Studies in Surface Science and Catalysis 141 NANOPOROUS MATERIALS III
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Studies
in S u r f a c e
A d v i s o r y Editors:
Science
and Catalysis
B. Delmon and J.T. Yates
Vol. 141
NANOPOROUS
MATERIALS !11
Proceedings of the 3 '~ International Symposium on Nanoporous Materials, Ottawa, Ontario, Canada, June 12-15, 2002
Edited by A. Sayari
University of Ottawa, Department of Chemistry, Ottawa, Ontario K1N 6N5, Canada
M. Jaroniec
Kent State University, Department of Chemistry, Kent, Ohio 44242, USA
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2002
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PREFACE Since the Breck Award winning discovery of the so-called M41S silica mesostructures in the early nineties, the field of ordered mesoporous materials has grown so dramatic that it has developed into a distinct research area. Remarkably, in the last ten years, over 3000 papers have been published on such materials. Impressive progress has been achieved in the use of self-assembly approaches and supramolecular templating techniques to generate a Wide variety of ordered inorganic, organic and hybrid mesostructures with tailored framework composition, pore structure, pore size, morphology and surface properties. The range, of synthesis conditions and the variety of templating surfactants and oligomers that have been explored is truly remarkable. Many fascinating discoveries have been made not only in the rational design of such materials at the molecular level, but also in a wide range of potential applications, for example in adsorption, catalysis, separation processes, environmental cleanups and optoelectronics. Among the most recent developments in this area is the extension of the amphiphile selfassembly techniques to the synthesis of ordered mesoporous organosilicas, which demonstrated that there are almost unlimited opportunities in tailoring surface and structural properties of mesoporous materials. Another important discovery is the use of ordered nanoporous silica and colloidal crystals to create new periodic mesoporous and macroporous materials, including carbons, polymers, metals, and alloys. Combination of different synthesis approaches such as amphiphile, colloidal crystal or microemulsion templating, micromolding and soft lithography led to materials with hierarchically ordered structures. International Symposia on Nanoporous Materials are intended to bring together investigators to discuss complementary approaches and recent advances concerning not only materials synthesized through supramolecular templating, but also a variety of other nanoporous materials such as clays, carbon molecular sieves, porous polymers, sol-gel and imprinted materials as well as self-assembled organic and organometallic zeolite-like materials. Judging from the remarkable success of the previous symposium "Nanoporous Materials II" (May 2000, Banff, Canada), and from the wide range of high quality abstracts and manuscripts submitted to the current meeting, the Organizing Committee is confident that the Nanoporous Materials III symposium will achieve its objective of gathering scientists interested in sharing their valuable findings related to a large variety of nanoporous materials. The contents of the current volume presents a sampling of more than 160 oral and poster papers that will be presented at the Symposium on Nanoporous Materials III held in Ottawa, Canada on June 12-15, 2002. The selected papers cover the three main themes of the symposium: (i) synthesis of mesoporous silicas and related materials (ii) synthesis of other nanoporous and nanostructured materials, and (iii) characterization and applications of nanoporous materials. Compared to the proceedings of the previous symposium, the current volume contains more contributions related to catalytic and environmental applications, which is a very positive trend. Although the present book does not cover all topics in the area of nanoporous materials, it reflects the current trends and advances in this field, which will certainly continue to attract the attention of materials scientists around the globe. Finally, on behalf of the Organizing Committee, we gratefully acknowledge the generous support of the Faculty of Science (University of Ottawa), the National Research Council of Canada (NRC), the Steacie Institute for Molecular Sciences (SIMS) and the University of Ottawa's Centre for Catalysis Research and Innovation (CCRI). Abdel Sayari February 18, 2002 Mietek Jaroniec
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vii
ORGANIZING COMMITTEE Chairman
Abdel Sayari
University of Ottawa, Ottawa, Ontario, Canada
Vice-Chairman
Mietek Jaroniec
Kent State University, Ohio, USA
Members
Markus Antonietti JeffBrinker Christian Detellier Kazuyuki Kuroda John Ripmeester
Max-Planck-Institute of Colloids and Interfaces, Germany University of New Mexico, New Mexico, USA University of Ottawa, Ottawa, Ontario, Canada Waseda University, Tokyo, Japan National Research Council, Ottawa, Canada
INTERNATIONAL ADVISORY COMMITTEE D. Antonelli G. Attard A. Cheetham J.H. Clark C. Crudden E. Derouane M. Fr6ba A. Galameau S. Inagaki K. Kaneko S. Komameni M. Kruk R. Kumar B. Lebeau Th. Maschmeyer C.Y. Mou G. Q. Max Lu A. Neimark E. Prouzet H.-K. Rhee W.J. Roth D.M. Ruthven R. Ryoo F. Schtith A. Stein M. St6cker B.-L. Su T. Tatsumi O. Yaghi M. Ziolek
University of Windsor, Ontario, Canada University of Southampton, United Kingdom University of California, Santa Barbara, Califomia, USA University of York, York, England University of New Brunswick, NB, Canada University of Liverpool, United Kingdom Justus-Liebig-University, Giessen, Germany Ecole Nationale' Sup6rieure de Chimie de Montpellier, France Toyota Central R&D Laboratories, Inc., Nagakute, Japan Chiba University, Chiba, Japan Pennsylvania State University, University Park, PA, USA Kent State University, Ohio, USA National Chemical Laboratory, Pune, India Universit6 de Haute Alsace, Mulhouse, France Delft University of Technology, The Netherlands National Taiwan University, Taipei, Taiwan The University of Queensland, Brisbane, Australia TRI/Princeton, New Jersey, USA Ecole Nationale Sup6rieure de Chimie de Montpellier, France Seoul National University, Seoul, Korea Exxon-Mobil Research and Engineering Co., New Jersey, USA University of Maine, Orono, Maine, USA KAIST, Taejon, Korea MPI ftir Kohlenforschung, Mtilheim, Germany University of Minnesota, Minneapolis, Minnesota, USA SINTEF, Oslo, Norway The University of Namur, Belgium Yokohama National University, Yokohama, Japan University of Michigan, Ann Arbor, Michigan, USA A. Mickiewicz University, Poznan, Poland
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ix
CONTENTS
Preface Organizing Committee
vii
Intemational Advisory Committee
vii
I.
Plenary Lectures
Recent Developments in the Synthesis and Chemistry of Periodic Mesoporous Organosilicas Tewodros Asefa, Geoffrey A. Ozin, Hiltrud Grondey, Michal Kruk, and Mietek Jaroniec Porous Materials: Looking Through the Electron Microscope O. Terasaki, T. Ohsuna, Z. Liu, M. Kaneda, S. Kamiya, A. Carlsson, T. Tsubakiyama, Y. Sakamoto, S. Inagaki, S. Che, T. Tatsumi, M. A. Camblor, R. Ryoo, D. Zhao, G. Stucky, D. Shindo and K. Hiraga
27
Molecular Imprinting - A Way to Prepare Effective Mimics of Natural Antibodies and Enzymes Giinter Wulff
35
II. Synthesis of Mesoporous Silicas Plugged Hexagonal Mesoporous Templated Silica: A Unique Micro- and Mesoporous Material with Internal Silica Nanocapsules P. Van Der Voort, P. I. Ravikovitch, A. V. Neimark, M. Benjelloun, E. Van Bavel, K.P. De Jong, B. M. Weckhuysen and E.F. Vansant
45
Imprinting of the Surface of Mesoporous Silicates using Organic Structure Directing Agents Kaveri R. Sawant and Raul F. Lobo
53
Synthesis and Characterization of Polymer-Templated Ordered Silica with Cagelike Mesostructure J.R. Matos, M. Kruk, L.P. Mercuri and M. Jaroniec
61
The Modeling of Wall Structure of Siliceous MCM-41 Based on the Formation Process Yasunori Oumi, Kazuhiko Azuma, Takuji Ikeda, Shintaro Sasaki and Tsuneji Sano
69
Pore Size Adjustment of Bimodal-mesoporous Silica Molecular Sieves Xiaozhong Wang, Tao Dou, Dong Wu and Bing Zhong
77
Alcothermal Synthesis of Large Pore, High Quality MCM-48 Silica Jihong Sun and Marc-Olivier Coppens
85
Studies of MCM-41 Obtained from Different Sources of Silica Icaro S. Paulino and Ulf Schuchardt
93
Synthesis and Characterization of Hexagonal Mesoporous Materials Using Hydrothermal Restructuring Method Kyoung-Ku Kang and Hyun-Ku Rhee
101
Synthesis of Highly Ordered Mesoporous Compounds with Control of Morphology Using a Non-ionic Surfactant as Template A. LOonard, J.L. Blin and B. L. Su
109
Towards a Better Understanding on the Mechanism of Mesoporous Formation via an Assembly of Cn(EO)mTMOS J.L. Blin, A. L~onard, G. Herrier, G. Philippin and B.L. Su
117
Mesoporous Silicas via Organic-Inorganic Hybrids Based on Charged Polymers Graham M. Gray and John N. Hay
127
Mesoporous Silicas of Hierarchical Structure by Hydrothermal SurfactantTemplating under Mild Alkaline Conditions Zhong-Yong Yuan, Wuzong Zhou, Bao-Lian Su and Lian-Mao Peng
133
III. Synthesis of Framework-Modified Mesoporous Silicas Synthesis and Characterisation of Super-microporous Aluminosilicates Prepared via Primary Amine Templating E. Bastardo-Gonzalez, Robert Mokaya and William Jones
141
A1-MCM-41 Synthesis Studies Using Al-Isopropoxide as A1 Source R. Birjega, R. Ganea, C. Nenu, Gr. Pop, A. Jitianu
.151
Mesoporous Aluminosilicates from Coal Fly Ash P. Kumar, N. Mal, Y. OumL T. Sano and K. Yamana
159
xi New Route for Synthesis of Highly Ordered Mesop0rous Silica with Very High Titanium Content Xiang-Hai Tang, Xin Wen, Shi-Wei Sun and Hai-Yan Jiang
167
Synthesis and Characterization of Ti-containing Mesoporous Alumina Molecular Sieves Chun Yang and Xi Li
173
IV. Synthesis of Surface-Modified Mesoporous Silicas Organizing One-Dimensional Molecular Wires in Ordered Mesoporous Silica Zongtao Zhang, Douglas A. Blom and Sheng Dai
183
Synthesis and Catalytic Properties of Organically Modified Ti-HMS Yong Yang and Abdelhamid Sayari
189
Synthesis and Characterization of Methyl- and Vinyl-Functionalized Ordered Mesoporous Silicas with High Organic Content Michal Kruk, Tewodros Asefa, Mietek Jaroniec and Geoffrey A. Ozin
197
Polyfunctionalized Silica Adsorbents Obtained by Using Dodecylamine as Template Inna V. Mel'nyk (Seredyuk), Yuriy L. Zub, Alexey A. Chuiko, Mietek Jaroniec and Stephan Mann
205
Characterization of Mesoporous Thin Films Formed with Added Organophosphonate and Organosilane Michael A. Markowitz, Eva M. Wong and Bruce P. Gaber
213
Improving the Hydro-Stability ofMCM-41 by Post-Synthesis Treatment and Hexamethyldisilazane Coating Jing Yang, Antje Daehler, Michelle L. Gee, Geoffrey W. Stevens and Andrea J. O'Connor
221
Adsorption of CO on Zn-Cu(I)/HMCM-41 Qihong Shi, Nongyue He, Fei Gao, Yibing Song, Yang Yu and Huilin Wan
229
V.
Synthesis of Mesoporous Metal Oxides
Design of Transition Metal Oxide Mesoporous Thin Films Eduardo L. Crepaldi, Galo J. de A. A. Soler-Illia, David Grosso, PierreAntoine Albouy, Heinz Amenitsch and ClOment Sanchez
235
xii Mesoporous Alumina as A Support for Hydrodesulphurization Catalysts Jiri Cejka, Nadezda Zilkovd, Ludgk Kalu$a and Miroslav Zdra~il
243
Preparation and XAFS Spectroscopic Characterization of Mesoporous Titania with Surface Area more than 1200 m2/g Hideaki Yoshitake, Tae Sugihara and Takashi Tatsumi
251
Mesoporous Zirconium Oxides: An Investigation of Physico-chemical Synthesis Parameters J.L. Blin, L. Gigot, A. L~onard and B.L. Su
257
Single Crystal Particles ofMesoporous (Nb, Ta)205 Junko N. Kondo, Tomohiro Yamashita, Tokumitsu Katou, Byongjin Lee, Daling Lu, Michikazu Hara and Kazunari Domen
265
VI. Synthesis of Other Nanostructured Materials and Nanoparticles Preparation of Exfoliated Zeolites from Layered Precursors - The Role of pH and Nature of Intercalating Media Wieslaw J. Roth and James C. Vartuli
273
Control of Mesopore Structure of Smectite-type Materials Synthesized with a Hydrothermal Method Masayuki Shirai, Kuriko Aoki, Kazuo Torii and Masahiko Arai
281
Synthesis, Characterization and Catalytic Application of Mesoporous Sulfated Zirconia Young-Woong Suh and Hyun-Ku Rhee
289
Synthesis of Mesoporous Silicoaluminophosphates (SAPO) Erica C. de Oliveira and Heloise O. Pastore
297
Synthesis and Characterization of Mesostructured Vanadium-Phosphorus-Oxide Phases Moises A. Carreon and Vadim V. Guliants
301
Novel Macroporous Vanadium-Phosphorus-Oxides Arrays of Spherical Voids Moises A. Carreon and Vadim V. Guliants
309
with Three-Dimensional
Engineering Active Sites in Bifunctional Nanopore and Bimetallic Nanoparticle Catalysts for One-Step, Solvent-Free Processes Robert Raja and John Meurig Thomas
317
xiii Using Au Nanoparticles-Surfactant Aqueous Solution for a Convenient Preparation of Mesoporous Aluminosilicates Containing Au-Nanoparticles Yu-Shan Chi, Hong-Ping Lin, Chinn-Nan Lin, Chung-Yuan Mou and BenZu Wan
329
The Use ofTemplated Mesoporous Materials as Tempates for the Development of Odered Arragements of Nanowire and Nanorods of Electronically Important Materials J. D. Holmes, T. R. Spalding, K. M. Ryan, D. Lyons, T. Crowley and M. A. Morris
337
Synthesis and Adsorption Properties of Novel Carbons of Tailored Porosity Z. Li, M. Kruk and M. Jaroniec
345
Flexible Metal-Organic Frameworks with Isomerizing Building Units D. V. Soldatov and J. A. Ripmeester
353
Dynamic Porous Frameworks of Coordination Polymers Controlled by Anions Shin-ichiro Noro and Susumu Kitawaga
363
Mesoporous Polymeric Materials Based On Comb-Coil Supramolecules Sami Valkama, Riikka Miiki-Ontto, Manfred Stamm, Gerrit ten Brinke and Olli Ikkala
371
VII.
Characterization of Nanoporous Materials
Electron Microscopic Investigation of Mesoporous SBA-2 Wuzong Zhou, Alfonso E. Garcia-Bennett, Hazel M. A. Hunter and Paul A. Wright
379
A Study of Morphology of Mesoporous Silica SBA-15 Man-Chien Chao, Hong-Ping Lin, Hwo-Shuenn Sheu and Chung-Yuan Mou
387
SBA- 15 versus MCM-41: Are they the same Materials? Anne Galarneau, HOlOne Cambon, Thierry Martin, Louis-Charles De M~norval, Daniel Brunel, Francesco Di Renzo and Franfois Fajula
395
Comprehensive Characterization of Iron Oxide Containing Mesoporous Molecular Sieve MCM-41 Zhong-Yong Yuan, Wuzong Zhou, Zhaoli L. Zhang, Q. Chen, B.L. Su, and Lian-Mao Peng
403
xiv Mesoporous Molecular Sieves of MCM-41 Type Modified with Cs, K and Mg Physico-Chemical and Catalytic Properties Maria Ziolek, Aleksandra Michalska, Jolanta Kujawa and Anna Lewandowska
411
Meso-ALPO Prepared by Thermal Decomposition of the Organic-Inorganic Composite. A FTIR Study Enrica GianottL Erica C. Oliveira, Valeria Dellarocca, Salvatore Coluccia, Heloise O. Pastore and Leonardo Marchese
417
Organic - Inorganic Phase Interaction in A1SBA-15 Mesoporous Molecular Sieves by Double Resonance NMR Spectroscopy Jean-Baptiste d'Espinose, Elias Haddad and Antoine G~dOon
423
Adsorption of Nitrogen on Organized Mesoporous Alumina Jiri Cejka, Lenka Vesel6, Jiri Rathousk~ and Arnogt Zukal
429
The Use of Ordered Mesoporous Materials for Improving the Mesopore Size Analysis: Current State and Future Michal Kruk, Mietek Jaroniec and Abdelhamid Sayari
437
Sorption Properties and Hydrothermal Stability of MCM-41 Prepared by pH Adjustment and Salt Addition Nawal Kishor Mal, Prashant Kumar and Masahiro Fujiwara
445
Acidity Characterization ofMCM-41 Materials Using Solid-State NMR Spectroscopy Qi Zhao, Wen-Hua Chen, Shing-Jong Huang, Yu-Chih Wu, Huang-Kuei Lee and Shang-Bin Liu
453
Acidity of Calcined AI-, Fe-, and La-containing MCM-41 Mesoporous Materials: An Investigation of Adsorption of Pyridine Nong-Yue He, Chun Yang and Zu-Hong Lu
459
Acid Properties of Ammonium Exchanged A1MCM-41 with Different Si/A1 Ratio Antonio S. Arafijo, Cristiane D.R. Souza, Marcelo J.B. Souza, Valter J. Fernandes Jr., and Luiz A. M. Pontes
467
Kinetic Evaluation of the Pyrolysis of High Density Polyethylene over HA1MCM-41 Material Antonio S. Arafijo, Valter J. Fernandes Jr, Sulene A. Araujo and Massao Ionashiro
473
Electrorheological Response of Mesoporous Materials under Applied Electric Fields Min S. Cho, Hyoung J. Choi, Wha-Seung Ahn and Myung S. Jhon
479
XV
VIII. Catalytic Applications of Nanoporous Materials Synthesis and Characterization of TiO2 Loaded Cr-MCM-41 catalysts E.P. Reddy, Lev Davydov and Panagiotis G. Smirniotis
487
Photocatalytic Ethylene Polymerization over Chromium Containing Mesoporous Molecular Sieves Hiromi Yamashita, Katsuhiro Yoshizawa, Masao Ariyuki, Shinya Higashimoto and Masakazu Anpo
495
Catalytic Reduction of Nitric Oxides on A1- containing Mesoporous Molecular Sieves W. Li, Y. Zhang, Y. Lin and X. Yang
503
Catalytic Oxidation of alpha-Eicosanol to alpha-Eicosanoic Acid Over Ti, Zr and Mn Doped MCM-48 Molecular Sieves Changping Wei, Yining Huang, Qiang CaL Wenqin Pang, Yingli BL and Kaiji Zhen
511
Preparation of Pd/A1-MCM-41 Catalyst and Its Hydroisomerization Properties for Long Chain Alkane Compounds Shui Lin, Han Ning, Sun Wan-Fu, Liu Wei-Min and Xue Qun-Ji
517
Alkylation of Phenol with Methyl tert-Butyl Ether over Mesoporous Material Catalysts Xiang-Hai Tang, Xin-Liang Fu and Hai-Yan Jiang
525
Isopropanol Dehydration over Nanostructured Sulfated MCM-41 Antonio S. Araujo, Joana M.F.B. Aquino, Cristiane D.R. Souza and Marcelo J.B. Souza
531
Effect of Si/A1 Ratio and Pore Size on Cracking Reaction over Mesoporous MCM-41 Wen-Hua Chen, Qi Zhao, Hong-Ping Lin, Chung-Yuan Mou and ShangBin Liu
537
Hydrogenation and Mild Hydrocracking of Synthetic Crude Distillate by Ptsupported Mesoporous Material Catalysts Hong Yang, Craig Fairbridge, Zbigniew Ring, Randall Hawkins and Josephine M. Hill
543
Carbon-Carbon Bond Forming Reactions Catalyzed by Meso- and Microporous Silicate-Quaternary Ammonium Composite Yoshihiro Kubota, Yusuke Nishizaki, Hisanori Ikeya, Junko Nagaya and Yoshihiro Sugi
553
xvi A Selectivity of Zeolite Matrices in the Cu(II) Reduction Process Vitalii PetranovskiL Valerij Gurin, Nina Bogdanchikova, Miguel-Angel Hernandes and Miguel A valos
561
Reduction of Binary Silver-Copper Ion Mixture in Mordenite: an Example of Synergetic Behavior Vitalii Petranovskii and Nina Bogdanchikova
569
Preparation, Characterization and Catalytic Properties of CuPC/Y Nanocomposite Huaixin Yang, Ruifeng Li and Kechang Xie
575
IX.
Environmental Applications of Nanoporous Materials
Environmental Applications of Self-Assembled Monolayers on Mesoporous Supports (SAMMS) Glen E. Fryxell, Yuehe Lin, Hong Wu and Kenneth Kemner
583
A Possible Use of Modified Mesoporous Molecular Sieves in Water Treatment Processes Izabela Nowak, Barabara Kasprzyk, Maria Ziolek and Jacek Nawrocki
591
Organized Mesoporous Titanium Dioxide - A Powerful Photocatalyst for the Removal of Water Pollutants Jiri Rathouslc~, Mark~ta Slabovd, Katerina Macounovd and Arnogt Zukal
599
Mesoporous Materials for Heavy Metal Ion Adsorption Synthesized by Displacement of Polymeric Template V. Antochshuk, M. Jaroniec, S.H. Joo and R. Ryoo
607
Organically-modified Mesoporous Silica Spheres with MCM-41 Architecture as Sorbents for Heavy Metals M. Etienne, S. Sayen, B. Lebeau, and A. Walcarius
615
NO and NO2 Gas Sensors Based on Surface Photovoltage System are Fabricated by Self-ordered Mesoporous Silicate Film Hao-Shen Zhou, Takeo Yamada, Keisuke Asai, Itaru Honma, Hidekazu Uchida and Teruaki Katsube
623
xvii
XO
Other Applications of Nanoporous Materials
Polymerisations in Mesoporous Environments James H. Clark, Duncan Macquarrie, Valerie Sage, Katie Shorrock and Karen Wilson
631
Incorporation ofNano-sized zeolites into a Mesoporous Matrix, TUD-1 Z. Shah, W. Zhou, J.C. Jansen, C. Y. Yeh, J.H. Koegler and Th. Maschmeyer
635
Formation and Stabilization of Gold Nanoparticles in Organo-Functionalized MCM-41 Mesoporous Materials and their Catalytic Applications Chitta Ranjan Patra, Anirban Ghosh, Priyabrata Mukherjee, Murali Sastry and Rajiv Kumar
641
Entrapment and Stabilization of Cadmium Sulphide (CdS) Nanoclusters Formed Inside Propylthiol Functionalized MCM-41 Mesoporous Materials Anirban Ghosh, Chitta Ranjan Patra, Priyabrata Mukherjee, Murali Sastry and Rajiv Kumar
647
SnO2 Nanoparticles in the Pores of Non-structured SiO2 and of Si-MCM-41: Comparison of their Properties in Gas Sensing Yuecel Altindag, Andrei Jitianu and Michael Wark
653
Spontaneous Nitride Formation in the Reaction of Mesoporous Titanium Oxide with Bis(Toluene) Titanium in a Nitrogen Atmosphere. M. Vettraino, X. He, Michel Trudeau and David Antonelli
661
Isolation and Characterization of Amorphous Solids from Oil Sands Fine Tailings Abdul Majid, Steve Argue, Irina Kargina, Victor Boyko, Gerry Pleizier and Jim Tunney
669
Author Index
675
Subject Index
679
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Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
Recent developments in the synthesis and chemistry o f periodic m e s o p o r o u s organosilicas
9
a*
Tewodros Asefa, a Geoffrey A. Ozln, ' Hiltrud Grondey, a Michal Jaroniec b
KIRlk, b
and Mietek
a Materials
Chemistry Research Group, Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada b Department of Chemistry, Kent State University, Kent, Ohio 44242, USA
Synthetic routes have recently been developed to an entirely new class of organicinorganic hybrid nanocomposite materials called periodic mesoporous organosilicas (PMOs) containing bridging organic groups integrated within a well-ordered mesoporous silica-based framework structure. Some of the interesting properties of these types of materials have been demonstrated but real challenges still remain including the scope of the synthesis approach and breadth of the compositional domain, reactivity and stability of the materials, their chemical and physical, electrical and mechanical properties, as well as potential applications. In this review, we will describe our past and current research concerning new synthetic strategies, unique properties and advantageous features of alkane, alkylene, aromatic, heteroatom-containing, chiral, and star-like organic functional group containing PMOs as advanced materials for diverse applications. Some of the effects of size and kind of organic groups on the order and integrity of the structure of the materials are discussed. Various synthetic routes, such as lithiation, Grignard, hydroboration, Pd-catalysed Heck coupling and alcoholysis reactions that we used to make the molecular poly(trialkoxysilyl)organic precursors are also briefly described.
I. INTRODUCTION The synthesis of periodic mesoporous organosilica (PMO) materials containing organic groups in the framework of ordered and high surface area mesostructures is drawing increasing attention recently [1-30]. One of the main driving forces behind the synthesis of various PMOs is the traditional interest in the synthesis of ordered hybrid organic-inorganic nanocomposites wherein beneficial properties of one of the components are enhanced or new properties uncharacteristic of the individual components are created. Furthermore, the presence of organic functional groups in such ordered nanoporous materials also offers additional advantages that make the materials potentially useful as catalytic and chromatographic supports, chemical and biological
sensors and membranes. Consequently, research in the field of organic-inorganic hybrid nanocomposite materials has remained an active area of investigation for the last few decades and is likely to expand in the years as well. Enormous advances made since the first types of classical organically modified silicas (ORMOSILs) and organically modified ceramics (ORMOCERS) were reported [31-33]. The successes in coupling organic and inorganic groups at a molecular level for the synthesis of hybrid organicinorganic xerogels and amorphous materials [34-40] have also led to many advances in recent years and proved to have advantages over thesimple physical mixing of the constituents in their bulk states. However, until recently, there have been no welldeveloped approaches to create uniform pores of controlled size in these materials. In fact, their porous structure is highly dependent on the synthesis temperature and drying conditions [39,40]. With the work of Mobil scientists in 1992 and the first report on mesoporous silica (MCM-41) materials [41-44], a new research direction in organicinorganic nanocomposite materials emerged. The synthesis of these materials is carried out using inorganic precursors and surfactant templates and the method of supramolecular self-assembly. In many cases, the supramolecular templates can be removed without the collapse of the ordered composite, thus rendering ordered mesoporous structures. After the first papers on materials with siliceous frameworks [41-44], synthesis techniques and compositions of mesoporous materials developed further to include various other kinds of nanocomposite and nanoporous materials. The methodology, for instance, was modified to include ordered hexagonal, cubic and lamellar structures as well as disordered structures with uniform pores [41-50] while the composition field was expanded to include Pt, TiO2, M/Ge4Sl0, etc. [51-57]. Furthermore, by judicial choice of templates and swelling agents, control over the size, porosity and structure of the materials have been achieved [41,49,50,58,59]. The literature in the past 10 years also contains demonstrations on potential applications of these materials in catalysis, nanoelectronics, separation, host-guest chemistry and sensing [60-66]. However, many of these applications were not achieved with the periodic structure alone. The presence of electroactive, optically active or reactive functional sites within these high surface area and ordered mesoporous materials were required for these applications to be realized in practice. The introduction of terminal organic functional groups into periodic mesoporous silicas either through direct or indirect (post-synthetic) synthetic approaches has been successfully used as a way to functionalize this class of materials in the past few years [67-71 ]. However, these approaches have often resulted in materials with lower degree of structural ordering and low or moderate loading of organic groups. The recent approach of introducing organic functional groups into the framework of periodic mesoporous organosilicas (PMO) provided a way to overcome these drawbacks [ 1-30]. PMOs are a new class of organic-inorganic hybrid nanocomposites with uniformly distributed organic functional groups in the framework of the materials. They are synthesized like MCM-41 materials through in a one-pot surfactant-templated supramolecular self-assembly procedure but from the hydrolysis and condensation of poly(trialkoxysilyl)organic precursors ([(R'O)3Si]xR, x = 2 ,3) [1-30]. The enormous choice of polysilylated molecular precursors having various types of bridging organic functional groups with for example electroptic, catalytic and hydrophilic/hydrophobic
properties and the self assembly of these with various kinds of supermolecular templates are providing researchers with large varieties of periodic mono- and multi-functional organic-inorganic nanocomposite materials. The ability for molecular integration of organic and inorganic groups in the framework of these materials may have advantages over direct and indirect (grafting)methods that may create non-uniformly distributed organic groups protruding into the void spaces. Benefits include tailorable physical properties [ 19,20], uniform distribution of functional groups, possibility of controlling the loading of the functional groups using co-condensation approach, and unique chemical properties. Some of these bridging organic groups have also proven to be accessible for chemistry and can be further transformed chemically [1,18]. Interesting chemical differences between organics in the framework and in the channels has been reported and used to have advantages in the preparation of a new sub-class of bifunctional and multifunctional PMOs [8]. The dependence of thermal, mechanical, dielectric and adsorptions properties on the nature of the organic groups in these hybrid materials will likely result in new products, processes and devices made out of PMOs in the near future [3,19-21 ]. The ability to synthesize film and various curved PMO morphologies [7,19-21] may lead to advances in catalysis, chromatography and membrane science and technology. Advances made in the synthesis and characterization of the properties of PMO materials in just less than two years have inspired investigations into new kinds of functionalized PMO materials and a search for commercial applications. However, only a few types of organic functionalized PMOs have been reported so far. They include methylene, ethane, ethylene, acetylene, thiophene, benzene and bithiophene containing PMOs, [ 1-30]. There is also a notable scarcity of detailed investigations of the structural integrity, and thermal and chemical stability of Si-C bonds for various kinds of organic groups in PMOs [3,8]. Herein, we review some developments in our research group mainly concerning the synthesis of various PMO materials and possible applications. Particular attention will be given to PMO materials having heteroatom and side-arm starlike organic groups, which are envisioned to enhance the reactivity and accessibility of bridging organic groups in PMOs enabling judicious surface modification that leads to the tailoring of function. 2. E X P E R I M E N T A L 2.1. Materials. HC1 and NH 3 solutions were obtained from BDH. Methanol was supplied by ACP. Hexanes, pentanes, diethylether and tetrahydrofuran (THF) were purchased from ACP and were dried with Call2 and molecular sieves before every use. The THF and the diethylether were further distilled over Na/benzophenone. All commercially available bis(triethoxysilyl)organic compounds were obtained from Gelest. All other chemicals were received from Aldrich. 2.2. PMO PRECURSORS 2.2.1. Commercially available PMO precursors. Scheme 1 shows most of the commercially available precursors (either triethoxysilylated or trichlorosilylated) that we
obtained from Gelest and used for the preparation of PMOs. The precursors were used as received without further purification.
Scheme 1. Commercially available PMO precursors used for the synthesis of PMO materials (Si denotes -Si(OEt)3 or-SIC13).
Si~Si
Si~si
1
Si~"x,~"X, Si
2
3
4
~---Si S
~
S i ~ S i
S i ~ N H
Si Si 6
S i ~ N H 7
2.2.2. Synthesis of PMO precursors.
We have utilized various synthetic routes to prepare polyalkoxysilylated PMO precursors that are not commercially available. Representative synthetic routes are shown in Scheme 2 and some examples of PMO precursors and PMOs that we have prepared from these precursors are shown in Scheme 3. Some of these precursors have been synthesized before and used to prepare hybrid org~ic-inorganic xerogels (materials with a relatively broad pore size distribution) by many groups over the past few years [34-40]. Most of the aromatic precursors were synthesized using Grignard reactions while the methine precursor is made through silylation-alcoholysis and the anthracene and ferrocene precursors through lithiation. 2.2.3. Synthesis of organomethylene PMO precursors. (Scheme 4 and 5). Bis(trialkoxysilyl)organic precursors containing methylene groups in the backbone with side-arm functionality have been prepared through lithiation of a bis(triethoxysilyl)methane precursor under nitrogen atmosphere in a freshly distilled anhydrous solvent followed by coupling reactions of the resulting carbanion with various electrophiles (Scheme 4) [72]. [CAUTION: t-BuLi is strongly pyrophoric and should be handled extremely carefully]. This route resulted in a new class of side-arm bridging organosilane precursors. A representative synthetic procedure for the preparation of bromomethylene (BM) and 1,1-bis(triethoxysilyl)-2-(p-bromophenyl)ethane (or pbromobenzyl-methylene) (BBM) PMO precursors are given below. Similar reactions were applied or could be applied to the synthesis of the other precursors shown in Scheme 5. Crude products were used as precursors when distillation resulted in decomposition.
Synthesis of [1,1-bis(triethoxysilyOmethyl] lithium salt (la). A commercially available bis(triethoxysilyl)methane (BTM) (1) was lithiated following a literature procedure [72] with a slight modification. Typically, to 150 mL freshly distilled dry THF was added
bis(triethoxysilyl)methane (BTM, 1) (3.0 g, 8.8 mmol) and the solution was stirred under nitrogen for 5 minutes. Then 5.2 mL of 1.7 M t-BuLi (8.8 mmol BuLi) was added to the above solution dropwise over 10 minutes a t - 78 ~ under a nitrogen atmosphere. The solution was stirred for 1 hr a t - 78 ~ The resulting carbanion lithium salt (la) was subsequently quenched with bromine or 4-bromobenzyl bromide (see below) a t - 78 ~ (Scheme 4 and 5). Scheme 2. Synthetic routes to polysilylated PMO precursors. 1) Grignard Mg / XSi(OR')3 r_
X--R--X
(R'O)3Si~R--Si(OR')3
2) Alcoholysis R'OH
X3Si~R--SiX 3
=
(R'O)3Si--R--Si(OR')3
3) Hydrosilation
~--R---~
(R'O)3Si~
HSi(OR')3,.
, ~R~si(OR')3
H2PtC16 4) Pd-coupling Heck Reaction '----"
X-Ar--X
+ 2
Pd (I) / NEt3
~
\Si(OR,)3
(- HX)
5) Hydroboration
(R'O)3Si-----~__ Ar .v_-
OR)3 BH3.THF
3
or
( ~Si(OR,)3 )
~___ Si(OR')3
l
i(OR')3
H~-----'--~Si(OR,)3
~R'O)3SiJ ~ S i ( O R ' ) 3 6) Lithiation (R'O)3Si~Si(OR')3
t-BuLi / THF .~ -78 C
(R'O)3S i ~ S i ( O R ' ) 3 Li §
R"X
r- (R'O)3Si'~Si(OR')3
R"
7) Silylation of acid halides (acyl halides) C13Si~/SiC13 R~COC1
+ 2 HSiC13
r-
I R
EtOH
(EtO)3Si~/Si(OEt)3
R
Scheme 3. Commercially unavailable precursors used for PMO synthesis (Si stands for
Si(OEt)3). Si Si I Si H
Si~Si
8
9
10
Si Si
Si~ ~-S- S i S
s i ~ S / ~ -Si
11
Si
Si [
Si
.... Si ~/] 12
Fe ~
Si~-.~Si
13
14
Si Si~.v,,'~/'~Si
Si 15
Si H3C~~~ L
H3C
Si H3CO~
--CH3 16
si 20
Si
Si
17
18
F
F
F
F
s. si 21
y "OCH3 Si 19
s si 22
Synthesis of 1,1-bis(triethoxysilyl)bromomethane[(EtO)3SiCHBrSi(OEt)3](BM). To
the carbanion solution (la) was added, a slight excess of bromine (1.5 g, 9.4 mmol) a t - 78 ~ The solution was stirred a t - 78 ~ for 30 min and then at room temperature for 2 hrs. The solvent was removed and the residue was extracted with dry hexane and then filtered. After pumping the solvent off, the residue was vacuum distilled giving bis(triethoxysilyl)bromomethane (3): bp 110-112 ~ / 0.04 mm Hg; IH NMR (300 MHz,
CDC13) 6; 1.21-1.25 (t, 18 H, CH3), 6; 2.18 (s, 1H, CHSi), 6; 3.90-3.96 (q, 12H, CH20); 13C NMR (75.48 MHz, CDC13) 6; 10.13 (CHSi), 6; 18.38 (CH3), 6; 59.62 (CH20); El-MS (m/z) 419 (23%, M+), 375 (25%, [M+- 44]), 163 (100%, [(EtO)3Si+]). Scheme 4. Lithiation of bis(triethoxysilyl)methane and subsequent coupling of lithiated carbanions with organic electrophiles. EEtOXs EtO .... iV
si~Eo Et .....OEt
t-BuLi/THF_ -78 ~
(1)
(la)
(1)
EtO QEt EtO~.\qi &OEt EtO....... ~S_ ......OEt L1
EtEtOksi si~EoEt EtO...... V + .....OEt Li
R'X ~
EtO\ ~Et EtO--~:i ~:i--,OEt EtO....... " ~ ......OEt R'
(2)
(la)
EtO\ EtO~si EtO...... V
~)Et X2 EtO\ ?Et Si~.,OEt .._ EtO,..-. . . . . OEt .....OEt "- EtO......~ly~l""OEt__ Li+ X
R ' M g X EEtONsi Si~.EOEt ._ EtO...... " ~ .....OEt R'
(3)
(la)
R' = D (lb); Br (lc); p-CH2-C6H4Br (ld); p-CH2-C6H4-Si(OMe)3 (le); COOH / COO-Li + (lf); CH2CH2CH2NH2 (lg); Br-camphor (lh); C6F5 (li) (Grignard reaction was also used after lithiation); CH2(CH2)n-i(CF2)mCF3 (lj); CH2NHC6H4NO2 (lk) (See Scheme 5 also).
Synthesis of 1,1-bis(triethoxysilyl)-2-(p-bromophenyl)ethane (p-bromobenzylmethylene precursor) [(EtO)3SiCH(CH2-p-C6H4BOSi(OEt)3] (BBM). To la was added a THF (10mL) solution ofp-bromobenzylbromide (2.2 g, 8.8 mmol) a t - 78 ~ After similar work-up as above, BBM was obtained: bp 144-148 ~ / 0.04 mm Hg; 1H NMR (300 MHz, CDC13) 6 0.55-0.58 (t, 1 H, CHSi), 8 1.18-1.23 (t,18H, CH3), 8 2.89-2.92 (d, 2H, CHzPh), fi 3.78-3.82 (q, 12H, CH20), ~ 7.17-7.20 (d, 2H, ArH), 6; 7.36-7.39 (d, 2H, ArH); ~3C NMR (75.48 MHz, CDCI3) fi 10.80 (CHSi), 6; 18.46 (CH3), 6;29.47 (CHEPh), 8 58.66 (CH20), 6130.84, 131.10 (CH aromatic); EI-MS (m/z) 508 (5%, M+), 464 (100%, [M +- 44]).
Scheme 5. Synthesis of PMO precursors with organomethylene groups in the framework. lh
(R'O)3.Si~Si(OR')3
(R'O)3Si'~Si(OR')3
~'H2
3c'a%dpLboTm~
]
(R'O)3Si~ISi(OR')3
lo
O~---O-Li§
Si(OMe)3 CO2 C1CH2
(R'O)3Si~-~Si(OR')3 [
/Si(OMe)3
(~U2Br !(R' !................................... O)3Si~Si(OR' [ )3 ] 1CH NH"(R'O)3Si'~Si(OR')3 i "+ [ C( 2)3 2 CH2(CH2)2NH2
~~1
[.................Li..........~p...j
CF3(CF2)m(CH2)nC1 /
D20
/ NHCH2CI / ~
Br2~
(R,O)3Si~.~Si(OR,)3
~I CH2(CH2)n-1 (CF2)mCF3 (R'O)3Si Si(OR')3 lj
(R'O)3Si~-Si(OR')3 NO2 ~FsMgBr CH2 I IH (R,O)3Si~...Si(OR,)3 (R'O)3Si'~Si(OR')3 D F~F lk lb li F F NO2 F 2.3. Self-assembly of BM and other PMOs under acidic conditions For a typical synthesis under acidic conditions, a solution of cetyltrimethylammonium bromide (0.34 g, 0.93 mmol), HC1 (7.18 g, 36 wt%, 70.8 mmol) and water (13.4 g, 0.74 mol) was prepared at room temperature (Scheme 6). To this solution was added 3.85 mmol (or 7.70 mmol Si) of the required precursor and the mixture was stirred for 30 mins. After aging at 80 ~ for 4 days, the product was isolated by filtration, washed with copious amounts of water, and dried under ambient conditions. (For bromomethine a light brownish powder was obtained with a typical yield of 0.56 g).
2.4. Self-assembly of B B M and other P M O s under basic conditions
A solution of cetyltrimethylammonium bromide (0.67g, 1.84 mmol), ammonium hydroxide (14.18 g, 35 wt%, 0.14 mol) and water (26.73 g, 1.48 mol) was prepared at room temperature (Scheme 6). To this solution 2.94 mmol (or 5.88 mmol Si) of precursor was added (for 50% BBM PMO, 2.94 mmol BBM and 1.47 mmol of TEOS) and the solution was stirred for 30 minutes during which time a precipitate formed. After aging at 80 ~ for 4 days, the product was filtered, washed with copious amounts of water, and dried under ambient conditions resulting in a fine powder (typical yield of 0.49 g of white powder was obtained for bromobenzyl-methylene PMO). Scheme 6. Synthesis and solvent-extraction of BM and BBM PMOs
EE~~si si~EoEt EtO " x ~ .....OEt
CTABr/ +/H20 . . . . . .
oS
Lo
O""Si~ Si''''O
Br
Br
lc
lc'
EtO\ ~Et EtO.---Si qi-,-OEt EtO"" x ' ~ ......OEt +
02,
.OEt EtO~S,i"OEt
CTABr/ OH-/H20
HCI / MeOH ..... =
BM PMO
......o
H2
HC1/ MeOH = BBM PMO
OEt
Br ld
Br ld'
2.5. Surfactant-extraction of P M O s
The surfactant was removed from the samples using solvent-extraction in an HC1/methanol solution. Typically ca. 0.3 g of as-synthesized powder was stirred for 6 hrs at 55 ~ in a solution of 4 g of conc. (36 wt %) HC1 and 170 g methanol. The product was then isolated by filtration, washed with methanol, and dried in air. A typical weight loss of ca. 30 % was obtained for each sample after a single solvent extraction. The surfactant removal was confirmed by solid-state 13C CP MAS NMR. 2.6. Instrumentation and characterization techniques
The as-synthesized and surfactant extracted materials were characterized using powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), 13C, 29Si CP MAS (29Si MAS as required) and 79Br MAS NMR, N2 adsorption, thermogravimetric analysis (TGA), and elemental analysis (EA).
10 Powder X-ray diffraction (PXRD) patterns were measured with a Siemens D5000 diffractometer using Ni-filtered Cu-K~ radiation with )~ = 1.54178 A. The high temperature in-situ measurements were done as reported in Ref. [8]. TEM images were recorded on a Philips 430 microscope operating at an accelerating voltage of 100 kV. Nitrogen adsorption measurements were carried out at 77 K on a Micromeritics ASAP 2010 volumetric adsorption analyzer. Before the measurements, the samples were degassed under vacuum at 100 or 140~ Weight change curves were obtained under nitrogen or air atmosphere on a TA Instruments TGA 2950 thermogravimetric analyzer (TGA) using a high-resolution mode with a maximum heating rate of 5 ~ min -1. Solution phase nuclear magnetic resonance spectra were taken with a Varian VXR 300 spectrometer. Solid-state NMR spectra 13C CP-MAS (100.6 MHz), 29Si CP-MAS (79.5 MHz) and 79Br MAS NMR (100.3 MHz) were obtained with a Bruker DSX400 spectrometer. Experimental conditions: 13C CP-MAS NMR (6.5 kHz spin rate; 2.5 ms contact time; 3 s recycle delay; 10,000-20,000 scans); 13C NQS (non-quaternary suppression) CP-MAS NMR experiment, 6.5 KHz spin rate, 3 s recycle delay, 50 ~ts dephasing delay, 5,000-10,000 scans; 29Si MAS NMR (6.5 KHz spin rate; 100 s recycle delay; 800-1000 scans). 29Si CP MAS NMR experiments: 6.5 kHz spin rate, 3 s recycle delay, 10 ms contact time, zr/2 pulse width of 6.0-7.5 ~ts, 200-5,000 scans. 79Br MAS NMR (6.5 KHz spin rate; 0.1 s recycle delay; 50,000-100,000 scans). 3. R E S U L T S A N D D I S C U S S I O N 3.1. Precursor synthesis Since the reports on the first PMOs [1,2,13,18]; we have been actively investigating the possibility of incorporating other organic functional groups into the framework of PMOs. Some of the challenges are briefly described here. As most PMO precursors were not commercially available, their synthesis under inert atmosphere was routinely required. Most of the poly(trialkoxysilyl)organic precursors discussed in this article were prepared through lithiations and Grignard reactions while some are commercially available. We found that the synthesis through lithiation usually resulted in higher yields than the analogous Grignard reactions. The required precursors were usually isolated and purified by distillation under vacuum to avoid possible decomposition due to their rather high boiling points under standard conditions. The reaction of lithiated carbanion solution (la) with electrophiles was found to be quite favorable and resulted in good yields when the reaction was carded out rigorously under an inert atmosphere. 3.2. Synthesis of PMOs 3.2.1. Linear unsaturated and saturated organic bridge-bonded PMOs So far, the literature reported PMOs have rigid and/or short organic groups. These were found to result in rather well ordered mesoporous structures. For instance, we reported the synthesis of the shortest organic bridgebonded PMOs (1) with 2-D hexagonal mesostructures (Figure 1) containing methylene bridges that are isoelectronic with oxygen atoms in MCM-41 materials [3]. We also recently expanded the synthetic route to include cubic [10] and biphasic (hexagonal and cubic together in the same material) methylene PMOs. Figure 1 shows the powder X-ray diffraction (PXRD) pattems and TEM images for various methylene PMO structures. The thermal
11 transformation of bridging methylene into terminal methyl groups in these PMO materials is particularly interesting and provides a novel route to new organic functionalized mesoporous materials with high loadings of functional groups. Similarly, we prepared an ordered mesoporous methine PMO (8) with the methine groups bridging three silicate units in a mesoporous structure (Scheme 3). The precursor for this PMO (8) was prepared by silylating chloroform in the right stoichiometry followed by alcoholysis as reported by Corriu et al. [72]. (A) Methylene PMO (hexagonal and cubic)
(C) Hexagonal methylene PMO
(B) Methylene PMO (biphasic)
(D) Cubic methylene PMO
Figure 1. PXRD patterns of hexagonal, cubic and biphasic methylene PMOs (A, B) and TEM images hexagonal and cubic methylene PMOs (C, D) PMOs with two carbon bridging organic groups were among the first to be reported. They included ethane (2), ethylene (3) and acetylene (12) PMOs [1,2,13-18]. 2D hexagonal, 3-D hexagonal and cubic structures were reported for 2 [ 13-16], both 2-D
12 hexagonal and disordered structures were reported for 3 [1,18], while most of the Si-C bonds were found to cleave for 12 [2]. The ability to vary both the composition and the structures of the materials will offer numerous opportunities for applications. Ethane PMOs were also reported to adopt various curved morphologies [25,26], which will likely be useful in separation applications. Furthermore, accessibility and reactivity of ethylene group in an ethylene PMO were demonstrated with bromination [1,18] and hydroboration [ 12] reactions giving more opportunities for further functionalization. For instance, the 13C NMR and 29Si CP MAS NMR spectra for ethylene PMO before and after hydroboration (Figure 2) indicate that some of the ethylene carbons are available for reaction with the retention of the periodic structure (see PXRD patterns in Figure 2A).
(B) t3c CP MAS NMR spectra
(A) PXRD patterns lOOOl~,
- Jll, ~, ~
"
Solvent-extracted
~
//
Hydroborated (Spin i~'ate= 8"()KHz
l] 1
tlydmborated (SpinRate = 6.5 KHz)
Hydroborated
20(0) (C) '~Si CP MAS NMR spectrum
/
II~
e
S~
borated " " * " ' ~ ' ( " ' ' " ' 1
150 I
";"
"
I
. . . . .
|
. . . .
....
100
~'1
. . . .
50
I'"
0
I
o -so -too .1so Chemical shift (ppm)
Chemical shift (ppm)
Figure 2. A) PXRD pattems of ethylene PMO before and after hydroboration; B) 13CCP MAS and (C) 29Si (CP) MAS NMR spectra of ethylene PMO before and after hydroboration. PMOs containing a bridging group with one or two carbon atoms as well as other rather short and rigid organic groups were observed to self-assemble quite readily and form well-ordered structures. Further investigations in our group revealed that poly(trialkoxysilyl)organic precursors having bridging organic groups with four or more carbon chains did not self-assemble and gave only amorphous gels and disordered materials. For instance, 2-butylene (16), hexylene (5), 1,4-diethylbenzene (6), and N,N'dipropylethylenediammine (7) bridge bonded materials when templated with surfactants were all found to be amorphous with no low angle Bragg reflections. However, three
13 carbon chain precursor materials with bridging organic groups like 1,1dimethylvinylidene (4) were found to give partially ordered PMO materials. These observations indicate that the more flexible and non-polar bridging organic groups partially or fully prevent the development of mesoscopic order during surfactant templated self-assembly. Attempted synthesis of 2-butylene with large head group surfactants (cetyltriethylammonium bromide) [73] also failed to produce well-ordered PMO materials.
3.2.2. Aromatic and organometallic bridge bonded PMOs various kinds of aromatic and organometallic PMOs [2,9] with ordered mesoporous structures and some of these with a certain degree of additional spatial ordering of the aromatic groups in the pore walls due to re-re stacking, (as indicated by Xray diffraction data [9]) have been synthesized and reported. For instance, we have prepared 1,4-phenylene (9) [2], 1,3,5-phenylene (14) 2,5-thiophene (10) [2], 2,2'bithiophene (11) [2], 1,4-xylyl (17) [9], 1,4-(2,5-dimethylphenylene) (18) [9], 1,1'ferrocene (13) [2], and 1,4-(2,5-dimethoxyphenylene) (19) PMOs [9]. In addition, ptoluyl (20), 1,4-perfluoroaryl (21) and 2,5-pyridine (22) PMOs have also been prepared but their structures were not well defined and their stability was poor. The synthesis conditions for most of the aromatic PMOs should be chosen carefully in order to avoid or minimize cleavage of Si-C bonds. For example, significant cleavage of Si-C bonds was observed in thiophene, bithiophene and ferrocene PMOs. Aromatic groups within these PMOs are potentially useful as good supports and carriers for metal or organometallic guest molecules, which may ultimately be useful as catalysts (for example pyridine is a good ligand for anchoring metals). Some of the organics are also known to have high affinities for many other organic molecules, a property that could make this class of PMOs useful for separation and environmental remediation applications. Moreover, conjugated aromatic and organometallic containing PMOs based on anthracene (15) and ferrocene (13) could find applications as sensors as they contain optically active and electroactive moieties. 3.2.3. Organomethylene bridge bonded PMOs Recently, we have prepared a new class of PMOs with star-like organic bridging groups based on a methylene backbone. It is known that methylenesilica PMOs were amongst materials with well ordered mesoporous structures and interesting properties and we have chosen these groups as platforms to attach various functional groups (Schemes 4 and 5). These reactions were carried out by taking advantage of the lithiation reactions that bis(trialkoxysilyl)methane undergo resulting in the corresponding carbanion (la) allowing one to replace one of the protons by other active organic molecules. This opened an avenue to a diverse group of functionalized PMOs including heteroatom containing organic groups. Some examples of PMOs whose synthesis was attempted by this route include bromomethylene (lc), p-bromobenzyl-methylene (ld), p-benzylmethylene (le), carboxylic acid-methylene (If), (+)-bromocamphor-methylene (lh), and perfluoaryl-methylene (li) PMOs. The synthesis of PMOs with these groups would afford some of the first mesoporous materials with highly reactive bridge bonded organic groups in the framework and protruding into the void spaces. Each of these materials is anticipated to have their own special properties and particular applications. For example,
14 the (+)-bromocamphor is a chiral group and could be useful for chiral reactions, catalysis and separations. The benzyl-methylene PMO is a new type of aromatic PMO with three surrounding silicate units unlike most of the previously discussed aromatic PMOs. The perfluoraryl (li) and perfluoroalkyl (lj) functional groups will make the PMO materials more hydrophobic and "Teflon-like" and could lead to a new class of low dielectric constant materials. Our preliminary investigation on perfluoraryl-methylene PMOs (li) was promising but more work is needed to determine their stability and properties. The other reactions in Scheme 5 that will not be discussed in detail here but are worth mentioning involve those carrying basic (lg), acidic (If) and optically non-linear (lk) functional groups on the methylene bridging group. Each of these PMOs would be interesting as an advanced material for various applications. (A) Methine PMO (solvent-extracted)
(B) p-Benzyl-methylene PMO (solvent-extracted)
~
IlL
9 .... 2
~.~t,~ ~ 4
.
. 6
.
.
.
t
. 8
.
l(i
0 5
1o
20 (degrees)
10
15
20
2A
.~0
2e (degrees)
(C) Other PMOs (solvent-extracted) 3OO0
ga, 2~m .,.., ~ l~D.~v~. I ,
/
Bromocamphor-methylene PMO
[
-Dime, y vm leae. , o
0 .....
-.~-~.~..-~,o.~.. ,. . . . . 5
,
..
.
!0
..
, 15
-~ ............. 20
20 (degrees)
Figure 3. PXRD pattems of various PMOs with organomethylene groups in their frameworks. The synthesis of the aforementioned PMOs was performed and PXRD patterns of most of the resulting materials after template extraction using acid/methanol washing showed at least one low angle Bragg peak indicating the presence of an ordered mesoporous structure (Figure 3). For some of these materials, the porous structure was additionally studied using nitrogen adsorption. The sample synthesized using the
15 bromocamphor-methylene precursor was clearly mesoporous (see adsorption isotherm in Figure 4A) and exhibited primary mesopores of diameter about 3.6 nm, a very large BET specific surface area (1310 m 2 g-l) and a significant total pore volume (0.99 cm 3 g-l). However, as will be discussed below in more detail, this PMO was found to contain only a small amount of bromocamphor-methylene bridging groups, whereas its predominant bridging group was methylene. The presence of very limited amount of bromocamphor groups can be inferred from a small magnitude of TGA weight loss (Figure 4B). Therefore, this sample can be referred to as methylene/bromocamphor-methylene PMO. The attempted synthesis of benzyl-methylene PMO (le) in which a bridging organic group connected to three silicon atoms resulted in a mostly microporous material with the BET specific surface area of 670 m 2 g-i and a total pore volume of 0.41 cm 3 g-1. The surfactant-extracted sample exhibited a small weight loss related to the decomposition of the residual surfactant at about 200~ followed by a very large weight loss that can be primarily related to the decomposition and thermodesorption of large organic groups in the PMO structure. The synthesis of a 1,1-dimethylvinylidene PMO (4) was also attempted and the resultant solvent-extracted sample was mesoporous (pore diameter of about 2.6 nm) and exhibited an appreciable adsorption capacity despite the fact that a certain amount of surfactant was not removed, as seen from TGA. As will be shown below, an appreciable degree of cleavage of Si-C bonds took place during the synthesis of this material, and therefore it is expected to have a significant amount of pendent rather than framework organic groups (the retention of organic groups can be inferred from TGA data; additional evidence for preservation of Si-C linkages is presented below) The presence of bridging organic groups in these PMOs, were investigated using solid-state NMR spectroscopy. The 13C CP MAS NMR (Figure 5), for instance, indicates the presence of small percentages of bromocamphor-methylene and a large quantity of methylene not containing camphor groups probably because the crude product was used as a precursor in this case. Further, large percentages of functional groups were observed in methine (8) and 1,1-dimethylvinylidene (4) PMOs. The methine PMOs also showed similar thermal~ stability to those of methylene PMOs and the surfactant template in these materials was easily removed with calcination in air at 350 ~ Template removal in acid/methanol solution for 1,1-dimethylvinylidene PMO causes hydrochlorination of the vinyl carbons unless carried out under very mild acidic solution. The 29Si CP MAS NMR spectrum (Figure 6) of the sample synthesized using the bromocamphor-methylene precursor showed essentially only T sites, thus revealing no Si-C bond cleavage. However, most of the other materials exhibited a cleavage of Si-C bonds, which was significant for instance for 1,1-dimethylvinylidene and methine PMO, in which case as much as about 50% of these bonds could have been cleaved. In what follows, we will focus on similarly prepared bromo- and p-bromobenzylfunctionalized methylene PMOs where bromo and organobromo sites are attached to bridging methylene groups. Organohalides are well known to undergo various reactions such as lithiation, Grignard and metal-catalyzed coupling and the organohalide functional groups in PMOs are likely to undergo similar reactions allowing further surface, chemical and physical modifications and making catalytic activity, ion-exchange and easily surface-modifiable materials possible. This property makes these materials the first examples of highly reactive PMOs due to the ease of substitution of bromides by other interesting functional groups and ligands.
16 100 -'-' 700 f melhylenei ' i A) " '~0 Lbr~176176 600 / m e t h y l e n e ~ l , 1-dimethyl- " r.~ ~ 500
90 80
..~methyiene/bromocamphorlene
~
._~ 1,1-dimethyl"x,.X.... . .vinylidene ................
(B)
400 "~
300
PMO \ \
60
200 t~ 100 <
7o
F
\\ benzyl
benzyl
\
50 _
0.0
i
i
I
i
0.2
0.4
0.6
0.8
Relative Pressure
1.0
I
I
I
I
200
400
600
800
1000
Temperature (~
Figure 4. (A) Nitrogen adsorption isotherms and (B) weight change curves under air atmosphere for methylene/bromocamphor-methylene, 1,1-dimethylvinylidene and benzyl PMOs. The PXRD pattems of as-synthesized and surfactant extracted bromomethylene (BM) and p-bromobenzyl-methylene (BBM) PMOs (Figure 7) reveal a low angle Bragg peak consistent with the presence of an ordered mesostructure. The former was synthesized under acidic conditions and the latter with 50% TEOS under basic conditions. Attempted synthesis of BM PMO under basic conditions failed to give a high yield of ordered material possibly because of the formation of soluble salts that remained in the supematant. The PXRD pattern of BBM PMO containing 50% TEOS, made under basic conditions showed an intense (100) reflection as well as visible (110) and (200) Bragg peaks indicating the presence of long-range mesoscopic order (Figure 7B). The cocondensation of the BBM precursor with TEOS might have accounted for better selforganization resulting in a more well ordered material, but there might also be some chances that phase separation took place forming an ordered phase that was not particularly rich in the bridged organosilicate groups. The structure of the material synthesized from 100% BBM precursor under the same conditions was a somewhat disordered material under the same conditions. The positions of the main XRD reflection (dl00) for surfactant-extracted BM and BBM/TEOS PMO were ca. 34.3 and 42.7A, respectively. The structure of surfactant-extracted BBM PMO when pyrolyzed in a nitrogen atmosphere and monitored in situ by PXRD revealed that the ordered structure was maintained up to 900 ~ (Figure 7C). A broadening of the main PXRD reflection, and unit cell dimension (ao = 2d100/~/3) from 49.3 to 35.8 A were observed during calcination of the material from RT to 1000 ~ The retention of PXRD intensity up to at
17 least 700~ may be related to the loss of the organic groups, thereby increasing the electron contrast between the framework and the void space, while the slow decrease in d-spacing from 200 to 1000 ~ was consistent with the condensation of residual silanol SiOH groups, and loss of the organic framework groups. The TEM images of these materials also showed some ordered hexagonal structure with some fraction of amorphous regions. The 13C, 298i CP MAS (29Si MAS as needed) and 79Br MAS NMR spectra of BM and BBM PMOs are shown in Figures 8, 9, and 10, respectively and the results are summarized in Table 1. Table 1. Chemical shift assignments for BM and BBM PMO materials Sample 13C CP MAS (ppm) 29Si MAS (ppm) 79Br MAS (ppm) 10 -72 ppm- T3 -300 ppm (broad) BM-PMO (-2 and 26) -67 ppm- T2 CHBr -57 ppm- T1 -365 ppm (sharp) B r 14 - 100 ppm - Q4 -300 ppm (broad) 29 -101 ppm - Q3 CHBr 120 -91 ppm - Q2 -365 ppm (sharp) B r BBM-PMO 131 -73 ppm- T3 142 -67 ppm - T2 -57 ppm- T1 The 13C CP MAS NMR spectra of BM PMO (Figure 8) prepared under acid conditions showed a peak at 10 ppm corresponding to (O3Si)2CHBr carbons, a Chemical shift consistent with solution NMR results (see Experimental). An additional peak at 26 ppm observed in the spectrum was attributed to (O3Si)2CHC1 and resulted from nucleophilic substitution of Br by C1 during self-assembly of BM under acid (HC1) conditions and to some extent during solvent extraction in an HC1/MeOH solution. Due to the many similarities in reactivity between the two halogen atoms, Br and C1, both bromomethylene and chloromethylene groups in the PMO could show similarities under most instances. However, slight differences in reactivity due to steric and electronic differences cannot be ruled out and can be taken advantage of for making multifunctional materials where one reacts selectively over the other under certain conditions. The 13C CP MAS NMR spectra (Fig 8B) also showed peaks consistent with p-bromobenzylmethylene groups. Peaks at 14 and 29 ppm were assigned to (O3Si)2CHCH2-p-C6H4Br and (O3Si)2CHCH2-p-C6H4Br carbons while peaks at 120, 131, and 142 ppm were attributed to aromatic carbons. A minor peak or shoulder at 1 ppm was also observed in both materials that corresponds to methylene carbons that were formed probably due to radical substitution of Br by H in BM PMO and cleavage of the p-bromobenzylmethylene group in BBM PMO. 29Si MAS and CP MAS NMR spectra (Figure 9) indicated that the Si-C bonds of the materials remained intact during the synthesis and surfactant extractions. The 29Si (CP) MAS NMR spectra also showed that BM PMO has only T sites while BBM PMO has a T:Q ratio of ca. 1:1 consistent with the ratio of starting alkoxy/poly(trialkoxy)organosilanes.
18 (A) Bromocamphor-methylene PMO
(B) 1,1-Dimethylvinylidene PMO
Solvent-extracted,
Solvent-extracted, "C CP MAS
/~ ...... i 150
......
,
. . . .
~ . . . .
100
50
, ' '
ppm
Solvent-extracted,
.
.
.
.
I
.
.
.
.
!
150
~
~'-
leO
"
I
.
.
.
.
i
50
'''J
.......
' '
""
!
. . . .
150
O
Chemical shift (ppm)
t
" ' '
100
"
I
50
. . . .
!
"
'
0
Chemical shift (ppm)
(C) Methine PMO
Calcined, 350 "C, ~3CCP MAS i .....
....
100
'"
"
"
"
I
. . . .
~ .... 50
!
. . . .
~'"
ppm
l
'
As-synthesized, I~CNQS CP MAS
~
leo
As-synthesized, ~3CCP MAS . . . .
1
. . . .
1O0
I
50
. . . .
!
"
"
0
Chemical shift (ppm)
Figure 5. 13C CP MAS NMR spectra of various PMOs with organomethylene and methine groups in their frameworks.
19 (A) Bromocamphor methylene PMO (solvent-extracted) (B) 1,l-Dimethyvinyledene PMO (solvent-extracted)
[
~.~
9 .v~
.~,-
,
I
"
~
' '
'
I
"
'
"
0
"
0 -5tj -100 Chemical shift (ppm)
-50 -100 Chemical shift (ppm)
(C) Methine PMO
9 reed " 350 "C
I'"'"
'
"
'
i
. . . .
-50
I
. . . .
-100 ppm
ctant-extracted
i--'-
O
'
'
'
I
. . . .
I
. . . .
-50 -10O Chemical shift (ppm)
Figure 6. 29Si CP MAS NMR spectra of various PMOs with organomethylene and methine groups in their frameworks.
Investigations using 79Br MAS NMR spectroscopy for the as-synthesized materials revealed the presence of bromides corresponding to both organobromine groups as well as bromide counterions for the surfactant head groups (Figure 10) (as expected for the BM sample because of the self-assembly under acidic conditions, where surfactant is known to be incorporated with counterions [45]). Interestingly, the peak at - 300 ppm corresponding to bromides of the organobromine groups appeared to have a larger line width than the peaks a t - 365 a n d - 398 ppm corresponding to bromide ions from the CTABr surfactant. This is most likely due to the fact that the organobromine groups are covalently attached and are therefore more rigid than the bromide counter-ions. The
20 presence of more than one sharp peak corresponding to Br- ions (surfactant counterions) is consistent with the presence of various kinds of bromides in different environments in the materials. Most of these sharp peaks corresponding to free bromide ions of the surfactant disappear or their intensity decreased significantlyupon solvent-extraction, as expected. (A) Bromomethylene PMOs
(B) p-Bromobenzyl-methylene PMOs
jt~l
4oo~
d,,,
3()IH~
~"
=.
I ~ ~,,~., S o l v e n t - e x t r a c t e d
'~176 t
)
"-"
)' ~.~
As-synthesized
j
:'~
'10~
k,.
...1............ 10
Solvent-extracted
20
As-synthesized
0 ..................................................................... r ........................." ~ ' ~ 10
30
20 C)
.......................................i
20
20 (")
(C) p-Bromobenzyl-methylene PMOs dloo
3000~
....___~ 20000
900~ 800 *c
700~ j
6oo*c 500"C 400"C 350"C 300 "C 200 ~'C 25 "C
-~,~
10000
o
............
;, .............. ~, ....
~
"
io
...... i2
....
'1'4'
20( ~) Figure 7. Powder X-ray diffraction pattems of (A) bromomethylene PMOs; ( B ) p bromobenzyl-methylene PMOs (BBM PMO); (C) surfactant extracted BBP PMOs monitored during high temperature in-situ pyrolysis under nitrogen from RT to 1000 ~ Elemental analysis (EA) of the materials indicated that the wt% of Br was 20.05 and 25.25 for the as-synthesized and surfactant extracted BM PMO, respectively and 24.23 for surfactant extracted BBM PMO samples. Moreover, 5.70 and 3.01 wt % C1 was obtained for as-synthesized and surfactant-extracted BM-PMO material proving the substitution of some of the bromides by chlorides, which is consistent with the 13C CP
21 MAS NMR spectra. The EA results indicated that the Br:C1 ratio for solvent-extracted material is c a . 4:1. Combining the NMR and elemental analysis results, we estimated (Ol.5SiCHBrSiOi.5)o.73(Oi.5SiCHC1SiOl.s)o.18(Ol.sSiCHzSiOi.5)o.09 to be the composition of the surfactant extracted material.
(A)
Bromomethylene PMO
(B) p-Bromobenzyl-methylene PMO
Solvent-extracted, t3
,,
"
"
"
"
'
R
Solven~ 2(10
150 ,
l~t0 5'0 ('~ Solvent-extracted, ~', ? ~
6o
l[o
lbo
sb i
6
~] As-synthesized, As-synthesized ~ 150
50
r------
0 Chemical shift (ppm)
..... i . . . . . . . . .
200
:!
150
!
100
.............l ..............
50 Chemical shift (ppm)
i ....
0
Figure 8. 13C CP and NQS CP MAS NMR spectra of (A) bromomethylene PMOs, assynthesized (a) and surfactant extracted (b); (B) bromobenzyl-methylene PMOs, assynthesized (a) and surfactant extracted (b, c). Both BM and BBM PMO samples exhibited rather moderate specific surface areas of 500 and 650 m 2 g-i, respectively, and total pore volumes (0.23 and 0.39 cm 3 g-i, respectively). BM PMO was microporous with average pore diameter around 1.6 nm, whereas BBM PMO was mesoporous with a very broad pore size distribution centered at around 3 nm. Adsorption properties of a porous silica obtained from the BBM PMO sample via calcination deviate from those of MCM-41 silicas. This may be due either to a prominent wall corrugation for the BBM PMO sample (or introduced during calcination) or to the presence of some disordered domains in the material, as suggested by TEM images and additionally supported by the fact that the isotherm for BBM is somewhat atypical for a 2-D hexagonal material. TGA for BM PMO corroborates the elemental analysis data, suggesting that bridging groups account for an appreciable fraction of the mass of the sample. Moreover, the comparison of TGA data for as-synthesized and surfactant-extracted BM PMO reveal an appreciable weight loss attributable to the surfactant decomposition, thus strongly suggesting that this is a supramolecularlytemplated material despite its microporous nature and unprecedented content of heteroatoms in the structure. BBM PMO sample exhibits a moderate weight loss in the temperature range for decomposition of the bridging groups. Apparently, its content of
22 bromobenzyl groups is significantly lower than that expected from the composition of the synthesis gel. (A) Bromomethylene PMO
(B) p-Bromobenzyl-methylene PMO (Solvent-extracted)
xtracted (~Si MAS)
-so
-ioo -1 o MAS NMR
Solvent-extracted ("~SiCP MAS)
6
-so
:ioo ,1so ~'Si CP MAS NMR
~-s~esized
("Si CP MAS)
6 I)
-50
-i00
.;o
.~bo
-1~o
-1.50 Chemical shift (ppm)
Chemical shift (ppm)
Figure 9. 29Si (CP) MAS NMR spectra of (A) bromomethylene and (B)p-bromobenzylmethylene PMOs. (A) Bromomethylene PMOs
(B) p-Bromobenzyl-methylene PMO
,
t-extracted
; .... -Ioo
~
I,
nt-extracted
2g
- '40O
a,.
nthesized
ynthesized .
!
Chemical shift (ppm)
i}
20o
.
.
.
.
44H~
~
Chemical shift (ppm)
Figure 10. 79Br MAS NMR spectra NMR spectra of (A) bromomethylene and (B)pbromobenzyl-methylene PMOs.
23 Attempted in-situ reactions of BM PMO with ammines, Na28204, NaHS.xH20 and other reagents indicated that the CHBr groups are chemically accessible. However, the Si-C bonds were found to cleave causing the collapse of the structures under some of these reactions. Further investigations will be required to find the optimum conditions to leave the Si-C bonds and the structure intact during some of these in-situ reactions. 100 "7
~0 250
r./3
(A) 90
p-bromobenzylmethylene PMO
200
~.
150 9 r~
"~ <
100
~
50
~
.,.~ O
bromomethylene PMO
~:
,
(B)
i
,
,
", p-bromobenzyl\,m,ethylene PMO
80 70
omethylene PMO
6o "'..s~nt-extracted 50
as-synthesized'" .................... 0 0.0
. 0.2
.
. 0.4
. 0.6
Relative Pressure
40 0.8
1.0
0
i
i
i
i
200
400
600
800
1000
Temperature (~
Figure 11. (A) Nitrogen adsorption isotherms and (B) weight change curves under air atmosphere (except for the one for as-synthesized material, which was recorded under nitrogen) for BM and BBM PMOs. Successful in-situ reactions in ordered PMOs and bifunctional PMOs (BPMOs) have been reported as new routes to funtionalized PMOs and BPMOs which otherwise might be hard to make by direct synthesis [1,8,12,18]. With these approaches, dibromo, borylated, epoxide, diol and alcohol functionalized materials have been synthesized. The borylated and epoxide functionalized PMOs particularly are important materials as they undergo further transformations to yield several other functional groups and under mild conditions. For instance, they are proven to undergo chemical transformations into alcohol functionalized mesoporous materials [8]. Preliminary experiments [12] also indicated that the amine-functionalized mono- and bifunctional PMO materials can form by treating a borylated bifunctional PMO material with hydroxylamine-o-sulphonic acid. Amine functionalized mesoporous materials can be useful materials as base-catalyst, acid scavengers and sensors. 4. CONCLUSIONS In this paper we have reviewed recent work, mainly from our laboratory, on a new class of hybrid organic-inorganic mesoporous materials called periodic mesoporous organosilica (PMOs) containing bridge bonded organic groups integrated into the frameworks. These materials show a number of unique properties including high loading of bridging organic groups, large free void space and a uniform distribution of functional
24 groups in the framework. Some of the functional groups are accessible to further chemical modification. The presence of bridging organic groups in PMO frameworks is expected to result in interesting physical and mechanical properties as well as useful chemical functionality that make PMOs potentially useful for diverse applications including catalysis, sensing, separations and nanoelectronics. 5. ACKNOWLEDGMENTS GAO is a Canada Research Chair in Materials Chemistry. He acknowledges the financial support of the National Science and Engineering Council of Canada (NSERC) for financial support of this project. MJ acknowledges the financial support from NSF Grant CHE-0093707. The authors would also like to thank Prof. Mark. J. MacLachlan, Stephen Knauer, Chiaki Yoshina-Ishii, Galina Temtsin, Prof. Shmuel Bittner, Dr. Masakatsu Kiroki, Dr. Srebri Petrov, Dr. Neil Coombs, Wesley Whitnall, and Rebecca Voss for their contributions and helpful discussions. REFERENCES 1. T. Asefa, M. J. MacLachlan, N. Coombs and G. A. Ozin, Nature, 402 (1999) 867. 2. C. Yoshina-Ishii, T. Asefa, N. Coombs, M. J. MacLachlan and G. A. Ozin, Chem. Commun., (1999) 2539. 3. T. Asefa, M. J. MacLachlan, H. Grondey, N. Coombs and G. A. Ozin, Angew. Chem. Int. Ed., 39 (2000) 1808. 4. M.J. MacLachlan, T. Asefa and G. A. Ozin, Chem. Eur. J., 6 (2000) 2507. 5. T. Asefa, C. Yoshina-Ishii, M. J. MacLachlan and G. A. Ozin, J. Mater. Chem., 10 (2000) 1751. 6. T. Asefa, N. Coombs, t3. Dag, H. Grondey, M. J. MacLachlan, G. A. Ozin and C. Yoshina-Ishii, Mater. Res. Soc. Symp. Proc., 628 (2000) CC3.9. 7. O. Dag, C. Yoshina-Ishii, T. Asefa, M. J. MacLachlan, H. Grondey and G. A. Ozin, Adv. Funct. Mater., 3 (2001) 213. 8. T. Asefa, M. Kruk, M. J. MacLachlan, N. Coombs, H. Grondey, M. Jaroniec and G. A. Ozin, J. Am. Chem. Soc., 123 (2001) 8520. 9. G. Temtsin, T. Asefa, S. Bittner and G. A. Ozin, J. Mater. Chem., 11 (2001) 3202. 10. T. Asefa, M. Kruk, N. Coombs, S. Petrov, M. Jaroniec and G. A. Ozin, unpublished results. 11.13. Dag and G. A. Ozin, Adv. Mater., 13 (2001) 1182. 12. T. Asefa, M. Kruk, M. Jaroniec and G. A. Ozin, unpublished results. 13. S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna and O. Terasaki, J. Am. Chem. Soc., 121 (1999) 9611. 14. S. Guan, S. Inagaki, T. Ohsuna and O. Terasaki, J. Am. Chem. Sot., 122 (2000) 5660. 15. S. Guan, S. Inagaki, T. Ohsuna and O. Terasaki, Microporous Mesoporous Mater., 44 (2001) 165. 16. S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna and O. Terasaki, Stud. Surf. Sci. Catal., 129 (2000) 155. 17. A. Fukuoka, Y. Sakamoto, S. Guan, S. Inagaki, N. Sugimoto, Y. Fukushima, K. Hirahara, S. Ijima and M. Ichikawa, J. Am. Chem. Soc., 123 (2001) 3373.
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26 47. 48. 49. 50.
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Studies in Surface Science and Catalysis 141 A. Sayariand M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
27
Porous materials: L o o k i n g through the electron microscope O. Terasaki l*, T. Ohsuna2, Z. Liu 3, M. Kaneda l, S. Kamiya 1, A. Carlsson 1, T. Tsubakiyama 1, Y. Sakamot01, S. Inagaki 4, S. Che 5, T. Tatsumi 5, M. A. Camblor6, R. Ryoo 7, D. Zhao 8, G. Stucky9, D. Shindo 3 and K. Hiraga 2 1 Department of Physics and CIR, Tohoku University, Sendai 980-8578, Japan 2 Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan 3 Insititute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan 4 Toyota Central R&D Laboratories, Inc., Nagakute, Aichi, 480-1192, Japan 5 Division of Materials Science & Chemical Engineering, Yokohama National University, Yokohama 240-8051, Japan 6 Industrias Quimicas del Ebro, Poligono de Malpica calle D, no 97, 50057 Zaragoza, Spain 7 Materials Chemistry Laboratory, School of Molecular Science-BK21, Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea 8 Department of Chemistry, Fudan University, Shanghai 200433, P. R. China 9 Department of Chemistry & Biochemistry and Materials Department, University of California, Santa Barbara, California 93106, USA
Porous and confined materials with periodic micro and mesoscale pores have attracted much attention, both from fundamental and applied scientists. Increasingly, we are encountering new, structurally complex materials, whose structures cannot be solved/characterized by the usual methods available. Knowledge of their structures is essential for thorough understanding of their properties and further development. Therefore, the development of methods appropriate for structural analysis of these materials is a matter of urgency. Electron microscopy may be the best technique for this purpose, as long as we are interested in both local structures and average periodic structures. Here we describe general aspects of the structural analysis of periodic nanoporous materials and our latest achievements in this field.
*To whom correspondence should be addressed:
[email protected]
28 1. INTRODUCTION Porous crystals, both microporous (i.e., zeolites), and mesoporous, contain periodically arranged pores and cavities, from less than 20 A for microporous systems and between 20 and 500 A for mesoporous systems. They are of industrial use as molecular sieves for chemical separations, ion-exchangers for detergents, heterogeneous shape-selective catalysts, 16wdielectric materials and also as molds for synthesis of novel materials within their spaces. Knowledge of the structures of these crystals can provide important insights into their properties and can lead to the design of desirable materials. More than 130 different framework-type structures (zeolites) have been reported as microporous materials, and many new crystals remain unreported, because their structures have yet to be solved. Many ordered mesoporous silicas have also been synthesized using amphiphilic molecules as templates. They are not always a single phase, due to the kinetically controlled synthesis procedure. These porous crystals have the following obstacles, which make their structural analysis difficult: (i) Most as synthesized microporous materials are microcrystalline, with particle dimensions of 1 micron or less. They are thus too small for structural analysis by single-crystal X-ray diffraction (XRD) experiment. In the absence of single crystal data, the structural analysis and refinement must be carried out by powder XRD experiments. If the reflections in the powder XRD profile overlap significantly, solving the crystal structure from these data can be extremely difficult without reasonable initial structural models as zeolites may contain several hundred atoms in a unit cell. (ii) It is hard to determine even the crystal class of mesoporous solids uniquely by powder XRD, as only a few reflections (with large broadening) are observed in the XRD pattern. Furthermore, the mesoscale order is extremely sensitive to the synthesis conditions and mesoporous materials contain local structural fluctuations and sometime tend to form intergrowths. These obstacles can be overcome with HRTEM images obtained from thin regions coupled with digital Fourier analysis. The amplitudes and phases of crystal structure factors (CSFs) obtained from a set of HREM images can be used to solve the three-dimensional (3d) structures. Here, we will report our approach using electron crystallography (EC)[ 1,2,3,4].
2. CRYSTAL MORPHOLOGY OF MESOPOROUS MATERIALS Recently, mesoporous crystals have been synthesized under well-controlled conditions and display nice crystal morphologies [3,4,5]. However, local structural fluctuations in mesoporous materials are common and produce only a small number of reflections with large peak widths on the powder XRD patterns (we must observe XRD patterns more carefully with higher S/N ratio in order to observe reflections with weak intensity), even though the materials may show regular crystal morphology. This situation is illustrated in Figures 1 and 2, which show the SEM images and powder XRD patterns for cubic organic-inorganic hybrid [5], MCM-48 [6], SBA-1 [7] and 3d-hexagonal material [to be submitted]. All SEM images show very beautiful crystal morphologies, which are commensurate with the point group symmetries. However, it is difficult to determine crystal classes uniquely from the XRD patterns, although we can index all peaks in these patterns if we have definite information from other experiments or if we assume the crystal classes.
29
Figure 1. SEM images of (a)eubic hybrid, (b)MCM-48, (c)SBA-1 and (d)3d-hexagonal material.
(a)
i
U 201"
201"
(d)
(c)
i i
i f
/
/
1
20/~
20/"
Figure 2. XRD pattems of (a)hybrid material, (b)MCM-48, (c)SBA-1 and (d)3d-hexagonal material.
30 3. WHAT ARE THE ADVANTAGES OF ELECTRON CRYSTALLOGRAPHY? Using a CCD camera or imaging plate, we can now measure ED intensities or HREM image contrasts quantitatively over wide ranges in intensity, as they have larger dynamic range and better linearity of output to input electrons than photographic film. The advantages of using electrons as probes for structural study are: (i) electromagnetic lenses can be used for image formation and (ii) electrons interact with matter approximately 104 times stronger compared with X-rays. The latter feature allows us to use very small crystals, down to only several tens unit cells in size, to obtain single crystal structural information. A natural extension based on the latter is to collect electron diffraction intensity data from thin single crystals and then apply "direct methods" for phase recovery in CSFs. HREM images carry both phase and amplitude information of CSFs, and this is the most important advantage of using the electron crystallography (EC) for porous silicas, compared to the traditional single crystal X-ray diffraction. The phase is a most important parameter to build the crystal structure and we can apply weak phase object (WPO) approximation for samples up to a few hundreds A thick at 300 keV. The novel materials, which are synthesized within the pores of silica mesoporous crystals, sometimes show new hierarchical structures with order at both atomic and mesoscales [8,9]. We must develop an adequate method to solve/characterize the structure of these new materials because there is no such method yet.
4. OBSERVATIONS The samples were investigated with a JEM-3010 (operating at 300 kV, Cs=0.6 mm, structural resolution 1.7 A) and JEM-4000EX (operating at 400 kV, Cs=l.0 mm, structural resolution 1.7 A). Images were recorded with films and a CCD-camera (Model 794, Gatan, size 1024x1024, pixel size 25x25~tm 2) using low dose conditions. Diffraction patterns were also recorded using both films and the CCD-camera.
5. RESULTS AND DISCUSSION
5.1. Microporous Materials We can now measure ED intensities easily by using a CCD camera or imaging plate. Applying "direct methods" to the collected ED intensities of many independent reflections from different zone axes, we have been able to solve the previously unknown structure of zeolite SFE from a very small crystal [3]. The conditions for applying this method are carefully studied and they are dependent on the symmetry of crystals [T. Ohsuna et al., Annual Japanese Zeolite Association Meeting, 2000]. As zeolites have low density, kinematical treatment in diffraction is a good approximation for analysis of the ED intensity distribution, if specimens are thinner than a few hundreds A for 300 kV electrons. This is not a very difficult condition for zeolites if we can synthesize them as a single/pure phase. Since HRTEM images of zeolites have low resolution and only a few HRTEM images are available, a potential density map derived from Fourier reconstruction of the HRTEM images is usually blurred (low resolution). Hence, it is difficult to find atom positions in the potential map directly. We have introduced a new enhancement method for retrieving atom
31 positions and obtaining a reasonable framework structure from the blurred density map by using a Patterson map to give a vector connecting a pair of atoms. The map is obtained from electron diffraction intensity and is independent of the phases. Further refinement was carried out for T-atom positions together with O-atom positions using a similar technique as the DLS program. Whole procedure was carried out by a software package developed by one of the authors (TO) for the present problem. A new polytype of zeolite Beta, which is coded as BEC, was solved by this method [4, 10]. Two HREM images of BEC, taken with the [100] and [001] directions, were used for Fourier reconstruction. Figure 3 shows an HREM image of[100] incidence and the structure solution obtained is shown in Figure 4. The framework has three straight channels with 12-MRs window along [100], [010] and [001] directions, and the same stacking of BEC in HREM image of ITQ-14 can be seen as indicated by an arrow in Figure 5. Most of the image of Figure 5 is the same as that of ordinary Beta, which is mixture of polytype A and polytype B.
5.2. Mesoporous Materials We have developed a new method for solving the structure with mesoscale ordering without assuming any structural models based on electron crystallography, using the crystalline nature of 3d mesoporous materials. The resolution for the structure is primarily limited by the quality of the HREM images, which depends mostly on the long-range mesoscale ordering. Further progress may give better resolution and finer details will be obtained, however no change in conclusions will be necessary about the structure because the validity of a solution does not depend on the resolution. From observed images we can choose thin areas, which are free from dynamical scattering, and a large domain of a single phase from a multi-phase mixture. Figure 6 shows an HREM image of SBA-12 revealing domain structures, from which we could find a sufficiently large domain to solve a new 3dstructure, and this will be reported separately[ 10]. An HREM image of MCM-48 taken with [111] incidence and corresponding Fourier diffractogram, which is obtained from a thin region and clearly indicates extinction conditions for reflections, are shown in Figure 7a. From observations under extinction conditions from the diffractograms and the point group symmetry deduced from the morphology shown in Figure 2, the space group of MCM-48 was uniquely determined to be Ia 3d. Two dimensional (2d) data of CSFs were obtained from the images of each zone axis, and they were merged into a 3d-data set after normalizing by common reflections. The 3d-structure solution of MCM-48 was uniquely obtained from the 3d-data set of CSFs by inverse Fourier transformation, that is, Fourier Sum. The result shows that the amorphous wall-surface follows exactly the G-surface, one of periodic minimal surfaces, and the wall separates the structure into two enantiomeric channel systems, which are not interconnected with each other (Figures 8a and 8b). Other new 3d-structures of silica mesoporous crystals recently determined by EC will be presented at the meeting, together with structural time evolution, which might be related to "phase transition", for example, 2d-hex t o Pm 3n phase [Annual Japanese Physical Soc. Meeting, 2000 & 2001, to be submitted] or to Ia 3d phase. B
5.3. Novel Mesoporous Materials Once the 3d-structures of silica mesoporous crystals are solved, they are used as a template (or mold) for the synthesis of new nanostructured materials[8,9,12]. Carbon and Pt nanowirenetworks were synthesized within pores of silica MCM-48. Pores of MCM-48, both right and
32
!ool]
'~' ,~ [010] ~i~z,~!ii~iiii~ili~' ~~
Figure 3. HREM image of BEC [100]. "~'~ [010] [100]
Figure 4. Structure solution ofBEC [ 100].
!ii!iii!!! i. . ...... r. ii!9...!!iii!! i!!il ill ii
Figure 5. HREM image Of ITQ-14.
iliii!
9 ili
Figure 6. HREM image of SBA-12.
i i~
33
Figure 7(a). HREM image and Fourier diffractogram of MCM-48.
Figure 7(b). HREM image and Fourier diffractogram of Carbon network, CMK-4.
Figure 9. Pt nanowire-network.
Figure 8. The channel system of MCM-48.
34 left hand channel systems, are equally and uniformly filled by carbon, and therefore the images of MCM-48 and carbon network give reverse contrast (Figures 7a and 7b). Fourier diffractograms of the images give same diffraction patterns as expected from Babinet's principle. Pt-network was formed inside the channels of MCM-48 without destroying the channel geometry [8,9]. Figure 9 shows Pt nanowire-network when the incident beam is parallel to the [ 100] direction of MCM-48. Since the MCM-48 has a chiral channel structure, the Pt-networks formed inside such channels are also chiral[9].
6. CONCLUSIONS As shown here, we have overcome some obstacles and succeeded in obtaining the structure solutions of porous materials by the technique of electron microscopy, that is, EC. It was shown that Fourier reconstruction of the HREM image was powerful approach for the materials, and that recent improvement of synthesizing high quality crystals made it possible to solve the structures by EC. Once 3d-structure of porous crystals have obtained, we can use the crystals as mold/templates for the synthesis of novel advanced materials, which requires us new development for structure analysis appropriate to the new materials.
7. ACKNOWLEDGMENTS Financial supports from CREST, Japan Science and Technology Corp. (OT) and Creative Research Initiative Program (RR) are greatly acknowledged.
REFERENCES
1. A. Carlsson, M. Kaneda, Y. Sakamoto, O. Terasaki, R. Ryoo and H. Joo, J. Electron Microscopy, 48 (1999) 795. 2. Y. Sakamoto, M. Kaneda, O. Terasaki, D.Y. Zhao, J.M. Kim, G. Stucky, H.J. Shin and R. Ryoo, Nature, 408 (2000) 449. 3. P. Wagner, O. Terasaki, S. Ritsch, J. G. Nery, S.I. Zones, M.E. Davis and K. Hiraga, J. Phys. Chem. B, 103 (1999) 8245. 4. T. Ohsuna, Z. Liu, O. Terasaki, K. Hiraga and M. A. Camblor, submitted. 5. S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna and O. Terasaki, J. Am. Chem. Soc., 121 (1999) 9611, and 122 (2000) 5660. 6. M. Kaneda, T. Tsubakiyama, A. Carlsson, Y. Sakamoto, T. Ohsuna, O. Terasaki, S.H. Joo and R. Ryoo, J. Phys. Chem. B., in press. 7. S. Che, Y. Sakamoto, O. Terasaki and T. Tatsumi, Chem. Mater., 13 (2001) 2237. 8. H.J. Shin, R. Ryoo, Z. Liu and O. Terasaki, J. Am. Chem. Soc., 123 (2001) 1246. 9. O. Terasaki, Z. Liu, T. Ohsuna, H.J. Shin and R. Ryoo, Microsc. and Microanal., in press. 10. Z. Liu, T. Ohsuna, O. Terasaki, M.A. Camblor, M.J. Diaz-Cabafias and K. Hiraga, J. Am. Chem. Soc., 123 (2001) 5370. 11. Y.Sakamoto, I. Diaz, O.Terasaki, D.Zhao, JP. Pariente, JM Kim & GD. Stucky, submitted. 12. S. Jun, S.H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, T. Ohsuna and O. Terasaki, J. Am. Chem. Soc., 122 (2000) 10712.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
Molecular imprinting enzymes
-
a
35
w a y to p r e p a r e effective m i m i c s o f natural a n t i b o d i e s a n d
Gfinter Wulff Institute of Organic and Macromolecular Chemistry Heinrich-Heine University Dfisseldorf, D-40225 Dfisseldorf, Germany
Molecular imprinting in synthetic polymers is used to prepare catalytically active imprinted polymers. Using different transition-state analogues of alkaline ester hydrolysis and suitable functional monomers, catalysts with strong esterolytic activity and enzyme-analogous properties are prepared. The polymers are obtained either as block polymers, and by standard suspension or mini-emulsion polymerization techniques. Through implementing the last two methods regular beads of 10 - 400 lam and minigels of 100 - 300 nm particle diameter are obtained. It is also possible to obtain soluble nanogels with diameters of 5 - 15 nm, i.e. in the same dimension as natural enzymes.
1. INTRODUTION The design of nano-scale specific arrangements in synthetic polymers can be obtained by an information transfer from low-molecular-weight compounds during the formation of the polymer. Thus the constitution and the configuration of complex arrangements in polymers can be controlled by specific interactions of the polymerizable monomers with a suitable template molecule. After polymerization of the monomer-template complex the original structure of the supramolecular arrangement will be stabilized and frozen. The removal of the template furnishes polymers with defined structures by transfer of structural information from the template to the polymer. If we look at the transfer of properties from the template to the resulting macromolecule, the situation is rather complicated since a large number of stereogenic centers are formed which are covalently linked with each other. Different possibilities exists: Through repetitive property transfer along a long chain it is possible to obtain structurally defined linear polymers which show a defined tacticity and in cases with optically active templates also optical activity (for reviews see [ 1, 2]). It is also possible to get an information transfer to a plain surface in a twodimensional manner (see [3, 4]). The case in which a crosslinking reaction is performed during the transfer is most interesting because a real three-dimensional transfer of properties (molecular imprinting) is possible in this instance. After removal of the template distinct cavities of a defined dimension of 0.5 - 2 nm, a predetermined shape, and an arrangement of functional groups in a predetermined stereochemistry can be obtained.
36 2. THE MOLECULAR IMPRINTING PROCEDURE
Scheme I Schematic representation of the molecular imprinting procedure [5]. Scheme I shows the principle of molecular imprinting, an approach we have introduced quite some years ago [5, 6]. Polymerizable vinyl monomers containing functional groups are attached to suitable template molecules(T) either by covalent or by non-covalent interaction. Subsequent copolymerization in the presence of solvent and large concentrations of crosslinking agent produces relatively rigid macroporous polymers. Removal of the template molecules (see Scheme I) leaves behind cavities in the polymer whose shape and whose arrangement of the
Scheme II Different types of interaction in the molecular imprinting technique.
37 functional groups are determined by the template molecules. The relation of template and imprint resembles very much the key-and-lock principle postulated for enzyme catalysis by Emil Fischer more than 100 years ago [7].
Scheme IIl Crosslinked polymers prepared from the template monomer 1 and removal of the template [6]. Template monomer 1 in Scheme II is shown as an illustration for covalent interaction, in which two molecules of 4-vinylphenylboronic acid have been bound to phenyl c~-Dmannopyranoside as the template by boronic ester linkages to the free hydroxyl groups. A 1 : 9 ratio of monomer 1 and ethylene dimethacrylate was copolymerized in an inert solvent to yield a macroporous polymer. From this the template could be split off to the extent of 9 0 - 95% providing chiral cavities each bearing a pair of boronic acid groups (Scheme Ill). Polymers of this type exhibit an excellent ability for racemic resolution of the racemate of the employed template. The original template enantiomer is preferably incorporated under batch equilibration and separation factors et (ratio of KD/KL) as high as 4 - 6 are achieved. These materials can also be used as chromatographic supports in h.p.l.c, columns giving resolutions higher than Rs = 4.3 (see Fig. 1) [8].
Gradient elution in h.p.l.c.: 70~ flow: l ml/min; gradient: acetonitrile incl. 5% aqueous NH3 (25%) and water incl. 5% aqueous NH3 (25%) 9:1 to 5:5
'
IfO
'
I 14
!
t / m i n -'-'-P
Figure 1
H.p.l.c. separation of the racemate of phenyl-c~-mannopyranoside on an adsorbent produced according to Scheme III.
38 These sorbents can be prepared conveniently and possess excellent thermomechanical stability. Even when used at 80~ under high pressure for a long period, no leakage of the stationary phase or decrease in selectivity during chromatography was observed [4]. Investigations as to the mechanism of this separation showed that the orientation of the functional groups within the cavities is the dominating factor for the separation; the shape of the cavity is of secondary importance [9]. In the meanwhile many other research groups all over the world have entered the field. Aside of covalent bonds as binding site interaction also metal-coordination has been used (for reviews see [10, 11] and Scheme II b). An interesting extension of the concept of molecular imprinting was introduced by Mosbach and his coworkers [12, 13] who used only non-covalent interactions during imprinting and the succeeding equilibration studies. In this case mostly hydrogen bonding and electrostatic interaction are used for the binding interaction (see Scheme IIc). A wide range of examples for different chemical classes of templates are known that have been used for molecular imprinting in crosslinked polymers. For general reviews on molecular imprinting see [books, 14, 15] and numerous review articles [4, 10, 13, 16-20]. Molecularly imprinted polymers have been used with great success as separation media (mostly in chromatography). Of special interest is the enantiomeric resolution of racemates. Further applications are as immunosorbents and chemosensors. The cavities in imprinted polymers have also been used as microreactors for selective reactions and, more interestingly, as the active sites of catalytically active polymers (see reviews [4, 10, 14-20]). In this paper the enzyme-like catalysis by molecularly imprinted polymers will be discussed as well as the influence of the state of dispersion on the catalytic activity. It is possible to obtain molecularly imprinted highly crosslinked microparticles in the order of 5 - 15 nm, i.e. in the same order of magnitude as natural enzymes, which are soluble in certain solvents.
3.
CATALYSIS WITH AMIDINE-CONTAINING MOLECULARLY IMPRINTED POLYMERS
Lemer et al. and Schultz et al. [21, 22] have shown that antibodies prepared against a stable transition state analogue of a reaction show considerable catalytic activity. Thus, antibodies prepared against a phosphonic ester (as a transition state analogue for alkaline ester hydrolysis) enhance the rate of ester hydrolysis 103 - 104 fold. Similarly, imprinting should also be an excellent method for preparing active sites of enzyme analogues. Various research groups have used this concept to prepare catalytically active polymers for ester hydrolysis, but their first results have been disappointing [23-25]. For a comprehensive review on catalytically active imprinted polymers see [26]. We have applied amidine functional groups for binding and catalysis. It was shown that N,N'-diethyl-4-vinyl benzamidine (2) is an advantageous binding site monomer for molecular imprinting since it shows a strong ionic, double bridged interaction between the amidine group and phosphonates, phosphates, and carboxylates [27, 28]. Phosphonic monoester 3 was used as the transition-state analogue for the alkaline ester hydrolysis [29]. The addition of two
39 equivalents of the binding site monomer 2 furnished a bisamidinium salt 4. By the usual polymerization, processing, and removal of the template, active sites were obtained with two amidine groups each. At pH 7.6 the imprinted polymer accelerated the rate of the hydrolysis of ester 5 more than 100 fold compared to the reaction in the same medium without polymer. In this case the formation of the diacid was measured. Was the released phenol measured instead a 235 fold acceleration was found. This shows that some product inhibition occurs. A polymer with statistically distributed amidine groups only showed a 20 fold acceleration.
% /OH __~ 0
EtN'~c\NHEt 2
caa ~ _
CH3
./," -- " ~',N
O
CH3 CH3
5
P\
\~N_H._67~
,,,,,,J
3
HOzC~ C \ o - - ~
-i*i O,~e~,O . ~ / c..
~N a -0
CH3 4
% ~,OH
o
o
Pho/P\oPh
Pho/C\oPh
Pho/C\NHPh
6
7
8
Further investigations showed that the molecularly imprinted polymers show typical enzyme analogue properties like Michaelis-Menten kinetics, competitive inhibition etc. From the obtained kinetic data a Michaelis constant, Km, of 0.60 mM, and a kcat of 0.8 " 10 -4 min ~ were obtained. The template molecule 3 is a powerful competitive inhibitor (Ki = 0.025 mM), which is bound more strongly than the substrate by a factor of 20. It is remarkable that such strong binding of the substrate and template occurs in water/acetonitrile (1:1). In order to avoid product inhibition the hydrolysis of carbonates and carbamates was investigated as well. Imprinted polymers were prepared from a complex obtained by addition of molar fractions of diphenylphosphate 6 and amidine 2. After the usual bulk polymerization the template 6 was removed and cavities with one amidine group each were obtained [30]. The hydrolysis of diphenylcarbonate 7 and diphenylcarbamate 8 was investigated in the presence of acetonitrile/buffer solution and in acetonitrile/buffer with the imprinted polymer. Substrate hydrolysis was treated as usual as a pseudo-first-order reaction and rate constants k of the initial reactions were determined. The ratio kimpr/ksol showed enhancements of 588 in the case of carbonate and 1400 - 3860 in the case of carbamate. The highest enhancements with respect to nonimprinted polymers containing statistically distributed amidine groups was 10 (carbonate) and 5.8 (carbamate) for these bulk polymers.
40 These values seem to be the highest accelerations published until now for molecularly imprinted catalysts. The activity for carbamate hydrolysis in this example is in the same order,of magnitude compared to catalytic antibodies [31]. In a similar fashion polymer catalysts were prepared for the hydrolysis of cholesterol carbonates [32].
4.
THE INFLUENCE OF THE TYPE OF DISPERSION ON THE CATALYTIC ACTIVITY
In the vast majority of cases, macroporous structures have been used for imprinted polymers. Macroporous polymers are obtained if polymerization of the monomers is carried out with a relatively high content of cross-linking agents (5 - 90%) in the presence of inert solvents (also known as porogens). During the polymerization phase separation takes plase and, after removal of the porogen and drying, a permanent pore structure remains. The relatively large inner surface area (50 - 600 m2g1) and large pores (about 10 - 60 nm) ensure that the specific microcavities formed by the imprinting process (between 0.5 - 2 nm in diameter depending on the template) are readily accessible and smaller molecules can diffuse freely inside the pores. If high levels of crosslinking agent are used, the cavities retain their shape quite well after removal of the templates.
a. Irregularly broken bulkpolymers, 5 - 500 ~tm, insoluble Figure 2
b. Spherical beads by suspension polymerisation, 2 - 500 lam, insoluble
c. Spherical beads by emulsion polymerisation, 100- 500 nm, insoluble
Different types of dispersion of molecularly imprinted polymers.
Usually the polymers are prepared in ampoules in the form of macroporous blocks which must then be crushed, ground, and sieved to obtain a desired particle size. Thus, irregularly broken bulk polymers with particle diameters of 5 - 500 I.tm can be obtained (see Fig. 2a). The preparation of these particles is a time consuming and energy-wasteful process. In addition, the properties of the resulting irregular particles may not be ideal with regard to flow, reproducibility, and scale-up procedures, whereas suspension polymerization methods produce relatively uniform spherical beads which are far more suitable (Fig. 2b). For these reasons, suspension polymerization has also been considered by others [33, 34], however, because of the used relatively weak interactions between the imprint substances and the binding site monomers in these cases simple suspension polymerization in water could not be applied. The new amidine monomer 2 allowed us to use the well-established suspension polymerization technique since the interaction between amidine and phosphonate, phosphate, or carboxylate is very stable.
41 Classical aqueous suspension polymerization techniques proceeded to smoothly give beads of 8 - 375 ~tm diameter, depending on the polymerization conditions used (e.g. mean diameter 31.3 ~tm; index of polydispersity of 1.16; surface area 277 m2/g; mean pore radius 6.3 nm, see also Table 1) [30]. The amidine-phosphate complex (from 2 and 6) or its components do not appear in the aqueous phase in the course of polymerization. The free, imprinted active sites were obtained by removal of the template. Table 1
Kinetic parameters for diphenyl carbonate hydrolysis with imprinted beads [30,36]
Particle size (index ofpolydisp.)
specific surface area (m2/g)
Hydrolysis of 7 relative reaction rate kimpr/ksol kimpr/kstat
Sample
porogen
water phase composition
SP 2
cyclohexanol/ dodecanol 9:1
20% NaC1 8% starch
375 ~tm (1.16)
288
168
24
SP 3
cyclohexanol/ dodecanol 9:1
0.2% PVP 0.1% PVA
31.3 ~tm (1.23)
234
150
23
M-1
-
2% PVP 1% PVA
149 nm
31
71
25
M-2
-
2% PVP 1% PVA
230 nm
20
54
17
The imprinted beads possess the same catalytic activity as bulk-type imprinted polymers if they are prepared with the same porogen, e.g., cyclohexanol-n-dodecanol (see Table 1). Although, the rate constants of diphenyl carbonate hydrolysis are apparently higher for polymers prepared on the basis of acetonitrile as porogen, it is impossible to use it in suspension polymerization due to the miscibility of acetonitrile with water. The enhancement with respect to non-imprinted polymers containing statistically distributed amidines is much higher with beads compared to bulk polymers. Enhancements of up to 24 are obtained. Thus, the beads show a much higher selectivity. It is also possible to obtain much smaller gel-type crosslinked minigels by emulsion polymerization by a method described first by Landvester et al. [35]. In this case no porogen is used and non-porous particles between 100 - 500 nm ~article diameter were obtained (minigels) (see Fig. 2c). The surface area is smaller (15 - 35 m / g ) since there is nearly no inner surface area present. These particles are insoluble in all solvents used but in some cases they can be solubilized in colloidal form. The same chemical composition as used with bulk polymerization and suspension polymerization was used in this case with the exception that no porogen was present (see Table 1) [36]. Although, the polymers prepared by suspension polymerization had much higher surface areas compared to the minigels (around 10 fold), the observed rate enhancements kimpr/ksol are only reduced by 1/3 to 1/2 and the kimpr/kstat are in the same order of magnitude (see Table 1).
42 Even more interesting would be the synthesis of imprinted microgels. Microgels are defined in polymer chemistry as unimolecular, crosslinked polymer particles possessing a size comparable to the statistical dimensions of noncrosslinked macromolecules (5 - 15 nm) which can exist as stable solutions in appropriate solvents. It should be possible to introduce into these microgels (or nanogels) imprinted cavities obtaining soluble particles with dimensions comparable to those of enzymes (see Fig. 3a and b). For this reason we have systematically investigated the synthesis of highly crosslinked, molecularly imprinted microgels [37].
a. Natural enzymes, e.g. chymotrypsin =:> radius of gyration 5- 15nm molecular weight 30000 - 500000 =:> soluble
Figure 3
b. Intramolecularly crosslinked macromolecules (nanogels, microgels) =:>radius of gyration of less than 15 nm in solution molecular weight 50000 - 200000 =:> soluble
c. Macrogels obtained by usual crosslinking polymerisation Three-dimensional infinite network insoluble
lntramolecularly crosslinked macromolecules.
The problem in this synthesis is that usually under these conditions under intermolecular crosslinking three-dimensional infinite networks of macrogels are obtained (Fig. 3c). In special solvents (e.g. cyclopentanone) at low monomer concentration (e.g. 1%) it is possible to obtain highly crosslinked (nominal degree of 70%) soluble microgels with molecular imprinting. They were characterized through GPC, viscosimetry, and membrane osmometry, and were found to be highly crosslinked macromolecules with a molecular weight comparable to the one of proteins (see Table 2). Molecular recognition experiments clearly pointed out the presence of selective functionalized cavities within the microgels. Recognition experiments can be performed in homogeneous solution, after which the microgels are conveniently separated by ultracentrifugation or by precipitation. At present experiments are undertaken with transition state analogue imprinted soluble microgels which are imprinted with a complex of 6 and 2 and which are catalytically active in hydrolyzing the diphenylcarbonate (7) [38].
43 Table 2
Synthesis and characterization of molecularly imprinted nanogels [37]
Solvent
% yield Mw Mn Mw/M. Mn(osm) q a-value monomer % mixture Cyclopentanone 1 88 4 . 0 104 9 . 2 103 4.3 3.5 " 10~ 7.9 1.1 Cyclopentanone 2 91 5.1 " 105 2 . 2 104 23 n.d. 11.8 1.2 Cyclopentanone 3 92 Partially gelated nld. 1.4 DMF 1 76 4.2104 1.2104 3.4 4.9105 5.5 1.2 ACN/toluene 1:1 1 85 8.5 " 104 1.3 " 104 6.8 7 . 0 105 n.d. n.d. Monomer mixture consisting of 70% ethylene dimethacrylate, 25% methyl methacrylate, and 5% monomer 1. Radically initiated polymerisation ( AIBN ) at 80~ for 4 days. Mw = weight averaged- Mn = number averaged molecular weight from GPC. Mn(osm) = membrane osmometry; r I = intrinsic viscosity; a-value = equilibratio of the microgel after template removal with the racemate of phenyl a-mannopyranoside. Mimicking natural enzyme action is quite a demanding task. Molecular imprinting has brought quite some progress in this direction. Typical enzyme properties like Michaelis-Menten kinetics, competitive inhibition, induced fit etc. were observed. The catalytic activity of natural enzymes, though, is much higher by several orders of magnitude but the catalysts obtained are rather stable and can be easily prepared. Soluble nanogels in which each particle possesses one active site will be of special interest since enzyme analogy will be relatively high in this case.
5. ACKNOWLEDGEMENT This work was supported by Deutsche Forschungsgemeinschafi and Fonds der Chemischen lndustrie.
REFERENCES
[1] [2]
G. Wulff, Angew. Chem. 101 (1989) 22; Angew. Chem. Int. Ed. Engl., 28 (1989) 21. G. Wulff, in: A. D. Schliiter, Ed., Synthesis of Polymers, Wiley-VCH Verlag, Weinheim, 1998, pp. 375-401. [3] G. Wulff, B. Heide, G. Helfmeier, J. Am. Chem. Soc. 108 (1986) 1089. [4] G. Wulff, Angew. Chem. 107 (1995) 1959; Angew. Chem. Int. Ed. Engl. 34 (1995) 1812. [5] G. Wulff, A. Sarhan, K. Zabrocki, Tetrahedron Lett. (1973) 4329. [6] G. Wulff, W. Vesper, R. Grobe-Einsler, A. Sarhan, Makromol. Chem. 178 (1977) 2799. [7] E. Fischer, Ber. Dtsch. Chem. Ges. 27 (1894) 2985. [8] G. Wulff, M. Minarik, J. Liquid Chromatogr. 13 (1990) 2987. [9] G. Wulff, S. Schauhoff, J. Org. Chem. 56 (1991) 395. [10] S. Mallik, S. D. PlunkeR, P. K. Dhal, P. D. Johnson, D. Pack, D. Shnek, F. H. Amold, New J. Chem. 18 (1994) 299.
44 [ 11] P.K. Dhal, in: B. Sellergren, Ed., Molecularly Imprinted Polymers - Man Made Mimics of Antibodies and Their Application in Analytical Chemistry, Elsevier, Amsterdam, 2001, pp. 185-201. [12] B. Sellergren, M. Lepist6, K. Mosbach, J. Am. Chem. Soc. 110 (1988) 5853. [ 13] K. Mosbach, O. Ramstr6m, Biotechnology 14 (1996) 163. [ 14] R.A. Bartsch, M. Maeda, Eds., Molecular and Ionic Recognition with Imprinted Polymers, ACS Symposium Series, Vol. 703, Washington, 1998. [15] B. Sellergren, Ed., Molecularly Imprinted Polymers. Man-Made Mimics of Antibodies and Their Application in Analytical Chemistry, Elsevier, Amsterdam, 2001. [ 16] G. Wulff, in: F. Diederich, P. J. Stang, Eds., Templated Organic Synthesis, Wiley-VCH, Weinheim, 1999, pp. 3 9 - 73. [17] K. Mosbach, Trends Biochem. Sci. 19 (1994) 9. [ 18] K.J. Shea, Trends Polym. Sci. 2 (1994) 166. [19] T. Takeuchi, J. Matsui, Acta Polym. 47 (1996) 471. [20] M.J. Whitcombe, E. N. Vulfson, Adv. Mater. 13 (2001) 467. [21] R.A. Lerner, S. J. Benkovic, P. G. Schultz, Science 252 (1991) 659. [22] P. G. Schultz, Angew. Chem. 101 (1989) 1336; Angew. Chem. Int. Ed. Engl. 28 (1989) 1283. [23] D. Robinson, K. Mosbach, J. Chem. Soc., Chem. Commun. (1989) 969. [24] B. Sellergren, K. J. Shea, Tetrahedron Asymmetry 5 (1994) 1403; B. Sellergren, R. N. Karmalkar, K. J. Shea, J. Org. Chem. 65 (2000) 4009. [25] K. Ohkubo, K. Sawakuma, T. Sagawa, J. Mol. Cat. A 165 (2001) 1; and earlier papers of this group. [26] G. Wulff, Chem. Rev. in press. [27] G. Wulff, A. Biffis, in: [ 15], pp. 71 - 111. [28] G. Wulff, K. Knorr, Bioseparation, in press. [29] G. Wulff, T. Gross, R. Sch6nfeld, Angew. Chem. 109 (1997) 2049; Angew. Chem. Int. Ed. Engl. 36 (1997) 1961. [30] A.G. Strikowski, D. Kasper, M. Grfin, B. S. Green, J. Hradil, G. Wulff, J. Am. Chem. Soc. 122 (2000) 6295. [31] P. Wentworth, A. Datta, S. Smith, A. Marshall, L. J. Partridge, G. M. Blackburn, J. Am. Chem'. Soc. 119 (1997) 2315. [32] J.-M. Kim, K.-D. Ahn, G. Wulff, Macromol. Chem. Phys. 202 (2001) 1105. [33] K. Hosoya, K. Yoshizako, N. Tanaka, K. Kimata, T. Araki, J. Haginaka, Chem. Lett. (1994) 1437. [34] L. Ye, P. A. G. Cormack, K. Mosbach, Anal. Commun. 36 (1999) 35. [35] K. Landvester, N. Bechthold, F. Tiarks, M. Antonietti, Macromolecules 32 (1999) 2679. [36] A. Strikowski, B. S. Green, G. Wulff, unpublished results. [37] A. Biffis, N. B. Graham, G. Siedlaczek, S. Stalberg, G. Wulff, Macromol. Chem. Phys. 202 (2001) 163. [38] B.-O. Chong, G. Wulff, unpublished results.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
45
Plugged Hexagonal Mesoporous Templated Silica: A unique micro- and mesoporous material with internal silica nanocapsules. P. Van D e r Voort a'! , P. I. Ravikovitch b, A.V. Neimark b, M. Benjelloun a, E. Van Bavel a, K.P. De Jong c, B. M. Weckhuysen c and E.F. Vansant a. a University of Antwerp (UIA), Dept. of Chemistry, Universiteitsplein 1, B-2610 Wilrijk, Belgium. i Corresponding author; email
[email protected]
b Center for Modeling and Characterization of Nanoporous Materials, TRI Princeton, P.O. Box 625, Princeton, NJ 08542, USA. $cUniversity of Utrecht, Dept. of Inorganic Chemistry and Catalysis, Debye Institute, Sorbonnelaan 16, 3508 TB Utrecht, The Netherlands. Following the development of purely mesoporous templated silicas, it is a desirable next step to create innovative catalytic support materials, consisting of a stable composite matrix with combined micro- and mesoporosities and a sufficient stability to withstand most industrial treatments. We show in this paper the development of a hexagonal plugged material, with combined micro- and mesopores and a tunable amount of both open and inkbottle mesopores. The ratios of these different pore types are variable in a wide range. The obtained materials are much more stable than the conventional micellar templated structures known so far. 1.
INTRODUCTION
Following the pioneering publications on the synthesis of mesoporous, semi-crystalline silicas [1-4], intensive research efforts have been devoted to the development of new mesoporous support materials of ordered structure. The research is motivated by the fact that such materials fill the gap in catalytic chemistry between the crystalline microporous zeolites and amorphous, disordered mesoporous supports like silica gel [5]. Due to their controlled pore size and a very narrow pore size distribution, the ordered mesoporous materials have a large potential as catalytic support in fine chemistry [5], pharmaceutical industry [7], as well as for the production of special polymer materials [6]. Heterogenizing the synthetic procedures in these fields of chemistry forms an important tool in achieving the goals of green, sustainable production processes and end-of-pipe waste reduction [7]. It is desirable to create innovative catalytic support materials, consisting of a stable composite matrix with combined micro- and mesoporosities. Such materials will offer significant supplementary advantages of an improved diffusion rate for transport in catalytic processes (faster reactions); better hydrothermal stability [8]; synthesis of multifunctional catalysts, which can process a large variety of feedstocks; capabilities of encapsulated waste in the micropores; controlled leaching rates for a constant and gradual release of an active component, etc. Here, we present a very simple synthesis procedure of a plugged hexagonal mesoporous material with very thick walls, high stability and controllable and tunable micro- and mesoporosities.
46 2.
EXPERIMENTAL
A plugged MTS material is prepared by dissolving 4 g of Pluronic P123 (non-ionic triblock copolymer, EO20POToEO20)in an acidic water/HC1 solution. Subsequently, an amount of TEOS (between 5 and 25 g) is added. The solution is stirred for 4-8 hours at a fixed temperature between 40 and 80~ and subsequently aged at ambient pressure for 17 h at 80120~ The white solid was filtered, washed and calcined at 550~ Detailed experimental conditions can be found in [9]. X-Ray Diffractograms were recorded on a Philips PW1840 powder diffractometer, using Ni-filtered Cu Ka radiation. Porosity and surface area studies were performed on a Quantachrome Autosorb-1-MP automated gas adsorption system. The calcined samples were degassed for 17 h at 200~ TGA measurements were recorded on a Mettler TG50 thermobalance. Mechanical pressing tests were performed in a unilateral press with a typical 13 mm dye (Specac). Hydrothermal tests were performed by placing the sample on a grid in an autoclave, which is filled with liquid water underneath the grid. The entire system is placed in an oven for 17 h in the temperature range 120-160~ exposing the sample to steam at autogeneous pressure. Other hydtrothermal experiments were performed using a fixed bed reactor, using a nitrogen flow, saturated with a certain percentage of water vapor.
3. RESULTS AND DISCUSSION
3.1. Nitrogen isotherms Changing the synthesis parameters in a controlled way allows the reproducible synthesis of a broad variety of materials. The adsorption-desorption isotherms of three distinctly different materials are shown in Figure 1. The isotherm in figure 1A is typical for the SBA-15 material [3], a two-dimensional p6mm structure formed by open cylindrical mesopores > ca. 5 nm in diameter. The desorption isotherm corresponds to the vapor pressure of the equilibrium meniscus in the open cylincrical pore, while the adsorption isotherm corresponds to the limit of stability of the adsorption film [ 10]. It should be noted that the material contains significant amounts (up to 30% of the porosity) of intrawall micropores (< ca. 3 nm) located in the pore walls, as evaluated by the NLDFT method [11]. The isotherm in Figure 1C shows the isotherm of a material with regular cylindrical pores that are accessible only through permeable microporous plugs. This is evident from the desorption branch of the isotherm and the shape of the hysteresis loop. If a pore is plugged, desorption is delayed until the vapor pressure is reduced below the desorption pressure from a pore aperture (ink-bottle efffect). However, if the pore aperture is below a critical diameter, decrease in the vapor pressure causes the fluid in the larger pores to become thermodynamically unstable before the desorption pressure for the pore aperture is reached [ 12]. For nitrogen at 77 K this instability occurs at p/p0: 0.42-0.45. The isotherm in Figure 1B is remarkable. It exhibits the following characteristic features: 1) adsorption in intrawall micropores at low relative pressures; 2) multilayer adsorption in regular mesopores and capillary condensation in narrow intrawall mesopores; 3) a one-step capillary condensation, indicating uniform mesopores; 4) a two-step desorption branch indicating the pore blocking effects (sub-step at the relative pressure of ca. 0.45). The adsorption-desorption behavior is consistent with a structure comprising both open and closed cylindrical mesopores. This interpretation is fully supported by the non-local density functional theory (NLDFT) of adsorption and hysteresis in cylindrical pores [ 10]. The mesopore size distribution and the total amount of micropores are calculated from the
47 adsorption branch of the isotherm by the NLDFT method [11]. The fractions of open and closed mesopores, as indicated schematically in Figure 2, have been determined from the pore size distributions (see Table 1). Details of calculations will be presented elsewhere.
Figure I : Nitrogen adsorption-desorption isotherms of (A) SBA-15, all open mesopores, (B) plugged material, with combined open and closed mesopores and (C) material with exclusively closed mesopores.
Figure 2. Nitrogen adsorption-desorption isotherm (77K) of a plugged hexagonal mesoporous templated silica and the mesopore size distribution calculated by the NLDFT method [ 10].
48 3.2. X-Ray Diffractogram The X-Ray Diffraction pattem in Figure 3 shows the characteristic reflections for a 2D hexagonal pore ordering in the p6mm space group [3]. The plugged mesoporous material therefore has the same structure as the SBA-15 hexagonal material.
Figure 3 : X-Ray Diffractogram of the plugged hexagonal mesoporous silica sample.
3.3. Plugged hexagonal mesoporous templated silica The data in Figures 1-3 point towards a composite material with a combined micro- and mesoporosity, as schematically represented in Figure 4. The rather thick walls ( - 4 nm) of the large cylindrical mesopores are perforated with micropores. Moreover, the cylindrical mesopores themselves are 'plugged' with amorphous silica nanocapsules, which are also microporous. These nanocapsules are created by large excess of the silica source (TEOS) that is used in the synthesis and by rapid hydrolysis of the silicon alkoxide at the very low pH used in the synthesis. The micropores in the silica walls can be explained by the penetration of hydrophilic poly(ethyleneoxide) chains of the triblock copolymer in the silica wall, as already suggested by Kruk et al [13]. The microporosity of the plugs may have a different origin. It is known that Pluronic triblock copolymers are in fact polydisperse mixtures of several triblock copolymers with a wide range of molecular weights, and that they contain appreciable amounts of diblock copolymers and even free PO chains. Some of these components, especially the low molecular weight ones, may not be involved in the actual templating of the mesopores, but still act as templates for the disordered nanocapsules, inducing a complementary porosity. The mesopores themselves are created by the so-called charge compensating templating mechanism of the entire triblock copolymer. The most important characteristics of these materials are summarized in Table 1. The table evidences the large variety in sample characteristics that can be obtained. The thickness of the mesoporous walls is typically 3-4 nm, which is excellent, compared to a typical wall thickness of 1 nm for the well-known MCM-41 structure. Extremely high total pore volumes can be obtained. The contribution of micropores (with contributions of both micropores in the walls and micropores in the silica nanocapsules) has an unprecedented high value. Micropore volumes up to 0.3 cm3/g can be obtained (40% of the total pore volume), which is
49 considerably higher than the micropore volumes of any composite material known so far. Both the ratio micropore/mesopore volume as the ratio open/closed mesopores is tunable in a wide range.
Microporous pore wall Microporous silica plugs 9
9
./ 9
9
............. 9...................................
......
9
9
9...... 9...............
9 9 ................................ 9
9
9 9
9
9
9
9
~
.
_
9
:l
6-8 nm
~ 1 7 96
9
3-4 nm
9
Open mesopore Closed mesopore Figure 4 9Schematical representation of the plugged hexagonal mesoporous templated silica (PHMTS) Table 1" Structural characteristics of 4 selected samples, PHMTS : Plugged Hexagonal Mesoporous Templated Silica, a0- lattice spacing, Vtot- totfil pore volume (micropores and mesopores), gmi- micropore volume, Vine- mesopore volume, Dads-- pore diameter from the adsorption branch, Does- - pore diameter from the desorption branch, Dgeom-- pore diameter from geometrical considerations using Vm,, Vine and 2.2 g/cm 3 for the silica skeleton density, hw pore wall thickness, hw = a0 - Daos; Vine(open)- volume of open mesopores, Vine (closed)volume of closed mesopores Sample
a0
(nm) SBA-15
ll.31
PHMTS-1
Vtot
SBET
Vmi
Vine
Dads
Ddes
Dg.... hw (ads) Vme(open) Vme (closed)
(cm3/g) (mVg)(cm3/g)(cm3/g)(nm)
(nm) (nm)
1.25
(nm)
(cm3/g)
(cm3/g)
950
0.14
l.ll
7.3.
7.59
7.76
4.01
l.ll
0
I 1.08 1.03
1040
0.29
0.74
6.79
7 . 0 3 7.12
4.30
0.23
0.51
PHMTS-2
9.58
0.71
880
0.26
0.45
6.08
no
5.89
3.50
< 0.01
0.45
PHMTS-3
10.16 0.83
945
0.30
0.53
7.03 7.03
6.94
3.13
0.38
0.15
.........
50
3.4. Transmission electron microscopy Using a Philips CM200 microscope, we have investigated extensively the PHMTS and SBA-15 samples in bright field transmission mode. In figure 5 we show representative images for these materials. Both micrographs provide side-on views of the ordered mesopore system. While"the mesopores in SBA-15 run smoothly over several micrometers of length, the PHMTS displays smaller domain sizes for the ordered mesopores. Moreover, the wall thickness yaries more strongly for the latter material, which may be caused by the presence of silica plugs inside the mesopores. Most recently, 3D-TEM techniques have been developed to image mesopores in three dimensions [14,15]. In a future paper, we will present evidence from 3D-TEM for the different pore systems in SBA-15 and our novel material PHMTS [9].
Figure 5" TEM images of SBA-15 (left) and PHMTS (fight).
3.5.
Stability
Table 3 presents the intrinsic thermal, mechanical ~and hydrothermal stabilities of some of the most important MTS materials [16]. Table 3 reveals that all materials are poorly resistant to (mild) hydrothermal treatments. The SBA-15 is the best. resistant of the conventional mesoporous silicas ; the PHMTS material is by far the most stable. It still has a very significant surface area and pore volume after 5 days of hydrothermal treatment or after a 24 h treatment in an autoclave in pure steam (sample placed above the water on a grid). Most materials collapse after a thermal treatment at 750~ with two exceptions: MCM-41 and PHMTS. Resistance toward mechanical (unilaterial) pressure is again best for PHMTS, followed by SBA-15. The thick walls of SBA-15 are further stabilized and supported in the PHMTS structure by the silica plugs, resulting in an extremely high mechanical resistance. The reported 10 tons/13 mm 2 was the highest pressure that could be obtained in our press. Pure silica based materials are obviously stable in neutral and acid conditions, but decompose in alkaline conditions.
51
Table 3 : Intrinsic stabilities of MTS materials ; SA = surface area (mZ/g), PV = pore volume (ml/g). Thermal stabilities after treatment in furnace, ambient atmosphere for 17 h at indicated temperature. Hydrothermal stability at x% water vapour at y temperature for z hours of treatment. Mechanical pressure, structure is collapsed if the XRD peak < 25% of the original peak and / or the typical diffraction peaks are no longer present. Chemical resistance : stirring for 24 h in an aqueous solution with indicated pH. l. Pressures are expressed as tons per 13 mm 2 pellet ; 1 ton/13 mm 2 corresponds to 740 bar. Treatment
MCM- 41 [PV SA
Themaal T :550~ 1027 T :650~ 970 T :750~ 879 T :850~ 795 Hydrothermal 25%/400/50 892 25%/400/120 864 100%/100/24 106 Mechanical 25% pressure I "ftonsI Chemical pH= 1 + pH = 7 + pH= 13 .
MCM- 48 [PV SA
HMS SA [ PV
SBA- 15 SA I PV
0.90 0.76 0.68 0.53
1433 1248 108 -
1.14 0.73 <0.05
1021 957 213 . .
0.81 718 0.58 561 <0.05 63 . .
0.66 0.59 <0.05
1357 1318 197
1.00 0.93 <0.05
915 830 228
0.57 0.48 <0.05
[ 4 tons
[ 5 tons
+ + .
+ + .
533 445 281
,PHMTS : [S~ :[)~
0.72 9!3::i i t 0!86 0.63 667 :: I 0.66 <0.05 451 0i48 !387 '~0~39 i 71:11' ::' ': : " " ...... : I 0.57 , 0.49 [~8~:: 1.0.47
~:!lOi~i I ~'
I 8 tons + +
.
4. C O N C L U S I O N S Using triblock copolymers as surfactant and TEOS as the silicon source, a whole variety of materials can be prepared, varying from pure SBA-15 (hexagonal stacking of pores, all pores are open) to hexagonal systems with exclusively ink-bottle pores and materials with combined open and closed pores, referred to as PHMTS (Plugged hexagonal mesoporous templated silica). PHMTS is characterized by very large but narrow distributed pores with very thick and microporous pore walls. Moreover, the pores are plugged internally with microporous silica nanocapsules. Their stability (thermal, hydrothermal, mechanical) is extremely high, compared to other commonly used mesoporous silicas. These materials might have unprecendented applications as adsorbers, desorbers and catalytic supports.
Acknowledgements This research was funded by a grant from the University of Antwerp (Special Research Fund) and by the F.W.O. (Flemish Fund for Scientific Research). The authors are grateful to Mrs. Sandra Kemp for making the TEM images.
5. REFERENCES 1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature 359 (1992) 710. 2. P.T. Tanev, M. Chibwe and T.J. Pinnavaia, Nature 368 (1994) 321; P.T. Tanev and T.J. Pinnavaia, Science 267 (1995) 865.
52 3. D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B. Chmelka and G.D. Stucky, Science 279 (1998) 548; D. Zhao, Q. Huo, J. Feng, B. Chmelka and G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. 4. S.A. Bagshaw, E. Prouzet and T.J. Pinnavaia, Science 269 (1995) 1242. 5. A. Corma, Chemical Reviews 97 (1997) 2373. 6. B.M. Weckhuysen, R.R. Rao, J. Pelgrims, R.A. Schoonheydt, P. Bodart, G. Debras, O. Collart, P. Van Der Voort and E.F. Vansant, Chemistry A European Journal 6 (2000) 2960. 7. J.H. Clark, Green Chemistry 1 (1999) 1. 8. A. Karlsson, M. St6cker and R. Schmidt, Microp. and Mesop. Mater. 27 (1999) 181. 9. P. Van Der Voort, P. I. Ravikovitch, A.V. Neimark, M. Benjelloun, E. Van Bavel, K.P. De Jong, A.H. Janssen, B. M. Weckhuysen and E.F. Vansant, in preparation. 10. A.V. Neimark, P.I. Ravikovitch and A. Vishnyakov, Phys. Rev. E 62 (2000) R1493. 11. P.I. Ravikovitch and A.V. Neimark, J. Phys. Chem. B 105 (200.1) 6817. 12. P.I. Ravikovitch and A.V. Neimark, Langmuir, submitted. 13. M. Kruk, M. Jaroniec, C.H. Ko and R. Ryoo, Chem. Mater. 12 (2000) 1961. 14. A.J. Koster, U. Ziese, A.J. Verkleij, A.H. Janssen and K.P. De Jong, J. Phys. Chem. B 104 (2000) 9368. 15. A.H. Janssen, A.J. Koster and K.P. De Jong, Angew. Chem. Int. Ed. 40 (2001) 1102. 16. K. Cassiers, T. Linssen, M. Mathieu, M. Benjelloun, P. Van Der Voort, P. Cool and E.F. Vansant, Chem. Mater., in press.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
I m p r i n t i n g o f the surface directing agents
of mesoporous
53
silicates u s i n g
organic
structure
Kaveri R. Sawant and Raul F. Lobo Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716, USA
A new protocol has been developed for incorporating the tetrapropylammonium cation (TPA +) into the walls of the mesoporous material SBA-15 with subsequent calcination to form a novel porous material. This modified material has been extensively characterized using powder x-ray diffraction (PXRD), nitrogen sorption studies, NMR spectroscopy, thermogravimetric analyses (TGA), SEM and TEM. As a'result of this treatment, the original mesoporous structure of SBA-15 is retained with a slight decrease in the unit cell dimension from 107 to 105 A. The average pore diameter is increased from 48 to 68 A although the total pore volume remains the same. 29Si MAS NMR spectra indicate that the silica phase is further condensed and densified. As a result a concomitant loss in BET surface area from 770 to 390 m2/g is observed. The incorporation ofTPA + into the amorphous siliceous walls of SBA-15 is confirmed by 13C-{1H} CP MAS NMR spectroscopy and by TGA. 1. INTRODUCTION The M41S family [1] and the related SBA-15 [2] mesoporous materials are a class of ordered porous silicates with pore diameters in the range of 20-500 A (see review [3]). Despite their large pore dimensions compared to that of zeolites, mesoporous materials are restricted in their use as catalysts for large organic molecules by their poor thermal stability and broad distribution of acid sites. A combination of the large pore dimensions of mesoporous materials with the strong acid sites present in zeolite-like structures would be highly advantageous leading to a novel and probably useful catalytic material. Hydrocarbon reactions involving large organic molecules that cannot enter the small pores of a zeolite catalyst may be catalyzed by the new material having a mesoporous framework structure with accessible strong acid sites on the surface of these large pores. Various strategies have been reported in the literature to combine some of the features of different porous materials [4-8]. Partial "recrystallization" of the surface of MCM-41 resulted in a mesoporous material with surface-tectosilicate structures giving enhanced acidity and catalytic cracking activity [4]. However, the walls of MCM-41 have a thickness of - 10 A and can contain, at most, very small organized units of silica/alumina. A dual templating method has been used for the synthesis of macroporous silicates with zeolitic microporous frameworks [5] and MCM-41/ZSM-5 composites [6]. In addition, another technique for the synthesis of macroporous materials with zeolitic microporous frameworks by self-assembly
54 of colloidal zeolites has been reported [7]. Finally, to obtain a mesoporous material with acid sites on the 'surface, a procedure has been reported for the syntheses of organically functionalized mesoporous materials with sulfonic acid groups [8]. This study reports a new protocol that has been developed for the synthesis of a novel mesoporous silicate that is obtained by reorganizing the structure of the walls of the mesoporous material SBA-15. SBA-15 is an all-silica mesoporous material consisting of long cylindrical pores ordered in a hexagonal packing arrangement with pore diameters in the range of 46-100 A [2]. The wall thickness is in the range of 31-64 ,~. The amorphous silica phase is itself structured and contains fine microporous channels interconnecting the large mesopores [10-11]. The use of a mesoporous precursor such as SBA-15 with a relatively large wall thickness o f - 59/k would allow structures comparable in size to the unit cells of zeolites like silicalite-1 thus increasing stability and catalytic activity. TPA + which serves as the organic structure directing agent (SDA) for the zeolite silicalite-1 was used to "imprint" silicalite-l-like domains within the thick amorphous pore walls of the mesoporous material SBA-15. Glycerol is used as a solvent, which also helps in the recrystallization of the silica phase. After the treatment the TPA + containing samples are calcined to remove the organic and obtain the porous novel material. These treated samples before and after calcination are referred to as TPA+-SBA - 15 and TPA+-SBA - 15 calcined respectively (Figure 1).
N
N
N
Amorphous Silica Phase SBA-15 (a)
O = TPA + ~
TPA+_SBA_I 5 (b)
Organized Silica Structure TPA+-SBA - 15 calcined (c)
Figure 1. Synthesis Strategy: (a) SBA-15 is treated with TPAOH and glycerol to obtain (b) the new material containing the incorporated TPA + (TPA+-SBA-15) which is then calcined to remove the TPA + to obtain (c) the final porous material (TPA+-SBA - 15 calcined). As a result of this treatment, the original mesoporous structure of SBA-15 is retained with a slight decrease in the unit cell dimension from 107 to 105 ,~. The average pore diameter is increased from 48 to 68 ~ although the total pore volume remains the same. 29Si MAS NMR spectra indicate that the silica phase is further condensed and densified as a result of this treatment with a corresponding loss in BET surface area from 770 to 390 m2/g. The incorporation of TPA + into the amorphous siliceous walls of SBA-15 is confirmed by 13C{1H} CP MAS NMR experiments and by TGA. The final objective is to synthesize an aluminosilicate with accessible acid sites on the inner surface of the mesopores.
55 2. M A T E R I A L S A N D M E T H O D S 2.1. Materials SBA-15 was prepared following a protocol adapted from the original work by Zhao et al [2]. 4.0 g of the EO20PO70EO20 triblock copolymer P-123 (BASF) is added to a mixture of 19.08 g of 37 wt. % HC1 and 97.1 g of deionized water at 35 ~ with stirring. After the polymer dissolves, 8.5 g of tetraethyl ortho-silicate (Aldrich) is added and stirred at 35 ~ for 20 h. The mixture is heated at 80 ~ statically for 24 hrs in a Teflon container. The product is filtered and washed w i t h - 3 L of deionized water and dried overnight at room temperature. The materials were calcined by heating in air from room temperature to 300 ~ in 5 hrs, then holding at 300 ~ for 12 hrs.
In a typical preparation, 1 g of calcined SBA-15 was impregnated with a liquid mixture of 0.89 g of glycerol and 0.1 g of aqueous (TPAOH) (40% solution) [2]. This mixture was then heated in a Teflon container at 120 ~ for 10 days under static conditions. The treated material was then washed with 1 L of deionized water and dried overnight at room temperature. The calcined form of the treated material is obtained by heating in air at a rate of 1~ to 500 ~ then holding at 500 ~ for 6 hrs. 2.2. Measurements The PXRD patterns were obtained using a Philips X'pert system with Cu KV radiation (8 = 1.54186 A~). SEM images were recorded using a Hitachi S-4000 field emission microscope. Nitrogen adsorption and desorption isotherms were obtained at 77 K using a Micromeritics ASAP 2010 volumetric adsorption analyzer. Before the measurements, the samples were outgassed overnight at 200 ~ 29Si MAS and 13C-{1H} CP MAS NMR spectra were recorded using a Bruker MSL 300 MHz spectrometer operating at resonance frequencies of 59.627 and 75.468 MHz respectively under conditions of magic-angle spinning at 3 kHz at room temperature; ~/3 pulse lengths of 4 gs were used to acquire one-pulse 29Si spectra employing a 90 s relaxation time between each scan. The 29Si MAS spectra and the 13C-{1H} CP MAS NMR spectra were referenced to 2, 2-dimethyl-2-silapentane-5 sulfonate (DSS) and hexamethylene benzene (HMB) respectively. TGA were performed using a Cahn TG-121 microbalance with a heating rate of 5 ~ from room temperature to 800 ~ in air. Transmission electron micrographs were taken on a JEOL 2010F field-emission electron microscope operating at 200 kV. The samples for TEM were prepared by dispersing a large number of particles of the materials through a slurry in acetone onto a lacey carbon film on a Cu grid. 2.3. Methods The BET specific surface area was calculated in the relative pressure range between 0.06 and 0.2. The single point total pore volume was determined from the amount desorbed at a relative pressure of 0.95. The pore size distribution curve came from the analysis of the desorption branch of the isotherm. The pore size distributions were determined using the Halsey equation for multilayer thickness and the calculation procedure proposed by Barrett, Joyner and Halenda (BJH) [9]. The wall thicknesses were calculated as wt = ao- DBm (ao = average hexagonal unit cell dimension and DBJH = pore diameter). The inner surface area of the primary mesopores (Sp) was geometrically calculated by assuming that the pores are perfect cylinders with diameter = DBJH.
56 3. RESULTS AND DISCUSSION A new protocol has been developed for the incorporation of TPA + into SBA-15 with subsequent calcination to form a novel porous material. Starting with the calcined form of SBA-15 as a mesoporous precursor, we aim to transforming the thick amorphous pore walls into domains of silicalite-l-like structures using TPA + as the organic SDA. For this treatment TPAOH was used to provide the TPA + ions and also act as a base. Glycerol (and some water) were used as solvents, also helping in the recrsytallization of the silica phase. The PXRD patterns at low angles indicate that the original hexagonal mesoporous structure of SBA-15 (as indicated by the dl00, dll0 and d200 peaks) is retained with a slight decrease in the unit cell dimension from 107 to 105 A (Figure 2). SBA-15 ...... TPA-SBA- 15 calcined
r.~
k_
0
~
0.5
1
7
1.5
.
2 2.5 2 theta (degrees)
...... : ........ ~.-=-::7...2 ............ ~. . . . .
3
3.5
.-22.. . . . . . . . . . . ;
4
Figure 2. PXRD patterns of SBA' 15 and TPA+-SBA - 15 calcined. As desired, the wide-angle PXRD patterns for the material do not have the characteristic peaks of silicalite-1. This result is as expected because the domains of silicalite-l-like structures formed are probably too small to give rise to measurable PXRD peaks [10]. As the wall thickness of SBA-15 is - 59 A the organized silica structures formed within its pore walls can only be comparable in size to a few unit cells of ZSM-5. PXRD patterns of ZSM-5 crystals consisting of different number of unit cells were calculated using the Cerius 2.0 simulation software (Figure 3). The broad peaks appearing as a result of either one, two, three unit cell cubes or a sum of the above cannot be detected when present in a largely amorphous silica environment of SBA-15. Hence this direct evidence for the formation of silicalite-1 structures cannot be obtained.
57
Figure 3. Calculated PXRD patterns of ZSM-5 crystals consisting of different number of unit cells. SEM images indicate that at the micron level the particle morphology is unaffected by the treatment but at the submicron level the surface of the particles is modified. Although the objective was to transform the structure of the silica phase primarily at the inner surface of the mesopores, the external surface of the SBA-15 particles is readily accessible to the TPA + molecules and the subsequent reaction is unavoidable. TEM images of the sample before and after treatment indicate that the hexagonal mesoporous structure is indeed retained with a significant increase in average pore diameter (Figure 4).
(a)
(b)
Figure 4. TEM images (a) perpendicular and (b) through the pores of TPA+-SBA - 15 calcined. Nitrogen adsorption and desorption experiments show that although the total pore volume (Vt) remains the same the pore structure is significantly modified with a-~20 A increase in the approximate pore diameter (Figure 5). This information along with the unit cell size dimensions was used to show that the wall thickness decreases b y - 2 2 A. The BET surface area (SBET) decreases b y - 5 0 %. This loss in surface area corresponds to the contribution to the total surface area by the fine microporous channels existing in the original SBA-15 material [11-12]. From sorption experiments on similar treated samples at the intermediate
58 stage containing the organic SDA (TPA+-SBA-15), it is shown that there is a definite increase in porosity on removal of the organic, which can be attributed to the additional micropores formed by the TPA + molecules. .,z
500
O
o
~SBA-15
~SBA-15
400
-"TpA-s~A-~,ca,c./ / :....."
r./3
o
6
-" T P A - S B A - 1 5
calc.
300
<@
o ~~ 2 0 0
~
O
O
o
>
~ &2 o
100 I
0 0
I
I
I
0.2 0.4 0.6 0.8 1 Relative Pressure (P/Po)
0 0
50 100 150 Pore Diameter (A)
200
Figure 5. Nitrogen sorption isotherms and pore size distributions of SBA-15 and TPA+-SBA 15 calcined. The d~00 spacings and the unit cell dimensions obtained via PXRD and the sorption data including the average pore diameters, BET total surface areas, inner surface areas due to the primary mesopores and total pore volumes alongwith the wall thickenesses are summarized in Table 1. Table 1.
~X~and_s0~tion~c~har~acter!stics,ofthemater!~a! at yarious~stag~es:................................................................................................................ Sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SBA-15 TPA-SBA-15 calc.
dl00 a0 DBJI~ wt SBzT Sp Vt (~) .........................(~) .........................(~) .............................. .(~)...............(m2g-.1)......(m2 g~!)............(cm3._g~!) .... 90.9 107 48 59 770 520 . 0.73 89.2 105 68 37 390 380 0.73
29Si MAS NMR spectra show a significant loss of Q2 and Q3 surface species as a result of the treatment (Figure 6 (a), (b) and (c)). Quantitative information regarding the Q2, Q3 and Q4 species in the original SBA-15 and in the treated materials is summarized in Table 2. This information along with the sorption results suggests that the pore walls of SBA-15 are condensed and densified by the treatment. TGA of the TPA-SBA-15 samples reveal a 3-4 % weight loss event at an intermediate temperature of 300 ~ which can be attributed to the presence of TPA + incorporated into the mesoporous framework. 13C-{~H} CP MAS NMR spectra of the TPA+-SBA-15 samples further confirm this (Figure 6 (d), (e) and (f)). Apart from the presence of glycerol, these spectra show that the TPA + molecules are present in a silicalite-l-like environment indicated by the two peaks at 16.31 and 11.43 ppm.
59 Q4
Q3 Peaks of TPA +
(a)
(d)
Q2
(b)
j
~
~
(e)
Glycerol
(f)
(c)
-80
-120 (ppm)
-160
88 66 44 22 (ppm)
0
Figure 6. 298i MAS NMR spectra of (a) SBA- 15, (b) TPA+-SBA - 15 and (c) TPA+-SBA - 15 calcined and 13C-{1H} CP MAS NMR spectra of TPA+-SBA-15 (d) deconvoluted (e) simulated and (f) experimental. Table 2.29Si MAS NMR characteristics of the material at various stages. Sample _ (%) ........................................................... SBA-15 4 TPA+-SBA - 15 TPA+-SBA - 15 calcined -
(%) .................................................... (0/0_).... 32 64 11 89 > 90
In this study, TPA + was chosen as the organic SDA to form organized silica structures similar to the model zeolite silicalite-1. This treatment has since been extended to the use of other SDA's such as tetramethyladamantammonium to "imprint" the more complicated cuplike structures of the zeolite MCM-22 on the inner surface of SBA-15. In conclusion, we have successfully developed a protocol for the incorporation of the organic SDA, TPA +, for the zeolite silicalite-1, into the mesoporous material SBA-15. As a result of this treatment the framework mesoporous structure of the material is retained with an increase of 20 A in the average pore diameter. The silica phase is further condensed and densified with a decrease of 22 A in the wall thickness. The incorporation of TPA + is confirmed by ~3C-{1H} CP MAS NMR experiments and thermogravimetric analyses. The domains of silicalite-1 formed are too small to be detected by XRD. In order to detect the formation of silicalite-l-like structures catalytic tests must be performed on aluminum
60 containing samples. Similar initial results have been obtained for A1-SBA-15 as for the allsilica materials 4. ACKNOWLEDGMENTS We would like to acknowledge BASF for providing the P-123, Dr C. Ni for the TEM images and NSF for funding this research (NSF Career Award CTS-9733066). REFERENCES
1.
C.T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710; 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. McCullenm, J. B. Higgins and J. L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 2. D. Zhao, Q. Huo, J. Feng, B. F. Chmelka and G. D. Stucky, J. Am. Chem. Soc., 120 (1998) 6024. 3. J.Y. Ying, C. P. Mehnert and M. S. Wong, Angew. Chem. Int. Ed., 38 (1999) 56. 4. K.R. Kloetstra, H. van Bekkum and J. C. Jansen, Chem. Commun., (1997) 2281. 5. B.T. Holland, L. Abrams and A.Stein, J. Am. Chem. Soc., 121 (1999) 4308. 6. L. Huang, W. Guo, P. Deng, Z. Xue and Q. Li, J. Phys. Chem. B, 104 (2000) 2817. 7. Y.J. Wang, Y. Tang, Z. Ni, W. M. Hua, W. L. Yang, X. D. Wang, W. C. Tao and Z. Gao, Chem. Lett., (2000) 510. 8. D. Margolese, J. A. Melero, S. C. Christiansen, B. F. Chmelka and G. D. Stucky, Chem. Mater., 12 (2000) 2448. 9. E.P. Barrett, L. G. Joyner and P. P. Halenda, J. Am. Chem. Soc., 73 (1951) 373. 10. J. L. Schlenker and B. K. Peterson, J. Appl. Cryst., 29 (1996) 178. 11. M. Kruk, M. Jaroniec, C. H. Ko, R. Ryoo, Chem. Mater., 12 (2000) 1961. 12. S. Jun, S. H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna, O. Terasaki, J. Am. Chem. Soc., 122 (2000) 10712.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
61
Synthesis and characterization o f p o l y m e r - t e m p l a t e d o r d e r e d silica with cage-like m e s o s t r u c t u r e Jivaldo R. Matos, a Michal Kruk, Lucildes P. Mercuri a and Mietek Jaroniec Department of Chemistry, Kent State University, Kent, Ohio 44242, USA
Advances recently made in our research group in the synthesis and characterization of ordered silicas with large cage-like mesopores are overviewed with a focus on materials templated using a B50-6600 poly(ethylene oxide)-poly(butylene oxide)-poly(ethylene oxide) (EO39BO47EO39) triblock copolymer, as proposed by Yu et al. The mesopore diameter of these silicas can be controlled by changing the initial synthesis temperature and by applying an additional higher-temperature treatment, but the excessively long treatment time may lead to a loss of the pore entrance size uniformity. Not only tetraethyl orthosilicate (TEOS), but also a cost-effective sodium silicate can be used to assemble these large-pore silicas, but TEOS provides more flexibility in terms of the structure tailoring. All the obtained silicas exhibited appreciable microporosity, as expected from the fact that they were templated by a polymer with EOn blocks. The pore entrance size in selected samples was determined by monitoring changes in gas adsorption properties after the surface modification with judiciously chosen organosilanes and it was shown that the pore openings can be made uniform in diameter and either primarily in the micropore or in the mesopore range depending on the synthesis temperature. There is also evidence that the surface-modificed silicas with cage-like mesopores may exhibit molecular sieving effects, because the pore entrance size within subnanometer range can be attained.
1. INTRODUCTION The last eight years brought the discovery of ordered silicas and organosilicas with cagelike mesopores [1-3] and a remarkable progress in their synthesis, characterization and application [4-40]. These materials can be synthesized using both cationic surfactants [116,35-37] and non-ionic oligomer or block-copolymer templates [17-29], many of which are commercially available. The pore (cage) diameter attainable for silica structures spans from the lower end of the mesopore range [4] to about 12 nm [27-29,40], and the materials can be prepared in the form of powders, thin films [10-12,14-16,19], and fibers [18]. Ordered silicas with cage-like mesopore structures (OSCMSs) have already been successfully frameworkmodified in order to incorporate heteroatoms of catalytic properties [30-32]. OSCMSs were also used to grow well-aligned carbon nanotubes [33] and to template the formation of 3Permanent address: Instituto de Quimica da Universidade de Sao Paulo. C.P. 26.077, 05599970, Sao Paulo, SP, Brazil.
a
62 dimensionally (3-D) ordered carbon frameworks stable upon template removal [34]. However, some important aspects of the synthesis and characterization of OSCMSs have been addressed only recently. First, the structural types of many of these materials were tentatively assigned on the basis of X-ray diffraction (XRD) and transmission electron microscopy (TEM) [1,2,17], but require verification and confirmation. In addition, details of their pore structure, and connectivity as well as framework properties, require proper elucidation. This is prompted by findings that one of OSCMSs (SBA-2) is not a pure phase and that the pores in these materials are 2-dimensionally connected, although the porous structure itself is 3-dimensional [8], and that some ordered block copolymer-templated silicas exhibit microporosity in their pore walls [41]. Recently, some of the OSCMS structures, including SBA-1, SBA-6 and SBA-16, have been solved using the electron crystallography [38], and their original structural assignments [1,17] were confirmed, but solutions of other structures are yet to be published and may differ from the structures originally suggested. For the first time, the electron crystallography provided detailed information about the connectivity of mesopores in the OSCMS structures, including the size of pore entrances, which were found to be much smaller than the pore (cage) diameters. Second, methods suitable for reliable determination of the pore diameter and the pore entrance size need to be identified or developed. The aforementioned electron crystallography method is a major step in this direction, but this approach is only tractable for highly ordered materials with appreciable sizes of ordered domains. A more practical method, but restricted only to the determination of pore diameter, was developed for spherical pores based on nonlocal density functional theory (NL DFT) of gas adsorption data [40]. This work provided strong indications that the pore diameters of OSCMSs are substantially larger than those estimated using standard adsorption methods [42], and somewhat larger than those assessed using a method [43] calibrated for cylindrical rather than spherical pores. The presence of intrawall pores in many OSCMSs, both templated by alkylammonium surfactants and oligomers/polymers, was also suggested. Third, convenient approaches for the synthesis of OSCMSs with tailored pore size and cage opening size need to be identified. The standard method is to use surfactants of different sizes [4,9,38], but there were also indications that in the case of polymer-templated syntheses, the temperature control allows one to tailor the pore diameter [26]. Forth, cost-effective syntheses of OSCMSs need to be developed. The use of oligomers and polymers as templates is a major step toward this goal, but another is to substitute the expensive tetraethyl orthosilicate (TEOS) by a cheaper silica source, such as sodium silicate. This has recently been accomplished for oligomer-templated OSCMSs, but the possibility to assemble OSCMSs using block copolymers, which would potentially lead to large-pore materials, was only mentioned [20]. Our recent studies provided important contributions to the aforementioned four issues in the OSCMS synthesis and characterization. This includes (i) the evidence that a triblockcopolymer-templated OSCMS has a strongly microporous framework [29], (ii) the development of a simple and practical method for determination of the pore entrance size in OSCMSs [39], (iii) the finding that not only the pore diameter, but also the pore entrance size can be controlled by changing the synthesis temperature in block copolymer-templated syntheses [39], and (iv) the elaboration of the synthesis of large-pore OSCMS using a costeffective sodium silicate [29]. These developments are discussed hereafter.
63
2. E X P E R I M E N T A L
FDU-1 silicas were synthesized as proposed by Yu et al. [27,28]. The synthesis mixture composition was 1 TEOS : 0.00735 B50-6600 : 6 HC1 : 155 H20, where B50-6600 is a poly(ethylene oxide)-poly(butylene oxide)-poly(ethylene oxide) triblock copolymer (EO39BOa7EO39).The initial synthesis step was performed at room temperature, 313 K or 333 K. In some cases, an additional period of heating at 373 K was employed. A typical synthesis was as follows [29]. 1.83 g ofB50-6600 was dissolved in 110 g of 2 M HC1 by stirring for 12 hours at room temperature, and a homogeneous mixture was obtained. Subsequently, 7.61 g (8.t6 mL) of TEOS was added and the stirring was continued for another 24 h at room temperature, during which precipitation took place. The precipitate was filtered, washed with water, dried at room temperature, and calcined under nitrogen, which was then switched to air at 813 K. Alternatively, sodium silicate (Aldrich; 14% NaOH, 27% SiO2, and 59% H20) was used as a silica source instead of TEOS. In this case, the synthesis mixture composition was 1 SiO2 : 0.778 NaOH : 0.00735 B50-6600:6 HC1 : 162 H20, and the polymer mixture in HC1 solution was cooled down to 288 K before the addition of the silica source. Nitrogen adsorption isotherms were measured on a Micromeritics ASAP 2010 volumetric adsorption analyzer. Before these measurements, samples were degassed under vacuum for at least 2 hours at 473 K. Weight change curves were recorded under nitrogen atmosphere on a TA Instruments TGA 2950 thermogravimetric analyzer using a high-resolution mode with a maximum heating rate of 5 K min -1. The BET specific surface area [42] was evaluated from data in the relative pressure range from 0.04 to 0.2. The total pore volume [42] was estimated from the amount adsorbed at a relative pressure of 0.99. The external surface area and the sum of the primary mesopore volume and micropore volume were evaluated using the cts plot method [42,44] in the C~s plot range from 1.75 to 2.2. The micropore volume and the sum of the primary mesopore surface area and external surface area were determined in the ~s range from 0.9 to 1.2. The standard reduced adsorption ~Xs is defined as the amount adsorbed at a given relative pressure divided by the amount adsorbed at a relative pressure of 0.4 for the reference adsorbent. Macroporous silica LiChrospher Si-1000 was used as a reference adsorbent [44]. The pore size distribution (PSD) was calculated using the Kruk-JaroniecSayari (KJS) method [43], which was calibrated for cylindrical pores and consequently, the pore diameters obtained for cage-like pores of FDU-1 are expected to be somewhat underestimated [29,40]. The statistical film thickness curve suitable for calculation of PSD was reported elsewhere [44]. The pore diameter is defined as a maximum o f K J S PSD.
3. R E S U L T S AND D I S C U S S I O N Yu et al. recently reported that OSCMS (referred to as FDU-1) with a structure identified as cubic Im3m can be synthesized in a wide range of conditions using B50-6600 poly(ethylene oxide)-poly(butylene oxide)-poly(ethylene oxide) triblock copolymer template [27,28]. These authors reported that the pore diameter of FDU-1 is 12 nm, but did not discuss whether it is possible to tailor it, and did not elucidate the pore entrance size. The cage-like nature of pores of FDU-1 was clear from the shape of the adsorption-desorption hysteresis loop on the nitrogen adsorption isotherm [28], because the hysteresis loop was broad, featured sharp capillary condensation and evaporation steps, and the latter step was located at a relative pressure corresponding to a lower limit of adsorption-desorption hysteresis (0.4-
64 0.5). We have synthesized a series of samples using the same synthesis mixture composition and similar synthesis conditions as those employed by Yu et al. We used one-step and twostep procedures (the latter with an additional higher-temperature treatment) and varied the synthesis temperature and time. Yu et al. reported that FDU-1 forms at temperatures up to 333 K and indeed silicas with adsorption properties similar to those reported for FDU-1 were obtained when the initial synthesis temperature was between room temperature and 333 K [29]. The pore diameter of these silicas was increased from 8.7 to 9.5 nm by increasing the temperature of the initial step from room temperature to 313 K. Further pore diameter enlargement to 11.2 nm was achieved when this step is followed by heating at higher temperature (for instance 373 K). However, in this case, an excessively long treatment led to the loss of uniformity of the pore entrance size, as seen from the fact that desorption from the pores started to take place well below the lower limit of adsorption-desorption hysteresis (relative pressure of 0.4-0.5; see data for samples A and C in Figure 1), suggesting the development of entrances of diameter above 5 nm, unlike in the case of good-quality FDU-1 (see data for sample B and [27-29]). When the first step of the synthesis was carried out at room temperature, the structural degradation was not observed after 12, or sometimes even 24 hours of heating at 373 K (see data for sample B), but was evident for longer heating times (see data for samples A and C). It was already reported that the increase in the synthesis temperature allows one to enlarge the pore size of polymer-templated ordered mesoporous silicas with 2-D hexagonal structure [ 17]. However, the indication that this behavior extends over polymer-templated OSCMSs was briefly reported just recently [26] for SBA-16 silica [ 17]. Our study provided additional evidence for the viability of this method of the pore size control for polymer-templated OSCMSs (see PSDs for samples D-F in Figure 2). 6oo
|
|
i
,
r
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i
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i
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i
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i
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Relative Pressure
0
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i
i
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Figure 1. Nitrogen adsorption isotherms for samples synthesized using B50-6600 as a template and TEOS (A-C) or sodium silicate (D-E) as a silica source. Samples A-D were synthesized at room temperature and subsequently heated (A-C only) for 1 day (A,B) or 4 days (C). Samples E and F were synthesized at 318 K, which in the case of sample F was followed by heating at 373 K for 6 hours (data for samples A and D-F taken from Ref. 29). It was reported that SBA-15 silica templated by poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer exhibited some microporosity [45], although
65 its origin was not identified. We have confirmed the occurrence of microporosity in SBA-15, and explained its formation as an effect of penetration of EOn blocks of the template in the silica pore walls [41 ]. On the basis of this explanation, we suggested [41 ] that the occurrence of microporosity in the pore walls is a general behavior that extends over SBA-15 and, most likely, other silicas templated by polymers with EOn blocks. The presence of microporosity in SBA-15 pore walls causes that the large, cylindrical mesopores of this material are connected, rather than isolated as in the case of MCM-41. Consequently, it was possible to synthesize 3D ordered platinum and carbon replicas of SBA-15 [34,41]. As expected, FDU-1 silicas were also found to be microporous. This is seen from C~s-plots, whose linear regions that preceded the increase in adsorption related to capillary condensation intercepted the Amount Adsorbed axis significantly above the origin. The corresponding micropore volumes ranged from 0.18 to 0.27 cm 3 g~ for different samples [29]. It can be expected that other OSCMSs templated by oligomers and polymers with EOn blocks also exhibit microporosity in their pore walls. .
.
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.
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.
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Pore Size (nm)
12
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Standard Adsorption c~s
Figure 2. Pore size distributions for samples D-F (see caption to Figure 1) and the C~s-plot for FDU-1 silica synthesized from TEOS at room temperature (G). Data for D-F taken from [29]. The problem of determination of the pore entrance size in OSCMSs is crucial from the point of view of their prospective applications as selective adsorbents, size- or shape-selective catalysts, supports for chemically bonded functional groups, templates, immobilization media, and so forth. The OSCMS pore entrance sizes are too small to be elucidated from the position of desorption branches of gas adsorption isotherms using known concepts in adsorption data analysis [40,47,48]. Moreover, no reports have indicated that the pore opening sizes can be reliably deduced from PSDs, so the OSCMS pore entrance dimensions had been unknown until the electron crystallography method was elaborated [38]. So far, this powerful approach allowed for elucidation of the pore entrance size in several OSCMS samples, including SBA-1, SBA-6 and SBA-16 silicas. However, this method requires a powerful TEM instrumentation, and is suitable for samples that are highly ordered and exhibit an appreciable size of ordered domains. Therefore, it is inapplicable for some important silicas with cage-like pores that are not ordered [48,49], and in any case, it does not appear to be practical for routine characterization of OSCMSs. Recently, we have developed a practical method for determination of the pore entrance size in OSCMSs and related materials with organosilica framework [39]. This novel method is based on the monitoring of changes in
66 pore accessibility brought about by surface modification with a series of organosilanes with a gradually increasing size. In practice, OSCMS is surface-modified with several judiciously chosen organosilanes, and gas adsorption isotherms of the resulting materials are measured. The size of the smallest organosilane whose introduction leads to a complete pore blockage (which manifests itself in the reduction of the adsorption capacity at higher relative pressures by one or more orders of magnitude) is related to the pore entrance size, taking into account the size of gas molecules used for adsorption studies. The application of this novel methodology allowed us to determine that for FDU-1 silica synthesized at room temperature, the pore entrance diameter was above about 1.2 nm, an average diameter was below about 1.4 nm, and there were essentially no entrances of diameter above 1.9 nm. The pore entrance size in FDU-1 synthesized at room temperature and subsequently heated at 373 K for 6 hours was substantially higher, as it was estimated to be above 1.9 nm, but below 2.9 nm with no evidence of any pore entrances of diameter above the latter value. This showed that for the same polymeric template, the OSCMS pore entrance size can be enlarged nearly two-fold simply by applying the higher-temperature treatment. It is important to note that for FDU-1 synthesized at room temperature, we observed that an increase in the length of alkyl chain of the surface modifier by just one carbon atom transformed a porous structure from wellaccessible to largely inaccessible to gaseous nitrogen [39]. This behavior strongly indicates the pore opening size in the FDU-1 sample considered was uniform and that the aforementioned modified sample with still accessible pores exhibited the entrance sizes in a subnanometer range. Thus, it is expected that the surface-modified FDU-1 that would exhibit molecular size selectivity can be readily synthesized [39]. Our method for the pore entrance size elucidation does not provide some important structural information, such as the shape of the entrance and the average connectivity in the structure, but is applicable for both ordered and disordered silicas [48,49]. In addition, our method allows one to probe the pore entrance dimensions within the entire sample, whereas the electron crystallography probes materials on a local level, allowing one to solve structures even for single particles, but does not provide an averaged information for the whole material, which our method does. So, the two approaches are complementary, especially as the first one can be used for routine analysis, but provides information about the pore entrance size only, whereas the second one provides the solution of the entire structure, but is not practical for the routine analysis. Yu et al. [27,28] originally used TEOS as a silica source for FDU-1 synthesis. We have demonstrated that when this reagent is substituted by a cost-effective sodium silicate [29], silicas with adsorption properties analogous to those of FDU-1 (see Figure 1) and with similar specific surface areas and pore volumes can be synthesized. Thermogravimetric studies of assynthesized samples assembled using TEOS and sodium silicate showed no appreciable differences in the triblock copolymer template content and its decomposition temperature range, additionally suggesting that sodium silicate can be used to assemble the FDU-1 silicas [29]. However, the synthesis from sodium silicate was successful only under more restricted conditions, when compared to the original procedure. In particular, when sodium silicate was used, no structure with uniform mesoporosity was obtained at 333 K, and the application of the additional heating step at 373 K led to the structural degradation after very short periods of time. For instance, a sample synthesized at room temperature and subsequently heated at 373 K for 6 hours already showed signs of structural degradation [29]. We found that the synthesis of FDU-1 from TEOS provides more flexibility in terms of the pore diameter tailoring, as the latter can be adjusted by 2.5 nm (from 8.7 to 11.2 nm) by varying the synthesis temperature in one-step and two-step procedures, whereas the range of pore diameters attainable using sodium silicate was restricted to within 1.8 nm (8.5-10.3 nm) [29].
67 4. CONCLUSIONS When B50-6600 template is used, ordered silicas with large cage-like pores can be synthesized using not only TEOS, but also a cost-efficient sodium silicate. The pore diameter of these materials can be controlled by choosing an appropriate initial synthesis temperature and employing an additional higher-temperature treatment. The pore entrance diameter can also be controlled in a similar manner and is primarily in either the micropore or mesopore range depending on the synthesis conditions. The pore entrance size of one of the FDU-1 silicas discussed herein was shown to be decreased to the subnanometer range by employing a simple surface modification procedure, which allows one to obtain a new kind of a mesoporous zeolite analogue. Extended high-temperature treatments led to the loss of uniformity in the pore opening size, which manifests itself in the formation of pore openings of diameter above about 5 nm. FDU-1 exhibits a high amount of micropores in the mesopore walls, which appears to result from the inherent tendency of EOn blocks to be occluded in silicate pore walls during formation of ordered silicas.
5. A C K N O W L E D G M E N T S M. J. acknowledges support by NSF Grant CHE-0093707. J. R. M. and L. P. M. thank for support Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP, Brazil), fellowships under grants 99/11170-5 and 99/11171-1. The authors also thank Dr. Rene Geiger from the Dow Chemical Company for providing B50-6600 triblock copolymer.
REFERENCES
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Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 Elsevier Science B.V. All rights reserved.
69
The m o d e l i n g o f wall structure o f siliceous M C M - 4 1 b a s e d on the formation process Yasunori Oumi, a Kazuhiko Azuma, a Takuji Ikeda, b Shintaro Sasaki, a and Tsuneji Sano a School of Materials Science, Japan Advanced Institute of Science and Technology, Tatsunokuchi, Ishikawa 923-1292, Japan; E-mail:
[email protected]
a
b Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-003, Japan
The Monte Carlo and molecular dynamics methods in combination with a continuous electron density representation were applied to the framework modeling of siliceous MCM-41. The X-ray diffraction intensities calculated using a computer simulation technique were strongly influenced by an atomic disordering in the wall structure. Namely, the layer structure in the wall was shown to be fitted better than the amorphous structure. It was also found from the electron density maps that the wall structure of MCM-41 is not uniform, namely a large difference in the charge density exists. This indicates that an understanding of the formation process of MCM-41 is needed for the modeling:
I. INTRODUCTION Mesoporous materials such as MCM-41, MCM-48 and SBA-15 have attracted much attention due to their high surface areas and well-ordered large pore systems [1-4]. Consequently, there are a large number of papers concerning the geometrical structural properties. Although X-ray diffraction(XRD), transmission electron microscopy and adsorption analysis are used for getting the reliable information of the mesoporous structure, a satisfactory structural model of the mesoporous materials is still not constructed. However, only few reports concerning the wall structures of the mesoporous materials have been reported [5, 6], because the atomic array in the wall is considered to be short-range ordering. In order to expand further the high performance applications of the mesoporous materials such as hosts, adsorbents or catalysts, an understanding of the surface property and structure of pore wall as well as the geometrical structure must be needed. Recently, computer simulation techniques such as the molecular dynamics and Monte Carlo methods are employed for modeling of MCM-41 [7-12]. Solovyov et al. proposed the heterogenated surface of the pore wall of MCM-41 using the Rietveld method in combination with a continuous electron density representation [ 13-14]. From such a viewpoint, we have tried to characterize the wall structure of MCM-41 by a new model based on its formation process, and report the preliminary result of MCM-41 modeling in this paper.
70 2. SYNTHESIS OF MCM-41 AND SIMULATION 2.1. Synthesis of MCM-41 Siliceous MCM-41 was prepared following the procedure described in the literature [ 15]. The starting synthesis gel was prepared using colloidal silica, cetyltrimethylammonium bromide (CTAB), sodium hydroxide, aqueous ammonia and water. This synthesis gel with molar composition 6 SiO2 9CTAB 91.5 Na20 90.15 (NH4)20:250 H20 was hydrothermally treated at 97 ~ for 4 days. The synthesis pH was threefold adjusted at 10.2 using dilute acetic acid from time to time. The MCM-41 prepared was calcined at 773 K for 20 h to decompose the surfactant. 2.2. Measurements The synchrotron XRD measurements were carried out at room temperature on a highresolution powder diffractometer at BL 15XU SPring-8 [ 16]. The goniometer equipped long Sollar slit of 1~ aperture to decrease axial divergence. The powder was mounted into a rotating 0.3 mm diameter capillary. Conditions of an measurement were as follows: ~. = 1.549604(3) A calibrated by NIST Si powder, scan range of 0.2 ~ < 20 < 10~ step width: 0.01 ~ counting time per step: 1 s. 29Si MAS NMR spectra were acquired using a Varian VXP-400 spectrometer and zirconia rotors at 79.4 MHz with 2000 pulses and 30s recycle delays. Chemical shifts are given as ppm from external tetramethyl silane. Textural properties were determined at-196 ~ using nitrogen adsorption in a conventional volumetric technique by a Belsorp 28SA spectrometer. 2.3. Simulation method The modeling of MCM-41 was conducted in Monte Carlo (MC) and molecular dynamics (MD) methods. The MC calculations were performed using sorption module in the Cerius2 developed by Molecular Simulation Inc., USA. MC method employs the canonical (NVT) ensemble and the burchart-UNIVERSAL potentials parameter set [17-19] for calculating the interaction between molecules. The all MC calculations performed for 300,000 cycle steps at 20 K. The MD calculations were carried out with the MXDTRICL program developed by Kawamura et al [20]. The Verlet algorithm [21] was used for calculation of the atomic motions, while the Ewald method [22] was applied for the calculation of the electrostatic interactions. Temperature and pressure were controlled by means of scaling the atom velocities and unit cell parameters under three-dimensional periodic boundary conditions. The calculations was performed for 4,000 - 12,000 steps with a time steps of 1.0 • 1015 s at 298.0 K. The two-body central force interatomic potentials, Eq. 1, was used for all MD calculations.
Uu=z, zje2/rjj + fo(b~+bj)exp[(a~+aj-r~j)/(b, +bj)]
(1)
Where Zi is the atomic charge, e is the elementary electronic charge, rij is the interatomic distance, and f0 is a constant. In this equation, the first and second terms refer to Coulomb and repulsive interactions, respectively. The parameters ai and bi represent the size and stiffness of the atoms, respectively, in the repulsive interactions. Table 1 shows the potential parameters employed in this study. The details and effectiveness of the two-body interatomic potential for reproducing the experimentally determined crystal structure of ZSM-5 as well as other conditions of the simulations have been reported elsewhere.
71 Both XRD pattern and 29Si MAS 298i MAS NMR module in Cerius2.
NMR spectra were simulated by the XRD module and
All calculations were performed on Silicon Graphics Indigo2 workstations, while the static visualization was done with the Cerius2 program developed by Molecular Simulation Inc., USA. The dynamic visualization was made, with RYUGA codes developed by Miura [23].
Table 1 Potential parameters of Si and O atoms for MD calculations Atom O Si
Zi -1.2 2.4
ai 1.735 1.012
bi 0.126 0.084
2.4. Calculation of electron density projection Electron density projection was simulated by frg developed by Sasaki. The electron density projection along the c axis is determined by Fourier synthesis in the equatorial plane. Two dimensional structure of the hexagonal system is probably centrosymmetric and has dyad axes (space group :p6mm), where the F(hkO)'s are real and F(hkO) = F(khO). Since the amplitudes(IF01 on a relative scale) are obtained from the intensity data, the projection can be calculated by assigning the phase factor (+ 1 or -1) to them. The electron density projected on the point (x, y) was defined here by the average of the density at (x, y, z) along z axis over a repeat distance c = 10 A (assumed for the standard t~-helix): c
=op(xyz)dz =
F o(hkO)cos{2zc(hx + ky)}
(2)
where V is the unit cell volume.
3. RESULTS AND DISCUSSION 3.1. Modeling of the framework structure of MCM-41 The three kinds of MCM-41 framework models were constructed using MC and MD methods. For the modeling of MCM-41, the dummy atoms arranged in a unit lattice were employed as a micellar rod. The lattice parameters and pore size of the synthesized MCM-41 obtained by XRD and N2 adsorption were used as initial parameters for the modeling. Fig. 1 shows the modeling schemes of MCM-41. The models of (b) and (c) are based on the assumption that the pore wall has a layer structure, whereas the wall structure of the model (a) is amorphous. 400 SiO2 molecules were totally generated in MC calculation. In the case of the model (a), 400 SiO2 molecules were generated simultaneously and uniformly distributed after MC calculation. And the dummy atoms were finally removed. This model was designated as the "random model". In the case of the model (b), three different dummy units with different sizes were constructed for formation of MCM-41 with the layer structure and integrated into the MCM-41 structure, designated as "layer model". The MD calculations were performed to stabilize the MCM-41 structure of the (a) and (b) models. In the case of the model (c), the MCM-41 structure was formed through successive accumulation of dummy models. Before integration, MD calculation of the model unit was performed at each step for stabilization of the model unit. In this way, integration were performed one by one, finally the stabilized MCM-41 model was obtained (phased layer model).
72
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removal of dummy atoms
MC
removal of MC dummy atoms '~ % ~,~ ~ .* L ~ '
9
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~
t ===============_===,==,=~ ~ . + %<>. .,"
Fig. 1 Modeling ofMCM-41 structure. (a) random model, (b) layer model, (c) phased layer model 3.2. X-ray diffraction pattern of the framework structure of MCM-41 The XRD simulations of the three constructed models were performed. Fig. 2 shows the XRD patterns of the constructed models and the synthesized MCM-41 as a reference. The XRD pattern of the synthesized MCM-41 presented a typical four-peak' pattern with a very strong peak at a low 20(d (100)) and three weaker peaks at a higher 20 (d (110), d (200), d (210)). Layer models exhibited a typical XRD pattern with four peaks. However, the peak broadening of the 100 reflection was observed. As it is considered that the intensity of the 100 reflection is very sensitive to the relative contrast between wall and mesopore densities, the observed broadening is probably due to imperfections in pore arrangement than to a finite particle size. On the other hand, in the case of the random model, intensities of the 110 and 210 peaks were relatively week. From the fact that the experimental XRD was
100
1
(a)
110
200
210
b)
~ 9
_=
I
2
t
I
3
I
I
I
I
I
I
4 5 6 2 0 (degree) Fig. 2 Simulated X-ray diffraction patterns of MCM-41. (a) random model, (b) layer model, (c) phased layer model, (d) experiment
73 fitted better to simulated pattern by both layer models than the random model, it found that the peak intensities are influenced by the atomic location in the wall structure. 3.3. 29Si MAS NMR spectra of MCM-41 In order to obtain information about the chemical state of Si in the MCM-41 wall structure, the 29Si MAS NMR spectra were simulated using the phased layer model. Fig. 3 shows 29Si MAS NMR spectra of the synthesized MCM-41 and the phased layer model. In the 29Si MAS NMR spectrum of the synthesized MCM-41, the broad peak a t - - - - l l 0 ppm, assigned to strongly polymerized Q4 Si species, as well as the weak shoulder peak a t - -100 ppm, assigned to Q3 Si species, was observed. In the simulated 29Si MAS NMR spectrum from the phased layer model, the broad peak centered a t - 1l0 ppm was observed. This suggests that there is no difference in the chemical state of Si between the synthesized MCM-41 and the phase layer model. 3.4.
4
-50
Coordination of Si in the framework of
MCM-41 The radial distribution function between Si and O atoms in the MCM-41 wall structure was investigated in order to clarify the difference in the
- 1O0 ppm
Fig. 3 29Si MAS NMR spectra of MCM-41. (a) experiment, (b) phased layer model
/y--
lO 40
(c)
50/
i
- 150
/
0 o
~2 30 0
r(c)
,.Q
~ 2o . ,...(
~
~10
0 1.5
l
1.6
1.7
1.8
1.9
Distance between Si and O (A)
2.0
0
I
1
2
I
I
4
I
6
Distance between Si and O (A)
Fig. 4 Radial distribution function between Si and O Fig. 5 Coordination number of Si. atoms. (a) random model, (b) phased layer model, (a) random model, (b) phased layer (c) MFI type silicalite model, (c) MFI type silicalite
74 coordination state of Si. As shown in Fig. 4, sharp peaks were observed at 1.68 and 1.71 _Ain the radial distribution function for MFI type silicalite. On the other hand, in the radial distribution functions for the random model and the phased layer model, the broad peak was observed around 1.7/~. The bond distances, l(Si-O), evaluated by both models were longer that than from MFI type silicalite. Fig. 5 shows that the coordination number of O surrounding Si atom calculated from the radial distribution function. The coordination number of Si increased with an increase in the Si-O distance. In the l(Si-O) ranging from 1.65 A to 3.0A, the coordination number of Si in MFI structure was almost constant 4, indicating the coordination state of Si in the MFI structure is tetrahedral, while the coordination number for the random model and the phased layer model was not constant. This strongly suggests that the coordination state of Si in the MCM-41 wall structure is not uniform. 3.5. Electron density image of the framework structure of MCM-41 The electron density images were calculated using the integral intensities of Bragg reflections. Fig. 6 shows the electron density maps for the synthesized MCM-41. It is noted that the electron density is not continues. The electron density at the intersections of wall was lower than that at the other parts. Fig. 7 shows that the electron density images calculated using the random model, the layer model and the phased layer model. The wall electron densities for both layer models were not continuous, whereas the image for the random model was continuous. The wall electron densities for both layer models were qualitatively similar to the simulated one using the synthesized MCM-41. The non uniformity of the wall electron density ofMCM-41 have been reported by Solovyov et al. [13-14].
(a)
(b)
(c)
Fig. 6 The electron density projections for synthesized MCM-41. (a) 2-D, (b) 3-D, (c) mosaic
4. Conclusion From all above results, it was found that the peak intensity of XRD pattem derived from the hexagonal structure of MCM-41 is dependent on the modeling method of the wall structure. It was also found that the wall structure ofMCM-41 is not uniform.
75
(a)
(A)
(B)
(C)
Fig. 7 The electron density projections for (A) random model, (B) layer model and (C) phased layer model. (a) 2-D, (b) 3-D, (c) mosaic
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. U. Cisela, F. Schtith, Microporous Mesoporous Mater., 27 (1999) 131. 4. A. Corma, Chem. Rev., 97 (1997) 2373. 5. A. Sayari, Stud. Surf. Sci. Catal., 102 (1996) 1. 6. P. Behrens, Angew. Chem. Int. Ed. Engl., 35 (1996) 515. 7. B.P. Feuston, J. B. Higgins, J. Phys. Chem., 98 (1994) 4459. 8. M.W. Maddox, J. P. Oliver and K. E. Gubbins, Langmuir, 13 (1997) 1737. 9. R.D. Oldroyd, J. M. Thomas, G. Snakar, J. Chem. Soc., Chem. Commun., (1997) 2025. 10. S. Schacht, M. Janicke, F. Schtith, Microporous Mesoporous Mater., 22 (1998) 485. 11. C. A. Koh, T. Montanari, R. I. Nooney, S. F. Tahir, R. E. Westacott, Langmuir, 15 (1999) 6043. 12. R. G. Bell, in 9M. M. J. Treacy, B. K. Marcus, M. E. Bisher, J. B. Higgins (Eds.), Proceeding of the 12th IZC, Baltimore, Materials Research Soc., (1999) 839. 13. L. A. Solovyov, S. D. Kirik, A. N. Shmakov, V. N. Romannikov, Microporous Mesoporous Mater., 44-45 (2001) 17. 14. V. B. Fenelonov, A. Yu. Derevyankin, S. D. Kirik, L. A. Solovyov, A. N. Shmakov, J.-L. Bonardet, A. Gedeon, V. N. Romannikov, Microporous Mesoporous Mater,, 44-45 (2001) 33.
76 15. J. M. Kim, L. H. Kwak, S, Jun, R. Ryoo, J. Phys. Chem., 99 (1995) 16742. 16. T. Ikeda, A. Nisawa, M. Okui, N. Yagi, H. Yoshikawa, S. Fukusima, to be submitted. 17. L.A. Castonguay, A.K. Rappe, J. Am. Chem. Sot., 114 (1992) 5832. 18. A.K. Rappe, K.S. Colwell, Inorg. Chem., 32 (1993) 3438. 19. The Zeoliotes and Aluminophosphates force field of Erik de Vos Burehart, Ph.D. Thesis, (1992) 'Studies on Zeolites: Molecular Mechanics, Framework Stability and Crystal Growth', Table I, Chapter XII. 20. K. Kwamura, in 9Moleucular Dynamics Simulations, eds. F. Yonezawa, Springer, (1990) 88. 21. L. Verlet, Phys. Rev., 98 (1967) 159. 22. P. Ewald, Ann. Phys., 64 (1921) 253. 23. R. Miura, H. Yamano, R. Yamauehi, M. Katagiri, M. Kubo, R. Vetrivel, A. Miyamoto, Catal. Today, 23 (1995) 409.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
77
Pore size adjustment of bimodal mesoporous silica molecular sieves Xiaozhong Wang, ab* Tao
D o u , a Dong
Wu band Bing Zhong b
Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, 030024 China e-mail" wanuxiaozhonu~.tvut, edu.cn
a
...
,....._..
f
bState Key Laboratory of Coal Conversion, Institute of Coal Chemistry, the Chinese Academy of Sciences, Taiyuan, 030001 China Bimodal mesoporous silica (BMS) molecular sieves were synthesized using quaternary ammonium surfactant at room temperature with lower pH values, and their pore sizes were tailored using a simple method by controlling the size of the structure-directing surfactant or incorporating an auxiliary organic solvent such as 1,3,5-trimethylbenzene (TMB) into the reaction systems, which has been used previously to control the pore sizes of MCM-41 mesoporous materials. It was shown that as the surfactant alkyl chain length or the amount of TMB used was increased, the enlargement of the primary mesopore size of BMS materials was accompanied concurrently by the decrease of its secondary mesopoe size, and the degree of the primary mesopore size enlargement (ca. 1.0nm) is far smaller than that of MCM-41 mesoporous materials prepared under similar synthesis conditions, but in contrast the degree of its secondary mesopore size decrease is more sharp (ca.6.4nm). These results may be in connection with the characteristic framework structure of our BMS materials. When the amount of TMB used exceeded a certain degree, the primary mesopore structure of BMS materials was still present but its secondary mesopore structure got collapsed. INTRODUCTION The synthesis of inorganic frameworks with specified and organized pore networks is of potential importance in catalysis[l], separation technology[2] and biomaterials engineering [3]. Since the first synthesis of mesoporous MCM-41 materials[4.5], there has been intense activity in the design and synthesis of a variety of mesoporous solids with different structural features. Such features as pore size, pore size uniformity, interparticle porosity, and stability (thermal and hydrothermal) of these mesoporous molecular sieves were shown to be controlled by a proper choice of synthesis conditions[4-9]. At present, the surfactanttemplated synthetic procedures have been extended to include a wide range of compositions, and a variety Of conditions have been developed for exploiting the structure-directing functions of surfactant. These solids allow fasten diffusion of large organic molecules than the zeolitic and aluminium phosphate-based microporous sieves. These structural characteristics
78 make them potentially useful as catalysts for fluidized catalytic cracking and for the manufacture of fine chemicals. However, the information feedback from the practical industrial process shows that the catalyst used in the large molecules reaction requires a reasonable distribution of two-grade or multi-grade pores, and therefore over a long period of time direct synthesis of inorganic porous materials with two-grade or multi-grade pore distribution is researched for by zeolite chemists. In earlier investigations, we showed that careful controlling alkalinity affords a novel porous materials with well-defined bimodal mesopore size distribution at ambient conditions [11~.! 1]. The materials contain randomly distributed hexagnoal and stripe-like mesoporous channels with uniform pore size and exhibit very large surface areas and pore volumes. The secondary mesopore structure of BMS materials may be formed via the development of incomplete condensation of SiO2species around the adjacent surfactant micelle. The bimodal mesoporous structure of thus formed should be able to be tailored by applying the similar methods with which were used usually in the pore size adjustment of M41S materials. So far, relatively little work has been reported for the pore size mediation of bimodal mesoporous silica mesostructures and no convincing mechanism for such bimodal mesoporous structure formation has been put forward. Undoubtedly, the ability to control framework bimodal mesoporous distribution can be of great value in designing BMS materials as catalysts, ' adsorbents and sensor materials. Accordingly, in the present work we have examined the effect of the structure-directing surfactant size and the addition of auxiliary organic solvent such as 1,3,5-trimethylbenzene on the pore size characteristic framework cross-linking of BMS molecular sieves. At the same time we give a full account of the trend of pore size adjustment of BMS materials and extend on our hitherto proposed formation mechanism. 2. EXPERIMENTAL SECTION
2.1. Synthesis The synthesis procedure for BMS materials was described elsewhere[10.11]. For the purposes of probing the effect of surfactant alkyl chain length and auxiliary organic solvent such as 1,3,5-trimethylbenzene (TMB) on pore size distribution of BMS materials, these samples were prepared typical using CI4H29N(CH3)3Br(C14) and C16H33N(CH3)3Br(C16) as templates. Tetraethylorthosilicate (TEOS) was used as a source of silica, and the pH values of the reaction mixture was adjusted with aqueous ammonia. In each case the reaction mixtures had the following molar composition: 1.0 SiO2:0.2 CI4H29N(CH3)3Br : 0.09 NH3" H20 : 115H20 1.0 SiO2:0.2 CI6H33N(CH3)3Br : 0.3 NH 3"H20 : (0-0.84) TMB : 115H20 The number of molar of ammonia in each reaction mixture was varied relying on surfactants alkyl chain length. When TMB was used as an auxiliary structure director, it was added to the surfactant solution and stirred for 15min before the addition of TEOS. All of the BMS reaction products were washed repeatedly with distilled water in a centrifuger, dried in air at 353K and finally calcined in air at 2K min"~to 823K for 6 h to remove the template.
79 2.2. Characterization
The powder X-ray diffraction pattems (XRD) were recorded using a D/max-2500 powder diffractometer with Cu-K a radiation (40kV, 100mA),0.02~ size and 1 s step time over the range 1~ 0 <8 ~ N2 adsorption isotherms were measures at -196"(2 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 ~ 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 DISCUSSION 3.1. Effect of surfactant alkyl chain length We have prepared BMS materials by the similar assembly procedure using quaternary ammonium surfactants with different chain length as structure director at room temperature. Figure 1 provides the representative X-ray powder diffraction patterns for the as-synthesized and calcined BMS derivatives formed using C~4 and C~6as structure directors. Each sample exhibites a single very low angle reflection, which seems to display less ordering in the mesostructure with a large unit-cell size. However, it can not be simple regarded as an average of pore-pore correlation distance due to the particularity of bimodal mesoporous framework structure of BMS materials. As the surfactant alkyl chain length is increased, the interplanar spacing dl0 0 of BMS samples is also of the trend of shifting gradually toward lower 2 0 angle, which is consistent with the synthesis results of MCM-41 mesoporous materials 8000
A
4000
B
= 4000 E 2000
b
b
a
a
0 0
2
4
6
8
0 10
0
O_Theta
2
4
6
8
10
2-Theta
Figure 1. Powder X-ray diffraction pattems of as-synthesized (A) and calcined (B) BMS samples prepared using C~4 (a) and C~6 (b) surfactant as templates prepared in similar conditions. There are no significant changes upon calcinations, except for the expected increase in XRD peak intensity and lattice contraction due to the higher removal of the contrast-matching surfactant. This result is consistent with the retention of the framework bimodal mesoporous structure upon complete removal of the surfactant from the framework.
80
C14
1200 "7
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800
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0
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0
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r,c]
1200800 C16
_
~
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<
400 i
0
0
i
,
~t
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0.2 0.4 0.6 0.8
1
Relative Pressure (P/Po)
10
100
1000
10000
Pore Size (/~)
Figure 2. N 2 adsorption isotherms and corresponding pore size distribution curves of BMS samples prepared using C~4 and C~6surfactant as templates Figure 2 shows the N 2 adsorption isotherms and corresponding BJH pore size distribution curves for calcined BMS samples. The samples exhibit type IV isotherms as expected for mesopores but with a characteristic hysteresis loop lifted up sharply in the P/Po region of 0.81.0. This may indicate that a change in the texture has occurred on the mesoporous frameworks of the product and suggest the presence of noticeable amount of secondary mesopores, i.e. filling of the framework-confined mesopores occurred at p/po=0.2-0.4 and p/po=0.8-1.0, respectively. This can be confirmed from BJH plots in Figure 2 (right), and two samples all show a well-confined bimodal mesopore distributions. Table 1 summarizes the effect of template sizes on the structural properties of BMS materials. It is especially Table 1. Structural properties of calcined BMS materials prepared using C~4 and C~6 surfactant. Primary mesopore Secondary mesopore SBET(m2g"l) Vs(cm3g"~) Ds(nm) Sample dl00(nm) SBET(m2g"l) Vp(cm3g"l) Dp(nm) 260.1 1.39 24.3 Ci4 4.25 981.3 0.6 2.45 243.1 1.18 18.9 Cl6 4.4 1064.6 0.66 2.6 noteworthy from the results in Figure 2 and Table 1 that the increase of the surfactant chain length causes the adsorption step at the position of p/po--0.2-0.5 to be shifted to higher relative pressure, but at the same time the adsorption step at the position of p/po=0.8-1.0 to be shifted to lower relative pressure. This suggests that an increase in primary framework pore size of BMS materials was accompanied concurrently by a decrease in its secondary framework pore size.
81 3.2. Effect of auxiliary structure director We consider another the effect of adding TMB into the reaction systems on the framework bimodal mesoporous structure of BMS materials. Figure 3 provides the X-ray diffraction patterns for BMS derivatives assembled from C~6 with TMB added as the auxiliary structure director. Structures formed from C~4 showed qualitatively equivalent diffraction features. The XRD patterns of as-synthesized BMS samples all contain a weak, relatively broad reflection at lower 2 0 angle. The qualitative form of the patterns is not affected by the presence of TMB. However, the positions of the intense reflection are dependent by the presence of TMB. It is worth noting that the degree of lattice contraction resulting from the calcination for the samples with TMB/TEOS molar ratio exceeding 0.58 is larger than that of other samples, this may indicate that a part of texture collapse has occurred. This can be confirmed from Figure 4, which provides corresponding N2 adsorption-desorption isotherms and pore size distribution
16000 12000 12000
f
8000
e
4000
C
f e
8000
d "
.......
0 0
C
4000
b
b
2
4
6
2-Theta
a
8
a
0 10
0
2
4
6
8
10
2-Theta
Figure 3. Powder XRD pattems of as-synthesized(A) and calcined(B ) BMS samples prepared using different TMB/TEOS molar ratio: a 0.065; b 0.194; c 0.322; d 0.451; e 0.58; f0.84. curves of BMS samples. Table 2 summarizes the relative structural parameters of series BMS samples. Clearly, the basal spacings represented by the main diffraction line are not correlated directly with the BJH primary pore sizes, even though the dl00 peaks of samples shift gradually to lower 2 0 angle and at the same time the primary mesopore sizes of BMS materials also increase gradually following the increase of TMB used in the reaction systems. The trend of bimodal mesoporous size adjustment resulting from the incorporation of TMB into reaction systems is similar with the results of increasing the surfactant chain length in the synthesis of BMS materials. The role of TMB as an auxiliary structure director on the primary mesopore size of BMS materials is also similar with its effect on the pore size of MCM-41 mesoporous materials, however, the degree of the primary pore size increase (ca. 1.0nm) is far smaller than that of MCM-41 materials. In contrast, it makes the secondary mesopore size of BMS materials contracted obviously (ca.6.4nm). After increasing TMB/TEOS ratio to exceed 0.58, the nitrogen adsorption isotherms of these samples vary significantly, showing that the
82
1200
A
.__J
800 400
I
0 1200 800
I
I
S
400 0
I
1200
I
I
C
m
~0
~D 9
<
800 4O0
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0 1200
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I
"
"
D
800 O
400
1200
E
i
800 400 0 1200 800 400
f
0
I
0
1
I,
0.2 0.4 0.6 0.8
1
Relative Pressure (P/Po)
10
100
1000
10000
Pore Size (A)
Figure 4. N 2 adsorption isotherms and corresponding pore size distribution curves for a series of BMS samples prepared at different TMS/TEOS mole ratio: A. 0.065; B. 0.194; C. 0.332; D. 0.451; E. 0.58; F. 0.84. structure has been affected. The sharp at p/po=0.8-1.0 decrease in nitrogen adsorption volume and shift of the step in nitrogen adsorption to lower P/Po imply that a large number of the secondary mesopores were destroyed and the pore size became smaller.
83 Table 2. Structural parameters for selected calcined BMS samples under study. Primary mesopore Sample A B C D E F
dloo(nm) SBET(mEg1) 4.25 4.56 4.51 5.26 4.55 4.80
1067.8 939.5 1106.9 937.6 1096.9 1138.3
Vp(cm3g"1) 0.69 0.66 0.76 0.71 0.87 0.85
Secondary mesopore
Dp(nm)
SBET(m2g "l) Vs(cm3g"1) Ds(nm)
2.88 3.15 3.27 3.20 3.28 3.50
270.2 260.9 288.8 271.9 \ \
1.05 0.95 1.07 0.87 \ \
17.0 15.8 14.6 12.5 \ \
The further experimental results show that BMS materials can be also synthesized using quaternary ammonium surfactant with alkyl chain length more than Cls, however, the chain length lower than C12 gives not a bimodal mesoporous structure but disordered mesostructure. The synthesis process of all BMS materials was completed in a short time and the viscosity of the reaction mixtures increased with time, and eventually set into jelly-like monoliths. These results suggest that under our synthesis conditions (pH ca.9.5), the relative rate of TEOS hydrolysis and condensation reactions may be almost equal[12], and, hence, the gel point is reached before the precipitation formed. After gelation, the effect of further extension reaction time on the products framework structure is negligible. In such a short reaction time, the surfactant micelles formed in the initial reaction mixtures may possess different size and shape, and the hydrolysis of TEOS and the condensation of the hydrolyzed products are incomplete, and thus the hydrolyzed SiO2 species are not enough to condense around every surfactant micelle. This may make some adjacent surfactant micelle interconnect and lead to the formation of mesopore framework of alternating hexagonal pore and stripe-like pore. The expansive micelles resulting from the increase of surfactant chain length or the addition of TMB into the reaction systems may require more hydrolyzed SiO2 species to ensure the formation of bimodal mesoporous framework structure in the prerequisite for two-grade mesopores expanded simultaneously. Obviously, at constant component concentration and thus hydrolyzed SiO2 species, it is inevitable results that the increase of primary mesopore size is accompanied by the decrease of secondary mesopore size. The further increase of TMB concentration may change the pH values of reaction systems and thus change the relative rate of TEOS hydrolysis and condensation reactions. The right proportion which is benefit for the formation of BMS materials between the hydrolyzed SiO2 species and surfactant micelles is destroyed. This may make the product's framework structure between some adjacent surfactant micelles susceptible to collapse during thermal treatment. As for the fact that the number of molar of ammonia in each reaction mixture was varied relying on surfactants alkyl chain length may be relevant with their different critical micelle concentration. Such a scenario would explain the pore size adjustment features of BMS materials, but further study are needed to fully understand the nature of the observed phenomena.
84 4. CONCLUSION The current study confirms that the methods used usually to control the pore size of MCM-41 mesoporous materials by adjusting the size of structure directing surfactant or the amount of auxiliary organic cosurfactant such as TMB added into the synthesis systems are also suitable for modifying the pore size distribution of BMS mesoporous materials. Because of the characteristic framework mesopore structure of our BMS materials, the increase of the primary framework mesopore size was accompanied simultaneously by the decrease of the secondary framework mesopore size, and the degree of increase of the primary mesopore size is far lower than that of decrease of the secondary mesopore size. These results suggest that the formation of bimodal mesoporous framework structure not only relies on an interaction between SiO2 species and the surfactant micelles, but also on a proportion of the hydrolyzed SiO2 species to surfactant micelles, and the pH adjustment of reaction systems and the relative rate of TEOS hydrolysis and condensation reactions resulting from the pH adjustment play a critical role in the synthesis of BMS molecular sieves. The controllability of bimodal mesoporous framework structure of BMS materials could make them more attractive for the practical applications. 5. ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Grant No.20073029). REFERENCES
1 P.T. Tanev, M. Chibeve and T.J. Pinnavaia, Nature, 368(1994)321. 2 R.M. Barrer, Hydrothermal Chemistry of Zeolites. Academic, London, 1982. 3 G. Guillemin, J.L. Patat, S. Foumie and M. Chetail, J.Biomed.Mater.Res., 21(1989)557. 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 and J.S. Beck, Nature, 359 (1992) 710. 6 Q. Huo, D.I. Margolese, U. Ciesla, P. Feng, T.E. Gier, P. Sieger, R. Leon, P.M. Petroff, F. Shuth and G.D. Stucky, Chem.Mater., 6(1994)1176. 7 M. Kruk, M. Jaroniec and A. Sayari, J.Phys.Chem.B., 101 (1997)583. 8 Q. Huo, D.I. Margolese and G.D. Stucky, Chem.Mater., 8(1996)1147. 9 A. Sayari, P. Liu, M. Kruk and M. Jaroniec, Chem.Mater., 9(1997)2499. 10 X.Z. Wang, T. Dou and Y.Z. Xiao, Chem.Commun., 1998,1035. 11 X.Z. Wang, T. Dou, Y.Z. Xiao and B. Zhong, Studies in Surface Science Catalysis, 135 (2001)199. 12 E.J.A. Pope and J.D. Mackenzie, J.Non-Cryst.Solid., 87(1986) 185.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
85
A l c o t h e r m a l s y n t h e s i s o f large pore, high quality M C M - 4 8 silica Jihong Sun and Marc-Olivier Coppens* Chemical Reaction Engineering, DelftChemTech, Delft University of Technology, Julianalaan 136, 2628BL Delft, The Netherlands
A two-step method to synthesize high-quality MCM-48 with tunable pore size is presented. CTAB is used as a template and various alcohols are used as solvents in the second synthesis step. The samples were studied by means of XRD, SEM, and nitrogen sorption. This new synthesis method allows expanding the pore size from 2.0 nm to 2.5 nm by using ethanol as a solvent during the second synthesis step. The pore size enlargement is accompanied by an improvement of the pore size uniformity and the degree of order of the secondary MCM-48 material. The mechanism by which the pore size expansion occurs is explained in terms of the interactions between the solvent and the hydrophobic core and hydrophilic shell of the micelles formed by the template molecules.
1. INTRODUCTION The discovery of the M41S family of silicas with uniform and ordered mesopores has triggered a considerable research effort to synthesize porous materials with a tailored pore structure and long range structural order. Members of the M41S family are typically prepared via a liquid-crystal templating mechanism in which the templates consist of surfactant molecules ordered in different self-assembled arrays: hexagonal (MCM-41), cubic (MCM-48) or lamellar (MCM-50). MCM-41 contains a hexagonal array of one-dimensional pores; the diameter of its pores can be controlled between about 1.5 to 10 nm [1, 2]. The cubic MCM-48 (Ia3d), on the other hand, contains bi-continuous three-dimensional arrays of pore channels [3-5]. This facilitates molecular diffusion, which is useful in potential applications of MCM-48 in catalysis [6-9], adsorption [10] and separations [11 ], either by itself or as the starting material in the synthesis of multi-structured porous materials [ 12-14]. From this point of view, it is very interesting to synthesize well-ordered MCM-48 with a wide range of pore size.
To whom correspondence should be addressed. E-mail:
[email protected] Tel: +31-15-2784399 Fax: +31-15-2784452
86
However, MCM-48 is particularly difficult to synthesize due to the very sensitive synthesis conditions [ 15]. The conventional synthesis methods for expanding the pore size of MCM-48 proceed via a hydrothermal route, mainly changing the pH of the reaction medium [1, 2, 16], the catalyst [3], the reaction time [17, 18] and the temperature [19, 20]. Because MCM-48 easily looses its cubic structure when boiled in water even for a short time, several methods have been proposed to improve its hydrothermal stability. These include the addition of inorganic or organic salts to the synthesis medium [21-23], and the increase of the pore wall thickness via secondary synthesis [24] or post-synthesis hydrothermal restructuring [25]. It has recently been reported that post-synthesis treatment and tailoring the alkyl chain length of the cationic surfactant by using mixtures of cationic-anionic [27] and cationicneutral surfactants [28, 29] lead to an increased pore size of MCM-48. Meanwhile, gemini surfactants [30-32] added as swelling agents to the synthesis gel were found to favor the formation of MCM-48 with enlarged pores by adjusting the different surfactant chain lengths. Inspired by our earlier work on SBA-15 [33], we report on a new method via an alcothermal route to synthesize MCM-48 with considerably improved long-range structural order and increased pore size.
2. SYNTHESIS AND CHARACTERIZATION
2.1 Synthesis method The synthesis consists of two steps. In the first step, the surfactant cetyltrimethylammonium bromide (CTAB) was added to a mixture of deionized water and ethanol. Aqueous ammonia and tetra-ethyl orthosilicate (TEOS) were subsequently added at room temperature. The composition of the synthesis mixture was as follows: TEOS: CTAB: H20: ethanol: NH4OH = 1:0.4:174:54:12.5 (molar ratio), similar to the procedure described by Schumacher et al. [19]. After stirring the mixture continuously at room temperature, it became a white gel. This gel was filtered using a Bfichner funnel and repeatedly washed with distilled water. In the second step, the white gel was divided into two parts. One part was dried at 120~ for 3 hours, subsequently calcined in air by heating at a rate of l~ to 550~ and maintaining this temperature for 6 hours, leading to the "primary" MCM-48 material. Another part was immersed into a solution containing 5 wt% CTAB in various solvents ("base case": ethanol). The resulting gel was aged for 2 days in an autoclave at 100~ Finally, the product was filtered and washed with distilled water. After drying and calcination in a way similar to the procedure followed with the primary gel, a series of "secondary" MCM-48 materials was formed. 2.2 Characterization Powder X-ray diffraction (XRD) studies were performed using a Philips XRD spectrometer (PW1840) with CuK~l radiation, operating at 40 kV and 50 mA. Nitrogen adsorption/desorption isotherms were measured with a Micromeritics ASAP2000 sorption analyzer, utilizing the Barrett-Joyner-Halenda (BJH) method to evaluate the pore volume and pore size distributions for the desorption portion of the isotherm. Scanning electron
87 microscope (SEM) images were recorded using a Philips XL20. Samples were deposited on a sample holder with an adhesive carbon foil and sputtered with gold.
