Studies in Surface Science and Catalysis 97 ZEOLITES: A REFINED TOOL FOR DESIGNING CATALYTIC SITES
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Studies in Surface Science and Catalysis Advisory Editors: B. Delmon and J.T. Yates Vol. 97
ZEOLITES: A REFINED TOOL FOR DESIGNING CATALYTIC SITES Proceedings of t h e International Zeolite S y m p o s i u m , Qu(~bec, O c t o b e r 15-20,1995
Canada,
Edited by
Laurent Bonneviot Departement de Chimie, CERPIC, Facu/te des Sciences et de Genie, Universite Lava/, Quebec, Canada G 1K 7P4
Serge Kaliaguine Departement de Genie Chimique, CERPIC, Faculte des Sciences et de Genie, Universite Lava/, Quebec, Canada G 1K 7P4
1995 ELSEVIER A m s t e r d a m - - L a u s a n n e - - N e w Y o r k - - O x f o r d --- S h a n n o n --- T o k y o
ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 P.O. Box 521, 1000 AM Amsterdam, The Netherlands
ISBN 0-444-82130-9 91995 Elsevier Science B.V. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science B.V., Copyright & Permissions Department, P.O. Box 521, 1000 AM Amsterdam, The Netherlands. Special regulations for readers in the U.S.A.- This publication has been registered with the Copyright Clearance Center Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the U.S.A. All other copyright questions, including photocopying outside of the U.S.A., should be referred to the copyright owner, Elsevier Science B.V., unless otherwise specified. No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. This book is printed on acid-free paper. Printed in The Netherlands
CONTENTS Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PL-1
M-l-1
One and Two-Dimensional Solid-State NMR Investigations of the ThreeDimensional Structures of Zeolite-Organic Sorbate Complexes C.A. Fyfe, H. Grondey, A.C. Diaz, G.T. Kokotailo, Y. Feng, Y. Huang, K.C. Wong-Moon, K.T. Mueller, H. Strobl and A.R. Lewis . . . . . . . . . . . . .
XI XII
1
The Use of Small, Weakly Basic Probe Molecules for the Investigation of BrCnsted Acid Sites in Zeolites by NMR Spectroscopy E. Brunner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11
Determination of the Environment of Titanium Atoms in TS-1 Silicalite by Ti K-edge X-ray Absorption Spectroscopy, 29Si and 1H Nuclear Magnetic Resonance- L. Le Noc, C. Cartier dit Moulin, S. Solomykina, D. Trong On, C. Lortie, S. Lessard and L. Bonneviot . . . . . . . . . . . . . . . .
19
An in situ 13C MAS NMR Study of Toluene Alkylation with Methanol over H-ZSM-11 - I.I. Ivanova and A. Corma . . . . . . . . . . . . . . . . . . . . . .
27
KL-1
Strategies for Zeolite Synthesis by Design- M.E. Davis . . . . . . . . . . . . . .
35
M-2-1
Use of Modified Zeolites as Reagents Influencing Nucleation in Zeolite Synthesis - S.I. Zones and Y. Nakagawa . . . . . . . . . . . . . . . . . . . . . . . . .
45
Templating Studies Using 3,7-Diazabicyclo[3.3.1]nonane Derivatives: Discovery of New Large-Pore Zeolite SSZ-35 - Y. Nakagawa . . . . . . . . . .
53
Synthesis of ZSM-48 Type Zeolite in Presence of Li, Na, K, Rb and Cs Cations - G. Giordano, A. Katovic, A. Fonseca and J.B. Nagy
61
Out-of-Plane Bending Vibrations of Bridging OH Groups in Zeolites: A New Characteristic of the Geometry and Acidity of a Broensted Site - L.M. Kustov, E. Loeffler, V.L. Zholobenko and V.B. Kazansky
63
In situ FTIR Microscopic Investigations on Acid Sites in Cloverite G. MUller, G. Eder-Mirth and J.A. Lercher . . . . . . . . . . . . . . . . . . . . . . .
71
Broensted Sites of Enhanced Acidity in Zeolites: Experimental Modelling - M.A. Makarova, S.P. Bates and J. Dwyer . . . . . . . . . . . . . . . .
79
Modelling of Structure and Reactivity in Zeolites - C.R.A. Catlow, R.G. Bell, J.D. Gale and D.W. Lewis . . . . . . . . . . . . . . . . . . . . . . . . . . .
87
M-l-3
M-l-4
M-2-2
M-2-3
M-2-4
M-2-5
M-2-6
PL-2
VI
Tu-l-1
Tu-l-2
Tu-l-3
Tu-l-4
KL-2
Tu-2-1
Tu-2-2
Tu-2-3
Tu-2-4
Tu-2-5
Computational Studies of Water Adsorption in Zeolites - S.A. Zygmunt, L.A. Curtiss and L.E. Iton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
101
Modeling of Adsorption Properties of Zeolites - A. Goursot, I. Papai, V. Vasilyev and F. Fajula . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
109
Loading and Location of Water Molecules in the Zeolite Clinoptilolite Y.M. Channon, C.R.A. Catlow, R.A. Jackson and S.L. Owens . . . . . . . . . .
117
Withdrawal of Electron Density by Cations from Framework Aluminum in Y-Zeolite Determined by A1 XAFS Spectroscopy D.C. Koningsberger and J.T. Miller . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
125
Geometry of the Active Sites in Zeolites under Working Conditions F. Fajula . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
133
Characterization of Hexagonal and Lamellar Mesoporous Silicas, Alumino- and Gallosilicates by Small-Angle X-Ray Scattering (SAXS) and Multinuclear Solid State NMR - Z. Gabelica, J.-M. Clacens, R. Sobry and G. Van den Bossche . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
143
Characterization of a Cubic Mesoporous MCM-48 Compared to a Hexagonal MCM-41 - R. Schmidt, M. St6cker and O.H. Ellestad . . . . . . . .
149
Synthesis and Characterization of Transition Metal Containing Mesoporous Silicas - S. Gontier and A. Tuel . . . . . . . . . . . . . . . . . . . . . .
157
Bimodal Porous Materials with Superior Adsorption Properties C.J. Guo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
165
MCM-41 Type Silicas as Supports for Immobilized Catalysts - D. Brunel, A. Cauvel, F. Fajula and F. DiRenzo . . . . . . . . . . . . . . . . . . . . . . . . . . . .
173
Tu-2-6
Synthesis of Mesop0rous Manganosilicates Mn-MCM-41, Mn-MCM-48 and Mn-MCM-L at a Low Surfactant/Si Ratio - D. Zhao and D. G o l d f a r b . . 181
W-l-1
Alkane Oxidation Catalyzed by Zeolite Encapsulated Ruthenium Perfluorophthalocyanines - K.J. Balkus Jr., A. Khanmamedova and M. Eissa . . . . . .
189
W-l-2
MOCVD in Zeolites Using Mo(CO) 6 and W(CO)6 as Precursors - S. Djajanti and R.F. Howe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
W-l-3
Tailored Synthesis, Characterization and Properties of ZnO, CdO and SnO 2 Nano Particles in Zeolitic Hosts - M. Wark, H.-J. Schwenn, M. Warnken, N.I. Jaeger and B. Boddenberg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
205
VII
Host-Guest Interactions in Zeolite Cavities - A. Zecchina, R. Buzzoni, S. Bordiga, F. Geobaldo, D. Scarano, G. Ricchiardi and G. Spoto . . . . . . . .
213
PL-4
Diffusion in Zeolites - D.M. Ruthven
223
Th-l-1
Frequency Response Study of Mixture Diffusion of Benzene and Xylene Isomers in Silicalite-1 - D. Shen and L.V. Rees . . . . . . . . . . . . . . . . . . . .
235
2D EXSY 129Xe NMR: New Possibilities for the Study of Structure and Diffusion in Microporous Solids - I.L. Moudrakovski, C.I. Ratcliffe and J. Ripmeester . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
243
Diffusion of C10-C24n-paraffins and perfluorotributylamine in clay catalysts- B. Liao, M. Eic, D.M. Ruthven and M.L. Occelli . . . . . . . . . . .
251
Sorption Properties of Dealuminated Large Crystals of ZSM-5: A New Approach to the Description of Isotherms- J. Kornatowski, M. Rozwadowski, W. Lutz and W.H. Baur . . . . . . . . . . . . . . . . . . . . . . .
259
Convective Methods to Investigate Multi-component Sorption Kinetics on Microporous Solids- A. Micke and M. Btilow . . . . . . . . . . . . . . . . . . .
269
KL-4
Zeolites as the Key Matrix for Superior deNOx Catalysts - T. Inui . . . . . . .
277
Th-2-1
Catalytic Properties of Palladium Exchanged Zeolites in the Reduction of Nitrogen Oxide by Methane in the Presence of Oxygen: Influence of Hydrothermal Ageing - C. Descorme, P. G61in, M. Primet, C. L6cuyer and J. Saint-Just . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
287
The Effect of Preparation and Steaming on the Catalytic Properties of Cu- and Co-ZSM-5 in Lean NO x Reduction - P. Ciambelli, P. Corbo, M. Gambino, F. Migliardini, G. Minelli, G. Moretti and P. Porta . . . . . . . .
295
Adsorption Sites for Benzene in the 12R Window Zeolites: A Molecular Recognition Effect- B.L. Su . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
303
Lewis Basic and Lewis Acidic Sites in Zeolites - M. Huang, S. Kaliaguine and A. Auroux . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
311
Effect of the Framework Composition on the Nature and the Basicity of Intrazeolitic Cesium Oxides. Correlation Activity/Basicity - M. Lasp6ras, I. Rodriguez, D. Brunel, H. Cambon and P. Geneste . . . . . . . . . . . . . . . . .
319
KL-3
Th-l-2
Th-l-3
Th-l-4
Th-l-5
Th-2-2
Th-2-3
Th-2-4
Th-2-5
...........................
VIII Th-2-6
F-l-1
F-l-2
F-l-3
F-l-4
KL-5
F-2-1
F-2-2
F-2-3
F-2-4
P-2
P-6
Hydrothermal and Alkaline Stability of High-Silica Y Zeolites Generated by Combining Substitution and Steaming - W. Lutz, E. LOftier and B. Zibrowius . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
327
Electron Spin Resonance and Electron Spin Echo Modulation Spectroscopy of Ni(I) in SAPO-5 and SAPO-11 - M. Hartmann, N. Azuma and L. Kevan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
335
Selective Acidic, Oxidative and Reductive Reactions over ALPO-11 and Si or Metal Substituted ALPO-11 - P.S. Singh, R. Bandyopadhyay, R.A. Shaikh and B.S. Rao . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
343
Characterization and Catalytic Performance of PdSAPO-5 Molecular Sieves - T.-C. Xiao, L.-D. An and H.-L. Wang . . . . . . . . . . . . . . . . . . . . .
351
FTIR Study of the Acidic Properties of Substituted Aluminophosphates V. Zholobenko, A. Garforth, L. Clark and J. Dwyer . . . . . . . . . . . . . . . . .
359
Selective Oxidation with Redox Metallosilicates in the Production of Fine Chemicals- P. Ratnasamy and R. Kumar . . . . . . . . . . . . . . . . . . . . .
367
Novel Model Catalysts Containing Supported MFI-type Zeolites N. van der Puil, E.C. Rodenburg, H. van Bekkum and J.C. Jansen . . . . . . .
377
Oligomerization of Butenes with Partially Alkaline-Earth Exchanged NiNaY Zeolites - B. Nkosi, F.T.T. Ng and G.L. Rempel . . . . . . . . . . . . . .
385
Isomerization of C 8 Aromatic Cut. Improvement of the Selectivity of MOR- and MFI- Catalysts by Treatments with Aqueous Solutions of (NH4)2SiF6 - E. Benazzi, J.M. Silva, M.F. Ribeiro, F.R. Ribeiro and M. Guisnet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
393
Contribution of Framework and Extra-framework A1 and Fe Cations in ZSM-5 to Disproportionation and C 3 Alkylation of Toluene - J. (2ejka, N. Zilkov~i, Z. Tvart)~kov~i and B. Wichterlov~i . . . . . . . . . . . . . . . . . . . . .
401
NO x Adsorption Complexes on Zeolites Containing Metal Cations and Strong Lewis Acid Sites and their Reactivity in CO and CH 4 Oxidation: a Spectroscopic Study - L.M. Kustov, E.V. Smekalina, E.B. Uvarova and V.B. Kazansky . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
409
Cracking of 1,3,5-triisopropylbenzene over Deeply Dealuminated Y Zeolites- E.F. Sousa-Aguiar, M.L. Murta VaUe, E.V. Sobrinho and D. Cardoso . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
417
IX
P-7
P-11
P-14
P-15
P-16
P-17
P-18
P-19
P-20
P-21
P-25
P-26
P-27
Hydrogenation of Styrene and Hydrogenolysis of 2-Phenylethanol T. Sooknoi and J. Dwyer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
423
Catalyst Deactivation of High Silica Metallosilicates in Beckmann Rearrangement of Cyclohexanone Oxime - T. Takahashi, T. Kai and M.N.A. Nasution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
431
IR Studies on the Reduction of Nitric Oxide with Ammonia over MFIFerrisilicate - T. Komatsu, Md. A. Uddin and T. Yashima . . . . . . . . . . . . .
437
The Role of the Na and of the Ti on the Synthesis of ETS-4 Molecular Sieve - P. De Luca, S. Kuznicki and A. Nastro . . . . . . . . . . . . . . . . . . . .
443
On the Potential of Zeolites to Catalyse the Aromatic Acylation with Carboxylic Acids - E.A. Gunnewegh, R. Downing and H. van Bekkum
447
Elimination of Methanol from Dimethylacetal over Aluminophosphate Molecular Sieves and Zeolites - S.-M. Yang and K.-J. Wang . . . . . . . . . . .
453
Deuteration of Zeolitic Hydroxyl Groups in the Presence of Platinum Evidence for a Spillover Reaction Pathway - U. Roland, R. Salzer and L. SUmmchen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
459
IR Investigation of CO Adsorption at Low Temperature: A Key Tool to Characterize the Porosity of Matrix Embedded Zeolite Catalysts Z.M. Noronha, J.L.F. Monteiro and P. G61in . . . . . . . . . . . . . . . . . . . . . .
465
Identification of Active Ti Centers in TS-1 as Revealed by ESR Spectra of UV-Irradiated Samples - A. Ghorbel, A. Tuel, E. Jorda, Y. Ben TaCit and C. Naccache . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
471
Microwave Crystallization of Titanium-Containing Cloverite - R. Fricke, H.-L. Zubowa, J. Richter-Mendau, E. Schreier and U. Steinike . . . . . . . . . .
477
Aqueous Silicate Chemistry in Zeolite Synthesis - C.T.G. Knight, R.T. Syvitski and S.D. Kinrade . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
483
Effect of Hydrolysis Conditions of the Silicate Precursor on the Synthesis of Siliceous MCM-48 - J. Lujano, Y. Romero and J. Carrazza . . . . . . . . . .
489
Sorption of Light Alkanes on H-ZSM5 and H-Mordenite - F. Eder, M. Stockenhuber and J.A. Lercher . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
495
P-28
P-29
P-31
P-34
P-36
P-41
P-42
P-43
P-45
Acidity and Reactivity of Steamed HY Zeolites Obtained by Progressive Extraction of Extraframework A1 Species- L. Mariey, S. Khabtou, M. Marzin, J.C. Lavalley, A. Chambellan and T. Chevreau . . . . . . . . . . . .
501
L.
Adsorption of Nitrogen and Methane on Natural Clinoptilolite Predescu, F.H. Tezel and P. Stelmack . . . . . . . . . . . . . . . . . . . . . . . . .
507
Dealumination and Acidity Measurement of HEMT Zeolites Modified by Steaming and Leaching - O. Cairon, S. Sellem, C. Potvin, J.M. Manoli and T. Chevreau . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
513
The Synthesis of UTD-1, Ti-UTD-1 and Ti-UTD-8 Using CP*2CoOH as a Structure Directing Agent- K.J. Balkus Jr., A.G. Gabrielov and S.I. Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
519
Zeolite Crystallization on Mullite Fibers - V. Valtchev, S. Mintova, B. Schoeman, L. Spasov and L. Konstantinov . . . . . . . . . . . . . . . . . . . . .
527
Synthesis and Characterization of V-Beta Zeolite - S.-H. Chien and J.-C. Ho . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
533
Titanium Boralites with MFI Structure Characterized Using XRD, IR, UV-Vis XANES and MAS-NMR Techniques - D. Trong On, M.P. Kapoor, S. Kaliaguine, L. Bonneviot and Z. Gabelica . . . . . . . . . . . .
535
Acidity and Structural State of Boron in Mesoporous Boron Silicate MCM-41 - D. Trong On, P.N. Joshi, G. Lemay and S. Kaliaguine . . . . . . .
543
The Role of Na and K on the Synthesis of Levyne-Type Zeolites C.V. Tuoto, J.B. Nagy and A. Nastro . . . . . . . . . . . . . . . . . . . . . . . . . . .
551
Author Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
557
Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
561
Studies in Surface Science and Catalysis
567
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xI FOREWORD Once upon a time the catalytic use of zeolites was exclusively in the field of acid catalysis. Today zeolites also find applications as catalysts in a wide array of chemical reactions. These encompass base catalyzed reactions, Redox reactions and catalytic reactions on transition metals and their complexes in confined environments. The concepts of Bronsted or Lewis acid-base pairs are abundantly illustrated in the literature and better understood in terms of structural and electronic properties of zeolites. By contrast properties of chemically modified silicates, aluminosilicates and aluminophosphates are not yet fully explored. The list of oxydo-reduction reactions performed in the presence of these new materials is indeed continuously growing. For example the selective catalytic reduction of nitrogen oxides or the numerous oxidations employing hydrogen peroxide could be cited. In this context much effort is currently made in order to get a better insight into the nature of the sites involved. Thirdly, the zeolite lattice may be used as a host for encapsulated complexes or metallic clusters allowing to control the nuclearity of these active species and the steric constraints imposed on the reactants. The molecular sieve and shape selectivity effects have always constituted the most fascinating aspects of zeolite properties. The recent developments leading to increasingly large pore sizes with VPI-5, cloverite and more recently mesoporous molecular sieves have broadened the spectrum of these applications. Indeed larger and larger reactant and product molecules can be accommodated in these lattices. These new adsorbant/adsorbate systems create additional needs for experimental data and theoretical descriptions of transport properties, in particular of mono- and multicomponents diffusion coefficients in the zeolite pore lattice. All these questions represent the forefront and current trends of zeolite research. To various extends they deal with the specific factors of the zeolites which allow the fine tuning of the geometric and/or electronic properties of the active sites. It was indeed very rewarding for us, as organizers of this symposium, to realize that all these questions were actually discussed in the papers submitted to the selection committee and that they were widely represented in the selected papers. A feature general to most of these contributions is the combined use of a variety of analytical techniques. Some of these techniques are at the frontiers of the latest analytical developments such as multiple scattering EXAFS and bidimensional MAS-NMR. It is also worth mentioning that the on-going refinements of molecular modelling can now rely on more and more accurate quantum mechanics calculations such as the density functional theory (DFT) improved by introducing high level electron correlation. We wish to thank each and every one of the contributors to the Qu6bec International Zeolite Symposium, who gathered coming from not less than 27 countries to share their recent findings and ideas in a research field so liable to yield future fundamental developments as well as potential technical innovations.
Laurent Bonneviot and Serge Kaliaguine
XII PAPER SELECTION COMMITTEE
Y. BEN TA,M~T L. BONNEVIOT E.M. FLANIGEN Z. GABELICA M. GUISNET W.O. HAAG S. KALIAGUINE H.G. KARGE G.T. KOKOTAILO
L.B. McCUSKER W.J. MORTIER
J.M. NEWSAM T. TATSUMI H. van BEKKUM
Institut de Recherches sur la Catalyse, Villeurbanne, France Universit6 Laval, Qu6bec, Canada UOP, Tarrytown, NY, USA Facult6s Universitaires Notre-Dame de la Paix, Namur, Belgium Universit6 de Poitiers, Poitiers, France Mobil Research and Development Co., Princeton, NJ, USA Universit6 Laval, Qu6bec, Canada Fritz-Haber Institut der Max Planck, Gessleschaft, Berlin, Germany University of Pennsylvania, Philadelphia, PENN, USA and University of British Columbia, Vancouver, Canada Eidgentissische Technische Hochschule, Ztirich, Switzerland Exxon Chemical Europe Inc., Machelen, Belgium Biosym Technologies Inc., San Diego, CA, USA The University of Tokyo, Tokyo, Japan Delft University of Technology, Delft, The Netherlands
ACKNOWLEDGEMENTS
The organizers owe a huge debt of gratitude to Mrs. H61~ne Michel who contributed so efficiently to the Qu6bec Symposium organization as well as to the preparation of these proceedings.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
O n e and t w o - d i m e n s i o n a l solid-state N M R investigations of the threed i m e n s i o n a l structures of zeolite-organic sorbate c o m p l e x e s C.A. Fyfe, H. Grondey, A.C. Diaz, G.T. Kokotailo, Y. Feng, Y. Huang, K.C. Wong-Moon, K.T. Mueller, H. Strobl and A.R. Lewis Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, B.C. V6T 1Z1, Canada
1. I N T R O D U C T I O N A most important characteristic of molecular sieve systems which is common to their applications as catalysts, sorbents and in gas separation is the size and shape selectivity toward adsorbed organic molecules conferred by the molecular dimensions of their channel and cage systems [ 1]. Because of their small crystallite dimensions, powder rather than single-crystal XRD techniques must be used. While it is possible to define framework topologies and structures powder X-ray diffraction, particularly if Rietveld analysis and synchrotron radiation are used [2,3], it is very difficult to reliably determine the structures of organic sorbate/framework complexes which would yield important information on the detailed nature of the interactions. Important exceptions in this regard are the single crystal XRD studies of van Koningsveld and co-workers who determined detailed high-quality structures of the highloaded forms of p-xylene and p-dichlorobenzene in zeolite ZSM-5. These are the only reliable zeolite/sorbate structures to date [4,5]. High resolution solid state NMR has emerged in recent years as an important complementary technique to XRD in the investigation of zeolite structures, being particularly sensitive to short to medium range geometries and orderings [6]. For some years we have worked to develop new approaches in the application of solidstate NMR techniques together with XRD studies to the investigation of zeolite structures with the aim of ultimately being able to determine the 3D structures of their complexes with sorbed organic molecules. In this paper, we outline the development of these techniques and their current standing.
2. RESULTS AND DISCUSSION In high-resolution solid state NMR, the widths of the signals from dilute spin-1/2 nuclei are determined by the degree of crystallinity and the perfection of the local ordering. This can be achieved in the case of zeolites by investigating high-quality, completely siliceous systems where there is only the Si(4Si) local environment present. As illustrated in Figures 1A and B for ZSM-12 [7] and ZSM-5 [8], respectively, sharp resonances are observed whose numbers and relative intensities reflect the number and occupancy of the crystallographically inequivalent T-sites in the unit cell. In the case of A1PO4 molecular sieves, there is exact alternation of A1 and P, giving completely and perfectly ordered frameworks in the as-synthesized materials, as shown in Figure 1C for the 31p spectrum of VPI-5 [9]. This result is quite general for perfectly
crystalline and ordered solids. These spectra may be used to monitor various structural transformations, for example, those induced by temperature as in the case of ZSM-5 [ 10], or in the case of A1PO4 materials by the hydration/dehydration of octahedral A1 sites. Of particular importance, they yield information on the interaction of organic sorbates with the molecular sieve framework. For example, Figure 2 shows the 29Si spectrum of ZSM-5 with p-xylene present at a loading of two
A
I
I
I
-107
I
I
I
I
PPM from TMS
-113
A
Cl
0 I
I
I
I
-108
I
I
I
I
I
I
B
c~
C
,r
I
-118
PPM from TMS
C
J
I
25
~
I
0 Frequency
A
I
-25
.JO
-75
(ppm from 83% H3PO4)
Figure 1. 29Si MAS spectra of (A) ZSM-12 and (B) ZSM-5. (C) 31p MAS spectrum of VPI-5 with spinning sidebands indicated by brackets.
--I
-105
!
I
I
I
I
I
I
I
PPM / TMS
I
I
I
I
I
-120
Figure 2. 29Si MAS spectra of ZSM-5 loaded with 2 molecules per u.c. of (A) p-dichlorobenzene, (B) p-chlorotoluene, and (C) p-xylene.
molecules per unit cell (u.c.). Comparison with Figure 1B shows that the number of T-sites has decreased from 24 to 12 indicating a change in symmetry from monoclinic to orthorhombic. Further, the similarities between the spectra in the presence of p-xylene and pdichlorobenzene and p-chlorotoluene (Figure 2) indicate that the interactions, at least in this case, are based on the size and shape of the organic molecule since the CH3 and C1 substituents have the same steric factors but the molecules differ in most other aspects [ 11]. The difficulty in using these spectra further is that the assignment of the resonances to the different T-sites is generally not known, although there may be some information from the intensities if the site occupancies are different. In the case of ZSM-12 and ZSM-5, all of the site occupancies are the same and no assignments are possible. This problem can be solved by using two dimensional homonuclear correlation experiments such as COSY and INADEQUATE to establish the three-dimensional (Si-O-Si) connectivity pattern with the framework when the topology is known [12]. Figure 3 shows such an experiment on ZSM-12 [7]. This yields the assignments of the resonances shown in the figures. The above experiments are based on the scalar Si/Si J-coupling which operates through the bonding network. Similar information may be obtained in the A1PO4 systems from CP and TEDOR experiments as shown in Figure 4 [9]. Although these are based on the heteronuclear dipolar coupling which is a through-space interaction, they are selective for 3tp_ O-2VA1 connectivities because the interaction is very strongly distance dependent and these P/A1 distances are the shortest. Knowledge of the assignments may now be used to gain additional information on the details of the structures and the various changes which they can undergo. For example, zeolite ZSM- 11 is found to undergo a temperature induced transition from a
T3
T+ Ts
T1 T7
T5 T2
, ,
// /J
/
// T1T2 / / /
//
--T+T --.--,.~-.4.' ST // T3T5
0~,,
,/ T3TI
"~
,," T4T6 /
It
I
I
I
I
I
-108.0-109.0 -110.0 -111.0-112"0 -113.0
PPM Figure 3.
29Si INADEQUATE
experiment on ZSM-12.
i
1-------7----.-
I
I
I
I I I I I I I I I I
"
tr
O
o7 "
E o E
q tT.
I I
'
i, . . . . . . .
I
o
9
I
,
'10 Frequency
I
-20
,
I
-30
9
I
,
.40
I,
i_
-so
(ppm from 85% H3PO4)
Figure 4. Two-dimensional 27A1 ~ alp TEDOR experiment on VPI-5. The connectivities are displayed in the dashed box and spinning sidebands are indicated by asterisks. high-temperature form with seven T-sites which matches the proposed framework topology (tetragonal, space group I~,m2) to a lower symmetry form at room temperature (twelve T-sites with equal occupancy). 2D INADEQUATE experiments on both forms yield the assignments of the resonances shown in Figure 5 (see ref. [13] for more details) and from the relationship between the two spectra it can be deduced that the space group of the room temperature form is I4 (tetragonal) and that the phase transition involves the loss of the mirror plane. Information of this type is useful for further investigation of the structure by diffraction techniques. The last step in the extension of these solid state NMR techniques is to apply them to the investigation of the three dimensional structures of zeolite-sorbate complexes. This can be done by using experiments such as cross-polarization and REDOR which are based on the through-space dipolar interaction. Because of the strong distance dependence, the distances between the T-sites in the framework (whose identifies are now known) and nuclei on suitably isotopically substituted substrates may be determined, yielding the 3D structure of the zeolitesorbate complex. To test the validity of this approach we have investigated a number of such experiments applied to the high-loaded form of zeolite ZSM-5 containing p-xylene where the answer is known from the high-quality single crystal structure of van Koningsveld and coworkers [4].
A
I
B
T=302 K 133'
T=342 K
4Z
I
65
3 7
4
2 I
I
I
I
I
-110
I
PPM
2
I
I
I
I
- 118
I
I
I
I
I
- 109
PPM
2
I
I
I
I
-117
Figure 5. 29Si MAS spectra of ZSM- 11 at (A) 302 K and (B) 342 K. Below each spectrum is the deconvolution in terms of Lorentzian curves. In the present paper, the application of the CP technique with protons as the source nuclei will be described as representative of this class of experiments. In order to localize the polarization source as much as possible, experiments were carried out with the two specifically deuterated p-xylenes (1) and (2). CH 3
CD3
CH 3
CD3
(1)
(2)
D
D
Since the CP process is greatly dependent on molecular motions, these must be well understood for the system being studied. In the present work, these were investigated by wide line deuterium NMR of the sorbed organics. It was found that at 6 molecules/u.c., the methyl groups in the organic substrate have rapid C3v rotational motion while the aromatic rings are essentially rigid but a proportion show some low frequency "ring-flips" around the 1,4-axis. The effect of the distance dependence can be seen qualitatively from a comparison of the CP spectra with that from a simple one-pulse experiment as shown in Figure 6. The structure is orthorhombic with 24 T-sites of equal occupancy and the assignments of the resonances come as previously from 2D INADEQUATE experiments [14]. In the CP spectrum some signals are obviously enhanced compared to the others. The resonances due to the T-sites 1, 2, 10, 12 and 16 are quite well resolved and these were used in the study.
CP/MAS
MAS 13,Z314'6'8 I /I 20,~SlIznll
:"),2~ 19,9
7
18
I
I
'
I
-110
'
$
I
-112
'
I
12 1716
'
I
'
3 4
I
-114 -116 -118 PPM from TMS
'
I
"
I
-120
Figure 6. (a) 29Si CP/MAS spectrum of the complex of p-xylene (2) in ZSM-5 at a loading of 6 molecules per u.c. The contact time was 5 ms, with a recycle delay of 5 s. (b) 29Si MAS spectrum of the complex of p-xylene in ZSM-5 at a loading of 8 molecules per u.c. The spin-dynamics of the cross-polarization process from I to S nuclei as a function of time are described by Equation (1) [ 15]. S(t)
= Sma x (1 - Tcp/T1p(H)) -1
(exp(-t/Tlotn)) - exp(-t/Tcv))
(1)
Smax represents the theoretical maximum signal intensity obtainable from the polarization transfer, T1oca) the proton TIp value and Tcp the cross-polarization time constant. Thus the S signal intensity as a function of time should consist of an exponential growth controlled by the cross polarization transfer and an exponential decay due the T1o process. Of particular interest, Tcp can be related to the second moment of the IS dipolar interaction, (Ao~2)Is, as in Equation (2) and is proportional to rls6 as in Equation (3), [16]. 1
Tce
-
C (A(o 2 )IS
(2)
~ ( A('02 )II 2
1
Tcp
o~ (A002 )IS oc
2
7i~s E ris6
(3)
(a)
Si 12 x
12
--~--
Si 3 10 _..~tlB
Si 17
9
'si ~o-Si 1 Si12
" " ' I t .... ----D.--
8
Si17 Si 16
-"1"-. ---o--e
m
I
M
c o
Si 3
'
r&,
i
i
SI 16
qmo
Si 1
c
m
Si 10
O~ 0
10
20
30
Contact
(b)
40
time
50
60
(ms)
Si 1
1.2
Si 17 Si 10 1.0
A
=~--...._ m ~ ' m ~ ' " " . ~
=
Si 3 Si 16
r
. . ~ . . . . - . =. ~ t . ~ . . - 2 ~ ~ . . . .- .
0.8
I=
Si 12
>., i m
c o
0.6
"-=-"
:' F
C m
0.4
,S
/
. . . . x .... "'"Q""
0.2
-"-P-
e~
......s~ ~0
II
Si 1 Si 12
Si 17 Si~6
---=---si3 0.0
o
~o
2o Contact
so time
4o
so
eo
(ms)
Figure 7. Variation of the intensity as a function of the contact time for (a) the complex of pxylene (2) in ZSM-5 at a loading of 6 molecules per u.c., and (b) the complex of p-xylene (1) in ZSM-5 at a loading of 6 molecules per u.c.
Using these equations, the cross-polarization results can be related to the sorbate-lattice distances in the complex. Figure 7 shows the experimental CP curves for the well-resolved resonances of Figure 6 for complexes of p-xylenes (1) and (2) [17]. Qualitatively, it can be seen that silicons 12, 3 and 17 are much more efficiently polarized and hence closer to the ring protons than silicons 16, 10, 1 (Figure 7a), and that silicons 10, 1, 17 and 3 are closer to the methyl protons than silicons 16 and 12 (Figure 7b). More quantitatively, the data can be compared directly with the XRD data. The structure is found to have two sites, one at the channel intersections and one in the zig-zag channels (Figure 8), and the lattice sorbate distances and related second moments can be calculated. The CP curves in Figure 7 can be fitted by deriving the Tip(H) values from the clearest decays and using these data to deduce the Tcp values are shown in Table 1. Figure 9 shows the plot of the experimental Tcp values versus the calculated second moment values, validating the general approach used.
Si-2 Si-3
Si-4
Si-5 Si-13
Si-6
/ C
C~ C--
'
,.c
/
Si-17
,
rc
i
ic c
i '
/,, C C
I
I
CH3
Figure 8. View approximately along [001] of the single crystal structure of p-xylene in ZSM-5 at a loading of 8 molecules per u.c. Oxygen and hydrogen atoms are omitted for clarity.
(a)
(b)
180 T
450 T
160--
Sil0
Sil
4 0 0 - |-
140 -120 -"7
350 --
Si 17 Si3.
300 "7
100 -13. O
80--
~ 13. O
"
Si12
~ 12
~16
~
250
-
200
60 -
150
40-
100
| si 1/ t - / " s, ,6
" 0
~ 250
0 500
750
1000
Calc. second moment (Hz 2)
0
10000
I 20000
I 30000
Calc. second moment (Hz 2)
Figure 9. Plot of experimental Tcp values versus the calculated second moment (Ao32 )IS values for (a) the complex of p-xylene (2) in ZSM-5 at a loading of 6 molecules per u.c., and (b) the complex of p-xylene (1) in ZSM-5 at a loading of 6 molecules per u.c. Thus, in the one case where the structure is well described, the CP technique yields a result in good agreement with the known structure. Similar results have been obtained using the REDOR [18] technique. It is thus felt that with proper precautions and with particular attention to the motions of different parts of the organic guest molecules, solid-state NMR spectroscopy will be a viable technique for the determination of the structures of zeolite/sorbate complexes and that it will yield valuable complementary information to that from diffraction studies of these systems. It should be possible to combine the results from solid-state NMR with molecular dynamics calculations in an interactive manner.
3. A C K N O W L E D G E M E N T S C.A.F. acknowledges the support of the NSERC of Canada in the form of operating and equipment grants, Y.H. and K.T.M. for Postdoctoral Fellowships, and K.C.W-M. and
10 H.S. for Postgraduate Scholarships. A.C.D. thanks INTEVEP S.A. for a Postgraduate Scholarship. REFERENCES
1.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
15. 16. 17. 18.
(a) Meier, W.M. In Molecular Sieves. SCI Mong., 1968. (b) Breck, D.W. Zeolite Molecular Sieves; Wiley Interscience: New York, 1974. (c) Smith, J.V. Zeolite Chemistry and Catalysis; Rabo, J.A., Ed. ACS Monograph Series 171, American Chemical Society: Washington, DC, 1976. (d) Barrer, R.M. Zeolites and Clay Minerals as Sorbents and Molecular Sieves; Academic Press: London, 1978. W.I.F.; Harrison, W.T.A.; Johnson, M.W. High Resolution Powder Diffraction. Materials Science Forum; Catlow, C.R.A., Ed.; Trans Tech. Publication: Andermannsdorf, Switzerland, 1986; Vol. 9, pp. 89-101. The use of synchrotron x-ray sources may permit single crystal studies on much smaller samples. Eisenberger, P.; Newsam, J.B.; Leonowicz, M.E.; Vaughn, D.E.W. Nature, 1984, 309, 45. van Koningsveld, H.; Tuinstra, F.; van Bekkum, H.; Jansen, J.C. Acta Cryst., 1989, B45, 423. van Koningsveld, H.; Jansen, J.C. Private communication, to be published. Fyfe, C.A.; Gobbi, G.C.; Murphy, W.J.; Ozubko, R.S.; Slack, D.A.J. Am. Chem. Soc., 1984, 106, 4435. Fyfe, C.A.; Feng, Y.; Gies, H.; Grondey, H.; Kokotailo, G.T.J. Am. Chem. Soc., 1990, 112, 3264. Fyfe, C.A.; Strobl, H.; Kokotailo, G.T.; Kennedy, G.J.; Barlow, G.E.J. Am. Chem. Soc., 1988, 110, 3373. Fyfe, C.A.; Mueller, K.T.; Grondey, H.; Wong-Moon, K.C.J. Phys. Chem., 1993, 97, 13484. (a) Fyfe, C.A.; Kennedy, G.J.; Kokotailo, G.T.; Lyerla, J.R.; Fleming, W.W.J. Chem. Soc., Chem. Commun., 1985, 740. (b) Hay, D.G.; Jaeger, H.; West, G.W.J. Phys. Chem., 1985, 89, 1070. Fyfe, C.A.; Strobl, H.; Gies, H.; Kokotailo, G.T. Can. J. Chem., 1988, 66, 1942. Fyfe, C.A.; Feng, Y.; Grondey, H.; Kokotailo, G.T.; Gies, H. Chem. Rev., 1991, 91, 1525. Fyfe, C.A.; Feng, Y.; Grondey, H.; Kokotailo, G.T.; Mar, A. J. Phys. Chem., 1991, 95, 3747. (a) Fyfe, C.A.; Grondey, H.; Feng, Y.; Kokotailo, G.T.J. Am. Chem. Soc. 1990, 112, 8812. (b) Fyfe, C.A.; Grondey, H.; Feng, Y.; Kokotailo, G.T. Chem. Phys. Lett., 1990, 173, 211. (c) Fyfe, C.A.; Feng, Y.; Grondey, H.; Kokotailo, G.T.J. Chem. Soc., Chem. Commun., 1990, 1224. Mehring, M. Principles of High Resolution NMR in Solids. Second edition, 1983, Springer, Berlin. Pines, A.; Gibby, M.G.; Waugh, J.S.J. Chem. Phys., 1973, 59, 569. Fyfe, C.A.; Diaz, A.C.; Grondey, H.; Fahie, B. To be published. (a) Guillon, T.; Schaefer, J. J. Magn. Reson., 1989, 81,196. (b) GuiUon, T.; Schaefer, J. Adv. Magn. Reson., 1989, 13, 55.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviotand S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
11
The use of small and weakly basic probe molecules for the investigation of Bronsted acid sites in zeolites by N M R spectroscopy Eike Brunner Universit/it Leipzig, Fakult/it for Physik und Geowissenschaften, Linn6str~e 5, D-04103 Leipzig, Germany Small and weakly basic probe molecules were used for the characterization of Bronsted acid sites in zeolites by variable-temperature 1H and 13C NMR spectroscopy. It could be shown that the induced ~H NMR chemical shift A& of free surface hydroxyl groups caused by the adsorption of probe molecules as, e.g., CO and C2C14reflects the strength of acidity of the hydroxyls. The deprotonation energy AEDp of surface hydroxyl groups can be determined from A& if the influence of rapid thermal motions and/or exchange processes is suppressed. Furthermore, the use of the ~H NMR chemical shift &H..M of surface hydroxyl groups influenced by adsorbed probe molecules M as a sensitive qualitative measure for the strength of acidity is suggested. The geometry of the adsorption complexes formed by CO molecules hydrogen bonded to Bronsted acid sites could be determined by NMR spectroscopy.
1. INTRODUCTION The catalytic behaviour of H-zeolites with respect to Bronsted-acid catalyzed reactions is determined by the strength of acidity, the concentration, and the accessibility of the bridging hydroxyl groups which are known to act as Bronsted acid sites [1,2]. The strength of gas phase acidity of a surface hydroxyl group TO-H is defined as the inverse value of the Gibbs free energy change n GDp of the deprotonation
TO-H
~. T O -
+ H +
(1)
It could be shown [3] that AGDp is the sum of the deprotonation energy AEDp (heterolytic dissociation energy) and a constant contribution for surface hydroxyl groups in zeolites. Therefore, AEDpis a convenient measure for the strength of gas phase acidity. However, the spectroscopic measurement of the deprotonation energy ~EDp is still a subject of discussion. Both, IR and 1H MAS NMR spectroscopy allow the direct investigation of bridging hydroxyl groups (see, e.g., [1,2]). Very often the potential energy of the O-H stretching vibrations is approximated by the Morse potential. A dissociation energy Do can be calculated (see, e.g., [4,5]) from the wavenumbers Yon and v02 of the fundamental stretching vibration and the corresponding first overtone, respectively, according to
12
hcv oa Do ~"
(2)
4X
with
1 2YouZ = ~
V02
(3)
3VoH - v02
h denotes Planck's constant and c the speed of light in empty space. It has however been demonstrated by Kazansky [4,5] that Do is close to the value for the homolytic dissociation energy of the surface hydroxyl groups which strongly deviates from AEDp. Jacobs and Mortier [6,7] have found that VOH of free bridging hydroxyl groups (i.e., bridging hydroxyl groups which are not influenced by additional electrostatic interactions with the zeolite framework) correlates well with the intermediate Sanderson electronegativity. A corresponding correlation could also be found between the 'H NMR chemical shift ~H of free bridging hydroxyl groups and the intermediate Sanderson electronegativity. Therefore, Pfeifer et al. [1,8,9] have suggested to make use of 8H as a measure for the strength of acidity of free surface hydroxyl groups. This suggestion was confirmed by quantum chemical calculations [10] which have shown that AEDp and bn are linearly correlated for hydroxyl groups bound to T-atoms (B, A1, Si or P) whose first coordination sphere consists of oxygen atoms only. The slope of AEDp amounts to - 84 _ 12 kJ mol-' ppm -1. Since chemical shift differences can be measured with an experimental error of __+0.1 ppm it is possible to determine differences in the deprotonation energy of free surface hydroxyl groups with an accuracy of + 8 kJ mo1-1. Recent quantum chemical calculations [ 11] revealed that the deprotonation energy of terminal SiOH groups amounts to 1400 ___25 kJ mol-~ which excellently agrees with the experimentally determined value of 1390 kJ mol -~ [12]. Since the ~H NMR chemical shift of terminal SiOH groups amounts to 2.0 + 0.1 ppm the deprotonation energy AEDp can be calculated [13] according to
AF-,Dp -
kJmo1-1
1570-
84
8H
.
(4)
ppm
Paukshtis and Yurchenko [12,14] have developed another method for the determination of differences in the deprotonation energy of TO-H groups. This method is based on the measurement of the induced wavenumber shift Av = VOH...M - VOH, where VOH...Mdenotes the wavenumber of the stretching vibration of the surface hydroxyl groups influenced by adsorbed probe molecules M. Provided that (-Av) ~ 400 cm-' (weak hydrogen bonding) the deprotonation energy can be calculated according to the formula
13
A EDp
A ~ SiOH =
kJmo1-1
"-'~DP
_ 1
kdmo1-1
log
A
A._____~_v
(5)
A v s~on
with A = 0.00226 [12] and AEDpSiOH = 1400 kJ mo1-1 (see above). The deprotonation energy of different types of surface hydroxyl groups in zeolites was determined successfully by this method by the use of CO as the probe molecule M [15,16]. On the other hand, it is known that the formation of hydrogen bonds leads to a considerable broadening of the stretching vibration bands of surface hydroxyl groups. This leads to a relatively large experimental error for Av which limits the accuracy of the measurement especially of small differences in the deprotonation energy. It is known that the interaction between surface hydroxyl groups and adsorbed probe molecules M causes an induced 1H NMR chemical shift A8 = 8H..M - 8H, where 8H.M denotes the chemical shift of the surface hydroxyl groups influenced by the probe molecules. It could be shown [ 17] that ~H and VoH are linearly correlated at least in limited ranges. The slope of ~H amounts to 0.0147 ppm/cm-' for surface hydroxyl groups in zeolites and to 0.0092 ppm/cm-' for hydrogen bonded protons in various solids. It can therefore be supposed that the correlation between A b and (-A v) is given by
a8
ppm
_
(-av) -1
cm
(6)
with B-values between 0.0092 and 0.0147. If this is true it should be possible to make use of A ~ instead of Av in eq. (5).
2. EXPERIMENTAL NMR and diffuse reflectance FTIR measurements have been carried out on identical samples which were prepared in the following manner: Glass tubes were filled with the hydrated zeolite (bed depth: ca. 8 ram) and heated up to 673 K with a heating rate of 10 K/h under permanent evacuation. At this temperature the samples were further evacuated for 24 h at a final pressure of 10.2 Pa. Then the samples were cooled to 77 K and loaded with definite amounts of probe molecules (CO or C2C14). 13C enriched substances were used for the 13C NMR spectroscopic investigation of the adsorption state of the probe molecules. After loading the samples were sealed. NMR spectroscopic investigations have been carried out on a Bruker MSL 500 spectrometer at low temperatures since small probe molecules as CO can exhibit rapid thermal motions at room temperature. The lowest temperature achievable under magic angle spinning conditions in our laboratory yet amounts to 123 K. In contrast, static 13C NMR investigations were carried out at temperatures down to 4.5 K. All NMR chemical shifts are given relative to tetramethylsilane (TMS).
14 3. RESULTS AND DISCUSSION 3.1. General remarks The influence of small and weakly basic probe molecules on the low-temperature ~H MAS NMR spectra of zeolites was firstly investigated by White et al. [18]. Table 1 summarizes induced chemical shift values for a variety of probe molecules. Table 1 Induced ~H NMR chemical shift A8 of bridging hydroxyl groups in zeolite H-ZSM-5 loaded with different amounts of probe molecules M per framework A1 atom (A1F) measured at a temperature of 123 K. The data are taken from ref. [18]. probe molecule M
N2
CO
A 8/ppm (1 M/A1 F)
-
1.8
AS/ppm (2 M/A1 F)
0.3
1.8
C2H6
0.6
C2H4
C2H2
2.7
3.5
2.7
3.9
Convenient probe molecules should exhibit the following properties: (i) The induced 1H NMR chemical shift A8 caused by the probe molecules should be high since its relative experimental error is then low. (ii) A 8 should remain constant for coverages higher than 1 probe molecule per surface hydroxyl group. Otherwise the dependence of n 8 on the coverage has to be taken into account which complicates the use of A8 as a measure for the strength of acidity. (iii) The molecules must be chemically stable even in the presence of acid sites in zeolites. Conditions (i) and (ii) are obviously fulfilled for CO and C2H4. However, it turned out that C2H4 rapidly chemically reacts at room temperature in H-zeolites which makes its use as a probe molecule difficult. Therefore, CO and C2C14 (instead of C2H4) were chosen as promising candidates. 3.2. The interaction of CO molecules with bridging OH groups in zeolites The data collected in Table 2 (see next page) show that the induced 1H NMR chemical shift A8 caused by the interaction of surface hydroxyl groups in H-ZSM-5 with CO molecules qualitatively reflects the strength of acidity of the hydroxyls. However, it is doubtful that the induced chemical shift of bridging hydroxyl groups in the large cavities of zeolite 0.3 HNa-Y is smaller than that of the AIOH groups on non-framework A1 species in H-ZSM-5. Furthermore, the induced wavenumber shift of the free bridging hydroxyl groups in H-ZSM-5 amounts to ca. 330 cm -~ which is in good agreement with the results reported in refs. [15,19,20]. According to eq. (6) one would then expect an induced ~H NMR chemical shift of ca. 3.0 - 4.9 ppm which is in contradiction to the experimentally observed value of only 2 ppm. It could be shown that the CO molecules exchange rapidly between the bridging hydroxyl groups even at 123 K [18,21] and that the induced chemical shift A 8 has not reached its maximum value [22] which explains this contradiction. Rigid 13CO molecules should exhibit a broad signal with the typical shape of a line dominated by the chemical shift anisotropy. The principal values of the chemical shift tensor of rigid ~3COare 81~ = 822 = 8~ = 305 ppm and 8 3 3 = 81 ~---- - 48 ppm [23], i.e., the chemical
15 shift anisotropy amonts to Aoc = 353 ppm. The corresponding line shape could be found [24] for the 13C NMR signal of 13CO molecules physisorbed on silicalite measured at 4.5 K. Table 2 Experimental data for the ~H NMR chemical shifts 8i~ and 8H..CO as well as the induced chemical shift A8 of surface hydroxyl groups in zeolites. The measurements were carried out at 123 K. It should be mentioned that bridging hydroxyl groups influenced by an additional electrostatic interaction with the zeolite framework are not considered here. zeolite
hydroxyl group structure
H-ZSM-5 hydrothermally treated
0.3 HNa-Y
8H/ppm
bH...co/ppm
A8/ppm
SiOH
2.0
2.0
0.0
A1OH (on non-framework A1)
2.9
3.9
1.0
SiOHA1 (free)
4.2
6.2
2.0
SiOHAI (in the large cavities)
3.9
4.8
0.9
The 13C NMR spectrum of ~3CO molecules adsorbed on a zeolite H-Y measured at 123 K consists of a single narrow line at ca. 185 ppm indicating that the CO molecules still move isotropically. At a temperature of 4.5 K the spectrum exhibits a width corresponding to that expected for rigid 13CO molecules (see Fig. 1). It is important to mention that the transition from the narrow line observed for 123 K to the broad signal shown in Fig. 1 takes place at temperatures between 60 and 80 K. The 1H MAS NMR spectroscopic investigation of the interaction between surface hydroxyl groups and CO molecules should therefore be carried out at temperatures below 60 K which will be possible in the near future.
__...
S / ppm
[
I
600
400
r
l
200
r
I
0
I
-200
1
-400
Figure 1. 13C NMR spectrum of 13CO adsorbed on zeolite H-Y (Si/A1 = 2.5) measured at 4.5 K. The sample was loaded with ca. 26 molecules 13CO per unit cell.
16 It is remarkable that the characteristic line shape shown in Fig. 1 can only be observed for CO coverages which are not higher than the concentration of accessible bridging hydroxyl groups. It could be shown by IR spectroscopic investigations that all CO molecules are then adsorbed on bridging hydroxyl groups. The characteristic features of the spectrum shown in Fig. 1, namely the two maxima at the left edge of the spectrum and the "step" at the right edge vanish for coverages higher than the concentration of accessible bridging hydroxyl groups. In the latter case IR spectroscopic investigations reveal the existence of physisorbed CO besides CO molecules adsorbed on bridging hydroxyl groups. Therefore, it has to be concluded that the characteristic shape of the spectrum shown in Fig. 1 is not caused by a superposition of two different signals due to 13CO molecules in different adsorption states. It is known from IR spectroscopic investigations as well as quantum chemical calculations [25-27] that the adsorption of CO molecules on bridging hydroxyl groups leads to the formation of linear adsorption complexes O-H CO where the C atom is preferentially attached to the proton. Therefore, the heteronuclear magnetic dipole interaction between the proton and the 13C nucleus of the ~3CO molecule has to be taken into account besides the chemical shift.anisotropy. The spectrum numerically calculated for this complex with a H-C hydrogen bond distance rH_c of 0.21 ___ 0.01 nm and AOc ~ 370 ppm excellently agrees [24] with the experimentally observed spectrum. This H-C hydrogen bond distance is in complete accordance with the predictions of quantum chemical calculations [25-27]. Furthermore, it is remarkable that AOc increases compared with physisorbed ~3CO. It can be stated that variable-temperature 13C NMR spectroscopy allows the study of the geometry and the thermal motions of the adsorption complexes formed by CO molecules hydrogen bonded to surface hydroxyl groups.
3.3. The influence of C2Cl 4 on the IH MAS NMR spectra of zeolites Fig. 2 shows the 1H MAS NMR spectrum of zeolite 0.3 HNa-Y loaded with 8 molecules C2CI4 per unit cell which is less than the actual concentration of ca. 17 bridging hydroxyl groups per unit cell. Obviously, a part of the signal at ~H = 3.9 ppm due to bridging hydroxyl groups in the large cavities is shifted to a position of ~H c2c~4 = 5.5 ppm.
Figure 2. 1H MAS NMR spectrum of zeolite 0.3 HNa-Y loaded with 8 molecules C2C14 per unit cell measured at 123 K.
. . . .
i
8
. . . .
i
7
. . . .
i
6
. . . . .
i
. . . .
5
i
. . . .
4
/ ppm
|
3
. . . .
i , , , , , i
2
. . . .
1
17 The spectrum measured at room temperature exhibits only one line at 4.5 ppm instead of the two signals at 3.9 and 5.5 ppm which shows that the C2C14molecules exchange rapidly between the bridging hydroxyl groups at 293 K. In contrast, the presence of the two well resolved signals at 123 K evidences that the exchange processes of the C2C14 molecules between the bridging hydroxyl groups are slow. Furthermore, it could be shown that the induced chemical shift becomes independent of the temperature for T < 150 K, i.e., it can be assumed that the maximum value of A8 is reached. Fig. 3 demonstrates that the correlation between A8 and (-Av) is given by a straight line with a slope of B = 0.01 [28] as it was predicted above (cf. eq. (6)). Therefore, the induced chemical shift A6 measured at 123 K for samples loaded with C2C14 is not only a qualitative measure for the strength of acidity of free surface hydroxyl groups. It can also be used for the calculation of the deprotonation energy according to eq. (5) since Av and n VSiOHcan be replaced by A~ and A~SiOH , respectively. An induced chemical shift of AbsioH = 0.75 ppm was determined for SiOH groups [28]. The induced chemical shift A 6 amounts to 1.6 ppm for bridging hydroxyl groups in the large cavities of 0.3 HNa-Y. From eq. (5) it follows that AEDp ~ 1250 kJ mo1-1 which is close to the value of ca. 1240 kJ mol -~ obtained with bH = 3.9 ppm by using eq. (4). Furthermore, it can be suggested to make use of 8H C2C14as a sensitive qualitative measure for the strength of acidity since both 8H and n 8 reflect the strength of acidity of free surface hydroxyl groups.
2,2
-
2,0
9
1,8
9
9
1,6
E Q. 1,4 ,,o 1,2 1,0 0,8 0,6 n
60
80
100
120
140
-Av / cm
160
180
200
220
-1
Figure 3. Correlation between the induced 1H NMR chemical shift A8 and the induced wavenumber shift nv caused by adsorbed C2C14 for free surface hydroxyl groups in zeolites.
18 ACKNOWLEDGEMENT The author wishes to express his gratitude to Prof. Dr. Dr. H. Pfeifer and Dr. B. Staudte for helpful discussions and to Mr M. Koch and Ms H. Sachsenr6der for excellent experimental assistance. Financial support by "Deutsche Forschungsgemeinschaft" (SFB 294 "MolekiJle in Wechselwirkung mit Grenzflfichen") is highly appreciated. REFERENCES .
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.
H. Pfeifer, NMR Basic Principles and Progress, Vol. 31, Springer, Berlin 1994., p.31. H.G. Karge, Stud. Surf. Sci. Catal., 65 (1991) 133. J. Sauer, J. Mol. Catal., 54 (1989) 312. V.B. Kazansky, Kinetika i Kataliz, 21 (1980) 159. V.B. Kazansky, Stud. Surf. Sci. Catal., 85 (1994) 251. P.A. Jacobs, Catal. Rev.-Sci. Eng., 24 (1982) 415. P.A. Jacobs and W.J. Mortier, Zeolites, 2 (1982) 226. H. Pfeifer, D. Freude and M. Hunger, Zeolites, 5 (1985) 274. D. Freude, M. Hunger and H. Pfeifer, Z. Phys. Chemie NF, 152 (1987) 171. U. Fleischer, W. Kutzelnigg, A. Bleiber and J. Sauer, J. Am. Chem. Soc., 115 (1993) 7833. J. Sauer and J.-R. Hill, Chem. Phys. Lett., 218 (1994) 333. E.A. Paukshtis and E.N. Yurchenko, Usp. Khim., 52 (1983) 426. E. Brunner and H. Pfeifer, Z. Phys. Chemie, in press. E.A. Paukshtis and E.N. Yurchenko, React. Kinet. Catal. Lett., 16 (1981) 131. L. KubelkovS., S. Beran and J.A. Lercher, Zeolites, 9 (1989) 539. M.A. Makarova, A. Garforth, V.L. Zholobenko, J. Dwyer, G.J. Earl and D. Rawlence, Stud. Surf. Sci. Catal., 84 (1994) 365. E. Brunner, H.G. Karge and H. Pfeifer, Z. Phys. Chemie, 176 (1992) 173. J.L. White, L.W. Beck and J.F. Haw, J. Am. Chem. Soc., 114 (1992) 6182. L.M. Kustov, V.B. Kazansky, S. Beran, L. Kubelkovfi and P. Jir~, J. Phys. Chem., 91 (1987) 5247. I. Mirsojew, S. Ernst, J. Weitkamp and H. Kn6zinger, Catal. Lett., 24 (1994) 235. E. Brunner, J. Mol. Struct., in press. M. Koch, E. Brunner, D. Fenzke, H. Pfeifer and B. Staudte, Stud. Surf. Sci. Catal., 84 (1994) 709. A.J. Beeler, A.M. Orendt, D.M. Grant, P.W. Cutts, J. Michl, K.W. Zilm, J.W. Downing, J.C. Facelli, M.S. Schindler and W. Kutzelnigg, J. Am. Chem. Soc., 106 (1984) 7672. M. Koch, E. Brunner, H. Pfeifer and D. Zscherpel, Chem. Phys. Lett., 228 (1994) 501. S. Bates and J. Dwyer, J. Phys. Chem., 97 (1993) 5897. P. Geerlings, N. Tariel, A. Botrel, R. Lissillour and W.J. Mortier, J. Phys. Chem., 88 (1984) 5752. K.M. Neyman, P. Strodel, S.P. Ruzankin, N. Schlensog, H. Kn6zinger and N. ROsch, Catal. Lett., 31 (1995) 273. H. Sachsenr6der, E. Brunner, H. Pfeifer and B. Staudte, in preparation.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
19
D e t e r m i n a t i o n o f the e n v i r o n m e n t o f titanium a t o m s in TS-1 silicalite by Ti Kedge X - r a y A b s o r p t i o n S p e c t r o s c o p y , 29Si and 1H N u c l e a r M a g n e t i c R e s o n a n c e . L. Le Noc a, C. Cartier dit Moulin a'b, S. Solomykina a, D. Trong On a, C. Lortie a, S. Lessard a and L. Bonneviot a aFacult6 des Sciences et G6nie, Universit6 Laval, G 1K 7P4 Sainte Foy (Qc), Canada bLaboratoire pour l'Utilisation du Rayonnement Electromagn6tique, CNRS-CEA-MENJS, B~timent 209d, 91405 Orsay Cedex, France The titanium environment and the modification engendered by the framework when this element is incorporated in a silicalite of orthorombic MFI structure were investigated using Ti K-edge X-ray Absorption, 298i and IH MAS NMR spectroscopy. A multiple scattering analysis of the EXAFS signal reveals that the titanium occupies a substitutional site characterized by Ti-O-Si bond angle, Ti-O and O-Si average distances consistent with a lattice expansion. The defects concentration remains constant and independent of the Ti loading. The proton NMR spectra of S-1 is characterized by narrow resonance lines whose relative intensities are modified in TS-I which indicate either the presence of new TiOH groups and/or a redistribution of the internal SiOH groups.
INTRODUCTION Microporous titanium silicalites of MFI or MEL structures, TS-1 and TS-2 respectively, have attracted many scientists owing to their unique capability to catalyze the oxyfunctionalization reactions of alkanes using H202 as oxidant. Despite the number of new titanium silicalites with larger and larger pore sizes such as Ti-beta, titanium mesoporous materials such as Ti-MCM-41, Ti-MCM-48 or high surface area amorphous TiO2-SiO2, no other materials were active for this demanding reaction [1-4]. The presence of micropores might be necessary for oxyfunctionalization to take place, though a specific local environment around titanium might as well be required for these reactions. These materials exhibit an IR band at 960 cm 1. It is assigned to a local vibration mode of the asymmetric stretching of a SiO4 unit linked to either a Ti 4+, a V 4+ or a Cr 3+ ion in amorphous or crystallyzed systems [57]. Consequently, this band is not an intrisic characteristic of titanium silicalite materials. The Ti isomorphous substitution for Si in these systems is a widely shared belief based on a conjunction of facts obtained from a panel of techniques such as XRD [8], IR, UV-visible, XPS [9], EXAFS [10], XANES [10] rather than on a direct proof. Recently, using the multiple scattering (MS) theory to analyze the second shell of neighbours around the titanium
20
in a TS-1, some of us calculated an average Ti-O-Si angle of about 160 ~ consistent with the isomorphous substitution proposal [~1]. Refined MS calculations on TS-1 free of Ti extraframework species, 29Si and H MAS NMR investigations of the framework modifications in presence of Ti are reported here. EXPERIMENTALS Three titanium silicalites with a Ti/(Ti+Si) atom ratio of 0.005, 0.010 and 0.015 were synthesized from a gel containing a mixture of TEOS, TEOT and TPAOH in propanol. Water was added. The hydrothermal crystallization was performed in a Teflon-lined stainless steel autoclave at 175~ for 4 days. The solid was filtered, washed and calcined at 5Q0~ The XRD recorded on a Rigaku D-Max IIIVC X-ray spectrometer revealed a crystallinity of about 95%. These samples exhibit features that characterizes TS-1 materials without extraframework Ti species, i) an MFI orthorhombic structure with a unit cell expansion of 25 A 3 per Ti, ii) an IR band at 960 cm -t and iii) no UV bands in the 270-400 nm UV range where the electronic transition of TiO2 anatase arises and iv) the characteristic X-ray near edge features of tetrahedral environment [ 10]. The X-ray Absorption measurements at the Ti K-edge were performed at the radiation synchrotron facility of the LURE (France). The white radiation was monochromatized using a Si(111) two crystals monochromator and harmonic rejection was obtained by detuning the crystals. The EXAFS signals were measured in transmission mode from 4800 to 5800 eV with 2 eV steps on samples pressed into pellets. The samples were dehydrated under vacuum at 500~ and transferred to a leak tight XAS cell under controlled atmosphere. The signal was extracted and normalized using a software package developed by Michalowicz [12]. The Fourier transforms were calculated after applying a parametrized Kaiser window function (t 3.7) between 3.16 and 14.20 A -1. The first shell was analyzed using the single scattering (SS) theory combined to an automated mean square minimization curve fitting process [12]. The second shell analysis that necessitated MS calculations were performed using the FEFF6 code from Rehr et al [ 13]. These calculations are usually performed on a priori known structures. Therefore the structures were generated using Cerius 2 TM, a molecular modeling package from Molecular Simulations. A trial and error approach was adopted to reach the structure that led to the best fit of the experimental data. The NMR measurements were also performed on TS-1 dehydrated at 500~ and transferred under controlled atmosphere into the rotors. No rehydration occured during the experiments as it was probed by tH NMR. The MAS NMR spectra were obtained at room temperature using a Bruker ASX 300. The rotors were spun at a rotation frequency of 4kHz. For 298i NMR, an onepulse sequence with a n/2 pulse of 5ps was used. The cycle time of 60s allows a full relaxation of the spins of all the silicon species. IH spectra were recorded following an echo sequence (n/2-x-n) where the echo time x is equal to the inverse of the spin rate, n/2 is 51as and the cycle time is fixed to 2.5s (full relaxation). The shifts are referenced to an external TMS standard.
21
~
:, , i , , . , i , , ~, i , ,', _ i r , , I , ,
9X - 0 . 4
_
:.i-
1.2
..'" ,,
,
i
4
'..
',..."r
i,
'..
....,- ......
,
6
I t i
8
,
12 I. ' ' . ' '
",..,. ....... 9.........
i
l,
10
,
I
~"
I '' ' ' I ....
~, ,.r,___
_
.
, , ...... i'1
12
....
~
14
0
.e
i
I
2
o
I . . . .
4
3
-
5
R(A)
k(A -i)
Figure 2. Module of forward transforms (uncorrected for phase the experimental (solid line) and FEFF6 (dots) model shown in fig.
Figure 1. Normalized EXAFS spectra of [1.5] TS-1 (solid line) compared to SS and MS-FEFF6 calculations using a [Ti(OSi)4] cluster with a geometry given in the text.
.4 pl i
Fourier shift) of the MS1.
........ , ....... .... ;
I II I I I'I I I'I I I I'"I I I'I I I I'I I l,lal i
f..t.,
"g o 2
4 1
2.s
3
RCA)
3.s
1
ltliltltlilll tt lit ill= ---Ii -
I
4
2.5
3
R C A ) 3.s
4
Figure 3. Imaginary part of forward Fourier transforms of the second shell of experimental (solid line) and MS-FEFF6 calculations (dots) for sets of Si-O distance and Ti-O-Si angle, a) 1.638 A and 161 ~ (as in fig. 1 and 2) and b) 1.62 A and 163 ~ respectively.
RESULTS
The X-ray near edge region for the three dehydrated TS-1 not shown here are identical and similar to those shown in the litterature for extraframework Ti free TS [10]. In addition, the similarity of the XANES of TS-1 and a model compound, the hexadecaphenyloctasiloxyspiro(9,9)titanium(IV) (HDPOSST), made of a tetrahedral [Ti(OSi)4] core strongly suggests that the titanium in the silicalite framework adopts the same symmetry and coordination compatible with a substitutional site [11]. The SS analysis of the first shell EXAFS
22
contribution led to a Ti-O distance of 1.80 +0.01 /~ also consistent with a tetrahedral environment. The multiple scattering calculation was proved to be necessary to reproduce the EXAFS signal of the HDPOSST [11]. This was attributed to widely open Ti-O-Si angles spanning in the 156-173 ~ range. A satisfying fit was obtained for calculations restricted to the tetrahedral [Ti(OSi)4 ] core of this molecule (Rmax = 4/~). A similar quality of fit was also obtained using average values 1.782/~ for Ti-O distance and 166 ~ for Ti-O-Si angle instead of the actual values. This demonstrated that the technique provides average values. A [Ti(OSi)4] cluster of atoms was also used to simulate the experimental EXAFS data of TS-1 samples (Fig. 1 and 2). To improve the match with the experimental data, the Ti-O distance and the Ti-O-Si angle were optimized resulting in a new set of 1.805 A and 163 ~ Further refinements were obtained by trial and error changing the O-Si distance as well. This led to a new O-Si distance of 1.638 ,~ and a Ti-O-Si angle of 161 ~ The accuracy of the process was estimated to be + 0.01 A on the distance and • 1~ on the angle. The Debye-Waller factors (DWF) that rely on the thermal agitation and structural disorder were found to be ol 2 = 0.00155 for the first shell and ~22 = 0.0045 for the second shell (S02 = 0.85) for the model compound [11]. For the TS-1 samples the DWF of the second shell had to be increased up to 0.0089. Keeping ol 2 and or22 the same as for the model compound would have led to 3 Si, assuming a [Ti(OSi)3(OH)] type of site. The 29Si MAS NMR was performed to identify the structural defects from vacancies of silicon implying nested OH or broken Si-O-Si bridges. The 29Si NMR spectra (fig.4) of the titanium free silicalite (S-l) and the TS-1 materials exhibit an intense and broad signal at -113 ppm assigned to Q4 [Si(OSi)4] species and a less intense signal at -103 ppm assigned to Q3 [Si(OSi)3OH ] species. No geminal silanol species were observed in the TS-1 or S-1 samples, even when cross polarization was used. The number of Q3 species is hardly constant and nearly equal to 8 per unit cell (8.2, 7.9, 7.2, 7.8 +1 for S-I, [0.5] TS-1, [1.0] TS-1 and [1.5] TS-1 respectively). Assuming a number of at least 3 Ti-O-Si bridges per Ti, the [Si(OSi)3(OTi)] species would already increase the Q3 signal by about 50% if they were to resonate at the same frequency. Consequently, the [Si(OSi)3(OTi)] species contribute to the Q4 signal intensity. The mformatlon taken from the Q signal is therefore exclusively related to the number of silanol species found at a nearly constant concentration whatever the titanium loading. The zeolites have been dehydrated during 12 hours at 500~ in vacuum. An analysis by mass spectrometry of the desorption under a flow of pure helium gas between 25 and 500~ (heating rate of 4~ demonstrates that all the physisorbed water is evacuated at 100~ and the water from the condensation of neighbouring silanols is eliminated after two hours at 500~ It can be pointed out that some trace of organic template is also observed and disappears at 400~ The structural defects observed by IH NMR correspond to terminal SiOH and possibly TiOH hydroxyls which are isolated or difficult to eliminate for structural reasons (lattice constraints, bad conformation of neighbouring hydroxyl groups,...). For the pure silicalite S-l, three components can be seen at 1.8 ppm (a), 2.2 ppm (b), and around 3.2 ppm (c) with the ratio 1/3 for each line (fig.5). A strong steaming (175~ in an autoclave in presence of water: conditions of crystallization) was performed leading to the monoclinic MFI structure. This has been confirmed by 29Si MAS NMR: 17 peaks or 9
9
3
9
23 10 TS11.5%
1.0
~TS10.5%
8
Q4 $1
0.8 A [, . ! . . . . [ ~ 5 t~ 0.6 ~ - 9 0 -100
56 .
4
3
2
,
,
-11
i""
r
y_=
L
e-
4
0.4
2
0.2 -
,,,,
0.0 0
!
5
4
3
2
1
chemical shift (ppm/TMS)
Figure 4. IH MAS-NMR spectra of the silicalites S-l, [0.5] TS-1 and [1.5] TS-I" spectra normalized on the weight of sample.
-80
-90
,
I
I
I
-100
-110
-120
-130
chemical shift (ppm/TMS)
Figure 5. 29Si MAS-NMR spectra of the silicalites S- 1 and [1.5] TS- 1.
shoulders with a total intensity of 24 are clearly visible instead of the Q4 broad line. The Q3 signal is absent. The receiver gain used for the IH NMR is 32 times greater than for the silicalite with 8% of defects. Surprisingly, these experiments show that the terminal silanols can resonate within a large window (1.5-4.5 ppm), far from the common value of 2 ppm. After one year of air exposure, the monoclinic silicalite progressively evolves toward the orthorhombic structure, containing 6% of Q3. The corresponding 'H NMR spectra exhibit the same components as the original silicalite with a higher proportion of the species giving rise to line c. It is unprobable that vacancies of silicon could have been created at room temperature in this structure, so the line c can not be assigned to strongly interacting nested OH around vacancies. Furthermore, it is worthwhile to notice that contamination by water lead to a well defined signal at 3.5 ppm, broader than those observed for the monoclinic silicalite and narrower than the line c for the orthorhombic silicalite. Finally, all the defects a, b and c seem to be natural defects of the structure. They probably correspond to terminal silanols in different conformations, arising from non-intact Si-O-Si bridges. The line a corresponds to defects which are easier to eliminate than the defects of type b and c. An ambiguity resides the high chemical shift of the line c (which could be due to strong interacting silanols), the difficulty to eliminate these defects and the fact that they do not seem to originate from silanol vacancies. The lH NMR spectra of the titanium containing silicalites show the same components as the pure silicalite. In this case, the proton can be a silanol proton or a titanol proton. The anatase TiO2 is characterized by two lines, one at 6.4 ppm from Ti-OH-Ti bridges and another
24 one from terminal TiOH groups at 2.3 ppm [16-17]. For anatase free TS-1, we only expect terminal TiOH species in the region of 2 ppm, their effective chemical shift depending on the structural environment as it is the case for the silanol protons. There is indeed no evidence of a signal from anatase clusters at about 6 ppm. A continuous increase of line a intensity with an increasing titanium content in the silicalite is observed (fig.5). Conversely and to a lesser extent, it seems that the line b decreases, while no significant change occurs for line c. The precision on the Q3 defects number does not allow to correlate the proton and Q3 signal intensities with the titanium loading. Moreover, these defects may be differently distributed among the type a, b and c, possibly explaining that the increase of line a intensity is not proportionnal to the Ti loading. Anyhow, the NMR data can be interpreted both ways as the result of formation of titanol species, or increase of type a silanol defects. It seems that no specific structural defects are produced in presence of titanium. DISCUSSION Within the range of 0.005-0.015 Ti/(Si+Ti) ratios, the TS-I investigated here exhibit the expected XRD, IR and UV-visible characteristics of TS-1 materials free of extraframework species. Their XANES spectra similar to the XANES spectrum of the model compound HDPOSST confirm this statement and bring further supports for the Ti isomorphous site occupancy [11]. The average Ti-O-Si angle decreases from 166 ~ in the molecular HDPOSST compound to 161 ~ in the TS-1, getting close to the average Si-O-Si angle of 154 ~ in the titanium free silicalite. This reveals the existence of a local pressure exerted on the Ti site by the lattice. The average Ti-Si distance of 3.38 A is however longer than the average Si-Si distance of 3.10 A and correspond to a local expansion of 37/~3 per Ti. The lattice is obviously relaxed since an actual 25 A 3 lattice expansion is measured. The 29Si NMR already reveals that the S-I framework relaxes from monoclinic to orthorhombic by SiO-Si bond breaking and narrowing of the Si-O-Si bond angle distribution. No further relaxation seems to occur when Ti is incorporated in the lattice according to the constant Q3 concentration. This situation could be explained by a local relaxation on the Ti site implying only a small rearrangement of the lattice. From this model, a [Ti(OSi)3(OH)] local (open) model is more probable because of the broken Ti-O-Si bond than the [Ti(OSi)4] (closed) model. In addition, according to the synthesis route that randomly distribute the titanium in the gel before cristallization, it is more likely that there is no site discrimination for the substitution. Unfortunately, since the coordination number and the DWF are correlated, it is impossible to differentiate between the closed and the open Ti site models from EXAFS. Moreover, the proton NMR does not bring any definitive proof of the existence of TiOH since a new SiOH distribution might as well explain the increase of line a in the spectra (fig.5).
25 CONCLUSION The MS calculations at the Ti K-edge EXAFS and the MAS NMR investigations at the IH nuclei lead to the conclusion that the titanium ions are located in framework substitutional sites of the TS-1 materials. It is more likely that Ti is randomly located in the structure. However, it seems that the relaxation of the lattice might occur by T-O-T bridge breaking preferably involving more Ti sites that are obviously more constrained than silicon sites with a roughly constant lattice defect concentration.
29Si and
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
T. Blasco, M.A. Camblor, A. Corma and J. P6rez-Pariente. J. Am. Chem. Soc. 115 (1993) 11806. P.T. Tanev, M. Chibwe and T.J. Pinnavaia, Nature 368 (1994) 321. J.S. Reddy, A. Dicko and/~. Sayari, Prepr., Am. Chem. Soc., Div. Petrol. Chem. 40 (1995) 225. R. Hutter, D.C. Dutoit, T. Mallat, M. Schneider and A. Baiker, J. Chem. Soc., Chem. Commun. (1995) 163. M.R. Boccuti, K.M. Rao, A. Zecchina, G. Leofanti and G. Petrini, Stud. Surf. Sci. Catal. 48 (1989) 133. P.R. Hari Prasad Rao and A.V. Ramaswamy, J. Chem. Sot., Chem Commun. (1992) 1245. Y. Chapu, A. Tuel, Y. Ben Taarit and C. Naccache, Zeolites 14 (1994) 349. R. Millini, E. Massara Previde, G. Perego and G. Bellussi, J. Catal. 84 (1994) 501. D. Trong On, L. Bonneviot, A. Bittar, A. Sayari and S. Kaliaguine, J. Mol. Catal. 74 (1992) 223. S. Bordiga, S. Coluccia, C. Lamberti, L. Marchese, A. Zecchina, F. Buffa, F. Genoni, G. Leofanti, G. Petrini, G. Vlaic, J. Phys. Chem. 98 (1994) 4125. C. Cartier, C. Lortie, D. Trong On, H. Dexpert and L. Bonneviot, Physica B 208-209 (1995) 653. A. Michalowicz, Logiciels pour la chimie: EXAFS pour le Mac, Soci6t6 frangaise de chimie (1991) 116. P.A. O'Day, J.J. Rehr, S.I. Zabinsky and G.E. Brown Jr., J. Am. Chem. Soc. 116 (1994) 2938. G.E. Maciel, D.W. Sindorf, J. Am. Chem. Sot., 102 (1980) 7606. G. Engelhardt, D. Michel, High-Resolution Solid-State NMR of Silicates and Zeolites, Wiley Interscience, New York (1987). V.M. Mastikhin, A.V. Nosov, React. Kinet. Lett., 46 (1992) 123. S. Haukka, E.L. Lakomaa, A. Root, J. Phys. Chem., 97 (1993) 5085.
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Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
A n in situ
13C l~lmS N M R
27
study of toluene alkylation with m e t h a n o l over H -
ZSM-11 Irina I. Ivanova # and Avelino Corma Instituto de Technologia Quimica, UPV-CSIC, Avd. los Naranjos s/n, 46022 Valencia, Spain
13C MAS NMR has been performed in situ to investigate the reaction of toluene with methanol (MeOH)over H-ZSM-11 catalyst. The identification of the main reaction products and intermediates and the quantification of the NMR results provide new information concerning the nature of the methylating agents, the primary products of alkylation and the whole reaction network. During the initial stages of the reaction, methanol partially converts to dimethyl ether (DME).The reaction then proceeds via two parallel reaction pathways. The first pathway which is the major one includes toluene methylation with DME or MeOH to yield o- and p-xylenes in the statistical ratio of two to one. The primarily formed xylenes undergo isomerization and further alkylation to tri- and tetramethylbenzenes. The observation of diphenylmethane-like species as reaction intermediates suggests transalkylation of alkylaromatics. The second reaction pathway includes toluene alkylation with the fragments formed upon methanol conversion to hydrocarbons and thus leads to various alkyltoluenes. The latter, in turn, undergo dealkylation, fragmentation and transalkylation to yield mainly xylenes, ethyltoluenes and polymethylbenzenes.
1. INTRODUCTION Toluene alkylation with methanol over pentasil type catalysts has been thoroughly studied during the last decade [1-12]. Attention was focused mainly on the application of these catalysts in the selective production of p-xylene [1-7]. Mechanistic studies have been limited because the reaction pattern is affected significantly by diffusion [4], and the products observed at the outlet of convensional continuous flow reactors, do not reflect the real kinetic situation on the catalyst. Information on this situation can now be obtained by means of recently developed in situ spectroscopic techniques [8-12]. This study aims to clarify the mechanism of toluene alkylation with methanol using in situ 13C M.AS NMR technique. The attention is focused on the following main aspects: 1) identification of the reaction pathways operating under the NMR/batch conditions; 2) identification of the reaction intermediates which may play the role of alkylating agents; 3) identification of the primary and secondary reaction products.
# On leave from Moscow State University,Russia
28 2. EXPERIMENTAL 2.1. Materials The study was performed on H-ZSM-11 catalyst with Si/AI ratio equal to 23. The catalyst was characterized using XRD, electron microscopy, FTIR, solid state 27A1 and 29Si MAS NMR, sorption of n-hexane and NH3 TPD. Methanol 13C (99.9% - enriched) used as labelled reactant was obtained form ICON Services Iiac. Natural abundance 13C toluene and dimethyl ether were obtained from Aldrich Chemical Company and Linde Technische gase, respectively.
2.2. Controlled-atmosphere 13CMAS NMR measurements Controlled atmosphere NMR experiments were performed in sealed glass NMR cells containing a catalyst and an adsorbate, and fitting precisely into MAS rotors. In a typical experiment, the powdered catalyst was packed into the NMR tube (Wilmad, with constriction), and evacuated to a final pressure of 10 -5 Tort after heating for 10 h at 573 K. Thereafter, the catalyst was cooled down to 298 K and the reactants were subsequently adsorbed. Toluene and methanol were dosed gravimetrically, whereas dimethyl ether was dosed volumetrically. Quantitative adsorption was ensured by cooling the sample to 77 K. After loading the reactants, the NMR cells maintained at 77 K to avoid local overheating were carefully sealed to achieve proper balance and high spinning rates in the MAS NMR probe. 13C MAS NMR measurements were carried out on a Varian VXR-400S WB spectrometer operating at 100.6 MHz. Spinning rates were within 4.5 - 5 kHz. Quantitative conditions were achieved using high-power gated proton-decoupling with suppressed NOE effect (3 ~ts 90 ~ pulse, recycling delay 6 s). Some non-lH-decoupled spectra were recorded and the observed multiplet patterns were analyzed to identify reaction products and intermediates. The assignments of some signals were confirmed by the direct adsorption of model compounds on the same catalyst in a separate set of experiments (e.g. in case of xylene isomers). Variable-temperature (298 - 387 K) and cross-polarization (contact time 5 ms) experiments were performed to distinguish between the species with different mobilities. The reaction was carried out by heating the sealed NMR cells outside of the spectrometer in the temperature range of 373 - 673 K. The 13C MAS NMR measurements were performed at lower temperatures (298 or 387 K) after quenching of the sample cells. After collection of the NMR data, the NMR cells were returned to reaction conditions. Two different reaction protocols were used. In the first one, the temperature was increased in a stepwise manner from 298 to 673 K, the NMR spectra being recorded after heating for 10 min at each temperature step. In the other experiments, the sample was rapidly heated to a final temperature (433 K) and maintained at this temperature for various periods of time (2 - 1400 min). 13C NMR spectra were recorded over the time course of the reaction. The first experimental mode showed more clearly a sequence of products, while the second one facilitated a comparison of reaction kinetics. The list of experiments is reported in Table 1. Quantification of the NMR results requires a number of definitions which are listed below: Conversion of methanol 13C at time t (Xt)was determined as, Xt = (1 - Im,t/Im,0)'100 [%], where Im,t corresponds to the integral intensity of the NMR line assigned to methanol in the NMR spectrum observed after heating for t min, Ir,0 corresponds to the integral intensity of methanol resonance in the initial spectrum. Yield of reaction product p at time t (Yp,t) was determined as methanol 13C conversion into this product,
29
Yp,t = (Ip,t/Im,0)" 100 [%], where Ip,t corresponds to the integral intensity of the product p resonance in the NMR spectrum observed after heating for t min. Table 1 List of experiments Experi- Catalyst
Reactants (molec./u.c.) Toluene MeOH13C
Reaction protocol
DME
Tspectra
ment
(g)
registr. (K)
A
0.02
20
13
-
stepwise temp. rise: 298-673 K
298
B
0.03
14
-
7
stepwise temp. rise 298-483 K
298
C
0.03
17
9
-
progressive increase of reaction
298
D
0.02
17
9
-
progressive increase of reaction
time; Treact.=433 K 387
time; Treact.=433 K
3. RESULTS AND DISCUSSION 3.1. Identification of the reaction products and intermediates. Reaction network 13CMAS NMR spectrum observed after adsorption of reactants over H - Z S M - 1 1 at 293 K (Fig. 1, Sample A) shows 5 narrow resonance lines at ca. 21, 125.5, 128.4, 129.2 and 137.8 ppm corresponding to unlabelled toluene and a broad signal at ca. 51 ppm corresponding to methanol 13C [13]. After heating the NMR cell to 373 K, methanol starts to convert to DME, evidenced by the appearance of the broad NMR line at ca. 60.3 ppm, as was also observed for H - Z S M - 5 zeolites when methanol was the only reactant [14, 15]. The line corresponding to DME reaches its maximum intensity at 413 K and then remains unchanged until alkylation begins. Alkylation starts at 433 K. The primary products are o-xylene and p-xylene confirmed by the resonance lines at 19.3 and 20.7 ppm, respectively. In the aromatic part of the spectra, formation of o-xylene is evidenced by the appearance of weak resonances at 126 and 129.8 ppm; The lines corresponding to p-xylene overlap with toluene resonances. Interesting, that observation of the primary products depends on the reaction protocol used (Table 1). This is discussed further in w 3.3. Further heating to 483 K results in the observation of new resonances at ca. 41.3, 30, 24 and 14 ppm. The latter three lines are related to conversion of methanol or DME to hydrocarbons [14, 15]. The narrow resonance at 24 ppm is assigned to i-butane, the major product of methanol conversion. The broad lines at ca. 30 and 14 ppm correspond to very rigid species as confirmed by experiments with cross-polarization, and may be attributed to overlapping species stemming from various longer chain hydrocarbons. The NMR line at 41.3 ppm can not be associated with the products of methanol conversion and is more likely due to diphenylmethane-like species [13] formed from toluene and/or xylenes. After heating to 523 K methanol consumption is terminated. The lines corresponding to long chain hydrocarbons disappear, indicating their cracking. As a result, weak resonances corresponding to alkyltoluenes are observed, pointing on toluene alkylation with the fragments formed upon cracking [16]. The most intensive resonances are attributed to sec-
30 butyltoluenes (42.1, 31.6, 22.2, 12.3 ppm), n-propyltoluenes (38.3, 24.6, 14 ppm), isopropyltoluenes (34.3, 24.6 ppm) and ethyltoluenes (29, 16 ppm) [13]. Formation of alkyltoluenes is accompanied by growing of the resonances corresponding to i-butane (24 ppm) and propane (16-17 ppm).
Sample A
~.
,~
673
K
523
K. . . . . . . . .
433
K
/
21.2
" " '-
"-
" -'- -- , 4
\
S
___-/AX..__./x~
. . . . . . . . . . .
. . . . .
,,
373 .
'
140
'
"
"
l
130
'
'
"
"
I
120
.
.
S I '
,, , ,
.
K .
.
]9
~
.....
....
/ ~\
_0_' .
I
"
'
'
'
I
60
'
'
'
"
I
"
'
'
'
I
40
'
'
'
'
1
'
'
'
'
I
20
'
'
'
"
9
ppm
Figure 1. 13C MAS NMR spectra observed before and after reaction of methanol 13C and toluene over H-ZSM-11 at progressively increasing temperatures. *- denotes spinning sidebands. Isomerization of xylenes begins in the temperature range of 523 - 573 K as evidenced by the appearance of the growing NMR lines at ca. 21.2, 126.4 and 130 ppm, corresponding to m-xylene [13]. At this reaction step, the lines corresponding to trimethylbenzenes and tetramethylbenzenes mainly 1,2,4-trimethylbenzene, and 1,2,4,5-tetramethylbenzene (growing shoulder at ca. 19 ppm [13]) also begin to appear. Further heating to 623 and 673 K leads to an equilibrium mixture of xylenes (p:o:m=23.7:52.8:23.5), formation of more polymethylbenzenes, and to fragmentation and dealkylation of butyl- and propyltoluenes to give methyl- and ethylsubstituted aromatics. Meanwhile the line corresponding to diphenylmethane disappears. The reaction network presented in Figure 2 rationalizes our experimental observations. At the initial stages of the reaction, methanol readily condenses to give DME and water. Both
31
methanol and DME may be responsible for further reaction proceeding via two parallel pathways: 1) toluene methylation and 2) conversion to hydrocarbons. The first reaction pathway leads to o- and p-xylenes as the primary products. At higher temperatures, o- and p-xylene isomerize to m-xylene, to give an equilibrium mixture of xylenes. The timing of the appearance and disappearance of small amounts of diphenylmethane like species is indicative of their intermediary role in isomerization. This gives further evidence that some xylene formation on zeolites may occur via a bimolecular transalkylation mechanism [17-19]. Xylenes in their turn undergo further alkylation or transalkylation to yield tri- and tetramethylbenzenes.
J
/
IT~ 1~73~K MeOHI ~--"" ( DME (o-,p-XylenesI "',~ .~ 473K...~ t_ ~-..... 523-673K IHydrocarbons ) I Diphenylmethanes)-~~ ~ 523-673K 1 I Tri-andtelramethylbenzenes 1 IEthyl-rpropyl-andbutyltoluenes '~ 673K .~ I Xylenes(eq.mixt.),ethyltoluenesandpoly'nelhylbenzenes I Figure 2. Reaction network.
The second route leads to small amounts of i-butane and some longer chain hydrocarbons, which upon cracking may yield lower alkanes and carbenium ion like species. Toluene acts as a trap for these species, as was observed for benzene in the course of the cracking of propylene oligomers [16]. As a result, butyl-, propyl-, and ethyltoluenes are formed. At higher temperatures, alkyltoluenes undergo dealkylation, fragmentation and transalkylation to yield mainly xylenes, ethyltoluenes and polymethylbenzenes. The final products observed at high temperatures (673 K) in our NMRfoatch experiment correspond to those observed in continuous-flow reactor tests [3], suggesting that the reaction proceeds in a similar way under both experimental conditions. The advantage of the in situ NMR techniques is the possibility to tailor specifically the reaction protocol which enables one to minimize superimposing of the parallel and consecutive reaction steps. As a result, the individual steps of the reaction can be followed in separate experiments and the reaction intermediates can be identified. The optimal conditions for the investigation of the individual reaction steps were determined to be 433-453 K for alkylation, 473-523 K for methanol conversion to hydrocarbons, 523-573 K for isomerization and hydrocabons cracking, and above 573 K for transalkylation dealkylation and fragmentation. The alkylation step will be considered in more detail.
32
3.2. A l k y l a t i n g agents
It is currently accepted that toluene ring-alkylation with methanol proceed via chemisorption of methanol on the acid sites, followed by formation of surface active species such as methoxy groups [21-23] or methoxonium ions [10, 11, 20], which can further react with weakly adsorbed toluene. On the other hand, it is known that DME is always present in abundance on the onset of xylenes formation [9, 10, 21] and there are indications that observation of DME can be associated with ring alkylation [8, 24]. Our results (Figs. 1,2) are also in favour of DME intermediary role in alkylation. Two other experiments were performed to check this hypothesis. In the first one, DME was adsorbed together with toluene over H - Z S M - 1 1 catalyst and the reaction was carried out as in experiment A (Table 1, Sample B). The aliphatic regions of the 13C MAS NMR spectra are presented in Fig. 3. The initial spectrum obtained after adsorption of reactants shows a broad NMR line at ca. 60.3 ppm and narrow line at ca. 21 ppm, assigned previously to DME and methyl group of toluene, respectively. Heating to 453 K results in a partial conversion of DME to p - and o-xylenes confirmed by the resonances at 20.7 and 19.3 ppm, respectively. No resonance line corresponding to MeOH is detected, possibly because the latter is rapidly converted back to DME at 453 K, as was observed in experiment A. It must also be noted that the NMR line corresponding to small amounts of MeOH can be broadened to beyond detection limits. After heating to 483 K DME consumption is terminated. The final products observed at this temperature are p-, o-xylenes and i-butane (24 ppm). This experiment shows clearly that DME can also serve as an alkylating agent under experimental conditions studied. 80 i-butane
xylenes .,,-,
453
,/~
60-
483 K
40
-
K toluene
DME
DME
9
Sample B
20 1:3..
298 K ,
9 '
.... ,.... ,.... ,,,35 ....,....,....,....,....,....,....,....,....,...., 65 55 25 21 ppm ,
b
I 20
~
I 40
'
I 60
'
I 80
'
10
Methanol conversion (%)
,
Figure 3. 13C MAS NMR spectra observed before and after reaction of DME and toluene over H - Z S M - 1 1 .
Figure 4. Variation of products yields as a function of methanol conversion.
To verify further whether DME is responsible for xylenes formation in course of toluene reaction with methanol, a kinetic study was performed. For that, sample C was prepared and the reaction was carried out at 433 K using the second reaction protocol (Table 1). The NMR data were then quantified, as defined in the experimental section, and product yields were plotted versus conversion to give selectivity plots [25], as shown in Fig. 4. The shapes of the selectivity patterns suggest that at low reaction temperature the major reaction pathway leading to xylenes include DME as an intermediate. On the other hand, a
33 small part of the xylene species is formed directly from methanol, as evidenced by non-zero initial selectivity to xylenes determined from the initial slope of the corresponding selectivity plot. Therefore the following reaction scheme operating in coarse of toluene alkylation is concluded: MeOH ~--- ~ DME ,~ To Iu e y "
Xylenes It should be noted, however, that contributions of these pathways to the overall xylene formation may vary with the experimental conditions (batch/flow, pressure, temperature, etc.). NMR results presented in this paper do not permit the identification of the surface active species, formed from DME and MeOH engaged in these reaction pathways and, therefore, hinder the suggestion of a more detailed reaction mechanism. The surface active species will be discussed in a further contribution [26].
3.3. Primary products of aikylation Basic principles for electrophilic substitution in the aromatic ring [27] and the extensive investigations of the electrophilic aromatic substitution in monoalkylbenzenes in homogeneous systems [28, 29] conclude to initial orto/para orientation of methyl substituent in course of toluene alkylation with methanol. When the reaction takes place on zeolite catalysts, isomerization and transalkylation reactions are usually superimposed on the primary alkylation, leading to a mixture of all xylene isomers [1-7]. In addition, product diffusion limitations within zeolite constrains may affect the ratio of xylene isomers [3, 4], preventing therefore the determination of the primary reaction products. The question of the primary products can be best answered by means of in situ techniques, which do not suffer from the above limitations. An in situ 13C MAS NMR specroscopy appears to be a suitable technique, as it allows one to distinguish unambiguously between carbon atoms of methyl groups in xylene isomers (Fig. 1). However, care should be taken while quantification of the corresponding NMR lines, as molecular dynamics of xylene isomers within zeolite pores can profoundly affect spectral features, leading to significant line broadening or even loss of intensity of one or several xylene lines. Thus, our additional experiments with the model mixtures of p- and o-xylenes adsorbed on H-ZSM-11 (not shown) indicate that a part of p-xylene molecules, corresponding to 6 molecules per u.c. of zeolite, may not be observed at room temperature, while at temperature higher than 373 K all the molecules become 'visible'. A detailed discussion of these phenomenon will be presented elsewhere [30]. For the determination of the initial ratio of the primarily formed xylene isomers, NMR experiments were performed at 387 K (Table 1, Sample D). The estimated ratio of o- and pxylenes was of two to one, indicating statistical ortho/para orientation. In conclusion, alkylation of toluene with methanol at low reaction temperatures was found to obey general concepts of electrophilic aromatic substitution [27].
4. CONCLUSIONS 1. The main reaction pathways identified in the course of methanol reaction with toluene over H-ZSM-11 under the NMRAgatch conditions are: 1) toluene methylation and 2)
34 methanol conversion to hydrocarbons followed by toluene alkylation with the fragments subsequently formed. 2 Both, methanol and dimethyl ether formed in abundance at the initial stages of reaction, may play the role of alkylating agents in toluene methylation. Under the experimental conditions studied, dimethyl ether is the main alkylating agent. 3. The primary products of toluene methylation are o- and p-xylenes, formed in the statistical ratio of two to one. ACKNOWLEDGMENTS The authors thank CICYT (project MAT 94-0359-C02-01) for financial support. I.I.I. Ivanova thanks ITQ for research postdoctoral position. REFERENCES .
2. 3. .
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.
N.Y. Chen, W.W. Kaeding and F.G. Dwyer, J. Am. Chem. Soc., 101 (1979) 6783. P.B. Weisz, Pure Appl. Chem., 52 (1980) 2091. W.W. Kaeding, C. Chu, L.B. Young, B. Weinstein and S.A. Butter, J. Catal., 67 (1981) 159. L.B. Young, S.A. Butter and W.W. Kaeding, J. Catal., 76 (1982) 418. J. Nunan, J. Cronin and J. Cunningham, J. Catal., 87 (1984) 77. J.-H. Kim, S. Namba and T. Yashima, Zeolites, 11 (1991) 59. K. Beschmann, L. Riekert and U. Muller, J. Catal., 145 (1994) 243. M.D. Sefcik, J. Am. Chem. Soc., 101 (1979) 2164. A. Philippou and M.W. Anderson, J. Am. Chem. Soc., 116 (1994) 5774. G. Mirth and J.A. Lercher, J. Phys. Chem., 95 (1991) 3736. G. Mirth and J.A. Lercher, J. Catal., 132 (1991) 244. G. Mirth and J.A. Lercher, J. Catal., 147 (1994) 199. E. Breitmaier and W. Volter, Carbon-13 NMR Spectroscopy, VCH Verlag, Weinheim, 1987. M.W. Anderson and J. Klinowski, J. Am. Chem. Soc., 112 (1990) 10. E.J. Munson, A.A. Kheir, N.D. Lazo and J.F. Haw, J. Phys. Chem., 96 (1992) 7740. I.I. Ivanova, D. Brunel, J.B. Nagy and E.G. Derouane, J. Mol. Catal., 95 (1995) 243. D.H. Olson and W.O. Haag, ACS Symposium Series, 248 (1984) 275. M. Guisnet and N.S. Gnep, A.S.I. Ser. E. Nato, 80 (1984) 571. A. Corma and E. Sastre, J. Catal., 129 (1991) 177. H. Vinek, M. Derewinski, G. Mirth and J.A. Lercher, Appl. Catal., 68 (1991) 277. T.R. Forester and R.F. Howe, J. Am. Chem. Soc., 109 (1987) 5076. J. Rakoczy and T. Romotowski, Zeolites, 13 (1993) 256. A. Corma, G. Sastre and P. Viruela, Stud. Surf. Sci. Catal., 84 (1994) 2171. E. Dumitriu, V. Hulea and M. Palamaru, Bul. Inst. Politch. Iasi., Sect. 2: Chim. Ing. Chim., 39 (1993) 127. A.N. Ko and B.W. Wojciechowski, Progr. React. Kinet., 12 (1983) 201. I.I. Ivanova and A. Corma, to be published. A. Schriesheim, in "Friedel-Crafts and Related Reactions", G.A. Olah (Ed.), Vol II, Interscience, New York, 1964. R.H. Allen and L.D. Yats, J. Amer. Chem. Soc., 83 (1961) 2799. M.S. Stock and H.C. Brown, Advan. Phys. Org. Chem., 1 (1963) 35. I.I. Ivanova, D. Brunel and A. Corma, to be published.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
35
S t r a t e g i e s for Zeolite S y n t h e s i s by D e s i g n M. E. Davis Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
The thermodynamics, kinetics and a proposed synthesis mechanism for the crystallization of a prototypical high-silica zeolite, namely ZSM-5, are presented. This information is used to develop strategies for the design of zeolite syntheses and a few of these strategies are outlined. 1. I N T R O D U C T I O N
Zeolites; can they be synthesized by design? Currently, the answer to this question is no [1-3]. However, great strides are being made to reach this lofty goal of zeolite synthesis by design and I outline some of the progress below. Several factors complicate the design of zeolite synthesis. First the molecular-level of understanding of the self-assembly processes occurring during zeolite crystallizations is unknown. Second, since analogies to covalent organic synthesis [4] can not be made, the large number of organic reaction mechanisms provides little help for zeolite synthesis. With covalent organic synthesis, it is possible to systematically construct a final product by using numerous irreversible reaction steps involving different reagents and chemistries with separation between steps. Such is not the case with zeolite synthesis. With zeolites, the separation of desired from unwanted products is almost always difficult. Thus, a zeolite synthesis medium must be designed to spontaneously self-assemble into the correct architecture and atomic ordering in a single step with high yield. In view of this daunting task, the notion that zeolite syntheses can be accomplished by design has been questioned by many. I will demonstrate that this is not the case for zeolite synthesis, and that strategies for zeolite synthesis by design are currently being developed. The routes by which zeolites are crystallized from an amorphous oxide are complex and involve numerous simultaneous and interdependent equilibria and condensation steps. For this reason and others, zeolite syntheses are not well understood except that there does not appear to be a universal mechanism describing all crystallizations [1]. In this paper, emphasis ~s placed on high or pure-silica zeolite syntheses mediated by organic molecules [1,2,5]. These types of syntheses are the most amenable to design as will be described below. A typical synthesis of this kind involves the addition of water-soluble organic species into aqueous alkaline suspensions of silica. The silica dissolves and these inorganic species interact via noncovalent bonding with the organic
36 molecules to ultimately form the final organic-inorganic composite structures. The crystallization is a kinetically controlled process [1,4]. As stated above, a strategy for the design of this type of synthetic process is much different than those used in creating materials with complete covalent bonding. Whitesides et al. [4] have outlined the differences in producing new organic entities by either covalent or noncovalent synthesis. Table 1 is modified after Whitesides et al. and also contains information relevant to zeolite synthesis. The key additional factors with zeolite synthesis in comparison to totally organic preparations are the interactions between the organic and inorganic species and the condensation chemistry of the inorganic oxide. Table 1 Comparison of covalent and noncovalent syntheses (modified from ref. 4) Covalent
Noncovalent (zeolite)
Constituent bond types in the assembly process
covalent
ionic, hydrophobic, hydrogen (between organic and inorganic)
Bond strengths (kcal/mol)
25-200
0.1-5 (between organic and inorganic)
Stability of bonds in product
kinetically stable
kinetically reversible (includes silica chemistry)
Number of interactions in the assembly steps
few
many
Contributions to AG
usually dominated by AH
AH and AS can be comparable
Importance of solvent effects
secondary
primary
Other characteristics
cooperative behavior important
In this paper, I first review the salient results t h a t lead to a basic u n d e r s t a n d i n g of how high-silica zeolites are synthesized. Using this information, I then outline strategies for the synthesis of high-silica zeolites by design.
37 2. T O W A R D S A M E C H A N I S T I C U N D E R S T A N D I N G ZEOLITE SYNTHESIS
OF H I G H - S I L I C A
In the p a s t few y e a r s t h e r e has been significant a d v a n c e m e n t in the u n d e r s t a n d i n g of how the crystallization process of pure silica ZSM-5 (from now on denoted Si-ZSM-5) occurs. Here, I will use this example to highlight the g e r m a n e mechanistic issues for the synthesis of high-silica zeolites in general. In the original report on the synthesis of Si-ZSM-5, a mechanism of assembly was proposed and involved t e t r a p r o p y l a m m o n i u m cations (from now on denoted TPA) preorganizing silicate species to form the zeolite channel intersections [6]. This work and others led to the use of the term "template" to describe the role of the organic species. As pointed out by Davis and Lobo [1], the specificity between organic guest and zeolite host is as yet not sufficient to invoke true templating in the sense t h a t this term is used in biological contexts. R a t h e r we suggest t h a t the organics can act as structure-directing agents, i.e., they can dictate the final outcome of a zeolite synthesis. Figure 1 illustrates a schematic of w h a t I believe is a reasonable proposal for the TPA-mediated synthesis of Si-ZSM-5. The relevant experimental findings t h a t support this mechanistic picture are discussed below.
o
N
N. ) ~
H,
%"
H
,,I 2(" H
2
~ H " ~ I'HI
H H
Si Si
1 s
OI ~O--'Si SiI~"3"s,'Si
,,':4" ,,].w" -'-
H ~O "H 9 SO ~H
hydrophobichydration / .
.
.
.
,
,
y
i!iii:~!'ilil. ~"" :"
,
,
I
',F
J
.
o" ~ hb" T' ~ FFI~I-II ' " " "i h yorop ,,,,; .o lc ,l~-~;,i,i , ' h drat]on ~ ~ ' ' , spheres com~
,
7o~, I
,u
,
,
nucleation
70~.
,
spe~:
i
I
9
f Figure 1. Schematic Diagram of Proposed Mechanism
t agg:eW~ahibo y
38
2.1. Thermodynamics Recently, H e l m k a m p and Davis [2] have calculated e s t i m a t e s of the t h e r m o d y n a m i c p a r a m e t e r s for t h e T P A + F - m e d i a t e d s y n t h e s i s of TPAF-Si-ZSM-5 using experimental AH and AS data for quartz, amorphous silica (glass) and Si-ZSM-5 and the e x p e r i m e n t a l l y m e a s u r e d change in enthalpy for reaction (2) from Patarin et al. [7]. The synthesis of TPAF-Si-ZSM5 from quartz can be considered as follows (T=298K): quartz --~ Si-ZSM-5 AGI=AHI-(TAS)I = (5.5)-(1.4)=4.1 kJ (mol SiO2)-1
(1)
Si-ZSM-5 + TPA+F-(aq) --+ TPAF-Si-ZSM-5 + H20 AG2=AH2-(TAS)2 = (-6.2)-(1.7)=-7.9 kJ (mol SiO2)-1
(2)
quartz + TPA+F-(aq) ----> TPAF-Si-ZSM-5 + H20 AGtot=-3.8 kJ (mol SiO2)-1 When using glass instead of quartz as the starting silica, the glass-to-Si-ZSM-5 reaction yields AG3=-3.0 k J (mol SiO2) -1 and a AGtot=-10.9 k J (mol SiO2)-l. Since it is not possible to transform quartz into Si-ZSM-5, the presence of the organic structure-directing agent renders this synthesis possible (Si-ZSM-5 has been prepared from TPA and quartz [8]). For reaction (2), the favorable enthalpic term (AH_<0) is related to the van der Waals interactions between the occluded TPA molecules (or, more precisely, the TPAF ion pairs) and the SiZSM-5 framework. Additionally, there is a smaller, favorable contribution to the reaction energetics from the entropic term (TAS>0) and reflects the release of ordered water molecules from the hydrophobic hydration spheres of the TPA molecules as they are encapsulated by silicate species. TPA in Si-ZSM-5 likely represents a nearly optimum case for the organic-inorganic van der Waals interactions in zeolite synthesis. When the enthalpic contribution to AG decreases, i.e., a "looser fit" of the organic guest in the zeolite host, it is possible t h a t the entropic factor arising from the release of ordered w a t e r from the hydrophobic h y d r a t i o n sphere of the organic species m a y play a more i m p o r t a n t role in driving the s y n t h e s i s process. The observation t h a t t e t r a e t h a n o l a m m o n i u m (approximately the same size as TPA but known to not have a hydrophobic hydration sphere due to the strong hydrogen bonding with water) does not produce a zeolite product at reaction conditions where TPA would quickly yield Si-ZSM-5 supports the above statements [9]. Zones and coworkers [10,11] as well as Davis and co-workers [9,12,13] have clearly demonstrated t h a t organic species that serve as structure-directing agents in the synthesis of high-silica zeolites are intermediate in hydrophobicity. That is, m o l e c u l e s t h a t are h y d r o p h i l i c do not elicit i n t e r a c t i o n s w i t h hydrophobically hydrated silica while those that are very hydrophobic tend to aggregate and phase separate in the aqueous synthesis medium [9]. One m u s t exercise c a u t i o n w h e n a t t e m p t i n g to e x t e n d the above thermodynamic analysis of zeolite synthesis involving F- to the more standard
39 ones t h a t use OH- and/or to the addition of heteroatoms (term used here to denote non-silicon t e t r a h e d r a l atoms) such as a l u m i n u m . Pure-silica syntheses with F- can result in TPAF ion pairs being occluded in the ZSM-5 voids w i t h o u t the f o r m a t i o n of siloxy groups. W h e n using O H - a s the mineralizer, only TPA § is e n c l a t h r a t e d and the charge balance is by either Si-O- or A1-O-Si g e n e r a t e d anionic sites [14]. Thus, in order to extend the a f o r e m e n t i o n e d t h e r m o d y n a m i c a n a l y s i s to zeolite s y n t h e s i s in general, coulombic interactions m u s t be accounted for as well. Recently, Koller et al. have provided a good working model for the structure of the Si-O- site in highsilica zeolites [14]. W h a t is now needed to generalize the t h e r m o d y n a m i c analysis given above are the interaction energies between TPA and Si-ZSM-5 and TPA-ZSM-5 (contains A1) analogous to t h a t m e a s u r e d by P a t a r i n et al. [7] for TPAF-Si-ZSM-5.
2.2. Proposed Si-ZSM-5 Synthesis Mechanism A reasonable model for the m e c h a n i s m of the TPA-mediated synthesis of Si-ZSM-5 is schematically illustrated in Figure 1. Initially, the hydrophobic h y d r a t i o n sphere formed around TPA is partially or completely replaced by silicate species when a sufficient amount of soluble silicate species is available [9,12,13]. Favorable van der Waals contacts between the alkyl groups of the organic species and the hydrophobic silicate species likely provide the enthalpic driving force while the release of ordered w a t e r to the bulk aqueous phase provides an additional entropic driving force for the process [2]. It is through t h e s e o r g a n i c - i n o r g a n i c i n t e r a c t i o n s t h a t the geometric c o r r e s p o n d e n c e between the structure-directing agent and the zeolite pore architecture t h a t is the h a l l m a r k of s t r u c t u r e direction arises [1,2,8-13,15,16]. These composite species have been identified by 1H-29Si CP NMR between the protons on the TPA and the encapsulating silicates in an otherwise d e u t e r a t e d synthesis medium [12]. Additionally, silica e n c l a t h r a t e d TPA has been t r a p p e d by silylation techniques [9], and small-angle n e u t r o n s c a t t e r i n g results suggesting the p r o m p t incorporation of TPA molecules into amorphous silicate s t r u c t u r e s when TPA and soluble silicate species are mixed together [17] are consistent with this postulated first step. The availability of soluble silicates influences the r a t e at which these composite species are formed [15]. The use of a monomeric silica source such as t e t r a e t h y l o r t h o s i l i c a t e or the presence of small amounts of alkali-metal cations in the synthesis mixture t h a t facilitate the dissolution of condensed silica sources, e.g., fumed silica, colloidal silica, precipitated silica, leads to an enhanced rate of nucleation [9]. After the formation of the silica enclathrated TPA species, these composites combine to form entities of size ~ 50-70 A. These units have been identified by in situ SAXS [17,18] and cryo-TEM [19] m e a s u r e m e n t s . Additionally, units revealing the ultimate s t r u c t u r e of the final zeolite but in the size range of 80-100 A have been observed by TEM for other zeolites [20,21]. Thus, these data suggest t h a t the nucleation centers for zeolite synthesis should be smaller t h a n 100 A in size. For Si-ZSM-5, I propose t h a t the 50-70 A entities observed by various techniques are in fact the nucleation sites. These sites may form in solution (homogeneous nucleation) and/or on the reactor wall and/or on the surface of inorganic oxide particles t h a t are in the s y n t h e s i s m e d i u m ( h e t e r o g e n e o u s n u c l e a t i o n ) d e p e n d i n g upon the s t a r t i n g r e a g e n t s and synthesis conditions. Additionally, Dokter et al. suggest from in situ SAXS
40 experiments that there is an intermediate step in the formation of the 50-70 ,~ units that involves the condensation of the TPA-silicate species into aggregates by a reaction-limited cluster-cluster mechanism leading to 70 A entities with mass fractal characteristics [18]. This step makes good physical sense and is consistent with other reaction limited growth processes and nucleation activation energies not being related to diffusion but r a t h e r chemical interactions of silicate species (-90-100 kJ/mole) [15]. After the formation of the mass fractal aggregates, there is a re-organization leading to densification of the -- 70 A entities (presumably to minimize surface energies). Both the aggregation and densification processes will involve silicate bond breaking and making steps mediated by the mineralizing agent and/or alkali-metal ions. It is interesting to note t h a t for crystal nucleation from aqueous electrolyte solutions, critical nuclei sizes have been measured, are normally 2-3 unit cells and are specified as microcrsstals with the same unit cells as the final bulk crystals [22]. Thus, a 50-70 A entity of ZSM-5 would be 2-3 unit cells and this size range correlates well with other critical nuclei sizes. Dokter et al. [18] and Regev et al. [19] both suggest that after the formation of the 50-70 ,~ species, ZSM-5 crystal formation occurs via the aggregation of the 50-70 /~ entities. Alternatively, we have proposed that the surfaces of the 50-70 A nuclei serve to "template" the crystal growth process by using the enclathrated TPA species as building units. Thus, the growth occurs in a layer-by-layer fashion [12,13]. Although aggregation of the 50-70 A entities into larger structures has been suggested from SAXS data, this crystal growth mechanism can not account for the layer-by-layer growth that is necessary for the formation of ZSM-5/ZSM-11 intergrowths [23] and could only occur with homogeneous nucleation. Thus, we suggest that crystal growth can occur in a layer-by-layer fashion [12,13]. This proposal for crystal growth may be more generally applicable to zeolite synthesis since layer-by-layer crystal growth is also implied by the intergrowth structures of zeolite beta [24], SSZ-26/SSZ33/CIT-1 [25-28], FAU/EMT [29-30], OFF/ERI [31], MAZ/MOR [31] and other non-zeolitic, crystalline oxides [32]. In fact, we have shown with two distinctly different zeolite systems, i.e., FAU/EMT [30] and SSZ-33/CIT-1 [27,28], that the control of the layer stacking sequence can be performed in a systematic and designed fashion t h r o u g h the purposeful m a n i p u l a t i o n of the organic structure-directing agents. If a layer-by-layer growth mechanism is feasible, then two questions immediately arise: (i) is the layer-by-layer growth truly single layer additions or are the building entities larger? and (ii) do the composite organic-inorganic species function as the building entities or do organic molecules adsorb onto the growing crystal surface from solution and then organize inorganic building units? To answer the first question, the upper bound on the number of layers possible in the growth step will be the minimum size of the observed stacking units in the crystal. Recently, Pan and co-workers have shown via high-resolution TEM images that the minimum size of stacking units can in fact be one layer for the n a t u r a l zeolite tschernichite (analogue of zeolite beta) [33]. Thus, single layer building units are possible. The answer to the second question is less clear. Vaughan has proposed that inorganic cations, e.g., Na § K § play a role in determining the s t r u c t u r e of the inorganic building units while the organics function to influence the orientation in which these structures assemble to create the longrange order [20]. For aluminosilicate materials like FAU/EMT where Si/A1
41 3.5-5.0, this type of mechanism appears plausible [29]. Recently, Stevens and Cox postulated a similar growth mechanism for zeolite beta by proposing that the organic molecules adsorb onto the growing crystal surface [34]. Since two equi-energetic orientations were possible, and since polymorphs A and B are postulated to have the same lattice energy [35], a high degree of faulting was predicted [34] (as is experimentally observed [24,33]). For high- and/or puresilica compositions it is hard to imagine otherwise that at least a portion of the units necessary for crystal growth would have to be silicate enclathrated organic species. These species are necessary for the formation of nuclei and it is most likely that they also participate in the growth process. Twomey et al. have shown that when Si-ZSM-5 is removed from its crystallization medium, nucleation and growth can be restarted [15]. These authors suggest that organic-inorganic entities are continually being produced and upon formation have to choose between nucleating new crystals or incorporating into those that already exist and are growing. From the experimentally measured crystal size distributions, Twomey et al. suggest that incorporation and growth are favored under typical conditions [15]. These results are consistent with silicate enclathrated organic species acting as building entities for crystal growth. However, these building units may be used in combination with "free" organic molecules that adsorb on the crystal surfaces and also participate in concert in the growth process. Evidence to support this conjecture is that ZSM-5 can be synthesized with occluded tetramethylammonium cations (TMA has been shown to not be enclathrated by silicate species [12 ]) if the synthesis mixture contains a sufficient amount of TPA (in addition to TMA) to induce nucleation and sustain crystal growth (Si/TPA=17; TPA/TMA>l)[36]. Thus, if this mechanism is close to the true molecular level assembly process then structure-direction appears to critically influence the ultimate fate of the crystallization at the nucleation stage. 2.3. K i n e t i c s
Since the kinetics of nucleation are typically slower than the kinetics of crystal growth in high- and/or pure-silica syntheses, the kinetics of nucleation play a significant role in determining the phase produced. Goepper et al. have shown that alkali-metal cations influence the rate of nucleation and crystal growth of pure-silica zeolites [37]. Additionally, Burkett and Davis have observed an isotope effect in pure-silica syntheses in the presence and absence of alkali-metal cations [9]. Pure-silica zeolites crystallize faster in H20 than D20 at low alkali-metal ion concentrations. This result supports the premise that O - - H (O--D) bond breaking is also important to zeolite crystallizations. With higher alkali-metal ion concentrations and/or the use of monomeric silica sources, the crystallization proceeds rapidly and the isotope effect becomes less obvious [9]. If the alkali-metal ion concentration is large, these cations compete with the organic species for interactions with the silicate species and ultimately form layered products [38]. This behavior can be rationalized using the bond valence approach of Brown [39]. That is, the best valence match with sodium ions is silicate ions that give layered structures rather than tectosilicates [39]. Since the nucleation step is typically the rate limiting process in highand/or pure-silica zeolite syntheses, the Ostwald ripening model would predict that increasing crystallization rates should be observed with increasing
42 stability of the crystallization material [8]. Recently, Harris and co-workers have shown that rate of crystallization (rate determining step is nucleation in this case) of nonasil does in fact increase as the calculated stabilization energy increases as predicted by the Ostwald ripening model [40]. It is suggested that as the organic molecular size more closely matches the nonasil cage size that there is an increase in van der Waals interactions t h a t lead to the energetic stabilization [40] (recall thermodynamic a r g u m e n t s described previously for TPAF-Si-ZSM-5). The one exception to the trend reported by Harris and coworkers was t h a t the largest organic molecule used (should have had the tightest fit according to the model calculations) had a slow crystallization rate. F u r t h e r work is necessary in order to u n d e r s t a n d this single anomalous experiment. 3. STRATEGIES F O R SYNTHESIS BY D E S I G N
If the thermodynamic, kinetic and mechanistic descriptions of high-silica syntheses of zeolites described above are phenomenologically correct, then this information can be used to develop strategies for zeolite synthesis by design. Two aspects of the design process are described below: (i) the design of pore architectures and (ii) the design of framework atom positioning. In order to develop a zeolite pore architecture by design, the proper choice of organic structure-directing agent and inorganic composition m u s t be made. The organic molecule m u s t have intermediate hydrophobicity and be able to organize w a t e r molecules as has been described by Zones, Davis and coworkers [9-13]. Next, the size and rigidity of the molecule m u s t be controlled. It is clear t h a t as the size of the organic molecule is increased, the specificity for producing zeolite pore architectures also increases [10]. Very recently, Zones has shown t h a t there is a good correlation between structure direction and the rigidity of the organic structure-directing agent as m e a s u r e d by the number of tertiary and quaternary connectivities in the molecule for a series of organics with C/N§ [11]. Thus, size, rigidity and hydrophobicity s i m u l t a n e o u s l y m u s t be correctly specified in order to p r e p a r e a good structure-directing agent by design. For the inorganic compositions, it is clear t h a t for pure-silica preparations a small a m o u n t of alkali-metal cations is necessary for practical kinetics [37]; however, large a m o u n t s will lead to layered compounds [2,38,39]. Since the Si-O-Si angle can vary greatly with little difference in energy [41], it is not surprising t h a t the enthalpies of formation for numerous pure-silica zeolites are all within 2RT (twice the thermal energy) at typical synthesis conditions [42]. Thus, numerous pure-silica structures are energetically feasible and the choice of the product formed is made by the kinetic p a t h w a y specified by the organic s t r u c t u r e - d i r e c t i n g agent. If heteroatoms, e.g., A1, B, Zn, are added into the inorganic mixture, they can strongly influence the s t r u c t u r e s produced even in a m o u n t s as small as SIO2/M203 = 50 [4,10]. We ascribe this behavior at least in part to the loss of flexibility in the Si-O-M angle as compared to the Si-O-Si bonding [4,10,11]. The loss of energetically feasible angles with the addition of heteroatoms creates a reduction in the possible atomic a r r a n g e m e n t s when compared to pure-silica structures. One fairly clear consequence of heteroatom addition is the shift from u n i d i m e n s i o n a l pore s y s t e m s (highly p r o b a b l e w i t h p u r e - s i l i c a
43 compositions) to multidimensional pore architectures [10]. A stellar example of the design of a zeolite pore architectural is by Zones in the synthesis of SSZ-26 [43]. Zones et al. prepared a polycyclic, rigid, diquaternary structure-directing agent to synthesize a multidimensional, large pore zeolite. Using this structure-directing agent and a low-alkali-metal cation c o n t a i n i n g a l u m i n o s i l i c a t e r e a c t i o n composition, a new multidimensional large-pore zeolite was synthesized [25,26,43]. This is the first example of a zeolite with a new pore architecture that was crystallized by the purposeful design of the structure-directing agent. For certain applications of zeolites, e.g., catalysis, it could be important to be able to place particular atoms into specific framework positions by design. Based on the aforementioned mechanism of high-silica zeolite crystallization, Li et al. synthesized ZSM-5 and ZSM-5/ll intergrowths using an organic structure-directing agent that contained a covalently attached silicon atom in order to test whether the silicon atom linked to the organic could be incorporated into a framework position [44]. Li et al. successfully implanted the silicon atom into a framework position as verified by solid-state NMR experiments. Thus, a strategy for designing atomic arrangements is now available in the attachment of target atoms onto specified positions of the organic structure-directing agent [44]. Currently, a completely designed synthesis of a zeolite has not been accomplished. However, the future is bright in this area. The design of structure-directing agents and synthesis compositions is occurring. A more complete design in the sense of the entire framework structure and atom positionings will require f u r t h e r molecular-level insights into the crystallization mechanism. RE~~CES 1. 2. 3. 4. 5. 6. 7. 8. ,
10. 11. 12. 13.
R.F. Lobo and M.E. Davis, Chem. Mater., 4 (1992) 756. M.M. Helmkamp and M.E. Davis, Annu. Rev. Mater. Sci., 25 (1995) 161. M.E. Davis, CHEMTECH, Sept. (1994) 22. G.M. Whitesides, E.E. Simanek, J.P. Mathias, C.T. Seta, D.N. Chin, M. Mammen and D.M. Gordon, Acc. Chem. Res., 28 (1995) 37. B.M. Lok, T.R. Cannan and C.A. Messina, Zeolites, 3 (1983) 282. E.M. Flanigen, J.M. Bennett, R.W. Grose, J.P. Cohen, R.L. Patton, R.M. Kirchner and J.V. Smith, Nature, 271 (1978) 512. J. Patarin, H. Kessler, M. Soulard and J.L. Guth, ACS Symp. Ser., 398 (1989) 221. R.A. van Santen, J. Keijsper, G. Ooms and A.G.T.G. Kortbeek, Stud. Sur. Sci. Catal., 28 (1986) 169. S.L. Burkett and M.E. Davis, Chem. Mater., submitted. R.F. Lobo, S.I. Zones and M.E. Davis, J. Incl. Phenom., in press. S.I. Zones and M.E. Davis, in Synth. Microporous Materials: Zeolites, Clays, Nanocomposites, (eds.) M. Occelli and H. Kessler, Marcel Dekker, NY, in press. S.L. Burkett and M.E. Davis, J. Phys. Chem., 98 (1994) 4647. S.L. Burkett and M.E. Davis, Chem. Mater., in press.
44 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. 39. 40. 41. 42. 43. 44.
84
H. Koller, R.F. Lobo, S.L. Burkett and M.E. Davis, J. P h y s . Chem., submitted. T.A.M. Twomey, M. Mackay, H.P.C.E. Kuipers and R.W. Thompson, Zeolites, 14 (1994) 162. H. Gies and B. Marler, Zeolites, 12 (1992) 42. L.E. Iton, F. Trouw, T.O. Brun, J.E. Epperson, J.W. White and S.J. Henderson, Langmuir, 8 (1992) 1045. W.H. Dokter, H.F. Van Garderen, T.P.M. Beelen, R.A. van Santen and W. Bras, Angew. Chem. Int. Ed. Engl. 34 (1995) 73. O. Regev, Y. Cohen, E. Kahet and Y. Talmon, Zeolites, 14 (1994) 314. D.E.W. Vaughan, Stud. Sur. Sci. Catal., 65 (1991) 275. M. Tsapatsis, T. Okubo, M. Lovallo, and M.E. Davis, MRS Symp. Ser., in press. I.N. Tang and H.R. Munkelwitz, J. Coll. Inter. Sci., 98 (1984) 430. G.R. Millward, S. Ramdas, J.M. Thomas and M.T. Barlow, J. Chem. Soc., Faraday Trans., 79 (1983) 1075. M.M.J. Treacy and J.M. Newsam, Nature, 332 (1988) 249. R.F. Lobo, M. Pan, I. Chan, H.X. Li, R.C. Medrud, S.I. Zones, P.A. Crozier and M.E. Davis, Science, 262 (1993) 1543. R.F. Lobo, M. Pan, I. Chan, R.C. Medrud, S.I. Zones, P.A. Crozier and M.E. Davis, J. Phys. Chem., 98 (1994) 12040. R.F. Lobo, S.I. Zones and M.E. Davis, Stud. Sur. Sci. Catal., 84 (1994) 461. R.F. Lobo and M.E. Davis, J. Am. Chem. Soc., 117 (1995) 3766. S.L. Burkett and M.E. Davis, Microporous Mater., 1 (1993) 265. J.P. Arhancet and M:E. Davis, Chem. Mater., 3 (1991) 567. D.E.W. Vaughan in Multifunctional Mesoporous Inorganic Solids, (eds.) C.A.C. Sequeira, M.J. Hudson, Elsevier, Amsterdam (1993) 137. C.N.R. Rao and J.M. Thomas, Acc. Chem. Res., 18 (1985) 113. R. Szostak, M. Pan and K.P. Lillerud, J. Phys. Chem., 99 (1995) 2104. A.P. Stevens and P.A. Cox, J. Chem. Soc., Chem. Commun. (1995) 343. S.M. Tomlinson, R.A. Jackson and C.R.A. Catlow, J. Chem. Soc., Chem. Commun. (1990) 813. J.A. Rossin and M.E. Davis, Ind. J. Technol., 25 (1987) 621. M. Goepper, H.X. Li and M.E. Davis, J. Chem. Soc., Chem. Commun. (1992) 1665. S.I. Zones, Microporous Mater., 2 (1994) 281. I.D. Brown, in Structure and Bonding in Crystals, (eds.) M. O'Keefe and A. Navrotsky, Academic Press, NY (1981) 1. T.V. Harris and S.I. Zones, Stud. Sur. Sci. Catal., 84 (1994) 29. M.D. Newton and G.V. Gibbs, Phys. Chem. Miner., 6 (1980) 305. I. Petrovic, A. Navrotsky, M.E. Davis and S.I. Zones, Chem. Mater., 5 (1993) 1805. S.I. Zones, M.N. Olmstead and D.S. Santilli, J. Am. Chem. Soc., 164 (1992) 4195. H.X. Li, M.A. Camblor and M.E. Davis, Microporous Mater., 3 (1994) 117.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
45
Use of Modified Zeolites as Reagents Influencing Nucleation in Zeolite Synthesis s. I. Zones and Y. Nakagawa Chevron Research and Technology Company Richmond, California 94802-0627 USA 1. INTRODUCTION New molecular sieve structures continue to be generated and the majority of these reactions rely on the participation of an organic component which is often found within the crystallized inorganic host lattice. There continues to be much debate regarding the role of various types of organic components in the crystallization process. Are there structure-directing roles that these organics fulfill in such areas as forming nuclei, redistributing solubilized silicate populations, aiding in crystal growing, or a number of other critical functions during the synthesis which are difficult to measure experimentally [ 1]? One of the more important results to impact this area of research has been the finding of Navrotsky and co-workers that the novel silicate zeolite structures formed in the presence of organic components do not differ much from each other in enthalpy of formation [2,3]. The enthalpy values are also not far removed from either very dense phases or amorphous silicates. A model of a guest/host complex that resides in an energy well as compared to a non-ordered gel state, while attractive in explaining the important role of the organics in zeolite synthesis, is not well supported by this calorimetry research. In fact, an energy balanced calculation for the formation of TPA*F-/MFI from quartz suggests that the entropic contributions may be the most important factors to consider in the crystallization process [4]. The burden of explaining the effectiveness of the organic reagents then shifts to the issue of zeolite synthesis kinetics and nucleation events. One of our recent studies addressed nucleation selectivity as related to a "goodness-of-fit" for nuclei which resemble the final crystalline product [5,6]. Bell and Chang also developed a model of zeolite crystallization from clathrated organic guests [7]. In this study we wish to continue this discussion on nucleation rate and phase selectivities in the presence of organic structure-directing agents, but with the focus on using highly porous zeolites as inorganic reactants. We have demonstrated that FAU materials are interesting reagents for delivery of A1 to a synthesis gel [8], and similarly, boron-beta zeolite has proven to be an even more impressive reagent for crystallizing novel borosilicate sieves [9]. We have expanded upon this work by modifying some of these reagents and examining the effects of modification of these reagents.
2. EXPERIMENTAL Two types of zeolite synthesis reactions were studied. In the first type, reactions were performed using calcined boron-beta zeolite as borosilicate source [ 10], a quaternary ammonium hydroxide compound, sodium hydroxide, water, and additional boron from sodium borate
46 decahydrate. In the second type of reaction, Cabosil M5 or "N" silicate was mixed into a solution of the two bases mentioned above and aluminum was provided as FAU zeolite [ 11 ], which in most instances was modified by ion-exchange with a transition metal. Care was taken to monitor the pH of the exchange to avoid precipitation from the hydrolysis of the hydrated cation. Table 1 describes the reactant ratios and reaction conditions used in the synthesis studies. The quaternary ammonium compounds used in this study are illustrated in Table 2. The zeolite synthesis reactions were run in Teflon cups for 23 mL Parr stainless steel reactors, and were heated with or without tumbling in Blue M convection ovens [ 12]. Crystallized products were analyzed by X-ray diffraction on a Siemens D-500 instrument. Elemental analyses were determined by ICP methods at Galbraith Laboratories, Knoxville, Tennessee (U.S.A.). Table 1 Reaction Conditions for Molecular Sieve Synthesis Reaction Type Conditions
Borosilicate
High OH A1
Low OH A1
OH/SiO2
0.25
0.90
0.30
Na/Si
0.10
0.90
0.12
Organic/Si
0.15
0.08
0.18
A1/Si
0.00
0.07
0.07
B/Si
0.05-0.10
0.00
0.00
H20/Si
44
32
28
Temp., ~
150
135
160
0
43
43
2-4
2-8
6-9
rpm Time, Days
Table 2 Organocations Used in the Study
Structure
Code
.3)~
T06
+
(CH3)3N~ (CH3)~
T20 ~1CH313
F40
Structure
Code B100
~ 3 I§ CH 3 ~
~
Me 3
M46
~ ~ F~1CH313 B15
B08 N(CH3)3
47
3. RESULTS AND DISCUSSION 3.1. Studies with boron-beta zeolite In our previous study we demonstrated that calcined boron-beta zeolite had limited stability under hydrothermal conditions and as such, could be converted to other zeolites in the presence of certain organocations [9]. Use of this reagent led to rapid nucleation in synthesis reactions, reducing crystallization times to a matter of hours as compared to weeks for synthesis of some zeolites, such as SSZ-24. No amorphous phase intermediate is detected in these conversion reactions. In fact, if the boron-beta alone is heated long enough in water, thereby becoming mostly amorphous, when it is used as a synthesis reagent, the rate and phase selectivity features are lost [13]. The reaction selectivity can sometimes be changed in the presence of additional borate. This demonstrates an interesting synergy between dissolved borate or borosilicates and the high surface area borosilicate zeolite. Is the re-dissolution of the boron-beta an important step in the rapid nucleation? Since borate has high solubility in basic solution, the role of the extra borate added may be to shift the equilibrium toward species which are needed in the synthesis of zeolites such as SSZ33, which appear to require some amount of lattice substitution in order to form [ 13]. We know that the boron-beta zeolite must eventually break down as part of the reaction process because products such as SSZ-24, which contain only 4- or 6-ring subunits, are obtained. The starting boron-beta zeolite contains 5-rings as well, therefore we do not believe that the conversions are a result of "local" transformations involving existing subunits at the surface of the beta-zeolite. Table 3 shows the results of conversion experiments run with and without extra borate. The most spectacular result is the shift in product from SSZ-24 to SSZ-33 upon changing the borate concentration in the presence of organocation T20. SSZ-24 is a large-pore, one dimensional zeolite, whereas SSZ-33 has a multidimensional, 12-10 pore system. F40, which was used to discover SSZ-26 [15] will make SSZ-33 provided there is sufficient boron available in the conversion reaction. Use of boron-beta zeolite is critical in this conversion, as evidenced by the failure of F40 to form SSZ-33 if more conventional, soluble sources of silica and boron are used. The organocation used by Lobo and Davis to discover CIT-1 (B 15) can affect, to some degree, conversion of boron-beta zeolite to CIT- 1, the pure polymorph B of the mixture which constitutes SSZ-33 [16]. B08, the organocation which crystallizes SSZ-33 from soluble systems, does not convert boron-beta into the expected product. We were at first surprised by the lack of reactivity observed in this reaction. Table 3 Conversion of Boron-Beta to Other Organo-Zeolites Organocation
Product
Product (B4Q'/Added)
T06
SSZ-24
SSZ-13
T20
SSZ-24
SSZ-33
F40
SSZ-33
SSZ-33
B08
Beta
Beta
B 100
SSZ-35
SSZ-35
B 15
CIT- 1/Beta
M46
Beta
Beta
48 Recently we showed that the boron-beta product from the reaction with B08 is not strictly "unreacted" [ 13]. We found that the pore system of the product was filled with B08 and that this was true even if the reaction was carried out at 85~ Our interpretation is that the organocation rapidly fills the beta pore system before nucleation to other structures can begin. We believe that the availability of the zeolite reactant surface is an important part of this transformation synthesis. This is similar to our earlier observations that FAU is not a viable reactant in zeolite synthesis if the pore access is blocked [ 11 ]. In Table 2, the entry M46 is similar in size to the adamantyl or tricyclodecane templates, but M46 is structurally much more flexible. As such, it very effectively inhibits conversion of boron-beta to other phases. In fact, using a mixed template system where one component is M46, all conversions were inhibited with the exception of the M46/TPA system, where we did observe formation of ZSM-5. Since calcined boron-beta zeolite has such attractive properties (enhanced rates, novel products) as a reagent for molecular sieve synthesis, we wondered if there were any other zeolites which would also behave in this capacity. Since we believe that high surface area materials are desirable, multidimensional materials would be the first likely candidates. We therefore considered using borosilicates SSZ-33 and B-ZSM-5. Neither of these materials show any reactivity in comparable conversion reactions. Part of the problem may be due to the stability of these zeolites under hydrothermal conditions. Both calcined SSZ-33 and B-ZSM-5 were found to be stable to prolonged aqueous heating, and are extremely steam stable. This again supports the hypothesis that the boron-beta zeolite breaks down under the reaction conditions prior to recrystallization of the new phase. 3.2. Reactions with modified Y zeolite (FAU) A number of years ago Bourgogne, Guth, and Wey demonstrated that Y zeolite can be used as an aluminum source in reactions to crystallize ZSM-5 [ 17]. More recently, we showed that Y zeolite can be used to prepare novel materials such as Al-rich beta which is difficult to make by other routes [ 18]. Other novel materials become accessible by using a very high Si/A1 version ofY [ 19]. Might the selectivity be changed further in our zeolite synthesis reactions if we ion-exchanged the Na in the Y zeolite for metals which would not easily re-exchange back out in basic solution? To test this, we prepared a series of ion-exchanged modified Y zeolites which are shown in Table 4. We envisioned that the rate of delivery of A1 (as AI(OH)4 or A1-O-Si-O units) might be slowed due to the blockage caused at the site by the potentially insoluble hydrolyzed metal. Table 4 Ion-Exchanged FAU as Reagent in Zeolite Synthesis Metal
% Exchange for Na
Solution pH
Surface Area m2_/g
Fe III (b)
80
2.10
346
Cr Ill (b)
75
3.10
532
Co II (c)
64
7.35
674
Ni (b)
69
6.60
664
Ag (b)
65
6.80
Zn (c)
71
6.40
600
Cu II (b)
75
4.38
656
(a) Exchange carried out at 23~ for 16 hours with M+z/A1 set at 5 for z=3 and 7.5 for z=2. (b) Nitrate salt (c) Acetate salt
49 Figure 1 shows a kinetic profile of crystallization of SSZ-13 (CHA) from T06 using the exchanged Y zeolites. We have shown that for this "high OH" type reaction (see Table 1), the pH increase above the buffering capacity of silicate is a good measure of extent of crystallization [8,11 ]. In this reaction the adamantyl organocation is found inside the chabazite cages and typical product SiO2/A1203 values are 10-12. 13.00 -
(~
.-.--~ ............
~_____.
,,~......................
,|
/
/ / ... / / /
I
lZSO-
_-
,' .~.~-
t
' ----'~
.....
~> .....
@--
*
iI / 12.00
-
@
~
@
~
~
To Point
_ _ ' " ' " ~ - ~ " " " ' " ' . ". . .". . ~. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
"@
,
0
25
50
75
100 200
Hours at 135~
Figure 1. Conversion of metal-exchanged Y zeolites (see Table 4) to SSZ-13. Rates are determined by pH increase during crystallization. The hydrothermal crystallization uses organocation T06 from Table 2 and "high OH" conditions in Table 1. Typically, no crystallization begins before about 10 hours and pH values remain at T o, the value before heating. Three groups of kinetic response were observed. Relative to the control (Na-Y), Co-Y or Cu-Y reaction rates were just as fast, indicating no retardation of crystallization. A second group consisting of Ni-Y, Ag-Y, and Zn-Y showed a slower growth rate. Here the pH-profile suggests that nucleation is not retarded, but that the overall crystallization is slower. These metals may affect the availability of A1 in solution. It is unclear as to why the Ag reactions plateau at a lower final pH. Fe and Cr, two trivalent metals, when exchanged into Y zeolite, completely inhibit the reaction! No transformation is observed, even at 200 hours ofreaction time. This may in part be due to the reduced surface areas for these latter two reagents (Table 4). Fe-Y shows the largest decrease of the prepared materials, and the fact that it has also lost 60% of its available micropore volume suggests that Fe has precipitated in the pores. In Table 1, a set of aluminosilicate reactant conditions at "low OH" is described. These conditions were used successfully in the synthesis of SSZ-26 [20], and the products of this type of reaction typically have SiO2/A1203 of 20-30. The reactant SiO2/A1203value is 35 (using Y zeolite with a value of 5 as a reagent), which is still in a range where SSZ-13 can be made from the adamantyl-type cations. We have shown that as the SiO2/A1203 reactant values change using T06, a progression of SSZ-13 (highest [A1]), SSZ-23, and SSZ-24 (lowest [A1]) can be produced [21].
50
In the same study it was pointed out that the use of methylene blue dye appeared to suppress the crystallization of SSZ-13 and allowed us to crystallize pure aluminosilicate SSZ-23. SSZ-23 is a novel (unsolved) structure which we believe has a constrained 10-ring pore system with sizable cavities (to accommodate the adamantyl cations) and higher surface area than other known intermediate pore zeolites (near 500 m2/g). If Co-Y is used under the "low OH" conditions, a Co-containing SSZ-13 is crystallized. However, if Cr-Y is used, Cr-SSZ-23 is crystallized in a region which would normally produce SSZ- 13 ! Based on the results in Table 1, we would not have expected to see any conversion at all. Therefore, in both "high" and"low" OH-type experiments, Cr-Y appears to be an inhibitor for SSZ13 formation (although the mechanism of inhibition might not be the same in the two reaction types). Since we were curious about how much of an inhibitory effect the Cr-Y had in these types of reactions, we ran a series of reactions where varying amounts of both Cr-Y and Co-Y were used as a source of aluminum. The results are graphed in Figure 2 and three product regions emerge. 0.80
0.70
0.60
0.50
| ~" 0.40 0 (.) 0.30
0.20
SSZ-24 Region
|
0.10 , 00
0.10
gi~ I
0.20
0.30
0.40
Cr+31AI
Figure 2. Zeolite synthesis from Co and Cr-exchanged Y zeolite as A1 source and under "low OH" conditions (Table 1). The amount of Co and Cr are inversely adjusted in the experiments and the mixture of zeolite products recovered is plotted. Organocation used was T20 (Table 2). SSZ- 13 (CHA) forms only in the presence of very minor amounts of Cr-Y; chromium seems to be a very effective inhibitor of its crystallization. When Co-Y is the major contributor, surprisingly, SSZ-24 forms. In this instance, the combined Co-Y and Cr-Y do not participate in the reaction; the SSZ-24 is crystallizing from the silica, added alkali and organocation. The metalexchanged Y materials are uninvolved "spectators" and FAU is detected in the isolated solids. (In an analogous experiment, this same behavior was observed for template B08 producing SSZ-31 and unreacted metal-Y). As Cr-Y becomes the major A1 source, SSZ-23 progressively dominates over SSZ-24 as the crystallization product. However, this SSZ-23 zeolite product contains both the chromium and aluminum from the starting reagent.
51 In the "high OH" reaction Cr-Y gave no product. The Cr-Y inhibits SSZ- 13 formation which needs some A1 to nucleate and grow, but the OH/Si level in these experiments is too high to nucleate all-Si SSZ-23 [22]. Under the "low OH" conditions, the SSZ-13 inhibition remains but SSZ-23 crystallizes. The fascinating change here is that the nucleation of SSZ-23 is not likely to be an allSi event favorable at these OH/Si ratios because SSZ-24 would be the more likely crystallization product. Therefore, relatively low amounts of A1 must be available at this early stage and by the end of the reaction, all of the Cr-Y is consumed, (only SSZ-23 is seen in the XRD), and A1 is transferred into the new structure. Catalytic activity is seen for the (chromium) aluminosilicate product, indicating incorporation of A1 into framework positions. Chromium is found associated with the SSZ-23 which is also recovered as a green solid! We do not have evidence at this time indicating the location of the chromium in the product, and there is no reason to believe that it is a framework component or in cationic sites in the organo-zeolite. 4. CONCLUSIONS Highly porous zeolites such as FAU and boron-beta are useful reagents for delivering A1 and B to growing organosilicate lattices during synthesis. The high surface areas of these reagents allow for rapid interaction with the synthesis solution providing for attractive nucleation rates. The way in which components are delivered to the nucleation site (by an unknown mechanism) can also alter the phase selectivity in the experiment. By ion-exchanging the FAU with transition metals whose hydrolysis products have limited solubility in basic solutions, the synthesis selectivity is altered even further. 23 Cr-Y was found to be a good inhibitor for SSZ-13 crystallization in the presence of adamantyl derivatives. The A1 could, however, be eventually transported into a growing SSZ-23 structure. This approach in zeolite synthesis may hold further promise for generating novel structures, compositions, or controlled lattice substitution in zeolite synthesis products. ACKNOWLEDGMENTS
We thank Lun Teh Yuen and Greg S. Lee for synthesis of organocations and help in running the zeolite syntheses. Chevron Research is gratefully thanked for permission to present this work and for continued research support. REFERENCES
1. 2. 3. 4. 5.
6. 7. 8. 9.
M.E. Davis, Chemtech (Sept. 1994), 22. I. Petrovic, A. Navrotsky, S. I. Zones, and M. E. Davis, Chem. Mater. 5 (1993) 1805. N.J. Henson, A. K. Cheetham, and J. D. Gale, Chem. Mater. 6 (1994). M.M. Helmkamp and M. E. Davis, Ann. Rev. Mater. Sci. (1995 in press). T.V. Harris and S. I. Zones, "Zeolites and Related Microporous Materials: State of the Art 1994," Ed. J. Weitkamp, H. G. Karge, H. Pfeifer, W. Holderich, (Elsevier, Amsterdam) (1994) 29. M.E. Davis and S. I. Zones, Third International Symp. on Synthesis of Microporous and Layered Mater. Ed. M. L. Occelli, H. Kessler in press. C.D. Chang and A. T. Bell, Catal. Lett. 8 (1991) 305. S.I. Zones, J. Chem. Soc. Farad. Trans. 86 (1990) 3467. S.I.Zones, L. T. Yuen, Y.Nakagawa, R.A. VanNordstrand, andS.D. Toto, inProc. 9thlnt'l. Zeol. Conf., Ed. R. von Ballmoos, J. B. Higgins, M. M. Treacy (Butterworth-Heinemann, Stoneham, Massachusetts) (1993) 163.
