of these bands does not change significantly even the sample calcined at 600^C indicating that the sulfate species are hydrothermally stable. These FT-IR spectra are very similar to that observed for sulfate species on Zr02.^ The sulfated zirconia in Meso-LCZ[0.5] could act as a sink for SO2 shuttling the SO2 away from the active sites. In our case, the absence of any significant deactivation of the Meso-LCZ[0.5] catalyst over 72 h could be due to sulfated zirconia, on which zirconium oxide adsorbs preferentially the sulfur oxide from the gas stream spilling over onto the solid. Another possible mechanism could be spilling over of SOx adsorbed species from the Co ions active sites to the Zr02 surface. This apparently prevents contamination of the catalyst active sites. A similar effect was described by Lampert et al. for zirconia supported PdO.^
644
1300
Time (h)
Fig. 3. Methane conversion at 50(FC in the presence of 26 ppm SO2 over: a) MesoLCZ[0.5], b) mesoporous silica supported perovskite c) bulk LaCoOa perovskite.
1200 1100 1000
900
800
Wavenumber (cm"')
Fig. 4. FT-IR spectra of the MesoLCZ[0.5] sample after calcination for 8h at a) 400, b) 500 and c) 600°C.
4. CONCLUSION We describe the synthesis of a new type of ternary mesoporous La-Co-Zr oxides. These materials yield high surface area, monodispersed mesopore size with a worm-hole type pore structure, good thermal stability and high component dispersion. They exhibit unusually high catalytic activity and resistance to SO2 poisoning in the complete methane oxidation as compared to bulk LaCoOs perovskites and mesoporous silica supported LaCoOi perovskite. This new synthesis approach is not limited to the preparation of mesoporous La-Co-Zr oxides, and could be extended to many other mesoporous mixed oxides such as La-Mn-Zr oxides and La-Co (or Mn)-Ce oxides.
REFERENCES M. A. Pena, J. L G. Fierro, Chem. Rev., 2001,101, 1981-2017. D. Trong On and S. Kaliaguine, A«gw. Chem. Int. Ed. 2002, 41, 1036-1040. D. Trong On, D. Desplantier-Giscard, C. Danumah, S. Kaliaguine, Applied Catalysis A'.General, 2001, 222, 299-357. V. S. Nguyen, V. Szabo, D. Trong On, S. Kaliaguine, Micro. Mesop. Mater., 2002, in press. F. Babou, G. Coudurier, J. C. Vedrine. J. Catal, 1995,152, 341. Hoyos, L. J.; Praliaud, H.; Primet, M. Applied Catalysis A: General, 1993, 98, 125; Lampert, J. K.; Kazi, M. S.; Farrauto, R. J. Applied Catalysis B: Environmental, 1997, 7^,211.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
645
Decomposition of VOCs using mesoporous TiOz in a silent plasma Won-hae Hong^, Kyung-soo Choi^, Geon-joong Kim^ and Dong-wha Park^ ^Department of Chemical Engineering, Inha University, 253 Yonghyun-Dong, Nam-gu Inchon, 402-751,Korea ^APSYS, 301 Incheon Center, Incheon city college, Dowha-Dong Nam-Gu, Incheon, Korea The decomposition of benzene was carried out using a silent plasma (dielectric barrier discharge: DBD)-catalyst hybrid system. Several types of catalysts such as TiOi, and V205/Ti02 had been selected. It was found that benzene decomposition efficiency decreases in the following order: 1 wt% V205/Ti02 catalyst > Ti02 catalyst > only plasma discharge. In addition, presence of catalysts improved the CO2 selectivity and suppressed the formation of N2O. 1. INTRODUCTION Many researchers have reported that nonthermal plasma chemical reactions are effective for decomposing most volatile organic compounds (VOCs)[l]. Chemical reactions in silent plasma reactors, such as surface-discharge reactors, dielectric-barrier discharge reactors, and packed-bed type discharge reactors, lead to removal of NOx, decomposition of VOCs, ozone generation, flue gas cleaning, and in door cleaning[l-3]. Compared to other technologies, a silent plasma chemical processing has many practical advantages; relatively low-temperature processing, achieving high decomposition efficiencies of dilute hazardous air pollutants (HAPs)[3]. However, silent plasma reactor has many disadvantages, including low energy efficiencies, poor selectivity to CO2, and byproduct formation[ 1,4-5]. To overcome this problem, we tried the combination of silent plasma and catalyst (hybrid reactor) in decomposing benzene in air stream and controlling discharge byproducts. 2. EXPERIMENTAL
•
a
^
Oscilloscope
MFClj] High Voltage Power Supply
Fig. 1. Schematic diagram of experimental setup
Mesoporous Ti02 catalyst was obtained by following synthesis. H2O, ethanol (EtOH), TiCU, and HCl were mixed and refluxed at 373K for Ih when mole ratio was TiCl4:H20:EtOH = 1:2:4. To this solution, 1.51g of Poly(alkylene oxide) block copolymer HO(CH2CH20)2o(CH2CH(CH3)0)7o(CH2 CH20)2oH (designated as E02()P07oE02o; Pluronic P-123, BASF) was dissolved
646 with vigorous stirring and refluxed at room temperature for 2h. The resulting sol solution was soaked in glass wool and dried at 338K in air for 1-7 days, during which the inorganic precursor hydrolyses and polymerizes into a metal oxide network. And BASF in the prepared glass wool was removed by 0.2M NaOH solutions. V205/Ti02 catalysts were prepared by impregnation technique. The schematic diagram of experimental system is shown in Figure 1. The reactant gas of 200 ppm benzene balanced with air was introduced into the non-thermal plasma reactor with a gas flow rate. The gas flow rate was adjusted with mass flow controllers. The rectangular type DBD plasma reactor consisted of parallel-plate electrodes. These electrodes were made of copper plate (150 mm x 100 mm). The parallel-plate electrodes were covered with a dielectric barrier, such as glass plate. The glass wool such as each attached Ti02 and V205/Ti02 was inserted between two glass plates in the plasma reactor to investigate the effects of the catalyst. The arrangement of silent plasma reactor had an effective discharge area of 140 mm x 90 mm. The AC high voltage with a frequency of 60 Hz was applied to one electrode, and the other electrode was grounded. Discharge voltage was varied from 10 to 14 kV. The discharge voltage and the current were measured by digital oscilloscope (Tektronix, TDS 3012). The concentrations of benzene at inlet and outlet in the DBD plasma reactor were measured by gas chromatograph (Agilent 6890). The on-line byproduct analysis in the outlet was performed by Fourier transform infrared spectrometer (FT-IR, BOMEM inc. MB-104). The experimental conditions are shown in table 1. Table 1 Experimental conditions Discharge voltage Electrode gab Initial C6H6 concentration Residence time Catalyst
lOkV- 14kV 5 mm 200 ppm 5s~ 15s Ti02 lwt%V20s/Ti02
3. RESULTS AND DISCUSSION Figure 2 shows TEM image and Figure 3 shows N2 adsorption and desorption isotherms
* W * > 50nm
0.2
0.4
0.6
0.8
1.0
Relative Pressure, (PIPJ
Fig. 2. TEM mesoporous Ti02
image
of
Fig. 3. Nitrogen adsorption-desorption isotherms and pore-size distribution plot (insets) for Ti02
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and pore-size distribution for TiOi. A mesostructured Ti02 powder was formed, based on ^^^•-^— TEM results (Fig. 2). This mesostructured Ti02 § 80 o powder exhibited an average pore diameter of 85A, •k o 60 • a BET surface area of 203 m^/g. (Fig. 3). Many c o microdischarge emitting blue light were observed •55 40 —•— No catalyst from 10 to 14kV. The current increased with o Q. increasing discharge voltage. And breakdown in i 20 • ^4—iwt%VjO/rioJ o dielectric layer is started at 10 kV. The discharge Q 10 11 12 13 14 was slightly changed when the catalyst was attached. Discharge voltage (kV) Figure 4 shows that the decomposition efficiency of Fig. 4. Decomposition efficiency of CeHe as a function of discharge voltage for different C6H6 as a function of discharge catalysts. After adsorption equilibrium, plasma voltage (CeHe concentration: 200ppm, discharge was kept during 20 min. We have known flow rate: 400 ml/min) that catalysts such as Ti02, V205/Ti02, improved the decomposition efficiency for CeHe in the silent plasma and C6H6 decomposition efficiency greatly increased with increasing discharge voltage. These results mean that increasing of discharge voltage leads to increase the intensity of high-energy electrons and the activation of the catalyst. The identification and suppression of secondary hazardous byproducts which accompany C6H6 decomposition are as important as the achievement of high C6H6 decomposition efficiency. The elimination of benzene from gas stream leads to the formation of inorganic byproducts such as CO, CO2, and N2O. The formation of these byproducts depends on reaction conditions and various catalysts. Figure 5 shows FT-IR spectra in the DBD plasma reactor without catalyst. Byproducts such as CO, CO2, and N2O, were produced by a discharge. It means that 200 ppm benzene balanced with air is oxidized into CO and CO2 by plasma discharge. And the amount of CO and CO2 increased with increasing discharge g 100 >» o
(a)
' Yf^
0)
(a)
X-
(b)
-A. r^-
u c
u c cs
(c)
CO,
E c
CO
^
4000
3000
1 CO 1000
Wavenumber (cm'^)
Fig. 5. Typical FT-IR spectra in DBD plasma reactor without catalyst for benzene decomposition experiments (a) before discharge, (b) after discharge (12 kV), (c) after discharge (14 kV) (benzene concentration: 200 ppm, gas flow rate: 400 ml/min).
4000
'A. f ^
-ji—A r^Y-
•^
-^Kj:—\ COj
N,0
2000
yy-v
--.
3000
N,0
'
CO,
03^1
2000
1000
Wavenumber(cm'^)
Fig. 6. Typical FT-IR spectra in DBD plasma reactor for benzene decomposition experiments (a) without catalyst, (b) Ti02, (c) lwt% V205/Ti02 (benzene concentration: 200 ppm, discharge voltage: 12 kV, gas flow rate: 400 ml/min).
voltage. A significant amount of ozone also is generated by dielectric barrier discharge. Figure 6 shows FT-IR spectra in the DBD plasma reactor with catalyst placed in the discharge zone. After adsorption equilibrium, plasma discharge was kept during 20 min. The use of the catalysts such as Ti02 and V205/Ti02 resulted higher CO2 selectivity than when no catalyst was used. This result means that catalysts in the silent plasma reactor play an important role for the improvement of CO2 selectivity. The amount of N2O increased with increasing discharge voltage in DBD reactor without catalysts, as shown in Figure 5. These results indicate that N2 molecules nearly are not dissociated by electron impact. However, electron impact converts N2 molecules into metastable N2 molecules. And decomposition efficiency of C6H6 with catalysts was higher than without catalysts. But amount of N2O was not increased. From these results, it may be suggested that the suppression of N2O formation is brought by the properties of catalysts. 4. CONCLUSIONS Silent plasma chemical decomposition of CeHa was investigated with a DBD-catalyst hybrid system in air stream. The catalyst of Ti02, V205/Ti02, obtained the highest decomposition efficiency in low C6H6 concentration. And its decomposition efficiency decreases in the following order: 1 wt% V205/Ti02 catalyst > Ti02 catalyst > only plasma discharge. The discharge byproducts are CO, CO2 and N2O. The selectivity of CO2 with catalyst was higher than when no catalyst was used. The use of catalysts also suppressed N2O formation. The combination of plasma and catalyst are useful for VOCs treatment. ACKNOWLEDGEMENTS This work was supported by the Inha University post-doctorial fund in 1999. REFERENCES 1. Atsushi Ogata, Toshiaki Yamamoto, IEEE Trans. Ind. Appl., 35 (1999) 1289 2. Azuchi Harano, Masayoshi Sadakata, The Society Chem. Eng., 31 (1998) 700 3. Aihua Zhang, Shigeru Futamura, J.air&Waste Manage. Assoc, 49 (1999) 1442 4. Toshiaki Yamamoto, J. Electrostatics, 42 (1997) 227 5. Atsushi Ogata, Toshiaki Yamamoto, IEEE Trans. Ind. Appl., 37 (2001) 959
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
649
Preparation of mesoporous 12-tungstophosphoric acid HPW/SiOi and its catalytic performance 'Zhirong Zhu*, 'Wenkui Lu, ^Colin Rhodes 'shanghai Research Institute of Petrochemical Technology, Shanghai 201208, China. FAX: +86-21-68482283. E-mail: zhuzhirong(5),vahoo.com ^Cardiff University, Cardiff CF10 3TB, Wales UK. The included HPW/Si02 with mesoporous structure is prepared with nonionic surfactant AEO as a template, and Keggin structure of HPW included in Si02 matrix is retained, hi addition, it shows much stronger acidity than Al-containing MCM-41. The included HPW/Si02 is of high catalytic activity and selectivity, especially a good stability for reaction of esterification. 1. INTRODUCTION Supported 12-tungstophosphoric acid (HPW), as an important heteropolyacid catalyst, is of greater value for practical applications than pure HPW in reactions relating to surface area and pore structure [1]. Si02 is the most common and efficient support thanks to its relative inert, large surface area and available resources. MCM-41 was considered to be an ideal support for HPW in some acid-catalyzed reactions owing to its large surface and special pore structure [2]. However, HPW on the surface of supports is easy to lose during reactions, especially in polar reactants or products. It was reported that the HPW included in Si02 matrix from hydrolysis of tetraethyl-orthosilicate (TESO) shows high catalytic activity and selectivity, especially a good stability for many reactions such as hydrolysis, esterification and alkylation [3]. The mesoporous materials with pore diameters of 2-8 nm greatly enlarge the accessibility of zeolite materials for molecules of reactants to perform catalytic organic syntheses. However purely siliceous MCM-41 materials show a very limited application in catalysis due to the lack of its acidity and capacity of ion-exchange. Although mesoporous molecular sieve, as an acid catalyst, has been prepared by incorporation of Al into its framework [4, 5], its acidity is weaker than ordinary zeolites [6]. On the other hand, the incorporation of Al in framework may result in lowing uniform mesoporous structure [7]. Therefore it is important to discover a new way to obtain acidic mesoporous materials to perform reactions catalyzed by strong acids. Recently other acidic mesoporous materials were prepared by post-synthesis, generally with modification of mesoporous surface [8, 9]. In this paper, the included HPW/Si02 with mesoporous structure was prepared with alkyl alcohol polyoxyethylene ether (AEO) as a template. The catalytic performance of included HPW/Si02 was measured through reactions of esterification.
650
2. EXPERIMENTAL 2.1. Preparation Tetraethyl-orthosilicate was added into HPW aqueous solution containing nonionic surfactant AEO C16H33 (C2H50)60H at the ratio of 5, 10 or 20 wt.% HPW/Si02, and stirred at 323K for 24 h. After included HPW/Si02 obtained above was dried at 393K, and was extracted by acetone and ethanol. Then it was pretreated at 523K and 673K respectively. 2.2. Characterization The XRD characterization was carried out by using D/MAX-2400 diffractometer with Cu target Ka-ray. FT-IR spectrum of samples was obtained with Pekin-Elmer 2000 FT-IR equipment and a self-supported wafer of sample. The surface area and pore distribution were determined by N2 adsorption at 77K, with automatic Microporous 2500 apparatus. The pore size distribution was calculated from the desorption branch of N2 desorption isotherm using the conventional Barrett-Joyner-Halenda(BJH) method [10]. Temperature peogrammed desorption (TPD) of NH3 was conducted with automatic Altamira-100 Characterization System (USA). The sample was pretreated in helium at 673 K for 2 h, and TPD was carried out in the helium flow of 25 ml/min from 373 K to 973 K with a heating rate of 12"C/min. 2.3. Catalytic test The synthesis of dioctyl phthalate was performed under atmosphere pressure at 383K in a multi-necked flask with magnetic stirrer. The mixture of either 1.2 g HPW/Si02 catalyst powder or 0.24g hexadydrate HPW, 15.6 g n-octylanol and 7.5 g phthalic anhydride reacted for 4 h. The products of esterification reaction were analyzed by GC with FID. 3. RESULTS AND DISSCUSSION 3.L Preparation and structural characterization Though most of mesostructured materials are synthesized in the basic medium, some mesoporous silica has been successfully synthesized with nonionic surfactant Triton X-100 or PO-EO, under acidic condition [11, 12]. 12-tungstophosphoric acid dissociates into 12-tungstophosphate anions with negative charges and protons to make the synthetic medium acidic (PH<2). Under this acidic condition, TEOS is positive charged above the iso-electronic point of silica [13]. So the 12-tungstophosphate and silicon precusor may form neutral ion pairs, and consequently the hydrogen bonding action between AEO surfactant and ion pair leads to the formation of the included HPW/Si02with mesoporous structure. XRD patterns indicate that there is neither peaks relating to pure HPW crystal, nor other related crystalline phases in the included HPW/Si02. However, a wide peak of about 2 "/2 0 may be observed, which is generally considered to reflect a kind of regular mesoporous structure. Figure 1 shows that the included HPW/Si02 is constructed with mesopores with 2.3-3.5 nm pore diameters. Besides, the pore size distributions vary slightly with the HPW/Si02 ratio increasing from 5 wt % to 20 wt %. FT-IR spectra of samples in Figure2 show that there are absorption bands corresponding to P-0, W=0, W-O-W, in line with the characteristics of the HPW heteropolyanion vibrations, which indicate that Keggin structure of HPW included in Si02 matrix is retained [14]. On the other hand, a blue shifted ca. 18 cm' of the bands for outer groups (W=0, comer- and edg-bridging W-O-W) is observed with respect to the crystalline HPW at 799 cm"', which
651
Pore Diameter (nm)
Fig. 1. Pore size distributions of HPW/Si02 (a)5% , (b) 10% , (c) 20% HPW/ SiOz
Fig. 2. FT-IR spectra of the included HPW/SiOs (1)20% , (2) 5% HPW/Si02
indicates a strong interaction between HPW and Si02. It is due to this interaction that the included HPW/Si02 shows better stabiHty than supported HPW/Si02. 3.2. Acidic property NH3-TPD of samples in Figure 3 shows two peaks at 473K ~ 483K and 873K ~ 883K, respectively corresponding to weak acidic sites and strong acidic sites on mesoporous HPW/Si02. The number of two kinds of acidic sites is in proportional to the HPW content of the included HPW/Si02. In addition, the strength of whether weak acidic sites or strong acidic sites slightly increases with the HPW content. With respect to Al-containing MCM-41, most of the adsorbed NH3 desorbs from its surface above 673K, indicating weak-mild acidity without the strong acidic sites [15].
373
573
Fig. 3. NH3-TPD patterns of HPW/SiOz
(a) 20% , (b) 10% , (c) 5% HPW/ SiOz
3.3. Catalytic performance test The results of esterification reaction in Table 1 show that when HPW was loaded on silica source at the same content (20% HPW/Si02), pure HPW and the supported HPW on MCM-41 are of slightly higher catalytic activity that the included HPW/Si02. When
652
catalysts were reused for this reaction, the catalytic activity of the supported HPW on surface of MCM-41 was decreased. However, the catalytic activity of the included HPW/Si02 remains stable, without an obvious decrease. On the other hand, the included HPW/Si02 is of the highest selectivity of monooctyl phthalate. Table 1 Results of catalytic test for esterification included HPW/Si02 supported HPW/Si02 pure HPW conversion selectivity conversion selectivity conversion selectivity 76% 87% Fresh use 59% 67% 89% 97% 61% 97% 58% 90% First reuse 57% 57% 98% 90% Second reuse 53% 57% 98% 91% Third reuse The data in table are phthalic anhydride conversions and selectivities of monooctyl phthalate. 4. CONCLUSION The mesoporous structure HPW/Si02 is synthesized with nonionic surfactant AEO as a template, and Keggin structure of HPW included in Si02 matrix is retained. In addition, it shows much stronger acidity than Al-containing MCM-41. The included HPW/Si02 is of high catalytic activity and selectivity, especially a good stability for reaction of esterification.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
I. V. Kozhevnikov, Catal. Rev. Sci. Eng., 37(1995)311. T. Blasco, A. Corma, A. Martinez and P Martinez-Escolano, J. Catal., 177(1998)306. Y. Izumi, K. Hisano and T. Hida, Appl. Catal. A, 181(1999)277. M. Busio, J. Janchen and J. H. C. Van Hoff, Microporous Mater., 5(1995)211. K. M. Reddy and C. Song, Catal. Lett., 36(1996)103. A. Corma, V. Fomes, M. T. Navaano and J. Perez-Pariente, J. Catal., 148(1994)569 Z. Luan, C. Cheng, H. He and J. Klinowski, J. Phys. Chem., 99(1995)10590. S. A. Antonio and J. Mietek, Stud. Surf. Sci. Catal., 129(2000)187. W. V. Rhijn, D. De Vos, W. Bossaert, J. Bullen, B. Wouters, P Grobet and P Jacobs, Stud. Surf Sci. Catal., 117(1998)183. 10. R T. Tanev and L. T. Vlaev, J. Colloid Interface Sci., 160(1993)110. I L L . Sierra, B. Lopez and J. L. Guth, Microporous and Mesoporous Mater., 39(2000)519. 12. D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. R Chmelka and G. D. Stucky, Science, 278(1998)548. 13. R. K. Her, The Chemistry of Silica, Wiley Press, New York, 1979. 14. C. Rocchiccioli-Deltecheff, Inorg. Chem.. 22(1983)207. 15. R. Mokaya, W. Jones, Z. Luan, M. D.Aalba and J.Kino, Catal. Lett., 37(1996)783.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
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Catalytic properties of heteropolyacids supported on MCM-41 mesoporous silica for hydrocarbon cracking reactions Jorge Norberto Beltramini The NanoMaterials Centre, Department of Chemical Engineering, The University of Queensland, Queensland 4072, Australia. FAX: +61-7-33654199. E-mail: iorgeb@cheque.uq.edu.au It is known that MCM-41 structures have very weak acid sites because of the lack of the bridging hydroxyl groups present in zeolites. Strong acidity however is required for the potential use of these materials in some specific applications such as: cracking and hydrotreating of heavy residue molecules, cracking of waste plastic, etc. The acidity enhancement of the MCM-41 materials was assessed using the n-hexane and polyethylene cracking reactions. MCM-41 samples were impregnated using heteropolyacid (HPA) such as tungestophospheric acid. The catalyst samples were characterized also by x-ray diffraction and benzene adsorption. 1. INTRODUCTION Recently an important new class of molecular sieve materials, called M41S, has been reported in the literature. These materials have interesting properties such as large pore volume and size and high thermal stability. Clearly, there is an excellent potential for these materials to be applied commercially in catalysis, specially in those applications where a large pore than those offered by the conventional zeolite is required. It has been show that the MCM-41 materials were unlikely to possess strong acid sites as those present in zeolites (1). The impregnation of heteropolyacids (HPA's) on MCM-41 materials as a mean of increasing acidity was lately reported (2). HPA's are well known superacid specially if supported on high surface area support. They own their high acid strength in catalyzing an organic reaction to the role their conjugate bases play in activating the reactant species. When HPA's are used as heterogeneous catalysts and specifically in gas phase reactions the nature of the reactants often determines whether the reaction take place on the surface or in the bulk of the catalyst catalyst performance. In this paper the acidity enhancement of the MCM-41 materials with supported HPA's was assessed using n-hexane and polyethylene cracking reactions. MCM-41 samples were impregnated using HPA's such as tungestophosphericc acid (HPW). Then the samples were characterized by x-ray diffraction, FTIR and benzene adsorption. 2. EXPERIMENTAL 2.1. Catalyst preparation The MCM-41 samples used were synthesized using the hydrothermal crystallization technique (3). Cetylmethylammonium bromide was dissolved in distilled water and an
654
aqueous solution of tetramethylammonium hydroxide was added while stirring, followed by a silica solution. Then the hydrogel was crystallized at 160 °C for 120 hours in a pressure vessel. The crystalline product was filtered, washed, dried at 100 °C and finally calcined at 560 °C for 18 hours. During impregnation the HPA was dissolved in distilled water and the calcined MCM-41 sample was added. The mixture was stirred for 18 hours and then left at 100 °C in an oven until the water evaporated. This procedure gave a loading of 16.6 % by weight of HPA on the supported catalyst. 2.2. Catalyst characterization X-ray powder diffraction (XRD) patterns were taken using a Siemens D5005 instrument. A Perkin-Elmer 16F PC FTIR was used for the analysis of samples. Benzene adsorption measurements were carried out on a Hidden Analytical IGA gravimetric system. 2.3. Catalytic tests The n-hexane cracking reaction was carried out in a stainless steel microreactor, introducing n-hexane by a bubbler system. Reactor effluent was analyzed on line by a GC system equipped with a FID detector and a KCl alumina capillary column. Polyethylene cracking was carried out in a glass batch reactor. The reactor was connected to a liquid condenser and a gas collection bag. Gaseous product was analyzed using a GC with FID and a KCI/AI2O3 capillary column. Liquid product was analyzed also using a GC with FID and SE30 capillary column. 3. RESULT AND DISCUSSION MCM-41 with all silica composition was synthesized with standard unit cell parameters of 60 A. Characterization results from x-ray diffraction, FTIR and benzene adsorption, showed that the material had an all silica composition with an aluminium impurity level lower than 10 ppm and with typical hexagonal unit cell symmetry. The properties of the calcined MCM-41 synthesized can be shown in Table 1. Table 1 Properties of MCM-41 sample synthesized Crystallization dioo(A) ao{A) Time (h) 110 52.8 61.7
Vp (cc/g)
P/Po
dp (A)
5(A)
0.72
0.36
50.9
10.9
Upon impregnation with tungstophosphoric acid (HPW) the unit cell of the MCM-41 material shrank by 12 A. This reduction appeared to be occurring primarily in the wall structure, as the drop in the benzene adsorption capacity did not fully account for the extent of the unit cell shrinkage. 3.L Normal hexane cracking The catalytic activity of HPW unsupported and supported on MCM-41 can be seen in Figure 1. MCM-41 was found to be inert for the catalytic conversion of n-hexane at any temperature. When impregnated with HPW, however, the resulting catalyst showed a high cracking activity of n-hexane at a low temperature and for a short time on stream.
655
- P u r e HPW -HPW/MCM-41
Time (h)
Fig. 1. N-hexane cracking conversion vs time on stream at 300 °C The n-hexane cracking activity of the impregnated MCM-41 as a function of the temperature and time on line is shown in Figure 2. The activity reached a maximum at 200 °C and dechned at higher temperatures for a catalyst cycle of near 20 minutes and total amount of n-hexane of 6.8 mg. 60 n * —^m
50
r" 1 30 £
-
^
X
•»v
"m-
^""-""-^^..^^ ^-
V
"** ^
o
O 20
200 -300
C c
350
C
-400
C
^*'*'^^-.....^ ^^^^^^ *v
5c
10
^ •
^.^
^^***~"^ "
"''-'.
0 Tim e
(h)
Fig. 2. N-Hexane cracking conversion vs time on stream for HPW/MCM-41 at different temperatures From the product distribution of unsupported HPW and supported HPW on MCM-41 it was found that the main effect of supporting the HPW on the MCM-41 was the increase in the selectivities of propane and pentane and reduction in butane selectivity. Very little methane and ethane was produced, suggesting a low level of protolytic cracking. High levels of propane, butanes and pentanes suggested high levels of bimolecular hydrogen transfer activity. On the other hand, the fast deactivation rate, as shown in Figure 1, was most likely due to the high level of hydrogen transfer and fast polymerization of olefins, resulting in fast coking rate. Very small amounts of olefins have been produced in the gas phase, which reflected the high acidity of the catalysts. The reason for the presence of an optimum temperature, where the catalyst activity was maximum, could be explained through the instability of the HPW at high temperatures as reported elsewhere (4). Both x-ray powder diffraction and infra red spectroscopy have shown evidence for the HPA instability starting at 300 °C. Characterization of the spent catalyst showed additional reduction in the benzene adsorption capacity.
656
3.2. Polyethylene cracking The cracking of high density polyethylene (HDPE) was studied on thermal cracking, pure MCM-41 and HPW impregnated MCM-41. The product composition of the cracking reaction can be taken as direct indication of the catalyst activity and selectivity. The quantitative yields of products in the Ci - C25 fraction as function of the carbon number are shown in Figures 3. 35 -«—HPW/MC
30
1
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15 10 5
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/ ''"^ ' // // / / 1
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1
Carbon Number of Components
Fig. 3. Yield (wt%) of product of cracking of high density polyethylene over different catalysts at 450 °C. As can be seen the lightest product was obtained over the HPW impregnated MCM-41 followed by those obtained over pure MCM-41. The product distribution of polyethylene cracking strongly indicates a carbenium ion mechanism. The levels of isobutane and isobutene are very high relative to the levels of methane and ethane as compared with pure MCM-41 test runs. It is proposed that the formation and stabilization of carbenium ion in the pores of the MCM-41 be due to the adsorption between the polyethylinic fragments with the surface of the channels where the HPW is adsorbed . 4. CONCLUSION It has been shown that addition of HPW to all silica MCM-41 catalyzes the cracking reaction of hydrocarbons. The product distribution of cracking reactions strongly indicates a carbenium ion mechanism, associated with acid sites created by the HPW lining the channels of MCM-41.
REFERENCES 1. A.Corma, M. Iglesia and F. Sanchez, J. Chem. Soc. Chem. Comm., 1635, (1995). 2. J.A. Diaz, J.P Osegovic and R.S. Drago, J. Catal., 183, 83, (1999). 3. Y. Izumi, N. Natsume, H. Takamine, I. Tamaoki and K. Urabe, Bull. Chem. Soc. Japan, 62, p. 2159,(1989). 4. Blasco, T, Corma, A, Martinez, A. and Escolano, P, J. Catal. 177 (1998), 306.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
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Preparation, characterization and catalytic activity of heteropolyacids supported on mesoporous silica and carbon Zhenbo Zhao^'^ , Whaseung Ahn^ and Ryong Ryoo'^ ^Catalysis Laboratory, School of Chemical Science and Engineering, Inha University, Incheon, 402-751, Korea ^Department of Light Industry and Textile Engineering, Jilin Institute of Technology, Changchun, 130012, R R .China '^National Creative Research Initiative Center for Functional Nanomaterials and Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon, 305-701, Korea HPA was impregnated on mesoporous silica (SBA-15, MCM-41), commercial silica (ML369) and activated carbon, as well as on mesoporous carbons (CMK-1, CMK-3). The resulting materials were investigated by means of XRD, BET and catalytic probe reactions of liquid-phase esterification of hexanoic acid with propanol-1 and acylation of 2-methoxynaphthalene with acetic anhydride. The catalytic performance was optimal with loading of around 40wt% HPA. H3PW12O40 (PW) was more active than H4SiWi204o (SiW) for esterification, irrespective of the supports used. Solvent used in the impregnation and hydrophobic/hydrophilic nature of the support could influence the performance of the catalyst in these acid catalyzed reactions. INTRODUCTION Heteropolyacids(HPAs) have stimulated considerable research in both heterogeneous and homogeneous catalysis [1-3]. Among them, 12-tungstophosphoric acid, H3PWi204() (PW), the strongest and the most stable acid in the Keggin series of HPAs, has attracted the most attention. The main drawback of such materials for catalytic application is their low specific surface area. Thus, direct dispersion of the bulk HPAs on a mesoporous silicate support such as MCM-41 [4], commercial silica, and activated carbon [5] has been attempted. SBA-15 [6] possessing larger pore size and CMK-1,3 [7] are mesoporous materials newly emerged recently, and have not been tested as a carrier for supporting HPAs. Here we report the comparison of mesoporous silica (SBA-15, MCM-41), commercial silica (ML369), activated carbon and mesoporous carbon (CMK-1, CMK-3) supported HPAs as catalysts for the liquid phase esterification and acylation reactions. 2. EXPERIMENTAL MCM-41 was synthesized according to the literature recipe [4]. SBA-15 was synthesized by using the triblock copolymer, EO20-PO70-EO20 (Pluronic 123, BASF) as the surfactant and the tetraethylorthosilicate (TEGS, 98% Aldrich) as silicon source [6]. The supported HPAs catalysts were prepared by impregnation of HPAs on various carriers following the procedure
658
of Kozhevnikov et al [2]. Typically, the required amount of HPAs was dissolved in various solvent and a proportional amount of the support material were added. The mixture was stirred for 18h at room temperature. Subsequently, solvent was removed in a rotary evaporator, yielding the HPA-impregnated catalysts. The material was dried and mildly calcined at 403K and stored in a desiccator until use. XRD patterns were obtained with a CuKa X-ray source (Rigaku Miniflex instrument, 45OW). N2-adsorption isotherms were obtained at 77K using a Micromeritics ASAP2000. The samples were outgassed at 403K and 0.1 Pa for 12h before measurements were performed. Specific surface areas were obtained with the BET equation; the mean pore sizes by the BJH method. Esterification of hexanoic acid with propanol-1 was carried out at reflux temperature in a glass vessel equipped with a magnetic stirrer and a Dean Stark trap for water removal. Toluene was used as a solvent. Acylation of 2-methoxynaphthalene with acetic anhydride was carried out at 323 K using chlorobenzene as a solvent. Analysis was performed with GC (ShimadzuGC-14A) equipped with Shimadzu column Hicap CBP1-M25-025 with a flame ionization detector. 3. RESULTS AND DISCUSSION As shown in Fig.l, the introduction of 12-tungstophosphoric acid, H3PW12O40 (PW) to mesoporous silica SBA-15 or mesoporous carbon CMK-3 resulted in little decreases in intensities of the XRD reflections of the mesostructures, which suggests that the structural order of the host materials is maintained, ki addition, no peaks corresponding to HPA were detected indicating highly dispersed state of HPA impregnated. The preparation conditions such as the heteropoly anionic species, physicochemical properties of the supports, the impregnation solvent, and the loadings of HPAs were shown to have pronounced influences on catalytic performances (shown in Figs. 2-5). H3PW12O40 (PW) was more active HPA than H4SiWi204o(SiW)for esterification, irrespective of the supports used. Catalytic performance was optimal with loading of around 40wt% HPA. Apparently, the higher the HPA loadings, the higher would be the catalytic activity due to generation of increased active sites, but loadings of 50% or more must have caused blockage in the support channels and also resulted
20%
PW / S B A - 1 5 ( a q )
_3_0^o_ P W / S B A - 1 5 ( a q ) 40%
20%
PW/CM
K-3(aq1
3 0%
PW/CM
K-3(aq)j
40%
P W / C M K - 3 (a q
P W / S B A - 1 5(aq)
4
2 theata
6
4
6
2 theata
Fig. 1. XRD Patterns after different loadings of H3PW,204o on SBA-15 and CMK-3
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Table 1 Surface areas and porosities of SBA- 5 impregnated with PW BET surface area BJH average diameter Samples (A) (m'/g) 64.1 30%PW/SBA-15(aq) 425 61.0 40%PW/SBA-15(aq) 292 59.8 50%PW/SBA-15(aq) 207_
Pore volume (cc/g) 0.60 0.44 0.31
in poor dispersion. As shown in Table l,when the amount of PW introduced to SBA-15 was 30wt%, 40wt%, 50wt%, the surface area and pore volume varied correspondingly as 425, 292, 207mVg and 0.60,0.44,0.3 Iml/g, respectively. These changes can be explained by the large molecular size of the Keggin anion(1.2nm in diameter) and its interaction with the amorphous wall of the host material. In the hexanoic acid esterification, following catalytic activity order of PW supported on different silica impregnated in aqueous phase was obtained: PW/SBA-15 > PW/MCM-41 > PW/silica. This difference can be explained in terms of the larger pore size of SBA-15 than MCM-41 and bigger surface areas of the mesoporous materials than
y^
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-a
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80 H
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/ •
/
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—•—40% SiW/SBA-l5(aq) —•—40% PW/SBA-15(aq) 1
^ o
r
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Reaction time / min
Fig. 2. Catalytic performance of different HPAs on SBA-15.
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/
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^
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Fig. 4. Effect of impregnating solvent for HPA on catalytic performance (SBA-15 support)
. Reaction time / min
. _ ^ . ^ - -
- . - 4 0 % PW/SBA-15(CH,0H) —•—40% PW/SBA-15(aq) 120
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- • - — 4 0 % PW/silica369(aq) - « — 4 0 % P W / M C M - 4 1 (aq) - - * - - 4 0 % PW/SBA-I5(aq)
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Fig. 3. Effect of different silica supports on catalytic performance.
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180
240
300
Reaction time / min
Fig. 5. Effect of HPA loadings on catalytic performance (SBA-15 support)
1 |
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Table 2 Conversion of hexanoic acid over various PW supported catalysts (loading: 40% PW reaction time: 3h) CMK-1 CMK-3 Activated Carbon O.lMHCl 42.9 54.4 61.4 CH3OH 33.7 36.8 58.4 commercial silica. As shown in Table 2, commercial activated carbon was a better carrier than mesoporous carbons when impregnated in methanol or HCl but aqueous impregnation resulted in poor conversion; the former having the larger surface area (ca. 1500 mVg) than mesoporous carbons (ca. 1000 mVg) and due to more hydrophobic nature of activated carbon. Water or methanol was equally acceptable solvent for HPA for mesoporous carbon CMK-3, and this shows different surface nature of mesoporous carbons from commercial one. For acylation, again mesoporous materials produced better catalytic performance than commercial silica, and HPA on CMK-3 with larger pores performed slightly better than commercial carbon or CMK-1 (Fig.6 and Fig.7).
40% PW/CMK-3(aq) -40%PW/MCM^l(aq) 4()%PW/SBA-15(aq) 40% PW/silica(ML369)(aq)
Reaction time/min Fig. 6. Comparison of different supports on acylation activity
• - 40% PW/CMK-1 (0.1M I ICl) • 40% PW/CMK-3(0.1M HCl) ^ 40% PW/Carbon(0.1M I ICl)
Reaction time / min
Fig. 7. Comparison of different carbon supports on acylation activity
ACKNOWLEDGEMENTS This work was supported by grant 2000-1-30700-3 from the basic research program of the Korea Science & Engineering Foundation. REFERENCES 1. M. Misono, N. Norjiri, Appl. Catal. 64 (1990) 1 2. I.V.Kozhevnikov, A.Sinnema, R.J.J.Janse, K.Pamin, H.Van Bekkum, Catal. Lett. 30 (1995) 241 3. T. Blasco, A. Corma, A. Martinez, P J. Martinez-Escolana, J. Catal., 177 (1998) 396 4. C.T. Kresge, J. E. Leonowicz, W.J. Roth, J. C. Vartuli, J. S. Beck, Nature, 359 (1992) 710 5. Y. Izumi, et al., J. Catal., 84 (1983) 402 6. D. Zhao, J. Feng, Q. Huo, W. Melosh, G.H. Fredrichson, B. F. Chmelka, G. D. Stucky, Science, 279 (1998) 548 7. R. Ryoo, S. H. Joo, S. Jun, J. Phys. Chem. B, 103 (1999) 7743
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Novel SBA-15 supported heteropoly acid catalysts for benzene alkylation with 1-dodecene Hai-Ou Zhu^, Jun Wang^'*, Chong-Yu Zeng^ and Dong-Yuan Zhao^ ^Jiangsu Key Laboratory of Chemical Engineering and Technology, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China ^Chemistry Department, Fudan University, Shanghai 200433, China Phosphotungstic acid (PW) catalysts supported on the mesoporous molecular sieve SBA-15 have been prepared, characterized and evaluated in the alkylation reaction of benzene with 1-dodecene. SBA-15 supported PW catalysts exhibit much higher catalytic activity, stability and selectivity compared with HY zeolite. It is proposed that the high dispersion of PW on SBA-15, high surface area, mesoporosity and specific acidity of the catalyst could responsible for its catalytic performances. 1. INTRODUCTION Linear monoalkylbenzene (LAB) is the primary raw material for detergent. The manufacture of LAB conventionally involves HP as a catalyst, which is a source of pollution and equipment corrosion. Thus, many studies on solid acid catalysts have been carried out to solve this problem, among which zeolite type catalysts are mostly measured, and only a few and isolated literatures have dealt with heteropoly acid (HPA) catalysts [1]. Pure HPA is known to possess the strong bronsted aicidity, and has been widely investigated in numerous acid catalyzed reactions [2]. However, owing to its very low surface area and high solubility in polar solvent, supported HPA catalysts attract much research attention recently [3,4]. Since the newly synthesized silica mesoporous molecular sieve, SBA-15 [5], has a very high surface area, large pore volume and satisfied stability, with the uniform pore size being large enough to implant HPA molecules, we consider it as a potential qualified support for HPA. In this work, the much higher catalytic activity, stability and selectivity in the benzene alkylation with 1-dodecene are achieved over the SBA-15 supported PW catalyst, compared with HY zeolite. Catalyst performances are discussed based on their physicochemical characteristics. 2. EXPERIMENTAL SBA-15 supported PW catalysts, m%PW/SBA-15, were prepared by an impregnation method, where m stands for the percentage of PW in the catalyst by weight. Before introducing into the reactor, the catalysts were dried at 333 K and then calcined at required temperatures, and HY zeolite was calcined at 773 K for 5 h. * Corresponding author. E-mail address: junw0212@yahoo.com.
662 1
1
1
i PW 90%PVWSBA-1^5
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I
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70%PW/SBA-15
JJIf-—
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60%PW/SBA-15
U
1 ,Jv^- , 4
8 12 Pore diameter /nm
16
Fig, 1. Pore size distribution 50%PW/SBA-15.
50%PW/SBA-15 40%PW/SBA-15 30%PW/SBA-15
K. V 20
for
0
20%PW/SBA-15 10%PW/SBA-15 SBA-15
5
10
15 20/"
20
25
30
Fi£;. 2 . XRD patterns for PW/SBA-15
catalysts.
Catalytic tests for the alkylation of benzene with 1-dodecene were carried out in a glass flask reactor equipped with a magnetic stirrer and a condenser. The reaction temperature was 353 K, with typically a 5 (ml/ml) ratio of benzene to 1-dodecene and a 20 (ml/g) ratio of 1-dodecene to catalyst. Reaction products were analyzed by the gas chromatograph with FID as the detector furnished with a 30 m SE30 capillary column. Physicochemical properties of catalysts were measured by X-ray powder diffraction (XRD, Bruker D8 Advance), temperature programmed desorption of ammonia (NH3-TPD, home-made) and N2 adsorption (Coulter Omnisorp lOOCX) techniques. 3. RESULTS AND DISCUSSION Table 1 shows the BET surface area (SB!:T), mean pore diameter (d) and pore volume (Vp) of the three selected samples. It reveals that the catalyst surface area and pore volume decrease gradually with the increase of PW loading. However, even at a high PW loading of 50%, the material still retains a rather large surface area (> 300 m^.g"'), with the pore size only decreasing slightly. Moreover, Fig. 1 indicates a very narrow pore size distribution for 50%PW/SBA-15, demonstrating a uniform mesoporosity of this catalyst. Table I Surface area and porosity of PW/SBA-15 catalysts Catalyst SBHT/m .g' SBA-15 540 30%PW/SBA-15 433 50%PW/SBA-15 315
d/nm 6.6 6.4 6.4
Vp/ml.g-'
0.94 0.69 0.47
Fig. 2 displays XRD patterns for various PW/SBA-15 catalysts. It can be clearly seen in Fig. 2 that the SBA-15 support employed here possesses a typical hexagonal mesoporous structure. When PW is loaded on SBA-15, no clear diffraction peak from PW crystal phase appears until the PW loading is as high as 70%. This indicates that PW can highly disperse on the surface of SBA-15 support with the mesoporosity unaltered by the loaded PW. This can be reconfirmed by the result in Table 1 and Fig. 1.
663 100
80
O
60 1
8
-o
•o ^ c g (0 (D
100
200
300
400
500
Temperature /°C
Fig. 3. NH3-TPD profiles for (1) 20%PW/SBA-15; (2) 40%PW/SBA-15; (3) 60%PW/SBA-15 and (4) HY.
—••
i
>
•
40
•
\
\
•
20
' - ' 1
—»—40% PW/SBA-15 1 —•—HY
2
3 4 5 Reaction cycle
6
Fig. 4. Comparison of catalytic stability between PW/SBA-15 and HY catalyst (1-dodecene/catalyst = 5 (ml/g)).
The catalyst acidity is shown in Fig. 3. Only one broad ammonia desorption peak in a temperature range from about 100 °C to 300 °C is observed for PW/SBA-15 catalysts, while two peaks for HY zeolite are found with the maximum at about 220 °C and 350 °C, respectively. This phenomenon directs the existence of weak and medium acid sites on PW/SBA-15, and medium and strong acid sites on HY zeolite. It is further revealed that both acid strength and acid number increases with the increase of PW loading. The stabilized conversion of 1-dodecene and product selectivity of PW/SBA-15 and HY catalysts are shown in Table 2. 2~6-P (2~6-phenyldodecane) and 2~6-dodecene are the only products detected here, with the later being considered as the unconverted reactant when calculating the reaction conversion. 2-P is the mostly desired isomer due to its better emulsibility and biodegradability. It can been seen in Table 2, by introducing the heteropoly acid into inactive pure SBA-15, all supported catalysts exhibit considerable catalytic activities, and the activity increases with the increase of PW loading up to 60%. Further increase of the PW loading results in a decrease of activity. The highest activity of 89.7% is found on 60%PW/SBA-15 catalyst, which is higher than that of HY by 28%, meanwhile, its selectivity for 2-P (37.3%) is also higher than that of HY by 7%. Fig. 4 compares the catalytic stability between PW/SBA-15 and HY. It reveals a much slower deactivation rate for PW/SBA-15 than that for HY catalyst. At the fourth reaction cycle, the activity of HY decreases sharply to 20%, while 40%PW/SBA-15 still exhibits a high activity of 90%. Benzene alkylation with 1-dodecene proceeds via the carbenium ion mechanism. While the secondary dodecylcarbenium ion is electrophilically attacked by benzene to produce 2-P, it also tends to react with other olefins to generate polymer, which is the source of coke, or transform into other dodecene isomers by intramolecular double bond isomerization and finally give 3~6-P products via attacking to benzene. Based on the above consideration, the high activity of the SBA-15 supported PW catalyst could be ascribed to its larger number of acid site over that of HY zeolite (Fig. 3), as well as its mesoporous channel, in which the acid site is much more easier of approach by the reactant and thus more favorable to the generation and diffusion of products in comparison with the 12-membered ring channel of HY zeolite. However, at very high PW loadings (> 60%), the activity of PW/SBA-15 begins to drop off. This is supposed to arise from the poor dispersion of PW on SBA-15 support due to the occurrence of PW crystal phase on its surface (Fig. 2).
664
Table 2 Conversion of 1-dodecene and product selectivity over PW/SBA-15 and HY catalysts Catalyst Conversion Product selectivity /% /% 2-P 4-P 3-P 5-P 61.7 30.7 13.7 16.6 HY 17.1 6.4 3.0 68.3 16.9 10%PW/SBA-15 5.7 45.8 44.1 21.7 12.9 20%PW/SBA-15 12.0 38.8 21.6 13.9 78.5 30%PW/SBA-15 14.6 14.1 84.5 40%PW/SBA-15 38.3 21.5 13.8 85.2 37.7 21.7 14.3 14.4 50%PW/SBA-15 89.7 37.3 22.0 14.3 60%PW/SBA-15 13.8 37.8 22.1 14.3 77.6 70%PW/SBA-15 13.6 32.3 49.3 21.3 11.6 10.4 90%PW/SBA-15
6-P 21.9 2.7 9.3 11.2 12.4 11.9 12.6 12.2 7.4
On the other hand, Y zeolite has a three-dimensional channel with the pore diameter of 0.74 nm and the interval supercage of 1.3 nm. This microporosity implies that the produced coke on medium and strong acid sites of HY tends to block the pore channel entrance and/or cover the acid sites inside the channel, resulting in a rapid deactivation, as shown in Fig. 4. By contrast, the mesoporosity of PW/SBA-15 is proposed to be responsible for its much stable catalytic performance. 2-P is the slimmest one among the produced isomers, and one tends to use zeolite as the target catalyst to improve the selectivity for 2-P due to its shape selectivity. However, we observed here an higher selectivity for 2-P on the mesoporous PW/SBA-15 catalyst than that on HY. It is known that 3~6-P are secondary reaction products in the whole reaction network, with the double bond isomerization of 1-dodecene taking place first. This isomerization reaction occurs simultaneously and competitively with the desired direct alkylation of benzene with 1-dodecene at same acid sites. Consequently, the catalytic selectivity for 2-P relates not only to the size of catalyst pore, but also to the catalyst acidity itself. It is thus deduced here that the weak and medium acidity on PW/SBA-15 can be more favorable to the direact alkylation of benzene with 1-dodecene with the produce of 2-P, compared with the strong acid sites on HY zeolite which is considered to be more beneficial to the double bond isomerization of 1-dodecene, leading to more 3~6-P products. ACKNOWLEDGEMENTS This work was supported by Jiangsu Natural Science Foundation (BK99122) and NSF (29925309) of China, and by Jiangsu High Technology Project (BG2001044) of China.
REFERENCES 1. 2. 3. 4. 5.
J.A. Kocal, V.V. Bipin and I. Tamotsu, Appl. Catal. A: General, 221 (2001) 295. L. Marosi, G. Cox, A. Tenten and H. Hibst, J. Catal., 194 (2000) 140. T. Blasco, A. Corma, A. Martinez and P Martinez-Escolano, J. Catal., 177 (1988) 306. L. Pizzio, P Vazquez, C. Caceres and M. Blanco, Catal. Lett., 77 (2001) 233. D. Zhao, J. Feng, Q. Huo, N. Melosh, GH. Fredrickson, B.F. Chmelka and G.D. Stucky, Science, 279 (1998) 548.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. Allrightsreserved
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Aluminum containing periodic mesoporous organosilicas: synthesis and etherification Jin-Won Kim^, Hyung Ik Lee*', Ji Man Kim^, Xingdong Yuan^'^ and Jae Eui Yie^* ^Catalyst and Surface Laboratory, School of Chemical Engineering and Biotechnology, Ajou University, Suwon, 442-749, Korea ^Functional Materials Laboratory, Department of Molecular Science & Technology, Ajou University, Suwon, 442-749, Korea '^Department of Petrochemical Technology, Fushun Petroleum University, Fushun, 113001, China Aluminum has been successfully incorporated within the frameworks of periodic mesoporous organosilicas (Al-PMO) by co-condensation of bis(triethoxysilyl)ethane and dibutoxyaluminotriethoxysilane. The Al-PMO materials exhibit highly ordered 2-d hexagonal structures, high surface areas, and narrow pore size distribution in the mesoporous range. The Al-PMO catalysts result in excellent catalytic activity and selectivity for etherification reaction between 2-naphthol and ethanol, which is comparable with those of beta zeolite. 1. INTRODUCTION Recently, periodic mesoporous organosilicas (PMO) have been synthesized by condensation of bridged silsequioxane in the presence of structure-directing agents, and attracted much attention due to their well-ordered mesostructures and noble framework structures [1,2]. The presence of organic groups within the frameworks is expected to give hydrophobic character and hydrothermal stability to the mesoporous materials. These properties are very important for the applications under hydrothermal conditions and organophilic reactions systems, compared with those of normal mesoporous silicas such as MCM-41 and MCM-48. However, the PMO materials constructed with organosilica frameworks (Si-PMO) are of limited use in catalysis, due to the lack of acidity and ion exchange sites. Incorporating other elements such as Al, Ti, Mn, Fe, V, etc. into the organosilica frameworks can improve the properties, which are important for the applications as catalysts and adsorbents. So far, there are a few reports on the modification with organic functional groups and on their applications [3]. It seems to be difficult to incorporate heteroatoms within the PMO frameworks with conventional methods that have been generally used in preparation of metal containing MCM-41. Etherification reaction of 2-naphthol is very important because the products have been extensively used in the fine chemical industry [4]. For example, 2-naphthyl methyl ether has been used in perfumery, which is traditionally manufactured from 2-naphthol and methanol in the presence of sulfuric acid. However, the drawbacks of such a process include corrosion, safety hazards, separation procedures, and environmental problems due to the use of sulfuric
666
acid. A PMO material with solid acid properties is expected to be an excellent heterogeneous catalyst for this reaction due to its mesoporosity and hydrothermal stability. In the present work, incorporation of aluminum into the PMO frameworks has been successfully carried out by co-condensation between bis(triethoxysilyl)ethane (BTSE) and dibutoxyaluminotriethoxysilane (DBATES) in the presence of structure-directing agents, and the possibility of the materials for catalytic applications to etherification are investigated. 2. EXPERIMENTAL Al-incorporated PMO materials (Al-PMO) were synthesized by modified procedures described elsewhere [1] using BTSE as the framework source, DBATES as the aluminum source and octadecyltrimethylammonium chloride (ODTMACl) as the structure-directing agent. A typical gel compositions was 1 BTSE : 0.067 DBATES : 0.57 ODTMACl : 2.4 NaOH : 350 H2O : 10 EtOH : 0.012 HCl. BTSE and DBATES were prehydrolyzed and oligomerized under acidic conditions before mixing with surfactant solution. To investigate the effect of aluminum source on the materials, the Al-PMO materials (Si/Al — 30) were synthesized by using various kinds of aluminum sources such as A1(N03)3, Al(i-OC3H7)3 and NaA102. The resulting mixture was magnetically stirred at room temperature for 20 hr, and subsequently heated in an oven at 368 K for 20 hr. The precipitate was recovered by filtration, washed with doubly distilled water and dried at 373 K for 6 h. The as-made products were refluxed in an excess acidified ethanol with HCl to remove the surfactant. The products were obtained by filtration, washed with ethanol and dried at 373 K for 10 h. The solvent-extraction procedures were repeated three times. The Al-PMO materials were characterized by powder X-ray diffraction (XRD), N2 adsorption, FT-IR, solid-state MAS ^^Al NMR spectroscopy, thermogravimetric analysis (TGA). Etherification reactions between 2-naphthol and ethanol were carried out in a down flow fixed bed reactor at 453 K. The reaction conditions were 0.1 g of catalysts, ethanol/2-naphthol = 10/1, and reactants flow rate = 1.0 cc/h. 3. RESULTS AND DISCUSSION Figure 1 shows XRD patterns for the Si-PMO and the Al-PMO obtained by using DBATES as the aluminum source, before and after surfactant extraction. The XRD patterns for PMO materials before extraction (Figure la and Ic) give a very intense diffraction peak and two or more weak 2 4 6 8 peaks, which are characteristic of 2-d hexagonal 20/ degree (P6mm) mesostructure [5]. There are no significant Fig. 1. X-ray diffraction patterns for (a) changes upon removal of surfactant except for the as-made, (b) washed Si-PMO, (c) as- expected increase in XRD peak intensity. The AlPMO after surfactant removal (Figure Id) gives made and (d) washed Al-PMO.