3. RESULTS AND DISCUSSION 3.1 Synthesis of large pore and high quality MCM-48
500 t
S}f
400
(420)(332)
Sample 2 ,o.o
(400) (321) ] J'l
o,=.,
/
300
o
200
I
1•/
0
2
4
6
8
10
20/~ Figure l XRD patterns of MCM-48
0.0
8.0
f'II
1~176 0
~......
4.0 .~
1.0
10.0 Pore diameter(nm)
t
!
1
1
1
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0) Figure 2 Nitrogen adsorption isotherms and corresponding pore size distribution of MCM-48
The XRD patterns for the calcined samples (Figure 1) show the characteristic cubic order of MCM-48 [1, 2]. The XRD pattern of the secondary material (sample 2), where ethanol was used as the solvent in the second step, shows a significantly increased intensity of the (211) peak, compared to the primary material (sample 1), as well as three higher order (220), (420) and (332) peaks. In addition, there are seldomly observed (321), (400), (422), and (431) peaks [15]. The higher peak intensities indicate that the second step extends the mesoscopic order [ 1, 2]. Meanwhile, the d2II spacing of sample 2 has increased from 3.1 nm to 3.4 nm, suggesting increased unit cell parameters. The nitrogen sorption isotherms and corresponding pore size distributions (Figure 2) indicate that the mesopore size increases from 2.0 nm to 2.5 nm, most likely by swelling of the organic structure-directing entities in the primary material [33]. The sharpness of the nitrogen condensation step (P/P0 = 0.2---0.4) and the well-defined XRD peaks provide strong evidence of the very high quality of this material. The hysteresis loop of sample 2 at a relative pressure P/P0 = 0.45~-0.9 is due to inter-particle porosity. The SEM image in Figure 4a reveals that sample 2 consists of agglomerated, irregularly shaped and partially cross-linked particles, about 0.8-1 pm in size, with macro-pores in between.
88 Table 1 Pore structure parameters of MCM-48 materials aa Sb Sample Solvent (nm) (mE/g) 1 2
Ethanol
7.8 8.3
1100 950
Vc
D d
(cma/g)
(nm)
He (nm)
0.75 0.73
2 2.5
1.5 1.4
a a = dhkl (h2+k2+12)l/2 is a cubic lattice parameter calculated from XRD. b S is the specific surface area obtained from nitrogen adsorption and desorption, c V is the specific pore volume, o D isthe mean pore diameter, e 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 1.4 nm, hardly changed from that in the primary MCM-48 (sample 1: around 1.5 nm). This indicates that the unit cell enlargement can be almost entirely attributed to an increase in pore size. Furthermore, the pore walls of sample 2 are thicker than those of typical high quality MCM-48 (around 0.8---1.0 nm) as reported in the literature [34]. To investigate the hydrothermal stability of the materials, 0.1 g of calcined secondary MCM-48 (sample 2) was boiled for up to 24 (211) hours in 100 ml of distilled water, using a Pyrex flask equipped with reflux condenser. XRD patterns were obtained after filtering and drying each boiled sample in an oven at "~'~ ~ L _ 12 hrs 150 ~ (Figure 3). Loss in XRD peak N 3 hrs intensity, as compared to that of the same sample before the boiling treatment, was used to judge the hydrothermal stability. Figure 3 Ohr shows that even after boiling in water for 24 hours, the XRD pattern of the secondary MCM-48 still shows the higher order (211) and (220) peaks, indicating long-range 0 2 4 6 8 10 mesoscopic ordering. The high hydrothermal 20/~ stability is a result of the thicker pore walls (1.4 nm) than for conventional MCM-48 Figure 3 XRD patterns of sample 2 (0.8-1 nm). However, after hydrothermal after boiling for different times in water. treatment, the XRD peak intensity is a little decreased compared to that without treatment in boiled water. This result suggests that the secondary MCM-48 (sample 2) using ethanol as solvent in the secondary synthesis step is of higher hydrothermal stability than the conventional MCM-48 (sample 1), which easily loses its cubic structure after boiling in water for a short period of time (< 8 hours). Therefore, the pore size enlargement is not accompanied by a loss in hydrothermal stability.
89 3.2 The effect of the type of alcohol on the pore expansion
Figure 4 SEM images of secondary MCM-48 materials, prepared using different solvents in the second synthesis step: a: ethanol; b: 1-hexanol, c: benzene
The XRD and nitrogen sorption results show that ethanol acts as a swelling agent: addition of ethanol in the second synthesis step apparently expands the CTAB liquid crystal within the pores of the MCM-48 primary gel particles. This is similar to what we recently observed for the swelling of the P-123 template in an analogous two-step synthesis of highly ordered SBA-I 5 [33]. Figure 5 shows the XRD patterns of samples 2-5 prepared using alcohols with an increasihgly long alkyl chain as solvents in the second synthesis step. While the diffractogram of sample 2, for which ethanol was used as a solvent, shows 8 intense reflections (also shown in Figure 1), the diffractograms of samples 3-5, using 1-propanol, 1butanol and 1-hexanol as solvent respectively, show a gradual shift of the (211) peak to low angles, a further broadening and weakening of this peak, and a disappearance of all other peaks in the region 28 = 3-10 ~ This indicates that the swelling action of these longer alcohols leads to extensive structural degradation and loss of order. When non-polar benzene is used as a solvent, the template dissolves and the ordered structure of the primary gel collapses: sample 6 exhibits the lowest degree of structural ordering of all samples. The SEM images show that the particle size of sample 4, for which hexanol was used as a solvent, is around 0.6 ktm (Figure 4b), which is smaller than that of sample 2 (Figure 4a), where ethanol was used (0.9 pm). When benzene is used (Figure 4c), the individual particle structure largely disappears, as the particles agglomerate.
90 The above results indicate that, in contrast to ethanol (sample 2), both benzene (sample 6) and alcohols with longer alkyl chains (samples 3-5), are unsuitable solvents in the preparation of large pore MCM-48. The effects of different alcohols on the phase behavior and microstructure I can be explained in terms of polarity and affinity to the different segments of the cubic liquid crystal structure. It is well known that ionic surfactant CTAB dissolved in water can spontaneously associate to form =~ micelles, with the hydrophobic block ~, localized in the micelle core and the hydrophilic block forming a corona (shell) around the core [1, 2]. Since ethanol is polar and completely miscible with water, it may participate 0 2 4 6 8 10 in the formation of the interface between the hydrophobic and the 2 0/o hydrophilic domains [35]. On the other hand, ethanol has more Figure 5 XRD patterns of the secondary MCM-48 tendency than water to penetrate into prepared by using different solvent. the hydrophobic region of the sample 2: ethanol; sample 3: 1-proapnol; micelles, and hereby increase the sample4: l-butanol;sample5: 1-hexanol; volume of the hydrophobic core. sample6:benzene Both the core and the corona of the cubic liquid crystal phase formed during the first synthesis step are swollen in ethanol [36], resulting in an improvement of its degree of order, as evidenced from the XRD patterns in Figure 1. The physical properties of 1-propanol are similar to those of ethanol, while those of 1butanol differ: it is less soluble in water (up to -7.5 wt % soluble in water at 25 ~ [37]. The XRD patterns indeed indicate that the structural order is largely lost (see Figure 5), while the position of the (211) peak is shifted to a lower angle, due to the expansion of the liquid crystal structure with an increased alkyl chain length. This is different from what we observed for SBA-15, where the primary hexagonal order was maintained upon enlarging the pores using a similar treatment [33]. This may be due to the lower stability of the cubic liquid crystal phase [38], even in propanol. l-Hexanol greatly differs in its properties from ethanol and propanol" it is polar but insoluble in water [37]. Because of this, it almost exclusively enters the non-polar hydrophobic domain of the CTAB liquid crystal and expands it, but does not swell the hydrophilic domain. As a result, the cubic phase becomes unstable, and the MCM-48 structure collapses (sample 5). Treatment with benzene as a solvent, being non-polar and immiscible with water, leads to a complete disappearance of the (211) peak in the XRD pattern (see Figure 5: sample 6). The mesopore channels formed in the first synthesis step collapse, dueto a very strong interaction of benzene with the hydrophobic core of the micelles formed by CTAB, which is dissolved. ._
r~
91 This results in a destruction of most of the liquid crystalline structure, so that a disordered amorphous material is formed. As follows from this study, the relation between the CTAB phase behavior and the solvent polarity indicates that the latter has a significant effect on the cubic liquid crystal and its templating action.
4. CONCLUSION It was shown that restructuring of a primary MCM-48 gel using CTAB in ethanol yields a secondary MCM-48 with larger pores and a higher degree of order, while maintaining thick pore walls around 1.4 nm, which increases the hydrothermal stability. From nitrogen sorption, it was determined that the surface area of the secondary MCM-48 materials is around 950 m2/g and the average pore size increases from 2.0 nm to 2.5 nm. The XRD characteristics displayed eight well-resolved diffraction peaks corresponding to the cubic Ia3d structure, when ethanol is used as a solvent during the second synthesis step, which implies that the structure of the secondary MCM-48 is highly ordered. However, the degree of order of the secondary materials sharply decreased with increased carbon number of the alkanol solvent from 2 to 6 carbons. The expansion mechanism of MCM-48 was briefly discussed in terms of the polarity of the alcohol and its affinity to the different segments of the cubic liquid crystal structure formed by CTAB during the first synthesis step. Alcohol may permeate into the hydrophobic core and/or hydrophilic shell of micelles, and thereby induces a structural rearrangement of the liquid crystal phase. Non-polar benzene has the tendency to destroy the liquid crystals, because it is a good solvent for CTAB. Further studies are required to investigate other synthesis parameters such as the acidity, synthesis period and temperature during the second synthesis step. ACKNOWLEDGEMENT We would like to thank P. Boeser for measuring the nitrogen adsorption isotherms.
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3.
4. 5. 6. 7.
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M. Kruk, M. Jaroniec, R. Ryoo, J. M. Kim, Chem. Mater., 11 (1999) 2568. R.H. Hall, C. E. Fowler, B. Lebeau, S. Mann, Chem. Commun., (1999) 201. C.M. Bambrough, R. C. T. Slade, R. T. Williams, S. L. Burkett, S. D. Sims, S. Mann, J. Colloid Interface Sci., 201 (1998) 220. C. Thoelen, K. van de Walle, I. F. J. Vankelecom, P. A. Jacobs, Chem. Commun., (1999) 1841. R. Ryoo, S. H. Joo, S. Jun, J. Phys. Chem. B, 103 (1999) 7743. J. Lee, S. Yoon, T. Hyeon, S. M. Oh, K. B. Kim, Chem. Commun., (1999) 2177. R. Ryoo, S. H. Joo, S. Jun, T. Tsubakiyama, O. Terasaki, Stud. Surf. Sci. Catal., 135 (2001) 07-0-01. A. Sayari, J. Am. Chem. Soc., 122 (2000) 6504. J. Xu, Z. Luan, H. He, W. Zhou, L. Kevan, Chem. Mater., 10 (1998) 3690. J. C. Vartuli, K. D. Schmitt, C. T. Kresge, W. J. Roth, M. E. Leonowicz, S. B. McCullen, S. D. Hellring, J. S. Beck, J. L. Schlenker, D. H. Olson, E. W. Sheppard, Chem. Mater., 6 (1994) 2317. M.L. Pena, Q. Kan, A. Corma, F. Rey, Micro. Meso. Mater., 44-45 (2001) 9. K. Schumacher, M. Grun, K. K. Unger, Micro. Mater., 27 (1999) 201. L.Z. Wang, J. L. Shi, J. Yu, W. H. Zhang, D. S. Yan, Mater. Lett., 45 (2000) 273. R. Ryoo, S. Jun, J. Phys. Chem. B, l01 (1997) 317. J.M. Kim, S. Jun, R. Ryoo, J. Phys. Chem. B, 103 (1999) 6200. D. Das, C.-M. Tsai, S. Cheng, Chem. Commun., (1999) 473. R. Mokaya, J. Phys. Chem. B, 103 (1999) 10204. S. Jun, J. M. Kim, R. Ryoo, Y.-S. Ahn, M.-H. Han, Micro. Meso. Mater., 41 (2000) ll9. F. Chen, F. Song, Q. Li, Micro. Meso. Mater., 29 (1999) 305. R. Ryoo, S. H. Joo, J. M. Kim,, J. Phys. Chem. B, 103 (1999) 7435. M. Kruk, M. Jaroniec, Chem. Mater., 12 (2000) 1414. P. van der Voort, M. Mathieu, F. Mees, E. F. Vansant, J. Phys. Chem. B, 102 (1998) 8847. M. Benjelloun, P. van der Voort, P. Cool, O. Collart, E. F. Vansant, Phys. Chem. Chem. Phys., 3 (2001) 127. M. Mathieu, E. van Bavel, P. van der Voort, E. F. Vansant, Stud. Surf. Sci. Catal., 135 (2001) 06-0-04. M. Morey, S. O'Brien, S. Schwarz, G. D. Stucky, Chem. Mater., 12 (2000) 898. J.H. Sun, J. A. Moulijn, J. C. Jansen, T. Maschmeyer, M.-O. Coppens, Adv. Mater., 13 (2001) 327. K. Schumacher, P. I. Ravikovtch, A. D. Chesne, A. V. Neimark, K. K. Unger, Langrnuir, 16 (2000) 4648. R. Ivanova, P. Alexandridis, B. Lindman, Colloids and Surfaces A., 183-185(2001), 41. Q. Huo, D. I. Margolese, G. D. Stucky, Chem. Mater., 8 (1996) 1147. D.R. Lide (ed), Handbook of chemistry and physics (81st), CRC Press, 2000. P. Alexandridis, U. Olsson, B. Lindman, Langmuir, 14 (1998) 2627.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
93
Studies o f M C M - 4 1 obtained from different sources o f silica Icaro S. Paulino and Ulf Schuchardt* Instituto de Quimica, Universidade Estadual de Campinas, P.O. Box 6154, 13083-970 Campinas-SP, Brazil.
An efficient and simple method has been used to synthesize MCM-41 at room temperature under stirring. The mesoporous materials were obtained using three different silica sources, tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), tetrapropoxysilane (TPOS), ammonium hydroxide and cetyltrimethylammonium bromide (CTMABr) as template, in various reaction times The route of the synthesis was monitored by X-ray diffraction (XRD) which showed that a highly ordered mesoporous material was obtained after just 15 min. The effect of the silica source on the properties of the MCM-41 samples obtained is also discussed wing nitrogen sorption and X-ray diffraction
1. INTRODUCTION The synthesis of MCM-41 type mesoporous materials has been the subject of intensive research in the last years. Since the discovery of the M41S family of mesoporous materials in the beginning of the 90s [1-3] intensive studies have been conducted by various research groups with the objective to understand the synthesis mechanism as well as the role of synthesis (time, pH, temperature, surfactant/silica ratio, choice of the surfactant kind and morphofogy control) on the formation of the mesoporous structure. MCM-41 exhibits a hexagonal array of one-dimensional mesopores, whose diameters can be tailored form 1.5 to 10 nm with a narrow pore-size distribution. The pore walls are composed of amorphous silica, as indicated by X-ray diffraction measurements. The structure of this material is shown schematically in Figure 1.
Figure 1. Schematic representation of the hexagonal array of one-dimensional mesopores of MCM-41.
94 The discovery of this new class of molecular sieves allowed the development of new heterogeneous catalysts. Besides the easy access to the channels, MCM-41 shows the possibility of isomorphic exchange in the structure, leading to the formation of materials with acid, basic or redox properties. The incorporation of various metals into the MCM-41 framework has granted them catalytic properties, allowing their use in hydroxylation and dehydrogenation of.aromatic compounds [4,5], epoxidation of unsaturated fatty esters [6] and oxidation of bulky organic molecules with aqueous H202 or tert-butyl hydroperoxide (TBHP) as oxidant [7,8]. Due to the open structures and large pore-size, together with their adsorption properties, MCM-41 has been used as support of metallocenes to generate catalysts for heterogeneous phase polymerization [9,10]. The physicochemical properties of MCM-41, as pore size distribution, hydrothermal stability, pore arrangement and silanol content depend very much on synthesis methods, which allow the preparation of a wide range of materials with different pore sizes, surface areas and numbers of peaks in XRD experiments [ 11 ]. Differences in preparation methods also may result in materials with different wall structures, but some of them show structural instability. In this work we report a simple and reproducible synthesis of MCM-41 type mesoporous materials via an efficient and rapid method, at room temperature, using three different silica sources, tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), tetrapropoxysilane (TPOS). The effect of the silica source upon properties of the MCM-41 samples are also discussed using nitrogen sorption and X-ray diffraction.
2. EXPERIMENTAL
2.1. Synthesis Tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), tetrapropoxysilane (TPOS), cetyltrimethylammonium bromide (CTMABr) were purchased from Aldrich and aqueous ammonia (25 wt. %) was obtained from Baker. All chemicals were used asreceived. Mesoporous molecular sieves were synthesized at room temperature from a mixture of appropriate quantities of CTMABr added as a solution in aqueous NHaOH. The silica source, TMOS, TEOS or TPOS, was then added to obtain a final mixture with the following molar composition: 1 Si : 33 NHaOH : 330 1-120:0.12 CTMABr. The mixture was kept under stirring for 15 min up to 20 h. The obtained solid was separated by filtration, dried at 373 K and finally calcined at 813 K for 4 h under a flow of nitrogen and subsequently for 6 h under a flow of synthetic air (heating rate 1 K minl).
2.2. Characterization 2.2.1. Powder X-ray diffraction (XRD) The X-ray diffractograms were recorded on a Shimadzu XRD6000 diffractometer using Cu-Kct radiation generated under 40 kV, 30mA. Diffraction data were recorded between 1.5 ~ and 10~ (20) at a interval of 0.02 ~ with a scanning rate of 1~ min 1. The slits used were 0.5 ~ 0.5 ~ 3 mm for divergence, scattering, reception, respectively.
2.2.2. Infrared Spectroscopy (IR) The FT I.R. spectra were obtained on a Perkin Elmer 1600 M-80 spectrometer between 400 and 4000 cm ~ in the transmission mode, using KBr pellets. Sixteen scans were accumulated at a 4 cm 1 resolution.
95
2.2.3. Thermogravimetric Analysis (TGA) The thermogravimetric analyses were performed under a nitrogen flow of 100 mL min l on a DuPont 951 TGA thermobalance between 293 and 1,273 K, using a heating rate of 10K mln . 9
-|
2.2.4. Pore-structure Analysis (ASAP) Adsorption and desorption isotherms for nitrogen were obtained at 77 K using a Micromeritics analyzer ASAP 2010. The samples were degassed at 423 K for 12 h before measurements were performed. Specific surface area values were obtained using the BET (Brunauer-Emmett-Teller) equation 9
2.2.5. Electron Microscopy (SEM) Scanning electron microscopy was performed on a DSM940A Zeiss microscope or on a JSM-T300 Jeol microscope, under an accelerating voltage of 20 kV.
3. RESULTS AND DISCUSSION The FT-IR spectra of the MCM-41 materials show bands between 1400 and 450 cm -~ due to fundamental vibrations of the framework [ 12], a band in-1650 cm l relative the O-H bond deformation. The spectra also presented a large band around 3000 cm l, relative to O-H stretching, which impedes the visualization of the characteristic bands of silanol groups [13]. We also observe the bands at 1489, 2844 and 2916 cm -1 due to the surfactant, which disappears upon calcination. The thermogravimetric analysis reveals a weight loss of about 12 % that is due to water desorption (< 200 ~ and silanol groups condensation (> 200 ~ The characteristic structural element of MCM-41 is a hexagonal array of the pores. The powder X-ray diffraction (XRD) patterns obtained from the non-calcined MCM-41 is typical of a well ordered structure and shows four Bragg peaks at low reflection angles between 2 and 7~ (20) that can be indexed to a hexagonal lattice as (100), (110), (200) and (210). The XRD patterns indicate that there are no crystalline phases present, because no reflections at higher angles are observed. However, as suggested by Cheng et al. [ 14], the term crystalline can be used only with respect to the periodic array of channels that represent the single element of order in the material. We performed some studies varying the reaction time to verify the structural development of MCM-41 with time. In studies of reaction times, Ortlam et al. [ 15] compared the temporal development of the X-ray reflection intensities of as-synthesized and calcined MCM-41 materials. They observed that the hexagonal structure of MCM-41 may be formed after reaction times of about lh at 104~ and that 90% of the hexagonal channel arrangement is formed after 5-6 h, whereas the fully developed mesopore structure requires 72 h. The XRD patterns obtained in this work from non-calcined MCM-41 recorded after different times are shown in Figure 2. We can observe that the structural arrangement of MCM-41 is formed after 15 min and maintained with prolonged reaction times. A rapid hydrolysis process of TMOS can explain this behavior TEOS and TPOS, assisted by the controlled pH and an efficient condensation under these synthesis conditions. However, these materials had demonstrated a very poor thermal stability, since the structure collapsed during the calcination process 9 Only crystallization times higher than 2 h allow the formation of MCM-41 that presented high thermal stability. Another feature of these diffractograms is the increasing of peaks intensity with reaction time and their shift to lower 20 values, indicating an growth of the unit cell.
96
However, when the MCM-41 samples were submitted to the calcination process, the peak intensities of the XRD patterns decrease and the 20 positions shift to a higher value indicating a contraction of the lattice. This process is caused by the removal of the surfactant from the channels, and subsequent condensation of silanol groups on the walls. We can observe the disappearance of peak (210); this is probably due to a reduction of the order of the structure. The XRD patterns for the MCM-41 as-calcined are shown in Figure 3. The observed 20 angles, the relative Miller indices (hkl) and the unit cell parameter ao calculated by linear regression of equation (1) are listed in Table 1. ao2= 4d2(h 2 + k 2 + hk)/3
(1)
(100)
20h 2h lh 0.5h 0.25 h
;
'
~
'
~
'
;
'
~
'
1'0'
1'2
2O Figure 2. XRD patterns of the as-synthesized samples with different reaction times, using TPOS as silica source.
TPOS TEOS
0)
TMOS
;
'
~
'
~
'
;
'
~
'
1'0
2O Figure 3. XRD pattems of the calcined MCM-41 using TMOS, TEOS and TPOS as silica source.
97
Table 1 X-ray data of MCM-41 synthesized with TMOS, TEOS or TPOS as silica sources. Source of silica TMOS
TMOS-calc.
TEOS
TEOS-calc
TPOS
TPOS-cal
hkl
Observed 20 (o)
dhkl (nm)
100 110 200 210 100 110 200 210 100 110 200 210 100 110 200 210 100 110 200 210 100 110 200 210
2.26 3.86 4.46 5.86 2.50 4.28 4.96
3.91 2.29 1.98 1.54 3.53 2.05 1.78
2.20 3.80 4.38 5.80 2.44 4.22 4.86
4.01 2.32 2.02 1.52 3.62 2.09 1.82
2.16 3.76 4.34 5.72 2.42 4.18 4.84
4.09 2.35 2.03 1.54 3.65 2.11 1.83
Unit cell parameter (XRD) ao (nm) 4.50
4.08
4.63
4.17
4.71
4.21
It is important to observe that an increase of alkoxy group shifts of the 20 positions to smaller values, and consequently lager unit cell parameters are obtained. After the calcination process of the samples an expected contraction of 9 % (Aao = 0.4 - 0.5 nm) of the unit cell parameter was observed. This unit cell contraction is near to that observed by Cai et al. [ 16] in similar synthesis conditions. Nitrogen adsorption measurements were carried out for the calcined samples as another method to confirm the highly ordered MCM-41 structure. The adsorption and desorption isotherms of nitrogen of each sample show the typical type IV isotherm, as defined by the International Union of Pure and Applied Chemistry (IUPAC) [ 17]. However, the isotherm obtained for MCM-41 synthesized using TMOS as silica source can be considered type I, since it presents very small inflection characteristics of capillary condensation processes. The nitrogen adsorption isotherms obtained at 77 K for calcined MCM-41 are shown in Figure 4. We observe three stages of nitrogen adsorption and desorpfion in the isotherms. The adsorptions at very low relative pressure, p/po, correspond to monolayer-multilayer adsorption on the pore walls with no pressure transitions and no inflection points indicating complete absence of micropores [ 18]. The increase of the absorbed volume at low pressures is followed by a steep with an inflection point at intermediate relative pressures, which is due to capillary condensation inside the mesopores. The last stage is a plateau at high relative pressures
98 associated with multilayer adsorption on the extemal surface of the crystals [19]. This indicates that the mesoporous were completely filled. The fact that inflections of the isotherm (TPOS) are sharper indicates that the MCM-41 synthesized with TPOS presentes a narrow pore size distribution. This can be related to the fact that TPOS is hydrolyzed more slowly in comparison to TEOS and mainly to TMOS. 700 600
TPOS ,.0~9~0~4--0~- 0 --0--0--0 - - 0 - 0 ~ 0
500 E
~
TEOS
400 300-
< 200"6 > 100-
~~"
~
c)_..~o
o
TMOS /o--9--9--2~-P~' +
-I-
T
[__~ Adsorption DesorptionI
0
0,0
'012'014 0',6 ' 0',8 RelativePressure(P/Po)
'
1,0
Figure 4. Nitrogen adsorption isotherms obtained at 77 K of MCM-41 calcined using TMOS, TEOS and TPOS as silica source. An important characteristic of nitrogen adsorption is that the specific pore volume, specific surface area and average pore diameter become larger with increasing carbon chain length of the alkoxy group as shown in Table 2. The MCM-41 synthesized using TMOS as silica source, presents the smallest specific pore volume, specific surface area and average pore diameter. On the other hand, the MCM-41 synthesized using TPOS as silica source, presents the largest specific pore volume, specific surface area and average pore diameter. The Brunauer-Emmett-Teller (BET) method [20] was used to calculate the specific surface area. Considering that the MCM-41 channels are cylindric, the diameters D of the mesoporous were calculated following the equation 2 given by Gurvitsch method: D4v/S = 4 Vmes/SBET
(2)
where Vines is the mesoporous volume estimated from the N2 adsorption isotherm and Sser is the BET surface area. The most used method to calculate the pore size distribution is based on the BarrettJoiner-Halenda (BJH) model [21 ]. However, we did not apply this model to calculate average pore diameter due to an underestimation of the pore diameter. This is occurred due to the instability of the liquid nitrogen meniscus inside the mesopores. The wall thickness of the MCM-41 was calculated by the difference between unit cell parameters ao, determined by X-ray diffraction, and the pore diameter (eq. 2) obtained by equation 3. e:ao-D
(3)
99 The values of wall thickness are in agreement with the literature, except for MCM-41 synthesized with TMOS that presented e = 1.83 nm. However, this value is only approximation, since there is no type IV isotherm. We therefore must be cautious with the values of e obtained using different analysis techniques. Same of the properties of the calcined MCM-41 samples (prepared by the use of different tetraalkoxisilane) obtained by nitrogen sorption, X-ray diffraction and density analysis are shown in Table 2. Table 2 Properties of the MCM-41 samples as-calcined prepared by use of different tetraalkoxisilane obtained by nitrogen sorption and X-ray diffraction. Source of silica
Specific pore volume
Specific surfacea r e a
c m 3 g-i
m 2 g-i
0.39 0.72 0.84
685 1002 1127
TMOS TEOS TPOS
Average pore diameter(N2sorption) n m 2.25 2.86 2.99
Wall thickness (nm)
Density (g cm_3)
1.83 1.31 1.22
2.00 1.96 1.81
Scanning electron microscopy was used to determine the particle size, particle morphology and the particle size distribution of the synthesized MCM-41. The particle size of all samples range from 0.5 gm to 2.0 gm with an average size of 1.1 pan. While the particles of MCM-41 synthesized with TEOS and TPOS presents a morphology more defined, the MCM-41 synthesized with TMOS presents fused particles. This can to be attributed to the fast hydrolysis process of TMOS, which may cause the particle coalescence. The scanning electron micrographs are shown in Figure 5.
(a)
(b)
(c)
Figure 5. Scanning electron micrographs of MCM-41 as-calcined using (a) TMOS, (b) TEOS and (c) TPOS as silica source.
4. CONCLUSIONS In this work we report the synthesis of MCM-41 type mesoporous materials via an efficient and rapid method at room temperature using the three different silica sources, TMOS, TEOS and TPOS. We observe that the structural arrangement of MCM-41 is formed after 15 min, but only after crystallization times of more than 2 h MCM-41 with high thermal stability is formed. We observe that with the the increase of chain alkoxy group a higher unit cell parameter, specific surface area, specific pore volume and average pore diameter is obtained.
100 Particularly, it appears clear that the use of TPOS produces an improvement in the structure of MCM-41. Because of these characteristics, the MCM-41 is an excellent support for various 9 catalysts, where are used in transesterification, oxidation and polymerization reactions of olefins, in our research group.
5. ACKNOWLEDGEMENTS The authors thank FAPESP and CNPq for financial support for this work (grant number 99/02649-5) and Profs. Mafia do Carmo Gongalves and Heloise de Oliveira Pastore for assistance.
REFERENCE
1. T. Yanagisawa, T. Shimizu, K. Kuroda e C. Kato, Bull. Chem. Soc. Jpn., 63 (1990) 988. 2. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.P. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins e J.L. Schlenker; J. Am. Chem. Soc. 114 (1992) 10834. 3. C.T. Kresge, M.E. Leonowicz, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. 4. C. Lee, W. J. Lee, Y. K. Park, S. Park, Catalysis Today, 61 (2000) 137. 5. S. Wong, H. Lin, C. Mou, Applied Catalysis A: General, 198 (2000) 103. 6. M.A. Camblor, A. Corma, P. Esteve, A. Martinez and S. Valencia, Chem. Commun., 795 (1997). 7. W.A. Carvalho, P. B. Varaldo, M. Walau and U. Schuchardt, Zeolites, 18 (1997) 408. 8. W.A. Carvalho, M. Walau and U. Schuchardt, J. Mol. Catal. A, 144 (1999) 91. 9. Y.S. Ko, T. K. Han, J. W. Park e S. I. Woo, Macromol. Rapid Commun., 17(1996) 749. 10. I. S. Paulino, A. P. de Oliveira Filho, J. L. de Souza and U. Schuchardt, Stud. Surf. Sci. Catal., 130 (2000) 929. 11. S. Biz and M. L. Occelli, Catal. R e v . - Sci. Eng., 40 (1998) 329. 12. G. Cent, S. Parathoner, F. Trifir6 A. Aboukais, C. F. Aissi and M. Guelton, J. Phys. Chem., 96 (1992) 2617. 13. M. D. Alba, Z. Luan and J. Klinowski, J. Phys. Chem., 100 (1996) 2178. 14. C-F. Cheng, W.Zhou, D. H. Park, J. Klinowski, M. Hargreaves and L. F. Gladden, J. Chem. Soc., Faraday Trans. 93 (1997) 359. 15. A. Ortlam, J. Rathousky, G. Schulz-Ekloff and A. Zukal, Microporous Mater., 6 (1996) 171. 16. Q Cai, W-Y Lin, F-S Xiao, W-Q Pang, X-H Chen and B-S Zou, Microporous Mesoporous Mater. 32 (1999) 1. 17. K. S. W. Sing, D. H. Everett, R. A. Haul, L. Moscou, R. A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure Appl. Chem., 57 (1985) 603. 18. S. Storck, H. Bretinger and W. F. Maier, Appl. Cat. A: General, 174 (1998) 137. 19. P. J. Branton, P. G. Hall and K. S. W. Sing, J. Chem. Soc., Chem. Commun., 1257 (1993). 20. S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc., 59 (1937) 1553. 21. E. P. Barrett, L. G. Joyner and P. P. Halenda, J. Am. Chem. Soc., 61 (1951) 373.
b t U d l e s i n b u r t a c e :::iclence a n a t s a t a l y s l s 1/41
A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
Synthesis and characterization of hexagonal hydrothermal restructuring method
101
mesoporous
materials
using
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
Pure siliceous MCM-41 samples were prepared by the usual hydrothermal synthesis method and also by the hydrothermal restructuring method. The hydrothermal restructuring procedure was carried out by the pH control and the rehydrothermal treatment. The restructuring method gave almost 90% of yield of calcined Si-MCM-41 on the basis of the weight of silica in the reaction mixture. All the samples were characterized by using X-ray diffraction (XRD), TEM, and N2 physisorption. The XRD patterns of all the samples exhibited the well-defined reflections and the (100) reflection of the restructured samples showed no shift after calcination because the wall of Si-MCM-41 was densely packed by the restructuring procedure. During the restructuring procedure, the values of d-spacing and unit cell parameter of Si-MCM-41 were increased. TEM analysis revealed that the restructured sample has a highly ordered hexagonal array. According to the N2 physisorption results, the restructured samples possessed a small pore size compared with that of the sample without being treated. Both the pH control and the rehydrothermal treatment have exercised influences on the structure of Si-MCM-41.
1. INTRODUCTION Mesoporous materials (M41S) have been synthesized by researchers at Mobil in recent years [1]. These materials consist of three different types of structure; hexagonal arrayed structure of MCM-41, 3D arrayed structure of MCM-48 and lamellar arrayed structure of MCM-50. These mesoporous materials exhibit unique characteristic properties. First, they possess a uniform pore size in the nano range (3-10 nm). Secondly, it is easy to control the pore size by using alkyl chain structure directing agents of different lengths [1 ] or micelle swelling agent (like trimethylbenzene) [2]. Mesoporous molecular sieves have also attracted much attention because of their unique properties [1]. Since the discovery of mesoporous materials, these materials have been applied as catalyst supports, adsorbents, column materials for separation, and hosts for large molecules [3]. The research interest in this field has been focused on their synthesis mechanism, development of synthesis procedure such as morphology control, enhancement of stability, characterization, synthesis of new materials based on MCM-41 synthesis concept, and technical applications [1,3-5]. Among them, the development of synthesis method is considered to be one of the most important subjects.
102 The aim of this work is to improve the textural properties of Si-MCM-41. To achieve this goal, we have developed a hydrothermal synthesis using pH control and rehydrothermal treatment and investigated the effect of hydrothermal restructuring method by applying various analysis techniques. As a result, it is found that both the rehydrothermal treatment and forced pH control result in an increase in the wall thickness. Therefore, the hydrothermal restructuring method is proven effective to improve the structural properties of mesoporous materials.
2. EXPERIMENTAL
2.1 Synthesis The Si-MCM-41 sample was prepared by using the usual hydrothermal synthesis method at 383 K. A sodium silica solution was prepared by combining aqueous NaOH solution with Ludox HS-40 (SiO2 40 wt% colloidal silica in water, Dupont). The resulting mixture was heated under stirring until clear. The template solution was then prepared with distilled water and cetyltrimethylammonium bromide (CTMABr) at 303K in an isothermal water bath. The sodium silicate was slowly added to the template solution under vigorous stirring. The composition of the resultant gel was SiO2 : 0.25 CTMABr : 0.7 Na20 : 60 H20. The gel obtained was stirred at room temperature for 1 h. The CTMA-silicate mixture was heated in an autoclave reactor without stirring to 383 K for 24 h. The precipitated product was hotfiltered, washed with distilled water and dried in an oven at 373 K overnight. The product was calcined in air at 823 K for 5 h by using a muffle furnace.
2.2. Hydrothermai restructuring method The restructured Si-MCM-41 samples were prepared with SiO2 : 0.25 CTMABr : 0.7 Na20 : 60 H20 by following the same mixing procedure as before and the hydrothermal reaction proceeded for 1 day. The value of pH was monitored by a digital pH meter. The reaction mixture was then cooled to room temperature, and the pH of the mixture was adjusted to -8 using strong acidic solution under vigorous stirring for more than 1 h. The resultant gel became homogeneous white solution. After stirring for lh, the pH of the mixture was adjusted to -10 using thick NaOH solution under vigorous stirring for 1 h and distilled water of 1'0 % by volume was added under vigorous stirring. This mixture was heated again at 383 K for 24 h. The pH adjustment and subsequent heating were repeated two more times. Finally, the solid product was hot-filtered, washed and dried in an oven at 373 K for overnight. The product was calcined in air at 823 K for 5 h by using a muffle furnace. 2.3. Characterization The yield of Si-MCM-41 is defined by the ratio of the weight of pure silica (SiO2) phase to the total weight of SiO2 in the reaction mixture. The weight of Si-MCM-41 was measured after calcination process. The phase identification of the solids were performed by using Xray diffractometer (Rigaku, D/MAX-II A) equipped with an Ni-filtered monochromatic Cu Ka (;~=1.54056 A) radiation from a tube at 30 kV and 40 mA. The morphology of the samples was examined by TEM (Jeol model JEM-2000EXII). The specific surface area and the average pore diameter were determined by nitrogen physisorption with the BET method at the liquid nitrogen temperature using a Micrometrics ASAP 2010 automatic analyzer.
103
Usual Synthesis Procedure ..
,"*
:
I
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
] #
.....................................................................................
] "**,
Cooling to room temperature, pH control with acid to about 8
1 "
IL :
~
9
Stirred at room temperature for 1 h
ID4'I, i l l l l l l i
:
~i pH control with NaOH solution and addition of water
,millER
~]
:"
:...: :.......:....:.::......:....w :.:E.: :: :.: :...
- ..................... Kept in a convection oven maintained at 383 K lEniN
: 9
IL
: ...: :.: :..:..: :.:.....:: :
]
....................
:
:.
pH control
ii l l l l l l l l l l l l l l , I l l i l l
i
~ rehydrothermal " treatment
Ilillll,llllIIIlllllllllllllllllllllllllll*
Filtration, washing and drying at 373 K for 24 h i
.....Calcination at823 K in air for 5 h ..... Fig. 1. Procedure for the hydrothermal restructuring method
3. RESULTS AND DISCUSSION The yield of Si-MCM-41 was calculated by the weight of calcined sample divided by the amount of SiO2 in the reaction mixture. As the restructuring procedure was repeatedly carried out, the yield of Si-MCM-41 was increased. In case of the usual hydrothermal synthesis, the product yield was about 65 %. The unreacted silica could be actually observed in the filtration solution. In case of the restructured samples, however, the yield of product reached the level of above 90 %. Such a high yield is attributed to the forced pH control which brings about a shift in the reaction equilibrium. It is well recognized that Si-MCM-41 is synthesized in basic medium. In this study, the usual synthesis procedure was carried out under basic condition with a pH value of about 11 [1,4]. The pH of the mother liquor, however, was adjusted to -8 by adding strong acidic solution. When the pH is-8, the solid product disappeared from the reaction mixture and the reaction gel became sticky. This phenomenon caused by the change in pH was similarly observed in the sol-gel process. This viscous solution was not maintained long and turned soft. This indicates that the forced pH control brought about a shift in the reaction equilibrium and promoted the progress of Si-MCM-41 synthesis reaction. XRD patterns of the Si-MCM-41 samples synthesized in the present work are presented in Fig. 2. The patterns for all Si-MCM-41 samples consist of three distinguishable peaks, which can be indexed to different (hkl) reflections of a hexagonal structure [1]. The XRD patterns in parts (c) and (d) of Fig. 2 consist of one very intense line, three weak lines, and one very
104 weak line, which can be indexed to (100), (110), (200), (210) and (300) diffraction lines, respectively, and these represent the characteristics of the hexagonal structure of MCM-41. Figure 2 (a) shows the XRD lines for the Si-MCM-41 samples which were obtained after heating the initial reactant gel mixture in an autoclave at 383 K for 24 h. Here one can clearly observe a strong XRD line broadening and lattice contraction after calcination. These changes are similar with those reported previously by other research groups [6]. In general, the XRD patterns of mesoporous materials shift to the region of higher angle and concurrently the value of d-spacing of mesoporous materials decreases after calcination. This phenomenon is caused by desorption of water, condensation of silica, and loss of structural uniformity. However, the lattice contraction and peak broadening in the samples synthesized by applying the restructuring method were negligible or did not occur at all. The presence of a (300) diffraction line for restructured Si-MCM-41 samples indicates that the high structural uniformity was maintained after calcination. Therefore, it is evident that the restructured SiMCM-41 sample has a high structural stability and the silica species would be completely condensed during the course of restructuring.
(a)
(b)
A~
~ 2
/
._~
calcination
4
6
8
10
2
4
2
4
2O
(c)
6
8
6
8
2O
10
(d)
% cr
#. 2
4
6
20
8
10
20
Fig. 2. XRD patterns of Si-MCM-41 samples: (a) Si-MCM-41 before applying the restructuring procedure; (b) Si-MCM-41 after applying the restructuring procedure once; (c) Si-MCM-41 after applying the restructuring procedure twice; (d) Si-MCM-41 after applying the restructuring procedure three times 9 The as-synthesized samples were washed with distilled water and dried in oven at 373 K, and the calcination was performed in air at 823 K.
105 Figure 3 shows the pore size distributions for the mesoporous samples obtained by the nitrogen adsorption isotherm at liquid nitrogen temperature using the Barrett-Joyner-Halenda (BJH) analysis [1,2,4]. Type IV isotherm, typical of mesoporous materials, is observed and as the relative pressure increases (P/P0>0.25), each isotherm exhibits a sharp inflection, characteristic of capillary condensation within the mesoporous [ 1]. This feature indicates that both samples possess a good structural ordering and a narrow pore size distribution, and also that there has been neither structural nor phase change during restructuring. Furthermore, it is noticed that the restructuring treatment results in a shift in the pore size distribution to the region of lower pore size.
(a)
./'
600
~0
~ 500 400
o 200
ni)i null ,
0.0
"U--n__l__l--l--l--I--l--l--l--I i
0.2
,
i
0.4
Relative
(b)
,
Pressure
i
,
0.6
i
0.8
i
i
i
,
1.0
i ,I
100
(P/Po)
Pore diameter (,h)
600
~
~o 5o0
.~
~
p ~ - i
9
9
I
ii
400
300
i
200
lO0
~n-i-n- 9149
o , 0.o
i
0.2
,
l
i
0.4 Relative
I
i
0.6 Pressure
(P/Po)
i
0.8
,
i
i
1.0
10
,
,
1
,
,
i
| il
,
100 Pore
diameter
i
i
,
,
, ,
(A)
Fig. 3. Nitrogen sorption isotherms and pore size distributions of calcined Si-MCM-41 samples" (a) Si-MCM-41 before applying the restructuring procedure, (b) Si-MCM-41 after applying the restructuring procedure three times. The results of TEM imaging of Si-MCM-41 are shown in Fig. 4. Part (a) shows the image taken in the direction perpendicular to the pores and part (b) the image of viewing down the pore axis. The images along these directions have often been used to identify the hexagonal MCM-41 type phases. These images of Si-MCM-41 revealed a highly mesoporous structure consisting of cylindrical pores arranged on a hexagonal lattice.
106 (a)
(b)
Fig. 4. TEM images of the calcined sample after applying the restructuring procedure three times. The view directions are along (a) the (100) direction and (b) the pore axis. Table 1 presents the structural data of Si-MCM-41 samples. The surface areas of both samples are larger than 800 m2/g, being typical of MCM-41 materials. As given in Table 1, the interplanar spacing dl00 for the calcined parent sample, which did not undergo any restructuring procedure, is 33.82 A and this value ig typical for MCM-41 silicates synthesized using cetyltrimethylammonium bromide as a templating surfactant [ 1, 2]. For the restructured sample which is synthesized with the same reactant composition, the increase in d-spacing was larger than that for the sample synthesized by the usual hydrothermal method. In the present work, the value of 2 0 was reduced by 0.47 while the d-spacing was increased by 7.43 A. This result is related to the shift of XRD patterns to the left-hand side. Of particular relevance to the present study is the pore wall thickness of the Si-MCM-41 materials; the average pore diameter and the pore wall thickness calculated by using the BJH method are given in Table 1. In case of the restructured sample, desorption and adsorption average pore size was decreased whereas the wall thickness was increased. Concerning the textural properties, the pore diameter increases in general if the XRD line shifts to the left. When surfactants of different alkyl chain lengths and different micelle swelling agents are used for the synthesis of Si-MCM-41, the samples synthesized would have different pore diameters [1,2]. As the pore size increases, the (100) reflection shifts to the left (i.e., the value of 2 0 decreases) and the d-spacing increases. The textural data from XRD register an increase after the hydrothermal restructuring treatment. The nitrogen sorption data, however, shows that the pore size was decreased after the restructuring treatment. This was certainly caused by the forced pH control and the subsequent hydrothermal treatment. During the step of pH control, the unreacted silica species in mother liquid are dissolved and during the reheating process, these dissolved silica species take part in the MCM-41 synthesis reaction again.
107 Table 1. Structural properties of Si-MCM-41. Si-MCM-41: usual systhesis method
Si-MCM-41: restructuring procedure three times
Surface area (m2/g)
955
878
20
2.61
2.14
d- value (dl00;A)
33.82
41.25
Unit cell parameter (a0A)
39.05
47.63
BJHdes average pore diameter (A)
32.2
29.8
BJHdes wall thickness(A)
6.85
17.83
BJHads average pore diameter (A) BJHads wall thickness (,~)
33.9
33.6
5.15
14.03
ao = the lattice parameter, from the XRD data using the formula ao = 2dlo o~f3, wall thickness = ao - pore diameter
4. CONCLUSIONS The Si-MCM-41 prepared by the usual hydrothermal synthesis method has been treated by the restructuring method, which consists of the forced pH control step and the subsequent rehydrothermal treatment step. It is found that the restructuring treatment developed in this study can substantially improve not only the yield but also the quality of Si-MCM-41. Indeed, the XRD patterns and the nitrogen sorption data of the treated samples present textural properties different from those of the parent sample. The lattice contraction and peak broadening was negligible or disappeared after calcination. The values of both d-spacing and unit cell parameter are increased and the pore size is decreased. These results were indirect evidence for the effectiveness of hydrothermal restructuring method. The hydrothermal restructuring method gives rise to an improvement in textural properties of mesoporous materials, which is achieved by the condensation of silica species within the pore wall, leading to an increase in the pore wall thickness. In the rehydrothermal procedure, it is evident that the time for the hydrothermal crystallization is extended and the pore. wall is strengthened into a thicker condensed silica frame as a result of the increase in the amount of dissolved silica species during the pH control step. In brief, the forced pH control brings about a shift in the equilibrium of Si-MCM-41 synthesis reaction through the additional dissolution of unreacted silica species. On the other hand, the reheating process promotes the condensation of silica. Therefore, it is obvious that the two steps in the restructuring procedure have a synergistic effect for the increase in the wall thickness.
108 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 also partially by the Brain Korea 21 Program of the Ministry of Education.
REFERENCES 1. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359, (i997) 710., J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, C. T-W Chu, D. H. Olsen, E. W. Sheppard, S. B. McCullen and J. L. Schlenker, J. Am. Chem. Soc., 114, (1992) 10834., C. T. Kresge, M. E. Leonowicz, W. J. Roth and J. C. Vartuli (Mobil Oil Corp.), U.S. Patent 5098684, (1992). 2. J. S. Beck, U.S. Patent 5057296, (1991)., N. Ulagappan and C. N. R. Rao, J. Chem. Soc. Chem. Commun. (1996) 2759. 3. N. Ulagappan and C. N. R. Rao, J. Ame. Chem. Soc., 116, (1996) 10785., R. Burch, N, Cruise, D. Gleeson and S. C. Tsang, J. Chem. Soc. Chem. Commun., (1996) 951., T. M. Abdel-Fattah and T. J. Pinnavaia, J. Chem. Soc. Chem. Commun., (1996) 665., J. Chui, Y. Yue, Y. Sun, W. Dong and Z. Gao, Stud. Surf. Sci. Catal., Vol. 105, (1997) 69., U. Junges, W. Jacobs, I. Voigt-Martin, B. Krutzsch and E Schuth, J. Chem. Soc. Chem. Commun., (1995) 2283., K. R. Kloestra and H. Van Bekkum Stud. Surf. Sci. Catal., Vol. 105, (1997) 431. 4. J. S. Beck, J.C. Vartuli, G. J. Kennedy, C.T. Kresge, W. J. Roth and S. E. Schramm, Chem. Mater., 6, (1994) 1816., A. Monnier, E Schuth, Q. Huo, D. Kumar, D. Kumar, D. Margolese, R. S. Maxwell, G. D. Stucky, M. Krishnamurty, E Petroff, A. Firouzi, M. Janicke and B. E Chmelka, Science, 261, (1993) 1299., G. D. Stucky, A. Monnier, E Schuth, Q. Huo, D. Margolese, D. Kumar, M. Krishamurty, E Petroff, A. Firouzi, M. Janicke and B. F. Chmeka, Mol. Cryst. Liq. Cryst., 240, (1994) 187., A. Firouzi, F. Atef, A. G. Oertli, G. D. Stucky and B. F. Chmelka, J. Am. Chem. Soc., 119, (1997) 3596. 5. N. Coustel, F. D. Renzo and F. Fajula, J. Chem. Soc. Chem. Commun. (1994) 967., D. Khushalani, A. Kuperman, G. A. Ozin, K.Tanaka, J. Garces, M. M. Olken and N. Coombs, AdV. Mater. 7, (1995) 842., A. Sayari, P. Liu, M. Kruk and M. Jaroniec, M. Chem. Mater. 9, (1997) 2499. 6. C. Y. Chen. S. L. Burkett, H.-X. Li and M. E. Davis, Microporous Mater., 2, (1993) 27.
Studies in Surface Science and Catalysis 141 A. Sayariand M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
109
Synthesis o f highly ordered mesoporous compounds with control o f m o r p h o l o g y using a non-ioni~z surfactant as template A. L6onard #, J.L. Blin and B.-L. Su* Laboratoire de Chimie des Mat6riaux Inorganiques, ISIS, The University of Namur, 61, rue de Bruxelles, B-5000 Namur, Belgium phone : +32-81-72-45-31, Fax: +32-81-72-54-14, e-mail :
[email protected] Highly ordered hexagonal mesostructures (CMI-1 compounds) can be obtained under mild acidic conditions by working at low concentrations of non-ionic decaoxyethylene cetyl ether [C16(EO)i0]. The present work shows that it is possible to gain control at the nanometer scale over the packing symmetry of the channels, as well as at the micrometer level over the morphology of the particles by varying the surfactant / silica molar ratio and the hydrothermal treatment conditions. Very high loadings of silica precursors typically afford highly ordered hexagonal CMI-1 compounds whereas an increase of the surfactant / silica molar ratio results in materials with a more disordered channel array. In a parallel way, very low molar ratios of surfactant / silica lead to ropes, gyroids and toroids whereas spheres are the most stable shape with the lower quantities of silica. From this point, it appears thus that not only the structure but also the morphologies encountered for MCM-41 type mesoporous silica can be reproduced with a non-ionic templating agent. 1. INTRODUCTION A more environmental-friendly way to prepare large-pore mesoporous materials consists in using polyoxyethylene alkyl ether surfactants as templates because of their lower toxicity and good biodegradability [ 1-6] with respect to their ionic analogues like for example cetyltrimethylammonium bromide, the template generally used in the preparation of MCM-41 [7,8]. Besides, it appears that the recovery of the template is easier and so a further reutilization could be envisaged. Until short ago, the use of these surfactants afforded only disordered wormhole-like structures unless working in very strong acidic media [2] or adding transition metallic cations to the micellar solution [9]. Another way to proceed was to remove the methanol released from the hydrolysis of TMOS by using a rotary evaporator like proposed by Attard et al.[10]. We however recently showed that it was possible to obtain directly highly ordered hexagonal structures of channels (CMI-1 compounds) under mild acidic conditions by working at low concentrations of decaoxyethylene cetyl ether [Cl6(EO)10] [11]. These materials possess very uniformly-sized openings, specific surface areas exceeding 900 mVg and consist of spheres with 1-2 ~tm diameter. A LCT-type cooperative mechanism was proposed to explain the formation of these molecular sieves. It is important to control the structure of the materials. Indeed, if they are to be applied in catalysis, the 3-dimensional structure of MSU is most appropriate whereas the # :FRIA fellow *: Corresponding author
110 production of low-branched polyethylene fibres [12] or the fabrication of semi-conducting wires [13] would require a regular array of long straight channels. Besides this, the morphology turns out to be crucial also. Indeed, spherical particles are the most suitable in chromatographic applications [14]. This work shows that it is possible to gain control over these two aforementioned factors by adjusting the Cl6(EO)10 / silica molar ratio and the hydrothermal treatment conditions. Different characterization techniques (SEM, TEM, XRD and nitrogen adsorption-desorption analysis) have been used to shed light on the morphological, structural, and textural features of the prepared CMI-1 compounds. 2. EXPERIMENTAL 2.1. Synthesis
A 10 wt.% micellar solution was prepared by dissolving 6.67 g of decaoxyethylene cetyl ether [Cl6(EO)10, Brij 56 | in 60 ml water. Sulfuric acid was then added to decrease pH to a value of 2. After homogenization at 70~ TMOS was added dropwise, the quantity depending on the desired surfactant / silica molar ratio (from 0.25 to 3.50). After further stirring during 1 hour, the synthesis gel was poured in teflon-lined cartridges sealed in stainless steel autoclaves. Hydrothermal treatment was performed at 40, 60 and 80~ during 3, 2 and 1 days respectively. The recovered gel was then extracted using a Soxhlet apparatus, dried under vacuum at ca. 60~ and calcined at 550~ under nitrogen and oxygen. 60 ml bidistilled (
water
~
6.67 g C16(EO)lo, [ (Bri~6| y
H2804
Micellar solution (pH 2, 70~ Gel Hydrothermal treatment at 40, 60 and 80~ during 3, 2 and 1 day(s) I
surfactant / TMOS molar ratio = R = 0.25-3.50
Ethanol extraction Drying[ Synthesis scheme of Calcination at 550~
I=:~1 Powders [
orderedcMi.1 materialsmeS~176176
2.2. Characterization
Information about structure was obtained by X-ray diffraction measurements with a Siemens D-5000 diffractometer and transmission electron microscopy was performed on a Philips Techna'f microscope with an acceleration voltage of 100kV. The powdery samples were embedded in an epoxy resin and sectioned with an ultramicrotome before being
111 deposited on carbon, coated copper grids. The textural properties of our compounds were assessed by nitrogen adsorption-desorption measurements. Analysis took place over a wide range of relative pressures on a Micromeritics ASAP 2010 or Tristar 3000. The pore diameter and the pore size distribution were determined by the BJH method [15] although it is well known that this method gives underestimated pore size values and that some new interesting methods have been developed recently by Jaroniec et al. [ 16]. However, this will not affect our systematic comparison as the same method was used for all of the experimental results. Morphological features have been investigated with the use of a Philips XL-20 scanning electron microscope. For conductivity purposes and in order to enhance the yield of secondary electrons, powders were first covered by a thin layer of gold by metallization. 3. RESULTS AND DISCUSSION 3.1. Information about structure 3.1.1. Determination of the arrangement by XRD
Only the lowest surfactant / silica molar ratios (R) have been investigated by XRD measurements because of the very small quantity of materials obtained as the molar ratio increases. As the walls of the mesoporous compounds are amorphous, the quality of the materials will be reflected by the regular repetition of the pore to pore distance which is characterized by a very strong feature at low angles. If the packing symmetry of the channels is regular in space, secondary reflections will appear on the diffractograms. For example, in the case of MCM-41, besides the sharp 100 peak, additional 110, 200, 210,...features will be visible on the diffractogram. In that case, it is possible to determine the cell parameter a0 = 2d~00/3 ~r2,which represents the sum of the pore diameter and the wall thickness. 1 day at 80~
2 days at 60~
3 days at 40~
'II- 5.3 nm
4 5.7 nm
5.7 nm
V••'l •-
I
R = 1.50
//1~0 7 9 nm
t~ 3.1nm
,,
~-" 5.3 nm
.==
~-5.4 nm
~1-- 5.5 nm
.t=t
R = 0.50
3.2nm
3.8nm ',~ l~2.7nm ~ _
_
2
_
/ 3.1nm
. . . . ,
4
.
.
.
6
.
20 (o)
.
-
8
10 2
.
.
.
.
20 (o)
.
.
8
lo
2
.
.
4
.
:
6
20 (o)
.
:
8
.
10
Figure 1" XRD patterns of samples prepared at different hydrothermal treatment conditions and with 2 surfactant / silica molar ratios (R) The multi-peak pattern characteristic of hexagonal materials can be clearly evidenced for the sample prepared at 80~ with a surfactant / silica molar ratio of 0.50. Using the Bragg law to calculate the d-spacings, the unit cell parameter can be determined to be equal to 6.2 nm. Increasing the surfactant / silica molar ratio does not influence the aspect of the diffractograms. However, if hydrothermal treatment is performed at 60~ the secondary
112 reflections are less well resolved and their intensity drops with the amount of added silica. If R exceeds 1.00, no secondary peaks can be evidenced any more, suggesting the appearance of a disordered network like MSU materials. At 40~ the 110 and 200 reflections drop in intensity with increasing R suggesting the appearance of a less ordered channel array. The hydrothermal treatment conditions as well as the surfactant / silica molar ratio (R) do not seem to have an effect on the unit cell parameter which remains between 6.1 and 6.8 nm. However, lower amounts of added silica have a strong influence on the structure of the samples. As R augments, the regular structure is progressively lost and materials are more likely to belong to the family of MSU rather than CMI-1. 3.1.2. TEM observations
Figure 2 shows the TEM pictures of the compounds that were obtained under different conditions of hydrothermal treatment and at molar ratios ( R ) o f 0.50 and 1.50. The molar ratios that were studied ranged from 0.25 to 3.50. From a general point of view, it appears that the compounds are well ordered if the R value remains below 1.50. The hexagonal "honeycomb-like" arrangement of the channels characteristic of CMI-1 is clearly visible on the TEM micro graphs . The inserted Fourier Transforms show hexagonally dispersed spots, confirming the hexagonal packing symmetry that was suggested from XRD measurements. However, if the surfactant / silica molar ratio increases beyond this value, the regular empilement is progressively lost, more rapidly at 60 than at 80 or 40~ Even if arranged channels can still be found on the grid, these zones become much more sparse when the amount of added silica decreases. This phenomenon is the most amplified at 60~ It appears thus that there is coexistence between regular CMi~-1 material and disordered MSU-type for the higher molar ratios. I day at 80~
2 days at 60~
3 days at 40~
R = 1.50
R = 0.50
Figure 2" TEM pictures and inserted Fast Fourier Transforms of samples prepared at different hydrothermal treatment conditions and with 2 surfactant / silica molar ratios (R) In all cases, the compounds evolve towards wormhole-like disordered structures as the added silica becomes less. Interestingly, the materials are not as well ordered at 60 than at 80 or 40~ whatever the value of R is, according to the results obtained by XRD and TEM. From all of these observations, it can be postulated that the amount of added inorganics plays a key role in the organization of our materials. Indeed, for low surfactant / silica molar ratios, i.e. for high contents of tetramethoxysilane, all the isolated micelles are covered by a shell of inorganic material and these supramolecular entities can self-assemble in
113 order to form the regular structure through a cooperative mechanism in agreement with our previous results [11 ]. The higher the ratio or the lower the silica content, the less regular the organization. One could imagine that there are not enough silica oligomers present in solution to perform a complete condensation of all these silica rods. In this case, the regular hexagonal packing symmetry, though still present at some places, does not prevail over the whole extend of the material. Places of disordered wormhole-like materials are then formed when the channels move one from each other in order to form a continuous silica framework. The first regular organization of CMI-1 materials was reported for samples prepared at a surfactant / silica molar ratio (R) of 1.50. Above results show that a very regular array of channels can also be obtained at a R value as low as 0.50 and so, the higher concentration of methanol released in this case (1.30 mol/1 compared to 0.44 mol/1 for the original preparation of CMI-1) seems not to disturb the formation of the regular structure. (Since it was previously reported that the threefold larger amount of released methanol could play a role of liquid crystal breaker [ 1,10]) Present observations strongly confirm our previous proposition that it is the interaction between the hydrolyzed silicic species and the hydrophilic heads of the surfactant that will determine the final structural geometry of the pores.
3.2. Nitrogen adsorption-desorption analysis Figure 3 depicts the isotherms and the pore size distributions (inserts) of materials obtained for R values of 2.50 and 0.50 for different hydrothermal treatment conditions. At 80~ all the isotherms are type IV, characteristic of mesoporous compounds. The capillary condensation step locates at around p/p0 = 0.54, whatever the molar ratio R is, suggesting constant pore diameters. This step however seems to be more steep for the lower values of R suggesting a better homogeneity in the pore sizes. This is verified on the pore size distributions which are very narrow and centered at 4.2 + 0.4 nm. For all of the amounts of added silica, the specific surface areas of mesoporous compounds are very high (Table 1). Table 1 : Textural and structural features of the samples as a function of surfactant / silica molar ratio (R) and hydrothermal conditions. Hydrothermal treatment
SBET(m2/g) 0.50 2.50 0.50 2.50
920 830 1182 2 days at 60~ 616 o15o 1096 3 days at 40~ [ 2.50 913 n.d. "no data, - "not observed, * 9from XRD data. 1 day at 80~
. ,
Pore diameter (nm) 4.1 4.3 3.4 3.8 3.8
Cell parameter ao (nm)* 6.2 n.d. 6.4 n.d. 6.1 n.d.
If treatment is performed at 60~ isotherms are type IV only for the molar ratios below 1.5 and get a shape located between type I and IV for the higher ratios, characteristic of supermicroporous compounds. If the surfactant / silica molar ratio varies from 1.50 to 3.50, the maximum of the pore size distributions passes from 3.4 to a value less than 2.0 nm (Fig.3). This shift toward supermicroporosity could be explained by a rearrangement of the micellar solution. Indeed, polymerization at 60~ is not as extended as at 80~ and so, to maximize interactions between silica species, a rearrangement could occur in the synthesis gel forming smaller micelles leading to a continuous silica network with shrinked openings. This phenomenon is accompanied by a loss of the regular ordering of the channels. As already
114 discussed in literature, the characteristics of the final framework result from an interplay between inorganic and organic species present in solution [1,11]. Evolution at 40~ is a bit more particular in the sense that secondary mesoporosity appears for the molar ratios that exceed 0.50. Indeed, isotherms are type IV for the high loadings of added silica whereas the adsorbed volume at high relative pressures strongly I day at 80~
800 o.o~ oo
R = 2.50 ~ 600 ~400
40011
2 days at 60~
3 days at 40~
0.03
1 oo!
3o0 o0
1000
0o,
200 800
600
800
200 /-
200 1/ "
200.
0.0 0.2 0.4 0.6 0.8 1.0
Relative pressure p/po
0.0 0.2 0.4 0.6
0.8 1.0
Relative pressure P/Po
0.0 0.2 0.4 0.6 0.8 1.0
Relative pressure p/Po
Figure 3 :Nitrogen adsorption isotherms and inserted pore size distributions of samples prepared at different treatment conditions for 2 surfactant / silica molar ratios (R). increases when there are less silica species in solution. The higher the R value, the more secondary mesoporosity appears. The extend of polymerization increases with temperature. For example, the masses of powdery materials obtained at 40, 60 and 80~ for R being equal to 0.25 are 1.30, 1.82 and 2.00 g respectively. For the higher surfactant / silica molar ratios at 40~ a rearrangement of the micellar solution like observed at 60~ would not be the best way to obtain maximum polymerization and instead of this, holes would remain in the structure. This hypothesis could explain the appearance of secondary mesoporosity and the constant pore diameters at 40~ Indeed, the maximum of the pore size distributions remains practically constant (3.5-3.8 nm). Nevertheless, rearrangements of the channels are likely to occur leading to more disordered wormhole-like structures. This explains the less good organization for the higher surfactant / silica molar ratios.
3.3. Morphological features The morphologies of the samples prepared at different surfactant / silica molar ratios and at variable conditions of hydrothermal treatment are shown in Figure 4. At 80~ if the loading of added silica is very high (R = 0.25), the majority of the sample is made of hexagonal-shaped ropes with a length of several microns. For a R value of 0.50, the toroids and gyroids prevail and when the surfactant / silica molar ratio is equal to 1.00, we can observe a mixture of toroids and spheres. Beyond this value, only spheres are present. There is thus a clear evolution from ropes to gyroids and finally towards spheres as the surfactant / silica molar ratio increases. These peculiar morphologies have already been encountered by Ozin and his coworkers who used an ionic templating agent and TEOS as inorganic source [17,18]. The syntheses were carried out at room temperature or 80~ and the preparations were done in a
115 quiescent state as agitation led to the same morphologies, but with more broken forms. They proposed that a silicate liquid crystal embryo with a hexagonal cross-section evolves into several morphologies with degrees of curvature that depend on the initial reaction conditions. 1 day at 80~
2 days at 60~
3 days a t 4 0 o c
R = 2.00
R = 0.50
R = 0.25
Figure 4 : SEM pictures of samples prepared at 80, 60 and 40~ and with surfactant / silica molar ratios of 0.25, 0.50 and 2.00. A lower acidity or an increase in temperature favour the preparation of spheres rather than gyroids [19,20]. They pointed out that higher acidic quiescent conditions afford rapid growth and polymerization of a silicate liquid crystal seed where polymerization induces local rigidification effects that dictate the curvature. In this case, there is a smooth and continuous deposition of silicate-micellar species on specific regions of the liquid crystal seed, which results in the formation of gyroids. When pH value is increased or when the syntheses are performed at 80~ there is a slower global silicification and the curvature results from surface tension forces. The slower polymerization at lower acidity makes thus surface tension the overriding shape-controlling factor and spheres minimize surface area and surface free energy. In our present study, pH value remained constant throughout all of the syntheses, but only the surfactant / silica molar ratio as well as the hydrothermal treatment conditions were changed. However, for the lower molar ratios, there are a lot of silica oligomers present in solution. So, we could deduce that, the more silica present in solution, the more polymerization will induce local rigidification effects, resulting in the specific local deposition of silica species affording ropes or gyroids. When the amount of inorganics present in solution is progressively decreased, i.e. for the higher molar ratios, the growth process will not be controlled by the polymerization any more. The more preferential mechanism will be the minimization of the surface tension at the surfactant / silica interface and thus the most stable resulting shape is the sphere. A similar evolution is observed at 60~ although the morphologies are not as well defined as at 80~ Spheres are the only morphology that exists at higher values of R. At 40~ the evolution of morphology is a bit more particular. For high loadings of tetramethoxysilane, toroids and ropes can be detected, just like at 60 and 80~ When the ratio is increased beyond 1.00 however, the characteristic morphologies can still be found but
116 have no smooth surfaces any more but rather a more broken appearance. At a ratio of 2.00, the surfaces of the particles become more and more broken and the shapes, though still suggesting the toroidal and gyroidal morphologies, start to be more stochastic. These observations are consistent with the results of the adsorption-desorption measurements. The fact that the particles are not as smooth any more on their surfaces is coherent with the appearance of secondary mesoporosity as the surfactant / silica molar ratio is increased. The more spongy appearance of the samples preiaared at 40~ comes from a less advanced polymerization at lower temperatures and, above all, at higher surfactant / silica molar ratios. 4. CONCLUSION The influence of the amount of added silica on the internal as well as external morphologies of mesoporous compounds has been evidenced. From a general point of view, the packing symmetry of the channels tends towards a wormhole-like one if less silica species are present in solution. A high concentration of TMOS typically leads to "exotic" morphologies already encountered for MCM-41 type materials. Also, hydrothermal treatment conditions have a drastic influence on structure, texture and morphology. ACKNOWLEDGEMENTS
This work has been performed within the framework of PAI/IUAP 4-10. Alexandre L6onard thanks FNRS (Fonds National de la Recherche Scientifique) for a FRIA scholarship. REFERENCES
1. G.S. Attard, J.C. Glyde and C.G. GSltner, Nature, 378 (1995) 366. 2. D. Zhao, Q. Huo, J. Feng, B.F. Chmelka and G.D. Stucky, Jr. Am. Chem. Soc., 120 (1998) 6024. 3. E. Prouzet and T.J. Pinnavaia, Angew.Chem.Int. Ed. Engl., 36(5) (1997) 516. 4. E. Prouzet, F. Cot, G. Nabias, A. Larbot, P. Kooyman and T.J. Pinnavaia, Chem., Mater., 11 (1999) 1498-1503. 5. S.A. Bagshaw and T.J. Pinnavaia, Angew. Chem.Int. Ed. Engl., 35(10) (1996) 1102. 6. P.T. Tanev and T.J. Pinnavaia, Science, 267 (1995) 865. 7. 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. Schenker, J. Am. Chem. Soc., 114 (1992) 10834. 8. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. 9. W. Zhang, B. Glomski, T.R. Pauly and T.J. Pinnavaia, Chem.Commun.,(1999) 1803. 10. N.R.B. Coleman and G.S. Attard, Microp. and Mesoporous Mater., 44-45 (2001) 73-80. 11. J.L. Blin, A. L6onard and B.L. Su, Chem. Mater. 13(10) (2001) 3542. 12. K. Kageyama, J.I. Tamazawa and T. Aida, Science, 285 (1999), 2113. 13. C.G. Wu and T. Bein, Chem. Mater., 6 (1994) 1109. 14. C. Boissi6re, A. van der Lee, A. E1 Mansouri, A. Larbot and E. Prouzet, Chem. Commun., (1999) 2047. 15. E.P. Barret, L.G. Joyner and P.P. Halenda, J. Am. Chem. Soc., 73 (1951) 373. 16. M. Jaroniec, M. Kruk and A. Sayari, Stud. Surf Sci. Catal., 129 (2000) 587. 17. H. Yang, N. Coombs and G.A. Ozin, Nature 386 (1997) 692. 18. G.A. Ozin, H. Yang, I. Sokolov and N. Coombs, Adv. Mater. 9(8) (1997) 662. 19. G.A. Ozin, C.T. Kresge and H. Yang, Stud. Surf. Sci. Catal. 117 (1998) 119. 20. H. Yang, G. Vovk, N. Coombs, I. Sokolov and G.A. Ozin, Jr. Mater. Chem., 8(3) (1998) 743.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
117
Towards a better understanding on the mechanism o f mesoporous formation via an assembly o f Cn(EO)m and T M O S J.L. Blin, A. L6onard #, G. Herrier #, G. Philippin and B.-L. Su* Laboratoire de Chimie des Mat6riaux Inorganiques, ISIS, The University of Namur (FUNDP), 61, rue de Bruxelles, B-5000 Namur, Belgium The present work deals with a systematic study of mesoporous materials synthesis. A series of polyoxyethylene alkyl ether surfactants such as Cl3(EO)n (n = 6, 12, 18), C16(EO)10, CI8(EO)I0 have been used. It is revealed that the surfactant conformation changed with the heating temperature. Indeed, at higher temperatures, a more extended molecular conformation can be obtained, which leads to materials with larger pore sizes. We have also shown that the interaction between template and silica disturbs the hexagonal array of micelles in solution leading to the formation of DWM-1 or DWM-2 compounds for concentrated micellar solution. We have also correlated the structural characteristics of the recovered mesoporous molecular sieves with the VH/VL ratio of the template.
1. INTRODUCTION Owing to their large internal surface area, open three dimensional structure, and adjustable chemical properties, microporous materials such as zeolites have widespreadly been used in chemical and petrochemical industry. However, because of their limited pore sizes, the treatment of more bulky molecules requires new solids able to provide catalysts or adsorbents with larger openings. The synthesis efforts have culminated in 1992 when Mobil researchers reported the preparation of several mesoporous silicates with unique pore structures [1, 2]. Tunable openings have been obtained by using cationic surfactants with variable hydrocarbon tail lengths as templating agents and by adding some auxiliaries like, for instance, organic swelling molecules. The synthesis of mesoporous molecular sieves consists of the condensation and polymerization of an inorganic precursor around the micelles of surfactant. 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, 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~ ~pathway, in which hydrogen bondings are responsible for the cohesiveness between the non-ionic recoverable and biodegradable surfactant (N~ and the inorganic precursor (I~ The first syntheses of mesoporous molecular sieves achieved by using such non-ionic polyoxyethylene alkyl ether [Cm(EO),] surfactants were reported by Attard et al. [8] with octaethylene glycol monododecyl ether [CI2(EOs)] and octaethylene glycol monohexadecyl ether [Cm6(EOs)]. However, the regular mesoporous obtained by this group [9] was only owing to the gentle removal of the large amount of methanol released from the hydrolysis of TMOS used as silica precursor. Indeed they found # : FRIA Fellow * : Corresponding author
118 that methanol played a role of liquid crystal breaker destroying the hexagonal H1 phase formed by the surfactant molecules in aqueous solution. Nevertheless, recently, via a new pathway [(N~ ~ which involves the formation of hydrogen bonds between a cationic metal (M "+= Li+, Co 2+, Mn2+, and Zn2+) complex form of a non-ionic polyoxyethylene surfactant (N ~ and the neutral inorganic precursor (I~ Pinnavaia et a/. [ 10] have successfully oriented the structure of the final silica compounds working at a very low concentration of around 1.8 wt.% in neutral media. Cubic SBA-11 and hexagonal SBA-!2 were obtained using CI6(EO)10 and Cls(EO)10 respectively at a weight percentage of 4-6, the syntheses being performed at room temperature in strong acidic media (pH << 1) [6]. Recently, it was reported that disordered wormhole-like mesoporous silicas, zirconium oxides [11] and titanium oxides [ 12] could be synthesized with neutral surfactants and two types of disordered wormhole-like mesostructures (DWM-1 and DWM-2) were distinguished. DWM-1 type materials are quite similar to MSU, while DWM-2 exhibit relative inhomogeneity in their pore sizes, explaining the absence of XRD reflections. However, behind this important and encouraging progress and the strong attraction of new materials synthesis, less knowledge has been acquired on the synthesis mechanism, despite the lot of results published and available in literature. Moreover, there are important divergences in scientific findings from different authors. These disparities mainly result from the different, even though slightly, synthesis conditions and have been objet of a dispute that is far from being settled. The present work deals with a systematic study of silica mesoporous materials synthesis. A series of polyoxyethylene alkyl ether surfactants such as C13(EO)n (n = 6, 12, 18), CI6(EO)I0, C18(EO)~0 have been used. The present research is expected to enlarge the amount of available informations on the synthesis mechanisms of mesoporous materials. 2. EXPERIMENTAL 2.1. Synthesis A micellar solution with a weight percentage of Cm(EO)n varying from 5 to 60 was prepared by dissolving the surfactant at a temperature below the cloud point value in an aqueous solution during 3 hours. The pH value of the micellar solution was then adjusted with H2804 to 2.0. The obtained medium was stirred for three hours at this temperature before adding drop by drop the silica source 9tetramethoxysilane (TMOS). The 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~ for 18 hours in order to remove all the surfactant and impurities.
2.2. Characterization The XRD patterns were obtained with a Siemens D 5000 diffractometer. The transmission electron micrographs were taken using a 100 kV Philips Techna'f microscope. For TEM observations, sample powders were embedded in an epoxy resin and then sectioned on an ultramicrotome. The morphology of the final phases was studied using a Philips XL-20 Scanning Electron Microscope (SEM) with conventional sample preparation and imaging techniques. Nitrogen adsorption- desorption isotherms were obtained at -196 ~ with a volumetric adsorption analyzer ASAP 2010 or TRISTAR 3000 both manufactured by Micromeritics. The samplrs were further degassed under vacuum for several hours before nitrogen adsorption measurements. The pore diameter and the pore size distribution were determined by the BJH method [ 13].
119 3. RESULTS AND DISCUSSION 3.1. Effect of the hydrothermal treatment duration and temperature This study was performed for compounds prepared with 50 wt.% of surfactant. This loading of surfactant corresponds to a region where the hexagonal array of the micelles (HI) is present in solution [14]. 3.1.1 Structure of the prepared compounds. The structural investigations show that the materials obtained with decaoxyethylene cetyl ether [C16(EO)10] and decaoxyethylene oleyl ether [Cls(EO)10] belong to the DWM-1 (or MSU) family. Indeed, only one peak is observed by XRD (Fig. l a). This reflection line is related to the sum of the pore diameter and the wall thickness. The TEM pictures confirm the wormhole structure of the channels (Fig. l b). In contrast, the use of polyoxyethylene (6) tridecyl ether [C~3(EO)6]affords materials that, in spite of a relatively homogeneous pore size distribution, do not show any peak on their XRD patterns. These compounds belong to the family of DWM-2. In any case, when working with a weight percentage located in the H1 domain, we could expect that the recovered materials should have the hexagonal array of their channels, similar to the "micellar mould". However, the XRD and TEM results show that the compounds belong to either DWM-1 or DWM-2 families. This led us to conclude that the addition of the silica source disturbs the micellar array formed in solution. The mechanism represented in Figure 2 has been proposed to explain the formation of materials when Cl6(EO)10 and Cls(EO)10 are used as template. When the TMOS is added to the aqueous solution of micelles having a hexagonal structure (HI), hydrogen bonding type interactions between the oxygen atoms of the ethoxy head groups of the surfactant and hydrogen atoms of the silica source occur. Then, the polymerization of silica disturbs the regular H1 micellar array and the channels slightly move in order to minimize steric and electrostatic energies. Thus, the regular organization of the preformed micelles is lost and the obtained materials will be disordered. Only one broad peak is detected on the XRD pattern, channels with a wormlike structure are observed by TEM and a H2 type hysteresis loop can be noted from nitrogen adsorption, which indicates that the recovered compounds are DWM-1 materials. However, when Cl3(EO)12 and C13(EO)6are used, the interaction between surfactant and silica, as well as the repulsion between hydrophilic headgroups lead to the formation of DWM-2 (See below). Table 1 sums up the different structures obtained with Cm(EO)ntemplates.
Table 1 : Different structures obtained with Cm(EO)n. CMI-1 Surfactant (wt.%)
CMI-3
DWM-1
DWM-2
C16(EO)I0(5-25) CIs(EO)10(< 30) C16(EO)I0(> 25) C13(EO)12(>- 15) C~s(EO)10 (-> 30) C13(EO)6(5-50) Cl3(EO)12 (< 15)
3.1.2 Textural features. The textural characteristics of the materials are summed up in table 2. From these, it is evident that increasing heating temperature favors the formation of large pore mesoporous molecular sieves. Sierra et al. [ 15] have reported the synthesis of mesoporous materials using
120 polyethyleneglycol-4,terocylphenylether with 9-10 ethoxy groups (Triton X100) as surfactant and sodium silicate as silica source. They have concluded that the pore size depends on the polycondensation of the silica, which is increased with temperature, and on the surfactant conformation. These authors explain the expansion of pore size with increasing pH from 2 to more than 8.5 by considering a change in the surfactant conformation and the arrangement of the polyoxyethylene chains on the surface of silica. However, no details have been given on how and in which manner the conformation of surfactant changes. In the present work, the variation of the pore diameter of mesoporous materials with increasing temperature can also be interpreted from the point of view of the change in the surfactant conformation. At low heating temperature, in an aqueous solution of polyoxyethylene alkyl ether, the hydrophilic part of the surfactant exhibits a contracted conformation due to its large number of hydrogen bondings with water molecules (Fig. 3a). Contacts and interaction of the ethoxy oxygens with the silanol groups of the silica are not favored by this conformation. The effective crosssectional area of the hydrophilic headgroup of the surfactant is important. According to Kunieda et al. [ 16], micelles with a high curvature and a small diameter are therefore formed. If the heating temperature is raised, the water molecules bound around ethoxy groups through hydrogen bonds disappear progressively and a more extended conformation is expected (Figure 3b). The curvature of the micelles will thus decrease. This conformation allows more interactions with silica but needs a larger area on the silica walls to be achieved. The consequence is an increase in the pore diameter. _
2
4
6
8
10
12
_
14
20 (o)
Figure 1 9XRD pattern (a) and TEM micrograph (b) of a typical DWM-l-type sample and TEM micrograph (c) of a DWM-2 sample (see Table 1). I ~ phase o f m i c e l l e s i n aqueous solution
Destruction o f the H l p h a s e d u e to 9
interactious at the interface o f s i l i c a , 9 source and P E O , , 99
9
9 1r 4o 9E r
~
.. , ~
9
_o _n
E O~
o
,,
-,,.
Figure 2 9Effect of the addition of silica to the hexagonal array of Cm(EO)nmicelles.
121 Table 2" Variation of the pore diameter (nm) with heating time and temperature Heating time (days) 0.5 1
2 3 4 5 6
60~
80~
100~
60~
4.5 5.5 5.6
5.0 6.5 6.3
6.8 11.7 10.3
1.7 1.7
9.4 6.7 5.8 5.2 4.5 4.8 7.9
-
7.3
9.1
15.6
.
8.2 9.3
8
CIs(EO)I0
C16(EO)I0
C13(EO)6
11
80~
100~
5.2
7.5
4.7 4.0 12.9 9.9 12.1 12.1
8.0 13.4 15.8 17.1 14.6 -
60~
80~
7.0 7.6
5.7 9.1 8.2 10.1
10.6
100~ 8.1 7.8 14.7 13.6 14.4
- : no data
I lip~
I
H,20 ~0 ~C C~ 2 I H2
~CH21
C H 2~ / C H 2 "0 " I
CH
/ C H2 2~0 "
2o
I lipophilic }--- o
Ci2
H,20 I ~0 ~C
I
~ ~o
H,20 18 H " C~ 2
] H2
C H 2~ / C H 2 "0 " I
~ ~o
,~0 ...CH .,,.CH /0 ...CH~. 0/.C H2,C OH cH?H2~o ~C H2~CH2 ~CH 2 2~0 2~CH2 ~CH 2 H2""
Figure 3 9Different conformations ofpolyoxyethylene alkyl ether a" at low temperature, b" at high temperature 3.2 Variation of the weight percentage of template
In the case of decaoxyethylene cetyl and oleyl ether, when the preparation was carried out at surfactant weight percentages that, according to the phase diagram, afford only isolated micelles in solution (10 wt.% of C16(EO)10for instance), the compounds exhibit a multi-peak XRD pattern (Fig. 4). The presence of secondary reflections, in addition to the first sharp peak, is suggestive of a high organization of the channels in our material. Indeed, these reflections can be indexed in a hexagonal system and are related to the 100, 110 and 200 peaks. CMI-I [7] and CMI-3 [17] materials are respectively obtained. The hexagonal arrangement is confirmed by the TEM analysis (Fig. 5b and c). The Fourier transform pictures of TEM images exhibit sixfold symmetry and the measured angles between two bright spots are very close to 60 ~. If the weight percentage of template is increased, the secondary reflections (110 and 200) decrease in intensity and f'mally disappear. This indicates the formation of the disordered structure (Fig. 5a).
122
r~
"'J
30wt.%
60~.%
20wt.%
45wt~%
~
B,I
20( ~) Figure 4 9Variation of the XRD patterns of samples prepared with the weight percentage of CI6(EO)10.
Figure 5" Variation of the TEM micrographs of samples obtained with the weight percentage of C16(EO)10 fl: 50 ; b and c : 10.
Pathway A of Figure 6 is used to explain the molecular sieve formation. It is analogous to that developed by Mobil's scientists [1]. First, isolated micelles of surfactant are formed since the concentration of surfactant in aqueous solution is low. When the silica is added as mentioned above, hydrogen bonding interactions between the oxygen atoms of the ethoxy head group of the surfactant and hydrogen atoms of TMOS appear. To complete polymerization of tetramethoxysilane, these rod-like supramolecular assemblies (templatesilica) have to pack together, which involves a highly ordered arrangement of the channels as it can be seen from the TEM micrographs. This involves a cooperative mechanism, i.e. hydroxyl groups of silica source produced by the hydrolysis reaction interact with hydrophilic chain heads of surfactant molecules and the polymerization of silica species at the interface of rod micelles attracts the rod micelles covered by the silica source together to form a regular arrangement, leading to CMI-n materials. Concerning the polyoxyethylene tridecyl ethers, evolution is quite different. For C~3(EO)12 only one reflection, characteristic of DWM-l-type materials is observed by XRD when the weight percentage of template is decreased, whereas no reflection is detected for C~3(EO)6. Stucky et al. [18] have shown that the molecular packing model used to describe the water-surfactant systems could be extended to mesoporous silicate structures. The surfactant packing parameter g is defined as V/a~ 1r where V is the volume of the molecule, as is the headgroup area and 1r is the critical length of the hydrophobic tail. Small values of g stabilize more curved surfaces such as MCM-41 (1/3 < g < 1/2), while larger values of g stabilize less curvature such as MCM-48 (1/2 < g < 2/3) or lamellar MCM-50 (g = 1). The formed mesostructure can be thus controlled by this parameter. Based on a method proposed by Stucky et al. [19], we have determined the ratio between the volume occupied by the hydrophilie and lypohilic parts of the surfactants (Vn/VL). The values are summarized in Table 3. \
123 Interaction at interface of silica source and PEO
J l
Agglomerates of single rod micelles owing to a cooperative pathway
Single rod micelles in aqueous solution ~ E
Ep
,,W,.EoA~~
"Q, o.-~n
~,., Eo Eo
o
.._ ,,o " , , ; T . e,~,.~ = OH
O11
O.U
EO E6EO EO
~
.~ 0
"
~'~'o
~~
0
l Figure 6 9Effect of the addition of silica to the isolated micelles of Cm(EO)n. In the case of ionic templates and in particular for quatemary ammonium salts CnTMAX (X = C1, 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 [20]. It was however reported that C22 alkyl chains could lead to MCM-41 if the methyls of the headgroup were replaced by ethyl entities [21]. No micelles are formed if the value of n is less than 6. In the present study we propose that the VH/VL ratio can play a similar role as n. When Cl3(EO)is is used as template the repulsion between the hydrophilic headgroups of the surfactant is more important. Considering the proposed mechanism in Figure 6 (pathway B), surfactant-silica interfaces will be strongly perturbed by these headgroup interactions leading to materials with a disordered MSU-type network of channels. This phenomenon is reflected in the VH/VL values, which are much higher for these two templates compared to C16(EO)10and C18(EO)10 that posses a similar ratio and the same final structures. For the C 1 3 ( E O ) 6 , whatever the weight percentage of template is, neither highly ordered CMI-n nor disordered DWM-1 compounds are obtained. The materials possess a relatively narrow pore size distribution but no peak can be detected on the XRD patterns suggesting thus a degree of structural order even less than DWM, 1-type compounds. Taking into account the ratio ofhydrophilic 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 be formed beyond this value whereas smaller hydrophilic portions of the template would not afford any structured materials. Additional syntheses implying
124 Table 3 : Contribution of the hydrophilic (Vn) and lypophilic (VL) part of surfactant, obtained structure and weight percental e below which this phase a 1~pears (wt.%).
C13(EO)6
CI6(EO)Io
CI3(EO)12
C13(EO)18
1.23
1.39
2.04
3.04
Obtained structure
Hexagonal
Hexagonal
Wormhole
Wormhole
wt.%
30
25
15
VH/VL
1.04
CI8(EO)I0
a : amorphous, * value not yet determined polyoxyethylene tridecyl ether with headgroups such that the VH/VLratio is in the same range as C16(EO)10 and Cls(EO)10 will be performed in order to verify this hypothesis. 4. CONCLUSION With use of the results gathered during this work, we tried to shed some light on the possible synthesis mechanisms of mesoporous compounds by using non-ionic polyoxyethylene alkyl ethers as surfactant. Whatever the template is, if the weight percentage is located in the range where the hexagonal array is detected on the phase diagram, the addition of TMOS disturbs the arrangement of the micelles in solution. Indeed, when the tetramethoxysilane molecules are added to the aqueous solution of micelles having a hexagonal structure, their interactions with surfactant molecules induce a displacement of the channels and the recovered materials have the DWM characteristics When decreasing the weight percentage of decaoxyethylene cetyl or oleyl ether, well ordered CMI-n materials can be prepared. In this case, isolated micelles are formed in solution and after the addition of silica, to reach a complete polymerization of tetramethoxysilane, the supramolecular template-silica assemblies have to pack together, which evolves toward a highly ordered arrangement of the channels. 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 Cl3(EO)12] or DWM-2 [for C13(EO)6] are obtained. The different behavior is explained considering the VH/rVLratio. Of course our interpretations will not last universally as there are so many syntheses variables that have to be taken into account in such preparations. It was also found that the texture strongly depends on the hydrothermal treatment conditions. The expansion of pore size with heating time and temperature can be explained by the change of surfactant conformation with temperature and time. ACKNOWLEDGEMENT
:
This work has been performed within the framework of PAI/IUAP 4-10. Alexandre L6onard and Gontran Herrier thank FNRS (Fond National de la Recherche Scientifique, Belgium) for a FRIA scholarship.
125 REFERENCES
1.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13 14. 15. 16. 17. 18. 19. 20. 21.
J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.TW. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schenker, J. Am. Chem. Soc., 114 (1992) 10834 C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710 P.T. Tanev and T.J. Pinnavaia, Chem. Mater., 8 (1996) 2068. TUD-1, Shen, Ziping; Jansen, and al., Mesoporous amorphous silicate materials and process for the seperation therof, Patent application, EU4741, (1998). Q. Huo, D.I. Margolese and G.D. Stucky, Chem. Mater., 8 (1996) 8 1147. S.A. Bagshaw, E. Prouzet and T.J. Pinnavaia, Science, 269 (1995) 1242. J.L. Blin, A. L6onard and B.L. Su, Chem. Mater., 13 (10) (2001) 3542. G.S. Attard, J.C. Glyde and C.G. GSltner, Nature, 378 (1995) 366. N.R.B. Coleman and G.S. Attard, Microporous and Mesoporous Mater., 44-45 (2001) 73. W. Zhang, B. Glomski, T.R. Pauly. and T.J. Pinnavaia, Chem. Comm., (1999) 1803. J.L. Blin, L. Gigot, A. L6onard and B.L. Su, Nanoporous III, accepted for publication. J.L. Blin, O. Provoost, A. L6onard and B.L. Su, to be published. E.P. Barret, L.G. Joyner, and P.P. Halenda, J. Am. Chem. Soc., 73 (1951) 37. D.J. Mitchell, G.J.T. Tiddy, L. Waring, T. Bostock and M.P. Mc Donald, J. Chem. Soc. Faraday Trans. 1 79 (1983) 975. L. Sierra and J.L. Guth, Microporous and Mesoporous Mater. 27 (1999) 243. H. Kunieda, K. Ozawa and K.L. Huang, J. Phys. Chem. B, 102 (1998) 83 I. G. Herrier and B.L. Su, Stud. Surf. Sci. Catal., 135 (2001) 08-0-02 G.D. Stucky, Q. Huo, A. Firouzi, B.F. Chmelka, S. Schacht, I.G. Voigt-Martin anf F. Schtith, Stud. Sure Sci. Catal., 105 (1997) 69. J.M. Kim, S.E. Park and G.D. Stucky, Stud. Surf. Sci. Catal., 135 (2001) 08-P-12 M. Kruk and M. Jaroniec, Langmuir, 15 (1999) 5279. R. Ryoo, C. H. Ko and I.S. Park, Chem. Commun., (1999) 1413.
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Studies in Surface Science and Catalysm 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
127
M e s o p o r o u s silicas via organic-inorganic hybrids based on charged p o l y m e r s Graham M Gray and John N Hay Department of Chemistry, School of Physics & Chemistry, University of Surrey, Guildford, Surrey, GU2 7XH, England. A range of polyviologens synthesised by the Menshutkin reaction have been successfully incorporated into silica hybrids using the hydrolytic sol-gel route. Hybrids were calcined at 650~ to remove the polymer, forming porous amorphous silica products. Nitrogen adsorptiondesorption studies have been undertaken on the hybrids and the calcination products. The hybrid porosities depend on the polymer content and type and the nature of the counter-ion. Calcination leads to silicas whose porosity depends on the nature and concentration of the polyviologen (PV) used. A significant effect of the counter-ion has been noted in these systems. 1. INTRODUCTION Synthesis of silica with controlled porosity has attracted much attention recently because of the range of potential applications for the products, including their use in catalysis and as catalyst supports, in separation processes and in sensors. One route to producing this porosity is the use of small molecules or polymers incorporated into the forming silica network as structure control agents, which are subsequently removed to leave the desired porosity [ 1-3]. A variety of templates has been studied for the control of porosity in amorphous silica synthesised by the hydrolytic sol-gel route including small molecules and polymers [4], supramolecular arrays [5] and bacteria [6], which can template the growth of inorganic species via electrostatic, van der Waals and hydrogen bonding interactions, or combinations thereof. In mesoporous silica synthesis using charged surfactant templates, electrostatic interactions are believed to play a key rrle in the initial organisation of the organic-surfactant array [5]. Both polymers and oligomers have been used to synthesise a wide range of organic-inorganic hybrids by the hydrolytic sol-gel route and the polymeric species or their supramolecular assemblies can act as templates for the resulting silica structure [7,8]. Surprisingly little attention has been paid to cationic polymers [8], despite the fact that cations and cationic surfactant assemblies are used to form porous crystalline zeolites and amorphous mesoporous silicas. There is also clear evidence of the importance of functionality in controlling the structure of natural organic-inorganic hybrid materials [6,9]. Synthetic charged polymers might therefore exert a structure directing effect on the growth of sol-gel silica networks. We have previously reported our investigations into the possibility of polyviologens (bipyridinium polymers) acting as non-bonding templates during the hydrolytic sol-gel process [ 10-13]. Polyviologens are an interesting class of cationic polymer and have found use in a wide variety of fields including herbicides, electrochromism and molecular electronics. Our interest in cationic polymers as potential templates stems from the use of cations and cationic surfactants for this purpose in the formation of zeolites and amorphous silicas. As part of our continuing investigation into these materials we report here preliminary results into the possibility of the associated counter-ion playing a r61e in this process.
128 2. EXPERIMENTAL 2.1 Chemicals p-Toluenesulfonyl chloride (98%) was purchased from Aldrich and used as received. Pyridine (AR), 4,4'-bipyridine (98%), 1,3-propanediol (98%), 1,5-pentanediol (97%), 1,3dibromopropane (98%), 1,5-dibromopentane (97%) and tetramethylorthosilicate (TMOS, 98%) were purchased from Lancaster and used as received. Methanol (AR), glacial acetic acid, ethyl acetate (AR), and hydrochloric acid (sp. gr. 1.18) were purchased from Fisher and used as received. Acetonitrile (HPLC, Fisher) was freshly distilled from Call2 under an atmosphere of nitrogen before use. 2.2 Instrumentation Infra-red (IR) spectra of the polyviologens and the sol-gel hybrids (as KBr discs) were recorded on a Perkin-Elmer 1750 Fourier transform IR (FT-IR) spectrometer. IH nuclear magnetic resonance (NMR) spectra were obtained using a Bruker AC-300 NMR spectrometer operating at 300.15 MHz, using CD3OD, D20 or DMSO-d6 as a solvent. Analyses for carbon, hydrogen and nitrogen were carried out on an Exeter Analytical EA440 machine. N2 adsorption-desorption isotherms at 77 K were obtained using a Micromeritics Gemini 2375 instrument. Samples were outgassed for at least 5 hours at 120~ under N2(g). Analysis of the surface area and porosity of the hybrids and the silica products resulting from calcination was based on the method derived by Brunauer, Emmett and Teller (BET) [ 14,15]. 2.3 Synthesis of ditosylate ester monomers A slightly modified version of the procedure reported by Marvel and Sekera [ 16] was used. The preparation of 1,3-propaneditosylate is given here as an example. Pyridine (20ml) and 1,3propanediol (1.704g, 1.62ml, 22.4mmoi) were added to a 100ml RB flask equipped with a magnetic stirrer. The flask was cooled to 0~ and p-toluenesulfonyl chloride (8.701g, 45.6mmol) added in portions over 5 minutes taking care to ensure that the temperature did not rise above 15~ The mixture was stirred for 3.5 hours below 20~ before addition of hydrochloric acid (15ml) in ice-cold water (50ml). The ester that crystallised was collected on a chilled Buchner funnel and sucked as dry as possible. Recrystallisation from methanol yielded a crystalline white powder, which was then dried under vacuum at 60~ for 24 hours. Yield = 4.86g; 56%, melting point 88-89~ The purity of the monomers was checked by IH NMR (tosylates in CD3OD, bromides in D20) and elemental analysis. 2.4 Polyviologen (PV) synthesis Equimolar amounts of 4,4'-bipyridyl and the corresponding monomer (ditosylate ester or dibromoalkane) were stirred in dry acetonitrile at 80~ for 120 h under an atmosphere of nitrogen according to the previously reported procedure [3]. The resulting polymers (e.g. I) were dried under vacuum at 80~ for 24 h. Analytical data for the polyviologens are presented in tables 1-4. PPrV is polypropylviologen and PPeV is polypentylviologen. I -I-
+
~___/ TsO-
129
2.5 Synthesis of silica-polyviologen hybrids Two methods were used to synthesise the hybrids. Method 1. The order of addition of reagents was as follows. 1. PV. 2. H20. 3. MeOH. 4. Acetic acid. 5. TMOS. Upon addition of methanol, the mixture was stirred to dissolve as much polymer as possible. Upon addition of TMOS the mixture became turbid and was stirred vigorously until homogenisation occurred. The mixture was covered with perforated parafilm and left for 48 hours at room temperature during which time it gelled. The sample was then dried at 80~ for 72 h before drying at 120~ for 48 h. Method 2. This method is similar to method 1 except the order of addition of reagents was as follows. 1. PV. 2. MeOH. 3. TMOS. 4. Acetic acid. 5. H20. The mixture was covered with perforated parafilm and left for 72 hours at room temperature before drying at 80~ for 10 days. Table 1. IR data for the polyviologens containing bromide counter-ions Assignment
PPrV-Br
stretch C=N stretch -CH2 deformation C-N + stretch
PPeV-Br
1640cm -1 1410 1360cm-1 1183cm -~
and
1638cm -1 1443 and 1349cml 1175cm -I
Table 2. IR data for the polyviologens containing tosylate counter-ions Assignment ......C = N
PPrV-OTs
S t r e t c i ~ ..............i 6 3 9 c m
C-C skeletal -S=O stretch C-N + stretch
PPeV-OTs
1 ~.........................................................i 6 3 8 c m
1 ~ ...................................................
1496 and 1598cm-
1494 and 1598cm-
1
1
1121 and 1350cm-
1123 and 1352cm-
1
1
1196cm -l
1196cm l
Table 3. ~H NMR data for the polyviologens containing tosylate counter-ions Chemical shift, 8 ...PPrv-OTS . 2.28 4.77 7.2-7.6 8.4-9.2 ......................................................
i~PeV'0Ts 2.26 4.65 7.1-7.8 8.6-9.4
............................................
Assignment 3H, CH3 +N-CHE dd, 2x4H, ArH m, 8H, hetero ArH
130
Table 4. ~H NMR data for the polyviologens containing bromide counter-ions Chemical shift, 6 PPrV-Br 4.7 7.9-9.2
PPeV-Br 4.65 7.8-9.2
Assignment +N-CH2 m, 8H, hetero ArH
3. RESULTS AND DISCUSSION
3.1. PV synthesis Pale off-white/yellow solids were obtained for poly(propylviologen ditosylate) (PPrV-OTs) and poly(propylviologen dibromide) (PPrV-Br). A bright yellow solid was obtained for poly(pentylviologen dibromide) (PPeV-Br). In the case of the poly(pentylviologen ditosylate) (PPeV-OTs) however, a tacky, viscous pale-yellow gel was obtained. Efforts to yield a solid by precipitation of the polymer out a solution of methanol with the careful addition of a miscible non-solvent such as chloroform, dichloromethane or diethyl ether were unsuccessful. The viologens show interesting lyotropic liquid crystalline behaviour [ 17-20], and it is possible that the gelation arises from physical interaction caused by the lyotropism. Infrared and IH NMR analysis (tables 2 and 3, above) were consistent with the formation of a viologen polymer. The addition of sodium dithionite to a solution of the polymer in methanol, which resulted in the characteristic deep blue-violet colour of the radical cation species (below, eqn 1.), also provided confirmation of the viologen structure.
--+N~-~N
m Equation 1
IH NMR spectroscopy showed that a small amount of methanol was contained within the gel despite attempts to remove it by drying under vacuum at elevated temperature. The small amount of methanol present was not deemed to be a problem, as the polymer would subsequently be dissolved in methanol during the sol-gel process.
3.2. Sol-gel hybrids The synthesis conditions for the sol-gel hybrids are reported in table 5. In each case a transparent or slightly translucent hybrid was produced suggesting a mainly nanoscale morphology.
3.3. Surface area analysis of the hybrids and calcined materials A number of factors (such as monomer concentration, r value and temperature) can influence the surface area, pore volume, porosity and pore sizes of silica xerogels. However, we have previously shown that despite the differences in r value used in the synthesis of these hybrids, it is possible to draw rational conclusions from observations of the effect of incorporation of the polyviologen in varying amounts into the sol-gel matrix [12]. The results of the present study, shown in tables 5 and 6, are broadly in agreement with our earlier predictions and observations
131 [ 12]. The previous work showed that mesoporous hybrids were formed, but with low porosities at high PV content. The present work confirms that incorporation of polyviologen within the silica matrix leads to a reduction in surface area, pore volume and porosity. All hybrids have an average pore diameter in the range 19-20)k, except for PPeV-OTs (50%), which has an average pore diameter of 22.5A. A full pore size distribution is still to be determined. Calcination of the hybrid materials at 650~ for 24 hours to remove the polyviologen leads to a decrease in surface area and total pore volume due to ageing of the, silica except when a high concentration of polyviologen was used (as seen in the previous study). In such cases, the removal of the PV has a greater effect than the ageing of the silica network resulting in an increase in the surface area and pore volume. In all cases, the average pore diameter decreases following calcination, with the most significant reduction in the case of the PPeV-OTs (50%) hybrid. Ongoing work will confirm whether or not the calcined silicas show high mesoporosities, as seen previously. Of interest is the difference in surface area and pore volume for the incorporation of polyviologens at similar loading but with different counter-ions. It is known that the choice of counter-ion can infuence the polymer properties [21-23]. Harris and co-workers have shown that replacing the tetrafluoroborate counter-ion in poly(pyridinium tetrafluoroborate)s with the triflate anion (known for its thermal stability) leads to a significant increase in thermal and thermo-oxidative stability [21,22]. Bhowmik and co-workers noted that by reducing the strong interaction between positive and negative charges compared to an inorganic counter-ion, the presence of a bulky organic counter-ion increased solubility in methanol and dimethylsulphoxide to the extent that formation of a lyotropic liquid crystalline phase was possible [23]. The presence of the different counter-ions used in this study (Br and TsO) is also likely to lead to differences in polymer packing due to the steric effect of a large bulky tosylate anion compared to the bromide ion. As the growing silica network forms around the polymer chains during the sol-gel process it is possible that such differences in packing of the polymer would be maintained in the resultant sol-gel hybrids. This could account for the marked differences in the surface area and pore volume of the sol-gel hybrids containing the same loading of polymer but with a different counter-ion. A more systematic study is required to confirm these results and, if possible, quantify the effect of different counter-ions. Table 5. Synthesis and surface area data for the sol-gel hybrids PV used (weight%)
Method
r
rs
ra
Gelation time
PPrV-Br (10%)
1
7.8
0.1
0.5
<16h
pale yellow, transparent
129
0.070
PPrV-OTs (10%)
2
4.1
7.3
0.99
22h
colourless, transparent
348
0.188
PPeV-Br (10%)
1
5.8
0.1
0.5
<16h
colourless, transparent
87
0.047
PPeV-OTs (10%)
2
3
2
0.99
<24h
yellow, translucent
394
0.214
2
0.99
<24h
yellow, translucent
64
0.040
PPeV-OTs 2 8.2 (50%) r - water:alkoxide ratio rs = solvent:alkoxide ratio ra = acid:alkoxide ratio
Appearance
BET surface area
Total pore volume
132 Table 6. Surface area data for the calcined materials PV used
BET surface area
Total pore volume (Vp)
PPeV-OTs (10%) PPeV-OTs (50%) PPrV-Br (10%)
250 385 56
0.132 0.210 0.030
4. A C K N O W L E D G E M E N T S The UK Engineering and Physical Sciences Research Council (EPSRC) is thanked for funding. Elemental analyses were carried out by Ms. N. Walker.
5. REFERENCES
1. Helmkamp, M. M.; Davis, M. E. Ann. Rev. Mater. Sci. 25 (1995) 161. 2. Burkett, S. L.; Davis, M. E. Chem. Mater. 7 (1995) 1453. 3. Davis, M. E.; Chen, C.-T.; Burkett, S. L.; Lobo, R. F. Mat. Res. Soc. Symp. Proc., 346 (1994) 831. 4. Raman, N. K.; Anderson, M. T.; Brinker. C. J. Chem. Mater. 8 (1996) 1682. 5. Dabadie, T.; Ayral, A.; Guizard, C.; Cot, L.; Robert, J. C.; Poncelet, O. Mat. Res. Soc. Symp. Proc., 346 (1994) 849. 6. Davis, S. A.; Burkett, S. L.; Mendelson, N. H.; Mann, S. Nature 385 (1997) 420. 7. Sommerdijk, N. A. J. M.; van Eck, E. R. H.; Wright, J. D. Chem. Commun. (1997) 159. 8. Sato, S.; Murakata, T.; Suzuki, T.; Ohgawara, T. J. Mater. Sci. 25 (1990) 4880. 9. Heuer, A. H.; Fink, D. J.; Laraia, V. J.; Arias, J. L.; Calvert, P. D.; Kendall, K.; Messing, G. L.; Blackwell, J.; Rieke, P. C.; Thompson, D. H.; Wheeler, A. P.; Veis, A., Caplan, A. I. Science 255 (1992) 1098. 10. Adeogun, M. J.; Hay, J. N. Polym. Int. 41 (1996) 123. 11. Adeogun, M. J.; Fairclough, J. P. A.; Hay, J. N.; Ryan, A. J. J. Sol-Gel. Sci. Tech. 13 (1998) 27. 12. Adeogun, M. J.; Hay, J. N. Chem. Mater, 12 (2000) 767. 13. Adeogun, M. J.; Hay, J. N. J. Sol-Gel Sci. Techol. 20 (2001) 119. 14. Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity;, Academic Press: London, 1967. 15. Lowell, S.; Shields, J. E. Powder Surface Area and Porosity, 3rd Ed.; Chapman and Hall: London, 1991. 16. Marvel, C. S.; Sekera, V. C. in Organic Syntheses Collect. Vol. III, Wiley: New York, 1955, pp.366-367. 17. Han, H. S.; Bhowmik, P. K. TRIP 3 (1995) 199. 18. Bhowmik, P. K.; Han, H. S. J. Polym. Sci., Polym. Chem. Ed. 33 (1995) 1745. 19. Bhowmik, P. K.; Akhter, S.; Han, H. J. Polym. Sci., Polym. Chem. Ed. 33 (1995) 1927. 20. Bhowmik, P. K.; Xu, W.; Han, H. S. J. Polym. Sci., Polym. Chem. Ed. 32 (1994) 3205. 21. Harris, F. W.; Chuang, K. C.; Huang, S. A. X.; Janimak, J. J.; Cheng, S. Z. D. Polymer 35 (1994) 4940. 22. Huang, S. A. X.; Chuang, K. C.; Cheng, S. D.; Harris, F. W. Polymer 41 (2000) 5001. 23. Bhowmik, P. K.; Burchett, R. A.; Han, H.; Cebe, J. J. J. Polym. Sci. Part A Polym. Chem. 39 (2001) 2710.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
133
M e s o p o r o u s silicas o f hierarchical structure b y h y d r o t h e r m a l s u r f a c t a n t - t e m p l a t i n g under m i l d alkali conditions Zhong-Yong Yuan, a,b Wuzong Zhou, c Bao-Lian Su a,. and Lian-Mao Peng b Laboratory of Inorganic Materials Chemistry, ISIS, The University ofNamur (FUNDP), 61 rue de Bruxelles, B-5000 Namur, Belgium. e-mail:
[email protected] a
b Beijing Laboratory of Electron Microscopy, Institute of Physics & Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 2724, Beijing 100080, China c School of Chemistry, University of St. Andrews, St. Andrews, Fife KY16 9ST, UK
Mesoporous silicas with hierarchical structures were synthesized under a mild alkali condition by cationic or mixed cationic-nonionic surfactant templating. When cetylpyridium bromide was used, a novel morphology of mesoporous silica of type MCM-41 was formed. It was obsearved that single nanotubes with diameter of about 5 nm and bundles of nanotubes grew from the (001) surface of the mesoporous silica particles, forming a paintbrush-like morphology. The synthesis process was investigated and the formation mechanism of such a surface nanotube patterning is discussed. When mixed surfactant of cetylpyridium chloride and Triton X-100 was used, a mixed ordered-disordered hierarchical mesostructure with arc rings of submicrometer scales was observed in the synthesized mesoporous silica which exhibits a narrow pore size distribution, high surface area and a large pore volume. Arc ropes consisting of ordered mesopores, as well as many spheres of submicrometer in diameter with shells of about 40 nm in thickness consisting of ordered arrays of mesopores, were observed, though the majority of the particles contained disordered mesoporous microdomains. This material lacks overall long-range ordering whereas local ordering of mesopores exists.
1. INTRODUCTION A variety of ordered mesoporous materials have been synthesized using supramolecular assembly of surfactant molecules as a template since the appearance of mesoporous M41S [1,2]. These materials have a range of framework compositions, morphologies, and porous structures. Correspondingly a number of models have been proposed in order to explain the formation of mesoporous materials and to provide a rational basis for the various synthesis routes [3,4]. Many efforts have been devoted recently to the macroscopic morphological control of mesoporous silicas, since it is now apparent that morphological control and texture of mesoporous silicas are extremely important for diverse industrial applications [5,6]. Syntheses of mesoporous silicas in various forms, such as films, fibers, spheres, monoliths, curved shapes, and designed patterns, have been achieved using the surfactant-templating method often under relatively wild conditions (such as strongly acid and basic media) [6-12]. On the other hand, mixed surfactants have been used as mixed micellar templates for the synthesis of mesoporous silicas with tailored properties [13-15]. The pore size of mesoporous silicas could be fine controlled by using the mixed surfactant with various ratios [13,16].
134
Mesoporous silica spheres have been synthesized with mixed cationic-nonionic surfactant under a strong acidic condition [ 14]. Cubic silica MCM-48 has been obtained in the presence of either mixed cationic-neutral surfactant [15] or mixed cationic-anionic surfactant micelles [17], respectively, under alkaline conditions. We have synthesized silica-based mesoporous materials with hierarchical structures under a mild alkaline condition by cationic or mixed cationic-nonionic surfactant templating. Surfactant cetylpyridium and Triton X-100 were used. NH4OH was used to tune the alkaline of the synthetic gel. A novel surface nanotube pattern grew on MCM-41 particle surface by single cationic surfactant templating. Whileas a mixed ordered-disordered mesostructure with arc rings of submicrometer scales was observed in mesoporous silica materials synthesized with mixed surfactants.
2. E X P E R I M E N T A L Cetylpyridinium bromide (CPBr) was used as a single cationic surfactant template. In a typical synthesis experiment, 5.76 g of CPBr was dissolved in 70 ml of water, 18.8 ml of NHnOH (25%) was then added under stirring, resulting in a clear solution. Then, 6.26 g of tetraethoxysilane (TEOS) was added slowly with stirring. The pH value of the final mixture was about 9.2. After further stirring for 30 min, the mixture was loaded into an autoclave and statically heated at 80 ~ for 72 h. The resultant solid product was recovered by filtration, washed with distilled water and dried in air at ambient temperature. The as-synthesized material was heated in air with temperature increased from room temperature to 540 ~ at a rate of 1 ~ and was then calcined at 540 ~ for 5 h. Cetylpyridinium chloride (CPC1) and Triton X-100 were used in the mixed surfactant templating. The typical synthetic conditions are as follows. 1.07 g of CPC1 and 2.43 g of Triton X-100 were dissolved in 15 ml of NH4OH (25%) and 50 ml of water. To this solution, 3.13 g of TEOS was added at room temperature with stirring. After further stirring about 45 min, the mixture was allowed to crystalline at 90 ~ for 72 h in a stainless-steel autoclave. Powder X-ray diffractograms (XRD) of the solids were recorded on a Rigaku D/max 2400 diffractometer using CuK~ radiation and scanning between 1.5~ and 10~ (20). N2 adsorptiondesorption isotherms were obtained at liquid nitrogen temperature on a Quantachrome Autosorb1 apparatus. The sample was outgassed at 300 ~ over 10 h prior to adsorption. The specific surface area was determined by the BET (Brunauer-Emmett-Teller) method and the pore-size distribution was obtained from the N2 adsorption isotherm using the BJH (Barrett-JoynerHalenda) method. Transmission electron microscopy (TEM) was carried out on a Jeol JEM2010 electron microscope operating at 200 kV. The specimens for the TEM studies were prepared by dispersing the particles in alcohol by ultrasonic treatment, and dropping onto a holey carbon film supported on a copper grid.
3. RESULTS AND DISCUSSION
3.1. Single cationic surfactant templating The X-ray diffraction patterns of mesoporous silicas synthesized with single surfactant exhibit at least four diffraction peaks in the 20 range of 2 to 7~ (Figure 1), reflecting a typical MCM-41 mesostructure [1,2]. After calcination, only a very small decrease of the hexagonal unit cell parameter from 5.10 to 5.00 nm was detected, suggestive of the high thermal stability of mesoporous silicas synthesized under the present condition [ 18]. All samples exhibit a surface area of more than 1000 m2/g, as calculated from low-temperature N2 adsorption-desorption
135
isotherms with typical irreversible type IV [19]. The nitrogen adsorption usually shows a typical rectangular hysteresis loop, as previously reported elsewhere [18]. This hysteresis shape represents a uniform tubular capillary pore calcined geometry with wide and narrow regions in the pore channels. The corresponding uniform pore size is about 3.1 nm. ~ as-synthesized A novel morphology of the surface when viewed down a direction I perpendicular to the pore axis has 10 2 4 6 8 been revealed. Many single silica 20 (degree) nanotubes and bundles of single Figure 1. Typical XRD pattems of mesoporous nanotubes appear on the (001) silicas by single cationic surfactant templating. surfaces of the particles (Figure 2). The diameter of a single nanotube is about 5 nm. The average diameter of the bundles of nanotubes is about 20 - 4 0 nm. All the silica nanotubes are parallel to the mesopore axis of the MCM-41 structure, though some nanotubes are bent. The lengths of the nanotubes were almost uniform in the same sample, and could be varied with the synthesis time (Figure 2). The surface nanotube patteming existed after 60-hour hydrothermal treatment, and the nanotubes could continue growing with the increase of crystallization time. The formation of the nanotubes was markedly enhanced after keeping for 72 h at 80 ~ The length of the nanotubes could reach above 250 nm after 80 h of hydrothermal treatment. Single nanotubes were also observed in Figures 2 and 3. Such a paintbrush-like morphology was maintained after calcination at 540 ~ for removing the organic templates [18]. According to a previous TEM study of MCM-41 specimens at different stages of formation, it was found that diffusion process of chemical spieces in the solution and in the surfactant-silica particles is important [20,21 ]. The initial nuclei of mesoporous silica were formed even before the hydrothermal treatment. However the surface of the mesostructured silicate-surfactant composite particle was rough and less condensed. The collected mixed gel, after roomtemperature stirring without any hydrothermal treatment, gave a well-defined X-ray diffraction pattern, characteristic of MCM-41, with a small dl00 spacing less than 3.48 nm. The hexagonally ordered mesopores were observed directly by TEM. During the hydrothermal treatment, the initial mesostructured silicate-surfactant composite particles could be restructured, forming better ordered array of mesochannels with larger dl00 spacing (4.09 nm), which can be confirmed by the XRD and TEM data. There are still silicate species and surfactant micelles in the precursor solution as detected by ~H, 13C and 29Si MAS NMR. Those partially hydrolyzed TEOS would continue to aggregate with the surfactant micelles to generate condensed rods during the mild hydrothermal treatment, and the condensation stabilizes the rods. While the spherical end cap of the cylindrical rod is relatively unstable due to the lack of geometric matching [22], 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, and thus the mesochannels would elongate from the initial silica particle surface during crystallization. On the other hand, due to the flexibility of surfactant micelle rods, the growth of mesochannels might be anisotropic. The growth direction of every nanotube could be different, even bent. And several nanotubes could linked together during growth, forming some small bundles of nanotubes on the surface. Thus, it
}
I
,
I
I
I
i
I
i
136
Figure 2. Typical TEM images of the as-synthesized mesoporous silicas of MCM-41 type, showing paintbrush-like (001) surface patterning: (a) short nanotube patterning by 72 hours of crystallization and (b) longer nanotubes after 80 hours of crystallization
Figure 3. TEM image of the as-synthesized mesoporous silicas showing several short nanotubes growing from the particle surface
137 is possible to form the present surface patterning of nanotubes. This proposed formation mechanism of surface nanotube patteming could be supported by the TEM images at the initial stage of nanotube growing (Figure 3). Figure 3 shows several short nanotubes growing from the particle surface, and its direction is parallel to the mesopore axis of the mesostructure. The particle surface is rough due to the growing of nanotubes. Such morphology was not found in the initial mesostructured silica particles before hydrothermal treatment. That is to say, hydrothermal treatment is necessary for the formation of this novel surface patteming. Another possible support is from very recent report of Mokaya [23] who investigated the effect of increasing the time of hydrothermal crystallisation during high temperature (150 ~ synthesis of MCM-41. A change of morphology from spherical particles to elongated or rope-like particles was found, and micrometre sized MCM-41 'ropes' made up of smaller silica fibres were obtained after long time of crystallization (168 h). A recent report of Lin et al. [24] also presented the mesoporous silica nanotubes obtained by post-synthesis hydrothermal treatment of their acid-made mesoporous silicas. The diameter of tubes was around 30-100 nm. The formation mechanism of the transformation of surfactantsilica interactions was proposed. Obviously, the mechanism of our nanotubes' formation should be quiet different with theirs. The present work can supply interesting information on the understanding of the mesostructure formation mechanism. Moreover, transition metal ions Ti and V have been introduced into the MCM-41 mesoporous materials, and the paintbrush-like morphology remained unchanged [ 18,25].
3.2. Mixed surfactant templating The molar composition of the synthetic gel with mixed surfactant templating is 1 TEOS : 14.6 NH4OH : 0.2 CPC1 : 0.25 TX-100 : 185 H20. The powder X-ray diffraction pattern of the as-synthesized material shows some overlapped reflections in the low 20 angle range with very low intensity (Figure 4a). However, after calcination, an intense peak corresponding to a dspacing of 4.1 nm presented together with another broad peak around 2.2 nm of d-spacing (Figure 4a). This pattern is characteristic of a mesophase, but it is different from those of either the ordered MCM-41 [ 1] or the disordered materials, for instance KIT- 1 [26] made with cationic surfactant in the presence of organic salts and MSU-n [27] prepared by nonionic surfactant templating. The latter materials generally display single dl00 reflections indicative of uniform worm-like mesopores without long-range periodicity [27]. Compared to the XRD patterns of MCM-41 with hexagonally ordered channels, the present calcined sample might have a novel pore structure, but with lower degree of structural ordering. The arising of the reflection peaks of 4.1 and 2.2 nm of d-spacing on the XRD pattern for the calcined sample suggests that during the calcination a relatively ordered array of well-defined mesopores was formed. It is believed that, in the as-synthesized material, mesopores filled with templating molecules already existed but in a disordered manner. The mesopores may be rearranged into a more ordered form during calcination to remove the templating molecules from the pores. The nitrogen adsorption-desorption isotherm of the calcined product and its corresponding BJH pore-size distribution curve are presented in Figure 4b. A typical irreversible type IV adsorption isotherm with a hysteresis loop, as identified by IUPAC [19], is seen. A well-defined step occurs in the adsorption curve at a relative pressure P/Po of ca. 0.35, which is indicative of the filling of framework-confined mesopores. The P/P0 position of the inflection points is clearly related to a diameter in the mesopore range and the step indicates the mesopore-size distribution. The pore-size distribution curve shows a narrow peak centered at 3.2 nm. The BET surface area is 1142 mR/g, and the single-point total pore volume at a relative pressure of 0.978 is 1.232 cm3/g. TEM images of the mixed-surfactant-templated silica samples are shown in Figure 5. It was found that the majority of the particles contain disordered mesoporous microdomains. Two arc
138 ropes, consisting of ordered mesopores, are present in one monolith (Figure 5a). We can also see in Figure 5b many spheres of about 390 to 700 nm in diameter, with shells of approximately 40 nm in thickness, which consist of ordered arrays of mesopores similar to those in Figure 5a. However, ordered array of mesopores was only observed in these arc rings. Such a disorderedordered mesostructure has not been reported in the literatures to date, to the best of our knowledge. Although some of vesicle structures have been reported to present occasionally under a similar mild alkaline condition [25], the shell of the vesicles previous reported was composed of disordered mesotubules.
(a) 80(} (b_,)_Adsorption 700~. -" Desorption j
f
"~ 60~
40 -~ I[ I ~
as.'s~~ynthesized .... 20
1oo
]
2
4
6 8 20 (degrees)
10
0.0
"2
Pore size (nm)
0.2 0.4 0.6 0.8 1.0 Relative pressure (P/Po)
Figure 4. (a) Powder XRD patterns of synthesized samples using mixed surfactant templates; (b) N2 adsorption-desorption isotherm and the pore size distribution (inset) of the calcined samples. If single cationic surfactant CPCI was used in the synthesis, one well-defined hexagonal mesoporous MCM-41 could be obtained [13,28]. On the other hand, lamellar mesophase may be produced when neutral surfactant alkylamine was used in this mild alkaline system. However, no significant yield of mesostructural products could be synthesised when nonionic surfactant Triton X-100 was used as the only template in the given synthetic system. If nonionic surfactant was initially protonated under low pH conditions, followed by base catalyzed neutralization in order to increase pH value in the solution to certain threshold values (between 5 and 6), mesoporous silicate MSU-2 possessing non-symmetrical worm-like pores could be synthesized. This synthetic route was labeled as N~ - (where N O stands for nonionic surfactant, M + for Na + or NH4+, and I- for hydroxylated tetraethyl orthosilicate) [29]. Sodium silicate solutions can also be used to synthesize mesoporous silicas by their addition to an acidic Triton X-100 solution (pH around 2), followed by a pH adjustment using NaOH or NHaOH to the value required for the polycondensation (usually between 6 and 7) [30]. It has been revealed that mesoporous silicas templated by cationic surfactants (S+) under basic conditions are produced by a S+I pathway where anionic silicate precursors (I) are assembled at the interface of rod-like micelles through electrostatic interactions, resulting in a hexagonal mesostructure [31]. Whereas mesoporous silicas templated by nonionic poly(ethylene oxide) (PEO)-based surfactants (N~ under neutral conditions are synthesized by a neutral N~ Otemplating pathway where neutral silica precursors (I~ are assembled at the interface of flexible worm-like micelles by hydrogen bonding, resulting in a disordered mesostructure [26].
139 However, in the present basic solution, the neutral surfactant N o should combine with the cationic surfactant S+ and with the anionic inorganic species to form a complex surfactant-silica mesophase, since these completely miscible surfactants exhibit complex phase behaviors in aqueous solutions [32] forming liquid-crystalline micellar mesophases cooperatively. Thus, the mesoporous silica synthesized by mixed cationic-nonionic surfactant templating under the present basic condition might form via a complex templating pathway: (S+N~ (M+ NH4+) and could be referred to a modified formation mechanism. The resultant ordered-disordered mesostructure could be affected by the ratio of the nonionic to cationic surfactants, and could be regarded as an intermediate structure between the ordered hexagonal MCM-41 and a disordered mesostructure. Since the nonionic/cationic surfactant ratio is larger than 1 in the present study, we believe that nonionic surfactant Triton X-100 must play an important role in the synthesis of the present complex mesostructure. In summary, mesoprous silicas with disordered-ordered hierarchical mesostructure has been synthesized by mixed cationic-nonionic surfactant templating under mild alkaline conditions. The present results demonstrate that mixed cationic-nonionic surfactant templating route can provide a suitable approach for the synthesis of silica-based mesoporous molecular sieves. It is certainly true that the synthesis has not yet been optimized. Further improvement of the synthetic method and studies of the relationship between the structure and the properties of these new mesoporous materials are currently carried out in these laboratories. =
Figure 5. TEM micrographs of calcined silicas showing disordered-ordered mesostructure.
140 4. ACKNOWLEDGEMENT This work is financially supported by the National Natural Science Foundation of China (NSFC), Chinese Academy of Sciences, and the Belgium Federal Government PAI-IUAP 4/10 project. 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.W. Sheppard, S.B. McCullen, J.B. Higgins, and J.L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 3. A. Sayari, and P. Liu, Microporous Mater., 12 (1997) 149. 4. J.Y. Ying, C.P. Mehnert, and M.S. Wong, Angew. Chem. Int. Ed., 38 (1999) 56. 5. D. Zhao, P. Yang, Q. Huo, B.F. Chmelka, and G.D. Stucky, Curr. Opin. Solid State Mater. Sci., 3 (1998) 111. 6. H. Yang, N. Coombs, and G.A. Ozin, Nature, 386 (1997) 692. 7. H. Yang, G.A. Ozin, and C.T. Kresge, Adv. Mater., 10 (1998) 883. 8. S.M. Yang, H. Yang, N. Coombs, I. Sokolov, C.T. Kresge, and G.A.Ozin, Adv. Mater., 11 (1999) 52. 9. S.M. Yang, I. Sokolov, N. Coombs, C.T. Kresge, and G.A. Ozin., Adv. Mater., 11 (1999) 1427. 10. Q. Huo, J. Feng, F. Schfith, and GD. Stucky, Chem. Mater., 9 (1997) 14. 11. H.-P. Lin, and C.-Y. Mou, Science, 273 (1996) 765. 12. P. Yang, D. Zhao, B.E Chmelka, and GD. Stucky, Chem. Mater., 10 (1998) 2033. 13. D. Khushalani, A. Kuperman, N. Coombs, and G.A. Ozin, Chem. Mater., 8 (1996) 2188. 14. L. Qi, J. Ma, H. Cheng, and Z. Zhao, Chem. Mater., 10 (1998) 1623. 15. R. Ryoo, S.H. Joo, and J.M. Kim, J. Phys. Chem. B, 103 (1999) 7435. 16. R. Ryoo, C.H. Ko, and I.-S. Park, Chem. Commun., (1999) 1413. 17. F. Chen, X. Yan, and Q. Li, Stud. Surf. Sci. Catal., 117 (1998) 273. 18. Z. Yuan, and W. Zhou, Chem. Phys. Lett., 333 (2001) 427. 19. K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscow, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603. 20. W. Zhou, and J. Klinowski, Chem. Phys. Lett., 292 (1998) 207. 21. W. Zhou, R. Mokaya, Z. Shan, and T. Maschmeyer, Prog. Nat. Sci.; 11 (2001) 33. 22. M. Adachi, T. Harada, and M. Harada, Langmuir, 16 (2000) 2376. 23. R. Mokaya, Microporous Mesoporous Mater., 44-45 (2001) 119. 24. H.P. Lin, C.Y. Mou, S.B. Liu, Adv. Mater., 12 (2000) 103. 25. Z.Y. Yuan, W.Z. Zhou, Z.L. Zhang, J.Q. Liu, J.Z. Wang, H.X. Li, and L.-M. Peng, Surf. Interface Anal. 32 (2001) 193. 26. R. Ryoo, J.M. Kim, C.H. Ko, and C.H. Shin, J. Phys. Chem., 100 (1996) 17718. 27. S.A. Bagshaw, E. Pouzet, and T.J. Pinnavaia, Science, 269 (1995) 1242. 28. Z.Y. Yuan, W. Zhou, L.M. Peng, J.Z. Wang, and H.X. Li, Stud. Surf. Sci. Catal., 135 (2001) 979. 29. S.A. Bagshaw, T. Kemmitt, and N.B. Milestone, Microporous Mesoporous Mater., 22 (1998) 419. 30. L. Sierra, B. Lop6z, H. Gil, and J.-L. Guth, Adv. Mater., 11 (1999) 307. 31. Q. Huo, D.I. Margolese, U. Ciesla, P. Feng, T.E. Gier, P. Sieger, R. Leon, P.M. Petroff, F. Sch/ith, and GD. Stucky, Nature, 368 (1994) 317. 32. P.M. Holland and D.N. Rubingh, Surfactant Sci. Ser., 37 (1991) 141.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
141
Synthesis and characterisation o f super-microporous aluminosilicates prepared via primary amine templating Emesto Bastardo-Gonzalez, a Robert Mokayab and William Jones a aDepartment of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.
bSchool of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, U.K.
Super-microporous aluminosilicate catalysts, possessing pores in the range 14 - 20A, can be prepared via primary amine templating alone or in combination with post-synthesis A1 grafting. Post-synthesis grafting of extra A1 also serves to remarkably improve the acidity and catalytic activity of the materials. Preparation of the super-microporous materials is made possible by circumventing the limitation on the lowest pore size obtainable, during supramolecular templating with primary amines, by improving the incorporation of A1. This is achieved by ensuring optimum dissolution of the A1 source and the formation of Al-rich aluminosilicate precursors prior to addition of the primary amine template. Performing postsynthesis grafting of extra A1 reduces the pore size of the resulting materials further thus increasing the possible pore size range. The super-microporous aluminosilicates are structurally as well ordered as any other primary amine templated mesoporous aluminosilicate materials. 1. INTRODUCTION The synthesis of porous inorganic materials, which may be used as heterogeneous catalysts, adsorbents or as hosts for composite materials is currently attracting a great deal of interest in materials science research. The ability to tailor the nanostructure and in particular the pore size of ordered inorganic solids is a key factor in their use in areas such as catalysis and molecular separation. For example the design and synthesis of materials that have a well defined pore structure similar to that of zeolites, but with larger pores has been a main research goal in solid acid catalysis over the past decade [ 1]. Recent advances, which include the synthesis of the M41S family of mesoporous solids [2], have to some extent achieved this goal. Many other varieties of mesoporous materials, with pores of size typically above 25A have been prepared [3]. Until recently, much less attention has been given to the synthesis of materials with pores in the super-microporous (10 - 20A) range. Solid acid materials in this pore size range are important since they bridge the gap between raditional microporous zeolites and mesoporous materials. Furthermore in addition to being able to handle molecules that are too large to be processed by zeolites, super-microporous solid acids are expected to exhibit interesting shape and size selectivity in the conversion of such molecules. Shape and
142 size selectivity is a desirable property that is generally lacking in mesoporous materials due to their larger pores. In general, supramolecular templating is used in the preparation of mesoporous materials while microporous materials are synthesised via molecular templating. Recently there have been several reports on the use of supramolecular assemblies of surfactants to template the formation of mesostructured materials with pores in the micropore and supermicropore range. Bagshaw and Hayman [4] used o~-hydroxy-bolaform surfactants to template the formation of super-microporous silicas. They obtained porous silica materials with pores in the 14 - 21 A range and pore symmetries bearing similarity to Lot, SBA-2 (P63/mmc) and M a (cram) phases. Zhao et al [5] reported the preparation of MCM-41 materials with tailored (narrow) pore openings in the super-microporous range. On the other hand Sun et al [6] used adamantanamine as templating agent for the synthesis of amorphous microporous silica and Serranno et al [7] employed cetyltrimethylammonium or dodeytrimethylammonium ions for the formation of microporous aluminosilicate materials. We have recently reported a preliminary account of the primary amine templated synthesis of super-microporous aluminosilicate materials whose pore size can be further tailored via postsynthesis grafting of A1 [8]. A further advantage of the post-synthesis grafting of extra AI is that it also serves to remarkably improve the acidity and catalytic activity of the materials. Here we report in detail the preparation, characterisation and properties of such primary amine templated super-microporous aluminosilicates. We have previously reported on the preparation of primary amine templated mesoporous aluminosilicates (AI-MMS, A1-HMS) [9-11 ]. Our previous work has shown that
the pore size of A1-MMS (A1-HMS) materials is to some extent dependent on the alkyl chain length of the primary amine template with longer amines resulting in larger pores [12,13]. The pore size was also found to be dependent on the Si/AI ratio, i.e., pore size reduced at low Si/A1 ratios [9-13]. This meant that pore sizes at the lower end of the mesopore range (-~20A) were only obtainable for highly aluminous samples and that pores in the super-microporous range (10 -20A) could not be achieved even at Si/A1 ratios as low as 7 [9-13]. Here we show that it is possible to circumvent the limitation on the lowest pore size obtainable by improving the incorporation of AI. This is achieved by modifying our previous synthesis procedure [ 11 ] so as to ensure optimum dissolution of the A1 source. Furthermore performing post-synthesis grafting of extra AI can reduce the pore size of the resulting materials further. 2. MATERIALS AND METHODS 2.1. Materials The synthesis procedure used was as follows; desired quantities of aluminium isopropoxide (AI(i-CaH70)3) were dispersed in 35 ml of isopropanol with vigorous mechanical stirring at 70~ for 15 minutes in order to dissolve the highest possible amount of the aluminium source. The resulting solution was cooled to room temperature and then added under magnetic stirring to a solution of 0.2 mol. tetraethyl orthosilicate (TEOS) in 80 ml of absolute ethanol. The resulting mixture was then heated at 70~ under vigorous stirring for 4 hours to obtain the polymerised Si-O-A1 species. After cooling to room temperature (under stirring) the inorganic precursor was added to a clear solution of 0.05 mol dodecylamine
143 (DDA) in a mixture of 80 ml of deionised water and 120 ml of absolute ethanol under slow stirring. The stirring was maintained for 5 minutes after which the resulting mixture was allowed to age at room temperature for 20 hours. The solid product was obtained by filtration, washed with two portions of 100 ml of ethanol, air-dried overnight at room temperature and finally calcined in air at 650~ for 4 hours. This procedure was used to prepare aluminosilicate materials with Si/AI molar ratios of 40:1, 20:1, 10" 1 designated AIMMSX where X is the synthesis gel Si/A1 ratio.
Grafting; Al-grafiing was performed on the A1-MMSX samples as follows; 1.28 g of a solution of chlorhydrol ([A12Cl(OH)5.2H20], 6.4 mole 1"l with respect to aluminium) was dissolved in 50 ml of distilled water and stirred at 80~ for 1 hour. 1.0 g of A1-MMSX was added to the solution and the resulting suspension stirred for another hour at 80~ After cooling to room temperature, the solid was filtered and washed with distilled water until free of CI" ions, dried in the oven at 100~ and finally calcined at 500~ for 4 hours. The resulting Al-grafted materials were designated A1A1-MMSX.
2.2. Characterisation Powder X-ray diffraction (PXRD) pattems were recorded using a Philips PW 1710 diffractometer. A graphite monochromator and CuKa radiation ()~ = 1.5418 A) were used with a Ni filter for KB attenuation. A step size of 0.02 degrees (2 theta) and a scan rate of 2 seconds per step were used. Elemental compositions were obtained using X-ray fluorescence (XRF). Surface area, pore volume and pore size were determined at -196~ by nitrogen sorption, using a micromeritics ASAP 2400 sorptometer or Coulter SA3100 sorptometer. Before analysis the samples were oven dried at 120~ and degassed at 200~ for at least 5 hours. The surface area was calculated using the BET method based on adsorption data in the partial pressure (P/Po) range 0.05 to 0.2 and the total pore volume was determined from the amount of N2 adsorbed at a P/Po = ca. 0.99. The Barrett-Joyner-Ha!enda (BJH) method and tplot analysis were used to determine the pore size and micropore surface area and pore volume.
Acidity measurement; The acid content was obtained by thermogravimetric analysis, following the adsorption of cyclohexylamine (CHA) on the samples under study. Samples were exposed to cyclohexylamine at room temperature for 16 hours and then heated in an oven at 100~ for 4 hours in order to remove any physisorbed amine. TGA curves were obtained for the CHA-containing samples using a Polymer Laboratories TG 1500 analyser with a heating rate of 20~ under nitrogen flow of 25 ml/min. The weight loss between 240 and 420~ was used to calculate the acid content assuming that each molecule of base interacts with one acid site. Catalytic evaluation; Two reactions were used to evaluate the catalytic activity of the study materials; cumene conversion and the alkylation of toluene. Conversion of cumene was performed in a tubular stainless steel, continous flow fixed-bed microreactor system with helium as carrier gas at 300oc and WHSV of 5.5. The catalyst bed (100 mg; 30-60 mesh) was first activated for 1.5 hours at, 500~ under helium (25 ml/min). During the reaction, a stream of cumene vapour in helium was generated using a saturator at room temperature. The reaction products were separated and analysed using a Carlo Erba HRGC 5300 gas chromotograph. Gas chromotographs were obtained automatically on
144 samples of the product stream which were collected at regular intervals using a Valco 6-port valve. The gas chromatographs were used to calculate the extent of cumene conversion which was principally to propene and benzene with only small amounts of t~-methylstyrene. The reaction products indicated that, under the reaction conditions used, Bronsted acid sites were the active sites. To evaluate both Bronsted and Lewis acis sites, the alkylation of toluene using benzyl alcohol or benzyl chloride was performed following a previously published procedure [14]; 100 mg of the catalyst (for benzyl alcohol) and 250 g (for benzyl chloride) were placed in a special pyrex tube and activated in a oven at 250~ for one hour. The tube containing the catalyst was cooled to the reaction temperature (50~ for benzyl chloride or 80~ for benzyl alcohol) and placed in a glycerol bath already at the reaction temperature. A pre-heated mixture made up of 20:1 molar ratio of toluene and the alkylating reagent was added to the catalyst with magnetic stilting. The reaction was carried out under a continuous flow of synthetic air. Reaction mixture samples were collected in vials every 30 minutes and analysed by gas chromatography. 3. RESULTS AND DISCUSSION The elemental compositions of the present AI-MMS materials are shown in Table 1. The bulk Si/A1 ratios of the samples indicate that Si and A1 are incorporated into the solid framework in proportions closely related to the synthesis gel composition. A significant observation worth pointing out is that A1-MMS40 and AI-MMS 10 are AI rich, i.e., a greater proportion of A1 in the synthesis gel is incorporated into the solid product compared to Si. This is a departure from our previous studies where we always obtained silica rich materials [9-13]. This is the first indication that the modified synthesis procedure used here, in which the dissolution of the AI source is optimised, does allow for greater A1 incorporation into the solid product. In our previous studies, the synthesis of A1-MMSX materials did not include the ahmainium isopropoxide dissolution step, i.e., the vigorous stilting of the A1 source in isopropanol at 70~ for 15 min was not performed. It is likely that the dissolution step enables a much higher proportion of the aluminium isopropoxide to dissolve thus forming AI species which readily interacted with the silica species after the addition of TEOS to form AIrich polymeric aluminosilicate species. Powder XRD pattems for the present AI-MMS samples are shown in Figure 1(a). The patterns are similar to those of comparable mesoporous materials [ 11-13,15], i.e., comprising of a single (100) peak. This is an indication that the present samples are as well ordered as any other primary amine templated mesoporous aluminosilicates. It therefore appears likely that the higher extent of A1 incorporation achieved here does not compromise structural integrity. The d spacing and textural properties of the A1-MMS materials are shown in Table 2. We note that the d spacing of each of the present samples is lower (by ca. 5%) than we have previously observed for equivalent (in terms of synthesis gel Si/A1 ratio) materials [11 ]. We attribute the apparently smaller lattice size to the higher extent of AI incorporation achieved here. A feature common to both the present set of samples and those from previous studies is that the d spacing reduces as the amount of A1 incorporated increases [9-13]; this emphasises the importance of A1 content in determining the lattice size. It is worth noting that the d spacings observed here are generally lower than those normally obtained for M41S type mesoporous materials [ 1-3].
145
Table 1. Elemental Composition, acid content and catalytic activity of super-microporous A1-MMS samples. Sample Si/A1 Acid content Cumene conversion (%) (mmole/g) 50 min 150 min A1-MMS40 39.0 0.36 35 28 A1A1-MMS40 22.8 0.57 53 47 AI-MMS20 22.0 0.56 47 41 A1A1-MMS20 14.9 0.75 59 54 A1-MMS10 9.0 0.73 59 51 A1A1-MMS 10 7.4 0.82 67 60 Table 2. Textural properties of super-microporous AI-MMS samples. Values in parenthesis micropore surface area and volume. Sample Basal (dl00) Surface Pore Spacing Area Volume (A) (m2/g) (cm3/g) A1-MMS40 32.4 1023 (488) 0.45 (0.18) A1A1-MMS40 32.0 643 (406) 0.33 (0.17) AI-MMS20 31.1 864 (497) 0.38 (0.19) A1AI-MMS20 30.2 562 (403) 0.28 (0.17) A1-MMS 10 30.5 684 (520) 0.31 (0.20) A1A1-MMS 10 29.8 358 (286) 0.24 (0.18) (a)
are Pore
Size 20.5 17.7 18.4 16.3 17.7 14.5
(b)
.m c-
t_
.m
E
0
2
4 2a/~
6
8
i
|
|
2
4
6
8
2~
Figure 1. Powder X-ray diffraction patterns of primary amine templated super-microporous aluminosilicates before (a) and after (b) post-synthesis grafting of A1.
146 The clearest indication that the greater AI incorporation attained here results in materials with super-microporous rather than mesoporous character is given by the textural parameters in Table 2. First we note that the surface area, pore volume and pore size obtained for the present samples are generally lower than previously observed for mesoporous A1MMS (AI-HMS) materials [ 11-15]. We have previously reported surface area of 1200, 1195 and 967 m2/g, and pore volume of 0.65, 0.52 and 0.49 cm3/g for mesoporous AI-MMS materials prepared (with dodecylamine as terriplate) at gel Si/A1 ratios of 40, 20 and 10 respectively [11]. The pore volume of the present samples is therefore at least 30% lower than that of analogous mesoporous materials [ 11]. A particular feature of the present samples is that they exhibit a high proportion of micropore surface area and volume as shown in Table 2. This is a significant finding because we have not previously observed any micropore surface area or volume in, for example, mesoporous AI-MMS40 and A1-MMS20 materials [ 11]. In the present samples the proportion of micropore surface area increases from 47% for AI-MMS40 to as high as 76% for A1-MMS 10 while the proportion of micropore volume rises from 40 to 65%. We attribute the high level of microporosity observed for the present A1-MMSX materials to the super-microporous nature of the pore channels they possess. The pore size of the materials (obtained using BJH analysis) is, as shown in Table 2, clearly in the supermicroporous range. This is also illustrated by the N2 sorption isotherms shown in Figure 2. The isotherms exhibit high adsorption at low (P/Po < 0.2) partial pressures which is characteristic of super-microporous materials. The isotherms do not exhibit the mesopore filling step (at P/Po > 0.2) normally observed for mesoporous materials. In Figure 2 we have, for comparison, also included sorption isotherms for equivalent (w.r.t. synthesis gel Si/A1 ratio) materials, which have been synthesised without the A1 dissolution step. Such materials
~, 400 ~
400
o~~ 300 i
300
~
200 J
i
1~ 100
"~
o
200 100
0.0
0.5
Partial pressure
1.0
(PIPo)
o
0.0
0.5
Partial pressure
1.0
(PIPo)
0.0
0.5
Partial pressure
1.0
(PIPo)
Figure 2. Nitrogen sorption isotherms of primary amine templated mesoporous aluminosilicates (top) and super-microporous aluminosilicates (middle), prepared at gel Si/A1 ratio of (a) 40, (b) 20, (c) 10. The bottom isotherms are for the super-microporous aluminosilicates after they were subjected to post-synthesis grafting of A1.
147 typically possess 15-20% less A1. As shown in Figure 2, the lower A1 content materials have larger pores and in some cases (top isotherm Figure 1(a)) exhibit a clearly defined mesoporefilling step. Post-synthesis grafting of A1 was found to increase the microporous character of the A1-MMS samples. The effect of Al-grafting on pore size and porosity is illustrated in Figure 2; the super-micropore filling step is shifted to lower partial pressures (lower pore size) after grafting. As shown in Table 2, the pore size of A1A1-MMS samples is lower than that of A1MMS materials by 2 to 3 A. This reduction in pore size is accompanied by a decrease in surface area and pore volume, which in turn leads to an increase in the proportion of micropore surface area and volume. Indeed sample A1AI-MMS 10 is virtually microporous, with a pore size of 14.5 A and a proportion of micropore surface area and volume above 75%. It is likely that the extra A1 is grafted onto the inner pore walls of the AI-MMSX materials thus reducing the pore size. This does not however affect the basal spacing (see Table 2) since the pore shrinkage occurs within existing pores. We note that structural ordering (as indicated by powder XRD - see Figure l(b)) is largely unaffected by the grafting process. Only modest reductions in the intensity of the basal peak are observed. We however do not think that this reduction in the intensity of the basal peak is due to degradation of structural integrity. It is more likely caused by changes in the scattering domain size (SDS), i.e., the SDS reduces during grafting. Indeed using scanning electron microscopy (SEM), we have observed that the particle size of A1A1-MMSX samples is smaller than that of the AIMMSX sample from which they are derived. This is illustrated in Figure 3 for A1-MMS20 and A1A1-MMS20 samples. It is noteworthy that although the particle size reduces, the spherical particle morphology is maintained after grafting. A uniform particle size is also retained after grafting. (a) A1-MMS20
(b) A1A1-MMS20
Figure 3. Representative scanning electron microscopy (SEM) micrographs of (a) A1-MMS20 and (b) A1A1-MMS20 illustrating the effect of post-synthesis Al-grafling on the particle size. Scale bar = 10 gm. In addition to tailoring the pore size, Al-grafiing also increases the acid content and catalytic activity as shown in Table 1. The increase in acid content after A1 grafting is greatest
148 for A1A1-MMS40 (-~ 60%) and lowest for AIAI-MMS 10 (- 12%). The introduction of extra AI clearly creates new acid sites. This is possible since the AI-MMS materials possess exposed and accessible silanol groups, which can act as anchoring points for the extra AI [ 16,17]. For extra acid sites to be generated, some of the extra A1 must be incorporated into the framework. We note that 27A1 MAS NMR spectroscopy indicated that the majority of AI in the A1A1-MMSX samples is in tetrahedral coordination. The proportion of octahedrafly coordinated (extra-framework) AI in A1A1-MMSX samples is however generally higher than that for corresponding AI-MMSX samples indicating that some of the extra AI is not incorporated into the framework. It is interesting to note that sample A1-MMS 10 which has the highest cation exchange capacity (CEC = 42 mEq/100g) takes up less 'extra AI' than AIMMS40 with a CEC of 12 mEq/100g. This implies that the take up of extra AI is not entirely an ion exchange process. Rather it appears that the extra AI is taken up chiefly via a grafting process that occurs on silanol groups [16,17]. This is consistent with our previous observation that the concentration of silanols in A1-MMS type materials decreases with A1 content [11]. It is therefore expected that A1-MMS40 would have more silanol groups (capable of anchoring greater amounts of extra A1) compared to AI-MMS10. Indeed under similar grafting conditions the pure silica material incorporates at least twice as much A1 as the A1-MMS samples. The solid acid catalytic activity of AI-MMS (AI-HMS) materials is a well studied topic. Here the discussion on catalytic activity is restricted to the differences between A1-MMS and AIAI-MMS materials. As shown in Table 1, all samples present considerable catalytic activity for the conversion of cumene. The percentage of cumene cracked (predominantly to benzene and propene) depends directly on the Si/A1 ratio and the acid content. Comparison of the activity at 50 and 150 min. indicates that the rate of deactivation is comparable for all samples. It is also clear that A1AI-MMS samples are more active than their corresponding AIMMS samples. The increase in activity, on grafting, is highest for A1-MMS40 and lowest for AI-MMS 10. The increase in activity for cumene conversion after grafting therefore mirrors the percentage (and absolute) increases in A1 content and acidity. Furthermore since the cumene conversion to benzene and propene occurs on Bronsted acid site, we may infer that there is an increase in the number of Bronsted acid sites after grafting. The catalytic activity of Bronsted and Lewis acid sites can be separately evaluated using the alkylation of toluene. Bronsted acid activity was tested using the alkylation of toluene with benzyl alcohol and Lewis activity was tested using the alkylation of toluene with benzyl chloride. In both cases 1,2 and 1,4 methyldiphenylmethane are the main products and the activity is pseudo-first-order rate for both reactions. In all cases Al-grafting was found to increase the extent of alkylation. In Figure 4, the effect of gratting extra AI is illustrated using sample A1-MMS20. The extent of alkylation over AIA1-MMS20, for both reactions, was found to be consistently higher at all reaction times. The variation of rate of alkylation with time suggests that Alkylation over A1A1-MMS20 may approach equilibrium quicker than happens for A1-MMS20. We note that similar trends were observed for A1-MMS40 and A1MMS10 and their grafted derivatives. The catalytic data in Figure 4 therefore shows that grafting of extra AI has beneficial effects not only for Bronsted acid catalysis but also for Lewis acid catalysed reactions.
149
30
(a)
(b)
25 o~ tO i,..
(D > c"
O (O
20 15 10
,
,
,
i
0 1 2 3 4 5 6
0 1 2 3 4 5 6
Time (hour)
Time (hour)
,
,
Figure 4. Conversion as a function of reaction time in the alkylation of toluene using (a) benzyl alcohol and (b) benzyl chloride on A1-MMS20 (dark symbols, 9 and o) and AIA1MMS20 (open symbols, [] and o). Primary amine templating of mesostructured aluminosilicates is known to provide solid acid catalysts whose activity is generally higher than that of materials prepared via quaternary ammonium ion templating [11,12,18]. The improved activity has been variously attributed to a number of factors including improved accessibility to acid sites, a unique pore structure and textural mesoporosity, which allow easy diffusion of molecules [11,12,15,18]. Recently, it has also been shown that the wormhole-like structure of primary amine templated aluminosilicates has 3-dimensional connectivity [19]. Three-dimensional connectivity is an important factor with respect to catalytic activity. This report therefore represents the first synthesis of super-microporous aluminosilicates with 3-dimensional connectivity. These super-microporous materials combine all the advantages of primary amine templated aluminosilicates and also offer the potential for size and shape selective catalysis of large substrates which is not possible for mesoporous materials. ACKNOWLEDGEMENTS EBG would like to thank CONICIT for the Venezuelan financial support. COT and ORS scholarships are also acknowledged. R.M. is grateful to the EPSRC for an Advanced Fellowship. REFERENCES
1. A. Corma, Chem. Rev., 97 (1997) 2373. 2. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710.
150 3. 4. 5. 6. 7.
J.Y. Ying, C. P. Mehnert and M. S. Wong, Angew. Chem. Int. Ed., 38 (1999) 56. S.A. Bagshaw and A. R. Hayman, Chem. Commun., (2000) 533. X.S. Zhao, G. Q. Lu and X. Hu, Chem. Commun., (1999) 1391. T. Sun, M. S. Wong and J. Y. Ying, Chem. Commun., (2000) 2057. D.P. Serrano, J. Aguado, J. M. Escola and E. Garagorri, Chem. Commun., (2000) 2041. 8. E. Bastardo-Gonzalez, R. Mokaya and W. Jones, Chem. Commun., (2001) 1016. 9. R. Mokaya and W. Jones, Chem. Commun., (1996) 981. 10. R. Mokaya and W. Jones, Chem. Commun., (1996) 983. 11. R. Mokaya and W. Jones, J. Catal., 172 (1997) 211. 12. R. Mokaya and W. Jones, Catal. Lett., 49 (1997) 87. 13. R. Mokaya and W. Jones, J. Mater. Chem., 8 (1998) 2819. 14. H. H. P. Yui and D. R. Brown, Catal Lett., 56 (1998) 57. 15. W. Z. Zhang, T. R. Pauly and T. J. Pinnavaia, Chem. Mater., 9 (1997) 2491. 16. R. Mokaya and W. Jones, Phys. Chem. Chem. Phys., 1 (1999) 207. 17. R. Mokaya and W. Jones, J. Mater. Chem., 9 (1999) 555. 18. T. J. Pinnavaia and W. Z. Zhang, Stud. Surf. Sci. Catal., 117 (1998) 23. 19. J. Lee, S. Yoon, S. M. Oh, C. H. Shin and T. Hyeon, Adv. Mater., 12 (2000) 359.
Studies in Surface Scienceand Catalysis 141 A. Sayariand M. Jaroniec (Editors) 9 2002 ElsevierScienceB.V. All rightsreserved.
151
A 1 - M C M - 4 1 synthesis using Al-isopropoxide as A1 source R. Birjega, R. Ganea, C. Nenu, Gr. Pop and A. Jitianu ZECASIN, Spl.Independentei 202, PO-Box 12 304, 77206 Bucharest, Romania. Fax: 40 1 3125241, e-mail: zecasin@ com.pcnet.ro *Institute of Physical Chemistry "I.G.Murgulescu", Bucharest, Romania. 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] a great deal of work was devoted to the possibility of isomorphous substitution of heteroatoms, especially aluminum, into the silica network, in order to modify the composition of the material. The aluminum substitution in silica MCM-41 is of particular interest, both for an academic or technological approach, as the expected acid properties and ion exchange capacity modifications, allowing the design of catalysts or catalyst support materials for further active species grafting. Numerous preparation strategies were described with a variety of chemical systems and different raw materials [3-10]. The synthesis of the ordered mesoporous materials implies a complex array of reactions in media where reactants organic and inorganic species and conditions can be continuously changed. In this work we focused on a series of A1-MCM-41 an aim to establish an appropriate Si/A1 atomic ratio domain of the reaction mixture to be used in relatively low concentration surfactant syntheses. The synthesis conditions were selected bearing in mind a possible application at large scale of such materials. The surfactant used was cetyltrimethylammonium bromide (C16TMABr). As silicon source a combination of alkali silicate, organic silicate and pure silica powder, was used. Aluminum isopropoxide, a monomeric alumina precursor was selected as alumina source. Janicke et al. [3] claimed that aluminum isopropoxide, as aluminum source is efficient in controlling Al-framework incorporation, while different results were reported by Luan et al. [7]. Janicke et al considered that syntheses employing aluminum isopropoxide yielded products required gentle heat treatment in an inert atmosphere of flowing nitrogen followed by calcination in flowing oxygen. However, we performed all the calcinations in air with low heating ramp l~
152 2. EXPERIMENTAL SECTION 2.1 Synthesis A series of mesoporous aluminosilicate A1-MCM-41 have been synthesized using cetyltrimethylammonium bromide (C16TMABr, Fluka) as surfactant. A1-MCM-41 syntheses of different Si/A1 mole ratios (5-120) at low surfactant concentration (CI6TMABr/SiO2 mole ratio= 0.15 ) using similar procedures to those reported [1-3] were performed. The nature of the silicon precursor is considered to be an essential factor for obtaining a high-quality MCM-41 mesoporous material. Although, it has been used as silicon source, various raw materials, the degree of condensation of the walls and the degree of pore ordering are influenced by a proper choice of the silicon precursor in connection with the synthesis conditions. Highly basic gels favor the lamellar phase, while weakly basic gels lead to the formation of amorphous silica. The alkalinity level of the synthesis gel plays an important role in the MCM-41 formation. Consequently, a combination of alkali silicate, organic silicate and pure silica powder is preferred to be used as a silicon source [4,5]. Therefore, we used a mixture of sodium silicate, tetramethylammonium silicate and fumed silica as an adequate silicon precursor for A1MCM-41 syntheses. Aluminum isopropoxide (Merck) was the aluminum source and tetramethylammonium hydroxide solution (25% TMAOH, Aldrich) was used as mineralizer. The aluminum-containing gels were prepared as follows: Firstly, a tetramethylammonium silicate solution (TMA/Si=0.5, 10% SiO2) was prepared by mixing appropriate amounts of 25% TMAOH solution and fumed silica (98% SiO2, Sigma). Then, the sodium silicate solution (27% SiO2, 9% Na20, Merck), water and fumed silica were added to the teramehylammonium silicate solution, under continuous stirring. Secondly, a 15% aqueous CI6TMABr solution was added to the above silicate mixture under vigorous stirring and a well-homogenized gel was obtained. Finally, an adequate amount of aluminum isopropoxide was added into the surfactant-silicate mixture. The mole chemical composition of the aluminosilicate gel was: SiO2:0.07Na20:0.085TMAOH:0.004-0.40A1203:0.15C16TMABr:60H20 A reference synthesis using only purely siliceous synthesis gel was prepared by the same procedure, without the addition of the aluminum source. After stirring for one hour at room temperature the synthesis gel having, a pH around 12 was loaded into a 500 ml Teflon-lined autoclave and heated at 100~ for 48 hours, under continuous stirring. After cooling to room temperature, the resulting product was repeatedly washed with distilled water until the pH reached 7.5, separated by filtration and dried in air, at room temperature. The surfactant was removed from the as-synthesized product by calcination in air (static conditions) with a heating rate of I~ from room temperature to 550~ and maintained at 550~ for 6 hours.
153 2.2 Characterization
The as-synthesis and calcinated samples were characterized by X-ray powder diffraction on a DRON-3 diffractometer using monochromated CuKa radiation. Diffractions patterns were recorded from 1~ to 30~ (20) with a resolution of 0.02~ and a count time of 20s at each point. The diffraction peaks were fitted assuming a Pearson-VII function for the peak profile. SiO2 a-quartz was used as internal standard as well as for the background extraction in the low angle region. The IR spectra were recorded between 1600 cm1 to 400 cm1 with a SPECORD M80 spectrophotometer using KBr pellets technique. The BET surface areas were measured by the N2 physisorption at 77K using a Grimm BET automatic surface analyzer. A preheating in a helium atmosphere at 200~ for 2 hours was performed in order to consider the percent of water in the calculation of the BET surface areas. The overall acidity of the samples was evaluated by a NH3-TPD. The procedure consists of: the preheated of the samples at 200~ for 2 hours in argon stream, the saturation of the surface with ammonia at 120~ in a mixture of ammonia-argon gas stream and the removal of the physically adsorbed ammonia. Finally, the temperature was linearly ramped up to 600~ and the total amount of desorbed ammonia was continuously trapped in sulfuric acid solution and quantified by titration with NaOH solution. For comparison, the acid content of the samples was also evaluated using a simple method of titration with a hydroxide solution, taking into account that one equivalent of base corresponds to one equivalent of proton (H§ associated to the framework aluminum sites [11,12]. 3. RESULTS AND DISCUSSION As figure 1 shows, except for the high aluminum content synthesis (Si/Al=l.25), the XRD patterns of the as-synthesized sample exhibit well -defined peaks of pseudohexagonal structure of MCM-41. The peaks are indexed in P6 symmetry with a lattice constant ao, actually the distance between the centers of two neighbor pores, calculated as ao=2dl00/~/3. Along with the pattern of an ordered channels disposition, the broad peak appearing between 20-25 ~ (20) usually assigned to an amorphous silica or silica-alumina ,is considered. Table 1 presents the structural data of the as-synthesized MCM-41 samples. o
A I/(A
I+ S i)
0.032 ".444
0
~ 2oC
uKa(*)
Figure 1. XRD pattems of as -synthesized MCM-41 content
samples with different aluminum
154 From the data presented in Table 1 one could notice the effect of aluminum incorporation into MCM-41 materials. For low contents of aluminum the diffraction intensity of the main peak is higher than that of the pure silica analogue suggesting a tendency of an improvement of the condensation of the silica structures in the presence of the aluminum. There is no a regular increases of the lattice constant ao with the aluminum content due probably, to the low aluminum amount and a high amount of silanol groups. For the last sample, the small ao value is explained by the formation of aluminum-rich dense phase, confirmed by SEM and BET measurements and therefore a low content aluminum MCM-41 is formed in this case. There is a linear relationship between the relative intensities of the 110 diffraction peak and the relative intensities of the 210 peak to the 200 peak. This result is in agreement with the model structure for MCM-41 proposed by Feuston & Higgins [13]. In the simulated patterns in accordance with the experimental patterns, the relative intensity of this peak decreases with increasing wall thickness. In the as -synthesis forms the presence of aluminum seems not to increase perceptive the wall thickness. One should also noticed that the full width at half maximum height of the 100 reflection FWHM increased with the aluminum content as a sign of a slight distortion of the long range order induce by aluminum incorporation. However, the A1-MCM-41 samples exhibit a slight decrease of the broadness of the 100 peak after calcination in contrast with the Si-MCM-41 analogue which 100 peak is broader after calcination. The Al-rich synthesis leading to a mixture phase composition thermally unstable is not included in this trend. A broad peak suggests also small particle size, confirms by SEM micrographs for the relative larger amounts of aluminum. Table 1. XRD data of as-synthesized MCM-41 samples Sam pie 1 2 3 4 5 6 7 8 9
A1 mole fraction 0 0.008 0.013 0.016 0.020 0.024 0.032 0.063 0.167 ,,,
ao (A) 51.14 50.97 51.77 50.61 51.31 51.44 49.72 51.26 47.92 ,,,,,
,,
MCM-41 I100 FWHM100Ill0 (a.u.) ( o) (a,u.) 8.32 0.25 0.88 7.65 0.28 0.46 9.32 0.26 0.68 7.32 0.28 0.72 7.00 0.28 0.75 7.76 0.27 0.52 6.08 0.30 0.66 5.67 0.32 0.25 4.61 0.25 0.79 ,,
,
Amorphous p e a k Ill0/I1001210/1200 d I FWHM (A) (a.u.) (o) 0.106 0 . 6 1 4.15 0.262 1.12 0.061 0.89 4.18 0.268 1.10 0.073 0.78 4.17 0.263 1.12 0.099 0.62 4.15 0.239 1.06 0.107 0 . 6 1 4.15 0.236 1.04 0.067 0.96 4.18 0.259 1.14 0.109 0.60 4.13 0.228 1.09 0.043 1.05 4.18 0.216 1.17 0.108 0.44 ,,,,
,
,
,
,
,,,,,,,,
,,,,,
,,
,
,
,,,,
It could be observed that the intensity of the 100 peak decreased with the intensity of the broad amorphous peak .It means that main contribution to the intensity of this peak is provided by the disordered material of the walls. As an intrinsic property of MCM-41 material is worth to give more attention to the analysis of this peak [14]. After the calcination procedure, the XRD diffraction pattems of the samples present the already reported behavior [1-7,5,15]: an enhancement of the peak intensities and a shrinking of the lattice as a result of surfactant removal and a subsequent condensation ofsilanol groups. Upon calcination a displacement towards higher angles
155 along with a decrease of the intensity of this maxim are observed. The result is to be connected with structural water removal. The structural data are collected in Table2. Table2. Structural data of calcinated MCM-41 samples Sample
1 2 3 4 5 6 7 8
A1 mole fraction 0 0.008 0.013 0.016 0.020 0.024 0.032 0.063
Lattice contraction (%) 7.16 5.80 9.06 6.69 7.99 6.85 6.86 9.60
I100eale/I100synt__FWHM100calc/FWHM100synt Weight loss (%) 2.432 1.089 48.74 2.568 0.908 49.15 2.629 0.926 49.60 2.384 0.976 48.55 2.611 0.980 48.00 2.428 1.009 47.56 2.253 1.000 46.44 2.317 0.933 44.84
Apart from sample 9, with high aluminum content (A1 mole fraction=0.167), which proves to be thermally unstable, all the samples exhibit well-defined typical MCM41 XRD pattems. The good quality of the materials is pointed out also by the high total weight loss and also by the relatively low contraction lattice. It was already mentioned that for the Al-containing MCM-41 samples calcination improves the restructuring process of the walls reflected by a sharper 100 diffraction peak in comparison with the assynthesis forms. The result could be attributed to an initial lower silanol groups amount. The 27A1 MAS NMR spectrum of as-calcined sample 7 (figure 2) proves clearly that almost all the A1 ( 94 %) are tetrahedrally coordinated ( 53.8 ppm) . The octahedral aluminum NMR signal (0 ppm) are influenced also a by the degree the sample hydration[ 16]. .
.
m
.
m
.
.
.,,=
I
-400
-2;o
"
o'
20'0
"
4o'o
ppm
Figure 2. The 27A1MAS NMR spectrum of calcinated sample with an A1 mole fraction = 0.032
156 The IR spectra of the as-calcinated samples are similar with those reported for siliceous MCM-41 [1] with four adsorption bands at 1080 cml , 800 cm1, 960 cm1 and 450-470 cm1. The T-O-T band at 1080 cm"1 usually indicating the MCM-41 formation presents a slight shift to lower wavenumbers as the A1 content increases. The 960 cm1 maximum assigned to Si-OH groups became less well-defined also with the aluminum amount increment. For the highest A1 content sample this maximum is entirely overlapped by the 1080 cm1 one (figure 3). ~ A I I ( A I+ S i) 0.032 c In E
. o
/..~
"
o12,
~ o
40'0
60'0
8(~0
10~)0
wavenum
12100 ber
14=00
16~)0
18=00
20'00
( c r n "')
Figure 3. IR-spectra of two calcinated A1-MCM-41 samples The measurements of the acidityare presented in Table 3. Table 3. The acidity measurements Sample
Ai mole fraction
1 2 3 4 5 6 7 8 9
0 0.008 0.013 0.016 0.020 0.024 0.032 0.063 0.167
,,
,,
,
,,
,
,
Acid measurements ............ NH3-TPD (mmol/g) NaOH titr~ion(mmol/g) 0.1100 0.0843 0.2000 0.1849 0.4383 0.1874 0.2960 0.1925 0.2800 0.2000 0.2850 0.2025 0.2992 0.2575 0.5038 0.3000 0.6042 0.1875 ,,
,,,,
,,,,
,,,,,,,
,
,,,,,
,,,
The above data show a difference between both methods of acidity evaluation. The NaOH titration could be more accurate correlated with framework A1, and consequently with Broensted acidity and therefore it displays a rather good linear relationship with the A1 content. The NH3-TPD measurements imply both Broensted and Lewis acid sites and therefore the overall acidity is higher. There is no obvious linearity of the ammonia acidity with the A1 content probably due to different silanol groups amount. The rich-A1 sample
157 which is a mixture of MCM-41 and a dense amorphous phase exhibits, as expected, a low NaOH acidity due to framework A1 and higher total acidity (ammonia-TPD) provided by the dense phase. The SEM images and the BET surface areas (figure 4) proved for the formation of high quality ordered mesoporous materials up to an A1 mole fraction of 0.063. The rich A1 amount samples exhibit upon calcination, a poorly resolved XRD pattern, a less wellresolved IR spectrum, a SEM image with large portions of non-porous material and a low BET surface area.
A1/(AI+Si) = 0 ....~ !~'~,~~!ii~iI
BET=1123 m2/g
A1/(Al+Si):0.032 BET=1090 m2/g
A1/(AI+Si):0.167 BET = 332 m2/g
Figure 4. SEM micrographs of representative MCM-41 samples
158 In conclusion, our studies emphasize that it can be obtain a good-quality A1-MCM41 material, with well-resolved XRD patterns, high BET surface areas and acid properties in an range of AI mole fraction (A1/(AI+Si)) up to 0.063. It seems also that the presence of low amounts of A1 in the framework prevents 'the deterioration of the long-range structural order as in the case of siliceous MCM-41 occurred [17].
3. ACKNOWLEDGEMNTS We are grateful to Dr. Teresa Blasco ( ITQ, Valencia) for the record.
27A1MAS NMR spectrum
REFERENCES
1. 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. 2. K.T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, U. S. Pat., 5,098,684 (1992). 3. M. Janicke, D. Kumar, G. D. Stucky, B. T. Chmelka, Stud. Surf. Sci. Catal., 84 (1994) 243. 4. F. Schfith, Ber. Bunsenges Phys. Chem., 99 (1995) 1306. 5. C.T. Cheng, D. H. Park, J. Kinowski, J. Chem. Soc. Faraday Trans., 93 (1997) 193. 6. R. Schmidt, D. Akporiaye, M. Stk6cker, O. H. Ellestad, J. Chem. Soc., Chem. Commun., (1994) 1493 7. Z. Luan, C. -F. Cheng, W. Zhou, J. Klinowski, J. Phys. Chem., 99 (1995) 1018. 8. K.R. Kloestra, H. W. Zandergen, H. van Bekkum, Catal.Lett., 33 (1995)157. 9. H. Kosslick, G. Lischke, B. Parlitz, W. Storek, R. Frike, Applied Catalysis A: General, 184 (1999) 243. 10. W. B6hlmann, D. Michel, Stud. Surf. Sci. Catal., 135 (2001), 06p 17. 11. R. Mokaya, W. Jones, J. Catal., 172 (1997) 211. 12. R. Mokaya, W. Jones, J. Mater. Chem., 9 (1999), 533. 13. B. P. Feuston, J. B. Higgins, J. Phys. Chem., 98 (1994) 4459. 14. M. Ookawa, Y. Yogoro, T. Yamaguchi, K. Kawamura, Stud. Surf. Sci. Catal., 135 (2001) 06002. 15. M. T. Keene, R. D. M. Gougen, R. Denoyel, R. K. Harris, J. Rouquerol, P. L. Llewellyn, J. Mater. Chem., 9 (1999) 2843. 16. J. M. Kim, J. H. Kwak, S. Jun, R. Ryoo, J. Phys. Chem., 99 (1999) 16742. 17. R. Ryoo, J. M. Kim, J. Chem. Soc., Chem. Commun., (1995) 711.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
159
M e s o p o r o u s aluminosilicates from coal fly ash P. Kumar*, N. Mal, Y. Oumi~, T.
S a n o 1 and
K. Yamana
Ceramic Section of Chemistry & Food Department, Industrial Research Institute of Ishikawa, Kanazawa, Ishikawa 920-0223, Japan. E-mail:
[email protected] 1School of Materials Science, Japan Advanced Institute of Science & Technology, Tatsunokuchi, Ishikawa 923-1292, Japan
For the first time supematant of the coal fly ash has been used to prepare aluminum containing MCM-48. It was found that most of the Si and A1 components in the fly ash could be effectively transformed into MCM-48 when a surfactant mixture containing cationic cetyltetramethylammonium bromide, CTMABr and tetraoxyethylene dodecyl ether, C12(EO)4 is used as template. It has been observed that the fusion plays an important role in enhancing the hydrothermal condition for synthesis of these mesoporous materials. High concentration of Na ions present in the supernatant of fused fly ash is found to be not critical in the formation of A1-MCM-48 when prepared under controlled pH condition. 1. INTRODUCTION Although there are many articles concerning synthesis of mesoporous materials [1-2] most of them have been severely biased to MCM-41 [3-4]. The bias may be attributed largely to the fact that the synthesis of MCM-48 required very specific synthesis conditions [5]. MCM-48 (cubic, space group Ia3d) with its highly branched and interwoven threedimensional networks of the mesopore channels are believed to be much more resistant to pore blockage while being used as absorbents and catalyst supports than the onedimensional channel of a hexagonal MCM-41 [1,6]. Presently however, both the economic and environmental costs for large-scale manufacture of these materials are high due to the cost and toxicities of both templates and preferred silica source. A variety of silica sources are generally used to prepare these materials including fumed silica and silicon tetraethoxide. The industrial manufacture of mesoporous materials is likely to be economically prohibitive if silicon alkoxides and fumed silica in particular are selected. Coal combustion in the world accounts for approximately 37% of the total electricity production and in turn, results in the production of a huge amount of fly ash as waste material [7]. Nearly 600 million metric tons of fly ash are produced annually in the world, with the global recycling rate being only 15% [8]. Since the major chemical compounds contained in fly ash are SiO2 and A1203 (60-70 wt% and 16-20 wt%, respectively), resource recovery is one of the most important issues of waste management at present [9-
160 11 ]. Very recently we have reported our preliminary investigation on the synthesis of A1MCM-41 and SBA-15 type of materials and their characterization [12]. In this paper we report the studies carried out on the hydrothermal synthesis condition of the aluminum containing MCM-48 type materials using coal fly ash as a silicon and aluminum source and their characterization. Synthesis of both MCM-41 and MCM-48 is related in the sense that they can be prepared under identical hydrothermal condition subject to the type and concentration of surfactant used [ 1]. In addition to that various synthesis routes to MCM-48 were developed in order to overcome the synthesis shortcomings [ 13]. These synthesis results showed that the crystallinity of the MCM-48 went through an optimum as a function of time. The MCM-48 products were obtained as an intermediate between transformations from a hexagonal or disordered surfactant-silica mesophase to a more stable lamellar mesophase [14]. One report [15] suggested that the transformation of the MCM-48 mesophase to lamellar can be quenched by adjusting the pH of the reaction mixture. Another report [ 16] claimed that the mixed surfactant approach resulted into high quality MCM-48 as an energetically favored mesophase. Finally, it has been reported that the use of gemini surfactants induce the formation of cubic structure even using fumed silica as silicon source [17]. All these studies indicate that the formation of MCM-48 type materials is possible under certain synthesis conditions. Our aim in this study is to prepare MCM-48 type materials so we focus on three factors, a) single surfactant concentration, b) mixed surfactant concentration and c) intermediate pH adjustment. 2.
EXPERIMENTAL
2.1.
Materials Coal fly ash used in this study was obtained from Nanao-Ota power plant, Hokuriku and used as obtained. The chemical composition of fly ash revealed apart from the main constituents such as silica (67.5%) and alumina (18.7%), the other impurities such as Fe203, CaO, MgO, 1(20, TiO2, Cr203, P205 Na20, K20 and SO3 with 3.6%, 2.0%, 0.7%, 0.9%, 0.8%, 0.9%, 0.3%, 0.2%, 0.4%, 0.7%, respectively. The specific surface area (BET) and cation exchange capacity (CEC) of the coal fly ash were found to be 4.5 mE/g and 0.8 meq/100g, respectively. 2.2. Synthesis of AI-MCM-41 and AI-MCM-48 The silica and aluminum source was the supernatant from fused fly ash powder [ 18]. The concentrations of Si, A1 and Na in supernatant measured by atomic adsorption spectroscopy (Perkin Elmer AS-800) are 11000, 380 and 35000 ppm, respectively. The detail synthesis procedure for MCM-41 was followed from our previous study [12]. MCM48 materials in this study were synthesized using both single surfactant and a surfactant mixture. While CTMABr was used as a single surfactant, the mixture was prepared by using CTMABr and C12(EO)4 (Aldrich). All batches were prepared using a synthesis gel with the following molar composition: CTMABr/ C12(EO)4/H20/Si = 0.35-0.55/0.150.25/100/1. The obtained gel was stirred for thirty min., the synthesis mixture was placed in a Teflon lined stainless steal autoclave and heated at 373 K for 4 days under static condition. Intermittently, the reaction mixtures were cooled to room temperature after one day at 373 K and subsequently added with acetic acid to adjust the pH at 10.2. The MCM48 product was finally filtered after an additional heating at 373 K for another 3 days. To
161 remove the surfactant, the as synthesized sample was calcined in air under static conditions at 813 K for 6 hours, with a linear temperature ramp of 0.5K / min and two plateaus of 60 minutes each at 423 and 623 K. For a comparison an aluminum containing MCM-41 and MCM-48 were prepared following the procedure as reported in the literature [1, 13]. All the materials obtained were further calcined as explained above.
2.3. Analysis and Characterization Powder X-ray diffraction (XRD) patterns were measured using CuK~ radiation by using MAX18X. cE The chemical composition was analyzed with Li]2B404 method by using X-ray fluorescence (XRF) technique (Philips PW2400), BET surface area by N2adsorption at liquid nitrogen temperature (Belsorp 28SA) and morphology by scanning electron microscopy (SEM) using Hitachi S-4100. Transmission electron microscope (TEM) image was obtained by using JEOL 2010. FT-IR spectra of the self supporting wafers were measured by JEOL JIR-7000. TG-DTA analysis was performed using Rigaku TG-8120. Solid-state nuclear magnetic resonance resonance 29 Si and 27A1 NMR spectra were obtained on a Varian VXP-400. 3. RESULTS AND DISCUSSION Because larger amounts of Si and A1 species can be dissolved by fusion method, we adopted fusion approach in this study. Figure 1 summarizes the XRD patterns of the fly ash (a) and the fused fly ash (b) at 823 K. It can be seen that the major crystalline phases present in the fly ash are quartz, mullite and aluminosilicate glass, which are present as the amorphous phase. On the other hand, a ! i m ; w w i large amount of sodium silicate exists in b the fused fly ash (Figure l b), which implies Ss that fusion is effective in extracting silicon from quartz. Most of the quartz species have reacted with the NaOH and resulted s s s into the disappearance of the respective s s s~Ms peaks in fly ash. SsS s iLlij,~slls sS Ss I ~' M S S S Figure 2 shows the XRD patterns of .,.., different MCM phases of calcined samples Q a prepared under different surfactant / silica ratio. It can be seen that low concentration of surfactant (CTMABr) results into MCM41 type materials as suggested from the Q XRD pattern (Figure 2a) clearly displaying at least four reflections that are consistent M MMM M O Q with indexing to a hexagonal cell, typical of an MCM-41 product. The observation 10 20 30 40 50 60 70 of three higher angle reflections other than 20 (degree) the d]00 indicates that the product is likely Figure 1. X-ray diffraction profiles of the to possess the symmetrical hexagonal pore fly ash (a) and the fused fly ash (b) at 823 structure typical of MCM-41. A further K. M = mullite, Q = quartz, and S = increase in surfactant concentration sodium silicate resulted into mesophases, poor in
162
Table 1 Physical properties of the raw material and the calcined mesoporous materials Sample Surf i Surf2 SBET/ Si/A1 Pore d 100 d 211 ao 3 /SiO2 /SiO2 m 2 g-i volume /nm /nm /nm
Pore size 4 /nm
/ c m 3 ~-1
Fly ash A1-MCM-41 (a) A1-MCM-41 (b)
0.20 0.35
-
4.5 842 738
2.9 13.8 18.5
. 0.75 0.57
A1-MCM-41 (c)
0.55
-
731
65.0
0.57
A1-MCM-48 (d)
0.55
0.15
639
62.3
A1-MCM-48 (e)
0.55
0.18
848
59.4
1 2 3 4
.
. 4.24 3.56
. -
4.9 4.1
3.2 2.9
3.56
-
4.1
2.7
0.55
-
3.17
7.8
2.5
0.82
-
3.04
7.4
3.0
cetyltrimethyl ammonium bromide tetraoxyethylene dodecyl ether unit cell parameter, using 2d100/~ for MCM-41 and d211 q5 for MCM-48 Dollimore-Heal method
hexagonal structural order as indicated from the gradual disappearance of diffraction peaks assigned to (110), (200) and (210) reflections (Figure 2b, 2c). The surfactant / silica ratio higher than 0.55 resulted into to a featureless XRD pattern (not shown). By increasing the concentration of CTMABr in the synthesis gel, a phase transitions from hexagonal to lamellar passing through an intermediate state of cubic structure is reported [1-4]. But using the supernatant as silica source it was not observed, in other words MCM-48 formation was not facilitated under the synthesis condition using CTMABr 211 alone. Figure 2d and 2e shows the 5 XRD patterns of the surfactant-silica mesophase obtained from the starting mixtures of CTMABr/ C12(EO)4 = 0.55/0.15 and 0.55/0.18, respectively. _= It can be seen the presence of neutral surfactant has resulted into mesophase, identical to the cubic MCM-48. We observed that the optimum condition ~ b for MCM-48 using supernatant as 110 silica source was 0.55/0.18 as it showed the sharpest XRD patterns. 4 6 8 Table 1 summarizes characteristics of 20 / degree the calcined mesoporous materials Figure 2. XRD profiles of the different obtained. calcined M C M type materials. CTMABr/SiOE 9a - 0.22, b - 0.35, c, d and e -- 0.55; C12(EO)4/SIO2: d - 0.15 and e - 0.18 9
!
9
!
,
4•
>,
e-
i
|
--
163 From this synthesis experiment the XRD pattern in Figure 2e, a highly ordered MCM-48, without any trace of lamellar phase peaks was recovered. The high ordered array of these materials could be inferred from the presence of a well def'med set of diffraction peaks between 3 ~ and 6~ the XRD patterns assigned to the (211), (220), (321), (420), (422) and (431). Gel representing higher than 0.18 of C12(EO)4 resulted either into unidentified mesophase or without any XRD pattern. The (211) reflection is found at approximately 3.6 nm for all the as-synthesized samples. This correspond to a unit cell size of 8.7 nm. For the calcined samples the same reflection occurs at-3.1 nm, which gives a unit cell length o f - 7 . 5 nm. Hence, as should be expected, there has been a shrinkage of the unit cell ( - 1 3 % ) during calcinations which probably Figure 3. N2 adsorptionis due to silanol condensation. This magnitude desorption isotherms of different of unit cell shrinkage was in the range of values samples (c, d and e), prepared normally reported in the literature using other under different conditions. silicon source, that are in the 5-15% range [1417]. The same tendency is observed for the (220) reflection suggesting that the supernatnt of coal fly ash could be used as a source material. The N2 adsorption-desorption isotherms of different samples (c, d and e) are shown in Figure 3. It corresponds to a reversible type IV isotherm which are characteristic for mesoporous materials. An inflection point is observed at relative pressures between 0.25 and 0.3. This corresponds to the filling of the mesopores, and the sharp increase in the adsorbed volume indicates a uniform pore-size distribution. It can be seen (Table 1) how the presence of neutral surfactant facilitates the formation of MCM-48. The presence of a small hysteresis loop in sample c, indicates the formation of lamellar phase which is very similar to the studies that has been reported at high surfactant/ silica ratio [19]. Figure 4 shows SEM images of the mesoporous materials prepared from the Figure 4. SEM images of fly ash and samples c ,d and e supernatant and the original fly prepared from supernatant of coal fly ash.
164 ash. Morphologically, the fly ash consists essentially of aluminosilicate glass spherules that disintegrate during fusion treatment and are converted into amorphous sodium silicate and sodium aluminosilicate. The SEM images of mesoporous materials are generally Figure 5. SEM images A1-MCM-41 (1) and A1-MCM-48 found in a range of (2) using pure chemicals. spherical top, ribbon, torous shape d particles [20]. In the present case, the particles of the materials obtained have a spherical habit at around less than 1 lxm although some agglomerates are also visible (sample c). The presence of C12(EO)4 as neutral surfactant resulted into more uniform spheres (sample d and e, respectively). In Figure 5 we have compared the SEM images of MCM-41 and MCM-48 by using pure chemicals such as TEOS and Al(NO3)3 as silicon and aluminum sources, respectively. The major particles of MCM-41 (sample 1) are constructed from elementary units that have ribbon-like habits. Formation of such ribbon-like particles are reported for MCM-41 when prepared under aged condition [21]. MCM-48 (sample 2) exhibited spherical raspberry-like pattern. Since both these samples are prepared from reaction mixtures differing in the surfactant / SiO2 ratio and content of water, this substantial difference in particle morphology are not unexpected. TEM images of micro-sectioned samples (Figure 6) also showed well ordered hexagonal arrays of mesoporous channel (sample a) and pores arranged on the cubic plane (sample e) confirming that the materials indeed possess the pore system symmetries that were inferred from the X-ray patterns and N2 isotherms. A mixed surfactant approach has been reported in the literature for the preparation of mesoporous materials [19]. In many cases, two different surfactants are completely miscible and form liquid crystalline miceller mesophase cooperatively. The structure in the micelle packing in the liquid crystalline mesophases may be determined by various effects such as their head to tail packing parameters, electrostatic interaction and hydrogen bonding between the head groups of the two different kinds of surfactant molecules [22]. This phase behavior becomes more complicated when silica and alumina sources are present in the Figure 6. TEM image of A1-MCMform of supernatant of coal fly ash. Supernatant is a highly alkaline solution of silicate and aluminate 41 (sample, a)and A1-MCM-48 (anions) and are strongly attracted by electrostatic (sample, e).
165 interaction surrounding the head groups of the CTMABr, which may lead to the high concentration of the anions on the surface of the surfactant micelles. The neutral surfactant has no strong interaction with the anions, and consequently its incorporation to the micelles will bring a dilution of the anions at the surface. This ~9 low surface concentration may further lead to a certain contraction of the micellar surface resulting in a phase transition from hexagonal to cubic. At this stage we are not advancing any explanation about the complexities of phase behavior of the fly ash supematant-surfactant mesostructures in the 10 5 0 aqueous solution, however we believe that ppm C12(EO)4 acts more as a diluents and based on our Figure 7. 27A1-MAS-NMR observation facilitated the formation of MCM-48 structure. spectra for fly ash and calcined Another factor that affected the formation A1-MCM-41 (sample a) prepared of cubic phase was the pH of the supematant- from supematant of fused fly ash surfactant mesostructure. Generally, a high pH solution. * Denotes the spinning condition is a major driving force for the side bands. transformation to lamellar [23]. In our case the pH adjustment to 10.2 during the synthesis arrested this transformation and also helped to improve the product yields. This is agreement with the report where the pH adjustment was mentioned as a means for quenching the transformation of the MCM-48 mesophase to lamellar [16]. We note that the BET surface area and pore volumes of the samples (Table 1) prepared in this work are slightly lower than those that may be obtained from more reactive silica sources. The values obtained are nevertheless similar to those reported in the literature and indicate high overall total porosity. Based on these observations, the possibility to use the dissolved silica and aluminum present in the supematant of fused fly ash as a precursor for the synthesis of MCM-48 are fairly good. But use of the high Si containing fly ash would be a better choice. One of the most important features of our study using coal fly ash is the aluminum incorporation into the framework of the synthesized materials [12, 18]. The information about the chemical state of A1 in the materials obtained using 27A1 MAS-NMR spectra of the original coal fly ash (a) and calcined A1-MCM-41 (b) were measured (Fig. 7). Fly ash did not show any peak, indicating the lack of any tetrahedrally (Td) coordinated framework aluminum. But a very strong single peak assigned to the Td framework aluminum at ca 54 ppm was observed for the sample prepared from fly ash without any Oh non-framework (0 ppm) aluminum. Even at such high aluminum concentration (Si/A1 = 13.8), absolutely no Oh peak was observed. This provides a direct evidence of A1 incorporation in the MCM-41 framework. Similarly, in the case of A1-MCM-48 (sample d and e) Td aluminum (spectra not shown) were observed though the presence of high concentration of surfactants has a dramatic effect on the aluminum incorporation as revealed from 27A1 MAS-NMR studies. ,
I
,
i
,
,
I
,
I
i
i
l
!
|
!
|
166 4. CONCLUSIONS In summary, it is shown that the supematant of coal fly ash can be used as a raw material for the synthesis of aluminum containing MCM-48. The use of surfactant mixture between cationic hexadecyl trimethylammonium surfactants and neutral tetraoxyethylene dodecyl ether surfactants has greatly facilitated the synthesis of MCM-48 performed under controlled pH condition. A high aluminum incorporation in tetrahedral position is revealed in the mesoporous materials which may be useful for certain catalytic applications. The experimental data produced here suggest that the coal fly ash could be a suitable source of silicon / aluminum with a low economy and environmentally friendly reagent for the preparation of well ordered mesoporous materials. REFERENCES
1. C.T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. 2. S. Inagaki, Y. Fukushima and K. Kuroda, J. Chem. Soc., Chem. Commun., 1993, 680. 3. A. Corma, Chem. Rev., 97 (1997) 2373. 4. A. Sayari, Y. Yang, M. Kruk and M. Jaroniec, J. Phys. Chem. B, 103 (1999) 3651. 5. J.M. Kim, S. K. Kim and R. Ryoo, J. Chem. Soc., Chem. Commun., (1998) 259. 6. C.L. Landry, S. H. Tolbert, K. W. Gallis, A. M. Monnier, G. D. Stucky, P. Norby and J. C. Hanson, Chem. Mater., 12 (2001) 1600. 7. C. Zevenbergen, J. P. Bradley, L. P. V. Reeuwijk, A. K. Shyam, O. Hjelmar and R. N. J. Comans, Environ. Sci. Technol., 33 (1999) 3405. 8. G. Belardi, S. Massimilla and L. Piga, Resource, Conservation and Recycling, 24 (1998) 167. 9. A. Singer and V. Berkgaut, Environ. Sci. Technol., 29 (1995) 1748. 10. S. Rayalu, N. K. Labhasetwar and P. Khanna, U.S. Patent No. 6027708 (22 February 2000). 11. N. Shigemoto, S. Sugiyama, H. Hayashi and K. Miyaura, J. Mater. Sci., 30 (1995) 5777. 12. P. Kumar, N. K. Mal, Y. Oumi, K. Yamana, and T. Sano, J. Mater. Chem., 11 (2001) 3279. 13. M. L. Pena, Q. Kan, A. Corma and F. Rey, Microporous Mesoporous Mater., 44-45 (2001) 267. 14. A. Corma, Q. Kan and F. Rey, J. Chem. Soc., Chem. Commun., (1998) 579. 15. J. Xu, Z. Luan, H. He, W. Zhou and L. Kevan, Chem. Mater., 10 (1998) 3690. 16. R. Ryoo, S. H. Joo and J. M. Kim, J. Phys. Chem. B, 103 (1999) 7435. 17. P. Van Der Voort, M. Mathieu, F. Mees and E. F. Vansant, J. Phys. Chem. B, 102 (1998) 8847. 18. P. Kumar, Y. Oumi, K. Yamana and T. Sano, J. Ceram. Soc. Japan, 109 (2001) 968. 19. G. Oye, J. Sjoblom and M. Stocker, Microporous Mesoporous Mater., 27 (1999) 171. 20. K. Schumacher, M. Grun and K. K. Unger, Microporous Mesoporous Mater., 27 (1999) 171. 21. M. Grun, K. L. Unger, A. Matsumoto and K. Tsutsumi, Microporous Mesoporous Mater., 27 (1999) 207. 22. J. L. Palous, M. Turmine and P. Letellier, J. Phys. Chem. B, 102 (1998) 5886. 23. R. Ryoo and J. M. Kim, J. Chem. Soc., Chem. Commun., (1995) 711.
Studies in Surface Science and Catalysis 141 A. Sayariand M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
167
N e w route for synthesis o f highly ordered mesoporous silica with very high titanium content Xiang-Hai Tang*, Xin Wen, Shi-Wei Sun and Hai-Yan Jiang College of Chemistry, Nankai University, Tianjin 300071, P. R. China
Meso-structured titanosilicates with variable amounts of titanium have been hydrothermally synthesized under static conditions via a new route. By a separate hydrolysis procedure during gel preparation, this approach effectively prevents titanium ions from formation of indissoluble titanium species in the mixture at very high titanium content. The physicochemical properties of these materials were characterized by means of XRD, FT-IR, UV-Vis DRS and catalytic oxidation of 4-methylphenol with dilute hydroperoxide. In contrast to literature reports, this novel method can lead to mesoporous titanosilicates with a highly ordered MCM-41 hexagonal phase even in a gel with n(Yi)/n(Si) up to 1/4 and a relatively low template content. It reveals that these materials are chemically homogeneous. During crystallization, titanium ions were partially incorporated into the MCM-41 framework. The addition of TBAOH resulted in a more cross-linked and ordered wall structure.
1.
INTRODUCTION
The synthesis and characterization of hetero-atom containing molecular sieves have been a hotspot in the field of zeolite synthesis since the last two decades [ 1-5]. These materials are the most promising candidate catalysts for the environmentally benign industrial processes. Among them titanium-containing molecular sieves (e.g., TS-1, TS-2 and Ti-beta) have attracted considerable attention. However, their catalytic performance in reactions involving in bulky molecules is greatly limited due to the micropore nature, as their pore size is usually less than 2 nm. Fortunately, the pore diameter of solid materials with regular voids has been largely expanded since the discovery of M41S family materials. Up to now, mesoporous titanosilicates such as Ti-MCM-41 [6], Ti-HMS [7] and Ti-MCM-48 [8] have been successfully synthesized, but titanium content in these solids is usually very low. More recently, E1 Haskouri et al. [9] used a complexing polyalcohol (2,2,2-nitrile-triethanol) as a hydrolysis retarding agent for Yi species and reported that meso-structured titanosilicates could be prepared from a gel of n(Yi)/n(Si) up to 1/4. However, the framework of these materials was lack of long-range order, the BET surface area of the product with the highest n(Ti)/n(Si) of 1/1.9 in solid was merely 595 m2/g. It seems that these materials are analogues * To whom correspondence should be addressed. Email:
[email protected].
168 to Ti-HMS, and most of the titanium atoms locate outside the framework, which are lack of oxidation activity. Generally, the catalytic performance of mesoporous titanosilicate in selective oxidation reactions is dependent on the framework titanium and its amount. Hence it is of importance to develop new approaches for the preparation of high quality mesoporous titanosilicate. Here we demonstrate the synthesis and characterization of high titanium content MCM-41 titanosilicates with a highly ordered hexagonal phase. By our new approach high quality products could be prepared from a gel with n(Ti)/n(Si) up to 1/4 and a relatively low template content.
2.
MATERIALS AND METHODS
2.1.
Materials Tetraethyl orthosilicate (TEOS) and tetrabutyl orthotitanate (TBOT) were employed as silicon and titanium sources, respectively. Cetyltrimethylammonium bromide (CTABr) was used as template. Both liquid ammonia and tetrabutylammonium hydroxide (TBAOH) were used as hydrolysis agents. Firstly, TEOS, liquid ammonia and distilled water were mixed to prepare solution A, while TBOT was dropwise added to a mixture of TBAOH, ethanol and distilled water to prepare solution B. The two solutions were vigorously stirred in closed vessels respectively at ambient temperature till the solutions became homogeneous and transparent. Secondly, a gel was prepared by mixing solutions A and B, then the gel was boiled to evaporate alcohols and part of water. Finally, the gel was slowly added to a solution of CTABr, liquid ammonia and distilled water, and further stirred at ambient conditions for 3 h to obtain a mixture with a molar composition of (1-x)SiO2:xTiOz:yCTABr:O.3yTBAOH: mNH3:zH20 (where x=0-~0.2, y=0.1~0.7, z=100--520, m=6~l 1). The resultant was sealed in a PTFE-lined stainless steel autoclave and heated at 383 K for 72 h. The as-synthesized product was separated by centrifugation at 9000 rpm, washed twice with distilled water and later freeze-dried overnight to remove water from the solid product. To obtain the template free sample, the as-synthesized product was heated in air from ambient temperature to 823K at 7 K/min, kept for 4h, then raised temperature to 923K at 5 K/min and kept for lh. 2.2.
Characterization Powder X-ray diffraction (XRD) patterns were obtained in the 20 range 1-10 ~ with a Rigaku D/MAX 7A diffractometer using the Cu Ka radiation operated at 40 kV and 40 mA. FT-IR study was performed using a Bruker Vector-22 with a resolution of 4 cm ~ and 30 scans. The KBr technique with a sample to KBr weight ratio of 1"150 was used. UV-vis diffuse reflectance spectra (UV-vis DRS) were measured with a Shimadzu UV-240 spectrophotometer. Spectra were recorded in the 190-800 nm wavelength range against a MCM-41 standard. The catalytic activity of the calcined sample was tested by selective oxidation of 4methylphenol with dilute hydroperoxide. The reaction products were analyzed with a Hewlett-Packard HP G 1800A GC-MS instrument.
3. 3.1.
RESULTS AND DISCUSSION Material synthesis
169 Table 1 summarizes the gel molar compositions for preparation of various samples. Generally, hydrothermal syntheses of microporous zeolites and mesoporous materials are carried out in alkaline media. Unfortunately, most transition metal ions tend to hydrolyze and form insoluble species like metal hydroxides even in a neutral solution. To overcome that disadvantage and to increase the efficiency of metal source in the synthesis, it is essential to develop a new method to prevent the metal source from the formation of insoluble species. TBAOH, a strong base and an ionic surfactant, is commonly used as a structure-directing agent in zeolite synthesis. The hydrolysis of TBOT in a TBAOH-ethanol-H20 solution resulted in a mixture of soluble titanium species and effectively prevented or suppressed the formation of Ti(OH)4 and TiO(OH)2. Indeed, no visible precipitation occured 9000 during gel preparation even in systems with very high amounts of titanium. It a, is worthy of mention that our products are quite different from those previous ~ 6000 ~ _ F reported by E1 Haskouri et al. [9]. In our case, most of the samples exhibit at .9 least three well-resolved reflections in ~ 3000 .;> the 20 range between 2-6 ~ (Figure 1), B which can be indexed to an ordered hexagonal lattice typical of MCM-41 0 6 11 16 [10]. It reveals that following the new 20( ~) synthetic route highly ordered MCM-41 titanosilicates could be hydrothermally Figure 1. XRD patterns of the calcined samples synthesized within a relatively wide A-F in Table 1 (Offsets: vertically by 1000 CPS, composition range. horizontally by 1.0 degree). , ,,,,~
;j
Table 1. Influence of gel composition on crystallization of mesoporous titanosilicate. Sample Gel molar composition Phase Crystallinity ~ (%) n(SiO~) n(TiO?) n(CTABr) n(TBAOH) n(NH3) n(HzO) A 1.00 0.00 0.50 0.02 9.20 130 MCM-41 100 B 0.95 0.05 0.50 0.03 9.20 130 MCM-41 99 C 0.90 0.10 0.50 0.03 9.20 130 MCM-41 90 D 0.85 0.15 0.50 0.05 9.20 130 MCM-41 86 E 0.82 0.18 0.50 0.05 9.20 130 MCM-41 80 F 0.80 0.20 0.50 0.06 9.20 130 MCM-41 68 G 0.85 0.15 0.50 0.05 6.00 130 MCM-41 69 H 0.85 0.15 0.50 0.05 11.00 130 MCM-41 70 I 0.85 0.15 0.05 0.05 9.20 130 Amorphous J 0.85 0.15 0.10 0.05 9.20 130 MCM-41 72 K 0.85 0.15 0.70 0.05 9.20 130 MCM-41 76 L 0.85 0.15 0.50 0.05 5.70 80 MCM-50 M 0..85 0.15 0.50 0.05 7.10 100 MCM-41 85 N 0.85 0.15 0.50 0.05 36.80 520 MCM-41 85 a. Counted on MCM-41 phase compared to sample A after calcination.
170 A careful XRD examination of the as-synthesized and calcined products was also performed at high angle area. However, no distinct peaks were observed in the 20 range between 20-80 ~ which implies the absence of bulky crystalline TiO2 (i.e., anatase and rutile) and titanium silicate. This result suggests that these materials are probably chemically homogeneous. 3.2.
Framework IR Shown in Figure 2 is the framework IR spectrum of as-synthesized sample D. The band at 1080 cm ~ can be ascribed to asymmetric stretching of [SiO4] tetrahedra, while the bands at 459 cm ~ and 800 cm ~ can be assigned to bending of framework Si-O bonds. To our surprise, two weak bands centered at 560 cm ~ and 1237 cm ~ respectively are also observed, which are rarely seen in previous literatures concerning mesoporous materials. The former band characterizes the double-rings' vibrations of external [SiO4] tetrahedra in MFI or MEL structure [ 11 ]. In our study, TBAOH was used as a hydrolysis agent, therefore it is possible that TBA + templated the formation of MFI or MEL structure during crystallization. It suggests that at least part of the [TO4] (T=Ti, Si) tetrahedra within the walls of the assynthesized products is highly cross-linked. This has been demonstrated very recently by two groups separately but using tetraethylammonium or tetrapropylammonium hydroxide as a hydrolysis agent [12,13]. The presence of TBA + resulted in a more ordered and condensed wall structure than those of conventionally prepared ones. However, no absorption related to TiOz or TiO(OH)2 species is detected, which indicates that Ti well disperses in the assynthesized titanosilicate with very high amounts of titanium. An IR band around 960 cm ~ is also observed in Figure 2, which is often assigned to a lattice defect and is correlated with the substitution for silicon with metal ions in the silica framework [1-8]. However, recent work suggests that this band corresponds to a Si-O vibration in a Si-OR (R=H, tetraalkylammonium) group in siliceous materials (e.g., Al-beta and pure MCM-41 silica) [ 14,15]. Nevertheless, there is a controversy on the origin of this IR band, we attentively exclude its appearance as an evidence of the incorporation of Ti into the framework of MCM-41.
1.2
0.8
f ............................................................... ,'-:'. 0.9
.~ 0.6
.:~.4
E r,r
O
DO
< 0.3
~.2 0.0
I
I
I
I
I
I
I
I
0'000 Wavenumber (cm")
Figure 2. Framework IR spectrum of as-synthesized sample D
300 400 2(nm)
500
Figure 3. UV-vis diffuse reflectance spectrum of calcined sample D
171
3.3.
UV-vis DRS UV-vis diffuse reflectance spectroscopy (DRS) is an especially useful technique to characterize the local titanium environment in titanosilicates [ 16]. Figure 3 depicts the UV-vis DRS of calcined sample D. Two absorption bands can be distinguished between 190-400 nm. The band centered at 230 nm corresponds to tetrahedrally coordinated Ti species that substitute for Si in the silica framework, whereas the band around 285 nm probably arises from five- and six-coordinated Ti species [17]. As tetrahedrally coordinated Ti can be hydrated and generates five- and six-coordinated Ti species, it suggests from the UV-vis DRS that more than half amount of titanium may reside in the mesoporous framework, though we can not exclude the existance of partially polymerized Ti species in the walls. However, the absence of a distinct absorption around 330 nm indicates that no anatase-like phases are formed in such a Ti-rich mesoporous titanosilicate upon calcination [16]. 3.4.