52 10. S.I. Zones, L. T. Yuen, and S. D. Toto, U.S. Patent 5,187,132 (1993). 11. S.I. Zones, J. Chem. Soc. Farad. Trans. 87 (1991) 3709. 12. Y. Nakagawa and S. I. Zones, "Molecular Sieves, Synthesis of Microporous Materials, Vol. 1," Ed. M. L. Occelli, H. E. Robson (Van Nostrand-Reinhold, New York) (1992) 222. 13. S.I. Zones and Y. Nakagawa, Microporous Mater 2 (1994) 543. 14. R.F. Lobo, M. Pan, I. Y. Chan, R. C. Medrud, S. I. Zones, P. A. Crozier, and M. E. Davis, J. Phys. Chem. 98 (1994) 12040. 15. S.I. Zones, M. M. Olmstead, and D. S. Santilli, J. Amer. Chem. Soc. 114 (1992) 4195. 16. R.F. Lobo, S. I. Zones, and M. E. Davis, "Zeolites and RelatedMicroporous Materials: State of the Art 1994," Ed. J. Weitkamp, H. G. Karge, H. Pfeifer, W. Holderich (Elsevier, Amsterdam) (1994) 461. 17. M. Bourgogne, G. L. Guth, and R. Wey, U.S. Patent 4,503,024 (1985). 18. S.I. Zones and Y. Nakagawa, U.S. Patent 5,340,563 (1994). 19. S.I. Zones and Y. Nakagawa, U.S. Patent 5,225,179 (1993). 20. S.I. Zones and D. S. Santilli, Proc. 9th Int'l. Zeol. Conf., Ed. R. von Ballmoos, J. B. Higgins, M. M. Treacy (Butterworth-Heinemann, Stoneham, Massachusetts) (1993) 171. 21. S. I. Zones, R. A. VanNordstrand, D. S. Santilli, D. M. Wilson, L. T. Yuen, and L. D. Scampavia, "Zeolites: Facts, Figures, Future, "Ed. P. A. Jacobs, R. A. vanSanten (Elsevier, Amsterdam) (1989) 299. 22. S.I. Zones, European Patent 213,018 (1987). 23. We have found that the ion-exchange properties of boron-beta zeolite are not as straightforward and hope to report on these findings in the near future.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
53
Templating Studies Using 3,7-Diazabicyclo[3.3.1]nonane Derivatives: Discovery of New Large-Pore Zeolite SSZ-35 Y. Nakagawa Chevron Research and Technology Company Richmond, California 94802-0627 USA In our continuing studies of rigid, polycyclic organocations as structure-directing agents for zeolite synthesis, we have focused on the 3,7-diazabicyclo[3.3.1]nonane skeleton. Various derivatives of this family can be used as templates in zeolite synthesis reactions, leading to the formation of large-pore zeolites. For example, the N,N,N'-trimethyl-diazabicyclononane derivative, A, results in crystallization of the novel zeolite, SSZ-35, over a broad range of inorganic reaction conditions. We believe that SSZ-35 is a multi-dimensional zeolite possessing at least one 12-ring channel system. Larger derivatives of A resulted in the crystallization of unidimensional large-pore zeolite, SSZ-24. 1. INTRODUCTION Many of the recent discoveries of new high-silica zeolitic frameworks have been achieved through the use of novel organocationic templates as structure-directing agents. Large-pore, unidimensional zeolite SSZ-24 (AFI-type) was first discovered by Zones using trimethylammoniumsubstituted adamantanes. ~ This was followed by the discovery of large-pore multi-dimensional zeolite SSZ-26, which was made using a rationally designed propellane-based template. 2-4 ZSM18 was first discovered using an unusual tris-pyrrolidinium cation. 5,6 The success of this approach has encouraged us to search for new organic structure-directing agents which we hope will lead to the discovery of new zeolitic structures. We have reported on the use of Diels-Alder chemistry to efficiently prepare a series of structurally related polycyclic organic templates which were effective in crystallizing zeolites ZSM-12, SSZ-31, SSZ-33 and SSZ-37. 7,8 In this study, three large organic templating agents based on a 3,7-diazabicyclo[3.3.1 ]nonane skeleton were synthesized and their effectiveness as structure-directing agents were explored. One derivative in particular, Template A, has led to the discovery of a new multi-dimensional zeolite, S 5 Z - 3 5 . 9
2. EXPERIMENTAL Template Synthesis: Templates A and B were synthesized using the double-Mannich reactions described in references 9 through 11. Methylamine hydrochloride was heated in the presence of glacial acetic acid and paraformaldehyde, and the resultant mixture was added to the appropriately N-substituted 4-piperidone. After purification by extraction and fractional vacuum distillation, the intermediate 9-keto-3,7-diazabicyclononane was subjected to Wolff-Kishner reduction in a mixture of hydrazine, potassium hydroxide and triethylene glycol. The resulting 3,7-diazabicyclononane was treated with methyl iodide to afford the corresponding quaternary ammonium iodide salt, which was recrystallized prior to ion-exchange.
54 Sparteine-derived Template C was prepared by neutralization of commercially available (-)-sparteine sulfate pentahydrate (Aldrich) using aqueous NaOH, followed by extraction into dichloromethane. 12,13 The free diamine was quarternized in chloroform using methyl iodide, and the resulting mono-methylated salt was recrystallized from a mixture of acetone and diethyl ether with a small amount of added methanol. The iodide salts wer~ converted into the hydroxide form by ion-exchange using Bio-Rad AG lX8 (20-50 mesh, hydroxide form) in a minimum amount of water. The molarity of the resulting template solutions was determined by titration using phenolphthalein as an indicator.
Zeolite Synthesis: Zeolite synthesis reactions were performed in 23 mL Teflon cups for Parr 4745 reactors and were heated in Blue M ovens as previously described. 8 Reagents and ratios for the various synthesis reactions will be described in more detail in following sections. Products were analyzed by X-ray diffraction using a Siemens Model D500 diffractometer (using CuK ). SEM micrographs were taken on a Hitachi S-570 instrument. Elemental analyses of solids were performed at Galbraith Laboratories using ICP methods. C and N levels were determined on a Carlo Erba unit.
Template Structures: CH3
CH3~ N + ~ I
~N~ X-
CH 3
N~
CH3 CH3 ~ .N+ / CH 3
A
XB
CH3 CH3
N
X
C
3. RESULTS AND DISCUSSION Our previous studies using rigid, polycyclic organocations derived from Diels-Alder chemistry demonstrated their effectiveness in directing toward crystallization of interesting large-pore molecular sieves. 7,8 These studies also illustrated how other factors greatly affect the observed product selectivity: a given template may give rise to several different products depending on the starting gel composition. The three new templates discussed in this study were designed to capture similar space-filling characteristics of the successful Diels-Alder templates. We chose a large carbon skeleton to ensure that the organocations would not make clathrasil-type structures, and also incorporated two nitrogens in the template, thus keeping the C/N ratio within a reasonable range (< 8), in order to retain its water solubility.
3.1. SSZ-35 Zeolite Synthesis We were gratified to discover that the first template we synthesized, Template A, did in fact lead to the crystallization of a novel new phase, SSZ-35. SSZ-35 can be prepared under a variety of conditions to afford purely siliceous SSZ-35, as well as boro- and aluminosilicate compositions. This is in contrast with the tricyclo-undecene Template (D) from our Diels-Alder series which gave three different zeolites (SSZ-31, SSZ-33 and SSZ-37) from three different gel compositions) The selectivity for making SSZ-35 is quite high for Template A, and the framework structure of SSZ35 apparently tolerates the levels and types of substitution observed. Under more traditional high
55
aluminum and alkali hydroxide conditions (A1/Si = 0.06; NaOH/Si = 0.055), the frequently encountered SSZ- 13 (CHA) phase was obtained. Typical reaction conditions for the synthesis of SSZ-35 are given in Table 1. 9'14 Table 1 Reaction Conditions for Synthesis of SSZ-35 Using Template A
SiO2/W203 oo 50 33 100 50
W
A/Si
M+/Si
OH/Si
B B A1 A1
0.20 0.22 0.33 0.20 0.20
0.05 0.08 0.04 0.05 0.05
0.25 0.25 0.33 0.25 0.25
Temperature 160~ 160~ 160~ 170~ 170~
M=Na, K Silica sources include Cabosil M-5 fumed silica and Ludox AS-30 colloidal silica. Boron sources include Na2B4OT10H20 and boric acid; Reheis F2000 dried aluminum hydroxide gel was used as a source of aluminum. Typical H20/Si ratios were from 35-50. Reaction times ranged from 10-12 days, however, by seeding the reaction mixture with as little as 0.5 wt % SSZ-35 crystals, complete crystallization could be achieved in 5 days. A typical preparation of borosilicate SSZ-35 is described below: Template A (3.2 grams of a 0.70 M solution), 3.3 g water, and 0.45 mL of 1.0 N NaOH were added to a 23-mL Teflon cup of a Parr 4745 reactor. Sodium borate decahydrate (0.045 g) was added and the mixture was stirred until homogeneous. Ludox AS-30 (DuPont, 1.36 g) was added to give a starting gel with a SiOJB203 of 28. The reaction was heated at 160~ (static) for 12 days, after which the settled solid was collected and washed thoroughly. XRD indicated that the product was SSZ-35, and the product was found to have a SiO2]B203 of 53. Characterization The XRD patterns for both as-made and calcined all-silica SSZ-35 are shown in Figure 1. The material is stable to calcination in air to 595~ and the distinctive pattern is not reminiscent of any previously-reported zeolites, therefore we believe that SSZ-35 represents a new framework topology. Unit cell parameters and symmetry for calcined SSZ-35 were determined from synchrotron data: ~5
Symmetry:
triclinic _a = b = c =
13.9719(24) * 18.1747(28) 7.3734(16)
alpha beta gamma
= = =
90.87 ~ 98.93 ~ 90.55 ~
Scanning electron micrographs for all-silica and aluminosilicate SSZ-35 are shown in Figure 2. The difference in crystal size due to introduction of aluminum into the framework is readily apparent.
56
'
"
'
'
'
,
,
,
I
....
9
,
Calcined SSZ-35
rl r
I-- O ~
;~"
uJ t.z
III
-----.._~
o
~_~
7'.2
xb.4
tb.6
~.a
2b.o
TWO -
THETA
2w
2~.4
~.e
3~,.e
36.0
9(DEGREES)
As-made SSZ-35
,i Q.
z
uJ i-.
o
~
q
32B
t
TWO
-
THETA
{OEGREES)
Figure 1. Powder XRD Patterns of As-made and Calcined SSZ-35
360
57
~3CMASNMR spectra confirmed that Template A was still intact within the channels of SSZ35. Weight losses of 13-17 wt % were observed above 250~ using TGA on the as-made product. For one-dimensional large-pore zeolites such as SSZ-24 and ZSM-12, weight losses above 250~ are typically 10-11 wt %; therefore, we anticipated that there would be a large void volume in the SSZ-35 framework. The BET area for SSZ-35 was found to be -- 500 m2/g and adsorption data (shown in Table 2) are comparable to other multi-dimensional frameworks with different dimensionalities. H-SSZ-35 was shown to have a Constraint Index ~6 of < 0.8, indicative of the structure having at least one large-pore channel system. Table 2 Adsorption Data for SSZ-35 (in cc/g; P/P Q = 0.15) Zeolite
N2
n-hexane
cyclohexane
SSZ-35 Beta SSZ-24 ZSM-5 SSZ-37
0.20 0.26 0.12 0.14 0.18
0.15 0.24 0.10 0.14 0.16
0.09 0.25 0.12 0.04 0.14
a) All-Silica SSZ-35
Channel system
3D; 1D; 3D; 2D;
? 12R x 12R 12R 10R x 10R 10R x 12R
b) Aluminosilicate SSZ-35 (SiO2/A1203 = 65)
Figure 2. Scanning Electron Micrographs of All-Silica and Aluminosilicate SSZ-35
58
3.2 Related Templates While the N,N,N'-trimethyl-3,7-diazabicyclo[3.3.1]nonane Template A is very selective for new zeolite SSZ-35, the corresponding N'-isopropyl derivative, Template B, exhibits a high selectivity for the SSZ-24 framework (although in an all-silica reaction, both SSZ-31 and SSZ-24 are seen as products). The even larger methyl sparteine derivative (Template C) also strongly directs toward SSZ-24 crystallization. Representative results using this family of templates are shown in Table 3. Table 3 Results of Various Screening Reactions using 3,7-Diazabicyclononanes
Template A B C
Reaction Type Boron-Containingt
All-Silica SSZ-35 SSZ-31 +/or SSZ-24 SSZ-24
Aluminum-Containingw
SSZ-35 SSZ-24 SSZ-24
SSZ-35 SSZ-24 + Layered SSZ-24
t Starting SiO2/B203 = 33 - 50 (Cabosil/Na2B407 or H3BO3; B-beta zeolite) w Starting SiO2/A1203 > 200 (Cabosil/Reheis F200; Tosoh Y390 high silica FAU) The observation that these larger templates resulted in the formation of a one-dimensional zeolite was somewhat surprising at first. We originally thought that larger templates were better candidates for crystallizing multi-dimensional zeolites. In trying to understand why only Template D gave SSZ-37, while other closely related analogs do not, we have learned from modeling studies that it is extremely important to consider how the templates occupy the channel system. 7 In the case of SSZ-37, two template molecules can effectively fill the interconnecting 12-ring channel system. 17 If the template is any larger, two molecules no longer fit within the defined space, and one template alone cannot provide sufficient stabilization of the channel. Although we have not yet determined the structure of SSZ-35, it is possible that Templates B and C (and even only slightly larger N-Me,N-ethyl derivative E) fail to give SSZ-35 for the same reason. If the length ('T': see Figure 3) of a structure-directing agent is too great, one-dimensional zeolites are formed. We have observed this tendency with several of our"linear" templates where the observed product is ZSM12,18 and Casci has reported similar results using bis-trimethyl ammonium templating agents. ~9 In the latter case, the "default" products are medium-pore zeolites EU-2 (ZSM-48) and ZSM-23, whose channel systems are complementary to the width and height of the bisquat salts. ~ N ~ CH3
//_ H3C ~ CH3 O
I+ CH3 X-
CH3
E
59 1 v
Figure 3. Important Dimensions of Structure-Directing Agents The dimensions of the 3,7-diazabiocyclononane templates (calculated h -- 7.5 ]k; w -- 5.0/~k) 20 are too large to be accommodated in ZSM-12 channels (MTW pore dimensions: 6.2 x 5.5 x 5.2 A), but they fill the SSZ-24 pore system (d = 7.5 A) optimally. In fact, the selectivity for making SSZ24 using Templates B and C is so high that we are able to prepare B-SSZ-24 directly from Cabosil and soluble sources of boron (Na2B407 or H3BO3),rather than having to start with calcined B-beta as described in Zones' earlier work using trimethylammonium adamantane derivatives. ~2,2~ Davis and Lobo observed the same phenomenon, which they attributed to the hydrophobicity of the methyl sparteine template. 13,22,23Although hydrophobicity may be a contributing factor, our belief is that direct synthesis of B-SSZ-24 is possible because the diazabicyclononane derivatives B and C prevent the formation of a cage-type structure such as SSZ-13 (CHA), which is the favored product from the adamantyl derivatives as lattice substitution is introduced. Although the charged nitrogen to carbon ratio of the methyl sparteine template is high, the presence of the second nitrogen in the molecule reduces the overall C/N to 8, thereby making it less hydrophobic. We have also been able to incorporate a small amount of aluminum directly into the SSZ-24 structure using Template C and Cabosil and Reheis F2000 aluminum hydroxide dried gel to give a product with a final SiO2/A1203 of 270. In addition, from a high silica Y-zeolite (Tosoh Y390) as a starting source of both silica and alumina, 24we obtained an SSZ-24 product with a SiOJA120 3 of 400. Although we have demonstrated that it is possible to prepare A1-SSZ-24 directly using Template C, the level of aluminum incorporation which is possible is still quite low. Aluminum exchange of B-SSZ-24 as described by Zones et al. still affords a more highly substituted aluminum-containing product. 25 4. CONCLUSIONS A new zeolite, SSZ-35, was discovered using the 3,7-diazabicyclo[3.3.1 ]nonane Template A. This zeolite can be prepared over a range of compositions and appears to possess a multidimensional channel system having at least one large-pore component. Related Templates B and C do not lead to the crystallization of this new phase, but rather, exhibit a selectivity for SSZ-24. We believe that these results can be explained by the inability of the 12-ring component of the SSZ35 structure to accommodate the larger Templates B and C. These templates therefore induce the formation of one-dimensional, large-pore zeolites such as SSZ-31 or SSZ-24. B-SSZ-24 can be made directly from soluble reagents using these templates, and does not require the use of calcined B-beta zeolite as a starting material. When substituting elements are introduced into the starting gel, products such as CHA often are favored, however, the steric requirements of Templates B and C are such that they cannot be accommodated within the CHA cage. Thus, the direct systhesis of B-and A1-SSZ-24 becomes possible.
60 ACKNOWLEDGMENTS
We thank Gregory S. Lee for synthesizing the organic templates described in this study and for assistance in running the zeolite screening reactions, R. A. Van Nordstrand for preliminary XRD work and Dr. R. C. Medmd, Dr. G. Zhang and G. Mondo in collecting the synchrotron data on SSZ-35. We also acknowledge the work of Dr. Medmd in determining the symmetry and unit cell parameters for both as-made and calcined SSZ-35. Dr. T. V. Harris provided assistance in modeling of the templates and Dr. S. I. Zones was involved in many helpful discussions. We also thank Chevron Research and Technology Company for continuing support of this program and permission to publish 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. 24. 25.
S.I. Zones, U.S. Patent No. 4 665 110 (1987). S.I. Zones, U.S. Patent No. 4 493 337 (1990). S.I. Zones, U.S. Patent No. 4 190 006 (1990). S.I. Zones, M. M. Olmstead, D. S. Santilli, J. Am. Chem. Soc., 114 (1992) 4195. S.L. Lawton, W. J. Rohrbaugh, Science, 247 (1990) 1319. J. Ciric, U.S. Patent No. 3 950 496 (1976). Y. Nakagawa, Studies in Surface Science and Catalysis, J. Weitkamp, H. G. Karge, H. Pfeifer, W. Holderich, eds., Part A, Elsevier (1994) 323. Y. Nakagawa, S. I. Zones, "Synthesis of Microporous Materials, Vol. 1," M. L. Occelli and H. Robson, eds., Van Nostrand Reinhold (1992) 222. Y. Nakagawa, U.S. Patent No. 5 316 753 (1994). S.A. Zisman, K. D. Berlin, F. K. Alavi, S. Sangiah, C. R. Clarke, B. J. Scherlag, J. Of Labeled Compounds and Radiopharmaceuticals, 27 (1989) 885. S.A. Zisman, K. D. Berlin, B. J. Scherlag, Org. Prep. and Proc. Int., 22 (199) 255. Y. Nakagawa, U. S. Patent No. 5 271 922 (1993). R.F. Lobo, M. E. Davis, Microporous Materials, 3 (1994) 61. We have since found other templates which make SSZ-35 and have broadened the synthesis ranges described in this paper: Y. Nakagawa, U.S. Patent No. 5 268 161 (1993) Y. Nakagawa, U.S. Patent No. 5 273 736 (1993). Synchrotron data was collected on X7A beam line, National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U. S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. V.J. Frilette, W. O. Haag, R. M. Lago, J. Catal., 67 (1981) 218. J.L. Casci, M. D. Shannon, P. A. Cox, S. J. Andrews, "Synthesis of Microporous Materials, Vol. 1," M. L. Occelli, H. Robson, eds., Van Nostrand Reinhold (1992) 359. S.I. Zones, Y. Nakagawa, J. W. Rosenthal, "Zeolites (Japan)," 11 (1994) 81. J.L. Casci, New Developments in Zeolite Science and Technology, Y. Murakami, A. Iijima and J. W. Ward, eds., Elsevier (1986) 215. Molecular dimensions were calculated using "Smallest cylinder program" which was developed at Chevron Research and Technology Co. by Paul Merz. S.I. Zones, L-T. Yuen, Y. Nakagawa, R. A. Van Nordstrand, S. D. Toto, "Proceedings from the Ninth International Zeolite Conference," R. Von Ballmoos, J. B. Higgins and M. M. J. Treacy, eds., Vol. 1, Van Nostrand Reinhold (1993)163. R. de Ruiter, J. C. Jansen, H. Van Bekkum, Zeolites, 12 (1992) 56. Although Davis and Lobo did not observe conversion of calcined B-beta zeolite to B-SSZ24 using Template C, we did obtain B-SSZ-24 from B-beta after 7 days at 150~ S.I. Zones, Y. Nakagawa, U. S. Patent No. 5 225 179 (1993). R.A. Van Nordstrand, D. S. Santilli, S. I. Zones, "Synthesis of Microporous Materials, Vol. 1," M. L. Occelli and H. Robson, eds., Van Nostrand Reinhold (1992) 373.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
61
S y n t h e s i s of Z S M - 4 8 T y p e Zeolite in P r e s e n c e of Li, Na, K , R b and Cs Cations G. Giordano 1, A. Katovic 1, A. Fonseca 2 and J.B. Nagy 2 1Dipartimento di Ingegneria Chimica e dei Materiali, Universit~t della Calabria, 1-87030 RENDE (CS), Italy 2Laboratoire de Catalyse, Facult6s Universitaires Notre-Dame de la Paix, 61 rue de Bruxelles, B-5000 NAMUR, Belgium
SUMMARY
This study investigates the formation of ZSM-48 zeolite starting from a hydrogel with or without A1 as a function of the alkali cations (Li, Na, K, Rb and Cs). Crystallization kinetics data, chemical composition, NMR characterization, morphology and crystal size are presented in order to give additional information about the synthesis of this catalyst precursor. INTRODUCTION Recently, the preparation of ZSM-48 type zeolite has shown a new interest due to its catalytic properties as additive in FCC catalyst. The presence of ZSM-48 in FCC catalyst increases both conversion and selectivity in C3-C 4 products [1]. The ZSM-48 possesses a 10MR unidimensional channel system with pore opening 5.3 x 5.6 A and it can be synthesized starting from silica or alumino-silicate gels in presence of various organic compounds. The nature of inorganic and organic cations affects the formation of ZSM-48 and also influences morphology, crystal size and the amount of structural defects. The aim of this work is to evaluate the effect that the alkaline cations (Li, Na, K, Rb and Cs) have on the synthesis of ZSM-48 zeolite in a system that contains hexamethonium ions (HM++). ~ The data about crystallization kinetics, morphology, crystal size, chemical analysis and NMR characterization are discussed with the goal to optimize the synthesis of ZSM-48 in HM systems. EXPERIMENTAL The synthesis was carried out at 200~ under static condition and autogenous pressure in Teflon-lined Morey-type autoclaves starting from the following hydrogel: 5 M20 - 2.5 HMBr 2 - x A1203 - 60 S i t 2 - 3000 H20 where HMBr stands for hexamethonium bromide, x = 0 or 0.5 and M = Li, Na, K, Rb or Cs. The samples were filtered, washed, dried and characterized by usual procedures (X-ray, SEM, EDX, A.A. spectroscopy and NMR).
62 RESULTS AND DISCUSSION
First of all, it can be observed that in absence of A1 the systems with Li, Na and K show similar crystallization kinetics. After 24 hours of reaction the crystallinity is higher than 90%. In presence of Rb ions a reaction time of about 48 hours is required for a complete crystallization. The Cs ions disfavour the formation of ZSM-48, since the first crystals are detected, by X-ray, only after 30 hours while with the other cations the crystallization is almost complete at that time. Moreover, the maximum of crystallinity (60%) is obtained after 50 hours of reaction, and at longer times co-crystallization with cristobalite occurs. The presence of A1 in the starting hydrogel leads to a longer crystallization time. Similar behaviour is observed for other zeolites such as the MFI type. In this case all the inorganic cations, except Cs ions, show similar behavior. In fact the nucleation time is identical for all four samples and after 48 hours of reaction all samples show more than 85% crystallinity. Also in this case, the presence of Cs ions disfavours the ZSM-48 formation. Indeed, after 4 days of reaction the crystallinity achieved is only 35%. However, in this case the formation of cristobalite is not observed for longer reaction times. Chemical analyses show that the amount of HM ions is close to one per unit cell for all examined samples. In the synthesis in absence of A1, only traces of alkaline cations are detected. This suggests that the synthesis of the siliceous ZSM-48 can be obtained also in absence of inorganic cations, and hence only a component is required that solubilizes and mobilizes the siliceous species. On the other hand, the amount of organic cations detected is justified by the large amount of structural defect observed by 29Si-NMR analysis. In the samples synthesized in presence of A1, both inorganic and organic cations are found in the final products. The amount of inorganic cation observed is not sufficient to act as counteractions of A1 species, and this suggests that also the hexamethonium ions play a role in stabilizing the negative charges. Probably the organic cations stabilize the framework by a pore filling action and act as neutralizing agent of the negative charges and of the defect groups. The number of A1 per unit cell found in the final products is always less than 1, and this is in agreement with the framework suggested for the ZSM-48 zeolite. In fact when the amount of A1 in the initial reaction mixture increases, the system derives toward the formation of EUO type zeolite. The 27A1-NMR data show that the aluminium is incorporated in the ZSM-48 zeolite mainly in the tetrahedral coordination. The spectra of 29Si-NMR show that the amount of SiOM (M = organic and/or alkali cation) defect groups is higher in absence than in presence of A1. Indeed, in presence of Li, Na, K and Rb, some 18 SiOM/u.c. are detected without A1, and ca 12 SiOM/u.c. with A1 incorporated in the structure. These values are higher than those reported previously in the presence of Na and a high amount of HM §247ions [2]. In presence of Cs, the amount of SiOM/u.c. is smaller (13/u.c.) and the 29Si-NMR spectrum shows a higher resolution. REFERENCES 1. F. Di Renzo, G. Giordano, F. Fajula, P. Schultz and D. Anglerot, French Pat. 2.698,863 (1994). 2. G. Giordano, N. Dewaele, Z. Gabelica, J.B. Nagy, A. Nastro, R. Aiello and E.G. Derouane, Stud. Surf. Sci. Catal., 69 (1991) 157.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
63
O u t - o f - p l a n e B e n d i n g Vibrations of Bridging OH G r o u p s in Zeolites: A New Characteristic of the G e o m e t r y and Acidity of a B r o e n s t e d Site L.M. Kustov*, E. Loeffler**, V.L. Zholobenko*, and V.B. Kazansky* *N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, Leninsky prosp. 47, 117334 Russia **Adlershofer Umweldschudztechnik, Berlin, Rudower Chaussee 5, D-0-1199 Germany
Abstract Out-of-plane bending vibrations of bridging OH groups in zeolites were studied by diffuse-reflectance IR spectroscopy in the region of the combination bands of the out-of-plane bending and fundamental stretching vibrations (3800 - 4200 cm -1). Similar to the in-plane bending vibrations, the out-of-plane bending modes were found to be a very sensitive characteristic of the bridging OH groups in zeolites. These frequencies increase with increasing acidity of the OH groups and with formation of H-complexes with adsorbed molecules.
1. INTRODUCTION The observation of the bending, torsional, and other low-frequency vibrations of the structural hydroxyl groups in oxide adsorbents and catalysts encounters a number of problems because of the unfavorable background due to the framework vibrations in the region of 200 - 1000 cm-1, as well as due to the low concentration of OH groups. There are however two possible ways to overcome this difficulty: (i) the use of modern highly sensitive spectroscopic methods for studying OH groups, for instance, Fouriertransform IR, diffuse-reflectance IR, inelastic neutron scattering, etc. and (ii) the analysis of the combination bands. In our previous studies [1 - 3], we have analyzed the combination modes (v + (3) of the fundamental stretching and in-plane bending vibrations in the DRIR spectra of zeolites and other oxide materials using highly sensitive diffuse-reflectance IR spectroscopy. The bending vibrations of the bridging
64 OH groups were shown to be much more sensitive to the structural environment of hydroxyls as compared to the stretching vibrations. Thus, two types of the isolated bridging hydroxyl groups characterized by the frequency of the fundamental stretching vibration at 3660 cm -1 have been resolved in the region of the combination bands of the stretching and in-plane bending vibrations. The range of the stretching frequencies for various types of OH groups is quite narrow (3550 - 3800 cm -1), i.e., about 250 cm -1, whereas the corresponding range of the in-plane frequencies is much wider and extends from 600 cm-1 (for MeOH groups connected with rare-earth cations) to 1640 cm -1 (for water molecule). The in-plane bending frequencies were also proposed as an additional measure of the acidic properties of these OH groups. In studying structural OH groups in SAPOs [4], we succeeded in observing a combination band of the fundamental stretching and out-of-plane bending vibrations of the bridging OH groups. The combination band for SAPO-34 was found at 3940 cm -1, which was attributed to the structural OH groups characterized by the fundamental stretching frequency at 3600 cm -1 Thus, the out-of-plane bending frequency calculated as the difference of the two observed frequencies is equal to 340 cm-1. The quantum-chemical calculations [5, 6] agree fairly well with the experimental data and show that the bending modes are more sensitive to changes in the local structure at the Broensted site compared to the stretching modes. The calculations allowed the authors to estimate the outof-plane bending frequency for the bridging OH groups in zeolites (y - 400 cm-1). Recently, the fundamental in-plane bending and out-of-plane bending modes for the bridging OH groups in zeolites have been observed using inelastic neutron scattering spectroscopy at 1090 cm-1 and 420 cm-1, respectively [ 7 - 11], in perfect agreement with the theoretical prediction and our studies of the combination bands. In the present paper, we attempted to use the similar approach to investigate the combination bands of the stretching and out-of-plane bending vibrations of structural OH groups in different types of zeolites.
2. E X P E R I M E N T A L H-forms of the zeolites X (Si/AI = 1.25), Y (Si/AI = 2.35), erionite (Si/AI 3.5), mordenite (Si/AI = 21) and ZSM-5 (Si/AI = 21) were prepared by calcination of ammonium forms in a vacuum at 640 - 770 K depending on the thermal stability of the zeolites. The thermal-vacuum pretreatment at the final temperature was performed for 8 h, The rate of the temperature increase was 2 - 5 K/min. The diffuse-reflectance IR spectra were measured using a Perkin-Elmer 580B spectrophotometer as described in [1, 2]. Adsorption of N20, CO 2, n-
65 C8F18, and CF3H was carried out at room temperature pressures of 20 - 40 torr.
and adsorbate
3. R E S U L T S A N D D I S C U S S I O N DRIR spectra of the HNaY, HNaX, and HNa-ERI zeolites under study in the region of 3200 - 4000 cm -1 are presented in Fig. 1. The absorption bands in the region of 3550 - 3750 cm -1 have been reliably assigned to the fundamental stretching vibrations of the bridging hydroxyl groups [1, 2, 12]. However, in some cases, additional low-intensity lines at 3900 - 3950 cm-1 are also observed. These bands are present in the spectra of H-forms of X and Y zeolites and erionite, but they are absent in the spectra of the HZSM-5 zeolite and H-mordenite. Although X, Y, and erionite zeolites have so little in common as concern their structure, stability, and chemical composition, it is still possible to find at least one common feature distinguishing them from pentasils. Indeed, the socalled LF-bands in the low-frequency region (3520 - 3580 cm-1) are observed in the IR spectra of X, Y, and erionite zeolites. They have been attributed to the bridging OH groups, which are hydrogen-bonded to the neighboring oxygen atoms of the lattice in the double 6-membered rings (D6R structural elements) [1, 12]. For the pentasil-type zeolites, such as mordenite and ZSM5 zeolite, this type of H-bonding is not realized because of the absence of the D6R units in the structures of these zeolites. Hence, an assumption could be drawn that the bands near 3900 cm-1 are observed only in those cases when the bands at 3520 - 3580 cm-1 are present in the IR spectra. To confirm this hypothesis, HX and HY zeolites with different ion-exchange degree of Na + for NH4 -I- (o~) were investigated. As is seen in Fig. l b, for faujasites with 0c < 50%, unlike the samples with 0c > 70%, the intensity of the band at 3550 - 3580 cm-1 is much lower than that of the band at 3650 cm-1 Accordingly, the intensity of the high-frequency band at 3900 cm-1 decreases in a similar way. Thus, these data show that the band at 3900 - 3950 cm-1 is somehow connected with the bridging OH groups that are H-bonded to the oxygen atoms of the lattice in the D6R structural units (vOH - 3520 - 3580 cm-1). The high values of the frequencies corresponding to the observed maxima (-3900 cm-1), which are placed far beyond the region of the stretching vibrations of OH groups and the significant halfwidths of the bands (AH1/2=80 - 100 cm -1) allow us to assign them to a combination v + ~/, i.e., to a combination of the stretching vibration ( v ) o f the H-bonded acidic hydroxyl groups with a low-frequency vibration ~f. The y values derived by subtraction of the stretching frequency from the combination are presented in Table 1. They are ranged from 325 to 380 cm-1 depending on the zeolite. In our opinion,
66 3610
3640
3560 1 1
3555 ~~ 3660
,/ _,j! 3,<,s .J
.',
2
1
I
3580
tt, l // ///
\.j~ /
I 3740
//
/ / f 2 / /
/
I / ~
J
3928
Fig. 1. DRIR spectra of H-forms of zeolites in the region 3300 - 4000 cm-1 (a) HZSM-5 (1), H-MOR (2), and H-ERI (3) (b) HNaX (1), HNaY-45% (2), and HNaY-80% (3).
6?
Table 1 Spectroscopic parameters of the bridging OH groups forming a hydrogen bond with framework oxygen atoms in zeolites.
Zeolite
HNaX HNaY H-ERI
v, cm -1
3580 3555 3560
v +~/, cm -1
3905 3928 3940
T, cm -1
325 373 380
Table 2 Spectroscopic parameters of bridging OH groups in the HZSM-5 zeolite forming a hydrogen bond with adsorbed molecules.
Adsorbate
v, cm- 1
v + y, cm- 1
y, cm- 1
CF3H
3530
3910
380
CO 2 N20
3470 3450
3940 3925
470 475
68
this vibration should be classified as an out-of-plane bending vibration of hydrogen bonded OH groups in the bridging fragment: ~Si
mI
" ~ ' 0 ..--H ..... 0 .~AI ~ " Indeed, the frequencies of the in-plane bending vibrations of OH groups (6) are known to be about of 800 - 1100 cm -1 and corresponding combination modes are ranged from 4500 to 4700 cm -1 [1 - 3]. The frequencies of the out-of-plane bending vibrations of isolated OH groups in alcohols are reported to be 200 - 220 cm -1 [13]. When H-bonding takes place, the frequency increases to 300 - 400 cm -1, in parallel with strengthening of an H-bond [13]. Consequently, the v + 3' combination band for the isolated acidic OH groups (vOH = 3610 - 3660 cm -1) , which should have the frequency at 3800 - 3850 cm -1 seems to be hardly observed on the background caused by the absorbance of silanol groups (VOH = 3740 cm-1). An increase in the frequency of the out-of-plane bending vibration for Hbonded OH groups with the enhancement of their acid strength may be explained by the fact that the O - H " ' O fragment becomes more rigid. It should be also noted that for X and Y zeolites having identical structures, the y values are quite different. Moreover, in the HY zeolite, which is more acidic than the HX zeolite, OH groups are characterized by a larger y-frequency shift as compared with its assumed value for isolated OH groups ( ~ / - 200 - 220 cm-1). This is in agreement with the above-mentioned dependence found for alcohols [13] more acidic hydroxyls that form stronger hydrogen bonds, exhibit higher values of the y frequency. Thus, the frequency of the out-of-plane bending vibration of the bridging OH groups may be used as an additional measure of their acidic properties. In the case of H-erionite, an increase in the frequency of the out-of-plane bending vibration as compared with faujasites is observed as well. However, this comparison seems to be incorrect, because the structures of erionite and faujasites are quite different and, consequently, the O ...... O distances in the O--H...O fragments may be changed. From the data obtained, it is possible to suppose that the band at 3900 3950 cm-1 may be observed even for the zeolites, which do not contain structural elements of the D6R type, if their acidic OH groups form H-bonds with adsorbed molecules. This hypothesis is confirmed by our data on N20, CO 2, n-C8F18, and CF3H adsorption on the HZSM-5 zeolite. Admission of these adsorbates leads to the formation of weak hydrogen bonds with the bridging OH groups (z~v = 60 - 150 cm-1). The adsorption results also in the appearance of the low-intensity absorption bands at 3910 - 3940 cm -1 in the IR spectra (Fig. 2). The calculated frequencies of the bending vibrations (Y)in
d9
3470 3450
1
q
3530
d
4
4 , / /
3610
/
3700
".X/~
9
, /
/
,/
/ 1
~
.j" ! [ 3745 ~"l
J
I
2
', \
A,
1
3925 "\ 3910 o
q I ~3940
I I ', \ \\
Fig. 2. DRIR spectra of the HZSM-5 zeolite before (1) and after adsorption of trifluoromethane (2), nitrous oxide (3), and carbon dioxide (4) at room temperature.
70 accordance with the above hypothesis increase with a strengthening of an Hbond (see Table 1). Thus, the bands at 3900 - 3950 cm-1 revealed in IR spectra of zeolites should be attributed to the combination modes (v + T) of the stretching and out-of-plane bending vibrations of the bridging OH groups forming hydrogen bonds with lattice oxygen atoms. These frequencies could be used to calculate the frequency T of the out-of-plane bending vibrations. The data on the T frequencies seem to give an additional information about acidic properties of the OH groups and about the geometric characteristics of the hydrogen-bonded complexes.
REFERENCES 1. L. M. Kustov, V. Yu. Borovkov, and V. B. Kazansky, J. Catal., 72 (1981) 149. 2. V. B. Kazansky, L. M. Kustov, and V. Yu. Borovkov, Izv. Chim. Bulg. Akad. Nauk, 16 (1983) 44. 3. L. M. Kustov, V. Yu. Borovkov, and V. B. Kazansky, Zh. Fiz. Khim. (Russ. J. Phys. Chem.), 59 (1985) 2213. 4. S. A. Zubkov, L. M. Kustov, V. B. Kazansky, et al., J. Chem. Soc., Faraday Trans., (1991) 897. 5. J. Sauer, J. Mol. Catal., 54 (1989) 312. 6. J. Sauer, Chem. Rev., 89 (1989) 199. 7. M. J. Wax, R. R. Cavanagh, J. J. Rush, et al., J. Phys. Chem., 90 (1986) 532. 8. T. J. Udovic, R. R. Cavanagh, J. J. Rush, et al., J. Phys. Chem., 91 (1987) 5968. 9. H. Jobic, J. Catal., 131 (1991) 289. 10. W. P. J. H. Jacobs, H. Jobic, J. H. M. C. van Wolput, and R. A. van Santen, Zeolites, 12 (1992) 315. 11. W. P. J. H. Jacobs, J. H. M. C. van Wolput, R. A. van Santen, and H. Jobic, Zeolites, 14 (1994) 117. 12. J. W. Ward, In: Zeolite Chemistry and Catalysis, (J. A. Rabo, Ed.), ACS Monograph 171, American Chemical Society, Washington D.C., 1976. 13. J. Rossarie, J.-P. Gallas, C. Binet, and R. Romanet, J. Chim. phys. phys.chim. biol., 74 (1977) 202.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviotand S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
71
In situ F T I R microscopic investigation of the acid sites in cloverite G. MOiler, G. Eder-Mirth and J.A. Lercher Christian Doppler Laboratory for Heterogeneous Catalysis and University of Twente, Department of Chemical Technology, P.O. Box 217, 7500 AE Enschede, The Netherlands. The acidity of cloverite was studied by sorption of probe molecules with varying basicity. Cloverite possesses two types of structural hydroxyl groups (Ga-OH and P-OH groups) which exhibit high and moderate Br6nsted acid strength towards apolar hydrocarbon molecules. Sorption of polar base molecules on the Ga-OH groups, however, leads to a breaking of the GaO bond close to the Ga-OH adsorption site resulting in a concerted Lewis and Br6nsted type interaction. For small molecules (like ammonia), multiple sorption at these sites causes a local rearrangement of the lattice leading to a collapse of the microporous structure upon thermal desorption of the probe molecules. Small alcohols are chemisorbed quite differently on the Ga-OH groups of cloverite compared to other acidic molecular sieves leading to the formation of dimethylether and formaldehyde upon desorption. 1. INTRODUCTION Cloverite, a microporous gallophosphate has attracted great attention as it possesses unusually large micropores [ 1-3]. Because of its potential as sorbent and catalyst for the reactions of larger organic molecules, the structure was thoroughly characterized by various techniques [ 17]. A major drawback for its application, however, is the instability of the microporous structure in ambient atmosphere once the template is removed. This is also the main reason why rather limited information is available on the sorption and catalytic properties of the molecular sieve [ 14,6]. The aim of this contribution is to characterize the nature and strength of the acid sites of cloverite by sorption and reactive desorption of probe molecules. The molecules studied were the strong bases ammonia, pyridine and trimethylamine as well as alcohols with varying chain length (methanol, ethanol and propanol). 2. EXPERIMENTAL 2.2 Material
We used single crystals with hexagonal/square shape with a diameter between 80 and 140 tam. as well as polycrystalline material (particle size of ~ 1 tam). Both samples were provided by Dr. H. Kessler, Laboratoire de Mat6riaux Min6raux, Ecole Nationale Sup6rieure de Chimie, Mulhouse. Cloverite possesses two non intersecting 3-dimensional pore systems. The large pore system consists 20 membered rings with a free diameter of approximately 1.3 nm, the other pore
72 system is formed by 8 membered rings showing a free diameter of only -0.4 nm. At the intersections of the large pores, supercages are formed which exhibit a diagonal diameter of 2.9 nm [1-3]. The lattice of cloverite is neutral consisting of alternating PO4- and GaO4- tetrahedra. The terminal hydroxyl groups which are an integral part of the lattice are located in the large pore system (see fig. 1).
absorbanee [ar units] __
3702 C-aOH
3675 POH
.
.
-
.
.
..-.
" ;"'"
I 0 . 8
--
~~
0.6 0.4
. "',G a O H
POH .
I
i
.o.5! m .
0.2 0 3800
1
3300
I
2800
I
I
I
2300
1800
1300
wavenumbar [1/em]
Figure 1" Spectrum of the activated cloverite and graphic representation of a part of the unit cell (enlarged, the part where the structural hydroxyl groups are situated). In the representation of the part of the unit cell, only the T-atoms are connected
2.2 Sorption studies Activation, sorption and desorption were followed in situ by time resolved FTIR microscopy. The measurements on the single crystals were carried out in a vacuum cell equipped with i.r. transparent windows attached to the stage of a BRUKER i.r. microscope. The spectrometer used was a BRUKER IFS 88 (4 cmq spectral resolution). The polycrystalline material was pressed into self supporting wafers and measured in another high vacuum cell which was placed in the BRUKER IFS 88 spectrometer. This cell was connected to a B ALZERS QMG 410 mass spectrometer which was used to monitor the gas phase composition during temperature programmed desorption. The template was removed by heating the sample with 10 K/min up to 800 K in air, holding this temperature for two hours and subsequently evacuating the system to 10-6 mbar and cooling it to 300 K to perform the sorption experiments. The structure of the activated molecular sieve was stable in vacuum or inert gas atmosphere. The sorbates were introduced into the reaction cell via a gas inlet system at a temperature of 300 K. The desired partial pressure was held constant until sorption/desorption equilibrium was achieved and was then increased in steps from 10.3 to
73 1 mbar. Subsequently, the system was again evacuated to 106 mbar for 90 minutes, followed by temperature programmed desorption (TPD, heating rate 10 K/min, final temperature between 800 and 900 K). 3. RESULTS AND DISCUSSION 3.1 Acid sites in cloverite The i.r. spectrum of the activated cloverite (fig. 1) showed two absorption bands at 3702 and 3675 cm~, which are assigned to Ga-OH and P-OH groups, respectively [8]. Upon sorption of benzene, the i.r. bands of these OH groups were broadened and shifted to lower wavenumbers (A v = 250 crn~ for the Ga-OH and 220 cm~ for the P-OH group, compared to the 260 cm~ shift observed for SiOHAI groups of HZSM-5). Because the magnitude of the red shift of the OH bands after adsorption of a donor molecule is directly proportional to the acid strength of the OH group [9], this indicates high and moderate acid strength of the OH groups of cloverite. The absorption bands in the spectral region below 2500 cmx are assigned to overtones and combinations of the lattice vibrations of the gallium phosphate [ 10]. 3.2 Sorption of strong bases The nature and accessibility of the acid sites in cloverite was probed by sorption of three bases molecules, ammonia, pyridine and trimethylamine (TMA). These molecules have different proton affinities and kinetic diameters, with ammonia having the lowest proton affinity and the smallest kinetic diameter. The i.r. spectra of ammonia and pyridine sorbed on cloverite show characteristic absorption bands for Lewis (1616 c m "1 for ammonia and 1613 cm~ for pyridine) as well as Bronsted (1450 cm1 for ammonia and 1543 cm1 for pyridine) bonded species [11]. With increasing coverage, the absorption bands of both species increased parallel in intensity, while the intensity of both types of OH groups decreased. It should be emphasized that preferential sorption on one of the sorption sites (Ga-OH or P-OH groups) was not found. After equilibration at 0.1 mbar at 300 K, all hydroxyl groups were found to interact with base molecules. Bonding to cloverite was concluded to be strong, as desorption did not occur upon evacuation at 300 K. During temperature programmed desorption (TPD), pyridine and TMA desorbed first from the P-OH group (maxima of the rate of desorption around 800 K). The molecules interacting with the Ga-OH groups could only be partially desorbed up to 900 K in vacuum, but were completely removed upon treatment in flowing air at 800 K. This unusual high thermal stability and the simultaneous disappearance of the free hydroxyl groups and the appearance of the bands characteristic for Lewis and Br6nsted bonded species led us to conclude that the Bronsted acidity of cloverite cannot account alone for the strong sorption of the base molecules. For ammonia, the removal of the molecules by the above described thermal treatments (900 K in vacuum, 800 K in air) did not lead to the reappearance of the OH stretching vibration bands characteristic for the cloverite structure. Then, only a small band at 3675 cm~ characteristic for P-OH groups was observed. This spectrum was similar to that recorded after sorption of water at ambient temperature (followed by TPD) which led to a collapse of the microporous structure [2]. In agreement with Patarin et a/.[3] we therefore conclude that the same occurs in the presence of ammonia at elevated temperatures. An amorphization of cloverite after a stepwise thermal desorption of ammonia was confirmed by XRD measurements [3].
74 As pyridine and TMA have stronger gas phase basicities than ammonia and their sorption did not lead to an irreversible change/damage of the lattice, we conclude the size of the molecule to be a decisive factor for the destabilization of the cloverite structure. These results clearly indicate that a concerted interaction of Br6nsted and Lewis acid sites occurs upon sorption of the base molecules on cloverite. This concept of a concerted interaction of Lewis and Br6nsted acid sites implies that upon sorption, one of the Ga-O bonds next to the Ga-OH group is partially opened and that the primary interaction takes place between the then accessible Ga cation and the base molecule. This might induce a rehybridization of the Ga cation from tetrahedral symmetry first to a 5-fold coordination (like it is present in the "as synthesized" form) and then to complete octahedral (corresponding to 6fold) coordination (fig.2). The 6-fold coordination is possible for the smaller ammonia molecules, as they can enter also the small pore system (free inner diameter -0.4 nm) and coordinate to the Ga cations through the double 4 tings connecting the large with the small pore system (as shown in figs. 1 and 2). For the larger pyridine and TMA molecules a cleavage of the Ga-O bond still occurs and leads to a Lewis type coordination between the Ga cation and the base molecule. However, the additional interaction of a second molecule via the small pore system cannot be achieved leading only to a five fold coordination of the Ga upon sorption of these molecules.
Ga in 4ofold coordination (activated sample)
Ga{
Ga in 5-fold coordination
ammonia
G a in 6-fold coordination
ammonia
ammonia
Figure 2: Change in coordination of gallium upon sorption of ammonia
75
3.2 Sorption of alcohols While the sorption of the strongly basic molecules occurred in parallel on both types of hydroxyl groups, preferential interaction with the Ga-OH groups was noticed with the alcohols (methanol, ethanol and propanol). This effect was quite pronounced with methanol and decreased in importance with increasing molecular weight of the alcohol. Note that all Ga-OH groups interacted with an alcohol molecule at rather low partial pressures (10 .3 mbar), while the partial pressures required to cover all P-OH groups were decreased with increasing chain length (see table 1). Tablel Uptake characteristics of alcohols on Br6nsted acid sites Alcohol
O0,.on) at O(Ga.ori)= 1 [1]
Peq. to reach Oo,.ori)=1 [mbar]
Methanol
0
1
Ethanol
0.25
0.1
Propanol
0.6
0.01
O ...... coverage Peq.. ..... equilibrium pressure After equilibration with 10s mbar, all Ga-OH groups interacted with methanol molecules. The i.r. spectra of the sorbed species recorded during the uptake of methanol suggest the existence of two different sorption structures on the Ga-OH groups [8]. At low coverages, structure A which is characterized by bands of the CH stretching vibrations at 2828 and 2940 cm1, was dominant. After longer exposure (corresponds also to higher coverages) a shoulder rose at 2838 cm1 and a new peak appeared at 2950 cm~ . These bands were attributed to a second sorption structure of methanol (structure B). Under these conditions, the P-OH groups were not affected by methanol adsorption. At partial pressures higher than 10.3 mbar, methanol interacted also with the P-OH groups causing bands of CH stretching vibrations at 3000, 2962 and 2855 cm~ (structure C). These absorption maxima are close to those observed for methanol hydrogen bonded to weakly acidic silanol groups (3001, 2958 and 2856 cm1) [12]. Thus, we suggest a similar type of interaction for methanol sorbed on the P-OH groups of cloverite. It should be emphasized that the bands of the CH stretching vibrations of methanol interacting with the Ga-OH groups occurred at quite low wavenumbers, comparable only with the values observed for methanol sorbed on Rb- of Cs-cations in alkali exchanged X-zeolites [ 13]. This led us to conclude that the interaction of methanol with the Ga-OH groups cannot be ascribed to hydrogen bonding or protonation like in other acidic zeolites [ 14], but an additional interaction of the hydrogen bonded molecules with Lewis type sites has to be assumed. Based on these results and the evidences collected for the sorption of strong bases, we conclude that also upon sorption of methanol, a Ga-O bond neighboring the Ga-OH group is broken creating a strong Lewis acid site next to the hydroxyl group.
76 The i.r. spectra of ethanol and propanol sorbed on cloverite showed only absorption features characteristic for an alcohol molecule interacting via hydrogen bonds with acidic hydroxyl groups [ 14]. Thus, it is difficult to judge whether ethanol and propanol are polar enough to cause the same type of lattice opening as it was observed with methanol. Upon evacuating the system, almost complete desorption of methanol and ethanol from the P-OH groups was observed. However, about 30 ~ of the P-OH groups still showed interaction with propanol under these conditions. Such a behavior can only be explained, if we assume that propanol interacts with the P-OH and the Ga-OH groups simultaneously. This seems possible because of the steric vicinity of the two structural OH groups (average distance absorbaace [ark units] between them is 0,51 nm and the length I 28~g of the propanol molecule is 0,53 rim. See / ~ 2835~' also fig.1.). It should be noted that 0.15 J~% evacuation at ambient temperature did not affect the interaction of the alcohol molecules with the Ga-OH groups. 300 K In addition to unreacted methanol, "/;'|~ Y / / / ~ ~ . . ~ ~ / ~ ~ ~ ~ the formation of two different products, [~/. dimethylether and formaldehyde was observed during temperature ~ ~ ~ ~ ~ programmed desorption ( T P D ) o f methanol from cloverite. The i.r.spectra 1 ~ ~ recorded during TPD of methanol from 0 900 K polycrystalline cloverite are shown in fig.3. Dimethylether formation occurred 3000 ~ ~ ~ 2800 r at lower temperatures (maximum rate of waveaumber [l/m] desorption around 500 K) and is directly correlated to the disappearance of structure B from the surface. The other sorption species (mainly species A) Figure 3: I.r. spectra recorded during TPD of desorbed as formaldehyde at higher methanol from cloverite temperatures (maximum rate of desorption around 670 K). Upon increasing the temperature, ethanol and propanol desorbed partly unreacted from the Ga-OH groups. At temperatures above 500 K, dehydration of the alcohols took place leading to the formation of water, ethene and propene. Other reaction products were not detected by mass spectrometry. 4. CONCLUSIONS The structural hydroxyl groups of cloverite exhibit high and moderate Bronsted acid strength towards apolar hydrocarbon molecules. With polar molecules, these hydroxyl groups show unique interactions leading to a local rearrangement of the lattice including a breaking of the Ga-O bond close to the Ga-OH sorption site. Consequently, a concerted Lewis and Br6nsted acid type interaction is concluded to exist between the basic molecules and the Ga-OH/Ga cation sorption
77 sites. This type of interaction is paralleled by a change in the coordination of the Ga cation to first 5-fold (upon sorption of 1 probe molecule) and then, after sorption of a second molecule to complete 6-fold (octahedral) coordination. This 6-fold coordination can only be achieved, if the probe molecule is small enough (like water or ammonia) to enter the small pore system and to coordinate to the Ga cation via the double 4 tings. Desorption of the molecules from the octahedral coordination, leads to a collapse of the microporous structure of cloverite and results in the formation of an amorphous G a P O 4. Methanol sorbs preferentially on the Ga-OH groups leading to two different sorption structures. One coordinatively bound species to Ga cations which desorbs reactively as formaldehyde at elevated temperatures. The other structure is attributed to methanol hydrogen bonded to the Ga-OH groups and yields dimethylether upon desorption. ACKNOWLEDGEMENT
The work was partially founded by the Christian Doppler Laboratory for Heterogeneous Catalysis. The graphic displays shown in figures 1 and 2 were printed out from the I N S I G H T H molecular modeling system (BIOSYM Technologies Inc.). We thank Dr.Henri Kessler for providing the cloverite samples. REFERENCES
.
.
.
7. 8.
9. 10. 11. 12. 13. 14.
M.Esterman, L.B.McCusker, C.Baerlocher, A.Merrouche and H.Kessler, Nature 352 (1991)320. A.Merrouche, J.Patarin, H.Kessler, M.Soulard, L.Delmotte, J.L.Guth and J.F.Joly, Zeolites 12 (1992) 226. J.Patarin, C.Schott, A.Merrouche, H.Kessler, M.Soulard, L.Delmotte, J.L.Guth and J.F.Joly, Proc. 9th IZC; van Ballmoos, R. et al.; Butterworth-Heinemann, 1993, 263. R.L.Bedard, C.L.Bowes, N.Combes, A.J.Holmes, T.Jiang, S.J.Kirkby, P.M.Macdonald, A.M.Malek, G.A.Ozin, S.Petrov, N.Plavac, R.A.Ramik, M.R.Steele, and D.J.Young, Am. Chem. Soc. 115 (1993) 2300. B.Zibrowius, M.W.Anderson, W.Schmidt, F.F.Scht~th, A.E.Aliev and K.D.Harris, Zeolites 1993, 13,607. T.L.Barr, J.Klinowski, H.He, K.Alberti, G.Mtiller and J.A.Lercher, Nature 365 (1993) 429. S.Bradley, R.F.Howe and J.V.Hanna, Solid State Nuclear Magn. Res. 2 (1993) 37. G.Mt~ller, G.Eder-Mirth, H.Kessler and J.A.Lercher, J.Phys.Chem. submitted 1995. M.L.Hair and W.Hertl, J. Phys. Chem. 74 (1970) 91. V.V.Krymova, L.E.Kitaev, A.A.Kubasov, Z.V.Gryaznova, L.S.Eshchenko and V.V. Pechkovskii, Vestn. Mosk. Univ., Ser. 2. Khim. 20(5) (1979) 476. L.H.Little, "Infrared Spectra of Adsorbed Species", Academic Press, London, 1966. E.P.Parry, J. Catal. 2 (1963) 371. B.A.Morrow, J.Chem.Soc.Farad.I 70 (1974) 1527. G.Eder-Mirth, H.Wanzenb6ck and J.A.Lercher, ZEOCAT 95, accepted for publication. M.T.Aronson, R.J.Gorte and W.E.Farneth, J.Catal. 105 (1987) 455. G.Mirth, A.Kogelbauer and J.A.Lercher, Proc. 9th IZC; van Ballmoos, R. et al.; Butterworth-Heinemann, 1993, 251.
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Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
79
Brgfnsted sites of enhanced acidity in zeolites: experimental modelling M.A.Makarova*, S.P.Bates** and J.Dwyer Chemistry Department, UMIST, PO Box 88, Manchester M60 1QD, UK
Low-temperature interaction of BF3 with acid forms of zeolites is shown to take place in two different ways: as an amphoteric complex (I) and as an acid-base complex (11). These complexes are observed using FTIR spectroscopy and their assignment is based on theoretical calculations. In the case of BF3/H-ZSM-5 only the weaker bonded complex (11) is observed, whereas in the BF3/H-EMT system both types of complexes are present. Formation of amphoteric complex (I) affects the acid strength of the remaining uncomplexed Brensted hydroxyls (as shown by lowtemperature adsorption of CO on the samples pretreated with BF3). This provides for the first time experimental proof that enhanced Bronsted acidity may result from interaction of acid forms of zeolites with Lewis acids. In contrast, formation of complex (11)does not affect the acidity of the zeolite. The fact that formation of complex (I) was not observed in the case of the BF3/H-ZSM-5 system indicates the significance of AI content and/or topological factors for creation of enhanced acidity in zeolites.
1. INTRODUCTION The nature of enhanced Brensted acidity remains one of the key questions in mechanistic studies of the acid-base reactions on zeolites. The enhancement in acid strength is believed to be a result of the interaction between Bronsted sites and Lewis sites created during steaming procedures, since both are present, and since it is known that, in solution, enhanced Brensted acidity can be generated by adding Lewis acid to Bronsted acid [1]. However, as yet there is no direct proof that Lewis sites participate in the enhancement of Brensted acidity. The present study deals with reversible adsorption of a Lewis acid, BF3, on two zeolite materials in the H-form: EMT and ZSM-5, with the aim of creating a model system which demonstrates enhanced acidity without destruction of the zeolitic framework.
* Present address: Koninklijke/Shell Laboratorium, Amsterdam, Postbus 38000, 1003 BN Amsterdam, The Netherlands ** Present address: Laboratory for Inorganic Chemistry and Catalysis, Eindhoven University of Technology, P.O.Box 513, 5600 MB Eindhoven, The Netherlands
80 2, E X P E R I M E N T A L
Carbon monoxide was supplied by BOC (R grade) and boron trifluoride by Aldrich (99.95%+). Two zeolite samples, H-EMT (Si/AI = 4, Na/AI < 0.01) and HZSM-5 (Si/AI = 16, Na/AI < 0.01) were used. 29Si MAS NMR and 27AI MAS NMR proved the absence of dislodged aluminium. The FTIR spectroscopic technique was described in detail elsewhere [2]. Briefly, an infrared cell facilitated high temperature pretreatment of the samples in-situ, and was on-line with the adsorption rig. A self-supporting disk (m - 5-10 mg, p ~ 5 mg. cm 2) was heated at l~ 1 to 350~ under vacuum and held at this temperature overnight (pressure of 10STorr). For spectroscopic/adsorption measurements, the IR cell was lowered into a quartz Dewar filled with liquid nitrogen and small doses of BF3 or CO (0.4 - 1.0/~mol) were admitted to the cell. Adjustment of the sample position within the cell permitted placement of the sample in regions of different temperature. Thus, the temperature of the sample during adsorption/desorption of BF3 ranged from -40 to -100~ during CO adsorption and spectra registration it ranged from -100 to -120~ The spectra were collected using a Cygnus-100 Mattson FTIR spectrometer, at a scanning rate of 100 and a resolution of 2cm 1 in the wavenumoer interval of 2500 4000cm ~ . A single-beam spectrum of thecell (including the quartz Dewar and liquid nitrogen) was used as background. The shifts, ~vOH, due to perturbation with carbon monoxide were determined as peak-to-peak distances in difference spectra obtained by subtraction of the spectra of the samples before adsorption from their spectra after adsorption.
3. RESULTS 3.1 BF 3 interaction with H - E M T
FTIR spectra of the initial H-EMT material and that after low-temperature adsorption of BF3 are shown in Fig. 1. As a result of interaction, the intensity of the high frequency band decreases and new broader bands at 3267cm 1 and 3472cm 1 appear. These new bands most probably correspond to stretching vibrations of Bronsted hydroxyls involved in hydrogen-bonded interaction with adsorbed BF 3 molecules. The adsorption is completely reversible and prolonged pumping at room temperature restores the spectrum of the initial sample. The band at 3472cm 1 disappears after low-temperature evacuation, while the band at 3267cmJ is unchanged. This indicates that the two observed spectroscopic bands can be assigned to different types of adsorption complexes: a stronger complex (I) characterised by the OH stretching vibration at 3267 cm 1 and the weaker complex (It) corresponding to the band at 3472cm ~ . Formation of complex (I) affects the acid strength of the remaining hydroxyls. This conclusion is reached from the results of low temperature adsorption studies of carbon monoxide. The method is based on measurements of the shift in the position of the hydroxyls, AVON, due to their perturbation with CO (at =-120~ [3-5]. Briefly, the larger the shift, the more acidic the hydroxyls. The proton affinities of the hydroxyls and their shifts are related by: PATM / kJ.mol 1 = 2254.8- 442.5 log
(AVOH/
cm 1 )
81
b
,
~__~
S
i
b
/ C
e
1
38'00
'
3400 Naven
1
1
3000
I
1
2600
mbens
Figure 1. FTIR spectra of H-EMT on exposure to BF3: a) initial sample, b) after low temperature adsorption of excess BF3, c) after subsequent low-temperature evacuation.
The shift determined at the limit 0co-,0 (low coverage) reflects the acid strength of the strongest hydroxyls whereas the extrapolation to eco-,1 (high coverage) corresponds to their average acidity. In the present experiment, the fractional coverages with the sorbates were determined from the spectroscopic results as follows: 0BF3 is the decrease in I(HF) (intensity of the high frequency band) due to BF3 adsorption divided by I(HF) before any adsorption; - 0co is the decrease in I(HF) due to BF3 and CO adsorption divided by I(HF) after BF3 adsorption only; - 0 B F 3 + c o i S the decrease in I(HF) due to BF3 and CO adsorption divided by I(HF) before any adsorption. Consequently, Oco = (eBF3+CO- OBF3)/(1 - OBF3). -
82
400 Mathematical treatment of the experimental results on CO adsorption in terms of plots of A v OH VS. OBF3+COfor different starting 0BF3 is given in Fig.2. The different lines correspond to different starting 0BF3 and the crosses at the end of each line correspond to their values in the series of adsorptions of CO. The shifts due to CO perturbation, A YON, become larger as more BF3 is preadsorbed in the form of complex (I). This is direct evidence of the enhancement of the strength of Bronsted hydroxyls resulting from the interaction of the zeolite with BF3 without destruction of the zeolitic lattice. In general, it proves, for the first time the hypothesis that enhanced Brensted acidity can be formed by the interaction of Brensted sites with Lewis acids or Lewis acid sites.
380
360 T
o ~
3/+0
<1 320
300 a
280
260
0
0;2
016 0;8 0BF 3 + CO
Figure 2. Shift in the position of the HF hydroxylsdue to perturbation with carbon monoxide vs. the fractional coverages of the HF hydroxyls with both sorbates, BF3 and CO.
3.2 BF 3 interaction with H - Z S M - 5
FIIR spectra corresponding to low-temperature adsorption of BF3 on H-ZSM-5 are shown in Fig.3. Only one band at 3494cm 1 arises as a result of interaction, accompanied by the decrease in the hydroxyl band from Brensted hydroxyls at 3619cm 1. Therefore, low temperature adsorption of BF3 on H-ZSM-5 results in preferential formation of complex (11). Experiments on carbon monoxide adsorption for acidity measurements, similar to those described above, show no increase in the acid strength of the Bronsted hydroxyls as a result of formation of complex (11). The shift in their position after perturbation with CO, AVON, stays approximately the same, ~310cm 1 , independently on the amount of BF3 preadsorbed, and corresponds to the shift, observed previously for the unmodified H-ZSM-5 sample [5].
4. D I S C U S S I O N
Ab initio HF/SCF molecular orbital calculations (using the 3-21G basis set in
GAUSSIAN92 [6]) show that the most stable adsorption complex is based on an 8-ring cycle and involves zeolite - BF 3 amphoteric interaction :
83 This structure is assigned, Broensted therefore, to the spectroscopically ~O F t", " , acid - base observed complex (I). The predicted .~.H. \ ""r- I complex is characterised by a two-point i . .r-~_.. interaction: a strong H-bond between & I ' B L . _ ~ Lewis BF3 and the Brensted hydrogen and a strong B-O bond, 0.2 ~ longer than the bond lengths observed in boron trioxide and B(OH)3 [7]. No stable complex exists when the B atom interacts with the framework oxygen immediately adjacent to the aluminium atom (in contrast to calculations involving other bases that are predicted to coordinate via two points, such as methanol and ammonia [8, 9]). A complex of this type is not stable for this system due to repulsion effects between the framework and the out-of-plane F atoms.
/I
i b 0.2
S 0 p
b a r]
_ _ _
C e
'
sBbo
1 .....
s4bo
I
sobo
~Aavenumbens Figure 3. FTIR spectra of H-ZSM-5 on exposure to BF3. a) initial sample, b) after low temperature adsorption of excess BF3.
I
26bo
84 The HF interaction energy of the complex shown above is 238 kJ/mol. The small basis set used for these calculations is subject to a considerable Basis Set Superposition Error (BSSE) [10]. This artificially increases the interaction energy by 102 kJ/mol, according to the method detailed in Ref.11. The effects of electron correlation are included via single point Moller-Plesset calculations (MP2) at the SCF optimised geometries. This correction decreases the interaction energy by 5 kJ/mol. (This destabUisation is in contrast to previous MP2 calculations which stabilise the complexes of CO and methanol on a Bronsted acid-site model [8, 11]). The framework oxygen acting as a Lewis base to BF3 is within 4~ of sp 2 hybridisation, thus maximising electron donation. The hybridisation at this oxygen is likely to be intermediate between sp (180~ and sp 2 (120 ~ in the real zeolite, thus reducing the efficiency of electron donation. To estimate the size of this, calculations have been performed with this SiOH angle fixed at 150~ A reduction in SCF interaction energy of 20% is observed. Thus the final estimate of the interaction energy is 102 kJ/mol. The spectroscopic pattern for Bronsted hydroxyls interacting to give complex (11), is typical for hydrogen-bonding perturbation by weak bases such as CH 4 - C6H14 [12] or CHF 3 [13]. Thus the following structure can be proposed for complex (11): m
zAI / O - H...F3B si
"
In this pair BF3 behaves only as a weak base interacting with an acid, which in this case is the zeolite Bronsted hydroxyl. Enhancement of the acidity of the remaining, uncomplexed Bronsted hydroxyls in the case of formation of complex (I) on one hand, and the absence of any effect in the case of formation of complex (11), on the other, concurs with our assignments. A possible mechanism for enhancement of acidity could involve electron density withdrawal from appropriate zeolite oxygen atoms. The fact that complex (I) can be easily formed during BF3 adsorption on H-EMT but not on H-ZSM-5, may have the following reasons. Firstly, the space inside the 10rings of ZSM-5 channels may be insufficient for proper orientation of the BF3 molecule for the 2-point interaction necessary for complex (I). This is illustrated in Fig.4: BF3 has much more space and freedom in the 12-ring of EMT channels for the formation of the complex of preferred geometry. Secondly, it may arise from the different Si/AI ratio in the two zeolites used, which can result in different acidity of the hydroxyls and different bacisity of the framework oxygens, thus changing the affinity of the zeolite towards formation of complex (I). Finally, it is important to note, that behaviour of these two zeolite materials in terms of enhancement of Bronsted acidity due to interaction with Lewis acid BF3 is in remarkable parallel with enhancement of their acidity as a result of steaming. The increase of catalytic activity is much more prominent in the case of US-Y catalysts (steamed faujesite, structure similar to EMT)[14] as compared to steamed H-ZSM-5
[15].
85
Figure 4. Visualisation of a BF3 molecule adsorbed in the zeolite channels of a) ZSM-5; b) EMT
86 ACKNOWLEDGEMENTS
We thank Drs. P.Schultz and T.Des Courieres (Elf France) for H-EMT sample preparation; Prof. V.B.Kazansky (Zelinsky Institute of Organic Chemistry), Dr. A.M.Rigby (Shell Research), Drs. J.Dewing, V.Zholobenko and C.Cundy (UMIST) for useful ideas and discussions. We are also grateful to the EC (BRITE EURAM 4633) for financial support for M.A.M.
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C. Mirodatos and D. Barthomeuf, J.Chem.Soc.Chem.Commun., (1981), 39. M.A. Makarova, V.L. Zholobenko, K.M. AI-Ghefaili, N.E. Thompson, J. Dewing and J. Dwyer, J.Chem.Soc.Farad.Trans., 90 (1994) 1047. E.A. Paukshtis and E.N. Yurchenko, Usp.Khim., 53 (1983) 426. L. Kubelkova, S. Beran and J.A. Lercher, Zeolites, 9 (1989) 539. M.A. Makarova, K.M. AI-Ghefaili and J. Dwyer, J.Chem.Soc.Farad.Trans., 90 (1994) 383. M.J. Frisch, G.W. Trucks, M. Head-Gordon, P.M.W. Gill, M.W. Wong, J.B. Foresman, B.G. Johnson, H.B. Schlegel, M.A. Robb, E.S. Replogle, R. Gomperts, J.L. Andres, K. Raghavachari, J.S. Binkley, C. Gonzalez, R.L. Martin, D.J. Fox, D.J. Defrees, J. Baker, J.J.P. Stewart, J.A. Pople, Gaussian 92, Rev. E.1; Gaussian Inc., Pittsburgh PA, 1992. A.F. Wells, Structural Inorganic Chemistry, 5th Ed., Clarendon Press, Oxford, 1984. S.P. Bates and J. Dwyer, J. Mol. Struct. (THEOCHEM), 306 (1994) 57. E.H. Teunissen, R.A. van Santen, A.P.J. Jansen and F.B. van Duijneveldt, J.Phys. Chem., 97 (1993) 203. J. Sauer, Chem. Rev., 89 (1989) 199. S.P. Bates and J. Dwyer, J. Phys. Chem., 97 (1993) 5897. M.A. Makarova, A.F. Ojo, K. Karim, M. Hunger and J. Dwyer, J.Phys.Chem., 98 (1994) 3619. M.A. Makarova, unpublished results. F. Lonyi and J.H. Lunsford, J.Catal., 136 (1992) 566. R.M. Lago, W.O. Haag, R.J. Mikovsky, D.H. Olson, S.D. Hellring, K.D. Schmitt and G.T. Kerr, Stud.Surf.Sci.Catal., 28 (1986) 677.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviotand S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
87
M o d e l l i n g of s t r u c t u r e a n d r e a c t i v i t y in zeolites C.R.A. Catlow a, R.G. Bell a, J.D. Gale b and D.W. Lewis a aThe Royal Institution of Great Britain, 21 Albemarle Street, London WIX 4BS UK bDepartment of Chemistry, Imperial College of Science, Technology and Medicine, Exhibition Road, London SW7 2AZ 1.
INTRODUCTION Computer modelling techniques are now well established as powerful tools in the study of microporous solids [1-3]. In this article we highlight recent applications using both forcefield and electronic structure techniques to the study of problems in structure, sorption, synthesis and reactivity of both zeolites and microporous aluminophosphates. In particular, we describe how simulation techniques can yield detailed and accurate information of the structure and energies associated with framework modification by hetero-atoms; on the diffusion coefficients and migration mechanisms of sorbed molecules; on the nature of the interactions between host structures and templates used in zeolite synthesis; and on the use of electronic structure techniques in throwing light on the crucial questions of acid site-molecule interactions in solid acid catalysts. In the next section we summarise the range and status of the current computational methodologies before describing the recent illustrative applications. We conclude with consideration of the likely future direction for the field. 2.
METHODOLOGIES The full range of contemporary computer simulation techniques are available for the simulation of microporous systems, and detailed accounts are available in, for example, reference [1]. The present section concentrates on recent developments.
Forcefield methods (i.e. techniques which rest on the specification of interatomic potentials) have been used extensively in modelling the structures of microporous solids and in investigating docking and diffusion of sorbed molecules [1]. The full range of simulation techniques m Energy Minimisation (EM), Monte Carlo (MC) and Molecular Dynamics (MD) have been employed, as have combinations of these methods, as in the widely used 'docking' procedure developed by Freeman and coworkers [4] for identifying low energy sites for sorbed molecules. It is a truism that the reliability of such simulation depends
88 upon the quality of the interatomic potential models employed. Two types of model are used in current studies: first Born model potentials with a shell model treatment of polarisability, which considers the solid in terms of ions interacting via long-range (Coulombic) and short-range potentials. Models based on both formal [5] and partial charges [6-7] are available. Both perform well in modelling structures of zeolites [8] although interestingly, a recent careful evaluation of energy minimised compared to high quality crystallographic data [9] suggested that the earlier formal charge model of Sanders et al. [5] generates the most accurate structures. An alternative approach is provided by "molecular mechanics" forcefields which treat the solid in terms of a covalently bonded network with energy terms depending on the extent of stretching, bending and rotation of bonds from equilibrium values. Again, a number of such parameter sets are available for zeolites [10-11] which work well in modelling structures. Parameters may be derived for both types of forcefield using either empirical methods in which the parameters are adjusted via a least square fitting procedure in order to reproduce the structure and properties of model compounds as closely as possible. Recent work has, however, placed increasing emphasis on the use of ab initio electronic structure techniques to calculate the energy of a periodic array or cluster of atoms, whose geometry is then varied in a systematic manner with the resulting energy surface being fitted to the interatomic potential function. Examples of such an approach are the parameter sets derived by van Beest et al. [6], Purton et al. [7] and Hill and Sauer [11]. As noted, both Born and molecular mechanics models have enjoyed success in modelling zeolites [8-12] as might indeed be expected in view of the character of the Si-O bond which is intermediate between ionic and covalent. In general, Born model potentials are more flexible and allow the inclusion of cations in a more f r a m e w o r k h e t e r o - a t o m s and e x t r a - f r a m e w o r k straightforward manner. Molecular mechanics potentials may possibly represent more accurately the response of the framework to sorbed molecules. Parameters for molecule-framework interaction still rest largely on empirical studies of sorption enthalpies, as in the widely used set for hydrocarbons due to Kiselev [13], while for more complex molecules e.g. alcohols and amines, parameters are generally 'borrowed' from sets developed for organic/biological systems. And, although several successful studies have been reported employing such parameters, there is a clear requirement for more reliably based parameter sets derived from ab initio calculations. The need for such parameterisation will grow as studies extend to an increasingly wide range of microporous solids containing a greater variety of atom types. The most important recent technical developments in the field of simulation concern first the growing use of Grand Canonical Monte Carlo (GCMC) methods to simulate sorption isotherms (as in the recent work of Gorman et al. [14], and increasingly sophisticated 'docking' procedures for locating molecules, especially template species, within zeolite pores; further discussion of the latter will follow below. A further significant development concerns molecular dynamics techniques, which have been used extensively in modelling molecules in zeolites for a period of more than ten years; such
89 techniques are beginning to find a wider range of applications owing to the availability of massively parallel processors which are ideal for undertaking long time scale simulations of the diffusion of sorbed molecules. Such calculations are needed for reliable simulation studies of diffusivity in microporous solids. Overall, simulation studies using interatomic potentials are now a standard technique in zeolite science, with the trend in current work being towards the refinement of interatomic potential parameters and their extension to new types of interactions, and with continuing development and adaptation of simulation techniques to new computer architectures and new problems. The use of explicit electronic structure techniques has also grown considerably during recent years. As noted, such techniques are of importance in the derivation of high quality interatomic potentials. But of even greater significance is their increasing use in modelling fundamental features relating to reactivity including acid site-molecule interactions [2], [15-19] and reaction mechanisms of molecules sorbed at both Br~nsted acid centres [2] and extraframework transition metal ions. The full range of current semi-empirical and ab initio techniques have been employed in electronic structure studies of zeolites, and reviews of both approaches are given by Vetrivel in reference [1] and by Sauer and coworkers in references [1] and [3]. Semi-empirical techniques, if well parameterised, may prove effective in modelling structures and stabilities. Recent work has, however, emphasised the use of ab initio techniques, which, due to algorithmic and hardware developments, may be applied to increasingly large and complex systems. Calculations may be performed on clusters of atoms (as in the majority of studies of zeolitic systems) or on periodic arrays, as in the recent ab initio Hartree Fock studies of Hess and coworkers [20]. Other important features of recent work have concerned the use of large clusters as in the work of Sauer et al. [3], [11], [17]; the location of transition states for reaction mechanisms [2], [16]; and the use of Local Density Functional (LDF) in addition to Hartree Fock (HF) methodologies [16]. The value of the use of the latter techniques on large scale clusters will be evident from the recent work on acid site-methanol interactions described later in this paper. 3.
APPLICATIONS To illustrate the present range and status of computational techniques in zeolite science, we have chosen to highlight recent applications concerning problems in structure, sorption and diffusion, templating and molecule-acid site interactions - - examples which reveal both the strengths and limitations of current approaches. 3.1.
Structural studies of framework hetero atoms in zeolites Manipulation of microporous materials by inclusion of framework substituting metal atoms is an increasingly common method of modifying their properties (including catalytic behaviour). Simulations are an effective tool for studying the structures of such materials and are particularly useful when
90 applied together with experimental techniques such as EXAFS, as the two following recent examples show. 3.1.1. Structural properties of Fe/ZSM-5 Fe modified ZSM-5 has been widely investigated because of the modification effected to its catalytic properties[21]. Although it has been well established that the material can be prepared with predominantly framework substituted Fe, the determination of the detailed structure of the localised state created on substituting Si by Fe is not straightforward. The topic is, however, an ideal one for investigation by contemporary computer modelling techniques. As good interatomic potentials are available for interactions involving all the relevant species, we can generate accurate models for the substitutional site. The most stable structures involve protonation of the neighbouring oxygen site to give the structure shown in Figure 1, in which the Fe atoms form three shorter Fe...O bonds and one longer one to the protonated site. Lewis et al. [22] were able to show that this structure accords very well with earlier EXAFS data of Axon et al. [23]. Indeed, they found that the observed spectra were within experimental error of those calculated from the simulated structures. In addition, the simulations were able to give improved models for the location of the template in the pores of the zeolite with respect to the Fe substitutional. Moreover, they showed how the iron substitutional causes interesting and subtle distortion of the pore geometries which are almost certainly responsible for modifications in the relative diffusivity of sorbed molecules which in turn effect the observed changes in the shape selectivity of these catalysts. 3.1.2. Titanium environments in titanosilicates There is an increasing interest in titanium-containing microporous and mesoporous silicates, due to their catalytic performance for the oxidation of organic compounds under mild conditions as well as their ion-exchange properties [24-30]. In most of these materials, the titanium is located at tetrahedral framework sites [31-34]; however a recently-synthesised class of titanosilicates [35-37], including ETS-10, exists in which Ti 4+ is octahedrally coordinated.
The basic structure of ETS-10 was recently elucidated by Anderson and coworkers [38-40], employing a combination of 29Si NMR, electron microscopy and power X-ray diffraction (using distance-least-squares refinement of bond lengths and angles). The framework is characterised by possessing 12-ring channels running in two orthogonal directions, and also by two sets of Ti-O-Ti chains which run parallel to these channels. Each titanium atom enjoys six-fold coordination, having four Ti-O-Si linkages in addition to the two Ti-O bonds along the chain direction. It was not possible however to obtain a detailed description of the structure by these experimental means, since ETS-10 exists as a highly disordered intergrowth of two polymorphs which differ in the stacking sequence of the 12-ring pores, and also because only powder samples with small particle sizes could be obtained. The structural models assumed a high degree of crystal symmetry which, for instance, constrained all Ti-O bonds to be between 1.87 and 1.91A in length, and moreover were unable to take into account the
91 extra-framework alkali metals ions which are necessary to maintain the charge neutrality of the structure. A further refinement of the local structure around titanium was achieved using a combination of x-ray absorption spectroscopy and computer simulation [41]. Lattice energy minimisation calculations [42] were carried out in which extra-framework sodium and potassium ions were introduced into the model structure on different sets of randomly-distributed positions. The structures were then optimised subject to the constraint of the original model symmetry [38], and those with the lowest lattice energy were then further minimised without regard to any internal symmetry. The most notable feature of these calculations was the wide range of Ti-O bond distances which emerged. These simulated structures, including the alkali cation locations, were used as the basis for interpretation of both XANES and EXAFS spectra (recorded using the Ti Kedge) which had initially provided contradictory evidence, XANES suggesting six-fold coordination of Ti and EXAFS four-fold. H o w e v e r when multiple scattering effects were into account, the EXAFS spectra were able to be u n d e r s t o o d in terms of a distorted octahedral TiO 6 e n v i r o n m e n t , with alternating long and short Ti-O distances (respectively 2.11 and 1.71~) along the Ti-O-Ti chain direction, and four bonds of intermediate length (2.02~) in the TiO-Si linkages. The Ti-O-Ti bond angle was around 165 ~ This geometry, shown in Figure 2, is more reasonable than that originally proposed [38], in which the Ti-O-Ti chains were almost linear, in that it is comparable to that found in other chain titanate materials notably the non-linear optical material KTiO(PO 4) known as KTP [43-44].
Si i
Ti f~t
2.11
~::~---~
~
l o ~~'~ 2~2//
Ti ~. / 2 . 0 2
d'~
2.11 \
\
~
({ o ~
~_~4
Ti
_
~
--
~ :
x.~
~.,
1.82
165 ~
C)Si
Figure 1. Ti environment in ETS-10.
Figure 2. Calculated Fe geometry in Fe-ZSM-5.
Diffusion in zeolites Two complementary approaches are available for modelling the diffusion of molecules in the pores of microporous materials. The most direct is v i a the 3.2.
92 use of the Molecular Dynamics (MD) technique which allows diffusion coefficients to be obtained from the mean square displacements of molecules as a function of time; the method is, however, limited to rapidly diffusing species. In contrast, for slower migration we can use the transition state theory approach which requires the identification of saddle points for the migration process. Recent examples of both are discussed below. 3.2.1. MD simulation of n-butane and n-hexane in silicalite A recent detailed study of this problem was reported by Hernandez [45] using long time scale (1000ps) dynamical simulations. The calculated diffusion coefficients are reported in Table 1. As discussed in reference [45], these are in good agreement with experimental data. Table 1 Diffusion coefficients of n-butane and n-hexane in silicalite as a function of system temperature at a loading of 4 molecules/u.c.
(Ta)
Dtot
(K)
n-butane
n-hexane
Dx
Dy
Dz
(cm2s-lxl05)
(cm2s-lxl05)
(cm2s-lxl05)
209.4
0.93(2)
0.45(2)
2.23(2)
0.109(2)
287.2
2.93(2)
1.71 (2)
6.65(2)
0.415(3)
400.1
4.15(2)
3.75(2)
7.70(4)
0.980(4)
206.6
0.56(1)
0.123(6)
1.555(3)
0.0235(8)
314.1
1.42(2)
0.69(1)
3.42(4)
0.160(2)
380.8
2.17(1)
1.28(2)
4.45(3)
0.383(2)
The simulations also were able to provide insight into the mechanism of molecular migration. From analysis of individual trajectories it was clear that diffusion takes place by means of jumps, i.e. by means of a hopping mechanism. The molecules remain trapped for long times in potential energy minima located at the channel sections, and eventually gain sufficient energy to overcome the potential energy barriers posed by the channel intersections. When this happens the molecule undertakes a rapid jump which is completed in just a few ps, reaching a new minimum. It was observed that at intermediate loadings (4 molecules/u.c.) the jump lengths were of the order of 10~ for both n-butane and n-hexane, which corresponds roughly to the distance between channel sections across intersection regions, which is in good agreement with the average jump lengths determined from neutron scattering experiments by Jobic et al. [46]. Further details are again available in reference [45]. 3.2.2. Transition state approach to hydrocarbon diffusion in DAF-1 Despite the success and value of MD techniques, they are, as noted above, inherently limited in that the number of diffusion events may be insufficient
93 during the 'real time' sampled by these simulations. In these cases, which occur with more slowly diffusing species, then a transition state theory becomes more appropriate, with the transition state being identified by a form of constrained minimisation [47]. In this method, the diffusion pathway, or at least an initial trajectory, is pre-defined and a series of energy minimisation calculations is carried out at close intervals as the molecule moves in a stepwise fashion along a proposed migration trajectory. Thus the minimum energy profile may be obtained. In a recent study [48] this method was applied to the diffusion of the four isomers of butene in the microporous aluminophosphate DAF-1. This framework contains two separate parallel channel systems defined by 12-ring pores. One of these is one-dimensional, while the other consists of a threedimensional network of channels, cross-linked by smaller 10-ring pores. The two channel systems are connected to each another by only by 8-ring pores. It was found that the butene molecules were able to access the entire network of the three-dimensional channel system, with the highest diffusion barriers lying in the range 17 to 22 kJmo1-1. However none of the isomers wer(~ able to pass between the two distinct channel systems, thus rendering them completely independent. The migration route identified by the calculation is shown in Figure 3.
I u
Figure 3. Calculated diffusion trajectory of isobutene between the double 10rings in DAF-1 together with the associated energy profile.
3.3
Template-Host Interactions The use of organic bases in the synthesis of microporous materials as structure-directing agents or templates is widespread. Indeed, of all the factors influencing the formation of microporous structures, it is this templating action
94 that appears to be the most likely to allow access to, and control over, new structural types, particularly in the aluminophosphate family of materials. Moreover, the current focus of attempts at designing new materials is on the selection of templates which have specific features which are expected to be reflected in any structure formed [49]. However, the exact function of templates is not understood and the extent to which they are critical to the formation of such materials under debate [50]. In order to establish whether this effect can be quantified in any way and to determine if computational methods can be applied to design new microporous materials, we have studied extensively the interactions of large range of templates and framework types [48, 51]. In order to locate templates in frameworks and to determine their interactions we have applied the methodology of Freeman et al. [4] which, as we have noted, is based on a combination of molecular dynamics, Monte Carlo and energy minimisation techniques. For a selected molecule, a library of conformations are generated using molecular dynamics, which are then docked into a framework using a Monte Carlo procedure. Suitable conformations are accepted according to a supplied energy cut-off and subsequently energy minimised producing a library of low energy configurations. We have shown that the method can successfully locate templates at experimentally determined positions [48, 51], in studies of cyclic templates in NU-3, tetrapropylammonium in ZSM-5 as well as in simulation of the postulated positions of tri-quaternary amine used in ZSM-18 synthesis and for hexamethonium in EU-1. Furthermore we have demonstrated a correlation between the ability of a template to form a particular framework with the nonbonding interactions of that template with the framework, as demonstrated in Figure 4 and further in Table 2 where the technique is successful at selecting the correct template for a given framework. Simulations of this type can therefore provided a quantification of the experimentally noted "good fit" of templates in the frameworks they form [52]. The crucial r61e of non-bonding interactions between the framework and the template is further emphasised by the ability of the technique to distinguish subtle effects such as small changes in unit cell parameters. For example, zeolite NU-3, when synthesised with two different cyclic templates, forms with unit cell parameters that are noticeably different (by -1.5%) [53]. Calculations (Table 3) show that the most stable template / unit cell combinations are those that are found experimentally. Furthermore, the template which best matches that of the hydrated calcium ion found in the natural analogue Levyne [54] has a higher binding energy in the unit cell of the mineral form. These results provide a basis on which we may now begin to attempt to apply computational methods in assisting the synthesis of new materials. The success of this work will, furthermore, allow us to probe further the nature of templating and the synthesis of microporous materials in general; a correlation between crystallisation rate and template-framework interactions has also been demonstrated using similar computational methods [55].
95 Table 2 a) Non-bonded interactions energy of tetraalkyl ammonium cations in various siliceous frameworks. Templates predicted as effective are emboldened. *Experimental template. b) Packing energy, i.e. the interaction between template molecules, of tetraalkyl ammonium template / framework combination when two adjacent template molecules are included. From these two figures, we can select the most suitable template for each framework. i
a) ZSM-5
ZSM-11
Einter (kJ mo1-1)
Einter (kJ mo1-1)
Einte r (kJ tool -1)
TMA
-51.7
TMA
-38.7
TMA
-43.1
TEA
-92.1
TEA
-73.0
TEA*
-104.7
TPA*
-133.9
TPA
-119.9
TPA
-83.4
TBA
-165.5
TBA*
-159.5
TBA
-56.7
i
b)
Template / framework
AEpack (kJ mo1-1)
TPA / ZSM-5
-29.7
TBA / ZSM-5
+14.9
TBA / ZSM-11
-18.3
TPA / ZSM-11
-8.5
Table 3. The interaction energies of the templates N-methylquinuclidinium (N-MeQ) and 1-aminoadamantane (AmAdam) in the different LEV unit cells. Framework (template)
Lattice parameter (A) a
NU-3 (N-MeQ) [53]
NU-3 (AmAdam) [53]
Levyne [54]
13.0595
13.2251
13.338
Template
Einter / kJ mo1-1
N-MeQ
-96.3
AmAdam
-94.4
AmAdam
-95.4
N-MeQ
-90.0
AmAdam
-90.1
N-MeQ
-92.1
c
22.6061
22.2916
23.014
96
---e--- tetraalkylammonium cations A bis-quaternary amines v c~0-diamines [] saturated cyclic • unsaturated cyclic -40
9
V
=
-80 E / ld mol
-1 r A&
-120-
-160-
I
50
'
I
100
'
I
150
'
I
200
'
I
250
'
Molecular Volume / ~ 3 Figure 4. Interaction Energy of experimental framework / template combinations as a function of molecular volume. Straight line is the best fit through all datapoints. Although the overall correlation shows considerable scatter, if single homologous series are considered then the correlation between template size/shape and interaction energy is excellent, as is evident for the tetraalkylammonium cations (highlighted with a line) and the c~,c0-diamines. 3.4.
Acid site-methanol interactions There has been much debate concerning the nature of the adsorbed state of methanol at Bronsted acid sites within microporous materials, in particular the question of whether there exists a chemisorbed complex in which the proton is transferred from the framework to form the methoxonium cation. We have performed ab initio gradient corrected density functional calculations to examine this question based on cluster models containing both three and four tetrahedral sites (H3SiOHAI(OH)2OSiH3 and H3SiOHAI(OH)(OSiH3)2 respectively). The physisorbed complex of methanol, in which two hydrogen bonds are formed with the acid site, is found to be the most stable configuration with a binding energy of 64.1 and 58.3 kJmo1-1 for the Becke-Perdew (BP) and Becke-
97 Lee-Yang-Parr (BLYP) functionals in good agreement with the lower bound of experimental estimates. No minimum exists corresponding to the chemisorbed (protonated) species and this complex corresponds to a transition state for methanol mediated proton exchange between framework oxygens. The BeckePerdew functional closely reproduces the geometry of the physisorbed complex obtained by Haase and Sauer [17] at the MP2 level. Detailed results for the 3T cluster model are given in Table 4 Table 4 Properties of methanol adsorbed on the 3T cluster model for BP and BLYP gradient corrected functionals, with the Hartree Fock and MP2 results of Haase and Sauer [17] included for comparison (Ore = oxygen of methanol, Hm = h y d r o g e n of methanol, 02 = zeolite oxygen of OH group coordinated to methanol, Oz' = oxygen of zeolite hydrogen bonding to Hm). |l
i|
BP
BLYP
HF [17]
MI~ [17]
Binding energy (kJmol-1) *
69.86
63.75
49.4
79.0
r ( O m - Hm) (]~)
1,009
1.001
0.953
0.993
r(Om - C) (]~)
1.449
1.460
1.404
1.425
r(Oz - Hz) (]k)
1.051
1.035
0.971
1.033
r ( O m - Hz) (]k)
1.498
1.569
1.734
1.499
r(Oz' - Hm) (]~)
1.727
1.837
2.166
1.737
r(A1 - Oz) (]k)
1.931
1.945
1.904
1.896
Z(Hz - Oz - A1) (o)
110.49
110.95
Z ( H m - O r e - C) (o)
109.46
109.18
* Values prior to correction for zero point energy. ZPE correction for BLYP = 5.43 kJmo1-1. Calculated vibrational frequencies for this complex alone are unable to rationalise the experimental infra red spectra. However, large shifts are predicted for the framework hydroxyl to 2442-2675 cm -1, depending on the functional used, which accords with the observation of a broad band at 2400cm -1 when allowing for the fact that larger cluster models will further lower the calculated frequencies. Preliminary calculations have been performed to examine the effect of increasing the loading of methanol to two molecules per acid site. The a d d i t i o n a l m e t h a n o l molecule fails to stabilise the f o r m a t i o n of the m e t h o x o n i u m ion and the m i n i m u m energy configuration contains the two
98 molecules in an eight membered hydrogen bonded ring. The heat of adsorption for the second molecule is found to be only 8 kJmo1-1 less than that for the first. Although no evidence was found for the formation of a methoxonium ion within the framework of cluster calculations, a parallel s t u d y based on periodic boundary conditions for methanol in chabazite indicates that this may in fact be the stable species when long range interactions are taken into consideration [56]. 4.
CONCLUSIONS AND FUTURE DIRECTIONS This brief review has, we hope, illustrated the diverse range of problems that can now be investigated by computer modelling techniques. The future of the field will involve accurate and precise 'forcefield' calculations on highly complex systems using reliable parameterisations; and high level quantum mechanical calculations on reaction mechanisms. The c o n t i n u i n g developments in both hardware and software will lead to an increasingly prominent r61e for computational techniques in zeolite science. 5.
ACKNOWLEDGEMENTS We are grateful to J.M. Thomas, C.M. Freeman, M.J. Anderson, S.W. Carr, E. Hernandez, P. Voigt, J. Sauer and R.A. van Santen for contributions to and discussions concerning this work. We wish to thank BIOSYM Technologies, Unilever and EPSRC for their support. REFERENCES
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47. 48.
49. 50. 51. 52. 53. 54. 55. 56.
T. Ohsuna, O. Terasaki, D. Watanabe, M.W. Anderson, S. Lidin, Stud.Surf.Sci.Catal., 84 (1994) 413. M.W. Anderson, O. Terasaki, T. Ohsuna, P.J. O'Malley, A. Philippou, S.P. MacKay, A. Ferreira, J. Rocha, S. Lidin, Phil.Mag. B, in press. G. Sankar, R.G. Bell, J.M. Thomas, M.W. Anderson, P.A. Wright & J. Rocha, to be published R.A. Jackson, S.C. Parker, P. Tschaufeser in Modelling of Structures and Reactivity in Zeolites. C.R.A. Catlow (ed.), Academic Press, London 1992. N.K. Hansen J. Protas. G. Marnier. Acta.Cryst.B 47 (1991) 660. M.M. Eddy, T.E. Gier, N.L. Keder, G.D. Stucky, D.E. Cox, J.D. Bierlein, G. Jones, Inorg.Chem. 27 (1988) 1856. E. Hernandez and C.R.A. Catlow, Proc.Roy.Soc., 448, (1995) 143. H. Jobic, M.Bee and J. Caro, Proc. 9th Int. Zeolite Conf, (eds. R. von Balmoos, J.B. Higgins and M.M.J. Tracy) Butterworth-Heinemann, Boston, 1993. GCMC Solid Docker Module, verion 5.0, BIOSYM Technologies Inc., San Diego, 1994. R.G. Bell, D.W. Lewis, P. Voigt, CM Freeman, J.M. Thomas, CRA Catlow Stuc Surf Sci Catal, 84, 2075 (1994). S.I. Zones, M.N. Olmstead, D.S. Santilli, Journal of the American Chemical Society, 164 (1992) 4195. Davis, M. E.; Lobo, R. F. Chemistry of Materials 4 (1992) 759. D.W. Lewis, C.M. Freeman, C.R.A. Catlow, Journal of Physical Chemistry 1995, in press, H. Gies, B. Marler, Zeolites 12 (1992) 42. L.B. McCusker, Materials Science Forum 423 (1993) 133-136. S. Merlino, E. Galli, A. Alberti, Tschermaks Mineral. Petrogr. Mitt. 22 (1975) 117. T.V. Harris, S.I. Zones, Struc.Surf.Sci.Catal., 84 (1994) 29. R. Shah, J.D. Gale, M.C. Payne and V. Heine n manuscript in preparation.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 1995 Elsevier Science B.V.
101
Computational studies of water adsorption in zeolites S. A. Z y g m u n t a, L. A. C u r t i s s b, a n d L. E. Iton b aDepartment of Physics and Astronomy, Valparaiso University, Valparaiso, IN 46383 (email:
[email protected]) bArgonne National Laboratory, curtiss @ cmt.anl.gov)
Argonne,
IL
60439
(email"
We have performed high-level ab initio calculations using Hartree-Fock (HF) theory, Mr perturbation theory (MP2), and density-functional theory (DFT) to study the geometry and energetics of the adsorption complex involving H 2 0 and the BrCnsted acid site in the zeolite H-ZSM-5. These calculations use aluminosilicate cluster models for the zeolite framework with as many as 28 T atoms (T=Si,A1). We included geometry optimization in the local vicinity of the acid site at the MP2 and DFT levels of theory for the smallest cluster, while in the larger clusters this was done at the HF/6-31G(d) level of theory. We have also calculated corrections for zero-point energies, extensions to higher basis sets, and higher levels of electron correlation. Results for the adsorption energy and geometry of this complex are reported and compared with previous theoretical and experimental values.
1. I N T R O D U C T I O N BrCnsted acid chemistry is a dominant feature of the catalysis by zeolites in many important industrial applications. A reliable theoretical treatment of the proton affinity of H-ZSM-5 zeolite has been obtained in previous work using high-level ab initio calculations on large cluster models [1]. Proton transfer to a strong adsorbed base has also been studied theoretically in the interaction of ammonia with the BrCnsted acid site [2-4]. The interaction of weak bases, e.g., H 2 0 , presents a more equivocal situation. What kind of equilibrium structure is formed when H 2 0 is adsorbed at the Brr acid site in H-ZSM-5? Some e x p e r i m e n t a l evidence, most notably in the form of IR spectroscopy, has suggested that the acidic proton is transferred to H 2 0 and that an ion-pair structure is most stable [5,6]. One ab initio theoretical study [7] also gave evidence to support this conclusion. However, recent ab initio calculations have disputed this claim and have led to a reinterpretation of the IR spectra in a manner consistent with a neutral hydrogen-bonded adsorbate [8,9]. These studies have suggested that the ion-pair complex should be regarded as a transition state and not a true equilibrium geometry. However, almost all of these theoretical studies have used small clusters to represent the acid site, and none have accounted for electron correlation in their optimization of molecular geometries, so they are open to criticism on the basis of these limitations.
102
The calculations presented here extend our earlier study of the adsorption of H 2 0 on a 2 T atom cluster model of H-ZSM-5 [10]. They incorporate local geometry optimization, large cluster size, and electron correlation in a unified way. We have obtained results for the geometry and adsorption energy of the neutral H 2 0 adsorption complex in H-ZSM-5, and have found that it is more stable than the ion-pair structure. The calculated adsorption energy is roughly consistent with the published experimental value. 2. T H E O R E T I C A L M E T H O D S The theoretical calculations presented here are based on ab initio molecular orbital theory [11,12] and density-functional theory [13]. We used four aluminosilicate clusters of increasing size to model the BrCnsted acid site in HZSM-5. These clusters include 3, 8, 18, and 28 T atoms, and have a total number of 14, 34, 69, and 101 atoms, respectively. Each cluster includes one A1 atom and a c h a r g e - b a l a n c i n g proton to maintain a neutral zeolite framework, and is terminated by H atoms at the periphery. Their stoichiometries are H 9 S i 2 A 1 0 2 , H 1 9 S i 7 A 1 0 7 , H 2 9 S i 1 7 A 1 0 2 2 , and H 3 3 S i 2 7 A 1 0 4 0 , respectively. For the 3 T atom cluster, the adsorption complex was found by using full geometry optimization. This allowed us to calculate zero-point vibrational energy corrections directly. These corrections were then used as estimates for the complexes between H 2 0 and the larger cluster models. For the 8 T atom cluster, the constant-volume relaxation (CVR) method [1] was used. Atoms at the periphery of the cluster were fixed at positions determined from x-ray diffraction studies of H-ZSM-5 [14], while the central 0 3 S i O H A 1 0 3 atoms near the acid site were fully relaxed. In addition, the six intermolecular degrees of freedom between the framework and the adsorbate molecule were fully optimized. The H 2 0 and H3 O+ geometries were held fixed at the optimized geometries of the isolated molecules. This constrained relaxation scheme is a useful model for the effect of an adsorbate on the local structure of the acid site. After CVR optimization, the effect of more distant atoms was then included by embedding this 8 T atom cluster in successively larger fragments of crystalline H-ZSM-5 to obtain first the 18 T atom and then the 28 T atom cluster. This procedure yielded a cluster size energy correction term. Our procedure is open to the criticism that the intermolecular degrees of freedom were not re-optimized in the 18 and 28 T atom clusters, but we tested its accuracy by re-optimizing the intermolecular coordinates of the 18 T atom complex with H 3 0 + at the HF/3-21G level and comparing them to those obtained in the 8 T atom cluster. Changes in bond lengths and angles on the order of 3-5 % were noted, but the total energy after re-optimization decreased by less than 1 kcal/mol, so we believe that this procedure is justified. Both 3-21G and 6-31G(d) basis sets were used in the calculations involving the 8, 18, and 28 T atom clusters, while in the 3 T atom cluster an additional calculation was performed with the 6-311+G(3df,2p) basis set. This result for the smallest cluster model allowed us to find a basis set energy correction, which was then used as an estimate for the larger clusters based on G2(MP2) theory [15]. In the 3 T atom cluster, the effect of electron correlation was treated by MP2 theory, the non-local BLYP formulation of DFT [16], and quadratic configuration interaction (QCISD(T)) [17]. These results allowed us to find correlation energy corrections which were used as estimates for the larger clusters. They also
103
provided a test of the accuracy of MP2 theory and BLYP more computationally demanding QCISD(T) method. 3. R E S U L T S
AND
when
compared
to the
DISCUSSION
The reaction energies of H 2 0 i n t e r a c t i n g at the h y d r o x y l site of aluminosilicate clusters of 3, 8, 18, and 28 T atoms representing H-ZSM-5 are listed in Tables 1 and 2. The quantities calculated in this study include A E i o n , A E c o v , AErel, and AEdesorp. The AEion is the complexation energy of the ionic complex
(1)
AEion = E(Z-) + E(H3 O+) - E(Z-...OH3 +) where
Z- is the unprotonated zeolitic cluster and Z-....OH3 + is the ion-pair complex.
Table 1 H 2 0 reaction energies, AEcov and AEion (in
kcal/mol), for different cluster sizes a
AEcov
AEion
Method/Basis
3
8
18
28
3
8
18
28
HF/3-21G
-
31.7
34.8
34.2
-
154.6
154.2
154.6
HF/6-31G(d)
15.2
14.7
17.0
16.7
138.4
131.1
130.9
129.7
MP2/6-31G(d) b
22.1
20.2
-
-
150.1
138.1
-
-
aAll results are from the constant volume relaxation procedure as described text, except for cluster 3, which is a full geometry optimization. Cluster 3 is illustrated in Figure 1. Zero-point energies not included in values. bAt HF/6-31G(d) geometry for cluster 8.
Table 2 H 2 0 reaction energies, AErel and AEdesorp (in
kcal/mol), for different
in
cluster
sizes a
AErel
AEdesorp
Method/Basis
3
8
18
28
3
8
18
28
HF/3-21G
-
-14.1
-13.0
-10.4
-
31.7
34.8
34.2
-14.5
-16.9
-14.7
-13.3
15.2
14.7
17.0
16.7
-6.2
-12.4
-
-
22.1
20.2
-
-
HF/6-31G(d) M P 2 / 6 - 3 1 G(d) b -i
i-
aAll results are from the constant volume relaxation procedure as described text, except for cluster 3, which is a full geometry optimization. Cluster 3 is illustrated in Figure 1. Zero-point energies not included in values. bAt HF/6-31G(d) geometry for cluster 8.
in
104
The AEcov
is the complexation energy of the covalent (hydrogen-bonded) complex
(2)
AEcov = E(ZH) + E(H20) - E(ZH...OH2)
where ZH is the protonated zeolitic cluster and ZH .... OH2 is the covalent complex. The AErel is the energy difference between the covalent and ionic complexes
(3)
AErel = E(ZH...OH2) - E(Z-...OH3 +)
where a positive value indicates that the ionic complex is more stable. The AEdesorp is the energy required to remove the H 2 0 molecule from the most stable complex (ionic or covalent) AEdesorp = E(ZH) + E(H20) - min[E(Z-...H30+), E(ZH...OH2)]
(4)
The results in Tables 1 and 2 for these energies are based on calculations which include relaxation of the local region of the cluster near the hydroxyl site as described in the previous section. Results for the 3 T atom cluster, for which full optimizations were carried out, are also included in the table. The structures of the ion-pair and hydrogen-bonded configurations for the 3 T cluster are illustrated in Figure 1. A comparison of the calculated geometrical parameters at HF, MP2, and BLYP levels of theory using the 6-31G(d) basis set is shown in Table 3. At all these levels, the ion-pair structure is a transition state, while the covalent structure is a local minimum in the potential energy surface. In Table 3 note p a r t i c u l a r l y the significant influence of electron c o r r e l a t i o n on the h y d r o g e n - b o n d distances OH1 and OH2 in the covalent complex. Complexation energies for both structures are also strongly influenced by correlation. Also note that MP2 and BLYP give very similar results for the covalent structure.
('~
OH 9
', OH
.s
l
(a)
~
(~
,~, OH1 "'"
OH2 ""
sSa
(b)
Figure 1. Optimized structures of (a) ion-pair and (b) hydrogen-bonded (covalent) configurations for H 2 0 interacting with the hydroxyl site in an aluminosilicate cluster with three T atoms.
105
Table 3 Structure and Energetics of 3 T Atom Clusters at Various Levels of Theory (see Figure 1 for explanation of coordinates) a
HF/6-31G(d)
MP2/6-31G(d)
1.42 96.8 138.4
1.36 95.4 150.1
OH 1 OH 2 OAIO AEco v
2.01 1.75 98.5 15.2
1.81 1.61 97.2 22.1
1.76 1.56 97.2 22.3
AEre I
-14.5
-6.2
-4.0
ion-pair
structure
OH OAIO AEio n covalent
aBond
BLYP/6-31G(d)
structure
lengths in /~, angles in degrees, and energies in kcal/mol.
The HF/6-31G(d) calculations for the 8 T atom cluster also show that the lowest energy structure is a h y d r o g e n - b o n d e d adsorption complex between H 2 0 and the zeolite framework, in which the adsorbate is anchored to the framework by O...H linkages of 1.70 and 2.06 /~. These bond lengths are within 0.05 /~ of the HF results for the 3 T atom cluster shown in Table 3, and they are nearly the same as those found in a previous study [18] using a slightly different DZP basis set and a smaller 3 T atom zeolite cluster. The adsorbed complexes ZH...OH2 and Z-...OH3 + are shown in Figure 2 for the 8, 18, and 28 T atom clusters. The terminating H atoms at the periphery of these clusters are not shown in the figure. Table 4 shows our energy corrections to the calculated desorption energy of 14.7 kcal/mol in the 8 T atom cluster. Extending the cluster size to 28 T atoms [A(CS)] i n c r e a s e d the binding energy by 2.0 kcal/mol, and treating electron correlation in the 3 T atom cluster by MP2 theory [A(MP2)] gave an additional increase of 6.9 kcal/mol. Correlation effects beyond the MP2 level using the 631G(d) basis [A(QCI)] decreased the desorption energy by 0.9 kcal/mol in the 3T cluster. The zero-point energy correction [A(ZPE)] from the 3 T cluster gave a 2.9 kcal/mol decrease in desorption energy, and the correction from G2(MP2) theory for extension to a higher-level 6-311+G(3df,2p) basis set [A(BS)] gave an additional 5.5 kcal/mol decrease. This gives a best estimate of 14.3 kcal/mol for the desorption energy of H 2 0 in H-ZSM-5 at 0 K. Thermal effects will cause this value to decrease, but it is unclear if the usual ideal gas corrections used in theoretical enthalpy calculations [19] are valid in the restricted environment of the zeolite interior.
106
(a)
(c)
(e)
(b)
(d)
(f)
Figure 2. Z-...OH3 + and ZH...OH2 complexes for cluster size 8 (a,b), 18 (c,d), and 28 (e,f). Terminating H atoms not shown.
107
A previous theoretical estimate [20] for the desorption energy in a 2 T atom cluster was 11.2 kcal/mol, but this approach included no correction terms besides A(MP2) and only allowed for a single O...H linkage between the ZH framework and the H 2 0 molecule. Our earlier study [10], using a similar cluster model but including A(MP2), A(QCI), and A(BS) corrections, gave a value of 10.8 kcal/mole. Other theoretical work using a 3 T atom cluster [7] resulted in a value of 14.8 kcal/mol, but it did not account for the A(CS), A(QCI), and A(BS) terms calculated here. By comparison, the experimental enthalpy of adsorption, which was obtained from a Clausius-Clapeyron analysis of p(T) data between 357 and 435 K for a coverage of less than one molecule per acid site, was found to be 12+1 kcal/mol [5]. The result presented here, in light of the necessary thermal corrections, is consistent with experiment.
Table 4 Energy Corrections to AEdesorp (in kcal/mol) from 8 T Atom Cluster
AEdesorp (HF/6-31G(d))
14.7
A(CS) A(MP2) A(QCI) A(ZPE)
2.0 6.9 -0.9 -2.9
~x(Bs)
-5.5
AEdesorp (best estimate)
14.3
4. C O N C L U S I O N S Using aluminosilicate cluster models of up to 100 atoms, we have carried out a computational study based on ab initio molecular orbital theory of adsorption of H 2 0 at the acid site in the H-ZSM-5 zeolite. The computations incorporate local geometry optimization, large cluster size, and electron correlation in a unified way. The results indicate that the neutral H 2 0 adsorption complex is more stable than the ion-pair structure previously proposed, which is probably a transition state. The calculated adsorption energy is roughly consistent with the published experimental value. Our results show the usefulness of such computations in investigations of reactions at the acid site in zeolites. It is also evident that the MP2 and the BLYP methods give correlation corrections for the desorption energy which are less than 1 kcal/mol from that of the more sophisticated QCISD(T) technique. Since the BLYP method can be i m p l e m e n t e d with a lower computational cost than MP2, it may be useful in future adsorption studies. ACKNOWLEDGMENTS This work was supported by the U. S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences, under Contract No. W-31-109ENG-38. We acknowledge a grant of computer time at the National Energy Research Supercomputer Center. Acknowledgment is also made to the Donors of
108
the Petroleum Research Fund, adminstered by the American Chemical Society, for partial support of this research (S.A.Z.). REFERENCES
1. H. V. Brand, L. A. Curtiss, and L. E. Iton, J.
Phys. Chem. 97 (1993) 12773.
2. H. V. Brand, L. A. Curtiss, and L. E. Iton, J. Phys. Chem. 96 (1992) 7725. 3. E. H. Teunissen, A. P. Jansen, and R. A. van Santen, J. Phys. Chem. 99 (1995) 1873. 4. J. Sauer, P. Ugliengo, E. Garrone, and V. R. Saunders, Chem. Rev. 94 (1994) 2095. 5. A. Ison and R. J. Gorte, J. Catal. 89 (1984) 150. 6. M. T. Aronson, R. J. Gorte, and W. E. Farneth, J. Catal. 105 (1987) 455. 7. J. C. Sauer, C. Kolmel, F. Haase, and R. Ahlrichs, in Proceedings of the Ninth International Zeolite Conference, (Montreal 1992) R. von Ballmoos et al., eds., Reed Publishing, Stoneham, MA, Vol. I (1993) 679. 8. A. G. Pelmenschikov and R. A. van Santen, J. Phys. Chem. 97 (1993) 10678. 9. F. Haase and J. Sauer, J. Phys. Chem. 98 (1994) 3083. 10. S. A. Zygmunt, H. V. Brand, D. J. Lucas, L. E. Iton, and L. A. Curtiss, J. Mol. Struct. (Theochem) 314 (1994) 113. 11. W. J. Hehre, L. Radom, J. A. Pople, and P. v. R. Schleyer, Ab Initio Molecular Orbital Theory. (John Wiley, New York) 1987. 12. Gaussian 92, M. J. Frisch et al. (Gaussian, Inc., Pittsburgh, PA 15106) 1992. 13. R. G. Parr and W. Yang, Density-Functional Theory of Atoms and Molecules (Oxford Univ. Press, New York) 1989. 14. H. van Koningsveld, H, Van Bekkum, J. C. Jansen, Acta Crystallogr. B43 (1987) 127. 15. L. A. Curtiss, K. Raghavachari, and J. A. Pople, J. Phys. Chem. 98 (1993) 1293. 16. P. M. W. Gill, B. G. Johnson, J. A. Pople, and M. J. Frisch, Chem. Phys. Lett. 197 (1992) 499. 17. J. A. Pople, M. Head-Gordon, and K. Raghavachari, J. Phys. Chem. 87 (1987) 5968. 18. J. Sauer, H. Horn, M. Haser, and R. Ahlrichs, Chem. Phys. Lett. 173 (1990) 26. 19. J. E. Del Bene, H. D. Mettee, M. J. Frisch, B. T. Luke, and J. A. Pople, J. Phys. Chem. 87 (1983) 3279. 20. E. Kassab, K. Seiti, and M. Allavena, J. Phys. Chem. 95 (1990) 9425.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviotand S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
109
Modeling of adsorption properties of zeolites A. Goursot, I. Papai*, V. Vasilyev and F. Fajula URA 418 CNRS, Ecole Nationale Sup6rieure de Chimie de Montpellier 8, Rue de l'Ecole Normale, 34053 Montpellier C6dex 1, France
1. INTRODUCTION It is well k n o w n experimentally that d e h y d r a t e d A or X zeolites adsorb nitrogen when a gas mixture containing N 2 and 0 2 circulates through their large pores. It has been shown that this effect of N2 concentration, which has been attributed to the interaction b e t w e e n the zeolite cations and the nitrogen quadrupole, is dependent on the nature of the counterions [1-3]. Indeed, A or X zeolites exchanged with lithium or calcium have been reported as much more efficient for N2 a d s o r p t i o n than Na zeolites [2, 3]. A l t h o u g h extensive experiments have been performed in this field, there has been, up to now, no fundamental study of the different adsorption properties of N 2 and 0 2 in zeolites. Our aim is thus to investigate how N 2 and 0 2 interact with different cations and to delineate how large is the role of a purely electrostatic effect in the adsorption process. To this end, the interactions of N 2 and 0 2 with a positive point charge, with isolated Na + or Li + cations, with their dimers and with a small model cluster of zeolite have been analysed and compared. Moreover, the role of the ionic nature of the zeolite structure around the adsorption cationic site has also been investigated, using the embedding of a large n e t w o r k of point charges. All these calculations are first-principle calculations based on density functional theory (DFT).
2. M O D E L S
AND METHODS
As a general notation for the investigated systems the two molecules (N 2 and 0 2) and the two cations (Li + and Na +) will be generally referred to as Y2 and X+, respectively. The systems which correspond to the interaction of Y2 molecules with a positive point charge (q+) and with X+ will be denoted as q+Y2 and X+Y2. Those including the pentameric zeolite model will be denoted as m(X+)Y2 .
* permanent address : Institute of Isotopes of the Hungarian Acacademy of Sciences, H-1525 Budapest, P.O.B. 77, Hungary
110 Since the linear geometry was proven to be the most stable (cf. 3.1), the potential energy curves for X+Y2 (q+Y2) were obtained by varying the X-Y (q-Y) distance in the linear X...Y-Y arrangement with a Y-Y bond length fixed at the equilibrium value of the free Y2 molecules. These curves have been fully corrected for basis set superposition error (BSSE). The small zeolite cluster (Figure 1) we have chosen does not represent any specific site of zeolite X. Its use was necessary in order to analyse the role of the zeolite framework in the interaction between a cation and the N2 or 0 2 molecule. It also allowed to verify the replacement of the zeolite by a network of point charges. In this m(X +) cluster, the dangling bonds of the four Si a t o m s are saturated with H atoms with a fixed Sill bond distance of 1.50 A, and the Sill bonds are aligned with the corresponding SiO bonds of the X-ray structure of zeolite X [4]. The geometries of the m(X +) clusters have been optimized as described previouly [5]. Y2 was then approached to X+ in the OXO plane in a linear B-X...Y-Y arrangement, where B is the midpoint of the two oxygen atoms bonded to X+. Varying only the X-Y distance (r(X-Y)), i.e. fixing the geometry of the m(X +) clusters and the Y2 molecules at their equilibrium structure, the interaction energy has been evaluated as a function of r(X-Y). The BSSE corrections were calculated in the same way as for the X+Y2 systems. Assuming that the long range interactions were essentially electrostatic, Li+-N2 and Li+-O2 interaction energies were estimated when Li + was positionned either in a site II or in a site III of a faujasite framework and embedded in a network of point charges representing the atoms of 27 unit cells (18143). The point charges used for these calculations were +1.0, +1.245, +1.897, -1.0355 for Li, A1, Si and O centers, respectively. These values have been obtained through a fitting of the electrostatic potential of several model clusters of zeolite. The density functional calculations were performed within the LCGTO-DF formalism [6-8] using the deMon program package [9-11]. All the calculations, except for the geometry optimization of the m(X +) clusters was carried out at a nonlocal level, using Becke's exchange [12] and Perdew's correlation [13] functionals. Geometry optimization was performed at the local level of theory using the Dirac exchange term and the Vosko-Wilk-Nusair parameterization [14] for the correlation energy. The basis sets used are described in Ref. 5 and 15. The energy of the interaction of the Y2 molecules with one or several point charge(s) is calculated by including the related electrostatic terms in the Hamiltonian.
3. RESULTS AND DISCUSSION 3.1. Interaction of N2 and 0 2 with a point charge The interaction energies of N 2 and 0 2 with a single positive point charge in the linear arrangement are shown in Figure 2 as a function of the distance R between the point charge and the midpoint of the diatomic molecules. Although this model is an extreme simplification of the real Y2-zeolite interaction, the
111 fundamental principle of oxygen separation is clearly s e e n : the interaction energy of N 2 is significantly larger than that of 0 2, for the entire range of R. This trend can be easily related to the difference in the physical properties of N 2 and 0 2 molecules. Indeed, it follows from the general theory of long-range interactions [16] that the two leading terms in the interaction energy of a point charge and a h o m o n u c l e a r diatomic molecule are those associated w i t h the q u a d r u p o l e moment (0) (electrostatic term) and the dipole polarizability ((x) (induction term) of the diatomics. The angle d e p e n d e n t formulas for the electrostatic and induction energies are given by: OqR-3((3/2)cos2(l~1/2) and -(1/2)q2R -4(0c + (1/3)((x[ [- 0U_)(3cos2(D-1)), where R -- I R I; (Xll and c~J_ are the parallel and perpendicular component of the polarizability tensor; (x = (1/3)((x[ i + 2(X_L); (I) is the angle between the molecular axis of Y2 and the vector R pointing from the centre of mass of Y2 to the point charge. Since O has a negative value for N 2 and 0 2 , both terms are additive and contribute to the lower the total energy. Moreover, they reach their m a x i m u m absolute value for ~ =0., i.e. for a linear geometry. Considering the experimental values of O and 0[]} (O(N 2) = -1.093, 0 ( 0 2) =-0.299, (Xll (N2) = 14.75, (Xll (0 2) = 15.73 from Ref. 17, where both O and (Xll are in a. u.), one would expect the induction terms to be quite similar for N 2 and 0 2, whereas the quadrupole moments suggest that the electrostatic term is considerably larger for N2, resulting in favorable interaction energies for this molecule.
40
20-~ 0
O-
E
"
--~ -20 UJ
-40-60
~,,,i,,,~l~,,~l,,,
0
1
2
i,,,~1,,,,i,,
3
4
5
,
6
7
R (angs) Figure 1. The pentameric model cluster m(X+)Y2.
Figure 2. Potential energy curves of q+N2. (a) and q+O2 (b).
112 3.2. Interaction of N2 and 0 2 with Li + and Na + cations As expected from the results above, the geometry optimization yields linear structures for all four molecules. The equilibrium r(X-Y) distances are 2.19, 2.11, 2.62 and 2.56 A for Li+N2, Li+O2 , Na+N2 and Na+O2, respectively. The corresponding potential energy curves for X+N2 and X+O2 are displayed in Figure 3 and 4. The relevant parts of the q+N 2 and q+O 2 curves are also given for comparison. These curves have been evaluated for each system (including BSSE corrections), the r(Y-Y) distance being fixed at the equilibrium value of the free Y2 molecules (1.118 A for N 2 and 1.232 A for O2), which are very similar to those in the X+Y2 systems. Consistently with the results for q+Y2, Li+N2 is more stable than Li+O2 and Na+N2 is more stable than Na+O2 . In addition, it is seen that the dissociation energies of the Li+Y2 systems are always larger than the corresponding Na+Y2 values. Moreover, we see that, for a given Y2, the Li+Y2 and Na+Y2 curves become identical for large distances ( R > 4 A ), and that, in this region, they fall very close to the q+Y2 curves. The X+-Y2 interaction is thus very well represented by the point charge model for distances sufficiently larger than the equilibrium distance. It appears that the reason why the Li+Y2 systems are thermodynamically more stable than the corresponding Na+Y2 systems is that the repulsive short range forces become significant at much shorter distances for Li + than for Na +, introducing larger electrostatic and induction energies in the total interaction energy. This explains the fact that, replacing Na + by Li + increases the binding energy with N2 (3.7 kcalmole -1) more than the binding energy with 0 2 (2.5 kcalmole-1), favoring the N2-O2 separation.
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Figure 3. Potential energy curves of Na+N2 (a), Li+N2 (b) and q+N2 (c).
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Figure 4. Potential energy curves of Na+O2 (a), Li+O2 (b) and q+O2 (c).
113 Although N2 and 0 2 adsorption energies seem to differ essentially through the quadrupole dependent term, it is worth to investigate the relative weights of the electrostatic and induction contributions to their binding energies. With this purpose, we have considered the interaction of N2 and 0 2 with a Li+ dimer. The Y2 molecule can be situated either in a bridged geometry with each end bonded to a Li cation or in a top position, with one end bonded to a single cation. Several bridged structures have been optimized, for different Li-Li fixed distances. Their calculated geometries and binding energies (not corrected for BSSE) are presented in Table 1. These results show that the optimum Li-Li distance is 6.0 A for 0 2 and 5.5 or 6.0 ,a, for N2. In fact, these values correspond to distances between two cations located at site II and site III in the faujasite framework.
Table 1 Binding energies (B.E.) of N2 and 0 2 in bridged structures between two Li cations (distances in A, energies in kcal/mole) r(Li-Li)
r(Li-N)
B.E.(N2)
r(Li-O)
B.E.(O2)
5.3 5.5 6.0 6.5
2.10 2.20 2.45 2.70
12.3 13.2 13.1 11.5
2.05 2.15 2.40 2.65
2.8 4.0 5.1 4.8
The presence of the second Li cation, in these symmetrical structures, has a stabilizing effect, since it doubles the quadrupole energy term, but, at the same time, it has a destabilizing effect because the induced dipole created by symmetrically placed charges vanishes the induction term. A Y2 molecule on top on a Li + dimer has a preferred bent structure with a lowest energy for a (LiLlY) angle of around 160 ~ For a Li + - Li + distance of 6A, the corresponding binding energies are 15.2 and 10.9 kcal/mole for N2 and 02, respectively. The analysis of the binding energies with respect to different bending angles shows that these energy values are almost constant if the Y2 molecules approach the cation within a cone defined by a (LiLiY) angle of 110 ~ The binding energies will then differ from the above values by less than 0.5 kcal/mole. Comparison with the bridged structures shows that the top adsorption is more favored, especially for 0 2 9Larger binding energies for top structures means that, even for N2, the interaction energy with one Li cation includes a preponderant induction contribution (which is cancelled in the bridged geometries). Moreover, these results suggest that bridged geometries are probably not realistic structures for the N2 or 0 2 adsorption, except if the zeolite environment modifies differently the electrostatic and induction contributions.
114 3.3. I n t e r a c t i o n of N 2 and 02 with zeolite models 3.3.1 m ( L i +) a n d m ( N a +) Optimization of the structures of m(Li +) and m(Na +) clusters has led both Li + and Na + to form two equivalent XO bonds. The cation-(zeolite)O interaction is stronger in m(Li +) than in m(Na+), since the Li-O distance (1.88 A) is shorter than the Na-O distance (2.18 A). The potential energy curves (not corrected for BSSE) for the m(X+)Y2 clusters are depicted in Figure 5. These curves show that the binding energies for N2 are larger than those for 02 and that Li+-Y2 is still preferred to Na+-Y2, just as was found for q+-Y2 and X+-Y2 models. The binding energies corrected for BSSE indicate the same trend (3.7, 1.2, 2.6 and 0.9 kcal/mole for the Li+N2, Li+O2, Na+N2 and Na+O2 systems, respectively). Comparison with X+Y2 models shows that adsorption energies are smaller in the presence of the zeolite cluster but display the same relative ordering. The decrease of the calculated binding energies has to be related to the decreased positive charge attributed to the X cations, which is a consequence of charge transfer interactions between the aluminosilicate skeleton and its counterions. The net Mulliken charges are 0.79 for m(Li+)Y2 and 0.86 for m(Na+)Y2 , instead of a value of 0.95 for both Li+Y2 and Na+Y2 . These results indicate that the N 2 or 0 2 adsorption is, to first order, controlled by the Y2-cation interaction, whereas the zeolite framework plays a perturbative role, which can however modify the adsorption strength of these molecules.
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2,5 3 3,5 r(X-Y) (angs) Figure 5. Potential energy curves of m(Li+)N2 (a), m(Li+)O2 (b),
m(Na+)N2 (c) and
o
m(Na+)O2 (d).
-8
I
2,5
I
I
3 3,5 r(Li-Y) (angs)
Figure 6. Potential energy curves of embedded Li+O2 a (site II), b (site [II) and Li+N2 c (site II), d (site III).
115
3.3.2 E m b e d d e d Li + and Na + m o d e l s The role of the small pentameric model of zeolite in N 2 or 02 adsorption can be depicted as a screening of the binding interaction with respect to free cations. This picture is however incomplete due to the small size of the cluster. Indeed, the long-range interactions between N 2 or 02 and the charge distribution of the zeolite framework are not negligible, since we have seen above that the electrostatic contributions decrease as 1/R 3. In order to take them into account in a further more realistic study, we have thus investigated how the previous X+Y2 descriptions are modified when they are placed in a representation where all surrounding atoms are replaced by appropriate point charges. Moreover, another purpose for this study was to delineate if this environment could modify specifically the adsorption properties of some cationic sites, with respect to N 2 or 02 molecules. In a very simple first attempt, conventional charges used in Molecular Mechanics simulations have been used for the zeolite atoms (values indicated in 3.2). Li cations have thus been placed at the coordinates of two neighboring cationic sites identified in faujasite as site II and site III, accessible to incoming gaseous molecules. These sites, located in a supercage, have been approached by N 2 or 02 molecules following a direction of lowest energy, roughly perpendicular to the zeolite framework around the cation, i.e. one six-membered ring for site II and two four-membered rings for site III. The binding energy curves for N 2 and 02 as a function of their distances from the Li cations in site II and site III are shown in Figure 6 (no BSSE corrections). The four curves have a minimum at around 2.3 A, which is longer than the equilibrium distances found for isolated cations and slightly shorter than the Li-Y distances evaluated for the m(Li+)Y2 models. The presence of the point charge network does not change the trend that N 2 is more strongly bonded to the cation than 02. Moreover, the binding energy difference between these two molecules, evaluated at 3.2 (site II) or 3.5 kcal/mole (site III) is similar to the value obtained from the pentameric model clusters (3.2 kcal/mole without BSSE correction). This difference was calculated at 4.4 for an isolated Li cation. If the presence of the point charge environment does not modify the relative ordering of N 2 and 02 adsorption energies, it clearly induces a difference between the adsorption property of cations in site II and site III. Indeed, the energy curves on Figure 6 show that site III is more favorable for adsorption than site II. Further calculations including larger clusters as well as more investigations on possible point charge sets are needed to confirm these preliminary results. 4. CONCLUSIONS These quantum mechanical calculations on small model clusters inluding Li and Na cations have shown that the different N2 and 0 2 adsorption properties in zeolites can be explained by simple arguments. Indeed, they have demonstrated
116 that a classical description involving electrostatic and induction energies, which depend on the quadrupole moment and dipole polarisability of these molecules repectively, is adequate to explain the basic reason for a stronger N2 adsorption. Moreover, the calculations show that a small cation like Li + behaves as a better attractor than a larger cation like Na +, the presence of the core electrons being the factor which limits the stabilizing contribution of the electrostatic and induction terms in the interaction energy. This conclusion justifies thus the use of classical simulations based on energy expressions including Lennard-Jones terms and also electrostatic (quadrupole) and induction contributions. The study of models including the zeolite as a cluster of atoms or as a network of point charges has shown that the zeolite framework has a perturbative role, screening the binding of both N2 and 0 2 , while leaving the main trends valid. A proper point charge embedding may help to rationalize the influence of the structure of the zeolite on the adsorption properties of specific sites.
REFERENCES
1. D.W. Breck, Zeolite Molecular Sieves, R.E. Krieger Publishing Company, Malabar (1984). 2. V.N. Choudary, R.V. Jasra, T.S.G. Bhat, Ind. Eng. Chem. Res., 32 (1993) 548. 3. C.G. Coe, J.F. Kirner, R. Pierantozzi, T.R. White, U.S. Patent No 5 152 813 (1992) and references therein. 4. D.H. Olson, J. Phys. Chem., 74 (1970) 2758. 5. I. P~pai, A. Goursot, F. Fajula, J. Weber, J. Phys. Chem., 98 (1994) 4654. 6. D.R. Salahub, R. Fournier, P. Mlynarski, I. P~ipai, A. St-Amant, J. Ushio, Density Functional Methods in Chemistry, J. Labanowski, J. Andzelm Eds., Springer-Verlag, New York, (1991) 77. 7. H. Sambe, R.H. Felton, J. Chem. Phys., 62 (1975) 1122. 8. B.I. Dunlap, J.W.D. Conolly, J.R. Sabin, J. Chem. Phys., 71 (1979) 3396. 9. A. St-Amant, D.R. Salahub, Chem. Phys. Lett., 169 (1990) 387. 10. A. St-Amant, Ph.D. Thesis, Universit~ de Montr6al, (1991). 11. C. Daul, A. Goursot, D.R. Salahub, Numerical Grid Methods and Their Application to Schrodinger's Equation, Cerjan, C., Ed.; NATO ASI Series, Kluwer Academic Press, 412 (1993) 153. 12. A.D. Becke, Phys. Rev. A, 38 (1988) 3098. 13. J.P. Perdew, Phys. Rev. B, 33 (1986) 8822; erratum in Phys. Rev. B, 38 (1986) 7406. 14. S.H. Vosko, L. Wilk, M. Nusair, Can. J. Phys., 58 (1980) 1200. 15. I. Phpai, A. Goursot, F. Fajula, D. Plee, J. Weber, J. Phys. Chem. to be published. 16. A.D. Buckingham, Intermolecular Interactions: From Diatomics to Biopolymers, Pullman, B.; Ed.; John Wiley & Sons, Chichester, (1978). 17. F. Visser, P.E.S. Wormer, W.P.J.H. Jacobs, J. Chem. Phys., 82 (1985) 3753.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
117
L o a d i n g and l o c a t i o n of w a t e r m o l e c u l e s in the zeolite clinoptilolite Y.M. Channon a, C.R.A. Catlow a, R.A. Jackson b and S.L. Owens c. aDavy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London, W 1X 4BS. bDepartment of Chemistry, Keele University, Keele, Staffordshire ST5 5BG. cCompany Research Laboratory, British Nuclear Fuels P.L.C., Springfields Works, Salwick, Preston PR4 0XJ. 1. INTRODUCTION This study concerns the application of computer modelling techniques to the behaviour of water in the zeolite, clinoptilolite. Such techniques have been extensively applied to dehydrated zeolites [1]; but their application to hydrated systems has been limited. Clinoptilolite is a naturally occurring zeolite that is a member of the heulandite group of zeolites. It has a unit cell formula NaxCayAl(x+2y)Si36_(x+2y)O72.24H20, and is isostructural with heulandite having an Si/A1 ratio of approximately 4. Clinoptilolite has a 2-D microporous channel system which was first characterised for heulandite [2]. Two channels run parallel to each other and the c-axis: channel A, a 10-member ring and channel B, an 8member ring. Another 8-member ring, channel C, lies along the a-axis and intersects the A and B channels. Figure 1 clearly shows channel A and B.
Figure 1 Structure of clinoptilolite showing the channels A, the 10 - member ring and B, the 8-member ring.
118 Widely used in industry and employed in gas separation and ion exchange [3], the material has a strong affinity for aqueous caesium and strontium ions, even in the presence of other aqueous cations [4, 5]. This property is exploited via ion exchange in the nuclear industry, and clinoptilolite was used to decontaminate the surrounding area after the accidents at Chernobyl and Three Mile Island [6]. Investigation of the loading and location of water in the zeolite is of importance owing to the influence water molecules exert on the structural and chemical properties by binding certain extra framework cations at specific positions within the framework. Although the location of water and the strength with which it is bound to the structure have been studied experimentally [7, 8], the location of cations and water molecules, as well as the coordination of these species within the structure is still poorly understood. As noted there have been many computational studies on dehydrated zeolites [9, 10], but few on hydrated systems. In this paper we report the first attempts to model hydrated clinoptilolite, and focus on the networks of hydrogen bonded water molecules that form inside the siliceous and aluminium substituted material. 2. M E T H O D O L O G Y Our work is based on the use of 'force field' methods, i.e. techniques employing interatomic potentials. The models used for the latter are described before we summarise the main techniques involved in the study.
2.1 Interatomic Forces Interatomic potentials describe the total energy as a function of the nuclear co-ordinates. These potential functions can be analytic or numerical. There are two broad categories of potential models; first those based on the Born and other related models for ionic solids which describe the system in terms of ions interacting via short and long range terms. The second type is molecular mechanics forcefields; these describe explicitly the bond angles and torsions for each specific system with the energy being given as a function of bond angles, lengths, torsional planes and other cross terms and their associated deviations from equilibrium values. The present study uses the molecular mechanics forcefield cff91_czeo (distributed by BIOSYM Technologies [ 12]), which has been specifically designed to be used in zeolite simulations. This forcefield is used with the following simulation techniques. 2.2 Molecular Dynamics This technique iteratively solves Newton's classical equations of motion. As discussed by Allen and Tildesley [ 13] a standard Taylor series is used to update positions and velocities by a suitable algorithm such as those first described by Verlet [ 13, 14]. The integration time step At, is typically 10-15 seconds. After the solution of the Taylor series equation, the original co-ordinates are replaced by the new ones, the velocities are updated and the acceleration is corrected by calculating the gradients, with the new co-ordinates. The work carried out in this study used the DISCOVER code [ 12] which embodies the Verlet algorithm.
119
2.3 Monte Carlo Techniques These generate ensembles by searching the configurational space of a system by a series of random moves, following for example, methodology developed by Metropolis and Ulam [15]. In the present case a simpler approach is used in which molecules are inserted randomly into the zeolite, and configurations are accepted when their energies fall below a specified threshold. 2.4 Energy Minimisation This technique explores the potential energy surface to locate configurations of minimum energy, and as such yields only static information. Iterative numerical procedures are used to search for the minimum, employing both first and second derivatives of the energy function. The present study uses the code DISCOVER [ 12]. 2.5 Multi Docker Our aim is to locate low energy sites of sorbed water molecules in clinoptilolite. To achieve this the computational techniques use a combination of molecular dynamics, Monte Carlo and energy minimisation methods, all employing a molecular mechanics forcefield. Specifically we use the Docker code developed by BIOSYM Technologies [12] which employs the methodology described by Freeman et al [ 16]. A molecular dynamics simulation is carried out at 298K on a water molecule. This generates a library of possible conformers. A Monte Carlo algorithm is then employed to choose Table 1 Adsorption energy of water, for the lowest energy configuration, as a function of molecule number in system 1. Water Molecule Number Energy (kcal / mole) 1 -5.4 2 -11.4 3 -11.7 4 -5.5 5 -11.2 6 -11.5 7 -17.5 8 -12.3 9 -17.1 10 -12.2 11 -11.8 12 -12.3 13 -14.1 14 -12.5 15 -13.2 16 -12.1 17 -10.6 18 -12.5 19 -12.6 20 -10.3
120
randomly a conformation from the library and insert it at random within the zeolite framework. Each accepted configuration is subsequently energy minimised; the configuration with the lowest energy is then used as the starting structure and the process repeated until the zeolite contains the required number of sorbates. 3. R E S U L T S Our primary aim in the initial study was to identify the ways in which water molecules are accommodated within clinoptilolite and the nature of the resulting hydrogen bonding patterns. We present the results as applied to two systems.
3.1 System 1 Twenty water molecules of adsorption of water number and Table 1 lists molecules are adsorbed each other t
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Figure 2 Graph of adsorption energy for water molecules in system 1 and the framework via hydrogen bonds. The next three water molecules locate at the intersection of channel A and channel C, and interact with each other, and the framework but not with the first three (see figure 3). The addition of the seventh molecule joins the first six water molecules via a hydrogen bond, and the ninth water molecule locates in a position that creates a circular network of hydrogen bonded water molecules, figure 4. The location of the next ten water molecules is controlled by their interactions with the existing hydrogen bonded network. This is illustrated by figure 2, which shows very little variation in their relative adsorption energies. They all locate at positions near enough to the circular network to form small clusters with sorbates participating in the network as shown in figure 5.
121
Figure 3 Six water molecules in system 1
Figure 4 Large Circular Network of water molecules in system 1
Figure 5 Large network with small clusters of water molecules 3.2 System 2 Here we present preliminary results for a siliceous system containing an aluminium ion substituted for a silicon ion, with the framework charge balanced by an hydroxyl bridge oriented into channel B. Ten water molecules have been iteratively docked into this system. Figure 6 shows the minimum energy of adsorption of water as a function of molecule number, and table 2 lists the relative adsorption energy for each water molecule. The addition of the first seven water molecules takes place in the same intersection of channels B and C, forming linked clusters of hydrogen bonded water molecules that also interact with the framework via hydrogen bonds as illustrated by figure 7. At this point the channel intersection appears to be full and the location of the eighth water molecule is at the boundary of channel C. This is still close enough to interact with the large cluster via a hydrogen bond. The ninth and tenth water molecules adsorb into the adjacent C channel and begin to form a
122
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Water Molecule Number
Figure 6 Graph of adsorption energy for water molecules in system 2 Table 2 Adsorption energy of water, for the lowest energy configuration, as a function of molecule number for system 2. water Molecule Number Energy (kcal / mole)
1
-5.6
2 -11.4 3 -10.7 4 -12.7 5 -10.5 6 -11.6 7 -13.5 8 -11.9 9 -11.4 10 -10.1 new cluster. They hydrogen bond to each other and the eighth sorbate, so creating a large network of hydrogen bonded small clusters as shown in figure 8. 4. DISCUSSION From figure 3 we can see how the first six water molecules have located, as two non interacting trimers in system 1. The adsorption energy of the fourth sorbate is almost the same as the first, reflecting a site similar to the first with no other water-water interaction. The adsorption energy of the seventh water molecule which links the two trimers has the minimum for this system. The low relative adsorption energy of the ninth water molecule also highlights the stability arising from linking the water molecules together through hydrogen bonded networks. At this point there is a large circular network of hydrogen bonded water molecules. Further addition of water molecules results in the formation of
123
Figure 7 Seven water molecules in system 2.