667
(210) and (300) peaks, which indicates excellent textural uniformity of the material. TEM image also indicates that the material has a highly ordered 2-d hexagonal structure. Lattice parameters (a), calculated from dwo spacings, for the Si-PMO and Al-PMO materials are 4.67 nm and 4.96 nm, respectively. Line broadening and large lattice parameter of the Al-PMO material, compared with those of Si-PMO, may be due to the Al incorporation within the frameworks. Nitrogen adsorption isotherms indicate that the BET surface areas of the Si-PMO and Al-PMO materials are 1050 mVg and 1692 m^/g, respectively. The pore sizes for the materials obtained by BJH model are 2.6 nm and 2.9 nm. From the lattice parameters and pore sizes, framework thickness for the materials is very similar (2.1 nm). According to IR spectra, all the PMO materials after surfactant — I — — I — removal exhibit strong bands at 2920 and 2890 cm' 100 -100 -200 200 assigned to C-H stretching and deformation Chemical shifts(ppm) vibrations, 1410 and 1270 cm' corresponding to C-H Fig. 2. 'Al MAS NMR spectra for Al- deformation vibrations, which means the presence of PMO materials obtained with (a) organic bridging group within the frameworks. Figure 2 shows ^''AI MAS NMR spectra of the AlA1(N03)3, (b) NaA102 (c) Al(i-OC3H7)3 (d) DBATES (Si/Al = 30) and (c) PMO materials obtained with different aluminum DBATES (Si/Al = 8) sources. NMR peak around 50 ppm and 0 ppm can be assigned to a tetrahedrally coordinated aluminum species within the framework and an octahedrally coordinated extraframework aluminum species, respectively. The NMR results in Figure 2 clearly show that the extraframework aluminum species are present in the Al-PMO materials obtained with A1(N03)3 and NaA102. In case of DBATES and Al(i-OC3H7)3, there is no NMR peak around 0 ppm, indicating that all the aluminum species are incorporated within frameworks. However, a significant amount of octahedrally coordinated aluminum species appears as the Si/Al ratio decreases when Al(i-OC3H7)3 is used as the aluminum source, whereas DBATES results in only framework aluminum species till Si/Al = 8 (Figure 2e). The results show that aluminum incorporation into the PMO frameworks is highly dependent on the nature of aluminum source. Figure 3 shows TGA results under nitrogen atmosphere for the Al-PMO 100 200 300 400 500 600 material before and after surfactant TerriDerature / °C Fig. 3. TGA diagrams for the Al-PMO material (a) removal. Before solvent extraction, weight loss of 5 wt% below 120 °C is attributed before and (b) after surfactant extraction
668 to the loss of small amounts of residual water adsorbed to the materials. This is followed by a weight loss of 30 wt% from 120 to 250 °C due to surfactant decomposition. An additional weight loss of 5 - 7 wt% above 500 °C indicates decomposition of organic bridging group within the framework. Figure 3b shows that there is little weight loss in the temperature range for surfactant decomposition (120 - 250 °C), indicating that the surfactant within the mesopores can be removed completely through the solvent extraction. The weight loss above 500 °C also appears in Figure 3b. According to the TGA results, the Al-PMO materials synthesized in the present work may be used below 500 °C without loss of organic bridging group within the frameworks. Figure 4 shows catalytic activities of 80 the etherification reaction between 2naphthol and ethanol. All the 70 materials give 100 % selectivity for 2naphthylethylether. As shown in ^ 60 Figure 4, beta (Si/Al = 13.5), c 50 mordenite (Si/Al = 15), HY (Si/Al = g 3) and ZSM-5 (Si/Al = 30) zeolites 40 0) result in 66 %, 43 %, 4 % and 1 %, > c 30 respectively. The Al-PMO material o O (Si/Al = 30, DBATES) exhibits 58 % 20 conversion and 100% selectivity for 10 the etherification reaction. The catalytic activity and selectivity are comparable with those of beta zeolite h . ^KO ^ K)®^•N^' .^^^^ ^ that is the best one among various 6®^ ^^' ^\'.?^ ^'P^' kinds of solid acids catalysts in the Fig. 4. Catalytic activities of the materials for present work. etherification reaction between 2-naphthol and In summary, the highly ordered Alethanol. PMO material with framework aluminum can be successfully prepared using DBATES as the aluminum source. The material thus obtained is an excellent solid acid catalyst. The present synthetic strategy may be very useful for the rational design and preparation of PMO materials containing other elements within the frameworks. The authors are grateful for financial support by the Research Initiation Program at Ajou University (20012010) and Department of Molecular Science & Technology through Brain Korea 21 Project. REFERENCES 1. S. Inagaki, S. Guan, Y. Fukushima, T Ohsuna and O. Terasaki, J. Am. Chem. Soc, 121 (1999)9611. 2. T. Asefa, M. J. MacLachlan, N. Coombs and G. A. Ozin, Nature, 402 (1999) 867. 3. C. B. Mark, A. M. Michael, S. S. Mark and P G. Bruce, Chem. Mater., 13 (2001) 4760. 4. G. D. Yadav and M. S. Krishnan, Ind. Eng. Chem. Res., 37 (1998) 3358 5. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vatuli and J. S. Beck, Nature, 359 (1992) 710.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
669
Friedel-crafts alkylation over Al-incorporated mesoporous honeycomb Young Soo Ahn^, Hong Soo Kim^, Moon Hee Han^, Shinae Jun^, Sang Hoon Joo^, Ryong Ryoo and Sung June Cho^ ^Functional Materials Research Center, Korea Institute of Energy Research, Taeduk Science Town, Taejon 305-343, Korea. ^National Creative Research Initiative Center for Functional Nanomaterials and Department of Chemistry, Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea, ^Catalytic Combustion Research Center, Korea Institute of Energy Research ,Taeduk Science Town, Taejon 305-343, Korea. Catalytic activity of Friedel-Crafts alkylation of toluene with benzyl alcohol has been measured over the Al-incorporated mesoporous honeycomb. The honeycomb was fabricated using MCM-48 and pseudobohemite as inorganic binder and the incorporation of aluminum was performed either by direct implementation of AICI3 or by slurry mixing before the extrusion. Hydrothermal stability and compressive strength can be improved with the increase of the aluminum content. High catalytic activity of Friedel-Crafts alkylation was observed for the honeycomb containing Al initially in the slurry mixture. 1. INTRODUCTION The catalyst powder should be fabricated into a certain type of structure that can allow a facile diffusion of reactants to catalytically active sites. Honeycomb is the most common commercially available structure that accommodates catalysts at the surface of each small rectangular structure. Recently, the silica-based mesoporous molecular sieves has been investigated extensively as a substrate for catalytic conversion of large molecules inside its uniform pore of which the surface area is ranging from 2 to 30 nm [1]. Their high hydrothermal stability is comparable to those of conventional aluminosilicate zeolites. Ahn et al. showed that the honeycomb can be fabricated from the MCM-48 powder [2]. The integrity of such a mesoporous structure in the honeycomb can be retained during the hydrothermal treatment. Here, we report the results of the catalytic activity of the FriedelCrafts alkylation over the honeycomb containing aluminum that is incorporated different ways. 2. EXPERIMENTAL The MCM-48 silica powder was synthesized following the method described in the previous reports using a surfactant mixture of cetyltrimethylammoniumbromide and tetraoxyethylene dodecyl ether. The MCM-48 samples containing the surfactants as synthesized were further treated with an aqueous solution of NaCl, in order to improve the
670
hydrothermal stability. The samples were then dried in an oven at 100°C, washed with an ethanol-HCl mixture to remove as much surfactant as possible, and finally calcined in air under static conditions at 550°C. The bath composition of the slurry containing the MCM48 powder was controlled to the 80 wt % of MCM-48, 20 wt % of inorganic binder, 1 5 - 2 5 wt % of organic binder and the above 100 wt % of water on the basis of the total weight of MCM-48 and inorganic binder. The fabrication of the honeycomb follows a typical procedure consisting of powder mixing, wet mixing, aging, kneading, extruding and final sintering. The test method for the hydrothermal stability, the characterization and the catalytic activity of Friedel-Crafts alkylation of toluene and benzyl alcohol can be found in elsewhere [3]. 3. RESULTS AND DISCUSSION Employing pseudobohemite as inorganic binder increased the mechanical stability of the mesoporous honeycomb. The pseudobohemite contains Al itself, which can also be act as an acid catalyst. Fig. 1 shows the compressive strength and surface area depending on the calcination temperature. The increase of the aluminum incorporation in the honeycomb during the slurry formation decreased the surface area but improved the compressive strength. The incorporated Al may also act as inorganic binder like the pseudobohemite. In addition to the mechanical stability, the hydrothermal stability is a important factor for the design of mesoporous honeycomb. In this work, the hydrothermal stability was measured from the XRD patterns before and after the treatment of the honeycomb in the boiling water for 12 h. 660 680 700 720 740 The XRD diffraction patterns Calcination temperature (C) indicated that the mesoporous structure Fig. 1. Change of Surface area and connpressive strength was retained after the treatment and the as a function of the calcination temperature: (I ), 0 wt% AICI3; (m), 5.6 wt% AICI3; (t ) 11.2 wt% AICI3. Al incorporation led to more hydrothermal ly stable mesoporous honeycomb. It was shown that the hydrothermal stability was increased by the incorporation of alkali or alkaline earth ion to the mesoporous material. It seems that the incorporated Al increased the hydrothermal stability in addition to the mechanical stability. The local environment of Al in the mesoporous honeycomb was probed with ^^Al NMR depending on the incorporation methods of Al. Fig. 2 illustrates the NMR spectra of the mesoporous honeycombs. The direct implementation of an acid fiinction to the surface of the mesoporous channel was reported to be another viable method for the catalyst preparation. The spectral intensity of the peak corresponding to the tetrahedral Al site increased for the sample containing the direct implementation of AICI3. The calcination of the sample at 650 °C resulted in the similar ^^Al NMR spectrum to that of the mesoporous honeycomb containing AICI3 in slurry mixture initially. This suggested that the incorporation method of
671
150
100
50
-50
-100
Chemical Shift/ppm Fig. 2. (a) the Al impregnated mesoporous honeycomb, (b) the sample (a) calcined at 650 °C and (c) the mesoporous honeycomb containing Al in the slurry, after calcination at 700 °C.
Al did not affect the local environment of Al sites, which can be attributed to the large amount of inorganic binder, pseudobohemite. Fig. 3 shows the microstructure of the mesoporous honeycomb. During the fabrication of the honeycomb, all the components were mixed thoroughly to get homogeneous slurry for the extrusion, which can result in the breaking or destruction of crystalline shape. Indeed, the MCM-48 had a crystalline shape in powder form but the honeycomb had an irregularly tough surface microstructure due to the mixing step as shown in Fig. 3. The increase of Al incorporation led to the increase of mechanical stability and hydrothermal stability. However, in the scanning electron micrograph of the honeycomb sample, there is no significant difference in the surface structure. The catalytic activity of the Friedel-Crafts alkylation was measured over the honeycomb samples in a similar way reported in the literature. Fig. 4 shows the effect of the Al-incorporation method on the catalytic activity. The honeycomb without Al direct implementation or impregnation gave a comparable catalytic activity for the alkylation of toluene with benzyl alcohol. The conversion of toluene increased up to 40 % for 2.5h.
^
Fig. 3. Scanning electron micrographs of the honeycomb sample calcined at 700 °C: (a), 0 wt% AlCb; (b), 5.6 wt% AlCb; (C) 11.2 wt% AICI3.
672
Honeycomb Al imprcg Honeycomb Honeycomb pre Al
60
80
100 120 140 160
Time / min
Honeycomb Al impreg Pellet Al imprcg Honeycomb Pellet
60
80
100 120 140 160
Time / min
Fig. 4. Catalytic activity of the Friedel -Crafts alkylation between toluene and benzyl alcohol. The activity measurement was performed in a similar way reported in the literature [3]. The reason is that the pseudobohemite was added to the slurry as an inorganic binder for the extrusion of the honeycomb. The Al incorporation method affected the catalytic activity of the alkylation. The honeycomb containing the Al in the slurry mixture showed the better catalytic performance compared to that of the Al-implemented or -impregnated honeycomb. This might be due to the masking of the active surface by Al or the agglomeration of the impregnated Al. The mixing of AICI3 in the slurry mixture was more effective for catalyzing the honeycomb for the alkylation, which can be attributed the homogeneous distribution of the catalytically active sites. From comparison of the catalytic performance with those of pellet, it has been suggested that the large open area of the honeycomb provided the better catalytic activity for alkylation due to the thin wall thickness of the honeycomb. In summary, this work suggested that the integrity of mesoporous structure can be retained during the fabrication of honeycomb and the incorporation of aluminum without the pore blockage or masking. This means that the catalyzing the honeycomb can be done successfully on the honeycomb containing mesopores for the alkylation of toluene. This work was supported by National Research Laboratory Program and Creative Research Initiative Program in the Korean Ministry of Science and Technology, KOREA. REFERENCES 1. J. S. Beck et al., J. Am. Chem. Soc, 114 (1992) 10834. 2. Y. S. Ahn et al.. Stud. Surf. Sci. Catal., 135 (2001) 318. 3. S. Jun and R. Ryoo, J. Catalysis, 195 (2000) 237.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
673
Heterogenization of AICI3 on mesoporous molecular sieves and its catalytic activity Kyoung-Ku Kang and Hyun-Ku Rhee* School of Chemical Engineering and Institute of Chemical Processes, Seoul National University, Kwanak-ku, Seoul, 151-742, Korea. Aluminum containing mesoporous materials were prepared by direct hydrothermal synthesis and AICI3 immobilization. All the samples were characterized by well-established methods. According to the results of XRD and N2 physisorption, all the mesoporous molecular sieves, pure silica and aluminum substituted samples, have a long-range order structure. The catalytic performance of AICI3 immobilized mesoporous materials in the liquid phase alkylation of benzene is compared with those of other aluminum containing mesoporous materials. The AICI3 immobilized mesoporous materials are more active than other materials and the selectivity to the mono-alkylation product increases as the chain length of olefin molecules becomes large or as the pore size decreases. 1. INTRODUCTION Linear alkyl benzenes (LABs), which are used in the production of biodegradable surfactants, are synthesized commercially by benzene alkylation with linear alkenes. This reaction is usually carried out in the liquid phase in the presence of Lewis acid (AICI3 and ZnCh) or using Bronsted acid (HF and H2SO4). However, this reaction system suffers from several disadvantages such as the corrosive nature, potential environmental hazards and difficulties in separation, recycling and disposal of the spent catalysts. To overcome such problems, heterogeneous processing using solid acid catalysts is highly desirable and thus an extensive effort has been directed to the heterogenization of homogeneous catalysts using clay minerals and zeolites as supports. For example, heterogeneous Friedel-Craft catalysts based on AICI3 and ZnCb immobilized on montmorillonite and silica gel have been reported to show a high catalytic activity for the alkylation reaction [1, 2]. The H^ form zeolite beta has also been known to have a good catalytic activity for the liquid phase alkylation of benzene with light olefins [3]. In this study, alkylation of benzene has been carried out with three olefins, which have different chain lengths, using heterogeneous Lewis acid catalysts prepared by modification of Si-MCM-41 and Si-SBA-15 with AICI3. We have also prepared Al-MCM-41 and Al-SBA-15 by the direct synthesis method and compared their catalytic activities with those of the former. •Adress for correspondence: E-mail. hkrhcc(a)snu.ac.kr Fax. +82-2-888-7295 Tel. +82-2-880-7415 ** This work was supported by Grant No. 2000-1-30700-002-3 from the Basic Research Program of the Korea Science & Engineering Foundation and also partially by the Brain Korea 21 Program sponsored by the Ministry of Education.
674
2. EXPERIMENTAL 2.1. Preparation of mesoporous materials The Si-MCM-41 was prepared using a cationic surfactant (cetyltrimethyl ammonium bromide), as a template and sodium silicate solution as a silica source and following the synthesis procedure reported elsewhere [4]. The Si-SBA-15 was obtained by hydrothermal synthesis in the presence of PI23 (BASF: triblock copolymer) as template [5]. All the samples were washed, dried at 373 K and calcined in air at 823 K. The direct synthesis of Al-MCM-41 and Al-SBA-15 in aluminosilicate form was realized by applying almost the same procedure as for the pure silica, except for the addition for aluminum source. The remainder of synthesis procedure is the same as the one for pure silica materials. To obtain the HAl-MCM-41 and HAl-SBA-15 catalysts, the calcined Al-MCM-41 and Al-SBA-15 were converted to the H^ form through NH4^ ion exchange and subsequent calcination. 2.2. Immobilization of AICI3 Anhydrous AICI3 was dissolved in dry benzene. The pure silica samples were heated in a flask at 473 K for 24 h under vacuum condition. The dried Si-MCM-41 was cooled to room temperature under dry N2(g). The AICI3 solution and dried benzene were added to the silica samples. The resulting mixture was refluxed under nitrogen for 48 h, the solvent was eliminated by syringe, and the solid was repeatedly washed with dry solvent more than five times. All the immobilization processes was carried out in a glove box under dry N2(g). Finally, AICI3 immobilized MCM-41 and SBA-15 catalysts were dried at 373 K for 24 h. 2.3. Catalyst characterization All the samples were characterized by various analysis techniques. The small-angle X-ray scattering (SAXS) patterns were measured at room temperature using a Bruker GADDS diffractometer. The N2 adsorption isotherm was measured at liquid N2 temperature with a Micromeritics instrument (ASAP 2010). The specific surface area and pore size were calculated by using BET method and BJH algorithm. 2.3. Alkylation The alkylation of benzene was carried out in the liquid phase with magnetic stirring under refluxing condition for 1-3 h. Under the atmosphere of nitrogen, 100 mmmol of each of the alkenes (1-hexene, 1-octene and 1-dodecene) was added over a period of 30 min to a reactor containing 200 mmol of dried benzene and Ig of catalyst. The AICI3 immobilized catalysts were recycled. The conversion of alkene was analyzed by gas chromatography. 3. RESULTS AND DISCUSSION The SAXS patterns of pure silica and aluminum incorporated mesoporous samples exhibited well defined reflections of hexagonal structure as reported [4, 5]. The SAXS patterns for aluminosilicate MCM-41 and SBA-15 prepared by the direct synthesis procedure showed almost the same SAXS pattern and intensity as those of pure silica sample. The AICI3
675
immobilized mesoporous samples exhibit nearly the same SXAS patterns and the intensities remain almost the same as those for their parent pure silica samples as shown in Figures 1 and 2. These results indicate that the incorporation of aluminum has no influence on the hexagonal structure formed during the direct synthesis procedure. i AICI3-MCM-4I (Si/AI=25)
\
AI-MCM-41 (Si/AI=25) AI-MCM-41 (Si/AI=50) Si-MCM-41
- AICI3-SBA-15(Si/AI=25) •AI-SBA-15(Si/AI=25) •AI-SBA-15(Si/AI=50) -Si-SBA-15
(A C 4)
"c
.>0) "(3
0^
J
.1 2 theta
Fig. 1. SAXS patterns of MCM-41
Fig. 2. SAXS patterns of SBA-15
The results of N2 physisorption for all the mesoporous samples registered surface areas over 800 m^/g and narrow pore size distributions, being typical of mesoporous molecular sieves (c/ Table 1). The results of XRD and N2 physisorption analyses confirmed that the structural integrity of the mesoporous materials remained intact after heterogenization with AICI3. All the aluminum containing samples except for HAl-SBA-15 were found to be effective for the liquid phase alkylation of benzene with olefins as may be noticed from Table 1. The conversion of olefins over HAl-SBA-15 synthesized by the direct synthesis method is very low. Especially, the alkylation of benzene did not progress at all over HAl-SBA-15 with Si/Al=50. Since the SBA-15 was synthesized under acidic condition with 1.6 M aqueous HCl solution, the acidic condition caused the elution of aluminum to the reaction mixture. Therefore, the Al-SBA-15 prepared by direct synthesis contains less aluminum than the initial reactant gel and shows a lower activity. The selectivity to the mono-substituted alkyl benzene increased as the chain length of the olefin molecules becomes large or as the pore size decreases. It should also be noted that AICI3 immobilized mesoporous samples exhibited an enhanced catalytic activity in comparison to HAl-MCM-41 and HAl-SBA-15 with the same Si/Al ratio, respectively, and these catalysts could be re-used three times without loss of catalytic activity.
676
Table 1 The structural data and the catalytic reaction results Si/Al
BET surface area (m^/g)
BJH adsorption average pore size (A)
00
1107.1
37.7
-
-
-
Si-MCM-41'' Al-MCM-4r Si-SBA-15' Al-SBA-15'
1 -hexene
1-dodecene conversion / selectivity
(%)
50
963.8
39.5
49.7/73.1
39.8/77.1
41.3/95.7
25
1013.1
42.04
29.9/79.9
32.3/79.7
31.1 /93.2
00
891.4
63.2
-
-
-
50
851.1
60.9
25
824.6
58.7
13.1 /62.1
10.3/61.7
11.7/77.3
70.1/76.5
53.8/92.8
67.4/78.3
48.3/94.1
85.0/60.0
73.8/75.9
81.9/63.1
71.9/74.8
25 853.4 69.7/74.9 35.6 (fresh) AICI3MCM-41 25 62.9/77.0 (recycled) 25 748.3 61.5 83.9/61.3 (fresh) AICI3SBA-15 25 79.8/63.9 (recycled) The alkylation of benzene was carried out under rcfluxing condition for 3 h. " direct synthesis, ^ selectivity to linear alkyl benzene
4. CONCLUSIONS The SAXS patterns of pure silica and aluminum incorporated mesoporous samples exhibited well defmed reflections of hexagonal structure with their surface areas and pore sizes being typical of mesoporous molecular sieves. The results of SAXS and N2 physisorption analyses confirmed that all the samples have well developed hexagonal mesoporous structure. All the aluminum containing MCM-41 and AICI3 immobilized samples were found effective for the liquid phase alkylation of benzene with olefins. Among various samples the AICI3 immobilized catalyst is the most active and the selectivity to mono alkyl benzene increases as the chain length of olefin molecules becomes large or as the pore size decrease.
REFERENCES 1. V. V. Veselosky, A. S. Gybin, A. S. Lozanova, A. M. Moiseenkov, W. A. Smit, and R. Caple, Tetrahedron Lett., 29 (1989) 175. 2. J. H. Clark, A. P. Kybett, D. J. Macquarrie, S. J. Barlow, and P. Landon, J. Chem. Soc, Chem. Commun., 1353 (1989). 3. E. Armengol, A. Corma, H. Garcia, and J. Primo, J. Appl. Catal. A., 149 (1997) 177. 4. K-K. Kang and H-K Rhee, Stud. Surf. Sci. Catal., 141 (2002) 101. 5. D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, and G.D. Stucky, Science, 279(1998)548.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
677
Asymmetric dihydroxylation catalyzed by MCM-41 silica-supported biscinchona alkaloid Sang-Han Kim and Myung-Jong Jin School of Chemical Science & Engineering, Inha University, Inchon, 402-751, Korea MCM-supported bis-cinchona alkaloid complexed to osimium was examined as a chiral catalyst in asymmetric dihydroxylation of olefins. The desired diols were obtained in high yield with high enantiomeric excesses of up to 99%. High ordered MCM-41 silica was found to be better inorganic support for the catalytic system than amorphous silica gel. 1. INTRODUCTION Osmium-catalyzed asymmetric dihydroxylation (AD) of olefins has emerged as an attractive method for the synthesis of optically active diols. ^ Cinchona alkaloid-based osmium complexes are known to be the most effective catalysts for AD reaction in terms of both reactivity and enantioselectivity."^ However, for large scale synthesis the high cost and toxicity of the osmium catalyst must be taken into consideration. For this reason the development of chiral heterogeneous catalysts is a field of great interest. One approach which has been shown to be highly fruitful is the attachment of the catalyst to an insoluble polymer support, which then allows easy separation and reuse of the catalyst. Recently, silica gel and mesoporous silica have been successfully used as supports for the immobilization of the osmium catalysts."^"^ Our interest in the field led to prepare a MCM-41 silica-supported biscinchona alkaloid 3. Herein, we report our preliminary results on the AD reaction of olefins using the MCM-supported chiral ligand 3. 2. RESULTS AND DISCUSSION
E
0;Si' O
"'^>i> MeO
Reaction of MCM-41 with an excess of (3-mercaptopropyl)trimethoxysilane in refluxing toluene gave mercaptopropylsilanized MCM 2. The MCM-supported bis-cinchona alkaloid 3 was prepared by the reaction of l,4-bis(9-0-quininyl)phthalazine monomer 1 with
678
mercaptopropylsilanized MCM 2 under radical condition (Scheme 1). With the heterogeneous chiral Ugand 3 in hand, we then performed investigated the AD reactions of various olefins. The results are summarized in the Table 1. hi most cases, the products were obtained in high yields with high enantiomeric excesses. The AD reaction of stilbene catalyzed by 1 or 2 mol% of 3 and 1 mol% of OSO4 at room temperature for 24 h proceeded to afford the corresponding diol in 95% conversion with 99% ee (entries 3 and 4). Satisfactory e.e. was also obtained in the reaction of styrene. It has been known that styrene substrate gives low enantioselectivity. The MCM-supported ligand 3 offered somewhat better asymmetric induction than silica gelsupported bis-cinchona alkaloid.^ The improved stereochemical outcome of the reaction seems to be attributed to crystalline structure of MCM support. The MCM framework allows Scheme 1
^ ^ ^ K2CO3 KOH Toluene, reflux
MeO
N-N 1,4-bis(9-0-quininyl)phthalazine ]
AIBN,CHC1
an ordered array of chiral catalytic sites on the pore surface. The ordered array leads to elegant site-isolation,^ which may result in enhanced enantioselectivity. These results are comparable to those of its homogeneous counterpart. The MCM-supported was easily filteredft-omthe reaction mixture. It is noteworthy that the heterogeneous system the MCMsupported alkaloid-Os04 complex can be reused by the easy filtration from the reaction mixture with only moderate loss of reactivity and enantioselectivity. In conclusion, we have achieved excellent results in the heterogeneous catalytic AD using MCM-supported bis-cinchona alkaloid 3. Moreover, The MCM-41 could be served as a potential support for the heterogeneous chiral ligand. Efforts for the synthesis of further MCM-based chiral ligands are currently underway in our laboratory
679
Table 1 Heterogeneous AD of olefins using MCM-41-supported-bis-cinchona alkaloid 3^ MCM-41-supported Ugand 3 cat. OSO4 K3Fe(CN)6-K2C03 in/-BuOH-H20(l:l)
Entry
R
Time (h)
1
H
22
2
U
3
Yield (%)
[a]D(c, solvent)
% ee^
Config^
94
+32.4 (2.5, EtOH)
83
S
22
80
+28.4 (2.5, EtOH)
73
S
Ph
24
93
-92.0 (1.0, EtOH)
99
S,S
4'
Ph
24
93
-92.1 (1.0, EtOH)
99
S,S
5"^
Ph
24
82
-85.6(1.0, EtOH)
92
S,S
6
COzMe
24
94
+10.4 (1.1, CHCI3)
97.6
2R,3S
7
CH3
24
91
+32.0 (0.8, EtOH)
96
2S,3S
''The reaction was carried out at RT; Molar ratio of olefin/ OSO4/ MCM-41-supported ligand = 1/0.01/ 0.02. ^% Ee and absolute configuration were determined by comparison of [a]D with literature value."' "Molar ratio of olefin/ OsOV MCM-41-supported ligand = 1/0.01/0.01. '^Reaction was carried out with 3 which was used in entry 3 without further addition of OSO4. 3. EXPERIMENTAL 3.1. Preparation of l,4-Bis(9-0-quininyl)phthalazine 1 A-100 mL three-neck round-bottom flask equipped with a Dean-Stark-condenser was charged with 1.56 g (4.82 mmol) of quinine, 0.5 g (2.51 mmol) of 1,4-dichlorophthalazine, 1.02 g (7.38 mmol) of K2CO3, and 50 mL of anhydrous toluene. After 2 hrs reflux under nitrogen atmosphere, 0.42 g (7.38 mmol) of KOH pellet were added and then the reaction was continued for 20 h. The light orange solution was mixed with water and then extracted with EtOAc. Recrystallization from Et20 gave 1.75 g of white powder L
680
3.2. Preparation of MCM-41 silica 2 MCM-41 silica^ (1.0 g) was treated with 0.87 g of (3-mercaptopropyl)trimethoxysilane in 12 ml of anhydrous toluene. The mixture was heated at 110°C for 24 hours. The powder was collected by filtration and washed with methylene chloride. After drying in vacuo at 50 °C, mercaptopropylsilanized MCM 2 was obtained. Elemental analysis and weight gain showed that 2.9 mmol of (3-mercaptopropyl)trimethoxysilane was anchored on 1.0 g of MCM-41. 3.3. Preparation of MCM-41-supported bis-cinchona alkaloid 3 This derivatized MCM 2 (0.75 g) was suspended in chloroform and refluxed with 1,4bis(9-0-quininyl)phthalazine 1 (0.56 g) and a,a'-azoisobutyronitrile (AIBN, 26 mg), as radical initiator, for 48 hours. The powder was collected by filtration and washed with methanol and methylene chloride until the l,4-bis(9-0-quininyl)phthalazine in excess was completely removed. After drying in vacuo at 50 °C, MCM-41-supported alkaloid 3 was obtained. Elemental analysis and weight gain showed that 0.52 mmol of l,4-bis(9-0quininyl)phthalazine 1 was anchored on 1.0 g of the MCM 2. 3.4. Typical procedure for the asymmetric dihydroxylation using MCM-41-supported bis-cinchona alkaloid 3 To a mixture of MCM-41-supported bis-cinchona alkaloid 3 (45 mg, 0.02 equiv.), potassium ferricyanide (0.58 g, 3.0 equiv.), potassium carbonate (0.24 g, 3.0 equiv.), and OSO4 (1 mole %, 0.5 M in water) in 5mL of ^er/-butyl alcohol-water mixture (1:1, v/v) at room temperature, the olefin (5 mmol) was added at once. The reaction mixture gradually changed from a heterogeneous to a homogeneous solution in 22-24 h. Solid sodium sulfite (0.47 g) was added, and the mixture was stirred for an additional hour. The MCM 3 was removed either by filtration or centrifugation and washed with ether. The combined organic extracts were then evaporated; the residue was dissolved in CH2CI2 (20 mL), washed with brine (10 mL) and dried (Na2S04). The residue was purified either by chromatography or distillation. Enantiomeric excess of the diol was determined by comparison of [ajo with literature value.^ This work was supported by the Center for Advanced Bioseparation Technology, Inha University. REFERENCES 1. Jacobsen, E. N.; Pfaltz, A.; Yamamoto, H. In Comprehensive Asymmetric Catalysis II, Springer-Verlag, Berlin, 1999. 2. Sharpless, K. B. Tetrahedron 1994, 50, 4235. 3. Song, C. E.; Yang, J. W.; Ha, H, J. Tetrahedron: Asymmetry 1997, 8, 341. 4. Lee, H. M.; Kim, S. W.; Hyeon, T. H.; Kim B. M. Tetrahedron: Asymmetry 2001, 12, 1537. 5. Vanppen, D. L. A.; De Vos, D. E.; Genet, M. J.; Rouxhlet, P. G.; Jacobs, P. A. Angew. Chem. Int. Ed. Engl. 1995, 34, 560. 6. Ryoo, R.: Jun, S. J. Phys. Chem. B, 1997, 101,317.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
681
Roles of pore size and Al content on the catalytic performance of Al-MCM41 during hydrocracking reaction Wen-Hua Chen ^ Qi Zhao ^ Shing-Jong Huang ^ Chung-Yuan Mou ^, and Shang-Bin Liu ^^* ^Institute of Atomic and Molecular Sciences, Academia Sinica, P. O. Box 23-166, Taipei, Taiwan 106, R. O. C. ^ Department of Chemistry and Center of Condensed Matter Research, National Taiwan University, Taipei, Taiwan 106, R. O. C. The catalytic properties of Al-MCM-41 materials having varied Al contents and pore sizes, were evaluated by means of 1,3,5-triisopropylbenzene (1,3,5-TiPB) cracking reaction. It is found that, while the overall activity increases linearly with increasing Al content of the AlMCM-41, the catalytic ability per active site is mainly controlled by the dispersion of acid sites. 1. INTRODUCTION It is well known that the activity of a catalyst depends mainly on its acidity and masstransport limitations. The former is normally manipulated by the concentration and distribution of Al species, whereas the latter is controlled by steric constrains imposed on the structural porosity of the catalyst. In particular, Mesoporous aluminosilicate Al-MCM-41 materials, being less acidic compared to most microporous zeolites, possess highly ordered mesoporous channels and hence are most suitable for cracking large molecules during which only weak acidity is required.'"^ Al-MCM-41 materials, first discovered by Mobil researchers in 1992,"* typically possess prominent properties, such as high surface area (~ 1000 m^/g), hydrocarbon sorption capacities (> 0.7 ml/g), and thermal and hydrothermal stability. Moreover, the pore size of these materials can be tailored (in the range of 1.5-10 nm) and they can be prepared in a wide range of framework Si/Al ratios thus render the manipulation of their acidic and catalytic properties during material synthesis. The objective of this study is to investigate the roles of Al content and pore size on the catalytic performances of Al-MCM-41 during 1,3,5-TiPB cracking reaction. In particular, the variation of 1,3,5-TiPB initial activities, a parameter used to reflect the concentration of acid sites, for Al-MCM-41 materials with two different pore sizes and various Si/Al ratios were examined. Corresponding author (SBL: sbliu@sinica.edu.tw); the support of this work by the Nation Science Council, R O C. (NSC 90-2113-M-OO1-065 to SBL) is gratefully acknowledged
682
2. EXPERIMENTAL Powdered, particulate MCM-41 molecular sieves with varied Si/Al ratios (15-oo) and pore diameters (2.6 and 3.0 nm) were synthesized by the "delayed neutralization" procedure.^ Their structural features and physical properties were confirmed by powder XRD, SEM/TEM, N2 adsorption/desorption (77 K) and ^^Si NMR spectroscopy. The Characteristics of the samples were shown in Table 1. Reagent 1,3,5-TiPB (A.R. grade, ACROS) was purified by molecular sieve 4A before use. Catalytic reactions were conducted in a continuous flow, fixed-bed flow reactor under the standard conditions, namely Tr = 573 K; WHSV = 15.25 h ' ; pressure = 1 atm; carrier gas: N2; N2/EB = 2.0 mol/mol, and time-on-stream (TOS) = 0-3 h. The catalyst was prepared by mixing the palletized MCM-41 sample (10-20 mesh; ca. 1 g) with quartz (ca. 20-30 mesh). Prior to the reaction, sample was first activated in air at 723 K for 8 h; the reactor was then cooled under N2 stream down to the desired reaction temperature. The composition of the reactor effluents was analyzed by gas chromatography (Shimadzu GC-9A) using a packed column (5% SP-1200 + 1.75% Bentone 34 on 100/120 Supelcoport, 6 ft). All products were identified using the internal standard method.
Table 1 Characteristics and catalytic properties of the MCM-41 samples.
Si/Al
Pore size (nm)^
Pore volume {m\/gf
c, r Surface area
MCM-C16/15
15
2.62
0.98
1015
29.2
38.3
0.81
67.5
MCM-C16/46
46
2.64
1.06
1032
24.3
21.4
0.76
45.7
MCM-C16/60
60
2.58
1.02
1135
22.6
20.6
1.04
43.2
MCM-C 16/120
120
2.61
0.98
1093
17.4
20.4
0.79
37.8
MCM-C 16/370
370
2.58
0.98
1027
7.7
27.8
0.87
35.5
MCM-C 18/15
15
2.98
0.97
1061
16.5
28.1
0.75
44.6
MCM-C 18/37
37
3.04
1.21
1028
11.4
17.1
0.58
28.6
MCM-C 18/60
60
2.80
1.06
1019
2.0
20.8
0.46
22.8
MCM-C18/120
120
2.82
1.12
1118
5.1
13.4
0.93
18.5
MCM-C 18/370
370
2.85
0.98
1044
2.1
14.0
0.13
16.1
Samples
Deactivation parameters'^
"^ Data obtained by the BJH method based on the desorption curve of N2 adsorption/desorption isotherms (77 K). ^ Determined by N2 isotherms at p/po = 0.96. ^' Results obtained from data fitting of Eq. 1. "^ Represent initial conversion (TOS = 0 h); in unit of wt%.
683
3. RESULTS AND DISCUSSION The catalytic activities of various Al-MCM-41 samples were assessed by the conversion of 1,3,5-TiPB, which was catalyzed to produce mainly mono- and di-substituted isopropylbenzenes. Except for the pure siliceous MCM-41, which revealed the expected null activity, the 1,3,5-TiPB conversion obtained from the two series of Al-MCM-41 samples (pore sizes 2.6 and 3.0 nm; varied Si/Al ratio) during cracking reaction were found to obey the first-order exponential decay function: Xt=Xo + k e"
(1)
where Xt represents the conversion at a given time-on-stream (TOS) t, Xo and k are constants, and the exponent a is a parameter accounts for the deactivation rate (by coking).^ The related deactivation parameters derived are depicted in Table 1. The initial activities of 1,3,5-TiPB observed in various Al-MCM-41 samples were used to evaluate the acid properties and catalytic performances of the catalysts. The variations of 1,3,5-TiPB initial conversion (i.e., Xo + k; at TOS = 0 h) with Al content of Al-MCM-41 samples are depicted in Fig. 1 and Table 1. It is obvious that the initial conversions of 1,3,5TiPB decrease exponentially with the Si/Al ratio of the Al-MCM-41. For the two sample series respectively with the pore size of 2.6 or 3.0 nm, the initial conversion curves tend to reach a plateau at ca. 37 and 18 wt%, as their respective Si/Al ratio exceeding ca. 120. It has been shown^ that, while the concentration of the acid sites decreases with increasing Si/Al I • 1 • I ' l l ratio of the Al-MCM-41, the acidic strength OU" remains practically unchanged. Upon initial Al-MCM-41 1] reaction, the 1,3,5-TiPB reactants are • MCM-C16 (2.6 nm) 1 ^ n • MCM-C18(3.0nm) MCM-C18(3 0nmi |I immediately catalyzed to form products or carbonaceous residues, which tend to deposit C 60on the acid sites. Thus, the reaction is readily o \ diffusion controlled. For samples with Si/Al > 120, it is plausible that the feed reactants have % 40covered all of the active sites in Al-MCM-41, c resulting a plateau in the observed initial o o conversion of 1,3,5-TiPB. Note that this effect should also depend on the contact time or •(5 WHSV applied. 20Figure 2 displays the correlation between the 1,3,5-TiPB initial conversions and sample 1 « 1 • r^— Al concentration (expressed in terms of Al 100 200 300 400 molar fraction). A linear correlation is evident regardless of the sample pore diameter, Si/Al ratios indicating the overall catalytic activity increases with increasing Al content for each Fig. 1. Correlations of 1,3,5-TiPB sample. The results indicate that all acid sites initial conversion with Si/Al ratio of are well isolated and apparently having similar Al-MCM-41 for two different sample catalytic activity. This is thus in line with the series during cracking reaction. results obtained in our previous investigation
T
f-V
-- 1
684
on the acid properties of Al-MCM-41 using solid-state ^'P MAS NMR of the adsorbed trimethylphosphine oxide (TMPO) as the probe molecule^ It was found that the ^'P chemical shifts remain practically unchanged upon varying sample Si/Al ratios of Al-MCM-41 samples indicating that the strength of the acid sites is invariant with the sample Al content. In addition, by comparing the results obtained from samples with varied pore sizes but having the same Al content, it is clear that sample with smaller pore size has a higher initial conversion. This is ascribed due to the fact that more Al per unit surface area is available in larger-pore sample. Thus, the hydrocracking ability per acid site is mainly controlled by the dispersion of acid sites on the internal surface of the Al-MCM-41. 4. CONCLUSIONS
80 i
0.00
• •
Al-MCM-41 MCM-C16 (2.6 nm) MCM-C18 (3.0 nm)
0.02
0.04
0.06
0.08
Al cone. Fig. 2. Correlations of 1,3,5-TiPB initial conversion with the Al concentration of AlMCM-41 for two different sample series during cracking reaction.
We have demonstrated that the initial catalytic conversion of 1,3,5 TiPB during cracking reaction over Al-MCM-41 can be used to reflect the distribution of acid sites in the mesoporous molecular sieve. While the overall activity increases linearly with increasing total Al content of the Al-MCM-41, the catalytic ability per acid site is mainly dictated by the dispersion of acid sites. Moreover, it is conclusive that 1,3,5-TiPB cracking reaction is more favorable for Al-MCM-41 having the smaller pore size and a greater acid site concentration. REFERENCES 1. (a) Reddy, K. M.; Song, C , Catal. Lett. 1996, 36, 103. (b) Reddy, K. M.; Song, C , Catal. Today \996,3\, 137. 2. Chen, X. Y., et al., Catal. Lett. 1997, 44, 123. 3. Siahkali, A.G. et al., Appl Catal. A 2000, 192, 57. 4. C. T. Kresge, M. G. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature 1992, 359,710. 5. Lin, H. P. et al., Microporous Mater. 1996. 10, 111. 6. Chen, W. H. et al, Microporous and Mesoporous Mater., submitted (2002). 7. Zhao, Q. et al, Stud. Surf. ScL Catal 2002, 141, 453.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
685
HDS of FCC gasoline: Mesoporous modified support catalyst and its effects on the hydrogenolysis reaction selectivity Gonzalo H. Tapia ^, Teresa Cortez ^, Rene Zarate ^, Javier Herbert ^, Jose L. Cano ^ ^Mayan Crude Program, Mexican of Petroleum Institute, Eje Central Lazaro Cardenas 152 c.p. 07730, Col. San. Bartolo Atepehuacan, Mexico, D.F., e-mail: ghemand@www.imp.mx. A synthesis methodology strategy for the systematic control of surface properties has been developed for HDS catalyst using alkaline metals and rare earth metals. The strategy is useful for change the acid-base balance on catalysts surface. The mesoporous modified catalysts have been used to HDS of a selected heavy cut of FCC gasoline to study the activity and selectivity reactions, in order to adapt the conditions that allows to reduce the sulfur content with minimum octane loss. 1. INTRODUCTION In a near future scene, the sulfur reduction in fuels will be the most important action to take into account to get the ambient legislation, considering to reduce the SOx and NOx emissions[l]. In this way, the tightening regulations and the incrisely product demands, becomes the FCC naphtha hydrotreatment attractive to achieve the sulfur level target, considering it contributes with 80 - 90 Wt % of the total sulfur in the gasoline pool [2]. The primary method to remove sulfur is hydroprocessing This methodology will likewise play an essential role in reducing FCC gasoline sulfur. A modified catalysts systems with systematic control of surface properties has been applied for HDS catalyst using alkaline metals and rare earth metals. Recent results obtained [3,4] support the supposition that after addition of small amounts of alkali oxides the acidity and basicity balance of the material surface were modified. The strategy is useful for change the acid/basic sites balance on catalyst surface without change on textural properties. In the present study the authors tried to examine the effect of basic metal oxides added to mesoporous support on acid-base surface properties in order to improve the activity and selectivity to hydrogenolysis reactions, that allows to reduce the sulfur content in heavy cut of FCC gasoline with minimum octane losses. 2. EXPERIMENTAL The synthesis of the mesoporous modified support were performed under basic conditions using lanthanum and potassium nitrates, in order to modified their surface properties. The ions (La^"^ and K^), were incorporated to the support before extrusion. Active metals were added in simultaneous way using ammonium hydroxide solution. The resulting catalysts were characterized by X-ray powder diffraction (XRD) with a DRX Siemens instrument model D5005. Pore size distributions was analyzed with N2 adsorption in a Micromeritics ASAP 2405
equipment. Acid-base surface properties were determined by temperature programmed desorption of C 0 2 (TPD-C02) in a Zeton-altamira-AMI-3 equipment and FTIR of pyridine in a Nicolet 170-SX equipment. The activities of hydrogenolysis and hydrogenation were estimated in a fixed-bed stainless steel tubular reactor at pilot plant level, using a selected heavy cut of FCC gasoline. Catalysts were pelleted and particles were diluted with a-alumine before being charged into the reactor. The catalysts were presulfided before HDS reaction. The reaction conditions for the HDS reaction were temperature, 270-340 OC; total pressure, 19 Kg/cm2; Hzlfeed ratio, 850 scfib and LHSV, 4 H-'. The reaction products were separated in a gas-liquid separator to collect the liquid products. The gas and liquid products were analyzed by a chromatograph with sulfur detector using a commercial capillary column. To select pilot plant feed, FCC gasoline and their cuts were characterized. Physical and chemical properties were determined and sulfur compounds identified. The olefins and sulfur distribution of full range FCC gasoline were employed in order to select the optimal cut temperature. 3. RESULTS AND DISCUSSION
The XRD pattems showed that all the supports and their corresponding synthesized catalysts have structures corresponding to the y-alumina. This result suggest that the ions added are highly dispersed on the surface of the support and the major effect is on the surface properties. Figure 1 shows that the modified supports are constructed with mesoporous with very narrow size distribution and the pore diameters were systematically controlled from 60 to 120 A. A poresize maximum for the A1201 and A1203-La201supports at 68 Angstrom has been found. The pore size variation of the K' modified support indicates that exist different interaction between the support and the added ion. It can be seen that the pore volume decreases lightly with potassium addition, : : ""'r:o,.r suggesting that the support sinterization is favored by calcination temperature. Support modification IOR . with lanthanum ions ( ~ a " ) do no change the pore .. 01 size distribution, moreover, it was found that ~ a ~ ' ions bring about an increase in thermal stability of the 00 alumina support. Physical properties of the prepared 10 100 IWO catalysts were reduced after depositing P m ZI-F. A distribution of the metal species. The decreases in specific surface Fig. 1. Port mesoporous modified supports from area and pore volume result partly from the density alumina with N2 at 77.3 K. increase by depositing the metal species and partly from the pore blocking by the species. Support modifications with lanthanum and potassium ions affects the acid-base balance on catalysts surface. Acid-base properties were determinate by FTlR of pyridine and temperature programmed desorption of CO2. Figure 2 shows desorption of C02 patterns for unmodified and modified supports. The addition of basic metals oxides resulted in a marked increase in the CO2
,)k'" A-
j:l /
-,.st).bO
P
..
687 desorption. K2O exhibited the most pronounced effect for increasing the signal intensity. Quantitative evaluation is needed for the effect of the added basic metal oxides on CO2 desorption; thus the micromoles of CO2 desorbed were quantified . Acid site distribution in solid supports is usually determinated by adsorption-desorption studies of basic probes molecules . Pyridine is the most common used for oxides. Pyridine adsorption on the supports gives rise to an IR band at 1450-1453 cm"' due to a superposition of two absorption: piridine H-bonded and coordinatively bonded to a lewis acid site. Weak, intermediate strong
100
200
300
I " I
400 500 600 Temperature, °C
WAVKNIJMBK.R
Fig. 2. CO2 desorption of modified support with alkaline earth oxide and alkali metal.
Fig. 3. FT-IR spectra of modified support with Alkaline earth oxide and alkali metal.
In addition, AI2O3-K2O and Al2O3-La203 modified supports exhibit IR bands of less intensity at 1450-1453 cm' compared to unmodified support ("d" line in figure 3), due to a reduction in acid sites density. Similar effect occurs on the IR espectra for catalysts. Modification of support properties by adding Lanthanum and Potassium ions has a significant effect on the activity of cobalt-molybdenum catalysts in the applied reactions of HDS of the heavy cut of FCC gasoline. Pilot plant results of catalytic activity of the discussed modifications in HDS reactions are presented at figure 4 and 5. The effect of modification of the support with lanthanum and potassium ions on the activity of cobalt-molybdenum are similar. Catalyst modified with potassium and lanthanum revealed high HDS activity and low hydrogenation selectivity (measured like octane losses) when compared to a commercial catalyst. Modification of the support with lanthanum and potassium ions decreases acidity of the catalysts, which is the reason why the hydrogenations reactions are low, and the octane losses too. On the other hand, the high activity to hydrogenolysis reactions indicates an increase of the basicity of the catalysts surface.
688
98
.--
94 90 86 82 78 74
72
t*'
:—^ -. CoMo/Ab034.a203 ^ I CoMo/AI}0>«:0 HI, Commercial
-
1
270
280
1
I
290 300
I
I
310
1 320
330 340
Temperature, ' C
Fig. 4. HDS activity of modified and unmodified Catalysts with alkaline earth oxide and alkali metal.
270 280
290 300
310
Temperature, "C
320 330 340
Fig. 5. Octane losses of heavy cut of FCC gasoline after HDS. 19 kg/cm^ 4 H ' , 850 scf^ H2/HC ratio.
In the process of support modification, lanthanum and potassium ions react with the surface OH groups, causing a decrease of the support acidity[6-7], they also react with Lewis centers[89]. In conclusion, a change in acid-base properties of a support causes similar changes in the acidic and basic properties of the surface of catalysts obtained on the basis of thus modified support. 4. CONCLUSIONS Modification of support with alkaline metals (potassium) and rare earths metals (lanthanum) decreases the acidity of the Cobalt-molybdenum catalysts, thus increasing their basicity without changes on textural properties. The effect of this modification on the activity of cobaltmolybdenum are similar. Catalyst modified with potassium and lanthanum revealed high HDS activity and low hydrogenation selectivity (measured like octane losses) when compared to a commercial catalyst. To reduce octane loss by HDS on the heavy cut of FCC gasoline, is convenient to use modified catalysts, which have high activity and low hydrogenating function.
REFERENCES 1. M. Seris, Outlook for European Demand of ULS gasoline and diesel and consequences for US imports. NPRA Annual Meeting, march 17-19, San Antonio Texas, 2002. 2. W.K. Shiflett and L.D. Krenzke. Consider improved catalyst technologies to remove sulfur. Hydrocarbon processing, February 2002, 41-43. 3. T. Horiuchi, H. Hidaka, T. Fukui, Y. Kubo, M. Horio, K. Suzuki, T. Mori. Applied Catalysis A: General 167 (1988)195-202. 4. M. Lewandowski and Z. Sarbak. Applied Catalysis A: General 173 (1988)87-93 5. S.W. Golden, D.W. Hanson and S.A. Fulton. Use better fractionation to manage gasoline sulfur concentration. Hydrocarbon processing, February 2002, 67-72. 6. L.Vordonis, P.G. Koutsoukos, A. lycourghiotis. J. Catal. 98 (1986) 296 7. L.Vordonis, P.G. Koutsoukos, A. lycourghiotis. Colloids and Surface 50 (1990) 353 8. R. Fiedorow, I. G. Dalla Lana. J. Phys. Chem. 84(1980)2779. 9. M. Lewandowsky and S. Zarbak. Applied Catal. 168 (1998) 179-185
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
689
Synthesis of mesoporous carbon nanotubes and their application in gas phase benzene hydrogenation Dong Cheng Han, Zhi Qing Zhu, Ai Min Zhang , Jian Zhong Zhu, Jia lu Dong, Department of Chemistry, Nanjing University, Nanjing, P.R. China The multi-wall carbon nanotubes with pore diameter of 30-50 nm, synthesized by chemical vapor deposition over Co-La catalyst via decomposition of acetylene at 700 °C, were employed as carrier of Ni-loading catalysts and exhibited excellent conversion of benzene and selectivity for cyclohexane in the gas phase benzene hydrogenation under atmospheric pressure. 1. INTRODUCTION Since their discovery in 1991 [1], carbon nanotubes have been presented as a very promising material in a wide range of potential applications [2]. Many researchers have reported their mechanical properties, superior thermal and electric properties. These exceptional properties of carbon nanotubes have been corroborated for devices such as field-emission displays, scanning probe microscopy tips and micro-electronic devices. With the large-scale synthesis of carbon nanotubes, attention is now being directed to their potential application in various fields of materials. Catalysis is a nanoscale phenomenon that has been the subject of research and development for many decades, but only recently become a nanoscale science of materials and chemistry involving more investigations on the molecular level. In the field of heterogeneous catalysis, various carbon materials have been used to disperse and stabilize metallic particles [3]. However, the carbon nanotubes, different from general carbon materials, exhibit exceptional properties such as uniform pore diameter, high length-diameter ratio, ability of very high H2 uptake [4], and large specific surface area, and the hydrophobic or hydrophilic character of the surface can be controlled by chemical treatment or modification [5]. These properties especially the unusual ability of H2 uptake suggest enormous potential applications of carbon nanotubes as novel materials for the catalyst carrier in hydrogenation reactions. The hydrogenation of benzene to cyclohexane is used probe the activity for reactions taking place on metal sites. Moreover, this reaction has arose practical interest as special attention has recently been focused on the hydrogenation of aromatic compounds and there in an increasing demand for suppression of the benzene content in petroleum fuels and especially in gasoline and diesel in near future. In this study, we characterized as-synthesized and treated CNTs with TEM, XRD, BET, and TG/DTA. Then prepared Ni-supported catalyst by impregnation in ethanol solution, and investigated the catalytic performance of nickel-supported carbon nanotubes in the reaction of gas phase benzene hydrogenation under atmospheric pressure. * Corresponding author.. E-mail address: zhangaamm@yahoo.com.cn
690
2. EXPERIMENT 2.1. Preparation of carbon nanotubes and Ni-supported carbon catalyst The multi-walled carbon nanotubes were obtained by chemical vapor deposition of acetylene over Co-La catalyst at 973K following the procedure reported previously [8]. The main impurities coexistent with multi-walled carbon nanotubes were metal particles and amorphous carbon. In order to remove these impurities, the as-synthesized carbon nanotubes were first suspended in concentrated HNO3 solution with stirring and refluxing at 333K for some hours. After filtering, washing and drying, the treatment with nitric acid was repeated twice for surface oxidation of carbon nanotubes. Subsequently, the dried oxidized carbon nanotubes were impregnated with NiN03-6H20 dissolved in ethanol with stirring for 5 h, dried at 323K for 15 h and heated in vacuum at 409K for 2 h. Finally, the samples were oxidized in air and reduced in H2 atmosphere. 2.2. Characterization The morphometries of carbon nanotubes were observed with the JEM-200 CX transmission electronic microscope (TEM). The specimens for TEM were first mulled in agate bowl, then dispersed in aqueous solution containing 50% alcohol by ultrasonic treatment and dropped onto holey grids. The specific surface area was measured by the method of nitrogen physisorption at liquid nitrogen temperature using a Micrometritics ASAP 2000 apparatus. XRD patterns were taken with a D/MAX X-ray diffraction instrument by using CuKa in the voltage of 40 kV and current of 50 mA. 2.3. Activity for hydrogenation of benzene Gas phase hydrogenation of benzene was carried out using a tubular 4 mm ID flow micro-reactor at 473 or 453K. 50mg of the sample with the particle size of 20/40 meshes was put into the U-shape quartz tube. Before reaction, the catalyst was pretreated by heating in nitrogen flow at a constant rate of 10 Kmin'^ to 773K and held at this temperature for 2 h. After reduction in hydrogen flow for 2 h, the reactor was cooled down to reaction temperature. Then, the mixture of H2 saturated with benzene was passed through the reactor at a constant flow rate of 15 ml/min. Reactants and hydrogenated products were analyzed by on-line gas chromatograph GC-1102 with FID detector and Proparak QS column. 3. RESULTS AND DISCUSSIONS 3.L Purification and characterization of carbon nanotube For large-scale synthesis of carbon nanotubes, the as-prepared carbon nanotubes usually contain a large amount of impurities such as metal particles, amorphous carbon and multi-shell carbon nanocapsules. These impurities bring about a serious impediment to the detailed characterization of carbon nanotubes and catalytic properties, so it is very important to purify the carbon nanotubes in order to obtain ideal catalysts. The TEM image of the as-synthesized carbon nanotubes is shown in Fig. 1. They are multi-walled carbon nanotubes and have outer diameter of about 30 nm, inner diameter of about 5 nm and length of several tens of |xm. From Fig. 1, we notice that there are many dark metal agglomerates around carbon nanotubes. The purified carbon nanotubes after treatment with acid are shown in Fig. 2. Those dark metal agglomerates have been removed and the
691
walls of carbon nanotube become thinner than before. Which is due to the occurrence of oxidation on the surface of carbon nanotubes at the same time of removing the impurities with concentrated nitric acid. The oxidized carbon nanotubes were favored to support active metals. The specific surface areas of as-prepared nanotubes were about 100 mVg but the value was increased to about 120 VOL'I g after treatment with acid. It has shown that the part of the nanotubes originally closed has opened. The XRD pattern shows only one sharp peak at 26= 26.06, indicating that the carbon nanotubes have a uniform pore size and graphited-well structure and there are no impurities.