Catalytic property The catalytic activity of these solids in the selective oxidation of 4-methylphenol in the presence of dilute hydroperoxide was tested in a glass flask. In a typical reaction, 1.0 g of calcined sample D, 1.0• .2 mol of 4-methylphenol, 1.0• .2 mol of H202 and 100 mL of distilled water were mixed and stirred at 353 K. The results obtained after 5 h of reaction indicated that the catalyst was highly selective. A conversion of 37.6% based on 4methylphenol was reached and 4-methyl catechol was the sole product. Such a preliminary result reveals that sample D is highly catalytically active, which suggests that Ti ions are tetrahedrally incorporated into the MCM-41 framework. In summary, our results unambiguously demonstrate that, by a separate hydrolysis procedure during gel preparation, titanium can be effectively prevented from formation of indissoluble titanium species in the synthesis mixture at very high titanium content, thus highly ordered MCM-41 titanosilicates can be hydrothermally synthesized even from a gel with n(Ti)/n(Si) up to 1/4 and a relatively low template content. During crystallization, titanium ions were partially incorporated into the MCM-41 framework. The addition of TBAOH resulted in a more cross-linked and ordered wall structure. With the advantages of large pore diameter and ordered framework structure, the as-synthesized mesoporous titanosilicates may find special usage in catalysis field.
4.
ACKNOWLEDGMENTS
The financial support of this research by both the National Natural Science Foundation of China (through Grants No. 29733070 and No. 50102001) and the Natural Science Foundation of Tianjin, China (through Grant No. 13608111) is gratefully acknowledged.
REFERENCES 1. G. Perego, G. Bellussi, C. Corno, M. Taramasso, F. Buonomo and A. Esposito, Stud. Surf. Sci. Catal., 28 (1986) 129. 2. M. S. Rigutto and H. van Bekkum, Appl. Catal. A, 68 (1991) L 1. 3. D.P. Serrano, H. X. Li and M. E. Davis, J. Chem. Soc., Chem. Commun., (1992) 745.
172 4. M.W. Anderson, O. Terasaki, T. Ohsuna, A. Philippou, S. P. Mackay, A. Ferreira, J. Rocha and S. Lidin, Nature, 367 (1994) 347. 5. X.-H. Tang, R.-Z. Zhu, L.-R. Pan and H.-X. Li, Chem. J. Chinese Universities, 21 (2000) 517. 6. A. Corma, M. T. Navarro and J. Perez-Pariente, J. Chem. Soc., Chem. Commun., (1994) 147. 7. P.T. Tanev, M. Chlbwe and T. J. Pinnavaia, Nature, 368 (1994) 321. 8. K.A. Koyano and T. Tatsumi, Chem. Commun., (1996) 145. 9. J. E1 Haskouri, S. Cabrera, M. Gutierrez, A. Beltr~n-Porter, D. Beltr~.n-Porter, M. D. Marcos and P. Amor6s, Chem. Commun., (2001) 309. 10. C.T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. 11. E.M. Flanigen, H. Khatami and H. A. Szymanski, Adv. Chem. Ser., ACS, 101 (1971) 201. 12. Y. Liu, W. Zhang and T. J. Pinnavaia, Angew. Chem. Int. Ed., 40 (2001) 1255. 13. Z. Zhang, Y. Han, L. Zhu, R. Wang, Y. Yu, S. Qiu, D. Zhao and F. Xiao, Angew. Chem. Int. Ed., 40 (2001) 1258. 14. C.B. Dartt, C. B. Khouw, H.-X. Li and M. E. Davis, Microporous Mater., 2 (1994) 425. 15. T. Blasco, A. Corma, M. T. Navarro and J. Perez-Pariente, J. Catal., 156 (1995) 65. 16. A. Tuel, Microporous Mesoporous Mater., 27 (1999) 151. 17. F. Geobaldo, S. Bordiga, A. Zecchina, E. Giamello, G. Leonfanti and G. Petrini, Catal. Lett., 16 (1992) 109.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
Synthesis
and
Characterization
173
of
Ti-containing
Mesoporous
Alumina
M o l e c u l a r Sieves Chun Yang a.b*and Xi Li a a College of Chemistry and Environmental Science, Nanjing Normal University, Nanjing, 210097, P.R.China u National Laboratory of Molecular & Biomolecular Electronics, Southeast University, Nanjing, 210096, RR.China
Ti-containing mesoporous alumina molecular sieves with a structure of MSU-2 have been synthesized using metal alkoxides as titanium and aluminum sources and Triton X-100 as templating surfactant, respectively. The structure of the products and the state of Ti species were characterized by XRD, TEM, N2-sorption, UV-vis DRS and Laser Raman Spectroscopy. It is found that the surface area and the pore volume of sample increase when Ti is incorporated, suggesting an effect of Ti on stabilizing the alumina framework. It is also shown that most of Ti exists in the framework as an isolated mononuclear species at Ti/Al_<6%. As Ti content increases, polymerized Ti species and even TiO2 domains form. But these domains grow so difficultly within mesoporous walls that they graft to the surface of alumina walls at a higher Ti content level, leading to some degree of pore blocking. However, up to Ti/AI = 33%, no well-defined bulk TiO2 is tbund, meaning that Ti species can be dispersed better in mesoporous alumina than in conventional TiO2-A1203 composite.
1. INTRODUCTION Because of their large pore size different from traditional molecular sieves, mesoporous molecular sieve materials have attracted considerable attention for their potential use in catalytic transformation of large molecules, material preparation and as hosts lbr supramolecular assembly. Except silica and aluminosilicate, various metal oxides with mesostructure have also been synthesized. Mesoporous alumina molecular sieve is one of them. It has been reported that the mesoporous aluminas can be synthesized by several templating routes [1-5], and the pore sizes of the products are 2-10 nm. Among these routes, a nonionic one, which employs polyethylene oxide (PEO) and aluminum alkoxide as templating surfactant (N ~ and inorganic aluminum precursor (I~ respectively, was adopted by
174 Pirmavaia's group to successfully synthesize the mesoporous aluminas with hexagonal-like symmetry and wormhole channel motifs [4,5]. These materials are named as MSU-X. The advantage of this N~ ~ pathway is that the surfactant is inexpensive and biodegradable. "l'i-containing aluminas (titania-alumina composites) have been widely used as catalysts, catalyst carriers or ceramic materials. Much research work has focused on the study of these materials [6-8]. Special attention has been riveted on the state of Ti species and the dispersity of Ti atoms on alumina surface or in alumina matrix. However, the properties of these materials are limited because they possess only textural porosity and lack the selective framework pore structure characteristic of a molecular sieve. Mesoporous aluminas have regular channels on the scale of nanometer and large specific surface area; therefore, the preparation of mesostructured Ti-containing aluminas by incorporating Ti atoms into the mesoporous aluminas is expected to endow the materials with fi'amework-confined pore structure and higher Ti dispersing degree. These new composites will undoubtedly possess greater space for application in catalysis and material area. In the present work, Ti-containing mesoporous aluminas with mesostructure of MSU-2 were synthesized. Their structures were characterized by XRD, TEM, N2adsorption/desorption. The information from UV-vis Diffuse Reflectance and Laser Raman spectroscopy showed that Ti species were dispersed better in mesoporous alumina than in conventional TiO2-A1203 composite.
2. EXPERIMENTAL
2.1. Synthesis of samples The synthesis of MSU-2 mesoporous alumina was carried out according to the method in Ref [5]. Aluminum tri-sec-butoxide and Triton X-100 were used as aluminum source and templating surfactant, respectively. The molar composition of the reaction mixture was lsurfactant : 5A1 : 20H20. Ti-containing mesoporous aluminas were synthesized following the same method but using titanium isopropoxide as a titanium precursor. A specific procedure was as follows: a calculated amount of aluminum tri-sec-butoxide and titanium isopropoxide were mixed and stirred for 6 h. Upon completion of mixing, adding Triton X-100 into the mixture and introducing deionized water slowly under stirring to make the molar composition of reaction mixture be 1Triton X-100 9 5A1 9 xTi " 20H20, where x is 0.1, 0.3, 0.95 and 1.65, corresponding to Ti/Al = 2%, 6%, 19% and 33% (molar ratio). The resulting gel was diluted with sec-buyl alcohol and allowed to react at room temperature for 19 h. After the reaction, the solid product was filtered and washed with absolute ethanol, and followed by drying sequentially at room temperature for 16 h and at 100~ for 6 h. With increasing Ti content, the sample was designated in turn as [Ti]-Al203-1, 2, 3 and 4. The surfactant in all samples was removed by calcining the samples in air at 500~ for 6 h after increasing the temperature to 500~ at 1~ of heating rate.
175 2.2. Characterization XRD (X-ray powder diffraction) patterns were taken on a D/max-rC diffractometer using Cu Kcz radiation. Low temperature nitrogen sorption measurements were carried out on Micromeritics ASAP 2000 porosimeter. The surface areas were determined by the BET method. The pore volumes and the pore size distributions were obtained from single-point and the BJH methods, respectively. Before the measurement, the calcined sample was degassed at 300~ for 4-5 h until the vacuum of system was above 5x 10.3 mmHg. TEM (Transmission election microscopy) were performed with a JEOL JEM-2000EX microscope working at 120 kV. The samples fbr TEM were prepared by suspending the powder in absolute ethanol and by depositing a few drops of the suspension on a Cu grid covered with a thin carbon film. UV-vis DRS (UV-vis diffuse reflectance spectra) were obtained on a Shimadzu UV-240 spectrophotometer. BaSO4 reference was used for the measurements. Laser Raman Spectra were measured on a Bruker RFS-100 spectrometer equipped with a Nd:YAG laser operating at a wavelength of 1064 nm. Tile number of scans was 100 and the resolution of the spectrometer was 4 cm -~
3. RESULTS AND DISCUSSION 3.1. XRD and T E M
Typical XRD patterns of MSU-2 alumina and Ti-containing sample are shown in Fig.1. Only a broad single peak ((100) peak), characteristic of disordered, hexagonal-like packing oi" channels [4], is observed in the range of 20 = 0.50-8~ The intensity of the peak greatly increases after removing the template (Fig la and b). However, the location of the peak can not be determined accurately because the d spacing is so large that the XRD line overlap with tile scattering at low angle and no whole (100) peak is observed. This is in agreement with Pinnavaia's observation that d~00of the alumina sample prepared with the same surfactant is greater than 9 [5]. XRD pattern of Ti-containing sample is similar to that of pure alumina (Fig l c). Introduction of Ti, however, decreases the intensity of XRD line. When Ti content is increased to Ti/AI = 33%, the intensity of" (100) peak is much lower than that of pure alumina (not shown here). This seems to suggest that a part of Ti have become extra-framework species at this content level, thereby resulting in the decrease in purity of mesophase and the lowering in diffraction intensity of (100) peak. But no TiO2 phase is detected by wide-angle XRD in the sample. The feature of channel structure shown by XRD is confirmed by TEM images in Fig 2. It can be seen that the particle of sample contains a large number of channels and the packing of channel system appears to be random and disordered though more or less regular in diameter. Since PEO-bascd surfactants adopt spherical to long "wormlike" micellar structures in
176
Fig.1 XRD patterns of samples a) as-synthesized AlzO3, b) calcined AI203, c) calcined [Ti]- A1203-2
Fig.2
TEM images of calcined samples (amplified by 300 thousand times) a) AI203, b) [Ti]- 11203-2
aqueous solution [9,10], the disordered channel system of MSU-2 samples should be mainly attributed to the formation of wormlike micelles in addition to a weak interaction between the organic templates and the inorganic species in N~ ~ assembly pathway. This character of channel packing still remains after Ti atoms are introduced.
177 3.2. N2-adsorption/desorption Fig.3 provides N2-adsorption/desorption isotherms of calcined samples. For pure alumina (Fig.3a), a broad step resulting from the condensation of adsorbate within framework mesopores occurs above P/Po = 0.5 and is accompanied by a large hysteresis loop. These features denote that the sample possesses a large pore size and broad distribution of diameter. The isotherm of Ti-containing sample with less Ti content is similar to that of pure alumina (Fig.3b). However, when Ti content is increased to Ti/AI = 33% (Fig.3c), the pore filling step in the adsorption curve becomes more narrow and shifts to a lower P/P0 value. The hysteresis loop becomes also smaller. These reveal a decrease in pore size and a narrowing in distribution of diameter, and can be more clearly seen from Fig.4. The data for pore diameter in Table 1 further show that pore size increases when less Ti are incorporated; but it decreases as Ti content increases, similar to the case of Ti-containing mesoporous silica [ 11 - 13]. Moreover, it can be found l'rorn Table 1 that the surface area and the pore volunle increase after Ti are incorporated. This phenomenon has also been observed tbr some TiO,-
o ~ B
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.
. r~ / !
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.
.
.
i
_
i
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.,
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Fig.3 N2-adsorption/desorption isotherms of calcined samples, a) A1203, b) [Ti]-A1203 -2, c) [Ti]- A1203-4
o.o
~-=-~
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",
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.r\
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',\ ~o
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,' ,;o
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,
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/ nm
Fig.4 Pore size distribution of calcined samples. A1203, - - [Ti]-A1203_2 ' [Ti]- A1203-4
178 Table 1 Somc paranactcrs of [Ti]-Ai20~ mcsoporous molecular sieves SaEv Pore volume Pore size sample Ti/Ai Ti wt% (m2. .gl) (mL. g~) (nm) .
.
.
AI203 [Ti]-AI203-1 [Ti]-AI203-2 [Ti]-AI203-3 [Ti]-AlzO3-4
.
.
.
.
2% 6% 19% 33%
.
.
.
1.8% 5.2% 13.8% 20.4%
.
232 325 367 359 376
0.42 0.59 0.55 0.57 0.48
4.1 4.5 4.1 3.8 3.4
AI203 composites and mesoporous silicas [8,11-13]. It has been reported that MSU-X aluminas are not thermal stable [14]. The structural collapse occurs during the calcination, leading to the loss of surface area and porosity. One improving approach is to dope MSU-X aluminas with rare earth metal ions, such as Ce 3' or La 3+, which dramatically enhance the surface area and the pore volume of the calcined samples. The stabilization mechanism was proposed to be the replacement of A13+by rare earth cations, which reduces the lability of the framework wall of MSU-X aluminas. For our samples here, a similar increase in surface area and porosity occurs after introducing Ti, meaning that Ti4+ cations are also effective in stabilizing the l'ramcwork wall of MSU-X almninas against collapse at elevated temperature. On the other hand, this fact also signifies an entrance of partial Ti 4+ cations into the framework by substituting A134. In the pore size distribution determined from the desorption isotherm (Fig.4), pure alumina sample displays textural pores with the diameter greater than 100 nm (Fig.4,---.line); but these pores disappear after Ti are introduced (Fig.4, . line). This fact also evidences that there exist a small amount of amorphous particles caused by collapse in pure alumina, resulting in large interparticle pores. These particles decrease or disappear upon the incorporation of Ti. However, for the sample with Ti/A1 --- 33%, both the pore diameter and the pore volume noticeably decrease (see Table 1), indicating some degree of pore blocking. This may be due to a removal of a part of Ti species from the framework to form TiO2 clusters depositing on the surface of pore wall at such a high Ti content level. Some small particles of TiO2 clusters departing from the surface may create interparticle pores and result in a small peak at 50-70 nm in pore distribution curve of [Ti]-AI203-4 sample ( Fig.4, ---line). 3.3. UV-vis D RS UV-vis DRS of pure alumina and Ti-containing alumina are shown in Fig.5. Pure assynthesized alumina exhibits two absorption bands at ca. 230 nm and ca. 280 nm, respectively; but they disappear after the calcination (Fig.5a and b), suggesting these two bands caused by Triton X-100 templating surfactant because of the presence of large rt bond, such as aromatic ring, in its molecules. Similarly, for as-synthesized Ti-containing sample (Fig.5c), two shoulder bands on both sides of the main absorption band also result from the template, while the main band around 240 nm indicates the incorporation of photoactive Ti species into the sample. Upon the calcination, the shoulder bands disappear and the main band widens due to
179
a -.._...
200
!
|
300
400
,
,
500
k/nm Fig.5 UV-vis DRS of samples a) as-synthesized A1203, b) calcined A1203, c) as-synthesized [Ti]-A1203-2, d) calcined [Ti]-A1203-2
200
!
I
300
400
500
)~/nm Fig.6 UV-vis DRS of calcined samples a) [Ti]-AI203-1, b) [Ti]-AI203-2, c) [Ti]-A1203-3, d) [Ti]-A1203-4
the increase of relative Ti content (Fig.5d). UV-vis spectroscopic investigations of Ti-containing mesoporous silicas and 'l'iO2-SiO 2 composites [11,13,15-18] have assigned the UV band at ca. 330 nm to bulk TiO2 (anatase), and the band centred at 220 nm to isolated Ti atoms in tetrahedral sites. The latter shills towards greater wavelength with increasing Ti content, which is attributed to the formation of polymerized Ti species (Ti-O-Ti species) or isolated Ti species with higher coordination numbers. Pacheco-Malagon and co-workers studied TiO2--AI203 composites prepared by hydrolysis of metal alkoxides [ 19]. Their results showed that for the sample with Ti/A1 >_ 7.5%, Ti species in the composites aggregated into oxide domains with radius >__0.8 nm and an evident absorption band appeared at 300-320 nm in UV-vis spectra. When Ti content was
180 increased to Ti/A1 > 17.5%, the further growth of titania domains in Al203 matrix was difficult. Thus, a fraction of Ti dispersed on A1203 support or departed from the support as individualized TiO2 crystalline particle s, leading to a significant pore blocking. The UV-vis spectra of our calcined samples with different Ti content are shown in Fig.6. For the samples with Ti/A1 < 6% (Fig.6a and b), an UV absorption band centred at ca.225 mn is presented, indicating most of Ti atoms existing in the framework as an isolated mononuclear TiOx species. For Ti/A1 - 19 (Fig.6c), the UV band shifts towards greater wavelength, suggesting the formation of polymerized Ti species and even TiO2 domains with increasing Ti content. When Ti/A1 ratio is as high as 33% (Fig.6d), the TiOz domains further grow so that a part of absorption edge shifts over 300 nm. Combining the data of pore volume and pore diameter, we may conclude that at such a high Ti content level, a fraction of TiO2 clusters remove from the alumina framework to become extra-wall dispersed TiO2. However, as shown in Fig.6, up to Ti/A1 = 33%, no well-defined absorption band is observed above 300 nm, suggesting that the size of TiO2 domain in our Ti-containing samples is smaller than that in TiO2-AI20~ composites reported by Pacheco-Malagon et al. That is to say, the dispersity of 'l'i species in mesoporous aluminas is higher than that in composite oxides. On the other hand, it is also seen from above results that the growth of TiO2 domains in the mesoporous alumina framework is difficult and consequently limited although their sizes are smaller. This probably arises from the thinner wall of mesoporous alumina as compared with the alumina matrix in composite oxides.
d
b
1000
800
600 cm
*
400
200
-1
Fig.7 Raman spectra of calcined samples a) [Ti]-A1203-4, b) [Ti]-a1203-3, c) [Ti]-a1203-2, d) [Ti]-al203-1,
181 3.4. Laser Raman spectroscopy
The Raman spectrum of anatase shows absorptions at 639, 516, 400 and 144 cm 1 [18. 20]. Among these absorptions, the band at 144 cm 1 has the greatest intensity. In Ti-containing zeolites, the presence of multinuclear TixOyclusters also provokes this band in spectrum. Therefore, it is considered as a sensitive indicator of polymerized TiO 2 clusters [20]. The study about TiO2-A1203 composite oxides by Luo et al. [8] showed that, when TiO2 content reached 10%, i.e., Ti/A1 ratio was ca.7%, a distinct 144 cm-' band (shifting to 156 cm") appeared in the Raman spectrum of the sample. This indicates the formation of TiO2 clusters in the form of anatase. Raman spectra of our Ti-containing samples are depicted in Fig.7. When Ti/A1 ratio is 19 or higher, small TiO2 clusters form in the samples, as confirmed by a very weak absorption at 144 cm -~ (designated by asterisk), in accordance with above results from UV-vis DRSI In addition, the Ti content in our sample is much higher than that in TiO2-A1203 composite reported by Luo et al. when the 144 cm ~ band appears; while the intensity of the band is lower tbr our samples than that for the composites. This suggests once again that Ti species in mesoporous alumina have a higher dispersity.
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1. Q-S. Huo, D. I. Margolese, U. Ciesla, P-Y Feng, T. E. Gler, R Sleger, R. Leon, R M. Petroff, F. Schuth, G. D. Stucky, Nature. 368 (1994) 317 2. F. Vaudry, S.Khodabandeh, M. E. Davis, Chem. Mater., 8 (1996) 1451 3. M.Yada, M. Machida, T. Kijima, Chem. Commun., (1996) 769 4. S.A. Bagshaw, E. Prouzet, T. J. Pinnavaia, Science, 269 (1995) 1242 5. S.A. Bagshaw, T. J. Pinnavaia, Angew. Chem. Int. Ed. Engl., 35 (1996) 1102 6. Z-B. Wei, Q. Xin, X-X Gao, E.L. Sham. R Grange, B. Delmon, Appl. Catal., 63 (1990) 305 7. M.Anpo, T. Kawamura, S. Kodama, K. Maruya, T. Onishi, J. Phys. Chem., 92 (1988) 438 8. S. Luo, L. Gui, Mat. Res. Sco. Symp. Proc., 371 (1995) 303 9. B. Chu, Langmuir, 11 (1995) 414 10. Z. Lin, L. E. Scriven, H. T. Davis, Langmuir, 8 (1992) 2200 11. O. Franke, J. Rathousky, G. Schulz-Ekloff, J. Starek, A. Zukal, Stud. Sure Sci. Catal., 84 (1994) 77 12. 13. 14.
15. 16.
A. Corma, M. T. Navarro, J. Perez-Pariente, F. Sanchez, Stud. Sure Sci. Catal.. 84(1994) 69 S. Gontier, A. Tuel, Zeolites, 15 (1995) 601 W-Z. Zhang, T. J. Pinnavaia, Chem. Commun., (1998) 1185 W-Z. Zhang, M. Froba, J-L. Wang, P. T. Tanev, J. Wong, T. J. Pinnavaia, J. Am. Chem. Soc., 118 (1996) 9164 S. A. Bagshaw, F. Di Renzo, F. Fajula, Chem, Commun., (1996) 2209
182 17. 18. 19. 20.
B. Notari, Stud. Surf. Sci. Catal., 37 (1988) 413 Z. Liu, R. J. Davis, J. Phys. Chem., 98 (1994) 1253 G. Pacheco-Malagon, A. Garcia-Borquez, D. Coster, A. Sklysrov, S. Petit, J. J. Fripiat, J. Mater. Res., 10 (1995) 1264 ,l. Klaas, G. Schulz-Ekloff, N. l. Jaeger, J. Phys. Chem. B, 101 (1997) 1305
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
183
Organizing one-dimensional molecular wires in ordered m e s o p o r o u s silica Zongtao Zhang 1, Douglas A. Blom2, and Sheng Dai 1. 1Chemical Sciences and 2Metal & Ceramic Divisions, Oak Ridge National Laboratory, Oak Ridge, TN 37831
We have successfully developed a methodology to assemble molecular wires inside channels of MCM-41. The new method can be generalized to prepare other interesting molecular wires based on charge stabilization between the walls of MCM-41 and molecular wires. 1. INTRODUCTION Recent breakthroughs [1-5] in nanomaterial synthesis have resulted in a novel methodology for preparing mesoporous inorganic materials with unprecedentedly large surface areas and highly ordered mesostructures. Mesoporous silicon and transition-metal oxides have been prepared. The essence of this new methodology is the use of molecular self-assemblies of surfactants or related substances as templates during the formation of oxides. The size of the oxide mesopores can be precisely tailored from 1.2 to 20 nm, based on the use of various surfactant or block copolymer assemblies. The perfect periodic mesopore structures of these materials suggest that they could serve as generic nanoscale reactors for manufacturing and replicating technologically important nanomaterials. Organic polymeric nanowires, carbon nanowires, and semiconductor materials have been synthesized inside the pore channels of mesoporous materials, such as MCM-41. [6-8] Here we report a new methodology to organize a one-dimensional molecular wire Of cationic halogen-bridged mixed valence complexes [Pt(II)(en)2][Pt(IV)C12(en)2] utilizing the interaction from oppositely charged inner walls of the MCM-41 host material. Pseudo-onedimensional, halogen-bridged mixed valence complexes [ptIt(en)2][ptlVX2(en)2](C104)4 (en = 1,2-diaminoethane; X = C1, Br, I) are attracting much attention because of their unique intense intervalence charge-transfer absorption, [9-12] semiconductivity, [13] and large third-order nonlinear optical susceptibilities. [14] So far, two methodologies have been developed to synthesize these one-dimensional molecular wires. The most common method is the crystallization of mixed valence complexes. Kimizuka and coworkers [15] have recently developed a novel self-assembly approach to synthesize such one-dimensional wires. Soluble 1D complexes were synthesized through the amphiphilicity-directed supramolecular organization. The assembly of 1D mixed-valence [ptll(en)2][PtC12IV(en)2] chain in the mesopore channels of MCM-41 represents another new methodology for fabricating 1D molecular wires. To whom all correspondences should be addressed.
184
2. MATERIALS AND METHODS
The preparative protocol is schematically shown in Scheme I. The synthesis of the MCM'41 mesoporous silica was based on a procedure reported in the literature. [16,17] The external surface of the MCM-41 was then functionalized with trimethylsilyl groups according to a method previously described. [18] This functionalization method leaves the inner wall of the MCM-41 material intact, because of the protection from surfactant (cetryltrimethyl ammonium bromide-CTAB) while the external surface is capped with the inert hydrophobic trimethylsilyl groups. This capping of the extemaI surface is important for directing charged hydrophilic complexes into the mesopore channels. The CTAB templates inside the mesopores of the solid were then removed by refluxing with an HC1/CHaOH/H20 solution. The functionalization of the inner wall of the above mesoporous material with thiol ligand was achieved by refluxing 0.35 g of the externally grafted MCM-41 in 100 ml of toluene containing 2.0 g of 3-mercaptopropyltriethoxysilane for 48 h. The thiol groups on the inner wall were then oxidized to the corresponding sulfonic acid by impregnating the solid briefly with 20% HNO3, [19] followed by careful addition of concentrated HNO3 and stirring for 24 h at room temperature. The resulting products were filtered, washed with deionized water, and then placed in 100 ml of aqueous NaC1 solution (0.1M) for the ionexchange reaction between protons and sodium ions. The ion-exchanged product was dried under vacuum at 80~ Both [Ptn(en)2]C12 and trans-[PtIVC12(en)2]C12 were prepared according to a method reported in the literature. [20] The ion-exchanged functionalized MCM-41 (0.1 g)was added to 2.0 ml of water and EtOH solution containing [ptII(en)2]C12
185
60000
40000 E D 20000
C B A
o
9
~
9
1
9
~ i:
9
8
i ~
" 0
Figure 1. XRD pattems of a series of MCM-41 materials: (A) as-synthesized MCM-41; (B) as-synthesized MCM-41 externally functionalized with trimethylsilyl groups; (C) surfactant- removed MCM-41 extemally functionalized with trimethylsilyl groups, (D) MCM-41 whose inner walls are functionalized with sulfonates and external surfaces with trimethylsilyl groups, (E) [ptII(en)2][ptrVc12(en)2] wires assembled inside the channels of functionalized MCM-41. and trans-[ptrVC12(en)2]C12 ([Pt]totaZ = 100 mmol). The solvent was then evaporated using a rotary evaporator. The control blank samples were prepared via an identical procedure using the capped MCM-41 material with the inner wall unfunctionalized. 3. RESULTS AND DISCUSSIONS The powder X-ray diffraction pattems of samples were recorded using a SIEMENS D5005 X-Ray diffractometer. Powder X-ray diffraction (XRD) patterns for the series of the MCM-41 samples are shown in Fig. 1. The XRD patterns for each of the three samples exhibit four reflection peaks, which can be assigned to the reflections from the (100), (110), (200), and (210) planes of a hexagonal MCM-41 material. The presence of these four reflection peaks represents the preservation of the mesoscopic order during various stages of the preparation. The formation of one-dimensional molecular wires inside mesoporous material channels does not influence the hexagonally ordered mesostructure. However, the (100) peak position slightly shifts during different stages of preparation, and the final (i 00) peak of the MCM-41 loaded with the molecular wire reflects at a d spacing of 40.43 A (a0 = 46.68 A). From the N2 adsorption-desorption isotherm, the calculated specific surface area (BET) decreases from 1176 mZ/g for the surfactant-removed MCM-41 starting material to 970 mZ/g for the corresponding MCM-41 material with the inner wall functionalized with a
186 sulfonate group. This decrease in the surface area can be attributed to a partial pore filling by the grafting groups. Scanning transmission electron microscopy (STEM) investigations were performed using a HD-2000 Scanning Transmission Electron Microscope (probe size = 0.8 nm) operating at 200 kV. The key point with high annular dark-field imaging is that the intensity of the Rutherford scattered beams is directly proportional to Z2, where Z is the atomic number of the scattering element. Thus, heavy atoms (such as platinum) stand out very clearly on a light background of silicon and oxygen. Figure 2 shows the bright-field and dark-field TEM images recorded on the same area of the functionalized MCM-41 material loaded with the platinum complexes. The bright-field image clearly illustrates that the pore structure is regular with well-ordered hexagonal arrays, and the pore diameter is estimated to be 25 A. The direct proof for the presence of the platinum complexes within the channels of MCM'41 comes from dark-field imaging. As seen in Fig. 2B, it is apparent that part of the image shows highly bright contrast, indicating the presence of heavy elements inside the silica channels. Energy-dispersive X-ray emission (EDX) measurements on the same range clearly show signals of platinum, confirming the presence of the platinum complexes inside the mesopores. Figure 3 shows a comparison among UV-vis diffuse reflectance spectra of the ftmctionalized MCM-41 materials loaded with [ptn(en)2]C12, [ptrVC12(en)2]C12, and the mixture of [ptn(en)2]C12 and trans-[ptrVC12(en)2]C12, respectively. Onr~" the spectrum of the MCM-41 sample loaded with a mixture of [PtH(en)2]C12 and trans-[Pt C12(en)2]C12gives an absorption band around Lm~x= 436 nm, which is very close to that for a single crystal of [ptIt(en)2][ptIVc12(en)2](C104)4 (Lmax at 456 nm).[21] This indicates that [PtII(en)2] and II IV [PtIV C12(en)2] complexes are self-assembled into a halogen-bridged Pt~-Pt molecular wire II IV similar to that in solid-state [Pt (en)2][Pt C12(en)2](C104)4. The slight blue shift of the absorption band from the [ptII(en)2][PtIVC12(en)2] complex assembled inside the channels of MCM-41 relative to that from the solid-state [ptII(en)2][PtIVC12(en)2](C104)4 can be attributed to the difference in the wire lengths of the two systems. The wire length of the former is shorter than that of the latter. It is interesting that the UV-vis diffuse reflectance spectrum of the control blank sample has no bands near 400 nm. Accordingly, no molecular
A
B
Figure 2. TEM images ofMCM-41 loaded with [ptll(en)E][ptlVc12(en)2] wires: (A) bright-field; (B) dark-field images recorded under the same region.
187 wires are present in the control blank sample. The control blank sample was impregnated with a mixture of [ptii(en)2]C12 and trans-[ptWc12(en)2]C12 in the same way as that for the
Figure 3. Diffuse-reflectance UV-vis spectra of (A) functionalized MCM-41 loaded with [ptlI(en)2]C12; (B)) functionalized MCM-41 loaded with [ptWc12(en)2]Cl2; (C) Control blank loaded with a mixture [ptII(en)2] and [ptIVc12(en)2] (D) functionalized MCM-41 loaded with a mixture [ptIl(en)2] and [ptrVC12(en)2]. functionalized MCM-41. The only difference is that the inner wall of the control blank sample is not functionalized with sulfonate groups. Therefore, the counteranions (sulfonates) on the inner walls of the mesopores play a crucial role in the assembly of the [ptlI(en)2][ptrVC12(en)2 ] w i r e s inside MCM-41. The color of the MCM-41 material containing [Ptn(en)2][ptIVc12(en)2 ]wires is yellow, which results from the intervalence charge-transfer electronic transition in the supramolecular system consisting of chlorobridged chains of (ptlLc1-ptW-c1-)n and internally grafted counteranions aligned in a parallel fashion. No similar color was observed for the control blank sample. In conclusion, the assembly of one-dimensional mixed-valence platinum complexes within a functionalized ordered mesoporous silica is described. The driving force for the formation of such one-dimensional molecular complexes is the coulombic association interaction between cationic mixed-valence complexes and anionic functionalized silica walls. Functional groups on the inner walls of MCM-41 exert a remarkable influence on molecular self-assembly. [22] A new methodology has been developed to assemble 1D molecular wires in the solid state. 4. ACKNOWLEDGEMENTS
This work was conducted at the Oak Ridge National Laboratory and supported by the Office of Basic Energy Sciences, U.S. Department of Energy, under contract No. DE-AC0500OR22725 with UT-Battelle, LLC.
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,,
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C.T. Kresge, M.E. Leonowicz, W. J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. (a) D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka and G.D. Stucky, Science, 279 (1998) 548. (b) Q. Huo, D.I. Margolese, U. Ciesla, P. Feng, T.E. Gler, P. Sieger, R. Leon, P.M. Petroff, F. Schuth and G.D. Stucky, Nature, 368 (1994) 317. P.T. Tanev, T.J. Pinnavaia, Science, 267 (1995) 865. J.P. Ying, C.P. Mehnert and M.S. Wong, Angew. Chem. Int. Ed., 38 (1999) 56. X. Feng, G.E. Fryxell, L.Q. Wang, A.Y. Kim, J. Liu and K.M. Kemner, Science, 276 (1997) 923. C.G. Wu and T. Bein, Science, 264 (1994) 1757. R. Leon; D. Margolese; G. Stucky and P.M. Petroff, Phys. Rev. B, 52 (1995) 2285. V.I. Srdanov, I. Alxneit, G.D. Stucky, C.M. Reaves and S.P. DenBaars, J. Phys. Chem. B, 102 (1998) 3341. M. Robin and P. Day, Adv. Inorg. Radiochem., 10 (1967) 247. S. Kida, Bull. Chem. Soc. Jpn., 38 (1965) 1804. M. Yamashita, N. Matsumoto and S. Kida, Inorg. Chim. Acta., 31 (1978) L381. R.J.H. Clark, Chem. Soc. Rev., 19 (1990) 107. Y. Hamaue, R. Aoki, M. Yamashita and M. Kida, Inorg. Chim. Acta., 54 (1981) L 13. Y. Iwasa; E., Funatsu; T., Hasegawa; T. Koda; M. Yamashita, Appl. Phys. Lett. 1991, 59,2219. N. Kimizuka, N. Oda, T. Kunitake, Inorg. Chem., 39 (2000) 2684. (a) S. Dai, M. C. Burleigh, Y.H. Ju, H.J. Gao, J.S. Lin, S. Pennycook, C.E. Barnes and Z.L. Xue, J. Am. Chem. Soc. 122 (2000) 992. (b) S. Dai, M.C. Burleigh, Y.S. Shin, C.C. Morrow, C.E. Barnes and Z.L. Xue, Angew. Chem. Int. Ed. Engl., 38 (1999) 1235. C.Y. Chen, H.X. Li, M.EI Davis, Microporous Mater., 2 (1993) 17. F. de Juan and E. Ruiz-Hitzky, Adv. Mater., 12 (2000) 430. M.H. Lim, C.F. Blanford and A. Stein, Chem. Mater., 10 (1998) 467. F. Basolo, J.C. Bailar and B.R. Tarr, J. Am. Chem. Soc., 72 (1950) 2433. Y. Wada, T. Mitani, M. Yamashita and T. Koda, J. Phys. Soc. Jpn., 54 (1985) 3143. N. Kimizuka, Adv. Mater., 12 (2000) 1462.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
189
Synthesis and catalytic properties of organically modified T i - H M S Yong Yang and Abdelhamid Sayari* Department of Chemistry and Centre for Catalysis Research and Innovation, University of Ottawa, Ottawa, Ontario, KIN 6N5, Canada
It is demonstrated that the hydrophobicity of Ti-HMS catalysts can be enhanced by organic modification via either direct synthesis or post-treatment. In comparison to unmodified Ti-HMS, the catalysts obtained by direct synthesis, in which methyl and dimethyl groups were incorporated directly in the synthesis gel, exhibited comparable Ti-content but higher surface area and stronger hydrophobicity; the catalyst obtained by post-synthesis methylation of surface silicon also showed higher hydrophobicity but much lower Ti-content. Characterization results and catalytic oxidation data of 2,6 di-tert-butylphenol (DTBP) showed strong support for the occurrence of hybrid Ti-HMS-based materials.
1. INTRODUCTION Ti-substituted mesoporouse molecular sieves such as Ti-MCM-41 and Ti-HMS have received much attention [1-4] because of their potential to oxidize bulky organic substrates that cannot be oxidized efficiently over microporous titanosilicates such as TS-1 and TS-2 [2, 5-7]. Compared with Ti-MCM-41, the structure of Ti-HMS is less organized but likely more stable as a result of their thicker walls [2,4]. Surface modification by incorporating various organic functionalities is believed to not only enhance the hydrothermal stability [8,9] but also induce hydrophobicity and thus a considerable improvement of the activity of Ti-MCM41 or Ti-HMS in the liquid phase oxidation reactions in the presence of aqueous H202 under mild conditions [5,6]. Several methods have been used to obtain hybrid Ti-substituted MCM41/HMS: One approach is the surface modification by post-synthesis grafting of different types of organic groups onto the silica network. Silylation of the surface can efficiently eliminate the terminal silanol groups, and enhance the hydrophobicity [5,10,11]. Hybrid materials can also result from a direct synthetic approach via the co-condensation of tetraethyl orthosilicate and organoalkoxysilane in the presence of surfactants [6,7,12-14]. In addition, hybrid mesoporous materials were also prepared by sol-gel method [15-18]. In this work, we for the first time incorporated methyl and dimethyl groups within Ti-HMS network by direct synthesis and by post-synthesis methylation. Characterization results by different techniques showed the successful incorporation of organic species in Ti-HMS and the enhancement of the surface hydrophobicity, thus resulting in improved catalytic performance for DTBP oxidation.
190
2. EXPERIMENTAL 2.1. Materials and synthesis The silica source was tetraethyl orthosilicate (TEOS, Si(OCH2CH3)4), purchased from Aldrich. The surfactant n-dodecylamine was also obtained from Aldrich. Methyltriethoxysilane (MTEOS, CH3Si(OC2Hs)3) and dimethyldiethoxysilane (DMDEOS, (CH3)2Si(OC2Hs)2) were supplied by Gelest. Titanium (IV) ethoxide (TEO, Ti(OC2Hs)4) was purchased from Aldrich. Isopropanol (IPA) was supplied by J. K. Baker. Methyl magnesium bromide (3 M in ether) was purchased from Strem Chemicals. All chemicals were used as received. An unmodified Ti-HMS sample was synthesized by the following procedure: a solution A was prepared by mixing 1.0 g of Ti(OC2Hs)4, 37.5 g of TEOS, 52.5 g of ethanol and 10.5 g of isopropanol. A second solution B was prepared by mixing 8.75 g of dodecylamine, 3.5 ml 1M HC1 and 115 g of water. Solution A was added slowly to solution B under vigorous stirring. Stirring was maintained at room temperature for 18 h. The mixture molar composition was: 1.0TEOS : 0.26DDA : 6.5EtOH : 1.0IPA : 0.025TEO : 37.4H20. The obtained mesostructure product was then filtrated and washed. The calcination was carded out at 540 ~ for 1 h in nitrogen and 4 h in air. The CH3-Si and (CH3)2-Si substituted Ti-HMS (designated as Ti-(CH3)HMS and Ti(CHa)2HMS) were prepared using the same procedure as for unmodified Ti-HMS except for replacing 37.5 g of TEOS by either 25 g of TEOS and 10.7 g of CHaSi(OC2Hs)3 or 25 g of TEOS and 8.9 g of (CHa)2Si(OC2Hs)2. Their reaction system compositions were: 0.67TEOS : 0.33MTEOS : 0.26DDA : 6.5EtOH : 1.0IPA : 0.025TEO : 37.4H20, and 0.67TEOS : 0.33DMDEOS : 0.26DDA : 6.5EtOH : 1.0IPA 90.025TEO : 37.4H20, respectively. The obtained mixtures were stirred at room temperature for 18 h. The mesostructured products were then filtrated and washed. The removal of the surfactant from the pore channels of assynthesized products was carded out by three ways. The first was extraction of 10 g of assynthesized product by 500 ml of anhydrous ethanol under refluxing conditions for 1 h and repeating this procedure twice. The second was extraction of 1 g of as-synthesized product using 20 ml 1 M HC1 solution under stirring for 1 h. The third was calcination of assynthesized product at 540 ~ for 1 h in nitrogen and 4 h in air. Another modified sample Ti-CHa/HMS was obtained by post-treatment of calcined TiHMS with a Grignard reagent in two steps as described by Yamamoto and Tatsumi [9]. In the first step, 1 g of calcined Ti-HMS was treated with refluxing BuOH at 140 ~ for 48 h to give BuO(Ti-HMS). This material was filtrated and dried at 200 ~ in flowing N2 overnight. The second step was the treatment of obtained BuO(Ti-HMS) by refluxing 100 ml 0.2 M CHaMgBr solution in ethyl ether at 35 ~ for 72 h to produce Ti-CHa/HMS. Subsequently, Ti-CHa/HMS was filtrated, washed with 1M HC1 and water, then dried at 200 ~ overnight. 2.2. Characterization The chemical compositions were determined by ICP analysis. Powder X-ray diffraction (XRD) patterns were obtained on a Siemens D5000 diffractometer using CuKa radiation with 0.15418 nm wavelength, a step size of 0.02 ~ 20, and a counting time per step of 2 s over a 1o < 20 < 10~ range. N2 adsorption experiments were performed at 77 K using a Coulter Ominorp 100 gas analyzer. Prior to adsorption, the samples were degassed at 473 K for 2 h. BET surface areas were calculated from the linear part of the BET plot at P/P0 = 0.05 - 0.15. Pore size distributions were calculated using the Horvath-Kawazoe method. FTIR analyses
191 were performed using KBr pellets that were prepared under 2 x 104 kPa pressure and a Nicolet spectrometer (Magna-IR 550) with a resolution of 2 cm-~. Each absorption spectrum reported is the average of 64 scans. XPS spectra were recorded using a V.G. Scientific Escalab Mark II system. UV-visible spectra were collected on a Perkin Elmer spectrometer using magnesium oxide for background correction. 2.3. Catalytic reaction The DTBP oxidation was conducted in a magnetically stirred three-necked flask connected to a condenser. In a typical example, 0.1 g of catalyst was added into a solution containing 1.03 g of DTBP, 1.7 g of 30 wt% H202 and 7.8 g of acetone at 62 ~ The mixture was then kept at this temperature under stirring for 2 h. The catalyst was filtrated and the reaction products were analyzed by a gas chromatograph (HP 5890 II) equipped with a capillary column and a FID detector.
3. RESULTS AND DISCUSSION 3.1. Characterization The surface compositions and Si/Ti ratios for Ti-HMS-based samples as determined from XPS data are listed in Table 1 along with the elemental analysis data. For all samples the Si/Ti ratio in the starting gel was 40. It is clearly shown that a significant increase of Si/Ti ratio from the starting gel to the final products was obtained. For unmodified Ti-HMS, the Si/Ti ratio in the final product was 69, CH3-Si and (CH3)2-Si modified Ti-HMS samples exhibited comparable Si/Ti ratios to unmodified Ti-HMS with 60 and 54 in final products although the surface Si/Ti ratios were much higher (114 in both cases). However, the Ti-HMS sample modified by post-synthesis treatment showed a much greater change in Si/Ti ratio (136). This was attributed to leaching of Ti during the treatment. It is also important to notice the carbon concentration in all samples; clearly all modified samples have much higher carbon concentrations than the unmodified Ti-HMS. Among the three modified samples, Ti-(CH3)2HMS has the highest surface concentration of carbon (21.1%), indicating a relationship between the hydrophobicity and the surface composition. Figure 1 presents the XRD data for all samples, which indicate the occurrence of typical mesostructured HMS phases [6]. Both unmodified and modified samples featured dominant d~00 reflections with a broad shape. The unmodified Ti-HMS and the sample modified by post-synthesis treatment showed similar lattice spacing of c a . 44 A. Ti-(CH3)HMS and Ti(CH3)2HMS have much lower lattice spacings of 35.4 and 33.9 A. It is known that hybrid Table 1. Surface concentrations and Si/Ti ratios for Ti-HMS samples a. Surface composition (mole % ) Si/Ti Sample C O Si Ti Surface ~ Bulk e Ti-HMS u 3.7 66.6 29.3 0.43 68 69 Ti-(CH3)HMS c 18.7 53.8 27.3 0.24 114 60 Ti-(CH3)2HMS c 21.1 51.0 27.8 0.27 114 54 Ti-CH3/HMS c 10.4 60.9 28.5 0.21 136 136 Si/Ti = 40 for all samples in the starting gel, b Sample calcined, c Samples extracted by ethanol, d Measured by XPS, e Measured by ICP.
192 mesoporous silicas prepared via direct cocondensation exhibit decreasing pore sizes. This decrease is dependent on the size of grafted ligands and the surface coverage. The current hybrid Ti-HMS samples show significant contractions of the lattice as C compared to the methyl substituted Ti-MCM41 reported in the literature [5,12]. b The nitrogen adsorption-desorption isotherms for these samples and the corresponding pore size distributions are shown in Figure 2. The isotherm for the unmodified Ti-HMS is of type IV and shows a distinct 1 3 5 7 9 capillary condensation step in the region of P/P0 = 0.3 - 0.5 indicating the occurrence of a 2-theta (degree) mesoporous structure. Ti-(CH3)HMS and Ti(CH3)2HMS also showed a nitrogen Figure 1. XRD patterns of (a) Ti-HMS, (b) condensation step, but at much lower P/P0 (< Ti-(CH3)HMS, (c) Ti-(CH3)zHMS, (d) Ti0.2). For the Ti-CH3/HMS, the capillary CH3/HMS. condensation step became broader but the meso-structure was retained. Data in Figure 2B show that unmodified Ti-HMS has a very sharp pore size distribution centred at 3.1 nm. Ti-(CH3)HMS and Ti-(CH3)zHMS have comparable pore sizes (1.8 and 1.7 nm) but much smaller than unmodified Ti-HMS. TiCH3/HMS has the largest pore size centered at 3.8 nm. The physical properties of unmodified and modified Ti-HMS samples are presented in Table 2. As seen, all samples exhibited typical surface areas and pore sizes for mesoporous materials. Ti-(CH3)HMS and Ti-(CH3)2HMS have surface areas over 1000 m2/g but small pores and thick wall as much as 2.5 and 2.1 nm, respectively. It is common that one-pot synthesized hybrid mesoporous silicas exhibit thicker pore walls compared to their r
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Figure 2. N2 adsorption-desorption isotherms (A) and pore size distributions (B) of (m) TiHMS, ()Ti-(CH3)HMS, (...:..)Ti-(CH3)2HMS, (---)Ti-CH3/HMS
193 Table 2. Physical properties of unmodified and modified Ti-HMS samples. dl00 SBET a Pore size b Wall thickness c Pore volume a Sample (nm) (m 2 g-I) (nm) (nm) (cm 3 g-l) Ti-HMS 4.40 859 3.1 1.8 0.86 Ti-(CH3)HMS 3.54 1196 1.8 2.5 0.66 Ti-(CH3)2HMS 3.39 1087 1.7 2.1 0.58 Ti-CH3/HMS 4.38 749 3.8 1.3 1.01 a Measured from N2 adsorption isotherm at P/P0 = 0.05 - 0.15, b Determined by HorvathKawazoe plot, c Calculated by subtracting the pore size from the unit cell parameter ao (ao = 2dloo/x/3 ), a Total volume at P/P0 = 0.996. corresponding pure unmodified silicas [13,14]. However, the hybrid mesoporous materials obtained by post-synthesis treatment, TiCH3/HMS, has the largest pore size and the thinnest wall as well as the highest pore volume. UV-Vis spectra for various Ti-HMS samples are shown in Figure 3. The band occurred at 210 - 230 nm can be assigned to Ti in tetrahedral (210 nm) and octahedral (230 nm) framework position [1,4,19]. The presence of a broad shoulder at about 270 nm in the case of Ti-(CH3)HMS and Ti-(CH3)2HMS is assigned to hexacoordinated Ti species containing Ti-O-Ti bonds [19]. It is clear the intensity of the band for sample Ti-CH3/HMS is considerable lower than other samples; this might be attributed to the lower Ti-content in this sample. The absence of any absorption at about
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Figure 3. UV-vis spectra of (a) Ti-HMS, (b) Ti-(CH3)HMS, (c) Ti-(CH3)2HMS, (d) Ti-CH3/HMS.
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Figure 4. FTIR spectra of Ti-(CH3)HMS (A) and Ti-(CH3)2HMS (B). (a) as-synthesized, (b) extracted by ethanol, (c) extracted by 1M HC1, (d) calcined.
194 330 - 340 nm indicates that all samples are free from oxide impurity anatase [1]. Our XPS results also lend support to this observation indicating that tetrahedral Ti is the dominant state on all samples. The presence of Ti species and organic moieties in the framework was investigated by FTIR. FT-IR spectra of dry KBr-pellets of organically modified samples after different treatments are shown in Figure 4. A band at 960 cm -1 is clearly observed in extracted and calcined samples. This peak has been associated with Si-O-(Ti) or Ti=O stretching frequency that is usually considered to be the evidence of isomorphous substitution Si by Ti [4,6]. This observation is also consistent with our UV-vis data in which a band at 210 - 230 nm was found. The peaks that appear at 2910, 2850 and 1500 c m -1 in the spectrum for the assynthesized sample belong to surfactant. They essentially disappear after extraction and calcination. The successful incorporation of (CH3)-Si and (CH3)2-Si was confirmed by four peaks at ca. 2970, 1267, 850, and 800 cm-1 in extracted samples, which can be assigned as CH3 asymmetric mode, CH3 deformation, CH3 rocking and Si-C stretching mode [6,14], respectively. The 13C MAS NMR spectra of Ti-HMS, Ti-(CH3)HMS and Ti-(CH3)2HMS show different peaks.. Ti-HMS gave only peaks attributable to the surfactant, whereas Ti(CH3)HMS exhibited a characteristic peak at 8-6.0 ppm assigned to CH3-Si. Ti-(CH3)2HMS exhibited a characteristic peak at 8-2.0 ppm for CH3-Si-CH3. As for the 29Si MAS NMR spectra for calcined Ti-HMS and extracted Ti-(CH3)HMS and Ti-(CH3)2HMS as well as TiCH3/HMS, in general, they showed three bands centered at chemical shifts of-92,-100 a n d 110 ppm attributable to Si(OSi)x(OH)a.x framework units where x = 2 (Q2), x = 3 (Q3), and x = 4 (Q4), respectively (Table 3). This was clearly observed on Ti-HMS and Ti-CH3/HMS. Additional peaks are observed in modified samples. For Ti-(CH3)HMS, apart from Q3 and Q4 peaks, a peak characteristic of CH3-Si-(OSi)3 (T3 8 -65.2 ppm) was obtained. For Ti(CH3)2HMS, the spectrum exhibited a characteristic peak attributed to (CH3)2Si(OSi)2 (D 2 8 Table 3. Modified Ti-HMS Silicates: 29Si NMR Data. Sample Chemical shift (ppm) Ti-HMS -92.0 (QZ) -100.2 (Q3) -109.6 (Q4) Ti-(CH3)HMS -56.3 (T2) -65.2 (T3) -90.0 (Q2) -102.2 (Q3) -110.6 (Q4) Ti-(CH3)2HMS -19.3 (D 2) -101.0 (Q3) -110.6 (Q4) -53.8 (T2) Ti-CH3/HMS -62.3 (T3) -93.2 (Q2) -101.1 (Q3) -109.9 (Q4)
Line width (Hz) 140 315 480 615 527 171 671 459 320 300
308 308 457 327 542 557
Intensity (%) 2.5 27.3 70.2 5.5 23.3 0.6 26.2 44.4 21.6 21.0 57.4 1.0 6.1 0.9 24.3 67.8
195 Table 4. Catalytic properties of modified and non-modified Ti-HMS samples for the oxidation of DTBP in the presence of H 2 0 2 . a Catalyst H202/DTBP Conversion (%) Quinone selectivity (%) Ref Ti-HMS (calc.) b 3 17.2 72.2 cw. b Ti-(CH3)HMS(ext.) b 3 33.8 63.1 CW. Ti-(CH3)HMS(calc.) 3 21.8 74.8 cw. Ti-(CH3)2HMS (ext.) 3 49.4 67.4 cw. Ti-CH3/HMS 3 29.6 65.4 cw. TS-1 (calc.) 3 23.1 2.7 20 Ti-HMS (Si/Ti = 50) 3 22.4 84.7 20 TS- 1 (Si/Ti = 100) 6 3 -20 Ti-HMS (Si/Ti = 76) 6 15 93 20 Ti-HMS (Si/Ti = 50) 6 55 58 21 a Reaction conditions: catalyst (100 mg), 30 wt% H 2 0 2 (1.7 g), solvent: acetone (7.8 g), reactant: di-tert-butylphenol, DTBP (1.03 g), reaction temperature: 62 ~ time (2 h). b calc." calcined, ext.: extracted by ethanol, cw.: current work. 19.3 ppm). These NMR results clearly demonstrated that the CH3-Si and (CH3)2-Si moieties were kept intact during the synthesis and were successfully incorporated into the silicate framework. 3.2. Catalytic oxidation of DTBP Table 4 lists the catalytic performance data of unmodified and modified Ti-HMS samples for the oxidation of DTBP in the presence of H202 and some literature data. It is seen that both unmodified and modified Ti-HMS samples are active for DTBP oxidation. Under the same reaction conditions, all modified samples gave higher conversion than unmodified TiHMS. The selectivity to quinone was comparable ( 6 0 - 75 %) for all catalysts. The catalytic performance for the oxidation of large organic compounds like DTBP is greatly dependent on many factors such as synthesis approach, surface area, Ti-content, surface hydrophobicity, reaction temperature, and solvent used. It is worth noticing that the extraction method to eliminate the surfactant contained in the samples is critical. Under the same reaction conditions, the sample after extraction by 1 M HC1 solution gave no activity for DTBP oxidation (not shown in Table 4). Although the FT-IR spectra show this extraction removed the surfactant and retained most methyl groups. The loss of activity is probably attributed to the collapse of meso-structure. Indeed, our XRD and N2 adsorption data indicated this extracted sample is lack of ordered meso-structure. The optimum surfactant-removal method seems to be the use of ethanol under mild conditions in the absence of acid. Among modified Ti-HMS samples, those extracted by ethanol exhibited slightly higher activity than their calcined counterparts. For example, extracted Ti-(CH3)HMS gave an overall conversion of 33.8 % whereas its calcined counterpart gave a conversion of 21.8 %. This can be attributed to the difference in the hydrophobicity of these two catalysts. Calcination completely eliminated the organic moieties on the sample leading to a drop of hydrophobicity. As shown by FT-IR data, bands corresponding to the organic groups disappeared. It is inferred that the hydrophobic property is favorable to oxidation in water containing liquid phase.
196 It should be noted that the Ti content also influences the catalyst activity. Our NMR data show the existence of organic groups in Ti-CH3/HMS that was obtained by post-synthesis modification (T 3 8-62.3ppm). However, as compared to other Ti-HMS modified through direct synthetic method, this sample has much lower catalytic activity for DTBP oxidation that may be caused by its very low Ti-content (Si/Ti = 136). TS-1 and Ti-HMS have already been employed in the oxidation of DTBP [20,21]. The former was found to be ineffective because of its microporous structure [2]. Our current unmodified Ti-HMS exhibited comparable catalytic performance for DTBP oxidation as the Ti-HMS reported in the literature [2,20]. Furthermore, our modified samples exhibit improved catalytic performance. The structural modification and hydrophobicity enhancement are likely responsible for the enhanced catalytic performance. 4. CONCLUSION Organically modified Ti-HMS catalysts were prepared either by direct synthesis using the combination of silica sources organoalkoxysilane and TEOS or by post-synthesis modification. Different characterization results indicated that the organic groups were successfully incorporated within Ti-HMS. Being more hydrophobic than their unmodified counterpart, the organically modified Ti-HMS samples exhibited enhanced catalytic activity in the oxidation of DTBP in the presence of aqueous H202 under mild conditions. REFERENCES 1. 2. 3. 4. 5.
A. Corma, M. T. Navarro, J. P6rez-Pariente, Chem. Soc., Chem. Commun., (1994) 147. P.T. Tanev, M. Chibwe, T. J. Pinnavaia, Nature, 368 (1994) 321. A. Sayari, K.M. Reddy, I. Moudrakovski, Stud. Surf. Sci. Catal., 98 (1995) 19. A. Sayari, Chem. Mater., 8 (1996) 1840. A. Corma, M. Domine, J. A. Gaona, J. L. Jord~, M. T. Navarro, F. Rey, J. P6rez-Pariente, J. Tsuji, B. McCulloch, L. T. Nemeth, Chem. Commun., (1998) 2211. 6. A. Corma, J. L. Jord~, M. T. Navarro, F. Rey, Chem. Commun., (1998) 1899. 7. L. Mercier, T. J. Pinnavaia, Chem. Mater., 12 (2000) 188. 8. K.A. Koyano, T. Tatsumi, Y. Tanaka, S. Nakata, J. Phys. Chem. B, 101 (1997) 9436. 9. K. Yamamoto, T. Tatsumi, Chem. Lett., (2000) 624. 10. J. Bu, H.-K. Rhee, Catal. Lett., 66 (2000) 245. 11. T. Tatsumi, K. A. Koyano, N. Igarashi, Chem. Commun., (1998) 325. 12. A. Bhaumik, T. Tatsumi, J. Catal., 189 (2000) 31. 13. A. Bhaumik, T. Tatsumi, Catal. Lett., 66 (2000) 181. 14. J. Joo, T. Hyeon, J. Hyeon-Lee, Chem. Commun., (2000) 1487. 15. H. Kochkar, F. Figueras, J. Catal., 171 (1997) 420. 16. F. Figueras, H. Kochkar, S. Caldarelli, Microporous Mesoporous Mater., 39 (2000) 249. 17. C. A. MOller, M. Maciejewski, T. Mallat, A. Baiker, J. Catal., 184 (1999) 280. 18. C. A. Mtiller, M. Schneider, T. Mallat, A. Baiker, J. Catal., 189 (2000) 221. 19. J. S. Reddy, A. Sayari, Appl. Catal., 128 (1995) 231. 20. J. S. Reddy, A. Dicko, A. Sayari, In Synthesis of Microporous Materials" Zeolite, Clays and Nanostructures, Occelli, M. L., Kessler, H., Eds.; Marcel Dekker (1995), page 405. 21. T. J. Pinnavaia, P. T. Tanev, W. Wang, W. Zhang, Mater. Res. Soc. Symp. Proc., 371 (1995) 53.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
Synthesis and c h a r a c t e r i z a t i o n o f m e t h y l - and v i n y l - f u n c t i o n a l i z e d m e s o p o r o u s silicas w i t h h i g h organic content
197
ordered
Michal Kruk, a Tewodros Asefa, b Mietek Jaroniec a and Geoffrey A. Ozin b
a
Department of Chemistry, Kent State University, Kent, Ohio 44242, USA
b Materials Chemistry Research Group, Department of Chemistry, University of Toronto, Toronto, Ontario, M5S 3H6, Canada
Methyl- and vinyl-functionalized ordered mesoporous silicas were synthesized via a cocondensation method from tetraethyl orthosilicate (TEOS) and methyltriethoxysilane (MTES) or vinyltriethoxysilane (VTES) in the presence of cetyltrimethylammonium bromide surfactant as a structure-directing agent using the synthesis procedure we have recently elaborated. The surfactant was removed via solvent extraction. The organosilane was incorporated in proportions corresponding to its content in the synthesis mixture. The materials with up to 70 molar % of VTES and 50% of MTES exhibited at least one peak in their powder X-ray diffraction pattems, and up to 3 peaks were observed for loadings of 33 and 43%. The (100) interplanar spacing, the primary pore volume and the pore diameter tended to systematically decrease as the organic group loading increased, whereas the specific surface area was relatively constant, although it appreciably decreased for high loadings of VTES. In particular, the pore diameter shifted from the mesopore to the micropore range for higher loadings of vinyl groups, allowing functionalized ordered microporous materials to be obtained. The synthesis of vinyl-functionalized silicas was found to be highly reproducible. It was concluded that vinyl-functionalized silicas can be synthesized in a reproducible way with retention of ordered structure and without phase separation for up to about 65% organic group loading, which is the highest hitherto attained loading of pendent organics in ordered silicas.
1. INTRODUCTION After its introduction in 1996 [ 1,2], the synthesis of ordered organic-functionalized silicas via co-condensation of organosilica and silica precursors has received much attention [3-24] and is currently an attractive altemative of post-synthesis functionalization method [3,4]. The co-condensation pathway is highly convenient in allowing a full utilization of reagents and thus being cost-effective [ 14]. It also allows one to achieve high loadings of organic groups on the silica surface. Early studies by Mann and coworkers resulted in the synthesis of an ordered silica in which up to 40% of silicon atoms were functionalized with organic groups [6]. Mann et al. also predicted that 40% is likely to be the highest functionalization level attainable for such organic-functionalized silicas [1]. These predictions were based on the
198 expectation that sufficiently large population of silicon atoms should be fully cross-linked (that is, connected via oxygen atoms with four other silicon atoms) within the framework. Clearly, the presence of pendent organic groups limits the degree of framework cross-linking, which was envisioned to lead to structural instability of frameworks that exhibit a high population of pendent groups. In accord with this expectation, the co-condensation route to the synthesis of ordered silicas with the functionalization level above 40% (based on the percentage of silicons that carry organic groups) has been unsuccessful in most cases. Macquarrie and coworkers [14,16] carried out a detailed study intended to the introduce loading of 50%, and although this loading was actually achieved, no structural ordering was observed. Babonneau et al. also achieved 50% functionalization level, but there was strong evidence that the material phase-separated into an ordered silica-rich phase and disordered organosilica-rich phase [ 17]. Others used synthesis mixture compositions that can potentially lead to 50% functionalization level, but no evidence of quantitative incorporation [ 11] or even formation of a cross-linked periodic product [23] was provided. However, there are two recent successful attempts to introduce organic group loadings of 50% or more in ordered silicas synthesized via co-condensation. Mori and Pinnavaia [24] synthesized HMS-type materials that exhibited one peak on their XRD pattern for functionalization levels up to 50%, and a distinct shoulder even for the loading of 60% for mercaptopropyl groups. These loading levels are based on the synthesis mixture composition, but the authors noted that the stoichiometric incorporation of the organosilica precursor was achieved. The resulting highly functionalized silicas exhibited almost no silanol groups and thus had an appreciable degree of framework cross-linking. We succeeded in the synthesis of vinyl-functionalized silicas that exhibit one XRD peak even for the functionalization level of 70% (estimated from the synthesis mixture composition), and provided strong evidence that phase separation did not take place for loadings up to as high as 62% (determined using 29Si MAS NMR spectroscopy) [25]. These loadings of organic groups are very high and in fact comparable to those achievable for periodic mesoporous organosilicas (PMOs) [26-32] and bifunctional PMOs (BPMOs) [33,34]. However, the degrees of framework cross-linking in the vinylfunctionalized silicas with extremely high organic group loadings are exceptionally low, bec.ause an appreciable population of silanols is present [25]. As demonstrated elsewhere, vinyl groups in ordered mesoporous materials can be transformed into alkylborane, alcohol, epoxide and diol groups [35,36]. Because of this, the silicas with high loadings of vinyl groups [25] are expected to be useful precursors for the synthesis of ordered materials with unprecedented loadings of various organic functional groups. Herein, the successful synthesis of vinyl- and methyl-functionalized silicas with very high group loadings [25] is discussed and an evidence for a remarkable reproducibility of the synthesis of vinyl-functionalized materials is provided.
2. EXPERIMENTAL Vinyl- and methyl-functionalized silicas were synthesized as described in Ref. [25], but the vinyl-functionalized samples discussed therein and herein come from different batches. The synthesis mixture composition was x/100 TEVS (or x/100 TEMS) : (100-x)/100 TEOS : 0.24 CTAB : 16.1 NH4OH : 128.7 H20, where TEVS, TEMS, TEOS, and CTAB denote triethoxyvinylsilane, triethoxymethylsilane, tetraethyl orthosilicate, and cetyltrimethylammonium bromide, respectively. The percentage of the organosilane expressed as a molar percentage of silicon atoms that originate from this precursor is denoted as x. A typical
199 synthesis procedure was the following. A solution of 0.54 g (1.48 mmol) CTAB, 5.7 g of 30 wt.% (0.10 mol) NHaOH and 10.6 g (0.59 mol) of water was prepared in a plastic bottle. 6.23 mmol of the silica source was added as an appropriate proportion of TEOS and TEVS. For instance, in the case of 43% TEVS (x = 43), 0.51 g (2.70 mmol) of TEVS and 0.74 g (3.53 mmol) of TEOS was added. The resultant mixture was stirred at room temperature for 30 min and subsequently aged at 353 K for 4 days. The product was filtered, washed thoroughly with water, and dried under ambient conditions. The surfactant extraction was carried out by stirring about 0.2 g of as-synthesized material in HC1 (5 g, 35 wt.%) / methanol (100 g) solution at 313 K for 6 hours. Subsequently, the product was filtered, washed with methanol and dried in air. The solvent-extracted vinyl-functionalized silicas are denoted as VINxE, where x is the content of organosilica precursor in the synthesis mixture, as defined above. X-ray diffraction patterns were recorded on a Siemens D5000 diffractometer using Nifiltered Cu Kot radiation. Nitrogen adsorption isotherms were measured on a Micromeritics ASAP 2010 volumetric adsorption analyzer. Before the measurements, samples were degassed under vacuum for at least 2 hours at 413 K or 573 K (for methyl-functionalized silicas) or 373 K (for vinyl-functionalized silicas). Weight change curves were recorded under nitrogen or air atmosphere on a TA instruments TGA 2950 thermogravimetric analyzer using a high-resolution mode with a maximum heating rate of 5 K min-:. The BET specific surface area [37] was evaluated from data in the relative pressure range from 0.01 to 0.02. The total pore volume [37] was estimated from the amount adsorbed at a relative pressure of 0.99. The primary pore volume and external surface area were evaluated using the Ors plot method [37,38] in the Ors plot range from 1.5 to 2.0. The standard reduced adsorption as is defined as the amount adsorbed at a given relative pressure divided by the amount adsorbed at a relative pressure of 0.4 for the reference adsorbent. Macroporous silica LiChrospher Si-1000 was used as a reference adsorbent [38]. The pore size distribution (PSD) was calculated using the Kruk-Jaroniec-Sayari (KJS) method [39]. The statistical film thickness data suitable for these calculations were reported elsewhere [38]. The pore diameter is defined as a maximum of the KJS PSD.
3. RESULTS AND DISCUSSION Shown in Figure 1 are typical XRD patterns for vinyl-functionalized silicas with loading levels from 25% to 65%. At least one peak was observed for all samples, but three peaks were observed for some samples with 33% and 43% loadings [25], and three or more peaks appeared for samples with 25% loading [25,36]. The interplanar spacings corresponding to the main XRD peaks are listed in Table 1. These values systematically decreased as the loading of the organic groups increased (which is a known phenomenon [3]) and were within +0.2 nm from those determined for the corresponding samples from different batches discussed in Ref. [25], which shows that the unit-cell size can be fine-tuned by the selection of the loading level. In the case of methyl-modified silicas, at least one peak was observed in the XRD pattern for loadings up to 50% (the behavior of samples with loading levels between 50% and 100% was not studied) [25]. It should be noted that the above loadings are based on the composition of the synthesis mixture, but we have shown elsewhere that the incorporation of the organosilica is close to stoichiometric for the loadings of 50-65% [25]. Shown in Figure 2 are nitrogen adsorption isotherms for vinyl-functionalized silicas. These isotherms gradually change from Type IV to I as the loading of the organic groups
200 increased, suggesting the concomitant decrease in the pore diameter. This decrease can be clearly observed on the pore size distributions shown in Figure 2 (pore diameters are listed in Table 1). In fact, the pore diameter decreases by more than 1 nm as the vinyl group loading increases from 25% to 65%. The samples with lower loadings are mesoporous, whereas those with higher loadings are already microporous. The primary pore volume systematically decreased as the content of organic groups increased. However, the specific surface area was quite constant for loading levels up to 50%, and dropped only for very high loadings. The structural parameters determined for the samples described herein are very close to those for samples from other batches synthesized. In particular, the materials reported in Ref. [25] exhibited BET specific surface areas, primary pore volumes and primary pore diameters that did not differ by more than 80 m 2 g-l, 0.06 cm 3 g-1 and 0.3 nm, respectively, from those for the corresponding samples described herein. In fact, the differences were in most cases much smaller than these maximum values. These results suggest that the synthesis of highly loaded vinyl-functionalized silicas that we have developed recently [25] is highly reproducible. This is important in the light of the fact that vinyl groups can be transformed to other organic 9groups [35,36] and thus the materials discussed herein may serve as robust precursors for the synthesis of a wide range of organic-functionalized ordered mesoporous or microporous materials with remarkably high loadings of surface organic groups. It should also be noted here that adsorption data provide strong evidence that there is no phase separation in the vinyl-functionalized silicas for loadings up to 65%, whereas samples synthesized with higher loadings show signs of phase separation or are disordered [25]. In the case of methylfunctionalized silicas synthesized under the same conditions, there is some evidence of phase separation even for Ioadings below 50% [25]. Similar to the vinyl-functionalized silicas, methyl-functionalized silicas also tended to exhibit a decrease in the interplanar spacing, primary pore volume, but their specific surface area was relatively constant as the organic group loading increased [25].
60000-
(100)
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2-theta (deg) Figure 1. Powder XRD pattems for solvent-extracted vinyl-functionalized silicas.
',
io
201 Table 1. Structural properties of the vinyl-functionalized silicas.
Sample
d,oo (nm) 3.30 3.20 2.91 2.76 2.67
VIN25E VIN33E VIN43E VIN50E VIN65E
SBETI) (m 2 g850 850 890 820 610
Vt
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(c
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Sex
WKJS
(m 2 g-l) 30 40 50 40 30
(nm) 2.8 2.4 2.1 1.8 1.7
d l 0 0 - XRD (100) interplanar spacing, SBET - BET specific surface area, V t - total pore volume, Vp - primary pore volume, and w~s - primary pore diameter evaluated using the KJS method.
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1.5
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2.5
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3.5
Pore Size (nm)
Figure 2. Nitrogen adsorption isotherms and pore size distributions for solvent-extracted ordered vinyl-functionalized silicas. Vinyl-functionalized silicas with loading levels above 25% exhibited low-pressure hysteresis on their nitrogen adsorption isotherms (Figure 2), that is, the hysteresis loops extend beyond the lower relative pressure limit at which the hysteresis is usually observed (0.4 for nitrogen adsorption at 77 K) [37,40]. The reason for this behavior is not clear at present, but may be related to several factors, such as some degree of swelling of the pore structure, the entrapment of nitrogen molecules in the palisade of organic groups or changes in adsorption capacity related to the orientation of organic groups on the surface during adsorption. These last two factors are likely to play a role in the development of low-pressure hysteresis in typical silicas with organic-functionalized surfaces [41]. However, the highly loaded vinyl-functionalized silicas were found to exhibit remarkably low degrees of framework cross-linking [25], which makes it possible to envision swelling of the porous structure, the latter being highly unlikely in highly cross-linked silica-based materials. The low-pressure hysteresis was not observed for methyl-functionalized silicas even with loadings
202 as high as 50%. However, this observation does not provide much insight into the elucidation of the origin of hysteresis in vinyl-functionalized silicas, as the methyl-functionalized silicas have smaller, rigid surface groups, and higher degree of framework cross-linking, which makes swelling due to any of the possible reasons mentioned above unlikely. Shown in Figure 3 are weight change curves for solvent-extracted vinyl-functionalized silicas. The initial weight loss below 400 K can be attributed to the removal of water and solvents left after the extraction. The subsequent minor weight loss centered at about 500 K can be attributed to the decomposition and thermodesorption of the residual surfactant [25,42]. A minor weight gain under air above 500 K is likely to be due to the oxidation of a fraction of vinyl groups, which is followed by the weight loss related to their final combustion and removal from the surface at higher temperatures (the major weight loss at around 600 K) [25]. Water release related to the condensation of silanols is also likely to contribute to the latter weight loss. Under nitrogen atmosphere, the weight loss related to the organic group decomposition and thermodesorption was much smaller than that under air and took place at higher temperatures that is, was centered at about 800 K. Moreover, this moderate weight loss was followed by the weight gain for materials with higher vinyl group loadings, which can be attributed to the reaction of these materials with nitrogen, which is similar to the behavior of PMOs and BPMOs [33,42]. The nature of this reaction is, however, unclear at present. It is also interesting to note that the weight loss under air in the temperature range of the vinyl group decomposition is well correlated with the content of vinyl and silanol groups determined from 29Si MAS NMR spectroscopy [25]. No such correlation can be found in the case of weight loss under nitrogen atmosphere, most likely because of the formation of carbonaceous residue that is retained even at temperatures up to 1270 K. This residue may also contain some nitrogen, as there is ample evidence of the reaction with nitrogen at around 1100 K and it is unclear whether all the nitrogen is subsequently released upon further heating. 100
.
.
.
.
.
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ir .........
92 Yz ~9
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600
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1000 1200
Temperature (K)
80
........ ......
VIN50E VIN65E
I
i
i
400
600
800
1000 1200
Temperature (K)
Figure 3. Weight change curves under air and nitrogen atmospheres for ordered vinylfunctionalized silicas.
203 4. CONCLUSIONS Ordered silicas with unprecedented loadings of reactive vinyl groups can be readily synthesized reproducibly via co-condensation of TEVS and TEOS under basic conditions in the presence of CTAB as a structure-directing agent. The repeated syntheses carried out under identical conditions afforded materials with highly reproducible (100) interplanar spacings, primary pore sizes, BET specific surface areas, and primary pore volumes. The pore diameter gradually decreased as the relative amount of the organosilica precursor in the synthesis mixture increased. These features, along with the fact that the synthesis procedure is reproducible, allow a fine-tuning of pore dimensions of organic-functionalized silicas on the borderline between the mesopore and micropore ranges. The ease of the derivatization of the vinyl groups on the surface of ordered porous hosts, which was demonstrated earlier, opens prospects for the use of the vinyl-functionalized silicas reported herein as precursors for the synthesis of ordered microporous materials with extremely high loadings of organic functional groups.
5. A C K N O W L E D G M E N T S M. J. acknowledges support by NSF Grant CHE-0093707. G. A. O. is a Government of Canada Research Chair in Materials Chemistry. He acknowledges the Natural Sciences and Engineering Council of Canada (NSERC) for financial support.
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204 19. D. Margolese, J. A. Melero, S. C. Christiansen, B. F. Chmelka and G. D. Stucky, Chem. Mater. 12 (2000) 2448. 20. M. A. Markowitz, J. Klaehn, R. A. Hendel, S. B. Qadriq, S. L. Golledge, D. G. Castner and B. P. Gaber, J. Phys. Chem. B 104 (2000) 10820. 21. A. Bhaumik and T. Tatsumi, Catal. Lett. 66 (2000) 181. 22. J. Joo, T. Hyeon and J. Hyeon-Lee, Chem. Commun. (2000) 1487. 23. A. Itoh and Y. Masaki, SYNLETT (1997) 1450. 24. Y. Mori and T. J. Pinnavaia, Chem. Mater. 13 (2001) 2173. 25. M. Kruk, T. Asefa, M. Jaroniec and G. A. Ozin, J. Am. Chem. Soc., submitted. 26. S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna and O. Terasaki, J. Am. Chem. Soc. 121 (1999) 9611. 27. B. J. Melde, B. T. Holland, C. F. Blanford and A. Stein, Chem. Mater. 11 (1999) 3302. 28. T. Asefa, M. J. MacLachlan, N. Coombs and G. A. Ozin, Nature 402 (1999) 867. 29. C. Yoshina-Ishii, T. Asefa, N. Coombs, M. J. MacLachlan and G. A. Ozin, Chem. Commun., (1999) 2539. 30. T. Asefa, M. J. MacLachlan, H. Grondey, N. Coombs and G. A. Ozin, Angew. Chem. Int. Ed. Engl. 39 (2000) 1808. 31. M. J. MacLachlan, T. Asefa and G. A. Ozin, G. A. Chem. Eur. J. 6 (2000) 2507. 32. G. A. Ozin, G. Temtsin, T. Asefa and S. Bittner, J. Mater. Chem. 11 (2001) 3202. 33. T. Asefa, M. Kruk, M. J. MacLachlan, N. Coombs, H. Grondey, M. Jaroniec and G. A. Ozin, J. Am. Chem. Soc. 123 (2001) 8520. 34. M. C. Burleigh, S. Dai, E. W. Hagaman and J. S. Lin, Chem. Mater. 13 (2001) 2537. 35. R. Anwander, I. Nagl, M. Widenmeyer, G. Engelhardt, O. Groeger, C. Palm and T. Roser, J. Phys. Chem. B 104 (2000) 3532. 36. T. Asefa, M. Kruk, M. J. MacLachlan, N. Coombs, H. Grondley, M. Jaroniec and G. A. Ozin, Adv. Funct. Mater. 11 (2001) 447. 37. S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982. 38. M. Jaroniec, M. Kruk and J. P. Olivier, Langmuir, 15 (1999) 5410. 39. M. Kruk, M. Jaroniec and A. Sayari, Langmuir, 13 (1997) 6267. 40. M. Kruk and M. Jaroniec, Chem. Mater. 13 (2001) 3169. 41. C. P. Jaroniec, M. Kruk, M. Jaroniec and A. Sayari, J. Phys. Chem. B, 102 (1998) 5503. 42. M. Kruk, M. Jaroniec, S. Guan and S. Inagaki, J. Phys. Chem. B 105 (2001) 681.
Studies in SurIace Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
205
Polyfunctionalized silica adsorbents obtained by using d o d e c y l a m i n e as template I.V. Mel'nyk (Seredyuk) a, Yu. L. Zub a, A.A. Chuiko a, M. Jaroniec b and S. Mann c a Institute of Surface Chemistry of NAS of Ukraine, 17, General Naumov Str., Kyiv 03164 Ukraine
b Department of Chemistry, Kent State University, Kent, Ohio 44242, USA c School of Chemistry, University of Bristol, Bristol, BS8 1TS, U.K.
Mesoporous materials containing bi- (thiol/amine) and tri- (thiol/amine/alkyl or aryl) functional surface layer are synthesized by using three- and four-component mixtures of organosilanes and 1-dodecylamine as a template. The resulting materials were thoroughly characterized by adsorption, X-ray diffraction, transmission electron microscopy, infrared spectroscopy, thermogravimetry, and solid state 13C and 298i NMR spectroscopy. It is shown that these materials exhibit disordered structure with small hexagonal domains, high specific surface area (-400-700 mZ/g) and high thermal stability.
1. I N T R O D U C T I O N Silica-based mesoporous molecular sieves [1] have attracted attention of many researchers because of their potential applications in catalysis, adsorption, chromatography, environmental cleanup and nanotechnology [2,3]. A characteristic feature of these materials is reactive surface silanols, which can be used to link various ligands in order to tailor their surface properties [4,5]. An attractive and effective altemative for introduction of the desired surface functionality into mesoporous materials is a one-step self-assembly of organosilanes (RO)3Si(CH2)nR', where R'- functional groups of different nature, in the presence of a proper template [6,7]. This one-step synthesis provides functionalized nanoporous materials (FNM) of well-developed porosity and surface area [8]. So far, FNM with a single functionality have been mainly synthesized. However, some applications require nanoporous materials with multifunctional surface properties. Therefore, it is not surprising that there were some attempts to synthesize FNM with bifunctional surface layer [9]. Also, our recent communication [10] reports the synthesis of nanoporous materials with multifunctional surface layer containing thiol- and amino groups or thiol, amino and alkyl (or aryl) groups. This synthesis was carried out in the presence of 1-dodecylamine (DDA) as template. The current work provides some additional details about recently reported synthesis of FNM [10] and focuses on their characterization by using thermal analysis, powder X-ray diffraction, TEM, IR and solid-state ~3C and 298i NMR.
206 2. M A T E R I A L S AND M E T H O D S
Tetraethoxysilane, Si(OC2H5)4 (TEOS, 98%); methyltrimethoxysilane, (CH30)3SiCH3 (MTMS, 98%); phenyltriethoxysilane, (C2HsO)3SiC6H5 (PTES, 98%); 3-aminopropyltriethoxysilane, (C2HsO)3Si(CH2)3NH2 (APTES, 99%); bis-[(3-trimethoxysilyl)propyl]amine, [(CH30)3Si(CH2)3]2NH (BTMPA, 97%, Fluka); N-[3-trimethoxysilyl)propyl]ethylenediamine, (CH30)3Si(CH2)3NH(CHz)NH2 (TMPED, 97%); 3-mercaptopropyltrimethoxysilane, (CH30)3Si(CH2)3SH (MPTMS, 95%); 1-dodecylamine, CH3(CHz)llNH2 (DDA); anhydrous methanol and ethanol from Aldrich (except BTMPA) were used to synthesize FNM with multifunctional surface layer. Two slightly different recipes were employed to synthesize the samples studied. The first recipe was employed to prepare the samples I-VII (see Table 1) by using one, two or maximum three-component mixtures of silanes. According to thisrecipe a mixture of 0.1 M TEOS and 0.02 M of proper organosilanes was added to the 0.03 M solution of DDA in water/ethanol (50/60 cm 3) under continuous stirring. The molar ratio of the reacting components was the following: 0.1 TEOS : 0.02 (RO)3SiR' (total quantity of organosilanes) : 0.03 DDA : (2.2 - 2.8) H20. In the case of BTMPA the presence of two (CH30)3Si(CH2)3parts in the silane molecule was taken into account in preparing the mixture composition. Approximately after two minutes a white precipitate began to form. The resulting mixture was allowed to stand for 24 hrs at room temperature. Next, the precipitate was filtered, immediately washed with 50 cm 3 of ethanol and dried in air for 24-48 hrs. A part of the sample, which was obtained by hydrolysis of TEOS only in the presence of DDA was dried at ambient temperature and calcined in air at 540~ for 4 hrs. The other recipe was employed to prepare the samples VIII-XIII (see Table 1) by using four-component mixtures of silanes. In this case, a mixture of 0.1 M TEOS and 0.02 M of proper organosilanes was added to the 0.03 M solution of DDA in 60 cm 3 ethanol under continuous stirring. After that about 40 cm 3 of water was added slowly (about 2 min) without stooping stirring. During 5 min a visible precipitate was formed, which was subsequently washed with methanol at boiling temperature for 3 hrs (300 cm 3 of methanol per 10 g of the sample). The sample was washed three times and dried under vacuum at 105~ for 4 hrs. The elemental analysis was carried out in the analytical laboratory at UMIST (U.K.) and allowed us to calculate the contents of the functional groups present in the resulting materials (see Table 1). Powder X-ray diffraction patterns were collected using a Scintag XDS2000 diffractometer with Cua radiation. TEM images were recorded on a JEOL JEM-4000 FX electron microscope operated at 350 kV. The samples were dispersed ultrasonically in ethanol and a drop of the suspension was air-dried on a carbon-coated grid. IR spectra were recorded on a Nicolet 5 PC FT spectrometer by using samples pressed with KBr. High-resolution solidstate NMR spectra were obtained on a Varian UNITYplus spectrometer at room temperature with a 7 mm zirconia rotor at the magic angle with the spinning frequency of 4.4 kHz. ~3C CP MAS experiments (75.43 MHz, ~H 90 ~ pulse width 20.0-30.0 ~ts, a CP contact time of lgs, 640-2816 acquisitions, pulse delay 1 s) and 298i DP MAS NMR spectra (single pulse, 59.58 MHz, 90 ~ pulse width 3.00-15.00 ~ts, 200-940 scans, pulse delay 60-300 s) were performed using TMS as standard. Thermal analysis was performed in the range of 20-800~ with a heating rate of 5~ min 1 (in air stream; E. Paulik, J. Paulik, L. Erdey System, Q-1500D). Nitrogen adsorption isotherms for all the samples were measured a t - 1 9 6 ~ on a Micromeritics ASAP 2010 volumetric adsorption analyzer. Before adsorption measurements
207 the samples were degassed at 200 ~ The BET specific surface area [11] was calculated in the relative pressure range between 0.01 and 0.02 in order to exclude the points in the range of capillary condensation [12]. The total pore volume was determined from the amount adsorbed at a relative pressure of 0.99 [13]. The pore size distributions were determined using the Kruk-Jaroniec-Sayari (KJS) approach [14] and the Barrett, Joyner and Halenda (BJH) algorithm [ 15].
3. RESULTS AND DISCUSSION Elemental analysis data for C, N and S were used to calculate the concentration of alkyl (or aryl), aminopropyl and mercaptopropyl groups, respectively. These concentrations, denoted as CNH2, CSH and C, are summarized in Table 1. As can be seen from this table the reported one-step synthesis afforded nanoporous materials with high content of functional groups. Elemental analysis data indicate that the N/S ratio in the prepared samples was close to that in the initial mixture of silanes used. The carbon content was higher than that predicted on the basis of the mixture composition, which could be caused by partial retention of methanol in the microstructure and formation of methoxy groups on the surface during template extraction. The XRD patterns for all the samples before and after extraction of the surfactant exhibit a single peak [10] that indicates the existence of rather worm-like porous structure with relatively narrow pore size distribution [16,17]. Some XRD patterns recorded for the samples before and after template extraction show a wide flattened peak located at 10-15 ~ 20 angle. Its occurrence may be connected with dispersion of X-rays caused by presence of a small amount of additional amorphous silica. Others observed a similar behavior [18]. The values of the interplanar spacing dl00 evaluated on the basis of the XRD peaks are given in Table 1. A comparison of the d~00values for the samples before and after template extraction indicates that there is slight framework shrinkage for the samples synthesized by using three- and fourcomponent mixtures of silanes (samples VII-XIII). An opposite effect is observed for the samples obtained from two-component silane mixtures (samples II-IV). Also, there is apparent that for samples V and VI, which were synthesized by using three-component silane mixtures, the template extraction caused a collapse of the porous structure as evidenced by a significant reduction of the specific surface area (Table 1). It should be mentioned that under the same conditions the synthesis of FNM from TEOS and organosilane with amine group was unsuccessful. In contrast, the presence of organosilane with alkyl or phenyl group in the synthesis mixture created favorable conditions for formation of the nanostructured phase. In the case of all four-component mixtures of silanes the nanostructured phases were formed and these structures were survived the template extraction process as evidenced by the XRD data for the samples VIII-XIII (see Table 1). TEM microphotographs obtained for samples I, VII and XIII (not shown) support the presence of worm-like channels of similar width. They resemblance nanoporous materials with analogous pore structure [17]. Spherical particles were mainly formed under synthesis conditions used. In the case of sample I the size of particles was about 1.5 microns, whereas for samples VII and XIII the particle size was between 0.5 and 1 microns. The latter samples contained also some non-spherical particles, which could be attributed to non-framework amorphous silica as indicated above.
208 Table 1. Concentration of surface functional groups and adsorption properties of the FNM studied. !
Sample Surface functional groups
CSH C1 dloo, nm mmol/g mmol/g mmol/g (B/A)2 .
I
Calcined
-
-
0.67
3.89/3.06
550
0.36
2.5
II
-CH3
-
-
5.98
3.18/3.23
650
0.52
2.6
III
-C6H5
-
-
17.1
2.74/2.79
670
0.38
2.1
IV
-(CH2)3SH
-
0.61
9.09
3.38/3.51
660
0.31
1.8
V
-(CH2)3SH/-(CH2)3NH2 1.16
0.97
1.53
3.24/-
80
0.04
1.7
VI
-(CH2)3SH/-(CH2)3NH(CH2)2NH2 -(CH2)3SH /=[(CH2)3]2NH -(CH2)3SH/-(CH2)3NH2 /-CH3 -(CH2)3SH/-(CH2)3NH2 /-C6H5 -(CH2)3SH/-(CH2)3NH-
2.20
0.97
2.17
3.08/-
76
0.14
2.8
0.79
0.44
3.45
3.75/3.74
490
0.48
3.0
0.80
0.70
2.45
3.19/3.12
450
0.24
2.1
0.65
0.67
5.87
3.16/3.16
520
0.24
1.8
1.46
0.71
2.66
3.32/3.22
400
0.32
2.1
-(CH2)3SH/-(CH2)3NH- 1.55 (CH2)2NH2/-C6H5 -(CH2)3SH 0.65
0.70
7.05
3.00/2.96
420
0.19
1.8
0.54
3.99
3.24/3.19
620
0.37
2.3
0.57
6.71
3.17/3.19
610
0.34
2.2
VII VIII IX X
CNH2
SBET3 Vp
w m Eg-1 cmag-1 nm
(CH2)2NH2/-CH3 XI XII XIII
/:[(CH2)3]2NH/-CH3 -(CH2)3SH /=[(CH2)3]2NH/-C6H5
0.58
C refers to alkyl or aryl groups only (amino- and mercaptopropyl groups calculated from %N and %S are not included). 2 B - before extraction, A - after extraction with methanol. 3 BET surface area calculated in the range of relative pressures from 0.01 to 0.02. Nitrogen adsorption isotherms at-196~ were used to calculate the BET surface area (S), pore volume (Vp) and pore width (w) at the maximum of the pore size distribution (see Table 1). The majority of nitrogen adsorption isotherms were of Type I according to the IUPAC classification [19]. Type IV was observed only for samples having pores close to 3 nm (Figure 1). Type I isotherms level off already at quite low relative pressures and are characteristic of microporous materials (and often also for materials with pore sizes on the borderline between micropores and mesopores. Type IV isotherms feature capillary condensation steps and are characteristic of mesoporous materials (pore diameters greater than 2 and smaller than 50 nm). As can be seen from Table 1 the nanostructured samples
209 possess high surface area and well-developed porosity in the borderline between micro- and mesopores. The maximum of the pore size distributions, which are quite narrow, is located between 1.7 and 3.0 nm. 300 n 250 I-03 o 200 0 v
.o
150
<
100
O
C O
E <
50
[
0.0
Sample VII Sample X! --o-- Sample XII Sample XIII . . . . . . . . .
i
0.2
I,
0.4
!
. . . . . . . .
0.6
i
.,.
0.8
1.0
Relative Pressure
Figure 1. Nitrogen adsorption isotherms at-196~
for selected samples.
A characteristic feature of IR spectra for all synthesized samples is the presence of intensive absorption band in the 1060-1195 cm 1 region related to the valence vibrations of bonds Si-O-Si in three-dimensional siloxane skeleton having Si-Cn-R' groups [20]. The absorption bands related to the presence of water are in the region of 1630-1640 cm -~ and higher---3100 cm 1. In addition, the IR spectra exhibit a set of absorption bands related to the valence vibrations of the C-H bond (2850-3080 cm-~). For all samples extracted with methanol there are three rather weak absorption bands at 2856, 2930 and 2965 cm -~ that can be attributed to the CH vibrations in =Si-OCH3 groups, which can be formed during extraction process with methanol. Normally, these groups should disappear after heating samples at 100~ In our case heating at 150 and 200~ for lhr did not cause a complete disappearance of these absorption bands. A similar behavior was reported elsewhere for aerogels [21]. Note that the aforementioned bands were not visible for sample I, which was calcined at 540~ for 4 hrs. The a b o v e IR investigations were confirmed by ~3C CP MAS NMR studies. The ~3C NMR spectra for the samples extracted with methanol show the presence of methoxy groups and methanol (see exemplary spectra for samples VIII and IX in Figure 2). Moreover, these spectra provide evidence on the effectiveness of the template extraction with methanol. For instance, the spectra for sample II (not shown) exhibit week signals related to the template. For other samples these signals are not present. First of all, the C NMR spectra allow us to identify the presence of organic groups in a given material, e.g., spectra for samples VIII and IX show clearly the presence of =Si(CH2)3SH and =Si(CHz)3NH2 groups as well as -SiCH3 (sample VIII) and =SiC6H5 (sample IX) groups in the porous structure.
210
o
,t.-
I ~,
1+1'
3' o'J
1 20
1 O0
80
60
40
20
9
,9
ppm
SiC6H5
9
J!
A
180
Ill
ItICH3___CH2OHI 9
200
-20
g
,t--
-
0
160
140
120
,
100
'b,+~t | I
80
60
40
20
0
-20
ppm
Figure 2.13C CP MAS NMR spectrum for samples VIII (top panel) and IX (bottom panel); numbers at signals refer to the carbon atoms in alkyl segments of the following groups" -SiC1H2C2H2C3H2SH and =SiCI'H2C2'H2C3'H2NH2. Let us analyze 29Si DP MAS NMR spectra for selected samples. All spectra exhibit the well resolved signals arising from silicon surrounded via siloxane [Qn: Si(OSi)n(OH)a-n, n = 2 4] and organosiloxane [Tin: R1Si(OSi)m(OH)3-m, m = 2 - 3] bonds (see Table 2). The Q4, Q3 -
211 and Q2 signals were observed at-110, -100 and-92 ppm. There are T 2 and T 3 signals present
on the spectra for all samples listed in Table 2, which refer to the organosiloxane units containing alkyl segment. Additional T 2 and T 3 signals are present for organosiloxane units having phenyl group. The left superscripts 1 and 2 were added to distinguish these signals. All synthesized samples have appeared to have relatively high thermal stability. The thermogravimetric weight change curves exhibit the weight loss (-10 %) at 80-130~ which is related to thermodesorption of physically adsorbed water. The weight loss occurring at higher temperatures (>270-300~ reflects the thermal decomposition of the surface layer, which contains various organic groups.
4. C O N C L U S I O N S A series of functionalized nanoporous materials containing the surface layer with methyl, phenyl, thiol and amine groups was prepared via one-step synthesis using dodecylamine as template. The resulting materials possessed high surface area and pore volume as well as highly developed worm-like structure of nanochannels. Their pore size distributions are narrow and located on the borderline between micro- and mesopores. A distinct feature of these materials is the presence of different functional groups on the pore walls, which was evidenced by IR, 13C CP MAS and 29Si DP MAS NMR spectroscopy. The possibility of tailoring the FNM structure and the surface functionality make these materials very promising for various applications including environmental cleanup, catalysis and separations. Table 2. Position of signals in ppm on the 298i DPMAS NMR spectra for selected materials; values in brackets denote % distribution of groups in the samples studied. Sample
IV VIII IX XII XIII
1T2
1T3
-56.3 (3.8) -57.5 (4.7) -57.8 (5.1) -59.9 (5.4) -56.5 (4.9) -58.8 (6.1)
-63.9 (13.6) -65.4 (8.8) -65.5 (14.9) -66.2 (8.1) -65.6 (13.6) -66.5 (8.4)
2T2
2T3
-77.1 (2.1)
-81.3 (2.0)
-77.0 (1.7)
-80.7 (2.0)
Q2
Q3
Q4
-92.1 (3.4) -92.7 (3.5) -94.3 (4.6) -92.5 (4.6) -91.6 (3.9) -92.7 (3.9)
-101.0 (29.3) -101.7 (29.8) -100.9 (29.1) -100.7 (28.2) -101.3 (30.3) -101.4 (31.8)
-110.1 (49.9) -110.0 (43.1) -110.0 (46.3) -110.1 (49.0) -110.3 (48.0) -110.2 (46.1)
For Si atoms connected to -CH3, - (CH2)3SH and - (CH2)3NH2 groups. 2 For Si atoms connected to -C6Hs.
5. A C K N O W L E D G M E N T
I.V.M., Yu.L.Z. and A.A.C. thank the NATO grant SFP-978006 for a partial support of this work. The NSF grant CTS-0086512 (M.J.) is gratefully acknowledged. Also, we thank
212 Dr. David C. Apperley at the University of Durham for the solid state NMR data and Dr. H.Honda at the Tsukuba Research Laboratory (Sumitomo Chemical Co.) for TEM images. REFERENCES
1. 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. 2. N.K. Raman, M. T. Anderson, C. J. Brinker, Chem. Mater., 8 (1996) 1682. 3. A.A. Kovalenko, V. G. Ilin, A. P. Filippov, Theor. Exper. Chem. (Russ.), 36 (2000) 135. 4. J. Lin, Y. Shin, Z. Nie, J. H. Chang, Li-O. Wang, G. E. Fryxell, W. D. Samuels, G. J. Exarhos, J Phys. Chem. A, 104 (2000) 8328. 5. J.V. Ving, C. P. Mehnert, M. S. Wong, Angew. Chem., Ed. Int. Engl., 38 (1999) 56. 6. S.L. Burkett, S. D. Sims, S. Mann, Chem. Commun., (1996) 1367. 7. D.J. Macquarrie, Chem. Commun., (1996) 1961. 8. S. Mann, S. L. Burkett, S. A. Davis, C. E. Fowler, N. H. Mendelson, S. D. Sims, D. Walsh, N. T. Whilton, Chem. Mater., 9 (1997) 2300. 9. S.R. Hall, C. E. Fowler, B. Lebeau, S. Mann, Chem. Commun., (1996) 201. 10. Yu. L. Zub, I. V. Seredyuk, A. A. Chuiko, M. Jaroniec, M. O. Jones, R. V. Parish, S. Mann, Mendeleev Commun., 11 (2001) 208. 11. S. Brunauer, P.H. Emmett and E. Teller, J. Am. Chem. Soc., 60 (1938) 309. 12. R. Ryoo, I.-S. Park, S. Jun, C. W. Lee, M. Kruk and M. Jaroniec, J. Am. Chem. Soc., 123 (2001) 1650. 13. M. Kruk and M. Jaroniec, Chem. Mater., 13 (2001) 3169. 14. M. Kruk, M. Jaroniec, A. Sayari, Langmuir, 13 (1997) 6267. 15. E. P. Barrett, L. G. Joyner, P. P. Halenda, J. Am. Chem. Soc., 73 (1951) 373. 16. C.-Y. Chen, S-Q. Xiao, M. E. Davis, Microporous Mater., 4 (1995) 1. 17. S. A. Bagshaw, E. Prouzet, T. J. Pinnavaia, Science, 269 (1995) 1242. 18. F. De Juan, E. Ruiz-Hitzky, Adv. Mater., 20 (2000) 430. 19. K. S. W. Sing, D. H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, T. Pure Appl. Chem., 57 (1985) 603. 20. L. P. Finn, I. B. Slinyakova, Colloid. J. (Russ.), 37 (1975) 723. 21. Li Wei, R. J. Willey, J. Non-Crystal Solids, 212 (1997) 243.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
213
Characterization o f m e s o p o r o u s thin films formed with added o r g a n o p h o s p h o n a t e and organosilane Michael A. Markowitz, Eva M. Wong and Bruce P. Gaber Laboratory for Molecular Interfacial Interactions, Code 6930, Center for Bio/Molecular Science and Engineering, Naval Research Laboratory, Washington, DC 20375
The effects of added N-trimethoxysilylpropyl-N,N,N-trimethylammonium (TMAC) and pinacolyl methylphosphonate (PMP) on the order and porosity of mesoporous thin films formed with bis(triethoxysilyl)ethane (BTSE) were examined by XRD and TEM. The as-prepared and ethanol extracted films formed with TMAC and the combination of TMAC and PMP exhibited P6mm type hexagonal mesostructure with a large unit cell. Calcination of the TMAC functionalized sample yielded a film with 2-D cubic pore ordering. Addition of PMP during film formation did not affect mesopore order but increasing the amount of added PMP resulted in d-spacing shrinkage.
1. INTRODUCTION The unambiguous and rapid detection of chemical and biological warfare agents is a major research focus [1-14]. The proliferation of the capability to develop and deploy biological agents has increased the risk of their use against military and civilian populations. We have been engaged in the development of a passive chemical agent detector to be incorporated into the uniform of military service men and women. Such a device would enable a more facile determination of an individual's exposure to specific chemical agents than is currently possible. The approach we have taken is to utilize the techniques of template-directed materials synthesis and molecular imprinting to form robust materials that are selective for specific chemical molecules [9, 15-17]. Molecular imprinting is the process of forming binding or reaction sites into a polymer that are selective for a molecules shape and functional group spacing [18-20]. Typically, this process involves the complementary hydrogen bonding or electrostatic interactions between the molecule to be imprinted and functionalized monomers prior to polymerization. Condensation of the polymers followed by subsequent removal of the imprint molecule results in the formation of sites selective for the imprint molecule. Two issues of major importance to the formation of imprinted materials are site accessibility and homogeneity. By coupling molecular imprinting with template-directed synthesis, we aim to form robust porous materials that permit rapid access to a homogeneous population of imprinted sites that will retain their homogeneity over the course of their operational lifetime in a variety of environments. To date, we have demonstrated the formation of imprinted silicates that are selective for pinacolyl methylphosphonate (PMP), the hydrolysis product of the nerve agent soman, and the affects of various added
214 organosilanes on that selectivity [9, 15]. Based on these results, we have begun the process of forming imprinted mesoporous thin films that could be more readily fabricated into individual passive chemical agent detection badges. In addition, the imprinted films are expected to have higher capacity than the imprinted particles and display rapid adsorption kinetics. The processing and fabrication of silica based mesoporous thin films using surfactant aggregates as structure directing agents have garnered recent interest [21-25] for applications where powder samples cannot be readily utilized and a thin film geometry is essential, such as membranes and sensors [26-28]. The first stage of this research is to characterize the effects the organosilane and organophosphonate additives have on the mesoporous character of the films. Herein, we describe the effects of the addition of the quaternary amine functionalized organosilane n-trimethoxysilylpropyl N,N,N -trimethylammonium chloride (TMAC) as well as the combined addition of TMAC and pinacolyl methylphosphonate (PMP) on the mesopore ordering of spin-coated films formed with bis(triethoxysilyl)ethane (BTSE).
2. MATERIALS AND METHODS 2.1. Materials Ethanol and hydrochloric acid were purchased from Aldrich Chemical Co. NTrimethoxysilylpropyl-N,N,N-trimethylammonium chloride (TMAC) and bis(triethoxysilyl)ethane (BTSE) were purchased from Gelest Co. Pluronic P123 was purchased from BASF. Water used was deionized and distilled to 18 Mf~'cm. Film solutions were prepared by stirring 0.925 mL BTSE, 6.21 mL EtOH, 0.45 mL H20, 20 ~tL 1 M HC1, and 0.259 g P123 for one hour at room temperature. N-trimethoxysilylpropyl - N,N,N trimethylammonium chloride (TMAC) was added in the ratio of 1:0.05 (Si:TMAC) 90 minutes prior to spin coating. The final molar ratio of the components for the TMAC functionalized film was 1:22:5:0.004:0.009:0.05 (Si:EtOH:H20:HCl:P123:TMAC). Films formed with PMP (added along with TMAC 90 min prior to spin coating) had final component molar ratio of 1:22:5:0.004:0.009:0.05:X, where X = 0.05, 0.10, and 0.20 PMP. Films were made by filtering the solution through a 0.22 ~tm polyethylene filter followed by spin coating 0.4 mL of prepared solution at 2000 rpm for 30 seconds onto ~ 6.25 cm 2 sections of (100) Si wafers. All films were aged at room temperature for 2 days before extracting or calcining. Extraction was performed by placing the films in a Soxhlet extractor with refluxing ethanol for 24 hours. Calcination was performed by heating the samples in air from room temperature to 100~ at 0.5~ holding at 100~ for 2 hours then ramping from 100~ to 250~ at 0.5~ and holding at 250~ for 2 hours. 2.2. Characterization X-ray diffraction measurements were performed on a Rigaku Rotaflex Series Model RU200B 0-20 rotating anode diffractometer using Cu K~ radiation. Transmission electron microscopy was performed on a Hitachi H8100 TEM operating at 200 kV. Fragments of the film were scraped from the substrate and suspended in ethanol. Drops of the ultrasonicated suspension were placed onto holey carbon TEM grids.
215 3. RESULTS AND DISCUSSION Shown in Figure 1 are transmission electron micrographs of spin-coated films containing 5 mol% TMAC relative to Si (Figure l a) and TMAC (5 mol%) plus 5 mol% PMP relative to Si (Figure l b). Both films are highly porous but the TMAC only film appears to be more ordered.
Figure 1. Transmission electron micrographs of BTSE films formed with a) added TMAC (5 mol% relative to Si) and b) added TMAC and PMP (each 5 mol% relative to Si). In order to better assess the effects of TMAC and PMP on mesopore ordering, the films were examined by XRD. In Figure 2, XRD patterns of as-prepared, ethanol extracted, and calcined BTSE films formed without added TMAC or PMP are presented. Both the as-prepared and extracted samples were determined to have porosity ordered in 2-D hexagonal arrays with the long axis parallel to the substrate as indicated by the absence of the (110) reflections [29]. The d-spacing decreased by 4% after ethanol extraction. Following calcination, XRD revealed that the films were no longer hexagonally ordered. As shown in Figure 2, indexing of the x-ray diffraction patterns for the calcined film shows the ordering to be 2-D cubic. This is supported by transmission electron microscopy analysis of a calcined film (Figure 3). Examination of a cross section of the film (Figure 3a) reveals cubic pore ordering and contraction with a d-spacing of -6.5 nm and pore diameters of 2-3 nm. Analysis of another cross section of the same film (Figure 3b) shows the long axis of the pores with regular spacing, pore diameters of 3 nm, and a wall thickness o f - 5 nm.
216
Figure 2. XRD of BTSE films formed by spin coating onto (100) Si wafers.
Figure 3. Transmission electron micrographs of calcined BTSE films formed by spin coating onto a (100) Si wafer.
217 Figure 4 shows XRD patterns of as-prepared, ethanol extracted, and calcined BTSE films formed in the presence of TMAC. The TMAC (5 mol% relative to Si) was added to the BTSE solution 90 minutes prior to spin coating. As can be seen from the indexing of the XRD patterns, the relative effects of extraction and calcination on pore ordering are similar to those observed for the BTSE only films. Both the as-prepared and extracted films have 2D hexagonal ordering while the calcined film has cubic ordering. For all three films, the addition of TMAC resulted in an increase in the d-spacing of 17-19% over that observed for the corresponding BTSE only film. TMAC Functi0nalized (90 min)
i
O
o ~
20 d(nm) (hkl) 1.10 8.0 (200) 1.56 5.6 (220) 2.10 4.2 (400) 3.14 2.8 (440)
o -~
~
il s'ii
/i H i !
,~.
..... i~,t ~...... ~ ~c -= .___ -~ n,"
calcined
~ ~
20 d(nm)(hkl) 0.82 10.8 (100) 1.56 5.6 (200) 2.34 3.7 (300) 3.16 2.8 (400)
~, o ~, ~ o ~o ~',4,..~~
extracted
o.78 11.3 (loo)
I1
~ o
1.62 2.22
oo
5.5 3.9
(200) (300)
asoreoare 1
2
3
I
I
I
I
I
I
4
5
6
7
8
9
10
Two Theta (degrees)
Figure 4. XRD patterns of as-prepared, ethanol extracted, and calcined BTSE films formed with TMAC (5 mol% relative to Si) by spin coating onto (100) Si wafers. The effects of added PMP on mesoporous film formation were also examined by xray diffraction. The XRD pattems of the as-prepared, ethanol extracted, and calcined films are presented in Figure 5. As was the case with the BTSE only and BTSE-TMAC films, the as-prepared and extracted films exhibit 2D hexagonal ordering with strong (100) reflections at 20 - 0.94 ~ and 1.34 ~ respectively. In contrast to the addition of TMAC only, the addition of TMAC and PMP results in shrinkage of the d-spacings of the films relative to that observed for films formed with only BTSE and TMAC. As shown in
218 TMAC/PMP Added (90 min) 6" (3 ,v i
20 d(nm)(hkl) 1.88 4.7 (100) 3.72 2.3 (200)
6"
(3
H \..........J~_i?<~::<.:::--= x so ...............................c a ! c ! n e d .......
~" ~" ~ ~_
(/) r
_=
..... x 50
I1)
iz
20 d(nm)(hkl) 1.34 6.6 (100) 2.60 3.4 (200)
O
o
v
20 d(nm) (hkl) 0.94 9.4 (100) 1.78 4.9 (200) 2.64 3.3 (300)
6" o
x 2oI 2
extracted
3
4
5
i
as prepared I
I
6
7
8
I
9 10
Two Theta (degrees)
Figure 5. XRD patterns of BTSE films formed with added TMAC (5 mol% relative to Si) and PMP (10 mol% relative to Si) by spin coating onto (100) Si wafers. Table 1, this shrinkage is dependent on the amount of PMP added. The biggest decreases are observed for films formed with 5 and 10 mol% of PMP relative to Si. In the case of the extracted and calcined films, the d-spacing shrinkage observed for films formed with 5 mol% PMP is 11-38% and 30-50% for films formed with 10 mol% PMP. An increase in d-spacing was observed for the as-prepared film formed with 20 mol% PMP although the d-spacing was still smaller than that for the film formed without PMP. Table 1 dl00 spacing, Si:P 123:TMAC (1:0.009:0.05) PMP, mol% (rel. to Si)
As prep, nm
Extracted, nm (Ad %)
Calcined, nm (Ad %)
0
11.32
10.8 (5.3)
8.03 (29.1)
5
9.39
8.32 (11.3)
5.81 (38.2)
10
9.39
6.59 (29.9)
4.70(50.0)
20
10.51
6.13(41.7)
4.55 (56.7)
219 Despite the shrinkage, the films remain highly ordered. These results demonstrate that PMP and TMAC are being incorporated into the films without significantly affecting the mesopore ordering.
ACKNOWLEDGEMENTS
Eva M. Wong is a NRC/NRL Postdoctoral Research Associate. This research was funded by the Office of Naval Research through an NRL Accelerated Research Initiative.
REFERENCES .
2. 3. 4.
.
o
8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
L. Ember, Chem. Eng. News, September 15, 1997, 26. Y-C. Yang, Acc. Chem. Res., 32 (1999) 109. D. A. Henderson, Science, 283 (1999) 1279. J. J. Pancraizio, J. P. Whelan, D. A. Borkholder, W. Ma and D. A. Stenger, Ann. Biomed. Eng., 27 (1999) 697. J. E. Kolakowski, J. J. DeFrank, S. P. Harvey, L. L. Szafraniec, W. T. Beaudry, K. Lai, J. R. Wild, Biocatal. Biotrans., 15 (1997) 297. B. D. Di Sioudi, C. E. Miller, K. Lai, J. K. Grimsley and J. R. Wild, ChemicoBiological Interactions, 119-120 (1999) 211. A. L. Jenkins, O. M. Uy and G. M. Murray, Anal. Chem., 71 (1999) 373. D. Y. Sasaki and T. M. Alam, Chem. Mater., 12 (2000) 1400. M. A. Markowitz, G. Deng, B. P. Gaber, Langrnuir, 16 (2000) 6148. M. S. Nieuwenhuizen and J. L. Harteveld, Sens. Actuators B, 40 (1997) 167. W. Trettnak, F. Reininger, E. Zinterl and O. S. Wolfbeis, Sens. Actuators B, 11 (1993) 87. E. B. Cogan, G. B. Birrell and O. H. Griffith, Anal. Biochem., 271 (1999) 29. J. J. Rippeth, T. D. Gibson, J. P. Hart, I. C. Hartley and G. Nelson, Analyst, 122 (1997) 1425. S. M. Kanan and C. P. Tripp, Langrnuir, 17 (2001) 2213. M. A. Markowitz, G. Deng, M. C. Burleigh, E. M. Wong and B. P. Gaber, Langmuir, 17 (2001) 7085. M. A. Markowitz, P. R. Kust, G. Deng, P. E. Schoen, J. S. Dordick, D. S. Clark and B. P. Gaber, Langmuir, 16 (2000) 1759. M. A. Markowitz, P. R. Kust, J. Klaehn, G. Deng and B. P. Gaber, Anal. Chim. Acta, 435 (2001) 177. K. Haupt and K. Mosbach, Trends Biotech., 16 (1998) 468. G. Wulff, Angew. Chem. Intl. Ed. Engl., 34 (1995) 1812. M. I. Whitcombe and E. N. Vulfson, Adv. Mater., 13 (2001) 467. T. Yamada, K. Asai, K.A.Endo, H. S. Zhou and I. Honma, J. Mat. Sci. Lett., 19 (2000) 2167. M. Ogawa, H. Ishikawa and T. Kikuchi, J. Mater. Chem., 8 (1998) 1783.
220 23. 24. 25. 26. 27. 28. 29.
R. Hemandez, A.-C. Franville, P. Minoofar, B. Dunn and J. I. Zink, J. Am. Chem. Soc., 123 (2001) 1248. D. Grosso, A. R. Balkenende, P. A. Albouy, M. Lavergne, L. Mazerolles and F. Babonneau, J. Mater. Chem., 10 (2000) 2085. S. Pevzner, O. Regev and R. Yerushalmi-Rozen, Current Opinion in Colloid & Interface Science, 4 (2000) 420. J. Y. Ying, C. P. Mehnert and M. S. Wong, M.S. Agnew. Chem. Int. Ed., 38 (1999) 56. H. Fan, Y. Lu, A. Stump, S. T. Reed, T. Baer, R. Schunk, V. Perez-Luna, G. P. Lopez and C. J. Brinker, Nature, 405 (2000) 56. G. Wimsberger, B. J. Scott and G. D. Stucky, Chem. Commun., (2001) 119. D. Zhao, P. Yang, N. Melosh, J. Feng, B. F. Chmelka, B.F. and G. D.Stucky, Adv. Mater., 10 (1998) 1230.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
221
I m p r o v i n g the H y d r o - s t a b i l i t y o f M C M - 4 1 b y P o s t - S y n t h e s i s T r e a t m e n t and H e x a m e t h y l d i s i l a z a n e Coating Jing Yang, a Antje Daehler, b Michelle L. Gee, c Geoffrey W. Stevens b and Andrea J. O'Connor b a School of Materials, China University of Geosciences, Beijing 100083, China b Department of Chemical Engineering, University of Melboume, Victoria 3010, Australia c School of Chemistry, University of Melbourne, Victoria 3010, Australia
The stability of siliceous MCM-41 in various aqueous solutions including distilled water, 2(N-morpholino)ethane sulphonic acid(MES) pH 6 buffer, tris-(hydroxymethyl)aminomethane (TRIS) pH 8 buffer and potassium phosphate pH 6 buffer is presented. The MCM-41 was hydrothermally-synthesised by sodium hydroxide (NaOH), cetyltrimethylammonium bromide (CTAB), fumed silica and distilled water. The structural stability of normal MCM-41 is compared to its stability after post-synthesis hydrothermal treatment alone and after postsynthesis hydrothermal treatment combined with coating with hexamethyldisilazane (HMDS). The combination of post-synthesis hydrothermal treatment and HMDS coating has the benefits of improvement to the silica stability plus a robust hydrophobic surface coating. This protects the silica surface from water and buffer solutions but, in contrast to HMDS coating alone, causes little change to the pore volume. After immersion in MES buffer for seven days or in TRIS buffer for one day, no significant change in the surface area or pore volume was observed in the coated MCM-41.
1. INTRODUCTION Since M41S mesoporous materials were reported in 1992 [1], the hydro-stability of MCM-41 has been the key problem preventing its application in industry. Many potential applications, such as separation processes, often involve contact with aqueous solutions or water vapour, which has been shown to cause structural degradation in MCM-41 [2-5]. A complete loss of structure was observed in MCM-41 after three months exposure to an atmosphere containing 60% humidity at room temperature [2]. The structure was not recovered upon recalcination. It has been demonstrated that the synthesis conditions influence the material stability, with recent results indicating that hydrothermally-synthesised samples are more stable in water than those synthesised at room temperature although they still underwent structural change when exposed to a basic or acidic solution after several hours at room temperature [6]. Silylation, for example, using trichloromethylsilane (TCMS), trimethylchorosilane (TMCS) or HMDS, has been used to produce a hydrophobic coating on the surface of MCM-41 to improve its stability [3, 7] . However, the coating causes a reduction in the surface area and pore diameter of the MCM-41. For example, the surface area of an HMDS-coated MCM-41 sample decreased by 22%, while the pore volume decreased from 0.89 to 0.66 cm3/g or even lower [7]. The surface area of a TMCS-coated MCM-41
222 sample decreased by 25%, as the pore volume simultaneously decreased from 0.91 to 0.79 cm3/g or even lower [3]. Disilazane reagents have been found to be effective in the modification of the surface of mesoporous silica materials. In contrast to using TCMS or TMCS, the use of HMDS eliminates the problem of forming a layer not covalently bound to the surface which could be removed by leaching in water [8-11]. It also avoids the necessity of a two-step reaction, as HMDS reacts directly with surface silanol groups with no need for an initial amine reaction [7]. An alternative method to improve the stability of MCM-41 is post-synthesis hydrothermal treatment [ 12, 13]. This method has the benefit that the treated samples have even higher surface areas and larger pore sizes than the untreated MCM-41, which can be beneficial for adsorption of large molecules such as proteins. Heating a postsynthesis hydrothermally treated MCM-41 sample in boiling water for 22 h was found to cause little structure damage [13]. Addition of salt and adjustments to the pH of the synthesis mixture can also improve hydrothermal stability; for example, no loss of structure was observed after 12h in boiling water for an MCM-41 sample prepared with three intermediate pH adjustments and a salt addition [14]. However, the long-term stability of such treated materials in water and buffer solutions, which are of relevance to biochemical separations and may exhibit chemical interactions with the silica, has not been proven. Thus, in this study we have investigated the stability of MCM-41 samples with and without post-synthesis treatment and HMDS coating, over periods of up to seven days in water and three different buffer solutions.
2. EXPERIMENTAL 2.1. Chemicals Sodium hydroxide (NaOH), cetyltrimethylammonium bromide (CTAB), fumed silica, hexamethyldisilazane (HMDS), potassium phosphate buffer (K2HPO4 and KHaPO4), MES buffer (2-[N-morpholino] ethane-sulfonic acid and 2-[N-morpholino]ethane-sulfonic sodium salt), and TRIS buffer (tris(hydroxymethyl)amino methane hydrochloride) were used. The NaOH was from Merck Pty Ltd, all other chemicals were from Aldrich. All chemicals were of analytical reagent grade. All water used was distilled. 2.2. Normal Synthesis The preparation procedure was as follows [3, 12]. CTAB (16.4 g) was dissolved in 50 g distilled water. NaOH (2.54 g) and 40 g distilled water were mixed at room temperature. The CTAB solution was then added to the NaOH solution, stirring for 30 minutes. Fumed silica (15.90 g) was added to the mixed CTAB and NaOH solution. Finally, 30 g distilled water was introduced, stirring at 75 ~ for 1 h to obtain a gel. It was then transferred into a PTFE-coated autoclave and heated to 120 ~ for 2 days. The as-synthesised sample was filtered and dried prior to calcination at 550 ~C for 9 h in air or in nitrogen for 1 h followed by air for 8 h with a heating rate of 1 ~C/min. 2.3. Post Synthesis Hydrothermal Treatment The post-synthesis hydrothermal treatment was carried out by replacing the mother liquor with distilled water after the normal synthesis [12, 13]. The as-synthesised sample (3 g) with 50 mL distilled water was put into a PTFE-coated autoclave, and kept in the oven at 135 ~ or
223 150 oC for 24 h. Finally, the solid product was filtered, washed, dried and calcined in air as in the normal preparation. 2.4. HMDS Coating HMDS surface coating was applied to the post-synthesis hydrothermally treated MCM-41 samples by gas phase reaction [9] following calcination and characterisation. Before coating, the MCM-41 sample and HMDS were isolated by a valve and the HMDS was degassed via a series of freeze-thaws under a vacuum. The MCM-41 was heated at 250 ~ overnight at a pressure of less than 0.01 atm, after which the HMDS vapour and the MCM-41 were allowed to react for 16 h, whilst cooling to room temperature. 2.5. Stability Tests The stability of the normal, post-synthesis treated and HMDS-coated MCM-41 samples was tested by immersing the samples in distilled water, MES buffer at pH 6, TRIS buffer at pH 8 and potassium phosphate buffer at pH 6 at room temperature for 1 day or 7 days. MCM41 (1 g) was mixed with 10-50 mL solution with gentle stirring. After immersion, the samples were dried in an oven at 110~ for several hours and characterised again. The buffer solutions were made according to methods described by Dawson [15]. The concentration of the TRIS buffer solution was 50 mmol/L. The MES buffer and potassium phosphate buffer solutions had a concentration of 100 mmol/L. 2.6. Characterization X-ray powder diffraction (XRD) data was obtained using a Philips PW 1800 diffractometer with CuK~ radiation (wavelength 0.154056 nm). Approximately 0.3 g of MCM-41 powder was placed in a sample holder and data was recorded for 20 angles between 1 ~ and 8 ~ at a step size of 0.02 ~ 20 and a count time of 5 seconds. Gas adsorption analysis measurements using nitrogen at 77 K were conducted on a Micrometritics ASAP 2000. Before analysis, the 0.1-0.3 g of each sample used was evacuated at 250 ~ overnight to remove water from the sample. Surface a r e a s (SBET) were calculated using the BET model [ 16, 17]. 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 [18-20]. The wall thickness (t) was estimated as the unit cell dimension minus the BJH pore diameter. Although the BJH model is known to underestimate the pore diameter of MCM-41, which will cause an overestimation of the wall thickness by this method, it is useful for comparison purposes [21].
3.
RESULTS AND DISCUSSION
3.1 Characterization of MCM-41 The quality of the MCM-41 strongly depends on the synthesis conditions. Figure 1 shows the XRD spectra of MCM-41 samples prepared by the different methods. Table 1 shows the results of XRD and nitrogen adsorption measurements on the MCM-41 samples used in the stability tests. It was seen that the dl00 spacing and therefore the unit cell (a0) values of the assynthesised MCM-41 sample shrank after calcination in air. The shrinkage rate was about 14%. Calcination in N2 followed by air improved the BET surface area by 10% relative to
224
(]00)
(200) o
1
2
3
4
5
6
7
2-theta (degrees)
Figure 1. XRD spectra of MCM-41 samples (a) normal synthesis (sample M2b); (b) post-synthesis treated (sample M2c); (c) HMDS-coated (sample M2d). 700
-
600
~ 500 400 o
300
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
200
--BE-post-synthesis treated MCM-41(sample M2c)
100
--4r--HM DS coated MCM-41(sample M2d)
0 0
0.2
0.4
Relative
0.6
Pressure
0.8
1
1.2
(PIPo)
Figure 2. Nitrogen adsorption/desorption isotherms of MCM-41 samples. calcination in air alone. The BET surface area and pore volume of the HMDS-coated MCM41 decreased, while the wall thickness increased, as expected if a coating was bound to the pore walls. However, after the as-synthesised MCM-41 was treated hydrothermally, its di00, a0, SBET, V and the BJH poresize were found to increase relative to the normal MCM-41 samples as described by Chen et al. [12, 13], shown in Table 1, Figures 1 and 2. The SBETof the postsynthesis treated sample increased by up to 15% compared to that of the normal sample calcined in air, while the V increased by up to 27%. The hydrotherma! treatment temperature has been found to affect the pore size of the MCM-41. The sample treated at 150 ~ for 24 h had a larger dl00 spacing, SBETand pore volume than the sample treated at 135~ It seems that the higher treatment temperature could improve the pore size of MCM-41 sample, but the structure has been found to deteriorate when the temperature exceeds 180~ with the intensity of (100) peak decreasing considerably and the (110), (200) peaks also diminishing[ 12].
225 Table 1. Characterisation results for the MCM-41 samples dloo ao SBET No. Description (nm) (nm) (m2/g)
V
DBJH
(cm3/g)
(nm)
(nm)
2.57 2.58 2.56 2.81
1.44 1.36 1.45 1.61
Initial samples M 1a M 1b M 1c M 1d M 1e
M 1f
M2a M2b M2c M2d
As-synthesised Normal, calcined in air Normal, calcined in N2 then air M 1c coated with HMDS Post-synthesis treated at 150 ~C, calcined in air Post-synthesis treated at 135 oC, calcined in air
4.04 3.47 3.41 3.47 3.83
4.67 4.01 3.94 4.01 4.42
. 922 1014 905 1060
3.70
4.27
1038
0.89
2.74
1.53
As-synthesised Normal, calcined in N2 then air Post-synthesis treated at 150 ~C And calcined in air M2c coated with HMDS
3.82 3.58 4.19
4.41 4.13 4.84
1036 1097
0.83 1.01
2.66 2.97
1.47 1.87
4.03
4.66
909
0.82
2.82
1.84
4.06 4.41
929 1042
0.71 0.90
2.58 2.80
985 951 863 1063 892 879
0.75 0.71 0.66 0.95 0.75 0.81
2.59 2.56 2.66 2.90 2.84 2.94
0.76 0.87 0.80 0.72
2.71 2.90 2.87 2.57
.
. 0.73 0.81 0.71 0.93
.
Samples after immersion in water for one day Mlb-W M 1e-W
Mlb in water, 1 day M 1e in water, 1 day
3.52 3.82
Samples after immersion in buffer solution for one day M 1c-M M 1c-T Mle-P M2c-M M2c-T M2d-T
M 1c in MES buffer, 1 day M 1c in TRIS buffer, 1 day M l e in phosphate buffer, 1 day M2c in MES buffer, 1 day M2c in TRIS buffer, 1 day M2d in TRIS buffer, 1 day
3.42 3.21 3.57 3.89 3.89 4.03
3.95 3.71 4.13 4.50 4.50 4.65
Samples after immersion in MES buffer solution for seven days M2b-M7 M2c-M7 M2d-M7 Mld-M7
M2b in MES M2c in MES M2d in MES MldinMES
buffer, buffer, buffer, buffer,
7 days 7 days 7 days 7 days
3.36 3.83 3.82 3.52
3.88 4.42 4.41 4.07
950 957 894 948
3.2 Stability of MCM-41 in Water for One day After the MCM-41 samples were immersed in water at room temperature for one day, the XRD and gas adsorption results showed no significant changes for either the normal or postsynthesis treated samples (Table 1). Thus, the MCM-41 synthesised here was found to be stable in water for 24 h.
226 700
Q. Ir
600
o~ 500
E 0 =9 400
Q .L0
f-~-pos-t.synthesis treated (sample M2c) .............] i i--l i--post-synthesis treated after 24 h in MES buffer ! = i j -" post-synthesis treated after 24 h in TRIS bufferi
0
= 300
"0 0
E 2O0
I
I---~-- HMDS coated (sample M2d) ',
0
100
= i
L...*_._.H._MDS_C_0a_!_ed__ __affe_r2.4 h in_TR!S ~ffer ......................! 0
0.2
0.4
0.6 0.8 Relative Pressure (PIP0)
1
1.2
Figure 3. Nitrogen adsorption/desorption isotherms of the post-synthesis and coated MCM41 before and after in MES and TRIS buffer for 24 h.
3.3 Stability of MCM-41 in Buffer Solutions Potential applications of MCM-41 in biochemical separation processes involve exposure not only to water, but also to basic and acidic buffer solutions. So testing the stability of the samples in different buffer solutions was also necessary. Table 1 and Figure 3 show the results of the one day stability tests. It was found that the coated MCM-41 was stable in TRIS buffer solution tested for one day, whereas the post-synthesis treated sample and the normal sample were not stable in the TRIS or phosphate buffers, with SBET and V of the samples all decreasing. The coated MCM41 kept its structural stability, large surface area and high pore volume after immersion in phosphate buffer solution for twelve days [8]. The uncoated MCM-41 samples were more stable in the MES buffer for one day than in the TRIS or phosphate buffer solutions. After immersion in TRIS buffer for one day, the surface area of the uncoated post synthesis treated MCM-41 decreased by up to 19%, while the pore volume dropped by up to 26%. The TRIS buffer was at pH 8, which may account for its greater effect on the samples than the pH 6 buffers, due to the slightly increased solubility of silica at this pH [22]. The difference between the stability of MCM-41 in MES and phosphate buffers of the same concentration and pH may be attributed to the chemical interactions of the buffer anions with the silica. The results of stability tests on the MCM-41 in MES buffer for seven days are shown in Table l, Figures 4 and 5. It was confirmed that the coated sample did not change significantly after contacting the MES buffer solution for seven days, with less than 3% change in the properties listed in Table 1. However, the uncoated samples including the postsynthesis treated samples degraded in unit cell dimension, BET surface area (by 8% - 13%) and pore volume (by 8% - 14%). The pore size distribution of the coated sample was maintained whereas that of the post-synthesis treated sample showed a decrease in pore volumeand diameter.
227 700
600 r
500
400 300 =e 200 = ~
~
! ~ POSt:synthe sis-t-reate d-M c M- ] 41(sample M2c) I !J ~syntehsis tre,tte MCM-! !-=-post-syntehsis treated 1 4 1 41 after7days in MES M E buffer i ':--A-HMDS coated MCMiI -~ 4 1 ( s a m p l e M 2 d ) , - o - H M D S coated MCM-41 after Ii ! ..... in_M_ES_`..b.u.ffe[_for7 . d a y s . . . . . . . . !
~_=ill~ . . , h J l r ' ~ i~'~
100
0
0
0.2
0.4 0.6 0.8 Relative Presure (PIP0)
1
1.2
Figure 4. Nitrogen adsorption/desorption isotherms of MCM-41 before and after immersion in MES buffer for 7 days. 12
A
E r l0 v E
a)
..~
--41~-after in M E S -C>--before
buffer
i
b)
in M E S b u f f e r
1
/
8
o
after m MES
i
,
t
f o r e in M E S " .................................
i
o
o .~ 2
0
1.5
2
2.5
Pore
3
3.5
Diameter
4
(nm)
4.5
5
1.5
2
2.5
3
3.5
4
4.5
Pore Diameter (nm)
Figure 5. BJH curves of the MCM-41 before and after immersion in MES buffer for 7 days; (a) post-synthesis treated MCM-41 (sample M2c); (b) HMDS-coated MCM-41 (sample M2d).
4
CONCLUSIONS
In this work, it was found that both normal and post-synthesis hydrothermally treated MCM-41 were unstable in different buffer solutions. The post-synthesis hydrothermal treatment caused an increase in the unit cell dimension, BET surface area and the pore volume relative to the untreated MCM-41, whilst the characteristic XRD pattem and type IV nitrogen adsorption isotherm were retained. The structural stability of the materials which were both hydrothermally treated and HMDS-coated was superior to the untreated MCM-41 in aqueous solutions over extended exposure times. The combination of post-synthesis hydrothermal treatment and HMDS coating has the benefits of improvement to the silica stability plus a robust hydrophobic surface coating. This protects the silica surface from water and buffer solutions but, in contrast to HMDS coating alone, causes little change to the pore volume. Hence, in this work we have investigated the stability of normal MCM-41 as well as samples which have been post-synthesis hydrothermally treated or HMDS-coated or both.
228 Tests have been performed using distilled water and three different buffers solutions commonlyused in biochemical processes for periods of up to seven days. The HMDS coating has been demonstrated to produce the most stable MCM-41 samples.
5
ACKNOWLEDGMENTS
Jing Yang gratefully acknowledges the support of China Scholarship Council and the Ministry of Education, People's Republic of China. REFERENCES
1. 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. 2. X. S. Zhao, F. Audsley, G. Q. Lu, Journal of Physical Chemistry B, 102 (1998) 4143. 3. X. S. Zhao, G. Q. Lu, Journal of Physical Chemistry B, 102 (1998) 1556. 4. T. Tatsumi, K. A. Koyano, Y. Tanaka, S. Nakata, Journal of Porous Materials, 6 (1999)13. 5. A. Stein, B. J. Melde, R. C. Schroden, Advanced Materials, 12(2000) 1403. 6. M. M. L. Ribeiro Carrott, A. J. Estevao Candeias, P. J. M. Carrott and K. K. Unger, Langrnuir, 15 (1999) 8895. 7. R. Anwander, I. Nagl, M. Widenmeyer, G. Engelhardt, O. Groeger, C. Palm, T. Roeser, J. Phys. Chem. B, 104 (2000) 3532. 8. J. Kisler, PhD thesis, Univeristy of Melbourne, Australia, 2001. 9. J. Kisler, M. Gee, G. W. Stevens, A. J. O'Connor, Chemistry of Materials, submitted. 10. C. P. Tripp, M. L. Hair, Langmuir, 8(1992) 1120. 11. C. P. Tripp, M. L. Hair, Langmuir, 8(1992) 1961. 12. L. Y. Chen, T. Horiuchi, T. Mori, K. Maeda, Journal of Physical Chemistry B, 103 (1999) 1216. 13. M. Knak, M. Jaroniec and A. Sayari, Microporous and Mesoporous Materials, 27 (1999) 217. 14. R. Ryoo and S. Jun, Journal of Physical Chemistry B, 101 (1997) 317. 15. R. M. C. Dawson, D. C. Elliott, W. H. Elliott, K. M. Jones, Data for Biochemical Research, 3rd edition, Oxford University Press, Oxford, 1986. 16. S. Brunauer, P. H. Emmett, E. Teller, Journal of the American Chemical Society, 60 (1938) 309. 17. S.J. Gregg, K. S. W. Sing, Adsorption, Surface Area and Porosity, 2nd edition, Academic Press, London, 1982. 18. P. J. Branton, K. S. W. Sing, J. W. J. White, Chem. Soc., Faraday Trans., 93(1997) 2337. 19. M. Kruk, M. Jaroniec, A. Sayari, Langrnuir, 13(1997) 6267. 20. M. Kruk, M. Jaroniec, J. M. Kim, R. Ryoo, Langmuir, 15(1999) 5279. 21. M. Kruk, M. Jaroniec, A. Sayari, Journal of Physical Chemistry B, 101 (1997) 583. 22. R. K. Iler, The Chemistry of Silica, John Wiley & Sons, New York, 1979.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
229
Adsorption o f CO on Zn-Cu(I)/HMCM-41 Qihong Shia, Nongyue He, b'c* Fei Gao, a Yibing Song,a Yang Yu a and Huilin Wan d a
Department of Chemistry, Shantou University, Shantou, 515063, Guangdong, P. R. China
b Key Lab of New Materials and Technology of China National Packaging Corporation, Zhuzhou Engineering College, Zhuzhou 412008, P. R. China c Key Laboratory for Molecular and Biomolecular Electronics of Ministry of Education, Southeast University, Nanjing 210096, P. R. China d The State key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, P. R. China Using mesoporous acidic HMCM-41 as parent, the Zn-Cu(I)/HMCM-41 catalysts, which can reversibly adsorb and release CO, were successfully prepared in laboratory scale by means of the solid-state ion exchange together with introducing Zn as an assistant to improve the dispersion degree of the active component Cu(I) on the surface. With increase of the loading amounts of Zn and CuC1 from 0% to 9.0% and 25.0% respectively, its CO adsorption amounts increased from 10.6 pmol/g to 183.0 pmol/g correspondingly. The FT-IR in situ characterization for CO adsorption demonstrated that there existed two dynamic equilibriums between surface carbonyl complexes: Cu(CO)3+<=>Cu(CO)2++ CO and Cu(CO)2+r Cu(CO) + + CO. The equilibriums can be shifted reversibly by changing the temperature and pressure. At the same time, adsorption and release of CO is accompanied, which is the possible CO source for carbonylation reactions. Our recent research reveals that a starting alcohol was protonated at the acid sites of modified microporous Y, 13 or ZSM-5 and transformed into carbonium ions, from which the carboxylic acids (-
1. INTRODUCTION Although tertiary monocarboxylic acids can be synthesized in liquid media, due to the difficult of separation, use of strong acid and the waste treatment, it is very necessary to develop heterogeneous catalysts to replace liquid catalysts [ 1]. Recently, we have synthesized metal-containing and enzyme-like zeolites with both solid acid reaction centers and metal carboxylic active sites and successfully synthesized the C5 carboxylic acid using butanol and
230 carbon monoxide as starting materials on these two bifunctional catalysts under mild conditions (-523 K, atmospheric pressure) [2-4]. However, the small pore size (<1.0 nm) of [3 and HZSM-5 parent zeolites limited the synthesis of tertiary monocarboxylic acid with a longer carbon chain, for example, C9 carboxylic acid, which is a wonderful paint for car. We have conducted some studies on mesoporous materials 5-9] and found that MCM-41 mesoporous materials showing to be wonderful catalysts for reaction of bulk molecules [ 10]. For this reason, by modifying the MCM-41 mesoporous material, carefully selecting the assistant and optimizing the synthesis conditions, we obtained the Zn-Cu(I)/HMCM-41 bifunctional catalysts with enough large pore size, carbon monoxide molecules can reversibly adsorb on and escape from it, promising to develop into a kind of catalyst allowing the catalytic reaction concerned with carbon monoxide and bulk organic molecules.
2. MATERIALS AND METHODS 2.1. Materials
All the chemicals were analytically pure and was used as supplied. The HMCM-41 sample with a SIO2/A1203 ratio of 32 was prepared as described previously [6]. The Zn-Cu(I)/HMCM-41 sample was prepared as below: Given amount of HMCM-41 sample was added into given absolute alcohol containing given amount of ZnC12 under stirring until the alcohol was completely vaporized at ambient temperature, followed by dring at 393 K in an oven for 6 h. Then the sample was transferred into a muffle and was calcined at 843 K for 4.5 h to obtain the Zn/HMCM-41 sample. Given amount of Zn/HMCM41 was mixed with given amount of CuC1 in a mortar and through ground by hand with a pestle, then calcined in a quartz tube in a nitrogen flow at 623 K for 4 h to get the ZnCu(I)/HMCM-41 samples. 2.2. Measurements and Methods
The CO-TPD-MS investigation was conducted on a temperature programmed desorption instrument adapted from the Blazers GSD 3000 02 mass spectrometer (MS). 100 mg of sample with particle size of 40-60 mesh was placed into a micro-activity reaction set and pretreated in a He flow of 30 ml/min at 373 K for 2 h. Then the mixture of CO and He (He : CO=I l:l)was introduced into the set and the temperature was increased from 373 K to 673 K, followed by decreasing the temperature to 295 K to allow the adsorption of CO. After the removal of the physically adsorbed CO, the MS spectra were recorded accompanying the temperature was increased from 295 K to 673 K at a rate of 10 K/min. The in situ FT-IR spectra were acquired on a PE-Spectrum 2000 in situ FT-IR spectrometer with a resolution of 2 cm -1. The samples were ground by hand with a pestle in a mortar for 5 min and were then pressed at 4 tons to give a self-supporting pellet (10 mm in diameter). The wafer was placed inside the in situ infrared cell to be treated for 2 h under a vacuum condition of 1• 10.3 Pa at 673 K. Then the temperature was decreased to 323 K and 6.63• 10.3 Pa of pure CO was introduced. After adsorption equilibrium was reached, a FT-IR spectrum was taken on the sample, followed by taking another FT-IR spectrum after evacuation for 10 min. Then the temperature was increased to 373 and 473 K to take the FT-IR spectra before and after evacuation similarly.
231 3. RESULTS AND DISCUSSION 3.1. Effect of the loading amount of CuCI and Zn on the adsorption of CO on ZnCu(I)/HMCM-41
Listed in Table 1 are the loading amounts of Zn and CuC1 and CO adsorption capacity on a series of samples. It was found that after loading CuC1 or Zn, the CO adsorption amount was significantly increased in comparison with the parent HMCM-41 sample 15. However, the CO adsorption capacity on CuCl-loaded sample 14 was much greater than that on Znloaded sample 13. Table 1. The influence of the loading amounts of Zn and CuC1 on CO adsorption capacity of the ZnCu(I)/HMCM-41. Samples 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Zn loading content (wt%) 3.0 3.0 3.0 6.0 6.0 6.0 9.0 9.0 9.0 12.0 12.0 12.0 6.0 0.0 0.0
CuC1loading content (wt%) 20.0 25.0 30.0 20.0 25.0 30.0 20.0 25.0 30.0 20.0 25.0 30.0 0.0 25.0 0.0
CO adsorption capacity Qn ( ~ mol/g) 68.84 80.77 71.05 75.38 114.04 84.03 139.71 182.99 142.54 107.91 87.52 83.34 26.05 59.57 10.61
On the other hand, comparing the data for samples 2, 5, 8, 11 and 14 possessing same loading amount of CuC1, it was demonstrated that the CO adsorption capacity increased more after loading Zn than the sample 14 which was only loaded CuC1. From the data we also found that the positive influence of Zn on the CO adsorption capacity increased from the 3.0% to 9.0% of the loading amount of Zn and decreased from 9.0% to 12.0%, i.e., the most optimized loading amount of Zn is 9.0 under our investigation. It seemed that the loaded Zn occupy part of adsorption sites by the CuC1 and, therefore, the CO adsorption capacity decreased from 9.0% to 12.0% of loaded Zn. For those samples with same Zn loading amount and varying loading amount of CuC1 (20.0%, 25.0% and 30.0%), the CO adsorption capacity behaviors are different from each other. When the loading amount of Zn is _<9.0%, the CO adsorption capacities on samples with 25.0% are greatest. Whereas, due to that a fraction of adsorption sites were occupied by Zn species, the CO adsorption capacity decreased with the increase in the loading amount of CuC1 for those samples with 12.0% of Zn (see samples 10-12).
232 The influence of Zn species on the CO adsorption capacity of CuCl-loaded samples is worthy of some discussion. It is well known that under same composition conditions, the population of reactive sites and, therefore, the activity, increases with the decrease in size and the increase in surface area of particles of same weight. Because there is a strong interaction between the Zn species and CuC1 species on Zn-Cu(I)/HMCM-41 samples prepared by solidstate ion exchange [ 11 ], the sintering and agglomeration of Cu species can be prevented and, therefore, Cu(I) species highly dispersed on the surface of channel wall of HMCM-41. This leads us to the conclusion that adding Zn species can increase the dispersion degree and, therefore, increase the population of reactive sites resulting from Cu(I) species. Thus the CO adsorption capacity increased for those samples loaded both Zn and CuC1 species.
3.2. In situ FT-IR spectra of CO adsorption on Zn-Cu(1)/HMCM-41 sample The investigation of In situ FT-IR spectra of CO adsorption on Zn-Cu(I)/HMCM-41 sample was investigated on the typical sample 8 in Table 1 which showed good adsorption behavior.
Figure 1. IR spectra of CO absorbed on ZnCu(I)/HMCM-41 before and after evacuation at different temperatures. (a) and (b): Before and after evacuation at 323 K; (c) and (d): Before and after evacuation at 373 K; (e) and (f): Before and after evacuation at 473 K; (g): Before evacuation at 573 K.
Shown in Figure 1 are the In situ FT-IR spectra of CO recorded for the sample before and after evacuation at different temperature. Five bands were detected at 2138 cm -1 2142 cm -1 2148 cm -1, 2152 cm -l and 2153 cm -1. These bands were assigned to Cu(CO) + (2135 cm-l), Cu(CO)2 + (2148 and 2151 cm -1) and Cu(CO)3 + (2138 and 2140 cml), respectively [12]. It was shown that under our sample prepared by solid-state ion exchange approach, Cu(CO)3 + was formed on sample at low temperature (323 K). When the temperature was increased or the CO partial pressure was decreased, all the bands shifted towards high wavenumber direction, Cu(CO)3 + was destabilized and its special band gradually vanished. If the temperature was increased to 573 K, all the bands owing to the adsorbed CO molecules completely disappeared (spectrum g in Figure 1), meaning CO can not be adsorbed at such a high temperature. When the temperature was increased to 373 and 473 K respectively, correspondingly Cu(CO)2§ and Cu(CO)3 + were formed respectively. Upon decrease of temperature from 473 K to 373 and 323 K, the spectra were recovered to the same as that obtained before increasing temperature. It seemed that some reversible and dynamic equilibriums existed among Cu(CO)3 +, Cu(CO)2+ and Cu(CO)3 + upon the changes in
233 temperature and pressure. It is believed that a carbonylation reaction takes place accompanying the adsorption and release of CO as below: C u ( C O ) 3 + ~z~ C u ( C O ) 2 + + C O Cu(CO)2 + ~
C u ( C O ) I + q- C O
(I) (II)
Recently we observed that the solid-state acid centers in bifunctional Cu(I)/HY, Cu(I)/HI3 or Cu(I)/HZSM-5 can work in cooperation with the above metal carbonyl centers under mild conditions (523 K and atmospheric pressure) to convert the alcohol into tertiary monocarboxylic acid (-
234 temperature upon the addition of Zn, and can work as source of CO for the heterogeneous carbonylation reaction. However, because carbonyl Cu(I) species can only exist on Cu(I)/HMCM-41 samples at 336 K, its function as a catalyst for heterogeneous carbonylation reaction is much limited. These results show us that the addition of Zn can stabilize the carbonyl Cu(I) species on HMCM-41 samples and, therefore, makes it possible to conduct the heterogeneous carbonyl reaction in a relative wide range of temperature.
Figure 3. CO-TPD-MS curves of CO adsorbed on Cu(I)/HMCM-41 (a) and Zn-Cu(I)/HMCM-41 (b).
4. ACKNOWLEDGMENTS Authors thank the kind help by associate professor W. Weng and Dr. L. Yang for the COTPD-MS measurement in Xiamen University and the financially support by the State key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, P. R. China. REFERENCES
1. 2. 3. 4.
A.G. Stepanov, M. V. Luzgin and V. N. Romannikov, et al., J. Catal., 164 (1996) 411. Q. Shi, X, Zhao and Z. Zhang, et al., Chinese Journal of Catalysis, 20 (1999) 125. Q. Shi and C. Li, Chinese Journal of Catalysis, 21 (2000) 113. Q. Shi, X. Zhao and C. Li, in: M. M. J. Treacy, B. K. Macus, M. E. Bisher and J. B. Higgins eds. Proceedings of the 12th International Zeolite Conference Vol II, Baltimore: Material Research Society, 1998. 1177. 5. 19. N. He, C. Yang, Q. Dai, J. Wang, C. Yuan and Z. Lu, J. Thermal Analysis and Calorimetry, 61 (2000) 827. 6. N. He, S. Bao and Q. Xu, Stud. Surf. Sci. Catal., 105 (1997) 85. 7. N. He, C. Yang, L. Liao, C. Yuan, Z. Lu, S. Bao and Q. Xu, Supramolecular Science, 5 (1998) 523. 8. N. He, C. Yang, Q. Dai, Y. Miao, X. Wang, Z. Lu and C. Yuan, Incl. Phenom. Macro. Chem., 35 (1999): 211. 9. N. He, D. Li, M. Tu, J. Shen, S. Bao and Q. Xu,. J. Thermal Analysis and Calorimetry, 58 (1999) 455. 10. N. He, S. Bao andQ. Xu, Appl. Catal. A: General, 169 (1998) 29. 11. Q. Sun, Y. Zhang and H. Chen, J. Catal. A, 169 (1997) 92. 12. K. I. Hadjiivanov, M. M. Kantcheva and D. G. Klissurski, J. Chem. Soc., Faraday Trans., 92 (1992) 4595.
Studies in Surtace Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
235
D e s i g n of transition m e t a l oxide m e s o p o r o u s thin films Eduardo L. Crepaldi, a* Galo J. de A. A. Soler-Illia, a David Grosso, a Pierre-Antoine Albouy, b Heinz Amenitsch c and ClEment Sanchez a a Laboratoire de Chimie de la MatiEre CondensEe, UniversitE Pierre et Marie Curie- CNRS, 4 place Jussieu, 75252, Paris CEDEX 05, France. E-mail: crepaldi @ccr.jussieu.fr. b Laboratoire de Physique des Solides, Universit6 Paris-Sud, 91405 Orsay Cedex, France. c Institute of Biophysics and X-ray Structure Research, Austrian Academy of Sciences, Steyrergasse 17/VI, 8010 Graz, Austria. Mesostructured transition metal (Ti, Zr, V, Fe) oxide-based hybrid thin films have been prepared reproducibly, displaying 2D-hexagonal (p6m), 2D-centred rectangular (c2m) or lamellar structure. A combination of time resolved Synchrotron-SAXS and interferometry was applied to study the earlier stages of film formation during dip-coating. The results showed that an intermediary worm-like structure was formed just after the drying line, giving rise to the organised mesophase upon evaporation of residual solvent and HC1. Many variables concerning the solution composition and the processing of the films have been optimised. Water (into the solution and from air moisture) and solution acidity clearly play a major role in the resulting degree of organisation. By carefully adjusting these variables it is possible to combine both high organisation and excellent optical quality. TiO2 and ZrO2-based hybrids show thermal stability up to 300-350 ~ The elimination of the template can be conducted efficiently and gives rise to high surface area mesoporous films. For the other metal oxide hybrids the inorganic framework is much more fragile, and require a precise sequence of post-treatments to be stabilised.
1. INTRODUCTION Hybrid organic-inorganic nanocomposites are extremely versatile in composition, processing, and optical and mechanical properties [ 1]. One of the most striking examples of those composites are the mesostructured hybrid networks, which are precursors to mesoporous solids [2]. The construction of these networks can be tailored by the adequate use of sol-gel methods, tuning the hydrolysis-condensation of the metallic cations with the selfassembly of the organic counterparts. A great deal of work has been focused on silica-based materials, which can be processed as powders, films or fibers, displaying a great mesostructural variety [2, 3]. On the other hand, transition metal (TM) oxide-based mesostructured hybrids have been less explored. This is mainly due to the higher reactivity of TM relative to Si, so the retarding of the condensation process is essential in these synthesis procedures. The evaporation-induced self assembly (EISA) [4] process represents an
236 alternative way for the design of such mesostructured hybrids. Besides being very effective for the preparation of silica-based materials [3], this process is specially adequate for the design of TM-based mesostructured hybrids, since its principle is based on the dilution of the system with an organic solvent (usually an alcohol), which can contribute to the reduction of the reactivity of TM towards hydrolysis and condensation. The preferential evaporation of such a solvent results in the concentration of the system in non-volatile species (surfactant, inorganic precursor) and water. By the adequate set up of the parameters involved one can tune the self assembly of the amphiphilic molecules in supramolecular templates, together with the condensation of the inorganic moieties around them. Recently, we communicated the preparation and characterisation of titania and zirconia mesoporous films by EISA, using poly(ethylene oxide)-based non-ionic surfactants or block copolymers as structure directing agents [5, 6]. Here, we describe the application of a combination of Syncrotron-SAXS and interferometry analyses to evaluate the mesostructure formation during dip-coating. Furthermore, the variables involved in the deposition method (surfactant and acid concentration, relative humidity), which are crucial for the formation of the mesostructure and for the optical quality of the films, were investigated. In addition, we demonstrate that the used method can be extended to the formation of mesostructured hybrid films based on other transition metals. Finally, the sequence of treatments applied to stabilise the mesophases and remove the template is described for the different systems.
2. MATERIALS AND METHODS 2.1. Film formation Anhydrous metal chlorides (TIC14, ZrC14, WE14, or FeC13) were used as inorganic precursors. Non-ionic poly(ethylene oxide)-based surfactants [C16H33(EO)nOH] Brij 56 (n = 10) or Brij 58 (n = 20), or triblock copolymer Pluronic F127 [OH(EO)106(PO)70(EO)106OH] (PO = propylene oxide, kindly donated by BASF AG) were applied as structure directing agents. In a typical preparation, the inorganic precursor was added to a solution of the template in anhydrous ethanol at 0 ~ To this final mixture a controlled quantity of water was added. The final dipping solutions contain 1 MClx : t template : 40 EtOH : h H20 (where t = 0.15 Brij 56, 0.05 Brij 58 or 5• -3 Pluronic F127; and h = 0-20). Films were prepared by dip-coating glass or silicon substrates at a constant withdrawal rate ranging from 1 to 5 mm s-1 at room temperature (20-25 ~ The relative humidity (RH) inside the dip-coater chamber was controlled between 10 and 80%. The as-prepared films were submitted to a sequence of treatments in order to stabilise the structure and remove the surfactant, which varied according the system, but the most used procedure may be describe as follows: drying overnight at low humidity (< 10%), 12 hours at 60 ~ and calcination in air or N2 at temperatures ranging from 150 to 500 ~ for 2 hours. In some cases a treatment combining 03 and ultraviolet light (using a Bioblock Scientific VL-215 G equipment) was used for removing residual organic compounds. 2.2. Characterisation X-Ray Diffraction (XRD) diagrams were collected in the 0-20 mode using a conventional goniometer PW 1820 Philips (Cu-K~ radiation). 2D-XRD patterns were collected in transmission mode using an equipment described elsewhere [7], for films deposited in 8 lam
237 thick silicon substrates. TEM, SEM coupled to EDS, and FT-IR spectrophotometry investigations were performed as described before [6]. In-situ structural evolution of films during dip-coating has been performed in transmission mode using the synchrotron radiation 0~ = 1.541 /k) of the Austrian SAXS beam-line of ELETTRA (Italy). The diffracted X-rays were collected on a CCD camera every 2 s. Simultaneously, the advancement of the evaporation and thus the thickness evolution was followed by interferometry. Better details on such an experiment involving both techniques have been published somewhere else [8].
3. RESULTS AND DISCUSSION 3.1. Time resolved SAXS and interferometry in-situ investigation For the studied systems, upon dip-coating, the evaporation of the solvent is mainly governed by h and RH, the nature of the metallic cation having a small influence; much faster evaporation rates are observed for "dry" systems. In fact, water plays many roles, influencing the evaporation rates, the selective evaporation of EtOH and HC1 due to the azeotropic mixtures EtOH/H20 and HC1/H20, the polarity of the medium and the condensation rate of the inorganic species [9]. Besides, a quite generalised process of film formation was observed for the different systems. A representative example is shown in Figure 1.
Figure 1. Time resolved Synchrotron-SAXS and interferometry analyses of a Brij 58templated TiO2-based mesostructured film [10]. Conditions: h = 10, RH = 45%.
238 Five regions could be identified during the evaporation process. The fast decrease in the film thickness observed in the Region 1 can be assigned to the preferential evaporation of ethanol [8]. In Region 2, most of the solvent evaporates, and mesophases were not detecfed. Few seconds after the drying line (Region 3), when the film reaches practically its final thickness, a broad diffraction ring can be observed, which is characteristic of a worm-like phase [7]. Even though at this point only residual amounts of solvent are still lying in the film, the structure displays high mobility, and during the Region 4 the micellar aggregates aligns (within the film/substrate and film/air interfaces) resulting in a phase transition from wormlike to 2D-hexagonal, as evidenced by the defined diffraction spots in the SAXS patterns [7]. In the Region 5, residual amounts of solvent and HC1 depart from the film, allowing condensation, which in turn results in a contraction of the structure. Due to the adhesion of the film to the substrate, this contraction is preferential in the direction normal to the substrate. As a result, the distortion of the 2D-hexagonal structure (space group p6m) leads to a 2D-centred rectangular one (space group c2m). For simplicity, as the 2D-hexagonal structure can be described using the parameters of the 2D-centred rectangular one, the last will be used. The sequence of these events varies slightly depending on the nature of the metallic cation involved. For ZrO2-based hybrids, the structure is "frozen" in the Region 3, probably as a result of the formation of a relatively rigid ZrO2_xClx.nH20 framework. In this case, a postprocessing is necessary to re~ich the organised 2D phase (see below) [6]. For VOx-based systems, the intermediate worm-like phase was not detected, probably as a function of a very fast transition from the isotropic solution to the organised 2D-mesophase. The size of the domains for as-prepared films is mainly dependent on the template (Figure 2a and b). Brij 56 or 58-templated coatings shows interplanar distances (d(02) and d(ll)) between 60 and 70/k, corresponding to interpore distances between 69 and 80/k, depending on the humidity. Pluronic F127-tempiated films show much larger domains, with interplanar distances between 130 and 150 ,~, also dependent on the relative humidity.
Figure 2.2D-SAXS patterns acquired for as-prepared (a) Pluronic F127-templated TiO2-based and (b) Brij 58-templated VOx-based hybrid films. The inset in part (a) is a scheme of the 2Dcentred rectangular unit cell (c2m). (c) TEM image along the [001] zone axis of a Brij 58templated ZrOz-based mesostructured hybrid treated at 60 ~ Insets in part (c) are 4 times magnified image and moduli of Fourier transform of the indicated region. TEM analysis confirms the structure. In Figure 2c one can visualise long range organised, hexagonally distributed pores, which are slightly elliptical, indicating a small degree of
239 structural contraction after treatment at 60 ~ The 2D-XRD pattern acquired for such a film showed well defined (02) and (11) spots, and a diffraction ellipse of low intensity [6]. The presence of such an ellipse, that was not observed in the SAXS patterns of as-prepared ZrO2based films, indicate that the thermal treatment results in a partial degradation of the structure.
3.2. The role of acidity and water: structural organisation
versus
optical quality
The solutions prepared from the metal (Ti, Zr, V) chlorides are very acidic. Varying h and the humidity inside the dip-coater chamber allow to tune the degree of organisation of the hybrid mesophase. For TiO2-based systems, "dry" conditions conduct to very low or no organisation. Even increasing h until 20, the organisation is still low for low RH (< 30%). The maximum organisation degree was reached for high RH and h between 10 and 15. These results indicated that the evaporation rate is a very important parameter. For "dry" systems (RH < 30%), the drying process takes place in less than 1 minute, while it can be very long for "wet" (RH > 50 %) systems, reaching 30 minutes. In Figure 3 the degree of organisation observed by XRD for heated films is plotted versus RH and h. The organisation attained during the formation process is kept after thermal treatment. For ZrO2-based systems [6], the profile of organisation is slightly different from that observed for TiO2-based system. The main difference lies in the behaviour in low humid conditions. A maximum of organisation is observed for films deposited from solutions with h = 20 and at RH = 10%.. Despite the high degree of organisation attained for TiO2-based systems in "wet" conditions, the films processed as such are of low optical quality, being translucent and presenting small white spots randomly distributed in the surface. However, high organised coatings presenting excellent optical quality can be formed by processing the film at low humidity (between 25 and 30%, with h = 10) and exposing such a film (just after the drying process) to a humid atmosphere (RH -~ 50%) for few (5-10) minutes. Films so obtained are transparent and SEM analysis confirmed that they are crack-free, presenting a completely homogeneous surface. A treatment at very high humidity (>80 %) for few seconds is necessary to produce high optical quality ZrO2-based mesostructured films [6]. VOx-based hybrid films can only be processed at very low humidity. The coatings quickly absorb water from high humid atmosphere, first swelling (with a high increase of the interplanar spacings) and then collapsing after some hours. This feature can be attributed to the presence of vanadyl species in solution (detected by ESR, data not shown) and in the deposited films (detected by FT-IR, intense band at about 990 cm -1, characteristic of the V=O bond). These species have low tendency to condense. The low stability of the amorphous vanadium oxide hydrates is well known. Confirming the low condensation of the vanadyl ions, the EDS Figure 3. Plot of the (200) XRD peak analysis of a fresh film showed the presence of intensity for Brij 58-templatcd TiO2- 0.9 C1/V. The low degree of condensation seems based mesostructured films as a function to make the structure mobile, and high degrees of of h and RH after thermal treatment at organisation can be attained for films deposited 220 ~ in air for 2 hours from solutions with h between 3 and 10 (see
240 6
Figure 2b). All processing of such films must be conducted at low humidity (< 30 %). As-prepared films are of excellent optical quality, being (002) ~. 4 21.1 A transparent and displaying a slight blue colour. After ageing overnight at low humidity, the films display a green coloration, indicating the partial oxidation of V(IV) to V(V) species. 2 (003) "~ 14.2A For iron oxide-based systems, low humid A 1(o~176 (005) conditions are also necessary. As-prepared films absorb water from the atmosphere, loosing o i i i i i immediately the organisation. A treatment at 60 ~ 2 4 6 8 10 12 2e/degrees for 2 days is enough to produce stable coatings. Following the conditions described in the Section Figure 4. XRD pattern (0-20 mode) for a 2, no mesophases were obtained. However, the Brij 56-templated iron oxide-based decrease of the pH (by the addition of 1 HC1/Fe, hybrid film displaying lamellar structure. with h = 4-10) resulted in the formation of mesostructured lamellar phases. A XRD diagram for such phase is shown in Figure 4. The high degree of organisation for TiO2, ZrO2 and VOx-based systems attained for very acidic solutions and the formation of mesophases of iron oxide-based only after the acidification of the solution, indicate that the assembly of the mesostructured transition metal oxide organised by non-ionic poly(ethylene oxide)-based templates occurs by an (S~ +)(X-I +) pathway, as previously proposed for silica-based materials [11].
3.3. Stabilisation and removal of the template TiO2 and ZrOz-based hybrids can be directly thermally treated for elimination of the template, displaying stability up to 350 ~ For these materials, the thermal treatment results in a uniaxial contraction in the direction normal to the substrate [5-7]. As an example, the pores of a Brij 58-templated ZrO2 film are almost circular with diameter around 35 A after treatment at 60 ~ (see Figure 2c), becoming elliptical with dimensions of about 35• after treatment at 300 ~ (see Figure 5a). The wall thickness also decreases with the thermal treatment, from 35-37 A at 60 ~ to approximately 20 A after 300 ~ Pluronic F127templated films display bigger pores and thicker pore walls (Figure 5b) than those templated by Brij surfactants. Titania mesoporous films treated at 350 ~ present semicrystalline walls, as observed by HRTEM (see inset in Figure 5b). After the treatment at 300-350 ~ residual carboxylates are still existing in the coatings, as evidenced by FT-IR. Therefore, a combination of ultraviolet light and ozone (UV/O3) treatment was applied to convert the coatings in pure mesoporous titania or zirconia. The thickness of these films after treatment was determined by ellipsometry to be around 400 nm for titania [5] and estimated by SEM to be around 700 nm for zirconia [6]. Nitrogen adsorption measurements for Brij 58-templated ZrO2 mesoporous films (treated at 220 ~ followed by UV/O3 treatment and detached from the substrate, 65 mg) showed an average pore diameter of 28 A and a surface area of 192 m 2 g-l, higher than that previously reported for a bulk zirconia, 150 m 2 g-l, which presented bigger pores (58 A) [ 12]. The stabilisation of the VO• hybrids can be carried out by drying at RH = 10% overnight, and thermally treating in N2 (with a 1 ~ min -~ heating rate) at 60 ~ for 4 hours followed by 2 hours in temperatures ranging from 160 to 250 ~ A strong preferential
241 contraction in the direction normal to the substrate was observed, reaching 57% (do-d/do) by 220 ~ (see Figure 5c). Hybrid films so-treated present stable mesophases with framework consisting basically of vanadium oxide (EDS analysis showed 0.05 C1/V). However, the elimination of the surfactant by UV/O3 treatment causes the collapse of the mesostructure, probably as a result of the oxidation of V(IV) to V(V). The thermal treatment of lamellar iron oxide-based hybrids at temperatures high enough to decompose the surfactant (higher than 150 ~ results in the collapse of the structure, which is expected for a layered material.
Figure 5. TEM images for (a) Brij 58-templated ZrO2 film treated at 300 ~ and (b) Pluronic F127-templated TiO2 film treated at 350 ~ (the inset is a HRTEM image showing the presence of anatase nanocrystallites). (c) low angle XRD patterns for Brij 58-templated VOxbased hybrids.
4. CONCLUSIONS We propose here a generalised method for the preparation of mesoporous transition metal oxide mesoporous thin films, that can be adapted for many different metal cations by the tuning of the conditions based on the specific chemical behaviour of the inorganic species. The in-situ analysis of the film formation during dip-coating contributed for a better understanding of the parameters governing the evaporation and condensation rates, and so of the formation of the mesostructure. A set of post-treatments has been developed to combine high structural organisation and good optical, as well as to stabilise the mesophase and remove the template. For TiO2 and ZrOz-based systems, the application of such methods resulted in the formation of high organised, crack-free coatings displaying pore diameters in the mesoporous range and high surface area. For VOx-based films further work is necessary to eliminate the template without collapsing the mesophase.
5. ACKNOWLEDGMENTS Financial support from the French Ministry of Research, CNRS, CNPq (Brazil, grant # 200635/00-0), CONICET and Fundaci6n Antorchas (Argentine Republic) are gratefully acknowledged.
242 REFERENCES
1. a) D. Loy (ed.), Special Issue on Hybrid Materials, Mater. Res. Soc. Bull., 26 (2001) 364. b) C. Sanchez and B. Lebeau, Mater. Res. Soc. Bull., 26 (2001) 377. c) C. Sanchez, G. J. de A. A. Soler-Illia, F. Ribot, L. Lalot, C. R. Mayer and V. Cabuil, Chem. Mater., 13 (2001) 3061. 2. a) C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. b) 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. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 3. a) N. K. Raman, M. T. Anderson and C. J. Brinker, Chem. Mater., 8 (1996) 1682. b) H. Yang, N. Coombs, I. Sokolov and G. A. Ozin, Nature, 381 (1996) 589. c) P. J. Bruinsma, A. Y. Kim, J. Liu and S. Baskaran, Chem. Mater., 9 (1997) 2507. d) N. Melosh, P. Lipic, F. S. Bates, F. Wudl, G. D. Stucky, G. H. Frederickson and B. F. Chmelka, Macromolecules, 32 (1999) 4332. e) D. Zhao, P. Yang, N. Melosh, J. Feng, B. F. Chmelka and G. D. Stucky, Adv. Mater., 10 (1998) 1380. f) D. Grosso, A. R. Balkenende, P.-A. Albouy, M. Lavergne, L. Mazerolles and F. Babonneau, J. Mater. Chem. 10 (2000) 2085. 4. C.J. Brinker, Y. Lu, A. Sellinger and H. Fan, Adv. Mater., 11 (1999) 579. 5. D. Grosso, G. J. A. A. Soler-Illia, F. Babonneau, C. Sanchez, P.-A. Albouy, A. BrunetBruneau and A. R. Balkenende, Adv. Mater., 13 (2001) 1085. 6. E.L. Crepaldi, G. J. A. A. Soler-Illia, D. Grosso, P.-A. Albouy and C. Sanchez, Chem. Commun. (2001) 1528. 7. M. Klotz, P.-A. Albouy, A. Ayral, C. M6nager, D. Grosso, A. Van der Lee, V. Cabuil, F. Babonneau and C. Guizard, Chem. Mater., 12 (2000) 1721. 8. a) D. Grosso, A. R. Balkenende, P.-A. Albouy, A. Ayral, H. Amenitsch and F. Babonneau, Chem. Mater., 13 (2001) 1848. b) C. J. Brinker, Y. Lu, A. Sellinger and H. Fan, Adv. Mater., 1999, 11,579. c) S. Cabrera, J. E1 Haskouri, J. Alamo, A. Beltran, D. Beltran, S. Mendioroz, M. D. Marcos, and P. Amoros; Adv. Mater., 1999, 11,379. d) M. H. Huang, B. S. Dunn and J. I. Zink, J. Am. Chem. Soc., 2000, 122, 3739. 9. G . J . A . A . Soler-Illia, E. Scolan, A. Louis, P. A. Albouy and C. Sanchez, New J. Chem., 25 (2001) 156. 10. D. Grosso, F. Babonneau, C. Sanchez, G. J. de A. A. Soler-Illia, E. L. Crepaldi, P.-A. Albouy, H. Amenitsch, A. R. Balkenende, A. Brunet-Bruneau, J. Sol-Gel Sci. Technol., (2001 ) in press. 11. a) Q. Huo, D. I. Margolese, U. Ciese, P. Feng, T. E. Gier, P. Sieger, R. Leon, P. M. Petroff, F. Schtith and G. D. Stucky, Nature, 368 (1994) 317. b) D. Zhao, Q. Huo, J. Feng, B. F. Chmelka and G. D. Stucky, J. Am. Chem. Soc., 120 (1998) 6024. 12. P. Yang, D. Zhao, D. I. Margolese, B. F. Chmelka and G. D. Stucky, Chem. Mater., 11 (1999) 2813.
otuu~e~ m aureate aclence anta ~atalysls 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
243
M e s o p o r o u s alumina as a support for hydrodesulfurization catalysts Jifi (~ejkal, Nad6~da Zilkovfil, Lud6k Kalu~a 2 and Miroslav Zdra~il 2 1j. Heyrovsk~ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolej~kova 3, CZ-182 23 Prague, Czech Republic, e-mail:
[email protected] Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, Rozvojovfi 136, CZ-165 02 Prague 6, Czech Republic New hydrodesulfurization catalyst based on molybdenum sulfide supported on mesoporous alumina is described. Mesoporous aluminas possessing pore sizes from 3.3 to 4.5 nm and surface areas between 400 and 450 mZ/g were synthesized using aluminium sec-butoxide, 1-propanol and stearic acid as structure directing agent. The hydrodesulfurization catalysts were prepared by conventional impregnation using ( N H 4 ) 6 M o 7 0 2 4 and with the loading of 15 and 30 wt. % of molybdenum oxide. The catalyst with loading of 30% of molybdenum oxide exhibited almost twice higher catalytic activity in the model reaction of thiophene compared to conventional hydrotreating catalyst containing 15% MOO3. In contrast to conventional y-alumina, significantly higher loading of MoO3 can be achieved with mesoporous aluminas, which results in a significantly higher thiophene conversion. In addition, it was shown that the resulting hydrodesulfurization activity strongly depends on the preparation procedure of the final catalyst. 1. INTRODUCTION The first successful synthesis of mesoporous materials in 1992 by Mobil researchers [ 1] represents one of the most exciting discoveries in the field of material science in the last decade of the twenties century, which opened a new area in the synthesis and further investigation of properties and possible applications of this new type of molecular sieves. These mesoporous molecular sieves posses extremely high surface areas (> 1 000 mZ/g) and significantly larger but well-defined pore sizes in the range of 2.0 - 30 nm exhibiting relatively narrow pore size distribution [2]. Especially the pore sizes overcame the severe limitation of catalytic applications known for traditional zeolite based molecular sieves with their pore sizes smaller than 1.0 nm. Although at first all-siliceous and aluminosilicate mesoporous molecular sieves were synthesized, many mesoporous molecular sieves of other chemical compositions like aluminophosphates, vanadophosphates, zirconia or various metal sulfides were recently synthesized. This required to employ different synthesis approaches and sometimes also
244 other structure directing agents [2,3]. In addition, these mesoporous molecular sieves attracted a particular attention as possible catalysts or catalyst supports [4,5,6]. Recently a new family of mesoporous aluminas, exhibiting a narrow pore size distribution of pore sizes from 2.0 to 10.0 nm and surface areas in the range of 250-700 mZ/g, has been successfully synthesized. Triblock copolymers (e.g. Pluronic 123 or Triton X-114) were used by Zhang and Pinnavaia [7] or P6rez-Pariente et al. [8] as structure-directing agents. The anionic route employing long-chain carboxylic acids like lauric or stearic acid was also successful as it was demonstrated by Vaudry et al. [9] and (~ejka et al. [10]. In addition, triethanolamine in combination with hexadecyl trimethylammonium bromide was used by Cabrera et al. [ 11 ]. As the surface areas of mesoporous alumina are significantly larger compared to conventional alumina supports, these molecular sieves seem to represent an attractive supports for a number of active phases for different catalytic reactions. Molybdenum oxide dispersed over various supports is very important catalyst or catalyst precursor in a number of industrially relevant reactions. For example, hydrodesulfurization of oil fractions represents probably the most important reaction in which alumina is employed as a support for MoO3 as the precursor of Mo, CoMo and NiMo sulfide catalysts. Alumina supported molybdena catalysts belong to the class of so-called monolayer catalysts, which means that the amount of molybdena on alumina increases with increasing surface area of alumina. The active sulfide species should be finely distributed on an alumina support and it is evident that the textural parameters of the support (surface area, pore size distribution) influence significantly the catalytic behavior of these catalysts. The objective of this contribution was to synthesize mesoporous alumina with a high surface area and pore size at least of 3.0 nm and to modify this molecular sieve by conventional impregnation method for hydrodesulfurization of thiophene as the model reactant. 2. MATERIALS AND METHODS 2.1. Materials The following reagents were used for the synthesis of mesoporous aluminas: stearic acid (Aldrich), 1-propanol (Fluka), aluminium sec-butoxide (Aldrich). 2.2. Synthesis of mesoporous alumina The synthesis of organized mesoporous aluminas, MA, was carried out in the following way: 5.1 g of stearic acid, which was used as a structure directing agent, was dissolved in 80 ml of 1-propanol and a small amount of distilled water (added to efficiently hydrolyse the source of aluminium). This mixture was stirred for 30 min followed by an addition of 13.7 g of aluminium sec-butoxide. The resulting gel was transferred after stirring for another 30 min into 90 ml Teflon-lined stainless steel autoclaves. The synthesis was performed at 100 ~ for 48 hours under static conditions. After the synthesis, the autoclaves were cooled down and the solid product was recovered by filtration, washed thoroughly by ethanol and dried at 60 ~ overnight. The calcination was carried out at first in a stream of nitrogen at 410 ~ for 2 hours
245 (temperature increase - 0.5 ~ MA2, respectively).
followed by 10 hours at 420 or 440 ~ (MA1 and
2.3. Catalyst preparation The modification of organized mesoporous aluminas MA1 and MA2 was carried out via conventional impregnation method. Alumina support was mixed with the solution of the corresponding amount of ammonium heptamolybdate and was left standing for about 60 minutes at ambient temperature. The mixture was dried at 50 or 100 ~ The catalysts were not calcined. For comparison as the reference MoO3/A1203 catalyst of BASF M8-30 was employed. This catalyst is industrially applied in the hydrorefining of crude benzene by the BASF-Scholven process. It contained 15 wt. % of MOO3. 2.3. Instrumentation Nitrogen adsorption isotherms of calcined mesoporous aluminas were measured a t 196 ~ with a Micromeritics ASAP 2010 volumetric adsorption instrument. The ASAP 2010 was equipped with 133 kPa transducer. Before the measurements, all samples were degassed at 300 ~ for at least 24 h. X-ray powder diffraction (XRD) patterns of as-synthesized, calcined and modified mesoporous aluminas were recorded on a Siemens D 5005 diffractometer in the Bragg-Brentano geometry arrangement with CuK~ radiation (k - 0 . 15418 nm). 2.4 Hydrodesulfurization The catalytic activity of molybdena catalysts supported on mesoporous alumina was tested in a hydrodesulfurization of thiophene (TH), which was carried out in the gas phase using a fixed bed flow reactor with a total pressure of 1 MPa. The feed rate of thiophene was 0.43 mmol/h while feed of hydrogen was 1.10 mol/h. Prior to the catalytic test the catalyst was sulfided under "in-situ" conditions with a mixture of H2S and hydrogen with molar ratio 1 : 10 under atmospheric pressure at 400~ for 1 h. The rate of temperature increase was either 6 or l~ The thiophene conversion was measured in the temperature range between 250 and 400 ~ with the temperature steps of 30 ~ and was calculated in the following way: xvH = (n~ - nxH)/n~ where n0TH and nvH represent the initial and final number of moles of thiophene, respectively. The conversion of thiophene was checked by gas chromatography (HP 5890 Series II). The water content in the catalysts depended on the last temperature treatment during catalyst preparation and was determined by the calcination of a part of each sample at 400 ~ for 1 hour in the flow of air. The relative catalytic activity was normalized in two ways, i) to catalyst weight W subtracting the content of water defined above and ii) to mol of molybdenum. 3. RESULTS AND DISCUSSION
3.1. Characterization of mesoporous aluminas The mesoporous alumina samples used in this study as a support for molybdenum oxide were synthesized with stearic acid as a structure directing agent. In contrast to results of Vaudry and coworkers [9] it seems based on our results that pore size of mesoporous aluminas can be varied (at least in some extent) with the length of the hydrocarbon chain of the relevant carboxylic acid [10]. Fig. 1 (left side) presents the X-
246 ray diffraction patterns of mesoporous aluminas MA1 and MA2 in the range of 1-10 of two theta. Both samples of mesoporous aluminas exhibit only one diffraction line at very low angle of two theta. These data are consistent with other studies on mesoporous aluminas showing that the pore ordering of calcined mesoporous aluminas is always worse in comparison with organized mesoporous silica. However, it is necessary to stress that although mesoporous aluminas exhibit only one diffraction line, it is without any doubt that, based on nitrogen adsorption isotherms, these materials are not layered but they form uniform pores. Some comparison with e.g. silica samples called KIT-1 can be expected as these molecular sieves exhibit only three broad diffraction peaks which indicate a short-range structural ordering with very uniform pore sizes [ 12]. Fig. 1 (right side) displays the adsorption isotherms of nitrogen for mesoporous aluminas MA1 and MA2. Both of these adsorption isotherms are characterized by a relatively steep increase in the adsorbed amount which starts at about p/p0 = 0.35 and 0.5, respectively and finishes at p/p0 = 0.5 and 0.65, respectively. Then the adsorption isotherms exhibit practically horizontal plateau up to the relative partial pressure of 0.95. This nearly horizontal plateau indicates that these mesoporous aluminas do not posses any pores lar~er than 8 nm. Mesoporous alumina MA1 exhibited the BET surface area of 445 m / g while MA2 397 m2/g. The pore size of MA1 was 3.3 nm and that of MA2 was 4.5 nm. Although the same synthesis procedure was used for the preparation of these two samples, the difference in the surface area is probably connected to the slightly different calcination procedure. In contrast to mesoporous silicas of MCM-41 type, mesoporous aluminas are extremely sensitive to the calcination procedure and relatively small differences in calcination procedures could result in large differences in textural parameters [ 10].
/
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2 4 6 8 2 theta (degree)
-
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,~
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z
relative pressure P/Po
X-ray diffraction patterns (left side) and nitrogen adsorption isotherms (right side) ofMA1 (A) and MA2 (B), respectively.
247
3.2. Modification od mesoporous aluminas The molybdenum oxide catalysts supported on mesoporous alumina were prepared by the conventional impregnation of mesoporous alumina with ammonium heptamolybdate. The natural pH of ammonium heptamolybdate is about 7.0 which seems to be the appropriate value for modifying mesoporous aluminas. Our further experiments (not presented here) have shown that in the case of slurry impregnation with molybdic acid when the pH is about 2.6, mesoporous aluminas are not sufficiently stable and they partially collapse. This leads not only to the substantial decrease in BET surface area (at least 50 % decrease) but also to the significantly lower hydrodesulfurization activity of this catalyst [ 13 ].
250 13.. I'Or')
200
E
d E 150 0 >
100 !,,,.,.
0
50 Z
I
0.0 Figure 2.
i
w
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i
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i
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0.4 016 0.8 1. Relative pressure P/Po
Nitrogen adsorption isotherms of parent mesoporous alumina MA1 (e), MA1 modified with 15 wt. % of MoO3 (A) and MA1 modified with 30 wt. % of MoO3 (0).
However, when the modification is properly carried out by conventional impregnation procedure the situation is completely different. As it is depicted in Fig. 2 for parent mesoporous alumina MA1 and that one modified with 15 and 30 wt. % of molybdenum oxide, the changes in the surface area and in the adsorbed volume are not so dramatic and in the case of MA1 modified with 15 wt. % of molybdenum oxide the surface area is even slightly higher (BET = 475 mZ/g). After the modification with 30 wt. % of molybdenum oxide the resulting surface area is about 402 m2/g.
3.3. Testing of hydrodesulfurization activity It is well-known that the catalytic activity in hydrodesulfurization reactions over molybdenum oxide supported on conventional aluminas strongly depends on the amount and form of molybdenum oxide which can be dispersed in the form of monolayer over alumina support. Conventional aluminas posses surface areas in the range of 150-250 m2/g and this surface areas enable to disperse of about 15 wt. % of
248 molybdenum oxide. Further increase in the amount of molybdenum oxide leads to the formation of bulk species where only the surface parts are active in hydrodesulfurization reactions. Such catalysts exhibit activities comparable to those having only 10-15 % of molybdenum oxide. Thus, it is evident that mesoporous aluminas offer a very interesting advantage due to their significantly higher surface areas. It is expected that considerably higher activity can be found with mesoporous alumina based catalysts because of significantly larger amount of molybdenum oxide. Fig. 3 depicts the dependence of thiophene conversion in the temperature range between 250 and 400 ~ for commercial BASF catalyst (15 % of MOO3) and two samples of mesoporous alumina modified with 30 % of MoO3 differing in support (MA1 or MA2), temperature of drying (50 or 100~ and temperature ramp during sulfidation (1 or 6 ~ It is evident that the rating of these catalysts is independent on the reaction temperature. While the thiophene conversion over BASF catalyst is
100 80-
X
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60-
I
250
'
'
'
'
I
300
'
'
'
'
I
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'
'
'
'
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temperature,~ Fig. 3. Temperature dependence of thiophene conversion for a commercial BASF catalyst (11), MA1 modified with 30 wt. % of MOO3, dried at 100~ sulfided with the ramp 6~ (O) and MA2 modified with 30 wt. % of MOO3, dried at 50~ sulfided with the ramp l~ (A). about 40 % at 400 ~ both mesoporous alumina based catalysts exhibit higher conversion reaching almost 70 % for MA2 catalyst with 30 wt. % of MoO3 prepared under drying at low temperature and with slow sulfidation. For comparison of the individual catalysts, reaction temperature of 370 ~ and the pseudo-first order rate constants calculated from thiophene conversion at this temperature were used. In our previous work [ 14] we have shown the validity of the pseudo-first order kinetics under our experimental conditions. The relative rate constants are normalized to the catalyst weight, kw, or to the mol of molybdenum, kMo, with BASF catalyst as the reference.
249 It was assumed that similar hydrodesulfurization activity should be achieved over the commercial BASF catalyst and that one prepared with mesoporous alumina but possessing the same amount of molybdenum oxide present in the form of monolayer. This was really achieved as it is given in Table 1 that the pseudo-first rate constants for commercial and our catalyst containing 15 wt. % MoO3 are practically the same (kw = 1.00 and 1.04 and kMo = 1.00 and 1.04, respectively). The main difference between two these catalysts is that although for commercial catalyst the loading is close to the complete monolayer, with mesoporous alumina exhibiting much higher surface area this loading is significantly lower compared to the possible monolayer. However, it is also seen from Table 1 that substantial increase in the activity was achieved increasing the loading of mesoporous alumina to 30 wt. % MOO3. The catalyst over alumina MA2 containing 30 wt. % MoO3 exhibited almost twice higher activity kw than the reference catalyst containing 15 wt. % MOO3. The relative activity kMo of this mesoporous alumina catalyst was close to unity (0.9) indicating that mesoporous alumina was able to disperse 30 wt. % of MoO3 in active form comparable to the form in the reference catalyst. The area per one molybdenum atom at filled monolayer is estimated in the range of 0.20 to 0.30 nm2/g which suggests that surface areas between 417 and 625 mZ/g are necessary to disperse about 30 wt. % of molybdenum oxide in the form of monolayer. It is evident that these values were achieved with mesoporous aluminas under investigation. Table 1 Surface areas and relative HDS activities of catalyzStS investigated Catalyst MoO3 (wt. %) BET area (m/g) kw BASF 15 210 1.00 MAla(719) 15 475 1.04 MAla(719) 30 402 1.48 MA2b(743) 30 423 1.80 a Dried at 100~ sulfided with the ramp 6~ b Dried at 50~ sulfided with the ramp 1~
kMo 1.00 1.04 0.74 0.90
4. CONCLUSIONS Mesoporous aluminas synthesized in 1-propanol from aluminium sec-butoxide using stearic acid as structure-directing agent possessing surface areas between 400 and 450 mZ/g and pore sizes 3.3 and 4.5 nm were used as a support for the preparation of hydrodesulfurization catalysts. Conventional impregnation procedure using ammonium heptamolybdate was used for the preparation of the catalysts. No crystalline phase of molybdenum oxide was found which indicates that significantly higher amount of this oxide can be spread on the walls of mesoporous alumina. The way of preparation (drying under mild conditions, no calcination, slow sulfidation) of the final catalyst is a crucial step in obtaining highly active catalyst.
250 The activity in thiophene hydrodesulfurization of this new type catalysts normalized to catalyst weight was significantly higher in comparison with the reference commercial catalyst possessing 15 wt. % of molybdenum oxide. This is due to the ability of mesoporous alumina to disperse in the form of monolayer at least two times higher amount of the active phase (min. 30 wt. % of MOO3) as compared to conventional alumina. 5. ACKNOWLEDGEMENT The work of J.(~. and N.Z. was kindly supported by the grant of NATO (Science for Peace - 974 217) and by the Ministry of Education, Youth and Sport (ME 404/2000). L.K. and M.Z. gratefully acknowledge financial support by the Grant Agency of the Czech Republic (grant No. 104/01/0544). REFERENCES
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3.
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5. 6. .
8. .
10. 11. 12. 13. 14.
C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature 359 (1992) 710. J.C. Ying, C.P. Mehnert and M.S. Wong, Angew. Chem. Int. Ed. 38 (1999) 56. G.D. Stucky, Q. Huo, A. Firouzi, B.F. Chmelka, S. Schacht, I.G. Voigt-Martin, F. Schfith, in: Progress in Zeolites and Microporous Materials (H.Chon, S.-K. Ihm, Y.S. U h - Eds.), Stud. Surf. Sci. Catal. 105 (1997) 3. A. Sayari, Chem. Mater. 8 (1996) 1840. A. Corma, Chem. Rev. 97 (1997) 2373. J. (~ejka, A. Krej~i, N. Zikov~., J. D6de~ek and J. Hanika, Micropor. Mesopor. Mater. 44-45 (2001) 499. W. Zhang and T.J. Pinnavaia, Chem. Commun. (1998) 1185. V. Gonzalez-Pena, I. Diaz, C. Marquez-Alvarez, E. Sastre and J. P6rez-Pariente, Micropor. Mesopor. Mater. 44-45 (2001) 203. F. Vaudry, S. Khodebandeh and M.E. Davis, Chem. Mater. 8 (1996) 1451. J. (~ejka, N. Zilkov/l, J. Rathousk2~ and A. Zukal, Phys. Chem. Chem. Phys., 3 (2001) 5077. S. Cabrera, J. E1 Haskouri, J. Alamo, A. Beltr/m, D. Beltr~in, S. Mendioroz,M.D. Marcos and P. Amor6s, Adv. Mater. 11 (1999) 373. R. Ryoo, J.M. Kim, C.H. Ko and C.H. Shin, J. Phys. Chem. 100 (1996) 17718. L. Kalu~a, M. Zdra~il, L. Vesel~i and J. Cejka, in preparation. E. Hillerov/l and M. Zdras Appl. Catal. A 138 (1996) 13.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
251
Preparation and X A F S spectroscopic characterization o f m e s o p o r o u s titania with surface area m o r e than 1200 m2/g Hideaki Yoshitake a, Tae Sugihara b and Takashi Tatsumi b aGraduate School of Environment & Information Sciences, Yokohama National University, Yokohama 240-8501, Japan bGraduate School of Engineering, Yokohama National University, Yokohama 240-8501, Japan
Mesoporous titania with BET surface area over 1200 m2.g"l was synthesized by primary amine templating followed by acidic extraction of the template. Dodecyl-, tetradecyl- and hexadecylamines gave intense single XRD patterns at 20 = 2-3~ reflections for higher indices, suggesting the wormhole structure. The pore size changed according to the length of template molecule. The chemical vapour deposition (CVD) of titanium tetraisopropoxide followed by its decomposition with water vapour was carried out in template-extracted titania and the improvement in thermal stability was evaluated by the decrease in S~Er by the calcination. The local structure of Ti was investigated by X-ray absorption spectroscopies. The K-edge EXAFS spectra measured at various temperatures demonstrated that large atomic vibrations in mesoporous titania are considerably suppressed in the powder before removing template. The preedge peaks in the XANES region showed the coexistence of 5- and 6-coordinated Ti. The ratio of intensity of these peaks was calculated to compare the degree of framework destruction.
1. INTRODUCTION Amphiphilic template directing synthesis of mesoporous oxide, originally reported on silica by Mobil scientists [ 1], was soon adapted to syntheses of the transition metal oxides. These non-silica oxides with ordered pore structures were expected to be useful for catalytic supports, electronic materials, optical devices etc. Titanium dioxide is a material of great interest for its many applications. Although a stable mesoporous titania was prepared by phosphate templating method, a considerable amount of phosphorus remained even after the calcination [2]. Antonelli has successfully synthesized mesoporous titania using primary amines as structure directing agents [3]. These surfactant templating syntheses provide periodic mesostructures with a uniform pore size in addition to a large surface area. Although these characteristics are probably preferable for most of the applications, the required physical properties depend on particular applications. A large surface area is necessary in most cases while it is usually accompanied by high surface reactivity and, consequently, such materials lack thermal and chemical stabilities. BET surface area of mesoporous TiO2 was 710 m2-g-1 but a heat treatment in dry air at 623 K led to the surface
252
area decrease by ca. 50 m2-gl/h [3]. This instability in air will restrict the industrial applicability. It has been widely recognized that MCM mesoporous silicas are more or less unstable in moist environment. The structure of MCM-48 and MCM-41 was collapsed by mechanical compression through in the presence of adsorbed water [4, 5]. These phenomena are likely caused by defects and/or hydroxyl groups on the surface, which initiate the destruction of the wall structures. Trimethylsilylation of MCM-41 improved the stability in moisture and under compression [6]. This was explained by the enhancement of hydrophobicity which suppressed the concentration of water in the pore and the adsorption of water. The other reason could be the block of surface active sites by an inactive group (-CH3). It has been recently reported that, when partially oxidized cetyltrimethylammonium glycotitanate complex is treated with Si2H6, the mesostructure is reinforced so that the template extraction is possible to give mesoporous TiO2 (or perhaps silicotitanate) [7]. This method is suggestive of the stabilization of the unstable mesoporous TiO2 by masking the active site with inert surface species. We report in this paper the synthesis of primary amine-directed mesoporous titania with BET surface area over 1200 m2.gl. Since a good thermal stability is not expected for the oxide with such extremely high surface area, chemical vapour deposition of a Ti compound was carried out to keep the chemical composition unchanged. The surface area after calcination in air was improved by this CVD treatment. Finally, the local structure of Ti was analysed by XAFS spectroscopies. 2. MATERIALS AND METHODS 2.1. Materials Dodecylamine CI2H25NH2, tetradecylamine CI4H29NH2, and hexadecylamine C16H33NH2 were purchased from Tokyo Kasei Kogyo Co., Ltd. Titanium tetraisopropoxide Ti(Oipr)4 (Cameleon Reagent, purity > 99 %) and p-toluenesulphonic acid (Tokyo Kasei Kogyo Co., Ltd. purity > 99%) were commercially available. The reagents were used without further purification. 2.2. Synthesis A typical procedure was as follows. 50 g of water was added to a mixture of titanium tetraisopropoxide (10.0 g) and dodecylamine (3.3 g) at 273 K. After the addition of 0.1 M HC1 (2.0 cm3), the mixture was allowed to stand overnight at room temperature and transferred to a Teflon container in an oven at 333 K. After 96 h, the solution was filtered and washed with methanol and diethyl ether. The white solid collected was dried at 373 K for 24 h, transferred into a pyrex test tube, which was evacuated at 453 K. After evacuation for 2 h, the tube was sealed. The powder in the tube was then heated at 453 K for 10 d with keeping the temperature at the other end between 273 and 373 K. The resulting solid from hydrothermal treatment in a reduced pressure is denoted as "thermally treated." This solid was treated with p-toluenesulphonic acid, to extract the template. The powder was then dried at 373 K. Chemical vapour deposition of titanium tetraisopropoxide was carried out in a flow reactor. Argon passed through liquid Ti(Oipr)4 was introduced to a pyrex tube containing the template-extracted powder kept at 353 K. After the deposition for 24 h, the gas was switched to moist nitrogen. The decomposition of deposited titanium alkoxide was completed by 12 h. The powder was finally treated in the flow of a dry air at 393 K for 2 h.
253
2.4. Characterization X-ray diffraction patterns of thermally treated and extracted powders were recorded using an XL Labo diffractometer (MAC Science Co., Ltd.) with Cu K~ radiation at 40 kV and 20 mA. Nitrogen adsorption-desorption isotherms were recorded by BELSORP 28SA (BEL Japan Inc.) after the sample was treated at 473 K in vacuo. X-ray absorption spectra were measured at BL-9A soft X-ray station of Photon Factory, High Energy Accelerator Research Organization, Tsukuba, Japan (Proposal #2000G269), with a ring energy of 2.5 GeV and a stored current around 300 - 450 mA. Si(lll) double-crystal monochrometer was used. The incident X-ray was focused and the higher harmonics were removed by the total reflection on a Rh-Ni composite mirror. All the measurement was carried out in a conventional transmission mode with gas ion chamber detections.
3. RESULTS AND DISCUSSION Figure 1 shows the X-ray diffraction pattems of mesoporous titania after template extractions. No diffraction patterns were observed above 20 = 10~ suggesting amorphous nature of the pore wall. A single intense peak without accompanied by reflections for higher indices implies a wormhole-like framework structure. The shift of the peak position according as the chain length of template molecule was observed. The structural parameters calculated from the data of XRD and nitrogen adsorption experiments are listed in Table 1. These amine templates directed narrow pore-size distributions in which the most probable diameter was observed at 2.6, 2.8 and 3.0 nm, respectively. The structure is sensitive to vapour pressure in the hydrothermal treatment prior to extraction of the template. The optimum pressure we found for SSET was 0.61 Pa. The BET surface area of TiO2 in an ordered structure changed from 940 to 1256 mZ/g according to the surfactants and the synthetic conditions. The wall thickness is 0 - 0.9 nm for dodecylamine templating if a degraded 2d-hexagonal structure was assumed. This thickness corresponds to from zero to a few atomic layers. On rutile(110) surface, which is the most frequently studied single crystal surface of titania, 5-coordinated Ti 4§ makes lines along with [001 ] direction in separating each other by 0.295 nm and the rows of oxygen atoms on the 6-coordinated Ti4§ situate among them [8]. The surface area ofrutile(ll0), an imaginary solid, is easily calculated with these structural data, i.e. aBET = 1440 m2/g. The observed surface area, 1256 m2/g, is 87 % of the "theoretical limit" of the surface area. The mesoporous TiO2 prepared by above templating method can be considered to be approximately an "all surface solid." The framework structure was presumed to be fragile so that we tried to stabilize the surface through blocking the reactive sites. Titanium tetraisopropoxide was deposited on mesoporous TiO2 which was templated with dodecylamine and heated at P(H20) = 3.33 kPa before the template removal (sample listed at the sixth column in Table 1). The deposited powder was treated by water vapour to decompose the alkoxide followed by dry nitrogen to remove generated 2-propanol. The peak position in XRD was changed slightly (d = 3.3 nm) after this CVD treatment. On the other hand, the peak intensity was not decreased significantly. Although the pore diameter was unchanged, a decrease in the BET surface area was observed (SBET = 890 m2"g1). Since the reduction of pore volume happened simultaneously, the deposited molecules likely blocked mainly the micropores on the wall. On the basis of the data from XRD and nitrogen adsorption, we conclude that CVD treatment results in little modifications in the chemical composition and the framework structure. The surface area was measured as a function of calcination time. The results are plotted in Figure 2. Untreated sample suffered from serious decrease by the calcination while the change in the surface area of treated TiO2 was much more moderate and retained more than 500 m2/g after calcination for 24 h. The decrease in pore diameter and the increase in
254
d-spacing were also more moderate in CVD-treated TiO2 than untreated one. These results demonstrate that CVD of titanium tetraisopropoxide improves the stability of the meso framework structure.
1200
'"
',,
looo C16
~'~,%~~~VD
800
a5
600 I,
C14
400 no CVD
wiB
200 C12 0
0
2
4
6
8
0
10
2 theta / o
Figure 1. XRD pattems of mesoporous titania synthesized with dodecyl- (C12), tetradecyl- (C14) and hexadecylamines (C16) templates.
I
I
10
20
t cat/11 Figure 2.
Decrease of BET surface area by
calcination in mesoporous titania with and without CVD.
T(calcination) = 573 K.
Table 1. d-Spacing, surface area, pore size andpore volume of mesoporous titania. amine Ti/amine T / K P(H20)/kPa d / n m SBer/m 2"81 2Rp/nm C12 2 453 0 3.11 1139 2.6 C14 2 453 0 3.59 1033 2.8 C16 2 453 0 3.74 942 3.0 C12 2 453 0 3.11 1139 2.7 C12 2 453 0.61 3.11 1256 2.9 C12 2 453 3.33 3.11 1150 3.1 C12 2 453 19.9 3.11 1158 3.0 C12 2 453 101 ..... 154 5.9 ---: Diffraction peaks were too broad to determine the position.
Vp I m 2- g-i 0.527 0.559 0.542 0.527 0.653 0.550 0.572 0.071 ....
It is well known that atoms on the surface are weakly bound and the thermal oscillations of surface chemical bonds are considerably larger than those in the bulk solid. If the wall of a mesoporous material is thin enough, we can detect the change in the vibration of the mesoporous framework induced by the template micelles in the pores or the adsorbates on the pore wall surface. These effects could offer the useful information on the interactions in the micelle-oxide complex because the vibration spectra cannot be changed unless the interaction at the template-solid interface and in the micelles are as comparably strong as in the oxide wall. In order to explore such vibrations, the local structure of Ti of this almost "all surface
255 titania" was investigated by Ti K-edge EXAFS spectroscopy. The spectra were measured at various temperatures and a part of the data is shown in Figure 3. The k3z(k) functions for TiO2 prior to and following template extraction agreed well at 50 K. At room temperature, the amplitude for the powder after template extraction was considerably suppressed while that before template extraction was changed little. A clear explanation for this difference is that the large atomic vibrations in "all surface titania" are suppressed by filling templates in the pores. The interactions aggregating the surfactant and that working at the interface between micelle and the pore wall surface are large enough to suppress the atomic vibrations in the TiO2 wall.
f e
d c
b a
4
6
8
10
12
k / 0.1 nni I Figure 3 Ti K-edge EXAFS oscillation of mesoporous titania. Thicker and thinner lines are measured at 298 and 50 K, respectively.
4950
i
i
4960 4970 4980 4990 5000 E/eV
Figure 4 Ti K-edge XANES of TiO2 in anatase (a), after heat treatment (b), template-extracted (c), after CVD (d), after calcined at 573 K (e) and after calcined at 673 K (f).
We further investigated the singularity in the local structure of Ti besides lattice vibrations. A relatively strong preedge peak appeared around 4968 nm in the XANES regions as shown in Figure 4. The s-d transition is forbidden by dipole selection rule but, when the s-p orbital mixing occurs in an absorbing site, the transition becomes allowed. The intensity and the position in the preedge absorption depend on the degree of p-d mixing as well as the oxidation state. In the comparison with the peak for anatase (Figure 4, a), the bandwidth was large and the shape is asymmetric in those for mesoporous titanias (Figure 4, b-f). Thus the peaks were deconvoluted into elemental peaks at 4967.6 and 4968.8 eV, which were assigned to 5-coordinated Ti and Oh (6-coordinated) one of anatase, respectively [9, 10]. The ratio of relative intensities of theses peaks are 1.3, 1.4, 1.2, 1.4 and 0.85 for TiO2 with template, after extraction, after CVD, after calcination at 573 K and after calcination at 673 K, respectively. The unchanged ratio of/(4967.6 eV)/I(4968.8 eV) demonstrates that the population of 5-fold Ti is not affected by template extraction, CVD and the calcination at 573 K even though the mesoporous framework is changed by those
256 treatments. It is to be noted that 5-coordinated Ti is rarely observed in a stable TiO2 and it is rather stable in mesoporous titania; it is less sensitive than the framework structure to the chemical and thermal processes. 4. ACKNOWLEDGEMENT The grant from the Asahi Glass Foundation is gratefully acknowledged for a partial support of this research. REFERENCES
1. C.T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck, Nature, 710 (1992) 359. 2. V.F. Stone, R. J. Davis, Chem. Mater., 10 (1998) 1468. 3. D.M. Antenolli, Microporous Mesoporous Mater., 30 (1999) 315. 4. T. Tatsumi, K. A. Koyano, Y. Tanaka, S. Nakata, Chem. Lett., (1997) 469. 5. R. Ryoo, J. M. Kim, C. H. Ko, C. H. Shin, J. Phys. Chem., 100 (1996) 17718. 6. K.A. Koyano, T. Tatsumi, u Tanaka, S. Nakata, J. Phys. Chem., B, 101 (1997) 9436. 7. D. Khushalani, G. A. Ozin, A. Kuperman, J. Mater. Chem., 9 (1999) 1491. 8. H. Onishi, Y. Iwasawa, Surf. Sci. Lett., 313 (1994) L783. 9. F. Farges, G. E. Brown Jr., J. J. Rehr, Geochim. Cosmochim. Acta, 60 (1996) 3023. 10. E Farges, G. E. Brown Jr., A. Navrotsky, H. Gan, J. J. Rehr, Geochim. Cosmochim. Acta, 60 (1996) 3039.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
M e s o p o r o u s zirconium o x i d e s ' a n
257
investigation o f physico-chemical synthesis
parameters J.L. Blin, L. Gigot, A. L6onard # and B.L.Su* Laboratoire de Chimie des Mat4riaux Inorganiques, ISIS, The University of Namur, 61, rue de Bruxelles, B-5000 Namur, Belgium A systematic kinetic study of mesoporous zirconia formation has been performed in order to optimize the synthesis conditions without addition of structure stabilizing agents such as sulfate or phosphate anions. We have investigated in particular the effect of synthesis time and temperature. On the basis of TEM, SEM XRD and N2 adsorption-desorption results, a synthesis mechanism has been proposed. Low temperature or short duration afford supermicroporous materials and continuation of hydrothermal treatment makes the walls separating adjacent pores break down allowing the transformation to mesopores. The obtained materials have a uniform pore size and their surface can reach 400mVg. However, if hydrothermal treatment is performed at too high temperatures or for too long durations, mesoporous compounds are no longer obtained, but thermodynamically more stable crystalline zirconium oxides with very low specific surface area, namely the tetragonal and monoclinic forms, are the final phases that are more likely to appear.