Figure 8 Ten water molecules in system 2.
small cyclic clusters interacting with each other via the large circular network. From these results it can be postulated that until the zeolite has been loaded with six water molecules, it is the confining effect of the structure that is the dominant influence; after this has been attained the dominant influence is the previous water structure, directing the next sorbate to form small trimer and tetramer clusters that are linked to each other via a large network of hydrogen bonds. Consider now system 2. Figure 6 has a totally different pattern from figure 2 and indeed until the addition of the sixth water molecule no distinguishable pattern of cluster formation is observed. The sixth water molecule locates at a position that forms a tetramer and trimer linked to each other by hydrogen bonds; but it is the seventh water molecule that has the lowest adsorption energy for this system. At this point two tetramers form, linked by a water molecule with hydrogen bonds forming a network of two clusters (see figure 7). The channel intersection is now full with water molecules, and the eighth water molecule must locate away from the intersection. It adsorbs at the boundary of channel C, and still along channel B, close enough to the large cluster to interact via a hydrogen bond. This water molecule acts as a link to a new cluster that begins to form on the addition of the ninth and tenth water molecules. From the results of system 1, it can be seen that when a new cluster begins to form there is a marked increase in the adsorption energy, although this increase does not occur if the initial cluster molecule is hydrogen bonded to another water molecule. Such is the case for the ninth molecule which begins a new cluster in the adjacent channel C, but is stabilised by the hydrogen bond to the eighth water molecule and thence the large cluster. The network of hydrogen bonds that has been built up for system 2 is quite different from that built up for system 1, which must be due to the influence of the aluminium and hydroxyl bridge. What both systems do have in common, however, is that the low adsorption energies
124 correspond to the formation of trimeric or tetrameric clusters. The system is even more stable if these small clusters can be linked together with hydrogen bonds to form large networks. The lack of interaction between water molecules and the hydroxyl bridge in system 2 is currently under investigation. It is thought that this may be due to the low loading of a small molecule into a structure containing channels of large dimensions, but it may of course, reflect inadequacies in the interatomic potentials. 5. CONCLUSIONS This study is only the first stage of developing an understanding of the behaviour of water molecules in clinoptilolite. The dominant influences in the location of each water molecule at low loading, and in zeolites in general, is the structure of the existing water network, although of course, the zeolite exerts an influence on the location of the sorbed molecule. These points are illustrated by the different hydrogen bonded water patterns built up, and the eight ring channels preferentially adsorbing water molecules over the ten ring channel. The formation of hydrogen bonded trimers and tetramers strongly influences the energy of the sorbed molecule. The linking of these small clusters also affects their stability. Future studies will be concerned with water behaviour at higher loadings, and different chemical potentials, using the Grand Canonical Monte Carlo methodology. Further studies of the interaction of water molecules with acid sites are also planned. 6. ACKNOWLEDGEMENTS We are grateful to British Nuclear Fuels p.l.c, for supporting this work. We are grateful to BIOSYM Technologies for supplying the Catalysis and Sorption software. 7. REFERENCES 1 C.R.A. Catlow (ed.), Modelling of structure and Reactivity in Zeolites, Academic Press, London, 1992. 2 A.B. Merkle and M. Slaughter, Am. Mineral. 53 (1968) 1120. 3 M.W. Ackley, R.F. Giese and R.T. Yang, Zeolites, 12 (1992) 780. 4 L.L. Ames, Jr. Am. Mineral., 45 (1960) 689. 5 L.L. Ames, Jr. Am. Mineral., 46 (1961) 1120. 6 L.J. King,, D.O. Campbell, E.D. Collins, J.B. Knauer and R.M. Wallice, in Proceedings of the 6th International Zeolite Conference, (eds. Olson, D. and Bisio, A.), Bt~tterworths, Guildford, (1984) 660. 7 K. Koyama and Y. Takeuchi, Z. Kristallogr., 145 (1977) 216. 8 R.L. Ward and H.L. McKague, J. Phys. Chem., 98 (1994) 1232. 9 C.J.J. den Ouden, R.A. Jackson, C.R.A. Catlow and M.F.M. Post, J. Phys. Chem., 94 (1990) 5286. 10 R.A. Jackson and C.R.A. Catlow, Mol. Sim., 1 (1988) 207. 11 M. Born and J.R. Oppenheimer, Ann. Physik., 84 (1927) 457. 12 BIOSYM Technologies Inc., 9685 Scranton Road, San Diego, CA 92121 U.S.A. 13 M.P. Allen and D.J. Tildesley, Computer Simulation of Liquids, Clarendon Press, Oxford, 1987. 14 L. Verlet, Phys. Rev., 159 (1967) 98. 15 N. Metropolis and S.J. Ulam, Am. Stat. Ass., 44 (1949) 335. 16 C.M. Freeman, C.R.A. Catlow, S. Brode and J.M. Thomas, Chem. Phys. Lett., 186 (1991) 23.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviotand S. Kaliaguine(editors) 9 1995 Elsevier Science B.V. All rights reserved.
125
Withdrawal of Electron Density by Cations from Framework Aluminum in Y Zeolite Deterlnined by A1 XAFS Spectroscopy D. C. Koningsberger a and J. T. Millerb "Laboratory of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, P.O. Box 80083, 3508 TB Utrecht, The Netherlands bAmoco Research Center, 150 W. Warrenville Rd., Naperville, IL 60566-7011, USA
The local AI structure and charge density in Y zeolites have been determined by low energy, AI XAFS spectroscopy. The whiteline intensity indicates that the AI charge density in Y zeolites decreases with increasing charge of the exchanged cation and correlates with the acidity of the zeolite. This result is consistent with the model that polyvalent cations withdraw electron density from the hydroxyl groups increasing their acidity.
1. INTRODUCTION For most hydrocarbon reactions, alkali metal zeolites, for example Na-Y, are relatively unreactive. Exchange of alkali ions by polyvalent cations like Ca, or La results in increased acidity and catalytic activity. The catalytic activity increases with the increasing charge on the cation [1-3]. Early explanations for the enhancement in activity by cations exchanged into Y proposed that strong electrostatic fields are present within the pores of the zeolite. Because of the rigid structure of the zeolite, the charge on the cation is not fully compensated by the negative charge localized on the aluminum-oxide tetrahedra [4]. Polyvalent cations in low coordination sites were proposed as acid centers [5]. An alternative explanation suggested that cations generate acidic hydroxyl groups by hydrolysis of coordinated 1-120 [6,7]. The acidic hydroxyl groups were confirmed by infrared spectroscopy [8]. Exchange of the alkali ions by ammonium ions with subsequent calcination to produce H-Y generates Bronsted acid sites and enhanced catalytic activity compared to Na-Y. In the absence of non-framework AI, however, the catalytic activity of H-Y is low. The activity of H-Y is greatly enhanced by exchange of a small amount of La ions [9,10]. The polyvalent cations, present in the 13 cages [11], are thought to withdraw electron density from the hydroxyl groups increasing their acidity [9,10,12]. The acid strength and catalytic activity of the Bronsted sites are determined by the polarizing effect of the cation and increases with increasing charge of the cation. In the present study, the effect of the cation on the charge density of the A1 ion and the AI-O bond distance has been determined by AI XAFS spectroscopy. As the charge on the cation increases from Na to H to Ca to La, the electron density on the A1 decreases. The
126 charge on the Al ion parallels the acidity of the catalysts. By contrast, the A1-O bond distance is independent of the type of exchanged cation and was about 1.7 A.
2. EXPERIMENTAL Na-Y was a commercial zeolite (LZY-54) purchased from UOP. The sample crystallinity was confirmed by XRD and had a unit cell dimension of 24.676 A, ca. 56 A1/unit cell. The preparation of H-Y has previously been given [13]. Briefly, Na-Y was repeatedly ammonium exchanged and carefully calcined to give H-Y. The crystallinity by XRD was 98% based on Na-Y as a standard, and the unit cell dimension was 24.643/~, or 52 Al/unit cell (11.5 wt% Al and 0.13 wt% Na). H-CaY (and similarly H-LaY) were prepared by ion exchange of Na-Y three times by a ten fold excess of 0.5 M Ca(NO3)2. The Ca-Y was wash, dried at 100C overnight and calcined at 300C for 3h. The Ca-Y was ion exchanged three times with a ten fold excess of 2 M NH4NO3, washed, dried and calcined at 300C for 5 h. The elemental analysis for H-CaY was 11.6 wt% Al, 1.5 wt% Ca, and 0.2 wt% Na, and for H-LaY was 12.4 wt% Al, 12.4% La, and 0.4 wt% Na. Standard methods were used to determine the Si and Al NMR, and the number of acid sites were determined by NI-I3 TPD [14]. EXAFS measurements were performed at the sot~ X-ray XAFS station 3.4 of the SRS at Daresbury (UK). This station is equipped with a quatrz, double crystal monochromator, and harmonic contamination of the X-ray beam was minimized by collimating mirrors. The estimated resolution was 1.5 eV at the A1 K-edge (1559 eV). The data were collected simultaneously with a fluorescence and an electron yield detector. Datum was collected with a k-space scan mode (start, 3 sec.; end, 30 sec.), and six scans were averaged in order to minimize both high and low frequency noise. Sample preparation, reference compounds, experimental conditions and standard procedures for analysis of XAFS data have previously been reported [13]. The near edge spectra were determined from the electron yield data, and the fluorescence data were used to generate the EXAFS function. The data were analyzed using the latest version of the Utrecht University XAFS Data Analysis Program (XDAP) which allows for fitting in r-space.
3. RESULTS X-ray diffraction and N2 micropore volumes indicate that the four Y zeolites are highly crystalline. The Si NMR also indicate that there is little change in the Si/AI ratio of the H-Y H-CaY and H-La-Y compared with Na-Y. Although the ratio of the peak intensities of HLaY are unchanged, the resonances are significantly broadened. The Si/A1 ratio of all catalysts is about 2.5 [ 15]. The Al NMR of Na-Y indicate that all of the Al is in tetrahedral coordination. For H-Y, 85% of the Al are in tetrahedral coordination with about 15% in octahedral coordination. The Al NMR of H-CaY indicate that 85% of the A1 is in tetrahedral coordination with 10% in octahedral coordination. The remaining 5% A1 seems to be in a distorted tetrahedral coordination which appears as a shoulder (about 55 ppm) of the main tetrahedral resonance. For H-LaY, 55% of the Al is in a symmetric tetrahedral coordination,
127
i.e., a resonance at 60 ppm. Approximately, 10% of the A1 is octahedral, 0 ppm, while the remaining 35% of the AI occurs as a broad resonance centered at 30 ppm. Since the Si NMR indicate little loss of structural A1, this 30 ppm peak may be due to structural, tetrahedral AI located near La ions resulting in a distorted electronic coordination about the AI. For each AI in the lattice, charge balance requires one equivalent of univalent cation. The elemental composition of H-Y, H-CaY and H-LaY indicate that there are 4.2, 4.3 and 4.6 mmol/g of AI, respectively. In addition, the univalent cation charge (due to Na, Ca or La) for H-Y, H-CaY and H-LaY are 0.06, 0.8 and 2.8 mmol/g. The number of protons required for charge balance for each, therefore, is estimated to be 4.1.3.5 and 1.8 mmoVg. The number of acid sites which strongly chemisorb NH3 in H-Y, H-CaY and H-LaY were 3.9, 3.6, and 1.9 mmol/g, respectively, and is equivalent to the number of protons in each catalyst. In H-CaY and H-LaY the cations, Ca +2 and La +3, do not chemisorb ammonia and are not strongly acidic. The x-ray absorption near edge spectra (XANES) of Na-Y (solid line), H-Y (dotted line), H-CaY (dashed line), and H-LaY (dotted-dashed line) are given in Figure 1. There is a clear distinction in the white line of the four samples with the intensity increasing in the order Na-Y < H-Y < H-CaY < H-LaY. A
N3 E
it;
L_
0
J t'.
~ ",
i..' \~ "-, ~." \ ' ; : ,
e,-
~c-2
/h
O e~
'.- .... ",,.
I
L_
I
... .......
.
J
-- .2"--:,'"
0
,',1 L_ !
X 0
.__j 0
-5
I
1
I
1
0
5
10
15
20
Energy (eV) Figure 1. X-ray Absorption K-edge of AI (Normalized) for Na-Y (solid line), H-Y (dotted line), H-CaY (dashed line) and H-LaY (dot-dash line). Figure 2a and 2b show the average EXAFS spectum and the Fourier transform of Na-Y (solid line) and H-CaY (dotted line), respectively. The high signal-to-noise ratio of the EXAFS data allows for the detection of higher coordination shells. The node positions for the two catalysts are nearly identical and indicate little difference in the A1-O bond distance which is confirmed in the Fourier transform. The lower amplitude of the first shell AI-O peak in the Fourier transform of H-CaY indicates a larger distortion in the AI-O coordination in
128
comparison to that in Na-Y. Similarly, Figure 3a and 3b compare the EXAFS spectrum and Fourier transform for H-CaY (solid) and H-LaY (dotted). Again, the node positions of the two catalysts indicate little difference in the Al-O bond distance. The amplitude of the first shell A1-O peak of H-LaY is slightly lower than that of H-CaY indicating a slightly larger distortion of the A1-O coordination.
0.4
0.3
0.3
0.2
0.2 ~ , 0.1
~ 0.1 =,?. I--0
0
'c. -0.1
-0.1
u. -0.2
"0"22
3
4
5
6
7
8
9
"0"30
k (l/A)
1
2
3
4
R (A)
Figure 2. a) Raw EXAFS data, and b) Fourier transform (k 1 : Ak = 2.7-8.2 A"1) of Na-Y (solid line) and H-CaY (dotted line).
0.4
0.3
0.3
~E 0 2
a
0.2
b
ol
c.9 -0.1
-o.1 " 0 "2 2
,,~ -o 2
3
4
5
6
k (l/A)
7
8
9
-0 301 - ' - ' ~ ~ ~ ~ ~ ~
~
R (A)
Figure 3. a) Raw EXAFS data, and b) Fourier transform (k ~ 9Ak = 2.7-8.2 A "~) of H-CaY (solid line) and H-LaY (dotted line). The first shell A1-O peak in the Fourier transform of the zeolite samples were fit in rspace (both magnitude and imaginary parts) using a non-linear multiple shell fitting routine [13]. All catalytsts were fit over the same data range (k ~ weighted, AR = 0.5 - 2.0 A, Ak = 2.7- 8.3 A'~). The number of independent parameters (N~p = 2*Ak*AR/x + 1) was 6.3, and the
129 degrees of freedom (Nfrce = N,,ap - Nat) were 2.3. The first shell fit in r-space is given in Figure 4a for H-LaY. The data and the fit were inverse Fourier filtered over the same r-range and are compared in Figure 4b. The quality of the fits is typical for all catalysts. The coordination parameters are given in the Table. Systematic errors are minimized by using the same background subtraction and normalization procedures for all data sets and by calibration with the reference phase shift and back-scattering amplitudes. Although fitting in r-space increases the useful data range, at present, it is not possible to calculate the limits of accuracy. For all Y zeolites, the AI-O coordination number is 4 in agreement with tetrahedral coordination. In addition, the AI-O bond distance for all catalysts is similar, around 1.7 A, also consistent with an A1-O tetrahedral coordination. The disorder (DebyeWaller factor) in the A1-O coordination increases in the order Na-Y < H-CaY < H-LaY.
Table Coordination Parameters Parameters
N
Ac~2
R
AEo
(xl 0 3) (,~2)
(A)
(eV)
Coordination: AI-O
E
Na-Y
4.0
-0.004
1.70
1.4
H-CaY
3.9
-0.002
1.71
-0.5
H-LaY
4.1
0.000
1.70
-0.3
1
0.3
o02
I-.
A ~
0
0
"~ -0.1 -0 2
-0.3 ' 0
1
1
9
i
1
2 3 R (A)
4
-0 12
3
4
5
6
7
8
9
k (llA)
Figure 4. Results of fit in r-space (k 1 9Ak = 2.7-8.2 A "l', AR "- 0.5-2.0 A) for H-LaY (data: solid line, and fit: dotted line), a) Fourier transform, and b) Fourier filtered data from 4a.
130 4. DISCUSSION As shown in Figure 1 the white line intensity, representing a ls to 3p transition, is sensitive to the type of cation exchanged into the zeolite. The white line intensity is the highest for H-LaY and increases in the order Na-Y < H-Y < H-CaY < H-LaY. Previously it was shown for H-Y and Na-Y that the whiteline intensity was higher for H-Y than Na-Y [13]. For these catalysts, the order of the electron densities derived from the white line intensities is in agreement with theoretical calculations for zeolite clusters. For example, the positive charge on the A1 in a protonated aluminosilicate ring, H § A1SiO3(OH)6", is 1.52 while on a symmetrically coordinated Na-aluminosilicate ring, Na § AISiO3(OH)6, the AI charge is 1.43, or 0.09 electrons less on the protonated cluster [16]. The whiteline intensities in this study, therefore, indicate that the electron density on the A1 is lowest, or positive charge of the AI ion is highest, for H-LaY and increases in the order H-LaY < H-CaY < H-Y < Na-Y. Structural determination of the A1-O bond distance in H-LaY, H-CaY and Na-Y indicate that the average distance is very similar, 1.70, 1.71, and 1.70 A, respectively. Previously, the AI-O bond distance in Na-Y was reported as 1.62 A [13]. The longer distance reported here is due to better data quality and the different fitting procedure, i.e., fitting in r-space. The current fluorescence data show a more linear response with increasing energy allowing for more accurate background subtraction. In addition, for AI in zeolites, the maximum data range for EXAFS analysis is limited up to about 8.5 A~ due to the overlap of the Si K-edge. Fourier filtering for fitting in k-space, however, limits the useful data range to about 3.5 A l [ 13]. By fitting in r-space, the useful data range can be increased by about 2 A "l improving the accuracy of the Al-O bond distance determination. Applying this new procedure to the previous data obtained on Na-Y results in an A1-O bond distance of 1.68/~ in general agreement with the results from this study. Reanalysis of the previous EXAFS data of H-Y results in an AI-O distance of 1.67/~. The current results indicate that within the limits of accuracy the AI-O bond distance is not sensitive to the type of cation exchanged into the zeolite. The whiteline intensity indicates that the cation has a dominating effect on the electron density of the aluminum ions. The order of the electron density parallels the acidity of the catalysts. That is, the electron density of the aluminum ion is the lowest and the whiteline intensity is highest for the most highly acidic zeolite, e.g., H-LaY. Since the cations are coordinated to lattice oxygen ions, it is likely that the cations are withdrawing aluminum electron density through these coordinated oxide and hydroxide ions. The low electron density on the aluminum ion also suggests that the electron density of the lattice oxide ions is lower due to coordination with the cation. Since the acid sites in H-CaY and H-LaY are due to the protons, the correlation of the whiteline intensity with the acidity suggests that a higher acid strength in ion-exchanged Y is due to withdrawal of electron density from the hydroxyl groups by the nearby polyvalent cations consistent with the model for enhanced acidity proposed by Lunsford [9,10,12].
131 5. CONCLUSION Low energy A1 XAFS spectroscopy is a powerful technique for directly measuring the local AI-O bond distance and charge on the AI ion. The average AI-O bond distance determined by EXAFS spectroscopy is independent of the type of exchanged cation in zeolite Y. On the other hand, the whiteline line intensity is very sensitive to changes in the charge on the AI ions induced by exchanged cations and correlates with the catalyst's acidity. As the charge on the cation increases, the charge on the A1 decreases suggesting that polyvalent cations withdraw electron density from the AI through the oxygen ions. The correlation of the whiteline intensity with the acidity suggests that the origin of the enhanced acid strength is due to the withdrawal of electron density from acidic hydroxyl groups by the nearby polycation.
REFERENCES 1. R.C. Hansford and J.W. Ward, J.Catal., 13, (1966) 316. 2. P.E. Eberly Jr. and C.N. Kimberlin, Adv. Chem. Ser., 102, (1971) 374. 3. M.L. Poustma, Zeolite Chemistry and Catalysis, ACS Mono. 171, J.A. Rabo (ed.), ACS, Washinton, D.C., (1976) 437. 4. J.A. Rabo, P.E. Pickett, D.N. Stamires, and J.E. Boyle, Proc. 2nd Int. Cong. Catal., Editions Technip, Paris, II ( 1961) 2055. 5. P.E. Pickett, J.A. Rabo, E. Dempsey and V. Schomaker, Proc. 3rd Int. Cong. Catal., W.M.H. Sachtler, G.C.A Schuit and P. Zwietering, eds., North-Holland Publishing Co., Amsterdam, I (1965) 714. 6. C.S. Plank,, Proc. 3rd Int. Cong. Catal., W.M.H. Sachtler, G.C.A Schuit and P. Zwietering, eds., North-Holland Publishing Co., Amsterdam, I (1965) 727. 7. P.A. Jacobs, J.B. Uytterhoven, J. Chem. Soc., Faraday, 69, (1973) 373. 8. J.W. Ward, J. Catal., 10, (1968) 34. 9. J.H. Lunsford, Fluid Catalytic Cracking II, Concepts in Catalyst Design, ACS Sym. Ser. 452, M.L. Occelli (ed.), ACS, Washington D.C., (1991) 1. 10. R. Carvajal, P.-J. Chu and J.H. Lunsford, J. Catal., 125, (1990) 123. 11. A.K. Cheetham, M.M. Eddy and J. M. Thomas, J. Chem. Soc., Chem. Commun., (1984) 1337. 12. P.O. Fritz and J.H. Lunsford, J. Catal., 119, (1989) 85. 13. D.C. Koningsberger and J.T. Miller, Catal. Lett., 29, (1994) 77. 14. B.L. Meyers, T.H. Fleisch, G.J. Ray, J.T. Miller, and J.B. Hall, J. Catal., 110, (1988) 82. 15. Engelhardt, G.; Michel, D. High Resolution Solid-State NMR of Silicates and Zeolitesi John Wiley and Sons, 150 (1987). 16. R.A. van Santen, B.W.M. van Beest and A.J.M. de Man, Guidelines for Mastering the Properties &Molecular Sieves, D. Barthomeuf (ed.), Plenum Press, New York, (1990) 201.
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Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 1995 Elsevier Science B.V.
133
Geometry of the Active Sites m Zeolites under Working Conditions F. Fajula Laboratoire de Matrriaux Catalytiques et Catalyse en Chimie Organique, URA 418 CNRS, ENSCM, 8, rue de l'Ecole Normale, 34053 Montpellier Cedex, France I. INTRODUCTION Due to the large use of zeolitic materials in major acid-catalyzed industrial processes, the characterization and description of the acid sites, particularly BrOnsted-type acid sites, in zeolites has received continuous attention. Among heterogeneous catalysts, zeolites constitute a unique class of solids because the acid centres are crystallographycally well defined. BrOnsted sites in zeolites can be attributed to acidic protons covalently bound to one of the oxygens linked to lattice aluminium atoms for which they provide charge compensation. In principle, the position of the active catalytic sites in the tridimensional network as well as their local geometry in terms of bond angles, bond lengths and environment might be fully determined and their acid strength and reactivity predicted. Considerable progress in the understanding of zeolite acidity has been gained from extensive spectroscopic, quantum chemical and structure modeling studies (see [1-3] for recent reviews), However, the catalytic significance of theoretical considerations has been established definitely only in a limited number of cases. The reasons for such a situation are multiple. The real links between structural acid sites and actual active sites is not straightforward. Reactivity depends indeed on intrinsic acidity but also on the properties of the conjugate ion generated upon protonation and on the contribution of the zeolite framework oxygen atoms in the stabilization of intermediates and transition states. These factors are not always easy to delineate and take into account in calculations and model studies. On the other hand, there are several evidences that H-form zeolites with their full complement of charge compensating protons exhibit little, if any, activity for hydrocarbon conversion reactions such as cumene and hexane cracking [4-6]. Secondary treatments generating extra-framework phases definitely enhance activity by creating new catalytic centers [6-8]. Similarly, although Lewis sites are almost systematically detected on activated zeolites, their direct or indirect implication in the catalytic events remains a matter of debate [9-11 ] and little is known regarding their nature, strength and location. Another main limitation arises from the fact that the actual working surface of a zeolite may be deeply modified by the presence of adsorbed and chemisorbed species, not
134 necessarily participating directly to the catalytic process. These species may behave as spectators, being chemisorbed on other sites, act as poisons or introduce additional diffusion limitations. In the last decade great efforts have been dedicated to adapt conventional spectroscopic techniques, among which infrared and NMR spectroscopy being the most popular, for studying catalytic reactions under conditions that recreate, or at least mimic, actual working conditions. The merits of this approach for the description of the reaction mechanisms at the molecular level and the detection of unrevealed reaction intermediates are well established and have been widely reviewed [12-14]. The in situ protocol should prove also well suited for the identification of the active sites and monitoring the changes of their local geometry during catalytic reactions or acidity measurements. It is my aim, at this symposium devoted to the <
>, to consider briefly some recent examples of such studies and to stimulate discussion about them. The aspects concerning the fate of the bridging structural A1 -OH-Si groups being largely documented, they will not be considered here. The examples I choose deal with the direct spectroscopic observation of peculiar behaviours of zeolites in the presence of sorbates, such as the change in the coordination of aluminium in protonic zeolite Beta, the generation of very strong Lewis acidity on ZSM-5 upon adsorption of olefins and the manifestation of the synergistic effect betwen Bronsted and Lewis sites in Faujasite and Mazzite. 2. REVERSIBLE CHANGE OF THE COORDINATION OF ALUMINIUM Changes in the environment of silicon and aluminium atoms upon hydration-dehydration or adsorption of bases have been evidenced several times for aluminophosphates and silicoaluminophosphates [ 15-18]. The easy adjustment of the lattice in the presence of adsorbates favors the attack of the AIO4 tetrahedra by the bases and generates penta and hexacoordinated framework aluminium species. Such a mechanism is supposed to be at the origin of the Lewisaciditj in these materials [ 19]. A reversible AI'V/AIv' transformation in aluminosilicate zeolites has been suggested to occur in the protonic form of zeolite Beta [20]. 27A1 NMR studies of hydrated H-BEA revealed that 20% approximately of the aluminium atoms were octahedrally coordinated. Replacement of the protons by ammonium ions, by treatment of the zeolite with gaseous ammonia at 100~ or by sodium and/or pgtassium cations, by standard exchange procedures, led to the disappearance of the AlW NMR signal and to an increase of the intensity of the AIIv one (Fig. 1). Material balances demonstrated that no aluminium was extracted from the solid during these treatments. Density functional calculations performed on pentameric cluster models of zeolite Beta with optimized geometry [21] showed that, due to the strong electron affinity of protons, the structure of the AIO4 units was considerably distorted with Si-O and AI-O bonds of the Si-OH-AI bridges elongated and weakened. Moreover the least stable site was found to be the one associated with two four-membered tings in the framework. On these bases, and by analogy with what has been reported for
135
d
L
_
ppm Figure 1.27A1 MAS NMR spectra of zeolite B e t a a) hydrated H-form, b) sample a saturated with ammonia, c) sample b acid-exchanged, d) sample c potassium exchanged [20].
Figure 2 . 2 7
ml MAS
NMR
spectra of zeolite H-Beta. Sealed ampules : a) hydrated sample, b) sample dehydrated at 300~ c) sample b with ethanol adsorbed, d) sample c after reaction at 200~ [24].
bl
d)
It
'
pp.~
136 VPI-5 [22], the AlXaNMR signal in protonic zeolite Beta was thus attributed to framework aluminium atoms linked to 4 lattice oxygens, the oxygen of a hydronium ion and the oxygen of a water molecule. The unique behaviour of zeolite Beta, and particularly the formation of hexacoordinated 27 framework aluminium, has been further investigated by AI MAS NMR by following the changes in the environment of the aluminium nuclei during the dehydration reaction of ethanol into ethylene and water. The reaction was carried out in 5 mm o.d. sealed capsules which were placed directly in an aluminium-free home-made NMR probehead that could spin the capsule at speeds up to 4 kHz [23,24]. NMR spectra were recorded for the parent hydrated zeolite in the protonic form, for the zeolite dehydrated at 300~ under vacuum, for the dehydrated zeolite equilibrated with 10 Torr of ethanol or pyridine and finally, after heating the zeolite sample loaded with ethanol for 30 min at 200~ Figure 2a shows the characteristic 27A1MAS NMR spectrum of hydrated zeolite Beta [20] with signals at 55 and 0 ppm due to tetracoordinated and hexacoordinated aluminium atoms, respectively. The removal of adsorbed water molecules causes a strong distortion of the electric field symmetry around aluminium. The increase of the quadrupolar coupling constants leads to a broadening of the signals which are no longer detected under our conditions (Fig. 2b). Adsorption of ethanol (or pyridine) induces a relaxation of the structure (also evidenced by infrared spectroscopy) and allows to regain the resonance line of tetrahedral aluminium at 55 ppm (Fig. 2c) while that of octahedral aluminium is still not observed. The latter signal develops after ethanol had been dehydrated, producing nascent water molecules which coordinate distorted AIOaH units leading to a more stable octahedral environment (Fig. 2d). These observations are qualitatively consistent with our assignment of the O ppm line to framework hexacoordinated aluminium and confirm the role of adsorbed water in generating it. Additional work is needed to reinforce this hypothesis and provide more quantitative data. The formation of hexacoordinated aluminium has been also postulated in steam dealuminated mazzite after a complexation treatment with acetylacetone [25]. The above results imply some flexibility of the zeolite lattice [26] or, at least, the presence of crystallographic sites able to suffer major local distortions without loss of long range crystallinity. As regards implications for catalysis using Beta zeolite, the change in symmetry of some aluminic sites will certainly modify their chemical feature. This may be of little importance in the case of the conversion of hydrocarbons (at high temperatures and with a low surface coverage by water) but could become significant for reactions involving polar substrates in the presence of solvents. 3. GENERATION OF LEWIS ACIDITY BY ADSORPTION OF HYDROCARBONS Medin et al [27] were the first to report on infrared studies of adsorbed acetonitrile and ammonia on H-ZSM-5 showing that olefms pre-chemisorbed at room temperature generated strong Lewis sites which were not detected in the freshly pretreated zeolite. As illustrated in
137 Figure 3a, the infrared spectrum of CD3CN adsorbed on the clean surface in the frequency -1 range of CN vibrations reveals the sole presence, at 2295 cm , of molecules coordinated to acidic hydroxyl groups. In the presence of pre-chemisorbed propylene, a second -1 signal at 2370 cm develops (Fig. 3b) that the authors attributed to new Lewis sites strongly interacting with the base. The formation of the new sites was / completely reversible as they disappear / after desorption at 500~ and readsorption of base. A very similar 0~ effect was observed when using I ammonia as probe. This unusual l 2~70 manifestation of Lewis acidity has been (U I I t.~ 50 explained by the cleavage of AI-O ! b). bonds in bridged alkoxyl groups and I inversion of the AIO4 tetrahedra to which the base molecules coordinate T 2~5 (Fig. 4). Support to this hypothesis was provided by quantum-chemical 100 L I ', 1 2 2400 2~00 2200 21 O0 calculations showing that the process Wavenumbers depicted by figure 4 would be Figure 3. Infrared spectra of CD3CN adsorbed energetically unfavorable for R = H but on H-ZSM-5 ' a) fresly pretreated sample, would become possible when R is an b) sample containing pre-adsorbed propene [27]. alkyl group. Such an interpretation was disputed by Bystrov [28] who explained the above results by invoking the formation of nitrilium cations in which acetonitrile is complexed to carbenium ions, the latter acting as very strong Lewis sites. The hypothesis of Bystrov has been elegantly confirmed by Jolly et al [29] who investigated the surface acidity of ZSM-5 in the presence of a series of pre- (or co-) adsorbed olefins choosen so as to generate carbenium ions with different structures. As expected, different nitrilium complexes were obtained using methylcyclopentene (signal at 2376 cm , Fig. 5c, d) and cyclohexene (signal at 2385 cm , Fig. 6a), due to interaction of acetonitrile with tertiary and secondary carbenium ions, respectively. The method was also able to detect the two carbenium ions, cyclohexyl and methylcyclopentyl, involved in the cyclohexene isomerization reaction (Fig. 6b,c) as well as those involved in the rearrangement of adsorbed propene oligomers. Two main conclusions can be drawn from this study. Firstly, nitriles constitute very efficient probes for the identification of adsorbed carbenium ions which are the intermediates of most hydrocarbon acid-catalyzed reactions. Secondly, under working conditions, the acidity of the surface is significantly modified. The strong Bronsted acid sites of ZSM-5 do
138 R
I X
o / \ X
Si
/I
I
1.6
b s o
1.4;
;
/
/
\
o
-,
!
,o
0
/
/
0
X/ Si
/I
O O I \
I
\
O "o" / \
/~
~
2376
\
I ~ ~ ^
/~
l
X "AI...B
/7
OO
1
'.'---~
o
+ B,-
IX
\
A
I
0 AI
O O
Figure 4. Proposed mechanism for the generation of Lewis acidity according to Medin et al. [27].
R
d) c)
0.4
b)
0.2 0.0
4000
~
3500
~
3000 2500 Wavenumbers (cm-1)
"
a)
2000
Figure 5. Infrared spectra of methylcyclopentene adsorbed on H-ZSM-5 9 a) freshly pretreated sample, b) aider adsorption of CD3CN, c) immediately aider adsorption of methylcyclopentene, d)aRer 1 h [29]. 0.8 0.7-
2300
0.6-
c) b)
0.50.4-
a) 0.3. 0.2
2soo
24bo
-2~
22bo
2~00-
Wavenumbers (cm-1)
Figure 6. Isomerization of cyclohexene on ZSM-5. Infrared spectra of CD3CN adsorbed after a) 1 h at RT, b) 1 h at 50~ c) 5 min. at 200~ [29].
139 not exhibit sufficient strength to form nitrilium ions [29,30] whereas the intermediate carbenium ions they generate do. Moreover, not only the strength, but also the nature of the acidity is modified. Both factors should be considered when dealing with complex reactions such as, for example, coke formation or paraffin/olefin alkylation.
4. SYNERGY BETWEEN LEWIS AND BRONSTED SITES.
4.1. The 3600cm
-1
infrared signal in faujasites
Several infrared studies of steam dealuminated faujasite have pointed out the enhanced acid strength (they are able to protonate acetonitrile) of hydroxyl groups characterized by a -1 stretching vibration frequency of 3600 cm [5, 8, 11, 31 ]. They are supposed to correspond to OH groups pointing to the large cage, perturbed by cationic tetrahedral aluminium present as A1OH+ entities in the beta cages. This signal is not observed on faujasites dealuminated by isomorphous substitution, free from extra framework species. The higher efficiency of these sites for the conversion of hydrocarbons has been demonstrated by using an infrared cell as catalytic reactor, operated under dynamic conditions at a temperature of 400~ [6,1 l!i Activity for n-hexane cracking was definitely associated with the presence of the 3600 cm OH groups. Regular high frequency OH hydroxyls and Lewis sites were inactive. All the accessible OH groups were found, by contrast, active for the conversion of cyclohexene, a -1 less demanding reaction. A direct relationship betwen the intensity of the 3600 cm signal and n-hexane cracking activity did not exist however since poisoning experiments using 2,6-1 lutidine and pyridine revealed that the 3600 cm OH groups were heterogeneous, only some of them being acidic enough to initiate the reaction.
4.2. Formation of iminium ions on mazzite. A recent study of the surface acidity of dealuminated mazzite using infrared and ultraviolet spectroscopy of adsorbed pyridine provided a direct evidence for the presence of very strong acid sites associated with Br6nsted/Lewis pairs [32]. At very low coverages in base, infrared bands at 2914, -1 2848, 1496 and 1462 cm and an ultraviolet signal at 205 nm, Figure 7. Dihydropyridistinct from those attributable to A i ~ dinium ion formed pyridinium ions and coordinated Oby reaction of pyridine pyridine, were detected. These on a BrOnsted/Lewis spectral features were pair of sites [32]. characteristic of a conjugated system containing aliphatic CH 2 groups and have been thus assigned to iminium L (dihydropyridinium) ions (Fig. 7). Their formation results
140 from a nucleophilic attack and protonation of pyfidine molecules adsorbed on Lewis sites and requires, therefore, the presence of paired Lewis/BrOnsted sites. Since loss of the aromaticring resonate energy is an energetically demanding process we can expect that the sites on which iminium ions form exhibit a very strong acid character. Additional experiments using heat-flow calorimetry and X-ray photoelectron spectroscopy confirmed the presence of such very strong sites [33]. The formation of iminium ion is not specific to mazzite and does not require, apparently, a particular spatial arrangement of the acid sites. An infrared signal at 1462 cm-1 that could characterize adsorbed pyridinium species has also been observed after high-temperature desorption of pyridine on a series of zeolites with different structures [32], on amorphous silica-alumina [34] and on clays [35]. Proton mobility or migration of cationic species with strong Lewis character may thus also account for this phenomenon. NMR and modeling studies are underway to clarify this point. 5. CONCLUDING REMARK. Experimental approaches aiming at a description of the fate of the active sites of acidic zeolites in the presence of adsorbed reactants or probe molecules may provide a less conventional picture of the surface than the one traditionally emphasized through well established structural and physical aspects of zeolite chemistry. These experiments are certainly promising to refine our understanding of the interaction between reactants and products with the inorganic framework at a molecular scale. In the present state of the art, the information available in the literature is, however, rather limited and essentially qualitative. The technical difficulties associated with the characterization of actual catalysts under non-ideal spectroscopic environments constitute undoubtly, to day, a major limitation. Considerable progress can be nevertheless expected in the future on account of the extensive efforts dedicated to the design of spectroscopic probes simulating reactor conditions. REFERENCES 1 J. Sauer, Stud. Surf. Sci. Catal., 84 (1994) 2039. 2. R.A. van Santen, Stud. Surf. Sci. Catal., 85 (1994) 273. 3 V.B. Kazansky, Stud. Surf. Sci. Catal., 85 (1994) 251. 4. S.J. DeCanio, J.R. Sohn, P.O. Fritz and J.H. Lunsford, J. Catal., 101 (1986) 132. 5. R. Carvajal, P.J. Chu and J.H. Lunsford, J. Catal., 125 (1990) 123. 6. S. Jolly, J. Saussey, J.C. Lavalley, N. Zanier, E. Ben~zi and J.F. Joly, Ber. Bunsenges. Phys. Chem., 97 (1993) 313. 7. R.M. Lago, W.O. Haag, R.J. Mikovsky, D.H. Olson, S.D. Hellring, K.D. Schmitt and G.T. Kerr, Proceedings, 7th Intern. Zeolite Conf., Kodansha, Tokyo, p 677, 1986. 8. F. Lonyi and J.H. Lunsford, J. Catal., 136 (1992) 566. 9. Y. Hong, V. Gruver and J.J. Fripiat, J. Catal., 150 (1994) 421. 10. G.R. Bamwenda, Y.X. Zhao and B.W. Wojciechowski, J. Catal., 150 (1994) 243. 11. S. Jolly, J. Saussey and J.C. Lavalley, J. Mol. Catal., 86 (1994) 401.
141 12. J.A. Rabo and G.J. Gajda, Catal. Rev. Sci. Eng., 31 (1990) 385. 13. J.F. Haw, NMR Techniques in Catalysis, A.T. Bell and A. Pines, Eds., Dekker, New York, 1994, P 139. 14. I.I. Ivanova and E.G. Derouane, Stud. Surf. Sci. Catal., 85 (1994) 357. 15. L.M. Kustov, S.A. Zubkov, V.B. Kazansky and L.A. Bondar, Stud. Surf. Sci. Catal., 69 (1991) 303. 16. A. Stein, B. Wehrle and M. Jansen, Zeolites, 13 (1993) 291. 17. P.J. Grobet, J.A. Martens, I. Balakrishnan, M. Mertens and P.A. Jacobs, Appl. Catal., L21 (1989) 56. 18. R. Vomscheid, M. Briend, M.J. Peltre, P. Massiani, P.P. Man and D. Barthomeuf, J. Chem. Soc., Chem. Commun.,6 (1993) 544. 19. M. Derewinski, M. Briend, M.J. Peltre, P.P. Man and D. Barthomeuf, J. Phys. Chem., 97 (1993) 13730. 20. E. Bourgeat-Lami, P. Massiani, F. Di Renzo, P. Espiau, F. Fajula and T. Des Couri6res, Appl. Catal., 72 (1991) 139. 21. I. Papai, A. Goursot, F. Fajula and J. Weber, J. Phys. Chem., 98 (1994) 4654. 22. L.B. McCusker, Ch. Baerlocher, E. Jhan, and M. B01ow, Zeolites, 11 (1991) 308. 23. F. Rachdi, J. Reichenbach, L. Firlej, P. Bernier, M. Ribet, R. Aznar, G. Zimmer, M. Helme and M. Mehring, Solid State Commun., 87 (1993) 547. 24. W. Buckermann, L.C. De Menorval, F. Figueras and F. Fajula, J. Phys. Chem., Submitted. 25. W. Buckermann,B.H. Chiche, F. Fajula and C. Gueguen, Zeolites, 13 (1993) 448. 26. J.A. Rabo and G.J. Gajda, NATO ASI Ser. B, Physics, Vol. 221 (1990) 273. 27. A.S. Medin, V. Yu, V.B. Kazansky, A.G. Pelmentschikov and G.M. Zhidomirov, Zeolites, 10 (1990) 668. 28. D.S. Bystrov, Zeolites, 12 (1992) 328. 29. S. Jolly, J. Saussey and J.C. Lavalley, Catal. Let., 24 (1994) 141. 30. J.F.Haw, M.B. Hall, A.E. Alvaro-Swaisgood, E.J. Munson, Z. Lin, L.W. Beck and T. Howard, J. Amer. Chem. Soc., 116 (1994) 7308. 31. G. Garralon, A. Corma and V. Fornes, Zeolites, 9 (1989) 84. 32. B.H. Chiche, F. Fajula and E. Garrone, J. Catal., 146 (1994) 460. 33. D. McQueen, B.H. Chiche, F. Fajula, C. Guimon, A. Auroux, F. Fitoussi and Ph. Schulz, J. Catal., Submitted. 34. K.H. Bourne, F.R. Cannings and R.C. Pitkethly, J. Phys. Chem., 74 (1970) 2197. 35. S. Bodoardo, F. Figueras and E. Garrone, J. Catal., 147 (1994) 223.
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Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
143
Characterization of Hexagonal and Lamellar Mesoporous Silicas, Alumino- and Gallosilicates by Small-Angle X-Ray Scattering (SAXS) and Multinuclear Solid State N M R Zelimir Gabelica 1, Jean-Marc Clacens 1, Roger Sobry2 and Guy Van den Bossche 2 1Laboratoire de Catalyse, FacultEs Universitaires Notre-Dame de la Paix, 61 rue de Bruxelles, B-5000, Namur, Belgique 2Laboratoire de Physique Exp6rimentale, Universit6 de Liege, Institut de Physique, b~timent B-5, B-4000, Li/~ge, Belgique
SUMMARY
Various mesoporous silicas and the corresponding alumino- or gallosilicates have been synthesized using a series of literature or home made recipes. The efficiency of the A1 or Ga incorporation markedly depends on the trivalent source and the evolution (aging) of the so formed Si-M(III) gel-type phases at different starting pH values, prior to the addition of the surfactant-structuring compounds. Low temperature (e.g. < 100~ syntheses yielded mesoporous compounds with hexagonal topology (MCM-41 type), involving thin multilayered Si walls (29Si-NMR data), eventually partly substituted for by A1 or Ga, (27A1and 71Ga-NMR). Such structures do remain stable after calcination in air at 600~ (TG-DSC and sorption data). The calcination either results in the expulsion of the (too voluminous) Ga(III) ions from the framework, or to a structural rearrangement of the remaining A1 species. The resulting solids still exhibit quite strong Br6nsted type acidic sites (NH3-TPD data). When the same gels are crystallized at 150~ fro 2 days, lamellar frameworks (MCM50 type) are preferentially stabilized. They readily collapse upon heating but the final amorphous phase still involves both A1 and/or Ga cationic (acidic) species randomly distributed in various structural environments. Small-angle X-ray scattering (SAXS) proved to be the choice method to unambiguously identify different topologies. The actual roughness (smoothness) of the various as-synthesized or calcined phases could be directly related to the fractal dimension derived from SAXS spectra. The presence of peaks at very small diffraction angles allowed us to estimate that the channel lengths of the thermally stable mesoporous materials essentially extend in the 150-200 nm range. EXPERIMENTAL
The different synthesis procedures adopted in this study were either literature recipes [1-3], sometimes slightly or thoroughly changed by home performed modifications [4]. All
144 the as-synthesized and calcined phases were characterized by using a combination of conventional techniques: powder XRD (topology, purity), atomic absorption and/or EDX (extent of trivalent incorporation), 27A1-, 71Ga- and 29Si- high resolution solid state MAS NMR (coordination and structural quantitative repartition of Ga and A1 bearing species, as well as the various Qx configurations of the Si atoms), TG-DTA-DSC-DTG combined with in situ n-hexane sorption (thermal stability, template content, pore volume determination, crystallinity), BET (surface area) and NH3-TPD (acidity). SAXS data were collected with a Kratky camera (Cu Ks radiation) using an experimental setup specifically built in the Liege University physics department [5]. RESULTS AND DISCUSSION
Gels prepared according to recipes as described in the literature [2,3] and left to crystallize at high temperature (i.e. 150~ for at least 2 days, yield well crystalline mesoporous lamellar (L) type phases. They exhibit good crystallinity (XRD data, high template content), medium surface area (< 500 ma/g) but their lamellar structure collapses as soon as they are heated under conditions currently used to remove the templates from mesoporous materials (600~ air flow). Conversely, thermally stable phases exhibiting a hexagonal (H) topology and a high surface area (> 1000 m2/g) could be prepared by crystallizing the same gels under milder conditions (at temperatures < 100~ or by modifying the gel composition (addition of TMA and/or under a careful pH adjustment and control). In both H and L structures, the degree of A1 incorporation dramatically depends on the nature of the Si and A1 starting materials, their solubility and their final degree of (de)polymerization at the end of the aging process at controlled pH, prior to any template addition. For equivalent experimental conditions, gallium tends to incorporate the final Si framework more quantitatively than aluminum. However, when both trivalents are added simultaneously (as oxides in the presence of various solubilizing additives such as NH3), their degree of incorporation is equivalent (Table 1). In the as-synthesized products, Ga is only partly sited in tetrahedral positions in both the H and L structures, as ascertained by 71Ga-NMR [6]. However, part of it remains either as a dispersed amorphous phase intermixed with the mesoporous compound, or incorporated in its framework, but randomly partitioned throughout the less well defined coordination sites, at least not detectable by NMR. In contrast, all the AI(III) ions are found located in both tetrahedral (T) and octahedral (O) framework sites, in various proportions that could be quantified [4]. Their repartition was shown to strongly depend on the synthesis conditions. After template removal upon calcination, the aluminosilicate phases generally show a better thermal resistance than the corresponding Ga analogs, although a partial dealumination and a marked framework (O) and (T) A1 redistribution is observed by 27A1-NMR. When gallium bearing L type phases are calcined, some residual Ga sited in T positions is still detected by NMR. These phases involve both Q3 and Q4 silicon configurations in various proportions, as seen by 29Si NMR, thus suggesting that both terminal silanols and Si(4Si) structural entities are present. The relative amount of Q3 and Q4 configurations could be quantified, provided the appropriate repetition times between 2 NMR pulses are selected in each case (the Si nuclei in the as-synthesized and calcined phases do relax very differently
145 [4]. Assuming that Q3 essentially corresponds to the terminal hydroxylated surface while the Q4 configurations reflect the non defected bulky Si structure, the calculated Q3/Q4 ratios could be related to the actual thickness of the framework (essentially siliceous) walls. It could be concluded that the lattices of the L type phases involve at least 5 or more Si layers, in which Ga still can find a stable T coordination within the internal layer(s). Table 1A Synthesis conditions Code
Source of trivalent element
T(~
/ time
Synth. cond. [ref.] (1)
1/9 2/11
A1203 Na Aluminate
150 / 65 h 150 / 65 h
[3 a] [3 a]
3/12
---
150 / 65 h
[3 b]
4/14
Na Gallate
150 / 65 h
[3 a]
5/15 6/20
Ga203 Na Aluminate (NH 3)
150 / 65 h 150 / 65 h
[3 a] [3 c]
7/21
Ga203 (NH 3)
150 / 65 h
[3 c]
8/22
A1203 + Ga203 (NH 3)
150 / 65 h
[3 c]
15/7 9/17-1
A1203 (TMA) ---
150 / 65 h 150 / 65 h
[1,5] [4,5]
10/17-2
---
100 / 6 days
[4,5]
11/18-1
A1 Sulfate
150 / 65 h
[4,5]
12/18-2
A1 Sulfate
100 / 6 days
[4,5]
13/19-1
Ga Sulfate
150 / 65 h
[4,5]
14-19-2
Ga Sulfate
100 / 6 days
[4,5]
(1) Letters a, b and c refer to variants in the synthesis recipes described in reference [3]
146 Table 1B Some characteristics of the final mesoporous phases obtained by using various synthesis conditions Final product Code
Nature (1)
1/9 2/11
L L
% incorporation Si
A1
88 (nd) (2)
100 18
Si/M (at.) Ga 24 (nd)
3/12
L
87
---
4/14
L
100
---
81
5/15
L
100
---
84
6/20
L
(nd)
93
45 43 (nd)
7/21
L
100
---
94
26
8/22
L
100
80
76
20
15/7
H
100
41
9/17-1
L
(nd)
---
36
10/17-2
H
(nd)
---
11/18-1
L
100
16
12/18-2
H
88
100
13/19-1
L
(nd)
---
98
(nd)
14/19-2
H
87
---
96
90
311 44
(1) L: lamellar phase; H: hexagonal phase (2) (nd) not determined Similarly, all the as-synthesized H type phases also exhibit a quite broad NMR resonance that could be deconvoluted into two lines of variable intensity, one located a t - 100 ppm and better corresponding to Q3 configurations, the other a t - 110 ppm, assigned to Q4. It is actually not clear to which NMR line should be attributed the Si atoms being interacting with the bulky terminal -N(CH3) 3 groups of the CTABr templates and thus forming Si (3Si, IN) configurations, so that the estimation of the actual wall thickness from only the NMR data in the precursors is dubious, especially when the spectra are broad. After calcination, the spectra are better resolved and more Q3 configurations were detected, suggesting the presence of major silanol terminations in the final calcined hexagonal frameworks, in line with a Hoffman type degradation of the tetraalkyl ends of the template. An estimation of the Q3/Q4 ratios could be achieved. It still suggests the presence of multilayered walls of Si atoms, roughly composed of a maximum of 3-4 Si layers. Undoubtedly the walls of the H phases are thinner than those detected in the L type phases. Irrespectively to the smaller AI(III) ions, Ga(III) species are probably too bulky as to still
147 remain sited in the "peripheral" T positions of the hexagonal structure. Their expulsion is possibly more readily achieved because of the presence of important strains within the thin layers. Finally, the SAXS spectra of the various as-synthesized and calcined compounds strongly confirm the above detected structural features and, in most cases, bring more detailed structural characteristics about the thickness of the walls, the fractal slope, the smoothness of the external envelope and the length of the hexagonal channel systems. The presence of peaks at very small diffraction angles allowed us to evaluate the channel lengths of the thermally stable hexagonal mesoporous materials. Depending on experimental condition and on the presence of trivalent ions in the framework, the mean length essentially extends in the 150-200 nm range. The detailed SAXS study is discussed elsewhere [5]. REFERENCES
1. S.J. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonovicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCuller, J.B. Higgins and J.L. Schlenker, J. Amer. Chem. Soc., 114 (1992) 10834. 2. C.Y. Chen, H.X. Li and M.E. Davis, Microporous Materials, 2 (1993) 17. 3. K.M. Reddy, L. Moudrakovski and A. Sayari, J. Chem. Soc. Chem. Commun. (1994) 1059. 4. J.M. Clacens and Z. Gabelica, in preparation. 5. R. Sobry, G. Van den Bossche, J.M. Clacens and Z. Gabelica, in preparation. 6. Z. Gabelica, C. Mayenez, R. Monque, R. Galiasso and G. Giannetto, in: Synthesis of Microporous Materials: Vol. I: "Molecular Sieves" (M.L. Occelli and H. Robson, Eds.), Van Nostrand Reinhold, New York, 1992, 289.
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Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
149
CHARACTERISATION OF A CUBIC MESOPOROUS MCM-48 COMPARED TO A HEXAGONAL MCM-41 Raft Schmidta, b , Michael St6cker a and Ole Henrik Ellestada, b a SINTEF, P.O. Box 124 Blindem, N-0314 Oslo, Norway. b University of Oslo, Department of Chemistry, P.O. Box 1033 Blindem, N-0315 Oslo, Norway.
ABSTRACT A cubic MCM-48 and a hexagonal MCM-41 material were synthesised. The pore ordering was confirmed by X-ray powder diffraction and HREM studies. The pore size of the materials was determined to 2.9 nm for MCM-41 and 2.5 nm for MCM-48 by N 2 adsorption and by measuring the freezing depression of water enclosed in the pores by 1H NMR. The self diffusion coefficient of water confined in the pores of MCM-48, determined by 1H NMR spinecho measurements, was found to be significantly lar~er compared to that of MCM-41.29Si MAS NMR showed a significant higher number of Q J species (Si(3OSi)OH) for MCM-48 in the as-synthesised state compared to MCM-41. INTRODUCTION The discovery of a new family of mesoporous materials, denoted as M41S, was recently reported by researchers from the Mobil Oil Co-operation [ 1,2] and has dramatically expanded the range of defined pore sizes ( < 1.3nm) found in crystalline microporous materials (e.g. zeolites) into the mesopore regime ( > 2 nm). High hydrocarbon sorption capacity and high thermal stability are additional interesting properties of these materials. They are synthesised from gels, containing besides silica optionally different other metals (e.g. A1.). The structure of the porous solid is hereby defined by the aggregation of surfactant molecules (such as C16H33(CH3)3NBr) [1]. Already in the first publications [1,2] it was reported that purely siliceous materials with hexagonal ordered pores (MCM-41), layered materials (MCM50) and materials with a three dimensional pore system (MCM-48) could be synthesised by this mechanism, indicating the diversity of this new family of mesoporous materials. However, most of the academic interest following this new invention, was concentrated on MCM-41 [37] and only very few characterisation data of the cubic member of the M41S family (MCM48) have been published [5 - 7], even though the three dimensional pore system of MCM-48 may make this material even more interesting for industrial applications as MCM-41. Here we report on the synthesis of a MCM-48 and a MCM-41 material and the characterisation by
150 XRD, HREM, N 2 adsorption, 29Si MAS NMR and 1H NMR of water enclosed in the pores [8]. Additionally the self diffusion-coefficients of water enclosed in the pores of these mesoporous materials were determined by 1H NMR spin-echo measurements. EXPERIMENTAL A purely siliceous MCM-48 material was synthesised according to the following procedure (similar to the one described by Monnier et al. [5]): 0.7 g of NaOH was dissolved in 17 g of distilled water. Then 7.22 g of tetraethylortosilicate (TEOS) (98%, Jansen) was added to the gel while stirring. After 5 min 27.8 g of the template solution containing 25 wt% C16H33(CH3)3NC1 in distilled water (Fluka) was added. The molar composition of the gel was 1:0.65:62, respectively for TEOS : C16H33(CH3)3NC1 :H20. The resulting gel was then stirred for another 15 min, loaded into a stoppered Teflon bottle and heated without stirring at 100~ for 72 hr. After cooling to room temperature, the resulting solid was recovered by filtration, extensively washed with distilled water, and dried in air at ambient temperature. The template was removed by calcination at 540 ~ for 1 hr in flowing nitrogen followed by 6 hrs in flowing air with flow rates of 100 ml/min, respectively. A siliceous MCM-41 material was synthesised according to a procedure given by Beck et al. using C16H33(CH3)3NBr as template[2] and processed according to procedure described above. The obtained solid materials were characterised by X-ray powder diffraction (Siemens D5000 diffractometer, CuKo~ radiation) and High Resolution Electron Microscopy using a JEOL 2000 FX instrument applying primary magnifications between 68 000X and 210 000X. The TEM samples were prepared by crushing the material in ethanol in an agate mortar and dropping this dispersal of finely grounded material onto a holey carbon film. This produced free sheets of the material suitable for TEM work. Tilting of the specimen for accurate crystal alignment was usually not necessary. N 2 isotherms were measured at 77 K with a Carlo Erba instrument using a conventional volumetric technique. The samples were outgased at 200~ for 1 hr. The mean pore size was calculated from the N 2 isotherm using the Kelvin equation correcting for the mulfilayer thickness. The water-saturated samples were studied by 1H NMR in the temperature range 273 to 183 K using a Varian VXR 300 S NMR spectrometer, operating at 300 MHz proton resonance frequency [8]. All measurements were performed with a 90 ~ pulse of 16 Its. Each spectrum was composed of 16 transients. The samples were temperature equilibrated for 5 min at each temperature before any measurements were performed. The spectra were accumulated applying a temperature cycling, i.e., heating and cooling of the sample. Further T 2 measurements of water enclosed in the pores were performed at 268 K using Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence with varying "cbetween 16 and 400 Its. The self diffusion coefficient of pore water was extracted from the obtained T 2 data using a model proposed by LeDoussal et al. [11]. 29Si MAS NMR spectra were recorded on the same spectrometer, equipped with a Jakobsen high spinning speed MAS probe using 7 mm zirconia rotors. Frequency of 59.6 MHz, sweep width: 1400Hz, pulse width 8 Its (90 ~ degrees pulse: 8.3 ~ts), repetition time: 300 sec, acquisition time: 1 sec, number of scans: 1000, MAS spinning speed: 4.5 kHz. The lines were referenced to the low field signal of ZSM-5 at -109.6 ppm (as a secondary reference).
151
RESULTS AND DISCUSSION
Well ordered MCM-48 and MCM-41 materials, confn'rned by XRD (Figures 1 and 2) and HREM (Figures 3and 4) were prepared.
t
d-value~ hki [4.~] 100 2.3 2.0
,--,
5 c6
d-values 4.19 3.59 2.73 2.48 2.25 2.15 2.06 1.98 1.84 1.63 1.56 1.47
110 200
21o II
1:53~ .
300 220
>, "~ r-"~ t
~
0
2
4
6
8
hid
[nm]
10
Degrees 2-theta
0
2
4
211 220 321 400 420 332 422 431 521 611/532 541 444 543
6
8
10
D~j rees 2-theta
Figure 1. X-ray powder diffraction data of the as-synthesised MCM-41
Figure 2 X-ray powder diffraction data of the as-synthesised MCM-48.
Figure 3. HREM image of as-synthesised MCM-41 in the direction of the pores.
Figure 4. HREM image of as-synthesised MCM-48 with pores on the 110 plane.
152 The X-ray diffractogram of the as-synthesised MCM-48 could be indexed according to a cubic space group with Ia3d symmetry, and a unit cell size of 102 A, which is in agreement with our HREM investigations. A representative electron micrograph of MCM-48 viewed along the [ 110] axis is shown in Figure 4. The unit cell size of the hexagonal MCM-41 was determined to 46/~ (Figure 1) and the hexagonal pore organisation was confirmed by HREM (Figure 3).The pore structure of MCM-48 was generally observed over the whole sheet investigated by HREM, even though small variations e.g. of the pore wall thickness could be registered. These variations result in the displacement of one single pore at the time, and not in the disturbance of the whole local structure. For MCM-41 local variations often result in a disturbance of the (hexagonal) pore organisation over a significant larger area than for MCM48. However, the pore structure of both materials is found to be quite unaffected by the removal of the template by a calcination at 540 deg C. The BET surface area was determined to be 1100 m2/g. and 1080 m2/g. for the MCM-48 and MCM-41, respectively. In accordance with the N 2 isotherms recorded for MCM-41 (Figure 5), the N 2 isotherm of MCM-48 showed a sharp increase in the adsorbed volume of N 2 due to capillary condensation of N 2 in the pores of this material (Figure 5).
~41~ I--v m
E
[
.~400 Figure 5. Nitrogen isotherm at 77 K on MCM-41 and MCM-48. Filled symbols denote adsorption, open symbols denote desorption.
0
E __=200 0 > |
0i 0
i
I
I
I
I
0,2
0,4
0,6
0,8
1
P/Po However, even though both materials were synthesised using C 16H33(CH3)3N+ as template the sharp increase in the adsorbed volume of N 2 due to capillary condensation occurs at different relative pressures P/P0; reflecting different pore sizes of the MCM-41 and MCM-48
153 material. Using the relative pressure (at the inflection point of the capillary condensation) of P/P0 = 0.35 for MCM-41 and P/P0 = 0.26 for MCM-48 and using the Kelvin equation correcting for multilayer thickness, the mean pore sizes were calculated to be 2.9 nm and 2.5 nm for MCM-41 and MCM-48, respectively. The pores of the MCM-41 and MCM-48 material were further characterised by 1H NMR temperature studies of water enclosed in the pores. The 1H NMR spectra of confined pore water of MCM-48 vs. temperature (from 265 to 183 K) are illustrated in Figure 6 and show clearly a sharp intensity decrease with decreasing temperature due to the freezing of pore water.
i
d >,
0
rr" Z "1, ..i. w
280
260
240
i
!
|
220
200
180
Temperature [ K] Figure 6. 1H NMR spectra of water enclosed in the pores of MCM-48 as a function of temperature. A mathematical model [8] was fitted to the 1H NMR signal intensity vs. temperature of water enclosed in MCM-41 and MCM-48 (Figure 7, solid line). Using this model a transition temperature of 224 K reflecting the freezing of the pore water was determined for MCM-48. For the MCM-41 material a transition was observed at 225 K while cooling and 228 K while wanning up. This observed hysteresis in freezing/melting transition (which is not observed for the MCM-48) is due to the larger pore size of the MCM-41 compared to MCM-48 and not due to the destruction of the pores during the freezing/melting process [9]. The effect of the pore size on the occurrence of freezing/melting transition is discussed in detail in a recent publication [10]. Using the freezing point depression (determined by warming up the water saturated samples form 183 K to 273 K, while recording the 1H NMR signal) the pore sizes of MCM-41 and MCM-48 were calculated to 2.9 + 0.08 nm and 2.7 + 0.16 nm, respectively. It was assumed that the observed freezing point depression (AT = 237.15 K - transition temperature) is correlated to the pore size by AT =
K/ R p - tl
9with Kf = 49.5 + 0.19 K*nm-1
154
and tf = 0.35 + 0.036 nm [9]. The pore sizes determined by N 2 adsorption and 1H NMR method are found to be in good agreement.
"L
MCM-41
"'7, :3
t'-
MC 48
Figure 7. 1H NMR signal intensity of pore water confined in MCM-41 and MCM-48. Filled symbols denote heating, open symbols denote cooling of the sample.
%
r
rr"
-
N
Z
"1-
280
260
240
220
200
180
T~rature [ K] The water saturated MCM-41 and MCM-48 samples were further studied by measuring the apparent spin-spin relaxation (T22) of the confined pore water at 268 K. The temperature of 268 K was chosen to ensure that water not enclosed in the pores is frozen and though does not contribute to the measured spin-spin relaxation time. Of particular interest is the dependence of the apparent relaxation rate, 1/T22, on the inter pulse time 2z. This behaviour has been discussed in the literature [11] and arises because of the motional diffusion of the water molecules through the magnetic field gradient created by the susceptibility difference between the solid matrix and the pore water. The apparent relaxation rate (1/T22) extracted from the observed CPMG echo envelope is therefore not only dependent on the inherent spinspin relaxation time of the enclosed pore water (T2), but also strongly affected by the fluid molecules moving (characterised by the diffusion coefficient D) in the inhomogeneous magnetic field within the pore: 1/T22 = 1/3 * ?2DG2z2 + 1/T 2, with ~/ the nuclear magnetogyric ratio, D the self diffusion coefficient of the pore fluid, G the magnetic field gradient G, and the inter pulse spacing z. For a porous medium this equation has to be modified to take into account the motional restrictions imposed by the pore walls. Applying the model derived by LeDoussal et al. [ 11] the self diffusion coefficient of the pore water can
155 be extracted from the measurements of the apparent spin spin relaxation times as a function of x. This is explained in detail for MCM-41 materials in a recent work by Hansen et al. [ 12]. The self diffusion coefficient for water enclosed in the pores was found to be 4.53 + 0.27 10-9 cm2/s for MCM-41 and 2.21 + 0.06 10-8 cm2/s for MCM-48 whereas the self diffusion coefficient (D) of bulk water at 300 K is approximately 2.5 10-5 cm2/s. This demonstrates a strong reduction of the diffusion coefficient of pore water compared to bulk water. In a previous study of MCM-41 materials D was found to decrease with decreasing pore size [12]. However, in this study a significantly higher self diffusion coefficient of pore water was calculated for MCM-48 compared to MCM-41 even though MCM-41 posses larger pores. This suggests a larger mobility of pore water in the three dimensional pore system of MCM-48 compared to the uni-dimensional pore system of MCM-41. However, other effects on D, as e.g. different particle sizes, validity of the model for the calculation of D, etc. may influence the result. This will be a subject of future studies. Through 29Si MAS NMR a significant larger number of Q3 species (Si(3OSi)OH) represented by the peak at around -100 ppm compared to Q4 (Si(4OSi) species represented by the peak around -110 ppm was observed for MCM-48 than for MCM-41 in their as synthesised state (Figure 8). A Q3/Q4 ratio of around 0.45 and 1 was found for MCM-41 and MCM-48, respectively.
-110 -100~"~
41
~ I
I
-50
-70
I
I
~-48 I
I
-90 -110 -130 -150 ppm
Figure 8.29Si MAS NMR spectra of MCM-41 and MCM-48. Resonances are assigned as follows" around - 100 ppm: Q3 _ species; around- 110 ppm" Q4_ species.
156 CONCLUSION Well ordered MCM-48 and MCM-41 were synthesised. The overall pore organisation of MCM-48 is much less affected by local variations (e.g. wall thickness) as for MCM-41. The pore size of the template free MCM-41 was found to be larger (around 2.9 nm) compared to MCM-48 (around 2.5 nm) synthesised with the same cationic template ( C 16H33(CH3)3N+)A good agreement between the determination of the pore size by N 2 adsorption and ~H NMR temperature studies of "pore water" was found. The self diffusion coefficient of water confined in the pores of MCM-48, determined by 1H NMR spin-echo measurements, was found to be significantly larger compared to that of MCM-41. A significant higher number of Q3 species (Si(3OSi)OH) was found for MCM-48 compared to MCM-41 in their assynthesised state. ACKNOWLEDGEMENTS A research grant from the Norwegian Research Council (Deminex program) is gratefully acknowledged by R.S.. The authors are indebted to E.H. TCrstad and A.Olsen for preparing the HREM micrographs and E. Hansen for recording the 1H NMR spectra and D. Akporiaye for fruitful discussions. 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-U. Chu, D.H. Olsen, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J.Am. Chem. Soc., 114 (1992) 10834. 3. C.Y. Chen, H.X. Li and M.E. Davis, Microporous Materials 2 (1993) 17. 4. R. Schmidt, D. Akporiaye, M. St/Scker and O.H. Ellestad, J. Chem. Soc., Chem. Commun.(1994) 1493. 5. A. Monnier, F. Schiith, Q. Huo, D. Kumar, D. Margolese, R.S. Maxwell, G.D. Stucky, M. Krishnamurty, P.;Petroff, A. Firouzi, M. Janicke and B.F. Chemelka, Science 261 (1993) 1299. 6. 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. Olsen and E.W. Sheppard, Chem. Mater. 6 (1994) 2317. 7. Q. Huo, D.I. Margolese, U. Ciesla, P. Feng, T.E. Gier, P. Sieger, R. Leon, P.M. Petroff, F. Schiith and G.D. Stucky, Nature 368 (1994) 317. 8. D. Akporiaye, E.W.Hansen, R. Schmidt and M. St/Scker. J. Phys. Chem., 98 (1994) 1926. 9. R. Schmidt, E.W. Hansen, M. St/Scker, D. Akporiaye and O.H. Ellestad, J. Am. Chem. Soc. in press. 10. R. Schmidt, M. St6cker, E.W. Hansen, D. Akporiaye and O.H. Ellestad, Microporous Materials 3 (1995) 443. 11. P. LeDoussal and P.N. Sen, Phys.Rev.B. 46 (1992) 3465 12. E.W. Hansen, R. Schmidt, M. St6cker and D. Akporiaye, Microporous Materials, submitted.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
157
Synthesis and Characterization of Transition Metal Containing Mesoporous Silicas S. Gontier and A. Tuel Institut de Recherches sur la Catalyse. C.N.R.S. 2, av. A. Einstein 69626 Villeurbanne Cedex France Titanium and Vanadium mesoporous silicas (MS) have been synthesized at ambient temperature using a neutral templating route with primary alkyl amines as surfactant. Spectroscopic characterization of the samples showed that transition metal cations were highly dispersed in the silica framework. The metal content, the surfactant chain length and the amine/SiO 2 ratio greatly influenced the properties of the final product. These materials were found to be active as catalysts in oxidation reactions with alkyl peroxides at mild temperature. 1. I N T R O D U C T I O N There has been an extensive interest during the last years for the synthesis of transition metal containing molecular sieves, due to their remarkable properties as catalysts in oxidation reactions with organic peroxides [1,2]. A very beautiful example is TS-1, the titanium substituted silicalite-1, that catalyzes many oxidation reactions in the liquid phase with aqueous hydrogen peroxide. However, as far as zeolitic supports are concerned, reactions are limited to small substrates with a kinetic diameter smaller than 7~. Recently, the discovery of a novel family of silica-based mesoporous molecular sieves M41S by Mobil researchers [3] and a group from Waseda University [4] opens new perspectives in the field. Corma et al. [5]
were the first to synthesize a Ti
mesoporous silica analog to MCM-41 and to show that the material was active as catalyst in the oxidation of bulky substrates. In a similar manner, Franke et al. [6] also prepared Ti-MCM-41, but they did not report any catalytic data over these solids. Reddy et al. [7] also reported the possibility of preparing vanadium-containing MCM-41, active in oxidation reactions.
158 Very recently, Tanev et al. [8] have shown that mesoporous silicas (MS) could be prepared using a neutral templating route with primary alkyl amines as surfactant. A great advantage of the recipe was that the template could be removed from the mesopores by a solvent extraction, which is of great interest for environmental protection. We have followed a similar procedure to prepare Ti and V-containing mesoporous silicas. The influence of synthesis parameters, e.g. the metal content, the amine chain length or the synthesis time have been examined. These materials have shown very interesting properties as catalysts in oxidation reactions with organic peroxides as compared to zeolitic materials. 2. E X P E R I M E N T A L Ti or V-MS were prepared at ambient temperature by mixing a first solution containing tetraethyl orthosilicate (1 mole), ethanol (6.5 moles), isopropyl alcohol (1 mole) and the metal precursor (tetrabutyl orthotitanate or vanadyl acetylacetonate) to a second solution containing the alkylamine (0.3 mole) in water (36 moles). The resulting solution was homogeneized, stirred for about 30 min and aged for different periods at room temperature under static conditions. Solids were then filtered, washed several times with distilled water and air dried. Calcination of the samples was performed at 650 ~C in air for 6 h. When the organics were removed by a solvent extraction, 1 g of dried solid was dispersed in 100 ml of ethanol and the mixture refluxed for about 1 h. The solid was recovered by filtration, washed with cold ethanol and the procedure repeated once. A Ti-Beta sample was synthesized following the recipe of Camblor et al. [9] and contained about 1.5 wt % Ti and 0.4 wt % AI. A TS-1 sample was also synthesized following the patent literature [10]. Samples have been characterized using X-Ray diffraction (Philips PW 1710, CuKa radiation), IR spectroscopy (Perkin Elmer 580), U.V-Vis spectroscopy (Perkin Elmer Lambda 9) and EPR (Varian, E9). N 2 adsorption/desorption isotherms were carried out on a Catasorb apparatus. 3. R E S U L T S AND DISCUSSION
3.1. Synthesis and Characterization A series of samples have been synthesized with dodecylamine (n = 12 carbon atoms) and varying the amount of metal precursor in the gel. Gels were aged for 12 h
159 before recovering as-synthesized solids by filtration. As shown in Table i, the amount of metal incorporated in the silica matrix is very close to that introduced during the preparation. Table i Characterization of the different samples Sample Gel .
.
M~
oc
Si/Ti Product .
S(m2/g) .
m
.
973
r
V(cm3/g)
.
.
26.5
0.52
Ti-MS
i0()
85
i086
28
0.62
Ti-MS
50
45
i 066
29
0.65
I1-M~
30
3i
894
28
0.54
Ti-MS
20
i 9.5
667
25
0.36
V-Mb
i00
108
998
28
0.55
V-MS
50
62
i0 i5
28
0.64
V-MS
30
36
794
28.5
0.48
9 p()~) is the mean pore diameter V(cm3/g) is the porous volume measured at P/P0 = 0.5 in the N 2 isotherm. This suggests that all the Si and Me precursors were in the solid phase as the yield in Ti or V-MS was always very high (> 95 %). For relatively low Me contents (< 2 wt %), U.V-Vis as well as EPR spectroscopies showed that the cations were highly dispersed in the solid. Ti-MS materials exhibited a U.V. absorption band around 240 nm. The absorption edge decreased by about 10 nm to 230 nm upon calcination in air. For V-MS, EPR parameters were characteristic of vanadyl ions in an axially symmetric crystal field (A ![ = 190 G, g [] = 1.94, A__k_ = 72 G and gA_ = 2.00). The totality of the EPR signal disappeared upon calcination of V-MS samples in air. Dried calcined samples were white, but their color rapidly turned to bright yellow upon exposure to air, suggesting a change in the cation coordination. This was clearly evidenced in U.V-Vis and 51V NMR spectra. In dry samples, V 5 + cations are more likely in a tetrahedral environment but water molecules can easily enter their coordination sphere to give hexacoordinated cations. The process was perfectly reversible as original U.V-Vis and NMR spectra were restored after outgasing the samples at 200~
for 3 h. This clearly showed the relatively
160 high hydrophilic character of mesoporous silicas as compared to substituted zeolites like TS-1 or VS-1. All calcined samples exhibited relatively high surface areas, typically 1000-1200 m2/g and a mean pore diameter close to 28/~. Both slightly increased with the metal incorporation up to about 2 wt % of metal in the solid. Beyond that limit, for higher Me contents, the surface area as well as the pore diameter decreased. In the same time, U.VVis and NMR spectra revealed the presence of extrawall dispersed oxide species in the samples. However, the decrease in pore diameter could also arise from a loss of thermal stability of the structure for high Me incorporations. Similar observations have been made by Franke et al. [6] for Ti-MCM-41, and the authors interpreted the decrease in pore dimensions to the presence of extrawall species. A sample has been prepared with Si/Ti = 100 in the precursor gel and aged at room temperature for different periods ranging from a few minutes to 18 h. Indeed, Chen et al. [11] had reported that even though an X-Ray powder pattern typical of MCM-41 could be obtained after heating a silica-alumina gel at 70~
for 3 h, the
material was not thermally stable and collapsed upon calcination in air at 540 ~C. Table 2 clearly shows that using the present synthesis route, Ti-MS was obtained after a few minutes after mixing of all the reagents. There was no significant evidence for modifications occuring during the aging period ; the Ti content as well as the BET surface area and the pore dimension remained unchanged. The same observations could be made with vanadium-containing samples. Table 2 Characterization of Ti-MS samples recovered at different time intervals t(h)
Si/Ti
S(m2/g)
~p()~)
0
93
1081
28
0.58
1
85
1212
28
0.62
2
96
1091
28
0.60
3
94
1112
28
0.63
6
92
1174
28
0.62
18
85
1085
28
0.62
For the definition of ~p and V, see Table 1
V(cm3/g)
161 it has been widely reported that the pore dimension of MCM-4i synthesized with aikyitrimethyiammonium cations depended on the number of carbon atoms of the alkyi chain. For pure silica materials, data from Beck et ai. [3] showed an almost linear increase in the pore diameter from i8 ~ with octyiamine to about 37 /~ with hexadecyiamine. Very recently, Tanev et ai. [8] also reported that the pore dimensions of mesoporous silicas prepared using a neutral templating route increased with the amine chain length. For both Ti and V-substituted materials, we also observed that the dimension of the mesopores increased from about 25 ~ for n = i0 carbon atoms to 35 for n = i6. The corresponding isotherms are shown in Fig. i.
(c) o') A
I...03
1I~176
/
(b)
E "6 ..Q L_
o
Ca)
"13 <
E
0'
'
' 0.2
'
' 0.4
'
' 0.6
'
0'8.
'
I 1.0
plpo Figure i. N 2 adsorption/desorption of Ti-MS synthesized with CnH2n + i N H 2. n = i0 (a), n - i2 (b) and n - 16 (c)
162 These values are somewhat higher than those of Beck et al. [3] but also than those of Tanev et al. [8] who prepared their samples following a very similar recipe. As for MCM-41, the use of an amine with 18 carbon atoms led to materials with the same pore dimensions as those prepared with hexadecylamine. Another interesting parameter in the synthesis of mesoporous silicas is the surfactant/silica (Surf/SiO2) ratio. Beck et al. [3] have reported that hexagonal phases could only be prepared for Surf/SiO 2 < 1.1. For higher ratios, a cubic phase MCM-48 or unstable lamellar materials were obtained. We have synthesized a series of Ti-MS with Si/Ti = 100 in the gel and varying the Surf/SiO 2 ratio from 0.13 to 1.1. Whilst the surface area was high for relatively low ratios (< 0.5), it decreased to 350 m2/g for Surf/SiO 2 = 1.1 (Table 3). X-Ray diffraction showed that the cubic phase similar to MCM-48 was never formed, at least at a significant level. In contrast to the surface area and porous volume, the pore diameter increased continuously with the amount of amine introduced in the gel. These experiments clearly demonstrated that the best materials were obtained for Surf/SiO 2 ratios close to 0.3. Table 3 Characterization of Ti-MS synthesized with hexadecylamine and various Surf/SiO 2 ratios Surf/SiO 2
Si/Ti
S(m2/g)
~p(*.)
V(cm3/g)
I).13
81
968
34
0.59
I).27
85
1045
34
0.70
I).54
76
887
35
0.39
[).81
90
526
36.5
0.21
1.1
41
354
37.5
0.14
For the definition of ~p and V, see Table 1
3.2. Catalytic experiments Because of the high dispersion of Ti and V cations in mesoporous silicas, these materials are potentially interesting catalysts for the oxidation of large substrates with alkyl peroxides in the liquid phase. Ti-MS have been used as catalysts in the epoxidation of cyclohexene with both aqueous H202 and tert-butyl hydroperoxide (TBHP). The performances are compared with those of TS-1, that is known to be inactive in these
163 reactions because of restriction limitations, and Ti-Beta prepared following the recipe given by Camblor et al. [9]. Results are summarized in Table 3. With hydrogen peroxide, the activity of Ti-MS is very high as compared to those obtained over TS-1 and Ti-Beta. With TBHP, activities over Ti-MS and Ti-Beta are similar but the nature of the products formed differ considerably" because of the presence of aluminium in Ti-Beta, the diol derivatives is the major product formed whilst a selectivity in epoxide close to 9(1% is reached over Ti-HMS. This makes these materials particularly interesting for the selective epoxidation reactions with organic peroxides in the liquid phase. Table 3 Oxidation of cyclohexene over various Ti-containing catalysts Catalyst
Oxidant
COx(%)
SEp(%)
SD(%)
TS-1
H20 2
16
97
3
Ti-Beta
H20 2
77
2
98
Ti-MS
H20 2
95
79
21
Ti-Beta
TBHP
92
90
10
Ti-MS
TBHP
97
88
12
COx is the conversion in oxidant, SEp the selectivity in epoxide and SD that in diol. Reaction conditions "0.1 mole cyclohexene, 20 ml ethanol, H202/cyclohexene = (}.2, T = 7{1~C. Data obtained after l h30 of reaction. The same catalysts have been used for the oxidation of substituted anilines. Over all Ti-containing catalysts, aniline was converted selectively into azoxybenzene with a selectivity close to 95 %. The activity over Ti-MS was comparable to that over Ti-Beta, and both were very high as compared to that obtained over TS-1. Over V-MS materials, the only product formed was nitrobenzene, but the catalysts were only active with TBHP and conversions relatively low. Interesting differences between Ti-MS and Ti-Beta were observed during the oxidation of 3-chloroaniline.
CI
,.,,,vmw2r'2~
CI
0
CI
164 The selectivity in 3-3' dichloroazoxybenzene was only 6() % over Ti-Beta after 2 h 30 of reaction whereas it was already 95 % over Ti-MS after 15 min. In both cases, all the H 2 0 2 introduced at the beginning of the reaction had been consummed. This example clearly evidenced the advantage of mesoporous systems with respect to zeolites in the liquid phase oxidation of organic molecules. 4. R E F E R E N C E S
1
G. Perego, G. Bellussi, C. Corno, M. Taramasso, F. Buonomo and A. Esposito, Stud. Surf. Sci. Catal., 28 (1986) 129.
2 3
B. Notari, Catal. Today, 18 (1993) 163. 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.
4
S. Inagaki, Y. Fukushima and K. Kuroda, J. Chem. Soc., Chem. Commun. (1993) 680.
5
A. Corma, M.T. Navarro and J. Perez-Parient6, J. Chem. Soc., Chem. Commun. (1994) 147.
6
O. Franke, J. Rathousky, G. Schulz-Ekloff, J. Starek and A. Zukal, Stud. Surf. Sci. Catal., 84 (1994) 77.
7
K.M. Reddy, I. Moudrakovski and A. Sayari, J. Chem. Soc., Chem. Commun.
8 9
(1994) 1059. P.T. Tanev and T.J. Pinnavaia, Science, 267 (1995) 865. M.A. Camblor, A. Corma and J. Perez-Parient6, Zeolites, 13 (1993) 82.
10
M. Taramasso, G. Perego and B. Notari, US Pat, 4 410 5[)1 (1983).
11
C-Y. Chen, H-X. Li arid M.E. Davis, Micorporous Mater., 2 (1993) 17.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 1995 Elsevier Science B.V.
165
Bimodal porous materials with superior adsorption properties C. J. G u o
National Centre for Upgrading Technology Western Research Centre, CANMET, 1 Oil Patch Dr., Devon, Alberta, T0C 1E0, Canada ABSTRACT Porous materials with bimodal (micro and meso) pore size distribution have been prepared at ambient conditions. The materials contain randomly distributed mesoporous channels with uniform pore size and exhibit very large surface areas and pore volumes. The adsorption properties for water and some large organic molecules of the materials were determined. The water sorption capacities of the materials are about 300% higher than those of conventional zeolite molecular sieves such as zeolite 13X. The materials also have a great capacity to adsorb large molecules (e.g., > 60 wt. % tetralin) that zeolites exclude. The open structure and large pore size also result in high rates of adsorption and desorption. The superior adsorption properties of these new materials indicate their great potential for applications in adsorption and catalysis. Large-scale production of the mesosieve materials is expected to be straightforward and economical. INTRODUCTION There is a large and growing demand for adsorbent materials for use in various industries. The major commercial adsorbent materials on the market today are aluminas, molecular sieve zeolites (A, X, and mordenite), silica gels, carbons, and organic polymers. Compared with the other adsorbents, zeolite molecular sieves have significantly higher sorption capacities at low partial pressures. The maximum sorptive capacity of zeolite 13X which has the highest capacity among zeolites, is 25 to 36% [1]. A key determinant of the adsorption properties of a porous solid is its microstructure. A porous solid with high adsorption strength at low partial pressure, high capacity at high partial pressure and with rapid adsorption and desorption rates would be an ideal material for adsorption applications. CANMET has recently developed a novel method to make porous material with just these features [2]. The water sorption capacity of this new material is about 120 wt. %, which is 200 to 300 % higher than most of the commercial adsorbents currently on the market. The preparation and projected large scale manufacturing procedures are simple since only ambient
166 conditions are involved in the original preparations. In this paper, the adsorptive properties of the uniquely structured materials and some characterizations of their micro structures are presented. MATERIAL CHARACTERIZATION Researchers in Mobil R&D corporation had developed crystalline mesoporous molecular sieves designated as M41S using high temperature (100 to 200 ~ hydrothermal conditions (4,5). The present materials were all prepared using ambient conditions (0 to 25 ~ and atmospheric pressure) and short times (5 minutes to 4 hours) followed by a thermal treatment procedure. Silicate and aluminate sources common in zeolite synthesis and commercial surfactants such as Ammonyx KP (oleyl dimethyl benzyl ammonium chloride, Stepan Co., IN) were used in the preparations. The details of the preparation procedure and compositions are not described here. The major features of these materials are high porosity, low particle density, high surface area, and high pore volume. The surface area (S) and pore volume (V) of the materials, as well as the relative amounts of micropores and mesopores can be varied as required (from 100 to 1200 m2/g for S, and from 0.1 to 1.3 cm3/g for V) by controlling the original mixture composition. Commercial manufacturing of the material in a continuous mode is easily achievable because of the simple procedures required: ambient conditions, a short amount of time, and a final thermal treatment in a regular furnace. Figures 1 shows a SEM (Scanning Electron Microscopy) micrograph of a typical
Figure 1. A SEM micrograph of the mesoporous materials.
Figure 2. A TEM mesoporous particle.
image
of
167
sample of the new materials. It is clear that the material is composed of small particles with diameter of less than 0.5 micron. These small particles themselves are highly porous as indicated by their high surface area. Figure 2 is a TEM (Transmission Electron Microscopy) image of the same sample. It is evident that different from most zeolites and crystalline mesoporous sieves such as MCM-41 which has a well organized hexagonal pore structure, the present materials contain randomly distributed channels with uniform mesopore size (about 3 nm for this particular sample). The micropores are not visible on the TEM image but are believed to be in the walls of the mesopores. As can be seen from the pore size distribution from nitrogen adsorption in Figure 4, in the mesoporous range, most of the pore volume is contributed by pores with a diameter of 3 nm. Comparing the results of figures 2 and 4, there is a good agreement on the channel size of the material between the two independent characterization methods. This agreement implies that the new materials, which are not really crystalline, have randomly distributed channels with a uniform mesopore diameter of 3 nm. The name mesosieve is used for these new materials in the rest of the paper. In addition to the regular mesopore size, the materials also have very large surface area and large pore volume. Table 1 lists some typical physical properties of the materials compared with zeolite 13X. The framework density was measured using helium and the particle density (or mercury density) is calculated from the framework density and N 2 saturation capacity. The materials are stable under severe acidic and thermal conditions; there was no pore structure change and only a 3 % surface area decrease was observed after a sample was placed in 70% nitric acid for five days and then calcined at 510 ~ Calcination at 800 ~ for two hours would not significantly change the pore size distribution pattern although a less than 20% decrease in surface area was observed. Table 1. Some physical properties of a mesosieve and zeolite 13 X Sample ID
Skeletal density (g/cm3)
Particle density (g/cm3)
Surface area (m2/g)
Total pore volume (cm3/g)
Mesosieve 11195A
2.39
0.63
1075
1.36
Zeolite 13X
2.03
1.31
790
0.32
ADSORPTION P R O P E R T I E S OF THE MATERIALS Figure 3 compares an N2 adsorption isotherm of one sample of the new materials (Figure 3a) with that of zeolite 13 X (Figure 3b). As is evident from the figure, the new mesosieve exhibited a type IV isotherm and 13X a type I isotherm. At a relative pressure of 0.001, 85 % of 13X total capacity is occupied because of the strong adsorption potential in
168
1
i
i
i
l
i
+
I
T
................ i .......I........I....... I........t.......I........t.......T
!" - I :
'dt
~ 'i
........ ,........ ! ~l....... .+........ , , , + / ~ ~ ~ ,
+
!........ l..-,..+,.
++B} + ............... i +....... i ........ +....... +........ +....... i ........ l
....... !........ t ....... j .... //,,.. ..... l ........ l ....... +........ +....... +........ i ..............
I I ........ +!...... ~ ,
! ,P" i
i
~
l ~ ~
i
i
i
l
i
~
+....... Ii...,
'~"i
. . . . . . . .
....... +........ i....... + v o = ] _ p
'
'
;
"
~ S
I
I
i
i
l d,, = 28.
%
o
./-----i ........ i ....... i ........ i ....... i-~
:
,
%
, o.,
,
,
=
,
o
%
\~2 ~
,
, , ,
%
%
.
:.
I. . . . . . .
t+ .t....t .
--
:j!io,-.o,,~
I I"]:
%
"o
~v
,.,=--,--!
-.,
o
%
l
l
i
Zeolite 13X:i .... , ....
.......
+Vp
=
0.32
l
..+....... +s=798
r=o,,./
+'-'L--'-~--,., -o
+
l
i
L , o L I ! L ! ! ! ! %
l
'[ .......]
,
.... l---t.J...
.
-+ ....... +........ +....... i ........ +....
!
.... il
1235
=
,,
. ;7 ...,.. 7"i 711 IT 71i+.~ . ; . '
!
.....
".
.......................................
:
i
-,....
l ....... ] ........ +....... +........ +....... +........ l:~,..~i
........ ~
~I
.
,~.......j_..
%
+....
i
i do = 5 -
+
+
+
+
%
%
%
%
7{
+ !
Relative pressure F i g u r e 3. N i t r o g e n sample
13X
sorption
isotherms
of a mesosieve
sample
(3a) a n d a z e o l i t e
(3b).
the zeolite micropores. The sudden jump in adsorption capacity at middle relative pressure on the isotherm of the mesosieve is a result of capillary condensation in the mesopores. The high "knee" in Figure 3a also indicates the presence of a significant micropore volume in these materials. It can be seen that the first "knee" sorption capacity of the mesosieve is already as high as the saturation capacity of zeolite 13X. The total sorption capacity of the new material is actually about four times that of 13X as can be seen from Figure 3. Such large differences in pore volume will be reflected in adsorption capacities.
......
..J ....... -~- ...... i ........ i ......... . ................ ] ................ + ................
i ................. o.~
i
i
i
i
i
i
~ . . . . . . . ~. . . . . . . . ~. . . . . . . . i . . . . . . . . i . . . . . . . . ".-. . . . . . . .'.L. . . . . . . . . . . . . . . .
.............
~. . . . . . . .
i
i
{
{
i
i
i
t
1. . . . . . . .
i ........
; ........
i ........
~.......
~. . . . . . . .
! ........
l
;H-K~
i
i
i
!
i
i
i
i
i
i
~,:,::, i iiii!i 0:m II ', ',',:', "~ 0 . . ; ~ '., ',;:; . . . . . . . . . . . II
~,,:,,::! Ii li
/ .~
,~i
0.~. ....... ~ .I--~--,
. . . . . . . ~. . . . . . . . i . . . . . . . . i . . . . . . . . i . . . . . . . . .; . . . . . . . -!.. . . . . . . . .;. . . . . . . .
To
9
;
i
..,1 ....... : ....... ~ o.= ..................
i
i
+
+
+
i
i
i
i
U
' ........
i ........
9.
.
.
i .
I , I i i I
i I iii i|lll
. . . . . . . . . .. .. . . . .
i I , I,
i
i
.
.
.
.
.
0:04
.
It
IIIII
II
I IIII I IIII
i I illl i I i ill i i i i1|
I all I
i
i
i
Figure
4. The
pore
models
size d i s t r i b u t i o n (HK and DFT).
I
I
I i IIII i i illl
i
I I ]J
I IIIII I i i iii i i illi
+ i i
* I i I i I
i
i
I I I
D F T ....... .......
I!'!1
. . . . . . . . . . . . . . . .
I
Iiiii
I I IIII
'
P"I '''"' I I IIII I i illl i ! {ill
I
I
I
'
' ' '''"'
I I !
I I !
I I !
I
I IIIII I I llil I I ~ III
II. ,I,,,,,, ,,,,,,
,
, , ,,,,,,
I i i
I i i
I IIIIIII I I IIII , iiiiiiii ,I i i i i i |111111 ii i i i i i
', ]
'~" I
~..I
I00
I i i i
,,.i IIIII 131111
IOO0
Pore diameter(A) of the
mesosieve
as d e s c r i b e d
I
"!
I IIIIII i i i i i ii i iiiiii
I IIIIl~llllhllllllllll I~11+~ I ~ 1 I I I I IIIII I I III IIIIIIII111111111 I I I I I 1 ~ 1 ~ ~ 1 II i ii iiiii!111 IIIIIIllllllllLqll I II IIIIIIIIrll II I III Iil IH II
IO
',
IIIII
III
Pore diameter(A)
adsorption
| ! ~ I
II IIIII
9
i
, .
,i,,,i
II I
i
| iI||l I i IIII
I I l IIII
o.~ , ..... i,,~-O',',', .... ~ ...............
o.~
I I
.
I IIIIII - I - . - - . 4 - - ~ l l Il ~ I I I I ~ 1111.1.
, ,,, ,, III
0.00
| I
, I I
i llll
,,
~ ;,
...................
..i ....... i ....... J........
.... ~........ +........ +........ ~........ " ....... + ....... , ........
"........
i i iii
,,
',',',:',:,, ........... ... . . . . . . . .
by two
169 In Figure 4, the pore size distribution of a typical mesosieve is described using two adsorption models, the Horvath-Kawazoe (HK) model [6] and the density function theory (DFT) [7], using nitrogen adsorption data. The DFT claims applicability to both microporous and mesoporous ranges although its accuracy in modelling nitrogen adsorption in micropores of diameter smaller than 1 nm is questionable. It is clear that the new materials have a bimodal pore distribution, with a significant presence of both micropores and mesopores. The micropores are in the range of 0.6 to 1.0 nm whereas the mesopores are all larger than 2.5 nm. Such bimodal pore size distributions are a very positive feature for the materials used as adsorbent since the micropores provide a strong adsorption potential at low partial pressures, whereas the mesopores assure very large sorption capacity at higher relative pressures. To demonstrate this effect, adsorption experiments were conducted to measure both the total capacity and adsorption rate at low partial pressures and short times. Figure 5 compares the saturation capacities for water, cyclohexane, and tetralin of several mesosieve samples with those of zeolites 5A, ZSM5, and 13X. The sorption capacities were determined by exposing a dry sample to the saturated vapor of an adsorbate at 23 ~ and weighing its equilibrium mass at the same temperature. Compared with 13X particularly, the mesosieves showed more than 300% higher water sorption capacity, more than 400% higher cyclohexane sorption capacity, and close to 300% higher tetralin sorption capacity. Compared with zeolite 5A, the mesosieve water capacity is 350% higher, cyclohexane capacity 1000% times higher, and tetralin capacity 3400% higher. Such extraordinarily high sorption capacities are a result of the large mesopores and high pore volumes of the mesosieves. Another possible advantage of the micro-mesopore combination is that a molecular sieving effect on molecules of size 0.6 to 1.0 nm can be provided by the micropores whereas the mesopores 140 assure a small diffusion resistance and a large volume for molecular transport. 120 El Water I [ ] Cyclohexane
In addition to the saturation capacity, the sorption rate of a particular adsorbate in the mesosieves is significantly higher than that in zeolite adsorbents. In Figure 6, two adsorption uptake curves from the adsorption of water in a mesosieve and in zeolite 5A, both in 1/32" extrudate form, are compared. This experiment included placing a dry sample in water-saturated air at room temperature and monitoring the sample mass change with time. The mesosieve shows faster water uptake rate and higher capacity. In Figure 7,
"~100
"o
[] Tetralin
i!~d ill.'
~o 80
~U,i
-o 60 C
=
~
40
o
/ 5A
ZSM-5
13X
MS-A
MS-B
~Hm
MS-C
Sample ID
Figure 5. Comparison of adsorption saturation capacities for three vapors of three mesosieve samples (MS) with that of zeolites 5A, ZSM5 and 13X.
170 the desorption curves of water desorbing from a mesosieve sample and a 13X sample under the same conditions are illustrated. The faster desorption in the mesosieve is evident: at the end of two-hour desorption, more than 60% of the total amount adsorbed is desorbed from the mesosieve, whereas only 26% is desorbed from 13X; this is on the top of a 300% higher total water loading. The desorption experiment was conducted by exposing a water-saturated sample to air and monitoring its mass change at room temperature. Obviously, the open pore structure and the presence of mesopores resulted in the high capacities and faster diffusional mass transfer rates. 35 f
70
A 30
~2s 'U o
"~ 40 m " 30 ,,,3 9 ,,9 "-o 20
t~ 2 o 0 Ir
9a 15
m
e~ 0=
10 5
I 013195A --4113Xzeolite
'ID , 960 .e oM ,, 50 "o
~"
MS(run 2) 11 10
20 30 Time ( m i n . )
40
50
Figure 6. Water adsorption c u r v e s o f a m e s o s i e v e and a zeolite 5 A s a m p l e .
0
20
40
60
80 100 120 140
T i m e (rain.)
Figure 7. W a t e r desorption c u r v e s of a m e s o s i e v e and a zeolite 1 3 X sample.
CONCLUSIONS Highly porous materials with bimodal (microporous and mesoporous) pore size distribution were prepared at ambient conditions and using short preparation times. The materials contain randomly distributed channels with uniform mesopore size. Large-scale production of the materials is expected to be easy and economical. The important features of the new materials are: 1. Unique pore structure, possibly micropores connected to uniform mesopores. 2. High surface area (up to 1200 m2/g and can be varied). 3. High pore volume (up to 1.3 cm3/g and can be varied). 4. Small particle size and easily formed into extrudates or tablets. 5. High stability under severe thermal and acidic conditions. 6. Low apparent density (0.6 to 0.7 g/cm3).
171 7. Noncrystalline. The new mesoporous materials exhibit extraordinarily high adsorption capacities for both water and organic compounds. Their water sorption capacities are 300 % higher than zeolite 13X and they adsorb large molecules like tetralin (which conventional zeolites exclude) in large quantity ( > 60 w. %). The materials also show greater rates of adsorption and desorption than zeolites. The unique pore structure of the materials indicate great potential for applications in adsorption and catalysis, particularly for applications that require 1) large pores and high surface area and 2) both micropores and mesopores, small mass transfer resistance and large pore volumes. REFERENCES
.
.
.
.
7.
R. H. Perry and D. H. Green (eds.), Perry's Chemical Engineer's Handbook, McGraw-Hill, Inc., 16-9, 1984. C. J. Guo and C. Fairbridge, CANMET division report WRC 93-48 (CF), November, 1993. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. J. S. Beck, C. T-W. Chu, I. D. Johnson, C. T. Kresge, M. E. Lex~nowicz, W. J. Roth and J. C. Vartuli, US Patent 5108725, April, 1992. G. Horvath and K. Kawozoe, J. Chem. Eng. Jpn., 16 (1983) 470. J. P. Olivier and W. B. Conklin, "The international Symposium on the effects of surface heterogeneity in adsorption and catalysis on solids", Kazimierz Dolny, Poland, July, 1992.
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Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 1995 Elsevier Science B.V.
173
M C M - 4 1 t y p e silicas as s u p p o r t s for i m m o b i l i z e d c a t a l y s t s Daniel Brunel*, Anne Cauvel, Francois Fajula and Francesco DiRenzo Laboratoire de Mat~riaux Catalytiques et Catalyse en Chimie Organique, CNRS-URA 418, Ecole Nationale Sup6rieure de Chimie, 8, rue de l'Ecole Normale, F-34053 - Montpellier C6dex 1 - Fax: + 33 - 67 14 43 49 Abstract MCM-41 type silicas were covalently grafted with various functional alkoxysilanes (RO)3Si(CH2)3 X with X = C1, NH(CH2)2NH2 and NHC(O)NSalpr. The functions of the first two organic moieties already attached were further transformed into respectively 2-(NHCH2)Pyr, 4-(NH(CH2)2NHCO)Pyr also known as organic ligands of transition metals. The grafted mesoporous silicas were c h a r a c t e r i z e d by IR, MAS-NMR s p e c t r o s c o p i e s , thermogravimetry, nitrogen adsorption and elemental analysis . The coupling reactions of organic ligands do not modify the covalent grafting bonds. Pore opening shrinks at increasing organic coverage. The diameter of the channels accessible to nitrogen varies from 33 to 13 /~. A total surface lining by concentrically oriented organic chains is suggested by the decrease in the nitrogen adsorption enthalpy as a function of the organic coverage. 1. INTRODUCTION MCM-41 molecular sieves are a new class of mesoporous aluminosilicates featuring cylindrical regular mesopores of monodispersed diameter w i t h potential applications in catalysis and adsorption [1,2]. They are obtained by precipitation of amorphous silica-alumina in the presence of cationic surfactants. The pores of the MCM-41 type materials are templated by the surfactant micelles. While the hydrocarbon chain length of the surfactant rules the pore diameter, the conditions of the hydrothermal synthesis, in particular the alkalinity of the parent hydrogel, influence the wall thickness [3]. An increase in the pore spacing leads to an improved thermal stability of MCM-41-type materials. Pure silica MCM-41s can also be prepared and they feature a better stability than their silica-alumina analogs. MCM-41-type silicas were obtained with a pore diameter of 33/~ and wall thickness higher than 10/~. MCM-41 type silicas are an ideal raw material for the design of new mesoporous solids having adjustable chemical and physicochemical properties [4] in view of applications in the field of adsorption and fine chemical catalysis. This objective can be reached by lining of the mesopore surface with covalently attached transition metal liganding moieties. The volume available inside the MCM-41 mesopores is much larger than in the channel volume of the usual
174 zeolite catalysts. The insertion and grafting of bulky, sterically-hindered functional molecules is hence possible, without a decrease of the accessibility of the catalytic groups. As the mesoporous surface of the MCM-41 type silicas presents the same surface silanol groups than the traditional amorphous silicas, it can be modified by applying usual methods of silica functionalization. They deal with the covalent linkage of organic groups to the silica surface by condensation of organotrialkoxysilanes with the silanol groups [5-22]. We report here the anchoring of organic groups to MCM-41 type silicas using functional propyltrialkoxysilanes. The organic moieties were f u r t h e r transformed by coupling reactions with suitable organic ligands of transition metals. Our goal in preparing these materials was to obtain mesoporous solidbound ligands that yield systems with potential for catalytic oxidation of organic substrates. As most of the monodentate ligands used to immobilize transition-metal catalysts suffer from the disadvantage of the metal complex leaching into the solution, there is considerable interest in covalently attaching multidentate chelating agent to a support surface. For this purpose, we selected the following l i g a n d s : (pyridino-2-methyl)amine, (2isonicotinamidoethyl)amine and pentadentate Schiff base as SalDPT [23,24]. In the case of the two first ligands, mild coupling organic reactions were carried out to perform the coupling of the liganding moieties while avoiding alteration of the covalent grafting bonds. The different modifications were controlled at each step using various characterization techniques. This study allowed the determination of the new adsorption properties of the modified mesopore surfaces. 2. E X P ~ A L 2.1. MCM-41 silica.
Pure MCM-41 mesoporous silica (MPS) was prepared in the presence of cetyltrimethylammonium hydroxyde according to the method described in the literature [1,2]. The MPS material consists in SiO2 containing 0.003 A1/(AI+Si) and 0.0014 Na/(AI+Si). X-ray powder diffraction pattern matches well with the patterns reported by J.S. Beck et al [2], with d loo = 40.0/~. The hexagonal lattice parameter is equal to 46.2 /~. The nitrogen sorption isotherm of MPS is a classical example of type IV isotherm [24], showing monolayer and multilayer adsorption on a mesoporous surface of very high area and a sharp, reversible step at P/P0 0.38, characteristic of capillary condensation within the mesopores (vide infra). No hysteresis loop is observed due to the low filling pressure of the small mesopores.
2.2. Functionalization procedure 3-chloropropylsilylated-MPS (C1-MPS), 3-(2-aminoethyl)aminopropylsilylated -MPS (NH2EtN-MPS) and 3-(bis-[3-(salicyliden-amino) propyl])carbamatopropyl silylated-MPS (SalDPT-amido-MPS). A suspension of freshly activated MCM-41 silica in toluene was refluxed and stirred for 1.5 hr. with the corresponding functional organoalkoxysilane under dry nitrogen atmosphere. After distillation in a Dean-Stark collector of a fraction of toluene containing volatile compounds, the mixture was again heated at toluene refluxing temperature for 1.5 hr. The distillation and heating sequences were repeated. After cooling, the solid was extracted in a soxhlet
175 apparatus overnight with ethyl ether and dichloromethane, then evacuated under vacuum at 200~ for 6 hr. Elemental analy~i~: C1-MPS: C 6.34%, C1 3,29%; NH2EtN-MPS" C 15,23%, N: 6.06%; SalDPT-amido-MPS: C 26.36%, N 5.17%.
2.3. Modification of the functional group 2.3.1.3-(2-pyridinomethyl)aminopropylsilylated-MPS (Pyr MeN-MPS) A suspension of activated C1-MPS material in toluene was refluxed and stirred in an excess of 2-(aminoethyl)pyridine for 6 hr. After separation, the modified solid was extracted according to the previous procedure, then dried under vacuum at 180~ overnight. Elemental analysis: C 14.62%, N 3.89% 2.3.2. 3-(2-isonicotinamidoethyl)aminopropylsilylated-MPS (IsoNicEtN-MPS) Activated NH2EtNH-MPS in suspension in dichloromethane was refluxed and stirred with an excess of a mixture (1/2) of isonicotinic acid and N,N'dicyclohexyl carbodiimine for 6 hr. The modified solid was then separated, washed in succession with water, ethanol, then treated according to the previous procedure. Elemental anolysi~: C 23.05%, N 6.98%. 3.3. Characterization Analyses of the modified solids were made using 13C MAS-NMR, Infrared and UV Spectroscopies, Thermogravimetry, Nitrogen Adsorption and Desorption Isotherm and Elemental Analyses. RESULTS AND DISCUSSION The grafting reactions on the MPS support were studied first by infrared and 13C MAS-NMR spectroscopies. Infrared spectra of the modified mesoporous silicas show a band at 2940 cm -1 characteristic of-CH2- stretching vibration whereas the band at 2970 cm-1 associated with the CH3 of the ethoxy group has a much lower intensity than in the grafting agent spectrum. Moreover the grafted MPS spectra exhibit lower silanol band intensity at 3741 cm -1 than the parent MPS. The release of ethanol, detected and characterized by GCMS during the process confirms the formation of the Si-O-Si linkage according to the following reaction scheme: ~-OH
,,, (EtO)3Si(CH2)3X
~
NN[_O\ S i / ~ / ~ X]---O/
X
+ 2 EtOH
X= CI, NH(CH2)2NH 2, NHC(O)SalDPT 13C MAS-NMR spectroscopy allows the determination of the structure of the grafted hydrocarbon chains. The assignments of the 13C NMR signals of the modified mesoporous silicas are reported in Table 1.
176 Table 1 13C CP-MAS NMR spectral features of the grafted mesoporous silicas Grafted species
Ca
C~
Cy
N(CH2)nN Aromatic C
-=SiCH2aCH2~CH2Y-CI
8.9
26
46
=_Si(CH2)3-NH(CH2)2-NH2
9.5 22.5
38
51
-Si(CH2)3-NHC(O)-SalDPT
9.5
22
40
45
118 131
-_-Si(CH2)3-NHCH2Pyr
10
19
38.5
52
125 138 149
=_Si(CH2)3-NH(CH2)2-NHC(O)Pyr 10
22
40
50
124
ppm
145
CO,CN
161
158
163
165
Thus, the eventual modification of the structure of the function can be monitored by the change in the 13C CP-MAS-NMR spectra resulting from the coupling reactions. This is examplified on Figure 1 showing the intensity of the signal assigned to the CH2-C1 group on the C1-MPS spectrum (Fig.la) at 47.2 ppm which is very low on the spectrum of the resulting Pyr MeNH-MPS (Fig. lb). Moreover this later spectrum exhibits other signals characteristic of the 2-pyridinomethylamino group. The disappearance of the residual ethyl arms during the coupling t r e a t m e n t probably results from nucleophilic assistance of siloxane coupling between adjacent chains.
=
S i ( O E t )
( C H 2 )
••-•• 3-CI
~.\ \ i ~ ~
-(CH1)3 - N I I - C I I .
1~o
.
_
,
.
.
1
-
,
6
--~
-
.
|
-
14o
.
.
,
- . -
,oo
,
-
.-
,
60
-
. - !
.
.
.
.
.
,
iO
PPM
Figure 1. 13C CP MAS-NMR of a)C1-MPS b) PyrMeN-MPS
PPM
Figure 2. a)13C CP MAS-NMR of SalDPT-amido-MPS. b) J-Mod. 13C NMR of SalDPT in CDC13 solution
On the other hand, Figure 2 illustrates the identification of the anchored
177 26,0 ~ SalDPT group attached with c a r b a m a t o p r o p y l silane chain (Fig 2a) by comparison with the s p e c t r u m of SalD PT zo.( material in CDC13 solution (Fig 2b). The diffuse r e f l e c t a n c e s p e c t r u m of SalD PT-amido-MPS presented on figure 3 is consistent with the s t r u c t u r e of the salen group linked by a carbamate function (~,= 254 14.( and 317 nm). The b a n d at ~,= 400 n m would K-M be c o n s i s t e n t w i t h excitonic t r a n s i t i o n r e s u l t i n g from a t i g h t ordered packing of 8. salen molecule. The i n f r a r e d s p e c t r u m of this solid is also in a good a g r e e m e n t with the proposed structure. This latter technique as well as 13C MAS-NMR spectroscopy also z. 200 300 ~0 500 indicate t h a t the coupling of organic ligands zi nm of t r a n s i t i o n m e t a l to the grafted molecules Figure 3. Diffuse reflectance spectrum does not a l t e r the covalent siloxane bonds of SalDPT-amido - MPS. between silica and organic moieties. Table 2 reports the organic content and coverage of the grafted mesoporous solids deduced from the elemental analysis and thermogravimetry.
Table 2. Organic coverage and content of modified MPS.
Materials Cl-MPS NH2EtNH-MPS PyrMN-MPS
Molar ratio C/Cl 6.1
chain nm 2. 0.6
10.3
-(ctl2) 3 -Nll-(Cll2)2-Ntt 2
C/N 2.7
1.3
23.5
"(C!]2)3 "NI]'CIt2 --~(,_))
C/N 4.4
0.9
26.7
C/N 3.8
1.0
33.5
C/N 5.9
0.6
40.7
Organic chain -(oil2)3 -ct
{or~) k SiO2
w
-(Cll2) 3 -NII-(CII2)2-Nlt
IsoNicEtN-MPS
SalDPT-amidoMPS
~,-~
oII -((;lt2)3 -Nn-C -N
llO
* surface area of the mineral support The organic coverage of the modified m a t e r i a l was c a l c u l a t e d from elemental analysis d a t a t a k i n g into account mass % chlorine for chloroalkyl
178 chain and mass % nitrogen for amino or amidoalkyl chain containing or not pyridino group. The organic content is obtained from thermogravimetric data as the mass loss at T > 200~ It is noticeable that the organic coverage is lower for C1-MPS than NH2EtNH-MPS. Probably the amino groups enhance the substitution reaction at the Si atom [25]. The low organic coverage of C1-MPS corresponds to a C/C1 ratio higher than the stoichometric value. It is likely that the C/C1 ratio is increased by the presence of EtO- groups linked to the Si atoms belonging to the grafted chain or to the surface. This hypothesis is consistent with the i.r. and 13C NMR results. It is noteworthy that C/N ratio confirms a complete coupling reaction whatever the previously grafted organic function. The nitrogen sorption isotherms of the functionalized MPS give informations on their texture and surface state. Figure 4 shows the isotherms of some materials grafted with chains of different lengths. Data derived from the sorption isotherms of all samples are reported in Table 3.
jf
8O0
n_
~=m600
j
v
w= 4 0 0 o
/
~
c 200 _1 o
.,,,..,.,.,,..~l~ ~ .
. -
450
400
300
l
200
100
0
,,~
o
~,
'
a.'2
]',, RS~..ATIV[
'
9
o'.6 PRESSURE
o'.~ ,
'
'
~
o
o
(J=/;=o)
.
0.2
.
.
o
u
Ri[LPTIV~
.
o
6
PRESSURE
o.~. ,
(P/Pro)
Fig.4. Nitrogen sorption isotherms of a) parent MPS b) (1) CI-, (2) PyrMN-, (3) SalDPTamido-, (4) IsoNicEtN-MPS. Table 3. Textural and thermodynamic parameters deduced from nitrogen sorption isotherms mesoporous mesoporous diameter BET Materials surface volume (cc/g) (A) parameter C (m2/g) MPS
920
0.76
33
100
C1-MPS
851
0.57
27
60
PyrMN-MPS
577
0.35
24
50
NH2EtNH-MPS
586
0.29
20
22
SalDPT-amido-MPS
608
0.31
20
19
isoNicEtN-MPS
436
0.22
13
28
179 The surface area and mesopore volume decrease with increasing length of the grafted chain, but the characteristic features of the MPS isotherms are essentially preserved. All isotherms are of type IV, indicating the preservation of the mesoporous system during the grafting reaction or the modification of the organic chain. However, the characteristic step of the sorption isotherm, corresponding to the Kelvin filling of the mesoporosity, becomes less sharp at increasing organic content, suggesting that the pore size distribution is widening. The pressure at which the step of the isotherm occurs decreases with increasing length of the organic molecules. The diameter of the organiclined mesopores can be evaluated by the ratio dmeso = 4V / S between mesopore volume and mesoporous surface area. The average pore diameter are reported in table 3. The pore opening shrinkage fits fairly well with the increase in the organic chain lengths. This trend is consistent with the variation of surface area and mesoporous volume. The application of the BET equation to the nitrogen sorption isotherms provides some useful information on the energy of the interaction between a probe molecule -nitrogen- and the surface. The BET parameter C is roughly connected to the nitrogen adsorption heat according to the following equation: C= exp[ (Eads- E1 ) /RT] where E1 is equal to liquefaction heat of nitrogen. Values of the parameter C for all samples are reported in Table 3. All organic-lined MPS feature much smaller values of the parameters C than the parent silica. The adsorption heat of nitrogen is indeed lower on organic surfaces than on hydroxyl-rich surfaces. As a consequence, the low C values for modified MPS can be considered as an indication of regular surface coverage by the organic groups. 4. CONCLUSION The MCM-41 type silicas provide suitable supports for anchoring organic moieties to the inner surface of a mesoporous system. The organic lined mesopores of regular, well-controlled diameter can be obtained by standard functionalization methods. The grafted function can be further transformed into well-defined transition metal liganding moities without loss of either the organic chain content or the regular mesoporous structure. Nitrogen sorption isotherms are useful tools to characterize the properties of such potentially hydrophobic materials. The strong decrease of the nitrogen adsorption heat with the organic coverage is a significant test of the modification of the surface properties. The decrease of the value of the C parameter of the BET equation indicates t h a t the mineral surface is no longer accessible to adsorbed molecules. These results confirm the unique properties of the composite materials prepared by grafting organic molecules to the inner surface of MCM-41 type silicas. Potential applications would be relevant to both specific oxidative catalysis induced by the anchored transition metal complexes and the hydrophobic property peculiar of the lined organic moieties. AI~OWLEDG~ The authors are grateful to ELF and CNRS for financial support. They thank Annie Finiels for her help in taking part of the 13C NMR spectra in
180 CDC13 solution. Anne Cauvel is indebted to ADEME (Agence de 1' Environnement et de la Maitrise de r Energie) for a doctoral grant. R~'ERENCI~ 1 J.C. Beck, C.T.-W. Chu, I.D. Johnson, C.T. Kresge, M.E. Leonowicz, W.J. Roth and J.C. Vartuli, to Mobil Oil Corporation, WO91/11390 (1991). 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. McCuUen, J.B. Higgins and .L. Schlenker, J. Am. Chem. Soc., 114, 10834 (1992). 3 N. Coustel, F. Di Renzo and F. Fajula, J. Chem. Comm., Chem. Comm., 967 (1994). 4 A. Cauvel, D. Brunel, F. DiRenzo and F. Fajula, "Organic Coatings" 53rd International Meeting of Physical Chemistry, Paris, 2-6 jan. 1995. 5 W. Hertl, J. Phys. Chem., 72, 1248 (1968). 6 B. Arkles, Chemtech, 766 (1977). 7 D.R. Fruge, G.D. Fong and F.K. Fong, J. Am. Chem. Soc., i01, 3697 (1979). 8 L. Yu Fu, X. Yong-Xia, X. Don-Peng and L.J. Guang-Liang, J. Polym. Sci., 19, 3069 (1981). 9 D.W. Sindorf an G. Maciel, J. Am. Chem. Soc., 105, 3767 (1983). 10 E.J.R. SudhSlter, R. Huis, G.R. Hays and N.C.M. Alma, J. Coll. Interf. Sci., 103, 554 (1985). 11 R. Rosset, Bull. Soc. Chim. Fr., 1128 (1985). 12 W.H. Pirkle, T.C. Pochapsky, G.S. Mahler, D.E. Corey, D.S.Reno and D.M. Alessi, J. Org. Chem., 51, 4991 (1986). 13 J.W. De Haan, H.M. Van den Bogaert, J.J. Ponjed and L.J.M. Van de Ven, J. Coll. Interf. Sci., 110, 591 (1986). 14 U. Nagel and E. Kinsel, J. Chem. Soc. Chem. Comm., 1098 (1986). 15 K. Soai, M. Watanabe and A. Yamamoto, J. Org. Chem., 55. 4832 (1990). 16 E.I.S. Adreotti and Y. Gushikem, J. Coll. Interf. Sci., 142, 97 (1991). 17 H.U. Blaser, Tetrahedron Asymmetry, 2, 843 (1991). 18 P. Herman, C. del Pino and E. Ruitz-Hitsky, Chem. Mater., 4, 49 (1992). 19 B. Pugin and M. Miiller, in "Heterogeneous Catalysis and Fine Chemicals III", M.Guisnet et al. Eds., Elsevier Science Publishers, Stud. Surf. Sci. Catal., 78, 107 (1993). 20 Y.G. Akopyants, S.A. Borisenkova, O.L. Kalya, V.M. Derkacheva and E.A. Lukyanets, J. Mol. Catal., 83, 1 (1993). 21 M. McCann, E.M. Giolla and K. Maddock, J. Chem. Soc. Dalton Trans., 1489 (1994). 22 R.S. Drago, J. Gaul, A. Zombeck and D.K. Straub, J. Am. Chem. Soc., 102, 1033 (1980). 23 D.E. De Vos, F. Thibault-Starzyk and P.A. Jacobs, Angew. Chem. Int. Ed. Engl., 33, 431 (1994). 24 S. Brunauer, L.S. Deming, W.S. Deming and E. Teller, J. Am. Chem. Soc., 62, 1723 (1940). 25 A. Cauvel, D. Brunel, F. DiRenzo, P. Moreau and F. Fajula, in Proceeding of Zeocat'95, Stud. Surf. Sci. Catal. in press.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviotand S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
181
Synthesis of mesoporous manganosilicates Mn-MCM-41, Mn-MCM-48 and Mn-MCM-L at a low surfactant/Si ratio Dongyuan Zhao and Daniella Goldfarb Chemical Physics Department, Weizmann Institute of Science, Rehovot, Israel 76100
Mesoporous manganosilicate molecular sieve, Mn-M41 S, having hexagonal (Mn-MCM-41), cubic (Mn-MCM-48), lamellar (Mn-MCM-L) structure, were synthesized in low surfactant/Si ratio (0.12) and characterized by X-ray powder diffraction, transmission electron microscopy (TEM), themogravimetric analysis (TGA) and electron paramagnetic resonance (EPR). The phase transformations trends: hexagonal ~ lamellar ~ cubic -~ hexagonal or hexagonal hexagonal, lamellar mixture ~ cubic ~ lamellar were observed by variations of the base or acid content of the gel or reaction temperature respectively. The results show at low surfactant/Si ratio, Mn-MCM-41 can be synthesized both in acid and base medium, and in a wide range of temperature (21-100~ Mn-MCM-48 can be obtained only in basic medium and in high temperature (100-120~ lamellar phase is formed in high temperature (>130~ and caused the collapse of the structure after calcination at 540~ Addition of Mn ions induces the formation of cubic phase also at low surfactant/Si ratio (0.12). By variations of the base content of the gel, AI-MCM-48 also can be synthesized. EPR results suggest that Mn ions can be incorporated into the amorphous silica wall of M41S or in the interface region of the polar head groups of the template. After calcination, Mn ions in Mn-MCM-41 and Mn-MCM-48, have high mobility and are not located within the inorganic wall.
1. INTRODUCTION Recently a new family of mesoporous molecular sieves designated as M41S was synthesized in the laboratories of the Mobil Oil Company [ 1-3 ]. The M41S materials possess a regular array of uniform mesopores, which can be systematically varied in size from around 20 to 100 A [411 ]. These materials bridge the gap between microporous (zeolites) and macroporous materials (e.g. amorphous aluminosilicates). The members of the M41S family include MCM-41, having a hexagonal arrangement of pores, MCM-48, displaying a cubic structure and other members such as lamellar and cubic octamer, that are not as easily categorized [2-4,6]. The pore size of the M41S materials, their high surface areas (up to 1000 m2g-1), their distinct adsorption properties (pore condensation without hysteresis) [3b], and their thermal stability render them as prim candidates for industrial applications. Modifications in the composition of the silica based M41S materials were so far limited to the incorporation of aluminum, vanadium and titanium into MCM-41 [ 12,15,16]. Similar substitutions have not yet been reported for the silica base cubic phase, MCM-48, which have been
182 synthesized with a high surfactant/Si ratio (1-2) [2,4]. In this work we report on the synthesis of mesoporous manganosilicate materials Mn-M41 S, having hexagonal (Mn-MCM-41), cubic (Mn-MCM-48) and lamellar (Mn-MCM-L) structures, at a low surfactant/silica ratio (0.12), and present a preliminary account on the location of the Mn 2+ cations as obtained from Q-band EPR spectroscopy. We show that the addition of Mn ions induces the formation of the cubic phase also at a low surfactant/Si ratio (0.12) and that by the variation the base content of the gel, one can control the structure formed.
2. EXPERIMENTAL Materials: The synthesis mixture was prepared using sodium silicate, N brand, 27% silica, or tetraethyl orthosilicate (TEOS), Aldrich; Catapal alumina, Condea; Aluminum-isopropylate, Merck; AICI3, Merck; cetyltrimethylammonium chloride (25 wt.%) (CTAC) or bromide (CTAB), Aldrich. Other inorganic materials were MnCI 2 .6H20, Merck, HCI and NaOH. All chemical were used as received. Synthesis: Two Synthesis procedures, differing mainly in the silica source, were employed [2]. In the first, 15-30 ml of 2 M NaOH solution were added under constant stirring to 14.5 ml CTAC or CTAB. The mixture was then combined with 0.1-20 ml of a 0.4 M MnCI 2 solution and 20.5 ml tetraethyl orthosilicate. The composition of the final gel was SiO 2 .xMnO-yNa20 zCTAC(CTAB).wH20 9 , where 0.0004<x<0.09, 0.16
183
Table 1 Synthesis and characterization data of the mesoporous manganese materials Sample
SiO 2
No.
(mmol)
A1CI 3 (mmol)
CTAC CTAB (mmol)
M n 2+ (mmol)
NaOH (HCI) (mmol)
Mn/Si molar ratio
Tem.
product
(~
d-value(A) (first peak)
MCM-41
39.2(32.1) c
MCM-L MCM-48 MCM-48 MCM-41
36.8 39.2(34.6) c 36.0(31.4) c 37.6(30.4) c
100 100 RT
MCM-41 MCM-48 MCM-41
41.3(32o7) c 37.6(31.0) c 39.2(32.1) c
4.3x10 "3 4.3x10 "3
150 100
MCM-L MCM-41
36.0 43.5(33.9) c
0.09 4.3x10 "3 4.3x10 "3 4.3x10 "3 4.3x10 "3
100 100 100 100 100/RT 100
MCM-48 MCM-48 MCM-41 MCM-41 MCM-41 MCM-41
1 2 3 4 5
92 a 61.5 a 52.4 a 92 a 92 b
-
11 11 50.2 11 11
0.4
50 50 55 62 8
4.3x10 "3
RT 150 100 100 100
6 7 8
86 a 92 a 92 a
5 -
11 11 11
0.4 0.4 0.4
55 55 55
4.7x10 "3 4.3x10 "3 4.3x 10 "3
9 10
92 a 92 a
-
11 11
0.4 0.4
55 30
11 12 13 14 15 16
92 a 92 a 92 b 92 b 92 b 92 b
2 -
11 11 11 11 11 11
8.4 0.4 0.4 0.4 0.4
60 65 0 10 d 25 d 41 d
Silica sources: a, tetraethylorthosilicate; b, sodium silicate; c, after calcined at 540~
37.8(32.1) c 37.7(33.2) c 38.4(34.0) c 43.1(40.5) c 45.3(40.0) c 47.7
d, HCI.
3. RESULTS AND DISCUSSION Using the procedures described in the experimental section all three structures, Mn-MCM41, Mn-MCM-48 and Mn-MCM-L could be generated with a wide of Mn/Si molar ratio (0.0004-0.09). Some examples of the materials synthesized, the specific gel compositions, the reaction conditions, the phases obtained and the corresponding d spacings are given in Table I. Typical X-ray diffraction (XRD) patterns of as-synthesized Mn-MCM-41, Mn-MCM-48 and Mn-MCM-L are shown in Fig. 1. Owing to the long range order of the pores they show diffractions in the 20 range of 2-8 ~ similar to XRD patterns previously reported for M41S materials with hexagonal, cubic and lamellar structures [2,5]. Mn-MCM-41 and Mn-MCM-48 are highlystable and their pore structures withstand calcination at 540 o C. The XRD patterns of the calcined materials are similar to those of the as-synthesized materials indicating, how-ever, a reduction in the pore size of--7 A and --5 A in Mn-MCM-41 and Mn-MCM-48 respectively (see Table 1). The BET surface area of calcined Mn-MCM-41 and Mn-MCM-48 was 1340 and 950 mEg-1 respectively, in good agreement with previously reported values [2,3]. Calcination of Mn-MCM-L caused the collapse of the structure as expected [2]. The transmission electron micrograph (TEM) of an as-synthesized Mn-MCM-41 sample, shown in Fig 2a, exhibits a regular array of channels in a hexagonal arrangement [4,7]. The repeat distance between the channels is about 40 A, which is in excellent agreement with the position of the first peak in the XRD pattern (d100=39. 2A, a=2d100/~/3=45.3A) [4,7]. The micrograph also shows that the size of the pores is--30/k with inorganic wall thickness or-~15
184
8vI~ i~
ll
r4)
c
100 36.0 200 17.8
~
211 37.6
421 20.2
II
~n
22o 32.8 332 ,9.r
II
~/V'
32, 2,.5 ,22 ,8.8
I
| 0
~ ,tJ ~
400 23.1
431
18.0 ~
.......
i+i/~
.... , ?
II
A
/I 11
B
1~
/ \ <'' / ~
/tY
\_J,
,1o ,...
' _
39.2
,,o ~2.7 200 ,9.8
M~ I 2
~
I 4
i
I 6
i
I 8
9
l 10
degrees 20
Figure 1. Powder X-ray diffraction patterns of as-synthesizedd Mn-MCM-41, Mn-MCM-48 and Mn-MCM-L.
Figure 2. Transmission electron micrographs of as-synthesized a, Mn-MCM-41 and b, Mn-MCM-48.
A. The frequency and diversity of the fringe patterns obtained from as-synthesized Mn-MCM48, shown in Fig. 2b, suggest the existence of three-dimensional order in more than one projection as expected for an analog of the cubic Ia3d liquid-crystal phase [3b]. This unique pattern is believed to be the [ 111] projection of Mn-MCM-48. Mn-MCM-41 could be produced over a wide range of temperature (21-100~ At room temperature it was detected already after 5 min. of reaction. The hexagonal structure was the first structure observed and we did not detect a precursor lamellar phase at earlier times, as reported by Monnier et al [5] for MCM-41. At a low surfactant/Si ratio, Mn-MCM-48 could be synthesized only at high temperature (100--120~ and the synthesis of pure Mn-MCM-L required either a high temperature (>130~ or a high surfactant/Si ratio. At a fixed gel composition the structure of the Mn-M41S materials obtained depends on the temperature. The XRD patterns of the reaction products as a function of temperature, shown in Fig. 3, exhibit to following order: hexagonal ~ mixture (hexagonal, lamellar) ~ cubic ~ lamellar. Namely, at room temperature pure Mn-MCM-41 was produced and an increase in the temperature transformed it to a lamellar or cubic phase. This indicates that the temperature plays an impor-
185
150~ -
130~ .._
oo t.=_,
120~ ~ _ _ ~ _
100~ 90~ 70~
~.~-
~
50~
~ . . . . ~ ~ 1 ~ 1 ~ 2 4
I~ ~ 6 degrees 29
RT ''
! 8
'
'i
Figure 3. Powder X-ray diffraction patterns of as-synthesized products obtained from the reaction gel (92 mmol TEOS" 11 mmol CTAC 55 mmol NaOH: 0.4 mmol MnCI2:10.6 mol H20 ) as a function of temperature, the reaction time for each sample was 72 hr.
tant role both in terms of template organization and silanol condensation and supports the basic liquid-crystal templating mechanism proposed earlier for M41S materials [2,3,7]. Furthermore, the transformations observed above suggest that the precursor to the M41S structure is a collection of individual silicate surfactant micellar rods. At a fixed temperature and surfactant/Si ratio the structure formed could be controlled by varying the base or acid content of the gel. The structures obtained were, however, a function of the silica source used. When tetraethyl orthosilicate was used, increasing the NaOH concentration while keeping content of all other components constant, resulted in the trend hexagonal ~ lamellar ~ cubic ~ hexagonal (in different samples). In contrast, when sodium silicate was used, the cubic phase was not produced and the order observed with increasing NaOH concentration was hexagonal ~ lamellar. This difference may arise from a faster silicate hydrolysis rates in sodium silicate, resulting in enhanced silanol condensation and the formation of a thicker amorphous silica wall that inhibits the transformation to the cubic phase. In general Mn-MCM-41 could be synthesized under a broader range of conditions as compared to Mn-MCM-48. For example, the base concentration ranges 0.16
186 Table 2 Elemental analyses of the mesoporous manganese materials wt% Sample No. 7 8 9
Sit 2
Mn
42.5 54.9 41.9
0.38 0.24 0.43
C
H
37.5 28.6 41.5
8.60 6.75 9.09
N 2.20 1.74 2.31
Si/Mn molar ratio
C/N molar ratio
N/Si molar ratio
102 210 89
19.9 19.2 21.0
0.22 0.14 0.24
product MCM-48
MCM-41 MCM-L
B
A a
a
C
d
f
Figure 4 Q-band EPR spectra, recorded at A, RT; B, low temperature(150K) of a, assynthesized Mn-MCM-41; b, calcined Mn-MCM-41; c, calcined Mn-MCM-48; d, Mn 2§ impregnated onto MCM-41 materials; e, alter d calcined at 540~ f, 0.1 mM MnCI2 solution in water.
Thermogravimetric analysis (TGA) of Mn-MCM-41, Mn-MCM-48 and Mn-MCM-L in air showed three distinct stages of weight loss at 35-150~ 150-400~ and above 400~ The first stage is due to the desorption of water, the second, which is accompanied by weak exotherms, is due to the combustion and decomposition of organic species (CTAC or CTAB) and the third stage is related to water losses via condensation of silanol groups to form siloxane bonds [7]. The weight loss for Mn-MCM-41, Mn-MCM-48, and Mn-MCM-L is 41.8%, 50.9% and 53% respectively. The composition of these samples as obtained from chemical analysis is given in Table 2. Both the chemical analysis and the TGA data show that at same composition
187 of the gel less template interacts with the inorganic species to form the hexagonal phase as compared to the cubic and lamellar phase. Also, the Mn concentration in the final product was usually similar or higher than in the reaction gel. A better understanding of the role of the Mn ions in determining the structure of the phase formed can be obtained from the location of the Mn ions in the final product. To characterize the Mn ions location we measured X-band and Q-band EPR spectra of the M41S materials before and after calcination. All as-synthesized materials besides those synthesized at RT with HCI did not show any EPR signals either at RT or at 150K. We attribute the absence of the EPR signal to the presence of Mn(IlI) species, rather than Mn(II), since the latter is oxidized to Mn(III) in basic media in air [ 17]. Calcination of these materials, however, did produce a typical spectrum of Mn(II) indicating that during calcination at least part of the Mn(III) was reduced to Mn(II). In contrast, in as-synthesized Mn containing aluminophosphate molecular sieves, which is synthesized under acidic conditions, typical spectra of Mn(II) were readily observed [ 18]. The X-band EPR spectra were poorly resolved due to the presence of forbidden transitions [18], therefore, Q-band spectra were recorded. The Q-band EPR spectrum of Mn-MCM-41 synthesized at RT in the presence of HCI exhibits a typical Mn(II) spectrum with a hyperfine coupling, a, of 81 G and g ~ 2.00, as shown in Fig. 4a. Low intensity forbidden transitions are evident in both the RT and low temperature spectra indicating that the Mn(II) species are rather immobilized at RT [19]. Upon calcination the hyperfine coupling changed to 91 G and the forbidden transitions, still apparent in the low temperature spectrum of the calcined sample, were averaged out in the RT spectrum. This indicates that the Mn(II) species become more mobile after calcination. The same spectrum was obtained from calcined Mn-MCM-41 which did not exhibit Mn(II) EPR signals prior to calcination. The spectrum of calcined Mn-MCM-48, shown in Fig. 4c, is very similar to that of a MnCI 2 solution in 1:1 water/glycerol at both RT and 150K (Fig. 4d) indicating that fully mobile, hydrated Mn 2+ ions are present within the pores. For comparison, samples where MnCI 2 was impregnated onto as-synthesized pure silica MCM-41 were prepared. The spectrum of the as-prepared sample differed significantly from that Mn-MCM-41, whereas that of the calcined impregnated sample was similar to that of calcined Mn-MCM-41. This suggests that in calcined Mn-MCM-41, Mn(II) ions are situated on the surface (internal and external) and not within the silica wall. Moreover, the larger immobility of the Mn(II) species in the as-synthesized Mn-MCM-41 can be attribute either to its location within the wall or in the interface region of the polar head groups the template and the silica walls. In the calcined samples of both Mn-MCM-41 and Mn-MCM-48, however, their high mobility provides further evidence that they are not located within the inorganic wall.
4. CONCLUSIONS Mesoporous manganosilicate, Mn-M41S, materials with a having hexagonal, cubic and lamellar structures, were synthesized at a low surfactant/Si ratio (0.12). For a fixed gel composition the phase formed could be controlled by the reaction temperature and at a constant temperature, the phase generated was dependent on the NaOH content. The addition of Mn ions induced the formation of cubic phase also at a low surfactant/Si ratio. Prior to calcination the Mn 2+ ions in Mn-MCM-41 are highly immobilized whereas in calcined Mn-MCM-41 and Mn-MCM-48, the Mn 2§ ions are hydrated and highly mobile located within the pore and not in the inorganic wall.
188 5. ACKNOWLEDGMENT We thank Dr. Lev Margulis for performing the TEM measurements.
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Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
ALKANE OXIDATION CATALYZED BY ZEOLITE RUTHENIUM PERFLUOROPHTHALOCYANINE S
189
ENCAPSUI~TED
Kenneth J. Balkus, Jr.*, Alia Khanmamedova and Mona Eissa
University of Texas at Dallas, Department of Chemistry, Richardson, TX 75083-0688 United States SUMMARY Ruthenium(H) perfluorophthalocyanines (RuF~c) have been encapsulated in the supercages of zeolite NaX by crystallizing the molecular sieve around the RuF~Pc complexes. These zeolite ship-in-a-bottle complexes were found to be effective catalysts for the oxidation of cyclohexane using t-butylhydroperoxide (t-BOOH). High conversions to cyclohexanone as the major product was achieved with nearly 3000 turnovers per day. The linear n-hexane was oxidized to a mixture of products with selectivity towards hexan-3-one. The intrazeolite RuFfle complexes show no signs of deactivation in contrast to the non-fluorinated analog (RuPc). Preliminary mechanistic studies suggest a P-450 type pathway may be operative. 1. INTRODUCTION We have previously reported the encapsulation of ruthenium(II) perfluorophthalocyanines (RuFl~C) inside zeolite NaX [1-3]. This is an example of a ship-in-a-bottle complex which is physically trapped in the zeolite X supercages but not necessarily bound to the oxide surface. One might anticipate hybrid like activity from such composites that reflects the solution behavior of the complex in combination with the zeolite's properties. The synthetic faujasite type zeolites and metallophthalocyanines (MPc) are well matched because the complex fits snugly in the X or Y supercage (-~12A diameter) and cannot escape through the restricted apertures (7.4A). Additionally, MPc complexes are amenable to either the template or synthesis method of encapsulation [4]. The RuF~rPc complexes were entrapped in NaX using the synthesis method which involves crystallizing the zeolite around the complex. This approach has many advantages including well defined intrazeolite species without contamination from uncomplexed metal or ligand. RuF~Pc is an effective catalyst for the room temperature oxidation of alkanes using tbutylhydroperoxide (t-BOOH), however, the complex forms a ~-oxo dimer which inhibits the activity [1-3]. Encapsulation inside zeolite NaX, site isolates the RuFfle and precludes dimerization. The result is a catalyst that is at least an order of magnitude more active than the homogeneous system. Perfluorination of the phthalocyanine ring contributes to the observed reactivity but also dramatically enhances the complex stability. The non-fluorinated RuPc bleaches and deactivates within a few hours when exposed to t-BOOH. This is a problem observed with other ship-in-a-bottle complexes such as NaY entrapped FePc which requires slow controlled addition oft-BOOH to prevent rapid deactivation [5]. Another approach is to
190 embed the FePc-NaY in a polymer matrix in order to regulate the amount of peroxide that can diffuse to the zeolite which results in a more stable and active catalyst [6]. Our strategy was to simply build a more stable metal complex by adding fluorine substituents to the Pc ring. This approach mirrors the efforts of Ellis and Lyons who have demonstrated that perhalogenation of a porphyrin [7,8] or phthalocyanine ligand periphery [9] enhances the oxidative stability and catalytic activity of the corresponding iron complexes. We have previously prepared perfluorinated phthalocyanine complexes (MF16Pc) of Fe [10] as well as Co and Cu [11] in both zeolites NaX and NaY. However, the isoelectronic ruthenium (II) complex, RuF16Pc has proven to be a more stable and active catalyst for the oxidation of alkanes regardless of how much peroxide is present. In this paper, results for the RuF~6Pc and RuF~6Pc-NaX catalyzed room temperature oxidation of n=hexane will be presented. Preliminary, mechanistic studies suggest the reaction is radical in nature but may involve a P-450 type pathway may be involved.
2. EXPERIMENTAL
2.1 Catalyst Preparation The synthesis and characterization of ruthenium(H) perfluorophthalocyanine as well as the synthesis of NaX in the presence of guF~6Pc have previously been described [1,2]. The loading in the RuF~6Pc-NaX catalyst was one complex per--16 unit cells or 125 supercages. This corresponds to --25% of the metal complexes that was added to the initial synthesis gel becoming encapsulated in the resulting NaX zeolite. The low loading of intrazeolite metal complex is desired in order to maintain diffusion pathways within the zeolite.
2.2 Catalysis The oxidations of cycloalkanes were carried out in sealed glass vials (15 mL) under a nitrogen atmosphere at 25~ Reactions were run with an alkane to t-BOOH molar ratio of l:l or 1:0.5. In a typical reaction the vial was charged with 1.5 mL acetone, 6 mmol alkane, 6 (or 3) mmol of 90% tertbutylhydroperoxide (t-BOOH) and 0.10 grams of RuFl6Pc-NaX or 0.001 grams of RuF~6Pc. The reaction was stirred at room temperature and samples were taken through a rubber septum by syringe. Products were analyzed by gas chromatography using a HI) 5880 capillary GC equipped with a 6 fl 100/6 carbowax on chromosorb W-HP column and a flame ionization detector. Products were verified by known standards and/or GC-MS. Turnovers are based on the mmoles of product (ketone + alcohol) per mmole of RuF~6Pc per unit time. The peroxide efficiency was determined from the molar ratio of alkane oxidation products to the total amount of peroxide consumed. Conversion is based on peroxide taking into account that 2 mmoles oft-BOOH are required for I mmole of cyclohexanone
191 3. RESULTS AND DISCUSSION 3.1 Catalytic Oxidation of n-Hexane We have previously shown that both RuF~6Pc and RuF~6Pc-NaX are effective catalysts for the room temperature oxidation of cycloalkanes [ 1-3]. Encapsulation of the metal complex in zeolite NaX dramatically enhances the catalyst activity with turnovers approaching 3000 per day for cyclohexane oxidation. The linear n-hexane is oxidized to a mixture of ketones and alcohols at a much slower rate but nevertheless both the free RuF~6Pc and RuF~6Pc-NaX are catalysts for this transformation. Figure 1 shows a plot of total turnovers (K+A/RuF~6Pc) for the homogeneous and heterogeneous systems at the different substrate to peroxide ratios.
1400 -o o
~1200 o
,
-0
8OO
6OO O 9
.~O " ~ ~
4OO
2OO 0
I-
I
I
0
2O
4O
Figure 1. Plot of Turnovers
I
I
6O 8O T I M E (hours)
I
100
(K+A/RuFI6PC)for an n-hexane to t-BOOH ratio of 1"1 ( - A -
RuF~6Pc, ---O---RuF~6Pc-NaX) and 1:0.5(---A--- RuFl6Pc, ---O--- RuFl~c-NaX). The activity of the free complex reflects the rapid formation of the ;~-oxo dimer as previously identified in the cyclohaxane oxidations [1-3]. In contrast, the R u F ~ c - N a X sample at the higher peroxide concentration is more than five times as active. It is interesting to note that the more peroxide is added initially, the greater the turnovers. This is quite different from the previously reported FePc-NaY systems which show an increase in conversion as peroxide is added but the catalysts deactivate at high peroxide concentrations [5]. The principal n-hexane oxidation products are the ketone and alcohol in the C2 and C3 positions with no evidence of terminal C-H activation. Figure 2 shows a plot of mole % products versus time for the R u F ~ c - N a X at an n-hexane to peroxide ratio of 1:1. Initially, the main products are the alcohols with oxidation at the C3 position being favored. No measurable
192 quantities of oxidation products involving the C 1 position were detected. The formation of the ketones involves a sequential oxidation of the alcohols as previously observed for the cycloalkanes [ 1-3]. After the first few hours the free RuF~c,Pc complex produces considerably more alcohol than ketone. However, after 4 days there is relatively little difference in the product distribution between the homogeneous and zeolite catalyzed reactions. The higher amount of ketones relative to alcohol products in the zeolite based systems has been rationalized in terms of the ability of the zeolite to adsorb the polar alcohol effectively increasing the concentration at the
-
__
\
.
_\,~,
- ~. ~
-so
-415
\
-
Ill
2-~m
-2.
-1O
$.d
411
Time O r )
Figure 2. Plot of mole % products versus time for the oxidation of n-hexane catalyzed by RuF~c,Pc-NaX with a substrate to t-BOOH ratio =1"1. active sites. The C-H bond energies in the 2 and 3 positions are nearly the same, however, the steric demands of the C3 are greater than C2 [12]. Therefore, the product distribution of nhexane oxidation could be used as a measure of steric constraints at the active site, where one might expect steric hinderance to favor oxidation at C2. However, there are also examples of ruthenium based homogeneous catalysts that lack any significant steric considerations, yet produce n-hexane C2/C3 product ratios much greater than one [ 13]. Therefore, one has to be careful when interpreting product distributions. The n-hexane molecule should have a smaller kinetic diameter than cyclohexane and therefore, should not experience dramatic steric effects in NaX. In the case of both FePc and FePc-NaY the C2/C3 ratio was approximately one for nhexane oxidation [5]. Only longer chain alkanes favored oxidation at the C2 position for the FePc-NaY system. The RuF~c,Pc based catalysts exhibit C2/C3 ratios less than one. After the first few hours the C3 products are favored nearly 2 to 1 but after several days this decreases to 1.5 to 1. Within experimental error the C2/C3 ratio for the homogeneous catalyst and the zeolite encapsulated complexes are the same which is similar to the FePc system.
193
100
o0
. . . . , , ,
-
- - - 0
j.
90
m.