Fig. 1. TEM image of the as-synthesized CNTs
Fig. 2. TEM image of the purified CNTs
3.2. Activity of Ni-supported carbon nanotubes for hydrogenation of benzene Three Ni-supported carbon nanotubes samples were obtained, with the Ni loads being 5.0%, 10.0% and 12.0 wt %, respectively. The activities of benzene hydrogenation measured at 200 °C under atmospheric pressure were shown in Figure3, in which the conversion of benzene as a function of reaction time over these catalysts. It is obviously that the conversion of benzene increases with the increase of Ni load. The products analysis showed didn't detected any other hydrogenated products except cyclohexane, the selectivity of cyclohexane achieved 100 % over these samples. Hydrogen spillover is a phenomenon that occurs in many heterogenous catalytic reactions and has received significant attention recently[6]. The term spillover is applied to the transport of active species from one surface to another in which the second surface does not form the active species under the same condition. It has been claimed that hydrogen spillover plays an important in aromatic hydrogenation on supported metal catalysts. When the carbon nanotubes with high ability of H2 uptake was employed as carrier of metal, the hydrogen in the inside of nanotubes was activated by nearby Ni metal, will react with benzene molecules adsorbed on the carbon nanotube sites in the form of carbonium ions are hydrogenated by the spillover hydrogen. Here the interparticle region of nickel and carbon nanotube may be an excellent acceptor of H species spilled over the metal nickel particle. In addition the carbon nanotubes have the trend for adsorbing benzene due to their organophilic property. So the high activities of benzene hydrogenation and high selectivity for cyclohexane could be attributed to the following contributions: the first is the contribution of metal nickel in Ni/CNTs; the second is that acid sites created on carbon nanotubes during the pretreatment with nitric acid; the third is that the carbon nanotubes adsorb preferential for the benzene due to their organophilic property; and forth is that the high ability for hydrogen uptake so that to supply much more H2 as precursor of spillover hydrogen. In the case they are also easy form the saturated cyclohexane instead of unsaturated cyclohexene.
692
The influence of the reaction temperature on the conversion of benzene over Ni/carbon nanotubes is shown in Fig.4. It can be seen that the conversion of benzene increase with the decrease of reaction temperature. Some researchers reported the optimal temperature of most high conversion of benzene is 200 "C due to the thermodynamic limitation. In our experiment we found the conversion of benzene is higher at the temperature of 180°C than that of 200 °C and they did not show the decline tendency for long time (from Fig.4). CD
100
CD N CD JD
80 _60 •5.0 wt% •10.0 wt% 12.0 wt %
p ^40 *CO CD
20
> c o
O
0 0
50
100 150 200 250 300 350 Reaction time (min)
Fig. 3. Conversion of benzene in benzene hydrogenation reaction as a function of reaction time over three different Ni load catalysts at 473K. 5 e
CD
120 100
f 80 ^^60 i 40 § 20 c
R
0
•• • • 200 "C -»-180°C 100
200
• •
•
300
400
Reaction time (min) Fig. 4. Conversion of benzene in benzene hydrogenation reaction over 12 % Ni/CNTs catalyst as a function of the reaction time at different temperature. REFERENCES 1. lijima S. Nature;354 (1991) 56. 2. R.F.Service, Science 281 (1998) 940. 3. Tans,S.J., Devoert, M.H., Dai, H., Thess, A., Smalley, R.E., Geerligs, L.J., and Dekker, C, A^a/wre, 386 (1997) 474. 4. Dillon, A.C., Jones, K.M., Bekkedahl, T.A., Kiang, C.H., Bethune, D.S., and Heben, M.J., A^amre, 386 (1997) 377 5. Fan, S., Chapline, M.G., Franklin, N.R., Tombler, T.W., Cassell, A.M., and Dai, H., Science, 283(1999)512. 6. D. Duprez, Stud. Surf Sci. Catal. 112 (1997) 13.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
693
Characteristics and Reactivities of Cobalt Based Mesoporous Silica Catalysts for Fischer-Tropsch Synthesis W. S. Yang, H.W. Xiang*, Y.Y. Xu, Y.-W.
Li
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001 P. R. China, E-mail: hwxiang(a),sxicc.ac.cn Performance in Fischer-Tropsch synthesis (FTS) and characteristics were investigated using Co-based HMS, MSU-1, and SBA-12 mesoporous silica catalysts. Due to the surface compound resistant to reduction for Co/MSU-1 and Co/SBA-12 and the decreasing number of the active site for Co/HMS, Co/SBA-12, Co/MSU-1 respectively, FTS activities of the catalysts decreased for Co/HMS, Co/SBA-12, and Co/MSU-1 respectively, and Co/HMS showed the lowest methane selectivity about 8.0 (wt)%. L Introduction Catalyst productivity and selectivity to C5+ hydrocarbons arc critical design criteria in the choice of FTS catalyst [1], and an active cobalt catalyst is usually prepared by depositing a cobalt salt on a support (Si02, AI2O3, TiOj, etc.) with second metal (Re, Rh, Ru, Pt, etc.) and another oxide (ZrOj, Th02, etc.) [2,3]. Recently, mesoporous silica with pore size ranging from 2 to 50 nm has been extensively applied as supports for catalysts [4-9], and Co/HMS catalyst showed the good FTS stability [9]. Now the present article tries to investigate the catalyst structure and FTS reactivities of Co/HMS, Co/MSU-1, and Co/SBA-12. 2. Experiment 2.1 Catalyst preparation HMS was prepared in the presence of ethanol and isopropyl alcohol as the co-solvent by our previous report[9] and the literature [10] (5'H,.T^890 mVg; Kp=1.07 cmVg); MSU-1 was synthesized by the hydrolysis of TEOS in the presence of AEO9 and water according to literature [11] (5i3i.j=761 mVg; Kp=0.49 cmVg); mesoporous SBA-12 was obtained by using TEOS, Brij 76 (Aldrich), water and HNO, from the literature [12] (5BET=728mVg; Vp=OAS cmVg). All catalysts (15% cobalt loading) in this study were prepared by aqueous incipient impregnation and cobalt nitrate hexahydrate as the cobalt source; the incipient wetness impregnation was performed at a single step, and followed by air-drying (room temperature for 12 h), then drying (313 K for 24h), and calcination (723 K for 4) [9]. 2.2 Catalysts characterization and FTS tests The X-ray diffraction (D/max diffractometer with Cu-Ka radiation; A = 0.154 nm) and N2 adsorption (micromeritics ASAP 2000 system) were used for the characterization of catalysts. XPS spectra (Reference energy value C 285.00 eV) were collected with a PHI-5300 ESCA spectrometer, and a Al x-ray source was used. The temperature programmed reduction (TPR) was carried out by passing a mixture of 5% H2 in N2 (temperature increase rate: 5K/min), after
694 5 A molecular sieve removed the water of the effluent gas passing through the micro-reactor, a TCD monitored the effluent gas with Nj as a reference, and the weight of the catalyst was 80 mg. The dihydrogen temperature programmed desorption (H2-TPD) was performed in a tubular quartz reactor, after loading the 150 mg sample. The catalyst was reduced with Hj at 673 K for 6 h and then cooled down to the room temperature in flowing H2; the Ar gas was adjusted for the sample, and the H2-TPD results were recorded at the temperature increase rate: lOK/min. For the reaction test, firstly the catalyst was crushed and sieved into 20-40 meshes, and then 5ml catalyst was loaded and reduced in situ for 16 h (P=0.2MPa, T=612> K, H2/CO=2.0, GHSV=500 h') with pure hydrogen in a integral fixed-bed reactor made of stainless steel. After reduction, syngas was introduced and the pressure was adjusted to 2.0MPa. The analysis of the outlet gases CO2, CO, H2, N2, and C, to Q hydrocarbons was done by off-line GC; the solid and liquid hydrocarbon products were analyzed after the end of the test. CO conversion was defined as the percent of the converted CO at total CO, and CO2 mole selectivity was defined as the percent of the CO converted into CO2. Hydrocarbon distribution was the percent of the component / weight at the total hydrocarbon product. 3. Result and discussion 3.1 Activity and hydrocarbon distribution of the catalysts for FTS FTS reactivities of three catalysts were shown in table 1. The gradually increasing CO conversion and the increasing methane selectivity for the cobalt catalyst were found with the increasing reaction temperature, which agreed well with FTS thermodynamics [13]. Co/MSU-1 catalyst was inactive in FTS, while Co/IIMS catalyst showed the highest activity with the lowest methane selectivity, besides it was found that the C5.,x hydrocarbon selectivity of Co/I IMS, about 70 wt% at total hydrocarbon product, was higher than that of Co/MSU-1 and Co/SBA-12. Generally Co/HMS catalyst presented the best reactivities, and indicated a potential application for liquid fuel production. Table 1 Effect of temperature on reactivities of F-T synthesis CO conv. CO2 sel. Hydr. Distr. (wt%) (mol%) (mol%) c, C.s-11 C12-1K C 19.25 C?.4 C7; 35.34 30.09 13.10 2.91 76.98 8.78 9.77 0.63 473 88.18 483 38.11 31.04 12.52 2.52 7.48 8.33 0.87 Co/HMS 90.89 34.83 28.28 15.37 4.90 7.70 2.59 493 8.93 43.04 25.55 9.41 2.84 2.39 96.26 503 11.81 7.81 27.84 33.21 18.84 6.39 4.82 493 63.78 -0.00 8.90 26.54 31.29 18.77 4.74 Co/MSU-1 77.50 0.59 503 12.33 6.33 31.66 28.91 13.65 4.05 86.71 1.43 15.20 6.52 513 73.40 -0.00 11.50 7.81 26.96 31.11 15.38 7.24 493 84.09 22.77 29.09 15.87 11.87 Co/SBA-12 503 0.17 12.06 8.35 513 88.06 16.56 8.67 33.23 26.15 10.79 4.60 1.71 3.2 N2 adsorption-desorption for the samples HMS, MSU-1, SBA-12, and three oxidized catalysts presented the characteristic type IV shape isotherm like MCM-41 [4]. Among three used catalysts, the only Co/MSU-1 and Co/SBA-12 performed the typical IV shape isotherm. So it was concluded that the mesoporous framework of Co/MSU-1 and Co/SBA-12 was partially retained and was more Catals
695
Stable than that of Co/HMS, corresponding to the literatures [11] and [12]. 3.3 XRD for the samples Three supports exhibited the low angle reflection, and transmission electron micrograph (HRTEM: JEM-200CX, not shown in this article) further confirmed the regular array of channels for SBA-12 like the literature [12] and the worm-like channels for HMS and MSU-1 [11]. The used Co/MSU-1 and Co/SBA-12 presented the small angle pattern with lower intensity strength due to the partial collapse of their mesoporous framework in FTS, while the small angle diffraction of the used Co/HMS disappeared due to the complete collapse of its mesoporous framework, and it was previously reported that the mesoporous structure collapsed completely after 24.00 hours from the beginning of FTS for Co/HMS [9], so the better structure stability for Co/SBA-12 and Co/MSU-1 was further confirmed. All oxidized catalysts contained C03O4, and the used catalysts showed the reflection of wax product and the metal cobalt. More importantly the oblivious CoO phase from XRD was found for Co/MSU-1 and Co/SBA-12, which was maybe connected with the lower FTS activity due to the lower reduction than Co/HMS. 3.4 XPS data for the oxidized catalysts Table 2 XPS data for the oxidized catalysts Catalysts Cobalt binding energy (eV) % Co (2py2) Surface Co/Si atomic ratio 780.81 0.04 Co/HMS 0.98 Co/SBA-12 5.82 780.32 0.26 Co/MSU-1 10.32 779.98 0.48 XPS data for the oxidized catalysts were presented in table 2. The presence of C03O4 phase for the catalysts was further confirmed from the binding energy; both the gradually increasing surface cobalt and increasing Co/Si ratio were found with the order of Co/HMS, Co/SBA-12, and Co/MSU-1, and it was suggested there was the oblivious surface structure difference such as the dispersion of cobalt among three catalysts needed to be further confirmed by the other techniques. 3.5 TPR and H^-TPD for the samples
300 400 500 600 700 800 900 1000
T/K
300 350 400 450 500 550 600 T/K
Fig. 1 TPR and M2-TPD spectroscopy for the catalysts (A. TPR; B. Hj-TPD)
a. Co/HMS; b. Co/MSU-1; c. Co/SBA-12 Two reduction peaks at low temperature for three catalysts corresponded to two step
696
reduction: C03O4 -^ CoO -^ Co [14], but cobalt reducibility shown in Fig. 1 were obliviously different. The reduction peak at 742K for Co/HMS should be related to the further reduction of CoO and Co-Si compound from the literature [15]. TPR peak at higher temperature for Co/SBA-12 and Co/MSU-1 indicated that the more difficult reduction of CoSi complex was formed in the preparation and calcination. Thus the reduction degree of Co/MSU-1 and Co/SBA-12 was lower than that of Co/HMS, which was further confirmed by the weak diffraction of CoO for the XRD patterns of the used catalysts. Generally it was concluded that the lower reducibility of Co/MSU-1 and Co/SBA-12 was maybe responsible for the lower FTS activity. H2-TPD spectroscopy presented a single peak at about 353 K and the similar peak shape. The peak area decreased with the order of Co/HMS, Co/SBA-12, and Co/MSU-1.It was suggested that the available cobalt active sites for the catalysts proportional to the peak area decreased with the same order, and this result agreed well with the activity difference shown in table 1. 4. Conclusion The cobalt based mesoporous silica catalysts were prepared, and FTS performance and characteristics of the catalysts were investigated. (1) It was found by Nj adsorption-desorption and XRD pattern that the mesoporous framework of Co/SBA-12 and Co/MSU-1, except Co/HMS, was partially kept after FTS. (2) The difference of the cobalt reducibility and the cobalt active sites uncovered by TPR and Hj-TPD spectroscopy confirmed the different FTS activity among three catalysts. (3) Co/HMS showed the lowest methane selectivity about 8.0 (wt)%, and indicated a potential application for liquid fuel production. Financial supports from the Key R&D Project (China) G1999022402 and Shanxi Science Foundation (China) 20021024 are highly acknowledged REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
E Iglesia, Appl. Catal. A 161(1997) 59. R. Oukaci, A. H. Singleton, J. G. Goodwin Jr., Appl. Catal. A 186(1999) 129. B. Ernst, L. Hilaire, A. Kiennemann, Catalysis Today, 50(1999) 413. C. T. Kresgc, M. E. Lconowicz, W. J. Roth, ct al.. Nature 359(1992) 710. P. T. Tanev, M. Chibwc, T. J. Pinnavaia, Nature 368(1994) 321. R. T. Yang, T. J. Pinnavaia, W. B. Li, et al., J. Catal. 172(1997)488. S. Kim, S. U. Son, S. I. Lee, et al., J. Am. Chcm. Soc. 122(2000) 1550. D. H. Yin, W. H. Li, W. S. Yang, et al., Micr. Meso. Mater. 47(2001)15 W. S. Yang, H. Y. Gao, H. W. Xiang, et al.. Acta Chimica Sinica 59(2001) 1870. P T. Tanev, T. J. Pinnavaia. Science 267(1995) 865. S. A. Bagshaw, E. Prouzct, T. J. Pinnavaia. Science 269(1995) 1242. D. Y. Zhao, Q. S. Huo, J. L Feng, et al., J. Am. Chem. Soc. 120(1998) 6024. R. B. Anderson, The Fischer-Tropsch Synthesis, New York, Academic Press, 1984. B. A. Sexton, A. E. Hughes, T. W. Tumey, J. Catal. 97(1986) 390. A. Kogelbaucr, J. G. Goodwin, Jr., R. Oukaci, J. Catal. 160(1996) 125.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
697
Physicochemical characteristics of Ti-PILC as a catalyst support for the reduction of NO by NH3 Ho Jeong Chae,^ In-Sik Nam^* and Suk Bong Hong^ ^Department of Chemical Engineering/School of Environmental Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea ''Division of Chemical Engineering, Hanbat National University, Taejon 305-719, Korea A pillared interlayered clay (PILC) intercalated by titania has been prepared as a catalyst support in an attempt to overcome the drawbacks of titania. The morphological, thermal, and surface properties of Ti-PILC have been particularly examined to use as a support for a NO SCR catalyst. The Ti-PILC prepared here was found to exhibit higher surface area and stronger acidity and thermal stability than the titania used as a common catalyst support. 1. INTRODUCTION The anatase type of titania has been widely employed as a catalyst support in the field of heterogeneous catalysis. However, titania as a catalyst support suffers from several disadvantages such as limited surface area and pore structure, weak mechanical strength, and poor thermal stability. Especially, the anatase type of titania reveals the poor thermal stability at high temperatures, which is of vital importance to determine the catalyst life. To overcome these drawbacks of titania, a composite material, titania with silica or alumina has been developed [1,2]. In the present study, the physicochemical characteristics of Ti-PILC have been examined as an alternative catalyst support to titania. The selective catalytic reduction (SCR) of NO by NH3 over V2O5 supported on Ti-PILC has been examined as a probe reaction to evaluate the performance of Ti-PILC as a catalyst support. 2. EXPERIMENTAL Ti-PILCs were prepared with Ti/clay ratios (mmol Ti/g of clay) in the range from 2 to 20 and are referred to as Ti-PILC(w), where m is mmol Ti per gram of clay used for the preparation of Ti-PILCs. Final products were calcined in the temperature range 300800 ""C for 5 h. JRCl (100% anatase) and P25 (70% anatase + 30% rutile, Degussa) as reference titania were employed for the comparative study. A series of vanadia/Ti-PILC catalysts were prepared by the conventional impregnation method. N2 adsorption experiments were carried out by a Micromeritics ASAP 2010 analyzer. Powder XRD patterns were measured on a MAC Science M18XHF diffractometer with CuKii radiation. TPD of NH3 was recorded on a fixed-bed, flow-type apparatus attached to a VG QMS quadrupole mass spectrometer. The IR spectra of adsorbed pyridine were measured on a Perkin-Elmer 1800 FT-IR spectrometer, using self-supported wafers of Ti-PILCs prepared here. XANES spectra were taken at the Ti K-edge using the 3C1 E-mail: isnam(a)Dostech.ac.kr. Fax: 82-562-279-8299.
698
beam line at the Pohang Acceleratory Laboratory (PAL) in Pohang, Korea. The catalytic activity and sulfur tolerance of V2O5 catalyst supported on Ti-PILC for NO SCR reaction were examined in a fixed-bed, continuous flow reactor. The concentration of NO was analyzed on-line by a Thermo Electron 42C chemiluminescence NO-NO2 analyzer. Details of the reactor system and the operating conditions employed are given elsewhere [3]. 3. RESULTS AND DISCUSSION 3.1. Morphological and textural properties of Ti-PILC One of major characteristics required for a promising catalyst support is its textural property such as surface area and pore structure. The advantage of PILCs over the conventional catalyst support is the diversity of their physical and structural characteristics with respect to the method of preparation. As shown in Table 1, the BET surface area and pore volume of Ti-PILC catalysts were found to considerably increase with increasing the content of titania, as well as with varying the method of the catalyst preparation. Especially when the freeze-drying method is applied, Ti-TILC with a considerably high surface area (> 200 mV') was obtained after intercalation of titania into the interlayer of the clay. Even after the calcination at 800 °C, in addition, Ti-PILC still maintains the surface area higher than 120 m^g'. It should be noted here that our Ti-PILC contains not only micropores but also meso- or macropores. Our recent TEM and PSD studies have shown that the macropores formed by the freeze-drying method are induced by the delamination of the layers [4]. Table 1 Physicochemical properties of Ti-PlLCs prepared in the present study Ti02 SA." Pore Vol." Cal. Temp. S.A. of Ti-PILC(IO)'''-' (wt%) (m^g') (cm^g-') CO (m^g') 25 100 320 KNB' 0.08 146 300 261 15 Ti-PILC(2)'^ 0.17 223 30.7 500 Ti-PILC(5)' 47.5 199 182 0.23 600 Ti-PILC(IO)' 47.5 0.27 156 230 700 Ti-PILCF(10r^ 49.3 Ti-PILC(20y 0.29 128 169 800 JRC-1 65 100 50 100 P25 ^The natural bentonite from Kyongju, Korea. ^ Determined after calcination at 500 ^C. "^ The values in parentheses are the Ti concentration (mmol Ti/g clay) used for their preparation. '^Prepared by the freeze-drying method. '^Material obtained from a different batch. 3.2. Acidic and thermal properties of Ti-PILC The surface acidity is one of the most important properties required for a NO SCR catalyst. From the NH3 TPD profiles in Figure lA, it can be seen that the pillaring of titania into the clay significantly enhances the surface acidity of the catalyst, compared to the original clay and commercial titania. To further identify the nature of acid sites, the IR spectra of adsorbed pyridine on the catalyst surface have been measured and are shown in Figure IB. The parent montmorillonite exhibit no IR bands associated with the pyridine adsorbed at Bronsted or Lewis acid sites. For P25 titania, in addition, only one band around 1455 cm"' typical of Lewis acid sites are detected. However, the IR
699
L
^^ c
4 c O)
L (£l
E 3
/ /
(OZ/>
L+
^
P\
3
B
Spectrum of the TiPILC(IO) sample after pyridine adsorption clearly shows the presence of both Bronsted and Lewis acid sites on its surface, which is mainly due to the formation of Ti-0Si (or Al) bonds by pillaring of TiOz- This strongly suggests that the Ti-PILC prepared here may have high potential as a support for the NO SCR catalyst.
\
\
// ^^^^^^^\ ^ ^"^ /-^-——-^ ^ ^ ^^ '
' ^-^-A \ //">o^^/''^-VX \T/ \
(b)
(A
\ ^ \A ^
O
(a)
if
.V-,
r>
_c
(a)
100
200
300
400
500
1700
Temperature {°C)
1600
1500
1400
Wavemunber (cm'^)
Fig. 1. (A) NH3 TPD profiles and (B) IR spectra of pyridine adsorbed: (a) KNB, (b) Ti02(P25), and(c)Ti-PILC(10). Another fatal drawback of the anatase-type titania in applications to a variety of catalytic processes may be its weak thermal stability, due to the transformation into the rutile phase at elevated temperatures. Figure 2 shows the powder XRD patterns of JRC-1 and P25 tinania, and Ti-PILC(IO) treated at different temperatures. It is clear that in cases of JRC-1 and P25, the phase transformation of anatase into rutile begins at 600 °C and the formation of rutile phase becomes evident as the thermal temperature increases. Thus, there is no indication of the presence of anatase phase in these titanias after calcination at 800 "C. As seen in Figure 2, however, the majority of Ti-PILC(IO) still remains as an anatase phase even after calcinations at the same temperature. This again shows the high applicability of Ti-PILC to a variety of the catalytic process, particularly to the high temperature reactions. .anatase
5
1
rutile
rutile
(A)
icy
rutile
jpi / ^ (c)
JLLUJLLI.
A ^ (•) 20
40 60 2-meta
80
Fig. 2. Powder XRD patterns of (A) Ti-PILC, (B) P25 TiOz, and (C) JRC-1 TiOz after calcination at different temperatures: (a) 300, (b) 500, (c) 600, (d) 700, and (e) 800 °C.
700
A similar result can be also observed from the XANES spectra of Ti-PILC(IO) materials treated as a function of temperature (Figure 3). Based upon the shapes of the bands appearing in the pre-edge and edge regions, it is clear that the anatase phase with octahedral Ti is predominant for Ti-PILC(IO) even after calcination at 800 ""C. 3.3. Reduction of NO by NH3 NO SCR reaction has been employed as a probe reaction to evaluate the performance of TiPILC as a catalyst support. The catalytic data listed in Table 2 Energy (eV) Energy (eV) Fig. 3. XANES spectra of references and Ti-PILC reveal that the VzOs/Ti-PILCClO) as a function of temperature: (a) Ti foil, (b) catalyst exhibits stronger sulfur tolerance as well as higher initial anatase TiOz, (c) rutile Ti02and Ti-PILC(IO) after activity than the commercial heating at (d) 100, (e) 300, (f) 500, (g) 600, (h) V205-W03/Ti02 catalyst [5]. This 700, and (i) 800 T . can be attributed to the higher acidic properties of Ti-PILC and high redox ability of V205/Ti-PILC [3]. Table 2 also shows that the sulfur tolerance of freeze-dried V205/Ti-PILC is distinctive, which is mainly due to its unique pore structure of the catalyst support. Table 2 NO conversion with respect to the reactor on-stream time^ SO2 Deactivation Time (hr) Catalyst 5 15 25 40 0 0.68 0.57 0.50 0.46 V2O5/Ti-PILCF(10) 0.82 0.45 0.83 0.65 0.51 0.40 VjOs/Ti-PILCClO) 0.46 0.52 0.62 0.54 0.38 VjOs-WOj/TiO. 0.32 0.77 0.55 0.38 V205/Ti02
55 0.40 0.32 0.31
^Reaction conditions: space velocity, 100,000 hr^ reaction temperature, 250 "C; NO = NH3 = 500 ppm; O2 = 5%; SO2 = 5000 ppm. 4. CONCLUSION A series of Ti-PILCs have been prepared, characterized and evaluated as NO SCR catalyst support. These materials exhibit high surface area and acidity, strong thermal stability and multi-modal pore structure as compared to the well-known P25 titania. It is found that the NO removal activity and sulfur tolerance of V205/Ti-PILC is superior to those of the commercial V205(W03)/Ti02.
REFERENCES 1. H. K. MatraHs, M. Ciardelli, M. Ruwet and P. Grange, J. Catal., 157 (1995) 368. 2. B. M. Reddy, I. Ganesh and E. P. Reddy, J. Phys. Chem. B, 101 (1997) 1769. 3. S. W. Ham, H. Choi, I.-S. Nam and Y. G. Kim, Ind. Eng. Chem. Res., 34 (1995) 1616. 4. H. J. Chae, I.-S. Nam, H. S. Yang, S. L. Song and I. D. Hur, J. Chem. Eng. Japan, 34 (2001) 148. 5. I.-S. Nam and H. J. Chae, H. S. Yang, S. L. Song and I. D. Hur, Korean Patent No. 2000-0020980 (2000).
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
701
Performance of double wash-coated monolith catalyst in selective catalytic reduction of NOx with propene H.-G. Ahn and J.-D. Lee Department of Chemical Engineering and Nanotechnology
Center, Sunchon
National
University, School of Applied Materials Engineering, #315 Maegok-dong, Suncheon-city, Jeonnam, 540-742, Korea. Highly dispersed AU/AI2O3 and Pt/A^Oa catalysts were applied to the lower layer of double wash-coated monolith catalyst for selective catalytic reduction (SCR) of NOx with C3H6. Hmordenite, Cu-mordenite or MCM-41 was coated as the upper layer. The catalytic performance was investigated in the presence of oxygen. The double wash-coated catalysts were more active than the catalyst with only zeolite or without upper layer. Temperature window of the double wash-coated catalyst was broadened, and catalytic performance was remarkably improved. The role of each layer and a reaction mechanism were discussed. The combined noble metal monolith catalyst with zeolite was effective in removing NOx by SCR with hydrocarbons. 1. INTRODUCTION Nitrogen oxides (NOx) in the exhaust of both automobile and stationary sources arc of critical concern because these byproducts are toxic environmental pollutants that lead to acid rain and ozone formation. Due to these effects, scientific and technological challenges have been poured to remove them. To alleviate NOx emission, variety of approaches has been applied such as direct catalytic decomposition of NOx, selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), and so on [1]. Especially, SCR was much attracted to us because it has much advantage. Addition of reducing agent is required for the selective conversion of NO to N2 in the presence of O2. However, most catalysts have the narrow temperature window and low conversion of NOx at lower temperature. Obuchi et al. [2] have * This work was supported by Jeonnam Regional Environmental Technology Development Center of Yosu National University.
702
applied double-layered catalysts to SCR of NOx with hydrocarbon. The results showed that the combination of Pt/SiOz with H-ZSM-5 showed high performance at lower temperature. The oxidation of NO to NO2 occurs in the lower layer, and C3H6 adsorbs in the upper layer, so the lower layer catalyst may require a properly active constituent. On the other hand, gold was scarcely employed in heterogeneous catalysis because of its less affinities to any chemical species. It was however reported that gold nanoparticle on metal oxides was highly active in oxidation of carbon monoxide, ethylene, and benzene even at low temperature [3,4]. In this study gold supported on AI2O3 was applied to the lower layer of double wash-coated monolith catalyst, and the upper layer was H-mordenite, Cu-mordenite, and MCM-41. The catalytic performance was examined in SCR of NO with C3H6 in the presence of oxygen, and the role of each layer was discussed. 2. EXPERIMENTAL Mini-size honeycomb type monolith (M) as a support was prepared by cutting out of a honeycomb with 400 cell/in^. Diameter of the monolith was ca. 20 mm (12 g). The monolith samples was coated by first immersing it into 50 (w/v)% solution of aluminum or cobalt nitrate, followed by drying and calcining at 600 '^C for 3 h. Loading of AI2O3 or C03O4 was ca. 10 wt% with respect to monolith. Gold and platinum in this layer were coated by deposition using NH4HCO3 and impregnation method, respectively. The upper layer was coated by immersing the lower layered monolith in well-mixed water slurry composed of a zeolite and colloidal silica (Aldrich Chem.), followed by drying and calcining at 500 °C for 3 h. Hmordenite (HM), Cu-mordenite (CuM), and MCM-41 were respectively used as the upper layer. The weight of coated zeolite was ca. 0.25 g. We denote the double layer catalysts as HM//AU/AI2O3/M (upper//lower layer). Also, HM or CuM was directly coated on bare monolith. Gold particle was observed using TEM (Phillips). Catalytic activity of the mini-size monolith catalysts was measured by using a flow type reactor under atmospheric pressure. Reactant was composed of 5000ppm NO, 2.5 mol% O2, and 5000ppm C3H6 balanced with helium at a flow rate of 60 ml/min. The concentration of NO and NO2 was analyzed with chemiluminescence NOx analyzer (Eco Physics), and C3H6, N2, N2O, and CO was analyzed by gas chromatography (Shimadzu). 3. RESULTS AND DISCUSSION Gold particle on AU/AI2O3/M with only lower layer was examined with TEM. Coated gold
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particles were uniformly dispersed on AI2O3/M, and its average size was about 5nm. Gold and platinum particles could be highly dispersed on monolith similarly to on powder AI2O3 [3,4]. From SEM image for the double wash-coated catalysts, the zeolite in upper layer of all catalysts was well coated by colloidal silica as a binder. Catalytic activity in NO+C3H6+O2 reaction was investigated over HM/M, CuM/M, and MCM-41/M coated on bare monolith. Their activities were very poor. NO conversion on AU/AI2O3/M and Pt/AbOs/M catalysts with only lower layer was very low because oxidation of C3H6 proceeded preferentially. Fig. 1 shows variation of NO and C3H6 conversion with reaction temperature over AU/AI2O3/M, MCM-41/M, and MCM-4I//AU/AI2O3/M. Combination of AU/AI2O3 with MCM-41 led to increase the activity considerably. Fig. 2 shows variation of NO and C3H6 conversion with reaction temperature over HM//AU/AI2O3/M and CUM//AU/AI2O3/M. Mordenite (especially CuM) in upper layer of AU/AI2O3/M was effective in increasing the activity. The maximum activity on CUM//AU/AI2O3/M was obtained at ca. 350 °C that was lower than 450 °C on AU/AI2O3/M. Fig. 3 shows effect of reaction temperature on conversion respectively over Ft/AhOsM, HM//Pt/Al203/M, and CuM//Pt/Al203/M that in which platinum was used instead of gold as noble metal of lower layer. Conversion of NO or C3H6 was greatly enhanced by coating mordenite (especially CuM) as upper layer. In all experiments, NO conversion began to decrease when C3H6 was nearly consumed in the course of reaction. o o
100
80
52 >
-#^4- • -O-A- O
Au/AI/M(N()) MCM41/M(NO) MCM41//Au/Al/M(N0) Au/Al/M(C3H(,) MCM41/M(C'3H(,) MCM41//Au/A1(C,HJ
80
60
>
#• eB-
HM//AuyAI/M(NO) CuM//Au/Al/M(NO) HM//Au/AI/M(C:3H6) CuM//Au/Al/M(C3H6)
40
X
u
60
o U
c o U vo
-
o =^ 20
20
o
o
0
200
300
400
500
Reaction temperature [^C]
100
200
300
400
500
600
Reaction temperature [^C]
Fig. 1. Effect of reaction temperature on
Fig. 2. Effect of reaction temperature on
conversion over AU/AI2O3/M, MCM-41/M,
conversion over HM//AU/AI2O3/M and
and MCM-4I//AU/AI2O3/M.
CUM//AU/AI2O3/M.
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The activity of AU/C03O4/M coated with HM or CuM in upper layer was poor because of high activity of AU/C03O4 in oxidation. It was therefore considered that the role of zeolite in upper layer is selective permeability among the reactant components and/or capacity for C3H6 adsorption.
MCM-41
coated
on
AU/AI2O3/M do not adsorb selectively C3H6 to pass through the upper layer, and C3H6 react rapidly with NO2 formed by NO+O2
reaction
in
lower
layer.
Improvement of catalytic performance of Au (or Pt)/Al203/M coated with Cumordenite may be explained by proper
200
300
400
500
600
Reaction temperature PC] Fig. 3. Effect of reaction temperature on conversion over HM//AU/AI2O3/M and CUM//AU/AI2O3/M.
permeability and adsorption capacity. In other words, the upper layer is considered to be a membrane that has substantially different permeability and adsorption capacity to the various reactants and products. 4. CONCLUSIONS Catalytic performance of double wash-coated monolith catalysts was examined for SCR of NOx with C3H6. The double wash-coated catalysts were more active than the catalyst with only zeolite or without the upper layer. Temperature window of CUM//AU/AI2O3/M and CuM//Pt/Al203/M was broadened and shifted towards lower temperature. It was known that two-functional monolith catalyst was effective in controlling NOx in exhaust gas by SCR with hydrocarbons. REFERENCES 1. M. Iwamoto, T. Zengyo, A.M. Hernandez, H. Araki, Appl. Catal. B, 17 (1998) 259. 2. A. Obuchi, I. Kaneko, J. Uchisawa, A. Ohi, A. Ogata, G.R. Bamwenda, S. Kushiyama, Appl. Catal. B., 19(1998) 127. 3. M. Haruta, N. Yamada, T. Kobayashi, and S. Ijima, J. Catal. 115, 301 (1989). 4. H.-G. Ahn and D.-J. Lee, Research Chemical Intermediates, in press.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved
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Copper loaded MCM-41. An alternative catalyst for NO reduction in exhaust gases? - Study of its acidic and redox properties Marcelo S. Batista, Martin Wallau, Rogerio A. A. Melo* and Ernesto A. Urquieta-Gonzalez'" * Departamento de Engenharia Quimica, Universidade Federal de Sao Carlos, Caixa Postal 676, 13565-905, Sao Carlos - SP, Brasil MCM-41 and ZSM-5 were exchanged with copper cations and tested as catalyst for the NO reduction with propane. ZSM-5 contains highly active isolated copper cations on ion exchange sites while inactive CuO was formed in the pores of MCM-41. 1. INTRODUCTION From its discovery, MCM-41 is being widely studied as catalysts and catalyst support [1], as a possible alternative to microporous zeolites in the processing of bulkier molecules or in processes which do not require shape-selectivity. Since the last decade [2], a wide range of transition metal exchanged zeolites are studied as catalysts for NO reduction by hydrocarbons. In this process molecular sieves might be important catalysts for the reduction of harmful nitrogen oxides (NOx) emitted by internal combustion engines. Here we will describe the properties of copper exchanged MCM-41 as catalyst for the reduction of NO with propane and compare the obtained results with that observed for Cu/ZSM-5, in order to judge the potential of MCM-41 type catalysts in environmental applications. 2. EXPERIMENTAL The precursor Na/ZSM-5 and Na/MCM-41 were prepared by conventional hydrothermal synthesis [3,4] and the copper containing catalysts by ion exchange of the parent sodium form at 25 °C using a solution of copper acetate (20 mmol/L; pH = 5.5) and a Cu/Al ratio of 1.3, subsequently drying at 110 °C for 12 h and calcining for 2 h at 520 °C. Also a physical mixture of CuO and Na/ZSM-5 zeolite was prepared, which was calcined in air for 2 h at 520 °C. The samples were denoted as Cu/ZSM(x/y) or Cu/MCM(x/y), x meaning the Si/Al ratio and y the copper content in weight %. The sample prepared by physical mixture is indicated by adding the letter M. The samples were characterised by XRD, nitrogen sorption (BET), UVA^IS, and H2-TPR. The catalytic reduction of NO with propane was developed in a fixed be reactor using 50 mg catalyst mixed with quartz powder (150 mg) activated for 1 h at 520 °C in air flow. A mixture of 0.3 % NO, 0.32 % C^Hg and 1.7 % O2 in helium, a GHSV of 42,000 h"^ and temperatures varying from 100 to 500 °C were used.
Present address: Centre Universitario do Sul de Minas, Faculdade de Engenharia Quimica, Av. Cel. Jose Alves. 37010-540 Varginha - MG, Brasil ^ corresponding author: FAX: +55-16-260-8266. E-mail: urquieta@power.ufscar.br ' Financial support: CNPq (461444/00-3; 300373/01-5), FAPESP (98/02495-5), FAPEMIG (TEEC- 1241/01).
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3. RESULTS AND DISCUSSION The elemental composition of the molecular sieves before and after ion-exchange is given in Table 1. It can be seen, that the exchange degree for the former is less than the theoretically expected Cu/Al ratio of 0.5, but more than ten times higher for the latter. This behaviour is probably due to the large hydration sphere of the Cu^^ cations which hampers the adsorption onto the microporous Na/ZSM-5 but not onto the mesoporous Na/MCM-41. The Cu/Al ratio of 5.7 observed for the Cu/MCM-41 suggests the CuO formation on its surface, while for copper exchanged ZSM-5, Cu^^ cations on ion exchange sites seem to be more likely. Table 1. Elemental composition of the studied molecular sieves. Sample Si/Al Exchange x time [h] Cu/Al 0.46 Cu/ZSM(11/4.8) 11 3 X 24 Cu/ZSM(23/0.7) 3x6 0.28 23 0.15 Cu/ZSM(11/1.6)M 11 5.7 2 X 24 23 Cu/MCM(23/9.0)
Cu content [%] 4.8 0.7 1.6 9.0
Table 2. Physical-chemical properties of Na/MCM-41 Table 2 reveals that before and after ion exchange. after copper exchange the pore diameter (dp) and the sample ao [A] dp [A] Sobs, fm^/gl Scai.'^ fm-/g] measured specific surface Na/MCM-41 45 30 910 819 area (Sobs.) of the Cu/MCM(23/9.0) 45 26 682 561 mesoporous MCM-41 are *Scal. = Sgeom./(Vgeom.p) = 8dp/[2«r/ - 2flr;dp)p] significantly decreased. As the unit cell parameter ao remains unchanged, the decrease in the pore diameter is rather due to the deposition of CuO species on the pore walls than to the degradation of the mesopore structure. It can be seen from the calculated specific surface area (Scai), obtained supposing an ideal hexagonally mesoporous structure and using the observed unit cell parameter and pore diameter demonstrated in Table 2, that the decrease in the specific surface area results from the decrease in the pore diameter. Furthermore, one should consider the higher density of CuO (6,5 g/cm"^ for crystalline CuO) in comparison to amorphous (Al,Si)0: (« 2.17 g/cm ) which also influences the specific surface area. The XRD patterns of the Cu/ZSM(11/4.8), Cu/ZSM(11/1.6)M and crystalline CuO are shown in Fig. 1 and for Na/MCM-41 and Cu/MCM(23/9.0) in Fig. 2. Although solid state ion exchange might be occurred in Cu/ZSM(11/1.6)M, the presence of reflections of CuO (Fig. Ic) in the pattern of Cu/ZSM(11/1.6)M (Fig.lb) suggests that in this sample Cu"^ cations on ion exchange sites are unlikely. By XRD no crystalline CuO can be observed in Cu/ZSM( 11/1.6), suggesting that in this catalyst, the copper cations are located on ion exchange sites. Although the high Cu/Al ratio in Cu/MCM(23/9.0) strongly indicates the presence of non ionic copper oxide species, no reflections, which could be attributed to CuO, are observed for this sample. It was outlined by Carniti et al. [5], that small CuO crystals (< 3 nm) may not be detected by XRD. Therefore, elemental analysis and XRD results suggest for Cu/MCM(23/9.0) the presence of finely dispersed CuO. It was discussed above that nitrogen sorption did not indicate degradation of the MCM-41 structure after ion exchange. Therefore the decreased intensity of the XRD reflections of the MCM-41 after ion exchange (Fig. 2b) is rather due to the adsorption of the radiation by the deposited copper species than to structural
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degradation. This is also suggested by the decreased intensity of the broad peak around 23°(20), typical for amorphous material, which should be increased by structure degradation.
Fig. 1. XRD pattern: (a) Cu/ZSM(11/4.8); Fig. 2. XRD pattern: (a) (b) Cu/ZSM(11/1.6)M; (c) CuO. (b) Cu/MCM(23/9.0).
Na/MCM-41;
215 . 260
A
(A 750 —
(c) ^^-~:rr^^
11A \
\
a>)
_,
(a) 600 >t ( n m )
200
400
600
Temperature [°C]
Fig. 3. UV/VIS spectra: (a) Cu/ZSM(23/0.7); Fig. 4. H2-TPR: (a) Cu/ZSM(11/1.6)M; (b) (b) Cu/ZSM(11/4.8); (c) Cu/MCM(23/9). (c) Cu/ZSM( 11/4.8); Cu/MCM(23/9.0); (d) Cu/ZSM(23/().7). A UV/VIS band around 890 nm confirms the presence of CuO in Cu/MCM(23/9.()) (see Fig. 3c). The UV/VIS bands observed for all copper exchanged samples (Fig. 3) at 215 and 260 nm are due to charge transfer transitions and the broad band around 750 - 800 nm, can be attributed to d-d transition of Cu^^ in octahedral symmetry [6]. The results of the H2-TPR (Fig. 4) reveal that in Cu/MCM(23/9.0), as well as in Cu/ZSM(11/1.6)M, the copper(II) cations are reduced in one step, as it is typical for CuO,
708
thus confirming that this compound is the only copper species present in those samples and that no solid state ion exchange occurred after thermal treatment of the physical mixture of CuO and Na/ZSM-5. Small CuO crystals in Cu/MCM(23/9.0) are indicated by the decreased reduction temperature, which is in accord with the absence of X-ray reflections attributable to CuO mentioned above. The H2-TPR (fig. 4) shows four and three peaks for the Cu/ZSM(11/4.8) and Cu/ZSM(23/0.7), respectively. Following the results reported by T^ ui ^ r» j .• . . ^ r.• • ^civ/i c Wichterlova et al. [71, we , ., , , . ,^ Table 3. Reduction temperatures for Cu cations in ZSM-5. attributed these peaks, as -— T: r-^^—i n TT 1 7 n" it is demonstrated in Cu/ZSM Cup^VCup- CupVCup" CuJVCu.^ Cu.VCu,/' Table 3, to the step wise 01/4.8) 210 °C 400 °C 600 °C 800 »C reduction of two different (23/0-7) 220:C 490:C 60(rC : kinds of copper cations (Cua and Cup). The Cua"^ specie in Cu/ZSM(23/0.7) is probably reduced at temperatures above 900 °C and it was not observed under the used conditions. Wichterlova et al. [7] identified both species as isolated copper cations on ion exchange sites. Cua, co-ordinated to two aluminium atom, possesses a high positive charge density, is difficult to reduce and preferentially observed on ZSM-5 with low copper content. Cu|i, coordinated to one aluminium atom possesses a low positive charge density, is easier to reduce and preferentially observed on ZSM-5 zeolites with high copper loading [7]. The are observed frequencies (TOF) depicted turnover against the reaction -Cu/ZSM(23/0.7) -Q-Cu/ZSM(11/4.8) 16 r-Cu/ZSM(11/1.6)M -•-Cu/MCM(23/9.0) temperature in Fig. 5. It can be seen, that t'^ Cu/ZSM(11/1.6)M and Cu/MCM(23/9.0) l^o are nearly inactive for the reduction of N O . l a On M C M - 4 1 large amounts of copper J e acetate are impregnated during ion ^ 4 exchange which after thermal treatment results in the formation of catalytic so iso 250 350 inactive C u O , as it is also present in T«mp«nrtur« ['C] Cu/ZSM(11/1.6)M. The catalytic activity of the Other two ZSM-5 samples, increases Fig- 5. Turnover frequencies for the continuously from 300 to 500 °C. This reduction of NO (GHSV: 42,(XK) h" ). beahivour indicates that isolated Cu"^ cations are the catalytic active species for the reduction of NO. The presented results might suggest lower activity of Cu"^ in Cu/ZSM(11/4.8). However, over this catalyst, 100 % of the reducing agent propane are already consumed at 450 °C, preventing further increase of NO conversion. Doubling the GHSV to 84,000 h ', similar TOF are observed for Cu/ZSM(23/0.7) and Cu/ZSM( 11/4.8), concluding that the different Cu*^^ species found in these catalysts do not differ in their catalytic activity. REFERENCES 1. F. Schuth, Stud. Surf. Sci. Catal. 135 (2001) 1. 2. E. Kikuchi, K. Yogo, Catal. Today 22 (1994) 73. 3. M.S. Batista, Master Thesis, DEQ/UFSCar, Sao Carlos, Brazil, 1997. 4. R.A.A. Melo, M.V. Giotto, J. Rocha, E.A. Urquieta-Gonzalez, Mater. Res. 2 (1999) 173. 5. P. Carniti, A. Gervasini, V.H. Modica, N. Ravasio, Appl. Catal. B 28 (2000) 175. 6. C. Dossi, A. Fusi, S. Recchia, R. Psaro, G Moretti, Microporous Mesoporous Mater. 30 (1999) 165. 7. B. Wichterlova, J. Dedecek, Z. Sobalik, A. Vondrova, K. Klier, J. Catal. 169 (1997) 194.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. Allrightsreserved
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Studies on synthesis and activity for selective catalytic reduction of NO over Pt supported MCM-48 Jae-seung Yang, Sung-Chul Lee and Suk-Jin Choung School of Environmental and Applied Chemistry, KyungHee University, Suwon, Kyungkido, 449-701 Korea In this study, in order to overcome drawbacks in the zeolite catalytic system for selective catalytic reduction of NOx, platinum metal was dipped in MCM-48. To confirm MCM-48 structure, various characterization methods such as XRD, B.E.T. surface areas, and XPS were carried out. Additionally simulated flue gases mixing and activity test were performed. Similar to specific charactertics of alumina supported platinum metal catalyst, NO reduction activity was improved in low temperature range under 300 °C on Al-MCM-48, and more improved activity was obtained when platinum was dipped over aluminum substituted MCM48 than silica type MCM-48. 10wt% water vapor injected on Pt/Al-MCM-48catalyst, and it was found that the NO reduction activity was not interfered by water vapor contents. When compared with the activity performance of Pt-ZSM-5, still Pt/Al-MCM-48 has superior resistance to water vapor and sulfur contents in flue-gases than Pt-ZSM-5. 1. INTRODUCTION There are several draw-backs such as sudden catalyst deactivation at high temperature range, difficulty in storage and transportation of NH3, and formation of ammonium sulfate in commercial SCR process with NH3 as reducing agent. To overcome those problems, the use of hydrocarbons as an alternative reducatnt has been studied. The S.C.R process-using hydrocarbon as a reducing agent under excess oxygen condition was already proposed at 1988 by Toyota, Japan. Many kinds of catalysts based on zeolite were tested until now (e.g Cu/ZSM-5[1], Co-Pt/ZSM-5[2]). However it also have several problems like diffusion resistance and hydro-thermal stability. To overcome these weaknesses, we tried new SCR catalyst using MCM-48 that is mesoporous molecular sieve. MCM-48 has been known as mesopore molecular sieve, which has a little diffusion resistance and relatively good thermal stability. From our experiment, we could expect that the disadvantages of common zeolite catalyst for S.C.R. would be overcome by using MCM-48.
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2. EXPERIMENTAL MCM-48 and Al/MCM-48 were synthesized by the conventional procedure given in Ref [3]. Pt/MCM-48 has been prepared by incipient wetness method. The powder X-ray diffraction(XRD) measurement was collected using M18XHF(MAC Science) with Cu Ka radiation at 1.0 deg/min scan speed over a 1.5°<20<1O°. N2 adsorption isotherms were measured using ASAP2000 (Micrometrics). The specific surface area of the samples was determined from the linear portion of the BET plots (P/PO = 0.05-0.20). TPD was carried out using home-made equipment in flowing He from 100°C to 550 °C. XPS data was collected an a Physical Electronics Model 5700 ESC A spectrometer operated at 350W using monochromatic Al Ka X-ray. Reactions were carried out in plug flow 1/2" -i.d. quartz reactor. To stimulate effluent gases of actual SCR process, NO, O2 and propylene were fed to reactor. Syringe pump was set up to test water vapor effect. The effluent gas was analyzed by on-line gas chromatography (GowMac 580-TCD, GowMac 750-FID) and gas analyzer (HORIBACLA210SS). 3. RESULTS AND DISCUSSION The structure of MCM-48 and Pt supported MCM-48 materials were characterized by Xray diffraction in an angular range between 1.5 and 10°(26 ). X-ray diffraction of MCM-48 showed characteristic peaks of cubic phase with a strong (211) and an additional (220) (321) (420) and (332) and the product is also manufactured due to the literature review. Al/MCM-48 materials were synthesized for acidic site as changing Si/Al ratio. The effect of the substituted Al amount on the structure of MCM-48 was examined in . It was showed that AlMCM-48 structures, though showed characteristic peaks of MCM-48, gradually deviated from that of pure Si-MCM-48 as increasing substituted amounts except Al-MCM-48(40). In the case of Al-MCM-48(40), it didn't show characteristic peaks of Si-MCM-48. Si-MCM-48 has high surface area of lOOOm^/g, which is a typical characteristic of mesopore materials, and pore volume is very large. As discussed XRD results, Al-MCM-48 structures were deviated from that of pure Si-MCM-48 when Al substituted. Pore structure of MCM-48 that has cubic system expanded as increasing Al amounts. From the results of XRD and N2 adsorption isotherms, pore wall thickness can be calculated. Pore wall thickness decreased as increasing Si/Al ratio. It's due to the transformation of pore structure by Al substituted and it might cause negative effect to NO reduction. Synthesized catalyst was characterized by X-ray photoelectron spectroscopy (XPS). display Pt-4f XPS spectra. From the deconvolution of Pt-4f XPS spectra of Pt/MCM48 catalyst, three-doublet were obtained. Many kinds of the XPS spectrum of Pt-4f have been showed at literature review. The XPS spectrum of Pt-4f showed at 70.8eV, 72.93eV and 74.59eV that could be assigned to Pt, Pt-0 and Pt-02, respectively.