1. INTRODUCTION Since the discovery of MCM-41 in 1992 by Mobil scientists [1,2], numerous studies concerning the preparation conditions, synthesis mechanism, characterization and use of these materials as catalysts and catalyst supports in various reactions [3-6] have been reported. Indeed, pure siliceous hexagonal MCM-41 can be used in inclusion chemistry, for example for the encapsulation of conducting quantum wires, the silicate framework providing the insulding part of the device. The incorporation of metal atoms within the framework of silica mesoporous materials has expanded the field of applications of these mesoporousmolecular sieves reaching beyond the size-limitations imposed by microporous zeolites. For instance, silica MCM-41 doped with aluminum possess acidic catalytic sites and can be employed for the cracking of n-heptane or polyethylene [7]. Transition metal oxides, which induce basic, acidic or redox catalysis properties, can either be added to the synthesis mixture in the form of a metal alkoxide in order to copolymerize with the silicon alkoxide or be grafted or exchanged after the precipitation of the pure silica. In 1996, doped silica mesoporous molecular sieves with zirconium which are suitable catalysts for oxidation reactions such as that of cyclohexane or norbornylene by H202 and TBHP, have been reported [8]. The Zr-MCM-41 materials can also catalyze the decomposition of isopropanol, with a selectivity towards # :FRIA Fellow * : Corresponding author
258 propene reaching 99% [9]. Owing to their optical and electronic properties such as luminescence, scientists recently tried to develop the synthesis of pure mesoporous transition metal oxides [ 10-15]. Due to the large field of applications ranging from catalysis to ceramics, among the non-silica mesoporous oxides, zirconia are of particular interest. Zirconium oxides exhibit high thermal stability (up to 550~ in the phosphate stabilized form, the stabilization being conferred to the structure by treating the as-prepared compounds with H3PO4) and ion exchange properties [ 16, 17]. ZrO2 can catalyze or is a catalyst support for various reactions such as the catalytic reduction of aldehydes and ketones with 2-propanol or the hydrogenation of aromatic carboxylic acids [ 18]. At high temperature, zirconium oxide becomes a superionic conductor and can be employed in oxygen sensors and as solid electrolyte in high temperature fuel cells. To develop particular catalytic properties such as high conversion and selectivity, the synthesis of mesoporous ZrO2 with high specific surface area and narrow pore size distribution is of great importance both from scientific and industrial point of view. In 1995, Hudson et al. [19] reported for the first time the synthesis of porous zirconia, using alkyltrimethylammonium halide as surfactant and zirconyl chloride as zirconium source. Then using amphoteric [20], anionic [21], or neutral [22] surfactants and, depending on the synthesis pathway, zirconyl chloride or zirconium propoxide as zirconium precursors, hexagonal, cubic or disordered mesoporous zirconia were successfully obtained. However, as it is the case for most mesoporous transition metallic molecular sieves, it is very difficult to preserve the structure after the surfactant removal. In this work, we have investigated the effect of some preparation parameters, in particular, heating time and temperature on the synthesis of pure zirconia molecular sieves in order to shed some light on the possible synthesis mechanism of such compounds. The samples were obtained without addition of sulfate or phosphate anions. 2. EXPERIMENTAL 2.1. Synthesis Cetyltrimethylammonium bromide (CTMABr) was first dissolved in water with stirring at 40 ~ C to obtain a clear micellar solution of 20 wt.% in surfactant instead of 16.7 wt.% as reported previously. Then zirconyl chloride was added to the solution and the pH value of the gel was adjusted with sodium hydroxide solution (1.4 M) to around 11.5 in order to induce the hydrolysis and the polycondensation of the inorganic precursor around the formed micelles. The surfactant / zirconia molar ratio is fixed at 7.5. After stirring for several minutes, the homogeneous gel with the molar composition of 1 CTMABr : 0.13 ZrO2 : 102 H 2 0 was sealed in Teflon autoclaves. The hydrothermal treatment temperature and duration vary respectively from 40 to 80 ~ and from 1 to 11 days. 2.2. Characterization The XRD patterns were obtained with a Philips PW 170 diffractometer, using CuKct (1.54178 A) radiation, equipped with a thermostatisation unit (TTK-ANTON-PAAR, HUBER HS-60). The transmission electron micrographs were taken using a 100 kV Philips Techna'f microscope. For TEM observations, sample powders were embedded in an epoxy resin and then sectioned with an ultramicrotome. The thin films were supported on copper grids previously coated by carbon to improve stability and reduce the accumulation of charges. The morphology of the final phases was studied using a Philips XL-20 Scanning Electron Microscope (SEM) with conventional sample preparation and imaging techniques. Nitrogen
259 adsorption- desorption isotherms were obtained at -196 ~ over a wide relative pressure range from 0.01 to 0.995 with a volumetric adsorption analyzer ASAP 2010 or TRISTAR 3000 both from Mjcromeritics. The samples were further degassed under vacuum for several hours before nitrogen adsorption measurements. The pore diameter and the pore size distribution were determined by the BJH (Barret, Joyner, Halenda) method. 3. RESULTS AND DISCUSSION
3.1. Structural investigations The XRD pattems of some samples prepared at 40, 60 and 80~ are depicted figure 1A, B and C respectively. It is observed that at low temperature (40~ or for short times of hydrothermal treatment at higher temperature (< 4 days at 60~ < 2 days at 80~ except a broad band, located in the range of 25~ ~, analogous to the one observed for amorphous silica, no peak is detected on the XRD patterns. This indicates that these ZrO2 materials are amorphous. After 4 days at 60~ peaks located at 20 = 28 (3.2 nm), 30 (2.9 nm), 31.5 (2.8nm), 34.5 (2.6 nm) and 50 ~ (1.8 nm) begin to appear. The intensity of these reflections increases with increasing synthesis temperature and time, whereas the d values (in nm) of these diffraction lines remain practically constant (XRD patterns nor shown Fig.l). By comparing the XRD pattern of our samples with those published by del Monte et aL [23], it is obvious that the monoclinic and tetragonal forms of zirconia are detected. This is in agreement with what observed by Fripiat et al. [24] and the results reported previously by our group [25]. Thus the crystallization of amorphous ZrO2 occurs. A
~0~
d
~~t
d
200,
20
~o~~ c c 1 ~ ~ i~ . ~; ~ r~
2o
"~
10
b
C
200
2o
b
10
1
200,
100, .i .
.
.
.
la,,
0 O- f o
20
3"0
40
20 (o)
50
60
.
. . . . 10 20
. . 30
. . 40
20 (o)
. 50
60
. . . . . . . . . 10 ~ 30 ~ ~
20 (o)
Figure 1 : X-ray diffraction pattems of the samples obtained at : A : 40~ a : 1, b :2, c : 6 and d : 11 (days); B : 60 ~ a : 1, b : 2, c : 3 and d : 6 (days); C : 80~ a : 0.5, b : 1, c : 1.5 and d : 2 (days),
260
Figure 2 : TEM micro graphs of samples prepared at 60~ a : 2 and b : 6 days. At elevated synthesis temperature, the peaks belonging to tetragonal and monoclinic zirconia are already detected for shorter heating time, It is therefore clear that the higher the temperature, the more quickly the transformation of amorphous ZrO2 to crystalline phases. Most of the compounds have a disordered structure with a large number of wormholelike channels lacking a long range packing order as reported for DWM (Disordered Wormlike Mesostructure) [26] compounds (Fig. 2a). When the transformation of the amorphous wall of the mesoporous materials into tetragonal or monoclinic zirconia structure occurs, some crystalline particles are detected on the TEM micro graphs (Fig.2b). If synthesis time or temperature is increased, the crystallization of the wall is more and more important and only very small particles of crystalline zirconia are detected on the TEM micrographs. As no small angle diffraction peak is detected, the compounds belong to the family of DWM-2. Comparing to DWM-1, this kind of materials exhibits inhomogeneity in the pore size, which involve the lack of reflection in the small angle region.
3.2. Nitrogen adsorption analysis The materials, synthesized in our conditions, exhibit a type IV isotherm (Fig. 3), according to the classification of BDDT. The adsorption branch of the isotherm can be decomposed in three parts: the monolayer- multiple adsorption of N2 on the wall of the mesopores, the capillary condensation of nitrogen within the mesopores and then the saturation. Thus, the zirconia molecular sieves obtained in the present study are mesoporous. However it should be noted that for a given duration, if the temperature is raised, the capillary condensation takes place at higher values. For example, the relative pressure at which the capillary condensation occurs varies from p/p0 = 0.50 to p/p0 = 0.60 if the hydrothermal treatment temperature is changed from 40 to 80~ This suggests that materials obtained at higher temperature have larger pore diameters, as the position of the inflection point of the isotherm is related to the size of the channel aperture. We have also reported [25] that for short heating time (< few hours at 80~ nitrogen adsorption - desorption isotherms of all obtained samples are not well defined since they exhibit a linear region from p/p0 = 0.1 to 0.4 before reaching a plateau. This kind of isotherm is located between the type I, related to microporous materials, and the type IV, characteristic of mesoporous materials. In agreement with Dubinin [27], we can conclude that these materials possess supermicropores, i.e. pores with sizes ranging from 1.5 to 2.0 nm. A hysteresis loop intermediate between H2 and H1 type is observed for the samples obtained in our conditions. H2 type hysteresis loop has a steep desorption branch and a more or less sloping adsorption branch. It may arise from the same types of open capillaries as are responsible for H1 type hysteresis, characteristic of MCM-41 kind of materials. This indicates that the effective radii of narrow entrances are all
261
A
5.6 nm
B
i
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,
-
,
-
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-
,
-
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,
-
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.
.
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-
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,
.,
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,
nm
.
a
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0.0
0.2
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Relative pressure p/Po
10
~o 20 go ~o Pore diameter (nm)
Figure 3 9 Nitrogen adsorption isotherms (A) and pore size distribution (B) of samples obtained at 80~ a" 0.5, b" 1, c 91.5 and d 92 days. of equal size. H2 type hysteresis loop is typical for wormhole structured materials such as DWM [26]. Figure 4 depicts the variation of the specific surface area with heating time and temperature. A t 60~ (curve b of Fig. 4): After 4 days of hydrothermal treatment, the value of the specific surface area decreases from 375 to 318 m2/g. Referring to the typical 4 steps crystallization curve observed for zeolites (steps I : the nucleation, II :the growth of crystals, III : the crystallization and IV : the amorphisation) [28] or mesoporous molecular sieve synthesis (step I : the hydrolysis of inorganic source in aqueous solution, step II : the polycondensation of inorganic source around micelles, step III "the continuation of polycondensation and the formation of mesostructures and step IV : the destruction of the latter) [29] this can be attributed to the destruction of the structure, i.e. the step IV. Neither step I nor step II are detected. These two steps are already achieved during the preparation of the gel. The destruction of the structure can be related to the crystallization of the amorphous wall of the zirconia mesoporous molecular sieve.
262 600 500 N
400
.... ::::::::::::::::::::::::
........
......................... .a...,..
. .......
......
0
300 0
'-
"
200
r~
9~ O
b
i 100
g
ra~
0
'
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,~
'
6
'
8
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'
12
Hydrothermal time (days) Figure 4 9Variation of the specific surface area with heating time and temperature a" 40, b" 60 and c" 80~
t~ A t 80 ~ (curve c of Fig. 5) 9the variation of the specific surface area with heating time is analogous to that reported for 60~ but drops atter a very short time, indicating the destruction of the mesostructure. r At 40~ (curve a of Fig. 4): The value of the specific surface area increase from 320 to 385 mVg. This part of the curve correspond to the second step of the crystallization curve, the mesoporous zirconia begins to be formed. Then, the specific surface area remains constant to 380 mVg, step III is reached.
3.3. Discussion From the results obtained in the present study and those reported previously, an electrostatic pathway, based on a supramolecular assembly of charged surfactants with charged inorganic precursors, is employed for the preparation of zirconia mesoporous materials (Fig. 5). In the presence of sodium hydroxide, the hydrolysis and the polymerization of zirconia precursor around the preformed micelles of surfactant in aqueous solution, takes place. In the first step, the synthesized material is supermicroporous, then it becomes mesoporous if the hydrothermal treatment duration is prolonged. This transformation is favored by an increase of the temperature (step2). In a paper dealing with the use of amine as expander and postsynthesis treatment to increase the pore sizes of MCM-41 silicas, Sayari et al [30] have concluded that there is a possibility that walls between pores may break during the process of pore expansion, in such a way that pairs or triplets of adjacent pores transform into single but larger pore. In our case, we can also consider such a mechanism to explain the transformation from a supermicroporous molecular sieve to a mesoporous one. The obtained materials exhibit specific surface areas up to 400 m2/g. However, the channels are not well organized, the compounds have a structure with wormlike channels such as reported for DWM samples [26]. If synthesis time or temperature are further increased, crystalline particles of ZrO2 appear in our compounds, as is proved from the TEM micrographs. The presence of tetragonal and monoclinic zirconia is confirmed by XRD, the peaks characteristic of theses structures are pointed on the XRD pattern (step 3). Thus too long heating time or too
263 high synthesis temperatures lead to the crystallization of the mesoporous walls. The structure collapses and only an interparticular porosity remains (step 4). 4. CONCLUSION The present study reveals that zirconia molecular sieves can be obtained via an electrostatic assembly, using cetyltrimethylammonium bromide as surfactant and zirconyl chloride as inorganic precursor. The optimization of the synthesis conditions leads to the formation of mesoporous zirconia without addition of some phosphate or sulfate anions. The template was removed by ethanol extraction. The different formation steps have been clearly evidenced. A synthesis mechanism to describe the evolution of materials formed in autoclave has been postulated. In a first step the material is supermicroporous, then a breakdown of the wall leads to the formation of the mesoporous molecular sieves with high specific surface area (400 mVg). However the prepared compounds belong to the family of DWM-2, i.e. materials with a disordered wormhole-like structure and inhomogeneous channel size distribution. Finally, too high temperature or too long hydrothermal treatments are responsible of the crystallization of the walls and the structure collapses. No mesoporosity is detected any longer. Particles of crystalline zirconia are observed by TEM and tetragonal and monoclinic ZrO2 are detected by X-ray diffraction analysis Step 2 MESOPOROUS
Stepl SUPERMICROPOROUS
ZrOC12,SH20 ~-Mieellar solution of CTMABr
ads
time
polymerization ofzirconia
T~
k,
p/po . . . . ,J
Step 3 Begimming of the
time
crys~lUne pbue formation
T~
Step 4
~ Crystallization
d~, gads
p/po
/ Collapse of the st
Figure 5 9Proposed mechanism for mesoporous zirconia synthesis.
p/p0
264 ACKNOWLEDGEMENT
:
This work has been performed within the framework of PAI/IUAP 4-10. Alexandre L6onard thanks FNRS (Fond National de la Recherche Scientifique, Belgium) for a FRIA scholarship. REFERENCES
1.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.
28. 29. 30.
J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.TW. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schenker, J. Am. Chem. Sot., 114 (1992) 10834 C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710 P.L. Llewellyn, Y. Ciesla, H. Decher, R. Stadler, F. Schtith and K.K. Unger,. Stud. Surf. Sci. Catal., 84 (1994) 2013. J. Aguado, D.P. Serrano, M.D. Romero and J.M. Escola, Chem. Comm., (1996) 765. A. Corma, M. Iglesias and F. Sanchez Catal. Lett., 39 (1996) 153. P.T. Tanev, M. Chibwe and T.J. Pinnavaia, Nature, 368 (1994) 321. A. Corma, Chem. Rev., 97 (1997) 2373. A. Tuel, S. Gontier and R. Teissier, Chem. Comm., (1996) 651. D.J. Jones, J. Jimenez, A. lopez, P. Torres, P. Pastor, E. Rodriguez and J. Rozi6re, Chem. Comm., (1997) 431. Q. Huo, D.I. Margolese, U. Ciesla, P. Feng, T.E. Gier, P. Sieger, R. Leon, P.M. Petroff, F. Schtith and G.D. Stucky, G.D, Nature, 368 (1994) 317. D.M. Antonelli and J.Y.Ying, Angew. Chem. ,Int. Edn. Engl., 35 (1996) 426. S.A. Bagshaw and T.J. Pinnavaia, Angew. Chem. Int. Edn. Engl., 35 (1996) 1102. Z.R. Tian, W. Tong, J.Y. Wang, N.G. Duan, V.V. Krishnan and L.S. Suib, Science 276 (1997) 926. U. Ciesla, S. Schacht, G.D. Stucky, K.K. Unger, K.K. and F. Schtith, Angew. Chem. Int. Edn. Engl., 35 (1996) 541. P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka and G.D. Stucky, Nature, 396 (1998) 152. Y. Inoue Y. and H. Yamazaki, Bull. Chem. Soc. Jpn., 60 (1987) 891. A. Clearfield, Inorg. Chem., 3 (1964) 146. T. Yokoyama, T. Setoyama, N. Fujita, M. Nakajima and T. Maki, Appl. Catal. A, 4 (1992) 149. J.A. Knowles and M.J. Hudson, Chem. Comm., (1995) 2083. A. Kim, 2 P. Bruinsma, Y. Chen, L.Q. Wang and J. Liu, Chem. Comm., (1997), 161. G. Pacheco, E. Zhao, A. Garcia, A. Sklyarov and J.J. Fripiat, Chem. Comm., (1997) 491. P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka, B.F. and G.D. Stucky, Chem. Mater., 11 (1999) 2813. F. Del Monte, W. Larsen and J.D. Mackenzie, J. Am. Ceramic Soc., 83 (2000) 1508. G. Pacheco, E. Zhao, A. Garcia, A. Sklyarov and J.J. Fripiat, J. Mater. Chem., 8 (1998)219. J.L. Blin, R. Flamant and B.L. Su, I. J. Inorg. Mater., 3 (2001) 959. J.L. Blin, A. L6onard. and B.L. Su, Chem. Mater., 13 (10) (2001) 3542. M.M. Dubinin in : Progress in Surface and Membrane Science, 9 (D.A. Cadenhead, ed) Academic Press, New York, (1975), p. 1. D.W. Breck, Zeolite Molecular Sieves, John Wiley & sons, New York, (1974). J.L. Blin, C. Otjacques, G. Herrier and B.L. Su, I. J. Inorg. Mater., 3 (2001) 75. A. Sayari, M. Krtak, M. Jaroniec and I.L. Moudrakovski, Adv. Mater., 10 (1998) 1376.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
265
Single crystal particles o f mesoporous (Nb, Ta)205 Junko N. Kondo, a Tomohiro Yamashita, a Tokumitsu Katou, a Byongjin Lee, a Daling Lu, a' b Michikazu Hara a and Kazunari Domen a, b a Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midoriku, Yokohama 226-8503, Japan
b Core Research for Evolutional Science and Technology, Japan Science and Technology, 21-13 Higashiueno, Taito-ku, Tokyo, 110-0015, Japan
Mesoporous Nb-Ta mixed oxide, (Nb, Ta)2Os, with whormhole mesopore structure was prepared by using a block co-polymer template and metal chlorides in ethanol. The mesoporous (Nb, Ta)205 calcined at 673 K for 20 h showed 140 mZ-gl of BET surface area and 3.0 nm of pore size. The amorphous wall of the mesoporous (Nb, Ta)205 was crystallized by further calcination at 923 K for 1 h. The BET surface area and pore size estimated by N2 adsorption isotherm of the crystallized (Nb, Ta)205 were 48 m2.g-~ and 10.0 nm, respectively. Detailed analysis of transmission electron microscope (TEM) and electron diffraction (ED) revealed that each particle of sub-micron size was a mesoporous single crystal. The pore size and crystallinity observed by TEM were in good agreement with N2 adsorption-desorption isotherms and powder X-ray diffraction patterns. Similar mesoporous (Nb, Ta)205 single crystal particles were also obtained by using amorphous precursor prepared by ligand-assisted templating method. Therefore, mixing two similar elements, Nb and Ta, is suspected to be beneficial for crystallization with sustaining the mesoporores.
1. INTRODUCTION Tantalates are found to be highly active photocatalysts for overall water decomposition under UV irradiation [1]. These tantalates are prepared by solid-state reactions and are completely crystallized, where electron mobility is high. The mesoporous metal oxides with high surface area and crystallized wall structures are expected to be advantageous not only for photocatalysis but also as various catalysts. For the purpose of development of new types of photocatalyst, we synthesized some mesoporous tantalum oxides [2-5], and studied their photocatalysis. In the studies on crystallization, we found that mesoporous (Nb, Ta)205 prepared by mixing Nb and Ta metal sources was crystallized with remaining mesopores in crystallized lattice structure, although pure Nb205 and Ta205 resulted in destruction of mesopores upon crystallization. In this study, production of mesoporous (Nb, Ta)205 single crystal particles is reported, and relation of the amorphous precursors and the crystallized product is discussed.
266 2. MATERIALS AND METHODS 2.1. Materials
Mesoporous Nb2Os,Ta205 and (Nb, Ta)205 were synthesized by two methods, a method using a block co-polymer surfactant as a template [4-7], and an improved ligand-assisted templating (denoted as LAT method) [2, 3, 8]. In the block co-polymer templating method, 0.01 mol of NbC15 or TaC15, or 0.005 mol of both TaC15 and NbC15 were added to 10 g of ethanol containing 1 g of poly (alkylene oxide) block copolymer, Pluronic P-123. After vigorous stirring for 30 min, the resulting sol solution was transferred to a Petri dish for aging at 313 K for 6-10 days. The surfactant was removed by calcination at 673 K for 20 h. In LAT method, octadecylamine (6.15 mmol) was mixed in Nb(OEt)5 or Ta(OEt)5, or Ta(OEt)5 and Nb(OEt)5 (12.30 mmol in total) under Ar gas atmosphere, and warmed to 323K for 10-30 min. Then was added deionized water (25 mL). The precipitate was then washed with water and ethanol. Aging was carried out at 353 K for 1 day, 373 K for 1 day, and 453 K for 7 days, successively. The product was washed with the deionized water, ethanol, and diethyl ether. The powder was then dried at 373 K for 12 h in the atmosphere. The surfactant-containing sample (1 g) was treated with trifluoromethane sulfonic acid in dimethoxyethane at 195 K for 1 h with stirring, followed by warmed to the ambient temperature. The powder Was washed with 2-propanol, deionized water, ethanol, and diethyl ether, and then dried in evacuation at 373 K within 12 h. 2.2. Measurements
X-ray diffraction (XRD) patterns were obtained on a Rigaku R1NT 2100 diffractometer using Cu K.ctradiation. The TEM images were obtained using a 200 kV JEOL JEM201 OF. Nitrogen-gas adsorption-desorption isotherms were measured by Coulter Omnisorp 100CX and SA-3100 systems. Differential thermal analysis (DTA) and thermogravimetry (TG) were performed using a Shimadzu DTG-50 in air at a heating rate of 5 or 10 K'min -1. 2.3. Methods
The BET specific surface area was calculated in the relative pressure range between 0.05 and 0.2. The pore-size distributions were determined by BJH (Barrett-Joyner-Halenda) analysis using the adsorption branch.
3. RESULTS AND DISCUSSION
Crystallization of a mesoporous transition metal oxide was first attempted by using Ta205 prepared by LAT method. The BET surface area and the pore size of the as-prepared wormhole mesoporous sample after chemical extraction of the surfactant were 410 m2"g~ and 3.3 nm, respectively [3]. The wall thickness of the as-prepared sample estimated by simple subtraction of the pore size from d(100) value was 1.1 nm, which is considerably thin. When the mesoporous Ta205 was calcined at 673 K for 20 h before crystallization, N2 adsorption isotherm as shown in Figure 1(a) was observed, which is analogous to type IV pattern. The BET surface area decreased to 330 m2.g-l, and the pore size was broadly distributed to 3.0 nm. The type IV isotherm as well as TEM observation still evaluate the sample calcined at 673 K as a mesoporous material. The crystallization condition of Ta205 sample was
267 determined by TG-DTA analysis together with XRD observation of samples calcined at various temperatures. By calcination at 1023 K for 1 h for crystallization, the BET surface area estimated from the N2 adsorption isotherm shown in Figure 1(b) decreased to only 16 m2.g-~, and the sample is no more regarded as mosoporous material, although a clear XRD pattern of orthorhombic Ta205 was obtained. 100
(a) __..
Figure 1. N 2 adsorption-desorption isotherms of mesoporous Ta20 5 prepared by LAT method after calcination at 673 K for 20 h (a) and 1023 K for 1 h (b). Filled and open symbols correspond to adsorption and desorption branches, respectively.
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P/Po Next, crystallization of mesoporous Ta205 prepared by using block co-polymer template was conducted because the thick wall of the product prepared by this method [6, 7] was expected to sustain mesopores even after crystallization. N2 adsorption-desorption isotherm of the mesoporous Ta205 after calcination at 673 K for 20 h for the complete template removal and that after further calcination at 1023 K for crystallization are compared in Figure 2A. BET surface areas of those samples were 123 and 23 m2.g-~, respectively. The smaller surface area of the amorphous Ta205 prepared here compared with that prepared by LAT method is attributed to the thick walls. Similarly to the result in Figure 1, mesoporous Ta2Os, which consists of thick walls, is neither considered to possess mesopores after crystallization. 80"7
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Figure 2. N 2 adsorption-desorption isotherms ofmesoporous Ta20 5 (A) and (Nb, Ta)205 (B) prepared using block co-polymer template after calcination at 673 K for 20 h (a) and after calcination at crystallization temperatures (1023 K (A) and 923 K (B) for 1 h, (b)). Filled and open marks corrspond to adsorption and desorption branches, respectively. The BET surface area of the mesoporous material of pure Nb205 prepared by block copolymer templating method was remained at 45 m2.g-~ after crystallization at 873 K, and the
268 crystallized mesoporous Nb205 was once expected to be formed. However, only a ring ED pattern was obtained from a crystallized particle instead of a sharp spot ED pattern of a single crystal (Details are mentioned below). Therefore, the each particle is regarded as "an aggregates of small crystals, and the mesoporosity is attributed to the interparticle space. We then mixed Ta with Nb for the purpose of decreasing the crystallization temperature of homogeneously mixed oxides due to their same oxidation number (V) and the same ionic radii (0.78 A) in hexa-coordination in oxides. The crystallization temperatures, which were determined by exothermal DTA peak above 773 K, of Nb205 (848 K) and Ta205 (1018 K) agree well with those for non-porous materials. The gradual and continuous change in crystallization temperature was observed depending on the Nb/Ta ratio between pure Nb205 and Ta205 (not shown), which is indicative of the homogeneous mixing of the Nb and Ta in mixed oxides. The Nb/Ta ratios were also confirmed to be "as prepared" by elemental analysis of the samples before and after crystallization using TEM apparatus in c a . 5 n m ranges. Although Nb-Ta mixed oxides at different Nb/Ta ratios were prepared and studied, results are focused on thesample at Nb/Ta = 1 are shown in this study (denoted as (Nb, Ta)205 hereafter). The as-prepared (Nb, Ta)205 indicated type IV adsorption isotherm pattern typical to mesoporous materials, and the BET surface area was 140 m2.g-1 (Figure 2B(a)). The pore size distribution was centered at 3.0 nm. The presence of only (100) diffraction peak at d(100) = c a . 7.0 nm in low-angle XRD pattern (Figure 3) indicates the wormhole mesoporous structure, which is also confirmed by TEM images (not shown). '
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Figure 3. Low-angle (A) and high-angle (B~(RD patterns of mesoporous (Nb, T a r o 5 after calcination at 673 K for 20 h (a) and after crystallization at 923 K for 1 h (b). The crystallization temperature of mesoporous (Nb, Ta)205 was determined as 923 K, and over 90 % of the particles were crystallized within 1 h (confirmed by ED analysis as indicated below). The crystallized (Nb, Ta)205 still showed type IV adsorption isotherm pattern (Figure 2B(b)), and 48 m2"g-I of BET surface still remained. The peak top of the pore size distribution was shifted to c a . 10.0 nm. Therefore, mesopores were expected to be sustained in the crystallized (Nb, Ta)205 sample which showed sharp XRD peaks as observed in Figure 3B(b). In order to clarify whether the mesopores in the crystallized (Nb, Ta)205 exist in the crystal lattice domain or they exist as interparticle space, careful and detailed TEM observation was carried out.
269
Figure 4. TEM image of a crystallized mesoporous (Nb, Ta)2 0 5 and ED patterns from the whole particle (top) and 4 different areas.
A TEM image and ED patterns from a whole particle and several areas (ca. 200 nm in diameter) indicated as dotted circles are shown in Figure 4. The wormhole mesoporous structure is observed in the image, and the sharp spots in ED pattern from the whole particle indicates that the particle is not a polycrystal but only one crystal domain exists in the particle. Furthermore, the ED patterns from several different areas are coincident with that obtained from the whole particle. This is a clear evidence that a mesoporous (Nb, Ta)205 particle is a single crystal. The presence of mesopores in crystallized lattice was clearly observed in high resolution images as shown in Figure 5 [2, 3]. The direction of lattice fringes though out mesopores was the same.
Figure 5. High resolution TEM image of mesoporous (Nb, Ta)205 single crystal particle.
50 particles in sub-micron size of the crystallized mesoporous (Nb, Ta)205 were analyzed in the same manner, and 45 particles resulted in the same images and ED patterns as those
270 observed in Figure 4. Therefore, crystallinity of the sample was estimated as ca. 90 %. The rest of the particles remained amorphous without showing any ED spots. Assuming that the mixing of Nb and Ta is effective, same strategy would be successful for LAT method. Mesoporous (Nb, Ta)205 was prepared by mixing equivalent amount of Nb(OEt)5 and Ta(OEt)5 in LAT method. 100
(a)
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Figure 6. N 2 adsorption-desorption isotherms of mesoporous (Nb, Ta)2 0 5 prepared by LAT method after calcination at 673 K for 20 h (a) and 923 K for 1 h (b). Filled and open symbols corrspond to adsorption and desorption branches, respectively.
P/Po
The mesoporosity of the samples was evaluated by N2 adsorption-desorption isotherms as shown in Figure 6. The BET surface area of the (Nb, Ta)205 after calcination at 673 K was 206 m2.g-~, and there was no peak in the pore size distribution which spread to 4.0 nm. The similar mesoporous (Nb, Ta)205 material was prepared by LAT method after calcination at 673 K to the case of pure Ta2Os. However, the effect of mixing Ta to Nb in the mixed oxide was observed after crystallization. The sample was crystallized at lower temperature calcination (923 K) than Ta205. The N2 adsorption-desorption isotherm of the crystallized sample indicated type IV pattern typical to mesoporous material. It is noticed that the crystallized (Nb, Ta)205 material prepared by LAT method and that prepared by using block co-polymer template resulted in similar material judging from the isotherms. Therefore, we also observed TEM images and ED patterns. A TEM image in Figure 7 (middle) demonstrates the presence of mesopored in a crystallized (Nb, Ta)205 particle, and the ED pattern from the whole particle indicates that the observed particle is not a single crystal but consists of a few crystal domains. Because the ED patterns in Figures 3 and 7 are obtained along different zone axes, they should not be necessarily coincident. However, the ED pattern in Figure 7 is clearly a set of spot-patterns of a few single crystals, although the presence of mesopores in the lattice image was confirmed in high magnification image (left). It is noted that small particle (< 100 nm in diameter) were single crystals. Therefore, the size of the single crystal domains of the (Nb, Ta)205 material is considered to be smaller when it is prepared by LAT method than that prepared by block co-polymer templating method. Interestingly, considerably different mesoporous (Nb, Ta)205 with amorphous wall produced similar crystallized mesoporous material. We therefore, tentatively regard the nature of the element is an important factor rather than the preparation method in the present study. The effect of mixing Nb to Ta in oxide is probably due to the low surface tension of Nb2Os, which also decreased the surface tension of mixed oxide and prohibited the pore collapse and formation of aggregates. Similar phenomenon was observed for Nb205 ultrafine 1Mrticles [9].
271
Figure 7. High resolution TEM image and an electron diffraction pattern of crystallized mesoporous (Nb, Ta)20 5 particle (middle) prepared by LAT method. There occurred a drastic change in material appearance. The wall thickness of the as-prepared mesoporous (Nb, Ta)205 by LAT method and block co-polymer templating method were 1.1 and 4.0 nm, respectively, while that of the crystallized sample is estimated as c a . 10 nm (see Figure 5 and 7). The pore size of the amorphous mesoporous (Nb, Ta)205 prepared by both methods expanded from c a . 3 to c a . 10 nm. Therefore, the material transfer of (Nb, Ta)205 consisting the wall upon crystallization is more for the precursor prepared by LAT method, resulting in the smaller crystal domains, i.e. poor formation of single crystal particles. All the amorphous (Nb, Ta)205 precursors had wormhole mesoporous structure, and the crystallized mesoporous (Nb, Ta)205 also consisted of wormhole mesopores. Although detailed phenomena occurring during crystallization are of interest, non-ordered mesoporous structure of the material prohibited the clarification of the crystallization process. Therefore, we prepared a hexagonally ordered mesoporous (Nb, Ta)205 by optimizing the preparation method using a block co-polymer in order to proceed in-situ observation by TEM during crystallization. Briefly, the amount of metal source, TaC15 and NbC15 was decreased to 0.0025 mmol each (0.005 mmol in total), a half of the original amount. A small amount of water was added before aging for the improvement of the ordered structure of mesopores [10].
Figure 8. N 2 adsorption isotherm (A) and low-angle (B~(RD pattern of hexagonally ordered mesoporous (Nb, Ta)205.
272 The type IV adsorption isotherm was observed (Figure 8A), and the BET surface area and pore size were 193 m2-g~ and 5.5 nm, respectively. The wall thickness estimated by the pore size and the repeat distance obtained from d(100) value (Figure 8B) assuming hexagonal mesopore structure was c a . 2.0 nm. As shown in Figure 9, hexagonally ordered mesoporous structure was observed by TEM together with ED pattern (inset). The pore size and the wall thickness mentioned above agreed with the values estimated from a high resolution TEM image. Now crystallization of this material is under examination.
Figure 9. TEM image and an electron diffraction pattern of a hexaganally ordered mesoporous (Nb, Ta)20 5 particle.
4. ACKNOWLEDGMENTS This work was supported by the Core Research for Evolutional Science and Technology (CREST) program of the Japan Science and Technology (JST) Corporation.
REFERENCES
1. 2.
H. Kato and A Kudo, Chem. Phys. Lett., 295 (1998) 487. Y. Takahara, J. N. Kondo, T. Takata, D. Lu and K. Domen, Chem. Mater., 13, (2001) 1194. 3. J.N. Kondo, Y. Takahara, T. Takata, D. Lu and K. Domen, Chem. Mater., t3, (2001) 1200. 4. B. Lee, J .N. Kondo, D. Lu and K. Domen, Stud. Surf. Sci. Catal., 135, 07-P-15 (2001). 5 B. Lee, J. N. Kondo, D. Lu and K. Domen, Chem. Commun., submitted. 6. P. Yang, D. Zhao, D. I. Margolese, B. F. Chmelka and G. D. Stucky, Nature, 396 (1998) 157. 7. P. Yang, D. Zhao, D. I. Margolese, B. F. Chmelka and G. D. Stucky, Chem. Mater. 11 (1999)2813. 8. D.M. Antonelli and J.Y. Ying, Chem. Mater., 58 (1996) 874. 9. P. Nair, J. Nair, A. Raj, K. Maeda, F. Mizulami, T. Okubo, and H. Imitsu, Mater. Res. Bull., 34 (1999) 3. 10. T. Katou, J. N. Kondo, D. Lu and K. Domen, in preparation.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
273
Preparation o f exfoliated zeolites from layered precursors: The role o f pH and nature o f intercalating media Wieslaw J. Roth and James C. Vartuli Corporate Strategic Research, ExxonMobil Research and Engineering, Annandale, NJ 08801. The ability to modify interlayer separation and arrangements in lamellar solids by appropriate chemical treatment has been exploited to generate novel porous materials. Treatments such as intercalation or delamination may be quite demanding and sensitive to various factors. This is illustrated by the zeolitic layered material, MCM-22 precursor, which can be swollen with a cationic surfactant but requires a high pH environment. The resulting product, swollen MCM-22 precursor, may then be converted to the pillared mesoporous material, MCM-36, or to a delaminated solid with randomly oriented layers. This report deals with the issues associated with swelling of the layered MCM-22 precursor, which is the critical step. We focus on the problem of swelling efficiency when the required surfactant medium with high pH is a mixture of a base (NaOH or tetraalkylammonium hydroxide) and cationic surfactant chloride. Only the combination with tetrapropylammonium hydroxide produced swelling, while the methyl and ethyl homologues did not. This indicates dependence on the nature of cations present in the swelling medium. We also discuss in detail the criteria for identification and quantification of the swollen product and avoidance of potential impurities. 1. INTRODUCTION Two-dimensional (layered) materials are characterized by relatively weak interlayer bonding [ 1]. The layers can in principle be separated by appropriate treatment, for example, by intercalation of guest molecules between the layers. This potential to manipulate the interlayer separation and spatial arrangement of the sheets has been exploited to generate novel materials [2,3]. Pillared clays and layered oxides with permanent props between the layers exemplify new catalysts with increased and controlled pore sizes prepared by this approach. While zeolites are considered rigorously 3-dimensional solids, one of them, MCM-22 [4] was recognized as existing in a lamellar form, called MCM-22 precursor [5,6], prior to any treatment that chemically locks the layers into the typical rigid zeolite solid. The precursor is composed of 25 A sheets stacked in registry. Each sheet by itself can be considered a zeolite because of its MWW internal connectivity and pore system. The 3-D zeolite framework is generated as the layers condense upon calcination, with concomitant contraction of the repeat distance in the stacking direction by about 1.5 A. The existence of the layered MCM-22 precursor was exploited to make a novel material, pillared zeolite, MCM-36, which combines strong acidity with mesoporous character [6,7]. This field, initiated by MCM-22, has been recently expanded to ferrierite, as the corresponding layered precursor was discovered [8].
274 The layer separation in MCM-22 precursor by intercalation, referred to as swelling or delamination, proved very challenging due to strong interlayer bonding. It was eventually accomplished with a solution of a cationic surfactant, hexadecyltrimethylammonium chloride, having high pH equal to 13.8 as the result of partial substitution of halide for hydroxide anions [6]. This work focuses on usage of a small base/surfactant salt combination as the alternative to the initially used reagent. Apart from the attempted replacement of this exotic chemical, the reported study illustrates the difficulty and pitfalls associated with swelling MCM-22 precursors. The report describes identification of the swollen product, detection of the unswollen phase (if present), and the possible presence of mesoporous impurities. Finally we propose a rationale for the observed relationship between the nature of the base cation and the success/failure of the swelling treatment. 2. EXPERIMENTAL SECTION The synthesis procedures are described elsewhere [6,7]. Briefly, MCM-22 precursor with silica/alumina molar ratio of approximately 23/1 was synthesized hydrothermally in the presence of hexamethyleneimine as the structure directing agent. The surfactant hydroxide solution (pH = 13.8) was obtained by contacting 29 % hexadecyltrimethylammonium chloride (CI6TMA-C1) solution (Akzo) with an anion exchange resin IRA-400(OH) - about 30 % of the C1 anions were replaced. The high pH surfactant mixtures used in subsequent experiments were prepared by adding concentrated MOH solution (M is Na or tetraalkylammonium cation) to the above 29 % C16TMA-C1 solution until pH 13.8 was reached. Swelling experiments were carried out with less than 10 % solid content at 95-100~ for 1-2 days. In the case of sodium, amorphization of the zeolite occurred and the experiment was repeated at room temperature. X-ray powder diffraction scans (XRD) were recorded using a Scintag diffractometer. 3. RESULTS AND DISCUSSION 3.1 Delamination of MCM-22 Precursor with Surfactant Hydroxide, R(CH3)3N-OH The separation of layers in MCM-22 precursor was first achieved by intercalation of long chain quatemary ammonium cations, CI6H33(CH3)3N§ by a unique treatment developed for that purpose. The swelling of layered materials, like clays and silicates, is possible under relatively mild conditions with organic amines [ 1-3,9]. All of the known treatments failed with the MCM-22 precursor. The latter proved swellable only under rather severe high pH environment with no other cations present to compete with the intercalating cationic surfactant species. The appropriate reagent was obtained by performing anion exchange with the solution of Ct6H33(CH3)3N-C1, resulting in replacement of ca. 1/3 C1 with OH anions (hereto designated CI6TMA-OH).
Non-covalent but relatively strong interlayer bonding is the apparent reason for the described difficulty in swelling the MCM-22 precursor. The in-registry alignment of the layers concluded from the distinct, albeit broadened, interlayer hkl reflections in the XRD pattern also supports the existence of some relatively strong chemical linkage. Hydrogen bonding between silanols on the surface of each layer with possible involvement of the templating HMI
275 molecules is the primary candidate. This is further supported by the observation that formation of the oxygen bridges between layers upon calcination involves unit cell contraction by ca. 1.5 A, consistent with conversion of SiOH into Si-O-Si moieties. In this context the high pH requirement for successful swelling may be rationalized by the following chemical reaction: Si-O-+ H20
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30
Figure 2. Expanded X-ray powder diffraction pattem, from Figure 1, of MCM-22 precursor and the CI6TMA-OH swollen derivative showing features critical for diagnosing surfactant intercalation and layer separation.
276 In this case high pH promotes elimination of hydrogen bonding by deprotonation of silanols. The generated negatively charged SiO- centers simultaneously repel each other and attract the intercalating long chain surfactant cations. An interlayer bonding scheme involving pairs of opposing silanol groups has been proposed in the report on the structure of the layered silicate KHSi205 [ 10]. 3.2 Identification of The Swollen MCM-22 Precursor Another challenge associated with delamination of MCM-22 precursor was proving successful swelling and the development of diagnostic tools for distinguishing between complete and partial swelling. This was non-routine although X-ray diffraction seemed to provide some obvious answers. Successful swelling was found to result in expansion of the crystallographic unit cell in the c-direction by about 25-30 A (corresponding to 50-55A repeat; the expansion is consistent with thickness of the CI6TMA+ bilayer). Based on that, the following effects could be expected in the XRD pattern:
1. general hkl reflections shift to lower 20 angles or disappear altogether, 2. hk0 reflections remain invariant, 3. a prominent 001 peak emerging around 50-55 A d-spacing, possibly accompanied by higher order peaks at appropriate positions. In practice, only the last two have been rigorously obeyed (see Figure 1). The first prediction was impossible to analyze over the entire region because the XRD pattern of MCM-22 precursor and its swollen derivative appeared too complex and with broadened peaks to permit unambiguous peak deconvolution and assignment. Subsequently experience showed that the XRD region up to 10 degrees 20 is sufficient to discern the efficiency of swelling/layer separation. The diagnostic features are marked in Figure 2. The disappearance of the 002 reflection at 6.5 ~ is obvious. The merging of the 101 and 102 peaks observed in MCM-22 at 20 angles 8~ and 10~ into a broad band and a new peak at-~5.5 ~ are still empirical but consistently observed in samples deemed successfully swollen. The independent criteria corroborating swelling were TEM examination (showing exfoliated layers) and successful preparation of the pillared derivative MCM-36 (proven by XRD, TEM, unique sorption features) [6]. 3.3 Detecting Presence of Unswollen Phase The primary concern regarding the swelling of MCM-22 precursor was determining if all precursor was exfoliated. The dominating 001 reflection at ca. 55 ~ could not be to used to establish complete swelling. The peak intensity appeared too sensitive to factors such as water content, possible preferred orientation, particle size, etc, to provide anything but a qualitative measure. The criterion for estimating the amount of unswollen phase was empirical and involved judging the extent of peak separation in the range 8-10 ~ A band without a trough in the middle suggests negligible unexfoliated component. And vice versa- the magnitude of a dip, if any, in the band indicates contribution from unswollen phase. This criterion carries over to the pillared species, MCM-36, which can be appraised in the similar manner.
277
3.4 Ruling out MCM-41 and Mesoporous Impurities An obligatory feature of the XRD pattern of surfactant intercalated MCM-22 precursor is the prominent low angle 001 line, which usually occurs at d-spacing >50 A. This may be thought sufficiently distinct from 40-45 A, typically seen for MCM-41 or related materials, which may form under comparable conditions [ 11]. Nonetheless, the possible presence of these mesoporous phases cannot be dismissed outfight. The synthesis conditions are conducive to M41S generation. In particular, high pH may result in partial dissolution of the MCM-22 precursor thus supplying silicate, which may combine with the surfactant and generate M41S. The absence of M41S phase at this stage can be determined by calcination of a small portion of the swollen product. We observed that swollen MCM-22 precursor converted to MCM-22 upon calcination. When M41S impurity was present the low angle line in XRD was maintained upon calcination and/or the product had increased BET and sorption compared to MCM-22. The initial studies in this area also relied on extensive TEM examination of samples and no M41 S-like patterns were observed. 3.5 Delamination Attempts with MOH/C16TMA-CI Mixtures As discussed above the delamination of MCM-22 precursor was achieved by treatment with a cationic surfactant solution under conditions of high pH generated by partial substitution of chloride with hydroxide. Subsequent studies explored swelling using solutions obtained by mixing the surfactant chloride solution and a base, such as NaOH or tetraalkylammonium hydroxide, as the high pH source. The corresponding XRD patterns (see Figure 3 and Table 1) show that nature of the cation accompanying the hydroxide determines whether swelling occurs or not. Among the four hydroxides investigated: sodium, tetraalkylammonium - methyl (TMA; XRD not shown), ethyl (TEA) and propyl (TPA), only the last allowed swelling with the surfactant. (/) r
no trough
1E
..(3 i._ .m
016
Z" v
1=:
(1) C: m
!
"
\.
A
.A
A .....
TPA-O./OI0TM
-O,J
L-
,I,,I
m
TEA-OH/C16TMA-CI~..~j
002.--------~ ~ tl
i
1
i
5
i
trough
i
i
10
2 + (degrees)
i
i
15
i
i
20
Figure 3. XRD patterns of MCM-22 precursor after treatment with different swelling solutions.
278 This behavior may be related to the cation size in the following manner. TPA cations may be excluded from the interlayer region as too large. This would allow the surfactant molecules to migrate in with concomitant swelling. In contrast; TEA and the smaller cations appear small enough to diffuse in between the layers and to affect ability of the surfactant cations to enter and/or cause swelling. The interlayer separation in MCM-22 precursor is estimated around 5 A or greater since condensation upon calcination, which is accompanied by contraction, produces a 10-member ring aperture (4.1 x5.5 A). There is also a possibility that cation interaction with the SiO- moieties on the surface of the layers is the responsible or contributing factor. The following series reflects the ability of cations to enter the interlayer space and/or interact with SiO-moieties in the MCM-22 precursor: N a, TMA, TEA > C I6TMA > TPA Swelling is possible when interaction with CI6TMA is favored, which occurs when TPA but not the other cations are present. The product compositions shown in Table 1 indicate that a significant pickup of the organic species occurs even without swelling. Apparently, in the unswollen products surfactant molecules accumulate on the surface and possibly between the layers in a horizontal orientation. Table 1 Properties of the MCM-22 precursor before and after treatment with swelling mixtures MCM-22 Precursor
MCM-22 precursor treated with
NaOH/ TEA-OH/ TPA-OH/ C16TMA.OH C16TMA-CI C16TMA-CI C16TMA-CI unswollen
XRD features
002 peak, deg. 8-10 deg. region 5.5 deg. peak Composition, wt. % SiO2 AI203
Molar ratio
6.5 -6.2 -6.2 separated 101 and 102 peaks no no no 73.80 5.3
48.00 6.1
54.60 6.4
swollen
absent band without trough present 38.90 4.7
44.20 3.3
Na
1.4
1
0.35
0.2
0.02
N
1.85
2.12
2.13
2.29
2.34
C
9.5
26
24.2
35.6
32.2
SiO2/AI203
23.7
13.4
14.5
14.1
22.8
(Na+N)/AI C/N
1.9 6 **
1.6 14.3
1.3 13.3
1.9 18.2
2.6 16.1
12.9 0.16
32.5 0.60
30.4 0.50
43.8 1.01
39.9 0.84
Estd. organic content wt% * Organic/solid weight ratio
*Sum of wt % of C+N+H; hydrogen content approximated at twice the molar amount of carbon **Fixed based on composition and used to calculate % C.
279 The silica/alumina molar ratios of the swollen products in Table 1 show an interesting trend. All of the MOH/CI6TMA-C1 treated samples show the value -14/1, which is much lower than the original 23/1. This indicates significant dissolution of silica and probable destruction of some portion of the crystalline product. In comparison, the CI6TMA -OH swollen product retained its original composition. This aspect of the use of MOH/CI6TMA-C1 mixtures for swelling suggests the need for caution and may warrant closer attention. CONCLUSIONS The layers in MCM-22 precursor can be separated by treatment with a hexadecyltrimethylammonium (C16TMA) hydroxide. The product, consisting of alternating layers of MCM-22 and surfactant bilayer (with thickness 25 and 25-30 A, respectively) can be identified and quantified based on unique XRD features. High pH mixtures obtained by mixing the surfactant halide and sodium or tetraalkylammonium hydroxide were investigated as altemative swelling media. It was found that only the tetrapropylammonium hydroxide/ surfactant combination resulted in the swollen product. Possible reasons are exclusion of TPA from the interlayer region based on size or less favorable interaction with SiO moieties. REFERENCES
1. "Intercalation Chemistry", M.S. Whittingham and A. J. Jacobson (eds.), Academic Press, 1982. 2. A. Clearfield in "Advanced Catalysts and Nanostructured Materials, Modem Synthetic Methods", W. R. Moser (editor), Academic Press, 1996, 345. 3. K. Otsuka, Chem. Mater., 9 (1997) 2039. 4. M.E. Leonowicz, J.A. Lawton, S.L. Lawton, and M.K. Rubin, Science, 264 (1994) 1910. 5. S.L. Lawton, A.S. Fung, G.J. Kennedy, L.B. Alemany, C.D. Chang, G.H. Hatzikos, D.N. Lissy, M.K. Rubin, H.C. Timken, S.E. Steuemagel, and D.E. Woessner, J. Phys. Chem., 100 (1996) 3788. 6. W.J. Roth, C.T. Kresge, J.C. Vartuli, M.E. Leonowicz, A.S. Fung, and S.B. McCullen, in "Catalysis by Microporous Materials" (Studies in Surface Science and Catalysis, vol. 94), H.K. Beyer, H.G. Karge, I. Kiricsi, J.B. Nagy (eds.), Elsevier, 1995, 301. 7. W.J. Roth and J. C. Vartuli, in "Nanoporous Materials II" (Studies in Surface Science and Catalysis, vol. 129), A. Sayari, M. Jaroniec and T.J. Pinnavaia (eds.), Elsevier, 2000, 501. 8. (a) A. Corma, U. Diaz, M.E. Domine, and V. Fornes, Angew. Chem. Int. Ed., 39 (2000) 1499; (b) A. Corma, V. Fornes, S.B. Pergher, T.L.M. Maesen and J.G. Buglass, Nature, 393 (1998) 353. 9. M.E. Landis, B.A. Aufdembrink, P. Chu, I.D. Johnson, G.W. Kirker and M.K. Rubin, J. Am. Chem. Soc., 113 (1991) 3189. 10. J.A. Malinowskij and N. W. Below, Dokl. Akad. Nauk SSSR, 1979, 99. 11. (a) C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359, 710 (1992). (b) 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., 144 (1992) 10834.
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Studies in Surface Science and Catalysis 141 A. Sayariand M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
281
Control of mesopore structure o f smectite-type materials synthesized with a hydrothermal method Masayuki Shirai, a Kuriko Aoki, Kazuo Torii, b and Masahiko Arai c a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan
a
b Supercritical Fluid Research Center, National Institute of Advanced Industrial Science and Technology, Nigatake, Miyagino, Sendai, 983-8551, Japan c Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan
Mesoporous smectite-type (MST) materials containing catalytically active magnesium or cobalt divalent cations in octahedral sheets (MST(Mg) or MST(Co)) were synthesized from water glass and metal chloride with a hydrothermal method. Mesopore structure of MST materials were controlled by the calcination of the mixture of alkylammonium chloride molecules and silicate fragments synthesized with the hydrothermal method.
1. INTRODUCTION Thermally stable mesoporous materials may be useful catalysts and supports [1 ]. Porous smectite-type materials having catalytically active divalent cations in the octahedral sheets are synthesized by a hydrothermal method without adding any template [2, 3]. Magnesium divalent cations in the lattice of the smectite-type materials synthesized showed high activities for the formation of dimethyl carbonate and ethylene glycol from ethylene carbonate and methanol [4]. Cobalt divalent cations in the lattice of the smectite-type materials synthesized showed high activities for hydrodesulfurization of thiophene [5, 6]. For the increase of the number of active sites, the enlargement of surface areas of MST materials is preferable. The enlargement of pore volumes and pore diameters is also desirable for easy diffusion of large molecules and for preventing blockage of pores by carbonaceous materials during reactions. In this paper, we report the control of pore properties (surface area, pore volume, and pore diameter) of smectite-type materials containing catalytically active species in lattice.
282 2. EXPERIMENTAL 2.1. Preparation of smectite-type materials containing Mg ~§ in octahedral sheets Smectite-type materials containing magnesium divalent cations in octahedral sheets were prepared with a hydrothermal method [7]. A Si-Mg hydrous precipitate was obtained by adding an aqueous solution of magnesium chloride to an aqueous water glass solution of controlled pH with an aqueous ammonium solution. The Si/Mg ratio was fixed at 8/5.8. After filtration and washing the precipitate with distilled water, a slurry was prepared from the Si-Mg hydrous oxide precipitate and an aqueous ammonium solution. Following a hydrothermal reaction, the resulting slurry (Slurry Si-Mg) was filtrated and calcined, and then the final product (MST(Mg)T, T: hydrothermal temperature) was obtained. We prepared other smectite materials containing magnesium divalent cations in lattice with dialkyldimethyl quaternary ammonium chloride containing 75% octadecyl, 24% hexadecyl, and 1% octadecenyl groups as alkyl groups (trade name: 2HT-75, Lion Akzo Co., Ltd.). Following the hydrothermal reaction, 2HT-75 was added to Slurry Si-Mg. The final product (MST(Mg)T+2HT75) was obtained by calcination of the mixture of 2HT-75 and Slurry Si-Mg. 2.2. Preparation of smectite-type materials containing Co 2+in octahedrai sheets Smectite-type materials containing cobalt divalent cations in octahedral sheets were also prepared with a hydrothermal method [8]. A Si-Co hydrous precipitate was obtained by adding an aqueous solution of cobalt chloride to an aqueous water glass solution of controlled pH with sodium hydroxide. The Si/Co ratio was fixed at 8/5.8. After filtration and washing the precipitate with distilled water, a slurry was prepared from the Si-Co hydrous oxide precipitate and an aqueous sodium hydroxide solution. Following hydrothermal reaction, the resulting slurry (Slurry Si-Co) was filtrated and mixed with aqueous ammonium chloride (NH4C1) solution. After filtration and calcination, the final product (MST(CO)T, T: hydrothermal temperature) was obtained. Other smectite materials containing cobalt divalent cations in lattice were prepared with dimethyldistearyl ammonium chloride (C18d). C18d was added to Slurry Si-Co after the hydrothermal treatment. The final product (MST(CO)T+C 18d) was obtained by the calcination of the mixture of C18d and Slurry Si-Co.
3. RESULTS AND DISCUSSION 3.1. Structure of MST(Mg) and MST(Mg)+2HT75 Figure 1 shows XRD patterns of MST(Mg)s23 and MST(Mg)523+2HT75 samples calcined at 873 K. Peaks at ca. 8~ 20 ~ 28 ~ 35 ~ 53 ~ and 61 ~ (the X-ray patterns were obtained with a Shimadzu XD-D 1 instrument using a Kct source (~ - 1.5418 A)) are assigned to (001), (02, 11), (003), (13, 20), (24, 31, 15) and (06, 33) peaks characteristic of smectite structure. All
283 MST(Mg) and MST(Mg)+2HT75 samples prepared in this study shows similar XRD patterns, which show that all the samples have smectite-type structures.
._~'~ (a)
"
"-"
c-
,
0
I
I
I
I
I
I
10
20
30
40
50
60
70
2 0 / degree Figure 1. XRD patterns of smectite-type materials containing magnesium divalent cations in octahedral sheets hydrothermally treated at 523 K and calcined at 873 K; (a) MST(Mg)523 and (b) MST(Mg)523+2HT75. The nitrogen adsorption-desorption isotherms were measured at 77 K on MST(Mg) and MST(Mg)+2HT75 samples and their pore diameters evaluated from desorption isotherms with the BJH method are shown in Figure 2. The MST(Mg) samples are micro- and mesoporous materials and the distribution of pores depends on the hydrothermal temperature. The MST(Mg)473 sample had much micropores than the MST(Mg)s23 sample. All smectites prepared in this study have large surface area and high pore volume even after calcination at 873 K. The surface areas of natural smectite clays are less than 20 m2g-1 [9]. Pillared clays are thermally stable above 773 K, having surface areas of 200-500 m2g1, because small oxide particles (pillars) induce interlayer porosity in montmorillonite [10]. Small fragments of smectite would intercalate between silicate layers (smectites fragments intercalated in smectite layers) in MST materials synthesized with the hydrothermal method, and the micropores would be formed between the layers [7]. The surface area and pore volume are enlarged by adding the quaternary ammonium chloride after the hydrothermal treatment. MST(Mg)+2HT75 samples were mesoporous materials and had no micropores. MST(Mg)T+2HT75 samples prepared had higher surface area and larger pore volume values than those of the MST(Mg)T samples that were prepared at the same hydrothermal temperature. The size distribution of silicate fragments in MST(Mg) would be similar to that of MST(Mg)+2HT75 because the hydrothermal conditions
284 for both samples were the same. The dispersion of silicate fragments during drying and calcination would relate to the pore structure of smectite materials synthesized. Bulky dialkyldimethyl ammonium cations would be adsorbed on the exchangeable sites on silicate layers and the orientation of the silicate fragments would be changed. After calcination the silicate fragments would become higher pillars which form higher pore volumes and make larger mesopores in MST(Mg)+2HT75 samples. There is no micropore in MST(Mg)+2HT75 samples because all silicate fragments become pillars or disperse in mesopores. Higher temperature of the hydrothermal reaction would increase the size of silicate fragments. MST(Mg)523 would have larger silicate fragments (higher pillars) compared with those of MST(Mg)473 , and then the pore size of MST(Mg)523+2HT75 became larger than that of MST(Mg)473+2HT75.
0.06
0.06 ~,
,_.<
0.05
MST(Mg) 473
0.05 f
575 m2g 1
MST(Mg)523 401 m2g 1
0.04
'~
0.04
~ E
0.03
o
0.02
0.02
a_ O.Ol ~
0.01 I
0.5 cm3g 1
0.4 cm3g 1
0.03
b
0 10
100 Pore diameter
0/.. 10
1000
"
0.06
MST(Mg) 4 7 3 + 2 HT7.= ._ 741 m2g1
'o~ 0.04
b
n
0.01 0 10
A 100
Pore diameter/ ~,
,
, , .-.
o.o5
MsT(Mg) 523+2HT75
~<'~ 0.04
1.0 cm3g "1 ~~
0.03 0.02
1000
0.06
0.05
Q) E o>
100
Pore diameter / ~,
/A
0.9 cm3gl
o,03
"~
0.02
n
0.01
1000
516 m2g ~
0
10
100 Pore diameter
1000 /
Figure 2. Pore size distributions of smectite-type materials containing magnesium divalent cations synthesized at 473 and 523 K in the presence or absence of 2HT-75 using the same calcination temperature of 873 K.
285 3.2. Structure of MST(Co) and MST(Co)+2HT75 The XRD pattems of MST(Co)523 and MST(Co)523+C18d samples calcined at 823 K were similar to those of smectite-type materials containing magnesium divalent cations in lattice (Figure 3). No peak ascribed to cobalt oxide (Co304) was seen after calcination at 823 K, suggesting that cobalt cations were incorporated in the lattice for MST(Co) and MST(Co)+C18d materials and the both samples were composed of only silicate fragments having smectite structure.
~
o~
~
t'-
(a) I
I
I
I
I
I
10
20
30
40
50
60
70
20/degree Figure 3. XRD patterns of smectite-type materials containing C o 2+ in octahedral sheets hydrothermally treated at 523 K and calcined at 823 K; (a) MST(Co)s23 and (b) MST(Co)523+C18d. The pore size distributions of smectite-type materials containing cobalt divalent cations from desorption isotherms with the BJI-I method are shown in Figure 4. All samples have large surface areas and pore volumes after calcination at 823 K. The pore formation mechanism of smectite-type materials containing cobalt divalent cations would be the same as that containing magnesium divalent cations. The pore properties of MST(Co) and MST(Co)+C18d samples are also related to the size and orientation of small silicate fragments having smectite structure between silicate layers. Small silicate fragments having smectite structure would be intercalated between silicate layers, and micropores would be formed between the layers in MST(Co) samples. The surface areas, pore volumes and pore sizes of the MST(Co)T+C 18d materials are much larger than those of the MST(Co)T materials, even though sizes of silicate fragments of MS~(CO)T and MST(Co)T+C18d materials would
286 be similar. The silicate fragments would become pillars between layers and the space between the fragments would become larger mesopores in MST(Co)+C18d samples. The formation of the large mesopores by pillaring of silicate fragments would lead to the increase of the surface area and pore volume as described in the section 3.1 (MST(Mg)+2HT75). The mechanism of pore formation in the present MST materials is different from those of MCM-41 and FSM-16, for which pore sizes are the same as templates materials (micelle of surfactant in solution) [11, 12].
0.03
0.0,3 . . . . . . . . .
MST(C'o~:~"
z~
MST(Co) sz3
"
371 m2g4 0.2 cm3g4
~,~ 0.02
g
Z~
205 m2g-1 0.2 cm3g-1
g 0.02 E
--~ 0.01
s O.Ol
p.
~5
n EL ....
0
10
0.04
'~
0.03
E
D s
,
,
, , ,,
100
........ ~ 0.04
516 m2g"1 . cm3g1
0.03
1000
MST(Co) ~aa+Cl"Sd" " 409 mag -1 1.1 em3g -1
E 0.02 .o
0.01 010
Pore diameter //~.
0.05
MST(Co)423+C18d
0.02
EL
100
1000
Pore diameter / ,~
0.05 ~-~
,
0_ ,
, 100 Pore diameter / A
.......
1000
0.01
%
1O0 Pore diameter / .~
1000
Figure 4. Pore size distributions of smectite-type materials containing cobalt divalent cations synthesized at 423 and 523 K in the presence or absence of C18d using the same calcination temperature of 823 K. Based on the above-mentioned pore formation mechanism of MST samples (silicate fragment pillared smectite), we have developed a method for the control of pore structure of MST(Co) materials. The mixture of Slurry Si-Co hydrothermally treated at 423 K, Slurry SiCo at 523 K, and C18d molecules was calcined at 823 K to prepare MST(Co)423,523+C18d. The mesopore size and surface area of MST(Co)423,523+ C18d were between those of MST(Co)423+C18d and MST(Co)523+C18d. The pore distribution of MST(Co)423,523+C18d was narrower than that of the physical mixture of MST(Co)423+C18d and MST(Co)523+C18d. Mesopores of around 65 A size of MST(Co)423+C18d sample would be formed with (smaller) silicate fragments hydrothermally synthesized at 423 K, and mesopores of around 100 A of
287
MST(Co)523+C18dwould be formed with (larger) silicate fragments at 523 K. Mesopores of around 80 A size of MST(Co)423,523+C18d would be formed with the mixture of the smaller and larger silicate fragments.
0.05 .... , . . . . . . z (a) MST(Co) 423+O18d (c) MST(Co)423,523+C1 8d ~i~,7 0.04 - 516 m2gt (a) 486 m2g-1 1.2 cm3g-1 I~1:= 1.1 cmag-1 ~~)
o
"~
o
/ .,,'
_=oO>o
'A
/7/\'
E
0
~ 20
_.~~_, ~ / J 40
,
(b) MST
, ,\
/ ," /
t3.
(b)
,,credo'
"~'~ \
' ,
,
W
409 m2g1
'~\ the mixture of '~,~L__ (a)and!b)
60 80 100 Pore diameter /
,~
300
Figure 5. Pore size distributions of MST(Co)+C 18d samples.
ACKNOWLEDGEMENT This study was partially supported by Industrial Technology Research Grant Program in 01B67003d from New Energy and Industrial Technology Development Organization (NEDO) of Japan.
REFERENCES 1. A. Corma, Chem. Rev., 97 (1997) 2373. 2. M. Shirai, K. Aoki, K. Torii and M. Arai, Appl. Catal. A, 187 (1999) 141. 3. M. Shirai, K. Aoki, Y. Minato, K. Torii and M. Arai, Stud. Surf. Sci. Catal., 129 (2000) 435. 4. B. M. Bhanage, S. Fujita, Y. Ikushima, M. Shirai, K. Torii and M.Arai, submitted. 5. M. Arai, Y. Minato, K. Torii and M. Shirai, Catal. Lett., 61 (1999) 83. 6. K. Aoki, Y. Minato, K. Torii, M. Shirai and M. Arai, Appl. Catal. A, 215 (2001) 47.
288 7. M. Shirai, K. Aoki, T. Miura, K. Torii and M. Arai, Chem. Lett., (2000) 36. 8. M. Shirai, K. Aoki, K. Torii and M. Arai, submitted. 9. D. E. W. Vaughan and R. J. Lussier, Proc. Fifth Intern. Conf. Zeolites (Ed. L. V. C. Rees), (1980) 94. 10. T. J. Pinnavia, Science, 220 (1983) 365. 11. T. Yanagisawa, T. Shimizu, K. Kuroda and C. Kato, Bull. Chem. Soc. Jpn., 63 (1990) 988. 12. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Wartuli and S. J. Beck, Nature, 359 (1992) 710.
~tuales in ~urrace ~clence ana t~atalysls 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
289
Synthesis, characterization and catalytic application o f m e s o p o r o u s sulfated zirconia Young-Woong Suh and Hyun-Ku Rhee School of Chemical Engineering and Institute of Chemical Processes Seoul National University, Kwanak-ku, Seoul 151-742, Korea
Mesoporous sulfated zirconia with high surface area was prepared by various surfactantassisted methods. The organic-zirconia mesophase composite was synthesized using the interaction between the positive-charged surfactant molecule and zirconium sulfate, but a structural collapse resulted from the calcination at 550 ~ The as-synthesized composite was then treated with sulfuric acid or phosphoric acid before calcination. The calcined samples were also post-treated with sulfuric acid again by ion exchange or impregnation method in order to develop sulfated zirconia with high surface area. In addition, we have synthesized mesoporous zirconia following the synthetic method of MSU-4 and also prepared the mesoporous sulfated zirconia by reflux method without using the surfactant. The prepared catalysts were applied to the alkylation of isobutane with 1-butene to produce highly branched paraffins and their catalytic activity was compared with those of the conventional sulfated zirconia catalysts.
1. INTRODUCTION It was in 1962 that Holm and Bailey [1] made the first report on the synthesis and catalytic application of sulfated zirconia (SZ) for both isomerization and alkylation reactions in the temperature range from 25 ~ to 400 ~ Sulfated zirconia was not widely studied until 1979 when Hino and Arata [2] reported the isomerization of n-butane over sulfated zirconia at 25 ~ in conjunction with Hammett acidity function measurements of the material. Although the superacidic character is uncertain [3], sulfated zirconia catalysts have received much attention because of their low-temperature catalytic activity in hydrocarbon conversion reactions such as n-butane isomerization and the alkylation of isobutane with butenes. Sulfated zirconia have been suggested as a replacement for the toxic liquid and solid supported acids, which are currently used as catalysts in these industrial processes [4,5] but can cause equipment corrosion and environmental pollution. Sulfated zirconia is typically prepared by contacting an amorphous hydrous zirconia, obtained by alkaline hydrolysis of the zirconia precursor, with a solution of ammonium sulfate or sulfuric acid, followed by calcination at temperatures varying from 500 to 650 ~ The standard preparation method generates solids with surface areas in the range of 70 to 120 m Z/g. A great deal of attention has been attracted by the idea that zirconia-based materials with much higher surface area may be prepared in the presence of surfactants. In addition,
290 mesoporous sulfated zirconia may be prepared in one step by introducing sulfate ions during the synthesis or by using sulfur-containing surfactants. Sayari and co-workers [6,7] extended the surfactant-templating technique to the synthesis of mesostructured zirconium oxide. The use of Zr(SO4)2 precursor in the presence of longchain quaternary ammonium salts or primary amines as templates led to the formation of hexagonal or lamellar ZrO2 phase, respectively. Both hexagonal and lamellar structures collapsed upon removal of the surfactant either by high temperature calcination or by solvent extraction [7]. However, the hexagonal form was successfully stabilized by post-synthesis treatment with potassium phosphate at room temperature followed by air calcination at 350 ~ It did obtain a surface area exceeding 500 m2/g. A similar approach for the synthesis of zirconium oxide was independently developed by Ciesla et al. [8,9]. Using both zirconium sulfate and cetyltrimethylammonium chloride, they obtained a thermally unstable hexagonal phase. The material was stabilized by treatment with phosphoric acid. After calcination, the product obtained a surface area of 230 m2/g. Knowles and Hudson [10,11] also prepared a mesoporous, high surface area zirconium oxide in a basic medium. At a pH from 11.4 to 11.7, zirconium species formed a gel, which afforded zirconium oxide samples with surface areas in the range of 238 to 329 m~/g. They proposed a mechanism that the positive surfactant ions were first exchanged with the protons in zirconium hydroxide gel and, subsequently, the inorganic structure was condensed by controlled heating and scaffolding. Sachtler and co-workers [12] prepared mesoporous ZrO2 using Zr isopropoxide in the presence of hexadecane amine as template. After synthesis, the surfactant was successfully removed by extraction with ethanol at 80 ~ A material with BET surface area of up to 347 m2/g and a pore radius of 18.5 A was obtained. They found that, as in the conventional sulfated zirconia [ 13], the treatment of mesoporous zirconium oxide with H2SO4 improved its thermal stability. The material did not show higher activity in n-butane isomerization compared to the conventional sulfated zirconia catalysts. However, the beneficial effect of the mesopores was best demonstrated in the alkylation of 1-naphthol with 4-tert-butylstyrene. Based on the above-mentioned literature survey, we focus on the synthesis of thermally stable mesoporous sulfated zirconia catalysts. Three synthetic routes to pursue this goal have been studied: (1) the organic-zirconia mesophase composite is post-treated with sulfuric acid or phosphoric acid; (2) PEO-based nonionic surfactant is first used to synthesize zirconium oxide on the basis of the synthetic method of MSU-4 [14]; (3) mesoporous sulfated zirconia is prepared without the surfactant by using zirconium sulfate as a zirconium precursor. The prepared catalysts are applied to the alkylation of isobutane with 1-butene and the catalytic activities of mesoporous sulfated zirconia catalysts are compared to those of the conventional sulfated zirconia catalysts.
2. EXPERIMENTAL
2.1. Synthesis of organic-zirconia mesophase composite Cetyltrimethylammonium bromide (CTAB; 2.5 g, 6.87 retool) was dissolved in water (85 g), and to this solution Zr(SOa)2"4H20 (4.55 g, 0.0128 mol) dissolved in water (25 g) was added [~. This led to a colorless precipitate. The mixture was stirred at room temperature for 2 h and then placed in an autoclave for hydrothermal reaction, and kept at 100 ~ in oven for 2 days. The solid product obtained was filtered and dried at 100 ~ overnight.
291
2.2. Post-treatment of as-synthesized samples The samples were treated with sulfuric acid or phosphoric acid before calcination to enhance the thermal stability. The as-synthesized samples were added to each acidic solution (0.5 M) and the solution was stirred for 5 h. After filtration, the colorless product was dried at 100 ~ overnight and calcined in air at 550 ~ for 5 h. The calcined samples were also treated with an acidic solution again by ion exchange or impregnation method. Then, the remaining phosphate or sulfate functional group was removed by evacuation at room temperature.
2.3. Synthesis of MSU-4-type zirconia MSU-4-type zirconia was synthesized in an open container from aqueous mixtures of zirconium precursor (Zr n-propoxide or zirconium sulfate), surfactant, and sodium mineralizer. The molar composition of each reaction mixture was Tween 20 (polyoxyethylene sorbitan monolaurate), 0.02; zirconium source, 0.16; NaF, 0.004, and H20 or EtOH, 55.6. In a typical preparation, 1.028 g of Tween 20 was first dissolved in 50 mL of water in using zirconium sulfate or EtOH in using Zr n-propoxide at 35 ~ Once surfactant dissolution was complete, 5.667 g of 40 wt% zirconium sulfate solution or 3.744 g of 70 wt% zirconium npropoxide was added. The reaction mixture was stirred slowly at 35 ~ for 24 h. Sodium fluoride (0.84 mL of a 0.238 M NaF solution) was then added to the zirconium/surfactant solution. The solution so obtained was aged under slow shaking at 35 ~ for 2 days. A white colloidal suspension appeared progressively. The powder was filtered, dried, and calcined in air at 200 ~ for 6 h and then at 620 ~ for 6 h. The heating rate was 5 ~
2.4. Synthesis of mesostructured zirconia prepared without surfactant The samples of hydrous zirconia were prepared via the conventional route. 28-30 % ammonia was slowly added to 100 ml of 0.4 M solution of zirconyl chloride or zirconium sulfate under vigorous stirring until the target pH value of 10.0 was attained in each preparation. The pH adjusted precipitated slurries were then stirred under reflux at 90 ~ during the 20 h digestion period. Afterwards, the hydrous zirconia was recovered via vacuum filtration and washed with excess water to remove chloride ions, and was then dried at 100 ~ All hydrous zirconia samples were subsequently sulfated by contact with 0.5 M sulfuric acid (5 ml/g) for 1 h under slow stirring. The sulfated zirconia was recovered via vacuum filtration without washing and dried at 100 ~
2.5. Characterization The structure and order of mesoporous zirconia samples were determined by X-ray powder diffraction (XRD) and small-angle X-ray scattering (SAXS) analyses. The XRD patterns were obtained by using CuKot radiation (~=1.54056A). SAXS patterns were recorded on BRUKER GADDS operating at 3 kW of X-ray. TE.M images were obtained on a JEOL 2000EXII electron microscope with an LaB6 filament as a source of electrons operated at 200 kV. Infrared spectra were recorded on a Nicolet Impact 410 FT-IR instrument (in the range of 4000 to 500 cm -1) using the self-supported wafer. The specific surface areas and average pore distributions were determined by nitrogen physisorption at liquid nitrogen temperature using a Micrometrics ASAP 2010.
2.6. Catalytic experiment Alkylation of isobutane with 1-butene was carried out in a fixed-bed flow reactor at atmospheric pressure. The reaction in the gas phase condition can lead to a low concentration
292 of the reactants in the reaction system so that the deactivation rate of catalysts is slow. Isobutane was first charged into the reactor at room temperature to avoid fast polymerization of olefins. Once the desired reaction temperature was stabilized, the reaction was started by feeding 1-butene into the reactor. An isobutane/1-butene volumetric flow rate ratio of 10 was used and the olefin space velocity, WHSV, was kept at 3 h -~ in these experiments. The reactants and the products were quantitatively analyzed by gas chromatography using an FID and a 50 m HP-1 capillary column. Data at different times on stream were obtained from the samples extracted directly from the reactor. The sulfated zirconia catalysts were activated in the reactor by calcining it in air at 550 ~ for 3 h. Air flow was then replaced by nitrogen and the catalyst temperature was lowered to room temperature.
3. RESULTS AND DISCUSSION
3.1. Organic-zirconia mesophase composite We synthesized the organic-zirconia mesophase composite on the basis of the procedure reported by Ciesla et al. [8,9]. As a result, the XRD patterns showed two reflections at very low angles, corresponding to d spacings of 4.28 and 2.49 nm, respectively. These reflections can be indexed as (100) and (110), respectively, assuming a hexagonal unit ceil. However, the calcination of this composite resulted in a structure collapse. To stabilize the structure, the surfactant composite was treated with phosphoric acid [8,9]. Figure 1 shows XRD patterns of the sample treated by different methods. When the assynthesized material was treated with H2SO4 solution followed by calcination at 550 ~ the structure was collapsed as may be noticed from curve e of Fig. 1. On the other hand, the XRD pattern of the sample obtained after the phosphoric acid treatment is similar to that of the organic-zirconia mesophase composite except for a small shift (0.1 nm) of the reflections (cf. curve a of Fig. 1). After calcination at 550 ~ the materials show highly intensive (100)
5
j
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i
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Figure 1. XRD pattems of the samples post-treated by different methods; (a) H3PO4treated sample, (b) calcined H3PO4-treated sample, (c) calcined H3PO4-treated sample followed by the impregnation of H2SO4, (d) calcined H3POa-treated sample followed by the ion-exchange of H2SO4, (e) calcined H2SOn-treated sample.
293 reflections, and weak (110) and (200) reflections as shown by curve b of Fig. 1. During calcination a strong shrinkage of the pores occurs, which can be seen in the shift of the (100) reflections to a smaller d spacing of more than 1 nm. The BET surface area of this calcined H3PO4-treated sample is found to be 370 mZ/g. In the present study, our aim is to develop sulfated zirconia with high surface area. Therefore, H3PO4-treated sample should be treated again with sulfuric acid by ion exchange or impregnation method. In case of using ionexchange method, a substantial structure collapse is observed as may be seen in curve d of Fig. 1 whereas the materials impregnated with sulfuric acid still exhibits an intense (100) reflection (cf. curve c of Fig. 1), which is similar to that of the calcined H3PO4-treated samples. Figure 2 shows the FT-IR spectra of several samples. H3PO____4-treatedsample obtains a broad intense band around 1036 cm -1 and two sharp bands for the C-H vibration at 2900 Figure 2. FT-IR spectra of (a) as-synthesiz- and 2846 cm -1. The former indicates the ed sample, (b)H3PO4-treated sample, and presence of phosphate ions while the latter (c) calcined H3PO4-treated sample, shows that the surfactant is not extracted after H3PO4_treatment. 3.2. MSU-4-type zirconia First of all, the mesoporous MSU-4 silica was synthesized on the basis of the recipe reported by Prouzet et al. [14]. Small-angle X-ray scattering (SAXS) pattern of this silica resembles those obtained with MSU-X materials with a single correlation peak due to the 3D wormhole porous framework structure. The pattern for the calcined sample contains a broad diffraction peak centered at d spacing of 5.0 nm. Following the synthesis method for MSU-4 silica, we have synthesized the MSU-4-type zirconia. After subtracting the background from the pattern shown in Fig. 3a, the resulting pattern contains two diffraction peaks at 2.94 ~ and 29.85 ~ of 20, respectively. The first diffraction peak indicates some mesoporosity with d spacing of 3.0 nm in analogy to the mesoporous silica. The second peak at 29.85 ~ is
Figure 3. SAXS pattern (a) and SEM image (b) of the calcined MSU-4-type zirconia. The inset of (a) shows the 2-dimensional image of X-ray pattern.
294
Figure 4. TEM images of calcined MSU-4-type zirconia with magnification of (a) 25,000 and (b) 150,000, respectively. characterized by the tetragonal phase of calcined zirconia [15]. This suggests that MSU-4type zirconia is mesoporous but the crystalline pore ,walls are tetragonal, which is supported by the results reported by Sachtler and co-workers [12]. Figure 3b shows the SEM image of the calcined MSU-4-type zirconia synthesized with Tween-20, clearly indicating well-defined elementary spherical particles with a mean particle size of 0.8 ~tm. This is further confirmed by the TEM images. As shown in Fig. 4a, the calcined zirconia sphere retained the spherical morphology with non-uniform sizes of 500 to 800 nm. Also, large aggregates of a framework with wormhole structure are observed in Fig. 4b and this is analogous to those obtained from MSU-X silica prepared by several PEO surfactants [14, 16-17]. However, this MSU-4-type zirconia is different from the mesoporous zirconia prepared by PEO-based nonionic block copolymers and zirconium chloride, exhibiting a hexagonal or cubic mesostructure with a semi-crystalline framework [18,19]. Apparently, the difference was caused by the synthesis medium; the hexagonal mesoporous zirconia was prepared by using a metal halide as the inorganic precursor, while the MSU-4-type zirconia with wormhole structure was synthesized by using metal alkoxide in a non-aqueous medium (e.g., EtOH), instead of aqueous one. Therefore, the question is which medium, acidic or neutral, is used for the synthesis of mesoporous zirconia. 3.3. Mesostructured zirconia prepared without surfactant In recent years, Risch et al. [20,21] have synthesized the mesoporous sulfated zirconia by digesting the freshly precipitated hydrous zirconia under reflux at 90 ~ Then, the hydrous zirconia was prepared by zirconyl chloride as a zirconium source. This sample showed a type IV isotherm with hysteresis loop, indicative of mesoporoisty [22]. In order to introduce the sulfate groups during the synthesis of mesoporous hydrous zirconia, we have utilized zirconium sulfate as a zirconium source. According to the discussion for the nitrogen adsorption isotherm of type II [22], this sample exhibits a textural porosity, as evidenced by the adsorption step and the uptake of N2 above a partial pressure of 0.80. The surface area determined by the BET method is close to 145 mZ/g. Figure 5 shows the TEM images of the sulfated zirconia prepared by zirconium sulfate at pH = 10 under
295
Figure 5. TEM images of the sulfated zirconia prepared by zirconium sulfate as a zirconium source under reflux condition. Right image is obtained by the magnification of the selected part in left image. reflux condition. As may be seen in Figure 5a, this material contains aggregates of particles having sizes from 5 to 10 nm, and these a~gregates were also observed by SEM. Figure 5b represents pore channels aligned in the same plane with no preferred direction and a distance between channels measured to be 10 A. This distance is relatively small, when compared to the one observed in a hexagonal structure. These characteristics of the sample prepared by zirconium sulfate are very much different from those of the sample prepared by zirconyl chloride. It is most likely that such difference is caused by the environment of zirconium ions which results from the complexing ability of sulfate and chlorine groups [23].
3.4. Catalytic alkylation of isobutane with 1-butene The gas phase alkylation of isobutane with 1-butene was chosen as the test reaction to investigate the catalytic activity of the mesoporous sulfated zirconia synthesized in this work. Recently, the sulfated zirconia (SZ) catalyst has also been applied to this reaction, which was carried out in a continuous flow reactor [24,25]. The experimental results in this work were obtained at a short period of reaction time (7 rain) to observe the initial reactivity before the fast deactivation occurred. As a result, the data for the conventional SZ catalysts are found to be in good agreement with the published data of Das et al. [25]. The results obtained over the mesoporous SZ catalysts of this work exhibit an increase of about 30% in the 1-butene conversion. This indicates that the mesoporous SZ catalysts are more active than the 9conventional SZ catalysts for the alkylation of isobutane with 1-butene. Also, the product distribution is found different between the two catalysts. The mesoporous SZ catalyst produces a larger amount of C5-7 hydrocarbons than the conventional SZ catalyst. Among C8 hydrocarbons produced over the mesoporous SZ catalyst, the selectivity to trimethylpentanes is 67.8%, which is much higher than the selectivity of 15.4% to dimethvlhexanes.
4. CONCLUSIONS The organic-zirconia mesophase is synthesized by using the interaction between CTAB and zirconium sulfate. This composite is also treated with phosphoric acid to prevent the structural collapse caused by calcination at 550 ~ For the purpose of developing sulfated
296 zirconia with high surface area, the calcined composite is post-treated with sulfuric acid by impregnation method, which can lead to the formation of mesoporous sulfated zirconia. The MSU-4-type zirconia is also prepared successfully in non-aqueous media by following the synthesis method for MSU-4 silica. This mesoporous material has a wormhole structure and a spherical morphology with non-uniform sizes. The sulfated zirconia is synthesized without using surfactant but by using zirconium sulfate, which yields a material having a textural porosity and pore channels with a distance of 10A. When applied to the alkylation of isobutane with 1-butene, the mesoporous sulfated zirconia synthesized in this work is found to be more active than the conventional sulfated zirconia.
ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of SK Chemicals and the partial aid from the Brain Korea 21 Program sponsored by the Ministry of Education.
REFERENCES 1. V.C.F. Holm and V. C. F. Bailey, U.S. Patent 3,032,599. 2. M. Hino, S. Kobayashi and K. Arata, J. Am. Chem. Soc., 101 (1979) 6439. 3. T.-K. Cheung and B. C. Gates, Chemtech, 27 (1997) 28. 4. M. Misono and T. Okuhara, Chemtech, 23 (1993) 23. 5. T. Yamaguchi, Appl. Catal., 61 (1990) 1. 6. A. Sayari, P. Liu and J. S. Reddy, Mater. Res. Soc. Symp. Proc., 431 (1996) 101. 7. J.S. Reddy and A. Sayari, Catal. Lett., 38 (1996) 219. 8. U. Ciesla, S. Schacht, G. D. Stucky, K. K. Unger and F. Schtith, Angew. Chem., Int. Ed. Engl., 35 (1996) 541. 9. U. Ciesla, M. FrSba, G. Stucky and F. Schtith, Chem. Mater., 11 (1999) 227. 10. J. A. Knowles and M. J. Hudson, J. Chem. Soc., Chem. Commun., (1995) 2083. 11. J. A. Knowles and M. J. Hudson, J. Mater. Chem., 6 (1996)89. 12. Y. Y. Huang, T. J. McCarthy and W. M. H. Sachtler, Appl. Catal., 148 (1996) 135. 13. X. Song and A. Sayari, Catal. Rev.-Sci. Eng., 38 (1996) 329. 14. E. Prouzet, F. Cot, G. Nabias, A. Larbot, P. Kooyman and T. J. Pinnavaia, Chem. Mater., 11 (1999)1498. 15. P. S. Kumbhar, V. M. Yadav and G. D. Yadav, Chemically Modified Oxide Surfaces, Gordon and Breach, 1989, p. 81. 16. S. A. Bagshaw, E. Prouzet and T. J. Pinnavaia, Science, 267 (1995) 865. 17. E. Prouzet and T. J. Pinnavaia, Angew. Chem., Int. Ed. Engl., 36 (1997) 516. 18. P. Yang, D. Zhao, D. I. Margolese, B. F. Chmelka and G.D. Stucky, Nature, 396 (1998) 152. 19. P. Yang, D. Zhao, D. I. Margolese, B. F. Chmelka and G. D. Stucky, Chem. Mater., 11 (1999) 2813. 20. M. Risch and E.E. Wolf, Appl. Catal. A, 172 (1998) L1. 21. M. Risch and E.E. Wolf, Catal. Today, 62 (2000) 255. 22. G. Leofanti, M. Padovan, G. Tozzola and B. Venturelli, Catal. Today, 41 (1998) 207. 23. J. Livage, M. Henry and C. Sachez, Prog. Solid State Chem., 18 (1988) 259. 24. A. Corma, A. Martinez and C. Martinez, J. Catal., 149 (1994) 52. 25. D. Das and D.K. Chakrabarty, Ener. & Fuels, 12 (1998) 109.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
297
Synthesis o f m e s o p o r o u s s i l i c o a l u m i n o p h o s p h a t e s ( S A P O ) Erica C. de Oliveira and Heloise O. Pastore* + Grupo de Peneiras Moleculares Micro- e Mesoporosas, Instituto de Quimica, Universidade Estadual de Campinas, CP 6154, CEP 13083-970, Campinas, SP, Brasil, Mesoporous silicoaluminophosphates with hexagonal organization were prepared from phosphoric acid, aluminum sulfate, and fumed silica or tetraethylorthosilicate as sources of silicon. Hexadecyltrimethylammonium bromide was the structure directing agent. The material was characterized by X-ray diffraction, pore analysis, 298i nuclear magnetic resonance with magic angle spinning, elemental analysis and derivative thermogravimetry.
1. INTRODUCTION The synthesis of aluminophosphate-based mesoporous materials have been subject to studies by several research groups since the preparation of the first silicate-based mesoporous molecular sieves [1,2]. As with the microporous aluminophosphates, the possibility of structure substitution by heteroelements in the originaly neutral framework, is advanced. Silicon substitution in the aluminophosphate framework generates anionic sites in the material [3], hence potencial acidic character that can be active in acid-catalysed reactions. Recently, we have reported the successful formation of aluminophosphate-based mesoporous materials using aluminum sulfate as a source of aluminum and without introduction of hydrofluoric acid as a mineralizing agent [4], and that of magnesiumaluminophosphates from aluminum isopropoxide [2]. This work describes two synthetic routes for the synthesis of mesoporous silicoaluminophosphates with hexagonal organization using two different sources of silicon, and hexadecyltrimethylammonium bromide as a structure directing agent. 2. EXPERIMENTAL SECTION The mesoporous silicoaluminophosphates samples were prepared by two synthetic routes, one of them with fumed silica and the other with tetraethylortosilicate, both starting from phosphoric acid (Merck 85%), aluminum sulfate (Merck), water, cetyltrimethylammonium bromide (CTAB, Alfa Aesar 99%), and tetramethylammonium hydroxide (TMAOH) (Alfa Aesar, 25 wt.% aqueous solution) was added until pH = 10,5. The reaction mixture was aged for 24 h at room temperature, transferred to an autoclave and kept at 70 ~ C for 48 h. The reaction mixture had the following composition: A1203:1.27 P205:0.76 SiO2:2 CTAB: 7.35 TMAOH: 409.6 H20. It was aged at room temperature for 24 h, and *Corresponding author e-mail:
[email protected] +This work was financed by Fundagao de Amparo/l Pesquisa no Estado de Sao Paulo, FAPESP, grant number 99/10391-8.
298 hydrothermally treated at 343K, for 2 days. After filtering and thorough washing with distilled water, the solids were extracted in a Soxhlet apparatus with a 0.15 m o l d m 3 HC1 ethanol/heptane solution 50:50 (V/V), or alternatively with a 0.30 mol.dm 3 isopropylammine/ ethanol solution. The extracted samples were heated to 773 K, at 1 K.min l, under dry argon and remained for 20h under dry oxygen at that temperature. The material after calcination was ion-exchanged three times with NH4C1 (1 mol.dm 3) at room temperature. The materials were characterized by powder X-rays diffraction (Shimadzu, XRD 6000, CuKa, 30 kV, 40 mA, 2 ~ 20 min-l), derivative thermogravimetric analysis (TA Instruments 5100, heating rate 10 K min l until 1273 K, under 100 ml min -I argon), 29Si CP MAS NMR was obtained at 99,31 MHz (Bruker AC 300/P, MAS speed of 4,5 kHz, contact time of 50 ms, and TMS as a reference), N2 adsorption (ASAP 2010, Micromeritics at 77K after thermal treatment at 298 K until residual pressure of 10-4 Pa) and chemical analysis by ICP-AES (Perkin-Elmer 300-DV, 213,618 for phosphorous and 308,215 for aluminum). 3. RESULTS AND DISCUSSION Both materials presented a X-rays diffractogram characteristic of a hexagonal organization of pores, Figure 1. The use of TEOS in the synthesis as source of silicon afforded a material more organized material than the one obtained from fumed silica. The Soxhlet extraction with acidic solution caused complete structure collapse for the fumed silica product while the product from TEOS suffered only partial disorganization indicating again the better organized structure of the sample prepared with TEOS. This was also the result in extraction with the alkaline solution as well as the calcination that promotes another partial disorganization to independent pores as shown by the presence of only the (100) signal in the X- rays diffraction, Figura 2.
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Fig. 1. X-rays diffractograms: (A) of as-synthesized samples used in this work, source of silica:(a) fumed silica; (b). TEOS; (B) X-rays powder diffractograms of samples prepared in this work, (a) from fumed silica, acid extracted; (b) from TEOS, acid extracted; (c) from TEOS, basic extracted; (d) from TEOS, basic extracted, calcinated. The elemental analysis of samples prepared here revealed that more silicon was incorporated in the structure than was primarily in the reaction gel (Table 1). The Si/A1 molar ratio was approximately 0.5 indicating that silicon substituted for phosphorus in the framework. The mass fraction of silicon indicated from this analysis in the sample is approximately 11.0 wt.% while in the synthesis gel this fraction was 6.8 wt.%.
299
Table 1 Elemental analysis for samples prepared in this work. Source of silica %A1 %P Fumed silica 22.45 + 0.44 12.56 + 0.12 TEOS 23.80 + 0.21 13.50+ 0.05
PzOs/A1203 0.49 0.49
SIO2/A1203 1.16 0.92
N2 adsorption at 77K shows an isotherm that is a mixture of types I and IV [5], Figure 3. The BET surface area is 638 mZ.gl and the pore volume 0.27 cm3.g~, both smaller than observed for other mesoporous materials [6] and mesoporous ALPO materials [2,4]. The derivative thermal analyses of samples after calcination, Figure 4A, and after NH4+ exchange, Figure 4B show signals that correspond to the water loss at 323-453 K and signals at 580-620 K that correspond to weak P-OH sites. ..-... 180 F-- 160
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A B Fig. 4 Derivative thermogravimetry analysis. A. Samples obtained to TEOS after the calcination, B. samples in A. after ion exchanged with NH4C1. In the case of NH4+-exchanged sample, the derivative thermal analysis, Figure 4B, shows that the material holds approximately the same percentage of water as the calcined material, the P-OH sites are in the same region observed before in calcinated samples, and additionally, ammonia release occurs at around 473-523 K and 773-843 K, indicating two different acidic sites. This analysis indicated that the mass fraction of silicon in acidic sites in
300 the product is 5.9 wt.%, and the ratio Si/A1 is approximately 0.24. Comparing the results with the ones of the elemental analysis it is observed that a little more the half of the silicon content of the sample are acidic. Probably some of the silicon sites are not accessible or not acidic. The 29Si CP MAS NMR spectrum, Figure 5, shows only one signal at-94 ppm with a shoulder at approximately-87 from TMS, typically assigned in the literature [7] to Si(3OA1) and Si(4OA1), respectively. This suggests that no SiO2 island was formed. r'-"- Si(3OAI) 9
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-t4o
is,!
Fig. 5. Solid-state 29Si CP MAS NMR of the calcined sample prepared using TEOS. CONCLUSIONS This work shows that mesoporous silicoaluminophosphate (SAPO) was synthesized using aluminum sulfate, phosphoric acid, CTAB, TMAOH, TEOS or fumed silica, in aqueous systems. The sample obtained from TEOS presented a better organization than the sample prepared using fumed silica. The TEOS sample showed a surface area of 638 m2.g-t with a pore volume of 0,27 cm3.g!. As in meso-ALPO and meso-MAPO [2], acid Soxhlet extraction causes structure collapse only less important in meso-SAPO as compared with the former two. An alkaline extraction however preserves the struture and aides in organics elimination. The thermal decomposition of [NHa]-SAPO shows two acid sites, one in the range of 423-573 K and another between 773-843 K, showing probably two sites for silicon localization, with different acidites. REFERENCES
1. C.T. Kresge, M. E Leonowicz, W. J. Roth, J. C. Vartulli, J. S. Beck, Nature 359 (1992) 710. 2. N.C. Masson, H. O. Pastore, Microporous and Mesoporous Mater. 44-45 (2001) 173. 3. B. Chakraborty, A. C. Pulikottil, S. Das, B. Viswanathan, Chem. Commun. (1997) 911. 4. E.C. Oliveira, N. C. Masson, A. J. S. Mascarenhas, H. O. Pastore, submitted. 5. T. Kimura, Y. Sugahara, K. Kuroda, Chem. Mater. 11 (1999) 508. 6. A. Caudel, D. Brunel, F. DiRenzo, E. Garrone, B. Fubini, Langmuir 13 (1997) 2773. 7. L.S. de Saldarriaga, C. Sadaniaga, M. E. Davis, J. Am. Chem. Soc. 109 (1987) 2686.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
301
Synthesis and characterization of mesostructured vanadium-phosphorus-oxide phases Moises A. Carreon and Vadim V. Guliants* Department of Chemical Engineering. University of Cincinnati, Cincinnati, OH 45221-0171, USA
Mesostructured vanadium phosphorus oxide (VPO) phases with improved thermal stability and displaying desirable compositions for the partial oxidation of lower alkanes were synthesized using cationic (alkyl trimethyl ammonium salts), anionic (alkyl sulfonates and phosphates) and primary alkyl amines structure-directing agents.
1. INTRODUCTION The vanadium-phosphorus-oxide (VPO) system is the only industrial catalyst for selective oxidation of n-butane to maleic anhydride [ 1]. The current synthesis approaches for the VPO system offer a limited control over structural and compositional parameters that define its catalytic performance in oxidation catalysis [2]. Structure-directed self-assembly represents a promising method for the synthesis of mesoporous VPO phases with desirable structural, compositional and catalytic properties. Several single mesoporous inorganic oxides with very interesting structural properties have been synthesized by this route [3-8]. However, only few reports exist on the synthesis of mesostructured VPO materials. Iwamoto et al. [9] reported the synthesis of mesostructured hexagonal vanadium-phosphorus oxide materials using different alkyltrimethyl ammonium surfactants (C~2-C16). However, the mesostructure in these materials was lost during calcination. Doi and Miyake [10] reported the synthesis of a novel hexagonal mesostructured VPO compound from VOHPOa0.5H20 by surfactant intercalation and subsequent hydrothermal treatment. However, these materials suffered from poor thermal stability and low phosphorus content that are detrimental for their use as heterogeneous catalysts. Recently, Mizuno et al. [11] described the synthesis of hexagonal, cubic and lamellar mesostructured vanadium-phosphorus oxides. However, the structural order in these materials was lost upon calcination. In all of these studies, only poorly stable VPO mesophases with unoptimized (i.e., phosphorus-deficient) compositions were obtained. We report here the novel synthesis and characterization of new hexagonal and cubic vanadium phosphorus oxide (VPO) phases that display improved thermal stability, desirable compositions and pore architectures for the partial oxidation of lower alkanes.
302 2. MATERIALS AND METHODS 2.1. Materials VOSO4 (Aldrich), VO[OCH(CH3)2]3 (Alfa-Aesar) and VO(acac)2 (Alfa-Aesar) were used as vanadium sources. H3PO3 and H3PO4 (85%) obtained from Fisher Chemicals were used as phosphorus sources. The pH of synthesis solutions was adjusted with NH4OH and HC1 (Fisher Chemicals). Alkyltrimethyl ammonium bromides, CHa(CHE)nN(CHa)aBr, n=11,13,15,17 (Aldrich) were used as cationic surfactants. Monododecyl phosphate, CH3(CH2)llOPO(OH)2, and sodium hexadecane sulphonate, CHa(CH2)IsSOaNa, (Lancaster) as well as dodecyl sodium sulfate salt, CHa(CH2)llOSOaNa (Aldrich) were used as anionic surfactants. Primary alkyl amines, CH3(CHE)nNH2, n= 11,15,17 were obtained from Lancaster. 2.2. Synthesis Hexagonal and cubic VPO phases were synthesized using ionic and amine surfactants by reacting aqueous solutions containing vanadium (VOSO4, VO[CHO(CH3)2]3 or VO(acac)2) and phosphorus (H3PO3 or H3PO4) sources. In a typical synthesis, an aqueous solution containing phosphorus and vanadium sources was added to an aqueous surfactant solution. The pH of the resultant solution was adjusted with HC1 (1M) to below 1.0. The sol was homogenized by stirring for 6 hours at 343 K, precipitated with NH4OH at 2.5
303 agreement with cubic mesostructured vanadium phosphorus oxide [ 11]. No other reflections were observed at 100<20<30 ~ indicating the formation of a single mesophase in all cases. 211
..................................
200
620 ---
400 420
d "
cl e-
e-
i'
2
3
4
5
6
7
8
9
10
2 theta
Figure 1. XRD patterns of as-synthesized mesostructured VPO phases prepared using cationic surfactants: a) CH3(CH2)llN(CH3)3Br, b) CH3(CH2)13N(CH3)3Br, c) CH3(CH2)IsN(CH3)3Br and d) CH3(CH2)17N(CH3)3Br.
The XRD patterns of mesostructured VPO synthesized with anionic surfactants (alkyl sulfates and phosphates) are shown in Figure 2. The VPO phase prepared using dodecyl phosphate showed 2 reflections at 34.9 and 20.9 h corresponding to the (100) and (110) reflections of the hexagonal mesostructure, respectively. The sample synthesized using dodecyl sodium sulfate showed 3 reflections at 36.7, 18.06 and 12.5 A corresponding to the (100), (200) and (210) planes of the hexagonal mesostructure. The sample with a C16 hydrocarbon chain length showed the presence of 7 reflections at 40.6, 35.7, 30.9, 20.6, 18.1, 13.7 and 12.0 A corresponding to the (210), (211), (200), (400), (332), (620) and (444) planes of the cubic structure, respectively [11, 13]. The XRD patterns of mesostructured VPO synthesized using alkyl amine surfactants are shown in Figure 3. For short chain lengths, 3 reflections were observed at 34.0, 21.5 and 16.6" h corresponding to the (100), (200) and (210) planes of the hexagonal structure. In the case of C16 surfactant, 2 reflections were observed at 37.8 and 13.5 .h which correspond to the (210) and (620) planes of the cubic structure. For the long chain alkyl amine Cls, 4 strong reflections at 39.2, 31.6, 20.8 and 12.6 h were observed corresponding to the (210), (211), (400) and (620) planes of the cubic mesostructure [11,13,14].
304
Figure 2. XRD patterns of as-synthesized mesostructured VPO phases prepared using anionic surfactants: a) CH3(CH2)llOPO(OH)2, b) CH3(CH2)llOSO3Na and c) CH3(CH2)15SO3Na.
Figure 3. XRD patterns of as-synthesized mesostructured VPO phases prepared using alkyl amine surfactants" a) CH3(CH2)llNH2, b) CH3(CH2)IsNH2 and c) CH3(CH2)I7NH2.
305 For cationic and anionic surfactants the degree of crystallinity as well as the number of low angle reflections increased with hydrocarbon chain length leading to the formation of highly ordered mesostructures. On the other hand, for alkyl amine surfactants the extent of crystallization was low and relatively independent of the hydrocarbon chain length. The low 20 angle d-spacings of mesostructured VPO phases prepared using different surfactants (cationic, anionic and alkyl amines) as a function of the hydrocarbon chain length are shown in Figure 4. The shift of the low angle peak toward lower 20 angles with increasing hydrocarbon chain length was observed for all surfactants indicating the formation of larger mesopores. Anionic surfactants, such as alkyl phosphonates and alkyl sulfates showed larger d-spacings mainly because of the larger size of anionic headgroups as compared to cationic and alkyl amine headgroups [15].
Figure 4. Low 20 angle d-spacings (A) of mesostructured VPO phases synthesized using cationic, anionic and alkyl amine surfactants as a function of the hydrocarbon chain length.
For all surfactants, the synthesis pH had a significant impact on the formation of mesostructured VPO phases. Hexagonal phases were synthesized in the pH range of 2.203.18. Cubic phases were observed only in the 3.30-3.43 range. Lamellar and amorphous phases were observed at pH>4.0. A similar formation sequence as a function of the solution pH has been reported previously for other mesostructured vanadium phosphorus oxides and associated with changes in the structure of surfactant mesophases [ 11,15]. The XRD patterns for the as-synthesized and calcined mesostructured VPO phases prepared using VOSO4 and H3PO4 as vanadium and phosphorus sources and a CH3(CH2)lsN(CH3)3Br surfactant are shown in Figure 5. The as-synthesized mesostructured VPO phase (Figure 5a) shows the presence of 3 intense reflections at low 20 angles, 2.2 ~ 3.8 ~ and 4.5 ~ corresponding to d-spacings of -39 ~ (100), 23 ,~ (110) and 19.8 A (200),
306 respectively which are in agreement with the formation of hexagonal VPO phase [9-11]. Small Angle X-Ray Scattering (not shown here) confirmed the presence of these reflections. For the calcined hexagonal VPO phase (Figure 5b), the reflection at 20 = 2.35 ~ (dspacing=37.4 A) suggests that the mesostructure is retained even after calcination in air at 673 K. A slight decrease in the d-spacing is attributed to the template removal. XRD reflections were not detected in the 10o<20<30 ~ range, indicating the formation of a single VPO phase.
Figure 5. XRD patterns of (a) as-synthesized and (b) calcined mesostructured VPO phases. The enhanced thermal stability was observed for both V 4+ and V 5+ sources. In the case of V 4+, the synthesis was conducted under acidic pH conditions that favor the formation of the V-P-O phases. In fact, the VPO phases are hydrolytically unstable at pH>5 [ 17], which may explain the limited stability of VPO phases synthesized previously at pH-7.5 [10]. In the case of V 5+, the improvement of thermal stability of mesostructured VPO phases can be explained by the self-assembly conditions at the pH in the vicinity of the isoelectric point for V(V) ions. For vanadium oxide species, the isoelectric point is around pH-~0.5 [18]. The strongly acidic conditions used in our syntheses near the isoelectric point of the V(V) ions enhanced the condensation of vanadium and phosphate ions and formation of the V-O-P linkages around at the interface with surfactant mesophases favoring the formation of hybrid organic-inorganic mesostructures. Moreover, the synthesis conditions near the isolectric point are also favorable for the structural organization of VPO species, since the condensation rate of inorganic species is slow enough to allow these species to form the V-O-P linkages. Disordered and thermally unstable mesostructured VPO phases were observed for the pH regions far away from the isolectric point (pH>4.0). A similar behavior has been observed for the formation of the silica mesostructures [19].
307 The TEM image of the as-synthesized mesostructured VPO prepared using VOSO4 and H3PO4 as vanadium and phosphorus sources and a CH3(CH2)IsN(CH3)3Br surfactant (Figure
6) displays a hexagonal array of cylindrical -38 ~ pores in agreement with the XRD data. The EDS elemental analysis showed a P/V molar ratio - 1.0, which is optimal for achieving superior catalytic performance in the oxidation of n-butane [ 1,2].
Figure 6. TEM image of as-synthesized hexagonal VPO phase.
The surface compositions and the average oxidation states of vanadium in mesostructured VPO phases were determined by XPS and double titration method, respectively [ 12]. For the sample prepared using VOSO4 and H3PO3 as the vanadium and phosphorus sources, respectively, and a cationic CH3(CH2)lsN(CH3)3Br surfactant at pH=2.87, a very phosphaterich surface (P/V-1.7) was obtained which is known to stabilize the +4 oxidation state of vanadium [20]. The average oxidation state was higher for the calcined (+4.3) than for assynthesized samples (+4.1) probably due to oxidizing activation conditions (air, 673K). The XPS results are in agreement with previous studies indicating a surface P/V ratio higher than the bulk value, i.e. P/V=I.0 [20]. Specific: surface areas for calcined phases were in the 45-60 m2/g range. Although these areas are higher than those reported for conventional VPO catalysts (5-20 m2/g), much higher surface areas were expected for the mesoporous phases. Remaining occluded surfactant species as well as the presence of amorphous regions were responsible for these relatively low surface areas. In fact, the XPS experiments for the sample prepared using VOSO4 and H3PO3 as vanadium and phosphorus sources and a cationic CH3(CH2)IsN(CH3)3Br surfactant at pH=2.87, confirmed the presence of the organic material in the calcined mesostructured VPO structure. According to the XPS data, the calcined mesostructured VPO phases contained-30 % of the original carbon present in the as-synthesized sample. Optimal synthesis conditions for achieving complete template removal are currently under investigation. Mesostructured hexagonal and cubic VPO phases can be prepared employing cationic, anionic and alkyl amine surfactants. The formation of well-defined mesostructures depends highly on the nature of the surfactant headgroup. Shorter hydrocarbon chain surfactants (C12C~6) favor the formation of hexagonal phases, while longer chain surfactants (C~8) lead
308 mainly to the formation of cubic mesostructures. These mesostructured VPO phases are promising as novel catalytic systems for the partial oxidation of lower alkanes.
ACKNOWLEDGEMENT The authors would like to thank Mr. A.M. Hirt (Materials Research Laboratories, Inc., Struthers, OH) for the XPS data. This work was supported by the University of Cincinnati Research Council and the Wright-Patterson AFRL/DAGSI grant. REFERENCES
1. G. Centi, Catal. Today, 5 (1993) 16. 2. V.V. Guliants, J.B. Benziger, S. Sundaresan, I.E. Wachs, J.M. Jehng, J.E. Roberts, Catal. Today, 28 (1996) 275. 3. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartulli, J.S. Beck, Nature 359 (1992) 710. 4. H.P. Lin and C. Mou, Science, 273 (1996) 765. 5. P.T. Tanev, Y. Liang and T.J. Pinnavaia, J.Am. Chem. Soc., 119 (1997) 8616. 6. P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka, G.D. Stucky, Chem. Mater., 11 (1999) 2813. 7. A. Sayari, Y. Yang, J. Phys. Chem. B, 104 (2000) 4835. 8. M. Kruk, Y. Sakamoto, O. Terasaki, R. Ryoo, C. Ko and M. Jaroniec, J. Phys.Chem B, 104 (2000) 292. 9. T. Abe, A. Taguchi, M. Iwamoto, Chem. Mater., 7 (1995) 1429. 10. T. Doi and T. Miyake, Chem. Commun., (1996) 1635. 11. N. Mizuno, H. Hatayama, S. Uchida, A. Taguchi, Chem. Mater., 13 (2001) 179. 12. B.K. Hodnett, P. Permanne and B. Delmon, Applied Catal., 6 (1983) 231. 13. Q. Huo, D.I. Margolese, U. Clesla, P. Feng, T.E. Gier, P. Sieger, R. Leon, P. Petroff, F. Schuth and G.D. Stucky, Nature 368 (1994) 317. 14. A. Sayari, J. Am. Chem. Soc. 122 (2000) 6504. 15. R.G. Laughlin, The aqueous phase behavior of surfactants, Academic Press, London, 1994. 16. M.S. Wong and J.Y. Ying, Chem. Mater., 8 (1998) 2067. 17. V.V. Guliants, J.B. Benziger, and S. Sundaresan, Chem. Mater., 6 (1994) 353. 18. C.J. Brinker, G.W. Scherer, Sol-gel science, Academic Press, London, 1990. 19. J.M. Kim, Y. Han, B.F. Chmelka, G.D. Stucky, Chem. Commun., (2000) 2437. 20. P. Delichere, K.E. Bere, M. Abon, Applied Catal. A: General, 172 (1998) 295.
Studies in Surface Scienceand Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
309
Novel macroporous vanadium-phosphorus-oxides with three-dimensional arrays of spherical voids Moises A. Carreon and Vadim V. Guliants* Department of Chemical Engineering. University of Cincinnati, Cincinnati OH, 45221-0171, USA
Macroporous vanadium phosphorus oxide phases with unique compositional, structural and morphological properties have been synthesized by employing close-packed hexagonal arrays of polystyrene spheres as a template. The macroscale-templated synthesis produced VPO phases with unprecedented high surface areas, desirable macroporous architecture, as well as optimal bulk composition and preferential exposure of the surface (100) planes of the catalytically active and selective phase VO2P207 for the partial oxidation of n-butane.
1. INTRODUCTION Mixed metal oxides possess interesting and promising catalytic properties for the selective oxidation of lower alkanes [1]. For example, Mo-V-Nb and Sb-V oxides are catalytically active in the oxidative dehydrogenation and selective oxidation of ethane and ammoxidation of propane [2]; and the vanadium-phosphorus-oxides (VPO) are selective in the oxidation of n-butane to maleic anhydride [3]. Conventional synthesis methods for mixed metal oxides, both wet chemistry and solid-state, offer very limited control over desirable structural and compositional properties such as the phase, bulk and surface compositions, preferential exposure of active and selective surface planes, surface areas and pore architectures, which define their catalytic properties in selective oxidation of lower alkanes. Therefore, there is a critical need for novel routes of assembling hierarchically designed mixed metal oxides, which display remarkable ordering on micro- (<3 nm for the surface region structure and composition), nano- (3-100 nm for the bulk and phase compositions) and macro- (>100 nm for pore architectures) scales. Macroscale-templated synthesis of nanocrystalline mixed metal oxides represents an attractive approach for the hierarchical design of catalytic materials. Several single-element macroporous oxides with very interesting structural properties have been synthesized by self-assembly using colloidal sphere templates. Stein et al. [4-8] reported the synthesis of highly ordered macroporous TiO2, ZrO 2, A1203, SiO2, Fe203, Sb406 , W O 3 , MgO, Cr203, Mn203, NiO, ZnO, CaCO 3 and Co304. Pine et al. [9,10] described the synthesis of ordered macroporous ZrO2, TiO 2 and SiO2. Velev et al. [11-13] reported the synthesis of macroporous silica via colloidal crystallization. Wijnhoven et al. [14,15] described the synthesis of ordered macroporous TiO2 and SiO2.
310 Recently, we reported the first successful example of a hierarchically designed macroporous mixed vanadium-phosphorus-oxide (macro-VPO) with desirable structural and compositional properties for selective oxidation of n-butane [ 16]. Here, we present a detailed study of these novel macroporous vanadium-phosphorus oxide phases.
2. MATERIALS AND METHODS
2.1. Materials V20~ (Aldrich) and VO[CHO(CH3)2] 3 (99 %, Alfa Aesar) were used as vanadium sources. HaPO 3 and HaPO 4 (85 %, Fisher Chemicals) were used as phosphorus sources. Ethanol and isobutanol (Aldrich) were used as solvents. NHEOHHC1 was used as reducing agent. Monodispersed polystyrene spheres (--- 400 nm diameter) were synthesized by emulsion polymerization process described elsewhere [6]. The ordered closed-packed colloidal array of spheres was obtained by centrifugation of polystyrene sphere suspensions for 12 h at-1000 rpm. 2.2. Synthesis of macroporous VPO In a typical synthesis, the close-packed array of polystyrene spheres was deposited on filter paper in a Buchner funnel under vacuum and impregnated with a phosphoric or phosphorus acid solution in anhydrous ethanol. Then a solution of vanadium source in anhydrous ethanol was added dropwise to the polystyrene spheres under suction. Then, the composite was dried in air overnight. The polystyrene spheres were removed from the assynthesized macroporous VPO composite by either calcination in air at 723 K for 12 h (heating rate=5~ or Soxhlet extraction for 5 days using a mixture of acetone and tetrahydrofuran (1:1 volume ratio). Typical synthesis compositions on weight basis were: Ethanol/Vanadium precursor =1-10, Vanadium precursor/spheres = 2-6. In all experiments P/V molar ratio was kept constant (1,1) which is the optimal bulk composition for the partial oxidation of n-butane. When V205 was used as a vanadium source the synthesis procedure for macro-VPO phases was as follows. VzO 5 was refluxed in isobutanol or ethanol for 16 h. HaPO 3 or HaPO 4 dissolved in isobutanol or ethanol and centrifuged polystyrene spheres were added to this resultant blue/green slurry. The slurry was refluxed for another 20 h. The resultant blue slurry was filtered, washed with small quantity of isobutanol and dried in air at 393 K. The synthesis conditions are shown in Table 1.
2.3. Synthesis of conventional VPO phases. For comparison, 3 VPO phases were synthesized using conventional synthesis methods. In all the syntheses the P/V molar ratio was kept at 1.1. Aqueous VPO precursor The so-called "aqueous" VPO precursor was prepared according to the synthesis procedures proposed by Yamazoe et al. [ 17]. A solution of NH2OHHC1 (5 g) and H3PO4 (85 wt.%, 13.94 g) in 150 ml of deionized water was heated under stirring to 353 K. V205 (10 g) was slowly added to this solution, and a color change from orange to blue/green due to reduction of VS+was noted. The solvent was evaporated in air and a blue-green product dried
311 at 393 K. Soluble VO(H2PO4) 2 impurity was removed from the V O H P O 4 0 . 5 H 2 0 product by boiling in water.
Organic VPO Precursor An organic VOHPO40.5H20 precursor was prepared according to a modified published procedure [18]. V205 (20 g) was reduced by refluxing in isobutanol (220 ml) for 14 h. Anhydrous orthophosphoric acid (H3PO4) (27.88 g) dissolved in isobutanol (20 ml) was added slowly over a period of 2 h to this blue/green suspension which was refluxed for another 20 h. The resultant blue slurry [VOHPO40.5H20] was filtered, washed with small quantities of isobutanol and acetone, and dried in air at 393 K.
Phosphite VPO Precursor Phosphite precursor VOHPO3.1.5H20 was synthesized according to a procedure described elsewhere [19]. V205 (10 g, 55 mmol) was refluxed in absolute ethanol (200 ml) for 16 h. A color change from orange to green indicated reduction of V 5+. The slurry was cooled to room temperature and H3PO3 (10 g) dissolved in absolute ethanol (80 ml) was added. The mixture was refluxed for another 20 h and the blue slurry obtained was cooled, filtered and washed with absolute ethanol. The solid was dried at 393 K for 16h. Each of the conventional VPO precursors described above was activated at 673 K for 8 days to obtain the "equilibrated" active catalytic phase (VO)2PzO 7 . 2.4. Characterization
Powder X-ray diffraction (XRD) patterns were recorded on a Siemens D-500 diffractometer using Cu Kc~ radiation with step size of 0.02~ The N2 BET specific surface areas were determined using a Micromeritics Gemini 2360 analyzer. Scanning electron micrographs were recorded on a Hitachi S-3200N SEM. Chemical analyses were carried out for P and V at Galbraith Laboratories, Inc., Knoxville, TN. Infrared spectra were collected on a Bio-Rad FTS-60 IR spectrometer.
3. RESULTS AND DISCUSSION Table 1 shows the typical synthesis conditions, specific surface areas and crystalline phases for conventional and macroporous VPO (macro-VPO) phases. Using conventional synthesis methods (aqueous, organic and phosphite routes) only low specific surface areas in the 7-17 m2/g range were obtained. On the other hand, macro-VPO phases showed much higher surface areas in the 44-75 m2/g range after the removal of polystyrene spheres. It is important to mention that the surface areas of the macro-VPO phases are consistent with a theoretical estimate for the 20 nm cubic crystals of (VO)2PzO 7. Our findings indicated that it was possible to obtain different crystalline VPO phases by appropriately choosing the VPO sources and a template removal method (calcination or Soxhlet extraction). There is a general agreement in the literature that vanadyl pyrophosphate, (VO)zP207, is the catalytically active and selective phase in the oxidation of n-butane to maleic anhydride over the VPO catalysts [1,3,20,21]. Table 1 shows that the macroporous VPO sample 5 contains (VO)2P207 as the only crystalline phase. However, all
312 other VPO phases (VOPO42H20 , VOHPO42H20 and ~-VOHPO42H20 ) present in macroporous samples 1-4 are immediate precursors for (VO)2P207. For instance, VOPO42H20 can be transformed to (VO)2P207 by reduction in alcohol and subsequent thermal treatment in N2. VOHPO42H20 and 13-VOHPO42H20 can be transformed to (VO)2P207 by calcination at 773 K in N2. Furthermore, the ICP elemental analysis revealed that these novel macro-VPO phases display optimal bulk compositions ( P / V - 1.05) for a superior catalytic performance in the oxidation of n-butane to maleic anhydride. Table 1. Typical synthesis conditions, crystalline phases and specific surface areas for amacro and bconventional VPO phases. For macro-VPO phases: Alcohol/Vanadium precursor ratio (wt:wt) was 1, 3, 5, 10 and 10 for samples 1, 2, 3, 4 and 5 respectively. Vanadium precursor/PS sphere ratio (wt:wt) was 2, 2, 6, 6 and 5 for samples 1, 2, 3, 4 and 5 respectively. Ethanol was used as solvent for samples 1, 2, 3 and 4 and isobutanol for sample 5. Sample
VPO sources
General Description
Crystalline Phase
Calcined 723 K
VOPO 4 2H20
Surface Area (m2/g) 64
Soxhlet extracted
WOPO4 2H20
50
Calcined 723 K
VOPO4 2H20
41
Soxhlet extracted
75
Calcined 723 K
VOHPO 4 4H20 13-VOHPQ 2H20 (VO)2P207
44
Aqueous VPO
(gO)zPzO 7
7
Organic VPO
(VO)2P207
17
Phosphite VPO
(VO)zPzO7
10
,
a
1
a
2
a3 a4 a 5
b6 b7 b8
VO[CHO(CH3)2]3 H3PO3 VO[CHO(CH3)2]3 H3PO3 V/O5 H3PO3 V205 H3PO3 V205 H3PO4 V205 H3PO4 V205 H3PO4 V205 H~PO~
Figure 1 shows the highly ordered and monodispersed closed-packed arrays of polystyrene spheres used as a template in the synthesis of macroporous VPO phases. In order to obtain these highly ordered structures, polystyrene suspensions were centrifuged at 9001000 rpm for 12-24 h. Formation of macroporous VPO phases involved two main steps: (1) the self-assembly of appropriate vanadium and phosphorus species at the surface of polystyrene spheres, followed by (2) the condensation of the inorganic framework around the spheres upon drying. Then, the template was removed from the inorganic-organic composite by either calcination or Soxhlet extraction. Figure 2 shows a typical SEM image of macroporous VPO after the template removal by calcination. An ordered pore structure displaying interconnected pores (200 nm diameter) inside spherical - 400 nm cavities left after template removal is evident. The wall thickness
313 estimated from SEM was - 90 nm. The average crystal size determined by the Scherrer equation [22] was 20 nm indicating that the macroporous wall was only four crystals thick. This relatively large size of nanocrystal building blocks is probably responsibly for somewhat less ordered appearance of the macro-VPO structures. These novel VPO phases offer a possibility to improve the transport of reactant molecules through the macroporous structure and are promising as novel partial oxidation catalysts.
Figure 1. SEM image of colloidal crystal arrays o f - 400 nm polystyrene spheres used as a template for macroVPO.
Figure 2. SEM image of macroporous Vanadium-Phosphorus-Oxide (macro-VPO) calcined in air at 723 K. Figure 3 shows the XRD patterns for macro-VPO (sample 5) and conventional VPO phases (samples 6, 7 and 8). The XRD patterns of the macro-VPO and conventional VPO phases shows the presence of (VO)2P2O 7 as the only crystalline phase. The (100) surface planes of (VO)2P2O 7 have been proposed to contain the active and selective surface sites for
314 n-butane oxidation to maleic anhydride. Previously, the intensity ratio of the interplanar (100) and (042) X-ray reflections of vanadyl pyrophosphate (I10o/Io42) has been employed as an indicator of the preferential exposure and the stacking order of the surface (100) planes [3]. The conventional VPO phases exhibited low intensity ratios (0.4, 0.9 and 1.5 for phosphite, aqueous and organic conventional VPO, respectively) indicating that the surface (100) planes were not dominant in these phases. On the other hand, the macroscale-templated synthesis yielded much higher intensity ratios, I~00/I04z= 2.5, suggesting that macroporous VPO phases expose the surface (100) planes of vanadyl pyrophosphate to much greater degree than the conventional VPO catalysts.
2.50
A
C
0.90 im
b
e~ c
....
:a
[
I
I
1
I
I
1
I
10
15
20
25
30
35
40
45
__
.
50
2 theta/degrees Figure 3. XRD pattems of a) phosphite VPO, b) aqueous VPO, c) organic VPO and d) macroporous VPO. Numbers on the left-hand side indicate the intensity ratio of the interplanar (100)* and (042)** X-ray reflections of ( V O ) z P z O 7 (I100/I042). Figure 4 shows the IR spectra for as-synthesized, Soxhlet extracted and calcined macroVPO. As-synthesized macro-VPO shows the characteristic bands for VPO phases. The stretching frequencies of V-O-V and V=O appear at 683-642 and 1044-972 cm ~ respectively [23]. The stretching vibrations for P-O-P, P=O, PO3, P-OH and P-H are present at 930, 1198, 1126-1090, 3371 and 2350-2250 cm -~ respectively [24]. The sharp peak at 1640 cm ~ has been associated with surface adsorbed water. The two bands at 1440 and 1495 cm ~ correspond to the C-H symmetric and asymmetric deformation vibrations of polystyrene, respectively. Also, C-H stretching vibrations are present at around 2920-2850 cm l.
315 Soxhlet-extracted and calcined macro-VPO phases show similar vibration modes for vanadium and phosphorus species. However, the vibration modes of the organic template (bands at 1440 and 1495 cm ] and 2920-2850 cm -~) were absent, indicating that the polystyrene spheres were completely removed.
a
A
5 0 C
.=_ E C L_
1-
3000
2500
2000
1500
1000
Wavenumber (cm-1) Figure 4. Infrared spectra of a) as-synthesized, b) Soxhlet extracted and c) calcined macroporous VPO. * Polystyrene vibration modes. The macroscale-templated route produced vanadium-phosphorus-oxide phases with unprecedented high surface areas (75 m2/g), desirable pore architecture, as well as optimal bulk compositions and preferential exposure of the surface planes of the active and selective catalytic phase for the selective oxidation of lower alkanes. The proposed method offers a possibility to control and fine tune structural, compositional and morphological properties of VPO phases that are critical for achieving superior catalytic performance. This study has demonstrated that the macroscale self-assembly route holds a great promise for the rational design of mixed metal oxides with desirable structural, morphological and compositional properties with promising catalytic properties for selective oxidation of lower alkanes.
316 REFERENCES
1. F. Trifiro, Catalysis Today, 21 (1998) 41. 2. S.A. Holmes, J. A1-Saeedi, V.V. Guliants, P. Boolchand, D. Georgiev, U. Hackler, Catalysis Today, 67 (2001) 403. 3. V.V. Guliants, J.B. Benziger, S. Sundaresan, I.E. Wachs, J.M. Jehng, J.E. Roberts, Catalysis Today, 28 (1996) 275. 4. B.T. Holland, C.F. Blanford, A. Stein, Science, 281(1998) 538. 5. B.T. Holland, L. Abrams, A. Stein, Journal of American Chemical Society, 121 (1999) 4308. 6. B.T. Holland, C.F. Blanford, T. Do, A. Stein, Chemistry of Materials, 11 (1999) 795. 7. H. Yan, C.F. Blanford, B.T. Holland, W.H. Smyrl, A. Stein. Chemistry of Materials, 12 (2000) 1134. 8. H. Yan, C. F. Blanford, B.T. Holland, M. Parent, W.H. Smyrl, A. Stein, Advanced Materials, 11 (1999) 1003. 9. A. Imhof, D.J. Pine, Nature, 389 (1997) 948. 10. G. Subramanian, Vinothan, N. Manoharan, James D. Thome, David J. Pine, Advanced Materials, 11 (1999) 1261. 11. O.D. Velev, T.A. Jede, R.F. Lobo, A.M. Lenhoff, Nature, 389 (1997) 447. 12. O.D. Velev, T.A. Jede, R.F. Lobo, A.M. Lenhoff, Chemistry of Materials, 10 (1998) 3597. 13. O.D. Velev, E.W. Kaler, Advanced Materials, 12 (2000) 531. 14. J.E. Wijnhoven, W.L. Vos, Science, 281 (1998) 802. 15. J. Wijnhoven, S. Zevenhuizen, M. Hendriks, D. Vanmaekelbergh, J. Kelly, W. Vos, Advanced Materials, 12 (2000) 888. 16. M.A. Carreon and V.V. Guliants, Chemical Communications, (2001) 1438. 17. H. Morishige, J. Tamaki, N. Miura and N. Yamazoe, Chemistry Letters, (1990) 1513. 18. H.E. Bergna, US. Patent, 4 769 477, 1988. 19. V.V. Guliants, J.B. Benziger, S. Sundaresan, Chemistry of Materials, 7 (1995) 1485. 20. G. Centi, Catalysis Today, 5 (1993) 16. 21. F. Cavani, F. Trifiro, Applied Catalysis A, General, 85 (1992) 115. 22. B.D. Cullity, S.R. Stock, Elements of X-Ray Diffraction, Prentice Hall, Upper Saddle River, NJ, 2001. 23. G. Socrates, Infrared Characteristic Group Frequencies, Wiley, New York, 1994. 24. T. Abe, A. Taguchi, M. Iwamoto, Chemistry of Materials, 7 (1995) 1429.
Studies in Surface Science and Catalysis 141 A. Sayariand M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
317
Engineering active sites in bifunctional nanopore and bimetallic nanoparticle catalysts for one-step, solvent-free processes
Robert Raja, a'b* and John Meurig Thomas a'c a
Davy Faraday Research Laboratory, Royal Institution of Great Britain, 21 Albemarle Street, London W1S 4BS, U.K (e-mail:
[email protected])
b Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, U.K c Department of Materials Science and Metallurgy, Cambridge University, Cambridge CB2 3QZ, U.K. The design, atomic characterization, performance and relevance to clean technology of two distinct categories of nanocatalysts are described and interpreted. The first category consists of extended, crystallographically ordered inorganic solids possessing nanopores (apertures, cages and channels), the diameters of which fall in the range c___~a0.4 to c__~a1.5 nm, and the second of discrete bimetallic nanoparticles of diameter 1 to 2 nm, distributed more-orless uniformly along the inner walls of mesoporous (c_._a_a3 to 10 nm diameter) silica supports. The bifunctionality of the former class of catalysts consists of Bronsted acid sites together with redox active ones. M H ions, since they have protons loosely bound to an adjacent framework oxygen atom are the loci of the Bronsted sites, and the MllI ions are demonstrably the redox active sites. These catalysts have proved effective in the one-step, solvent-free, ammoximation of cyclohexanone to cyclohexanone oxime and ~-caprolactam with a mixture of air and ammonia. A range of bimetallic nanocatalysts (Cu4RUl2, Pd6Ru6, Ru6Sn, RusPt and Rul0Pt2), encapsulated in mesoporous silica, have also been studied for the direct one-step hydrogenation of dimethyl terephthalate (to 1,4-cyclohexanedimethanol), of benzoic acid (to cyclohexanecarboxylic acid), of naphthalene (in the presence of adsorbed sulfur) to (predominantly) cis-decalin, as well as for the solvent-free, selective hydrogenation of cyclic polyenes at low temperatures.
1. INTRODUCTION More than three quarters of the organic molecular products that are manufactured industrially entail the processes of either hydrogenation or oxidation; and with the impending arrival of the so-called hydrogen economy and the parallel drive towards clean technology this fraction will inevitably rise in the near future, the most desirable agents of conversion being molecular hydrogen and air (or oxygen). But increasing use of these agents requires the further development of robust, new, highly active and selective catalysts that, ideally, should effect single-step conversions under relatively mild and solvent-free conditions [1].
318 Apart from those that, inter alia, simulate the behavior of enzymes in their specificity shapeselectivity, regioselectivity and ability to function under ambient conditions, many of these new nanocatalysts are also viable as agents for effecting commercially significant processes in a clean, solvent-free, single-step fashion. In particular, oxidations using these nanocatalysts may be effected in air; they do not require aggressive oxidants like concentrated nitric and sulfuric acids, dichromates, permanganates or periodates [2], nor cryogenic or other engineering designs to produce nitrogen-free oxygen. So far as hydrogenations are concerned, it is relevant to note that the chemical industry is turning increasingly for its feedstocks to biological molecules extracted from the plant kingdom rather than to the constituents of oil in the production of high added value materials. Because such molecules are too large to enter the inner surfaces of microporous catalysts, it is appropriate for mesoporous catalysts (dia. in range 30-100 A) to be used so as to facilitate access of reactants to, and diffusion of products away from, the catalytically active sites that are (ideally) distributed in a spatially uniform manner over the high-area solid. With microporous and mesoporous solids it is readily possible to "place" or "engineer" active centres in atomically well-defined, spatially isolated fashions at accessible locations at their internal areas, which typically fall in the range 500 to 1000 m2g-~. The greater the number of active sites per unit area, the greater, obviously, is the overall catalytic activity. And by ensuring that the sites are spatially isolated the greater are the chances of securing high intrinsic catalytic activity per site.
2. EXPERIMENTAL
2.1. Preparation of bifunctional microporous catalystsThe microporous materials were synthesized from their precursor gels with a requisite amount of a suitable organic template and a carefully chosen amount of M u ions to substitute for the A1In in the framework. The bifunctional nanopore catalysts used for the ammoximation experiments are: (MIIMm)A1PO-36, M II- Mg; M nI = C o , Mn; {conventionally represented Co0.04Mg0.04A10.92PO4-36} they are structurally well-defined [3] possessing pore apertures of 6.5 x 7.5 .~, and a surface area (overwhelmingly internal) of ca 700 m2g1. X-ray absorption spectroscopy has established [4] that of the 4 atom percent of the framework A1uI isomorphously replaced by M ions, approximately 50 percent are in the M ~ and 50 percent in the M III state. M II ions, since they have protons loosely bound to an adjacent framework oxygen atom, are the loci of Brrnsted acid active sites. The M nl framework ions, on the other hand, are 'redox' active sites, capable of activating hydrocarbons and oxygen [5]. In (MIIMIII)A1PO-18, M - Co, which we have also studied for the ]gurposes of elucidating the nature of the catalysis, all the Co ions are in the Co m state [4]; and the pore diameter is so small that only air, H202 and ammonia (or hydroxylamine when formed [6]) may gain access to the interior surface of the sieve.
2.2. Preparation of encapsulated bimetallic nanoparticle catalysts: The catalytic materials were prepared following standard procedures [7-9]. The mesoporous silica was loaded with the cluster {for example, [Ph4P]2[RusPtC(CO)Is] or [PPN]2[RuloPt2C2(CO)28]} by making a slurry in diethylether/dichloromethane, stirring for 48 h under nitrogen, and filtering. The purple {for [Ph4P]2[RusPtC(CO)15]} and brown {for [PPN]2[RuloPt2C2(CO)28]} solids were washed with dry ether and dried in vacuum. The
319 loading was calculated for 10 wt % of Ru/Pt metal core. However, a small amount of non adsorbed compound [PPN]2[Ru10Pt2C2(CO)28]was found in the filtrate. Both products were characterized by F.TIR and the clusters were shown to be intact within the pores of MCM-41. The mesopore-encapsulated clusters [Ph4P]2[RusPtC(CO)Is] and [PPN]2[Ru]0Pt2C2(CO)28] were activated by heating at 195~ in v a c u o for 2 h. As in earlier preparations [7], the FTIR spectra recorded after activation showed no residual peaks corresponding to the carbonyl stretching frequencies. 2.3. Electron microscopy The electron microscopy characterizations were carried out on a VG HB501 fieldemission STEM microscope, and the catalysts dispersed on a holey carbon film supported on a copper grid from a suspension in hexane, as described in ref. [7]. 2.4. Catalysis The catalytic reactions were carried out in a high-pressure stainless steel catalytic reactor lined with Poly Ether Ether Ketone (PEEK). Dry hydrogen (20 bar) {in the case of hydrogenations} or air (30 bar) {in the case of ammoximation} was pressurised into the reaction vessel and, using a mini robot liquid sampling valve, small aliquots of the sample were removed to study the kinetics of the reaction, without perturbing the pressure in the reactor [10]. The products were analysed (using a suitable internal standard) by gas chromatography (GC, Varian, Model 3400 CX) employing a HP-1 capillary column (25 m x 0.32 mm) and flame ionisation detector. The identity of the products was confirmed by injecting authenticated samples and further by LC-MS (Shimadzu, QP 8000) employing a 7cyclodextrin dialkyl column (Chiraldex, 20 m x 0.25 mm).
3. RESULTS AND DISCUSSION 3.1. Bifunctional catalysts for the ammoximation of cyclohexanone The conversion of cyclohexanone 1 to the oxime 2 and its subsequent Beckmann rearrangement to e-caprolactam 3 are vital stepping stones in the manufacture of Nylon-6 [ 11 ] (scheme 1). On an industrial scale, one popular procedure in converting 1 to 2 o
NOH
--(OH2) 5 I~1
. . . . . . . .
1
2
3
Scheme-1 is to employ hydroxylamine sulfate, the sulfuric acid thus liberated being neutralized by ammonia [12,13], with the consequential production of large quantities of (low value) ammonium sulfate [14]. The traditional industrial route for effecting the Beckmann rearrangement (2----~ 3) is by use of a strong mineral acid such as oleum (Scheme 2).
320
2 + (NH4)2SO4 + H20
1 + (NH2OH).H2SO4 + 2NH 3 oleum 2
~ ~
NH3 H2SO4
OH
3 + 1/2 (NH4)2SO4
Scheme-2 We have investigated the effectiveness of a bifunctional nanopore catalyst, designated MnMIIIA1PO-36 (see Fig. 1), where MIII - Co, Mn, in the framework of which a few Mg II ions have replaced some A1111ions, achieves the conversion of cyclohexanone to its oxime and e-caprolactam in a one-step, solvent-free manner in the liquid phase using a mixture of air an ammonia. The dimensions of the nanopores in MA1PO-36 are just large enough to permit ingress of any of the molecules, cyclohexanone, cyclohexanone-oxime or e-caprolactam. In MIIMUIA1PO-18, M - Co, which we also studied to elucidate the nature of the catalysts, all the Co ions are in the Co IH state; and the nanopore is so small (0.38 nm) that only air, ammonia (and hydroxylamine when formed) may gain access to the interior surface of the sieve. M!! :M:it!AI P ~ t 8
M IIM iI!AIP~ 36!
A9
!::
i:'
oMg,,Oco,,eco,,,e,i oP e o OH
B
Fig. 1A In MIIMUIA1PO-36 (M - Co, Mn), the framework M nI ions are the redox active centres (A1), whereas M II ions have associated ionizable OH bonds attached to the framework and these are the Br6nsted (B1) acid sites. Mg II ions in the framework also have neighbouring ionizable OH ions (B2 sites). Fig. 1B In MIIIA1PO-18 all the framework M Ill ions are again redox active centres: there are no Co II (or Mn II) framework sites. Mg II framework ions again have
Our designed bifunctional nanocatalysts, l~IIIMIIIA1PO-36, perform very well in consecutively converting cyclohexanone to its oxime and c-caprolactam because: 9 hydroxylamine (NH2OH) is readily formed in situ inside the pores from NH3 and 02 at the M IxI redox active site; and 9 the NH2OH converts cyclohexanone to its oxime both inside and outside the pores, and, likewise, at the BrBnsted active sites, cyclohexanone-oxime is isomerized to ecaprolactam inside the nanopores of the catalyst. Confirmation of the essential correctness of the above interpretation of the mode of operation of the nanocatalysts comes from the following facts: 9 deliberate increase in the concentration of Bronsted sites in Co(Mn)A1PO-36 significantly enhances the rate of production of caprolactam;
321 9 no caprolactam is ever produced with MA1PO-18 catalysts, even when the Bronsted active center concentration is increased, solely because the oxime is too large to gain access to these centers via the 0.38 nm pore apertures; 9 with air (or Oz) as oxidant, the smaller-pore ComA1PO-18 nanocatalyst gives higher rates of conversion of cyclohexanone to the oxime than with CoHCoIIIA1PO-36 because of the higher concentration of the redox active centers in the former; 9 when a bulky oxidant, such as tertiary butyl hydroperoxide, is used with a CoIIIA1PO-18 nanocatalyst, no conversion at all of cyclohexanone takes place, because the redox active centers are inaccessible; and 9 a kinetic study shows that NH2OH is initially formed at a rapid rate but is then converted to 2 in the presence of cyclohexanone. Furthermore, experiments carried out in the absence of cyclohexanone proved unequivocally the formation of NH2OH from NH3 and 02 at the redox (Co III) site. 25
conversion
conversion
o
,
m~m~m ---------m~'nn~ - ~
23-
-/
v / IZ
. ~
"
~"N'%
I/-/
5_] i/~"
/T----T ,jv/r"
-
NOH
5 "I~
o~hers 9~
0
5
15
10 t/h
20
NOH
25
O0
~1~~_4_~__~____, T
05
'
I
iO
'
I
15
'
I
2O
'
others I
25
~-
Fig. 2 Kinetic plot (left) showing the conversion of cyclohexanone and the formation of cyclohexanone-oxime and s-caprolactam as a function of time, in the presence of air and ammonia. The plot on the fight shows an expanded view of the initial part of the reaction, where the presence of hydroxylamine (formed in situ from NH3 and 02) at the M nI active (redox) site, is unequivocally established. We may categorically rule out the 'imine' mechanism [ 15] for the formation of the oxime 2, according to which C6H]0=NH is a necessary intermediate. Since this species is also too large to enter the pores of the ComA1PO-18 (which smoothly yields 2 from 1 with a mixture of air and NH3), the dominant mechanism entails direct conversion of 1 with NH2OH to 2; Further, peroxydicyclohexylamine (PDCA), believed by some workers [ 15] to be the key byproduct in the imine mechanistic path, was not observed. Our results also demonstrate that the MA1PO catalysts that we have developed for this and other oxidations [16,17] function in a genuinely heterogeneous manner [18] and not seemingly s o - because the active entities (e.g. Co III or Mn In ions in this instance) do not leach out and then simply adhere to the molecular sieve where they would operate as loosely bound homogeneous catalysts. If the Co III or Mn III were leached out we would have seen
322 appreciable conversion using TBHP as an oxidant with ColIIA1PO-18, yet there was none. Moreover, if the Mg II ions were leached out they would have catalyzed the Beckmann
Fig. 3 Bar chart summarizing the relative performances of the bifunctional A1PO catalysts for the ammoximation of cyclohexanone in the presence of ammonia and different oxidants (air and TBHP). Catalyst A = MnnMnnlA1PO-36; B = MgIIMnmA1PO-36; C = conlA1PO-18; D = MgIICoInA1PO-18; Reaction conditions: cyclohexanone : TBHP -= 3 : 1 (mol); catalyst - 0.5 g; A i r - 3.5 MPa; cyclohexanone : NH3 = 1 : 3 (mol); T - 328 K; cyclohexanone _---50 g; mesitylene (internal standard) - 2.5 g;
rearrangement of (2) to e-caprolactam (with MgnCoHIA1PO- 18), but again none of the latter is formed. In a separate experiment, using Mg~MnmA1PO-36, the solid catalyst was filtered off from the reaction mixture (when hot) after 4 h and the reaction was continued with the resulting filtrate for a further 16 h. No further conversion to e-caprolactam was observed, and the filtrate analyzed by ICP/AAS analysis revealed only trace amounts of Mn and Mg (<3 ppb and < 5 ppb, respectively). Finally, we took an equimolar mixture of cyclooctane and III II cyclohexanone along with NH3 and air over a Co Mg A1PO-18 catalyst. Unsurprisingly, there is no conversion whatsoever of the cyclooctane molecule, as it is too large to gain access into the interior of the catalyst where the vast majority of the active sites are located. The NH3, however, gains ready access to such active sites, where NHzOH is first produced before subsequently reacting with cyclohexanone to produce the oxime (2) outside the molecular sieve. When the active sites are deliberately leached [18] using acetic acid as a solvent, considerable oxidation of the cyclooctane now occurs (the main products being cyclooctanol, cyclooctanone and cyclooctanoic acid), in addition to the formation of NHzOH from N H 3. The NHzOH, now, reacts not only with cyclohexanone to produce the corresponding oxime, but also with the cyclooctanone to give its oxime. Clearly acetic acid as a solvent results in homogeneous as well as heterogeneous catalysis.
323 3.2. Engineered bimetallic active centres for selective hydrogenations An inherent advantage possessed by mesoporous silicas, of the type exemplified in Figure 4, is that it is readily possible to insert into such pores mixed-metal carbonylates that are quite bulky or to graft carefully constructed, chirally biased metal complexes. We have taken advantage of this fact to insert cluster carbonylate salts such as [Ru6C(CO)16CuC112[PPN]2, where PPN stands for bis(triphenylphosphino) iminium into MCM-41 silica. These salts can be shown by ex situ HREM studies to be encapsulated in a spatially uniform manner along the inner surfaces of the mesopores. (The Si-OH groups undergo hydrogen-bonding with the O - C - M bond of the carbonyls). In situ EXAFS and FTIR studies are used to chart the progressive conversion, by gentle thermolysis, of the carbonylate salts into the denuded, bimetallic (carbided) nanoparticle catalysts. Separate Cu and Ru K-edge EXAFS studies [19], for example, yield a good picture of the active (approximately 15 A. diameter) catalyst, which has a composition Cu4Rul2C2. A key point so far as their catalytic behaviour is concerned, is that these bimetallic nanoparticles display no tendency to sinter, aggregate or fragment into their component metals during use as hydrogenation catalysts. (They exhibit high turnover frequencies for the hydrogenation of 1-hexene, diphenylacetylene, stilbene, cis-cycloctene and (R)-(+)-limonene [9]). Fig. 4 Computer graphics model of Cu4RUl2nanocatalyst clusters anchored (originally in their carbonylated form via pendant silanol groups) to the inner walls of mesoporous silica. The nanocatalyst is akin to Pd6Ru6, Ru6Sn and RusPt clusters (see text). A typical reactant polyene (2,5-norbomadiene) and H2 are also shown in the mesopore channel, which has a diameter of ca 30 ./t.
Recent work [20] using density functional theory reveals that the energy-minimized structure of the Cu4Rul2C2 nanoparticle, anchored catalyst is a rosette-shaped entity in which twelve exposed Ru atoms are connected to a square base composed of relatively concealed Cu atoms. These are, in turn, anchored by four oxygen bridges to four Si atoms of the mesopores lining. It is also noteworthy that, in this computed structure, which also satisfies the EXAFS experimental information, the carbidic carbon atoms have moved from their original interstitial positions to well-defined surface sites. A number of solvent-free selective hydrogenations of polyenes using a bimetallic Ru6Sn nanoparticle catalyst have also been performed (see Figure 5). The selective hydrogenation of a polyene such as 1,5,9-cyclododecatriene is quite an important procedure in the synthesis of organic and polymeric intermediates such as laurolactam [21], 12-aminod0decanoic acid and dodecanedioic acid, which are important monomers for nylon 12, nylon 612, copolyamides, polyesters and for various materials used as coatings. Hitherto, Raney nickel, Pd, Pt, Co, and mixed transition-metal complexes have been used for these hydrogenations, and all the reactions entail the use of organic solvents (such as n-heptane or benzonitrile [21,22]) and often in the presence of efficient hydrogen donors (such as
324 9,10-dihydroanthracene). Our procedures emphasize that there are solvent-free routes available for such processes. Here we also describe the promising performance of two, related new bimetallic nanocatalysts for the hydrogenation of: (i)
benzoic acid to cyclohexanecarboxylic acid;
(ii) dimethyl terephthalate (DMT) to 1,4-cyclohexanedimethanol (CHDM), and iii) naphthalene in a highly selective manner to cis-decalin.
Fig. 5 A Solvent-free selective hydrogenation of 2,5-norbornadiene using a variety of bimetallic nanocatalysts. T (K) = 333 K; t (h) = 10; H2 pressure = 30 bar; catalyst = 25 mg; 2,5-norbornadiene --- 50 g; Fig. 5 B The effect of temperature in the solvent-free selective hydrogenation of 1,5,9-cyclododecatriene: A comparison between Ru6Sn and Pd6Ru6 after 8 h (reaction time) at 353 K and 413 K, respectively is shown. H2 pressure = 30 bar; catalyst = 25 mg; 1,5,9- cyclododecatriene --- 50 g. The bimetallic nanoparticle catalysts that we would like to focus upon here are discrete, anchored clusters RusPt and Ru~0Pt2, prepared by the gentle decarbonylation of the parent, mixed-metal precursor anions: [RusPtC(CO)15] 2 and [Ru10Pt2C2(CO)28]2" after first inserting them, along with their counterions into mesoporous silica, in a manner closely akin to the preparation of anchored Ru6Sn [7], Ru6Pd6 [8], and of Ru12Cu4 [9], nanoclusters described previously. A preliminary test of the performance of these catalysts was carried out using cyclohexene as the reactant. The results (Table 1) leave little doubt that the catalytic activity and selectivity of both RusPt and Ru~0Pt2 are exceptional. In the hydrogenation of naphthalene again the performance of these two catalysts is exceptional (Table 1): only the fully hydrogenated naphthalenes (decalins) are formed, and the ratio of the cis to the trans forms is high (ca 5.5 to 8.6 depending upon the precise conditions). Moreover, these catalysts are unimpaired in their performance even when substantial amounts of a well-known 'sulfur poison' is deliberately introduced m in sharp contrast to the behavior of otherwise good bimetallic nano-catalysts (Pd6Ru6, Ru6Sn and CuaRu12) previously tested by us [8].
325
Table-1" H y d r o g e n a t i o n o f cyclohexene and naphthalene - C o m p a r i s o n o f catalysts Catalyst
Substrate
RusPh/MCM-41 RUl0Pt2/MCM-41 Pd6Rur/MCM-41 Ru6Sn/MCM-41 Cu4Ru~JMCM-41 RusPh/MCM-41
cyclohexene cyclohexene cyclohexene cyclohexene cyclohexene naphthalene
Ru~0Pt2/MCM-41
naphthalene
Pd6Rur/MCM-41
naphthalene
Ru6Sn/MCM-41
naphthalene
Cu4Rulz/MCM-41
naphthalene
T
Conv
TOF
(h)
(mol %)
(h")
A
3 2 3 3 3 8 8** 8 8** 8 8"* 8 8"* 8 8**
96.9 98.0 55.0 26.5 21.7 31.5 32.0 44.7 45.0 7.0
6341 14201 3015 1734 1950 792 805 1660 1671 19 238 -
100 100 100 85 80 -
9.5
Product distribution (mol %)
-
B
C
84 15 86.4 13.3 89.4 10.2 88.5 11.3 50 34 No Reaction 48.2 45 No Reaction No Reaction No Reaction
14.7 20.2 15 6.5
Reaction conditions: cyclohexene z 50 g; naphthalene ,~ 8 g (dissolved in 55 g of hexadecane); catalyst = 50 mg; H2 pressure = 20 bar; temp (cyclohexene) = 353 K; temp (naphthalene) = 373 K; A = cyclohexane; B = cis-decalin; C = trans-decalin; D = others; TOF = [(mOlsubstr)(molclu~ter)-lhl]; ** 200 ppm of sulfur was introduced (benzothiophene).
Table-2: H y d r o g e n a t i o n o f benzoic a c i d - Comparison of catalysts Catalyst RusPtl/MCM-41 Ru~oPt2/MCM-41 Ru6Sn/MCM-41 Pd6Ru6/MCM-41 Cu4Rulz/MCM-41
Conv (mol %) 61.2 78.5 15.9 44.5 21.8
TOF (h l) 167 317 24 126 48
Product distribution (mol %) A B C 86.5 13.3 99.5 9.0 42.5 48.3 61.5 39.2 79.6 21.2
Reaction conditions: benzoic acid ~ 2.5 g (dissolved in 75 ml of ethanol); catalyst = 50 mg, H2 pressure = 20 bar, temp = 373 K; t = 24 h. A = cyclohexanecarboxylic acid; B = cyclohexene-1carboxylic acid; C = 1,3-cyclohexadiene-2-carboxylic acid; TOF = [(mOlsubst0(molcl~ter)-lh-1]. In the hydrogenation o f benzoic acid the efficacy o f the two RuPt nanocatalysts again surpasses that o f the three quite distinct bimetallic catalysts that we have previously described [7-9] (Table-2), both in terms o f activity and selectivity for cyclohexane carboxylic acid. The hydrogenation o f D M T (see Scheme 3) to the valuable product C H D M , is carried out industrially in two steps using two reactors [23]. The first, requiting a temperature in the range 160~ to 180~ and a pressure o f 30-48 MPa, uses a supported Pd catalyst. This highly exothermic process yields the intermediate, dimethyl hexahydroterephthalate (DMHT). This, in turn, is converted at cA 200~ and 40 bar H2 over a copper chromite catalyst, to the required C H D M . With our catalysts, one-step conversions o f D M H T to C H D M are effected in high yield even after a mere 4h contact time at 100~ a pressure of 20
326
MeO -~1 ~ -OMe O O dimethylterephthalate DMT
~ ~ -OMe DMHT 0 O dimethylhexahydroterephthalate
H2
MeO
v
HO2HC~ H 2 O H
CHDM
_
1,4-bis(hydroxymethyl)cyclohexane (1,4-cyclohexanedimethanol)
Scheme-3
bar H2 and ethanol as a benign solvent (Table 3). It is noteworthy that the yield of C H D M passes through a maximum, there being cross-linked products such as 4methyloxymethlhydroxymethyl cyclohexane and bis (4-hydroxymethylcyclohexyl) ether formed during the conversion. It is also of interest that other bimetallic nano-catalysts that are less efficient than RusPt or R10Pt2 as straightforward hydrogenation catalysts (see Tables 1 and 2), such as Rul2Cu4, are especially effective in producing good yields of CHDM (see Table 3). Kinetic studies (Figs 6 and 7) reveal the changes more explicitly. In Fig. 6, for example, the C H D M further reacts in the presence of this catalyst to form coupling byproducts as elaborated in Table-3. In Fig. 7, however, the product distribution is markedly different and the formation o f by-products is relatively suppressed. The synergistic role of the copper in driving the reaction further (conversion of DMHT to CHDM) is clearly evident here, in stark contrast with what is observed in Fig. 6 using the Ru~0Pt2 catalyst. Table-3: Hydrogenation of Dimethyl Terephthalate ( D M T ) - C o m p a r i s o n of Catalysts Catalyst RusPh/MCM-41
Ru~oPh/MCM-41
Pd6Ru6/MCM-41
Ru6Sn/MCM-41
Cu4Ru~JMCM-41
t (h) 4 8 24 4 8 24 4 8 24 4 8 24 4 8 24
Conv (mol %) 7.5 18.4 36.9 23.3 39.5 67.2 6.2 15.5 23.7 . 5.3 8.0 . 10.4 14.2
TOF (h-') 155 191 138 714 605 443 98 125 64 .
. 54 27
.
. 102 45
A 58.7 55.3 61.2 42.6 53.6 51.4 32.9 22.5 18.2 . 77.2 81.0 . 22.2 25.3
Product distribution (mol %) B C D 33.5 6.9 5.2 39.4 38.7 52.3 9.0 37.1 48.5 16.8 51.1 4.2 74.2 81.5 . . 22.6 18.6 . . 54.5 23.0 63.2 11.3
Reaction conditions: DMT ~ 2.5 g (dissolved in 75 ml ethanol); T = 373 K; H2 pressure = 20 bar; ToF = [(mols,bst0(molciust~r)lh~]; catalyst z 50 mg; A - dimethyl hexahydroterephthalate (DMHT); B = 1,4-cyclohexanedimethanol (CHDM); C = mixture of methyl 4-methyl-1-cyclohexanecarboxylate and 1-hydroxymethyl-4-methylcyclohexane; D = mixture of 4-methoxymethyl-l-hydroxymethylcyclohexane and bis(4-hydroxymethylcyclohexyl) ether.
327
01 7 .I
l 60 -1
t
"-~
- - - - - conversion of DMT - - e - - C (see Table-3 for details) - - & - - DMHT - - v - - CHDM - - e - - D (see Table-3 for details)
16
~u_..--4~m~"
/
~e / m~
14
"~
~
,
5
j
,
10
~ 15
conversion of DMT C (see Table-3 for details) DMHT CHDM D (see Table-3 for details)
___.._~~
~
----------
12
30
0 : _ -7 0
--m---e---A---v---e--
,
0-
~-7--'* 20
6
25
g--~O--4k u 5
! 10
9
i 15
~e
u 20
'
t/h
t/h
Fig. 6 Kinetics of hydrogenationof DMT using Rul0Pt2 as catalyst.
19
Fig. 7 Kinetics of hydrogenationof DMT using CUaRUl2 9
It is also remarkable that the cis/trans ratio of the CHDM formed is 80:20 for the Cu4RUl2 catalyst, and 65:35 in the case of both the RuPt catalysts. It augurs well here that, in the case of the former catalyst, these values are extremely significant for industrial processing (88:12) [24]. Interestingly, the Pd6Ru6 catalyst produces a 50:50 mixture (cis/trans) of CHDM. The Ru-Pt catalysts have been re-used 3 times without appreciable loss in catalytic activity or selectivity. Further, experiments, analogous those reported earlier [7], were carried out to rule out the possibility of leaching, and analysis of the resulting filtrate at the end of reaction (24 h, for DMT hydrogenation) by ICP and AAS revealed only trace amounts (< 5 ppb) of dissolved metal ions (Pt, Ru). 4. ACKNOWLEDGMENTS We thank our co-workers, listed in the references and the EPSRC (UK) for continued support. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9.
J.M. Thomas, R. Raja, G. Sankar, B.F.G. Johnson and D.W. Lewis, Chem. Eur. J., 7 (2001) 2973. M.T. Reetz and K. T611ner, Tetrahedron Lett., 36 (1995) 9461. P.A. Wright, S. Natarajan, J.M. Thomas, R.G. Bell, P.L. Gai-Boyes, R.H. Jones and J. Chen, Angew. Chem. Int. Ed. Engl., 31 (1992) 1471. P.A. Barrett, G. Sankar, C.R.A. Catlow and J.M. Thomas, J. Phys. Chem., 100 (1996) 8977. J.M. Thomas, R. Raja, G. Sankar and R.G. Bell, Nature, 398 (1999) 227. A. Zecchina, G. Spoto, S. Bordiga, F. Geobaldo, G. Petrini, G. Leofanti, M. Padovan, M. Mantegazza and P. Roffia, New Frontiers in Catalysis, Elsevier, Amsterdam, 1993. S. Hermans, R. Raja, J. M. Thomas, B. F. G. Johnson, G. Sankar and D. Gleeson, Angew. Chem. Int. Ed., 40 (2001) 1211. R. Raja, S. Hermans, D. S. Shephard, S. Bromley, J. M. Thomas, B. F. G. Johnson and T. Maschmeyer, Chem. Commun., (1999) 2131. D . S . Shephard, T. Maschmeyer, G. Sankar, J. M. Thomas, D. Ozkaya, B. F. G. Johnson, R. Raja, R. D. Oldroyd and R. G. Bell, Chem. Eur. J., 4 (1998) 1214.
328 10. J.M. Thomas, R. Raja, G. Sankar and R.G. Bell, Acc. Chem. Res., 34 (2001) 191. 11. G.W. Parshall and S.D. Ittel, Homogeneous Catalysis: The Applications and Chemistry of Catalysis by Soluble Transition Metal Complexes, 2na ed., Wiley-Interscience, New York, 1992. 12. Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, Germany, 2001. 13. Kirk-Othmer Encyclopedia of Chemical Technology, Wiley, New York, 1982. 14. G. Bellussi and C. Perego, Cattech, 4 (2000) 4. 15. J.S. Reddy, S. Sivasanker and P. Ratnasamy, J. Mol. Catal., 69 (1991) 383. 16. J.M. Thomas and R. Raja, Chem. Commun. Feature Article, (2001) 675. 17. R. Raja, G. Sankar and J.M. Thomas, Angew. Chem. Int. Ed. Engl., 39 (2000) 2313. 18. R. Raja, G. Sankar and J.M. Thomas, J. Am. Chem. Soc., 121 (1999) 11926. 19. J.M. Thomas and G. Sankar, J. Synchrotron Rad., 8 (2001) 55. 20. S. Bromley, C.R.A. Catlow, G. Sankar, J.M. Thomas, T. Maschmeyer and B.F.G. Johnson, Chem. Phys. Lett., (2001) in press. 21. J. Hanika, I. Svoboda and V. Ruzicka, Coll. Czech. Chem. Commun., 46 (1981) 1039. 22. M. Gerst and C. Ruchardt, Tetrahedron Lett., 34 (1993) 7733. 23. P. Appleton and M.A. Wood, (Eastman Chemical), US 5414159 (1993). 24. P. Werle and M. Morawietz, Ullmanns Encyclopedia of Industrial Chemistry, WileyVCH Verlag, Weinheim, Germany, 2001.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
329
U s i n g A u nanoparticles-surfactant aqueous solution for a convenient preparation o f m e s o p o r o u s aluminosilicates containing Au-nanoparticles Yu-Shan Chi a, Hong-Ping Lin b*, Chinn-Nan Lin c, Chung-Yuan Mou a and Ben-Zu Wan c a. Department of Chemistry and Center of Condensed Matter Science, National Taiwan University, Taipei, Taiwan. b. Institute of Atomic and Molecular Sciences, Academia Sinica, EO. Box 23-166, Taipei, Taiwan 106. c. Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan.
A stable Au nanoparticles-quaternary ammonium surfactant aqueous solution with the Au/surfactant molar ration up to 0.06 has been conveniently prepared after a typical reduction of tetrachloroaurate salt (HAuCI4). That solution was used directly to synthesize the mesoporous aluminosilicates containing Au-nanoparticles. The calcined mesoporous aluminosilicates containing Au-nanoparticles with the advantages of high surface, large pore size, pore volume high dispersity of Au nanoparticles, demonstrated the catalytic activity of the CO oxidation at 80~
1. INTRODUCTION In recent years, gold nanoparticles deposited on various oxides supports were found to exhibit enhanced catalyst activities, especially for the low temperature oxidation of CO [ 1, 2]. One finds the catalytic activity depends on the size and shapes of Au nanoparticles, the properties of the support, and the interaction between support and Au nanoparticles [3]. All of these factors are controlled by the precursors and preparation method. Thus there are many catalyst preparation strategies for developing the novel catalysts of Au-nanoparticles supported on oxides matrixes. In the literatures [4-6], the chemical vapor deposition and precipitation methods using various Au precursors are the two common strategies to prepare supported Au nanoparticles catalysts. However, precursors of organic Au precursors in CVD are expensive, and the precipitation procedure is sometimes a little complicated. Using simple Au precursors and a convenient preparation method still remains desirable. In this paper, we used the quaternary ammonium surfactants as both the stabilizing-agents of Au-nanoparticles and the structural template of mesoporous MCM-41 aluminosilicates [7, 8]. Therefore, the preparation of Au nanoparticles containing mesoporous aluminosilicates was achieved via a simple one-pot preparation process. We first prepared the stable surfactant encapsulated Au nanoparticles in aqueous solution with a simple reduction oftetrachloroaurate
330 salt (HAuC14). Then that Au nanoparticles-containing surfactant solution was used directly as templating the synthesis of mesoporous material. After calcination, the mesoporous aluminosilicates containing Au nanoparticles are obtained. The Au-containing materials were then used as catalyst in the CO oxidation reactions.
2. MATERIALS AND METHODS 2.1. Materials The tetrachloroaurate salt (HAuC14) and sodium borohydride (NaBH4) are from Aldrich. The organic template, cetyltrimethylammonium bromide, CI6TMAB, And the silica source, sodium silicate (29 % SiO2, 12 % Na20), are from Acr6s. The source of aluminum is sodium aluminate (NaA102) from Riedel-de HaEn. Sulfuric acid (H2SO4) was purchased from Merck. All chemicals were used without further purification 2.2. Synthesis 2.2.1. Synthesis of Au nanoparticles-surfactants aqueous solution: Briefly, 17.70 g of a 0.020 M tetrachloroaurate salt (HAuC14) aqueous solution was mixed with 1.53 g cetyltrimethylammonium bromide (CI6TMAB) and 11.0 water to give a yellow-colored clear solution. That solution was reduced with 2.10g of 1.10 M aqueous sodium bonohydride (NaBH4) aqueous solution dropwise at room temperature, and formed a red-brown colored Au-nanoparticles/surfactants aqueous solution.
2.2.2. To prepare the Au-eontaining mesoporous MCM-41 aluminosilieates To the above Au-nanoparticles/surfactants aqueous solution, a desired amounts of sodium silicate and sodium aluminates were added. After a neutralization procedure (the final pH value of the gel solution is about 8-10), a red precipitate gel solution was formed instead of white one in the case of Au-free mesoporous MCM-4 laluminosilicates [9]. The molar ratios of the gel composites are 1.0 SiO2 : 0.028 NaA102 : 0.71 CI6TMAB : (0-0.042) HAuC14 : (0-0.285) NaBH4 : 0.6 NaOH : 0.24 H2SO4 : 300 H20. Then, the gel solution was then transferred into an autoclave, and statically via a 100~ hydrothermal reaction for 2 days. Finally, filtration, washing, drying and calcination at 560~ in air resulted in the Au-containing mesoporous aluminosilicates products. 2.3. Measurements The nano-structural characterizations of the Au nanoparticles and mesoporous aluminosilicate were recorded on Philip CM-200 high-resolution transmission electron microscope with an operating voltage of 200 keV. The powder X-ray diffraction patterns (XRD) were taken on a Scintag X1 diffractometer using Cu-K,~ radiation. 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 isotherm using the BJH (Barrett-Joyner-Halenda) method. The UV-vis spectra of the Au nanoparticles-surfactant aqueous solution after a proper dilution and Au nanoparticles/mesoporous aluminosilicates were recorded on the Hitachi U-3010 UV/Vis Spectrophotometer at ambient temperature.
331 3. RESULTS AND DISCUSSION
Fig. 1A shows the UV-vis spectra of the Au-quaternary ammonium surfactant (C 16TMAB) aqueous solution with Au/surfactant ratio of 0.02, 0.04 and 0.06. One can clearly see all of the absorption maximum center at 524 nm, which is ascribed to the surface plasmon resonance of Au nanoparticles. The almost identical spectra indicate that the extent of size growth is small with the increase of Au/surfactant ratio. Moreover, the resulting Au nanoparticles-surfactant aqueous solution were stable for over one week without the formation of bulk Au precipitation and any apparent red-shit in UV-Vis spectra. Under the HR-TEM observations, the size of the Au nanoparticle is measured at about 3-5 nm (Fig. 1B). Apparently, the Au nanoparticles could be conveniently obtained in the surfactants water solution through a simple reduction process.
Figure 1. The UV-vis spectra (A) and the HR-TEM micrograph (B) of the Au nanoparticles-surfactant aqueous solution of various Au/surfactant molar ratio. In Fig. B, the Au/surfactant molar ratio = 0.04. Because the quaternary ammonium surfactant is a typical template in MCM-41 materials, we suggest that the mesoporous materials may be synthesized using the Au-surfactant aqueous solution directly as substitute of the normal surfactant solution for making MCM-41 aluminosilicates. After delayed neutralization, hydrothermal reaction, and high-temperature calcination procedures for preparing the typical MCM-41 aluminosilicates [9], as-synthesized and calcined samples, both red colored were obtained and their XRD patterns are shown in Fig. 2A. We find all the samples possess a broad peak at low angle indicting the mesostructures, and two peaks at high angle range resulting from the Au nanostructures. With the HR-TEM observations, the nanochannels of mesoporous materials and Au nanoparticles (dark spots) in the as-synthesized sample were clearly confirmed (Fig. 2B). The average size of Au nanoparticles is about 4-7 nm, which is a little bit bigger than that of Au in the starting aqueous solution. This shows the agglomeration of Au nanoparticles is not significant during the two-day hydrothermal reaction for stabilizing the silica mesostructures. After high temperature calcination, the HR-TEM micrograph of Fig. 2C shows that the mesostructures are still retained and the Au nanoparticles are nearly spherical and isolated
332 from each other. Because the size of the Au nanoparticles (> 3 nm) is larger than that of aluminosilicates nanochannels (2.5 nm), the Au nanoparticles are not confined within single channels of mesoporous aluminosilicates. Under higher magnification, the distinct Au lattice fringe images of the calcined Au/mesoporous aluminosilicate complex are seen in Fig. 2D. Although the sizes of the Au nanoparticles increased a little upon calcination, the small increase of Au nanoparticles after the surfactants removal at high temperature indicates the mesoporous aluminosilicates are good protecting matrix of the Au nanoparticles that prevents a serious Au nanoparticles agglomeration.
Figure 2. The XRD patterns and HR-TEM micrographs of the as-synthesized and 560 and 700 ~ calcined Au nanoparticles containing mesoporous aluminosilicates of Au/SiO2 wt % = 8 %. A. XRD patterns; B. The HR-TEM micrograph of the as-synthesized sample; C. The HR-TEM micrograph of the calcined sample at 560 ~ D. The HR-TEM micrograph of sample C. Although the parallel nanochannels of the MCM-41 aluminosilicates were revealed in the as-synthesized and calcined Au nanoparticles/mesoporous aluminosilicates, only one broad XRD peak exists in both samples and the intensity of the d~00decrease dramatically compared to the Au-free MCM-41 ones. The disappearance of the higher order (110) and (200) peaks of hexagonal structure and the low (100) XRD intensity indicate that the incorporations of Au
333 nanoparticles interfered with the long range order of MCM-41 nanochannels. So, one would expect that most of Au nanoparticles should be contained within the mesoporous aluminosilicates instead of deposition on the outer surface of the MCM-41 particles [ 10]. The images in Fig. 2 B-D indeed show that most all the Au particles are embedded within the MCM-41 materials. To avoid the sampling drawback in TEM observation, we also calculated the size of Au nanoparticles from the XRD peaks width in high angle by using the Sherrer equation. The estimated dimension is about 8 nm in agreement with the average value from the direct HR-TEM measurement. Accordingly, one can know that the mesoporous MCM-41 aluminosilicates containing Au nanoparticles have been synthesized using such a simple composites and economic procedure in aqueous solution. When using the reflectance UV-Vis spectra for further examinations on the Au nanoparticles properties (Fig. 3A), one can clearly observed that the there exist the broad band centered at 529 nm, 517 nm, and 515 nm, for the Au/MCM-41 composites before and after calcination at 560, 700 ~ respectively. However, HR-TEM examinations and XRD patterns calculations revealed that the growth of Au nanoparticles upon calcination, and the extent of the size-rise increases with the calcination temperature. So, it is evident that the Au/MCM-41 composite presents a red shift of resonance band with the decrease of Au nanoparticles size.
Figure 3. The UV-vis spectra and HR-TEM micrographs of the as-synthesized at 560 and 700 ~ calcined Au/mesoporous aluminosilicates composites of Au/SiO2 wt %-- 8 %. A. UV-Vis spectra, B. The HR-TEM micrograph of the 560~ calcined sample.. This optical behavior, in opposite to the Au nanoparticles embedded in stabilizing organic agents, was suggested due to the interface interactions between the Au nanoparticles and the mesoporous aluminosilicate matrixes [11]. It was believed that the red shift in this Au/MCM-41 composite resulted from the electron transfer from Au nanoparticles to aluminosilicate after the removal of surfactants that prevent the direct interactions between Au nanoparticles and aluminosilicate host. Moreover, the interface interactions are more significant with the smaller size of nanoparticles, which lead to a larger red shift of Mie resonance band [12]. For further investigation of the Au incorporations, the HR-TEM microscope was used to
334 provide the direct observations on the Au nanoparticles/mesoporous aluminosilicates composites. In Fig. 3B, one can clearly see that the Au nanoparticles are embedded in the microparticles (site I, in Fig. 3B) or attached to the outer surface of the particulate (site II, in Fig. 3B) of the mesoporous aluminosilicates. The Au nanoparticles coment incorporated in the mesoporous aluminosilicates matrixes could be easily adjusted by using differem Au/surfactant ratio aqueous solution. With the XRD characterizations and HR-TEM observations, the Au/SiO2 wt % in the gel composites can be extended up to 12 % without any significant damage of the aluminosilicates mesostructures. Analyzing the TEM micrographs of samples with different Au contem, we found that the percentage of Au nanoparticles larger than 10 nm increases with the increase of Au content, but the extem of size increasing of Au nanoparticles with the increase of Au content is not large. After analyzing the N2 adsorption-desorption isotherms of the Au nanoparticles containing mesoporous aluminosilicates samples with different Au content, the basic physical properties were listed in Table 1. The Au/mesoporous aluminosilicates preserved the advantages of high surface area (~ 1000 m2/g), thick wall thickness (1.8-2.0 rim) and large pore (2.0-2.5 nm) and porosity (0.6-0.7 cm3/g) as in the metal-free mesoporous MCM-41 aluminosilicates. Thus, the high-loading incorporations of the Au nanoparticles have not exerted considerable pore-blocking effects on the nanochannels of the mesoporous aluminosilicates matrixes. Table 1. The physical properties and CO oxidation percentage of the Au-nanoparticles contained mesoporous aluminosilicates with various Au/SiO2 ratios. Au/SiO2 (wt%) 0.4 4.0 8.0 12.0 0
XRD di00 ~nm~ 3.58 3.77 3.77 3.75 3.90
Pore size Cnm) 2.00 2.22 2.53 2.46 2.50
Thickness a BET S.A. Porosity b CO conversion c (nm)
2 (m /g)
3 (cm /g)
/%
2.14 2.13 1.82 1.87 2.00
988 956 920 915 1046
0.64 0.74 0.73 0.68 0.78
-~ 0 8 16 0.5 ---
a. The value --- 2dl00/x/3-pore size. b. N2 adsorption volume at P/P0 = 0.9 c. The samples were treated in H2/N2 (1/9), 30mL/min, 600~ for 2hrs prior to the catalytic reactions. The reaction condition: in a CO/(CO+air) = 1% gas, WHSV - 9xl 04 ml/hrg, T = 80~
pressure = 1 atm.
As a result, it was suggested that the Au nanoparticles-containing mesoporous aluminosilicates should be considered as a highly dispersed Au-nanoparticles catalyst, which possesses the catalytic capability. For investigating the activity of this catalysis, the CO oxidation reaction was chosen as the test reaction. The activity data for CO oxidation of the Au nanoparticles containing mesoporous aluminosilicates with the Au/SiO2 wt % ratio in the range of 4 to 12 % are also listed in Table 1. The low CO oxidation activity (less than 16 % CO conversion) of the Au/MCM-41 aluminosilicates is ascribed mainly to two reasons: (1). The Au nanoparticles are still too large
335 (> 5 nm) in the samples synthesized from this one-pot process. For the insulator support, the Au nanoparticles possessing high activity should be controlled in a dimension as small as possible [13]. (2). The silica is not an efficient support on the CO oxidation [13] as those of 3d transition metal oxides of group VIII [14,15]. Thus, reducing the Au-nanoparticle size and using the proper transition metal oxides, the mesoporous materials containing Au nanoparticles possess the proper mesoporous supporter and a high-performance catalyst for CO oxidation should be obtained. Nevertheless, advantages of high surface area, large porosity, great pore size and high thermal stability that may be useful for applications in catalytic reactions of large molecules. To summarize, the main concepts of the preparation of Au nanoparticles/mesoporous materials are schematically shown in equation (1). After reduction, the hydrophobic Au nanoparticles are formed and preserved in the surfactant aqueous solution by surrounding alkyl long chain of quaternary ammonium surfactants, which are the typical templates of MCM-41 mesoporous materials. Combining with the aluminosilicate species, the surfactant-enclosed Au nanoparticles were mostly embedded in the surfactant-templated mesoporous aluminosilicates. After calcination, the mesoporous aluminosilicates containing Au nanoparticles can be successful obtained.
~
Silica Condense
~ -t- Sodium
~ ~
Calcine ~9
"
Au nanoparticles;
(1)
9Surfactant
4. CONCLUSION In summary, the Au-surfactants aqueous solution was found to be a good templating precursor for the convenient preparation of the Au-mesoporous aluminosilicates or metal oxide composites. Consequently, using other quaternary ammonium surfactant of different chain length could tune the pore size of the Au nanoparticles containing mesoporous aluminosilicates. With further modifications by using different surfactants as the stabilizing agent for various metal nanoparticles and structural templates of other metal oxides [ 16,17], we believe that various high-performance mesoporous metal oxides materials containing metal nanoparticles could be prepared by the simply one-pot synthesis procedure reported here.
ACKNOWEDGMENTS This research was financially supported by the National Science Council of Taiwan (NSC 89-2113-M-002-028) and the CTCI Foundation is gratefully acknowledged for supporting the HR-TEM facility and partial financial support.
336 REFERENCES
1. M. Haruta, Catal. Today, 36 (1997) 153. 2. M. Valden, X. Lai and D. W. Goodman, Science, 281 (1998) 1647. 3. A. Sanchez, S. Abbet, U. Heiz, W. D. Schneider, H. Hakkinen, R. N. Bamett and Uzi Landman, J. Phys. Chem. A, 103 (1999) 9573. 4. A. I. Kozlov, A. P. Kozlov, H. Liu and Y. Iwasawa, Applied Catalysis A, 182 (1999) 9. 5. A. P. Kozlova, S. Sugiyama, A. I. Kozlov, K. Asakaru and Y. Iwasawa, J. Catal. 176 (1998) 333. 6. M. Okumura, S. Tsubota, M. Iwamoto and M. Haruta, Chem. Lett., (1998) 315. 7. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. 8. T. Yanagisawa, T. Shimizu, K. Kuroda, and C. Kato, Bull. Chem. Soc. Jpn., 63 (1990) 988. 9. (a) H. P. Lin and C. Y. Mou, Science, 273, (1996). 765. (b) H. P. Lin, S. Cheng and C. Y. Mou, Microporous Mater., 10 (1997) 111. 10. J. P. M. Niederer, A. B. J. Amold, W, F. Hoelderich, B. Spliethol, B. Tesche, M. Rertz and H. Boenneman, Stud. Surf. Sci. Catal., 135 (2001) 316. 11. W. Chen, W. Chai, G. Wang and L. Zhang, Appl. Surf. Sci., 174 (2001) 51. 12. U. Kreibig and M. Vollmer, Optical Properties of Metal Clusters, Vol. 25, Springer Series in Materials Sciences, Springer, Berlin, 1995. 13. J.-D. Grunwaldt, C. Kiener, C. Wogerbauer, and A. Baiker, J. Catal., 181, (1999) 223. 14. W. Vogel, D. A. H. Cunningham and M. Haruta, Catal. Lett., 40 (1996)175. 15. M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M. J. Genet and B. Delmon, J. Catal., 144 (1993) 175. 16. P. Yang, D. Zhao, D. I. Maargolese, B. F. Chmelka and G. D. Stucky, Nature, 396 (1998) 152. 17. U. Ciesla, S. Schacht, G. D. Stucky, K. K. Unger and F. Schuth, Angew. Chem. Int. Ed. Engl. 35 (1996) 541.
Studies in Surtace Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
337
The use o f templated m e s o p o r o u s materials as templates for the d e v e l o p m e n t o f ordered arrangements o f nanowire and nanorods o f electronically important materials. J. D. Holmes, T. R. Spalding, K. M. Ryan, D. Lyons, T. Crowley and M. A. Morris* Dept. of Chemistry, University College Cork, Cork, Eire *main author; Tel: +353 214902180; Fax: +353 214274097; e-mail;
[email protected]
In this paper is detailed recent results which show how ordered mesoporous materials may be used as templates for the development of ordered arrays of nanowires. In particular it is shown how pore size control of the mesoscopic materials can be used to define wire widths. Further, methods are discussed for how macroscopically ordered films of the materials can be prepared at substrate surfaces so that homogeneous arrangements of wires can be achieved. The implication of these techniques for future electronic devices is discussed.
1. INTRODUCTION The application of Moore's Law has dominated the computer industry since the conception of the personal computer. It states that the processor speed (and the reciprocal of the interdevice distance since they are intimately linked) doubles every 12-18 months. Thus, the device density increases and the feature sizes decrease. Predictions suggest that around 2072010 both metal and semiconductor circuitry will reach nanosized dimensions i.e. less than 10nm [ 1]. For this reason there is a plethora of work detailing the preparation of nanosized particles. However, how these particles can be arranged into useful architectures remains something of a problem [2,3]. One method adopted is to use gas phase deposition methods to grow nanowires and rods within the pores of directionally ordered mesoporous materials [3,4]. However, these methods are not likely to be easily incorporated into mass device production methods because they are slow, involve high processing temperatures and expensive. We have recently shown that a simple laboratory supercritical fluid (SCF) based technique involving the decomposition of a precursor at the SCF conditions in the presence of a mesoporous material can be used to controllably yield g-quantities nanowires of semiconducting materials within the pores of the solid [6,7,8]. The technique has been extended to allow preparation of metallic and magnetic wires and these results are reported herein. Coupled to the advances in nanowire growth methods there must also be similar progress in methods for the synthesis of 'designed' mesoporous systems particularly for thin mesoscopic film preparation. Clearly, many of the techniques used for solid preparations are incompatible with film methods when 'castable' solutions are required so that films can be spray or dip coated and still show a distribution of ordered and directional mesopores. Of course, the microelectronics industry require high stabilities, mechanical strengths and
338
adhesion strengths so that films can be polished and withstand other process conditions. This paper summarises work in these laboratories in this important area. 2. EXPERIMENTAL Mesoporous Solid/Film Formation: In all preparations of meso~copic materials polyoxide surfactants were used (tri-block Synperonic type or single-block-alkane head Brij type surfactants usually- obtainable from Uniquema, Belgium and ICI, UK respectively). These allow much greater ranges of pore sizes to be achieved and here it was possible to design materials with pore sizes in the range of 3.0 to 10 nm. We also adopted the philosophy of using high concentrations of surfactant so that the solid is directly templated from the surfactant-solvent micellar (liquid-crystal) arrangement given by the phase diagram. This ensured that the structure of the mesoporous solid could be reliably predicted. In a typical solid preparation (carried out at 313K) a viscous gel prepared from lg of surfactant, 1.8g of tetramethoxysilane (TMOS) and l g of 0.5 M HC1 was condensed for several days. The resultant solid obtained was calcined at 723K for 24 hours. This method was adopted for film production by using controlled amounts of water and acid in the mixture so as to yield a clear sol-gel type material whose viscosity was controlled by addition of ethanol as a solvent. A typical preparation involved mixing 3 g of water/acid mixture (0.5 molar HC1) and 3 g of ethanol to for example 2-6 g of C18H37(OCH2CH2)20OH (manufactured under the trade name Brij 78 and supplied by ICI plc) for 5 minutes at room temperature. 10.8 ml of TEOS were added and the mixture stirred for 30 minutes. A solid could be prepared by drying the resultant gel at 70C for 3 hours and then calcining at 723k for 4 hours- the result in this case was a hexagonally ordered solid. Films of 0.2 to 1.0 micron could be developed by either dip or spray coating, drying and a similar calcination procedure. The result was a well-adhered film, which, like the solid, had a well-defined hexagonal structure. Both solids and films were characterised by Cu K~ x-ray diffraction (Philips 3710 PWD system calibrated to the (111) reflection from an ultra pure powdered silicon sample), transmission electron microscopy (TEM - JEOL 1200 EX system at 80 kV acceleration voltage), 29Si MASNMR (Chemagnetics CMX Lite 300 MHz apparatus), N2 gas uptake at 77K (Micromeritics ASAP 2010 volumetric analyser) and elemental analysis by x-ray fluorescence spectroscopy- XRF- (Philips Minipal). For the MASNMR studies data were taken at 5kHz spinning speeds and with 30 ~ 4s pulse width and pulse delay times of 60 to 10,000s depending on the sample. Chemical shifts are quoted relative to tetramethylsilane and referenced using tetramethyoxysilane. For TEM, powder samples or flakes removed from a substrate by scraping with a diamond knife, were redispersed in chloroform and a drop of the mixture was placed on a carbon-coated copper TEM grid. More complete experimental information is given in references 6-9. 3. RESULTS AND DISCUSSION The preparations used here resulted in well-ordered mesoscopic materials. Typical results (for the preparation detailed above using P85 - PEO26PPO39PEO26 triblock copolymer) are shown in figure 1. There is a hexagonal arrangement of pores and TEM suggests a pore wall thickness of 3nm and a pore size of 5nm. This is in close agreement with XRD analysis that suggests a pore-to-pore distance of 8.3nm. Pore sizes can be controlled to some extent by
339
simply changing the surfactant. However this method is somewhat crude as many applications such as selective species trapping may require very f'me control of the pore size. This was achieved by using mixtures of surfactants and typical data is summarised in table 1. In these experiments pore-to-pore distances were measured by PXRD. TEM data showed that the pore wall thickness remained approximately constant and this was verified using neutron scattering experiments[9]. Note that these materials all exhibited ordered structures but cubic and lamellar structures were observed for some of the products. It can be seen that this method can be used to control pore sizes to within 0.1nm and it is believed that this is the first report of this degree of control.
Fig. 1: PXRD profile (Intensity versus 2-theta angle) of mesoporous material prepared as detailed in the text. Inset shows the TEM image of the same sample. Image is 300 nm in width. Table 1: Variation of pore-to-pore distance in a series of mesoporous materials prepared with mixtures of Synperonic surfactants at molar ratios indicated. Mole % Surfactant ratio
P123:P85
100:0 80:20 60:40 40:60 20:80
9.99 9.73 9.42 9.13 8.86
d value in nm P85:P65 8.55 8.34 8.10 7.89 7.65
P123:P65 9.99 9.52 8.94 8.44 7.92
The films produced from the sol-gel precursors prepared as given above proved to results in well-adhered and robust films. Typical TEM data from a Brij 78 surfactant templated film (see above) are shown in fig. 2. The TEM data was collected from flakes of films prepared by scraping a coated glass substrate (3cm x 3cm) and the hexagonal arrangement can be clearly seen. The cast film is very well ordered. Fig. 3 shows the results of an omega scan of the glass-coated substrate. The 20 angle was set to the main (100) reflection (1.55 ~ i.e. a pore-to-
340
pore distance = 5.79nm) from the mesoporous film and the 0 angle scanned. Somewhat unexpectedly a very strong (at 0 = 0.776 ~ and sharp (FWHM = .021 ~ omega peak is observed (rocking-curve). The width is extremely close to that recorded for a (111) reflection from a silicon single crystal (FWHM = .016 ~ suggesting that a very well ordered film is formed and that the pores are macroscopically aligned to the surface. This was confirmed by carrying out a series of 0-20 plots as a function of azimuthal angle. The data showed quite distinct peaks of the (100) peak intensity as a function of azimuthal angle. The strongest peaks were observed at angles of 10 and 190~ measured from the edge of substrate aligned (at zero degrees) to be parallel to the x-ray beam direction. Data is shown in fig. 3 indicating how the peak intensity changes as a function of azimuthal angle. As the position of maximum intensity is moved away from there is a decrease in the intensity of the features until peak intensity is lost completely. The data also indicates very strong (110) peaks at about 1.8~ compared to the (100):(110) peak intensity observed for powder hexagonal mesoscopic materials (see typical data reported in references 6,7 and 8 for example). The data suggests that the pores are aligned across the whole of the substrate and further that the (110) planes are parallel to the surface. This is confirmed when the edges of flakes are examined (see figure 2). The appearance of very strong peaks at 0 and 190 ~ most probably indicate that the pores lie perpendicular to the x-ray direction (across the substrate) since this would give the greatest scattering of x-rays as it maximises the density of scattering planes (modelling studies are currently being carried out). It is believed that this is the first observation of this degree of macroscopic ordering in mesoporous films.
Fig.2: TEM images of (left) powder particles and (right) the edge of a flake scraped from film deposited onto glass. The right image is about 400 nm across whilst the left image is about 0.4 micron in width. As can be seen, the edge of the flake shows misalignment of pores due to damage made during removal. The pores can be readily filled with semiconductor, metal and oxide materials so as to form embedded nanowires. In typical preparations sufficient precursor material to allow filling of the pores of the mesoporous solid was pressurised to SCF conditions using a stainless steel tube and piston. CO2 was normally used to displace oxygen free solvent and the inorganic precursor into a reaction cell placed inside a tube furnace that could be heated to temperatures between 473 and 1473K (+/- 1K). Typically, easily degradable precursors were u s e d - e.g. diphenylsilane- and for silicon wires the precursor was suspended in hexane and then reacted at 375 bar, 773K for 15 minutes [6,7]. The product solid was harvested by filtration and vacuum distillation of the solvent. The wires grow selectively within the pore system. 29Si
341
MASNMR data shows this conclusively (rigA). The mesoporous material solid material prepared as described above using PEO26PPO39PEO26 (Synperonic PE/P85 supplied by Uniquema, Belgium) displayed two principle chemical shifts o f - 1 0 3 . 4 and -111.4 ppm, respectively assigned to species Q3 (Si(OSi)3(OH)) and Q4 (Si(OSi)4)[10,11]. Upon inclusion of the silicon into the pores there is complete loss of the Q3 feature consistent with the filling of the pores and loss of these surface sites. Higher field features become apparent after the reaction a t - 8 0 ppm in positions reminiscence of elemental silicon. It should be noted that there is no degradation of the mesoporous arrangement and XRD and TEM shows the structure to be intact [6,7,8] iiill -
,moo,
folio i
t., ,:m,t ~,m~,111o ..........
Doltltlel Thet,
Figure 3: XRD profiles recorded from films prepared as described. Above is the omega scan and to the right a series of images at various azimuthal angles. Uppermost is data from the position of maximum intensity (190 ~) and below it at data +/- 10 and +/- 20 ~ from this angle (data superimposed).
A
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s
'"'
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~:,
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Fig. 4: 29Si MASNMR data from mesoporous material before (A) and after (B) filling with silicon nanowires. In (A) a distinct shoulder due to the presence of Q3 sites can be clearly seen, in (B) this
342
shoulder is absent in (B) but instead the appearance of a feature at high field due to elemental silicon is now present.
The wires are crystalline in nature. Data is shown in fig.5 for germanium wires grown in identical conditions as silicon wires except using diphenylgermane as precursor. The x-ray diffraction data reveals the presence of features associated with crystalline germanium and detailed analysis of the features shows that the wires are orientated in the (100) direction within the silica pore system. N
(11 l)j 9 it
(220) (13 I i )
., (4ool
Fig. 5: PXRD profile from germanium wires in mesoporous silicon. Main reflections are indicated. The insert is the expansion of the (220) peak that shows a sharp feature (from planes perpendicular to the wire direction) superimposed on a broader peak (from planes parallel to the wire direction). The very sharp (400) feature suggests that the wires are aligned in the (100) direction. By judicious control of conditions and precursors it is possible to grow nanowires of many other materials and two examples are given here. Fig. 6 shows copper nanowires grown from the supercritical decomposition of copper acetate in CO2 (at 773K and 5000 psi). Both TEM and XRD data are shown. The unit cell parameter (a = 0.3629nm) calculated for the copper wires shows a slight expansion to that expected for bulk copper (a = 0.3615nm) [12]. Similar lattice expansions were obtained for wires grown from various other compounds such as copper acetylacetonate and copper nitrate and it would seem likely that the narrow width of the wires and the consequent decrease of orbital overlap and bond weakening compensates for any lattice contraction expected as a result of surface tension effects. Also shown in figure 6 are copper oxide wires (CuO as shown by XRD) grown in identical conditions except for the presence of water in the reaction mixture. 4. SUMMARY Mesoporous materials provide robust templates for the synthesis of nanowires of semiconductors, metals and oxides. It is shown that the use of polyoxide templates as surfactants can allow simple control of mesopore sizes over a very wide range. The preparation methods are facile and allow simple control and prediction of the structure of the mesoporous materials. The preparation method used can be adapted to form a route to the synthesis of sol-gel materials which can be used to cast ordered mesoporous films at surfaces.
343 Not only is the local porous structure ordered, but also detailed x-ray diffraction analysis shows that the films are highly macroscopically ordered.
Fig. 6: Left: copper wires grown as described in text. Right: CuO wires grown as described in the text.
By decomposition of inorganic precursors in the presence of the mesoporous systems in supercritical conditions it is possible to grow dense wires of materials within the pores of the porous matrix. It is suggested that the very high mobility of species in SCF conditions allows the control of growth so that the wires are largely defect free (since high quality XRD profiles are observed from the products). The high pressures and high mobility possible aid the mass transport of reactive moieties through the channel network thus allowing rapid formation of the wires. The simple chemical based methods used here allow bulk quantities of materials to be prepared in short process times. The importance of the methods used here to future developments in microelectronics is clear. In further work, we are using the experimental controlled variation of pore size in the mesoporous solids to direct allow direct control of semiconducting wire width. Currently we are preparing a series of varied width silicon wires and measuring their electro-optical properties by techniques such as uv-visible absorption and emission spectroscopies to examine if important electronic properties can be tuned by these methods. REFERENCES
1. 2. 3. 4.
L J Brus, J. Phys. Chem., 98 (1994) 3575 G Schmidt, M Baumle, M Geerkens, I Heim, C Osemann, T Sawitowski; Chem. Soc. Rev.; 28 (1999) 179 P P Nguyen, D H Pearson, R J Tonucci, K Babcock; J. Electrochem. Soc.; 145 (1998) 247 O Dag, G A Ozin, H Yang, C Reber, G Bussiere; Adv. Materials; 11 (1999) 474
344
5. 6. 7. 8. 9. 10. 11. 12.
A H Whitehead, J M Elliott, J R Owen, G S Attard; Chem. Commun.; (1999) 331 N R Coleman, M A Morris, T R Spalding, J D Holmes; J. Am. Chem. Sot.; 343 (2001) 1 N R Coleman, N O'Sullivan, K M Ryan, M A Morris, T R Spalding, J D Holmes; J. Am. Chem. Sot.; 23 (2001) 7010 N R Coleman, J D Holmes, T R Spalding, M A Morris; Chem. Phys. Letts.; 23 (2001) 187 Detailed description of the preparations is given: - M A Morris, K M Ryan, D Lyons, J D Holmes; submitted to Langmuir O Dag, G A Ozin, H Yang, C Reber, G Bussiere; Adv. Materials; 11 (1999) 474 A Steel, S W Carr, M W Anderson; Chem. Materials; (1995) 1829 JCPDS file set 04-0836
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
345
Synthesis and a d s o r p t i o n properties o f novel carbons o f tailored p o r o s i t y Zuojiang Li, Michal Kruk and Mietek Jaroniec Department of Chemistry, Kent State University, Kent, Ohio 44242, USA
It has recently been demonstrated that mesoporous carbons with uniform pores of tailored size can be synthesized from cost-efficient mesophase pitch and colloidal silica precursors using the colloidal imprinting (CI) method. Herein, this novel synthesis approach is discussed and compared with other available templating pathways for synthesis of mesoporous carbons. It is shown that CI allows one to obtain more uniform pores of size closer to that of the colloidal silica particles than in the case of where colloidal silica is lused as a template for polymerization of carbon precursors, followed by carbonization. This indicates that aggregates rather than single silica particles act as templates in the latter procedure. A similar phenomenon may be responsible for the formation of secondary mesoporosity/macroporosity in some of colloidal-imprinted carbons.
1. INTRODUCTION Recently, there has been a great interest in the silica-templated syntheses of mesoporous carbons, whose viability was originally demonstrated by Knox et al. [ 1]. There are currently several methods for these syntheses: (i) disordered mesoporous silica gel templating, (ii) ordered mesoporous silica templating, (iii) colloidal silica templating, (iv) colloidal crystal templating, and (v) colloidal silica imprinting. In the first of these methods, the porous structure of a disordered mesoporous silica gel can be infiltrated with a carbon precursor that is subsequently carbonized and the silica template is dissolved in order to liberate the carbon framework [1,2]. These carbons retain the particle shape of the silica template used [1] and tend to exhibit broad pore size distributions (PSDs) [2]. The second approach is a highly successful extension of the first strategy, and involves the use of ordered mesoporous silicas as templates, which allows one to synthesize ordered mesoporous carbons [3]. Initially, MCM-48 silica, which exhibits porous structures consistent of two disconnected interwoven three-dimensional porous systems [4,5], was used as a template [3,6-10]. MCM-48 was impregnated with sucrose in the presence of sulfuric acid,, the resulting mixture was carbonized, and the template was dissolved in order to isolate the ordered carbon with uniform mesopores of about 3 nm in width, accompanied by micropores. MCM-41 silica with a 2-dimensionally hexagonally ordered structure of cylindrical one-dimensional pores was found unsuitable as a template because of the lack of connections between the pores [7,11 ]. However, SBA-15 silica [12] that exhibits 2-D hexagonal array of pore channels akin to those of MCM-41, but connected by pores in the walls, as originally revealed during our collaborative work [13], was found to be an excellent template for synthesis of ordered
346 mesoporous carbons [14,15]. The latter include carbons that exhibit 2-D hexagonally arranged microporous rods [16-19] or pipes [15]. So far, the pore sizes of these carbons were reported in the range from 2 to 6 nm [60]. 3-D ordered carbon of cubic symmetry was also obtained [20] using SBA-1 silica template [21]. Several other mesoporous carbons that exhibit lower degree of structural ordering, in most cases giving rise to an occurrence of a single peak on the XRD pattern, were also synthesized [11,22,23] using templates such as HMS silica with wormlike porous structure [24], silica mesocellular foams (MCFs) [25], and silica particles with solid cores and mesopore shells [26]. The use of MCF templates afforded carbons with large uniform pores of size similar to those of the MCF template (and thus potentially tailorable in the size range from about 20 to 40 nm) [25], uniform pores of diameter about 4 nm templated by the walls of MCF, and most likely also micropores in the carbon framework. The third silica-templated method for synthesis of mesoporous carbons is based on the idea of the use of colloidal silica particles as templates [27-29]. In this method, carbon precursor is polymerized in the presence of colloidal silica particles, allowing one to obtain mesoporous (and often also macropor0us, as seen from the reported gas adsorption isotherms) carbons with very high specific surface areas and pore volumes. However, it was noted [27,29] that in many cases, pore dimensions of these carbons did not correspond to the size of colloidal silica particles used, indicating that agglomerates of these particles effectively templated the pore formation. Hereafter, it will be discussed that even in the cases where the correspondence between the attained pore diameter and the colloidal silica particle size was claimed [28,29], the PSD of the resultant carbon is too broad to reflect the actual particle size distribution. The forth method that may be useful in the synthesis of mesoporous carbons is based on the templating with silica colloidal crystals [30-33]. Because of the fact that the standard colloidal crystal templates are ordered arrays of silica spheres at least 100 nm in diameter, this approach is primarily suitable for synthesis of macroporous carbons [30,31,33]. However, some success has recently been reported in the synthesis of colloidal crystal templates with silica spheres of diameter down to as little as 30 nm [32]. Such templates are highly promising for synthesis of ordered carbons with pores in the upper mesopore range, but gas adsorption data reported so far [32] for a carbon synthesized by using crystal consisting of 50 nm silica colloids suggest that the pore size of this carbon is larger than the size of the colloids used. This points out to the need for improvement of this carbon replica synthesis procedure to fully benefit from the development of colloidal crystal templates of small particle size. It should be noted that further downscaling of the particle size of colloidal crystal templates is likely to be difficult. The fifth approach for synthesis of silica-templated mesoporous carbons that was recently developed by Li and Jaroniec [34], is based on the finding that under suitable conditions, silica colloids are imprinted in the mesophase pitch particles, which after carbonization and silica dissolution results in the formation of well-connected, highly uniform mesopores. This process, which is called the colloidal imprinting (CI), allows one to obtain colloid-imprinted carbons (CICs) with pore diameters tailored from about 7 to at least 24 nm by using colloidal particles of different sizes. CICs have primarily non-microporous frameworks that are suitable for graphitization or introduction of microporosity in a controlled way using judiciously chosen activation methods. The CI method employs relatively cheap and commercially available starting materials (mesophase pitch and colloidal silica), which is promising from the point of view of a large-scale production of CICs. Herein, selected aspects of the CI synthesis of mesoporous carbons are discussed with the focus on the distinction between this method and the colloidal silica templating approach proposed earlier [27-29].