80 ~
0
,D 9
70
9
60 0
50
C) 40 c) 30 20 100 0
I
I
I
i
I
20
40
60
80
100
Time (hours)
Figure 3. t-butylhydroperoxide conversion for an n-hexane to t-BOOH ratio of 1"1 (---O--RuF:6Pc, - I RuF:6Pc-NaX) versus time. The peroxide conversion is nearly the same for the different substrate to t-BOOH ratios. Figure 3 shows a plot of peroxide conversion versus time for the R u F ~ c and RuF:d)c NaX samples at the 1:1 ratio. The free metal complexes exhibit higher rates of t-BOOH conversion based on t-butanol formation. The peroxide efticiencies are quite low for both catalysts. The yields of ketones and alcohols based on peroxide consumed for the R u F ~ c NaX catalyst are around 17% and -~13% of the peroxide ending up as hexane oxidation products for the free complex. This is in contrast to the R u F ~ c catalyzed oxidations of cycloalkanes which exhibit peroxide efficiencies as high as 97% [ 1-3]. 3.2 Mechanistic Studies
We have previously noted that the RuF:~c and RuF:~c-NaX catalyzed oxidation of cycloalkanes was radical in nature [I=3]. This is based on the addition of hydroquinone, a radical trap, which completely inhibited the reaction. Furthermore, the addition of AIBN (2,2'azobisisobutyronitrile), a flee radical initiator, to the reaction accelerated the rate. These results are consistent with a radical type mechanism which may involve ruthenium oxo species. A mechanism has been proposed for RuCI2(FPh3)3 catalyzed oxidations of alkanes using t= BOOH which is similar to a cytochrome P-450 type pathway [14]. In this mechanism the Ru(II) complex first reacts with t-BOOH to form R u ( I ~ - O and t=butanol as shown in equation I. The ruthenium oxo complex then abstracts a hydrogen atom from the alkane to give a caged radical pair which can subsequently escape as the alcohol product regenerating Ru(H) as shown in equations 2 and 3. The fact that we do not observe any alkane coupling
194
gu(II)
+
Ru(IV)=O
t-BOOH
-~
Ru(IV)=O
+
~
Ru(m)-OHR.
(2)
-~
Ru(II)
(3)
Ru(m)-OHR.
RH
+
+
t-BOH
R-OH
(1)
products that would be expected in an autoxidation, suggests reactions 1-3 are possible. However, the effectiveness of radical traps and initiators is also consistent with a chain mechanism. If hydrogen abstraction is the rate determining step, then would expect to observe a kinetic isotope effect. Therefore, we decided to evaluate the kinetic isotope effect for oxidation of cyclohexane versus cyclohexane-dl~. The dueterated substrate was oxidized much slower and the selectivity towards the ketone was never better than 50%. Additionally, the peroxide efficiency in the Cd)~2 oxidation was nearly an order of magnitude less than observed for the normal cyclohexane [1-3]. The kinetic isotope effect was calculated according to published procedures [ 15,16] and was based on decreasing concentrations of Cd-I~2 and Cd)12 converted to ketone and alcohol at less than a 5% conversion. We observe an isotope effect of ka/kD =4.1 which suggests that indeed hydrogen abstraction is rate determining. Free radical reactions typically have kinetic isotope effects less than 2, while high valent metal oxo complexes generally exhibit isotope effects from --2-5 with even higher values observed when symmetrical transition states are involved in the C-H bond cleavage [12]. A kinetic isotope effect of 9 was reported for the FePc-NaY/polymer system which was proposed to involve a linear transition state [6]. The value of 4.1 for RUFlc,Pc-NaX suggests a nonlinear or asymmetric transition state but not a radical chain mechanism. The mechanism for the RuF~c,Pc based systems is radical in nature but probably involves a Ru(IV)=O species. Now if we examine the n-hexane reactions they appear more like an autoxidation with higher peroxide conversions and low efficiency as well as very little difference between the homogeneous and heterogeneous systems. We might reconcile these observations if we consider that a Ru(IV)=O species could react with t-BOOH as shown in equation 4 to form a hydroxy complex and an RO2" radical that could initiate a chain mechanism. The peroxide could also react with the metal complex itself and form an alkoxy radical as in equation 5. It very well may be that mechanistic pathway varies with substrate and Ru(IV)=O
+
t-BOOH
-~
Ru(III)-OH
+
RO2"
(4)
Ru(II)
+
t-BOOH
-~
Ru(III)-OH
+
RO.
(5)
reaction conditions. The reactants, products and solvent will adsorb differently in the NaX zeolite with more polar molecules having preference. Acetone which is used a solvent and standard, competes effectively with the other components of the mixture for the zeolite pores. If we remove acetone from the n-hexane oxidation mixture then we observe a C2/C3 product ratio much greater than 1. This would be consistent with more of the substrate being able to access the zeolite cavities resulting in a C2/C3 ratio indicative of steric factors at work. As our understanding of the RuF~c,Pc-NaX system improves, the mechanistic details of these oxidation reactions will become clear. However, we anticipate a complex picture will begin to emerge that involves a delicate balance of several variables that affect the reaction pathways.
195 4. CONCLUSIONS We have shown that zeolite NaX encapsulated RuFI#Pc complexes catalyze the oxidation of cycloalkanes and n-hexane where the major products are ketones. The intrazeolite RuFlc,Pc represents a significant improvement over similar zeolite ship-in-a-bottle oxidation catalysts because of the high activity and stability to high concentrations of peroxide. Further mechanistic studies are in progress, however, a cytochrome P-450 type pathway is consistent with the results so far.
ACKNOWLEDGMENTS We thank the National Science Foundation and the Robert A. Welch Foundation for their financial support. We also thank the American Chemical Society for a Project SEED award to RL. REFERENCES .
2. 3. 4. .
6. .
8. 9. 10. 11. 12. 13. 14. 15. 16.
K.J. Balkus, Jr., M. Eissa and R. Lavado, Stud. Surf. Sci. Catal., In Press. K.J. Balkus, Jr., M. Eissa and R. Lavado, ACS Symp. Ser., In Press. K.J. Balkus, Jr., M. Eissa and R. Lavado, J. Am. Chem. Soc., In Press. K.J.Balkus, Jr and A.G.Gabrielov, in Inclusion Chemistry with Zeolites, Nanoscale Materials by Design, N. Herron and D. Corbin (Eds), Kluwer, (1995) 159. R.F.Parton, L.Uytterhoeven and P.A.Jacobs, Stud. Surf. Sci. Catal., 59 (1991) 395. R.F.Parton, I.F.J.Vankelecom, M.J.A.Casselman, C.P.Bezouhanova, J.B.Uytterhoeven and P.A.Jacobs, Nature, 370 (1994) 541. P.E.EUis, Jr. and J.E.Lyons, Catal. Lett., 3 (1989) 389. P.E.Ellis, Jr. and J.E.Lyons, Coord. Chem. Rev., 105 (1989) 181. J.E.Lyons and P.E.Ellis,Jr., Appl. Catal. A : Gen., 84 (1992) L1. A.G.Gabrielov, K.J.Balkus, Jr., S.L.Bell, F.Bedioui and J.Devynck, Micropor. Mater., 2 (1994) 119. K.J.Balkus, Jr., A.G.Gabrielov, S.L.BeU, F.Bedioui, L. Roue and J.Devynck, Inorg. Chem., 33 (1994) 67. A.M. Khenkin and A.E. Shilov, New J. Chem., 13 (1989) 659. M. Bressart, A. Morvillo, and G. Romanello, J. Mol/Catal., 77 (1992) 283. S.-I. Murahashi, Y. Oda, T. Naota and T. Kuwabara, Tet. Lett., 34 (1993) 1299. R.P. Bell, Chem. Soc. Rev., 12 (1974) 513. C. Paquot, R. Perron and J. Dulieu, Bull. Soc. Chim. Fr., 60 (1973) 3729.
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Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviotand S. Kaliaguine(editors) 9 1995 Elsevier Science B.V. All rights reserved.
197
M O C V D in Zeolites Using Mo(CO)6 and W ( C O ) 6 as Precursors Samitha D. Djajanti and Russell F. Howe Department of Physical Chemistry, University of New South Wales, Sydney, NSW 2052, Australia
1. I N T R O D U C T I O N The vapour phase adsorption of volatile carbonyl complexes has been used for some time as a method to load transition metals into zeolite supports. Metals loaded in this way include Mo, Cr, W, Re, Co, Fe and Ni. There have been a few catalytic studies reported of such metal loaded zeolites, but the majority of published work has involved the spectroscopic investigation of the interaction between the metal carbonyl complex and the zeolite host, and of the decomposition of the adsorbed complexes on heating, which generally proceeds via distinct subcarbonyl intermediates [1-4]. From the viewpoint of catalysis, the chemistry of carbonyl adsorption and decomposition is less important than the reactivity and dispersion of the metal or oxide species produced by complete decomposition of the carbonyl precursor. This aspect of intrazeolite metal carbonyl has not been widely explored. The possibility of using sequential adsorption and decomposition of different metal carbonyl precursors to produce intrazeolite bimetaUie metal clusters has received little attention. A second question concerns the production of the metal or metal oxide species dispersed in zeolites with larger pore sizes which might be useful for catalysis involving larger reactant or product molecules. In this paper we examine these two questions further. The possibility of bimetallic cluster formation has been studied with the metal carbonyls Mo(CO)6 and W(CO)6 in zeolite Y, using particularly EXAFS to characterize the species produced by the decomposition of the adsorbed carbonyls. We will describe also our application of the MOCVD approach to load molybdenum into larger pore zeolites cloverite and MCM-41. Cloverite is a gallophospate molecule sieve with a supercage of approximately 3nm diameter accessed through four-leaf clover shaped windows which are partially blocked with terminal hydroxyl groups, leaving a free dimension of approximately 0.6nm.[5]. The potential of cloverite as a host for nanoscale host-guest chemistry has been discussed in detail by Ozin et al [6]. MCM-41 is one of a new class of mesoporous silicate structures first prepared by Mobil workers from surfactant templates[7]. In the case of MCM.41, the pore structure consists of a regular hexagonal array of parallel cylindrical pores approximately 4nm in diameter. By partially substituting silicon by aluminium in the MCM-41 synthesis it is possible to produce a mesoporous aluminosilicate analog [8].
198 2. E X P E R I M E N T A L Zeolite Y was a commercial sample obtained from Toyo Soda Co. in the sodium exchanged form, with Si/AI = 2.75 and was used without further treatment. Mo(CO)6 (98%) and W(CO)6 (99%) (Aldrich) were purified by vacuum sublimation prior to use. Cloverite was synthesized following two different methods with different templates (quinuclidine(97%, Aldrich)[5] and piperidine (>99.0%, Fluka)[9]) using Ga(NO3) 3 hydrate (Aldrich, 99.9 %; water content determined using oxine) and H3PO 4 ( 90 %, M.B)as starting materials. In the second method GaCI 3 solution was used to prepare GaOOH by titration with NaOH solution [ 10]. MCM-41 in pure silica and aluminium substituted forms were synthesised according to a modified method described by Ryoo [ 11 ]. Ludox HS-40 (DuPont)was used as silica source and sodium aluminate was used as source of aluminium. The template used was cetyltrimethylammonium chloride (CTACI) solution (25w% in water, Aldrich), with the gel composition: CTAC1 : 6 SiO 2 : 0.15 (NH4)20 : 1.5 Na20 : 250 H20. Calcinations were done by raising the temperature to 550C at a rate of 1 degree per minute in air, and holding at that temperature for 10 hours. Synthesized samples were characterised by XRD, FTIR and NMR. The sodium exchanged form was obtained by ion exchange of Al-MCM-41 (Si/Al =7 in mixed solution) with sodium chloride solution 0.02 M. Gravimetric measurements of carbonyl adsorption and decomposition were carried out on a vacuum microbalanCe. Samples, in pellet form, were activated up to 450 C, and held for 1 hour at this temperature under oxygen. After cooling to room temperature, the sample was exposed to metal carbonyl vapour, and the weight changes on subsequent decomposition monitored. EXAFS samples were prepared by activating the pelletised sample (~100rag dry weight/cm 2) under vacuum up to 450 C, then exposing to metal carbonyl vapour at room temperature for at least 6 hours to achieve saturation adsorption before evacuation. Samples were then heated to 200C or higher under dynamic vacuum to achieve complete decomposition of the adsorbed carbonyl (analogous FTIR experiments showed that this procedure completely removed carbonyl ligands). For multiple dosing, the sample was exposed to metal carbonyl vapour again at room temperature and all further steps were repeated. EXAFS measurements were carried out by transmission on the Australian National Beam Line Facility BL20B at the Photon Factory, Tsukuba, Japan, (except for multiple doting of molybdenum experiments, which used BL 10B). The collected data were analysed using the University of Washington UWXAFS 3.0 and FEFF5.05 programs[ 12].
3.RESULTS AND DISCUSSION
3.1 Bimetallic Clusters in Zeolite Y. Exposure of dehydrated NaY zeolite to Mo(CO) 6 vapor at room temperature causes rapid uptake of ca 2 molecules per supercage. The adsorbed carbonyl is irreversibly held, and heating to 200 C in vacuo causes complete decarbonylation, giving a material containing 14 w% Mo. A further 2 molecules of carbonyl can then be adsorbed, which on decarbonylation increases the Mo loading to ca 25 w%; this process can be continued further, although the capacity to adsorb Mo(CO)6 gradually decreases as the supercages fill with molybdenum. Yong and Howe
199 [ 13] prepared zeolites containing up to 53w% Mo by sequential adsorption of 14 doses of Mo(CO)6. Figure 1 shows Fourier transforms of Mo Kedge EXAFS data for MoY zeolites containing 1,2 and 4 successive doses of Mo(CO)6, decarbonylated at 200C. As reported previously[14], decomposition of Mo(CO)6 in NaY at 200C produces highly dispersed d. molybdenum species in which the average Mo-Mo ,P coordination number (the second peak in the c Fourier transform at r = 0.25nm uncorrected) is about 1.0, suggesting that the original loading of 2 Mo per supercage is retained on decomposition of , the carbonyl. The first peak in the Fourier 4 6 transform (0.16nm uncorrected) is due to Mo-O R, Angstroms bonding to the zeolite lattice (Mo-O coordination number = 2) [ 14]. The striking feature of the new data presented in Figure 1 for multiple doses of molybdenum is that the Fourier transform is Fig. 1 FT of Mo EXAFS: Mo(CO)6 in unchanged. The affinity of molybdenum for oxide NaY after decomposition at 200C; a. 1 ions of the zeolite lattice evidently overrides any dose; b. 2 doses; c. 3 doses; and d. 4 tendency to form metal clusters, and doses molybdenum in the multiple dosed samples appears to line the walls of the supercage. Some sintering as evidenced by an increase in the relative intensity of the Mo-Mo peak could be induced by heating to 400C. Gravimetric experiments showed that the stoichiometry of adsorption and decomposition of W(CO)6 in NaY is similar to that of "12 ~2 Mo(CO)6. In particular, multiple adsorption and g decomposition cycles could be used to build up o the tungsten content of the zeolite in steps of 2 FLL atoms per supercage. As shown in Figure 2 however, the W LIii-edge EXAFS data indicate that the dispersion of tungsten is very different from that of molybdenum. After decomposition o of a single dose of adsorbed W(CO)6 in NaY at 200C the dominant peak in the EXAFS Fourier 2 4 6 transform at about 0.27nm (uncorrected) is R, Angstroms attributed to a W-W interaction. Detailed fitting of the data to determine structural parameters is not yet complete, but from the relative intensities Fig.2 FT ofW EXAFS: W(CO)6 in NaY aider decomposition at 200C; a. 1 dose; b. of the 0.27nm and shorter distance peaks (which 2 doses; c. 4 doses and d. 4 doses at 400C are attributed to W-O interactions) it would appear that relatively large tungsten clusters have
~2
200 been formed. This conclusion is supported by the observation of a further peak in the Fourier transform at about 0.37nm(uncorrected) not seen in molybdenum zeolites. Evidently the interaction of tungsten with zeolite oxide ions is 3 insufficiently strong to prevent inter-supercage migration of W during decomposition of the a2 adsorbed W(CO)6 at 200C, forming what L appears to be a heterogenous distribution of larger tungsten clusters. Addition of further doses of W(CO)6 causes some changes in both relative intensities and positions of peaks; heating subsequently to 400C further enhances 0 2 4 6 the major W-W peak in the Fourier transform R, Angstroms relative to the W-O peaks, suggesting further sintering is taking place. The heterogeneity of this system will make structural analysis difficult. Fig.3 FT of a.Mo EXAFS and b.W Figure 3 shows Fourier transforms of the Mo EXAFS for NaY loaded with Mo(CO)6 K-edge and W Liii-edge EXAFS respectively for then W(CO)6 a NaY zeolite which was first loaded with 2 Mo per supercage then with one dose of W(CO)6 decomposed at 200C. Comparison of the Mo EXAFS with that obtained from one or two doses of Mo(CO) 6 alone (Figure 1) shows several differences. The relative intensity of the peak at r = 0.25nm (uncorrected) previously assigned to Mo-Mo is significantly enhanced, suggesting that this peak now contains a contribution from Mo-W interactions (which has yet to be confirmed by detailed fitting of the EXAFS). Also, the Mo-O peak at r = 0.16nm (uncorrected) is split into two components following addition of tungsten. The corresponding W EXAFS data show that "13 in comparison with the zeolite containing only -~ 2 W, addition of W to the Mo loaded zeolite produces a higher dispersion of W with a greatly reduced contribution from the W-W peak at r = 0.3 nm (uncorrected). Our provisional interpretation of these results is that pre-loading the zeolite with molybdenum ( 2 Mo atoms per supercage) A~/X,-x/'~rx produces a better dispersion of tungsten on 0 6 subsequent adsorption and decomposition of RoAngstroms W(CO)6 because of a direct Mo-W interaction in the supercage. If this is correct, Fig.4 FT of a.Mo EXAFS and b.W EXAFS of the opposite experiment in which tungsten is NaY loaded with W(CO)6 then Mo(CO)6 loaded first followed by molybdenum should lead to a different result, since the tungsten is not distributed homogeneously throughout
201 every supercage. Figure 4 shows Fourier transforms of Mo K-edge and W Liii-edge EXAFS from such an experiment( W(CO)6 adsorbed and decomposed at 200C followed by Mo(CO)6 ). The W data in this case are much more closely similar to those of the zeolite containing W alone, with major contributions from W-W peaks at r > 0.3nm (uncorrected). For Mo, on the other hand, the dispersion appears to be lower than in the zeolite containing only Mo, as judged by the enhanced contribution of the r = 0.25nm peak (uncorrected). This result is also consistent with a direct Mo-W interaction if the presence of a heterogeneous distribution of tungsten clusters in only some of the zeolite supercages is presumed to attract Mo to those particular supercages.
3.2 Molybdenum in Cloverite Since cloverite is unstable in air following removal of the template [ 15], infrared and gravimetfic experiments on Mo(CO)6 adsorption and decomposition were performed on samples calcined in-situ up to 480C in oxygen Weight loss measurements and infrared spectra both indicated that this calcination procedure removed more than 75% of the template. Nevertheless, the stoichiometry of Mo(CO)6 adsorption and decomposition were found to depend on which template was used in the cloverite synthesis Figure 5 shows the results of gravimetric experiments with both types of cloverite Prolonged exposure of cloverite derived from quinuclidine to Mo(CO)6 vapour at 18 room temperature gave a ~-- 16 iiiiiiiiii total uptake of only 4 14 iiilriiill wt%. Ozin et al [6] o 12 iiiiiiiiii report a total supercage ~ i ~ i H H H '~ 10 !@iiiili pore volume for calcined ~ 8 ::xx:::: .......... cloverite (as measured by oxygen adsorption) of ~ 4 iiiiiiiiii ca. 24 cm 3 per 100g. --- 2 iiiiiiiiii ~__~ ~ Complete filling of this 0 : : : : - : volume with Mo(CO)6 Ads Evac 100 C 200 C 400C would thus correspond quinuclidine template D piperidine template to an uptake of ca 47 wt%. The observed uptake for cloverite is only 8 % of this value, Fig. 5 Adsorption of Mo(CO)6 into cloverite suggesting that there is very little penetration of Mo(CO)6 into the supercages, and that adsorption occurs mainly on the external surface. The adsorption is only partly irreversible, the weight losses on subsequent heating in vacuo leave only 0.2 wt% Mo in the sample. In contrast, the uptake of Mo(CO)6 into cloverite derived from piperidine is 36 % of that expected for complete filling of the supercages, indicating that in this case supercage adsorption does occur. This adsorption is however also only partly irreversible. The 2 wt% Mo remaining after heating in vacuo to 200C or above corresponds to only 35% of the Mo initially adsorbed. An explanation for the differences between the quinuclidine and piperidine derived cloverite is offered in terms of the extent of dehydroxylation of the cloverite lattice during calcination. Ozin et al [6] undertook a detailed thermal analysis study of the decomposition of
202 the quinuclidine template, and identified three thermal events in the temperature range between 350 and 550C corresponding to desorption of untracked quinuclidine, pyrolysis of the quinuclidinium cation and evolution of HF and structural H20. These events did not cause any significant loss of structural integrity. The TGA-MS data reported in [6] show in fact a continuous evolution of water due to dehydroxylation between 300C and 800C, suggesting that the structural hydroxyl groups which partially block the supercage entrance in the as synthesized cloverite are still present to a significant extent after calcination at 480C. We have measured infrared spectra of cloverite samples calcined at 480C; these show a broad weak band in the region 3100-3300 cm -1 assigned by Ozin et al to the structural hydroxyl groups. Thermal analysis data are not available for the piperidine derived cloverite, but infrared spectra of this material after calcination at 480C show no evidence of the 3100-3300 cm -1 band, suggesting that dehydroxylation occurs more readily in this case. The partial desorption of Mo(CO)6 from cloverite on heating in vacuo is similar to that observed previously with AIPO-5 [16] . Ion exchanged cations have been shown to play an important role in the anchoring of adsorbed metal carbonyl complexes in zeolite Y [ 1]. The absence of such anchoring sites in aluminophosphate or gallophosphate molecular sieves restricts the applicability of the MOCVD method for these materials, at least for metal carbonyl precursors which require thermal decomposition at elevated temperatures. Mo K-edge EXAFS measurements on the Mo loaded cloverite show that the molybdenum is highly dispersed after decarbonylation, with no evidence of Mo-Mo interactions.
3.3 Molybdenum in MCM-41. Gravimetric measurements of Mo(CO)6 adsorption and decomposition were undertaken on 3 different MCM-41 materials: the all silica form, a sample containing aluminium substituted to a Si:A1 ratio of 7, and the same sample ion exchanged with sodium after calcination. Figure 6 summarizes the results of these experiments. The surface area of
~(':f~ ..-,
~~
Ifilifiii ~::;,i:~.::i~!]
i,iiiiii
;::h':: ....
e~ ~iii!i!i!i ,~] i i [ ! -
il
|
Ads Si-~l
i i I ! ~i'~I!~ "i!
Evac
:-."~
|
!
200 C ~ Si/AI= 7
Fig. 6 Adsorption of Mo(CO)6 into MCM-41
t ...............
..... ~;"
Reads
|
Evac
........
~
200C
~ Si/AI=7;Na-exdmaged
i |
203 MCM-41 after removal of the template is approximately 1000 m2 g-1. Monolayer coverage of this surface with Mo(CO)6 would correspond to an uptake of up to 150wt%. Exposure of silica MCM-41 to Mo(CO)6 at room temperature gives an actual uptake of around 20wt%, but this is almost completely removed by evacuation at room temperature, and the amount of molybdenum remaining after subsequent heating to 200C is less than 0. l wt % . Attempts to increase the molybdenum loading by adsorbing and decomposing further doses of Mo(CO)6 were unsuccessful. The aluminium substituted MCM-41 will contain acid protons after calcination of the cetyltrimethylammonium template, if the aluminium is substituting for silicon in the MCM-41 lattice. It is difficult to prove conclusively that all of the aluminium is in the lattice, but the 27A1 NMR spectrum of this material showed only tetrahedral aluminium. Aluminium substitution more than doubles the amount of Mo(CO)6 which is adsorbed on exposure of MCM-41 to the vapour at room temperature.This adsorption is however still almost completely reversible; the amount of molybdenum retained after heating to 200C in vacuo is ca. 0.5 %, and this figure was not increased by adding subsequent further doses of Mo(CO)6. Ion exchange of sodium into the aluminium substituted MCM-41 further increases the amount of Mo(CO)6 adsorbed at room temperature, to about one third of full monolayer coverage, but the amount of molybdenum retained on evacuation and heating to 200C remains extremely low. In this case, however, adsorbing and decomposing a second dose of Mo(CO)6 does significantly increase the Mo loading (to about 2.7wt% ). The surface chemistry of the all silica MCM-41 with regard to Mo(CO)6 adsorption closely resembles that of silica gel, which irreversibly adsorbs less than 1 wt% Mo [ 17]. The silanol groups lining the walls of the MCM-41 channels have a low affinity for Mo(CO)6 , and evacuation removes all of the adsorbed carbonyl. Infrared spectra have confirmed that physically adsorbed Mo(CO)6 is the only species present in this material. Substitution of aluminium into the silicate lattice will enhance the acidity of the hydroxyl groups (as in silica alumina gels), and this does appear to enhance the affinity of the MCM-41 for Mo(CO)6. The binding of Mo(CO)6 is however still much weaker than in the analogous zeolite systems (e.g.HY [18]), and desorption of the intact complex is still favoured over decarbonylation. Incorporation of sodium ions into the channels provides cation anchoring sites for the carbonyl complex which are likewise much weaker than the corresponding sites in NaY. The Na exchanged MCM-41 offers some hope of increasing the Mo content by many repeated cycles of adsorption and decomposition, but this will be a much less effective method than in NaY zeolite. The best chance of achieving high loadings of metal within the MCM-41 channel system is to decompose the adsorbed complex before it can escape the channels. We have recently attempted to do this by rapid heating of MCM-41 samples loaded with physically adsorbed Mo(CO)6 in sealed glass tubes of minimal volume. Preliminary XPS analyses of such materials show that high loadings of metal homogeneously dispersed through the MCM-41 pores are obtained, and further characterization is in progress. 4. C O N C L U S I O N S The results presented here have illustrated some of the chemistry involved in chemical vapour deposition of transition metals in micro- and mesoporous molecular sieves from carbonyl precursors. The loading and dispersion of the metal achieved depends on the size and
204 shape of the pore openings, the nature of adsorption sites within the pores, the volatility of the precursor complex and its ease of decomposition. Use of the MOCVD method to prepare novel catalysts or host-guest materials for other applications clearly warrants further investigation.
5. A C K N O W L E D G M E N T S We acknowledge Professor Ryong Ryoo for assistance with synthesis of MCM-41 and EXAFS measurements EXAFS measurements on the Australian National Beam Line Facility were made possible by a grant from the Access to Major Research Facilities Program funded by the Australian Government. Financial support also from the Australian Research Council and AusAID ( to SDD) is gratefully acknowledged.
6. R E F E R E N C E S 1. R.F.Howe in Tailored Metal Catalysts, ed. Y. Iwasawa, Reidel, Dordrecht, 1986 2. S. Ozkar, G.A. Ozin, K. Moiler and T. Bein, J. Am. Chem. Soc., 112 (1990), 9575 3. Y. Okamoto, T. Imanaka, K. Asakura and Y. Iwasawa, J. Phys. Chem. 95 (1991), 3700 4. G. Coudurier, P. Gallezot, H. Praliaud, M. Primer and B. Imelik, C.R. Acad. Sci. Ser. C., 282 (1976), 311 5. M. Estermann, L.B. McCusker, C. Baerlocher, A. Merrouche and H. Kessler, Nature, 352 (1991),320 6. R.L. Bedard, C.L. Bowes, N. Coombs, A.J. Holmes, T. Jiang, S.J. Kirkby, P.M. Macdonald, A.M. Malek, G.A.Ozin, S.Petrov, N. Plavac, R.A. Ramik, M.R.Steele, and D. Young, J.Am.Chem. Soc., 115 (1993), 2300 7. C.T.Kresge, M.E. Leonowicz, W.J.Roth, J.C. Vartuli, and J.S.Beck, Nature, 359 (1992), 710 and 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 8. C.Y.Chen, H.X. Li and M. E. Davis, Microporous Mater., 2 (1993), 17 9. Q. Huo and R. Xu, J. Chem. Sot., Chem. Commun., (1992), 1391 10. S.Bradley, Thesis, Univ. Calgary, 1991 11. R.Ryoo and J.M.Kim, J.Chem. Sot., Chem.Commun., (1995), 711 12. E.A. Stem, M. Newville, B. Ravel, Y. Yacoby, and D. Haskel, preprint submitted to Elsevier Science, (1994) (UWXAFS); and J.J. Rehr, J. Mustre de Leon, S.I. Zabinsky and R.C. Albers, J. Am. Chem. Soc., 113 (1991), 5135 13. Yong,Y-S and R.F.Howe, J.Chem. Soc., Faraday Trans.l, 82 (1986), 2887 14. J.M.Coddington, R.F.Howe, Y-S.Yong, K. Asakura, and Y. Iwasawa, J.Chem. Soc. Faraday Trans., 86 (1990), 1015 15. S. M. Bradley, R.F. Howe, and J. V. Hanna, Solid State Nucl. Magn. Reson.,2 (1993), 37 16. R.F. Howe, M. Jiang, S.T Wong and J.H Zhu, Catalysis Today, 6(1989), 113 17. R.F. Howe, Inorg. Chem., 15 (1976), 486 18. S. Abdo and R.F. Howe, J. Phys. Chem, 87 (1983), 1713
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
205
Tailored synthesis, characterization and properties of ZnO, CdO and SnO 2 nano particles in zeolitic hosts
M.Wark 1, H.-J. Schwenn 2, M. Wamken 2, N.I. Jaeger 2 and B. Boddenberg I
1 Lehrstuhl fiir Physikalische Chemie II, Universit/it Dortmund, Otto-Hahn-Str.6, D-44227 Dortmund, Germany 2 Institut ftir Angewandte und Physikalische Chemie, Universit~it Bremen, D-28359 Bremen, Germany
ABSTRACT The successful incorporation ofnanometer-sized ZnO-, CdO- and SnO 2 particles in zeolites is presented. It is demonstrated that the particles are formed in mesopores which were created during the process. The sizes of the embedded particles are strictly depending on the extent of the formation of mesopores and can be adjusted by the experimental conditions such as the Si/Al ratio of the supporting zeolite. 119Sn-MAS-NMR is applied to probe the host-guest interactions of faujasite encaged SnO 2 nano particles. 1. I N T R O D U C T I O N While the encapsulation of sulfide clusters or nano particles in zeolitic hosts has been extensively studied in view of their potential application in opto-electronic devices [1], much less information is available on the specific preparation of oxide dispersions supported within zeolite matrices [2-4], although they are interesting as catalysts for oxidation reactions [4]. SnO 2 or ZnO thin films are furthermore widely used as sensor materials for reductive gas atmospheres or ammonia [5]. Highly dispersed oxide particles promise to show improved sensitivity as could be demonstrated in the case of zeolite stabilized CdO nano particles which show a drastically increased reactivity towards CO 2 [3]. In the following the influences of the zeolite type and the preparation conditions on the growth of ZnO and CdO nano particles in zeolitic matrices are studied systematically. 2. E X P E R I M E N T A L ZnO and CdO nano particles were prepared by conventional ion-exchange of A, X and Y zeolites in aqueous solutions of the metal acetates (c < 0.1 M), and in case of faujasites by subsequent treatment with NaOH solutions of different concentrations.
206 SnO2 loaded zeolites were generated either by ion exchange in aqueous SnC12 solutions of pH 3-4 for 4 days at 293 K, by repeated impregnation with small amounts of SnC12 solutions for 2 hours at 353 K, or by chemical vapour deposition (CVD) with SnC14. CVD was carded out as follows: after dehydration of the zeolite in a stream of dry nitrogen at 673 K, dry nitrogen loaded with SnC14 was led through the zeolite powder at temperatures between 373 and 673 I~ Subsequently, non chemisorbed SnC14 was removed by flushing with pure dry nitrogen. Finally, the zeolite was treated in a stream of wet nitrogen at the same temperatures at which the chemisorption was performed in order to hydrolyse the fixed SnC14 In a final step all samples were calcined under flowing oxygen at 673 K for 4-24 hours. The characterization was carded out by X-ray diffraction (Guinier-Haag camera), diffuse reflectance UV-VIS spectroscopy, transmission electron microscopy (Phillips EM 420, acceleration voltage: 120 kV), adsorption measurements, and solid state NMIL As UV-VIS spectrometer a Varian Cary 4, equipped with a diffiase reflectance attachment (Praying Mantis) and a specially designed gas cell was used. The absorption is expressed in units of the Kubelka-Munk function. Reflecting standards (LOT, reflection 75 and 99 %, resp.) were used to reference the absorption of the unloaded zeolites. The NMR experiments (27A1,298i and 119Sn) were recorded under MAS conditions at room temperature in a magnetic field of 9.4 T (Bruker MSL 400). After dehydration of the samples in vacuum (p < 1 Pa) at 673 K, adsorption and desorption isotherms were measured at room temperature using cyclopentane as probe molecule. Samples containing tin dioxide nano particles were additionally studied by impedance spectroscopy. The impedance spectra were recorded with a Hewlett Packard Impedance Analyzer HP 4192 A in a frequency range of 10-107 I-Iz under vacuum and different gas atmospheres (02, HE) using a high temperature measuring cell (up to 673 K). 3. R E S U L T S A N D D I S C U S S I O N 3.1. Zinc and cadmium oxide dispersions in zeolites The treatment of zeolite samples, in which 20 or 40 per cent of the sodium ions were exchanged by zinc or cadmium ions (table 1), with diluted sodium hydroxide solutions leads to the precipitation of the corresponding metal hydroxides in the zeolitic pores. The hydroxides can be converted into the oxides by calcination of the samples in a stream of oxygen or air at 673 K for 5 hours. The cadmium containing samples change their color from white to yellow during this treatment. 3.1.1.Diffuse reflectance UV-VIS spectroscopy Whereas zeolite samples containing zinc or cadmium only in the cationic form exhibit no absorption at wavelengths higher than 200 nm~ samples containing dispersions of zinc or cadmium oxide clusters start to absorb at wavelengths in the near UV- or visible region, respectively. In figure 1 the absorption behaviour of dispersions of zinc oxide in different zeolitic hosts is shown. With the exception of the reflectance spectrum obtained for the dispersion in A-zeolites which resembles more a molecule spectnun, all other spectra show the form typical for solid state semiconductors, but in comparison to bulk ZnO the absorption edges are blue-~ified for
207
F(R)
1-
'\
(e)
0.5-
x O_
200
250
300
350
400
wavelenght / Figure 1" Diffuse reflectance spectra of zinc oxide dispersions in different types of zeolites, (a) NaA (Si/Al = 1.0), (b) NaX (Si/A1 = 1.3), (c) NaY (Si/A1 = 2.9), (d) NaEMT (Si/AI = 3.8) and (e) bulk Zn0, physically diluted with NaY. all samples. This can, as a consequence of the well known quantum-size (Q-size) effect [6], be attributed to sizes of the zinc oxide particles smaller than about 6 nm (table 1). It is observable that the positions of the absorption edges resulting from the main diameters of the encapsulated nano-particles depend on the Si/AI ratio of the zeolitic hosts. In A-zeolites the absorption spectrum exhibits a relatively sharp maximum at 215 nm. By comparing this result with data published in the literature, the formation of extremely small clusters with the composition [Zn40] 6+ can be assumed. Kunkely and Vogler had found an absorption maximum at 216 nm for such cluster ions stabilized in aqueous solutions by acetate counterions [7]. The clusters are formed during the dehydration step even without a prior treatment of the samples with NaOH solution, because the zinc ions attempt to maintain a favorable coordination sphere by substituting the water molecules by oxygen atoms of the zeolite framework. The best possible coordination can be achieved by the formation of [Zn40] 6+ cluster ions in the sodalite cages. Only in A-zeolites (Si/AI = 1.0) there are enough A1 atoms in the sodalite units to balance the charge of the clusters. In faujasites, however, the treatment of the ion exchanged zeolites with NaOH solution is essential for the formation ofnano particles of zinc or cadmium oxide. The higher the Si/AI ratio of the zeolitic hosts the less pronounced is the blue-shift of the absorption edges and the bigger are the formed nano-oxide particles. The diameters of the zinc oxide nano particles can be calculated from the blue-shifts by an "effective mass approximation" established by Brus [8], to approximately 3 nm for the dispersion in zeolite X, and 4-6 nm for that in the zeolites Y and EMT. The results are in good agreement with particle sizes estimated from TEM micrographs and indicate that mesopores must be created during the formation of the nano oxide particles because they exceed the width of the supercages in
faujasites.
208
3.1.2.Adsorption isotherms Evidence for the growth of the oxide particles within the zeolite framework was obtained from the observation of hysteresis loops in the adsorption/desorption cycles of cyclopentane isotherms. The hysteresis loops range from relative pressures P/P0 ~ 0.3 to P/P0 ~ 0.85 corresponding to average pore size diameters of 2-10 nm [9]. The volumes of the micro- (Vmi) and mesopores (Vine) per gram of dry zeolite can be obtained from the total amount of adsorbed probe gas a 0 at the relative pressure P/P0 = 1, the amount aj at the junction of both branches of the hysteresis loop, and the molar volume of the liquid probe molecule V with the equations Vmi = a).Vand Vine= (a0-aj)V. For dispersions of zinc oxide nano particles in different zeolites the values listed in table 1 have been calculated. Table 1: Si/Al ratios, micropore (Vmi) and mesopore volumes (Vine) of the zeolites and diameters of the formed zinc oxide nano-particles of ZnO dispersions in different types of faujasites. Sample ZnNaX ZnO/NaX ZnNaY ZnO/NaY ZnNaEMT ZnO/NaEMT
degree of ion Si/Al ratio exchange/% 20 1.3 20 1.25 40 2.9 40 2.55 40 3.8 40 3.35
treatment with NaOH . . . . . 0.1M,0.3h,293K . . . . . 0.1M,0.3h,293K . . . . . 0.1M,0.3h,293K
particle size/nm . 1-2 . 3-4 . 4-6
E 9 E e /(cm~g"1) /(cm~ "1) 0.32 0.000 0.30 0.008 0.29 0.000 0.25 0.031 0.30 0.000 0.24 0.052
Additionally it was found by 29Si-MAS-NMRthat the Si/Al ratio of the zeolites decreases with the NaOH treatment. This observation indicates that in a first step of the mesopore formation the hydroxyl ions attack at the silicon centers of the zeolites. In this context it is important to mention that a parent zeolite containing no zinc or cadmium but only sodium ions showed no mesopore formation under the same conditions. This demonstrates that the simultaneous precipitation of metal hydroxides favours the formation of mesopores. A more detailed discussion of the mechanism of the mesopore formation will be presented in a further paper [ 10]. Table 2: Formation ofCdO nano particles and mesopores of the zeolite in dependence of the treatment of CdNaX zeolites (degree of ion exchange: 20 %) with NaOH solution. Sample
treatment with
NaOH
CdNaX CdO/NaX CdO/NaX CdO/NaX CdO/NaX H
particle
E 9
. . . . . . 0.1M,0.3h,293K 1-2 0.1M,0.3h,353K 7-9 0.1M,2h,293K 10-12 0.75M,0.3h,353K ,~ 30 ill
i
E e
size/am /(om 1) /(cm~;"1) 0.31 0.31 0.27 0.25 0.22 i
0.000 0.000 0.005 0.016 0.029 i
209 The extent of mesopore formation and the diameters of the formed oxide particles can furthermore be varied by the conditions of the ion-exchange (pH value) [3] and of the treatment with NaOH solution. The latter is shown as an example in table 2 for CdO dispersions in zeolite X. The higher the concentration of the NaOH solution, the longer the time of treatment or the higher the temperature the higher the extent of mesopore formation and consequently the larger the oxide particles formed.
3.2. Dispersion of SnO 2 particles in faujasites 3.2.1.Preparation The formation of S n O 2 llano particles depends both on the preparation method and the Sn loading. The modification of NaY zeolite (Si/AI = 2.9) with Sn (II) ions (SnC12 x 2 H20 ) by conventional ion exchange causes strong acidic conditions depending both on the solvent and the concentration of the solution. A suitable pH value is required to minimize the damage of the zeolite framework. If the zeolite is exposed to exchange solutions of pH > 3 only small losses in crystallinity, expressed by the BET surface, of 5-10 % were observed. But even under these conditions the removal of alumina l~om the zeolite framework leads to the appearance of a signal of extra framework aluminum in the 27A1-MAS-NMR (~i = -1 ppm, referenced to 0.1 m A1C13 solution) and the formation of mesopores, identified again by hysteresis loops in adsorption/desorption isotherms with cyclopentane as probe molecule. Due to these conditions at high loadings (> 4 wt %) SIlO2 nano particles with a broad size range of 2-20 nm are formed as can be inferred from TEM micrographs. The particles are inhomogeneously dispersed and numerous agglomerations are obvious probably on the outer surface of the zeolite. At lower loadings (< 4 wt %) the particle size range is somewhat smaller (2-10 nm) and the dispersions are more homogenous. In contrast to that the impregnation method reveals a relative homogenous SnO 2 dispersion with small particles of 2-5 n m By this preparation conditions the creation of large mesopores as well as the formation of large agglomerates is prevented. By CVD with SnC14 tin loadings up to 2 wt.% can be achieved trader a tolerable loss of crystallinity of 10-15 %. In the UV-VIS spectra of samples prepared by CVD the absorption of the tin species cannot unambigously be distinguished from the zeolite absorption leading to just a weak absorption below 2 = 250 nm~ and on TEM micrographs no particles were observed indicating that the tin species are very highly dispersed, i.e. that possibly formed SnO 2 nano particles are smaller than 1 nm in diameter. 3.2.2.Determination of particles sizes The determination of the sizes of highly dispersed tin dioxide nano particles is in principle possible from X-ray diffractograms, UV-VIS spectra, and transmission electron microscopy. X-ray difl~actograms offaujasites modified with SnO 2 allow the "fingerprint" identification of both the zeolite framework and the formed SnO 2. The SnO 2 reflections (110), (101) and (201) are detectable if the Sn loading is > 2 wt.% indicating that the particles consist of crystalline SnO2, but the reflections are partially superimposed by reflections of the zeolite lattice which prevents a particle size determination according to the Debye-Scherrer equation. Electron diffraction patterns reveal no single reflections of SnO 2, but rings which indicate that the particles must be randomly distributed.
210 F(R) d
200
300
400
500
w a v e l e n g t h / nm
Figure 2: Diffuse reflectance spectra of(a) bulk SnO2, physically diluted with NaY (30 wt.% Sn), and dispersions of SnO 2 in Y-zeolites, prepared by: (b) and (e) ion exchange (1.1 and 4.4 wt.% Sn, resp.), (d) impregnation (4.6 wt.% Sn) and (e) CVD (1.4
wt.% Sn). The analysis of diffuse reflectance UV spectra of SnO2 modified faujasites (figure 2) is possible only for loadings higher than about 1.2 wt.% and is moreover complicated by the absorption of the base zeolite materials which must be taken into account at wavelenghts smaller than 300 nm Therefore special precautions, e.g. use of suitable reference substances, must be taken to separate the absorption of the zeolite from that of the embedded material (in this case: SnO2) [ 11]. In contrast to the relative sharp absorption edge in the spectrum of bulk SnO2 which was physically diluted with the parent NaY zeolite (figure 2a), in the spectra of zeolite stabilized SnO 2 dispersions a blue-shitted absorption with a pronounced tailing is obvious due to either a superposition of the absorption edges of differently sized nano particles or to changes in the electronic structure which may result from changes in the crystal structure or from interactions with the host. The UV-VIS spectrum of the sample obtained by impregnation (figure 2d) which consists of particles with a rather narrow size range, shows a more pronounced blue-shill as well as a steeper slope of the absorption flank with less tailing supporting the interpretation, that a superposition of particles with different diameters influences the spectra. Furthermore there are no indications for changes in the crystal structure of the SnO 2 particles from X-ray or electron diffraction. But as an additional effect on the spectra interactions of the embedded particles with the host must be taken into account as revealed by ll9Sn-MAS-NMR spectroscopy (see below). Due to the uncertainty in the interpretation of the absorption edges of the SnO 2 nano dispersions, for this systems no size determination according to the "effective mass approximation" can be performed in contrast to the zinc or cadmium oxide dispersions. Therefore we conclude that for zeolite encapsulated SnO 2 nano dispersions the transmission electron microscopy is the only reliable method for particle size determinations. Consequently all statements in chapter 3.2.1. are based on TEM results.
211 3.2.3.119Sn-l~IAS-NMR The ll9Sn MAS-NM~ of bulk SnO 2 physically mixed with parent NaY zeolite (10 wt.% SnO2) and of a zeolite sample containing relatively large SnO 2 particles (2-20 nm) is shown in figure 3. The sample containing bulk SnO 2 exhibits the usual spectrum for SnO 2 with a central band at 8 - -602.8 ppm, referenced to (CH3)4Sn, and the typical rotation side-bands [12]. The width at half height for the central band is about 800 Hz.
"--500
'-600
' 70 O -
D pxltl
Figure 3: 119 Sn-MAS-N]VIR spectra of bulk SnO 2 physically mixed with zeolite Y (lower spectrum) and SnO 2 nano particles embedded in Y-zeolite (upper spectrum). For SnO 2 particles encapsulated in the zeolitic host the same signals are found but with a distinct decrease in the signal to noise ratio of the signals. Furthermore the width at half height of the central band is increased to about 1900 Hz. In the original spectrum of the dispersion a broad, non-structurated hump between -500 and -700 nm below the signal was found probably resulting from interactions of the smallest particles. It was surpressed by cutting off the first points in the FID (free induction decay) to clarify the effects on the linewidth. For particles smaller than 3-4 nm only the broad hump without a distinct signal of SnO 2 could be identified. Sheng et al. reported a broadening of the linewidths for SnO 2 which was co-precipitated with AI203 in comparison to non-diluted bulk tin dioxide. They assumed that the [19Sn spin-lattice relaxation time (T1) increases due to dilution effects or a strong interaction between Sn and A1 [13]. Our spectra of bulk SnO 2 physically mixed with the unloaded zeolite contradicts the assmnption that the degree of dilution is important. Moreover, our results confirm that both the number of defects and the interactions with the host which increases both with decreasing particle size of the encapsulated SnO 2 particles, influence the linewidth oft he NMR signal.
3.2.4.Impedance spectroscopy of tin dioxide dispersions Impedance spectroscopy reveals for SnO 2 modified zeolite samples with high loadings a conductivity process which is caused by charges hopping or tunneling between neighbouring SnO 2 nano particles. This conductivity process response is extremly sensitive to changes of the surrounding gas atmosphere. The mechanism of the conductivity will be discussed in detail in a forthcoming paper [14].
212 4. C O N C L U S I O N S 9It is possible to stabilize dispersions of zinc and cadmium oxide or tin dioxide nano particles in the pores offaujasites. The sizes of the encapsulated particles can be determined from UVVIS spectra or transmission electron micrographs. The first method fails for SnO2 dispersions, because the absorption of the embedded particles coincides with the absorption of the parent zeolite. 9The formation ofnano particles is combined with a formation ofmesopores in the zeolite and depends therefore on the pH values of the exchange solutions and the stability of the zeolite Si/AI ratio) against the used reactants, for example NaOH solution. 9The incorporated nano particles interact strongly with the zeolitic host, observable, for example, by an increase in the linewidths of I19Sn-MAS-NMR signals and with each other as indicated by impedance spectroscopy. 5. A C K N O W L E D G E M E N T S Dr. M. Wark is member of the graduate college "Dynamische Prozesse an FestkOrperoberfl~ichen" supported by the Deutsche Forschungsgemeinschait. We thank Drs. H. Wiggers and U. Simon (University of Essen, Germany) for recording impedance spectra and Drs. J. Rathousky and A. Zukal (Heyrowsky Institute Prague, Czech Republic) for the measurement of adsorption/desorption isotherms and Dr. 1L Grof~e (University of Dortmund, Germany) for discussions concerning the NMR spectroscopy. 6. R E F E R E N C E S 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
G.D. Stucky, Stud. Surf. Sci. Catal. 85 (1994), 115 and references therein S.0zkar, G.A. Ozin, IC MSller and T. Bein, J. Amer. Chem Soc. 112 (1990), 9575 M. Wark, H.-J. Schwenn, G. Schulz-Ekloffand N,I. Jaeger, Bet. Bunsenges. Phys. Chem. 96 (1992), 1727 F. Roessner, A. Hagen, U. Mroczek, H.G. Karge and I~L-H. Steinberg, Stud. Surf. Sci. Catal. 75 (1993), 1707 W. G6pel, J. Hesse and J.N. Zemel (eds.), Sensors, VCH Weinheim, 1989 H. Weller, Adv. Mater. 5 (1993), 88 H. Kunkely and A. Vogler, J. Chem Soc.; Chem. Commun. (1990), 1204 L.E. Brus, J. Phys. Chem 90 (1986), 2555 M. Wark, G. Schulz-Eklofl~ N.I. Jaeger and A. Zukal, Stud. Surf. Sci. Catal. 69 (1991), 189 M. Wark, H. Kessler and G. Schulz-Eklofl~ in preparation J. Klaas, K. Kulawik, G. Schulz-Ekloff and N.I. Jaeger, Stud. Surf. Sci. Catal. 84 (1994), 2261 N.J. Clayden, C.M. Dobson and A. Fern, J. Chem Soc.; Dalton Trans., (1989), 843 T.-C. Sheng, P. Kirszensztejn, T.N. Bell and I.D. Gay, Catal. Lett. 23 (1994), 119 H.-J. Schwenn, U. Simon, H. Wiggers, G. Schulz-Ekloff and N.I. Jaeger, in preparation.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
213
Host- Guest Interactions in Zeolite Cavities. A. Zecchina, R. Buzzoni, S.Bordiga, F. Geobaldo w, D. Scarano, G. Ricchiardi, G. Spoto, Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universit~ di Torino, via P. Giuria 7, I-10125 Torino Italy. Abstract When molecules of low-medium proton affinity (B = N2, CO, C2H4, C2H2, propene, methylacetylene, acetonitrile and H20) are adsorbed on acidic zeolites, the resulting host-guest interactions in the cavities and in the channels are dominated by hydrogen bonding in neutral Z--H-..B adducts between the Bronsted groups Z---H (Z = zeolite framework) and the base. If the interaction is sufficiently strong and the protonated products stable (ethylene, acetylene, propene and methylacetylene) the hydrogen bonded precursors are slowly consumed with formation of olygomeric products entrapped into the zeolite cavities. Acetonitrile gives a stronger hydrogen bonding interaction not followed by protonation.Water initially gives neutral complexes; however when more than one water molecule per protonic site is present, HsO2§ species are formed. When molecules of high proton aff'mities are adsorbed (NH3 and Py), B--H§ - ionic pairs readily appear. The modifications induced on the spectrum of B - - H § by interaction with Z- can be used to probe the basicity of the negatively charged framework. At higher filling conditions BhH+...B dimers are also observed. The basic IR spectroscopy of all these hydrogen bonded systems is discussed in detail. 1. Introduction. At is well known, the zeolites cavities are often considered as special regions of the intracrystalline space where molecules are forced and guided to react together following special paths dictacted by: i) the forces acting inside the cavities; ii) the distribution of sites on the internal surfaces; iii) the spatial restrictions imposed by the dimension and shape of the cavities; iv) the defined organization of the intersecting channels imposing limitations to the chemical and diffusion paths. All these points can be collected under the synthethic definition of hostguest interactions so fruitfully used to understand many properties of supramolecular and enzymatic systems. Among the great variety of known zeolitic structures, in this contribution we shall restrict our analysis to two of the most acidic ones: H-ZSM-5 and H-MORD. This choice is dictated by the importance of these microporous solids in many acid-catalyzed reactions of great economic importance and by the abundance of data present in the specialized literature. However, we hope that many of the considerations contained in this paper can be sufficiently general to be usefully extended to other systems of different structure.
Dipartimento di Scienza dei Materiali ed Ingegneria Chimica Politecnico di Torino, corso Duca degli Abruzzi 24, 1-10129 Torino Italy
214 In particular we shall examine the following problems: i) the interaction of single molecules with increasing proton affmities with the acidic sites and the structure of the formed hydrogen bonded precursor species, ii) the subsequent proton tranfer process and the effect of the simultaneous presence of other molecules in the channels and in the cavities in favouring the proton tranfer (proton transfer as a cooperative process), iii) the role of cavities and channels dimensions in determining the structure of the protonated species, iv) the role of acidic sites concentration and v) the type of interaction occurring between the protonated species and the negatively charged walls of the zeolite cavities. The discussion of these items will be mainly based on the IR spectra of interacting species, not only because of our more specific experience in this field, but also because IR spectroscopy is an extremely sensitive tool for the study of the forces (even the weakest ones like those of the van der waals and hydrogen bonding type) acting among the molecules and between the molecules and the internal surfaces. 2. Bronsted acidity and hydrogen bonding: a short review of the perturbations induced in the IR spectra of the adsorbate and of the adsorbent.
When a base B interacts with a Bronsted acid H - - Z the formation of hydrogen bonded 1:1 adducts Z--H---B is constantly observed. If the internal modes of A and B moieties are (for the time being) not considered, only three relevant vibrational modes of the hydrogen atom (corresponding to the three degrees of freedom of the hydrogen mass) must be considered [ 1, 2]: v(Z--H...B) ~i(Z--H-..B )
Z - H stretch Z - H bending (in and out of plane)
7(Z-- H--.B) J When the Z moiety has internal structure, the two bending modes of the unperturbed Z - - H species can have distinct frequencies and the same happens after hydrogen bonds formation. A further important (low frequency) mode is the v( Z - - H---B) mode: Z....B stretch. With respect to the unperturbed v(Z--H) stretching frequency, the hydrogen bonding interaction causes: i) a downward shift Av ~: AH (interaction entalpy), ii) a parallel increase of the half-width (FWHM), because the band can be now better expressed as v( Z--H-.-B ) + nv (Z...B) and iii) a parallel increase of the intensity I [1,2]. Notice that it has been experimentally observed that the FWHM of the perturbed band is roughly 3/4 Av [ 1, 2]. The shifts Av can vary from few tens of cm -1 (extremely weak perturbations) to 300-400 cm 1 (weak-medium hydrogen bonds) to 1000-2000 cm-' (strong and very strong hydrogen bonds). Simultaneously the half width of the perturbed Z---H stretch can gradually increase from a few tens to 1500 cm -'. In this last case the band is so broad and covers a so large frequency interval that the possibility of mixing with other modes (either with fundamental or overtones and combination character) becomes very important, making the profile analysis a complicated matter [3]. It is useful to recall that the v(Z--H.--B) usually falls in a spectroscopic region where the zeolites are fully transmitting the IR radiation. The in and out of plane bending modes are shifted above the frequency of the unperturbed molecule (with broadening). The shift (and the associated broadening) is usually smaller than that observed for the stretching mode. A schematic illustration of the evolution of the v, ~i and ~/ modes with the increase of the strength of the hydrogen bonding interaction is illustrated in Fig. 1 (the frequencies of the unpertur-bed and perturbed modes are in rough agreement with
215 the literature data obtai-ned on zeolites and in homogeneous conditions) [1-4]. In this scheme, the complex effects on the A A AI band shape of the broad v(ZmH.--B) mode, deriving from resonance effects with the overtones and combinations of the 8 and 1, fundamen-tals are omitted. These effects should be particularly relevant in spectra d-e, be-cause the frequencies of the v(Z--H..-B) and of the 2x8 modes fall in the same range and consequently are suita-ble to give a strong Fermi-type reson-ance with appearance of an Evans win-dow. Similarly in spectra f-g, the effect of anharmonic mixing of the v(Z--H---B) with the corresponding 8 and 1, modes (now heavily superimposed) is not 2 oo lo'oo 4000 considered at all, although it is known wavenumber c m -1 that it deeply alters the band shape [3]. As a final point let us remark that all the Figure 1. Schematic illustration of the evolution of the considerations developed so far, did not v, 8 and "ymodes of the (ZH...B) groups with the increase consider the internal structure of the of the strength of the hydrogen bonding interaction. base B, with the associated (internal) overtones and fundamental modes which could either couple or direcOy mix with the v, 8 and ~/fundamentals and so contribute to make the band profile of the v(Z---H---B) mode more and more complicated. In conclusion the sequence of spectra reported in Fig.2 is highly simplified and must be simply considered as a useful frame for the understanding of the very basic manifestations of hydrogen bonding when the proton affinity of the base is gradually increased. On zeolites, the 8 and 7 modes cannot be always observed because they usually fall in the region where the skeletal modes of the zeolite are strongly absorbing. Moreover, due to their lower frequency, they can easily mix with these modes (zeolite framework stretching and bending modes). Finally the A.-.B stretch, usually in the 20-200 cm -1 interval [ 1-3], cannot be observed at all with the usual FI'IR spectrometers. As it is well known, the typical Bronsted sites of H-ZSM-5 and H-MORD [ZH = A1OHSi (Z = A1OSi )] are characterized by stretching bands at 3609 cm-' (H-ZSM-5) and 3612 cm -1 (HMORD) [5, 6]. Basic molecules entering the channels, form hydrogen bonded species with the acidic groups shifting their stretching frequency to lower values, the shift being proportional to the interaction energy and to the FHWM. As far as H-ZSM-5 is concerned, this is shown schematically in Fig. 2a for the series of bases of increasing proton affinities N2, CO, C2H2, C2H4, propene, methylacetylene, acetonitrile and H20 [7,8,9,10]. In this representation, the frequency and the FWHM of the peaks are in agreement with the experimental results.When the shift Av is reported against the FWHM, the diagrams reported in Fig.2b are obtained, which are extremely similar to those well known in homogeneous solutions. Entirely similar data have been obtained on H-MORD. The data reported in Fig 2a, 2b have been obtained after the completion of the reaction 2) Z w H + B ~ Z--H---B
A_v
8A YA
AA
aA
216
to
~ o. o o o.. o~ -,o • /ox
8 ~5
J
!
3500
3000
2500
wavenumber cm
2000
~
-1
Fig. 2a: Evolution of the v(Z---H...B) band caused by hydrogen bonds with bases (B) of increasing proton affinities. i.e.under conditions where the channels and cavities are far from being completely filled by the adsorbates. It must be recalled that the shape of the Z--H...B stretching bands of the CH3CN and H/O adducts are heavily modified by the presence of a prominent Evans window at 2630 cm ~ caused by a Fermi resonance with 900 the 8(Z--H-..B) mode localized a t - 1315 cm -1 (vide infra) as expected on the basis of 750 the previous discussion and as already demonstrated in ref [9,10] (two false bands being actually observed as shown schemati600 cally in Fig. 2a). The band corresponding to water is not the only one present in the "1450 stretching region: for Fig.2a we have simply LL selected the most intense, corresponding to 300 the strongest O--H...O hydrogen bond (vide
infra). 150
2~o
480
88o
~v
c m "1
88o
lo'oo
Figure 2b. Relation between the frequency shifts (Av) of the acidic OH groups and the FWHM.
Let us recall that the case of H20 has been the source of an interesting debate, since some autors think that beside hydrogen bonded species also the protonated ones can be present [4 and references therein]. The problem is not of simple solution either from the theoretical and from the spectroscopic point of view because the various types of hypothesized species have very similar energy [4] and have quite complicated IR spectra (owing to the presence of two OH
217 groups in the base) which can be distinguished only with some difficulty. 3. From the hydrogen bonded precursors to the protonated species: the ethene, propene and acetylene cases. After the formation of the hydrogen bonded precursors with the x-electrons of unsaturated hydrocarbons, a slow reaction is usually observed leading to the formation of protonated species. This species are: - ethene: saturated oligomers Z-CH2CH3, Z-CHzCH2CH2CH3 etc. The Z-C bond is partially covalent [8] - propene: saturated oligomers. Also in this case the Z-C bond is quite covalent [8]. - acetylene: Z-..[(CH=CH)n-CH=CH2] + polyacetylenic c h a i n s w i t h blue-violet color where the positive charge is delocalized on the whole chain [8]. On H-ZSM-5 the length of the chains is dictated by the distance between the intersecting channels (which is also the distance between he growing oligomeric chains initiated at different sites). We can clearly see here how the structure of the zeolite is determining also the structure of the reaction products. The protonation speed is faster in the case of propene, which is also forming the strongest hydrogen bonded precursor of the whole series of investigated hydrocarbons. The protonation reaction can be suppressed by lowering the temperature of the sample to - 200 K: under these conditions the hydrogen bonded precursors are stable. On H-MORD the situation is very similar with only one exception concerning the interaction with propene. In fact when the interaction is made at RT, the high number of olztgomefic chains (consequential to the high concentration of Bronsted sites present in our mordenite samples characterizes by Si/Al = 5), quickly formed at the channels mouths, is sufficient to block the penetration of propene in the internal spaces. The spectrum of the hydrogen bonded precursor cannot consequently be observed with sufficient intensity. This obstacle can be overcome by conducting the experiment at -200 K, i.e. at a temperature where the protonation reaction is suppressed: the schematic spectrum represented in fig 2a has been obtained in this way [ 11]. It is worth to consider that even with propene (which gives the fastest reaction at RT) the number of hydrocarbon molecules inserted into the Z - - H bond and into the growing chain is not exceeding (in average) the number o f - 1 per second [8]: this indicates that a high activation energy barrier is present for both protonation and insertion reactions. That the proton transfer reaction is slow, can be easily understood on the basis of the moderate strenght of hydrogen bonding interaction between the hydrocarbon and the acidic group in the precursor species. Despite this unfavourable kinetic factor, we can always observe the formation of a quantity of saturated and unsaturated oligomers sufficient to fill all the available internal space. These oligomeric species a r e thermodynamically very stable and cannot be depolymerized by decreasing the hydrocarbon pressure at RT. When the the Z--H---NCCH3 complex is considered, we notice that the involved hydrogen bonding interaction is stronger than that found before for the hydrocarbons: however no traces of protonated species Z--..*I-I--NCCH3 is observed at low Idling conditions [9]. We think that this is due to the low stability of the iminium species and to the insufficient stabilization of this positive species by hydrogen bonding with the weak Z- base. In principle the situation could change in presence of excess CH3CN (which is a base stronger than Z-) because the CH3CN--+H--NCCH3 species could be stabilized by a stronger hydrogen bonding interaction. We did not obtain definite proofs that this is really happening when the pores are completely f'dled by CH3CN. This does not means that nothing significant is happening in presence of an excess of
218 adsorbate: in fact the spectroscopic properties of the hydrogen bonded species are greatly influenced possibly because of the modification of the dielectric constant occurring during the pore filling [ 12].
4. On the formation of H2n+IOn + species through cooperative effects. The IR spectra of increasing doses of H 2 0 adsorbed on H-ZSM-5 are illustrated in Fig 3 (difference spectra). The full line spectra correspond closely to those already published [ 10] for H+/H20 ratios comprised in the 0-1 interval, the only difference being represented by appearance of a clear peak at 1315 cm -1 (not reported before). Another important feature HzO/H-ZSM-5 appearing upon water dosage (not J~ reported for sake of brevity), is ',i 'J' \ observed at 875 cm -1, i.e. in the small I, i| ; characteristic transmission window 1: ' h separating the two families of it j~11 , ' " .~..r r . i Ill stretching modes of the TOn building %1%~ units [12]. The negative peaks at 3610 and 3745 cm -~ correspond to consumed 'Ill ~ s'" '%'%# li.l 11 species (Bronsted sites and silanols ii,. s i ~ ** , | stretching modes). The reported specIll, %..,,.' , "~ t i t ~ 9 , , ,,~ tra are better explained on the basis of .oI~1 ~ .""* II~tl \ " " ' ' e, the dominant hydrogen bonded struc.." , -- :, ture: el t "14~ ~" " . " " ~# l, I II llt H I
.
...a
9
-',.,I
ia'"
OI
I
I
I
I
3500
3000
2500
2000
1500
i
w a v e n u m b e r c m "1
In particular: i) the narrow peaks in the 3680-3697 cm -~ interval belong to Figure 3. Background subtracted spectra of increasing the v(OH) of the external OH (a) doses of H20 adsorbed on H-ZSM-5 outgassed at 673 K. group of adsorbed water in two slightly different locations; ii) the broader absorption at - 3550-3530 cm -1 ( FWHM --- 200 cm-1), partially overlapped to the negative peak of the Bronsted sites, is the stretching mode of the second OH group of water (e) interacting with the negative oxygen group of the framework via a weak hydrogen bonding; iii) the two apparent bands at 2900 and 2450 cm -] belong to a nearly symmetric absorption centred at 2650 cm -1 with b'WHM - 800 cm -] (see Fig. 2a) and are generated by an Evans window at the centre of the adsorption caused by a Fermi resonance effect with the 8 mode of the [}(OH) group (Br~3nsted group) at 1315 cm -1 (following the empirical v(OH..)-do_o correlation well established for hydrogen bonding interactions in homogeneous phases [ 1-3], a do+ distance in the 2.6-2.65 A interval is inferred, in complete agreement with the structure proposed by Sauer et al. [ 14,4]); iv) the absorption at 1670-1620 cm -~ (highly asymmetric on the high frequency side)
219
corresponds to the bending mode of water; v) the band at 1315 cm -~ is the in plane bending ~5 mode of the Brrnsted group (at - 1050 cm -~ in vacuo) [15,16,4] upward shifted by the strong hydrogen bonding interaction; vi) the band at 875 cm -1 is the ? mode of the Br~3nsted group (at400 cm -1 in vacuo) upward shifted by the strong hydrogen bonding. When further doses of water are adsorbed (corresponding to the progressive filling of the channels and cavities and also with the formation of weak hydrogen bonding interactions with the external silanols" broken spectra) four main effects are observed, i.e." i) the disappearance of the 1315 cm -~ band; ii) the simultaneous disappearance of the Evans window; iii) the disappearance of the 875 cm -1 band; iv) the formation of a broad continuum in the 2250-1300 cm -1 interval, associated with an extremely broad absorption due to very strong hydrogen bonds typical of solvated protons [ 17]. Of course also the manifestations of liquid water (bands 3300 and 1620 cm -1) are clearly emerging. All these facts indicate that for H20/H + ratios > 1, a proton tranfer occurs through the agency of further water molecules and that the situation becomes similar to that of a concentrated acid solution, as it can be easily verified by comparing these spectra with those of concentrated HC1, H2SO4 etc [18]. The two background oscillations labelled with an asterisk, appear any time an adsorbate is interacting with the framework and do not depend upon the structure of the adsorbed molecule. A detailed discussion of their origin is given elsewhere [ 19]. A similar series of spectra has also been found on H-MORD" however in this case, due to the higher concentration of protons, the final spectrum obtained at high pore filling conditions is similar to that of an even more concentrated acid solution characterized by H20/H + =_ 3 [ 18]. Coming back to the H-ZSM-5/H20 system, it is worth to remark that the formation of the broad background in the 2000-1300 cm -~ range, already starts before the full disappearance of the free Bronsted v ( Z - - H ) band: this likely means that in the vicinity of the H20/H + - 1 stoichiometric ratio, free Brrnsted groups, monomeric hydrogen bonded species, dimeric H502 + and polymeric HsO2+-n(H20) species are in mutual equilibrium.
5. The proton transfer occurring with strong bases (NH3 and Py): the IR spectroscopy of NH4+-'-Z" and PyH§ - (Z = zeolite) ionic pairs. Strong bases like NH3 and Py interacting with Z - - H , immediatly gives the protonated NHn+--'Z- and PyH+---Z- ionic pairs. The strength of the interaction within the moieties of the ionic pairs will depend upon: i) the acidic character of the N-H groups of NH4 + and PyH+; ii) the basic strength of Z-. As Z - - H is a strong acid, the coniugated Z- base will be consequently weak; the same can be concluded about the acidity of NH4 + and PyH +. On this basis the hydrogen bonding interaction between the two moieties of the ionic pairs is expected to be rather weak. However as this interaction is sufficient to perturb the spectra of NH4 + and PyH +, we can use the NH4 + and PyH + species as probes of the framework basicity of Z- (exactly like we have used weak B bases as probes of the Brrnsted acidity). The spectra of increasing doses of NH3 adsorbed on H-MORD are illustrated in Fig. 4 (spectrum 7 is corresponding to NH3/ H + - 1). Similar spectra have been obtained on H-ZSM-5 (not illustrated for brevity). As far as the spectra 1-6 are concerned, we notice: i) the progressive consumption of the Brtinsted sites band at 3610 cm -1 (negative band); ii) the proportional growth of an intense peak with maximum at 1440 cm -1 (and a shoulder at -1420 cm -1) assigned to the v4 mode of the NH4 + (with T2 symmetry in the free ion); iii) the formation of a weak absorption at 1680 cm -1 assigned to the v2 mode of NH4 + (with E symmetry and only Raman active in the free ion) iv) the proportional growth of a complex band in the 3400-3150 cm -1 interval, with maximum at 3254 and
220 shoulders at 3370 and -3200 cm -1, corresponding to the v(N-H) of NH bonds of NH4 + not perturbed by hydrogen bonding interaction; v) the parallel growth of a broad and 1complex absorption in the 3150 2250 cm -1 interval with apparent maxima at -3000 and -2800 cm -1 (or minima at 3120 and 2880 cm -1) t.and a shoulder a t - 2650 cm-1; the O low frequency tail of this band t-" t~ seems to extend down to 2000 cm -1 t2 O where an apparent maximum at ffl ! r~ 0 2150 (or a minimum at 2250 cm -1) I, 1 I I is clearly observable. This broad and L._ v2 + V4 , ' complex absorption is the spectros2 XV 4 copic manifestation of the v ( N m H§ -) modes shifted to lower frequency by the hydrogen bonding perturbation. The complexity of this band is not necessarily the result of I I I I I 3000 2500 2000 1500 the presence of different types of interacting groups. In fact many of -I wavenumber cm the components previously mentioned, are clearly generated by Fermi resonance effects with low frequenFigure 4. Background subtracted spectra of increasing doses cy modes. For instance the first of NH3 adsorbed on H-Mord outgassed at 673 K. overtone of the mode at 1440 cm -1 corresponds closely with the apparent minimum at 2880 c m -1 (Evans window); similarly the other minimum at 3120 is explained by a second Evans resonance with the 1440 + 1680 c m -1 combination. For sake of brevity, in this paper we shall 00t go further in the interpretation of the NH4 § spectrum. Let us simply remark that the observed frequencies and the assignments closely correspond to theoretical calculations [4 and ref. therein) even if we are not yet in the position to choose among the different sitings of the NH4 § Other features (labelled with an asterisk) are also observable in the 2000-1750 c m -1 interval: these features are always observed upon adsorption of molecules and are consequently assigned to framework overtones modifications [19] not dependent upon the structure of the adsorbed molecule. With spectra 8 and 9 we start to clearly observe the modes of neutral NH3 (Vl: 3375, v3: 3315, v4:1625 cm -1) which is a base stronger than Z-. The presence of neutral NH3, not only means that the titration of the acid sites is reaching the final point, but also that the reaction
NH3/H-Mord
4)
Z-.-.NH4 + + NH3 ~ Z- + H3NmH +... NH3
is taking place with formation of new hydrogen bonded dimers (first step of NH4 § solvation with NH3). Also this problem will not be analyzed in detail for sake of brevity. This phenomenon is similar to that observed for water and once more suggest that the structure of the species present in the channels and cavities, is greatly infuenced by cooperative effects among the molecules.
221 As a final point, we illustrate the interaction of Py with H-ZSM-5 leading to the full consumption of Z - - H groups and stoichiometric formation of PyH +... Z- ionic pairs (Fig .5). For sake of brevity only one spec-trum obtained after exposure to excess Py and successive prolon-ged outgassing at RT is reported. Due to the absence of Py excess, the positive bands belong mainly to the PyH§ - ionic pair 0.8 only ( while the negative ones Py-H-ZSM-5 mainly correspond to consumed BrSnsted sites and to a small amount of silanols). We can divide the sprec-trum into two regions (3300-2000 and 1700-1300 cm-1). In the first region a very broad v(~t-w...z) band centred at --2800 cm -1 0.4(FWHM --_ 500-600 cm -1) is cundoubtely the v(NmH+---Z -) cstretching mode of the Py~H+-.-Z c~ ionic pair. As shemati-cally I I o reported in the figure, the bands ffl indicated with broken lines and modulating the v(Py~H+...Z -) stretch, are Fermi resonances with 0.0the combination and overtones of the ring (8a, 8b, 19a and 19b) and the ~5(Py--H+.--Z-) modes falling in the 1700-1300 cm -1 interval [20]. The detailed assignment will be I I I I I published elsewhere [19]. For the -1 time being, let us stress once more w a v e n u m b e r cm the importance of Fermi resonance effects, in explaining the high complexity of the spectra of Figure 5. Background subtracted spectrum of Py on Hstrongly hydrogen bonded systems. ZSM-5. The spectrum has been obtained after exposure Similarly to what observed with to Py excess and successive prolonged outgassing at RT. NH3, when higher filling conditions are investigated, PyH+...Py dimers are formed, characterized by a v (PymH+...Py) absorbing at lower frequency (because Py behaves as a base stronger than Z-). In a separate paper [19] we demonstrate that the band a t - 2200 cm -~ must be assigned to a small amount of the dimeric species. Conclusions. The host guest interactions in acidic zeolites are dominated by hydrogen bonding in neutral ZH...B complexes (B: weak base like N2, CO, C2H4, C2H2, propene, methylacetylene, acetonitrile and water) or in ionic Z-...+I-I~B pairs (B: strong base like NH3 and Py). The IR spectroscopy of these complexes is quite intriguing because of the presence of many types of resonance effects which modify the bands profile. In the case of H20, the cooperative effect of further water
222 molecules, favours the proton transfer and the formation of solvated HsOE+-nH20 clusters. The modifications induced on the spectrum of B--H + by interaction with Z- can be used to probe the basicity of the negatively charged framework.
Aknowledgements This work has been supported by the MURST and CNR (Progetto Strategico Tecnologie Chimiche Innovative)
References 1. G. C. Pimentel and A. L. McClellan, "The Hydrogen bond", W.H. Freeman and company Ed., London, 1960. 2. D. Hadzi and S. Bratos, in "The Hydrogen Bond/II Structure and Spectroscopy", P. Schuster, G. Zundel C. Sandorfy Eds., North-Holland Amsterdam, 1976. 3. C. Sandorfy, in "The Hydrogen bond/II Structure and Spectroscopy", P. Schuster,G. Zundel C. Sandorfy Eds., North-Holland Amsterdam, 1976. 4. J. Sauer, P. Ugliengo, E. Garrone and V.R. Saunders, Chem. Rev., 94 (1994) 2095. 5. A. Zecchina, S. Bordiga, G. Spoto, D. Scarano, G. Petrini, G. Leofanti, M. Padovan, C. Otero Are~in, J. Chem. Soc. Faraday Trans., 88 (1992) 2959. 6. S. Bordiga, C. Lamberti, F. Geobaldo, A. Zecchina, G. Turnes Palomino and C. Otero A r e s , Langmuir, 11 (1995) 527. 7. T. Yamazaki, I. Watanuki, S. Ozawa and Y. Ogino, Bull. Chem. Soc. Jpn., 61 (1988) 1039. 8. G. Spoto, S. Bordiga, G. Ricchiardi, D, Scarano, A. Zecchina and E. Borello, J. Chem. Soc. Faraday Trans., 90 (1994) 2827. and S. Bordiga, G. Ricchiardi, G. Spoto, D. Scarano, L. Camelli, A. Zecchina and C. Otero Are~in, J. Chem. Soc. Faraday Trans., 89 (1993) 1843. 9. A.G. Pelmenschikov, R. A van Santen, J. J~chen and E. Meijer., J. Phys Chem., 97 (1993) 11071. 10. A. Jentis and G. Warecka, M. Derwinski and J. A. Lercher, J. Phys. Chem., 93 (1989) 4837. 11. F. Geobaldo, C. Lamberti, D. Scarano, G. Spoto and A. Zecchina, in preparation. 12. S. Bordiga, R. Buzzoni, A. Zecchina and G. Spoto, in preparation. 13. D. Scarano, A. Zecchina, S. Bordiga, F. Geobaldo, G. Spoto, G. Petrini, M. Padovan and G. Tozzola, J. Chem. Soc. Faraday Trans., 89 (1993) 4123. 14. J. Sauer and M. M/iser, J. Phys. Chem., 98 (1994) 3083. 15. W.P.J.H. Jacobs, J.H.M.C. van Wolput and R.A. van Santen, Zeolites, 14 (1994) 117. 16. H. Jobic and M. Gzjzek, J. Phys Chem., 96 (1992) 1540. 17. J. M. Williams, in "The Hydrogen Bond/II Structure and Spectroscopy", P. Schuster,G. Zundel C. Sandorfy Eds., North-Holland Amsterdam, 1976. 18. R. Buzzoni, S. Bordiga, G. Ricchiardi, G. Spoto and A. Zecchina, J. Phys. Chem. submitted 19. S. Bordiga, R. Buzzoni, G. Ricchiardi, C. Lamberti, G. Bellussi, A. Zecchina, Langmuir to be submitted 20. V. P. Glazunov and S.E. Odinokov, Spectrochimica Acta, 38A (1982) 399.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
223
Diffusion in Zeolites Douglas M. Ruthven Department of Chemical Engineering, University of Maine, Orono, ME, 04469
Abstract
Recent developments in the study of intracrystalline diffusion in zeolites by novel macroscopic methods and the results obtained by some of these methods are reviewed. For many systems there is a significant discrepancy between the macroscopic and microscopic (QENS, PFG NMR) diffusivity values. A possible explanation is suggested.
1. INTRODUCTION The earliest systematic studies of diffusion in zeolites (1) were reported in the 1930s yet, despite more than sixty years of study our understanding of this subject remains fragmentary and incomplete in many important aspects. The early expectation that, because of their structural regularity, zeolites would provide simple model systems for the study of micropore diffusion has been fulfilled to only a limited extent. While it is true that many of the broad features of the diffusional behaviour of these systems can be understood, in a general way, in terms of simple concepts (e.g. the size of the sorbate molecule relative to the channel diameter) the details are often confusing with many anomalies and inconsistencies. For many systems it is still unclear whether the apparent complexity is real or merely the result of poor experimentation and incorrect interpretation of the experimental data. For example, it is now well established that many of the earlier uptake rate measurements were corrupted by the intrusion of processes other than intracrystalline diffusion (external mass transfer resistance, heat transfer etc. (2-6)yet the reported intracrystalline diffusivity values continue to be quoted in the secondary literature. The "window effect", which is discussed in greater detail below, provides another example. The anomalous variation of diffusivity with chain length for linear alkanes in zeolite T reported by Gorring (7) has been quoted in many of the standard reference texts (8-11)and a detailed theory has been derived to account for this behaviour (12'13). Yet more recent experiments in a series of very similar zeolites (14) failed to reproduce the claimed effect, thus casting doubt on the validity of the interpretation of the original experiments. Diffusion in zeolites has been reviewed in detail by K/irger and Ruthven (is) and Rees 06). Therefore, in the present paper no attempt is made to provide an overall review; instead a number of recent topics related to the measurement and understanding of intracrystalline diffusion have been selected for more detailed discussion.
224 2. EXPERIMENTAL METHODS
A wide range of different methods have been developed for the measurement of intracrystalline diffusion. These can be divided into macroscopic methods (in which a transport flux is measured) and microscopic methods (in which the movement of the molecules is tracked directly)--see Table 1. The development of the PFG NMR technique in the mid-1970s 0719) represented a major milestone in that this approach provided the first reliable microscopic measurements of self-diffusivity. That the NMR values were orders of magnitude larger than most of the then accepted macroscopic values provided the first clear indication that the validity of much of the macroscopic data had to be questioned. Re-examination of the experimental conditions led to the conclusion that the impact of heat effects and extra-crystalline diffusion was much more significant than had been originally assumed C26). This stimulated the development of a range of more sophisticated macroscopic methods aimed at minimizing extraneous effects. Some of the recently reported results obtained with four such techniques are summarized below. Table 1
Summary of Experimental Techniques for Measuring Micropore Diffusion Macroscopic Methods /
Microscopic Methods NMR- Relaxation Times
Steady State
Transient
QENS
Membrane Permeation
Uptake Rate
PFG NMR
Effectiveness Factor
L
l
Chromatography DkeetMethods ZLC
IR Method Freq. Response
In comparing diffusivities measured by microscopic and macroscopic methods caution is needed. Most microscopic techniques measure the self-diffusivity ( ~ ) while most macroscopic techniques (except for tracer methods) measure the transport diffusivity (D). These quantities are different and for highly non-linear systems the difference may be more than an order of magnitude. A more meaningful comparison is between the self-diffusivity and the thermodynamically corrected diffusivity (Do), which is related to the transport diffusivity by the Darken expression: D = D O (denp/denC)T
(1)
where (denp/denC)T represents the gradient of the equilibrium isotherm in logarithmic
225 coordinates. For a Langmuir isotherm D = Do/(1-0 ) where 0 represents the fractional saturation, so the strong concentration dependence of D is evident. Although precise coincidence between and D O can be expected only in the low concentration limit, rough agreement (within a factor of 2 or 3) is to be expected over the entire concentration range (15). 2.1 Embedded Crystals Caro et al. (20) devised a technique to prepare oriented arrays of large silicalite crystals which were then coated with an impermeable copper film by a sputtering process. By careful abrasion, different faces of the crystal could be exposed, thus allowing diffusion in different directions to be measured. Diffusion (of n-hexane)in the transverse direction is much faster than diffusion along the crystal length, as is to be expected from the dimensiom of the crystals. However when diffusivities are estimated from the half-time of uptake the transverse diffusivity is found to be about three times the value for the longitudinal direction. These diffusivity values are close to the self-diffusivities obtained from PFG NMR measurements which show a similar degree of non-isotropy. However, this agreement is somewhat misleading since, for a proper comparison with NMR selfdiffusivities these values should be reduced by about an order of magnitude to allow for the Darken correction factor. Furthermore, the shape of the uptake curves does not conform well to the simple diffusion model, suggesting that processes other than simple diffusion may be significant. Experimental data showing the wide range of reported diffusivity values for this system are included in Table 2. Table 2 Diffusivity Data (~, D O xl06 cm2.s "1) for Linear Alkanes in Silicalite at 300K.
Method
~2H6
C3H 8
C6H14
Reference
Hernandez et al. (60) MD 6-7 --6 Caro et al. (37) PFG NMR 54 40 Heink et al. 21) 0.5 PFG NMR 1-12 Jobic et al. (22) 5 QENS 20 12 Kapteijn et al. (57) Membrane 2-6 0.65 Paravar and Hayhurst (25) 0.01 Membrane 0.02 0.07 van den Begin et al. (23'40) 0.2-0.3 FR 30 6 Hufton and Ruthven (24) 0.01 ZLC 1-1.5 Hufton and Danner (38) GC 1.5 0.4 Caro et al. (2~ 0.18 Uptake 10-310-7 Various authors (263~ Uptake M ) = Molecular dynamic calculations; PFG NMR = Pulsed field gradient nuclear magnetic resonance; QENS = Quasi-elastic neutron scattering; ZLC = Zero length column; GC = Gas chromatography; FR = Frequency response 2.2 ZLC (Zero Length Column) The ZLC method, introduced in the 1980s (31'32), is a very simple technique aimed at eliminating the intrusion of extraneous heat and mass transfer processes in a chromatographic flow system.
226 A very small quantity of sample (< 1 mg) is equilibrated with a low concentration of sorbate in an inert carrier (He or Ar) and then purged at a high flow rate under conditions such that the desorption rate is controlled by diffusion out of the crystal, rather than by convection. Since in a desorption experiment the baseline concentration is zero, with a sensitive detector (e.g. FID or mass spec.) the desorption rate can be measured accurately even at very low concentration levels. Consistency between limiting diffusivity values (Do) determined by the ZLC method and the NMR self-diffusivities has been demonstrated for several systems--see for example Figure 1. The original ZLC method is limited to the measurement of the limiting diffusivity at low sorbate concentrations (within the Henry region of the isotherm). However, in a recent modification, by the use of an isotopically tagged tracer (tracer ZLC)(a33) the method was extended to allow the measurement of tracer exchange (self) diffusivities over the entire concentration range. The sample is equilibrated with a known partial pressure of sorbate containing a proportion of the labelled species and the flow is then switched to a stream containing the same mole fraction of sorbate, all in the unlabelled form. The desorption of the labelled species is followed by mass spectrometry. For hydrocarbon sorbates the use of deuterated and non-deuterated forms is convenient although other kinds of isotopic labelling could also be used. A key advantage of this method is that thermal effects are eliminated although there remains the possibility of external diffusion resistance. Some of the recent results obtained by this method are summarized in Figure 2. For propane--5A (33) we find excellent agreement between the tracer ZLC data and the traditional ZLC values at low concentration. There is also reasonable agreement with the PFG NMR data--the trends being similar but with some difference in the absolute values. For methanol-NaX (35) the agreement with the NMR data is almost quantitative and the somewhat unusual maximum in the trend of diffusivity with loading is confirmed by both techniques. However, for propane and propene in NaX (34) there is a serious discrepancy, with the PFG NMR (36'37) data yielding much higher diffusivities, a much greater difference between paraffinic and olefinic sorbates and the opposite trend with loading. iO-s
Xe 0
zl0
ZLC A
CO 2
9
NMR
~
9 o
IO-B
Figure 1. Comparison of ZLC (Do) (44) and PFG NMR 05) ( ~ ) diffusivities for Xe and CO 2 in large crystals of 5A zeolite. From Ruthven (43). The recent FR data of Onyestyak et al. (4x) are also indicated ( - - - ) .
I I 10-7
10-8
I
2.0
3.0
,
I
4.0 103/T (K -) )
5.~
6.0
227
(b)
Ca) 1 E-07
'
3x10 -~
PFG
-I
r--:- - I - - ' -
-I
'
I
--'
I
lxlO -II ZLC ( t r a c e r )
.<
%
1 E-O8 o
c
lOO .... ZLC
o9 Tracer
Propane- 5A
.[] 9 .PFG
ZLC
NMR
85~ C 1 E-Os
l X 1{3-12
0
0.5
1
1.5
2
2.5
3
3.5
,
{3
q (m01ecule/cage)
I
,
2
I
4
,
I
G
,
I
8
,
~
,
1o
q (molecules / cage)
(c)
D ~
"~~ropane
PFG NMR 30oK
Cyclopropane
~
~b Propene
Tracer ZLC 358K
o o /
O
~ _ g t _ . - - - ---~~
Propane a Propene o
I 1
1 2
I 3
I 4
I 5
6
q (molecule/cage)
Figure 2. Comparison of self-diffusivities measured by PFG N M R and tracer ZLC for Ca) Propane-5A (33 '37) ; (b) Methanol-NaX (35) ; (c) Propane and propene in N a X (34 '36 '37) .
228 2.3 Frequency Response The frequency response technique, in which the volume of a system containing a dispersed sample of adsorbent under a known pressure of sorbate is subjected to a sinusoidal perturbation, has been applied to several zeolite systems by Yasuda (39) and Rees C4~ In essence the diffusional time constant is deduced by matching the resonance peak in the spectrum of the phase lag of the pressure response as a function of frequency, to the theoretical curve for a diffusion controlled process. Being a quasisteady state method this approach has some advantages and for many systems the diffusivity values obtained are close to the NMR data--see Table 2. However, this is not always true and for the CO2-5A system (Figure 1) the frequency response data yield much smaller diffusivities than the PFG NMR or ZLC values. The argument that the cyclic nature of the perturbation eliminates the intrusion of thermal effects must be treated with caution. For rigid linear molecules (p-xylene, 2-butyne) in silicalite Shen and Rees C42'47~ observed a bimodal response and they interpreted the two peaks as indicative of two different diffusion processes corresponding to transport in the straight and sinusoidal channels. There is some NMR evidence to support the view that such molecules cannot easily re-orient within the channels so this explanation is certainly plausible. However, Sun and Bourdin C46)have shown that an alternative explanation is also possible. If the heat balance equations are included in the model the predicted response becomes bimodal and the heat transfer parameters required to match the experimental data appear to be physically reasonable. 2.4 IR Methods Karge C5~ introduced a novel method of following the uptake in a transient sorption experiment by monitoring the intensity of an IR band for the adsorbed species. The approach is novel but is of course limited by the usual difficulties associated with uptake rate measurements. Results for the diffusion of benzene and p-xylene in silicalite are similar to the data obtained by Richard and Ruthven C5"0 and do not show the large difference in diffusivity observed by Shen and Rees C48'49~(see Table 3). Table 3 Diffusivity Data (~, Doxl0acm2.sl) for Benzene and p-Xylene in Silicalite at 395K Method FR Sq. Wave FTIR ZLC
Benzene
p-Xylene
Reference
0.1 0.5 0.1 0.5
1-10
Shen and Rees (48'49) van den Begin C23) KargeCS~ Ruthven C51)
0.1 0.8
An important new experimental method based on the use of a sensitive IR detector to measure the temperature of the adsorbent sample was introduced by Meunier, Grenier and their co-workers at the CNRS laboratory (52"s3~. Rather than attempting to eliminate heat effects the temperature response is used, with the heat balance, as a measure of the sorption rate. The advantage is that the time constant associated with the IR detector
229 2.50
o
2.00
Step A (N~ 48.380.2 Pa ..... Experiment M o d e l w i t h Doe-- 3.6 - 2.3 W m - K -~
I 0 -Iz
2-1
rns
0)
293 K 1.5o 05 d.)
f:~ 1.oo 0
0.50
0.00
1 --]-L---~-V~t
....
I '
t'/2 (s!/~)
Figure 3. Temperature response (measured by IR) for sample of NaX zeolite subjected to a step change in methanol pressure. From Grenier e t a l . (52). is extremely small, so relatively fast processes can be followed. A typical response curve, for the system methanol--NaX, is shown in Figure 3. The initial uptake is effectively determined by the diffusion process while the tail at long times is due to heat dissipation. The time constants for both the diffusion and heat transfer processes can therefore be determined with reasonable confidence even from a single response curve. Of course the heat transfer parameters should depend only modestly on the nature of the sorbate so, with the accumulation of experimental data for different sorbates under different conditions the heat transfer characteristics can be established, thus allowing the diffusional time constant to be measured with greater confidence. This permits additional information concerning the nature of the mass transfer resistance (surface barrier/internal diffusional) to be deduced from a detailed analysis of the form of the response curve. For the methanol-NaX system, results obtained by this method were shown to be consistent with both ZLC and PFG NMR data 02). 2.5 IR-Frequency Response In a recent development of the IR temperature monitoring approach the system was adapted to allow frequency response measurements in which both the pressure and sorbate temperature are followed in response to a sinusoidal perturbation of the system volume (54~s). The main advantage of this approach is that it allows a more confident discrimination between different mass transfer mechanisms (surface barrier/internal diffusion). 3. ZEOLITE MEMBRANES The construction of a coherent zeolite membrane that is sufficiently thin to give reasonably high permeation rates, while maintaining the high intrinsic selectivity which is theoretically possible for a diffusion controlled system, has been a long-time goal of zeolite researchers. The group at Delft University has made considerable progress.
230 4O
30
----
20
10
0 0.0
0.2
0.4
0.8
0.6
1.0
Figure 4. Concentration dependence of Fickian diffusivities for light alkanes at 292K. Line is D / D o = 1/(1-0). From Kapteijn et al. (57~.
2.00 S -::-
methane
-
14
i E
(b)
4
e
10
a
n ,r v
n-butane
(a)
@ ~ , ~
3
Q ,.-
2
|- 9 ca.
vE ~t
~
,7"
,==
~w'-
--
1.20
o
o
o
vE
,.,o
~',
2
9
0.80
4t M.
~
n-butane
Q.
2 ~ 0 0
9
1
/~~
,., ,.,i,
40
80
. . . . 120
i,
180
2
J
" ~"
0 0
Time
'
40
0.40
methane -~--='--=
80
end value=26 P~
"-:
:
120
-
-~
160
'
0.00
!00
(s)
Time (s)
Figure 5. Transient permeation for silicalite membrane (a) single components at 300K, 50kPa; (b) binary mixture at 300K, 50kPa for each component. From Bakker et al. 06) Their silicalite membranes are still too thick to provide economically viable fluxes but they have already provided a wealth of fundamental information on diffusion rates. The diffusivities for several light alkanes in silicalite, derived from membrane permeation measurements show the usual pattern of concentration dependence in approximate conformity with the Darken equation (Figure 4). The corrected diffusivities are compared, in Table 2, with values derived from other experimental methods. In contrast to the data in Table 3 (for benzene and p-xylene) which show reasonable consistency between several different macroscopic techniques, the data for the light alkanes (Table 2) show several very large anomalies. Molecular dynamic calculations and QENS yield very high self-diffusivities ( - 10s cm2.s "1 at 200K. The diffusivities derived from the recent membrane permeation measurements are of order 10 -6 cm2.s "1 and this is very much higher than the values obtained by Paravar and Hayhurst (25) from single crystal permeation measurements. Recent ZLC (24) and chromatographic data (38) yield similar diffusivity values which are also consistent with the recent membrane data. The PFG NMR values vary over more than an order of magnitude, the lower range of these values being consistent with the membrane and ZLC data. The values from the
231 frequency response measurements (23) are substantially larger and comparable with the higher range of the PFG NMR data and the QENS and MD values. The binary data (57), a representative example of which is shown in Figure 5b are even more interesting. In a single component system at a given pressure methane permeates substantially more rapidly than butane. However, in the mixtures we see that this is reversed. The reason is that the more strongly adsorbed butane essentially blocks the adsorption of methane leading to a large difference in the fluxes. These data can be quantitatively predicted from the single component diffusivity values using the generalized Stefan-Maxwell equation (58) thus providing convincing evidence for the validity of that model to predict binary and multicomponent diffusion in zeolite crystals and zeolite membranes. Clearly in assessing the perm-selectivity of a zeolite membrane a proper interpretation of the single component data is necessary. Naive comparisons based on single component permeation rates will lead to erroneous conclusions. 4. THE WINDOW EFFECT The unusual trend of diffusivity with chain length for linear alkanes in zeolite T, reported by Gorring in 1973(7) was noted in the introduction to this paper. Since we had observed no such anomalies for the diffusion of these compounds in zeolite A, zeolite X or silicalite and since Gorring's diffusivity values are substantially smaller than the currently accepted values for comparable systems we decided to attempt to repeat these measurements (14). Zeolite T is a synthetic intergrowth of offretite and erionite and since the composition is somewhat variable we made our measurements on four well characterized samples spanning the range from near offretite to near erionite. Measurements were made by both gravimetric and ZLC techniques with consistent results. For all four samples at all temperatures a monotonic decrease of diffusivity with chain length was observed with no indication of the pronounced maxima and minima reported by Gorring (see Figure 6). In the light of what we now know about the problems associated with the measurement of intracrystalline diffusivity by the uptake rate method it seems probable that the complex trend observed by Gorring resulted from the combined effect of several different rate processes.
5. CONCLUSION A review of the diffusivity data in Table 2 as well as other recent compilations of diffusivities measured by different experimental techniques shows that: (a) In general, for zeolite A, there is reasonable agreement between macroscopic and microscopic diffusivity data, when only the more recent results are considered. (b) For silicalite and zeolite X we commonly see significant differences between microscopic and macroscopic values with the macroscopically measured values being substantially smaller than the microscopic values. The usual sequence is: QENS -- PFG N M R - - FR > ZLC--- Membrane > Uptake
232
10-10
1.0
10-11
o
Rest~st~
0.6
7 v
Surface
0.8
r Offretite o Intermediate D Erionite
0.4
t0-12
~
--- Diffusion
0 2 10-13
-
4
,
,
8
,
,
-
,
-
,
-
12 16 Carbon Number
Figure 6. Variation of diffusivity for linear alkanes in o~etite-edonite at 573K with carbon number tt*).
,
,
20
0
1
2
3
t/tl/2
Figure 7. Theoretical uptake curves for a one dimensional system with diffusion/barrier controL
This pattern is not, however, universal. For example frequency response data for CO 25A yield diffusivities that are smaller than the ZLC values by more than an order of magnitude. All of the more obvious explanations for the discrepancy between the PFG NMR data and the macroscopic values have been examined in detail05,59). While it is clear that in many of the earlier macroscopic measurements the discrepancy can be attributed to the intn~ion of thermal effects or extracrystalline diffusion, this is not true of the more recent macroscopic measurements, particularly the membrane and ZLC data. In view of the consistency of the values obtained, for several systems, by different macroscopic techniques one has to conclude that the values are probably genuine. To obtain sorption desorption rates which are slow enough to be macroscopically measurable it is necessary to use large zeolite crystals (-- 100#m). As the crystal size increases so does the probability of fault planes and other semi-regular defect structures. If these are spaced by distances of several #m the medium length scale diffusivity measured by NMR techniques will reflect mainly the rather rapid transport between the higher level barriers formed by the fault planes while the macroscopic measurements will yield the (lower) 'effective' difftmivity over the active dimension of the crystal. Similarly, if there is no barrier at the crystal surface, the frequency response measurement will reflect primarily the higher dif~sivity associated with transport in the ideal structure up to the first fault plane. Such ideas are not new and have been used to interpret the pore blocking effect of relatively small numbers of strongly adsorbed molecules (61). However, recent calculations reveal that only a small number of such barriers is needed to explain these effects--see Figure 7. For a one dimensional system three fault planes in the half thickness of the crystal will lead to an uptake curve which has almost the same shape as the curve for an ideal diffusion controlled system, although the apparent diffusivity will now be determined by the rate of passage through the barriers associated with the
233 defects rather than by diffusion in the ideal structure. Such a conclusion is of course tentative and requires verification by more detailed experimental studies. A more definite and perhaps even more significant conclusion from recent macroscopic diffusion measurements is that the widely quoted "window effect" is not confirmed and we are forced to conclude that the original results may have arisen from a fortuitous combination of heat and mass transfer effects
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A. Tiselius, Z. Phys. Chem. A169, 425 (1934) and A174, 401 (1935). L-K. Lee and D.M. Ruthven, J. Chem. Soc. Faraday Trans I 75, 2406 (1979). D.M. Ruthven, L-K. Lee and H. Yucel, AIChEJ1 26, 16 (1980). D.M. Ruthven and L-K. Lee, AIChEJ1 27, 654 (1981). D. Prinz and L. Riekert, Ber. Bunsenges, Phys. Chem. 90, 413 (1986). M. Biilow and P. Struve, J. Chem. Soc. Faraday Trans. 1 80, 813 (1984). R.L. Gorring, J. Catalysis 31, 13 (1973). S.M. Csisery, Zeolites 4, 202 (1984). N.Y. Chen, W.E. Garwood and F.G. Dwyer Shape Selective Catalysis in I n d ~ l Applications p. 55 Dekker, N.Y. (1989). 10. J. Rabo Zeolite Chemistry and Catalysis ACS Monograph Series 171, 692 (1976). 11. R.M. Barrer, Zeolites and Clay Minerals p 299 Academic Press, London (1978). 12. J. Nitsche and J. Wei, AIChEJ1 37, 661 (1991). 13. J. Wei I and EC Research 33, 2467 (1994). 14. C. Cavalcante, M. Eic, D.M. Ruthven and M. Occelli, Zeolites--in press. 15. J. K~irger and D.M. Ruthven, Diffusion in Zeolites, John Wiley, N.Y. (1992). 16. L.V.C. Rees, Proc. Tenth Internat. Zeolite Conf., Garmisch, July 1994, p. 1133. J. Weitkemp et al. eds. Elsevier, Amsterdam (1995). 17. J. K/irger and J. Caro, J. Chem. Soc. Faraday Trans I 1363 (1977). 18. J. K/irger and W. Heink, Exp. Technik Phys. 19, 453 (1971). 19. H. Pfeifer, Phys. Rep (Phys. Letters C) 26, 293 (1976). 20. J. Caro, M. Noack, J. Richter-Mendau, F. Marlow, D. Petersohn, M. Griepenstrog and J. Kornatowski, J. Phys. Chem. 97, 13685 (1993). 21. W. Heink, J. K/irger, H. Pfeifer and K.P. Datema, J. Chem. Soc. Faraday Trans. I. 22. H. Jobic, M. Bee and J. Caro, Proceedings, Ninth Internat. Zeolite Conf., Montreal, July 1992, Vol. 2 p. 121, R. von Ballmoos, J.B. Higgins and M.M.J. Treacy eds., Butterworth, Stoneham MA (1993). 23. N. van den Begin, PhD Thesis, Imperial College, London (1989). 24. J.R. Hufton and D.M. Ruthven, I and EC Research 32, 2379 (1993). 25. D.T. Hayhurst and A.R. Paravar, Zeolites 8, 27 (1988). 26. N. van den Begin, L.V.C. Rees, J. Caro and M. Billow, Zeolites 9, 287 (1989). 27. K. Beschmann, S. Fuchs and L. Riekert, Zeolites 10, 798 (1990). 28. P. Wu and Y.H. Ma, Proc. Sixth Internat. Zeolite Conf., Reno, July 1983 Eds. D. Olson and A. Bisio p. 251, Butterworth, Guildford U.K. (1984). 29. Y.H. Ma, Proc. Seventh Internat. Zeolite Conf. Tokyo 1986, Y. Murakami, A. Lijima. and J.W, Ward eds., Kodansha p. 531 Elsevier, Tokyo (1986). 30. M. Billow, H. Schlodder, L.V.C. Rees and R. Richards, Ibid p. 579 (1986). .
2. 3. 4. 5. 6. 7. 8. 9.
234 31. M. Eic and D.M. Ruthven, Zeolites 8, 472 (1988). 32. D.M. Ruthven and M. Eic, Am. Chem. Soc. Symp. Ser. 368, 302 (1988). 33. J. R. Hufton, S. Brandani and D.M. Ruthven, Proc. Tenth Internat. Zeolite Conf., Garmisch July 1994, J. Weitkamp, H.G. Karge, H. Pfeifer and W. Holdereich eds., p 1323, Elsevier Amsterdam (1994). 34. J.R. I-I'afton, S. Brandani and D.M. Ruthven, Microporous Materials--in press. 35. S. Brandani, D.M. Ruthven and J. K/irger, Zeolites--in press. 36. U. Hong, J. K~irger, B. Hunger, N.H. Feoktistova and S.P. Zhdanov, J. Catalysis 137, 243 (1992). 37. J. Caro, M. Billow, W. Schirmer, J. K/irger, W. Heink and H. Pfeifer, J. Chem. Soc. Faraday Trans. 1 81 2541 (1985). 38. J.R. Hufton and R.P. Danner, AIChEJ1 39, 963 (1993). 39. Y. Yasuda, J. Phys. Chem. 80, 1867 (1976); 86, 1913 (1982). 40. N.G. van den Begin and L.V.C. Rees, Proc. Eighth Internat. Zeolite Conf., Amsterdam 1989 p. 915, P.A. Jacobs and R.A. van Santen eds. Elsevier Amsterdam (1989). 41. G. Onyestyak, D. Shen and L.V.C. Rees--AIChE Meeting, San Francisco Nov. 94. 42. D. Shen and L.V.C. Rees, Zeolites II, 684 (1991). 43. D.M. Ruthven, Zeolites, 13, 594 (1993). 44. Z. Xu, M. Eic and D.M. Ruthven, Proc. Ninth Internat. Zeolite Conf., Montreal, July 1992, Vol. 2 p. 147, R. von Ballmoos, J.B. Higgins and M.M.J. Treacy eds. Butterworth, Stoneham MA (1993). 45. J. K/irger, H. Pfeifer, F. Stallmach, N.N. Feoktistova, and S.P. Zhdanov, Zeolites 13, 50 (1993). 46. L.M. Sun and V. Bourdin, Chem. Eng. Sci., 48, 3783 (1993). 47. D. Shen and L.V.C. Rees, J. Chem. Sci., Faraday Trans. I. 89, 1063 (1993). 48. D. Shen and L.V.C. Rees, Proc. Ninth Internat. Zeolite Conf., Montreal, July 1992 Vol. 2 p. 45, R. von Ballmoos, J.B. Higgins and M.M.J. Treacy eds., Butterworth, Stoneham MA (1993). 49. D. Shen and L.V.C. Rees, Zeolites 11, 666 (1991). 50. W. Niessen and H. Karge, Microporous Materials 1, 1 (1993). 51. D.M. Ruthven, M. Eic and E. Richard, Zeolites 11, 647 (1991). 52. Ph. Grenier, F. Meunier, P.G. Gray, J. K/irger, Z. Xu and D.M. Ruthven, Zeolites 14, 242 (1994). 53. T. Torresan and Ph. Grenier, Chem. Eng. J. 49, 11 (1992). 54. V. Bourdin, Ph. Grenier, F. Meunier and L.M. Sun, AIChEJ1--in press. 55. V. Bourdin, L.M. Sun, Ph. Grenier and F. Meunier--Chem. Eng. Sci.--in press. 56. W.J.W. Bakker, G. Zheng, F. Kapteijn, M. Makkee, J.A. Moulijn, E.R. Guns and H. van Bekkum in Precision Process Technology p. 425, M.P.C. Weijnen and A.A.H. Drinkenberg eds., Kluwer (1993). 57. F. Kapteijn, W.J.W. Bakker, G. Zheng, J. Poppe and J.A. Moulijn, Chem. Eng. J.--in press. 58. R. Krishna and L.J.P. van den Broeke, Chem. Eng. J.--in press. 59. J. K/irger and D.M. Ruthven, Zeolites 9, 267 (1989). 60. E. Hernandez, M. Kawano, A.A. Shubin, C.M. Freeman, C.R.A. Catlow, J.M. Thomas and R.L. Zamaraev, Proc. Ninth Internat. Zeolite Conf. p. 695, R. von Ballmoos, J.B. Higgins and M.J. Treacy, eds., Butterworth, Stoneham (1993).
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviotand S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
235
Frequency Response Study Of Mixture Diffusion Of Benzene And Xylene Isomers In Silicalite-1 Dongmin Shen and Lovat V.C. Rees Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, UK
The diffusion of the single components and mixtures of benzene and two xylene isomers in silicalite-1 has been studied by a frequency response method. The responses of benzene, p-xylene and o-xylene single component in the same silicalite-1 show that the diffusivities of these three molecules increase in the sequence of o-xylene, benzene and p-xylene. The FR spectra of benzene/p-xylene and benzene/o-xylene binary mixtures in silicalite-1 have shown two distinct response peaks. The diffusion coefficients of benzene obtained from the mixture diffusion are from 5.6x10 ~3 to 3.3x10 -12 m2 s1 in the temperature range of 355-435 K, which are consistent with those obtained previously from the single component study. The diffusivities of o-xylene in silicalite-1 obtained from the mixture diffusion were found to be two orders of magnitude smaller than those of benzene. A film resistance has been observed in diffusion of the mixtures.
INTRODUCTION One of the major applications of zeolites in catalysis is aromatic alkylation, in which molecular diffusion properties have been found to play a significant role in the overall reactions, especially in the selectivity of products. Many studies on the single component diffusion of aromatic molecules have been made using various methods. However, the knowledge of mixture diffusion of these aromatics is still limited. The frequency response technique based on the response of a sorbent-sorbate system over a large dynamic time scale has shown its ability to distinguish the different dynamic processes which occur within this wide frequency range [ 1-3]. For MFI type zeolites, the diffusivities of benzene obtained using the frequency response method were found to be in good agreement with literature data [4]. The diffusivity of p-xylene obtained by the FR method is one to two orders of magnitude faster than that of benzene in silicalite-1. The co-diffusion of benzene and p-xylene binary mixtures investigated by the FR method [5] showed that a film resistance in the mixture diffusion existed and the diffusivity of p-xylene decreased with increasing benzene loadings. In this study, the influence of another second component, o-xylene, on the co-diffusion of benzene in silicalite-1 will be investigated. The diffusivities of benzene, p-xylene and o-xylene in silicalite-1 will also be compared.
236 THEORY
The FR apparatus is schematically shown in Fig. 1. The principal features of the FR ID apparatus have been described previously [6,7]. In the FR method a dose of sorbate is brought into sorption equilibrium with the sorbent. The two electromagnets which drive the moving disc are activated in turn via a computer at a given frequency. The copper bellow attached to the :::::::::::::::::: ~1 disc, which is part of the sorption gas phase Fig. 1 Frequency response apparatus. volume, is expanded or contracted to produce 1, sorbates dosing system; 2, moving the ca +1% square-wave modulation of the gas disk; 3, electromagnets; 4, bellows; phase equilibrium volume, Ve. The pressure 5, vacuum system; 6, magnet driving response to the volume changes is then followed unit; 7, pressure transducer; 8, vacuum by the on-line computer through a high accuracy, taps; 9, pressure display unit; 10, data fast response pressure transducer. A frequency acquisition interface; 11, IBM range of 0.01 to 10 Hz is scanned over 23 compatible PC; 12, sample vessel. increments. The frequency response parameters (phase lag and amplitude) are derived from the equivalent fundamental sine-wave perturbations by a Fourier transformation of the volume and pressure square waves. The phase lag, r = Cz- r where Cz and Cn are the phase lags determined in the presence and absence of sorbent respectively. The amplitude ratio P J P z is determined, where PB and Pz are the pressure responses to the volume perturbations in the absence and presence of sorbent respectively. The frequency response spectrum of a system can be described by the in-phase and outof-phase components, fc(c~ and fs(co), respectively [ 1]:
(PB/Pz) cos ~bz_B -1 =re(co)
(1)
(PSPz) sin Cz-B=fs(co)
(2)
The two components, fc(co) and fs(co), when two independent diffusion processes coexist, may be described by [8]:
f: (co) = Kz6s: (Dz/r 2, o9) + KIZ63j (Dzz/r2, co),
wherej = c, s
(3)
where K/ and K n are constants which are respectively related to the intensities of the characteristic functions of the two diffusion processes. The characteristic functions, 63c and 63s, which are derived from an isotropic sphere are given by:
83c ( D/r 2 , o9) = 3/1"/[( sinhr/- sinr/)/( coshr/- cosr/)]
(4)
83s( D/r 2 , co) = 6/77 { 1/2 [( sinhr/+ sinr/)/( coshr/- cost/)] - l/r/}
(5)
237 where r/= (2r2/D) 1/2, o0 = 2~rf, the angular frequency, and D is the transport intracrystalline diffusion coefficient. If a surface resistance is considerable, A and C represent the adsorption species on the external surface and within the micropores, respectively, and the adsorption-desorption rate on the external surface may be characterised by k_A, equation (3) can be modified to [9]:
fj (o~) : K, aa,,. (~. D,/r~. co) + '1, a4, ((.. D./r~. co),
wherej : c, s
(6)
where the parameter ~, which characterises the surface resistance, is defined by:
~. = a k_A r2/Dj
(7)
and
a83c ((, D/r 2, co) = (a k_A/CO)2 { a + c 83c(D/r2, o0)}/0
(8)
ota3s ((, D/r 2, co) = (a k_~/~o)[1-(a k_A/co){(a k_A/co)+C 4s(D/r 2, o0)}/0]
(9)
where
0= {(a k_A/co)+C63s(D/r 2, co)}2 + {a+c 6~c(D/r2, o0)}2
(10)
a and r are defined by: a~
dP
e
~l-C
C=__
e
a
dP
e
(11)
(12)
= 10 -2
and r ~ 1 are usually satisfied with most of zeolites. The "corrected" diffusion coefficient, Do, is obtained by applying the Darken equation to the transport diffusion coefficient, D, measured by the FR method.
EXPERIMENTAL
Zeolite samples (30--50 mg) were outgassed at 350 oc overnight. The temperature was raised to 350 ~ at 2 ~ per minute by a programmable tube furnace. The sorbate was admitted to the outgassed sample and allowed to come to pressure equilibrium at the temperature of the diffusion measurements. The equilibrium pressure was measured by the pressure transducer. The gas/zeolite system was scanned over a range of frequencies from 0.01 to 10 Hz. The pressure response data were recorded on an on-line computer at each
238 frequency step over 5 square-wave cycles after the periodic steady-state had been established. The pressure response to the volume change over the whole frequency range was measured in the presence and absence of sorbent sample to eliminate time constants associated with the apparatus. Two different sizes of silicalite-1 samples were used in this study. Silicalite-l(A) is spherical crystal of 2r = 14.4 ~tm and silicalite-l(B) is nearly cubic shape of dimensions 4x3x4 lam3. The silicalite-1 samples were dispersed in a plug of glass wool in the sorption vessel to eliminate the effects of heat and mass transfer. Benzene, p-xylene and o-xylene with purity of 99.9+%, 99+% and 99+%, respectively, were supplied by Aldrich.
RESULTS AND DISCUSSION The phase lag shifts of the response curves for single component diffusion in silicalite-l(B) at 395 K and 0.862 Torr of equilibrium pressure are compared in Fig. 2. The positions of the phase lag curves reflect the kinetic time constants involved in the corresponding kinetic processes. A fast diffusion process will generate a response curve at a high frequency, vice versa a slow diffusion process will give a response curve at a low frequency. Fig. 2, therefore, clearly demonstrates that the diffusion of p-xylene in silicalite-1 under the same experimental conditions is faster than that of benzene, which is faster than that of o-xylene. The faster diffusion and lower activation energy of p-xylene than benzene in silicalite-1 may be due to its linear methyl-groups, which may decrease the pitching of the p-xylene molecule as observed by molecular dynamic simulations[10]. On the other hand, the methyl-groups in the o-xylene molecule increase o-xylene molecular kinetic diameter and lead to slow diffusion. Fig. 3 shows the response characteristic function curves for diffusion of o-xylene, benzene and their mixture in silicalite-l(B) at 395 K. The response curves for o-xylene appear at very low frequency outside the frequency window of 0.01-10 Hz because of the slow diffusion process of o-xylene in 10-ring pore sizes of silicalite-1, while the response for benzene occurs inside the frequency range at ~I0 the same conditions. This result indicates that diffusion of benzene is faster than that of o[] xylene by about two orders of magnitude. $ 5 rO D The response curves for the o-xylene/benzene 13mixture show two distinct peaks within the 0 frequency range. It seems that the diffusivities of benzene and o-xylene in the 0.1 1 0.01 mixture do not change significantly, Frequency / Hz compared with the diffusivities of the single Fig. 2 Comparison of the phase lag shifts for components. This may be because very low (rl) benzene, (O) p-xylene and (r o-xylene coverage of these sorbates (ca. 1-2 molecules diffusion in silicalite-l(B) at 395 K and per u.c.) are used in the FR study and the 0.862 Torr of equilibrium pressure. interference and blockages between these .
.
.
.
.
.
.
.
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.
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.
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.
.
.
.
!
.
.
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239
0.8
0.8
(a)
0.6
0.6
0.4
0.4
0.2
0.2
o.o ~
'
~
n
"
0.6
(b)
u3 r
o 0.4 El. u3
9 0.2 0.0
0.0
...... I
o
I
~'
(a)
(I.) 0.6 ~ d ~ ~ O.
0.4
...................
(13 n~ 0.2 0.0
..............................
(c)
0.6
(b) I
...........................
0.6
0.4 " \ 0 . 2 .............. ~ '"~-~....................... o0
0.01
0.1
1
10
Frequency / Hz Fig. 3 (1"1) KSin and (O) KSout characteristic functions for diffusion of (a) 0.431 Torr o-xylene, (b) 0.431 Tort benzene and (c) 0.144 Torr o-xylene/0.431 Torr benzene mixture in silicalite-l(B) at 395 K. Dashed and dotted lines denote the two different diffusion processes of o-xylene and benzene. Solid lines are overall fits.
(J.01
0.1
1
10
Frequency / Hz Fig. 4 (D) KS~, and (O) KSout characteristic functions for diffusion of (a) 0.431 Torr benzene, (b) 0.862 Torr p-xylene and (c) 0.431 Torr p-xylene/0.144 Torr benzene mixture in silicalite-l(A) at 395 K. Dashed and dotted lines denote the two different diffusion processes. Solid lines are overall fits.
molecules inside the three-dimensional channels of silicalite-1 are negligible. An intersection between the in-phase and out-of-phase characteristic functions occurred in the mixture response, as can been seen from Fig. 3(c). This intersection indicates that there is an external surface resistance and its time constant is comparable with the diffusion process. This surface resistance is most probably due to a film resistance caused by the multi-components because no surface barriers are observed in the response curves of the single components. This film resistance in the diffusion of the mixture is caused by the concentration gradient in the gas phase created by the difference in diffusion rates of the components inside the crystals. Fig. 4 shows the response spectra of the in-phase and out-of-phase characteristic functions for diffusion of benzene, p-xylene and their mixture in silicalite-l(A) of larger crystals at 395 K. As can be seen from Fig. 4(a), a large diffusion time constant, which shifts the response curves to a low frequency, is expected for benzene diffusion in the large crystals of silicalite-l(A) and the response curves appear outside the frequency window. On the other hand, the full response curves are obtained within the frequency range, Fig. 4(b), for diffusion of p-xylene in this large silicalite-1 (A) sample. It has been previously reported[5] that the bimodal response curves for p-xylene diffusion in silicalite-1 cannot be described by one simple diffusion process and when two independent diffusion processes which take place simultaneously are assumed the response curves can be reproduced with two diffusion coefficients, which are associated with the
240 diffusion in the straight and sinusoidal channels of silicalite-1. Sun et al.[11] have given other possible interpretations considering heat dissipation interference on diffusion or considering finite reversible mass exchange between the straight channels where diffusion takes place and the sinusoidal channels where the sorbate is stored[ 11-13 ]. This bimodal behaviour of p-xylene in silicalite-1 cannot be satisfactorily interpreted by the models available at present. A model which considers anisotropic diffusion in the two channels of silicalite and a finite heat exchange between gas phase and sorbent should be developed to explain such bimodal behaviours. In Fig. 4(c) the two peaks observed for diffusion of the benzene and p-xylene mixture are different from those obtained for pure p-xylene diffusion. The effect of benzene concentration in the mixtures on the response curves has been reported previously[5]. The intersection of the in-phase and out-of-phase characteristic functions is also observed in the diffusion of benzene/p-xylene mixtures. This intersection is also due to the existence of a film resistance, because no surface barriers are observed for the diffusion of pure benzene and pxylene single components in the same silicalite-1, as can be seen in Fig. 4. The FR results show that p-xylene diffuses faster than benzene in silicalite-1 by 1---2 orders of magnitude measured by the FR method in pure single component systems and in the mixtures of benzene and p-xylene at 375-435 K, while o-xylene diffuses more slowly than benzene in silicalite-1 by two orders of magnitude in the pure single component systems and in the mixtures of benzene and o-xylene. Fig. 5 shows the response of the characteristic functions for the system of .......... iai t silicalite-1 and a mixture of benzene (0.431 Torr) and o-xylene (0.144 Torr) at the temperatures of 355, 375, 395 and 415 K, respectively. It was found that as the (b) temperature increases an intersection between the in-phase and out-of-phase characteristic function curves occurred at ~oo g~ higher frequencies and the intersection area increased. These intersections indicate the existence of a surface film resistance in the mixture diffusion as discussed above. There are three possibilities for this surface film resistance: (i) the rate for benzene diffusion through an o-xylene film is comparable to the rate of benzene diffusion inside channels; (ii) 0"801 01 1 10 the rate for o-xylene diffusion through a Frequency / Hz benzene film is comparable to the rate of Fig. 5 (D) K(~in and (O) K~out characteristic o-xylene diffusion inside channels; or (iii) the functions for the mixture of 0.431 Torr rates for both molecule diffusion through the benzene and 0.144 Torr o-xylene in 50 mg films are comparable to their diffusion rates. of silicalite-l(B). (a) 355 K, (b) 375 K, (c) Case (ii) may be ruled out, because the 395 K and (d) 415 K. Dashed and dotted experimental data cannot be fitted by the lines denote the diffusion processes of model. When the amount of o-xylene in the o-xylene and benzene, respectively. Solid mixture was reduced from 25% to 10%, the lines are overall fits intersection between the characteristic
241
1.2
. . . . . . . .
,
. . . . . . . .
,
. . . . . . . .
10 "I
~
O
r
1.0 0.8 ~'~ c~
0.6
i
10. I
E
d 10" 1
10-1s 0.1
I
2'.3
14 " is
i.
2'.7
18
z9
103 K/T
Frequency/Hz
Fig. 6 (I-I) KSin and (O) KSo.t characteristic functions for the mixture of 0.431 Tort benzene and 0.047 Torr o-xylene in 50 mg ofsilicalite-l(B) and 395 K.
Fig. 7 Temperature dependence of the corrected diffusivities of (1"!) benzene and (O) o-xylene in silicalite-1 derived from the binary mixture diffusion at low coverages. ( I ) diffusivities of benzene in silicalite-1 obtained from benzene single component measurements.
function curves disappeared as shown in Fig. 6. This result also proves that the o-xylene film has a strong effect on the benzene diffusion. Thus, case (i) is considered to be significant. It is also worth noting that the response of o-xylene shifts to a higher frequency when only 10% of o-xylene exists in the mixture. These experimental data were reproduced by eq. 6 for two independent diffusion processes with a surface film resistance using the parameters in Table 1. The strong film resistance at high temperatures may indicate that benzene diffusivities increase more rapidly with temperature than the rate of benzene diffusion through the film. Table 1 FR parameters obtained from the mixture diffusion of benzene and o-xylene in silicalite-1 Benzene
o-Xylene
T/K
D I /m 2 s -1
Kx
k_ a / s -1
DII /m 2 s "1
KII
k_ A/s -1
355
2.5 x l 0 -13
0.28
-
3 x l 0 -15
1.74
oe
375
4.6 x l 0 -13
0.17
2000
4 X10 "15
0.88
395
7.0 x l 0 -13
0.25
750
9 xl0 -15
0.64
415
10 xl0 -13
0.1
650
13 xl0 -15
0.26
The temperature dependencies of the diffusivities of benzene and o-xylene derived from the mixture/silicalite-1 system at low coverages of 1-2 molecules per unit cell are given in Fig. 7. The diffusion coefficients of benzene in the mixtures are slightly higher (but within experimental errors) than those measured by the FR method for the pure single component previously [14], and have nearly the same activation energy of 27 kJ moll. The diffusion coefficients of o-xylene obtained from the mixture are about two orders of magnitude smaller
242 than those of benzene. The activation energy of diffusion for o-xylene is 34.4 kJ mol"1 and it is much larger than that for benzene. The diffusivities of p-xylene in silicalite-1 is about 3-4 orders of magnitude faster than those of o-xylene.
CONCLUSION Two response peaks for diffusion of the binary mixtures of benzene/o-xylene and benzene/p-xylene in silicalite-1 were observed in the FR rate spectra. A external surface resistance effect on the response curves has frequently been observed in the diffusion of the binary mixtures, but not in the diffusion of single components. This surface resistance is, therefore, considered to be due to the existence of a film resistance in the mixtures. The frequency responses of single components and binary mixtures in silicalite-1 have shown that the diffusion of p-xylene is about 1-2 orders of magnitude faster than that of benzene and about 4 orders of magnitude faster than that of o-xylene. The diffusivities of benzene in silicalite-1 obtained from the mixture study are in reasonable agreement with those obtained from pure single component study.
REFERENCES
7 8 9 10
11 12 13
14
Y. Yasuda, Heterog. Chem. Rev., 1 (1994) 103. R. G. Jordi and D. D. Do, Chem. Eng. Sci., 48 (1993) 1103. L. M. Sun, F. Meunier, Ph. Grenier and D. M. Ruthven, Chem. Eng. Sci., 49 (1994) 373. L.V.C. Rees and D. Shen, in Sym. Charact. Porous So#ds, COPS III, Eds. J. Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger, Elsevier, Amsterdam, (1994) p563. D. Shen and L.V.C. Rees, in Proc. of the 9th Int. Zeo#te Conf., Montreal 1992, Eds. R. von Ballmoos, J.B. Higgins and M.M.J. Treacy, Butterworth-Heinemann, Boston, (1993) p45. N. G. Van Den Begin and L. V. C. Rees, in Zeo#tes: Facts, Figures, Future, Eds. by P.A. Jacobs and R.A. van Santen, Elsevier, Amsterdam, 1986, p915. L.V.C. Rees and D. Shen, Gas Separation & PurL, 7(1993) 83. Y. Yasuda, Y. K. Matsumoto, J. Phys. Chem., 93 (1989) 3195. Y. Yasuda, Y. Suzuki and H. Fukada, J. Phys. Chem., 95 (1991) 2486. T. Inui and Y. Nakazaki, Zeo#tes, 11 (1991) 434. L. M. Sun and V. Bourdin, Chem. Eng. Sci., 48 (1993) 3783 D. D. Do, R. G. Jordi and D. M. Ruthven, J. Chem. Soc. Faraday Trans., 88 (1992) 121. R. G. Jordi and D. D. Do, J. Chem. Soc. Faraday Trans., 88 (1992) 2411. D. Shen and L.V.C. Rees, Zeo#tes, 11 (1991) 666
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 1995 Elsevier Science B.V.
243
2D EXSY 129Xe NMR- New Possibilities for the Study of Structure and Diffusion in Microporous Solids I.L. Moudrakovski, C.I.Ratcliffe and J.A.Ripmeester Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario, K1A 0R6, Canada Some new applications of 129Xe NMR spectroscopy are presented which use the dynamic properties of adsorbed xenon to learn about structure of pores, xenon exchange and diffusion. Exchange rates, obtained with 2D EXSY NMR techniques, were measured for diffusion paths with widely varying length scales as demonstrated for intraparticle exchange in Na Mordenite and interparticle exchange in mixed NaX and NaY zeolites. The observation of distinct clusters of up to 8 xenon atoms in AgA zeolite and intercluster exchange has given information on the location of silver ions and residual water. The observation of anisotropic chemical shifts in sieves such as ALPO- 11, ALPO-5, ALPO-8, VPI-5, SSZ-24 and ZSM- 12, along with 2D-EXSY NMR data, has allowed modeling of the distribution of xenon atoms in these low-dimensional pore systems. 1.
INTRODUCTION. Since the first application to the study of microporous solids [1,2], 129Xe NMR has become a very popular technique for the study of the internal structure of zeolites and other solids with void spaces. Xenon is a relatively large, easily polarizable atom, and due to its chemical inertness and good NMR sensitivity it is a very attractive probe for adsorption studies. To date, much of the work reported has relied on empirical correlations between the isotropic chemical shift of adsorbed xenon and the size of the sorption site. This approach has considerable limitations, requiting a knowledge of the location of the xenon atoms within the pore structure, and any dynamic processes which average the NMR parameters associated with different sorption sites. Here we offer a different approach for the study of microporous solids with 129Xe NMR spectroscopy which makes use of the dynamical characteristics of the atom within the pore system. In this contribution we demonstrate the applicability of 2D EXSY NMR [3,4] to xenon exchange between different sorption zones within solid microporous samples on time scales which range from milliseconds to seconds, and on quite different length scales. The model systems chosen to illustrate the effects of xenon exchange on microscopic and macroscopic scales were Na-Mordenite and mixtures of NaX and NaY zeolites. AgA zeolite provides an example of a system with very slow xenon exchange. 1D spectra show distinct populations of xenon clusters in different cages. Intercage exchange rates can be directly obtained from the 2D EXSY data. Combined 1D and 2D 129Xe NMR data sets for xenon in solids with one-dimensional channels (ALPO-11, ALPO-5, VPI-5, ALPO-8, SSZ-24 and ZSM-12) indicate fast exchange between inequivalent sites in micropores of reduced dimensionality.
244 2.
EXPERIMENTAL The solids studied include Na-Mordenite (Strem), LZY-52 zeolite pellets and zeolke X beads (Aktrich), ~ 1 1 and AI.PO-5 (UOP, kindly supplied by S. W~lson), VPI-5 ( Ca[Tech., kindly supplied by M. Davis), SSZ-24 (Ct~vron, kindly supplied by S.S. Zones); highly siliceous ZSM-12 was synthesized as reported elsewhere [5]. AgA zeotke was prepared by ion exchange of NaA (Union Carbide) with 1 M water solution of AgNO3. Before xenon adsorption, samples were placed in 10 mm OD pyrex tubes and outgased on a vacuum line at 110~ C for AgA and 400~ C for all other zeolkes. Calitrated anaotmts of xenon were condensed into the tubes before these were flame-sealed. 2D 129Xe NMR data sets were collected on a Bruker AMX 300 NMR ~ m e t e r at a frequency of 83 MHz with a 7r/2-tl-~t2-t~x-~/2-t2 sequence in TPPI mode (Fig. 1A) [6]. 80 and 256 points were acquiredin the tl and t2 dormins respectively. Recycle delays ranged from 200 rm to 100 s depending on the sample. Mixing times varied in the range lms- ls. Xenon gas at low pressure was used as an external reference. 3. DETERMINATION SPECTRA
OF
KINETIC
PARAMETERS
FROM
2D
EXSY
Chemical exchange within a time of the order of the mixing time t~ gives rise in the 21~EXSY spectrum to off-diagonal cross-peaks between the resonances of the sites in exchange. The cross-peaks provide infomaation about the exchange pathways and the integrated intensities are related to the rates of exchange. The coupled differential equations that descaSbe the rmgnetization m along the z-axis d u n g the e ~ n t , can be written in the form m =-Rm (1) where R , for the case without cross-relaxation, is a sum of the kinetic and relaxation r m t r ~ s [3,7-9]. Fontal solution ofeq. 1 at t =tm (tin= mixing time) is given by equation M(tm) = e - t " M ( 0 ) (2) where M(O) is a column matr~ of equilibrium magnetization of each site. The most eff~nt approach to solving this system is by the application of rmtrix methods [7,8], and the rmtrix R is given by In A X (In A) X -1 R = ---
tm
=
tm
(3)
where A~ = ][#(t~/Mf is the e ~ n t a l
tmtrix of normalized intensities, X is the square matrix of eigenvectors of A, such that X "~A X = A = diag(~ ) and ~- are the eigenvalues of A. N ~ solution of equation (3) gives a complete set of the rate constants for the system in exchange [8,9]. 4.
RESULTS AND DISCUSSION
4.1
Xenon exchange between particles of zeolites NaX and NaY.
NaY and NaX zeolites have slightly different adsorption properties for xenon and, because the exchange of atoms between the particles is slow on the NMR time scale, it is possible to observe resolved lines from xenon inside the pore structure of each zeolite (Fig. 1). The rate of interparticle xenon exchange depends on the size of the particles and the temperature, and rate constants can be obtained from the 2D-EXSY experiments. The complete absence of cross-peaks at a mixing time of 2 ms indicates that no significant exchange between the two regions occurs during this period. Cross-peaks become visible at 10 ms, and their intensity grows up to 30 ms, above which it stays constant. Solution of equation (3) for given t,, gives us the rate constants for exchange between the regions. Dependence of the rate constants on the average particle
245
~r2
l t
trnix
t l
~.rz
= 2
rnsec
t=.
~:~
l~t
t2
trnix
A
size is presented on Fig. 2. Assuming that exchange is controlled by intercrystaUine diffusion, for spherical particles the exchange rate constant can be represented by the formula [10] 15 D ,#,..i k i = < R2 >
= 15 m s e c
(4)
where <,R2> is a mean-square particle radius, De#.,i is an effective diffusion coefficient that depends on the diffusion coefficients of the xenon inside the zeolite crystaUites composing the i-th domain and in the space between the crystallites. From plots of the exchange rate constants vs. reciprocal square of the particle radius we can estimate tmix = 30 m s e c tmix = 10 m s e c the values of the effective diffusion constants De#. in each adsorption region: 13+4x10 9 m2/s for NaX, 10:l:3x10 9 m2/s for NaY. Data on xenon diffusion in zeolites obtained by other methods is scarce. The self-diffusion constant for xenon in NaX zeolite has been found to be 7x10 9 m2/s at ppm room temperature from PFG Fig. 1. A- The pulse sequence for 2D-EXSY experiment. studies [11]. The difference B - 2D-EXSY t29Xc NMR spectra of xenon adsorbed between this self-diffusion on the mixture of NaX and NaY zeolites with average particle constant and the effective size 220 ~tm. The mixing times are shown above the spectra. diffusion constant obtained from 2D-EXSY experiments should be attributed to the fact that the xenon atoms are present only part of the time inside the zeolites crystaUites. Diffusion in the space between the crystallites is much faster than intracrystalline diffusion, which gives an increase to the observed constant. For the sample with average particle size of 220 l.tm we have obtained the temperature dependence of the 2D-EXSY spectra. Arrhenius plots of the diffusion constants in the NaX and NaY regions pemak a determination of the pre-exponential factors and activation energies of the
@
9
246 diffusion for both regions: preexponential factors are 5.8x10~ and 6.1x108 mE/s, and activation energies are 3.6 and 4.4 kJ/mol for xenon in NaX and NaY r e - - e l y . The Plq3 rreastnen'ents for xenon in NaX gave 8xl0 8 mE/s for the preexponential factor and 6 kJ/rnol for the activation energy [ 11]. The discrepancies with our data_ for NaX are mall and readily understandable in the light of the simplifica~ns made.
Rate c o n s t a n t , 1/sec 200 150 1O0 5O
Y
o. . . . . . . 0
200
;. 400
600 xlO 6
,xl 800
1/
2
>, cm
-2
Hg2 Plot of the xmJn exchangerate ~ as a fimaion of recipr~al n ~ n square averagewidthof the NaX and NaY ~ . 4.2.
Xenon exchange between the different adsorption zones of Mordenite The equihqxium dismqmtionof xenon between the main channels and side pockets of Mordenite has been studied earlier [12,13], but data concerning the kinetics of exchange are not available. The 2DEXSY technique is again very suitable for obtaining this sort of infonmtion. In contrast with the fore'm" example of rmcroscopic exchange, here we deal with diffusion in the microscopic regime. Fig 3. shows 2D-EXSY ~:'gxe NMR ~ of the xenon adsort~ in Na-mordenite recorded with different ~ g times. At short mixing times of the order of 1 rm and shorter, only diagonal peaks corresponding to the two distinct adsorption sites are observed. The low field signal con'esponds to xenon in the side pockets. At longer mixing times cross-peaks appem"in the ~ ~ g exchange between the sites. Qualitatw"ely the picture of exchange is the same as for the ~ of zeolites, but here there are some different and interesting features. First consider the shape of the peaks in Fig.3; the low field (sidepocket) diagonal peak is elongated along the diagonal, whereas the ~ from the xenon in the rmin channels has a syn-rmufcal mtmd shape. These features restflt from dism'butions of slightly different
tmix.
=
200 ~ts
tmix. =
tmix. = 8 ms
2 ms
100
/
200
/
0
f
300
300
ng.3
'
200
'
100
~ X e 21~EXSY NMR ~
j~o'
z~o'
~00
300
"
zoo
9
1oo
of xenon adsort~ in Na Mordenite at different mixing times
247 adsorption sites. The adsorption properties of the side-pockets in particular can differ because of small inequivalences of the sodium position in the lattice and near the side-pocket 8-rings. B Such distributions in adsorption properties leads to a dispersion of the nuclear shiekting for xenon in distinct sites. This mechanism of xenon line broadening in mordenite was suggested previously on the tltsis ofMAS NMR ~ [121. The elongated 2D lineshape tmambiguously tells us that during the rnixing time t~ ~ is no direct exchange of xenon between different side-pockets. Exchange of xenon between the main channels and any of the side-pockets, however, does occur. Fig. 4 shows e ~ n t a l and simtthted 100 2D ~ of the 1~Xe in Na-rnordenite. The s'malated ~ c t r m n was obtained assuming a Gaussian distri-bution in the 300 200 100 chemical shift of the xenon in the F2, ppm side-pockets, with the distribu-tion Fig.4 Experimental (A) and simulated (B) 2D-EXSY width taken from the cross-section in the stack-plot spectra for Xe in Na Mordenite (paran~ters for F2-domain of the experimental stx~calculation: k ~ p . ~ = 0.065 I/s, lq~_,~p. = 0.03 I/s, rum. The good correlation between the Gaussian distribution with width of 40 ppm) sinaflated and e ~ n t a l supports the va!id_i_tyof the n'ode[ Rate constants for xenon exchange, obtained from the analysis of integrated intensities in 2D spectra recorded with different mbdng times, are as follows: kM,--~ -- 0.065_+0.022 s-1, k~,,,,m 0.030!0.012 s-1. It is interesting to note that the order of magnitude for the rate constants is the same as for the exchange between particles of large pore zeolites discussed above. This exan~le highlights the great ingx)rtance of acc~tmting for xenon exchange processes in porous media and points to a potential source of error when correlations of pore sizes with the xenon chemical shift are made [ 14].
4.3
Adsorption and dynamics of xenon in AgA zeolite. Intercage exchange in A zeolites takes place through the 8-rings which separate the supercages. In AgA zeolite activated at low t e x t u r e (110~C in our case), xenon exchange is very slow as distinct populations of up to 10 different Xe~ clusters can be observed; Xe7 and Xe8 clusters appear to be present in two different states Xe7" + Xe7"" and Xes" + Xes": In many restx~s the situation for xenon in this system resembles the case of NaA z~lite [4]. The dynamics of Xe exchange between the cages can be studied in detail by 2D-EXSY NMR. Fig. 5 shows typical spectra obtained with two mixing times for samples of different average loading. The spectra reveal well resolved cross-peaks between the signals from different xenon
248 clusters, which unambiguously c o ~ ,< n > = 3.3 at./cage < n > = 2.4 at./cage exchange of xenon atoms between adjacent a3 4 4 2 cages. Rates of exchange 5 " 3 6 i,'t, ,i'i F,, 2 1 can be obtained directly ~' t e-,, ____ ~ _ j ; "'u_,i ',,,I '~,,Q/.~___ /~,~ ~_,i" t i I '~_ by solving equation (3) together for all sites, t . = 1 O 0 ms ~. 0 t . = lOO taking into account the discrete cage populations [4]. We should emphasize that for this system with such a multisite exchange the complete set of rate constants can be estimated from a single F1 t . = 500 ms e~nt by using the t.:5OOms ~ ,~ 0 0 2D EXSY technique. Rate constants o 0 for xenon transfer between the cages of AgA zeolite depend on the number of xenon atoms in the cage, and for the cages with xenon occupancy lower than 6 230 180 130 ppm 230 180 130 are of the order of 1 s~ F2 (Fig. 6). The increase of the rate constants with Fig.5 129Xe2D EXSY NMR of xenon in AgA zeolite depending occupancy can be on the average loading and mixing time t~ explained as a result of decreasing sorption energy with rise of population [4]. From the variable temperature 2D exchange experiments the activation energy of xenon transfer between the different cages can be estimated as Ea = 45 + 10 kJ/mol, noticeably lower kn, s "1 than in NaA zeolite. Higher k, and lower "r" Ea for the intercage exchange in AgA than in NaA is most likely a consequence of higher hindrance at the intercage window w!,,0 by the sodium cations in the latter case. 2 4 6 8 n, ruTbe" d ) l n m a~-ns in o~age Another point of interest is that all the lines in the spectrum of xenon in AgA are Fig.6 Rate constants for xenon transfer shifted approximately 20 ppm to lower between the cages of AgA zeolite. field than for NaA.
ms
o
oaOo
o 0 9
00o
o000
249 4.4.
Adsorption of xenon in moleodar sieves with one-dimensional pore structure. To better tmdersta~ the relationship between lmXe NMR ~ ~ the pore sm/ctm'e it is irr~rtant to start with well-eaab~hed systen~. We have sadied with static, tragic angle spinning (MAS) and 2I~EXSY NMR the adsorption of xenon in ~ 5 , AI.J:~I 1, VPI-5, ~ 8 , ZSM-12 and SSZ-24. These rnok~a~ sieves are expected to be good nxxtel systems ~ of a) the absence of cations, whose presew.e can corr~licate the specwa b) their well established and relatively shrole crystal and pore structure. We have observed anisotropy of the ~29Xe c ~ shift which we relate to the structtwe of adsorption sites in ~ molecular sieves. The deper~nce of the anisotmpy on Xe loading in AI20-11 was analyzed in terms of a statistical dismbution of three types of xenon sites ppm (which have either 0, 1, or 2 neighboring sites occupied by other Xe, i.e., OXeO, XeXeO or XeXeXe), each with a characteristic shielding tensor, with fast exchange over 100 the three site types at room t e ~ [15]. The anisotropy is bading dependent for all the systen~ mxler 150 study, however detenmmtion of the anisotropies of the c o ~ n e n t s was possible only for AI20-11. The stall size of the channels in AI.PO-11 rmkes the n ~ of xenon types small, but when the size of the channels 150 100 50 ppm imxemses, more types of ~lsorption site can be realized and the analysis of anisotropy txx~ornes a t m c h bigger problem It is obvious, however, that the observed fig. 7 2I) EXSY spectrum ofhigh-baded anisotropy is not just a result of moulding of the xenon s m i l e of xenon in ZSM-12. electron cbud by the shape of the channels, as suggested Line in high field is from Xe in the previously by Springuel-Huet [16], ~ s e the interpart~le space anisotropy is observed even for xenon adsorbed in VPI5 - mo~ sieve with armch larger channet While exchange averaging within channels must be rapid in order to give the single anisotropic lines observed, 2D EXSY spectra show that slower exchange processes are occurring:
tm = 0 . 5 m s
tm = 3 0 m s
tm = 10 m s
FI 90 I00 ii0
''"il''''l''''l''''l'''' 110 100 90 ppm
Fag. 8
'
I
'
[
100
'
i
'
I
,
F2
i , , , i , , , , 1 , , , , i , , , , ' l , , , ,
~0
100
90
ppm
21~EXSY specam oflow-loaded sample of xenon in ZSM-12 with different mixing times.
250 a) Fig 7 shows a spectrum for a high loading of xenon in ZSM- 12. Intense cross-peaks between the anisotropic signal from xenon inside the channels and the high field line from xenon in the intercrystalline space indicate a significant degree of exchange between these two regions. The anisotropy is preserved in the cross-signal lineshape ( which again shows that xenon in each crystaUite has a single frequency due to rapid intra-channel exchange), b) There is also slow interparticle xenon exchange ( perhaps tlrAiated by the gas phase ). Fig. 8 shows spectra of a low xenon loading in ZSM-12 with different mixing times. At longer mixing times the intrazeolite signal develops off-diagonal intensity in a pattern which indicates exchange of xenon between crystallites with different orientations, i.e. exchange between components of the shielding tensor
5.
Conclusions
The resuks of the present study graphically demonstrate the utility of 2D-EXSY 129XeNMR in studies of adsorption and diffusion in molecular sieves. The technique is highly suitable for studying the porous structure of zeolites and permits a quantitative description of inter- and intra-crystallite mass transfer processes. Based on the experimentally rrr.asured rates of xenon exchange, it is possible to obtain inforrmtion on diffusion as well as the possible paths of exchange, and the distribution and availabih'ty of adsorption sites. Themxxtynamic characteristics of the adsorption system can also be detem'fined from variable temperature 2D-EXSY experiments. The experiments show that exchange processes are an important factor in xenon NMR spectra in porous which must be accounted for when correlations with the porous structure are made.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11 12. 13. 14. 15. 16.