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- Pure Si-MCM-48 AI-MCM-48(Si/AI=80) AI-MCM^(Si/AI=60) AI-MCM-48(Si/AI=40)
78
76
74
72
68
Bindina Energy /eV
Fig. 1. XRD patterns of MCM-48 and Al substitution MCM-48
Fig. 2. Pt-4f X-ray Photoelectron spectra ofPt/Si-MCM-48
1=1=
m 0 Vol% — • — 2.2 Vol% ^ A ^ 4 . 2 Vol% — • — 6 . 2 Vol%
-Pt(1wt%)/Si-MCM-48 - R(1wt%yAI-MCM-48(80) - Pt(1wt%VAI-MCM-48(60) - Pt(1wt%)/AI-MCM-48(40)
j^ : . • ' • " ^
4 Temperature ("C")
Fig. 3. NO conversion activity of Pt( 1 wt%)/ MCM-48 and Al-MCM-48 MCM-48
Fig. 4. NO conversion activity of Pt( 1 wt%)/ MCM-48 as changing O2 concentration
From the XPS results, over than 70% of Pt in MCM-48 structure existed as Pt-O form, 8% of Pt was Pt-metal, and 18.8% of Pt was Pt-02 form. In other words, most of Pt was participated in NO reduction reaction as a form of Pt-O as an active sites. De-NOx activity of Pt(lwt%)/MCM-48 and Pt(lwt%)/Al-MCM-48 were shown in . Pt(lwt%)/MCM-48 showed NO conversion around 210°C, increasing NO conversion as increasing temperature, then maintained maximum NO conversion until 500°C. To test NO activity changing by Si/Al ratio, NO activity test of Pt( 1 wt%))/MCM-48 and Pt(lwt%)/Al-MCM-48(x) were performed. It was predicted that NO conversion might be excellent as increasing Si/Al ratio because Al formed extra acidic site for NO reduction. However, actually, Pt(lwt%)/Al-MCM-48(80) showed more NO activity than any other catalysts. From these results, we could found that the increase of Al amounts does not affect
712
significantly on the catalytic reduction of NO; rather optimal Al amounts would induce best NO conversion. De-NOx activity of Pt(lwt%)/MCM-48 was tested as changing O2 concentration. As increasing O2 concentration from OVol% to 4.2%, NO reduction activity improved and C3H6 consumption increased with the same proportion of NO consumption. While, in the case of 6.2Vol% O2 concentration, with increasing temperature, NO conversion of Pt(lwt%)/MCM-48 was found to increase, passing through a maximum NO at the temperature range between 180°C and 220 °C, then decrease at high temperature. It's due to the C3H6-O2 reaction proceeded preferentially at higher temperature on Pt(lwt%)/MCM-48. NO conversion increased as increasing Ft loading amounts, whereas maximum NO conversion temperature decreased as increasing Ft loading amounts. This results are well coincide with the results of Cho et. al.[4] that maximum conversion temperature was shifted to low temperature range as increasing Ft loading amounts. Cho et. al.[4] reported that this excellent low-temperature activity may be attributed to the high activity of Ft surface toward both the oxidation of hydrocarbon and decomposition of NO. So, we could be achieving not only inducing excellent activity but also shifting maximum NO conversion temperature by control Ft loading amounts. Water addition effect was also studied. 10wt% of water vapor was injected to reactor. Water did not exert a bad influence on catalytic activity. This result reveals that Ft/MCM-48 had higher water resistance than conventional zeolite catalysts. 4. CONCLUSION Ft supported MCM-48 catalysts showed higher NO reduction activity than zeoilte base catalyst. Especially, Al substituted A1-MCM48 showed excellent NO conversion. It's the results that substituted Al form additional acidic site and it works for active site for NO reduction. Ft supported M41S catalyst didn't affected by water vapor addition. It means that M41S catalyst have higher water resistance than zeolite catalyst. M41S materials have advantages that they have high surface area; so high loading of active metal can be doped on MCM-48 without significant limitation. Also, reaction gases can more easily access pore than other zeolite catalyst. In conclusion, we could expect that metal supported MCM-48 might be a highly possible candidate for a new catalyst of hydrocarbon SCR process. REFERENCE 1. W.Held, A.Koning, T.Richter and L.puppe, SAE paper 900496(1990) 2. Akira shochi, Kenji, Katagi, Atushi Satuma and Tadashi Hattri, Applied Catalysis B 24(2000) 97 3. Gisle Oye, Johan Sjoblom and Michale Stocker, J. dispersion science and technology, 21(2000)49 4. Cho B.K and Yie J.E, Applied Catalysis B 10(1996) 263
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. Allrightsreserved
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Synthesis of titania-pillared clays and their application as catalyst supports for selective catalytic reduction of NO with ammonia Sung-Chul Kim, Ju-Ki Kang, Dul-Sun Kim and Dong-Keun Lee Department of Chemical Engineering/Environmental Protection, Environment and Regional Development Institute, Gyeongsang National University, Chinju, Kyongnam 660-701, Korea. Ii02-pillared clays were synthesized by pillaring Ti02 onto the pure bentonite. Successful intercalation of Ti02 could be achieved, and the physical properties such as dooi spacing, surface area and pore volume were influenced by the concentration of HCl, TiCU and Ti/clay ratio. The dooi spacing and surface area of the Ti02-PILCs increased up to 29.8A and 389m^/g, respectively. TPD and FTIR analyses showed the wide presence of the Br0nsted acid site which is essential to the SCR of NO with NH3. When Fe203 was incorporated into the Ti02-PILCs, the Br0nsted acidity increased significantly, and the complete conversion of NO could be achieved with the Fe203/Ti02-PIl>Cs at the temperature window between 375-400°C. 1. INTRODUCTION As a unique two dimensional zeolite like material, pillared interlayer clay(PILC) has attracted increasing interest for its adsorption[1,2] and catalytic properties[3-5]. Ti02PILC has large pore sizes that allow further incorporation of active ingredients without hindering pore difrusionf6] and has high thermal and hydrothermal stability among pillared clays[71. Selective catalytic reduction(SCR) of nitrogen oxides with ammonia is well-known for stationary source application, and V205/Ti02-based catalysts are most commonly used for practical applications. The presence of Br0nsted acid sites for the adsorption of ammonia is important to the SCR reaction. Intercalating Ti02 between the Si02 tetrahedral layer is a unique way of increasing acidity of the Ti02 support. In this research we synthesized Ti02-PILCs, and the prepared Ti02-PILCs were used as catalyst supports for selective catalytic reduction of NO with ammonia. 2. EXPEREMENTAL The starting clay for the preparation of Ti02-PILCs was a purified bentonite powder whose particle size was about Ifoii As the pillaring agent was used a solution of partially hydrolyzed Ti-polycations which had been prepared by adding TiCU into HCl solution. An aqueous solution of bentonite was then added to the pillaring agent to have a Ti/clay ratio in the range 5-10m mole Ti/g of clay under rigorous stirring. After being filtered and washed with distilled water, the samples were dried at 120°C for 24hr and
714
Table 1. Summarized properties>oftheTi02-PILCs Ti conc.(mmole/g clay) Sample 0 Bentonite Ti02-PILC-A 5.0 Ti02-PILC-B 6.5 Ti02-PILC-C 8.0 Ti02-PILC-D 10.0
Surface area( I iiVg) 33 194 227 284 382
then calcined at 300 °C for 6h. The prepared pillared clays were finally impregnated with Fe203 and V2O5. Selection catalytic reduction of NO with ammonia was carried out in a continuous fiow tubular reactor, and NO concentration was continuously monitored by a chemiluminescent analyzer. X-ray dilTraction(XRD) patterns of the prepared samples were analyzed with a Rigaku Miniflex instrument, and BET surface areas were obtained from the liquid nitrogen adsorption isotherms. 3. RESULTS AND DISCUSSION 3.1. Characterization of the TiOz-PILCs I igure 1 shows the XRD patterns of the clay and the synthesized Ti02-pillared clays. The d()()i peak of bentonite from XRD pattern was at 29=7.8°. Upon intercalation the d()()i peak shifted toward lower 29 values, corresponding to the increase in the spacing. The dooi spacing of the samples ranged from 27.9 to 28.5A. Table 1 summarized the properties of the prepared ri02-PILCs. The surface areas were strongly influenced by the Ti/clay ratio. The ri02-PlLC samples showed high surface area higher than 190m^/g (the surface area of bentonite was 33m^/g). The liquid nitrogen adsorption isotherms showed a typical type- II behavior, which indicated the presence of both micropores and mesopores in the Ti02-PILC samples.
I'igure 1. XRD patterns of the clay and the prepared 'Ii02-PlLCs ((A) clay, (B)Ti02-PILC-A, (C)Ti02-PlLC-B, (D) ri()2-PILC-C, (i:) ri02-PILC-D).
3.2 Acidity analysis of the Ti02-PILCs Ihe NH3 TPD spectra of the unpillared clay and the Ti02-PlLCs are shown in Figure 2. While the desorbed amount of NH3 from the unpillared clay was almost negligible, two distinct desorption peaks at around 220 °C and 330 °C were obtained from the Ti02- PILCs which did clearly chemisorb substantial amount of the strongly bound NH3. Moreover the intensity of the higher temperature desorption peak was much weaker than that of the lower temperature peak.
715
100
200
300
400
1750
1650
1550
1450
1350
TemperatureCC)
Waven u m ber(cm~'')
Figure 2. TPD spectra of NH3 on the Ti02-PILCs ((A)Ti02-PILC-A, (B)Ti02PILC-B, (C)Ti02-PILC-C, (D)Ti02PILC-D).
Figure 3. FTIR spectra of the adsorbed NH3 on the Ti02-PILCs ((A)Ti02-PlLCA, (B)Ti02-PILC-B, (C)Ti02-PILC-C, (D)Ti02-PILC-D).
FTIR spectra of the adsorbed NH3, as shown in Figure 3, could be used to investigate the nature of acid sites. The band at around 1450cm'' came from the asymmetric bending vibration of NH4* on Br0nsted acid sites and the asymmetric bending mode of ammonia on Lewis acid sites appeared at approximately 1620cm"'. For all the Ti02PILC samples there existed substantially more Br0nsted acid sites than Lewis site. By comparing with the FYIR spectra, the NH3 desorption peaks at around 220°C and 330 "C could be deduced to be due to the NH3 adsorbed on Br0nsted and Lewis acid sites, respectively. ri02 alone does not show strong acidity. The bulk mixed oxides, especially Si021 i02 mixed oxide, developed a greater acidity than individual phases[8]. The acid sites in the Ti02-PILCs are believed to locate mainly at the interface between silicate layers and Ti02 pillars. Since the presence of acid sites for the adsorption of NH3 is important to SCR reaction with NH3, the Ti02-PILC samples were used as catalyst supports. 3.3. SCR reaction on the FezOj and V2O5 impregnated TiOz-PILCs Figure 4 showed the plot of NO conversion versus reaction temperature with Fe203 and V2O5 impregnated Ti02-PILC catalysts. The loading of Fe203 and V2O5 was kept to be 5% as weight percentage. When Fe203 was impregnated on the Ti02-PILC-D having the highest values of surface area and pore size, complete conversion of NO could be achieved in the temperature window of 375-400°C. At this temperature window the ri02-PILC-A alone showed just 259^0 NO conversion. The incorporation of Fe203 must have enhanced the SCR activity remarkably. The activities of Fe203 impregnated Ti02-PILC catalysts were much higher than the corresponding ones of V2O5 impregnated Ti02-PILC catalysts.
716
o z
- • — FejOj/TiOj-PILC-A o
Fe203n"i02-PILC-B
-T-
Fe^Ojn-JOj-PILC-C
^ , - Fe^Oj/TiOj-PILC-D 200
225
25C 275
300
325
350
Temperature(°C)
375
400
425
200 225 250 275 300 325 350 375 400 425
Temperature(°C)
(A) (B) Figure 4. Temperature dependence of NO conversion with Fe203(A) and V205(B) impregnated Ti02-PILC catalysts( 1 OOOppm NO, lOOOppm NH3, 2% O2, 8% H2O, balance Ar). 4. CONCLUSION Ti02-pillared clays were synthesized under different HCl and TiCU concentrations. All the prepared Ti02-PILCs had both the micropores and mesopores. The surface area, pore volume and dooi spacing increased with increasing TiCU and decreasing HCl concentration. The prepared Ti02-PILCs showed the presence of both the BrOnsted and Lewis acid sites which were believed to be formed mainly at the interface between silicate layers and IIO2 pillars. Moreover there existed substantially more Br0nsted acid sites than Lewis sites. When Fe203 or V2O5 was impregnated onto the Ti02-PILCs, the acidity, especially BrOnsted acidity, increased significantly. This wide presence of BrOnstcd acid sites made the iron-doped ri02-PILCs successful catalyst for the SCR of NO with ammonia. REFERENCES 1. R.T. Yang and M.S.A. Baksh, AlChe J., 37 (1991) 679. 2. L.S. Cheng and R.T. Yang, Ind. Rng. Chem. Res., 34 (1995) 2021. 3. A. Vaccari, Catal. Today, 41 (1998) 53. 4. J. Barrault, M. Abdellaoui, C. Bouchoule, A. Majeste, J.M. Tatibouet, A. Louloud, N. Papayannakos and N.H. Gangas, Appl. Catal. B, 27 (2000) L225. 5. H.L. Delcastillo, A. Gil and P Grange, Catal. Lett., 36 (1996) 237. 6. J. Sterte, Catal. Today, 2 (1988) 219. 7. A. Bernier, L.F. Admaiai and P Grange, Appl. Catal., 77 (1991) 219. 8. K. Shibata, L Kiyoura, J. Kitagawa, I. Sumiyoshi and K. Tamabe, Bull. Chem. Soc. Jpn., 46 (1973) 2985.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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Hydrogenation of aromatics on Pt/Pd bimetallic catalyst supported by alcontaining mesoporous silica Soon-Yong Jeong Applied Chemistry & Engineering Division, Korea Research Institute of Chemical Technology, Yusung, P. O. BOX 107, Taejon, 305-600 Korea Aluminum substituted mesoporous molecular sieves (Al-MMS) were prepared using H2SiF6 as a silicon source with the mole ration of Si02:Al203:HF:NH40H:CTMABR:H20=l : 0.0066-0.02: 6.75: 7.19: 0.32: 220. Aluminum substituted silica-based mesoporous molecular sieve with good crystal Unity and narrow pore size distribution could be prepared at atmospheric pressure in several hours. In order to prepare the catalysts for aromatic hydrogenation, platinum and palladium precursors were impregnated on Al-MMS. The activity of Pt-Pd/Al-MMS was compared with that of Pt-Pd/DAY (dealuminated zeolite Y). Pt-Pd/Al-MMS catalyst has more activity than Pt-Pd/DAY, and has much less cracking activity due to mild acidity, indicating high yield of liquid. These above results well illustrate the application of Pt-Pd/Al-MMS as catalyst for selective aromatic hydrogenation. 1. INTRODUCTION As a consequence of the strict environmental regulations directed lower hazard emissions from vehicle exhausts, hydrotreating processes are playing an important role in the modern refinery strategies. Particularly, new diesel fuel specifications will include a reduction of sulfur to 0.05 wt% maximum and of aromatics content, while the cetane number will be set to a minimum value of 40 (1). A reduction of the aromatic content results in reduced particulate and NOx emissions from diesel motors. The concern about particulate emissions is related to the possible health hazards of the particulates. In order to solve these problems, numerous studies have been conducted in developing new catalysts and processes for aromatics saturation with improved activity and stability at moderate temperatures (2). The mesoporous aluminosilicate MCM-41 has been shown to be a suitable support for hydrogenation of aromatics. Particularly, Pt and/or Pd containing Al-MCM-41 was already claimed as a catalyst for the low temperature hydrogenation of benzene (3) and for the hydrogenation of aromatics in diesel and kerosene feeds (4). In this work, we report an alternative synthesis method for aluminum containing mesoporous molecular sieves (Al-MMS) in the presence of fluoride. Fluoride has been successfully used to extend the pH range over which anionic silica precursors can be utilized to create organized periodic structures (5,6). We investigated the catalytic activity of Pt/Pdsupported Al-MMS catalysts for the hydrogenation of aromatics. For this purpose, we studied the hydrogenation of naphthalene as a model compound in a batch reactor at 300 °C reaction temperature and 5.0 MPa pressure. Also, we compared the hydrogenation activities of catalysts prepared by two supports such as Al-KIT (mesoporous moleculat sieve synthesized by Ryoo and coworks (7)), and dealuminated zeolite Y
718
2. EXPERIMENTAL The substrate mixtures with the mole ratio of Si02: AI2O3: HF: NH4OH: CTMABr : H2O = 1 : 0.0066-0.02: 6.75: 7.19: 0.32: 220 were used to prepare a series of Al-MMS with different aluminum contents. 20g of H2SiF6 solution (10wt% Si02) was prepared by gradually dissolving 2g of Si02 (grade 62, 60-200 mesh, Aldrich) to 18g of 25% HF (48%, Merck). Various amounts of A1(N03)3 (99%, Aldrich) were added to the H2SiF6 solution. Upon complete dissolution, pH of the solution becomes between 1.4 and 2.0. These solutions were added to lOOg of 4wt% CTMABr (cetyltrimethyl ammonium bromide; 99%, BDH) in distilled water, and stirred during 1 hour. The above solution was added to 30g of NH4OH. A white gel was formed within 10 sec. The resulting solid product was recovered by filtration. The dried sample was calcined at 600 for 4 hours to remove the surfactant. For comparison purposes, the following materials were also used as supports: dealuminated zeolite Y (CBV 760, PQ Corp., Si/Al=30). All supports were impregnated with the required amount of platinum and palladium by incipient methods. Pt(NH3)3Cl (99.99%, Alfa) and Pd(NH3)3Cl (99.99%, Alfa) were used as precursors of platinum and palladium. The nominal Pt content and Pd content in all catalysts were 0.5 wt% and 0.5 wt%. The activity test for naphthalene hydrogenation were performed in 0.45 L batch stirred reactor. lOOg of a 5wt% solution of naphthalene dissolved in n-hexadecane are added to the reactor, and the system is pressurized with H2 at the operating pressure. For the simulation of diesel fuel, 200 ppm(w) of sulfur as dibenzothiophene (DBT) were added to the naphthalene solution. The hydrogenation experiments were carried out at 300°Oeaction temperature and 5.0 MPa total pressure. 3. RESULTS AND DISCUSSION Al-substituted mesoporous molecular sieves were prepared hydrothermally with fluoride ion. The surface area of Al-MMS samples prepared varies from 811 to 971 m^/g, and the incorporation of aluminum decreases the specific surface area and pore volume compared with a pure silica analog. No clear influence of Al content on the pore diameter was observed within the Al range examined. The corresponding maxima in the pore size distribution curves indicate the uniform mesopore of ca 2.8 nm diameter The acidic properties of the catalysts were tested by NH3 TPD between 90 and 600 °C. The desorption curves obtained are compared in Figure 1. All mesoporous molecular sieves have only one large peak at 150°C, indicating medium acidity and a large amount of acid sites. As Al content increases, the amount of acid sites increases. On the other hand, dealuminated zeolite Y has two large peaks at 150 and 320 °C, indicating medium and strong acidity, and a large amount of acid sites at two acidic sites. In order to prepare catalysts for aromatic hydrogenation, platinum and palladium precursors were impregnated on Al-MMS, Al-KIT-60, and dealuminated zeolite Y. Their catalytic activities were measured from hydrogenation reaction of naphthalene. The general reaction scheme proposed for the hydrogenation of naphthalene is presented is Fig. 2.(8). The hydrogenation occurs in a sequential manner, with the rate of tetraline hydrogenation being of an order of magnitude less than that of naphthalene hydrogenation. Table 1 represents compositions of products after hydrogenation of naphthalene using various catalysts. Tetraline was the major product under the operating conditions used in this work. Only at high conversion the hydrogenation of tetraline into cis and trans decaline occurred at a significant extent. Unknown products mainly consist of low molecule weight hydrocarbons
719
obtained from the cracking of n-hexadecane. Amount of unknown products is dependent on a type of catalyst. In the hydrogenation of naphthalene using PtPdDAY-30 catalyst, the percent weight of unknown products increases up to 6.24 wt%, indicating the strong cracking activity of PtPdDAY-30. Its cracking activity is related to the strong acidity of PtPdDAY-30 shown in Fig. 1. Consequently, when diesel fuel is hydrotreated using this catalyst, liquid yield will be reduced. According to the reaction scheme in Fig. 2, the hydrogenation of naphthalene to tetraline is a reversible reaction, while the dehydrogenation rates of decalines are negligible(8). However, in the range of temperature used in this work the equilibrium concentration of naphthalene is negligible (9) and the formation of tetraline can be considered as an irreversible process. Taking this into account, and in order to determine the kinetic rate constant for the process, and to be able to compare the activity of the different catalysts on the bases of this parameter, we have considered that the system can follow a pseudo first-order kinetic equation with respect to the conversion of naphthalene, which after integration results in the equation. -ln(l-X) = k, t This equation was almost fitted with the experimental results obtained for the different catalysts. Figure 3 show that the above assumption is adequate, in agreement with previous published results (10). The pseudo first order rate constants for the hydrogenation of naphthalene, ki, obtained on various catalysts are given in Table 2. The rate constant of PtPdKIT-60 is similar to that of PtPdMMS-50, and the rate constants of Pt/Pd supported on mesoporous materials are almost same within experimental error except the rate constant of PtPdMMS-75. The activity of PtPdDAY-30 is lower than activities of Pt/Pd supported on mesoporous materials.
Naphthalene
^00
t-SHjkj .
Qs-decaline
^3K,k3^ Trans-decaline
Fig. 2. General reaction scheme for the hydrogenation of naphthalene.
200
300
400
500
600
700
Temperature( °C )
Fig. 1. NH3 temperature programmed desorption(TPD) of Al-MMS-75, Al-MMS50, Al-MMS-25, Al-KIT-60, and DAY-30.
Fig. 3. Pseudo-first-order kinetic plots for naphthalene hydrogenation on various catalysts.
720
The rate of constants for the hydrogenation of tetraline into cis- and trans-decaline, k2+k3, calculated assuming an irreversible pseudo-first-order consecutive reaction, and the product selectivity defined as the ratio ki/(k2 -t- ks) are presented in Table 2. The hydrogenation rates of the second ring of naphthalene are one order of magnitude lower than the hydrogenation rates of naphthalene to tetraline. At this temperature, the pseudo-first-order rate constants for tetraline hydrogenation obtained on the different catalysts follow the next order, i.e. PtPdMMS-50>PtPdDAY-30>PtPdKIT-60> PtPdMMS-75> PtPdMMS-25. Pt/Pd supported on the DAY-30 and Al-MMS-50 gives a higher selectivity towards the hydrogenation of the second ring of naphthalene, as can be seen from the ratio between the first, ki, and second hydrogenations, k2+k3. These above results well illustrate that application of Pt-Pd/Al-MMS as catalyst for selective aromatic hydrogenation. Table 1 Compositions of products after hydrogenation of naphthalene using various catalysts(wt%) PtPdMMS-75 PtPdMMS-50 PtPdMMS-25 PtPdKIT-60 PtPdDAY-30 Naphthalene 0.02 0.02 0.01 0.01 0.05 Tetralin 3.46 2.68 4.87 3.18 0.42 0.04 0.23 0.19 1.01 Cis-decaline 0.53 0.52 1.65 0.05 1.52 Trans-decaline 1.07 2.46 6.24 Unknown 0.86 0.66 1.57 94.91 93.98 92.33 93.64 n-Hexadecane 91.58 Operating conditions : 300°C, 5.0MPa, 200ppm sulfur, Ig catalyst, lOOg feedstock(5wt% naphthalene, 95wt% n-Hexadecane), Ihour. Table 2 Kinetic rate constants for the hydrogenation of naphthalene (ki), and tetraline to cis-and transdecaline (k2+k3) obtained on the different Pt/Pd-supported catalysts ki(hr-') k,/(k2+k3) Sample Name (k2:+k3 ) X 1 0 V ) PtPdMMS-75 3.56 4.89 7.28 PtPdMMS-50 4.33 12.39 3.52 PtPdMMS-25 0.34 4.75 139.71 4.51 PtPdKIT-60 9.01 5.01 1.20 9.62 PtPdDAY-30 1.25 REFERENCES 1. Van den Berg, J. P., Lucien, J. P., Germaine, G., Thielemans, G. L. B. Fuel Process. Technol. 35, 119(1993). 2. Corma, A., Martinez, A., Martinez-Soria, V. J. Catal. 169, 480(1997). 3. Armor, J. N. Appl. Catal. 112, N21(1994). 4. Heinerman, J., Vogt, E., PCT Int. Pat. Appl. W094126846(1994). 5. Jeong, S. Y, Suh, J. K., Lee, J. M., Kwon, O. Y, J. Colloid and Interface Sci 192, 156(1997) 6. Voegtlin, A. C , Ruth F., Guth J. I., Patarin, J., Huve, L., Microporous Materials, 9, 95(1997). 7. Ryoo, R., Jun, S., Kim, J. M., Kim, M. J. M. Chem. Commun. 2225(1997). 8. Girgis, M. J., Gates, B. C. Ind. Eng. Chem. Res. 30, 2021(1991). 9. Frye, C. G., Weitkamp, A. W. J. Chem. Eng. Data, 14, 372(1969). 10. Lin, S. D., Song, C. Catal. Today 31, 93(1996).
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
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High loading of short W(Mo)S2 slabs inside the nanotubes of SBA-15. Promotion with Ni(Co) and performance in hydrodesulfurization and hydrogenation. L. Vradman*, M. V. Landau*, M. Herskov^itz*, V. Ezersky^, M. Talianker^, S. Nikitenko^ Y. Koltypin^ A. Gedanken^. * Blechner Center for Industrial Catalysis and Process Development, Chemical Engineering Department, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel. ^ Materials Engineering Department, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel. ^ Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel. Layered nanoslabs of a M0S2 and WS2 phases with a well-defined hexagonal crystalline structure were inserted into the nanotubular channels of SBA-15 at loadings up to 60 wt%. Sonication of a slurry containing SBA-15 in a W(Mo)(CO)6-sulfur-diphenylmethane solution yielded an amorphous W(Mo)S2 phase inside the mesopores that was transformed into hexagonal crystalline W(Mo)S2 nanoslabs by further sulfidation. The nanoslabs were distributed exclusively inside the mesopores in a uniform manner (HRTEM, local quantitative microanalysis), without blocking the pores (N2-sorption). The Ni(Co) promoters were introduced into the W(Mo)S2/SBA-15 composites by impregnation from an aqueous solution of nickel (cobalt) acetate. The activity (based on the volume of the catalyst loaded into reactor) of the optimized Ni-W-S/SBA-15 catalyst in hydrodesulfurization (HDS) of dibenzothiophene (DBT) and hydrogenation (HYD) of toluene was 1.4 and 7.3 times higher, respectively, than that of a sulfided commercial C0-M0/AI2O3. The HDS activity of Co-MoS/SBA-15 catalyst was 1.2 times higher than that of commercial catalyst. After promotion with Co, the directly introduced M0S2 slabs and M0S2 slabs prepared by sulfidation of Mooxide monolayer spread over SBA-15 displayed similar HDS performance. 1. INTRODUCTION Since the discovery of MCM-41 and related materials [1], many attempts were done to employ them as supports for catalytic phases dispersions [2-5]. However, it was shown that the main problem is to combine the formation of a well-defined nanocrystalline catalytic phase at high loading (>30 wt%) inside the mesopores with high accessibility of the nanocrystals to the reacting molecules (low blocking extent). It was shown previously [6, 7] that ultrasonication of the Mo(CO)6 solution in decalin in presence of MCM-41 support yielded closed packed monolayer of Mo-oxide spread at silica surface without blocking the
722
mesopores. In the present study the ultrasonication method was employed for direct introduction of M0S2 and WS2 crystalline phase into mesopores of SBA-15 material. 2. EXPERIMENTAL The catalysts were prepared by sonication of a slurry containing W(C0)6 or Mo(CO)6, elemental sulfur and SBA-15 in diphenylmethane at 90 °C for 3 h under argon with a highintensity ultrasonic probe [6]. The dried solid was transferred to the tubular reactor and sulfided in situ with a 1.5% dimethyldisulfide (DMDS)-toluene mixture at 320 °C and 5.4 MPa under hydrogen flow for 24 h. The Ni(Co) component was introduced into the W(Mo)S2/SBA-15 composite after sulfidation by impregnation with an aqueous solution of nickel (cobalt) acetate and drying under vacuum at room temperature. Reference Mo-oxide monolayer spread over SBA-15 (M0O3/SBA-I5) and Co-Mo-0/SBA-15 samples were prepared by ultrasonication as described in [6]. After sulfidation, the activity of the catalysts in HDS of DBT and HYD of toluene were measured as described in [6] and [8], respectively. 3. RESULTS AND DISCUSSION The pore volumes and BET surface areas of the different samples are listed in Table 1. The surface areas and pore volumes, normalized to SBA-15 contribution [6], were high for all loaded samples, which is evident for the small pore blocking effect. XRD data showed the amorphous nature of the ultrasonically deposited W(Mo)S2 phases. Treatment with the DMDS-toluene mixture under hydrogen led to the formation of small crystals of hexagonal WS2 and M0S2 phases. Direct evidence for the location the W(Mo)S2 phase nanocrystals within the SBA-15 nanotubes was obtained by HRTEM. The micrographs (Figure 1) clearly show the nanoparticles occluded within the nanotubes at the side view (a, c, e) and at the front view (b, d, f) of the hexagonally ordered nanotubes. Parallel fringes running across the nanoparticlc images have a periodicity of 6.2 A, which corresponds to the well-known distance between the atomic layers packed along the c-axis in the hexagonal WS2 or M0S2. An examination of 15 different 85x85 |am areas of the sample indicated no W(Mo)S2 phase outside the SBA-15 particles. Thus the nanocrystals were located only inside the mesopores of SBA-15 support. Table 1 Texture of the samples derived from N2-sorption. Sample Pore volume cm^/g SBA-15 20wt%WS2/SBA-15 60wt%WS2/SBA-15 32wt%MoS2/SBA-15 50wt%MoS2/SBA-15 42 wt% M0O3/SBA-I5 before sulfidation after sulfidation (47 wt% M0S2)
Normalized
BET surface area m'/g
Normalized
1.0
1.0
800
1.0
0.68
0.85
509
0.80
0.28 0.52
0.70
230 424
0.72
0.76
0.34
0.68
296
0.78 0.74
0.51 0.42
0.88 0.79
394 332
0.78
0.85
723
Fig. 1. HRTEM micrographs of the 60 wt% WS2/SBA-I5 (a, b) and 50 wt% M0S2/SBA-I5 (c, d) both prepared by direct insertion of sulfide, 47 wt% M0S2/SBA-I5 prepared by sulfidation of oxide monolayer (e, f).
724
Table 2 Comparison of catalysts performance in dibenzothiophene HDS and toluene HYD. Catalyst
W(Mo) (wt%)
Ni(Co) (wt%)
Slab length (nm)*
Stacking number*
C0-M0/AI2O3
17.6
4.5
-
Ni-W-S/SBA-15 Co-Mo-S/SBA-15
44.5 30.0
5.7 9.2
Co-Mo-O/SBA-15
28.0
8.9
^HYD
kHDS
TONHDS
-
(h-^) 0.6
(h-^) 38
(h-^) 1.26
3.6
3.2
4.4
54
0.90
3.5 3.4
2.6 2.4
(commercial) 50 1.23 1.29 47 0.74 7.7 5.8 26.9 16.6 5.1 28 Ni-W/Si02 16.4 0.9 26.4 0.78 7.3 2.4 28 Ni-W/AbOs * Average value obtained from HRTEM statistics performed as described elsewhere [9]. Increasing the Ni content in the Ni-W-S/SBA-15 catalyst increased both HDS and HYD activity up to Ni/W ratio of about 0.4 followed by a slight decrease at Ni/W ratio of 0.8. The optimal Co/Mo ratio was found to be close to 0.5 for both M0S2/SBA-I5 samples prepared by direct insertion of M0S2 or sulfidation of Mo-oxide monolayer. Furthermore, after promotion with Co, the HDS activity of both catalysts was similar (Table 2). This is a result of the same texture of the sulfided samples (Table 1) as well as the same structure and dispersion of the M0S2 nanocrystals as follows from HRTEM (Figure 1) and XRD data. Table 2 compares the HDS and HYD performances of the ultrasonically prepared catalysts with a commercial C0-M0/AI2O3 catalyst (KF-752, Akzo Nobel Chemicals) and with optimized Ni-W catalysts deposited on conventional y-AbOs and Si02 supports by impregnation [9]. The HDS activity of the SBA-15 supported Ni-W and Co-Mo catalysts was higher than that of commercial Co-Mo catalyst and Ni-W catalysts supported on conventional supports as a result of higher loading of W(Mo)S2 in the SBA-15. Ni-W-S/SBA-15 catalyst displayed HYD activity close to silica-supported Ni-W due to the lower stacking number [9] and much higher compared with the C0-M0/AI2O3 and Ni-W/Al203. Thus, the high loading Ni-W-S/SBA-15 has excellent potential for application in deep HDS of petroleum feedstocks.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
U. Ciesla, F. Schuth, Micropor. Mesopor. Mater., 27 (1999) 131. F. Schuth, A. Wingen, J. Sauer, Micropor. Mesopor. Mater., 44-45 (2001) 465. J. Sauer, F. Marlow, B. Spliethoff, F. Schuth, Chem. Mater., 14 (2002) 217. M. Froba, R. Kohn, G. Bouffaud, Chem. Mater., 11 (1999) 2858. A. Ghanbari-Siahkali, A. Philippou, J. Dwyer, M. W. Anderson, Appl. Catal. A, 192 (2000) 57. M. V. Landau, L. Vradman, M. Herskowitz, Y. Koltypin, A. Gedanken, J. Catal., 201 (2001)22. A. Gedanken, X. Tang, Y. Wang, N. Perkas, Yu. Koltypin, M. V. Landau, L. Vradman, M. Herskowitz, Chem. Eur. J., 7 (2001) 4546. L. Vradman, M. V. Landau, M. Herskowitz, Catal. Today, 48 (1999) 41. L. Vradman, M. V. Landau, Catal. Letters, 77 (2001) 47.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
725
Cr-MCM-41 for selective dehydrogenation of lower alkanes with carbon dioxide Ye Wang^, Yoshihiko Ohishi*', Tetsuya Shishido^, Qinghong Zhang^ and Katsuomi Takehira'' ^State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen 361005, China ''Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-hiroshima 739-8527, Japan Cr-MCM-41 synthesized by both direct hydrothermal (DHT) and template-ion exchange (TIE) methods is studied for dehydrogenation of lower alkanes including C2H6, C3H8 and i-C4Hio with CO2. Both methods lead to Cr species highly dispersed on the wall surface of MCM-41 and exhibited similar catalytic behaviors. Selectivity higher than 90% to each alkene has been achieved, and the presence of CO2 enhances the conversion of alkane. 1. INTRODUCTION Supported chromium oxide is used as catalyst for the dehydrogenation of C3H8 or i-C4Hio to alkene in industry [1]. There exist many reports on the development of catalysts for the oxidative dehydrogenation of lower alkanes with O2 since the highly endothermic dehydrogenation process consumes a large amount of energy [2]. However, the selectivity to alkenes with O2 is generally low due to the formation of COx. Recently, a few studies have reported the coupling of CH4 [3] and the dehydrogenation of C2H6 [4] or CsHg [5] with CO2. On the other hand, MCM-41, a typical mesoporous silica, which possesses a hexagonal array of uniform mesopores and high surface area may result in high concentration of uniformly distributed active sites if an appropriate method is used to introduce the catalytically active sites to MCM-41. Cr-MCM-41 has been synthesized by either the DHT method [6, 7] or an impregnation method [8], and has been applied to the liquid phase oxidation with H2O2 [9] and the oxidative dehydrogenation of CaHg with O2 [8]. However, the selectivity and yield to C3H6 in the latter reaction were very low. In this study, we apply the Cr-MCM-41 synthesized by both the DHT and the TIE methods to the dehydrogenation of lower alkanes with CO2. In the
726
TIE method, the Cr source is introduced to MCM-41 by an ion-exchange between the template cations embraced in the as synthesized MCM-41 and the Cr^^ in the aqueous solution. This method has been used for the synthesis of Mn- [10, 11], V- [12] and Fe-MCM-41 [13], and the metal ions introduced by this method show different coordination environments with those by the DHT method [11-13].
2. EXPERIMENTAL Cr-MCM-41-DHT was prepared by hydrothermal synthesis at I50°C for 48 h using the synthesis gel containing sodium silicate, chromium nitrate and hexadecyltrimethyl- ammonium bromide. Cr-MCM-41-TIE was synthesized by exchanging the template cations embraced in the as synthesized MCM-41 with the Cr^^ ions in aqueous solution at 80°C for 20 h. After hydrothermal synthesis or template ion exchange, the solid was recovered by filtration and then washed with deionized water, followed by drying at 40"C in vacuum and calcination in a flow of air at SSO^'C for 6 h. XRD and N2 adsorption at 77 K were measured to obtain information about the mesoporous structure. The diffuse reflectance UV-Vis and UV-Raman (exciting source 325 nm) spectroscopic studies were performed to characterize the coordination environment of Cr species. The catalytic reactions were carried out with a conventional fixed-bed flow reactor using lower alkanc and CO2 as rcactants. No reaction occurred without catalyst.
3. RESULTS AND DISCUSSION 3.L Properties of catalysts XRD measurements showed that the diffraction lines of (100), (110), (200) and (210) at 20 degrees of ca. 2.2, 3.6, 4.3 and 5.7" indexed to the hexagonal regularity of MCM-41 were observed for the Cr-MCM-41 samples by both the DHT and TIE methods, suggesting that the hexagonal array of mesopores of MCM-41 was sustained after the introduction of Cr with both methods. The peak intensity was not significantly changed with increasing Cr content to 1.7 wt% (Si/Cr= 50), but a further increase in Cr content to 3.4 wt% (Si/Cr= 25) decreased the peak intensity. The porous properties obtained from N2 adsorption measurements at 77 K are shown in Table 1. Narrow pore size distribution around 2.5-3.0 nm was observed for all the samples. The surface area and pore volume gradually decreased with an increase in Cr content up to 1.7 wt%, and became remarkably low for the sample with Si/Cr ratio of 25, indicating the decrease in structural regularity at high Cr content. The change of color of all the Cr-MCM-41 samples from pale green to pale yellow after
727
Table 1 Properties of Cr-MCM-41 synthesized with both the DHT and the TIE methods Sample^ MCM-41 Cr-MCM-41-DHT( 100) Cr-MCM-41-DHT(50) Cr-MCM-41-DHT(25) Cr-MCM-41-TIE( 100) Cr-MCM-41-TIE(50) Cr-MCM-41-TIE(25)
(m'g')
Pore vol. (mlg')
Pore dia. (nm)
1025 878 780 629 961 885 624
0.89 0.79 0.70 0.36 0.92 0.83 0.85
2.7 2.7 2.7 2.7 2.7 2.7 2.5
SBET
Color of sampl e As synthesized White Pale green Pale green Pale green Pale green Pale green Pale green
Calcined White Pale yellow Pale yellow Pale yellow Pale yellow Pale yellow Pale yellow
^The number in parenthesis are the Si/Cr atomic ratio. calcination suggests that Cr^^ in the as-synthesized sample was transformed to Cr^^ during calcination. This indicates that most of the Cr species after calcination exist as chromate species on the wall surface of MCM-41 but not as Cr^^ in the framework of MCM-41. Fig.l shows the diffuse reflectance UV-Vis spectra of Cr-MCM-41 synthesized by the two methods. UV bands at 280 and 370 nm were mainly observed for both kinds of samples. These bands could be assigned to O-Cr(VI) charge transfers of a chromate species. A weak shoulder around 440 nm was also observed particularly for the TIE samples, probably suggesting the existence of polychromate species. An intense band at 980 c m ' ascribed to the dehydrated monochromate was observed in the UV-Raman spectra for the DHT samples recorded at 200"C in N2 atmosphere, whereas bands around 1000-1200 cm' were observed in addition to that at 980 cm"' for the TIE samples. Thus, only monochromate species exist in the DHT samples, while the TIE samples contain monochromate and polychromate species.
300 400 500 600 700 800 Wavelength /nm Fig. 1. UV-Vis spectra of Cr-MCM-41. (a) DHT, Si/Cr=50;(b) DHT, Si/Cr=25; (c)TIE, Si/Cr=lOO; (d) TIE, Si/Cr=50;
1 2 3 Cr content /wt% Fig. 2. Dehydrogenation of C3H8 over Cr-MCM-41 by DHT(a,c)and TIE(b,d) methods. W=0.4g, P(C3H8)=12.2 kPa, P(C02)=68.9 kPa, T=823 K, F=50ml/mi n.
728 3.2. Catalytic properties of Cr-MCM-41 Fig. 2 shows the effect of Cr content on the catalytic properties of CsHg dehydrogenation with CO2 over both DHT and TIE samples. Both series of catalysts showed similar performances; C3H8 conversion increased almost linearly with an increase in Cr content and C3H6 selectivity was kept at 92-95%. This result indicates that the monochromate and the polychromate species exhibit similar catalytic effect on the dehydrogenation of C3H8 with CO2. Table 2 Table 2 shows that although CjHg could be converted to C3H6 in the absence of CO2, but C3H8 conversion increases remarkably with the partial pressure of CO2, suggesting that CO2 plays a cmcial role in the dehydrogenation of C3H8 to C3H6. The dehydrogenation of C2H6 and i-C4Hio occurred also effectively on the same catalyst with CO2 as shown in ja5ie 2.
Dehydrogenation of lower alkanes over Cr-MCM-41 P(C02)
Alkane
Alkene
/kPa
conv.%
select./%
9.8
93.2
Catalyst
Akane
TIE (50)
C3H8
0
TIE (50)
C3H8
68.9
17.4
95.5
DHT(50)
C3H8
0
9.4
90.0
DHT (50)
C3H8
68.9
17.0
93.1
DHT (50)
C2H6
68.9
11.5
99.7
DHT (50)
i-C4Hio
68.9
18.3
90.4
^= 823 K, W= 0.4 g, P(alkane)= 12.2 kPa, F= 50 ml/min.
REFERENCES 1. B.M. Weckhuysen, I. E. Wachs and R. A. Schoonheydt, Chem. Rev., 96 (1996) 3327. 2. G. Centi and F. Trifiro, Appl. Catal. A: General, 143 (1996) 3. 3. Y. Wang, Y. Takahashi and Y Ohtsuka, J. Catal., 186 (1999) 160. 4. K. Nakagawa, C. Kajita, K. Okumura, N. Ikenaga, M. Nishitani-Gamo, T. Ando, T. Kobayashi and T. Suzuki, J. Catal., 203 (2001) 87. 5.1. Takehara and M. Saito, Chem. Lett., (1996) 973. 6. N. Ulagappan and C.N.R. Rao, Chem. Commun., (1996) 1047. 7. Z. Zhu, Z. Chang and Larry Kevan, J. Phys. Chem. B, 103 (1999) 2680. 8. J. Santamaria-Gonzalez, J. Merida-Robles, M. Alcantara-Rodriguez, P. Maireles-Torres, E. Rodriguez-Castellon and A. Jimenez-Lopez, Catal. Lett., 64 (2000) 209. 9. A. Sakthivel, S.E. Dapurkar and R Selvam, Catal. Lett., 77 (2001) 155. 10. M. Yonemitsu, Y. Tanaka and M. Iwamoto, Chem. Mater., 9 (1997) 2679. 11. Q. Zhang, Y Wang, S. Itsuki, T. Shishido and K. Takehira, J. Mol. Catal. A: Chem., (2002) in press. 12. Q. Zhang, Y Wang, Y Ohishi, T. Shishido and K. Takehira, J. Catal., 202 (2001) 308. 13. Y Wang, Q. Zhang, T. Shishido and K. Takehira, J. Catal., 209 (2002) 186.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved
729
Methane reforming on molybdenum carbide on Al-FSM-16 Masatoshi Nagai, Toshihiro Nishibayashi, and Shinzo Omi Graduate School of Bio-applications and Systems Engineering, Tokyo University of Agriculture and Technology, 2-24 Nakamachi, Koganei, Tokyo 184-8588, Japan Methane reforming on the carbided 12% Mo/Al-FSM-16 catalysts with Si/Al ratios of 30, 50, and 80 was performed at 973 K under atmospheric pressure. The characterization was carried out by N2 adsorption, XRD, and ^^Al MAS NMR. AI-FSM-16 with a Si/Al ratio of 30 exhibited an implantation of aluminum into the Si02 structure of FSM-16. The 873 K-carbided 12% Mo/Al-FSM-16 catalyst was more active than the oxidized catalyst and the catalysts carbided at a higher carbiding temperature. The largest amounts of hydrogen and benzene were formed using the catalyst with the Si/Al ratio of 80. P-M02C on the catalyst was formed during the carbiding and methane reforming. 1. INTRODUCTION Recently, methane reforming has been extensively studied for effectively utilizing natural gas resources. Mo/ZSM-5 catalysts are very active in methane reforming. The implantation of aluminum into mesopore FSM-16 is expected to be used as a catalyst support by generating acid sites [1]. Mesoporous materials having a high surface area and heat tolerance promoted the reaction with a fast molecular diffusion in the mesopores. In this study, 12% M0O3/AI-FSM-I6 is carbided by a temperature-programmed reaction in a stream of 20% CH4/H2 [2], and analyzed by N2 adsorption, NMR, and XRD. The effects of the Si/Al ratio and preparation procedure on the structure were studied. The catalytic activity is determined during methane reforming using the 12% Mo/Al-FSM-16 catalysts with three different Si/Al ratios. 2. EXPERIMENTAL Sodium silicate and sodium aluminate (Si/Al=30, 50, and 80) were added to a small amount of water and the mixture was stirred at 333 K for 3 h. The solution was dried at 353 K (and 413 K) to yield a sodium aluminosilicate glass which was then calcined at 973 K for 3 h. The layered sodium silicates containing aluminum at three Si/Al ratios were dispersed in an aqueous solution of [Ci6H33N(CH3)3]Cl and stirred at 343 K. The solid products were separated from the solutions by suction filtration (or decantation), and dried at 353 K (Method II). The sample (Method I) was dried at 353 K and subsequently calcined at 813 K in air . The 12 wt% M0O3/AI-FSM-16 catalyst was prepared by an incipient wetness method after Al-FSM-16 (Method I or II) was added to an aqueous solution of (NH4)6Mo7024-4H20. The resulting product was dried at 353 K, calcined at 573 K, and carbided by the temperature-programmed reaction in 20% CH4/H2 at a flow rate of 66.7 ml min' from 573 to 873 (-1073) K at a rate of 1 K min''. The catalyst was maintained at this temperature for 3 h. The BET surface area of the samples was measured at 77 K using a Beckman-Coulter adsorption apparatus. The structure of the samples before and after pretreatment and carburidation was measured by XRD analysis. Diffraction patterns were determined using a RAD-II (Rigaku Co.) equipped with Cu-Ka radiation with slits of (ds) 1/2°, (rs) 0.3 mm, and (ss) 1/2°. The solid state MAS NMR spectra were measured on a JEOL JNM-EX400 spectrometer. ^^Al MAS NMR spectra were recorded at 6 kHz spinning. Methane reforming was carried out using a continuous-flow quartz reactor (0.03 g) in streams of methane and helium with a 15 mlmin"' rate at 973 K. The reaction mixtures were analyzed using a Balzer quadrupole mass spectrometer.
730
3. RESULTS AND DISCUSSION
500^
3.1. Preparation of Mo/Al-FSM-16 The N2 isotherms of the Al-FSM-16, 12% Mo/Al-FSM-16 (I), and 12% Mo/Al-FSM-16 (II) are shown in Figure 1. The predominant increase in the adsorptions of Al-FSM-16 and 12% Mo/Al-FSM-16 (II) were observed at P/Po = 0.25 ~ 0.4, which was Y^^ characteristic of the N2 adsorption in mesopores, while these characteristic 0.2 1.0 peaks were not observed for the 12% Mo/Al-FSM-16 (I) after molybdenum P/Po loading. This result indicated that the Fig. 1. N2 Adsorption/desorption is oth erms loading of molybdenum destroyed the of(#)Al-FSM-16, (x)Mo/Al-FSM-16(l), and structure of 12% Mo/Al-FSM-16 (I), while the structure of the 12% ( • ) Mo/Al-FSM-16(11). Volume(gas)l.56Xl0 3 Mo/Al-FSM-16 (II) was uniformly maintained even after calcination at 813 K. The BET surface areas of the Al-FSM-16 are 715, 799, and 1275 m^ Table 1 g"' for the Si/Al ratios of 30, 50, and 80, BET surface area, spacing dioo, and unit cell respectively, showing that the surface dimension ao (100) of each sample area increased with the increasing Si/Al ratio. After drying the sample (II), the XRD Surface area Sample surface area after loading the dioo/nm ao/nm /m^g' molybdenum species decreased from 715 to 514 m^ g ' in Table 1, but 1444 FSM-16 3.73 4.31 maintained the structure of the support. 3.84 4.43 Al-FSM-16 715 The surface area of the sample (I) l2%Mo/Al-FSM--16(1) 269 decreased much more than that of the 12%Mo/Al-FSM-16(1) 247 sample (II). The decrease in surface carbide at 973 K area of the sample (I) was due to l2%Mo/Al-FSM-16(II) 514 3.73 4.31 plugging of the molybdenum oxides in the micropores of Al-FSM-16 and destroying the FSM-16 structure. The XRD patterns of FSM-16, Al-FSM-16, and 12% Mo/Al-FSM-16 (I, II) are shown in Figure 2. The (100), (110), and (200) phases were observed for FSM-16, but only the (100) phase was seen for Al-FSM-16. The Al-FSM-16 exhibited the implantation of the aluminum atom into the SiOz structure of FSM-16 by having an irregular structure. The surface area of FSM-16 was two times greater than that of Al-FSM-16, supporting the result of the structure regularity by XRD. Thus, the structural regularity was likely to affect the surface area. The 12% Mo/Al-FSM-16 (II) was prepared by calcination after loading the molybdenum compound which resulted in retaining the structure of the (100) phase. Since carbization of the sample (II) slightly decreased the surface area, the structure of the support was not changed before and after the carbization. In Table 1, FSM-16 and Mo/Al-FSM-16 (II) had the same unit cell dimensions as the value (4.31 nm) in the literature [3]. This result showed that Al-FSM-16 and 12% Mo/Al-FSM-16 maintained the mesoporous structure of the 16 carbon chains. The pore sizes of Al-FSM-16 were 2.8 and 4.2 nm. The former pore size was due to FSM-16 and the latter due to the formation of bridging of the aluminum with silica in the preparation of Al-FSM-16. The 12% Mo/Al-FSM-16 (II) contained micropores of 2.8 nm more than Al-FSM-16. The XRD pattern of the impregnated 12% Mo/Al-FSM-16 is shown in Figure 2e. Al-FSM-16 had a peak of Si02. 12% Mo/Al-FSM-16 carbided at 973 K had the peak of P-M02C but no peaks for the oxide form. P-M02C had agglomerated outside the pores
731
of the support in flowing 20% CH4/H2 at high carbiding temperatures. This result showed that molybdenum oxides on the surface of the Al-FSM-16 were loaded more than that inside of the micropores. 3.2. Properties and structure The ^^Al MAS NMR spectra for the Al-FSM-16 with the three Si/Al ratios are shown in Figure 3. The Al-FSM-16 sample (filtration) had a peak at 50 ppm, which is ascribed to four coordinated sites, while the Al-FSM-16 sample (decantation) had the peak at 8 ppm for the six coordinated sites. The formation of six-coordinated alumina is due to the more basic solution of sodium silicate and sodium aluminate at a pH of about 12.4. These compounds were precipitated and changed to the six-coordinated compounds containing aluminum sources. The ratio of the six-coordinated octahedral to four-coordinated tetrahedral aluminum increased with the decreasing aluminum content (high Si/Al ratio). This result suggested that the implantation of aluminum in the Si02 body required a certain amount of aluminum in the feed solution. The uniform implanting of aluminum into the SiOi structure needs an excess amount of aluminum. For Al-FSM-16 with Si/Al = 80, the large ratio of the hexahedral aluminum to the tetrahedral aluminum was observed more than those with Si/Al = 30 and 50 from the decantation preparation. The decantation cannot completely remove the dissolved feed (sodium aluminate). The XRD analysis confirmed maintaining of the hexagonal structure after the molybdenum oxides were loaded and subsequently carbided in a stream of 20%CH4/H2. Thel2%Mo/Al-FSM-16 (II) with good hexagonal structure exhibited a higher surface area than 12% Mo/Al-FSM-16(I).
Vi 1
(a)
1^ 1 _0
0
5000
-^-. \ ^ \ <=•
(b)
v=
v^_^
;3
2500 (c)
\ •4—»
\ \\ \
2000
\i\
(d)
1500 ^"'--..-.A.w-,
2
4
6 2 0 I deg.
.__..]
8
10
" "(eY 1500
(0
w^ 20
2000
vu 40 60 2 0 /deg.
80
100
Fig. 2. XRD pattems of (a) FSM-16, (b) Al-FSM-16(11) (Si/AH30), (c) 12% Mo/AlFSM-16(I), (d) 12% Mo/Al-FSM-16(Met- hod II), (e) 12% Mo/Al-FSM- 16(11) (f) after reaction. (o)Si02 and (•) I3-M02C.
3.3. Methane reforming Figure 4 shows the methane reforming on the 973 K-carbided catalysts with the three different Si/Al ratios. The largest amounts of hydrogen and benzene were formed at the Si/Al ratio of 80. The catalyst with the Si/Al ratio of 80 was the most active based on the surface area. This is because the aluminum atom in the support was involved in the reforming reaction because of the high activity per surface area. Furthermore, the catalysts carbided at 873 K exhibited a high activity during methane reforming, or possibly the interaction of
732
alumina with silica. Furthermore, the methane reforming was carried out over the oxidic catalyst with the Si/Al ratio of 80. This lag time is required for carbidization with methane in the feed. M0O3 on the surface was changed from a to P-M02C. The molybdenum species changed from molybdenum oxide to molybdenum carbide even after 10 min from the run-start since the molybdenum carbide was formed. l.UUh-UU/ i 8.00E-008
3
•
4.00E-008
8
2.00E-008
1.20E-011 9.00E-012 0 ppm
-100
-120
Fig. 3. 27A1 MAS NMR spectra of Al-FSM-16 with prepared with Si/Al ratios of 30, 50 and 80 4. CONCLUSION
X
(\
C
100
3
6.00E-008
^
200
(a)1
6.00E-012 3 9
,, •,.^.,....„«grj
. , "" ,
(b)
\v^
3.00E-012
i0
...J 20
. x _ l , _ _ l 40 60
.
1 80
;
1 100
f ^ 120
Time on stream /min Fig. 4. Formation of (a) H2 and (b) CeHe over 12% M02 C/Al-FSM-16. Si/Al=30 (O), 50( ) and 80(#)
The BET surface areas of Al-FSM-16 increased from 715 and 799 to 1275 m^g"' with the increasing Si/Al ratios from 30, 50, to 80, respectively, and those of the 12% M0O3/AI-FSM-I6 were 269, 306, and 735 m^g•^ The ^^Al MAS NMR spectra of Al-FSM-16 showed that the sample by filtration mainly had four coordinated sites and the sample by decantation contained six coordinated sites. The uniform implanting of aluminium into the SiOi structure needs excess aluminium. In methane reforming on the oxidized catalyst, the induction period was observed until the catalyst was carbided on the surface. This activation period was due to the time for changing M0O3 to P-M02C on Al-FSM-16.
REFERENCES 1. S. Inagaki, Y. Fukushima, A. Okada, T. Kurauchi, K. Kuroda, C. and Kato, Proc. 9th. Int. Zeolite Conf, 1 (1992) 305: S. Inagaki, Y. Yamada, Y Fukushiam, Prog. Zeolite Microporous Mat. (1997) 109. 2. K, Oshikawa, M. Nagai, and S. Omi, J. Phys. Chem. B, 105 (2001) 9124 . 3. S. Inagaki, A. Koiwai, N. Suzuki, Y. Fukushima, and K. Kuroda, Bull. Chem. Soc. Jpn., 69 (1996) 1449.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
733
Preparation of Carbided WO3/FSM-I6 and Al-FSM-16 and Its Catalytic Activity M. Nagai, K. Kunieda, S. Izuhal, and S. Omi Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan The tungsten carbides on FSM-16 were prepared in carburizing of WO3/FSM-I6 with 20% CH4/H2 and characterized using nitrogen adsorption, XRD, TEM, and NMR. The relationship between the surface properties and the catalytic activity of tungsten carbides on FSM-16 in cyclohexene hydrogenolysis at 250°C was discussed. 1. INTRODUCTION Mesoporous silica and silica alumina materials (FSM-, Al-FSM-16) can be applied to use as catalysts because of high surface area, high pore sizes, and high stability of the structure up to 600 °C. Tungsten carbides are reported to be similar to catalytic activities of Pt catalysts for the hydrocracking of hydrocarbons and hydrogenation of CO and olefins. In this study, the tungsten carbide supported on FSM-16 catalysts were prepared in carburizing in a stream of CH4/H2 after the preparation of tungsten oxides. The catalysts were characterized by N2 adsorption, XRD, TEM, and NMR. The relationship between the surface properties and catalytic activity of the tungsten carbides supported on Al-FSM-16 for cyclohexene hydrogenolysis at 250°C was discussed. 2.