347
2. EXPERIMENTAL
CICs were synthesized from a synthetic Mitsubishi mesophase pitch (H/C ratio of 0.6, softening point of 510 K) and several kinds of colloidal silica particles, including Ludox AS40, AS-30 and HS-40. The following synthesis procedure was employed [34]. The mesophase pitch was ground and fraction of particles below 45 gm in size was separated by sieving and used for the CIC synthesis. Thus prepared mesophase pitch particles were dispersed in ethanol, mixed with an excess of colloidal silica solution in a closed flask and stirred at about 323 K for 5 hours. Subsequently, the flask was opened to allow for an evaporation of the solvent under stirring for 3-5 hours. The resultant mixture was placed under nitrogen atmosphere and kept at 533 K (which is slightly above the softening point of the mesophase pitch) for 30 minutes, which was followed by heating with a ramp 2-5 K rain 1 to 1173 K, which was maintained for 2 hours to carbonize the mesophase pitch. Then, silica was dissolved in 3 M NaOH solution at about 368 K. In the case of the sample synthesized using Ludox AS-40 colloidal silica, the mesophase pitch particles were pretreated by heating them in silicone oil for 30 min at 523 K, and washing with toluene and acetone at room temperature. The resultant samples are denoted CIC x, where x stands for the symbol of the colloidal silica used fro their synthesis (AS-40, AS-30, or HS-40). Nitrogen adsorption isotherms were measured on a Micromeritics ASAP 2010 volumetric adsorption analyzer. Before the measurements, samples were degassed under vacuum 473 K. Weight change curves were recorded under air atmosphere on a TA instruments TGA 2950 thermogravimetric analyzer. The cq plot analysis [35,36] was performed using the reference adsorption isotherm on Cabot BP 280 carbon black [36]. The pore size distribution (PSD) was calculated using the Barrett-Joyner-Halenda (BJH) method [37], in which the statistical film thickness curve was based on the aforementioned reference data adjusted [2,34] using the reference statistical film thickness curve for silicas (the latter was calibrated using MCM-41 silicas [38]).
3. RESULTS AND DISCUSSION For a material composed of ideal spheres of a uniform size, one can evaluate the sphere diameter on the basis of the specific surface area (S) and density (p). Namely, one can readily show that the sphere diameter is equal to 6/(S.p). The manufacturer reported that the approximate specific surface areas of Ludox AS-40, AS-30 and HS-40 colloidal silicas are 135,230, and 220 m 2 g-l, respectively. Under assumption that these specific surface areas are accurate and that the colloidal particles are spherical and monodisperse, and their surfaces are smooth, one can evaluate the particle diameters of 20.2, 11.9, and 12.4 nm for AS-40, AS-30 and HS-40 silicas, respectively, using the above equation. It was assumed that the silica density is 2.2 cm 3 g-l, which is a typical value for amorphous silica [39]. So, it can be expected that these colloidal silicas are suitable as porogens for mesopores of diameter similar to the particle sizes estimated above. An exact correspondence is not expected because of many reasons, including the inaccuracy of the specific surface area assessment, the departures from the spherical shape of silica particles, surface roughness, inaccuracy in the assumed density, and the shrinkage of silica particles upon heat treatment. The last of these factors would lead to the formation of pores of size smaller than that of the colloidal particles. On the other hand, the specific surface area estimated by the manufacturer was most likely
348 calculated from nitrogen adsorption data, which leads to its overestimation in the case of silicas [38,40]. The overestimation of the specific surface area leads to underestimation of the particle size when the equation mentioned above is used. Moreover, any surface roughness and particle shape irregularity would cause that the specific surface area of colloidal silica particles assessed using gas adsorption methods would be higher than that for ideally spherical and smooth particles of the same diameter. Therefore, it is expected that the colloidal silica particle size assessed on the basis of their specific surface area is somewhat underestimated. These considerations lead to the conclusion that the shrinkage upon heating and the surface area overestimation are likely to be two major sources of differences between the size of colloidal silica particles estimated from their specific surface area and the size of the pores formed via their imprinting or templating. However, these factors are likely to offset each other to some extent, and moreover, the differences are expected to be quite systematic, which gives good prospects for prediction and tailoring the pore size of CICs simply on the basis of the specific surface area of colloidal silica particles used, if only the particle size is actually replicated in this synthesis approach. Shown in Figure 1(A) are nitrogen adsorption isotherms for two CICs synthesized using Ludox AS-40 and AS-30 colloidal silicas of different particle size. Sharp steps of capillary condensation were observed, whose position shifted to a higher relative pressure as the size of the colloidal particles used increased. The adsorption-desorption hysteresis loops are rather broad, which suggests that entrances to the pores are significantly more narrow than the pores themselves. On the basis of this behavior and transmission electron microscopy observations [34], CICs appear to exhibit porous structures similar to silica mesocellular foams [25]. It is also noteworthy that CIC synthesized using AS-40 colloidal silica is the only CIC sample among those discussed herein, whose structural properties were enhanced by pretreating mesophase pitch in silicone oil [34]. This treatment led to a significant increase in the adsorption capacity without any change in the pore diameter. This suggests that this pretreatment would allow one to improve adsorption capacity of CICs synthesized using different colloidal silicas. The pore diameters of AS-40 and AS-30 CICs is likely to be close to 20 and 12 nm, respectively, on the basis of the specific surface area of colloidal silica particles. The pore diameters were estimated to be 24 and 13 nm [34] from nitrogen adsorption data using a method developed for cylindrical pores. This is a reasonable agreement taking into account the possible inaccuracy of these pore size predictions and a limited accuracy of the gas adsorption evaluation of the pore diameter for large mesopores. In order to compare the structure of carbons synthesized using CI approach and the colloidal templating method proposed by i-Iyeon et al., CIC was synthesized using HS-40 colloidal silica particles. Its adsorption isotherm is compared in Figure 1(B) with that of a mesoporous carbon synthesized by Hyeon et al., for which the authors concluded that isolated silica particles (about 12 nm in size) act as templates. The comparison of the PSDs for these two carbons is shown in Figure 2(A). It can be seen that CIC exhibited a narrower PSD centered at about 15 nm, whereas the PSD for the other carbon sample is broader and shifted to somewhat larger pore sizes. These results suggest that the CI method is advantageous for the synthesis of mesoporous carbons with uniform pore diameter controlled by the size of the colloidal particles used.
349 ,
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Figure 1. Nitrogen adsorption isotherms for selected colloid-imprinted carbons (CICs). The isotherm for the CIC HS-40 sample synthesized using Ludox HS-40 colloidal silica is compared with the isotherm for the SMCI-I.5 carbon synthesized by others using a different method that employed the same colloidal silica as template. Data for samples CICs AS-40 and AS-30 are taken from [34], whereas data for SMCI-I.5 were reported in [29]. ,
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Standard Adsorption c~s
Figure 2. (A) The comparison of pore size distributions for two carbons synthesized via a silica templating [29] and colloidal-imprinting method [34] using colloidal silica Ludox HS40 as a porogen. (B) A representative C~splot for a colloid-imprinted carbon. The better ability of the CI approach to form uniform carbon pores that reproduce the size of colloidal silica particles is likely to be related to the inherent differences in the two methods discussed. In the case of colloidal templating approach of Hyeon et al., polymeric precursor of carbon framework is polymerized in solution in the presence of colloidal silica particles. On the basis of the nitrogen adsorption data, one can conclude that there is a transition from the single silica particle templating (which is reflected in a relatively smaller
350 pore sizes attained) to templating by aggregates of silica particles (which is reflected in the formation of very large pores) as the relative amount of silica to the polymeric carbon precursor is increased. It appears that as the relative amount of silica is increased, there is less and less of the carbon precursor available to coat particular silica particles and consequently, the polymeric precursor structure formed confines groups of adjacent silica particles rather than the isolated particles. This leads to the broad PSD for the resulting carbons and to the lack of correspondence between the pore size of the carbon and the particle size of the colloidal template. The sample compared above with CIC was the one that exhibited tl~e most prominent single particle templating behavior among the carbon samples reported in [29], and yet the comparison indicates that even for this sample, the templating by silica aggregates has a significant contribution to the mesopore formation. On the other hand, the CI procedure tends to be reproducible as far as the pore size attainable using a particular colloidal silica particles [34]. This can be attributed to imprinting of individual colloidal silica particles in the mesophase pitch. The fact that the silica particles can be readily removed after carbonization resulting in a mesoporous carbon structure with good pore connectivity suggests that the colloidal silica particles in the mesophase pitch are in contact with one another. On the other hand, the pore sizes of CICs are uniform and pore entrance sizes are not excessively wide, which strongly suggests that the mesophase pitch intimately encloses the colloidal silica particles. This is an important feature of the CI approach. However, in some cases, there is evidence of formation of secondary mesopores and macropores of size larger than that of the colloidal-imprinted mesopores. This is clearly observed for HS-40 CIC. The origin of these pores is not well-understood at present, but as the mesophase pitch itself is primarily non-porous, the formation of secondary pores is likely to be related to the templating by aggregates of colloidal silica particles rather than by individual particles. The secondary pores may also be pores between small colloidalimprinted carbon particles. Further studies will be required to gain control over the formation of the secondary mesoporosity/macroporosity in CICs. The CI approach is suitable for formation of mesoporous carbons with very small relative population of micropores, which abundantly occur in many other kinds of mesoporous carbons. As can be seen in Figure 2(B), the initial part of the as-plot for CIC does not exhibit any prominent deviations from linearity. Minor deviations from linearity for the initial part of the c~s-plot are similar to those observed on the c~s-plots for carbons, whose surfaces were more heterogeneous than those of the reference adsorbent used in the as-plot analysis [41,42]. % This would suggest that the CIC surface is more heterogeneous than the surface of a nongraphitized carbon black Cabot BP280 and other similar carbon blacks [43]. The expected differences in the surface properties between CICs and the reference adsorbent used make it difficult to accurately determine the micropore volume of CICs. However, a line drawn through the part of the plot that is located below the onset of capillary condensation (below -1.5 for HS-40 CIC) would intersect the Amount Adsorbed axis only slightly above the origin. This indicates that the micropore volume of CICs is relatively small.
4. CONCLUSIONS The CI method allows one to synthesize mesoporous carbons, whose pore diameter reflects the size of the colloidal silica particles used as a porogen, and thus is narrowly defined and tailorable. The comparison of the PSD for CIC and colloidal silica templated
351 carbon synthesized using an approach reported earlier indicates that the pore dimensions of the former are more uniform in size and much closer to the size of the colloidal particles used. It is thus concluded that CI is much more effective than the colloidal silica templating procedure proposed by others in affording mesoporous carbons of well-defined pore diameters. However, in some cases, CICs exhibit bimodal PSDs, as the pores commensurate with the dimensions of the colloidal silica particles are accompanied by significantly larger mesopores and macropores. The origin of the secondary pores is not clear, but their size may reflect the size of aggregates of colloidal silica particles rather than the particles themselves. Low-pressure nitrogen adsorption data for CICs suggest that their surfaces are slightly less homogeneous than surfaces of ungraphitized carbon blacks are. The as plot analysis indicates that CICs exhibit detectable, but relatively very small micropore volumes, which are difficult to quantify because of the apparent differences in the surface properties between CICs and non-graphitized carbon blacks, which are typical reference adsorbents in adsorption analysis of porous carbons.
5. ACKNOWLEDGMENT Professor T. Hyeon (Seoul National University, Korea) is gratefully acknowledged for providing the nitrogen adsorption isotherm data for silica-templated carbons reported in [29].
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Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
353
Flexible M e t a l - O r g a n i c F r a m e w o r k s with Isomerizing Building Units D. V. Soldatov, J. A. Ripmeester Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Canada K 1A 0R6 The utilization of isomerizing building units for constructing flexible metal-organic frameworks is discussed as a prospective strategy for creating new supramolecular materials. The utilization of reversible and controllable structural changes, performed on a molecular level, is a powerful but still not well explored instrument for triggering changes in the bulk properties of supramolecular solids. Such changes include changing the shape or geometry of molecules that can isomerize or oligomerize, and the set of target response properties may involve microporosity in all its diversity as well as other types of functionality. Our recent developments, traced in more detail below, include: (1) The first smart sorbent, the microporosity of which may be generated in situ and reversibly under external control, is represented by a system of two conjugated forms of a copper 13-diketonate. (2) The design of new isomerizable host molecules, which are versatile host receptors when in the trans form but incapable of inclusion when in cis form, is represented by the now emerging modified metal dibenzoylmethanates.
1. Flexible Metal-Organic Frameworks Since the first materials having supramolecular organization were identified half a century ago, [1-3] extensive efforts have resulted in whole generations of inclusion compounds, micro- and mesoporous sorbents, coordination polymers and other supramolecular materials. [4-6] The last 15 years has been a general tendency of materials science to move towards innovation and design, [7] and supramolecular chemistry has had an especially significant impact after its academic relevance was emphasized with a Nobel Prize. [8] Contemporary ideas in supramolecular design can be seen in such concepts as Smart Materials, [9] Molecular Machines [10] and Crystal Engineering. [11] The implied output targets are functional materials, that is, materials bearing a certain functionality, thus facilitating their implementation as elements in specific processes. [9,12-14] The remarkable potential for rational design of practically useful materials was demonstrated for the inorganic sorbents: zeolites, [15] zeosils, [16] and their analogs. [17] Yet, there is a clear understanding that the utilization of organic structural fragments will expand the horizons for choice in design tremendously. Metal complexes beating organic
354 ligands are especially attractive, as coordination with an endless variety of ligands can be obtained with simplicity and control. Wemer complexes were the first metal complexes utilized for creating supramolecular materials in the 50's and 60's. [ 18,19] About ten different metals, 15 anionic groups and > 100 (!) organic ligands were successfully incorporated in this host type to give a choice of hundreds of new host receptors. Detailed studies made it evident that for every guest a characteristic host receptor, specifically selective to the particular guest, could be found. The physico-chemical behavior of some of these complexes is similar to that of zeolites and it is these observations that inspired the idea of "organic zeolites" - materials mimicking inorganic sorbents. [20,21 ] Other large classes of metal-organic hosts, developed later on, demonstrated a similar remarkable variety and versatility with respect to potentially useful properties: porphyrin-based materials [22,23] and numerous coordination polymers based mainly on cyanide [24,25] and multipyridyl [26-28] types of bridging ligands. As we move from the past to the present, many authors have implied that priorities in the design of new metal-organic frameworks have changed. Kitagawa & Kondo [29] distinguish three generations of microporous frameworks: (1) "unstable to the loss of inclusion"; (2) "stable frameworks, reversibly lose and re-adsorb guest species without undergoing a change in phase or morphology"; (3) "dynamic structures, which change their frameworks responding to external stimuli". The overwhelming majority of reported materials belong to the "first generation". Special efforts were recently directed towards the construction of robust ("second generation", "stable") host frameworks, but only a few of these were found, in most cases by accident (see e.g. [20,29-35]). The frameworks we will refer to as flexible (also referred to as "third generation", "dynamic" or "soft" frameworks by different authors) certainly have the best prospects as materials but they are the most challenging from the point of view of purposeful design. The concept of flexible frameworks implies the existence of two or more forms of a microporous framework that are structurally and energetically similar to each other. In some situations the microporous form transforms into a metastable non-porous modification but they are so alike that even a minor external factor can switch them over to the other form (some authors call these two forms "host" and "apohost" [13,36]). In other cases the microporous form can change continuously over wide limits responding to such external factors as temperature, guest pressure, guest shape, size, polarity and functionality. Finally, discrete changes may be followed by a modification rather than a loss of microporosity, or the full range of changes may be a more complex combination of the situations listed above. A few examples of flexible frameworks have been reported. [21,35,37-44] 2. Metal Complexes as Changeable Building Units In most cases the presence of functionality in a material implies change; functional materials first of all are changeable materials. Change is not the only requirement but it is a
355 fundamental element in functional materials design. Metal complexes are very convenient building units as many their properties can vary dramatically without a change in gross composition. These properties range from those originating from molecules (color, dia/paramagnetism, chemical reactivity) to those solely attributable to macroscopic structure (microporosity, heterogeneous catalysis, ferromagnetism). Below we exemplify four types of change metal complexes may undergo on molecular level: conformational isomerism, oligo/polymerization, molecular isomerism and contact stabilization. Conformational isomerism of molecules in the solid phase is a known and well-studied topic. Zorkii and Razumaeva [45] applied statistical analysis to data on 330 crystals having two or more symmetrically independent molecules in the same structure; about 20% contained molecules with different conformations. Conformational changes in host complex molecules upon inclusion of different guests have been traced [21,46,47] including those where the color of the resulting solids changed [44,48,49]. Oligo- and polymerization is possible for complex molecules where the coordination is unsaturated. Metal(II) acetylacetonates [50] give a lucid illustration of this effect: in the solid they were isolated as dimeric (Fe), trimeric (Mn, Ni, Zn), tetrameric (Fe, Co) and polymeric (Cd) species. The monomer-trimer equilibrium for nickel acetylacetonate in solution is a well-known example in Cotton & Wilkinson's textbook; [51] similar behavior was observed for other related complexes. [52,53] The coexistence of both monomeric and trimeric forms in solution confirms that the forms have similar energies and therefore the formation of the desired form may be controlled by an additional external factor. This idea recently was realized for nickel dibenzoylmethanate (DBM). [54] The complex may exist both in monomeric and trimeric forms (Figure 1).
Figure 1. Monomeric and trimeric molecular structures of nickel dibenzoylmethanate
~,..
[Ni(DBM)2]
[Ni3(DBM)6]
The monomeric form exists as a brown diamagnetic solid stable at room temperature. Upon heating to a temperature higher than 202~ it transforms into a trimeric form that is green and paramagnetic. The exciting thing is that the trimerization can also be accomplished alternatively under much milder conditions using a suitable template: with benzene, an
356 inclusion compound [Ni3(DBM)6]*2(benzene) forms readily, with the trimeric nickel dibenzoylmethanate species thus existing at room temperature as the main component of a thermodynamically stable phase. Isomerization has very good prospects of being utilized. Bond-stretch isomerism [55] is the simplest; host molecule bond lengths may respond sharply to guest exchange [56] and this may dramatically affect the color of the resulting materials. [57,58] Linkage [59,60] and diastereomeric [61,62] isomerism represent more complex situations. Finally, stereoisomerizm (cis-trans) has the most potential: stereoisomers usually are similar in energy (because they have the same set of coordination bonds) but can form very different materials (due to dramatic differences in molecular geometry). [63,64] In order to cause cis-trans isomerization, a number of methods may be Used. Thermally [64-66] and photoinduced [67-69] isomerization can be considered as methods controlled by solely physical parameters. Our particular interest covers the cases where the isomers are switched and the desired state stabilized by a templating agent, that is, some species included in the final material but that does not change the composition of the isomerizing complex itself. The range of such templating species exemplified in the literature varies from inorganic ions, [70] through polar molecules like water, [71] to typical organic molecules like o-xylene. [72] Finally to be noted is the phenomenon called contact stabilization, [73,74] the extreme situation when a complex molecule can exist only in certain clathrate matrices while decomposing upon removal of the guest template. A good example of this is a copper complex that, depending on external conditions, forms either green or blue inclusion compounds with the guest 4-methylpyridine, but decomposes upon any attempt to remove this guest. [58,74] In the next two sections we discuss two examples that illustrate the potential utilization of molecular isomerism in designing materials and organizing processes. 3. The First Smart Sorbent
The
[CuL2] complex (L = L - = CF3COCHCOC(OCH3)(CH3)2 ) [35,75-78] has a
molecular structure with the Cu(II) chelated in the plane by two 13-diketonate ligands (Figure 2). Peripheral oxygen or fluorine atoms provide secondary, apical coordination to the metal centers of neighboring molecules that results in a 3D polymeric assembly, taking on either the ct or 13form.
357
)
trans (50%)
+
cis (50%)
trans ( 1 0 0 % )
a-form
p-form
Figure 2. Isomer distribution for the two polymorphs of [CuL2]. The a form has both trans and cis isomer units (left) whereas in the [3 form they are all trans (right). The polymorphs differ on both molecular and macroscopic levels: the a-form contains both cis and trans complex units which assemble in a dense structure, whereas the [3-form contains only trans complex units which assemble in a micropore structure. (or reverse) (Figure 2) Therefore, the a-to-]3 (or reverse) transition 1) involves r isomerization on molecular level and 2) switches on (or off) microporosity, an integral property of the material. Being thermodynamically metastable with respect to the stable a form, the [3 form reveals excellent kinetic stability. [35,76] The ]3-form includes an extremely wide range of organic and inorganic species [35,75] and reveals typical zeolite-like behavior [76,78] thus making it a useful and universal sorbent with approximate pore diameter of 6A, 6• pore volume of 20%, and specific surface ppm o area of-380 m3/g. 110 The material appears to be an efficient, zeolite-like sorbent material, capable of sorption over a wide range of guest pressures, able to absorb significant quantities reversibly, and with an excellent degree of predictability. However, unlike for zeolites and other common sorbents, this sorbent may be "switched off" by being temporarily transformed into the dense (a) form at any time. This property gives the material an unprecedented potential as a programmable ("smart") sorbent. [78]
S gO
105
ao
s..~ "~
100
9 .~4~/~~176 eooO4. / "
95
/~
.
9
E#'"
00 85
0
i
i
i
i
i
I
0.1
0.2
0.3
0.4
0.5
0.6
x
Figure 3. 129Xe chemical shift versus guest:host molar ratio, x, with methylene chloride as guest. Each point was accumulated for 1 min. [78]
358 The unusual behavior of the [CuL2] sorbent required special methods to follow transient changes in the microporosity of the material. ]29XeNMR spectroscopy appeared to be an excellent technique for probing microporosity, [79,80] and the recently developed optical polarization methods for xenon have improved the method to a degree where it can be used for rapid analysis. [81-83] The spectrum of the 13 form of [CuL2] unequivocally indicated the presence of microporosity by a resonance line at---80 ppm (with respect to the gas line at 0 ppm). Remarkably, the shift of the line shifted reversibly up to-~110 ppm as the available pore space was progressively filled with a guest (Figure 3). For the methylene chloride guest, the linear correlation between the shift 5xe and the guest:host molar ratio x was obtained: ~Sxe(ppm) = 40.7x + 87.4, thus giving a method for establishing quantitative control over the availability of micropore space in the material. [78] Figure 4 shows how the process of in situ generation of microporosity and its subsequent destruction proceed with time. The flow of adsorbate (methylene chloride) containing a small quantity of optically polarized xenon transforms the ~ form into the 13 matrix filled with guest adsorbate. Shutting off the flow immediately results in the availability of pore space to xenon that increases with time (left). A temperature pulse causes the signal intensity to decrease until its complete disappearance due to collapse of the microporous material back to the a form (right). The processes of molecular isomerization and phase interconversion that trigger the sorption potential of the material are both reversible and controllable in situ by the application of appropriate external stimuli. Evidently, the performance of this first smart sorbent is a feature that can be displayed only by flexible framework materials.
_
..... |
. . . . . . . . . . ~. . . . '|
120
n
i
100
"
w
-. . . . . . . . . . . . . |
80
'
'i
" l
60
"
low
off
flow
on
~.
i
'
,
] 20
u
w
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'
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I
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'
_
I
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_
.
"
.
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.
'
.
.
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.
.
.
.
i
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Figure 4. In situ transformation between ct and 13 forms of [CuL2] monitored by 129Xe NMR spectroscopy: a to 13transformation in the flow of methylene chloride (left) and [3 to cx transformation at 70~ (right). Spectra accumulated over a 5 rain period.
359 4. Modified Metal Dibenzoylmethanates This newly emerging class of host materials is formed by octahedral complexes of the general formula [MAz(DBM)2] (M is a metal(II), A is a pyridine-type ligand, and DBM is dibenzoylmethanate, C6HsCOCHCOC6H5-). [44,49,84] About 40 complexes of this type have been synthesized, with four different metal centers and 17 A-ligands. The host molecule is trans configured in the more than 140 inclusion compounds of 40 structural types isolated up to the present, while the molecule in the cis configuration was found only in dense modifications. This fact indicates that the ability to create supramolecular structures is a property of the trans isomer, thus giving a direct relationship between the isomeric state of the molecule and the integral properties of the resulting material. A salient example is provided by the two complexes [M(Pyridine)2(DBM)2] (M - Ni and Zn). [84] The nickel complex is trans configured in all seven solids isolated, and it is a versatile host receptor. The zinc complex is cis configured and it does not form any inclusions, forming a dense, stable solid. Our research is currently directed at finding complexes of this type that can exist in both cis and trans forms and could be suitable candidates for switchable materials. Recently, two such forms were prepared of a cadmium complex [85], and this inspires a certain optimism.
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atumes m ~urrace ~clence and Catalysis 141 A. Sayariand M. Jaroniec (Editors) 9 2002 Elsevier ScienceB.V. All rightsreserved.
363
Dynamic porous frameworks o f coordination polymers controlled by anions Shin-ichiro Noro and Susumu Kitagawa* Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
In this manuscript, we report in syntheses, crystal structures, and porous functionalities of novel coordination polymers, {[Cu(TiF6)(4,4'-bpy)2]'xH20}, (3a'xH20) (3-D Regular Grid) and {[Cu(4,4'-bpy)2(H20)2].TiF6} . (3b) (2-D Interpenetration), which show unprecedented dynamic structural transformation responding to guest molecules.
1.
INTRODUCTION
The research of self-assembled coordination polymers containing transition metal ions and organic bridging ligands has been practiced by many researchers during the past decade [ 1-8]. Especially, coordination polymers having regular microporous channels are of great interest due to their unique properties such as physical gas adsorption [9-12], chemical adsorption [13,14], ion-exchange [15], heterogeneous catalysis [16-18], and so on. The porous coordination polymers have advantages to provide not only light materials with high porosity but also desirable regular networks. Wemer complexes [19], Prussian blue compounds [2022], and Hofmarm clathrates and their derivatives [23] are widely known as porous materials that can reversibly adsorb small molecules. There are also a great many examples of porous organic frameworks that are sustained by hydrogen bonds [24-26]. Previously, we have defined the porous compounds in the three categories as shown in Scheme 1 [6]. The first 1st Generation
adsorption
-%_
~
~
~
2nd Generation ~> ~"
e!
Stable Frameworks ........ I
.....
desorption
3rd Generation Dynamic Channeies Responding r 5timuti
o ,o0 0
adsorption desorption Scheme 1
i~ncapusu~ating
364 generation compounds provide microporous channels with guest molecules, which are broken by the removal of all guest molecules. The second ones have rigid vacant channels formed after the removal of guest molecules. The third ones bear flexible channels, which change their own frameworks responding to external physical stimuli, such as electric or magnetic field and light, and a chemical stimulus by guest molecules. A large number of dicarboxylate- or tricarboxylate-bridged porous coordination polymers have been so far synthesized and investigated about their porous functions. These carboxylate-bridged porous coordination polymers tend to provide rigid framework because of the two site-binding mode of anionic carboxylate groups, therefore classified as the second generation compounds. Recently, several coordination polymers have been prepared, where these frameworks change reversibly on removal/clathration of guest molecules or anions [14,27-33]. The porous coordination polymers of 4,4'-bipyridine (4,4'-bpy) have relatively flexible frameworks based on the single site-binding of neutral pyridyl groups, potentially affording the third generation compounds evolving from the second generation ones [9,12,16,34]. On this background, we have challenged to develop a new type of coordination polymer chemistry of 4,4'-bpy. Recently, we have reported in syntheses and dynamic porous functionalities of a series of Cu--4,4'-bpy-AF 6 (A = Si, Ge, P) coordination polymers [35], in which a conversion of 3-D networks, {[Cu(AF6)(4,4'-bpy)2]-8H20}n (A = Si (la'SH20), Ge (2a-8H20)) (3-D Regular Grid), to interpenetrated ones, {[Cu(4,4'-bpy)2(H20)2]-AF6} n (A = Si (lb) and Ge (2b)) (2-D Interpenetration), took place by immersed in water in the solid state. Moreover, l b showed unprecedented dynamic anion-exchange properties and is classified as the third generation compound. As an key point to construct such a dynamic porous system, we noted counter anions, which have not only a role to neutralize overall charge in the solid but also to regulate frameworks, therefore we called this anion a framework-regulator. On the other hand, a pair of a metal and a ligand is regarded as a framework-builder because frameworks owes to topology and geometry of both ligands and metal cations. Cu(II) complexes could be relevant for crystal engineering by such framework-builder/-regulator, liable to undergo Jahn-Teller effect, resulting in a (4+2) coordination. In the presence of 4,4'-bpy ligand, the AF6 anions tend to sit the axial sites of the Cu(II) ion. By utilizing this tendency, the control of the framework by anions could be carried out. In this manuscript, we succeeded in synthesizing novel porous coordination polymers, {[Cu(TiF6)(4,4'-bpy)2].xH20 }, (3a.xH20) (3-D Regular Grid) and {[Cu(4,4'bpy)2(H20)z]'TiF6}~ (3b) (2-D Interpenetration), which were crystallographically characterized and investigated about dynamic porous functions.
2.
EXPERIMENTAL SECTION
2.1. Syntheses of bpyh(H2Oh]'TiF6}n (3b)
{[Cu(TiF6)(4,4'-bpy)2]-xH20}n (3a'xH20)
and
{[Cu(4,4'-
The compounds of 3a-xH20 and 3b were synthesized as follows: a hot aqueous solution
(50 mL) of Cu(BFa)2"xH20 (711 mg, 3.00 mmol) and (NH4)2TiF6 (594 mg, 3.00 mmol) was added to a hot aqueous solution (50 mL) of 4,4'-bpy (936 mg, 6.00 mmol). A color of the resultant suspension was purple and gradually changed to sky-blue. The obtained sky-blue powder of 3b was filtered, washed with acetone, and dried under vacutma to give the microcrystals (yield: 1184 mg, 66 %). The crystals of 3b suitable for the X-ray analysis were obtained as follows: a EtOH solution of 4,4'-bpy was diffused to an aqueous solution of
365 Cu(BF4)fxH20 and (NH4)TiF 6 in the straight glass tube. Sky-blue crystals of 3b were obtained together with purple crystals of 3a-xH20 after a few weeks. Although purple crystals and powder perhaps form a similar 3-D porous network to la-8H20 and 2a'8H20 from the result of the XRPD measurement, a good quality of single crystals was not obtained. The homogeneity of the powder sample of 3b was confirmed by comparison of the observed and calculated XRPD patterns obtained from the single-crystal data. This powder sample contains guest H20 molecules, because of the presence of a vacant space generated by a slight defect of the overall structure. Anal. Calcd for {[Cu(4,4'-bpy)2(H20)2]-TiF6.1.3H20}n (3b-1.3H20): C, 40.25; H, 3.59; N, 9.42. Found: C, 40.22; H, 3.81; N, 9.38. IR (KBr pellet): 3366 bin, 3106 w, 3083 w, 1645 w, 1609 s, 1536 m, 1490 m, 1413 m, 1322 w, 1221 m, 1067 m, 1012 w, 850 w, 813 m, 730 w, 680 m, 637 m, 526 s, 470 m (cml).
Table 1.
Crystallographic Data for 3b.
formula fw crystal system a,A c,A V, t13 space group Z p(calcd), g.cm3 F(000) /~(MoKcz), cm "~ diffractometer radiation (~,, A) temprature, ~ GOF no. of obsd data no. of variables R" (I > 2.00o(I)) Rwb (all data)
C2oH2oCuF6Na02Ti 573.84 tetragonal 11.301(1) 15.733(2) 2009.3(4) P4/ncc (No. 130) 4 1.897 1156.00 15.40 AFC7R 0.71069 23 1.06 570(I > 2.00o(I)) 81 0.042 0.070 b R~ -- [(Xko (IFo[-IFc])2/~,wFo=)] ~j2.
R = ~,llFol-IFcl]/~,lFo[. .
2.2.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
X-ray Structure Determination In compound 3b, data collections were carried on a Rigak~ AFC7R automated diffractometer with a graphite monochromated Mo-Kct radiation. Unit cell constants were obtained from a least-squares refinement using the setting angles of 25 well-centered reflections in the ranges 22.95 < 20 < 29.83 o. Azimuthal scans of several reflections indicated no need for an adsorption correction. The structure was solved by direct methods using the MITHRIL90 program [36] and expanded using Fourier techniques [37]. The nonhydrogen atoms were refined anisotropically. All hydrogen atoms were included but not refined. All calculations were performed using the teXsan crystallographic software
366 package of Molecular Structure Corporation [38]. determinations are summarized in Table 1.
3. 3.1. (a)
(c)
Crystal data and details of the structure
RESULTS AND DISCUSSION Crystal Structure of 3b An ORTEP view around a Cu(II) center of 3b is shown in Figure l(a) with numbering (b)
(d)
Figure 1. (a) ORTEP drawing around a Cu(II) center of 3b at the 30 % probability level. The hydrogen atoms are omitted for clarity. Selected bond distances (A): Cu(1)-N(1)= 2.040(4), Cu(1)-O(l*) = 2.384(6) [Symmetry Code : (*) x, y-l, z]. (b) ORTEP drawing of a 2-D network of 3b along the b-axis. The hydrogen atoms are omitted for clarity. (c) View of the interpenetration mode of 3b along the ab vector. The two types of 2-D layers lying parallel and perpendicular to the paper plane are represented by the stick and cylindrical bond models, respectively. The counter TiE62" anions and the hydrogen atoms are omitted for clarity. (d) ORTEP view showing the micropore cross section of the network of 3b along the c-axis. The counter TiF62 anions and the hydrogen atoms are omitted for clarity.
367 scheme. The Cu(II) atom has anelongated octahedral environment with four nitrogen atoms of 4,4'-bpy ligands in the equatorial plane and two oxygen atoms of 1-I20 molecules in the axial sites. The bond distances of Cu-O and Cu-N of 3b are similar to those of lb and 2b. The Cu(II) centers are bridged by 4,4'-bpy ligands to form a 2-D sheet having square grids with comer angles of ca. 89 and 91 ~ as shown in Figure 1(b). Each 2-D sheet lying in (a-b)c and (b-a)c planes affords a doubly-interpenetration mode (2-D Interpenetration) to make microporous channels with dimensions of ca. 2 A x 2 A along the c-axis (Figures l(c) and l(d)). These channels are filled by free TiF62"dianions, which interact with the coordinated 1-120 molecules by hydrogen bonds (2.665(4) A), whose value is apparently shorter than those of lb and 2b (2.702(3) and 2.686(4) A, respectively), relevant for a size of AF6 anions. This complex is isostructural with the Cu(II) and Zn(II) compounds reported previously [35,39].
3.2.
Dynamic Structural Transformation by Solvent and Anions The summary of framework transformation by solvent and anions are listed in Scheme 2. An interesting feature of this complex is that the 3-D structure of 3a-xH20 (3-D Regular Grid) are transformed into the 2-D interpenetrated structure of 3b (2-D Interpenetration) in the solid phase. When the mixture of Cu(BF4)2"xH20 and (NH4)2TiF6 reacted with 4,4'-bpy in a hot H20 solution, purple powder of 3a'xH20 immediately precipitated, where identification was carried out by the XRPD measurement (Figure 2(a)). Further stirring of suspension with purple powder of 3a-xH20 made a color changed from purple to sky-blue. The IR measurements show that a Ti-F stretching band of the sky-blue sample have the different frequency from that of the purple sample (from 570 to 526 crn~). Moreover, as shown in Figures 2(b) and 2(c), the XRPD pattern of the sky-blue powder is in good agreement with the simulated pattern calculated from the crystallographic data of 3b, clearly indicating that the 3D porous coordination polymer, 3a'xH20, is transformed into the 2-D interpenetrated network, 3b.
~ NH4PF6 NH4NO3 (NH4)2GeF6
H20
(NH4)2TiF6 2-D Interpenetration
{[Cu(4,4'-bpy)2(H20)2]'TiFs}n(3b)
~
sky-blue
3-D Regular Grid
2-D Interpenetration
{[Cu(TiFsX4,4'-bpy)2]-xH20}n(3a-xH20) purple
{[Cu(4,4'-bpy)=(H20)2]'SiFs}n (1b) sky-blue
Scheme 2
368
(a) (a)
(b)
(b)
.. II&.J,
L.,,.
(c)" ~ (c)
t
(d)
I IIilitLIll.i,~I
,,J,,Jl,J,ld..,.L,,, ,,.,,,,
I,,
I
3
10
,,
,I
I
I
!-
20
30
40
50
26 / *
I 60
1000
I
I
I
800
,
I
600
'J
I
,
I
400
v / c m -I
Figure 2 (left). XRPD patterns of (a) immediately obtained purple solid 3a-xH20, (b) sky-blue solid 3b obtained by long immersing pure 3a-xH20 in a hot I-I20 solution, and (c) simulation of 3b. Figure 3 (fight). IR spectra of (a) lb, (b) solid obtained by immersing lb in a H:O solution containing excess amount of (NI-{4)2TiF6,(c) solid obtained by immersing 3b in a I-I20 solution containing excess amount of (NI-{a)2SiF 6, and (d) 3b. The black and dotted arrows show SiF62 and TiF6 2" stretching bands, respectively. Several anion-exchangeable porous coordination polymers have been hitherto reported
369 [ 15,34,40], in which the microporous frameworks are maintained during the anion-exchange, so called the second generation compounds. We have investigated about the anion-exchange properties of lb, illustrating the third generation system [35]. In the same way, we also examined about the anion-exchange properties of 3b. When microcrystals of lb were immersed in (NH4)2TiF 6 (excess) solution, the color of the compound unchanged. However, as shown in Figure 3, the IR spectrum of a resultant powder clearly shows a new TiF6 2" band (525 cm"~) in addition to original SiF62 bands (746 and 483 crnl), indicating that the compound has partially undergone the anion-exchange. This compound maintains crystallinity during the anion-exchange process as illustrated by sharp peaks observed in the XRPD pattern, which is in a good agreement with that of a original sample lb. The complete exchange of the counter anion is not attained. This is possibly because the TiF6 2" anion is larger than SiF62 and is readily trapped in the channel near the surface by a strong hydrogen bonding interaction with coordinated 1-120 molecules. Therefore, interpenetration into a deeper region of the anion is prevented. Indeed, no anionexchange from TiF62 to SiF62 occurred in 3b as mentioned below. On the other hand, when 3b was immersed in aqueous solution in the presence of excess amount of NH4PF6, NH4NO3, ('NH4)2SiF6, and (NH4)2GeF 6 anions for a few days, no anion-exchange occurred. This is associated with the size of TiF62 anions: the anion'is too large to go through the small channel windows (ca. 2 A x 2 A). Moreover, hydrogen bonds with coordinated I-I20 molecules may support the strong trap ofTiF62- anions to the channels.
4.
CONCLUSION New dynamic porous coordination polymers, {[Cu(TiF6)(4,4'-bpy)2]-xH20}, (3a'xH20) and {[Cu(4,4-bpy)2(HzO)2].TiF6s, ' " (3b) (2-D Interpenetration), have been synthesized, crystallographically characterized, and investigated about their dynamic porous functions. Interestingly, the 3-D network 3a-xH20 is transformed into the 2-D interpenetrated network 3b. As compared with l b, the 2-D interpenetrated network 3b shows no anion-exchange properties, because of the larger size of the TiF6 2- anion tharl SiF62- one. On the other hand, the partial anion-exchange from lb to 3b was observed. Future works are in progress to create a novel dynamic porous coordination polymer responding to the external stimulus such as light, pressure, heat, and electric field.
REFERENCES O. M. Yaghi, H. Li, C. Davis, D. Richardson, T. L. Groy, Ace. Chem. Res., 31 (1998) 474. M. J. Zaworotko, Chem. Soc. Rev., (1994) 283. M. J. Zaworotko, Chem. Commun., (2001) 1. P. J. Hagrman, D. Hagrman, J. Zubieta, Angew. Chem., Int. Ed. Engl., 38 (1999) 2638. C. Janiak, Angew. Chem., Int. Ed. Engl., 36 (1997) 1431. S. Kitagawa, M. Kondo, Bull. Chem. Soc. Jpn., 71 (1998) 1739. A. J. Blake, N. R. Champness, P. Hubberstey, W.-S. Li, M. A. Withersby, M. Schr0der, Coord. Chem. Rev., 183 (1999) 117. B. Moulton, M. J. Zaworotko, Chem. Rev., 101 (2001) 1629. M. Kondo, T. Yoshitomi, K. Seki, H. Matsuzaka, S. Kitagawa, Angew. Chem., Int. Ed.
370
10. 11. 12. 13. 14. 15. 16. 17. 18. 9. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.
38. 39. 40.
Engl., 36 (1997) 1725. M. Kondo, T. Okubo, A. Asami, S. Noro, T. Yoshitomi, S. Kitagawa, T. Ishii, H. Matsuzaka, K. Seki, Angew. Chem., Int. Ed. Engl., 38 (1999) 140. M. Kondo, M. Shimamura, S. Noro, S. Minakoshi, A. Asami, K. Seki, S. Kitagawa, Chem. Mater., 12 (2000) 1288. S. Noro, S. Kitagawa, M. Kondo, K. Seki, Angew. Chem., Int. Ed. Engl., 39 (2000) 2082. S. S.-Y. Chui, S. M.-F. Lo, J. P. H. Charmant, A. G. Orpen, I. D. Williams, Science, 283 (1999) 1148. H. J. Choi, T. S. Lee, M. P. Suh, Angew. Chem., Int. Ed. Engl., 38 (1999) 1405. B. F. Hoskins, R. Robson, J. Am. Chem. Sot., 112 (1990) 1546. M. Fujita, Y. J. Kwon, S. Washizu, K. Ogura, J. Am. Chem. Sot., 116 (1994) 1151. T. Sawaki, T. Dewa, Y. Aoyama, J. Am. Chem. Sot., 120 (1998) 8539. J. S. Seo, D. Whang, H. Lee, S. I. Jun, J. Oh, Y. J. Jeon, K. Kim, Nature, 404 (2000) 982. R. M. Barter, ACS Advances in Chemistry Series 121: Molecular Sieves, Eds. W. M. Meier and J. B. Utyyerhoeven, American Chemical Society, Washington, DC, 1974, 1. R. E. Wilde, S. N. Ghosh, B. J. Marshall, Inorg. Chem., 9 (1970) 2512. H. J. Buser, D. Schwarzenbach, W. Petter, A. Ludi, Inorg. Chem., 16 (1977) 2704. K. R. Dunbar, R. A. Heintz, Prog. Inorg. Chem., 45 (1997) 283. T. Iwamoto, Inclusion Compounds, vol. 5, Eds. J. L. Atwood, J. E. D. Davies, and D. D. MacNicol, Oxford, New York, 1991, 177. T. Dewa, Y. Aoyama, Chem. Lea., (2000) 854. T. Dewa, T. Saiki, Y. Imai, K. Endo, Y. Aoyama, Bull. Chem. Soc. Jpn., 73 (2000) 2123. T. Tanaka, K. Endo, Y. Aoyama, Chem. Lea., (2000) 1424. D. V. Soldatov, J. A. Ripmeester, S. I. Shergina, I. E. Sokolov, A. S. Zanina, S. A. Gromilov, Y. A. Dyadin, J. Am. Chem. Soc., 121 (1999) 4179. K. S. Min, M. P. Suh, J. Am. Chem. Soc., 122 (2000) 6834. K. S. Min, M. P. Suh, Chem. Eur. J., 7 (2001) 303. L. C. Tabares, J. A. R. Navarro, J. M. Salas, J. Am. Chem. Soc., 123 (2001) 383. O.-S. Jung, Y. J. Kim, Y.-A. Lee, J. K. Park, H. K. Chae, J. Am. Chem. Soc., 122 (2000) 9921. C. J. Kepert, T. J. Prior, M. J. Rosseinsky, J. Am. Chem. Soc., 122 (2000) 5158. S. O. H. Gutschke, D. J. Price, A. K. Powell, P. T. Wood, Eur. J. Inorg. Chem., (2001) 2739. O. M. Yaghi, H. Li, J. Am. Chem. Soc., 118 (1996) 295. S. Noro, R. Kitaura, M. Kondo, S. Kitagawa, T. Ishii, H. Matsuzaka, and M. Yamashita, J. Am. Chem. Soc., in press. C. J. Gilmore, MITHRIL - an integrated direct methods computer program. University of Glasgow, Scotland, 1990. P. T. Beurskens, G. Admiraal, G. Beurskens, W. P. Bosman, R. de Gelder, R. Israel, and J. M. M. Smits, The DIRDIF-94 program system, Technical Report of the Crystallography Laboratory, University of Nijmegen, The Netherlands, 1994. Crystal Structure Analysis Package, Molecular Structure Corporation, 1985 & 1999. R. W. Gable, B. F. Hoskins, R. Robson, Chem. Commun., (1990) 1667. O. M. Yaghi, H. Li, J. Am. Chem. Sot., 117 (1995) 10401.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
371
M e s o p o r o u s Polymeric Materials Based On C o m b - C o i l Supramolecules Sami Valkamaa, Riikka M/~ki-Onttoa, Manfred Stammb, Gerrit ten Brinke a'c and Olli Ikkalaa Department of Engineering Physics and Mathematics, Helsinki University of Technology, P.O.Box 2200, FIN-02015-HUT, Espoo, Finland b Institut ffir Polymerforschung "Dresden e.V.", Hohe Strasse 6, D-01069 Dresden, Germany c Materials Science Center, Dutch Polymer Institute, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
a
In this work we present a procedure to achieve nanoscale mesoporous materials. Previously we have shown using hydrogen bonded amphiphiles that polymeric comb-coil supramolecules leading to lamellar-within -cylindrical assembly allow preparation of "hairy tubes", i.e. nanoscale empty tubes with polymer brushes at the wall. Here the concept is generalized: We use comb-coil supramolecules with lamellar-within-lamellar structure based on coordination bonding. Polystyrene-block-poly(4-vinylpyridine), PS-block-P4VP, is used with zinc dodecyl benzene sulphonate Zn(DBS)2, which coordinates to the lone electron pair of pyridine nitrogen of the latter block. Due to the physical nature of coordination bonding part of supramolecular template can be removed after the structure is formed by selective dissolution resulting in empty space between lamellae with polymer brushes at the walls. 1.
INTRODUCTION
Biological systems allow several examples for functional membranes, such as the cell walls with their transport proteins. Numerous biomimetic concepts have thus been pursued. Synthetic functional membranes have major technological applications e.g. in purification and biotechnological applications. Nanoporous materials (pore size 20-500 A) have been prepared using various methods. One method is based on using block copolymers via photocrosslinking and ozonolysis[1]. Another, in turn, uses assemblies of surfactants and block copolymers in the synthesis of inorganic materials[2-4]. Different kinds of degradation processes have also been presented to obtain nanoporosity[5, 6]. A new application has been to prepare so-called low dielectric material for electronics, based on self-assembly and selective removal of materials[7, 8]. Self-organization leads to nanoscale polymeric structures based on competing interactions and incorporation of several schemes of self-organization[9-13]. Previously we have introduced a concept where amphiphilic molecules are physically bonded selectively to one block of a block-copolymer and they self-organize to form structure-within-structures [1416]. The scheme also allows the preparation of mesoporous materials[17]. The starting material has been diblock copolymer polystyrene-block-poly(4-vinyl pyridine), PS-blockP4VP, with a stoichiometric amount of pentadecyl phenol, PDP, hydrogen bonded to the latter block. The block lengths have been selected to render a lamellar-within-cylindrical morphology, where the P4VP/PDP-blocks form cylinders within the rigid glassy PS-medium
372 and where the P4VP/PDP-complexes, being of a comb-like architecture, self-organize as lamellae within the cylinders. Due to the physical nature of the hydrogen bonding the cylinders can be emptied afterwards by using selective solvent to flush the amphiphiles away resulting nanoporous material with polymer brushes at the walls [17]. The same procedure has also been successfully used to prepare polymeric nanofibers[ 18]. In this article we generalize the concept to prepare other geometries. We describe an alternative structure-within-structure morphology, i.e. lamellar-within-lamellar, and use coordination bonding instead of hydrogen bonding used previously. The underlying idea is that, in general, coordination may allow construction of supramolecules when hydrogen bonds cannot be formed. The present PS-block-P4VP is a feasible model compound as it allows to test both interactions. We use PS-block-P4VP with zinc dodecyl benzene sulphonate Zn(DBS)2, which coordinates to the lone electron pair of pyridine nitrogen of the latter block (see Scheme 1). We show that the concept used previously [17] can also be applied to cover potentially stronger interactions between polymer backbone and amphiphiles i.e. coordination bonding. Due to the physical nature of coordination bonding part of supramolecular template can be removed after the structure is formed by selective dissolution resulting in empty space between lamellae with polymer brushes at the walls.
P4VP (/CH2~cH/)n ~
:5
coordination bon~...,,~ -_---
PS (/CH2~cH/)m
6
Zn(DBS)2 Scheme 1. 2.
MATERIALS AND METHODS
2.1. Materials PS-block-P4VP (Polymer Source Inc.) had Mw = 41,400 g/mol and 1,900 g/mol, respectively, for the PS and P4VP blocks and Mw/Mn = 1.07. Dodecyl benzene sulphonic acid (DBSA) was of purity 90% (Tokyo Kasei) and the main remaining impurity consisted of different chains lengths: (CnH2n+l)(CmH2m+l)CH-Ph-SO3H, with n+m+1= 10... 14. ZnO was of purity 99.0% and acquired from J. T. Baker B. V. Zinc dodecyl benzene sulphonate Zn(DBS)2 was synthesized in ethanol from DBSA anal ZnO according to ZnO + 2DBSA --->Zn(DBS)2 + H20. The detailed description of the procedure is given elsewhere[19]. In contrast to the earlier work, the product was additionally purified by recrystallizing it three times from acetone by adding water dropwise and the purity was assured by NMR-spectroscopy. After purification, the alkyl chains do not contain branches. Finally, the Zn(DBS)2 was dried in vacuum (10 .2 mbar) at 80 ~ for 24 hours.
373 The complexes PS-block-P4VP[Zn(DBS)2]o.9o were prepared by dissolving both components, PS-block-P4VP and Zn(DBS)2, in analytical grade chloroform. Mole fraction 0.90 of Zn(DBS)2 was used. The solvent was evaporated at 60 ~ on a hot plate; thereafter the samples were vacuum dried at 60 ~ for at least 12 hours.
2.2. Dynamic rheological orientation ARES (Rheometric Scientific Inc.) rheometer was used in oscillating mode with parallel plate geometry with gap of 1 mm. The sample was heated up to 200 ~ and then annealed at 170 ~ for 1 h. The shear was performed at 170 ~ for 16 h using 0.1 Hz and 50 % strain amplitude for 7 h. More details of the rheological experiments can be found elsewhere [17, 2O]
2.3. FTIR FTIR spectroscopy was used to study the interaction between the polymer and amphiphile molecules. All infrared spectra were obtained using a Nicolet Magna 750 FTIR spectrometer. The minimum number of scans was 64 and the resolution was 2 cm -l. Analysis was made from pressed pellets, which were prepared by grinding the samples with potassium bromide.
2.4. Small angle X-ray scattering (SAXS) measurements The Bruker NanoSTAR equipment used consists of a Kristalloflex K760-8- 3.0kW X-ray generator with cross-coupled G6bel mirrors for Cu K~ radiation resulting in a parallel beam of about 1 mm 2 at sample position. A Siemens multiwire type area detector was used. The sample-detector distance was 0.65 m (for more details see [ 17]).
2.5. Preparation of "hairy objects" After the dynamic shear orientation, the SAXS intensity patterns and FTIR were measured near room temperature. The pieces of the sample were immersed into analysis-grade methanol at room temperature for at least 12 h to remove Zn(DBS)2 within the lamellae. To verify that Zn(DBS)2 has been removed, the SAXS intensity patterns and FTIR measurements were performed again and compared to the original measurements. 3.
RESULTS AND DISCUSSION
Evidence for the complex formation between Zn(DBS)2 and PS-block-P4VP was obtained based on the FTIR bands characteristic for the aromatic carbon-nitrogen stretching. Figure 1 shows the FTIR spectra of PS-block-P4VP[Zn(DBS)2]o.9o complex in the 1650-1550 cm -l region. The spectra of similarly treated pure PS-block-P4VP and Zn(DBS)2 are also depicted for reference. Due to the coordination between Zn(DBS)2 and lone electron pair of the nitrogen in pyridine ring a shifted absorption band appears at ca. 1620 cm -1 (Figure 1). Previous studies showed that the 1596- 1597 cm -~ aromatic carbon-nitrogen stretching band of P4VP is shifted to 1615 - 1617 cm -I when the pyridine group participates in metal-ligand n-bonding[ 19, 21 ].
374 I
I
~
I
~
I
'
I
'
I
a)-
1640
1620
1600
1580
Wavenumber cm
1560
-1
Figure 1. FTIR spectra for a) Zn(DBS)2 b) PS-block-P4VP[Zn(DBS)2]o.9o and c) PS-blockP4VP. In b) the aromatic carbon-nitrogen stretching band is shifted to ca. 1620 cml due to the pyridine group participating in metal-ligand n-bonding. Oscillatory shear flow was used in order to enhance the organization of nanostructures and reduce the amount of grain boundaries. In contrast to the previous studies [17, 20] the P4VP[Zn(DBS)z]0.90 complex has higher softening temperature than P4VP(PDP) probably because the Zn-pyridine coordination may cause some crosslinking between the chains. Therefore the resulting material PS-block-P4VP[Zn(DBS)2]o.9o is rather stiff even at relatively high temperatures such as 170~ Previous studies with P4VP[Zn(DBS)2] showed that the material is ordered up to 200 ~ [19] in contrast to order-disorder-temperature of 67~ for P4VP(PDP)I.0 [22]. Although the material was sheared long time in high temperatures, only small signs of macroscopic orientation was observed. However, the shear has notable effect on the material allowing more facile removal of the amphiphiles. Small angle X-ray scattering was used to analyze the mesomorphic behavior of the samples after the shear flow. The SAXS intensity pattern of unwashed sample in Figure 2 shows the first intensity maximum at ca. ql = 0.03 A 1 (corresponds the long period value of Lp = 210 A), which comes from the larger structure between the blocks of PS and P4VP[Zn(DBS)2]0.90. Also the second order intensity maximum is presented as a shoulder at ca 2q~ = 0.06 A 1, indicating that the structure is lamellae. The broad band at q2 - - 0 . 2 0 A l, in turn, corresponds to the inner lamellar structure in the P4VP[Zn(DBS)2]0.90 phase with a long period of 32 A [23]. In the corresponding homopolymer complex where a slightly less pure Zn(DBS)2 was used, the second order peak at 2q2 becomes observable [19], indicating lamellar structure.
375 . . . . . . . .
100
!
. . . . . . . .
!
. . . . . . . .
!
. . . . . . . .
/'3
!
. . . . . . . .
!
. . . . . . .
,~
unwashed
10 1
0.1 0.01
. . . . . . . . ' ............................................ 0.00 0.05 0.10 0.15 0.20 0.25 0.30
q(1/A) Figure 2. SAXS intensity patterns for PS-block-P4VP(Zn(DBS)2)o.9 before and after amphiphile (Zn(DBS)2) removal with methanol. The larger structure is 210 A and the inner structure is 32 A. The magnitude of the scattering vector is given by q - (4~/X)sin0 where 20 is the scattering angle and X = 1.54 A. The scattering intensity is in a logarithmic scale. The advantage of using physically bonded (in this case coordinated) supramolecule template PS-block-P4VP[Zn(DBS)2]o.9oinstead of conventional block copolymer molecules is that the formed structures can be emptied easily, as part of the template, e.g. the oligomeric Zn(DBS)2, "flows" out from the inner structure in a suitable solvent. This is presented in the Figure 2, which shows SAXS intensity patterns for the sample before and after washing procedure. After washing the sample with methanol, which is a suitable solvent for both P4VP and Zn(DBS)2 but not for PS the inner lamellar structure is lost. This results in the disappearance of the SAXS intensity maximum of the inner lamellar structure (q2 = 0.20 A l) and simultaneously a strong increase in the intensity of the larger structure is observed which is a clear indication that a substantial part of the Zn(DBS)2 has been removed (Figure 2). The removal of amphiphile also results in a color change of the sample from transparent to white. Further evidence for the amphiphile removal can be observed from the FTIR spectra, which is illustrated in the Figure 3. The characteristic band for pyridine coordination at 1620 cm -1 is disappeared in the washed sample, indicating that a substantial amount of Zn(DBS)2 is removed from the material.
376 l
I
"
I
,
l
I
9
,
1720
I
1680
,
1640
I
,.
,
1600
Wavenumber cm
I
1560
,
,
1520
-1
Figure 3. FTIR for PS-block-P4VP[Zn(DBS)2]o.9o before (a) and after (b) amphiphile (Zn(DBS)2) removal with methanol. In the washed sample (b) there is no evidence of characteristic peak at 1620 cm 1 corresponding to pyridine-metal ligand interaction. The resulting structure was lamellar-within-lamellar although the volume fraction was chosen to be in the cylindrical regime of PS-block-P4VP(PDP) - phase diagram [16] (i.e. 75.3 w% of PS). This indicates that the phase diagram of PS-block-P4VP[Zn(DBS)2]x complexes is highly asymmetric. The lamellae are not highly macroscopically oriented but surprisingly we found that the removal of the amphiphiles was successful. We believe that proper shear flow conditions and further annealing can improve the macroscopic order. The search for right parameter to find structures other than lamellar and to tailor the dimensions of the mesoporous structures are under investigation. In conclusion, we have demonstrated that different kinds of physical interactions can be used for the preparation of self-organized hollow structures in a glassy rigid PS-medium. We show that also stronger interactions than hydrogen bonding [ 17], e.g. coordination bonding, between diblock copolymer and amphiphile are usable. The structures are formed by selforganization of supramolecules (Figure 4 a). Part of the supramolecular template, Zn(DBS)2, can be conveniently removed at the end after the structure has been formed (Figure 4 b), thus overcoming the need of use degradation or corresponding methods to make mesoporous materials. This concept permits a relatively easy way to increase the functionality of the material i.e. surface area per volume unit. Such materials could be further developed to nanoscale electrical or biotechnological applications.
377
a)
b) lamellar-with/n-lamellar
hairy lamellae
Figure 4. Schematic picture of the procedure towards mesoporous materials, a) Original lamellar-within-lamellar structure and b) after the removal of Zn(DBS)2 the lamellae selforganization remains due to the rigid glassy PS. Since the P4VP block can be expected to still cover the wall of the otherwise empty lamellae, we call them "hairy lamellae". 4.
ACKNOWLEDGEMENTS
Dr. Evgeny Polushkin is gratefully acknowledged for assistance with the SAXS measurements in Groningen. Dr. Roland Vogel and Dr. Werner Haselbach are gratefully acknowledged for assistance with the rheological experiments in Dresden. The work has been supported by Finnish Academy and Technology Development Centre (Finland). 5.
REFERENCES
1. S. Stewart and G. Liu, Chemistry of Materials, 11 (1999) 1048. 2. E. Kr~imer, S. F6rster, C. G61tner and M. Antoinetti, Langmuir, 14 (1998) 2027. 3. J.K. Ying, C. P. Mehnert and M. S. Wong, Angewandte Chemic International Edition, 38 (1999) 56. 4. S. F6rster and M. Antonietti, Advanced Materials, 10 (1998) 195. 5. M. Bognitzki, H. Hou, M. Ishaque, T. Frese, M. Hellwig, C. Schwarte, A. Schaper, J. H. Wendorff and A. Greiner, Advanced Materials, 12(9) (2000) 637. 6. T. Thurn-Albrecht, R. Steiner, J. DeRouchey, C. M. Stafford, E. Huang, M. Bal, M. Tuominen, C. J. Hawker and T. P. Russell, Advanced Materials, 12 (2000) 787. 7. D. Mecerreyes, E. Huang, T. Magbitang, W. Volksen, C. J. Hawker, V. Y. Lee, R. D. Miller and J. L. Hedrick, High Performance Polymers, 13(2) (2001) S11. 8. D. Mecerreyes, V. Lee, C. J. Hawker, J. L. Hedrick, A. Wursch, W. Volksen, T. Magbitang, E. Huang and R. D. Miller, Advanced Materials, 13(3) (2001) 204. 9. M. Antonietti, J. Conrad and A. Thtinemann, Macromolecules, 27 (1994) 6007. 10. M. Antonietti, A. Wenzel and A. Thtinemann, Langmuir, 12(8) (1996) 2111. 11. M. Antonietti, J. Conrad and A. Thtinemann, Trends in Polymer Science, 5 (1997) 262. 12. G. ten Brinke and O. Ikkala, Trends in Polymer Science, 5 (1997) 213. 13. C. Ober and G. Wegner, Advanced Materials, 9(1) (1997) 17. 14. J. Ruokolainen, R. M~ikinen, M. Torkkeli, T. M/ikel~i, R. Serimaa, G. ten Brinke and O. Ikkala, Science, 280 (1998) 557. 15. J. Ruokolainen, M. Saariaho, O. Ikkala, G. ten Brinke, E. L. Thomas, M. Torkkeli and R. Serimaa, Macromolecules, 32 (1999) 1152.
378 16. J. Ruokolainen, G. ten Brinke and O. T. Ikkala, Advanced Materials, 11 (1999) 777. 17. R. M/iki-Ontto, K. de Moel, W. De Odorico, J. Ruokolainen, M. Stamm, G. ten Brinke and O. Ikkala, Advanced Materials, 13(2) (2001) 117. 18. K. de Moel, G. O. R. Alberda van Ekenstein, H. Nijland, E. Polushkin, G. ten Brinke, R. M/iki-Ontto and O. Ikkala, in press Chemistry of Materials (2001). 19. J. Ruokolainen, J. Tanner, G. ten Brinke, O. Ikkala, M. Torkkeli and R. Serimaa, Macromolecules, 28 (1995) 7779. 20. R. M~ikinen, J. Ruokolainen, O. Ikkala, K. de Moel, G. ten Brinke, W. De Odorico and M. Stature, Macromolecules, 33 (2000) 3441. 21. L. A. Belfiore, A. T. N. Pires, Y. Wang, H. Graham and E. Ueda, Macromolecules, 25(5) (1992) 1411. 22. K. de Moel, R. M~iki-Ontto, M. Stamm, O. Ikkala and G. ten Brinke, Macromolecules, 34 (2001) 2892. 23. Preliminary TEM measurements confirm that the structure is lamellar-within-lamellar. To be published.
. . . . . .
,,,
lJt.t
~..,,ul l.u,v~,
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1"1" 1
A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
379
Electron microscopic investigation o f mesoporous SBA-2 Wuzong Zhou*, Alfonso E. Garcia-Bennett, Hazel M.A. Hunter and Paul A. Wright School of Chemistry, University of St Andrews, St Andrews, Fife, KY16 9ST, UK.
Microstructure of mesoporous SBA-2 has been investigated by using transmission electron microscopy and scanning electron microscopy. The material consists of two phases based on hexagonal close-packed and cubic close-packed supercages. These two phases coexist in domains and synthesis of monophasic specimen has not so far been achieved. Three morphologies, i.e. solid spheres, hollow spheres and flat plates, have been recorded and their formation mechanisms are discussed.
1. INTRODUCTION Mesoporous silica SBA-2 was first reported in 1995 [1]. The material was believed to consist of discrete supercages in a hexagonal close-packed (hcp) arrangement and the space group was determined to be P63/mmc. In 1998, based on transmission electron microscopic (TEM) studies, we proposed [2] that two types of mesopores, a group of straight pores along the [100] direction and another group of zigzag pores parallel to the [001 ] zone axis, connect the supercages in the hcp structure (Fig. l a). We also revealed a new phase designated STAC-1, which has a structure with cubic close-packed (ccp) supercages. The latter phase is also connected by two-dimensional mesopores (Fig. 1b). Since then, very few reports dealing with the structures of these materials have been released, and, up to date, the above two models provide the best approaches to the real structures of the hcp and ccp phases. However, uncertainty exists regarding the pore connectivity. Once well ordered materials are prepared, better structural models will be developed from electron microscopy using the so-called direct determination method [3, 4]. Nevertheless, the co-existence of the hexagonal and cubic forms is not in doubt. One of the difficulties in the structural studies of these materials is that it is hard to obtain large domains of the monophasic hcp or ccp phase. The TEM images we used in our previous report [2] show large enough monophasic domains of these two phases for image simulations in order to determine the mesopore systems. However, there are usually some stacking faults showing a mixture of the ABCABC and ABAB ordering along the c axis of the hexagonal unit cell (Fig. 2). In fact, the real structure contains much smaller domains and their orientations can be random. The present work is therefore focused on these domain structures and on the microstructure-related morphologies of the particles.
380
Fig. 1 Schematic drawing of the channel structures of (a) the hcp phase and (b) the ccp phase in mesoporous specimen SBA-2. For comparison with the hcp phase, a hexagonal unit cell is also chosen for the ccp phase.
2. EXPERIMENTAL
The synthetic method is the same as that reported previously [2]. Gemini quaternary ammonium surfactant was used as template. The ratio of surfactant, TMAOH (tetramethylammonium hydroxide), TEOS (tetraethyl orthosilicate) and water was 0.05 : 0.5 : 1 : 1.50. The reaction pH was adjusted to 11 with 1M HCI. Aider 2 h stirring at room temperature, the specimen was recovered by filtration, washed with distilled water, and dried in air at room temperature. The powder sample was calcined at 500 ~ to Fig. 2 TEM image of a large domain of the ccp phase. Stacking faults are indicated by remove surfactant molecules. Initial characterization of the specimens white arrows. was by X-ray powder diffraction (XRD) method using a Philips PW 1830 diffractometer equipped with a secondary monochromator. A 20 range from 1.5 to 8 ~ was normally scanned over 2 h. TEM images were obtained on a Jeol JEM-~)0 CX and a Jeol JEM-2010 electron microscopes, both operating at 200 kV. Specimen was prepared by spreading the powder on a holey carbon f i l l supported on a Cu grid, followed by transferring it into the chamber of the microscope. Structural images were recorded at magnifications from 24,000X to 80,000X. Scanning electron microscopic (SEM) images were recorded on a Jeol JSM-5600 scanning electron microscope operating at various accelerating voltages from 1 kV to 30 kV. The powder sample was deposited on a double-sided carbon adhesive disc sitting on a specimen stub. The specimen was then directly transferred into the SEM chamber without any coating treatments. An accelerating voltage with minimum beam charge was then chosen.
3. RESULTS AND DISCUSSION
XRD profiles of the samples agreed with the previous results for SBA-2 [1 ] and may be indexed onto a hexagonal unit cell with a = 4.90 and c = 8.04 nm. However, some variation in
381 the peak intensities indicated possible existence of the ccp component [5]. Three principal morphologies were found in the specimen after calcination. One is large hollow sphere with about 50 to 150 ~tm in diameter and the thickness of shells is about 1 to 2 ~tm directly measured from the SEM images of some holes on the hollow balls (Fig. 3a, b). The second morphology is small solid sphere with the diameter in a range of 2 to 3 lxm. Some individual solid spheres can be seen on the surface of the hollow ball. The third morphology is sheet-like plate as shown in Fig. 3c. The particle becomes transparent under the electron beam, indicating that it is very thin along the incident beam direction. It was also noticed that these flat plates usually have sharp edges. Some small spherical particles, 2 to 3 ~tm in diameter, can also be seen on the surface of the plate (Fig. 3c). TEM images of most small spheres show a multi-domain structure (Fig. 4). Some domain boundaries are highlighted in Fig. 4b. It can be seen that these domains intergrow together with random orientations. The projections of domains 1, 2 and 6 can be considered to be [100] of the hcp phase. However, their c axes rotate around the a axis as shown in Fig. 4b. Domain 5 shows mainly the ccp phase with a few stacking faults, the image contrast pattern is similar to that shown in Fig. 2. Domain 4 is also a ccp phase viewed down the [110] direction of the cubic unit cell. This domain structure is beneficial to the formation of the spherical morphology.
Fig. 3 (a) and (b) SEM images of the synthesized SBA-2 specimen, showing two principal morphologies, hollow ball and solid sphere. The diameter of the hollow ball shown is about 150 ~tm and that of the small spheres is about 1 to 2 ~tm. The cross section of the shell of the hollow ball is marked by two white arrows in (b). (c) TEM image of a fiat plate obtained at a low magnification with some solid spheres on the surface.
382
Fig. 4 (a) A TEM image of part of a small spherical particle. (b) A copy of (a) with the domain boundaries marked by white lines.
Fig. 5 Enlarged TEM image of the domain 2 in Fig. 4b. The sequence of layer-packing is indicated.
Fig. 6 TEM image of a solid sphere showing a single ccp phase when viewed down the [110] direction of the cubic unit cell.
383 A close examination of individual domains in Fig. 4 reveals that stacking faults are very common inside the domains. For example, the area 2 in Fig. 4b looks like a monophasic domain with the projection along the [100] zone axis of the hcp phase. However, an examination of sequence of the layer-arrangement along the c axis enables us to find many stacking faults so that it becomes a mixed phase of the hcp phase and ccp phase. Consequently, identification of this domain to either the hcp phase or the ccp phase is not justifiable (Fig. 5). This structural feature is similar to the intergrowth of zeolites FAU/EMT [6,7]. The hcp/ccp irregular intergrowth happens often because the lattice energies of these two phases are very close. Refinement of the synthetic conditions in order to produce either pure hcp or pure ccp phase is difficult, but not impossible. In the same specimen presented above, we occasionally observed indeed some particles that seem to be monophasic. For example, Fig. 6 is a TEM image from a small solid sphere. The structure has been identified as the ccp phase and the view direction is along the [ 110] zone axis. No domain structure can be seen in this particle. Direct TEM examination on the hollow balls is difficult due to their large size and the spherical shape. To can'y out TEM structural studies, the hollow balls were selected under optical microscope. The specimen was then ground for a few minutes and most hollow spheres were crushed into fragments. TEM images of these fragments show again a multidomain property and a uniform thickness (Fig. 7). The size of domains in the particle shown in Fig. 7a is about 5 nm or more and they do not have regular shapes. The domains in Fig. 7b, on the other hand, show a regular but distorted hexagonal shape with domain size of about 2 nm in diameter. These domains are close-packed on the shell plan, forming a larger hexagonal pattern. A possible formation mechanism is that in the ab plans of the hcp phase exist some clusters of ccp phase as shown in the inset of Fig. 7b. These clusters are partially ordered in the ab plans to form hexagonal pattern. The thin particle shown in Fig. 3c is observed from a specimen before grinding. Its more regular shape and lower thickness distinguish itself from the fragments of the hollow balls. In fact, the flat plate shown in Fig. 3c was most likely to be originally a part of larger sheet. Some TEM images showed indeed much larger plates with several cracks. TEM images at a high magnification show that the sheet-like particle seems to be monophasic, although some local defects are still visible (Fig. 8). Selected area electron diffraction (SAED) pattern from an area of a few micrometer in diameter (see the inset of Fig. 8) confirms its monophasic property and shows a hexagonal pattern. Therefore the incident beam was perpendicular to either the (001) plane of the hcp phase or the { 111 } planes of the ccp phase. According to the models proposed in our first paper about SBA-2 [2], the ideal mesopore networks in the hcp and ccp phases are both 2-dimensional instead of 3-dimensional (Fig. 1). In the case of latter, the 2-dimensional network contain mesopore-connected supercages is the (111) plane of the cubic unit cell, and there are no other mesopores acting as bridges between them. Consequently, the interaction of the micellar network in between these (111) planes must be much weaker in comparison with the intraplane interaction. It is therefore not surprising to see that the flat plates are perpendicular to the [111] zone axis of the cubic unit cell.
384
Fig. 7 TEM images of some fragments from hollow spherical particles. A multi-domain structure can be easily observed. Examples of typical domains in (a) and (b) are highlighted. The inset of (b) shows schematic drawing of a ccp cluster in the hcp network.
385
Fig. 8. TEM image at high magnification obtained from a sheet-like particle as seen in Fig. 3c. The inset is the corresponding SAED pattern.
4. CONCLUSION According to the SEM and TEM observations, synthesized SBA-2 specimens have three morphologies. Most solid small spheres consist of irregular domains with random orientations. This morphology must relate to a spherical micelle packing arrangement. Although each domain shows structural homogeneity, it lacks long-range ordering and otten contains irregular intergrowth of the hcp and ccp components with size in a nanometer scale. Large domains and even single domain spheres were occasionally observed, implying that the formation of monophasic spheres is possible. We believe that hollow spherical balls of silicate form from assembly of micellar and silicate condensation on surface of some bubbles. These particles therefore can move to the liquid surface during the reaction process and were indeed observed by optical microscopy. During calcination, hollow balls undergo considerate damage, which resulted in the formation of irregular openings that enabled us to measure the thickness of the shells (Fig. 3b). TEM images revealed that the shells of the hollow particles have also a domain structure (Fig. 7). In addition to the stacking faults along the c axis of the hexagonal unit cell as shown in Fig. 5, some very small 3-dimensional domains were also observed (see Fig. 7b). The flat plates, which probably formed in the liquid/air interface, are monophasic and the orientation is well selective to be normal to the [ 111 ] axis of the cubic unit cell. If our model
386 for the STAC-1 [2] is correct, all the mesopores in the flat plates would be parallel to the planes and there would be no pores across the plates. Further studies are being carried out in these laboratories. During the preparation of this report, we have performed part of systematic investigations of the synthetic conditions for SBA-2 and found that we were already approaching the goal of producing single-phase materials. Introducing bubbles into the reaction system, we obtained much larger yield of hollow balls of silicate.
REFERENCES
1. Q. Hue, R. Leon, P. M. Petroff and G. D. Stucky, Science, 268, 1324 (1995). 2. W. Zhou, H. M. A. Hunter, P. A. Wright, Q. F. Ge and J. M. Thomas, J. Phys. Chem., 102, 6933 (1998). 3. O. Terasaki, personal communication, (2001). 4. Y. Sakamoto, M. Kaneda, O. Terasaki, D. Y. Zhao, J. M. Kim, G. Stucky, H. J. Shin and R. Ryoo, Nature, 408, 449 (2000). 5. H.M.A. Hunter, A. E. Garcia-Bennett, I. D. Shannon, W. Zhou and P. A. Wright, J. Mater. Chem., in press (2001) 6. J. M. Thomas and G. R. Millward, J. CherrL Soc. Chem. Commun., 1380 (1982). 7. J.M. Thomas, O. Terasaki, P. L. Gai, W. Zhou and J. Gonzalez-Calbet, Accounts Chem. Res., 34, 583 (2001).
~tuales m ~urrace ~clence ana ~ataiysls 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
387
A study o f m o r p h o l o g y o f m e s o p o r o u s silica S B A - 15 Man-Chien C h a o a, Hong-Ping Lin b , Hwo-Shuenn Sheu c and Chung-Yuan M o u a a. Department of Chemistry and Center of Condensed Matter Research, National Taiwan University, Taipei, Taiwan, 106. b. Institute of Atomic and Molecular Sciences Academia Sinica, P. O. Box 23-166, Taipei, Taiwan 106. c. Synchrotron Radiation Research Center, Hsinchu, Taiwan.
The mesoporous silica SBA-15 in various morphologies micrometer-sized fibers, millimeter-scaled ropes and macrospheres have been conveniently prepared by controlling the chemical composition. For reducing the size of particles of the SBA-15 materials, a delayed agitation process was found to lead to nanometer-sized fibers. We propose that it generates numerous nucleation seeds at the interface of TEOS and surfactant water solution and leads to very small fibers. With a proper aging time of 20 minutes, silica nanotubes-bundles with diameter of about 100 nm were obtained. In addition to the normal mesopores, the SBA-15 silica nanotubes possess extra textural porosity.
1. INTRODUCTION Micelle-templated mesoporous silica (MMS) [1,2] are of great interest to scientific community because of their tunable mesopore structures which lead to many applications such as catalyst supports, adsorbent and solid templates. In applications of the mesoporous silica such as catalysis, its morphology is an important controlling factor [3,4]. When a silica source is combined with a surfactant, the self-assembly process is complicated involving surfactant self-assembly in solution, mesophases transformation, and silica speciation reactions. All the factors influence the morphology of the mesoporous materials obtained. This has been amply demonstrated in MCM-41 materials [5]. Tuning the chemical composition, using proper inorganic precursors or applying physical field have achieved morphology and size controls on the mesoporous materials [6-8]. Recently the highly ordered SBA-15 [2], synthesized by using triblock copolymer EO20PO70EO20, was found to exhibit rich morphologies. [9,10] The acid-made SBA-15 particles appear to be softer(weaker surfactant/silicate interaction), stickier(more surface
388 silanol), and resulting in richer morphologies. Furthermore, the interface between the insoluble organic TEOS and aqueous copolymer solution appears to offer a new way of morphological control through multiphase assembly [ 10]. Basically, the formation process of MMS materials follows the sequence: nucleation assembling growth ~ aggregation. The particle size of MMS will be dependent on the number of nucleation seeds during and the aggregation capability of the surfactant-silica clusters. The more the nuclei, the smaller the particle sizes. On the other hand, a decrease of growth and aggregation would also help the formation of smaller particles (or fibers). Recently, Mann and coworkers devised a growth quenching procedure in the alkaline synthesis of MCM-41 to obtain nanoparticles of mesoporous silicas [ 11]. In this report, we present several methods of morphological control of the SBA-15. We tuned the TEOS/triblock copolymer ratios or added a proper amount of multivalent salts in the EO20POToEO20-TEOS-HC1-H20 reaction composites to increase the aggregating ability of the triblock copolymer-silica species nanocomposites. Thus, the SBA-15 mesoporous silicas in macro-scaled form (e.g. centimeter-sized sphere, millimeter-sized ropes and micrometer-sized fibers) were facilely prepared. Moreover, a delayed-agitation procedure was conveniently used to create rich silica nucleation seeds at the interface between the TEOS and surfactant aqueous solution. These induced the formation of nanotubes and fine microparticles of the SBA-15 mesoporous silica.
2. MATERIALS AND METHODS 2.1. Materials The tri-block copolymer is (ethylene oxide)20-(propylene oxide)70-(ethylene oxide)20, (EO20PO70EO20; P123) from Aldrich as the mesostructure-templating species. The silica source is tetraethylorthosilicate (TEOS; 98% from Acr6s), and hydrochloride (HC1, 37%) is from Acr6s. All chemical agents were used as received. 2.2. Synthesis The micrometer-sized fibrous mesoporous SBA-15 silicas were prepared according to the typical synthetic process reported by Stucky et al. [2]. 1.0 g triblock copolymers P123 and 9.44 g of 37% aqueous hydrochloride acid were dissolved in 30.0 g water to form a clear solution. Then 2.30 g TEOS was added to that solution under stirring condition then further stirred for 5-24 hr at the 40 ~ The gel chemical compositions in molar ratio is 1.0 P123:(64-160) TEOS: 555 HCI: 11584 H20. We differ from ref.[2] mainly in that higher acid concentration is used here. The millimeter-sized silica ropes were prepared according to the above procedure and same composition except for an extra addition of (2.0-4.0)g of Na2SO4 or Na3PO4. With the same synthetic procedure, the centimeter-sized mesoporous SBA-15 silica sphere was obtained from a higher TEOS content system with TEOS/EO20PO70EO20 weight ratio in the range of 3.5 to 5.0 under a stirring rate of about 500 rpm. For the preparation of SBA-15 silica nanotubes, a delayed-agitation procedure was performed. In this process, the TEOS was added into the surfactant-acid aqueous solution without agitation, and that two-phase solution (TEOS is on the upper layer) then stood statically for equal or longer than 20 minutes. After stirring the reaction mixture at high speed,
389 a white precipitate was suddenly formed. The gel solution was further stirred for 18-24 hr. The chemical composition are the same as that for silica fiber (TEOS/EO20POy0EO20 weight ratio = 2.30). While using the composition for macrosphere formation (TEOS/ EOz0PO70EO20 weight ratio = 4.0), microparticles were formed instead. After filtration, washing with water and drying at room temperature, we recovered the SBA-15 mesoporous silica products. The surfactant templates were completely removed after calcination. 2.3. Measurements X-ray powder diffraction (XRD) patterns were recorded on Wiggler-A beamline ()~ = 0.1326 nm) of the Taiwan Synchrotron radiation research center at Hsinchu, Taiwan. N2 adsorption-desorption isotherms were obtained at 77 K on a Micromeritics ASAP 2010 apparatus. Before the analysis, the calcined samples were outgases at 250~ for about 6 h under 10-3 torr condition. The pore size distribution was obtained from the analysis of the adsorption branch by using the BJH (Barrett-Joyner-Halenda) method. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were taken on the Hitachi S-800 and H-7100 with the operating voltages of 20 and 100 keV, respectively.
3. RESULTS AND DISCUSSION Figure 1 shows several representative SEM, TEM and optical microscope imagines for SBA-15 mesoporous silicas synthesized from different reaction composites. Using the reaction composites similar to that of typical one [2], the micrometer-sized silica fibers were obtained and the length is in tens micrometers (Fig. 1A). Under higher magnification (Fig. 1B), one can clearly find the fibers are nodular which seems to be formed from sticking linearly many sub-micron particles. The nodular shape is different from the rope-like domain observed in ref. [2] where [HC1] ~ 2.0 M. This could be ascribed to the higher acidity ([HC1] ~ 2.5 M) in our synthesis composites. However, stirring is also an important factor. With the addition of a proper amount of NazSO4 or Na3PO4, the fibrous mesoporous SBA-15 products in millimeter size were obtained, and the longer one is about 0.5 mm (Fig. 1C). Under a higher magnification (Fig. 1D), one can clearly see the morphology is rope-like and the SBA- 15 ropes consist of fibers of micron diameter. Using microtome TEM technique to examine the nanostructures of the fibers (Fig. 1E), it is shown the SBA-15 nanochannels are well ordered and aligned with the direction of fibers. Therefore, the millimeter-sized SBA-15 silica ropes is regarded as a hierarchical structure similar to the silica ropes synthesized by this laboratory from C~sTMAB-TEOS-HNOg-H20 composite [12,13]. Thus, we suggest that the addition of the multivalent salts promoted the elongation of the EO20POy0EO20-silica micelles. The long micelles are then shear-aligned into the millimeter-sized SBA-15 silica ropes. In contrast, the addition of univalent salts (NaC1, NaBr or NaNO3) did not help the formation of millimeter-sized silica ropes. The above explanation is further corroborated by a recent study of the effects of salts on the micellization of pluronic solution. [ 14] Pandit et al. [ 14] reported that salt solutions help the elongation of the micelles of Pluronic copolymers by increasing the hydrophobic domain. The power of the micelle formation is in the order: Na3PO4> Na2SO4>NaC1, with NaC1 solution being the least almost as effective as pure water.
390
Figure 1. The SEM, TEM and optical microscope images of the SBA-15 mesoporous silicas, synthesized from different reaction composition, in various morphologies: A. The SEM micrograph of micrometer-sized fibers (TEOS/EOz0POv0EO20 weight ratio - 2.30); B. SEM micrograph of sample A in higher magnification; C. The optical micrograph of millimeter-sized silica ropes (TEOS/Na2SOa/EOz0POv0EO20 weight ratios = 2.30/2.0/1.0).; D. SEM micrograph of sample C in higher magnification; E. Microtome TEM micrograph of sample C.; F. Photograph of the centimeter-sized sphere (TEOS/EO20PO70EO20 weight ratio = 4.O). To examine the effect of aggregation, one may use more TEOS at high acidity to promote the cross condensation between surfaces of silica particles. When the TEOS/EOz0POv0EO20 weight ratio was adjusted into the higher range of 3.5-5.0, we saw the silica-EOz0POToEO20 particles mutually aggregated together during the reaction process and then a centimeter-sized sphere was formed (Fig. 1F). We found the sphere has interestingly high elastic property and mechanical stability [ 15]. In strong acidic condition, the larger silica oligomers have greater binding strength with EOz0POv0EO20 micelles and stronger aggregation capability. However, further increasing the TEOS/EO20POv0EO20weight ratio higher than 7.0, most of TEOS were hydrolyzed and formed the template-free amorphous silicas in acidic condition [16]. The macro-sphere was no longer produced at such high TEOS content.
391 Besides the compositional adjustments on the cooperation assembly of the silica-EOz0POy0EO20 composites, controlling the number of the nucleating seeds in the gel solution is also an essential determining factor on the morphology of the mesoporous materials. According to previous reports [17,18], the silica nuclei can be progressively created at the interface of the hydrophobic TEOS and aqueous surfactant solution via a surfactant-catalyzed hydrolysis of TEOS. Based on this concept, we performed a delayed-agitation method to induce more nucleation seeds in the synthesis of SBA-15 mesoporous silicas. It is hoped that the growth and aggregation processes will be retarded relatively because of transport limitation. In Fig. 2A, we see bundles of nanotubes of SBA- 15 were obtained after the two-phase reaction mixture stood statically for a 20-minute and then followed by a sudden stirring at high speed.
Figure 2. The SEM and TEM micrographs of the SBA-15 mesoporous silicas prepared by the delayed-agitation process. A. The SEM micrograph of SBA-15 nanotubes (TEOS/EO20POToEO20 weight ratio - 2.3; aging time = 20 rain); B. TEM micrograph of sample A.; C. The SEM micrograph of microparticles (aging time - 1 hr); D. The SEM image of microparticles (TEOS/EO20POToEO20 weight ratio = 4.0; aging time = 20 rain). The TEM micrograph shows nanotubes consisting of about ten nanochannels with the diameter at about 100 nm (Fig. 2B). To our knowledge, this may be the smallest dimension SBA-15 silica ever made. Prolonging the aging time of the reaction mixture to about 1.0 hr, the
392 SBA-15 product is in inhomogeneous microparticles instead of the nanotubes (Fig. 2C). To explain the above results, we propose that the nucleation seeds of the SBA-15 be continuously generated at the interface of TEOS-EOz0PO70EO20 solution. In the early nucleation stage, the number of nucleation seeds would increase with the aging time. However, the nucleation seeds would also aggregate with each other or grow into larger ones. Thus aging-time control crucially determines the homogeneity and the particle dimension of the final SBA-15 products. From many tests on aging-time, we found 20-minute aging can produce the smallest silica nanotube-bundle. In order to show further the effect of increasing nucleation seeds on the SBA-15 particle size, the delayed-agitation process was also applied to the composites for centimeter-sized sphere as well. One could obviously find the SBA-15 morphology transformed into the microparticles (Fig. 2D) instead of the macro-sphere (Fig. 1 F). The size reducing also occurred in the system for silica fibers or ropes. Fig. 3A shows the XRD patterns of the as-synthesized mesoporous SBA-15 aforementioned in different morphologies and dimensions. All of the SBA-15 samples possess distinct 2-3 peaks indexed to the well-ordered hexagonal structure. The almost identical dspacings of 8.8 nm for these samples reflect the same reaction temperature of 40 ~ and similar composition [2]. When examining their N2 adsorption isotherms (Fig. 3B), it is clear that all samples have a sharp capillary condensation at P/P0 - 0.60-0.70 corresponding2 to pore sizes around 6.0 nm. The BET surface areas of these samples are about of 450-550 m/g. However, the adsorption behavior of the SBA-15 silica nanotubes is worth mentioning. In that sample, there exists further increase of N2 condensation at P/P0 >0.9 (indicated by an arrow in Fig. 3B), which is attributed to the filling of textural pores [3]. The textural porosity results from the aggregation of the small-sized bundles (sample IV). The samples with larger domain (sample I, II, III) show little or no textural porosity.
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Figure 3. The XRD patterns and N2-adsoption isotherms of the SBA-15 mesoporous silicas in various morphologies and dimensions. A. XRD patterns; B. N2-adsoption isotherms. I. Micrometer-sized fiber; II. Millimeter-sized rope; III. Centimeter-sized sphere; IV. Nanotubes.
393 Besides of morphology control, we also used the post-synthesis hydrothermal treatment to tune the pore size and porosity of these SBA-15 mesoporous silicas [19]. After 100~ hydrothermal treatment for one day, pore size (-7.5 nm), surface area (- 650 m2/g) and porosity (---0.9 cm3/g) increased in all of the SBA-15 samples but the morphologies were still preserved. Combining with the hydrothermal treatment, the SBA-15 mesoporous materials with desired morphologies, dimensions and porosity could be easily prepared for potential applications.
4. CONCLSION In conclusion, the controls of nucleation, growth and aggregation are shown to be fundamental factors in tailoring the morphologies of the mesoporous materials. Adjusting the chemical composition or performing the delayed-agitation process can help us conveniently obtain the SBA-15 mesoporous silicas in different morphologies and dimensions. It should be a versatile mesoporous material with potential applications in catalyst, separations, sensors, and nano-materials fabrications. Besides, the control of interfacial nucleation could help one to understand other sol-gel processes such as biomineralization or in designing better methods for creating new inorganic-organic nanocomposites.
ACKNOWEDGMENTS
This research was financially supported by the National Science Council of Taiwan (NSC 89-2113-M-002-028). We also acknowledge the CTCI Foundation for supporting HR-TEM work.
REFERENCES
1. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. 2. P. Yang, D. Zhao, D. I. Maargolese, B. F. Chmelka and G. D. Stucky, Science, 279 (1998) 548. 3. T. R. Pauly, Y. Liu, T. J. Pinnavaia, S. J. L. Billinge and T. P. Rieker, J. Am. Chem. Soc. 121 (1999) 8835. 4. (a) S.T. Wong, H. P. Lin, C. Y. Mou, 2000, Applied Catalyst A 198 (2000) 103 (b) H. P. Lin, S.T. Wong, C. Y. Mou, and C.Y. Tang, J. Phys. Chem. B, 104 (2000) 7885. 5. C.Y. Mou, H. P. Lin, Pure and Applied Chemistry, 72 (2000) 137. 6. P. T. Tanev, T. J. Pinnavaia, Science, 271 (1996) 1267. 7. D. Zhao, P. Yang, Q. Huo, B. F.Chmelka and G. D. Stucky, Current Opinion in Solid State and Materials Science, 3 (1998) 111. 8. Ch. Danumah, S. Vaudreuil, L. Bonneviot, M. Bousmina, S. Giasson and S. Kaliaguine, Microporous and Mesoporous Mater., 44-45 (2001) 241. 9. D. Zhao, J. Sun, Q. Li and G. D. Stucky, Chem. Mater., 12 (2000) 275. 10. D. Zhao, P. Yang, B.F. Chmelka, and G. D. Stucky, Chem. Mater., 11 (1999) 1174.
394 11. C. E. Fowler, D. Khushalani, B. Lebeau and S. Mann, Adv. Mater., 13 (2000) 649. 12. H. P. Lin, C. P. Kao, S. B. Liu and C. Y. Mou, J. Phys. Chem B, 104 (2000) 7885. 13. H. P. Lin, S. B. Liu, C. Y. Mou and C. Y. Tang, Chem. Comm., (2000) 583. 14. N. Pandit, T. Trygstad, S. Croy, M. Bohorquez, and C. Koch, J. Colloid and Interface Sci., 222 (2000) 213. 15. C. P. Kao, H. P. Lin and C. Y. Mou, J. Phys. Chem. Sold., 62 (2001) 1555. 16. J. H. Jung, K. Nakashima and S. Shinkai, Nano Lett., 3 (2001) 145. 17. Q. Huo, D. Zhao, J. Feng, K. Weston, S. K. Burano, G. D. Stucky, S. Schachi and F. Schuth, Adv. Mater., 12 (1997) 974. 18. H. Yang, N. Coombs, I. Sokolov, G. A. Ozin, Nature, 381 (1996) 589. 19. R. Ryoo, C. H. Ko, M. Kruk, V. Antochshuk and M. Jaroniec, J. Phys. Chem. B, 104 (2000) 11465.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
395
SBA-15 versus M C M - 4 1 : are they the same materials? Anne Galameau, H616ne Cambon, Thierry Martin, Louis-Charles De M6norval, Daniel Brunel, Francesco Di Renzo and Francois Fajula Laboratoire de Mat6riaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 ENSCM/CNRS, Ecole Nationale Sup6rieure de Chimie de Montpellier, 8 rue de l'Ecole Normale, 34296 Montpellier cedex 5 - FRANCE. e-mail:
[email protected].
SBA- 15 materials have been used instead of MCM-41 in different applications for the last three years because of their large pores easy to synthesize. Large pore MCM-41 are obtained by adding swelling agents during the synthesis, which gives harder syntheses than SBA-15 due to the use of unstable nanoemulsions. But are SBA-15 the same material as MCM-41 differing only in their method of synthesis? The answer depends on the synthesis temperature of SBA-15. SBA-15 synthesized up to 100~ possess micropores which lead to an overestimation of their surface areas, whereas higher synthesis temperatures allow to eliminate micropores producing SBA-15 materials with properties close to MCM-41. Proper equations are provided to evaluate the amount of micropores, the true surface area and the true wall thickness. The micropores which interconnect the mesopores in SBA-15 have a strong influence in adsorption measurement, nitrogen adsorption, 129Xe NMR and in surface functionalization.
1. INTRODUCTION In 1998, a new synthesis of ordered hexagonal mesoporous silica, named SBA-15 [ 1], was proposed using triblock poly(ethylene oxide) - poly(propylene oxide) -poly(ethylene oxide) copolymers as templates. SBA-15 materials are large-pore (> 50 A) ordered silicas more stable and easier to form than large pore MCM-41 [2] with a pore size in the same range (same pressure of nitrogen pore filling). But have SBA-15 the same properties as MCM-41 ? MCM-41 templated by swelled alkylammonium micelles present a pore size equivalent to SBA-15 but feature much larger volume. Nevertheless, classical BET surface areas are the same for SBA-15 and MCM-41 with the same pore diameter. This indicates that the evaluation of pore size by the classical D-4V/S Gurvitch equation does not hold for SBA- 15. The presence of microporosity on SBA-15 could justify the results, although t-plot analysis gives opposite results [3]. A surface of mesopores pitted by large non-uniform micropores could render unreliable the evaluation of surface areas by classical BET equation and of microporosity by the t-plot analysis, due to the non-possibility of monolayer-multilayer adsorption in micropores and to the unvalidity of the usual reference isotherms to this kind of surface, respectively. Proper equations to evaluate microporosity, wall density and surface area of SBA-15 are provided. Calculated surface areas of SBA-15 are much lower than BET
396 surface areas for solids presenting microporosity. This difference of surface area evaluation can be a large source of errors in different applications such as catalysis or adsorption implying, for instance, grafting densities calculations. Microporosity renders also unreliable the correlation between 129Xe NMR chemical shift and pore size like it has been noticed for MCM-41 [4, 5]. The amount of microporosity in SBA-15 can be controlled by the synthesis temperature and syntheses performed at higher temperatures than temperatures used in literature (temperatures between 35 and 100~ can offer SBA-15 without microporosity and produce materials with properties close to MCM-41.
2. MATERIALS AND METHODS 2.1. Materials
SBA-15 materials have been synthesized according to the methods described in literature (for synthesis temperature of 100~ [1]: 1 g of Pluronic P123 [(EO)20(PO)70(EO)20, Aldrich] 15 g H20, 30 g HC1 2 M, 2.1 g tetraethylorthosilicate (TEOS, Aldrich). The mixture has been maintained at 35~ for 24 h and then for 2 days at a given temperature between 35 and 130~ under static conditions in a teflon-lined autoclave. SBA-15 with pores of 50, 80 and 100/~ are synthesized at 60, 100 and 130~ respectively. Reference MCM-41 materials were synthesized at 115~ by using cetyltrimethylammonium bromide (CTAB, Aldrich), 1,3,5-trimethylbenzene (TMB, Aldrich) pyrogenic silica (Aerosil 200V Degussa), sodium hydroxide (Prolabo) and deionized water in molar ratios 1 SiO2 / 0.26 NaOH / 0.1 CTAB / 20 H20 / x TMB. MCM-41materials with pore size of 37, 50 and 115 A pore size were synthesized using TMB/surfactant ratio of 0, 2.6 and 13, respectively [6]. All materials were filtered, washed with water and dried at 80~ for 24 h. The solids were then calcined in air at 550~ for 8 h. Two different types of surface functionalization leading to the best surface coverage for MCM-41 [7] were performed, using two different octylsilane as grafting agents: chlorodimethyloctylsilane and trimethoxyoctylsilane. Calcined materials were first outgassed under vacuum at 180~ The amount of grafting agent added as modifier corresponds to a density of 5 grafting agent/nm 2 of calcined materials. In type 1 surface functionalization, chlorodimethyloctylsilane was added to a stirred suspension of material (1 g) in anhydrous refluxing toluene (30 mL) containing pyridine (1 pyridine / grafting agent). The reagents were stirred for 15 h at 120~ In type 2 surface functionalization, trimethoxyoctylsilane (Aldrich) was added to a stirred suspension of material (1 g) in anhydrous toluene (30 mL) at room temperature. After 1 h of stirring in flowing nitrogen, H20 (1 H20 / grafting agent) and catalysts, p-toluenesulfonic and ammonium fluoride (0.05 / grafting agent), were added. The mixture was stirred 1 h at 20~ and 4 h at 60~ The resulting water and methanol were removed by azeotropic distillation at 120~ The solids were then recovered by filtration, washed with different solvents and dried at 80~ overnight. 2.2. Measurements
Powder X-ray diffraction (XRD) data were obtained on a CGR Th~ta-60 diffractometer with Inel drive, using monochromated Cu Kot radiation. The adsorption/desorption isotherms of nitrogen or argon at 77 K were measured using a Micromeritics ASAP 2000 instrument. Each sample was outgassed at 250~ for calcined materials or at 180~ for functionalized
397 materials until a stable static vacuum of 3x10 -3 Torr was reached. Pore diameter was measured by the Broekhoff and de Boer (BdB) method which has been demonstrated as one of the best method for MCM-41 materials [8]. Aerosil silica was used as non-porous reference material for the t-plot analyses. BET surface area and the CBET parameter were calculated using adsorption data in the relative pressure range from 0.15 to 0.26 included in the validity domain of the BET equationenon adsorption was performed at room temperature on previously in-situ outgassed (400~ materials by adding different amounts of xenon corresponding to pressures between 200 and 1000 Torr. The tubes were then sealed and 129Xe NMR spectra were recorded with a Bruker AC250 spectrometer at room temperature, at the resonance frequency of 69.19 MHz using a re/2 pulse of 18 ~ts and repetition time of of 2 s. Chemical shifts are referenced to that of gaseous xenon.
3. RESULTS AND DISCUSSION 3.1. Nitrogen sorption at 77 K The nitrogen isotherms of SBA-15 and MCM-41 materials with similar pore sizes are reported in Figure 1. In all cases the isotherms are of type IV and exhibit hysteresis loop of HI, typical of materials withpores of constant cross-section (cylindrical or hexagonal). The pore-filling step in adsorption and desorption curves is sharp, corresponding to a 600 ' ' ' I ' ' ' I ' ' ' I ' ' 'I'' " narrow pore size distribution. SBA-15 and MCM-41 materials share these overall features 400 I/ i but differ when their isotherms are 200 " (b)/ I quantitatively examined. ' The slope of the J isotherm after the low-pressure adsorption step 000 MCM-41 -'-'-I .i is much lower in the case of SBA-15, 800 corresponding to a lower surface area of the mesopore surface, and the pore volume as 600 (a)/ / measured at the top of the mesopore-filling step 400 is also much lower for SBA-15 than for MCM41. Both features strongly suggest that SBA-15 200 pores are separated by silica walls thicker than Or the walls of MCM-41. As wall thickness is 0.2 0.4 0.6 0.8 1 0 inversely proportional to surface area [9], these P/Po observations should give much lower BET surface area for SBA-15 than for MCM-41. But, Figure 1" Nitrogen isotherm at 77 K surprisingly BET surface area calculations show of MCM-41 with 50 A (a) and similar surface areas (-900 m2/g) for both 115/~ (b) pore size and SBA- 15 materials except for SBA-15 synthesized at with 50 )~ (c) and 96 A (d) pore size. 130~ where the BET surface area is -500 me/g. This unexpected trend of the BET surface area requires a careful examination of the measurement technique. The CBET parameters from the BET equation have been calculated and negative or unusual high positive values are observed for SBA-15 synthesized at 60 and 100~ whereas usual CBET values for MCM-41 (CBET "~ 90) are obtained for SBA-15 synthesized at 130~ [3]. These values reveal that BET equation is not valid for SBA-15 -
398 synthesized at 60 and 100~ and strongly suggests the presence of micropores in these two materials. T-plot analyses show effectively micropores for SBA-15 synthesized at 60~ but no micropores for SBA-15 synthesized at 100~ which is in contradiction with the previous observations. To evaluate the amount of micropores in SBA-15 and to calculate the true surface area, the true wall thickness and the true wall density, we used in the calculations unambiguous data: the cell parameter (a) from XRD, the total pore volume (Vp) taken at the end of the pore-filling step and the pore diameter (DBdB) calculated by BdB method on the desorption branch of the isotherm. For MCM-41, by using a model of hexagonal honeycomb, it has been shown that DBdB is close to the geometrically calculated pore diameter [8] and can then be expressed as: DBdB = 1.05 a [Vp/(Vp+l/Psi)] 1/2
(1)
where Psi is the density of amorphous silica (2.2 g.cm-3). For MCM-41, Wp is equal to the mesopore volume (Vmes). In the case of materials containing micropores, like SBA-15, the total pore volume (Vp) is the sum of the micropore volume (Vg) and the mesopore volume (Vmes), so equation (1) becomes: DBdB = 1.05 a [Vmes/(Vp+l/Psi)] 1/2
(2)
Hence the true mesopore and micropore volumes can be Calculated by the following equations: Vmes= (DBdB/1.05a) 2 (Vp+ 1/Psi)
(3)
Vla = Vp_Vmes
(4)
The average density Pw of the walls between mesopores of SBA-15 is the result of the contributions of micropore volume and silica volume and can be expressed as: 1/pw = V~t + 1/Psi
(5) In this model of hexagonal honeycomb structure for SBA-15, the wall thickness [8] and the surface area [9] of the mesopores are given by the following equations: t = a - 0.95 DBdB Smes = 4.104/pw t [(1-t/a)/(2-t/a)]
(6) (7)
with t and a expressed in A. By calculation (Table 1), we found that SBA-15 synthesized at 60~ exhibits a micropore volume as high as the mesopore volume, SBA-15 synthesized at 100~ features a non-
399 negligeable amount of micropores (whereas no micropores were identified by t-plot) with a micropore volume equal to 33% of total pore volume and SBA-15 synthesized at 130~ has no micropores. True mesopore surface areas Smes are much lower than BET surface areas for SBA-15 synthesized at 60 and 100~ and in good agreement with the BET surface area (-~500 m2/g) for SBA- 15 synthesized at 130~ Table 1 Total pore volume Vp, mesopore volume Vmes (eq. 3), micropore volume Via (eq. 4) (mL/g), BET surface area, mesopore surface area (eq. 7) (m2/g), wall density law (eq. 5), wall thickness (A) (eq. 6) and pore size for SBA-15 synthesized at 60, 100 and 130~ Vp SBET DBd B Vmes Via Pw t Sines 60-SBA 100SBA 130SBA
0.76 1.19
931 912
49 77
0.34 0.79
0.42 0.40
1.15 1.17
44 33
263 422
1.23
514
96
1.25
0
2.20
15
550
This confirms that the porosity of SBA-15 synthesized at high temperature corresponds to an array of constant-diameter mesopores, with no contribution from microporosity and exhibits the same features as a large pore MCM-41. The origin of the microporosity has been proposed to result from the sharing of the hydration spheres of poly(ethylene oxide) chains between micelles, which disappears at high synthesis temperature, when the hydration sphere volume of ethylene oxide chains decreases [3]. The micropores of SBA-15 are non-homogeneously organized around the mesopores (no supplementary XRD peaks have been distinguished on XRD pattern) and low pressure argon isotherms strongly suggest non-uniform pore size (no step at low pressure has been observed in argon isotherms, like for zeolites). The size of the micropores are estimated to be between 10 and 30 A [ 10]. SBA- 15 offer a new kind of surface containing the openings of a new kind of "micropores" which could imply adsorption properties significantly different of what is known and leads, for instance in this study, to the unvalidity of t-plot analysis. 3.2. Xenon adsorption and 129Xe NMR 129Xe NMR of adsorbed Xe at different pressures on various materials, like zeolites [ 11 ], has been used to characterize their porosity. The resulting Xe chemical shift (SXe) is, in first approximation, depending on Xe interaction with the solid surface and on the interaction between Xe molecules: 8Xe = 8interaction Xe-surface + 8interaction Xe-Xe The first term (Sinteraction Xe-surface) depends on the pore size and on the type of surface. It can be evaluated by extrapolation at p = 0 of the plot 8Xe as a function of Xe pressure. The second term (Sinteraction Xe-Xe) depends on the pore size and on Xe pressure. High Xe pressures and small pores increase the number of collisions between Xe molecules, hence, 8Xe increases. It has been found on microporous amorphous silica [12] that the plots 8Xe as a function of Xe
400
pressure are linear, 8Xe(p=0) and the slope decrease as pore size increases. 8Xe becomes independent of Xe pressure for pore larger than 13 A. For MCM-41, 8Xe have been plotted as a function of Xe pressure. A slight positive slope has been obtained for MCM-41 with 20/~ pore size. For larger pore size, 129Xe NMR spectra are practically independent of Xe pressure and a correlation between 8Xe and pore diameter has been established at a Xe pressure of 1000 Torr (Figure 2).
100 80
El
I:
";-
-,,.
o
0,30~
o
1
&
4o
! o
20 0
150~ ,,,
0
I , , ,
20
I , , ,
40
It,,
60 D (A)
I , , ,
80
I , ,
I00
,
120
Figure 2. 129Xe NMR chemical shift of adsorbed Xe at 1000 Torr versus pore diameter. Plain circles are representing MCM-41 materials and the curve, the resulting relationship between 8Xe and pore diameter. Empty triangles, squares and circles, are corresponding to the ~SXeof the first, second and third 129Xe NMR peaks found for each SBA-15 and have been plotted as a function of their BdB pore diameter (corresponding synthesis temperature is indicated).
For SBA-15, at 1000 Torr Xe pressure, several 129Xe NMR peaks have been observed. For materials prepared at low temperature (50-60~ three peaks have been obtained: a first small one at --90 ppm and two others a t - 8 0 and ~-70 ppm. For solids synthesized at higher temperature (larger pore size), only the second and third peaks at lower chemical shifts are obtained. The first peak should be relevant of micropores and the second and third of the mesopores. Even if the position of the first peak is not yet well understood and if the presence of the later two peaks is not clear yet, Figure 2 shows that data of SBA-15 synthesized at temperatures between 50 and 100~ significantly differ from the relationship of MCM-41, whereas data of SBA-15 synthesized at higher temperatures do. This can be explained by the presence of micropores in SBA-15 obtained at temperatures between 50 and 100~ Indeed Xe has an average residence time in all the pores of SBA-15 with a residence time of Xe in micropores higher than in mesopores, which will shift 8Xe relative to mesopores towards higher chemical shifts. Moreover these observations strongly suggest an interconnection between the micropores and the mesopores. Further works are under study. 3.3. Surface functionalization Two kinds of surface functionalization, providing the best surface coverage for MCM-41 [7], have been performed on SBA-15 materials. Two different octylsilanes were used: (1) chlorodimethyloctylsilane and (2) trimethoxyoctylsilane. For MCM-41, the surface coverage is almost independent of pore size with a grafting density of nl-l.5 grafts/nm2 (CBET~18) and n2-~1.8 grafts/nm2 (CBET"23) for method (1) and (2), respectively (Table 2). CBET is a good indicator of surface coverage on silica materials, a lower CBET is diagnostic for a better
401 coverage [7]. The maximum of grafting density has been obtained on silica gel with nl=l.9 grafts/nm 2 (CBET=16) and n2=2.5 grafts/nm 2 (CBET=23). The surface coverage is more efficient with method (1) although the number of grafted species is lower. Method (2) is a surface polymerization (horizontal) between grafted chains and requires a strict homogeneous surface to avoid anarchical polymerization (vertical) which can be evidenced by a higher CBET relative to the presence of non-grafted silanols. Table 2 Pore volume V (mUg), BET surface area SBET (mUg), CBET, grafting density n (greffons/nm 2) and n* corrected by the surface calculated by eq. 7 for methods (l) and (2), for MCM-41 with 35, 50 and 115 A pore size and SBA-15 synthesized at 60, 100 and 130~
Samples
V1
SBET1 CBETI nl
nl*
V2
SBET2 CBET2 n2
MCM-35 0.27 nd nd 1.36 0.24 nd MCM-50 0.52 560 17 1.48 0.44 574 MCM-115 1.53 562 20 1.42 1.31 544 60-SBA 0.25 275 30 0.95 3.36 0.15 204 100-SBA 0.62 424 20 1.16 2.50 0.48 412 130-SBA 0.78 327 18 1.44 1.44 0.66 336 nd: not determinated (Pore sizes are so reduced that BET equation is
nd 22 28 43 35 24 no more
1.52 1.90 1.79 1.23 1.61 2.20 valid)
n2*
4.35 3.47 2.20
For SBA-15 synthesized at 60 and 100~ the grafting induces larger decrease in BET surface areas (900 to 200 or 400 m2/g) than for MCM-41 pointing out again some problems about the validity of BET surface areas for these calcined materials. Incorrect surface areas induce false grafting densities (n) which are calculated by m 2 of initial calcined materials. Corrected values can be obtained by using the surface areas reported in Table 1 by eq. 7, but the resulting grafting densities (n*) exhibit values too high to be correct which can be explained by a part of octylsilanes grafted in the micropores. The surface coverages are lower (CBET higher) than for MCM-41 revealing non grafted silanols on the surface so a different homogeneity of surface. For SBA-15 synthesized at 130~ the grafting density and the surface coverage are analogous to MCM-41. 1
'
''
I
'
'
'
I'
'
'
I
'
''
I
0.8
.,--'ll"
.6
.,0"'"
o.4 f
:.
0.2
ol, 0
' 20
:"
"
'
'
'
I'
.... m ......
D
[]
,
'
Figure 3. Ratio of volume of grafted samples per g of silica (V) to initial volume (Vp) versus pore diameter. oCircles are representing MCM-41 samples and square SBA-15 samples. Filled points are relative to the grafting by method (1) and empty point by method (2). Filled and dotted curves are the losses of relative volume for a cylender of pore diameter decreasing 120 of I0 and 12 A, respectively.
.
D
40
'
60
~80
DBdB(A)
100
402
To compare all the different samples, we can normalize the volumes and calculate the volumes per g of silica (instead of g of material) reported per volume of initial materials. In this way, Figure 3 shows that MCM-41 samples grafted by methods (1) or (2) followed a similar evolution as a loss of volume of cylindrical pores where the diameter will be decreased by 10 or 12 A, respectively. SBA-15 synthesized at 130~ (D = 96 A) shows a loss of volume similar to MCM-41, with a slightly lower value revealing a barely higher grafting density than MCM-41. For SBA-15 synthesized at 60~ (D = 50 A), the losses of volume strongly differ from MCM-41, showing that some of the micropores have been grafted and some micropore volume filled. The loss of volume after grafting by method (2) suggests a large part of anarchical polymerization as evidenced by the high value of CBET (Table 2). In the case of SBA- 15 synthesized at 100~ the loss of volume after grafting by method (1) is close to MCM-41 and the surface coverage is similar (similar CBET) to MCM-41 (Table 2) showing that micropores are not blocked but are large enough to be grafted in a similar way as mesopores. This observation will induce the presence of larger micropores in SBA-15 synthesized at 100~ than in SBA-15 synthesized at 60~ With the grafting method (2), higher loss of volume and lower surface coverage (higher CBET) than MCM-41 (Table 2) are consistent with the occurence of anarchical polylerization in the pores. This later could be induced by some surface heterogeneity due to the presence of micropores on this surface which does not allow a perfect horizontal polymerization of the grafted chains due to the presence of variation in surface curvature on the surface. In conclusion, SBA-15 materials synthesized at temperatures lower than 130~ possess micropores (or small mesopores) connecting well-ordered mesopores which confer to the materials different adsorption properties and different behaviours towards grafting than MCM-41. On the contrary, strong similarities have been found between MCM-41 and SBA15 synthesized at 130~ where no micropores are produced. REFERENCES I. D.Y. Zhao, Q. S. Huo, J. L. Feng, B. F. Chmelka and G. D. Stucky, J. Am. Chem. Soc., 120 (1998) 6024. 2. J.S. Beck et al., J. Am. Chem. Soc., 114 (1992) 10834. 3. A. Galarneau, H. Cambon, F. Di Renzo and F. Fajula, Langmuir, (2001) in press. 4. R. Ryoo and J. M. Kim, J. Chem. Soc., Chem. Commun., (1995) 711. 5. S.J. Jong, J. F. Wu, A. R. Pradhan, H. P. Lin, C. Y. Mou and S. B. Liu, Stud. Surf. Sci. Catal., 117 (1998) 543. 6. D. Desplantier-Giscard, A. Galarneau, F. Di Renzo and F. Fajula, Stud. Surf. Sci. Catal., 135 (2001) 06-P-27. 7. T. Martin, A. Galarneau, D. Brunel, V. Izard, V. Hulea, A. C. Blanc, S. Abramson, F. Di Renzo and F. Fajula, Stud. Surf. Sci. Catal., 135 (2001) 29-0-02. 8. A. Galarneau, D. Desplantier, R. Dutartre and F. Di Renzo, Microporous Mesoporous Mater., 27 (1999) 297. 9. F. Di Renzo, D. Desplantier' A. Galarneau, F. Fajula, Catal. Today, 66 (2001) 75. 10. R. Ryoo, C. H. Ko, M. Kruk, V. Antochshuck and M. Jaroniec, J. Phys. Chem. B, 104 (2000) 11465. 11. J.-L. Bonardet, J. Fraissard, A. Gedeon, M.-A. Springel-Huet, Catal. Rev. Sci. Eng, 41 (2) (1999) 115. 12. A. Julbe, L. C. de Menorval, C. Balzer, P. Davis, J. Palmeri and J. A. Dalmon, Porous Mater., 6 (1999) 41.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) @2002 Elsevier Science B.V. All rights reserved.
403
C o m p r e h e n s i v e characterization o f iron oxide containing m e s o p o r o u s molecular sieve M C M - 4 1 Z.Y. Yuan, a'* W. Zhou, b'* Z.L. Zhang, a Q. Chen, c B.-L. Su d and L.-M. Peng a.~ Beijing Laboratory of Electron Microscopy, Institute of Physics & Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 2724, Beijing 100080, China a
b School of Chemistry, University of St. Andrews, St. Andrews, Fife KY 16 9ST, United Kingdom, e-mail:
[email protected] CDepartment of Electronics, Peking University, Beijing 100871, China d Laboratory of Inorganic Materials Chemistry, University ofNamur (FUNDP), 61 rue de Bruxelles, B-5000 Namur, Belgium
We have synthesized iron oxide-containing mesoporous silica MCM-41 by a direct route. This material has a significant microporousity, which might be related with the loading of iron oxide in the MCM-41 mesopores, though no crystalline phases of iron oxide were detected by XRD. Intensity deviations and extra bright intensity within the pores, as well as local superstructural phases within a large area, were observed in the TEM images. The position and area of such a crystalline phase could change during the successive electron beam exposure and resulted in a formation of a cycle. These phenomena imply the possible existence of small Fe203 nanocrystals loaded in the mesoporous silica materials with regular and oriented arrangement. EELS spectrum of the sample proves the existence of iron, but the chemical environment of iron in the MCM-41 silica should be different from that of bulk amorphous porous iron oxide. The small iron oxide crystals in the MCM-41 sample might be linked with silica wall through oxygen. Another possibility is that iron oxide reacts with silica during synthesis, causing the partial crystallization of the MCM-41 framework. In other words, microcrystals of iron oxide may exist in the silica matrix.
1. INTRODUCTION Since the exciting discovery of the novel family of molecular sieves M41S was reported by Mobil's researchers in 1992 [1,2], these materials have received considerable interest due to their potential applications in the area of catalysis, separation, and advanced materials. MCM-41, the well-known hexagonal member of this family, exhibits regular mesopores between about 1.5 and 10 nm, very high surface area (typically 1000-1200 m2/g), high hydrocarbon sorption capacity, and thermal stability. Various transition-metal atoms can be introduced into the network of MCM-41 in order to generate potential catalysts, which could be more active compared to microporous systems [3-6]. MCM-41 has also been proved to be
404 a suitable support for preparing metal or metal oxide-based catalysts [4,7,8]. The welldefined mesoporous structure of MCM-41 implies that the material could be a suitable host for quantum semiconductor structures of low dimensionality [8-10]. Iron-containing zeolite and zeolite-like molecular sieves are of great interest, because they show interesting catalytic properties. For example, iron-containing zeolite Y was used in the process for the catalytic reduction of NOx in exhaust gases [ 11]. Catalytic synthesis of carbon nanotubes, with a fullerene-like structure, has been reported with the use of a zeolite Y catalyst which contains iron or cobalt [12], or a Fe-loaded mesoporous silica [13]. The synthesis of iron-containing MCM-41 was first reported by Yuan et al. [ 14]. Fe incorporation in the silicate "framework" was evidenced on the basis of FTIR and EPR data. FeMCM-41 has also been investigated by some other groups for possible applications, exhibiting many significant catalytic properties recently [15,16]. Nanoparticles of Fe203 were claimed by Abe et al. [ 17] to encapsulate into the uniform pores of MCM-41, which had a wide bandgap from 2.1 to 4.1 eV owing to the quantum size effect. Very recently Fe203 nanoparticles were synthesized within mesoporous MCM-48 silica phases by using multiple cycles of wet impregnation, drying, and calcination procedures [ 18]. Herein a direct synthesis route for iron oxide-modified MCM-41 is described. Small Fe203 crystals exist in the channel network of MCM-41 silica, as revealed by high resolution transmission electron microscopy (HRTEM).
2. E X P E R I M E N T A L
The mesoporous MCM-41 materials were prepared using cetylpyridinium bromide (CPBr) surfactant as a templating material. All chemicals were obtained from Beijing Chemicals Corp. Under stirring, tetraethylorthosilicate (6.75 ml) was added dropwisely to an aqueous solution containing 5 g of CPBr and 18.8 ml of ammonia solution (-25%) in 65 ml distilled water. After stirring for several minutes the mixture becomes cloudy, indicating the onset of some silica precipitation; a solution of 0.121 g iron(Ill) nitrate (Fe(NO3)3-H20) in 5 ml was then added, which co-precipitate with the silica and become incorporated into the silica. After stirring more than 30 min, the mixture was loaded into an autoclave and statically heated at 90 ~ for 3 days to complete crystallization of the MCM-41 material. The resultant solid product was recovered by filtration, washing with distilled water and drying in air at room temperature. In order to remove the organic species in the mesopores, the as-synthesized material was calcined in air from room temperature to 540 ~ with a rate of 1 ~ followed by a further calcination at 540 ~ for 5 h. The powder X-ray diffractograms (XRD) of the solids were recorded on a Rigaku D/max 2400 diffractometer using CuKo~ radiation (~, = 0.154 nm). N2 adsorption-desorption isotherms were obtained at liquid nitrogen temperature on a Quantachrome Autosorb-1 apparatus. The sample was degassed at 300 ~ for 10 h in vacuum prior to adsorption. The specific surface area was determined by the BET (Brunauer-Emmett-TeUer) method and poresize distribution was obtained with the N2 adsorption branch using the BJH (Barrett-JoynerHalenda) method. Local structures of the mesoporous crystals were studied using high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) on a Philips CM200 FEG (field emitting gun) equipped with electron energy loss spectroscopy (EELS) and working at a voltage of 200 kV. EELS experiments were performed in image mode using the GIF (Gatan imaging filter) system and a 0.5 and 1.0 eV/channel energy dispersion, and a 3 mm selected area aperture in the GIF system. The
405 specimens for the HRTEM studies were prepared by dispersing the particles in alcohol by ultrasonic treatment, and dropping onto a holey carbon film supported on a copper grid.
3. RESULTS AND DISCUSSION
The X-ray diffraction pattems of the resultant materials are depicted in Figure 1. The diffractograms of both assynthesized and calcined samples exhibit four sharp diffraction peaks at low 20 range (1.5 - 10~ reflecting a typically well-aligned MCM-41 structure [1,2]. The diffraction peak of (100) did not change atter calcination except for an increase of the peak intensity, indicating no decrease of the unit cell parameter (a0 = 4.719 nm). This suggests that the nanostructure of the iron containing MCM-41 silica possesses a high thermal stability. The XRD patterns show no additional peaks at the high 20 range of 10-100 ~ indicating that no crystalline iron oxide phase has been formed outside the pore structure, even atter calcination. However, iron oxide clusters might be synthesized within the pores and too small for X-ray detection. N2 adsorption-desorption isotherm for the calcined sample and its corresponding pore size distribution curve calculated using the BJH method are presented in Figure 2. As shown in Figure 2, a typical irreversible type IV adsorption isotherm with a hysteresis loop, as identified by IUPAC [19], is observed. A sharp step occurs in P/Po range between 0.3 and 0.4, which indicative of the filling of N2 molecules in the mesopores. The P/Po position of the inflection points is clearly related to a diameter in the mesopore range and the step indicates the mesopore size distribution. From a plot of the pore size distribution in Figure 2, we can see a narrow pore size distribution centered at 3.1 nm. The pure silica MCM-41 was also
~J
])i i)\
calcined
,
jl ~
.~," \ . 2 \ . . . . . .I
I
2
4
.
as-synthesized
~,, .
I
,
I
9
I
6 8 10 20 (degree) Figure 1. XRD patterns of iron oxide containing MCM-41 samples. + Adsorption --'-- D e s o r p t i o . n ~ ~
60(
~
50(
4oo
i il
o..ot }t
& 30t3
.15 ~o.~o
2
4
6
8
10
Pore diameter (nm) 1
0.0
9
I
~
i
,
1
'
i
0.2 0.4 0.6 0.8 Relative pressure (P/P0)
,
"!
1.0
Figure 2. N2 adsorption-desorption isotherm at liquid N2 temperature for the sample and its corresponding pore-size distribution curve
(Inset). synthesized with the same condition without the
406
Figure 3. Micropore size distribution plots resulted from (a) MP method and (b) HK method. addition of iron species, and presented the similar XRD and N2 adsorptin data. BET surface area of the present FeMCM-41 sample is 981 m2/g and pore volume is 0.925 cm3/g, which are slightly lower than those of pure silica MCM-41. However, further micropore analysis by means of t-method [20], MP method [21] and Horvath-Kawazoe (HK) method [22] shows that the sample has a significant microporosity. A V vs. t (the volume of gas adsorbed versus the statistical thickness of an adsorbed film) plot with interval slopes is calculated. The micropore surface area and micropore volume from the t-method are 596 m2/g and 0.377 cm3/g respectively. Micropore analysis result from the MP method shows a micropore size distribution centred at 1.37 tun, and from the HK method, the pore width is 1.22 nm (Figure 3). Such a large microporosity might be related with the loading of iron oxide in the MCM-41 mesopores. A combination of highresolution transmission electron microscopic image processing and selected area electron Figure 4. A typical TEM image of the calcined iron oxidediffraction has been containing MCM-41 and its electron diffraction pattern (Inset).
407 proved to be a suitable method to study small crystals, which can not be efficiently measured by single-crystal X-ray diffraction for structure determination [23]. In the present study, HRTEM and electron diffraction have been used to study the possible small crystals of iron oxide implanted in mesoporous MCM-41. It is significant to notice that the samples were relatively stable under the electron beam irradiation in comparison with pure silica MCM-41 and most of other doped MCM-41 materials. Figures 4 and 5 show a series of TEM images obtained by successive imaging at the same area and their electron diffraction images. A regular hexagonal arrangement of the pore openings is observed and the pore center to pore center distance is about 4.5 ran, in agreement with the XRD results very well. Since these photographs were recorded under conditions far from optimum (Scherzer focus), the contrast is reversed. The overfocus condition, however, does not alter the symmetry of the structure observed so that the hexagonal shape of the pores in the micrographs represents the true geometry of the pores [24]. A close look of these images enables us to view intensity deviations and extra bright intensity within the pores. Fr~Sba et al. [18] believed that the existence of extra intensity modulations should be related with the doping of iron oxide in MCM-48 molecular sieve silicas. The TEM images in Figures 4 and 5 also show local superstructure crystalline phases within a large area. The position and area of such a crystalline phase could change during the successive electron beam exposure and resulted in the formation of a cycle. Under the electron beam irradiation, the possible movement with slight tilting of the particle, and the ruggedness of the particle surface might result in some changes of the image contrast pattern. However, we believe that the significant contrast change observed in Figures 4 and 5 implies the existence of iron oxide nanocrystallites within the pores or in the silica framework. They may have regular and oriented arrangement. Additional strong evidence for the Fe203 loading is supported by the selected area electron diffraction of the same particle (Figure 4) and the inverse Fourier transform pattern (Figure 5d). There are superstructure reflections reflected in the electron diffraction patterns besides many diffraction spots appeared with uniform hexagonal pattern (Figure 5d), though no any additional X-ray diffraction peaks were observed for Fe203 nanocrystallites. It is evident to indicate the possible existence of onedimensionally ordered iron oxide nanoarrays accreted with the hexagonal mesoporous silica. Figure 6 shows the TEM image of the iron oxide containing MCM-41 sample viewed along a direction perpendicular to the pore axis and its inverse Fourier transform pattern. The intensity deviations and extra intensity within the pores are also visible on the images. Many spots appear in its inverse Fourier transform pattern, revealing that Fe203 crystals present in the pore structure and its orientation is almost along with the pore axis (Figure 6). To make sure of the formation of Fe203 microcrystals, electron energy-loss spectroscopy was carried out. Transmission electron energy-loss spectroscopy has been proved to be a powerful analytical tool to investigate chemistry and electronic structures in thin solid objects [25,26]. Since the iron oxide crystals are too small and dispersed within the mesoporous silica, the signal-to-noise ratio is very low, and the EELS experiments should be done by increasing the number of scans up to 50. Figure 7 shows the EELS spectrum of the present sample, in comparison with the spectrum of amorphous mesoporous Fe203 obtained from the similar method described in experimental section. It is interesting that the Fe-L2.3 edge of the present Fe203 containing MCM-41 silica is -15 eV lower in energy than that of the bulk porous amorphous Fe203, and its intensity is lower. Only one weak peak can be observed obviously instead of at least two from the bulk mesoporous Fe203, which could be attributed to the low iron content in the sample (Si/Fe ratio of 100). The chemical environment of iron
408 in the MCM-41 silica should be different from that of bulk amorphous iron oxide. The small iron oxide crystals in the MCM-41 sample might be linked with silica wall. Another possibility is that the small iron oxide crystals react with silica during synthesis, causing the partial crystallization of the framework. These possibilities might cause the decrease of the energy of Fe L2,3edges in the present MCM-41 materials.
Figure 5. (a) - ( c ) A serious of TEM images obtained by successive imaging at the same area in Figure 4 (the crystalline phase positions are circled in white), and (d) the inverse Fourier transform pattern of image (a) (some superstructure reflection spots are pointed with white arrows).
409 4. CONCLUSIONS Iron oxide containing mesoporous silica MCM-41 has been synthesized directly and characterized with various techniques. Comprehensive application of HRTEM, SAED, and ELLS revealed that microcrystals of iron oxide might exist in the silica matrix with regular and oriented arrangement, though the chemical environment of iron in the present mesoporous silica is different from that of bulk amorphous porous iron oxide. Further investigation of this material on the relationship of its microstructure and property as well as its possible applications is still in progress in these laboratories.
Figure 6. TEM image of the sample on a direction perpendicular to the pore axis and its inverse Fourier transform pattern (inset).
5. ACKNOWLEDGEMENT This research was supported by the National Natural Sciences Foundation of China (NSFC) and Chinese Academy of Sciences.
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.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Sot., 114 (1992) 10834. 3. A. Sayari, Chem. Mater., 8
Figure 7. EELS iron L2,3 edges for (a) bulk amorphous mesoporous Fe203 and (b) Fe203 containing MCM-41 silica.
410 (1996) 1840. 4. A. Corma, Chem. Rev., 97 (1997) 2373. 5. K.M. Reddy, I. Moudrakovski and A. Sayari, J. Chem. Sot., Chem. Commun., (1994) 1059. 6. A. Corma, M.T. Navarro and J. Porez-Pariente, J. Chem. Soc. Chem. Commun., (1994) 147. 7. A. Corma, A. Martinez, V. Martinez-Soria and J.B. Monton, J. Catal., 153 (1995) 25. 8. J.Y. Ying, C.P. Mehnert and M.S. Wong, Angew. Chem. Int. Ed., 38 (1999) 56. 9. H. Winkler, A. Birkner, V. Hagen, I. Wolf, R. Schmechel, H. von Seggern and R.A. Fischer, Adv. Mater., 11 (1999) 1444. 10. R. Leon, D. Margolese, G. Stucky and P.M. Petroff, Phys. Rev. B, 52 (1995) 2285. 11. K. Segawa, K. Watanabe and S. Matsumoto, Jpn. 93317649, 1993 [Chem. Abstr., 120 (1993) 142982s]. 12. A. Hernadi, A. Fonseca, J.B. Nagy, D. Bernaerts, A. Fudala and A.A. Lucas, Zeolites, 17 (1996) 416. 13. W.Z. Li, S.S. Xie, L.X. Qian, B.H. Chang, B.S. Zou, W.Y. Zhou, R.A. Zhao and G. Wang, Science, 274 (1996) 1701. 14. Z.Y. Yuan, S.Q. Liu, T.H. Chen, J.Z. Wang and H.X. Li, J. Chem. Soc., Chem. Commun., (1995) 973. 15. A. Wingen, D. Anastasieviec, A. Hollnagel, D. Werner and F. Schiith, Stud. Surf. Sci. Catal., 130 (2000) 3065. 16. N. He, S. Bao and Q. Xu, Appl. Catal. A, 169 (1998) 29. 17. T. Abe, Y. Tachibana, T. Uematsu and M. Iwamoto, J. Chem. Soc., Chem. Commun., (1995) 1617. 18. M. FrSba, R. KShn and G. Bouffaud, Chem. Mater., 11 (1999) 2858. 19. K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol and T. Siemieniew, Pure Appl. Chem., 57 (1985) 603. 20. G.D. Halsey, J. Chem. Phys., 16 (1948) 931. 21. R.S. Mikhail, S. Brunauer and E.E. Bodor, J. Colloid Interface Sci., 26 (1968) 45. 22. G. Horvath and K. Kawazoe, J. Chem. Eng. Japan, 16 (1983) 470. 23. A. Carlson, T. Oku, J.-O. Bovin, G. Karlsson, Y. Okamoto, N. Ohnishi and O. Terasaki, Chem. Eur. J., 5 (1999) 244. 24. V. Alfredsson, M. Keung, A. Monnier, G.D. Stucky, K.K. Unger and F. Schiith, J. Chem. Soc., Chem. Commun., (1994) 921. 25. R.F. Egerton, Electron Energy-Loss Spectroscopy in the Electron Microscope, Plenum, New York, 1986. 26. M.M. Disco, C.C. Ahn and B. Fultz, Eds., Transmission Electron Energy Loss Spectrometry in Materials Science, TMS, Warrendale, 1991.
~stUdleS m Surtace Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) O 2002 Elsevier Science B.V. All rightsreserved.
411
M e s o p o r o u s molecular sieves of M C M - 4 1 type modified with Cs, K and M g physico-chemical and catalytic properties Mafia Ziolek, Aleksandra Michalska, Jolanta Kujawa and Anna Lewandowska
A.Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, PL-60-780 Poznan, Poland; e-mail: ziolek@amu, edu.pl Siliceous, aluminosilicate, and niobosilicate mesoporous molecular sieves of MCM-41 type were modified with Cs, K, and Mg via an ion exchange and impregnation. The impregnation with Cs-acetate leads to a partial structural distortion of mesoporous sieves used. In spite of that Cs-impregnated NbMCM-41 exhibits the highest basicity, whereas, Cs template ion exchanged materials as well as Cs-impregnated AIMCM-41 show the acid-base properties. Kimpregnated MCM-41 is only less basic than Cs/NbMCM-41, and thanks to its stability during the modification , one can recommend K/MCM-41 mesoporous molecular sieves as an effective basic catalysts. I. INTRODUCTION The discovery of mesoporous materials of MCM-41 type in 1992 has given hope for their use as matrices for basic agents. Many organic syntheses require the catalytic reactions involving basic centres. Catalysis by basic zeolites is limited to relatively small molecules. The mesoporous molecular sieves can be of assistance in this field. The impregnation or ion exchange with cesium species commonly generates basicity. However, it is well known that SiO-Si bonds readily hydrolyse in strong basic media. Recently, C.Noda Perez et al. [1] have found that even in pH _= 8.5 the MCM-41 structure is not stable. Al-containing samples were less resistant than the purely siliceous one to basic media. However, Kloestra and van Bekkum [2] concluded that an increase in the framework stability occurs with lowering of Si/AI ratios, i.e. with the growth of AI content. It can be supposed that the nature of T-atom in the MCM41 frameworks influences their resistance to basic media interaction. The aim of this study was to use various mesoporous matrices (SiMCM-41, AIMCM-41 and NbMCM-41) for the generation of basic centres with Cs, K and Mg via an ion exchange procedure or an impregnation. A very low concentrated (- 0.02M) solutions of metal salts were applied. The obtained materials were characterised with X R , N2 adsorption/desorption, F T I ~ H2-TPR, TEM, and a test reaction (acetonylacetone cyclization [3]). 2. EXPERIMENTAL
2.1. Synthesis and modification Si- Nb- and AI- containing mesoporous molecular sieves ofMCM-41 type were synthesised according to the procedure described in [4] and modified in the preparation of NbMCM-41 according to [5]. Si/T atom ratio of 32 has been applied. The Cs, K, and Mg ion-exchange
412 (IE) was performed using stirring of the calcined mesoporous solid in aqua solution of cesium acetate (0.02 M), or potassium chloride, or magnesium chloride, respectively, at room temperature (RT). After stirring, the samples were filtrated, washed with 20 cm3 of distilled water and dried at 373 K for 5 h. The template ion exchange (TIE) has been also applied using mesoporous molecular sieves containing template (i.e. before the calcination). The modified mesoporous molecular sieves were also prepared by the impregnation. In this case the calcined sieves were used as parent materials and atter the impregnation the samples were not washed, only dried at 393 K for 1 h and calcined at 773 for 14 h. The percent of metal introduced to the MCM-41 samples was obtained from AAS analyses. 2.2. Sample characterisation
N2 adsorption/desorption studies were conducted at 77 K with Micrometrics ASAP 2010 apparatus. The samples were first outgassed at 573 K for 3 h.
Powder X-ray diffraction (XRD). XRD patterns were obtained on TUR 42 difffactometer with CuK~ radiation (10kV, 40 mA) and a step size 0,02 ~
The temperature-programmed reduction (TPR) of the samples was carried out using H2/Ar (10 vol.%) as reductant (flow rate = 32 cm3 min'~). 0.03 g of the sample was filled in a quartz tube, treated in a flow of helium at 673 K for 1 h, and cooled to room temperature. Then, it was heated at the rate of 10 K min"~to 1100 K under the reductant mixture. A thermal conductivity detector in the PulseChemiSorb 2705 (Micromeritics) instrument measured hydrogen consumption. Fourier-Transform Infrared Spectroscopy ~TIR). Infrared spectra were recorded with a VECTOR 22 (BRUKER) FTIR spectrometer. The samples were prepared by diluting of 0.001 g of the mesoporous molecular sieve in KBr. The spectra were scanned in the framework range ( 4 0 0 - 1500 cm~).
Transmission Electron Microscopy~EM). JEOL 2000 transmission electron microscope was used for the TEM image registration. 2.3. Test reaction The acid and base characteristic of the catalyst were evaluated using the probe reactionacetonylacetone (AcAc) cyclization- reported by Dessau [3] and applied by Alcaraz et al. [6]. In this reaction dimethylfuran (DMF) is produced on acidic centres, whereas, basic centres are involved in the formation of methylcyclopentenone (MCP). The reaction was conducted in a pulse-flow micro-reactor in which 2 cm3 of AcAc was passed continuously over 0.05 g of the granulated catalyst at 523 K in a nitrogen flow. The reaction products were collected downstream of the reactor in a cold trap and analysed by a gas chromatography (CHROM-%, Silicone SE-30 / Chromosorb column).
413 3. RESULTS AND DISCUSSION 3.1. Texture characterisation
The data calculated from N2 desorption isotherm of cesium and potassium containing molecular sieves are shown in Table 1. All of the materials are mesoporous with a high surface area and pore volume. Both parameters decrease drastically in two samples, Cs-NbMCM41(IE) and Cs/NbMCM-41, less significantly for all other impregnated materials and only slightly in the case of cation-exchanged ALMCM-41. Table. 1. Catalysts and their characterisation Cation exchange, [%] or impreg.
Catalyst
Surface area, BET [m2gq ]
Pore volume BJH [cm3gq ]
1022 750 752 1033 1000 946 791 1034 984 558 301
1.455 1.125 1.054 1.266 1.236 1 142 0.905
The above results were confirmed by XRD patterns which showed less intensive, smiled peaks for the impregnated materials and Cs-NbMCM-41 (IE) in comparison with those for the parent samples (as example - Fig. 1). However, when template ion exchange procedure has been applied the final materials, even in the case of Nb-containing matrix, exhibited well ordered hexagonal arrangement (Fig. 2).
[wt.%] MCM-41 K/MCM-41 Cs/MCM-41 A1MCM-41 K-AIMCM-41 (TIE) Cs-AIMCM-41 (IE) Cs/AIMCM-41 NbMCM-41 K-NbMCM-41 (TIE) Cs-NbMCM-41 (IE) Cs/NbMCM-41
5 5 -
73 77 5 80 79 5
1 193 1 121
0.423 0.236
120 a - MCM-41
100
a - NbMCM-41 b - C s - N b M C M - 4 1 (TIE)
:~ 60
c- Mg/MCM-41
t~
c - K - N b M C M - 4 1 (TIE)
d - K/MCM-41
._~ 60 t/) f-
.~ C
80
b - Cs/MCM-41
a
_
.
d - ag-
4o
N b M C M - 4 1 (TIE) a
40
2O
20 o
b
,
,
,
20, o
Fig. 1 . X R patterns of the impregnated MCM-41 materials.
o
,
I
4
,
20,
I,,
6
,
I
8
,
I
10
o
Fig. 2. XRD patterns ofNbMCM-41 modified with various cations.
It is worthy to notice that a TEM image (Fig. 3) of the material estimated on the basis of XRD and N2 adsorption/desorption isotherm as the most distorted one, i.e. Cs/NbMCM-41,
414 indicates the presence of parallel hexagonal ordered mesopores.Therefore, one can suggest that the decrease of pore volume and the intensity of XRD peaks are due to the presence of a bulky phase in the mesopores rather than to the destruction of mesopores in the material. The literature [7] described the effect of the adsorbate molecules located in the mesopores on the decrease of the XRD peak intensity. The same effect can be involved by the presence of bulky oxide species in the impregnated samples. So, one should consider rather the distortion of the mesoporous structure and not the destruction due to the modification. This distortion effect for the desired cation depends on the nature of T-element located in the framework. The presented results indicate that aluminosilica material exhibits the highest stability. The following order of the stability of modified mesoporous molecular sieves of M41S family can be presented: Me/AIMCM-41 > Me/MCM-41 > Me/NbMCM-41 where Me=K, or Cs, or Mg. Most probably two effects cause the destabilisation of the mesoporous molecular sieves. One, described earlier in [1], it is the interaction of basic alkali media with the solid. The second one is the location of a bulky phase in the pores leading to the changes of the texture parameters Therefore, the structure properties strongly Fig. 3. Transmission electron depend on the modification procedure, which, among micrographe of Cs/NbMCM-41. others, determines the species formed in the final material. It is well illustrated by the FTIR spectra shown, as example, for aluminosilica materials in Fig. 4. The impregnation followed by calcination leads to the formation of cesium oxide species which size is, of course, greater than that of cesium cation located in the extra framework position after the cation exchange procedure. The presence of a bulky phase in the mesopores of Cs/AIMCM-41 material causes the lowering of the IR band intensity in the spectrum registered in the framework range.
3.2. Surface properties
Fig. 4. FTIR spectra of aluminosilica based Materials.
We have considered the effect of the modification procedure on the surface properties of the matrix. Nb-containing support has been chosen for this study because one could observe the changes in the reduction properties of niobium. H2-TPR technique has been applied in this study. In the H2-TPR profiles the peaks at about 1000 K and higher temperatures were assigned earlier to the reduction of niobium located in the framework [8].
415 160 140
-120
f, 0
~ 80 .
-r-
60
40
200
400 600 800 Temperature, K
1000
Fig. 5. H2-TPR profiles of modified NbMCM-41 materials.
The number of H2-TPR peaks in this region corresponds to the number of various niobium species, i.e. various surroundings of No. From this observation one can indirectly conclude the uniformity of metal loading. The analysis of the TPR profiles presented in Fig. 5 suggests the highest uniformity of Cs loading in the sample prepared via the impregnation - only one main reduction peak in the TPR profile was registered. A higher number of peaks observed in the case of the other samples suggests various location of cesium cations in relation to niobium in the NbMCM-41 framework, or more precisely saying - various cationic surroundings of Nb framework species. This behaviour can determine the catalytic activity of the material discussed in the next paragraph. If cesium cations are not located near each Nb in the framework, a part of niobium species can act as active centres, increasing the acidity of the material.
3.3. Catalytic testing Table. 2. The results of the cyc!ization of acetylacetone.
In the transformation of acetonyloacetone the ratio of the Catalyst AcAc MCP/DMF conversion, selectivity ratio selectivities: methylcyclopentenone (MCP) / % dimethylfuran (DMF) determines Cs/MCM-41 21 oo the b a s i c - acidic properties of Cs-NbMCM-41 (IE) 3.5 0.4 the catalysts [3,6]. It has been Cs- NbMCM-41 (TIE) 21 0.7 stated that when MCP/DMF >> 1 Cs/NbMCM-41 5 oo the catalyst exhibits basic Cs-AIMCM-41 (IE) 26 0.2 properties, whereas MCP/DMF Cs-AIMCM-41 (TIE) 58 0.2 << 1 indicates the acidic Cs/AIMCM-41 6 1.4 character of the material. If this K/MCM-41 17 95 ratio is of the order of magnitude K-NbMCM-41 (TIE) 26 0.4 o f - 1 , the acidic-basic character K-AIMCM-41 (TIE) 64 0.3 of the catalyst can be concluded. Mg/MCM-41 20 18 Taking into account the above Mg-NbMCM-41 (TIE) 20 0.3 behaviour and the results Mg-AIMCM-41 (TIE) 13 0.1 presented in Table 2, the MgO 36 oo following conclusions can be drown: 1. The template ion exchanged mesoporous molecular sieves exhibit the highest activity and acidity (MCP/DMF << 1).
416 2. For the desired cation this behaviour is less pronounced when niobosilica matrix instead of aluminosilica one is used. 3. Siliceous and niobiosilica materials impregnated with Cs and K salts present the highest basicity (MCP/DMF >> 1), like MgO. 4. Magnesium impregnated matrices are less basic than Cs and K impregnated samples. 5. Aluminosilica material, even impregnated with cesium acetate, is not basic. Cs/AIMCM-41 reveals acid - basic properties (MCP/DMF -~1). 4. SUMMARY 9 The impregnation ofMCM-41 samples studied with Cs-acetate leads to the decrease of BET surface area and pore volume as well as the XRD peaks intensity which is the highest on NbMCM-41. However, a TEM image indicates that the honey comb structure and parallel ordered mesopores in Cs/NbMCM-41 are preserved, suggesting only a partial structural distortion. It hints that NbMCM-41 mesopores arrangement is rather stable and the surface area and pore volume decrease is due to the presence of bulky phase inside the pores. 9 The impregnation of MCM-41 with KCI or MgCI2 solutions causes only slightly changes in the ordering of the materials. 9 The traditional cation exchange procedure (IE) influences a little the mesoporous structure, whereas, the application of the template ion exchange (TIE) allow the total resistance of the hexagonal arrangement of the materials. 9 The basicity (measured by the test reaction) of the impregnated samples is higher than that one of the ion-exchanged materials and changes in the following order: Cs/NbMCM-41 ~ Cs/MCM-41 > K/MCM-41 > M g ~ C M - 4 1 9 Cs-NbMCM-41 (TIE) and the Cs/AIMCM-41 exhibit acid - base properties. The presented studies confirmed the earlier observation [1,2] that mesoporous molecular sieves of MCM-41 type modified via impregnation with Cs-acetate are highly basic. Moreover, this work indicates that the basicity of mesoporous molecular sieves can be enhanced if the N b containing matrix (thanks to its higher oxygen charge [9]) is applied instead of AIMCM-41, although the surface area and pore volume of NbMCM-41 drastically decrease after impregnation with Cs. To reduce this disadvantage, K-impregnated siliceous MCM-41 can be used. Its basicity is higher than that of Mg~CM-41 and slightly lower than that of Cs/MCM41 and Cs/NbMCM-41. REFERENCES
1. C.N. Perez, E. Moreno, C.A. Henriques, S. Valange, Z. Gabelica and J.L.F. Monteiro, Microporous and Mesoporous Materials, 41 (2000) 137 2. K. R. Kloestra and van Bekkum, Stud. Surf. Sci. Catal., 105 (1997) 431. 3. R.M. Dessau, Zeolites, 10 (1990) 205. 4. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, and J.S. Beck, Nature, 359 (1992) 710. 5. M. Ziolek, I. Nowak, Zeolites, 18 (1997) 356. 6. J.J. Alcaraz, B.J. Arena, R,D, Gillespie and J.S. Holmgren, Catal. Today, 43 (1998) 89. 7. B. Marler, U. Oberhagemann, S. Vortman, H. Gies, Microporous Materials, 33 (1999) 165. 8. M. Ziolek, I. Sobczak, A. Lewandowska, I. Nowak, P. Decyk, M. Renn and B. Jankowska, Catal. Today, 70 (2001) 169. 9. M. Ziolek, I. Sobczak, I. Nowak, P. Decyk, A. Lewandowska and J. Kujawa, Microporous and Mesoporous Materials, 35-36 (2000) 195.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
417
M e s o - A L P O prepared by thermal decomposition o f the organic-inorganic composite: A F T I R study Enrica Gianotti ~a)*,Erica C. Oliveira~b),Valeria Dellarocca~a), Salvatore Coluccia ~a),Heloise O. Pastore ~b)and Leonardo Marchese ~c) Ca)Dipartimento di Chimica IFM, Universi~ di Torino, v. P. Giuria, 7, 10125, Torino- Italy ~b)Instituto de Quimica, Universidade Estadual de Campinas, CP 6154, 13083-970, Campinas, SP, Brasil. ~c)Dipartimento di Scienza e Tecnologie Avanzate, Universitfi del Piemonte Orientale, "A. Avogadro", C.so Borsalino, 54, 15100, Alessandria - Italy.
Mesoporous ALPO was synthesised using cetyltrimethylammonium bromide (CTAB) as a structure-directing agent. FTIR spectroscopy was used to follow the formation of the ALPO mesophase by thermal decomposition of the aluminophosphate/surfactant composite and NH3 was used as probe molecule to monitor the surface acidity of the product. 1. INTRODUCTION Aluminophosphate-based microporous molecular sieves are known to exist in a wide range of structural and compositional diversity. In the search for new synthesis methods, that could afford channel systems with pores in the range of mesoporosity, phosphate-based molecular sieves, like cloverite[1] and VPI-5[2], have been prepared and displayed ring systems larger than the usual 12-T-atom found in large pore zeolites. Despite the large pores, the openings in these solids are not larger than 1.2 nm limiting their use to similar microporous systems of reactants. It was not until the advent of mesoporous silicates and aluminosilicates that the possibility of preparing aluminophosphates with pore apertures larger than the ones already known turned into a reality[3]. However, the main problem of the mesostructures firstly obtained is their low thermal stability when calcined or submitted to conventional neutral or acid solvent extraction for the removal of the structure-directing agent. Recently, we reported the synthesis of aluminophosphates and magnesium-aluminophosphates using cetyltrimethylammonium bromide (CTAB) as a structure-directing agent with the aim of obtaining large-pore mesoporous materials[4]. An alkaline extraction was proposed to prevent collapse of the mesostructure while promoting simultaneous ion exchange in metal-aluminophophates. The synthesis of aluminophosphate and magnesium-aluminophosphates-based mesoporous materials using aluminum sulfate as a source of aluminum and without introduction of hydrofluoric acid as a mineralizing agent was also explored[5]. In this paper, we report on the formation of an ALPO mesophase by thermal decomposition of the aluminophosphate-surfactant composite monitored by FTIR spectroscopy. NH3 was used as molecular probe of the surface acidity of the product.
*to whomcorrespondenceshouldbe addressed,
[email protected]
418 2. EXPERIMENTAL Mesoporous ALPO samples were synthesized[5] by adding solution 1, prepared by dissolving aluminum sulfate 18-hydrate in water, into solution 2, prepared by the dilution of phosphoric acid in water. After that, an aqueous suspension of CTAB was added, followed by 30 min of homogeinizing and the addition of TMAOH 25 wt % aqueous solution until the desired pH. The mixture was aged for 24h at room temperature, after which it was submitted to hydrothermal treatment at the 70~ C for 48h. The gel composition for a final pH of 8.50 was A1203 : 1.27 P2Os : 2 CTAB : 7.35 TMAOH : 410 H20. The sample was submitted to an alkaline extraction [4] followed by heating under argon until 773 K at 1K min ~ and 10h at that temperature under dry oxygen. The materials were characterized by powder X-rays diffraction (Shimadzu, XRD 6000, CuK~, 30 kV, 40 mA, 2~ 20 minl) and N2 adsorption (ASAP 2010, Micromeritics at 77K after thermal treatment at 298 K until residual pressure of 10-4 Pa). FTIR spectra on ~elletised sample were recorded using a Bruker IFS88 spectrometer at the resolution of 4 cm-, equipped with a high vacuum variable temperature infrared cell (LB-100 of the Infraspac of Novosibirsk) which was permanently connected to a vacuum line (ultimate pressure < 105 mbar). 3. RESULTS AND DISCUSSION The use of cetyltrimethylammonium bromide as surfactant, aluminum sulphate and orthophosphoric acid has allowed the preparation of mesoporous aluminophosphate with Xrays diffractogram characteristic of a hexagonal organization of pores, Fig.la. However, after extraction on alkaline solution and calcination, the samples show only the (100) diffraction in the X-ray diffractogram, Figure lb and c. N2 adsorption at 77K shows that the surface area of of the aluminophosphate is 760 m2g1 with an isotherm that is a mixture of types I and IV and a maximum pore volume of 0.32 cm 3 g-l. The study of the thermal decomposition of the template in mesoporous as-synthesized ALPO was followed by in situ infrared spectroscopy.
]~1000 cps o,.~ r~
-:i
',.._..-_..L
l
20/degrees Fig. 1 - X-rays diffractograms: (a) of as-synthesized sample used in this work; (b) extracted with alkaline solution, and (c) calcined after extraction.
419 Fig.2 shows the FTIR spectra of the, as-synthesised mesoporous ALPO after outgassing at increasing temperatures from 200~ to 500~ (curves a to e). After water desorption at 200~ a broad band in the range 3800-3200 cm -1 due to H-bonded P-OH and AI-OH groups is observed. At higher temperatures (curves b-e), dehydroxylation takes place and oxygensharing-AIO4 and -PO4 tetrahedra, along with isolated P-OH and AI-OH groups, are formed (Scheme 1). In fact, bands at 3670 cm 4, corresponding to the stretching mode of isolated POH, and bands at 3789 and 3720 cm 4, due to the stretching mode of free AI-OH groups, became more evident when the temperature was increased. Absorptions in the range of 3050-2800 cm 4 are due to the C-H stretching vibrations of the CH2 and CH3 groups of the surfactant. Bands of CH2 (2922, 2851 and 1458 cm -1) and CH3 (2964 and 2876 cm 4) of the hydrocarbons chains of the template decrease with increasing temperature up to 500~ (curve e) but disappear only after calcination at 550~ (curve f).
0 /
H..o/H_ -
H. __0 /
t
H
H
--O ~
t
-H20
I
H
0 /
,, a"-o P'o
O
O/
"a,
o / / \.-./ OYOo t)
_
_
-
~
o-O-
_
Scheme 1
. . . . .
N
,n.
/ / b
<
J r
I
3600
,
I
,
3200
I
,
2800
~,I/
I
1800
i
I
1500
Wavenumber c m
Fig.2 - FTIR spectra of mesoporous ALPO recorded after outgassing the sample at: 200~ (curve a), 300~ (curve b), 350~ (curve c), 400~ (curve d), 500~ (curve e), after calcination in 100 torr 02 at 550~ (curve f)
420 Similarly to mesoporous silicas, the organic template preserves its cationic form after synthesis as revealed by bands at around 3025 and 1482 c m assigned to -CH3 stretching and bending vibrations respectively in -N(CH3)3+ polar heads of the surfactant. These positive charges might be counterbalanced by either bromide ions, introduced during the synthesis, or PO and A10- groups present at the ALPO-surfactant interface. However, the existence of POand A10 groups cannot be inferred by the present IR results for POH and A1OH groups are abundantly present even on the as-synthesised sample treated at 200~ In Table 1 are reported the IR frequencies and assignments for the surface species present in mesoporous ALPO and in siliceous MCM-48 for comparison [6]. Table 1" Comparison of frequency values for vibrations in ALPO and MCM-48 Observed
Visolated
OH groups V . .H .. -bonded
CH3 groups in N(CH3)3+ polar heads of the surfactant CH3 groups in hydrocarbon chains of the surfactant CH2 groups in 9 hydrocarbon chains of the surfactant
ALPO 3670 (P-OH) 3720, 3789 (A1OH) 3660-3550 3025
(cm l) MCM-48*
3745 (SiOH) 3704-3696 3040
1482
1489, 1479
2964, 2876
2958, 2870
2922,2851
2925,2858
1458
1466
*values from ref.6
The acidity of mesoporous ALPO was monitored by NH3 adsorption at room temperature (Fig. 3). The adsorption of 1 mbar of NH3 (Fig. 3, curve b) produces bands at 3380, 3280, 1620 and 1460 cm 1, that increase in intensity ~ncreasing the NH3 pressure (Fig. 3, curves c and d). Simultaneously, the bands at 3670 c m , due to the stretching of P-OH, and the bands at 3720, 3789 cm 1, due to the stretching of A1-OH groups, decrease and completely disappear after the adsorption of 10 mbar of NH3 (Fig. 3, curve d). The bands at 3380, 3280, 1620 and 1460 cm 1 formed upon ammonia adsorption are assigned to the stretching and bending modes of NH4+ ions produced by a proton transfer from the surface hydroxyl groups to NH3 molecules. After outgassing the sample for lh at room temperature (Fig. 3, curve e), the bands due to NH4+ ions decrease in intensity and the band related to the P-OH stretching mode (3670 cm l) reappears, although of smaller intensity than before adsorbing NH3. This means that only a fraction of NH4§ ions is stable under these conditions. In the spectrum of the sample outgassed at 100~ (Fig. 4 , curve b) the NH4+ bands are less intense, the P-OH band is increased and the bands due to AI-OH groups (3720, 3789 cm -1) are also partially restored. Only after outgassing the sample at
421 350~ (Fig. 4, curve c), the NH4 + bands disappear and the bands due to AI-OH and P-OH stretching are completely reformed. This behaviour shows that P-OH groups are less acidic than AI-OH groups, in fact AI-OH are formed at temperature of NH3 desorption (350~ than P-OH groups (present just after NH3 desorption at room temperature). In siliceous mesoporous materials, MCM-41 or MCM-48, in which only Si-OH groups are present, NH4 + species are not formed. After NH3 adsorption, in fact, only weakly H-bonded NH3 complexes (SiOH...NH3) are present [7,8]. This indicates that P-OH and AI-OH groups in mesoporous aluminophosphates are more acidic than Si-OH groups in mesoporous silicas setting these materials as intermediate between Si-OH and zeolitic bridged OH groups, specific of silicate and/or aluminosilicate structures, respectively. This shows the potential of these molecular sieves for organic reactions where a mild acidity is necessary. Si-containing meso ALPO materials are presently under study, with the aim to propose a new class of catalysts with modulated surface acidity.
a
----wL d
I
3500
3000
2500
2000
1500
3500 Wavenumber c m t
Fig.3 - FTIR spectra of NH3 adsorption on mesoporous ALPO. Curve a: sample in vacuo, curve b: 1 mbar NH3; curve c: 5 mbar NH3; curve d: 10 mbar NH3; curve e: outgassed sample at room temnerature for 1 h.
,
I
3000
,
I
2500
,
I
2000
,
I
1500
Fig.4 - FTIR spectra ofNH3 desorption at increasing temperatures. Curve a: 25~ curve b: 100~ curve c: 350~ curve d: sample in vacuo before ammonia adsorption, for comparison.
ACKNOWLEDGEMENTS The Italian MURST (Progetto di Rilevante Interesse Nazionale, Cofin. 2000) and the Brazilian FAPESP (Fundagfio de Amparo ~ Pesquisa no Estado de S~o Paulo) are acknowledged.
422 REFERENCES
1. M. Estermarm, L.B. McCuster, C. Baerlocher, A. Merrouche, H. Kessler, Nature 352 (1991) 320. 2. M.E.Davis, C. Saldarriaga, C. Montes, J. Garces, C. Crowder, Nature 331 (1998) 698. 3. T. Kimura, Y. Sugahara, K. Kuroda, Micropor. Mesopor. Mater. 22 (1998) 115. 4. N.C. Masson, H. O. Pastore, Micropor. Mesopor. Mater. 44 (2001) 173. 5. E.C. Oliveira, N. C. Masson, A. J. S. Mascarenhas, H. O. Pastore, submitted to Colloids and Surfaces. 6. M.L. Pefia, V. Dellarocca, F. Rey, A. Corma, S. Coluccia, L. Marchese, Microporous Mesoporous Mat., 44-45 (2001) 345. 7. L. Marchese, E. Gianotti, T. Maschmeyer, G. Martra, S. Coluccia, J.M. Thomas, I1 Nuovo Cimento 19D (1997) 1707 8. E. Gianotti, V. Dellarocca, G. Martra, L. Marchese, S. Coluccia, T. Maschmeyer, in preparation.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
423
Organic-inorganic phase interaction in A1SBA-15 m e s o p o r o u s solids b y double r e s o n a n c e N M R spectroscopy Elias Haddad a, Jean-Baptiste d'Espinose b, Andrei Nossov a, Flavien Guenneau a, Claude Mignon a, and Antoine Grdron a* Laboratoire Syst6mes Interfaciaux h l'Echelle Nanom6trique (SIEN). CNRS-FRE 2312 aUniversit6 Pierre et Marie Curie, case courrier 196, 4 place Jussieu, 75252 Paris Cedex 05, France. Email :
[email protected] b Physique Quantique, ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France
Aluminum-incorporated SBA-15 mesoporous materials have been obtained by direct synthesis. The surfactant- aluminosilicate interaction during synthesis was studied by double resonance NMR and confronted with the structural properties of the materials obtained after calcinations. Continuous-flow laser-polarized 129XeNMR spectroscopy was applied for the first time to explore the porosity of the A1SBA-15 mesoporous molecular sieves. TRAPDOR experiments firmly established a strong interaction between segments of the PEO block of the surfactant with the silica-alumina framework. ~H Dipolar Dephasing revealed that the amount of segments rigidified by this interaction increased with the maturation time. The increased rigidity of the surfactant is to be linked with the increased mesoscopic ordering during maturation, resulting in the higher mesoporous surface obtained after calcinations. The invariability of the TRAPDOR effect proved that the strength of the interaction, that is the degree of interpenetration of the organic/inorganic phases remained the same irrespective of maturation time. Together with the dramatic decrease of the microporous volume with maturation time, this established that the origin of the microporosity of A1SBA- 15 is to be found in the incomplete hydrolysis of the TEOS precursor itself rather than in the incomplete PEOaluminosilicate phase separation. 1. INTRODUCTION We have synthesized acid A1SBA-15 mesoporous solids with regular channels and very high thermal and hydrothermal stability [ 1]. Incorporation of A1 was established by HETCOR double resonance l H - 27A1 NMR [2]. A1SBA-15 materials retain the hexagonal order and physical properties of purely siliceous SBA-15. They present higher thermal stability and catalytic activity in cumene cracking reaction than A1MCM-41 solids. To better understand the origin of these improved properties, textural results from N: porosity measurements are confronted with molecular scale double resonance MAS NMR results in order to discuss the incorporation of A1 and the interpenetration of the organic/inorganic phases during synthesis. Indeed, recent publications have evidenced the significant occurrence of a microporous "corona" around the internal surface of the mesopores in SBA-15 [3,4]. Considering that micropores result from the calcination of an incompletely hydrolyzed silicate precursor, it is of primary importance, if one wants to be able to control the extent of the microporosity, to
424 understand at the molecular scale why the silicate network did not fully condense: Is it because of the interpenetration of the hydrophilic part of the surfactant with the forming inorganic phase [3]? Or is it because the organometallic TEOS precursor was not fully mineralized prior to calcination? To address this question, samples of different hydrolysis levels were prepared by varying the maturation time. It was then possible to investigate phase separation and ordering in the parent material by NMR double resonance between the organic protons and the aluminum of the solid, the results were then related to the structural properties of the calcined final mesoporous A1SBA-15. This protocol was deemed preferable to comparing samples prepared by varying the temperature, as this would have affect simultaneously the hydrolysis kinetics and the hydrophilicity of the PEO fragments. 2. EXPERIMENTAL
2.1 Materials and synthesis Al-containing SBA-15 mesoporous solids were synthesized by using tetraethyl orthosilicate (TEOS), aluminum tri-tert-butoxide, and triblock poly(ethylene oxide)poly(propylene oxide)-poly(ethylene oxide) (EO20PO70EO20) Pluronic 123 copolymers. The synthesis conditions were described elsewhere [1]. After being stirred for 3 hours, the gel solution was transferred into a Teflon bottle and heated at 100 ~ for different reaction or maturation times 0, 16 and 48 h. The solid products were filtered (parent composites) and finally calcined (calcined samples) in air flow (9 L h -l) at 823 K for 4 h with a heating rate of 24 K h". In what follows, the samples are denoted AISBA-15. 2.3 Hyperpolarized 129XeNMR 129Xe NMR spectra were collected on a Bruker AMX 300 spectrometer operating at 83.03 MHz. Hyperpolarized (HP) xenon was produced in the optical pumping cell in the fringe field of the spectrometer magnet. The gas mixture containing 800 torr of He and 40 torr of Xe polarized to ca. 1% was delivered at 70 cc/min flow rate to the sample via plastic tubing. 256 FIDs were accumulated with 10~ts (re/2) pulses and 5s delays.
2.2 MAS-NMR Magic angle spinning nuclear magnetic resonance (MAS NMR) experiments were performed on a Bruker ASX500 spectrometer at 11.7 T. 27A1 one-pulse experiments were performed at 14 kHz with a selective pulse (
425 isotherms by the BET method, and pore size distributions from the desorption isotherms by the BJH method. To assess the presence of micropores, we used the modified as-plot analysis [6]. rd2
n
"
1H
~
"
~-.
27A1
I
Figure 1" Scheme of the TRAPDOR pulse sequence.
I
3. RESULTS AND DISCUSSION 3.1. Structural properties The structural properties of A1SBA-15 samples have been analyzed in earlier studies by Synchrotron XRD and transmission electron microscopy [7,8]. The five well-resolved XRD peaks associated with 2D p6mm hexagonal symmetry and the well ordered hexagonal arrays of mesoporous channels from TEM images indicate the high structural quality of these materials. The nitrogen adsorption-desorption isotherms of the A1SBA-15 studied samples have been used to obtain information about their mesoporosity. These isotherms, illustrated in figure 2, are similar to those reported earlier [8]. As the maturation time decreased, the N2 adsorption isotherms preserved the same shape with a significant decrease of the hysteresis loop towards lower P/P0 values. This led to smaller surface area and mean pore diameter. The pore size distributions (PSD) for the studied samples are shown in figure 3. They indicated that the mesoporous channels were very regular with a narrow gaussian distribution centered at 4.5, 7.0, and 8.2 nm respectively for samples at 0, 16 and 48 h. of maturation time. The textural characteristics of the A1SBA-15 samples are listed in table 1. 48 h
0,10'
48 h
0,08 8OO.
,<
"T,
oE
12I--
~
16h 0,06
600-
-'o> = 4oo-
//~ r a t i o n
~
/
~.,Q
"~ ._ 0,04
/
0,02
200
0
,
o,o
,
0,00
o:~
o',4
o',~
no matu
o:~
relative pressure (P/Po)
,:o
Figure 2" N2 adsorption-desorption isotherms for A1SBA-15 samples for different maturation times.
0
2'0
4'0 ' 6'0 ' 8'0 '1;0",~0
pore diameter (A)
.
.
1~0
Figure 3: BJH pore size distribution of A1SBA-15 samples for different maturation times.
426 The adsorption-desorption isotherms have been further used to evidence an additional microporosity. The specific mesoporous volume and the micropore volume were calculated using modified Gts-plots. The micropore volume evolved inversely to the maturation time. Table 1. Structural-textural parameters of the A1SBA- 15 samples studied. maturation time
a) b)
SteEr (mZ/g) '~
Pore volume (cm3/g)
Pore diameter (nm)
micropore volume b Vmi (cm3/g)
Mesopore volume b
Vme(Cm3/g)
none
525
0.60
4.5
0.045
0.536
16 h
648
0.96
7.0
0.010
0.696
48 h 908 1.41 8.3 0.002 0.972 total pore volume estimated from the amount adsorbed at P/P0 = 0.99 micropore and mesopore volumes evaluated from the modified c~-plot method.
3.2. Probing the evolution of porosity during maturation time by hyperpolarized (HP) 129XeNMR Fig 4 shows the 129Xe NMR spectrtra of HP xenon adsorbed on the A1SBA-15 samples. Both spectra exhibit the lines at 0 ppm from xenon gas in the voids between the particles of the solids and the lines shifted to lower field due to xenon, adsorbed in the pores. In the case of the nonmatured sample the latter part of the spectrum consists of two lines: at 86.6 ppm and a broader signal at ca. 65 ppm. Maturation during 48 hours leads to the spectrum, comprised of two lines: at 65.6 ppm and a low intensity broad signal, centered at ca. 60 ppm. Chemical shift of xenon in mesoporous materials depends on the pore size, the smaller value of the chemical shift corresponding to the bigger pore diameter [9]. Taking this into account, the 129Xe NMR data are in a good qualitative agreement with the pore size distribution observed for the samples during maturation. The higher value of the xenon chemical shift and broader lines observed in the spectrum of the nonmatured sample corresponds well to its smaller pore diameter and broader size distribution (Fig.). The results of more detailed study of xenon adsorption in this system will be published elsewhere. 3.3. Rigidification of the template: One-pulse 1H NMR and dipolar dephasing of parent
AISBA-15 While the 27A1 spectra were identical irrespective of maturation time, the IH resonances broadened during maturation (Figure 5). This suggested an increased homonuclear interaction that was indeed confirmed by IH Dipolar Dephasing experiments (Figure 6). Analysis of the time dependence of the dipolar dephasing of the CH and CH2 protons of the surfaetant revealed a common bi-exponential decay with two time constants (T2) of about 10 ms and 0.7 ms. This reflected different segmental mobility for the PO and EO backbone. More surprisingly, the relative weight of the faster relaxing component of the 3.5 ppm resonance increases from 60% to 75% for the samples with no maturation and with 48 h maturation respectively. Everything being equal, this meant that a larger part of the backbone became rigid as maturation proceeded, thereby increasing homodipolar interaction.
427
____j ~,~
'"I
'
I
'
I
'
120
I
tion
'
80
I
I
l
'
40
I
'
I
'
I
;
0
Figure 4: 129MeNMR spectra of liP Xe adsorbed on the nonmatured (lower trace) and matured for 48h (upper trace) AISBA-I 5 samples.
I
-40
(ppm)
70-
8 maturation
....
60
~A
time:
--Oh 48h
50
CHa Iit PO ]1
CH,andCH~
9: nomaturation
\It
" : ~"
4o 3o 20
>,
._
4
3
2
1
ppm 1H
lO
e-
~e "--
0
...... 0,0
'
01~
,i0
i
,~
2,'0
,
;~
,
3,'0
1: (ms)
Figure 5: IH one-pulse spectra of AISBA- 15 PEO-PPO-PEO parent composites synthesized with different maturation times.
Figure 6: ]H spin echo amplitude So as a function of dipolar dephasing time of AISBA- 15 parent composites synthesized with different maturation times. Amplitudes were obtained from integration of the two resolved lines of the ~H spectra (Figure 5). Best biexponential fits of the T2 decay are shown (see text).
3.4 Molecular scale interaction with the template: 1H{27Al} TRAPDOR of parent AISBA15 As the template rigidified, the mesoporous surface area of the forming aluminosilicate increased. The rigidification therefore reflected the larger interaction of the PEO blocks with the larger aluminosilicate mesoporous surface. However, this could also be due to an increased interpenetration between the organic polymer and the aluminosilicate network as the I+ cations
428 condense around the S+ chains interacting through the S+CI-I+ mechanism [I0]. This would translate eventually also in a larger organic-inorganic interaction but necessarily in a stronger interaction as the PEO fragments become imbedded within the forming aluminosilicate. To evidence such an increase in the strength of the interaction, the TRAPDOR effect on the CH and Ctt 2 protons of the surfactant due to dephasing by the 27A1 of the aluminosilicate was quantitatively followed for samples at different maturation time. It appeared that the TRAPDOR effect and thus the strength of the organic-inorganic interaction remained the same irrespective of maturation time and of the microporous volume of the final A1SBA-15 material (Figure not shown). This last result ruled out the possibility that the microporous "corona" in SBA- 15 material originates in the penetration of the hydrophilic PEO fragments of the template within the forming silicate framework.
4. CONCLUSION The occurrence of a dipolar interaction of constant strength between the CH and CH2 protons of the surfactant and the 27A1 of the aluminosilicate in the composite parent material irrespective of the microporous volume of the final A1SBA-15 solid precluded the possibility that the microporosity of SBA-15 originates from the occlusion of part of the template in the oxide. The increased rigidity of the PPO-PEO-PPO backbone evidenced by the increasing fraction of protons in strong homonuclear interaction must be attributed to the forming mesoporous surface and thus to a larger amount of PPO fragments interacting with it. Actually, the microporous volume decreased drastically with maturation time as hydrolysis proceeded therefore suggesting that it resulted instead from incomplete condensation due to the TEOS not being fully hydrolyzed. In other words, the appearance of the microporous "corona" is not related to the surfactant behavior but is only due to the hydrolysis kinetics of TEOS under acidic conditions.
REFERENCES
1. Y. Yue, A. Grdron, J.-L. Bonardet, N. Melosh, J.-B. d'Espinose, J. Fraissard, Chem. Commun. (1999) 1967. 2. J.B. d'Espinose de la Caillerie, Y.-H. Yue, E. Haddad, A. Grdron, Stud. Surf. Sci. Catal. 135 (2001) 1321. 3. M. Kruk, M. Jaroniec, C. H. Ko, R. Ryoo, Chem. Mater. 12 (2000) 1961. 4. M. Imprror-Clerc, P. Davidson, A. Davidson, J. Am. Chem. Soc. 122 (2000) 11925. 5. C. Grey, A. J. Vega, J. Am. Chem. Soc. ll7 (1995)8232. 6. V.B. Fenelonov, V.N. Romarmikov, A.Y. Derevyankin, Microporous Mesoporous Mater. 28 (1999) 57. 7. Y.-H. Yue, A. Grdron, J.-L. Bonardet, J.B. d'Espinose, J. Fraissard, Stud. Surf. Sci. Catal. 130 (2000) 3035. 8 Y.-H. Yue, A. Grdron, J.-L. Bonardet, J. B. d'Espinose, N. Melosh, J. Fraissard, Stud. Surf. Sci. Catal. 129 (2000) 209. 9 V.V. Terskikh, I.L. Mudrakovskii and V.M Mastikhin., J. Chem. Soc. Faraday Trans. 89, (1993), 4239 10. D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024.
Studies in Surtace Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
429
Adsorption of nitrogen on organized mesoporous alumina Jiri Cejka, Lenka Veselfi, Jiri Rathousk~ and Arnogt Zukal J. Heyrovsk3~ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolej~kova 3, CZ- 182 23 Prague 8, Czech Republic
Organized mesoporous aluminas were synthesized from aluminum sec-butoxide and lauric or stearic acid as structure-directing agent and characterized by adsorption of nitrogen and X-ray powder diffraction. Adsorption data analysis based on the comparison of adsorption isotherm of nitrogen at -196 ~ on mesoporous aluminas with the standard adsorption isotherm of nitrogen on a reference non-porous solid seems to be the promising approach to characterize these not completely regular materials. To obtain reliable standard nitrogen adsorption data for characterization of alumina porous structure, nitrogen adsorption isotherms on Degussa Aluminiumoxid C and a-alumina were measured at-196 ~ Standard nitrogen adsorption data were listed for 41 points in the range of relative pressures from 0.001 to 0.95. The applicability of these data was tested on organized mesoporous aluminas of various structural parameters using the comparison plot method. In contrast to c~-alumina the standard data on Degussa Aluminiumoxid C were proved to be suitable for the comparative analysis of adsorption isotherms. These data were transformed to the statistical film thickness curve and further used to calculate of Barrett-Joyner-Halenda pore size distributions.
1. INTRODUCTION Successful synthesis of mesoporous molecular sieves in 1992 [1,2] opened a new area in the synthesis, characterization and also application of molecular sieves. During the last 10 years a large number of papers have appeared describing their synthesis, characterization and applications [3]. This success has prompted the efforts to extend the synthetic strategy used for silica-based materials to the preparation of non-silica organized mesoporous oxides. As a result, a number of mesoporous materials based on aluminophosphates, zirconium oxide, vanadium oxide and also alumina was synthesized. In the case of the synthesis of organized mesoporous aluminas (OMA), the typical procedures used for the synthesis of silica-based materials have usually failed. However, Bagshaw and Pinnavaia employed the controlled hydrolysis of aluminum sec-butoxide in organic solvents and non-ionic polyethylene oxide surfactants as structure directing agents [4]. All mesoporous aluminas synthesized according to this procedure exhibited a wormlike channel motif possessing surface areas and pore sizes in the range from 420 to 535 ma/g and from 2.4 to 4.7 nm, respectively. Davis et al. synthesized mesoporous aluminas with surface areas up to 700 mZ/g in low-molecular-weight alcoholic solvents using aluminum sec-butoxide as the inorganic precursor and different carboxylic acids as structure-directing agents [5]. Pore size distributions of these samples were centered at pore diameter of about 2 nm, independent of
430
the hydrocarbon chain length of the carboxylic acid used. The larger pore diameters were reached due to subsequent heat treatment at 600 - 800 ~ which caused the coalescing of the pores. Cabrera et al. used triethanolamine in combination with hexadecyl trimethylammonium bromide for the synthesis of OMA with the pore size in the range of 3.3 - 6.0 nm and the surface area of 250 - 340 m2/g [6]. Valange et al. reported a variety of synthesis pathways leading to OMA based on hydrolysis and condensation of monomeric aluminum cations and/or oligomeric cationic species [7]. All materials were obtained in aqueous media in the presence of one-component (anionic or non-ionic) or mixed (cationic-anionic) surfactant micelles. The mean pore diameter of aluminas prepared varied between 0.8 and 6 nm, the surface area of these materials ranged between 300 and 820 m2/g. Zhu et al. prepared aluminas from aqueous solution of inorganic aluminum salt and polyethylene oxide surfactant Tergitol 15-S-5 [8]. The condensation of the aluminum species around surfactant micelles was induced by urea. The surface areas of these mesoporous aluminas ranged from 250 to 350 m2/g. OMAs of surface areas 480 - 565 m2/g and pore sizes 4.2 - 6.0 nm were prepared by Gonz/des-Pefia et al [9] using aluminium sec-butoxide as aluminium source and sec-butanol as solvent. As structure-directing agents Tergitol 15-S-9 and Triton X-114 were used. The variety of described procedures indicates that the synthesis of OMA represents more complex problem compared to the preparation of organized mesoporous silica. The final structure of pure alumina samples is influenced not only by the synthesis conditions but also by the conditions of their heat treatment. The pore ordering of calcined OMA was always worse in comparison with that of organized mesoporous silica. The transmission electron microscopy images of OMA show a partial pore arrangement [4]. In agreement with this, X-ray diffraction patterns display usually one reflection only. Although mesoporous aluminas exhibit only one reflection, based on nitrogen adsorption isotherms it is evident that these materials are not layered but forming uniform pores. Some comparison with e.g. silica samples called KIT-1 can be expected as these molecular sieves exhibit up to three broad diffraction peaks which indicate a short-range structural ordering with very uniform pore sizes [10]. Due to the mentioned features of OMA, gasadsorption appears to be very important for characterization of its porous structure. Adsorption isotherms of nitrogen at -195 ~ are most frequently used to characterize the porous structure of solid materials. Although methods of adsorption isotherm analysis are well-established, accurate and reliable evaluation of structure properties of porous solids is still a matter of discussion [l 1]. One of the promising approaches to adsorption data analysis is based on the comparison of the adsorption isotherm a~/po) on porous solid under study with the adsorption isotherm ar~KP/Po) on an appropriate reference solid, on which adsorption occurs identically as on a flat open surface [12]. In comparison plots the adsorption isotherm a(p/po) is transformed to a function a(arer) of the amount arer adsorbed on a reference solid at the same relative pressure. Based on this comparison, the main characteristics of porous structures can be evaluated [ 13]. Usually, one chooses a macroporous reference adsorbent, which has surface properties similar to those of the solid under study. With respect to the complexity of numerous forms of aluminum hydroxides, oxide-hydroxides and oxides, the reference alumina adsorbent must be carefully chosen. Since as-synthesized OMAs are mostly calcined at the temperature of 400 450 ~ it appears that the surface chemistry of reference material should be similar to that of y-type alumina prepared by the heat treatment of Al(OH)3 or A1OOH at intermediate temperatures. Standard isotherm data determined on the non-porous Degussa Aluminiumoxid
431
C have been found suitable for the analysis of isotherms on porous 7-aluminas [14]. Therefore, it can be supposed that these data will be suitable also for the OMA. Since it is of interest to investigate the changes of OMA porous structure due to the heat treatment, the high-temperature non-porous ~-A1203 cannot be excluded as a reference adsorbent. The aim of this contribution was to obtain reliable standard nitrogen adsorption data for characterization of mesoporous aluminas and to test them on materials synthesized using the modified preparation route by Davis et al. [5]. This route yields OMAs that do not contain micropores. In the present work due to adjusting the reaction mixture composition or subsequent heat treatment at temperatures varying from 400 to 800 ~ materials were prepared, which are characterized by mean pore diameters in the range from 2.5 to 5.7 nm and surface areas reaching up to 758 m2/g.
2. EXPERIMENTAL 2.1 Materials As reference materials the Aluminiumoxid C (denoted as DC, SBET-- 128.0 m2/g) obtained from Degussa and a-alumina (denoted as AA, SBET = 6.77 m2/g) prepared by high temperature treatment (1200 ~ 10 h) of tablets prepared from pseudoboehmite were used. The sample OMA/1 was synthesized as follows: 3.4 g of lauric acid (LA, Aldrich) was dissolved in 100 mL of 1-propanol (Fluka). After the addition of 3.1 ml of water the solution was stirred for 30 min. 13.7 g of aluminum sec-butoxide (Aldrich) was then added and the reaction mixture was stirred for another 20 min. The molar composition of the reaction mixture was 1 AI(OCH(CH3)C2Hs)3 : 3.1 H20 : 0.30 LA : 24 C3H7OH. The gel prepared was aged at 100 ~ for 50 h in a Teflon-lined autoclave under static conditions. Finally, after cooling the autoclave, the product was recovered by filtration, washed with ethanol and dried at 50 ~ overnight. The dried sample was calcined in nitrogen at 410 ~ for 2 hours and then in air at 420 ~ for 9 hours. The samples OMA/2 and OMA/3 were prepared by an additional calcination of the sample OMA/1 in air at 600 and 800 ~ respectively, for 6 hours. The molar composition of the reaction mixture for samples OMA/4 and OMA/5 was identical with that of the sample OMA/1, stearic acid (SA, Aldrich) being substituted for lauric acid. Both samples differ in calcination conditions. The sample OMA/4 was calcined in nitrogen at 410 ~ for 2 hours and then in air at 420 ~ for 9 hours. The sample OMA/5 was calcined in nitrogen at 410 ~ for 2 hours, and then in air at 420 ~ for 19 hours. The sample OMA/6 was obtained from the reaction mixture of the molar composition 1 AI(OCH(CH3)C2Hs)3 : 3.1 H20 : 0.30 SA : 36 C3H7OH. Calcination of the sample OMA/6 was performed similarly to the sample OMA/4. 2.2 Methods Measurements of nitrogen isotherms at -196 ~ on reference aluminas DC and AA were performed on an ASAP 2010 (Micromeritics) and modified Accusorb 2100E (Micromeritics) volumetri~z adsorption instruments. Nitrogen isotherms on OMA were measured with the ASAP 20 l0 only. The Accusorb 2100E was equipped with pressure transducers covering the 133 Pa, 5.3 kPa, and 133 kPa ranges; the diffusion pump of its vacuum system enabled to reach the ultimate vacuum of l0 -4 Pa. The ASAP 2010 was equipped with a 133 kPa transducer. Before the measurements, all samples were degassed at 300 ~ for at least 24 h.
432 X-ray powder diffractograms were recorded using a Siemens D5005 instrument operating in the Bragg-Brentano geometry arrangement with CuKa radiation.
3. RESULTS AND DISCUSSION The standard nitrogen adsorption data (adsorbed amount divided by the surface area) on reference aluminas DC and AA are listed in Table 1. These data and the standard isotherm reported earlier by Lippens et aL [ 15] are shown in Fig. 1.
40E
/
30-
0
E
2010 0
I
i'
i
i
0,0 0,2 0,4 0,6 0,8 p/po
1
Figure 1. Nitrogen adsorption isotherms at -196 ~ on Degussa Aluminiumoxid C (o) and a-alumina (+) together with the isotherm reported by Lippens et al. [15]
(•
It can be seen that the isotherms on aluminas DC and AA agree very well with each other at relative pressures higher than 0.25 where the multilayer adsorption takes place. At lower relative pressures, where the monolayer adsorption occurs, the adsorption of nitrogen on a-alumina is a little smaller, obviously due to the high temperature treatment of this material, which leads to the removal of adsorption centers with higher energy of adsorption. The nitrogen isotherm published by Lippens et al. was obtained by averaging adsorption data on several selected samples of aluminum hydroxides and oxides. In the region of relative pressures 0.1 - 0.4 this isotherm agrees well with isotherm on the alumina DC. The differences between both isotherms at higher relative pressures are most probably caused by the capillary condensation of nitrogen in mesopores, which were present in materials studied by Lippens et al. [ 15].
Fig. 2 presents nitrogen isotherms on OMAs prepared with lauric acid as the structure director. The isotherms on samples OMA/I, OMA/2 and OMA/3 are characterized by a relatively steep increase in the adsorbed amount starting at p/po of 0.4 - 0.6. Due to the absence of pores larger than 8 nm, all these isotherms end with nearly horizontal plateau. The adsorption isotherms on samples OMA/1 and OMA/2, processed by the method of comparison' plots up to the relative pressure of 0.95, are shown in Fig. 3. As the reference data, the isotherm on alumina DC was used. Both the comparison plots are characterized by two linear parts. The first one corresponds to the formation of a monolayer and the beginning of multilayer adsorption; the linear fit goes through the origin and its slope gives the total surface area STOT: a = STOTaref. (1)
433 The subsequent steep increase is caused by the capillary condensation of nitrogen in mesopores. The second linear part of the comparison plots corresponds to the plateau of the isotherm. As on this plateau the adsorption and desorption branches are identical, it can be supposed that the multilayer adsorption takes place here. For this reason, the comparison plots corresponding to the plateau can be approximated by the straight line: a
=
SEXT aref +
(2)
VMESO/V,
where SEXT is the extemal surface area of particles, which remains free after filling up the mesopores, VMESOis the volume of mesopores and v is the molar volume of liquid nitrogen at -196 ~
15-
I!'
' 9 10
"7
10
o
E E
E
~r /
...-"
v
5
0 ~-
0,0
'
0,2
,
0,4
,
0,6
,
0,8
1,0
P/Po Figure 2. Nitrogen adsorption isotherms at -196 ~ on samples OMA/1 ( ) , OMA/2 (v) and OMA/3 (a). The solid points denote desorption.
0
d,ze ...... ...... ,.... ..,.::;." ..
0
2'0
3'0
4'0
aref (!amoI m 2)
Figure 3. Comparison plots for samples OMA/1 ( ) and OMA/2 (v) in the region of relative pressures lower than 0.95. The solid points denote desorption.
The application of reference data on Gt-alumina AA in the comparative analysis of isotherms on OMA has proved inappropriate. This fact illustrates the comparison plot for the sample OMA/3, which was additionally treated at the highest temperature of 800 ~ Fig. 4 presents the first linear part of the comparison plots for this sample using either alumina DC or alumina AA as the reference adsorbents. It is obvious that only the use of the reference data on alumina DC provided the correct comparison plot going through the origin.
434
Table 1. Standard data for the adsorption of nitrogen at 77 K on Degussa Aluminiumoxid C (DC) and (z-alumina (AA) P/Po DC AA P/Po DC AA (lamol m "2) (~tmol m "2) (~mol m "2) (~tmol m "2) 0.001 4.642 2.550 0.300 14.251 14.283 0.005 6.264 4.146 0.320 14.602 14.637 0.010 7.096 4.953 0.340 14.953 14.937 0.020 8.039 6.201 0.360 15.304 15.290 0.030 8.648 6.985 0.380 15.655 15.617 0.040 9.108 7.652 0.400 16.006 15.997 0.050 9.481 8.083 0.420 16.357 16.378 0.060 9.797 8.464 0.440 16.708 16.705 0.070 10.073 8.827 0.460 17.174 17.086 0.080 10.318 9.111 0.480 17.513 17.439 0.090 10.539 9.462 0.500 17.864 17.820 0.100 10.741 9.810 0.550 18.805 18.881 0.120 11.091 10.393 0.600 19.859 19.997 0.140 11.442 10.883 0.650 21.068 21.276 0.160 11.793 11.400 0.700 22.493 22.745 0.180 12.144 11.862 0.750 24.233 24.404 0.200 12.495 12.352 0.800 26.465 26.445 0.220 12.846 12.787 0.850 29.540 29.438 0.240 13.198 13.168 0.900 34.330 34.308 0.260 13.549 13.512 0.950 44.063 42.143 0.280 13.900 13.930
0,4
5-
-
A
`7
B
E ,-- 0,3-
"7
"7 4tm
171
O
E 3E v 2
~E 0,2o a >
"o
":'""'/""""/""
0
5
10 15 are f (~tmol m2)
I
20
Figure 4. Comparison plots for the sample OMA/3 using standard adsorption data obtained on Degussa Aluminiumoxid C (A) and ~x-alumina (+).
0,10,0
4
6
D (nm)
I
8
Figure 5. Mesopore size distributions for samples OMA/1 (A), OMA/2 (B) and OMA/3 (C).
435 The reference isotherm obtained on alumina DC transformed to statistical film thickness curve was also used for calculations of mesopore size distributions using the Barrett-JoynerHalenda (BJH) method [ 16]. The resulting distribution curves for samples OMA/1, OMA/2 and OMA/3 are shown in Fig. 5. Table 2. Material parameters
Sample OMA/1 OMA/2 OMA/3 OMA/4 OMA/5 OMA/6
SBET
STOT
SEXT
VMESO
VBJH
DBJH
d
6
(m 2 g-l)
(m 2 g-l)
(m 2 g-i)
(cm 3 g-l)
(cm 3 g-~)
(nm)
(nm)
(nm)
489 313 223 758 397 707
475 303 222 769 382 696
2.0 1.9 2.0 36.1 25.2 5.9
0.527 0.477 0.409 0.681 0.633 0.588
0.566 0.511 0.422 0.722 0.671 (0.521)
3.3 4.4 5.1 3.7 4.7 (2.5)
5.0 7.0 8.8
1.7 2.6 3.7
The parameters of aluminas obtained by means of BET, comparison plots and BJH methods are summarized in Table 2. The values of SmT and VMESOfrom comparison plots are in satisfactory agreement with values of SBETand VBJHobtained using BET and BJH methods.
20_crrgz-~l "7 15-
O
i'
E 10t~
o 0,0
1
0,2
i
0,4
I
0,6
i
0,8
1
P/Po Figure 6. Nitrogen adsorption isotherms at - 196 ~ on samples OMA/4 (), OMA/5(v) and OMA/6 (zx). The solid points denote desorption.
In the reaction mixture of samples OMA/4, OMA/5 and OMA/6 lauric acid was replaced with stearic acid, which enabled to obtain aluminas with substantially larger surface areas exceeding 700 m2/g (Table 2, samples OMA/4 and OMA/6). The pore size of sample OMA/6 was assessed by means BJH method; as the Kelvin equation is not strictly valid for these relative fine mesopores, the pore diameter of sample OMA/6 is given in Table 2 in parenthesis. The conclusions, which can be drawn from the results obtained on the samples OMA/4, OMA/5 and OMA/6, are fully identical with those, which follow from results on samples OMA/1, OMA/2 and OMA/3. Therefore, the applicability of the standard adsorption data on the alumina DC was successfully tested on all the OMA samples.
The influence of the heat treatment of the OMA on its structure was investigated also by X-ray powder diffraction. All the diffractograms of samples OMA/1, OMA/2 and OMA/3 exhibited only one diffraction peak at very low 20. With increasing temperature of the heat treatment the position of the diffraction peak is shifted towards lower 20, which indicates some re-organization of the alumina structure connected to an increase in the pore size caused
436 by their coalescing [5]. The correlation length d corresponding to the maximum of the diffraction peak and pore wall thickness 6, which was determined by subtracting the pore diameter from the correlation length, are given in Table 2. The structure data obtained from nitrogen adsorption confirmed that the higher the calcination temperature, the larger the correlation distance and consequently the pore size. Simultaneously, the pore size increases from 3.3 to 5.1 nm. This increase in the pore size is accompanied by the increase in the pore wall thickness, which agrees with the coalescing mechanism. 4. CONCLUSIONS The standard nitrogen adsorption data on non-porous Degussa Aluminiumoxid C and a-alumina were obtained at -196 ~ in the range of relative pressures from 0.001 to 0.95. The applicability of these data was tested using the comparison plot method. Although some OMA samples were calcined at relatively high temperatures, only the standard data on Degussa Aluminiumoxid C have proved to be suitable for the analysis of nitrogen isotherms on OMA. ACKNOWLEDGEMENTS
This investigation was supported by the Ministry for Education, Youth and Sport of the Czech Republic (ME404) and by NATO (SFP-974217). REFERENCES
1. 2.
3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
C.T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. J.S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, 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. J.Y. Ying, C. P. Mehnert and M. S. Wong, Angew. Chem. Int. Ed., 38 (1999) 56. S.A. Bagshaw and T. J. Pinnavaia, Angew. Chem. Int. Ed. Engl., 35 (1996) 1102. F. Vaudry, S. Khodabandeh and M. E. Davis, Chem. Mater., 8 (1996) 1451. S. Cabrera, J. E1 Haskouri, J. Alamo, A. Beltrfin, D. Beltr~in, S. Mendioroz, M. Dolores Marcos and P. Amor6s, Adv. Mater., 11 (1999) 379. S. Valange, J.-L. Guth, F. Kolenda, S. Lacombe and Z. Gabelica, Microporous Mesoporous Mater., 35-36 (2000) 597. H.Y. Zhu, P. Cool, G. Q. Lu and E.F. Vansant, Stud. Surf. Sci. Catal., 135 (2001) 253. V. Gonz~les-Pefia, C. M~rquez-Alvarez, E. Sastre and J. P&ez-Pariente, Stud. Surf. Sci. Catal., 135 (2001) 204. R. Ryoo, J.M. Kim, C.H. Ko and C.H. Shin, J. Phys. Chem. 100 (1996) 17718. M. Jaroniec, M. Kruk and J. P. Olivier, Langmuir, 15 (1999) 5410. S.J. Gregg and K. S. W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982, p. 90. J. Rathousk~, G. Schulz-Ekloff and A. Zukal, Micropor. Mater. 6 (1996) 385. F. Rouquerol, J. Rouquerol and K. Sing, Adsorption by Powders and Porous Solids, Academic Press, London, 1999, p. 174. B.C. Lippens, B. G. Linsen and J. H. de Boer, J. Catal., 3 (1964) 32. J.H. de Boer, B. G. Linsen and Th. J. Osinga, J. Catal., 4 (1965) 643.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
437
The use o f ordered m e s o p o r o u s materials for i m p r o v i n g the m e s o p o r e size analysis: Current state and future Mietek Jaroniec, a Michal Kruk a and Abdelhamid Sayari b a
Department of Chemistry, Kent State University, Kent, Ohio 44242, USA
b Centre for Catalytic Research and Innovation, Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
In 1997 a simple approach to an accurate calculation of mesopore size distributions was proposed on the basis of model gas adsorption isotherms for ordered mesoporous materials (OMMs), whose pore diameters were evaluated independently from adsorption pore size analysis. This approach was originally developed for nitrogen adsorption at 77 K on silicas with cylindrical pores, and recently extended to argon adsorption at 77 and 87 K, as well as to adsorption on materials with hydrophobic surfaces. The method to calculate the pore size distributions (PSDs) from nitrogen data at 77 K has already attracted much interest and has been implemented by several researchers working in the field of OMMs. Herein, the current state and future perspectives in the development of accurate methods for PSD calculation using OMMs as model adsorbents are discussed. The development of these methods requires the availability of OMMs with as wide range of pore diameters as possible, and therefore the progress and challenges in the synthesis of such materials are overviewed. Recent advances in the development of reliable and independent methods for the OMM pore size assessment, necessary for elaboration of the aforementioned methods for the PSD calculation, are reviewed. The current status and prospects for the development of OMM-calibrated methods for the pore size analysis of different kinds of mesoporous materials are discussed.
1. INTRODUCTION The discovery of ordered mesoporous materials (OMMs) [1] has had a tremendous influence on the field of gas adsorption characterization of porous materials. Gas adsorption has become a key tool for characterization of OMMs [2,3], because it is a crucial method for calculation of mesopore size distributions. On the other hand, OMMs are first model mesoporous solids with pores of uniform size and well-defined shape, so they are suitable for evaluation of the applicability and accuracy of gas adsorption methods for characterization of porous solids [2-8]. The use of OMMs as model adsorbents allowed one to convincingly demonstrate that gas adsorption data actually provide an abundance of.information about structural properties of mesoporous materials. However, the studies of OMMs also strongly suggested that many currently used methods for determination of the structural parameters, such as the pore size distribution (PSD) and the specific surface area, from gas adsorption
438 data are highly inaccurate or sometimes even inherently inapplicable [2-8]. Therefore, there is a strong incentive to develop new methods for gas adsorption data analysis or, if possible, to modify the classical methods, in order to be able to fully realize the potential of gas adsorption for characterization of porous materials. To this end, the discovery of OMMs not only allowed us to find weaknesses in the hitherto known methods of adsorption data analysis, but also provided a way for refinement of some of these methods or for development of new ones. This is because OMMs can actually be used to generate model gas adsorption isotherms for mesopores of various sizes and shapes. In the case, when the pore size and geometry of the model OMMs can be accurately determined, and the degree of their structural ordering is high, the model gas adsorption isotherms generated in this way are benchmark results that can be used to assess the suitability of other methods for generating model gas adsorption data. These other methods include those based on advanced computational approaches, such as nonlocal density functional theory (NL DFT) [4] and computer simulations [9]. These methods require a largely arbitrary assignment of branches of computational adsorption-desorption hysteresis loops to the experimentally observed branches of the hysteresis loops, in addition to the necessity of using proper interaction parameters, models for the adsorbent and gas molecule structures, and so forth. The availability of experimental adsorption data for model mesoporous solids would certainly facilitate the refinement of these highly promising, advanced computational approaches and other, less sophisticated methods for adsorption isotherm modeling and PSD calculation. Herein, the current state and future challenges in the development of the pore size analysis methods with the aid of model OMMs are discussed.
2. PORE SIZE ANALYSIS BASED ON MODEL MESOPOROUS ADSORBENTS
The first practical approach for calculation of PSDs in a wide range of pore diameters using OMMs as model adsorbents was reported by us in 1997 [6]. This approach, often referred to as the Kruk-Jaroniec-Sayari (KJS) method, was based on the following ideas. The starting point is the synthesis of a series of OMMs with the same type of the porous structure, and with gradually increasing pore diameter within as wide range as possible. The subsequent step requires the determination of the pore size of these OMMs using an accurate and reliable method independent from gas adsorption methods of the pore size analysis. The final step is to acquire gas adsorption isotherms on these model materials and to use these model data to assess the feasibility of the pore size assessment from adsorption and desorption data, and to elaborate, if possible, a method to calculate PSDs from the branch or branches of isotherms that were found to be inherently suitable for the pore size assessment. This final step includes the following sub-steps: 9 the determination of the experimental relation between the pore diameter and the capillary condensation/evaporation pressures, 9 the critical assessment of the inherent suitability of these relations for the PSD determination, which leads to a selection of one branch or two branches of the isotherms as potentially suitable for the pore size analysis, 9 the approximation of the potentially useful relation(s) by Using appropriate expressions, and, if needed and possible, the extrapolation over the pore size range beyond that exhibited by the model adsorbents used,
439 9 the construction of model adsorption isotherms for pores of different sizes (which in addition to the information obtained in the previous step requires the elucidation of the statistical film thickness of the surface layer formed on the walls of pores of different sizes), 9 the implementation of the PSD calculation procedure based on the model gas adsorption isotherms generated as described above. So far, this approach has been implemented for cylindrical pores with silica-based and hydrophobic (hydrocarbon-based) surfaces in the case of nitrogen adsorption at 77 K [6,10], and argon adsorption at 87 K [11,12] and 77 K [13] (in the latter case, so far the method has been elaborated for silica-based surfaces only). Series of MCM-41 silicas with approximately cylindrical pores of size between 2 and 6.5 nm were used as model OMMs to establish the relations between capillary condensation/evaporation pressures and the pore size [6,11,13]. An equation that provides a relation between the MCM-41 pore diameter, X-ray diffraction (100) interplanar spacing, and the pore volume in a honeycomb porous structure [14,15] was used for an independent pore size assessment. In all cases, it was found that the capillary condensation pressure tends to gradually and systematically increase as the pore diameter of model OMMs increased, which provides strong evidence that adsorption branches of isotherms are suitable for the PSD calculations. On the other hand, the relation between the capillary evaporation pressure and the pore diameter was much more complicated in the pressure range of adsorption-desorption hysteresis, including much scatter of results and a systematic irregularity close to the lower pressure limit of adsorption-desorption hysteresis. In particular, there was much evidence that model materials with more uniform pores tend to desorb at higher pressures than materials of less uniform porous structure do, which suggests that the capillary evaporation is delayed by the presence of constrictions even in uniform channel-like pores of MCM-41 [6,16]. This is reminiscent of the well-known pore network effects that lead to a delayed desorption in solids with 3-dimensionally connected porous structures (see [3,16,17] and references therein). It was concluded on the basis of adsorption studies for model OMMs that adsorption branches of isotherms are suitable for PSD calculation, whereas desorption branches of isotherms are much less suitable even for channel-like pores (their unsuitability for PSD calculation for pore networks with constrictions is well known [3,16,17]). The relation between the pore diameter and the capillary condensation pressure was approximated and extrapolated by expressions similar to the well-known Kelvin equation, but with additional constant correction terms, which was intended to ensure a proper behavior for pores much larger than those of the model OMMs used [6,11 ]. The examination of the adsorption data for OMMs also led to the conclusion that the monolayer-multilayer formation before the onset of capillary condensation can be satisfactorily approximated for pores of different diameters by a common statistical film thickness curve (t-curve) based on an adsorption isotherm for a macroporous silica. This approximation is more crude for pores of diameter close to the micropore range [11,13,18], and in general less satisfactory for argon [11,13] than for nitrogen [6,19]. The aforementioned relations between the pore diameter and capillary evaporation pressure were used along with the t-curves in calculations of PSDs [6,10-13] using an algorithm based on the well-known Barrett-Joyner-Halenda (BJH) work [20]. A very good agreement was usually observed between PSDs assessed using the KJS approach discussed above for nitrogen and argon (both at 77 and 87 K). It should be noted that the use of OMMs to improve the PSD calculation methods, such as BJH, had been proposed by Naono et al. [21 ] before the development of the KJS method. However, Naono et al. did not follow many of the steps and sub-steps
440 mentioned above, which are needed for successful development of the OMM-calibrated PSD calculation procedures, and consequently, their method was found to produce significant errors in the pore size assessment [2]. The KJS procedure proposed for nitrogen adsorption at 77 K has already been implemented by several research groups working in the field of nanomaterials [22-26] in addition to its extensive use by the two groups that participated in the development of this approach [6,10-13,18,19,27]. However, the procedures based on the KJS concept have been elaborated so far only for materials with channel-like pores. So the KJS PSDs are expected to be less accurate for materials with cage-like pores. These expectations are confirmed by the results of the recently published work [28], which showed that the KJS procedure for channel-like silica pores [6] appreciably underestimates the size of cage-like pores, although it still offers a significant improvement in accuracy when compared to the standard adsorption methods of the pore size analysis for channel-like pores. In addition, there are also indications that the extrapolation of the experimental relation between the capillary condensation pressure and pore diameter for cylindrical pores over larger pore sizes using an empirical equation similar to the Kelvin equation for hemispherical meniscus [6,11] becomes less accurate as the pore diameter increases [29]. Therefore, the challenge is in the development of adsorption methods for an accurate determination of PSDs for the pore range as wide as possible (perhaps not only mesopores, but micropores and macropores), and for various pore shapes (including channel-like and cage-like structures), using OMMs and related materials as model adsorbents.