Ito, T., Fraissard, J." J.ChenxPhys. 7__6,5225 (1982) Ripm~ster, J.A." J.AnxChem.Soc. 104, 289 (1982) Jeener, J., Meier, B.H., Bachmann, P., Ernst, R.R.: J.ChenxPhys. 71, 4546 (1979) Larsen, R.G., Shore, J., Schmidt-Rohr, K., Emsley, L., Long, H., Pines A., Janicke M., Chn~lka B.F.: Chem.Phys.I~tt. 214, 220 (1993) Moudrakovski, I., Sayari, A., Ratcliffe, C., Ripme~ster, J., Preston, K.: J.Phys. Chem. 98, 10895 (1994) Drobny, G., Pines, A., Sinton, S., Weitkan~, D., Wemn~r,.D.: Faraday Syrup. Chem. So<:. 13, 49 (1979) Perrin, C., Gipe, R.K.: J.AnxChem.Soc. 106, 4036 (1984) Abel, E.W., Coston, T.P.J., Orrdl, K.G., Sik, V., Stephenson, D.: J.Magn.Res. 70, 34 (1986) Perrin, C., Dwyer T.J.: Chem.Rev. 90, 935 (1990) Dubinin,M_, Erashko, I., Kadlec, O., Ulin, V., Voloshchuk, A., Zolotarev, P.: Carbon 13, 193 (1975) Kikger, J., Pfeifer, H., Stalkmch, F., Feoktistova, N.N., Zlxlanov, S.P.: Zeolkes 13, 50 (1993) Ripmeester, J.A.: J.Magn.Res. 56, 247(1984) Ripmeester, J.A., Ratcliffe, C.I.: J.Phys. Chem. 94, 7652 (1990) Fraissard, J., Ito, T." 7_e.olites8, 350 (1988) Ripmeester, J., Ratcliffe, C." J.Phys. Chem. 99,619 (1995) Springuel-Huet, M., Fraissard, J.: Chero_Phys.Lett. 154,299 (1989)
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviotand S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
Diffusion of
C10-C24n-paraffins
251
and perfluorotributylamine in clay catalysts
Baoqiang Liao a, Mladen Eic a*, Douglas M. Ruthven a and Mario L. Occelli b aDepartment of Chemical Engineering, University of New Brunswick P.O. Box 4400, Fredericton, N.B., Canada E3B 5A3 bGeorgia Tech Research Institute, Zeolites and Clays Research Program Georgia Institute of Technology Atlanta, GA 30332, U.S.A.
Abstract. Pillared and expanded clays show both adsorptive and catalytic properties which are similar to those of zeolites, although their structure is quite different. Five different samples of expanded clays were prepared from solutions containing hydroxyaluminum oligomers (pillared montmorillonite), or colloidal particles such as SiO2xTiO 2 or SiO2xZrO 2 (expanded montmorillonites). The pore size of four montmorillonite samples expanded with colloidal particles was modified using two different drying techniques (air or supercritical drying). Supercritically dried (SCD) samples have much higher meso/macropore area, pore volume and porosity compared to the air-dried (AD) counterparts. Diffusion measurement were carried out using the Zero Length Column (ZLC) method to determine effective
diffusion time constants (Deff/R2) for a number of n-paraff'ms and perfluorotributylamine. Analysis of the diffusion results showed that both micropore and macropore diffusion can be the rate controlling step. I. INTRODUCTION Pillared clays constitute a new class of materials of considerable interest as potential catalysts in the petroleum and chemical industry. They are cost-effective and relatively easy to prepare, but generally suffer from limited hydrothermal stability in the presence of saturated steam and deactivate fairly quickly due to coke formation. One method of improving their hydrothermal stability and coke selectivity is by expansion using solutions containing colloidal SiO2xTiO2 or SiO2xZrO2 particles [1, 2]. Upon drying, dehydroxylation of the intercalating species forms heat stable metal oxide clusters that can permanently prop the silicate layers apart thus making the newly exposed interlayer surface area (200-500 rn2/g) available for sorption and catalysis. The nature of the space generated by the various pillaring reactions strongly depends on the morphology of the clay platelets, on the size and properties of the intercalating compounds (pillaring agent) and on the method of drying used to activate the clay [3]. *author to whom all correspondence should be addressed
252 In this study effective diffusion of n-paraffins (in C10 to C24 range) and perfluorotributylamine (PFTBA) was investigated using Zero Length Column (ZLC) method on four different samples of expanded clays (two were activated by air drying and the other two by supercritical drying using CO 2 and dimethyl ether, DME). One montmorillonite sample was also pillared with A13+ clusters. Diffusion results were analyzed and correlated with the pore structure of the samples. H. EXPERIMENTAL
1. Expanded Clays The source of clay material used in this study was Ca-bentonite (bentonite L grade, obtained from the Southern Clay Products Inc. of Gonzales, TX). Two types of pillaring agent were used for the clay expansion: a. colloidal particles with composition TiO2x5Si02 (TISI samples); b. colloidal particles with composition ZrO2x5SiO 2 (ZRSI) samples; The details of preparation and conditions used to dry each gel are given elsewhere [ 4 ]. 2. Pillared day Pillaring of the original montmorillonite sample was carried out using hydroxyaluminum oligomer (chlorhydrol) supplied by the Reheis Chemical company (pillared clay sample). Details of the preparation are given elsewhere [5]. 3. Surface Area and Pore Size Characterization Total surface area for each sample was determined by applying BET analysis on N 2 adsorption isotherms at 77 o K, obtained on an ASAP 2400 microbalance (Micrometrics Instrument Corporation). Surface area, pore volume and average size of macropores were determined by standard mercury penetration porosimetry using Micrometrics Model 9220 porosimeter [4]. 4. Zero Length Column Method The Zero Length Column technique is a chromatographic method based on the analysis of desorption curve obtained when a small sample (usually less than 1 mg) of single adsorbent particles (previously equilibrated with sorbate at a low concentration) is purged with an inert gas [6]. A relatively high purge gas flow rate is used in order to maintain a low sorbate concentration at the external surface of the particles, thus minimizing any external heat or mass transfer resistance. In general He is used as the carder and purge gas, but checks can be made with other gases such as Ar to confirm absence of any extracrystalline mass transfer resistance. The analysis of the ZLC desorption curve involves solving Fickian diffusion equation with appropriate initial and boundary conditions [6]. The solution of the desorption curve is given as"
253 |
2
R2
c = 2L)--' exp(-fl. D~jct/ ) co .=, [ft. + L ( L - 1)] where 13n is given by the roots of the auxiliary equation:
(1)
fin cot fin + L - 1 = 0
(2)
and L=
6.vR2
(3)
3(1- 6)KD,,#z
The effective diffusional time constant (Deft/R2) and Henry's constant (K) can be obtained from the numerical solution of equations 1-3. HI. RESULTS AND DISCUSSION 1.
Structure
Characterization
The total surface area of each sample was determined from nitrogen isotherms at 77 K using the BET method. The method of drying will strongly influence the nature of the space generated by the pillaring reactions. Supercritical drying generates large meso and macropore surface area and pore volume, as determined by mercury porosimetry, due to dispersion of clay platelets in the presence of large colloidal particles. This leads to formation of large regions of meso and macroporosity and a broad distribution of pore size (micropores to macropores). In contrast to that, when supercritical-drying (SCD) is replaced by air drying (AD) the samples lose meso and macroporosity and their surface area, as determined by mercury porosimetry, decreases drastically. These samples, both expanded with colloidal particles and pillared clays, are characterized by a relatively narrow distribution of pore size (mainly micropores) although some meso and macropores still exist. Table 1 gives a summary of surface areas and pore structure. Table 1 Surface Area and Pore Size Characterization of Pillared and Expanded Clays Sample
BET Surf. Area (m2/g)
Hg Surf. Area (m2/g)
Pore Volume (cm3/g)
Average Pore Dia. (~l)
Particle porosity (cm3/cm 3)
Pillared Clay TISI-AD TISI-DME ZRSI-AD ZRSI-CO 2
275 414 554 363 498
-58 268 50 223
0.18 0.47 4.90 0.58 4.80
-326 728 464 865
0.14 0.41 0.88 0.47 0.88
AD denotes an air dried sample. DME and CO 2 denote supercriticaUy dried samples using DME or CO 2 respectively.
254 2. Diffusion Measurements The clay materials are considered to have a biporous structure with micro and macropores and a different pore size distribution depending on methods used in their preparation. This property is reflected in the different mass transfer resistances involved in the diffusion of sorbate molecules. In order to evaluate the relative significance of both micro and macropore diffusion, two different particle sizes were used in the diffusion experiments. The results of these experiments for n-decane diffusion in the clay samples used in this study are summarized in Table 2. The ratio of the particle sizes was set to the average value for the two samples (R1/R2=2), which should give a ratio of the effective diffusion time constants (DodR 2) of four, if the diffusion is entirely controlled by the macropore diffusion. This was found to be true for the pillared montmorillonite sample, thus confirming that the mass transfer rate is entirely controlled by macropore diffusion. However, for the four expanded clay samples, ratio values of the effective diffusion time constants were generally much less than four, indicating a transition from a macropore controlled process to the combined micro/macropore controlled situation. For most of the expanded clay samples, (SCD samples in particular), it seems that the intraparticle (micropore) diffusion in the single clay platelets is the rate controlling, especially for the cases where Dcfr/R2 ratios are relatively low (between one and two).
Table 2. Summary of Diffusion Data for Decane (C 10H22) on Clay Samples with Different Particle Sizes (RI/R2 "" 2) TemperaOct~l 2 Ocff/R.22 Ratio of Sample ture x 103 x 103 time (oc) s-1 s-1 constants Pillared 225 115 4.47 3.88 Clay 200 0.581 2.32 3.99 175 0.27 1.12 4.2 TISI200 9.39 16.0 17 DME 175 3.00 4.40 15 TISI-AD 200 4.13 8.26 2.0 175 1.61 2.69 17 ZRSI-AD 200 2.95 7.03 2.4 175 115 2.39 2.1 ZRSI200 6.12 8.36 1.4 CO2 175 1.74 3.44 2 150 0.68 1.12 1.65 R1 = 115 /an and R2 = 57.5 /an (based on the average particle size). Two or three different particle sizes were used to study perfluorotributylamine (PFTBA) diffusion in the clay samples expanded with colloidal particles (TISI-AD and TISI-DME), as well as the pillared clay (montmorillonite). The results obtained showed virtually no effect of particle size on DcdR 2 values, thus confirming micropore controlled diffusion. A summary of
255 the results is presented in Table 3. Transition of the kinetic behaviour of PFTBA from macro to micropore control is not surprising since the kinetic diameter of the PFTBA molecule (10.4,4) is much larger than for an n-paraffin molecule (about 4.3,4 for n-paraffins with carbon number greater than four). The size of the PFTBA molecule is much closer to the size of micropores in the individual clay platelets (approx. 8 A x 15,4 ) which are defined by the height and lateral free separation between pillars. Table 3. Summary of diffusion data for PFI'BA on clay samples with different particles sizes Sample
Pillared Clay
TISI-AD
TISI-DME
ZRSI-CO2
ZRSI-AD
Temp.(C) 250 225 200 225 200 275 150 225 200 175 150 225 200 175 150 225 200
D(eff)/R12xl03 (second -1) 3.22 1.07 0.28 2.55 1.20 0.52 0.26 6.00 2.04 0.94 0.43 3.66 2.04 1.03 0.40 3.66 1.20
D(eff)/R22x103 (second -1) 2.80 0.81 0.24 3.26 1.61 0.46 0.20 4.10 1.80 1.24 0.50 4.16 1.70 0.64 0.32 2.82 0.81
Ratio of Time constants 0.87 0.76 0.87 1.28 1.34 0.89 0.8 0.68 0.88 1.31 1.17 1.14 0.83 0.62 0.8 0.77 0.67
RI=115 um; R2=57.5 um (Based on the average particle size) Figures 1 and 2 show Arrhenius plots for different n-paraffins and PFTBA on the pillared and one expanded clay sample, (data are based on average Deff/R 2 values obtained with different particle sizes). As expected the effective diffusion time constants for paraffins decrease as the chain length (carbon number) is increased. The decreasing trend is very noticeable in the C10 to C20 range, but becomes less pronounced for the carbon numbers greater than 20 (Fig. 3). Deff/R 2 values for supercritically dried samples (SCD) are somewhat higher compared to the air dried (AD samples), but the differences are not significant, especially for the longer chain paraffins (see Fig. 3). This is not surprising given the fact that most of the diffusion resistance is within the single clay platelet, which is of the same internal structure, due to the same origin of the expanded clay samples. Deff]R 2 values for C10 and C16 in the pillared montmorillonite were found to be several times smaller than for the expanded clays (Fig. 3), due to much lower porosity (larger macropore resistance).
256 Effective diffusion time constants for PFTBA are generally higher compared to nparaffins (except for decane) for all the samples studied (Figures 1 and 2). This may be due to the fact that the long chain paraffins have a more complex diffusional path through both the micro and macropore structure of the clays investigated in this study.
1.00E-02
-.
9n-C 10H22 I r~C16H34 i
1.00E-03 ~.)
i An'C20H42 ' I en.C24H50 !
~"
.
.
\
\R
\
~ a 1.00E-04
1.00E-05
J
.
1.8
1.6
,
2
2.2 1000/T (K^-I)
2.4
2.6
Figure 1. Arrhenius plots of D(eff)/R 2 for different nparaffins and PFTBA on pillared clay sample 1.00E-02
41 ]R
: .
.
.
. -
~,
[ on-C10H22 l
-~
Bn-C16H34 ii An-CZ0H42 i
<
1.00E-03
LU O0
\\
= \
\\.
<~
"~, \
~ i\
n,' ~"
1.00E-04
r~ w
1.00E-05
' 1.6
1.8
-!
!
2
2.2
.i 2.4
, 2.6
2.8
1000/T (K ^-1) Figure 2. Arrhenius plots of D(eff)/R 2 for different nparaffins and PFTBA on ZRSI-CO2 sample
257 1.00E--02 OTISI-DME 1TISI-AD &ZRSI-CO2 XZRSI-AD
1.00E-03 uJ
_=
1.00E-05
10
NfPiUared clay
I
I
~
t
t
t
12
14
16
18
20
22
24
Carbon number
Figure 3. Comparsion :of D(eff)/R^2 for n-paraffins on different clay samples at 200 C
IV.
CONCLUSIONS
1. Expanded clays of different structure were prepared by using different preparation and drying procedures (i.e., air vs supercfitical drying). 2. A large increase of meso and macroporosity was obtained for supercfitically dried (SCD) samples of expanded clays. 3. Diffusion of n-paraffins in pillared montmofillonite is shown to be entirely macropore controlled. 4. Combined micro/macropore diffusion is the rate controlling step for the diffusion of nparaffins in clays expanded by colloidal particles. Micropore diffusion seems to be the more important step in the overall mass transfer process. 5. Deft/R2 values of n-paraffins generally do not show very large differences for the diffusion in expanded clays prepared by different drying methods. 6. Deft/R2 values for n-decane and n-hexadecane in the pillared montmofillonite are several times lower compared to those in the expanded clays due to much smaller macropores (rate controlling) of the pillared montmofillonite. 7. Micropore diffusion entirely controls diffusion of PFTBA in both pillared and expanded clays.
258 ACKNOWLEDGEMENTS Financial support for this project provided by the Natural Science and Engineering Council of Canada (NSERC) is gratefully acknowledged. NOTATIONS c Co Deft
K
L R
t v z
sorbate concentration in gas phase initial steady state sorbate concentration effective diffusivity dimensionless Henry's constant defined by Eqn. 3 equivalent radius of particle time interstitial velocity of gas thickness of zero length column defined by eqn. (4) particle macroporosity
REFERENCES
1. M.L. Occelli, Proc. Int. Clay. Conf., Denver 1985; L.G. Shultz, H. van Olphen and F. A. Mumpton eds., p. 319 (1987). 2. S. Yamanaka, T. Nishihara and M. Hattori, Mat. Res. Soc. Symp. Proc., 111, 283 (1988). 3. M. L. Occelli, K. Takahama, M. Yokoyama, and S. Hirao, Synthesis of Microporous Materials, Vol. II, p. 81; M. L. Occelli and H. E. Robson eds., Van Nostrand Reinhold, New York, 1992. 4. M. L. Occelli, P.A. Peaden, G.P. Ritz, P.S. Iyer and M. Yokoyang, Microporous Materials 1, 99 (1993). 5. M.L. Occelli and R. M. Tindwa, Clays and Clay Minerals 31, 22 (1983) 6. M. Eic and D. M. Ruthven, Zeolites 8, 40 (1988).
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
259
Sorption properties of dealuminated large crystals of ZSM-5. A n e w a p p r o a c h to the description of isotherms. J. Kornatowskia, b #, M. Rozwadowski a, W. Lutz c, W.H. Baur b a Faculty of Chemistry, Nicholas Copernicus University, Gagarina 7, 87-100 Torufi, Poland b Institut for Kristallographie und Mineralogie, Johann Wolfgang Goethe-UniversitY,t, Senckenberganlage 30, 60054 Frank~rt am Main, Germany c Institut for Angewandte Chemie, Rudower Chaussee 5, 12489 Berlin, Germany
Sorption isotherms for H20, C6H6 and N 2 on silicalite and ZSM-5 after different dealumination treatments (severe steaming and a milder leaching) are described by equations derived from the Polanyi-Dubinin potential theory of volume filling of micropores. The calculated W 0 values correspond to the micropore volume actually filled with adsorbate, i.e. the calculations should include only those micropores which are active in the actual sorption process.
1. INTRODUCTION The influence of dealumination (hereafter designated as Deal) on the sorption properties of zeolites is not sufficiently appreciated [ 1,2]. In spite of many papers reporting on sorption properties of MFI type zeolites (e.g. refs. 4-54 in ref. 3 and refs. 3-39 in ref. 4), there are only a few papers dealing with large crystals [1,5] and with the analytical description of adsorption [6] or isotherms [1,7]. Sorption properties of MFI type zeolites depend on crystal size [8], morphology [9], temperature [3,5], Si/Al ratio [5,10], Al distribution [3,5], acidity and degree of ion exchange [5,8,10], the type of adsorbate [4,5,8,10] and the sorption history [4]. Therefore, we studied effects of Deal [2] on the sorption properties of ZSM-5 for various types of adsorbates of potentially different interactions with the zeolite framework: small molecules of H20 with a high dipole moment, larger cylindrical N 2 molecules with a quadrupole moment due to double bonding and large and flat C6H6 rings with n electrons. Formerly [ 1], we applied isotherm equations based on the Polanyi-Dubinin potential theory of volume filling of micropores to describe the isotherms of C6H 6 adsorption on ZSM-5. Now, we extend the application of the theory by using the actual values of the micropore volume taken from isotherms.
2. EXPERIMENTAL The same samples synthesized as large crystals and dealuminated previously [ 1,2] were used here. The Deal treatments of parent ZSM-5 were leaching with 1.25 N HCI solution for 2 and 5h at the boiling point (AD 2 and 5 samples) or with 0.3 M AIC13 solution at RT for 24h (SD #Now: Institut fiir Brennstoffchemie, Technische Universitdit, Worringerweg 1, 52074 Aachen, Germany.
260 Table 1 Sorption characteristics, Si/AI ratio [2] and degree of dealumination of the ZSM-5 samples BET Micro- Meso- and Sorption capacity at p/ps=0.8 Si/AI Sample surface pore macropores (chem. area volume surface H20 C6H6 N2 anal. m2/g cm3/g area m2/g mmol/g mmol/g molec./u.c. AAS) ZSM-5 AD 2 AD 5 SD SDUS US 4 US 3 US 2 US 1 Silicalite
367 362 355 365 362 357 355 354 338 358
0.1739 0.1723 0.1721 0.1748 0.1812 0.1803 0.1795 0.1765 0.1722 0.1788
9.2 5.6 3.2 6.4 8.2 8.4 6.1 6.3 8.0 6.0
1.90 1.75 2.25 2.10 0.80 0.90 0.40 0.85 0.35 1.60
1.43 1.43 1.52 1.51 1.48 1.50 1.55 1.48 1.40 1.42
29.5 29.0 28.8 29.3 30.7 30.7 30.3 29.7 29.1 29.6
39.5 93 4 120.3 67.0 93.4 67.0 79.9 140.7 129.4 -~660
Ddeal in % as defined from: chem. NH 3 analysis TPD
57 67 40 57 40 50 71 69
4 7 3 75 -100 295 288 80
samples) and more severe steaming (US samples; US 1 and 2 pretreated by AD, US 3 and 4 by ion exchange with NH4 + and SDUS by SD leaching; all US samples finally leached with 1.25 N HNO 3 at the boiling point for 2h; details in ref. 2). The applied Deal procedures [1,2] changed the samples largely into the H-ZSM-5 form [2]. Adsorption isotherms for H20 and C6H6 (new more precise measurements different from those in ref. 1) were determined in a vacuum device equipped with a McBain quartz spring balance and an MKS Baratron gauge at 298.2 K. Every point of the isotherms was measured under equilibrium conditions, i.e. when no gain of mass was observed for at least 12h at a constant relative pressure of adsorbate P/Ps. The N 2 sorption isotherms were measured in an Omnisorp 100, Coulter, device at 77 K. The samples were activated before all measurements in situ under stationary vacuum of~ 10-3 Pa at 700 K to a constant mass (~8h).
3. RESULTS AND DISCUSSION
The synthesis products were pure MFI type phases (XRD) [1,2] composed of large columnar crystals of mean length of ca. 300 ttm. The crystals were twinned [2] in the manner discussed and described elsewhere [11 ]. The Deal treatments caused no loss of crystallinity or other damages to the crystals [2]. The previous chemical analyses and TPD of ammonia [2] (Table 1) allowed to order the samples by increasing degree of dealumination (Ddeal): ZSM-5 < SD <_ AD2 < AD5 < SDUS < US4 < US3 < U S 2 < US1 The parent ZSM-5 zeolite shows a sorption capacity for H20 (Table 1, Fig. 1) only slightly higher than silicalite in spite of its much higher content of AI. We interpreted this formerly as a blocking of a part of the pores due to twinning in our large crystals [2]. Both the constant sorption capacity for C6H6 and N 2 and the constant micropore volume (Table 1) indicate that the low sorption capacity of large crystals of ZSM-5 has to have other reasons. One of them could be a localized strong adsorption of H20. A Deal by leaching removes up to ~2/3 of the originally present AI and causes only a small decrease of the sorption capacity for H20. This may support our suggestion [2] that a type of new acidic centres as e.g. silanol nests can
261 I "e"
.
I
I
i
(Z~)Z~E) Z S M - 5 nuuon ZSM-5 z~Az~z~z~ Z S M - 5 00000 ZSM-5 ooooo ZSM-5 ,A,AAAA
ZSM-5
iIIIi
ZSN-5 ZSM 5 + + + + + ZSM 5 x x x x x Silir
c~
o
E s
I
,
o
AD5 SD SDUS US2 US5 US4 AD2 US 1
I
~)
o. k..
o "u
.
|Q
~
" |
~
ijtl
g
i
~I
x
"
l/
t
$
u
4-
[] x
A
1
9
,
o
0.0
0.2
0.4
0.6
0.8
1.0
p/ps
Figure 1. Isotherms ofH20 sorption on dealuminated large crystals of ZSM-5 measured at RT. be formed under milder Deal conditions. A Deal by steaming lowers the sorption capacity considerably below that measured for silicalite in spite of the Ddeal being apparently not higher (analytical Ddeal , Table 1) than that for the leached samples [2]. This indicates a significant loss of hydrophilicity connected with the acidic OH groups at the AI centres. The effect is well pronounced already at the lowest relative pressures P/Ps (Fig. 1) and supports the statement [2] that steaming can lead to complete Deal of the zeolite framework. The isotherms of all samples have the same shape (type I, IUPAC classification) and they are divided into two groups: 1) isotherms for the steamed samples with a very low sorption capacity and 2) isotherms for the reference ZSM-5 and silicalite samples as well as the samples dealuminated by a milder leaching which show a sorption capacity ca. 3 times higher than in group 1. As the content of AI found analytically [2] is similar for the samples of both groups (e.g. AD 2 = SDUS, SD = US 4, AD 5 ~ US 1 ~ US 2), it indicates that steaming results in a high Deal of the framework but AI oxide species evolved are mostly not removable from the crystals even by leaching with 1.25 N HNO 3 at the boiling point. It is worth noting that the decreasing sorption abilities of particular samples expressed by the isotherms (Fig. 1) follow generally the order of the Ddeal mentioned above, i.e. the order of increasingly severe Deal treatments. Considering the relatively low initial content of A1 in the parent ZSM-5 sample (~2.5%, cf. Table 1) [2], strong effects in isotherms indicate a surprisingly high sensitivity of the sorption measurements. The sorption capacity for C6H6 (Table 1, Fig. 2) is nearly independent of the Ddeal. The nonpolar C6H6 molecules may be insensitive to the number of negatively charged A1 centres in the framework. The lack of an increase of sorption after Deal may mean that the framework
262
m
ii
II ,r m
o
E t-
AAAAA Z S M - 5 ~wuuwZSM 5 ZSM 5 +++++ZSM-5 xxxxxSilicalite
O
US4 AD2 US1
~ Z S M 5 BuanuZSM-5 z,,,,',,~,',,z~ Z S M - 5 O0000ZSM 5 ooeeeZSM 5
AD5 SD SDUS US2 US3
o.- i cb.0
0.2
0.4-
0.6
0.8
1.0
p/ps Figure 2. Isotherms of C6H6 sorption on dealuminated large crystals of ZSM-5 (RT). structure is restored [2] by the migration of Si and the healing of the vacancies formed a~er removal of AI. The shape of the isotherms differs considerably and the isotherms are divided into the same two groups as for 1-120 sorption. All leached samples have the same type I shape as ZSM-5 (group 2) and the steamed samples (group 1) change that to a stepwise S-type curve. This indicates that initially, at the lowest relative pressures P/Ps, the C6H6 molecules are bonded in the pores close to their openings at the surface layer of the crystals and the pores become clogged at a certain sorption level. This means the occurrence of a surface barrier. At higher P/Ps, those weak bonds are broken and an instant filling of pores almost to the full capacity occurs. This indicates that no centres able to interact with C6H6 are present in the framework of steamed ZSM-5. A surface barrier was proposed [3,12] to explain the S-shaped uptake curves observed for other sorbates on ZSM-5. As suggested in ref. 3, the surface barrier "may be related to the enrichment of the outer shell by aluminium accompanied by the corresponding enhancement of the amount of acid -OH groups". We cannot agree with that since, as opposed to ref. 3, we found the effect of a surface barrier only for the highly dealuminated samples in which any enrichment of framework A1 in the outer shell and thus an increase of acidity are not observed. Then, the surface barrier is likely due to an interaction of rt electrons of the aromatic ring with the non- or weakly acidic surface OH groups. In the not steamed samples, the effect of a surface barrier may be cancelled by drawing the C6H6 molecules into the channels due to interactions between rt electrons and the OH groups at the AI centres. Possibly, in the samples composed of powder ZSM-5 [3], the outer surface of the crystals is so large that it contains a sufficient number of surface OH groups that can overcome other interactions. Thus, the surface barrier effect can be observed in spite of the presence of the AI centres and their
263
32
.I
30
28
26
-- US1
24
...... US2 -- US4 - - SD + SDUS m-ZSM-5 ~ AD5 Silicalite
-- AD2
.... US3
'~'
relative pressure i i i I 0 0.2 0.4 0.6 0.8 1 Figure 3. Isotherms ofN 2 sorption on dealuminated large crystals of ZSM-5 measured at 77 K. 22
strongly acidic OH groups. The above suggestions about the surface barrier are strongly supported by the experimental finding that the observed sorption capacity for benzene is higher than the theoretically calculated maximum value (cf Table 2 VII and VIII, W 0 values). This also suggests a nonnegligible role of the external surface of the crystals on the final sorption effect of ZSM-5 for C6Ht, i.e. the occurrence of the interactions surface OH groups - C6H 6. The sorption capacity of all samples for N 2 (Fig. 3) is close to the well determined [5] value of~30 molecules/u.c, and nearly independent of the Ddeal (Table 1) similarly as seen for C6H6. The shape of isotherms for ZSM-5 and other samples of group 2 is nearly of type I with only a slightly pronounced S-type shape and a small, strongly inclined hysteresis loop. The isotherms for the steamed samples (group 1) show a sharp S-type shape connected with an abrupt increase of sorption from 24 to ~30 molecules/u.c. [5] with a large and uptight directed hysteresis loop within the P/Ps range of 0.10 to 0.30 characteristic of silicalite [5]. This change of shape and increase of the hysteresis follow gradually the Ddeal. The weaker hysteresis for our silicalite reflects the presence of some framework A1 (NMR) found analytically (0.15 mol %, ICP). The hysteresis effect is ascribed to the formation of a solid N 2 phase in the pores of silicalite [5]. Thus, the considerable and systematic weakening and inclination of the hysteresis loop with increasing content of AI at a nearly constant final sorption capacity indicate occurrence of a type of N 2 - A1 centres interactions which inhibit the formation of such solid phase. The samples of group 1 and AD2 show the second fiat and elongated hysteresis loop within the P/Ps range of~0.5 to 1. To our knowledge, this effect is observed for the first time and requires further examination. However, at P/Ps approaching 1, all isotherms except for those of the not dealuminated samples, i.e. ZSM-5 and silicalite (Fig. 3), exhibit a steep
264 increase of sorption usually characteristic of liquid phase condensation in the macropores. Thus, the second hysteresis may be due to such condensation in the "secondary pores" formed during Deal. They were found in small amounts by N 2 adsorption (Table 1). The pore size distribution (Table 1) and the effective pore diameter (Horvath-Kawazoe plots) determined from the N 2 sorption measurements show a sharp peak at ~5.5 A. Besides, a clearly lower amount of pores with diameters up to ~12 A and a low number of surprisingly uniform mesopores of~19-20 A can be inferred. The latter may reflect the formation of holes by damaging parts of the framework between neighbouring channel intersections during Deal. The BET surface area as well as micropore volume values (Table 1) show only small differences of less than 10% between parent ZSM-5 and all dealuminated samples. This finding together with a very low surface area of meso- and macropores (Table 1) support the deduction from XRD that Deal causes no loss of crystallinity. The fundamental equation of the Polanyi-Dubinin potential theory of volume filling of micropores is the Dubinin-Astakhov (DA) equation [ 1] W = W 0 exp [-(A/13E0)n] = W 0 exp [-k(A/13)n]
(1)
where W is the volume of the "liquid-like" adsorbate filling the micropores at the pressure p and temperature T, W 0 is the total volume of micropores, 13 is the constant characterizing the adsorbate, A is equal to RTln(Ps/p) and determines the adsorption potential, Ps is the saturation pressure of adsorbate vapour at temperature T, E 0 is the characteristic energy of adsorption, k is the structural parameter correlated with micropore dimensions, and n is the parameter connected with the micropor0us structure of the adsorbent. We applied the DA eqn. as well as its derivatives, the Dubinin-Radushkevich (DR) and Dubinin-Radushkevich-Stoeckli (DRS) eqns. [ 1] to describe the sorption isotherms for H20 and C6H6. In routine calculations, the value of W 0 is assumed as one of the adjustable parameters which should yield the best fit of the theoretical and experimental isotherms. The fit is determined by the determination coefficient DC which is equal to 1 in the case of an ideal fitting. For zeolites of a defined geometry of pores, application of such "open" W 0 value is not justified. We performed the calculations not only by such classical method but also by using several different actual values of micropore volume taken from the experimental isotherms and corresponding to: a) P/Ps ~ 0.1 (close to the applicability limit of the above eqns. to sorption isotherms); b) P/Ps ~ 0.85 (almost full saturation of zeolites by sorbates) and c) to the total geometrically calculated volume (the highest possible packing of the sorbate molecules in the pores). Table 2 gives the results of calculations with all three equations for different n values and by using all four possibilities for the W 0 value mentioned above. For clarity, only one leached (AD 5) and one steamed (US 2) samples are listed. As can be seen from the DC values in Table 2, the isotherms for C6H6 are generally better described (DC closer to 1) than those for H20. However, the actual W 0 values in case a), i.e. taken from the isotherms at P/Ps ~ 0.1 (Table 2 II and VI), yielded the best description (DC closest to 1) from all actual values used [cf. a), b) and c) above]. Table 3A shows the results of calculations with DA and DRS eqns. for all samples at the "open" W 0 value and Table 3B at the actual W 0 value taken from isotherms at P/Ps "~ 0.1. It is evident that the actual W 0 values match the classically calculated "open" W 0 values (Tables 2, 3), while no other values of W 0 [cf. b) and c) above] and especially not the total volume of micropores could give any reasonable results. Some discrepancies between the W 0 values calculated from DA, DR and DRS eqns. and the "isothermal" W 0 values may be due to both the occurrence of some amount of mesopores (19-20/k) found from the N 2 sorption and the twinning of the crystals.
265
Table 2 Description of the sorption isotherms of H20 (I-IV) and C6H6 (V-VIII) obtained at calculated (I, V) and constant actual W 0 values: a) taken from isotherms at P/Ps equal to ~0.1 (II, VI), b) ~0.86 (III, VII), c) at the theoretical maximum volume of the sorbate in micropores (IV, VIII) AD5
US2
Eqn n
W 0 cm3/g
DC
n
W 0 cm3/g
DC
DR DA DA DA DRS
2.0 3.0 4.0 0.2162 2.0
.0166 .0140 .0128 .9031 .0208
.9019 .8150 .7290 .9871 .9677
2.0 3.0 4.0 1.9870 2.0
.0056 .0046 .0042 .0056 .0057
.9942 .9754 .9330 .9942 .9944
II
DR DA DA DA DRS
2.0 3.0 4.0 1.9165 2.0
.0170 .0170 .0170 .0170 .0170
.9006 .6993 .4241 .9085 .9258
2.0 3.0 4.0 2.5011 3.0
.0050 .0050 .0050 .0050 .0050
.9761 .9619 .8453 .9889 .9954
III
DR DA DA DA DRS
2.0 3.0 4.0 0.6409 2.0
.0467 .0467 .0467 .0467 .0467
.9814 .5483
2.0 3.0 4.0 0.5711 2.0
.0210 .0210 .0210 .0210 .0210
.9418 .5428
IV
DR DA DA DA DRS
2.0 3.0 4.0 0.4091 2.0
.1098 .1098 .1098 .1098 .1098
.9859 .1043
2.0 3.0 4.0 0.3013 2.0
.1098 .1098 .1098 .1098 .1098
.9188 .2419
DR DA DA DA DRS
2.0 3.0 4.0 2.7253 3.0
.1152 .0950 .0855 .0989 .0970
.9832 .9926 .9719 .9939 .9936
2.0 3.0 4.0 2.1472 3.0
.1192 .0914 .0793 .1130 .0968
.9708 .9603 .9300 .9712 .9640
VI
DR DA DA DA DRS
2.0 3.0 4.0 2.4865 3.0
.1030 .1030 .1030 . 1030 .1030
.9699 .9829 .9104 .9928 .9874
2.0 3.0 4.0 1.9392 2.0
.1220 .1220 .1220 . 1220 .1220
.9704 .8912 .7461 .9704 .9704
VII
DR DA DA DA DRS
2.0 3.0 4.0 1.5035 2.0
.1380 .1380 .1380 .1380 .1380
.9488 .7847 .5642 .9611 .9521
2.0 3.0 4.0 1.7332 2.0
.1340 .1340 .1340 .1340 .1340
.9626 .8389 .6572 .9679 .9634
VIII
DR DA DA DA DRS
2.0 3.0 4.0 2.1800 3.0
. 1098 .1098 .1098 .1098 .1098
.9808 .9614 .8608 .9880 .9780
2.0 3.0 4.0 2.2354 3.0
. 1098 .1098 .1098 .1098 .1098
.9668 .9325 .8251 .9710 .9541
266 Table 3 Results of the description of the sorption isotherms for water and benzene obtained at calculated (A) and actual (B) W 0 values (at P/Ps ~ 0.1) for all investigated MFI type samples A
H20
C6H6
Sample
nDA (real) W 0 cm3/g nDRS
DC
nDA (real) W 0 cm3/g nDRS
ZSM-5
0.7691
.0250 .0152 .0126 .0109 9031 0208 0000 .0188 0069 0059 0000 0043 0037 .0031 0056 0057 ,0039 0037 0242 .0175
9791 9702 9338 9303 9871 9677 9676 9049 9349 9381 9691 8668 9969 9939 9942 9944 8992 9070 9983 9974
1.9495
.0140 .0140 .0150 .0150 .0170 .0170 .0190 .0190 .0050 ,0050 ,0040 0040 0030 0030 0050 0050 0040 0040 .0140 .0140
.9529 .9583 .9158 .8691 .9085 .9258 8901 8184 9152 9048 8254 8372 9844 9879 9889 9954 8987 8538 9771 9837
2.0 AD 2
2.3945 3.0
AD 5
0.2162
SD
0.0000
SDUS
1.2863
US 4
O.0000
US 3
1 1067
US 2
1 9870
US 1
2.1509
Silicalite
1.0373
2.0 2.0 2.0 2.0 2.0 2.0 3.0 2.0
ZSM-5
1.8722 2.0
AD 2
1.1632 2.0
AD 5
1.9165
SD
1.3914
SDUS
2.2032
US 4
1.8020
US 3
1.8399
US 2
2.5011
US 1
2.0059
Silicalite
1.9998
2.0 2.0 3.0 2.0 2.0 3.0 3.0 2.0
2.0 2.4423 3.0 2.7253 3.0 3.5999 4.0 2.5014 3.0 2.1825 3.0 2.0954 3.0 2.1472 3.0 2.0460 3.0 2.0886 3.0
2.5379 3.0 2.2311 3.0 2.4865 3.0 2.9804 3.0 1.9400 2.0 1.9436 2.0 1.9392 2.0 1.9392 2.0 1.8950 2.0 2.5716 3.0
DC
1337 1219 0942 0907 0989 0970 .0918 0893 1009 0941 1126 0989 1154 0997 1130 0968 1088 0933 1139 1024
9794 9777 9832 9833 9939 9936 .9948 .9938 9738 9714 9647 .9589 9636 .9580 9712 9640 .9580 .9519 9839 9914
.1030 1030 0980 0980 1030 1030 1000 1000 1190 1190 1220 1220 1220 1220 1220 1220 1150 1150 0990 0990
9705 9799 9823 9710 9928 9874 .9897 9901 9681 9690 9636 9638 9632 9631 9704 9704 9576 .9570 .9782 .9892
267 The regular coincidence of the W 0 values for both types of sorbates and different equations proved explicitly that the analytical description of the sorption isotherms should include only that part of the micropores which are really involved in the process of sorption. In other words, though the classical calculations yield always W 0 values several times lower than the geometrical volume of the pores, these calculated values correspond well to the volume of the micropores filled with the sorbate molecules and not the total volume of micropores. Applying the latter yields not only the wrong description discussed above but also decreased values for the apparent (as a mean value for all active and inert pores) adsorption potential.
4. CONCLUSIONS The structure of well crystallized large crystals of ZSM-5 is not damaged under severe dealumination treatments. The pore volume and BET surface area change very slightly upon dealumination of ZSM-5. The sorption measurements for various sorbates reflect very sensitively small changes in the degree of dealumination. They reveal the occurrence of a surface barrier in the sorption process of nonpolar molecules as C6H6 and N 2 which are able to interact via their electron cloud with the framework of zeolite. Equations derived from the Polanyi-Dubinin potential theory of volume filling of micropores can be applied to the description of dealuminated ZSM-5 samples especially in the case of nonpolar C6H6 molecules. The values of the micropore volume W 0 obtained from the experimental isotherms at P/Ps ~ 0.1 match those calculated from the equations. It indicates that W 0 corresponds to the actual volume of the micropores being really involved in the process of sorption and not to the total volume of the micropores.
Acknowledgments. The work was partially supported by the Polish Committee for Scientific Research (KBN) and the Bundesministerium fiat Forschung und Technologie (BMFT). REFERENCES 1. J. Kornatowski, M. Rozwadowski, A. Gutsze and K.E. Wisniewski, in Zeolites as Catalysts, Sorbents and Detergent Builders, H.G. Karge and J. Weitkamp (eds.), Elsevier, Amsterdam, 1989, Stud. Surf. Sci. Catal., 46, p. 567. 2. J. Kornatowski, W.H. Baur, G. Pieper, M. Rozwadowski, W. Schmitz and A. Cichowlas, J. Chem. Soc. Faraday Trans., 88 (1992) 1339 and refs. therein. 3. A. Zikgmova and M. Derewinski, Zeolites, 15 (1995) 148. 4. H. Karsli, A. Gulfaz and H. Yiacel, Zeolites, 12 (1992) 728. 5. U. M011er and K.K. Unger, in Characterization of Porous Solids, K.K.Unger, J.Rouquerol, K.S.W.Sing and H.Kral (eds.), Elsevier, Amsterdam, 1988, Stud.Surf.Sci.Catal., 39, p. 101. 6. R.J.M. Pellenq and D. Nicholson, J. Phys. Chem., 98 (1994) 13339. 7. A.V.Kiselev, Proc.5thlZC, Naples, 1980, L.V.C.Rees (ed.), Heyden, London,1980, p.400. 8. V.P. Shiralkar, P.N. Joshi, M.J. Eapen and B.S. Rao, Zeolites, 11 (1991) 511. 9. K. Foger, J.V. Sanders and D. Seddon, Zeolites, 4 (1984) 337. 10. V.R. Choudhary, K.R. Srinivasan and A.P. Singh, Zeolites, 10 (1990) 16. 11. J. Caro, M. Noack, J. Richter-Mendau, F. Marlow, D. Petersohn, M. Gripentrog and J. Kornatowski, J. Phys. Chem., 97 (1993) 13685. 12. A. Micke, M. Bialow and M. Korifik, J. Phys. Chem., 98 (1994) 924.
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Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
Convective Methods to Investigate Kinetics on Microporous Solids
269
Multi-component
Sorption
A. Micke and M. Btilow The BOC Group, Technical Center, 100 Mountain Avenue, Murray Hill, NJ 07974, USA Based on the Volterra integral equation technique, a universal method is proposed to calculate sorption kinetic curves of multi-component gas mixtures in porous solids under the conditions of convective methods. In particular, the simulation of concentration vs. time dependences as obtained by the Zero Length Column Chromatography and the F o u r i e r Transform Infrared Spectroscopy arrangements are considered. A simplified approach to simulate the kinetics as measured by the latter method is given. The applicability of the theoretical treatment presupposes an isothermal kinetic system and sufficiently small concentration changes. 1. I N T R O D U C T I O N The development of multi-component sorption kinetics on microporous solids becomes increasingly important because many purification and separation processes involve sorption kinetics under thermodynamically non-equilibrium conditions. As a basis of overall process optimization, kinetic processes have to be modeled and rate parameters have to be identified. There is a need for information on the diffusivity matrix of microporous sorption systems, due to the high density of sorption phase and, therefore, mutual influences of sorbing species. The complexity of the phenomena involved demands both experimental [1] as well as theoretical work [2]. In recent years, considerable attention was paid to the introduction of new experimental methods for investigation of mixture sorption kinetics. With regard to their mass balances, these methods can be classified as closed (cf. [3, 4, 5]) or open. The main class of the open methods comprises convective methods. Convective methods can be divided into those where the bed length either does or does not influence the measured characteristic response function. Respective techniques are the Finite Column Length Method (cf. [6]), the Zero Length Column Chromatography (ZLCC) [7, 8] and the F o u r i e r Transform Infrared Spectroscopy (FTIR) [9, 10, 11]. Both latter methods are equivalent with respect to their physical fundamentals. The manner in which the concentration vs. time dependences were monitored is different for each method. For the ZLCC method, the concentration change in the gas phase is observed. In FTIR technique the average concentration of the sorption phase is measured. Due to this peculiarity, this method offers a remarkable advantage for data evaluation, compared to the ZLCC method. However, mathematical modeling of both methods [7, 8, 11, 12, 13] exists for single component sorption kinetics, only. Presumably, because of that reason, the ZLCC method has not yet been applied to mixture sorption kinetics. The data evaluation utilized in the mixture case of the FTIR method still bases on simplified simulations of single component behavior [10, 11]. which leads to misleading data. Evaluation formulas widely utilized for single component sorption kinetics for the ZLCC method show both advantages and
270 disadvantages. On the one hand, they are easy to handle although care has to be taken to interpret the calculated data. On the other hand, this approach fails in case of mixtures. The Volterra integral equation approach as applied to the case of single component kinetics of the ZLCC method [8, 12] as well as to multi-component kinetics in batch systems [14], allows one also to simulate mixture sorption kinetics for convective arrangements. The following treatise shows in detail how the Volterra integral equation technique can be utilized to simulate completely the sorption kinetics of gas mixtures in convective systems. 2. M O D E L I N G Some of the convective methods which allows one to neglect the influence of bed length on sorption kinetics, appear to be suitable to favorably measure intracrystalline mass transport in microporous systems (cf. [7, 11 ]). This is mainly due to the exclusion of influences of sorption heat on mass transport as measured. In the case of the FTIR method, the in situ measurement of time-dependent average concentration in the sorption phase (cf [13]), represents an additional advantage over any other transient method. The open mass balance of convective methods is, however, a principal disadvantage. Therefore, the direct measurement of concentration in the case of the FTIR method compensates this disadvantage to a large extent. For both methods, one of the purposes of the approaches for the evaluation of experimental data as given in [7, 11], was to facilitate calculations. As shown for the ZLCC method in the case of single component sorption kinetics [8, 12], an oversimplified consideration of data treatment may lead to erroneous transport coefficients if the experimental conditions for a concrete sorption system do not meet the assumptions made for data evaluation [2]. Irrespective of the particular evaluation method applied,
carrier gas with s0rbate m ixture
O,.Q,~O @~O~-O
|174 |
~) = c o n s t
T = const
ev|
CBi(t )
a-i( t )
FTIR observation
ci(t)
ZLCC observation
Figure 1. Scheme of the experimental arrangement
the situation becomes even more complicated if the experiment is extended to mixture kinetics. This relates not only to data evaluation that is more complex but also to simplifications made for single component experiments such as a negligible concentration in gas phase compared to the concentration in sorption phase. The principal arrangement of convective sorption experiments as depicted schematically in Figure 1 comprises a carrier gas stream of constant velocity u (constant flow rate ~,) and constant temperature T, loaded by certain constant concentrations of a N-component gas mixture. The FTIR method observes the average sorbate concentrations t~ iti-~N
J i=l
directly. On the other hand, the ZI.CC
method utilizes the implicit way of measuring the outlet gas phase concentrations { c i }N . i-1
For an isothermal sorption column, the mass balance equation /)te_(y, t ) +
1 - e ~) _ ~)2 e ~--t~Y , t ) = D a x ~) y2 C(y . t ) - u .
~)
~. y
t) .
t >. 0
.y e [0 L ]
(1)
271 is utilized for the full description of above mentioned convective systems. The quantity C=~-l ci-~N denotes the vector of N partial sorbate concentrations c. in gas phase as function J i=l 1 of the space coordinate y of the column with length L and the time variable t. The variable ~={ ~ti }N stands for the vector of average partial sorbate concentrations ~. in the sorption i=l 1 phase at the column point x. The parameters D and u describe, respectively, the dispersion ax
and the convection in the column. The quantity E with IV I v IV I= f d z , ~ = I V I - I V I' v
s
(2)
V
stands for the void fraction of the column where V v and V S denote, respectively, the volume of the empty column and the one of sorbent particles. For the transport process inside the sorbing medium, the validity of the generalized second Fick's law is assumed, i.e. the partial sorbate concentrations a i fulfill N otai(x,t)=
~xx
ij~xxaj ( x ' t )
, i=l(1)N, t>0,
XeVs,
(3)
j=l i.e. in vector notation
a t (l=Vx ~)Vx (1 ,
(4)
where ~ = { Dij }Ni,j
=
1
denotes the matrix of diffusion coefficients and t~= {a i }Ni= 1 the vector
of the local partial sorbate concentration. To describe the sorbate concentration ~s(t)=~
= ~ Y ( ~ ( t ) ) , t > O,
x, t)lx E
(5)
FVs
at the outer surface Fvs of sorbent particles, non-linear sorption isotherms 3: = fi i= 1 are permitted. The average concentration ~t is related to the local sorbate concentration by ~y,t)=
f
~x,t)dx,
t>0,
ye [0,L],
(6)
Vs~y where v s ~ y stands for the volume of all sorbent particles that are located in the column section at y ~ [ 0, L ]. To simulate mixture sorption kinetic experiments utilizing the above system of equations, the general solution of the transport equation (3) for variable boundary conditions is calculated from that for constant boundary conditions by solving systems of Volterra integral equations of the second kind. The solution for mixtures under constant boundary conditions is reduced to the knowledge of that for a single component and the solution of an eigenvalue problem as proposed in [ 14]. The technique allows the calculation of the sorbate concentrations c to fit
272 experimental ZLCC multi-component data as well as the determination of the vector ~ of average concentration in order to evaluate VrlR mixture kinetic data. 3. M O D E L
ASSUMPTIONS
If the experimental arrangement allows one to neglect the influence of the bed length L on the concentration vector ~, the gas phase concentration will be fully described by the values of the function C at the entrance and at the end of the column, i.e. by ~ 0, t ) and ~ L, t ) which are, of course, time-dependent functions. The concentration at the entrance of the column can easily be determined in blank experiments, i.e. C~0,t)=~(t),t>0.
(7)
Thus, ~ L , t ) remains as the only function that describes gas phase behavior. Assuming sufficiently high smoothness with respect to the space co-ordinate y, the Taylor expansion of the function C at y = L into a polynomial yields
u{ }=U~y~L,t)L e(L,t)-~(t)
2 Oy 2 e ( L ' t ) + O
(L2)
'
(8)
where O( L 2 ) stands for the quadratic Taylor remainder. Neglecting this remainder and setting e(t)-e(L,t),
~(t)-~L,t),
(9)
etc., in order to shorten the formulas furtheron, the mass balance equation (1) yields ot~t)+--~-~-~t)=13
6(t)-~(t)
, t>0,
(10)
where the parameter u [3=L=IV
V
~, I-IV
S
I
(11)
is introduced. Using such an approximation, the dispersion Dax is not neglected but it is equated to Lu Dax = 2
(12)
that makes the equations system stable. Equation (10) represents the model transition from a finite length column to that with zero length properties. Furthermore, the matrix of diffusion coefficients is assumed to be independent of sorbate concentration. This assumption is valid as far as the concentration change during an experimental kinetic run is sufficient enough. By ensuring small changes in concentration during kinetic experiments, the constancy of the diffusivity matrix fl~ can be assumed, i.e. equation (4) can be rewritten as b ~)t t~= ~ Axa.
(13)
The set of equations (5), (7), (10) and (13) has to be solved to simulate the sorption kinetic process under consideration.
273
4. MODEL SOLUTION The principle of superposition ([ 15] p. 180) as the basis of the Volterra integral equation approach, has its mathematical expression as a relationship between average sorbate concentration and corresponding surface concentration. The vector A of average partial concentrations is determined by t
~ t)- a0 : f~'(t- s) { as(S)- a0 } ds,
(14)
0 1
where ffC =
- ~Ni, j = 1 d e n o t e s Hij
the matrix of normalized solutions for constant boundary
conditions. The matrix function ffChas the following properties. The general solution ~onst for constant boundary conditions is expressed by ~i~c~
- (~_ = fie(t) { (i0+ -(10_ } .
(15)
The corresponding vector (l c~ of local concentrations fulfills the transport equation (3) under constant initial and boundary conditions: (tc~
. t ) = tl0_ if t < 0
(i'c~
. t ) = ~ + if t > 0 '
(16)
where tl0_ and tl0+ stand, respectively, for the constant surface concentration prior to and during a kinetic experiment. In the case of single gas sorption kinetics, the matrix function ~ degenerates into a scalar function that is represented by the normalized solution I:I for constant boundary conditions. The solutions fI are well known from literature (cf. [16, 17]). For simple geometric shapes of particles, e.g. of a microporous solid, IZI is given as cos( k n ) = 0 for 0 = 0 (plate),
oo
- 'I: ) = 1 - 2 (0 + 1) E H(
~ , J0 ( kn ) = 0 for 0 = 1 (cylinder), e-k~'l:
(17)
sin( k n ) = 0 for 0 = 2 (sphere). n=0
As shown in [14], in general, ffCis of the form fie( t ) = I- ~-I h( A t ) ~3
(18)
i= 1" T h e q u a n t i t i e s A = { A i } Ni= 1 a n d ~ = { Bij }N i, j= 1 are, where h( A t ) = diag { 1 - ft( Ai t ) }N respectively, the eigenvalues and the transposed eigenmatrix of the diffusivity matrix fl), i.e. ( ~ - A i I)B.,T = O , i = I(1)N, where B.i =
Bji
= 1 are the transposed eigenvectors of the diffusivity matrix fl~.
(19)
274 Integration of the mass balance equation (10) leads to t
) f{o ~ s ) - e B ( S ) } ds
e..(t)-eo+~
~ t ) - ~ 0 =13
'
t>O
(20)
9
Using (14), & can be substituted by the surface concentrations ~s and equation (20) yields t
t
1-13 e-(t) - dO + - - f ~ ' ( t - S ) { gs(S)- (~ } ds = 13f { e..(s) - ~ ( s ) } ds o o
'
t,O
9
(21)
Due to relation (5), equation (21) can be rewritten as t
t
e_(t , - ~ + - - 7 - - f :5c'( t - s ,{ 5(e,,( t , ) - 3 : ( ~ )} ds=[~ f { e,(s ,-eB( s , }ds, t > 0 . o o
(22)
This equation system represents a system of non-linear Volterra integral equations of the second kind with respect to the concentration vector ~ The corresponding vector 0~of average concentration can be calculated as a function of e utilizing relations (5) and (14), i.e. t
~ t , - (10 =
f :rc'tt-s ){ 5(e_,( t ,
) - 5 ( e 0 )} ds.
(23)
0 To solve non-linear integral equation systems, a broad variety of methods exists (cf [ 18]). Quadrature methods were successfully applied to simulate sorption kinetic curves [ 19, 20]. 5. S I M P L I F I C A T I O N S In cases where the gas phase concentration can be considered as being independent of the sorbed amount (e.g. if e-->1 {small sorbent volume} or [3--->~ {high flow rate}) and where the concentration change is small enough to ensure a sufficiently accurate linear approximation of sorption isotherm, formula (23) can directly be utilized to explicitly calculate ~. The rigour of this presumption becomes evident from equation (20). For example, utilization of exponential functions to express surface concentration ~s (as applied in [10, 11]), would lead, due to (20), to a representation of ~ in terms of exponential functions. Therefore, it is apparent, that the above presumption contradicts the validity of the column mass balance equation (10). Nevertheless, cases may exist for which such an approach is sufficiently accurate. A linear isotherm in the case of multi-component sorption processes is considered, i.e. N fi(•) = Z G j cj or 5 ( C ) = ~ C i=l
(24)
275 is assumed to be valid for an appropriate ~ =
{ Gij }Ni,j = 1" Furthermore,
the gas phase
concentration vs. time dependence might be given as (25)
r t )- C0 = ( 1 - e-at )(Coo- e0 ), t > 0 ,
where o~ > 0 is an appropriate constant and the vectors e0 and Coo denote the respective concentrations at the initial and at the equilibrium state. As far as the sorption phase concentration has no detectable influence on the gas phase concentration, this relation (25) with respect to the gas phase concentration should reflect all apparatus effects (as finite valve opening, delays due to finite flux, etc.) superimposed upon the intrinsic sorption kinetics. Besides, a piecewise linear approximation instead of (25) may also be utilized within this approach. Under above premises, the vector ~t of the average sorbate concentration can be calculated explicitly as
t 0.( t )- 120: f ( 1 - e-at ) ~ ' ( t- s )ds ~ (Coo- r ), t > 0 . 0
(26)
The integrand in (26) is a matrix function, i.e. the integral stands for the componentwise integration of this matrix. The matrix function ~; = Hij i, j = 1 can be calculated explicitly for mixtures up to ternary ones. For the most often considered case of a binary mixture ( c f [ 14] part II), one obtains
__1__1( c2 c 2H(A 2 t ) - c
ftll(t)=l-cl
llq(A it)
)
1
fi12 ( t )=
-cc1~
I5121( t ) =
c 1 c2 c~-c2 (I2I( A2 t )- I-I( A1 t ) )
1 (
H22 ( t ) = l + c l _ c 2
1
c 1 I2I(A 2 t ) - c 2I=I(A 1 t)
, t > O,
(27)
)
where A1/2=2 D l l + D 2 2 +
(Dll+D22)2+4D12D21
, Cl/2 =
D21
=A1/2-Dll
Thus, the integrals in formula (26) can be calculated by oo
t f0 I=I'(A( t - s ))(1-e-aS)ds = 1- 2 ( 0 + l )
oo
e
e-k2nAt
kn2 (~n2A - ~) -
kn2 (--kn2A- o0
,(29)
276 where the constants 0 and k n are given in (17). An alternative series expansion is given in [ 17] for plates (p. 49), for cylinders (p. 67) and for spheres (p. 87). 6. C O N C L U S I O N S Although the physical fundamentals of the ZLCC and the FTIR methods are equivalent in many ways, data evaluation practiced hitherto within the frame of both methods is based on different sets of equations to calculate transport coefficients. Both approaches utilize one-track formulas to reach description of the kinetic system in a way as simple as possible. The widely applied ZLCC evaluation method uses asymptotic expansions for the transport parameter determination. The FTIR evaluation method utilizes a complete curve fit with a strt)ngly simplified mass balance in the gas phase of the kinetic ~ystem. The FTIR evaluation method presupposes a gas phase concentration independent of sorbate concentration but dependent on apparatus effects, while, paradoxically, the ZLCC setup is completely reversed. The gas phase concentration is presupposed independent of apparatus eff6cts but the transport parameters calculation just exploits the gas phase concentration dependence on the sorbate phase concentration. A uniform theoretical approach for both methods is presented. It allows treatment of the system dynamics not only for single component sorption kinetics but also for the general case of multi-component sorption kinetics by accounting for the same factors of influence for both arrangements. For the FTIR method an explicit set of formulas to determine mixture kinetic transport parameters has been derived for special cases.
REFERENCES 1. Suzuki M. (Ed.), Fundamentals of Adsorption, Kodansha, Tokyo, 1993. 2. Btilow M. and Micke A., Adsorption, 1 (1995) 29. 3. Hille J., Btilow M. and Micke A., in [ 1] p. 285. 4. Qureshi W. R. and Wei J., J. Catal., 126 (1990) 126 (part I); 147 (part II). 5. Yasuda Y. and Matsumoto K, J. Phys. Chem., 93 (1989) 3195. 6. Do D. D., Hu X. and Mayfield P. L. J., Gas Sep. Purif, 5 (1991) 35. 7. Eic M. and Ruthven D. M., Zeolites, 8 (1988) 40. 8. Micke A., Btilow M. and Kocirik M., Bet. Bunsenges. Phys. Chem., 98 (1994) 27. 9. Karge H. G. and Klose K., Ber. Bunsenges. Phys. Chem., 79 (1975) 454. 10. Niegen W. and Karge H. G., Microporous Materials, 1 (1993) 1. 11. Niegen W. and Karge H. G., Stud. Surf. Sci. Catal., 60 (1991) 213. 12. Micke A., Kocirik M. and Btilow M., Microporous Materials, 1 (1993) 363. 13. Shavit D., Voogd P. and Kouwenhoven H. W., Collect. Czech. Chem. Commun., 57 (1992) 698. 14. Micke A. and Billow M., Gas Sep. Purif, 4 (1990) 158 (part I)" 165 (part II). 15. Courant R. and Hilbert D., Methoden der mathematischen Physik, Bd I1, Springer, Berlin, 1937. 16. Carslaw H. S. and Jaeger J. C., Conduction of Heat in Solids, Clarendon Press, Oxford, 1959. 17. Crank J., Mathematics of Diffusion, Clarendon Press, Oxford, 1964. 18. Baker C. T. H., The Numerical Treatment of Integral Equations, Clarendon Press, Oxford, 1975. 19. Btilow M., Micke A., ZEUS - Zeolite Uptake Simulator - Software, VCH Verlagsgesellschaft, Weinheim, DECHEMA- Monografien, 118 (1990) 349. 20. Micke A., Computing, 42 (1989) 207.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
277
Zeolites as the Key Matrix for Superior d e N O x Catalysts Tomoyuki Inui Division of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan
ABSTRACT Comparison of the ability of NO decomposition was made between a bulk metal oxide catalyst and metal-containing zeolitic catalysts prepared by either an ion-exchange method or incipient crystallization method. The former can decompose NO but oxygen formed was uptaken by the surface and deactivated it, however, the later could maintain the decomposition reaction with releasing N2 and 02. The intermediate oxidation state of metal oxide can be existing only in its highly dispersed state on the zeolitic matrix. The combination of acid properties created by the metal with its intrinsic catalytic properties allow the chemical transformation of the added hydrocarbon into appropriate fragments which are then combusted, and which reduce the catalyst surface before the NO decomposition hereon. In the light of non-linear reaction mechanism on NO elimination, zeolite is the indispensable key factor to design deNOx catalyst for the diesel engine and lean-burn engine. 1. INTRODUCTION The exhaust gases from diesel engines and lean-burn engines contain a large excess oxygen, and the conventional three-way catalysts for gasoline engines do not work well for these exhaust gases. Because the reducing components such as carbon monoxide and hydrocarbons in the exhaust gas have burnt, the surface state of the active species of catalyst becomes a very oxidative one almost inactive in NOx oxidation or decomposition. Several years ago, it has been found out by some researchers that NO can be decomposed catalytically on appropriate metal-containing zeolites into N2 and 02 under the reaction condition without oxygen feed. For example, Iwamoto and coworkers [1] reported in 1990 that Cu-ion-exchanged ZSM-5 showed an extremely higher catalytic activity than other traditional catalysts. Inui and coworkers reported in 1991 [2] that Cu-silicate having MFI structure and some other Cu-incorporated zeolites such as Cu-A and Cu-X exhibit a steady NO decomposition activity. However, when a considerable concentration of oxygen exists in the feed gas, Cu in those zeolite catalysts is easily oxidized and the activity for decomposition of NO is not exhibited any more. Since then, it is widely known however, that when a low concentration of appropriate hydrocarbons exists in the reaction gas, the conversion of NO on those zeolitic catalysts can be maintained even under the coexistence
278 of oxygen [2,3]. Hamada and coworkers reported that this type of reaction could progress on simple protonated ZSM-5 and mordenite without supporting any other transition metal element [4] and even on merely y-alumina itself [5]. These observations were made with a very low space velocity, i.e., 3600 l/kg~zat.h, and it has been pointed out afterward that at higher space velocity conditions the NO conversion decreases markedly [3]. This indicates that the intrinsic catalytic activities of these catalysts are fairly low. So far, most of the catalysts used for this reaction are based on zeolites or zeolite-like microporous crystalline catalysts such as silicoaluminophosphates [6]. In this paper, the reasons why the microporous crystalline catalyst is the key factor for the deNOx catalyst is discussed through both experimental data and computer simulation, and the results would be provided for design and preparation of better catalysts and the way for suitable reaction operation. 2. DIFFERENCE IN CATALYTIC DECOMPOSITION OF NO ON CONVENTIONAL BULK CATALYSTS AND ON ZEOLITIC CATALYSTS In an early stage of our research on catalytic elimination of NO, we used a bulk catalyst composed of 8.0 wt% Fe3On/?-alumina, which was prepared by a coprecipitation method with a careful reduction, for NO decomposition [7]. A 5 ml (4.1 g) portion of the catalyst was packed in a Pyrex glass tubular reactor of 8 mm inner diameter. The catalyst-bed length was 6.2 cm. On the catalyst NO diluted with He was easily decomposed above c a . 200~ and formed N2; however, below 350~ the oxygen formed by the NO decomposition was immediately uptaken by the catalyst almost quantitatively and deactivates it as shown in Figure 1. The broken line in the figure drawn at 272 min on stream indicates the time that the oxygen, formed by NO decomposition, oxidizes Fe304 in the catalyst to Fe203 obeying the following stoichiometric equation. 2 NO + 4 Fe304
> N 2 + 6 Fe203
(1)
When the reaction gas was fed to the reactor with a lower flow rate,, the decomposition of NO and the deactivation of the catalyst, owing to the oxidation by the oxygen formed by NO decomposition, progressed longitudinally from the entrance of the catalyst bed. The deactivated zone, which could be differentiated from non-deactivated zone by a clearly different color, progressively moved forward, and no NO was detected in the effluent gas until the whole catalyst bed was oxidized. After that the NO conversion decreased suddenly to zero-conversion level. When a highly concentrated 02 was fed with NO, the catalyst was oxidized rapidly and no NO conversion occurred, even in the presence of a small concentration of hydrocarbons such as propylene in the feed gas. These facts indicate that both the NO decomposition rate and the oxidation rate of the partly-reduced metal-oxide catalyst are very fast and that the oxidation of NO (reaction Eq. (2)) on the oxidized catalyst surface would be very low, especially at the lower temperature range below 300 ~. NO + (1/2) 0 2
>
NO 2
(2)
279
1O0 -OO-O--O---O0
80 ~
0
60
> 0 o
9 2:
40200
I
0
60
,
I
,
I
,
I
120 180 240 Time ,on stream (min.)
300
Figure 1. NO decomposition on a bulk Fe304 catalyst supported on y-alumina under an O 2absent condition. 5 ml (4.1 g) of 8.0 wt% FeaO4/Y-alumina catalyst, 1400 ppm NO diluted with He, 293~ 1000 h -1 SV. As briefly mentioned in the introduction, a highly dispersed copper on the zeolite matrix could maintain the activity of NO decomposition without significant deactivation. Here, a comparison was made between Cu-ion-exchanged H-ZSM-5 (denoted as Cu/H-ZSM-5) and Cu-incorporated protonated MFI-type silicate, (denoted as H-Cu-silicate) to evaluate the performance for NO decomposition. Cu (1.8 wt%)/H-ZSM-5 and H-Cu (0.57 wt%)-silicate gave XRD patterns same as that of H-ZSM-5, and no indication for the existence of isolated copper oxides. Those catalysts had almost the same BET surface area, 380 and 373 m'Vg, respectively. The NO decomposition conversions on both catalysts under the O2-absent condition are compared in Figure 2. The NO conversion of Cu/H-ZSM-5 decreased gradually with an increase of the time on stream. That of H-Cu-silicate slightly decreased in the beginning and stabilized at ca. 12% level, which was about one third that of Cu/H-ZSM-5. The redox responses of both catalysts were measured by a pulse method using H E and 02 as the reducing and oxidizing reactants, respectively. For H-Cu-silicate, these responses were about one third those of Cu/H-ZSM-5, which was consistent with the difference in NO decomposition activity of both catalysts. The integrated amounts of NO converted till 8 h on stream for Cu/H-ZSM-5 and H-Cusilicate are 47 times and 54 times those calculated by the Eq. (3), respectively. This is clearly different from the result obtained with the bulk catalyst prepared by coprecipitation mentioned above. During the NO conversion, no N20 and NO 2 were detected, and almost all converted NO was detected as N 2, which was confirmed by replacing the carrier gas N 2 by He, indicating that the NO decomposition progressed as per Eq. (4). 2 Cu + 2 NO 2 NO
> N2 + 2 C u t > N2 + 0 2
(3) (4)
280
50 40 = o
"-
30
9 =
~
2o
0
I
0
I
2
I
I
,
I
I
i
4 6 T i m e on stream (h)
8
Figure 2. Comparison of NO decomposition activity between Cu/H-ZSM-5 (O) and H-Cusilicate (0). NO 4.0 %, N 2 96%, SV 2000 h -1, 500~ Under the O2-absent condition, this NO decomposition occurred on the active sites of the partly reduced copper oxide dispersed in MFI silicate crystals, and the activity was proportional to the redox capacity of Cu containing zeolite. Under the conditions where 02 is present with a considerable concentration, both catalysts allowed the oxidation of copper, and exhibited no NO conversion activity any more. The capacity for Cu ion-exchange for a typical H-ZSM-5 is larger than the amount of Cu which can be incorporated into the framework of MFI structure. This seems to be an advantage for Cu/H-ZSM-5; however, the position of Cu is not in the framework but in the space of pore channels as shown in Figure 3, which was obtained by molecular dynamic calculation [8]. Therefore it is presumed that
I
I
2A Figure 3. State of Cu species in Cu/H-ZSM-5 depicted by computer simulation.
281 this kind of Cu would be instable due to an easy sintering or evaporation when exposed to high temperatures and repeated redox-cycles [9], mainly owing to its high dispersion and the low melting point of copper itself. It was suggested from the results mentioned above that the zeolite, in which Cu is incorporated as much as possible, not by the ion-exchange method, but by the incipient incorporation at the stage of crystallization, has a potential of high performance for NO decomposition. The incorporation of Cu was then investigated for various kinds of zeolites. As a result, considerably higher concentrations, i.e., 3.7 and at least 8.6 wt% Cu could be uniformly incorporated into the crystals of zeolites X and A, respectively. The NO decomposition activities of these catalysts were measured in the same way as mentioned in Figure 2, and it was found at 500~ that the Cu (8.6 wt%)-NaA gave the same activity as H-Cu-silicate; however, it exhibited a maximum NO conversion (55%) at a lower temperature around 350~ This activity was the same as the activity of Cu/H-ZSM-5 obtained at 500~ The activity per Cu involved in Cu (8.6 wt%)-NaA was much lower than that of Cu-silicate, however, the maximum activity per catalyst volume of Cu (8.6 wt%)-NaA was 4.6 times that of Cu-silicate. In Figure 4, the effect of Cu concentration incorporated into NaA and a Cu catalyst supported on a SiO 2 gel (calcined at 800~ BET surface 360 m2/g) prepared by the incipient impregnation method, on the catalytic performance for NO decomposition are compared in both reduced and oxidized surface states.
100 809 o,,,~
60-
9
40-
r
9 Z
200
a
0
200
400 Temperature (~
I
600
I
800
Figure 4. Comparison of the catalytic performance between Cu-NaA with different Cu contents and Cu supported on SiO 2. Circle: Cu (18.0 wt%)NaA; Triangle" Cu (7.1 wt%)NaA; Square: 18 wt% Cu/silica (prepared by incipient impregnation); Full symbols: with H 2 reduction before use; Empty symbols: without H2 reduction, 4% NO/N 2, SV 500 h -1.
282 The catalytic activity for NO decomposition markedly increased when the catalysts were reduced before use. The activity of Cu NaA was much higher than that of the Cu supported on the SiO2 gel with the same copper loading. Thus, Cu containing zeolites exhibit much higher catalytic activity than the Cu catalyst supported on SiO2 gel, and the activity increases with the increase of Cu content. The activity of the Cu-incorporated zeolite is apparently lower than the ion-exchanged one, but with increase in the amount of Cu incorporation into the zeolite matrix, the activity per catalyst volume could be beyond that of the Cu-ion-exchanged H-ZSM-5. 3. COMPARISON OF REDOX PROPERTY OF CU CONTAINING ZEOLITE WITH Cu SUPPORTED ON SILICA GEL OR y-ALUMINA
The above three kinds of Cu catalysts with the same concentration (8.6 wt%) were compared for their redox properties. Figure 5 shows their DTA profiles in the temperature programmed reduction with H2. As shown in the figure, Cu-NaA displaid two distinct peaks appearing at 160 and 250~ and these profiles were confh'med repeatedly at least three times without any change.
3, 2I.
._A
o
Cu-NaA
I,,.'=4
I
Cu/SiO2
3< [.., Cu/T-A1203 100
200 300 Temperature (~
400
Figure 5. TPR profiles for Cu catalysts prepared by different methods. Cu content of each catalyst was 8.6 wt%. The weight decrease of the two peaks of Cu-NaA was equivalent and the total weight decrease corresponded to the weight difference between CuO and Cu. Therefore, the low temperature peak and the high temperature peak correspond to the change from CuO to Cu20 and from Cu20 to Cu, respectively. The Cu/SiO 2 catalyst showed two peaks, but these
283 peaks were not as distinct as in the case of Cu-NaA and they were shifted to higher temperature. Cu/y-A1203 catalyst showed only a single peak at around the same temperature range as the peaks of Cu/SiO 2. These indicate that the Cu loaded on non-zeolite matrix had a much lower degree of dispersion than Cu-NaA. These results suggest that only the Cu dispersed in the zeolite matrix has a potential to yield fairly stable Cu § ions under the appropriate reaction condition. 4. THE ROLE OF ACIDIC PROPERTIES OF Z E O L I T I C CATALYSTS FOR NOx REDUCTION The acidity of H-ZSM-5 [10,11] and H-BEA [11] are strictly proportional to the content of aluminium, and the acidity and intrinsic catalytic property based on the existence of A1 in the framework of these high silica zeolites can be changed by isomorphous substitution of other transition-metal elements for the A1 in ZSM-5 and BEA [12]. A typical comparison was carded out for the deNOx reaction on H-Fe-silicate, H-Cosilicate, and H-Ga-silicate under an excess oxygen condition and a low concentration of noctane [13]. The oxidation activity of these catalysts for n-octane was in the following order. H-Fe-silicate > H-Co-silicate > H-Ga-silicate n-Octane combusted on H-Fe-silicate at the lowest temperature almost completely, but on H-Ga-silicate many other unburnt hydrocarbons, namely aromatics, formed besides CO 2 and CO at the highest temperature range. On H-Co-silicate, n-octane almost combusted at the medium temperature range, but it formed hydrocarbons ranging C 1 - Clo in a lower extent. These combustion characteristics are clearly related to the acidity redox property of the metal incorporated in those metallosilicate catalysts and these combustion activities were consistently synchronized with the catalytic performance of NO elimination [13,14]. As a result, H-Co-silicate exhibited the highest performance for deNOx reaction among various kinds of metallosilicates as shown in Figure 6 [ 15]. lOOi
^
~
80 0
60
~
40
0 L)
20 ~ . -
200
300
w
v
400
500
600
Temperature (~ Figure 6. Effect of temperature on deNOx performance of H-Co-silicate. Catalyst: H-Co-silicate (Si/Co = 20), 1000 ppm NO, 10% 02 diluted with N 2, 560 ppm cetane, 30 000 h -1 SV. r-I NO conversion; O Cetane conversion to CO 2 and CO; I Cetane conversion to CO.
284 5. HIGH THERMAL RESISTANCE OF HIGH SILICEOUS MICROPOROUS CRYSTALLINE CATALYSTS
Silica gel and "f-alumina easily reduce their surface areas after high temperature calcination above 800~ however, high siliceous microporous crystals such as ZSM-5 and other metallosilicates having the same framework structure as ZSM-5, maintain their high surface areas even after calcination at 1000~ [16,17]. As summarized in Table 1, non-metal silicate having MFI structure was very stable against the high temperature calcination, and with increase of A1 content the decrease in BET surface area was observed. Metallosilicates (H-Fe-silicate and H-Co-silicates) were much more stable than H-ZSM-5. Cu ion-exchanged ZSM-5 was very unstable. Co ion-exchanged ZSM-5 was much stable in comparison with Cu ion-exchanged ZSM-5, however, more unstable than H-Co-silicate. Table 1 Change in BET surface area of various kinds of metaUosilicates by a high temperature calcination BET surface area (m2/g)
Catalyst
non-metal silicate H-ZSM-5 H-ZSM-5 H-Fe-silicate H-Co-silicate Cu/H-ZSM-5 Co/H-ZSM-5
(Si/Al<1500) (Si/A1 100) (Si/A1 40) (Si/Fe 100) (Si/Co 100) (Si/A1 20) (Si/A1 40)
calcined at 540~ for 3.5 h
calcined at 1000~ for 2 h
difference
392 353 367 363 371 385 364
393 321 327 361 357 257 332
- 0 32 40 2 14 128 32
6. CONCLUSION The decomposition of NO can progress even under the oxygen excess condition when the following main three factors are combined with as good balance; i.e., a proper metallosilicate catalyst, the addition of an appropriate hydrocarbon in the feed, and the temperature at which combustion of the hydrocarbon and decomposition of NO happen on the reduced catalyst surface. The most reasonable reaction mechanism of this reaction, the Microscopic Sequential Reaction (MSR) Mechanism [2,13,15] has been considered based on the redox mechanism and the non-linear real heterogeneous mechanism. In this mechanism, the hydrocarbon adsorbed on the surface oxygen active sites combusts with consuming adsorbed oxygen atoms at once and forms islands of reduced catalytic surface whereon NO decomposes before the active sites are oxidized with the excess oxygen. This reaction mechanism could be realized well on the metal dispersed zeolitic matrix, in particularly, HCo-silicate which is one of the best catalysts for deNOx in an oxygen excess gas, with the coexistence of fairly long carbon-chain saturated-hydrocarbons such as n-octane and ncetane.
285 REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
M. Iwamoto, H. Yahiro, T. Yoshioka and N. Mizuno, Chem. Lett., 11 (1990) 19671970. T. Inui, S. Kojo, M. Shibata, T. Yoshida and S. Iwamoto, Stud. Surf. Sci. Catal., 69 (1991) 355-364. M. Iwamoto, Stud. Surf. Sci. Catal., 84 (1994) 1395-1410. H. Hamada, Y. Kintaichi, M. Sasaki and T. Ito, Appl. Catal., 64 (1990) L1-L4. Y. Kintaichi, H. Hamada, M. Tabata, M. Sasaki and T. Ito, Catal. Lett., 6 (1990) 239244. Y. Takita, Preprints of 74th Annual Meeting of Catalysis Soc. Japan, Kagoshima, 1994, p. 54-55. T. Inui, J. Kohashi and H. Shingu, Preprints, 33rd Annual Meeting of Chem. Soc. Jpn, Fukuoka, 1975, I, p. 40. A. Miyamoto, M. Kubo, K. Matsuba and T. Inui, "Computer Aided Innovation of New Materials II", M. Doyaamaa et al. (Eds), Elsevier 1993, p. 1025-1028. S. Matsumoto, Abstracts of Intern. Forum on Environmental Catalysis '93, Tokyo, Feb. 4-5, 1993, p. 61. W.O. Haag, R.M. Lago and P.B. Weisz, Nature, 309 (1984) 590. T. Inui,, K. Matsuba and Y. Tanaka, Catal. Today, 23 (1995) 317-323. T. Inui, Stud. Surf. Sci. Catal., 83 (1994) 263-272. T. Inui, S. Iwamoto, S. Kojo, S. Shimizu and T. Hirabayashi, Catal. Today, 22 (1994) 41-57. K. Ishihara and T. Inui, J. Jpn, Inst. Energy, 72 (1993) 98-103. T. Inui, S. Iwamoto and S. Shimizu, Proc. 9th Intern. Zeolite Confer., Montreal, 1992, Butterworth-Heinemann, R. von Ballmoos, et al. Eds., 1993, p. 405-412. S. Kon, S. Iwamoto and T. Inui, Preprints of 69th Annual Meeting of Chem. Soc. Jpn, Kyoto, 1995, I, p. 61. S. Iwamoto and T. Inui, 74th Annual Meeting of Catalysis Soc. Japan, Kagoshima, 1994, p. 58-59.
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Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviotand S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
287
Catalytic Properties of Palladium Exchanged Zeolites in the Reduction of Nitrogen Oxide by Methane in the Presence of Oxygen: Influence of Hydrothermal Ageing.
Claude Descorme a, Patrick G61ina, Michel Primet a, Christine Lecuyerb and Jacques Saint-Just b
aLaboratoire d'Application de la Chimie b. l'Environnement UMR 9977 CNRS Universit6 Claude Bernard Lyon I, bat. 303 43, Blvd du 11 novembre 1918 F69622 Villeurbanne Cedex France bGaz de France, Direction de la Recherche 361 av. du Pdt Wilson BP33 F93211 La Plaine St-Denis Cedex France
Pd exchanged HZSM5 and HMOR catalysts were found highly active and selective in the catalytic reduction of NO by methane in the presence of oxygen at 773 K. Their specific catalytic behaviour is ascribed to the presence of Pd ions dispersed in the zeolite channels. Both catalysts are totally deactivated after steam ageing at 1073 K. The migration and the reduction of Pd ions into large metal Pd particles due to severe dealumination of zeolitic frameworks is thought to be responsible for the loss of activity.
1. INTRODUCTION In the past few years, many studies have been devoted to the catalytic reduction of nitrogen oxides with C2 and higher hydrocarbons in the presence of excess oxygen and now more and more are concerned with methane as reducing agent. The difficulty is that most of the catalysts used with non-methane hydrocarbons were found inactive with methane. Very recently cobalt, palladium and gallium exchanged ZSM-5 zeolites were found highly active and selective in the catalytic reduction of NO by methane in large concentrations of oxygen [ 1-11 ]. We wish to report here the catalytic properties of Pd loaded zeolites (ZSM-5, MOR and Y) in the catalytic reduction of NO with methane in oxidizing atmosphere. In order to evaluate their potential use in the treatment of exhaust gases on natural gas fueled vehicles, the influence of steam ageing on the physicochemical and catalytic properties of the most active catalysts is carefully examined and discussed.
288 2. EXPERIMENTAL Y and mordenite zeolites were provided in their sodium form by Zeocat (ZF-090 tbr Y sample and ZM-060 for MOR sample) and further exchanged with an aqueous solution of ammonium chloride to get the ammonium form. The ZSM-5 zeolite sample was purchased in the ammonium form from PQ Zeolites B.V. (CBV 5020, Si/A1 ratio of 25). All our Pd loaded zeolite samples were prepared by conventional exchange of the ammonium form of each zeolite sample in an aqueous solution of tetrammine Pd(II) nitrate at 353 K for 24 hours. After exchange the preparation was thoroughly washed with deionized water, filtered and dried at 393 K overnight. The elemental compositions determined by atomic absorption analysis are listed in table 1. Table 1 Elemental composition of the catalysts catalyst
Si/AI molar ratio
wt%Na
wt%Pd
Pd
A1 mol/uc
Pd-H-ZSM5 Pd-H-MOR Pd-H-Y
23.6 7.3 2.9
0.04 0.05 0.75
0.5 1.0 1.2
0.27 0.27 1.3
3.9 5.8 49
Pd exch. level(%) 7 4.6 2.6
In order to decompose Pd complexes and ammonium ions, the Pd exchanged zeolite samples were subsequently activated under flowing oxygen ramping the temperature from ambient up to 773 K at a rate of 0.5 K/min. The H-ZSM-5 and H-MOR based catalysts were steam aged according to the following treatment. After being calcined under oxygen flow at 773 K (temperature ramp rate of 0.5 K/min), the catalysts were purged for 1 hour with nitrogen before increasing further the temperature up to 1073 K at a rate of 5 K/min. 10% water was then admitted into the nitrogen flow in order to steam the samples for 6 hours. Water admission was stopped and the samples were purged for 1 hour under nitrogen at 1073 K before being cooled down to room temperature in a nitrogen stream. The catalytic activities for the conversion of NO and CH 4 were measured using an upstream U-shaped micro-catalytic quartz reactor in a steady-state plug flow mode. Typically 200 mg of catalyst were deposited onto the 16 mm diameter sintered glass disc of the reactor, activated in situ in oxygen flow at 773 K as described in the preparation section, purged for 1 hour at 773 K and cooled down to 523 K in helium flow before being contacted with the reaction mixture. The reaction mixture consisted of 2000 ppm NO, 1000 ppm CH 4 and 1000 ppm 0 2 (balance as helium), which corresponded to the stoichiometric mixture where the S ratio, defined as [oxidant]/[reductant] = ([O2]+0.5[NO])/2[CH4], equals 1. The total flow rate of the feed was 167 cm3/min (gas hourly space velocity, GHSV = 30,000 h-l). The temperature was ramped at a rate of 1 K/min up to 773 K and maintained for 10 hours at 773 K. The samples were then cooled down to 523 K before increasing the 0 2 concentration up to
289 6240 ppm (lean mixture, S = 3.62) and carrying out the same temperature profile for catalytic activity recording. The effluent gases were analyzed using gas chromatographs with TCD and FID detectors and NOx IR analyzers. The carbon and nitrogen balances were checked. The NO conversion was calculated based upon the NOx consumption: NO conversion = ([NO]o+[NO2] o - ([NO]+[NO2])) * 100% /([NO]o+[NO2]o) , where [NO]o and [NO2] o are the inlet concentrations in NO and NO 2 respectively and [NO] and [NO2] the NO and NO 2 concentrations after reaction. The CH4 conversion was calculated based upon the CH 4 consumption. The eventual formation of N20 was followed with the TCD gas chromatograph. X Ray Diffraction measurements were performed using CuKc~ radiation. TEM micrographs were obtained by direct observation of the steamed samples with a JEOL 100 CX microscope. Nitrogen adsorption and desorption isotherms were carried out on a home made apparatus. IR spectra were recorded at a resolution of 4 cm-1 on a FT-IR spectrometer Nicolet Magna 550 using self supported samples wafers for NO adsorption studies and pellets of samples diluted in KBr for examining zeolitic framework vibrations [12]. For the study of NO adsorption, the sample wafers were introduced into a home made cell allowing in situ studies and described elsewhere [13]. The samples were activated in situ in oxygen and contacted with the reaction mixture following exactly the same procedure as that described for the catalytic test (with 1000 ppm 02). The reaction was allowed at 773 K for 1 hour and the temperature decreased to room temperature before recording the spectrum of the adsorbed phase after reaction. The spectra of the activated sample and the gas phase were subtracted from that obtained after reaction in order to focus on the changes of the catalyst surface after reaction.
3. RESULTS- DISCUSSION The catalytic activity of Pd 2+ exchanged zeolite catalysts was tested in the presence of 1000 ppm and 6240 ppm 02, which corresponds respectively to stoichiometric and lean conditions. The results of NO and CH4 conversions reported in table 2 were obtained after 10 hours run at 773 K. Table 2 Catalytic activity of fresh catalysts catalyst
% NO conversion Stoichio Lean
% CH 4 conversion Stoichio Lean
Pd-H-ZSM5 Pd-H-MOR Pd-H-Y
28 11 2
62 34 5
33 24 3
57 82 7
290 The Pd-H-ZSM-5 catalyst was found to be the most active in the catalytic reduction of NO, its activity being comparable to that reported in the literature for Co- and Ga-ZSM-5 catalysts [ 1-9], the differences in experimental conditions being accounted for. Similarly to the reduction of NO by other hydrocarbons, the Pd-H-ZSM-5 catalyst exhibited a significant activity in the oxidation of methane too. Increasing 0 2 concentration had very little influence on both NO reduction and CH 4 oxidation. Pd-H-MOR was less active than the ZSM-5 based catalyst. However, its activity in the catalytic reduction of NO is fairly enhanced in lean mixture conditions, which makes this catalyst of interest. It is remarkable that, for these two catalysts, the presence of N20 in effluents was not detected. Pd exchanged ZSM5 and MOR catalysts were found highly active and selective in the catalytic reduction of NO in the presence of 0 2. Moreover both catalysts did exhibit a fairly constant activity with reaction time. On the contrary, the Pd-H-Y catalyst had no activity in both reactions. The specific activity of Pd loaded H-ZSM-5 and H-MOR catalysts in the selective reduction of NO in the presence of oxygen is thought to be due to the presence of isolated Pd ions atomically dispersed in the zeolite channels of ZSM-5 and MOR zeolites, as suggested for Co exchanged zeolites [3 ]. In order to check this point, the adsorption of the reaction mixture onto the studied catalysts has been investigated by infrared spectroscopy. Figure 1 shows the IR changes of Pd exchanged H-ZSM-5, H-MOR and H-Y samples after reaction at 773 K in the stoichiometric mixture ([O2]=[CH4] = 1000 ppm, [NO]=2000 ppm).
1836
0.7 A b
0.6
s o r
0.5
b a n
0.4
c e
0.3
C
0.2
2000
~9bo Wavenumbers
~800 (cm- 1 )
Figure 1. Infrared Spectra of Pd Exchanged Catalysts After reaction at 773 K: (a) H-MOR; (b) H-ZSM-5; (c) H-Y. The spectra of the activated sample and the gas phase have been subtracted.
It turns out that both ZSM-5 and MOR based catalysts exhibit two strong features at about 1880 and 1835 cm -1, while the Y sample presents no absorption in the same region. These bands were not observed when reacting the Pd free zeolitic supports with the reaction feed, which suggests to ascribe them to the stretching vibration mode of NO coordinated to Pd ions. Their frequencies fall in the region characteristic of nitrosyl complexes supported on zeolites [ 14]. The presence of a doublet could be ascribed either to symmetric and asymmetric
291 vibrations of a dinitrosyl complex, as proposed for Cu-exchanged ZSM-5 [15] or to two distinct mononitrosyl complexes. Significant amounts of water have been trapped by the samples due to residual water traces in the reaction feed. The alternative interpretation consists in considering two distinct mononitrosyl Pd species differing by the presence or not of water in their coordination sphere. The striking feature is that the intensity of the doublet is strictly proportional to the Pd loading in ZSM-5 and MOR samples, while it is absent for the Y sample, inactive in the catalytic reduction of NOx by methane. The presence of the doublet can be therefore considered as a characteristic of the specific activity of Pd catalysts in the reduction of NOx. The property of the Pd exchanged Y catalyst to inhibit the formation of nitrosyl complexes could be ascribed to the possible migration of Pd ions in small cavities, inaccessible to reactants and/or not suitable to allow the formation of nitrosyl complexes. This hypothesis is corroborated by the significant increase of activity in NOx reduction of a H-Y sample partially Ca-exchanged and subsequently Pd-exchanged [ 16]. Aider steaming at 1073 K, the activity of the Pd exchanged ZSM-5 and MOR samples in the catalytic reduction of NO was measured as described with the freshly activated samples at 773 K. The results are reported in table 3. Table 3 Catalytic activity of aged catalysts catalyst
% NO conversion Stoichio Lean
% CH 4 conversion Stoichio Lean
Pd-H-ZSM5 Pd-H-MOR
2 1
53 49
2 2
86 90
Obviously the aged catalysts were no longer active in the reduction of NO by methane whatever the oxygen concentrations were. On the other hand they revealed to be active in the oxidation of methane into CO2, their activity being strongly dependent on the oxygen concentration. This suggested strong modifications of the catalysts upon steaming, which we further investigated, focusing both on the zeolitic support - texture and chemical composition and on the active phase - oxidation state and dispersion of Pd. The crystallinity of aged catalysts (ZSM-5 and MOR) has been determined from X ray diffraction measurements, considering the fresh catalysts as a reference, and the results were reported in table 4. It turned out that the H-ZSM-5 framework exhibited an excellent resistance against steaming, as indicated by the small loss of crystaUinity (less than 8% loss), while the MOR structure was significantly affected by steaming, loosing 38% crystallinity. The microporous volume of aged catalysts varied in the same way as the crystalline fraction. Although exhibiting different resistances against steam ageing, ZSM-5 and MOR structures both lose totally their catalytic activity in the reduction of NO by methane. This suggested that the ability of the zeolite structure to retain its crystallinity is certainly not the main factor to maintain the catalytic property in NOx reduction. -
292 The framework AI composition (AIF) of fresh and aged ZSM-5 and MOR based catalysts was measured by 29Si and 27A1 NMR and IR spectroscopy and the results compared in table 4 with the global AI composition (AIT) given by chemical analysis (C.A.). Both IR and NMR measurements agreed to indicate a sharp decrease of framework AI compositions of aged samples while the total AI composition remained constant. It was therefore concluded that steam ageing induced a severe dealumination of ZSM-5 and MOR frameworks. As a result, this should lead to the sharp disappearance of cationic exchange sites and consequently affect the Pd ion sites responsible for the specific catalytic behaviour in the NOx reduction by methane. Table 4 Crystallinity, framework and global AI composition of fresh and aged catalysts catalysts
%C
Vmicr o
XRD cm3/g Pd-H-ZSM-5 Fresh Aged Pd-H-MOR Fresh Aged
100 92 100 62
115 91 127 111
AIF/uc
29Si NMR
27A1NMR
AIT/uc
IR
C.A.
4.3 b 0.4 b
3.9 3.6 6.6 6.3
0.4 a 6.5 1.6
0.7 a
a calculated relatively to the fresh sample as a reference for which the AI composition is given by chemical analysis. b using a calibration based upon the shift of the 1070-1090 cm -1 framework vibration band [17]. In order to confirm the latter hypothesis, aged samples have been examined both in X ray diffraction and electron microscopy. Figure 2 shows X ray diffractograms of fresh and steamed Pd-H-ZSM-5 samples between 5 and 60 ~ 20. For the aged catalyst, it could be observed, besides the peaks characteristic of the zeolitic structure, an additional peak at 40 ~ 20 , which indicated the presence of Pd metal particles of large size. This peak could also be observed in the case of aged MOR sample. It was concluded that steam ageing induced the formation of large metallic Pd particles on both ZSM-5 and MOR catalysts. Electron micrographs of these samples shown in figure 3 confirmed the presence of large facetted Pd metal particles: the ZSM-5 sample exhibited particles homogeneously dispersed throughout the zeolitic crystals and ranging between 100 and 300 nm in size while the MOR sample presented only very few metal particles of larger size (approaching 1 ~tm). As a conclusion, it is suggested that, upon severe steaming ageing, the formation of Pd metal particles proceeds as follows. Extensive dealumination of the zeolitic framework leads to the disappearance of cationic sites (H + and Pd 2+) and therefore the zeolite structure looses its ability to anchor Pd ions throughout the whole zeolitic channel structure. Consequently, Pd ions are allowed to move along zeolite channels and aggregate probably in the form of PdO
293
~000
-
.. 0 :1.000
a
-
I
'
~.0
I
I
20
30
'
I
40
'
I
50
60
Figure 2. X Ray diffractograms of Pd-H-ZSM-5 catalysts after activation in oxygen at 773 K (a) and after steam ageing at 1073 K (b).
.... -\~ 9 Pd
"~
m~,~ ili!!,
iii!i
~i~i
~ii~
~i~i~~',,~I
A
......~ili!ii!iii!!!~'
i:iiiii
iiiI
ZSM-5
200 nm
200 nm
Figure 3. Electron micrographs of aged Pd exchanged ZSM-5 (A) and MOR (B) catalysts.
294 particles, which can deposit and grow on the outer zeolite surface. Above 1023 K, it has been demonstrated that PdO particles reduce into metal Pd particles [18]. Therefore, the aged catalysts exhibit catalytic properties of supported Pd metal catalysts, inactive in the NOx reduction by methane in the presence of oxygen but very active in the total oxidation of methane into CO 2.
REFERENCES 1
2 3 4. 5 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
Y. LI and J.N. ARMOR, Appl. Catal. B, Environmental, 1 (1992), L31 K. YOGO, M. IHARA, I. TERASAKI and E. KIKUCHI, Chem. Lett., (1993), 229 Y. LI and J.N. ARMOR, AppL Catal. B, Environmental, 2 (1993), 239 Y. LI, P.J. BATTAVIO and J.N. ARMOR, J. Catal., 142 (1993), 561 Y. LI and J.N. ARMOR, AppL Catal. B, Environmental, 3 (1993), 55 Y. LI and J.N. ARMOR, J. Catal., 145 (1994), 1 T. TABATA, M. KOKITSU and O. OKADA, Catal. Lett., 25 (1994), 393 E. KIKUCHI and K. YOGO, Catal. Today, 22 (1994), 73 R. BURCH and S. SCIRE, Appl. Catal. B, Environmental, 3 (1994), 295 F. WlTZEL, G.A. SILL and W.K. HALL, Jr. CataL, 149 (1994), 229 A. FAKCHE, B. POMMIER, E. GARBOWSKI, M. PR/MET and C. LECUYER, French Patent Application 9308006, Gaz de France (1993) E.M. FLANIGEN, Zeolite Chemistry and Catalysis, ed. Rabo, J.A., ACS Monograph 171, Washington, D.C., 1976 N. ECHOUFI and P. GELIN, J. Chem. Soc. Faraday Trans., 88 (1992), 1067 K. NAKAMOTO, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 3rd ed., John Wiley and Sons, New York, 1978, p. 295 M. IWAMOTO, H. YAHIRO, N. MIZUNO, W-X. ZHANG, Y. MINE, H. FURUKAWA and S. KAGAWA, J. Phys. Chem., 96 (1992), 9360 Unpublished results L.O. ALMANZA RUBIANO, PhD Thesis, Claude BERNARD University - LYON 1, 1993 R.J. FARRAUTO, R.C. HOBSON, T. KENNELY and E.R. WATERMAN, AppL Catal. A, General, 81 (1992), 227
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
295
The effect of preparation and steaming on the catalytic properties of Cuand Co-ZSM-5 in lean NOx reduction P. Ciambelli a, P. Corbo b, M. Gambino b, F. Migliardinib, G. MineUi c, G. Morettic and P. Porta c aDiparfimento di Ingegneria Chimica e Alimentare, Universit~ di Salerno, 84084 Fisciano (SA), Italy. blstituto Motori del CNR, Via G. Marconi 8, 80125 Napoli, Italy. CCentro di Studio del CNR "SACSO", c/o Diparfimento di Chimica, Universit~ "La Sapienza", Piazzale A. Moro 5, 00185 Roma, Italy.
Overexchange of H-ZSM-5 zeolite (Si/AI=80) with copper and cobalt was easily performed at 50 and 80~ With copper acetate solution it was related to the formation of a precipitate identified as copper hydroxyacetate, Ctt2(OH)3(OAc)-H20. Under the same conditions the overexchange with cobalt acetate solution did not cause the formation of any precipitate. The initial catalytic activity in lean NOx reduction of CuZSM-5 catalysts was not influenced by the formation of copper hydroxyacetate, but for both Cu-ZSM-5 preparations a change of the nature of copper species was evidenced after temperature programmed reaction test at spark ignition engine exhaust. Similar modifications were found for Co-ZSM-5 samples after catalytic test at the exhaust of a methane fuelled engine. The very demanding conditions of experiments with engine exhaust gas resulted in lower performances of both Cu- and Co-ZSM-5 catalysts in comparison to those reported for less severe conditions.
1. INTRODUCTION The removal of NOx by hydrocarbons (SCR) has been recently reviewed by Shelef [1]. Cu-exchanged ZSM-5 zeolite is the most active and selective catalyst for this reaction. Nevertheless, durability tests carried out on overexchanged Cu-ZSM-5 (SilA1 = 80) at the spark ignition engine exhaust in lean conditions, showed fast deactivation, likely due to the formation of small CuO particles in the channels of ZSM-5 zeolite, favoured by the water present in the exhaust gas [2]. Cu-ZSM-5 is ineffective for NOx SCR with methane in the presence of excess oxygen [3], whereas Co-ZSM-5 is a selective catalyst [4]. It has been found that 2 % water significantly decreases the NO conversion on Co-ZSM-5 at T_<450~ but this effect diminishes at T>500~ [4]. A similar effect has been found on Co-ferrierite [5]. The high hydrothermal stability of Co-ZSM-5 with respect to Cu-ZSM-5 in N20 decomposition has been reported [6]. Copper and cobalt exchanged ZSM-5 catalysts
296 were prepared by several exchanges of ZSM-5 zeolites with low Si/A1 ratio (close to 10) using 0.05M metal acetate solutions at 80~ for 24 h. Calcination of the catalysts at 750~ in 2% water vapor has a modest effect on Co-ZSM-5 catalysts, whereas the activity of Cu-ZSM-5 catalysts is dramatically reduced [6]. In the present work the effect of the preparation procedure on the catalytic properties of Cu- and Co-exchanged ZSM-5 with Si/AI=80 in the actual lean conditions of a spark ignition exhaust is investigated. 2. EXPERIMENTAL
2.1 Catalyst preparation Two samples of copper and cobalt ZSM-5 catalysts were prepared by ion exchange using 0.1M and 0.05M acetate solutions at 50~ and 80~ respectively. The starting material was H-ZSM-5 zeolite with Si/AI=80 prepared as described in [2]. The list of catalysts and their main features are reported in Table 1. Copper and cobalt content were obtained by atomic absorption analysis. Table 1. List of catalysts and their main features. Sample(a) t (~ [M2+] (mol/1)(b)
time (h)
M (wt%) (b) pHi/pHf (c)
CuZ(80) CuZ(80) CoZ(80) CoZ(80)
2 0.5 (d) 2 2
3.89 3.99 0.96 2..54
648 665 167 450
50 80 50 80
0.1 0.05 0.1 0.05
5.5/5.3 5.6/5.1 7.0/5..6
(a) The samples are identified by the Si/Al ratio and by the % of exchange; (b) M= Cu or Co; (c) pH values of the solutions at the start and the end of the ion exchange process; (d) After 2 h of ion exchange at 80~ the sample and the solution are dark brown due to formation of hydrated CuO.
2.2 Catalyst characterization X-ray powder diffraction (XRD) patterns were obtained with a Philips automated PW 1729 diffractometer. Scans of 0.01 ~ 20 were taken using Ni filtered CuKc~ radiation. Diffuse reflectance (DRS) spectra were recorded using a Cary 5 spectrometer with diffuse reflectance accessory in the wavelength range 200-2500 nm. X-ray photoelectron (XPS) and X-ray excited Auger spectra were obtained with a LeyboldHeraeus LHS-10 spectrometer, operating at constant transmission energy (E0=50 eV), using Mg Kot radiation (hv=1253.6 eV) for Cu-ZSM-5 catalysts, and A1 K~ radiation (hv=1486.6 eV) for Co-ZSM-5 catalysts [7]. The spectra were recorded at room temperature and with X-ray flux of 240 W (12 KV, 20 mA). The surface compositions of samples were obtained on the basis of the peak area intensities of the Cu(2p3/2), Co(2p) and Si(2p) emissions using the sensitivity factor method [8]. The Fermi level of the samples was determined using a binding energy value of 103.2 eV for Si(2p) of the ZSM-5 zeolite eV according to ~ e r t et al. [9].
297 2.3 Catalyst activity tests The Cu-ZSM-5 catalysts were tested at the exhaust of a spark ignition engine (1350 cm3 displacement) fuelled with commercial unleaded gasoline. The engine operated at air/fuel mass ratio A/F=17, 2000 rpm, 17 kW. The exhaust gas composition was: 02=4%, CO2=11%, H20=12%, HC (as propane)=410 ppm, NOx=1220 ppm, CO=1310 ppm, N2=balance. Co-ZSM-5 catalysts were tested at the exhaust of a spark ignition heavy-duty engine (10 1 displacement) using methane as a fuel. This engine operated at air/fuel mass ratio A/F=21, 1250 rpm, 13 kW. The exhaust gas composition was: O2=3.5%, CO2=9.5%, H20 =15%, CI-I4=1800 ppm, NOx--950 ppm, CO=650 ppm, N2=balance. Both engines were installed on test bench and coupled to an electric dynamometer. Details of the experimental apparatus are described in [2]. The catalytic properties were evaluated with temperature programmed (TP) test (from 25 to 550~
3. RESULTS AND DISCUSSION 3.1 Characterization of fresh Cu- and Co-ZSM5 catalysts Overexchanging H-ZSM-5 zeolites (Si/AI=80) with copper or cobalt ions is strongly favoured, even at 50~ in spite of the short time (only a 2 h single cycle of exchange) in comparison to the much longer time usually employed [3-6]. Similar results for CuZSM-5 were reported by Parfillo et al. [ 10]. The reasons for this phenomenon are not yet clear at all; the presence of M(OH) + and polymeric Mx(OH)y(2X-y)+ species in the solution [ 11] could lead21[o exchange levels higher than 100%, the latter corresponding to the exchange of 1 M ion per 2 A1 atoms of the zeolite framework. Moreover, the extent of overexchange seems to depend on the Si/A1 ratio, probably involving the presence of structure defects (Si vacancies which lead to the formation of nested silanols) [ 12]. At 80~ Cu overloading of H-ZSM-5 is much more favoured (Table 1), but this is related to the fact that copper acetate solution with concentration higher than 0.01M at 80~ results in the formation of a green-pale blue solid identified by chemical analysis, XRD, TG, DTA, IR and XPS as a copper hydroxyacetate, Cuz(OH)3(OAc)-I-I20 [13]. The hydroxyacetate has a layered structure and can be formed, in the concentration range investigated (0. 05-0.1M), at temperature > 60~ [ 13]. During ion exchange performed in these conditions it could be adsorbed on the external surface or entrapped at the channels intersection of the ZSM-5 zeolite and therefore be responsible, in park of the overexchange reported very often for Cu-ZSM5. Moreover the formation of Cu2(OH)a(OAc)-FL20 can be indicative of the presence of polymeric species in the copper acetate solution as suggested in [ 11]. As reported in Table 1 the copper hydroxyacetate at 80~ can be transformed in copper oxide. It is important to consider that under the same experimental conditions the ion exchange with cobalt acetate solution does not lead to the formation of any precipitate. However, as reported in Table 1, the H-ZSM-5 zeolite with Si/AI=80 is easily overexchanged also in the case of cobalt. Polymeric species in solution and the presence of structure defects in the H-ZSM-5 can be invoked to explain this result, as for of Cu-ZSM-5. The XRD spectra of all fresh catalysts present the typical pattern of the parent HZSM-5 zeolite. DRS spectra 2q*'ftheo,as prepared (hydrated) Cu-ZSM-~.samples show that the d-d transitions of Cu (3d) lie in the range expected for Cu in octahedral
298 environment of O-containing ligands (range 700-1000 nm) [14]. O2" to Cu 2+ charge transfer transitions occur in the range 220-260 nm as one broad and intense band. It should be noted that Cu-ZSM-5 samples with Cu exchange levels as high as --650% are stable under treatment in air at 550~ for 2 h. Segregation of CuO particles can be excluded on the basis of XRD and DRS evidences. The Cu-ZSM-5 samples treated in air at 550 ~ show a blue shift in the d-d band maximum with respect to the as prepared samples pointing to a more planar type of the Cu species coordinaUon [ 14]. The XPS analysis confirms the presence of dispersed ions in the as prepared samples. It appears that on the fresh Cu-ZSM-5 samples the chemical state of copper is identified by the Cu(2p3/2) and Cu(LMM) transitions as intrazeolitic Cu + ions [19] with EB(2p3/2) ~ 933.5 eV and EK (LM~+) ~ 913.5 eV. The presence of Cu + species is due to the reduction of the hydrated Cu species under X-rays irradiation in vacuum [see Ref. 9 and references therein]. Bulk Cu20 particles, if present, could be easily identified by XPS because the kinetic energy of the Auger Cu(LMM) transition is =__2.5 eV higher than that of intrazeolitic Cu+ species. In Figure 1 we report the DRS spectra in the Vis-UV region for the CoZ(80) catalysts with 450% of exchange. The as prepared sample shows the well +z~nown band around 550 nm associated with the transition 4T lg(4F) - 4Tlg(4p)of Co ions in octahedral symmetry [ 15]. After the treatment in air at 550~ for 2h the spectrum exhibits2+bands in the region 500-700 nm associated with the transition 4A2(F) - 4T I(P) of Co ions in tetrahedral symmetry [ 16]. The large absorption in the region 250-350 nm, due to oxygen ligand to metal charge transfer transitions, has been observed in Co-APO-5 after treatment in 02 at high temperature [ 17]. _
2 +
.
.
.
3.2 Catalytic properties of Cu-ZSM-5 catalysts. The results of TP tests performed on the Cu-ZSM-5 catalysts are shown in Figures 23. The NOx conversion curve presents the typical profile obtained in all similar experiments carried out with simulated or actual exhaust gases [1,2,18]; i.e., at temperature higher than 400~ the HC oxidation by oxygen seems to be the predominant reaction with respect to the reduction of NOx by hydrocarbons, causing the decrease of NOx conversion. In these experiments no significant activity difference was observed between the two catalysts prepared by ion exchange at 50 and 80~ In particular, for both catalysts the NOx conversion curve exhibits a maximum of about 25% at 400~ while CO and HC conversions are almost complete after 350~ This behaviour suggests that the formation of the copper hydroxyacetate, observed when the ion exchange is carried out at 80~ does not affect the overall number of active sites of the catalyst. This result is in agreement with the previous finding that copper content does not affect the NOx conversion in the range from 200 to 650% Cu over-exchange [ 18]. The comparison with literature data obtained in the absence of water in the feed gas shows that the performance of Cu-ZSM-5 catalysts is more or less reduced in the treatment of actual engine exhaust.
299
Absorbance 1
after CH4 - NOx temperature
0,8
programmed
test
0,6 a i r - 5 5 0 *C - 2h 0,4
0,2
~"~-----~
0 200
! 300
as p r e p a r e d
I
I
I
I
I
400
500
600
700
800
X/nm
Figure 1. DRS spectra of COZ(80)450 catalyst as prepared, after treatment in air at 550 ~ and after TP catalytic test at the methane engine exhaust.
3.3 Characterization of Cu-ZSM-5 catalysts after catalysis After the TP catalytic test XRD spectra of both Cu-ZSM-5 catalysts show the presence of very weak and broad peaks at 20 ~36 ~ and ~39 ~ due to the formation of CuO particles. The zeolite crystallinity remains essentially unchanged after the catalytic tests, as judged by XRD, suggesting that a fraction of copper is present at the external surface of the zeolite crystallites as CuO particles larger than the size of the channels of the ZSM-5. Moreover, the DRS spectra show a clear absorption edge in the used catalysts at ~ 700 nm, due to the electronic band structure of the CuO particles, confirming in a more evident way the results obtained by XRD. The presence of CuO particles in both Cu-ZSM-5 catalysts after TP tests is also confirmed by XPS analysis. The two catalysts show the presence of a new Cu Auger peak at EK (LMM) ~ 917 eV suggesting the presence of both intrazeolitic Cu + species and segregated Cu20 species [ 19]. Considering that Cu20 is formed under X-ray irradiation from dispersed particles of CuO the XPS results confLrm the DRS data. 3.4 Catalytic properties of Co-ZSM-5 catalysts As regards the catalytic properties of Co-ZSM-5 catalysts, the sample COZ(80)167, containing 0.96% of cobalt, was completely inactive until to 400~ and showed very low NOx and CH4 conversions (about 10%) at 500~ High conversions were obtained on this catalyst only for CO, but at temperatures higher than 400~ (90% of conversion at 500~ An appreciable activity was observed only for the sample at higher Co concentration, COZ(80)450 (prepared by ion exchange at 80~ and containing 2.54 wt % of cobalt) (Figure 4). A NOx maximum conversion of about 20% was measured at 400~ while the catalyst was active for CO already after 200~
300
100 CO
o~ =.o
75 50
f Nox
l_
r o r
25
i 200
400
Temperature,
600
*C
Figure 2. NOx, HC and CO conversions on CUZ(80)648 in TP catalytic test at the gasoline engine exhaust. W/F=0.05 gscm3. Gas feed composition: see Experimental. 100
75
.~
50
--,t--CO
25 O 0 0
200
400
600
Temperature, ~ Figure 3. NOx, HC and CO conversions on CUZ(80)665 in TP catalytic test at the gasoline engine exhaust. W/F=0.05 gscm-3. Gas feed composition: see Experimental. The difficulty in converting CH4 was confirmed also on this catalyst, as the conversion of this hydrocarbon did not exceed 50% at 550 ~ The comparison with literature results confirm that the performance of lean NOx reduction catalysts is strongly reduced when tested in the actual conditions of engine exhaust. 3.5 Characterization of Co-ZSM-5 catalysts after catalysis The XRD spectra of the Co-ZSM-5 catalysts after the catalytic test do not show significant differences with respect to the fresh samples. The DRS specmma of COZ(80)450 after the TP test is reported in Figure 1. The large absorption in the2range 50q;800 nm suggests the presence of small oxidic CoOx clusters with the Co and Co species in tetrahedral and octahedral symmetry, respectively. This assignement is consistent with the presence of small Co304clusters entrapped in the channels of the ZSM-5 zeolite (the presence of large particles of Co304 can be excluded on the basis of the XRD results). Similar results are obtained for CoZ(80)167 catalyst.
301
100 CO
0.,,9,
75
=.o
50
HC
t~
> to r
25
.
0
200
.
.
.
400
i
600
Temperature, *C
Figure 4. NOx, HC and CO conversions on COZ(80)450 in TP catalytic test at the methane engine exhaust. W/F=0.10 gscm3. Gas feed composition: see Experimental. The chemical state of cobalt is identified by the Co(2p3/2) and Co(LMM) transitions as dispersed Co 2§ ions [20] with EB(2p3/2) ~ 781.5 eV and EK (LMM) ~ 769.5 eV. After the TP test EB(2p3/2) values remain constant at = 781.5 eV, while EK (LMM) values increase of 0.5-1.0 eV. According to the cobalt chemical state plot the formation of bulk Co304 should involve a chemical shift in the Auger transition ~ 4 eV. This result is in agreement with the DRS data (Fig. 1) which suggest the formation of small defective Co304 clusters entrapped in the channels of the ZSM-5 zeolite. The values of the IM(2p)/Isi(2p) intensity ratios (M-Cu or Co) of the as-prepared catalysts are greater than those corresponding to the chemical composition, indicating that the external surface is enriched in copper and cobalt species. After the TP test the ratio values decrease by a factor of 3 to a M/Si molar ratio close to the value given by chemical analysis, pointing to a better dispersion of metallic ions in the zeolite matrix. The Ic(ls)/Isi(2p) intensity ratios remain about constant after the catalytic tests suggesting that the coke deposition occurs in limited amount. 4. CONCLUSIONS H-ZSM-5 zeolite with Si/AI=80 is easily overexchanged with copper and cobalt ions using metal acetate solutions. The reason for this is related very likely both to the presence in solution of polymeric Mx(OH)y(2X-y)+ species and the structure defects (Si vacancies) in the H-ZSM-5. Moreover, in the case of Cu-ZSM-5, copper overexchange can be due to the formation of a copper hydroxyacetate, favoured by both higher temperature and concentration of copper acetate solution. The characterization of both Cu-ZSM-5 catalysts gives evidence for some modifications of the nature of copper species, but does not allow to conclude that different modifications are induced by the preparation method. The formation of copper hydroxyacetate does not result in modifying the initial catalytic activity in the test at the gasoline engine exhaust. Nevertheless, preliminary results of durability test indicate that the deactivation rate is strongly affected by the preparation method of overexchanged Cu-ZSM-5 catalysts. Work is in progress to elucidate this aspect.
302 A different result has been obtained with Co-exchanged ZSM-5. In fact no precipitate has been formed during the exchange with cobalt acetate solutions. The characterization of the samples after the catalytic test has shown the formation of small defective CoOx clusters, probably Co304 in the channels of ZSM-5. It has been found that Co-overexchanged ZSM-5 (Si/AI=80) is a selective catalyst for the reduction of NOx at the exhaust of a methane fuelled engine. ACKNOWLEDGEMENTS The authors gratefully acknowledge support of this research by G.S. Gilardini Silenziamento S.r.1. (Torino, Italia) REFERENCES
1. Shelef, Chem. Rev. 95, 209 (1995). 2. Ciambelli, P. Corbo, M. Gambino, S. Iacoponi, G. Minelli, G~ Moretti and P. Porta, 1st Intern. Conference on Environmental Catalysis, Recent Research Reports, Pisa, Italy, May 1995, p. 32. 3. Witzel, G.A. Sill and W.K. Hall, J. Catal. 149, 229 (1994). 4. Li, P.J. Battavio and J.N. Armor, J. Catal. 142, 561 (1993). 5. Li and J.N. Armor, J. Catal. 152, 376 (1994). 6. N. Armor and T.S. Farris, Appl. Catal. B: Environmental 4, L l l (1994). 7. Fierro, R. Dragone, G. Moretti and P. Porta, Surf. Interface Anal. 19, 565 (1992). 8. Wagner, L.E. Davis, M.V. Zeller, J.A. Taylor, R.M. Raymond and L.H. Gale, Surf. Interface Anal. 3, 211 (1981). 9. Griinert, N.W. Haayes, R.W. Joyner, E.S. Shpiro, M.R.H. Siddiqui and G.N. Baeva, J. Phys. Chem. 98, 10832 (1994). 10. Parrillo, D. Dolenec, R.J. Gorte and R.W. McCabe, J. Catal. 142, 708 (1993) 11. Kuroda, A. Kotani, H. Maedaa, H. Moriwaki, T. Morimato and M. Nagao, J. Chem. Soc. Faraday Trans. 88, 1583 (1992). 12. Woolery, L.B. Alemany, R.M. Dessau and A.W. Chester, Zeolites 6, 14 (1986). 13. Moretti, P. Porta, G. Minelli, N. Masciocchi, manuscript in preparation. 14. Schoonheydt, Catal. Rev.-Sci. Eng. 35, 129 (1993). 15. Pepe, M. Schiavello, G. Minelli and M. Lo Jacono, Z. physik Chem. Neue Folge 115, 7 (1979). 16. Angeletti, F. Pepe and P. Porta, J. Chem. Soc. Farad. Trans. 1 73, 1972 (1977). 17. Kurshev, L. Kevan, D.J. Parrillo, C. Pereira, G. Kokotailo and R.J. Gorte, J. Phys. Chem. 98, 10160 (1994). 18. Ciambelli, P. Corbo, M. Gambino, G. Minelli, G. Moretti and P. Porta, Catal. Today, in press. 19. Moretti, Zeolites 14, 469 (1994). 20. Dillard and M.H. Koppelman, J. Colloid Interface Sci. 87, 46 (1982).
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine(editors) 9 1995 Elsevier Science B.V. All rights reserved.
303
Adsorption sites for benzene in the 12R window zeolites: A molecular recognition effect Bao Lian SU"t Laboratoire de R6activit6 de Surface, Universit6 Pierre et Marie Curie, URA 1106 CNRS, 75252 Paris cedex 05, France Adsorption sites of benzene with and without coadsorption of a m m o n i a have been studied by infrared spectroscopy over NaEMT and KL zeolites and c o m p a r e d with other zeolites containing 12R w i n d o w s such as NaX, NaY d e a l u m i n a t e d NaY (NaYd), HY and NaBeta zeolites. When benzene is present alone, only one kind of adsorption site has been observed in NaEMT and KL zeolites as observed in NaYd and HY zeolites. It has been attributed to the interaction of x electron cloud of benzene with the Na + ions in NaEMT and the K + ions in KL. No significant adsorption of benzene on the 12R w i n d o w s has been detected. This is in contrast to what has been found in NaX, NaY and NaBeta zeolites. However, upon coadsorption of a m m o n i a , a migration of adsorbed benzene molecules from the cationic sites to the 12R aperture sites has been evidenced in NaEMT as observed previousely in NaX, NaY, NaYd and HY zeolites. This has not been observed in KL. The absence of the benzene adsorption on the 12R windows in KL even in the presence of ammonia suggests that factors, other than chemical (basicity of the framework oxygen atoms), should intervene in the location of benzene on 12R windows. 1. I N T R O D U C T I O N Besides the location of benzene on accessible cations, the 12R w i n d o w of faujasites and Beta structures is also an adsorption site and the amount of benzene ~Present address: Laboratoire de Catalyse, Facult6s Universitaires Notre Dame de la paix, 61 Rue de Bruxelles, B-5000, Belgium
304 adsorbed either on the cations or on the atoms of oxygen is governed by an equilibrium which depends on the intreaction strength of benzene with one site or the other [1-4]. A strong Lewis acidity of the cation (Li as opposed to Cs) favours the interaction of the cation with the ~ electron cloud of the aromatic. A high basicity of the oxygen atoms in the 12R windows can reinforce the interaction of these atoms with the CH group of benzene, stabilizing the location of benzene molecule in the 12R window [1-4]. This basicity Can be expressed by the negative charge of the framework oxygen atoms which can be calculated by using Sanderson electronegativity equalization principle [5]. NaEMT and KL zeolites contain also the 12R windows. However, recent research on the benzene adsorption made by infrared spectroscopy over NaEMT zeolite [6] illustrated only one main adsorption site i.e. the cations. This was also observed over KL zeolite at a coverage of I molecule per unit cell (m/u.c.) by the deuterium NMR [7, 8] and at various loading levels by infrared spectroscopy [9]. The location of benzene could be modified by co-adsorbing a compound able to compete with benzene on one site or on the other [3, 4]. For instance, in faujasites, HCI reacting with framework oxygen atoms in the CsX displaces the aromatics from the 12R window to the cations and NH3 adsorbed on the cations in the NaY moves the benzene from these sites to the 12-R windows [3]. This shift of benzene molecules from one site to another might result from the changes in the charges of the cations and the framework oxygen atoms, which is caused by the adsorption of an acid or a base compound on zeolites. The question then arises as to what the location of benzene molecules in the 12R window sites will be and whether or not the 12R window sites in NaEMT and KL zeolites should become the adsorption sites for benzene upon coadsorpfion of ammonia. The present paper describes the changes in the location of benzene adsorbed in zeolites containing 12R windows upon NH3 adsorption and brings more insights on the dominating factors for benzene adsorption. The samples are NaEMT and KL. The first one has tridirectional network of 12R chanels and the second one has parallel non intersecting 12R pores. 2. EXPERIMENTAL
NaEMT was prepared as in ref. [10-12] and KL was provided by Union Carbide. Their formulas from chemical analysis are Na21(A102)21(SiO2)75 and
305 K8.25Na0.15(A102)8.4(SiO2)27, respectively. Infrared spectra in the CH out-of-plane vibration range of 1700-2200 cm -1 are recorded using a Fourrier-Transform PerkinElmer spectrometer PE1750 with a Data Station 3600. The zeolite wafers (15 mg and 18 mm in diameter) are activated in situ in the infrared cell made of pyrex with two CaF2 windows in a flow of dry oxygen up to 723 K for NaEMT and 773 K for KL with a heating rate of 100 K per hour. The temperatures are maintained for 6h under the dry oxygen flow and then for 6-8h under vacuum. Increasing and known amounts of benzene are introduced at room temperature as described previously [1-4, 6, 9]. After recording the spectra, known amounts of NH3 are successively added into the infrared cell. All infrared spectra are recorded at room temperature after lh equilibration. 3. RESULTS AND DISCUSSION -NaMET The location of benzene upon co-adsorption of NH3 in NaEMT zeolite are studied on two different wafers having benzene loadings, 4.25 and 20.7 molecules per unit cell (m/u.c.), respectively. The changes in the infrared absorbance spectra of the CH out-of-plane vibration bands of adsorbed benzene in the range of 17002200 cm -1 are given in Figure I (A at benzene loading of 4.25 m/u.c, and B for 20.7 m/u.c.). Only one pair of bands vibrating at 1848 and 1986 cm q, giving a shift in wavenumber around 26-33 cm -1 compared to liquid benzene (1815-1960 cm -1) has been observed. These bands have previously been attributed to the benzene molecules adsorbed on the cations [1-4, 6]. It was known that benzene molecules adsorbed on the 12R windows of zeolites can normally give a higher shift in wavenumber around 50-100 cm q depending on the cation present, the zeolites and the benzene loadings [1-3]. Almost no band in this range for any benzene loadings in NaEMT has been detected (Figures 1A, curve a and 1B, curve a). This is in line with what we have previously observed [6]. The progressive introduction of ammonia with known quantities in the IR cell makes the appearance of such bands while the intensities of the doublet at 1848 and 1986 cm -1 decrease. The two paires of band will be refered to in what follows as LF (1848-1986 cm -1) and HF (19742014 cm-1), respectively. For high levels of ammonia introduced the HF pair of band for two different benzene loadings is the most intense. The results suggest that benzene formely adsorbed on cations in the large cages of EMT is desorbed
306
i
..
oI,,
,-"
201/, 107L,I ']',.,,.,., /~ 1990 ~50
lo7,, A I
j u, )i;.i I
L.LI C.~ Z
.,
iJ
d C
w
,
|
2000
,
.1850
cm-1
2200
2000
1850
cm-1
Figure 1, Changes in the infrared absorbance spectra of the CH out-of-plane vibration bands of adsorbed benzene on NaEMT zeolite in the range of 1700-2200 cm -1 with increasing amount of introduced ammonia at different benzene loading A: 4.25 m / u.c. and B: 20.7 m/u.c. and moved to the 12R windows in the presence of increasing amounts of ammonia. The 12R windows become adsorption sites for benzene in the presence of NH3. - KL
A KL wafer, saturated with 2.3 m/u.c, of adsorbed benzene is used to study the changes in the spectra of adsorbed benzene upon coadsorption of increasing amounts of ammonia (Figure 2). When benzene is present alone (Figure 2, curve a), only one pair of bands vibrating at 1986 and 1845 cm -1, giving a shift in wavenumber around 26-30 cm -1 compared to liquid benzene, has been found. This pair of bands corresponds to the LF bands as observed in NaEMT zeolite and has been therefore attributed to the adsorption of benzene on K + cations. No HF bands
307
1993
;
A
1996
=1855 /~
/&
II
1992
1984 1856
7
ISS2
1844 !
2000
!
1850
20
,
cm'l
40
60
80
n (NH3) M l u . c .
Figure 2, Changes in the absorbance
Figure 3, Changes in wavenumber of
spectra of adsorbed benzene upon
CH out-of-plane vibration bands of
coadsorption of increasing amounts of ammonia
adsorbed benzene as a function of the amount of NH3 introduced in the infrared cell; (a): HF (1)5+1)17), (b): LF (1)10+1)17)
have been observed. Considering the large size of a benzene molecule and the dimensions of the different cavities or channels of KL zeolite, only the 12R main channels are accessible to benzene molecules. These molecules will therefore interact with the K + cations in the site D located in the 12R main channels. With successive addtion of NH3 in the infared cell, also only this pair of LF bands is present and is broadened. No HF pair of bands is formed at higher wavenumbers in the presence of NH3. It shows that this progressive introduction of NH3 in the infrared cell induces only a decrease in the absorbance of the LF bands and a shift of the LF bands to higher wavenumber (Figure 3), which reveals the desorption of a part of benzene molecules rather weakly adsorbed on the zeolites since the aromatic is more and more evidently seen by infrared spectroscopy in the range of 2800-3200 cm -1 in the gas phase of the infrared cell as the amount of introduced ammonia is increased.
308 Table 1, Location of benzene without and with coadsorption of NH3 on series of zeolites, their negative charges (-80) of the framework oxygen atoms and the dimension of the 12R windows in these zeolites
Location of benzene Zeolites
-8o (a)
on cations
on 12R windows
C6H6
C6H6+NH3
C6H6
C6H6+NH3
NaX (b)
0.410
yes
yes
yes
yes
NaY (c)
0.350
yes
yes
yes
yes
KL (d)
0.350
yes
yes
no
no
NaEMT (d)
0.318
yes
yes
no
yes
NaYd (b)
0.275
yes
yes
no
yes
NaBeta (e)
0.240
yes
nd (g)
yes
nd
HY (b)
0.234
yes
yes
no
yes
(a): (b): (c): (d): (e): (g):
calculated from Sanderson's equalization principle (1-5). ref. [4] ref. [3] from this work ref. [13] no data
4. GENERAL DISCUSSION The results of benzene adsorption and aromatic/ammonia competitive adsorption (summerized in table 1) show that by contrast with the cases of NaEMT described here and NaY, dealuminated NaY (NaYd), HY and NaX studied previously [4, 6], KL does not adsorb benzene in its 12R windows. This indicates that in the presence of ammonia, the migration of the benzene molecules from the cations to the 12R apertures is not a general trend. Theoretical calculations and higher basicity of framework oxygen atoms in KL zeolite lead to the idea that these
309 12R apertures are potential adsorption sites for benzene despite the fact that no such location was observed by deuterium NMR after adsorption of 1 m/u.c, of benzene [7, 8]. The present work shows that only one main adsorption site i.e. cations has been observed on NaEMT and KL zeolites when benezene is present alone. The NaEMT zeolite consists of the same building units as NaY or NaX zeolites and the average negative charge of the famework oxygen atoms in this zeolite is -0.318, lower than that of NaY (-0.350) or NaX (-0.410). This lower basicity of the oxygen atoms in the 12R windows in NaEMT zeolite compared to that of NaX and NaY can explain the absence of the interaction of the CH group of benzene with the oxygen atoms in the 12R windows as observed in NaYd and HY zeolites [4]. The lack of the adsorption of benzene on the 12R windows in KL zeolite, however, can not be completely explained by the basicity of the framework oxygen atoms, since the average negative charge of the framework oxygen atoms of KL zeolite is similar to that of NaY. According to theoretical calculation [7, 8] and regarding the basicity of this zeolite, the oxygen atoms of the 12R window in KL zeolite are expected to be adsorption sites for benzene. The fact that the experimental results are opposite to the expectation indicates that the parameters, other than chemical factors should participate in affecting the location of benzene. L zeolite has paralel cylinder channels and without interconnection. It is possible that this kind of geometry renders the adsorption of benzene on the 12R aperture difficult, which means that there is no a suitable geometrical compatibility between benzene molecule and 12R aperture in KL. 5. CONCLUSIONS The present results reveal that three factors govern the location of benzene: i) the Lewis acidity of cations, ii) the basicity of oxygen atoms in the 12R windows and iii) the structural compatibility between the benzene molecule and the 12R windows. We believe that the adsorption of benzene is rather governed by a molecular recognition effect where the substrat and the absorbent should involve adapted chemical and structural properties like in the enzyme-substrat system. 6. ACKNOWLEDGEMENT This work was carried out under supervision of Dr. Denise Barthomeuf.
310 REFERENCES
@
0
.
5.
.
.
10. 11. 12. 13.
A. De Mallmann and D. Barthomeuf, in "New Developments in Science and Catalysis" (Y. Marakami, A. Iijima and J. W. Ward, eds), Stud. Surf. Sci. Catal., 28 (1986) 609 A. De Mallmann and D. Barthomeuf, in "Innovation in Zeolite Materials Science" (P. J. Grobet, W. J. Mortier, E. F. Vansant, G. Schulz-Ekloff, eds), Stud. Surf. Sci. Catal., 37 (1988) 365 A. De Mallmann and D. Barthomeuf, J. Chem. Soc., Chem. Commun., No. 2 (1989) 129 A. De Mallmann, PhD thesis, University of Paris, 1989, France R. T. Sanderson, Chemical Bonds and Bond Energy, Academic Press, New York, 1976 Bao Lian Su, J. M. Monoli, C. Potvin and D. Barthomeuf, J. Chem. Soc., Faraday Trans. I, 89 (1993) 857 J. M. Newsam, B. G. Silbernagel, A. R. Garcia and R. Hulme, J. Chem. Soc., Chem. Commun., (1987) 664 B. G. Silbernagel, A. R. Garcia, J. M. Newsam and R. Hulme, J. Phys. Chem., 93 (1989) 6506 Bao Lian Su and D. Barthomeuf, Zeolites, 15 (1995) D. Dougnier, J. Patrin, J. L. Guth and D. Anglerot, Zeolites, 12 (1992) 160 F. Delprato, L. Delmont, J. L. Guth and L. Huve, Zeolites, 10 (1990) 546 F. Delprato, PhD thesis, University of Haute Alsace, Moulhouse, France, 1990 S. Dzwigaj, A. De Mallmann and D. Barthomeuf J. Chem. Soc., Faraday Trans. I, 86 (1990) 431
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
311
Lewis basic and Lewis acidic sites in zeolites Minming Huang a, Serge Kaliaguine a, Aline Auroux b aD6partement de G6nie Chimique et CERPIC, Universit6 Laval, Qu6bec, Canada blnstitut de Recherches sur la Catalyse, CNRS 2 Avenue Einstein, 69626 Villeurbanne Cedex, France
INTRODUCTION Acidic and basic sites should be regarded as paired concepts always conjugated to each other. As pointed out by Barthomeuf [1] and Kazansky [2], this general relationship should be also true in the case of zeolites. The extraframework protons and cations in zeolites are the Br6nsted and Lewis acid sites, respectively, while the framework oxygens are their conjugated basic sites. The alkali exchanged zeolites are designated as basic zeolites [3]. Indeed, the acid-base pairs in cation exchanged zeolites are Lewis acids (cations) and Lewis bases (framework oxygens) pairs [4,5]. Many factors are known to affect the Lewis acidity and Lewis basicity in cation exchanged zeolites. They are the bond angles and bond lengths [6], the location of A1 [3,6], the crystallographic sitting of the oxygen [3-6], the framework Si/A1 ratio [3-6], the electronegativity of both the framework and extraframework atoms [4,5], and so on. According to our understanding, these factors can be roughly divided as two groups: the short range and the long range affecting factors. In the case of alkali exchanged zeolites, both XPS [5,7] and IR [4] results of our previous works suggested that the basic strength are remarkably affected by the short range interaction of the adjacent cations. The basicity in these materials is however a rather local property. The question may be also raised concerning the Lewis acidity, namely, the relative importance between the short range and long range factors influence on the Lewis acidity in zeolites. This paper is an attempt to answer the above question. The new experimental material in this work comes mainly from microcalorimetric results obtained using pyrrole, sulfur dioxide and ammonia as the probes. Some key results of our previous works are cited again in order to make a clear comparison. To examine the interaction between basic sites and adjacent cations, a series of alkali exchanged zeolites was employed in the experiments. To examine the long range effect of zeolite crystal, a series of Na-zeolites with different degree of crystal collapse, prepared using the high energy ball milling method [8], was also employed in this work. EXPERIMENTAL Alkali exchanged zeolite samples were prepared by ion exchange. For comparison HX [4] and H-ZSM-5 [9] zeolites were also prepared. Table 1 lists the unit cell compositions of all samples determined by Atomic Absorption. A series of ball-milled samples was further prepared by the high energy ball milling [8] of NaA, NaX and NaY zeolites. Each milled sample is hereafter designated by the milling time (min) noted behind the sample name. The crystallinity of the milled samples was monitored by X-ray diffraction. Taking the crystallinity of parent samples (NaA, NaX and NaY) as 100%, the relative crystallinity
312
of milled samples was measured as the intensity ratio of X-ray diffraction peaks between milled and parent zeolites. A longer milling time yielded more loss in the zeolite crystallinity. The relative crystallinity is 40%, 12%, 0%, 22%, 52%, 9% and 0% for Y-30, Y-60, Y-120, X-30, A-10, A-30 and A- 120, respectively. Table 1 Unit-cell Composition of Zeolite Samples NaA
Nal 1.9(ALOE)11.9(SIO2)12.1NaA
HX LiX NaX KX RbX CsX
H49.7Na29.3(A102)79(SiO2) 113 Li54.3Na31.1(A10 2)85.4(Si02) lO6.6 Na85.4(A102)85.4(SiO2) 1o6.6 K48.3Na37.1(A102)85.4(SiO2)1o6.6 Rb37.7N 347.7(A102)85.4(Si O 2)106.6 Cs28.8N356.6(A102)85.4(SIO2)106.6
LiY NaY KY RbY CsY
Li27.sNa26.7(A102)54.5(SiO2)137.5 Na54.5(A102)54.5(SIO2)137.5 K4o.oNal 4.5(A102)54.5(SiO2)137.5 Rb34.3NaEO.E(A1OE)54.5(Si02) 137.5 Cs37.oNa l 7.5(A10 2)54.5(SiO 2)137.5
Lewis basic and Lewis acidic strength distribution of all the samples were measured using calorimetric and volumetric gas-solid titration [10,11 ]. The adsorption temperature for both ammonia and sulfur dioxide was maintained at 353 K, while it was 338 K for pyrrole adsorption. Before adsorption the samples were evacuated at 673 K overnight. In order to calculate the irreversibly chemisorbed amount (Virr), the sample was pumped at the end of the adsorption, and a second adsorption was then performed at the same temperature. Virr was determined by the difference between the primary and secondary isotherms. RESULTS AND DISCUSSION Lewis basic and Lewis acidic sites in zeolites Figure 1 (Cs-X) shows a representative IR spectrum [4] of pyrrole chemisorbed on Cs-X zeolite. The characteristic band for pyrrole chemisorbed on Lewis basic sites is the one located at 1470 cm -1. The broad band around 3175 cm 1 is the NH stretching frequency of the pyrrole molecule. The higher the basic strength, the stronger the chemisorption bond formed between the basic sites and the hydrogen atom of the NH group of the pyrrole molecule. Then the NH bond becomes looser after adsorption and the corresponding stretching frequency is shifted to a lower frequency. As an amphoteric probe [12], pyrrole can also interact with the acid sites. Figure 1 (H-X) shows the IR spectrum of pyrrole chemisorbed on H-X zeolite. Instead of 1470 cm 1, the characteristic band for pyrrole chemisorbed on acid sides is located at 1490 cm 1. In this case the pyrrole acts as a proton acceptor through the ~ orbital of its ring. The ~ bonding is weaker than t~ bonding, then the chemisorption of pyrrole on acid sites results in a relatively small NH shift (around 3400 cm 1) from the free molecule. The representative spectrum of pyrrole adsorbed on Lewis acid sites may be seen in the case of Li-ZSM-5 (Figure 1). The 1490 cm l band was detected again. IR spectra of
313
chemisorbed pyridine confirmed that there is no BrOnsted but Lewis acid sites on this sample. Consequently, the 1490 cm -1 band in Figure 1 (Li-ZSM-5) can only correspond to the pyrrole chemisorbed on Lewis acid sites. A weak 1470 cm 1 band can be also observed in the spectrum of Li-ZSM-5, which suggests that both Lewis acid and Lewis basic sites are detected simultaneously by pyrrole adsorption. A better example of the simultaneous detection of both Lewis acidic and Lewis basic sites by pyrrole adsorption is the case of K-ZSM-5 (Figure 1), where the intensities of the 1470 and 1490 cm -1 bands became comparable. cml
~s3o
1,,ao
K-ZSM-5 Li-ZSM-5
m . io _
H-X
.o <
0 .Q
<
Cs-X
Figure 1. IR spectra of pyrrole chemisorbed on zeolites Lewis basic strength in zeolites Alkali exchanged X zeolites and also a Cs exchanged Y sample were examined by microcalorimetry of pyrrole chemisorption. IR results [4] had revealed that pyrrole only adsorbed on the basic sites of these samples. The differential heat of pyrrole chemisorption on these samples is then a direct measurement of basic strength in thermodynamic scale (Figure 2). A sharp decrease in Qaiff is generally observed in the beginning of these tests, which should correspond to the adsorption on a few very strong sites. After the sharp decrease a relatively slight decrease in Qaiff or plateau is observed, corresponding to the heat released during adsorption on the predominant sites. Finally, the evolved heat falls drastically into the physisorption domain (the heat of pyrrole liquefaction at 353 K is 41.67 kJ/mol) [14]. Careful analysis indicated that the slow decrease region of these curves usually contains two plateaus. The two plateaus or two slow decrease regions are very apparent in the case
12
9- ~
c_.
o
Io
2o
30
(o
50
K
o
~
Io
,4.--
Aamt of 1 ~ o ~ P~ole
(molecule/u.c.)
Figure 2. Differential heat of pyrrole adsorption of CsX. The Qdiff for the first plateau of CsX is centred at 146.1 kJ/mol while it is 116.9 kJ/mol for the second one. The two plateaus can be also distinguished in the cases of RbX, KX, LiX and CsY. However, in the case of NaX, only one slow decrease region can be claimed. The dn/dQ versus Qdiff curves were then drawn based on the above data. Clearly, corresponding to the presence of two plateaus in Figure 2, there are two peaks or two maximum values in Figure 3 for LiX, KX, RbX, CsX and CsY samples, and only one peak for NaX sample. The Qdiff responsible for the maximum of dn/dQ is referred to Qmax" It is found that the first Qmax for KX, RbX and CsX samples are nearly
314
117
1
3
Ls
J
c~
126
A )
129
~x
146
csY
-- 50
~ 1 2 4 i i 10o 15o
Qd,. (kJ/mol)
Figure 3. dn/dQ versus differential heat of pyrrole adsorption
the same as that of NaX (around 116.6 kJ/mol). The only Qmax for NaX should be the differential heat characteristic of the strength of basic sites associated with Na cations. The appearance of the same Qmax then suggests the existence of the same basic sites associated with Na cations for KX, RbX and CsX samples. This is reasonable since all these three samples still contain a significant amount of Na cations (see Table 1). Consequently, the second Qmax observed in KX, RbX and CsX samples should correspond to the differential heat characteristic of the strength of basic sites associated with another kind of cations, the K, Rb and Cs cations, respectively. Therefore the calorimetric titration using pyrrole as probe evidences the coexistence of basic sites with two different strengthes, which are associated with the two different cations. CsY sample also includes two kinds of cations (Table 1), as the change in dn/dQ also displays two maximum values. Since the basic site associated with Cs cation should possess stronger base strenght, the Qmax with high value (123.7 kJ/mol) for CsY is assigned to the differential heat characteristic of the basic sites associated with Cs cations, while the Qmax of 104.8 kJ/mol characteristic of basic sites associated with Na cations in Y zeolite. The situation of LiX is different. The first Qmax at 109.9 kJ/mol may correspond to the basic sites associated with Li cations in X zeolite, however, the second Qmax at 123.0 kJ/mol is too high to correspond to the basic sites associated with Na cations in X zeolite, (116.6 kJ/mol). The Li cation possesses the highest charge/radius ratio
among alkali cations and will produce a high polarizing effect. Depending on the local structural environment of a cation site, the high polarizing effect of Li cation may produce a contraction of the T-O-T angles of zeolite framework around some Li § sitting. This effect may lead to an increase of the basicity of the associated oxygens. The same samples were examined by XPS and FTIR using pyrrole as probe in our previous works [4,5]. The coexistence of two Nls peaks in XPS and two NH-stretching bands in IR corresponding to pyrrole chemisorbed on two different basic sites was also observed, provided the sample contains two kinds of alkali cations. All the above results allowed us to conclude that the basic sites in alkali-exchanged faujasite are the framework oxygens adjacent to alkali cations, and the basicity is determined mainly by the local environment [7]. Table 2 compares all these characteristic parameters of chemisorbed pyrrole on alkali exchanged X zeolites, which were measured by different methods, namely the Qmax, the N]s binding energy and the NH-stretching frequency. The negative charges on oxygen calculated by the EEM (Electronegativity Equivalence Method) [4] are also shown in this table. They are a theoretical indication of the basic strength.