EXPERIMENTAL
2.1. Preparation of FSM-16, Al-FSM-16, and Tungsten Catalysts Mesoporous silica (FSM-16) was synthesized in similar way of the reference [1,2J. The layered sodium silicates (Na20/Si02 = 2; oxidized at 800 °C ) were dispersed in cetyltrimethylammonium chloride (CMACl) in an aqueous solution and stirred at 70 °C for 3 h. The pH of the suspension solution was 12.4. The pH of the suspension was adjusted at 8.5 by the addition of an aqueous HCl solution. The solid product (FSM-16) was dried and calcined at 540 °C for 3 h in air. In synthesis of Al-FSM-16 (Si/Al ratio of 20, 30, 50, and 80), a mixture of sodium silicate and sodium aluminate (heated at 800 °C) was added into the CMACl solution and stirred for 3 h at 70 °C. The solution was added to adjust the acidity to pH=8.5. TheNa* in the products was exchanged with NH4* ions. The dehydrated products were calcined in air at 540 °C for 3 h. In synthesis of tungsten carbides supported on FSM16, the FSM-16 sample was added in an aqueous solution (and DMF solution) of ammonium metatungstate (12% WO./AIMPS (0.05 g)),^and heated in air at 450 °C for 1 h. The products were carbided by heating from 350 to 700 °C in a stream of 20 % CH4/H2 (4 Ih"') at a rate of 1 °Cmin'' and kept at this temperature for 3 h. 2.2. Characterization The BET surface area and pore size distribution of the catalysts were measured at 196 °C using a conventional apparatus for volumetric adsorption of N2 after the samples were degassed at 200 °C. The X-ray diffraction patterns were obtained by using a Rigaku RAD-II diffractometer with Cu-Ka radiation. The peaks were identified on the basis of the reference [1]. Solid state MAS NMR were measured on a JEOL JNM-EX400 spectrometer. ^*^Si and Al MAS NMR spectra were recorded at spinning of 6 kHz. The morphology of the sample was determined by a JEOL JEM-2010 transmission electron microscopy operation at 200 kV. The sample was located on a copper grid and transferred to an analysis chamber in the TEM equipment.
734
2.3. Catalyst Activity The hydrogenolysis of cyclohexene was carried out in a differential microreactor at 250°C in atmospheric pressure [3]. The carbided catalysts was pretreated by heating at 540°C for 1 h in a stream of He. Cyclohexene was introduced into the reactor by bubbling a stream of pure hydrogen at a rate of 15 cm^ min' through a cyclohexene saturator, and maintained in an ice bath at 0.4 °C. The concentration of cyclohexene in the feed stream (total flow rate: 15 ml min"') was maintained by adjusting the H2 flow rate (7.5 ml min"') through the saturator. Sampling was performed by injecting a sample from a sampling loop into the gas chromatograph. The reaction products were analyzed by a FID gas chromatography on a 1 m stainless-steel column packed with VZ-8 at 60 °C. 3. RESULTS AND DISCUSSION 3.1. Preparation of FSM-16 and Al-FSM16 The TEM of the calcined sample illustrates the regular array of mesoporous channels in a hexagonal arrangement as shown in Fig. 1. The ordered pores were clearly determined. In this analysis, pore diameter D=2.5 nm and wall thickness t=1.6 nm. The largest intensity was detected at (100) plane peak with d,oo==3.68 nm spacing and hexagonal unit cell mdex a^f=425 nm based on ao = 2d,oo/V 3 . This is in excellent agreement with the position of the first peak (d_LO()=3.68 nm) in the XRD pattern (a - 2d,oo/'V3 = 4.1 nm). The BET surface area of the prepared Al-FSM-16 was 825 m~ g' (FSM-16; 1043 nrg"'). From the adsorption/desorption isotherm, the N^ adsorption isotherm was a IV type of Nj adsorption with an increased adsorption at a relative ?/PQ = 0.25, indicating the condensation of micropores due to N2 filling into mesopores. Also, hystcrisis was observed due to the formation of micropores of Al-FSM-16. Furthermore, the intensity of (100) peak and djoo were a maximum at pH = 8.5, while the intensity at (100) peak decreased to 0.5 times lower than those at pH = 2 to 5 to an advance in dehydration and condensation of silica. The XRD analysis confirmed that Al-FSM-16 (Si/Al ratio = 30) held a clear decrease in d,oo spacing and peak intensity at (100) plane. This is due to the disturbance caused by the existence of aluminum in forming the mesopores. In NH3-TPD analysis (Fig. 2), the Na' sample without NH4^ ion-exchange had no desorption of NH3, but a strong peak was observed at 170" °C for the NH4' ion exchanged sample. The peak at 170°C
Fig. 1. TEM image of calcined FSM-16.
e
15
h ^
100
200
300
400
500
600
Temperature /"C Fig. 2. NH,-TPD of ( • ) NH4*-exchanged and (O) Na* Al" FSM-16 (Si/Al=30).
735
indicated that the Al-FSM-16 held some acidity and weaker than a Y zeolite. The Na^ ions were electrically bonded to protons, thus lowering the acidity of the resulting catalyst. The XRD peak of the NH4" sample was low in the Na' sample only. The NMR analysis was done to study whether aluminum materials can be incorporated (loaded) into these materials. The aluminum coordination states of the AlFSM-16 were determined by ^^Si and ^^Al 7J:MAS NMR MAS NMR spectra. The '^Si spectra of the FSM-16 and WO3/FSM-I6 samples held a broad peak at -100 ppm together with shoulder peaks at -90 and 110 ppm (Fig. 3A, B). For mesoporous silica alumina, NH4^ and H' ions (Fig. 4C, D) are present near the tetrahedral Al with a negative charge and act as a Bronsted acid. The Al-FSM-16 prepared by the former method had many four coordinated sites and that by the later method had many six coordinated sites. The '^Al MAS NMR spectra for the Al-FSM-16 with various Si/Al ratios showed that the ratio of six coordinated aluminum to four coordinated aluminum increased with decreasing aluminum content, and resulted in increasing the surface area of the samples. The six coordinated aluminum increased with increasing the Si/Al ratio of the samples. Implanting of aluminum into SiOj structure in uniformity needs excess aluminum resources.
ppm
Fig. 3. ''Si MAS NMR spectra of (A) WO3/AI-FSM-I6 (Si/Al=30) and (B) AlFSM-16 (Si/Al=30).
3.2. Tungsten Carbide Supported Catalysts The BET surface area and micropore volume decreased after tungsten carbides were loaded on Al-FSM-16 due to plugging the micropores of the samples. The phase ppm (100) characteristic of mesopore in the XRD pattern was observed for the 12 wt% Fig. 4 ''Al MAS NMR spectra of (C) AlW/FSM-16 catalyst prepared at pH = 8.5. FSM-16 (Si/AN20) and (D) NH/ exchanged Furthermore, WO3 was precipitated in the Al-FSM-16 (Si/Al=20). solution when pH in the solution was lowered to pH=2.0. This result showed that the tungstate ions were likely to be condensed by lowering the pH, resulting to a considerable formation of polytungsten acid ions and lower XRD intensity. When WC/FSM-16 was prepared in DMF solvent by the incipient wetness method, the mesopore structure of the WC/FSM-16 was not broken so much as those in an aqueous solution of polytungstate ions. The XRD intensity of phase (100) was more sharp than those in the samples mixed in aqueous solution.
736
In W/FSM-16, ''Si MAS NMR spectroscopy (Fig. 3 A), signals were detected at Q2 (at about -90), Q, (at about 102) and Q4 (at about -108). The result was compared to FSM-16. It was found that the ratio of Q3 Si was higher than Q4 Si, while vice versa in FSM-16. The chemical shift of silica were not seen, resulting the product of Si-O-W bond undetermined. These results suggested that the W/FSM-16 with the Si-O-W bonds was not formed in the sample. 3.3. Hydrogenolysis of Cyclohexene The reaction products were cyclohexane and methylcyclopentane. The reaction rates for cyclohexene hydrogenolysis and the formation rates at 250°C are shown in Fig. 5 and Table 1. The 20% WC/FSM16 sample was the most active of the samples such as WO3 supported on AljO^, FSM-16, and Al-FSM-16. The WC/FSM16 catalyst was deactivated and reached a steady state after 500 min, but the other catalysts were deactivated rapidly and diminished by 500 min.
300 c pi 0
B
•
^
250 700 150
«1
c
100
«3 i)
50
0
0
• « A
0
OOn n
100
200
300
400
500
600
Time on stream /min
Fig. 5. Hydrogenation of cyclohexene over (o) W0,/FSM-16, (•) WO./AIA, (•) 3% W/FSM-16, (A) 12% W/FSM16, and (D) 20% W/FSM-16 carbided at 700°C.
Table 1 The hydrogenolysis of cyclohexene at 250 °C Sample
Cyclohexane (A) Methylcyclopentane (B) (jimol min"' g ) (iimol min g')
Al-FSM-16(Si/A130) 86.9 Al-FSM-16(Si/A150) 48.0 Al-FSM-16(Si/A180) 40.3 3%WC/FSM-16 28.1 10%WC/FSM-16 155.6 20%WC/FSM-16 206.3 W0,/FSM-16 31.6 WO./Aip, 48.0
14.3 7.9 6.5 4.9 0 17.8 5.1 10.4
B/A 0.16 0.16 0.16 0.17 0 0.09 0.16 0.22
In Table 1, the carbided catalysts had a better selectivity for the hydrogenation of cyclohexene from cyclohexene and a less selectivity for cyclohexene hydrogenolysis. Furthermore, the rate of cyclohexene hydrogenolysis (per catalyst-gram) on the carbided catalyst decreased with increasing carbiding temperature from 500 to 900 °C. This result indicated that the partially carbided tungsten catalyst was more active than fully carbided catalyst. REFERENCES 1. S. Inagaki, Y. Fukushima, and K. Kuroda, J. Chem. Soc, Commun. (1993) 680. 2. Z. Luan, et al. J. Phys. Chem., 99 (1995) 1018. 3. M. Nagai, K. Koizumi, and S. Omi, Catal. Today, 35 (1997) 393
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
737
AMI study on the catalytical isomerization of 1-Hexene to 2-Hexene on the surface of aluminosilicate molecular sieves MinPu"'''
Zhi-HongLi'
Shang-Ru Zhai" Dong Wu'*
Yu-Han Sun '
^State Key Laboratory of Coal Conversion, Shanxi Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, P. R. China ^School of Science, Xi'an Jiaotong University, Xi'an 710049, R R. China The catalytic isomerization mechanism of 1-hexene to 2-hexene on the surface of aluminosilicate molecular sieve has been studied using the semiempirical method of Austin Model l(AMl) of quantum chemistry. The transition state of the isomerization reaction was optimized and verified by the only virtue frequency. The intrinsic reaction coordinates inferred the reaction paths were calculated. The energy barriers of the isomerization forward and reverse are 159.810 and 177.707 kJ/mol, respectively. 1. INTRODUCTION Isomerization of hexene has been frequently used as a probe to characterize the structure of aluminosilicate molecular sieves'"^ and a carbonium ion mechanism for the isomerization of hexene was suggested.'*'^
However, the theoretical studies of the catalytic rearrangement of
hexene over the surface of aluminosilicate molecular sieves has not been seen in literatures until now.
It is necessary to explain the isomerization mechanism of hexene in theory for
developing new catalyst.
In this paper, the catalytic isomerization of 1-hexene to 2-hexene
over the surface of aluminosilicate molecular sieve has been studied with AMI semiempirical method.^ 2. CALCULATION DETAILS The initio structure of all the molecules of the reaction was established by molecular mechanical method in HyperChem Pro 6.0 program package.
The molecular geometry was
fully optimized with AMI method using analytical energy gradients. The default settings of the gradients and self-consistent-field (SCF) convergence were used.
The intrinsic reaction
coordinate (IRC) was calculated by Bemy reaction coordinate optimization.
The BrOnsted
acidic site of the aluminosilicate molecular sieve was represented by a protonated Si-O-Al 3T National Natural Science Foundation (No.29973057); National Key Basic Research Special Foundation (NO.G20000480). Fax +86-351-4041153. E-mail: wudong@public.tv.SX.cn
738
cluster. GAUSSIAN 98 computer program are used to finish the quantum chemical calculations. All works were implemented on a Pentium III computer. 3. RESULTS AND DISCUSSION The theoretical researches showed that the mechanism of isomerization was very complicated. Fig. 1 gives the structural models of the single molecules, i.e. 1-hexene molecule (R), 2-hexene molecule (P) and the aluminosilicate molecular sieve (C). The calculation results indicated that R was first adsorbed on the surface of C and formed a supermolecule (RC) with C. RC was the stable point in energy contours. Then, RC transformed to another supermolecule (PC) through an 8-fold ring of transition state (TS). PC was the complex of P adsorbed on the surface of C. Finally, PC desorbed to form the single molecule of P and C. Fig. 1 also gives the structure models of RC, TS and PC. It can be found that the change of the isomerization of hexene was really the exchanges of H atom on the catalysis C.
RC
TS Fig. 1. Molecular Structure of R, P, C, RC, TS, PC
PC
Table 1 gave the optimized molecular geometry parameters of R, P, C, RC, TS and PC.
It
can be seen that the corresponding bond length and bond angle of R was very close to that of RC, and so do P with PC. The bond length Rc(i)-C(2) in RC is 0.1333nm indicating it is a double bond.
Rc(i)-C(2) changes to 0.1381 nm in TS. The value is between the double and
single bond length meaning that the double bond is weakened. it became a single bond. the reaction.
Rc(i)-c(2) is 0.1475 nm in PC,
Similarly, the bond length Rc(2)-C(3) had corresponding changes in
Corresponding changes can be also seen in RO(8)-H(13) and RO(IO)-H(12)- This
739
displayed the transferring of acidic site proton. The changes of C-C and 0-H bond length verified that the isomerization of 1-hexene to 2-hexene is the shifting of double bond and transferring of acidic site proton. Table 1 Geometry parameters of R, C, RC, TS, PC and P
R rc(i)-a2) rC(2)-C(3) rC(3)-C(4) rC(4)-C(5) rC(5)-C(6)
C
0.1331 0.1484 0.1515 0.1514 0.1507
rC(l)-H(13) rC(3)-H(12) 0C(1)-C(2)-C(3) 0C(2)-C(3)-C(4) 6c(3)-a4)-C(5)
Qc(4)-a5)-a6)
0.1124 123.38 112.19 111.22 111.50
rSi(7)-0(8) rO(8)-AI(9) rAI(9)-O(i0)
ro(io)-si(ii) rO(8)-H{I3)
0.1737 0.1824 0.1974 0.1805 0.0948
rO(10)-H(12) Qsi(7)-0(8)-AI(9) 6o(8)-Al(9)-O(10) 6AI(9)-O(10)-Si(ll) 0A1(9)-O(1O)-H(12) 6n(12)-O(10)-Si(in
97.29 117.83 98.52 121.63 130.91
RC
TS
PC
0.1333 0.1483 0.1516 0.1515 0.1507 0.2372 0.1130 123.96 113.24 113.81 112.89 0.1804 0.1972 0.1828 0.1738 0.0955 0.2426 98.26 113.76 97.16 110.04 124.71
0.1381 0.1394 0.1486 0.1520 0.1507 0.1628 0.1603 126.01 120.37 113.27 112.24 0.1790 0.1909 0.1902 0.1888 0.1040 0.1055 98.18 109.71 98.13 127.21 128.25
0.1475 0.1338 0.1483 0.1516 0.1507 0.1123 0.2416 124.09 123.41 112.58 112.22 0.1738 0.1828 0.1972 0.1804 0.2441 0.0955 97.16 114.51 98.27 120.26 129.85.
_P 0.1476 0.1336 0.1482 0.1517 0.1507 0.1118 123.90 123.65 111.63 111.37
r: nm, 6: ° The second force constant matrix was calculated to verily the transition state.
Its eigenvalue has the only
virtue frequency (752.20 cm"').
Fig. 2 gave the
vibrational mode of TS at the virtue frequency.
The
vibrational directions indicated that the reactant was 1 hexene and the product was 2-hexene.
^^
This result
confirmed the calculated transition state belonged to the isomerization path of 1-hexene to 2-hexene.
In order
to further analyze the reaction mechanism the intrinsic
,
^
^' pjg 2. Vibrational mode of the TS virtue frequency
740
reaction coordinates were calculated from transition state to reactant and the product, respectively. Fig. 3 is the energy curve of the intrinsic reaction path. The left site and right site represented the energy of RC and PC, respectively. This also indicated the optimized saddle point was the transition state of hexene isomerization. According to the calculation results the adsorption energy of R to C or P to C was small. The activation energy from RC to TS was 159.810 kJ/mol, and that from PC to TS was 177.707 kJ/mol. The results were close to the experiment report.^ The energy barrier of the catalytic rearrangement is relatively low so that the 1hexene is easy to form 2-hexene on the surface of molecular sieve. In order to improve the calculation accuracy, the single energy correction of the reactant, product and transition state has been done at the level of ab initio HF/6-31G** and DFT B3LYP/6-31G**. Table 2 gives the relative energies of reactant, product t/SQRT(AMU)'BOHR and transition state. The results calculated with DFT Fig. 3. The energy curve in IRC path method accorded with AM 1 method. Table 2 Relative energies of R+C, RC, TS, PC and P+C / kJ.mof AMI HF/6-31G** B3LYP/6-
R+C
RC
TS
PC
P+C
0.485 17.047 30.813
0 0 0
159.810 244.957 161.363
-17.898 -20.355 -20.587
-14.287 -0.206 10.602
4. CONCLUSION The calculation results using AMI semiempirical molecular orbital method show that 1hcxene could be transferred to 2-hexene over aluminosilicate molecular sieve.
The 1-hexene
first adsorbed on the surface of catalysts, and then formed 2-hexene through a shift of double bond after overcoming a energy barrier with 160 kJ/mol. REFERENCES 1. X.-J. Li, J. H. Onuferko, and B. C. Gates, J. CataL, 1984, 85, 176 2. A. K. Talukdar, K. G. Bhattacharyya, T Baba, Y. Ono, Appl. Cata. A: Gen., 2001, 213,239 3. K. Dallmann, R. Buffon, J. Mol. Catal A-Chem., 2001, 172 : 81 4. M. Neurock, R. A. van Santen, Catal. Today, 1999, 50, 445 5. D. M. Brower, J. Catal, 1962, 1, 22 6. A. Coma, A. Lopez A, I. Nebet, J. Catal, 1982, 77, 159 7. J. J. P Stewart, J. Comp. Aided Mol. Design, 1990, 4, 1 8. M. C. Clark, B. Subramaniam, AICHE J., 1999, 45, 1559
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
741
Isomerization and hydrocracking of n-decane over Pt/MCM-41//MgAPO-n composite catalysts S.P.Elangovan^ and Martin Hartmann* University of Kaiserslautem, Department of Chemistry, Chemical Technology, P.O. Box 3049, D-67653 Kaiserslautem, Germany The hydroconversion of «-decane has been investigated over Pt/MCM-41//MgAP0-n (n = 5, 11) composite catalysts. A high selectivity for w-decane isomerization has been observed over a 50 : 50 physical mixture of 1.0 Pt/MCM-41 with MgAPO-11, while hydrocracking is dominant over the catalysts composed of Pt/MCM-41 and MgAPO-5. The high isomer selectivity of the Pt/MCM-41//MgAPO-11 catalyst is explained in terms of shape selectivity. 1. INTRODUCTION Bifiinctional metal/acid zeolite catalysts are used in various industrial processes, viz. isomerization of C5/C6 alkanes, dewaxing and isomerization of Cg aromatics. Platinum and palladium ion-exchanged zeolites are known to give high isomerization yield at medium conversion. However, due to faster cracking of the branched isomers, hydrocracking becomes dominant at high conversion levels. The exact value of the isomerization maximum is expected to be dependent on the balance between the two catalytic functions, viz. the density and the strength of the Bronsted acid sites and the nature, amount and dispersion of the metal. Weisz [1] already showed in 1962 that the two catalytic functions can be well separated and e.g. two different supports can be used for the acid and the metal function. However, only little attention has been given to the use of these type of composite catalyst in alkane isomerization [2-3]. In the present study, the mesoporous molecular sieve MCM-41 was employed as a support for the metal clusters, while MgAPO-5 and MgAPO-11 were tested as acid functions. 2. EXPERIMENTAL MCM-41 was synthesized using tetradecyltrimethylammonium bromide (C^TMABr) as described elsewhere [4]. MgAPO-5 and MgAPO-11 were synthesized as described in our previous publication [5]. The materials were characterized by X-ray powder diffraction (XRD), nitrogen adsorption and atomic absorption spectroscopy (AAS). The density and strength of the acid sites of MgAPO-5 and MgAPO-11 were studied by temperatureprogrammed desorption of pyridine. The metal incorporation was carried out by impregnation with Pt(NH3)4Cl2 to obtain catalysts with a 2.0, 1.0, 0.67 and 0.5 wt.-% of platinum. The two catalytic functions were physically mixed, pressed without binder, crushed and sieved to obtain particles with a diameter of 0.25 to 0.355 mm. The catalytic experiments were carried out in a fixed-bed flow-type apparatus at a hydrogen pressure of 1 MPa. The pressure of the feed hydrocarbon was adjusted to 10 kPa. The modified residence time W / F„.decane amounted to 400 g h m o l " ' . The conversion was varied by varying the reaction temperature.
*- To whom correspondence should be addressed. Fax : +49 631 205 4193 E-mail: hartniann(Q)jhrk.uni-kl.de f- On leave from Sri Venkateswara College of Engineering, Pennalur-602 105, Tamil Nadu, India.
742
3. RESULTS AND DISCUSSION The MgAPO-5 apd MgAPO-11 microporous molecular sieves possess specific surface areas of 380 and 230 m^/g, respectively. The nMg/ HAI ratio was determined by AAS to 0.02 and 0.01 for MgAPO-5 and MgAPO-11, respectively. The results show that the AEL structure (MgAPO-11) has a lower capacity to incorporate magnesium compared to the API structure (MgAPO-5). Similar findings have been reported in a recent study [6]. The substitution of aluminum by magnesium in the neutral AlP04-n framework creates a negative charge in the framework that is balanced by a bridging proton imparting Br0nsted acidity in the MgAPO-n samples. The density and the strength of these acid sites has been probed by temperature-programmed desorption of pyridine (not shown). Both samples exhibit a sharp maximum at ca. 150 °C, while for MgAPO-5 a second maximum at 500 °C with a shoulder at 580 °C is observed. The low temperature peak appears at the same position for AIPO4-5 and AIPO4-II and has been assigned to surface hydroxyls [7]. The high-temperature peak shows a broad distribution of Br0nsted acid sites of different strength. The absence of such a high-temperature peak in MgAPO-11 is probably related to the lower magnesium content of the sample. 30 -^>-•--m~ -•H V^
MgAPO-5 0.5PVMCM-41 0.67 Pt/MCM-41//MgAPO-5 (25:75) ; 2.0 Pt/MCM-41//MgAPO-5 (25:75) : 1.0 Pt/MCM-41//MgAPO-5 (50:50)
20
2
0)
CD
E o
i 2 10
o6-uO'tt^tTl»» 200
250
300
350
Reaction temperature / **C
400
200
250
300
350
400
Reaction temperature / °C
Fig. 1. Conversion of n-decane (left) and isomer yield (right) as a function of the reaction temperature over different Pt/MCM-41//MgAPO-5 composite catalysts. Figure 1 (left) shows the conversion of Ai-decane over bifunctional composite catalysts composed of Pt/MCM-41 as the metal function and MgAPO-5 as the acid function, denoted as Pt/MCM-41//MgAPO-5, mixed in different ratios without changing the overall metal content. For comparison, the monofunctional catalysts 0.5Pt/MCM-41 and MgAPO-5 are also displayed. In comparison to the monofunctional catalysts, the bifunctional composite catalysts show significantly higher catalytic activity. For the composite catalysts, Az-decane conversion starts at 200 °C and almost complete conversion is reached between 330 and 350 °C. The catalytic activity increases with increasing number of acid sites, viz. with increasing fraction of MgAPO-5 in the composite catalyst. At a reaction temperature of 300 °C, the conversion of Ai-decane over the catalyst 2.0Pt/MCM-41//MgAPO-5(25:75) amounts to 65 %, while for 1.0Pt/MCM-41//MgAPO-5(50:50) and 0.67Pt/MCM-41//MgAPO-5(75:25) the conversion
743
reaches ca. 55 and 30 %, respectively. Over the monofunctional catalyst Pt/MCM-41, the conversion is below 5 %. However, only a low isomer yield (Yjso. < 10 %) is found (Figure 1 (right)), when MgAPO-5 is employed as the acid function. A similar low isomer yield is also observed for 0.5Pt/MgAPO-5, which is ascribed to fast cracking of the formed multibranched isomers [5]. In Figure 2 (left), the results of «-decane conversion over the Pt/MCM41//MgAP0-ll composites are depicted. The Pt/MCM-41// MgAPO-11 composites exhibit almost similar catalytic activity as compared to composites containing MgAPO-5, but a significantly higher isomer yield of above 35 % (Figure 2 (right)) is observed. The experimental results are in complete agreement with the well known reaction mechanism for isomerization and hydrocracking of «-alkanes [8]. With increasing conversion, the isomer yield passes through a maximum indicating that the branched hydrocarbons are consumed by hydrocracking. In essence, the n-decane feed is isomerized (i.e. branchings are created) until a configuration is reached at which hydrocracking (i.e. cleavage of carbon-carbon bonds, so called ionic p-scission) occurs. 100 90
r W/F
80
Pn2
0.5Pt/MCM-41 2.0 Pt/MCM-41//MgAPO-5 (25:75) ; 0.67 Pt/MCM-41//MgAPO-5 (75:25) ; 1.0 Pt/MCM-41//MgAPO-5 (50:50) '
= 400 = 1 MPa
70 h 60
/
50 h
i
E o
40 c
o O
Q)
W 20
30 h 20 10 Oii P < H | » m m • • ^ • • 200 250 300
0(!M;M?4 350
Reaction temperature / °C
400
250
300
350
400
Reaction temperature / °C
Fig. 2. Conversion of w-decane (left) and isomer yield (right) as a function of the reaction temperature over different Pt/MCM-41//MgAP0-ll composite catalysts. Table 1 summarizes the cracked product distribution expressed in moles of cracked products formed per 100 mol of Ai-decane. The distribution of the cracked products, in particular the low selectivity for methane and ethane is indicative of ideal hydrocracking with an ionic mechanism of cleavage. Pure primary cracking occurs when for 100 moles feed cracked the number of moles formed does not exceed 200 [9]. Over Pt/MCM-41//MgAPO-5 composites only a low selectivity to methane and ethane is not observed, which rules out hydrogenolysis on the platinum clusters. Table 1 also depicts the amount of branched isomers in the C4 and C5 fraction the composite catalysts tested in this study. For composites containing MgAPO-5, the C4 and C5 products are mainly branched and their relative amount changes only slightly with the degree of conversion. In contrast, over MgAPO-11 composites mainly ^-isomers are formed at low conversion level, while the formation of branched isomers increases with conversion. The observed high selectivities for linear cracking products are probably due to hydrocracking of methylnonanes via type C p-scission [10]. Over MgAPO-5 composite catalysts, ca. 70% of the cracked products are formed via type A P-scission, which accounts for the high selectivities for branched isomers [11]. However, the
744
low selectivity for branched isomers observed over composites containing MgAPO-11 may be attributed to contributions of type B (Bl and B2) and type C p-scission. Table 1 Distribution of the cracked products at a cracking yield of Ycr. of 15 % 0.67Pt/ l.OPt/ 2.0Pt/ 0.67Pt/ Catalyst 2.0Pt/ l.OPt/ MCM-41// MCM-41// MCM-41// MCM-41// MCM-41// MCM-41// MgAPO-5 MgAPO-5 MgAPO-5 MgAPO-11 MgAPO-11 MgAPO-11 (25:75) (50:50) (75:25) (75:25) (25:75) (50:50) Number of moles/100 moles cracked 11 7 21 15 3 0 Ci 7 2 8 5 6 0 C2 57 62 50 52 33 9 C3+C7 76 90 84 102 75 106 C4+C6 39 84 39 52 65 46 Cs 5 4 1 0 4 5 Cs 7 4 1 6 1 0 C9 206 216 217 209 207 199 Total % iso in 66.8 48.2 27.5 79.9 10.9 6.5 C4 48.7 86.8 75.3 31.8 8.7 14.5 Cs 4. CONCLUSIONS The hydroconversion of w-decane has been studied over bifunctional composite catalysts containing Pt/MCM-41 as the metal function and a microporous aluminophosphate MgAPO-5 and MgAPO-11, respectively, as the acid function. It follows from these studies that the two catalytic functions can be tuned separately to obtain well balanced catalyst in order to achieve high selectivities for isomerized or cracked products. The use of the 12-membered ring aluminophosphate MgAPO-5 (dp = 0.73 nm) results in isomer yields below 10 %, while with MgAPO-11 (pore diameter of 0.39 x 0.63 nm) a maximum isomer yield of ca. 45 % is achieved at a reaction temperature of 300 °C. Over MgAPO-11, the formation of dibranched isomers is suppressed, which results in a reduced cracking rate and, hence, in a higher isomer yield.
REFERENCES 2. 3. 4. 5. 6.
8. 9. 10 11
P.B. Weisz, Adv. Catal., 13 (1962) 137. R. Parton, L. Uytterhoeven, J.A. Martens and G.F. Froment, Appl. Catal., 76 (1991) 131. G. Kinger, D. Majda and H. Vinek, Appl. Catal., 225 (2002) 301. M. Hartmann, S. Racouchot and C. Bischof, Microporous Mesoporous Mater., 27 (1999) 309. M. Hartmann and S.P. Elangovan, Chem.-Ing.-Tech., accepted. R. Fernandez, M.V. Giotto, H.O. Pastore and D. Cardoso, Microporous Mesoporous Mater., 53(2002)135. J. Das, V.V. Satyanarayana, D.K. Chakraborthy, S.N. Piramanayagam and S.N. Shrinji, J. Chem. Soc, Faraday Trans., 88 (1992) 3255. J. Weitkamp, Ind. Eng. Chem. Prod. Res. Dev., 21 (1982) 550. J.A Martens and P.A. Jacobs, Zeolites 6, (1986) 334. J. Weitkamp, P.A. Jacobs and J.A. Martens, Appl. Catal., 8 (1983) 127. J. Weitkamp, P.A. Jacobs and J.A. Martens, Appl. Catal., 20 (1983) 283.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
745
Catalytic Properties of Mesoporous Aluminosilicates and Lanthanum Containing Mesoporous Aluminosilicates studied by w-Xylene Isomerisation Martin Wallau, Rogerio A. A. Melo* and Ernesto A. Urquieta-Gonzalez^' ^ Departamento de Engenharia Quimica, Universidade Federal de Sao Carlos, Caixa Postal 676, 13565-905 Sao Carlos - SP, Brasil Mesoporous aluminosilicates were synthesised with and without lanthanum. Their acidic properties were characterised by TPD of ammonia and by the isomerisation of w-xylene and the results compared with those obtained for a microporous ZSM-5 zeolite. It was found that the catalytic activity of the mesoporous molecular sieves increases with increasing aluminium content. On the other hand , the deactivation of the lanthanum containing catalysts was lower. However, ZSM-5 was more active and showed only slight deactivation. 1. INTRODUCTION Ordered mesoporous (alumino)silicates, firstly described in the early nineties, possess pore diameters in the range of 20 to 100 A and they are therefore discussed as an alternative to microporous zeolites for the catalytic transformation of bulky substrates [1]. However, the catalytic activity and stability observed for mesoporous aluminosilicates are lower than that observed for zeolites [1]. It is known that the presence of lanthanum cations improves the stability and the catalytic properties of zeolites [2]. Here we will describe the catalytic properties of mesoporous aluminosilicates and lanthanum containing mesoporous aluminosilicates using the isomerisation of m-xylene as a test reaction. 2. EXPERIMENTAL SECTION The mesoporous aluminosilicates (Al-MCM-41; sample I-III) and lanthanum containing mesoporous aluminosilicates (La/Al-MCM-41; sample IV-VI) were prepared under autogeneous pressure (T = 373 K; t = 240 h), using fumed silica and sodium trisilicate hydrate as silicate sources, tetramethylammonium hydroxide pentahydrate (TMAOH) and cetyltrimethylammonium bromide (CTMABr) as mineraliser and template, respectively, and aqueous sodium aluminate and lanthanum chloride. The molar composition of the synthesis gels is given by equation 1, and x and y can be found in Table 1. 1 Si02 : 0,07 NazO : 0,27 CTMABr : 0,14 TMAOH : 100 H2O : x AI2O3 : y LazOj (1) The obtained solids were treated firstly under nitrogen flow at 393 K for 3 h and subsequently calcined under flow of dry air at 813 K for 12 h. The calcined Al-silicates (I-III) were exchanged three times for 8 hours with NH4CI (1 mol/L) and subsequently calcined at 773 K in air for 5 h. The La/Al-silicates (IV-VI) were treated with HCl (0.1 mol/L) instead of NH4CI and then dried at 383 K.
Present address: Centro Universitario do Sul de Minas, Faculdade de Engenharia Quimica, Av. Cel. Jose Alves, 37010-540 Varginha - MG, Brasil ^ corresponding author: FAX: +55-16-260-8266. E-mail: urquieta@power.ufscar.br ' Financial support by CNPq (proc. 461444/00-3; 300373/01-5) and FAPEMIG (proc. TEEC - 1241/01).
746
The samples were characterised by Table 1 X-ray diffraction, ^^Si and ^^Al MAS Al- and La-content of the synthesis gels. NMR spectroscopy, nitrogen sorption, Sample x AI2O3 y La203 elemental analysis, temperature0,008 (I) programmed desorption of ammonia 0,017 (11) (NH3-TPD) and isomerisation of (III) 0,050 m-xylene. The catalytic properties of the 0,005 (IV) 0,050 studied MCM-41 were determined in a 0,019 (V) 0,050 fixed bed reactor with a WHSV of 4 h'' 0,038 (VI) 0,050 at 623 K and compared to that obtained for a H-ZSM-5 (Si/Al = 35) under the same conditions. Indexing the observed XRD peak in hexagonal symmetry, the unit cell parameter could be calculated from the dioo interplanar distance as: ao = 2dioo/3''^"^. The specific surface area (SBET) and the pore diameter (dp) were determined from the nitrogen sorption using the BET and BJH method, respectively. The activity factor was calculated as Fa = (10 • WHSV • %XTmax) / FW, where %XTmax is the m-xylene conversion observed after 10 minutes on stream and FW the formula weight of m-xylene. 3. RESULTS AND DISCUSSION The X-ray patterns of sample II, III and VI are shown in Fig. 1. The patterns show very broad peaks around 2 and 4°(20) for the Al-MCM-41 and only one peak with relative low density for the La/Al-MCM-41. Although the little number, low Sample III intensity and broadening of the X-ray Sample II peaks are usually attributed to a low order of the mesopore structure, such conclusions might be erroneous for MCM-41 [ 3 , 4 ] . E, g. the decreased XRD intensity observed for Sample VI might be explained by the enhance of the radiation adsorption by the relative high lanthanum loading (5 %) in this sample. Also the decrease of the specific surface area (SBHT) with increasing Al- and La-content, which 20(°) can be observed from Table 2, cannot Fig. 1. XRD pattern of calcined Alnecessarily be attributed to a less (sample II, III) and La/Al-MCM-41 ordered structure. It is obvious that the (sample VI). pore diameter, the thickness and the density of the pore walls must be considered for the comparison of the specific surface area of different MCM-41 samples. The unchanged content of aluminium after ion exchange indicates its incorporation into the framework structure, which is confirmed by the NMR results presented elsewhere in these proceedings [5]. The data presented in Table 2 reveal further a low exchange degree of the sodium cations. Here one should keep in mind that the negative framework charge of the as made mesoporous solids are also charge balanced by the organic surfactant molecules. Their ammonium groups act during the calcination process as proton source, so that the calcined mesoporous Al-silicates will partially be in the proton exchanged form. Further one should consider the greater wall thickness of the mesoporous silicates in comparison to that of zeolites. Estimating the wall thickness as the difference between unit cell parameter ag and pore diameter dp, values around 2.0 nm can be calculated from the data given in Table 2. Assuming a constant Si/Al-ratio through these wall cross-sections, one would expect that approximately one half to three fifth of the aluminium sites would be placed in the wall core
747
instead of the wall surface and therefore not accessible for ion exchange. It should be emphasised that for the Al-silicates studied here, the Na/Al-ratio after ion-exchange varies between 0.45 and 0.55 which would agree with approximately one half to three fifth of the aluminium incorporated in inaccessible sites in the wall core. On the other hand, the pronounced decrease of the lanthanum content after ion exchange clearly indicates its presence on extra-framework sites as it is discussed in more details elsewhere [5]. Table 2 Physico-chemical properties of the calcined mesoporous solids Normalised sum formula of the solid dp* before after Sample SBJRT* [nm] [nm] [mVg] ion exchange ion exchange n.d. 29.2 1054 Nao.014sio.98Alo.0202 Nao.011Sio.98Alo.02O2 (I) 45.2 27.2 910 Nao.027sio.96Alo.0402 Nao.022Sio.96Alo.04O2 (II) Nao.078sio.90Alo.1002 (III) 49.3 28.3 900 Nao.036Sio.92Alo.08O2 (IV) n.d. 24.2 832 Lao.oo 15Nao.06Sio.9Alo. 102 Lao.0007Nao.03Sio.9Alo. 102 Lao.013Nao.05Sio.92Alo.08O2 Lao.oo 1 Nao.02Sio.93Alo.07O2 (V) n.d. 24.4 853 Lao.020Nao.04Sio.94Alo.06O2 Lao.004Nao.03Sio.95Alo.05O2 iVTL, 48.5 25.6 715 *determined before ion exchange. 60
A \
V ^
-•-Sample (1) -«v Sample (III) -D-Sample (V) -•-H-ZSM-5(35)
- ^ Sample (II) -^Sample (IV) • o Sample (VI)
S" 1 0,4.
c
8v c
k 420
495 570 645 720 795 870 945 1020 1095 Temperature [K]
Fig. 3. NHi-TPD of selected catalysts.
1 0-
- -I 50
l
l
100 Time on stream [min]
150
Fig. 4. Conversion of m-xylcnc catalysed by Al , La/Al-MCM-41 and H-ZSM-5.
Due to clarity only selected patterns are shown in Fig. 3, which illustrates the results of the NH3-TPD, which are also summarised in Table 3. The TPD results reveal that the studied mesoporous Al-silicates possess less acidic sites than H-ZSM-5 and for the mesoporous Alsilicates, the number of the acidic sites increases with increasing aluminium content while their strength decreases. The comparison between sample (II) and sample (VI), which possess similar aluminium content and number of acid sites, reveal that the presence of lanthanum decreases markedly the strength of the catalytic sites. Following the work of Kosslick et al. [6] one can attribute for sample (I) and (II) strong Lewis sites, and for sample (III) to (VI), as well as to H-ZSM-5, Bronsted sites of different strength. However, one should consider that extra-framework lanthanum cations in sample (V) and (VI) can act as Lewis acid sites. Fig. 4 reveals that the initial activity of the aluminium rich mesoporous Al-MCM-41 (sample III) and La/Al-MCM-41 (sample IV) are slightly higher than that of the ZSM-5 zeolite, however, they deactivate more rapidly. It is further observed that after approximately one hour on stream the aluminium rich Al-silicate and the La/Al-silicates show in spite of their different aluminium content nearly the same activity, thus suggesting that the presence of extra-framework lanthanum cations enhances the catalytic stability of these catalysts.
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The isomerisation of w-xylene can occur Table 3 by two mechanisms: (a) monomolecular Acidic and catalytic properties of A1-, isomerisation and (b) bimolecular La/Al-MCM-41 andH-ZSM-5. transalkylation. Due to steric hindrance esorpTe NH3 bimolecular transalkylation will not occur desorbed mp. , [mmol-h''g''] sample over microporous ZSM-5 and xylene [mmol/g] [Kl isomerisation proceeds exclusively through 035 0.36 920 ~) methyl shift of the intermediate benzenium 0.64 0.63 816 (11) ion. In contrast, Morin et al. [7] found that 2.24 1.0 603 (III) over mesoporous MCM-41 xylene is 1.81 n.d. 663 (IV) isomerised exclusively by successive 1.51 n. d. 693 (V) bimolecular transformations: xylene 1.39 0.67 650 (VI) disproportionation to trimethyl- benzenes H-ZSM-5 1.71 1.03 744 (TMB) and toluene followed by transalkylation between TMB and the xylene reactants. AI-MCM-41 (Sample I-III) and La/Al-MCM-41 with low lanthanum content (Sample IV) contain principally acid sites. On these sites benzylic carbocations are created, which may form a cationic transition state by electrophilic attack of xylene. This intermediate may disproportionate to TMB and toluene but also form coke by electrophilic attack of further xylene molecules, thus explaining the observed deactivation. On La/Al-MCM-41 with increased lanthanum content CH3) (Sample V and VI) the La cations on ion exchange sites create Lewis acid/Bronsted basic (L/B) pairs. Here a anionic transition state may be created by nucleophilic attack of a benzyl anion, formed by proton abstraction on the Bronsted basic site, on xylene nuclei polarized by the Lewis acid (La'^^) schematised in scheme 1. This transition state will not attack xylene, thus avoiding deactivation by coke formation. Scheme REFERENCES 1. A. Corma, V. Forncs, M.T. Navarro, J. Pcrcz-Paricntc, J. Catal., 148 (1994) 569. 2. P.B. Venuto, Microporous Mater., 2 (1994) 297. 3. B. Marlcr, U. Oberhagemann, S. Vortmann, H. Gics, Microporous Mater., 6 (1996) 375. 4 .S. Schacht, M. Janickc, F. Schuth, Microporous Mesoporous Mater., 22 (1998) 485. 5. M. Wallau, R.A.A. Melo, E.A. Urquieta-Gonzalez, Stud. Surf Sci. Catal., in press. 6. H. Kosslick, G. Lischkc, B. Parlitz, W. Storck, R. Fricke, Appl. Catal. A, 184 (1999) 49. 7. S. Morin, P. Ayrlault, S. E. Mouahid, N. Gncp, M. Guisnct, Appl. Catal. A, 159 (1997) 317.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
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Diels-Alder reaction catalyzed by ordered micro- and mesoporous silicates Yoshihiro Kubota, Hiroyuki Ishida, Ryo Nakamura and Yoshihiro Sugi Department of Chemistry, Faculty of Engineering, Gifb University, Gifli 501-1193, Japan Ordered micro- and mesoporous silicates such as mordenite, Y-zeolite, zeolite beta, and MCM-41 in their proton forms effectively enhance the reactivity and stereoselectivity in the Diels-Alder reaction of cyclopentadiene with 3-buten-2-one (MVK). All-silica periodic mesoporous materials such as [Si]-MCM-41 and FSM-16 also significantly promoted the reaction. 1. INTRODUCTION High-silica, ordered micro- and mesoporous materials are recognized to be extremely useful as catalysts and sorption materials because they have hydrophobic pores with uniform size, large surface area, large adsorption capacity, and high thermal stability [1,2]. From the environmental points of view, silica-based solid catalysts could be utilized for recyclable, waste-minimum, non-hazardous, and energy-minimum reaction systems since they can be readily separated and recovered. The Diels-Alder reaction is a cycloaddition of a 1,3-diene with a dienophile such as a,P-unsaturated carbonyl compounds [3]. It is well documented that this reaction is catalyzed and controlled by homogeneous Lewis acids [4-7]. In addition, it has been reported that this reaction is accelerated by the effect of 'hydrophobic interaction' in aqueous media [8-10], by use of adsorbent [11,12] or inorganic solid catalysts [13-17], and by encapsulation inside organic hosts [18,19]. The rate-acceleration in the acid-catalyzed reaction is due to the lowering of LUMO of dienophile by acids [5]. In the case of adsorbent, on the other hand, some speculations on the rate enhancement by adsorbent are given although the exact mechanism of the promotion effects is not well understood [12]. Concerning the silica-based solids, there are a few reports on the catalysis by modified clays [13,14], Y-zeolite [15], and MCM-41 [13], in which the main interest is introducing metal cations such as Fe^^, Cr^"^, Cu^, and Zn^^ to give Lewis acidity. Very recently, the Diels-Alder reaction of anthracene with p-benzoquinone over different types of aluminosilicate MCM-41 (Al-MCM-41) was reported and it was suggested that the main active sites on Al-MCM-41 are Lewis acid sites [17]. In spite of this knowledge, some information is still missing. Firstly, only little attention has been paid to the effect of high-silica large-pore (= 12-ring) zeolites such as beta. Secondly, the promoting effect of all-silica mesoporous material has not been carefully examined. Therefore, in the present work we planned to examine the catalytic performance of some large-pore zeolites as well as MCM-41 for the reaction of cyclopentadiene (1) with 3-buten-2-one (MVK; 2) Micro- and
(Eq. 1), and also to examine the effect of all-silica materials on the same reaction.
o
^y^ ^
||
Se"""'
C0CH3
Q 2
C0CH3 3-endo
3-exo
(1)
750
2. EXPERIMENTAL 2.1. Materials The catalysts used in this work are large-pore microporous silicates such as mordenite (MOR), Y-zeolite (FAU), zeolite beta (BEA), SSZ-24 (AFI), ZSM-12 (MTW), and periodical mesoporous silicates such as MCM-41 and FSM-16. Mordenite as H-form (denoted H-MOR) and Y-zeolite as Na-form (Na-FAU) are supplied from Tosoh Company (HSZ-690HOA) and The Catalysis Society of Japan (reference catalyst; JRC-Z-Y5.3), respectively. Na-MOR was prepared by treating the H-MOR in a 0.34 mol dm"' NaCl solution (50 cm per 1 g of the zeolite) at room temperature for 48 h. H-FAU was prepared by treating the Na-FAU in a 1.0 mol dm'^ NH4NO3 solution (100 cm^ per 1 g of the zeolite) at 80°C for 2 h twice, followed by calcination at 550°C. BEA, AFI, MTW and MCM-41 were synthesized hydrothermally by known methods [20-24] and transformed into their H-forms (the same method as that for FAU was applied except for AFI [21]). All-silica MCM-41 (denoted [Si]-MCM-41) was synthesized from a starting gel of all-silica composition. FSM-16 synthesized from kanemite and hexadecylammonium chloride by typical method [24] was supplied from Toyota Central R&D Labs.,Inc. Davison #57 was used as amorphous Si02. All materials were characterized by powder X-ray diffraction (Cu-Ka radiation), N2 adsorption and elemental analysis. 2.2. Reaction procedures Typical reaction procedure is as follows: freshly distilled 1 (10.0 mmol) and 2 (5.0 mmol) were dissolved in hexane (5 cm^) at 0°C. Within one minute, 500 mg of solid catalyst was added to this mixture, and the whole mixture was stirred at 0°C. After 1 h, a solution of A^-phenylmaleimide (10 mmol) in chloroform was added to quench the reaction in order to inhibit further reaction during working-up (the overwhelmingly higher reactivity of A^-phenylmaleimide than that of 2 has been experimentally confirmed [25]). Filtration, evaporation of solvent, and purification by column chromatography gave the product as a mixture of endo and exo isomers. The structures were confirmed by ' H and '^C NMR. The endo/exo ratio was determined by GC, which was further confirmed by ' H NMR. 3. RESULTS AND DISCUSSION Talble 1 Diels-Alder reaction over micro-and mesoporous aluminosilicates Entry 1 2 3 4 5 6 7 8 9
Catalyst
SiOj/AljOj"
None Na-MOR '^ H-MOR' Na-FAU' H-FAU '^ H-BEA'* H-AFI '^ H-MTW '^ H-MCM-41^
128.0 128.0 5.2 10.0 105.2 90.4 87.0 67.4
Yield'of 3 / % 4 4 82 5 77 71 28 20 97
endo:exo ratio 85.6 : 14.4 95.6 :4.4 97.9:2.1 94.0 : 6.0 96.2 : 3.8 95.0 : 5.0 98.4: 1.6 86.5: 13.5 97.2 : 2.8
' Carried out as described in the text. Determined by ICP analysis. '^ Isolated yields. Ion-exchange has been done as described in the text. ^ JRC-Z-Y5.3 (Catalysis Soc. Jpn.). ' HSZ-690HOA (Tosoh Co.).
Table 1 shows the results of the reaction of 1 with 2 in hexane catalyzed by large-pore zeolite or MCM-41 (aluminosilicate version). The yield of adduct 3 was only 4% in the absence of catalyst (entry 1). The Na-MOR did not promote the reaction and only resulted in
751
a low yield of the adduct (entry 2), whereas the use of H-MOR gave the adduct in a good yield (entry 3). The same trend was observed upon using FAU catalysts (entries 4 and 5). These results mean that the materials having only sodium ion with no acidic proton as counter-cations show no rate-acceleration effect. Nitrogen adsorption measureme;it indicated that the porosity of Na-form zeolite in this work was similar to that of corresponding H-form zeolite, suggesting that the lack of activity with the use of Na-MOR or Na-FAU was mainly due to the absence of acid sites. H-BEA exhibited relatively high activity (entry 6), whereas H-AFI and H-MTW showed only low activity (entries 7 and 8). This suggests that larger cavity is preferable to catalyze this reaction. In fact, H-MCM-41 showed particularly high activity and 3 was formed in very high yield (entry 9). No loss of activity was observed for H-MOR as well as H-MCM-41 even at the third use, indicating that the catalysts are reusable. Although Br(t)nsted acids have not commonly been used for this reaction as homogeneous catalysts, we believe that the MOR, FAU and BEA are successfully working as Br(j)nsted acids. On the other hand, H-MCM-41 should be mainly working as Lewis acid [17,26,27]. It is generally known that acid causes not only rate-acceleration but also enhancement of endo/exo ratio due to the increased secondary interaction between HOMO (diene) and LUMO (dienophile) [5,28]. In light of this fact, it is reasonable that the endo/exo ratio increased in the case that the reaction was facilitated by the effect of protonated porous silicates. It is interesting to note that the endo/exo ratio was relatively high even when Na-MOR or Na-FAU was used, although reaction was not accelerated at all in either case (entries 2 and 4). This observation is consistent with the results in ref 16. One possible explanation is that confinement of substrates inside pores or adsorption of them on non-acidic surface of the catalyst could alter the geometry of substrates or transition states, affecting the stereoselectivity, even if the reaction was not facilitated by acid. Particularly high endo/exo ratio given by H-AFI and low ratio by H-MTW are still unclear. It was surprising that all-silica mesoporous materials showed comparable or higher activity than protonated zeolites. Figure 1 shows the time-course of the yield of adduct 3 formed by using [Si]-MCM-41, FSM-16 and amorphous Si02. Positive effect was observed for every silicate, suggesting that weak acidity of silanol can promote the reaction. The presence of silanols in every silicate was confirmed by IR and Si MAS NMR spectroscopy. Significantly higher reaction rate and endo/exo 100ratio were observed for [Si]-MCM-41 and FSM-16 than for amorphous Si02, indicating the superiority of periodical mesoporous structure. The difference in BET surface area (5BET) does n ot seem to o 50H be the only reason for the large difference in catalytic performance. It is suggested that the confinement 14.4) of substrates inside pores positively affect the reaction rate. Further investigation on the larger effect by mesoporous silica than amorphous Fig. 1. Time course of the reaction catalyzed by pure-silica materials, silica is in progress. (a) [Si]-MCM-41 {S^^j =1013), (b) FSM-16 (S^^:J = 1062), (c) SiOj (S^i:r = 370), and (d) No catalyst.
752 4. CONCLUSIONS Ordered micro- and mesoporous silicates, such as MOR, FAU and BEA as well as MCM-41 in their protonated aluminosilicate forms, exhibited significant enhancement effects on the yield and stereoselectivity in the Diels-Alder reaction of cyclopentadiene with 3-buten-2-one (MVK). The effects were larger in non-polar media. Periodical mesoporous silicates had significant effect for promoting the reaction even when all-silica composition. Superiority of periodical structure over amorphous was clearly observed. Although introduction of stronger Lewis acid sites should be necessary in order to catalyze the reaction of less reactive substrates, the periodical structure of silica should play some positive role when utilized as a reaction field. The information could be useful for the development of reusable heterogeneous catalysts for the syntheses of fine-chemicals under mild conditions in appropriate organic media.