3. SYNTHESIS OF M O D E L M E S O P O R O U S ADSORBENTS
Since the time the KJS approach for cylindrical pores was developed [6], a limited progress has been made in the synthesis of OMMs with extra-large cylindrical pores. In particular, we are aware of not reports on the successful synthesis of alkylammoniumsurfactant-templated MCM-41 silica with high degree of structural ordering and pore sizes above 7 nm, that is, above the limit achieved before using hydrothermal restructuring approaches [30-32]. On the one hand, the discovery of triblock-copolymer-templated SBA-15 silicas [33] appeared to extend the upper limit of pore diameters attainable for 2-D hexagonally ordered materials with cylindrical pores to about 30 nm. However, it has soon become clear that the actual pore diameter limit for SBA-15 is most likely less than half of this value, as the larger pore materials have some i~ore structure ordering, but are foam-like [34]. In addition, SBA-15 exhibits a 3-dimensionally connected porous structure, that is, the large, uniform mesopores of this silica are connected by much narrower pores (micropores and narrow mesopores) in the pore walls [35,36], which is related to inherent properties of the templates with poly(ethylene oxide) blocks [35,36] that are capable of becoming occluded in the silica pore walls. The 3-D pore connectivity is a general feature of SBA-15 synthesized under various conditions [37]. Until recently, most of the claims about the possibility of synthesizing non-microporous (that is, with 2-D pore system) SBA-15 did not have any good basis or were in disagreement with the results obtained using reliable inverse replication methods [38], while other claims deserve further scrutiny (see discussion in [38]). In particular, there is good evidence that high-temperature calcination of SBA-15 at above 1173 K leads to the closure of the connecting pores [39]. This is accompanied by a prominent structural shrinkage, so the pore size of the resultant SBA-15 with 2-D pores tends to be in the upper range of diameters attainable for high-quality MCM-41. A way to circumvent this problem by using an SBA-15 silica with somewhat larger pores and with a small content of
441 connecting pores was proposed, and a silica was obtained with pore diameter of about 9 nm and with adsorption properties distinctly different from those of typical SBA-15 and similar to those expected from an extra-large-pore MCM-41 [38]. However, the contention about a complete elimination of pore connections for such a material still needs to be verified using inverse carbon or platinum replication methods [38]. The synthesis of SBA-15 in the presence of salts is also promising from the viewpoint of elimination of the connecting porosity in SBA-15 [23], but the final verification by using inverse replication methods is lacking. Finally, the hydrothermal treatment at 403 K was claimed to afford SBA-15 without micropores, which would imply the lack of connections between the pores [40], but SBA-15 synthesized under very similar conditions have already been found to exhibit highly connected porous system with large holes in the walls of ordered pores [41 ]. Some progress has also been made in the synthesis of small-pore silicas with 2-D hexagonal structures. In particular, the use of mixtures of surfactants with two short alkyl chains afforded highly ordered MCM-41 with the XRD (100) interplanar spacing as low as 2.7 nm [18]. Moreover, silicas that exhibit pore diameters tailorable in the micropore range and a single, but very narrow XRD peak corresponding to the (100) interplanar spacing down to as low as 2.3 nm, can readily be synthesized from commercially available reagents [42]. Further studies are required to verify whether these remarkable ordered microporous silicas exhibit well-ordered 2-D hexagonal structures, or less ordered pore structures. A significant progress has recently been achieved in the pore size tailoring of MCM-48 silica [1] that exhibits a 3-D connected structure of uniform pore channels. In particular, large-pore MCM-48 (pore diameter above 4.5 nm) was reported [43,44]. However, the pore size range attainable for MCM-48 is still much more restricted than that attainable for MCM41. Also, a noticeable progress has been made in the synthesis of OMMs with cage-like pores of tailorable size. This includes the work on SBA-1 and SBA-6 silicas with the same cubic Pm3n structure [45], whose pore diameter can be tailored in a range about as wide as that achievable for MCM-41 by choosing surfactants of various structures and different alkyl chain length, or by adding proper amount of micelle expanders [45,46]. However, SBA-1 and SBA-6 exhibit structures with two kinds of mesoporous cages of different size [45], which would make their prospective application as model adsorbents more difficult. Oligomer- and polymer-templated silicas with cage-like mesoporous structures of cubic Im3m and 3-D hexagonal P63/mmc symmetry can also be synthesized with pores of various sizes [33,47,48], and are thus promising as prospective model adsorbents. However, because of the inherent tendency of the poly(ethylene oxide) blocks of oligomer/polymer templates to be occluded in the walls of silicas synthesized using these templates, the aforementioned cage-like silicas are likely to exhibit micropores in their siliceous frameworks in a way similar to that proven for SBA-15 [35-37]. Finally, there emerges an opportunity to synthesize ordered materials with pore sizes on the borderline between the mesopore and macropore ranges using colloidal crystals as templates [49], which promises to provide model extra-large-mesopore adsorbents.
4. A S S E S S M E N T OF PORE SIZE FOR M O D E L M E S O P O R O U S ADSORBENTS
The most promising methods for an independent assessment of pore dimensions of model OMM adsorbents are those based on equations that provide relations between structural parameters in the OMMs structure [14,15], and on the electron crystallography method [45]. A geometrical equation for the pore size assessment of MCM-41 was originally reported by two research groups [14,15], whose contributions were submitted for publication in 1996
442 within just two months from one another. The derivation of this equation was based on the consideration of a relation between the (100) interplanar spacing, dl00, the primary mesopore volume, Vp, and the pore diameter, Wd, in 2-D hexagonal structure of uniform cylindrical pores. This equation assumes the form: Wd = cdl00[pVp/(l+pVp)] 1/2, where 9 is the density of the pore walls and c is a constant dependent on the pore geometry (1.213 for circular pores). Others [7,50-52] later proposed the same equation. This equation was extended to materials with 2-D hexagonal porous structure akin to that of MCM-41, but with porosity in the pore walls, in which case Wd = Cdl00[Vp/(1/p+Vp+Vmi)]1/2, where Vmi is the volume of pores in the walls [53]. Others also proposed an analogous equation [54]. This equation is suitable for the pore diameter determination of SBA-15 and MSU-H silicas [29,54]. More recently, geometrical equations suitable for the pore size determination for various OMM structures have been reported. These include equations for cubic Ia3d structure of MCM-48 [55], 3-D hexagonal P63/mmc structure [28], cubic Im3m structure of SBA-16 [28,56], other cubic structures with cage-like pores [28], and 2-D hexagonal structure of CMK-3 carbon, which consists of an array of connected rods [37]. So, it is now possible to determine the pore diameter of many OMMs on the basis of the interplanar spacing and primary mesopore volume (in some cases the volume of pores in the walls is also needed) using simple geometrical relations. It needs to be noted that in general, the geometrical relations for the OMM pore size determination are based on assumptions that the structure is infinite in two or three dimensions, and the material is composed only of an ordered phase of a given structure. In addition, the relation for a 2-D hexagonal array of rods neglects the presence of connections between these rods [37], whereas the relations for structures with cage-like pores [28,56] neglect the pore connectivity and assume the spherical pore shape. A more realistic model of 3-D OMM structures can be elucidated using the electron crystallography [45,57], which reveals the actual structural complexity and does not require assumptions about a particular simple structure type that are necessary in derivations of geometrical equations for the pore size of 3-D OMMs. However, the most convenient implementation of the electron crystallography technique for OMM has much in common with the aforementioned equations, because it requires the ratio of the pore volume to the pore wall volume in order to determine the threshold in the 3-D potential map, or in other words, the surface that divides the pores from the walls in the structure [57]. This is analogous to the case of the geometrical equations where the pore diameter/unit cell size ratio in a simplistic structural model is calculated from the pore volume/pore wall volume ratio. So far, the electron crystallography method involved the determination of the pore wall volume as a reciprocal of the wall density evaluated using helium picnometry [45,57], and in the case of polymer-templated silicas, no provisions were made for taking into account the presence of micropores in their pore walls. The microporous nature of these walls results in the difference between the actual volume of the microporous pore wall and the pore wall volume estimated from the framework density. This is expected to lead to an error in the determination of the threshold of the 3-D density map from the pore volume/pore wall volume ratio for these materials.
5. CONCLUSIONS The use of OMMs as model adsorbents for elaboration of accurate methods for the PSD calculation is a highly promising approach. Because of its simple nature and the recent advances in the synthesis and independent pore size assessment of OMMs with different
443 structure types, there emerge opportunities in extending this approach to the pore shapes different from cylindrical. On the other hand, model OMMs are still available only with pore dimensions in limited intervals of the mesopore range, which hinders the development of accurate PSD calculation methods for very large mesopores. It would be desirable to extend the synthesis of ordered materials to the largely uncharted 1.3-2.0 nm part of the micropore range, thus providing better foundations for the PSD analysis in this important pore size interval.
6. ACKNOWLEDGMENTS The donors of the Petroleum Research Fund administered by the American Chemical Society are gratefully acknowledged for support of this research.
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Studies in Surface Scienceand Catalysis 141 A. Sayariand M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
445
Sorption properties and hydrothermal stability of MCM-41 prepared by pH adjustment and salt addition Nawal Kishor Mal, a'* Prashant Kumarb and Masahiro Fujiwara a a
AIST Kansai, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, JAPAN
b Ceramic Section, Industrial Research Institute of Ishikawa, Kanazawa 920-0223, JAPAN
MCM-41 with average pore size of ca. 4 nm was prepared with intermediate pH adjusted to 10.5, 9.75 and 9.25 for three times and a salt addition. Water and benzene sorption data show that hydrophobicity of MCM-41 increased after decreasing the pH of synthesis gel and the absence of occluded SiO2 type species inside the pores of MCM-41. TG analysis predicts that hydroxyl groups in as-synthesized catalyst decreased by 48 % after lowering the pH of the synthesis gel from 10.5 to 9.25.29Si MAS NMR shows the presence of small amount of Q3 species in the as-synthesized samples. Hydrothermal stability of the samples was drastically improved after lowering the pH to 9.25, which shows at least three peaks in XRD after 8 days of hydrothermal treatment at 373 K. It seems that lowering of the pH causes further polymerization, a kind of silylation of pore walls or deposition of precipitate silica on pore walls and, therefore, decreases the hydroxyl groups and improves the hydrothermal stability.
1. INTRODUCTION The improvement of the structure and hydrothermal stability of MCM-41 materials is of great importance due to their applicability as catalyst [1,2], absorbents [3], and host for various kinds of molecules [4,5]. Following the discovery of M41S family of mesoporous molecular sieves by Mobil [6,7] many attempts have been made to improve the structure by direct hydrothermal synthesis at low temperature (i.e. 373 K) [8,9] to give smaller pore size (diameter < 4.0 nm) and at high temperature (i.e. > 423 K) to give larger pore size (diameter > 4.5 nm) MCM-41 materials [10-12]. According to Chen et al. [13], MCM-41 can maintain its structure up to 1123 K in anhydrous conditions. Kim et al. [14] also reported that MCM-41 shows stability in air and oxygen containing water vapor system at 1170 K. However, hydrothermal stability of MCM-41 is poor in water [15], especially in boiling water the structure gets collapsed within 2 days [ 16]. Recently, new variety of mesoporous silica such as SBA-15 [16], KIT-1 and MSU-G [16-18] has been synthesized, they exhibit greater hydrothermal stability due to much thicker walls compared with Si-MCM-41 but are less wall ordered. The hydrothermal stability ofMCM-41 was improved after three times intermediate pH adjustment and salt addition during crystallization [8] or post synthesis restructuring of as-synthesized samples in water [19] or mother liquor [20]. Post synthesis silylation is effective to enhance the hydrothermal stability due to increase ofhydrophobicity [21].
446 In the present investigation, we have found that hydrothermal stability of MCM-41 was dramatically improved prepared using different molar composition better than described by Ryoo et al. [8], since the stability and the characteristics of the MCM-41 are strongly affected by the synthesis conditions [ 13]. The finding is supported by the characterization of parent and hydrothermally treated samples by using XRD, sorption study, TG analysis and 29Si MAS NMR.
2. MATERIALS AND METHODS 2.1. Materials The reactants used in this study were sodium silicate (52.5% SiO2, 25% Na20, Wako chem.), cetyltrimethylammonit~m bromide (96%, Kanto Chem.) (CTMABr), tetramethylammonium hydroxide (25% aqueous, Aldrich Chem.) (TMAOH), potassium chloride (KC1, Wako Chem.) and H2SO4 (96%, Wako Chem.). In a typical synthesis, 22.86 g of sodium silicate was dissolve in 100 g of water to give clear solution under stirring. 48.86 g of TMAOH and 38.72 g of CTMABr were dissolved in 100 g of water by stirring at 308 K to give a clear solution. Both the clear solutions were mixed together and stirred for 2 h. Finally, 13.28 g of H2SO4 in 51 g of H20 was then added and stirred for 2 h to become pH ca. 10.5. The molar composition of the gel was 1 SiO2 : 0.51 CTMABr : 0.67 TMAOH : 0~46 Na20 : 0.75 KCI : (0.65 + X) H2SO4 : 80 H20 (pH = 10.5), where KC1 was added after first heating period to 373 K for 24 h and X (0.1 - 0.5) the amount of H2SO4 was added during intermediate pH adjustment for three times. The gel was then heated in a polypropylene bottle, without stirring, to 373 K for 24 h. The mixture was then cooled to room temperature and H2SO4 was added to adjust the pH ca. 10.5. i 1.83 g of KC1 was then added and heated again to 373 K for 24 h. This procedure for pH adjustment and subsequent heating was repeated twice (i.e. carried out three times in all), except no KC1 was added during second and third pH adjustment. The resultant product was filtered, washed with distilled water, dried at 378 K for 24 h and calcined at 823 K for 6 h. Two other MCM-41 samples were prepared under similar condition with different pH adjusted to 9.75 and 9.25. Catalysts prepared with pH adjusted to 10.5, 9.75 and 9.25 are denoted to be as sample 1, 2 and 3, respectively. 2.2. Surfactant extraction The surfactant (template) from as-synthesized samples was removed by treatment of 1 g of catalyst in 60 g of dry ethanol and 1 ml of HC1 (1 M) at 353 K for 24 h under vigorous stirring. After filtration samples were washed with eti,anol and dried at 378 K for 24 h. This procedure was repeated once. 2.3. Hydrothermal treatment 0.4 g of calcined MCM-41 in 400 g ofwater was heated in a propylene bottle to 373 K for 4 and 8 days. The Sample was then filtered, dried at 378 K for 24 h and calcined at 773 K for 90 min. 2.4. Characterization XRD patterns were obtained with a Shimadzu XRD-6000 diffractometer. BET surface area and pore size were obtained from N2 adsorption isot~aerms measured at 77 K using Bellsorp
447 28 instrument. Prior to N2 adsorption, the samples were degassed at 473 K for 5 h. The sorption measurement was carried out gravimetrically in a electrobalance (Chan, USA) at 298 K and at fixed p/p0 ratio of 0.5 each of water and benzene as adsorbents after equilibrium for 3 h. FT-IR spectra of template extracted samples were obtained with a JASCO FT/IR-230 using KBr pellets (3 mass% catalyst). Thermogravimetric analyses (TGA) of as-synthesized samples were obtained with a Seiko, SSC/5200. Samples were heated at the rate of 5.0 K/m from 293 to 1073 K. 29Si MAS NMR spectra were recorded at 11.75 T on a Varian INOVA 500 NMR spectrometer with a CPIMS probe. Data were acquired at 99.3 MHz and 10 s recycle delays. The chemical shifts are given in ppm using tetramethylsilane (TMS) as a standard material.
2.5. Methods The BET surface area [22] w~,,s calculate{' in the relative pressure range between 0.04 and 0.2. The primary mesopore size (WKjs) was ,~alculated using adsorption branch of isotherms according to method describe elsewhere [23] that is; WKjs = cd(pVv)V2/(1 + pVI,) 1/2, where C = 1.213, p - - 2.2 gcm -3, d is the lattice spacing of d~00, and Vv is the primary mesopore volume. Total pore volume was determined from the amount adsorbed at relative pressure of 0.99 [22]. The pore size distributions were calculated from the adsorption branches of the nitrogen adsorption isotherms using Barrett-Joyner-Halenda (BJH) method [24].
3. RESULTS AND DISCUSSION
3.1. Synthesis, structure and sorption properties Absence of the C-H stretching vibration band at 2933 and 2855 cm 1, and bending vibration band at 1460 cm 1 in the FT-IR spectra of surfactant-extracted samples confirm the complete removal of surfactant. XRD profiles of all the MCM-41 samples, with pH adjusted to 10.5, 9.75 and 9.25, after cal,c,ination an~! surfactant-extraction are shown in Fig. 1. Four peaks in the XRD patterns of all samples are clearly observed, which are characteristics of long range ordering of the MCM-41 structure. Intensity of dl00 peak of surfactant-extracted samples is slightly higher than that of calcined samples. The XRD and adsorption characteristics of the calcined and surfactant-extracted samples are shown in Table 1. As clear from Table 1, lowering the pH from 10.5 to 9.25, the d~00 spacing of calcined samples were ~
~ - - - ~ ~ 4 6 20 (degree)
B
, ,,, i
8
10
2
4 6 20 (degree)
8
10
Figure 1. XRD profiles of calcined (A) anO surfactant-extracted (B) samples obtained with pH adjusted t o (a) 10.5, (b) 9.75 and (c) 9.25.
448 Table 1. XRD and adsorption characteristics of the samples under study, a Sorption capdloo (nm) Sample As-synt- Calci- ao SBET Vp WKJS acity (wt%) Vbenzene /pH hesized ned (nm) (m2g"1) (cm3g"1) (nm) H20 Benzene (cm3ga) 1 (10.5) 3.91 3.75 4.33 1020 0.82 3.65 18.6 73.8 0.85 2(9.75) 4.12 4.03 4.65 960 0.80 3.90 15.3 70.6 0.81 3 (9.25) 4.15 4.12 4.76 911 0.75 3.94 13.4 66.4 0.77 Surfactant-extracted samples 1 3.91 4.09 b 4.72 1116 0.91 4.05 32.2 75.6 0.93 2 4.12 4.23 b 4.88 1082 0.87 4.16 24.1 71.2 0.89 3 4.15 4.18 b 4.83 1021 0.78 4.03 16.2 68.2 0.81 aParenthesis indicates the repeated pH adjustment of the synthesis gel; dloo:X-ray diffraction (100) interplanar spacing; ao: unit cell parameter = 2d~oo/3V2;S~ET:BET specific surface area; Vp: Primary meosopore volume; WKjs:Averagepore size; Vb...... : pore volume measured using benzene as adsorbent at P/Po= 0.5 and 298 K. bnot calcined. increased from 3.75 to 4.12 nm, respectively. An increase in dl00 spacing after lowering the pH of the synthesis gel was reported elsewhere [25]. The lattice contraction after calcination of samples 1, 2 and 3 were 4.1, 2.2 and 0.7%, respectively. MCM-41 synthesized without pH adjustment shows lattice contraction 20-25% [13]. In contrast, lattice contraction was not observed in samples synthesized by repeated pH adjustment [26]. Unit cell expansion after surfactant-extraction of samples 1, 2 and 3 are 4.6, 2.7 and 0.7%, respectively. This result emphasized that the amount of hydroxyl groups at low pH (9.25) adjusted sample was much lower than at high pH adjusted samples. However, the surface area and pore volume of the samples were decreased after lowering the pH. It is likely that lowering of pH from 10.5 to 9.25 causes the formation of more polymerized pore wall and/or deposition of silica on the pore walls inside and outside the pore uniformly and therefore results in an increase in the dl00 spacing and a decrease in the pore volume. The sorption capacity for water shows that surfactant-extracted samples are more hydrophilic than calcined form because silanol groups were present in surfactant-extracted samples. The amount of water absorbed in surfactantextracted sample 3 (16.24 %) is 97.5% lower than surfactant-extracted sample 1 (32.21%) ~oo]
A
~i
2.8 ( n m )
B
)IW'~uA~A4 9A- 9C
:: ,0ol 300 l
o
. . ~
~
<
"~'
jl__. ........u........................
200 o N o 0.0
0.2
0.4 P/Pc
0.6
0.8
1.0
9
0 0
2
4
- . . . ~ . , . -, . ,-, , 6 8 10 12 14 16 18 20 Pore size (nm)
Figure 2. N2 sorption isotherms (A) and pore size distribution curve (B) of calcined MCM-41' (a) sample 1, (b) sample 2 and (c) sample 3.
449 and 7.2% more than calcined form of sample 3 (13.42%). It clearly indicates that hydrophilicity of MCM-41 drastically decreased after lowering the pH to 9.25. Comparable sorption data of benzene and pore volume (based on benzene adsorption and N2 adsorption) indicate that the pore of MCM-41 is free from any occluded SiO2 type species. N2 sorption isotherms and pore size distribution of calcined samples 1, 2 and 3 are shown in Fig. 2. N2 sorption isotherms show a feature of a type IV isotherm with sharp capillary condensation at p/p0 ca. 0.3. Pore size distribution curve shows narrow pore size distribution for all samples with peak pore diameter at 2.80 nm.
3.2. Thermogravimetric analysis (TGA) TG analyses of as-synthesized samples 1, 2 and 3 are shown in Fig. 3. All samples show four distinct weight losses in TG diagram [13, 27, 28]. Weight loss below 423 K corresponds to desorption of physisorbed water and ethanol, between 423 and 623 K 1 O0 corresponds to breakage, decomposition and combustion of residual organic. Weight loss 8O above 623 K is attributed to water losses x: resulting from dehydroxylation reaction [ 13, 27, ~: 60 28]. The TG analysis of sample 1, 2 and 3 are given in Table 2. The amount of sufactant in a samples 1, 2 and 3 are 35, 32.4 and 30.4%, 400 600 800 1 000 respectively. Loss of water due to Temperature (K) dehydroxylation, between 623 and 1073 K, in sample 3 is 48 % less than sample 1 and 26.9% Figure 3. TG profiles of as-synthesized less than sample 2. samples: (a) sample 1, (b) sample 2 and (3) sample 3. Table 2. Thermogravimetric characteristics of as-synthesized samples. Weight loss (%) Sample 293- 1073 K 42.2,--623K 1 46.8 35.0 2 42.6 32.4 3 40.2 30.4
623- 1073 K 7.3 5.2 3.8
3.3. XRD and Sorption properties after hydrothermal treatment XRD profiles of calcined samples 1, 2 and 3 after hydrothermal treatment for 4 and 8 days are shown in Fig. 4. All the samples show intense d~00peak and two higher order peaks after 4 days of hydrothermal treatment. After 8 days of hydrothe:mal treatment structures of samples 1 and 2 are severely degraded but sample 3; still shows intense dl00 peak and two higher order peaks. As far as we know, this is the be~t result, so far reported in literature regarding hydrothermal stability of MCM-4! prepared by direct hydrothermal synthesis. It is worth to note that structure of MCM-41 obtained without salt addition was completely lost during hydrothermal treatment for 12 h [8]. Structure of MCM-41 obtained by salt addition was retained without any significant loss after heating in boiling water for 12 h [8]. Small loss of structure ofMCM-41 (pore size > 4.5 art,) ~vas observed b~,,Kr :k et al. [20] after
450
B
i Y.
i, Y~
2
4
-
6
8 .....
]'0
,..~
2
4
20 (degree)
6
8
]'0
20 (degree)
Figure 4. XRD profiles of calcined samples after hydrothermal treatment at 373 K for 4 days (A) and 8 days (B) (a) sample 1, (b) sample 2 and (c) sample 3. heating in boiling water for 22 h. However, direct comparison between our results and reported ones [8, 20] is not possible because in our case hydrothermal treatment at 373 K was carried out in closed vessel, statically, under autogenous pressure for 4 and 8 days. N2 sorption isotherms and pore size distribution curves of sample 3 after hydrothermal treatment for 4 and 8 days show a type of IV isotherms and little broad pore size distributions as shown in Fig. 5. After 8 days of hydrothermal t~eatment the peak pore diameter of sample 3 shifted from 2.8 to 3.4 nm (Fig. 5B). XRD and sorption characteristics of samples after hydrothermal
600 500
a
j
-~ 300
3._4 (nm)
~.I.'~'/"'/
.;::~'::-'~ ./J
400
;>
0.6-
A
~o~
0.4-
7
b.__.:.s~:~#~l'~"
~ ~ "
b
" , . . .... . . . . . . . . . . .
2.8 3.4
200.
.~
0.2
100
e 0.0
0.2
0.4
0.6 P/Po
0.g
1.0
~- 4 0
2
4
a 6
8 10 12 14 16 18 20 Pore size (nm)
Figure 5. N2 sorption isotherms (A) and pore size distribution (B) of sample 3 after hydrothermal treatment for: (a) 4 and (b) 8 days. treatment for 4 and 8 days are given in Table 3. Lattice parameter (a0) of all the samples after 4 days of hydrothermal treatment was decreased, but after 8 days of hydrothermal treatment an increase in a0 was observed. Surface area and pore volume further decreased after extending hydrothermal treatment from 4 to 8 days, but sample 3 still possesses good surface area (685 m2g1) and pore volume (0.58 cm3g~). Water absorption capacity ofhydrothermally treated samples was increased by 2 to 4 fold, probably due to hydrolysis of Si-O-Si bond by water leading to increase hydrophilicity.
451 Table 3. XRD and adsorption characteristics of the samples after hydrothermal treatment. Sorption dloo ao SBET Vp WKJS capacity (vvt %). Vbenzene Sample day (nm) (nm) (m2g1) (cm3g-1) (nm) H20 Benzene (cm3g-1) 1 4 3.59 2 4 3.84 3 4 3.95 1 8 3.69 2 8 4.00 3 8 4.14 a see foot note of Table 1. 3.4.
29Si M A S
4.15 4.43 4.56 4.26 4.62 4.78
523 637 759 480 576 685
42.0 43.3 38.8 64.5 48.2 42.4
3.27 3 56 3 73 3 00 3.48 376
0.59 0.64 0.70 0.37 0.48 0.58
52.2 56.0 61.3 39.9 50.0 63.6
0.64 0.66 0.71 0.45 0.52 0.63
NMR.
29Si MAS NMR of as-synthesized and calcined forms of sample 3 are presented in Fig. 6. Comparison of these two spectra indicates that the intensity of Si(-OSi)3(-OH) (i.e. Q3, at -99 ppm) in as-synthesized sample is very small due to low pH and salt effect, therefore improving the hydrothermal stability because of decrease in silanol groups.
-80
~i~o PPM
.....
-8o
......
-l~0
PPM
Figure 6.29Si MAS NMR spectra of sample 3' (a) as-synthesized and (b) calcined form.
4. CONCLUSIONS MCM-41 obtained by low pH adjusted to 9.25 and salt addition shows a long range ordering of hexagonal array (XRD), improved hydrothermal stability and hydrophobicity. Low pH adjustment causes the further polymerization of pore walls and/or deposition of precipitate silica on the pore walls. The salt effect causes a drastic decrease in silanol groups in as-synthesized sample. Water absorption capacity of hydrothermally treated samples was 2 to 3 fold more than parent samples. Pore size dis:ributions of hydrothermally treated samples are relatively broader than parent sampies.
452 5. ACKNOWLEDGMENT NKM is grateful to AIST, Tokyo for STA fellowship.
REFERENCES
1 2 3 4. 5 6.
A. Sayari, Chem. Mater., 8 (1996) 1840. A. Corma, Chem. Rev., 97 (1997) 2373. P.J. Branton, P. G. Hull, K. S. W. King, J. Chem. Soc., Chem. Commun., (1993) 1257. J.H. Clark and D. J. Macquarrie, J. Chem. Soc., Chem. Commun., (1998) 853. J.F. Diaz and K. J. Balkus, J. Mol. Catal. B, 2 (1996) 115. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli andJ. S. Beck, Nature, 359 (1992) 710. 7. 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. 8. R. Ryoo and S. Jun, J. Phys. Chem. B, 101 (1997) 317. 9. K.J. Edler and J. W. White, Chem. Mater., 9 (1997) 1226. 10. C.- F. Cheng, W. Zhou and J. Klinowski, Chem. Phys. Lett. 263 (1996) 247. 11. C.- F. Cheng, W. Zhou, D. H. Park, J. Klinowski, M. Hargreaves and L. F. Gladden, J. Chem. Soc. Faraday Trans., 93 (1997) 359. 12. A. Corma, Q. Kan, M. T. Navarro, J. Perez-Pariente and F. Rey, Chem. Mater., 9 (1997) 2123. 13. C.- Y. Chen, H.- X Li and M. E. Davis, Micropor. Mater., 2 (1993) 17. 14. J. M. Kim, J. H. Kwak, S. Jun, R. Ryoo, J. Phys. Chem., 99 (1995) 16742. 15. L. Y. Chen, S. Jaenicke and G. K. Chuah, Micropor. Mater., 12 (1997) 323. 16. R. Ryoo, J. M. Kim, C. H. Ko and C. H. Shin, J. Phys. Chem., 100 (1996) 17718. 17. D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka and G. D. Stucky, Science, 279 (1998) 548. 18 S. S. Kim, W. Zhang and T. Pinnavaia, Science, 282 (1998) 1302. 19 L. Chen, T. Horiuchi, T. Moil and K. Maeda, J. Phys. Chem. B, 103 (1999) 1216. 20 M. Kurk, M. Jaroniec and A. Sayari, Micropor. and Mesopor. Mater., 27 (1999) 217. 21 K. A. Koyano, T. Tatsumi, Y. Tanaka and S. Nakata, J. Phys. Chem. B, 101 (1997) 9436. 22 S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc., 60 (1938) 309. 23 M. Kruk, M. Jaroniec and A. Sayari, (a) Langmuir, 13 (1997) 6267, (b) J. Phys. Chem., 101 (1997) 583, (c) Chem. Mater., 9 (1997) 2499. 24. E. E Barrett, L. G. Joyer and P. R Halenda, J. Am. Chem. Soc., 73 (1951) 373. 25. A. Wang and T. Kabe, J. Chem. Soc., Chem. Commun., (1999) 2067. 26. R. Ryoo and J. M. Kim, J. Chem. Soc., Chem. Commun., (1995) 711. 27. R. Schmidt, D. Akporiaye, M. Stocker and O. Ellestad, Stud. Surf. Sci. Catal., 84 (1994) 61. 28. R T. Tanev and T. J. Pinnavaia, Chem. Mater., 8 (1996) 2068.
otuulr
111 O U l l t l ~ t :
L 3 ~ I ~ I I L ; ~ i:IrlO
L,atalysls
1/-41
A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
453
Acidity characterization o f M C M - 4 1 materials using solid-state N M R spectroscopy Qi Zhao, a Wen-Hua Chen, a Shing-Jong Huang, a Yu-Chih and Shang-Bin Liu a'*
WU, b
Huang-Kuei Lee, b
a Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan 106, R.O.C. b Institute of Materials Science and Manufacturing, Chinese Culture University, Taipei, Taiwan 101, R.O.C.
A comprehensive study has been made on the acid properties of the mesoporous H-A1MCM41 molecular sieves with varied Si/A1 ratios using solid-state NMR techniques, namely by 31p MAS NMR of the adsorbed trimethylphosphine and trimethylphosphine oxide probe molecules in conjunction with elemental analysis. As the consequence of such combined techniques, which rendered not only qualitative (types and strengths) but also quantitative (concentrations) determinations of acid sites, one type of Lewis acid site and two different Bronsted acid sites were identified for H-A1MCM-41. Furthermore, based on the results obtained from ~H and 27A1MAS NMR of the species associated with structural frameworks, possible formation mechanisms for various acid sites were also proposed.
1. INTRODUCTION Mesoporous aluminosilicate materials (i.e., the M41S family), first discovered by Mobil researchers in 1992 [1], typically possess pore size in the range of 1.5-10 nm, which place themselves in the regime between traditional microporous zeolites and amorphous materials. In particular, mesoporous MCM-41 molecular sieves are known to possess several important characteristics, such as large surface area, high porosity and thermal/hydrothermal stability, which render many potential applications [2-4], for example, as adsorbent, support or catalyst. Acidity, that requisite for the overall catalytic performance of the MCM-41 materials as catalysts can either be achieved by isomorphous substitutions of framework Si by A1 atoms during synthesis or by post-synthesis surface modification. Although the former method is known to create Bronsted and Lewis acid sites in mesoporous aluminosilicates, the origin of Bronsted acid sites and related roles played by the structural framework A1 remain controversial [5-8]. Conventionally, the characterization of overall acid properties (namely type, strength and concentration) of a porous catalyst normally invokes adsorption of basic probe molecules (such as pyridine, ammonia or acetone) followed by spectroscopic [9, 10] and/or thermal analysis [10, 11] methods. Among them, multinuclear solid-state NMR spectroscopy has been shown as the unique, non-destructive technique for the investigation of the porous host adsorbents, organic and inorganic guest adsorbates, and the related guest/host interaction [12]. Owing to the high sensitivity and wide chemical shift range possessed by the 31p nucleus, 31p MAS NMR of the adsorbed phosphorous-containing organic compounds, such as trimethylphosphine (TMP) and
454 trimethylphosphine oxide (TMPO), has been demonstrated to be the most powerful technique for probing the acid properties of porous solid acid catalysts [10, 13]. The objective of this study is to investigate the nature and formation of acid sites in HA1MCM-41 mesoporous materials by means of 31p MAS NMR of the adsorbed TMP and its oxides. In particular, the roles of structural framework A1 on the characteristics of acid sites are also examined in conjunction with ICP, and 1H and 27A1MAS NMR.
2. MATERIALS AND METHODS 2.1 H-AIMCM-41 Sample Preparation Na-A1MCM-41 samples with particulate morphology and having varied Si/A1 ratios were synthesized according to the procedures reported elsewhere [14]. The as-synthesized samples were ion-exchanged with 1M NHnNO3 solution at 333 K for 12 h. After repeating the ionexchange procedures for three times, the resultant NHa-A1MCM-41 samples were subsequently filtered, washed and calcined (at 823 K in air for 5 h) to obtain H-A1MCM-41 samples. Due to the loss of partial extra-framework A1 species during the ion-exchange procedures, the Si/A1 ratios (16, 40, 57 and 71) of the final H-A1MCM-41 products are found to increase slightly compare to that originally specified for the corresponding Na-A1MCM-41 samples. 2.2 Characterization Methods Multinuclear solid-state MAS NMR experiments were carried out on a Bruker MSL-500P instrument at room temperature (298 K). Prior to the adsorption of probe molecule, each sample was subjected to dehydration treatment at 623 K for 26 h under vacuum (10 .5 torr). For the 3~p MAS NMR studies, different procedures were applied when preparing TMP and TMPO loaded samples. In the case of TMP, a known amount of the adsorbate was adsorbed onto the sample directly on a vacuum manifold. Whereas in the case of TMPO, a known amount of dry TMPO (100%, Alfa) in CH2C12 mixture solution was introduced into the samples via a gastight syringe under N2 glovebox. To ensure uniform adsorption of TMPO in the sample, the sample vessel was put over an ultrasonic vibrator for 0.5 h then placed at ambient temperature for at least 12 h, then, the solvent was slowly extracted at 323 K under vacuum. Finally, the sample loaded with adsorbed probe molecules were transferred into a zirconia MAS rotor under N2 glovebox and sealed by a gastight Kel-F cap. 31p MAS NMR spectra were acquired at frequency 202.46 MHz using a single pulse sequence under the following conditions: pulsewidth, 2 Its (ca. zt/5 pulse); recycle delay, 5 s, spinning rate, 8-10 kHz. Single pulse sequence was also used during 27A1MAS NMR experiments under pulsewidth 1 Its (<~/6) and spinning rate 10 kHz. On the other hand, ~H MAS NMR spectra were obtained from dehydrated samples by a rotor-synchronized spin-echo pulse sequence operating at spinning rate of 5 kHz and recycle delay of 15 s. Tetramethylsilane (TMS), aqueous 85% H3PO4, and 1M AI(H20)6 3+ solutions were taken as the external references for 1H, 31p, and 27A1 chemical shifts, respectively. To afford the quantitative (i.e. concentration) determination of the acid sites, each adsorbate-loaded H-A1MCM-41 sample was also subjected to elemental analyses by ICP/AES and ICP/MS.
3. RESULTS AND DISCUSSION 3.1.27A1 MAS NMR It is well known that the acid properties of aluminosilicates are closely related to the nature and concentration of the A1 species present in the samples. Figures 1 displays the 27A1MAS NMR
455 spectra of the hydrated H-A1MCM-41 samples. In general, the resonance peak correspond to the framework (tetrahedral-coordinated) A1 (at ca. 55 ppm) is much stronger than the extraframework (octahedral-coordinated) A1 (at 0 ppm) in each sample. However, since only the 1/2*1/2 central transitions was excited during the experiment, not all of the A1 species are visible in the 27A1 MAS NMR spectrum. Presumably, while the presence of water in the samples should resulted in an effective narrowing of the resonance for the framework A1 species, the extraframework A1 species which have lower symmetry normally possess larger quadrupolar coupling constants and hence resulting broad, unresolved resonance. To verify the relative concentration of tetrahedral vs. octahedral A1 species, samples adsorbed with acetylacetone [15] were also examined (not shown). Consequently, the amount of extra-framework A1 species was found to decrease with decreasing sample A1 content, as expected. 3.2. l H M A S N M R
1H MAS NMR spectroscopy has been widely used for the characterization of hydroxyl OH groups in dehydrated aluminosilicate samples [12]. The spectra obtained from H-A1MCM-41 with varied Si/A1 ratios are depicted in Fig. 2. As determined by spectral analysis using Gaussian deconvolution method, four distinct silanol peaks can be inferred, they can be respectively assigned to isolated (1.9 and 2.1 ppm; SiOH), hydrogen-bonded (2.7 ppm) or germinal (3.5 ppm) OH groups [12, 16]. However, characteristic peaks correspond to OH groups associated with the extra-framework A1 (i.e., A1-OH) species and bridging OH (i.e. Si-OH-A1 or Bronsted acid sites) were not observed. Presumably, if these species were present, resonance peaks should occur at 2.6-3.6 ppm (A1-OH) and above 3.6 ppm (Si-OH-A1) [17]. The results obtained from 1H MAS NMR therefore inferred the absence of Bronsted acid sites in the H-A1MCM-41 samples, obviously contradicting the statement that bridging hydroxyl groups exist in the H-A1MCM-41, as proposed earlier by Corma and co-workers [6]. A question then arises: does this mean only Lewis acid sites exist in the H-A1MCM-41 samples? This point is further verified by the 31p MAS NMR experiments (see below).
_: : ~
.........
HMCM-41/71~
'' ~ HMCM-41/40] 9 _ _JILL HMCM-41/1~ I .........
150
I" .......
100
'1'
50
......
"1
0
.........
I .........
-50
I
-100
Chemical shift (ppm)
Figure 1. 27A1 MAS NMR spectra of fully hydrated H-A1MCM-41 samples.
10 I
"
5 0 Chemical shift (ppm) i
. . . . . . . . .
i
. . . . . . . . .
-5 I
Figure 2. IH MAS NMR spectra of HA1MCM-41 with varied Si/A1 ratios. Top" Gaussian deconvolution spectrum of HMCM-41/71 sample.
456 3.3. 31p MAS NMR of Probe Molecules Adsorbed on H-AIMCM-41 As mentioned earlier, conventional methods for characterizing acidity of solid acid catalysts normally invoke either directly by IR or 1H MAS NMR spectroscopic observation of the acidic hydroxyl OH groups or by adsorption of basic probe molecules measured by titration, calorimetric, thermal gravitation or desorption, and spectroscopic techniques [5, 6]. Probe molecules containing elements with unpaired electrons (N, O, P etc.) are commonly used, for example, ammonia, pyridine, acetone or trimethylphosphine (TMP). However, while most of the aforementioned techniques are HMCM-41/71 A capable of differentiating Bronsted and Lewis acid sites in the aluminosilicate samples, quantitative information regarding to the concentration and strength of acid sites often .. cannot be obtained. Recent developments in HMOM-41/40 A / \ 31p MAS NMR of phosphorous-containing adsorbates on aluminosilicate materials have shown its capacity in providing both qualitative and quantitative information of acid sites [10]. However, in terms of the complexity involved in sample preparation procedures and the capability in resolving i.,,,,..,I,l.,==.,,,,l.,=,.====l,,=,===,,l different acid sites, the weaker base phosphine 150 100 50 0 -50 oxides (such as TMPO), are more preferable than TMP [18]. In any case, the adsorbate Chemicalshift(ppm) probe molecules tend to interact with acid sites to form hydrogen bonded complexes. As the Figure 3. 31p MAS NMR spectra of H3~ result, the higher the observed P chemical A1MCM-41 samples loaded with TMPO. shift would represents the stronger O--H Bottom: Gaussian deconvolution spectrum bonding strength of the complex and hence the for sample HMCM-41/16. Symbol * higher acid strength of the acid sites. denotes spinning sidebands.
Table 1. Assignment and..distribution, of acid sites for various H-A1MCM-41 samples 31p NMR Chemical Shift and Acidity Assignmentsb
Samples .
.
.
Si/A1a .
.
.
.
Bronsted acid . . . .
Lewis acid
69 ppm
64 ppm
55 ppm
HMCM-41/16
16
15.7
25.3
4.7
HMCM-41/40
40
10.0
26.4
11.6
HMCM-41/57
57
4.6
24.4
37.3
HMCM-41/71 71 3.0 27.6 34.0 aobtained from elemental analysis by ICP. bValues denote molar percentage of detected acid sites to total A1 contents in samples (i.e., amount acid sites/A1).
457 The 3~p MAS NMR spectra of adsorbed TMPO on various H-A1MCM-41 are displayed in Fig. 3. Four characteristic peaks at chemical shifts 69, 64, 55 and 48 ppm can be identified (Table 1). As the samples were exposed to humidity, the peak intensity of the 55 ppm line vanished rapidly with the elongation of contact time while the 48 ppm peak increased accordingly (not shown). Thus, the peak at 55 ppm may be assigned to TMPO adsorbed on Lewis acid sites, which diminishes in the presence of water molecules. The peak at 48 ppm can be attributed unambiguously due to physically adsorbed TMPO [ 10]. On the other hand, the peaks at 69 and 64 ppm, whose intensities remained nearly unchanged upon exposure to humidity, can be assigned to Bronsted acid sites that possess different acidic strengths. The fact that both Lewis and Bronsted acid sites exist in the HA1MCM-41 samples are also supported by the results obtained from samples loaded with TMP probe molecule. In this case, three distinct peaks were visible at -4, -50 and-58 ppm, which can be respectively assigned due to TMP associated with Bronsted (-4 ppm) and Lewis acid sites (-50 and-58 ppm). Considering that the isolated silanols (Si-OH) do not possess any acidity [17], the results obtained from 3~p MAS NMR in Fig. 3 therefore reveal that the Bronsted acidity in H-A1MCM-41 materials mainly arise from isolated Si-OH groups which interact with the three-coordinated A1 species. Although several interaction schemes have been proposed in the existing literatures [5, 7, 8], our findings seem to favor the hypothesis made by Trombetta et aL [5]. That is, as the base probe molecules interact with the silanol groups, negative charge will be enriched on the oxygen atoms of the silanol groups due to donation of extra electrons from the base molecule, thus promoting the coordination of the silanol groups with the adjacent three-coordinated A13+ sites. Consequently, Bronsted acid sites similar to the bridging hydroxyl groups (Si-OH-A1) in zeolitic materials may be formed, as shown schematically in Fig. 4. Samples loaded with TMPO were also subjected to further elemental analyses using ICP, by which quantitative information regarding to the formation probability of Bronsted vs. Lewis acid sites can be obtained, as depicted in Table 1. The effects of sample A1 content on the relative distribution of Lewis and Bronsted acid sites can thus be inferred. It is noted that, regardless of the amount of A1 presented in the samples, the amount of weaker Bronsted acid sites (at 64 ppm) remained practically unchanged, as also revealed by the change in molar percentage of acid sites to total A1 contents in Table 1. On the other hand, the formation of the stronger Bronsted acid sites (69 ppm) is seemingly more favorable at higher A1 contents. Assuming that majority of the A1 present in the samples were associated with the structural framework, the Bronsted acid sites at 64 ppm should be originated from silanol groups associated with the adjacent to three-coordinated framework A1 sites. Since the 27A1 MAS NMR results showed marked increase in the extra-framework A1 signal upon increasing sample A1 content, we speculate that the formation of Bronsted acid sites at 69 ppm are likely due to association of silanol groups with the extra-framework A1 sites. The formations of Lewis acid sites are therefore more favorable for H-A1MCM-41 having lower A1 contents. Taking into account that, in this case, less extra-framework A1 species are present, the formation of Lewis acidity should thus mainly arises from framework tri-coordinated A1 sites whereas the two Bronsted acid sites observed in H-A1MCM-41 were respectively related to framework and extraframework A1 species. Moreover, the fact that the observed 31p chemical shifts remain practically unchanged upon varying sample Si/A1 ratios also suggests that the acid strength is invariant with the sample A1 content.
458
/H 0
] o"
Si.
0 3+A(
;og .......o
+L (a Lewis base)..._ "-
o
L : ' ~/H .0 +/ I 3A ~ I Si..
......o
.._ "-
L I H 18- / 3 /O ~ 3 + / ~,," A1 ...... o
Figure 4. Schematic formation of Bronsted acid sites on H-A1MCM-41 associated with Lewis bases (L). In conclusion, we have demonstrated that both qualitative and quantitative information of the acid sites presented on H-A1MCM-41 samples can be monitored by means of solid-state 31p MAS NMR of the adsorbed phosphorous-containing probe molecules such as TMPO. The formation mechanisms for acid sites in H-A1MCM-41 were also proposed.
4. ACKNOWLEDGMENTS
The support of this work by National Science Council, R.O.C. (NSC89-2113-M-001-102) is acknowledged. REFERENCES
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9V l V l l b ,
l~ t t l l l l
I~.~r
11"!" 1
A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
459
Acidity of calcined AI-, Fe- and La-containing MCM-41 mesoporous materials: An investigation of adsorption of pyridine Nong-Yue He a'b*, Chun Yang c, Zu-Hong Lu a
a
Key Laboratory for Molecular and Biomolecular Electronics of Ministry of Education, Southeast University, Nanjing 210096, P. R. China
b Key Laboratory of New Materials and Technology of China National Packaging Corporation, Zhuzhou Engineering College, Zhuzhou 412008, P. R. China c Department of Chemistry, Nanjing Normal University, Nanjing 210024, P. R. China
A FT-IR of adsorption of pyridine on Fe- and La-containing MCM-41 mesoporous molecular sieve materials was conducted in comparison with Al-containing analogues. Besides weak acid sites, both Bronsted and strong Lewis-acid sites were observed. Interaction of pyridine with strong Lewis-acid sites (1456 cm -l) is stronger than that with Bronsted-acid sites. It seems that the pyridine adsorbed on Bronsted and Me n+ weak Lewis-acid sites obstructed pyridine from adsorbing on strong Lewis-acid sites. It was shown that a band at 1497 cm l was overlapped by the 1492 cm "1 band and ascribed to the pyridine adsorbed on strong Lewis-acid sites. At similar Me203 species content, the acid density sequence is: A1SiMCM-41 > FeSiMCM-41 >LaSiMCM-41. 1. INTRODUCTION Due to the channel diameter within 1.5-10 nm, the synthesis of mesoporous molecular sieves family M41S by Mobil in 1992 threw new light on the conventional concept of synthesis of zeolites [ 1]. For its potential application, the studies on this family of novel materials have been very active in the last decade [2, 3]. It is well known that heteroatoms can modify the physico-chemical properties of zeolite molecular sieves, thus numerous papers about heteroatom-containing mesoporous materials have been published [4-13]. However, though a Fe-containing MCM-41 sample with Si/Fe molar ratio of 75 has been reported, Fe, and La, two common elements widely used in microporous molecular sieves, have not yet been studied in detail. We have successively incorporated Fe(III) and La(III) into the framework (channel wall) of MCM-41 (designated as FeSiMCM-41 and LaSiMCM-41) using CI6H33(CH3)3NBr (CTAB) as template and water glass as silicon source [14]. In our previous publications, some very interesting properties of these materials were reported. FeSiMCM-41
460 is a very active alkylation catalysts [15]; SiMCM-41, A1SiMCM-41 and LaSiMCM-41 mesoporous molecular sieve materials show a very strong photoluminescent (PL) effect [ 16]; Fe-loading SiMCM-41 not only allows the catalytic deposition of carbon to synthesis carbon nanotubes, but also can orientate the growth direction of the carbon nanotubes and control the diameter of the formed carbon nanotubes [ 17]. Recently we investigated the change of the state of Fe and La species in FeSiMCM-41 and LaSiMCM-41 before and after calcination to remove template and studied the acidity of mesoporous materials FeSiMCM-41, LaSiMCM-41, SiMCM-41, A1SiMCM-41 and HA1SiMCM-41 with microcalorimetric studies of the adsorption of ammonia and temperature programmed ammonia desorption method (NHa-TPD) [18-19]. However, the microcalorimetric studies of the adsorption of ammonia and NHa-TPD method can not differentiate Bronsted-acid from Lewis-acid. Because the surface characteristics, such as catalysis, sorption behavior and so on, of a solid material show close relationship with the nature of its surface acid sites, it is very important to investigate the Bronsted-acid and Lewis-acid of mesoporous materials. Generally, pyridine is applied as probe molecule to obtain information about Bronsted-type acid sites and Lewis-type acid sites. In this paper, we will report the Bronsted-acid and Lewis-acid properties of A1SiMCM-41, FeSiMCM-41 and LaSiMCM-41 by means of Fourier transform infrared (FT-IR) following adsorption of pyridine on these samples. 2. EXPERIMENTAL
2.1. Samples The synthesis of samples has been reported previously [ 14]. The resulting solid products were recovered by filtration, extensively washed with distilled water, and dried in air at ambient temperature, followed by calcination at 813 K in air for 6 h to remove template. 2.2. Characterization The low-angle powder X-ray diffraction patterns were recorded for calcined samples on a Rigaku (D/max-Y A) X-ray diffraction instrument with Cu-Kcz radiation to verify their hexagonal phase structure. The adsorption of nitrogen at 77 K was conducted on a Micromeritics ASAP2000 instrument to analyze the diameters of the investigated samples. The compositions of samples were obtained on a Jarrell-Ash 1100 inductively coupled plasma quantometer. For each of the investigated samples the adsorption of pyridine was conducted after activation. The samples (10 mg) were ground by hand with a pestle in a mortar for 5 min and were then pressed at 4 tons to give a self-supporting pellet (15 mm in diameter, 6 mg/cm2). The wafer was placed inside a glass cell equipped with CaF2 window. After degassed at 723 K for 5 h to reach the vacuum of 5xl0 3 Pa, the samples were cooled to room temperature, then pyridine gas was introduced into the cell. After equilibration of samples with pyridine at room temperature for 5 min, the temperature was increased to 423 K at a heating rate of 5 K/min and was remained at that temperature for 0.5 h while the samples were degassed. Then the samples were cooled to room temperature to allow the FT-IR spectra to be recorded. Similarly the samples were evacuated at 523, 623 and 723 K for 0.5 h and
461 cooled to room temperature to acquire the corresponding FT-IR spectra respectively. Spectra were recorded on a Nicolet 510P FT-IR spectrometer with a resolution of 2 cm -~. 3. RESULTS AND DISCUSSION
3.1. physieo-chemical properties of samples All the as-synthesized samples showed a XRD pattern (not shown here) matching with that first reported by Kresge et al. for hexagonal mesoporous molecular sieves materials [1]. After calcination to remove template, all the XRD patterns of investigated samples remained typical patterns of hexagonal phase with increase in diffraction strength and a shift to lower 20 direction (partiallyl shown here). Shown in Figure 1 are the XRD patterns for a few typical calcined samples, all the patterns show a very strong (100) diffraction peak and 2-3 weak peaks at relatively higher 20 positions. The composition and pore sizes of the tested samples are shown in Table 1.
Figure 1. XRD patterns of calcined (a) AISiMCM-41(118), (b) FeSiMCM-41 (124) and (c) LaSiMCM-41(147).
Table 1. Physico-chemical properties of calcined samples. .
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Samples a
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s'uri;ace
Molar composition of ca.!cined'samples 'Pore sizeb sio2/A1203 SiO2/Fe203 SiO2/La203 (nm)
AISiMCM-41 (118)
118
AISiMCM-41 (58) AISiMCM-41 (32)
area (m2/g)
3.3
l 107
58
3.3
1084
32
3.3
1049
FeSiMCM-41 (124)
124
3.3
998
FeSiMCM-41 (62)
62
3.3
947
FeSiMCM-41 (32)
32
3.3
932
LaSiMCM-41 (147)
147
3.3
974
LaSiMCM-41 (83)
83
3.3
932
LaSiMCM-41 (48) 48 3.3 893 aNumbers in parentheses are SiO2)Me ratios, Me=AI, Fe and La for the corresponding samples, respectively. b From BJH method.
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462 Compared with microporous zeolites, all the MCM-41 type mesoporous molecular sieve samples show a much larger pore size (-3.3 nm) and larger surface area.
3.2. Adsorption of pyridine NH3-TPD (Temperature-Programmed-Desorption) technique is a conventional technique to detect the acidity of a solid catalyst, but it can only acquire the overall information of the acid strength and amount and is difficult to detect the acid type and subtle change in acid density and strength. NH3-microcalorimetric measurement can investigate the subtle change in acid density and strength but still is difficult to distinguish the Bronsted-type acid from Lewis-type acid [20]. Usually, pyridine was chosen as the probe molecule since it enables a distinction between weak and strong Lewis- and Bronsted-type acid sites, i.e., hydroxy groups that interact with pyridine by hydrogen bonding or electron-pair donation and hydroxy groups that form pyridinium ions upon adorption [21-23]. In the FT-IR investigation of pyridine adsorption on samples, the bands at-1545,-~1490 and 1625 cm 1 were assigned to pyridinium ions formed on Bronsted-acid sites and those at 1600 and 1449 cm 1 hydrogen-bonded pyridine, the band at 1455 cm ~ was attributed to the the presence of a small fraction of strong Lewis-acid sites [23, 24]. In other publications, the band at-~1449 cm l was associated with the adsorption of pyridine on Me n+ (metal cations) such as Na +, K +, Cs +, La3+ and so on [25-27]. In this paper, we mainly described the changes in bands at-1545,-1490, 1455 and-1449 cm ~. Shown in Figure 2-4 are the FT-IR spectra of pyridine adsorption on AISiMCM-41, FeSiMCM-41, LaSiMCM-41 samples, respectively. At similar MezO3 species content, the acid density sequence is: A1SiMCM-41 >FeSiMCM-41 >LaSiMCM41. From Figure 2, as previously reported by Jentys and co-workers for Al-containg MCM-41 samples [23], we can find that bands assigned to pyridinium ions formed on Bronsted-acid sites (-1491, -1546 cm I) and to pyridine adsorbed on strong Lewis-acid sites (1456 cm 1) were observedclearly on all the AISiMCM-41 samples and the adsorption intensity Figure 2. Difference FT-IR spectra of pyridine increases with the AI content, meaning adsorbed on calcined (A) AISiMCM-41 (118), (B) both the Bronsted-acid and Lewis-acid AISiMCM-41(58) and (C) AISiMCM-41 (32) at densities increased with the increase in AI (a) 423 K, (lo)523 K, (c) 623 K and (d) 723 K. content.
463
Wavenumber (cm -1) Figure 3. Difference FT-IR spectra of pyridine adsorbed on calcined (A) FeSiMCM-41(124), 03) FeSiMCM-41(62) and (C) FeSiMCM-41 (32) at (a) 423 K, (lo) 523 K, (c) 623 K and (d) 723 K.
Wavenumber (cm "1)
Figure 4. Difference Fr-IR specWa of pyridine The presence of band a t - 1 4 4 9 cm -1 in adsorbed on calcined (A) LaSiMCM-41(147), (B) LaSiMCM-41 (83) and (C) LaSiMCM-41 (48) at Figure 2 is different from the observation by Jentys et a1.[23]. They reported that the 1449 (a) 423 K, (b) 523 K, (c)623 K and (d) 723 K. cm ~ band was only observed on the samples with a SIO2/A1203 ratio <100. For our samples, this band, as a shoulder band overlapping with band 1456 cm -l, also appeared on the sample with a SiQE/A1203 ratio =118 (see the spectrum a in Figure 2A). The intensity of this band increased with the increase in A1 content. When the SIO2/A1203 ratio decreased to 32, owing to the acid density increase, this band even developed into a detectable band only partially overlapping with the 1456 cm t band. Its intensity significantly decreased when temperature was increased from 423 K to 523 K, indicating the acid strength of the corresponding acid site is weak. In accordance with the assignment by Parry [24], Jentys et al. assigned this band to the hydrogen bonding of pyridine to weakly acidic SiOH groups when investigated the nature of hydroxy groups in MCM-41 samples [23]. In other publications [25-26], Ward ascribed this band to metal ions such as Na +, K +, Cs +. When investigating the LaCla-loaded NaY samples, Karge et al. also detected a band at 1449 cm -I due to La 3+ ion [27]. In this research, although needing more detailed investigation, we ascribe this band to the Na + and Laa+(for La 3+, in reference to Figure 4). The relative concentration of acid sites can be determined from the intensities of bands. The acid density ratios of the weak acid of Me n+ (1449 cm l ) ion sites to strong Lewis-acid sites (1456 cm -I) were different each other for these samples. In contrast to that the acid concentration from Me n+ is much smaller than that of strong Lewis-acid sites for A1SiMCM-41 samples, the intensity of band at 1449 cm -1 (a shoulder band at -1450 cm -I for FeSiMCM-41 in Figure 3 , - 1 4 4 7 cm -1 for LaSiMCM-41 in Figure 4) was stronger than that
464 at 1456 cml.when degassed at 423 k. This change in relative acid concentration will be further discussed below. The difference in wavenumbers (meaning different acid stength) for the bands due to pyridine on M_n+ e , - 1 4 4 7 , - 1 4 5 0 , - 1 4 4 7 cm l for AI-, Fe-, La-containing samples respectively, is suggested to result from the difference in the kinds of Me n+. It is because that not only Na + cation, but also a small amount of A13+, Fe 3+ or La 3+ cations were possibly contained in samples [ 19]. The behavior of bands at-1456 cm ~ and-1546 cm ~ was similar for all the samples in Figure 2-4. Although the intensities for both bands decreased with the increase in degassing temperature, the intensity of-1546 cm 1 band decreased more significantly than that of-1456 cm l. After degassing at 723 K, no 1546 cm 1 band was detected for all samples, whereas the band at 1456 cm -z still showed intensive adsorption for all samples, meaning the interaction between pyridine probe molecular and strong Lewis-acid sites (1456 cm "1) is stronger than that between pyridine and Bronsted-acid sites (1546 cm ~) for our samples. The relationship between the intensity of bands and Me contents (Me=A1, Fe and La) is not similar in Figure 2-4. In the applied SiO2/Me203 range, for A1SiMCM-41 samples, the intensities of all bands increased with AI content (Figure 2), whereas intensities of all bands for Fe- and La-containing samples decreased with the Fe or La content (Figure 3-4). These phenomena can be easily understood when taking the states of Me species in samples into consideration. In our previous report [19], we found that Fe and La species, especially La species, can be incorporated into framework channel wall of the synthesized MCM-41 samples. However, because Fe-O bond (0.197 nm) or La-O (0.254 nm) is much longer than Si-O bond (0.161 nm), the incorporation of Fe(III) or La(III) into channel wall to replace Si-O bond will severely distort the tetrahedron MeO4 and is, therefore, more difficult than the incorporation of A1 species. Thus the content of Fe(III) or La(III) species in framework was very limited, no matter what was the total content of these species in the as-synthesized samples. Whereas the length of A1-O bond (0.175 nm) is only slightly longer than that of Si-O bond, making it easy for A1 to insert into framework, in accordance with observation b y Schmidt et al..[28]. Moreover, because the Fe-O or La-O bond is longer, not only the incorporation of Fe(III) or La(III) into framework (channel wall) is limited, but also the stability of Fe(III) or La(III) species in channel wall is influenced. By means of ESR (electron spin resonance) and M6ssbauer spectroscopy techniques [ 14, 19], it was found that the Fe(III) species in FeSiMCM-41 samples gradually transformed from tetrahedrally coordinated state to octahedrally coordinated state upon calcination at 813 K in air to remove template, and that most of Fe(III) species migrated from framework to the outer surface of channel wall. XRD study also showed that La(III) species transformed from framework state to highly dispersed non-framework state in stead of XRD detectable phase for the La(III) loaded SiMCM-41 sample prepared by mixing La203 and SiMCM-41[19]. Therefore, the intensity of band at -1450 (Figure 3) and -1447 cm l (Figure 4), owing to counterions, decreased with the Fe or La species content, giving rise to a lower intensity ratios of-1450 cm -~ to 1456 cm -~, 1447 cm -1 to 1457 cm 1, respectively for FeSiMCM-41 and LaSiMCM-41 samples. Unlike Fe and La species, however, although part of A1 species also changed from tetrahedrally coordinated state to octahedrally coordinated state upon calcination to remove template [14, 28], A1
465
content in channel wall of both synthesized and calcined A1SiMCM-41 samples increased with the total A1 content. Thus the intensity, i.e. the acid concentration of A1SiMCM-41 samples increased with A1 content, whereas intensities of all bands for Fe- and La-containing samples decreased with the Fe or La content. As a result of this phenomenon, the concentration of counterions will be also increase with A1 content [see band at 1447 cm -~ in Figure 2] and decease with Fe or La species content (1450 and 1447 cm -l in Figure 3 and 4, respectively). It is interesting that when the degassing temperature for LaSiMCM-41 samples was increased from 423 K to 523 K (see Figure 4), the intensity of band at-1456 cm -I increased with a decrease in 1447 cm -I band. It seems that the pyridine molecules adsorbed on Bronsted-acid and weak Me n+ Lewis-acid sites somewhat obstruct pyridine molecules from adsorbing on strong Lewis-acid sites. When the pyridine molecules on Bronsted-acid and weak Me "+ sites were removed upon evacuation, the strong Lewis-acid sites were mostly exposed to pyridine molecules and, therefore, the intensity of band at-~1456 cm l increased. The situation was also fit for the A1SiMCM-41 (118) sample (see curve a and b in Figure 2A). Other samples could not be discerned owing the severe overlapping of 1456 cm l band with 1449 cm l band. Finally, the nature of the acid sites associated with the band at -1492 cm ~ is worthy of some consideration. This band was usually assigned to pyridinium ions formed on Bronsted-acid sites [23-24]. However, under our investigation, band at-1497 cm "l was found to be overlapped in 1492 cm -~ band. Figure 2-4 demonstrated that when the band at 1546 cm-l-disappeared, the 1492 cm -~ also disappeared indeed, but a band at-~1497 cm -1 appeared. Because this band simultaneously presented with the 1456 cm -1 band assigned to Strong Lewis-acid sites, it is suggested to the pyridine adsorbed on strong Lewis-acid sites. 4. ACKNOWLEDGEMENT This research is supported by the Chinese 863 High-Tech Project, 973 National Key Fundamental Research Project, of P. R. China, Natural Science Foundation of P. R. China, the High-Tech Program of Jiangsu Province, P. R. China, the Visiting Professor Program and Excellent Teacher's Fundation of Ministry of Education, P. R. China, and the Hangke Fund of Southeast University.
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Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
467
Acid properties of ammonium exchanged A1MCM-41 with different Si/A1 ratio Antonio S. Araujo *'t, Cristiane D. R. Souza l, Marcelo J. B. Souza l, Valter J. Fernandes Jr l, and Luiz A. M. Pontes 2 1Federal University of Rio Grande do Norte, Department of Chemistry, CP 1662, CEP 59078-970, Natal, RN (Brazil), *
[email protected] 2 UNIFACS- Department of Chemical Engineering, Av. Cardeal da Silva, 747, CEP 40.220-141, Federa~ao, Salvador, BA (Brazil). The mesoporous A1MCM-41 catalysts with Si/A1 ratio of 15, 30, 45 and 60, were synthesized by the hydrothermal treatment at 373 K, of a gel prepared from aluminum nitrate, sodium silicate, silica gel, water and cethyltrimethylammonium bromide as organic template. The materials were recovered by filtration, drying and calcination at 823 K. They were characterized by X-ray diffraction, thermal analysis, infrared spectroscopy and scanning electron microscopy. The H-A1MCM-41 acid form was obtained from ion exchanged with a 0.1 mol.L -1 ammonium chloride solution followed by calcination. The catalytic activities of the materials were evaluated to n-heptane cracking at temperatures of 623, 673 and 723 K and weight hourly space velocity of 29.1 h -l. The products were analyzed by coupled gas chromatography and mass spectrometry. The catalysts presented activity to cracking of n-heptane, showing selectivity to C3 (propane and propene). A higher activity at 723 K and Si/A1 ration of 45 was observed. Neural network modeling showed that to a simulated Si/A1 = 51, a maximum yield is obtained. The acid properties was evaluated as the catalytic actitvity for n-heptane craching. 1. INTRODUCTION Actually many researches have been applied in mesoporous materials field [1-3]. The use of these materials in chemical engineering, chemistry and catalysis its increasing in the last years. High surface area, well defined mesoporous array and possibility of generate surface acidity are any potential characteristics of the mesoporous materials. The silica based MCM-41 nanostructured is the main material of the M41S family, discovered by Mobil Company. This material has practically no acidity, with respect of Br6nsted and Lewis acid sites, and its application is limited to catalytic applications. Many researches have been showed who the aluminum incorporation [4-6] in the MCM-41 possibility the conception of the aluminossilicate A1MCM-41 and possibility the existence of the Br6nsted acidity. The generation of the Br6nsted acidic sites in the
468 mesoporous surface favor the catalytic activity and selectivity for many industrial reactions as isomerization, alkylation, cracking and hydrocracking [7-9]. In this field a important parameter is the silicon/aluminum (Si/A1) ratio. The variation of the Si/A1 ratio to a optimized value, promote a increasing in the superficial acidity and, consequently, a good potential for catalytic reactions. The aim of this work is study the influence of the Si/A1 ratio and temperature in the catalytic activity of the n-heptane cracking reaction. A neural network modeling [10] was used with the objective of simulate the best Si/A1 and temperature to obtain the optimized degrees of conversion of the n-heptane in natural gas. 2. EXPERIMENTAL
For the A1MCM-41 hydrothermal synthesis, the following reactants were used: silica gel (Riedel), sodium silicate (Riedel), aluminum nitrate (Merck), distilled water and the cethyltrimethylammonium bromide (CTMABr; Merck), as surfactant. The chemicals were mixed in order to obtain gels with the molar composition: 4 SiO2. (1 + x/2) Na20. x Al(NO3)3. CTMABr. 200H20, with Si/A1 molar ratio of 15, 30, 45 and 60. The pH of the hydro gels were adjusted [ 1 l] to 10 with 30% acetic acid aqueous solution, and then they were autoclaved at 373 K, by 4 days. The materials were washed with deionized water, filtered, dried at 363 K, and calcined at 823 K for 4 h in N2 atmosphere and then for 1 h in air. The temperature was increased at heating rate of 2.5 ~ The obtained materials were characterized by the following techniques: X-ray diffraction (Rigaku), thermal analysis (Mettler TGA/SDTA-851) and chemical analysis (EDXS00). The acid form of the material was obtained by exchanging the A1MCM-41 with a 0,1 mol.L I ammonium chloride solution at 80~ under reflux for 6 h, followed by filtration drying and calcination at 823 K using N2 flowing at 100 mL.min ~, for 2 h. The acid properties of the materials were evaluated by adsorption of n-buthylamine at 363 K and its dessorption followed by thermogravimetry, from 363 to 773 K, at heating rate of l0 ~ in a Mettler TGA/SDTA-851 thermobalance. The catalytic tests were accomplished in a fixed bed continuous flow reactor [ 12], at temperatures of 473, 513 and 553 K, and WHSV (Weight Hourly Space Velocity) of 29 h l. A mass of 0.1 g of catalyst was used and a hydrocarbon / inert ratio of 2.25. The products were analyzed by gas chromatography in a CG17A coupled in a mass spectrometer QP5000 using a Petrocol DH 50.2 column. 3. RESULTS AND DISCUSSION During the synthesis, the initial pH values of the hydro gel were extremely basic (pH 14). In order to control the self-assembly process of silica species, the pH was adjusted and maintained at about 9.5-10 during the synthesis. The pH was checked each day, and two adjustments were required for MCM-41 samples. According to this procedure, good quality MCM-41 mesostructures were obtained. The table 1 shows the gel compositions, acidity parameters and respective x-ray difractogram parameters. The catalytic testes accomplished in a fixed bed continuous flow reactor shown that the H-A1MCM-41 materials presents good activity to the catalytic cracking of the nheptane. The products obtained were: propane, propene, butane, isobutane, 2-methylbutane and pentane. The chromatogram of the figure 1.a is a blank run without catalyst and shows
469 that 723 K, the thermal cracking reaction is not observed. The figure 1.b shows the typical products of the catalytic cracking over H-A1MCM-41 material. Table 1. Gel compositions, acidity parameters and respective x-ray difractogram parameters to the A1MCM-41 samples. Si/A1 ratio
Total Acidity (mmol/g)
20
Gel composition
d(100) nm
4SIO2.1.13Na20.0.27Al(NO3)3.CTMABr.200H20 4SIO2.1.06Na20.0.13AI(NO3)3.CTMABr.200H20 4SIO2.1.04Na20.0.08AI(NO3)3.CTMABr.200H20 4SIO2.1.03Na20.0.06AI(NO3)3.CTMABr.200H20
15 30 45 60
0.175 0.158 0.215 0.096
2.02 2.05 2.00 2.00
4.42 4.31 4.48 4.48
r,/3
b)
0
5
10
15
Retention time (min) Figure 1. Typical chromatogram of products analysis in the n-heptane cracking over HA1MCM at 723 K. a) Blank run without catalyst: 1 = nitrogen; 2 = n-heptane and b) with HA1MCM-4141 (Si/A1 = 45): 1 = nitrogen; 2 = propane; 3 = propene, 4 = butane; 5 = isobutane; 6 - ciclobutane; 8 - 2-methyl butane; 9 = pentane; 11 - heptane.
470 The acid properties of the materials were evaluated by adsorption of n-buthylamine at 363 K and its dessorption followed by thermogravimetry, from 363 K to 773 K, at heating rate of 10 ~ It was observed that the introduction of aluminum in the MCM41 generates acid sites on its surface. The main products obtained were propane, propene, isobutane and butane. The figure 2 presents a comparison of the propane selectivity in the n-heptane cracking over H-A1MCM-41 (Si/A1 = 15, 30, 45 and 60) and the propane selectivity using the same reaction with HY zeolite (Si/A1 = 2.5) to several temperatures. It was observed a gradual increasing of the propane selectivity with the temperature for all catalysts. The maximum selectivity to propane over H-AIMCM-41 was obtained with Si/Al=45 at 723 K.
Figure 2. Selectivity of the propane in n-heptane cracking reaction over H-A1MCM-41 (Si/A1 = 15, 30, 45 and 60) and HY zeolite (Si/A1 = 2.5) to several temperatures. The experimental obtained results were applied to generation of an empirical model using neural networks algorithms, by the retro propagation technique (backpropagation) [ 10]. The neural networks can be applied in many science fields to modeling of complex systems, e.g. processes with catalytic reactions. This networks are parallel systems constituted of single process units, the neurons. This neurons as used to calculate specifical math functions. The figure 3 presents the values of degree of conversion versus temperature to several samples of H-ALMCM-41 (Si/A1 = 15, 3 0 , 4 5 and 60). It was observed that the
471 degree of conversion of the n-heptane increase with the temperature for all samples. The catalysts shown high activity for the process with selectivity for C3 products. According to the Si/A1 ratio, the catalytic activity increased in the following order: 45 > 15 > 30 > 60. These results are directly correlated with the acidity of the materials. A higher activity at 723 K and Si/A1 ration of 45 was observed. Neural network modeling showed that to a simulated Si/A1 = 51, a maximum yield is obtained.
Figure 3. Degree of conversion versus temperature of the n-heptane cracking over HA1MCM-41 (Si/A1 = 15, 30, 45 and 50). 4. CONCLUSIONS The catalytic activity for the H-A1MCM-41 materials, with different Si/A1 ratios, was evaluated by reactions of n-heptane catalytic cracking. The obtained results shows that aluminum incorporation in the mesoporous array produces a acid solid with catalytic cracking potential applications. The catalysts showed high activity for the process with selectivity for C3 products. According to the Si/A1 ratio, the catalytic activity increased in the following order:- 45 > 15 > 30 > 60. These results are directly correlated with the acidity of the materials. ACKNOWLEDGEMENTS The authors acknowledge the support from the Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq) and Ag6ncia Nacional do Petr61eo (ANP).
472 REFERENCES
1.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
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. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. C.T. Kresge, M. E. Leonowicz, J. S. Beck, J. C. Vartuli and W. J. Roth, Nature, 359 (1992)710. X.S. Zhao, G. Q. Lu and G. J. Millar, Ind. Eng. Chem.Res., 35 (1996) 2075. B. Lindlar, A. Kogelbauer and R. Prins, Microp. Mesop. Mater., 38 (2000) 167 S. Biz, M. L. Occelli, A. Auroux and G. J. Ray, Microp. Mesop. Mater., 26 (1998) 193. S.C. Shen and S. Kawi, J. Phys. Chem. B, 103 (1999) 8870 A. Liepold, K. Roos and W. Reschetilowski, Chem. Eng.Sci., 51 (1996), 3007 A. Corma and D. Kumar, Stud. Surf. Sci. Catal., 117 (1998) 201. T . J . Pinnavaia, P. T. Tanev, W. Zhang, J. Wang and M. Chibwe, US Patent 5.855.864 (1999). S. Haykin, Neural Networks, Prentice-Hall, Inc., (1997). A.S. Araujo and M. Jaroniec, Termochim. Act., 363 (2000) 175. A.S. Araujo, M. J. B. Souza, T. B. Domingos and A.O.S. Silva, React. Kin. Catal. Lett. 73 (2001) 283.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
473
Kinetic evaluation o f the pyrolysis o f high density p o l y e t h y l e n e over H - A 1 M C M - 4 1 material Antonio S. Arat]jo *'a, Valter J. Fernandes Jra, Sulene A. Araujo b, Massao Ionashiro b Federal University of Rio Grande do Norte, Department of Chemistry, CP 1662, 59078-970, Natal, RN (Brazil)
a
b Institute of Chemistry, UNESP, CP 355, 14801-970, Araraquara, SP (Brazil)
The A1MCM-41 material with Si/AI=50 was synthesized by hydrothermal method, using cethyltrimethylammonium as template. The protonic H-A1MCM-41 acid form was obtained by ion exchange with ammonium chloride solution and subsequent calcination. The characterization of the material by several techniques showed that a good-quality MCM-41 material was obtained. High-density polyethylene (HDPE) has been submitted to thermal degradation alone, and in presence of the exchanged H-A1MCM-41 catalyst at a concentration of 1:1 in mass (H-A1MCM-41 / HDPE). The reactor was connected on line to a gas chromatograph connected to a mass spectrometer. This process was evaluated by thermogravimetry (TG), from 350 to 600~ under helium dynamic atmosphere, with heating rates of 5.0; 10.0 and 20.0 ~ From TG curves, the activation energy, calculated using a multiple heating rate integral kinetic method, decreased from 225.5 KJ.mol -~, for the pure polymer (HDPE), to 184.7 KJ.mol -~, in the presence of the catalyst (H-A1MCM-41 / HDPE).
1. INTRODUCTION Pyrolysis methods for the catalytic recycling of waste is a promising way to convert polymer materials into low molecular weight chemicals which can be used as raw materials for the chemical and petrochemical industry. Catalytic pyrolysis of polyolefins is of great interest because of their potential use as fuels or chemical resource [ 1]. The mainly catalysts used were Y and ZSM-5 molecular sieves [2-5], silicoaluminophosphate [6], silica-alumina [7], sulfated zirconium oxide [8], mesoporous materials [9,10] and "used" FCC catalysts [11 ]. The applications of mesoporous M41S materials opened new perspectives in the absorption and catalysis fields because they present an organized wide pores system. They can be applied to conversions of molecules of voluminous size, like polymers. Recent studies show the application of mesoporous MCM-41 as active catalyst for polyethylene degradation [ 10]. The degradation of polymer was done aiming the use of polymeric residues as source of energy to reduce the carbon monoxide concentration strongly, providing a reduction of toxicant gases, allowing the obtaining of hydrocarbons of commercial interest, specifically in the range of fuels (light gases, gasoline and diesel oil). The use of acid catalysts can enhance the thermal degradation of synthetic polymers which may be monitored by thermogravimetry [6]. In this work the degradation of high
474 density polyethylene (HDPE) was processed in the presence of acid H-A1MCM-41 and the results of the catalytic and thermal degradation were compared. The kinetics of the process was monitored by TG, using integral dynamic curves at multiple heating rates, and the activation energy was estimated from the Flynn and Wall kinetic model. 2. EXPERIMENTAL
The aluminum MCM-41 material was synthesized by the hydrothermal treatment of a gel with molar composition: 4SIO2.1.04Na20:0.08AI(NO3)3.CTMABr.200H20, where CTMA represents the cethyltrimethylammonium template. In a typical synthesis, a mixture containing sodium silicate (Riedel-de Harn), silica gel (Riedel-de Harn), aluminum nitrate (Merck) and destiled water were homogenized at 60 ~ for 2 h, under continuous stirring. To this mixture, an aqueous solution of CTMABr (Merck) was added, and stirred for 1 h at room temperature. The reactive hydrogel were placed into the autoclave and submitted to a hydrothermal treatment at 100 ~ for a period of 4 days. Each day, the pH was adjusted to 10 with a 30% acetic acid solution. For the structure stabilization, sodium acetate (Carlo Erba) was added to the product (salt/CTMABr = 3), and the material was heated for 1 day more. The obtained material was washed with a 2%vol. HC1/EtOH solution, recovered by filtration and dried at 100~ for 2 h, and then calcined at 550 ~ with nitrogen for 1 h, and for an additional time of 1 h in air, at a heating rate of 2,5 ~ The characterization of the material from X-ray diffraction (Rigaku), thermal analysis (Mettler TGA/SDTA-851), scanning electron microscopy (Stereoscan-440), infrared spectroscopy (Bomen-102) and nitrogen adsorption isotherm analysis (Micromeritics-ASAP 2000) characterized the typical MCM-41 nanoporous structure. The acid form of the material was obtained by exchanging the A1MCM-41 with a 0,1 mol.L-1 ammonium chloride solution at 80~ under reflux for 6 h, followed by filtration drying and calcination at 550~ using N2 flowing at 100 mL.min 1, for 2 h. The activity of the catalyst for the pyrolysis of HDPE is related with the acid sites present on the surface of the H-A1MCM-41 material. The kinetic parameters for decomposition of the HDPE alone and with catalyst (HA1MCM-41/HDPE) were determined in a simultaneous TG/DTA system (Mettler SDTA851), in the temperature range from 300-600 ~ with heating rates of 5.0; 10.0 and 20.0 ~ and atmosphere of argon flowing at 60 mLmin-1. Applying the Flynn and Wall multiple heating rate kinetic model, the values of activation energy to the processes were obtained. High-density polyethylene (HDPE), in powder form (50 mesh), was obtained from Palmman of Brazil. The calcined H-A1MCM-41 was added to the HDPE at a concentration of 50 wt%. The sample (H-A1MCM-41 / HDPE) was transferred to a tubular quartz microreactor, with a 3-way valve, heated at 600~ under static argon atmosphere. The reactor was connected on line to a Shimadzu GC 17-A gas chromatograph coupled to a QP-5000 mass spectrometer, and the products were analyzed with a capillary column type Petrocol DH-50.
3. RESULTS AND DISCUSSION
In the HDPE thermal degradation process one can observe'normally two different reaction types: the polymeric combustion and the thermal cracking. The thermal cracking produces hydrocarbons in the range of C5 to C26. The combustion produces CO2 and water.
475
C21 Cz,~
(a)
C7
11)
<j .m
C 6
C13
5O
t.-
o,1 I
t'm
0
.
0
1
25
J t~ 5O
Retention time (min)
tl 1 05
~. v
C7 ~
1
13 C14
50
0
0
25
50
Retention time (min)
Figure 1. Chromatograms of the reaction products of the pyrolysis of polyethylene: (a) without catalyst; (b) mixed to H-A1MCM-41.
476 The use of atmosphere of argon under the HDPE and H-A1MCM-41/HDPE eliminates the oxidant atmosphere and consequently the combustion reactions. The CG/MS analysis showed selectivity to C3 - C~4 hydrocarbons to the degradation of H-A1MCM-41/HDPE and C9 - C26 to polymer without catalyst. Typical chromatograms are shown in Figure 1. According to the chromatograms shown in the Figure 1, it is observed that the pyrolysis of the high-density polyethylene produces hydrocarbons in a wide range of carbon number, typically from C9 to C26. When H-A1MCM-41 catalyst is added to the polymer, it is observed that these hydrocarbons convert to products with a small number of carbon atoms, specifically in the range of C3-C4 (fraction of liquid petroleum gas), C5+ fraction, C6-C9 (gasoline) and Cl0-Cls (diesel oil). The acid H-A1MCM-41 showed to be very active to the pyroysis of polyethylene, being a promising catalyst to this process. It is proposed that when occurs the thermal degradation of the polymer, the hydrocarbons fractions access the mesoporous of the HA1MCM-41, interact with its acid sites, and promote the high hydrocarbon chain cracking to low hydrocarbons, typically in the range of C3 to C14. A general mechanism for polymer cracking can be proposed by Holmstrong et al. [12], in which, after the carbon chain, several reactions of recombination (R) or disproportionation (D) occurs, as follows: R-*CH2 R-*CH2 R-*CH2 R-*CH2
+ + + +
R'-*CH-CH2-R" --> R-CH2-CHR'-CH2-R" R'-*CH-CH2-R" --> R-CH3 + R'-CH-CH-R" *CH2-R" --) R-CH2-CH2-R' *CH2-R" -~ R-CH3 + CH2=CH-R '
(R) (D) (R) (D)
In general, the obtained compounds by HDPE pyrolysis were paraffin, olefins and aromatics, and they were grouped as C,, where "n" represents the number of carbons. The velocity of the catalytic cracking of polymer, depends on the conversion (or), temperature (T) and time of reaction (t). In each process, the reaction velocity is given as a function of conversion f(a) and can be determined from experimental data. From thermogravimetric curves for the mixture (H-A1MCM-41/HDPE) at three different heating rate (Figure 2), graphics of degree of conversion (%) as a function of temperature, were obtained, as shown in the Figure 3. One well-defined weight change state is viewed in the degradation process. Through TG curves, were determined the initial, medium and final temperature for the HDPE pyrolysis in presence of catalyst. The use of H-A1MCM-41 solid acid catalyst to the polymer degradation requires information concerning the kinetic parameters, mainly the energy activation relative to the process. Reliable methods for determining the activation energy using dynamic integral TG curves at several heating rates have been proposed by Flynn and Wall [13], where it was demonstrated that the heating hate and the absolute temperature can be related as follows: dlogfl
~0,457~
--L--Y-J E
01)
477 and, inserting the R value 8.314 J.mol~.K ~, an expression obtained for E Clog fl E ~ -4,35 9~ 31/T
100
9
(02)
,~, ,x
...... 5 ~ . . . . 10 ~ - 20 ~
',l
80
o~
'
~" t-
60
I 9
i
.
I
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i
,
i
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20 .
,
0
.
,
150
.
300
.
.
450
,
.
.
600
9
750
900
Temperature (~
Figure 2. Thermogravimetric curves of degradation of H-A1MCM-41/HDPE.
100
80
C .0_ i,.
tO
o
(a)
60
(b) 40
(c) 2o
0 '
350
,t,
,
I
400
'
I
450
'
I
500
'
I
550
'
""
-
600
Temperature ~
Figure 3. Conversion ofH-A1MCM-41 /HDPE in function of temperature at different heating rate: (a) 5 ~ (b) 10 ~ and (c) 15 ~
478 Thus, it was calculated the activation energy related to degradation of a the HDPE in presence of the H-A1MCM-41 catalyst, using the slope of the logarithmic heating rate curves as a function of the reciprocal temperature. The activation energy observed for the polymer degradation without catalyst was 225.5 KJ.mol -~ against 184.7 KJ mol -~ in the presence of the H-A1MCM-41. These results indicate that this material may have acted as a cracking catalyst for the HDPE, enhancing the generation of light products of potential industrial use. The low value of activation energy for evidences that the H-A1MCM-41 mesoporous material is efficiency of the for the degradation process. 4. CONCLUSIONS The products resulted from HDPE pyrolysis by acid H-A1MCM-41 are distributed in a narrow range of carbon, C2-C5, C6-C9 e Cl0-Cl4, typically LPG, gasoline and medium distillate, evidencing that the pyrolysis mechanism is a function of the pore system and the acid properties. The activation energy for the process, as determined from multiple heating rate TG curves and kinetic model, decreased from 225.5 KJ.mol -I (H-A1MCM-41/HDPE) to 184.7 KJ mo1-1 (HDPE), evidencing that the mesoporous H-A1MCM-41 acted as a good catalyst for pyrolysis of polyethylene. ACKNOWLEDGEMENTS
The authors acknowledge the support from the Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq), and Fundagao de Amparo h Pesquisa do Estado de Sao Paulo (FAPESP). REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
(a) Characterization of Municipal Solid Waste in the United States, EPA Report, 530-R94-042, 1994. (b) E. D. Amico and M. Roberts, Chem. Week, 4 (1995) 12. A.R. Songip, T. Masuda, H. Kuwarara and K. Hashimoto, Appl. Catal. B: Environmental, 2 (1993) 153. M.M. Taghiei, Z. Feng, F.E. Huggns and G.P. Huffman, Energy Fuels, 8 (1994) 1228. V.J. Fernandes Jr., A. S. Arafijo and G.J.T. Fernandes, Stud. Surf. Sci. Catal., 105 (1997) 941. P.N. Shrrtt, Y.H. Lin and A.A. Garforth, Ind. Eng. Chem. Res., 36 (1997) 5118. V.J. Femandes, A.S. Araujo, G.J.T. Fernandes, J.R. Matos and M. Ionashiro, J. Therm. Anal. Calorim., 64 (2001) 585. Y. Ishihara, H. Nambu, T. Ikemura, and T. Takesue, Fuel, 69 (1990) 978. X. Xiao, W. Zmierczak and J. Shabtai, Preprints of ACS - Div. Fuel. Chem., 4 (1995) 4. M.A. Uddin, Y. Sakata, A. Muto, Y. Shiraga, K. Koizumi and K. Murata, Microporous Mesoporous Mater., 21 (1998) 557. J. Aguado, J.L. Sotelo, D.P. Serrano, J.A. Calles and J.M. Escola, Energy Fuels, 11 (1997) 1225. E. Dwyer and D.J. Rawlence, Catalysis Today, 18 (1993) 487. A. Holmstrong and E.M. Sorric, J. Appl. Polym. Sci., 18 (1974) 761. J.H. Flynn. and W.A.Wall, Polym. Lett., 4 (1969) 323.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
479
Electrorheological response o f m e s o p o r o u s materials under applied electric fields Min S. Cho a, Hyoung J. Choi a, Wha-Seung Ahn b, and Myung S. Jhon c a
Department of Polymer Science and Engineering, Inha University, Inchon, 402-751, Korea
b Department of Chemical Engineering, Inha University, Inchon, 402-751, Korea c Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 152133890, USA
As a novel candidate for electrorheological (ER) fluids, mesoporous MCM-41 particles suspended in silicone oil, were examined. MCM-41 particles, with a well-defined channel structure analyzed by X-ray diffractometry and transmission electron microscopy, was synthesized. ER fluids were then prepared by dispersing these synthesized MCM-41 particles in silicone oil. These ER fluids exhibit viscosity enhancement and peculiar viscoelastic properties under applied electric fields. We found that the small amount of moisture absorbed in hydrophilic MCM-41 is responsible for the particle polarization at high electric field strengths. The static yield stress of ER fluids were also measured and correlated with a universal scaling function suggested by Choi et al. 1. INTRODUCTION The discovery of the mesoporous materials has opened a new class of molecular sieves exhibiting ordered arrangement of uniform, nanometer size pores. Especially, the silicabased mesoporous materials have been extensively studied in various catalytic reactions [ 1-3]. Other interesting applications, using this unique pore structure and high surface areas of mesoporous materials, are the removal of mercury or heavy metals from contaminated solutions [4], nanometer electronic material [5, 6], and immobilization of small enzymes in the mesopore structure [7]. Recently, mesoporous particles have been also used [8] as a new candidate material for electrorheological (ER) fluids. The ER fluids are typically composed of a suspension of micron-sized particles in a nonconducting fluid, and their rheological response can be changed by an imposed external electric field [9, 10]. Under an applied electric field, ER fluids make rheological property change from a liquid-like state to solid-like state; a stress is required to break the chainlike or columnar structure, and flows afterwards. This stress, referred to as a yield stress, is electric field strength dependent and is the key characteristic parameter for an ER fluid. ER fluids also exhibit increase in their shear viscosities of several orders of magnitude at low shear rates. The solidified ER fluids under an electric field are referred as a Bingham fluid. The Bingham fluid model, popular rheological model for many ER fluids, is described below [ 11 ].
480
~,=0
x<x
(1) Y
Here, x is the shear stress, Xy is the yield stress, ~, is the shear rate, and ~1 is shear viscosity. ER suspensions can be classified as either an intrinsic polarizable (anhydrous) or extrinsic polarizable (hydrous) system. The former system, which are polarized in an electric field due to electrons within the polymer molecule, usually contains semiconducting polymer panicles such as polyanilines and its derivatives [12-15], poly(acene quinone) radicals [ 16], polymer/clay nanocomposites [17], phosphate celluose [18], inorganic particles including PZT [19], and carbonaceous particle [20]. While the latter system uses additives such as water and alcohol. It is generally believed that the anhydrous ER systems are better than the hydrous counterparts in practical applications due to high thermal stability and better dispersion quality. Commercial zeolites have also been tested as a suspended panicle for ER fluids [21,22]. Recently, Cho et al. [22] examined the rheological and dielectric properties of the ER fluids prepared from commercial zeolites 3A, 5A and 13X suspended in silicone oil and found that the ER fluid using zeolite 13X showed the best ER performance. It is noted that porous solid materials including zeolite have been used as adsorbents, catalysts and catalytic supports owing to their high surface areas. However, their applications are limited due to the relatively small pore openings. The pore enlargement was one of the challenge in zeolite research [23], and a myriad of synthetic efforts to enlarge the pore size led to the success in preparation of ordered mesoporous materials with a broad range of pore sizes up to 30 nm [24]. A recent discovery of a new family of crystalline mesoporous molecular sieve materials M41S have attracted considerable interest. They are either a hexagonal phase, a cubic phase, or a unstable lamellar phase which are known as MCM(Mobil Composition of Matter)-41, MCM-48 and MCM-50, respectively [ I, 25]. In our study, the synthesized MCM-41 was dispersed in silicone oil (5 - 20 wt%) and evaluated their ER fluid properties. We also examined the mesoporous materirals as a potential candidate for a dispersed phase in ER suspensions.
2. MATERIALS AND METHODS 2.1. Materials
MCM-41 was synthesized using a 25 wt% aqueous solution of hexadecyltrimethylammonium chloride (HTAC1, Aldrich), 28 wt% ammonia (NH3) solution, and sodium silicate [26]. HTA-silicate gel with a chemical composition of 4SiO2:IHTACI: 1Na20:0.15(NH4)20:200H20 was placed into a polypropylene bottle and heated to 370 K in an oven for 3 days. EDTANa4 and acetic acid treatments were performed during the synthesis to enhance crystallinity and hydrothermal stability. The precipitated product was filtered, washed with distilled water and dried at 370 K for 12 hours in an oven, followed by calcination in 02 at 823 K for 10 hours. 2.2. Measurements
X-ray diffraction (XRD) pattern of the calcined MCM-41 sample was obtained with a powder X-ray diffractometer (Philips, PW-1700) using a CuKa radiation (40kV, 25mA).
481 Nitrogen adsorption isotherm and specific area were determined using Micromeritics ASAP 2000 automatic analyzer. A series of ER fluids were prepared by dispersing the MCM-41 particles in anhydrous silicone oil with the particle concentrations of 5, 10, and 20 wt%. ER properties of the fluid were then measured at (25 + 0.1) ~ using a rotation type rheometer (MC120, Physica, Germany) equipped with a high voltage generator. Measuring unit was a concentric cylinder having 1.06 mm gap between a set of bob and cup, and DC high electric field strength perpendicular to the flow direction was applied to the measuring unit through the cup; the inner wall of the cup is the positive electrode, the bob is grounded. Initially, an electric field was applied to the ER fluid for 3 minutes before the measurement in order to establish the equilibrium internal structure of particles. Flow curves for each ER fluid were obtained in a controlled shear rate mode. Static yield stresses (smallest stress to fluidize an ER fluid under the applied electric field) were measured in controlled shear stress (CSS) experiments.
3. RESULTS AND DISCUSSION Figure l(a) shows the XRD pattern for the calcined MCM-41. Regular hexagonal channel structure of MCM-41 was clearly implicated by the strong (100) peak followed by three [(110) (200) and (211)] peaks in the diffractogram for the 20 ranges of 2 - 7~ (Fig. 1(a)). BET surface area of the sample was ca 950 mZ/g with average BJH pore diameter of 31A. TEM image of Fig. l(b) shows uniform hexagonal shaped pore structure of MCM-41. Various particle morphology of MCM-41 is reported and surface textures exhibit a range from smooth through pitted [27]. According to our Scanning electron microscope (SEM) analysis (not shown), MCM-41 crystals prepared were mostly made of particles of 8 0 - 120 nm in diameter, and existed as agglomerated groups [8].
Figure 1. XRD pattern (a) and TEM image (b) for MCM-41 used in our experiment.
482 Figures 2(a) and (b) show the shear stresses and shear viscosities as a function of shear rates under the various applied electric field strengths for MCM-41 ER system with the particle concentration of 10 wt% in silicone oil. Typical ER characteristics, shear stress increase with electric field strength and also decrease or reaches plateau at low shear rate region [12,13,15], are observed as shown in Figs. 2. MCM-41 is hydrophilic but has no polarizable species such as electrons and ions, so that the MCM-41 ER fluid is believed to be activated by absorbed moisture. The moisture content in MCM-41 was ca 6 wt% from thermogravimetric analysis (TGA).
Figure 2. Shear stress (a) and shear viscosity (b) as function of shear rates for MCM-41 ER fluids (10 wt% MCM-41 particles in silicone oil) under different applied electric field strengths. (c) An optical microscopic photo of an ER fluid (5 wt% MCM-41) under zero electric field strength. (d) Optical microscopic photo of the same fluid as (c) under the electric field strength of 3kV/mm. Black parts in (c) and (d) are electrodes, which are separated by 0.5 mm.
483 As shown in Fig. 2(d), the variations in flow properties of ER fluids are due to the particle chain structures formed by the applied electric field. The instantaneous transition from liquid to solid structure stems from the particle microstructures. An applied deformation over certain level breaks the particular chains and these can be reformed as long as electric field is applied. The energy consumption during destruction of particle structures is responsible for the viscosity enhancement. The ER effect disappears under high deformation rate (shear rate), because there is no enough time for broken particles to reform structures at such high shear rates (Figs. 2 (a) and (b)). The particle chains are maintained under sufficiently small strain, and they are stretched and rearranged by the deformation and produces the viscoelasticity of solidified ER fluids under an applied electric field. 10 5
(a)
(b) /k
A
10 2
v v v v v v v v V v
Q.
A
L___,
~~
~ O
03 t~
0
oolO
0
I~. 104
0
0
1
(D t-C0
0
0
0
0
0
0
0
[]
10 0 10 -2
,
zX 20 wt% o 10 wt% [] 5 wt%
.
, ....
,I
10 -1
,
,
......
13
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. . . . . . .
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101
. . . . .
,,,I
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Shear rate [sec -~]
. . . . . .
ul
10 a
[] []
I
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. . . . . . . .
[]
V
3.0 kV/mm 2.0 kV/mm o 1.0 kV/mm a. 0.5 kV/mrn
O
[] . . . . . . . .
10a0-1 1
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i
. . . . . . . .
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Frequency [sec -~]
Figure 3. (a) Shear stress as function of shear rates for MCM-41 ER fluids for three different particle concentrations (5, 10 and 20 wt%) at 3 kV/mm. (b) G' as a function of deformation frequency with strain of 0.0015 for 10 wt% MCM-41 ER fluid.
Shear stress data of MCM-41 ER fluids having three different particle concentrations (5, 10, and 20 wt%) are shown in Fig. 3(a). A sample with 5 wt% MCM-41 exhibited little ER effect ; the slope of shear stress versus shear rate in log plot was almost 1.0, (i.e., Newtonian fluid behavior). The samples with 10 wt% and 20 wt% of MCM-41 demonstrate similar flow behaviors as given in Fig. 2. An observed, large ER property above the "critical particle concentration" is noteworthy. More careful measurement on this observation will be provided in future communication. Dynamic tests employing an oscillatory shear were also conducted to study the viscoelastic properties of the solidified ER fluid under an applied electric field. Linear viscoelastic response is obtained at strain of 0.0015 from the strain amplitude sweep measurement, which measures stress as a function of sinusoidal strains at a constant frequency. The storage modulus (G'), the in-phase stress component with the strain, is observed to be larger than the loss modulus (G"), the out-of-phase stress component. These values were independent of the strain in the linear viscoelastic region. However, as we
484 increased the applied strain, G" becomes larger than G', and these moduli sharply decreased. This phenomenon can be explained in terms of the elasticity of ER fluid, which is generated by particle chain structures in an imposed electric field [28, 29]. When the fibril structures of the suspended zeolite particles sustain the applied strain, the elasticity is dominant in the linear viscoelastic region. However, as the strain is increased, the deformation begins to distort the structure, and the structure breaks down beyond a certain degree of deformation, and finally the elasticity of ER fluid disappears abruptly [20]. Martin et al. [30], reported that they could not observe any linear viscoelastic region and, therefore, claimed that the energy of ER fluid was stored in G". Based upon our experimental result, however, it seems thestrain amplitude less than 0.01 will guarantee that the measurements are in the linear viscoelastic region. Figures 3(b) shows the plot of G' as a function of frequency with small strain in linear viscoelastic region. G's either remained constant or slightly increased as deformation frequency increased up to 100 sec j. Since the relaxation time for deformation was too large, it is expected that the internal chain structures of ER fluids are not destroyed by deformation under the given conditions. The increase in G' with the applied electric field strength indicate that the ER fluid becomes more elastic with the increasing electric fields strength under linear viscoelastic conditions. 10 3
.
.
.lOW 9
10 2
Q.. '# 01
.
.
,~
10 3
20wt%
s,ooe- /
9
10 4 9 20 wt% 9 10 wt%
....
10 2
10 ~
/
t
9
10 0
10 ~ 10 -~
,
,
,
10 0
I ......,d
101
10 -1 10 -1
10 0
^
101
10 2
E
E [kV/mm]
Figure 4. (a) Xs of each MCM-41 ER fluid with various electric field strengths. (b) the universal correlation. The solid line represents the calculated value from Eq. (3). The static yield stress (Xs) data of the MCM-41 ER fluids are plotted in Fig. 4(a) and these are correlated with a universal scaling relationship between electric field strength (E) and Xs via Eq. (2) suggested by Choi et al. [31 ]"
x, (E) - ~E 2
~/E/Er/
tanh ~/E/E ~:
'
(2)
485 where Gt depends on the dielectric constant of the fluid and particle concentraion, and Ec is the critical electric field strength, which is related to the particle conductivity and particle concentration. Equation (2) indicates that % is approximately proportional to E 2 for E << Ec, while switching abruptly to E ~5 for E >> Ec. We simplified Eq. (3) by using a generalized scaling function [31 ]: = 1.313133/2 tanh ~ ,
(3)
where 1~- E/E c and ~ = zs(E)/'C(Ec). The Xs data ofMCM-41 ER fluids are collapsed onto a single curve using Eq. (3), as shown in Fig. 4(b). E c is 0.4 kV/mm for 10 wt% MCM-41 and 0.7 for 20 wt% MCM-41.
4. CONCLUSIONS In this study, MCM-41 mesoporous molecular sieve was synthesized and its ER characteristics was examined. The synthesized MCM-41 had well-defined channel structure from XRD and TEM analyses. Its suspension in silicone oil showed typical ER properties and moisture absorbed in hydrophilic MCM-41 is the polarization species at high electric field strengths. The static yield stresses were measured in CSS mode, and these were related to applied electric field strengths by Zs "- E TM for 10 wt% MCM-41 ER fluid and % ~ E 1"67 for 20 wt% MCM-41 ER fluid.. The linear viscoelastic properties ( G' and G") of MCM-41 ER fluid under various electric field strengths were also measured at small strain. The elasticity of solidified ER fluids increased with applied electric field strength under the linear viscoelastic condition.
5. A C K N O W L E D G M E N T S This study was supported by research grants from the Korea Science and Engineering Foundation (KOSEF) through the Applied Rheology Center (ARC), an official KOSEFcreated engineering research center (ERC) at Korea University, Seoul, Korea.
REFERENCES
1. 2. 3. 4. 5. 6. 7. 8.
U. Ciesla and F. Schfith, Microporous Mesoporous Mater., 27 (1999) 131. M. ICruk, M. Jaroniec and A. Sayari, Microporous Mesoporous Mater., 35 (2000) 545. A. Sayari, Chem. Mater., 8 (1996) 1840. X. Feng, G. E. Fryxell, L. Q. Wang, A. Y. Kim, J. Liu and K. M. Kemner, Science, 276 (1997) 923. C.G. Wu and T. Bein, Science, 264 (1994) 1757. C.G. Wu and T. Bein, Science, 266 (1994) 1013. J.F. Diaz and K. J. Balkus Jr., J. Mol. Catal. B: Enzymatic, 2 (1996) 115. H.J. Choi, M. S. Cho, K. K. Kang and W. S. Ahn, Microporous Mesoporous Mater., 39 (2000) 19.
486 9. R. Tao and Q. Jiang, Phys. Rev. Lett., 73 (1994) 205. 10. H. Yamada, Y. Taniguchi and A. Inoue, Int. J. Mod Phys. B, 15 (2001) 947. 11. I. S. Sim, J. W. Kim, H. J. Choi, C. A. Kim and M. S. Jhon, Chem. Mater., 13 (2001) 1243. 12. H. J. Choi, T. W. Kim, M. S. Cho, S. G. Kim and M. S. Jhon, Eur. Polym. J., 35 (1997) 699. 13. H. J. Choi, M. S. Cho and K. To, Physica A, 254 (1998) 272. 14. H. J. Choi, J. W. Kim, M. S. Suh, M. J. Shin and K. To, Int. J. Mod. Phys. B, 15 (2001) 649. 15. H. J. Choi, M. S., Cho J. W. Kim, R. M. Webber and M. S. Jhon, Int. J. Mod. Phys. B, 15 (2001) 988. 16. H. J. Choi, M. S. Cho and M. S. Jhon, Int. J. Mod. Phys. B, 13 (1999) 1901. 17. J. W. Kim, S. G. Kim, H. J. Choi and M. S. Jhon, Macromol. Rapid Commun., 20 (1999) 450. 18. S. G. Kim, H. J. Choi and M. S. Jhon, Macromol. Chem. Phys., 202 (2001) 521. 19. W. Wen, N. Wang, W. Y. Tam and P. Sheng, Appl. Phys. Lett., 71 (1997) 2529. 20. J.W. Kim, H. J. Choi, S. H. Yoon and M. S. Jhon, Int. J. Mod. Phys. B, 15 (2001) 634. 21. D. J. Klingenberg, P. Pakdel, Y. D. Kim, B. M. Belongia and S. Kim, Ind. Eng. Chem. Res., 34 (1995) 3303. 22. M. S. Cho, H. J. Choi, I. J. Chin and W. S. Ahn, Microporous Mesoporous Mater., 32 (1999) 233. 23. M. Kruk, M. Jaroniec and A. Sayari, Microporous Mesoporous Mater., 35/36 (2000) 545. 24. Ch. Danumah, S. M. J. Zaidi, G. Xu, N. Voyer, S. Giasson and S. Kaliaguine, Micropor. Mesopor. Mater., 37 (2000) 21. 25. W. Zhang, M. Fr i5ba, J. Wang, P. T. Tanev, J. Wong and T. J. Pinnavaia, J. Am. Chem. Soc., 118 (1996) 9164. 26. R. Ryoo, C. H. Ko and R. F. Howe, Chem. Mater., 9 (1997) 1607. 27. Q. Huo, D.I. Margolese, U. Ciesla, P. Feng, T.E. Gier, P. Sieger, A. Firouzi, B.F. Chmelka, F. Schiith, and G.D. Stucky, Chem. Mater., 6 (1994) 2317. 28. M. S. Cho, Y. J. Choi, H. J. Choi, S. G. Kim and M. S. Jhon, J. Molecular Liq., 75 (1998) 13. 29. S. G. Kim, J. W. Kim, M. S. Cho, H. J. Choi and M. S. Jhon, J. Appl. Polym. Sci., 79 (2001) 108. 30. J. E. Martin, D. Adolf and T. C. Halsey, J. Colloid Int. Sci., 167 (1994) 437. 31. H. J. Choi, M. S. Cho, J. W. Kim, C. A. Kim and M. S. Jhon, Appl. Phys. Lett., 78 (2001) 3806.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
487
Synthesis and characterization o f TiO2 loaded C r - M C M - 4 1 catalysts E.P. Reddy +, Lev Davydov, and Panagiotis G. Smirniotis* Reaction Engineering and Catalysis Research Laboratory, Chemical Engineering Department, University of Cincinnati, Cincinnati, OH 45221-0171, USA. + Presenter; * Corresponding Author: E-mail:
[email protected] Chromium substituted mesoporous MCM-41 material (Si/Cr = 80) was synthesized by the incorporation of chromium ions during synthesis. This was then loaded with titania via sol-gel method to explore the photoactivity in visible light. This prepared catalyst was characterized by different physico-chemical techniques such as BET surface area, nitrogen physisorption, oxygen chemisorption, XRD, XPS, TPR and Raman spectroscopy. The BET surface area, however, was lower than those commonly found in our siliceous MCM-41. This is due to the partial pore breakage, as recorded by the pore size distribution analysis. The chromium metal dispersion found in the majority of the specimens studied was quite high (in the vicinity of 50%) and decreased with the loading of TiO2. The XRD analysis showed the patterns of CrMCM-41, 25%TiO2/Cr-MCM-41 similar to those of siliceous MCM-41, however, the intensity of the d~00 peak is decreased, while the loading of titania. Raman studies of CrMCM-41 and 25% TiOz/Cr-MCM-41 indicate that the chromium is well dispersed inside the MCM-41 framework, and decreasing intensity of Raman peaks at 144 cm ~, 397 cm -~, 518 cm ~ and 641 cm -~ upon loading of titania on Cr-MCM-41 indicates that the titania was interacting with chromium, and where as the peaks are associated to anatase phase of loaded titania. TPR studies revealed a change in the reduction temperature of Cr(IV) in the titanialoaded Cr-MCM-41. The reduction temperature of Cr(VI) was found to depend on the nature of the chromium species in the MCM-41 matrix. This significantly contributes to the remarkable photocatalytic activity of TiOz/Cr-MCM-41, and this does not happen in other transition metals incorporated MCM-41. The surface composition and binding energy of Cr 2p3/2 peak of the Cr-MCM-41 and 25%TiO2/Cr-MCM-41 was analyzed by XPS and showed considerable diffusion of chromium ions to the surface upon loading of titania. The binding energy value of Cr 2p3/2 was decreased upon loading of TiO2 on Cr-MCM-41, indicates that Cr is strongly interacting with TiO2. Eventually, two separate surface electronic levels corresponding to Si-O-Cr and Ti-O-Cr regions were found by XPS analysis for TiOz/CrMCM-41. 1. INTRODUCTION A great deal of recent research focused on a new family of molecular sieves, designated as MCM-41, has been discovered by Mobile scientist [1-3]. This material possessed a uniform arrangement of hexagonally shaped mesopore structure. Moreover, by changing the liquid
488 crystal template (LCT) mechanism, the pore size may be varied from 1.5 to 10 nm by changing the surfactant chain length [4]. Their high thermal and hydrothermal stability, uniform size and shape of the pores, and large surface area, make them of interest as sorbents and catalysts [ 1-4]. Pure silica possesses a neutral framework, which limits its application as a catalyst or as a support for preparing novel heterogeneous catalysts. Consequently, isomorphous substitution of silicon with transition metals was an excellent strategy in creating catalytically active sites and anchoring sites for catalytically active molecules in the design of new heterogeneous catalysts. Therefore, various transition metals such as Ti [5], V [6], Fe [7], Mn [8] and Cr [9] incorporated molecular sieves having redox catalytic properties have been synthesized by hydrothermal method. Among these materials, chromium incorporated microporous as well as mesoporous molecular sieves are particular interest because chromium compounds are widely used as stoichiometric oxidants in organic synthesis [10] and as homogeneous catalysts [11] in the presence of alkyl hydroperoxides. Moreover, Cr (VI) typically catalyzes oxidations via an oxometal mechanism in which chromyl (CrO2+2) species are the active oxidants. Only one report is available in the open literature concerning the photocatalytic oxidation of organics over chromium incorporated MCM-41 [ 12] Very recently we reported that the titania doped Cr-MCM-41 is very active catalyst for the liquid phase photocatalytic oxidation of organics at atmospheric conditions [13]. However, the exact nature of chromium in MCM-41 needs to be known. The explanation of the distribution, oxidation state, and co-ordination of these Cr species on titania doped Cr-MCM-41 is fundamental important for understanding thecatalytic role of Cr in 25%TiO2/Cr-MCM-41. In this present work, a systematic characterization study of Cr-MCM-41 and 25%TiO2/CrMCM-41 was undertaken by using different physico-chemical techniques such as BET surface area, nitrogen physisorption, oxygen chemisorption, XRD, diffuse reflectance UVVis, XPS, TPR, and Raman spectroscopy and evaluated these catalyst by visible light irradiated photocatalytic oxidation of formic acid. 2. EXPERIMENTAL SECTION
2.1. Materials The sources of silica, titania and chromium were Ludox HS-40 (Aldrich, 40 wt.-% colloidal silica in water), tetraisopropyl othotitanate (TIPOT, Fluka, p.a.) and chromium nitrate (Cr(NOa)a.9H20 Fisher, 99.97% purity) respectively. As a quaternary ammonium surfactant compound hexadecyltrimethylammonium bromide (HDTMABr, Alfa Aesar, 99 +%) was used. Other compounds of the synthesis were tetramethylammonium hydroxide (TEAOH, Fluka 40 wt.-% solution in water) and ammonium hydroxide (NH4OH, 29 wt.-% solution in water). All chemical were used without further purification. 2.2. Synthesis procedure Chromium substituted MCM-41 with an atomic Si/Cr ratio of 80 was synthesized as previously reported [14] using Ludox HS-40 as the source of silica. The following is the typical preparation procedure: 35 grams of Ludox was added to 14.55 ml of water under stirring, and 18.2 ml of 40 % TEAOH added. Independently, 18.25 g of the template was dissolved in 33 ml of water, and subsequently 7 ml of 28 % NH4OH was introduced. Finally, the above two solutions containing Ludox and template were mixed together. The
489 corresponding amount chromium nitrate dissolved in water was added drop-wise from a pipette to the resulting mixture. The final mixture was stirred together for 30 minutes, then transferred into telfon bottle and treated under autogenous pressure without stirring at 90 100~ for 3 days. The resulting solids were filtered, washed, dried, and calcined at 550~ for 10 hours under airflow. The temperature profile was 2 ~ up, 15 ~ down.
2.3. Impregnation procedure 1.5 g of Cr-MCM-4 l was impregnated with a solution of TIPOT in---100 ml of isopropanol giving Ti loading 25 wt.-%. The system was dried while stirring at ambient temperature. It was then placed in the oven to dry at 100~ for 1 hour. They were then transferred into a boat-like crucible and calcined at 450~ for 3 hours with a temperature ramp of 2 ~
2.4. Characterization The specific BET surface area of Cr-MCM-41 and 25%TiO2/Cr-MCM-41 materials were measured by nitrogen adsorption at-196~ by using a Micromeritics Gemini 2360. HorvathKawazoe maximum pore volume and adsorption average pore diameter measurements of these materials were performed with a Micromeritics ASAP 2010 using adsorption of N2 a t 196~ All samples were degassed at 250~ under vacuum before analysis. Oxygen uptake measurements of Cr-MCM-41 and 25%TiO2/Cr-MCM-41 materials performed at 370 ~ with a Micromeritics ASAP 2010 Chemi system. The powders were characterized by UV-Vis spectrophotometer (Shimadzu 2501PC) with an integrating sphere attachment ISR1200 for their diffuse reflectance in the range of wavelength of 200 to 800 nm. BaSO4 was used as the standard in these measurements. X-ray diffraction (XRD) studies were obtained on a Nicolet powder X-ray diffract meter equipped with a CuK~ radiation source (wave length 1.5406/~) to assess their crystallinity. Raman spectra were obtained at room temperature using excitation line from Coherent 906 Ar + (514.5 nm) and K-2 Kr + (406.7 nm) ion lasers, collecting backscattered photons directly from the surface spinning (-~2000 rpm) solid samples in 8-mm diameter pressed pellets. Conventional scanning Raman instrumentation equipped with a Spex 1403 double monochromator, with a pair of gratings with 1800 grooves/mm, and a cooled Hamamastsu 928 photomultiplier detector was used to record the spectra under the control of a Spex DM3000 micrometer system. Temperature programmed reduction (TPR) experiments were carried out in a gas flow system equipped with a quartz micro-reactor, using custom-made set-up attached with TCD detector. Approximately 100 mg of sample was pretreated in 23 ml/min flowing of He at 350~ for 1 h. After pretreatment, the materials were tested in 6 vol% H2/He, 25 cm3/min and increasing the temperature from 100~ to 800~ at 5~ and kept the temperature at 800~ for 2 h. XPS analyses were conducted on a Perkin-Elmer Model 5300 X-ray photoelectron spectrometer with MgK~ radiation at 300 W. Typically, 89.45 and 35.75 eV pass energies were used for survey and high-resolution spectra, respectively. The effects of the sample charging were eliminated by correcting the observed spectra for a C 1s binding energy value of 284.5 eV.
490 3. RESULTS AND DISCUSSIONS
The BET surface areas, pore volume, pore diameter and metal dispersion values of MCM41, Cr-MCM-41, 25%TiOz/MCM-41 and 25%TiOz/Cr-MCM-41 are depicted in table 1. One can clearly see that the lowering of surface area as well as increase of pore volume and pore diameter values with the introduction of chromium inside the MCM-41 framework as compared to the siliceous MCM-41. Since the same surfactant template was used for the synthesis of both siliceous and Cr substituted MCM-41 materials, one should expect to obtain nearly same pore diameter for both the materials. The above difference is mainly due to the partial blockage of the hexagonal tubular walls of the MCM-41 structure. The presence of chromium salt changes the ion strength of the gel during synthesis, which may hinder the action of the template and results in the formation of lower surface area as well as bigger pore diameter and pore volume. Cr-MCM-41 lost its surface area, pore volume and pore diameter with the 25% loading of titania. It is due to the partial blockage of the pores, as we explained in our earlier papers [ 13] smaller percentage of titania deposition shows negligible loss of the surface area, where as higher coverages lead to substantial loss of in surface area. This clearly indicates the higher loading of titania on Cr-MCM-41 fill up some of the pores leading to their partial blockage. Table 1. BET surface areas, pore volume, pore diameter and metal dispersion of MCM-41, 25%TiOz/MCM-41, Cr-MCM-41 and 25%TiOz/Cr-MCM-41 ............ Catalyst ............ i~ET SA ......Pore volume Pore diameter(nm) % of dispersion a (m2/g) (cm3/g) , (O/Cr) MCM-41 940 0.94 4.2 Not detected 25 %TiO2/MCM-41 667 0.56 3.4 0.1 Cr-MCM-41 825 1.08 5.23 54.37 25%TiOz/Cr-MCM-41 623 0.72 4.62 22.25 a %'"'Ofdispersion fraction of Cr'atoms at iiae su'rface, assuming OadjC"rs~f= 1. As shown in Table 1, MCM-41 and 25%TiO2/MCM-41 did not show any metal dispersion as expected, where as the Cr- substituted MCM-41 revealed higher Cr-dispersion when compared to the titania loaded Cr-MCM-41. This is due to the partial blockage of the Cr active sites by the TiO2 loading, making them inaccessible to co-ordinatively unsaturated sites. Davydov et al [ 13] already explained that the loading of titania on siliceous MCM-41 does not show any metal dispersion, this suggests that the loaded titania does not chemisorb any oxygen atoms, moreover, it blocks the accessible co-ordinatively unsaturated sites of transition metals incorporated MCM-41 s. UV-vis absorption spectra of Cr-MCM-41 and TiOz/Cr-MCM-41 were recorded in the range of wavelength of 200 to 800 nm (Figures not shown). Neat Cr-MCM-41 exhibit three types of absorption peaks at "-275 nm, "-380 nm, corresponding to Cr+6 and shoulders at--470 nm corresponding to and Cr+3 species [9]. The same material, but loaded with 25% TiO2 exhibits higher absorption in the UV range due to the presence of titania. All the materials still retain high absorption in visible light (up to 600 nm)and have distinct shoulder at--370 nm.
491 X-ray diffraction (XRD) patterns of MCM-41, Cr-MCM-41 and TiO2/Cr-MCM-41 recorded from 20 = 2 ~ to 7 ~ are shown in Figure 1. The XRD reflections 100, 110, 200, and 210 of Cr-MCM-41 and TiOz/Cr-MCM-41 are determined at the same location as that of siliceous MCM-41 reflections [2,3], which can be indexed to hexagonal lattice structure. The intensities of these peaks lower, when compared to the MCM-41. One can suggest that the presence of Cr ions obstructs the structure-directing action of the template by changing its ionic strength [15]. One more interesting point is that we could not detect any peaks associated to Cr or chromium oxides. This indicates that the chromium ions are either dispersed in the MCM-41 framework or stays outside the framework as an amorphous phase. The XRD of 25%TiOz/Cr-MCM-41 was also recorded in the range of 20 = 20 ~ to 50 ~ in order to assess the crystallinity of TiO2 loading on to the Cr-MCM-41. It showed the 25%TiOa/CrMCM-41 exhibited low crystallinity of titania, may be due to an intimate contact with chromium ions or uniform distribution of titania on the pore walls of the MCM-41. These XRD results are in perfectly agreement with Raman and XPS results that we explained in latter paragraphs.
144
lOO
I-.,, 5 v
A /
5
25%TiOJMCM-41 397 518
cQ}
r _.=
25%TiOJCr-MCM-41 ~ % T i O 2M / -C 4 r-M ~C
2
~
~,
~
~ 7
2e
Figure 1. XRD patterns of MCM-4 l, CrMCM-41 and 25%TiO2/MCM-41.
Cr-MCM-41
1+0
360
4~0
6~0
Raman shift (cm "1)
Figure 2. Raman spectra of Cr-MCM-41, 25%TiOz/MCM-41 and 25 O~TiO2/Cr-MCM-41
Raman Spectra of MCM-41, Cr-MCM-41, 25%TiO2/MCM-41 and 25%TiO2/Cr-MCM-41 was presented in Figure 2. Raman spectra associated to Cr-MCM-41 did not show any peaks allied to Si-O, Cr-O bending or stretching modes, shows that the Cr is well dispersed in side the MCM-41 framework. However, in the case of 25%TiOz/MCM-41 and 25% TiO2/CrMCM-41' shows fours bands at 144 cm -~, 397 cm -~, 518 cm ~ and 641 cm -l indicate the existence of titania (anatase) particles [16]. The intensity of these four peaks corresponds to 25%TiOz/MCM-41 higher than 25%TiO2/Cr-MCM-41. Therefore, One can easily understand that the loaded titania is directly interacting with the Cr ions incorporated inside the MCM-41
492 framework. The results obtained from these spectra are well agreeing with the XPS results explained in the latter paragraphs. TPR profiles measured for Cr-MCM-41 and 25%TiO2/Cr-MCM-41 are shown in Figure 3. The reduction behavior of Cr is different in both the cases, though the amount chromium is same. There are two major peaks were observed in the case of Cr-MCM-41, according to Uhm et al [ 17] the peak at 434~ corresponding to reduction of Cr(VI) to Cr(III), and the peak at 800~ was associated to the hydroxyl groups leaving the surface of amorphous silica, since it observed even in the siliceous MCM-41. The TPR profile of 25%TiO2/Cr-MCM-41 show marked difference, when titania is loaded onto the Cr-MCM-41. The reduction temperature of Cr (VI) to Cr (III) increased from 434~ to 502~ this may due to a higher degree of its interaction with the titania loading. In addition to that chromium is expected to achieve the tetrahedral coordination, when incorporated during the synthesis of MCM-41 [ 15]. Therefore, this may also contribute to the peculiar interaction of incorporated chromium and loaded titania leads to increase the reduction temperature of Cr(VI). The interaction between chromium ions and titania was clearly observed by XPS analysis, which is discussed in latter paragraphs.
/
502
~'~
800 5
G)
v
200
,
!
400
600
800 Isothermal I = Temperature (~
Figure 3. TPR profile of: a) 25%TiO2/CrMCM-41; b) Cr-MCM-41
526
528
530
532
534
536
Binding energy (eV)
Figure 4. XPS of 0 l s core level for : a) 25%TiO2/Cr-MCM-41; b) Cr-MCM-41.
The XPS spectra of O 1s core level for Cr-MCM-41 and 25%TiO2/Cr-MCM41 are shown in Figure 4. Only one type of oxygen photoelectron peak at 532.7 eV belongs to SiO2 [18] was observed for Cr-MCM-41, where as the deconvoluted spectra of O ls core level peak corresponding to 25%TiO2/Cr-MCM-41 show three types of peaks, which was attributed due to the overlapping contribution of oxygen from silica, titania and chromium. As shown in figure 4, one can clearly see that the binding energies values of O ls at 529.3 eV, 530.2 eV, and 532.7 eV are belongs to the oxygen atoms that are bound to Cr(O)x, TiO2 [ 18] and SiO2 respectively. Moreover, the peak intensity of O 1s is decreased when titania loaded onto Cr-
493 MCM-41. This shows that the titania interacting with incorporated Cr ions and inducing into the surface of MCM-41. The XPS of Si 2p core level spectra belongs to Cr-MCM-41 and 25%TiO2/Cr-MCM-41 is shown in Figure 5. The binding energy value of Si 2p is found at 103.2 eV, which agrees well with the values reported in the literature [20]. The intensity of Si 2p is very high in the case of Cr-MCM-41 when compared to that of 25%TiO2/Cr-MCM-41. This indicates that the loading of titania is covering the surface of Cr-MCM-41, at the same time, it inducing the chromium from the framework of MCM-41. As shown in Figure 6, the intensity of Cr 2p is more predominant in the case of 25%TiOJCr-MCM-41 than in the case of Cr-MCM-41, but the binding energy of Cr 2p3/2 is decreased in the former case. In the case of Cr-MCM-41 the binding energy of Cr 2p3/2 is 579.4 eV; it probably corresponds to the Cr(IV). With loading of titania, the binding energy of Cr 2p3/2 decreases from 579.4 to 577.2 eV, it is an indication of either decrease of oxidation state of chromium or may be due to the interaction between loaded titania and chromium.
Cr 2P3/2
Si 2p
,.-..,
.i >" r
a
t,-
_.=
tin
97.5
lo;.o'1o~,.~
~o;.o'1o7.5
Binding e n e r g y (eV)
Figure 5. XPS of Si 2p core level for: a) 25%TiO2/Cr-MCM-41; b) Cr-MCM-41.
575
580
585
590
Binding energy (eV)
Figure 6. XPS of Cr 2p core level for 9 a)25%TiO2/Cr-MCM-41; b) Cr-MCM-41
The relative dispersion of chromium inside and out side the MCM-41 framework was also estimated from the XPS measurements of Cr-MCM-41 and 25%TiO2/Cr-MCM-41. The surface atomic concentration ratios of Cr/Si and Cr/Ti were taken as a measure of the relative dispersion of chromium oxide; inside and outside of the MCM-41 before and after the titania loading. The ratio between Cr/Ti - 0.136 and Cr/Si = 0.236 for 25%TiOJCr-MCM-41, where as the Cr/Si ratio is 0.002 in the case of Cr-MCM-41. As a matter of fact, the Cr/Si atomic ratio values clearly indicate that the chromium ions species was well dispersed inside the MCM-41 framework in the latter case. When loading of titania, Cr/Si ratio increased from 0.002 to 0.236 and the Cr/Ti - 0.136 indicates that Cr species induced form the framework to the surface and interacted with titania in the form of Ti-O-Cr, which also was observed on
494 Cr/TiMCM-41 [21] and on Cr/TiO2 [22]. The XPS results are in perfectly agreement with XRD and Raman results explained in earlier paragraphs. 4. A C K N O W L E D G M E N T S
The authors are grateful to the Young Investigator Award of the United States Department of Army ( Grant DAAD 19-00-1-0399) and NATO Science for Pease Program (Grant SfP974209) for the their support of this work. We also acknowledge funding from the Ohio Board of Regents (OBR) that provided matching funds for equipment to the NSF CTS9619392 grant through the OBR Action Fund #333. REFERENCES ~
2. .
.
5. .
7. .
9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
J.S. Beck, US Patent No. 5,057,296 (1991). C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. 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. C.Y. Chen, H.X. Li, M.e. Davis, Microporous Mater., 2 (1993) 17. W.Z. Zhang, J. Wang, P.T. Tanev, T.J. Pinnavaia, J. Chem..Soc., Chem. Commun., (1996) 979. K.M. Reddy, I. Moudrakovski, A. Sayari, J. Chem. Soc., Chem. Commun. (1994) 1059. Z.Y. Yuan, S.Q. Liu, T.J. Chen, J.Z. Wang, H.X. Li, J. Chem. Soc., Chem. Commun., (1995) 973. D. Zhao, D. Goldfrab, J. Chem. Soc., Chem. Commun., (1995) 875. N. Ulagappan, C.N.R. Rao, J. Chem. Soc., Chem.Commun., (1996) 1047. G. Cainelli, G. Cardillo, Chromium Oxidations in Organic Chemistry, Springer Publishers, Weinheim (1984). J. Muzart, Chem. Rev., 92 (1992) 113. H. Yamashita, K. Yoshizawa, M. Ariyuki, S. Higashimoto, M. Che, M. Anpo, Chem. Commun., (2001) 435. L. Davydov, E.P. Reddy, P. France, P.G. Smimiotis, J. Catal., 203 (2001) 157. A. Sayari, P. Liu, M. Kruk, M. Jarinoiec, Chem. Mater., 9 (1997) 2499. Z. Zhu, Z. Chang, L. Kevan, J. Phys. Chem. B, 103 (1999) 2680. G.T. Went, S.T. Oyama, A.T. Bell, J. Phys. Chem., 94 (1990) 4240. J.H. Uhm, M.Y. Shin, Z. Zhidong, J.S. Chung, Appl. Catal. B, 22 (1999) 293. B.M. Reddy, I. Ganesh, and E.P. Reddy, J. Phys. Chem. B, 101 (1997) 1769. B.M. Reddy, B. Chaodhary, E.P. Reddy, A. Fernandez, J. Mol. Catal. A,162 (2000) 431. C.U.I. Odenbrand, S.L.T. Andersson, L.A.H. Andersson, J.M.G. Brandin, G. Busca, J. Catal., 125 (1990) 451. Z.Zhu, M. Hartmann, E.M. Maes, R.M. Czemuszewicz, L. Kevan, J. Phys. Chem. B, 104 (2000) 4690. K. Kohler, C.W. Kohler, A.V. Zelewsky, J. Nickl, J. Engweiler, A. Baiker, J. Catal 143 (1994) 201.
Studies in Surface Science and Catalysis 141 A. Sayariand M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
495
Photocatalytic Ethylene Polymerization over C h r o m i u m Containing Mesoporous Molecular Sieves Hiromi Yamashita*, Katsuhiro Yoshizawa, Masao Ariyuki, Shinya Higashimoto, and Masakazu Anpo* Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Gakuen-cho 1-1, Sakai, Osaka 599-8531, Japan
The photocatalytic reactivities for ethylene polymerization on chromium-containing mesoporous molecular sieves (Cr-HMS) have been investigated. The characterizations with several spectroscopic measurements, such as XAFS, ESR, UV-VIS, and photoluminescence have indicated that Cr-HMS involves tetrahedral chromium oxide (Cr-oxide) moieties which are highly dispersed and incorporated in the framework of molecular sieve with two terminal Cr=O. In the presence of ethylene, Cr-HMS exhibited photocatalytic reactivity for the ethylene polymerization not only under UV light irradiation but also visible light irradiation. The photocatalytic reactivity of the Cr-HMS mesoporous molecular sieves was found to be much higher than those of the Cr-silicalite microporous zeolite and imp-Cr/HMS prepared by the impregnation method. The efficient dynamic quenching of the photoluminescence of the Cr-oxide moieties in the excited state in the Cr-HMS by the addition of ethylene molecules was found to indicate that the charge transfer excited state of a tetrahedral Cr-oxide moieties and large pore size plays a significant role in the photocatalytic reactions.
1. INTRODUCTION The highly dispersed transition metal oxides incorporated within the framework of zeolites and molecular sieves show unique reactivities not only for various catalytic reactions, but also for photocatalytic reactions under UV light irradiation [1-3]. The unique and efficient photocatalytic systems incorporating the transition metal oxides (Ti, V, Mo, etc.) have been designed and developed using the cavities and frameworks of zeolites and mesoP0rous molecular sieves [4-6]. Recently, we have found that chromium-containing mesoporous and zeolite catalysts can exhibit the photocatalytic reactivities for the NO decomposition and partial oxidation of alkanes and alkenes not only under UV light irradiation but also visible light irradiation [7-9]. The highly dispersed chromium oxide (Cr-oxide) supported on silica is an industrially important catalyst for ethylene polymerization [10,11]. Recently, it has reported that chromium acetyl acetonate complexes grafted on mesoporous molecular sieves (MCM-41) can exhibit the efficient reactivity for ethylene polymerization at higher than 373 K after the calcination at higher than 773 K [ 12, 13].
496 In the present study, we have investigated the photocatalytic reactivity of the chromium-containing mesoporous molecular sieves (Cr-HMS) for the ethylene polymerization not only under UV light irradiation but also visible light irradiation. The characterization of the local structure of the active sites and their role in the photocatalytic reaction have been investigated at the molecular level by means of dynamic photoluminescence, XAFS, ESR, UV-VIS, and XRD measurements along with an analysis of the reaction products.
2. E X P E R I M E N T A L
Cr-HMS mesoporous molecular sieves (Si/Cr=50, 100, 500) were synthesized using tetraethylorthosilicate and Cr(NO3)3 9H20 as the starting materials and dodecylamine as a template [7,8,14,15]. The chromium-silicalite (CrS-1) microporous zeolite (Si/Cr=500) was hydrothermally synthesized using tetraethoxysilane and Cr(NO3)9H20 as starting materials and tetrapropyl ammonium hydroxide (TPAOH) as a template in accordance with previous literature [16]. Imp-Cr/HMS zeolite (Si/Cr=50) were prepared by impregnating HMS with an aqueous solution of Cr(NO3)3.9H20. Calcination of the sample was carried out in a flow of dry air at 773 K for 5 h. Prior to spectroscopic measurements and photocatalytic reactions, the catalysts were degassed at 723 K for 2 h, heated in 02 at the same temperature for 2 h and then finally evacuated at 473 K for 2 h to 10-6 Torr. The photocatalytic reactions were carried out with the catalysts (100 mg) in a quartz cell with a flat bottom (80 ml) connected to a conventional vacuum system (10 -6 Torr range). The photocatalytic reactions were carried out under UV light (~> 270 nm) or visible light (k>450 nm) irradiation at 273 K using a high pressure mercury lamp through water and color filters. The photocatalytic polymerization of ethylene was carried out in the presence of ethylene (3.0 mmolg-cat -~) and the formation of polyethylene was confirmed by the IR measurement. XRD patterns were obtained with a Shimadzu XD-D1 using Cu K_ radiation. XAFS (XANES and EXAFS) spectra were obtained at the BL-9A facility of the Photon Factory at the National Laboratory for High Energy Physics (KEK-PF) in Tsukuba. The Cr K-edge absorption spectra were recorded in the fluorescence mode at 295 K with a ring energy of 2.5 GeV and the Fourier transformation was performed on k3-weighted EXAFS oscillations by a procedure described in previous literature [7,17]. UV-VIS spectra were recorded at 295 K with a Shimadzu UV-2200A spectrophotometer. ESR spectra were recorded with a JEOL2X spectrometer (X-band) at 77 K. The in situ photoluminescence spectra the catalysts were measured at 77 K with a Shimadzu RF-501 spectrofluorophotometer. IR measurements were carried out at 295 K using a JASCO FT-IR 7600 spectrometer with the catalysts before and after photocatalytic reaction.
3. RESULTS AND DISCUSSION
Figure 1 shows the XRD patterns of the Cr-HMS mesoporous molecular sieve and impCr/HMS. The results of the XRD analysis indicated that the Cr-HMS mesoporous molecular
497 sieve has the structure of the HMS mesoporous molecular sieve having pores larger than 20 A and the Cr-oxide moieties are highly dispersed in the framework of molecular sieves, while no other phases are formed [7,8,14,15 ]. Figure 2 shows the XAFS spectra of the treated Cr-HMS and imp-Cr/HMS. Cr-HMS exhibits a sharp and intense preedge peak which is characteristic of Cr-oxide moieties in tetrahedral coordination having terminal Cr=O [7,8,18]. In the FT-EXAFS spectrum, only a single peak due to the neighboring oxygen atoms (Cr-O) can be observed indicating that Cr ions are highly dispersed in Cr-HMS. From the curve fitting analysis of the FT-EXAFS spectrum, it has found that there are two oxygen atoms (Cr=O) in the shorter atomic distance of 1.57 A and two oxygen atoms (Cr-O) in the long distance of 1.82 A. The imp-Cr/HMS exhibits a weak preedge peak in the XANES spectra and an intense peak due to the neighboring Cr atoms (Cr-O-Cr) in the FT-EXAFS spectra, indicating that the catalyst consists of a mixture of tetrahedrally and octahedrally coordinated Cr-oxide species (Cr203like cluster).
r-HMS
:5
:5
o
o
8 aCr= O A Cr-O R N :5 4 . / / 1.57 2.1
v 0
..O
mp-Cr/HMS
f-
Cr-O b ~Oo-cr
N
c
0
I
5
0
I
1'0
2e / Degree
15
20
5990
6030
Energy / eV
0
2
4
6
Distance / ,&
Fig. 1. XRD patterns. Fig. 2. Cr K-edge XANES spectra (A,B) and (a) Cr-HMS,(b) imp-Cr/HMS (Si/Cr=50). Fourier transforms of spectra EXAFS(a,b). (a) Cr-HMS,(b) imp-Cr/HMS (Si/Cr=50). R: atomic distance (A), N" coordination number.
The ESR technique was also applied to investigate the coordination state of the Croxide moieties by monitoring the Cr 5+ ions formed under UV irradiation of the catalyst in the presence of H2 at 77 K. As shown in Fig. 3, after photoreduction with H2 at 77 K, Cr-HMS exhibits a sharp axially symmetric signal at around g=l.9 (g//= 1.889, g_/=1.979), attributed to the isolated mononuclear Cr 5+ ions in tetrahedral coordination [19]. On the other hand, imp-Cr/HMS exhibits a broad signal at around g=2.02 indicating the presence of Cr203 clusters. Figure 4 shows the diffuse reflectance UV-VIS absorption spectra of the Cr-HMS catalysts. Cr-HMS catalysts exhibit three distinct absorption bands at around 270, 370, 480
498 Cr5
gl-- 1.979
1.2
(a) Cr-HMS
~ g / / = 1.889 t-
._o I
g = 2.02
I
100 mT
t-
0.8
U.. e,-
~; 0.4 -i V
.
0 I
200
300
I
400
I
500
I
600
I
700
800
Wavelength/nm
Fig. 3. The ESR spectra of the photoreduced Fig. 4. Diffuse reflectance UV-VIS spectra (a) Cr-HMS and (b) imp-Cr/HMS (Si/Cr=50) of Cr-HMS (A-C) and HMS (D). (A) in the presence of H2 at 77 K. Si/Cr=50, (b) Si/Cr=100, (c) Si/Cr=500.
n m which can be assigned to the charge transfer from O 2" to Cr 6+ of the t e t r a h e d r a l l y coordinated Cr-oxide moieties [20]. The absorption bands assigned to the absorption of the dichromate of C r 2 0 3 cluster cannot be observed above 550 nm, indicating t h a t t e t r a h e d r a l l y coordinated Cr-oxide moieties exist in an isolated state. Cr-HMS evacuated at 473 K exhibited a photoluminescence spectrum at around 550750 nm upon excitation of the absorption (excitation) band at around 250-550 nm. Figure 5 shows the photoluminescence spectra of Cr-HMS observed at 77 K upon the excitation at 370 nm. The photoluminescence bands upon the excitation at 280, 370 and 500 nm were observed at the same position, while the intensities of spectra depend on the wavelength of excitation; the larger intensity was observed with the excitation at 370 nm. In the excitation spectrum of Cr-HMS monitored at 640 nm, three excitation bands are observed at 270, 370 and 490 nm, which are corresponding to the absorption bands observed in the UV-VIS absorption spectra shown in Fig. 4. No change in the positions of these absorption bands is observed with changing the monitoring wavelength of photoluminescence. These results suggest that the photoluminescence occurs as the radiation decay process from the same excited state independently to the excitation wavelength. These absorption and photoluminescence spectra are similar to those obtained with well-defined highly dispersed Cr-oxides anchored onto Vycor glass or silica [21-24] and can be attributed to the charge transfer processes on the tetrahedrally coordinated Cr-oxide moieties involving an electron transfer from 0 2- to Cr 6+ and a reverse radiative decay, respectively. These results indicate that the Cr-HMS mesoporous molecular sieve involves Cr-oxide moieties in tetrahedral
499 %t~
,o 1.2 0
5
E E ~
0.8
i-" eet-
0
0.4
t~
550
600
650
700
750
D
800
0 0
Wavelength / nm
60
120
180
240
300
Reaction time / min
Fig. 5. Effect of the addition of
Fig. 6. The reaction time profiles of photocatalytic polymerization of ethylene on the Cr-HMS (a:Si/Cr=50, c:Si/Cr=500), imp-Cr/HMS (b:Si/Cr=50), and CrS-1 (d:Si/Cr=500) under UV light irradiation (
ethylene on the photoluminescence spectra of the Cr-HMS (Si/Cr=50). Amount of added ethylene: a) 0, b) 5, c) 10, d) 15 lamol'g-cat-1, e) degassed after d).
X>270nrn).
coordination having two terminal Cr=O, being in good agreement with the results obtained by XAFS, ESR and UV-VIS measurements. The estimated model for the local structure of the Cr-oxide moieties and the charge transfer excited state are shown in the following scheme.
0202~ Cr6+ ff O/
~
hv "
hv'
I~
020~Cr5( O~ %
As shown in Fig. 5, the addition of ethylene onto the Cr-HMS led to an efficient quenching of the phosphorescence in its yield, its extent depending on the amount of ethylene added. The observation of efficient quenching with the ethylene addition indicates that the charge transfer excited state of the tetrahedrally coordinated isolated Cr-oxide moieties, (Cr5+--O-) *, easily interact with ethylene under light irradiation. UV light irradiation (~>270 nm) of the Cr-HMS in the presence of ethylene led to the photocatalytic polymerization at 275 K. Figure 6 shows the reaction time profile of photocatalytic polymerization of ethylene. As shown in Fig. 6, the ethylene uptake increases almost linearly to the irradiation time. The reaction immediately stopped when irradiation was ceased. Figure 7 shows the IR spectra of the Cr-HMS in the presence of ethylene. The
500 formation of polyethylene on the Cr-HMS after the UV irradiation was confirmed by monitoring CH2 streching bands (2854 cm-', 2926 cml) of polyethylene [13]. The formation of these reaction products was not detected in the dark conditions nor in irradiation of the HMS itself without Cr-oxide. These results clearly indicate that the presence of both Cr-oxide moieties included within HMS as well as UV light irradiation are indispensable for the photocatalytic reaction to take place and the Cr-HMS can act as an efficient photocatalyst for the ethylene polymerization under UV irradiation. As shown in Fig. 6, the photocatalytic reactivity of the Cr-HMS was found to be much higher than those of the CrS-1 microporous zeolite and imp-Cr/HMS prepared by the impregnation method. These results indicate that the charge transfer excited state of a tetrahedral Cr-oxide moieties and large pore size plays a significant role in the photocatalytic reactions.
2925 cm 1
0
&
1.2
0
E E ~
0.8
b 0.4
3000
2900
2800
Wavenumber / cm "1
0 0
60
120
180
240
300
Reaction time / min
Fig. 7. IR spectra of Cr-HMS (Si/Cr=50) in the presence of ethylene, a) before light irradiation, b) after UV light irradiation (
Fig. 8. The reaction time profiles of photocatalytic polymerization of ethylene on the Cr-HMS (Si/Cr=50) under a) UV light
~>270 nm)for 1 h.
irradiation ( ~,>270 nm) and b) visible light irradiation ( L>450 nm).
Figure 8 shows the reaction time profile of photocatalytic polymerization of ethylene on the Cr-HMS under visible light irradiation (L>450 nm). The Cr-HMS also shows photocatalytic reactivity even under visible light irradiation, although the reaction rate under the visible light irradiation is smaller than under UV light irradiation (L> 270 nm). These results indicate that Cr-HMS can absorb visible light and act as an efficient photocatalyst for the photocatalytic polymerization of ethylene under not only UV light but also visible light irradiation.
501 4. CONCLUSIONS It has been found that Cr-HMS molecular sieves contain tetrahedrally coordinated Croxide moieties in the framework having two terminal Cr=O and that the charge transfer excited state of the Cr-oxide moieties are responsible for the efficient photoluminescence and photocatalytic reactivities. The present results have clearly demonstrated that the Cr-HMS with mesoporous structure and tetrahedrally coordinated Cr-Oxide moieties can exhibit the efficient reactivity for the photocatalytic polymerization of ethylene under UV light irradiation. The Cr-HMS can also absorb visible light and act as a photocatalyst even under visible light irradiation. This photocatalytic system with tetrahedrally coordinated Cr-oxide moieties dispersed on mesoporous silica seems to be a good candidate to use abundant visible or solar light energy for the useful chemical synthesis.
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) with helpful advice from Prof. M. Nomura.
REFERENCES
1. B. Notari, Ad. Catal., 41 (1996) 253. 2. A. Corma, Chem. Rev., 97 (1997) 2373. 3. M. Anpo and M. Che, Ad. Catal., 44 (1999) 119. 4. M. Anpo and H. Yamashita, in "Surface Photochemistry", (ed) M. Anpo, J. Wiley & Sons, Inc., Chichester, 1996, pp. 117-164. 5. H. Yamashita, J. L. Zhang, M. Matsuoka, and M. Anpo, in "Photofunctional Zeolites: Synthesis, Characterization, Photocatalytic Reactions, Light Harvesting", (ed) M. Anpo, NOVA Science Publishers, New York, 2000, pp. 129-168. 6. S. Higashimoto, R. Tsumura, S. G. Zhang, M. Matsuoka, H. Yamashita, C. Louis, M. Che, and M. Anpo, Chem. Lett., (2000)408. 7. H. Yamashita, M. Ariyuki, S. Higashimoto, S. G. Zhang, J. S. Chang, S. E. Park, J. M. Lee, Y. Matsumura, and M. Anpo, J. Synchrotron Rad., 6 (1999) 453. 8. H. Yamashita, M. Ariyuki, S. Higashimoto, Y.Ichihashi, Y. Matsumura, M. Anpo, J. S. Chang, S. E. Park, and J. M. Lee, in Proc. 12th Intern. Zeolite Conf., (Baltimore, USA), 1998, pp. 667-672. 9. H. Yamashita, K. Yoshizawa, M. Ariyuki, S. Higashimoto, and M. Anpo, Stud. Surf Sci. Catal., 135 (2001) A28P07. 10. B.M. Weckhuysen, I. E. Wachs, and R. Schoonheydt, Chem. Rev., 96 (1996) 3327. 11. Z. Tvaruzkova, B. Wichterlova, J. Chem. Soc., Faraday Trans. 1, 79 (1983) 1591. 12. R.R. Rao, B. M. Weckhuysen, R. A. Schoonheydt, Chem. Commun., (1999) 445.
502 13. B.M. Weckhuysen, R. R. Rao, J. Pelgrims, R. A. Schoonheydt, P. Bodart, G. Debras, O. Collart, P. V. D. Voort, and E. F. Vansant, Chem. Eur. J, 6 (2000) 2960. 14. W. Zhang, P. T. Tanev, and T. J. Pinnavaia., J. Chem. Sot., Chem. Commun., 979 (1996). 15. S.G. Zhang; M. Ariyuki, S. Higashimoto, H. Yamashita, and M. Anpo, Microporous and Mesoporous Materials., 21 (1998) 621. 16. H.O. Pastore, E. Stein, C. U. Davanzo, E. J. S. Vichi, O. Nakamura, M. Baesso, E. Silva, and H. Vargas, J. Chem. Sot., Chem. Commun., (1990) 772. 17. H. Yamashita, M. Matsuoka, K. Tsuji, Y. Shioya, and M. Anpo, J. Phys. Chem., 100 (1996) 397.14. 18. M.S. Rigutto and H. V. Bekkum, Appl. Catal., 687 (1991) L 1. 19. B.M. Weckhuysen, R. A. Schoonheydt, J. M. Jehng, I. E. Wachs, S. J. Cho, R. Ryoo, and E. Poels, J. Chem. Sot., Faraday Trans., 91 (1995) 3245. 20. B.M. Weckhuysen, R. A. Schoonheydt, D. E. Mabbs, and D. Collison, J. Chem. Soc., Faraday Trans., 92 (1996) 2431. 21. B.M. Weckhuysen, A. A. Verberckmoes, A. L. Buttiens, and R. A. Schoonheydt, J. Phys. Chem., 98 (1994) 579. 22. M. Anpo, I. Yakahashi, and Y. Kubokawa, J. Phys. Chem., 86 (1982) 1. 23. M.F. Hazenkamp and G. Blasse, J. Phys. Chem., 96 (1992) 3442. 24. W. Hill, B. N. Shelimov, I. R. Kibardina, and V. B. Kazanskii, React. Kinet. Catal. Lett., 31 (1986) 315.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
503
Catalytic reduction of nitric oxide on Al-containing mesoporous molecular sieves W. Li*, Y. Zhang, Y. Lin, X. Yang Department of Environmental Science & Engineering, Tsinghua University, Beijing 100084, China A series of mesoporous aluminosilicate materials were synthesized at room temperature, and tested for nitric oxide (NO) reduction by propene in the presence of oxygen on their Cu ion-exchanged forms. The experimental results revealed that NO reduction activity was not decreased but slightly increased above 400~ on Cu-A1-MCM-41 in the presence of water vapor as compared to that on Cu-ZSM-5, which indicated Cu-A1-MCM-41 was more water resistant than Cu-ZSM-5. In addition, NO reduction activity was also investigated on the Ni, Co or Mn ions doped Cu-A1-MCM-41 samples in the absence or the presence of water vapor. It was found that NO conversion on Ni doped Cu-A1-MCM-41 was increased below 350 ~ in the absence of water vapor, and NO conversion was also enhanced above 400 ~ upon introducing water vapor into the feed gas. 1. INTRODUCTION Selective catalytic reduction of nitric oxide with hydrocarbons (HC-SCR) in the presence of oxygen is one of the major challenges in the automobile exhaust after-treatment for lean-burn gasoline engines and diesel engines. Since Iwamoto et al [1] reported a high NO reduction activity with hydrocarbons on Cu-ZSM-5, a number of metal ion-exchanged zeolites have been widely investigated for reducing NOx with ethylene, propene, propane, methane and other hydrocarbons [2]. Besides, several metal oxide catalysts such as Sn/ZrO2, Cu/A1203 and A1203 were also reported to be active for HC-SCR reaction. Among all the catalysts investigated, Cu ion-exchanged ZSM-5 shows the best NO conversion. However, most zeolite-base catalysts, namely Cu-ZSM-5, Co-ZSM-5, Ga-ZSM-5, Ce-ZSM-5, Mn-ZSM-5, Fe-ZSM-5, In-ZSM-5, are very sensitive to water vapor and sulfur species and quickly deactivated, possibly because of the irreversible dealuminization of the zeolite structure and the sintering of metal active species [3]. Therefore, some new zeolite materials
*To whom correspondence should be addressed E-mail:
[email protected]
504 such as Cu-exchanged IM5 [4] and Cu-exchanged SAPO [5] were also tested and found to be very active for the reaction. As we know, Besides the zeolite acidity, the pore structure was a key factor for the NO reduction activity [6]. Indeed, Tabata et al [7] proposed that the superior activity of the large-pore zeolite Co/beta could be ascribed to the ease of diffusion of reactants, products and inhibitors such as water and SO2 in its channels. By contrast, the lower activity of Co/ferrierite was found for C3H8-SCR reaction, probably because the diffusion in its small pores was hindered [7]. On the other hand, MCM-41 type materials developed by Beck et al [8] have the regular uniform mesoporous structures and high surface areas, may provide a better dispersion of active metal components on their surface and prevent the diffusion limitation for the catalytic reduction of NO. Jentys et al [9] and Shen et al [10] recently reported a high activity for NO reduction by propylene on Pt/MCM-41. In this presentation, aluminum containing MCM-41 mesoporous materials with high hydrothermal stability were synthesized, and their Cu ion-exchanged forms were tested for catalytic reduction of NO with propylene in the presence of excess oxygen. Special attention was paid on water vapor effect on the catalytic performance of those mesoporous materials. 2. EXPERIMENTAL 2.1 Materials
A1-MCM-41 was synthesized at ambient temperatures using cetyltrimethylammonium bromide (CTAB, Beijing chemical reagents company) as a template. A fixed amount of CTAB and NaOH were dissolved in deionized water under stirring and slightly heating at 50~ then a required amount of tetraethyl silicate (TEOS, Huabei special chemical reagents center) was slowly added to the above solution. After stirring for 15 min, an appropriate amount of aluminum sulfate solution was introduced into the solution under strong stirring. The reaction mixture had the following molar composition: TEOS:A12(SO4)3:CTAB:NaOH:H20 = 1:x:05:0.2:10, where x = 0.2, 0.1, 0.05, 0.02. The pH value of the suspension was then repeatedly adjusted to the value of 12 with tetramethylammonium hydroxide (TMAOH, Beijing Daxing Xingfu chemical company) solution. After the mixture gel was placed at room temperature for 24h, the resulting solid was filtered, washed with deionized water, and dried at 60~ The thus obtained samples were calcined in nitrogen at a heating rate of 2 ~ to 550 ~ and shifted to oxygen atmosphere at 550~ for another 2h. The available sample was ion-exchanged twice with an aqueous solution of NH4NO3, then dried and calcined at 550~ to obtain the acid form zeolite, H-MCM-41-y, where y is the SIO2/A1203 ratio of the sample. The above H-MCM-41-y was further ion-exchanged with a copper acetate solution for 4 times, followed by drying at 100~ and calcining at 550~ to obtain Cu-A1-MCM-41-y catalysts. Cu-ZSM-5 was prepared by ion-exchanging H-ZSM-5 (Nankai University Plant, Tianjin) with a copper acetate solution under the similar conditions. Ni, Mn and Co doped Cu-A1-MCM-41-y catalysts were prepared by impregnating the above Cu-A1-MCM-41-y sample with their nitrate salt aqueous solution.
505
2.2. Characterization of the catalysts X-ray diffraction patterns were obtained on Rigaku D/max RB X-ray diffractometer using Cu K a radiation. Nitrogen adsorption and desorption isotherms were determined at 77K by means of Quantachrome AUTOSORB-I surface area analyzer, from which BET surface areas were calculated and the pore size distributions were determined using the procedure proposed by Barrett, Joyner and Halenda (BJH). Elemental analysis was done with X-ray fluorescence analyzer on Shimadzu XRF-1700 spectroscopy. 2.3. Catalytic and adsorption measurements NOx reduction experiments were carried out in a fixed bed reactor. In a typical experiment, 0.10g of a catalyst was introduced into the reactor with a feed gas of 1000ppm NO, 3100ppm C3H6, 3% 02 and helium as balance gas, and the total flowrate of 150ml/min. 4~8% water vapor by volume was once supplied by passing helium gas through water bubbler. NOx gases from the reactor outlet was continuously analyzed by a NO/NOx Chemiluminescence Analyzer (Thermal Electronics Model 42CHL). Other reactants and products were analyzed by gas chromatography with a 5A molecular sieve column and a Porapak Q column with a thermal conductivity detector. NO conversion was calculated based on the difference between the inlet and outlet NO concentration.
A B C D 1;
1'2
20( ~ )
Figure 1. XRD pattern of A1-MCM-41 samples with the various SIO2/A1203 ratios after calcination at 550~ in oxygen. The SIO2/A1203 ratios were (A) 50; (B) 20; (C) 10; (D) 5. 3. RESULTS AND DISCUSSION
3.1 Characterization of the catalysts Figure 1 shows the small-angle XRD pattern (2-10 ~ 2 0 ) of the A1-MCM-41 samples with different SIO2/A1203 ratios after calcination at 550~ It is observed that all the samples show one peak around 2 ~ 2 0 associated with dl00 plane assigned to the typical MCM-41 materials [11 ]. The height of the peak remained high for the samples with the Si02/A1203 ratio being 50 to 10. A further decrease in the SIO2/A1203 ratio would lower the peak intensity, indicating a poor ordered wall structure, which is in good agreement with previous report [12]. The main peak shifts towards higher d-spacing with a decrease in the SIO2/A1203 ratio,
506 indicating an increase in the interplanar distance for the A1-MCM-41 material, which is due to the replacement of shorter Si-O bands (0.160nm) by longer A1-O bands (0.175nm) in the A1-MCM-41 structure. Similar results were reported by Corma et al [13]. Figure 2A and 2B show the NE adsorption/desorption isotherm and the pore size distribution calculated based on BJH method according to adsorption branch of A1-MCM-41-10 and A1-MCM-41-50, respectively. The steps of the isotherms at relative pressure (P/P0) between 0.2-0.35 in both Figure 2A and 2B are associated with the condensation of nitrogen in primary mesopores. Figure 2A shows the pore size distribution of primary mesopores based on BJH calculation method for A1-MCM-41-10, and the BET specific surface area is 1100 mE/g; Likewise, Figure 2B shows the pore size distribution for A1-MCM-41-50, and its BET specific surface area is 1150 m2/g. It is noteworthy that there is a sharp step at high relative pressure in the isotherm in Figure 2A characteristic of H1 type isotherm for slit-shaped secondary mesopores [ 14], which is caused by the condensation of nitrogen within the existent secondary mesopores formed by crystal aggregates [15]. This type of isotherms is attributed to nanostructural materials with uniform mesopores[16]. A similar isotherm was previously reported by Occelli et al [15] and Cesteros et al [17] on A1-MCM-41.
2000 .~
4"
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(A)
~.
.
Pore Diameter {am)
~
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.
,
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.
.
.
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.
.
.
,
.
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Relative pressure (P/Po)
.
~
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.
.
.
,
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9 ,
,
.
,
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.
.
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.
-
l'ore D|am~er (rim) ,
9
9
0.6
,
,
0.8
-
.
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Relative pressure (P/Po)
Figure 2. N2 adsorption-desorption isotherm and pore size distribution from BJH (inset) for the sample A1-MCM-4 l- 10 (A) and A1-MCM-41-50 (B). The low thermal stability of MCM-41 type materials compared with conventional zeolites such as ZSM-5 is a critical problem that will affect the practical application of the MCM-41 type materials. Therefore, several methods have been used to improve the hydrothermal stability of the MCM-41 type materials. Here, the pH value was repeatedly adjusted in the gel solution to improve the hydrothermal stability followed the method proposed by Ryoo et al [16]. Figiare 3 shows the XRD patterns of A1-MCM-41-10 heated at 550~ in oxygen for 2h and successively treated in boiling water for 12h. The XRD pattern for the sample heated in boiling water shows only a slight decrease in its peak intensity compared to the sample calcined at 550~ in oxygen. The decrease in XRD intensity is due to the disintegration of MCM-41 structure in hot water and silicate hydrolysis. The better hydrothermal stability
507 obtained on the A1-MCM-41-10 sample is using the repeated pH adjustment, which is structure order and textural uniformity as show that no significant changes occurred acetate solutions.
.~
believed to be related by the synthesis procedure consistent with the improvement in the long range reported by Ryoo et al[16]. Further experiments for the sample being ion-exchanged with copper
~ 20(
~ ' l~, ' ,'2 ~ )
Figure 3 XRD pattern of the A1-MCM-41-10 at different treatment: (A) As-synthesized; (B) Calcined at 550~ in oxygen; (C) Calcined at 550~ in oxygen followed by heating in boiling water for 12h. Table 1 shows the results of chemical compositions determined by X-ray fluorescence analyzer. It was found that the molar ratio of SIO2/A1203 in the final solid samples except for that on A1-MCM-41-10 was only slightly higher than that in the gel precursors, because some A1 species might be lost during the synthesis procedures. Table 1. Chemical compositions of Cu 2+ ion-exchanged A1-MCM-41 and Cu 2+ ion-exchanged ZSM-5. SIO2/A1203 Cu loading 7 Sample (wt %) .......................Gel ...........a........................................................................................... S o ' i i d 'b ................... A1-MCM-41-5 5 7.4 Cu-A1-MCM-41-10 10 9.9 5.5 MCM-41-20 20 25.6 Cu-A1-MCM-41-50 50 55.7 5.7 Cu-ZSM-5-50 50 54.5 4.8 ............... ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a Calculated value of the molar ratio of SiOJAI203 in the synthesis gel. bAnalyzed value for Cu and SIO2/A1203in the final solid samples calcined at 550 ~
3.2. Catalytic experiments NO conversion into nitrogen over Cu-A1-MCM-41-10 and Cu-A1-MCM-41-50 as function of temperature in the absence or presence of water vapor was illustrated in Figure 4. It is observed that adding water vapor into the feed gas led to an increase in the activity on
508 Cu-A1-MCM-41-10 at lower temperatures; while at higher temperatures (i.e., above 400~ the NO conversions remain almost unchanged. For the sample with a higher SIO2/A1203 ratio as 50, the temperature for the maximum NOx conversion was shifted to lower temperature range, and an increase in the NO reduction activity was found at 350 ~ on Cu-A1-MCM-41-50 in the presence of water vapor as compared to that in the absence of water vapor, i.e., 48.7% viz. 37.6%. The different behaviors between the two samples are possibly due to the depression of total oxidation of propylene by water vapor at the temperature investigated, and hence provide different type or amount of organic intermediates for NOx reduction. The detailed reason is under investigation and possibly related with the different pore structures and the acid sites or acid strength on the sample. Jentys et al [9] recently reported that the similar enhancement of the water vapor on the activity of the NOx reduction by propene on Pt/MCM-41 impregnated with tungstophosphoric acid, but a decrease in the activity was still observed on Pt/MCM-41 without dopants in the presence of 2.5 vol. % water vapor. 60 50 .~ 40 o
+
A,W/O H20
+
A,With H20
---/k-- B,W/O H20 ~-
30
B,With H20
~ 2o z 10 0
~ 50
150
250
350
450
550
Temperature/oc
Figure 4. NOx conversion versus temperature on Cu-A1-MCM-41-10 (A) and Cu-A1-MCM-41-50(B) in the absence of water vapor (open symbols) and in the presence of water vapor (solid symbols). Since Cu-ZSM-5 have been widely reported as a highly active catalyst for NO reduction by propene [2], Cu ion exchanged ZSM-5 was also prepared and tested under the same conditions with that for Cu-A1-MCM-41-50. The results in Figure 5 shows the dependence of NOx conversion on water vapor over Cu-ZSM-5. Although the NOx conversion was higher on ZSM-5 without H20, NOx conversion was greatly affected by the presence of water vapor. For example, at 400~ NOx conversion was decreased from 85% to 71% upon introducing 4% water vapor into the feed gas; A further increase in water vapor content to 8%, led to more serious deactivation and a lower NO conversion of 54%, as expected. By comparing Figure 5 with Figure 4, it is clear that Cu-A1-MCM-41 was more water resistant than Cu-ZSM-5, particularly at the temperature above 400~ The further investigation for understanding the enhancement effect is in progress in our laboratory.
509 100 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
+ A ~
80
B
i .--a-c
~
60 J'
= o ~
I 40 i
Z
20 ~ 0 '
.
100
. 200
.
.
. 300
.
.
. 400
.
.
. 500
600
Temperature/~C
Figure 5. Water effect on NOx conversion over Cu-ZSM-5-50" (A) without water; (B) with 4% water; (C) with 8% water. Figure 6 shows NO conversion as a function of temperature on the Co, Mn and Ni modified Cu-A1-MCM-41-10 catalysts. By the comparison with the data in Figure 4, it revealed that doping of Co and Mn decreased NO conversion greatly, and meanwhile more NO2 gases were detected. However, the temperature corresponding to maximum NO conversion was shifted from 425 ~ for 45.4% to 350 ~ for 48.2% after doping Cu-A1-MCM-41 catalyst with Ni ions. It is clear that the maximum NO conversion was slightly increased, and NO conversion below 350 ~ was increased remarkably in the absence of water vapor. For example, NO conversion at 350 ~ was increased from 20% to 48.2% after doping Ni ions on Cu-A1-MCM-41-10. A slight decrease was also found above 400 ~ in the absence of water vapor. However, upon introducing water vapor into the feed gas NO conversion on Ni doped Cu-A1-MCM-41 was found to be enhanced above 400 ~ despite a decrease in NO conversion was observed below 400 ~
60! -~A 5o! 4O
..........................
A B +C
"~ g 30
z~ 20 10
100
200
300
400
500
600
Temperature/o c
Figure 6. NO conversion as a function of temperature without water vapor on Co (A), Mn (B), Ni (C) doped Cu-A1-MCM-41-10 catalysts, and on the Ni doped sample in the presence of 4% water vapor (D).
510 4. CONCLUSION A series of mesoporous aluminosilicate materials were synthesized at ambient temperatures, and their mesoporous structures were intact after calcination at 550~ in oxygen or heating in boiling water as detected by XRD, N2 adsorption-desorption analysis. NO reduction by propene was conducted on these Cu ion-exchanged mesoporous materials. It is found that in presence of water vapor NO reduction activity was not decreased but slightly increased above 400~ on Cu-AI-MCM-41 as compared to that on Cu ion-exchanged ZSM-5, which implied Cu-A1-MCM-41 was more water-resistant than Cu-ZSM-5. Furthermore, addition of Ni ions to those Cu-A1-MCM-41 samples led an increase in NO conversion below 350 ~ and a slight decrease above 400 ~ in the absence of water vapor. It is worth noting that NO conversion on Ni doped Cu-A1-MCM-41 was also enhanced above 400 ~ in the presence of water vapor. The results suggest that the Ni and Cu modified A1-MCM-41 type materials are potential catalysts for NO reduction by propene in the presence of water vapor with further attempts on improving the NO reduction activity. 5. ACKNOWLEDGMENTS National Natural Science Foundation of China (NO. 29907003) are gratefully acknowledged. REFERENCES
1. M. Iwamoto, H. Yahiro, Y. Mine, S. Kagawa, Chem. Lett., (1989) 213. 2. Y. Traa, B. Barger, J. Weitkamp, Micropor. Mesopor. Mater., 30 (1999) 3. 3. J.Y. Yang, Ct P. Lei, W. M. H. Sachtler, H. H. Kung, J. Catal., 161 (1996) 43. 4. A.E. Palomares, F. Marquez, S. Valencia, A. Corma, J. Mol. Catal. A, 162 (2000) 175. 5. T. Ishihara, M. Kagawa, Y. Mizuhara, Y. Takita, Chem. Lett, (1991) 1063. 6. C.Yokoyama, M.Misono, Catal. Today, 22 (1994) 59. 7. T. Tabata, H. Ohtsaka, L. M. F. Sabatino, (2 Bellussi, Micropor. Mesopor. Mater., 21 (1998)517. 8. J.S. Beck, J. C. Vartuli, W. J. Roth etal, J. Am. Chem. Soc., 144 (1992) 10834. 9. A. Jentys, W. SchieBer, H. Vinek, Catal. Today, 59 (2000) 313. 10. S.-C. Shen, S. Kawi, Catal. Today, 68 (2001) 245. 11. P.T. Tanev, M. Chibme, T. J. Pinnavaia, Nature 368 (1994) 321. 12. R. B. Borade, A. Cleatfield, Catal. Lett., 31 (1995) 267. 13. A. Corma, V. Fornes, M. J. Navarro and J. P. Pariente, J. Catal. 148 (1994) 569. 14. S. J. Gregg, and K. S. W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982. 15. M. L. Occelli, S. Biz, A. Auroux, G. J. Ray, Mesopor. Mesopor. Mater., 26 (1998) 193. 16. M. Kruk, M. Jaroniec, and A. Sayari, Langrnuir, 13 (1997) 6267. 17. Y. Cesteros, Ct L. Hailer, Mesopor. Mesopor. Mater., 43 (2001) 171. 18. R. Ryoo, S. Jun, J. Phys. Chem. B, 101 (1997) 317.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
511
Catalytic oxidation o f alpha-eicosanol to alpha-eicosanoic acid over Ti, Zr and M n doped M C M - 4 8 molecular sieves Changping Wei a*, Yining Huang b, Qiang Cai c, Wenqin pangC, Yingli Bi d and Kaiji Zhen d aDepartment of Chemistry Engineering, Jilin Institute of Technology, Changchun 130012, P.R.China bDepartment of Chemistry, The University of Western Ontario, London N6A 5B7 Canada CKey Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130023, P.R.China dDepartment of Chemistry, Jilin University, Changchun 130023, P.R.China
A series of MCM-48 mesoporous molecular sieves doped with Ti, Zr and Mn were synthesized by hydrothermal crystallization and characterized by XRD, UV, EDX and N2 adsorption. These samples were used as catalysts to perform the catalytic oxidation of a-long chain eicosanol to the corresponding a-eicosanoic acid. The experimental results show that MCM-48 molecular sieves doped with Ti, Zr and Mn can be used as a catalyst for the title reaction and have highter catalytic activity than pure MCM-48 for the conversion. 1. INTRODUCTION Since 1992, a new family of mesoporous molecular sieves has been discovered[I-2]. Because of its poor thermal stability, the MCM-50 attracts less research attention. But the MCM-41 and the MCM-48 structures are excellent candidates for catalysis and separation processes. Ti- and V-substituted MCM-41 and Ti-substituted hexagonal mesoporous silica such as Ti-HMS have been synthesized[3-7]. These mesoporous molecular sieves can be used as catalysts for oxidation of bulky molecules which could not enter the micropores of zeolites such as TS-1, TS-2 and Ti-beta. As a catalyst, MCM-48 characterized by a three-dimensional channel system has several advantages over MCM-41 which has a one-dimensional channel system. For instance, the three-dimensional pore system should be more resistant to blockage by extraneous materials than the one-dimensional pore system. Thus, MCM-48 mesoporous molecular sieves may find industrial and biochemical applications in catalysis, separation, and encapsulation[8-11 ]. In this work, we synthesized MCM-48 mesoporous molecular sieves doped with Ti, Zr and Mn by hydrothermal crystallization using surfactants, TEOS and several different transition metal salts as starting materials. The products were characterized by XRD, UV, EDX, and N2 adsorption. The catalytic performance of M (M = Ti, Zr and Mn)-MCM-48 for the oxidation of a-eicosanol to a-eicosanoic acid has been tested. * Corresponding author, E-mail:
[email protected]; Fax: 86-0431-5952413.
512 2. EXPERIMENTAL
2.1 Synthesis of M-MCM-48 molecular sieves The M-MCM-48 (M = Ti, Zr and Mn) molecular sieves were synthesized[12] hydrothermally with TEOS (Tetra-Ethyl-Ortho-Silicate), transition metal salts, CTAB (Octadecyl-Trimethyl-Ammonium-Bromide), NaOH and distilled water. The procedure was following: NaOH was dissolved in distilled water, then transition metal salts and the CTAB were added. When the solution became homogeneous, TEOS was added and the resulting solution was transferred to an autoclave and heated at 373 K for three days. The products were washed with distilled water, dried at ambient temperature and calcined at 773 K for 4h. 2.2 Characterization of M-MCM-48 molecular sieves The X-ray diffraction patterns of the M-MCM-48 were recorded on a SCINAG XDS2000 Diffractometer with Cu-K~ radiation. The UV diffuse reflectance spectra were recorded on a UV-3100 (HITACHI company) spectrometer. EDX analysis were carried out on a HIACHI-8100 transmission electron microscope operated at 200 KV. Nitrogen adsorption and desorption isotherms at 77K were measured using a Micromeritics ASAP 2400 Instrument. The data were analyzed by the BJH (Barrett-JoynerHalenda) method using the Halsey equation for multilayer thickness. The pore-size distribution was obtained from the analysis of adsorption branch of the isotherm. 2.3 Test of the catalytic properties The catalytic reactions of eicosanol were carried out in a 4-neck flask equipped with a magnetic stirring bar, a thermometer, an oxygen inlet and a condenser. Reactions were carried out at 413 K for 5h. 0.1-0.2 g catalysts (100 mesh) were used. The a-eicosanol was purified before used. Conversion of ct-eicosanol and the yield of a-eicosanoic acid was calculated according to a stearic acidity. The stearic acidity was determined as following: 1.0 g products were dissoved in 70 ml hot ethanol. To the solution 6 drops of phenol phthalein were added, followed by titration with 0.2 M KOH. Then the excessive 0.2 M KOH was added, followed by titration with 0.2 M HC1. The stearie acidity was calculated based upon the titer.
3 RESULTS AND DISCUSSION
3.1 Characterization of M-MCM-48 molecular sieves The X-ray diffraction pattems of M-MCM-48 (M = Ti, Zr and Mn) (Figure 1) are in agreement with those of typical MCM-48 materials[13]. All as-syntheiszed samples exhibited a very strong low angle peak at around 2.30 ~ two weak peaks at 2.70 ~ and 4.40 ~ corresponding to diffraction planes of (211), (220) and (332), respectively. The XRD patterns of calcined M-MCM-48 looked similar to those of as-synthesized samples except that the refletion peaks shifted to the higher 20 angle slightly. The existence of metal atoms in MCM-48 framework was confirmed by UV and EDX analysis. For example, the UV spectra of the Si-MCM-48 and the Ti-MCM-48 are shown in
513 Figure 2. The band at 210 nm was assigned to isolated framework titanium in tetrahedral coordination, and the band at 230 nm was assigned to framework titamium in octahedral coordination[4]. A band at ca. 270 nm was attributed to extraframe titanium[ 14]. EDX spectrum of Ti-MCM-48 is shown in Figure 3. Both UV and EDX results indicated that the titamium atoms exist in the MCM-48 framework.
.
Before calcination
l
After calcination
g~ t-,r
!
1
3
5
7
9
2
t
t
4
~
.I
6
1
l
t
8
_
10
2O
Figure 1 XRD patterns of 2% M-MCM-48 3.2 Influence of reaction conditions on catalytic oxidation activity
We first carded out the gas phase (non-catalytic) oxidation and the results indicated that the product selectivity is low. The reaction is also uncontrollable. Over other catalysts such as simple metal oxides, the conversion of higher a-carboxylic alcohol to the corresponding carboxylic acid is also very low (20%). However, the selectivity of aeicosanoic acid was greatly enhanced when M-MCM-48 were used as catalysts. The effect of temperature on catalytic activity over Ti-MCM-48 was studied and results were given in Table 1. The optimum reaction temperature was 413K. The same conclusion can be drawn for Zr-MCM-48 and Mn-MCM-48. The product of the oxidation of a-eicosanol at 413K was extracted from the reaction system for composition analysis. The results show that after 5h, the highest yield of aeicosanoic acid was obtained (Table 2). Further increasing reaction time did not result in a higher yield. This probaly is due to decarbonation of the acid caused by heating for a longer time.
514 3.3 Influence of M content on catalytic oxidation activity The effect of M content on catalytic activity was also examined. Table 3 gives results of the catalytic oxidation of eicosanol over Ti-MCM-48. The yield of the desired product, aeicosanoic acid, increases gradually with increasing Ti content and reaches a maxium at a loading level of 1% Ti-MCM-48. Further increase in the Ti content results in a decrease in the yield. Table 4 shows the effect of M content on catalytic activety over Zr-MCM-48 and MnMCM-48. The yield and the selectivity of a-eicosanic acid both increase with increasing M content.
0.3 0 0.4
o
*m'
0.2
-t3- MCM-48 -o- TI-MCM-48
0.1 0
~
200
250
300
350
450
400
500
Wavelengths/nm
Figure 2 UV spectra of Ti-MCM-48
-siv,, ,,
Ti Ka
" L
0
'
I
I
I
1
2
3
4
"
I,
I
'
'
'
5
6
7
8
9
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Figure 3 EDX spectrum of Ti-MCM-48 Table 1. Effect of reaction temperature on Y, CH3(CH2)IsCOOH, over 1% Ti-MCM-48 T (K) 393 403 413 423 433 Y*(%) 18.4 40.2 54.4 38.8 21.4 *" Yield of a-eicosanic acid. Reaction time: 5h.
515 Table 2. Effect of reaction time on Y ( C H 3 ( C H 2 ) I s C O O H ) o v e r 1% Ti-MCM-48 t (h) 3.0 5.0 7.5 10.0 Y* (%) 27.0 54.4 54.0 46.4 *: Yield of a-eicosanic acid. Reaction temperature: 413K. Table 3. Effect of Ti content on yield of a-eicosanic acid over xTi-MCM-48 xTi (%) 0 0.1 0.5 1.0 2.0 5.0 Y* (%) 14.9 47.8 51.6 54.4 52.2 46.8 *" Yield of tx-eicosanic acid. Reaction temperature: 413K. Reaction time: 5h. Table 4. Effect of M content on catalytic oxidation activity over xM-MCM-48 xM (%) Selectivity of tx-eicosanic acid (%) Yield of tx-eicosanic acid (%) 2.0Zr 51.3 48.4 5.0Zr 64.4 60.4 8.0Zr 87.7 84.3 1.0Mn 35.9 33.7 2.0Mn 39.3 37.6 5.0Mn 52.3 48.9 Reaction temperature: 413K. Reaction time: 5h.
q13
8
3200.0
1975.0
1250.0
762.5
400.0
Wavenumbers/cm 4
Figure 4 IR spectra of products obtained over 2% M-MCM-48 catalysts In the IR spectra of reaction products, a band at 719 c m "1 c a n be assigned to the C - O H bending vibration of cx-eicosanol. The peak at 1720 cm l can be attributed to the carboxylic
516 group in a-eicosanoic acid. The intensity ratio, 11720/ 1719 indicates qualitatively yield of the reaction. Fig.4 clearly shows that for a given M content catalytic reactivity is: Zr-MCM-48 > Ti-MCM-48 > Mn-MCM-48 > Si-MCM-48, which is consistent with the results of chemical and GC-MS analysis. In Summary, the M (M = Ti, Zr and Mn)-MCM-48 can be used as a catalyst for the selectively catalytic oxidation of a-eicosanol to a-eicosanoic acid. Further studies on the catalytic properties of M (M = Ti, Zr and Mn) are in progress.
4 CONCLUSION MCM-48 molecular sieves doped with Ti, Zr and Mn can be synthesized by hydrothermal crystallization. M-MCM-48 molecular sieves can be used as a catalyst for the oxidation of a-eicosanol to its corresponding acid. The M contents have important effect on the catalytic activity. M-MCM-48 exhibits higher Catalytic activity than pure MCM-48 for the conversion. ACKNOWLEDGEMENTS This work was financially supported by the Committee of Science and Technology of Jilin Province, China, National Nature Science Foundation of China, and China Scholarship Council. REFERENCES 1. C.T. Kresge, M.E. Leonowicz, W.J. Roth et al., Nature 359 (1992) 710. 2. J.S. Beck, J.C. Vartuli, W.J. Roth et al., J.Chem. Soc., Chem. Commun. (1994) 147. 3. K.M. Reddy, I. Mondrakovski et al., J. Chem. Soc., Chem. Commun. (1994) 1059. 4. A. Corma, M.T. Navarro and J.P. Pariente, J. Chem. Soc., Chem. Commun. (1994)147. 5. P.T. Tanev, M. Chibwe and T.J. Pinnavaia, Nature, 368 (1994) 321. 6. T. Blasco, A. Corma, M.T. Navarro and J.P. Pariente, J.Catal. 156 (1995) 65. 7. N. Vlagappan, C.N.R.Rao, J.Chem. Soc., Chem. Commun. (1996) 1064. 8. V. Alfredsson, M.W. Anderson, Chem. Mater. 8 (1996) 1141. 9. S. Anderson, S.T. Hyde, K. Larsson, Chem Rev. 88 (1988) 221. 10. M. Morey, A. Da~cidson, H. Eckert, Chem. Mater. 8 (1996) 486. 11. M.J. Hudson, J. Knowles, Chem. Mater. 6(1) (1996) 89. 12 W. Changping, C. Qiang et al., Chem. J. Chinese Universities, 19(7) (1998)1154 13. S. Kawi, M. te, Catalysis Today 44 (1998) 101-109. 14. K. A. Koyano, T. Tatsumi, Chem. Commun. (1996) 145.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
517
Preparation o f Pd/A1-MCM-41 Catalyst and Its H y d r o i s o m e r i z a t i o n Properties for long chain alkane c o m p o u n d s Shui Lin a,b, Han Ning a'b, Sun Wan-Fu c*, Liu Wei-Min a and Xue Qun-Ji a a Lanzhou Institute of Chemical Physics, The Chinese Academy of Sciences, Lanzhou 730000, P R China b Great Wall Lubricating Oil Group Company, SINOPEC, Beijing 100085, P R China c Fushun Research Institute of Petroleum and Petrochemicals, Fushun 113001, P R China
The mesoporous material A1-MCM-41 with various Si/A1 ratios was synthesized rapidly by microwave under acidic conditions using cetylpyridinium bromide (CPBr) as template and tetraethylorthosilcate (TEOS) as silica source. The hydrogen forms of A1-MCM-41 were obtained via ion exchange. Pd/A1-MCM-41 catalysts were prepared by impregnation method. The catalytic activity and selectivity of as-synthesis catalysts for tridecane hydroisomerization were also evaluated.
1. INTRDUCTION The hydroisomerization of long-chain paraffins for improving the properties of gasoline (high octane number), diesel (low pour and cloud points) and lubricant base stocks (low pour points and high viscosity) is very importance, especially, for producing high quality lubricant base oil. This process usually use novel metal (Pt or Pd) supported on zeolite (Beta, SAPO-5 or Y) as catalyst[I,2], this kinds of catalyst are good for hydroisomerization of small molecule alkanes (~< C6), for hydroisomerization of long-chain alkanes, however, there exist undesirable cracking due to their relative strong acid sites. In order to suppress the cracking reaction and keep high hydroisomerization selectivity, the alternative choice is to find a new support with suitable porosity and acidity. Since the discovery of the new class ofmesoporous molecular sieves in 1992, there has been a growing interest in their potential catalytic applications. Because of their relatively mild acid sites and the possibility to vary the Si/A1 ratio in a wide range without significant changes in pore structure, these materials are very attractive model catalysts for transformation of bulky compounds, especially, for the hydroisomerization of long-chain alkanes. De Rossi et al.[3] studied the hydroisomerization of normal paraffins over a series *Corresponding author: Fushun Research Institute of Petroleum and Petrochemicals, Zip: 113001, Liaoning, P R China. Fax" 86-431-6429551" E-mail" Sunwanfu@fripp,conl,cn
518 of catalysts and found that the selectivity for isomers is higher and cracked products is lower on Pt/MCM-41 as compared to other materials. In this communication, The mesoporous material A1-MCM-41 with various Si/A1 ratios were synthesized rapidly by microwave under acidic conditions using cetylpyridinium bromide (CPBr) as template and tetraethylorthosilcate (TEOS) as silica source. The catalytic activities of Pd/A1-MCM-41 catalysts for tridecane hydroisomerization were evaluated at 3.5MPa in a fixed bed reactor packed with 20ml of catalyst. The reaction products were analyzed by gas chromatography. Comparing with microporous zeolites such as USY, Pd/A1-MCM-41 has higher hydroisomerization selectivity.
2. EXPERIMENTAL
2.1. Synthesis AI-MCM-41 Mesoporous aluminum-containing MCM-41 with different Si/A1 ratio was snthesized by microwave under acidic conditions. The preparation procedure is as follow: Preparing mixture solution A: Distilled H20, diluted H2SO4 and surfactant (cetyltrimethylammonium chloride, CTAC) were mixed together with stirring, then adding sodium aluminate at temperature about 50~ with intensive stirring until homogeneous. Mixture Solution B: Sodium waterglas(40 wt% SiO2) was dissolved by ethanol with stirring. Adding mixture B into Mixture A under an appropriate rate at different temperature with stirring. The homogeneous reaction gel was sealed in a cylindrical PTFE container and heated by a 700W microwave oven for 20 minutes. The solid product was recovered by filtration, washed with deionized water, dry at 120~ and calcinated at 560~ for 6h. The hydrogen forms of A1-MCM-41 (HA1-MCM-41) were prepared by ion exchange A1-MCM-41 with 1 M aqueous solution of NH4NO3 at 70~ followed by the deammonation at 400~ in the atmosphere. 2.2. Preparation of Pd/HAI-MCM-41 The synthesized HA1-MCM-41 samples were extruded into a strip form with a binder. Pd/HA1-MCM-41 catalysts were prepared by the impregnation with Pd (NO3)2 and dried at 120~ for 4 hours, then calcinated at 500~ for 3 hours, respectively. 2.3. Characterization of the samples Microwave oven (model CEM-2000) power is 700W with temperature programmed. X-ray diffraction (XRD) was carried out with a Ragaku D/max 2500 using Cuko radiation. Texture parameters were investigated by nitrogen sorption measurements with a Micromeritics ASAP 2400 automatic N2 adsorption instrument. The external and intemal surface area and volume were determined using a comparison plot. The pore size distribution of the synthesis samples was calculated by a geometrical method. The samples were degassed at 300 ~ for 8 hours. Differential thermal analyses (DTA) were performed on a Du Pond thermal analyzer from ambient temperature to 1000~ with 10 mg of the sample, a heating rate of 10~ and an air flow. The acid properties of HA1-MCM-41 and Pd/HA1-MCM-41 were measured using Nicolet 560 FT-IR with pyridine adsorption/desorption. Elemental compositions of the samples (the contents of A1203 and SiO2 of bulk analysis) were determined by chemical analysis. The catalytic activities for
519 tridecane hydroisomerization were evaluated at 3.5MPa in a fixed bed reactor packed with 30ml of catalyst. The catalyst first were reduced with a Hz gas at 300~ for 5 hr at 3.5MPa, then the tridecane was introduced into reactor at the rate of 45mL/h, WHSV=I.5, H2/oi1=600:1, temperature is 300~176 reaction products were analyzed by gas chromatography.
3. RESULTS AND DISCUSSION 3.1. Characterization of the support The texture and structure of the synthesized mesoporous materials were examined using XRD, BET and DTA. The XRD patterns are shown in figure 1. From figure 1 we can see, the X-ray diffractogram of A1-MCM-41 structure present typical pattern with a strong peak at low angle assigned to dlo0 reflection(figure 1a) and the structure well-preserved after calcination in air at 700~ for 5 h (figure l b). The intensity of dl00 peak of A1-MCM-41 decreased with the amount of A1 incorporated into framework of MCM-41 increased. The DTA pattern shows three distinct peaks at 1 2 3 4 5 temperatures of 100~ 280 and 900~ The peak at 2Theta [deg. ] 100~ is attributed to water evaporation in the sample. The decomposition of the template results Fig1. XRD patterns of as synthesized in middle-peak in the DTA profile, the high-temperature peak at 900~ is attributed to zeolite under 500 ~ (a) and 700 ~ (b) framework collapse of the synthesis sample. The properties of different Si/A1 ratio of A1MCM-41 are listed in table 1. ---
.
Table 1. The textural and structureal of the different Si/A1 ratio A1-MCM-41 . Sample . . 1 2 3 Si/A1 ratio 100 75 50 Surface area (mZ/g) 671.2 650.4 639.5 Pore volume (ml/g) 0.695 0.684 0.675 Mean pore size (nm) 4.2 4..2 4.0 .
.
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As the amount of A1 incorporated increased, the surface area, pore volume of A1MCM-41 decreased gradually This is probably due to calcinated partial collapse of the hexagonal structure caused by the instability associated with the presence of increasing amounts of framework A1. The mean pore sizes were evaluated using N2 adsorption isotherms by BJH method. The adsorption/desorption isotherm plots and resulting hysteresis loop are shown in the
520 Figure 2. It can be seen that there is a sharp step at intermediate relative pressures typical of IUPAC type IV isotherm. The step restricted to narrow range of p/p0, implies the existence of a narrow range of pores in the vicinity of 4.0nm. The "d" value and adsorption/desorption isotherm plots are in agreement with the literature [4,5] Purely siliceous MCM-41 has no Broensted acidity, but when some trivalent cations such as A1, Fe incorporate into framework of MCM-41 [6,7], it will creates moderately acidic sites. IsothermPlot 600550500~450-
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Figure 2. Adsorption/desorption isotherm plots of different Si/A1 ratio of A1-MCM-41 (a): Si/AI=100, (b): Si/Al=75, (c): Si/AI=50, (d): Si/A1-25. These A1 incorporation amount and methods influence the nature of A1-MCM-41 such as concentration of acid sites, pore structure and surface area. The acidity and pore
521 structure of the catalysts has a major effect on hydrocracking and hydroisomerization. Lubricating oils of good quality should have lower pour points and higher viscosity indexes. Effective producing lubricating oil catalysts should have higher activity to transform n-alkanes to isoalkanes, because the isoalkanes have lower pour points. The catalysts with high hydrogenation ability and moderate acidity are desirable for hydroisomerization of long chain hydrocarbons [8]. Figure 3 gives the acid property of different Si/A1 ratio HA1-MCM-41 From Figure 3 we can know, with the content of A1 increasing, 0.25 the total amount of acidity the zeolites increase strikingly, / ~ , , , , ~ , ~ CB+L especially for Lewis acid. When 0.2 " Si/A1 ratio of the zeolite ,.........4~.-- . ~ ' ' ' ' ' ~ CL decreases to 25, the Lewis o.15 amount of acidity increases E almost 50% comparing with that E of Si/A1 ratio of the zeolite is 0.1 100. 0.05
3.2. Pd/HAI-MCM-41
HA1-MCM-41 was prepared by ion exchange A1-MCM-41 with 1M aqueous solution of NH4NO3, followed by the deammonation at 400 ~ in the atmosphere. The temperature of aqueous solution of NH4NO3 influences the exchange degree of the A1-MCM-41. Therefore, the suitable exchange temperature should be chose by the experiment. The effect of different temperature on the exchange degree of the zeolite has been investigated as shown in Figure 4. From Fig 4 we can see, as the ion exchange temperature increase, the more Na were released from the zeolite, When the steaming temperature is at 70 ~ the content of NaO of the zeolite is'only 0.12 wt%, continue rise the temperature to 90~ the content of NaO only decrease a little bit, and the framework of
CB
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60
80
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Figure 4. The relationship between content of NaO and ion exchange temperature
100
522 the zeolite collapsed partially due to severe desodium(according to the XRD pattern, not shown), as a result, the amorphous phase increased in the zeolite. Pd/HA1-MCM-41 catalysts with different Si/A1 ratio were prepared by the impregnation with Pd (NO3)2, the properties and composition of the Pd/HA1-MCM-41 catalysts were listed in table 2. From table 2 we can see that as the Si/A1 decrease, the total amount of acidity increase, while the surface area decrease. This is probably due to partial collapse of the hexagonal structure caused by the instability associated with the presence of increasing amounts of framework aluminum. Table2. The properties and comPos!t!on of the Pd/HA!-MCM?41 catalyst s .... Catalysts 1 2 3
4
0.35
0.35
0.35
0.35
Si/A1 ratio
100
75
50
25
Surface area m2/g
668.5
644.3
637.0
613.5
Total acid amount mmol/g
0.135
0.161
0.164
0.198
Content of Pd
wt %
3.3. Catalytic activity The performance of the Pd/HA1-MCM-41 catalysts with different Si/A1 ratio for conversions of tridecane was evaluated in a 30ml fixed bed reactor. The catalysts (1, 2, 3 and 4) hydro-conversion was shown in Figure5. From the Figure we can see: the order of conversion of tridecane of the four catalysts is 4>3>2>1. This order is the same as total acid amount of these four catalysts (see table 2), that is, the more amount of A1 incorporated into framework of the MCM-41 catalyst ~upports, the higher hydro-transformation activity of the catalysts were obtained. The product distribution data of n-tridecane hydro-transformation over Pd/HA1-MCM-41 catalysts with different Si/A1 ratio were summarized in table 3.
Figure 5. Influence of catalyst with different Si/A1 ratio on coversion of tridecane
523 From table 3 we can know, at around 60% conversions, all Pd/HA1-MCM-41 catalysts showed high hydroisomerization selectivity of more than 86 wt.% It seems that hydroisomerization reaction is favorable than hydrocracking, due to the mild acidity of HA1-MCM-41. Table3. Product selectivity for hydroisomerization of n-tridecane over different catalysts Catalyst Conversion Mono-C~3 Di- C13 Tri- C13 Branch C13 % (wt%) (wt%) (wt%) (wt%) 1 59.3 57.0 18.3 11.1 86.4 2 59.8 58.3 19.9 10.5 88.7 3 60.2 58.8 17.5 10.2 86.5 4 59.5 60.2 20.2 11.4 91.8 Among the catalysts, Pd/HA1-MCM-41 catalyst with Si/A1 ratio 25 has the highest hydroisomerization selectivity, while the catalyst withSi/A1 ratio 100 is the lowest one. In other words, the higher the total acid amount of the Pd/HA1-MCM-41 catalyst support was the higher the conversion and isomerization yield were obtained over the Pd/HA1-MCM-41 catalysts. From this point of view, Pd/HA1-MCM-41 catalyst with lower Si/A1 ratio will be good catalyst for producing high quality lubricating base oil.
4. CONCLUSIONS The mesoporous material A1-MCM-41 with various Si/A1 ratios was synthesized successfully under the conditions mentioned in this paper. The hydrogen forms of A1-MCM-41 were obtained via ion exchange. The ion exchange temperature affects the acid property of the supports drastically.The hydroisomerization of n-tridecane was carried out in a fixed bed reactor at 350~ and 3.5MPa over Pd/HA1-MCM-41 catalysts, the experimental data show that the Pd/HA1-MCM-41 catalyst with Si/A1 ratio 25 has higher hydroisomerization selectivity for transformation tridecane. The catalytic activity decreased in the order of Pd/HA1-MCM-41 (Si/Al=25)> Pd/HA1-MCM-41 (Si/AI=50)> Pd/HA1-MCM-41 (Si/Al=75)> Pd/HA1-MCM-41 (Si/AI=100). The more amount of A1 incorporated into framework of the MCM-41 catalyst supports, the higher were the reactivity and isomer yield obtained.
REFERENCES
1. R. A. Meyers, Handbook of Petroleum Refining Processes, McGraw-Hill, New York (1996). 2. C. Bischofand M. Hartmann, Stud. Surf. Sci. Catal., 135(2001). 3. K. J. Del Rossi, G. H. Hatzikos, A. Huss, US Patent 5,256.277 assigned to Mobil Oil Corp. (1993). 4. K. J. Edler and J. W. White, J. Chem. Soc. Chem. Commun. 155(1995). 5. T. Chiranjeevi, Prashant Kumar, M. S. Rana, G. Murali Dhar, and T. S. R. Prasada Rao, Stud. Surf. Sci. Catal., 135(2001).
524 6. R. Mokaya and W. Jones, J. Mater. Chem., 9(1999)555. 7. R. Mokaya, J. Catal., 186(1999)470. 8. F. Alvarez, F. R. Ribiero, G. Perot, C. Thomazeau and M. Guisnet, J. Catal., 162(1996)179.
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A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
525
Alkylation o f phenol with methyl tert-butyl ether over mesoporous material catalysts Xiang-Hai Tang*, Xin-Liang Fu and Hai-Yan Jiang College of Chemistry, Nankai University, Tianjin 300071, P. R. China
Alkylation of phenol with methyl tert-butyl ether (MTBE) for the synthesis of tert-butylphenol (TBP) and 2,4-di-tert-butylphenol (2,4-DTBP) has been studied over mesoporous material catalysts with MCM-41 structure. Influence of reaction conditions as well as modification with superacid on the catalytic properties of aluminosilicate MCM-41 was evaluated. The results were compared with those of Y zeolite. It revealed that the alkylation was governed by the acidity and the pore structure of the catalyst. The distribution of products was a function of temperature. Increasing temperature promoted the selectivities toward p-TBP and 2,4-DTBP while dealkylation of 2,4-DTBP was observed at high temperature. At low temperature, the lower the space velocity was applied, the higher the phenol conversion and the 2,4-DTBP selectivity were obtained. The ratio of height to diameter of catalyst bed had almost no effect on this reaction in the range of 2-6. A mild acidity can meet the requirement for the alkylation of phenol with MTBE, while a strong acidity boasts the side-reactions such as dimerization and coke formation which results in a fast deactivation of the catalyst. Aluminosilicate MCM-41 was found to be a promising catalyst for phenol alkylation with MTBE by taking the advantage of its mild acidity and large pore diameter.
1. INTRODUCTION Alkyl phenols are valuable fine chemicals, which are vastly employed in chemical industry, pharmacy and pesticide manufacturing [1]. The syntheses of alkyl phenols by alkylation of phenol with alcohols and olefins have been extensively studied in many literatures. The most commonly used catalysts are liquid mineral acids (e.g., HF, H2SO4, H3PO4, etc.), acid solids (e.g., A1C13, ZrC14, BF3, SbC15, etc.), metal oxides and mixed oxides (e.g., 7-A1203, A1203-SIO2, TiOz-WO3, etc.) as well as resins, whereas the major disadvantages are quite obvious: (1) the exhausted catalysts usually cause pollution problems; (2) the alkylation reactions are non-selective thus undesirable by-products are produced. However, there are relatively few publications concerning zeolitic materials as catalysts in these reactions. This is particularly true in the catalytic syntheses ofbutylphenols. * To whom correspondence should be addressed. Email:
[email protected]
526 Molecular sieve is widely used in petrochemical industry due to its regular pore aperture and tunable catalytic activity. MCM-41 silica is a newly discovered ordered mesoporous material. The MCM-41 structure possesses a hexagonal array of uniform mesopores in the range of 2-10 nm, a large surface area (>700 m 2 g-l) and a large pore volume [2]. By incorporation of aluminum into the silica framework, acid sites can be generated, nevertheless they are weakly acidic and can only be compared with those present in amorphous silicaaluminas [3]. However, to those reactions requiting a mild acidity, aluminosilicate MCM-41 is a catalyst of choice [4,5], moreover, it can also be used as a support for catalytically active materials. Here for the first time we demonstrate the alkylation of phenol with MTBE on aluminosilicate MCM-41 catalysts. The catalytic performances of these catalysts are also compared with those of Y zeolite.
2. EXPERIMENTAL 2.1. Materials All chemicals used were A.R. grade and commercially purchased from companies without further treatment. Aluminosilicate MCM-41 was prepared by the hydrolysis of tetraethyl orthosilicate (TEOS), aluminum isopropoxide [AI(i-OPr)3] and cetyltrimethylammonium bromide (CTAB) in ammonia solution, the final molar composition is 1.0 TEOS:0.05 AI(i-OPr)3:0.50 CTABr: 9.2 NH3:130 H20, then the mixture was hydrothermally treated at 383 K for 72 h. The solid product was filtered, washed with distilled water and dried at 393 K overnight. The assynthesized sample was calcined in a muffle at 873 K in air for 3 h to burn off the occluded organics. Finally, it was ion-exchanged twice with a 0.3 mol 1-l NH4NO3 solution at 368 K for l h to remove the extra-framework A1. Sample thus obtained is denoted as MC-0. The MC-0 powder was mixed with boehmite, dilute nitric acid and distilled water for giving a proportion MC-0:alumina of 65:35 in the final solid (MC-l) after calcination at 773 K for 3 h. A superacid-supported sample MC-2 was prepared as follows. Dried MC-1 was wetimpregnated with a 10% v/v TiC14/ethanol solution (1 g/2 ml) and exposed in a moisture air ovemight, then soaked with a 0.25 mol 1-1 (NH4)2S208 solution. After dried in air it was heated at 473 K for 2 h and later calcined at 823 K for 3 h. H-form Y zeolite (atomic ratio Si/Al=2.5) was purchased from Huahua Group Ltd., P. R. China and further calcined at 923 K for 3 h (HY-0). An alumina-bonded Y (denoted HY-1) was also prepared similarly to MC-1 with a proportion HY-0:alumina of 65:35. 2.2. Characterization Powder X-ray diffraction (XRD) pattems were obtained with a Rigaku D/MAX ),A diffractometer using the Cu Kot radiation operated at 40 kV and 40 mA. Elemental analysis was performed on a Shimadzu X-Ray Fluoresence Spectrometer VF320, data were collected and analyzed with a Data Processor DP-32 workstation. The reaction products were analyzed with a Hewlett-Packard HP G 1800A GC-MS instrument. A SE-30 capillary column (50 m, 0.2 mm I.D.) was equiped. 2.3. Catalytic testing The alkylation was carried out under atmospheric pressure in a down-flow fixed-bed
527 reactor with a 16 mm I.D. Catalysts (20-30 mesh) were loaded in the thermal static part of the reactor. In a typical run 8.0 g of catalyst was loaded and tested for 6 h on stream. A mixture of phenol and MTBE at an n(MTBE)/n(phenol) ratio of 2/1 was pumped into the reactor. The products were trapped in a condenser at the reactor outlet.