315
Table 2 Characteristic parameters of pyrrole chemisorbed on basic sites of alkali exchanged X zeolites Cation Li Na K Rb Cs
Nls (eV)
VN (cm ~)
Qmax (kJ/mol)
Electric charge on oxygen
400.3 399.8 399.1 398.7 398.3
3295 3280 3230 3215 3175
110 117 126 129 146
-0.4044 -0.4094 -0.4533 -0.4659 -0.4861
The fact that basic sites in alkali exchanged zeolites become characteristic of the conjugated cation demonstrates the short range interaction between basic and acidic sites. This short range interaction can be also seen from the change in Ols binding energy with the counter cations. Table 3 shows that the larger the electropositivity of the counter cation, the lower the framework oxygen binding energies (O1s). However, these changes are strongly correlated with the Si/A1 ratio Table 3 O1s binding energy of alkali exchanged zeolites Cation Zeolite H Li Na K Rb Cs Change range
(eV)
Binding Energy X
Y
ZSM-5
532.2 532.0 531.6 531.2 530.9 530.9
/ 532.3 532.4 532.2 531.9 531.4
533.1 533.3 533.2 533.2 533.2 533.1
1.3
0.9
0.2
of zeolites. The most significant shift was found in X zeolites, (Si/A1 = 1.25). This shift is smaller for Y zeolites. Finally, the change in binding energy with the alkali cations becomes negligible in ZSM-5 zeolites, (Si/A1 = 40.7). These differences illustrate that the influence of extraframework cations (Lewis acid) is just limited to the nearest framework atoms. When the Si/A1 ratio approaches one in the case of X zeolites, almost every framework oxygen is the nearest neighbour atom of an alkali cation. The short range interaction between alkali cations and framework oxygens then causes a shift of O~s to the lower energy side. When the Si/A1 ratio becomes higher, a portion of framework oxygens cannot find alkali cations as their nearest neighbour, the influence of alkali cations become thus negligible. Then these atoms keep high binding energies. On the other hand, the long range effect of the zeolite crystal on the Lewis basic strength was examined for series of progressively milled Na zeolites. Table 4 lists the microcalorimetric results of SO 2 chemisorbed over parent and milled Na-zeolites. The amount adsorbed under a given equilibrium pressure (V0.2 at P = 0.2 torr) and the irreversibly chemisorbed amount (Virr) are listed in the first two columns of the Table. The integral heat of adsorption corresponding to this volume is Qint,irr, then the ratio of Qint,irt/Virr is a measure of the average strength of Lewis basic sites. The differential heat of adsorption at initial (Qinit) and end (Qend, P=0.5 torr) points of the measurements are also shown in this Table. Clearly, the collapse of the zeolite crystal caused a reduction in the
316
number of basic sites (the reduction in Vo.2 and Virr), but only a slight reduction or sometimes almost no reduction at all in the strength of these sites. The low crystallinity dependence of Lewis basic Table 4 Thermodynamic Results of S O 2 chemisorption at 80~ Sample
Vo.2
Virr
molecule/u.c, NaA A-10 A-30 A-120 NaX X-30 NaY Y-30 Y-60 Y-120
2.4 1.2 0.3 0.06 27.3 5.8 7.9 4.1 3.6 0.9
1.1 0.7 0.2 0.06 12.9 2.6 1.3 1.4 1.3 0.6
Qint,irr/Virr
Qinit
Qend*
kJ/mol
kJ/mol
kJ/mol
120.9 118.9 108.0 100.0 104.8 111.1 103.9 102.6 102.6 88.2
235.2 209.0 162.8 141.5 201.0 187.0 162.0 154.2 129.7 106.5
49.9 54.1 40.0 32.9 50.8 59.9 64.6 69.9 62.1 59.7
* Differential heat of adsorption measured at P = 0.5 torr. strength can be seen more clearly from the site distribution in SO 2 chemisorption shown in Figure 4. Except for the completely collapsed (amorphous) samples A-120 and Y-120, the distribution curves of milled samples possess the same shape and nearly the same maximum value as those of their parent samples.
Lewis Acid Site Distribution (%) IOO~-
100
80 I\ A-120 t x/ 601'
/NaA
A.3O
A/"
IOO
Y-60
80
A
NaX
so
u
/
60 40
20
-25 -50 -75 -100 -125 >125
-25 -50 -75 -100 -125 >125
0
-25 - ~
-75 -100 -125 >i25
Differential Heat Interval (kJ/moi) Figure 4. Site distribution of SO 2 adsorption on Na zeolite samples
Lewis acidic strength in zeolites In contrast to the basic strength, Lewis acidic strength is strongly crystallinity dependent. Table 5 shows the microcalorimetry results of ammonia chemisorption over parent and milled Na-zeolites.
317
Table 5 Thermodynamic Results of NH 3 chemisorption at 80~ Sample
Vo.2
Virr
molecule/u.c,
Qint,irr/Virr
Qinit
Qend*
kJ/mol
kJ/mol
kJ/mol
NaA A-10 A-30 A-120
2.0 1.1 0.2 0.08
1.0 0.4 0.07 0.04
97.1 81.5 78.9 18.2
102.2 91.0 87.5 21.8
60.0 55.9 45.4 11.4
NaX X-30
29.5 5.6
12.4 2.3
102.9 88.7
116.0 96.6
67.3 44.2
NaY Y-30 Y-60 Y-120
20.7 11.9 3.8 2.1
2.4 1.2 1.0 0.38
107.3 91.0 56.4 47.6
126.0 96.3 78.2 66.8
67.0 64.8 35.7 40.6
* Differential heat of adsorption measured at P = 0.5 torr. Obviously, the Lewis acidity decreases in both strength and number after collapse of the crystal. More importantly some weak acid sites are actually created on the milled samples, the portion of which keeps growing with the milling time. This can be seen from the distribution of differential heats of NH 3 adsorption (Fig. 5). The maximum value in the distribution curves of milled samples is shifted to the lower adsorption heats, and thus the proportion of weak Lewis acid sites is increased. The XRD and BET results suggest the formation of amorphous material after milling. The question then becomes why an amorphous aluminosilicate possesses Lewis acid sites
Lewis BaseSite Distribution(%) 100
---
1oo
!oo
80
80
NaY
/
60
A-10
60
40
4020~ . - - A - 1 2 0
ol
NaX
40
o
-75-100-125-150-175-200
>200
-75-10o-125-ISO-175
-200 >200
60
20
~ 0
0
~
~
-7s-!00-12s-1so-I)S-200 ~200
DifferentialHeat Interval(kJ/mol) Figure 5. Site distribution of NH 3 adsorption on Na zeolite samples (sodium cations) with lower strength than the crystalline zeolite material of same composition. A similar fundamental question about the difference in Bronsted acidity between amorphous aluminosilicates and crystalline zeolites has been noticed for a long time and discussed by many authors. In general, as noted by Mortier and Schoonheydt [15], Rabo and Gajda [16], the long-range
318
stabilization effect of crystal zeolites results in the electronic structures of the A1-O and Si-O bonds becoming more equivalent. Then the interaction of O with A1 will weaken the bridging O-H bond and therefore increase the Br0nsted acidity. A similar trend was also claimed by Beran and Dubsky [17] using a CNDO theoretical calculation for Na-zeolites. According to these calculations, the O bonded to zeolite framework A1, forms stronger donor-acceptor bond with Na +, the ionicity of the O-Na bond or the positive charge on Na is then increased compared with the case of amorphous aluminosilicates. The increase in ionicity of the bond between the sodium cation and the bridging oxygen in zeolites is then inducing a stronger Lewis acidity. After milling this bonding becomes stronger (less ionic) due to the crushing of crystals, and then the Lewis acidity becomes weak. Thus the Lewis acidity distribution change after milling is an experimental confirmation for Beran and Dubsky's theoretical prediction. REFERENCES 1. D. Barthomeuf, J.Phys.Chem., 88 (1984) 42. 2. V.B. Kazansky, in Catalysis and Adsorption by Zeolites, G. Ohlmann, et al. Eds., Elsevier Scientific Publishing, Amsterdam, 1991, p. 117. 3. D. Barthomeuf, A. de Mallmann, in Innovation in Zeolite Materials Science, J. Grobet, et al. Eds., Elsevier Scientific Publishing, Amsterdam, 1988, p.364. 4. M. Huang, S. Kaliaguine, J.Chem.Soc.Faraday Trans., 88 (1992) 751. 5. M. Huang, A. Adnot, S. Kaliaguine, J.Catal., 137 (1992) 322. 6. D. Barthomeuf, G. Coudurier, J.C. Vedrine, Material Chem. Phys., 18 (1988) 553. 7. M. Huang, A. Adnot, S. Kaliaguine, J.Am.Chem.Soc. 114 (1992) 10005. 8. P.A. Zielinski, R. Schulz, S. Kaliaguine, A. Van Neste, J.Materials Research, 8 (1993) 2985. 9. R. Borade, A. Sayari, A. Adnot, S. Kaliaguine, J.Phys.Chem., 94 (1990) 5989. 10. A. Auroux, In Catalysis Characterization; B. Imelik, C. Vedrine, Eds.; Plenum Press; New York, 1994; p. 611. 11. N. Cardona-Martinez, J.A. Dumesic, Adv. Catal. 38 (1992) 149. 12. R.A. Jones, G.P. Bean, The Chemistry of Pyrrole, Academic Press, London, 1977. 13. D.B. Akolekar, M. Huang, S. Kaliaguine, Zeolites, 14 (1994) 519. 14. A. Gervasini, A. Auroux, J.Phys.Chem. 97 (1993) 2628. 15. W.J. Mortier, R.A. Schoonheydt, Prog.Solid State Chem. 16 (1985) 1. 16. J.A. Rabo, G.J. Gajda, Catal.Rev.-Sci.Eng. 31 (1990) 385. 17. S. Beran, J. Dubsky, J.Phys.Chem. 83 (1979) 2538.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
319
Effect of the framework composition on the nature and the basicity of intrazeolitic cesium oxides. Correlation activity / basicity. M. Lasperas, I. Rodriguez, D. Brunel, H. Cambon and P. Geneste Laboratoire de Materiaux Catalytiques et Catalyse en Chimie Organique, URA 418 du CNRS, Ecole Nationale Superieure de Chimie de Montpellier, 8, Rue de I'Ecole Normale, 34053 Montpellier Cedex 1, France.
Abstract
Basic CsNaX and CsNaY zeolites have been impregnated with various Ioadings of cesium acetate in order to improve, after calcination, the basic character of the solids and to study their application in fine organic chemistry. The characterizations of the two kinds of solids, CsNaX n Cs and CsNaY n Cs zeolites, show that their cristallinity is largely retained after modification and that their behaviour is totally distinct. The nature and the basicity of intrazeolitic cesium oxides has been determined by stepwise thermodesorption of CO2 (TPD). The nature arises from linear relationships between the number of CO2 molecules desorbed per unit cell, up to 550~ and the impregnated cesium loading (atoms per unit cell) for cesium Ioadings before 16 Cs and 9 Cs for modified CsNaX and CsNaY zeolites, respectively. These linear relationships suggest that active cesium species are homogeneously located inside the cages. The values of the slopes, 0.44 and 0.23 for the CsNaX n Cs and CsNaY n Cs zeolites, respectively, account for the proposed nature of intrazeolitic species: Cs20 inside the CsNaX zeolite and (0s20)2 inside the CsNaY zeolite.The variation of the desorption maximum from 250~ to 150~ indicates that the basicity of the occluded oxide Cs20 is higher than the basicity of the intrazeolitic cluster (0s20)2. The volumetric measurements for the CsNaY n Cs solids by N2 adsorption parallel the CO2 desorption showing that, for Ioadings higher than 13 cesium atoms per CsNaY unit cell, the results can be explained by diffusional constraints in relation with the cluster size. A good agreement is obtained between these results and the base activity of the modified zeolites from measurements of the initial rates of the model Knoevenagel reaction, in liquid phase. Relationships activity/basicity have been established which lead to the evaluation of the activities of intrazeolitic cesium oxides. 1.
INTRODUCTION
The zeolitic host framework is able to accomodate guest oxide molecules [1, 2, 3]. More generally, nanostructures dispersed in the cages of Y zeolite, with a structural geometry superimposed by the host framework, have been studied. They
320
include metal and metal carbonyl clusters, metal oxides and metal sulfides. These clusters are composed of only a few atoms that strongly interact with the zeolite host and, consequently, their properties must be considered totally distinct from that of the bulk solid. Up to now little is known about the nature and thus the activity of encapsulated species as a function of the zeolite composition that is, for example, of the Si/AI ratio for X and Y zeolites which possess the same structure. On one hand, Gates et aL [4] show that the chemistry of guest iridium carbonyl clusters varies with the host zeolite, it parallels the chemistry of the metal carbonyl clusters in basic or in neutral medium as the host solid is NaX or NaY zeolite. On the other hand, the zeolite composition of CsNaX and CsNaY zeolites seems to have no effect on the nature of intrazeolitic cesium oxides [2]. Enhancement of the basic character of exchanged alkali-zeolites is effected by generation of cesium oxide species inside the cages of X and Y zeolites. Such intrazeolite complexes have numerous applications in the growing field of heterogeneous base catalysis, including C-C bond formation from condensation reaction of activated methylene derivatives with benzaldehyde, in liquid phase [5]. The aim of this work is to show that, even for zeolites which possess the same large pore structure, the nature of occluded oxides is different. Consequently, the basicity determined by CO2 TPD and also by initial rates in the model Knoevenagel reaction depends on the composition of the parent zeolite. 2. EXPERIMENTAL
CsNaX and CsNaY zeolites were prepared by conventional cation-exchanged procedures with cesium acetate. The chemical compositions of the unit cells were determined by elemental analysis, Na50 Cs36 AI86 Si106 0384 (41.9% exchange) and H5 Na17 Cs33 AI55 Si137 0384 (60% exchange), respectively. The post synthetically modified CsNaX and CsNaY were prepared by impregnating CsNaX and CsNaY zeolites with various Ioadings of cesium acetate and by calcination at 550~ during 6 h. The X-ray analyses indicate that cristallinity is largely retained for all the samples. The specific surface areas were evaluated using the BET method and the micropore volumes were calculated by the BJH method for automatic measurements and by the Sing method for the manual ones [1]. The characterization of the basic sites on the surface of these solid catalysts was done by TPD using carbon dioxide as an acidic probe molecule. The experimental conditions to ensure reproducible results were described elsewhere [1]. Before determining the initial rates of the Knoevenagel reaction activation of the solids was performed in situ in the same conditions as for the TPD study. 3. RESULTS 3. 1. Characterization of the modified zeolites Our recent results on characterization [1] indicate that post modification of exchanged CsNaX and CsNaY zeolites by impregnation with cesium acetate and activation under a n air flow lead to solids with good cristallinities. Thermogravimetric weight losses are in agreement with the theoretical amounts of cesium acetate Ioadings. The following results are given with theoretical amounts calculated for the activated solids. The X-ray analyses do not reveal any evidence
321
for the presence of another phase, e. g. an oxide phase, indicating the formation of small clusters within the zeolite cages. The CO2 TPD for modified CsNaX zeolites, based on the decomposition temperature of carbonates, has been shown to be a convenient method for measuring total basicity and base strength distribution [1]. The changes in TPD profiles with the cesium loading are shown in Figure 1 for the CsNaX n Cs and CsNaY n Cs, respectively. These profiles are obtained from substracting, at each temperature, the value obtained for the exchanged zeolite from the value for the impregnated solid. Two desorption peaks are identified for the modified CsNaX, one at 250~ and the other at a higher temperature (>600~ For the modified CsNaY zeolites, one major peak is observed at 150~ 20
6 ,1
o
LI1
o
T"
X 5"
x15 .NO
E ~10 O O
~3"
O O "02-e
"0
x~
5
o
o
a
121
|E.L,I-.M|
0
200
300
400
500
750
Temperature (~ I~1 CsNaX4Cs m CsNaX9Cs !~1 CsNaX 11 Cs F'J CsNaX 16 Cs W CsNaX 26Cs
200
IIK.L,~J~Itl
300
9 1=1,11..41 ,,
400
~
500
w|Er-JP.~|
750
Temperature (~ I:::i CsNaY 4 Cs I I CsNaY 7 Cs I~ CsNaY9Cs [3 CsNaY13Cs !~! CsNaY 18 Cs E] CsNaY 24 Cs
Figure 1 Amount 9 of desorbed CO2 v e r s u s cesium loading a- CsNaX n Cs zeolites b- CsNaY n Cs zeolites As shown on Figure 2 the variation of the amount of desorbed CO2, up to 550~ as a function of the cesium loading is very different for the two kinds of zeolites. For the X zeolites this amount increases linearly with the cesium content up to 16 cesium atoms per unit cell and remains constant for higher Ioadings. For the Y zeolites this amount goes through a maximum between 9 and 18 cesium atoms per unit cell. In the linear part of the correlation, up to 16 and 9 cesium atoms per unit cell for the CsNaX n Cs and CsNaY n Cs, respectively, the slopes of the straight lines (0.44 and 0.23) indicate that one CO2 molecule is desorbed for 2 cesium atoms in the first case, and for approximately 4 cesium atoms in the second case. Figure 3 describes the variation of the micropore volume with the cesium loading. For CsNaX n Cs zeolites a linear correlation is obtained. The CsNaY n Cs micropore volumes are higher than the CsNaX n Cs ones, up to 9 cesium atoms per
322 unit cell but it can be observed that the decrease is higher for the former than for the latter solids. Above 9 cesium atoms per unit cell the reverse is obtained and micropore CsNaY n Cs volumes become very small.
O
.
510
X
0.20 X
X
"~ 0.15" X
X
::3
0~4 ~
~ o
0.10-
s
0.05-
2
0
O 0
9
I
0
5
"
I
"
I
10
15
"
I
'
20
0.00
I
9
0
25
Encapsulated Cs (atoms / u. c.)
.
5
9
10
,-Ir"-
15
9
20
25
Encapsulated Cs (atoms / u. c.) Figure 3 Variation 9 of the micropore volume with the cesium loading, x CsNaX n Cs + CsNaY n Cs
Figure 2 : Desorbed carbon dioxide as a function of the cesium loading, x CsNaX n Cs + CsNaY n Cs
3. 2. Knoevenagel condensation reaction The Knoevenagel condensation between ethyl cyanoacetate (11) and benzaldehyde (I)leads to ethyl trans-o~-cyanocinnamate (IV) (Scheme 1).
(t) ~ C =O
/
+
/CN CH 2
~\ .~
~CO2Et
(i)
(Jl)
/ HO
CH
(lil)
(t)
\
/
CN
C-----C H/
\CO2Et
(iv) Scheme 1
/CN CH \ CO2Et
323
Good selectivities (> 95%) are obtained using stoichiometric concentrations of reactants which almost suppress the successive Michael condensation between ethyl cyanocinnamate (IV) and ethyl cyanoacetate (11). The initial rates were determined in DMSO at 80~ with 0.3 mol/I concentrations of each reactant using 0.2 g of catalyst. Figures 4 and 5 show the variations of the initial rate and of the amount of desorbed CO2 up to 550~ v e r s u s the cesium loading for the two kinds of solids. Concerning the CsNaX n Cs solids we see that the initial rate increases up to 11 Cs atoms per unit cell. Then, a change in the slope and therefore, in the catalyst activity is observed. Initial rates with CsNaY n Cs solids are much smaller than with the former zeolites. Curves going through a maximum are obtained. 50
350
~o O X
E3
9
9 300 ~"
40
-~ -
o E
O 0
250 B 200 o
30
--
150
20
B
"O
:D
100
.Q 9 }"'-
o 03
10
r v
x
50
a 0
, 5
0
, , , 9 14 18 Encapsulated Cs (atoms / u. c.)
, 22
---" o(.o
0 27
Figure 4 Variation 9 of initial rates and amounts of desorbed CO 2 with cesium loading on CsNaX n Cs zeolites 9 Desorbed CO 2 up to 550~
4.
9 Initial rates
DISCUSSION
Previous results [1] showed that the 0 0 2 adsorption on the exchanged CsNaX and CsNaY zeolites occurs on the remaining sodium cations. Thus, the increase of this adsorption on the modified zeolites can be assigned first to the adsorption onto the basic oxides and moreover, on the basic function of these species. The peaks at 250~ and 150~ are devoted to the active cesium oxides with an intracristalline location whereas location of cesium oxide over the external surface of the zeolite leads to CO2 desorptions at temperatures higher than 550~ Effectively, Figure 2 shows that, for Ioadings higher than 16 Cs atoms per unit cell on the CsNaX n Cs solids, the amount of desorbed CO2 up to 550~ is nearly constant. However, a second desorption peak appears at high temperatures explained by an external
324
O
16
70
14
60
m
~
x 12 ~
0
E
5o 40
8
0 0
6
30
"0
~
4
o
2
0
.
g
10
v
..Q
_~.
20 10
3
0 m
3 = n
~
x 0
0
0
5
10
15
20
25
Encapsulated Cs (atoms / u. c.) Figure 5 Variation 9 of initial rates and amounts of desorbed CO 2 with cesium loading on CsNaY n Cs zeolites, 9 Desorbed CO 2 up to 550~
9 Initial rates
deposit of cesium oxide (Figure la). The unique peak described for the CsNaY n Cs zeolites (Figure l b) seems to indicate that all the oxide species are located inside the cages of these solids. Consequently, inside the (z-cages of CsNaX zeolite, the basic species are assumed to be cesium oxide Cs20 (slope = 0.44) which upon CO2 sorption would give a carbonate form, the structure of which has no longer been determined. Our results show clearly that another basic oxide is generated inside o~-cages of CsNaY zeolite, which is proposed to be a cluster (0s20)2 (slope = 0.23). Moreover, the variation of the desorption maximum from 250~ to 150~ for the modified CsNaX and CsNaY zeolites respectively, indicates that the encapsulated oxides (Cs20) generated inside the cages of the host CsNaX zeolite are more basic than those (0s20)2 inside the CsNaY zeolite. These results are confirmed by the volumetric measurements (Figure 3). The higher value of the micropore volume for CsNaY than for CsNaX can be explained, as for NaY and NaX zeolites, by the lower number of extraframework cations in the former than in the latter solid [1, 2, 6]. Micropore volumes decrease faster but remain higher for CsNaY n Cs than for CsNaX n Cs up to 9 cesium atoms per unit cell with increasing the cesium loading and decreasing the available void volume. For higher Ioadings the reverse is observed; the high decrease in available CsNaY n Cs pore volume and the conservation of cristallinity by XRD indicate a diffusional limitation. Thus, the variation of the micropore volume accounts for the formation of a larger cluster inside the Y than the X zeolite. The Knoevenagel condensation between ethyl cyanoacetate and benzaldehyde
325
(scheme 1) was chosen as a model reaction. In fact, the activation of the methylene group by electroattracting substituants enables soft conditions for the comparison of the solids [7]. The kinetic and mechanism studies in DMSO [8] show that the reaction is first order in each reactant and that the determining step is the condensation between the adsorbed reactants. The activity data are in good agreement with the TPD results (Figures 4 and 5). Modified CsNaX n Cs zeolites are much more efficient catalysts than CsNaY n Cs solids and therefore, as indicated by CO2 desorption temperatures, encapsulated cesium oxides in the former hosts are more basic than those in the latter ones. On CsNaX n Cs zeolites, base catalysis by intrazeolitic Cs20 leads to a linear initial rate increase until 5.5 species per unit cell. Above this value it is proposed that catalysis by species located on the external surface (Figure 1) takes place. On CsNaY n Cs zeolites no external species are formed. Thus activity (Figure 5) is related with the number of occluded oxides until 4.5 oxides per unit cell. Above, the same limitation as that shown by TPD and volumetric measurements is observed. 140 % "-X 120 r .m
E "~ O
E
10
x
04 O
x
100
8
e.w
E --
80
m
6
O
60
E
v
4
(D
40
...+-J~--~ 20 ..E c 0. 0
--~
2
c"
2'o
2s
CO2(mol/g ) x 105 Figure 6 Activity 9 of encapsulated cesium oxides
0
o
1'o 1'5 2'o 2's
30
Cs2CO3 (mole) x 104 Figure 7 Activity 9 of cesium carbonate
x CsNaX n Cs - 4- - C s N a Y n Cs Catalytic activities of intrazeolitic cesium oxides have been determined (Figure 6). Linear correlations are obtained between the initial rates and the amount of desorbed CO2 for Ioadings up to 11 and 9 cesium atoms per unit cell for CsNaX n Cs and CsNaY n Cs zeolites, respectively. These correlations lead to the calculation of the turn over frequency taking into account that the number of desorbed CO2 mole per gram reflects the number of basic sites : 340 and 110 mol / I / m i n for the intrazeolitic Cs20 or (Cs20)2 site, respectively. In the same conditions the activity of cesium carbonate is about 30 mol/I / min per mol (Figure 7). Our results show the effect of the composition of the host zeolite on the nature
326
and the basicity of the guest oxides which differ from that of the bulk oxide [9,2]. Taking into account that the number of exchanged cesium cations is nearly the same in CsNaX and CsNaY zeolites, the change arises from the number of sodium cations. It may result in somewhat different occupancies of cation sites II and III which affect the supercage environment and thus, the cage free volume, the electric field and the basic character [10]. The formation of the intrazeolitic oxide species in the activation step seems to be governed by the stability of the ionic oxide in interaction with the negatively charged oxygen framework and with the extraframework cations. The difference in basicity of the two species Cs20 and (0s20)2 can be explained by the coordination of the oxide [11]. Such a difference was not observed until now for intrazeolitic cesium oxides [2]. A curve similar to that described for CsNaY n Cs zeolites is explained rather by physicochemical changes in the occluded cesium oxide particules. The same conditions of activation of the solids allow us to propose the same oxidation state for the oxide species. 5.
CONCLUSION
A good agreement between the physical characterization of intrazeolitic cesium oxides by CO2 TPD and N2 volumetry and their catalytic activity in the model Knoevenagel condensation leads to the conclusion that the nature and the basicity of these oxides depend mainly on the composition of the host zeolite. Two different structures are proposed Cs20 and (0s20)2 inside the o~-cages of CsNaX and CsNaY, respectively, the former being more efficient than the latter. In relation with the cluster size, diffusional limitations can take place in the second case. REFERENCES
1. M. Lasp~ras, H. Cambon, D. Brunel, I. Rodriguez and P. Geneste, Microporous Mater., 1 (1993) 343; M. Lasp~ras, H. Cambon, D. Brunel, I. Rodriguez and P. Geneste, Microporous Mater., (1995) submitted. 2. P. E. Hathaway and M. E. Davis, J. Catal., 116 (1989) 263; P. E. Hathaway and M. E. Davis, J. Catal., 116 (1989) 279; J. C. Kim, H.-X. Li and M. E. Davis, Microporous Mater., 2 (1994) 413. 3. H. Tsuji, F. Yagi and H. Hattori, Chem. Lett., (1991) 1881; F. Yagi, N. Kanuka, H. Tsuji, H. Kita and H. Hattori, Stud. Surf. Sci. CataL, 90 (1994) 349. 4. S. Kawi, J. R. Chang and B. C. Gates, J. Am. Chem. Soc., 115 (1193) 4830. 5. I. Rodriguez, H. Cambon, D. Brunel, M. Lasp~ras and P. Geneste, Stud. Surf. Sci. CataL, 78 (1993) 623. 6. S. S. Tamhankar and V. P. Shiralkar, J. Inclusion Phenomena and Molecular Recognition in Chem., 17 (1994) 221. 7. A. Corma, V. Forn~s, R. M. Martin - Aranda, H. Garcia and J. Primo, Appl. Catal., 39 (1990) 237. 8. I. Rodriguez, Thesis~ Montpellier, U. S. T. L., (France), June 1995. 9. A. Jentys, R. W. Grimes, J. D. Gale and C. R. A. Catlow, J. Phys. Chem., 97 (1993) 13535. 10. D. C. Doetschman, D. W. Dwyer, J. D. Fox, C. K. Frederick, S. Scull, G. D. Thomas, S. G. Utterback and J. Wei, Chem. Physics, 185 (1994) 343. 11. G. Pacchioni, J. Amer. Chem. Soc., 116 (1994) 10152.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
327
H Y D R O T H E R M A L AND A L K A L I N E STABILITY OF H I G H - S I L I C A Y Z E O L I T E S G E N E R A T E D BY C O M B I N I N G SUBSTITUTION AND STEAMING W. Lutz 1, E. L0ffier 2 and B. Zibrowius 3 1 Institut for Angewandte Chemie Berlin-Adlershof e. V., Rudower Chaussee 5, D- 12484 Berlin, Germany 2 Analytik - Umwelttechnik - Forschung - GmbH, Rudower Chaussee 5, D- 12484 Berlin, Germany 3 Institut for Brennstoffchemie und physikalisch-chemische Verfahrenstechnik der RWTH Aachen, Worringerweg 1, D-52074 Aachen, Germany Y-type zeolites dealuminated by steaming (DAY-T) are more stable in water and alkaline solutions than those dealuminated by substitution (DAY-S). However, DAY-S zeolites have a higher adsorption capacity and are more hydrophobic. A dealumination by substitution and subsequent steaming yields a zeolite DAY-ST that combines the high chemical stability of DAY-T with the high adsorption capacity and hydrophobicity of DAY-S. I. I N T R O D U C T I O N Dealuminated Y zeolites (DAY) are applied in technical processes as catalysts or carriers of catalytic active ingredients. Recently, they have become more and more interesting as incombustible adsorbents of organic pollutants from water and air. By steaming a NH4Y zeolite, a stable molecular sieve DAY-T is formed [1]. Mesopores as well as non-framework aluminium generated during this thermochemical process (T) reduce the sorption capacity of the zeolite. Since the degree of dealumination is limited to values of Si/A1 _<30, the hydrophobicity of DAY-T is insufficient for many applications. The substitution (S) of the framework aluminium by silicon in zeolite NaY using SiCI4 vapour leads to a molecular sieve DAY-S that contains neither a significant amount of mesopores nor of non-framework aluminium [2]. Hence, the sorption capacity as well as the selectivity in sorption processes are higher than those for DAY-T. Unfortunately, under hydrothermal and alkaline conditions DAY-S is chemically less stable than DAY-T. In this paper we report on a new dealumination process that consists in a substitution using SiC14 (S) followed by steaming (T). The zeolite DAY-ST combines the desired sorption behaviour and hydrophobicity with high chemical stability in water and alkaline solutions. The properties of DAY-ST are compared with those of the DAY-T and DAY-S zeolites as well as with their modifications DAY-Tex and DAY-Salum, which were additionally acid leached and surface aluminated, respectively.
328 2. EXPERIMENTAL The DAY-T samples (U. Lohse, Berlin) were prepared by steaming NH4Y zeolite at temperatures between 723 - 1073 K for 4 hours [3]. The effect of dealumination increased with rising temperature as well as with the degree of ammonium exchange. The obtained Si/AI ratios varied between 3.5 and 29. In order to obtain the DAY-Tex samples the non-framework A1 was removed from DAY-T by leaching in HCI at pH 2.5 at room temperature up to 80 hours. Whereas under these conditions no significant further dealumination took place, at pH 1 at the boiling point of the acid a dealumination of the framework up to Si/A1 ratios of 95 was observed (US-EX). The DAY-S samples (Degussa AG, Hanau) were prepared at elevated temperatures in SiC14 vapour in a technical process similar to that described by Beyer et al. [2]. The substitution of framework silicon by aluminium produced Si/A1 ratios from 7.6 up to 150.
The DAY-Salum zeolites were obtained by stirring DAY-S in as-dissolved 0.001 - 0.025 M solutions of sodium aluminate for 5 up to 120 minutes at room temperature using liquid/solid mass ratios of slurries between 10 to 100. No insertion of aluminium into framework position was detectable. The new DAY-ST modification was obtained by steaming of DAY-S samples generated by the substitution process described above. The samples were treated between 773 and 973 K in water vapour of 1 atm for 4 hours. Si/A1 ratios above 100 were obtained irrespective of the parent Si/A1 ratios (7.6, 12.5, 28, 55) and the conditions of steaming. The hydrothermal stability of DAY-ST and of all other modifications was checked by a treatment in water at 433 K under autogenous pressure for 80 h using a liquid/solid mass ratio of 100. The alkaline stability was checked by a treatment in 0.01 M solution of sodium hydroxide for 20 minutes at room temperature using 1 g zeolite per 100 ml solution. The parent and the modified samples were characterized by means of nitrogen sorption uptake at 77 K, IR spectroscopy, wet chemical analysis, and by molybdate measurements. The Si/A1 ratios of the samples were determined from the framework vibrations in the IR spectra [4].
329
3. RESULTS AND DISCUSSION 3.1. Hydrothermal stability of DAY zeolites
Nitrogen uptake The DAY-T zeolite shows a typical N 2 uptake with respect to the sorption
nitrogen
20
uptake mmoFg DAY-T
15
volume as well as to the nitrogen condensation due to the presence
of
10
mesopores (Fig. 1). The hydrothermal
5
untreated treated
treatment causes only a small decrease of the sorption capacity. However, the
0
0.2
0.4
0.6
0.8
P/P0
number of structure defects rises. A steeper increase in the isotherm in the Fig. 1 Sorption uptake of DAY-T zeolite (Si/Al=29) range of higher loadings and the more pronounced shape of the hysteresis are observed. The removal of the non-framework aluminium from DAY-T leads to an in-
2o nitrogenuptake mmokg D
15-
A
Y
crease in the sorption capacity (Fig. 2). DAY-Tex contains more micropores
10-
per mass unit than DAY-T. During the
5-
untreated
treated
acid leaching the mesopore volume in-
f
0-
o
creases as well. Due to the absence of
o12
o14
o16 P/P0
the non-framework aluminium, DAYTex is less stable under hydrothermal Fig. 2 Sorption uptake ofDAY-Texzeolite (Si/Al=95) conditions. Recently we have reported preliminary results on the stabilizing effect of nonframework aluminium [5, 6]. The substitution of the framework aluminium by silicon produces a DAY-
20 nitrogenuptake mmoFg 15
I
DAY-S
S zeolite with an intact micropore system and, as opposed to DAY-Tex, with
10 ~
only few mesopores (Fig. 3). In con-
5f
trast to these desired properties, the
0-II
hydrothermal stability of DAY-S is un-
0
untreated
treated
012
0~4
016
018
fortunately low. Due to the absence of P/P0 non-framework aluminium the micro- Fig. 3 Sorptionuptake ofDAY-S(Si/AI=150) pore system of the sample is nearly totally destroyed under the severe test conditions.
330 After the alumination of DAY-S, the DAY-Salum zeolite thus generated shows
2o
nitrogenuptake mmol/g DAY-Saluminate d
15-
a remarkably higher hydrothermal stability (Fig. 4). The surface layer protects the
10-
/~'--
treated
zeolite against the attack of water under hydrothermal conditions. The volume of
0 0
the mesopores formed is negligible. The amorphous
~
untreated
f
o12
o14
aluminosilicate precipitated
on the crystal surface reduces the sorp-
o16
o18
1
P/P0
Fig. 4 Sorption uptake of DAY-Salum zeolite (Si/Al=145)
tion capacity only to a low extent since the amount of this type of non-framework aluminium is in DAY-Salum much smaller than in zeolites dealuminated by steaming [5]. Compared with DAY-T generated by
2o
direct steaming bf NH4Y, the new DAY-ST
modification
(steamed
nitrogen uptake mmol/g
I
DAY-ST
at
973 K) contains less mesopores and exhibits a higher micropore volume (Fig. 5). Furthermore, without any disadvantages for the hydrothermal stability the
10 untreated
5
treated
0 0
i
i
i
0.2
0.4
0.6
01s
Si/Al ratio of DAY-ST is significantly p/p0 higher. Thus, the DAY-ST zeolite is a Fig. 5 Sorption uptake ofDAY-STzeolite (Si/Al=155) real high-silica molecular sieve stabilized by non-framework aluminium. Besides, during the steaming an annealing of the framework occurs. In order to get significant information about the hydrothermal stability of the samples severe conditions are used in the test. Although such conditions are rare in technical processes, the results give hints to the long-term behaviour of the zeolites. The state of the framework of the parent and the hydrothermally treated samples was characterized by nitrogen isotherms as well as by IR measurements in the range of the framework sensitive vibration bands. The thus obtained information on the micropore volume and its loss by the formation of mesopores as well as X-ray amorphous material for all highsilica zeolites investigated is compiled in Table 1.
331
Table 1 Properties of high-silica Y-zeolites DAY-T
DAY-Te~
DAY-S
DAY-S21,,m
DAY-ST
substitution
substitution
+ alumination
+ steaming
preparation by steaming
steaming
substitution
+ acid leaching
non-framework aluminium ++
-
_
+
+
145
155
framework Si/A! 29
95
0.20
0.25
150
nitrogen micropore volume [mUg] 0.31
0.28
0.24
15
11
loss of micropore volume [%] 22
41
7
3.2. Alkaline stability of DAY zeolites 50 dissolvedSiO2/ mg per 100ml
Wet chemical analysis The alkaline stability of high-silica Y
4o
zeolites strongly depends on the presence
3o 1
of
20 ~
non-framework
aluminium.
The
DAY-Saluminated- - ~ -
samples DAY-T, DAY-Salum, DAY-ST (steamed at 973 K) show different solu-
10 i o
bilities, but all of them are sufficiently stable in alkaline solutions. In contrast to this result, the samples DAY-Tex and DAY-S
easily dissolve under alkaline
DAY-T
0
50
dissolvodSIO2/mgper 100ml
100
150
i 100
150
Si / AIratio
'~ 1
40 30
conditions (Figs. 6 and 7). From the results for hydrothermal sta-
2010 ~
~
DAY-Textracted
bility, we expect a high alkaline stability for DAY-ST. As a possible reason for the observed behaviour we suspect an insuf-
0 0
50 Si / AIratio
Figs. 6/7 Alkaline stability of DAY zeolites with (6) and without (7) significant amounts of non-framework aluminium layer caused by an insufficient residue of ficient thickness of the formed surface framework aluminium after the substitu-
332 tion process even for DAY-S with Si/AI=7.6. DAY-ST samples produced by steaming of DAY-S with Si/A1 ratios in the range of only 4 to 5 should exhibit a higher stability in alkaline solutions. Further investigations are in progress.
3.3. Influence of the steaming temperature on the properties of DAY-ST
IR spectroscopic characterization of DA Y-ST zeolites The wavenumbers of the framework sensitive IR vibration bands in the range between 450 and 1100 cm -1 characterize the degree of dealumination of DAY zeolites. Tab. 2 gives the values for the asymmetrical and the symmetrical TO stretching vibration before and after the steaming of the DAY-S samples. Only the treatment at 973 K produces a DAY-ST zeolite with a Si/A1 ratio of about 150, corresponding to wavenumbers of 1080 and 836 cm "1, irrespective of the Si/A1 ratio of the parent DAY-S sample. In the series investigated, the best result with respect to a high dealumination degree (Si/AI=150) combined with a high hydrothermal stability was reached for the DAY-S sample with Si/AI=7.6 steamed at 973 K.
Table 2 IR spectroscopic characterization of DA Y-S and DA Y-ST zeolites Zeolite
NaY
2.4 Si/Al IR vibration 1021 */790.* bands [cm-1]
DAY-S
7.6 1074/829
12.5
28
1077/832
1077/834
55
150
1081/833
1080/837
DAY-ST at 873 K
-
1077/835
1078/835
1079/835
1080/836
1080/836
at 973 K
-
1078/836
1080/836
1080/836
1080/837
1079/836
* asymmetrical TO stretching vibration ** symmetrical TO stretching vibration (resolution + lcm-1)
Nitrogen uptake after a hydrothermal treatment Fig. 8 shows the remarkable loss of the sorption capacity of DAY-S with the increasing Si/A1 ratio of the framework. This result agrees with our earlier observations of this type of zeolite [6]. However, by means of the subsequent steaming of samples and, especially, with rising steaming temperature, the stability of the generated DAY-ST zeolites increases remarkably. Even those samples (Si/AI=28, 55, and 150) which contain framework aluminium in too low amounts to generate a protective layer of aluminosilicate are stabilized. In those samples the higher stability should result from a healing of the framework defects.
333
nitrogen uptake after the hydrothermal treatment ml/g
160
120 -
DAY-ST 973 K DAY-ST 873 K
80-
DAY-ST 773 K 40-
P/Po = 0.5
DAY-S
I 20
0
I 40
! 60
I 80
i 100
I 120
I 140
160
Si/A1ratio of the parent DAY-S zeolite Fig. 8 Dependence of hydrothermal stability of DAY-ST zeolites on the steaming temperature 3.4. The non-framework aluminium- nature and stabilizing effect
The non-framework aluminium of both DAY-T and DAY-ST stabilizes the zeolites in water and alkaline solutions by blocking the terminal OH groups and the energy-rich Si-O-Si bonds at the crystal surface where the attack of hydroxyl ions starts [6]. The layer of sodium aluminosilicate generated on DAY-Salum zeolites acts in the same way [5]. We believe that the formation of the non-framework A1 of DAY-T and DAY-ST, which can be characterized as thermodynamically stable X-ray amorphous aluminosilicate, is the driving force of the hydrothermal dealumination. Therefore, for DAY-T the dealumination process stops at Si/AI ratios of <_ 30 after the generation of a closed layer of aluminosilicate. Higher Si/A1 ratios
~ortionof reacted silica / %
100
DAY ~
can
only
be
achieved by a stepwise treat-
-. DAY-ST
ment where each but the last
DAY-~t,tnit,,
steaming is followed by acid leaching in order to remove the non-framework A1 formed in
///mono-and dimenc
t mlicaunits
the preceding step [7]. How-
DAY-S DAY-T+ x _ / +,
0
2
4
6
ever, in each step the amount
sili.... uts , polymenr ,
8
,0
(,+
time / minutes
of micropores is decreased. ,4
For example, the micropore volume of a typical US-EX
Fig. 9 Molybdate curves [8] of high-silica Y zeolites
amounts to only 60% of that of
334 the parent Y zeolite. In the case of DAY-ST the loss in the micropore volume is significantly reduced. By means of the molybdate method, monomeric silicate units typical of the existence of an M-rich aluminosilicate are found in addition to the polymeric framework silica in all high-silica zeolites which contain non-framework aluminium, namely DAY-T, DAY-ST, and also DAYalum (Fig. 9). Whereas the non-framework M of DAY-Salum contains easily exchangeable sodium ions, the DAY-T and DAY-ST modifications contain aluminium cations for the compensation of the negatively charge aluminosilicate bulk. Since the cations are not exchangeable in salt solutions we assume that the samples contain oligomeric aluminium cations, e.g. of the tridecameric type All3 {[IVA104VIAl12(OH)24(H20)1217+}, instead of Al3+ ions. In sodium hydroxide these cations are soluble and may be exchanged by sodium cations. 4. CONCLUSIONS Non-framework aluminium in dealuminated Y zeolites increases the chemical resistance to water under hydrothermal conditions as well as to alkaline solutions. A protective surface layer is generated in DAY-T during the thermochemical dealumination of NH4Y in steam. In DAY-ST, the surface layer is formed during the steaming of DAY-S obtained by silicon substitution. The zeolite DAY-S, which is almost free of non-framework aluminium, is significantly less stable. The described dealumination technique of combined substitution and steaming generates a hydrophobic molecular sieve with a high sorption capacity and a high stability in water and alkaline solutions. Another method of the stabilization of DAY-S zeolite consists in the subsequent surface alumination in an alkaline solution of sodium aluminate. ACKNOWLEDGEMENT The authors are grateful to the Degussa AG, Hanau and Dr. U. Lohse, Berlin for providing the zeolite series DAY-S and DAY-T, respectively. REFERENCES 1 2 3 4. 5 6 7 8
C.V. Mc Daniel et al., Molecular Sieves, Soc. Chem. Ind., London, 1968, p. 168 H. Beyer et al., J. Chem. Soc., Faraday Trans. I, 81 (1985) 2889 U. Lohse, E. Alsdorfand H. Stach, Z. Anorg. Allg. Chem., 477 (1978) 64 H. Fichtner-Schmittler et al., Z. Phys. Chem., Leipzig, 271 (1990) 69 W. Lutz, E. Lrffler and B. Zibrowius, ZEOLITES 13 (1993) 685 W. Lutz, B. Zibrowius and E.Lrffler, Stud. Surf. Sci. & Catal. 84, Elsevier, 1994, p. 1005 J. Kornatowski et al., J. Chem. Soc., Faraday Trans. I, 88 (1992) 1339 R.K. Iler, The Chemistry of Silica, Wiley, New York, 1979
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
Electron Spin Resonance and Electron Spectroscopy of Ni(I) in SAPO-5 and SAPO-11
335
Spin
Echo
Modulation
Martin Hartmann, Naoto Azuma and Larry Kevan Department of Chemistry, University of Houston, Houston, Texas 77204-5641, USA
The various Ni(I) species formed by dehydration, reduction and adsorbate interactions in silicoaluminophosphates-11 and-5, synthesized by incorporation of Ni(ll) into the synthesis mixture (NiAPSO-n (n=5,11) and formed by partial ionexchange of H+ by Ni(ll) (NiH-SAPO-n) were studied by electron spin resonance (ESR) and electron spin echo modulation (ESEM). Differences in the ESR spectra between Ni(I) in ion-exchange and framework sites can be seen when polar and nonpolar adsorbates are added. Analysis of the deuterium modulation from deuterated adsorbates show different coordination geometry for Ni(I) complexes with methanol and ethylene in NiH-SAPO-n and NiAPSO-n. The contrasting ESR and ESEM characteristics of Ni(I) in NiH-SAPO-n and NiAPSO-n suggest that nickel is indeed in a framework site. The effect of an increase of the channel size between SAPO-11 with 10-ring channels and SAPO-5 with 12-ring channels is seen by an increase in the number of coordinated methanol and ethylene molecules. 1. INTRODUCTION
Silicoaluminophosphates (SAPO-n) are microporous inorganic oxides, comparable to the well known zeolites, or aluminosilicate molecular sieves [1]. Zeolites have been widely used for adsorption and heterogenous catalysis, whereas the catalytic properties of the SAPO-n molecular sieves are only now being explored. SAPO-5 and SAPO-11 are both novel structures, which have no zeolite analog. The SAPO-5 molecular sieve is composed of 4-ring, 6-ring and 12-ring straight channels, whereas in SAPO-11 4-ring, 6-ring and 10-ring channels are formed. This leads to channels of 7.3 A and 6.3 A diameter in SAPO-5 and SAPO-11, respectively [2]. Because the SiO 2 group is electrically neutral and in SAPO-n (n=5,11) the number of AIO2 groups is slightly greater than the PO2+ groups, the SAPO-n framework is slightly negatively charged. The framework negative charges are, after calcination, balanced by H+. The H+ ions can to some extent be exchanged by Ni(ll) ions and the exchanged product is denoted NiH-SAPO-n.
336
It has been shown that Ni(I) ions can be active sites in catalytic reactions such as acetylene cyclomerization and ethylene and propylene oligomerization [3-5]. Ni(I) ions formed by reduction of Ni(ll) can be stabilized on silica [6] and in zeolites [7]. If the transition metal ion can be incorporated substitutionally into the molecular sieve lattice it is surmised that the stabillity and the catalytic properties of such a framework ion might be advantageous compared to a nonframework site. However, it has been experimentally difficult to verify that the transition metal ion incorporated by hydrothermal synthesis is actually in a framework site. By direct comparison of Ni(I)ions in ion-exchange sites (NiH-SAPO-n) and synthesized samples in which the nickel ion is presumed to be incorporated into a framework site (NiAPSO-n) more definite evidence can be obtained for the actual incorporation into a framework site. Electron spin resonance (ESR) spectroscopy and electron spin echo modulation (ESEM) analysis are powerful tools for the characterization of the transition metal ion location and their interaction with various adsorbate molecules [8]. 2. EXPERIMENTAL
NiH-SAPO-5 and NiH-SAPO-11 were prepared by solid state ion exchange with NiCI 2 starting from H-SAPO-5 and H-SAPO-11. The starting materials were synthesized as decribed in the patent literature with some modification developed in our laboratory [9]. NiAPSO-5 and NiAPSO-11 were synthesized according to the procedures developed in our laboratory. The details of the synthesis can be found elsewhere [10,11 ]. The nickel contents of these samples were determined by electron probe microanalysis with a JEOL JXA-8600 spectrometer. The nickel content in all samples was about 0.1 atom %. All ESR spectra were recorded with a Bruker ESP-300 X-band spectrometer at 77 K. The magnetic field was calibrated with a Varian E-500 gaussmeter. The microwave frequency was measured by a Hewlett Packard HP 5342A frequency counter. ESEM spectra were measured at 4 K with a Bruker ESP 380 pulsed ESR spectrometer. Three pulse echoes were measured by using a (~d2-x-~2-T-~2) pulse sequence as a function of time T to obtain the time domain spectrum. To minimize 27AI modulation from zeolitic aluminium in measurements of phosphorus modulation the x value is fixed to 0.27 ItS. The phosphorus and deuterium modulations were analyzed by a spherical approximation for powder samples in terms of N nuclei at distance R with an isotropic hyperfine coupling Ais o [12]. The best fit simulation of an ESEM signal is found by varying the parameters until the sum of the squared residuals is minimized. D20, CH3OD, CD3OH, and ND 3 were obtained from Union Carbide, Linde Division. C2D 4 (99% D) was obtained from Cambridge Isotope Laboratories. All gases were purified before use by repeated freeze-pump-thaw cycles.
337
3. RESULTS AND DISCUSSION
The ESR spectra of the hydrated SAPO materials do not show any ESR signal at 77 K. Thus, the Ni species exist in the form of Ni(ll). Dehydration of the samples at 723 K leads to the formation of an isolated nickel(I) species A with gl1=2.487 and g_L =2.106. The location of the Ni(I) species after dehydration depends on the dehydration temperature in NiH-SAPO-5 and NiH-SAPO-11 [13,14]. The location was determined by 31p electron spin echo modulation. For dehydration below 573 K Ni(I) is in a main channel near the SII position and for dehydration above 773 K the Ni(I) is located in the center of a hexagonal prism (SI). Table 1 ESR g Values of Ni(I)in Various Matrices NiH-SAPO-11 or NiAPSO-11 adsorbate
gsl
g_L
dehydrated reduced + D20 + CH3OH
2.463 2.460 2.135 2.496
2.099 2.099 2.062, 2.046 2.078
+ C2H4 + NH3 aNiH-SAPO-5
NiH-SAPO-5 or NiAPSO-5 gil
g_L
2.519 2.481 2.150 2.574 a 2.385 b
2.111 2.110 2.061,2.051 2.063 2.132
2.688 2.497, 1966 2.705 2.336 2.075 2.325 2.621 c 2.217, 2.032 2.634 c bNiAPSO-5 Conly in NiH-SAPO-n
2.489, 1.959 2.073 2.204, 2.034
After oxidation the samples are again ESR silent, but reduction with H2 at 573 K for 45 min leads to the formation of a nickel (I) species with somewhat similiar g values (Table 1). ESEM measurements of 31p modulation of Ni(I) species produced by hydrogen reduction in NiH-SAPO-n and NiAPSO-n show differences between these species with very similar ESR parameters (Table 1). There is a significant difference in the 31p modulation pattern between Ni(I)in ion-exchange sites and Ni(I) in framework positions (Table 2). It is deduced that Ni(I) in NiH-SAPO-5 and NiHSAPO-11 is located in the center of a hexagonal prism site (SI) since the number of the nearest phosphorus atoms ( N ) i s 5.2 and 5.3, respectively (Table 2). After hydrogen reduction Ni(I) occupies the same positions as after dehydration at temperatures above 773 K. The ESEM data for Ni(I) in NiAPSO-5 and NiAPSO-11 are consistent with nickel replacing a phosphorus in the original alternating AI-P framework. The simulation data (Table 2) show 8.8 and 10.5 nearest P atoms at a distance of 0.5 nm. If nickel were in an aluminum site the results should show four nearest P at a distance of about 0.31 nm. Thus the 31p modulation gives further evidence for nickel incorporation into a framework position.
338
After exposure to polar adsorbates such as water, methanol or ammonia the g values change as indicated in Table 1 in less than 5 min. However, for less polar adsorbates such as ethylene the development of new g parameters takes a longer time and even a higher temperature (353 K) in the case of SAPO-11. Table 2 Three Pulse ESEM Simulation Parameters for 31p Modulation of Ni(I) sample NiAPSO-5 NiH-SAPO-5 NiAPSO-11 NiH-SAPO-11
shell 1 2 1 2 1 2 1 2
N 8.8 24.5 5.2 10.5 10.5 30.1 5.3 12.3
R/nm 0.51 0.97 0.33 0.72 0.52 0.95 0.34 0.70
A/MHz 0.1 0 0.56 0 0.1 0 0.5 0.12
Subsequent ESEM analysis after coordination with various adsorbates give the results in Table 3. Major parameters are the number and the distance from the nickel(I) species to the interacting adsorbate deuteriums. It can be seen from Table 3 that the number of coordinated adsorbates to Ni(I) in SAPO-5 versus SAPO-11 is larger for methanol and ethylene, reflecting the increase in channel size. Before any adsorbate is added, the hydrogen reduced samples are evacuated at 573 K for 10 min so that only isolated Ni(I) species remain. Then the adsorbate is exposed to the sample for 5 min at room temperature before the sample is quenched to 77 K. Table 1 summarizes the ESR parameters of adsorbates with reduced NiAPSO-n and NiH-SAPO-n. Differences can be seen with adsorbed ammonia and methanol. With ammonia an axial species B (gH=2.33 and gL=2.078) is found in all samples (NiH-SAPO-n and NiAPSO-n), but an additional orthorhombic species C (g1=2.63, g2=2.21 and g3=2.03) can only be seen in the ion-exchanged samples (NiH-SAPO-n) at higher ammonia pressures. The nine visible 14N hyperfine splitting lines with a splitting constant of 14.4 G show an interaction of four equivalent nitrogens from NH3 with Ni(I). In the case of species B also nine hyperfine lines are detected, but the hyperfine splitting constant is only 7.4 G, which is consistent with more distant N nuclei than in species C. The fact that the formation of species C is not possible with Ni(I) in NiAPSO-n suggests that the Ni(I) species in NiAPSO-n are in a different environment than in NiH-SAPO-n. Both type of species can also be obtained in NiCa-X zeolite [7], but the appearance of these spectra is connected to the initial Ni(ll) concentration. Methanol adsorption shows other evidence for a Ni(ll) location difference between NiAPSO-5 and NiH-SAPO-5 whereas in NiAPSO-11 and NiH-SAPO-11 no difference from the g values (gli=2.496 and g.L=2.078) can be seen. In NiH-SAPO-5 species D is assigned with g11=2.574 and g_L=2.063, in contrast, NiAPSO-5 shows a sharp signal E with gt1=2.385 and g.L=2.132. Since these g values obviously differ from those of
339
isolated Ni(l), methanol coordination is indicated and supported by the ESEM data (Table 3). In the ion-exchanged NiH-SAPO-n materials the coordination geometry of methanol for the synthesized NiAPSO-n materials is distinctively different as shown in Figure 1. Table 3 Three Pulse ESEM Simulation Parameters for Deuterium Modulation of Ni(I) NiH-SAPO-11
NiAPSO-11
Adsorbate
shell
N
R/nm
Adsorbate
shell
N
R/nm
CH3OD
1
1
0.28
CH~OD
1
1
0.24
CD~OH
1
3
0.36
CD3OH
1
3
0.32
C2D4
1
4
0.34
C2D4
1 2
2 2
0.27 0.47
N
R/nm
NiH-SAPO-5
NiAPSO-5
Adsorbate
shell
N
R/nm
Adsorbate
shell
CD3OH
1 2
6 3
0.34 0.40
CD3OH
1 2
6 3
0.33 0.36
C204
1
4
0.35
C2D4
1 2
4 4
0.31 0.55
In the case that nickel substitutes for phosphorus in the framework [11], Ni(I) can be regarded as a locally negative site. The adsorbate coordination geometry of methanol in NiAPSO-5 and NiAPSO-11 is consistent with this. In NiAPSO-11 Ni(I) interacts with one methanol with an OD bond orientation as shown in Figure l(d). This OD bond orientation geometry is typical for the coordination of small polar molecules to anions [15]. A similar OD bond orientation is found in NiAPSO-5 (Figure l(b)), but due to the bigger channels two methanols are so coordinated. One additional molecule is also indirectly coordinated. The Ni(I)-D distances can be found in Table 3. The methanol coordination in NiH-SAPO-5 and NiH-SAPO-11 is different and typical for exposed cations in ion-exchange sites [15]. The methanol is oriented with the partially negatively charged oxygen toward the nickel ion. In NiH-SAPO-5 due to the increased channel size Ni(I) is coordinated to three methanol molecules compared to one methanol in NiH-SAPO-11. The data for adsorbed CH3OD and CD3OH give complementary information consistent with the geometry shown in Figure 1.
340
V'
§
NiH-SAPO-5 / methanol
(a)
oi.
"",,'"
I
NiH-SAPO-11 / methanol
[
H H
,, ,,
NiAPSO-5 / methanol
..-" ,y
j . o ..... \.
~H
(c)
o
NiAPSO-11 / methanol
H
"-.I H
~""c~__~." /
O,~
c~ i
io:"" ..H~ ,,. ~ ...
H
to "
'',
"o
",.C I---H
,,,o.'~'o.~, .""9 \
""'FI~ U
(d)
H*~C
I "~'H
H
Figure 1. Schematic diagram of Ni(I) coordinated to methanol in (a) NiH-SAPO-5, (b) NiAPSO-5, (c) NiH-SAPO-11 and (d) NiAPSO-11 These results show substantial differences between synthesized and ionexchanged materials. Similiar results have been found in previous work for manganese ion incorporation into SAPO-11 [16]. ESEM analysis of adsorbed ethylene also shows evidence for a difference between Ni(I) in ion-exchanged and framework sites of Ni(I) species showing almost identical ESR spectra (Table 1). Figure 2 shows the geometry for the coordination of ethylene to Ni(I)in ion-exchanged NiH-SAPO-n versus synthesized NiAPSO-n materials. For the ion exchanged samples the ~-bond of one ethylene is coordinated with its molecular plane perpendicular to a line towards the nickel atom (Figure 2(a) and 2(c). This is the typical geometry that has been found for ethylene coordination to a variety of transition metal ions in ion exchange sites in molecular sieves [15].
341
NiH-SAPO-5 / ethylene O -o~ O,.(~"
(a)
.
~~""~
-o
NiAPSO-5 / ethylene
NiH-SAPO-11 / ethylene O ~o~'~
%, ~""~? o
//
(c)
NiAPSO- 11 / ethylene 0
(b)
/
D"'C/C'~o
~o\//o~/~'~ /o,
./
NI-~""..
~o\ ~ - o\~ NI
~
......... "~o2.~o%o, o o
(d)
...."
/o- \o
.,,,
I o
Figure 2. Schematic diagram of Ni(I) coordinated to ethylene in (a) NiH-SAPO-5, (b) NiAPSO-5, (c) NiH-SAPO-11 and (d) NiAPSO-11
The ESEM data for NiAPSO-5 and NiAPSO-11 show a different coordination geometry for C2D4 with Ni(I). One ethylene in NiAPSO-11 and two ethylenes in NiAPSO-5 interacts with Ni(I) as shown in Figure 2(b) and 2(d) showing that ~bonding is not present. In NiAPSO-11 and NiAPSO-5 (Figure 2(d) and 2(b)) the distances to the more distant D nuclei are in excellent agreement with the known ethylene structure assuming weak o-bonding. Since this type of bonding is only present in NiAPSO-5 and NiAPSO-11 it can be assumed that ~-bonding is not possible due to steric hinderance which is evidence for Ni(I) being in a framework site. The coordination of two ethylene molecules to Ni(I) in NiAPSO-5 in contrast to only one in NiAPSO-11 is consistent with a larger coordination number for the larger channel in SAPO-5. 4. CONCLUSIONS This work shows comparative ESR studies of Ni(I) reduction and adsorbate interaction between synthesized NiAPSO-n (n=5,11) and ion-exchanged NiH-SAPOn in order to differentiate the Ni(I) location in these two preparations. Thermal and hydrogen reduction produces isolated Ni(I) in NiH-SAPO-5 and NiAPSO-5. Although Ni(I) has almost the same g-values, differences can clearly be detected by 3~p
342
NiAPSO-n and ion-exchanged NiH-SAPO-n materials suggest different site positions for Ni(I) in these materials. For coordination with ammonia two different Ni(I)-(NH3) 4 complexes are found in NiH-SAPO-n, but only one of them in NiAPSO-n. The Ni(I) complex generated after adsorption of methanol in NiH-SAPO-5 differs by ESR from that of NiAPSO-5. ESEM shows a different coordination geometry in NiH-SAPO-5 versus NiAPSO-5 which is consistent with Ni(I) framework incorporation in NiAPSO5. In NiH-SAPO-11 and NiAPSO-11 the ESR spectra show the same g values but the ESEM data show a different coordination for the one methanol molecule. The adsorption of ethylene on Ni(I) produces a ~-complex in the ion-exchanged materials and a weak o-bonded complex in the NiAPSO-n materials. The different ESR spectra and ESEM results show that the local environment of Ni(I) in synthesized NiAPSO-n is clearly different from that of ion-exchanged NiAPSO-n. Therefore the results support that Ni(I) occupies a framework position in NiAPSO-11 and NiAPSO5, with unique adsorbate coordination geometries. ACKNOWLEDGEMENT: This research was supported by the National Science Foundation and the Robert A. Welch Foundation.
REFERENCES ~
.
~
4. 5. ~
7. ~
9. 10. 11. 12. 13. 14. 15. 16.
S.T. Wilson, B. M. Lok, C. A. Messinam, T. R. Cannon and E. M. Flanigan, J. Am Chem. Soc., 104 (1982), 1146. J. M. Bennet, J. P. Cohen, E. M. Flanigan, J .J. Pluth and J. V. Smith, Am. Chem. Soc. Symp. Ser., 218 (1983) 109. V. B. Kazansky, I. V. Elev and B. N. Shelimov, J. Mol. Catal., 21 (1983) 265. L. Bonneviot, D. Olivier and M. Che, J. Mol. Catal., 21 (1983) 415. B. W. Moiler, C. Kemball and H. F. Leach, J. Chem. Soc. Faraday Trans., 1 79 (1983)453. W. Bogus and L. Kevan, J. Phys. Chem., 93 (1989) 3223. R. A. Schoonheydt, I. Vaesen and H. Leeman, J. Phys. Chem., 93 (1989) 1515. C. W. Lee, X. Chen and L. Kevan, J. Phys. Chem., 95 (1991) 8626. X. Chen and L. Kevan, J. Am. Chem. Soc., 113 (1991) 2861. N. Azuma, C.W. Lee and L. Kevan, J. Phys. Chem., 98 (1994) 1221. M. Hartmann, N. Azuma and L. Kevan, J. Phys. Chem., 99 (1995) 0000. L. Kevan, In: Time Domain Electron Spin Resonance; L. Kevan, R.N. Schwartz (Eds.); Wiley: New York, 1979, Chapter 8. N. Azuma and L. Kevan, J. Phys. Chem., 99 (1995) 5083. N. Azuma, M. Hartmann and L. Kevan, J. Phys. Chem., 99 (1995) 6670. L. Kevan, Acc. Chem. Res., 20 (1987) 1. G. Brouet, X. Chen, C. W. Lee and L. Kevan, J. Am. Chem. Soc., 114 (1992) 3726.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
343
Selective acidic, oxidative and reductive reactions over ALPO-11 and Si or Metal substituted ALPO-11
Puyam S Singh, Rajib Bandyopadhyay, R. A. Shaikh and B. S. Rao* Catalysis Division, National Chemical Laboratory, Pune 411008 (INDIA).
Aluminophosphate molecular sieves are microporous crystalline inorganic solids. ALPO-11 and derivatives of ALPO-11 containing Si, V, Co, Mn have been synthesized using n-dipropyl amine as a template. The addition of these elements in ALPO-11 considerably influences the physical and catalytic properties of the materials synthesized due to the generation of new catalytic centers. The samples were characterized by XRD, SEM, ESR, UV-Visible and sorption measurements. The acidic characteristics of the samples were evaluated in the hydrocarbon reactions, like toluene methylation, m-xylene isomerisation and isobutanol dehydration. The product pattern obtained in the methylation of toluene and isomerisation of m-xylene with Si or Metal substituted ALPO-11 is similar to that obtained for zeolites. For these two reactions, ALPO-11 did not show any catalytic activity. The studies distinguish two types of acid centers in ALPO-11 and SAPO-11. ALPO-11 being, very weakly acidic due to terminal hydroxyl group or weak Lewis acid sites, is not having catalytically active centers for alkylation and lsomerisation, but is active for the isobutanol dehydration. Further studies indicate that metal substituted ALPO-11 generates additional few stronger acid centers due to the substitution of M 2 + in place of A13+ in the AEL structure. These acid centers are stronger than the acid centers of SAPO-11, however acidity of these molecular sieves is much weaker as compared to that of a zeolite. Metal substituted ALPO's also exhibit activity in the hydrogenation of nitrobenzene and oxidation of benzyl alcohol, thus demonstrating the reductive and oxidative properties due to the introduction of new metallic sites. 1. INTRODUCTION ALPO-11 belongs to a class of novel microporous crystalline solid with potential use as an adsorbent and catalyst[ 1]. Incorporation of silicon or metal in ALPO- 11 is of particular interest in catalytic reactions as it exhibits acidic, oxidative and reductive properties. Incorporation o f silicon or metal in ALPO-11 have been reported[2,3]. However, much literature is not available on the physico-chemical and catalytic properties of these
344 materials. Presence of acid sites makes SAPO- 11 a ~ood catalyst for acid catalyzed reactions such as alkylation and isomerisation of aromatics whereas incorporation of metal in ALPO-11 exhibits acidic, oxidative and reductive properties. Co2 +, Mn 2+ and [V = 0] 2 + are likely to enter aluminium site, hence MeAPO's are expected to have different acidity[4].
2. EXPERIMENTAL
ALPO-11 and derivatives of ALPO-11 containing Si, Co, Mn and V were hydrothermally synthesized according to the known procedures[2]. The gel composition and final chemical composition of calcined products are given in Table 1. SAPO-11 and MeAPO-11 are prepared by adding silica or metal source to the aluminophosphate gel. Some metal impregnated samples also were prepared by treating aqueous solution of metal salts with ALPO-11 according to the general procedure[5]. The as-synthesized and calcined samples were characterized by XRD (Rigaku, model D/MAX-VC, with CuK~radiation, Ni filter, X = 1.5404~). The surface area was measured for ALPO-11 and derivatives of ALPO-11 on Omnisorb surface analyzer by the single point Brunauer-Emmett-Teller(BET) method using N 2. X-band ESR spectra were recorded at 300 K and 77 K (BRUKER-E 2000) with a rectangular cavity ST9424 using a standard sample, weak pitch variation, g = 2.0029. UV - Visible spectra were recorded on Shimadzu (UV 2101PC) U V - Visible spectrophotometer in the reflectance mode using Barium sulphate as reference. The catalytic experiments were performed in a fixed bed vertical downflow integral silica reactor, taking 2 g of the catalyst particles of 10-20 mesh. Products were analyzed by gas chromatography (15A Shimadzu, with SE-30 column (non-polar silicon fluid; capillary column and FID as detector, the carrier gas being nitrogen) and GCMS (QP 2000A, Shimadzu SE 52, capillary column with non-polar silicon fluid) 3. RESULTS AND DISCUSSION Table 1 includes the unit cell composition of all samples. More cobalt, as compared to manganese and vanadium is incorporated in CoAPO-11. XRD powder pattern of assynthesized ALPO-11, SAPO-11 and MeAPO-11 are in very good agreement with the hterature[2,3] and do not indicate any variation in lattice size with substitution. All the samples possess particles of spherical agglomerates (3~t- 5~t)consisting of elongated crystals. The UV-Visible diffuse reflectance spectra (figures are not shown) of the as synthesized and calcined samples suggests the conversion of Co 2 + to Co 3 § in CoAPO-11 which is reversible. The ESR studies of framework Mn 2 + in as synthesized and calcined sample do not reveal much changes in the MnAPO-11.
345 Table 1 Molar composition of the reaction mixture for the synthesis of ALPO-11, SAPO-11 MeAPO-11 and unit cell compositions of crystallized samples Synthesis batch composition Unit cell composition of crystallized samples (on anhydrous basis) Sample A1203 P20 5 SiO2 ALPO-11 1 1 0 SAPO-11 1 1 0.6 CoAPO-11 1 1 0 MnAPO-11 1 1 0 VAPO-11 0.93 1 0 aMO = Co 2 + O, Mn 2 + O, [VO] 2 + 0
MO a 0 0 0.20 0.15 0.07
H 2 0 DPA 40 1 40 1 40 1 40 1 40 1
A120.4P19.6080 H3.2Si3.2A119.6 P 17.2080 H 1.4Co 1.4A118.8P 19.8080 H0.6Mn0.6A119.0P20.4080 H0.6V0.6A120.6P 18.8080
However, unlike CoAPO-11, the reversible conversion of V 4 + to V 5 + is maximum in case of VAPO-11. These observations are in agreement with the reported literature[6-12]. Table 2 summarizes equilibrium sorption capacities obtained from sorption measurements at P/Po = 0.8 and 298 K of different sorbates in AEL type molecular sieve Table 2 Sorption properties of AEL type molecular sieves, P/Po = 0.8; temp. = 298 K Sample ALPO-11 SAPO-11 CoAPO- 11 MnAPO-11 VAPO-11
Amount sorbed, wt.%, (molecules/unit cell) a H20 14.58 (19.78) 17.56 (23.68) 11.29(15.55) 11.69(15.91) 8.04 (10.93)
n-C6H14 7.20 (2.04) 6.83 ( )1.93 5.43 (1.56) 5.40 (1.54) 4.14 (1.18)
C6H12 5.10 (1.48) 4.94 (1.43) 3.49 (1.03) 3.23 (0.94) 3.39 (0.99)
Surface area (m2/g) 245 214 182 146 140
aFigures in parentheses indicate molecules/unit cell. materials and specific surface area(BET), obtained from low temperature (78 K) nitrogen sorption. In general, it is observed that the sorption capacities of sorbates are maximum in ALPO-11. However, the sorption capacity of water increases (from 19.8 to 23.7 molecules/unit cell) almost by 16.16% in SAPO- 11, as compared to ALPO- 11. The incorporation of silicon in the lattice, which is responsible for Bronsted acid sites in the samples, generates more hydrophilicity. The equilibrium sorption capacities of both water and hydrocarbons decrease with the substitution of metal in the lattice and specific surface areas decreases from 245mZ/g to 140mZ/g of VAPO-11. The decrease in porosity of the sample is due to occluded metal oxide inside the pores. Ion-exchange using a CaC12 solution removes some metal, however the porosity is not significantly improved even after the exchange treatment.
346
3.1 Catalytic data (A) Acidic properties: The acidic sites formed in ALPO-11 and substituted ALPO's due to the replacement A1 or P are different than in zeolites, which mainly consist of Bronsted sites. The activity of these acid sites of ALPO and substituted ALPO is demonstrated in three different reactions. (i) Alkylation of toluene: Bronsted sites of solid acid catalysts like zeolite catalyze the toluene methylation reaction. Table 3a includes Table 3a Alkylation of toluene with methanol over related AEL type materials Catalyst
ALPO-11
SAPO-11
CoAPO-11 MnAPO-11 VAPO-11
Product distribution (wt. %) Non aromatics Benzene Toluene para-Xylene Z Xylene 124 TMB LTMB Conversion a (wt. %) p-Xylene/Z Xylene 124 TMB/YI'MB
-0.02 99.01 0.47 0.93 0.02 0.04 0.99 0.50 0.50
0.12 0.30 75.53 7.74 17.60 4.84 6.45 24.47 0.44 0.75
0.03 0.07 91.50 2.63 6.25 1.97 2.15 8.50 0.42 0.92
0.03 0.07 94.03 2.22 5.34 0.50 0.53 5.97 0.42 0.94
0.02 0.15 97.09 1.07 2.60 0.12 0.14 2.91 0.41 0.86
Experimental conditions: Temperature = 450~ WHSV = 2.4h-1, TOS = 1.0h, Mol ratio, T o l / M e O H = 2:1, Pressure = 1 atm. a(wt. of toluene converted X 100)/wt. of toluene fed. Table 3b meta-Xylene isomerisation over related AEL type materials Catalyst
ALPO- 11
SAPO- 11
CoAPO- 11 MnAPO- 11
VAPO- 11
Product distribution (wt. %) Benzene Toluene p-Xylene Z p-Xylene + o-Xylene 124 TMB YI'MB Conversion a
. . 0.10 0.50 0.87 0.03 0.03 1.00
p-Xylene/ Zp-Xylene + o-Xylene
0.57
(wt. %)
.
. . 0.43 15.31 23.07 0.33 0.33 23.83 0.66
.
. 0.21 3.21 4.44 0.08 0.08 4.73 0.72
.
.
. 0.20 1.64 2.41 0.05 0.05 2.66
0.15 0.55 0.99 0.02 0.02 1.16
0.68
0.55
Experimental conditions" Temperature = 450~ WHSV = 2.4h -1, TOS = 1.0h, Pressure = 1 atm. a(wt. of toluene converted X 100)/wt. of m-xylene fed.
347 Table 3c Dehydration of isobutanol over AEL type related materials. Catalyst
ALPO- 1 la
SAPO- 11
CoAPO- 11 MnAPO- 11 VAPO- 11
Aliphatic product distribution (wt. %) Propane Propene Isobutane n-Butane Butene = 1 Isobutene tsans-Butene = 2 cis-Butene = 2 YButene YPropene/YButene Isobutanol Conv.(wt.%)
. . 0.03 1.23 . . 5.71 74.11 11.86 7.06 98.74 0.0003 70.00
.
. . 0.02 1.00 . . . 8.66 65.68 15.87 8.77 98.98 0.0002 13.86
Experimental Conditions" Temp. = 200~ atm., aTemp. = 300~
.
. 0.18 0.64 . . 6.92 62.21 18.74 11.31 99.18 0.0018 7.05
. .
.
. 0.03 0.21 . . 6.60 61.59 19.86 11.71 99.76 0.0003 7.12
-0.11 7.12 62.82 18.74 11.21 99.89 -2.56
WHSV = 2.4h -1, TOS = 2.0h, Pressure = 1
the catalytic data of the SAPO-11 and MeAPO-11. No activity was noticed with ALPO-11 and in SAPO-11 the observed activity decreases rapidly. However, MeAPO-11 indicated less activity than SAPO- 11 inspite of the fast deactivation as in SAPO- 11. As already pointed out ALPO-11 does not show any acidity and the acidic sites of MeAPO's is less than that of SAPO. The conversions are in agreement with the acidity of the catalysts. The product pattern of SAPO-11 and MeAPO-11 is similar to that of a zeolite. The high para isomer amongst xylenes and 124 TMB (trimethyl benzene) isomer in the trimethylbenzenes suggest the shape selective nature. In ALPO-11 and substituted ALPO-11's the shape selectivity is attributed to the restricted transition shape selectivity due to the elliptical pore structure of ALPO's (0.67nm X 0.40nm) which is different from that of the ZSM-5 pore (0.56nm X 0.54nm) where product shape selectivity is considered[13,14], p-Xylene being the primary product formed, can isomerlze to other isomers on highly acidic substances leading to lower Psara xylene selectivity. However, SAPO or MeAPO are slightly acidic and hence the fast 1 omerisation of primary product, para xylene is restricted thus contributing to high para isomer in the product. 124 TMB is formed selectively. (ii) Isomerisation of meta-Xylene: This is yet another acid catalyzed reaction where in meta isomer is converted to para and ortho isomers, which are valuable in fibre industry. It is reported that m-xylene isomerisation in medium pore zeolite proceeds via 1,2 methyl shifts[15]. Data presented in Table 3 indicates the pattern of conversion and para selectivity when ALPO's and substituted ALPO's are used as catalysts. The major differences in the toluene methylation and this reaction is the high para xylene selectivity and only 124 TMB formation. In the former reaction, the TMB is formed due to the successive alkylation of xylenes leading to high 124 isomer whereas in this, the formation is by the disproportionation of xylene to toluene and TMB. Due to the restricted transition shape selectivity, only 124 isomer is formed. (iii) Dehydration of isobutanol: This reaction can be catalyzed by all types of acid sites. Table 3c reflects the activity of the catalysts. ALPO-11 though not active at 200~ is active at 300~ with 70% conversion of isobutanol. However, ALPO-11 did not indicate any activity for the other above two reactions. This indicates the difference in the acid sites. SAPO's and MeAPO's are more active than ALPO-11 at 300~ and no liquid product is observed.
348 Isobutene is the major component in the products. (B) Oxidative dehydrogenation of benzyl alcohol: The oxidation of benzyl alcohol by oxidative dehydrogenation yields benzaldehyde as the main product[16a, b]. Thus the results in Table 4a conclude that in the absence of air, major product is that by dehydration leading to dibenzyl ether which undergoes further Table 4a Catalytic and oxidative dehydrogenation of benzyl alcohol Catalyst
SAPO-11
CoAPO-11
MnAPO-11 VAPO- 11
8.97 17.31 30.40 29.61 13.71
7.07 27.70 30.15 19.23 15.85
27.16 6.17 33.13 33.54 --
20.74 0.90 64.02 4.96 9.38
4.13 34.92 47.41 5.48 8.06
9.96 -80.86 6.24 2.94
(i)In absence of air Product distribution b (wt. %) Toluene Benzyl alcohol Benzaldehyde Bibenzyl ether Others
2.01 59.29 20.21 10.25 8.24
(ii)In presence of air Product distribution (wt. %) Toluene Benzyl alcohol Benzaldehyde Bibenzyl ether Others
2.95 52.38 24.50 7.57 12.60
Experimented conditions: Temp. = 250~ 10rml/min. oGC values.
WHSV = 0.5, TOS = 6hrs., Flow rate of air =
Table 4b Hydrogenation of nitrobenzene Catalyst
CoAPO- 11
MnAPO- 11
VAPO- 11
Product distribution b (wt. %) Nitrosobenzene Aniline Nitrobenzene Azobenzene Azoxybenzene
0.32 4.80 86.44 5.64 2.80
0.30 1.04 95.92 2.16 0.58
1.09 6.98 81.71 7.49 2.73
xperimental condition: Temperature = 250~ WHSV = 1, Flow rate of H 2 = 10 ml/hr. C values. reaction to benzaldehyde and toluene. In presence of air, benzyl ether or toluene formation is less and benzaldehyde is the major product (Figure la). The high activity of VAPO 11, followed by CoAPO-11 in the benzaldehyde formation is according to the reversible oxidation of metal cations as per the spectral studies. (C) Reduction of nitrobenzene: The introduction of metal cation in ALPO creates new catalytic centers which are mainly due
349
HIGHER
H+1 I
BOILERS
~0
OH
X
p
I
,=
O
Figure la-Oxidation of benzyl alcohol
0
~~_~-~'- ~ NO 2
NO / I ~
NHOH
~~-~-~<~ Figure lb: Reduction of nitrobenzene
NH 2
!
350 to metal function, active in the oxidation and reduction reactions. Thus, nitrobenzene when contacted with substituted ALPO's, various products of reduction, like, nitrosobenzene, azobenzene, azoxybenzene and aniline are observed(Table 4b). The reaction can be visualized as per the mechanism (Figure lb). In both oxidation and reduction reaction ALPO-11 is not active. SAPO-11 is active only in oxidation reaction. 4. CONCLUSIONS 1. 2. 3. 4. 5.
SAPO-11 and MeAPO-11 have been synthesized hydrothermally. The sorption and spectroscopic studies indicate that the acidity of SAPO-11 is lower than that of zeolite and the metal cation of as-synthesized change to higher oxidation state when the material is calcined, except for Mn. The acidic properties reported in the hydrocarbon reaction indicate that the acidic site of SAPO-11 is different than ALPO-11, the latter being able to catalyze the isobutanol dehydration reaction. High para selectivity amongst the products due to restricted transition state shape selectivity is correlated to the pore geometry of AEL type materials. Introduction of metal ions like Co, Mn or V creates new oxidation reduction centers.
REFERENCES
0
.
4. 5. 6.
0
o
9. 10. 11. 12. 13. 14. 15. 16.
W. M. Meier & D. H. Olson, Atlas of zeolite structure types, 2nd Edn (Butterworths, London) (1987). (a)B. M. Lok, C. A. Messina, R. L. Patton, R. T. Gajek, T. R. Cannan and E. M. Flanigen, J. Am. Chem. Soc. 106 (1984) 6092; (b)B. M. Lok, C. A. Messina, R. L. Patton, R. T. Gajek, T. R. Cannan and E. M. Flanigen, US Patent 4,440,871 (1984). C. A. Messina, B. M. Lok and E. M. Flanigen, US Patent, 4,554,143 (1985) J. J. Pluth, J. V. Smith and J. W. Richardson, J. Phys. Chem. 92 (1988) 2734. M. Narayana, R. Y. Zhan and L. Kevan, J. Phys. Chem. 88 (1984) 3990. R. A. Shoonheydt, R. D. Vos, J. Pelgrims and H. Leeman in zeolites,: Fact, figures and future, edited by P. A. Jacobs and R. A. Van Santen, Part B (Elsevier, Amsterdam) (1989) 559. B. Kraushaar-Czarnetzki, W. G. M. Hoogervorst, R. R. Andrea, C. A. Emeas and W. H. J. Stork, J. Chem. Soc., Faraday Trans., 87 (1991) 891. L. E. Iton, I. Choi, J. A. Desjardins and V. A. Maroni, Zeolites, (1989) 535. D. Goldfarb, Zeolites 9 (1989) 519. G. Brouet, X. Chen, C. W. Lee and L. Kevan, J. Am. Chem. Soc., 114 (1992) 3720. C. J. Ballhausen and H. B. Gray, Inorg. Chem., 1 (1962) 111. P. S. Singh, R. Bandyopadhyay and B. S. Rao, J. Mol. Catal., (submitted for publication). G. N. Long, R. J. Pellet and J. A. Rabo, US Patent, 4,528,414 (1985). J. A. Martens, J. Perez-Pariente, E. Sastre, A. Corma and P. A. Jacobs, Appl. Catal., 45 (1988) 85. M. L. Poutsma, Zeolite Chemistry and Catalysis, (ed. J. A. Rabo) ACS, Washington D.C. (1976) 437. a) P. V. Venuto and P. S. Landis, Advances in Catalysis, Academic, New York, 18 1968) 259; (b) K. Ganesan and C. N. Pillai, J. Catal., 119 (1990) 8.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviotand S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
351
Characterization and catalytic p e r f o r m a n c e of P d S A P O - 5 molecular sieve Tian-Cun Xiao a, Li-Dun An b and Hong-Li Wang b,* aThe Center of Environmental Sciences, Shandong University, Jinan 250100, Shandong,China
bLanzhou Institute of Chemical Physics, CAS, Lanzhou 730000, China ABSTRACT A palladium catalyst supported by SAPO-5 molecular sieve was obtained by direct cocrystallization from a SAPO-5 gel mixture and a palladium salt, which has been compared with a Pd/SAPO-5 catalyst formed by conventional impregnation method. The PdSAPO-5 catalyst was found to have the characteristics of high and even dispersion of palladium, modification of the acidic sites on SAPO-5 by the introduced Pd, and higher activity and selectivity for methanol conversion. A catalytic site comprising a Bronsted acid site and an adjacent Pd is proposed. Keywords: PdSAPO-5 molecular sieve, palladium dispersion, MTO reaction. 1. INTRODUCTION Molecular sieve catalysts with supported noble metal have been widely used in chemical processes. Most of these catalysts are prepared by impregnating the molecular sieve with a metal salt solution or exchanging the cation in the molecular sieve with the metal ions, followed by removing the solvent, drying, calcination and reduction. Other methods to support metal on the molecular sieves are known such as by chemical vapor deposition. But the above methods are time consuming. Besides, during the preparation, some loss of the metal will occur and cause environmental pollution. New techniques need to be developed. Aluminophosphate molecular sieves are new materials first disclosed by Union Carbide, which exhibit some properties characteristic of zeolite and also show unusual physicochemical traits ascribed to their unique chemical composition [1,2]. Studies have shown that SAPO-5 molecular sieve can be synthesized in a relatively wide range of pH values. Recently, we have synthesized palladium catalyst supported by SAPO-5 by adding the palladium salt to the gel mixture of SAPO [3]. In this work, we present the characterization of such catalyst with regard to palladium dispersion, acidic site modification and catalytic activity for methanol to olefin. The conventionally prepared Pd/SAPO-5 will also be studied for comparison. *Also affiliated with the State Key Laboratoryof Catalysis, Dalian Institute of Chemical Physics, CAS, Dalian 116023, China.
352 2. EXPERIMENTAL
2.1. Sample preparation The gel mixture was obtained by a ratio of 1.2EhN:AI203:0.3SiO2:P2Os:65H20 (in mol.) and it is divided into two groups, one is crystallized to get a SAPO-5 molecular sieve, the other is mixed with palladium salt and cocrystallized. The detailed description is given in Ref.[3]. 2.2 Characterization The XRD patterns of all materials were obtained on a D/MAX-12 kV X-ray Diffractometer. IR spectra were recorded on a Hitachi 270-30 infrared spectrometer. The IR cell permitted the recording of the IR spectra in situ during all sample treatments. The sample was pressed into self-supporting wafers and activated at 450 ~ and pressure below 10.5 Torr for 2 h, then cooled down to room temperature under vacuum. The methanol steam is 100 Torr in the recycle system and the reactive vapor in the IR cell is recycled by a circulation pump. Catalytic measurements were performed in a fixed-bed flow reactor at atmospheric pressure using 2.4 g catalyst. The conditions were: WHSV=0.5 g methanol-g-l.cata1-1 -h-1. CM~o~=25 % in nitrogen. The products were analyzed using gas chromatography. Samples for 129XeNMR spectroscopy were pretreated by the following procedure: typically, about 0.5 g of the sample was loaded into a glass tube NMR cell, attached to a grease-free glass manifold, in which it was outgassed to 10.5 Torr at 450 ~ over 5 h, then cooled to room temperature. The sample was loaded with a certain amount of xenon. 129Xe NMR spectra were recorded with a Bruker MSL-400 k spectrometer. The chemical shifts are referred relative to the extrapolated shift of bulk xenon at p =0. A second standard of xenon adsorbed in NaY in equilibrium with the gas at p=590 Torr was used for calibration. All ~3C MAS NMR measurements with the cross-polarization (CP) technique to enhance the sensitivities were performed on a commercial NMR spectrometer. The catalysts adsorbed methanol (13C enriched) or used in the reaction for a certain time are cooled to room temperature and then transferred to the sample probe under nitrogen atmosphere with high purity. The measurements were realized at a spinning rate of ca. 3000 Hz/s. 3.RESULTS AND DISCUSSION 2.~
._ 5
l 10
15
~0
25
30
35
4O
45
20' Figure 1. XRD patterns of a. SAPO-5, b. PdSAPO-5 and c. Pd/SAPO-5.
353 The X-ray diffraction patterns of the calcined and reduced samples are shown in Fig. 1. It can be seen that the PdSAPO-5 and SAPO-5 have the same diffraction profiles, suggesting that addition of palladium salt does not alter the structure of SAPO-5. No palladium metal is detected in the XRD measurements of the calcined and reduced PdSAPO-5 samples. While on Pd/SAPO-5 with the same loading of palladium as PdSAPO-5, a sharp peak of 20=40.34 appears in the XRD patterns (Fig. l-c), revealing that there exists palladium particle supported on the SAPO-5 molecular sieve. These results give some evidence that palladium in PdSAPO5 is highly dispersed in the channel of SAPO-5. The results of 129Xe NMR spectra further support the above conclusion. Fig.2 gives the NMR spectra of xenon adsorbed on different samples. The NMR spectrum of xenon adsorbed on SAPO-5 contains only a single resonance line, indicating that the elements of S, A1 and P are evenly dispersed in the framework(4), which is in agreement with that of NMR spectra of 29Si, 27A1 and 31p [5,6]. The peak is however not symmetrical which may arise from some residual template produced during calcination. However, on PdSAPO-5 catalyst, a resonance line with high symmetry at 77.5 ppm was detected, showing that the palladium in PdSAPO-5 has changed its electronic state in the channel of SAPO-5, but it does not alter the distribution of AI, S and P. These results reveal that the palladium particles are evenly and highly dispersed in the channel of SAPO-5 [7,8]. That the resonance line of 129Xe on PdSAPO-5 is symmetrical may result from the presence of palladium in SAPO-5 which can catalyze the removal of the template. 67.5
t x-..L_ ~.5
"-
! i :!40
9
1
120
.
1
i00
~
I
9
80
l
60
l
l
4o
L
1
.
]
2o
pllm
Figure 2. NMR spectra of 129Xe adsorbed on a. SAPO-5, b. Pdsapo-5 and c. Pd/SAPO-5.
-
LI
1
I
1
50
150
250
350
450
Tcal~omtu~ ('C) Figure 3. NH3-TPD profiles of a. SAPO-5, b. PdSAPO-5 and c. Pd/SAPO-5.
On Pd/SAPO-5 catalyst, a complex unsymmetrical wide line is detected. The main peak is at 67.5 ppm, two shoulder resonance lines corresponding to ca. 73 ppm and 60 ppm are also present. These results demonstrate that the supported palladium on SAPO-5 by impregnating method has caused great changes in the electronic state of SAPO-5 possibly owing to different distribution of palladium particle on SAPO-5. That is to say, large palladium particles ( > 10 nm) that are too big to enter the channel of SAPO-5 are supported on the external surface of SAPO-5,only a few palladium particles with high dispersion reside in the channel of SAPO-5.
354 From the results of 129Xe of PdSAPO-5 and Pd/SAPO-5, it can be concluded that in PdSAPO-5, palladium is highly dispersed, while the palladium supported on SAPO-5 by impregnation method cannot be well dispersed on the SAPO-5 molecular sieve. The acidity of different samples is tested by NH3-TPD and is shown in Fig.3. It can be seen that different acid sites are present on SAPO-5. They are the weak, and medium sites corresponding to the desorption temperatures of 168 ~ and 305 ~ respectively. For PdSAPO-5 catalyst, the NH 3 desorption temperature shifts downward, while the magnitude of the peak remains almost the same as that of SAPO-5, suggesting that Pd in PdSAPO-5 decreases the acidity of SAPO-5, but it does not change the number of the acidic sites. The reason for these lies in that Pd acts as an electron donor to the acid site. The NH3-TPD profiles reveal that the desorption temperature and the amount of desorbed NH3 in Pd/SAPO5 are almost the same as those of SAPO-5, showing that palladium supported on SAPO-5 by impregnation method has little effect on the acidic property of SAPO-5. This may be due to that the impregnated palladium is mainly distributed on the external surface of SAPO-5 and thus has little effect on the acidic site in the channel of SAPO-5. Table 1 Activities and selectivities of the catalysts in MTO reaction at different temperature
T(~ 200 250 270 300 350 400
SAPO-5(%) Cony. Selec. 40 31 56 50 82 65 100 70 100 75 100 79
PdSAPO-5(%) Cony. Selec. 36 40 60 55 99 70 100 80 100 88 100 95
Pd/SAPO-5(%) Cony. Selec. 42 35 58 44 84 62 99 69 100 78 100 82
The MTO reaction characteristics for the catalysts SAPO-5, PdSAPO-5 and Pd/SAPO-5 are shown in Table 1. It is seen from the above results that the presence of palladium in PdSAPO-5 favors the reaction of MTO and improves the selectivity to C-, while palladium supported by impregnating method on SAPO-5 has less effect on the MTO reaction. It is probably because that palladium in PdSAPO-5 is highly dispersed and it can adjust the acidic sites of SAPO-5. While palladium supported by impregnating method exists as large particles on the external surface of SAPO-5 and it has little effect on the acidic nature of SAPO-5. The IR spectra of methanol adsorbed on different catalysts, are shown in Fig.4. It can be concluded that at room temperature, the methanol cluster prevails over SAPO-5 [9]. With the temperature being raised to 100 ~ the adsorbed state of methanol is not changed. However, when the temperature is increased to 140 ~ two new sharp peaks at ca. 3250 cm -~ and 1380 cm -1 appear respectively showing that at this temperature, some methanol cluster is converted to methoxy groups, which interact strongly with the acid site of SAPO-5 [9]. IR spectra of the adsorbed methanol on PdSAPO-5 (Fig. 4-d) reveals that methanol adsorbed on PdSAPO-5 exists mainly as methanol clusters and methoxoniums (3150-3300 cm -1, 13651408 cm-1). With the temperature being raised to 120 ~ some of them are converted to methoxy groups, showing that the presence of palladium on PdSAPO-5 gives higher activity in MTO reaction.
355
e
e
/ /
40o0
l
,
t
32oo 2800
9
!
I
2oo0
,6o0
noo
c m -~
Figure 4. IR spectra of methanol asdorbed on the catalysts at different temperature, a.SAPO -5 at room temperature, b.SAPO-5 at 120 ~ c. SAPO-5 at 140 ~ d. PdSAPO-5 at room temperature, e.PdSAPO-5 at 120 ~
200
150
1O0
50
0
-50
Figure 5. ~3C CP MAS NMR spectra of methanol adsorbed on the catalysts at different temperature, a. on PdSAPO-5 at room temperature, b.on SAPO-5 at room temperature, c. on SAPO-5 at 120 ~ for 30 min., d.at 300 ~ for 30 min., and e. at 400 ~ for 2 h.
After the catalyst is exposed to ethanol for a certain time and then transferred to the NMR probe under nitrogen atmosphere, the 13C CP MAS NMR spectra are shown in Fig.5. It can be seen that on SAPO-5, only a single resonance line of 13C NMR at 50 ppm is observed. While on PdSAPO-5, in addition to the peak of 50 ppm, a small shoulder peak at ca. 47 ppm is also detected, suggesting that methanol has two adsorbed states on the PdSAPO-5 molecular sieves. One state (50 ppm) results from the methanol adsorbed on the acidic sites [10], another from the methanol adsorbed on highly dispersed palladium particles (47 ppm). Since palladium acts as an electron donor, the chemical shift of 13C of its adsorbed methanol moves upfield. When SAPO-5 is exposed to methanol at 120 ~ for 2 h, the ~3C NMR spectra of methanol indicate that dimethylether (DME, 60 ppm) is produced on the catalyst. With the temperature being increased to 160 ~ the 13C NMR spectra reveal that besides DME, some hydrocarbon (10-40 ppm) is formed and a peak at 80 ppm also appears, which can be assigned to dimethyl-ethyloxonium. After the catalyst is exposed to methanol at 400 ~ for 2 h, new lines at ca. 100-150 ppm can be seen, suggesting that some coke is deposited on the SAPO-5 catalyst [10]. From the above results, the process of MTO reaction on the catalysts studied can be inferred as follows: B-OHWCH3OH B-OCH
3 +CH3OH
--- > --- >
B-OCH
3+H20
CH3OCH
3+B-OH
(1) (2)
356 The yielded CH3OCH3 goes on reacting with methanol on SAPO-5 catalyst in the following way as first suggested by van den Berg et al. [11]H
/o,I CHaOCH3, CHaOH Si
I-I3C CH3
BASIC SITE
I
A1
\0/+I
,
CH 2-
CH 3
'I'H3OCH3'2ovoas \zcr
CI~=Cn: +
cr cr ocr
-H +
' 131 (~§ CEH3 However, on PdSAPO-5, besides by way of reactions(I) and (2), CH3OCH3 probably could also be produced through the synergistic action of an acid site and an adjacent Pd as schematically shown below: CHaOCH3
H
I
CH30[ H
I
crr
Pd+ 2CHjOH
I
-//7//////////////////////
>
H
Pd
I
I
crr-
o
H-OCH[ 3
Y//////////////////////Z
03 fOCH 3 9
CH3 PdH
I
crrr
I
> CHjOCI-t3+ H
I
Pd (3)
I
"7 / / / / / / / / / / / / / / / / / / / / / / / / / ~ / A
So more DME is yielded on PdSAPO-5, and thus giving rise to a higher activity for methanol conversion. Its higher selectivity for ethylene is possibly because that palladium atom in the channel of SAPO-5 can act as a basic site and a center for dehydrogenation, compound (3) is thus easily produced, and more olefin is obtained. The higher activity and selectivity of the PdSAPO-5 may be brought about by a catalytic site comprising a Bronsted acid site and a Pd atom nearby which plays the role of an electron donor. 4.CONCLUSIONS 1.Addition of palladium salt to the gel mixture of SAPO-5 does not depress the formation of SAPO-5. In the reduced PdSAPO-5 catalyst, palladium is highly dispersed in the channel of SAPO-5, while palladium supported by impregnation method exists on the external surface of SAPO-5, and so has little effect on the electronic state of SAPO-5.
357 2.Palladium in PdSAPO-5 does not decrease the number of acidic sites, but reduces the acidic strength of SAPO-5 somewhat. 3.The activity and selectivity to C--- on PdSAPO-5 are higher than those of SAPO-5 and Pd/SAPO-5. This maybe due to the presence of the highly dispersed palladium adjacent to the acid sites can act as a basic site, so more intermediates, i.e., DME and dimethylethyloxonium are yielded. REFERENCES
1. B. M. Lok, C. A. Messina, R. L. Patton, R. J. Gejak, T. R. Canon and E. M. Flanigen, J. Am. Chem. Soc., 106 (1986) 6093. 2. E. M. Flanigen, B. M. Lok, R. L. Patton and S. T. Wilson, Stud. Surf. Sci. Catal., 46 (1989) 103. 3. XIAO Tian-cun, Ph.D. Thesis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, 1993. 4. M. E. Davis,C. Saldarriaga, C. Montes and B. E. Hanson, J. Phys. Chem., 92 (1988) 2557. 5. WANG Xing-qiao, LIU Xiu-Sheng, SONG Tian-you, HU Jian-zhi, and QIU Jian-qing, Chem. Phys. Lett., 157 (1989) 87. 6. C. S. Blackwell and R. L. Patton, J. Phys. Chem., 92 (1988) 3965. 7. L. C. de Monoral, J. P. Fraissard and T. Ito, J. Chem. Soc. Faraday Trans.,78 (1982) 403. 8. J. P. Fraissard, T. Ito, L. C. de Menoral and M. A. Springuel-Huet, "Metal Microstructures in Zeolites" (P. A. Jacobs et. al eds.) p179, Elsevier, Amsterdam, 1982. 9. K. H. Schnabel, R. Fricke, I. Girnus, Jahn Elke, Loeffler Elke, B. Parlitz and C. Peuker, J Chem. Soc. Faraday Trans., 87 (1991) 3569. 10. E. J. Munson, A. A. Kheir, N. D. Lazo and J. F. Haw, J. Phys. Chem., 96 (1992) 7740. 11. J. P. ran den Berg, J. P. Wolthuizen and L. H. C ran Hoof, Proc. 5th Int. Conf. Zeolites (L. V. C. Rees, Ed.), Heyden, London, 1980.
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Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
359
FTIR s t u d y of the acidic p r o p e r t i e s of s u b s t i t u t e d aluminophosphates V. Zholobenko, A. Garforth, L. Clark, J. Dwyer Chemistry Department, UMIST, PO Box 88, Manchester M60 1QD, UK.
INTRODUCTION Isomorphously substituted crystalline aluminophosphates represent a new class of molecular sieves possessing mild Broensted acidity [1-3]. Introduction of transition metals (Co, Mn) into A1PO4 frameworks leads to an unusual combination of acidic and redox properties of these catalysts. It is generally accepted that alummophosphates containing bivalent cations (MeAPO) have negatively charged frameworks where each Me+e-ion potentially gives rise to the formation of one Broensted acid site. The generation of acidic sites in MeAPO was supported by test reaction studies, Py adsorption and TPD of ammonia and of reactive amines and alcohols [1-5]. Unlike crystalline aluminosilicates, however, the corresponding bridged OH-groups have not been directly observed [2,6]. This work presents a comparative study of the acidic properties of AlPO4-5, AlPO4-11, CoAPO-5, MgAPO- 11 and SAPO- 11 using adsorption of ammonia and Py monitored by FTIR. Butene-1 isomerisation has been carried out over the above series of catalysts in order to evaluate the effects of structure and acidity on catalytic performance.
EXPERIMENTAL Materials. Samples of AIPO4-5, AlPO4-11, CoAPO-5, MgAPO- 11 and SAPO11 were synthesized according to the procedure described in [7]. Chemical analysis of as-made molecular sieves was carried out using A A . Prior to FTIR and catalytic studies all materials were calcined at 500oC for two hours in 02. XRD. XRD analysis of as-made powdered samples was performed usmg a Scintag diffractometer scanning at lo/min (2-theta range from 5 to 40~ FT/R. The acidity of the catalysts was assessed usmg thermodesorption of ammonia monitored by an FTIR spectrometer. Py adsorption was also employed to monitor Lewis and Broensted acidity. The detailed experimental procedure was described recently [8,9]. Butene-1 isomerisation. All samples were activated at 500oC for 2h in the flow of pure oxygen. Then the catalyst (100 rag) was cooled to the desirable reaction temperature (200-300oC), and a N2 flow containing 15% of butene-1 (99%, BOC) was
360 passed through the reactor. A complete analysis of the reaction products (H2, C1-C8) was conducted using on-line GC (Varian 3400). RESULTS AND DISCUSSION
XRD and Chemical Analysis The characterisation of synthesized molecular sieves by XRD and SEM indicate high cryst~llinity of both as-made and calcined materials. X-ray diffraction patterns agree with those reported in the literature for alummophosphates structures 5 and 11 [7,10] (structure types AFI and AEL). The results of chemical analysis presented in Table 1 suggest, in accordance with [2], incorporation of Co and Mg into Al-sites in CoAPO-5 and MgAPO- 11, and introduction of Si into P-sites. Table 1. Chemical analysis results of aluminophosphate molecular sieves
Sample
Elemental composition
CoAPO-5
( 0.025 Co.0.478 A1.0.498 P)O2
M gAPO- 11
( 0.035 Mg.0.459 A1.0.506 P)O2
SAPO-11
( 0.056 Si.0.507 A1.0.437 P)O2
FTIR
AIPO-5 and A1PO-11 were used as reference materials which, owing to the electroneutrality of their frameworks, should not exhibit Broensted acidity. Indeed, only a weak band at 3677 cm 1 is observed in the OH-region for these samples which is due to P-OH or A1-OH groups [2,11]. Interaction of these OH-groups with ammonia is weak and NH3 can be removed upon evacuation at 150~ (Fig.l). At the same time, the intensity of the absorbance band of Py-H + complexes is very low (Fig.2). Therefore we may conclude that both samples do not contain Broensted acid sites in appreciable amounts. A1+3substitution by Me § in the frameworks of A1PO4-s is believed to generate acidic bridged hydroxyls [2,5]. In spite of this the only band observed m the hydroxyl region of the IR spectrum of MgAPO-11 is the band at 3678 cm 1 assigned to nonacidic hydroxyls. The difference spectrum of the sample after ammonia adsorption, however, shows a broad negative peak around 3500 cm 1 (Fig.3) and an intense positive peak at 1452 cm 1 from NH4+-ions. Ammonia can be completely desorbed from the sample at 350~ and the intensity of the band due to Py-H § at 1545 cm -1 is significantly higher in comparison with A1PO4-11 (Fig.2).. The absorption band at 1448 cm 1 can be due to Py-Mg +2 complexes [12]. Thus, MgAPO-11 clearly exhibits Broensted acidity.
361
0.5 A
~176 ~176 s
o
rO
b
n c0 e
!
3800
3700
3600
3500
!
3400
3300
A bO.8 s
oo
6
aO
4
_--~_
C eo
2
"-
I
~.600
_
4
_,__
-
T
~.550
Ir
~500
i
~.4~50
~.400
Wavenumbers
Figure 1 (top). OH-region of the FTIR spectra of ALP04-11. 1- initial sample, 2- 1 after NHs adsorption at 150~ 3- 2 after ammonia desorption at 150~ 4- (2-3). Figure 2 (bottom). FTIR spectra of ALP04-11 (1), MgAPO-11 (2), CoAPO-5 (3), and SAPO-11 (4) after adsorption of Py at 150~C.
362 It is generally accepted that substitution of A1+s ions by Co+3 does not change the electroneutraliW of the aluminophosphate framework, and therefore, does not lead to the formation of acid sites. On the contrary, introduction of Co+2is considered to result in the formation of Broensted acid sites and an increase in catalytic activity [1,4]. IR spectra for both oxidized and reduced samples show only a weak band at 3678 cm 1. However, as for MgAPO-11, the adsorption of ammonia and Py reveals Broensted acidity (Fig.4). The results for both samples are similar, i.e. most of ammonia can be desorbed at 250~ and a broad negative peak appears in the difference spectra after NH3 adsorption. Py-H + complexes at 1545 cm x are also observed (Fig.2). The band at 1449 cm-1 can be assigned' to Py-Co +2 [12]. It should be noted that adsorption of ammonia or Py onto the oxidized sample results in a clear colour change from green to grey-blue. SAPOs have been extensively studied in the literature. In accordance with the majori W of publications, five OH-bands are present in the IR spectrum of SAPO11, at 3794, 3745, 3677, 3626 and 3530 cm x (Fig.5). The first three bands are due to weak or non-acidic P-OH and A1-OH groups, and the last two can be attributed to bridged A1-OH-Si hydroxyls. For SAPO-37 these two bands are usually attributed to acidic hydroxyls in supercages and small cages of the faujasite structure [2,13,14]. Broad bands at ~3530 cm ~ were also found for SAPO-5 [11,15], SAPO-34 [16] and SAPO-11 (this work). It is likely therefore, that the presence of broad bands at 35003550 cm-1 is a common feature of substituted crystalline aluminophosphates rather than the faujasite structure. Acidic hydroxyls interact with ammonia and Py forming NHt + ions and Py-H+(Fig.2). As for MgAPO-11 and CoAPO-5, ammoma can be desorbed at 350~ Fig.6 presents difference spectra of A1POt- 11, SAPO- 11 and CoAPO-5 after ammoma adsorption. For SAPO-11 and CoAPO-5 samples, the spectra show broad negative peaks at ~3500 cm" which are indicative of acidic hydroxyls interacting with ammonia. SAPO-11, CoAPO-5 and MgAPO-11 retain ammonia up to 350oC, whereas A1PO4-5 and -11 only up to 150oC. Py adsorption studies also support the presence of Broensted acid sites in substituted aluminophosphates. However, narrow absorption bands of isolated bridged OH-groups are absent in the IR spectra of CoAPO and MgAPO. This is in agreement with the results reported by M.Peters et al. [6]. The authors suggested that the reduction of Co§ to Co§ in CoAPO-5 and11 might be accompanied by the formation of terminal P-OH groups interacting with Co+e, i.e. P-(OH) ""Co% Butene-1 isomerisation Isomerisation of butene-1 proceeds via donation of a proton from the acid catalyst to the olefin, resulting in the formation of a secondary carbenium ion. This species can either isomerise or interact with a second butene molecule. The direct skeletal isomerisation can only occur via formation of the unstable primary carbenium ion [15]. Dimerisation reactions lead to the formation of branched Cs hydrocarbons, which undergo skeletal isomerisation and cracking. Catalytic results presented in Table 2 demonstrate that a double bond shift reaction is catalysed by
363
A 0.25
b s 0.20 o
rO.~5 b a 0.10n
3
c O.05e
0 00,
3800
-
3700
.
.
.
.
3800
. . . . . . . . . . .
.
.
.
.
.
3500
~.
.
|
3400
.
.
.
.
3300
. . . . . .
}
AO.5 b
F i 0.2-
3
n c e
O.~0.0_~-_
...........
~-3BbO 3700 "3600 'IN 35 bO
34b0
.....3 3 b 0
Wavenumbers Figure 3 (top). OH-region of the ~ spectra of MgAPO-11. 1- initial sample, 2- 1 after NH3 adsorption at 150~ 3- 2 after ammonia desorption at 250~ 4- (2-3).
Figure 4 (bottom). OH-region of the FTIR spectra of CoAPO-5. 1- initial sample, 21 after NH3 adsorption at 150~ 3 - 2 after NH8 desorption at 150~ 4 - (2-3).
364 0.4-I
A b s
0.3
o
ro.2 b 8
no.:[ c e
0.0 ~
. . . .
w
. . . . . .
3800
I
. . . . . . . .
3700
l
. . . . .
3600
i-
3500
3400
.......... r -3300 !
A
o.~o
! !
b
J
I~
4
o 0.05 r I b a n
O.O0-
c
'
-0
2
05i !
. . . .
3Bbo
3600 ~
.
.
.
.
3 4 0' 0
3 2 0~ 0 .
.
.
.
.
Wavenumbers
Figure 5 (top). OH-region of the FTIR spectra of SAPO-11. 1- initial sample, 2 - 1 after NH~ adsorption at 150~C, 3- 2 after ammonia desorption at 250~C, 4- (2-3).
Figure 6 (bottom). Difference FTIR spectra after ammonia adsorption at 150~C on 1ALP04-5, 2- CoAPO-5, 3- SAPO-11.
365 both strong and weak acid sites. Indeed, catalytic performance of AIPO4-11, MgAPO11, SAPO-11 and HZSM-5 (used for reference purposes) at 200~ is very similar. Clearly, butene-1 isomerisation at this temperature cannot be used as a test reaction to assess the strength of Bmensted acid sites. At 300oC, MgAPO-11, SAPO11 and HZSM-5 demonstrate high selectivity towards formation of iso-butene, oligomers and cracked hydrocarbons. Reactions leading to these products require much stronger acid sites than isomerisation of n-butenes. Based on performance of the catalysts at 300~ we can estimate strength of Broensted sites in the studied catalysts: HZSM-5>SAPO- 11 = MgAPO- 11 > A1PO4-s. Surprisingly, CoAPO-5 shows low activity at 200~ at higher temperature, product distributions on CoAPO-5 and ALP04-11 are similar. Therefore, strong acid sites detected in CoAPO-5 using ammonia and Py do not contribute to the catalytic reaction. Previously, based on the study on n-butene cracking, E. Flanigen et al. concluded [1] that the incorporation of metal in the structure 5 imparts low acidity. These results are yet to be explained, but could be related to the destruction of Broensted acid sites in the course of the catalytic process. Table 2. The results ofbutene-1 conversion. A1PO4-11 Conversion, % iso-butene trans-butene-2 cis-butene-2 Olig.+Crack.**
86.0 0.1 54.5 31.3 0.1
MgAPO- 11
CoAPO-5
SAPO- 11
Reaction Temperature=200oC 86.0 21.3 86.5 0.5 0.0 1.4 53.8 8.7 52.2 31.0 12.3 30.3 0.7 0.3 2.6 Reaction Temperature=300oC 83.3 73.6 86.7 14.1 0.2 24.3 40.7 39.2 32.4 26.9 33.3 21.6 1.6 1.1 8.4
Conversion, % 80.0 iso-butene 1.0 trans-butene-2 47.4 cis-butene-2 31.4 Olig.+Crack.** 0.2 *- HZSM-5 (Si/Al=60) ** - Total amount of oligomers and cracked products (wt.%).
HZSM-5 * 81.9 0.0 50.0 31.5 0.4 97.1 15.6 7.3 4.9 69.3
CONCLUSIONS Acidic properties of isomorphously substituted alummophosphates have been studied. In spite of the absence of isolated bands of acidic OH-groups in the IR
366 spectra of CoAPO-5 and MgAPO-11, these catalysts clearly demonstrate Broensted acidity. Strength of the acid sites decreases as following SAPO-11 --MgAPO-11 > CoAPO-5 >> A1PO4-s. The results of butene-1 isomerisation at 300oC support the presence of strong Broensted acid sites in SAPO- 11 and MgAPO- 11. ACKNOWLEDGMENT
The authors gratefully acknowledge financial support from EEC BRITE EURAM grant 4633 and thank Dr. A.F. Ojo for preparation of SAPO-11 and MgAPO- 11. REFERENCES
1. E.M.Flanigen, B.M.Lok, R.L.Patton, S.T.Wflson, New Developments in Zeolite Science and Technology, 1986, Tokyo, p.103-112. 2. E.M.Flanigen, R.L.Patton, S.T.Wilson Stud. Sur. Sci. Catal., v.37, p.13-27 (1988). 3. J.A.Martens, P.A.Jacobs, Stud. Sur. Sci. Catal., v.85, p. 653-685 (1994). 4. B.Craushaar-Czarnetzki, W.G.M.Hoogervorst, R.R.Andrea, C.A.Emies, W.H.Y. Stork, J. Chem. Soc., Faraday Trans., v.87, p.891-895 (1991). 5. D.J.Pan~o, C.Pereira, G.T.Kokotailo, R.J.Gorte, J. Catal., v.138, p.377385(1992). 6. M.P.J.Peters, J.H.C. van Hooff, R.A.Sheldon, V.L.Zholobenko, L.M.Kustov, V.B.Kazansky, Proc. 9th Intern. Zeolite Conf., Montreal, v.1, p.651-658 (1992). 7. B.M.Lok, C.A.Messina, R.L.Patton, R.T.Gajek, T.R.Cannan, E.M. Flamgen, US Patent No 4440871 (1984); S.T.Wflson, B.M.Lok, E.M. Flanigen, US Patent No 4310440 (1982); S.T.Wflson, E.M. Flamgen, US Patent No 4567029 (1986). 8. V.L.Zholobenko, M./LMakarova, J.Dwyer, J.Phys. Chem., v.93, p.59625964(1993). 9. M.A.Makarova, A.Garforth, V.L.Zholobenko, J.Dwyer, G.J.Earl, D.Rawlence, Stud. Sur. Sci. Catal., v.84(A), p.365-372 (1994). 10. S.T.Wflson, B.M.Lok, C.A.Messina, T.R.Cannan, E.M. Flamgen, Am. Chem. Soc. Syrup. Ser., v.218, p. 79-106 (1983). 11. L.Kubelkova, S.Beran, J.A.Lercher, Zeolites, v.9. p.539 (1989). 12. J.W.Ward, J.Catal., v.14, p.365 (1965). 13. S.Dzwigaj, M.Briend, A.Shikholeslami, M.J.Peltre, D.Bartomeuf, Zeolites, v.10, p.157 (1990). 14. M.A.Makarova, A.F.Ojo, K.M.Al-Ghefai~, J.Dwyer, Proc. 9th Intern. Zeolite Conf., Montreal, v.2, p.259-266 (1992). 15. B.C. Gates, Catalytic Chemistry, John Wiley & Sons, New York (1991).
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
367
Selective oxidation with redox metallosilicates in the production of fine chemicals Paul Ratnasamy and Rajiv Kumar National Chemical Laboratory, Pune 411 008, INDIA Fax: + 91-212-334761; email: [email protected]
Titanium-, vanadium-, tin- and arseno-silicate molecular sieves oxidize a variety of organic substrates e.g. aromatics, alkanes, olefins, unsaturated alcohols, aldehydes, amines and thioethers etc. using dilute hydrogen peroxide (30% aq.) exhibiting shape-, regio- and chemo-selectivity. The efficiency in H 2 0 2 utilization follows the order: Ti- > V- - Sn- As-silicates. Large pore V-zeolite (V-NCL-1) oxidizes even bulky aromatic compounds like xylenes, trimethyl benzenes, naphthalene etc. A significant enhancement in conversions and para-selectivity has been achieved in the oxidation of hydrophobic organic compounds under triphasic conditions in the absence of any co-solvent. Heterolytic cleavage of H 2 0 2 is preferred by Ti-silicates while homolytic cleavage also occurs over others. 1. INTRODUCTION During the sixties and seventies zeolites were used mainly in acid-catalyzed reactions in the petroleum/petrochemical industries. Beginning in the early eighties they were also used to replace the conventional acid catalysts like A1C13, H2SO4 etc. in the fine chemicals industry [ 1,2]. The advent of TS-1 opened new vistas in the selective oxidation of organic compounds especially in the fine chemicals industry, where the use of H 2 0 2 as the oxidant can be economically viable and environmentally desirable [3,4]. In recent years various other metal ions with redox characteristics, e.g. Fe, V, Sn, As etc., could be incorporated in different medium and large pore zeolitic silicate networks providing an opportunity to carry out diverse selective oxidation reactions in a confined micro-environment around catalytic active sites (Table 1). Ti-, V- and Sn-silicate molecular sieves can oxyfunctionalize / oxidize a variety of organic compounds such as alkanes, olefins, aromatics (benzene, toluene etc), phenols, amines, thioethers etc.[4-8]. In the present paper we discuss some recent results from our laboratory in this area relevant to fine chemical industry. 2. EXPERIMENTAL In Table 1 various metallosilicate analogues of different framework topologies are given along with their corresponding references, where details about their synthesis and characterization have already been given. The catalytic runs were carried out in batch reactors with or without any co-solvent. The products were generally analyzed by GC (HP 5980 using FID detector) and CGMS (Shirnadzu, QP 2000). In certain cases the products were isolated and characterized by NMR.
368 Table 1 Metallosilicate molecular sieves with redox properties. Metal ion Fe 3 + Ti 4+ V4+/5+ Sn 4 + As3 + As 5 +
Zeolite structure(s) a [9] MFI, MEL, MTI', ZSM-48, EUO, MOR, BEbA, FAU, SOD MFI[10,11], MEL[12,13], ZSM-48[14], BEA[15] MFI[16,17], MEL[18,19], ZSM-48120,21], MTW[21,22], NCL-l[23], BEA[24] b MFI[8], MEL[25], MTW[26] [27] MFI, MEL, ZSM-48, MTW [28] MFI, MEL, ZSM-48, MTW, NCL-1
a: The numbers in parentheses refer to the references at the end of the text. b: A1-Ti or A1-V analogue.
3.
RESULTS AND DISCUSSION
3.1. Hydroxylation of aromatic compounds
The direct hydroxylation of aromatic hydrocarbons to phenol or phenol derivatives can be carried out by means ofH20 2 and titanium-, vanadium-, tin- or As-silicate molecular sieves [3-8,28]. The oxidation of phenol with dilute H 2 0 2 in the presence of TS-1/TS-2 catalyst gives hydroquinone and catechol in nearly equal ratio. The selectivity of H 2 0 2 towards hydroxy products was found to be 70-85%. Infact, a commercial plant (Enichem, Italy) is already producing catechol and hydro quinone via hydroxylation of phenol over TS- 1using H 2 0 2 [3]. Similarly, V-, Sn- and As-sihcate molecular sieves can also hydroxylate phenol in the presence of dil. H20 2. The H 2 0 2 efficiency over these catalysts follows the order : TS-2 (70%) > VS-2 (56%) -- SnS-2 (54%) -- AS-2 (52%). The direct hydroxylation of benzene and substituted benzenes [4,6,7] such as toluene is also possible by TS-1 or VS-1. While, in the oxidation of toluene over Ti-zeolites, only aromatic ring hydroxylation takes place, V- and Sn-zeolites can, in addition, also oxidize side chains (Table 2). Table 2 Hydroxylation of benzene and toluene over TS-2 (Si/Ti = 77), VS-2 (Si/V = 79) and SnS-2 (Si/Sn=70) molecular sieves. Benzene H 2 0 2 efficiency (mole%) Products (mole%) Phenol p-Benzoquinone o-Cresol p-Cresol Benzyl alcohol Benzaldehyde Others
Toluene
TS-2
VS-2
TS-2
VS-2
SnS-2
42.2
18.2
37.8
49.5
36.4
88.0 09.0 . . . . . . . . . . . 03.0
90.0 07.0
. . . . 36.0 59.0
. . . . 20.0 17.0 08.0 52.0 03.0
04.1 08.8 14.2 71.4 01.5
. . . .
. . 03.0
. 05.0
. .
. .
Reaction conditions: Catalyst 0.1g; Substrate 1.0g; Substrate/H20 2 (mol) = 3.0; Solvent: acetonitrile 10g; T=60~ time=8h.
369 So far, V-MTW, V-NCL-1 and A1-V-BEA are the only vanadium-containing large pore molecular sieves available for the oxidation of bulkier molecules, which are too big to enter the channels of medium pore zeolites. O-xylene is oxidized to 2-methylbenzyl alcohol/aldehyde and almost no ring hydroxylation was observed. However, the oxidation ofm-xylene and 1,3,5-trimethyl benzene led to ring hydroxylation, resulting in the formation of corresponding phenols, in addition to the benzyl alcohols/aldehydes. Nearly 168 moles of naphthalene were converted per mole of vanadium in 18 h over V-NCL-1. The products were: 1-naphthol (4.0%), 2-naphthol (7.0), 1,4 naphthoquinone (22.0), phthalic anhydride (47%) and other dioxygenates (20%). Phthalic anhydride is formed by the further oxidation of 1,4 naphthoquinone which in turn was formed from 1-naphthol. This was confirmed by oxidizing 1-naphthol which gave 1,4 naphthoquinone (12.6%) and phthalic anhydride (81.%). 3.2. Alkane Oxidation
Table 3 compares the competitive oxyfunctionalization of an equimolar mixture of n-hexane and another alkane (alkane-II), selected from 3-methyl pentane (3-MP), 2,2-dimethyl butane (2,2-DMB) or cyclohexane, with vary!ng critical diameter, over TS-2 using dilute H 2 0 2. As the size of the alkane-II increases, its relative conversion (vis-a-vis n-hexane) decreases, the reactivity order being: n-hexane > 3-MP > 2,2-DMB > cyclohexane. In homogeneous reaction systems, the opposite trend is observed. Further, although the critical diameter of 2,2-DMB and cyclohexane are comparable, the former competes better with n-hexane compared to the latter, illustrating that not only size but also the shape/conformation of the reacting molecule plays a significant role in competitive oxidation. This is an example of reactant shape selectivity. TS-1 also behaves in a similar way. Table 3 Competitive oxidation of an equimolar mixture of n-hexane and another alkane-II over TS-2 (Si/Ti = 77) as catalyst and dilute H 2 0 2 (30 wt% aq.) as oxidizing agent. Alkane-II
3-MP 2,2-DMB cyclohexane n-hexane
Critical
Conversion, mol %
Diameter, nm
n-hexane
alkane-II
n-hexane/ alkane II
0.55 0.61 0.60 0.43
7.8 8.2 12.3 18.9
2.8 1.7 1.8 -
2.8 4.8 6.8 -
Reaction conditions: Catalyst 0.1g; Substrate 1.0g; Substrate/H20 2 (mole/mole) = 3; Solvent: acetonitrile 10g; T=80~ time=8h. The major difference between Ti- and V- zeolites is that while TS-1/TS-2 oxyfunctionalize n-hexane at 2 and 3 positions, VS-1/VS-2/[V]-ZSM-12 activate even primary C-H bonds (producing hex-l-ol/al) in addition to oxidation at 2 and 3 positions [21,22]. Over TS-2, VS-2 and [V]-ZSM-12 the distribution of products in relation to 1,2,3 regio isomers was as follows. TS-2: 1- (0%) < < < 2- (52%) > 3- (48%) (no activation at 1-position). VS-2: 1-(13%) < < 2- (45%) > 3-(42%) VS-12: 1- (6%) < < 2- (43%) < 3- (51%) (1,2,3 positions refer to numbering in n-hexane).
370 In medium pore TS-2/VS-2 molecular sieves the concentration of hex-2-ol/one was slightly more than that of hex-3-ol/one while over large pore [V]-ZSM-12 the reverse trend was observed. Further, over both TS-2 and VS-2, the secondary products, ketone (from sec. alcohol) or aldehyde (from primary alcohol), are also formed. However, the extent of the secondary reaction was significantly more predominant over VS-2 than over TS-2. The values of molar ratio -ol/-one over TS-2 and VS-2 were 0.77 and 0.36, respectively. 3.3. Epoxidation of olefins
TS-1/H20 2 catalyzed epoxidation of olefins with terminal C=C is faster than that with internal C = C bond(Table 4). Further, the epoxidation rate decreases with the increase in chain length of the olefin (1-hexene > 1-octene > 1 dodecene) (Table 4). The increased diffusional resistance experienced by longer chain olefins to reach the active sites in TS-1 pores may be the main reason for this observation [4,6,7,29,30]. The epoxidation of styrene by TS-1/H20 2 system resulted in the formation of epoxide and its further isomerization into phenylacetaldehyde [31]. Almost no or very little acetophenone was produced by isomerization of styrene epoxide, phenylacetaldehyde being the sole or major product. The high regio-selectivity in favor of phenylacetaldehyde may be attributed to the stabilization of the benzyl cation [2,31]. Protic and aprotic solvents influence the product distribution differently. In acetone, epoxide + phenylacetaldehyde selectivity was quite high (85-90%). However, in methanol, in addition to phenylacetaldehyde, alcoholysis also occurred to a large extent (45%) producing 2-methoxy-2-phenylethanol. Table 4 Epoxidation of various olefins over TS-2 (Si/Ti = 29) catalyst under identical conditions. Alkene
Conv., mole% Epo. Sel.,%
hex-1ene
hex-2ene
hex-3ene
oct-lene
dodec-1 -ene
cyclohexene
92.0 73.5
81.2 69.0
72.0 76.5
56.4 66.3
28.8 50.0
40.2 54.3
Reaction conditions: catalyst 0.1g; substrate 1.0g; H202/substrate = 1.1; solvent: acetonitrile, 10g; T = 60~ time = 6h. Chemoselective oxidation of unsaturated alcohols and aldehydes Recently, we have reported the chemoselective oxidation of unsaturated alcohols and aldehydes over TS-1 and VS-1 catalysts [32]. Allyl alcohol was converted faster than methallyl alcohol (Table 5). Further, over TS-1 while allyl alcohol gave > 95 % epoxide with minor amount ( < 5%) of allyl aldehyde i.e. acrolein, in the case of methallyl alcohol, the epoxide selectivity was about 70% only, the rest being aldehyde and minor amount of acid [32]. Over VS-1, the epoxidation was much less compared to the oxidation of the alcoholic group (Table 5). However, when acrolein and methacrolein were reacted over TS-1 and VS-1, only 10-15% epoxidation occurred (the corresponding acids were the other products) on both the catalysts (Table 5). The greater diffusional resistance as well as steric crowding at or around the active site due to methyl branching adjacent to the C=C bond in methallyl alcohol seems to be the main reason for the relatively low reactivity of its C = C vis-a-vis the CH2OH group.
3.4.
371 Table 5 Chemoselective oxidation of allyl alcohol (A), methallyl alcohol (B), acrolein (C) and methacrolein (D) over TS-1 (Si/Ti=27) and VS-1 (Si/V=56). TS-1 Conv.(mole%) Prod.(mole%) epoxide aldehyde acid others a
VS-1
A
B
C
D
A
B
C
D
95.0
62.0
71.2
65.3
27.0
21.6
56.7
54.0
95 5 -
70 18 8 5
10 50 40
9 41 50
20 60 10 10
5 80 15 5
10 60 30
15 30 55
Reaction conditions: Catalyst 0.1g; substrate 1.0g; H202/substrate (mol,) = 1.1; solvent" acetonitrile 10 g; T = 60~ time = 8h. a: mainly diols and minor amount of other high boiling unidentified products. Hydroxyl-assisted chemo- and stereo-selective oxidations Hydroxyl-assisted epoxidation using TS-1/H20 2 combination is both chemo and stereoselective (Fig. 1). For example, in the chemoselective epoxidation of 1-hydroxy3,7-dimethyl-2,6 octadiene (geraniol, species 1) the C = Cbond at2 position (2-ene) adjacent to the -OH group was selectively epoxidized giving 1-hydroxy-2-epoxy-3,7 dimethyl-6-ene (species 2), 6-epoxy isomer (species 3) being absent. The stereoselective epoxidation of cyclopent-2-en-l-ol (species 4)or cyclohex-2-en-l-ol (species 7) gave the corresponding epoxy alcohols in 75-80 % yield. The cis isomer was the major (90%) constituent in both the cases [4,33]. 3.5.
3.6. Carbon-Nitrogen Double Bond (C= N) Cleavage TS-1 can catalyze [34,35] the oxidative cleavage of various tosylhydrazones and imines to their corresponding carbonyl compounds in the presence of dilute H 2 0 2 (30%) as an oxidant (Table 6). The tosylhydrazones of butan-2-one and cyclopentanone gave the corresponding ketone in 84% and 80% yields, respectively, at 100% conversion. Similarly, the tosylhydrazones of menthone and ~-tetralone afforded the corresponding ketone in 60% and 70% yields, respectively, at 80% conversions. However, with aldehydes, further oxidation to carboxylic acid was observed. When methanol was the solvent, the acids were converted further to the corresponding methyl esters.. Table 6 Cleavage of C = N bonds in tosyl hydrazones of carbonyl compounds catalyzed by TS-1 / H 2 0 2 system. Tosylhydrazones of." butane-2-one cyclopentanone cyclohexenone acetophenone benzophenone menthone a-tetralone cyclohexenone benzaldehyde
Cony.,%
Reaction Time, h
Ketone Sel.%
100 100 100 100 66 80 80 80 60
4 10 10 6 18 18 18 10 15
84 80 75 73 70 60 70 65 60
372
F
2
3
OH
OH
-
.-
4 OH
\
1,,,,0
(90)
(10)
5
6
OH
. 7
OH
OH
o + (90) 8
L j,,,, ~ (10) 9
Figure 1: Hydroxyl-assisted chemo- and stereo-selective epoxidation catalyzed by TS-1 in the presence of dilute H20 2. Values in parentheses refer to percent selectivity. 3.7. Oxidation of Amines
Metal catalyzed oxidation of amines is of interest because of its relevance to the enzymatic degradation of nitrogen containing compounds in biological systems [36] and preparation of liquid crystals [37]. Recent findings from our laboratory have shown that TS-1/H20 2 exhibits a remarkable activity and selectivity in the liquid phase oxidation of aniline to symmetrical azoxybenzene [38]. In the oxidation of aniline Into azoxybenzene over TS-1 the yield of azoxybenzene was 87.8%. In the case of 4-methoxy aniline, the yield was rather low (10%). Recently Reddy et.al. [39]. have reported that primary aliphatic amines with ~-hydrogen atoms can be oxidized over titanium silicate molecular sieves in the presence of hydrogen peroxide as oxidant to give the corresponding oximes as the main product. TS-1, for example, oxidizes n-alkylamine with selective formation of the oxime (rather than the bulkier m'troso dimer) due to the sterically constrained environment of the 10-membered ring. The activity, oxime selectivity and peroxide efficiency gradually decrease when the
373 alkyl chain length increases, again due to the shape selective nature of TS-1. The byproducts in this reaction are alkylazo compounds and isomeric hydrazones. Low activity and selectivity are observed with cyclohexylamines. 3.8. Ammoximation of Carbonyl Compounds TS-1/TS-2 molecular sieves catalyze the conversion of carbonyl compounds to the corresponding oximes in the presence of ammonia and H 2 0 2 with high selectivities [3,4,6,7]. The classical methods employed in the manufacture of oximes are associated with the co-production of ammonium sulphate and the use of hazardous chemicals like oleum, hahdes and oxides of nitrogen. The ammoximation of aldehyde is faster than that of ketones. The oxime selectivity is quite high, (99+ 1%) in both the cases. The ammoximation of cyclohexanone, by this method offers an environmentally friendly alternative route to cyclohexanone oxime, an intermediate in the manufacture of Nylon-6. 3.9. Oxidation of Thioethers Through controlled, selective oxidation of organic sulfides/thioethers, quite useful products like sulfoxides and sulfones can be obtained. Recently, we have shown that both, TS-1/TS-2 as well as VS-1/VS-2 can very efficiently and selectively catalyze the oxidation of various thioethers to sulfoxides and sulphones using dilute H 2 0 2 [4,40,41]. The reactivity of thioethers as well as selectivity for sulfoxide decrease with the increase in their size according to the order: dimethyl thioether (time for complete conversion = 30 rain.) > > diethyl thioether (90 min.) > methylphenyl thioether (120 rain.) > ethylphenyl thioether (180 rain). The bulky diphenyl thioethers could not be oxidized under similar experimental conditions. The most reactive dimeth~,l thioether also exhibited the highest ( > 95%) selectivity for the primary product of oxidation i.e. dimethylsulfoxide. Dimethyl sulfone was the secondary product (5%) formed by the further oxidation of the sulfoxide. Under similar reaction conditions, VS-2 and VS-1 were more active compared to TS-2 and TS-1. For example, while VS-2 could completely convert methylphenyl thioether in 30 min. at 60 ~ with 84% sulfoxide yield, TS-2 needed atleast 120 rain. for complete conversion with 76 % sulfoxide yield. The remaining product was sulfone. 3.10. Triphase catalysis Oxidation of water-immiscible organic compounds by dilute H 2 0 2 using metallosilicate catalysts can be carried out either in a biphase condition (solid + one homogeneous liquid phase containing H20, or[ganic substrate and a solvent in which both the water and the substrate are soluble) or a trlphase condition (solid + water + the substrate which is immiscible with water). Recently, we have demonstrated [42] that a considerable enhancement in the activity andpara-selectivity in the oxidation of water immiscible organic compounds, catalyzed by the TS-1/H20 2 system, can be achieved under triphasic conditions in the absence of any co-solvent (Table 7). Under biphasic conditions, the organic solvent competes with the reactant for diffusion in the channels of the hydrophobic TS-1. Under triphasic conditions, the hydrophobic organic substrate competes more favorably with water for diffusion in TS-1. This phenomenon results in the higher activity and para-selectivity under triphasic conditions [421.
3.11 Oxidative dehydrogenation of alcohols using molecular oxygen. The use of molecular oxygen or air in the vapor phase oxidative dehydrogenation of aq. ethanol (ca. 10 wt%) to acetaldehyde over TS-1 catalyst, under fixed bed conditions, was first reported from our laboratories [43]. With the increase in the reaction temperature the conversion increased and the selectivity towards acetaldehyde decreased.The other
374 major products were acetic acid and ethylene (Table 8). Similarly, MFI type arseno-silicate (AsS-l) catalyzed the oxidative dehydrogenation of 2-butanol to 2-butanone using air [28] (Table 9). Table 7 Enhancement of oxidation activity and para-selectivity in triphasic media. TS-1, 353 K, substrate/H20 2 = 1 (mole/mole), catalyst/substrate (wt/wt) = 0.2. Biphase: solvent (acetonitrile)/substrate = 10 (wt/wt) and triphase: water/substrate = 10 (wt/wt); no solvent. Substrate
Phase
TON
Products, %
(h "l)
ortho
para
others
Toluene
Bi Tri
0.42 2.28
69.7 41.4
28.4 55.9
1.9 2.7
Anisole
Bi Tri
2.07 6.54
66.6 25.6
30.3 72.3
3.1 2.1
Table 8 Influence of temperature on the oxidative dehydrogenation of ethanol over TS-1 using molecular oxygen. O2/ethanol = 1 (mole/mole), WHSV = 1 h "1. Temperature / K
Ethanol conversion, wt% Products, wt% Acetaldehyde Acetic acid Ethylene Othes a
523
573
623
673
05
20
55
88
95 05 ---
87 07 03 03
81 08 06 05
68 14 08 10
a: HCHO, CO, CO2 and CH 4 Table 9 Oxidative dehydrogenation of 2-butgnol over ASS-1 using air. 02 / 2-butanol = 1 (mole / mole), WHSV (2-butanol) = 3.5 h "l. Temperature / K
2-Butanol conversion., wt% Products distribution, wt% 2-Butanone Butenes Others a
623
673
723
15
35
60
90 08 02
78 15 07
65 25 10
a: CO, CO2 and some higher boiling dimerized etheric products.
375 3.12. Mechanistic Aspects The redox potentials of some metal ions, ease of their reduction and catalytic activity in H 2 0 2 decomposition in aqueous solutions are given in Table 11 [44].
Table 10 Redox potentials of some transition metals. Reaction Co 3 + V5 + Fe 3 + Ti 4 + As 5 + Sn4 +
+ + + + + +
ee" ee" 2e2e-
--- > --- > --- > --- > --- > --- >
Co 2 + V4 + Fe 2 + Ti 3 + As 3 + Sn2 +
E (ev)
Reduction
H 2 0 2 decomp.
1.82 1.00 0.77 0.06 0.56 0.15
easy moderate moderate difficult moderate moderate
fast moderate moderate difficult moderate moderate
Catalytic oxidations with H 2 0 2 involve two types of H 2 0 2 cleavage / activation: (i) heterolytic (ionic) and (ii) homolytic (radical). In general, the heterolytic mechanism predominates in the case of those catalysts involvin~ metal ions (like TS-1) having low redox potential (difficult redox transition M n + ~ M~n-l) +/M(n-'2) + ), while the radical mechanism is predominantly operative i n cases (like V) where redox potentials are high with easy redox transition M n§ ~ M(n-1) +/M(n-2) 4. 4. ACKNOWLEDGEMENTS We thank members of Catalysis Division for providing many of the results. The work was partly funded by the European Commission (contract no. Cll-CT93-0361). REFERENCE
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3. 4.
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6. 7. 0
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10. 11. 12. 13. 14. 15. 16.
W. H61derich, M. Hesse and F. Naumann, Angew. Chem., Int. Ed. Engl. 27 (1988) 226. W. H61derich and H. Van Bekkum, Stud. Surf. Sci. Catal., 58 (1991) 631. B. Notari, Stud. Surf. Sci. Catal., 37 (1988) 413. P. Kumar, R. Kumar and B. Pandey, J. Indian Inst. Sci., 74 (1994) 293; P. Kumar, R. Kumar and B. Pandey, Synlett., to appear as an Account in April/May issue of 1995. P. Ratnasamy and R. Kumar, Catal. Lett., 22 (1993) 227. A.V. Ramaswamy and S. Sivasanker, Catal. Lett., 22, (1993) 239. A.V. Ramaswamy, S. Sivasanker and P. Ratnasamy, Microporous Materials, 2 (1994) 451. N.K.Mal, V. Ramaswamy, S. Ganapathy and A.V. Ramaswamy, J. Chem. Soc. Chem. Commun., (1994) 1933. P. Ratnasamy and R. Kumar, Catal. Today, 6 (1991) 329. M. Taramasso, G. Perego and B. Notari, U.S. Pat. 4,410,501 (1983). A. Thangaraj, R. Kumar, S.P. Mirajkar and P. Ratnasamy, J. Catal., 130 (1991) 1. J.S. Reddy and R. Kumar, J. Catal. 130 (1991) 140. J.S. Reddy and R. Kumar, Zeolites, 12 (1992) 95. D.P. Serrano, H.-X. Li and M.E. Davis, J. Chem. Soc. Chem. Commun., (1992) 745. M.A. Camblor, A. Corma and J. Perez-Pariente, Zeolites, 13 91993) 82. A. Miyamoto, D. Medhanavyn and T. Inui, Appl. Catal. 28 (1990) 89.
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Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
377
Novel model catalysts containing supported MFI-type zeolites N. van der Puil, E. C. Rodenburg, H. van Bekkum and J. C. Jansen Laboratory of Organic Chemistry and Catalysis, Delft University of Technology Julianalaan 136, 2628 BL Delft, The Netherlands.
Abstract The synthesis, characterization and catalytic testing of composite catalysts consisting of a supported catalytic phase and a silicalite-1 coating are reported. Si/Fe203/MFI and Si/Cr203/MFI composites were tested in the dehydrogenation of n-heptane at 500~ The composites showed selectivity for the formation of methylcyclohexane, while the uncoated metal oxide particles showed selectivityfor toluene. TiO2/Pt/silicalite1 composites were tested in the competitive hydrogenation of a mixture of 1-heptene and 3,3-dimethyl-1butene. The composite showed a high selectivity for the conversion of the linear olefin, which was not observed for a TiO2/Pt catalyst. These experiments indicate the feasibility and shape selectivity of the composite catalysts. 1. I N T R O D U C T I O N The synthesis of thin layers and coatings of different types of zeolites on supports such as aluminum, stainless steel, mullite, quartz has been reported recently [1]. The composites may be applicable as catalysts, membranes and as chemical interface in sensors. Coatings of silicalite-1 with a random orientation on sintered metal supports have been prepared and tested as membranes [2]. A prerequisite for optimization of the applications is the ability to prepare very thin and oriented layers of zeolites. In the case of MFI-type zeolites, various orientations of the supported crystals can be obtained under specific synthesis conditions [3]. Thin layers of laterally oriented crystals (b-direction perpendicular to the support surface) for sensor purposes were reported [4]. Axially oriented crystals of ZSM-5 (c-direction perpendicular to the support surface) on stainless steel provide a dust-free reactor set-up with advantageous heat transfer properties and a low pressure drop [5]. The objectives of the present study are the preparation and testing of well defined model catalysts, containing uniquely oriented crystallites of MFI on single crystal supports (silicon and rutile). In these composites the catalytic sites are present at the interface between support and zeolite, either resulting from the combination of two different oxide phases [6,7],or as a separate phase obtained by modification of the support surface. The support material serves to provide a well defined interface, which offers high stability for the catalytic sites. In this way it may be possible to stabilize catalytic sites exposed to the zeolite pores, which can not be obtained in the zeolite framework. Since the catalytic sites are created on the support before zeolite synthesis, no ion exchange capacity of the framework is needed. It is therefore possible to obtain a true monofunctional zeolite catalyst. Moreover, no catalytic sites will be present at the external surface of the zeolite phase. If the zeolite layer is not chemically bonded to the catalytic phase, only reactant selectivity is expected to occur. If however chemical bonding exists between the zeolite layer and the catalytic material, it is expected that additional transition state shape
378 selectivity will occur during reaction at the catalytic sites. In the ideal case, this might also result in an "end-on" effect by which only the ends of molecule chains are able to react. Here we report on the synthesis, characterization and catalytic performance of laterally oriented crystallites of silicalite-1 on different types of catalytic phases and/or supports. First composites consisting of a Si support, iron(III) and chromium(III) oxide particles and a silicalite-1 coating are reported, which were tested in the dehydrogenation/cyclization of n-heptane. Secondly, silicalite-1 coatings on TiO 2 supported particles of platinum and their performance in competitive hydrogenation reactions are described.
2. EXPERIMENTAL Silicon (100) wafers (10xl0x7 mm, one side polished, DIMES) and TiO 2 wafers (10xl0xl mm, both sides polished, Single Crystal Technology b.v.)were used as supports. Before use, the Si-wafers were treated by a special cleaning sequence in order to remove organics and metal contamination [3]. Metal oxide coatings on the polished side of the Si-wafers were obtained by a spincoating technique [8]. Ethanolic solutions of 0.1 wt% FeC13.6H20 (p.A. Janssen Chimica) and CrC13.6H20 (p.A. Janssen Chimica) were brought onto a rotating support, while flushing with nitrogen. The precursors were calcined in air at 450~ for 6 hours. The metal oxide coatings were studied by X-ray Photo-electron Spectroscopy (Phi 5400 spectrometer), ICP/AES (Varian SpectrAA 300)and Scanning Electron Microscopy (Philips XL20). Platinum particles on TiO 2 were obtained by sputtering on one side for 6-24 seconds at 300 mV and 160 Watt. Here TiO 2 was chosen as a support, since platinum does not stabilize on Si. At temperatures above 150~ platinum diffuses through the surface silicon oxide layer and forms a stable PtSi phase [9]. High Resolution SEM measurements of the sputtered samples were carried out with a Jeol JSM-6320F microscope. Zeolite synthesis conditions were refined to promote crystallization of the zeolite coating, and to avoid dissolution of the support phase. The silicalite-1 coatings were synthesized from TEOS (98%, Janssen Chimica), TPAOH (25%, Chemische Fabriek Zaltbommel) and deionized water. The molar oxide ratio in the gel was 6.6 SiO 2 : 1 TPA20 : 770 H20. Synthesis of the metal oxide composites took place at 150~ for 3 hours; the platinum composites were synthesized at 165 ~ for 5 hours and 15 minutes. Calcination of the metal oxide composites was carried at 500~ for 6 hours, while the platinum composites were calcined at 350~ for 12 hours or 500~ for 6 hours. After the low temperature calcination, the samples were treated in ozone at 120~ for one hour and subsequently reduced in H 2 at 100~ one hour. A cross-section of the metal oxide composites was analyzed with TEM (Philips CM-30 FEG). All model catalysts were tested in a batch reactor of 20 ml, suitable for analysis of amounts of products in the nano- and micro-range. Mixing of the products and reactants t o o k place by applying a small temperature difference over the reactor volume. The reactor set-up was fully automated. Analysis of the products was performed by online GC analysis on a Packard 438, equipped with a 25 m CP Sil5 column and a 25 m CP Wax 52 column in series. In all cases, the column temperature was kept constant at 40~ The Fe20 3 and Cr20 3 containing systems were tested in the non-oxidative
379
dehydrogenation of n-heptane at 500~ and atmospheric pressure. In each experiment five platelets of catalyst and 0.5 % n-heptane in N 2 were used. The platinum composites were tested in the (competitive) hydrogenation (1:1 volume ratio) of 1-heptene (99%, Aldrich) and 3,3-dimethyl-l-butene (95%, Aldrich). It is expected that for the zeolite coated catalysts the conversion of the linear molecule will be significantly higher than that of the branched molecule [12-14]. The reactions were carried out at 100~ atmospheric pressure in a mixture of H 2 and argon (1:9). The H2/hydrocarbon ratio was 17. In the experiments half a platelet was used of the TiO2/Pt catalyst which was sputtered for 12 seconds, as well as of the TiO2/Pt/silicalite-1 composite obtained from the same TiO2/Pt sample. After use the catalysts were analyzed by FTIR on a Bruker IFS-66 equipped with an A590 microscope.