REFERENCES 1. S.I. Zones and M.E. Davis, Curr. Opin. Solid State Mater. Sci., 1 (1996) 107. 2. D. Zhao, P. Yang, Q. Huo, B.F. Chmelka and G.D. Stucky, Curr. Opin. Solid State Mater. Sci., 3 (1998)111. 3. W. Oppolzer, ''Comprehensive Organic Synthesis,'' ed by B.M. Trost, Pergamon Press, Oxford (1991),Vol. 5, pp. 315-399. 4. R. Yates and P Eaton, J. Am. Chem. Soc, 82 (1962) 4436. 5. K.N. Houk, R.W. Strozier, J. Am. Chem.Soc, 95 (1973) 4094. 6. PV. Alston and R.M. Ottenbrite, J. Org. Chem., 40 (1995) 1111. 7. S. Kobayashi, I. Hachiya, M. Araki and H. Ishitani, Tetrahedron Lett., 34 (1993) 3755. 8. D.C. Rideout and R. Breslow, J. Am. Chem. Soc, 102 (1980) 7816. 9. W. Blokzijl, M.J. Blandamer and J.B.F.N. Engberts, J. Am. Chem. Soc, 113 (1991) 4241. 10. F Fringuelli, O Piermatti, F Pizzo, L. Vaccaro, Eur. J. Org. Chem., (2001) 439. 11. V.V. Veselovsky, A.S. Gybin, A.V. Lozanova, A.M. Moiseenkov and W.A. Smit, Tetrahedron Lett., 29(1988) 175. 12. R.D. Weinstein, A.R. Renslo, R.L. Danheiser, J.W. Tester, J. Phys. Chem. B, 103 (1999) 2878. 13. M. Onaka and R. Yamasaki, Chem. Lett., (1998) 259. 14. P Laszlo, Ace. Chem. Res., 19 (1986) 121. 15. J. Ipaktschi, Z. Naturforsch., 41b (1986) 496. 16. H.M. Najafi, M. Ghandi, F Farzaneh, Chem. Lett., (2000) 358. 17. T. Kugita, M. Ezawa, T Owada, Y Tomita, S. Namba, N. Hashimoto, M. Onaka, Micropor. Mesopor. Mater., 44-45 (2001) 531. 18. J. Kang and J. Rebek Jr., Nature, 385 (1997) 50. 19. K. Endo, T Koike, T. Sawaki, O. Hayashida, H. Masuda and Y Aoyama, J. Am. Chem. Soc, 119 (1997)4117. 20. M.K. Rubin, US Patent, 5,164,169 (1992). 21. R.F Lobo and M.E. Davis, Micropor. Mater. 3 (1994) 61. 22. S. Ernst, PA. Jacobs, J.A. Martens and J. Weitkamp, Zeolites, 7 (1987) 458. 23. C.-Y Chen, H.-X. Li and M.E. Davis, Micropor. Mater., 2 (1993) 17. 24. S. Inagaki, A. Koiwai, N. Suzuki, Y Fukushima, K. Kuroda, Bull. Chem. Soc, Jpn., 69 (1996) 1449. 25. J. Sauer, H. Wiest and A. Mielert, Chem. Ben, 97 (1964) 3183. 26. S. Biz and M.G. White, J. Phys. Chem. B, 103 (1999) 8432. 27. R. Mokaya and W. Jones, J. Catal., 172 (1997) 211. 28. PV. Alston and R.M. Ottenbrite, J. Org. Chem., 40 (1975) 1111, and references cited therin.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved
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Isospecific polymerization of propylene with Metal-MCM-41 Yasunori Oumi^, Ayako Hanai^, Toyoaki Miyazaki^, Hiroyoshi Nakajima^, Satoru Hosoda*', Toshiharu Teranishi^ and Tsuneji Sano^ ^School of Materials Science, Japan Advanced Institute of Science and Technology, Tatsunokuchi, Ishikawa 923-1292, Japan ; E-mail: t-sano(q)Jaist.ac.ip ^'Sumitomo Chemical Co., Ltd., Sodegaura, Chiba 299-0295, Japan Polymerizations of propylene were conducted using various Metal-MCM-41 prepared by the post-synthesis method. Ti-, Zr-, Hf-, Mn- and Zn-MCM-41 combined with alkylaluminiums were found to give isotactic polypropylenes with wide molecular mass distributions. 1. INTRODUCTION Since the discover of ordered mesoporous silicas such as MCM-41, MCM-48, FSM-16 and SB A-15 synthesized using surfactants, much effort has been paid to incorporation of various metals into these structures for heterogeneous catalysis and adsorption. There are a large number of papers concerning incorporation of Ti and V as well as Al atoms into the framework of mesoporous silicas by the direct hydrothermal synthesis and the post-synthesis methods. It is well known that Ti- and V-containing mesoporous silicas are active for selective oxidation of a wide variety of organic substrates with the environmentally friendly oxidant [1-3]. However, up to now no information on olefin polymerization has been reported. Of course, there are several papers concerning grafting organometallic complexes onto the surfaces of the mesopores for olefin polymerization [4-6]. In this paper, we describe for the first time the high potential of Metal-MCM-41, especially Ti-, Zr- and Hf-MCM-41 for isotactic polymerization of propylene. 2. EXPERIMENTAL Various metal-containing MCM-41 (Metal-MCM-41, Metal:Al, Ti, Mn, Zn, Ga, Zr, HO were prepared by the post-synthesis method. The parent siliceous MCM-41 was prepared following the procedure described in the literature [7]. The siliceous MCM-41 was calcined at 500°C for 10 h to decompose the surfactant (hexadecyltrimethylammoinium bromide). 1 g of calcined MCM-41 was dried at 280°C for 24 h under vacuum and then dispersed in 10 ml of dry toluene containing 3 mmol of corresponding metal compound under nitrogen. The metal compounds used for the post-synthesis were A1(CH3)3, Ti(OC4H9)4, Mn(CH3COO)2, Zn(C2H5)2, Ga(CH3)3, Zr(OC4H9)4 and Hf(OC4H9)4. The mixture was kept for 48 h at room temperature (A1-, Zn- and Ga-MCM-41) or refluxed at 110°C (Ti-, Mn-, Zr- and Hf-MCM-41). The product was filtered, washed with dry toluene several times, dried at room temperature and then calcined at 500°C in air for 5 h. Polymerizations of propylene were conducted in a 100 cm^ stainless steel autoclave equipped with a magnetic stirrer. After the reactor was filled with nitrogen, measured amounts of toluene as a solvent, alkylaluminium and Metal-MCM-41 evacuated at 400°C for 8 h were added to the reactor and aged at room temperature for 15 min. The reactor was evacuated at
754
liquid nitrogen temperature, and then 7 dm^ of propylene were introduced. Polymerization was started by quickly heating the reactor up to the polymerization temperature (40°C). The polymerization reaction was terminated by adding acidified methanol. The resulting polymers were extracted with boiling o-dichlorobenzene for 8 h. The weight-average molecular weight (Mw) and molar mass distribution (Mw/Mn, Mn: number-average molecular weight) of the polymers were measured at 145°C by gel-permeation chromatography using o-dichlorobenzene as a solvent. The melting points (Tm) of the polymers were measured on a calorimeter with a heating rate of 10°C/min. ^^C NMR spectra of the polymers were measured in 1,2,4-trichloro benzene/benzene-d6 (9/1 v/v) at 140°C. 3. RESULTS AND DISCUSSION The parent siliceous MCM-41 exhibited a typical X-ray diffraction (XRD) pattern with four peaks that indicates hexagonal structure as shown in Fig.l. All of the Metal-MCM-41 samples gave slightly lower quality XRD patterns than that of the parent MCM-41. The XRD patterns were found to be free from crystalline metal oxides such as Ti02 and Zr02. This was also confirmed from FTIR and UV-Vis spectra of the Metal-MCM-41. In the FTIR spectrum of Ti-MCM-41, there is no band at 710 cm'' assigned to Ti-O-Ti, while the characteristic vibration band assigned to Ti-O-Si linkage was observed at ca. 950 cmV The UV-Vis spectrum also showed no absorption band at ca. 330 nm corresponding to octahedrally coordinated Ti species, whereas an absorption band at ca. 220 nm due to a charge transfer between framework oxygen to tetrahedral Ti(IV) was observed [8]. In the UV-Vis spectrum of Zr-MCM-41, only an adsorption band due to the oxygen to Zr(IV) charge transfer
100 200 210
MCM-41 AI-MCM-41
Ti-MCM-41 Mn-MCM-41 Zn-MCM-41 Ga-MCM-41 Zr-MCM-41 Hf-MCM-41 3
5 7 2 theta (degree)
9
XRD patterns of various Metal-MCM-41 Fig. Table 1 Characteristics of various Metal-MCM-41 samples Metal-MCM-41 Bulk Si/Metal BET surface area Pore diameter Pore volume (mVg) (cm (liquid)/g) ratio {^^) 3.00 0.84 MCM-41 950 13.4 874 2.74 Al-MCM-41 0.63 30.9 2.74 Ti-MCM-41 908 0.77 862 3.00 46.1 Mn-MCM-41 0.76 714 2.74 Zn-MCM-41 8.9 0.68 12.4 2.74 Ga-MCM-41 735 0.62 2.52 Zr-MCM-41 718 6.1 0.61 2.74 0.61 Hf-MCM-41 13.5 743
755
transition was observed at ca. 210 nm [9]. The N2 adsorption isotherms on the Metal-MCM-41 were found to exhibit sharp inflective characteristics of capillary condensation at the relative pressure of ca. 0.3, although a slight reduction in the amounts of N2 adsorbed was observed. The BET surface areas, pore diameters and pore volumes calculated from the isotherms are summarized in Table 1. Next, polymerizations of propylene were conducted using the various Metal-MCM-41. Table 2 also summarizes the results of propylene polymerization. When the Metal-MCM-41 was combined with A1(/-C4H9)3, the Ti-, Mn-, Zn-, Zr- and Hf-MCM-41 displayed activity and gave selectively isotactic polypropylene (Run Nos. 3, 4, 5, 7, 8). No polymer was obtained when the Al- and Ga-MCM-41 as well as the parent siliceous MCM-41 were used. Table 2 Propylene polymerization with Metal-MCM-41/ A1(/-C4H9)3 catalyst^^ Tm Run Metal-MCM-4 Activity lO'^Mw Mw/Mn [mmmm] 1 No. (g-PP/Metal-mol • h) CC) (%) MCM-41 0 1 Al-MCM-41 2 trace Ti-MCM-41 153.7 42 44 38 1,296 3 4 Mn-MCM-41 154.9 7.0 40.3 17 27 39 Zn-MCM-41 159.0 1.35 5 28 63 Ga-MCM-41 6 trace Zr-MCM-41 160.3 18 34 45 190 7 Hf-MCM-41 157.4 19 12 40 293 8 a) Metal-MCM-41=0.5 g, Al (from Al(/-C4H9)3)/Metal=2, Toluene=30 cm\ Propylcne=7 dm-\STP), Temp.=40°C, Time=2h As the Ti-MCM-41 showed the highest activity among the Metal-MCM-41 prepared, an influence of organometallic cocatalyst on the polymerization performance of the Ti-MCM-41 was studied. As listed in Table 3, it was found that the polymerization activity is strongly dependent on the kind and amount of alkylaluminium used. Namely, the order of the cocatalytic activity was follows: A1(C2H5)2C1 > A1(/-C4H9)3 > MAO(methylalumoxane) > A1(C2H5)3 > A1(CH3)3. The polymerization activity decreased gradually with an increase in the amount of A1(/-C4H9)3. As the use of Zn(C2H5)2 an Ga(CH3)3 in place of organoaluminiums did not give polymer (Run Nos. 17, 18), the addition of organoaluminiums seems to be essential for formation of active species in the isotactic polymerization of propylene. Although the formation mechanism of active species is not clear at the present stage, we have now speculated that the active species are generated by dissociation of Si-O-Ti bond through reaction with alkylaluminium, resulting in formation of Ti-alkyl bonds. We further tried to check the possibility of elution of active Ti species from the MCM-41 framework into the liquid phase during aging before polymerization. The mixture of Ti-MCM-41 and A1(/-C4H9)3 was brought into contact in toluene, and propylene polymerization was conducted using the solution fraction. However, no polymer was obtained, indicating that the polymerization took place within the Ti-MCM-41 structure. To characterize polypropylenes obtained, the polymers were extracted using boiling o-dichlorobenzene, which is commonly used for extraction of polypropylene produced with the conventional heterogeneous Ziegler-Natta catalyst. However, the polymer could not be extracted completely from the Metal-MCM-41. About 40-60% of polymer was remained in
756
the mesopores of Metal-MCM-41. As it is well recognized that cross-linking of polypropylene hardly takes place during polymerization and extraction processes, this suggests strong interaction between the occluded polymer and pore walls. However, we could not explain the exact reason at the present stage and a further study is now in progress. Therefore, the boiling o-dichlorobenzene soluble parts were analyzed and some analytical data are listed in Tables 2 and 3. The melting point (Tm) and the molar mass distribution (Mw/Mn) of polypropylene produced with A1(/-C4H9)3 were >155°C and >25, respectively. The isotacticity [mmmm] pentad was 40-60, which was considerably smaller than that expected from the Tm value. Polymerization of ethylene was also conducted using the same catalyst system (Run No. 14). The polymerization activity was approximately 10 times larger than that of propylene. The Mw/Mn was found to be more than 100, indicating an existence of multi active species within the Ti-MCM-41. From all above results, it was concluded that the Ti-, Zr- and Hf-MCM-41 have the high potential for isotactic polymerization of propylene in spite of the absence of specific organic ligands. Table 3 Propylene polymerization with Ti-MCM-41 using various organometallic cocatalysts"^^ Run Cocatalyst Metal/Ti Activity Tm 10' Mw Mw/Mn [mmmm] No. (g-pp/Ti-mol • h) (°C) (%) 9 158.2 60 50 55 148 A1(CH3)3 10 A1(C2H5)3 222 50 0 11 A1(/-C4H9)3 0 " 44 42 2 1,296 153.7 3 38 '' 704 12 24 158.7 10 61 55 " 574 39 154.9 13 50 57 51 14b) " 124 46 134.2 2 5,370 944 12 15 A1(C2H5)2C1 50 26 158.0 30 161.4 426 16 M A O 50 30 16 43 trace 17 Zn(C2H5)2 50 18 Ga(CH3)3 trace 50 a) Ti-MCM-41 =0.5 g, Toluenc=30 cm\ Propylcnc-7 dm^(STP), Tcmp.=40°C, Timc=2 h b) Ethylene was used as monomer instead of propylene. Polymerization timc=l h REFERENCES 1. R T. Tanev, M. Chibwe and T. J. Pinnavaia, Nature, 368 (1994) 321. 2. A. Corma, M. T. Navarro and J. Perez-Paricnte, Chem. Commun.. (1994) 147. 3. T. Tatsumi, K. A. Koyano and N. Igarashi, Chem. Commun., (1998) 325. 4. Y. S. Ko, T. K. Han, J. W. Park and S. I. Woo, Macromol. Rapid Commun., 17 (1996) 749. 5. J. Tudor and D. O'Hare, Chem. Commun., (1997) 603. 6. K. Kageyama, J. Tamazawa and T. Aida, Science, 285 (1999) 2113. 7. J. M. Kim, J. H. Kwak, S. Jun and R. Ryoo, J. Phys. Chem., 99 (1995) 16742. 8. L. Marchese, T. Maschmeyer, E. Gianotti, S. Coluccia and J. M. Thomas, J. Phys. Chem. B, 101 (1997)8836. 9. K. A. Vercruysse, D. M. Klingeleers, T. Colling and P. A. Jacobs, Stud. Surf Sci. Catal., 117(1998)469.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. Allrightsreserved
757
Catalytic carbonylation of glycerin by urea in the presence of zinc mesoporous system for the synthesis of glycerol carbonate J-W.YOO and Z.MOULOUNGUI* Ecole Nationale Superieure des Ingenieurs en Arts Chimiques Et Technologiques Laboratoire de Chimie Agroindustrielle - UMR 1010-INRA-INPT/ENSIACET Phone: 33 5 62 88 57 27 , Fax: 33 5 62 88 57 30 Email: zephirin.mouloungui(a)ensiacct.fr The glycerin is transformed into glycerol carbonate by reaction with urea in the presence of a series of heterogeneous catalyst system including metallic sulfates or zinc organomettallic sulfate. Zinc sulfate exhibits characteristic catalytic behaviour in the carbonylation of glycerin into glycerol carbonate wich gives a good molar yield of 86% at 140°C, 30 mbar for 2 hours. 1. INTRODUCTION Glycerine is a major co-product from oleochemical processing whether this involves hydrolysis, saponification, transesterification of triacylglyccrols to produce soaps, fatty acids or fatty esters. The glycerine may be considered an oleochemical product. So we led development of new processes to set a new network of the valorization of glycerol [1,2,3]. The preparation of glycerol carbonate is the first link of a chain of the chemical transformation of glycerol. The glycerol carbonate is the cyclic carbonate compoud, such as ethylene carbonate and propylene carbonate. The cyclic carbonate structure contributes to high polarity, high fiash point, low toxicity and biodcgradability of these organic compound [4]. So the glycerol carbonate is a five membered cyclic carbonate, high polar compound with a broad solvent power to function as solvent, compatibilizcr and reactive diluent. In addition, owing to physical characteristics, the glycerol carbonate is held up by absence of product in the market even though it's great reactivity. Recently, we have devclopped a competition process of the synthesis of the anhydrous glycerol carbonate from the pure of crude glycerine by the reaction with urea in the presence of a series of solid metallic sulfate systems including zinc sulfate, zinc mineral or zeolite sulfate, zinc organosulfatc, zinc ion exchange resins (6). 2. EXPERIMENTAL METHODS 2.1. Preparation of heterogeneous catalyst systems A strong acid mesoporous resin in the H^ form (trade name «Bayer K2431»), pre-swollen in water, was exchanged into the Na^ form by percolating with 2N NaOH solution (5 volumes for 1 volume of resin). The exchanged resin was rinsed with demineralized water until an effluent was obtained with neutral pH. It was then exchanged into the bivalent metal form
758
Zn^^ by percolating an ZnS04 solution correspondoing to IN (5 volumes for 1 volume of resin). The exchanged resin was rinsed with demineralized water until the free ZnS04 was eliminated (10 volumes for 1 volume of resin). It was washed with ethanol and then diethyl ether. It was then dried under vaccum using a filter pump and stored in a dessicator. It contained 0,5 meq grams of Zn^^ per ml. 2.2. Surface area Specific surface area was measured by the BET method using N2. 2.3. Reaction procedure 27,6g (0,3 mole) of glycerin, 18,0 g (0,3 mole) of urea and 1,7 g of zinc sulfate (zinc ptoluene sulfate hydrate or resin in the Zn^^ form or Zn^^ mineral sulfate) were mixed in the reactor provided with a pressure sensor, a temperature sensor, a mechanical stirred, a jacket containing a heating oil bath and a vacum pump for extracting vapors of ammonia. The reaction mixture was brought to 140-150°C and the pressure was reduced to 40 mbar. After the reaction, the crude reaction mixture was analyzed by gas chromatography on a « Carbowax 20M » capillary column (12m) with tetraethylene glycol as the internal standard. The molar yield based on glycerol are indicated in the tables. 3. RESULTS 3.1. Surface area Specific surface areas of ZnS04 prcteatcd at 400°C and ZnS04 recovered after a run of the carbonation of glycerin by urea arc given in table 1. The surface areas arc equivalent. This implies that the ZnS04 exhibits characteristic catalytic behavior of mcsoporous catalyst in the carbonatation of glyccron by urea. The thermal experimental conditions contribute to these characteristics. Table 1 Specific surface areas of ZnS04 after reaction Treatment Surface area ^]2!/^^} 0,1 ZnS04. H2O
t-plot
Particle size (nm)
1,7
20-80
ZnS04
Pretreatment at 450°C,3h
18,8
29,5
20-80
ZnS04
Recovered after run
19,1
26,8
>80 20-80
Treated 104°C, 3h
>80
3.2. Catalytic carbonation reaction of glycerin with urea catalysts systems The tables 2, 3, 4 show that the reaction between glycerin and urea is catalyzed by the metallic catalyst. Experiments have show that the urea reacts with glycerin when heated in the presence of zinc catalyst according to the following mechanism with two steps in situ : (i
759
carbamoylation of glycerin to glycerol urethane on the zinc sulfate ii) carbonylation of the glycerol urethane to glycerol carbonate with abstraction of the ammonia (Figure 1,2). Table 2 Catalytic carbonatation of glycerin by urea in the presence of heterogeneous zinc catalysts Catalyst Opering condition Molar yield (%) P-S03"Na^
130°C
2,5 h
16
(PS03")2Zn^^
130°C
1,5 h
65
(PS03)2Zn^^
130°C 145°C
2,5 h 1 h
78 67
145°C
1,5 h
75
Table 3 Catalytic carbonatation of glycerin by urea in the presence of homogeneous zinc catalysts Catalyst Quantity (g) Time (h) Molar yield (%) 3 26 CH3C6H4-S03"Na^ 1,7 (CH3C6H4-S03")2Zn
0,5
1
38
0,5
3
60
1,7
1
81
1,7
1,25
85
Table 4 Catalytic carbonatation of glycerin by urea in the presence of zinc mineral sulfate systems Catalyst Pretreatmcnt Time (h) Molar yield (%) ZnS04.H20 2 83 ZnS04
450°C
3h
2
86
Alumina-silicate-ZnS04
500°C
3h
4
40
Zeolite Y.Na.ZnS04
700°C
3h
4
27
The second step has slower kinetic than the first but the presence of the catalyst prevents this step being a blocking. A metallic sulfate or zinc organometallic sulfate can be used. These solid sulfates have many Lewis acid sites which have a high activity. The sites arc responsible for most of the catalytic activity, hinging about very efficient activation of the
760
carbonation of the second step. In particular, a zinc sulfate may be used as a catalyst. This metallic catalyst dissolves in media after 30 mn. The catalytic behavior is similar as homogeneous catalyst. The formation of complexes glycerin-zinc sulfate is possible in glycerin solvent. After the formation of glycerol carbonate, the metal complexe is destroyed. We observed the nucleation of zinc sulfate. The chemical result assimilates this reaction as the carbonylation glycerin acid-catalyzed by solid, mesoporous biflinctional catalysts.
\r )
ZnS04
\
y-NH, O Glycerol urethane
Evolution of the glycerol carbonate concentration
V OH
OH
NH3
Glycerol carbonate
Evolution of the glycerol carbonate concentration 100 d
90
1
80
•5* % c 0
70 60 „^ 50
t
\ i? 1 0 ^ ^
^
40
30 20 10
oi 0
Reaction time (h)
Fig. 1. Evolution of the glycerol carbonate concentration during the reaction between glycerol and urea.
0,5
1
1,5
2
Reaction time (h)
Fig. 2. Evolution of the glycerol carbonate concentration during carbonylation of glycerol urethane.
AKNOWLEGMENTS The authors would like to thank ONIDOL (S.Claude, G.Vermeersch, D.Delplancke), ADEME, INRA for financial support and N.Caruel for technical support.
REFERENCES 1. Z.Mouloungui, C.Gauvrit, Ind.Crops Prod., 1998, 8, 1-15 2. J-W.Yoo, Z.Mouloungui, A.Gaset, FR 9703163 (1997), PCT/FR98/00451 (1998) 3. C.Vieville, J-W.Yoo, S.Pelet, Z.Mouloungui, Catalysis, 1998, 56, 245-247 4. H.Klein, T.Marquis, Coating World, May-June, 1997, 38-40 5. Z.Mouloungui, S.Pelet, EurJ.Lipd Sci.Technol, 2000, 103,216-222 6. Claude,Z.Mouloungui, J-W.Yoo, A.Gaset, USPat 6 025 504 (Feb 15, 2000)
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
761
Preparation of porous materials using gold and silver miners waste slime Hyoung Ho Lee^, Ji-Whan Ahn^, Hyung Seok Kim'' and Hwan Kim^ ^School of Materials Science and Engineering, Seoul National University, Seoul, 151-742, Korea ^Institute of Geoscience & Mineral Resources, Taejon, 305-350, Korea Porous materials were prepared from mine's waste slime. The pore-formation process was observed with the samples treated in different temperature and time conditions. The liquid phase which comes from the decomposition of the sanidine and the muscovite-3T, explains the foaming reaction of mine's waste slime. Furthermore, in order to induce liquid formation and therefore to activate the foaming reaction, the alkaline oxide was added and its effects were examined. 1. INTRODUCTION Because Mine's waste slime contains various forms of minerals, it is expected to have a large amount of foaming pores by firing due to the differences in the melting points of the constituents, so it seems to be the good candidate as the raw materials for porous products. Many researches have been conducted to utilize porous bodies for applications of clearing and purifying pollutants and wastewater[l-3]. However the researches hardly went on in detail about the reaction of many kinds of inorganic chemicals, the pore-generating mechanism, the control of porosity and the mechanical strength, and their relationships. In this study, with mine's waste slime, the porous materials are prepared and their characteristics are examined according to variable factors including the heat-treatment conditions, glass-inducing additives. 2. EXPERIMENTAL The chemical compositions of raw powders, the Duckem mine's waste slime, appears in Table 1. They are the mud-type materials which take the portion of about 4 0 % of the overall slime, and the 99% of them are smaller than 38 /fln. Table 1 XRF result of the Duckeum mine's waste slime (%) Si02
AI2O3
K2O
Na20
Fe203
CaO
MgO
84.72
6.79
5.00
2.77
0.58
0.36
0.26
To observe the phase changes with different heat-treatment conditions, the samples were heated at 1050, 1100, 1150, 1200, and 1250°C for two hours and analyzed by XRD. As for the observation of microstructure, the samples with no additives were sintered at 1200 and at
762
1250°C for various times up to 5 hours, and the fractured surfaces of the samples were examined by SEM. For the samples with 2.5 or with 5wt% Na20 addition, sintering temperatures are 1125 and 1150°C. The green body was prepared by compacting the powders at the pressure of 50kgf/cm^. The density, porosity and compressive strength of samples were measured. The density and the porosity were measured by the Archimedes' law and mercury porosimetry. The compressive strength was measured by using UTM(Universal Test Machine) at the speed of 0.5mm/min. 3. RESULTS AND DISCUSSION 3.1. The effects of heating conditions Fig. 1 shows the XRD analysis for the raw powder and heat-treated samples of mines waste slime. The raw powder turned out to contain a-quartz, the muscovite3T[(K,Na)(Al,Mg,Fe)2(Si3.iAlo.9)Oio(OH)2]. In the case of heat-treatment over 1150°C, a typical halo is found at around 20-30°, which represents the existence of the amorphous phases. This amorphous phase comes out with the decomposition of sanidine and the muscovite-3T, reaction (1) and (2). The composition of Na20-K20-Si02 is possible to make a liquid-phase above 750°C. (K, Na)(Si3Al)08 (Si02 - Al203)system + glass (K,Na)(Al,Mg,Fe)2(Si3.i Alo.9)0,o(OH)2 (Si02-Al203)system + glass + H2(g) • : Al^SI^O,, T : »A1'o ' O . (K.N«)(Al.Mg.Fe),(Si^,A1^^0„(0H), (Mu«covitfr3T) • : (K:.Na)(Si,Al)0 (Sanidine)
-LX-H
26
Fig. 1. XRD patterns of mining tailings sintered at different heating condition In the phase analysis of heat-treated samples, alkali or alkaline earth metal compound, the sanidine and the muscovite-3T were not detected. Therefore, it seems that the almost all these phases are consumed to produce the liquid-phase. On the other hand, in the result of microstructure examination, the pore-creation was hardly observed at 1150°C. But, above 1200°C, the foaming reaction happened very actively. The firing temperature required to activate the foaming reaction is expected to lower by adding the alkali oxides, either Na20 or K2O, in the starting material because liquid-phase could come out above 750°C from the phase equilibrium.
(1) (2)
763 To evaluate applicability as the porous materials, the basic physical properties of samples sintered at 1200 and 1250°C were measured. For each temperature, the degree of foaming progress was examined at the passage of 10, 20, 30min., 1, 2 and 5 hours. The density, the porosity and the compressive strength, and microstructure were shown in Table 2, Fig. 2, respectively. Table 2 Density and mechanical strength changes of sintered body according to heat treatment conditions Sintering Compressive strength Sintering time (min) Density (g/cm ) (kgf/cm^) temperature (°C) 2.37 1006 10 1.96 474 20 1200 1.85 431 30 1.78 276 60 1.66 200 120 10 184 1.48 136 1.25 20 128 1.20 30 1250 113 1.07 60 109 1.02 120 64 0.85 300
Fig. 2. SEM photographs of the fractured surface of mining tailings sintered at 1250°C with different heating time, (a) lOmin (b) 20min (c) 30min (d) Ihr (e) 2hrs (f) 5hrs At the above two temperatures, the density of the body was decreased, as heattreatment time got longer, however, its porosity was increased. Therefore it could be concluded that the foaming reaction was progressing continuously. In the case of heat treatment at 1250°C, there were over lOO/fln-sized pores. Though it satisfies the basic requirement of the high porosity in practical uses of porous materials, it has the ultimate weakness in the viewpoint of specific surface. So it is not proper for the practical applications.
764 At 1200°C as time goes by, the reaction speed becomes slower and the created pores are combined each other. Thus the number of pores decreases and their sizes get larger with time elapsed. The result of compression strength measurement was shown in Table 2. When the sample was sintered at HOO^C for a short time, its strength turned out to be over 400kgf/citf. However the mechanical strengths of samples heat-treated at 1200°C for a long time and at 1250°C are below 276kgf/citf. This is because of grown pores. 3.2. The effects of additives As it was found that the foaming reaction was mainly caused by the liquid formation of Na20-K20-Si02 system, Na20 was added to waste mine slime powder, 2.5, 5, 7.5 and 10wt%, to facilitate the foaming reaction and reduce the foaming temperature. When Na20 was added, the test piece showed the vigorous foaming behavior at over 1125°C, irrespective of the added amount. However, the samples of composition with the addition of more than 7.5 wt% Na20 and the ones heat-treated at 1175°C or higher couldn't maintain the structure because of excessive liquid formation and the growth of pores. In the XRD analysis, we could not detect the compound which includes Na as its component, so most of Na is thought to contribute to the liquid formation. Furthermore, with the increase in the added amount of Na20 and in the temperature for heat treatment, the intensity of halo which represents amorphous phase increased. For the compositions of 2.5 and 5wt% Na20 addition which had the most excellent foam characteristics, the physical properties of the test pieces were shown in Table 3. With Na20 addition, samples having similar porosities to those with no additive could be produced at temperature 100°C lower. Having similar density, the test piece with Na20 addition had a relatively higher compressive strength than the one with no additive, which shows that the addition of Na20 gives the favorable effect on the foaming reaction and the mechanical properties. Table 3 Density and mechanical strength changes of Na20 added specimens according to heat treatment conditions wt.%of Sintering o- . • .• /u N Density Compression XT ^ . . /v-x Sintering time (hrs) . , 3x . ., ,, r, 2x Na20 temperature ( C) ^I ^ ^ (g/cm strength (kgf/cm^) 1.81 ) 1125 5 1.10 219 10 085 106__ 2.5 1 0.93 200 1150 5 0.58 41 10 053 19^ 1 1.45 1125 5 1.38 203 10 099 74_ 1 0.97 139 1150 5 0.63 50 10 057 37
765 These are explicable in terms of the fine structures of the test pieces with Na20 additions shown in Fig 3. When NazO was added, as shown in the figure, the pores have relatively regular size, which has the strong resistance to destruction.
Imin
Imm
(a)
(b)
'^,.r
Iwin, (c)
(d)
• SSE (e)
*K-m^
Fig. 3. Photographs of the fractured surface of 2.5 wt% Na20 added mining tailings with different heating conditions, (a) 1125"C, 5h (b) 1125c, lOh (c) 1150"C, Ih (d) 1150c,5h(e) 1150c, lOh To examine the characteristics of the pores generated by foaming, the mercury porosimetry analysis of test pieces with the additions of 2.5 and 5wt% Na20 was made and the results were shown in Table 4. From these data, we can suggest that the pores in the 2.5wt% Na20 sample is more open than those in the 5wt% Na20 and it is because of the amount of amorphous phase. Therefore, by controlling the additives amount, and the conditions of heat treatment, it would be possible to control the properties of pores generated by foaming. Table 4 Mercury porosimetry results of 2.5 and 5 wt.% Na20 added specimens (sintered at 1125°C for5hrs) Apparent Bulk Total Porosity wt.% of Average pore density density pore area (open pore) added NajO diameter (j^m) (g/cm^) (g/cm^) (%) 7.3 49.33 2.5 246.953 1.10 2.16 8.0 7.14 5 25.710 1.38 1.49 REFERENCES 1. H. Jeong et al.. Ceramist, 3(6), p5, 2000 2. J. Yang, Ceramist, 3(6) p21-38, 2000 3. M. Kawase et al., Wat. Sci. Tech., 21, p77. 989
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Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
767
Preparation of mesoporous carbon by steam activation of commercial activated carbon in the presence of yttrium oxide W. Z. Shen*, J. T. Zheng, Y. L. Zhang, J. G. Wang, and Z. F. Qin State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, P.R.China, E-mail: shenwzh@sina.com Mesoporous carbon was prepared by steam activation of commercial activated carbon in the presence of yttrium oxide. The loading of yttrium nitrate (precursor of yttrium oxide) was 0.2, 0.6, 1.0 and 2.0 wt%. The weight lost and gases formed during heating were detected by using thermogravimetric analysis and mass spectroscopy. The surface area and the total volume of the mesoporous carbon were determined by the nitrogen adsorption. The pore size distribution was calculated by the BJH method. With the increase of activation temperature, the reaction became faster and the pore formed was wider and the pore size distribution became scatter. The pore diameter and volume increase with the increasing of the loading of yttrium oxide. 1. INTRODUCTION Activated carbon (AC) is widely used as adsorbent [1]. The micropore of AC plays an important role in the removal of small molecules. However, when they are used as catalyst supports and adsorbents for bulk molecules, not only the high surface area but also the high mesoporosity are required [2, 3]. Therefore, the development of mesoporous AC is essential. Several methods have been reported to prepare mesoporous carbon [4]. Among them, catalytic activation of carbon over metals and their compounds is the most common and convenient method [5, 6]. In this study, we choose the yttrium nitrate as catalyst precursor to prepare high content mesoporous carbon by steam activation. The results are discussed in terms of the effects of loading amount of yttrium nitrate, activation temperature and time upon the surface area, pore volume and pore size distribution of mesoporous carbon. 2. EXPERIMENTAL 2.1. Materials and pretreatment The raw AC (peach shell-based) was produced by Gao Yuan Adsorbent Co. (Shanxi, China). The yttrium nitrate was provided by Shanghai Chemical Reagent Co., China. The raw AC was heated at 120 °C for 4 h before impregnated with the yttrium nitrate aqueous solution, the loading amount were 0.2, 0.6, 1.0 and 2.0 wt% on the basis of AC, respectively, and dried at 120 °C for 5 h. 2.2. Activation The ACs containing yttrium nitrate were heated to desired temperatures at a rate of 6 °C/min under nitrogen (36 ml/min), then activated by steam. Finally the products were cooled to room temperature under nitrogen flow.
768
to room temperature under nitrogen flow. 2.3. Analysis and characterization TG-MS was carried out with a TGI51 (CAHN) combined with an MS (Omni Star) in Ar. The weight loss was recorded in the range of 30-910 °C at the rate of 10 °C /min. The nitrogen adsorption-desorption isotherms of the AGs were measured using AS AP2000 Gas Adsorption Analyzer (Micromertics Instrument Co.). The total surface areas, mesopore surface area and volume were calculated using the BET equation and BJH method. 3. RESULTS AND DISCUSSION 3.1. TG - MS The weight lost and the gases formed of the samples during the heating processes are shown in Fig. 1. When AC is heated to 910 °C, the weight lost is due to the removal of some adsorbed substances. The effluent gases produced from the AC loading yttrium nitrate, mainly contain CO, CO2, H2O and NO. The CO2 and CO come from the surface function groups. When impregnated with yttrium nitrate, the NO is formed by nitrate decomposition at the range of 150 °C and 400 °C. Therefore, the yttrium oxide plays the role in catalytic gasification during activation processes. 1E-10f
200
400 600 800 Temperature ( °Q )
1000
Fig. 1. The thermal gravity and the gases producing in heating process. 3.2. N2 adsorption-desorption isotherm Fig. 2 represents the N2 adsorption-desorption isotherms of the four samples with yttrium nitrate loading of 0.2, 0.6, 1.0, 2.0 wt%, respectively. There exists the hysteresis loop in every isotherm when the relative pressure is higher than 0.4 and these hysteresis loops are associated with mesoporous solids, where capillary condensation occurs [7]. The hysteresis loops are close to the shape of the H3 type, which was observed in the case of aggregates of plate-like particles giving rise to slit-shaped pores [7]. The isotherms are becoming steeper with the extending of activation time at the same temperature, or with the raising of the activation temperature at same time. Two specified conditions are compared, one is activated at 870 °C for 90 min, the other is activated at 800 ''C for 180 min. The isotherms of samples prepared in the latter condition are gentler than those of the former when the loading of yttrium nitrate is 0.2% or 0.6%, indicating that the mesopore volumes of the samples prepared in the latter condition are smaller. When the loading of yttrium is 1.0 % or 2.0 %, the two activation processes give almost overlapped isotherms, which show the pore volume is near equal to each other. At the higher relative pressures, the isotherm increases faster, suggesting the presence of wider pores and the content of mesopore markedly increase at higher yttrium nitrate loading.
769 5 900
300 _1000 -.800-180 "!• ^ 800 J _^___ 870-60 "O —'—870-90 0) o 600 J __-.__ 800-90
^ 200 0
c
0.4
u
0.6
1Q).0
P/P° Fig. 2. N2 adsorption-desorption isotherms, a: 0.2%; b: 0.6%; c: 1.0%; d: 2.0%
3.3. Pore size distribution The measured pore size distributions (PSD) of the samples are shown in Fig. 3. It can be seen that the PSD is strongly affected by the loading of yttrium nitrate and the activation conditions. Under the same activation conditions, the PSD is becoming wider with the 1.4 "B 1.2-1
800-180 870-60 870-90 800-90
• ^ 1.0
^0.8 o) 0.6-1 o 5 0.4 ^ 0.2 0.0-1 1.2
A
im
^
\jNJrr iSl-'^**^% **''^\
^'^.0 -SiO.8 9.0.6 o> 0 0.4
• 0 * -
800-180 870-60 870-90 800-90
"Xv
> 0.2^ 0.0-
870-90 800-90
800-180
"'^O^BO^-^
870-60 I
100
^^**--<
pore size (nm)
10 pore size (nm)
Fig. 3. The pore size distributions,
a: 0.2%; b: 0.6%; c: 1.0%; d: 2.0%
10
100
increasing of yttrium loading. There is a peak between 2.8 nm and 4.2 nm, whose intensity decreases with the increase of yttrium loadings. For the yttrium loading of 0.2%, the most pores are less than 10 nm. With the extension of activation time, the pores in the range of 4-20 nm are increased. The PSD of samples prepared at same activation condition is broader with the higher yttrium nitrate loading. In the case of 1.0% yttrium nitrate loading, the PSD of sample prepared at 800 °C for 180 min is significantly broader than that prepared at 870 "^C for 90 min between 4.1 nm to 21.7 nm, but relatively narrower from 21.7 nm to 68 nm. When
770
the loading yttrium nitrate increased to 2.0%, the PSD changes more radically than the others. First, the PSD shifts to more broader; secondly, there appears a plateau from 3.8 nm to 26.7 nm of the sample that prepared at 800 °C for 180 min; Thirdly, a new peak produces in the range of 14.5-81 nm, with the center of 26.3 nm. Therefore, It can be concluded that the increase in yttrium nitrate loadings, the raising of the activation temperature or extension of activation time make the PSD moving broader. 3.4. Role of yttrium oxide Both catalytic and non-catalytic activation exist during the steam activation process. The rates of the two activations increase with temperature. At lower temperatures, catalytic activation plays the major role in forming pores. While at high temperatures, the non-catalytic activation is fast and the yttrium oxide is easy to aggregate. Consequently, the pore formed at high temperature is wider with scattered PSD. The carbon located at the edge of the yttrium oxide takes the advantage to react with steam during the activation. A pit will be formed on the carbon surface during activation, the pit gradually grows and the pore is formed. The pore size is related to the particle size of yttrium oxide, i.e., the larger of the particle size of yttrium oxide, the broader of the formed pore. At the lower yttrium loadings, the chance of yttrium particles collide to aggregate is less and the pore formed is narrower. On the contrary, the chance to form large particles of yttrium oxide is more and the pore formed is wider at the higher loadings. When the activation temperature is high, the reaction rate between carbon and steam is faster and the advantage of catalytic activation is relatively decreased. Meanwhile, yttrium oxide also shows faster mobility for colliding; larger particles of yttrium oxide will be formed, which produce wider pores. At lower activation temperature, the reaction rate between carbon and steam is slower, and the catalytic activation plays the predominant role in forming pores. The aggregating of yttrium oxide is less at lower temperature; the formed pore is narrower and relatively concentrated. 4. CONCLUSION The pore volume and pore diameter are related to the loading of yttrium nitrate and activation conditions. The loading is higher, the volume is larger and the mean pore diameter is bigger. The highly dispersed yttrium oxide with lower loading leads to less pore volume. Higher temperature and longer activation time lead to wider pores and larger volume.
REFERENCES 1. R.C. Bansal, D. B.Donnet et al. (eds.). Activated Carbon, Marcel Dekker, New York, 1988. 2. L.R. Radovic (ed.). Chemistry and Physics of Carbon: Vol. 27, Marcel Dekker, Inc, New York 2000. 3. PA. Thrower (ed.). Chemistry and Physics of Carbon: Vol. 25, Marcel Dekker, Inc, New York, 1997. 4. T.Kyotani, Carbon, 38 (2000) 269. 5. A.Oya, S.Yoshida, J.A. Monge, A.L.Solano, Carbon, 33 (1995) 1085. 6. H.Tamai, T. Kakii, Y. Hirota, H.Yasuda, Chem. Mater., 8 (1996) 454. 7. K.S.W. Sing, D.H. Everett, et al., Pure and Appl. Chem., 57 (1985) 603.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
771
Study of Mesoporous Carbon with Function of Absorbing Microwave^ W. Xing
Z.-F. Yan*
State Key Laboratory for Heavy Oil Processing, The Key Laboratory of Catalysis, CNPC, University of Petroleum, Dongying, Shandong, P. R. China, 257061 A novel mesoporous microwave carbon absorbent was synthesized from petroleum coke by chemical activation. Surface properties and crystal structure of the absorbent were tested by N2 adsorption and XRD techniques respectively. Results showed that the absorbent possesses high surface area and a special crystalline structure. Electromagnetic parameters and coating microwave-absorbing test demonstrated that the absorbent is an efficient dielectric loss - type absorbent. 1. INTRODUCTION Radar absorbing materials (RAM) refer to a kind of material which can be used to absorb the emitted electromagnetic energy and to minimize the wave reflected in the direction of an energy radar receiver. Such materials are often required to provide specific reflectivity performance over a wide frequency range and to have two properties: the first is an impedance match between free space and a perfectly conducting surface; the other is that the incident electromagnetic wave can enter the RAM and be attenuated sufficiently [1]. Many efforts have been made to study the traditional ferrite absorbers, of which the application is constrained by their high density, instability and relative narrow waveband of absorbing [2, 3]. In this paper, a novel carbon-based microwave absorbent was synthesized from petroleum coke by thermo-activation, using alkali metal hydroxide and transition metal nitrates as activator Compared with traditional types of metal microwave absorbents, carbon absorbents have such merits as plain synthesis process, relatively low cost, high corrosion resistivity and low coating density that is fairly important for its coating application in airplane. The rules of real and imaginary part of the relative complex permittivity and relative complex magnetic permeability of the absorbent changing with the frequency were discussed in detail. The dielectric loss tangent (tan 5^) and the magnetic loss tangent (tan 6^) were determined using the measured value of the relative complex permittivity and the relative complex magnetic permeability of the absorbent. It has been found, by permittivity and permeability measurement and frequency scanning reflectivity testing, that the main microwave attenuation mechanism of the absorbent is dielectric loss.
Corresponding author. E-mail: /fyancat(^:hdpu.cdu.cn ^Financial supported by key teacher foundation of national education department of China.
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2. EXPERIMENTAL 2.1. Preparation of carbon absorbents Microwave carbon absorbents were prepared from petroleum coke (manufactured by Shengli Refinery Work, Sinopec Co. Ltd.) by chemical activation using alkali metal hydroxide as activator, transition metal nitrates as promoters. The activation of petroleum coke was then carried out in the presence of N2 flow of 30ml/min at a programmed temperature. After activation, the sample was transferred into deionized water, filtered and then washed again to remove any activator derivatives and any other impurities. 2.2. Parameter measurements N2 adsorption technique at 77.3 K was used to determine the surface properties of this absorbent. Results showed that the absorbent was a mesoporous material with high surface area. The average pore diameter and specific surface area of this carbon are about 2.4 nm and 2100 m^/g. Laboratory X-ray powder diffraction studies were performed at room temperature with a PDS120 diffractometer from NONIUS in transmission mode, equipped with a Ge(l 11) primary monochromator and a 120° 2 Theta position sensitive detector, using CuK-alpha-1 radiadon (Lambda = 1.540598 A) at 40 kV and 30 mA. Permittivity and permeability have been measured over a wide frequency band of 2 to 18 GHz, encompassing the S, X and Ku bands of military radar interest, using a HP8510B vector network analyzer. An HP8410C microwave vector network analyzer was used to determine the reflectivity by a frequency scanning test in the frequency range of 8~18 GHz. The specimen size was 180x180x1.5 mm, adhered to a 4 mm thick pure aluminum substrate. 3. RESULTS AND DISCUSSION 3.1. XRD Characterization The XRD spectrum of the absorbent is shown in Fig. I. As can be seen from Fig. 1 that the microwave absorbents possess special crystal structure. There are 4 sharp peaks visible, which can be indexed as (100, HI, 200, 210). Assuming a disordered rhombohedral (hexagonal) cell, this calculates to a= 0.143 and c= 0.351 nm which is comparable to the distances in graphite within and perpendicular to the layers. As a result, the structure of the absorbent somewhat likes the graphite's layer structure. Electrons in the layer can freely move, and more importantly, can generate an inductive current in the electromagnetic field. The induced current then causes a loss current and energy dissipation. That is to say, the electromagnetic wave energy is changed into heat energy in the absorbers, which is Fig. 1. The XRD pattern of carbon microwave absorbent one of mechanism in microwave attenuation
773
for this absorbent. 3.2 Permittivity and Permeability measurements The material properties of greatest importance in the microwave absorbing are the complex relative permittivity (e =e'-ie''), permeability {ii = fi'-ifi") and the calculated dielectric loss tangent (tan^^ =£"/£') and magnetic loss tangent (tan5^ = fi"/fJi'). The real part of the permittivity and permeability mostly determines how much of the incident energy is reflected at the air-sample interface, and how much enters the sample. Therefore, the larger the value of the real part of the complex permittivity and permeability, the more the incident energy will be reflected by the air-sample interface. However, it is worth noticing that the microwave energy reflected by the air-sample interface only takes up a little proportion of the total energy due to the inconductive property of absorbent, and that most of the microwave energy is reflected by the substrate conductive metallic materials. Therefore, the most important property in microwave absorbing is the imaginary part of permittivity and permeability, which predict the ability of the absorbent to convert the penetrating energy into heat, hi view of quantification, the dielectric loss and magnetic loss of microwave energy are proportional respectively to e'^and ^''of absorbents, which are expressed as follows: p,oc
£"
P,^
^l'
As a result, the ideal microwave absorbent should possess lowest f', fi', as well as highest e", ji", that is, possess highest tan^^, tan^^ . So, it is very instructive to determine the distribution of dielectric constant (£:), magnetic permeability ( ^ ) as well as the calculated dielectric loss tangent (tan5^ =£"!£'), magnetic loss tangent (tan5^ = fi"Iji') in a specific bandwidth, from which we can reveal the nature of microwave absorbing for this absorbent.
2 3 4 5 6 7 8 9 10 1112 1314 1516 17 18 Frequency/GHz
2 3 4 5 6 7 8 9
10 1112 1314 1516 1718
Freqency / GI Iz
Fig. 2. Permittivity versus frequency
Fig. 3. Permeability versus frequency
The curve of f', e'^and fx\ jx" as a function of frequency are shown in Fig.2 and Fig.3 respectively. The dielectric loss tangent (tan^^) and the magnetic loss tangent (tan5^) are also plotted in these figures using the measured value of the relative complex permittivity and the relative complex magnetic permeability of the absorbent. According to Fig. 2 and Fig. 3, it can be concluded that this absorbent has high dielectric permittivity and relatively
774
low magnetic permeability. As can be seen, £''declines slowly with the increase of frequency. e'' is fairly high and varies between 30 ~ 70, while /i^ is small and below 0.2. So, we can draw a conclusion that the energy of microwave losses mainly by dielectric loss. That is to say, dielectric loss mechanism is the main microwave absorbing mechanism for this carbon absorbent. More importantly, e' is relatively low when compared with e'', which leads to the high value of tan5^, ranging from 1.0-1.5, and will benefit to lower the microwave reflectivity. 3.3 coating test 0 :
^y^^
Ah. 18 -10
-15
(11.75, -15. 19)
-20
1
8
1
1
1
1
1
1
1
9 10 11 12 13 14 15 16 17 18 f/GHz
Fig. 4 Microwave reflectivity of the absorbent suspended in epoxy resin
Coating test indicated that absorptive capacity of the absorbent was perfect. The bandwidth corresponding to the reflection coefficient below -5dB is more than 8 GHz for absorbent coating (Mass ratio: carbon/epoxy resin=4/6) with 1.5 millimeter. The minimum reflectivity amounts to -15.19 dB at the frequency of 11.75 GHz. Besides, the coating density is determined to be about 1.0 kg/m^, which is much smaller than traditional ferrite absorbers and favors its application to airplane stealth.
4. CONCLUSION 1) A novel mesoporous carbon absorbent with function of microwave absorbing can be prepared by thermal activation using alkali metal hydroxide as activator. 2) Parameter measurements show that the absorbent possesses special crystal structure and that the absorbent is a typical dielectric loss material with high e' and tan5^.. 3) Coating test shows that the carbon is a perfect microwave absorbing material with low coating density. 4) Abundant mesoporous structure is expected to benefit the microwave absorbing for the as-prepared absorbent.
REFERENCES 1. T. R. George, E. B. Donald, D. S. William, K. K. Clarence, Radar Cross Section Handbook, Plenum Press, New York/London, 1970. 2. M. Z. Wu, Z. S. Zhao, H. H. He and X. Yao, Journal of Magnetism and Magnetic materials, 217 (2000) 89. 3. M. S. Pinho, M. L. Gregori, R. C. R. Nunes and B. G. Soares, Polymer Degradation and Stability, 73 (2001) I.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved
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Adsorption of lysozyme and trypsin onto mesoporous silica materials Jing Yang/ Antje Daehler,*^ Geoffrey W. Stevens*' and Andrea J. O'Connor^ ^School of Materials, China University of Geosciences, Beijing 100083, China ''Particulate Fluids Processing Centre, Department of Chemical Engineering, University of Melbourne, Victoria 3010, Australia. The adsorption of lysozyme and trypsin by the mesoporous materials MCM-41 and SBA15 with different pore volumes has been studied as a model protein adsorption system. The amounts adsorbed depend on the pore volumes and surface chemistry of the mesoporous materials as well as the binding mechanisms of particular proteins. The adsorbed amounts of lysozyme on MCM-41 were 90-110 mg/g over a wide range of lysozyme solution concentrations, whereas the adsorption isotherm of trypsin on MCM-41 fitted the Langmuir model and adsorbed amounts of up to 200 mg/g were recorded. 1. INTRODUCTION Mesoporous silica materials, MCM-41 and SBA-15, which have high surface area, high pore volume and a well ordered hexagonal pore structure''^, have been identified as attractive supports to adsorb and immobilize proteins and enzymes, such as cytochrome c, trypsin and papin, due to their narrow pore size distributions in the mesopore range."^"*' They provide many advantages over current dextran polymer and sol-gel based immobilization techniques, the most significant being their highly ordered rigid structures and their resistance to swelling and microbial attack due to their inert inorganic nature. These advantages allied with the range of pore sizes, high internal surface areas and the ability to modify the surfaces of such materials make them very promising materials for such applications."^ It has been found that enzymes immobilized on MCM-41 or SBA-15 can retain their catalytic activity, be recycled and can be stable at room temperature for at least a few weeks.^*^ Different results have been reported for the adsorption ability of enzymes on MCM-41 and SBA-15, indicating the importance of both steric and surface chemistry effects in determining the extent of binding. For example, Takahashi et al. showed the enzymes horseradish peroxidase and subtilisin were selectively adsorbed to MCM-41 and FSM-16, and were not adsorbed significantly to SBA-15.^ Yiu et al. found that the amount of enzymes adsorbed on MCM-41, SBA-15 and MCM-48 was related to the pore size of the molecular sieves. On this basis, larger pore mesoporous molecular sieves such as SBA-15 may have a greater potential for immobilizing enzymes than MCM-41.^'^ In addition, the stability of mesoporous molecular sieves in aqueous solutions is a significant limitation,'^''^ with SBA-15 showing somewhat improved stability over MCM-41 due to its thicker walls. In this work, the effects of contact time and solution concentrations on the adsorption of the proteins lysozyme and trypsin onto MCM-41 and SBA-15 with different pore volumes and pore sizes have been studied. These proteins were selected as they both have isoelectric points above 10, making them positively charged at pH 6, whereas the silica surfaces would be negatively charged. In addition they both comprise about 50% non-polar groups and 50%
776
polar groups. Their molecular dimensions are also similar but lysozyme is ellipsoidal (ca. 27 X 30 X 43 A) whereas trypsin is closer to spherical (37 x 37 x 42 A).'"^ 2. EXPERIMENTAL AR grade sodium hydroxide (Merck Pty Ltd.), cetyltrimethylammonium bromide (CTAB), fumed silica, hydrochloric acid, tetraethoxysilane (TEOS), lysozyme, trypsin, MES buffer (2[N-morpholino] ethane-sulfonic acid and its sodium salt) (Aldrich) were used. Pluronic PI23 was generously provided by BASF. All water used was distilled. MCM-41 was prepared hydrothermally using CTAB, NaOH, and fumed silica with post synthesis hydrothermal treatment to enlarge the pores, according to established methods. ' '^''^ The post-synthesis treatment was carried out by replacing the mother liquor with distilled water after the normal synthesis (50 mL for 3 g of as-synthesized sample) and holding it at 150°C for 48 h in a PTFE-coated autoclave. The sample was filtered and dried prior to calcination at 550° C for 1 h in nitrogen followed by 8 h in air with a heating rate of l°C/min. To synthesise SBA-15,^''^ Pluronic P123 (4.0 g) was dissolved in 30 g water and 120 g 2 M HCl solution. TEOS (8.5 g) was added and the resulting mixture was stirred for 30 min and then kept loosely covered at 35 °C for 20 h without stirring. It was then transferred into a polypropylene bottle and heated to 80 °C for sample SBA-15(a) and 90 °C for sample SBA15(b) for 24 h. The as-synthesized sample was filtered and dried prior to calcination at 550°C for 4 h in air with a heating rate of 1 °C/min. X-ray powder diffraction (XRD) data were obtained using a Philips PW 1800 diffractometer with CuKa radiation (wavelength 0.154056 nm). The data were recorded for 20 angles between \° and 8° at a step size of 0.02° 20 and a count time of 5 seconds. Nitrogen physisorption measurements at 77 K were conducted on a Micrometritics ASAP 2000. Surface areas (SBET) were calculated using the BET model.''^•^^ Total pore volumes (V) were estimated based on the amount of nitrogen adsorbed at a relative pressure of about 0.99 and adsorption average pore diameters (DBJH) were calculated using the BJH model for the adsorption branch of the isotherm.*^'"^^ Although the BJH model is known to underestimate the pore diameter of MCM-41, it is useful for comparison purposes.^'* Adsorption experiments were performed in 0.1 M MES buffer solution at pH 6. Solutions of lysozyme and trypsin of different concentrations (10 mL) were contacted with the mesoporous materials (0.05g) on a sample spinner at 20°C. The protein solution concentrations were determined after centrifugation from the absorbance at 280 nm using a Varian Cary 1E UV-visible spectrophotometer. 3. RESULTS AND DISCUSSION 3.1. Characterization of MCM-41 and SBA-15 The XRD spectra of the MCM-41 and SBA-15 samples showed the characteristic features for these materials, corresponding to hexagonal structures. Table 1 shows the properties of the samples. As expected, both SBA-15 samples had larger pore diameters than the MCM-41. However, sample SBA-15(b), prepared at 90 °C, had a larger BET surface area and pore volume than the MCM-41, whereas sample SBA-15(a) did not. 3.2. Adsorption of lysozyme and trypsin by MCM-41 and SBA-15 The adsorption of lysozyme onto MCM-41 and SBA-15 in O.IM pH 6 MES buffer solution was found to reach equilibrium within 48 h (Fig. 1). The slow approach to equilibrium is
777
consistent with hindered diffusion of the proteins into the pores due to the large size of the proteins relative to the mesopore sizes. It is interesting to note that the kinetics for both adsorbents were similar despite the SBA-15 having a significantly larger pore diameter. Table 1 Properties of MCM-41 and SBA-15 samples and amounts of lysozyme adsorbed after 2 - 48 h. V Initial cone. Lysozyme adsorbed (mg/g) DBJH ao SBET Sample (nm) (m'/g) (cmVg) (nm) (mg/mL) 2h 48 h 0.4 37 MCM-41 3.32 888 5.3 104 0.898 0.6 59 1.0 53 98 0.4 18 45 0.784 4.24 24 SBA-15(a) 10.1 836 0.6 49 30 1.0 58 0.4 80 0.993 0.6 4.86 SBA-15(b) 10.2 929 1.0 189 The amounts of lysozyme adsorbed onto MCM-41 were significantly higher than those adsorbed onto SBA-15(a) (Table 1), although the latter had the larger pore diameter. At pH 6, lysozyme has a net positive charge and silica has a negative surface charge, favouring adsorption of the lysozyme onto the surface. The fact that the MCM-41 sample adsorbed more lysozyme may be explained by the different synthesis routes for the two silica materials. MCM-41, prepared with an ionic surfactant, is expected to have a higher surface charge density than SBA-15 which is prepared with a non-ionic surfactant.^ In addition, the MCM-41 had a higher pore volume and BET surface area than SBA-15(a). In contrast, SBA-15(b) adsorbed more of both proteins than the MCM-41, presumably as a result of its larger pore volume and surface area reducing steric hindrance effects sufficiently to allow more protein to be adsorbed despite the lower surface charge density of this material. For trypsin (initial concentration 1.6 mg/mL), the adsorbed amount on MCM-41 was 165 mg/g, compared to 190 mg/g for SBA-15(b). The amount of lysozyme adsorbed onto MCM41 after 48 h was found to be consistently 90-110 mg/g over a wide range of solution 300
0.010
^
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-
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00
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D
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i D MCM-41
50 P
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<
• SBA-15(b)
0 • 0
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° .