3. RESULTS AND DISCUSSION 3.I. Material characterization Elemental analysis revealed that the atomic ratio Si/A1 in MC-0 was 23, which is a little higher than that in the synthetic gel. A TiO2 loading of 8.4% and a SO42 loading of 6.2% were observed in MC-2. The XRD patterns of MC-0, MC-1 7500 and MC-2 are shown in Figure 1. All samples exhibit at least three wellc..) resolved reflections in the 20 range .~5000 between 2-6 ~ which can be indexed to
an ordered hexagonal lattice typical of MCM-41 [2]. The intensities of the reflections decrease by 40% on MC-1 2500 A~ and by 50% on MC-2 as compared to B ~ those of MC-0, respectively. This is quite in accordance with the proportion 0 of the MCM-41 in these samples. 1 4 7 10 Meanwhile, the peaks of the MCM-41 20( ~) Figure 1. XRD patterns of (A) MC-0, (B) MCphase in MC-1 as well as in MC-2 move slightly towards high angle, which is 1 and (C) MC-2. probably due to the fact that the MCM-41 framework suffer more shrinkage upon further calcination [2]. A minor widening of the reflection peaks was also observed both on MC-1 and MC.2. It indicates that the MCM-41 phase in MC-1 as well as in MC-2 is less ordered than that in MC-0. Nevertheless, the mesoporous framework sustained after calcination even for several times. A careful XRD examination was also performed at high angle area. However, no distinct peaks were observed for MC-1 and MC-2 in the 20range between 20-80 ~ which implies the absence of crystalline 7-A1203 phase in both samples and the absence of crystalline TiO2 phase in MC-2. This result suggests that the titanium species are highly dispersed in MC-2. ~
3.2. Catalytic properties MTBE is carefully chosen as an alkylating agent for phenol butylation because itself as well as its cracking product can act as a good solvent for phenols. Factors that affect the alkylation of phenol with MTBE on the catalysts were evaluated. 3.2.1. Influence of temperature The analysis results of the products revealed that the distribution of the reaction products was a function of temperature. Figure 2 depicts the influence of temperature on the reactions over MC-0 and HY-0. On both catalysts, under the conditions of feed weight hourly space
528 velocity (WHSV) of 0.8 h -I and molar ratio n(MTBE)/n(phenol) of 2.0, the MTBE conversion increased with temperature before 413 K was reached, so did the phenol conversion. This is quite easy for understanding that the first step for the alkylation requires the cracking of MTBE, and increasing temperature accelerates the formation of C4+ and the overall reaction rate. Further increasing temperature resulted in MTBE fully converted while the phenol conversion began to decrease from 423 K. Butylphenols were the main alkylation products and minor methylphenols were formed at high temperature. As temperature increased the pTBP selectivity increased whereas the o-TBP selectivity decreased, however, the selectivity toward 2,4-DTBP changed following the trend of the phenol conversion. Indeed, a higher temperature favors the formation of para isomer from a thermaldynamic point of view, meanwhile, side-reactions are also boasted. At high temperature water, dimethylether and low hydrocarbons were detected in the products, which suggests that dehydration between formed methanol molecules, dimerization of C4+ species and dealkylation of 2,4-DTBP took place on both MC-0 and HY-0. However, the results also indicate that HY-0 is more active than MC-0, which may be due to a higher density of acid sites on HY-0. 100
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Figure 2. Influence of temperature on the performance of catalysts (A) HY-0 and (B) MC-0. (a: phenol conversion, b: p-TBP selectivity, c: 2,4-DTBP selectivity and d: o-TBP selectivity) 3.2.2. Influence of space velocity Tables 1 and 2 show the effect of feed space velocity on the alkylation over MC-0 and HY-0, respectively. The evaluation was performed at 413 K with a feed molar ratio n(MTBE)/n(phenol) of 2.0. The conversions of phenol and MTBE as well as the selectivities
Table 1. Alkylation of phenol with MTBE on MC-0 catalyst at various space velocity. WHSV Phenol conversion Selectivity (%) 2,4-DTBP (h -~) (%) TBP p-TBP o-TBP 0.80 92.8 45.4 40.1 5.3 53.1 1.05 86.4 51.7 35.6 16.1 45.5 1.65 74.1 61.7 36.6 25.1 36.5 2.58 72.0 62.4 36.0 26.4 35.8 3.12 64.5 63.4 33.6 29.8 34.9
529 Table 2. Alkylation of phenol with MTBE on HY-0 catalyst at various space velocity. WHSV Phenol conversion Selectivity (%) (h -~) (%) TBP p-TBP o-TBP 2,4-DTBP 0.95 91.1 55.4 48.4 7.0 42.3 1.80 85.3 60.3 51.9 8.4 37.1 2.01 83.2 65.7 56.5 9.2 31.5 2.27 71.0 70.3 61.8 8.5 29.7 toward p-TBP and 2,4-DTBP gradually decreased with increasing space velocity. This can be correlated to the change of contact time. We assume that 2,4-DTBP is a consecutive reaction product formed by alkylation of o-TBP and p-TBP with MTBE. At high space velocity sidereactions were markedly suppressed, which is supported by the observation of less multialkylated components in the products. However, the reaction mechanisms may be different on catalysts MC-0 and HY-0. The difference on selectivity toward o-TBP as well as 2,4-DTBP is significant. On MC-0 the selectivity toward o-TBP was much higher and quickly increased with increasing space velocity, whereas on HY-0 it was almost constant. The reason will be discussed in the following context. To elucidate the effect of external diffusion on this reaction, an experiment was performed by varying the height of the catalyst bed. On both catalysts MC-0 and HY-0, at 413 K with a feed molar ratio n(MTBE)/n(phenol) of 2.0 and a WHSV of 0.8 h -1, the ratio of height to diameter of catalyst bed had negligible influence on this reaction in the range of 2-6, i.e., the phenol conversion and the product distribution changed very little.
3.2.3. Influence of surface acidity To understand the mechanism of phenol alkylation with MTBE, reaction was carried out on catalysts with various surface acidity. Evaluation was performed at 413 K with a feed molar ratio n(MTBE)/n(phenol) of 2.0 and a WHSV of 0.68 h -~. The results are summarized in Table 3. Table 3. Alkylation of phenol with MTBE on different catalysts with various surface acidity. Catalyst Phenol conversion Selectivity (%) (%) TBP p-TBP o-TBP 2,4-DTBP HY-1 85.7 54.8 44.0 10.8 40.9 MC-1 84.6 49.7 37.0 12.7 48.7 MC-2 68.4 55.4 43.2 12.2 42.8 As titania was formed on the MC-1 matrix and interacted with $042"during the treatment, surperacidic sites could be generated on the surface of MC-2 [6]. On HY-1 Broensted acid sites are in the ascendant, while on MC-1 weak Lewis acid sites are dominant and a few weak Broensted acid sites are present. As previously reported [7], Broensted acid sites strongly interact with the aromatic ring while Lewis acid sites interact with oxygen to form phenolate complexes. In the former case alkylation can occur either at the oxygen or at the ring, while in the latter alkylation at the ring in the ortho position is favored. However, hardly any O-
530 alkylation products were detected. We assume that, if they were formed, they had been consumed through isomerization and transalkylation. A. Corma et al. found that, on partially ion-exchanged Y zeolite, at 303 K phenol reacted with tert-butanol in CC14 to form tert-butyl phenyl ether; at 353 K the activity increased with the strength of the acid sites, and weak strength acid sites favored the formation of 2,4-DTBP [8]. Interestingly, the selectivities toward o-TBP and 2,4-DTBP are significantly higher on the MC catalyst series than those on the HY catalyst series. It is worthy of mention that under the same reaction conditions the bondant (A1203) was much less activity than HY-0 and MC-0. Generally, cracking and isomerization require a strong acidity. The minority of o-TBP in the reaction products on Y zeolite catalysts might be due to the following reasons: (1) the consumption of o-TBP through isomerization to p-TBP and formation of 2,4-DTBP; (2) the shape-selective effect of the pore aperture on its formation. The latter factor hindered the formation of 2,4-DTBP on Y zeolite too. While on the MC catalyst series, the spacial hindrance on formation of intermediate complex and diffusion could be excluded, and the isomerization of o-TBP was less pronounced due to the weak acidity. However, it can be seen from Table 3 that phenol conversion on MC-2 was obviously lower. This can be correlated to the surperacidity on MC2. As strong acid sites are always responsible for the coke formation due to the strong interaction with the adsorbed molecules, the catalytic active sites are blocked and a fast deactivation is resulted in. Indeed, after a 6 h run, MC-2 became dark black while HY-1 and MC-1 were yellowish. It indicates that a mild acidity can meet the requirement for the alkylation of phenol with MTBE, strong acid sites are not necessary and may do harm to the catalyst. In summary, our results suggest that mesoporous aluminosilicate is a promising catalyst for the alkylation of phenol with MTBE by taking the advantages of its mild acidity and large pore diameter. Further work is still ongoing to improve the activity and the selectivities toward p-TBP and 2,4-DTBP with MCM-41 aluminosilicate.
4. ACKNOWLEDGMENTS The financial support of this research by the National Natural Science Foundation of China (through Grant No. 29873024) is gratefully acknowledged.
REFERENCES 1. T. Kirk and K. Othmer, Encyclopedia of Chemical Technology 3rd Ed., Wiley, New York, 1981. 2. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. 3. A. Corma, V. Fomes, M. T. Navarro and J. Perez-Pariente, J. Catal., 148 (1994) 569. 4. A. Sayari, Chem. Mater., 8 (1996) 1840. 5. R. Mokaya, W. Jones, Z. Luan, M. D. Alba and J. Klinowski, Catal. Lett., 37 (1996) 113. 6. H. Hino, S. Kobayashi and K. Arata, J. Am. Chem. Soc., 101 (1979) 6439. 7. E. Santacesaria, D. Grasso, D. Gelosa and S. Carra, Appl. Catal., 64 (1990) 83. 8. A. Corma, H. Garcis and J. Aprimo, J. Chem. Res., (1988) 40.
Studies m ~urtace ~mence and tdatalysls 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
531
Isopropanol dehydration over nanostructured sulfated M C M - 4 1 Antonio S. Araujo*, Joana .M.F.B. Aquino, Cristiane D.R. Souza and Marcelo J. B. Souza Federal University of Rio Grande do Norte, Department of Chemistry, CP 1662, 59078-970, Natal, RN (Brazil) The synthesis of MCM-41 and sulfate-MCM-41 is reported. The MCM-41 sample was prepared by the hydrothermal method using cethyltrimethylamine as template, and characterized by BET surface area, X-ray diffraction, infrared spectroscopy and thermogravimetry. The sulfate containing MCM-41 was prepared by the controlled impregnation of MCM-41 with 0.5 M sulfuric acid. From n-buthylamine adsorption data and thermogravimetry, MCM-41 has no considerable acidity, whereas the SO427MCM-41 presents medium acid sites. The materials were used as catalyst for isopropanol dehydration, in a fixed bed continuous flow reactor. The SO42-/MCM-41 presents catalytic activity for isopropanol dehydration at relatively moderate temperature 473 K, with selectivity to propene.
1. INTRODUCTION The newly discovery mesoporous molecular sieve MCM-41 possesses high surface area and a uniform hexagonal array [1,2], opening new opportunities in the hydrothermal synthesis and modification in order to obtain new acid materials for heterogeneous catalysis applications [3,5]. Some properties of a thermostable mesophase of basic zirconium sulfate with texture characteristics close to those of MCM-41 has been reported [6]. The peculiarities of the catalytic behavior of the mesophase are related to its acidic properties. The MCM-41 nanostructured materials present ordered channels, and disordered atomic arrangement similar to that of amorphous silica. The formation of the MCM-41 phase occurs according to the liquid crystal template (LCT) mechanism, in which tetrahedral SiO4 species react with the surfactant template under hydrothermal conditions. The number of acid sites can be modified on a wide scale by isomorphic substitution, by ion-ex-change or by treatment with acids. In this work the silica MCM-41 was treated with sulfuric acid solution in order to generate acid sites in its surface. The introduction of sulfate ions in molecular sieves has recently been studied [7] and revealed that they can be active catalyst for the synthesis of b-naphtyl-methyl-ether, from bnaphthol and methanol at 473 K. In this work, the synthesis of SO42-/MCM-41 was studied using the controlled impregnation method. The obtained material was applied to the isopropanol dehydration using fixed bed continuous flow reactor. 2. EXPERIMENTAL
The MCM-41 was firstly synthesized by the hydrothermal method of a gel with molar composition 4SIO2:1Na20:1C16H33(CH3)3NBr:200H20, with pH adjustment and salt addition.
532 The synthesis was carried out at 373 K by 4 days. Then, it was washed with distilled water, recovered by filtration and dried at 373 K for 1 day. The material was calcined at 823 K in nitrogen and then in air atmosphere. The material was characterized by XRD (Rigaku), infrared spectroscopy (Midac) and thermogravimetry (Mettler Toledo TGA/SDTA 851). BET surface area was measured using nitrogen adsorption at 77 K, on an ASAP 2010 (Micromeritics). For the sulfatation of MCM41, ca. 1 g of calcined material was treated with 30 ml of 0.5 N sulfuric acid, at room temperature for 2 h, and then heated at 343 K until complete evaporation. The sample was dried at 383 K for 10 h, in an oven, and subsequently calcined at 823 K for 5 h, under nitrogen atmosphere flowing at 30 mL.min -~. The presence of sulfate species in the MCM-41 material was verified by thermogravimetry. The acid properties were investigated by using nbutylamine as molecular probe, followed by TGA, according to procedures de-scribed in the literature [8]. The obtained material was tested as a catalyst for isopropanol dehydration, in a fixed bed continuous flow reactor [9], at temperatures of 473, 513 and 553 K, and W/F ranging from 1.3 to 4.1 g.s.mol -l, where W - mass of catalyst (g) and F = flow of reactant (mol.sl). The products were analyzed by gas chromatography using a Porapak Q-packed column. 3. RESULTS AND DISCUSSION
The thermogravimetric analysis of the uncalcined MCM-41 material in nitrogen atmosphere show three weight losses [8], in the following temperature ranges: (i) from 298 K to 443 K (4% thermodesorption of physically adsorbed water); (ii) from 443 K to 543 K (38% surfactant decomposition) and (iii) from 543 to 803 K (7% residual surfactant decomposition and silanol condensation). From the characterization of the synthesized MCM-41 by XRD, FT-IR and TG, it was verified that the hydrothermal method has been efficient to obtain the MCM mesophase. The FT-IR spectra of the Si-MCM-41 show a characteristic absorption band at 960 cm -1, due to Si-OH groups, and others at the 1080, 800 and 465 c m -1 regions, which are characteristics of the material. As shown inFigure 1, the XRD patterns for the Si-MCM-41 and SO42-/MCM41 present a very strong peak, at ca. 2.1 ~ 2 O, due to (100) index. Two weak peaks were also distinguished as peaks characteristic of the family, at 4.10 (110) and 4.8 ~ (200), suggesting hexagonal symmetry [1,2]. Thus the structure of the sulfate modified sample is still nanoporous and similar to MCM-41. The thermogravimetry measurements showed that the sulfate species interact with the MCM-41 surface, generating the catalytic acid sites. From TG curves, the sulfate groups decompose in two steps: i) from 473 to 668 K and ii) from 668 K to 774 K, generating the Bronsted and Lewis acid sites. From n-buthylamine adsorption, it was verified that pure MCM-41 has practically no acidity or very low acid sites density (0.1 mmol/g), whereas SO4-2/MCM-41 has ca. 1.2 mmol/g of total acidity. The adsorption parameters for the silica MCM-41 were: surface area of 780 m2.g~, pore volume of 0.68 cm3.g-I and pore width of ca. 3.7 nm. For the sulfated sample, a decrease in the surface area to 720 m2.g-l was obtained. However, the pore volume and width were practically the same. This confirms that stable SO4~/qVICM-41 can be synthesized by controlled impregnation methodologies.
533
Figure 1. X-ray diffraction patterns of MCM-41 and sulfate-containing MCM-41.
The proposed structure to the sulfated material is shown in Figure 2. The scheme considers that the material surface is totally dehydrated, which is obtained after calcination at 773 K, with the sulfate covalently bonded to the silicon via oxygen atoms. The negative charge of the oxygen is neutralized by one proton forming Bronsted acid sites (BA). Due to the inductive effect of the sulfate group, the strong Lewis acid sites (LA) can be generated on its surface. From the infrared analysis, the asymmetric and symmetric stretching of the S=O bond were determined in the 1215-1125 cm ~ and 1060-995 cm l regions, respectively.
Figure 2. Scheme proposed for the sulfate-containing MCM-41 material showing possible Bronsted acid (BA) and Lewis acid sites (LA). The catalytic tests shown that the MCM-41 without sulfate presents very low catalytic activity, with conversion of ca. 7% at the studied temperatures. On the other hand, the SO42 /MCM-41 was very active to the isopropanol dehydration, with ca. 78% of conversion, producing propene and isopropyl ether, as can be seen in Figure 3. The conversion attains a maximum at W/F equal to 4.1 g.s.mol -~, independently of the reaction temperature. The high activity of the SO42-/MCM-41 catalyst can be visualized as a function of the surface acidity generated by the sulfate groups. The selectivity was measured as the propene/ether ratio to
534 each temperature reaction as a function of the W / F . In Figure 4, it is observed that the selectivity to olefin is higher for low W/F values, with propene/ether ratio in the range of 1 to 1.8. For values of W/F superior to ca. 2.7, the propene/diisopropyl ether ratio decrease is around 0.8, being almost constant for the studied temperature. This is evidence that there is a relation between the contact time of the reactant with the particular pore system and diffusion associated with the acidity of the SO42/MCM-41. 90 80
o~ v ~
t--. 0 oo L_
> rO
70 60 50
- - = - - 473 K
40
- - e ~ 553 K
30
- - A ~ 513 K
'
10
i
1,5
'
i
2,0
9
i
'
2,5
i
3,0
,
i
3,5
'
i
4,0
'
4,5
W/F (g.s/mol) Figure 3. Isopropanol conversion as a function of W/F for different temperature on the SO42/MCM-41.
2.0 l~ .O t~
- - = ~ 473 K |
1.6
- - A ~ 513 K I
I
L_ L_
(1) r-
1.2
uJ t-
0.8-
Q. O --9 0.
0.4
0.0 1.0
!
'
1 15 ' 2:0
' 215
' 310
' 3'.5
' 410
' 4.5
W/F (g.s/mol) Figure 4. Propene/diisopropyl ether ratio in the isopropanol dehydration reaction as a function of the W/F at different temperatures for the S042/MCM-41.
535 Experimental kinetic data of the isopropanol dehydration over MCM-41 and 8042" /MCM-41 have been obtained in a fixed bed continuous flow reactor. For all experiments the following conditions were assumed: isothermal reaction in fixed bed, catalyst in powder form, uniform bed porosity, reactor profile as plug flow and stream in stationary state. Linking the values of residence times with conversion in a first order kinetic model [ 10], the obtained fit represents the variation of the conversion rate for a given temperature (Figure 5). The slop of this curve gives the rate constant of the process. 3.5 3.0
9 MCM-41 9 S042/MCM-41
% 2.5 ~ o "" 2.0
"i--..
1.5 1.0
. 1.32
. 1.36
.
. 1.40
. 1.44
1.48
, 1.52
103/T (103/T)
Figure 5. Arrhenius plots for determination of the activation energy for isopropanol dehydration over MCM-41 and SO42-/MCM-41. Table 1 summarizes the kinetic data obtained by the Arrhenius plot for the experimental reactor data. These results show a decrease in the activation energy to isopropanol dehydration in comparison with the pure MCM-41 with activation energy of 20 kJ.mo1-1 for MCM-41 and 36.73 for SO42-/MCM-41. The decreasing in the activation energy is affirmed by the increasing in the MCM-41 acidity by the incorporation of the sulfate groups in the mesoporous array. Table 1. Parameteres of Arhenius equation (T, k), apparent activation energy (Eat), preexponential factor (Ao) and respectives aciditiesfor the isopropanol dehydration over MCM41 and SO42-/MCM-41. MCM-41 Temperature (K) k.106 (s") 1000/T (K-') ln(k.106) Eat (kLmol:') 473 3.334 2.114 1.204 36.73 513 9.611 1.949 2.263 Acidity (mmnol/g) 553 12.667 1.808 2.539 0.1 SO4"2/MCM-41 Temperature (K) k. 106 (S-1) 1000/T (K l) ln(k. 106) Eat (kJ.mol -l) 473 5.365 2.114 1.680 20.02 513 9.737 1.949 2.276 Acidity (mmol/g) 553 11.112 1.808 2.408 1.2
536 4. CONCLUSIONS The sulfated MCM-41 material was very active to the process, with conversion to propene and diisopropyl ether. This activity is attributed to the high acidity generated by the sulfate incorporation on the MCM-41 structure generating Brrnsted and Lewis active sites. The isopropanol conversion increases with the temperature from 55 to 72 %, with the propene/diisopropyl ether molar ratio changing from 1 to 1.8. The possibility to modify the surface of the MCM-41 by treatment with sulfuric acid and subsequent calcination open new opportunities to generate strong active acid sites in stabilized nanostructured materials. ACKNOWLEDGEMENTS The authors acknowledge the support from the Conselho Nacional de Desenvolvimento Cientifico e Tecnolrgico (CNPq), and Agrncia Nacional do Petrrleo (ANP). REFERENCES
1.
C.T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992). J. S. Beck, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt moder, C. T. W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, Y. B. Higgins and I. L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. A. Sayari. Stud. Surf. Sci. Catal., 102 (1996) 1. 4. A. S. Araujo and M. Jaroniec, J. Colloid. Interf. Sci., 218 (1999) 462. 5. X. S. Zhao, G. Q. Lu and G. J. Millar, Ind. Chem. Res., 35 (1996) 2075. 6. V. N. Romannikov, V. B. Fenelonov, B. A. Paukshtis, A. Y. Derevyankin, V. I. and Zaikovskii, Microporous Mesoporous Mat., 21 (1998) 411. W. C. Li, Y. C. Chih and N. K. Na, Appl. Catal. A: Gen., 178 (1998) 1. 8. A. S. Araujo, V. J. Femandes Jr. and S. A. Verissimo, J. Therm. Anal. Calorim., 59 (2000) 1. A. S. Araujo, M. J. B. Souza, V. J. Fernandes Jr. and J. C. Diniz, React. Kinet. Catal. Lett., 66 (1999) 141. 10. A. S. Araujo, T. B. Domingos, M. J. B. Souza and A. O. S. Silva, React. Kinet. Catal. Lett., 73 (2001) 283. .
.
.
.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
537
Effects o f Si/A1 Ratio and Pore Size on Cracking Reaction over M e s o p o r o u s MCM-41 Wen-Hua Chen a, Qi Zhao a, Hong-Ping Lina, Chung-Yuan Mou b, and Shang-Bin Liu a'* a Institute of Atomic and Molecular Sciences, Academia Sinica, E O. Box 23-166, Taipei, Taiwan 106, R.O.C. b Department of Chemistry and Center of Condensed Matter Research, National Taiwan University, Taipei, Taiwan 106, R.O.C.
The hydrocracking abilities of mesoporous MCM-41 materials were studied using 1,3,5-triisopropylbenzene (1,3,5-TiPB) cracking as test reaction. Various MCM-41 samples with varied Si/A1 ratios (15 to ~ ) and pore sizes (1.57 to 3.04 nm), synthesized by the 'delayed neutralization' method, were examined. It is concluded that 1,3,5-TiPB cracking reaction over A1-MCM-41 is diffusion controlled and coking is responsible for catalyst deactivation. The roles of A1 content and pore size on the catalytic features of the samples were evaluated by the conversion of 1,3,5-TiPB, coke content and deactivation parameters.
1. INTRODUCTION The mesoporous MCM-41 materials, which consist of hexagonal arrays of uniform channels with tunable poresizes (1.5 - 20.0 nm), possess prominent properties, such as high surface area (~ 1000 m2/g), hydrocarbon sorption capacities (> 0.7 ml/g), and thermal and hydrothermal stability render many potential applications. For examples, as adsorbents during sorption/separation processes, as supports for electronic/optical devices, or as catalysts to catalyze large organic molecules whose molecular size are greater than typical pore size (ca. 7 A) of the microporous zeolites [1-4]. It is well known that the activity of a catalyst depends mainly on its acidity and mass-transport limitations. The former is normally manipulated by the concentration and distribution of A1 species contained in the catalyst, while the latter is controlled by steric constrains imposed on the structural porosity of the catalyst. In this context, A1-MCM-41 materials, being less acidic compared to most microporous zeolites and possess highly ordered mesoporous channels, are most suitable as catalysts for catalytic cracking of large molecules during which only weak acidity is required [5-8]. Nevertheless, catalyst deactivation due to coking remains as the major problem need to be resolved. Thus, in view of promoting the catalytic performance of A1-MCM-4 l, fabrication of catalysts with desirable catalytic activity while in the same time resistant to coking is an interesting task. The objective of this study is to investigate the effects of A1 content and pore size on the catalytic performances of A1-MCM-41 during hydrocracking reaction.
538 2. EXPERIMENTALS
2.1. Materials The powdered, particulate MCM-41 molecular sieves with varied Si/A1 ratios (15 -00 ) and pore diameters (1.57-3.04 nm) were synthesized by the so-called "delayed neutralization" procedure. Their structural features were confirmed by powder X-ray diffraction (XRD) and by scanning/transmission electron microscopy. The average pore size and surface area of the sample were shown in Table 1.
2.2. Cracking Reaction 1,3,5-triisopropylbenzene cracking was used as test reaction throughout this study. The reagent (1,3,5-TiPB; A.R. grade, ACROS) was used with further purification by molecular sieve 4A. All reactions were conducted in a continuous flow, fixed-bed flow reactor under the standard conditions, namely T r = 573 K; WHSV = 15.25 h-~; pressure = 1 atm; carrier gas: N2; N2/EB = 2.0 mol/mol, and time-on-stream (TOS) = 0-6 h. Palletized and pressed MCM-41 sample (10-20 mesh; ca. 1 g) was mixed with quartz (ca. 20-30 mesh) and packed into the reactor. 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 fl). All products were identified using the internal standard method. The carbonaceous residues retained in the fouled samples were determined by thermogravimetric analysis measurement (TGA; Netzsch TG209). Typically, ca. 10 mg of catalyst was heated from 298 to 1173 K at a rate 10 K/min under dried air. The coke content was determined from the weight loss between 573-973 K.
3. RESULTS AND DISCUSSION The catalytic activities of various A1-MCM-41 sasmples were assessed by the conversion of 1,3,5-TiPB, which was catalyzed to produce mainly mono- and di-substituted isopropylbenzenes. In general, the yields of major products were found to obey the order: 1,3-DiPB > 1,4-DiPB > cumene > 1,2-DiPB > propene >> benzene. Only trace amount of benzene yield was observed.
3.1. Effect of catalyst Si/Al ratio Although the structures of siliceous MCM-41 materials are normally more stable than aluminosilicate MCM-41, they lack ion-exchange capability due to electrically neutral framework charge. As the result, siliceous MCM-41 materials are expected to exhibit nearly null catalytic activity. On the other hand, isomorphous substitution of the framework Si by A1 would results more negatively charged framework, which in turn render the formation of acid sites requisite for catalytic reactions. To investigate the effect of A1 content on catalytic performance during hydrocarbon cracking, AI-MCM-41 samples of similar pore size (ca. 2.6 nm) but with varied Si/A1 ratios were prepared. The catalytic activities of various samples during cracking reactions are then evaluated in terms of conversion of 1,3,5-TiPB, as shown in Fig. 1.
539
Table 1. Characteristics and catalytic properties of the MCM-41 Samples. Fouled catalysts
Fresh catalysts Samples
Si/A1
Pore size Pore volume Surface area Coke (rim) a (ml/g) b (mZ/g)b contentc
Deactivation parameters
d
Xo
tc
a
Xo + t~
M15
15
2.62
0.98
1015
4.5
29.2
38.3
0.81
67.5
M20
20
2.68
0.94
927
4.1
23.1
33.0
0.51
56.1
M37
37
2.54
1.15
1064
4.3
22.1
25.6
0.47
53.3
M46
46
2.64
1.06
1032
4.6
24.3
21.4
0.76
45.7
M60
60
2.58
1.02
1135
4.3
22.6
20.6
1 . 0 4 43.2
M120
120
2.61
0.98
1093
3.7
17.4
20.4
0.79
37.8
M177
177
2.56
1.05
983
3.1
7.8
29.3
0.77
37.1
M370
370
2.58
0.98
1027
2.6
7.7
27.8
0.87
35.5
SM
~
2.66
0.94
1074
MCM-C10
37
1.57
0.81
1191
9.6
12.9
80.7
0.75
93.6
MCM-CI2
37
1.80
0.96
1291
4.4
40.0
50.3
0.50
90.3
MCM-C14
37
2.18
0.96
1150
4.6
35.5
42.1
0.45
77.6
MCM-CI6
37
2.54
1.15
1064
4.3
22.8
30.5
0.53
53.3
MCM-C18
37
3.04
1.21
1028
3.2
17.1
0.58
28.5
11.4
aData obtained by the BJH method based on the desorptlon curve of N2 adsorption/desorption isotherms (77 K). bDetermined by N2 isotherms at P/Po = 0.96. CObtained from the fouled catalysts by TGA, in unit of wt%. dResults obtained from data fitting of Eq. 1. eRepresent initial conversion (TOS = 0 h); in unit of wt%. Except for siliceous MCM-41 (Si/A1 - ~o ) which revealed the expected null activity, the 1,3,5-TiPB conversion curves obtained from various A1-MCM-41 samples were found to decay exponentially with time-on-stream (TOS) and can be fitted by the following equation: Y , = Y o + k e ~'
(1)
where Xt represents the conversion at a given time (TOS) t, Xo and k are constants, and the exponent et is a parameter accounts for deactivation rate. The results of the fittings are shown in Fig. 1 as solid curves and related deactivation parameters derived are depicted in Tablel. Coking is the prominent reason accounts for the deactivation of the catalysts and appears to be more pronounced during the initial stage of the reaction. Overall, the catalytic activities of
540
catalysts became more stable as TOS exceeds ca. 4 h. That the coke content (obtained by TGA at TOS = 6 h) decreases with increasing AI content of the AI-MCM-41 samples indicates that carbonaceous residues are likely deposited on the acid sites of the catalysts.
A
|
70
'{
6o
,~0
50
uE,
a.m
II I
i
9 9
I 4)'
M37 M60
M17'7
~
.,0
/k
M37O |
0
0
70
|
M46 M120
i
A ~ 70~ . . . . . . .
I
A
I I
e-
o
60
> C
o
40
0
m 50
C
30
C
I11
20 ""
~ 40
lO
I/ =
0
0
=
B]
_
2
,
6
T i m e - o n - s t r e a m (h)
Fig. 1. Variations of 1,3,5-TiPB conversion against time-on-stream during cracking reaction over various MCM-41 and A1-MCM-41 samples.
0
'
!
100
"''
|-
200
'
|
300
'
t
400
SilAI ratio
Fig. 2. Correlation of 1,3,5-TiPB initial conversion with Si/AI ratio of AI-MCM-41 samples.
Furthermore, the initial conversion of 1,3,5-TiPB (i.e., Xo + k; at TOS = 0 h) deduced from Eq. 1 was also found to decrease exponentially with the Si/AI ratio of the AI-MCM-41, as shown in Fig. 2. Eventually, the initial conversions reach a plateau value of ca. 37 wt% for samples with Si/AI >__120. However, we note that this effect should depend on the contact time or WHSV applied. Presumably, an increase in WHSV will shorten the contact time and hence result in a lower 1,3,5-TiPB conversion. It is hypothesized that, upon initial reaction, reactants 1,3,5-TiPB are immediately catalyzed to form carbonaceous residues, which tend to deposit on the acid sites. Progressive formation of coke on the acid sites therefore resulted in an overall reduction of catalyst acidity. As the result, the conversion of 1,3,5-TiPB maintained at a constant level for TOS > 4 h. In this context, this observation is thus in line with the notion that hydrocarbon cracking reactions over AI-MCM-41 catalysts, which is diffusion controlled, require only weak acidity.
3.2. Effect of catalyst pore size
To explore the effect of pore size on the catalytic performance, 1,3,5-TiPB cracking reaction were carried out on different AI-MCM-41 samples with varied pore diameters (namely, 1.57, 2.18, 2.80, 2.54 and 3.04 nm) but having the same AI content (Si/AI = 37). The resultant 1,3,5-TiPB conversions against TOS are shown in Fig. 3. Again, the solid curves in Fig. 3 represent fittings of deactivation curves for various samples by Eq. 1. The results of the fittings are also summarized in Table 1 together with the coke content. The variations of the
541 extrapolated 1,3,5-TiPB initial conversion with catalyst pore size are shown in Fig. 4. 100
,
,
~
"~ C
~k(~
75
~..1 \ --
~
0 ,m
E > = 0 o I-
~
' ~ k \\
[] 50
.
~
8O
-~
i
470
~
"-2a--_,-o~
om
o z___o
[ ] x7~- - ~ -
o
o m oc. .
0 m
c
"o m (I. I---
460
,.:
450
al
40
a. I-
20
,i
Time on stream
960
e-
- -~.
c
.o m
> C
(~
o
o
480 E
.-..
,_-h,[] '.'v. \
25
100
Q.
'-- " - - ' - - ' - -
\ x x
490
.=
SilAI=37 r-I MCM-CIO O MCM-C12 f MCM-C14 V MCM-C16 0 MCM-C18
. (h)
Fig. 3. Variations of 1,3,5-TiPB conversion against time-on-stream for various AI-MCM-41 samples with varied pore size durin~ crackin~ reaction.
1.5
2.0
2.5
3.0
Pore size ( n m )
Fig. 4. Correlations of 1,3,5-TiPB initial conversions and desorption temperature with pore size of AI-MCM-41 samples.
The consistent decrease in initial conversion of 1,3,5-TiPB, coke content, and deactivation rate (~) with pore size indicate that, at a given AI content, AI-MCM-41 with smaller pore diameters are more favorable in terms of their catalytic activity. Nevertheless, they also appear to deactivate more easily. The aforeobserved phenomena seem to associate with the adsorptive properties of samples. To verify this point, additional experiments were performed on samples subjected to special treatments. The samples were prepared by first adsorbing saturated amount of 1,3,5-TiPB by vapor transfer method, then placed under ambient conditions overnight to ensure homogeneous adsorbate distribution followed by TGA measurements as desorption tests. Each sample (ca. 10 mg) was heated to 1173 K at 5 K/min under dried N2, accordingly the temperature at which 1,3,5-TiPB completely desorbed can be determined. As shown in Fig. 4, the final desorption temperature was found to increase with decreasing pore size of AI-MCM-41, which is in line with the trend observed for initial conversion. The results therefore indicate that, for AI-MCM-41 with the same AI content, the increase in adsorption capacity of 1,3,5-TiPB with decreasing pore size therefore corresponds to enhanced catalytic activity observed during the initial stage of the reaction. It may be envisaged that the smaller the pore size, the greater the adsorption strength for the reactant, and consequently the more hindrance imposed on the reactant/product molecular diffusion, this promoting the catalytic activity during the cracking reaction. Finally, the notable differences observed in the overall catalytic features of the particular AI-MCM-41 sample with the smallest pore diameter (1.57 nm) deserve further discussion. Considering that the kinetic diameter of the 1,3,5-TiPB reactant is ca. 0.85 nm and the progressive deposition of carbonaceous residues during reaction, steric hindrance and possibly pore blocking are likely to occur in the channels of the mesoporous AI-MCM-41. Presumably, these effects should also be inter-connected and should be more pronounced for samples with
542 smaller pore size. As the result, the A1-MCM-41 sample with 1.57 nm pore size would be vulnerable to coking due to diffusion limitations, thus the observed high deactivation rate (ix) and total coke content (9.6 wt. %) compared to the other samples (Table 1). Moreover, except for sample with 1.57 nm pore size, the fact that the coke content and deactivation rate are nearly independent of pore size of A1-MCM-41 indicating that deactivation due to coking depends mostly on the sample A1 content.
4. CONCLUSIONS The effects of Si/A1 ratio and pore size on catalytic performances of mesoporous aluminosilicate MCM-41 molecular sieves during 1,3,5-TiPB cracking reaction have been investigated. The activity of the catalyst was found to decrease exponentially with time-on-stream regardless of the AI content and pore size possessed by the sample. Coking was found responsible for catalyst deactivation, the coke content and deactivation rate are found to depend on A1 content rather than pore size of the samples, except for the extreme case of small mesopores. The initial conversion of 1,3,5-TiPB was found to decay exponentially with Si/A1 ratio of the samples with similar pore sizes, whereas for samples with the same Si/A1 ratio, it decreases gradually with pore size. It is concluded that hydrocracking reaction over A1-MCM-41 is diffusion controlled and requires only weak acidity.
5. ACKNOWLEDGMENTS The authors thank Profs. Soofin Cheng and Ben-Zu Wan for helpful discussions. The supports of this work by the Chinese Petroleum Corporation (88-S-067) and by the Nation Science Council, R. O. C. (NSC89-2113-M-001-033 to SBL) are gratefully acknowledged.
REFERENCES
1. 2. 3. 4. 5.
M.E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature 359 (1992) 710. A. Coma, Chem. Rev. 97 (1997) 2373. P. Selvam, S. K. Bhatia and C. G. Sonwane, Ind. Eng. Chem. Res. 40 (2001) 3237. J.Y. Ying, C. P. Mehnert and M. S. Wong, Angew. Chem. Int. Ed. Engl. 38 (1999) 56. (a) K. M. Reddy and C. Song, Catal. Lett. 36 (1996) 103. (b) K. M. Reddy and C. Song, Catal. Today 31 (1996) 137. 6. X.Y. Chen, L. M. Hung, G. Z. Ding and Q. Z. Li, Catal. Lett. 44 (1997) 123. 7. K. Roos, A. Liepold, W. Roschetilowski, R. Schmidt, A. Karlsson amd M. Stocker, Stud. Surf. Sci. Catal. 84 (1994) 389. 8. A. Ghanbari-Siahkali, A. Philippou, J. Dwyer and M. W. Anderson, Appl. Catal. A 192 2000) 57.
5tuclles in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) Crown Copyright 9 2002 Published by Elsevier Science B.V. All rights reserved.
543
Hydrogenation and mild hydrocracking o f synthetic crude distillate by Pt-supported mesoporous material catalysts Hong Yang a, Craig Fairbridge a, Zbigniew Ringa, Randall Hawkinsa and Josephine M. Hill b aNational Centre for Upgrading Technology, Devon, AB, Canada, T9G 1A8 bDepartment of Chemistry, University of Calgary, Calgary, Alberta, Canada, T2N 1N4
The hydrogenation and mild hydrocracking activities of a Pt-supported mesoporous molecular sieve catalyst and a Pt-supported mordenite catalyst were studied in a bench scale fixed-bed reactor. The reaction temperatures ranged from 240 to 360~ with a total pressure of 10.3 MPa, and LHSV of 1.0. The feed was a hydrotreated middle distillate derived from Canadian oil sands. Feed and total liquid products were characterized by ASTM standard methods for physical properties, and by the GC-MS method for chemical compositions. The detailed chemical compositional results using GC-MS, show that supporting Pt on a mesoporous material (Pt/MM-alumina) results in a catalyst that is able to hydrocrack large molecules such as 3-ring naphthenes. At the same reaction conditions, this catalyst also has a higher hydrogenation activity and better mild hydrocracking selectivity than a mordenitesupported Pt catalyst (Pt/Mor-alumina). At a similar conversion level, the Pt/MM-alumina catalyst gave a superior diesel yield and a lower naphtha yield than the Pt/Mor-alumina catalyst, likely because of its larger pore structure and lower acidity.
1. INTRODUCTION Middle distillates derived from heavy oils and oil sands contain high concentrations of aromatics and naphthenes, and low concentrations of paraffins. The middle distillates often have lower cetane number compared to those derived from conventional petroleum. Deep aromatic saturation and ring opening of naphthenic compounds by hydrogenation and mild hydrocracking reactions would improve the combustion properties of the fuels. Existing hydrotreating catalysts do not perform these functions. Fuel quality improvements, therefore, depend heavily on the development of new catalysts. Furthermore, replacing the conventional catalysts in the existing hydroprocessing units with new functionality catalysts, is the most economic approach to improve product quality and avoid large capital investment. The synthesis of mesoporous molecular sieves with tunable pore structure and surface functionality has attracted significant interest [ 1-5]. Mesostructured materials are beginning to find application in the area of adsorption, catalysis and environmental protection [6-10]. In the present work, we tested a mesoporous molecular sieve-supported Pt catalyst for the
544 hydrogenation and mild hydrocracking of a middle distillate feed, and compared the activity to that of a mordenite-supported Pt catalyst. 2. EXPERIMENTAL
2.1 Catalyst preparation Pt-supported mesoporous aluminosilicate catalyst was prepared according to a patented procedure [10] using aluminum sulfate hydrate and sodium silicate solution (27% SiO2) as alumina and silica sources, and cetyltrimethylammonium bromide (CTABr) as template. (All chemicals were supplied by Aldrich.) In a typical preparation, a solution made of 5.2 g aluminum sulfate, 208 g water and 35.8 g cetyltrimethylammonium bromide, was added to a second solution containing 56.6 g sodium silicate, 80 g water with 2.4 g sulfuric acid. The mixture was stirred for 30 min and placed in a sealed glass bottle at room temperature ovemight. A solid product was recovered by filtration, washed with deionized water, air-dried at room temperature, and oven-dried at 110~ The synthesized solid was calcined at 540~ under NE/air for 5 hours to remove the template. Afterwards, the product was reacted at 70~ with ammonium nitrate solution for 4 hours. The ion-exchanged product was washed and centrifuged to remove all traces of ammonium ion before being heated at 120~ for 1 hour and 505~ for 4 hours, under air. This product was then impregnated with platinum using an aqueous solution of Pt(NH3)4C12 (54.35wt% Pt, Alfa Aesar, Ward Hill, MA) to produce a Ptloaded mesoporous material (Pt/MM) catalyst. Pt-supported mordenite catalyst was prepared from CBV21A (Zeolyst International Valley Forge, PA) by ion exchange with Pt(NH3)4C12 in aqueous solution. The mixture was reacted ovemight at room temperature to establish equilibrium and complete the ion-exchange process. The solid was washed and centrifuged free of chloride before being air-dried at room temperature. Samples were mixed with alumina to disperse the active metal/mesoporous material into a matrix suitable for catalyst testing on a complex petroleum feed. Alumina also acted as a binder to fabric catalyst extrudates of good mechanic strength. Catalyst extrudates (0.8 mm diameter) were made by mixing the Pt/MM and Pt/Mor with 80wt% pseudo-b6ehmite (Catapal B, Condea Vista, Houston, TX.). The extrudates were calcined at 400~ for 4 hours before use. The final catalysts were coded Pt/MM-alumina and Pt/Mor-alumina.
2.2 Feedstock preparation The feedstock used for hydrogenation and mild hydrocracking experiments was prepared by fractionation and hydrotreatment of a light gas oil from an ebullated bed hydrocracker (Syncrude Canada Ltd, Fort McMurray). The lighter portion of the gas oil was first distilled off using a single-column continuous distillation unit to give an initial boiling point of approximately 260~ The strategy was to fractionate the material to obtain a somewhat heavier fraction that could be cracked down into the diesel range by mild hydrocracking. Following distillation, the heavier fraction was severely hydrotreated to reduce the sulfur and nitrogen contents to less than 20 ppm in order to protect the noble metal catalysts.
545
2.3 Catalyst activity testing procedure Evaluation of the hydrogenation and mild hydrocracking activities of the Pt/MMalumina and Pt/Mor-alumina catalysts was carried out using an automated microreactor system. The fixed-bed stainless steel tubular reactor (30.5 x 0.635 cm) was operated in the continuous up-flow mode and heated by a three-zone electric furnace. The reactor was equipped with an axial thermowell housing one moveable thermocouple, and the temperature gradient along the catalyst bed was approximately 2~ The reaction conditions were, therefore, considered isothermal. A 6 ml volume of catalyst extrudes 3 to 4 mm in length were loaded into the reactor. Pre-heating and post-heating zones were filled with quartz particles of 20-48 mesh. The catalysts were reduced in situ in a hydrogen flow (350 ml/min, 0.69 MPa) at 400~ for 14 hours. After reduction, the reactor was cooled to 200~ and feed and hydrogen were introduced to the reaction system through a mixer coil and a pre-heater that was set at 150~ The reactions were conducted at 10.3 MPa pressure, 1.0 h 1 liquid space velocity, and with a hydrogen gas rate of 600 NL/L feed. The temperature was varied in the range 240~ to 360~ Once the density of the liquid products became stable (usually after 24 hours for each change of temperature), the liquid product was collected over a set mass balance time period.
2.4 Analytical methods Chemical compositions of feed and total liquid products were determined by lowresolution mass spectrometry using a modified Robinson method [11]. The method did not require prior separation of samples into saturate and aromatic fractions. The instrument used was a Hewlett Packard GC-MS equipped with an HP 5972 mass spectrometer, HP 7673GC/SFC injector and HP 5890 gas chromatograph, with Helium as the carrier gas. The column used for the analysis was a 30m x 0.250mm x 0.25~tm HP 5MS. The software used for calculating the chemical compositions of the total liquid products was supplied by PCMSPEC [12]. The boiling range of feed and total liquid products were given by ASTM method D2887. Density (g/mL, 15.6~ aniline point (~ and kinematic viscosity (cSt, mm2/s, 40~ were also analyzed by standard ASTM methods. Ignition quality of the total liquid product was estimated by two different methods: 1) a combustion test method using the Ignition Quality Tester (IQT TM) [13]; and 2) a correlation method ASTM D976-80 ~ where the cetane index (CO was calculated using density d (15.6~ and T50, the 50% volume recovery temperature of ASTM D86 distillation method, by the equation CI = 454.74 -1641.416 *d +774.74 *d2 -0.554 *T50 + 97.803 *(Log T50) 2 T50 was obtained by converting SD50, the 50% weight recovery temperature of the ASTM D2887 simulated distillation method, using the following equation: T50 = 0.77601 (SD50) 1"0395 [14]. The equipment used for measuring the surface areas and pore size distributions of the catalysts was a Micromeritics ASAP 2010C. The BET N2 adsorption, single point, and Horvath-Kawazoe methods were used to calculate the total surface area, pore volume and median pore diameter, respectively. For the ammonia temperature programmed desorption (TPD), samples (0.2 g) were placed in a quartz flow cell and attached to a gas handling and vacuum system (Advance Scientific Designs Inc.). The samples were heated to 500~ over 1 hour in flowing helium (60 ml/min). After 2 hours at 500~ the samples were cooled to 100~ and exposed to a stream of ammonia in helium (9.79 % NH3, Praxair) for 1 h. The samples were then purged with helium for 20 min before beginning. The samples were heated
546 at 10~ to 600~ in 25 ml/min He (STP), and held at 600~ for up to 20 minutes. A fraction of the gases exiting the sample cell was directed to a quadrapole mass spectrometer (UTI 100C) through a leak valve. The pressure in the mass spectrometer was maintained at 2.0 x 10-4 Pa and calibration of the mass 17 signal was performed at the beginning of each experiment using a standard gas mixture of NH3 in He. The contents ofPt, AI and Si in Pt/MM and Pt/Mor materials were measured by inductively coupled plasma-mass spectrometry (ICPMS) at the Alberta Research Council. The catalyst samples were first acid digested under microwave heating using a QWAVE-1000 microwave sample preparation system (Questron, Mercerville, NJ, USA), equipped with temperature and pressure regulation. The ICP-MS system used for analysis was a Perkin-Elmer Elan 5000 ICP quadrupole mass spectrometer (Thornhill, ON, Canada), equipped with a GemTip cross-flow nebulizer, Ryton spray chamber, plasma torch with a quartz injector, a Gilson four-channel peristaltic pump (Model Minpuls III) and a Gilson 212B auto-sampler. A detailed analytical procedure can be found elsewhere [ 15]. 3. RESULTS AND DISCUSSION 3.1 Characteristics of the catalyst materials Catalyst pore structure, surface area and acidity are factors that affect the performance of a catalyst. Table 1 summarizes the BET surface area, pore volume and pore diameter of the as-synthesized material, Pt/MM and Pt/Mor, as well as the prepared catalyst extrudates. The Pt contentand Si/A1 ratio of Pt/MM and Pt/Mor catalysts are also indicated in Table 1. The Ptloaded mesoporous molecular sieve (Pt/MM) had larger median aPOre diameter (26A), substantially higher surface area (978 mE/g) and pore volume (0.88cm/g) than P ~ o r , which had a pore diameter of 5.4A, surface area of 387 m2/g and pore volume of 0.24 cm3/g.
Table 1. Characterization of Pt-loaded mesoporous material and mordenite catalysts Catalyst samples
SaET
(m2 g-l) Mesorpore materials (MM) 1054 Ammonium ion-exchanged MM 1001 P~M, 978 P~M-alumina 479 CBV 21 A (Mor) 346 Pt/Mor 387 Pt/Mor-alumina 315
Pore volume Median pore (cm3 g-l) 1.185 0.950 0.881 0.467 0.196 0.238 0.328
diameter (A) 32.1 26.8 26.0 27.9 5.2 5.4 32.1
Pt
Si/A1
(wt%)
(mass)
0.97
16.2
0.82
10.7
Results of the acidity measurements (NH3-TPD) of ammonium ion-exchanged mesoporous material and CBV 21A, the original mordenite, are presented in Figure 1. Three maxima were observed on the curve of CBV 21A. Two peaks at low temperature around 160 and 200~ correspond to the presence of two different weak acid sites. The third peak at high
547 temperature, around 560~ is due to strong acid sites. The quantity and the strength of the acid sites on the mesoporous material are significantly lower than those of mordenite as observed in Figure 1. In fact, the high temperature desorption peak disappeared on the mesoporous material. The amount of ammonia desorbed at lower temperatures is also much less compared to mordenite. The higher acidity of mordenite could be due to its higher A1 content (Table 1). While the low temperature peaks are commonly assigned to weak Lewis and Br6nsted acid sites, the assignment of the high temperature peak (>550~ is still under discussion [1619]. In a study of coupling the NH3-stepwise temperature programmed desorption and FT-IR, Zhang et al. concluded that both Lewis and Br6nsted acid sites were responsible for the desorption that occurred around 180 to 250~ and that only Brfnsted acid sites caused the desorption at higher temperatures [ 16]. On the other hand, Kosslivk eta/. [19] suggested that peaks at temperatures above 550~ are mainly caused by ammonia desorbed from strong Lewis acid sites. Clearly, further studies are needed to solve these contradictions, which are beyond the scope of this study. 3.5
80.0 -I..............................................................................................
3.o
7o.o 6o.o
"~
i[
50.0 1 1.5
~ 40.0
~.0
~-
0.5
0.0 .
~0.0 )
. . . . . . . . , 100 200 300 400 500 600 isotherm . Temperature (~
Figurel. Ammonia TPD curves: a)mordenite CBV21 b) ammonium-ion exchanged mesoporous material.
3.2 Hydrogenation
and mild hydrocracking
0.0
f
i ....... ,.
0.0
10.0 20.0 30.0 40.0 ,,,~4~ .o C+ Conversion%
. . . . 50.0
Figure 2. Naphtha and diesel yields as function of the conversion of 343~ (o) Pt/MM-alumina, naphtha; (ra) Pt/MMalumina, diesel; (o) Pt/Mor-alumina, naphtha; (+)Pt/Mor-alumina, diesel.
activities
Hydrogenation and mild hydrocracking experiments were carried out between 300 and 360~ for Pt/MM-alumina and between 240 and 300~ for Pt/Mor-alumina. A lower temperature was used for the Pt/Mor-alumina because of its higher cracking activity at elevated temperatures. Table 2 presents the yields of naphtha, diesel and 343~ fractions, and physical properties of the feedstock and the products at various temperatures.. Figure 2 compares the diesel and naphtha yields versus the conversion of 343~ fraction for both catalysts. These results show that Pt/Mor-alumina had significantly higher undesirable cracking activity than Pt/MM-alumina. At the same temperature (300~ the conversion of
548 the 343~ fraction was 50.1% with a naphtha yield of 46.4wt% for Pt/Mor-alumina compared to 11.8% conversion of 343~ and 6.3wt% naphtha yield for Pt/MM-alumina. At the same conversion level, Pt/MM-alumina gave significantly higher diesel yield and lower naphtha yield. By interpolation and extrapolation of the conversion-yield curves, we calculated that at 30% 343~ conversion, Pt/MM-alumina would produce 65.4wt% diesel and 13.0wt% naphtha, whereas Pt/Mor-alumina would produce 53.1wt% diesel with 25.3wt% naphtha. In a mild hydrocracking process, an acceptable level of naphtha formation might be less than 20% with a conversion of 343~ of around 30% [19-20]. Therefore, Pt/MM-alumina is a better catalyst to convert heavy oil feedstock into the diesel fraction with a mild hydrocracking process. The selectivity of the mesoporous molecular sieve catalyst towards higher diesel yields relates to its weaker acidity and mesoporous structure, as compared to the strong acidity and the microporous structure of Pt/Mor-alumina catalyst. Table 2 showed that, over both catalysts, density and viscosity of the total liquid products decreased with reaction temperature. Since these properties are often correlated with paraffinic and aromatic contents in fuel, a decrease means a reduction in aromatic content; hence hydrogenation of aromatics to naphthenes and the formation of paraffins by ring opening was apparent under the conditions used in this study. Aniline point is another important parameter in characterization of petroleum fractions. It is a measure of aromatic content and molecular weight. In this study, aniline point first increased with temperature, reached a maximum, then decreased with temperature. These results indicate that product quality is improved by hydrogenation of aromatics at lower temperatures; however, as the temperature increases, the quality is deteriorated due to the formation of lighter molecules by overcracking. Table 2. Physical properties of feed and total liquid products (hydrogen pressure 10.3 MPa, LHSV 1.0, hydrogen flow 600NL/L feed)
wt% of fractions IBP-177 (naphtha) 177-343 (diesel) 343+ Conversion of 343~ Density (g mLl) H/C ratio Viscocity 40~ (cSt, mm2 s-1) Aniline point (~ Cetane IQT Cetane D613 CI(D976-80)
Feed 3.3 65.8 30.8
Reactiontemperatures (~ Pt/MM-alumina Pt/Mor-alumina 240 260 280 300 300 320 340 360 5.2 7.0 16.9 46.4 6.3 9.2 16.6 32.0 64.9 64.0 58.5 38.2 66.5 66.4 64.4 59.0 29.9 29.0 24.6 15.4 27.2 24.4 19.0 9.0 2.9 6.0 20.2 50.1 11.8 20.8 38.5 70.9
0.8628 0.8489 0.8428 0.8277 0.7975 1.85 1.94 1.92 1.94 2.01 5.49 4.66 4.06 2.89 1.62 75.3 51.4 50.4 50.3
0.8586 0.8541 0.8364 0.7920 1.89 1.89 1.95 1.81 5.10 4.73 3.12 1.33
81.6 52.3
81.4 52.3
79.1 50.7
74.4
76.8 49.1
77.8 49.1
76.7 46.7
68.3 38.5
53.6
54.8
57.4
54.4
51.2
52.4
56.6
46.5
549 Table 3. Chemical compositions (mass%) of feed and total liquid products (Hydrogen pressure 10.3 Mpa, LHSV 1.0, Hydrogen flow 600NL/Lfeed) Reaction temperatures (~ Pt/Mor-alumina Pt/MM-alumina Feed 300 320 340 360 240 260 280 300 75.0 79.7 86.6 89.5 Saturates 74.6 94.0 94.2 95.4 98.4 ll.4 11.8 1 3 . 2 14.0 Paraffins ll.0 1 2 . 3 13.0 14.7 19.3 23.7 25.5 28.6 31.8 Monocycloparaffins 23.1 30.6 31.4 34.5 41.1 26.3 27.9 29.3 27.5 Dicycloparaffins 26.3 34.3 33.5 33.5 30.9 13.9 14.6 15.7 16.3 Tricycloparaffins 14.3 16.4 16.1 12.3 6.9 25.0 21.0 14.9 12.3 Aromatics 25.4 6.1 5.9 4.7 1.7 14.6 10.8 4.9 2.8 Monoaromatics 16.4 0.6 0.3 0.3 0.4 4.4 2.6 0.2 0.0 Benzenes 5.4 0.0 0.0 0.0 0.0 5.2 4.0 2.1 1.0 Naphthenebenzenes 6.1 0.0 0.0 0.0 0.0 5.0 4.2 2.8 2.0 Dinaphthenebenzenes 5.0 0.6 0.3 0.3 0.4 5.4 5.1 4.4 3.7 Diaromatics 4.4 2.2 2.3 1.8 0.3 2.8 2.9 3.4 4.1 Triaromatic+ 3.0 2.6 2.5 2.0 0.2 1.4 17.5 41.4 51.4 Aromatics conversion % 76.1 76.9 81.6 93.3
Hydrocarbon groups
Four saturated hydrocarbon groups (paraffins [normal plus isoparaffins], monocycloparaffins, dicycloparaffins and tricycloparaffins) and five aromatic hydrocarbons (benzenes, naphthenebenzenes, dinaphthenebenzenes, diaromatics and triaromatics+) can be effectively identified by the GC-MS method used in this work. Table 3 presents detailed chemical compositional analyses of the feedstock and the liquid products at different reaction temperatures. The results indicate that Pt/MM-alumina has a higher hydrogenation activity than Pt/Mor-alumina. At the same reaction temperature (300~ the aromatics conversion was 76.1% for Pt/MM-alumina compared to 51.4% for Pt/Mor-alumina. The aromatic conversion increases with temperature and reaches 93.3% at 360~ for Pt/MM-alumina. Pt/MM-alumina also has a much higher ability to convert diaromatics and triaromatics+ compounds than Pt/Mor-alumina. The contents of diaromatics and triaromatic+ were reduced to near zero at 360~ over Pt/MM-alumina catalyst, whereas these values were almost constant over the Pt/Mor-alumina catalyst. Since Pt/MM-alumina has substantially higher surface area than Pt/Mor-alumina, for the same level of metal loading, we may expect a better metal dispersion in the former catalyst that would promote the hydrogenation reaction. Because of its high surface area, we could increase the metal loading on Pt/MM-alumina without causing agglomeration of the Pt atoms, making it, therefore, more active at lower temperatures. It is also worth to note that the density of Pt/MM-alumina is lower than Pt/Mor-alumina, due to its higher pore volume and, therefore, for the same space velocity, a smaller amount of Pt/MMalumina (4.09 g) than that of Pt/Mor-alumina (5.01 g) was loaded into the reactor. This advantage makes the mesoporous materials more attractive for industrial scale-up since less amount of metal and catalyst support are needed.
550 As for the saturated hydrocarbon groups, the results in Figure 3a (Pt/MM-alumina) and Figure 3b (Pt/Mor-alumina) show that paraffins and monocycloparaffins contents increase with reaction temperature for both catalysts. Monocycloparaffins can be produced by a number of different reactions pathways such as hydrogenation of monoaromatics or opening one ring of a dicycloparaffin. For the formation of two ring naphthenes, maximums were observed for both Pt/MM-alumina and Pt/Mor-alumina catalyst. These maximums indicate that both catalysts are able to convert molecules with sizes comparable to two-ring naphthenes into one-ring naphthenes. Over Pt/MM-alumina catalyst, the formation of the three-ring naphthenes first increased with temperature and reached a maximum at about 300 to 320~ At temperatures over 320~ the three-ring naphthenes decreased with temperature. However, three-ring naphthenes continuously increased with temperature over the Pt~or-alumina catalyst. The pore size of the Pt/MM is 26A, which is large enough for the diffusion of threering compounds [21] to the hydrogenation and hydrocracking sites located inside the mesoporous molecular sieve portion of the catalyst extrudates. The conversions of these bulky molecules are limited by the smaller pore diameter of Pt/Mor (5.4 A) that prevents the approach of bulky molecules to the active sites. 50.0 45.0 40.0 35.0 30.0 o 25.0 20.0
.........................................................................................................................................................................
...........b ................
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
(ae~
20.0
I,,,,,
15.0
"~ 10.0 5.0 0.0 280
D_____----o-------
10.0
~5.0
5.0 0.0
300
320 340 Temperature (~
360
380
,
220
240
,
r
260 280 Temperature (~
300
320
Figure 3. Distribution of saturated products over Pt/MM-alumina (a) and Pt/Mor-alumina (b): (+) paraffins; (o) monocycloparaffins; (o) dicyloparaffins; (t~)tricycloparaffins. The ignition quality of a diesel fuel depends mainly on its chemical composition. In general, normal paraffins have the highest cetane numbers and, for the same carbon number, isoparaffins have lower cetane numbers, followed by moncycloparaffins, alkylbenzenes, polycycloparaffins and polyaromatics. Results in Table 2 show that the Pt/MM-alumina catalyst produces products with a higher ignition quality as predicted by both IQT TM and ASTM D976-80. Higher saturate and lower aromatic contents in the case of Pt/MM-alumina are believed to be the major contributors to the higher quality liquid product.
551 4. CONCLUSIONS A Pt-supported mesoporous molecular sieve (Pt/MM-alumina) and a Pt/supported mordenite (Pt/Mor-alumina) were used as catalysts for hydroprocessing a middle distillate derived from Canadian oil sands. The catalytic testing results suggest that Pt/MM-alumina has more suitable pore structure and surface acidity for hydrogenation and mild hydrocracking of middle distillate, so as to create a better quality diesel fuel than Pt/Mor-alumina. Compared to Pt/Mor-alumina, Pt/MM-alumina produced higher aromatic conversion and diesel yield, with minimum formation of naphtha under the same reaction conditions. Detailed chemical compositional analyses of the feedstock and the total liquid products at several reaction temperatures showed that both catalysts were able to hydrocrack two-ring naphthenes to smaller naphthenes and paraffins. However, only Pt/MM-alumina could effectively convert three-ring naphthenes and three-ring aromatics. The liquid products obtained over Pt/MMalumina catalyst had better ignition quality as determined by Ignition Quality Tester and ASTM D976-80.
5. ACKNOWLEDGEMENTS Partial funding for this work has been provided by the Canadian Program for Energy Research and Development (PERD), the Alberta Research Council and The Alberta Energy Research Institute. The authors gratefully acknowledge Mr. Robert Garez for operating the catalyst testing unit and NCUT analytical laboratory staff for determining the feed and products properties. The authors are grateful to Mr. Ken Mitchell and Mr. David Sporleder, Shell Canada Limited, Calgary Research Center, for the IQT test. We wish to thank Dr. R.A. Kydd, Department of Chemistry, University of Calgary, for the TPD measurements. The authors also wish to thank Syncrude Canada for kindly supplying of the LC-Finer LGO. Hong Yang is thankful to the Natural Sciences and Engineering Research Council of Canada for partial financial support. 6. R E F E R E N C E S
1 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. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 2 C.K. 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, Science, 267 (1995) 865. 4 A. Tuel, Microporous Mesoporous Mater., 27 (1999) 151. 5 A. Sayari, M. Jaroniec and T.J. Pinnavaia, eds., Studies in Surface Science and Catalysis, Volume 129, Nanoporous Materials II, Elsevier, Amsterdam, 2000. 6 M.R. Apelian, T.F. Degnan, Jr., D.O. Marler and D. N. Mazzone, US Patent 5,227,353, 1993. 7 K.M. Reddy, B.L. Wei and C.S. Song, Catal. Today, 43 (1998) 261. 8 C.S. Song and K. M. Reddy, Appl. Catal. A: General, 176 (1999) 1.
552 9 M.J. Cheng, F. Kumata, T. Satio, T. Komatsu and T. Yashima, Appl. Catal. A: General, 183 (1999) 199. 10 C.J. Guo, C.W. Fairbridge and J-P. Charland, US Patent, 5,538,710, 1996. 11 C.J. Robinson, Anal. Chem. 43 (1971) 1425. 12 R.M. Teeter Software for calculation of hydrocarbon types, PCMASPEC, 1925 Cactus Court, #2, Walnut Creek, CA 94595-2505, USA., 1994. 13 L.N. Allard, G.D,Webster, T.W. Ryan, G. Baker, A. Beregszaszy, C.W. Fairbridge, A. Ecker and J. Rath, SAE Paper, 3591, 1999. 14 T.E. Daubert and R.E. Jr. Pulley, Chapter 1. Division of Refining. Technical Data Bookm Petroleum Refining, fifth edition. Washington DC: American Petroleum Institute, 1992. 15 S.L.Wu, Y.H. Zhao, X. B. Feng and A. Wittmeier, J. Anal. At. Spectrom 11(1996) 287. 16 W.M. Zhang, E.C. Burckle, P.G. Smirniotis, Microporous and Mesoporous Materials, 33(1999)173. 17 A.W. O'Donovan, C.T. O'Connor and K.R. Koch, Microporous Materials, 5 (1995) 185. 18 B.L. Meyers, T.H. Fleisch, G.J. Ray, J.T. Miller and J.B. Hall, J. Catalysis, 110 (1988) 82. 19 H. Kosslick, G. Lischke, B. Parlitz, W. Storek and R. Frichke, Appl. Catal. A, general, 184 (1999) 49. 20 E.P.Dai and C.N. Campbell, Mild hydrocracking of heavy oils with modified alumina based catalysts, P 127, Catalytic Hydroprocessing of petroleum and distillates, Eds, M.C. Oballa and S. S. Shih, Marcel Dekker, Inc. 1994. 21 E. Benazzi, L. Leite, N. Marchal-George, and H. Thouloat. 17th North American Catalysis Society Meeting, Toronto, 2001. Poster program P44.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
553
C a r b o n - c a r b o n b o n d forming reactions catalyzed by meso- and m i c r o p o r o u s silicate-quaternary a m m o n i u m composite Yoshihiro Kubota, Yusuke Nishizaki, Hisanori Ikeya, Junko Nagaya and Yoshihiro Sugi Department of Chemistry, Faculty of Engineering, Gifu University, Gifu 501-1193, Japan The Knoevenagel condensation of carbonyl compounds with active methylene compounds catalyzed by as-synthesized, ordered porous silicate-quaternary ammonium composite materials gave corresponding c~,13-unsaturated esters in high yields under very mild liquid phase conditions. The activity was as high as that of aminopropyl-functionalized porous silicates. In the case of other carbon-carbon bond forming reactions such as Claisen-Schmidt reaction and Michael reaction, the activity of the composite materials was much higher than that of aminopropyl- functionalized silicates. 1. INTRODUCTION High-silica, ordered porous materials including micro- and mesoporous materials have found great utility as catalysts and sorption materials [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. Solid base catalysis is less investigated than acid catalysis [3]. Knoevenagel condensation, which is catalyzed by a weak base catalyst, is one of carbon-carbon bond formation reactions. Traditionally, amines and other homogeneous bases are known to be effective catalysts for this reaction [4]. As for heterogeneous catalysis, there are examples of amino group-immobilized silicas (amorphous [5] or ordered [6]). Modified ion-exchange resins can catalyze this reaction [7,8], and in these cases catalytic active sites are also immobilized amino groups (In both cases, 'push-pull' type mechanisms are proposed.). Besides these types of catalysts, various solids such as mesoporous silicates in alkali ion form or alkali-impregnated mesoporous silicates [9], zeolites in alkali ion form [ 10], sepiolite [ 11], and hydrotalcites [ 12], are used as catalysts, although less mild reaction conditions are necessary in most cases. In the case of porous silicates, structure-directing agent (SDA)-free materials have been considered as catalysts, which is logical to utilize their large surface area inside pores. On the other hand, no attention has been paid to the catalytic activity of as-synthesized organic-silicate composites. We report here the high catalytic activity of as-synthesized mesoporous silicate (MCM-41) and large-pore microporous silicate (beta; BEA) for the Knoevenagel condensation (Eq. l) [ 13]. Additionally, the same catalysts were used for some other carbon-carbon bond forming reactions such as Claisen-Schmidt reaction (Eq. 2) [14] and Michael reaction (Eq. 3) [ 15], and proved to be active as well. R1
R3
~=:=O + (
R2
1
R4
2
catalyst=
R1
R3
R2~(
R
3
4
+
H20
(1)
554 2. EXPERIMENTAL SECTION 2.1. Materials
(HDTMA+)-[Si]-MCM-41 denotes as-synthesized pure-silica MCM-41 synthesized using hexadecyltrimethylammonium (HDTMA § cation as SDA [ 16,17]. To synthesize this composite, the procedure (2)-(b) of Ref. 16 was exactly followed. On the basis of elemental analysis, the amount of HDTMA § cation occluded in the (HDTMA§ is 1.65 mmol/gcomposite. (TEA§ denotes as-synthesized aluminosilicate beta synthesized using tetraethylammonium (TEA § cation as SDA [18]. This was synthesized from a gel having composition 1.0SiO2-0.3TEAOH-0.6TEABr-0.02NaOH-0.0084A1203-0.22N(CH2CH2OH)315H20. In a typical synthesis procedure, 11.91 g of TEABr (90 mmol) was dissolved in 14.5 g of de-ionized water. 18.93 g (45 mmol) of 35wt%TEAOH solution (Aldrich) and 0.276 g of sodium aluminate (Nacalai, 42.8%A1203, 33.7%Na20) were added with stirring. The stirring was continued for 10 min, and 22.53 g of colloidal silica (Ludox HS40, 150 mmol) and 5.01 g (33 mmol) of triethanolamine (Wako) were added to the homogeneous mixture. The gel was further stirred for 3 h to make it completely homogeneous. The mixture was then transferred to Teflon-lined autoclave (125 ml) and heated statically in a convection oven at 150 ~ for 8 d. The product was recovered by filtration and washed with deionized water, and then dried at room temperature. (TMBp2§ denotes as-synthesized pure-silica beta synthesized using 4,4'.... 9 trimethylenebis(1-methyl-l-(2-methylbutyl)plpendmmm) ( T M B P2§) cation as S D A. TMBP2§ was synthesized by the quaternization of 4,4'-trimethylenebis(1-methyl piperidine) with (S)-(+)- 1-iodo-2-methylbutane in ethyl acetate under reflux for 18 h, followed by conversion into dihydroxide form with ion-exchange resin (DIAION | SA10A(OH), Mitsubishi Chemical Co.). The hydroth_ermal synthesis was carried out statically from a gel having composition SiO2- 0.3TMBP2§ at 150 ~ for 20 days. The product was recovered by filtration and washed with deionized water, and then dried at room temperature. (TEA§ is an as-synthesized, pure-silica beta synthesized by the method of Camblor et al., in which the amount of defect site is very low [ 19,20]. This was synthesized as follows: 6.82 g (16.2 mmol) of 35%TEAOH solution (Aldrich) was gently stirred in a Teflon vessel. Then 6.25 g (30.0 mmol) of tetraethylorthosilicate (Tokyo Chemical Industry) was added and the mixture was stirred at room temperature for 18 h allowing evaporation of ethanol. To the resulting clear solution, 0.59 g (16.2 mmol) of HF (55% aqueous solution, Stella Chemifa) was added. The gel became semi-solid after the addition of HF. Manual stirring with a Teflon rod was necessary to make the gel homogeneous. The final composition of the synthesis mixture was SiO2-0.54TEAOH-0.54HF-10.7H20. The gel was divided into three parts and each part was transferred to Teflon-lined stainless-steel autoclaves (23 ml each) and heated to 150~ with rotation (66 rpm) using a convection oven equipped with a rotator. After 5 d, the product was recovered by filtration and washed with deionized water, and then dried at room temperature. Part of (HDTMA+)-[Si]-MCM-41 and (TEA§ were calcined at 550~ in air to give [Si]-MCM-41 and [A1]-BEA, respectively. Aminopropyl-functionalized [Si]-MCM-41 (denoted AP-MCM-41), which was used for comparison, was also prepared [5,21]. In a typical procedure, [Si]-MCM-41 (2.0 g) vacuum dried at 250~ for 1 h was suspended in anhydrous toluene (30 ml). To this suspension, 0.494 g (2.75 mmol) of 3-aminopropyltrimethoxysilane was added and the mixture was stirred under reflux for 2 h. Toluene containing methanol (ca. 10 ml) was distilled off and toluene (10 ml) was added again; the reflux was continued for another 0.5 h. The product was recovered by filtration and washed with deionized water, and then dried at room temperature to give 2.438 g of white powder. The content of amino group was estimated 1.31 mequiv./g based on elemental analysis.
555 2.2. Measurements X-ray diffraction data were recorded on a Shimadzu XRD-6000 diffractometer using CuKtx radiation and ~, = 1.5404 A. Elemental analyses were performed using ICP (JICP-PS-1000 UV, Leeman Labs Inc.). The scanning electron microscopy (SEM) images were recorded on a Philips XL30 microscope. ~H and 13C NMR spectra were obtained on a JEOL t~-400 FT-NMR spectrometer. 27Si MAS NMR spectra were recorded on a Varian UNITY Inova 400 FT-NMR spectrometer. Nitrogen adsorption measurements were carried out on a BELSORP 28SA gas adsorption instrument. A Shimadzu DTG-50 thermogravimetric analyzer was used to carry out the thermogravimetric analysis (TGA) and differential thermal analysis (DTA). 2.3. Reaction procedures The Knoevenagel condensation was typically carried out as follows: to a solution of a carbonyl compound (1, 2.5 mmol) and an active methylene compound (2, 2.6 mmol) in benzene (2 ml), solid catalyst (200 mg) was added and stirred for 1-6 h. After filtration, the catalyst was washed thoroughly with benzene and recovered. Ethyl ~~-cyano-ig!-phenylacrylate (3: R~=Ph, R2=H, R3=CN, Ra=CO2Et) was isolated from the filtrate by column chromatography (hexane/ethyl acetate = 10/1 ). In a typical procedure of the Claisen-Schmidt reaction, solid catalyst (120 mg) was added to a solution of aryl aldehyde (4, 1.0 mmol) and excess ketone (5, 10-68 mmol) and stirred for 6 h. After filtration, the catalyst was washed thoroughly with benzene and recovered. The products 6 and 7 were isolated from the filtrate by column chromatography (hexane/ethyl acetate=l/I). The typical procedure of the Michael reaction was as follows: under nitrogen atmosphere, solid catalyst (100 mg) was added to a solution of chalcone (8, 1.25 mmol) and ethyl malonate (9, 1.4 mmol) in benzene (2 ml), and stirred for 6 h. After filtration, the catalyst was washed thoroughly with benzene and recovered. The product 10 was isolated from the filtrate by column chromatography (hexane/ethyl acetate=8/l). All products were confirmed by means of IH, 13C NMR spectroscopy and GC.
O O Ar"~H + " ~ R 4
catalyst OH O - Ar/J"'~R
5
0 ph./~.,~L.ph + 8
O + Ar/'~"~R
6
C02Et ( CO2Et 9
(2)
7 o
,~Ph catalyst = Ph CO2Et CO2Et
(3)
10
3. RESULTS AND DISCUSSION Results of the reaction of benzaldehyde (1, R ~=Ph, R2=H) with ethyl cyanoacetate (2, R 3=CN, Ra=CO2Et) using various catalysts are listed in Table 1. (HDTMA+)-[Si]-MCM-41 showed high catalytic activity and the reaction proceeded smoothly under very mild conditions to give desired product 3 in high yields (Entries 1, 2). The reactions catalyzed by (TEA+)-[AI]-BEA and 2+ . . . . (TMBP)-[SI]-BEA gave 3 . m moderate yields, respectwely (Entries 3, 4). On the other hand, calcined, SDA-free materials such as [Si]-MCM-41 and [AI]-BEA showed no catalytic activity even at elevated temperature (Entries 5, 6). HDTMA+Br- and TEA+Br-, which are raw materials
556
for the hydrothermal synthesis, did not exhibit high activity (Entries 7, 8). When TEA+OH -, which should be stronger base than halides, was used as catalyst, the yield of 3 was still relatively low (Entry 9). These results suggest that the high activity emerges only when the silicate and quaternary ammonium parts f o r m a composite. However, (TEA+F-)-[Si]-BEA had no activity despite the fact that this is a composite material (Entry 10). Table 1 Condensation of benzaldehydewith ethyl cyanoacetate using various catalysts a Entry
Catalyst
1 2 3 4 5 6 7 8 9 10
(HDTMA+)-[Si]-MCM-41 (HDTMA+)-[Si]-MCM-41 (TEA+)-[AI]-BEA f (TMBP2+)-[Si]-BEA [Si]-MCM-41 [AI]-BEAf HDTMA+Br" g TEA+Br g TEA+OH h (TEA+F')-[Si]-BEA
Temp. (~ 20 20 20 20 80 80 20 20 20 20
Time (h) 1 6 6 6 6 6 6 6 6 6
Yield b of 3 (%) 82 97, 94c, 80a, 60e 51 49 0 0 6 6 24 0
'7 The reactionwas carriedout as describedin the text. bIsolatedyields. cThe 2nd use of catalyst, oThe 3rd use of catalyst. ~The 4th use of catalyst. f SIO2/A!203=105" g0.30mmolof eachcatalystwas used. h0.96mmol.
Solid-state 29Si MAS NMR spectra of the representative as-synthesized materials are shown in Fig. I. The resonances corresponding to Si(3-OSi, 1-OH), i._e. Q3, are obvious in the spectra of (HDTMA+)-[Si]-MCM-41, (TEA+)-[AI]-BEA and (TMBP2+)-[Si]-BEA, whereas only little Si(3-OSi, l-OH) resonance can be seen in the spectrum of (TEA+F-)-[Si]-BEA, which is consistent with the reported results [16,19,22-24]. Therefore, it is suggested that the actual catalytic sites are basic (SiO)3SiO- moieties in the composite materials. Metal oxides, hydroxide ions and organic amines are absent in this reaction system. It seems that the SiO- moiety is an effective base in a non-polar medium with the assistance of quaternary ammonium cation. This is essentially different situation from the case that hydroxide or alkoxide could be generated from metal cation in aqueous or alcoholic media and function as a base. The basic function is located on the side of parent silicate framework unlike the case in which mobile hydroxide or alkoxide could take part in the reaction mechanism as a base. Nitrogen adsorption measurement of active (HDTMA+)-[Si]-MCM-41 did not give Type-IV isotherm and BET surface area was 14 mE/g (Fig. 2b), whereas the typical Type IV isotherm and a large BET surface area (1013 mE/g) were obtained from catalytically inactive [Si]-MCM-41 as shown in Fig. 2a. This indicates that the large surface area and complete porosity are not indispensable in this reaction system. The reaction should be taking place at around pore-mouth of the silicates, not deeply inside the pore. The efficient catalysis by MCM-4 l-based material may be due to the more exposed catalytic sites at pore-mouth as compared to zeolite-based materials.
557 osi I SiO--S,i--OSi [
osi
I
k
SiO--Si--OSi
~ t
o- -,,
'
osi
f;
I s,o-s,-os,
1000
t
?si
I/ s~o ~ ~-,
800
~i-os '
% " L___
OSi
OSi
I
SiO--Si--OSi
A
oJ-'..,.
B
(a)
'7,
E
B
600
0 "O
I
SiO--Si--OSi
~.. 0
'
B
SiO--Si--OSi
]~
~
OSi
t-
SiO--Si--OSi
0
200
3
o -40
-60
-80
-100
-120
-140
-160
B B
B
E <:
-20
(b)
r
0.5
0
-180 -200
P/Po
Chemical shift (ppm)
Fig. 1. Solid-state 29Si MAS NMR spectra of as-synthesized (a) (HDTMA+)-[Si]-MCM-41, (b) (TEA+)-[AI]-BEA, (c) (TMBP 2+)-[A1]-BEA, and (d) (TEA+F)-[Si]-BEA.
B BEBIBB B BB
B 400 -
"0
OS
B
Fig. 2.
Nitrogen adsorption isotherm of (a) (HDTMA+)-[Si]-MCM-41 and (b) [Si]-MCM-41.
(HDTMA+)-[Si]-MCM-41 was reusable, although the activity significantly decreased after 3rd run (Table 1, Entry 2). It was confirmed by means of X-ray diffraction (Fig. 3), elemental analysis (Table 2), and TG-DTA (Fig. 4, 5) that the framework structure and organic content in the recovered catalysts were unchanged aider 1st run. Significant loss of activity after 3rd run may correspond to the slight change in organic content as seen in Fig. 4c. The changes in profiles of TGA and DTA after 4th run are prominent at 200-250~ (Fig. 4c, 5c), suggesting the loss of active sites are =. mainly from near surface. Unlike the recovered catalyst, the filtered reaction mixture .~" demonstrated no activity after adding fresh substrates. Therefore, the probability of -= homogeneous catalysis by any leached species (including amine) was excluded. -~ Lower reactivity was observed for bulkier tc esters of cyanoacetic acid, and higher temperature was needed to obtain high yield (Table 3, Entries 2, 3). Diethyl malonate hardly reacted with benzaldehyde (Entry 4), suggesting that either R 3 2 3 4 5 6 7 8 9 10 26 / degree or R4 should be cyano group to realize enough reactivity in this system. Malononitrile reacted Fig. 3. Powder X-ray diffraction patterns of with less reactive acetophenone while ethyl (HDTMA+)-[Si]-MCM-41 catalyst cyanoacetate did not (Entries 5-7). However, both (a) before use, (b) after 1st use, malononitrile and ethyl cyanoacetate reacted with (c) after 3rd use and (d) after 4th use. aliphatic ketone such as cyclohexanone (Entries 8-10). '
i
'
i
'
i
'
I
'
t
'
i
'
i
'
i
'
558
Table 2 Content of N, C and H in the (HDTMA+)-[Sil-MCM-41 catalyst before and after use
N (%)
C (%)
Before use
2.52
33.78
H (%) 7.1
After 1st use
2.56
34.61
7.04
After 2nd use
2.46
34.91
6.56
0-
-10
II ,,
-
o~...., -20 u~
_o -30 .,..,
"~....
e-
-40
-
()c
(b)
-50 200
300
400
5 0
6 0
......
~. ii 1~
/~I
" ...............
.........~.~.............. ).. ..",..~ ................................................... (a)
(a) 100
o
700
i
~oo
Temperature (~ Fig. 4.
r 0
,I,I 1 ~ I,~' ',,,
I
~;o ~;o ,oo Temperature (~
i
~oo
~oo
~;o
Thermogravimetric analysis of (HDTMA+) - Fig. 5. Differential thermal analysis of (HDTMA+) [Si]-MCM-41 catalyst (a) before use, (b) after [Si]-MCM-41 catalyst (a) before use, (b) after 1st use, and (c) after 4th use. 1st use, and (c) after 4th use.
Table 3 Knoevenagei condensation of I with 2 using (HDTMA+)-[Si]-MCM-41 a Entry
1
2
R1
RE
R3
Temp. R4
(~
Yield b of 3 (%)
1
Ph
H
CN
CN
20
94
2
Ph
H
CN
CO2But
20
60
3
Ph
H
CN
CO2Bu t
80
95
4
Ph
H
CO2Et
CO2Et
80
5
Ph
Me
CN
CN
20
37
6
Ph
Me
CN
CN
80
45 d
7
Ph
Me
CN
CO2Et
80
0c
0e
8
-(CH2)5-
CN
CN
20
85
9
-(CH2)5-
CN
COEEt
20
31
10
-(CH2)5-
CN
COEEt
80
68
a Reaction conditions: 1, 2.5 mmol;2, 2.6 mmol; solvent, benzene (2 ml); catalyst, 200 mg; period, 6 h. b Isolated yields, c A 8% yield of 3 was obtained when the reaction was carried out in toluene at 110~ d Polymericby-productwas formed, c A 8% yield of 3 was obtained when the reaction was carried out in toluene at 110~
559
Regarding the Knoevenagel condensation, there were no significant difference in activity between (HDTMA+)-[Si]-MCM-41 and aminopropyl-functionalized MCM-41 (AP-MCM-41) as shown in Table 4. It should be noted that the densities of active sites of (HDTMA+)-[Si]-MCM-41 and AP-MCM-41 are comparable based on the values described in Experimental section. Next we investigated more 'demanding' reactions, such as Claisen-Schmidt reaction and Michael reaction. Table 5 lists the result of the Claisen-Schmidt reaction of 4-nitro- benzaldehyde (4: Ar = 4-nitrophenyl) with acetone (5: R=Me) or acetophenone (5: R=Ph) catalyzed by the (HDTMA+)-[Si]-MCM-41 or AP-MCM-41. When the ketone component was acetone, the activity of (HDTMA+)-[Si]-MCM-41 was significantly higher than that of AP-MCM-41 (Entries 1, 2). The product 6 predominantly formed in both cases. The difference in activity between (HDTMA+)-[Si]-MCM-41 and AP-MCM-41 was much larger when the ketone component was acetophenone. The (HDTMA+)-[Si]-MCM-41 showed high activity at 60~ and the use of it gave the condensation product 7 in 73% yield, whereas AP-MCM-41 exhibited no activity (Entries 3, 4). Table 6 shows the result of the Michael reaction of chalcone (8) with ethyl malonate (9). In this reaction, the (HDTMA+)-[Si]-MCM-41 was again much more active than AP-MCM-41. The use of (HDTMA+)-[Si]-MCM-41 at 40~ gave 10 in 96% yield, while 10 was obtained only in 4% yield even at elevated temperature when AP-MCM-41 was used. Table 4 Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate a Yield b of 3 (%)
Entry
Catalyst
Temp. (~
Conv. (%)
1
(HDTMA+)-[Si]-MCM-41
20
96
92
2
AP-MCM-41
20
99
94
" The reaction was carried out in benzene for 2 h as described in the text. b Isolated yields. Table 5 Ciaisen-Schmidt reaction of 4-nitrobenzaidehyde (4: Ar = 4-nitrophenyl) with ketone (5) a Entry
R
Catalyst
Yield (o~)b
Temp. (~
Conv. (%)
6
7
(HDTMA+)-[Si]-MCM-41
20
98
78
3
1
Me
2
Me
AP-MCM-41
20
72
67
3
3
Ph
(HDTMA+)-[Si]-MCM-41
60
79
4
73
4
Ph
AP-MCM-41
60
0
0
0
" The reaction was carried out for 6 h as described in the text.
b Isolated yields.
Table 6 Michael reaction of chalcone (8) with ethyl malonate (9) a Entry
Catalyst
1
(HDTMA+)-[Si]-MCM-41
2
AP-MCM-41
Conv.
Yield b
(%)
of 10 (%)
40
98
96
80
8
4
Temp.
(~
'7 The reaction was carried out in benzene for 2 h as described in the text. b Isolated yields.
560 CONCLUSIONS Unexpectedly high catalytic activity of quaternary ammonium-ordered porous silicate composite materials for Knoevenagel condensation has been found and investigated. These composites were also active for other carbon-carbon bond forming reactions such as Claisen-Schmidt reaction and Michael reaction. Although these materials are not to be utilized for "shape- selective" purposes, they still should be useful for general heterogeneous catalysis, particularly for the synthesis of fine-chemicals under mild conditions in non-polar media. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
S.I. Zones and M.E. Davis, Curr. Opin. Solid State Mater. Sci., 1 (1996) 107. D. Zhao, P. Yang, Q. Huo, B.F. Chmelka and G.D. Stucky, Curr. Opin. Solid State Mater. Sci., 3 (1998) 111. H. Hattori, Chem. Rev., 95 (1995) 537. G. Jones, in: A.C. Cope (ed.), Organic Reactions Vol. 15, John Wiley & Sons, New York, 1967, pp.204-599. E. Angeletti, C. Canepa, G. Martinetti and P. Venturello, J. Chem. Soc., Perkin Trans. 1, (1989) 105. D.J. Macquarrie, Green Chemistry, (1999) 195. T. Saito, H. Goto, K. Honda and T. Fujii, Tetrahedron Lett., 33 (1992) 7535. W. Richardhein and J. Melvin, J. Org. Chem., 26 (1961) 4874. K.R. Kloetstra and H. van Bekkum, J. Chem. Soc., Chem. Commun., (1995) 1005. A. Corma, V. Fornes, R.M. Martin-Aranda, H. Garcia and J. Primo, Appl. Catal., 59 (1990) 237. A. Corma and R.M. Martin-Aranda, J. Catal., 130 (1991) 130. M.L. Kantam, B.M. Choudary, Ch.V. Reddy, K. K. Rao and F. Figueras, Chem. Commun., (1998) 1033. Y. Kubota, Y. Nishizaki and Y. Sugi, Chem. Lett (2000) 998. J. March, Advanced Organic Chemistry (3rd edition), John Wiley & Sons, New York, 1985, Chap. 16. E.D. Bergmann, D. Ginsburg and R. Pappo, in: R. Adams (ed.), Organic Reactions Vol. 10, John Wiley & Sons, New York, 1959, pp.179-555. C.-Y. Chen, H.-X. Li and M.E. Davis, Micropor. Mater., 2 (1993) 17. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T-W. Chu, D. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. M.K. Rubin, U. S. Patent, 5164169 (1992). M.A. Camblor, A.C. Corma and S. Valencia, Chem. Commun., (1996) 2365. M.A. Camblor, L.A. Villaenscusa and M.J. Diaz-Cabafias, Topics in Catalysis, 9 (1999). K. Tsuji, C.W. Jones and M. Davis, Micropor. Mesopor. Mater., 29 (1999) 339. C.W. Jones, K. Tsuji and M.E. Davis, Nature, 393 (1998) 52. C.W. Jones, K. Tsuji and M. Davis, Micropor. Mesopor. Mater., 33 (1999) 223. E.J.R. Sudhrlter, R. Huis, G.R. Hays and N.C.M. Alma, J. Coll.I. Sci., 103 (1985) 554.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
561
A s e l e c t i v i t y o f zeolite m a t r i c e s in the Cu(II) r e d u c t i o n p r o c e s s Vitalii Petranovskii, a Valerij Gurin, b Nina Bogdanchikova, a Miguel-Angel Hemandez c and Miguel Avalos a a Centro de Ciencias de la Materia Condensada, UNAM, Ensenada, B.C. 22800 Mexico; b Physico-Chemical Research Institute, BSU, Minsk, Belarus; c Institute of Sciences, University of Puebla, Puebla, Mexico.
Copper nanoparticles were produced within a series of protonated and alkaline metal forms of zeolites (mordenite, erionite and clinoptilolite) by the hydrogen reduction of corresponding Cu(II)-exchanged forms. Variation of the zeolite structure, the reduction temperature and the acidity of zeolites (regulated via cation types and in the case of mordenite via Si/A1 ratio from 5 to 103) are the main factors influencing the appearance of copper reduced forms. They were detected by means of optical absorption using Diffuse Reflectance Spectroscopy (DRS) technique as the plasmon resonance band of different shape and position and simulated with the Mie theory for the simple model of spherical copper particles embedded in a dielectric medium. The observed effects are interpreted as a difference in the acidity strengthand type of matrix playing a role of medium for the reduction process. The acidity control and zeolite type structure are proposed as the main tools for selectivity of copper reduction and its final state.
1. INTRODUCTION The diversity of natural and artificial zeolites [1,2] presents inexhaustible research field of nanomaterials synthesis with controlled chemical, catalytic, optical, magnetic, etc., properties [3-5]. In contrast with amorphous matrices like silica and nanostructured titania, alumina, etc., the crystallinity of zeolites a p r i o r i provides the special organization of the system, although this organization can be very complicated due to the complexity of real zeolite crystal lattices. Transition metal ions and nanoparticles within nanoporous media like zeolites are efficient modifiers of their properties. The multi-component metal-dielectric systems that appear as a result of metal incorporation into zeolites are of special interest from the point of view of metal-support interaction and understanding of mutual contribution into properties of different components to the whole system features. Unlike some matrices that are simpler in chemical composition, zeolites are not only mechanical support by geometrical and topological reasons, but also active ionic medium promoting rich chemistry of metal ions, incorporated together with other accompanying reactants like water, alkali ions or different acidic centers [1 ]. Thus, zeolite matrices combine the factors of nanoporosity and nanometerscale chemical reactivity with respect to incorporated foreign ions, clusters, and
562 nanoparticles. In the present paper, we touch some examples of zeolite systems with incorporated copper, present in the forms of both ions and small particles located in different positions, and study how the properties of the zeolites, on one hand, and copper ions and particles, on the other hand, manage the metal reduction process. This reaction could be considered as rather trivial processes (Cu 2§ + H2 ~ {Cu§ or Cu ~ + H+; Cu++ H2 Cu ~ + H+), however, they become very non-trivial (in the sense of kinetic and thermodynamic feasibility as well as in forms of final products) when carried out within the "zeolite medium". Zeolite peculiarities influence drastically the process, from a strong promotion to a complete stopping. In a recent publication [6] we have shown the leading role of Si/A1 ratio in mordenite (from 5 through 103) on the optical appearance of reduced copper species. Changing the chemical composition of mordenite framework leads to a non-monotonous dependence of the copper reducibility upon Si/A1 ratio. Zeolite structure type is another factor to control copper reducibility and state of final products. In this paper, we extend the study of copper reducibility to erionite and clinoptilolite, that are distinguished from mordenite by the geometry of intracrystalline voids. We elucidate, thus, new factors managing the copper reduction process. Copper chemical forms are discovered within the powder-like Cu-zeolites with optical absorption using diffuse reflectance spectroscopy (DRS) technique. To find of the control factors for copper reduction is of importance for production of Cu-zeolite catalysts (e.g., for selective catalytic reduction of NOx [7-9]), since no unambiguous general rules associating the catalytic activity and chemical composition of similar catalysts are known. The reason for this is a wide difference in Cu adsorption sites in zeolites for Cu § and Cu 2§ ions [ 10-12] and variable nanoparticle size, extending from the fewatom clusters [13,14] fitting the intracrystalline pores to colloidal size copper nanoparticles on the outer open surface atop microcrystals. Rather intensive studies in this field were concentrated mostly on ZSM-5 zeolite type [15,16], which does not have too wide variability in the properties to tune copper reduction process. It was shown earlier [17-19] that coordination, localization and stabilization of copper ions in the zeolitic materials depend strongly on the structure and composition of zeolite matrix. Covalent bond character of the metal-matrix interaction becomes stronger with increasing of Si/A1 molar ratio in zeolites [20]. The red-ox behavior of the system Cu2+/Cu+/Cu0 is known to be very sensitive to medium and complexation of copper by different ligands. For copper within zeolites this role is played by the solid matrix and we vary in this study both "ligand types" and acidity of the medium. A geometric factor of the cavity type play a secondary role, however, appearance of few-atomic clusters can be governed by it. Spectroscopic manifestation of the clusters in the case of copper is possible, but more troubled to observe because less stability refers to similar silver species and poorly studied properties of size-selected copper clusters [6,21-24]. The aim of the present work is to investigate, how variation of the zeolite structure, reduction temperature and acidity of zeolites (regulated via cation types and in the case of mordenite via Si/A1 ratio from 5 to 103) influence the appearance of copper reduced forms.
2. EXPERIMENTAL §
§
~
~
Synthetic zeolites K ,Na -erlonlte with Si/A1 ratio equal to 3.9 and protonated forms of mordenites with Si/A1 ratio varying from 5 to 103 were supplied by TOSOH Corporation,
563 Japan. Natural Ca2+,Na+,K+-clinoptilolite with Si/AI ratio equal to 3.5 was originated from Caimanes deposit, Cuba. Copper ion exchange was carried out from 0.1 M Cu(NO3)2 aqueous solution for one day. The excess of solution was removed; samples were dried under ambient conditions followed by heating in dry H2 flow at temperatures from 150 to 450~ for 4 h. Throughout the text and figures, samples are abbreviated as Mor, Eri, and Cli for an initial forms of mordenites, erionite and clinoptilolite, respectively, and CuMor, CuEri, and CuCli for the corresponding Cu-exchanged forms, with the value of Si/A1 for mordenites and temperature of the hydrogen reduction in ~ if applicable (e.g., CuEri-250 or CuMor5-250). The copper content in the prepared samples was determined by atomic absorption spectrometry using a Varian 1475 equipment. High-resolution adsorption measurements were performed on an Autosorb-1, Quantachrome equipment in the range of 10-6 < p/po < 1. Surface area SBET and micropore volume Vz were determined. Results of the measurements are shown in Table 1. Table 1. Copper content and adsorption characteristics of the samples under study. CuMor5 CuMor7.5 CuMor 15 CuMor 103 CuEri Cu wt % 0.79 0.26 0.26 0.44 1.25 SBET, m2/g 359 380 480 493 V~, cm3/g 0.200 0.174 0.238 0.261 -
CuCli 1.80 -
Diffuse reflectance spectra (DRS) were collected on a Varian Cary 300 spectrophotometer equipped with a standard diffuse reflectance unit using a barium sulfate reference.
3. RESULTS AND DISCUSSION The state of copper in zeolite matrices were estimated from DRS data, since DRS is an efficient method to study optical properties of opaque samples. This allows us to decode both, a spectral position and relative intensities contributed by some copper species, similar to conventional absorption spectra in transparent media (under low absorbance values) [25]. DRS spectra for the three mordenites with different Si/A1 ratio are given in Fig. 1, while for erionite and clinoptilolite samples the reduced and unreduced forms are presented in Figs. 2-3. The blank mordenite as well as erionite and clinoptilolite without copper absorb little in the range under study. Cu-exchanged mordenites reveal the broad absorption band at ~,>600 nm that can be assigned to the familiar d-d transition of Cu 2+ ions [26]. This broad band in the spectra does not show any structure, and we cannot speak about difference in Cu 2+ ion positions from these data, although, they should have some distribution between the available ion-exchanged sites [10-12]. But they are not resolved in the absorption spectroscopy results, and this resolution is worse in DRS measurements. The appearance of this long-wave band is similar for all mordenites (Fig. 1a-c), making evident that their various acidity [6] do not provide noticeable changes in the positions of Cu 2+ ion absorption band. The situation is changed principally for the reduced Cu-exchange mordenites. Under the small Si/A1 ratio we see no clear features in the DRS spectra that could be associated to the metallic copper for all temperatures of reduction. However, under Si/A1--15 and 103 the absorption feature is developed in the form of plasmon resonance band for metallic copper nanoparticles (see, e.g., [27-32]). This situation could be considered as a
564 0.8-
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~I
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!
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.
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C
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9
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........... .t
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u c
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i
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0.1 i . , ~ . , ~ . I o.=
eoo
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4;o
0.0 I . . . . . . . . .
860 400 eoo 800 W a v e I e n g t h, nm
e~o
Figure 1. DRS spectra of blank Mot (1), CuMor (2)and reduced at temperatures 150, 250, 350 and 450 ~ (3, 4, 5, 6 respectively),
I
8~o
rather trivial fact, if its appearance does not change with varying the matrix properties. Fig. lb,c shows that the shape of the plasmon resonance changes essentially under growth of Si/AI: from a step-like feature to the pronounced maximum, and a weak feature appears in the range . . . . . . . . . . of 400-500 nm. The latter can be attributed to the
features in the bulk copper band structure and are often easily observable in different types of ultrafine copper (particles, thin films) [32-35]. They will not be considered here in detail. The different reduction temperatures result in almost same shape of spectra, and, hence, this factor contributes only in the amount of the copper reduced. This result indicates that the type of matrix rather than the reduction conditions determines the state of the reduced copper nanoparticles. The temperature factor looks here as the kinetic one, and can be treated as occurrence of some activation barrier for the reduction reaction since we see some threshold variation under comparison of the DRS spectra for 250 ~ an those for the higher temperatures (especially, for Si/AI=103). Such temperature dependence can be quite understandable if to take into account that Cu 2§ ions are bound in zeolite matrix while H2 molecules usually unreactive under low temperatures. In the case of the Cu-exchanged reduced mordenite under Si/AI=7.5 one can point the rise of a structureless absorption and the disappearance of the long-wave band k>600 nm; i.e. the copper reduction process occurs also, however the products are difficult to decode. Those can be: i) few-atomic clusters like the species known in the case of Ag-exchanged mordenitesof similar composition [22-24]; ii) Cu § ions in different position in the mordenite lattice [36]; iii) some form of copper oxide (nanoparticles, nonstoichiometric clusters, etc.) [37]. The experimental data in Figs. 2 and 3 for the reduced forms of Cu-exchanged erionite and clinoptilolite support the above conclusion on the dominant role of the zeolite matrix in the formation of copper reduced form, and the little contribution of the reduction temperature, however the reduction is noticeable already at 250 ~ That may be associated with the lower acidity of Na §,K-enomte § " " and Na+,K§ 2§ clinoptilolite as compared with the above H § mordenites. In the case of erionite and clinoptilolite we can see intermediate shapes between the steplike one and well-pronounced maximum. No significant change of shape and position of maximum is observed for all the set of temperatures used. Under intermediate reduction temperatures (e.g. 250 and 350 ~ the initial oxidized Cu 2§ form still is observed in the spectra. The curves, corresponding to the highest reduction temperature (450 ~ already do not contain the noticeable long-wave band. That evidences the complete reduction of Cu(II)
565
1.0 0.3 -4
.--'"" "~".
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.-..,, ';
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,
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er ~
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o
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.
i
.
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.
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0.2
/
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.
,
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.
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Figure 2. DRS spectra of blank Eri (1), CuEri (2) and samples reduced at temperatures 150, 250, 350 and 450 ~ (3, 4, 5, 6 respectively).
o
O.4
/tI
800
nm
Figure 4. Calculated spectra of Cu particles in the media with ~0=2 (a) with size (from bottom to the top) 1, 2, 3, 5, 10 and 50 nm and with ~0=4 (b) with size (from bottom to the top) 1, 2, 5 and 10 nm.
600
800
nm
Figure 3. DRS spectra of blank Cli (1), CuCli (2) and samples reduced at temperatures 150, 250, 350 and 450 ~ (3, 4, 5, 6 respectively). under these conditions and the dominating influence of zeolite properties on the final state of reduced copper resulting in variation of the band position and the shape of this band. In order to simulate these observations we used the Mie theory for the simple case of individual spherical particles with size in the range of R<<)~ embedded in a transparent dielectric medium with dielectric constant eo. For the dielectric function of copper we take bulk values [38] with addition of the sizedependence for the imaginary part in the dielectric function e - el + ie2 through the effect of relaxation frequency on mean free path length of electrons, L, 3' = Ybulk+V~'L, and obtain e2 (LR) = e2 (~) +(3/4)(mpZvdm3R), where Vf denotes the value of Fermi velocity of electrons, while Vf = 1.3-10~Snm/s and "/bulk = 2.9-1013 S1 [39]. Fig. 4 displays the simulation results for the two selected values of medium dielectric constants in the range of
566 data expected for the zeolites. Particle size was tested in the interval from 1 nm through 10 nm, and the appearance of the plasmon resonance band can be analyzed in comparison with the above experimental spectra. It should also be stated that the literature data on the dielectric properties of zeolites are limited and poorly coherent [1,40,41], which prevents from drawing conclusions on a more precise values of this parameter. Also, the Si/A1 dependence is presupposed - the more Si/A1 ratio, the lower have to be to. The situation is complicated bythe complex composite nature of zeolite-metal-water samples. It is known that to of zeolites depends on the amount of adsorbed water and can vary for different zeolites in the range from 1.3 up to 6 [1]. Appearance of water in close vicinity of acidic centers promotes charge transfer by stabilizing ion-pair structures [40]. It is possible that such kind of interaction leads to the stabilization of charged Ag8§ species in mordenite [22]. But detailed discussion on the matter is out of the frame of the present work. The simulated spectra show the two principal effects influencing the shape of the plasmon resonance maximum. First, the rise of particle size makes the maximum more pronounced, like that in the reduced CuMorl03, and the position of this maximum remains practically unchanged. The latter observation could be consistent with the series of curves for Cu-exchanged zeolites reduced under different temperatures. However, the shape of the simulated spectra becomes smoother while different temperatures in zeolites provide unchanged shapes. Therefore, one can argue, that size of copper particles reduced under different temperatures is not changed: even at the minimum reduction temperature the effect of size-particle selectivity works effectively, and the matrix rather than the reduction conditions determines the size of copper particles. The second parameter controlling the simulated sl~ectra is dielectric constant of zeolite medium (Fig. 4a,b). Decrease of to not only shifts positions of the maximum to the blue region, but also changes the shape of the spectrum. Now it begins to resemble the step-like one, like the spectra of reduced CuMorl5, CuEri and CuCli samples. Also, under the smaller to the position of the maximum appears to be size dependent (Fig. 4a). Thus, the theoretical spectra simulate the general view and dependencies from the main parameters of the experimental spectra for reduced copper in three types of zeolites. The bigger nanoparticles appear in the high Si/A1 mordenites, while the smaller particles appear in erionite and clinoptilolite. These particles can be located in cleaved areas of zeolite crystals (a particle is surrounded by the matrix with to), while the particles of less size are expected to occur at an external surface of the microcrystals (surrounded by air from one side). In the case of mordenites with the lowest Si/A1 ratio used, the reduction to the metallic nanoparticles does not occur. Variation of zeolite structure and Si/A1 ratio that changes dielectric constant can manage the reducibility of copper. It should be noticed that we couldn't present here any direct evidence of the above conclusions with, e.g., electron microscopy data, since polycrystalline zeolite materials with low metal concentration are very troubled for similar studies. From the other hand, the method of particle size estimation used in the present paper based on shape and position of the Mie absorption peak can be utilized in similar situations when any direct size determination is problematic in some composite solid materials.
567
4. CONCLUSIONS Three types of zeolites (mordenite, erionite and clinoptilolite) were used for copper incorporation by means of ion exchange followed by hydrogen reduction. The complete process results in the formation of copper nanoparticles discovered by plasmon resonance in the range of 560-600 nm with different shape of curves and slightly deviated position of the maxima. In some cases (the low Si/AI ratio mordenite) the reduction process does not lead to the nanoparticle formation. Simulation with the Mie theory of the plasmon resonance provided by copper nanoparticles leads to interpretation that observable dependences of the plasmon resonance are effects of particle size and matrix dielectric constant. The selection of zeolite matrix can be considered as an efficient tool governing the copper reduction and able to regulate in this way catalytic activity of Cu-zeolite catalysts. The main conclusions from the experimental and theoretical study are the following: (i) The zeolite matrix plays the role of medium for reduction of copper and support for the final products; it also controls the final state of copper; (ii) The influence of zeolite structure type can be interpreted as possible variation in copper absorption sites and the contribution of the medium chemical composition in the copper reducibility; reduction temperature plays a secondary role in the reduced species formation; the acidity of zeolites (regulated here for mordenite by Si/A1 ratio) is a significant parameter determining the products of reduction process; (iii) Size of the reduced copper nanoparticles depends both on zeolite type and Si/A1 molar ratio and changes the optical appearance of the plasmon resonance. The maximum size (of the order of 10 nm) can be found for the high Si/A1 ratio mordenites while the particles formed in erionite and clinoptilolite enter to the range of few nanometers.
5. ACKNOWLEDGMENTS We are very grateful to Dr. E. Stoyanov for many helpful discussions. Thanks are given to J. Tamariz-Flores and I. Rodriguez-Iznaga for their experimental work and to F. Ruiz, E. Aparicio, E. Flores, J.A. Peralta and C. Gonzalez for the precious technical support. The research reported in this paper was supported by CONACYT, Mexico, through Grants No. 3894P-E and 32118-E.
REFERENCES I . D . W . Breck, Zeolite Molecular Sieves. Structure, Chemistry and Use, A WileyInterscience Publication, John Wiley & Sons: New York, 1974. 2. Atlas of Zeolite Structure Types, 5th revised edition, (Ch. Baerlocher, W. M. Meier and D. H. Olson, Eds.), 2000. 3. Ph. Moriarty, Rep. Progr. Phys., 64 (2001) 297. 4. G. A. Ozin, Chem. Commun., (2000) 419. 5. C. N. R. Rao, J. Mater. Chem., 9 (1999) 1. 6. V. Petranovskii, V. Gurin, N. Bogdanchikova, A. Licea-Claverie, Y. Sugi and E. Stoyanov, Mater. Sci. Eng. A (2001) in press.
568 7. P. Gilot, M. Guyon and B. R. Stanmore, Fuel, 76 (1997) 507. 8. C. Torre-Abreu, C. Henriques, F. R. Ribeiro, G. Delahay and M.F. Ribeiro, Cat. Today, 54 (1999) 407. 9. Z. Chajar, V. Le Chanu, M. Primet and H. Praliaud, Catal. Lett., 52 (1998) 97. 10. M. P. Attfied, S. J. Weige and A. K. Cheetham, J. Catal., 170 (1997) 227. 11. P. Gaezot, Y. Ben Taarit and B. Imelik, J. Catal., 26 (1972) 295. 12. M. W. Anderson and L. Kevan, J. Phys. Chem., 91 (1987) 4174. 13. Y. Kuroda, Sh. Konno, Y. Yoshikawa, H. Maeda, Y. Kubozono, H. Hamano, R. Kumashiro and M. Nagao, J. Chem. Soc. Faraday Trans., 93 (1997) 2125. 14. R. A. Schoonheydt, J. Phys. Chem. Solids, 50 (1989) 523. 15. M. LoJacono, G. Fierro, R. Dragone, X. Feng, J. d'Itri and W. K. Hall, J. Phys. Chem. B, 101 (1997) 1979. 16. W. Grunert, N. W. Hayes, R. W. Joyner, E. S. Shpiro, M. R. H. Siddiqui and G. Baeva, J. Phys. Chem., 98 (1994) 10834. 17. K.-P. Wendlandt, P. Vogt, W. Morke and I. Achkar, Stud. Surf. Sci. Catal., 69 (1991) 223. 18. J. Dedecek and B. Wichterlova, J. Phys. Chem. B, 101 (1997) 10233. 19. Y. Itho, S. Nishiyama, S. Tsuruya and M. Masai, J. Phys. Chem., 98 (1994) 960. 20. A. Yu. Stakheev and L. M. Kustov, Appl. Catal. A, 188 (1999) 3. 21. V. S. Gurin, N. E. Bogdanchikova and V. P. Petranovskii, Mat. Sci. Eng. C, (2001), in press. 22. V. S. Gurin, N. E. Bogdanchikova and V. P. Petranovskii J. Phys. Chem. B, 104 (2000) 12105. 23. N. Bogdanchikova, V. Petranovskii, S. Fuentes, E. Paukshtis, Y. Sugi and A. LiceaClaverie, Mater. Sci. Engineer. A, 276 (2000) 236. 24. N. Bogdanchikova, V. Petranovskii, R. Machorro, Y. Sugi, V. M. Soto and S. Fuentes, Appl. Surf. Sci., 150 (1999) 58. 25. A. Ishimaru, Wave propagation and scattering in random media, Academic Press, New York, 1978. 26. A. B. P. Lever, Inorganic Electronic Spectroscopy, Elsevier, Amsterdam, 1986. 27. U. Kreibig and M. Vollmer, Optical Properties of Metal Clusters, Springer-Verlag, Berlin, 1995. 28. M. P. Pileni, J. Phys. Chem., 97 (1993) 6961. 29. I. Lisiecki, F. Billoudet and M.P. Pileni, J. Phys. Chem., 100 (1996) 4160. 30. H. H. Huang, F. Q. Yan, Y. M. Kek, C. H. Chew, G. Q. Xu, W. Ji, P. S. Oh and S. H. Tang, Langmuir, 13 (1997) 172. 31. R. Doremus, Sh.-Ch. Kao and R. Garcia, Appl. Opt., 31 (1992) 5773. 32. H. Abe, K.-P. Charle, B. Tesche and W. Schulze, Chem. Phys. Lett., 68 (1982) 137. 33. B. G. Ershov, E. Janata and A. Henglein, Radiat. Phys. Chem., 39 (1992) 123. 34. J. Khatouri, M. Mostafavi, J. J. Amblard and J. Belloni, Chem. Phys. Lett., 191 (1992) 351. 35. J. F. Perez-Robles, F.J. Garcia-Rodriguez, J.M. Yanez-Limon, F.J. Espinoza-Beltran, Y. V. Vorobiev and J. Gonzalez-Hemandez, J. Phys. Chem. Solids, 60 (1999) 1729. 36. Y. Kuroda, H. Maeda, Y. Yoshikawa, R. Kamashiro and M. Nagao, J. Phys. Chem. B, 101 (1997) 1312. 37. Y.-J. Huang and H. P. Wang, J. Phys. Chem. A, 103 (1999) 6514. 38. R. Doremus, Sh.-Ch. Kao and R. Garcia, Appl. Opt., 31 (1992) 5773. 39. Yu. Ekmanis, Yu. Rud and I. Radchenko, Latv. J. Phys. Techn. Sci., 5 (1997) 3. 40. J. F. Haw, T. Xu, J. B. Nicholas and P. W. Goguen, Nature, 389 (1997) 832. 41. R. A. van Santen and G.R. Kramer, Chem. Rev., 95 (1995) 637.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
569
Reduction o f binary silver-copper ion mixture in mordenite: an example of synergetic behavior Vitalii Petranovskii and Nina Bogdanchikova Centro de Ciencias de la Materia Condensada, UNAM, Ensenada, B.C. 22800 M6xico; 9 Mordenite exchanged with mixture of (Ag+,Cu 2+)-ions was reduced in hydrogen flow. Variations of the Ag/Cu ratio and reduction temperature influence the appearance of silver and copper reduced species. They were detected by means of optical absorption using diffuse reflectance spectroscopy technique as the characteristic peaks of size selected Ag8 clusters and plasmon resonance band of copper nanoparticles. Changes in the temperature range of appearance and disappearance of distinct copper and silver species and variations of their relative stability were revealed. Formation of bimetallic particles was not detected. The observed effects are interpreted as a mutual influence of silver and copper ions and reduced species of silver and copper due to their concurrence for ion-exchange sites and relative positions in mordenite voids, respectively.
1. INTRODUCTION Nanotechnology fundamentally changes the way of new materials design. The ability to synthesize nanoscale building blocks with precisely controlled size and composition and the assembly of them into larger structures with unique properties is one of the great challenges. Zeolites are an example of ordered nanoporous materials with regular pores (cavities, channels) that are able to incorporate species of different chemical nature and regularly assemble them inside the intra-crystalline space [ 1-4]. It was shown previously that reduction of Ag-mordenites under certain conditions leads to self-assembling of Ag8 clusters [5]. More complicated chemistry was demonstrated by Cu-mordenite samples, leading to formation of a different kind of Cu reduced species [5,6]. Nanosize metal particles are intermediate between the bulk metals and individual atoms, being of considerable theoretical and practical interest. Main efforts are directed on the preparation of mono-metal supported nanoparticles. Recently papers concerning bimetallic system formation began to appear [7-11]. The properties of such kind of systems depend not only on the particle composition, but on the particle structure as well. Four types of structures would be expected for bimetallic particles: it might be i) a homogeneous or nearly homogeneous solid solution of the metals; ii) an intermetallic compound; iii) nanoheterogeneous "core-shell" structures or iv) aggregated nanodomains of individual metal clusters. The Cu-Ag system was selected because both metal clusters are known to be promising de-NOx catalysts [12-18]. Ag-mordenite and Cu-mordenite systems were carefully studied earlier [5,6,19-22]. Thermodynamic properties of the Cu-Ag system are well known [23].
570 Some attempts to investigate structure of non-equilibrium silica-supported binary Ag-Cu clusters were done using extended x-ray absorption fine structure (EXAFS) [24]. The atomic ratio of copper to silver was close to one in the material investigated in that work. The EXAFS results, which were obtained in the presence of hydrogen, indicate extensive segregation of the components in the silver-copper clusters. The pronounced segregation in the silver--copper clusters is readily understandable, since silver and copper are only slightly miscible in the bulk. The EXAFS results on the silver-copper clusters suggest that the copper-rich region is in the interior of the clusters, with the silver concentrating at the surface [24]. Such nanostructured composite materials are currently receiving a great deal of interest due to their unique mechanical, magnetic or optical properties. These alloys are thermodynamically unstable and for the Cu-Ag system the heat of mixing is in the moderate range of AHm < 6 kJ/g atom [23,25]. It is now well documented that ball milling can force the mixing of immiscible elements [26]. Nevertheless even this method permits to obtain nearly random mixing of copper and silver in the AgsoCus0 alloys under cryo-milling (85 K) only, whereas the mixing achieved by milling at 315 K is calculated to be around 70% [27]. Zeolite matrices are known to stabilize thermodynamically unstable molecules and compounds [2-5,28-34]. The aim of the present work is to investigate experimentally approaches to bimetallic Ag-Cu nanostructure formation inside the zeolite voids. Ag § Cu 2§ and Ag+-Cu2+ binary mixtures with different Ag/Cu ratios were supported on mordenite with Si/A1 ratio equal to 10 and reduced in the temperature range 323-673 K. The reduction was followed by means of optical absorption with diffuse reflectance spectroscopy technique of the powder'-like zeolite samples.
2. EXPERIMENTAL
Mordenite with Si/A1 ratio equal to 10 was supplied by TOSOH Corporation, Japan. Ion exchange was carded out for one day from two-component mixture of 0.1 N water solutions of AgNO3 and Cu(NO3)2, mixed in ratios 3:1, 1:3 and 1:9 respectively, thus saving the total normality of the mixed solution. The excess of solution was removed; samples were washed and dried under ambient conditions followed by heating in dry H2 flow in the temperature range 100-400 ~ for 4 h. Throughout the text and figures, samples are abbreviated as AgCuM, accompanied by the value of nominal Ag/Cu ratio and temperature of the hydrogen reduction in ~ if applicable (e.g., AgCuM(1:3)-200). The silver and copper content in the obtained samples was determined by a JEOL 5200 scanning electron microscope equipped with a Kevex Super Dry energy disperse spectroscopy (EDS) attachment. Quantification was done using the standard Magic 5 software. Diffuse reflectance spectra (DRS) were collected on a Varian Cary 300 spectrophotometer equipped with a standard diffuse reflectance unit using a BaSO4 reference.
3. RESULTS AND DISCUSSION
Monometallic Ag-Mordenite and Cu-Mordenite with the same Si/A1 ratio studied in the previous works [6,19] after similar ion-exchange treatment adsorb approximately 1 wt % of the Ag and Cu. Samples obtained in the course of the present work surprisingly demonstrate
571 much higher degree of ion exchange. Results of analysis are shown in the Table 1. Also, note the significant deviations from nominal Ag/Cu atomic ratios refer to observed ones. Copper adsorption seems to be constant, while silver is adsorbed more and more selectively with decreasing its content in the mixture. Table 1. Cu and Ag content (wt %) and observed Ag/Cu atomic ratio of the samples under study. Sample CuM [6] AgM [19] AgCuM(3"I ) A gCuM(!~3.) A.gCuM(1;9_) Ag 1.0 8.0 4.9 2.2 Cu 1.0 1.4 1.5 1.5 Ag/Cunominal 6:1 0.7:1 0.02:1 Ag/CUobserved 3.7:1 2:1 1.1" 1 DRS spectra for the AgM and CuM and for three samples with different Ag/Cu ratio reduced at different temperatures are presented in Fig. 1, a-f. In the spectra of AgM (Fig. 1a) the peaks at 320 and 285 nm are observed for all temperature of 0.6 {i~ CuM-150 Ag M- 100 1.5~..~ - - - CuM-250 - - - AgM-200 reduction. Only for AgM-400 [~ ........ CuM-350 ........ AgM-300 . . . . . AgM-400 intensity of the peaks start to 0.41 \ . . . . . CuM-450 1.0~ decrease. These bands are typical for the size-selected 0.5 . . . . , clusters Ag8 that are formed in the channels of mordenite 0.0, 9 O.Ol . . . . . . , . . . . . . . . . . , 200 400 600 800 200 400 600 800 [5,19]. Earlier, the distinct geometrical structure of this /~ ~ AgCuM(3:I)-100 [ ~ AgCuM(3:1 )-200 cluster was experimentally 7~ 1"5t /~t~ - - - AgCuM(I:3)-100 1.5 t - - - AgCuM(1:3)-200 established by EXAFS [35] and 10t ~ ........A~CuM~:9~-200 verified by quantum chemical e calculations [5,22]. CuM samples (Fig. l b) after ~ . -"'-..i.,. hydrogen treatment does not < show any noticeable reduction, 0.0 . 200 400 600 800 200 400 600 800 and traces of plasma resonance peak at 580 nm due to ~ AgCuM(3:1 )-300 ~ AgCuM(3:1 )-400 1.5 - - - AgCuM(l:3)-300 1.5AgCuM(I:3)-400 appearance of small copper ........ AgCuM(1:9)-300 ........ AgCuM(1:9)-400 particles in the range of several ~o. e , ~of nanometers [6] can be observed for CuM-450 sample only. 0.5 ;"~::-"~.. 0.5In distinction from CuM, .v..-.. 9 ~ .~..'r..:.':.':.',:'. AgCuM samples (Fig. 1, c-f) 00 0.0 ......... , .... demonstrate appearance of the 200 400 600 800 200 400 600 800 step-like feature, typical for Wavelength, nm copper-containing mordenites with low Si/A1 ratio [6], starting Figure 1. DRS spectra of AgM (a) and CuM (b) reduced from as low temperature as at different temperatures, and spectra of three AgCuM with different Ag/Cu atomic ratio, reduced at 100, 200, 200 ~ (Fig. l d). The shape 300 and 400 ~ (c, d, e, f, respectively). and position of this feature does
t
. . . . . . . .
572 0.20
not change with temperature of reduction and Ag/Cu atomic ~s a ratio, testifying that the c 0.10. . . . . . ..,.,,, formation process of these small metal particles is not 5 0.05sensitive to the variations of composition and reduction r 200 300 4oo soo ~oo 4oo soo 600 700 800 conditions. From this I.. o | ~AgCuM(3:I)-100 fresh " AgCuM(l:3)-400'fre'sh' observation one can conclude .Q 2.01 - S - AgCuM(3:1 )-100 stored 20 days 0.6 - - - AgCuM(1:3)-400 stored 150 days < t that zeolite matrix play the dominant role in the formation of copper reduced forms. While copper reduction is facilitated by the presence of ool . . . . . . . . o.o silver in the samples, formation 200 300 400 500 600 400 s6o 660 76o 800 of silver reduced species is Wavelength, nm observed for a much more narrow range of the reduction Figure 2. Spectra of AgM- 100 (a), AgCuM(3" 1)- 100 (b), conditions. Only sample CuM-450 (c) and AgCuM(1:3)400 for fresh and stored under ambient condition samples. AgCuM(3:1)- 100 demonstrate the spectrum comparable with the spectra of the reduced AgM- 100 - AgM-300, in spite of much higher content of silver in the former. For samples AgCuM(1:3)- 100 and AgCuM(1:9)- 100, the intensity of both peaks fall down in different way, while for AgCuM(I:9)-100 sample a band at 250 nm characteristic of Cu + ion [6] appears (Fig. l c). Still observed for all three AgCuM samples reduced at 200 ~ the peaks of Ag8 clusters can be seen for AgCuM(3:I)-300 sample only, and are completely absent for samples reduced at 400 ~ (Fig. l f). It is worthy of note the shape and position of Ag8 cluster peaks to be changeless for all Ag/Cu atomic ratios and again conclude that zeolite matrix play the leading role in the formation of silver reduced species. Formation of mixed Ag-Cu species cannot be concluded from the data obtained. The study of the bimetallic particle sols, produced by co-reduction of respective ions, shows ashoulder-like fall-off in the 450-600 nm wavelength region instead of the pronounced absorption bands characteristic of the individual copper and silver particles, indicating the bimetallic nanoparticles formation [36]. In our case, the only features in the spectra are the bands belonging to the individual silver and copper reduced species that can be obtained in the AgM and CuM independently [6, 19]. Oxidation of samples during storage under ambient condition was investigated. Increasing of stability of both copper and silver reduced species was observed (Fig. 2). Transformation of spectrum of AgM-100 after 20 days of storage is much more pronounced than corresponding changes of the AgCuM(3"I)-100 spectrum (Fig. 2a,b). Even higher stabilization is observed for copper particles. Plasma resonance feature after 20 days of storage completely disappears in the spectrum of CuM-450 (Fig. 2c), whereas it is still observable for AgCuM(I:3)-400 sample even after 150 days of storage (Fig. 2d). 2.0
i" 9
~
t
AgM-100 fresh
- - - A g M - 1 0 0 stored 20 days
,
.
.
.
.
.
.
.
"";
- "
-
"
- - -
"
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fresh CuM-450 stored 20 days CuM-450
~
, . . . . . .
,
.
573 4. CONCLUSIONS Mutual influence of Ag and Cu was observed starting from the ion-exchange treatment of mordenite. The degree of ion exchange increased in the case of treatment in mixed solutions. Drastic changes in the conditions of Ag8 cluster formation and their relative stability in the prepared binary AgCu-mordenite samples were observed. The effect of Ag upon the plasmon resonance band (560-600 nm) and other lower-wavelength bands associated with the reduced copper species were well pronounced too. Ag addition assists the stabilization of Cu+-ions and the appearance of small copper particles at significantly lower temperatures compared to pure Cu-mordenite. No evidence of binary alloy particles formations was observed. So, the cross-impact of Cu and Ag ions during reduction treatment leads to the appearance of mixture of definite reduced species of copper and silver. Synergetic effects consist in the changing of the reduction temperature of silver and copper, as well as changing the stability of some reduced species and their relative concentration.
5. A C K N O W L E D G M E N T S The authors are grateful to Drs. V. Gurin and E. Stoyanov for many helpful discussions. Thanks are given to L. Vellegas Martinez for the experimental work and to I. Gradilla, E. Aparicio, E. Flores, J.A. Peralta and C. Gonzalez for the precious technical support. The research reported in this paper was supported by CONACYT, Mexico, through Grants No. 3894P-E and 32118-E.
REFERENCES
1. G. Stucky and J. Mac Dougall, Science, 247 (1990) 669. 2. Yu.A. Alekseev, V.N. Bogomolov, T.B. Zhukova, V.P. Petranovskii, S.G. Romanov and S.V. Kholodkevich, Izv. Akad. Nauk USSR, Ser. Phys., 50 (1986) 418. 3. T. Sun, K. Serf, N.H. Heo and V.P. Petranovskii, Science, 259 (1993) 495. 4. P.L. Dubov, D.V. Korolkov and V.P. Petranovskii, Clusters and matrix isolated cluster superstructures. Ed. House of St. Petersburg State University, St. Petersburg, 1995. 5. V. Gurin, N. Bogdanchikova and V. Petranovskii, Mat. Sci. Eng. C, (2001) in print. 6. V. Petranovskii, V. Gurin, N. Bogdanchikova, A. Licea-Claverie, Y. Sugi and E. Stoyanov, Mater. Sci. Eng. A (2001) in print. 7. A.V. Loginov, V.V. Gorbynava and T.B. Boicova, Russ. J. Gener. Chem. 67 (1997) 189. 8. P. Mulvaney, M. Giersig and A. Henglein, J. Phys. Chem., 97 (1993) 7061. 9. G. De, G. Mattei, P. Mazzoldi, C. Sada, G. Battaglin and A. Quaranta, Chem. Mater., 12 (2000) 2157. 10. S. Link, Z. Wang and M E1-Sayed, J. Phys. Chem. B, 103 (1999) 3529. 11. M. Catalano, E. Carlino and G. De, Phil. Mag. B, 76 (1997) 621. 12. V.I. Parvulescu, P. Grange and B. Delmon, Catal. Today 46 (1998) 233. 13. T. Nakatsuji, R. Yasukawa, K. Tabata, K. Ueda and M. Niwa, Appl. Cat. B, 17 (1998) 333. 14. N. Aoyama, K. Yoshida, A. Abe and T. Miyadera, Cat. Lett., 43 (1997) 249. 15. J.A. Martens, A. Cauvel, F. Jayat, S. Vergne and E. Jobson, Appl. Cat. B, 29 (2001) 299. 16. P. Gilot, M. Guyon and B. R. Stanmore, Fuel, 76 (1997) 507.
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54 (1999) 407. 18. Z. Chajar, V. Le Chanu, M. Primet and H. Praliaud, Catal. Lett., 52 (1998) 97. 19. N. Bogdanchikova, V. Petranovskii, R. Machorro, Y. Sugi, V. Soto and S. Fuentes, Appl. Surf. Sci., 150 (1999) 58. 20. N.E. Bogdanchikova, E.A. Paukshtis, M.M. Dulin, V.P. Petranovskii, Y. Sugi, T. Hanaoka, T. Matsuzaki, X. Tu and S. Shin, Inorg. Mater., 31 (1995) 451. 21. N. Bogdanchikova, V. Petranovskii, S. Fuentes, E. Paukshtis, Y. Sugi and A. LiceaClaverie, Mater. Sci. Engineer. A, 276 (2000) 236. 22. V.S. Gurin, N.E. Bogdanchikova and V.P. Petranovskii, J. Phys. Chem. B, 104 (2000) 12105. 23. R. Najafabadi, D.J. Srolovitz, E. Ma and M. Atzmon, J. Appl. Phys., 74 (1993) 3144. 24. G. Meitzner, G. H. Via, F. W. Lytle and J. H. Sinfelt, J. Chem. Phys., 83 (1985) 4793. 25. T. Klassen, U. Herr and R.S. Averback, Acta Mater., 45 (1997) 2921. 26. C.C. Koch, in Mechanical Milling and Alloying, Ed. by R.W. Cahn, P. Haasen and E.J. Kramer. Materials Science and Technology, Vol. 15, VCH, Vienna, 1991, p. 193. 27. F. Wu, P. Bellon, A.J. Melmed and T.A. Lusby, Acta Mater., 49 (2001) 453. 28. S. Averkiev, L. Agroskin, V. Aleksandrov, V. Bogomolov, Y. Volgin, A. Gutman, T. Zhukova, V. Petranovskii, D. Poloskin, L. Rautian and S. Kholodkevich, Sov. Phys. Solid State, 20 (1978) 251. 29. V. Bogomolov, A. Zadorozhnyi, V. Petranovskii, A. Fokin and S. Kholodkevich, JETP Lett., 29 (1979) 373. 30. V.N. Bogomolov, E.K. Kudinov, T.M. Pavlova and V.P. Petranovskii, JETP Lett., 30 (1979) 382. 31. V.N. Bogomolov, A.I. Zadorozhnyi, L.K. Panina and V.P. Petranovskii, JETP Lett., 31 (1980) 339. 32. V.N. Bogomolov, V.P. Petranovskii, V.V. Poborchii and S.V. Kholodkevich, Sov. Phys. Solid State, 25 (1983) 1415. 33. Yu. Alekseev, V. Bogomolov, T. Zhukova, V. Petranovskii and S. Kholodkevich, Sov. Phys. Solid State, 24 (1982) 1384. 34. B.S. Zadohin, M.F. Limonov, Yu.F. Markov and V.P. Petranovskii, JETP Lett., 52 (1990) 285. 35. J. Ogden, N. Bogdanchikova, J. Corker and V. Petranovskii, Europ. J. Phys. D, 9 (1999) 605. 36. Yu.A. Fedutik, Yu.V. Bokshits and G.P. Shevchenko, in: Physics, Chemistry and Application of Nanostructures, Ed. by V.E. Borisenko, S.V. Gaponenko and V.S. Gurin, World Scientific, Singapore-New Jersey-London-Hong Kong, 2001, p. 287.
~itudles in Surtace Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
575
Preparation, characterization and catalytic properties o f C u P C / Y nano-composite Huaixin Yang, Ruifeng Li*, and Kechang Xie Institute of Special Chemicals, State Key Laboratory of CI Chemistry and Technology, Taiyuan University of Technology, Taiyuan 030024, China
Well-crystallized CuPC/zeolite Y nano-composite of average particle size less than 80nm (denoted as CuPC/nano-Y) has been successfully synthesized via zeolite synthesis route. The nano-zeolite host was confirmed by the XRD and TEM. The CuPC complexes entrapped in the nano-hosts were characterized by XRD, TEM, FTIR, TG/DTA, ICP and UV-vis. CuPC/Y samples with different particle sizes in micro-scale were also synthesized by adjusting the synthesis conditions. Under the mild situation (60~ l atm), CuPC in nano-Y exhibits the catalytic activity 2.1 times higher than CuPC in normal Y with particle size of 500nm for the liquid phase oxidation of cyclohexane using aqueous H202 (30wt. %) as the oxidant.
1. INTRODUCTION Metal phthalocyanines are a fascinating class of molecular complexes containing a central metal ion surrounded by a macrocyclic system and famous for their well-known photocatalytic and electrocatalytic properties [1-5]. The preparation of zeolite encapsulated metal phthalocyanines and its interesting catalytic, photocatalytic and electrocatalytic properties have been widely studied by different research groups [6-14]. Nano-crystalline "ship-in-the-bottle" materials are of a considerable advantage for carrying out various spectroscopic and electrochemical studies on zeolite-entrapped organic species, and more efficient for catalytic and photochemical reactions [ 15-19, 10]. This is due to their small size, stronger electrochemical signals, larger surface area, shorter diffusion pathways and higher surface-to-volume ratios. There are three main approaches to the preparation "ship-in-the-bottle" materials, namely the flexible ligand method, the template synthesis method and the zeolite synthesis method. For the preparation of nano-crystalline "ship-in-the-bottle" materials, the zeolite synthesis method provides two advantages over the other two methods. The nature of the intrazeolite species is well defined [8] and the possibility of causing frame degradation during the complexation could be avoided [ 19]. In this paper we report the synthesis of CuPC inside the cages of nano-zeolite Y by zeolite synthesis method and investigate their relative catalytic performance in cyclohexane oxidation.
576 2. EXPERIMENTAL
2.1 Sample preparation Water glass ([OH]=3.9M, [SIO2]=6.2M), sodium aluminate aqueous solution ([OH] = 10.0M, [A1203]=2.2M), aluminum sulfate and sodium hydroxide (fine-reagent grade), H2SO4 (98wt.%) were used as original materials. CuPC/nano-Y was synthesized by following the procedure: The initial solution of aluminosilicate gel was prepared by adding sodium silicate to an aqueous sodium aluminate solution stoichiometrically in 16Na20: A1203" 13SIO2: 320H20. The solution was shaken until a clear solution was obtained. This was left to age at 68 ~ for 4h, then cooled to room temperature. Other aluminum sulfate solution, H2SO4 solution and CuPC were added into this solution and homogenized by a mixer. The reaction composition, expressed as molar ratio, is 9Na20: A1203" 8.5SIO2: 230H20" 0.15CuPC. The mixture was then sealed into a stainless-steel autoclave (100-150ml capacity). The crystallization was carried out under autogeneous pressure at 90~ in an oven for 30h. The nano-Y with particle size of ca.80nm was synthesized as the above, except that no CuPC was added into the reaction mixture [20]. The CuPC/Y of particle size 500 nm has also been synthesized as following literature [21], denoted as CuPC/NY .The as-synthesized solid materials were filtered, washed and dried at 900C overnight, followed by extraction with acetone, pyridine and then acetone using Soxhlet technique until the solvent becomes colourless.
2.2 Sample characterization X-ray powder diffraction pattems (XRD) of the products were recorded by a Rigaku D/max-2500 diffractometer with a CuK~t radiation (Ni filtered). The crystal size was calculated from the peak broadening by Scherrer's equation using NIST 640A silicon as a standard by Jade 5.0. The Si/A1 ratio were also determined by X-ray diffraction, according to the following relationship: SiO2/A1203=(25.858-a0)/(a0-24.191). The morphology and crystallite size of the sample were examined with Hitachi-9000NA electron microscope operated at 250kv, in which the samples were dispersed on Cu grids coated with holy-carbon support files. The thermogravimeteric analysis (TG) and differential thermal analysis (DTA) in flowing air at 30ml/min were conducted at a heating rate of 10~ /min with a Dupont instrument series 99 thermal analyzer. The FTIR framework vibration spectra of the sample were recorded by a 1760X Perkin-Elmer FTIR spectrometer. The diffuse reflectance spectra (DRS) of samples were recorded on a Perkin-Elmer Lamda/bio 40 UV-vis spectrometer.
2.3 Catalytic test Oxidation of cyclohexane as a model reaction was carried out at 60~ for 8 h with stirring in a batch reactor (50ml), which contains 0.1 g CuPC/Y, 2mmol cyclohexane, 10 ml acetone as a solvent, and 2mmol 1-1202 was used as an oxidant. The products were analyzed by gas chromatography (GC-9A) with a flame ionization detector.
3. RESULTS AND DISCUSSION Fig. 1 gives the XRD patterns of the as-synthesized nano-Y and CuPC/nano-Y composite. They show the typical X-ray diffraction patterns identified as the Y-topological structure, with
577 the characteristic broadening of the lines typical in microcrystalline material. Based on Scherrer's equation, an average crystallite size of 45 __+2,5nm is calculated for the CuPC/ nano-Y composite from the change in bandwidth of the powder diffraction peaks by jade 5.0 programme. Quayle and co-workers reported that the formation of a large transition-metal complex ion in the supercage of zeolite Y could be confirmed by the change in the relative intensities of the peaks 220 and 311 reflections in the XRD pattern. Here I220>I311 is for zeolite Y and 1220
5
15
25
35
2 0 / ( ~) Fig. I.XRD patterns of the as synthesized nano-Y and CuPC/nano-Y composite (a) CuPC/nano-Y composite, (b) nano-Y The XRD analysis also indicate that the as synthesized CuPC/nano-Y composite has a Si/AI ratio of ca. 1.6, which suggests that the framework around the guest molecule CuPC is faujasite-Y. The analysis with ICP of the composite indicates that 0.44 wt.% of the as-synthesized product attributable to the confined guest, corresponding to ca. one MP complex for every 32 supercages. The TEM of the as-synthesized product (Fig. 2a) indicate the presence of well-defined crystals of 80nm without any patches of phthalocyanine complexes overlaid on their external surface after the strict extraction. The XRD, ICP and TEM results show that the nano zeolite Y is synthesized in the presence of the CuPC complex, the copper complexes are inside the zeolite cavities and not on the external surface of the zeolites. The successfully encapsulation of CuPC in nano zeolite Y can be further evidenced by FTIR spectroscopy, DRS spectroscopy and TG/DTA measurements. Framework FTIR spectroscopy The FTIR lattice vibrations of the extracted as-made sample shows that the as-synthesized sample is zeolite Y and has highly crystallization. It can be found that although some bands in low-wavenumber region attributed to the encapsulated complexes are overlapped by the
578
i: b i ~ i ~ ~ m ~-
~:.~ ~ ~
.
.
.
~~!
84
.
i
Fig.2. TEM imagines of the as synthesized samples at different crystallization conditions: (a) Aging temperature of the initial solution 68"(2, H20/SiO2 = 28(denoted as CuPC/nano-Y) (b) Aging temperature of the initial solution 68*(2, H20/SiO2=l 10 (denoted as CuPC/Y 100) (c) Aging temperature of the initial solution 35 ~ H20/SIO2=28 (denoted as CuPC/Y200) (d) Aging temperature of the initial solution 35*(2, H20/SiO2=110 (denoted as CuPC/Y450)
.r
\ \ (a)
/
0
\
I
I
I
I
200
400
600
800
Temperature (~ Fig.3. DTA curves of the as- synthesized CuPC/nano-Y and physical mixture (a) Physical mixtures of nano-Y and CuPC, (b) CuPC/nano-Y
579 framework vibration bands of zeolite matrix, the typical bands of the complexes in the range from 1200 to 1600 cm 1 can be clearly distinguished. These structure sensitive bands may be contributed to the C=C and C=N vibrations of the phenyl [ 12-13, 20]. The notable red shift are observed for bands corresponding to the introazeolitic Cu-PC, which could be accounted for by a change in molecular symmetry of CuPC in the encapsulated state. These results are in concurrence with the VU-vis data of these materials. Similar red shifts have also been reported for other intrazeolitic complexes [10] and also CuPC encapsulated in the zeolite Y of normal particle size [12-13]. Thermal analysis In order to understand the thermal stability of the nano-zeolite and complex influenced by the host/guest interaction, the as synthesized sample has been analyzed with TG/DTA. The DTAcurve of the CuPC/nano-Y composite shows the shift of the exotherm corresponding to the complexes combustion towards higher temperature compared with the physical mixture of CuPC and nano-Y. Since it could be expected that the encapsulated complex will be somewhat thermally stabilized, the observed shift can also be a proof for the encapsulation CuPC in nano zeolite Y. Exothermic peaks occurred about 850~ corresponding to the collapse of the crystal structure also show that stability of zeolite vastly increased. This is because that CuPC as space-filling agents plays important roles in crystallization of zeolites [ 11,14], it is helpful for the crystallization and stability of products. Ultraviolet spectroscopy Phthalocyanine exhibits two characteristic ligand based a - a * transitions referred to as the Q bands in the visible and near IR regions. The material under consideration exhibits these
(a)
...-"
II 300
I 400
I I I 500 600 700 800 Wavelength (nm) Fig. 4. DRS spectra of pjysical mixture and the samples after extraction (a) CuPC/nano-Y, (b) CuPC/NY, (c) Physical mixture of CuPC
580 bands in the region 550-760nm. Fig.4 shows the DRS spectra of CuPC/nano-Y sample, CuPC/NY and physical mixture of CuPC and nano-Y. There is an obvious red shift of the Q bands in the UV-vis electronic spectra of the copper complexes on encapsulation compare to physical mixture. This indicates that zeolitic environment is enhancing the electron transfer processes probably through a distortion of the molecule from planarity, which have been recorded by literature [6-14]. These adsorption thresholds are shifted towards longer wavelengths, as a consequence of the decreased size of the crystallites. Effects of particle size Several CuPC/Y materials of different particle sizes from 450-70nm with similar Si/AI ratio have also been synthesized by adjusting the temperature and water content in
Fig. 5. XRD patterns of the as-synthesized CuPC/Y samples of the different particle sizes (a) CuPC/nano-Y, (b) CuPC/YI00, (c) CuPC/Y200, (d) CuPC/Y450 this system. A list of as synthesized CuPC/Y samples, with each one's chemical composition and average particle size, is presented in Table I. The TEM imagines of the as-synthesized prodticts are shown in Fig. 2. It can be seen that particle sizes of the final products decrease as the result of the increasing of the aging temperature of the initial solution and also the reducing water content of the reaction mixture in the system. It also shows that the loading of the CuPC increases in the sample as the water content in the reaction mixture increases. That maybe because that the zeolite synthesis medium contained more dissolved phthalocyanine complexes which could be possible to be encapsulated in the matrix of the final products [ 1 l]. The integrity of the as synthesized zeolite hosts of different particle size are confirmed by the XRD and TEM. XRD, TEM, UV-vis, and TG/DTA also verify the encapsulation of the Cu-PC complex in the framework of the hosts. Fig. 5 gives the XRD profiles of the CuPC/Y materials of different particle sizes. It illustrates that XRD lines of the as-synthesized samples broaden as the result of the decreased crystal size. Two remarkable features are revealed in DRS spectra and TGA curves. The Q bands of the encapsulated copper complexes of the DRS spectra shift towards longer
581
wavelengths and the exothermic peaks corresponding to the complex combustion in the DTA curve also shift slightly towards lower temperature, as a consequence of the decreased size of the crystallites. The catalytic properties of the host/guest compounds were tested by oxidation of cyclohexane using 30% H202 as oxidant in acetone. The results of the catalytic experiments are depicted in table2. It can be seen that the TOF for catalysts with a similar Cu contents gets higher, as the host particle size decreases. This is due to the bigger surface area and shorter diffusion pathways.
Table 1 The compos!tion and TEM averag e parti...cle size of the as s.ynthesized .cupc/Y Zeolites H20/SiO2 Aging SIO2/A1203 Cu content TEM average temperature/~ /wt.% particle size/nm CuPc/Y 100 CuPc/Y400 CuPc/nano-Y CuPc/Y200
.
.
.
.
.
.
.
110 110 28 28 .
.
.
.
.
68 35 68 35
.
.
.
.
.
.
.
3.34 3.20 3.61 3.52 .
.
.
.
.
.
0.444 0.315 0.091 0.089 .
.
.
100 450 70 200
Table 2 Catalytic results of CuPC / zeolite Y of different particle sizes in cyclohexane oxidization Catalyst
Cu(wt.%) '
TOF
NaY-
-
-
CuPC/Y450
0.315
30
CuPC/Y 100
0.444
45
CuPC/NY
0.086
51
CuPC/Y200
0.089
75
CuPC/70
0.091
100
Reaction conditions: catalyst weight = 0.1 g, reaction time = 10 h; temperature - 60 ~ Cyclohexane = 2 g ; cyclohexane 9H202 (mol:mol) 1; solvent = acetone 10 ml; TOF =turnover frequence; moles of the converted cyclohexane per mole of metal in catalysts
4. CONCLUSION This work has resulted in the synthesis of a well-crystallized CuPC/zeolite Y nanocomposite of average particle size less than 70nm via zeolite synthesis method. In this way, CuPC complex has been successfully immobilized in zeolite Y without damaging the integrity of the host and causing impurity. The XRD shows the relative peak intensities of 220 and 311 reflections change as the result of accommodation of a large transition-metal
582 complex ion in the supercage of zeolite Y. The TG/TDA shows that stability of zeolite as well as the encapsulated complexes vastly increases. UV-vis shows that Q bands are shifted towards longer wavelengths, as a consequence of the decreased size of the crystallites. In addition, Cu-PC/Y samples with different particle sizes from 70-450nm have also been synthesized by adjusting the temperature and water content. CuPC in nano-Y exhibits the catalytic activity 2.1 times higher than CuPC in normal Y with particle size of 500nm for the liquid phase oxidation of cyclohexane due to the bigger surface area and shorter diffusion pathways.
5. ACKNOWLEDGEMENTS We thank Professor Dr. Jianqi Li of the Institute of Physics, the Chinese Academy of Science for TEM characterization and the National Natural Science Foundation of China (29973028) for fmancial support of this work.
REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
A. Harriman, G. Porter, M.G. Richol, J. Chem. Soc., Faraday Trans., 277 (1981) 1175. H. Schultz, H. Lehman, M. Rein, M. Hanack, Structure and Bonding, 74 (1991) 205. R. Guilard, C. Lecomte, K.M. Kadish, Structure and Bonding, 64 (1987) 205. S. Methitsuk, M. Ichikawa, T. Tamoru, J. Chem. Soc.,Chem. Commun., 1974, p. 158. K. Hiratsuka, K. Takushaski, G. Susaki, S. Toshima, Chem. Lett., 1979, 305. G. Mayer, W.D. Mohn, E. Schulz, Zeolites, 4 (1984) 13. K.J. Balkus Jr., J.P. Ferraris, J. Phys. Chem., 94(1990) 8019. K.J. Balkus Jr., A.G. Gabrielov, S.L. Bell, Inorg. Chem., 33(1994) 67. F.P. Ruby, M.J. Venkelecom, A. Cselman, C.P. Bezoukhanova, J.B.Vytterhoeven, P.A. Jacobs, Nature, 370 (1994) 541. M.P. Vinod, T.Kr. Das, A.J. Chandwadkar, K.Vijayamohanan, J.G. Chandwadkar, Materials Chemistry and Physics., 58 (1999) 37. R. Raja, P. Ratnasamy, Catal. Lett., 48 (1997) 1. E. Armengol, A. Corma, V. Fomes, H. Garcia, J. Primo, Appl. Catal. A, 181 (1999) 305. S. Seelan, A.K. Sinha, D. Srinivas, S. Sivasanker, J. Mol. Catal. A, 157 (2000) 163. S.S. Shevade, R. Raja, A.N. Kotasthane, Appl. Catal. A, 18 (1999) 243. F. Bedioui, J. Devynck, K.J. Balkus, J. Phys. Chem., 100 (1996) 8607. D.R.Rolison, C.A. Bessel, M.D. Baker, C. Senaratne, J. Zhang, J. Phys. Chem., 100 (1996) 8610. K. J. Balkus Jr., A.K. Khanmamedova, K.M. Dixon, F. Bedioui, Appl. Catal. A, 143 (1996) 159. S. Ernst, H. Disteldorf, X. Yang, Microporous and Mesoporous Mater., 22 (1998) 457. N.B. Castagnola, P.K. Dutta, J. Phys. Chem. B, 102 (1998) 1096. H.X. Yang, R.F. Li, B.B. Fan, K.C. Xie. Stud. Surf. Sci. Catal., 105(2001) 187. B.B. Fan, R.F. Li, W.B. Fan, J.H. Cao, B. Zhong, Acta Petrolel Sinica (Petroleum Processing Section) 16 (2000) 12. W. H. Quayle, J. H. Lunsford, Inorg. Chem., 21 (1982) 97. W. H. Quayle, G.Peeters. et al., Inorg. Chem., 21 (1982) 2226.
Studies in Surface Science and Catalysis 141 A. Sayariand M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
583
Environmental applications o f self-assembled monolayers on m e s o p o r o u s supports ( S A M M S ) Glen E. Fryxell, Yuehe Lin, Hong Wu Pacific Northwest National Laboratory Kenneth M. Kemner Argonne National Laboratory
The field of mesoporous ceramics has provided a versatile foundation upon which to create high efficiency environmental sorbents. These nanostructured materials provide a huge amount of surface area in a very small volume. Self-assembled monolayers provide a simple and direct method of chemically derivatizing ceramic oxide surfaces. The marriage of mesoporous ceramics with self-assembled monolayer chemistry has created a powerful new class of environmental sorbent materials. These nanoporous hybrid materials are highly efficient sorbents, whose interfacial chemistry has been fine-tuned to selectively sequester a specific target species, such as heavy metals (e.g. mercury, cadmium and lead), and oxometallate anions (e.g. chromate, arsenate). Details addressing the design, synthesis and characterization of SAMMS materials specifically designed to sequester radionuclides of importance to the post-Cold War DOE clean-up (such as plutonium and cesium), as well as evaluation of their binding properties are discussed.
1. INTRODUCTION The surfactant templated synthesis of nanoporous ceramics has generated significant interest in the application of these materials as catalysts, sensors and sorbent materials. The basic morphology associated with hexagonal mesoporous silica (e.g. MCM-41) provides exceptionally high surface area in a very small volume. Installation of a covalently anchored self-assembled monolayer across all of the pore surfaces allows for the creation of a highly selective chemical interface through the mesoporous matrix. Thus, self-assembled monolayers on mesoporous supports (SAMMS) came into being, the result of three successive generations of molecular self-assembly (aggregation of the surfactant molecules to form the micelle being the first generation, aggregation of the silicate metathesized micelles into the mesostructured greenbody being the second, and coating the nanoporous ceramic with an organized monolayer being the third) [ 1]. SAMMS have proven to be superior sorbent materials for the selective sequestration of environmentally problematic chemical species such as mercury, arsenic and chromium [2-4]. The high surface area of the mesoporous ceramic backbone
584 provides a support capable of high loading capacity. The dense monolayer coating fulfills the promise of that high capacity. The close proximity of the binding sites allows for multi-ligand chelation of the target ion in the monolayer interface, enhancing the binding affinity, as well as the chemical stability of the metal laden adduct. The open pores structure allows for facile diffusion into the mesoporous matrix, allowing for fast sorption kinetics. The rigid ceramic backbone provides robust physical stability, as well as preventing pore closure due to solvent swelling (a problem commonly encountered with polymeric ion exchange resins). Combining all of these attributes in a single sorbent design concept, results in SAMMS having unprecedented binding affinities, excellent chemical selectivity and exceptionally fast binding kinetics.
Figure 1. The genesis of self-assembled monolayers on mesoporous supports (SAMMS)
2. MATERIALS AND METHODS 2.1 Materials
MCM-41 was prepared as described previously [1 ]. 3-aminopropyl trimethoxysilane and (2-aminoethyl)-3-aminopropyl trimethoxysilane were obtained from United Chemical Technologies. The acetamidephosphonate ("Ac-Phos") diethyl ester was prepared in straightforward fashion by treating diethylphosphonoacetic acid with thionyl chloride, followed by excess trifluoroethanol. After distillation, the desired trifluoroethyl ester was obtained 58% yield. Exposure of trifluoroethyl ester to neat 3-aminopropyltrimethoxysilane resulted in a moderate exotherm and the clean production of the desired Ac-Phos silane. The self-assembled monolayers were deposited in MCM-41 (60-65A pores, 880-900 m2/g) by carefully prehydrating the ceramic with approximately 2 to 2.5 monolayers worth of water (approximately 1.6 mL of water for each 5.0 gof MCM-41), followed by treatment with a slight (ca. 10-15%) excess of silane (relative to available surface area, and the mixture held at toluene reflux for 4-6 hours. The mixture was then cooled to ambient temperature, filtered
585 and the collected solid washed copiously with 2-propanol. The SAMMS were then air-dried to constant weight in a a fume hood, and characterized using solid state 29Si and ~3C NMR, BET, EXAFS and XPS. The phosphonic ester was cleaved post-deposition using trimethylsilyl iodide in anhydrous acetonitrile for 18 hours at ambient temperature, followed by the addition of excess water and stirring for an hour. 2.2 Measurements NMR. Solid-state 29Si and 13C NMR spectra were determined at 19.944 and 25.2458
MHz, respectively using a Chemagnetics CMX-100 NMR quadruple channel spectrometer system. The probe was a 7-mm pencil-type probe with magic angle spinning at 4 kHz. 4microsecond 90-degree pulses were utilized for both 29Si and 13C, with proton decoupling power at 62.5 kHz. 29Si spectra were acquired using the Freeman-Hill T1 (tlfh) pulse sequence with a short recovery time, 50 microseconds, to suppress acoustic ringing. Pulse delays of 60 seconds were employed. Samples were prepared by loading a spinner with a Teflon spacer followed by c a . 65 mg of sample, 15 mg of tetrakis(trimethylsilyl)silane (TTMS), followed by an additional 65 mg of sample and a sample end spacer. Chemical shifts were referenced to internal TTMS. BET. The N2 adsorption/desorption isotherms were obtained from a Quantachrome BET instrument (Autosorb-6). The Brunauer-Emmett-Teller (BET) surface area of the pure silica is 886 m2/g, and the BET surface of the functionalized silica is approximately half this value, depending upon the degree of coverage. The pore size is estimated from the desorption branch, using a standard Barrett-Joyner-Halenda (BJH) method. ICP-MS. Solution metal concentration were determined using ICP-MS, on a Hewlett Packard HP4500. Solutions were acidified using either HC1 or HNO3. Radioeounting. 239pu (IV) aliquots were suspended in Ultima Gold (Packard Instrument, Meriden, Connecticut) scintillation cocktail, and alpha activities were measured using a 2550 TR/AB liquid scintillation counter (Packard Instruments). XPS. X-ray photoelectron spectroscopic (XPS) characterization of the SAMMS materials were made on a Physical Electronics Quantum 2000 Scanning ESCA (electron spectroscopy for chemical analysis) Microprobe. This system uses a focused monochromatic A1 Ka x-rays (1486.7 eV) source for excitation and a spherical section analyzer and has a 16 element multichannel detection system. 2.3 Methods
Solution batch contacts were performed by suspending a weighed amount of SAMMS solutions of the target species, and shaking the mixtures for 1-4 hours. Control experiments revealed that equilibrium was reached in less than 30 minutes, so these results represent true equilibrium conditions. The solution/solid ratio was varied from 100 to 5000 to explore the effect on binding efficiency. Analyte concentration, pH, and ionic strength were systematically varied. Experiments were also performed in which competing ions and complexants were included in the mrlange. After exposure, the mixture was filtered through a 0.2 micron filter and the supernatant subjected to ICP-MS analysis for the Cs samples and routine radiocounting methods for Pu. Kinetics were run by removing aliquots at 1 minute intervals. Cs laden SAMMS were subjected to XPS analysis.
586 3. RESULTS AND DISCUSSION
Actinide cations are large, "hard" Lewis acids and in recent years solution extraction methods aimed at actinide separations have been commonly employed the "CMPO" ligands. [5] We have incorporated the CMPO ligand design concepts into a mesoporous framework by combining an amide carbonyl with an adjacent phosphonate, resulting in Ac-Phos SAMMS. Both the ester and phosphonic acid forms of these SAMMS have proven to be highly efficient Pu (IV) sequestrants, as can be seen from the accompanying table of distribution coefficients (a distribution coefficient is a mass-weights partition coefficient, so the higher the K,l value, the more effective the sorbent is at removing the target species; a Kd value above 500 is considered good, one over 5000 is excellent and above 50,000 is exceptional).
Table 1. Ac-Phos SAMMS Distribution Coefficient (Ka) Values in the Presence of Competing Species Kd (Pu) Ac-Phos Kd (Pu) Ac-Phos Competitor Concentration acid form diethyl ester None 21206 10865 Fe(III) 100 ppm 21210 6029 AI(III) 100 ppm 28770 9923 Zr(IV) 100 ppm 20483 9691 Ni(II) 100 ppm 20477 11172 Ca(II) 100 ppm 19622 10037 Mn(II) 100 ppm 17762 8363 Mo(VI) 100 ppm 18933 8611 Cu(II) 100 ppm 18846 9299 Pb(II) 100 ppm 19940 11911 Cr(III) 100 ppm 18319 11326 Hg(II) 100 ppm 16281 10089 Phosphate 0.01M 20406 10457 Sulfate 0.01M 19847 10610 EDTA 0.01M 20459 59 Citrate 0.01M 23116 11583 [Pu] = 2000 cpm/mL 1M NaNO3 0.1 MHNO3 0.10 g SAMMS in 10mL
As can be seen from the data in Table 1, the acid form of Ac-Phos SAMMS displays excellent Pu selectivity, with very little or no competition from a wide variety of transition metal and alkaline earth cations. However, the ester form of Ac-Phos SAMMS does not bind Pu (IV) as strongly and experiences modest competition from ferric ion, but little else from other metal cations. Complexants can also provide competition by wrapping up the Pu (IV) cation and preventing it from being chelated by the sorbent. Even a powerful complexant like EDTA
587 does not compete with the binding of Pu by the acid form of Ac-Phos SAMMS (however, the neutral ester ligand is completely out-competed for the Pu (IV) cation by EDTA). This extremely high affinity between the Ac-Phos phosphonic acid and Pu (IV) can be explained by the close proximity of the silanes to one another within a self-assembled monolayer. This closeness allows multiple ligands to chelate the metal cation, thereby providing an even greater driving force for the "macromolecular chelation" of the metal than does EDTA. EXAFS studies of similar SAMMS materials laden with Eu (III) (an Am (III) mimic) clearly show that the Eu (III) cation is 8-coordinate and bound to four chelating ligands in either a square prism or square antiprism geometry. Thus, we believe that four (and possibly more) Ac-Phos phosphonic acids are chelated to the Pu (IV) cation in the Pu-laden SAMMS. Both the ester and acid form of Ac-Phos SAMMS were found to effective sorbents for Pu (IV). The phosphonic acid SAMMS was found to be exceptionally rapid at sequestering Pu, with equilibrium being reached in less than a minute. Clearly, diffusion into the mesoporous matrix is not a limiting factor in this chemistry. These sorption kinetics are very similar to what we've observed in our other studies looking at SAMMS as environmental sorbents.
Figure 2. Sorption kinetics of Pu (IV) by Phosphonic Acid SAMMS.
Initial forays into SAMMS design and synthesis focused on the incorporation of ligand fields into the SAMMS monolayer interface, and the use of those ligands to bind metal cations. However, the metal laden SAMMS are perhaps even more valuable as sorbents than are the original SAMMS themselves. This is because traditional anion exchange methods have centered around polymer-bound quaternary ammonium salts, which allow for little (if any) interaction between the cation and anion. As a result, the binding affinity and selectivity afforded by these materials is marginal, at best. Once the metal cation is installed in the monolayer interface, then anions can plug directly into the metal "socket", and chemical selectivity is possible by tuning the properties of the metal complex to match those of the
588 anion. For example, the Hg adduct of the thiol SAMMS forms an excellent sorbent for the removal of radioiodine [6]. Once again, this is an example of the pairing of a soft Lewis acid with a soft Lewis base, resulting in excellent chemical selectivity.
"~N~c{S'"'N.---J -~n
/OISi~ i~ Si--O C+2 /O-~~1~ ix Si--O
l,...... S / Fe(CN)6
r--4 /O~.!S ( !~ ---O\
Figure 3. Copper (II) E D A - Ferrocyanide SAMMS Additional chemical complexity can be built into the SAMMS interface by incorporating a stereospecific receptor to bind anions of a specific shape. We have accomplished this through the installation of cationic transition metal complexes on the pore walls of MCM-41 [7,8]. One such complex is the octahedral copper (II) ethylenediamine (Cu-EDA) species, which has C3 symmetry, and is dimensionally ideal for binding tetrahedral oxometallate
10000
1000 ..0 (:3. r'-
.o
100
i_ t,-
8 to
tO 0
v
_A
v A
0.1
20
40
60
80
1O0
120
Time (rain)
Figure 4. Kinetics of Cs sorption by Cu-EDA-Ferrocyanide SAMMS.
140
589 anions, such as chromate and arsenate [7,8]. In addition, it is possible to bind octahedral anions as well. Cesium is known to form insoluble ferrocyanide salts. Thus, we used similar methodology to bind the octahedral complex anion ferrocyanide (Fe(CN)6)4- to the Cu-EDA monolayer interface This results in a complex Cu-EDA-ferrocyanide anionic adduct that has proven to be an excellent material for sequestering cesium. Binding affinities are exceptionally high for Cs. All of the binding affinities we have measured to date have been in excess of 100,000, even in the presence of huge excesses of Na and K [9]. Analysis by XPS reveals virtually complete metathesis of Cs for Na in the ferrocyanide SAMMS, indicating a highly efficient ion exchange process.
4. CONCLUSIONS Thus, by changing the interfacial chemistry lining the pores of MCM-41 we are able to create wide diverse chemical specificities in SAMMS sorbent materials. The high surface area of the MCM-41 provides a foundation with the potential of a very high loading capacity. The high functional density of the monolayer allows this high loading capacity to be realized. Self-assembled monolayer chemistry allows for the ready installation of a wide variety of chemical interfaces, and therefore excellent chemical selectivity. The rigid open pores structure of MCM-41 makes diffusion into the porous matrix facile and rapid, thereby allowing for fast sorption kinetics. We have taken this combination of desirable attributes and joined them to create a new class of environmental sorbent materials that are important to the post-Cold War clean-up. In this capacity, SAMMS provides unprecedented capability for the rapid and selective sequestration of heavy metals, oxometallate anions and radionuclides.
5. ACKNOWLEDGEMENTS This work is supported by the Environmental Management Science Program, Office of Science, of the US Department of Energy (USDOE). Pacific Northwest National Laboratory (PNNL) is operated by Battelle Memorial Institute for the US DOE. The research described in this paper was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE's Office of Biological and Environmental Research and located at PNNL.
REFERENCES
1. 2. 3. 4.
X. Feng, G. E. Fryxell, L. Q. Wang, A. Y. Kim, J. Liu, Science, 276 (1997) 923. S.V. Mattigod, X. Feng, G. E. Fryxell, J. Liu, M. Gong, Separat. Sci. & Tech., 34 (1999) 2329. K. Kemner, X. Feng, J. Liu, G. E. Fryxell, L. Q. Wang, A. Y. Kim, M. Gong, S. V. Mattigod, J. Synch. Radiat., 6 (1999) 633. G.E. Fryxell, J. Liu, Designing Surface Chemistry in Mesoporous Silica in Adsorption at Silica Surfaces, edited by Eugene Papirer, Marcel Dekker, (2000) 665.
590 5. 6. 7. 8. 9.
N.C. O'Boyle, G. P. Nicholson, T. J. Piper, D. M. Taylor, D. R. Williams, G. Williams, Appl. Radiat. Isot., 48 (1997) 183. S.V. Mattigod, G. E. Fryxell, R. J. Seine, K.E. Parker, and F. M. Mann, Radiochimica Acta, (2001) in press. G.E. Fryxell, J. Liu, M. Gong, T. A. Hauser, Z. Nie, R. T. Hallen, M. Qian, K. F. Ferris, Chem. of Mater., 11 (1999) 2148. S. Kelly, K. Kemner, G. E. Fryxell, J. Liu, S. V. Mattigod, K. F. Ferris, J. Phys. Chem., 105 (2001) 6337. Y. Lin, G. E. Fryxell, H. Wu, M. Englehard, Environ. Sci. & Tech., 35 (2001) 3962.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec(Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
591
A possible use of modified mesoporous molecular sieves in water treatment processes Izabela Nowak, Barbara Kasprzyk, Maria Ziolek and Jacek Nawrocki
A. Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, PL-60-780 Poznan, Poland; e-mail: ziolek@amu,edu.pl
Mesoporous molecular sieves of MCM-41 type (siliceous, aluminosilicate and niobosilicate) modified with perfluoroacid and perfluorohydrocarbons were tested in the degradation of organic molecules. They appeared as useful adsorbents/catalysts for the removing of toluene, chlorobenzene and cumene from water.
I. INTRODUCTION Among the methods currently applied for the removal of organic impurities from water, the destructive oxidation with ozone or hydrogen peroxide belongs to those intensively studied. They are often assisted by adsorption onto porous solids, which can also play a role of the catalysts for the oxidation process [1]. Cooper and Burch [2] were the first (according to our knowledge) who studied mesoporous materials of M4IS family as alternative adsorbents and as potential catalysts for the catalytic ozonation process for portable water treatment purpose. They used siliceous and aluminosilica mesoporous molecular sieves and they have found very high adsorption capacities of these materials for the model compounds: cyanuric acid and pchlorophenol. In this study, we used NbMCM-41 (niobosilica synthesized according to [3]) mes0Porous molecular sieves besides aluminosilica (AIMCM-41) one as sorbents and catalysts in the ozonation of the model compounds: toluene, chlorobenzene and cumene. A choice of Nbcontaining material was approved by the earlier discovery of their high activity in the oxidation of organic sulfur compounds and cyclohexene with hydrogen peroxide [4]. Both NbMCM-41 and AIMCM-41 were treated with NH4§ cations followed by deamonation in the purpose of hydroxyl group generation. These samples as well as parent materials were modified with perfluoroacid and perfluorohydrocarbons, respectively, because, recently, two of us discovered a high activity of alumina modified with those compounds in the ozonation of organic materials [5]. It was known that perfluorinated hydrocarbons are considered as having 10 times better ozone solubility than water and they provide ozone with a higher level of stability [6,7].
592 2. EXPERIMENTAL
2.1. Synthesis and modification Niobium and aluminum were incorporated into the mesoporous molecular sieves of MCM41 type during the synthesis carried out due to the description presented in [2,3]. H-TMCM41 (where T=A1 or Nb) were prepared by using conventional ion-exchange procedure, i.e. stirring of the solid in aqua solution of NH4CI (0,1 M, pH 6-6,5) for 8 h. The prepared materials were recovered by filtration, washing with water and drying at 344 K for 2 h and then were calcined in helium flow at 573 K for 3 h (with a slow increase of the temperature of 3 K minl). The following catalysts were used (the last number in the catalysts symbols means Si/T ratio) for the modification with perfluoroacid (PFA): H-AIMCM-41-16; H-AIMCM-41-32; H-NbMCM-41-16 and H-NbMCM-41-32. The final samples were obtained as a result of a reaction carried out between perfluorooctanoic acid and above mentioned materials in aqueous solution for 4 h at a temperature of 333 K. Moreover, PFC/NbMCM-41-32 and PFC/AIMCM-41-16 were produced by the adsorption of Fluorinert product, perfluorinated hydrocarbons (PFC) FC-77, onto NbMCM-41-32 and AIMCM-41-16 surface at 343 K. Extraction with hexane was provided in order to remove weakly bonded hydrocarbons from the catalyst surface. The alkyl group surface coverage, assessed by means of elemental analysis, is presented in Tab. 1. 2.2. Sample characterization
Powder X-ray Diffraction (XRD). XRD patterns were recorded using a TUR 42 powder diffi,actometer with Cu K~ radiation (10 kV, 40 mA) and a step size 0,02 ~
Nitrogen Sorption. Nitrogen adsorption/desorption isotherms were measured at 77 K with a Micromeritics ASAP 2010 instrument. The samples were first outgassed at 573 K (383 K for samples containing perfluoroacids) for 3 h.
Thermogravimetric and Differential Thermal Analysis. Thermal Analysis was performed by simultaneous TG-DTA measurements in flowing helium using the SETARAM thermobalance TG-DTA 9. Approximately 10 mg of the samples were heated in a helium flow to 1273 K at a heating rate of 5 K min~ in platinum sample holders. Fourier-Transform Infrared Spectroscopy ~TIR). Infrared spectra were recorded with a Vector 22 (BRUKER) FTIR spectrometer. The samples were compressed, under low pressure, into self-supporting discs (5-10 nag cm"2) and placed in a special vacuum cell where they underwent evacuation at various temperatures under vacuum. Spectra were recorded at RT after activation at each temperature and the background spectrum was subtracted. Moreover, spectra of samples containing perfluoroacids and perfluorocarbons were measured by diluting to 1 wt. % in KBr.
Model compounds. A model compounds solution prepared by dissolving of 1,7 mg of toluene, 1,7 mg of cumene and 2,2 mg of chlorobenzene in 11 of distilled water was used in adsorption and ozonation experiments.
Adsorption experiments. Adsorption experiments were conducted in a closed reactor of 200 ml volume. 190 ml of model compounds solution was continuously stirred for 4 hours in the presence of 0,5 g of the dehydrated catalyst (443 K, vacuum, 4 h). The residual concentration of organic compounds present in the solution was measured every hour by means of GC-FID
593 analysis (Varian 3000, CW20M column). Liquid-liquid extraction of organic substances with hexane was carried out before chromatography analysis.
Ozonation experiments. Ozonation experiments were performed at room temperature (RT) in a reactor of 200ml capacity equipped with a magnetic stirrer. Only parent catalysts were dehydrated prior to the experiment, while fluorinated catalysts were introduced directly to the ozonation system. Ozone, generated from pure oxygen by means of a Sorbios ozone generator, was continuously introduced to 190 ml of model compounds solution for 20 min and was allowed to stay in a closed reactor for another 1 h and 40 min after switching the ozone generator off. The residual organic substances concentration was determined after 20, 60 and 120 min of the experiment. After 2 h (20 min of ozonation + 1 h and 40 min of ozone interaction with organic substances) ozone was introduced to the reactor again for 13 min and the same procedure was followed as in the first step of ozonation experiments. After 13 min of the ozonation process, the ozone generator was again switched off. The residual concentration of organic compounds present in aqueous solution was measured after this specific time as well as after another 47 min, and 1 h and 47 min. 3. RESULTS AND DISCUSSION 3.1. Characterization
The powder X-ray diffraction patterns of the 4,0 parent materials: Nb- and Al-containing samples are consistent with the X-ray powder diffraction patterns of MCM-41 reported in the literature. They are characterized by a narrow single peak 3,0 [100] centered at 20 ~ 2 ~. Moreover, the structures of all samples are not destroyed after :5 2,5. the modification with perfluoroacids or ai perfluorocarbons. As example, Figure 1 shows ~ 2,0 XR patterns of H-AlMCM-41-32 (parent " material) and PFA/H-AIMCM-41-32 (sample after ~ 1,5 modification). 1,0 Mesoporous molecular sieves of MCM-41 type exhibit very high surface areas and large pore 0,5 H-AIMCM-41-32 volumes even after modification (Table 1). 0,0 .... , . . . . Although, after the modification with 2 4 6 10 perfluoroacids a small decrease in both parameters 20, o occurs, a shape of adsorption/desorption isotherm Figure 1. XRD patterns of parent is the same. This can suggest only a slight and modified H-AIMCM-41-32. blockage of the pores with the organic molecules. The pore size distribution (PSD) looks very similar for all of the samples. As shown in Fig. 2 the perfluoroacid treated samples are quite uniform. The above-presented XRD and N2 adsorption results indicate that the samples prepared by post-modification with perfluoroacids exhibit good structural uniformity.
3,it
594
Table 1 Physico-chemical data of some mesoporous materials used.
Catalysts AIMCM-41-16 H-AIMCM-41-16 PFA/H-AIMCM-41-16 PFC/AIMCM-41-16 AIMCM-41-32 PFC/AIMCM-41-32 H-AIMCM-41-32 PFA/H-AIMCM-41-32 H-NbMCM-41-16 PFA/H-NbMCM-41-16 H-NbMCM-41-32 NbMCM-41-32 PFA/H-NbMCM-41-32
Concentration of pertluoroacid ~tmol m 2 wt.% 0,276 3,02 1,06 0,71 0,051 0,59 0,189 1,56 0,065 0,54
Surface area BET, m 2 g-I
Pore volume,
935 920 840 1014 959 897 865 863 1006 856
1,219 1,253 1,078 1,502 1,256 1,135 0,845 0,677 1,13 8 0,604
c m 3 g-1
The IR spectra in the skeletal r e g i o n - 1800-400 crnq (recorded in the wafer with KBr and presented in Fig. 3) reveal that perfluoroacid has been bonded to the surface of the mesoporous molecular sieves. The shift of the characteristic vC=O (from 1759 crn "1 for pure PFA to --1685 cm q for PFA/H-AIMCM-41-16) and vC-F (from 1204 to 1207 cm q and 1146 to 1147 cm q for pure PFA and PFA/H-AIMCM-41-16, respectively) bands suggests the chemisorption of PFA on the solid most probably via H + ion-exchange by C8F~7C0 +. 1200-
P F N H - A I M C M-41-16
1,0'
"7'0)1000"
E 0
8OO,
::i 0,8'
ei L 600" 0 t~ "0
m
oe'-
.~ o,6,
400.
E
>O
e 200. 0
ov) 0,4
- . o - - - - o - - Desorption
oo" o',2 o',4 o',6 o',8 lo
Relative pressure, P/Po Fig. 2. N2 ads./des, isotherms and pore size distribution (BJH-ads.) of H-AIMCM-41-16 (A) and PFA/H-AIMCM-41-16 (B).
P F N H - N bM C M-41-16
1500
'
1000
"
500
Wavenumber, cm "1
Fig. 3. FTIR spectra of PFA/H-AIMCM-4116 and PFA/H-NbMCM-41-16 (recorded with KBr as dilutent) in the skeletal region.
595
3.2. Thermal stability of modified samples Thermogravimetry (TGA) is a simple experimental technique, which allows one to determine the weight changes that accompany heat treatment of samples. The DTG curve for modified samples exhibit major weight loss peak at about 553 K. The amount of the desorbed compounds corresponds to the amount of perfluoroacid inserted into the parent material. The additional study performed by the use of FTIR shows that the intensity of bands assigned to v(C=O) at about 1720 cm "1and at-~1420cm "1 - (-CO2H) decreases by evacuation at 553 K and disappears at 573 K. Those studies confirm the stability of the modified samples up to 553 K as shown in Fig. 4 and agree with the thermogravimetric results. It is worthy to notice the difference in the v(C=O) band position in Fig. 3 and 4 which can be due to the various quantity of water in the samples (less amount of water when pumping of the sample is applied).
1 , 8 "
t vC.=CL
(-C02H) ,,
.... a
1,5-
5 e" 1,2, o cal 0 r .r
0,9.
< 0,6
1800
9
i
1600
9
1
1400
'
9
Wavenumber, cm1
Fig. 4. FTIR spectra of PFA/H-AIMCM41-16 after evacuation at various temperatures: RT (a), 373 K (b), 473 (c), 523 K (d), 553 K (e) and 573 K (f).
3.3. Adsorption experiments Residual organic substances concentration in aqueous solution after adsorption onto NbMCM-41-32 conducted for 4 h is shown in Fig. 5. This concentration decreases significantly after the first hour of the adsorption process. A 1 g of NbMCM-41-32 is capable of adsorbing approximately 0,155 nag of toluene, 0,119 mg of cumene, and 0,140 mg of chlorobenzene after 1 h contact time. Maximum adsorption capacity is achieved after 2 h contact time and was found to be 0,179 mg of toluene, 0,176 mg of cumene, and 0,219 mg of chlorobenzene per 1g of the catalyst (Fig. 5). Ozonation experiments in the presence ofNbMCM-41-32 were also conducted in order to compare the efficiency of the catalytic ozonation and the adsorption process only. When ozone is introduced to the reaction system, the concentration of organic compounds in aqueous solution decreases significantly. Approximately the same amount of organic compounds is removed from aqueous solution after 20 min in the case of the catalytic ozonation when compared with the results obtained for the adsorption process only after 2 h contact time (Fig. 5). Moreover, the decrease of the organic compounds concentration in aqueous solution proceeds in time when catalytic ozonation system is examined. 4,4 mg 03 11 introduced to the reaction cell in the first 20 min time is capable of oxidizing approximately 0,38 mg 1~ of toluene, 0,41 mg 1"1 of cumene, and 0,4 mg 1" of chlorobenzene after 20 min and 0,52 mg 1"l of toluene, 0,56 mg 11 of cumene, and 0,66 mg 11 of chlorobenzene after 2 h contact time. After this time, when 3,2 mg 1~ of ozone is again introduced to the reaction system, the concentration of organic compounds decreases to a greater extent (Fig. 5).
596 1,6 ~ 1,4 .E. 1,2
I
'
-- ~ - - .adsorption ---O----- Ozonation
1,5
F
1,2 0,9
:~ 0,8
E
"~ 0,6 111 u 0,4 o
(.1
0,3
0,2 0
0
!
,
,
J
!
50
100
150
200
25O
3OO
214
!
|
100
200
300
~me[min]
Time [min] . . . . . . . . . . .
Figure 5. Residual toluene (A), cumene (B) and chlorobenzene (C) concentration in aqueous solution after adsorption and ozonation in the presence of NbMCM-41-32
1,2 u
---0
0,6
A
0,6
o
0
0
0
l
,
,
,
50
100
150
200
,'
250
300
Time [min]
3.4. Regeneration A two step experimental procedure was carried out in order to investigate whether the adsorption capacity of NbMCM-41-32 is regenerable. The first step. After the adsorption process carried out for 1 h, 4,6 mg 1"1 of ozone was dosed continuously to the reaction system for 20 min. After switching the ozone generator off, the reaction was allowed to continue for another 40 min in a closed reactor under stirring. It was found that approximately 0,125 mg of toluene, 0,114 mg of cumene, and 0,137 mg of chlorobenzene were adsorbed on 1 g of NbMCM-41-32 during 1 h (Fig. 6). Ozone introduced to the reactor caused a 2,5 STEP I STEP II further decrease of organic compounds Adsorption Ozonation Adsorption Ozonation concentration in aqueous solution. .. 4,6mgO3&.. 4,6mg03/I ~ 0.47 mg 1"1 of toluene, 0,66 mg 1"1 of -..o ,. cumene, and 0,82 mg 11 of E. 1,5 chlorobenzene were removed from aqueous solution after a time of 2 h, by means of adsorption and subsequent ozonation process. Toluene n,' 0,5 --- 4 .-- Cumene The second step. In the second step of - - ~ - .Chlorobenzene experiment, a new portion of organic 0 50 100 150 200 250 300 substances was introduced to the Time [min] reactor so as to examine whether NbMCM-41-32 is still capable of Fig. 6. Degradation of organic substances a~er adsorption prior to ozonation process in the adsorbing the organic substances present in the aqueous solution. presence of NbMCM-41-32. Adsorption and subsequent ozonation
597 process were conducted as in the first step. 0,122 mg g-i of toluene, 0,123 mg g-i of cumene, and 0,112 mg g-1 of chlorobenzene were removed from the aqueous solution aider 1 h of the adsorption process only. However, 0,43 nag 11 of toluene, 0,66 mg 1"1 of cumene, and 0,66 nag 11 of chlorobenzene were removed after 2 h contact time as the result of the continuous introduction of ozone to the reaction system for 20 min. When comparing results obtained for the second step of the experiment with the first one, it can be suggested that regeneration of NbMCM-41-32 proceeds during ozonation step. It is supposed that ozone introduced to the reaction cell causes oxidation of organic moieties adsorbed on the NbMCM-41-32 surface and, as the result, recovers its adsorption capability in respect of organic substances present in aqueous solution
3.5. Degradation activity In order to compare the activity of different mesoporous materials as catalysts in ozonation of organic substances present in aqueous solution, several experiments were carried out. Parent mesoporous materials and perfluorinated catalysts shown in Tab. 2 were examined. As shown in Tab. 2, the catalytic ozonation process with various materials is effective towards particular compounds only when compare it to ozonation alone. Perfluorinated catalysts are effective for the removal of toluene and cumene from water. Ozonation in the presence of PFA/H-NbMCM-41-16 as well as PFA/H-A1MCM-41-16 and PFC/A1MCM-41-16 causes respectively 84%, 77% and 66% toluene degradation. Whereas, only 63% of this compound is removed from aqueous solution when ozonation alone is applied. The same situation takes place in the case of cumene. Approximately 82% of cumene is degraded when O3/PFA/H-NbMCM-41-16 system is examined. This compound is also removed to approximately the same extent from aqueous solution when O3/PFA/H-AIMCM41-16 (81%) and O3/PFC/AIMCM-41-16 (82%) systems are used. Ozonation alone causes only 72% cumene removal from aqueous solution. It is worth mentioning that only 0,276 l.tmol of perfluorooctanoic acid covers 1 m 2 of AIMCM-41-32 surface showing that even traces of perfluorinated organics are responsible for significant decrease of toluene and cumene concentration, possibly, by extending ozone lifetime and its stability in aqueous solution. Chlorobenzene, however, is removed to the Table 2 Oxidation of organic compounds by ozonation alone and ozonation in the presence of mesoporous catalysts. Degradation [%] of: cumene A B A B without catalyst 45 63 57 72 NbMCM-41-16 54 64 66 76 PFA/H-NbMCM-41-16 65 84 64,7 82 AIMCM-41-32 49 68 64 74 PFA/H-AIMCM-41-16 44 77 60 81 PFC/AIMCM-41-16 52 66 72 82 PFC/NbMCM-41-32 43 54 50 62 A - atter 133 rain (+ 3 mg 03 l'l'); B - after 240 rain Catalysts
toluene
chlorobenzene A B 48 60 51 53 37 55 59 70 40 55 43 57 45 54
598 greatest extent when non-modified AIMCM-41-32 is applied to the reaction cell. 70% of chlorobenzene is removed in the case of O3/AIMCM-41-32 system, while only 53-60% degradation level is achieved in the case of other investigated systems. Further study has to be undertaken in order to examine catalysts' efficiency in the catalytic ozonation in the basic pH range. It is suggested that parent catalysts such as NbMCM-41-32 and NbMCM-41-16 are responsible for hydroxyl radicals generation in aqueous solution as the result of decomposition of ozone. Such situation is more likely to occur in basic rather than acidic pH range. Perfluorinated catalysts are supposed to be active in the acidic pH range because they are regarded as being molecular ozone stabilizers in aqueous solutions. 4. SUMMARY Mesoporous molecular sieves of MCM-41 type modified with perfluoroacid and perfluorohydrocarbons are useful adsorbents for organic molecules. One can suggest the application of these materials as adsorbents and catalysts in the ozonation of organic compounds. It is worthy to notice the following observations: 9 Modification of H-AIMCM-41 and H-NbMCM-41 with perfluoroacid and perfluorohydrocarbons does not change significantly the surface area, pore volume and hexagonal arrangement of mesopores in MCM-41 samples. 9 Perfluoroacid is stable on AI- and No-containing surfaces up to ca. 553 K. 9 AI- and Nb-containing mesoporous molecular sieves are good adsorbents for organic compounds studied. However, the use of ozone significantly increases the level of organic substances removal. 9 It is supposed that thanks to the oxidation of organic moieties adsorbed on NbMCM-41 surface the catalyst is recovered and it is active in the following step of the ozonation. 9 The modification of mesoporous molecular sieves with PFA and PFC increases the degradation level of toluene and cumene, whereas for chlorobenzene the most effective sorbent and catalyst is non modified AIMCM-41-32.
ACKNOWLEDGEMENT This work was partially supported by the Polish Committee for Scientific Researches.
REFERENCES [ 1] B. Legube and N. Karpel Vel Leitner, Catal. Today 53 (1996) 61. [2] C. Cooper and R. Burch, Wat. Res. 33 (1999) 3689. [3] M. Ziolek and I. Nowak, Zeolites 18 (1997) 356. [4] M. Ziolek, I. Sobczak, I. Nowak, P. Decyk, A. Lewandowska and J. Kujawa, Microporous and Mesoporous Mater. 35-36 (2000) 195. [5] B. Kasprzyk and J. Nawrocki, Ozone Sci. Eng. (2001) accepted for publication. [6] D. Bhattacharyya, T.F. Van Dierdonck, S.D. West, A.R. Freshour, J. of Hazard. Mat. 41 (1995) 73. [7] F.A. Stich, D. Bhattacharyya, Environ. Progress 6 (1987) 224.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
599
Organized mesoporous titanium dioxide - A powerful photocatalyst for the removal o f water pollutants Jifi Rathousk3~, Mark6ta Slabovd, Katefina Macounovfi and Amo3t Zukal J. Heyrovsk~ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolej~kova 3, CZ-182 23 Prague 8, Czech Republic, e-mail:
[email protected]. 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. As the thermal stability of anatase materials is generally not sufficient, a novel ordered mesoporous titania was prepared, whose framework is composed of anatase nanocrystals stabilized by aluminum, cationic surfactant cetyltrimethylammonimn chloride being used as a structure directing agent. The character of the porous structure and the thermal stability of this material strongly depend on the content of aluminum. I. 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. The major difficulty faced with synthesizing such oxides is their facile crystallization and subsequent grain growth, which leads to the loss of the original mesoporous structure. However, there is a major stimulus behind the research into the new forms of organized TiO2, namely its potential for converting light to electrical energy (photochromics and photovoltaics) [ 1-4] or chemical energy (photocatalytic splitting of water, photooxidation of harmful organics and microorganisms) [5] by its solar-driven band gap excitation. Additionally, it is inexpensive, nontoxic, and water-stable, which makes it amenable for use in a wide range of processes with minimal environmental impact. Recently, two novel approaches have been developed, enabling to obtain mesoporous titanium dioxide with highly interesting structural properties [6, 7]. In the first approach amphiphilic poly(alkylene oxide) block copolymers were used as structure directing agents in non-aqueous solution for organizing the network-forming titanium dioxide species [6]. The organized mesoscopic oxide is formed through a mechanism involving block-copolymer self-assembly and complexation of the metal atom during restrained hydrolysis of the metal chloride (here TIC14). In the second synthesis route a novel ordered mesoporous titania was prepared, whose framework is composed of anatase nanocrystals stabilized by aluminum, cationic surfactant cetyltrimethylammonium chloride being used as a structure directing agent [7].
600 In the present communication these synthetic approaches will be analyzed with respect to the effects of decisive processing parameters on the properties of obtained materials and it will be shown that one of the materials prepared can be effectively used in the photocatalytic destruction of an important organic water pollutant, viz. 4-chlorophenol. The possibility to prepare titania in the form of porous thin films will be also addressed as it is advantageous from the viewpoint of the application to continuous effluent decontamination.
2. MATERIALS AND METHODS 2.1. Materials 2.1.1. Mesoporous titanium dioxide prepared using block copolymers First, 0.9 g of Pluronic P-123 was 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~ 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 gelled in air at 40~ for 7 days and calcined at 400~ for 5 h in air. Finally, the titania films were peeled off the support (samples I-A, I-B, I-C, I-D). 2.1.2. Mesoporous titanium dioxide stabilized by aluminum An Al1304(OH)247+ solution (O.12mol/L) was prepared by dissolving 13.1 g of A1C13.6H20 in 420 ml of water, and the pH was adjusted to 3.95 with concentrated ammonium hydroxide. This solution was slowly added to a solution of (NH4)2Ti(OH)2(C3HsO3)2 (Tyzor LA, DuPont) and cetyltfimethylammonitma chloride (CTAC1, Aldrich) as described below. The Ti/A1 ratios were 19, 3, 1, respectively (samples II-A, II-B, II-C): Sample II-A: 827 g of Tyzor (15.2 mmol Ti) and 8.82 g of CTAC1 solution (8 mmol) were combined. 6.62 ml (0.8 mmol) of the above-described aluminum-containing solution was diluted to 80 ml with water, and then was slowly added to the Tyzor/CTACI mixture at vigorous stirring. Sample II-B: 6.82 g of Tyzor (12 mmol Ti) and 8.82 g of CTAC1 solution (8 retool) were combined. 33.09 ml (4 mmol) of the above-described aluminum-containing solution was diluted to 80 ml with water, and then was slowly added to the Tyzor/CTAC1 mixture at vigorous stirring. Sample II-C: 4.54 g of Tyzor (8 mmol Ti) and 8.82 g of CTACI solution (8 mmol) were combined. 66.18 ml (8 mmol) of the above-described aluminum-containing solution was diluted to 80 ml with water, and then was slowly added to the Tyzor/CTAC1 mixture at vigorous stirring. Sample II-D (a reference sample without AI): 9.08 g of Tyzor (16 mmol Ti) and 8.82 g of CTAC1 solution (8 mmol) were combined. Then 80 ml of water was slowly added to the Tyzor/CTAC1 mixture at vigorous stirring. A white precipitate, which formed immediately in all the four reactions, was aged in a Teflon bomb at room temperature overnight, one day at 70~ and 2 days at 100~ and subsequently isolated by washing and centrifuging. The prepared materials were calcined at 450~ in air for 30 minutes. The ratios Ti/AI found by elemental analysis after calcination were 25, 11 and 6 for samples II-A, II-B and II-C, respectively.
601
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. Raman spectra were measured using a T64000 spectrometer (Instruments, SA, France) equipped with an Olympus BH2 microscope. The spectra were excited in a 180~ back scattering geometry by Ar+ laser (Innova 305, Coherent, USA), )~ = 514.5 nm. The Raman spectra were measured either in the standard macro-chamber set-up (without microscope) or in the micro-Raman geometry. In the latter case, the laser beam was focussed either to the surface of one selected grain of the sample or into its bulk. Adsorption isotherms of nitrogen (Linde, purity 5.6) were measured at -196~ with an ASAP 2010 instrument (Micromeritics). Each sample was degassed at 200~ for at least 20 hours until a pressure of 10.4 Pa was attained. Photocatalytic activity of the TiO2 samples was studied using a model pollutant 4-chlorophenol. Photodegradation of this compound was examined employing a tube photoreactor where TiO2 was dispersed in water. After illumination, 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. 93.
RESULTS AND DISCUSSION
3.1. Mesoporous titanium dioxide prepared using block copolymers The stock solution for the film preparation contains an ethoxide-modified titanium chloride, formed by the reaction: TIC14 + x EtOH ~ TiCl4.x(OEt)x + x HC1, where x ~ 2. The formed TiCIx(OC2Hs)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 ESCA 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 2 and 4 mg/cm 2, 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 (Fig. l) and nitrogen adsorption (Fig. 2). With medium (sample I-C, density of 6 mg/cm 2) and thick films (sample I-D, density of 8 mg/cm 2) the hydrolysis is far from complete. Consequently, larger pores are formed in
602 addition to smaller ones during the calcination of the non-hydrolyzed fraction. This leads to the formation of a bimodal porous structure (Fig. 2).
Fig 1. SEM image of the thinnest film I-A
Fig 2. Adsorption isotherms of nitrogen at 77 K on samples I-A, I-B, I-C and I-D. The solid points denote desorption.
The structure parameters of all the films studied are given in Table 1. X-ray diffractograms and the Raman spectra 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 a pure reference material (Bayer). This component does not seem, however, to exhibit any characteristic Raman signal, which would distinguished it from anatase. Table 1. Structure parameters of filmsprepared using block copolymers SBETb Sample da (mg/cm 2) (m2/g) I-A 2 105 I-B 4 94 I-C 6 127 I-D 8 104
Dpe
(nm) 4.4 4.6 4.0, 5.2 6.2,16.0
a density of the film, b BET surface area, r mean pore size (two values correspond to a bimodal porous structure).
603
3.1.1. Mesoporous titanium dioxide stabilized by aluminum X-ray diffractograms of all the four samples can be indexed as those of pure anatase. Table 2 surveys the particle size determined from line broadening. It is apparent that the particle sizes decrease with increasing aluminum content. Table 2. Basic structure parameters of the prepared materials Sample II-A II-B II-C II-D II-C500 II-C700 II-C 1000 II-D500
Ti/A1a (mol/mol)
Tb (~
Dcc (nm)
25 11 6 without A1 6 6 6 without A1
400 400 400 400 500 700 1000 500
25 11 6
SBETd
(mZ/g) 162 359 437 73 363 74 2.5 1.2
Dpe (nm) 3.7 2.3 1.9 3.6 2.5 6.1
a Ti/A1 molar ratio in the mesoporous material, b temperature of the heat treatment, c size of anatase crystals determined from XRD, d BET surface area, r mean pore size.
.
.
.
.
.
.
.
II-Cl(surface~ II-C2 --II-C4
--II-C5
_c
~~--'~--'~" 200
I ~-~'l 400
600
" ~ ' ~ ~ ~ ~ ! l-C6xll0'II-C7xl0~bulk) 800
Rarnan s h i f t / c m
-1
1000
1200
Fig 3. The micro-Raman spectra of Al-modified TiO2 (sample II-C). The laser focus was gradually moved from the surface (II-C 1) into the bulk of the material (II-C7). The spectra are offset for clarity but the intensity scale is identical for spectra II-C1 to II-C5. The intensity was multiplied by a factor of 10 for the spectra II-C6 and II-C7.
604 In accord with the X-ray diffractograms, the macro-chamber Raman spectra indicate that the bulk material contains essentially the anatase phase only (bands at 144, 196, 397, 517 and 639 cml). However, the micro-Raman set-up allowed to select precisely the sampled area on one individual grain, and by focussing the laser beam on the grain surface or underneath the surface, the spectra correspond to the Raman scattering from the grain's surface or to a signal convoluted from the surface and bulk, respectively. Figure 3 displays this kind of Raman depth profiling on a single grain of sample II-C. Curve II-C1 corresponds to the micro-Raman spectra of sample II-C if the laser beam was focussed exactly on the grain's surface. Spectra II-C2 to II-C7 correspond to the same illuminated spot, but the laser focus was gradually moved into the grain's bulk. This analysis evidences that the surface of grains has differem composition from the bulk material. The surface layer is characterized by ruffle bands at 246, 436 and 609 cm "l, which is in contrast to the absence of ruffle in the X-ray diffractograms. By comparative Raman measurements on many sample grains, we have confirmed that the described composition is characteristic property of the material. Moreover, the same morphology (with anatase core and rutile shell) was reproduced also with samples II-A and II-B. Adsorption isotherms of nitrogen at 77 K on samples with differing content of aluminum show its striking effect on the porous structure of materials obtained (Fig. 4, Table 2). The larger the content of aluminum the larger the surface area of the porous material and the smaller the pore size. While samples II-B and II-C exhibit pore sizes at the border between micro- and mesopores, samples II-A and II-D are typically mesoporous. The introduction of aluminum is also extraordinary beneficial for the enhancement of the thermal stability (Fig. 5). While the porous structure of sample II-D is totally destroyed at temperature as low as 500~ sample II-C undergoes only some structure changes (an increase in the pore size and decrease in surface area). For the collapse of its porous structure calcination at temperature as high as 900~ is needed. ,
II-C
II-C500
II-B -
't::p. 4
II.A
I
E 3-~ '
vt~ E E m
II-D
I1-C900 I
0.0
I
0.2
I
0.4
I
0.6
I
0.8
I
1.0
Fig. 4 Effect of aluminum content on the porous structure of samples II-A, II-B, II-C and II-D as detected by nitrogen adsorption at77K
0 ,, "'"'I---'
0.0
0.2
i ~---
0.4
016
Wpo
0'.8
1.0
Fig. 5 Effect of heat treatment of sample II-C on its porous structure as detected by nitrogen adsorption at 77 K
605
3.2. Photoeatalysis 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 [8]. 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 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 3. Table 3. Decomposition of 4-chlorophenol Sample I-A I-B Non-optimum films Bayer
Rate constant of the decomposition of 4-chlorophenol (10 4 s-l) 3.49 3.42 1.4-2.4 3.37
The mesoporous titania stabilized with alumimma exhibits lower photocatalytic activity, the rate constant being 0.4 x 10-4 s~ with sample II-C. The obvious reason for the decreased activity is the presence of the aluminum phase. 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. A novel ordered mesoporous titania was prepared, whose framework is composed of anatase nanocrystals stabilized by aluminium, cationic surfactant cetyltrimethylammonium chloride being used as a structure directing agent. This material exhibits an exceptionally high thermal stability. Its relatively low photocatalytic activity may be advantageous in some applications, such as cosmetics.
ACKNOWLEDGMENTS This work was supported by the Academy of Sciences of the Czech Republic (contract No. A4040804) and by the EC-COST Action (contract No. D 14/0002/99).
606 REFERENCES
1. 2. 3. 4.
5. 6. 7. 8.
M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Mueller, P. Liska, N. Vlaehopolous and M. Gr/itzel, J. Am. Chem. Soc., 115 (1993) 6382. A. Hagfeldt and M. Gr/ttzel, Chem. Rev., 95 (1995) 49. S.Y. Huang, L. Kavan, M. Gr/itzel and I. Exnar, J. Electrochem. Soc., 142 (1995) 142. C.G. Granqvist, A. Azens, J. Issidorson, M. Kharrayi, L. Kullman, T. Lindstr6m, G.A. Niklasson, C.G. Ribbing, D. R6nnow, M. Stromme Matttsson and M. Veszelei, J. NonCryst. Solids, 218 (1997) 273. N. Serpone and E. Pelizzetti, Photocatalysis, Fundamentals and Applications, J. Wiley & Sons, New York 1989. P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka and G.D. Stucky, Nature, 396 (1998) 152. A. ARia, S.H. Elder, R. Jir~sek. L. Kavan, P. Krtil, J. Rathousk~, A. Zukal, Stud. Surf. Sci. Catal., 135 (2001) 361. V.F. Stone and R.J. Davis, Chem. Mater. 10 (1998) 1468.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
607
M e s o p o r o u s materials for h e a v y metal ion adsorption synthesized by displacement o f p o l y m e r i c template V. Antochshuk a, M. Jaroniec a, S.H. Joo b and R. Ryoo b a
Department of Chemistry, Kent State University, Kent, Ohio 44242, USA
b Department of Chemistry and School of Molecular Science (BK21), Korea Advanced Institute for Science and Technology, Taeduk Science Town, Taejon, 305-701 Korea
A successful application of the template displacement synthesis to the polymer templated mesoporous silica SBA-15 resulted in the preparation of materials with high surface area and open porosity. It was shown for the first time that such procedure is applicable for neutral polymer templated silica-based materials. Some of the materials, particularly the sample with attached 1-allyl-3-propylthiourea functionality, exhibited promising properties towards mercury ion adsorption. The maximum mercury loading from aqueous solution is 0.16 g Hg2+/g or 0.8 mmol Hg2+/g. The mercury desorption was accomplished by washing the mercury-loaded samples with aqueous thiourea solution.
1. INTRODUCTION Pollution with heavy metals represents a serious threat for environment and constitutes a great concern for ecologists in developed countries. One of the approaches to this problem is the removal of harmful metal ions through binding them to the unsoluble matrixes, adsorbents [ 1-4], which later can be disposed or regenerated and reused. Such materials can be designed by combining advantageous properties of mesostructured silicas (high surface area, large pore size and outstanding transport properties) [5, 6] with metal-specific organic functionality [7-9]. Specificity of functional groups can be described in terms of the hard-soft acid-base theory [10]. According to this theory the common metal ions such as Na +, K +, Mg 2+ and Ca 2+ are classified as hard acids, but Hg 2+, Cd 2+, Ag + and Pb 2+ metal ions are soft acids, and consequently only the second group of ions would strongly interact, for instance, with sulfur-containing ligands, which are soft bases. This fact allows designing potential adsorbents for environmental cleanup via attaching soft bases (e.g. sulfur-containing ligands) to the silica surface. Such materials would selectively bind heavy metal ions from aqueous solutions and their regeneration can be achieved by utilization of acid, complexing agent solutions or their mixtures. In the most favorable case, there is 1:1 interaction between Hg 2+ ions and SH-ligands. The smallest, readily available, sulfur-containing silane that is suitable for mercury adsorption and subsequently, for silica modification, is 3-mercaptopropyltrimethoxysilane
608 [8, 9]. The attachment of mercaptopropyl groups to the silica surface would increase molecular weight of the material by 119.25 g/mol. The highest population of silanols on the silica surface is ca. 4.9 groups/nm2 that results in the maximum of ca. 6 mmol/g groups for most common silicas, and even less, ca. 4 mmol/g for mesoporous silicas [ 11]. Thus, in the case of the most optimal scenario one mol (200.59 g) of mercury(II) would be adsorbed on one mol (119.25 g) of the SH-ligands attached through one mol of silanols present on the silica surface, the mass of which should be at least 167 g (in order to assure the 6 mmol/g silanol concentration). This calculation shows that the highest attainable loading of mercury ions on mercaptopropyl functionalized silica would be ca. 0.7 g Hg2+/g of functionalized material. The actual number would be much lower if one takes into account several factors such as much lower concentration of silanols present on the high surface silicas (typically 1.5 times lower), difficulties in achieving the ligand coverage higher than 75% and the pore wall curvature restrictions. Taking into account the aforementioned factors one could provide a more realistic estimation of the maximum mercury adsorption for a hypothetical "superadsorbent" as ca. 0.5 g Hg2§ That number is very close to 0.505 g Hg2+/g, which was reported for the material showing an unprecedented 100% ligand coverage [9]. Two conclusions arise from the above calculations: (i) there is an evident limitation for the maximum adsorption capacity of materials with a given framework, which can be increased by using "lighter" matrices, for example, active carbon; (ii) there are other areas which could be improved, e.g., regeneration and stability of the adsorbing materials. The objective of this study was to explore the applicability of the template displacement synthesis (TDS) for preparation of modified materials by using polymer-templated silica, SBA-15. One of the goals is to synthesize functionalized silica-based mesoporous materials suitable for removal of heavy metal ions from aqueous solutions and to elucidate their adsorption performance with respect to mercury(II) ions.