3. RESULTS AND DISCUSSION
3.1. Metal oxide composite preparation and characterization From XPS analysis it is concluded that Fe203 and Cr203 was formed from the precursor materials on the surface of the silicon wafers. SEM measurements of the metal oxide layers indicate particle/cluster sizes of 50-500 nm for the Fe203 samples (see Fig.l) and 10-60 nm for the Cr203 samples. From these measurements it is concluded that the particles are hemi-spherical. Calculations based on the XPS results [10] and ICP/AES results showed that the surface coverage is approximately 8-10% for the Fe203 catalyst and 1-3.5% for the Cr203 catalysts. From XPS analysis before and after calcination of the chromium oxide catalysts, it was concluded that only part of the chromium precursor material was fixed to the support. Figure 2 shows a typical silicalite-1 layer which is grown on a metal oxide coated Si-wafer at 150~ 3 hours. The zeolite coating consists of more than a monolayer of crystals. The orientation of the first layer of crystals is lateral, and thus the b-direction is perpendicular to the support surface. On top of this layer, some axially oriented crystals are present. These crystals are not expected to influence the catalytic properties of the composites. From the thickness of the axial crystals, the layer thickness of the laterally oriented layer is estimated at 300 nm for each side of the wafer. Studies of the supported silicalite-1 systems proved the chemical bonding of the crystals to the support, which is assumed to take place by condensation between hydroxyl groups on the support surface and the zeolite crystals. Moreover, in this case it is possible that the crystals are bonded with the hydroxyl groups of the metal oxide particles. Calcination did not change the appearance of the layer or cause cracks. The total support coverage was estimated at a minimum of 95 %, thus leaving a limited amount of pin-holes. No evidence was found that at these holes uncovered support and catalytic material are exposed, and it is probable that a thin layer of zeolitic or amorphous SiO 2 is present. Since the wafers were suspended vertically in the synthesis mixture, it is expected that the lower horizontal side of the sandwich may contain particles which are not covered by the zeolite layer. The presence of the metal oxide particles after zeolite synthesis was confirmed by ICP/AES. Some of the metal oxide dissolved under synthesis conditions. XPS measurements of the external surface of the silicalite-1 coatings, however, did not show the presence of iron or chromium, which indicated that the dissolved metal oxide is not present in the MFI lattice [11]. For the Fe20 3 composites, TEM combined with elemental analysis showed the presence of iron at the interface between zeolite layer and
380
~i~i~!i!! ~i~ii~i '~i i~!!i~i 84 i i~!i!!i !i~!!ii~i~! ~i N i l~ ~ i~ ~ ~i iiiiiiii~!i~!~ii!i~!~!Wii~iiii~ ~i!!
;i!i iiiiii!!iii~iii i!! !ii~ '~'~'' i!i~ii!i!i~iiii!ii!i!!ii~ iii ~iii!!~~..........
~.,~ .......
Figure 1: Iron oxide particles on a Si support obtained by spin-coating and calcination.
Figure 2: Typical silicalite-1 layer on a Si support containing metal oxide particles. The average layer thickness is 300 nm.
Figure 3a: Platinum clusters on rutile (001) obtained after 12 s of sputtering. Average cluster size is 5 nm.
Figure 3b: Platinum clusters on rutile (001) obtained after 24 s of sputtering. Average cluster size is 20 nm.
381 support only. It is therefore concluded that the composites contain catalytic sites which are only accessible through the zeolite framework and are expected to show high reactant selectivity.
3.2. Platinum composite preparation and characterization Figures 3a and 3b show platinum species on the TiO 2 surface at different sputter times. The platinum cluster sizes increase from 2 to 20 nm, leaving parts of the support surface uncovered. At these parts the silicalite-1 layer is able to form chemical bonds with the surface OH-groups. From these pictures an estimation of the surface coverage by Pt of 85 % was made, from which the total amount of platinum sites was calculated. Zeolite coatings were grown on the TiO 2 supports which were sputtered during 6, 12 and 18 seconds. The silicalite-1 coatings have the same features as the coatings on the metal oxide particles, thus the bottom layer of crystals is almost continuous and has a lateral orientation. Since platinum does not have terminal functional groups, no chemical bonding or interaction with the zeolite layer is expected. XPS of the outer surface of the zeolite layer did not show any platinum.
3.3. Catalytic testing of the composites
3.3.1.Dehydrogenation of n-heptane over the metal oxide composites During dehydrogenation of n-heptane, heptenes, methylcyclohexane, toluene and various light components were formed. Figures 4 and 5 show the conversion of n-heptane as a function of time for the metal oxide particles and the metal oxides covered with silicalite-1. After a few cycles of heating to 500~ cooling to room temperature, part of the gold coating on the reactor wall had disappeared. This was accompanied by a sharp increase in the background conversion level, caused by the activity of the stainless steel reactor wall, responsible for a large amount of the cracking products and part of the dehydrogenation products. 4O
4O ~" 0
30
0
,,..4
20
~0
10
,~"
Fe203/siliealite-1
0
~O
30 20 10
f
Cr203/silicalite-1
0 0
10
20
30
40
50
time (min)
Figure 4: Conversion of n-heptane as a function of time for the Fe203 catalysts.
0
10
20
30
40
50
time (rain)
Figure 5: Conversion of n-heptane as a function of time for the Cr203 catalysts.
Table 1 gives the conversion levels, turnover numbers and product distribution of the catalysts at 30 minutes and at a conversion of 14.2%. The data have been corrected
382 for the background effects. It is shown that by coating of the catalyst sites, the conversion level decreased significantly. After 30 minutes and at equal conversions the composites gave a different product distribution than the metal oxide particles. For the composites the selectivity towards toluene had decreased, which was accompanied by an increased selectivity for the formation of methylcyclohexane. This selectivity is still not understood, and currently a matter of study.
Table 1. Dehydrogenation methylcylcohexane.
products of n-heptane
at 500~
after 30 min Catalyst
Fe20 3 Cr20 3 Fe203/MFI Cr203/MFI
TON (mole/mole/h-1) 51" 530** 20* 310"*
C7-" heptenes,
MCH:
at 14.2 % conversion
C7 (%)
MCH (%)
toluene (%)
C7 (%)
MCH (%)
toluene (%)
7.7 -
28.8 38.5 100.0 79.9
63.5 61.5 20.1
11.9 -
27.1 38.5 78.8 88.8
61.0 61.5 21.2 11.2
* calculations based on a particle size of 50 nm; ** calculations based on a particle size of 60 nm. After use the samples were analyzed by XPS, in order to find out if deactivation of the catalysts had taken place. It appeared that part of the chromium(III) species was reduced during use. This change towards Cr(II) or Cr(0) may enhance the activity of the catalyst [12]. No change in the oxidation state of the iron catalysts was observed.
3.3.2.Hydrogenation reactions by platinum composites The results of competitive hydrogenation of 1-heptene and 3,3-dimethyl-l-butene using the TiO2/platinum and TiO2/platinum/silicalite-1 catalysts are presented in Figures 7 and 8. Here the activity of the reactor wall was at a negligible level. Using the TiO2/Pt catalyst, both molecules were hydrogenated in competition at approximately the same rate. After coverage of platinum by silicalite-1, however, a large difference in conversion rates was observed. The same trends were observed for the hydrogenation of the single components. The results of the competitive hydrogenations are summarized in Table 2. The selectivities were calculated as the ratio of the initial reaction rates. From the conversion data of 1-heptene in a single component experiment, it appears that in the competition experiments the conversion of 1-heptene is not slowed down by the presence of 3,3-dimethyl-l-butene in the zeolite layer. Based on the kinetic diameter as well as the diffusivity of 3,3-dimethyl-l-butene, the adsorption of this molecule in MFI is low [13]. It is thus expected, in particular at this low partial pressure, that within the duration of these experiments no retardation of 1-heptene by 3,3-dimethyl1-butene is occurring. After use of the composite catalyst in single component experiments, it was observed by FTIR at 100~ that the zeolite layer was almost completely filled with 1-heptene and n-heptane, whereas only very small amounts of the branched molecules were found in the zeolite layer. It is therefore suggested that the conversion of the branched alkene takes place at pin-holes and not via the zeolite pores.
383 From Table 2 it is also apparent that the calcination procedure of the composites is an important variable. At high calcination temperatures, the activity increases and the selectivity decreases significantly, which is probably caused by cracks in the zeolite coating. Using mild calcination conditions, the selectivity remains high. 12
100
75 o
.,..~
~ 0
6
50 25
O o
0
1
2
3
3
0
time (h)
1
2
3
time (h)
Figure 8: Competitive hydrogenation of 1-heptene (o) and 3,3-dim ethyl-l-butene (A) over TiO2/Pt/MFI composite at 100~
Figure 7: Competitive hydrogenation of 1-heptene (o) and 3, 3-dimethyl-l-butene (A) over TiO2/Pt catalyst at 100~
Table 2. Competitive hydrogenation of 1-heptene and 3,3-dimethyl-l-butene at 100~ Catalyst
time ~ 1-heptene (min) (%)
~3,3-DMB-1 TON (%) (mol/mol/h -1)
TiO2/Pt TiO2/Pt/MFI(1)* TiO2/Pt/MFI(2)** TiO2/Pt/MFi(2) t TiO2/Pt/MFi(2) t
30 180 180 180 180
48.8 6.4 0.59 -1.10
47.2 49.4 10.0 10.7 --
* After 3 times of calcination at 500~ single component hydrogenation.
487 45 8.6 8.6 1.0
Selectivity
1.0 9-10 22-25 ---
6 hours;** after 1 time of calcination at 350~
12 hours; t
Separation of n-hexane and 2,2-dimethylbutane over a silicalite-1 membrane at 50~ resulted in a separation factor of 17.2 [14]. It was suggested that non-zeolitic micropores probably played a role, so that the separation factor in principle could be higher than the observed one. Competitive hydrogenation of linear and branched olefins over Pt/ZSM-5 catalysts was carried out for mixtures of 1-hexene/2,4,4-trimethylpentene-1 and 1-hexene/ 4,4-dimethylhexene-1 [15,16].In both cases a high selectivity for the conversion of the linear molecules was observed. In the first case a conversion ratio of approximately 12 was reported for reaction at 100~ the second case the selectivity was equal to 25 during conversion at 100~ composite catalysts thus show comparable selectivity. A difference in selectivity between a regular catalyst and the composite catalysts is expected on the basis of the particular catalyst configuration. Assuming comparable intrinsic reaction rates and sorption equilibria, the theoretical selectivity of a regular
384 catalyst containing homogeneously dispersed catalytic sites is equal to the square root of the ratio between the effective diffusivities [17]. In case of a shape selective diffusion barrier covering the catalytic site, as in the case of the composites, the theoretical selectivity is equal to the ratio of the effective diffusivities. Here the intra-zeolitic diffusivities at 100~ estimated at 10 -12 m2/s and 10 -18 m2/s for 1-heptene and 3,3dimethyl-l-butene, respectively. Thus if mutual interaction (retardation of 1-heptene by the branched alkene) would be absent, a theoretical selectivity of more than 104 might be possible. The relatively low selectivity can be due to pin-holes or sides of the sandwich, containing platinum directly exposed to the reactants or platinum covered with amorphous silica which only causes Knudsen diffusion effects. Additionally the presence of very small amounts of platinum on the external surface of the zeolite layer, which can not be detected by XPS may also be responsible. Adjustment of the zeolite synthesis procedure, resulting in less pin-holes, shorter synthesis times and a thinner zeolite layer are expected to further improve selectivity in catalysis.
4. C O N C L U S I O N S A novel type of catalyst containing supported oriented coatings of silicalite-1 has been achieved. With this concept, many types of catalytic material can be combined with the shape selectivity of a zeolite framework. This is demonstrated by the preparation of two types of MFI-covered supported catalysts. The first true monofunctional platinum/MFI catalyst is obtained from direct synthesis. The shape selectivity of the rutile-platinum-MFI composite was tested by competitive hydrogenation of a linear and a branched olefin. The composites give a high selectivity towards conversion of the linear olefin, which is comparable to or even higher than optimized regular zeolite catalysts.
Acknowledgements The authors thank Mr. Th. de Mooij from Jeol Europe and Dr. H. Otsuji from Jeol USA for the HRSEM measurements, and the Royal Dutch/Shell Laboratory, in particular Dr. Z. Chen and Dr. E.W. Kuipers for their advice in the development of the reactor and the spin-coating procedure. REFERENCES [1] J.C. Jansen, D. Kashchiev and A. Erdem-Senatalar, Stud. Surf. Sci. Catal., 85, (1994), 215-250. [2] E.R. Geus, H. van Bekkum, W.J.W. Bakker and J.A. Moulijn, Microporous Mat., 1, (1993), 131-147. [3] J.C. Jansen, W. Nugroho and H. van Bekkum, Proc. 9th Int. Zeolite Conf,, I, (1992), 247-254. [4] J.H. Koegler, H.W. Zandbergen, J.L.N.Harteveld, M.S. Nieuwenhuizen, J.C. Jansen and H. van Bekkum, Stud. Surf. Sci. Catal., 84, (1994), 307-314. [5] H.P. Calis, A.W. Gerritsen, C.M. van den Bleek, C. Legein, J.C. Jansen and H. van Bekkum, Can. J. Chem. Eng., 73(1), (1995), 120-128. [6] K. Tanabe, M. Misono, Y. Ono and H. Hattori, Stud. Surf. Sci. Catal., 51, (1989), 108-113. [7] M. Niwa, N. Katada and Y. Murakami, J. Catal., 134, (1992), 340-348. [8] E.W. Kuipers, C. Laszlo and W. Wieldraaijer, Cat. Lett., 17, (1993), 71-79. [9] S.P. Murarka, Silicides for VLSI Applications, Academic Press, (1983). [10] H.P.C.E. Kuipers, H.C.E. van Leuven and W.M. Visser, Surf. Interf. Analysis, 8, (1986), 235-242. [11] N. van der Puil, J.C. Jansen and H. van Bekkum, Stud. Surf. Sci. Catal., 84, (1994), 211-218. [12] T. Komatsu, J. Mol. Catal., 78, (1993), 57-66. [13] M.F.M.Post, J. van Amstel and H. van Kouwenhoven, Proc. 6th Int. Zeolite Conf., (1984), 517-527. [14] J.G. Tsikoyiannis and W.O. Haag, Zeolites, 12, (1992), 126-130. [15] J. Weitkamp, T. Kromminga and S. Ernst, Chem.-Ing.-Tech. 64 (12), (1992), 1112-1114. [16] R.M. Dessau, J. Catal., 89, (1984), 520-526. [17] W.O. Haag, R.M. Lago and P.B. Weisz, Disc. Faraday Soc., 72, (1981), 317-330.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
385
O L I G O M E R I Z A T I O N OF B U T E N E S W I T H P A R T I A L L Y A L K A L I N E EARTH EXCHANGED NiNaY ZEOLITES B. Nkosi, F. T. T. Ng, and G. L. Rempel Department of Chemical Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1 Summary A number of catalysts were prepared by partially exchanging NaY with alkalineearth cations followed by nickel promotion. These catalysts were used to oligomerize butenes in the liquid phase. For the catalysts with similar activities, the dimer selectivity was found to increase with increase in the cationic size of the alkaline-earth metals, but decreased with increase in the Sanderson's electronegativity function. This is attributed to the increased adsorption of olefins on the more acidic catalysts. Catalysts that showed good dimer selectivity were found to have better stability than those that had poor dimer selectivity, indicating that the deactivation was caused by fouling of pores by long chain oligomers. This improvement in catalyst stability imparted by the alkali-earth cations is probably due to the blocking of the sodalite cages of the NiNaY catalysts. The catalyst derived from partially exchanging sodium with barium cations was found to have high activity, good C8 selectivity and low deactivation rate. 1. Introduction
Fluid catalytic cracking of naphtha leads to the production of low molecular weight hydrocarbons. Low molecular weight alkenes in the C2-Ca range are a major product of this fluid catalytic cracking process. Oligomerization is used to convert these alkenes back to higher boiling liquid products. Butenes are oligomerized to octenes and higher alkenes for use in a variety of products. The octenes are useful for the synthesis of alcohols as well as for the blending of gasoline. For gasoline blending applications it is important that only dimers are formed from the oligomerization of butenes as higher molecular weight products have poor blending properties. These higher molecular weight products have also been reported to cause serious deactivation of NiNaY oligomerization catalysts(I). Other workers have also observed that the nickel exchanged zeolites undergo serious deactivation(2,3). In this study, an attempt has been made to improve the stability of these nickel exchanged catalysts by slowing down the deactivation of these oligomerization catalysts. The strategy used to address the deactivation phenomena was to partially exchange the sodium ions with alkali or alkali-earth metal ions. In this paper, only the results from the work on the partial exchange with alkaline-earth ions will be reported; the work on partial exchanging sodium ions with alkali ions will be reported elsewhere(4).
386
2. Experimental Unless otherwise stated, all catalysts were made by ion exchange procedures. All chemicals were purchased from Aldrich Chemicals and used without any further purifications. Linde Y (LZ-Y52, from Union Carbide) zeolite was ion exchanged, once, with 0.5M aqueous solutions of the appropriate alkali-earth chloride salts (e.g. MgCI 2, CaCI 2, SrCI2, BaCI2) at 80~ for 24 hours and then washed thoroughly with deionized water to get rid of residual ions. This was followed by a second exchange with 0.2M aqueous solutions of nickel chloride, at room temperature, for 16 hours. In the case of NiHY catalyst, LZ-Y82(from Union Carbide) was ion exchanged with a 0.2M aqueous solution of nickel chloride at 80~ for 24 hours. The loaded catalysts were subsequently washed with deionized water to get rid of occluded nickel salt. They were then dried, first in air, under ambient conditions followed by drying in the oven at 110~ for over 24 hours. Calcination was performed at 500~ under dry air for 16 hours. Fresh and spent catalysts were characterized for surface area by N2 adsorption using the BET method. An Autosorb-1 instrument from Quantachrome Corporation was used for the BET measurements. Nickel loadings were determined by X-ray fluorescence (XRF) using an Oxford Lab-X 1000 instrument. Sodium analysis was determined by atomic absorption analysis (AAS) using a Perkin Elmer 3100 instrument. Carbon content of spent catalysts was analyzed by the thermal gravimetric analysis/differential thermal analysis method (TGA/DTA) using an SDT 2960 from TA Instruments, Inc. Approximately 50 mg of spent catalyst samples were heated in a 5% 02 in helium flow up to 650~ at 20~ The compositions of the catalysts are shown in Table 1. Table 1: Composition of Alkaline-earth Exchanged NiNaY Catalysts Catalyst
Nickel Loading (wt%)
Sodium Content (wt%)
NiHY" NiNaY b NiNaY NiNaMgY b NiNaMgY NiNaCaY b NiNaCaY NiNaSrY NiNaBaY
3.97 0.76 4.85 0.90 4.67 0.86 2.72 3.09 0.87
< 0.14 7.19 2.23 5.62 1.37 3.27 1.73 1.72 1.25
catalyst prepared by ion exchange of nickel salt at 80~ for 36 hours. b prepared from a 0.02M of nickel chloride solution.
a
1-Butene oligomerization was performed in the liquid phase using a 300ml Parr autoclave. The autoclave was equipped with a cooling coil to control temperature
387 excursions such that temperature control was within 1~ of the desired set point. All the oligomerization reactions were performed in a batch reactor under the following standard conditions: Reactor temperature = 110~ pressure = 600psi; run time = 2 hours; isopentane : 1-butene wt ratio = 1:2.67. In a typical reaction, a liquid charge containing approximately 70 grams of 1-butene and 25 grams of isopentane is contacted with 3 grams of catalyst for 2 hours at 110~ and 600psi pressure. At the end of 2 hours the reaction products are cooled to 6~ The amount of products and unreacted butenes are weighed and subsequently analyzed by gas chromatography (GC). A 60 m DB-1 column was used to separate 1-butene and the reaction products. The oligomerization activity is determined from the GC results and the weight of recovered autoclave contents. The C8 selectivity is determined from the weight of all C8 isomers to the weight of all oligomers produced ( C4 isomers are not taken into account in ranking catalysts for activity and selectivity). 3. Results and Discussion
Product distributions for the oligomerization of butenes from a few selected catalysts are shown in Table 2. The results show that the C8 dimers produced are mainly branched isomers with negligible amounts of straight chain octenes present. These branched dimers would be suitable for gasoline blending purposes. Isomerization of 1butene to 2-butenes and trace amounts of isobutylene were observed. Close examination of the results reveal that the ratios of trans-2-butene:cis-2-butene(Tr-C4~: Cis-C4- ) decreases with increase in the size of the alkaline-earth cation( Trans:cis C4- ratio for Mg=l.95, Ca=l.90, Sr=1.82, and Ba=l.79). Trans -2-butene formation has been reported to be favoured over acidic catalysts compared with cis-2-butene (5). This trend indicates that the acidity of the catalysts is decreasing with cationic radius from Mg to Ba. Table 2: Typical Product Distribution obtained from Butene Oligomerization Reaction Using Selected NiY Catalysts under Standard Reaction Conditions. Catalyst
NiNaMgY NiNaCaY NiNaSrY NiNaBaY
C4
isomers (wt%)
C8 isomers (wt%)
>_C~
l-C4
Tr-C4
Cis-C~
DM-C6
M-C7
n-C~
_>C~
2.26 3.86 3.37 4.13
30.49 34.36 33.53 30.83
15.62 18.10 18.41 17.28
19.64 18.09 19.26 19.98
14.41 12.00 14.96 17.96
0.44 0.80 0.75 0.68
16.81 12.30 8.86 8.99
A summary of all the oligomerization data for a number of catalysts used in this study are shown in Table 3. The results show that generally for catalysts that have a nickel loading of less than 1% w/w, with the exception of the NiNaBaY catalyst, have lower activity than the catalysts with high nickel loading. This is attributed to the fact that when NaY is ion exchanged with Ni 2+ solutions, Ni 2+ ions preferentially exchange
388 with Na + ions in the sodalite and hexagonal prisms until about 36% of exchange capacity is reached (6). After all the exchange sites in these positions are exhausted, the Ni 2+ions begin to exchange for Na ~ cations in the supergages. It has been proposed that for Ni 2+ to be active for the oligomerization reaction, the cations have to be located in the supercages(6). It is probable that the low nickel loading for the zeolite exchanged with Ba 2~ could be due to the large size of the Ba 2~ cations which effectively block the hexagonal prism in the faujasite structure, confining the Ni 2+ cations to the supercages. Table 3: Oligomerization of Butenes Using Various NiY Exchanged with Alkali-earth Metal Chlorides a. Catalyst c
3.97NiHY 0.76NiNaY 4.85NiNaY 0.90NiNaMgY 4.67NiNaMgY 0.86NiNaC aY 2.72NiNaCaY 3.09NiNaSrY 0.87NiNaBaY
Activity (g /g. cat. /h)
C8 Selectivity b (%)
Coke Content (%)
Deactivation (%loss BET Area)
5.16 2.13 5.91 1.52 4.99 2.49 4.77 4.81 4.44
57.81 89.36 69.51 79.99 67.21 70.72 70.41 73.40 81.58
15.13 d 11.42 10.52 13.83 12.06 12.61 10.96 9.71
92.42 34.10 70.72 55.26 64.14 60.79 63.69 55.39 37.54
" Standard reaction conditions used. b as a % of all oligomers produced c The numbers designate the nickel % wt loading. d not measured Table 3 shows that for catalysts prepared from similar cationic types, but different nickel loadings, the dimer selectivity decreases with increase in activity. The catalysts prepared from NiNaCaY are exceptions to this generalization, it is not clear at this stage why the low nickel calcium catalyst has such a low selectivity. This decrease in selectivity with increase in activity is consistent with a consecutive reaction pathway, where dimers are the first intermediate products. Interestingly, the NiNaBaY catalyst has a good C8 selectivity even though its activity is reasonably high. Close examination of the data given in Table 3 for the catalysts prepared from the partially exchanged alkaline-earth metals at about similar activity (>4.44 g. olig./g.cat./h) reveal some interesting trends. The influence of cationic size on the dimer selectivity for catalysts with similar activities is given in Figure 1. The dimer selectivity of the catalysts increases with the cationic radius of the alkaline-earth metals, this is probably due to partial blocking of the sodalite cages and/or the hexagonal prisms by some of
389 of these bulky alkaline earth cations, preventing the formation of long chain oligomers. In fact, some of these cations have also been reported to have preference for occupying sodalite cages in these faujasite zeolites(7). The acidity of zeolites have been shown to be related to physicochemical properties, such as electrostatic field, electrostatic potential as well as electronegativity of the cations in the exchange sites (8,9). A plot of the dimer selectivity as a function of the Sanderson's electronegativity function of the alkali-earth cations in the catalyst is shown in Figure 2. These results show that the dimer selectivity is inversely related to the electronegativity of the alkali-earth metals, indicating that the more acidic the catalyst the less the dimer selectivity. To further probe the effect of acidity on dimer selectivity, a highly acidic NiHY catalyst was prepared. This catalyst was tested for the butene oligomerization reaction under standard conditions. Table 3 shows that this catalyst has the lowest selectivity of all the catalysts tested, thus confirming that high acidity leads to low C8 selectivity. This is ascribed to the enhanced olefin adsorption with increase in acidity and hence favour the formation of higher molecular products.
90
~ 85
B~
Ba
4--' >" .
_> ao +.~ ~
.E
65
C3
C3 60
0.60
BO
sr
L) (D 75 (I) CO 70 (1)
75
70 MQ ---~
_
o.ao
t 1.00
l 1.20
65 60
1.40
Cationic Radius(A)
Fig. I The Influence of Alkaline-earth Cationic size on Dimer Selectivity
0.60
I OZO
I i.O0
I 1~_0
i 1.40
1.60
Sanderson's e l e c t r o n e g a t i v i t y
Fig. 2 The Influence of the Sanderson's Electronegativity on Dimer Selectivity
Table 3 shows the deactivation of catalyts when used for the butene oligomerization reaction. Deactivation is defined as: Surface Area of Fresh Catalyst - Surface Area of Spent Catalyst x 100 Surface Area of Fresh Catalyst The results for surface area analysis of various catalysts before and after the oligomerization reaction are given in Table 4. These results show that all the catalysts are undergoing deactivation during the oligomerization reaction. It is noteworthy that the deactivation is very high for the highly acidic NiHY catalyst. The results in Table 4 show that the area inside micropores of spent catalysts is reduced quite dramatically, whilst that inside the mesopore does not change significantly indicating that the catalysts undergo deactivation by blocking of the micropores. Fig. 3 is a plot of the carbon content of spent catalysts as a function of
390 the Sanderson's electronegativity for catalysts with similar activity(_>4.44 g olig./g cat./h). Since the electronegativity function is related to the acidity of the zeolites (8,9), it appears, therefore, that the deactivation of these catalysts increases with increase in acidity. It has been generally accepted that high acidity leads to enhanced catalyst deactivation.
[Z o .Q
13
63 O
12
c-
11
jJ
J
~~ o~
g B
0.60
0.80
1.00
Sanderson's
1.20
1.40
1.60
electronegativity
Fig. 3 The Deactivation as a function of the Sanderson's Electronegativity.
One of the questions that need to be addressed is the cause and the nature of the carbon found in the spent catalysts. To address this question, a spent catalyst from NiNaCaY, which was previously used for 2 hours in a butene oligomerization reaction under standard reaction conditions, was subjected to a soxhlet extraction using hexane as solvent. This soxhlet extraction was performed over a 36 hour time duration. The Table 4 BET Surface Areas of Fresh and Spent Catalysts a BET Surface Area (m2/g)
Catalyst
Fresh Catalyst
3.97NiHY 4.85NiNaY 4.67NiNaMgY 0.86NiNaCaY 2.72NiNaC aY 3.09NiNaSrY 0.9NiNaBaY
Spent Catalyst
Total Area
Mesopore
Total Area
Mesopore
520.9 653.0 692.9 693.2 747.8 641.4 566.2
123.8 166.3 176.8 138.8 163.5 145.4 126.0
39.5 251.5 249.8 271.8 271.5 287.3 361.9
39.5 116.0 125.9 128.3 107.1 126.1 117.2
all samples were evacuated at 110~ for at least 5 hours; fresh catalysts were calcined at 500~ for 16 hours; spent catalysts were used in a batch reaction for 2 hrs under standard conditions; area is from a 5 point BET at P/Po<0. a
391
extract was analyzed by GC and found to consist of butene oligomers, C12 = and C16 =. Interestingly, when the soxhlet extracted catalyst was analyzed for carbon content using TGA, about 4.6% of residual carbon was found. These results suggest that fouling is caused by blocking of pores by long chain butene oligomers, since under our reaction conditions a graphitic type of carbon is not expected to form on the catalyst. The residual carbon species unable to be extracted by the soxhlet extraction procedure is probably oligomers longer than C~6. Fig. 4 shows a plot of the carbon content of the spent catalysts as a function of dimer selectivity. An inverse relationship is observed between the carbon content and the dimer selectivity. These results indicate that long chain oligomerization products may be responsible for the fouling of the catalysts. It is interesting to note that other workers also concluded from their results that C~6 and higher oligomers were responsible for catalyst deactivation (2, 10).
14 cO 33
13
El
12
f-
11
(9
M%a
13 33 El 0
12
~0) ~o E3ao
9
%
B
70
75
IDimer S e l e c t i v i t y
BO
(%)
Fig 4 The relationship between Dimer Selectivity and Coke Content
85
!
O.150
0.70
i
i
t
O J B O O.CJlO 1.00
Cationic
i
1,
|
1. i 0
1.2.0
1.30
1.40
Radius(A)
Fig 5 The Influence of Cationic Radius on Coke Content
Our results confirm that the acidity of the catalysts is one of the causes for the deactivation in this butene oligomerization reaction. A question that emerges from this is: Is the deactivation of the catalysts also influenced by factors other than acidity? One of the most important phenomena in zeolite catalysis is diffusion. The diffusivity of molecules in zeolites is influenced by physicochemical properties of the cations in the exchange sites, such as, electrostatic field, electrostatic potential, as well as the size of the pores. It should be mentioned that the influence of the electrostatic field on the diffusivity of alkenes in zeolites should not be underestimated, since the alkene double bond is rich in electrons and is, therefore, liable to be polarized. It should be noted that all these physicochemical properties are dependent on the size of the cations in the exchange sites. Ward (11) has reported that the electrostatic field and potential increases with decrease in the size of the cation for faujasite type zeolites exchanged with a variety of cations from alkali and alkaline-earth metals. To probe the influence of cationic size on the deactivation, cationic sizes of catalysts exchanged with different alkaline-earth
392 metals and having similar activities (_~ 4.44g/g/hr), were correlated with carbon content as shown in Fig. 5. These results show that the deactivation of the catalysts is inversely proportional to the cationic size of the alkaline-earth cation and hence indicate that diffusivity may be important in affecting the stability of these catalysts.
4. Conclusions.
Our results indicate that physicochemical properties of alkaline-earth cations in the exchange sites, such as, cationic radius and electronegativity can be used to predict catalytic properties of nickel promoted zeolites for the oligomerization reaction. An increase in electronegativity, i.e. acidity, increases the catalyst deactivation and a decrease in dimer formation. This is attributed to the increased adsorption of olefins on the more acidic catalysts. An increase in the alkaline-earth cationic radius leads to an increase in the dimer selectivity and a decrease in catalyst deactivation. We attribute this to the blocking of hexagonal prisms and/or sodalite cages by these alkaline-earth cations, and the influence of the physicochemical properties of the alkali-earth cations on the diffusivity of the alkenes. The catalyst prepared from partially exchanging sodium with barium showed the best properties, these properties included; high activity, good C8 selectivity and low deactivation rate. 5. Acknowledgements.
We would like to acknowledge financial support from NSERC Strategic Grants Research Program. References.
1. G. Podrebarac, M.A.Sc. Thesis, University of Waterloo, Waterloo, Ontario, Canada,(1992). 2. D. Kiessling, K. Hagenan, G. Wen&, A. Barth, and R. Schoellner, React. Kinet. Catal. Lett., 39, 89-93, (1989). 3. L. Fomi, R. Invemizzi, and L. Van Mao, La Chimica E L'Industria, 57, 577, (1975). 4. B. Nkosi, F. T. T. Ng, and G. L. Rempel, to be submitted for publication. 5. P. Berteau, S. Ceckiewicz and B. Delmon, Applied Catal., 31 (1987) 361. 6. A. K. Ghosh and L. Kevan, J. Phys. Chem., 94 (1990) 3117 7. W. M. H. Sachtler, Catalysis Today, 15, 419-429, (1992). 8. J.W. Ward, in "Zeolite Chemistry and Catalysis", J. A. Rabo (Ed.), ACS Monogr. 171 (1976) 118. 9. W. J. Mortier, J. Catal., 55 (1978) 138 10. J. Datka, J. Chem. Soc., Faraday Transactions I, 77 (1981) 2633 11 .J.W. Ward, J. Catal., 10 (1968) 34
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
393
Isomerization of C8 Aromatic Cut. Improvement of the selectivity of MORand MFI-Catalysts by Treatment with Aqueous Solutions of (NH4)2SiF6 E. Benazzil, J.M. Silva2, M.F. Ribeiro2, F.R. Ribeiro2 and M. Guisnet3 l lnstitut Frangais du P~trole, 1-4 av. de Bois Pr~au, BP 311, 92506 RueilMalmaison, France 21nstituto Superior TOcnico, 1. av. Rovisco Pais, 1096 Lisboa, Portugal 3Universit~ de Poitiers, 40 av. du recteur Pineau, 86022 Poitiers, France
SUMMARY
The treatment of MOR and MFI zeolites with ammonium hexafluorosilicate solutions leads to a preferential dealumination of the crystallite outer surface. The treatment has fully different effects on the activity and selectivity of bifunctional catalysts constituted of Pt/AI203 and MOR or H-MFI zeolites for the transformation of an o-xylene-ethylbenzene mixture at 410~ under hydrogen pressure. The activity of treated H-MOR catalyst is lower than that of non-treated catalysts but their isomerization selectivity is increased owing to an inhibition of disproportionation and transalkylation reactions. The decrease in activity can be explained by a partial blockage of the channels which could be caused by a silica deposit at the entrance of mordenite pores during the treatment. The inhibition of disproportionation and transalkylation is due to the decrease in acidity of the outer surface caused by its selective dealumination. On the other hand, the treatment of H-MFI with ammonium hexafluorosilicate causes a significant increase in the rates of ethylbenzene and o-xylene transformation which can be attributed to a decrease in the rate of coke formation on the outer surface of the crystallites. Furthermore, there is a decrease in the selectivity of xylenes disproportionation which involves bulky diphenylmethane intermediates but no change in the selectivity of ethylbenzene disproportionation which occurs on H-MFI through a dealkylation-alkylation mechanism. 1. INTRODUCTION
In the last few years numerous studies have been devoted to the improvement of the selectivity in the isomerization of C8 aromatic cut. Several techniques allowing the reduction of undesirable side-reactions over MOR-based catalysts and to improve p-xylene selectivity of MFI-catalysts have been reported [1,2]. We show here, that the selectivity for isomerization of MOR and MFI-catalysts can be improved by treating them with aqueous ammonium hexafluorosilicate (N H4)2SiF6 solutions.
394
2. EXPERIMENTAL
The mordenite sample, with an atomic Si/AI ratio of 5, was supplied by TOSOH Corporation in the sodium form. Three exchanges with ammonium nitrate (10M) were necessary to prepare the ammonium form, sample H-MOR/5-1. Th:e MFI sample, with an atomic Si/AI ratio of 28 was supplied by Conteka. The following procedure was used for treating the mordenite (H-MOR/5-1) and the MFI zeolite (HMFI/28-1) with (NH4)2SiF6 solutions. One gram of each sample was pretreated at 80~ in 25 ml of 0.8M aqueous solution of ammonium acetate (pH=6.7). Then, addition, under stirring and at a flow rate of 12ml/h, of an aqueous solution of (NH4)2SiF6 was performed. The concentrations of the fluorosilicate solutions were such as the number of silicon atoms was equal to 50%, 100%, (samples H-MOR/52, H-MOR/5-3) and 200% and 400% (samples H-MFI/28-2 and H-MFI/28-3) of the number of aluminum atoms present in the zeolite sample. Stirring was maintained for 3 hours at 80~ After treatment under N2 flow for 4h at 550~ the zeolites were characterized by X-Ray Fluorescence (XRF), X-Ray Diffraction (XRD) and X-ray Photoemission Spectroscopy (XPS). The catalysts were prepared by mechanical mixing of H-MOR or H-MFI samples with Pt/AI203 (0.3 wt% Pt), the MFI content was equal to 60 wt% and the MOR content to 20 wt%. The transformations of pure ethylbenzene and of the 80 wt% o-xylene + 20 wt% ethylbenzene mixture were carried out in a fixed bed reactor at 410~ total pressure 1.2 MPa with a H2/hydrocarbon molar ratio of 4. 3. RESULTS AND DISCUSSION 3.1. Characterization of the modified zeolites Treatment of the MOR and MFI type zeolites with aqueous ((NH4)2SiF6) solutions leads only to a slight increase of the total Si/AI atomic ratio, measured by XRF. On the other hand, the results show an increase of the Si/AI ratio of the outer surface of the zeolite crystals, determined by XPS (Table 1). The greater the concentration of the aqueous (NH4)2SiF6 solution the higher the Si/AI ratio of the outer surface of the zeolite crystals.
Table 1 Physicochemical characteristics of the parent and treated samples Samples Si/AIXRF Si/AIIR Si/AIxPS Cristallinity Toluene adsorption XRD (%) wt. % H-MOR/5-1 5.4 7.0 7.2 90 8.3 H-MOR/5-2 5.4 7.1 16 89 5.9 H-MOR/5-3 5.5 7.2 22 83 4.1 H-MFI/28-1 28 -27 95 11.9 H-MFI/28-2 29 -40 96 12.0 H-MFI/28-3 31 -43 95 11.2 These results indicate that (NH4)2SiF6 dealuminates preferentially the outer surface of the MFI and MOR crystals. Nevertheless, the toluene adsorption capacity
395
measurements evidence a pore plugging in the case of the treated mordenite samples whereas for the H-MFI samples this phenomenon does not occur. 3.2 Transformation of the o-xylene-ethylbenzene mixture over H-MOR samples Over a bifunctional, Pt/AI203 + H-Mordenite, catalyst operating under hydrogen pressure the following reactions take place (Figure 1). (I)
d d
9~
90
(2)
+C2H4 (3)
24
2d
d O.c, d Figure 1 : Main reactions involved in the C8 aromatic cut isomerization The xylenes are isomerized (reaction 1) via a monomolecular reaction. This monomolecular reaction, which occurs mainly through methyl shift in benzenium ions intermediates [3], requires only an acidic function whereas the ethylbenzene isomerization into xylenes involves both an acidic and a metallic functions (reaction 2). The first step of this latter reaction consists in the hydrogenation of ethylbenzene on platinum sites into ethylcyclohexenes intermediates followed by a skeletal isomerization on the acid sites and finally a dehydrogenation of the produced dimethylcyclohexenes into xylenes isomers [4]. Simultaneously, C8 alkylcyclohexanes and cyclopentanes and C3-C5 alkanes are formed through hydrogenation, hydroisomerization or hydrocracking reactions. Other secondary reactions are the dealkylation of ethylbenzene (reaction 3), the disproportionation of xylenes (reaction 4), the disproportionation of ethylbenzene (reaction 5) and the transalkylation between xylenes and ethylbenzene, (reactions 6-7). All these reactions occur on acid sites and lead to a high xylenes loss. Now, we are going to examine the effects of the selective dealumination of the external surface of the mordenite crystals on these different reactions. The treatment with ammonium hexafluorosilicate aqueous solutions, and particularly with the more concentrated one (sample H-MOR/5-3), has a strong effect on the
396
activity of the catalysts. In this case, the approach to thermodynamic equilibrium (A.T.E) of o-xylene does not exceed 83 wt% with H-MOR5/-3 against 100% with HMOR/5-1 and H-MOR/5-2. (Figure 2). A similar effect is observed concerning the ethylbenzene A.T.E. This reduction of activity cannot be only attributed to the elimination of the external acid sites. More probably, the decrease of both activity and toluene adsorption capacity (Table 1) is due to a partial blockage of the access to the monodimensional channels of the mordenite which could be caused by a silica deposit occuring during the treatment with ammonium hexafluorosilicate solutions.
100-
6"
/
~5o H-MOR/5-1 n
I,,I.I
~, <
60"
ii
I/
l
~4"
/
j J
.,,,,
9 H4VIC~/5-2
I C
I
0
s s//
o
g2
9H-tV~,5,3
0.05
9 H-MCR/5-2
0 H4VICR/S-I
t
I
I
I
0.1
0.15
0.2
0.25
9
o
I
I
I
70 80 90 o-xylene A.T .E ( wt. %)
6o
1/pph (h)
I
100
Figure 3 9Change of the disproportionation Figure 2:Change of the o-xylene A.T.E (approach to thermodynamic equilibrium) as a function of products as a function of o-xylene approach to whsv-1 (weight hourly space velocity)-1, thermodynamic equilibrium (A.T.E)
On the other hand, Figure 3 evidences that treatment of H-MOR/5-1 with an (NH4)2SiF6 aqueous solution causes a significant decrease in the selectivity to transalkylation and disproportionation reactions hence an increase in the yield of isomerization reactions (Figure 4). The selectivities to dealkylation reactions and hydrocracking reactions are not influenced by the treatment and remain in all cases at low levels less than 0.7 wt. % for dealkylation reaction of ethylbenzene and less than 1.5 wt. % for the hydrocracking reactions at the highest values of o-xylene and ethylbenzene approaches to thermodynamic equilibrium. lOO
~,~
~
, ~=
O H-MOR/5-1 gO
;
9 H-MOR/5-2
~
~O
9 H-MC~/5-3
w
(J
85
'
6O
t
70
'
I
80
'
I
gO
'
I
1(30
o-xylene A.T .E (wl. %) Figure 4 Change 9 of the (C8 aromatics + naphthenes) yield as a function of the o-xylene A.T.E
397
The decrease in disproportionation selectivity, hence the increase in isomerization selectivity, can be related to the elimination of the non shape selective sites of the outer surface of the zeolites crystallites. It is indeed well known that disproportionation of xylenes occurs via bulky diphenylmethane intermediates [5,7], (Figure 5) which are not easily accommodated in the zeolite pores. The same mechanism is also responsible for ethylbenzene disproportionation on H-MOR and for the transalkylation reactions between xylenes and ethylbenzene (reactions 6 and 7). Only a small part of these reactions can occur through the dealkylation-alkylation mechanism (reaction 8) proposed for ethylbenzene disproportionation over H-MFI [8]. Indeed in the case of H-MOR the formation of ethylene through ethylbenzene dealkylation is very slow. Owing to the large size of the diphenyl-alkane intermediates involved in disproportionation and transalkylation reactions pronounced steric constraints exist at the vicinity of the inner acid sites. It is therefore likely that a large part of these reactions occur on the acid sites of the outer surface of the mordenite crystals. This is why the selective dealumination of the outer surface reduces significantly the rates of the disproportionation and transalkylation reactions (Figure 3). R'
@ R,.v~
~
R
R
!
f
R
R
k
Ox R R
R, R'= H, CH3 and R=R' or R~:R' Figure 5 Disproportionation 9 and transalkylation mechanisms of alkylaromatics
398
3.3 Transformation of the o-xylene-ethylbenzene over H-MFI samples Important differences exist in the reactions involved on H-MFI and H-MOR bifunctional catalysts. These differences concern mainly ethylbenzene reactions. In the case of H-MOR catalysts the transformation of ethylbenzene is relatively slow and occurs mainly through disproportionation, transalkylation (with xylenes) and isomerization while in the case of H-MFI catalysts ethylbenzene is rapidly dealkylated. Disproportionation and transethylation reactions are at least 10 times slower than dealkylation and moreover there is no isomerization of ethylbenzene into xylenes. Because of this rapid dealkylation of ethylbenzene, disproportionation and transethylation reactions occur probably through a dealkylation-realkylation mechanism. Differences in the effect of the treatment with (NH4)2SiF6 aqueous solutions on the catalytic properties of H-MFI and H-MOR are also observed. While with H-MOR catalysts the treatment causes a decrease in the rates of o-xylene and ethylbenzene transformation the reverse results are observed with H-MFI catalysts. Both the rates of ethylbenzene and of xylene transformation are significantly increased. The activity of H-MFI/28-3 sample for the ethylbenzene transformation is about 3 times greater than that of H-MFI/28-1 (Figure 6). No change in the selectivity is observed : whatever the sample, the dealkylation to disproportionation ratio is equal to about 12 at 60% of the ethylbenzene conversion. Likewise, the values of A.T.E of o-xylene are, at identical contact times, greater with the treated samples than that with nontreated samples (Figure 7). A decrease in the selectivity to disproportionation products is also observed. The same observations were made with another sample of H-MFI sample (Si/AI=45) [9].
/
~ Q] 0
50
II
[]
[] H-MFIf28-] 9 H-M:I/~-2
9 H-MF 1/28-2
u
10 lJ, O, 0
: 0.02
'
: 0.04
'
:
'
0.06 I/v4~v
; 0.08
(h)
'
; 0.1
'
; 0.12
0~t,
0
, , ,, , , , ,, , , , ,, , , , ,, - - - ~ - - - ,,
Q02
Q04
Q05
008
Ol
Q12
1/V~v (h)
Figure 6 :Changeof the ethylbenzeneconversion Figure7:Change of the o-xylene A.T.E (approach as a function of whsv-1 (weight hourly space to thermodynamic equilibrium) as a function of velocity)-1 whsv-1 (weight hourly space velocity)-1 The increase in the rates of o-xylene and of ethylbenzene transformation is due neither to an increase in the acidity of the fresh samples nor to a better accessibility to their acid sites. Indeed, no increase in acidity is observed by NH3 TPD and toluene adsorption experiments show that there is no change neither in the
399
adsorption capacity and in the diffusion coefficient. Moreover, the treatment of H-MFI has practically no effect on the rate of pure m-xylene transformation under normal pressure [10]. The increase in activity is therefore related either to the presence of ethylbenzene in the feed or to the addition of Pt/AI203 to H-MFI samples or to both. The latter proposal is the most likely. Indeed, at high reaction temperature ethylbenzene causes generally a rapid deactivation of zeolite samples owing to a rapid coke deposit [6]. This coke formation can be explained by the rapid formation of coke maker molecules such as ethylene through ethylbenzene dealkylation or such as styrene through ethylbenzene dehydrogenation in the case of bifunctional catalysts. It can be proposed that styrene molecules formed on Pt/AI203 would be rapidly transformed (before the first activity measurement)into coke on the acid sites of the outer surface of the zeolite crystallites. This coke would block the access of ethylbenzene and of o-xylene to the inner acid sites. In the treated samples, this transformation hence the pore blockage would be considerably limited owing to the dealumination of the outer surface of the crystallites.
:
8
[]
3-
E] I-fM=l/~l
7/'
~ 0
'~
2.5-
H-MF
1/28-1
9 H-MF
I/'28-2
9 H-MF
1/28-3
2-
e-
,~
1.5-
O
~.
e ~L
,m
,_
10.5-
O
4O
(:0
eO
o-xyle-eAT.E (~t. ~
leo
0
'
0
'
'
',
20
'
'
'
I
40
J
'
'
I
60
'
~
'
I
~
'
80
~
I
100
E thylbenzene convers ion (wt. %)
Figure 8 : Conversion of o-xylene into Figure9 : Ethylbenzenedisproportionation as a disproportionation products as a function of functionofethylbenzeneconversion o-xylene approach to thermodynamic equilibrium (A.T.E) This phenomenon probably occurs also with H-MOR samples but is masked by the significant blocking effect that the treatment with ammonium hexafluorosilicate has on the acces to the inner acid sites of this monodimensional zeolite. The decrease in the selectivity to xylenes disproportionation reaction caused by the treatment of H-MFI samples (Figure 8) can be explained, like the one observed with H-MOR samples, by the dealumination of the outer surface. Indeed, this reaction involves bulky intermediates (Figure 5) which cannot be easily accommodated in the H-MFI zeolite pores and which formation occurs preferentially on the acid sites present on the external surface of the MFI crystals. Consequently, a decrease of their number leads to a decrease of the xylenes disproportionation selectivity. The treatment with ammonium hexafluorosilicate has no effect on the ethylbenzene disproportionation selectivity as shown in Figure 9. This confirms that
400
this reaction occurs through a dealkylation-alkylation mechanism (and not through the mechanism described in Figure 5) which does not involve bulky intermediates, hence can occur in the narrow pores of H-MFI zeolites. Finally, the dealumination of the outer surface of the zeolite crystals seems to lead to a slight increase in the para-selectivity. Indeed, the 1,3,5-trimethylbenzene to trimethylbenzenes ratio is higher with the treated samples than with the non treated sample and a slight increase of the p-xylene to m-xylene ratio is also observed. 4. CONCLUSION
The treatment of H-MOR and H-MFI zeolites with ammonium hexafluorosilicate has fully different effects on the activity and selectivity of bifunctional catalysts constituted of Pt/AI203 and H-MOR or H-MFI zeolites for the transformation of a C8 aromatic cut (ethylbenzene + o-xylene) under hydrogen pressure. The activity of H-MOR catalysts is decreased owing to a partial blockage of the access to the large channels while that of H-MFI is increased owing to a decrease in the production of coke on the crystallite outer surface caused by the formation of styrene resulting from ethylbenzene dehydrogenation. For both types of catalysts the selectivity to xylene disproportionation (which occurs through bulky diphenylmethane intermediates)is decreased, hence the isomerization selectivity increased. With H-MOR catalysts the selectivity for ethylbenzene disproportionation is also decreased while no change is observed with H-MFI catalysts. This difference of behaviour can be explained by differences in disproportionation mechanisms : through bulky diphenylmethane intermediates whose formation is restricted by the zeolite pores with H-MOR catalysts, via dealkylation-alkylation steps with H-MFI catalysts. REFERENCES
[1] T. Hibino, N. Niwa, Y. Murakami, Journal of Catalysis, 128, p. 551, (1991). [2] E. Benazzi, S. De Tavernier, P. Beccat, C. N~dez, A. Choplin and J.M. Basset, CHEMTECH, p. 13, October 1994. [3] M. Guisnet and N.S. Gnep, in F. Ramoa Ribeiro, A.E. Rodrigues, L.D. Rollmann and C. Naccache (Editors) Zeolites: Science and Technology (NATO ASI Series, Vol. 80), Martinus Nijhoff Publishers, The Hague, p.571, (1984). [4] N.S. Gnep and M. Guisnet, Bull. Soc. Chim. France, n~ p.429 and 435 (1977). [5] D.H. Olson and W.O. Haag, ACS Symp. Ser., 248, p. 275, (1984) : [6] H.G. Karge, J. Ladebeck, Z. Sarbak and K. Hatada, Zeolites, 2, p. 94, (1982). [7] N.S. Gnep and M. Guisnet, Appl. Catal., 1, p. 329, (1981). [8] J.A. Amelse, in J.W. Ward (Editor), Catalysis 1987 (Studies in Surface Science and Catalysis, Vol. 38), Elsevier, Amsterdam, p.165, (1988). [9] J.M. Silva, unpublished results. [10] J.M. Silva, M.F. Ribeiro, F.R. Ribeiro, N.S. Gnep, M Guisnet and E. Benazzi, React. Kinet. Catal. Lett., 154, p. 209 (1995).
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviotand S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
401
Contribution of framework and extraftamework A1 and Fe cations in ZSM-5 to disproportionation and C3 alkylation of toluene J. Cejka, N. 7,ilkov~i, Z. Tvarfi~kov/l and B. Wichterlov~i J. Heyrovslc~ Institute of Physical Chemi~ry, Academy of Sciences of the Czech Republic, CZ- 182 23 Prague 8, Czech Republic
Toluene disproportionation and its C3 alkylation with isopropanol has been investigated over series of alumo- and ferrisilicates of ZSM-5 structure. It has been shown that the reaction rate for toluene disproportionation is linearly proportional to the number of both Si-OH-A1 and SiOH-Fe groups, the latter sites exhibiting a lower activity in agreement with their lower acid site strength. A simultaneous presence of Lewis and Broensted sites does not si~ificantly affect toluene disproportionation, but enhances toluene dealkylation leading to a higher selectivity to benzene. On the contrary, while the rate of toluene disproportionation is controlled by the molecular sieve intrinsic activity, for toluene Ca alkylation the reaction rate is controlled by the rate of the product desorption, which is assumed to be faster on ferrisilicates because of a lower acidity of the Si-OH-Fe groups. Therefore, all the ferrisilicates exhibit a higher toluene conversion in the alkylation reaction compared to alumosilicates.
1.0 INTRODUCTION Isomorphous substitution of trivalent metal ions for silicon, with ionic radius close to that of tetravalent silicon and with preference for tetrahedral coordination, has appeared to be a powerful way for tailoring the acidity (via both strength and number) of molecular sieves [ 13]. However, with introduction of larger metal ions like Ga, Fe, In [4] a higher structural instability leading to formation of extraframework metal ion species can be expected. A lower acid site strength of Si-OH-Fe compared to Si-OH-A1 groups is well documented by IR vibrations of these groups, lower temperature of ammonia desorption and lower activity in reactions like paraffins cracking, isomerization etc. [2,4]. Moreover, for variously isomorphously substituted molecular sieves, both the different acidity of bridging OH groups and presence of extraframework metal ions can affect the rate of individual acid-base reactions of hydrocarbons and thus the selectivity of the resulting processes. However, contribution of the Lewis centers (extraframework metal ions) to the molecular sieve activity is so far a matter of discussion [5]. A suggested enhancement of the acid strength of the Broensted sites neighbouring the Lewis site by an electron transfer from the OH group to this
402 electron acceptor site [6,7] is not unambiguously proven up to now. However, the increase in activity of molecular sieves in acidic like reactions owing to the simultaneous presence of Broensted and Lewis sites, which is different for various hydrocarbon transformations, is also assumed to be connected with a mutual effect of the Lewis and Broensted centers on the hydrocarbon molecule as suggested in Refs. 5, 8-10. The aim of this contribution is to show how the different Broensted acidity of alumo- and ferrisilicates of MFI structure affects in a substantially different way toluene disproportionation and its alkylation with isopropanol. Moreover, the effect of extraframework Fe species on the reaction of toluene disproportionation has been investigated.
2.0 EXPERIMENTAL H forms of alumosilicates of ZSM-5 structure (with Si/AI ranging from 13.5 to 600 and crystal size 1-3 ~tm) were supplied by the Research Institute for Oil and Hydrocarbon Gases, Slovak Republic. Ferrisilicates (Si/Fe = 23-45) of the same structural type were synthesized according to the following procedure: a solution of 45 g tetraethyl orthosilicates (Fluka) in 10 ml of ethanol was added slowly to the solution of the appropriate amount of Fe(NO3)3.9H20 (1.4-2.4 g) in 20 ml of distilled water. The mixture was stirred for 90 minutes at ambient temperature. In a separate container 81 g of tetrapropylammonium hydroxide (TPAOH) was mixed with 80 ml of distilled water and this solution was added dropwise under vigorous stilxing to the solution of tetraethylorthosilicate and ferric nitrate. During the addition of TPAOH a pale yellow-green gel was formed which was further redissolved to give a clear liquid of the same colour ( p H - 1 1 ) . This liquid was filled in Teflon-lined stainless steel autoclaves and crystallization was carried out for 3 days at 443 K under agitation. The resulting white solid was washed repeatedly with distilled water, filtered and dried at 373 K. The synthesized material had a distinct powder X-ray diffraction pattern and appeared to be a single-phase compound in scanning electron micrograph (Fig. 1). The crystallinity of all the ZSM-5 zeolites was also checked by the framework IR vibrations (FT-IR Nicolet Magna 550). The X-ray diffraction data, IR framework vibrations and sorption capacities for Ar (5.6-5.8 retool/g) indicated that well-crystalline samples had been obtained. IR vibrations of OH groups and sorption of ammonia were investigated by using the same instrument, where the self-supporting wafer of the zeolite was activated at 690 or 920 K under vacuum. Besides the band at 3745 c m -1 corresponding to the terminal Si-OH groups the band at 3610 cm -1 indicated presence of Si-OH-AI sites in alumosilicates. With Fe analogs the band at 3630 cm 1 reflected presence of bridging Si-OH-Fe centers and the absorption increase around 3660-70 cm -1 showed some OH groups coordinated probably to extralattice Fe; no intensity increase was detected at 3610 cm 1 evidencing an absence of the Si-OH-A1 groups in ferrisilicates. The number of strong acid sites in molecular sieves (see Table 1) was calculated from the high-temperature peak of the temperature-programmed desorption (20 K/min) of ammonia performed in a helium stream of 5.0 ml/min on zeolites pretreated at 690 K for 1 hour. The zeolite was equilibrated in an ammonia stream (10 % ammonia in nitrogen) at 373 K for 0.5 h and then in a helium stream at the same temperature for 0.5 h.
403
Table 1 Acid sites of molecular sieves Activation temperature (K)
OH (TPD) (mmol/g)
B/L*
A B C D
690 690 690 690 920
0.44 0.53 0.42 0.66 0.08
10 50 15 0.25
(AI)ZSM-5 A
690 920 690 690
1.05 0.58 0.7 0.3
185 13 200 180
Fe(ZSM)-5 Fe(ZSM)-5 Fe(ZSM)-5 Fe(ZSM)-5
(A1)ZSM-5 B (AI)ZSM-5 C
B/L - ratio of Broensted to Lewis acid sites according to ammonia adsorption
Prior to IR spectra measurements, ammonia was sorbed at ambient temperature at a pressure of 10 Torr followed by desorption for 30 minutes. The ratio of Broensted and Lewis sites was estimated from the area of the band at 1611 c m -1 (NH3 coordinated to bridging OH groups) and the band at 1425 cm 1 corresponding to NH3 coordinated to electron acceptor sites (see Table 1). X-band ESR spectra of (Fe)ZSM-5 were monitored on an ESR spectrometer (ERS 220, DAW) at 80 K with 100 kHz modulation frequency; Mn 2+ ions were used as a standard for determination of g values. Toluene alkylation with isopropanol was performed in a vapour phase continuous down-flow glass microreactor under atmospheric pressure, at temperature of 520 K and WHSV 5.0 h -1. Nitrogen, used as a carrier gas, was saturated with toluene at 335 K at a level of 18.5 vol. % and separately with isopropanol at 306 K to establish toluene/isopropanol molar ratio of 9.6. Toluene disproportionation was carried out at 770 K and WHSV 2.7 h 1 with the same toluene concentration. The reaction products were analyzed using an "on-line" high resolution gas chromatograph Hewlett Packard 5890 Series H with a Supelcowax 10 capillary column (30 m length, inner diameter 0.2 ram, phase thickness 0.2 Bm) combined with FI and MS (5971A) detectors.
3.0 RESULTS
and DISCUSSION
All ferrisilicates of ZSM-5 structure exhibitedsmall crystals of the size less than 1 gm (see Fig. 1). Introduction of Fe into the silicate framework sites was evidenced by the IR vibrations of Si-OH-Fe groups at 3630 cm-1 and Fe 3+ ESR signals with g at 4.3 and 2.0 (Fig. 2, cf. res 2). The intensity of both the ESR signals did not significantly change under NH4+ ion
404
g4.3 ;AH80
g2.0 ;&H150
t
Fig. 1 Scanning electron microscope photograph of synthesized (Fe)ZSM-5B.
2422 20 18 r
16
o
14
Fig. 2 ESR spectrum of H-(Fe)ZSM-5A at liquid nitrogen temperature.
25
A
0~0~0_~..___0"
20
--O
~15 cO
L..
> r
o
o
9 12 10
~'10 i0
o
8
A A
6 4
|~1~1.---~1-----1
2 0
'
s'0
'
160
'
l g0
Time-on-stream (rain) Fig 3 Time-on-stream dependence of toluene conversion in toluene disproportionation; (AI)ZSM-SB (--ore); (AI)ZSM-5C (--&--); (Fe)ZSM-SA (--T--); (Fe)ZSM-SB ( - - " ~ )
'
200
O
,,
0,0
'
I
0,2
'
I
'
0,4
I
0,6
O,
I
0,8
OH (retool/g) Fig.4 Dependence of toluene conversion in toluene disproportionation on the number of OH groups; (AI)ZSM-5( A ); (Fe)ZSM-5( O );
405 exchange as well as after treatment of the molecular sieve in hydrogen up to 670 K. Therefore, the ESR signals reflect Fe ions planted in tetrahedral environment in the framework sites, which are stable against ion exchange and reduction to a divalent state [see ref. 11]. A broad low intensity signal at g 2.3 indicated the presence of some well dispersed Fe oxidic species.
3.1 Toluene disproportionation Toluene conversion during the reaction of toluene disproportionation exhibited a very low steady decrease depending on time-on-stream (T-O-S) for both AI- and Fe-silicates (Fig. 3). The selectivity values to benzene are slightly higher compared to the sum of xylenes and thus some dealkylation of toluene proceeded with (Fe)ZSM-5, resulting in a higher concentration of benzene and formation of C1 -Ca paraffins and olefins. The selectivity to para-xylene increased with T-O-S, however this increase corresponded to the decrease in toluene conversion owing to deactivation of the molecular sieve by coking. Figure 4 shows that the toluene conversion (after T-O-S of 15 min.) is linearly proportional to the number of Si-OHA1 or Si-OH-Fe groups. The ferrisilicates exhibit much lower activity per Si-OH-Fe site compared to Si-OH-A1 in agreement with their lower acid strength, evidenced by an IK vibration of their OH groups and a high-temperature peak of TPD of ammonia at 620 compared to 690 K, respectively. The dehydroxylation of Si-OH-Fe groups proceeded much easily compared to AI analogs. While under heat treatment at 920 K only 30 % of Si-OH-A1 groups were dehydroxylated, with (Fe)ZSM-5 90 % of Broensted sites was removed at the same conditions. The dehydroxylation of (Fe)ZSM-5 at 970 and 1050 K resulted in a decrease in toluene conversion, which was again roughly proportional to the number of remaining Si-OH-Fe sites. On the other hand, the product selectivity indicated an increase in the rate of toluene dealkylation, reflected in a higher concentration of benzene in the products in comparison with xylenes (see Fig. 5 and Table 2). Thus a more profound effect of the simultaneous presence of Broensted and Lewis sites was found for toluene dealkylation while toluene conversion in disproportionation roughly corresponded to the number of Si-OH-Fe sites.
3.2 Toluene alkylation with isopropanol Completely different feature of toluene conversion and product selectivity in dependence on T-O-S was observed for toluene alkylation with isopropanol compared to toluene disproportionation, over both A1- and Fe-silicates (Figs. 6A and B). A si~ificant increase in toluene conversion with T-O-S accompanied by an increase in selectivity to propyltoluenes was found. Moreover, among propyltoluenes the isopropyltoluenes concentration in the gaseous products increased considerably with T-O-S, while at the same time the concentration of n-propyltoluenes, being much higher at low T-O-S, decreased to a similar extent. As for the selectivity to para-isomers, at the be~nning of the reaction the para-isomers in iso- as well as n-propyltoluenes were of a higher concentration in the products, which decreases with T-O-S. All these T-O-S dependencies were very similar for A1- and Fe-silicates, but substantially more profound for A1 analogs. When "steady state" values of toluene conversion in the alkylation reaction are plotted against the number of Si-OH-A1 and Si-OH-Fe groups of the corresponding molecular sieves
406
8
, . ,......J--
o
//
9
,,j---
//"
e-
o
06-
v tO ,m u) L.
/
/
> E 0
0
21050
/
0:2
0:4
0:6
0,8
0 0,0
0:2
0:4
Fig. 5 Dependence of toluene conversion in toluene disproportionation on the number of OH groups for (Fe)ZSM-5 zeolites activated at 770 K ( 9 ) and for (Fe)ZSM-5D activated at higher temperatures ( o ).
._~
A
~
40
40
~0 0.
20
20
_= O F-
0
11)0
2()0
12
Fig.7 Dependence of toluene conversion in "steady state" in toluene alkylation with isopropanol on the number of OH groups; (AI)ZSM-5( e); (Fe)ZSM-5 ( 9).
60-
== 60t
0
1:0
80 A-"....jr.--d,~"
8o
.g E (!) > E O o q) E ~)
0:8
100
100
v i-
0:6
OH (mmol/g)
OH (mmol/g)
3()0
400
Time-on-stream (min)
Fig. 6 A,B Time-on-stream dependence of toluene conversion and propyltoluene selectivity; A - (AI)ZSM-5B B - (Fe)ZSM-5C
o
0
1~0
2~o
300
400
Time-on-stream (min)
toluene conversion isopropyltoluene n-propyltoluene sum of propyltoluenes
o 9 9 9
407 dehydrated at 690 K, the activity per one Broensted center for all the ferrisilicates investigated was higher than that of alumosilicates (Fig. 7). While with the AI analogs the toluene conversion exhibited nearly a constant value in the range of the concentration of OH groups investigated, for the Fe analogs the conversion slightly increased with the increasing number of the Si-OH-Fe sites. This clearly indicates that in addition to the intrinsic alkylation activity of the individual Si-OH-Me groups some other effect has to contribute substantially to the overall process. All these results imply that the process of toluene alkylation with isopropanol is not exclusively controlled by the intrinsic rate of the alkylation reaction on the Broensted sites, but on the contrary by the desorption/transport rate of the propyltoluene products from the inner molecular sieve channels. This is probably the reason why ferrisilicates exhibit a higher activity per Si-OH-Fe site compared to Si-OH-A1 as the desorption of propyltoluenes is easier from the less acidic former groups.
Table 2 Toluene disproportionation over H-(Fe)ZSM-5
Activation (K) Time-on-stream (min.) Conversion (%)
770 15 55 3.3 3.2
920 15 1.1
55 1.0
1050 15 55 0.8 0.6
Selectivity (%) p-Xylene m-Xylene o-Xylene
26.2 50.4 23.4
26.4 50.7 22.9
32.7 49.0 18.4
34.2 48.8 17.1
39.4 45.5 15.2
40.0 44.0 16.0
46.0 0.0 10.1 19.4 9.0 15.5
46.5 0.0 10.5 20.1 9.1 13.9
53.9 0.0 12.2 18.3 6.9 8.7
52.7 0.0 12.6 18.1 6.3 10.3
56.0 0.0 14.1 16.3 5.4 8.2
53.9 0.0 14.2 15.6 5.7 10.7
Selectivity (wt. %) Benzene Ethylb enzene p-Xylene m-Xylene o-Xylene C1-C4 aliphatics
408 4.0 CONCLUSIONS It can be summarised that (i) The reaction of toluene disproportionation is controlled by the number and acid strength of Si-OH-A1 or Si-OH-Fe groups, thus, by the intrinsic activity of the bridging OH groups. Extraframework A1 or Fe species, exhibiting Lewis acidity and simultaneously present with the Broensted sites in molecular sieves, contribute considerably to toluene dealkylation but not si~ificantly to toluene disproportionation. Thus the presence of these electron acceptor sites changes the selectivity of the toluene transformation. (ii) The reaction of toluene alkylation with isopropanol is controlled by the rate of desorption/transport of propyltoluenes from the active Si-OH-Me sites located in the inner channels of the molecular sieves. This is tmder~andable considering a relatively low reaction temperature, high boiling point of propyltoluenes and in addition the fact that propyltoluenes are bulky molecules. The desorption rate of propyltoluenes can be expected to be lower from Si-OH-A1 groups compared to Si-OH-Fe, however, this assumption should be independently experimentally evidenced. (iii) The effect of acid site strength on the desorption/transport rate of the products for reactions carrying out at relatively low temperature is shown here to be so dramatic that it can reverse the sequence of the activity of the sites of different acidity. If the reaction controlling step is this desorption/transport rate of the products, then the less acidic catalyst can be more efficient in the total conversion compared to that one exhibiting high acid site strength. ACKNOWLEDGEMENT
Financial support of the Grant Agency of the Academy of Sciences of the Czech Republic, project No. 440408, is highly acknowledged. REFERENCES
1. M. Tielen, M. Geelen, P.A. Jacobs, Acta Physica et Chemica Szegediensis, Proc. Int. Syrup. on Zeolite Catalysts, p. 1 (1985). 2. C.T.W. Chu, C.D. Chang, J. Phys. Chem. 89 (1985) 1569. 3. 1L Zahradnik, P. Hobza B. Wichteflovfi, J. Cejka, Coll. Czech. Chem. Commun. 58 (1993) 2474. 4. J. Cejka, A. Vondrovfi, B. Wichtedovfi, G. Vorbeck, R. Fricke, Zeolites 14 (1994) 147. 5. Y. Hong, V. Gruver, J.J. Fripiat, J. Catal. 150 (1994) 421. 6. J.1L Sohn, S.J. DeCanio, P.O. Fritz, J.H. Lunsford, J. Catal. 125 (1990) 123. 1L Carvajal, Po-Jen Chu, J.H. Lunsford, J. Catal. 125 (1990) 123. 7. I~M. Lago, W.O. Haag, R.J. Mikovsky, D.H. Olson, S.D. Hellring, K.D. Schmitt, G.T. Kerr, Proc. 7th Int. Zeol. Conf., (Eds. Y. Murakami et al.), Kodansha - Elsevier, p.677 (1986). 8. V.L. Zholobenko, L.M. Kustov, B.V. Kazansky, E. Loeffler, U. Lohse, G. Oehman, Zeolites 11 (1991) 132. 9. B. Wichterlov~, J. Nov~kovfi, L. Kubelkovfi, P. Jirfi, Proc. 5th Int. Zeol. Conf., Neapol, (Ed. L.V.C. Rees) Heyden, London, p. 373 (1980). 10. A. Corma, Stud. Surf. Sci. Catal. 49 (1989)49. 11. D. H. Lin, G. Coudurier, J.C. Vedrine, Stud. Surf. Sci. Catal. 49 (1989) 1431.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
409
NOx Adsorption Complexes on Zeolites Containing Metal Cations and Strong Lewis Acid Sites and Their Reactivity in CO and CH4 Oxidation: A S p e c t r o s c o p i c Study L. M. Kustov, E. V. Smekalina, E. B. Uvarova, and V. B. Kazansky N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, Leninsky prosp. 47, 117334 Russia ABSTRACT Complex formation and transformations of nitrogen oxides (NO, N20 ) on cations and strong Lewis acid sites in zeolites of the ZSM-5, MOR, LTA, and FAU types were studied by diffuse-reflectance IR spectroscopy. The NOD+ and N20 3 species yielded on cationic forms via NO disproportionation were shown to exhibit strong oxidizing properties with respect to CO and CH4 molecules. Unlike cationic forms, for the dehydroxylated zeolites or the zeolites containing strong Lewis acid sites, the latter were found to be responsible for the polarization of N20 (either produced from NO via disproportionation or adsorbed from the gas phase), which results in chemisorption of atomic oxygen that displays the oxidizing properties in the reactions of CO and CH4 oxidation to CO 2.
INTRODUCTION The exhaust gas purification from NOx is one of the important problems related to the ecology. The use of various heterogeneous catalysts for NOx abatement is reviewed, for instance, in [1]. Among the catalytic systems, the zeolite catalysts may be considered as an alternative to the available honeycomb ceramic oxide catalysts that are based on the process of selective catalytic reduction (SCR) of NOx using NH3. Carbon monoxide and hydrocarbons, such as methane and propane were proposed as efficient reducing agents for SCR of NO using zeolite catalysts (see, for example, [2 - 4]). Nitric and nitrous oxides are also often used as molecular probes for studying surface sites in oxide catalysts and supported metals by IR and ESR techniques [4, 5]. Cationic forms of zeolites are example of systems that are characterized by the formation of a variety of complexes with adsorption sites of different nature and by the occurrence of chemical transformations upon admission of NO [6, 7]. Such transformations can proceed not only on the
410
zeolites containing transition metals [8] and reduced noble metals [9] but also on the zeolites containing no d-elements [10]. It is known that NO disproportionation occurs on Na-chabasite, Na-faujasites, and CaY zeolites according to the overall reaction [11, 12]: 4 N O = N20 + N20 3 At the same time, there are indications that other products may be also formed, such as NO 2 and NO3". Metal cations were assumed to be the active sites for this reaction. However, the mechanism of this process and the influence of the zeolite structure and composition on this process were not yet virtually studied. There are some data on the SCR of NO using CO and methane or other hydrocarbons on zeolites (see, for instance, [2]), but the IR-spectroscopic data related to the investigation of these systems are scarce. In our previous papers [13, 14], we studied the interaction of nitrous oxide with strong Lewis acid sites in dehydroxylated zeolites and found that the latter are able to catalyze the reaction of N20 decomposition via the formation of a chemisorbed atomic oxygen species that is active in the oxidation of H 2, CO, CH 4, and some other molecules. The aim of this work was to study the complex formation and transformations of NO and N20 (disproportionation, reduction with CO and CH4) on cationic (alkaline and alkaline-earth) forms of zeolites that differ in their structure and chemical composition. The similar data were also obtained for the zeolites containing strong Lewis acid sites (dealuminated mordenite, dehydroxylated HZSM-5, and HZSM-5 modified with zinc oxide).
EXPERIMENTAL Na-, Ca-, and Mg-forms of A (Si/AI = 1.0), Y (Si/AI = 2.35), and MOR (Si/AI = 5.0) zeolites were studied. The ion-exchange degrees of Na for Ca or Mg were 50 - 90%. Ion exchange was carried out from aqueous solutions of nitrates. The samples were washed with distilled water, dried at 450 K, and pretreated under vacuum at 720 K for 4 h in IR cells. The rate of the temperature increase was 4 K/min. Also, HZSM-5 zeolite (Si/AI = 20), dehydroxylated at 1070 K for 5 h, H-MOR (Si/AI = 26), dealuminated with hydrochloric acid at 350 K, and HZSM-5 zeolite, modified by ZnO (2 wt %) or Ga (in the framework tetrahedral positions, Si/Ga = 30) were investigated. Diffuse-reflectance IR spectra were measured in the frequency range of 1600 - 4000 cm-1 using a Perkin-Elmer 580B spectrophotometer according to the procedure [15]. Nitrous and nitric oxides, methane, and carbon monoxide were adsorbed at 300 K and P = 1 - 100 torr.
411 RESULTS AND DISCUSSION 1. Adsorption of nitrous oxide Figure 1 shows IR spectra of nitrous oxide adsorbed on different zeolites. Adsorption results in the appearance of a set of bands attributed to the fundamental stretching vibration v as (2230 - 2245 cm-1), as well as to the com-
2240
l 2360
J
~
2590
2260 ~, 2220
2245 I
! 2820
A
I ~2455 590 2260 i 0
2230I .
.
.
.
[
~
"~2810
4.o,/r 2503
.
2800
Fig. 1. IR spectra of N 2 0 adsorbed onzeolites: 1 - NaMOR, 2 - NaA, 3 - NaY, 4 - CaY, 5 - dealuminated mordenite, 6 - the same, after admission of CO and heating at 470 K.
412
bination bands and overtones of adsorbed N20 molecules. The bands at 2420 2510 cm -1 are assigned to the combination of the symmetric stretching and overtone of the bending vibrations (vs0_l + 50-2), those at 2590 - 2600 - to the first overtone of the symmetric stretching vibration (vS0_2) and those at 2800 2820 cm -1 - to the combination of the antisymmetric stretching and bending vibrations (vas0_l + 50_]). The data show that N20 is adsorbed in the molecular form and the v as and (vs0_l + 50_1) frequencies barely change for the cationic forms, independently of the cation nature and type of a zeolite. More considerable changes are observed for the vS0_2 overtone frequency and the combination vs0_l + 50_ 2. Evidently, the metal cations are the centers responsible for the N20 adsorption. Then the distinctions in the IR spectra, especially in the region of 2400 - 2600 cm-1 ($0_2 + vS0 -1) may be explained by the presence of different localization sites for cations in the zeolites under study. For the NaA zeolite, localization in the centers of the 6-membered rings (8 of 12 sites) is most probable [16]. Accordingly, a single band is observed at 2455 cm -1 in the region of vs0_l + 50_2 and a single overtone band ( vS0_2 ) is revealed at 2590 cm -1. For the NaY and Na-MOR zeolites, the number of possible localization sites increases to 2 - 3 [16] (for instance, S I, S I, and SII for NaY, localization sites in the main and side channels of mordenite). In agreement with this site distribution, a more complex superposition of several bands is observed in the region of vs0_l + 80_2 and vS0_2. When passing from Naforms of Y and MOR zeolites to the alkaline-earth forms, the IR spectra in the region of 2400 - 2600 cm -1 become more simple, which may be accounted for by the more uniform distribution of the alkaline-earth cations in these zeolites, in agreement with the available data [16]. Thus, the frequencies of the symmetric stretching vibrations are most sensitive to the interaction with cations in zeolites, which may be explained by a particular, presumably, two-point geometry of the adsorption complex. In the case of the zeolites containing strong Lewis acid sites (ZnO/HZSM-5, dehydroxylated HZSM-5, HGaZSM-5, and dealuminated mordenite), adsorption of N20 gives rise to the shift of the v as frequency to higher frequencies (2280 2260 cm-1), compared to the gas phase and cationic forms. Further heating results in N20 decomposition with evolution of N 2 into the gas phase and chemisorption of atomic oxygen, as it was shown-previously [13, 14]. Admission of CO or CH 4 to the samples with preadsorbed N20 or chemisorbed oxygen, generated as described above, and heating to 400 - 450 K leads to the formation of CO 2 (v = 2360 cm -1). In the case of vel~' strong Lewis acid sites (ZnO/HZSM-5), the heating to 400 - 450 K is not necessary and oxydation occurs at room temperature. Adsorption of carbon monoxide or methane on the cationic forms of zeolites with preadsorbed N20 and further heating to 450 K does not result in the oxidation of these molecules, unlike the case of the zeolites containing strong Lewis acid sites.
413
2. NO a d s o r p t i o n The IR spectra of NO adsorbed at 300 K on CaY and Mg-MOR zeolites are presented in Fig. 2. Unlike NO adsorption on reduced metals or transition-metal ions, no IR bands attributed to NO adsorbed in the molecular form (v = 1900 1860 cm -1) were found for CaY and MgMOR. Instead, a set of bands with maxima ranging from 2260 to 1940 cm -1 was observed due to charging or disproportionation of adsorbed NO molecules [4, 5]. For MgMOR and other cationic forms of high-silica zeolites, the bands at 2160 - 2050 cm -1 predominate in the IR spectra. For CaY and other faujasites under study, the
"•221•0
223O
2280
230,
2360
1930
i1 -.
2060 120 k-t~^
\\\"
2260
k\
2120
j
2360 1 Ill I
255 2360 2585
Fig. 2. IR spectra of NO adsorbed on zeolites: 1 - NaY, 2 - MgMOR, 3 ZnO/HZSM-5. Solid lines: NO adsorption; dotted lines: after further admission of CH 4 and heating to 470 K.
414
bands at 1980 - 1940 cm -1 and those at 2260 - 2230 cm -1 are the most intense spectral features. The bands at 2260 - 2230 (and the corresponding vS0_2 band at 2505 cm -1) and 1980 - 1940 cm -1 can be assigned to N20 and N20 3 (or NO.NO2) species formed via disproportionation of NO, whereas the bands at 2160 - 2050 cm -1 could be attributed to positively charged NO + 5 species [3, 4]. According to [5, 6], other species, such as NO 3- and NO 2 are also formed as a result of NO disproportionation, but we failed to observe the corresponding bands in the DRIR spectra because of the unfavorable background below 1600 cm-1. Thus, for low-silica cationic forms, disproportionation of NO mainly takes place, whereas for the high-silica zeolites, positive charging of NO is more likely. The reason for this difference seems to be different cation density and pore structure of faujasites and pentasils (mordenite and ZSM-5). In the cavities of faujasites, the cation density is much higher than in the channels of the pentasil and short distances between neighboring Me(NO)x ensembles probably allow the easier NO transformation into N20 and N20 3 or NO 2. Further adsorption of ccarbon monoxide or methane at 300 K on CaY with preadsorbed NO results in a slow oxidation into CO 2 (v = 2360 cm -1) in both cases (Fig. 3). Simultaneously, the bands at 1980 - 1940 cm-1 disappear, whereas those at 2260 - 2060 cm -1 remain unchanged. Warming the samples with preadsorbed NO and methane up to 400 - 450 K accelerates CO 2 formation. Unlike the faujasite sample, MgMOR does not exhibit any oxidizing activity in the NO + CO or NO + CH 4 reactions. Slow oxidation starts only at T > 570 K. Hence, we may conclude that the oxidized products of NO disproportionation (in particular, N20 3 or NO2) are the active species responsible for the CO or CH 4 oxidation. In the case of the zeolites containing strong Lewis acid sites, NO adsorption does not result in the appearance of the N20 3 species (1950 - 1930 cm-1) and the N20 bands (2280 - 2230 cm -1) predominate in the IR spectra. Also, the bands assigned to NO c5+ species at 2120 -2150 cm -1 are observed. Probably, in this case, deeper oxidation of NO takes place yielding the species that cannot be observed in DRIR spectra (NO 3- and NO2). Further adsorption of CO, especially followed by heating at 370 - 400 K, causes the decrease in the intensity of the bands at 2280 - 2260 cm -1, attributed to N20 molecules coordinated to strong Lewis acid sites. Simultaneously, the band of CO 2 is revealed at 2360 cm -1. Further heating to 520 K results in the diminution of the band at 2120 - 2150 cm -1, assigned to NO ~5+, and in the parallel growth of the CO 2 band. We may assume that the oxidation of CO or CH 4 with NO (or, vice versa, the NO reduction with CO or CH4) on the zeolites containing strong Lewis acid sites proceeds as follows. First, NO disproportionation takes place that yields N20. Upon heating, N20 is decomposed with the chemisorption of atomic oxygen. The latter seems to be a species consumed for CO or methane oxidation to CO 2. It is possible that the strongly adsorbed N20 molecules also participate in the oxidation reactions.
415
CONCLUSION 1. It is found that N20 adsorption on alkaline and alkaline-earth forms of zeolites leads to weak molecular complexes that do not participate in the reaction of CO or methane oxidation. On the contrary, very strong adsorption complexes are formed upon N20 admission on the zeolites containing strong Lewis acid sites. These complexes are the precursors of the chemisorbed atomic oxygen species that are active in the oxidation processes. 2. Unlike N20, nitric oxide undergoes disproportionation on the cationic forms yielding various NOx species that differ in their stability and oxidizing activity, the most active entities being N20 3 and NO 5+. In the case of the zeolites containing strong Lewis acid sites, N20 formed as a result of disproportionation, and chemisorbed atomic oxygen are responsible for the oxidation activity of the zeolites.
REFERENCES 1. M. Iwamoto, Stud. Surf. Sci. Catal., 54 (1990) 121. 2. H. Bosch and F. Janssen, Catal. Today, 2 (1987) 369. 3. H. Hamada, Y. Kintaichi, and M. Sasaki, AppI. Catal., 64 (1990) L I. 4. A. A. Davydov, IR-Spectroscopy Applied to the Chemistry of Oxide Surfaces, Nauka, Novosibirsk, 1984. 5. Y. Kanno, Y. Matsui, and H. Imai, J. Incl. Phenom., 5 (1987) 385. 6. Y. Kanno, Y. Matsui, and H. Imai, J. Incl. Phenom., 3 (1985) 461. 7. A. A. Alekseev, V. N. Filimonov, and A. N. Terenin, Dokl. Akad. Nauk SSSR, 147 (1962) 1392. 8. M. Iwamoto and H. Yahiro, Chem. Lett., (1990) 1967. 9. H. Miessner and J. Burkhardt, J. Chem. Soc., Faraday Trans., 86 (1990) 2329. 10. C. C. Chao and J. H. Lunsford, J. Am. Chem. Soc., 93 (1971) 6794. 11. W. E. Adisson and R. M. Barrer, J. Chem. Soc., 70 (1955) 757. 12. C. C. Chao and J. H. Lunsford, J. Am. Chem. Soc., 97 (1971) 71. 13. V. L. Zholobenko, L. M. Kustov, and V. B. Kazansky, Proc. 9th Int. Zeolite Conf., Montreal, 1992, Butterworth-Heinemann, 2 (1993) 199. 14. V. L. Zholobenko, I. N. Senchenya, L. M. Kustov, and V. B. Kazansky, Kinet. Katal., 32 (1991) 151. 15. V. B. Kazansky, V. Yu. Borovkov, and L. M. Kustov, Proc. 8th Int. Congr. on Catalysis, Dechema, Weinheim, 3 (1984) 3. 16. W. J. Mortier, Compilation of Extraframework Sites in Zeolites, Guilford, 1982.
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Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
417
Cracking of 1 , 3 , 5 - triisopropylbenzene over deeply dealuminated Y zeolite E.Falabella S-Aguiar 1'2, M.L.Murta-Valle 1, E.V. Sobrinho 3, D. Cardoso 3. Petrobr/ls / CENPES - Ilha do Fund~o, Q7, 21949-900, Rio de Janeiro, Brazil. Fax: 55-21-5986626 2Escola de Quimica/UFRJ, Rio de Janeiro, Brazil, Tel: 55-21-5903192. 3DEQ / UFSCar, S~o Carlos, Brazil, Fax: 55-162-748266.
ABSTRACT Cracking of a rather volmninous molecule, 1,3,5-triisopropylbenzene, was carried out over several zeolites with different degrees of dealumination. Very crystalline zeolites were prepared via combined steam/acid leaching treatments, being afterwards characterized by various techniques. IR-OH region showed that highly condensed EFAL, rather than non-condensed EFAL, was removed during the acid leaching. Nevertheless, MAS/27A1 NMR clearly demonstrated that EFAL-free zeolites are never obtained, regardless of the pH of the acid leaching step. Mesopores surface areas, determined by t-plot increased with increasing number of treatments, as well as the strength of acid sites. Initial rate of cracking of 1,3,5-triisopropylbenzene was plotted against the number of A1 atoms per unit cell, a maximum being obtained for 11 A1/u.c.. Since this molecule has a kinetic diameter larger than 8.0 A, it will not penetrate the zeolitic micropores. After dealumination, mesopores are generated and the reactant is allowed to diffuse. Therefore, both accessibility and acidity seem to control the rate of reaction.
1. INTRODUCTION During the last three decades, extensive work has been carried out aiming at the correlation between the acidity of several dealuminated zeolites and their catalytic activity in the cracking of hydrocarbons. Various molecules such as 2,3dimethylbutane [1], n-pentane [2], n-hexane [3], n-heptane [4], isooctane [5], decalin [6] and cumene [7] have been used in model reactions. Nevertheless, the use of heavier feedstocks in FCC processes has greatly increased the interest for the cracking chemistry of more voluminous molecules. However, not much has been
418 published regarding model reactions with bulky molecules. Previous studies developed in our group [8] have emphasized the importance of external surface area of small crystallite zeolites in the cracking of molecules with kinetic diameter larger than 8.0A such as 1,3,5-triisopropylbenzene (TIPB), showing that acid strength and site density are not the only parameters controlling activity and selectivity during catalytic cracking. The aim of the present work is to demonstrate that the presence of mesopores generated by acid leaching may also play an important role in the cracking of the same molecule (TIPB).
2. EXPERIMENTAL Parent NaY (Si/AI=2.8) zeolite was ion-exchanged in a 11 wt% NH4C1 solution, at 70~ After the ion-exchange, steaming treatments took place in a cylindrical reactor containing 60 g of the zeolite, with 20 g/h flow rate of 100% saturated steam (200~ at 650~ for 90 minutes. Acid leachings were carried out at 70~ for 30 minutes, with pH between 1.0 and 2.5, generating DAZ samples. Treatments were repeated three times and the sodium values were reduced to values below 0.1% Na20. X-ray diffraction (XRD) took place in a Phillips PW1729 diffractometer with CuKo~ radiation and the relative crystallinity of the samples was estimated from the integrated areas of the peaks with Miller indexes 220, 311, 33 l, 333, 440, 533, 642, 660, 751 and 664. Chemical composition was determined by X-ray fluorescence (XRF) in a Phillips PW1407 spectrometer with CrKct radiation. X-ray photoelectron spectroscopy (XPS) was carried out in a VG-Scientific ESCALAB MK II spectrometer with MgKtx radiation, using bands of Al(2p), Si(2p) and Na(ls) to determine surface chemical composition. MAS/NMR spectra of 29Si and 27A1 were obtained on a Varian VXR 300 spectrometer working under a 7.05T magnetic field, being 27A1 spectra collected after impregnation with acetylacetone. Infrared measurements were done on a FTIR Nicolet 60 SXR spectrometer and the OH region (3400-3800 cm -1) was observed. Nitrogen adsorption isotherms were determined using a Micromeritics ASAP2400. From the isotherms, BET surface area, micropores volume (t-plot) and volume of mesopores (BJH) were calculated. Cracking of 1,3,5-triisopropylbenzene (FLUKA 92075) was carried out in a differential fixed-bed gas phase plug flow reactor, with a 15 mm bed containing 5 wt% zeolite diluted in kaolin. Reaction products were analyzed by means of GC-MS (HP5890 and HP5970-B), column HP-PONA (50 m, 0.2 mm, 0.5 pro) for liquid phase and A1203/KC1 column (50 m, 0.3 mm, 0.5 ~rn) for the gas phase.
3. RESULTS AND DISCUSSION Table 1 presents the main characteristics of the parent NaY zeolite and the dealuminated samples prepared thereof. It is clear that dealumination generated very crystalline samples, as a good indication that the process took place without extensive
419 destruction of the framework. It is also evident that progressive dealumination was performed along the cycles, since A o values decreased considerably whereas Si/A1 ratio in the framework (measured by MAS/29Si NMR) increased accordingly. The comparison between Si/A1 ratios obtained by XRF and XPS indicates that aluminum surface contents (XPS) in the samples obtained by acid leaching are lower than the values obtained by XRF and similar to those obtained by 29Si NMR (framework aluminum contents). This reveals a preferential outer surface leaching of the extra framework aluminum by acid treatment [9-11].
Table 1 Main characteristics of the NaY zeolite and dealuminated samples Sample XRD XRF XPS
29Si NMR
Cryst.(%)
Ao(A )
Si/Alg
Si/Als
Si/Alf
NaY
100
24.63
2.8
3.0
2.7
DAZ 1
110
24.50
3.8
4.0
5.6
DAZ 2
119
24.34
11.2
15.0
16.2
DAZ 3
126
24.29
27.8
35.0
33.5
*g=global, s=superficial, f=framework
Results of textural properties of the samples are depicted in table 2, as well as 27A1 NMR relative intensities of aluminum peaks after acetylacetone impregnation. Besides the increasing values of mesopore volume with the increasing dealumination conditions, one can also verify that micropore volume is growing, being larger than the value encountered for the NaY zeolite. This fact indicates that in addition to the characteristic micropores of the parent zeolite, dealuminated samples present supermicropores with diameter between 10 and 20A [9]. The 27A1 NMR spectra display two distinct peaks for tetrahedral (framework) and octahedral (non-framework) aluminum [9-12]. As showed in figure 1, all samples contain extraframework aluminum species (EFAL), regardless of the acid leaching step. Although acid leaching reduces EFAL contents, results of the table 2 indicate that dealuminated samples contain around 30% of EFAL, as an evidence that diffusional limitations in the removal of aluminum species are taking place. In fact, infrared spectra in the OH region after acid leaching revealed the disappearance of the band at 3690 cm l , which is ascribed to highly condensed EFAL located in the larger cavities [ 13]. However, the band at 3600 cm 1, related to low
420
Table 2 Textural properties and 27A1 NMR relative intensifies of the NaY zeolite and dealuminated samples 27A1NMR Sample MiPV a
MePV b
% tetrahedral
%octahedral
NaY
0.326
0.031
100
DAZ 1
0.326
0.092
68.5
31.5
DAZ 2
0.358
0.191
70.7
29.3
DAZ 3
0.360
0.188
64.8
35.2
a MiPV = volume of micropores (cm3/g) b MePV = volume ofmesopores (cm3/g)
DAZ 1
DAZ 2
DAZ 3
O
100
0
100
100
0
-100
100
0
-100
--9 p.p.m. Figure 1. MAS/27A1 NMR spectra for DAZ samples. (T) Tetrahedral, (O) Octahedral
condensation non-framework ahanina located at smaller cavities, remained almost intact. This confmn our assumption that non-accessible EFAL is hardly removed by acid treatment. Rates of 1,3,5-triisopropylbenzene cracking were calculated assuming differential behavior. Plots of rate of TIPB disappearance against time-on-stream were fit to the classical Voohries equation (r = r o t -n) [ 14], allowing one to estimate initial rates of reaction. Initial rates of TIPB disappearance were then plotted against the number of aluminum atoms per unit cell (A1/u.c.), according to NMR results. Such graph is
421 presented in figure 2, for three different temperatures (400, 425 e 450~ For all temperatures, a maximum is achieved for about 11 A1/u.c.. These results contradict previous published data which found a maximum for 30 A1/u.c. in the cracking of, nhexane [3],for instance. Although extensive published literature has often encountered a maximum of activity between 30 - 40 A1/u.c., it must be borne in mind that all model reactions employed in previous studies used small molecules such as linear paraffins (hexane, heptane) or light aromatics (cumene). Explanations for maximum occurrence were based upon changes in the strength, type and concentration of acid sites [3,9,15,16]. However, when voluminous molecules are used, the effect of the formation of mesopores (and thus changes in the external surface area of the zeolites) must not be disregarded. The increase in surface area would certainly be important for diffusion limited reactions, which is apparently the case of the cracking of TIPB. Thus, DAZ 2 and DAZ 3 zeolites possess similar values of micro and mesopore volumes, which, in turn, are higher than the values obtained for DAZ 1 (table 2). Nevertheless, DAZ 2 is more active than DAZ 3 because of the higher concentration of acid sites thereof. This fact confLrms that in the cracking of bulky molecules, both acidity and diffusion are playing an important role in the rate determining step.
3.60
3.20
2.80
~
2.40 -
2.00
1.60
~ 6
l 10
,
l f ~ 18 20 AI/u.c. (NMR)
~
1
l
28
30
Figure 2. Initial cracking rates of 1,3,5-TIPB as a function of A1/u.c. (NMR). (a) 400~ (o) 425~ and ( . ) 450~
422 4. CONCLUSIONS Cracking of TIPB was studied over several acid leached steam treated zeolites and initial rates of disappearance were plotted against the number of A1/u.c.. For three different temperatures, a maximum of activity was obtained for 11 A1/u.c., contradicting previous results. Apparently, as not yet observed in the cracking of smaller molecules, the accessibility of the larger molecule to the internal zeolitic pores is controlling the rate of reaction. Therefore, maxima are encountered for zeolites which display both high mesoporosity and considerable acid site concentration. In the case of acid leaching of ultra-stable samples this is a compromise between acidity and accessibility, since the combined treatments generate mesoporosity but also reduce the acid site concentration.
REFERENCES
1 2 3 4 5 6
G.R. Bamwenda, Y.X. Zhao, B.W. Wojciechowski. J. Catal. 150(1994)243. P.V. Shertukde, W.K. Hall, J.M. Dereppe, G. Marcelin. J. Catal. 139(1993)468. J.R. Sohn, S.J. DeCanio, P.O. Fritz, J.H. Lunsford. J. Phys. Chem. 90(1986)4847 A. Corma, J.Planelles, J.Sanches-Marin, F.Tomfis. J. Catal. 93(1985)30. R. Beamnont, D. Barthomeuf. J. Catal. 30(1973)288. E. Falabella S-Aguiar, M.P. Silva, M.L.Murta-Valle, D.F. Silva. ACS Division of Petroleum Chemistry, Inc. Preprints, 39-3(1994)356. 7 S.J. DeCanio, J.R. Sohn, P.O. Fritz, J.H. Lunsford. J. Catal. 101(1986)132. 8 E. Falabella S-Aguiar, M.L.Murta-Valle, M.P. Silva, D.F. Silva. Proc. of the 1st EUROPACAT, Montpellier, 1(1994) 104. 9 J. Scherzer in T.E. Whyte Jr.(Editor). The Preparation and Characterization of Aluminum Deficient Zeolites (ACS Symp.Series 248), New York, 1984, p.157. 10 G. Fleisch, B.L. Meyers, G.J. Ray, C.L. Marshall. J. Catal. 99(1986)117. 11 E.V. Sobrinho, D. Cardoso, E. Falabella S-Aguiar, J.G. Silva. XIV Simp. Iberoamericano Catal, Concepci6n, 1994, p.433. 12 G. Engelhardt in H.V. Bekkun (Editor). Solid State NMR Spectroscopy Applied to Zeolites, Elsevier, 1991, p.285. 13 R.D. Shannon, K.H. Gardner, R.H. Staley, G. Bergeret, P. Gallezot, A.Auroux. J. Phys. Chem. 89(1985)4778. 14 A. Voorhies. Ind. Eng. Chem. 37(1945)318. 15 D. Barthomeuf. Materials Chem. Phys. 17(1978)49. 16 H. Stach. Catal. Letters 13(1992)339.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine(editors) 9 1995 Elsevier Science B.V. All rights reserved.
423
H y d r o g e n a t i o n of Styrene and H y d r o g e n o l y s i s of 2- Phenylethanol
Mechanistic Study of the Side-Chain Alkylation of Toluene and Methanol Tawan Sooknoi and John Dwyer Centre for Microporous Materials, Department of Chemistry, UMIST, PO Box 88, Manchester, M60 1QD, United Kingdom
Introduction A mechanism reported previously for the side-chain alkylation of toluene with methanol suggests that formaldehyde is the alkylating agent, primarily forming styrene, which then undergoes hydrogenation to ethylbenzene with small amounts of hydrogen, created by decomposition of methanol. In the present work mechanisms involving hydrogenolysis of 2phenylethanol intermediates, or involving the direct alkylation of methanol, are suggested to be predominant routes to the formation of ethylbenzene. The latter mechanism appears to be especially promoted in the presence of excess of Cs over stoichiometric ion exchange requirement of zeolite X. It is proposed that the intermediate, 2-phenylethanol, is formed from the reaction between toluene and formaldehyde. The fact that it is not detected in the product mixture is attributed to its rapid dehydration to styrene and, as proposed in Figure 1, hydrogenolysis to ethylbenzene.
(1)
(U"
(2)
H
Figure 1 Formation of 2-phenylethanol and its reactions 1) Dehydration to styrene 2) Hydrogenolysis to ethylbenzene. In this work, the hydrogenation of styrene and the hydrogenolysis of 2-phenylethanol are demonstrated over Cs exchanged zeolite X under the same reaction conditions as used for the side-chain alkylation. We postulate the formation of ethylbenzene either from direct alkylation of toluene with methanol or from the reaction of both styrene and 2-phenylethanol with hydrogen produced from the decomposition of methanol. The overall mechanism is shown in Figure 2. The ethylbenzene/styrene ratio is used as a parameter to determine the relative reactivities of direct alkylation, hydrogenation and hydrogenolysis.
424
U
CH3
cH3oH MI
J
H2
"U"
T
H2
M5
M3
Figure 2 Overall mechanism for the side-chain alkylation reaction
Experimental procedure CsNaX was used in side-chain alkylation of toluene, hydrogenation of styrene and hydrogenolysis of 2-phenylethanol. Molecular sieve 13X (BDH | was ion-exchanged three times with 0.5 M CsC1 at 50 ~ and once with 0.5 M CsOH at room temperature. The solid material was separated into two portions. The first was washed several times with 0.5 M CsOH and left to dry at room temperature overnight. This material believed to contain clusters of CsOH (possibly CsHCO3/Cs2CO3) was defined as "CsNaX-I" catalyst. The second portion was washed with deionised water until no basicity was detected. The Cs cations in this material, designated as "CsNaX-2", were presumably exchanged ions. The reactions were carried out in a fixed bed down-flow reactor at 350 ~ C with helium as carrier gas. In the hydrogenation of styrene and the hydrogenolysis of 2-phenylethanol, hydrogen was also employed. Mixtures of toluene and methanol were continuously fed by syringe pump. In order to enhance the decomposition of methanol, and hence the amount of hydrogen, a toluene/methanol ratio of 0.2 was used. Styrene and 2-phenylethanol were fed as pure components. In addition, --15-20% styrene in toluene and --15-20% 2-phenylethanol in toluene were also employed to test the hydrogenation and hydrogenolysis reactions. Liquid products were collected in an acetone-dry ice bath every 30 minutes and separated using a Chromosorb 20M column at 140-180~ with helium flowrate 30 ml/min as carrier gas. Gas products were periodically detected by on-line gas chromatography using a 10 ft Molecular sieve 13X and a 6 ft Chromosorb 20M column at 40 ~ C. Helium was again used as carrier gas at the rate of 30 ml/min.
Results and Discussions Side-chain alkylation of toluene with methanol resulted in ethylbenzene, styrene and, in some case, traces of cumene and a-methylstyrene. The gas products contain carbon monoxide, carbon dioxide and small amounts of methane. No dimethylether was found which reflects the basicity of the catalysts. When CsNaX-1 was used, more ethylbenzene than styrene was produced and cumene, together with a trace of a-methylstyrene, was also found in the product mixtures. On the other hand, styrene was a major product over CsNaX-2 and additionally, no cumene or a-methylstyrene was observed. The conversion of toluene was 10% and 5% for CsNaX-1 and CsNaX-2, respectively, but the conversion of methanol over these catalysts was higher than that of toluene owing to the parallel decomposition of methanol to carbon monoxide. For hydrogenation of styrene 4% hydrogen was used in the carrier gas because, in side-chain alkylation, the hydrogen produced was found to be less than 4% of the total gas. The styrene hydrogenation over CsNaX-1 and CsNaX-2 catalysts using the same conditions gave small amounts of toluene, ethylbenzene and benzene. Most of the styrene remained unreacted. Although hydrogen was used as carrier gas, the yield of ethylbenzene was not significantly increased with both catalysts. However, when CsNaX-1 was used, there was an initial increase in hydrogenation products, followed by a rapid decrease in reactivity to a level similar to that observed with CsNaX-2. When feed mixtures of styrene and toluene were used (4% hydrogen), a dramatic increase in hydrogenation product was observed (Table 1) but the conversion of styrene was quite low.
425
Hydrogenolysis of 2-phenylethanol (4% hydrogen ), over both catalysts, gave product compositions similar to that obtained from hydrogenation of styrene but with a relatively higher yield of hydrogenolysis products. Interestingly, c~-methylstyrene, which results from the sidechain alkylation of e t h y l b e n z e n e with formaldehyde, and 3 - p h e n y l p r o p e n e were observed. About 10% of 2-phenylethanol is converted by direct reaction with hydrogen (Fig 1 scheme 2 and Fig 5 scheme a) and by decomposition to toluene (1), the rest is directly converted to styrene via dehydration (Fig 1 scheme 1). Even when hydrogen was used as carrier gas, the yield of hydrogenolysis products increased by only 2-3%. However, when a mixture of toluene and 2phenylethanol, at a composition roughly the same as that expected in the side-chain alkylation, was used (4% hydrogen), more than 90% of the 2-phenylethanol was h y d r o g e n o l y s e d at the beginning of the reaction subsequently the hydrogenolysis selectivity decreased and remained constant, at 50%, after 3 hours on stream. The product distributions are listed in Table 1. Table 1 Product distribution in side-chain alkylation of toluene, hydrogenation of styrene and hydrogenolysis of 2-phenylethanol (%mole).
Reaction S S Hs Hs Hts Hp Hp Htp
Catalyst H2* Conversion Benzene Toluene Ethylbenzene Styrene Cumene at-Methylstyrene CsNaX- 1 CsNaX-2 CsNaX-1 CsNaX-2 CsNaX- 1 CsNaX- 1 CsNaX-2 CsNaX- 1
4% 4% 4% 4% 4% 4%
11a 5a 1.6b 1.5b 8.4c 100b 100b 100c
trace 0.1 trace 0.5 0.1 trace 0.9
0.8 0.7 5.2 5.7 4.3 34.8
7.3 0.8 0.7 0.7 2.5 3.2 1.8 15.5
2.5 3.8 89.4 91.6 46.4
1.0 -
trace 1.5 1.4 1.9
Yields represent the average yield over 180 minutes time on stream, reaction temperature 350 ~ catalyst 1 g, contact time 0.015 min, W/F = 47 g.mol.h -1 (total feed), He carrier gas 25 ml/min S = Side-chain alkylation of toluene, H s = Hydrogenation of styrene, Hp = Hydrogenolysis of 2-phenylethanol, Hts = Hydrogenation of Styrene in the presence of toluene and Htp = Hydrogenolysis of 2-phenylethanol in the presence of toluene. aConversion based on Toluene b Conversion based on styrene or 2-phenylethanol CConversion based on styrene or 2-phenylethanol in the mixtures * %Hydrogen in Helium carrier gas (mol/mol)
Table 2 Ethylbenzene/Styrene ratio (mol/mol)
Reactions
Feed
Catalysts
Side-Chain alkylation Side-Chain alkylation Hydrogenation Hydrogenation Hydrogenation Hydrogenolysis Hydrogenolysis Hydrogenolysis
Toluene/methanol = 0.2 Toluene/methanol = 0.2 Styrene (pure) Styrene (pure) Toluene/styrene - 5 2-Phenylethanol (pure) 2-Phenylethanol (pure) Toluene/2-phenylethanol- 5
CsNaX-1 CsNaX-2 CsNaX-1 CsNaX-2 CsNaX-1 CsNaX-1 CsNaX-2 CsNaX-1
Hydrogen in cartier gas
-4% --4% --4% -4% --4% --4%
Ethylbenzene/ Styrene 3.16 0.20 --" 0.01 -
0.01
- 0.03 -0.04 -0.02 0.26
426 Table 2 shows the ratio of the yield of ethylbenzene to the yield of styrene in the product mixture for each reaction (averaged over the first 3 hours). In the hydrogenolysis reaction using mixtures of toluene and 2-phenylethanol and the hydrogenation reaction using mixtures of toluene and styrene, yields of the products are calculated, respectively, based on the amount of 2-phenylethanol and styrene in the feed mixtures. The ethylbenzene/styrene ratio for the side-chain alkylation is considerably higher for CsNaX-1 indicating that the formation of ethylbenzene is influenced by the Cs "clusters". It is suggested that the Cs "clusters" enhance either direct alkylation (Path M1, Fig 2 ) o r hydrogenation and hydrogenolysis (Path M4 and Ms, Fig 2). It seems clear that ethylbenzene is not produced, in the main, from hydrogenation of styrene but from the hydrogenolysis of the 2phenylethanol intermediate. A higher conversion of 2-phenylethanol to ethylbenzene by hydrogenolysis is obtained in the presence of toluene because the dehydration of 2phenylethanol, to give styrene, does not require the more active basic sites. The sites inducing only a weak electron donor-acceptor interaction can adequately promote dehydration of 2phenylethanol to styrene. For example, conversion of 2-phenylethanol over "silica-alumina" (1) results in styrene as product. In separate work (8), we also test dehydration of 2-phenylethanol over "silicalite-I" and over "silica gel" (Fig 3) -H20
H
H
Oxygen Bridge Sites (Silicalite) Si
Si
Si -H20 H
H
Na
-
/ O ~ Si
~ AI
/0 ~
H
/
Exchanged Cation Sites (NaX)
I
/0
OH
OH
0
I
I
I
-H20 -
Silanol Sites (Silica gel)
Silo / Silo / Silo
Figure 3 Three possible sites promoting the dehydration of 2-phenylethanol to styrene Presuming that 2-phenylethanol is more strongly adsorbed on the more basic sites, (Cs "clusters" or Cs exchanged sites) which promote hydrogenolysis to ethylbenzene, than on other electron donor-acceptor sites (e.g. oxygen bridges or exchangeable cations other than Cs), then, as the partial pressure of 2-phenylethanol is increased, relatively more conversion can take place on the more weakly adsorbing sites which, we presume, promote dehydration of 2phenylethanol to styrene, as observed with silicalite and silica gel. This may be the explanation for the reduced yields of ethylbenzene at the higher partial pressure of 2-phenylethanol (pure feed) as compared with the mixtures of 2-phenylethanol and toluene. This, of course, assumes that toluene acts largely as diluent. The assumption that 2-phenylethanol is more strongly adsorbed than toluene would result in inhibition of toluene sorption by the product (2phenylethanol) and also inhibition of the side-chain alkylation reaction rate.
427 Previous work (2) suggests that the side-chain alkylation can indeed be inhibited by the reaction products and we suggest that 2-phenylethanol is involved in this inhibition. Moreover, toluene, which is a reactant in the side-chain alkylation, is found to be the major product of 2phenylethanol hydrogenolysis (Table 1 and Fig 4), therefore, the side-chain alkylation of toluene seems unfavourable compared with ring alkylation to give xylene over acid catalysts.
CH3OH
Figure 4 Toluene and 2-phenylethanol cycle The observation that toluene is a major product from the hydrogenolysis of 2phenylethanol also provides evidence to support the effect of toluene/methanol ratio observed by Yashima (3) who reported that a higher toluene/methanol ratio (> 5) gave better results in sidechain alkylation than reactions using more methanol in the feed. We suggest that the lower conversion of toluene in the side-chain alkylation, when methanol concentration is high, is due to the competitive adsorption of methanol and the increase of hydrogen by decomposition of methanol which leads to more hydrogenolysis of 2-phenylethanol to give toluene and ethylbenzene. A high ratio of toluene/methanol in the side-chain alkylation reduces the concentration of methanol in the feed, diminishes the rate of direct alkylation and also reduces methanol decomposition to carbon monoxide and hydrogen so that less hydrogen is available for hydrogenolysis of 2-phenylethanol. This is reflected in a decrease in yield of ethylbenzene when feed methanol is decreased within the experimental range used (3) The mechanism for hydrogenolysis of 2-phenylethanol to toluene is proposed in Figure 5. Additionally, the presence of a-methylstyrene in products from hydrogenolysis of 2phenylethanol suggests that the catalysts are highly active in promoting side-chain alkylation of ethylbenzene with formaldehyde which results either from hydrogenolysis of 2-phenylethanol to toluene (Fig 5a) or from direct decomposition of 2-phenylethanol to give toluene as observed by Vivekanandan (1). The methanol from these reactions (Fig 5a) is rapidly decomposed to formaldehyde and to carbon monoxide which is also detected in small amounts during the reaction. The 2-phenylpropanol, which, we suggest, is an intermediate in the side-chain alkylation of ethylbenzene (Fig 5d), undergoes dehydration to a-methylstyrene as shown in Figure 5(e). No cumene (e.g. from hydrogenolysis of 2-phenylpropanol) is detected in the product mixtures, possibly due to its low concentration. However, direct alkylation to cumene is also not expected because the amounts of methanol produced by hydrogenolysis of 2phenylpropanol are small in proportion to the number of active basic sites which, presumably, results in rapid decomposition of methanol to formaldehyde and to carbon monoxide.The ethylbenzene/styrene ratio from the hydrogenolysis of 2-phenylethanol (4% hydrogen) in the presence of toluene using CsNaX-1 catalyst is similar to that for side-chain alkylation using CsNaX-2 catalyst whereas the ethylbenzene/styrene ratio from the hydrogenation of styrene (4% hydrogen) in the presence of toluene using CsNaX-1 catalyst is very much lower. This implies that, when Cs exchanged zeolites containing no "clusters" are used in side-chain alkylation, ethylbenzene is mainly produced from hydrogenolysis of the 2-phenylethanol (Path M4, Fig 2), and considerably less is produced from hydrogenation of styrene. This could also explain the lack of influence of transition metals in the catalysts, on selectivity of ethylbenzene and styrene (4,5,9) because the mechanism proposed here involves nucleophilic attack by a hydride anion generated from dissociation of hydrogen to H- and H + over the basic catalyst, and not over the transition metals.
428 | OH
HG / Basicsite
CH3
(a) .~ CH3OH +
~~'~2HH2 (d)
OH
~'-- ~ M . ~
(e) -HE0
2-phenylpropanol 120 Figure 5 Formation of ct-methylstyrene in hydrogenolysis of 2-phenylethanol According to the above reaction schemes (Fig 2, 3 and 5), it is impossible to obtain yields of ethylbenzene greater than yields of styrene from the side-chain alkylation of toluene with formaldehyde from the decomposition of methanol. Therefore, direct alkylation of toluene with methanol must take place in the reaction using catalysts containing Cs "clusters" in order to obtain the observed high yields of ethylbenzene (ethylbenzene/styrene ratio > 3). This is supported by the reaction over K exchanged zeolite X by Yashima (3) who used formaldehyde as alkylating agent and found that the ethylbenzene/styrene ratio is about 0.3 and is also supported by the work of Itoh (4) who found an ethylbenzene/styrene ratio less than 1 when KX was used as catalyst. In contrast, Hathaway (6) obtained higher ethylbenzene yields when CsNaY containing cesium acetate "clusters" were employed and, interestingly, Engelhardt (7) who used KX washed with KOH, as catalyst, obtained an ethylbenzene/styrene ratio higher than 10. In addition to this result, Engelhardt also found that the ethylbenzene/styrene ratio decreased with the number of washings of this catalyst with water. This strongly suggests that the "clusters" provide the active basic sites for the direct alkylation since extensive washing would remove K cations in excess of stoichiometry. In addition, the direct alkylation of toluene was also observed in the reaction between toluene and ethylene over Rb exchanged zeolite X.(3) Formation of cumene in the side-chain alkylation of toluene with methanol over CsNaX-1 is further evidence that direct alkylation of alkylbenzene with methanol takes place in the presence of Cs "clusters". Over strongly basic catalysts, ethylbenzene, produced from primary side-chain alkylation of toluene with methanol, undergoes secondary alkylation with methanol to form cumene directly (8). Some of the ethylbenzene can also react with formaldehyde, but in much smaller amounts, giving only traces of a-methylstyrene. We concluded that catalysts, for side-chain alkylation, perform two important roles; the first is the basicity which promotes proton abstraction of benzylic hydrogen to generate a "carbanion like" intermediate and the second is the stabilisation of this intermediate by the highly polar framework. Catalysts lacking the ability to stabilise this intermediate, will not be able to promote side-chain alkylation. For example, MgO which is a strongly basic solid catalyst cannot catalyse side-chain alkylation (6), presumably because the catalyst has no polar framework to stabilise the appropriated intermediates which could be generated by its basicity. For this reason zeolite X, which has a particularly high ion exchange capacity (the most polar framework), appears to be the most active catalyst for the side-chain alkylation. Cs exchanged zeolite X (CsNaX-2) seems to be sufficiently basic to generate benzyl "carbanion like" intermediates and to stabilise them within the highly polar framework. The negative charge of the intermediate is presumably delocalised into the benzene ring which, as suggested (10,11), is sitting on two (or more) Cs cations at the active site surrounding by a number of Cs and Na cations in the framework. This makes the benzyl carbanion sufficiently stable to facilitate reaction with more electrophilic species such as formaldehyde. Direct alkylation, therefore, is rarely promoted by Cs exchanged zeolites without Cs "clusters" and ethylbenzene is only produced from hydrogenolysis of 2-phenylethanol intermediates leading to
429 a low ethylbenzene/styrene product ratio. When Cs "clusters" are incorporated into Cs exchanged zeolite crystals (CsNaX-1), the benzyl carbanion is certainly generated by the strong basicity of the cluster, together with that of the negatively charged framework. Cs cations in "clusters" can stabilise "carbanion-like" intermediates by providing a stronger basic site localised on the "clusters". These species are sufficiently stable to facilitate proton abstraction of the benzylic hydrogen and localisation of the charge on the benzyl "carbanion" which is then more available to react with appropriate electrophilic species. In other words, Cs "clusters" in Cs exchanged zeolites generate more reactive benzyl "carbanion" intermediates than Cs exchanged zeolites without "clusters", and this results in the alkylation with both methanol and formaldehyde. Consequently, the yield of ethylbenzene in reactions using CsNaX-1 is higher than that of styrene since ethylbenzene is produced by both hydrogenolysis, and also by the direct alkylation with methanol which is promoted by incorporated Cs "clusters".
References 1. G. Vivekanandan, C. S. Swaminathan and V. Krishnasami, Ind. J. Chem., 32A, 215-220 (1993) 2. P. Beltrame, P. Fumagalli and G. Zuretti, Ind. Eng. Chem. Res., 32, 26-30 (1993). 3. T. Yashima, K. Sato, T. Hayasaka and N. Hara, J. Cat., 26, 303-312 (1972) 4. H. Itoh, A. Miyamoto and Y. Murakami, J. Cat., 64, 284-294 (1980). 5. C. Lacroix, A. Deluzarche, A. Kiennemann and A. Boyer, Zeolites, 4, 109-111 (1984) 6. P. Hathaway and M. Davis, J. Cat., 119, 497-507 (1989). 7. J. Englehardt, J. Szanyi and J. Volyon, J. Cat., 107, 296-306 (1987). 8. T. Sooknoi and J. Dwyer, Unpublished papers, Department of Chemistry, UMIST (1995). 9. C. Lacroix, A. Deluzarche, A. Kiennemann and A. Boyer, J. Chim. Phys., 81, 473479,481-485,487-490 (1984). 10. K. Tanabe, M. Misono, Y. Ono and H. Hattori, Stud. Surf. Sci. Cat., 51,233-234 (1989). 11. M. L. Unland and G.E. Baber, Catalysis of Organic Reaction, Marcel Delecker, 54 (1981).
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Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
431
Catalyst deactivation of high silica metallosilicates in Beckmann rearrangement of cyclohexanone oxime Takashige Takahashi, Takami Kai and M.N.A.Nasution Department of Applied Chemistry & Chemical Engineering, Faculty of Engineering, Kagoshima University, Kagoshima 890 Japan The vapor phase Beckmann rearrangemem was carried out over high silica ZSM-5 type metallosilicates to elucidate the effect of acid strength on ~ -caprolactam selectivity and catalyst deactivation rate. It was found that the indiosilicate which had the lowest acid strength was the best catalyst among the metallosilicates. When carbon dioxide and methanol were used as diluent gas and diluent solvent of cyclohexanone oxime, respectively, the deactivation rate decreased over indiosilicate. Furthermore, when the indiosilicate was modified with a precious metal, the catalyst deactivation significantly decreased. It was considered that the oxidation of coke on the surface was accelerated by the precious metal. 1. INTRODUCTION Although the vapor phase Beckmann rearrangement of cyclohexanone oxime (CHO) is known to be an attractive process to prepare ~ -caprolactam (CL) as the starting compound of nylon-6, the industrial process was not achieved due to the low selectivity to CL and rapid catalyst deactivation of the solid catalysts. Recently, some investigations were carried out to improve the CL selectivity. Sato et al. reported that a high silica HZSM-5 zeolite (SiO2/A120 3 ratio = 3200) was effective for the rearrangement reaction and the selectivity was 90 % or more [ 1]. They also reported that when an alcohol was used as the diluent solvent of CHO, the selectivity and catalyst life were significantly improved [2]. Furthermore, when the CHO vapor diluted with carbon dioxide was fed into the catalyst layer, the selectivity to the CL was higher than that diluted with nitrogen [3]. It was reported that the catalyst life and the selectivity to CL also increased by use of boria supported HZSM-5 zeolite [4, 5], MEL type zeolite [6], femerite zeolite [7] and bona deposited on an alumina or silica by a CVD method [8, 9]. The CL selectivity gradually increased by use of the new type catalysts, but the catalyst deactivation was not overcome so far. Many questions on the reaction mechamsm of the Beckmann rearrangement over the zeolites and on the deactivation mechanism of the zeolites still remained.
432 In the presem study, the rearrangement has been camed out over a ZSM-5 type metallosilicates including iron, gallium or radium in the crystal lattice to elucidate the effect of acid strength on the catalyst deactivation rate. We have also examined the effects of diluent solvent and diluent gas of CHO on the deactivation rate and CL selectivity. Furthermore, the rearrangement was camed out over an mdiosilicate modified with precious metals to decrease the catalyst deactivation. 2. EXPERIMENTAL
A high silica ZSM-5 type metallosilicate with gallium, iron or indium in the crystal lattice was synthesized by modifying the method described in a previous paper [ 10]. In the present study, the ratio of silica/metal changed from 500 to 3200. After the powder of the silicates was compressed into a small die, it was crushed to 32 -~ 48 mesh. The surface area was measured by a nitrogen adsorption method, in which Langmuir equation was used for calculation. The acidity and acid strength distribution were measured using ammonia temperature programmed desorption method. The impregnation method was used for the precious metal modification of zeolites with silica/metal ratio = 500. The vapor phase reaction of CHO has been carried out at atmospheric pressure in a flow type system. The reactor assembly was essentially similar to that reported in a previous paper [4]. 3.RESULTS AND DISCUSSION It has been reported that the catalytic activity of zeolites decreased with time on stream due to the deposition of coke on the strong acid sites in the reaction of CHO. Figure 1 shows the relationship between CHO conversion and time on stream over Z5(Ga)-500H and Z5(Ga)-1000H. Z5(Ga)-500H means proton exchanged gallosilicate whose Si/Ga ratio is 500. Since similar relationships between CHO conversion and time on stream were obtained over the metallosilicates, the effect of time on stream on CHO conversion is represemed by Equation (1). x(t)---x(O) 9exp(-b 9t)
(1)
where x(t) and x(0) are CHO conversion at any time on stream and initial conversion, respectively, b is deactivation coefficient and t is time on stream. The deactivation coefficients of various proton exchanged metallosilicates with the same Si/M ratio (500) are shown in Table 1. This table also shows the strong acid concentration and the maximum temperature of strong acid sites of the silicates measured by ammonia TPD method. The maximum
433
1.0 0.8 0.6
~
17 O tn
!
0.4 -
k.,
Reaction temp. = 623 K WlFA0 = 113 kg.s/mot Diluent gas = H2 Diluent solvent = Benzene
c 0.2
O Z5(Ga)-1OOOH 9Z5(Ga)- 500H
0 U
0.1 ~ 0
10
~
~ 20 30 40 50 Tim~ on stream [ m i n i
60
Figure 1. Catalyst deactivation of Z5(Ga)-500H and Z5(Ga)-1000H. Table 1 Strong acid concentration, deactivation factor(b) and maximum temperature of ammonia TPD Catalyst Z5(A1)500-H Z5(Fe)500-H Z5(Ga)500-H Z5(In)500-H
Strong acid conc. 1) [mmol/g] 0.058 0.046 0.044 0.039
Deact. factor [I/s] 2.7 x 10-4 2.4 x 10-4 1.8 • 10-4 4.0 • 10-5
Max. temp. [K] 758 749 733 706
1) Strong acid means the acid sites can retain ammonia at >680 K. Reaction temperature=623 K, Diluent gas=H 2, Diluent solvent=Benzene temperature of aluminosilicate is highest among the silicates. At the same time, the deactivation coefficiem is also the highest. On the other hand, the deactivation coefficient of the indiosilicate with the lowest maximum temperature is smallest among the silicates. This result indicates that the catalyst deactivation can decrease to control the acid strength of the strong acid sites m the metallosilicate. It was found that when the change of acid strength distribution of the used silicate was examined by ammonia TPD method, the strong acid concentration did not reduce as expected from the decrease in CHO conversion. Furthermore, the surface area of the used silicate was found to be almost the same as that of the fresh silicate. These results suggest that the coke on the surface should be removed on heating for 1 h at 773 K in a nitrogen stream or evacuating for 2 h at 473 K ~mder 1 torr. If the coke was removed from the
434
1.0 A
9
"
t
7_>_0.8 m 0.6 - Catalyst 9Z 5 ( I n ) - 500H =Reaction temp. = 623 K O Diluent gas = H2 solvent = Benzene O.4 -Diluent W/FAo = 120 kg .s/mo[ O ~
(J
0.2. 0
I 1
I 2
~- Evacuated at 473 K f o r 2h CHO Conversion
~
CL Selectivity
I I I I 5(0) I 2 3 4 T i m e on st ream [ h ]
I 3
Figure 2. Effect of evacuation on recovery of catalytic activity. silicate surface, the catalytic activity would be recovered by the evacuatton. Figure 2 demonstrates the effect of the evacuation of the used silicates on the CHO conversion. The catalytic activity decreases with the time on stream as shown in Figure 2. The reaction was terminated after 5 h of time on stream. The silicate taken out from a reactor was evacuated for 2 h at 473 K under 1 torr. After the silicate was recharged in the reactor, the reaction was started under the same conditions. Although the deactivation rate of the treated silicate is higher than that of the ti'esh silicate,the catalytic activity is recovered as expected. On the other hand, when the used mordenite was treated by the same conditions,the activity did not increased. This result indicates that the carbonaceous deposit on the metallosilicate should be low molecular weight. Recently, Ichihashi et al.[2] reported that when the solvent of CHO changed from benzene to methanol or other alcohols,the CL selectivity and the catalyst life of the high silica aluminosilicate were simultaneously improved. They also reported that when the carrier gas changed from hydrogen to carbon dioxide, the CL selectivity was improved [3]. The reaction of CHO was carried om under the same diluent gas and solvent over Z5(In)500-H. Figure 3 demonstrates the relationship between CHO conversion and CL selectivity and time on stream. The lactam selectivity and catalyst life are significantly improved under the reaction conditions. These results suggest that the oxygen atom in alcohol or carbon dioxide would be importm~t for increasing the catalyst performances. If the coke is removed with oxygen on the silicate surface, much higher selectivity and longer catalyst life will be expected on Beckmann rearrangement over the mdiosilicate modified by platinum. Figure 4 shows the relationship between CHO conversion and CL selectivity on time on stream over the
435
1.0
>" 0.95
.,=,..
O CHO Conversion
>
.m,.
u
9 CL Selectivity
-~ 0.90 U3
E O L_
O U
Catalyst : Z 5 ( I n ) - 5 O O R ~ O"'%M~O.." Reaction temp.= 623 K 0.85 -Diluent gas = CO2 Diluent solvent = n - Prol)anot WlFA0 ~ 118 kgis/mo{ 1 0.80 0 2 4 6 8 T i m e on stream [ h ]
10
Figure 3. Relationship between CHO conversion and CL selectivity over Z5(In)-500H.
1.00
0 CliO Conversion
I
,
i....i
>, 0.98
I
CL Selectivity
> o ~ .
u 0-96 151 m
- 0.94 -
Reaction temp. = 623 K Diluent gas = CO2 _ Diluent solvent = n - Propanot ~- 0.92 > W/FAo = 118 kg.s/mot
E O
. ~
E O
u 0.90
0
I
I
2
4
I
6 Time on st ream
I 8
10
[hi
Figure 4. Relationship between CHO conversion and CL selectwity over PtZ5(In)-500H. PtZ5(In)-500H. The selectivity to CL was constant throughout time on stream of 10 h. The CHO conversion gradually decreases with time on stream, but the deactivation coefficient calculated from Figure 4 is 1.29 • 10"6s. This value lS about two orders of
436 magnitude less than the aluminosilicate shown in Table 1. When CHO rearrangement was carded out over ruthenium or palladium modified indiosilicate under the same reaction conditions as shown in Figure 4, the CHO conversion and the selectivity to CL were almost same as the results shown in Figure 4. These results indicate that the precious metals with oxidation activity are effective to reduce the coke deposited on the metallosilicate surface. It is well known that an alcohol is easily dehydrated over acid catalysts. In this reaction system, since the metallosilicates with strong acid sites were used as the catalysts, the dehydration of n-propanol used as the diluent would occur. However, an ether or an olefin which would be produced by the dehydration was not detected by a gas chromatograph in this study, because the production rate would be too small to be measured. When CHO mixed with a small amount of water was fed into the reactor the catalyst deactivation rate decreased up to 0.3 wt% of water. This result suggests that water produced from n-propanol should play an important role in decreasing catalyst deactivation. The pentasil type indiosilicate with high silica ratio modified with precious metals could be effective catalyst for the vapor phase Beckmann rearrangement of CHO. Furthermore, when n-propanol and carbon dioxide were used as the diluent solvent and diluent gas, respectively, the catalyst deactivation rate was significantly depressed. The further investigations are required to develop a high CL selective catalyst with long catalyst life. REFERENCES
1. 2.
H. Sato, K. Hirose and M. Kitamura, Nippon Kagaku Kaishi, 1989 (1989) 548. M. Kitamura and H. Ichihashi, Prep. Acid-Base Catalysis II, The Organizing Committee of International Symposium Acid-Base, Sapporo (1993) p. 217. 3. H. Sato, N. Ishii, K. Hirose and S. Nakamura, Proc. 7th Int. Zeolite Conf., Y. Murakami, A. Iijima and J.W. Ward eds., Elsevier, Amsterdam, 1986, p. 755. 4. T. Takahashi, K. Ueno and T. Kai, Can. J. Chem. Eng., 69 (1991) 1096. 5. T. Takahashi, M. Nishi, Y. Tagawa and T. Kai, Microporous Materials, 3 (1995) 467. 6. J.S. Reddy, R. Ravishankar, S. Sivasanker and P. Ratnasamy, Catal. Lett., 17 (1993) 139. 7. K. Miura, T. Komatsu, S. Namba and T. Yashima, Prep. 64th Autumn Meeting of Chem. Soc. Japan (1992) p. 477. 8. H. Sato, S. Hasebe, H. Sakurai, K. Urabe and Y. Izumi, Appl. Catal., 29 (1987) 107. 9. H. Sato, K. Urabe and Y. Izumi, J. Catal., 102 (1986) 99. 10. T. Takahashi and X.Y. Yun, Research Report of Faculty of Engineering, Kagoshima University No. 26 (1984) 119.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
437
IR studies on the reduction of nitric oxide with a m m o n i a over MFI-ferrisilicate
T. Komatsu, M.A. Uddin and T. Yashima D e p a r t m e n t of C h e m i s t r y , Tokyo I n s t i t u t e of Technology, Ookayama, Meguro-ku, Tokyo 152 Japan
IR studies were c a r r i e d out on H - f e r r i s i l i c a t e for the c a t a l y t i c r e d u c t i o n of nitric oxide with a m m o n i a in the p r e s e n c e of oxygen. Adsorbed NO 2 or n i t r a t e species was found to be the r e a c t i o n i n t e r m e d i a t e to form n i t r o g e n through the r e a c t i o n with adsorbed a m m o n i a species.
1. INTRODUCTION M e t a l c a t i o n s c o n s t i t u t i n g the f r a m e w o r k of m e t a l l o s i l i c a t e s are e x p e c t e d to have unique c a t a l y t i c p r o p e r t i e s b e c a u s e unlike pure m e t a l oxide t h e y have a t e t r a h e d r a l c o o r d i n a t i o n with [SiO 4] t e t r a h e d r a and are s e p a r a t e d from e a c h other. For f e r r i s i l i c a t e with MFI s t r u c t u r e , we have r e p o r t e d its unique c a t a lytic p r o p e r t i e s for the oxidation of CO [1,2] and the o x i d a t i v e d e h y d r o g e n a t i o n of alkanes [3,4] as c o m p a r e d with s u p p o r t e d iron oxide and F e 3 + - e x c h a n g e d ZSM-5. In this work, for the f u r t h e r u n d e r s t a n d i n g of the c a t a l y t i c p r o p e r t i e s of Fe 3+ in f e r r i s i l i c a t e , we applied f e r r i s i l i c a t e to the s e l e c t i v e c a t a l y t i c r e d u c t i o n (SCR) of nitric oxide with a m m o n i a in the p r e s e n c e of oxygen. R e c e n t l y , Fe ions in z e o l i t e s have been studied for this r e a c t i o n using F e - e x c h a n g e d Y zeolites [5]. We studied here on the r e a c t i o n i n t e r m e d i a t e in the SCR of NO with a m m o n i a on M F I - f e r r i s i l i c a t e , which we have found to be a c t i v e and s e l e c t i v e for this r e a c t i o n [6]. 2. EXPERIMENTAL M F I - f e r r i s i l i c a t e was p r e p a r e d by a usual h y d r o t h e r m a l synthesis [2] S i / F e a t o m i c r a t i o of 44 and e x c h a n g e d with an aqueous solution of followed by the c a l c i n a t i o n at 773 K in air to obtain H - f e r r i s i l i c a t e . e x c h a n g e d ZSM-5 (Fe-ZSM-5) was p r e p a r e d by an i o n - e x c h a n g e m e t h o d aqueous solution of FeC13 (Si/AI=29, Si/Fe=315). Iron oxide supported
to have NH4NO 3 Fe 3+with an on MFI-
438
Table 1 Reduction of nitric oxide with ammonia. Catalyst
NO conversion/%
NH 3 conversion/%
N 2 yield/%
56.3 76.5 -10.7 29.9
60.4 87.9 52.6 31.3
58.8 83.2 22.8 28.5
ill
H-ferrisilicate Fe-ZSM-5 FeOx/Sil H-ZSM-5 i
.ll
i,
React i o n t e m p e r a t u r e : 773 K. C a t a l y s t weight: 0.10 g (H-ferrisilicate and FeOx/Sil) , 0.70 g (Fe-ZSM-5 and H-ZSM-5). R e a c t a n t : NO (0.10%), NH 3 (0.10%) and 0 2 (2.0%) in helium. Total flow rate: 500 ml min -1.
silicalite (FeOx/Sil) was prepared by impregnating H-ZSM-5 with an aqueous solution of Fe(NO3) 3 followed by the calcination at 773 K in air. IR s p e c t r a were obtained at room t e m p e r a t u r e with self-supporting sample wafers of 11 mg cm -2 thickness placed in a quartz vacuum cell. Samples were e v a c u a t e d at 773 K for 2 h before the IR m e a s u r e m e n t .
3. R I ~ U L T S AND DISCUSSION 3.1. SCR of nitric oxide on H-ferrisilicate The c a t a l y t i c a c ti v i ty and s e l e c t i v i t y of H-ferrisilicate for SCR of nitric oxide with ammonia in the presence of excess oxygen were studied using a conventional fixed-bed flow-type r e a c t o r under atmospheric pressure co m p ar ed with those of Fe-ZSM-5, FeOx/Sil and H-ZSM-5. Table 1 shows the results obtained at 773 K a f t e r 1 h on stream. Iron co n t en t s of the c a t a l y s t s ex cep t for H-ZSM-5 was adjusted to be the same by changing the c a t a l y s t weight. Nitrogen was produced on all the catalysts. Though H-ferrisilicate gave lower a c t i v i t y than Fe-ZSM-5, the difference b e t w een conversions of NO and NH 3 on H - f e r r i s i l i c a t e was smaller than that on Fe-ZSM-5, indicating the high sel ect i v i ty of H - f e r r i s i l i c a t e for the formation of nitrogen. In the case of FeOx/Sil, however, the NO conversion was apparently negative, which resulted from the
formation of NO through the r e a c t i o n b e t w e e n NH 3 and 0 2 [6]. Kinetic studies were carried out at 723 K with 0.01 g of H - f e r r i s i l i c a t e to know the r e a c t i o n order with r e s p e c t to the partial pressures of the r e a c t a n t s . The r a t e of N 2 formation was found to be ca. 0.8 order with NO, ca. 0.3 order with 0 2 and nearly zero order with NH 3. This suggests that the r a t e - d e t e r m i n ing step involves a r e a c ti o n b e t w e e n NO and atomic oxygen. 3.2. Adsorption of NO or NH 3 The kinetic result that the r a t e of N 2 formation was zero-order with the pressure of ammonia suggests that ammonia is strongly adsorbed on the surface
439
of H - f e r r i s i l i c a t e under the r e a c t i o n conditions. T h e r e f o r e , IR s p e c t r a of adsorbed NO w e r e t a k e n first w i t h o u t i n t r o duction of a m m o n i a to o b s e r v e weakly adsorbed species origin a t e d from NO. F i g u r e 1, a shows the IR d i f f e r e n c e spectrum obtained after H-ferrisili-
1810 1843 I 1868
1
(a)
c a t e was exposed to NO (20 Torr) at 298 K for 5 min. (b) Four absorption bands were o b s e r v e d at 1810, 1843, 1868 and 1915 cm-lo A f t e r 4 h of t h e NO exposure (b), 1810 and (c) 1915 cm-1 bands i n c r e a s e d in (d) t h e i r i n t e n s i t y , while 1843 and I I I I I 1868 cm -1 bands did not 2000 1800 1600 change significantly. There e x i s t e d a linear relationship W a v e n u m b e r / cm -1 b e t w e e n t h e i n t e n s i t i e s of the f o r m e r bands during prolonged Figure 1. IR s p e c t r a of H - f e r r i s i l i c a t e adsorption, which indicates a f t e r NO (20 Torr) a d s o r p t i o n at 298 K t h a t t h e s e bands are a t t r i b u t e d for 5 min (a) and 4 h (b) and subsequent to a s y m m e t r i c and s y m m e t r i c e v a c u a t i o n at 298 K for 10 min (c) and at s t r e t c h i n g modes of a dinitro573 K for 30 rain (d). syl species. We may assign t h e o t h e r two bands at 1843 and 1868 cm -1 to m o n o n i t r o s y l species. Similar bands have been r e p o r t e d [7] for the NO adsorbed on F e - Y z e o l i t e . When NO was p u m p e d out at 298 K for 10 min (c), t h e i n t e n s i t i e s of the bands at 1810 and 1915 cm -1 d e c r e a s e d and a new band at 1766 cm -1 appeared. The new band still r e m a i n e d a f t e r the e v a c u a t i o n at 573 K for 30 min (d), while the dinitrosyl bands t o t a l l y disappeared. The m o n o n i t r o s y l bands w e r e r e l a t i v e l y s t r o n g a f t e r the e v a c u a t i o n . The new band at 1766 cm -1 might be due to a m o n o n i t r o s y l species f o r m e d from t h e dinitrosyl species.
3.3. Adsorption of NO-NH 3 In order to know the r e a c t i o n i n t e r m e d i a t e for t h e NO-NH3-O 2 r e a c t i o n , t h e r e a c t i v i t i e s of the adsorbed species w e r e studied. A f t e r the adsorption of NO (20 Torr) at 298 K for 1 h (Fig. 2, a), 1 Torr of a m m o n i a was i n t r o d u c e d at 298 K. As shown in Fig. 2, b, 5 min of the a m m o n i a exposure gave bands at 1465, 1620, 1766, 1830 and 1930 cm-1. The 1465 and 1620 cm-1 bands are due to the adsorbed a m m o n i a on Brdnsted and Lewis acid sites, r e s p e c t i v e l y . The r a t i o of the i n t e n s i t i e s of 1930 to 1830 cm -1 bands was a l m o s t the s a m e as t h a t of t h e
440 two dinitrosyl bands (1915 to 1815 cm -1 bands). Therefore, it is indicated that the 1930 and 1830 cm-1 bands result from the shift of the dinitrosyl bands. This shift may be caused by the interaction of NH4 + ions with framework iron as an adsorption site for the dinitrosyl species. The subsequent evacuation at 298 K for 30 rain (c) eliminated the 1830 and 1930 cm -1 bands, increased the 1766 cm-I band intensity, and generated a band at 1843 c m - 1 These two bands were already assigned to the mononitrosyl species. These results suggest that the mono- and dinitrosyl species do not r e a c t with ammonia in the absence of oxygen.
3.4. Adsorption of NO-O2-NH 3
1810
1915
(a) ii 1830 1465 1766
~ .X
1620
O
(b)
<
I
i
I
i
2000
1800
1600
1400
_
(c)
Wavenumber / cm -1 Figure 2. IR spectra of H-ferrisilicate after NO (20 Torr) adsorption at 298 K for 1 h (a), following c o n t a c t with NH 3 (1 Torr) at 298 K for 5 min (b), and following evacuation at 298 K for 30 min (c).
The reactivity of the nitrosyl species with ammonia was examined in the presence of oxygen. Figure 3, a shows the IR difference spectrum of H-ferrisilicate before and after the exposure to NO (20 Torr) at 298 K for 1 h followed by an evacuation at 298 K for 1 h. The spectrum was essentially the same as that of Fig. 1, c, though the band intensity was slightly higher for the latter because of the longer adsorption period. When 0 2 (20 Torr) was introduced at 298 K for 10 min (b), the mono- and dinitrosyl bands decreased in their intensity and small bands appeared at 1576 and 1616 cm-lo The band at 1616 cm -1 may be assigned to NO 2 adsorbed species since its wavenumber is close to the s y m m e t r i c stretching frequency of gaseous NO 2 (1620 cm-1). The other band at 1576 cm -1 may be assigned to a nitrate species. We have found a similar band at 1630 cm -1 in the case of Cu2+-exchanged ZSM-5 exposed to the mixture of NO and 0 2 [8]. Using 15NO and 1802, the 1630 cm -1 band was assigned as NO 3 species, which was found to be the reaction i n t e r m e d i a t e for NO-NH3-O 2 r e a c tion on Cu-ZSM-5. As shown in Fig. 3, e, the nitrosyl bands disappeared after 12 h of the c o n t a c t with 0 2. When 1 Torr of ammonia was introduced after an evacuation
441
at 298 K (f), the two bands at 1576 and 1616 cm -1 i m m e d i ately disappeared and new bands arose at 1465, 1620 and 1700 cm-1 corresponding to the adsorbed a m m o n i a species. These results indicate that both NO 2 and n i t r a t e species r e a c t with a m m o n i a probably to form n i t r o g e n and w a t e r . It could be concluded t h a t the SCR of nitric oxide with a m m o n i a occurs through the f o r m a t i o n of adsorbed NO 2 or n i t r a t e species from NO and 0 2 followed by the r e a c t i o n of these adsorbed species with ammonia. It is c l e a r from Fig. 3 t h a t the f o r m a t i o n of the r e a c t i v e NO 2 or n i t r a t e species p r o c e e d e d much slowly than t h e i r subsequent r e a c t i o n with ammonia, which implies t h a t the f o r m e r r e a c t i o n is the r a t e - d e t e r m i n i n g step. This coincided with the kinetic r e s u l t s m e n t i o n e d above. REFERENCES
(a) (b)
(c) (d) 9 it)
<
(e) ~65
I 2000
I 1800
' 1600
I 1400
W a v e n u m b e r / cm -1 Figure 3. IR s p e c t r a of H - f e r r i s i l i c a t e a f t e r NO (20 Torr) was adsorbed and evac u a t e d both at 298 K for 1 h (a), following c o n t a c t with 0 2 (20 Torr) at 298 K for 10 min (b), 1 h (c), 2 h (d) and 12 h followed by e v a c u a t i o n at 298 K (e), and a f t e r c o n t a c t with NH 3 (1 Torr) at 298 K for 3 min (f).