20
^^
•^ 0.004 •
i
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1
1
40
60
80
100
Time(h)
Fig. 1. Lysozyme adsorption kinetics on MCM-41 and SB A-15(b).
•
•
°% D U
^ 1
0.000 i l
'
D D
! • trypsin 1D lysozyme
-'
0.00 0.05 0.10 0.15 Final solution concentration (mmol/L)
Fig. 2. Adsorption isotherms for lysozyme and trypsin on MCM-41 in 0.1 M MES buffer, pH 6, 48 h contact.
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concentrations including very dilute solutions (Fig. 2). By contrast, the data for trypsin adsorption onto MCM-41 fitted the Langmuir isotherm, with increasing amounts adsorbed as the solution concentration increased, indicating different binding mechanisms dominating for the two proteins despite their similar isoelectric points and hydrophobicity. Thus, MCM-41 may adsorb lysozyme selectively in low concentration solutions. 4. CONCLUSIONS The adsorption of proteins onto mesoporous materials depends on the pore volumes and surface chemistry of the mesoporous materials as well as the binding mechanisms of particular proteins. MCM-41 adsorbed more lysozyme than a sample of SB A-15 with a larger pore diameter but lower pore volume, possibly due to a combination of steric effects and a lower surface charge density in the SBA-15. Increasing the pore volume of the SBA-15 resulted in reduced steric hindrance effects and a higher protein adsorption capacity than the MCM-41. Different isotherm behaviour was found for lysozyme and trypsin on MCM-41, indicating different binding mechanisms and the potential for selective adsorption from mixed solutions. The support of a China Scholarship for J. Yang and the Selby Research Award for A.O'Connor are gratefully acknowledged. REFERENCES 1. J. S. Beck, etal.J.Am. Chem. Soc, 1992, 114, 10834. 2. D. Zhao et al. Science, 1998, 279, 548. 3. J. Deere et al. Stud. Surf. Sci. Catai, 2001, 135, 3694. 4. L. Washmon-Kriel et ai, J. Mol. Catal. B: Enzym., 2000, 10, 453. 5. J. Kisler et al, Micropor. Mesopor. Mater., 2001, 44-45, 769. 6. H. Takahashi et al., Micropor. Mesopor. Mater., 2001, 44-45, 755. 7. J. F. Diaz and K.J. Balkus. J. Molec. Catal. B-Enzymatic, 1996, 2, 115. 8. H.H.P. Yiu et al, J. Molec. Catal. B-Enzymatic, 2001, 15, 81. 9. H.H.P. Yiu.. Phys. Chem. Chem. Phys., 2001, 3, 2983. 10. H. Takahashi, et al, Chem. Mater., 2000, 12, 3301. 11. Kisler, J., et al, Mater. Phys. Mech., 2001, 4, 89. 12. J. Yang, et al. Stud. Surf. Sci Catal, 2002, 141, 221. 13. Daehler, A. et al, Proc. 6th World Congr. Chem. Eng., Melbourne, Aust., 2001. 14. Protein Data Bank (www.rcsb.org/pdb) data analysed using Swiss PDB Viewer v3.7b2. 15. L. Y. Chen, etal,J. Phys. Chem. B, 1999, 103, 1216. 16. X. S. Zhao, G. Q. Lu, J. Phys. Chem. B, 1998, 102, 1556. 17. M. Kruk, et al, Micropor. Mesopor. Mater., 1999, 27, 217. 18. M. Kruk, etal, Chem. Mater. 2000, 12, 1961. 19. S. Brunauer, et al, J. Am. Chem. Soc, 1938, 60, 309. 20. S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, 2"^^ ed., Academic Press, London, 1982. 21. P. J. Branton, et al, Chem. Soc, Faraday Trans., 1997, 93, 2337. 22. M. Kruk, et al, Langmuir, 1997, 13, 6267. 23. M. Kruk, et al, Langmuir, 1999, 15, 5279. 24. M. Kruk, etal, J. Phys. Chem. B, 1997, 101, 583.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. Allrightsreserved
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The effect of pore structure of activated carbon on the adsorption of Congo red and vitamin B12 W. Z. Shen*, J. T. Zheng, Y. L. Zhang, J. G. Wang, and Z. F. Qin State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, RR.China, E-mail: shenw^zh@sina.com The mesopore content of a commercial activated carbon was increased by catalytic activation over various metal oxides. The adsorption of VB12 and Congo red was measured to investigate the effect of pore structure upon the adsorption property. It was found that both the pore size distribution and the pore volume play key roles in determining the adsorption properties. The wider the pore size was, the shorter of time reaching equilibrium was. Higher pore volume gave higher adsorption capacity. 1. INTRODUCTION AC is widely used as adsorbents in either liquid or gas phase processes due to its highly porous texture and large adsorptive capacity. Adsorption capacity of AC is mainly dependent on the pore characteristics such as surface area, pore size and its distribution [1]. Only the micropore and the mesopore contribute to the selective adsorption. Generally, most commercial ACs are extremely microporous with high surface areas and are mainly applied to the processes involving gas molecules and small liquid molecules. When larger molecules are needed to remove from the solution, high mesopore containing AC is required. Tamai discussed the dye adsorption on mesoporous ACF [2]. Pelekani studied the importance of pore size distribution in the competitive adsorption between atrazine and Congo red [3]. In this study, the modified activated carbons based on the same original activated carbon were evaluated by the adsorption of VB12 and Congo red. The purpose was to investigate the effects of pore size and volume upon the adsorption capacity and rate. 2. EXPERIMENTAL 2.1. Materials The target molecules are VB12 (Beijing Medical Chemical Co., BR, RR. China) and Congo red (Tianjin Dye Factory, Ind. RR. China). Table 1 compares the primary properties of VB12 and Congo red. Several ACs with different pore size distributions and surface areas were produced from a commercial AC. The modification was performed as follows: raw AC (signed: R-AC); iron
780
catalyzed CO2 activation at 870 °C for 90 min (signed: Fe-AC); yttrium and cerium catalyzed steam activation at 680 °C for 360 min (signed: Y/Ce-AC); reactivated with steam at 735 °C for 120 min (signed: AC-735) and 870 °C for 90 min (signed: AC-870); yttrium oxide catalyzed steam activation at 870 °C for 60 min the yttrium nitrate loaded 0.6 wt% and 0.2 wt% (signed: Yl-AC and Y4-AC), at 800 °C for 180 min the yttrium loaded 2.0 wt% and 0.2 wt% (signed:Y2-AC and Y3-AC). Table 1 Primary properties of VB12 and Congo red Molecular weight (g/mol) 1355.38 VB12 650.73 Congo red
Width (nm) 1.835 2.62
Depth (nm) 1.412 0.74
Thickness (nm) 1.14 0.43
2.2. Analysis and characterization The VB12 and CR were analyzed using 7550 spectrophotometer (Shanghai Analyzer Company, China), with detected characteristic peaks at 361nm and 488nm, respectively. The nitrogen adsorption of the ACs was measured using an ASAP2000 Gas Adsorption Analyzer (Micromertics Instrument Co.). The total surface areas, mesopore surface area and volume were calculated by the BET equation and BJH method. Adsorption experiments were carried out using batch equilibrium technique. The initial concentrations were lOOmg/1 for Congo red and VB12, respectively. To keep Cong red ionized, ImM phosphate was buffered in the Congo red solution. A series of 200ml Congo red containing 0.30 g AC and 200ml VB12 with 0.10 g AC samples were sealed and shaken at room temperature. The adsorbed amounts were calculated by the difference of concentration. 3. RESULTS AND DISCUSSION 3.1. Physical characterization of adsorbents Table 2 summarizes the surface area, total volume, micropore and mesopore volume of samples. After catalytic activation modified, the surface areas and pore volumes of the ACs are increased. The pore size distributions calculated based on the BJH method are shown in Fig. 1. For all the cases, there is a representative peak between 3 nm and 4.5 nm. The pore ^ 1-6 [o) 1.4 "^"i 1.2
Y2-AC Y3-AC Y4-AC
Q 0.8 ^ 0.4
I 0.2 ^ o.oi
10
Pore size (nm)
Pore size (nm)
Fig. 1. Pore size distribution of adsorbents
size distribution of Fe-AC is different from others in 20 - 100 nm. There is no difference between R-AC and AC-735 in pore size distribution except the relative intensity in the range of 3 - 4.5 nm, where the R-AC has less volume than the AC-735. A larger distribution appears in 4.5 - 20 nm for AC-870. It is similar for Y3 and Y4 except the intensity of the former is
781
higher than that of the latter in 1 2 - 2 0 nm. As for Y2, a plateau appears from 4.5 to 30 nm and gradually weakens to 100 nm. However, the mesopore of Y/Ce-AC concentrates in 2.5 to 20 nm. Table 2 Structure parameters of activated carbons R-AC Fe-AC Y/Ce-AC Samples 1222 1531 SBET(m /g) 1535 1.37 VTotal(cmVg) 0.74 1.31 VMic(cmVg) 0.304 0.142 0.236 1.190 VMeso(cmVg) 0.428 0.890
Y4 AC-735 AC-870 Yl Y2 Y3 1771 1244 1506 1427 1507 1436 1.39 0.81 1.18 1.38 1.13 1.06 0.267 0.240 0.235 0.275 0.287 0.347 0.441 1.010 0.855 1.120 0.778 0.716
3.2. Adsorption of VB12 Fig. 2 gives the amount adsorbed VB12 as a function of adsorption times. The Y/Ce-AC exhibited the fastest adsorption and the Fe-AC showed the biggest amount adsorbed among all samples. This is due to its higher mesoporous volume and the more concentrated pore size distribution. Although AC-870 has larger mesoporous volume than Y/Ce-AC, its pore size distribution is broader. VB12 cannot be efficiently adsorbed on the pores of AC-870 because the pore distribution is so wide that adsorption can only take place on the surface. Y2-AC has similar volume to Y/Ce-AC, but it has more pores that locate in the range of 20 - 100 nm. The VB12 molecule is also adsorbed on the pore surface, and this weak deriving force resulted in slow adsorption rate and took long time to reach the equilibrium. Y3 and Y4 have similar pore volume and pore size distribution, but the adsorption curve is quite different. The reason maybe their different pore shapes. Therefore, Yl finally gave fewer amounts adsorbed than
-•"Y/Cfe -0-Y1
5
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15 20 25 •nrTB(h)
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-X-
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-«-
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5
10 15 2 D 2 5 3 0 3 5 4 0 0
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5
^^ * V2 -X-V3 I Y4
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Fig. 3. The adsorption of Congo red on adsorbents
that of Y4 due to its broader pore distribution, although it holds larger pore volume than that of Y4. The wider the pore is, the faster diffusion of VB12 molecule is, and the adsorption rate is faster, as a result, it can reach equilibrium in shorter time. The amount adsorbed is less and the adsorption rate is slower when there is more micropore in the AC. It was reported that the pores wider than lOnm affect the adsorption rate and the amount adsorbed is mainly determined by the pores between 2.0 - 4.0 nm [5]. In this study, it shows that the pores larger than 4 nm also determine the adsorption capacity. The most pores of Y/Ce-AC are larger than lOnm and more relative concentrated in the range of less than 20 nm, therefore, the adsorption rate and amount adsorbed is the highest among these samples (except Fe-AC). By contrast, the R-AC has the poor pore size distribution and less poor volume, and consequently it showed slow adsorption rate and fewer amounts adsorbed.
782
3.3. Adsorption of Congo red The amount adsorbed of Congo red as a function of adsorption time is shown in Fig.3. Congo red is larger in width and much less in depth and thickness than that of VB12. The adsorption behaviors are similar to that of VB12. There is no difference in the adsorption amount of Congo red on R-AC and AC-735, suggesting that the pores in 2 - 4 nm do not play a significant role in adsorption. The mesopore content of R-AC is the least among the samples and thus the Congo red adsorbed on it is also the least. The AC-870 has more pores in the range of 4 - 100 nm than R-AC and shows higher adsorption capacity. The highest amount adsorbed is observed over Fe-AC and the fastest adsorption rate obtained over Y/Ce-AC. The Yl, Y3 and Y4 have similar pore volumes and pores size distributions, consequently exhibited similar the adsorption behaviors. However, Y2 gives an increasing adsorption in the process due to its higher pore size distribution from 4 nm to 100 nm. The adsorption capacity of samples for Congo red only reaches 50 % of VB12, implying that the width of Congo red affects the adsorption properties on the samples. 4. CONCLUSION The least adsorbed amounts of the probe molecules and the slowest adsorption rate were observed over R-AC and 735-AC because of their lower mesopore content. The Y/Ce-AC gives the fastest adsorption rate and higher adsorption capacity, which is resulted from its more concentrated pore size distribution and larger mesopore volume. The broadest pore distribution and the highest pore volume in the range of 20-100 nm of Fe-AC resulted in the largest adsorption capacity for both of the two probe molecules. The Y2 with a pore size distribution ranging from 4 nm to 100 nm appears a gradually increasing adsorption with a longer time to reach adsorption equilibrium. In summary, the higher content of mesopore, the less of the diffusion resistance and the faster adsorption rate for the investigated AC. REFERENCES 1. L.R. Radovic (ed.), Chemistry and Physics of Carbon, Vol.27, Marcel Dekker, Inc, New York, 2000. 2. H. Tamai, T.Yoshida, M. Sasaki, H. Yasuda. Carbon, 37 (1999) 983. 3. C. Pelekani, V.L. Snoeyink, Carbon, 39 (2001) 25. 4. S.J. Gregg, and K.S.W. Sing (eds.). Adsorption, Surface Area and Porosity, Academic Press, London, 1982 5. Z.C. Liu. Preparation of Pitch-Based Spherical Mesoporous Activated Carbon and The Control of Structure, D.S. Thesis, Institute of Coal Chemistry, Taiyuan, P.R. China, 1999.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved
783
A Possibility of Block-Copolymer Templated Mesoporous Silica Films Applied to Surface Photo Voltage (SPV) type NOx Gas Sensor T. Yamada", H. S. Zhou*', H. Uchida^ M. Tomita", Y. Ueno', T. Katsube^ and I. Honma' * Energy Electronics Institute, National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan. ^ Department of Information and Computer Science, Faculty of Engineering, Saitama University, 255 Shimo-okubo, Saitama, Saitama 338-0825, Japan. ^' Environmental Information Systems Laboratory, NTT Lifestyle and Environmental Technology Laboratories, Nippon Telegraph and Telephone Corporation, 3-1, MorinosatoWakamiya, Atsugi, Kanagawa 243-0198, Japan. A self-ordered hexagonal and cubic-like mesoporous silica film has been successfully fabricated from a Metal-Insulator-Semiconductor device applied to a Nitrogen oxides (NOx) gas sensor based on the surface photo voltage system. These self-ordered mesoporous silica films are synthesized by using a nonionic triblock copolymer surfactant as a template in spin coating. The sensing characteristics as a NOx gas sensor are dependent on both mesostructures and exposure gases. 1. INTRODUCTION Nitrogen oxides (NOx) generated by combustion are harmful to one's health, and all the nations have regulated their concentrations in the environment. Therefore, development of a highly sensitive, highly responsive, and portable device for monitoring these gases are urgently required. The surface photo voltage (SPV) semiconductor characterization technique [1,2] has great potential for satisfying the requirements of such gas sensors. The basic principle of this characterization technique is based on the semiconductor surface voltage properties of the metal-insulator-semiconductor (MIS) structure [2]. By measuring semiconductor photocurrent, the SPV system sensitively detects variation in surface voltage, which is effected by physical adsorption and chemical interaction between the target gases and the sensitive layer [1]. Therefore enhancing sensor performance requires refinement of the metal and insulator layer of the MIS. Especially, the capacitance of the insulator layer depends on the gas adsorption performance, and improvement of gas adsorption performance leads directly to enhancement of SPV gas sensor characteristics.
' To whom all correspondence should be addressed (E-mail: hs.zhou@aist.go.jp)
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The large surface area created by nanosize pores in mesoporous materials enables improvement in the gas adsorption properties of SPV devices. This paper reports possible application of the self-ordered hexagonal and cubic-like mesoporous silica fihns [3-5] to SPV [5-8] type NOx gas sensor, the film being synthesized by the spin coating method and by use of nonionic poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) (PEO-PPOPEO) type triblock copolymer surfactant as a structure-directing agent. The NOx gas sensors are based on the SPV characterization system. The mesoporous silica film is assembled as an insulator gas adsorption layer of the MIS structure based on the SPV characterization system, 2. EXPERIMENTAL 2.1. Construction of SPV Device The SPV sensor system is fabricated from the MIS structure of semiconductors. Specifically, n-Si having SiOz and Si3N4 layers is used as a substrate. Hexagonal silica film by PI23 (EO20PO70-EO20) or cubic-like silica film by F127 (EOioo-POes-EOioo) is prepared on the Si3N4 layer of the substrate as a gas adsorption insulator layer. Subsequently, a thin Au electrode is deposited on the mesoporous film. The subject gas is dispersed into this thin Au electrode and reaches the top of mesoporous film. An Al electrode is fabricated on the rear side of the n-Si surface. Consequently, the mesoporous silica combined MIS structure of the SPV sensor is constituted of Au/ hexagonal or cubic-like mesoporous silica/ SisNV Si02/ n-Si/ Al (SPV-hex or SPV-cub). The sensing properties of each SPV device were estimated under NO (lOOppm, lOOsccm) and NO2 (50ppm, 1 OOsccm) gas exposure conditions. 2.2. Measurement System of SPV The measurement system for the SPV method was assembled, as shown in Figure 1. The basic sensing principle is based on the detection of the change in the potential of the semiconductor surface. A variety of insulator capacitances were induced due to gas adsorption, and charges were trapped in the insulator. The electric response of the sensor to NO gas or NO2 gas was measured under a cyclic flow of NO gas (100 ppm) and standard air or NO2 gas (50 ppm) and standard air. These gases were controlled by a mass flow controller and a multiport valve. The 100-ppm NO gas or 50-ppm NO2 gas was prepared by this mass flow control system from source NO gas and standard air or source NO2 gas and standard air. Both the NO gas and the standard air or both the NO2 gas and the standard air were supplied to the sensor at a constant flow rate of 100 standard cc/min (seem). In this measurement system, the bias voltage, which was controlled by a Lock-in amplifier, was applied between the Au and Al electrodes. An alternating modulated LED beam (k = 930nm, v = IkHz), which was also controlled by a Lock-in amplifier, was irradiated on the rear side of the semiconductor to induce an AC photocurrent. C^Ml The sensitivity to supplied NO gas and NO2 gas was estimated by the induced ^—-^ L r photocurrent through the MIS structure, ^ - ^ '"'respectively.
785
(a)
1
1
1
1
M
(B
1
1
xl(X) ^
xKXX)
1
29 n Fjg. 2. The XRD pattern of self-ordered hexagonal film by PI 23. (b) The TEM image of SBA-15 powder by P123. (c) The TEM image of self-ordered hexagonal film by P123.
V
26 n
Fig. 3. The XRD pattern of self-ordered cubiclike film by F127. (b) The TEM image of SBA-16 powder by F127. (c) The TEM image of selfordered cubic-like film by F127.
3. RESULTS AND DISCUSSION 3.1 Film Structure Figures 2 and 3 show the X-ray diffraction (XRD) patterns and transmission electron micrograph (TEM) images of the self-ordered hexagonal by PI23 (Figure 2) and cubic-like by F127 (Figure 3) structured films after calcination. In Figure 2, The XRD patterns and TEM images indicate the ordered hexagonal pore structure in the self-ordered hexagonal structured film. The film has a highly oriented hexagonal structure and the pore channels are parallel to the substrate surface [4,9]. Therefore, the self-ordered hexagonal structure film is estimated to be a one-dimensional hexagonal (IDH) structure. In Figure 3, the XRD pattern has only two diffraction peaks. Difficulties are encountered in inferring the exact structure of the film; however, the TEM images as shown in Figure 3(c) suggest that this film has a cubic structure. Also, based on the reported results for powders [10] and dip-coated film [3], both the SBA-16 powders and dip coated thick film have cubic mesostructures. Moreover, they were prepared by using the same triblock copolymer F127 as a structure-directing agent. In this study, this spin coated thin film also uses the same triblock copolymer. Therefore, we consider that its cubic structure was strained, because of the differences between the dip coating and the spin coating film processes. Consequently, the spin-coated film by F127 was determined to have a cubic-like structure with bicontinuous mesopores. 3.2 SPV Character Figure 4 shows the Current-Bias voltage curves (CB curve) of SPV-hex and SPV-cub, as measured at room temperature under standard air (lOOsccm) conditions and under NO conditions. These SPV samples exhibit clear bias shift by NO gas adsorption in mesoporous films and also indicate the gas accessibility Bias [V] dependence on mesostructures. The bias shift resulted ^.^ ^ Current-Bias Voltage Curve of from changes m the dielectric constant and charge in sPV-hex and SPV-cub under air and NO the insulator layer, which in turn were caused by condition.
786
physical adsorption and chemical interaction between the detected gases and the gas-sensitive film [5-8]. Figure 5 shows the CB curve of SPV-hex and SPVcub, as measured at room temperature under standard air conditions and under NO2 conditions. These SPV samples also exhibit clear bias shift and by NO2 adsorption in mesoporous films and indicate the dependence on mesostructures. The bias shift differences between exposure gases in each device are also observed in each figure. This result is strongly influenced dielectric constants of exposure gases.
800| <700| ^600
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I 200! 100
oy "-:=0.5 0 0.5 1 1.5
Bias [V] Fig.5. Current-Bias Voltage Curve of Self-ordered hexagonal and cubic-like structured SPV-hex and SPV-cub under air and mesoporous silicate film combined SPV type gas NO2 condition. sensors (SPV-hex and SPV-cub) have been successfully fabricated. These sensors have sensitivities for NO gas (100 ppm and lOOsccm) and NO2 gas (50 ppm and lOOsccm), and exhibit different sensing performances depending on the accessibility of the mesostructure to the gas. In the case of the SPV-hex film (IDH structure), there is poor accessibility to the mesopore for the gas, because the pore is only open on the side of film due to its highly ordered structure. In the case of the SPV-cub film (cubic-like structure), the gas could access the mesopore from both the top and the side of the film, due to its bi-continuous 3D pore structure. Thus, the self-ordered cubic structured mesoporous silicate film is more suitable for the SPV type gas sensors. 4. CONCLUSIONS
REFERENCES 1. W. Zhang, H. Uchida, T. Katsube, T. Nakatsubo, Y. Nishioka, Sensors and Actuators B 49(1999)58. 2. D. K. Schroder, Meas. ScL TechnoL 12 (2001) R16. 3. D. Zhao, P. Yang, N. Melosh, J. Feng, B. F. Chmelka, G. D. Stucky, Adv. Mater. 10 (1998)1380. 4. T. Yamada, K. Asai, A. Endo, H. S. Zhou, I. Honma, J. Mater. Sci. Lett. 19 (2000) 2167. 5. T. Yamada, H. S. Zhou, H. Uchida, M. Tomita, Y. Ueno, I. Honma, K. Asai and T. Katsube, Micro. Meso. Mater, in press 6. H. S. Zhou, T. Yamada, K. Asai, I. Honma, H.Uchida and T..Katsube, Jpn. J. Appl. Phys. 40(2001)7098. 7. T. Yamada, H. S. Zhou, H. Uchida, M. Tomita, Y. Ueno, T. Ichino, I. Honma, K. Asai and T. Katsube, Adv. Mater. 14 (2002) 812. 8. T. Yamada, H. S. Zhou, H. Uchida, M. Tomita, Y. Ueno, K. Asai, I. Honma and T. Katsube, lEICE Trans. Electronics E85-C (2002) 1304. 9. Y. Lu, R. Ganguli, C. A. Drewien, M. T. Anderson, C. J. Brinker, W. Gong, Y. Guo, H. Soyez, B. Dunn, M. H. Hung, J. I. Zink, Nature 389 (1997) 364. 10. D. Zhao, Q. Huo, J. Feng, B. F. Chmelka, G. D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
Proton conducting silica mesoporous/heteropolyacid-PVA/SSA composite membrane for polymer electrolyte membrane fuel cell
787
nano-
Young-Hwan Chu, Jung-Eun Lim, Hyun-Jong Kim, Chang-Ha Lee, Hak-Soo Han and YongGun Shul Department of chemical engineering, Yonsei University, Seodaemun-Gu, Shinchon-Dong, 134, Seoul, 120-749, Korea Silica mesoporous/heteropolyacid(HPA)-PVA/SSA hybrid membrane was prepared by solgel processes to make a proton conducting membrane. The method involves stabilization of HPA in inorganic-organic hybrid membrane. By adding HPA in the silica mesoporousPVA/SSA matrix, the membrane showed high proton conductivities of 10"^ S/cm at room temperature. Methanol crossover was decreased with increasing the contents of mesoporous material. This simple method can be applied to make a proton conducting membrane involving mesoporous structure. 1. INTRODUCTION The recent discovery of mesoporous silicates formed by the cooperative self-assembly of silicates and surfactants has opened up a new range of possibilities for separation engineering and catalysis involving large molecules. Nowadays, research work on mesoporous transparent hard spheres, glass sheets, hollow spheres, fibers, thin film and monoliths with controlled structure and pore size are still expanding and resulting in the discovery of new method through a combination of inorganic-organic cooperative assembly processes and emulsion or sol-gel chemistries. Recently, inorganic-organic nanocomposites are a remarkable family of isotropic, flexible, amorphous hybrid materials, which have been studied extensively because of their potential applicability to many industrial materials such as automobile, airplane, bridges, artificial tissues, contact lenses, bones, integrated circuits insulators and others. The proton conductor has been studied for a large number of inorganic and polymer materials because of the technological potential for application in fiiel cells, hydrogen separation, water electrolysis and other electrochemical devices. However, their use in devices is limited by numerous materials requirements. One of the major problems in low temperature proton conductor is the narrow temperature in temperature range of room temperature-100°C. In addition methanol crossover through the membrane results in a decreased performance in direct methanol fiiel cell due to the depolarization of the oxygenreducing cathode. To solve these problems, new synthetic route has been developed to make a inorganic-organic hybrid materials membranes for electrochemical devices such as fuel cells and batteries
788
2. EXPERIMENTAL 2.1. Synthesis To prepare silica precursor, 9.7 Ig of tetramethoxysilane (TMOS) was partially hydrolyzed by a substoichiometirc amount of water under acid conditions for 2hr at room temperature. Various concentration (0.49wt%, 0.98wt% and 1.47wt%) of HPA (silicophosphoric acid ; SiWA and tungstoposphoric acid ; PWA) are used as proton conductor in the silica mesoporous matrix. 20g of surfactant solution (cetyltrimethyl-ammoniumchloride solution, 25wt% ; CTAC) was mixed with various concentration (33wt%, 50wt%) of PVA/SSA to prepare template. Then a given silica precursor was dropped into the mixture of surfactant and PVA/SSA with stirring very vigorously at room temperature in a glass vessel. After aging about various times in room temperature, we prepared silica mesoporous/HPAPVA/SSA hybrid membrane by casting into the filter paper. Then the film was dried at room temperature for 12hr. After drying, membranes were heated at 125°C during 1 hour for crosshnking of PVA/SSA. 2.2. Charaterization X-ray diffraction patterns of siHca mesoporous/HPA-PVA/SSA membrane were obtained with a Cu-Ka X-ray source using a Rigaku instrument at room temperature. For the measurement of the conductivity, a conductivity cell was used to measure the designed membrane. The measurements were performed in a O.IM HCl electrolyte solution at a temperature of 25''C, using impedance measurement. It were made using Autolab(AUT30.FRA2). Impedance spectra were recorded between 100000 and 0.1 Hz. For the estimation of crossover rate of methanol, the oxygen supply to the cathode was adjusted by using the ilow-meter at the inlet of the cathode compartment. And 2.0M methanol solution were fed to the anode side. The on line G.C. was connected to outline to analyze methanol crossover rate in real time. Overall crossover rate of membrane was performed on the temperatures range from room temperature to 60°C. Fuel cell performance was measured by unit cell test station with cross-sectional area of Icm^ at room temperature for direct methanol fuel cell. 3. RESULT AND DISCUSSION To make a membrane, there is optimal range for membrane casting was about 2-4hr after mixing. It changed to bulk gel after 4hr aging. For convincing the mesoporous structure, silica mesoporous/HPA-PVA/SSA hybrid membranes were measured by X-ray diffraction patterns. As shown in fig 1., the prepared membranes showed typical low angle (110) diffraction associated with the nature of mesoporous structure. And the intensities of mesoporous material increase as increasing the HPA loading and the aging time. It suggests that mesoporous is dependent on the HPA contents and the aging time Fig. 2. shows proton conductivities of the prepared membranes. The conducdvifies was increased from 2.236*10"^ to 2.930*10"^ with increasing the HPA contents from 0.49wt% to 1.47wt%, respectively. When we change the different kinds of HPA in silica mesoporous
789
matrix, the conductivity changes depend on HPA composition. For example, conductivity of hybrid membrane containing the SiWA is 3.190*10'"^, and membrane containing PWA, it is 6.120*10'^ respectively. The conductivity of 10'^ S/cm order is due to the loading of HPA in the mesoporous matrix enabling transport of protons through mesoporous pore. Crossover test of methanol on silica mesoporous/HPA-PVA/SSA membranes has been tested in direct methanol fuel cell system at from 2(fC to 60°C. In the case Nafion 117 membrane, the permeability increased with increasing the operating temperature. As shown in fig 3., this trend was also observed for the prepared hybrid membranes. As increasing the HPA content in membrane, the methanol crossover rate of silica mesoporous/HPA-PVA/SSA membranes is decreased. In comparing with Nafion 117, silica mesoporous/HPA-PVA/SSA membrane remarkably reduced crossover rate of
HPA HPA HPA
f\
49% 98% 47%
_ • O - T -^
PVA/SSA PVA/SSA PVA/SSA PVA/SSA
33%. 33%, 50%. 50%.
2hr 3hr 2hr 3hr
\\
< ^ 'H
\
\
^
.-^ HPA content
Fig. 1. X-ray diffraction pattern of Silica mesoporous/HPA-PVA/SSA membrane (PVA/SSA : 50wt% aging time : 3hr).
i '" 1
60
S
40
S
20
i
- • O _.^^ - » -
Fig. 2. conductivity change of silica membrane as a function of HPA mesoporous /HPA-PVA/SSA contents.
HPA 0 49%. PVA/SSA 33%. 3hr HPA 1 4?%. PVA/SSA 50%. 3hr HPA 0 49%. PVA/SSA 50%. 3hr nadon 115
-'''
o
Temperature (°C)
Fig. 3. Methanol permeability of the silica mesoporous/HPA-PVA/SSA membranes.
L-nl il.-asity(mA/ m )
Fig. 4. Current-voltage charasteristic obtained got methanol/oxygen fuel cell utilizing silica mesoporous/HPAPVA/SSA membranes.
methanol up to 80%. Fig. 4 shows the current-voltage characteristic obtained for methanol/oxygen fuel cell utilizing the silica mesoporous-PVA/SSA membrane which contains 1.47% of HPA(PWA). The experiments were performed at room temperature. The highest
790
current densities obtained for the fuel cell studied were approximately equal to lOmA/cm^. From these results, we can suggest HPA in silica mesoporous-PVA/SSA matrix is promissing material as a proton conducting membrane. ACKNOWLEDGMENTS This research was funded by Center for Ultramicro-chemical Process System sponsored by KOSEF, BK21 program, and Korea Research Foundation Grant. (KRF-2001-005-E00030).
REFERENCES 1. RJ. Bruinsma, A.Y. Kim, J. Liu, S. baskran, Chem. Mater. 9 (1997) 2507 2. S.H. Tolbert, T.E. Schaffer, J. Feng, RK. Hansma, G.D. stuky, Chem. Mater. 9 (1997) 1962 3. M.C. Weinssenberger, G.C. Goltner, M. Antonietti, Ber. Bunsen Phys. Chem. 101 (1997) 1679 4. Q. Huo, J. Feng, F. Schuth, G.D. Stucky, Chem. Mater. 9 (1997) 14 5. H.-H. Huang, G.L. Wilkes, J.G. Carlson, Polymer 30 (1989) 2001 6. A.M. Grillone, et al., J. Electrochem. Soc. 146 (1999) 27 7.1. Honma, et al., Solid State Ionics 118 (1998) 29
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
791
Hydrothermal Synthesis of Titania Nanotube and its Application for DyeSensitized Solar Cell S. Uchida^ R. Chiba^ M. Tomiha^ N. Masaki^ and M. Shirai' ^Institute of Multidisciplinary Research for Advanced Materials(IMRAM), Research Building of Chemical Reaction Science, Tohoku University, Sendai 980-8577, Japan* ^Supercritical Fluid Research Center, National Institute of Advanced Industrial Science and Technology(AIST), Nigatake, Miyagino, Sendai, 983-8551, Japan Titania nano tube was successfully obtained by hydrothermal treatment of nano size Ti02 fine powder in 10 M NaOH solution for 20 h at 110 °C. The morphology of product was 8-10 nm in width and about 100 nm in length with tubular shape. The specific surface area was 270 m ^ g ' , much higher than that of starting material titania powder with 50 m ^ g ' . Photovoltaic properties of these titania film was also examined. 1. INTRODUCTION The TiO^ particles have been widely used for various applications such as pigment, photocatalyst, photo-electron conversion device, UV protection shield, Anti-Bacteria material, and so on. In order to improve their specific properties, much effort have been paid for morphology control of the Ti02 particles. Titania nano tube is one of the promising material for above use because of the high specific surface area and specially ordered structure. It was firstly synthesized by a two-step template process. Starting from the porous aluminum oxide, a polymer mold suitable for the formation of titanium dioxide nano tubes was obtained. Then, the tubular structure was formed by electrochemical deposition in the mold. After dissolution of the polymer, titania nano tubes in 70 - 100 nm inner diameter were obtained [1-3]. Similar replication process has been also reported by using SiO^ sheaths as a mold [4, 5]. The formation of titania nano tubes filled with up to 24.5% of Pt metal clusters is reported by sol-gel processing with an inorganic platinum salt [Pt(NH3)J(HC03)2 as structure-directing agent. Very small fibers of the salt with sizes in the nano meter range are coated with titanate species forming the tube walls [6]. Mono dispersed hollow nano cylinders consisting of crystallized titania particles have been prepared directly in a porous alumina membrane by a deposition technique using an aqueous solution system of titanium tetrafluoride [7]. Adachi et al. reported that titania nano tubes with high photocatalytic activity were synthesized by surfactant assisted templating mechanism using laurylamine / tetra-isopropyl-ortho-titanate (TIPT) with acetylacetone system [8]. They also examined titania nano tubes for dye-sensitized solar cells [9].
792
In contrast to these methods, direct formation process of the titania nano tube without a template were also reported by D. S. Seo et al. [10, 11] Nanotube-shaped Ti02 powder was prepared by a digestion of the powder obtained by the reaction of TiOCl2 and NH^OH solutions over 100 °C. Recently a notable simple method has been proposed by Kasuga et al., in which titania nano tubes with 8 nm in diameter and 100 nm in length were obtained by only treating TiOjSiOj gel or nano size Ti02 fine powders in 5-10 M NaOH solutions for 20 h at 110 °C. Synthesized titania nano tubes had a large specific surface area up to 400 m^g' [12-16]. In this study, Kasuga's synthesis method was traced to produce titania nano tubes. And then it was newly examined to apply for the electrode of dye-sensitized solar cell with aiming to improve the photoelectronic properties. 2. EXPERIMENTAL The Ti02 nano tube was synthesized based on the method developed by Kasuga as mentioned in introduction. The nano meter sized Ti02 powder (P-25, Nippon Aerosil Co., Ltd.) was specially used as the starting material considering future mass production of dyesensitized solar cell in low cost. The primary particle size was 30 nm in diameter. The crystal structure was the mixture of anatase form (70 %) and rutile. A typical experimental procedure is as follows. Firstly 8 g of NaOH and 20 ml of water were put into a tubular 30 cm^ of Tefion''^ cup to form 10 M NaOH solution. Then 0.2 g of Ti02 (P-25) powder was added in the solution. These were placed in the pressure resistable glass bottle (100 mL GL-45, Duran). After scaling the bottle, it was set into a dry oven at 110 °C for 20 h. After the reaction was completed, the product was separated from the solution by centrifuge, then rinsed with hydrochloric acid and pure water to remove the residual alkaline, and finally dried by freeze drier. Photovoltaic properties were measured fundamentally according to the method as previously reported [17, 18]. 3. RESULTS AND DISCUSSION In the preliminary experiments, the reactivity of TiO^ fine particles in the hot alkaline solution was investigated as shown in Fig. 1 [19-21]. Here the large field of amorphous phase was observed at higher NaOH concentration and at higher temperature. The critical NaOH concentration was 10 molkg'-HoO. The formation area of titania nano tube was also shown as a dotted line. It was almost overlapped to the amorphous phase. When the NaOH concentration was more than 45 molkg '-H2O, no nanotube-shaped product could be obtained. Hydrothermal synthesis was carried out to form the titania nano tube at NaOH concentration 10 molkg' and temperature of 110 °C for 20h. TEM images of the product are shown in Fig. 2. The morphology of products was quite similar as already reported. Numerous needle-shaped products about 100 nm in length are seen. The width of products is about 8 nm and the lattice fringes with 1.5 nm thickness are present on both side. The specific surface area measured by BET was 270 m~g''. This is much higher than that of 50 m"g'' before the hydrothermal
793
10
20
30
40
[NaOHJ/molkg-i-H20
Fig. 1 Formation diagram of products in the Ti02-NaOH-H20 system at various temperatures for 2h. Fig. 2 TEM image of titania nano tube treatment, and no spherical particle of and electron diffraction pattern starting material about 30 nm in diameter of selected area. can be seen. Therefore the formation of these needle-shaped products is ! ! ! ! ! ! considered to be proceeded by dissolution-reprecipitation mechanism. The selected-area electron diffraction < pattern from agglomerated tube products in TEM observation showed (101), ViX -\ Ti02 nano tube (112), (200), (211) and (204) diffractions of the anatase phase. • ^ " Ti02 nano powder(P-25) Finally the application of this titania i i i L _L J1 1. \l nano tube for dye-sensitized solar cell 400 600 200 800 was examined. Here, about 0.25 cm' Voltage / mV active area of the cells were constructed. The results of photovoltaic properties Fig. 3 Photovoltaic properties of solar cells with titania nano tube and titania nano powder of these titania film were shown in Fig. electrodes. 3. The titania film derived from titania nano tube showed Vn 704 mV, L 1.26 mA, r| = 2.9 %, FF = 0.66 as open-circuit voltage, short-circuit current density, energy conversion efficiency and fill factor, respectively. These results were nearly equal to those of titania nano powder (P-25); Vo^: = 767 mV, I^c = 1.20 mA, r| = 3.0 %, FF = 0.72. The slight increase of short-circuit current of titania nano tube is due to the increase of the amount of dye adsorbed. Relatively lower open-circuit voltage of titania nano tube is possibly the result of lower inter connectivity of the tubes.
794
4. CONCLUSIONS From the results of tests described above, the following conclusions may be drawn. Hydrothermal synthesis was carried out to form the titania nano tube. The morphology of product was about 8-10 nm in width and 100 nm in length with tubular shape. The specific surface area was 270 m^-g', much higher than that of starting material titania powder (50 m^-g'). Nevertheless, significant difference on photovoltaic properties of the solar cells with these titania films could not be observed. They were: (1) Titania nano tube; VQ^ = 0.704, I^^ = 1.26 mA, r| = 2.9 %, FF - 0.66 (2) Titania nano powder (P-25); V^c = 0.767,1,^, = 1.20 mA, T] = 3.0 %, FF - 0.72 For further improvement of the solar cell performance, optimum condition should be found out to avoid the aggregation of titania nano tubes. Acknowledgement The authors give special thanks to Mr. Aoyagi and Mr. Hayasaka in High-Voltage Electron Microscope Laboratory of Tohoku University for the TEM observation. REFERENCES 1. P. Hoyer, Langmuir, 12(6) (1996) 1411. 2. P. Hoyer, Adv. Mater., 8(10) (1996) 857. 3. X. H. Li, X. G. Zhang, and H. L. Li, Gaodcng Xuexiao Huaxuc Xuebao, 22(1), (2001) 130. 4. M. Zhang, Y. Bando, and K. Wada, J. Mater. Res., 16(5), (2001) 1408. 5. M. Zhang, Y. Bando, and K. Wada, J. Mater. Sci. Lett., 20(2) (2001) 167. 6. M. Wark, C. Hippe, and G. Schulz-Ekloff, Stud. Surf Sci. Catal., 129 (2001) 475. 7. H. Imai, Y. Takei, K. Shimizu, M. Matsuda, H. Hirashima, J. Mater. Chem., 9(12) (1999) 2971. 8. M. Adachi, Y. Murata, M. Harada, S. Yoshikawa, Chem. Lett., 8 (2000) 942. 9. M. Adachi, I. Okada, Y. Murata, T. Matsuda, Proc. 10th Int. Conf Unconventional Photoactive Systcms(UPS'Ol), September 4-8 , Les Diablerets, Switzerland, (2001) C-89. 10. D. S. Seo, J. K. Lee, and H. Kim, Han'guk Seramik Hakhoechi, 37(7) (2000) 700. 11. D. S. Seo, J. K. Lee, and H. Kim, J. Cryst. Growth, 229 (2001) 428. 12. T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, and K. Niihara, Langmuir, 14(12) (1998) 3160. 13. T. Kasuga, M. Hiramatsu, Eur. Pat. Appl., EP 832847 Al 1 Apr, (1998). 14. T. Kasuga, M. Hiramatsu, Jpn. Kokai Tokkyo Koho, JP 100152323 A2 9 Jun, (1998). 15. T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, and K. Niihara, Adv. Mater., 11(15) (1999)1307. 16. T. Kasuga, Mater. Integr., 12(1) (1999) 33. 17. S. Uchida, M. Tomiha and N. Masaki, Electrochemistry, 70(6) (2002) 244. 18. S. Uchida, R. Chiba, M. Tomiha N. Masaki and M. Shirai, Electrochemistry, 70(6) (2002) 52. 19. N. Masaki, S. Uchida, H. Yamane and T. Sato, Chem. Mat., 14 (1) (2002) 419. 20. M. Tomiha, N. Masaki, S. Uchida and T. Sato, J. Mat. Sci., 37 (2002) 2341. 21. N. Masaki, S. Uchida and T. Sato, J. Mat. Chem., 12 (2002) 305.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved
795
Preparation of Hydrophobic Ti-containing Mesoporous Silica by the F-modification and Their Photocatalytic Degradation of Organic Pollutant Diluted in Water Hiromi Yamashita*.
Hidetoshi Nakao,
Miho Okazaki
and
Masakazu Anpo
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Gakuen-cho 1 -1, Sakai, Osaka, Japan The Ti02 photocatalysts loaded on the fluoride-modified hydrophobic mesoporous silica were prepared and the adsorption properties and the photocatalytic degradation of an aqueous 2-propanol or 2-hexanol solution into CO2 and H2O have been studied. The amount of adsorption and the photocatalytic reactivities increased with increasing the F" content of these photocatalysts. 2-Hexanol diluted in water was adsorbed and degradated on the hydrophobic catalysts more efficiently than 2-propanol. 1. INTRODUCTION Recently, the environmental purification such as antibacterial, antifouling and deodorization has been investigated using Ti02 photocatalysts [1-6]. It is effective to use the hybrid materials, combined with adsorbents and photocatalysts for the purification of water [7-9]. Organic compounds can be decomposed into CO2 and H2O by Ti02 photocatalysts because of adsorbing them on it without diffusing into the liquid phase. Charcoal and zeolite, which have high surface area and porous structure, have been used as conventional adsorbents. In the system, the adsorption of organic compounds on the adsorbents needs to be appropriate. In the present study, therefore, the Ti02 photocatalysts loaded on the fluoride-modified hydrophobic mesoporous silica were prepared. Using these photocatalysts, the adsorption properties and photocatalytic reactivities for the degradation of various alcohols diluted in water have been studied. 2. EXPERIMENTAL The synthesis of the hydrophobic mesoporous silicas (denoted as HMS(F)) was performed using tetraethyl orthosilicate (TEOS), tetraethylammonium fluoride (TEAF) as the source of the fluoride and dodecylamine (DDA). TEOS dissolved in a mixture of 2propanol and ethanol and DDA (0-7.75g) dissolved in water with HCl (3ml) are mixtured, following to stirring at 295 K for 24 h. The precursor mixture was washed by distilled water.
796
K for 24 h, and then calcined at 823 K for 7 h. The ratio of TEAF to DDA was 0 (HMS), 0.25 (HMS(Fl)), 0.75 (HMS(F2)) and 1.25 (HMS(F3)). Furthermore, impTi/HMS(F) (10 wt% as Ti02) was prepared by impregnating HMS(F) with an aqueous titanium oxalate solution, then dried and calcined for 5 h at 773 K. The impTi/Si02 (aerosil silica), as the reference catalyst, was prepared by the same method. The photocatalytic reactions were carried out with the catalysts (50 mg) in the quartz tube with an alcohol (2-propanol or 2-hexanol) solution (2.6x10'^M, 25 ml). The sample was irradiated at 295 K using UV light (k> 280nm) from a 100 W high-pressure Hg lamp with stirring under O2 atmosphere in the system. The reaction products were analyzed by gas chromatography. The adsorption isotherms measurement was carried out stirring the catalysts (10 mg) dissolved in the solutions (5 ml) without the light irradiation at 295 K. The local structure of the titanium oxide species was investigated by UV-VIS, XRD and XAFS techniques. The hydrophobisity of the catalysts was studied measuring the H2O adsorption isotherms of the catalysts. 3. RESULTS AND DISCUSSION Figure 1 shows the adsorption isotherm of H2O molecules at 298 K obtained over various catalysts, indicating that the amount of adsorbed H2O molecules decreases with increasing the content of fluoride in the photocatalysts. This shows that fluoride-modified mesoporous silica can be hydrophobilized by the fluoride-modifications. The XRD patterns of impTi/HMS(F) exhibited only a single diffraction peak at low angle, indicating that impTi/HMS(F) has the structure of the HMS zeolite having mesoporous larger than 2 nm. The XANES spectra of these catalysts at the Ti K-edge exhibited the preedge 100
1UU' v ^
80
-
60
"
40
-
m
lighten
—•—2-PrOH / % /% ~*""CAcetone 02/% ~H 80
T
9 CL CVJ
0
c 0
0 > c 0 20 0
r-T^ \
^^L
xT^-*^ 20
P/Po Fig.1 The adsorption isotherms of H2O molecules at 298 K.
^d 40
y^
\>"
60
40 60 Time / h
20
80
Fig.2 The reaction time profiles of the photocatalytic degradation of 2-propanol diluted in water on the impTi/HMS(F3).
797
peak branched off into three distinct weak peaks. The FT-EXAFS spectra exhibited the existence of the two peak attributed to the neighboring Ti atoms and the neighboring O atoms, respectively. These XAFS (XANES and FT-EXAFS) resuhs indicate that the Ti02 loaded on the mesoporous silica exists in the mixture of anatase and rutile phases. The photocatalytic degradation of alcohol diluted in water was investigated using the impTi/HMS(F) photocatalysts. Figure 2 shows the reaction time profiles of the photocatalytic degradation of 2-propanol diluted in water on the impTi/HMS(F3). Some amounts of 2-propanol were adsorbed on the photocatalyst without light irradiation, and then the photocatalytic reaction proceeds under the UV-irradiation. The concentration of 2propanol decreased and that of acetone as the intermediate increased, and finally 2-propanol and acetone were degradated into CO2 and H2O. Figure 3 shows the adsorption properties and photocatalytic reactivities of impTi/HMS(F) photocatalysts for the degradation of 2-propanol in water. The amount of adsorbed 2propanol increased with increasing the amount of fluoride species, indicating that highly selective adsorption of organic compounds in their aqueous solutions can be realized by hydrophobilizing the support. Furthermore, it can be found that the more highly adsorption, the more efficiently photocatalytic reactivities were promoted. To account for the enhancement in the photocatalytic reactivities of the impTi/HMS(F) catalysts as compared to the non-modified impTi/HMS, the adsorption properties of 2propanol on these catalysts were studied. Figure 4 shows the adsorption isotherm of 2propanol on impTi/HMS(F3). Applied to the Langmuir equation, the derived adsorption constant on the impTi/HMS(F3) is K^,,, = 1.19 M '. Similarly, the adsorption constants of 2propanol were determined to be K^j^ - 0.64 M ' for impTi/HMS, K^^^ - 0.72 M ' for impTi/HMS(F 1), K,^,, = 0.92 M ' for impTi/HMS(F2). The enhanced adsorption constant by fluoride-modification of the catalysts coincides with the results of Fig. 2.
-V- 200
impTi/Si02
impTi/HMS(F1)
impTi/HMS
impTi/HMS{F3)
impTi/HMS(F2)
Fig.3 The adsorption properties and photocatalytic reactivities of impTi/HMS(F) photocatalysts for the degradation of 2-propanol in water.
0
0.5
1
1.5
2
2.5
3
[2-propanol]eq / mM Fig.4 Adsorption isotherm of 2-propanol over impTi/HMS(F3).
798
In the case of the degradation of 2-hexanol, the adsorption and photocatalytic reactivities were more increased with increasing the content of fluoride in the catalysts than 2-propanol due to the hydrophobic interaction between 2-hexanol molecules and the fluoride-modified photocatalyst. 4. CONCLUSIONS The Ti02 photocatalysts loaded on the fluoride-modified hydrophobic mesoporous silica exhibited the highly selective adsorption of organic compounds in the liquid phase, as shown in Scheme 1. More amount of adsorbing organic compounds on the impTi/HMS(F) led to the higher reactivity for photocatalytic degradation. 2-Hexanol diluted in water was more highly adsorbed on the catalysts and degradated more efficiently than 2-propanol.
0
•
0
H a O ^ ^^^Organic H2O , \ ^compound
?^
/^Organic compound
t Ti02
Hydrophilic
Ti02
Hydrophobic
S c h e m e t . The difference of the adsorption of H2O or organic compound on hydrophilichydrophobic adsorbents in the liquid phase.
ACKNOWLEDGMENT This work has been supported by the Grant-in-Aid Scientific Research from the Ministry of Education, Science, Culture, and Sports of Japan (Grants 12042271 and 13650845). The XAFS measurements were performed at the KEK-PF in the approval of the Photon Factory Program Advisory Committee (Proposal No. 2001G115). REFERENCES 1. N. Takeda, T. Torimoto, S. Sampath, S. Kuwabata, and H. Yoncyama, J. Phys. Chem., 99 (1995)9986. 2. C. Minero, G. Mariella, V. Maurino, and E. Pelizzetti, Langmuir, 16 (2000) 8964. 3. D. Brinkley and T. Engel, J. Phys. Chem., 104 (2000) 9836. 4. H. Yamashita, Y. Ichihashi, M. Anpo, and M. Che, J. Phys. Chem., 100 (1996) 16041. 5. H. Yamashita, S. Kawasaki, M. Anpo, and M. Che, J. Phys. Chem. B, 102 (1998) 5870. 6. H. Yamashita, M. Honda, and M. Anpo, J. Phys. Chem. B, 102 (1998) 10707. 7. M. Harada, M. Honda, H. Yamashita, and M. Anpo, Res. Chem. Intermed., 25 (1999) 757. 8. M. Harada, A. Tanii, H. Yamashita, and M. Anpo, Zeitschrifl fur Physikalische Chemie, Bd., 213(1999)59. 9. H. Yamashita, M. Harada, A. Tanii, M. Honda, and M. Anpo, Catal. Today, 63, (2000) 63.