2. M A T E R I A L S AND M E T H O D S
Tetraethyl orthosilicate, 3-aminopropyltriethoxysilane, 3-cyanopropyldimethylchlorosilane, allyl isothiocyanate and pyridine (anhydrous) were from Aldrich Chemical Co. (Milwaukee, WI). Toluene (p.a.), ethyl alcohol and isopropyl alcohol (anhydrous) were from Fisher Scientific (Pittsburgh, PA). All materials were used as received. The SBA-15 mesoporous silica was synthesized using Pluronic P123 triblock copolymer (EO20POToEO20) at 35~ following the procedure described elsewhere [12]. TEOS was used as a silica source and low-temperature preparation was followed by aging for 2 days at 90~ The solid product was filtered out without washing to obtain a material with templating polymer inside of mesopores. The uncalcined and unmodified sample of SBA-15 is denoted as S-U. The unwashed and uncalcined sample was subjected to the template displacement synthesis (TDS) [13, 14] in order to remove polymer from mesopores and to attach functional groups. The resulting samples are designated as follow: sample treated with tetraethyl orthosilicate, S-UT, sample treated with 3-cyanopropyldimethylchlorosilane, S-UCN, sample treated with 3-aminopropyltriethoxysilane, S-UNH2. The TDS procedure included dispersion of ca. 0.2 g of uncalcined silica in 10 ml of silane-toluene (1:3) mixture followed by refluxing for 8 h. The mixture was filtered and washed several times with small portions of toluene, isopropyl alcohol, and finally the modified mesoporous material was dried overnight in an oven at
609 90~ under vacuum. The sample treated with tetraethyl orthosilicate, S-UT, was subjected to the calcination in air for 4 h at 550~ and is designated as S-UTC. Further, the sample treated with 3-aminopropyltriethoxysilane, S-UNH2, was reacted with allyl isothiocyanate using toluene solution under reflux conditions for 2 h. The product was filtered out, washed with toluene and isopropyl alcohol, and dried at 90~ for 8 h under vacuum. The product of the reaction between amine and substituted isothiocyanate is a thiourea derivative [15] covalently bonded to the silica surface, designated as S-UNA. For comparison purposes, the polymer contained in as-synthesized material was removed by extraction procedure. The assynthesized product in the amount of 1.0 g was stirred in 20ml of EtOH:HzO:HCI:NH4C1 (90:10:0.5:0.5 by weight) solution for 20 min at room temperature, filtered, washed with 30 ml of the aforementioned solution followed by washing with 20 ml of deionized water and dried in oven at 90~ for 8 h. Further, this sample was subjected to calcination in air for 4 h at 550~ Extracted sample is denoted as S-E and subsequently calcined sample is S-EC. Mercury adsorption was studied under static conditions from aqueous solutions. All solutions were prepared by dilution of the 0.1 N standard Hg(II) nitrate solution, Aldrich Chemical Co. (Milwaukee, WI), up to 10 ml. In a typical determination 0.05 g of the sample was agitated for 1 h with 10 ml of the solution containing mercury nitrate of known concentration (ratios Hg2+:Ligand were 2:1, 1:1, 10:1). After filtration adsorbent was washed with deionized water and the filtrate was collected and diluted to 25 ml. Mercury(II) concentration was measured spectrophotometrically with dithizone as a complexing agent [16]. The mercury uptake was determined as the difference between the initial and equilibrium concentrations. In a similar fashion the mercury uptake experiments were repeated with regenerated adsorbent. For comparison purposes, adsorption experiments were performed for unfunctionalized mesostructured silica using the same amounts of the sample and solutions. Mercury desorption studies were performed for the samples loaded with mercury in the previous experiments via treatment with 10% thiourea solution in aqueous 0.05 M HC1. The regeneration was performed under static conditions by soaking 0.05 g of mercury-loaded adsorbent for 15 min with 10 ml of thiourea solution followed by filtration and rinsing of the sample with additional 10 ml of thiourea solution and 10 ml of deionized water. The content of carbon, nitrogen, sulfur and hydrogen in all modified samples was determined using a LECO Model CHNS-932 elemental analyzer (St. Joseph, MI). For each sample three measurements were performed with the relative error of less than 0.1%. TA Instruments model TA 2950 (New Castle, DE) analyzer was used to carry out highresolution thermogravimetric analysis. All thermogravimetric measurements were done in nitrogen atmosphere. The maximum heating rate in all cases was 5~ over a temperature range from 25 to 1000~ The accuracy in the weight change measurements was 0.1%. Nitrogen adsorption measurements were performed using a Micromeritics model ASAP 2010 adsorption analyzer (Norcross, GA). Adsorption isotherms were measured at -196~ over the interval of relative pressures from 10.6 to 0.995 using nitrogen of 99.998% purity. Before each analysis the sample was degassed for 2 hours at 110~ under vacuum of about 10-3 Torr in the degas port of the adsorption apparatus. Specific surface areas of the materials under study were calculated using the BET method [17, 18]. Differential pore size distributions were evaluated from the adsorption branches of nitrogen isotherms using the BJH method [19] with the corrected form of the Kelvin equation for capillary condensation in cylindrical pores [20, 21 ].
610 Solid state 298i single-pulse MAS and 13C CP-MAS NMR experiments were performed on a Bruker 400DMX NMR spectrometer (Bruker Instrument Inc., San Jose, CA) operating at resonance frequencies of 79.49 and 100.54 MHz for 298i and 13C, respectively. Measurements were performed at a room temperature using air as driving and bearing gas in a 7 mm zirconia rotor at the magic angle with the spinning frequency of 2.5 kHz. The relaxation delay between pulses in the 29Si NMR experiment was 200 seconds (it was taken as 5T~ for slowest relaxing site). The total number of scans was 40000 for ~3C and 800 for 298i, chemical shifts were externally referenced to TMS. 298i spectra deconvolution was performed in the absolute intensity mode and line broadening of 40 Hz was applied. The powder XRD spectra were acquired on a Rigaku D/MAX-Ill diffractometer using Cu Kc~ radiation.
3. RESULTS AND DISCUSSION The synthesis of SBA-15 mesosilica from TEOS and Pluronic P123 triblock copolymer at 35~ with further aging at 90~ resulted in the preparation of well ordered material with characteristic low angle XRD peak at 20 angle ca. 0.9 ~ (Fig. 1). The uncalcined product had a very low surface area and completely absent porosity. As it was shown [13, 14] a direct interaction of organosilanes with silica-surfactant nanocomposites results in the successful template displacement from the structure. Further exploration demonstrated that such displacement is possible for neutral polymer-templated systems such as SBA-15. The C,H,N,S analysis revealed a significant decrease in the amount of carbon in the samples exposed to silanes. It was noted before [13] that the template displacement synthesis results in somewhat higher surface coverage then conventional modification. Calculation of the ligand coverage (Table 1) shows that the ligand attachment typically exceeds 2.0 mmol/g. According the elemental analysis over 80% of the attached aminogroups were successfully converted into the 1-allyl-3-propylthiourea functionality. Nitrogen adsorption data showed a significant increase in the surface area and pore volume of samples subjected to TDS (Table 1 and Fig. 2 and 3), which indicates the opening of pores and surfactant expulsion. The BJH calculations (Table 1 and Fig. 2) demonstrate that upon calcination of the extracted sample there is a significant (0.7 nm) shrinkage of the mesoporous structure (from S-E to S-EC sample). In the case of template displacement Table 1. Adsorption and surface properties of mesoporous silica and functionalized samples. ,
Sample
,
BET surface area, m2/g
S-E ' 6"00 S-EC 870 S-UT 370 S-UTC 710 S-UCN 380 S-ENH2 235 S-UNA 155
9
,
,
,,
Total pore volume, cm3/g
Pore Ligand diameter bonding (BJH), density, nm mmol/g
0.91 1.03 0.49 0.72 0.62 0.41 0.25
9.5 8.8 7.8 8.0 8.5 8.5 7.4
,
,,
--2.16 -2.08 2.20 1.75
S-U
.i, 1
2
3
4
5
20 Figure 1. X-ray powder diffraction pattern for uncalcined SBA-15 mesostructuredmaterial.
611
-.
700
09 tm
600
"~o
400
I" I
500
Sc~ ~'r
~S-VT/'~/
200
?
f
S-UT
300
=
l
0.2
0.4
0.6
0.8
1.0 4.0
6.0
P/P0
~-= 0s ~ 04.~ 03 .~
surfactant with tetraethyl orthosilicate followed by calcination results only in an insignificant pore change (ca. 0.2 nm for
o.1"~ o.o ~.
, 0.0
8.0
Pore width
I0.0
12.0
(nm)
Figure 2. Nitrogen adsorption isotherms (A) for the SBA-15 sample after extraction (S-E), extraction and calcination (S-EC), template displacement with tetraethyl orthosilicate (S-UT) and template displacement with tetraethyl orthosilicate followed by calcination (S-UTC). Differential pore size distributions (B) calculated from nitrogen adsorption data.
700
t--,
- 0.7 ~"
B
A
600
0.6~'~
e~o
o 0 . 5 "~"
500 o ~9
O
400
0.4 "~
S-UNHz/
ul r
0.3 " ~
0.2 ~
200
~E 100
0.1 " ~
o ~"
0
.... 0.0
| .... 0.2
| .... 0.4
| .... 0.6
P/Po
.........
! .... 0.8
1.0
4.0
,,,~,., .... |,,r ...... 6.0
8.0
o o . 0 ~.~
| ......... 10.0
relacemen'
of the structure directing
\~0.2~
1o0
0
'ynt ~
06~
12.0
Pore width (nm)
Figure 3. Nitrogen adsorption isotherms (A) for the SBA-15 sample after extraction and calcination (S-EC), after template displacement synthesis with 3-aminopropyltriethoxysilane (S-UNH2) and after modification of SBA-15 with 3-aminopropyltriethoxysilane followed by subsequent reaction with allyl isothiocyanate (S-UNA). Differential pore size distributions (B) calculated from nitrogen adsorption data.
S-UT
vs.
S-UTC).
The
pore width of the S-UTC sample is noticeably smaller than that of the extracted S-E sample due to the attachment ethoxysilyl groups. The template displacement with larger silanes results in the
appropriate decrease in the pore size. Also, the pore size analysis proves that the size of side chains in the silane molecule is as important in reducing the pore size (Fig. 2, 3) and volume (Table 1) as the length of the main chain. A successful template displacement was proved by means of 13C and 29Si solid state NMR spectroscopy (Fig. 4 and 5). Chemical modification of the SBA-15 silica withsilanes results in the disappearance of signals for
polymer in the 13C NMR spectra (range 55-95 ppm) and the appearance of a new group of signals with chemical shifts between 10 and 60 ppm characteristic for 3-cyanopropyl and ethoxy functionalities, correspondingly (Fig. 4). 29Si single-pulse NMR spectra (Fig. 5) confirm an increased condensation of the framework as a result of the template displacement. A cjuantitative analysis of NMR spectra suggests a drastic decrease in the amount of silanols (Q" signal) on the surface from 37% for as-synthesized sample to ca. 20% for modified sample. The interaction of tetraethyl orthosilicate with the uncalcined SBA-15 results not only in the template displacement and chemical modification of the silica surface but also in the entrapment or some sort of complexing of additional silane molecules in the micropores of the material. This is evident from the unusual doubling of the number of signals (second pair at ca. 27 and 68 ppm) in the 13C NMR spectra of S-UT (Fig. 4) as well as a relatively small pore volume and BET surface area for this sample (Table 1) in comparison to the materials
612
S-U
S-UCN
S-UT
il}l} " ' 8 ' 0 . . . . . 6'0 . . . .
4'0'
' '2'0'
" "0
ppm
Figure 4. 13C CP-MAS NMR spectra for uncalcined mesoporous silica (S-U) and materials prepared via interracial reaction with 3-cyanopropyldimethylchlorosilane (S-UCN) and tetraethyl orthosilicate (S-UT).
~ | .,..... 50
i .... 0
~ .... -50
w .... -100
.v
ppm
Figure 5. 29Si single-pulse MAS NMR spectra for uncalcined mesoporous silica (S-U) and materials prepared via interfacial reaction with 3-cyanopropyldimethylchlorosilane(S-UCN)and tetraethyl orthosilicate (S-UT).
obtained with larger silanes. Also such entrapment of tetraethyl orthosilicate probably results in the formation of some sort of complex that causes the signal s~plitting in the 298i NMR spectra (Fig. 5) and appearance of a new line at -96 ppm between Q and Q 3 signals. It should be mentioned that the TGA analysis indicates that the extraction with acidified ethyl alcohol-water mixture does not result in a complete polymer removal and an additional calcination is necessary (Fig. 6) whereas the TDS process clearly removes even strongly bound template with silane molecules (Fig. 6 and 7). Based on the previous [13, 14, 22] and current studies, it is possible to state that the template displacement synthesis is applicable for the systems templated with neutral or charged surfactants as well as for the systems prepared under acidic or basic conditions. Also, this approach is applicable to titanium alkoxides [22]. Further reaction of amino functionalized SBA-15 sample with allyl isothiocyanate results in the preparation of the material with attached 1-allyl-3-propylthiourea groups (S-UNA). It is evident that some amount of unreacted ethoxy groups from the first TDS step involving aminopropyltriethoxysilane is present but their presence as evidenced by the TGA spectra (Fig. 7) in the final sample S-UNA proves that they do not interfere in the reaction with allyl isothiocyanate. The silica samples with attached thiourea were previously utilized for analytical purposes in relation to gold(III) and platinum(IV) ions [23] and because of the sulfur present in the structure they are able to coordinate soft metal ions and subsequently they should be useful for mercury ion adsorption. The binding of mercury through thiocarbonyl group
613 0.6
100
100
0.5
o'~ 70 9~
J
" ": :.
~
r,.)
S-E S-UT
: "----'. . . . . ~i
90
~ ~ ~
80
i" ' " - . . . .::~ ~\
' l 0.40 0.35 ...._S-UNH2.~.t 0.30
~
0.4 o~
0.25
7~ 0.3 ~
60
0.20
~ 60
3:
o.~5 .~
0.2
50
|
'.ii'" " ". . . . . . S-U . . . .
'~ i i! i ......
0.1
I
O.lO
" ""~
005
30 0.00
t0.0 | 0
200
400
600
800
1000
Temperature,~ Figure 6. TG and DTG curves for uncalcined material, S-U (dotted lines), extracted material, S-E (solid lines), and samples prepared by the template displacement synthesis with tetraethyl orthosilicate, S-UT (dashed lines) and 3-cyanopropyldimethylchlorosilane, S-UCN (open triangles).
0
200
400
600
800
1000
Temperature,~ Figure 7. TG and DTG curves for uncalcined material, S-U (dotted lines), and samples prepared by the template displacement synthesis with 3-aminopropyltriethoxysilane, S-UNH2 (dashed lines) and subsequently modified with allyl isothiocyanate, S-UNA (solid lines).
(>C=S) is expected to be weaker than through thiol group (-SH) and consequently the regeneration of the former material is possible with moderate complexing agents. The mercury adsorption from aqueous solutions of Hg(NO3)2 showed that the S-UNA material studied is able to adsorb up to 0.16 g Hg2+/g of material or ca 0.8 mmol Hg2+/g. It was possible to achieve regeneration of the adsorbent via washing of loaded samples with acidified 0.05 M thiourea aqueous solution. The regenerated material showed loading up to 0.10 g Hg2+/g of material. The adsorbent as well as mercury loaded material are thermally stable up to 120~ The preliminary research showed that there are several requirements such as pore framework of the starting material and grafting procedure that must be controlled in order to obtain an adsorbent with significant mercury uptake capacity. For instance, the SBA-15-based materials with analogous ligand concentration exhibit lower adsorption capacity than the corresponding MCM-41 adsorbent [24] due to the presence of complimentary micropores [25].
4. CONCLUSIONS A one-step modification-extraction procedure was effective for preparation of modified SBA-15 mesoporous silica. A successful application of this procedure to the polymertemplated mesoporous silica SBA-15 afforded materials with high surface area and open porosity. It was shown for the first time that such procedure is applicable for neutral polymer-templated silica-based materials. Some of the materials, particularly the sample with attached 1-allyl-3-propylthiourea functionality exhibited a promising properties towards mercury ion adsorption. The maximum mercury loading from aqueous solution is 0.16 g Hg2+/g or 0.8 mmol Hg2+/g. The mercury desorption was accomplished by washing loaded samples with aqueous thiourea solution.
614 5. ACKNOWLEDGEMENT Authors acknowledge the National Science Foundation grant CTS-0086512 for the support of this research. REFERENCES
1. 2. 3. 4. 5.
C. Kantipuly, S. Katragadda, A. Chow, H. D. Gesser, Talanta, 37 (1990) 491. D. Mohan, V. K. Gupta, S. K. Srivastava, S. Chander, Colloids Surf. A, 177 (2001) 169. R. Celis, C. M. Hermosin, J, Comejo, Environ. Sci. Technol., 34 (2000) 4593. S.J.T. Pollard, G. D. Fowler, C. J. Sollars, R. Perry, Sci. Total Environ., 116 (1992) 31. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. 6. 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. 7. Y. Lin, G. E. Fryxell, H. Wu, M. Engelhard, Environ. Sci. Technol., 35 (2001) 3962. 8. L. Mercier, T. J. Pinnavaia, Adv. Mater., 9 (1997) 500. 9. X. Feng, G. E. Fryxell, L.-Q. Wang, A. Y. Kim, J. Liu, K. M. Kemner, Science, 276 (1997) 923. 10. R. G. Pearson, J. Am. Chem. Soc., 85 (1963) 3533. 11. B. P. Feuston, J. B. Higgins, J. Phys. Chem., 98 (1995) 4459. 12. D. Zhao, Q. Huo, J. Feng, B. F. Chmelka, G. D. Stucky, J. Am. Chem. Soc., 120 (1998) 6024. 13. V. Antochshuk, M. Jaroniec, Chem. Comm., (1999) 2373. 14. V. Antochshuk, M. Jaroniec, Chem. Mater., 12 (2000) 2496. 15. Comprehensive Organic Functional Group Transformations, ed. A. R. Katritzky, O. Meth-Cohn, C. W. Rees, Pergamon, New York, Vol. 6, 1995. 16. AOAC Official Methods of Analysis, ed. W. Horwitz, The Association of Official Analytical Chemists, Washington, 1990. 17. S. Brunauer, P.H. Emmett and E. Teller, J. Am. Chem. Soc., 60 (1938) 309. 18. J. Roquerol, D. Avnir, C. W. Fairbridge, D. H. Everett, J. H. Hayness, N. Pemicone, J. D. F. Ramsay, K. S. W. Sing, K. K. Unger, Pure Appl. Chem., 66 (1994) 1739. 19. E. P. Barrett, L. G. Joyner, P. P. Halenda, J. Am. Chem. Soc., 73 (1951) 373. 20. M. Kruk, M. Jaroniec, A. Sayari, Langmuir, 13 (1997) 6267. 21. M. Kruk, V. Antochshuk, M. Jaroniec, A. Sayari, J. Phys. Chem. B, 103 (1999) 10670. 22. Z.-L. Hua, J.-L. Shi, L.-X. Zhang, M.-L. Ruan, X.-G. Zhao, J. Mater. Chem., 11 (2001) 3130. 23. V. N. Losev, N. V. Maznyak, A. K. Trofimchuk, V. K. Runov, Ind. Lab., 64 (1998) 365. 24. V. Antochshuk, M. Jaroniec, Chem. Comm., (2002) 258. 25. R. Ryoo, C. H. Ko, M. Kruk, V. Antochshuk, M. Jaroniec, J. Phys. Chem. B, 104 (2000) 11465.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
615
Organically-modified mesoporous silica spheres with MCM-41 architecture as sorbents for heavy metals M. Etienne a, S. Sayen a, B. Lebeau b*, A. Walcarius a* Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, LMCPE, UMR 7564, CNRS - Universit6 Henri Poincar6 Nancy I; 405, rue de Vandceuvre, F-54600 Villers16s-Nancy, France.
a
b Laboratoire de Mat6riaux Min6raux, LMM, UPRES-A 7016, CNRS - Ecole Nationale Sup6rieure de Chimie de Mulhouse - Universit6 de Haute Alsace; 3, rue Alfred Werner, F68093 Mulhouse, France. corresponding authors,
[email protected],
[email protected] A new hybrid organic-inorganic material with ordered mesoporous structure (MCM41) and spherical morphology (St6ber silica) has been synthesized by co-condensation of tetraethoxysilane (TEOS) and organoalkoxysilanes in water/ethanol solution containing ammoniac and in the presence of cetyltrimethylammonium bromide as templating agent. The attractive properties of such spherical ordered mesoporous silicas, MCM-41 type, functionalized with organic ligands (aminopropyl and mercaptopropyl) are demonstrated by providing batch, kinetic and voltammetric studies of their metalion-binding ability. Our efforts focus especially on the ion-exchange abilities of immobilised amine groups towards the chloro-complexes of mercury(II), and on the adsorption of uncomplexed mercury(II) species on the immobilised thiol groups.
1. INTRODUCTION After a pioneering work of Kolbe [ 1], St6ber et al. have systematically studied the synthesis of spherical silica particles from TEOS in alkaline hydro-alcoholic medium [2]. Afterwards, several investigations were directed to better controlling and understanding the factors affecting the production of such silica spheres, the characterization of their structure and the mechanisms responsible for their formation [3-5]. TEOS is hydrolyzed in solution and by-product can condense to form siloxane bridges. Condensation leads to the precipitation of spherical silica particles. These particles can find application as highly efficient packings in high-resolution separation such as highperformance liquid chromatography (HPLC) and electro-chromatography [6,7]. However, their modification by covalent grafting of organic groups is though to lead to low loading capacity because of the strongly restricted access to most of the silanol groups located in the microstructure [5]. By another way, scientists of the Mobil Oil Company have discovered in 1992 a new class of porous adsorbents that are ordered mesoporous silica-based materials [8,9]. The most common, labeled MCM-41 (MCM for Mobil Composition of Matter), presents an hexagonal arrangement of cylindrical pores. The synthesis occurs by synergetic self-assembly in solution containing silica precursors and surfactant molecules. Interaction between silicate species and surfactant molecules
616
leads to precipitation of an organised hybrid organic-inorganic solid. Surfactant molecules can be removed by calcination resulting in pure mesoporous silica. This discovery has opened a large field of research in material science [10-12]. Considerable interest has been focused on their functionalization by covalent coupling of organic moieties, because combining such rigid inorganic structure with specific functions could lead to attractivel applications such as toxic metal-ion extraction from aqueous media and catalysis [13,14]. Grafting of organosilanes has been largely used in this way [ 13,14]. More recently, Burkett et al. have reported the preparation of organically functionalized mesoporous silica by co-condensation of TEOS and organosilane precursor in the presence of surfactant templates [15]. The resulting materials were the first examples of covalently linked, ordered, hybrid inorganic-organic networks. Moreover, functionalization by co-condensation seems to be more homogeneous than by grafting [ 16] and allows a better control of organic function content, especially for high content [ 17]. Another part of this important research effort has been directed toward the control of the macrostructure morphology in order to make spheres [18], fibbers [19], and films [20]. Recently, Unger et al. have prepared a silica gel material with surfactants in ammoniacal medium [21-23]. The new material possess MCM-41 and St6ber silica particle properties, i.e. spherical morphology, large surface area (700-1500 m2/g), mesoporosity with narrow pore-size distribution and regular pore structure, which can be observed by low angle X-ray diffraction experiments. This new material has been modified with heteroatoms (A1, Cr, V, Ti, Fe, Nb and Ga) for catalytic applications [24-26]. In this paper, the preparation of ordered mesoporous spherical silica particles functionalized by co-condensation between TEOS and organosilanes, especially aminopropyl-triethoxysilane (APTES) and mercaptopropyltrimethoxysilane (MPTMS), is reported. The functionalization of nonporous St6ber silica particle by co-condensation has already been proposed in literature [27-29]. But up to now, this work is the first example of preparation of organically-modified mesoporous silica spheres. Mercury adsorption has been performed on both amino and mercapto functionalized silica. Electrochemical techniques have been largely used in order to characterize sorption kinetics of Hg(II) in batch reactors and to study the interaction between functionalized silica and mercury(II) at the electrode/solution interface by means of carbon paste electrodes modified with these organicinorganic hybrids. 2. EXPERIMENTAL SECTION Synthesis conditions were inspired by the work of Schumacher et al. [26]. 2.4 g (6.6 mmol) of cetyltrimethylammonium bromide (CTAB) was dissolved in 50 ml of water, 50 ml of ethanol and 13 ml of 28 % ammoniac solution. In order to obtain a TEOS/organosilane molar ratio of 90/10, 0.36 g (1.6 mmol) of APTES or 0.32 g (1.6 mmol) of MPTMS was mixed with 2.9 g (13.9 mmol) of TEOS. The resulting mixture was added to the surfactant solution under magnetic stirring. A white precipitate was observed after few minutes of reaction. Stirring was maintained for 2 h and the product was filtered, washed with ethanol and dried under vacuum (< 10.2 bar). Final products are named APTES-10% and MPTMS-10%, respectively. Template removal was achieved by refuxing the materials in 1 M HCl-ethanol solutions during 24 h, calcination being prevented because of the presence of the organic moieties. The solids were then filtered, washed with ethanol and dried. XRD experiments have been performed with the X'PERT PRO from Philips (copper anode, Ka1=1.54056 fit). TEM observations have been made by using a Philips CM20 microscope (200 keV, punctual resolution: 0.27 nm). N2 adsorption experiments have been performed at 77 K with a Coulter SA 3100 apparatus.
617
Mercury(II) determination for batch experiments has been performed by anodic stripping differential pulse voltammetry on rotating gold electrode (a - 4 mm, co - 500 rain 1, electrolysis at 0.3 V during 30 s) in 10-ml electrolyte solution (72 mM NaC1, 12 mM EDTA, 2.8 M HC104). Hg(II) adsorption experiments involving the aminated silica were carried out by dispersing 42 mg of APTES10 % in 10.4 ml solution containing 2.0x 10.4 M Hg(II) and 0.1 M HC1. Hg(II) adsorption on MPTMS10% was typically performed by using 20 mg og the material in 200 ml solution containing 3.0x 10-4 M Hg(II), 5x 10.3 M HNO3 and 0.1 M NaNO3. Equilibrium concentrations have been determined after 24 h of constant stirring. The kinetic studies were conduced in 200 ml solutions containing 3.0x10 4 M Hg(II), 5x10 3 M HNO3 and 0.1 M NaNO3.20 ml of MPTMS-10% material was added at t = 0 and consumption of mercury(II) was monitored by chronoamperometry using a rotating glassy carbon electrode (e - 4 mm, co - 2000 min l, applied potential = -0.5 V). Electrochemical measurements have been performed by using modified carbon paste electrodes. The preparation of the electrodes was described elsewhere [35]. Carbon paste was composed of 10 % of the modified silica, 60 % of carbon powder and 30 % of mineral oil. Accumulation of mercury was made during 3 min at open circuit in solution containing 10.5 M Hg(II) and 0.1 M HNO3. The electrode was then washed and immersed in the detection medium containing thiourea (5%) and 0.1 M HC1. Electrolysis was peformed at a potential of-0.5 V during 30 s and detection was made by anodic stripping differential pulse voltammetry. 3. RESULTS and DISCUSSION 3.1. Synthesis and characterisation of the new materials
Figure 1 shows XRD patterns obtained from the MPTMS-I 0% material before (pattern a) and after (pattern b) the extraction of the surfactant. Three peaks are observed on each curve, which correspond to diffraction plans that could be indexed 100, 110 and 200. The two XRD patterns are characteristic of a MCM-41 structure (hexagonal symmetry). Similar X-ray patterns were observed with APTES-10% material before and after surfactant extraction. Whatever the organosilane, a MCM4 l-type structure was obtained. Syntheses with a more important amount of organosilane (20 %) have been made. However, increase of the amount of organic functions leads to a less ordered material.
b l~ ii /!
/!
20000
/ I
~=
15000
/
'!
/ 10000
2
!
|
4
6
2 theta
,
| 8
, 10
(degrees)
Figure 1. X-ray patterns of MPTES-10% material. (a) As-synthesised and (b) surfactant free material.
618
Nitrogen adsorption measurements at 77 K have been run on the functionalized materials after surfactant extraction. The isotherm obtained from APTES-10% material is reversible and does not exhibit any hysteresis between the adsorption and desorption branches. Similar result was obtained with MPTMS-10% material. These curves can be classified as a type IV isotherm according to the IUPAC nomenclature [3 I]. Specific surface area and total pore volume of the two functionalized materials have been reported in Table 1. MPTMS-10% material exhibits a very important specific surface area close to 1600 m2/g that is much higher than the APTES-10% specific surface area (1040 m2/g). Table 1. Characterization of modified silica materials: porosity and loading. Material APTES- 10% MPTMS-10%
Specific surface area (m2/g) 1040 1580
Total pore volume
Organic group content (mmol/g)
0,64 0,78
1.1 (NH2) 1.0 (SH)
(cm3/g)
Two reasons can be proposed to explain this difference. First, surfactant extraction has been conducted in ethanol/HC1 medium so that the amine groups have been transformed into ammonium groups associated with chloride ions (-NH3 § CI). On the other hand, the thiol groups do not react with HC1. The concomitant formation of ammonium together with charge compensation with C1- can result in a porosity decrease. Secondly, determination of specific surface area depends on the interaction between the surface of the material and the nitrogen molecule. This interaction could be different between uncharged groups (-SH) and charged groups (-NH3 +, C1-). Charges can therefore interfere in specific surface area determination. Finally, elemental analysis (Table 1) shows that MPTMS-10% contains 1.0 mmol/g of sulphur and APTES-10% 1.1 mmol/g of nitrogen. The small excess in nitrogen in comparison with sulphur could be explained by a residual quantity of CTAB in mesopores.
Figure 3a. TEM photography of a APTES- 10% particles aggregate.
Figure 3b. TEM photography of an isolated APTES- 10% particle.
619
Transmission electron microscopy (TEM) enables to visualize the spherical shape of the assynthesised materials. Figure 3a shows that samples are made of particles with diameters ranging between 200 and 500 nm, being often associated in aggregates. All the particles have a spherical form. An isolated particle is visible on Figure 3b. The hexagonal array of uniform mesopores can be observed. This isolated particle has 290 nm diameter. Two peaks were observed on the distribution of particle size before ultrasonic treatment. The smaller particles size distribution corresponds to isolated particles and the bigger to the aggregates. Ultrasonic treatment results in an increase of the smaller particle size distribution frequency and a displacement toward smaller particle size of the aggregate. Ultrasonic treatment allows to break many of the aggregates and to liberate isolated particles. A recent publication reports the control of the MCM-41-type nanoparticle aggregation by quenching the condensation reactions (dilution followed by pH neutralisation) during the early stages of the synthesis of ordered mesoporous silica [30]. A similar procedure would be followed in the near future to obtain isolated ordered mesoporous silica spheres functionalized by co-condensation of TEOS and organosilanes. It was recently shown that grafting a thiol function to ordered mesoporous supports enhanced significantly the adsorption of mercury(II) species from aqueous solution [ 13]. The higher capacity of the organically modified mesoporous molecular sieves toward metal species, as compared to a silica gel grafted with the same ligand, was attributed to easier access to the binding sites located inside the uniform pore structure of the molecular sieve. Moreover, recent publications report the use of amino and mercapto functionalized sol-gel for selective separation of copper(II) [32] and the use of functionalized mesoporous silica with controlled macrostructure for heavy metal remediation [33]. Heavy metal sorption has been studied in order to compare properties of the new materials with those of the literature. New methodologies are proposed.
3.2. Heavy metal accumulation Three different approaches to study the interaction between functionalized silica and a heavy metal are presented: classical batch experiment, new batch studies of kinetics as well as the use of a carbon paste electrode modified with these materials. Mercury(II) is used as a model in order to investigate the accumulation by both complexation and ion-exchange.
3.2.1. Batch experiments Mercury(II) can be found in aqueous medium under cationic, neutral or anionic forms, depending on pH and chloride concentration. Then, it is liable to interact with the materials surface either by ion exchange or complexation. In acidic medium, APTES-10% exhibits positively charged ammonium groups. Batch experiment has been performed in solutions containing 0.1 M HC1 and 2.0x 10.4 M Hg(II). In these conditions, mercury speciation is 40 % HgCI2, 48 % HgC13- and 11% HgC142. Mercury can adsorb on silica surface by ion exchange between anionic chloro-complexes of mercury(II) and chloride ions associated with ammonium groups. The capacity results from an equilibrium between solid and solution and appears to be largely inferior to the ammonium loading. After 24 h equilibration, the mercury capacity was found to be about 102 mmol/g. On the other hand, the strong interaction between thiol groups immobilized on silica and mercury ions has been already reported by Pinnavaia et al. [13,36] in order to study the influence of organisation and mean pore size on the accessibility to the binding sites in the materials. Experiments were performed here with solution containing 2.0• .4 M Hg(II) at pH 1. Mercury is present in excess with respect to SH groups. After 24 h equilibration, an accessibility of 60 % was measured. It is closed to the results obtained by Brown et al. who have used copolymerized material with disordered assemblies of wormlike channels [36].
620
3.2.2. Kinetic experiments A new investigation method has been introduced in order to determine the apparent diffusion coefficient in the mesoporous materials. The model used is rather simple and considers homogeneous diffusion in spherical particles. The mercury concentration decrease is described by equation (1) where Q is the amount of mercury(II) adsorbed at t and Q0 the maximal amount of adsorbed mercury (previously determined by batch experiment). Equation (2) shows the mathematical model describing Q/Q0 versus the apparent diffusion coefficient D and the particle radius a [34]. Q
Amount of adsorbed mercury(II) at t
Q--~ Maximal amount of adsorbed mercury(II)
Q =6 Qo ~-
* ;r -~
(1)
2y](-1)",erfc -~+
~,~5-)
(2)
-
I!=1
a: Particle radius (cm); D: diffusion coefficient (cm2/s) [34] Two difficulties were encountered. First, particles of MPTMS-10% materials self-aggregate during synthesis. The MPTMS-10% solid has been treated with ultrasounds during 5 min before dispersion in solution. Granulometric measurement has been performed in the same conditions to appropriately evaluate the mean particle size. A median radius of 2.95 ~tm was obtained from the particle volume distribution. However, ultrasonic treatment does not permit to break all the aggregates. The second difficulty is related to the fast decrease in mercury(II) concentration, which occurs immediately after the addition of the MPTMS- 10% material, requiring a rapid determination of mercury during the first minute of the experiment. This problem was solved by using chronoamperometry at rotating glassy carbon electrode. The recorded kinetic profile can then be compared to the simulation curve (Fig. 4). The two curves correlate very well for a diffusion coefficient of 2x 10~2 cmZ/s. This result allows us to confirm the usefulnees of the method and to open a new field of research on the influence of the pore structure on the diffusion kinetics in mesoporous materials. 0,3 Simulation curve
L,,~,~.~
0,2
Q/Qo
/
0,0
0
Experimental curve
i
I
100
J
I
2O0
a
300
Time (seconde)
Figure 4. Simulation and experimental curve of mercury adsorption by MPTMS-10%.
621
3.2.3. Electrochemical experiments with modified carbon paste electrode Modified carbon paste electrodes have been already used in our group to investigate electrochemical sensor applications of pure [37, 38] and amine-modified silicas [35]. A MPTMS-10% modified carbon paste electrode has been prepared and used for the detection of mercury(II) in aquous solutions. Sorption occurs only at the electrode/solution interface without changing macroscopically the mercury concentration in solution, then this approach can bring important information on the sorption processes at the particle level. Figure 5 shows the response of the modified electrode obtained by differential pulse voltammetry before and after 3 rain preconcentration in a solution containing mercury at the 10-5 M concentration level. The feasibility of the method is demonstrated and could bring some useful fundamental information related to mass transfer processes at the electrode/ solution interface or could allow to develop new amperometric sensor-oriented applications.
51JA
b
, -0,6
i -0,5
,
i -0,4
,
i -0,3
,
i -0,2
,
i -0,1
,
i 0,0
,
i 0,1
,
i 0,2
, 0,3
potential (V) Figure 5. Voltammetric response of carbon paste modified by 10 % (weight) of MPTMS-10%, (a) before and (b) after accumulation in 104 M Hg(II) (and 0.1 M HNO3).
4. CONCLUSION A new material has been synthesised by co-condensation of TEOS and organosilanes in hydro-alcoholic solution containing ammoniac as catalyst and CTAB as templating molecule. Spherical particles with MCM-41 architecture have been obtained. Amine groups (APTES-10%) and thiol groups (MPTMS-10%) have been incorporated in the silica structure. APTES-10% material can adsorb 10.2 mmol/g of mercury(II) by ion exchange of chloride ions and anionic chloro-complexes of Hg(II) in 0.1 M HC1 solution. MPTMS-10% exhibits a very strong interaction with Hg(II). A capacity of 0.6 mmol/g has been measured in batch, corresponding to an accessibility of 60 %. MPTMS-10% material has been used as model for a new kinetic approach of the mercury adsorption in mesoporous materials. An apparent diffusion coefficient of 2x10 -12 cm2/s has been calculated. This material has been incorporated into a carbon paste electrode and the feasibility of the accumulation - electrochemical detection process has been demonstrated.
622
REFERENCES
1 2 3 4 5 6 7 8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
G. Kolbe, Ph.D. Thesis, Friedrich-Schiller-Universit/it Jena, Germany, 1956 W. St6ber, A. Fink, E. Bohn, J. Colloid Interface Sci., 26 (1968) 62. H. Giesche, (a) J. Eur. Ceram. Soc., 14 (1994) 189, (b) J. Eur. Ceram. Soc., 14 (1994), 205. A. Labrosse, A. Bumeau, J. Non-cryst. Solids, 221 (1997) 107. A. Walcarius, C. Despas, J. Bessi&e, Microporous Mesoporous Mater., 23 (1998) 309. S. Lfidtke, T. Adam, K.K. Unger, J. Chromatogr. A, 786 (1997) 229. K.K. Unger, D. Kumar, M. Griin, S. Lfidtke, Th. Adam, K. Schumacher, S. Renker, J. Chromatogr. A, 892 (2000) 47. 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. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature, 359 (1992) 710. J.Y. Ying, C.P. Mehnert, M.S. Wong, Angew. Chem. Int. Ed. Engl., 38 (1999) 56. D. Brunel, N. Bellocq, P. Sutra, A. Cauvel, M. Lasp6ras, P. Moreau, F. Di Renzo, A. Galameau, F. Fajula, Coord. Chem. Rev., 178-180 (1998) 1085. A. Corma, Topics Catal., 4 (1997) 249. L. Mercier, T. Pinnavaia, Adv. Mater., 9 (1997) 500. C.P. Jaroniec, M. Kruk, M. Jaroniec, A. Sayari, J. Phys. Chem. B, 102 (1998) 5503. S.L. Burkett, S.D. Sims, S. Mann, Chem. Commun., (1996) 1367. M.H. Lim, C.F. Blanford, A. Stein, J. Am. Chem. Soc., 119 (1997) 4090. Y. Mori, T.J. Pinnavaia, Chem. Mater., 13 (2001) 2173. Q. Huo, J. Feng, F. Schfith, G.D. Stucky, Chem. Mater., 9 (1997) 14. P. Yang, D. Zhao, B.F. Chmelka, G.D. Stucky, Chem. Mater., 10 (1998) 2033. M. Ogawa, J. Am. Chem. Soc., 116 (1994) 7941. M. Griin, I. Lauer, K.K. Unger, Adv. Mater., 9 (1997) 254. M. Griin, K.K. Unger, A. Matsumoto, K. Tsutsumi, Microporous Mesoporous Mater., 27 (1999), 207. D. Kumar, Schumacher K., C. du Fresne von Hohenesce, M. Grfin, K.K. Unger, Colloids Surf. A, 187-188 (2001) 109. K. Schumacher, C. du Fresne von Hohenesce, K.K. Unger, R. Ulrich, A. Du Chesne, U. Wiesner, H.W. Spiess, Adv. Mater., 11 (1999) 1194. K. Schumacher, M. Grfin, K.K. Unger, Microporous Mesoporous Mater., 27 (1999) 201. A. Matsumoto, H. Chen, K. Tsutsumi, M. Griin, K. Unger, Microporous Mesoporous Mater., 32 (1999) 55. J.Y. Choi, C.H. Kim, D.K. Kim, J. Am. Ceram. Soc., 81 (1998) 1184. E. Yacoub-George, E. Bratz, H. Tiltscher, J. Non-Cryst. Solids, 167 (1994) 9. A. van Blaaderen, A. Vrij, Adv. Chem. Ser., 234 (1994) 83. C.E. Fowler, D. Khushalani, B. Lebeau, S. Mann, Adv. Mater., 13 (2001) 649. S. Brunauer, L.S. Deming, W.S. Deming, E. Teller, J. Am. Chem. Soc., 62 (1940) 1723. H.-J. Ira, Y. Yang, L.R. Allain, C.E. Barnes, S. Dai, Z. Xue, Environ. Sci. Technol., 34 (2000) 2209. I.N. Nooney, M. Kalyanaraman, G. Kennedy, E.J. Maginn, Langmuir, 17 (2001) 528. J. Crank, The mathematics of diffusion (2"d Ed.), Clarendon Press, Oxford, 1975. M. Etienne, J. Bessi&e, A. Walcarius, Sensors Actuators, B76 (2001) 531. J. Brown, L. Mercier, T.J. Pinnavaia, Chem. Commun., (1999), 69. A. Walcarius, C. Despas, J. Bessi&e, Anal. Chim. Acta, 385 (1999) 79. A. Walcarius, J. Devoy, J. Bessiere, Environ. Sci. Technol., 33 (1999) 4278.
OttlUlt75
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A. Sayariand M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.
623
NO and NO2gas sensors based on surface photovoltage system fabricated by self-ordered mesoporous silicate film Hao-Shen Zhou a*, Takeo Yamada a'b, Keisuke Asai b, Itaru Honma a, Hidekazu Uchida r and Teruaki~Katsuber a Energy Materials Group, Energy Electronics Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, JAPAN b Department of Quantum Engineering and Systems Science, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8656, JAPAN c Department of Information and Computer Science, Saitama University, 255 Shimo-Okubo, Urawa, saitama, 338-8570, Japan
The first NO and NO2 gas sensors based on surface photo-voltage (SPV) semiconductor device system are fabricated by the metal/ SiO2 (self-ordered hexagonal mesoporous)/Si3N4/SiO2/Si structure. Size controlled silicate hexagonal mesoporous film is successfully synthesized by spin coating on a Si3N4/SiO2/Si silicon wafer using poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) (Pluronic P 123 =EO20PO70EO20) triblock copolymers as a template. The characteristics of mesoporous film are investigated in XRD, TEM. The sensing properties of the self-ordered hexagonal mesoporous SPV system have been investigated by exposing to the NO or NO2 gas and air repeatedly. The changes of the average value and phase of the AC photocurrent (Iph) have been observed between the NO or NO2 gas and air. The response of the alternatively photocurrent is resulted from the physical adsorption and chemical interaction between detected NO or NO2 gas and the self-ordered hexagonal mesoporous film.
1. INTRODUCTION The novel silicate / aluminosilicate mesoporous material M41S was synthesized by ionic surfactant template in 1992 [1], which had ordered and aligned pore in the mesoporous
* Correspondence author (E-mail:
[email protected])
624 materials region. The mesoporous material has attracted considerable interests because its orientation and uniformity are of benefit to the applications in molecular sieve, catalyst, and adsorbent [2]. Efforts to create sensor, electronic and photonic device [3,4], based on ordered molecules, have been inspired by these self-assembled mesoporous materials. Nitrogen oxide generated by combustion is not only dangerous and harmful to health, but also bring the acid rain to destroy the construction. So, it becomes very important to monitor the nitrogen oxide from the combustion source by a simple on site method. The properties of high surface area and bi-continuous in the mesoporous material make it an idea sensitive material as a gas sensor. Here, we report the first application of the self-ordered hexagonal mesoporous silicate film for NO and NO2 gas sensors. The NO and NO2 gas sensors are based on the surface photo-voltage (SPV) semiconductor characterization system. The self-ordered hexagonal mesoporous silicate film is synthesized using commercially available poly (ethylene oxide)poly (propylene oxide)-poly (ethylene oxide) (EO20-PO70-EO20) type triblock copolymer as a template by spin coating [5,6]. The characteristics of mesoporous film are investigated in XRD, TEM. The SPV system has been applied to the Metal/MesoporousSiO2/Si device structure (MIS) to investigate the sensing properties as NO and NO2 gas sensors.
2. SYNTHESIS AND CHARACTERISTICS OF MESOPOROUS FILM 2.1. Synthesis of Mesoporous Film The source sol solution is prepared from two solutions. One is the ethanol solution of PEO-PPO-PEO triblock copolymer with stirring for 2 hours at room temperature. The other is the silica sol-gel solution mixture of tetraethoxysilane (TEOS), ethanol, water and HC1 with stirring for several hours at a range from room temperature to 75~ The mole ratio of the chemicals is 1 TEOS: 0.005-0.018 EO20-PO70-EO20:15 H20:0.16 HCI: 39 ethanol. Here, we call silica sol-gel solution's stirring time as synthesis time and silica sol-gel solution's stirring temperature as synthesis temperature. After mixing these solutions, the mixed solution is stirred for another 2 hours at room temperature. Then, the coating solution is used for film deposition on glass substrates by spin coating. Finally, the calcination is carried out at 450~ for 4 hours. After these processes, we can get homogeneous and transparent mesoporous
silicate thin films. 2.2. Characteristics of Mesoporous Film The typical XRD pattern of calcined mesoporous silicate thin films using EO20-PO70-EO20 as template (Fig. 1) shows three well resolved peaks with d spacing of 4.81, 2.50 and 1.65nm, which index as (100), (200) and (300) reflections associated with one dimension hexagonal structure. Comparing to the XRD pattern of powder hexagonal mesoporous silicate, the (110), (210) and (220) reflections disappear. It suggests that the films have a highly oriented hexagonal structure and the pore channels are parallel to the substrate surface [7]. The (100)
625 peak reflects a d spacing of 4.81nm, corresponding to a large unit cell parameter {a = 2 ~ A/3 = 5.55nml)}.
Fig. l(a) X-ray diffraction pattern of calcined mesoporous silicate thin film from P123
Fig. 2(a) The d spacing depends on the polymerization time and temperature
Fig. l(b) TEM image of calcined mesoporous silicate thin film from DIOQ
Fig. 2(b) The d spacing depends on the molar ratio P 123 to TEOS
The size of mesoporous silicate thin film from EOz0-PO70-EO20 can be controlled by three synthesis parameters, the synthesis time, the synthesis temperature and the mole ratio of EOz0-POy0-EO20 to TEOS. Fig. 2(a) shows the d spacing dependence on the synthesis time and temperature. This figure shows that the d spacing increased with both synthesis time and temperature. Finally, Fig. 2(b) shows the d spacing tendency on the mole ratio of EOz0-PO70-EO20 to TEOS. It shows that the d spacing increased with decreasing the mole ratio. The d spacing is controlled
626 by this parameter with ordered mesostructtwe. However, in this parameter, the mole ratio decreases with relatively increasing amount of silica precursor TEOS. In the mesoporous silicate using surfactant as template, the thickness of silica framework can be controlled by the ratio of surfactant to silica precursor [8]. Hence, this parameter is the mole ratio of triblock copolymer to TEOS which might control the thickness of silica framework [9].
3. APPLICATION FOR NO GAS SENSOR
The mesoporous NO gas sensor is fabricated on the SPV system which is shown in Fig.3. The sensor device consists of a typical metal-insulator-semiconductor (MIS) junction structure. The substrate silicon wafer (n-type 30-50 f2cm) with a SiO2 layer and a Si3N4 layer is commercial available. The gas sensive layer (= self-ordered hexagonal mesoporous film) is spin-coated on the Si3N4 layer by above mentioned synthesis method using the EO20-POT0EO20 as a template to form a SiO2(meso)/Si3N4/SiO2/Si structure. The thickness of the coated mesoporous film is about 200nm. Finally, the metal (Au) gate layer for electrode is sputtered on the mesoporous film. The other side (back) of silicon wafer has two regions. One region consists of a deposited A1 metal layer for ohmic contact. The other one is an un-deposited transparent window for LED illumination. Electric response of the sensor to NO or NO2 gas is measured under cyclic gas flow between the NO or NO2 gas (100 ppm) and the standard air, which is controlled by mass flow controller (Aera Japan Limited: SG7S 1) and multi-port valve.
Exhaust Treatment
Mass Flow Controller Mass Flow Controller
Samplel
~7 I MassFlow
~
/_.._~l COntrOller
\1/
i
[~
I L~ Amplifier
SG-7S1 Fig. 3 The gas flow diagram and SPV gas sensor detection system. The sensing principle is based on the detection of semiconductor surface charge and potential's change caused by the physical adsorption and chemical reaction due to the interaction between gases and gas sensitive film [10,11,12]. The band diagram of a MIS and the corresponding band diagram with the surface potentials are shown in Fig. 4(a). A bias voltage is applied on the MIS structure by a Lock-in amplfier (Stanford Research systems:
627 SR830). A modulated alternatively LED beam (~=1 cm, 930nm, lkHz) is used to irradiate on the back of the semiconductor surface to generate the alternative photocarriers. These alternative photocarriers move and trap in the surface range of the silicon substrate by bias voltage to form AC photocurrent. When NO or NOz gas (mixed with air gas) is flowed through the measurement system, the AC photocurrent is effected by the physical adsorption and chemical interaction between the target gases and gas sensitive film. The accumulation, depletion and inversion cases exist at the semiconductor surface when the MIS structure is applied and swept from positive to negative voltages. The equivalent circuit of SPV gas sensor is shown in Fig. 4(b). Rs is the resistance associated with the interface traps in semiconductor layer. Ci and CD are the insulator capacitance and the semiconductor depletion layer capacitance, respectively. Cs and Rs are the capacitance and resistance associated with the interface traps and are functions of surface potential.
-
"--'_ L
~i
V>O-'-If-"
"
-
""
GIN
EcEF
Cd
q* ~'~'~'~ 'l . . . . . . . . E, L~I + + Ev
I
Rs
1 Fig. 4(b) The equivalent circuit for SPV gas sensor system.
Fig. 4(a) The scheme of energy band diagram.
The C-VG curve's shift resulted from the change of the oxide layer's dielectric constant and charges by the physical adsorption and chemical interaction between detected gases and the gas sensitive film. In the physical adsorption case: _ Qo _
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In the chemical interaction case, chemical interaction at the surface area of the mesoporous layer gives the net mobile ions and produces the change of the dielectric constant.
Qo AV~(~ - c~
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Ci(oN~
Coei
(ei + lxei)
Here, ~i is the dielectric constant of the insulator layer, di is the thickness of insulator layer. Ci is the insulator capacitance including Cm~so,C S i 3 N 4 , C s i o 2 . And the VGis the bias voltage. So, we can know the C-Vo or Iph-VG curve's shift as exposing to NO gas.
628 4. RESULTS AND DISCUSSION Fig. 5 shows the average value of alternative photocurrents (Iph) as a function of applied bias voltage measured in air and NO gas (=mixture of NO gas and air) condition. The shift of Iph curve to negative direction in 100 ppm NO gas condition indicates that the negative surface potential change is produced due to the reaction between NO gas and sensitive layer mesoporous film. The similar phenomenon also is observed in phase curve of the alternatively photocurrents (Iph) as a function of applied voltage measured in air gas and 100ppm NO gas (=mixture of NO gas and air gas). 4
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Fig. 6 The photocurrent-bias characteristics of the the mesoporous NO gas sensor from air condition to 100ppm NO
Fig. 5 The response reversibility of the mesoporous NO gas sensor measured repeatedly from air condition to 100ppm NO gas exposure 4
-
oe
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.
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Fig. 7 The response reversibility of the mesoporous NO2 gas sensor measured repeatedly from air condition to 50 ppm NO2 gas exposure
0
0.5
1 Bias[V]
1.5
2
Fig. 8 The photocurrent-bias characteristics of the the mesoporous NO2 gas sensor from air condition to 50 ppm NO2
Fig. 6 shows the response reversibility of the mesoporous NO gas sensor, which was measured repeatedly in air gas condition after exposed to the 100 ppm NO gas condition.
629 Although the response reversibility of the mesoporous NO gas sensor is still not so good, it should be optimized by improving the structure of gate catalytic metal layer to directly introduce detected gas to mesopores of the mesoporous layer. The results of the relationship between average photocurrents (Iph) and the applied bias voltage measured in air and NO2 gas (50 ppm) are shown in Fig. 7. Fig. 8 shows the response reversibility of the mesoporous NO2 gas sensor. The sensing result of NO2 gas based on hexagonal mesoporous SPV system is better than that of NO gas. The Au-SPV device without self-order hexagonal mesoporous layer has also been investigated for comparing. Although some response current can been observed on the first step of exposing to the NO gas. However, after the first gas adsorption step, the current exhibits an unrecoverable property. The surface area and the pore size of the mesoporous film synthesized by P 123 have not been investigated although those of mesoporous powder synthesized by P123 in the same condition have been valued by the N2 gas adsorption and desorption method. The pore size is about 6.3nm, and surface area is about 800m2/g including both mesopore and micropore [13,14]. The micropore surface area in the mesoporous material synthesized from tri-block copolymer is about 30% of the total surface area and the micropore size is about 0.8nm [14,15]. These micropores connect the hexagonal mesopore and make them to form a bicontinuous micro-mesoporous material. The NO gas can enter the hexagonal channels through the both micropore and mesopore. So, the micropores take a very important role in the sensing process.
5. CONCLUSION Mesoporous silicate thin films using commercially available EO20-PO70-EO20 type triblock copolymer as template by spin coating was synthesized. These films formed in one dimension hexagonal mesostructure by XRD pattern. We succeeded in controlling the size of mesoporous silicate thin films. The application of this self-ordered hexagonal mesoporous silicate film for NO and NO2 gas sensor has been successfully fabricated using a MIS[=SiOz(hexagonal meso)/Si3N4/SiO2/Si] device based on the surface photo voltage (SPV) system. The sensing properties of the self-ordered hexagonal mesoporous SPV system have been investigated by exposing to the 100 ppm NO (or 50 ppm NO2) gas and standard air repeatedly. The response of the alternatively photocurrent, which resulted from the physical adsorption and chemical interaction between detected NO (or NO2) gases and the selfordered hexagonal mesoporous film, has been observed. These results can be clearly explained by the characters of the hexagonal mesoporous silicate film, which has got the high surface area and bi-continuous mesopore structure. And this kind of mesoporous film indicates great possibility as a high sensitive and responsible gas sensor application.
630 6. ACKNOWLEDGMENTS Research Fund administered by the New Energy and Industrial Technology Development Organization of Japan is gratefully acknowledged for a partial support of this research.
REFERENCES
1.
,
3. 4. 5. 6. .
8.
9. 10. 11. 12. 13. 14. 15.
J.S. Beck, J. C. Vartuli, W. Ji Roth, M. E. Lenowicz, C. T. Kresge, K. D. Schmitt, C. TW. Chu, D. H. Olson, E. W. Sheppared, S. B. Mcculen, J. B. Higgins and J. L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834 X. S. Zhao, G. Q. (Max) Lu and G. J. Millar, Ind. Eng. Chem. Res. 35 (1996) 2075 H. S. Zhou and I. Honma, Chem. Lett. (1998) 973, H. S. Zhou, H. Sasabe and I. Honma, J. Mater. Chem. 8 (1998) 515 D. Zhao, Q. Huo, J. Feng, F. Chmelka and G. D. Stucky; J. Am. Chem. Soc. 120 (1998) 6024 D. Zhao, P. Yang, N. Melosh, J. Feng, B. F. Chmelka, and G. D. Stucky, Adv. Mater. 10 (1998) 1380 D. Kundo, H. S. Zhou and I. Honma, J. Mater. Sci. Lett. 17 (1998) 2089 M. Ogawa, J. Am. Chem. Soc. 116 (1994) 7941 M. Ogawa, T. Igarashi and K. Kuroda, Bull. Chem. Soc. Jpn. 7 (1997) 2833 E. A. Vasconcelos, H. Uchida, W. Zhang and T. Katsube, Jpn. J. Appl. Phys. 38 (1999) 2893 T. Sato, M. Shimizu, H. Uchida, and T. Katsube, Sensor and Actuators B, 20 (1994) 213 O.V.Fedosseeva, H. Uchida, T.Kasube, Y.Ishimaru, T.Iida, Sensor and Actuators B, 65 (2000) 55 R.Ryoo, S.Hoon, S.H.Joo, M.Kruk, & M.Jaroniec, Adv. Mater. 13 (2001) 677 M.Kruk, & M.Jaroniec, C.H.Ko, and R.Ryoo, Chem. Mater. 12 (2000) 1961 T. Yamada, K. Asai, H. S. Zhou, and I. Honma, to be submitted
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
631
Polymerisations in mesoporous environments James Clark, Duncan Macquarrie, Valerie Sage, Katie Shorrock and Karen Wilson Clean Technology Centre, Department of Chemistry, University of York, York YO 10 5DD, UK
The solid acid supported aluminium chloride is an effective cationic initiator for the polymerisation of hydrocarbons. Reactions are highly dependent on the nature of the active sites and the Lewis/Bronsted acid balance in particular.
1. INTRODUCTION Cationic polymerisation is one of the most important methods of forming hydrocarbon polymers. Aromatic monomers such as styrene and aliphatic monomers such as 1,3pentadiene are readily polymerised using strong Lewis acids, notably aluminium chloride. Among the more speciality hydrocarbon polymers, hydrocarbon resins (used in applications such as adhesives) are manufactured by reaction of mixed petroleum feedstreams with aluminium chloride or boron trifluoride. These reactions suffer from the need for an aqueous quench step to separate the organic products from the inorganic Lewis acids which adds a step to the process and generates large volumes of hazardous waste. As with all reactions catalysed by soluble Lewis acids, it would be desirable if cationic polymerisations could be carried out with simpler processes and producing less waste. In the light of this and the recent progress that has been made in developing effective methods for immobilising Lewis acids, notably on mesoporous solid supports ~, we have investigated the use of supported aluminium chloride in the polymerisation of aromatic, aliphatic and mixed monomers 2. As well as developing less wasteful and more efficient heterogeneous methods, we are also interested in understanding the importance of the nature of the acidity and the Lewis/Bronsted balance in particular. While aluminium chloride is a Lewis acid, adventitious water will in most "real" homogeneous situations lead to Bronsted acidity. Through the use of solid acids we have a better chance to study and control this acidity balance.
2. MATERIALS AND METHODS 2.1. Materials Supported aluminium chloride was prepared by our method described elsewhere 3 using either commercial or synthetic mesoporous silica and commercial aluminium chloride.
632 Once prepared the materials were dried under vacuum on a Schlenk line and stored in sealed containers under argon. 2.2. Measurements
Lewis and Bronsted acidities of the solids were routinely measured via spectroscopic titration by adsorbing pyridine onto the particulate solid which was then analysed by diffuse reflectance FTIR spectroscopy. Overall surface acidity was also approximately measured by adsorption of Hammett indicators. Polymerisation reactions were analysed by NMR and by GPC at 40~ with THF a solvent and a differential refractometer and UV detectors. 2.3. Methods
In a typical reaction the monomer free from any inhibitor, was outgassed with nitrogen before use. The required amount of the solid cationic initiator was then added and the polymerisation carried out with stirring under a nitrogen atmosphere.
3. RESULTS AND DISCUSSION For the polymerisation of styrene we have studied the effects of several variables:
The support. At a solid initiator loading of 0.53% (A1 loading on the silica of 0.8 mmol/g) the best conversion at 0~ was achieved using aluminium chloride supported on K100 support (average pore diameter 10 nm) compared to both smaller and larger pore commercial silica supports (Table 1). Table 1. Effect of the support on the immobilised aluminium chloride initiated polymerisation of StYrene . Silica support Conversion (%) Mz (g/mol) Mw (g/moi) Mn (g/mol) 'Polydisp. K60 22 7030 ' 4010 1'940 2.1 K100 57 6660 2220 840 2.6 Gasil (200A) 38 20750 9780 3270 3.0 As can be seen, larger pores lead to larger molecular weight products and to larger polydispersities as seems reasonable for a reaction that is largely in-pore.
Catalyst loading. Under the same conditions as above using the K100 support, we have found that conversion and to a lesser extent molecular weight is very dependent on the loading of A1 on the support (Table 2).
633 Table 2. Effect of A1 loading on the cationic polymerisation of styrene Catalyst loading Conversion Mz (g/mol) Mw (g/mol) (mmol/g) (%) 0.5 100 1080 760 0.8 50 8590 2800 1.2 41 12630 4250 1.67 39 10370 3510
.... Mn (g/mol)
Polydisp.
640 820 1120 1050
1.2 3.4 3.8 3.3
This is despite the known "optimum" loading for A1 on these supports of ca. 1.7 mmol/g. At such high loadings the material is mostly a solid Lewis acid as long as it is kept d r y the only source of protons being polarised surface OH groups which are largely reacted or blocked at high A1 loadings. This suggests that the balance of the Lewis and Bronsted acid sites can affect the efficiency of the polymerisation as well as the nature of polymer products. For the polymerisation of aliphatic monomers using supported aluminium chloride as a solid initiator we have also found a dependence on the nature of the surface acidity. For the polymerisation of 1,3-pentadiene the polymer yield steadily falls as the A1 loading is increased from 0.6 mmol/g to over 2 mmol/g. To further investigate this we have deliberately blocked the Bronsted sites by adsorbing the purely Bronsted base 2,6-di-tbutyl-4-methylpyridine. This shows that removal of Bronsted acid sites indeed results in a steady decrease in polymer yield but only to a limit. Once all of the Bronsted sites have been complexed and effectively removed from the reaction, a small but consistent amount of polymerisation remains. At a loading of 1.7 mmol A1/g, this corresponds to about 30% of the yield obtained in the absence of the pyridine. We are currently extending these studies to other solid acid initiators notably supported boron trifluoride complexes.
4. ACKNOWLEDGEMENTS We thank the EPSRC and ExxonMobil Chemicals for supporting this work.
REFERENCES
1. P.M. Price, J. H. Clark and D. J. Macquarrie, J. Chem. Soc. Dalton, (2000), 101. 2. For one of the first patents on the use of solid Lewis acids see L. Babcock, Int Pat., W098/30587, 1998; J. K. Shorrock, J. H. Clark, K. Wilson and J. Chisem, Org. Proc. Res. Dev., 5, (2001), 249. 3. J.H. Clark, K. Martin, A. J. Yeasdale and S. J. Barlow, Chem. Commun., (1995), 2037.
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A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
635
Incorporation o f nano-sized zeolites into a m e s o p o r o u s matrix, TUD-1 Z. Shah a, W. Zhou b, J.C. Jansen a, C.Y. Yeh c, J.H. Koegler d and Th. Maschmeyer a aApplied Organic Chemistry and Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands bSchool of Chemistry, University of St. Andrews, St. Andrews, Fife KY 16 9ST, UK. c ABB-Lummus Global Inc. 1515 Broad Street, Bloomfield NJ 07003, USA d ABB Corporate Research, Speyererstrasse 4, D69115 Heidelberg, Germany A hierarchically structured composite material with interconnecting meso- and micropores has been developed to optimize mass transfer. A generalized synthesis method has been setup, which allows various types of nano-sized zeolite to incorporate into a three-dimensional mesoporous matrix in a controlled manner. There is a great flexibility to tune the physicochemical properties and control the amounts of two components in the composite, while their bulky and surface properties will retain in the composite. The incorporation of beta zeolite shows high activity in the liquid phase alkylation of benzene to ethylbenzene. I. INTRODUCTION Zeolites with uniform microporous structures have been widely used in heterogeneous catalysis. To optimize their utilization, their primary particles should be no larger than a few tenths of nanometers, as in this case the diffusion path is relatively short and the accessibility high whilst preserving the beneficial properties of the zeolite, i.e. selectivity and high specific surface area. Packed beds of such small zeolite particles can not be used directly in industry due to the high pressure drop. They are normally mixed with binder and extruded into beads or pellets [ 1]. The binder offers physical ruggedness under operational conditions, but it hinders mass transport of reactants into and products out of the zeolite catalyst. Recently developed amorphous mesoporous materials are excellent media to support such nano-sized zeolites, due to their large pores, for instance, SBA-15 and TUD-1 with tunable pores up to about 25-30 nm [2, 3], thus overcoming the mass-transport barrier imposed by clay binders. So a composite with nano-sized zeolite particles homogeneously incorporated into a three-dimensional mesoporous material would be an improved class of catalytic materials. The well-defined pore structures in mesoporous materials such as M41S family [4, 5], MUS-n [6] and SBA-n [2] led to a great potential of applications in catalysis, separation [7], optical and electronic materials [8, 9]. But their practical use in catalysis is still deadly limited due to e.g. weak and low acidity resulting from their amorphous nature [ 10]. Thus the combination of crystalline
636 zeolite and well-organized amorphous mesoporous materials might broaden the application of mesoporous materials in catalysis. It is clear that such micro-mesoporous composites have both academic and industrial interests. Thus two main attempts have been reported to prepare such composites. The amorphous wall of MCM-41 was re-crystallized into zeolite, e.g. FAU [ 11] or MFI [ 12]. However, the reconstruction of the thin amorphous wall resulted in a lower integrity. Moreover, the amount of zeolite in this composite is severely limited. Another two-step syntheses have been reported, in which highly ordered mesoporous materials have been synthesized via self-assembly of preformed nano-sized cluster or nuclei of zeolite using surfactant micelles as templates [13-15]. With these methods zeolite is incorporated in mesoporous matrices, but the physicochemical properties of the zeolites, such as their compositions and crystal sizes, are difficult to tune. Moreover, the one-dimensional mesopore system in MCM-41 will naturally hinder the mass transfer, compared to a 3-dimensional mesoporous matrix. Here we report a new generalized approach to incorporate various nano-sized zeolites into a three-dimensional mesoporous matrix TUD-1, by adding the desired preformed zeolites into a precursor of TUD-1. This composite retains the surface and bulk properties of both the zeolite and the mesoporous TUD-1. This method shows a great flexibility in tuning physicochemical properties of both zeolites and mesoporous matrices. 2. EXPERIMENTAL
2.1 Synthesis A desired type of zeolite was homogeneously dispersed in demineralized water in a mass ratio of about 1/10~20. A mixture of tetraethyl orthosilicate (TEOS, 98% ACROS) and triethanolamine (TEA, 97% ACROS) was added into the above suspension whilst stirring. After stirring for about 2 h, tetraethylammonium hydroxide (TEAOH, 35% Aldrich) was added drop-wise to the above mixture under stirring. The vigorous stirring was continued until gelation of the synthesis mixture. In particular, a final synthesis gel has a mole composition of TEOS: 0.5 TEA: 0.1 TEAOH: 11H20, including a certain amount of zeolite (about 10-60% wt. zeolite in the final composite). Then following the simple procedure of TUD-1 synthesis [3], the solid gel was aged at room temperature for 6-24 h, dried at 100~ in air for 24 h, heated in an autoclave at 170~ for 2-24 h, and finally calcined at 570~ in air for 16 h. 2.2 Characterization X-ray powder diffraction (XRD) pattems were recorded using Cu-K~ radiation on a Philips PW 1840 diffractometer equipped with a graphite monochromator. The samples were scanned in a range of 0.5-40 ~ in 20 for about 30 min. In order to investigate the existence of zeolite, the composite was scanned from 5 to 50 ~ in 20 for 9 h to intensify the diffraction signal. Microporosity and mesoporosity were measured using argon adsorption and nitrogen sorption, respectively. Argon adsorption isotherms were recorded on Micromeritics ASAP 2010 at 87K. Nitrogen adsorption/desorption isotherms were measured on the Quantachrome Autosorb-6B at 77 K. Before measurements, all measuring samples were degassed at 350~ for 16 h. Micropore and mesopore sizes were calculated using Saito-Foley and BJH model from desorption branch, respectively.
637
High-resolution transmission electron microscopy (HRTEM) was performed on a Jeol JEM2010 electron microscope operated at 200 kV.
2.3 Catalyst test A composite with beta zeolite, denoted as beta-TUD-1, containing 16 wt % beta zeolite was ionexchanged with 0.5 M NH4NO3 solution at room temperature. After being washed and dried, it was mixed with 20% Nyacol alumina sol and calcined at 500~ The resulting catalyst contained 80% of beta-TUD-1, and thus had only 12.8 wt % of beta zeolite. It was ground to 0.8-1.4 mm in particle size before use. An 8-hour activity test was carried out in a differential recycle fixed-bed reactor with about 0.4 mol % of ethylene in benzene at 190~ For comparison, pure beta zeolite from the same source was extruded in the same way and tested under identical conditions. 3. RESULTS AND DISCUSSION
3.1 Synthesis In this synthesis method, nano-sized zeolite particles, typically about 50-60 nm, are added to a synthesis mixture of TUD-1, a mesoporous material. One of the challenges is to ensure the homogeneity of zeolite particles in the final mesoporous matrix. In general, the desired nano-sized zeolite powder is homogeneously dispersed in water. Silica sources such as TEOS and mesopore templates such as TEA were added into the above suspension under vigorous stirring. Then a base such as NH4OH or TEAOH is added drop-wise into the above mixture under stirring and consequently the synthesis mixture became a solid gel. Upon the vigorous stirring, homogeneously dispersed zeolite particles added before gelation will keep their homogeneity unchanged after gelation. The control of the gelation of silica sources has been well-addressed [ 16]. Under the synthesis conditions applied, the molar ratio of TEAOH/Si _<0.1 will favor the gelation, while the ratio > 0.2 often leads to a clear solution with zeolite powder at the bottom. The solid gel with a homogeneous dispersion of zeolite was aged and dried. At this stage no meso-structures formed. The dried gel (organic-inorganic hybrid) was then heated in an autoclave, and consequently meso-sized organic aggregates of TEA formed and eventually shaped the silica phase into the mesoporous structure [ 17]. After the removal of organic templates by calcination, a porous network with interconnecting mesopores and micropores is obtained. Zeolite particles in the solid hybrid gel will retain their homogeneity in the final mesoporous matrices after calcination. In general, there are many opportunities to choose zeolites, their compositions and their crystal sizes. The properties of the zeolite before incorporation are comparable to their properties in the composite. The high porosity of the mesoporous matrixes ensures a high access to the internal zeolites by external reagents. As an example, uncalcined beta zeolite with primary particle sizes of about 50-60 nm was incorporated into siliceous TUD-1, denoted as beta-TUD-1. 3.2 Structure and properties Figure 1 shows the X-ray diffraction (XRD) patterns of beta-TUD-1 (a) and beta zeolite added (b). Beta-TUD-1 shows a small but characteristic reflection peaks of beta zeolite at 22.4 ~ in 20,
638 which can be intensified (Figure l c) by extending scanning time from 0.5 h to 9 h. This peak obviously locates at the same position as that of the beta zeolite added (Figure l b). In addition, beta- TUD-1 also shows a peak at about 1.2 in 20, presenting the characteristics of a mesostructured material. This confirms the existence of beta zeolite in a meso-structured matrix. A high-resolution transmission electronic micrograph (HRTEM) image of the composite in Figure 2, clearly shows a three-dimensional foam-like or worm-like mesoporous matrix with some dark gray domains. These domains were estimated to be about 45-55 nm in size, close to the original particles added, but they are slight smaller, probably due to partial dissolution during synthesis and/or because their borders are vague due to being buried in the mesoporous matrix. The electron diffraction pattern of these domains (inset of Figure 2) gives a d-spacing of 1.17 nm, which is corresponding to the diffracted beam of the (101) or (011) planes of beta zeolite. It confirmed that they are beta zeolite crystals. These results indicate that nano-sized beta zeolite homogeneously dispersed in the mesoporous host.
Figure 1 XRD patterns of beta-TUD-1 (a), beta zeolite added (b) and beta-TUD-1 with long scanning time of about 9 h.
Figure 2 A HRTEM image of beta-TUD-l. Inset: the electron diffraction pattern of dark domains.
639 0.03 -l......~...........................................'...................................................................................................................... 6.0 b A
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Pore diameter (nm) Figure 3 Meso- and micropore size distribution plots of beta-TUD-1 (O) and TUD-1 without beta zeolite added 0)Nitrogen sorption showed a narrow mesopore peak around 9.1 nm, a surface area of about 710 m2/g and a pore volume of about 1.1 cm3/g in beta-TUD-1. Argon sorption showed a micropore peak around 0.64 nm, consistent with the characteristic pore size of beta zeolite, shown in Figure 3. However, it is difficult to get an accurate micropore volume due to the existence of an artificial peak or shoulder in the micropore region, resulting from the monolayer formation on the mesopore surface.[ 18] So a sample without the addition of beta zeolite was prepared with the same mesopore size and its porosity is also shown in Figure 3 for comparison. As shown in Figure 3a, the sample TUD-1 without beta zeolite also showed microporosity around 1 nm. The micropore volume of beta-TUD-1 is calculated by subtracting the micropore volume of TUD-1 from that ofbeta-TUD-1. Based on the micropore volume, the percentage of beta zeolite in the composite is estimated to be 16 wt %. Initial addition of uncalcined beta zeolite was 20 wt.% based on the final composite. The used beta zeolite lost about 20 wt.% due to the removal of template during the calcination. Taking the mass loss of zeolite during calcination into account, the expected content of zeolite beta in the final composite is about 16 wt.%, which is consistent with the value obtained from micropore volume. So it is believed that zeolite loss during the incorporation is not obvious. 3.3 Catalytic test The activity of the beta-TUD-1 was tested in the liquid phase alkylation of benzene to ethylbenzene. Prior to the test, the beta-TUD-1 was ion-exchanged to get proton form and shaped
with Nyacol alumina. The resulting catalysts containing about 12.8 wt % beta zeolite shows the first-order rate constant of 0.30 cm 3g-ls-l, whereas the catalyst with 80 wt % beta zeolite (the same material as that incorporated into TUD-1) showed the first-order rate constant of 1.70 cm3g~s~. Based on beta zeolite mass in these two catalysts, beta-TUD-1 has a higher activity than the pure beta zeolite (the ratio is about 1.1). But the 12.8% beta zeolite in TUD-1 showed the same activity as the commercial catalyst (containing 80% beta zeolite and 20% binder). From this result, it may be concluded that the integrity and intrinsic activity of the zeolite beta are not changed significantly
640 during the formation of the mesoporous phase. However, it is not easy to separate the intrinsic activity and the diffusion effects. 4. CONCLUSIONS Nano-sized beta zeolite can be homogeneously incorporated into a three-dimensionally randomly connected mesoporous matrix, retaining the features of both zeolite and mesoporous matrix. It is possible to independently tune physicochemical properties of both zeolite and mesoporous matrix. Many commercial zeolites can be easily incorporated into mesoporous matrices TUD-1 in the same way. ACKNOWLEDGMENTS
Thanks to ABB Lummus Global Inc. for financial support. Thanks to Mr. Johan Groen for gas adsorption. REFERENCES:
1. D.W. Breck, Zeolite Molecular Sieves, Krieger, Malabar, FL 1984, pp. 742-746. 2. D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka and G.D. Stuky, Science, 279 (1998) 548-552. 3. J.C. Jansen, Z. Shan, L. Marchese, W. Zhou, N. v.d. Puil and Th. Maschmeyer, Chem. Comm. (2001) 713-714. 4. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710-712. 5. 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. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834-10843. 6. A.S. Bagshaw, E. Prouzet and T.J. Pinnavaia, Science 269 (1995) 1242-1244. 7. D. Zhao, P. Yang, B.F. Chmelka and G.D. Stuky, Chem. Mater. 11 (1999) 1174-1178. 8. F. Marlow, M.D. McGehee, D.Y. Zhao, B.F. Chmelka and G.D. Stucky, Adv. Mater. 11 (1999) 632-636. 9. M. Gratzel, Current Opinion in Colloid and Interface Science, 4 (1999) 314-321. 10. A. Sayari, Chem. Mater. 1996, 8, 1840-1852. A. Corma, Chem. Rev. 97 (1997) 2373-2419. 11. K.R. Kloetstra, H. van Bekkum and J.C. Jansen J. Chem. Soc., Chem. Commun. (1997) 22812282. 12. L. Huang, W. Guo, P. Deng and Z. Xue, Q. Li, J. Phys. Chem. B 104 (2000) 2817-2823. 13. Z. Zhang, Y. Han, L. Zhu, R. Wang, Y. Yu, S. Qiu, D. Zhao and F. Xiao, Angew. Chem. Int. Ed. 40 (2001) 1258-1262. 14. Y. Liu, W. Zhang and T.J. Pinnavaia, Angew. Chem. Int. Edit. 40 (2001) 1255-1258. 15. W. Guo, L. Huang, P. Deng, Z. Xue and Q. Li, Micro. Meso. Mater. 44-45 (2001) 427-434. 16. J. Brinker and G. Scherer, Sol-gel science; the physics and chemistry of sol-gel processing, Boston, Academic Press, 1990. 17. Z. Shan, J.C. Jansen and Th. Maschmeyer, submitted. 18. S. Storck, H. Bretinger and W.F. Maier, Appl. Catal. A: General 1998, 174, 137-146.
~tuules m ~urrace ~clence ana tSatalysls 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
641
F o r m a t i o n and stabilization o f gold nanoparticles in organo-functionalized M C M - 4 1 m e s o p o r o u s materials and their catalytic applications Chitta Ranjan Patra, a Anirban Ghosh, a Priyabrata Mukherjee, a Murali Sastryb and Rajiv Kumara aCatalysis and bMaterials Chemistry Divisions, National Chemical Laboratory, Pune 411 008, India. (Fax : 91-20-5893952/5893761, Email :
[email protected] /
[email protected])
A new process for the synthesis and stabilization of gold nanoparticles within the cavities of propylamine functionalized mesoporous MCM-41 material is described wherein silanol groups present on the inner surface of the mesoporous silica reduce aqueous chloroaurate ions to form the Au nanoparticles, and the amine groups anchor the nanoparticles thus formed in the inner surface of the cavities. The nano-Au-NH2-MCM-41 hybrid material was characterized by XRD, UV-Vis spectroscopy, EDX, TG-DTA and N2 adsorption measurements. Further, this nano-Au-NH2-MCM-41 material shows promising catalytic activity and selectivity in the hydrogenation of styrene to ethylbenzene.
1. INTRODUCTION The synthesis of nano-clusters and their stabilization in various matrices is of current research interest mainly due the immense importance of these nanomaterials in catalysis, opto-electronical applications, templates for bio-minerals etc. [1-3]. In all the cases the hosts used are usually passive and do not participate actively in the reduction of metal ions to form nanoparticles, followed by their entrapment in the host matrix. We have recently found that when aqueous chloroaurate ions are treated with fumed silica, metallic gold particles of nanometer size are formed [4], where the reduction of Au(llI) to Au(0) is accomplished by the surface silanol groups, probably behaving in the similar way as the hydroxyl groups present in sugar moieties reduce the A u 3+ to A u ~ [5]. Here, we describe, the synthesis and entrapment of gold nanoparticles within the pores of Si-MCM-41 and propylamine-functionalized MCM-41 (NH2-MCM-41) mesoporous materials, where the reduction of the Au(IIl) ions occurs via surface hydroxyl groups on the cavities of MCM-41, while the grafted amine groups bind the gold nanoparticles to the silicate matrix. The MCM-41 supported gold nanoparticles show good catalytic activity and selectivity in the hydrogenation of unsaturated hydrocarbons like hydrogenation of styrene to ethylbenzene.
642 2. EXPERIMENTAL The S i-MCM-41 and NH2-MCM-41 materials were synthesized and characterized according to our published method [6,7]. Low angle X-ray diffraction (XRD) was recorded on a Rigaku D Max III VC instrument with Cu Ka radiation between 20 ranges 1.5~ and 10~ at a scan rate of l~ The specific surface areas of the samples were determined by the BET method using the adsorption of Na measured with an Omnisorb CX-100 instrument. Prior to the adsorption experiments, the samples were activated at 150~ for 6 h at 10.4 Torr. The formation of nano-Au-MCM-41 hybrid materials by in situ reduction of AuCI4 ions is described as follows. In a typical experiment, 1.0 g of each of the Si-MCM-41 and NH2MCM-41 materials were separately treated with 100 mL of 10.4 M HAuCI4 solution for 96 h followed by filtration, washing thoroughly with copious amounts of water andacetone and finally dried under vacuum. Thereafter, 0.5 g of each of the nano-Au-Si-MCM-41 and nanoAu-NH2-MCM-41 materials were separately stirred with 50 mL of distilled water for 12 h. The materials thus obtained after aqueous treatment (designated as Au-Si-MCM-41-w and Au-NH2-MCM-41-w respectively) were filtered, washed with water and dried under vacuum. The UV-Vis spectra of all the nano-Au-MCM-41 hybrid materials were recorded on a Shimadzu UV-2101PC spectrophotometer operating on reflection mode at a resolution of 2 nm using barium sulphate as a standard for background correction. Further, UV-Vis spectra of all the filtrates obtained in the above experiments were also taken on the same instrument using distilled water as a standard for background correction. In order to estimate the size of the gold nanoparticles formed by spontaneous reduction of chloroaurate ions, X-ray diffraction (XRD) measurements of the nano-Au-MCM-41 hybrid materials were carried out on a Philips PW 1830 instrument operating at 40 kV voltage and a current of 30 mA with Cu Ka radiation. A carefully weighed quantity of the nano-Au-MCM-41 hybrid materials were subjected to thermogravimetry and differential thermal analysis (TG-DTA) on a Seiko Instruments model TG/DTA 32 at a heating rate of 10~ EDX measurements of the materials were carried out on a Leica Stereoscan-440 scanning electron microscope equipped with Phoenix EDX attachment.
3. RESULTS AND DISCUSSION The surface areas of the Si-MCM-41 and NHa-MCM-41 materials were found to be 1000 and 600 m2 g~ with mean pore diameters of 4 nm and 3 nm respectively. After in situ reduction of the AuCI4 ions, the nature of the N2 adsorption isotherms remained the same but a decrease of ca. 8% and 20% in the surface areas of Au-Si-MCM-41 (920 m a g-l) and Au-NH2-MCM-41 (480 m2 g-i)respectively, was observed indicating filling of part of the mesopores by gold nanoparticles keeping the mesoporous structure intact. Energy dispersive X-ray analysis of the nano-Au-NH2-MCM-41 material indicates that the mesoporous material contains 3 wt.% of Au. After treatment of Si-MCM-41 and NHa-MCM-41 with AuCI4" ions for 96 h, it was immediately observed that both the materials had attained a deep pink color, clearly indicating the presence of Au nanoparticles adsorbed in the cavities of the silicate matrix. Au nanoparticles, due to their surface plasmon vibrations, have a characteristic absorption band in the visible region of the electromagnetic spectrum (around 5 2 0 - 550 nm), which is responsible for the striking violet to pink range of colors of the nanoparticles depending
643 upon the particle size [8]. Figure IA shows the UV-vis spectra recorded from the parent NH2-MCM-41 (curve a), from the nano-Au-NH2-MCM-41 (curve b), and from the Au-NH2MCM-4 l-w materials (curve c). Fig. 1B shows the UV-Vis spectra of the parent Si-MCM41 (curve a), the nano-Au-Si-MCM-41 (curve b), and the Au-Si-MCM-41-w materials (curve c). A strong absorption at ca. 540 nm is observed for both the mesoporous materials after treatment with HAuCI4 solution and is clear indication of reduction of the AuCI4 ions in both the NH2-MCM-41 (Fig. 1A, curve b) and Si-MCM-41 samples (Fig. IB, curve b). Note that this resonance is clearly missing in the parent NH2-MCM-41 (Fig. I A, curve a) and Si-MCM-41 materials (Fig. 1B, curve a). A
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Figure 1. A) UV-vis spectra of the parent NH2-MCM-41 (curve a), the nano-Au-NHz-MCM41 (curve b), and the Au-NH2-MCM-41-w samples (curve c). B) UV-Vis spectra of the parent Si-MCM-41 (curve a), the nano-Au-Si-MCM-41 (curve b), and the Au-Si-MCM-41w samples (curve c). The curves have been displaced vertically for clarity. An interesting observation is the presence of an additional resonance at ca. 725 nm in the case of the nano-Au-NH2-MCM-41 material (Fig. I A, curve b). This feature arises due to excitation of longitudinal surface plasmon vibrations and is a consequence of close-packing of the gold nanoparticles in open, string-like networks within the cavities of this material [9,10]. This may be a consequence of the fact that the amine groups anchor the gold nanoparticles to the inner surface of the silicate cavities while such an interaction does not occur in the case of the nano-Au-Si-MCM-41 material (Fig. IB, curve b). It has been reported that primary amines bind to gold nanoparticles via a weak covalent linkage [11] and is believed to be the binding mechanism in this study as well. Since there is no such organic moiety present in the Si-MCM-41 material for binding the Au nanoparticles, the stability of the gold nanoparticles formed by in situ reduction within the Si-MCM-41 material is a matter of interest. Both the nano-Au-NH2-MCM-41 and the nano-Au-Si-MCM-41 materials were treated with distilled water for 12 h, filtered and dried. The UV-Vis spectra of the washed samples nano-Au-NH2-MCM-41-w (curve c) and nanoAu-Si-MCM-41-w (curve c) are shown in Fig.l A and B, respectively. It is interesting to observe that the characteristic absorption at ca. 540 nm of gold nanoparticles is missing in Au-Si-MCM-4 l-w, clearly indicating that all the gold nanoparticles were leached out in the
644 aqueous phase. This is further supported by the UV-Vis spectra of the filtrates obtained from Si-MCM-41 material after treatment with HAuCI4 for 96 h and that of the filtrate obtained from the nano-Au-Si-MCM-41 material after treatment with distilled water (data not shown). Characteristic absorbance of Au nanoparticles at ca. 530 nm is observed for both the filtrates. Comparing the UV-Vis spectra of nano-Au-Si-MCM-41 material (Fig. 1B, curve b and c), it can be inferred that although, the entrapment of all the gold nanoparticles in the cavities of Si-MCM-41 was not complete, the leaching of the gold nanoparticles during washing was near complete. Moreover, a weak absorbance at ca. 530 nm for the filtrate obtained after aqueous treatment of the nano-Au-Si-MCM-41 material (not shown) is further indicative of the leaching of the gold nanoparticles in the aqueous phase, which was previously inferred from the UV-Vis spectra of the Au-Si-MCM-41-w material (Fig. 1B, curve c). The filtrate obtained from NHE-MCM-41 material after treatment with HAuCI4, and the filtrate obtained from nano-Au-NHE-MCM-41 material after aqueous treatment did not show any absorbance in this visible region, clearly suggesting that there is no leaching of Au in the case of NH2-MCM-41 materials. Further, the UV-Vis spectra of the material obtained from nano-Au-NHE-MCM-41 material (designated as Au-NHE-MCM-41-w) is given in Fig. l A, curve c, which is almost similar to that of the nano-Au-NH2-MCM-41 material, including the absorbance at ca. 725 nm indicating that even the close-packing of the gold nanoparticles in open, string-like networks is also stabilized in NHE-MCM-41.
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Figure 2. A) XRD patterns of NH2-MCM-41 (curve a), Si-MCM-41 (curve b), nano-AuNHE-MCM-41 (curve c) and nano-Au-Si-MCM-41 (curve d). B) The (11 l) Bragg reflection of Au nanoparticles at 20 - 38.2 ~ for the nano-Au-NHE-MCM-41 material. The solid line is a Lorentzian fit to the reflection. Figure 2A shows the low angle X-ray diffraction measurements of the samples NH2MCM-41 (curve a), Si-MCM-41 (curve b), nano-Au-NHE-MCM-41 (curve c) and nano-AuSi-MCM-41 (curve d). All the materials yielded the (110) and (200) Bragg reflections along with the main (100) reflection indicating high degree of ordered hexagonal mesopores even after in situ formation of gold nanoparticles within the cavities. The diffraction pattern obtained from the nano-Au-NHE-MCM-41 material is shown in Figure 2B in the region of the (l l l) Bragg reflection (20 = 38.2~ the solid line being the
645 Lorentzian fit to the (l 1 l) reflection. A similar pattern was obtained for the nano-Au-SiMCM-41 material (not shown). The mean size of the gold nanoparticles was estimated from the broadening of the (111) peak using the Debye-Scherrer equation [12], which yielded the value of ca. 3.5 + 0.5 nm for both the materials. This value is in good agreement with the maximum possible particle size that can be accommodated within the pores of the mesoporous Si-MCM-41 and NH2-MCM-41 materials, which we recollect, had average diameters of 4 nm and 3 nm, respectively. Esumi et al. [5] have shown that the reduction of chloroaurate ions by sugar balls (sugarpersubstituted poly (amidoamine) dendrimers, containing large amount of hydroxyl groups on the outer surface), occurs via the hydroxyl groups in the sugars. We believe that a similar mechanism is operative in this study and that the reduction of the chioroaurate ions within the pores of the silicate mesoporous material occurs through silanol groups present on the surface of the silicate material. The -NH2 functionality residing deep inside the mesoporous channel holds the gold nanoparticles. Figure 3 shows TGA (A) and DTA (B) of nano-Au-NHz-MCM-41 sample. A large weight loss, corresponding to ca. 10 % of the overall sample weight was observed at 100~ and is attributed to loss of adsorbed water in the sample. Another large weight loss, corresponding to ca. 5 % was occurred at ca. 300~ However, a fairly steady decrease in mass was observed till 600~ and amounts to a further 10 % weight loss. This loss is attributed to decomposition of organic matter. This TGA data are supported by the DTA data of the same material. An endothermic peak below 100~ indicates the loss of adsorbed water molecules from the material. An exothermic peak at ca. 300~ indicates the removal of lattice water and decomposition of propyl amine group inside the pores of the material.
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Figure 3. A) TGA curve and B) DTA curve of a carefully weighed sample of the nano-AuNHz-MCM-41 material We have also investigated the catalytic activity of the gold nanoparticles immobilized in the amine functionalized MCM-41 structure in a representative hydrogenation reaction. The catalytic hydrogenation o f styrene to ethylbenzene was carried out in a quartz vessel at 60 psi pressure of H2 and at room temperature wherein 0.1 g of the Au nano-NH2-MCM-41 sample was treated with 250 mg of styrene (Aldrich 99.8 %) in 8 mL of methanol solvent for
646 6 h under constant stirring. The products wereanalyzed by high-resolution capillary gas chromatography using a flame ionization detector. 100 % selectivity for the hydrogenated product (ethylbenzene)was observed with a total conversion of 30 mol. %. It may be pointed out here that the gold nanoparticles less than 5 nm show high catalytic activity [ 13,14]. Hence, nanoparticles synthesized within the organo-functionalized-MCM-41 materials having 2-5 nm pore size are thus ideal candidates for catalytic applications.
4. CONCLUSION It has been shown that nanoparticles of gold may be synthesized and entrapped within the pores of NH2-MCM-41 material by in situ reduction of aqueous chloroaurate ions. The reduction occurs via silanol groups on the inner surface of the mesopores. The Au-nanoNH2-MCM-41 material is an example of inorgano-organic nanocomposite wherein the matrix actively participates in the reduction of the metal ions as well as acts as a support for the nanoparticles thus formed. The dimension of the pores in the mesoporous material is in accordance with the size range of the gold nanoparticles where they show excellent catalytic activity. This makes the Au-MCM-41 nano-hybrid materials exciting systems for future applications in catalysis,
ACKNOWLEDGEMENTS Two of us, CRP and AG, would like to thank Council of Scientific and Industrial Research, India for granting research fellowships.
REFERENCES 1. W. Shenton, D. Pum and U. B. Sleytr, S. Mann, Nature, 389 (1997) 585. 2. M. Zhao, L. Sun and R. M. Crooks, J. Am. Chem. Soc., 120 (1998) 4877. 3. P. Mukherjee, R. Kumar and M. Sastry, PhysChemComm., 4 (2000) 1. 4. P. Mukherjee, C. R. Patra, A. Ghosh, R. Kumar and M. Sastry, Chem. Mater. (accepted). 5. K. Esumi, T. Hosoya, A. Suzuki and K. Torigoe, Langmuir, 16 (2000) 2978. 6. P. Mukherjee, R. Kumar and U. Schuchardt, Stud. Surf. Sci. Catal., 117 (I 998) 351. 7. P. Mukherjee, S. Laha, D. Mandal and R. Kumar, Stud. Surf. Sci. Catal., 129 (2000) 283. 8. S. Underwood and P. Mulvaney, Langmuir, 10 (1994) 3427. 9. C.G. Blatchford, J. R. Campbell and J. A. Creighton, Surf. Sci., 120 (1982) 435. 10. K. S. Mayya, V. Patil and M. Sastry, Langmuir, 13 (1997) 3944. 11. D. V. Left, L. Brandt and J. R. Heath, Langmuir, 12 (1996) 4723. 12. J. W. Jeffrey, Methods in Crystallography, Academic Press, New York (1971). 13. M. Haruta, Catal.Today, 36 (1997) 153. 14. G. C. Bond and D. T. Thompson, Catal. Rev.-Sci. Eng., 41 (1999) 319.
atuoles in ~urrace ~clence and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
647
E n t r a p m e n t and stabilization o f cadmium sulphide (CdS) nanoclusters formed inside propylthiol functionalized MCM-41 mesoporous materials Anirban Ghosh, a Chitta Ranjan Patra, a Priyabrata Mukherjee, a Murali Sastryb and Rajiv Kumara aCatalysis and bMaterials Chemistry Divisions, National Chemical Laboratory, Pune 411 008, India (Fax : 91-20-5893952 / 589376 l, Email :
[email protected] /
[email protected])
Cadmium sulphide nanoparticles were stabilized within the mesopores of thiol functionalized mesoporous MCM-41 by treatment with aqueous Cd 2§ ions to form molecularly dispersed Cd(II) species within the pores, followed by addition of S2 ions to the system to form the CdS nanoparticles and their simultaneous entrapment. These SH-MCM41 and nano-CdS-SH-MCM-41 hybrid materials were characterized by XRD, UV-Vis spectroscopy, EDX, TG-DTA and N2 adsorption measurements.
1. INTRODUCTION Surface modified MCM-41 [1,2], where the surface of MCM-41 contains organic functionality, such as propylamine, propylthiol, vinyl etc., are emerging as important materials as host for metallic and semiconductor nanoparticles [3-5]. Semiconductor nanoparticles are subject of intense investigation because of their interesting size dependent physicochemical and optical properties [6]. The main emphasis is on the control of the size of the nanoparticles, and their stabilization involving their in situ preparation within different matrices [7-9]. Generally, surfactant molecules containing polar head groups like -SH, NH2 etc. are used to stabilize the nanoclusters to prevent their agglomeration to get the quantum dots. The propylamine and propylthiol functionalized MCM-41 materials are particularly important in the sense that they have the polar head group commonly required to stabilize the nanoclusters. The importance of these organically modified MCM-41 material is that they can not only stabilize the nanoclusters via traditional polar groups (-SH, -NH2) but also provide a solid support of well-defined pores to the nanoclusters. Therefore, these nanocomposites can be visualized as a "three tier" molecule, where the outer sphere is the inorganic support, the second sphere is the organic moiety attached on the surface of the MCM-41 material and protruding inside the pores and the third one is the nanoclusters hooked to the polar head groups of the organic moiety present inside the pores. These threelayered nano composites may to lead to interesting non-linear optical properties and host guest interactions.
648 Realizing the importance of nanoclusters in various areas, we have used organofunctionalized mesoporous MCM-41 type materials as host for entrapment and stabilization of semiconductor nanoparticles. This paper reports our studies on the entrapment and stabilization Of cadmium sulphide (CdS) nanoclusters inside propyl thiol modified MCM-41 (SH-MCM-41) material. In order to find the utility of functional organic moiety (propylamine) in MCM-41, Si-MCM-41 was also included in the present work for comparison.
2. E X P E R I M E N T A L The syntheses of thiol functionalized as well as siliceous MCM-41 materials were carried out as described in the literature [2,10]. Tetraethyl orthosilicate (TEOS, Aldrich), 3mercaptopropyltrimethoxysilane (MPTS, Aldrich), cetyltrimethylammomium bromide (CTAB, SD Fine Chem) materials were used as received. In a typical synthesis of thiol functionalized MCM-41 (SH-MCM-41), the gel composition was 1.0 MPTS : 4 TEOS : 0.42 CTMABr : 0.96 NaOH : 272 H20 : 66 MeOH. Methanol was used in the initial gel mixture to reduce and control the fast hydrolysis of MPTS. The reaction mixture was first stirred at room temperature for 12 h and then heated in autoclave at 95~ for 36 h under static condition followed by filtration. Similarly, siliceous MCM-41 (Si-MCM-41) was prepared [10] where MPTS was not used. Both the solid samples was washed with distilled water and acetone, and dried at 950C for 4 h. The surfactant from the solid (1 g) was removed by solvent extraction with a mixture of solvents containing 85 g of methanol and 3.25 g of HCI (35.5%) for 24 h [2]. The surface area of the surfactant,free samples was determined by the BET method (N2 adsorption) using an Omnisorb CX-100 instrument. Prior to the adsorption experiment, the samples were activated at 150~ for 6 h at 10 "4 Torr. Chemical analysis of the extracted sample of SH-MCM-41 (for C, H and S) was done on a Carlo Erba EA1108 elemental analyzer. The synthesis and stabilization of CdS nanoparticles inside the cavities of SH-MCM-41 is described as follows. Dry samples (0.5 g) were stirred with 50 mL solution of 10 -3 M cadmium chloride for 6 h at room temperature. Thereafter, 50 mL of 10 -3 M sodium sulphide solution was added into the mixture slowly with stirring. The stirring is continued for 12 h at room temperature followed by filtration, washing thoroughly with distilled water and acetone and drying under vacuum. The UV-Vis spectra of the samples were measured on a Shimadzu UV-2101PC spectrophotometer operating on reflection mode at a resolution of 2 nm using barium sulphate as a standard for background correction. Low angle X-ray diffraction (XRD) patterns of the mesoporous SH-MCM-41 material and the CdS entrapped MCM-41 material (CdS-SH-MCM-41) were recorded on a Rigaku D Max III VC instrument with Cu Ka radiation between 20 ranges 1.5 ~ and 10~ with a scan rate of 1~ per minute. To calculate the size of the CdS nanoparticles, XRD pattern of the CdS-SH-MCM-41 sample was recorded on a Phillips PW 1830 instrument operating at the voltage of 40 kV and a current of 30 mA with Cu Ka radiation between 20 ranges 20 ~ and 30 ~ with a scan rate of l~ A carefully weighed quantity of the CdS-SH-MCM-41 was subjected to thermogravimetry and differential thermal analysis (TG-DTA) on a Seiko Instruments model TG/DTA 32 at a heating rate of 10~ EDX measurements of the materials were carried out on a Leica Stereoscan-440 SEM equipped with Phoenix EDX attachment.
649 3. RESULTS AND DISCUSSION Chemical analysis of the SH-MCM-41 material yielded a C : H : S ratio of 8.44 : 22.38 : 4.28, quite comparable with the corresponding theoretical values as 10.52 : 24.6 : 3.54. Thus, chemical analysis of the sample, after extraction of the surfactant, confirms that the intact propyl-thiol functionality is present in the organo-MCM-41 mesoporous material. EDX analysis of the CdS-SH-MCM-41 material revealed a composition of 1.2 wt.% of Cd, 31.52 wt.% of S and 67.27 wt.% of Si. The N2 adsorption measurements on the SH-MCM-41 and CdS-SH-MCM-41 materials yielded specific surface areas of 926 and 782 m 2 g-I, the mean pore diameter of the SHMCM,41 material being 4 nm. Nature of adsorption isotherm remained the same in both the cases. This decrease of ca. 15 % in surface area in the CdS-SH-MCM-41 material indicates filling in the part of the mesopore volume by CdS nanoparticles. However, there was no appreciable change in surface area, in the case of Si-MCM-41 (754 m 2 g-l) and CdS-SiMCM-41 (725 m 2 g-i) samples, indicating insignificant presence of CdS particles inside the MCM-41 mesopores.
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Figure 1. UV-Vis spectra of SH-MCM-41 (curve a), CdS-SH-MCM-41 (curve b) and CdSSi-MCM-41 (curve c) samples. The curves have been displaced vertically for clarity. After adding Na2S solution to the slurry of Cd(II)-SH-MCM-41 and Cd(II)-Si-MCM-41 materials in water followed by stirring for 12 h, both the samples acquired a bright yellow color, clearly indicating the presence of CdS. The UV-Vis spectra of the CdS-Si-MCM-41, CdS-SH-MCM-41 and the SH-MCM-41 material are shown in Figure 1. For CdS-SH-MCM41 (Fig. 1, curve b), the absorption occurs at ca. 430 nm, while the same occurs at ca. 480 nm for CdS-Si-MCM-41 (Fig. 1, curve c). These absorption maxima are determined by taking the derivative of the UV-vis spectra. The absorption maximum of CdS-SH-MCM-41 at ca. 430 nm corresponds to a particle size of ca. 3.5 nm [ 11], while that of CdS-Si-MCM-41 at ca. 480 nm corresponds to a particle size of ca. 7 nm. This band is absent in the spectrum of the parent SH-MCM-41 material (Fig. 1, curve a), as expected.
650 While in SH-MCM-41, with pore size of 4 nm, the CdS nanoparticles of ca. 3.5 nm size can be accommodated easily; the CdS particles of ca. 7 nm cannot be present inside the SiMCM-41 pore. Since there was no binding moiety in the Si-MCM-41 material (unlike SHMCM-41 material), it is quite likely that there is no anchoring of Cd 2§ ions occurred inside the mesoporous matrix, resulting in the formation of bigger CdS nanoparticles in the aqueous media, which then got supported on the silica surface. These results emphasize the necessity of -SH groups inside the mesopores of MCM-41 material, which acted as anchors for the Cd 2§ ions present in solution, while subsequent addition of S2 ions led to formation of CdS nanoparticles in situ. Figure 2A shows the low angle X-ray diffraction patterns of the thiol functionalized MCM-41 (SH-MCM-41) material (curve a) as well as the CdS entrapped SH-MCM-41 (CdSSH-MCM-41) material (curve b). Both the materials exhibited weak (110) and (200) Bragg reflections along with the main (100) reflection indicating hexagonal ordering of the pores in the mesoporous material even after entrapment of CdS nanoparticles within the pores.
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Figure 2. A) Low angle XRD patterns of SH-MCM-41 (curve a), and CdS-SH-MCM-41 (curve b). B) The (101) Bragg reflection at 20 = 28.2 ~ of CdS nanoparticles in the SH-MCM41 material (curve a), and in the Si-MCM-41 material (curve b). The solid lines represent the Lorentzian fit to the reflections and were used to estimate the nanoparticle size. The size of the CdS nanoparticles in both the samples were also calculated from the line width broadening of the (101) Bragg reflection at 20 = 28.2 ~ by applying Debye-Scherrer formula [12]. Figure 2B shows the experimentally measured (101)Bragg reflection of CdS nanoparticles in the SH-MCM-41 material (curve a) and in the Si-MCM-41 material (curve b), along with Lorentzian fit to the data. The size of CdS particles present in CdS-SH-MCM41 and CdS-Si-MCM-41 samples was found to be 3.7 nm and 7.6 nm respectively, which are in good agreement with those inferred from UV-Vis maxima. This result again proves the contention that the -SH groups inside the pores are responsible for size control of CdS nanoparticles, which evidently resides inside the channels. The TGA data recorded from the CdS-SH-MCM-41 material is plotted in Figure 3A, where a small weight loss, corresponding to ca. 3 % of the overall sample weight, is observed at ca. 100~ due to the loss of adsorbed water in the sample. Another large weight loss,
651 corresponding to ca. 10 wt. % is observed at around 300~ However, a fairly steady decrease in mass is observed till 600~ and amounts to a further ca. 5 % weight loss. This may be due to decomposition of organics. The TGA results are supported by the DTA data recorded for CdS-SH-MCM-41 material (Fig. 3B). An endothermic peak below 100~ was obtained due to evaporation of adsorbed water molecules. Another exothermic peak at ca. 300~ was obtained which was attributed to removal of lattice water and decomposition of proypl thiol group inside the cavities of the material.
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4. CONCLUSIONS It has been shown that semiconductor nanoparticles of cadmium sulphide can be synthesized and stabilized within pores of thiol functionalized MCM-41 material. The grafted thiol groups within pore walls of the MCM-41 serve as anchors for the CdS nanoparticles formed by treating the SH-MCM-41 first with cadmium ions followed by the addition of sulphide ions to the system leading to the formation of CdS nanoclusters of ca. 3.5 nm.size inside the pores commensurating with the pore diameter. The formation of organo-inorganic nano-hybrid materials opens up a new area with immense future applications, such as in opto-electronic devices, or in photocatalysis to mention a few.
5. A C K N O W L E D G E M E N T S AG and CRP would like to thank Council of Scientific and Industrial Research, India for granting research fellowships.
652 REFERENCES
1. X. Feng, G. E. Fryxell, L. Q. Wang, A. Y. Kim, J. Liu and K. M. Kemner, Science, 276 (1997) 923. 2. P. Mukherjee, S. Laha, D. Mandal and R. Kumar, Stud. Surf. Sci. Catal., 129(2000) 283. 3. Y. Xu and C. H. Langford, J. Phys. Chem. B., 101 (I 997) 3115. 4. T. Hirai, H. Okubo and I. Komasawa, J. Phys. Chem. B., 103 (1999) 4228. 5. P. Mukherjee, M. Sastry and R. Kumar, PhysChemComm, 4 (2000) 1. 6. A.P. Alivisatos, Science, 271 (1996) 933. 7. A. Milekhin, M. Friedrich, D. R. T. Zahn, L. Sveshnikova and S. Repinsky, Appl. Phys. A, 69 (1999) 97. 8. E. Lifshitz, I. Dag, I. Litvin, G. Hodes, S. Gorer, R. Reisfeld, M. Zelner and H. Minti, Chem. Phys. Lett., 288 (1998) 188. 9. J.R. Agger, M. W. Anderson, M.E. Pemble, O. Terasaki and Y. Nozue, J. Phys. Chem. B, 102 (1998) 3345. 10. P. Mukherjee, R. Kumar and U. Schuchardt, Stud. Surf. Sci. Catal., 117 (1998) 351. 11. V. L. Colvin, A. N. Goldstein, A. P.Alivisatos, J. Am. Chem. Soc., 114 (1992) 5221. 12. J. W. Jeffrey, Methods in Crystallography, Academic Press, New York (1971).
Studies in ~urtace Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
653
SnO2 nanoparticles in the pores o f non-structured SiO2 and S i - M C M - 4 1 : C o m p a r i s o n o f their properties in gas sensing Y~icel Altindag a, Andrei Jitianu b and Michael Wark a a Institute of Applied and Physical Chemistry, University of Bremen, FB 2, D-28334 Bremen, Germany, e-mail:
[email protected] b Romanian Academy, Institute of Physical Chemistry, 202 Splaiul Independentei, RO-77208 Bucharest, Romania
Nano-sized SnO2 particles were stabilized in SiO2 matrices directly during a sol-gel process using modified organic precursors as well as in the pores of Si-MCM-41 by chemical vapour deposition (cvd) or impregnation. The dimensions of the SnO2 particles found in the sol-gel samples vary between 3 and 20 nm depending on the use of the acidic or basic synthesis route. In Si-MCM-41 the combination of adsorption isotherms, transmission electron microscopy and DR-UV/Vis reveals the formation of a carpet of SnO2 clusters on the inner pore walls due to strong interactions of the tin oxide species with the silanol groups of the Si-MCM-41 matrix. The reversible detection of reducing and oxidizing gas atmospheres (CO, 02) is possible with all the samples, however, differences in the sensitivity of the samples result from different sizes of the formed SnO2 nanoparticles and their interaction with the matrices. Due to the smaller sizes of the embedded tin species Si-MCM-41 appears to be the most promising matrix for the hostage of gas sensing composites working at moderate temperatures.
1. INTRODUCTION Due to its semiconducting properties SnO2 is a very useful material for gas sensing [1 ]. Since air pollution is a serious problem and for protection against explosions, sensors based on SnO2 with applications in detection of CO, flammable gases like H2 and more recently, NO2 have been studied and developed [2-4]. It has been proven that the sensitivity towards the dangerous gases increases with decreasing crystallite size of the SnO2. Whereas most SnO2 sensors work electrochemically here the gas sensing properties of SnO2 nanoclusters in Si-MCM-41 and in non-structured SiO2 were monitored by diffuse reflectance spectroscopy (DR-UV/VIS), since it had previously been demonstrated that TiO2 or SnO2 nanoparticles stabilized in the pores of zeolites are very suitable composites for the optical detection of reductive gases [5,6]. The sol-gel processing is a very cheap method which is suitable to obtain metal oxide nanoparticles with high dispersion and narrow size distribution, in mono- and polycomponent systems [7,8]. Hampden-Smith et al. [9] wrote an excellent review, referring to the behavior of organic tin compounds in sol-gel processes.
654 In contrast to sol-gel materials mesoporous molecular sieves, like Si-MCM-41 [ 10], possess a much more regular pore system for the encapsulation of guest species like SnO2 nanoparticles providing presumably a better accessibility for the gases to be detected. However, their synthesis is more expensive since supramolecular arrangements of surfactant ar6 involved in their formation [ 11]. Aim of this study is the comparison of different host materials concerning the stability of SnO2 nanoparticles and the resulting gas sensing properties.
2. EXPERIMENTAL SECTION 2.1. Materials Reagents were obtained from the following sources: sodium metasilicate, Aldrich; cetyltrimetylammonium bromide (CTAB), Aldrich; ethylacetate (p.a.), Acros; isopropanol (p.a.), Fluka; tetraisopropoxy-tin(IV) isopropanol adduct (Sn(i-OPr)4), Gelest; SnCI4, Aldrich; SnC12 • 5 H20, Aldrich; dibutyl-tin-diacetate (Sn(C4Hg)2(ac)2), Fluka. 2.2. Syntheses The synthesis of the Si-MCM-41 was performed by the homogeneous precipitation method [12,13]. The embedding of SnO2 dispersions in the pores of the Si-MCM-41 was achieved following two different routes: (i) chemical vapor deposition (cvd) This method is, aside from the Si-MCM-41 used in this study, applicable to a variety of matrices containing free OH groups. 1 g Si-MCM-41 was placed in a flow reactor. After drying the sample in N2 at 673 K for 17 h the sample was loaded at 373-673 K for 15-60 min in a nitrogen stream saturated with SnCI4. Excess SnCh only physisorbed in the Si-MCM-41 pores was removed by flushing the reactor with dry nitrogen for 2 h at 373 K. To substitute the remaining chloride atoms of the Si-MCM-41-anchored Sn species by OH groups the samples were hydrolyzed at 373 K in a nitrogen stream saturated with water. (ii) impregnation Sn(IV) oxide containing samples were prepared by impregnating the Si-MCM-41 at ambient temperature with variously concentrated solutions of Sn(CaH9)2(ac)2 or SnCI4 x 5.H20 in isopropanol. After the removal of the solvent in a rotary evaporator the samples were dried at 533 K for 12 h and calcined in air at 873 K for 12 h. SnO2 nanoparticles in porous SiO2 matrices were prepared by the sol-gel method. A molar ratio TEOS:EtOH:H20 = 1:3.5:5.2 was used for the SiO2 network and different amounts of the tin-source Sn(i-OPr)4 were added. In the sol-gel process both types of catalysis generally possible were tested: the acidic and the basic route. As catalysts either HC1 (36,5 vol.%) or NH4OH (25 vol.%) were used. Syntheses with HC1 were performed in the following way: 3,6 ml water were mixed with 23,38 ml of ethanol. To this mixture one drop of HC1 was added to get a pH value of approximately 3. Subsequently, 22,35 ml TEOS were dropped slowly to this mixture and the whole was stirred for 1 hour. During the stirring the pH value increased to about 5. In the meantime, a second solution was made by dissolving 0.8711 g Sn(i-OPr)4 (to achieve a final Sn content of 5 wt.%) in 15,57 ml ethanol. After the stirring period this second solution was added to the first one. Again, the mixture was stirred for 30 min and, subsequently, a third
655 solution consisting of 1 drop of concentrated HC1 in 18 ml was added. From this sol a gel formation was obtained after 16 h. The gel was dried for 48 hours at 343 K and, subsequently, calcined in air at 823 K for 5 h (heating rate: 1 K/rain). For the basic catalysis the synthesis procedure is in principle the same with the exception that 2 drops of NH4OH are used instead of HC1. 2.3. Instrumentation The tin content of the samples was determined by atomic absorption spectroscopy (Carl Zeiss AAS 5 FL) after dissolving ca. 25 mg of a sample in 1 ml of HF (48 %), 5 ml of HNO3 (65 %) and 1 ml of HC1 (37) in a Teflon autoclave and heating the mixture in a microwave oven (MLS 1200) for 30 min at 350 W. Powder XRD patterns were collected on a Philips X'PERT MPD Alpha-1 diffractometer using Cu Kct radiation. Transmission electron microscopy (TEM) was carried out on a Philips EM 420 instrument at 120 kV. Adsorption isotherms of nitrogen at 77 K were taken with a Micromeritics ASAP 2010 apparatus. Diffuse reflectance UV/Vis spectra (DR-UV/Vis) were recorded on a Varian Cary 4 spectrometer equipped with a Praying Mantis and a reaction chamber (Harrick). All reflectance spectra were converted to F(R~) spectra using the Kubelka-Munk formalism [ 14] and an Oriel LOT 75% teflon standard as reference. In-situ measurements regarding reduction and oxidation with CO and O2 were continuously recorded at ~ = 620 nm, i.e. the wavelenght at which in response to reductive agents the greatest change in the reflectance can be observed [6]. The powdered samples were carefully hand-pressed to pellets of a thickness of about 3 mm and the pellets were mounted in the reaction chamber. In order to obtain identical initial conditions all pellets were pretreated by dehydration at 673 K and 10-s mbar over night. Thereafter they were oxidized in a stream of 50 vol.% O2 in Ar (80 ml min -l) for 2h. For reduction and re-oxidation the samples were exposed alternately to streams (80 ml min -l) of reductive (50 vol.% CO in Ar) and oxidative (50 vol.% 02 in Ar) atmospheres. The temperature on the pellet in the flow cell was 673 K. The samples were repeatedly reduced and oxidized to deduce the reversibility of the reflectivity changes. Gas mixtures with low CO concentration were used to determine the sensitivities of the different samples.
3. RESULTS AND DISCUSSION The AAS analysis proved that for the impregnation of Si-MCM-41 and for the SnO2/SiO2 sol-gel samples the amounts of Sn offered in the preparation procedures were completely incorporated in the obtained samples. From XRD pattern it became obvious that in case of the Sn-loading of Si-MCM-41 by cvd the content of Sn reachable without significant destruction of the host material is restricted to about 2 wt.%, whereas impregnation is possible up to a loading of 20 wt.% Sn. Figure 1 shows the N2 adsorption isotherms of the parent Si-MCM-41 in comparison to SnO2 loaded Si-MCM-41 and to SnO2/SiO2 samples obtained by the sol-gel processing. The texture parameters of all the samples, deduced from the isotherms, are given in Table 1. The total surface area STOT, external surface area SEXTand mesopore volume VME were evaluated using the comparison plot method [13]. The mesopore surface area SME was calculated according to the formula SME= STOT- SEXT. The Si-MCM-41 channels were characterized by
656 36
I
333027"7
a
24.
o
E E
be
18-
Figure 1:. Nitrogen adsorption isotherms on samples (a) Si-MCM-41, (b) SnO2/Si-MCM-41 cvd (1,1 wt.% Sn), (c) SnO2/Si-MCM-41imp (8 wt.% Sn), (d) SnO2/SiO2solgel,basic (5 wt.% Sn) and (e) SnO2/SiO2sol-gel, acid (5 wt.% Sn).
151296a0
e
010 012 014 016 018 110 P/Po
their hydraulic diameter DME, which was determined using a geometrical model of the MCM41 structure [ 15]. The type IV adsorption isotherms obtained on the Si-MCM-41 materials are typical for samples possessing regular mesopores. The adsorption at low partial pressures P/P0 is due to the monolayer adsorption of N2 on the inner walls of the mesopores and on the external surfaces, and the steep increase at p/p0 ~ 0.3 is due to the condensation of liquid N2 in the mesopores. With increasing loading of SnO2 in the pores of Si-MCM-41 the mesopore volume decreases. However, since the values are still high for the impregnated sample a blockage of a significant degree of the pore mouths can be ruled out; it is more probable that carpet-like two-dimensional SnO2 structures are formed onto the inner walls of the matrix. This is very likely, since the introduced Sn(C4H9)2(ac)2 or SnC14 precursors show a strong interaction with the silanol groups of the Si-MCM-41. In case of the sol-gel SnO2/SiO2 samples it is obvious from the adsorption isotherms that the basic catalysis leads to samples with much larger pores that the acidic route. The basic samples possess mesopores with a wide distribution of diameters (about 80% of the pore diameters are between 4 and 8 nm) and additionally some macropores, whereas the acidic samples contain exclusively micropores, and are thus more compact. Table 1 Texture parameters of the samples. The estimated ranges of errors are + 5 %. Sample
STOT/m2g "1
SEXT/m2 g-1
SME/m2g-I
VME/Cm 3g-I
DME/nm
Si-MCM-41
1036
173
858
0.685
3.2
SnO2/Si-MCM-4 limp
996
182
804
0.646
3.2
SnO2/Si-MCM-41 cvd
987
176
792
0.637
3.2
SnO2/SiO2sol-gel, acid
419
7
-
-
-
SnO2/SiO2sol-gel, basic
238
10
232
0.432
mainly 4-.8
657 The diffuse reflectance spectra show for all the samples a blue shift of the absorption edge in comparison to bulk SnO2 (Fig. 2). These blue shifts result from the size-quantization effect and indicate that the SnO2 particles have sizes in the nanometer regime [16]. The less pronounced blue-shift was found for the samples formed in a sol-gel process using HC1 as catalyst. Transmission electron micrographs reveal that the particles have diameters between 10 and 20 nm. Since these samples contain only micropores this indicates that the SnO2 particles are part of the SiO2 network formed in the sol-gel process. Under basic catalysis, however, smaller SnO2 particles are formed, which might be hosted in the mesopores of the SiO2 network. In agreement with TEM results the two steps in the F(R) spectra indicate that there is a broad and bimodal size-distribution with mean particle diameters around 3 and 5 nm.
For the Si-MCM-41 samples it is obvious that the loading by chemical vapor deposition (cvd) leads to the smallest SnO2 particles. Since the onset of the absorption at around )~ - 320 nm is red-shifted with respect to that of mononuclear SnO2 units formed in the supercages of the zeolite Y, for which the onset lies at around )~ = 240 nm [6], it is probable that in the wider pores of the Si-MCM-41 structure aggregates consisting of several Sn atoms are formed. The impregnation procedure leads the formation of even larger aggregates observable by the less pronounced red-shift of the spectrum compared to bulk SnO2. The formation of larger aggregates by impregnatin occurs due to the higher concentration of Sn achievable by this method and due to the presence of the solvent isopropanol which increases the mobility of the Sn precursors. Although the F(R) spectra of these sample look very similar to that of sol-gel SnO2/SiO2 samples formed by basic catalysis, no indication for the formation of 3-5 nm large particles could be found in TEM micrographs. This leads to the assumption that instead of 1.0
a c d
~ b
N
0.5
200
c
Sn0/Si02sol-gel,acid
d
physical mixture of
e
Sn0/Si-MCM-41cvd
Si-MCM-41 and bulk S n 0 2
E
0.0
Sn0/SiO2sol-gel,basic SnO/Si-MCM-41imp
9
!
300
,
i
4O0
'
I
500
Wavelenght / nm
Figure 2: DR-UV spectra of SnO2 nanoparticles in (a) SnO2/SiO2sol-gel,basic (5 wt.-% Sn), (b) SnOz/Si-MCM-41imp (8 wt.-% Sn), (c) SnO2/SiO2sol-gel,acid (5 wt.-% Sn), (d) physical mixture of Si-MCM-41 and bulk SnO2 (8 wt.-% Sn) and (e) SnOz/Si-MCM-41cvd (l,1 wt.%
Sn).
658
OH
< fO'"
OH
OH O
I
,-:sn ,~nL. "-:~n--o._JOj ~ "O'" \ o ~ ~ / o I o o
!i~liiil
wall
Figure 3" Sketch of two-dimensional Sn(IV)-oxide species anchored onto the inner walls of the Si-MCM-41 channels.
three-dimension particles a twodimensional carpet of SnO2 aggregates is formed on the inner walls of the pores (Figure 3). In a first step the Sn precursors are anchored on the silanol groups of the SiMCM-41 and afterwards during the hydrolysis of the samples the bound Sn(IV) oxide species are cross-linked with each other without dissolution of the bonds to the matrix.
In-situ reduction/oxidation studies In the presence of reducing gases a decrease in the reflectance of the SnO2-1oaded samples can be observed due to the formation of oxygen vacancies [6]. In Figure 4 the development of the reflectance with time at a fixed wavelength of L = 620 nm during reduction and reoxidation at 673 K is compared for five samples prepared by the different methods. All spectra show a typical decrease of the reflectance upon exposure to CO followed by an increase after switching the surrounding gas atmosphere to 02 indicating that oxygen vacancies formed during reduction can be more or less healed by the addition of oxygen. The decrease in the reflectance, which is proportional to the degree of reduction in the Sn species, i.e. the number of oxygen vacancies, is highest for the Si-MCM-41 sample in which the SnO2 particles were formed by impregnation. For SnO2 nanoparticles in zeolite Y in-situ moessbauer studies showed that with CO as reducing agent the Sn(IV) species are only reduced to Sn(II) whereas with I-I2the reduction continues to Sn(0) [6]. The response of the SnO2/Si-MCM-41 sample loaded by cvd to CO is much worse due to a stronger interaction of the smaller aggregates with the matrix. Similar differences have also been observed for SnO2 nanoparticles hosted in the cages of zeolite Y [6].
b
~ ~ e C m
E O r
d
v
n,,
"-.02 0
'
2'0
'
;0
'
6~0
'
8'0
T i m e I min
'~;0'1~0
Figure 4: Time dependent reflectance at ~, = 620 nm during alternating exposure of CO and 02 at 673 K for (a) a physical mixture of SiMCM-41 and bulk SnO2 (8 wt.% Sn), (b) SnO2/SiO2solgel, acid (5 wt.% Sn), (c) SnO2/SiO2sol-gel, basic, (5 wt.% Sn), (d) SnO2/Si-MCM41 imp (8 wt.% Sn) and (e) SnO2/Si-MCM-41 cvd (1,1 wt.%
Sn).
659 In case of the sol-gel SnOz/SiO2 samples distinct changes of the reflectance are only detected for the sample prepared via basic catalysis. This again indicates that here the SnO2 nanoparticles are encapsulated in the mesopores of the SiO2 network where they are highly accessible for the gas molecules. Furthermore, the size of the SnOz particles is much smaller than that in the samples synthesized by acid catalysis, offering a higher active surface. The behavior against CO of the samples obtained by the acidic route is relatively similar to that of bulk SnO2, which was only physically diluted with Si-MCM-41. In both cases relatively large and compact particles are present providing only a small degree of the Sn(IV) oxide units is at the surface and can react with the CO. If the samples SnOz/Si-MCM-41imp and SnOz/SiO2sol-gel,basic showing the best response towards CO are compared, it becomes obvious that the reversibility is best for the Si-MCM-41 sample; for the sol-gel sample the value of reflectance obtained after a complete cycle decreases continuously whereas it remains almost constant for the Si-MCM-41 sample. This documents that in the sol-gel sample during the partial reduction and re-oxidation a further agglomeration of the particles, which are only weakly bound to the matrix, takes place. In the Si-MCM-41, however, the carpet-like SnO2 network is tightly bound and is not altered during reduction and re-oxidation. Due to its high sensitivity and good reversibility the sample SnOz/Si-MCM-41 imp was chosen for further experiments aiming at the determination of the detection limit for CO under realistic conditions. For this study a flow of a CO:O2 = 1:1 mixture was used as starting point. Pulses of different CO:O2 ratios varying from CO:O2 = 1.05 to 1.25 were added to this flow for 60 s and the response in the reflectance of the sample was recorded. The result is shown in Figure 5. It can clearly be seen that the change in the reflectance increases proportional with an increasing amount of CO in the mixture and that already a deviation of only 5% from the standard mixture can clearly be detected. The response time was always only about 15 s and is even shorter for higher CO concentrations. This demonstrates that the SnO2 particles stabilized in high dispersion in the regular pores of a Si-MCM-41 possess a high potential for the development of sensors basing on optical detection. standard flow: 20 ml/min CO + 20 ml/min 02 + 80 ml/min Ar The numbers at the peaks give the CO flows during the pulses in ml/min.
E
tO C'q
rr
.
21
.
22 24 25
o
'
2'o
' 3'o
' 4'o
'
T i m e / min
' 6'o
'
r
'
8'o
Figure 5: Monitoring of pulsed deviations from a stoichiometric mixture of CO: 02 = l:l (18 vol.% CO, 18 vol.% 02 in 64 vol.% Ar) over a pellet of SnO2/Si-MCM-4 limp via changes in the reflectance.
660 4. CONCLUSIONS The formation of SnO2 nanoparticles via the sol-gel route depends strongly on the catalyst used in the synthesis. Whereas under acidic conditions relatively large particles, which become part of the network, are formed, the basic route leads to 3-5 nm large, almost spherical SnO2 particles hosted in mesopores of the SiO2 network. In the regular pores of SiMCM-41 two-dimensional Sn(IV) oxide structures are arranged along the walls showing a high stability and offering a high accessible surface. The high sensitivity, the short response times and the prospect of a more feasible miniaturization compared to electrochemically cells, which often need a reference gas atmosphere, makes the SnO2/Si-MCM-41 an interesting material for alternative sensing using optical detection.
ACKNOWLEDGEMENTS
We thank Prof. Dr. G. Schulz-Ekloff and Prof. Dr. N. Jaeger (University of Bremen) for valuable discussion, Dr. J. Rathousky and Dr. A. Zukal (Heyrovsky Institute of Physical Chemistry, Prague, Czech Republic) for adsorption measurements and the German Science Foundation (DFG, SCHU 426/9-3 and WA 1116/2) for financial support.
REFERENCES
1. W. G6pel, J. Hesse and J.N. Zemel (Eds.), Sensors, VCH Weinheim (1989). 2. F. Quaranta, R. Rella, P. Sicliano, S. Capone, M. Epifani, L. Vasenelli, A. Licciuli, A. Zocco, Sensors and Actuators B, 58, 350 (1999). 3. K. Wada and M. Egashira, Sens. and Actuators B, 62, 211 (2000). 4. C. Xu, J. Tamaki, N. Miura and N. Yamazoe, Sensors and Actuators B, 3, 147 (1991). 5. G. Grubert, M. Wark, N.I. Jaeger, G. Schulz-Ekloff and O. P. Tkachenko, J. Phys. Chem. B, 102, 1665 (1998). 6. M. Warnken, K. Lazar and M. Wark, Phys. Chem. Chem. Phys. 3, 1870 (2001). 7. C. Sanchez and F. Ribot, New J. Chem., 18, 1007 (1994). 8. C.J. Brinker and G.W. Scherer, Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, ,Academic Press, San Diego, p. 60 ff (1990). 9. M.J. Hampden-Smith, T.A. Wark and C.J. Brinker, Coord. Chem. Rev., 112, 81 (1992). 10. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vatuli and J.S. Beck, Nature, 359, 710 (1992). 11. J.Y. Ying, C.P. Mehnert and M.S. Wong, Angew. Chem. Int. Ed., 38, 58 (1992). 12. J. Rathousky, M. Zukalova, A. Zukal and J. Had, Collect. Czech. Chem. Commun., 63, 1893 (1998). 13. G. Schulz-Ekloff, J. Rathousky and A. Zukal, Microporous Mesoporous Mater., 27, 273 (1999). 14. G. Kortiim, Reflexionsspektroskopie, Springer, Berlin (1969). 15. G. Schulz-Ekloff, J. Rathousky and A. Zukal, in: Synthesis of Porous Materials, M. Occelli and H. Kessler (eds.), M. Dekker, New York (1996). 16. H. Weller, Angew. Chem. Int. Ed. Engl., 32, 41 (1993).
otumes m ~urmce ~mence ana t~atalysts 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
661
S p o n t a n e o u s nitride f o r m a t i o n in the r e a c t i o n of m e s o p o r o u s t i t a n i u m o x i d e with b i s ( t o l u e n e ) t i t a n i u m in a n i t r o g e n a t m o s p h e r e M. Vettraino, ~ X. He, :~Michel Trudeau, b and David Antonelli ~' '~Department of Chemistry and Biochemistry, The University of Windsor, 401 Sunset Avenue, Windsor, Ontario, Canada N9B 3P4 bM. L. Trudeau, Emerging Technologies, Hydro-Qu6bec Research Institute, 1800 Boul. Lionel-Boulet, Varennes, Quebec, Canada J3X 1S1
Recently we showed that mesoporous niobium oxide reduced with bis(toluene) niobium reacts at room temperature with dinitrogen, leaving a thin coat of sp 3 hybridized nitride on the low valent niobium oxide surface. In this report we show that mesoporous titanium oxide reduced with bis(toluene) titanium in the presence of dinitrogen leads to formation of a new mesoporous titanium oxide with surface nitride. All materials in this study were characterized by X-ray photoelectron spectroscopy, X-ray powder diffraction, nitrogen adsorption, elemental analyses, and superconducting quantum interference detector magnetometry. ~SN-MAS NMR labeling experiments confirmed that ambient dinitrogen was the source of the surface nitride species. Direct cleavage of dinitrogen is extremely rare and has never been reported on the surface of a porous oxide. These results demonstrate that this system may be useful in designing catalysts for nitrogen activation and incorporation of nitrogen in small molecules.
1. I N T R O D U C T I O N The fabrication of shape, porosity, and electronic properties of nanoscale materials is one of the flontiers of modern science [1-6]. Since the discovery of periodic mesoporous materials [7-10], a wide variety of new materials have been synthesized that exploit the ability of these nanostructured substrates to act as hosts for a broad range of inorganic and organic guest species which are too large to be included in the smaller pores of microporous zeolites. The discovery of stable mesoporous transition metal oxides [1112] further expands the range of possible host materials for such one-dimensional inclusion reactions. Recent work in our group has shown that the inorganic framework of mesoporous niobium oxide can act as an electron acceptor, making it the first fully reducible molecular sieve [13]. This property, necessary in the design of cathodic materials, is due to the capacity for variable oxidation states in the walls of the inorganic host and allows the individual pore channels to act as charge-balancing sheaths for nonstoichiometric phases of the type M"+/M~''+~> residing in the pores. Thus mesoporous Nb, Ti, and Ta oxides react with a variety of organometallic species of suitably low ionization potential including alkali metal naphthalene reagents [13],alkali fullerides [14], metallocenes [15], and bis(arene) complexes [16], to give reduced mesostructures with metallic, superparamagnetic, and insulating properties depending on the nature of the
662 organometallic dopant. Of special interest is the reaction of mesoporous niobium oxide with zero-valent early transition metal bis(arene) complexes, since reaction of mesoporous niobium oxide with bis(toluene) niobium leads to deposition of a thin layer of a metallic low valent oxide phase over the surface of the mesostructure [17]. Mesoporous materials with metallic or semiconducting electron transport properties are very rare [18-21] and are projected to be useful in the fabrication of nanoscale optoelectronic devices and fuel cells. These new low valent mesoporous niobium oxides offer metallic conductivity along with the very high surface area associated with mesoporous materials. The high reactivity of the low-valent mesostructure is immediately evident in a room temperature dinitrogen cleavage reaction. This is an extremely rare process and may represent the first step in the development of a new catalyst for the reduction of nitrogen to ammonia. In this report we will focus on new materials synthesized by the treatment of mesoporous titanium oxide with bis(toluene) titanium. Mesoporous titanium oxide is cheaper than its niobium oxide counterpart and bis(toluene) titanium is much more readily synthesized by metal vapor synthesis than bis(toluene) niobium.
2. M A T E R I A L S AND M E T H O D S 2.1. Materials
All chemicals unless otherwise stated were obtained from Aldrich. Samples of mesoporous titanium oxide (Ti-TMS 1) were obtained from Alfa-Aesar and used without further purification. Trimethylsilyl chloride was obtained fi'om Aldrich and distilled over calcium hydride. Mesoporous titanium oxide samples were dried at 100 ~ overnight under vacuum and then stirred with excess trimethylsilyl chloride in dry ether for 12 h under nitrogen before repeating the drying step. Bis(toluene) titanium was prepared by metal vapor synthesis with the assistance of Professor F. G. N. Cloake at the University of Sussex [22]. Labeled lSN 2 w a s obtained from Cambridge Isotopes and used without further purification. 2.2. M e a s u r e m e n t s Nitrogen adsorption and desorption data were collected on a Micromeritics ASAP 2010. Xray diffraction (XRD) patterns (CuK,0 were recorded in a sealed glass capillary on a Siemens D500 0-20 diffractometer. All X-ray photoelectron spectroscopy (XPS) data were obtained with a Physical Electronics PHI-5500 spectrometer using charge neutralization. All emissions were referenced to the Carbon C-(C,H) peak at 284.8 eV. The conductivity measurements were recorded on a Jandel 4 point universal probe head combined with a Jandel resistivity unit. The equations used for calculating the resistivity were as follows: for pellets of <0.5mm thickness: P
-
logn 2
x
I
For pellets of >0.5ram thickness the following equation is used: p = 2Jr(S)~ 1
/z Where p = resistivity; logn 2 = sheet resistivity; V = volts; I = current; t = thickness of the pellet; S = the spacing of the probes (0.1cm). The powder electron paramagnetic resonance (EPR) samples were prepared under vacuum and the data collected on a Bruker X-band ESP
663
300E EPR spectrometer. Magnetic measurements were conducted on a Quantum Design SQUID magnetometer MPMS system with a 5 Tesla magnet. All elemental analysis data (conducted under an inert atmosphere) were obtained from Galbraith Laboratories, 2323 Sycamore Drive, Knoxville, TN 37921-1700. Solid state NMR spectra were obtained from Spectral Data Services in Champaigne Illinois and recorded at 36.835 MHz at room temperature on an Oxford 363 MHz spectrometer with a superconducting magnet and a Tecmag console. Shifts are relative to external nitromethane at 0.0 ppm. 2.3. Methods To a suspension of mesoporous titanium oxide in dry toluene under nitrogen was added excess bis(toluene) titanium calculated on the basis of metal percent as determined from the elemental analysis data. The mesoporous solid immediately went from a light faun color to a deep gray-black. After several days and additional stirring to ensure complete absorption of the organometallic, the reduced material was collected by suction filtration and washed several times with toluene. The material was dried in v a c u o at 10.3 tort on a Schlenk line until all condensable volatiles had been removed.
3. RESULTS AND DISCUSSION A sample of trimethylsilated mesoporous titanium oxide, possessing a BET (Brunnauer, Emmett. Teller) surface area of 785 m2g-l, an HK pore size of 24,~, and an X-ray powder difflaction (XRD) pattern displaying one peak centered at d = 32 ,~, was treated with excess bis(toluene) titanium in toluene over several days until absorption was judged complete. The new black material was collected by suction filtration, washed with 350 300 250 ~200
s
~ 150 100 if)
50
0
0.2
0.4
0.6
0.8
1
PtPo
Figure 1. Nitrogen adsorption (I) and desorption (II) isotherms for samples of mesoporous titanium oxide before (upper) and after (lower) treatment with bis(toluene) titanium. excess toluene, and dried in v a c u o . The XRD pattern of this air sensitive solid displayed a broad peak centered at d = 32 A while the BET surface area dropped to 507 m2g -~ and
664 the HK pore size decreased to 19 A (Fig. 1). The elemental analysis of this new material gave 51.13% Ti, 2.56% C, 1.07% H, and 1.16% N as compared to 38.11% Ti, 3.12%C, 1.36% H, and <0.01%N in the starting material. From these data a molecular formula of TiO26C02H~ 0N00s can be calculated. The oxygen value was calculated by difference since oxygen analysis in the presence of transition metals is not generally accurate. The increase in Ti with only a slight increase in the C suggests that the surface of the pore channels are coated with a thin layer of low valent Ti. This is also consistent with the retention of the XRD peak at d = 32 A with a slight shrinkage of the HK pore size. The thermal decomposition of bis(toluene) titanium over mesoporous aluminum oxide has been reported previously to lead to nanoscale grains of Ti metal in the pores of the alumina [23], however our approach differs as decomposition is likely induced by oxidation of the organometallic Ti(0) complex by the Ti (IV) mesostructure since bis(toluene) titanium is thermally stable at room temperature [22]. The most salient feature of the elemental analysis, however, is the incorporation of 1.16% N in the material, since the only nitrogen source present in the synthesis conditions was N2, an extremely robust molecule that is notoriously difficult to activate. In a previous report we showed that samples of mesoporous niobium oxide treated with bis(toluene) niobium under argon led to the formation of low valent non stoichiometric mesoporous niobium oxide with evidence for metallic properties and the ability to cleave dinitrogen on the inner surface of the mesopores [ 17]. The nitrogen content of the nitrided materials was as high as 1.5%. The extension of this chemistry to Ti-based systems represent a significant step since bis(toluene) niobium is extremely difficult to synthesize and Ti is a much less expensive metal than Nb. This result also demonstrates that nitrogen activation can proceed without a prior synthesis step conducted under an argon atmosphere. In order to determine the composition of this new material, X-ray photoelectron spectroscopy studies were conducted. Since mesoporous materials possess very high 6o0
a)
120
b)
100
5O0
4O0
300
200
100
0
= 45
35
.
'It 25
Binding Energy (eV)
15
Binding Energy (eV)
Figure 2. XPS spectrum of mesoporous titanium oxide treated with bis(toluene) titanium showing (a) Ti 3p region and (b) region near Fermi level. Binding energy versus intensity in arbitrary units.
665 surface areas and thin walls of only ca. 15-30 A, this technique, normally limited to thin surface layers, gives an accurate picture of the entire sample and has thus been used extensively by our group in previous work to determine the degree of reduction of the porous framework. Figure 2a shows the 3p 1/2,3/2 region of the XPS spectrum of a sample of the material treated with bis(toluene) titanium. The 3p 1/2 emission falls at 37.5 eV, as compared to 36.8 eV in the material reduced with 1.0 equivalents of Li and 37.9 eV in the unreduced material. The gradual shift to lower binding energy on reduction of the framework has been commented on before; in this case the emission at 37.5 eV demonstrates reduction of the framework to a level of about Ti 3.6 +. There is no clear peak for a second Ti species corresponding to the Ti atoms originating from the organometallic, suggesting that the material may have a more homogeneous distribution of oxidation states for the Ti throughout the material than in mesoporous niobium oxide materials reduced with bis(toluene) niobium and subsequently treated with dinitrogen, which display two clear emissions in the Nb 312,512 region corresponding to Nb 4.8 + and Nb 2 +. The N ls region is similar to that in the Nb material, exhibiting emissions at 399.2 eV and 397.1 eV corresponding to two reduced nitride species on the surface [24]. While the N Is emission is sensitive to local coordination environment further studies will have to be conducted in order to determine the precise nature of the bonding and hybridization involved in these two species. Figure 2b shows the region near the Fermi level of the material reduced with bis(toluene) titanium. The large emission from 3-8 eV corresponds to the Ti-O sp valence band electrons while the smaller hump at the Fermi level, also observed in samples of mesoporous niobium oxide reduced with bis(toluene) niobium, is indicative of metallic behavior, common in low-valent early transition metal oxides such as VO, TiO, and NbO. Electron transport measurements using the 4-point method on pressed pellets of this material show surprisingly high conductivity values of 10-2 ohm -j cm-', comparable to those values measure for mesoporous niobium oxide reduced with bis(toluene) niobium, and over 1000 times greater than the Ti-based materials reduced with 1.0 equivalents of Li. The electron paramagnetic resonance (EPR) spectrum of this material displays one broad resonance centered at 2.0 g that can be assigned to free electrons in the mesostructure on the basis of previous work on the EPR spectra of reduced mesoporous Ti oxide species [13b]. Samples of mesoporous niobium oxide redtlced with bis(toluene) niobium show a second resonance at higher field, however this may be related to the second Nb species detected in the XPS spectrum, again suggesting that the Ti-based material has a more homogeneous distribution of electronic states throughout the material. Figure 3a shows a plot of the zero field cooled (ZFC) and field cooled (FC) molar magnetization versus temperature for the sample in Figure 2 over a temperature range from 4-150 K. The transition at around 20 K in the ZFC branch of the plot is indicative of spin glass or superparamagnetic behavior, also observed in cobaltocene and nickelocene composites of mesoporous niobium oxide. The magnetization decreases with increasing temperature according to the Currie law above 20 K, and shows strong evidence for temperature independent paramagnetism expected on the basis of the high conductivity and the emission at the Fermi level in the XPS spectrum [25]. Figure 3b shows the plot of molar magnetization versus T -~, exhibiting a Y-intercept at 1.32 x 10e emu, corresponding to the temperature independent paramagnetism. Below 20K the Currie law is not obeyed, the magnetic moment being somewhat lower than expected in this region. This is indicative of antiferromagnetic coupling in the material, supportive of
666 spin glass rather than superparmagnetic behavior. From the Currie constant a la~.f of 2.1 can be calculated, indicative of 1.3 unpaired electrons per stoichiometric unit [26]. -~ 6.00E-.02 ,E E 5.00E.-02
I--,--zFc I
0 4.50E-02
E
E
| 3.50E-02 .....
n
,,=_ 3.00E-02
~. 3.00E-02 r O0
._ E
j
4.00E-02
>" 4.00E-02
2.50E-02
~ 2.00E-02
2.00E-.02
= 1.50E-02
1,00E-02
u 1.00E-02 ,_ ,,,., | 500E-03
,,.
O.OOE+O0 0
i
,
,
50
1 O0
150
m
O.OOE+O0 0
,
,
,
,
0.01
0.02
0.03
0.04
Temperature (K)
0.05
l I T (K)
Figure 3. SQUID plots of a) ZFC and FC molar magnetization versus temperature (right) and b) molar magnetization versus inverse temperature (left). To confirm that the nitride in the material originated from dinitrogen, the synthesis was carried out on a high vacuum Schlenk line under a ~~N atmosphere. After three days stirring to ensure maximum absorption of the nitrogen, the sample was collected by filtration and dried in v a c u o . Figure 4 shows the ~SN-MAS spectrum of this labeled material clearly exhibiting two resonances at -355 and -377 ppm, consistent with aminoid sp 3 hybridized nitrogen. The peak positions are similar to those reported previously for mesoporous niobium oxide reduced with bis(toluene) niobium and differ
,
,
t
Figure 4. JSN-MAS NMR spectrum of mesoporous titanium oxide treated with bis(toluene) titanium under JSN,. quite substantially from those expected for a terminal nitride which can appear as far downfield as +840 ppm [27, 28]. For comparison, samples of ~SN-labeled as-synthesized mesoporous niobium oxide, believed to have Nb-N dative bonds in the structure, exhibit
667 a single strong resonance at -361 ppm [12]. The activation of dinitrogen by an oxide is extremely rare, normally requiring microwave radiation and argon plasmas [29,30]. We attribute the high reactivity of our material to low valent, low coordinate Ti centers on the surface of the material which formed by the electrochemical decomposition of bis(toluene) titanium on the inner and outer surface of the material. Some transition metals form a thin coating of nitride on the surface upon exposure to air, but these materials have low surface areas and are inert to further reaction of the nitride functionality. We believe that the high surface areas, uniform porosity, and high concentration of surface defects expected from the amorphous nature of the mesostructured walls may lead to enhanced downstream reactivity, useful in the development of catalytic nitrogen incorporation reactions. 4. A C K N O W L E D G E M E N T S The Petroleum Research Fund administered by the American Chemical Society and NSERC are acknowledged for funding of this work. The Ontario Premier's Research Excellence Award (PREA) program is also thanked. John Robinson is acknowledged for his help in obtaining XRD data. REFERENCES 1. M.E. Davis, Nature, 364 (1993) 391. 2. M. Antonietti, B. Berton, C. Goeltner and H. Hentze, Adv. Mater., 10 (1998) 154. 3. D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Frederickson, B. F. Chmelka and G. D. Stucky, Science, 279 (1998) 548. 4. B.T. Holland, C. F. Blanford and A. Stein, Science, 281 (1998) 538. 5. J . E . G . J . Wijnhoven and W. L. Vos, Science, 281 (1998) 802. 6. A. Imhof and D. J. Pine, Nature, 389 (1997) 948. 7. (a) C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710; (b) 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. Shepard, S. B. McCullen, J. B. Higgins and J. L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 8. (a) Q. Huo, D. I. Margolese, U. Ciesla, D. G. Demuth, P. Feng, T. E. Gier, P. Sieger, A. Firouzi, B. F. Chmelka, F. Schuth and G. D. Stucky, Chem. Mater., 6 (1994) 1176; (b) A. Firouzi, D. Kumar, L. M. Bull, T. Besier, P. Sieger, Q. Huo, S. A. Walker, J. A. Zasadzinski, C. Glinka, J. Nicol, D. Margolese, G. D. Stucky and B. F. Chmelka, Science, 267 (1995) 1138. 9. C.-Y. Chen, S.L. Burkette, H.-X. Li and M. E. Davis, Micropor. Mater., 2 (1993) 27. 10. P.T. Tanev, M. Chibwe and T. J. Pinnavaia, Nature, 368 (1994) 321. 11. a) D. m. Antonelli and J. Y. Ying, Current Opinion in Colloid and Interface Science, 1 (1996) 523; (b) D. M. Antonelli and J. Y. Ying, Angew. Chem., Int. Ed. Engl., 34 (1995) 2014. 12. (a) D. M. Antonelli and J. Y. Ying, Angew. Chem., Int. Ed. Engl., 35 (1996) 426; (b) D. M. Antonelli, A. Nakahira and J. Y. Ying, Inorg. Chem., 35 (1996) 3126. (c) D. M. Antonelli and J. Y. Ying, Chem. Mater., 8 (1996) 874. 13. a) M. Vettraino, M. Trudeau and D. M. Antonelli, Adv. Mater., 12 (2000) 337. b) M. Vettraino, M. Trudeau and D. M. Antonelli, Inorg. Chem., 40 (2001) 2088. 14. B. Ye, M. Trudeau and D. M. Antonelli, Adv. Mater., 13 (2001) 29.
668 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.
S. Murray, M. Trudeau and D. M. Antonelli, Adv. Mater., 12 (2000) 1339. X. He, M. Trudeau and D. M. Antonelli, Adv. Mater. 12 (2000) 1036. M. Vettraino, X. He, M. Trudeau and D. Antonelli, unpublished results. Z. R. Tian, J. Y. Wang, N. G. Duan, V. V. Krishnan and S. L. Suib, Science, 276 (1997) 926. K. K. Rangan, S. J. L. Billinge, V. Petkov, J. Heising and M. G. Kanatzidis Chem. Mater., 11 (1999) 2629. M.J. MacLachlan, N. Coombs and G. A. Ozin Nature, 397 (1999) 681. G. S. Attard, C. G. G61tner, J. M. Corker, S. Henke and R. H. Templer,' Angew. Chem. Int. Ed., 36 (1997) 1315. F. G. N. Cloke, Chem. Soc. Rev., (1993) 17. J.J. Schneider, Adv. Mater. 13 (2001) 529. J. Casanovas, J. M. Ricart, J. Rubio, F. Illas and J. M. Jimn6z-Mateos, J. Am. Chem. Soc., 118, (1996) 8071. P. A. Cox, The Electronic Structure and Chemistry of Solids, Oxford University Press, New York, 1987. C.-W. Chen, Magnetism and Metallurgy of Soft Magnetic Materials, Dover, New York 1986. C. E. Laplaza and C. C. Cummins, Science, 268 (1995) 861. G. L. Leigh, Acc. Chem. Res., 25 (1992) 177. R. Niewa and H. Jacobs, Chem. Rev., 96 (1.996) 2053. R. Niewa and F. J. DiSalvo, Chem. Mater., 10 (1998) 2733.
Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) Crown Copyright 9 2002 Published by Elsevier Science B.V. All rights reserved.
669
I s o l a t i o n a n d c h a r a c t e r i z a t i o n o f a m o r p h o u s s o l i d s f r o m oil s a n d s f i n e t a i l i n g s A_bdul Majid, Steve Argue, Irina Kargina, Victor Boyko, Gerry Pleizier and Jim Tunney Institute for Chemical Process and Environmental Chemistry, National Research Council of Canada, Ottawa, Ontario K1A 0R9, Canada The hot water process used by Suncor and Syncrude to extract bitumen from the Athabasca oil sands produces tailings with about 35 % more volume than that occupied by the bituminous sands before mining. This increase is largely the result of water hold-up in the fines fraction from the tailings, arising primarily from the "middlings' treatment circuit. Our recent work suggests that approximately 50% of these fines consist of amorphous clays. In this investigation we have developed fractionation methodology to separate amorphous solids from Syncrude fine tailings. This fraction has been 'characterized by elemental analysis, X-ray diffraction, XPS, SEM, infrared spectroscopy, and surface area measurement.
1. INTRODUCTION Oil from Alberta oil sands is currently being separated using the Clark Hot Water Process. This results in the production of large volumes of fluid wastes called fine tailings [ 1,2]. Fine tailings are made up of a complex system of clays, minerals, and organics. They show little tendency to dewater, even when subjected to mechanical dewatering procedures [3]. These clay tailings are acutely toxic to aquatic organisms and are currently being stored in large tailings ponds. The buildup of these partially settled clay tailings presents not only an environment problem but also a significant repository for non-recyclable water which eventually must be reclaimed. The reason for the intractability of the clay tailings has been a subject of considerable study [4-7]. Based on the results of published work it is generally believed that a combination of residual organics and fine clay particles contribute to the stability of fine tailings. The high water holding capacity of fine tailings has been attributed to the presence of amorphous minerals such as iron oxide and clays [8]. Our group at NRC has been actively involved in research dealing with various aspects of fine tailings [5-7, 9-15]. As a result we have developed a number of fractionation schemes to separate fine tailings into various components. In this investigation we have developed a fractionation methodology to separate amorphous solids from Syncrude fine tailings. This fraction has been characterized by elemental analysis, X-ray diffraction, XPS, infrared spectroscopy, and surface area measurement.
2. MATERIALS AND METHODS 2.1. Materials
Aqueous tailings samples, used in this investigation were obtained from the 17 m level of the Syncrude tailings pond. The physicochemical properties and handling procedures for these samples have been reported previously [7]. The recovery of amorphous solids was carried out from a fine tailings
670
sample as received, a clean sample of fine tailings after removing bitumen and hydrophobic solids and a sample of suspension fraction from clean fine tailings. Tiron (4,5-dihydroxy- l, 3-benzene-disulph0nic acid disodium salt) was obtained from Sigma Chemicals Inc. I t was used as 0.1 M-aqueous solution containing 5.3 g of anhydrous sodium carbonate. The final pH of the solution was adjusted to 10.5 with sodium hydroxide. The solution was kept in a polypropylene bottle and stored in a refrigerator for a maximum of one month. The solution pH dropped during storage and was adjusted to 10.5 before use. All other reagents were obtained from Aldrich and used as received. 2.2. Measurements PAS-FTIR (photoacoustic Fourier Transform Infrared Spectroscopy) spectra were collected using a MTEC Model 300 photoacoustic detector combined to a Bruker IFS 66/S FTIR spectrometer. 500 scans were collected at a resolution of 8 cm! in the rapid scan mode. 64 scans of Carbon black were used as reference and helium was the purge gas.
XPS was performed with a Physical Electronics (Perkin Elmer, Eden Prairie, MN, USA) model 550 instrument. Monochromatic Al K a radiation was used. The dry samples were pressed into indium foil for analysis. Survey spectra were collected using pass energies of 188 eV, while high resolution spectra were recorded with a 22 eV pass energy. An electron flood gun was used to neutralize the charge during the experiment. Binding energies were referenced to the carbon-carbon bond, which was assigned a binding energy of 284.6 eV. Atomic compositions were estimated using a standard program provided with the instrument. During analysis, the pressure inside the instrument was always below 5 x 10-9 torr (<0.7 ~Pa). Specific surface areas, pore volumes and pore size distributions were determined using a Micromeritics Gemini III 2375 apparatus. The density of the materials was determined by a pycnometry measurement with helium using Micromeritics Accupyc 1330 apparatus. Heavy metals were determined using inductively coupled plasma atomic emission spectrometry (ICP-AES). Cation exchange capacity (CEC) was measured using the BaCl2 method [ 16]. X-ray powder diffraction data were collected between 20 = 5~ 100~ with a scan rate of 2~ at room temperature on Scintag XDS 2000 with a theta-theta geometry and a copper X-ray tube. The diffractometer had a pyrolytic graphite monochromator in front of the detector. The samples were mounted on a zero background sample holder made of an oriented silicon wafer. 2.3. Procedures Removal of residual bitumen and hydrophobic solids from fine tailings was carried out using a procedure reported previously [7]. For the separation of amorphous solids a typical procedure involved mixing an aqueous sample of fine tailings or centrifuged suspension (300g) with 0.1 Malkaline Tiron solution (100 mL) in a 500 mL polypropylene bottle. The contents were agitated in an end-over-end fashion for seven days at 30 + 2 rpm. The treated sample was centrifuged at 500 G for 15 minutes to separate a clear dark black solution from the coarser solids. The sediment was mixed with deionized water (100 mL) and centrifuged again to separate residual Tiron extract adsorbed on the surface of the coarser solids. The washing of the coarser solids was repeated two more times in order to obtain cleaner solids. All washings were combined with the Tiron extract from the first step.
671
The separation of amorphous solids from Tiron extract was carried out by precipitation with a mineral acid (HCI). A typical treatment involved acidification of the extract to pH -1, when the amorphous solids precipitated as a gel, releasing free water. The contents were centrifuged and the wet sediment washed several times with distilled water. Finally, the wet cake containing the amorphous solids was dried at 80 ~ under vacuum.
3. RESULTS AND DISCUSSION The high water holding capacity of fine tailings has been attributed to the presence of clays and amorphous minerals of silicon, aluminum and iron [8]. The amorphous oxides may be cementing the clay particles. The removal of this coating by treatment with alkaline Tiron solution has been attempted. This treatment is known to remove the finely divided amorphous oxides of iron, aluminum and silicon [ 17]. Table 1 lists sample description, yield and carbon content of various samples of amorphous solids separated in this investigation. Compared with the sample of bulk fine tailings, Tiron extracted the highest amount of solids from the suspension fraction. Approximately 50 percent of the colloidal solids suspended in the suspension fraction appear to be amorphous. This supports Yong and Sethi's assertion [8] that fine amorphous colloidal solids contribute significantly to the stability of fine tailings. High levels of organic carbon indicate that there is a very important interaction between the organic and inorganic materials in these systems. Hydrous oxides are known to provide more sites for organic matter adsorption in soils than the surfaces of micaceous clay minerals [18]. Amorphous oxides are exceptionally stable in the presence of humified organic matter [ 19].
Table 1. Sample descripti0nand yield for tiron extracts Fraction Description Yield (w/w% of ............................... solids)* 1 Tiron extract from tailings as 3.6(26.5) received 2 Tiron extract from clean tailings 3.9 (34.0) 3 Tiron extract from suspension 52 (8.2) 4 Tiron extracted residual solids 91 (34.0)
LOI at 500~ (w/w%) 29.8
Organic carbon (w/w%)
20.5 34.5 4.1
14.4 16.8 1.5
16.3
* Amount of total solids in the feed is shown in parenthesis, LOI=Loss on ignition
Table 2. Physico-chemical characteristics of tiron extracts Fraction Density Surface Area CEC Average pore (g/cm 3) (m2/g) Cmol[+]Kg l size (A)
Elemental Analysis
SIO21A1203 Ratio
(w/w%) 1
2.61
98
16.1
2 2.60 105 17.9 3 2.58 103 19.3 4 2.83 10.6 7.4 -CEC="Cat'ion 'Exchange capacity ..................
6.6 6.2 5.7 70
Si 28 26 25 35
AI 17 16 16 9.4
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fe 2.8 4.8 3.9 1.1
1.6 1.6 1.5 2.0
672 Table 2 lists elemental analyses and physico-chemical properties of the solid fractions. The three samples of Tiron extracts had remarkably similar characteristics indicating a homogeneous nature of these materials in the fine tailings. The density of the Tiron extracts (2.6 + 0.02) is in the range for kaolinite, halloysite, quartz, allophanes and imogolite [20, 21]. This suggests that these solids may have a structural arrangement comparable with these materials. Surface area and pore size determine the accessibility to active sites and this is often related to catalytic activity and selectivity in catalyzed reactions. These materials have high specific surface areas that approach the values reported for silica-alumina gels [22]. Cation exchange capacity (CEC) values of the amorphous solids are higher than those reported for kaolinite minerals but within the range for halloysites [23]. Relatively low cost and micropores of the amorphous solids separated from fine tailings render these materials as excellent candidates for potential applications in heterogenous catalysis [24]. Micropores control shape selectivity and molecular exclusion which contribute most extensively to the total surface area and often host the majority of active sites. Elemental content of these solids suggests the presence of 3-5% iron supporting Yong and Sethi's hypothesis regarding the role of iron in the stability of fine tailings [8]. The value of silica to alumina ratios in these solids supports the presence of high silica allophanes [25]. The residual solids after Tiron extraction had considerably lower surface area and CEC, but higher density and pore sizes. These solids had higher silica content and lower amounts of aluminum and iron. Silica to alumina ratio of 2 for these solids suggests structural similarity with kaolinite.
3.1. X-Ray Diffraction The X-ray diffraction pattern of the Tiron extracts contained very broad peaks indicating the amorphous nature of the solids. Table 3 lists d-spacings and tentative assignments based on the comparison with the XRD patterns of the standards. The strongest peak overlaps with pattern for allophanes, amorphous silica, halloysites and muscovite suggesting that these materials may consist of a mixture of several components present in different proportions. Other peaks correspond to the standard X-ray diffraction patterns of halloysite and muscovite. Thus XRD pattern for the Tiron extracts indicates the presence of halloysites, muscovite and possibly allophanes and amorphous silica. The X-ray diffraction pattern of the residual solids after Tiron extraction contained sharp peaks which is consistent with the presence of crystalline materials. The comparison of the XRD pattern for these solids with the standards suggests the presence of quartz as a major mineral. Minor amounts of other minerals identified from XRD include: kaolinite, muscovite, corundum and calcite. These results suggest that the treatment of oil sands fine tailings with alkaline Tiron solution can effectively separate amorphous components from crystalline components.
Table 3: XRD data (d spac!ngs,, A) for,,,tiron extracts and,some standards* Tiron Halloysite Muscovite Allophane Amorphous Extracts silica 9.93 (58) 10 (100) 9.97 (100) 5.0 (38) 5.0 (65) 4.42 (69) 4.46 (70) 3.52 (100) 3.36 (100) 3.35 (40) 3.31 (100) 3.30 (100) 2.54 (55) 2.54 (35) 1.98 (28) 210 (45) ,,, 1.43 (37,) ,,. !.48 (30) , ......... - ................. * Relative intensities' in p~enthesis'"
673
3.2. Infra-red Spectroscopy Many difficulties arise in the determination of the physical and chemical characteristics of amorphous materials. By their very nature amorphous substances are difficult to detect and estimate, and frequently their presence is determined by implication rather than by direct measurement. The principal forms of amorphous inorganic materials which occur in soils are the oxides, or more usually, the hydrous oxides of iron, aluminum, manganese, or silicon, either separately or combined. X-ray diffraction data has suggested the presence of allophanes, amorphous silica, halloysites and muscovite. Identification of these minerals from infrared spectrum is difficult because of other overlapping absorptions. The Infrared spectra of Tiron extracts contained a very broad and strong absorption in the OH region centered around 3400 cm "I. Absorption in this region is characteristic of amorphous aluminosilicate like allophane and amorphous oxides [26]. The presence of an amorphous silicious component is indicated by the silicate stretching area between 1110 and 900 cm 1, where a strong and broad absorption is present in the infrared spectrum of Tiron extract [27]. Muscovite and halloysites also absorb in this region but cannot be distinguished from silicious materials. The infrared spectra of the residual solids after Tiron extraction contained characteristic absorptions for kaolinite, quartz and calcite which is consistent with XRD results [28].
3.3. Surface Analysis Table 4 lists a summary of XPS data for both Tiron extracts and the Tiron extracted residual solids. XPS provides information on the top l0 nm of the surface. Silicon, aluminum and carbon were detected on the surface of both solids suggesting that these elements may be an integral part of the mineral structure. The presence of significant amounts of carbon in this environment could be an indication for the existence of clay-organic complexes in these materials. Tiron extracts appear to contain a thin layer of iron strengthening the Yong and Sethi's concept regarding the association of amorphous alumino-silicates with iron oxide which acts as a cementing agent. The Tiron extracted residual solids did not show any iron on the surface.
Table 4. Elemental Concentrations by x p s Element Si AI Fe C Si:AI ratio
.... -. . . . .
.: . : .
. . . . . . . .
....:::::..
:.:
.
.
.
.
.
.
.
.
...,,:,:,
Concentrations (atomic %) Tiron Extract Tiron Extracted Solids 18 14 12 9 1 24 20 1.5 1.6 .........
.~.
_
:.
.
.
.
.
.
.
.~....:
.
.
.
.
:..
:.
:.-
:. . . .
. ...................
: .................................
~. . . . . . . . .
4. CONCLUSION Nano-particles of clay present in oil sands fine tailings are coated with a matrix consisting of natural organic matter and hydrous metal oxides of iron, aluminum and silicon. This coating can be effectively removed by dissolution in alkaline Tiron solution. Tiron solution also dissolves substantial amounts of allophanes and micas with trace amounts of crystalline clays such as kaolinite and quartz. These findings support the Yong and Sethi theory regarding the stability of fine tailings.
674 5. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
25. 26. 27. 28.
F.W. Camp, "The Tar Sands of Alberta, Canada"; Cameron Engineering Inc.:Denver, 1969. R.N. Young, A. J. Sethi, J. Can. Pet. Technol. 76 (1978) 76. F.W. Camp, Can. J. Chem. Engin. 55 (1977) 581. M.A. Kessick, Int. J. Min. Process, 6 (1980) 277. B.D. Sparks, L. S. Kotlyar and A. Majid, 1991, Pre-Print # 91-117, Pet. Soc. of CIM and AOSTRA meeting B.D. Sparks, L. S. Kotlyar and R. Schutte, Adv. Oil Sands Tailings Research, Alberta Energy, Oil Sands Research Division, 1995, 25. A. Majid and B. D. Sparks, Fuel, 75 (1996) 879. R.N. Yong and A. J. Sethi, J. Can. Petr. Tech. Oct-Dec, 1978, 76. A. Majid and J. A. Ripmeester, J. Separation and Process Technology, 4 (1983) 20. A. Majid and John A. Ripmeester, Fuel 65 (1986) 1714. A. Majid, V. P. Clancy, and Bryan D. Sparks, Energy & Fuels, 2 (I 988) 651. A. Majid, J. Bornais & R. A. Hutchison, Fuel Science & Technology International, 7 (1989) 507. B. D. Sparks, C. E. Capes, J. D. Hazlett and A. Majid, Proc. Int. Symp. on Tailings & Effluent Management, Halifax, Aug. 20-24 1989, 295. A. Majid and J. A. Ripmeeter, Fuel, 69 (1990) 1527. A. Majid and B. D. Sparks, J. Can Pet. Tech. 38 (1999) 29. W. H. Hendershot, and M. Duquette, Soil Sci. Soc. Am. J. 50 (1986) 605. K. Wada, and D. J. Greenland, Clay Minerals, 8 (1970) 241 D. J. Greenland, Soil Science, 111 (1971) 34. U. Schwertmann, Nature, 212 (1966) 645. S. I. Wada, and K. Wada, Clay Minerals, 12 (1977) 289. J. B. Dixon, and S. B. Weed, Soil Science Society Am. (1977). Y. Kitagawa, The American Mineralogist, 56 (1971) 465. R. E. Grim, 'Clay Mineralogy'. McGraw-Hill, New York, 1968. W. F. Maier, F. M. Bohnen, J. Heilmann, S. Klein, H. C. Ko, M. F. Mark, S. Thorimbert, I. C. Tilgner, and M. Wiedorn, in, " Applications of Organometallic Chemistry in the Preparation and Processing of Advanced Materials, (1995) 27. T. Henmi, and K. Wada, Morphology and Composition of Allophane, American Minerologist vol. 61,379-390, (1976). M. J. Wilson, 'Clay Mineralogy: Spectroscopic and Chemical Determinative Methods', Chapman and Hall, London, 1994 L. E. DeMumbrum, and G. Chesters, Proc. Soil Soc. Am, 28 (1964) 355 H. W. van der Marel and H. Beutelspacher, Atlas of Infrared Spectroscopy of Clay Minerals and their Admixtures: Elsevier Scientific Publishing Co. 1976.
675 AUTHOR INDEX
A Ahn, W.-S. Albouy, P.-A. Altindag, Y. Amenitsch, H Anpo, M. Antochshuk, V. Antonelli, D. Aoki, K. Aquino, J. M. F. B. Arai, M. Araujo, A. S. Araujo, S. A. Argue, S. Ariyuki, M. Asai, K. Asefa, T Avalos, M. Azuma, K.
479 235 653 235 495 607 661 281 531 281 467,473,531 473 669 495 623 1,197 561 69
Bastardo-Gonzalez, E. Benjelloun, M. Bi, Y Birjega, R. Blin, J. L. Blom, D.A. Bogdanchikova, N. Boyko, V. Brinke, G. Brunel, D.
141 45 511 151 109,117,257 183 561,569 669 371 395
Cai, Q. Camblor, M. A. Cambon, H. Carlsson, A. Carreon, M. A. Cejka, J. Chao, M.-C. Che, S.
511 27 395 27 301,309 243,429 387 27
Chen, Q. Chen, W.-H. Chi, Y.-S. Cho, M. S. Choi, H. J. Chuiko, A. A. Clark, J. H. Coluccia, S. Coppens, M.-O. Crepaldi, E. L. Crowley, T.
403 453,537 329 479 479 205 631 417 85 235 337
D
Daehler, A. Dai, S. Davydov, L. De Jong, K. P. De M6norval, L.-C. Dellarocca, V. d'Espinose, J.-B. Di Renzo, F. Domen, K. Dou, T.
221 183 487 45 395 417 423 395 265 77
Etienne, M.
615
Fairbridge, C. Fajula, F. Femandes Jr, V. J. Fryxell, G. E. Fu, X.-L. Fujiwara, M.
543 395 467,473 583 525 445
G Gaber, B. P. Galameau, A. Ganea, R. Gao, F
213 395 151 229
676 Garcia-Bennett, A.E. G6d6on, A. Gee, M.L. Ghosh, A. Gianotti, E. Gigot, L. Gray, G.M. Grondey, H. Grosso, D. Guliants, V.V. Gurin, V.
379 423 221 641,647 417 257 127 1 235 301,309 561
H
Haddad, E. Hara, M. Hawkins, R. Hay, J.N. He, N. He, N.-Y. He, X. Hernandes, M.-A. Herrier, G Higashimoto, S. Hill, J.M. Hiraga, K. Holmes, J.D. Honma, I. Huang, S.-J. Huang, Y. Hunter, H. M.A.
423 265 543 127 229 459 661 561 117 495 543 27 337 623 453 511 379
Ikeda, T. Ikeya, H. Ikkala, O. Inagaki, S. Ionashiro, M.
69 553 371 27 473
Jansen, J.C. 635 J~oniec, M. 1,61,197,205,345,437,607 Jhon, M.S. 479 Jiang, H.-Y. 167,525 Jitianu, A. 151,653 Jones, W. 141
Joo, S. H.
607
K
KaluZa, L. Kamiya, S. Kaneda, M. Kang, K.-K. Kargina, I. Kasprzyk, B. Katou, T. Katsube, T. Kemner, K. Kitawaga, S. Koegler, J. H. Kondo, J. N. Kruk, M Kubota, Y. Kuj awa, J. Kumar, P. Kumar, R.
Lebeau, B. Lee, B. Lee, H.-K. L6onard, A. Lewandowska, A. Li, R. Li, W. Li, X. Li, Z. Lin, C.-N. Lin, H.-P. Lin, S. Lin, Y. Liu, S.-B. Liu, Z. Lobo, R. F. Lu, D. Lu, Z.-H. Lyons, D.
243 27 27 101 669 591 265 623 583 363 635 265 1,61,197,345,437 553 411 159,445 641,647
615 265 453 109,117,257 411 575 503 173 345 329 329,387,537 517 503,583 453,537 27 53 265 459 337
M
Macounov/L, K.
599
677 Macquarrie, D. Majid, A. M/iki-Ontto, R. Mal, N. Mann, S. Marchese, L. Markowitz, M. A. Martin, T. Maschmeyer, T. Matos, J. R. Mel'nyk (Seredyuk), I. V. Mercuri, L. P. Michalska, A. Mokaya, R. Morris, M. A. Mou, C.-Y. Mukherj ee, P.
Nagaya, J. Nawrocki, J. Neimark, A. V. Nenu, C. Ning, H. Nishizaki, Y. Noro, S.-i. Nowak, I.
631 669 371 159,445 205 417 213 395 635 61 205 61 411 141 337 329,387,537 641,647
553 591 45 151 517 553 363 591
O O'Connor, A. J. Ohsuna, T. Oliveira, E. C. Oumi, Y. Ozin, G. A.
221 27 297,417 69,159 1,197
P
Pang, W. Pastore, H. O. Patra, C. R. Paulino, I. S. Peng, L.-M. Petranovskii, V. Philippin, G. Pleizier, G. Pontes, L. A. M.
511 297,417 641,647 93 133,403 561,569 117 669 467
Pop, G.
151
Q Qun-Ji, X.
517
R
Raja, R. RathouskS,, J. Ravikovitch, P. I. Reddy, E. P. Rhee, H.-K. Ring, Z. Ripmeester, J. A. Roth, W. J. Ryan, K. M. Ryoo, R.
317 429,599 45 487 101,289 543 353 273 337 27,607
Sage, V. Sakamoto, Y. Sanchez, C. Sano, T. Sasaki, S. Sastry, M. Sawant, K. R. Sayari, A. Sayen, S. Schuchardt, U. Shan, Z. Sheu, H.-S. Shi, Q. Shindo, D. Shirai, M. Shorrock, K. Slabovfi, M. Smirniotis, P. G. Soldatov, D. V. Soler-Illia, G. J. Song, Y. Souza, C. D. R. Souza, M. J. B. Spalding, T. R. Stamm, M. Stevens, G. W. Stucky, G.
631 27 235 69,159 69 641,647 53 189,437 615 93 635 387 229 27 281 631 599 487 353 235 229 467,531 467,531 337 371 221 27
678 Su, B. L. Sugi, Y. Sugihara, T. Suh, Y.-W. Sun, J. Sun, S.-W.
109,117,133,257,403 553 251 289 85 167
Wu, D. Wu, H. Wu, Y.-C. Wulff, G. X
Xie, K Tang, X.-H. Tatsumi, T. ten Brinke, G. Terasaki, O. Thomas, J. M. Torii, K. Trudeau, M. Tsubakiyama, T. Tunney, J.
167,525 27,251 371 27 317 281 661 27 669
U Uchida, H.
623
V Valkama, S. Van Bavel, E. Van der Voort, P. Vansant, E. F. Vartuli, J. C. Veselfi, L. Vettraino, M.
371 45 45 45 273 429 661
W Walcarius, A. Wan, B.-Z. Wan, H. Wan-Fu, S. Wang, X. Wark, M. Weckhuysen, B. M. Wei, C. Wei-Min, L. Wen, X. Wilson, K. Wong, E.M. Wright, P. A.
615 329 229 517 77 653 45 511 517 167 631 213 379
77 583 453 35
575
Y Yamada, T. Yamana, K. Yamashita, H. Yamashita, T. Yang, C. Yang, H. Yang, J. Yang, X. Yang, Y. Yeh, C. Y. Yoshitake, H. Yoshizawa, K. Yu, Y. Yuan, Z.-Y.
623 159 495 265 173,459 543,575 221 503 189 635 251 495 229 133,403
Z
ZdraZil, M. Zhang, Y. Zhang, Z. Zhang, Z. L. Zhao, D. Zhao, Q. Zhen, K. Zhong, B. Zhou, H.-S. Zhou, W. Zilkov~i, N. Ziolek, M. Zub, Y. L. Zukal, A.
243 503 183 403 27 453,537 511 77 623 133,379,403,635 243 411,591 205 429,599
679 SUBJECT INDEX A Acidity 141, 411,467, 453,459, 531 Activation energy 531 Activity 537 Adsorbents 583 Adsorbents for heavy metals 615 Adsorption 229 Adsorption characterization 345,437 Al-containing MCM-41 411,459, 591 Alcothermal treatment 85 Al-iso-propoxide 151 Alkylation 289 Alkylation of phenol 525 Alkylation of toluene 141 Allylpropylthiourea ligand 607 A1-MCM-41 151, 159, 467, 473,503 A1-MCM-48 159 A1-SBA-15 423 Aluminophosphates 297 Aluminosilicates 141 Ammoximation 317 Amorphous solids 669 Amorphous titania 251 Anion adsorption 437, 583
Basicity Bifunctional catalysts Bimetallic nanoparticles Bimodal mesoporous silica Butane oxidation Butene Butylphenol
Cadmium sulfide Cage-like mesostructures Carbon monoxide Carbons Carbonylation Catalysis Catalytic cracking Catalytic oxidation
411 317 317 77 301,309 289 525
647 61 229 345 229 329 467 511
Characterization 61, 77, 167, 173, 189, 197, 301,309, 575 Chemical vapor deposition 251 Chromium oxide 495 Claisen-Schmidt reaction 553 Clay 281 Clinoptilolite 561 Clusters 569 Coal fly ash 159 Colloidal silica 345 Colloidal spheres 309 Colloid-imprinted carbons 345 Comb-coil 371 Combined micro- and mesoporosity 45 Comparison plots 429 Composite materials 553 Coordination bonding 371 Coordination polymers 363 Copper 561,569 Cr-MCM-41 487 Crystallization 265 Crystal-to-crystal conversion 363 Cumene conversion 141 Cu-modified MCM-41 229
Dodecylamine Dynamic pores
205 363
Electron density Electron diffraction analysis Electron energy loss spectroscopy Electron microscopy Electronic materials Enzyme mimics Erionite Ethylene polymerization EXAFS Exfoliated zeolite
69 265 403 27 337 35 561 495 251 273
FDU-1
61
680 Fe-containing MCM-41 Film FT-IR spectroscopy Functionalized organosilica
459 623 167, 417 1
G Gas adsorption Gas sensors Gold nanoparticles
363 623,653 329, 641
H
Heavy metal ions 583 Heptane 467 Hexamethyldisilazane coating 221 Hierarchical structure 133 High resolution TEM 403 HY zeolite 525 Hybrid materials 127 Hybrid organic-inorganic materials 1, 615 Hydrocarbons 631 Hydrocracking reaction 537 243 Hydrodesulfurization 371 Hydrogen bonding 189 Hydrogen peroxide 317,543 Hydrogenation 189 Hydrophobicity 221 Hydro-stability 167,281 Hydrothermal synthesis 221 Hydrothermal post-synthesis 101 Hydrothermal restructuring 85,445 Hydrothermal stability
Imprinting In-situ UV/Vis spectroscopy Intercalation Iron oxide Isobutane Isopropanol dehydration
La-containing MCM-41 Layered precursors Long-range order
345 653 273 403,669 289 531
459 273 85
M
Macroporous materials 309 MCM-41 69, 93, 101,167, 221,229, 329, 395, 403, 437, 445, 453, 479, 531,537, 615, 653 MCM-41 aluminosilicate 525 MCM-48 85 Mercury ion adsorption 607 Mesophase pitch 345 Mesoporous molecular sieves 173 Mesoporous ALPO 417 Mesoporous alumina 243,429 Mesoporous carbons 345 Mesoporous catalysts 543 Mesoporous materials 53, 109, 117, 127, 183, 213, 257, 265, 281,289, 301,371, 379, 479, 503, 583,623,661 Mesoporous matrix 635 Mesoporous molecular sieves 495 Mesoporous organosilicates 1 Mesoporous oxides 235 Mesoporous silica 133 Mesoporous silica spheres 615 Mesoporous solids 337, 631 Mesoporous thin films 337 Mesoporous titania 251,599 Mesoporous titanosilicate 167 Mesostructures 297 Metal-organic frameworks 353 Methane 363 Methyl-functionalized silica 197 Methyl-tert-butyl ether 525 Michael reaction 553 Michaelis-Menten kinetics 35 Microporous materials 353 Microstructure 379 Mild hydrocracking 543 Mn-MCM-48 511 Modification with Cs, K, Mg 411 Molecular dynamics method 69 Molecular imprinting 35 Molecular sieve 511, 661 Molecular wires 183 Molybdenum oxide 243 Monolayers 583 Monte Carlo method 69 Mordenite 561,569
681 Morphology MSU materials
133 289
N Nanocomposites Nanogels Nanoporous silica Nanotubes Nanowires Nb-containing MCM-41 Neutral surfactants NH3 adsorption Nitrogen activation Nitrogen adsorption NMR NO• reduction
1 35 205 133 337 411, 591 109 417 661 93,429, 437 159, 205,423 503
Polyoxyethylene alkyl ether Polyviologen Pore expansion Pore size Pore size adjustment Porosity Porous materials Post-synthesis A1 grafting Post-synthesis treatment Powder XRD Primary amine templating Propylamine-MCM-41 Propylthiol-MCM-41 Pyridine Pyrolysis
O
O
Quaternary ammonium
Octylsilane grafting 395 Oil sands 669 Ordered mesoporous materials 151,437 Organosilica 641,647 Organophosphonate 213 Organosilane 197, 213 Orgnic-inorganic interactions 423 Oxidation of cyclohexane 575 Ozonation of organic compounds 591
R
Particle morphology 109, 387 Perfluoro-compounds 591 Periodic mesoporous organosilica 1 pH adjustment 445 pH control 101 Phosphine oxide 453 Photocatalysis 495,599 Pillared clay 281 Pillared layer 363 Plugged pore 45 Pollutant mineralization 599 Poly(alkylene) block copolymers 599 Polyethylene 473 Polyfunctionalized silica 205 Polymerization 631 Polymer-templated silica 61,607
109,117 127 85 437,537 77 395 27 141 289 205,437 141 641 647 459 473
Radionuclides Raman spectroscopy Rheology Room temperature synthesis
553
583 487 479 93
S Salt addition 445 SBA-15 45, 53, 387, 395,607 SBA-2 379 Selective oxidation 189 Self-assembly 183,235, 5'83,623 Self-assembly mechanism 117 Self-organization 371 SEM 379 Semiconductor nanoparticles 647 Ship-in-bottle synthesis 575 Si/A1 ratio 537, 561 Silicalite-1 53 Silicate 553 Silicoaluminophosphates 297 Silver 569 Single crystal particles 265 Small particles 561,569 Smart materials 353 Smectite 281
682 SnO2 nanoparticles 653 Sol-gel silica 653 Solid acids 141, 631 Solid-state NMR 453 Spherical particles 109 Stability 537 Stabilization by aluminum 599 Standard adsorption data 429 Stereoisomerism 353 Structure-within-structure 371 Sulfate 531 Sulfated zirconia 289 Supercritical fluids 337 Super-microporous materials 141 Supported aluminium chloride 631 Supramolecules 371 Surface acidity 417 Surface modification 189 Surface photovoltage system 623 Surfactants. 297, 301 Synthesis 61, 77, 133,301,309 Synthesis mechanism 257
TEM 27, 45, 159, 265,379, 403 Template-displacement modification 607 Thermal analysis 473 Thin films 213,235 Thiophene 243 Ti coordination 251 Ti-containing alumina 173 Ti-HMS 189 Ti-MCM-48 511 Titania 487, 661 Transition metal oxides 265 Transition metals 235 TUD-1 635 U UV-vis
167
V Vanadium-phosphorus-oxide Vinyl-functionalized silica Voltametric measurements
301,309 197 615
W Wall thickness Water resistance
93 503
X
XAFS XANES Xe NMR XPS XRD
495 251 395 487 487
Z
Zeolite incorporation Zeolite mimics Zeolite Zirconia Zn-modified MCM-41 Zr-MCM-48
635 353 53,575 257 229 511
683 STUDIES IN SURFACE SCIENCEAND CATALYSIS Advisory Editors: B. Delmon, Universit6 Catholique de Louvain, Louvain-la-Neuve, Belgium J.T.Yates, University of Pittsburgh, Pittsburgh, PA, U.S.A. Volume 1
Volume 2
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Volume 6 Volume 7 Volume 8 Volume 9 Volume 10 Volume 11
Volume 12 Volume 13 Volume 14 Volume 15
Preparation of Catalysts I.Scientific Basesfor the Preparation of Heterogeneous Catalysts. Proceedings ofthe First International Symposium, Brussels, October 14-17,1975 edited by B. Delmon, RA. 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 II. Scientific Basesfor the Preparation of Heterogeneous Catalysts. Proceedingsofthe Second InternationalSymposium, Louvain-la-Neuve, September 4-7,1978 edited by B. Delmon, R Grange, RJacobs and G. Poncelet Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Soci~t~ de Chimie Physique,Villeurbanne, September 24-28,1979 edited by J. Bourdon Catalysis by Zeolites. Proceedingsof 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.E Froment New Horizons in Catalysis. Proceedings ofthe 7th International Congress on Catalysis,Tokyo, June 30-July4,1980. Parts A and B edited by T. Seiyama and K.Tanabe Catalysis by Supported Complexes by Yu.I.Yermakov, B.N. Kuznetsov andV.A. Zakharov Physics of Solid Surfaces. Proceedings of a Symposium, BechySe, September 29-October 3,1980 edited by M. Ldzni~ka 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, Ecuily (Lyon), September 14-16,1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, R Meriaudeau, R Gallezot, G.A. Martin 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.Jin3 and G. Schulz-Ekloff Adsorption on Metal Surfaces.An Integrated Approach edited by J. B6nard Vibrations at Surfaces. Proceedings oftheThird 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
684 Volume 16
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Preparation of Catalysts I!1.Scientific Basesfor the Preparation of Heterogeneous Catalysts. Proceedings oftheThird International Symposium, Louvain-la-Neuve, September 6-9,1982 edited by G. Poncelet, RGrange and RA. 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 RA. Jacobs, N.I. Jaeger, R Ji~,V.B. Kazansky and G. Schulz-Ekloff Catalysis on the Energy Scene. Proceedings ofthe 9th Canadian Symposium on Catalysis, Quebec, RQ., September 30-October 3,1984 edited by S. Kaliaguine andA. Mahay Catalysis byAcids and Bases. Proceedings of an International Symposium, Villeurbanne (Lyon), September 25-27,1984 edited by B. Imelik, C. Naccache, G. Coudurier, Y. BenTaarit and J.C.Vedrine Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Uxbridge, June 28-29,1984 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 andApplication. Proceedings of an International Symposium, Portoro~-Portorose, September 3-8, 1984 edited by B. Dr~aj, S. HoEevar and S. Pejovnik Catalytic Polymerization of Olefins. Proceedings ofthe 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 ofthe Fourth International Conference, Bowness-on-Windermere, September 15-19, 1985 edited by D.A. King, N.V. Richardson and S. Holloway Catalytic Hydrogenation edited by L. Cerven# New Developments in Zeolite Science andTechnology. Proceedings of the 7th International Zeolite Conference,Tokyo, August 17-22, 1986 edited byY. Murakami, A. lijima and J.W.Ward Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H. Kn6zinger Catalysis andAutomotive Pollution Control. Proceedings of the First International Symposium, Brussels, September 8-11,1986 edited by A. Crucq andA. Frennet Preparation of Catalysts IV. Scientific Basesfor the Preparation of Heterogeneous Catalysts.Proceedings of the Fourth International Symposium, Louvain-laNeuve, September 1-4,1986 edited by B. Delmon, R Grange, RA. Jacobs and G. Poncelet Thin Metal Films and Gas Chemisorption edited by RWissmann Synthesis of High-silica Aluminosilicate Zeolites edited by RA. Jacobs and J.A. Martens Catalyst Deactivation 1987. Proceedings ofthe 4th International Symposium, Antwerp, September 29-October 1,1987 edited by B. Delmon and G.E Froment Keynotes in Energy-Related Catalysis edited by S. Kaliaguine
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Volume 45 Volume 46
Volume 47 Volume 48 Volume 49 Volume 50
Volume 51 Volume 52 Volume 53 Volume 54
Methane Conversion. Proceedingsof a Symposium on the Production of Fuelsand Chemicals from Natural Gas,Auckland,April 27-30,1987 edited by D.M. Bibby, C.D. Chang, R.E Howe and S.Yurchak Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-17, 1987 edited by RJ. Grobet,W.J. Mortier, E.F.Vansant and G. Schulz-Ekloff Catalysis 1987.Proceedings ofthe 10th North American Meeting ofthe Catalysis Society, San Diego, CA, May 17-22, 1987 edited by J.W.Ward Characterization of Porous Solids. Proceedings of the IUPACSymposium (COPS I), Bad Soden a.Ts.,Apri126-29,1987 edited by K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral Physics of Solid Surfaces 1987. Proceedings ofthe 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. Pdrot Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by Z. Padl Catalytic Processes under Unsteady-State Conditions by Yu. Sh. Matros Successful Design of Catalysts. Future Requirements and Development. Proceedings oftheWorldwide Catalysis Seminars, July, 1988, on the Occasion of the 30th 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,WL~rzburg, September 4-8,1988 edited by H.G. Karge and J.Weitkamp Photochemistry on Solid Surfaces edited by M.Anpo andT. Matsuura Structure and Reactivity of Surfaces. Proceedings of a European Conference, Trieste, September 13-16, 1988 edited by C. Mortenra,A. Zecchina and G. Costa Zeolites: Facts, Figures, Future. Proceedings of the 8th International Zeolite Conference, Amsterdam, July 10-14,1989. Parts A and B edited by RA. Jacobs and R.A. van Santen Hydrotreating Catalysts. Preparation, Characterization and Performance. Proceedings of the Annual International AIChE Meeting,Washington, DC, November 27-December 2,1988 edited by M.L. Occelli and R.G.Anthony New SolidAcids and Bases.Their Catalytic Properties by K.Tanabe,M. Misono,Y. Ono and H. Hattori RecentAdvances in Zeolite Science. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-19,1989 edited by J. Klinowsky and RJ. 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 andA. Bishara Future Opportunities in Catalytic and Separation Technology edited by M. Misono,Y. Moro-oka and S. Kimura
686 Volume 55
New Developments in Selective Oxidation. Proceedings of an International Symposium, Rimini, Italy, September 18-22,1989 edited by G. Centi and ETrifiro Volume 56 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 Volume 57A SpectroscopicAnalysis of Heterogeneous Catalysts. Part A: Methods of SurfaceAnalysis edited by J.L.G. Fierro Volume 57B Spectroscopic Analysis of Heterogeneous Catalysts. Part B: Chemisorption of Probe Molecules edited by J.L.G. Fierro Volume 58 Introduction to Zeolite Science and Practice edited by H. van Bekkum, E.M. Fianigen and J.C. Jansen Volume 59 Heterogeneous Catalysis and Fine Chemicals I1. Proceedings of the 2nd International Symposium, Poitiers, October 2-6,1990 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. P~rot, R. Maurel and C. Montassier Volume 60 Chemistry of Microporous Crystals. Proceedings ofthe International Symposium on Chemistry of Microporous Crystals,Tokyo, June 26-29,1990 edited by T. lnui, S. Namba andT.Tatsumi Volume 61 Natural Gas Conversion. Proceedings ofthe Symposium on Natural Gas Conversion, Oslo,August 12-17,1990 edited by A. Holmen, K.-J. Jens and S. Kolboe Volume 62 Characterization of Porous Solids Ii. Proceedings ofthe IUPAC Symposium (COPS II),Alicante, May 6-9,1990 edited by E Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing and K.K. Unger Volume 63 Preparation of CatalystsV. 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, RA. Jacobs, R Grange and B. Delmon Volume 64 NewTrends in COActivation edited by L. Guczi Volume 65 Catalysis andAdsorption by Zeolites. Proceedings of ZEOCAT 90, Leipzig, August 20-23,1990 edited by G. ()hlmann, H. Pfeifer and R. Fricke Volume 66 Dioxygen Activation and Homogeneous Catalytic Oxidation. Proceedings of the Fourth International Symposium on Dioxygen Activation and Homogeneous Catalytic Oxidation, Balatonffired, September 10-14, 1990 edited by L.I. Simdndi Volume 67 Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis. Proceedings ofthe ACS Symposium on Structure-Activity Relationships in Heterogeneous Catalysis, Boston, MA, Apri122-27,1990 edited by R.K. Grasselli andA.W. Sleight Volume 68 Catalyst Deactivation 1991. Proceedings ofthe Fifth International Symposium, Evanston, IL, June 24-26,1991 edited by C.H. Bartholomew and J.B. Butt Volume 69 Zeolite Chemistry and Catalysis. Proceedings of an International Symposium, Prague, Czechoslovakia, September 8-13,1991 edited by RA. Jacobs, N.I. Jaeger, L. Kubelkov~ and B.Wichtedovd Volume 70 Poisoning and Promotion in Catalysis based on Surface Science Concepts and Experiments by M. Kiskinova
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Catalysis andAutomotive Pollution Control II. Proceedings of the 2nd International Symposium (CAPoC 2), Brussels, Belgium, September 10-13,1990 edited by A. Crucq New Developments in Selective Oxidation by Heterogeneous Catalysis. Proceedings ofthe 3rd EuropeanWorkshop Meeting on New Developments in Selective Oxidation by Heterogeneous Catalysis, Louvain-la-Neuve, Belgium, April 8-10,1991 edited by R Ruiz and B. Delmon Progress in Catalysis. Proceedings of the 12th 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 CurrentApplications edited by S.D. Kevan New Frontiers in Catalysis, PartsA-C. Proceedings of the 10th International Congress on Catalysis, Budapest, Hungary, 19-24 July, 1992 edited by L. Guczi, F.Solymosi and RT(~t~nyi Fluid Catalytic Cracking: Science andTechnology edited by J.S. Magee and M.M. Mitchell, Jr. NewAspects of Spillover Effect in Catalysis. For Development of HighlyActive Catalysts. Proceedings oftheThird 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 II1. Proceedings ofthe 3rd 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 IntegratedApproach to Homogeneous, Heterogeneous and Industrial Catalysis edited by J.A. Moulijn, RW.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 oftheThird Natural Gas Conversion Symposium, Sydney, July 4-9,1993 edited by H.E. Curry-Hyde and R.E Howe New Developments in Selective Oxidation I1. Proceedings of the SecondWorld Congress and Fourth EuropeanWorkshop Meeting, Benalm~dena, Spain, September 20-24,1993 edited by V. Cortds Corberdn 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 andT.Yashima Zeolites and Related Microporous Materials: State of the Art 1994. Proceedings ofthe 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22,1994 edited by J.Weitkamp, H.G. Karge, H. Pfeifer andW. H61derich Advanced Zeolite Science and Applications edited by J.C. Jansen, M. St6cker, H.G. Karge and J.Weitkamp Oscillating Heterogeneous Catalytic Systems by M.M. Slin'ko and N.I. Jaeger Characterization of Porous Solids II1.Proceedings of the IUPAC Symposium (COPS III), Marseille, France, May 9-12,1993 edited by J.Rouquerol, E Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger
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Catalyst Deactivation 1994. Proceedings of the 6th International Symposium, Ostend, Belgium, October 3-5,1994 edited by B. Delmon and G.E Froment Catalyst Design forTailor-made Polyolefins. Proceedings of the International Symposium on Catalyst Design forTailor-made Polyolefins, Kanazawa, Japan, March 10-12,1994 edited by K. Soga and M.Terano Acid-Base Catalysis II. Proceedings ofthe International Symposium on Acid-Base Catalysis II, Sapporo, Japan, December 2-4,1993 edited by H. Hattori, M. Misono andY. Ono Preparation of CatalystsVl. 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, RA. Jacobs and R Grange Science andTechnology in Catalysis 1994. Proceedings of the SecondTokyo Conference on Advanced Catalytic Science andTechnology, Tokyo, August 21-26,1994 edited by YoIzumi, H.Arai and M. Iwamoto Characterization and Chemical Modification of the Silica Surface by E.EVansant, RVan DerVoort 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 Metalsand Alloys by V. Ponec and G.C. Bond Catalysis andAutomotive Pollution Control III. Proceedings of theThird International Symposium (CAPoC3), Brussels, Belgium, April 20-22,1994 edited by A. Frennet and J.-M. Bastin Zeolites:A RefinedTool for Designing Catalytic Sites. Proceedings of the International Symposium, Qu6bec, Canada, October 15-20,1995 edited by L. Bonneviot and S. Kaliaguine Zeolite Science 1994: Recent Progress and Discussions. Supplementary Materials to the 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22, 1994 edited by H.G. Karge and J.Weitkamp Adsorption on New and Modified Inorganic Sorbents edited by A. D~jbrowski andV.A.Tertykh Catalysts in Petroleum Refining and Petrochemical Industries 1995. Proceedings ofthe 2nd International Conference on Catalysts in Petroleum Refining arid Petrochemical Industries, Kuwait, April 22-26,1995 edited by M.Absi-Halabi, J. Beshara, H. Qabazard andA. Stanislaus 11th Intemational Congress on Catalysis - 40th Anniversary. Proceedings ofthe 11th ICC, Baltimore, MD, USA, June 30-July 5,1996 edited by J.W. Hightower, W.N. Delgass, E. Iglesia andA.T. Bell RecentAdvances and New Horizons in Zeolite Science andTechnology edited by H. Chon, S.I.Woo and S. -E. Park Semiconductor Nanoclusters - Physical, Chemical, and CatalyticAspects edited by RV. 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 ofthe 11th International Zeolite Conference, Seoul, Korea, August 12-17,1996 edited by H. Chon, S.-K. Ihm andY.S. Uh
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Hydrotreatment and Hydrocracking of Oil Fractions Proceedings ofthe 1st International Symposium / 6th European Workshop, Oostende, Belgium, February 17-19,1997 edited by G.E Froment, B. Delmon and R Grange Natural Gas Conversion IV Proceedings ofthe 4th International Natural Gas Conversion Symposium, Kruger Park, South Africa, November 19-23,1995 edited by M. de Pontes, R.L. Espinoza, C.R Nicolaides, J.H. Scholtz and M.S. Scurrell Heterogeneous Catalysis and Fine Chemicals IV Proceedings ofthe 4th 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 ofthe International Symposium,Antwerp, Belgium, September 15-17,1997 edited by G.E Froment and K.C.Waugh ThirdWorld Congress on Oxidation Catalysis. Proceedings of theThirdWorld 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 ofthe 7th 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 ofthe 4th 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 ofthe 13th 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 ofthe 4th International Conference on Carbon Dioxide Utilization, Kyoto, Japan, September 7-11,1997 edited by T. Inui, M.Anpo, K. Izui, S.Yanagida andT.Yamaguchi Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters. A patent-oriented survey by M. Sideris Catalysis andAutomotive Pollution Control IV. Proceedings ofthe 4th 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 1st International Symposium, Baltimore, MD, U.S.A., July 10-12,1998 edited by L.Bonneviot, E B(~land,C. Danumah, S. Giasson and S. Kaliaguine Preparation of CatalystsVII Proceedings of the 7th International Symposium on Scientific Bases for the Preparation of Heterogeneous Catalysts, Louvain-la-Neuve, Belgium, September 1-4,1998 edited by B. Delmon, RA. Jacobs, R. Maggi, J.A. Martens, R Grange and G. Poncelet Natural Gas ConversionV Proceedings of the 5th International Gas Conversion Symposium, Giardini-Naxos, Taormina, Italy, September 20-25,1998 edited by A. Parmaliana, D. Sanfilippo, E Frusteri,A.Vaccari and EArena
690 Volume 120A Adsorption and its Applications in Industry and Environmental Protection. Vol I: Applications in Industry edited by A. Dabrowski Volume 120B Adsorption and its Applications in Industry and Environmental Protection. Vol I1:Applications in Environmental Protection edited byA. Dabrowski Volume 121 Science andTechnology in Catalysis 1998 Proceedings oftheThirdTokyo Conference in Advanced Catalytic Science and Technology,Tokyo, July 19-24,1998 edited by H. Hattori and K. Otsuka Volume 122 Reaction Kinetics and the Development of Catalytic Processes Proceedings ofthe International Symposium, Brugge, Belgium,April 19-21,1999 edited by G.F.Froment and K.C.Waugh Volume 123 Catalysis:An IntegratedApproach Second, Revised and Enlarged Edition edited by R.A. van Santen, RW.N.M. van Leeuwen, JoA. Moulijn and B.A.Averill Volume 124 Experiments in Catalytic Reaction Engineering by J.M. Berty Volume 125 Porous Materials in Environmentally Friendly Processes Proceedings ofthe 1st International FEZA Conference, Eger, Hungary, September 1-4,1999 edited by I. Kiricsi, G. PdI-Borb~ly, J.B. Nagy and H.G. Karge Volume 126 Catalyst Deactivation 1999 Proceedings ofthe 8th International Symposium, Brugge, Belgium, October 10-13,1999 edited by B. Delmon and G.E Froment Volume 127 Hydrotreatment and Hydrocracking of Oil Fractions Proceedings of the 2nd International Symposium/7th European Workshop, Antwerpen, Belgium, November 14-17,1999 edited by B. Delmon, G.E Froment and R Grange Volume 128 Characterisation of Porous SolidsV Proceedings ofthe 5th 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.R Baselt Volume 129 Nanoporous Materials II Proceedings of the 2nd Conference on Access in Nanoporous Materials, Banff, Alberta, Canada, May 25-30, 2000 edited byA. Sayari, M. Jaroniec andT.J. Pinnavaia Volume 130 12th International Congress on Catalysis Proceedings ofthe 12th ICC, Granada, Spain, July 9-14, 2000 edited byA. Corma, F.V.Melo, S. Mendioroz and J.L.G. Fierro Volume 131 Catalytic Polymerization of Cycloolefins Ionic, Ziegler-Nat~a and Ring-Opening Metathesis Polymerization byV. Dragutan and R. Streck Volume 132 Proceedings of the International Conference on Colloid and Surface Science, Tokyo, Japan, November 5-8, 2000 25th Anniversary of the Division of Colloid and Surface Chemistry, The Chemical Society of Japan edited byY. iwasawa, N; Oyama and H. Kunieda Volume 133 Reaction Kinetics and the Development and Operation of Catalytic Processes Proceedings ofthe 3rd International Symposium, Oostende, Belgium,April 22-25, 2001 edited by G.F.Froment and K.C.Waugh Volume 134 Fluid Catalytic CrackingV Materials and Technological Innovations edited by M.L. Occelli and R O'Connor
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Zeolites and Mesoporous Materials at the Dawn of the 21st Century. Proceedings ofthe 13th International Zeolite Conference, Montpellier, France, 8-13 July 2001 edited by A. Galameau, E di Renzo, E Fajula and J.Vedrine Natural Gas ConversionVI. Proceedings ofthe 6th 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 nd completely revised and expanded edition edited by H. van Bekkum, E.M. Flanigen, RA. Jacobs and J.C. Jansen Spillover and Mobility of Species on Solid Surfaces. edited byA. Guerrero-Ruiz and I. Rodriguez-Ramos Catalyst Deactivation 2001 Proceedings ofthe 9th 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 Ao Gamba, C. Colella and S. Coluccia Nanoporous Materials lU Proceedings of the 3rd International Symposium on Nanoporous Materials, Ottawa, Ontario, Canada, June 12-15, 2002 edited by A. Sayari and M. Jaroniec
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