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
799
Synthesis of functionalised silicas for immobilisation of homogeneous catalysts S. A. Riddel, W. P. Hems, A. Chesney, S. R. Watson Synetix Chiral Technology, Synetix, Billingham, Cleveland TS23 ILB. Fax +44 (01642)522542. E-mail stewart_riddel@ici.com The synthesis of inorganic-organic hybrid silica materials has been developed for immobilisation of a range of homogeneous catalysts. The materials prepared have been characterised using a variety of techniques and it has been found that the presence of high concentrations of organic species in the materials has a dramatic and surprising effect on the porosity and structure of the final material. 1. INTRODUCTION The immobilisation of homogeneous catalysts to solid supports provides potential for extending the benefits of heterogeneous catalysts to homogeneous systems. These benefits may include, the ease of separation of catalyst and reaction products leading to improved efficiency and also the potential for re-activation and re-use of the supported catalyst leading to reduced costs. These solid supports may be prepared by a sol-gel type reaction between alkyl silicates, organo-functionalised silanes and water (1). O R R O
R O
R O
/
R O
O R
/
H 3O •
R O
R O
I
/
O
I
/
J n
Base
O R
Porous S 0 lid
D ry
I
\ O R O R
^ O
3
1
R
^ S R O "^ / R O
Formation of short chains
O R
I
O R
I.
i ^ ^ O
/
I
S i \ ^ O O R
^o
R O ^ S i — R
R Polym erisation a n d c r o s s l i n k in g of sm all c h a i n s
o R O — ) ' — O R
R O
^ 8 R O
/
i ^
O
-S /
I
O R i ^ ^ O
I
,
\ O R O R
The pore diameters required to allow significant improvement catalyst immobilisation vary with application and here we report on methods of tailoring pore size using pore
800
expanders, templates and non-functionalised silanes (2). We also consider the difficulties encountered during material characterisation. 2. EXPERIMENTAL The preparation of the amorphous, porous functionalised silica was by co-hydrolysis of a tetra-alkyl silicate and an organofunctional silane with a structure directing surfactant employed as a template. The materials were not calcined and the template was removed from the resulting solid by refluxing in a suitable solvent. The porosity of the materials was optimised using a pore expander and by introducing a nonfunctionalised silane in combination with an organofunctionalised silane during the solgel process. The pore size distribution was determined by Nitrogen adsorption and further analysis of porosity was carried out using Scanning Electron Microscopy (SEM). Wide Angle X-ray Diffraction (WAXS) patterns were taken with a Siemens D5000 diffractomcter to confirm the amorphous nature of the materials (3). 3. RESULTS AND DISCUSSION Nitrogen adsorption analysis indicated that the addition of a pore expander resulted in an increase in average pore diameter from 20A to 40A. However, SEM images suggest that this may not in fact be wholly accurate. X-ray Fluorescence analysis of the support material revealed that despite significant surface coverage by the organofunctionality introduced by the organofunctional silane only a small percentage of this was employed in subsequent catalyst immobilisation reactions. The introduction of the non-functionalised silane, however reduced the surface concentration of the functional groups and actually improved the catalyst immobilisation. The WAXS patterns, revealed the materials were primarily amorphous with no significant degree of order, this was supported by SEM images. A common broad band pattern was present in all the samples studied with small peaks about 23" and 79". Additional broad scattering was observed in the 30" - 50" range for the non-solvent extracted materials and this has been attributed to the presence of the template and/or pore expander within the pores.
Fig. 1 SEM image of organoFunctionalised silane.
In summary, the presence of large concentrations of organic species appears to have a significant effect on the structure and porosity of the catalyst support materials prepared. The SEM images revealed not an ordered structure as obtained in crystalline mcsoporous silicas but a sponge-like structure with a wide distribution of pore sizes. Although BET porosimetry found the pore size to be in the region of 40A , the range observed in SEM images suggests the presence of significantly larger pores. Further work on materials with reduced organic content will be necessary to fully understand the process behind the forming of such structures.
801
REFERENCES 1. C.J.Brinker,G.W.Scherer. "Sol-Gel Science" UK Edition, 1990 2. Patent Application Pending 3. S.Norval, S.Riddel Internal Report XrdW00366RL001
This Page Intentionally Left Blank
803
AUTHOR INDEX A
Brinker, C.J.
Abecassis-Wolfovich, M.
247
Ahn, H.-G.
701
Ahn, J.-H.
117
Ahn, J.-W.
761
Ahn, W.S.
509, 523^,657
Ahn, Y.-S.
259^,669
295,351
Bruce, I.J.
57,581
Brunei, D.
419
c Cagnol, F.
281
Campeanu, V.
609
12>,, 427, 465^,517 165,,497
Cano, J.L.
685
Cao, Y.
485
Albouy, P.A.
281
Chae, H.J.
697
Alifanti, M.
585
Chan, Y.-T.
113
Alvarez, T.C.
331
Chang, J.H.
447
Aida, T. Aiello, R.
281
Chang, J.-S.
101
Anpo, M.
593, 597,,795
Chang, Y.H.
89, 193
Araki, H.
23
Chang, Y.-Y.
201
Archer, J.R.
29
Chang. K.H.
469
73, 427,,465
Chao, M.-C.
569
Che, S.
431
Chen, C.-W.
617
Amenitsch, A.
Ariga, K. Asai, K.
81
B
Chen, H.
347
Chen, H.-G.
457
Babonneau, F.
281
Chen, H.-R.
457
Baccile, N.
281
Chen, J.D.
319
Badici, A.
133
Chen, W.-H.
681
Bae, B.-S.
65
Cheng, B.-W.
311
Bae, J.Y. Bando, K.K.
65
Cheng, C.-F.
359,,363
Cheng, S.
311 227,315,569 481
Barolo, C.
375
Chcon, J.H.
Bartha, E.
609
Chesney, A.
799
Barthel, E.
535
Chia, S.
413
Batista, M.S.
705
Chiba, R.
791
Bekyarova, E.
395
Chilukuri, S.
573
Belousov, O.V.
299
Cho, M.S
Bchramini, J.N.
653
Cho, S.I.
523 137,669
Besson, S.
535
Cho, W.-J.
117,489
Blanc, A.C.
419
Cho, Y.S. Choi, B.D.
527,637 141
Blin, J.L.
243,,443
Bodoardo, S.
379
Choi, H.
201
Boilot, J.-P.
535
Choi, H.J.
523
Bonelli, B.
319,,379
Choi, K.-S.
645
Bonneviot, L.
133,,439
Choi, M.K.
37
Borello, L.
375,,379
Choi, S.-H.
93
804 Choi. J.W.
209
Choi, C.K.
387
Chong, P.H. Choo, D.H.
501 213,217
Ezersky, V.
F Fajula, F.
165,419 9, 45, 97
Choung, S.-J.
709
Fan, J.
Chu, W.
231
Ferid, R.
Chu, Y.-H.
787
Ferreira, Y.K.
Chung, J.K.
621
Fiorilli, S.
Chung, J.S. Chung, S.H.
621 213,217
413 319
Fukuoka, A.
Coluccia, S.
593
Fukushima, T.
Coppens, M.-O.
157
Furukawa, H.
Cortez, T.
685 535 281
Cui, X.
117
Gacoin, T.
399
Galameau, A. Garcia, B. Garrone, E. Garshtein, E.
Daehler, A.
493, 775
23 73,23 577
G
Czuryskiewicz, T.
D
197 453, 379, 375
Franville, A.-C.
551
Craven, E.
331
Fubini, B.
Clark, J.H.
Crepaldi, E.L.
721
535 319,419 133 319,375,379,453 247
Ge, S.
129
Gedanken, A.
721
Damlc, C.
573
Darmstadt, H.
335
Gcdeon, A.
299
Dascalcscu, C.
629
Gee, M.L.
493
Datsubc, Y.
783
Gibaud, A.
Deng, F.
161,461
Gibot, P.
351 41
165
Gigot, L.
443
251,323
Gobetto, R.
453
Dominguez, J.M.
331
Golctto, V.
351
Dong, B.H.
461
Gong, Y.J.
461
Dong, J.I.
689
Graillat, C.
439
Doshi, D.
351
Grange, P.
585
Dou, T.
255
Grosso, D.
281
Di Renzo, F. Domen, K.
Duan, X.
383,589
Guinness, F.M. Guo, W. Guo, Z. Guterl, P.
Egashira, Y.
327
Eguchi, T.
359
Eic, M.
145
Elangovan, S.P.
741
Ha, C.-S.
El-Safty, S.A.
173
Haddad, E.
Erenburg, A.I.
247
Hahm, Y.M.
383,589
Haller, G.L.
Evans, D.G. Exarhos, G.J.
447
57 307,367, 489 589 41
H
Halligudi, S.B.
117,307,367,489 299 89, 193 371 573
805 Hamoudi, S. Han, C. Han, D.C. Han, H.S. Han, H.-S. Han, M.H. Han, Y. Han, Y.-S. Hanai, A. Hanaoka, T. Hara, M. Hartmann, M. Hattori, Y.
23, 473
177 689 371 787 669 565 355 753 173 251 285,741
61
Inagaki, Y. Ion, A.
23 609
Ishida, H.
749
Itsuki, S.
625
Izuhal, S.
733
J Janssen, A.H. Jaroniec, M.
271 49, 263, 343
Jeong, S.-Y.
717
Jia, X.
485
Jin, M.-J.
509, 677
He, J.
383,589
Jin, M.-K.
85
He,N.
129,347,485
Jong, K.P.
271
Hems, W.P. Herbert, J. Hernandez, R. Herrier, G. Hcrskowitz, M. Higashimoto, S. Hiraga, K. Hong, S.B. Hong, W.-H. Honma, I. Hosoda, S. Hou, C. Hsu, H.-T. Hu, Y. Huang, J. Huang, L. Huang, M. Huang, S.-J. Hung, S.-C. Hwang, Y.K.
799 685 413 243 721 593 275 697 645 81,783
753 149 539 593 149 367 413 681 105 77,201. 101
I
Joo, S.H. Joseph, T. Jun, S.
65
Jung, S.J.
621
Jung, S.M
585
Jung, Y.W.
205
K Kabe, A. Kaliaguine, S.
Ichikuni, N. Ikegame, M. Ikeue, K. Ilescu, M. Iljima, S. Imamura, M. Inagaki, S.
73 23, 145,335 473,561,609,641
Kamiyama, T.
275
Kan, Q.
149
Kanazawa, Y.
597
Kancda, M.
431
Kancko, K.
61,395
Kang, H.S.
89
Kang, J.-K.
633,713
Kang, K.-K.
23 359 517 597 41 395 363 1
573 37, 669
Jung, J.-I.
Kang, M. Ichikawa, M.
33, 49, 53 275, 299, 669
Kang, T.W.
673 53 527, 637
Kanoh, H.
61
Kasuya, D.
395
Katou, T.
251
Kevan, L.
65
Kida, K.
597
Kim, CM.
209
Kim, D.H.
141,355
806 Kim, D.-P.
547
Lafont, V.
Kim, D.S.
101
Landau, M.V, Le Bihan, L.
Kim, D.-S.
633,713
Kim, G.-J.
137 141,355
Lee, B.
469 501,645
Lee, C.H.
Kim, H. Kim, H J .
761 213,217
Lee, C.-H. Lee, C.I.
419 121,247,721 363 251,323 213 787 89, 193
Kim, H.-J.
787
Lee, C.W.
89
Kim, H.-K.
513
Lee, C.Y.
501
669,761
Lee, D.-K.
Kim, H.-S.
259
Lee, G.-S.
633
Kim, J.-D.
169
Lee, H.C.
213,217
Kim, J.-H.
505
Lee, H.H.
Kim, J.-K.
513
Lee, H.I.
Kim, J.M.
53 477, 665
Kim, J.W.
477, 665
Kim, H.S.
633,713
761 477, 665
Lee, H.-Y.
37
Lee, J.-D.
701 109
Kim, J.Y.
259
Lee, J.H.
Kim, K.-S.
505
Lee, J.-K.
Kim, K.Y.
523 621
Lee, J.S.
213,217
Lee, J.-S. Lee, J.-W.
101,235
Kim, M.H. Kim, P.
109,209,637
Kim, S.-C.
633,713
Lee, J.Y.
Kim, S.-H.
509, 677
Lee, K.H.
Kim, T.-S.
547
Lee, K.-M.
85 33 205 213,217 387
275,335
Lee, K.-P.
93
Kim, Y.H.
209
Lee, S.-C.
709
Kirik, S.D.
299
Lee, S.-J.
509
Kiyozumi, Y.K.
169
Lee, S.W.
193
Klcitz, F.
221,399
Lee, U,-H.
77
Klotz, M.
535
Lee, W.
85
Ko, A.-N.
617
Lee, Y.
193
Koltypin, Y.
721
Lee, Y.-S.
Kondo, J.N.
251,323
Kim, T.-W.
Leonard, A.
29,89 243,443
307
Li,G.
149
61
Li,Q.
307, 367
Korin, E.
247
Li,T.
569
Kostcr, A.J.
271
Li, W.
231 255
Kong, L. Konishi, T.
49 263, 343
Li, W.-H.
Kubota, Y.
749
Li, Y.-W.
Kunieda, K.
733
Li, Z.H.
Kuroda, K.
577
Lim, J.-E.
787
69
Lim, J.H.
141
137, 141,469 501,355 77 101,201
Lim, S.H.
125
Lin, H.-P.
105, 113,311,557,569
Kruk, M.
Kuwabara, M. Kwon, Y.K. Kwon, Y.-U.
L
693 461,737
Linden, M.
399
Liu, M.-C.
315
807 Liu, Q.
177
Liu, S.-B.
681
Liu, S.-T.
113
Liu, X. Liu, X.-M. Liu, Y.-H. Liu, Z.
N Nagai, M.
729, 733
9
Nakajima, H.
753
239
Nakamura, R. Nakao, H.
749
Nam, L-S.
697
311 221,275
795
Lorret, 0 .
419
Nguyen, S.V.
641
Lu(Max), G.Q.
605
Niki, M.
427
Lu,A.
399
Lu, D.
251,323
Lu, M.G.Q.
239
Lu, W.
649
Luo,Q.
161
Lutner, J.D.
339
Nikitenko, S.
721
Nishivayashi, T.
729
Nishiyama, N.
327
o Ocko, B. O'Connor, A.J.
M Macquarrie, D.J.
351 493,775
Oh,C.
189
Oh, J.H.
205
375,453
Oh, S.-G.
189
Malckian, A.
145
Oh, S.-S.
633
Malwadkar, S.
573
Oh,T.
387
Mann-Kipcrman, A.
121
Ohishi, Y.
725
Martra, G.
593
Ohkubo, T.
61
Masaki, N.
791
Ohsuna, T.
275
Matsubayashi, N.
363
Okabc, A.
Matsui, T.
363
Okazaki, M!
Matsuoka, M.
593
Omi, S.
McCuiicn, S.B.
339
On, D.T.
Mcl'gunov, M.S.
543
Onida, B.
543
Otero Arean, C.
379
Oumi, Y.
753
Ouyan, D.-F.
391
Mcrgunova, E.A. Mclo, R.A.A.
303, 705, 745
Mcng, X.
565
Minoofar, P.
413
Miyazaki, T.
753
Miyazawa, K.
69
427,465 795 729, 733 561,609,641 319,375,379,453
P
Mogilyansky, D.
247
Pak, C.
371
Mokaya, R.
435
Park, B.-G.
489
Moon, M.J.
621
Park, C.J.
109
Morin, C.
133
Park, D.H.
481
105, 113,311
Park, D.-H.
327
539,557,569,681
Park, D.W.
621
Mou, C.-Y. Moulongui, Z.
757
Park, D.-W.
Murata, K.
395
Park, J.C.
Murayama, H.
359
Park, J.-H.
189
Mutin, P.-H.
419
Park, J.-K.
513
645 527, 637
808 Park, J.Y.
489
Park, O.-H.
65
Park, S.-B. Park, S.E.
93 101, 137
Park, S.S.
481
Park, Y.G. Parmentier, J.
527, 637 41
s Sakamoto, Y. Samuels, W.D.
23,97 447
Sanchez, C.
281
Sano, T.
753
Saputra, H.
327
Parvulescu, V.
629, 585
Sastry, M.
573
Parvulescu, V.I.
609, 585
Sato, K.
363
Pasqua, L.
165,497
Patarin, J.
41
Schmidt, W. Schuth, F.
399 221,399
Peer, Y.
121
Seo, G.
505
Penazzi, N.
379
Seong, B.-S.
355
Peng, S.Y.
161 133
Separovic, F. Shen, S.
493
Perriat, P. Provoost, O.
443
Shen, W.Z.
Pu,M.
737
Q Qin, Z.F.
767, 779
Sheu, H.-S.
315
Shi, J.-L.
457
Shih, P.-C.
557
Shimazu, S.
359
Shin, S.-I.
189
Shin, Y.S.
447
Shindo, D.
R
9 767, 779
Shirai, M.
275
Shishido, T.
791 625, 725
Shmakov, A.N.
299, 543
Radu, D.C.
609
Ranjit, K.T.
65
Shul, Y.-G.
787
573
Slabova, M.
601
Rathman, J.F.
29
Smarsly, B.
295
Rathousky, J.
185,601
Soler Illia, G.J.dc A.A.
281 299
Rao, B.S.
Ratnasamy, P.
573
Raul, R.-S.
331
Rcincrt, P.
133,439
Solovyov, L.A. Song, C.-M.
153
Song, J.H.
505
Rcnzo, F.D.
319
Song, M.-G.
169
Rhee, H.-K.
673
Spitz, R.
439
Rhee, H.-L.
235
Stevens, G.W.
Rhodes, C.
649
Stevenson, S.A.
Riddei, S.A.
799
Stucky, G.D.
Rosenholm, J.
399
Su, B.L.
443,629, 181,243 749
493,775 339 45
Roth, W.J.
339
Sugi, Y.
Rotter, H.
247
Sugimoto, N.
23
Roy, C.
335
Sugiyama, H.
613
453
Russo, A. Ryoo, R. Ryu, S.Y.
Suh, Y.-W.
235
33,37,49,53,275
Sun, J.
157
299, 335, 657, 669
Sun, W.
289, 37
Sun, Y.H.
177 161,461,737
809 Park, J.Y. Park, O.-H.
489 65
Park, S.-B.
93
Park, S.-E.
101, 137
Park, S.S. Park, Y.G. Parmentier, J.
481 527, 637 41
s Sakamoto, Y.
23,97
Samuels, W.D.
447
Sanchez, C.
281
Sano, T.
753
Saputra, H.
327
Parvulescu, V.
629, 585
Sastry, M.
573
Parvulescu, V.I.
609, 585
Sato, K.
363
Pasqua, L.
165,497
Patarin, J.
41
Peer, Y.
121
Penazzi, N.
379
Schmidt, W. Schuth, F.
355 493
Peng, S.Y.
161
Separovic, F.
133
Shen, S.
Provoost, O.
443
Shen, W.Z.
Pu,M.
737
767, 779
R
505
Seo, G. Seong, B.-S.
Perriat, P.
Qin, Z.F.
399 221,399
9 767, 779
Sheu, H.-S.
315
Shi, J.-L.
457
Shih, P.-C.
557
Shimazu, S.
359
Shin, S.-I.
189
Shin, Y.S.
447
Shindo, D.
275
Shirai, M.
791
Shishido, T.
625, 725
Shmakov, A.N.
299,543
Radu, DC.
609
Ranjit, K.T.
65
Shul, Y.-G.
573
Slabova, M.
601
Rathman, J.F.
29
Smarsly, B.
295
Rathousky, J.
185,601
Rao, B.S.
787
Soler Illia, G.J.de A.A.
281
Ratnasamy, P.
573
Solovyov, L.A.
299
Raul, R.-S.
331
Song, C.-M.
153
Rcinert, P.
133,439
Song, J.H.
505
Renzo, F.D.
319
Song, M.-G.
169
Rhce, H.-K.
673
Spitz, R.
439
Rhee, H.-L.
235
Stevens, G.W.
Rhodes, C.
649
Stevenson, S.A.
Riddel, S.A.
799
Stucky, G.D.
Rosenholm, J.
399
Su, B.L.
443,629, 181,243
Roth, W.J.
339
Sugi, Y.
749
Rotter, H.
247
Sugimoto, N.
23
Roy, C.
335
Sugiyama, H.
613
453
Russo, A. Ryoo, R. Ryu, S.Y.
493,775 339 45
Suh, Y.-W.
235
s 37, 49, 53, 275
Sun, J.
157
>9, 335, 657, 669
Sun, W.
289, 37
Sun, Y.H.
177 161,461,737
810 Sung, I.-K.
547
Sunn, B.
413
Vradman, L.
121,721
W T Wallau, M.
197,303,705,745
Taguchi, A.
399
Wan, B.-Z.
391,539
Tajima, K.
517
Wang, H.-P.
153
Tak, Y.S
205
Wang, H.-W.
227
Takehira, K.
625,725
Wang, J.
Talianker, M.
721
Wang, J.G.
Tanaka, T.
363
Wang, L.
Tang, J.
347
Wang, L.-Q.
Tapia, G.H.
685
Wang, X.-Z.
Tarasaki, O.
97
Tatsumi, T.
125,431,531,613
Teranishi, T.
753
Terasaki, O.
221,275,431
Wang, Y. Watson, S.R. Werckmann, J.
661 767, 779 9,97 447 255 625, 725 799 41
Wiesner, U.
407 605
165,497
Williams, T.
Thang, H.V.
145
Wong, S.-T.
569
Thomson, S.J.
221
Woo, H.C.
621
Testa, F.
Tian, B. Tillcmcnt, O.
9,45 133
Wright, P.A.
57
Wu, C.-W.
69
Ting, C.-Y.
391,539
Wu, D.
161,461,737
Tomita, M.
783,791
Wu, P.
613
Trong, D.
23, 145
565
Tsai, J.-L.
227
Wu, S. Wu, T. Wu, W.-F.
391
Tu, B. Turncs Palomino, G.
9,97
149
379
X
u
Xiang, H.W.
693 149, 565
Uchida, H.
783
Xiao, F.-S.
Uchida, S.
791
Xing, W.
771
Ucmatsu, T.
359
Xu, Y.Y.
693
Ucno, Y.
783
Ueyama, K. Urquieta-Gonzalez, E.A.
327
Y
197,303 705, 745
Yamada, T. Yamashita, H. Yan, D.-S.
V
81,783 597, 795 457
Yan, J.-N.
457
van der Voort.
271
Yan, Z.-F.
153,239,771
Vartuli, J.C.
339
Yang, C.
129,347,485
Vinu, A.
285
Yang, H.
Vioux, A.
419
Yang, J.
Viscardi, G.
375
Yang, J.-S.
9 177,383,775 709
811
SUBJECT INDEX cumene cracking
149,311
cyclohexene hydrogenolysis acid-base pair iica acid-functionalized organosilica acidity acylation
9 15
D
137, 149,307,649,653 657
adsorption
285,527,581,775
alkylation
149,661
degradation of organic pollutant
725
deposition by ICPCVD
387 391
581
dielectric constant
amphiphilic
117
Diels-Alder reaction
antireflective coating
85, 89, 205
601
dehydrogenation of lower alkanes
amine functionalization anodic aluminum oxide
733
749
dye molecule
375,379
539
aromatic hydrogenation
717
asymmetric catalysis
509
asymmetric dihydroxylation
677
E electron tomography
271
electrorheological fluid
B
523
encapsulation
201,379
673
enzyme esterification
649, 657
bimodal pore structure
161
etherification
665
biocatalyst
513
cxtraframework species
303
block copolymer
407
carbidedW03/FSM-16
733
benzene alkylation
775
fabrication carbon nanotubes cathodic electrodeposition charge transfer complex
689,761 205 73
chemical coating of AI2O3
311
chemoselective oxidation
609
chromia aerosol
247
collagen colloid aggregation composite
9, 85, 547
Fischer-Tropsch synthesis
693
Friedel-Crafts alkylation
669
functional mesostructured materials
407
functionalization
1,37,473,477 531,799
29 193 15,29,93,367
gas adsorption
263, 343
grazing incidence small angle x-ray scattering
composite material
145
conduction polymer
523
grafting method
331
copolymer
113
green chemistry
551
co-solvent
9,209
counter-anion effect
431
counterion
133
cracking activity
307
cubic mesoporous material
77
351
H heteropoly acid hollow silica
125,653,657,661 189
812 hollow spheres
439
-Fe-MCM-41
625
HRTEM
275
-FSM-16
749
hydrocarbon cracking
653
- FSM-type silica
hydrocracking
681,741
hydrodechlorination
637
hydrodesulfurization
721
hydrogen spillover
689,761
hydrogenation
509,689,721,761
hydrophobic coating
577
-HMS
319^,453,. 485, 693
-MCM-41
129,303 3 3 r ,339,,573, 775
- MCM-41 composite
367
- MCM-48
41, 157
493
hydrothermal stability
137, 141, 177
259^, 339, 493, 669 - MCM-48-S
557 359
hydrotreating
653
-Mo/MCM-41
hydroxylation
589
- MSU-H
45,49
- MSU-X
I
461
-SBA-1
125,315
- S B A-15 immobilization
4 5 , 4 9 , 5 7 , 105
73, 93,469,501,505,551
109, 117, 121, 157
577, 775, 799 65,81, 125,481
incorporation infiltration
41
505, 581, 601, 721,775 - S B A-16
97, 101
- SBA-3
485
inorganic salt
9
in-situ XAFS
359,363
-Ti-MCM41
517, 593, 589
isomcrization
569,737,741
- Ti-MSU-G
613
K Knocvcnagcl condensation
485, 505
L low-k material
391
- Ti-HMS
605
-Ti-SBA-15
609
-Vinyl SB A-15
489
-V-MCM-41
617
- Zr-MCM-48
327
-ZSM-5/MCM-41 composite
367
mesoporous titanosilicate
565
mesoporous USY zeolite
363
mesoscopic morphology
173
mesostructured acidic phosphate M mechanical stability membrane mcsoporous alumina mesoporous carbon
285 89, 327 209,213,217 33,37,41,45,49,53 57, 97,335,657,769,771
mesoporous KL zeolite
347
mesoporous materials -Al-FSM-16
729, 733
15
metal incorporation
177
methane oxidation
641
methane reforming
729
methylphenothiazine
65
microcalorimetry
319
micro-mesoporous material
137
microporosity
105
microporous framework
141
microwave
101
modification
- Al-MCM-48
709
molybdenum carbide
-Al-SBA-15
543
monolithic nanostructure
-Cr-MCM-41
725
morphology
- Cr-MCM-48
371
morphology control
81, 281, 311, 465, 609 729 173 45, 109, 141, 189, 193,197 101,447
813 multiple-emulsion
189
N nanocasting nanocomposite nanofiltration nanoparticle -Ag
37 383 ,577 327
photocatalytic activity
227
photocatalytic degradation
795
photocatalytic epoxidation
597
photoinduced electron transfer
289
photoluminescence
65
plasma-catalyst hybrid
645
polyethylene fiber
517
57 , 121
polymeric film
85
93
polymerization
113,753
- Au-Pt
573
pore size
- Pd-Pt bimetallic
363
pore size analysis
- Pt/MCM-48
709
pore structure
161,343 193,209
109, 157,,681,779, 117,285 263
- Pt-Pd/Al-MMS
717
porosity
-CdS
535
post grafting
nanoreactor
121
post-synthesis
157
nanostructured SiC and BN
547
propene photometathesis
359
457
121 ,721
proteosilica
427
NaY zeolite
469
Pt nanowire
23
Nb/Fe mixed oxide
621
NbTa oxide
251
niobium oxide
323
nanotube
R
NO decomposition
593
repeated nanocasting
non-siliceous material
399
replica
399 33,41 45,49,53,97,399
o one-pot grafting method
453
optical response
169
SCR deNOx
organic additive
185
selective adsorption
organic bridged mesoporous material
231
self-assembly
311
silica nanosphere
organic modification organized mesoporous silica organo-modification organosilica
481,665 489 1
697, 701, 709,713 531 221,565 197
silica nanotube
181
silica/chlorophyll
577
single-wall carbon nanohoms
395
small angle neutron scattering
295,355
small-angle X-ray scattering sol-gel peptide mimic immobilization
427
sorption property
phase transformation
281
stability
295 161,,255,585 339 133, 165,,213,251
phase transition
125
phenol alkylation
477
steam stability
phosphinooxazolidine
501
supercritical extraction
247
photoactive molecule
413
supercritical grafting
435
surface functionalization
419
swelling agent
117
photocalcination photocatalyst
77,201 •,593,601
303, 307,,315,649 435
814
T 271
3D-TEM template length
209
textural property
461 153, 177,231
thermal stability
29,65,69,73,77,81
thin film
281,387,391,413,535,539 Ti-Zr oxide
231
V205-Ti02-Si02 catalyst
585
vanadia-silica composite
201
VOC decomposition
645
W wood cellular
447
Xe NMR
367
X-ray scattering
351
zeolite beta as SBU
307
zeolite coating
561
zeolite nanocrystal
561
zeolite seed
557
zero length column zirconia zirconium oxo-phosphate
145 235,239,443,457 221
STUDIES IN SURFACE SCIENCE AND CATALYSIS Advisory Editors: B. Delmon, University Catholique de Louvain, Louvain-la-Neuve, Belgium J.T. Yates, University of Pittsburgh, Pittsburgh, PA, U.S.A. Volume 1
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Preparation of Catalysts I. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 14-17.1975 edited by B. Delmon, P.A Jacobs and G. Poncelet The Control of the Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, with Special Emphasis on the Control of the Chemical Processes in Relation to Practical Applications by V.V. Boldyrev, M. Bulens and B. Delmon Preparation of Catalysts ll.Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Second International Symposium, Louvain-la-Neuve, September 4-7, 1978 edited by B. Delmon, P. Grange, P. Jacobs and G. Poncelet Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings of the 32"*^ International Meeting of the Soci6t6 de Chimie Physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon Catalysis by Zeolltes.Proceedings of an International Symposium, Ecully (Lyon), September 9-11, 1980 edited by B. Imelik, C. Naccache,Y. BenTaarit, J.C.Vedrine, G. Coudurier and H. Praliaud Catalyst Deactivation. Proceedings of an International Symposium, Antwerp, October 13-15,1980 edited by B. Delmon and G.F. Froment New Horizons in Catalysis. Proceedings of the 7*^ International Congress on Catalysis, Tokyo, June 30-July 4,1980. Parts A and B edited by T. Seiyama and K.Tanabe Catalysis by Supported Complexes by Yu.l.Yermakov, B.N. Kuznetsov and V.A Zakharov Physics of Solid Surfaces. Proceedings of a Symposium, Bechyrte, September 29-October 3,1980 edited by M. L^nidka Adsorption at the Gas-Solid and Liquid-Solid Interface. Proceedings of an International Symposium, Aix-en-Provence, September 21-23,1981 edited by J. Rouquerol and K.S.W. Sing Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium, Ecully (Lyon), September 14-16, 1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau, P. Gallezot, G.AMartin and J.C.Vedrine Metal Microstructures in Zeolites. Preparation - Properties - Applications. Proceedings of a Workshop, Bremen, September 22-24,1982 edited by P.A Jacobs, N.I. Jaeger, P. Jiru and G. Schulz-Ekloff Adsorption on Metal Surfaces. An Integrated Approach edited by J. B^nard Vibrations at Surfaces. Proceedings of the Third International Conference, Asilomar, CA, September 1-4, 1982 edited by C.R. Brundle and H.Morawitz Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.I. Golodets
816 Volume 16
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Preparation of Catalysts III. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Third International Symposium, Louvain-la-Neuve, September 6-9,1982 edited by G. Ponceiet, P. Grange and P.A. Jacobs Spillover of Adsorbed Species. Proceedings of an International Symposium, Lyon-Villeurbanne, September 12-16,1983 edited by G.M. Pajonk,S.J.Teichner and J.E. Germain Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July 9-13,1984 edited by P.A. Jacobs, N.I. Jaeger, P. Jiru, V.B. Kazansky and G. Schulz-Ekloff Catalysis on the Energy Scene. Proceedings of the 9*^ Canadian Symposium on Catalysis, Quebec, P.Q., September 30-October 3,1984 edited by S. Kaliaguine and A.Mahay Catalysis by Acids and Bases. Proceedings of an International Symposium, Villeurbanne (Lyon), September 25-27, 1984 edited by B. Imelik, C. Naccache, G. Coudurier,Y. Ben Taarit and J.C.Vedrine Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Uxbridge, June 2a-29, 1984 edited by M. Che and G.C.Bond Unsteady Processes in Catalytic Reactors by Yu.Sh. Matros Physics of Solid Surfaces 1984 edited by J.Koukal Zeolites:Synthesis, Structure,Technology and Application. Proceedings of an International Symposium, Portoroi -Portorose, September 3-8, 1984 edited by B.Dri^aj,S. HoCevar and S. Pejovnik Catalytic Polymerization of Olefins. Proceedings of the International Symposium on Future Aspects of Olefin Polymerization, Tokyo, July 4-6,1985 edited by T.Keii and K.Soga Vibrations at Surfaces 1985. Proceedings of the Fourth International Conference, Bowness-on-Windemnere, September 15-19, 1985 edited by D.A. King, N.V. Richardson and S. Holloway Catalytic Hydrogenation edited by L. Cerveny New Developments in Zeolite Science and Technology. Proceedings of the 7^^ International Zeolite Conference, Tokyo, August 17-22,1986 edited by Y.Murakami,A. lijima and J.W.Ward Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H. Kn6zinger Catalysis and Automotive Pollution Control. Proceedings of the First International Symposium, Brussels, September 8-11,1986 edited by A. Crucq and A. Frennet Preparation of Catalysts IV. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Fourth International Symposium, Louvain-la-Neuve, September 1-4,1986 edited by B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet Thin Metal Films and Gas Chemisorption edited by P.Wissmann Synthesis of High-silica Aluminosilicate Zeolites edited by P.A. Jacobs and J.A.Martens Catalyst Deactivation 1987. Proceedings of the 4*^ International Symposium, Antwerp, September 29-October 1, 1987 edited by B. Delmon and G.F. Froment Keynotes in Energy-Related Catalysis edited by S. Kaliaguine
817 Volume 36 Volume 37 Volume 38 Volume 39 Volume 40 Volume 41
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Methane Conversion. Proceedings of a Symposium on the Production of Fuels and Chemicals from Natural Gas, Auckland, April 27-30, 1987 edited by D.M. Bibby, C.D.Chang, R.F.Howe and S.Yurchak Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-17,1987 edited by P.J. Grobet, W.J.Mortier, E.F.Vansant and G. Schulz-Ekloff Catalysis 1987.Proceedings of the 10*^ North American Meeting of the Catalysis Society, San Diego, CA, May 17-22, 1987 edited by J.W.Ward Characterization of Porous Solids. Proceedings of the lUPAC Symposium (COPS I), Bad Soden a. Ts., April 2&-29,1987 edited by K.K.Unger, J. Rouquerol, K.S.W.Sing and H. Krai Physics of Solid Surfaces 1987. Proceedings of the Fourth Symposium on Surface Physics, Bechyne Castle, September 7-11, 1987 edited by J.Koukal Heterogeneous Catalysis and Fine Chemicals. Proceedings of an International Symposium, Poitiers, March 15-17, 1988 edited by M. Guisnet, J. Barrault, C. Bouchoule,D. Duprez, C. Montassier and G. P6rot Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by Z. Pa^l Catalytic Processes under Unsteady-State Conditions by Yu. Sh. Matros Successful Design of Catalysts. Future Requirements and Development. Proceedings of the Worldwide Catalysis Seminars, July, 1988, on the Occasion of the 30^*^ Anniversary of the Catalysis Society of Japan edited by T. Inui Transition Metal Oxides. Surface Chemistry and Catalysis by H.H.Kung Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an International Symposium, Wurzburg, September 4-8,1988 edited by H.G. Karge and J.Weitkamp Photochemistry on Solid Surfaces edited by M.Anpo and T. Matsuura Structure and Reactivity of Surfaces. Proceedings of a European Conference, Trieste, September 13-16, 1988 edited by C.MorterraA Zecchina and G. Costa Zeolites: Facts, Figures, Future. Proceedings of the 8*^ International Zeolite Conference, Amsterdam, July 10-14, 1989. Parts A and B edited by P.A. Jacobs and R.A. van Santen Hydrotreating Catalysts. Preparation, Characterization and Perfomriance. Proceedings of the Annual International AlChE Meeting, Washington, DC, November 27-December 2, 1988 edited by M.L. Occelli and R.G.Anthony New Solid Acids and Bases.Their Catalytic Properties by K.Tanabe, M. Misono,Y. Ono and H. Hattori Recent Advances in Zeolite Science. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-19, 1989 edited by J. Klinowsky and P.J. Barrie Catalyst in Petroleum Refining 1989. Proceedings of the First international Conference on Catalysts in Petroleum Refining, Kuwait, March 5-8,1989 edited by D.L.Trimm,S.Akashah, M.Absi-Halabi and A. Bishara Future Opportunities in Catalytic and Separation Technology edited by M. Misono,Y.Moro-oka and S. Kimura
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New Developments in Selective Oxidation. Proceedings of an International Symposium, Rimini, Italy, September 18-22, 1989 edited by G. Centi and F.Trifiro Olefin Polymerization Catalysts. Proceedings of the International Symposium on Recent Developments in Olefin Polymerization Catalysts, Tokyo, October 23-25,1989 edited by T.Keii and K.Soga Spectroscopic Analysis of Heterogeneous Catalysts. Part A: Methods of Surface Analysis edited by J.L.G. Fierro Spectroscopic Analysis of Heterogeneous Catalysts. Part B: Chemisorption of Probe Molecules edited by J.L.G. Fierro Introduction to Zeolite Science and Practice edited by H. van Bekkum, E.M. Flanigen and J.C. Jansen Heterogeneous Catalysis and Fine Chemicals II. Proceedings of the 2™^ International Symposium, Poitiers, October 2-6,1990 edited by M. Guisnet, J. Barrault, C. Bouchoule,D. Duprez, G. P6rot, R.Maurel and C. Montassier Chemistry of Microporous Crystals. Proceedings of the International Symposium on Chemistry of Microporous Crystals, Tokyo, June 26-29, 1990 edited by T. Inul.S. Namba and T.Tatsumi Natural Gas Conversion. Proceedings of the Symposium on Natural Gas Conversion, Oslo, August 12-17, 1990 edited by A. Holmen, K.-J. Jens and S.Kolboe Characterization of Porous Solids II. Proceedings of the lUPAC Symposium (COPS II), Alicante, May 6-9, 1990 edited by F. Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing and K.K.Unger Preparation of Catalysts V. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Fifth International Symposium, Louvain-la-Neuve, September 3-6, 1990 edited by G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon New Trends in CO Activation edited by L. Guczi Catalysis and Adsorption by Zeolites. Proceedings of ZEOCAT 90, Leipzig, August 20-23, 1990 edited by G. dhlmann, H. Pfeifer and R. Fricke Dioxygen Activation and Homogeneous Catalytic Oxidation. Proceedings of the Fourth International Symposium on Dioxygen Activation and Homogeneous Catalytic Oxidation, Balatonfured, September 10-14,1990 edited by L.I. Sim^ndi Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis. Proceedings of the ACS Symposium on Structure-Activity Relationships in Heterogeneous Catalysis, Boston, MA, April 22-27,1990 edited by R.K. Grasselli and A.W.SIeight Catalyst Deactivation 1991. Proceedings of the Fifth International Symposium, Evanston, IL, June 24-26, 1991 edited by C.H. Bartholomew and J.B. Butt Zeolite Chemistry and Catalysis. Proceedings of an International Symposium, Prague, Czechoslovakia, September 8-13,1991 edited by P.A. Jacobs, N.I. Jaeger, L.Kubelkov^ and B.Wichterlov^ Poisoning and Promotion in Catalysis based on Surface Science Concepts and Experiments by M. Kiskinova
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Catalysis and Automotive Pollution Control II. Proceedings of the 2^ International Symposium (CAPoC 2), Brussels, Belgium, September 10-13, 1990 edited by A. Crucq New Developments in Selective Oxidation by Heterogeneous Catalysis. Proceedings of the 3"^ European Workshop Meeting on New Developments in Selective Oxidation by Heterogeneous Catalysis, Louvain-la-Neuve, Belgium, April 8-10,1991 edited by P. Ruiz and B. Delmon Progress in Catalysis. Proceedings of the 12*^ Canadian Symposium on Catalysis, Banff, Alberta, Canada, May 25-28,1992 edited by K.J. Smith and E.C. Sanford Angle-Resolved Photoemission.Theory and Current Applications edited by S.D. Kevan New Frontiers in Catalysis, Parts A-C. Proceedings of the 10^^ International Congress on Catalysis, Budapest, Hungary, 19-24 July, 1992 edited by L. Guczi, F. Solymosi and P.T^t^nyi Fluid Catalytic Cracking: Science and Technology edited by J.S.Magee and M.M. Mitchell, Jr. New Aspects of Spillover Effect in Catalysis. For Development of Highly Active Catalysts. Proceedings of the Third International Conference on Spillover, Kyoto, Japan, August 17-20,1993 edited by T. Inui, K. Fujimoto,T.Uchijima and M. Masai Heterogeneous Catalysis and Fine Chemicals III. Proceedings of the 3"* International Symposium, Poitiers, April 5 - 8,1993 edited by M. Guisnet, J. Barbier, J. Barrault, C. Bouchoule,D. Duprez, G. P^rot and C. Montassier Catalysis: An Integrated Approach to Homogeneous, Heterogeneous and Industrial Catalysis edited by J.A. Moulijn, P.W.N.M. van Leeuwen and R.A. van Santen Fundamentals of Adsorption. Proceedings of the Fourth International Conference on Fundamentals of Adsorption, Kyoto, Japan, May 17-22, 1992 edited by M. Suzuki Natural Gas Conversion II. Proceedings of the Third Natural Gas Conversion Symposium, Sydney, July 4-9, 1993 edited by H.E. Curry-Hyde and R.F.Howe New Developments in Selective Oxidation II. Proceedings of the Second World Congress and Fourth European Workshop Meeting, Benalmcidena, Spain, September 20-24,1993 edited by V. Cortes Corber^n and S.Vic Bell6n Zeolites and Microporous Crystals. Proceedings of the International Symposium on Zeolites and Microporous Crystals, Nagoya, Japan, August 22-25, 1993 edited byT. Hattori and T.Yashima Zeolites and Related Microporous Materials: State of the Art 1994. Proceedings of the 10*^ International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22, 1994 edited by J.Weitkamp, H.G. Karge.H. Pfeifer and W. H6lderich Advanced Zeolite Science and Applications edited by J.C. Jansen, M. Stacker, H.G. Karge and J.Weitkamp Oscillating Heterogeneous Catalytic Systems by M.M. Slihko and N.I. Jaeger Characterization of Porous Solids III. Proceedings of the lUPAC Symposium (COPS III), Marseille, France, May 9-12, 1993 edited by J.Rouquerol, F. Rodrfguez-Reinoso, K.S.W. Sing and K.K.Unger
820 Volume 88 Volume 89
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Catalyst Deactivation 1994. Proceedings of the 6*^ International Symposium, Ostend, Belgium, October 3-5,1994 edited by B. Delmon and G.F. Froment Catalyst Design for Tailor-made Polyolefins. Proceedings of the International Symposium on Catalyst Design for Tailor-made Polyolefins, Kanazawa, Japan, March 10-12, 1994 edited by K.Soga and M.Terano Acid-Base Catalysis II. Proceedings of the International Symposium on Acid-Base Catalysis II, Sapporo, Japan, December 2-4,1993 edited by H. Hattori,M. Misono and Y.Ono Preparation of Catalysts VI. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Sixth International Symposium, Louvain-La-Neuve, September 5-8,1994 edited by G. Poncelet, J.Martens,B- Delmon, P.A. Jacobs and P. Grange Science and Technology in Catalysis 1994. Proceedings of the Second Tokyo Conference on Advanced Catalytic Science and Technology, Tokyo, August 21-26, 1994 edited by Y. Izumi, H.Arai and M. Iwamoto Characterization and Chemical Modification of the Silica Surface by E.F.Vansant, P.Van Der Voort and K.C.Vrancken Catalysis by Microporous Materials. Proceedings of ZEOCAT'95, Szombathely, Hungary, July 9-13, 1995 edited by H.K. Beyer, H.G.Karge, I. Kiricsi and J.B.Nagy Catalysis by Metals and Alloys by V. Ponec and G.C.Bond Catalysis and Automotive Pollution Control III. Proceedings of the Third International Symposium (CAPoC3), Brussels, Belgium, April 20-22, 1994 edited by A. Frennet and J.-M. Bastin Zeolites:A Refined Tool for Designing Catalytic Sites. Proceedings of the International Symposium, Quebec, Canada, October 15-20, 1995 edited by L. Bonneviot and S. Kaliaguine Zeolite Science 1994: Recent Progress and Discussions. Supplementary Materials to the 10^^ International Zeolite Conference, Gamnisch-Partenkirchen, Germany, July 17-22, 1994 edited by H.G. Karge and J.Weitkamp Adsorption on New and Modified Inorganic Sorbents edited by A.D4browski and V.A.Tertykh Catalysts in Petroleum Refining and Petrochemical Industries 1995. Proceedings of the 2"^ International Conference on Catalysts in Petroleum Refining and Petrochemical Industries, Kuwait, April 22-26,1995 edited by M.Absi-Halabi, J. Beshara, H. Qabazard and A. Stanislaus 11**^ International Congress on Catalysis - 40**^ Anniversary. Proceedings of the 1V^ ICC. Baltimore, MD, USA, June 30-July 5, 1996 edited by J.W. Hightower.W.N. Delgass, E. Iglesia and A.T. Bell Recent Advances and New Horizons in Zeolite Science and Technology edited by H. Chon,S.I.Woo and S. -E. Park Semiconductor Nanoclusters - Physical, Chemical, and Catalytic Aspects edited by P.V. Kamat and D. Meisel Equilibria and Dynamics of Gas Adsorption on Heterogeneous Solid Surfaces edited by W. Rudzihski,W.A. Steele and G. Zgrablich Progress in Zeolite and Microporous Materials Proceedings of the 1V^ International Zeolite Conference, Seoul, Korea, August 12-17, 1996 edited by H. Chon,S.-K. Ihm and Y.S.Uh
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Hydrotreatment and Hydrocracking of Oil Fractions Proceedings of the 1^ International Symposium / 6'^ European Workshop, Oostende, Belgium, February 17-19, 1997 edited by G.F. Froment,B- Delmon and P. Grange Natural Gas Conversion IV Proceedings of the 4*^ International Natural Gas Conversion Symposium, Kruger Park, South Africa, November 19-23, 1995 edited by M. de Pontes, R.L. Espinoza, C.P. Nicolaides, J.H. Scholtz and M.S. Scurrell Heterogeneous Catalysis and Fine Chemicals IV Proceedings of the 4*^ International Symposium on Heterogeneous Catalysis and Fine Chemicals, Basel, Switzerland, September 8-12, 1996 edited by H.U. Blaser, A. Baiker and R. Prins Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis. Proceedings of the International Symposium, Antwerp, Belgium, September 15-17,1997 edited by G.F. Froment and K.C.Waugh Third World Congress on Oxidation Catalysis. Proceedings of the Third World Congress on Oxidation Catalysis, San Diego, CA, U.S.A., 21-26 September 1997 edited by R.K. Grasselli,S.T.Oyama, A.M. Gaffney and J.E. Lyons Catalyst Deactivation 1997. Proceedings of the 7*^ International Symposium, Cancun, Mexico, October 5-8, 1997 edited by C.H. Bartholomew and G.A. Fuentes Spillover and Migration of Surface Species on Catalysts. Proceedings of the 4*^ International Conference on Spillover, Dalian, China, September 15-18, 1997 edited by Can Li and Qin Xin Recent Advances in Basic and Applied Aspects of Industrial Catalysis. Proceedings of the 13^^ National Symposium and Silver Jubilee Symposium of Catalysis of India, Dehradun, India, April 2-4, 1997 edited by T.S.R. Prasada Rao and G.Murali Dhar Advances in Chemical Conversions for Mitigating Carbon Dioxide. Proceedings of the 4*^ International Conference on Carbon Dioxide Utilization, Kyoto, Japan, September 7-11,1997 edited by T. Inui, M.Anpo,K. lzui,S.Yanagida and T.Yamaguchi Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters. A patent-oriented survey by M. Sideris Catalysis and Automotive Pollution Control IV. Proceedings of the 4*^ International Symposium (CAPoC4), Brussels, Belgium, April 9-11, 1997 edited by N. Kruse, A. Frennet and J.-M. Bastin Mesoporous Molecular Sieves 1998 Proceedings of the 1** International Symposium, Baltimore, MD, U.S.A., July 10-12, 1998 edited by L.Bonneviot, F. B^land, C.Danumah, S. Giasson and S. Kaliaguine Preparation of Catalysts VII Proceedings of the 7^^ International Symposium on Scientific Bases for the Preparation of Heterogeneous Catalysts, Louvain-la-Neuve, Belgium, September 1-4, 1998 edited by B. Delmon, P.A. Jacobs, R. Maggi, J.A.Martens, P. Grange and G. Poncelet Natural Gas Conversion V Proceedings of the 5*^ International Gas Conversion Symposium, Giardini-Naxos, Taormina, Italy, September 20-25, 1998 edited by A. Parmaliana, D. Sanfilippo, F. Frusteri, A.Vaccari and F.Arena Adsorption and its Applications in Industry and Environmental Protection. Vol I: Applications In Industry edited by A. D^browski
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Adsorption and its Applications in Industry and Environmental Protection. Vol II: Applications In Environmental Protection edited by A. D^browski Science and Technology in Catalysis 1998 Proceedings of the Third Tokyo Conference in Advanced Catalytic Science and Technology, Tokyo, July 19-24, 1998 edited by H. Hattori and K. Otsuka Reaction Kinetics and the Development of Catalytic Processes Proceedings of the International Symposium, Brugge, Belgium, April 19-21, 1999 edited by G.F. Froment and K.C.Waugh Catalysis:An Integrated Approach Second, Revised and Enlarged Edition edited by R.A. van Santen, P.W.N.M. van Leeuwen, J.A. Moulijn and B.A.Averill Experiments in Catalytic Reaction Engineering by J.M. Berty Porous Materials in Environmentally Friendly Processes Proceedings of the 1** International FEZA Conference, Eger, Hungary, September 1-4,1999 edited by I. Kiricsi, G. P^l-Borb^ly, J.B.Nagy and H.G. Karge Catalyst Deactivation 1999 Proceedings of the 8*^ International Symposium, Brugge, Belgium, October 10-13,1999 edited by B. Delmon and G.F. Froment Hydrotreatment and Hydrocracking of Oil Fractions Proceedings of the 2^ International Symposium/7*^ European Workshop, Antwerpen, Belgium, November 14-17,1999 edited by B. Delmon, G.F. Froment and P. Grange Characterisation of Porous Solids V Proceedings of the 5*^ International Symposium on the Characterisation of Porous Solids (COPS-V), Heidelberg, Germany, May 30- June 2, 1999 edited by K.K.Unger,G.Kreysa and J.P. Baselt Nanoporous Materials II Proceedings of the 2"^ Conference on Access in Nanoporous Materials, Banff, Alberta, Canada, May 25-30, 2000 edited byA. Sayari,M. Jaroniec and T.J. Pinnavaia 12*^ International Congress on Catalysis Proceedings of the 12*^ ICC, Granada, Spain, July 9-14, 2000 edited byA. Corma, F.V. Melo,S. Mendioroz and J.L.G. Fierro Catalytic Polymerization of Cycloolefins Ionic, Ziegler-Natta and Ring-Opening Metathesis Polymerization By V. Dragutan and R. Streck Proceedings of the International Conference on Colloid and Surface Science, Tokyo, Japan, November 5-8, 2000 25^^ Anniversary of the Division of Colloid and Surface Chemistry, The Chemical Society of Japan edited by Y. Iwasawa, N.Oyama and H.Kunieda Reaction Kinetics and the Development and Operation of Catalytic Processes Proceedings of the 3*^ International Symposium, Oostende, Belgium, April 22-25, 2001 edited by G.F. Froment and K.C.Waugh Fluid Catalytic Cracking V Materials and Technological Innovations edited by M.L. Occelli and P. O'Connor
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Zeolites and Mesoporous Materials at the Dawn of the 21"* Century. Proceedings of the 13*^ International Zeolite Conference, Montpellier, France, 8-13 July 2001 edited by A. Galameau, F. di Renso, F. Fajula ans J. Vedrine Natural Gas Conversion VI Proceedings of the 6*^ Natural Gas Conversion Symposium, June 17-22, 2001, Alaska. USA. edited by J.J. Spivey, E. Iglesia and T.H. Fleisch Introduction to Zeolite Science and Practice. 2"^ completely revised and expanded edition edited by H. van Bekkum, E.M. Flanigen, P.A. Jacobs and J.C. Jansen Spillover and Mobility of Species on Solid Surfaces edited by A. Guerrero-Ruiz and I. Rodriquez-Ramos Catalyst Deactivation 2001 Proceedings of the 9*^ International Symposium, Lexington, KY, USA, October 2001. edited by J.J. Spivey, G.W. Roberts and B.H. Davis Oxide-based Systems at the Crossroads of Chemistry. Second International Workshop, October 8-11, 2000, Como, Italy. Edited by A. Gamba, C. Colella and S. Coluccia Nanoporous Materials III Proceedings of the 3"^ International Symposium on Nanoporous Materials, Ottawa, Ontario, Canada, June 12-15, 2002 edited by A. Sayari and M. Jaroniec Impact of Zeolites and Other Porous Materials on the New Technologies at the Beginning of the New Millennium Proceedings of the 2^ International FEZA (Federation of the European Zeolite Associations) Conference, Taomnina, Italy, September 1-5, 2002 edited by R. Aiello, G. Giordano and F.Testa Scientific Bases for the Preparation of Heterogeneous Catalysts Proceedings of the 8*^ International Symposium, Louvain-la-Neuve, Leuven, Belgium, September 9-12, 2002 edited by E. Gaigneaux, D.E. De Vos, P. Grange, P.A. Jacobs, J.A. Martens, P. Ruiz and G. Poncelet Characterization of Porous Solids VI Proceedings of the 6^^ International Symposium on the Characterization of Porous Solids (COPS-VI). Alicante, Spain, May 8-11, 2002 edited by F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger Science and Technology in Catalysis 2002 Proceedings of the Fourth Tokyo Conference on Advanced Catalytic Science and Technology, Tokyo, July 14-19, 2002 edited by M. Anpo, M. Onaka and H. Yamashita Nanotechnology in Mesostructured Materials Proceedings of the 3"* International Mesostructured Materials Symposium, Jeju, Korea, July 8-11, 2002 edited by Sang-Eon Park, Ryong Ryoo, Wha-Seung Ahn, Chul Wee Lee and Jing-San Chung
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