1.
M.A. Uddin, T. K o m a t s u and T. Yashima, Microporous Mater., 1 (1993) 201.
2. 3. 4. 5.
M.A. Uddin, T. Komatsu and T. Yashima, ]. Catal., 146 (1994) 468. M.A. Uddin, T. Komatsu and T. Yashima, Chem. Lett., (1993) 1037. M.A. Uddin, T. Komatsu and T. Yashima, J. Catal., 150 (1994) 439. M.D. Amiridis, F. Puglisi, J.A. Dumesic. W.S. Millman and N-Y. Tops~be, J. Catal., 142 (1993) 572. M.A. Uddin, T. Komatsu and T. Yashima, submitted to J. Chem. Soc., Faraday Trans.. K. Segawa, Y. Chen, J.E. Kubsh, W.N. Delgass, J.A. Dumesic and W.K. Hall, J. Catal., 76 (1982) 112. T. Komatsu, T. Ogawa and T. Yashima, submitted to J. Phys. Chem..
6. 7. 8.
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Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
443
T h e R o l e of the N a and of the Ti on the Synthesis of E T S - 4 M o l e c u l a r Sieve Pierantonia De Luca a, Steven Kuznicki b and Alfonso Nastro a aDepartment of Chemical Engineering and Materials, University of Calabria, Arcavacata di Rende, 87030 Rende (CS), Italy bEngelhard Corp., Iselin New Jersey 08830-0770, USA
SUMMARY In this paper the synthesis, the crystallisation kinetics and the characterization of Titanium molecular sieve ETS-4 are reported. This ETS-4 material has an open structure, with tetrahedral and octahedral primary building units. The effect of varying of the single reagents amount in the reaction mixture on kinetic parameters, on gel preparation and on properties of the final products is discussed. INTRODUCTION In 1967 Young reported that the synthesis of charge bearing titanium silicates can be obtained under reaction conditions similar to aluminosilicate zeolite formation. In 1972 a naturally occurring alkaline titanosilicate identified as Zorite was discovered in the Siberian Tundra. While these materials were called titanium zeolites, no further reports on titanium silicates appeared in the open literature until 1983, when traces of tetrahedral Ti(IV) were reported in a analogue ZSM-5. The object of this research is to report the synthesis of the ETS-4 crystalline titanium silicates molecular sieve, discussing the effect of the individual chemicals in the formation of these crystalline materials. EXPERIMENTAL The general batch composition is xNa20-yTiO2-zHC1-4SiO 2-11H20 where 1.5 < x < 5.0; 0.2 < y < 1.6; z > 1.28x. The reagents used to prepare the initial gel were: alkaline sodium silicate solution (8% Na20; 27% Sit2; 65% H20), NaOH (50% wt), TiC14 (50% wt), HC1 (37% wt) and distilled water. The synthesis runs were carried out at 190~ The reaction batches were prepared mixing together an alkaline solution, sodium silicate, sodium hydroxide solution and water, with a mixture of titaniumtetrachloride, hydrochloric acid and water. The products of reaction were filtered, washed with distilled water, dried at 100~ overnight and characterised with a variety of techniques including XRD and SEM.
444
RESULTS
AND
DISCUSSION
The figure 1 shows the crystalline phases obtained varying the amount of TiO 2 and Na20 in the batch composition. The figure 2 shows the relative crystaUinity versus the reaction time in the systems with different amount of TiO2.
1.4 O
1.0
/ . / ~
O r.z.1
AMOR
ETS(pH =14.9)
0.6
O 0.2 2.0
3.0
4.0
5.0
MOLES OF Na20 Figure 1. Crystalline phases obtained from the system xNa20-yTiO2-zHCI-4SiO2110 H20 with z>_1.28x, reaction time 4 days, reaction temperature 190~ 100 m
80
w
:z;
60
,-.1
4o
ro
20
0
"
20
40 TIME, HOURS
60
80
l0 0
Figure 2. Values of the relative crystallinity versus the reaction time from the systems with 3.5 moles of Na20 and with different amount of TiO2.1.2 TiO2 (e), 1.0 TiO2 (ira), 0.8 TiO2 (A) and 0.4 TiO2 (~)
445
CONCLUSIONS - The synthesis of ETS-4 is influenced by the pH of the reaction batch. - The crystallisation domain of ETS-4 is also function of the Na/Si and Si/Ti ratios in the initial batch composition. - The relative crystallinity does not follow a sigmoid course, but a straight line. - The crystallisation of ETS-4 cannot be considered an autocatalytic reaction.
REFERENCES 1. 2. 3. 4.
Young, US Patent No. 3, 329,481 (1967). P.A. Sandomirskii and N.Y. Belov, Sov. Phys. Crystallogr., 24(6) (1979). G. Perego et al., Proc. 7th Int. Zeolite Conf. (1986) 129. S.M. Kuznicki, US Patent No. 4, 938,939 (1990).
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Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
447
On the potential of zeolites to catalyse aromatic acylation with carboxylic acids E.A. Gunnewegh, R.S. Downing and H. van Bekkum Laboratory Julianalaan
of Organic Chemistry 1 3 6 , 2 6 2 8 B L Delft
and Catalysis, Delft University
of Technology,
SUMMARY The intramolecular acylation of 4-phenylbutyric acid was investigated as a model reaction for Friedel-Crafts acylation catalysed by zeolites in the liquid phase. This reaction is catalysed by zeolite HBeta in 4-chlorotoluene as solvent. The catalytic ability of H-Beta was demonstrated by the fact that at reflux temperature the total turnover number (TON) was found to be 35. However, the acylation of toluene or butylbenzene with carboxylic acids was slow; the unbalanced adsorption equilibrium between the two reactants on the H-Beta (Si/AI= 12) may be a contributing factor in this case. The acylation of anisole by carboxylic acids or acid anhydrides however, was readily catalysed by zeolite H-Beta. 1. I N T R O D U C T I O N The synthesis of aromatic ketones is an important process in several areas of the fine chemicals industry. This is recognised in the fact that many synthetic fragrances of the musk type contain an acetyl group. Also for the synthesis of various pharmaceuticals an aromatic acylation step is involved. A major drawback of conventional catalysts such as metal chlorides for Friedel-Crafts acylation is that they are non-regenerable and used in more than stoichiometrical amounts. The hydrolysis of the complexes formed between the aromatic ketone products and the catalysts results in highly corrosive waste streams, the disposal of which leads to environmental problems. In recent years considerable attention has been paid to the potential of zeolites to act as heterogeneous and re-usable catalysts in many organic reactions [1]. In 1986 it was reported that lanthanide-exchanged zeolites, particularly CeY, catalyse the acylation of toluene by various carboxylic acids, which are most attractive as acylating reagents from an environmental point of view, at high reaction temperatures (150 - 200 ~ in an autoclave reactor [2,3,4].The water released in this reaction may be expected to deactivate the hydrophilic zeolite Y and the catalyst/reactant ratio was relatively high (0.3 g zeolite/mmol reactant), which means that high turnover numbers cannot be obtained. However, recent years have seen little further progress in the field of Friedel-Crafts acylation of aromatics with aliphatic carboxylic acids in the liquid phase. Most recent reports deal with the acylation of more reactive substrates, such as anisole [5], 2-methoxynaphthalene [6], thiophene [7] and benzofuran [6], using acid anhydrides [6,8] or acyl chlorides [5,7,9]as acylating reagents. Benzoic acids were applied to acylate resorcinol [10], toluene [11] and 1-naphthol [12] using H-Beta and
448
H-MCM-41, respectively, as the catalysts. In this paper we report on the intramolecular Friedel-Crafts acylation of 4phenylbutyric acid catalysed by zeolite H-Beta in an open reaction system. This model reaction is compared with the acylation of other substrates with carboxylic acids and acid anhydrides under the same reaction conditions. Moreover, the competitive adsorption of the reactants by the zeolite is studied. O *"
4-phenylbutyric acid
4-
o-tetralone
HRO
0
2. EXPERIMENTAL
2.1 Chemicals All chemicals were used as received from Aldrich or Janssen Chimica.
2.2 Zeolite H-Beta Zeolite H-Beta (Si/A1 = 12, determined by ICP-AES/AAS) was synthesised according to the Wadlinger procedure [13]. After calcination of the as-synthesised Beta twice at 540 ~ in order to completely remove the template, the zeolite was ionexchanged three times in a 1 M CH3COONH 4 solution in water. The resulting NH 4Beta was converted to the active H-Beta at 450 ~ The zeolite was characterised by X-ray diffraction.
2.3 Reaction procedure All reactions were carried out in a thermostatted liquid-phase reactor equipped with a reflux condenser and magnetic stirrer. A typical reaction run was as follows: 30 mmol 4-phenylbutyric acid ( 5 . 0 g ) a n d 1 g nitrobenzene (internal standard) were stirred in 40 ml chlorotoluene. To the hot reaction mixture 0.38 g activated H-Beta was added. Samples were taken periodically and analysed by GC on a CP Sil 5 CB column and on a VG 70SE mass spectrometer using 70 eV as ionisation energy.
2.4 Adsorption procedure Adsorption experiments were carried out at room temperature in a reaction vessel equipped with calcium chloride tube for protection against humidity. A bulky solvent and internal standard, incapable of adsorption by the zeolite, were used. To 15 ml 1,3,5-tri-isopropylbenzene were added 11 mmol toluene, 11 mmol octanoic acid and 4 mmol 1,3,5-tri-t-butylbenzene (internal standard). At t=0, 0.75 g activated zeolite HBeta was added to the solution. Samples were taken periodically and analysed by GC on a CP SIL 5 CB column.
449 3. RESULTS AND DISCUSSION
3.1 Intramolecular acylation of 4-phenylbutyric acid In Figure 1 the results of the intramolecular acylation of 4-phenylbutyric acid to ~-tetralone are presented. For the reaction carried out at 166 ~ in the liquid phase it 70 60
g ~
+
+
50
4o
-.~
:
o
2O 10
O0
0
-
i
25
50
75
I
100
i
125
t (h)
Figure 1. Effect of the temperature in the intramolecular acylation of 30 mmol 4-phenylbutyric acid to a-tetralone catalysed by 0.38 g H-Beta in 40 ml chlorotoluene; (o) 138 ~ (A) 150 ~ (+) 166 ~ was found that TON = 35, assuming each H + to represent a catalytic site. The ability of zeolite H-Beta to activate an aliphatic carboxylic group for acylation of an aromatic moiety is clearly demonstrated by this model reaction. A considerable temperature effect exists. After 50 h the reaction rate becomes very low, probably due to the formation of coke, deactivating the catalytic sites in the zeolite. Under the same conditions the reaction of toluene, o-xylene or butylbenzene with octanoic acid was investigated. However, the conversion of these substrates with the acid to the corresponding aromatic ketones was very low (2-4 %). Competitive adsorption experiments of the substrates toluene or butylbenzene together with octanoic acid reveal that the affinity of zeolite H-Beta towards the acid is much higher than the affinity towards the aromatic substrates. In Figure 2 the co-adsorption of octanoic acid and toluene in zeolite H-Beta from the non-adsorbing solvent 1,3,5-triisopropylbenzene is presented. Less than 0.01 mg toluene / mg H-Beta co-adsorbs in the zeolite, whereas the adsorption of the acid is 0.22 m g / mg H-Beta. As is shown in Table 1, for the competitive adsorption of octanoic acid and butylbenzene it was found that less than 0.01 mg butylbenzene / mg H-Beta and 0.26 mg octanoic acid / mg H-Beta were adsorbed. From this it can be concluded that, though the zeolite is
450
~"
0.30 t
~,
cctsnolo sold
+
0.20
++
F
g
o.~o
0.00
0
4
8
12
16
20
t (rain)
Figure 2. The competitive adsorption of equimolar amounts of octanoic acid and toluene in H-Beta using 1,3,5-tri-isopropylbenzeneas solvent capable of activating the carboxylic acid, formation of the aromatic ketone is slow due to the intrinsic low reactivity of toluene and butylbenzene and to the unfavourable, i.e. unbalanced, adsorption equilibrium of the acylating reagent and the substrate, as contrasted with the intramolecular acylation where substrate and acylating reagent are obviously present in equal amounts and moreover in close proximity.
Table 1. Co-adsorption (mg / mg H-Beta) of octanoic acid and aromatic substrates into H-Beta from 1, 3,5-tri-isopropylbenzene octanoic acid toluene butylbenzene anisole
0.22 O.26 0.25
aromatic substrate < 0.01 < O.01 0.04
It may be noted that the adsorption of the aromatic hydrocarbon substrate may be enhanced by (i) applying an excess of substrate and (ii) by applying a higher Si/A1 ratio. A test of the latter method is in progress. 3.2 Acylation of anisole with a carboxylic acid or an acid anhydride In another series of experiments anisole, an electronically activated substrate, was acylated over H-Beta using carboxylic acids and acid anhydrides as acylating reagents. The results are presented in Table 2. The main product of the reaction of anisole with a carboxylic acid or an acid anhydride is the para-acylated anisole (1). Side products are the phenyl ester (2) and acylated phenol (3). Also small amounts of cresols and methyl esters were detected. The acylation with the long chain octanoic acid proceeds more readily than the acylation with the short chain acetic acid, a phenomenon which
451
OH,O~
)
o
R
1
2
3
Table 2. Yields (mmol) of the Friedel-Crafts acylation of anisole (40 ml: substrate = solvent) with carboxylic acids (30 mmol) or acid anhydrides (30 mmol) catalysed by zeolite H-Beta (0.35 g)at 155 ~ The reaction time varied from 40- 70 h. Acylation with RCO-OH
Acylation with (RCO-)20
R
1
2
3
R
1
2
CH 3 CH(CH3) 2 C7H 15 C6H 5
3 13 14 5
1 3 3 7
< 1 < 1 - (*)
CH 3 CH(CH3) 2 C6H 5
23 38 23
< 1 2 4
(*)
3
-
(*)
< < 1 mmol of the ortho-acylated anisole was detected
was encountered earlier in the acylation of toluene [2]. In the reaction with carboxylic acid the amount of phenyl ester (2) and acylphenol (3) formed was substantially higher than in the reaction with acid anhydride. The detection of cresols and methyl esters is indicative of two side reactions leading to the formation of phenol: (a) transalkylation of anisole leading to cresol and phenol and (b) reaction of anisole with a carboxylic acid leading to a methyl ester and phenol, the latter being the most important for the production of phenol in the reaction mixture. The phenyl ester (2) is probably formed from carboxylic acid and phenol. The ketone 3 might be formed by Friedel-Crafts acylation of phenol. However, when phenyl octanoate (2) was reacted in anisole in the presence of H-Beta both 1 and 3 were formed, indicating that 3 may also be formed from 2, by the Fries-rearrangement. Upon reacting anisole with an acid anhydride the p-acylated anisole is formed first, accompanied by the production of the carboxylic acid. No methyl ester is detected until all the acid anhydride has reacted with anisole. Then side reaction (b) leads to the production of phenol, subsequently leading to compound 2. These findings are illustrated for the reaction of anisole with isobutyric anhydride in Figure 3: the reaction of anisole with isobutyric anhydride is completed within 2 hours, yielding 1 with practically 100 % selectivity. Then, anisole both reacts slowly further with the formed isobutyric acid to 1 and undergoes side reaction (b), leading to the formation of 2 and a slow decrease of selectivity to 1. The acylation of anisole with an acid anhydride was found to be much more rapid and selective than the acylation with a carboxylic acid.
452
80
100
t
>
40
80 A
ar
O
60
E E
v
"O
Q
40
o o m
20
< A
0
"
10
20
,"
m
,~ ,
m
40
$0
A,
IA 50
2 ,
,
e0
A
0
70
t (h)
Figure 3. Course of the reaction of anisole (40 ml) with isobutyric anhydride (30 mmol) catalysed by H-Beta at 155 ~ (o) selectivity to 1 4. C O N C L U S I O N Zeolite H-Beta (Si/A1 = 12) is able to activate aliphatic carboxylic acids in the Friedel-Crafts acylation of an aromatic substrate. However, acylation of non-activated aromatic compounds is difficult, the unbalanced adsorption equilibrium possibly reinforcing the intrinsic low reactivity. For the acylation of anisole, an example of an activated substrate, acid anhydrides are more suitable acylating reagents due to the absence of side reactions, as contrasted with acylation using carboxylic acids. ACKNOWLEDGEMENT This work was financially supported by the Dutch Innovation Oriented Program on Catalysis.
Research
REFERENCES 1. W.F. H61derich and H. van Bekkum, Stud. Surf. Sci. Catal., 58 (1991) 664 2. B. Chiche, A. Finiels, C. Gauthier, P. Geneste, J. Graille and D. Pioch, J. Org. Chem., 51 (1986) 2128 3. B. Chiche, A. Finiels, C. Gauthier and P. Geneste, Appl. Catal., 30 (1987) 365 4. C. Gauthier, B. Chiche, A. Finiels and P. Geneste, J. Mol. Catal., 50 (1989) 219 5. A. Corma, M.J. Climent, H. Garcia and J. Primo, Appl. Catal., 49 (1989) 109 6. G. Harvey and G. M~der, Collect. Czech. Chem. Commun., 57 (1991) 862 7. A. Finiels, A. Calmettes, P. Geneste, and P. Moreau, Stud. Surf. Sci. Catal., 78 (1993) 595 8. F. Richard, J. Drouillard, H. Carreyre, J.L. Lemberton and G. P~rot, Stud. Surf. Sci. Catal., 78 (1993) 601 9. D.E. Akporiage, K. Daasvatn, J. Stolberg and M. St6cker, Sud. Surf. Sci. Catal., 78 (1993) 521 10. A.J. Hoefnagel and H. van Bekkum, Appl. Catal., A97 (1993) 87 11. J-P. Bourgogne, C. Aspisi, K. Ou, P. Geneste, R. Durand and S. Mseddi, Fr. Pat. Appl. 90.11856 (1992), to PLASTO S.A. 12. H. van Bekkum, A.J. Hoefnagel, M.A. van Koten, E.A. Gunnewegh, A.H.G. Vogt and H.W. van Kouwenhoven, Stud. Surf. Sci. Catal., 83 (1994) 379 13. R.L. Wadlinger, G.T. Kerr and E.J. Rosinski, US Patent, 3,308,069(1967)
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
453
Elimination of methanol from dimethylacetal over aluminophosphate molecular sieves and zeolites Sze-Ming Yang and Kuo-Jenn Wang D e p a r t m e n t of Chemical Engineering, National Central University Chung-Li, Taiwan Abstract The formation of methyl vinyl ether from methanol elimination of dimethylacetal is highly selective over aluminophosphate molecular sieves and zeolites. The conversions over medium and small pore weakly acidic a l u m i n o p h o s p h a t e molecular sieves are high and the deactivations are slow. Introduction Methyl vinyl ether (MVE) is an important reagent for Diels-Alder and polymerization reactions. MVE can also be used as modifier for paint, polystyrene and ion exchange resin, and as plasticizer for nitrocellulose and a d h e s i v e s . MVE can be obtained from methanol elimination of dimethylacetal (DMA). Traditional homogeneous catalyst will cause polution problem and the lifes of phosphate and c a r b o n a t e salts are short and selectivities are low. Holderich et al [1-3] studied the reaction over zeolites such as pentasil, mordenite and SAPO-5 and found mildly acidic or basic zeolites are more selective than strongly acidic zeolites. Alum i n o p h o s p h a t e molecular sieves are mildly acidic. In this study we report the effects of acidity, pore size, reaction conditions on elimination of methanol from DMA over aluminophosphate molecular sieves and zeolites. Experimental A l u m i n o p h o s p h a t e molecular sieves, A1PO4-5, SAPO-5, A1PO4-11, SAPO-11, and SAPO-34 were synthesized according to the procedures similar to those reported in the literature [4]. Tetraethylorthosilicate was used as the silicon source and triethylamine, dipropylamine and isopropylamine were used as templates for the synthesis of SAPO-5, SAPO-11 and SAPO-34, respectively. The molar ratio of Si:AI:P in mother liquid = 0.1:1:1, 0.1:1:1, 0.3:1:1 for SAPO-5, SAPO-11 and SAPO-34. ZSM-5(Si/Al=27.5), mordenite ( S i / A l = 5 ) , beta ( S i / A l = 1 2 . 5 ) , Y (Si/A1=2.43) were obtained from Union Chemical Lab, ITRT. The stuctures of molecular sieves and zeolites after calcination were identified by XRD p a t t e r n s obtained from Siemens DS00 X-ray diffractometer. ~gSi NMR spectra were recorded on a Bruker MSL 200 spectrometer. Elemental analyses were obtained from a Kontron Plasmakon S-35 inductively coupled plasma atomic emission spectrophotometer. DMA in the saturator was carried to the reactor by a stream of nitrogen, the
454 products were analyzed by an online GC equiped with a two meter long 1/8" o.d. poropack Q column and a FID detector. Results and Discussion Characterization of SAPO-5, SAPO-11 and SAPO-34 X-ray diffraction patterns agree with those reported in the literature [4]. The molar ratios of Si:AI:P analyzed by ICP together with those in mother liquids are listed in table 1. 29Si MAS NMR spectra show a peak at 93.3 ppm for SAPO-5, 93 ppm for SAPO-11 and two peaks at 90 ppm and 93 ppm for SAPO-34. The results indicate that some silicon atoms substituted phosphor atoms into the framework structure and forming Si-O-A1 bond. Table 1 The results of elemental analysis
Si:AI:P (mother liquid) Si'AI:P (ICP)
SAPO-5
SAPO-11
0.1"1"1
0.1"1"1
0.089:1.098"1
0.077:1.095"1
SAPO-34 0.3"1:1 0.178"1.26"1
Elimination of methanol from dimethylacetal The selectivities of DMA over molecular sieves and zeolites at 200~ W H S V = 27hr -1 under atmospheric pressure approach 100%. Figure 1 shows the activities of various molecular sieves and zeolites after three hours on stream. The conversions over SAPO-34 and A1PO4-11 are above 70%, and the deactivation is slow. The initial activity of A1PO4-5 is also high, but the deactivation is fast. The activities of HZSM-5, H~, NaB, NaY, NaM are low. The activities of small pore SAPO-34 and weakly acidic medium pore A1PO4-11 are the highest. The large pore NaB, NaM, SAPO-5 and NaY are less active. The effect of pore size, acidity of the zeolites and reaction conditions are discussed in the following sections.
so I-
SAPO-34
AL~,-LL so
Al~,-5
Q
H-Be~ 40
Figure 1. The comparison of activities over aluminophosphate molecular sieves and zeolites after three hours on stream.
455
The effect of reaction temperature The effect of reaction temperature on the initial activities of the zeolites at WHSV = 27 hr -~ is shown in figure 2. Above 250~ the conversions and selectivities reach 100 % for AlPO4-5 and SAPO-5. The conversion over NaM reaches 100% at 300~ whereas the thermal reaction reaches 10% conversion at this temperature. L 00.0
m
~
~ BLeak
ALIO~ mmmmm ALF'04-5 xxxxx
80.0
o o o o o SAeO-5 AAAAA ~-ZSI15
~~
Na-llordenite Na-Se~a
60.0 Q o,,,,
= 40.0 r
x
O
20.0 +
0.0
!
t
I
I
1
~
i
100.0 150.0 200.0 250.0 500.0 ,.550.0 4.00
Tesp ( ' O Figure 2. The variation of activity with reaction temperature The effect of space velocity The effect of WHSV on the variation of activity with time on stream is shown in figure 3 for A1PO,-5. The selectivities of MVE reach 100% in both cases. When WHSV was increased to 114 hr -t over ALP04-5, initial deactivation rate increased. A Large amount of coke formed from the polymerization of MVE causes faster deactivation at higher WHSV. The effect of acid strength Large pore molecular sieves, A1PO4-5 and SAPO-5, differ by the acidic strength, both show high selectivities but the deactivation over more acidic SAPO5 is faster (figure 4a). Similar results are also observed over medium pore molecular sieves, A1PO4-11 and SAPO-11 (figure 4b). The conversions over Hfl and NaB are about the same, and the deactivation of acidic HZ is also faster (figure 4c). Milder acidity favors the slower deactivation. Strongly acidic HZSM-5 and basic NaM, NaB are not highly active for the elimination reactions of methanol from DMA. The activities of weakly acidic aluminophosphate molecular sieves are high. The results show that the active site is neither strongly acidic nor basic, probably acid base bifunctional as shown in scheme 1. Furthermore, the deactivation of more acidic molecular sieve is faster. It is attributed to the acid catalyzed polymerization reaction of MVE.
456
,
ALINI6Jj (llg-iiNhr ~ )
9ot aoPi
itmo-S ( l l g - f ~
4 )
70 bq v
60
--I
50
30 20 I0
0
60 I 2 0 1 8 0 2 4 0 3 0 0 3 6 0 4 2 0 4 8 0 5 4 0 6 0 0
Tim Cl[in) Figure 3. The variation of activity with space velocity over AIPO4-5
I
00'
,
90
A
q I Q
.
,
.=
.
00000 ~z~z~
&
7O 60 ! 5O 4O ~ 30 ~ 2O
.
.
I
ALI'O..-S
~_
~
i
.
.
.
.
.
iLIPO.-LL SAIq~-,LL
00000 ~r~r~r~r~r
b
A
I
0~'
0
'
'
I'
I
I
I
I
I
;
I
60 fZO 1 8 0 2 4 0 3 0 0 3 6 0 4 2 0 4 8 0 5 4 0
Tiane Ciin)
'
9
0
I
I
I'
'
I
I
I
I
I
60 I 2 0 1 8 0 2 4 0 3 0 0 3 6 0 4 2 0 4 8 0 5 4 0 6 0 0
T|,,e Ciia)
Figure 4&,b. The effect of acid strength on the activities
457
I00 QOQGQ
90
k-DefA H-&e~
80
v
60
o
50
m I.J
40
> r r
0
3O
,o!
2O
Ol
'
|
o
i
|
|
i
l
'
I
eo 8o 4oaoo3so4zo4 os4o6oo Tim Cllin)
Figure 4c. The effect of acid strength on the activities
j
H ~ s
/
/
u~
~ ~
B-/
' fl§
////I///~///////~HHH//#
QI~OII + gzfl~C \
l
,,
Scheme [ The effect of pore size Comparing the results of the large pore AIPO4-5 and medium pore AIPO4-11 in figure 5a, we find that ALP04-5 is less active than AIPO4-11, and the deactivation is also faster. Both AIPO4-5 and AIPO4-11 are weakly acidic. The effect must come from the different pore size. Similar results are also obtained over mildly acidic SAPO-5 and SAPO-11 as shown in figure 5b. The conversion over small pore SAPO-34 is high and the deactivation is slow. The deactivation is attributed to the polymerization reaction of MVE. The polymerization is more restricted in medium and small pore molecular sieves. High activities and Low deactivations are observed over weakly acidic, medium and small pore aluminophosphate molecular sieves.
458
I00 ,,,
,~!
0 O 0 0 C]
A|lt04"5 smH
70
]
~R
6O
~
5o
§ -'1"§
+-i-
m
~
40
~
30
20
A
1o
ol o
60 1 2 0 1 8 0 2 4 0 3 0 0 3 6 0 4 2 0 4,80 5 4 0
T i ~ Olin)
0
60 f 20180 24,0 300 360 420 480 540 600
Time (Win)
Figure 5. The effect of pore size on the activities Conclusion The selectivities of MVE from methanol elimination of DMA reach 100% over the zeolites studied at 200~ High conversions can be obtained over small and medium pore aluminophosphate molecular sieves. Strong acid sites and large pore sizes facilitate the coke formation and accelerate the deactivation of the catalysts. Neither strongly acidic sites nor basic sites are effective for this reaction. Coorporation of acid and base sites may be needed for this reaction. References 1. W. F. Hoelderich, in S. Yoshida, N. Takezawa and T. Ono ed. "Catalytic Science and Technology" Vol. 1, 31, Kodansha, Tokyo, 1990. 2. W. F. Hoelderich and N. Goetz, DE 37.22.891 BASF AG 1988. 3. W. F. Hoelderich, N. Goetz and L. Rupfer, DE 38.04.162, BASF AG, 1988. 4. B. M. Lok, C. A. Missina, R. L. Patton, R. T. Gajek, T. R. Cannan, E. M. Flanigen, US Patent 4,440,871, 1984.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
459
Deuteration of zeolitic hydroxyl groups in the presence of platinum Evidence for a spillover reaction pathway u. Roland, R. Salzer and L. Stimmchen Technische Universit~it Dresden, Institut for Analytische Chemie, D-01062 Dresden, Germany An overview of our experimental results on the H-D exchange of zeolitic hydroxyl groups in two-component P t ~ a Y - HNaY samples is given. The exchange process at room temperature has been studied by means of infrared spectroscopy. Three different deuteration pathways were taken into account when platinum was present : deuterium spillover, deuteration by deuterium from the gas phase and deuteration by heavy water. The experimental conditions have been modified to distinguish between these possible reaction pathways. The H-D exchange after deuterium loading in the absence of water and heavy water was prevented after CO pre-adsorption and hindered in the presence of a magnetic field directed perpendicularly to the main direction of diffusion (from the P t ~ a Y to the HNaY sample component). Deuteration after addition of heavy water was influenced neither by CO pre-adsorption nor by application of a magnetic field. In the absence of platinum no H-D exchange occurred after deuterium introduction. The results were explained by a reaction pathway via deuterium spillover, i.e. active species were formed on platinum (which is prevented by CO pre-adsorption), and diffuse as electrically-charged species on the zeolitic surface (which is hindered by an external magnetic field). 1. INTRODUCTION Although spillover, i.e. the formation of an active species on one phase and the subsequent diffusion of this species onto another phase, has been accepted as a phenomenon of great scientific interest in heterogeneous catalysis, its importance for technical catalysis has been doubted for a long time. Meanwhile the impact of spillover, especially that of hydrogen and oxygen, for the acceleration of solid state reactions and catalytic reactions has been frequently shown [1]. Particularly, hydrogen spillover plays a crucial role in hydroconversion reactions [2,3], in the lowtemperature reduction of oxides [4], the formation of hydrogen bronzes [5] and the H-D exchange of surface hydroxyl groups [6-9]. Even when the concentration of the spilt-over species is too low to play a significant role as a reactant, the catalytic properties (activity, selectivity, stability) can be markedly influenced by these species. This is due to the indirect actions of the spilt-over species such as formation and stabilization of active sites [1,10], removal of coke and coke precursors [11] and maintaining a distinct oxidation state of the catalyst [12]. In order to investigate the - still not fully clarified - fundamental mechanism of spillover processes (e.g., nature of the active species and their interaction with the. solid, origin of their special activity) technical catalytic reactions and the corresponding catalysts are rarely suitable because of their complexity. Therefore, it
460 is often difficult to define the role and the mode of operation of spillover in such catalytic systems. To overcome these problems model catalysts and model reactions have to be studied. We use a special geometrical disposition of the samples characterized by the macroscopic separation of the component donating the active species (Pt supported on NaY zeolite), and the component where the action of the spilt-over species is observed (HNaY zeolite). Model reactions have to be studied to test the presence of the spilt-over species. In such reactions the spilt-over species should act as reactant and the reaction should take place under the experimental conditions only due to these activated species. Other reaction pathways leading to the same reaction product should be negligible. We chose the H-D exchange of zeolitic hydroxyl groups as a test reaction. In the case of two-component Pt/NaY - HNaY samples the reaction mechanism via deuterium spillover can be described as follows (fig. 1) : 1. Deuterium is adsorbed and activated on platinum in Pt~aY. De De 2. The activated species (D*) spill over onto the NaY support \ Pt \ (~) (primary spillover step ). --(~ ] , , ~ ' ~ ) ~ , t ~ D D*" " 3. The spilt-over species diffuse on the ',z~-~ ~ ,".=----.~D~'~O~Ot, ol zeolitic surface. / / / / / A HNaY 4. Diffusion occurs also from Pt/NaY r/Pt/NaY ~ / ~ to HNaY in two-component samples
( secondary spillover step ).
i / / ~ ~ ~
5. An H-D exchange takes place with the hydroxyl groups of HNaY. Figure 1 Reaction steps in the H-D 6. Destruction of the spilt-over exchange via D* spillover species occurs by recombination or backspillover to platinum. The spillover reaction pathway - in comparison to other deuteration pathways by molecular deuterium or by heavy water - is characterized by the following features : The spilt-over species are more reactive for the H-D exchange than molecular deuterium, they are formed on the metal and have to diffuse to the reaction site (OH groups) and this diffusion occurs on the surface of the catalyst. Infrared microscopy, the investigation of the influence of a homogeneous magnetic field and the effect of CO pre-adsorption were used to demonstrate the occurrence of the spillover reaction pathway after deuterium application at room temperature to Pt-containing zeolite samples.
2. EXPERIMENTAL 2.1. Samples The samples consisted of NaY and NH4NaY zeolites (Chemie AG B itterfeld, Si/A1 ratio 2.4). Pt (0.5 wt.-%) was introduced by impregnation with an aqueous Pt(NH3)C12 solution, subsequent washing and drying as described elsewhere [9]. The zeolite powders were pressed into prisms, so-called one-component Pt/HNaY and HNaY samples (with a size of 4 x 4 x 25 mm 3) and two-component Pt/NaY- HNaY samples (4 x 4 x [5 + 20] mm3). The applied pressure was 300 MPa for 1 min. The geometry of the two-component samples is shown in fig. 2. The region where both zeolites could be mixed was less than 0.5 mm in thickness. 2.2. Activation procedure The activation of the zeolite samples included vacuum, oxygen and hydrogen treatments at 450~ [7,9]. After this procedure the expected diameter of the metallic
461 Pt clusters was 1 to 2 nm [13]. All samples to be compared were activated under identical conditions to achieve the same Pt dispersion.
2.3. H-D exchange experiments During the activation and the application of the reactant gases the samples were placed in quartz tubes connected to a vacuum system. The H-D exchange was observed either in-situ (infrared microscopic investigations) or ex-situ after pumping off the gas phase and sealing the samples (magnetic field studies). Deuterium (purity 99.5 % for the magnetic field investigations and 99.7 % for the other studies), hydrogen (purity 99.99 % and 99.999 %, respectively), CO (purity 99.997 %) and D20 (purity 99.8 %) were used for the exchange and poisoning experiments. To study the influence of a magnetic field on the kinetics of the H-D exchange two identically prepared two-component samples were simultaneously loaded with deuterium. During the exposure to deuterium one of the samples was placed in a homogeneous magnetic field which was directed either perpendicular or parallel to the sample axis (main direction of diffusion from the Pt/NaY to the HNaY sample component); further details are described elsewhere [6,7]. The strength of the magnetic field ranged from 0.1 T to 1 T. 2.4. Infrared spectroscopy An IR-Plan FTIR microscope Pt/NaY HNaY (Spectra Tech) combined with a 5 PC FTIR spectrometer (Nicolet) was used. .. ~.., For the investigations on the influence of the magnetic field a modified I Beckman 2A spectrometer was also I applied. For the microscopic I I I measurements the infrared beam A B C D E (diameter about 100 gm) was directed onto different points of the twoFigure 2 Measuring points at a twocomponent sample as represented in component Pt/NaY-HNaY fig. 2. Otherwise, the i.r. beam was sample directed onto the centre of the HNaY sample component. To evaluate the degree of H-D exchange from the areas of the OH and OD bands in the diffuse reflectance spectra the Kubelka-Munk function was used as already described [8,9]. 3. RESULTS
3.1. Deuteration after the application of heavy water The exposure of a two-component sample to D20 at room temperature led to an immediate adsorption of heavy water and a formation of OD groups in the whole Ptfree sample component without concentration gradient. Both effects could be observed in the infrared spectra [15]. The deuteration of OH groups by heavy water occurred also in the absence of Pt, i.e. in one-component HNaY samples. This process was not influenced by the presence of a homogeneous magnetic field B independent on its direction in relation to the sample axis. The deuteration of the hydroxyl groups via the heavy water reaction pathway was observed even in the case when CO was adsorbed before the exchange experiment was carried out.
462
3.2. Application of deuterium in the absence of platinum When D2 was applied to Pt-free samples at room temperature (e.g. for more than 24 hours) no formation of deuteroxyl groups was observed. Long term investigations of scaled samples showed the absence of OD groups in deuterium-loaded samples even after some months [8]. CO pre-adsorption and the application of a magnetic field did not change this result. 3.3. Deuteration after D2 application in the presence of platinum In the presence of Pt, i.e. in one-component Pt/HNaY samples and twocomponent P t ~ a Y - HNaY samples, the treatment with D2 at room temperature led to the formation of OD groups. All types of OH groups of the HNaY zeolite, corresponding to the different oxygen positions in the framework, were involved in the H-D exchange. The exchange process was shown to be reversible, i.e. hydrogen application to the deuterated sample led to the re-formation of OH groups [9]. By studying the kinetics of the exchange after deuterium loading of twocomponent samples at room temperature for 2 or 4 hours, pumping off the gas phase and sealing it could be shown that a reservoir of adsorbed spilt-over deuterium species was formed in the samples [7,9,14]. The adsorbed spilt-over species led to a further H-D exchange in the course of some weeks until isotopic equilibrium was achieved. By means of infrared microscopy the H-D exchange after deuterium application at room temperature was studied as a function of time and measuring position (points A to F in fig. 2) in two-component samples. A concentration gradient of the OD groups formed was observed within the HNaY component of the Pt/NaY - HNaY sample. Thus, it could be concluded that diffusion in addition to the deuteration of the OH groups itself are the rate determining steps in the spillover process as discussed elsewhere [9]. The application of a magnetic field B directed perpendicular to the 0,5 9 sample axis during the deuterium ~" 0,4 loading led to a lower degree of H-D ~" 9 9 exchange in isotopic equilibrium. The ~ 0,3 9 reference sample was simultaneously exposed to deuterium outside the ,~ 0,2 m a g n e t i c field. C o r r e s p o n d i n g ' 0,1 9 II1 experimental results are represented in ~" fig. 3, whereas the difference of the 0,0 ~ , degrees of H-D exchange in the presence of a (perpendicularly 0,0 0,2 0,4 0,6 0,8 1,0 1,g B inT directed) magnetic field A(B) and without a field A(0) is referred to A(0) as a function of B. When the magnetic Figure 3 Influence of a (perpendicufield was directed parallel to the main larly directed) magnetic direction of diffusion no difference in field on the H-D exchange the H-D exchange was observed [7]. The pre-adsorption of CO prior to the exposure to deuterium at room temperature (with and without pumping off the CO at this temperature) prevented the deuteration of the OH groups in all Pt-containing samples (Pt/HNaY and Pt/NaY - HNaY) [9,15]. No OD groups could be observed even after 24 hours of deuterium application. If the procedure was carried out using partially deuterated samples, the degree of exchange remained constant after hydrogen or deuterium application at room temperature. The experimental results, which have been achieved under different conditions for the three possible reaction pathways, are summarized in Table 1.
463 Experimental results with respect to the deuteration of zeolitic OH groups obtained under different conditions
Table 1
reaction pathway Deuteration
Occurrence of Occurrence of a Influence of CO Influenceof a H-D exchange concentration pre-adsorption magneticfield gradient
by heavy water via the gas phase
Ref.
yes
no
no
no
15, this work
no
-
no
no
6-8
yes
yes
yes
yes
6-9,15
(Pt absent) via spillover (Pt present)
4. DISCUSSION 4.1. Occurrence of an H-D exchange at room temperature The application of D2 at room temperature led to the formation of OD groups only in the presence of Pt. Without Pt no deuteration could be observed even after months. This is in agreement with results from the literature that the exchange in Pt free samples was observed only at higher temperatures (above 300oc) [16]. The influence of Pt (even at the remote Pt/NaY component) is a strong indication of a spillover reaction pathway. By contrast the deuteration by heavy water takes place already at room temperature directly from the gas phase, i.e. without an activating step of the reactant on Pt. A concentration gradient of the OD groups formed in the HNaY component of two-component samples (from measuring points C to F) was only observed after D2 application and did not occur after exposure to D20 [9]. This result can be easily explained by the diffusion of the spilt-over deuterium species from P t ~ a Y to HNaY. Investigations on the H-D exchange after CO pre-adsorption and in the presence of a magnetic field provided further evidence for the deuterium spillover pathway by influencing distinct steps of the spillover process (see fig. 1). 4.2. Influence of CO pre-adsorption It has been frequently observed that CO strongly suppresses the activity of metalcontaining catalysts for hydrogenation/dehydrogenation reactions and for the H-D exchange of hydroxyls [17]. Because of the strong adsorption of CO on the metal the activating sites get blocked thus preventing H2 or D2 adsorption. Taking into account the mechanism of deuteration via D* spillover the activation step (reaction step 1 in fig. 1) cannot take place when CO is pre-adsorbed. This can explain the absence of HD exchange in both sample components. In contrast, the deuteration by D 2 0 is still possible, because CO does not markedly block the OH groups themselves. 4.3. Influence of a magnetic field As discussed in detail elsewhere [6,7] a magnetic field lowers the diffusion constant of the electrically-charged spilt-over deuterium species when applied perpendicularly to the main direction of their diffusion. As experimentally shown this hindrance of the diffusion results in a smaller amount of D* and, consequently, in a
464 lower degree of H-D exchange in isotopic equilibrium. The electrical charge of the spilt-species must be connected with a charge transfer between the adsorbed D* and the zeolite. To understand the influence of the magnetic field a surface diffusion of D* and a main direction of this diffusion have to be considered. These features are represented by the spillover reaction mechanism (steps 3 and 4 in fig. 1). 5. C O N C L U S I O N The H-D exchange of zeolitic hydroxyl groups at room temperature has been studied as a test reaction for deuterium spillover. The separation of the activating phase (Pt supported on NaY) and the phase where the reaction was observed (HNaY) allowed to study the exchange as a function of time and distance from the Pt/NaY component. The spillover reaction pathway could be characterized by the observation of a concentration gradient showing the diffusion of the reacting species D* from Pt to HNaY. The diffusion of the electrically-charged spilt-over species was hindered by a magnetic field when it was directed perpendicularly to the main direction of diffusion. The absence of H-D exchange after CO pre-adsorption indicated that the reacting species were produced (by dissociation of D2) on the metal. ACKNOWLEDGEMENTS The authors thank Prof. B. Delmon and Prof. H. Winkler for many helpful comments and discussions related to these studies. Financial support from the Deutsche Forschungsgemeinschaft and the Leopoldina (LPD 1995) is gratefully acknowledged.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
B. Delmon; Heterogeneous Chemistry Rev. 1 (1994) 219 A. E1 Tanany, G.M. Pajonk, K.-H. Steinberg and S.J. Teichner; Appl. Catal. 68 (1988) 89 F. Roessner, U. Roland and Th. Braunschweig; J. Chem. Soc., Faraday Trans. 91 (1995) 1539 B. Delmon and M. Houalla; Stud. Surf. Sci. Catal. 3 (1979) 439 J.E. Benson, H.W. Kohn and M. Boudart; J. Catal. 5 (1966) 307 U. Roland, H. Winkler and K.-H. Steinberg; Proc. 2nd Int. Conf. on Spillover, Leipzig, 1989; p. 63 U. Roland, H. Winkler, H. Bauch and K.-H. Steinberg; J. Chem. Soc., Faraday Trans. 87 (1991) 3921 R. Salzer, J. Dressier, K.-H. Steinberg, U. Roland, H. Winkler and P. Klaeboe; Vibr. Spectroscopy 1 (1991) 363 U. Roland, R. Salzer and S. Stolle; Stud. Surf. Sci. Catal. 84 B (1994) 1231 D. Bianchi, G.E.E. Gardes, G.M. Pajonk and S.J. Teichner; J. Catal. 38 (1975) 135 J. Kapicka, N.I. Jaeger and G. Schulz-Ekloff; Appl. Catal. A 84 (1992) 47 R. Burch, R.J. Chapnell and S.E. Golunski; J. Chem. Soc., Faraday Trans. I 85 (1989) 3569 NT Jaeger, J. Rathousky, G. Schulz-Ekloff, A. Svensson and A. Zukal; Stud. Surf. Sci. Catal. 49 (1989) 1005 U. Roland, H.G. Karge and H. Winkler; Stud. Surf. Sci. Catal. 84 B (1994) 1239 S. Stolle, L. Stimmchen, U. Roland, K. Herzog and R. Salzer; J. Molecular Structure 348 (1995) R.V. Dmitriev, A.N. Detjuk, Ch.M. Minachev and K.-H. Steinberg; Stud. Surf. Sci. Catal. 17 (1983) 17 R.R. Cavanagh and J.T. Yates; J. Catal. 68 (1981) 22
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviotand S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
465
IR investigation of CO adsorption at low temperature: a key tool to characterize the porosity of matrix embedded zeolite catalysts. Zuy Magriotis Noronha a, Jos6 Luiz Fontes Monteiro a and Patrick G61inb aNUCAT-COPPE / PEQ, Universidade Federal do Rio de Janeiro, C. P. 68502, 21945-970 Rio de Janeiro, RJ- Brasil bLaboratoire d'Application de la Chimie/l l'Environnement UMR 9977 CNRS Universite Claude Bernard Lyon I, bat. 303 43, Blvd du 11 novembre 1918 F69622 Villeurbanne Cedex France
The physisorption of CO at low temperature on a catalyst of H-mordenite embedded into an amorphous silica alumina matrix was investigated by FT-IR. For the fresh catalyst, the interaction of CO with bridging zeolitic hydroxyls (vCO=2174 cm-1) was shown to feature the accessibility of zeolite channels, which was not affected by matrix binding. Upon severe steaming, the crystallinity of the MOR component was preserved by the matrix, while extensive dealumination of the framework occurred. The CO physisorption resulted in the formation of a solid-like phase inside the MOR channels, demonstrating the accessibility of zeolitic pores to the CO probe even after steaming.
I. INTRODUCTION The industrial use of zeolites often requires their agglomeration into granules using a binding agent. This might affect the accessibility of the zeolitic pore openings and modify the adsorptive properties of the zeolite. It is therefore of the utmost interest to develop experimental techniques, sensitive enough to evaluate the possible modifications of the zeolite when using binder. The task becomes even more difficult when the zeolite component represents the smallest fraction of the catalyst, the main component being an amorphous matrix, such as in the case of FCC catalysts, where the zeolite loading does not exceed at most 30% by weight. The conventional techniques such as the measurements of adsorptiondesorption isotherms for molecules of varying kinetic diameter turn out to be inefficient at evidencing tiny changes of the adsorptive properties of the zeolite. On the other hand, FT-IR spectroscopy of probe molecules has demonstrated its ability at characterizing the different modes of adsorption of a molecule on the zeolite surface. Particularly, the FT-IR study of the physisorption of CO at low temperature has been revealed as a powerful tool to characterize
466 Broensted acid sites in protonated zeolites [ 1-3]. The CO molecule specifically interacts with hydroxyl groups via hydrogen bonding, causing a shitt of the CO stretching frequency from that of the free molecule to higher values. The magnitude of the shitt can be used, with some restrictions, to probe the strength of the acid sites [3]. This specific behavior prompted us to carry out a thorough investigation of the physisorption of CO at low temperature on a catalyst of protonated mordenite (H-MOR) embedded into an amorphous silica alumina matrix in order to evaluate the technique in the quantitative measurement of the microporous zeolite volume. The influence of the hydrothermal treatment on the physico chemical properties of the embedded mordenite catalyst was also examined, focusing on the changes of the zeolitic microporous volume deduced from the FT-IR study of CO physisorption at low temperature.
2. EXPERIMENTAL The starting H-mordenite (HM) material was purchased from PQ Zeolites B.V. (Valfor CBV10AH, silica to alumina ratio of 14). The matrix embedded mordenite catalyst (HM/SA) was prepared by intimately mixing the mordenite material with a silica alumina gel containing 76 wt% SiO 2 and 24 wt% AI20 3, according to a procedure described elsewhere [4]. The resulting catalyst contained 15.6 wt% H-MOR and 84.4 wt% silica alumina matrix (determined on ignited basis from X Ray diffraction analysis). The silica alumina matrix (SA) was also prepared without further incorporating the mordenite. The HM, SA and HM/SA samples were steamed at 1123 K for 6 hours in nitrogen flow containing 84% water vapour. The steamed samples were denoted as HM St, SA St and HM/SA St respectively. X Ray Diffraction measurements were performed using CuKt~ radiation. The crystalline fraction of zeolitic samples was determined by summing the areas of the most intense peaks between 18.8 and 31.9 ~ 20. The HM sample was taken as a reference in the absence of matrix while a mechanical mixture of HM and SA samples containing 16.5% HM was considered for matrix embedded zeolitic samples. The framework A1 composition of zeolite crystals was determined from 27A1 NMR spectra and calibration data based on frequency shitts of framework vibration bands between 550 and 650 cm -1 [5,6]. Nitrogen adsorption and desorption isotherms were carried out on a home made apparatus. For IR measurements, self supported wafers of samples were introduced into an home made IR cell allowing in situ high-temperature treatments and low-temperature measurements [3]. All the samples were activated in vacuum at 673 K for 1 h, atter ramping the temperature from ambient at a rate of 5 K min-1, subsequently cooled down to liquid nitrogen temperature before admitting increasing doses of CO. The spectra were recorded at a resolution of 2 cm-1 on a Bruker IFS48 FT-IR spectrometer.
3. RESULTS- DISCUSSION The influence of the steam ageing on the crystallinity (%C) of HM wether or not incorporated into the silica alumina matrix has been examined in table 1. It could be observed that severe steaming induced a significant loss of crystaUinity of the mordenite structure (44% loss) when not embedded into the matrix. Mechanically mixing the zeolite to silica alumina
467 sample did not improve the resistance of the zeolitic structure at all (40% crystallinity loss). On the contrary, it turned out that the zeolite component retained almost entirely its crystallinity when embedded into the amorphous silica alumina matrix. This result confirmed the protecting effect of the matrix on the zeolitic structures against severe steaming treatments
[41. Table 1 Crystallinity and framework A1 composition (FAL) of fresh and steamed HM and HM/SA catalysts. sample
HM HM/SA HM St HM/SA St MM St d
zeolitic FAL (mol/uc) 27 A1NMR
IR
4.9 a
4.9 a 5.0 1.5 1.4
1.3
%C
loss of crystaUinity (%)
XRD 100.0 15.6 56.0 12.4 9.5
0b 44 c 20 b 40 b
ameasured by 29Si NMR b HM/SA as a reference. CHM as a reference. dSteamed mechanical mixture of riM and SA samples containing 16.5 wt.% HM. In addition, it could be observed from table 1 that the framework A1 composition of the zeolitic component decreased aIter steaming, whether the zeolite was or not embedded into the matrix, indicating the dealumination of the zeolitic framework upon steaming. It was concluded that the matrix prevented the zeolite structure from collapse but not from dealumination. The porosity of the fresh and steamed catalysts was characterized by conventional measurements of isotherms of adsorption-desorption of nitrogen at 77 K and the results were reported in table 2. Table 2 Microporous volume (Vmicro) of catalysts determined from N 2 adsorption. sample HM SA HM/SA HM St SA St HM/SA St
Vmicr o (ml/g) 0.2170 -0.0140 O.0035 -0.0020 -0.0040 0.0115
468 The negative values obtained for samples containing the amorphous matrix clearly suggested the inefficiency of the method to characterize with confidence the microporous volume of matrix containing catalysts. The results given in table 2 suggested that (i) the matrix was not constituted of micropores and consequently the microporous volume was only characteristic of the zeolitic component of the catalyst; (ii) that the steamed matrix embedded zeolite sample kept at least partly the microporosity of the flesh catalyst; (iii) the micropores of the steamed zeolite sample were at least partially blocked: this was in agreement with the partial collapse of the structure and to the strong dealumination of the framework which was expected to induce extraframework alumina species to deposit in the channels. The physisorption of CO at 120 K on flesh HM, SA and HM/SA samples was investigated by FT-IR and the spectra as a function of CO pressure were reported respectively in figures 1, 2 and 3A, where spectral regions characteristic of vOH and vCO vibrations were examined. For the HM sample (figure 1), it could be observed that CO mainly interacted with bridging hydroxyls, resulting in the concomittent formation of a broad intense vOH feature around 3310 cm-1 and a vCO band at 2174 cm-1. The composite nature of these bands might suggest different hydroxyls, siting in main channels and side pockets for example [7]. At higher CO pressures, the vCO spectral region was dominated by the development of new features at 2154 and 2138 cm -~ ascribed respectively to CO interacting with non acidic terminal hydroxyls and to a liquid-like CO phase condensed into zeolite channels [3, 8].
1.4
I
. . . . . .
~
0.6
I
i
d
Abs
0.4
0.0
3500 W a v e n u m b e r (cm- 1)
3000
_
2200 2100 W a v e n u m b e r (cm- 1)
Figure 1. IR spectra of CO adsorbed at low temperature on HM sample: (a) under vacuum; under CO pressure of(b) 0.01 Torr; (c) 0.1 Torr; (d) 3 Torr. The IR spectrum of the SA sample (figure 2) was dominated by a vOH at 3746 cm-1 characteristic of non acidic Si-OH groups. Accordingly, upon CO physisorption, an intense v CO band developed at 2157 cm "1, while the band corresponding to a condensed CO phase remained vey weak. As a result, the physisorption of CO on the HM/SA sample (figure 3A) turned out to be a combination of what was observed on the two components being examined separately. At low CO pressure, CO mainly adsorbed on bridging hydroxyls of the MOR
469 component, generating a strong band at 2174 cm -1 while high CO pressures induced the interaction of CO with non acidic hydroxyls present both on the zeolite outer surface and on the matrix surface.
--
1.6
0.1
......... t
t
Abs
'I\ 't
I 3500 W a v e n u m b e r (cm- 1)
0.9
3000
2200 2100 W a v e n u m b e r (cm- 1)
Figure 2. IR spectra of CO adsorbed at low temperature on SA sample: under CO pressure of (a) 0.01 Torr; (b) 3 Torr.
0.2
.....
I
I
0.5
.... I
I
A
B
b Abs
--
-
b 0.0
0.0 2200 2100 W a v e n u m b e r (cm -1)
,, 1 - ~ ' ' ~ 2200 2100 W a v e n u r n b e r (cm-1)
Figure 3. IR spectra of CO adsorbed at low temperature on fresh (A) and steamed (B) HM/SA sample: under CO pressure of (a) 0.01 Torr; (b) 3 Torr. The intensity of the 2174 cm -1 feature plotted as a function of CO pressure for HM/SA sample was found equal to 15+1% that of the original HM sample, i.e. reflecting the
470
exact composition of the catalyst in MOR component. This indicated that matrix incorporation did not affect the accessibility of acidic zeolitic hydroxyls to the CO probe. The IR spectra of CO physisorbed on the steam aged HM/SA catalyst were reported in figure 3B. The vCO band at 2174 cm "1 was no longer observed, in agreement with the diappearance of bridging hydro,'~yls of the mordenite due to extensive framework dealumination upon steaming. A sharp new intense vCO band developed at 2140 cm "1, specific of CO adsorbed on dealuminated HM (this band was not observed upon CO adsorption on the steamed SA sample). The sharpness of this new band indicated a strongly inhibited rotational behavior of the related adsorbed CO species, possibly a solid-like CO phase. The intensity of this band, related to the amount of zeolite in the catalyst, was found to be 4 times more intense than upon CO adsorption on the equivalent amount of steamed HM sample. It was concluded that the accessibility of zeolite pores was much preserved upon steaming when the zeolite was incorporated into the matrix. By contrast, the pores of the steamed HM sample were partially blocked because of the partial collapse of the structure and also because of the condensation of extraframework alumina deposits inside the channels.
REFERENCES
,
L. Kubelkova, S. Beran and J. Lercher, Zeolites 9, (1989) 539 E. Garrone, R. Chiappetta, G. Spoto, P. Ugliengo, A. Zecchina and F. Fajula, Proceedings of the 9th International Zeolite Conference, Montreal 1992, Eds. R. von Balmoos et al., Butterworth-Heinemann, 1993, p. 267 N. Echoufi and P. Gdin, Proceedings of the 9th International Zeolite Conference, Montreal 1992, Eds. R. von Balmoos et al., Butterworth-Heinemann, 1993, p. 275 P. Gdin and T. Des Couri+res, Applied Catalysis, 72 (1991), 179 L. Almanza, thesis, University Claude Bernard Lyon I, 1993 Z.M. Magriotis Noronha, J.L. Fontes Monteiro and P. Gdin, 8th Brazilian Seminar on Catalysis, sept. 13-15,1995, Rio de Janeiro, Brasil S. Bordiga, C. Lamberti, F. Geobaldo, A. Zecchina, G. Turnes Palomino and C. Otero Arean, Langmuir, 11 (1995), 527 T.P. Beebe, P. Gdin and J.T. Yates, Jr, Surface Science, 148 (1984), 526
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviotand S. Kaliaguine (editors) 1995 Elsevier Science B.V.
I d e n t i f i c a t i o n of a c t i v e Ti c e n t e r s in TS-1 as r e v e a l e d b y E S R irradiated samples
471
spectra of U V -
A. Ghorbel b, A. Tuel a, E. Jorda a, Y. Ben Taarit a and C. Naccache a** alnstitut de Recherches sur la Catalyse (conventionne rUniversit6 Claude Bernard) 2 avenue A. Einstein - 69626 Villeurbanne C6dex France. bOn leave from Facult6 des Sciences de Tunis, D6partement de Chimie.
ESR spectra of TS-1 UV-irradiated samples revealed the generation of tetrahedral trivalent titanium together with hydrogen atoms. A slight energy imput results in the rearrangement of the paramagnetic species to form a new paramagnetic center including a Ti (OH) Si bridge with the odd electron residing in an antibonding sp orbital of the OH group, the titanium ion reverting to its initial tetravalent state.
1. INTRODUCTION Titanium based catalysts have been widely used in selective oxidation of organic substrates. In the search for the active centers in these oxidation processes, Ti 3+ ions were identified using ESR [ 1-4]. More recently titanium isomorphously substituted silicalite-1 (TS-1) were shown to be highly active and selective for phenol hydroxylation, alcohol oxidation and olefin epoxidation and the oxidation of various substrates in the presence o f H 2 0 2 [5-7]. The structure of the active titanium component was thouroughly investigated via IR spectroscopy and an absorption at 960 cm" 1 was associated with the presence of titanium in T sites; a strong absorption at 230 nm was ascribed to the UV induced charge transfer from the oxide ~on " to T~"4+ in tetrahedral environment and an ESR study of CO-reduced TS-1 samples confirmed the tetrahedral siting of the titanium ions. Additional evidence for the T siting of titanium ions in TS-1 together with information on the Redox behaviour of these ions within the framework may be obtained directly from ESR studies of UV irradiated TS-1 samples. The aim of this work was to genrate ESR active species via UV irradiation at low enough temperature so that no structural disruption could be produced in the process. 2. EXPERIMENTAL TS-1 was synthesized following the procedure proposed by Taramasso et al. [5]. The titanium content as determined by chemical analysis was equal to 1.2 %, XRD spectra and IR lattice vibration showed the sample to be authentic TS-1 in particular the band at 960 cm-1 was very intense. In addition, a strong and sharp absorption at 230 nm is indicative of a UV induced charge transfer from the oxide ions to a tetrahedral Ti 4+ ion. For comparison a pure silicalite-1 was also prepared.
**To whom correspondance should be sent.
472 After air calcination to remove the template, samples were heated in oxygen and finally evacuated in quartz ESR tubes at 573 K. In a specially designed experiment, TS-1 was evacuated at 773 K then equilibrated with D20 and subsequently outgassed at 473 K. UV irratiation was performed at 77 K, using a mercury lamp, for 1 to 3 hour periods the samples were kept at the same temperature and the ESR spectra also obtained at 77 K on a E 9 Varian spectrometer equipped with a dual cavity operating in the X band mode with 100 Kc field modulation. The DPPH was used as reference to determine g values. 3. RESULTS Prior to UV irradiation, all samples were ESR silent. No paramagnetic species were present on the activated samples. UV irradiated silicalite, at 77 K, exhibited 2 isotropic lines centered at g = 2.002 with a hyperfine splitting of 506 Gauss (signal 1). These two lines disappeared rapidly on warming the sample. No new signal appeared. A new irradiation regenerated the same signal.
H 20 gauss
gl Figure 1. ESR signal of UV irradiated TS-1 (signal 2). UV irradiation of activated TS-1 samples showed, along with signal 1, an additional axially symmetric signal with g l 1 = 1.999 and gj.= 1.92 signal 2 of Figure 1. Upon progressively warming the sample, using a temperature controlled device within the ESR cavity, a new signal 3 developed at 133 K at the expense of signal 2. This new signal (Figure 2a) is consistent with a three component g tensor, each component being split into a doublet. The following values for the g and A tensors principal components were extracted: gl = 2.0018 g2 = 2.0003 g3 = 1.9991 A1 = 80 Gauss A 2 = 6 Gauss A 3 = 33 Gauss
473 On warming the sample up to room temperature, it became again ESR silent. TS-1 subjected to D20 treatment before final evacuation and subsequent UV irradiation gave rise to a reduced amplitude signal 1 and a triplet also centered at g = 2.002 with a hyperfine splitting of 76 Gauss (signal 1D) along with signal 2. Upon warming signal 1 and 1D disappeared while the far two most features of signal 3 developed to, however, a lower amplitude than D20-untreated samples, while the central part of signal 3 was considerably altered. As before, signal 2 disappeared concomitantly to the developement of the modified signal 3 (Figure 2b).
4. DISCUSSION
4.1. Identification of the paramagnetic species. Signal 1 is unambiguously assigned to atomic hydrogen trapped at the surface of the zeolite. Indeed the near free electron g value and the hyperfine splitting of 506 are fingerprints for hydrogen atoms. Futhermore the D20 treatement producing a H/D exchange led to the formation of H and D atoms as confirmed by the observation of the triplet (I D = 1) with a splitting of 76 in accordance with the nuclear magnetic moments ratio (laH/laD = aH/aD). Signal 2 appeared only on TS-1 and not on pure silicalite. Therefore it must be associated with the presence of titanium ions. In addition both g components are less than the free electron g value. As such, the signal is associated with an unpaired electron in a d orbital. This signal should be attributed to a 3d 1 center which may only be Ti3+. In addition gl 1 is larger than g_t_and is very close to ge. Such an ordering corresponds to the unpaired electron in adz 2 orbital. The degenerate d orbitals for a 3d 1 configuration experiencing a tetrahedral crystal field have the dz 2 as the ground state thus hosting the unpaired electron. Therefore the ESR signal 2 is indicative of the presence of Ti 3+ ions subjected to a tetrahedral crystal field, as one should expect if titanium ions are in framework T sites of the TS-1. This is in agreement with our previous findings obtained upon CO-thermal reduction ofTS-1 [8]. Signal 3 exhibited a nearly isotropic g tensor, which is indicative of an unpaired electron with a considerable s-character (spherical symmetry) and the latter could no longer be located on titanium d orbital. Futhermore each g component is split into a doublet indicating that the unpaired electron is interacting with a spin 1/2 nucleus. The only spin half element with 100 % natural abundance present in TS-1 is hydrogen. Therefore signal 3 should be associated with a center comprising a single proton. This interpretation is confirmed through the D20 exchange experiment which resulted in a reduced signal 3 intensity and the appearance of a signal with a reduced hyperfine splitting as expected considering the ratio of the two magnetic moments of the proton and deuteron. In addition the isotropic hyperfine splitting derived from the hyperfine tensor of signal 3 AHiSO = 1/3(80 + 33 + 6) ~ 40 Gauss indicates that the odd electron has only a partial s character and is therefore located in asp or spd type orbital. Hydroxyl radicals HO ~ are well known and their magnetic parameters have been reported to be g_l_ = 2.01 and gll = 2.007 in irradiated ice with AHiSO = 41.3 Gauss (9).
474
S
b
a
9
I
j
A2
Figure 2. ESR signal obtained upon warming up UV irradiated TS-1. Signal 3 (a); ibid Deutero TS-1 (b).
475 Hydroperoxyl radicals HO~ have also been reported to be generated by photolysis of hydrogen peroxyde in various organic solvents at 77 K, they exhibit an ESR signal with g l = 2.0085 g2 = 2.0085 and g3 = 2.027 without any hyperfine splitting due to hydrogen, indicating that the unpaired electron is largely residing on the non protonated oxygen [ 10]. Comparison of the magnetic parameters of signal 3 with any set of the above values precludes the assignment of signal 3 to either pure HO ~ or HO~ radicals.
4.2. Interpretation of the ESR results It is well established that upon UV irradiation of transition metal oxides an electron transfer is induced from the oxide to the transition metal ion resulting in a one electron redox process [4]. Under these conditions a hole-electron pair is formed, the odd electron being trapped in a non bonding d orbital. Therefore UV irradiation ofTS-1 reveals a similar behavior of the titanium ions resulting in the formation of Ti 3+ centers with a 3dl configuration by transfer of one electron of the Ti-O bond into an empty orbital of titanium, thus accounting for the formation of lattice Ti 3+ ions characterized by signal 2. Apparently this excited electronic state is stable only at low enough temperatures. Indeed upon warining up the sample to room temperature all paramagnetic centers disappear regenerating the initial ground state and showing that there was no irreversible modification of the zeolite sample. However at intermediate temperatures in the 133-213 K range the ESR data showed that the odd electron of the excited state, shown to reside initially in a d z 2 orbital of titanium, has acquired a s p character and is interacting with a single proton. It could be proposed that a nearby hydrogen atom (H*) may react with the electron hole thus annihilating it to form a regular Si OH, ensuring simultaneously the local charge balance disrupted by the initial reduction of the Ti 4+ into a Ti 3+ ion. The Ti 3+ center may be pictured as follows H O Ti (III)
Si
Raising the temperature provides the energy increment to transfer the unpaired electron from the dz 2 orbital of titanium into an antibonding (sp) orbital of the adjacent OH group, thus confering to the odd electron a significant s character compared to the free electron of HO* where the odd electron primarily resides an a Pz orbital of the oxygen. Such a paramagnetic center may be formed only on solids where the electron donor is adjacent to a hydroxyl group. The structure of the paramagnetic species responsible of signal 3 may be illustrated as below : H
Ti (IV)
Si
Indeed such a species may not survive warming up the sample to room temperature: a hydrogen atom would split from this paramagnetic center and regenerate the original TS-1 structure.
476 In conclusion this ESR study of UV irradiated TS-1 at very low temperatures, avoiding any possible change in the chemical and structural features of the zeolite, appeared as a very powerful method to probe the symmetry of Ti4+ ions and possibly other transition metal ions; Indeed under irradiation, non framework titanium would give rise to Ti3+ ions with magnetic parameters quite different from those associated with framework titanium. In particular, the gl 1 principal value wil be lower than the g_l_value for non framework Ti3+ species.
REFERENCES
[1]
R.D. Iyengar, M. Codell, J.S. Karra and J. Turkevich, J. Am. Chem. Sot., 88 (1966) 5055. [2] P.F. Cornaz, J.H.C. Van Hooff, F.J. Pluijm and G.C.A. Schuit, Discuss. Faraday Sot., 41 (1966) 290. [3] M. Che, C. Naccache, B. Imelik and M. Prettre, C.R. Acad. Sci., Set. C., 268 (1967) 1901. [4] P. Meriaudeau, M. Che, P.C. Gravelle and S.J. Teichner, Bull. Soc. Chim., France, (1971) 13. [5] A. Esposito, M. Taramasso, C. Neri and F. Buonomo, U.K. Patent, 2 116 974 (1985). [6] G. Perego, G. Bellussi, C. Como, M. Taramasso, F. Buonomo and A. Esposito in "New Developments in Zeolite Science and Technology" Y. Murakami, A. Iijima and J.W. Ward, Eds. Elsevier, Amsterdam, 1986 pp. 129-136. [7] P. Roffia, M. Padovan, E. Moretti and G. De Alberti, Eur. Pat. 208 311 (1987), Eur. Pat. 267 232 (1988) US. Patent 4 745 (1988). [8] A. Tuel, J. Diab, P. Gelin, M. Dufaux, J.F. Dutel and Y Ben Tfiarit, J. Mol. Catal., 63 (1990) 95. [9] J.A. Mc Millan, M.S. Matheson and B. Smaller, J. Chem. Phys., 33 (1960) 609. [ 10] R. Livingston, J. Ghormley and H. Zeldes, J. Chem. Phys., 24 (1956) 483.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
477
M i c r o w a v e crystallization o f titanium-containing cloverite R. Fricke*, H.-L. Zubowa, J. Richter-Mendau, E. Schreier 1, and U. Steinike Institute of Applied Chemistry and 1Humboldt University Berlin (WlP), Rudower Chaussee 5, D- 12484 Berlin, Germany The conditions for the synthesis of Ti-containing cloverite under microwave radiation are described and optimal conditions for the synthesis of large crystals are given. XRD, IR, DTA/TG and adsorption measurements characterize the obtained products as of high crystallinity. SEM pictures show the crystals to be of various habit; the growth of the different crystallographic faces could be demonstrated. INTRODUCTION Since the development of the titanium silicalite oxidation catalyst TS-1 [ 1] great attention has been devoted to the incorporation of titanium into zeolite structures to combine the properties of titanium with those of other well defined porous structures. Parallel to these studies there is an additional development that is focused on the production of more open structures of zeolite solids, i.e. the synthesis of super wide pore zeolites or mesoporous materials. From these the aluminophosphate molecular sieve VPI-5 as the first
18-ring structure and the M41S silica system receive the greatest attention, the latter due to the great variety of its pore diameters up to about 100 A [2]. Cloverite is a galliumphosphate molecular sieve which has two 3-dimensional pore systems, one containing small 8-ring pores (about 4 A) whereas the second one consisting of 20-ring pores (about 13.2 ~) with large 30 ~-cavities at their intersections [3]. Although this structure makes it interesting as a promising catalyst system it has, like the VPI-5 and JDF-20 aluminophosphates, the disadvantage of being instable after activation (detemplating). Like in the cases of other molecular sieves it can be expected that the incorporation (or the isomorphous substitution of Ga or P) of heteroatoms will influence properties of cloverite. The present contribution describes the synthesis and characterization of titanium-containing cloverite samples which mainly have been synthesized under microwave radiation. EXPERIMENTAL The samples have been synthesized in teflon-lined autoclaves under autogeneous pressure from gels having the following composition: 1.0 Ga203 : 1.0 P205 : 2.2 HF : 6.0 Q : 0.050.30 TiO 2 : 70-300 H20 (Q: quinuclidine). The crystallization has been carried out in a
478 programmable microwave (MDS 2000, CEM) allowing the simultaneous measurement of temperature and pressure within the autoclaves. Samples synthesized in a furnace were synthesized for comparison. The products were characteriszed by XRD (Cu Kot radiation), adsorption measurements (ASAP 2000M, micromeritics), DRIFT/FTIR (Mattson 5000 FTIR, Unicam, and IRE 180, ZWG). DTA/TG measurements on a TGA 92 (Setaram). SEM pictures were taken with a KYKY instrument. RESULTS AND DISCUSSION When starting with the investigation of the influence of heteroatoms on the crystallization of cloverite in a furnace and under microwave heating two general observations have been made for the conditions used: i) in accordance with literature [4] it has been found that the HF content is of great importance, however, in contrast to literature data (HF:Ga203=0.7) the best crystallization products were obtained when higher amounts of HF, i.e. HF:Ga203=2.2 [5] were introduced into the gel. ii) when applying microwave radiation it has been found that the content of water should be generally higher than when using a furnace. A ratio of H20:Ga203-200 (instead of 70) is therefore used for standard synthesis procedures.
Influence of the Titanium Content. A systematic attempt has been made to stepwise increase the contents of titanium within the gel. The XRD pattern compared to that of pure cloverite were taken as criterion for the identification of the synthesis products (Fig. 1).
1
2 3 ,.-
i
4.00
.
.
.
.
!
15.33
'
9
" ',
,i
9
~
~.,,.'...,~i,.,,,,,,s
I
.
.
.,,,~ . . . .
.
.
i
26.67
38.00 20
Fig. 1 XRD pattern of galliumphosphate samples containing titanium (TiO2:Ga203 = x), 1) x = 0.10, 2) x = 0.15, 3) x = 0.20
479
From these results it'is obvious that the degree of crystallization drastically decreased when the titanium content in the gel was increased above 0.1 mole TiO 2. At 0.15 or 0.20 mole TiO 2 the XRD pattern show already nearly no pattern of cloverite. A similar conclusion can be drawn from IR measurements when looking at the framework vibrations (Fig. 2). In accordance with literature the bands at 596 and 638 cm -1 with shoulders at 575 and 665 cm -1 where taken as evidence for the clovefite structure. The spectra clearly show that a titanium content up to 0.10 has no influence on the spectra whereas dramatic changes can be observed in the characteristic range when increasing the titanium content up to 0.20 mole TiO 2. 1 ~
15o.
~
lgoo
16oo
s&~
wavenumbers (cm -1) Fig. 2 IR spectra of galliumphosphate samples containing different amounts of titanium (TiO2Ga203=x),
1)x=0.10,
2) x = 0 . 1 5 ,
3) x = 0 . 2 0
It is concluded from these results that the presence of titanium has a strong influence on the crystallization process and that at titanium concentrations above 0.10 mole TiO 2 the formation of the cloverite structure is strongly distorted. In some cases the formation of GaPO4-a and/or another unidentified phase has been observed at higher titanium concentrations. If one accepts that the presence of titanium has an influence on the crystallization process then the question arises where the Ti is located in the solid synthesis product. In the titanium silicalite catalyst TS-1 Raman spectroscopy was very helpful and proved the isomorphous substitution of Si by Ti [6]. The Raman spectrum obtained from a template-containing Ticloverite sample is dominated by strong lines of the template molecule. Only two lines of medium intensity at about 320 and 445 cm -1 can be attributed to the cloverite structure. No indication of the presence or not of titanium in the lattice can be found under these circumstances. In any case it is, however, difficult to transfer the kind of conclusion of TS-1
480 to cloverite samples because in any case titanium, if incorporated into the framework, is coordinated in a completely different way (in the second coordination sphere) in cloverite than in silicalite. ESR measurements of the reduced samples show a Ti 3+ signal with g = 1.93 and g - 1.88 [5] which can, however, not be taken as evidence for framework titanium. A comparison of the unit cell volumes of pure and the Ti-containing cloverite samples show values of Vu.c.= 51.70 /~3 and 51.61 A 3, respectively, what can be taken as a hint on a possible location of titanium in the framework of cloverite. A further contribution to this problem is expected from l.W-Vis measurements which are undertaken at present. It should be mentioned at this occasion that it has been observed that the microwave samples in most cases show a higher crystallinity than those synthesized in a furnace. Bedard et al. [7] have shown by calculation of the XRD pattern and comparison with the experimental ones that in the latter case the peaks at 200 (not shown here) and 220 (2 | = 4.828 ~ are surprisingly low. This is not the case for the Ti-cloverite samples which show nearly exactly the pattern calculated by Bedard et al.. Crystallization m the microwave:Some parameters which might have an influence on the result of the crystallization process have been systematically varied to find optimal conditions for the synthesis. The results are briefly described below: 1. Variation of time (H20:Ga20 3 = 200, Tsynt h = 170~ After 1 h cloverite is obtained in good quality (microporous vol. = 0.23 cm3/g); prolonged microwave heating to 6 h leads to a decrease of the microporous volume. 2. Variation of temperature (H20:Ga20 3 = 200; t = 5 h) At 120-130~
a mixture of cloverite and GaPO4-a is obtained, sometimes cloverite has
been found to be already the only phase. At 150~
cloverite is formed having a
microporous volume of 0.26 cm3/g. 3. Variation of the water content (gradual temperature program*, see below) Using 70 mole H20 (the literature value for synthesis in a furnace) led to an amorphous product.Taking 120-160 mole water: the crystallization product was GaPO4-a. In the range of H20:Ga20 3 - 200-250 well crystallized cloverite samples up to about 60 lam were obtained; the microporous volume of the samples was 0.27 cm3/g. 4. *The gradual temperature program. In all the reported cases the crystal sizes have been found to be some lam. However, when the synthesis was carried out using an empirical temperature program that increases the temperature of synthesis stepwise from 120~ to 170~ large crystals up to 60 ~tm have been obtained. At present the complete mechanism for the formation of large crystals is not completely understood. It has been found that already at 120~
large crystals can be obtained although the crystallinity of the whole
synthesis product is not high (the microporous volume is about 0.20 cm3/g). When the temperature is further increased then the average crystal size is again decreased due to the
481 dissolution of the large crystals and the simultaneous formation of a lot of very small seed crystals. As at present this phenomenon is still under investigation the results will be reported later.
The morphology of the crystals. In their first report Esterman et al. [3] described the crystallization products of cloverite to be cubes. In the meantime further crystal forms were reported by the same group. The possibility to reproducibly synthesize also large crystals by microwave heating enables the investigation of the crystal growth of cloverite in more detail than at any time before in zeolite chemistry. Fig. 3 shows a sequence of three SEM pictures of Ti-containg cloverite.
Fig. 3 SEM pictures of Ti-containg cloverite crystals at various stages of growth The key result is shown in Fig.3c where, to our knowledge for the first time, is documented that the different faces of a molecular sieve crystal grow with different velocity. In the present case it is obvious that the square faces grow slower than the triangle or hexagonal faces. The results are 'windows' which could be observed not in all but in some of the samples. According to the rules in crystal physics those faces which grow faster disappear in the final crystal. This means that the final product should only contain squares, i.e. it is a cube. Following the crystal habit 'back' to the origin it is concluded that the growing process starts with the octahedron, i.e. the various crystal forms (cubooctahedra etc.) are obviously intermediate forms. It should be mentioned, however, that the crystal size itself is obviously not influenced by this change of the crystal habit because the large crystals generally show many different intermediate crystal forms. As already mentioned above the connection between the synthesis parameters and size and crystal habit is yet not fully understood.
Thermal behaviour. It is known from literature that cloverite is thermally stable up to more than 600~
[3, 7]. Fig. 4 shows the results of the DTA/TG measurements which are
characterized by an endothermic peak at about 140 ~ (desorption of water) and exothermic
482
T---0 m
.
.
.
.
.
.
,,
~r
F.'0,' t . t d ' . v )
9
'l' S~ 250
0 .TG
,oo
I.
......... -/)"11
o-
.
\
!
,
~ -30
\
i
$oo . %--..'# .
/i ,i,.; ~
~. . . . .
2O0 .
~
"
',. N " .
~ "~
~%.
I
.
. 3O0 .
1!1
'o"i v | : k _ _ _
~', ~
4OO
BOO
0
- - - -
I
.....
6OO ,
70O ,
-,oo
000 |
Fig. 4 DTA/TG measurement of (1) pure cloverite and (2) Ti cloverite (TiO 2:Ga203=0.10); heating rate was 10 K/min. in air (the DTA curve of (2) is shit~ed) peaks at 372, 433, 512, 587 (sh), and 666~ for pure cloverite and at 374, 442, 514, 560 (sh), and 652 ~ for the Ti-containing sample. The exothermic peaks are obviously caused by the desorption of HF and quinuclidine and/or its destruction products. The high temperature peak above 600 ~ may already indicate distortion of the cloverite structure. The results demonstrate small differences in the TG and DTA curves of both samples, it is, however, supposed that these are not (only) characteristic for the presence of titanium. REFERENCES 1. M. Taramasso, G. Perego, and B. Notari, US Patent 4 410 501 (1983) 2. 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. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. 3. M. Esterman, L. B. McCusker, C. Baerlocher, A. Merrouche, and H. Kessler, Nature, 352 (1991) 320. 4. C. Schott-Darie, H. Kessler, and E. Benazzi, Proc. Intern. Symp. Zeolites and Microporous Crystals, ZMPC 93,, T. Hattori and T. Yashima (eds.), Kodansha/Elsevier Tokyo, Amsterdam 1994, p. 3. 5. H.-L. Zubowa, E. Schreier, K. Jancke, U. Steinike, and R. Fricke, Collect. Czech. Chem. Commun., 60 (1995) 403. 6. W. Pilz, Ch. Peuker, V. A. Tuan, R. Fricke, and H. Kosslick, Ber. Bunsenges. Phys. Chemie, 97 (1993) 1037. 7. R. L. Bedard, C. L. Bowes, N. Coombs, A J. Holmes, T. Jiang, S. J. Kirkby, P. M. Macdonald, A. M. Malek, G. A. Ozin, S. Petrov, N. Plavac, R. A. Ramik, M. R. Steele, and D. Young, J. Am. Chem. Soc., 115 (1993) 2300.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
483
Aqueous Silicate Chemistry in Zeolite Synthesis Christopher T.G. Knight, a Raymond T. Syvitski bt and Stephen D. Kinrade b*
aSchool of Chemical Sciences, University of Illinois at Urbana-Champaign, 505 South Mathews Ave., Urbana, IL 61801, USA ~ of Chemistry, Lakehead University, Thunder Bay, Ontario P7B 5El, Canada
Solution state nuclear magnetic resonance (NMR) spectroscopy is ideally suited to the study of aqueous silicate solutions, and indeed is the only method available for systematically investigating the molecular level influence of synthesis parameters on silicate speciation and kinetics. It is thus of immense importance to our understanding of the processes by which zeolites and mesoporous solids are formed from solution. In this paper, we give a brief overview of what NMR can legitimately tell us about the mechanism of zeolite synthesis (while pointing out its limitations and the various pitfalls inherent in its application to the study of zeolites), and report experimental evidence in support of a clathration hypothesis of zeolite formation in media containing organic solutes. We also consider the existence (or otherwise!) of double 5- and double 6-ring polyanions, and note the novel chemistry of aqueous stannosilicates. INTRODUCTION Knowledge of the structure and dynamics of silicate species in bulk solution has increased significantly over the last 15 years owing to the advent of high-field NMR spectroscopy. Silicon-29 NMR, in particular, has been used to identify some 2 dozen rapidly interchanging silicate anions [1,2] along with a number of metallosilicate analogues [3-6]. Because these small (mono- to octameric) anions bear a superficial resemblance to the hypothetical secondary building units envisaged as repeat units in zeolitic networks, several researchers jumped to the appealing conclusion that these secondary building units actually exist in solution in the form of discrete and stable silicate anions, and that, given the above, zeolite growth occurs simply by sequential addition of these building units [8] (an idea originally mooted by Barrer in the 1950's [7]). A more sober appraisal of the actual structures present in solution has since revealed that, with one or two minor and almost certainly coincidental exceptions, secondary building units do not actually exist in solution [9]. Indeed, the idea "that silicate anions in solution play a key role in determining the final zeolitic structure" [5] has turned out to be thoroughly misleading. The species that do exist are far too labile to affect zeolite growth (except when associated with certain quaternary Consequently, the ammonium ions), and appear to be little more than spectator species.
tCurrent address: Dept. of Chemistry, University of British Columbia, Vancouver, B.C. V6T 1Z1, Canada *Corresponding author. Tel: (807)343-8683. Fax: (807)346-7775. E-mail: stephen'kinrade @lakeheadu.ca.
484 visually tempting but mechanistically challenging concept of a series of large, prefabricated silicate rings and cages joining together to produce a zeolitic framework must be abandoned [9]. The theory persists nonetheless, as witnessed by the repeated attempts to identify the chemically unlikely aqueous double 5-ring and double 6-ring polyanions [8]. Zeolite growth in media containing organic molecules such as tetraalkylammonium (TAA § ions is typically explained in terms of cation templating, according to which small (mono- or dimeric) silicate anions replace water molecules in the structured hydration spheres around organo-cations [ 10-12]. The silicate anions, if held in position for sufficient time, will join together into primary "clathration units". These are thought to order progressively into the final zeolite structure [11], or, alternatively, to participate in stepwise crystal growth through diffusion to the crystal surface [12]. The crystal lattice consequently is expected to reflect the structure of the cation's hydration sphere. Templating in this conventional sense, however, does not provide a satisfactory explanation of why, for example, in some syntheses pore spaces correlate well with cation geometry, while in others there is no correlation at all [12]. A vastly improved understanding of molecular interactions is required if we are ever to achieve the goal of producing zeolites by design. We submit that the best hope at present of understanding the genesis of solid silicate structures is by careful investigation of the bulk solution chemistry. Parameters which influence the stability of free silicate anions similarly affect the participatory species of zeolite nucleation and growth. Indeed, several inferences can already be made based on recent NMR findings: 1. Alkali-metal (M +) cations promote formation of higher polyanions, most likely by overcoming the electrostatic repulsion between silicate anions [13]; Goepper et al. [ 14] have observed the importance of small amounts of M § in accelerating crystallization of all-silica zeolites. The ion-paired cations stabilize open silicate structures in bulk solution by immobilizing pendant groups and large ring structures [13], and thus would be expected to affect structural elements similarly in an emerging crystal lattice. 2. The double 3-ring (Si60156-) and double 4-ring (SiaO20a-) polyanions tend to be favoured in solutions containing tetraalkylammonium cations [13]. These stabilized species are extremely long-lived in alkaline solutions [15], though less so under zeolite synthesis conditions [16], and are therefore unlikely to participate directly in zeolite nucleation or growth as such. There is evidence, however, that quaternary ammonium ions also stabilize, albeit less tenaciously, a host of other silicate polyanions [ 16]. These may well represent the true participatory species in the formation of zeolites (and MCM meso-porous sieves) in systems containing such organic cations. 3. The presence of non-ionic organic solutes enhances silicate polymerization by decreasing water activity and by reinforcing the stabilizing influence of quaternary ammonium ions [17,18]. 4. Solution state NMR experiments have demonstrated the existence of aqueous germano[3], alumino- [4], boro- [5] and gallosilicate [6] oligomers, all apparent analogues of the polysilicate anions. Their occurrence may have important implications in terms of the mechanism of zeolite growth. The corresponding solutions readily precipitate in the presence of alkali-metal cations and, unless stabilized by organic cations, these metallosilicate anions are 2 to 3 orders of magnitude more labile than the unsubstituted species [4]. 5. We lastly note that 5-coordinate silicate centres have been proposed as intermediates in the synthesis of silicate sieves in non-aqueous media based on solution 29Si NMR [19].
485 CLATHRATION OF POLYSILICATE ANIONS Silicon-29 NMR studies of aqueous TAA-silicate systems reveal their unique characteristics: 1. The double 3-ring and double 4-ring cages are frequently the dominant silicate anions in solution. The relative abundance of each of these species depends strongly on TAA + concentration (a critical "micelle" concentration is indicated), but, unlike the concentrations of other silicate anions, is unaffected by pH [16]. 2. In TAA-silicate solutions which favour double ring cages, the chemical exchange lifetimes of these species are orders of magnitude greater than for most other anions, and are a function of TAA +, Si and H + concentrations [16]. Kinetic studies indicate that TAA + cations stabilize additional cage-like species - though much less effectively - including a tricylic octamer and doubly-bridged cyclic tetramer [16]. 3. TAA + ions cause the 29Si resonances of TAA-stabilized polysilicate cages to shift to lower frequency; other resonances are unaffected. In addition, whereas resonances typically shift to higher frequencies as pH is increased, those corresponding to TAA+-stabilized species move to lower frequency. 4. Longitudinal (Ta) relaxation of 29Si nuclei in the stabilized double-ring species is anomalously rapid. Furthermore, added paramagnetic ions have comparatively little effect on their rates of transverse (T2) relaxation, indicating that the approach of these metal ions is somehow hindered [16]. Taken together these observations indicate that TAA+-stabilized silicate cages are protected from their immediate environment in a unique manner. The extent of this protection varies depending on the exact morphology of the silicate anion, and is greatest for the double ring cages. We propose that stable clathrate polyanions (not to be confused with the hydrated organo-cation "clathration units" proposed, e.g., by Chang and Bell [11]) form when TAA + cations are electrostatically attracted to, and surround, any cage-like silicate oligomers. If the organo-cations are optimally distributed, their individual hydration spheres combine to form one continuous hydrophobic hydration shell that encapsulates the central anion - the formation process being predominantly entropy-driven. The hydrophobic solvent region shields the anion from interaction with other solution species, hydrolysis is forestalled, and clathrated anions increase at the expense of all other species. The integrity of the protective shell depends on the central anion's size, shape and rigidity; the clathrated doublybridged cyclic tetramer and tricyclic octamer, for example, are thus only weakly stabilized. Organic solute molecules, e.g. co-solvents, associate with and thereby reinforce this hydrophobic shell, accounting for the equilibrium shifts noted. Indeed, organic solutes that have the greatest effect are those with the highest "hydrophobicity". Furthermore, the effect is most pronounced for TMA cations, as would be expected since they have the greatest charge to radius ratio and consequently would be subject to the strongest electrostatic interaction with the silicate anions. We have observed that added cryptand, crown-ether and polyols can mimic the effect of TAA + cations in sodium silicate solutions, suggesting that organo-M + complexes also yield clathrate anions, though with different relative stabilities. How does this relate to zeolite synthesis? The idea that clathrate polyanions can be incorporated wholesale from solution into the zeolitic framework appears, in itself, to be overly simplistic. Indeed, species which are sufficiently long-lived to survive diffusion through the double-layer region to the crystal surface are highly unlikely candidates for subsequent polymerization. Rather, we believe that it is the weakly clathrated species formed
486 near or directly at the crystal surface that are the key - i.e., open structures which are stabilized only temporarily. As these species participate in nucleation and subsequent crystal growth, the role of their remnant clathrate shells can best be likened to external scaffolding that stabilizes the loose, open structures. Ion-paired alkali-metal cations act similarly, albeit less effectively [13]. The probability of cation occlusion increases with the degree of cationanion association, i.e., at elevated pH levels and organic co-solvent concentrations. There is no requirement for organic ions to occupy cages; nor is the effect especially dependent on the type of organic ion (or organo-M + complex). Various ionic/non-ionic solute combinations can yield the same aqueous cage species and, thus, yield the same zeolite lattice. This clathrate hypothesis of zeolite formation thus is consistent with general experience in zeolite synthesis (including recent reports of zeolite omega being synthesized from TAA-free media containing polyols [20] or polyethers [21,22]). LARGE DOUBLE RING CAGES Over the years, sharp peaks have been noted to low frequency of the double 4-ring (Si80208-) resonance in 29Si NMR spectra of concentrated TAA-silicate solutions. These have been variously attributed to the double 5-ring (Si100251~ [23] and double 6-ring (Si1203012-) [24] cages and their single-ring counterparts [25], apparently solely on the grounds that if they were to exist, they might be expected to resonate in this region! Since the existence of these larger species has been frequently cited in attributing a mechanistic pathway for zeolite formation [26], we undertook a series of experiments to test the assignments. As a result, we believe that these signals correspond to variously protonated, strongly clathrated double 4-ring cages, the protons being trapped inside the clathration shell of the anion, in rapid exchange over the terminal oxygens of the cage. We find no evidence whatsoever of any other species. These conclusions are based on the following: 1. At least 4 peaks occur successively at nearly regular intervals to low frequency of the double 4-ring resonance as the pH is decreased such that the average charge per Si is less than - 1. See figure 1. 2. The peak separation between these peaks remains roughly constant as the pH is changed, indicating that the respective structures are similar to the double 4-ring. 3. The peaks evolve at the same rate as that of the double 4-ring when the equilibrium is perturbed. 4. There is no evidence for anything other than the double 4-ring species as witnessed by isomorphous replacement of Si sites with Ge. STANNOSILICATES We have successively prepared a variety of stannosilicate solutions. Both 29Si and 117/119Sn NMR experiments reveal the presence of many small stable stannosilicate polyanions which we are in the process of identifying. We show in figure 2 the first 2D heteronuclear correlated 29Si-117Sn NMR spectrum ever published. The 3 simplest stannosilicate species have been assigned on the basis of connectivity information, relative peak intensity and chemical shift considerations to singly Sn-substituted dimer, linear trimer (Sn occupying the middle site) and cyclic trimer. Interestingly, the 29Si line widths are comparable to those of unsubstituted silicate
487
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=
.
!
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-610 I
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Figure l. Expanded 29Si spectra at 296 K of solutions containing 1.0 tool kg -1 SiO2 and tetramethylammonium hydroxide, pH is adjusted using concentrated HC1. Chemical shift is referenced to the monomer peak which moves to low frequency as pH is decreased (i.e., the spectra are not offset). Peaks A through E are assigned to different protonation states of the double 4-ring anion, i.e., HnSisO2o-(8-n) with n = 0, 1, 2, 3 and 4, respectively.
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Figure 2. An expanded 29Si-117Sn NMR 2D correlation spectrum of a solution with 0.6 mol kg -1 SiO2 (22 % enriched in 29Si), 0.2 mol kg -1 SnO2 (93% enriched in ll7Sn), 2.4 mol kg -1 tetramethylammonium hydroxide, and 0.8 mol kg-1 HC1. The assigned stannosilicate structures are represented with filled circles representing tetrahedral silicate centres, solid circles representing tetrahedral stannate centres and lines representing the shared O linkages.
anions indicating that, unlike the other metal substituted systems reported, replacement of Si by Sn does not result in an appreciable increase in lability. This comparative stability of metallosilicates may well be an important variable in zeolite synthesis. The llTSn spectra further reveal a series of high frequency 1H-coupled resonances that we have tentatively assigned to the octahedral H2Sn(OH) 4 anion and small stannosilicate species containing this octahedral Sn centre. This opens up the exciting possibility of preparing novel Sn-substituted molecular sieves containing both Sn environments. CONCLUSIONS Information provided by solution NMR experiments indicates that organic-mediated synthesis of zeolites proceeds via open cage intermediate structures at the crystal surface that are stabilized kinetically by hydrophobic clathration shells. Although the initial results of 29Si NMR experiments on zeolite precursor solutions were originally misinterpreted, 29Si NMR continues to provide the clearest and most instructive picture of the chemistry of zeolite formation. If the goal of designed zeolite synthesis is ever to be attained, it will undoubtedly rest on the rational examination of the data provided by solution NMR studies.
488 ACKNOWLEDGEMENTS We gratefully acknowledge Kirk Marat (Director of the Prairie Regional NMR Centre, Winnipeg), and Dr. Stacey Zones (Chevron Research and Technology) for sharing his synthesis results. This work was supported by the Lakehead University Senate Research Committee, the U.S. National Institutes of Health (grant GM42208), and the Natural Sciences and Engineering Research Council of Canada. REFERENCES
.
3. .
5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.
R.K. Harris and C.T.G. Knight, J. Chem. Soc., Faraday Trans. 2, 79 (1983) 1525, 1539. C.T.G. Knight, J. Chem. Soc., Dalton Trans. (1988) 1457. S.D. Kinrade and T.W. Swaddle, Inorg. Chem., 27 (1988) 4253, 4260. C.T.G. Knight, R.J. Kirkpatrick and E. Oldfield, J. Am. Chem. Soc., 108 (1986) 30; 109 (1987) 1632. S.D. Kinrade and T.W. Swaddle, Inorg. Chem., 28 (1989) 1952. R.F. Mortlock, A.T. Bell and C.J. Radke, J. Phys. Chem., 95 (1991) 372. R.F. Mortlock, A.T. Bell and C.J. Radke, J. Phys. Chem., 96 (1992) 2968. R.M. Barrer, J.W. Baynham, F.W. Bultitude and W.M. Meir, J. Chem. Soc. (1959) 195. A.V. McCormick and A.T. Bell, Catal. Rev.-Sci. Eng., 31 (1989) 97. C. T.G. Knight, Zeolites, 10 (1990) 140. R.M. Barrer, in Zeolite Synthesis, M.L. Occelli and H.E. Robson (eds.), ACS Symposium Series 398, American Chemical Society, Washington, DC, 1989. C.D. Chang and A.T. Bell, Catal. Lett., 8 (1991) 305. S.L. Burkett, M.E. Davis, J. Phys. Chem., 98 (1994) 4647. S.D. Kinrade and D.L. Pole, Inorg. Chem., 31 (1992) 4558. M. Goepper, H.X. Li and M.E. Davis, J. Chem. Soc., Chem. Commun. (1992) 1665. C.T.G. Knight, R.J. Kirkpatrick and E. Oldfield, J. Chem. Soc., Chem. Commun. (1986) 66; J. Magn. Reson., 78 (1988) 31. R.T. Syvitski, M.Sc. Thesis, Lakehead University, Thunder Bay, 1994. W.M. Hendricks, A.T. Bell and C.J. Radke, J. Phys. Chem., 95 (1991) 9513. S.D. Kinrade, K.J. Maa and R.T. Syvitski, unpublished work. B. Herreros, S.W. Carr and J. Klinowski, Science, 263 (1994) 1585. S. Yang and N.P. Evmiridis, in Zeolites and Related Microporous Materials, J. Weitkamp, H.G. Karge, H. Pfeifer and W. Holderich (eds.), Elsevier, Amsterdam, 1994. S.L. Burkett, M.E. Davis, Microporous Mater., 1 (1993) 265. S.I. Zones, in Proc. Symposium on Synthesis of Zeolites, Layered Compounds and Other Microporous Solids, Anaheim, April 1995, in press. G. Boxhoorn, O. Sudmeijer and P.H.G. van Kasteren, J. Chem. Soc., Chem. Commun. (1983) 1416. A.T. Bell, in Zeolite Synthesis, M.L. Occelli and H.E. Robson (eds.), ACS Symposium Series 398, American Chemical Society, Washington, DC, 1989. P. Bodart, J.B. Nagy, Z. Gabelica and E.G. Derouane, J. Chim. Phys., 83 (1986) 777. R.A. van Santen, J. Keijsper, G. Ooms and A.G.T.G. Kortbeek, in New Developments in Zeolite Science and Technology, Y. Murakami, A. Iijima and J.W. Ward (eds.), Elsevier, Amsterdam, 1986.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
489
Effect of Hydrolysis Conditions of the Silicate Precursor on the Synthesis of Siliceous MCM-48 J. Lujano, Y. Romero and J. Carrazza INTEVEP S. A., P. O. Box 76343, Caracas 1070A, Venezuela
M C M - 4 8 is a mesoporous crystalline material of cubic (Ia3d) symmetry. This work examines the effect of specific synthesis variables such as concentration and mode of addition of both base and surfactant (CTAB) on its synthesis. When the silicate source (TEOS) is hydrolyzed with a dilute base solution and the surfactant solution is added at the early stages of hydrolysis, M C M - 4 8 was obtained at CTAB/SiO2 ratios higher than 0.65. Lower ratios led to the formation of a lamellar material. When the base concentration is high (Na20/H20 = 0.049) a poorly crystallized mixture of the cubic and lamellar materials was obtained, even at a CTAB/H20 ratio equal to 0.65. By varying the base concentration, both the degree of condensation of the silicate oligomers and the relative proportion of monomers in solution changes. Since the cubic phase is obtained with dilute base solutions, it is probably the result of the interaction of the surfactant with monomeric silicate species, while the lamellar material results from the CTAB interaction with the more condensed ones. The delay time between addition of the base and the surfactant, and the mode of addition of the base also influence the type of inorganic material obtained. Short delay times (less than 2 min), and a gradual addition of the base favor the formation of M C M - 4 8 , otherwise a mixture of the lameUar and cubic phases is observed. The interaction of the surfactant with diverse distributions of silicate species in solution could also explain these results. 1. INTRODUCTION Tensioactive agents have been known to modifiy the textural properties of amorphous materials (surface area or pore size distribution), by controlling the gelling and precipitation processes involved [ 1]. Recently, ithas been demonstrated that these agents can have an even greater role of in the prepraration of inorganic materials. Researchers at Mobil Oil Corporation have reported the synthesis of a new family of crystalline mesoporous materials, formed by promoting the ordered condensation of inorganic species in solution, with the aid of cationic surfactants (in particular, tetra-alkyl ammonium salts) [2,3]. One of these new materials, identified as M C M - 4 1 , shows an hexagonal array of non-connected cylindrical channels, while another (MCM-48) is a symmetric cubic system of interconnected
490 channels, with an Ia3d space group [4]. A lameUar material (MCM-50) has also been reported [4]. The pore sizes of these materials range between 20 and 100 A, and is controlled by the size of the hydrophobic tail of the surfactant used. The potential applications of these new materials in the fields of catalysis, separations and electro-optics among others, has generated substantial interest on learning more about the conditions governing their formation [3, 5-8]. It has been proposed that the inorganic phase is induced by the interaction of the head group of the cationic surfactant with the anionic silicates, either through supramolecular (miceUar) arrays that serve as templates for the solid [6] or through discrete silicate-surfactant interactions that drive the ordering of the inorganic species
[5,7]. It has also been suggested that the degree of condensation of the silicate oligomers in solution influence the phase formed [5, 9]. In basic media, the hydrolysis of silicon alkoxides and their further condensation follows a particular dynamic that leads to a distribution of silicate species in solution, with a maximum population of monomers in coexistence with oligomeric species of various degrees of polymerization [ 10]. This distribution, and the extent of condensation of the oligomers (by branching or cross-linking) depends on hydrolysis conditions [ 10,11 ]. This suggests that if alkoxides are used as a silicon source, the conditions governing the hydrolysis and condensation of the silicate species should also influence the phase formed. In this work we have addressed this hypothesis by examining the effect of specific synthesis variables such as concentration and mode of addition of both the base and the surfactant. We have focused our attention on conditions that have been reported to lead to the synthesis of MCM---48, since the formation of this material seems to be particularly sensitive to the silicon source employed [5,8].
2. EXPERIMENTAL 2.1. Materials The reactives employed were tetraethyl orthosilicate (TEOS) 98% from Aldrich, cetyl-trimethyl-ammonium bromide (CTAB) 99% from SIGMA, and sodium hydroxide 98% from PROLABO VENEZOLANA. All chemicals were used as received. 2.2. Synthesis schemes
Effect of the surfactant concentration: A basic solution (solution B) is poured slowly (addition of the base takes around 2 rain) into a continuously stirred mixture of H 2 0 in TEOS (solution A). The surfactant solution (solution C) is added immediately afterwards. The surfactant concentration was varied by changing the amount of CTAB dissolved in a fixed amount of H 2 0 (38 g) in solution C. The CTAB/H20 molar ratio was varied between 0.0041 and 0.021. The N a 2 0 / H 2 0 ratio in solution B was kept constant at 0.012. In all the experiments carried out in this work 11.2 g of TEOS, 1.08 g of NaOH and a total of 60 g of H 2 0 were used. In this series the H20 was distributed in the following manner: 2 g in
491 solution A (with the TEOS), 20 g in solution B (with the NaOH) and 38 g in solution C (with the CTAB). The overall composition of the gel obtained after combining solutions A, B and C can be expressed as 1 SiO2 : 0.25 N a 2 0 : X CTAB : 62 H 2 0 , where X varies between 0.16 and 0.82. The reactant gel is homogenized for 5 rain after addition of the CTAB, and it is then hydrothermaUy crystallized in a 60 m L Teflon lined stainless steel reactor at 100 ~ for 5 days. Effect of the base concentration: The same preparation scheme previously outlined was followed. In this case, however, the base concentration during hydrolysis of the TEOS, expressed as the ratio of N a 2 0 / H 2 0 , was varied between 0.014 and 0.049 by changing the amount of H 2 0 in solution B, while maintaining fLxed the amount of base. Solution C was added immediately after completing the addition of the base. The amount of H 2 0 in solution C was adjusted accordingly, to maintain the H20/SiO2 ratio in the resulting gel constant at 62. The overall gel composition can be expressed as 1 SiO2 : 0.25 N a 2 0 : 0.65 CTAB 962 H 2 0 .
Effect of the delay in addition of the surfactant during hydrolysis of the TEOS: For one of the compositions tested in the previous series that led to the formation of a "pure" cubic phase ( N a 2 0 / H 2 0 = 0.012 in solution B and C T A B A t 2 0 equal to 0.017 in solution C), the surfactant was added at either 2 or 10 rain after completing the addition of the base. These times were selected because from visual inspection it had been determined that, for our experimental conditions, the reaction between solution A and B, before addition of solution C, led to a clear solution for the first 10 min. Beyond this time the solution started to turn turbid, and at times longer than 20 min the presence of small silica particles became evident. Effect of the way the base is added: In one case the base was added gradually and slowly, which is the way it was done in the previously described experiments. In an separate case involving solutions of the same composition, the base was mixed with the TEOS at once, and the surfactant was added 2 rain later. The composition of the solutions for this series was: Na20/H20 = 0.012 in solution B and CTAB/H20 = 0.017 in solution C, for an overall gel composition of 1 SiO2 : 0.25 Na20 : 0.65 CTAB - 62 1-120.
2.3. Characterization of the samples by small-angle x-ray diffraction The spectra were gathered in an angular range 2 e between 0 ~ and 10 ~ The.light source was an Elliot rotating anode x-ray generator (Cu-ko~ line, ~. = 1.54 A, 30 kV x 25 mA). The detection system was a linear position sensitive detector (LPSD) coupled to a multichannel analyzer, with a A(20)/channel = 0.05 ~ 3. RESULTS AND DISCUSSION Figure 1 shows the low-angle x-ray diffraction patterns of the materials obtained at various surfactant concentrations in the gel. (Experiments were carded out by varying the C T A B / H 2 0 ratio in solution C, while maintaining constant the conditions for hydrolysis of the TEOS.) A crystalline material with
492 cubic symmetry (Ia3d) was obtained at CTAB/SiO2 ratios higher than 0.65. Lower ratios led to the formation of a lamellar material. The x-ray diffraction patterns of the cubic material is the same as the one reported by Vartuli et al. for MCM--48, although composition of the parent gel is different. In our case the material was prepared at a CTAB/SiO2 below 1.0 (0.65), while they report values higher than 1.0 [4,8]. On the other hand, the lamellar material obtained in this study does not show the same peak intensity ratios reported by Vartuli et al. for M C M - 5 0 [4]. Its pattern resembles more the one reported by Monnier et al. for a precursor in the synthesis of the MCM-41 [5]. In our case, however, a stable lamellar material was obtained from this series of experiments. When the crystallization time was increased to 8 days, a similar pattern was obtained, but with better defined and more intense peaks. Figure 2 shows the x-ray diffraction patterns for the materials resulting when the base concentration used to hydrolyzed the TEOS was varied, while maintaining constant the surfactant concentration. (The N a 2 0 / H 2 0 ratio in solution B was varied by changing the amount of H20 for a given amount of NaOH. The amount of H 2 0 in solution C was adjusted to maintain the same CTAB/H20 ratio in the overall gel.) When the base concentration is high (Na20/H20 = 0.049) a poorly crystallized mixture of the cubic and lamellar materials was obtained. Neither the proportion nor the crystaUinity of these two phases is affected by an increase in the crystallization time to 8 days. A decrease in the base concentration leads to an increase in the proportion of the cubic material, up to a N a 2 0 / H 2 0 ratio of 0.014, in which case only the cubic phase can be detected by the x-ray diffraction. Since in this series the overall gel composition remains constant, the behavior of the surfactant in solution is expected to be the same in all cases, and thus, it is not considered to be the reason for the differences observed in Figure 2. What I
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Figure 4. Effect of the way the base is added during hydrolysis of the TEOS.
changes, is the distribution of silicate species in solution (from mainly monomeric to oligomeric). By varying the base concentration, the relative rates of hydrolysis and polycondensation reactions are affected. As a consequence, the degree of condensation of the silicate oligomers, and the relative proportion of monomers in solution changes. Since the cubic phase is obtained with dilute base solutions, it is probably the result of the interaction of the surfactant with monomeric silicate species, while the lamellar material results from the CTAB interaction with the more condensed ones. The fact that the proportion of these two inorganic phases does not vary with the crystallization time, suggests that their formation occurs independently of each other. The interaction of the surfactant with different distributions of silicate species could also explain the results shown in Figure 3. In this case surfactant solutions of the same concentration (CTAB/H20 = 0.0165 for solution C) have been added at two different times (either 2 or 10 min after concluding the gradual addition of the base) into identical hydrolyzed-TEOS solutions ( N a 2 0 / H 2 0 = 0.014 and Na20/SiO2 = 0.25). By waiting different times, the surfactant is interacting with diverse distributions of silicate species in solution, resulting from the various stages of the TEOS hydrolysis and silicate condensation.At 2 min, these processes are at their early stages and the concentration of monomers should be high, leading to the formation of the material with cubic symmetry. After 10 min, these processes have advanced considerably, and a larger fraction of oligomeric species should be present, which in accordance with the previously discussed results, leads to a mixture of the cubic and lamellar materials. The way the base is added has also proven to influence the type of inorganic material obtained. (See Figure 4.) A preparation scheme in which the base is added gradually favors the formation of the material with cubic symmetry. Adding the
494 base all at once leads to the presence of both phases. The sudden mixing of the base and TEOS solutions should cause a fast hydrolysis of the TEOS and condensation of the silicate species, allowing the presence of oligomeric species, and thus the formation of the lamellar material. 4. CONCLUSIONS This study indicates that the formation of MCM-48 is controlled not only by the composition of the overall silicate/surfactant gel, but also by the way this gel is prepared. Hydrolysis conditions that favor the presence of silicate monomers in solution, and an adequate amount of surfactant to effectively interact with them are required to form this material. On the other hand, once condensed silicate species are formed, their interaction with the surfactant would lead to the formation of a lamellar material. Our results suggest that the fo..-znation of the lamellar and cubic materials follow separate non-interfering pathways. REFERENCES
1. A. M. Youssef, N. A. Youssef, S. A. Ismail, Bull Soc. Chim. Fr., 128 (1991) 362. 2. J.S. Beck, U. S. Patent N ~ 5,057,296 (1991). 3. C.T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck, Nature, 359 (1992) 710. 4. 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, Stud. Surf. Sci. Catal., 84 (1994) 53. 5. A. Monnier, F. Schiith, Q. Huo, D. Kumar, D. Margolese, R. S. Maxwell, G. D. Stucky, M. Krishnamurty, P. Petroff, A. Firouzi, M. Janicke, B. F. Chmelka, Science, 261 (1993) 1299. 6. C.Y. Chen, S. L. Burkett, H. X. Li, M. E. Davis, Microporous Mater., 2 (1993) 27. 7. 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, B. E Chmelka, Science, 267 (1995) 1138. 8. 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. 9. A. Steel, S. W. Carr, M. W. Anderson, J. Chem. Soc., Chem. Commun., (1994) 1571. 10. L. L. Hench, J. K. Wost, Chem. Rev., 90 (1990) 33. 11. J. K. Bailey, M. L. Mecartney, Colloids and Surfaces, 63 (1992) 151. 12. The authors greatly appreciate the aid of Dr. R. Vargas, and of the Laboratory of Molecular Structures at the Center of Biophysics and Biochemistry of the Venezuelan Institute of Scientific Research (IVIC), for gathering the x-ray diffraction spectra presented in this article.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
495
Sorption of light alkanes on H - Z S M 5 and H - M o r d e n i t e F. Eder, M. Stockenhuber and J. A. Lercher University of Twente, Department of Chemical Technology, Christian Doppler Laboratory for Heterogenous Catalysis, 7500 AE Enschede, The Netherlands 1. ABSTRACT The sorption of light n- and iso-alkanes on H-ZSM5 and H-MOR was studied by means of calorimetry, gravimetry and in situ infrared spectroscopy. Alkanes sorb preferentially localized on the strong Bronsted acid sites of H-ZSM5 and H-MOR. Two n-alkane, but only one iso-alkane molecule can be simultaneously accommodated on the Bronsted acid sites of H-ZSM5. In contrast, only one alkane molecule interacts with a strong Bronsted acid site of H-MOR, and, moreover, only a third of these sites are strongly interacting at all with alkanes because of steric reasons. For both zeolites, the heat of adsorption increases with increasing size of the hydrocarbon. The linear relationship between the enthalpies and the entropies of sorption suggests that the molecular motions of the sorbed molecules are restricted in proportion to the strength of interaction with the zeolite. 2. INTRODUCTION As zeolite catalysts are widely used for conversion of hydrocarbons in petrochemical industry, transport, sorption and reaction of such molecules in acidic molecular sieves has attracted significant interest [ 1]. Several concepts have been proposed to describe the interaction of hydrocarbons with zeolites and, in particular, with the acid sites. Such models either emphasize the local properties of the zeolites to explain the sorption behavior [2,3,4] or use primarily global properties [5,6] for the same purpose. Hydrocarbon sorption is usually thought to depend more on global than on local properties of the zeolite. With respect to this point, Derouane et al. [5,6] described sorption and diffusion in dependence of the relation between the size of the sorbed molecules and the size and shape of the zeolite pores. It was concluded that the strength of interaction is high when the size of the sorbed molecule and the pore size of the zeolite are similar. For a given zeolite structure the chemical composition, i.e., the polarizability, which is a consequence of it, also influences markedly the strength of interaction between the hydrocarbon and the molecular sieve [7,2]. Mortier et al. [3,4] identified the average polarization of the lattice, i.e., the averaged softness or hardness as the further important factor, that influences the hydrocarbon sorption. The softer, the less polarized the lattice of a molecular sieve is, the stronger is the interaction with alkane molecules. Localized sorption between the apolar alkane and the zeolite occurs as the hydroxyl group induces a dipole in the hydrocarbon. Thus, the strength of the interaction will be determined by the acid strength of the hydroxyl group and the polarizability of the CH groups of the alkanes. The current communication addresses the description of the interaction of alkanes on acidic ZSM5 and MOR. Gravimetry, calorimetry and i.r. spectroscopy are used as the main experimental means.
496 3. EXPERIMENTAL
3.1. Material H-MOR (Si/AI= 10) is a dealuminated mordenite obtained from the Japanese Catalysis Society (for a detailed charcterization see ref [8]). Dealumination was carded out by a procedure described previously [9]. H-ZSM5 (Si/AI=35) was supplied by MOBIL Corp. 3.2. Gravimetry and calorimetry The measurements were performed with a SETARAM TG-DSC 111 instrument. A quantity of about 15 mg of the NH4-form of the samples was charged into the quartz sample holder of the balance and transferred into the hydrogen form by heating in vacuum to 823 K for one hour. After cooling to 323 K, the alkanes were stepwise introduced into the closed system and equilibrated with the surface. The equilibration was confirmed by observing the heat flow and the changes in weight. If significant changes were not observed the system was considered to be equilibrated. The experiments were carded out in the pressure range from 103 to 13 mbar. The pressures were recorded with a BARATRON pressure transducer. The adsorption isotherms of all the hydrocarbons were determined simultaneously with the differential enthalpies of adsorption. The isotherms were fitted to a sum of Langmuir type adsorption isotherms taking into account the differences in adsorption stoichiometries. The concentration of acid sites used in the calculations was taken from ammonia adsorption from ref.[ 10]. From the equilibrium constants and the heats of adsorption, entropies of adsorption were calculated. 3.3. Infrared spectroscopy The studies were performed with a BRUKER IFS-88 spectrometer. The zeolites were pressed to self supporting wafers and introduced into the heatable sample holder of a vacuum cell. The samples were activated by heating in vacuum (p < 10.6 mbar) to 823 K and cooled to adsorption temperature. The sorption experiments were carded out from 303 K to 373 K and in pressure range from 10-3mbar to 3 mbar. The alkane partial pressure was raised stepwise and after every pressure increase equilibration was observed by time resolved infrared spectroscopy. The decrease in intensity of the OH stretching vibrations band of the Bronsted acid sites (3610 cm~) was used to determine the coverage of the acid sites. The integral intensities of the CH-stretching (2800-3000 cm~) vibrations of the adsorbed alkanes (calibrated by gravimetric measurements) were used to determine the amounts adsorbed. 4.RESULTS
4.1 Adsorption on H-ZSM5 The values for the sorption enthalpy of the hydrocarbons are compiled in Table 1. For H-ZSMS, the sorption enthalpies of n-alkanes were constant up to a coverage of two molecules per Bronsted acid site. At higher coverages, the values decreased (see Fig. 1). For iso-alkanes, however, the heat of adsorption decreased already at a coverage of one iso-alkane molecule per acid site. Infrared spectra show that only the band of the Bronsted acid Si-OH-AI groups at 3610 cm~ decreased in intensity upon adsorption of the hydrocarbons. At sufficiently high partial pressures (e.g., 1 mbar of n-hexane at 303K) the hydroxyl band disappeared indicating that all Bronsted acid sites of H-ZSM5 interacted with the alkanes. At low coverages the uptake of hydrocarbons corresponded exactly to the fraction of free
497 hydroxyl groups that disappeared. This suggests that one molecule H [kJ/mol] is adsorbed per acid site. When 100 more than 30% of the acid sites were covered, the number of n80 alkanes adsorbed per acid site exceeded markedly this 60 stoichiometry indicating that hydrocarbon molecules adsorbed 40 on Si-OH-A1 groups already 20 interacting with one alkane molecule, or were adsorbed delocalized, while other Bronsted O0 0.5 1 1.5 2 2.5 3 acid sites were still free. Coverage [molec./site] Upon sorption of the alkane molecules, a broad band Figure 1 Adsorption of n-hexane on H-ZSM5 characteristic of hydrogen bonded (perturbed) OH groups appeared at lower wavenumbers. The wavenumber difference between this band and the band for the free Si-OH-AI groups increased with increasing size of the hydrocarbons indicating an increase in the strength of the direct interaction with the acid sites [ 11 ]. For a given n-alkane the wavenumber of the band of the perturbed OH, however, also decreased with increasing coverage. Because of the uniform acid strength of the hydroxyl groups in H-ZSM5 [ 10] and in agreement with observed stoichiometries of adsorption, we attribute this red shift of the perturbed Si-OH-AI group to an increasing fraction of OH groups interacting with more than one molecule. Such multiple interactions enhance the perturbation. The asymmetric form of the band of the perturbed hydroxyl groups allowed to separate two contributions, i.e., the band between 3570 and 3500cm 1 (depending on the substance adsorbed) attributed to sorption of one and the band between 3445 cm 1 and 3485 cm 1 attributed to sorption of two molecules per acid site. The 1:1 stoichiometry contributed most at low coverages, the 2:1 stoichiometry at high coverages. It should be emphasized that such a sorption behavior was only observed for n-alkanes on H-ZSM5. The band of the perturbed OH groups after adsorption of iso-alkanes had a symmetric form and the absorption maximum was independent of the coverage.
....
.-. ,~,..-,~,-n~--"-
__mm
,
L
__
1
,
i
,
m _ _
1
,
m[_
1
~,~
-ram
,
i
,
Table 1 Heats of adsorption on and maximum sorption capacity at 323K on H-ZSM5 and H-MOR H-ZSM5 H-MOR AHa~ [kJmol "1]
Loading [mmol/g]
AHa,~ [kJmol "l]
Loading [mmol/g]
propane
- 46
1.47
- 41
0.9
n-butane
- 58
1.39
- 50
0.75
iso-butane
- 52
0.67
-52
0.64
n-pentane
- 70
1.28
- 59
0.65
iso-pentane
- 64
O.71
-61
O.61
n-hexane
- 82
1.24
-69
0.61
498 4.2. Sorption on H-MOR The values of the enthalpies of sorption of the alkanes on H-MOR are compiled in Table 1. Up to a coverage of 0.35 to 0.45 alkane molecules per strong acid site the heat of adsorption on H-MOR was nearly constant. At higher coverages the heat of adsorption decreased suggesting that only about 40% of the acid sites are accessible to alkane molecules. The intensity of the band of the Bronsted acidic Si-OH-AI group at 3610 cm1 decreased upon the sorption of alkanes to a minimum of two third of its original value. The asymmetric form of this band suggests the existence of two kinds of acid sites characterized by two bands with maxima at 3610cm 1 and 3 590cm -1, respectively (see Fig.2). They do not differ in acid strength (as seen by sorption of ammonia [ 10]) but in location and environment. The fraction of the acid sites characterized I -i .I 3610 cm 3590 cm by the band at 3610 cm1 that .3 r- i disappeared upon hydrocarbon sorption caused a band characteristic of hydrogen bonding some 90 to 115 cm~ lower than the wavenumber of the :5 .5free Si-OH-AI group. As with H,m ~ .4ZSM5, the wavenumber decreased with the chain length of the adsorbed 1,1 .3o molecules. In contrast to the observations on H-ZSM5 the shift was independent of the coverage for a given alkane. The concentration of .1hydrocarbon molecules adsorbed 36'00 3s00 3g00 corresponded well to the fraction of Wavenumber [cm~] hydroxyl groups in interaction. Thus, Figure 2. Adsorption of n-hexane on H-MOR. The lines we conclude that one alkane is adsorbed per accessible Bronsted acid correspond to increasing equilibrium pressures of nsite. hexane.
-
il
5. DISCUSSION I.r. spectroscopy indicates that most of the hydrocarbons adsorbed are bound via hydrogen bonds to the Bronsted acid sites of the two zeolites studied. This bonding involves induction of polarity in the alkane by the hydroxyl group. The strength of the bonding (indicated by the difference of the band of the perturbed and the unperturbed OH groups [ 11 ]) depends upon the extent of the polarization of hydrocarbon C-H bonds and the strength of the Bronsted acidic OH group. The difference in wavenumbers between the free and the perturbed Si-OH-AI group was at least for the larger hydrocarbons quite similar for H-ZSM5 and H-MOR. Thus, we conclude that the Si-OHAI groups in the H-ZSM5 and H-MOR investigated have a similar acid strength with respect to hydrocarbons. Previous characterization by sorption of strong bases [ 10] indicated also a rather similar acid strength of the two zeolites. With increasing size of the sorbed alkanes the strength of hydrogen bonding to Bronsted acid sites (monitored via the difference in wavenumbers of perturbed and unperturbed Bronsted acid
499 OH groups) and the sorption enthalpy increased. This indicates that the polarizability of the alkanes increases with increasing carbon numbers of the alkanes. It is striking, however, that in contrast to the strength of the direct hydrogen bonding to the Bronsted acidic OH groups, the increase in enthalpy is more pronounced with H-ZSM5 than with H-MOR. In absolute terms, the heats of adsorption of all n-alkanes investigated was at least 5 to 13 kJ/mol higher on H-ZSM5 than on H-MOR. This indicates that the main reason for the higher sorption enthalpy with H-ZSM5 are the more intense lateral interactions of the hydrocarbons with the lattice. Such an interpretation agrees well with previous communications reporting that the van der Waals interaction between alkane molecules and the lattice of zeolites increases with the chain length of the sorbed alkanes [5,12,13]. We would like to attribute this higher strength of lateral interactions between the alkanes and the zeolite lattice to the smaller pore size of H-ZSM5 [5,6] and/or to the higher polarizability of its lattice ("softer character" of H-ZSM5 [3,4]) which should cause stronger interactions with soft bases like alkanes. A linear relationship between the enthalpies and the entropies of sorption was found suggesting one common adsorption structure for all n-alkanes and that the molecular motions of the sorbed molecules are restricted in proportion to the strength of interaction with the zeolite. Different slopes for H-MOR and H-ZSM5 suggest that the structure of the zeolite has a distinct influence on the molecular motions of the adsorbed molecule. The same enthalpy of adsorption results in higher entropy of adsorption for H-ZSM5 compared with that for H-MOR indicating again a more pronounced interaction of the alkanes with the lattice H-ZSM5. With respect to the sorption stoichiometry at higher loadings, a unique behavior was found for the n-alkanes in H-ZSM5. n-Alkanes are adsorbed localized in a stoichiometry of up to two molecules per acid site of H-ZSM5. Indeed, the maximum sorption capacity of iso-alkanes on HZSM5 is approximately a factor of two smaller than the maximum sorption capacity of n-alkane molecules. In contrast, the gravimetric and calorimetric results and the correlation between the intensity of the CH i.r. bands of the sorbed alkanes and OH band of the acid sites indicate that the n- and iso-alkanes are sorbed in a stoichiometry of one molecule per accessible acid site in H-MOR. Therefore, the maximum uptake of n-alkanes in H-MOR was significantly lower than in H-ZSM5. Because localized sorption of two alkane molecules per acid site was not observed for any of the large pore zeolites, we conclude that this unique sorption of n-alkanes in H-ZSM5 is due to the excellent fit between two adlineated n-alkanes and the channels or the intersections of the zeolite. This view is supported by the fact that the adsorption enthalpy was the same, if one or two molecules were adsorbed. As the addition of a further molecule to an already sorbed molecule must lead necessarily to a lower strength of the direct interaction, the constant enthalpies point to a strong stabilization of the sorption complex by the surrounding. This seems possible for the medium size pore ZSM5, but not for the large pore H-MOR. Finally, as shown by i.r. spectroscopy, all Bronsted acid sites of H-ZSM5, but only one third of the acid sites of H-MOR strongly interacted with the alkane molecules. Because the acid sites showed uniform acid strength, we conclude that the alkanes cannot interact with some the acid sites in H-MOR because of sterical reasons. These acid sites are concluded to be located in the side pockets of the H-MOR channels [ 14,15]. The entrance of the side pocket has an diameter of 0.39 x 0.57 nm, the opening at the end of the pocket has a diameter of 0.39 x 0.28 nm, which is substantially smaller than the aperture of the large pores (0.67 x 0.7 nm). The critical diameter of linear alkanes was calculated to 0.45 nm [ 16,17]. Hence acid sites in the side pockets must be very difficult to reach for the alkane molecules. A similar effect was .observed for the adsorption of benzene and cyclohexane on H-MOR [18].
500 6. CONCLUSIONS The present report shows the distinct differences between sorption of light alkanes on zeolites H-ZSM5 and H-MOR. While all Bronsted acid sites of H-ZSM5 are accessible for alkane molecules, only one third of the acid sites of H-MOR (i.e., the fraction located in the main channels) strongly sorb these molecules. With H-ZSM5 the n-alkanes are sorbed locally at the acid sites in a maximal stoichiometry of two molecules per acid sites, the bulkier iso-alkanes in a one to one stoichiometry. With H-MOR one molecule of n- and iso-alkanes is sorbed locally per accessible acid site. This localized direct interaction (hydrogen bonding) between the alkanes and the strong Bronsted acid sites is of similar strength with both zeolites. The hydrogen bonding occurs as the charge at the Bronsted acid site induces a dipole in the alkane. The interaction gains strength with increasing chain length of the hydrocarbon due to (i) enhanced polarization and (ii) stronger lateral interactions of the hydrocarbons with the lattice oxygen. For a given hydrocarbonH-ZSM5 forms tighter bound surface complexes than H-MOR reflected in a larger heat release and a more pronounced entropy loss due to sorption. ACKNOWLEDGEMENTS Financial support of the Christian Doppler Society is gratefully acknowledged. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
W.O. Haag, Stud. Surf. Sci. Cat., 84 (1994) 1375. A. Corma, G. Sastre, R. Viruela and C. Zicovich-Wilson, J. Catal., 136 (1992) 521. K.A. Genechten and W. Mortier, Zeolites, 8 (1988) 273. W. Mortier, Stud. Surf. Sci. Catal., 37 (1988) 253. E.G. Derouame, J.B. Janos, C.Fernandez, Z. Gabelica, E. Laurent and P. Maljean, Appl. Catal., 40 (1988) L 1. E.G. Derouane, J.-M. Andre and A.A. Lucas, J. Catal., 110 (1988) 58. R.G. Pearson, J. Am. Chem. Soc., 85 (1963) 3533. M. Sawa, M. Niwa and Y. Murakami, Zeolites, 10 (1990), 532. G.I. Kapustin, T.R. Brueva, A.L. Klyachko, S. Beran and B. Wichterlova, Appl. Catal. 42, (1988) 239. M. Stockenhuber and J.A.Lercher, in preparation. M.L. Hair and W. Hertl, J. Phys. Chem., 74 (1970) 91. J.O. Titiloye, S.C. Parker, F.S. Stone and C.R.A. Catlow, J. Phys. Chem. 95 (1991) 4038. B. Smit and J.I. Siepmann, J. Phys. Chem., 98 (1994) 8442. W.M. Meier, Z. Kristallogr. 115 (196 l) 439. K. Shiokawa, M. Ito and K. Itabashi, Zeolites, 9 (1989) 170. E.G. Derouane, "Intercalation Chemistry", Academic Press, New York, (1984) 101. R.M. Barrer, "Zeolites: Science and Technology", NATO ASI Series E, Martin Nijhoff, The Hague, 80 (1984) 227. V.L. Zholobenko, M. A. Makarova and J. Dwyer, J. Phys. Chem. 97 (1993) 5962.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
501
Acidity and reactivity of steamed HY zeolites obtained by progressive extraction of extraframework AI species L. Mariey, S. Khabtou, M. Marzin, J.C. Lavalley, A. Chambellan and T. Chevreau Catalyse et Spectrochimie, URA CNRS 0414 - ISMRA-UNIVERSITE - 14050 Caen Cedex, France Two series of steamed HY zeolites progressively leached with HCI(1N), were studied by FTIR. The numbers of each zeolitic hydroxyl type were determined by analysis of the complex v(OH) massif. The numbers of zeolitic hydroxyls remain nearly constant for a wide range of acid leaching, during progressive extraction of AI extraframework species, however, the relative quantities of strongly acidic hydroxyls go through a maximum for a mild leaching. The catalytic activity of these zeolites for benzene hydroconversion confirms their acidities. 1.
INTRODUCTION
Acid leaching of steamed HY zeo;ites generates zeolitic hydroxyls due to the partial exchange between AI cations and protons. IR spectroscopy studies have shown that perturbation of framework hydroxyls (HF and LF) by extraframework species led to stronger acid hydroxyls, characterized by bands near 3600 and 3525 cm -1 [1-4]. Such a perturbation would occur through inductive effects due to aluminic cationic species localized in sodalite cages [5]. According to this assumption, the progressive extraction of extraframework species should allow optimisation of the number of highly acidic hydroxyls. The v(OH) massif of steamed HY zeolites is complex [6, 7] and results from the superposition of bands due to framework and extraframework OH groups. Recently, we published a method allowing the distinction and counting of each type of zeolitic hydroxyls [8]. This method has been applied in the present study to quantify the remaining OH groups after progressive extraction of extraframework AI species. Results are correlated to the catalytic activity of the leached samples towards benzene hydroconversion, a reaction found sensitive to the Br6nsted acidity [3]. 2. E X P E R I M E N T A L
SECTION
The HY catalysts studied stemmed from the LZY62 zeolite (Union Carbide) on which a double steaming was carried out, the first at 810 K and the second at 810 or 860 K [3]. Two series of zeolites were obtained by progressive acid leaching with HCI(1N) at 350 K. The leaching strength, LX, is given by the HCI volume (expressed
502
in cm 3) per gram of dehydrated zeolite i.e. the number of H+ millimoles per g. The two series are named S(810)LX and S(860)LX (X = 0 until 11.5). Chemical analysis of the powders and eluents allows the AI extractions to be followed. Table 1 gives the approximate molar formulae of the studied samples according to [3]. The SiO2 amount is always less than 5 SiO2 per u.c. except for the strong leaching (L 11.5). The data are expressed per unit cell of the crystalline zeolite. The FTIR spectra were recorded from self-supported wafers of activated zeolites at 720 K, under reduced pressure (10 -3 Pa, wafer of 10 mg). Benzene hydroconversion was carried out in a microreactor, under hydrogen flow (PH2 = 40 bars) at 600 K [3]. Table 1 Approximate molar formulae Sample
Crystalline u.c.
S(810)LX S(860)LX
Hy(AIO2)18(SiO2)174 Hy(AIO2)I 5(SIO2)177
AI01.5 unit in the extraframework phase L0 L2.5 L3.75 L5 L7.5 L8.75 48.2 32.1 52.4 -
29 31.1
24.3 30
11.8 14.8
10.7
3. RESULTS AND DISCUSSION 3.1 Effect of the progressive acidic leachings Figure 1 shows the variation in the v(OH) massif of the progressively leached zeolites S(860)LX. After steaming and before leaching, the intensity of the v(OH) massif is low (sample L0), some unidentified AI cations counterbalance the main part of the negative charges due to the framework AI atoms. The limited acid leaching, L3.75, gives rise to a clear increase of the v(OH) massif resulting from AI n+ <--> nH+ exchanges. The subsequent formation of zeolitic hydroxyls is characterized by the classical HF and LF bands and the two additional bands at 3600 and 3525 cm -1 described as zeolitic OH groups perturbed by extraframework species [1-4]. The progressive leachings modify the relative intensities of these four bands, therefore, counting these OH groups allows the quantitative description of these acidic solids. IR spectra analysis of the v(TOT) bands in the 1100-600 cm -1 range reveals a framework attack in the case of L l l . 5 , [9], explaining the appearance of the very strong SiOH band after such a treatment (Fig. 1). IR spectra of the series S(810)LX are similar except for the unleached sample which is more hydroxylated in agreement with the decrease of the steaming temperature. 3.2. Quantitative determination of the zeolitic hydroxyls Pyridine adsorption at 470 K, followed by a brief evacuation, allows the discrimination between acidic and non-acidic (or no accessible) OH components. Details for deconvolution of the v(OH) massif and molar extinction coefficients are given in [8]. Figure 2 shows the two steps of deconvolution, i) decomposition of the
503
(3600!OH
~-1
:
O
=
to
~
'
!/\;(3s2s)o.
i
ii
I
~
I
'
•
cD
~q
1.3.75
Wavenumber / c m -1
i
I c m "1
-~-
_
_..,
3800
Figure 1. Massif v(OH) of the series Figure 2. Zeolite S(810)L3.75 - A : v(OH) massif of steamed S(860)LX zeolites - B, C : decomposition of perturbed and unperturbed massif, after pyridine adsorption at 470 K acidic massif perturbed by pyridine adsorption and obtained by difference of spectra 2A - 2C (Fig. 2B), ii) decomposition of the non-acidic massif due to extraframework OH groups and unperturbed LF hydroxyls (Fig. 2C). Since the band half-width decreases when the dealumination level increases, two HY zeolites free of extraframework species were used to fix the shape of HF and LF bands : HY (6.8) for the series S(810)LX, [8], and the leached zeolite S(910)L15 for the series S(860)LX, their Si/AIf ratios were 6.8 and 15, respectively. The molar extinction coefficients used were : s = 7.5 #mo1-1 cm s = 4.7 #mo1-1 cm ~LF = 5.6 ~mo1-1 cm ~pyH+ = 1.8 ~mo1-1 cm (band at 1542 cm-1). - Acidic zeofitic hydroxyls
Deconvolution of the massif due to acidic hydroxyls (Fig. 2B) shows that it results exclusively from framework hydroxyls, HL, LF, (3600)OH and (3525)OH ; their numbers versus the amount of HCI used are reported in Fig. 3. As expected, they strongly increase after the first leaching. The following leachings only slightly increase their total number in spite of the deep extraction of extraframework aluminium atoms. The latter reaches 80 % in the case of the sample S(860)L8.75 (Table 1). Whereas the numbers of HF and LF acidic OH groups slightly increase, the number of strong acidic hydroxyls (bands at 3600 and 3525 cm -1) seems to be maximum for the L5 leaching. In all cases, the intensities of 3600 and 3525 cm -1 bands are almost equal. The total number of acidic hydroxyl groups, never higher than 5 OH per u.c., must be equal to the amount of pyridine species formed as
504 El Zeolilic acid Oi ! [] (i ! F)OI ! [] (!.i;)O! i [] (3600+3525)OI !
,3 3
o
E 2
k
t-.... 0
t
3.75
i
5 cm31g 7.5
t
I
8.75
t
11.5
Figure 3. Numbers of various acidic hydroxyls of the zeolites S(860)LX (massif perturbed by pyridine adsorption). observed from the intensity of the pyH + band at 1542 cm -1 which validates the method used : the difference N(pyH+)-N(acidic OH) is always lower than 0.4 per u.c. Similar features are observed in the series S(810)LX. However, the decrease of the zeolitic hydroxyl number already begins for the leaching L7.5. This less dealuminated series is therefore less stable toward acid leaching, and of interest for the higher number of strongly acidic OH groups (3.6 per unit cell, instead of 2.4 for S(860)L5). - Total zeofitic and non-zeofitic hydroxyls
Hydroxyls unperturbed by pyridine (Fig. 2C) are constituted by i) a part of LF groups and ii) extraframework non acidic OH groups, characterized by a broad band at 3607 cm -1 [8]. Quantitative results reported in Table 2 show that the contribution of the unperturbed LF hydroxyls to the total amount (Fig. 3) is not negligible. As for nonacidic extraframework OH groups at 3607 cm -1, we are unable to determine their amount due to their unknown molar extinction coefficient. The leaching effect on hydroxylation of the amorphous phase occurs as soon as the sample is in contact with the acidic solution and, simultaneously, attack of this hydroxylated phase begins. The amorphous phase dissolution depends on the amount of protons introduced, decreasing progressively the intensity of the broad band on which the zeolitic OH bands are superposed and hence modifying the relative apparent intensities of the HF and 3600 cm -1 bands (Fig. 1). The total number of acid and non-acid zeolitic hydroxyls and the sum of HF and LF groups are reported in Figure 4 for the series S(860)LX. Variations similar to that reported in Figure 3 are observed. We can note that the N(3600+3525)OH / N(HF+LF)OH ratio is maximum for the leaching L5 and corresponds to the optimization of the strong acidic hydroxyl number. We conclude that only a small part of the acid is used to exchange some AI cations of the solid : c.a. 5 H+ per u.c. appear. The other part partly dissolves the aluminic amorphous phase, with formation of AI cations in the liquid and without attack of the framework (except for the strong leaching L11.5) : in the case of the
505
Table 2 Deconvolution of the non-acidic v(OH) massif for the zeolites S(860)LX LX
0
3.75
7.5
8.75
11.5
(LF)OH / u.c. (3607)OH band intensity
0.21 2.72
0.78 4.88
0.69 4.00
0.34 2.55
0 0.61
_
1o.o
d
0
3
0.4
~-
6
; ~
0.4 4
2 O.2
0.2 2
1
cm3 / g
0L 0
3.75
5
7.5
J 8.75
11.5
o
'
o 0
2.5
3.75
o 5
7.5
Figure 4. Numbers of acidic and non-acidic zeolitic hydroxyls ; a :total zeolitic OH ; b : (HF + LF)OH ; c : variation of the ratio N(3600+3525)OH / N(HF + LF)OH ; A : zeolites S(860)LX ; B : zeolites S(810)LX.
sample S(860)L5, 71 H + per u.c. were introduced, 5 protons have generated 5 zeolitic hydroxyls and the remaining have partly hydroxylated this phase and have dissolved 22 extraframework AI atoms. Under limiting leaching conditions, regardless of the amount of neutral solid species remaining, the equilibria between AI cations in the zeolite and in the liquid limit the appearance of zeolitic OH groups. With the second series of solids, S(810)LX the results are similar. Moreover, attempts to extract the AI extraframework phase at pH 1 or 2, in a large excess of H +, at room temperature have failed. At pH = 1, the zeolitic framework of the S(860)L0 sample was completely destroyed ; at pH = 2, only 50 % of the extraframework amount were extracted. 3.3. B e n z e n e
hydroconversion
Over steamed HY zeolites, the benzene hydroconversion is a complex reaction which leads to i) direct hydrogenation products : cyclohexane and its abundant isomer, methylcyclopentane ii) light aikanes resulting from cracking : propane, isobutane, butane, cyclopentane and methyl-2 butane iii) alkylation products : mainly toluene, ethylbenzene and traces of xylenes. This reaction has already been described in [3] for HY zeolites steamed at various temperatures and the catalytic
506
100%
iiiiiiiii!iiiiii iiii ii i ii i iii i i i iiiii ii!!!!!i!!!i i!iii!i!ii!!ii i!i !i i !!ii i i i i i i i i !!ii!!ii!i!!!ii!il
~oo,o ii!!iiiii!i!iiiii!ii!iii!ii!i!!i!ii!!ii!i!ii!!i!iii!iiii!!!!!!i!!!i!iii!i!!!!!! ~iiiiiiiiiiiiiiiiiiiiiiiiii~iii~iiiiiiiiiiiii!~iii!~ii~iiiiiii~!~!~ii!iii!iiiii~ ~ ,oo~ iiiiiiiiiiiiiiiiiii~{~iiiiiiiiiiiiii!iiiiiiiiiiiiill
!ii!iii!i!iiiii!iiiiiiiiiiiiii!iiiii!ii!iiiii !iiiiiiiiiiiiiiiiiiiii iiiiiiiiiiiiiiii!ii!iiiii!i!i •!i•i••iiii•i•i•••ii•i•i•ii•i•ii•i•i•i•i•i••ii•••ii•••!i•i••ii ~i~ii~i~iiiii!iii!ii~ii~ii~i~iiiii!ii~i~!iiiii!i!i~!i!~ iiiiiiiiiiiiili~iiiii~i..t.:~iiiiiiiiiiiiiiiiiiiiiiiiiiiiiil,
••i!i•ii•!•i•!i•i!i!i!ii!i•!•••i!ii•!i!i•i•ii•i•i•i•••••
40% iii•i!i!ii!ii!i{i!i!iii•!!ii!iii•ii•!i!iiiii!!i••i•i!i!!•!i!•iiiiiiiiiiiii!!•!!!!• 20% ii•iiii!••i•}i}i{i••••!••!ii!i••i•!!!!i!i!i!i••!!!!!!•!•!!!!!!!{•!•!•!!•:•
453
1344 ~-A'~
2086
time /Ini,)
2828
470
1213 [~-~--J
2048
2791
time In|in
Figure 5. Benzene hydroconversion ;i) total conversion A S(860)L0 9 ; B S(860)L3.75 9 ; C S(860)L7.5. 9
490
1233 ~_~C ]
1975
2719
,i .... / mi,,
t9 ;ii) selectivity.
activities (alkylation and hydrogenation rates at their maximum) have been correlated with the intensities of the band due to the strongly acidic (3600)OH groups. For the catalysts studied here, the number of 3600(OH) groups is almost equal to the half-sum of (3600+3525) hydroxyls. Figure 5 shows the total conversion and the selectivity obtained with three catalysts S(860)LX, at 600 K. As in [3], for each sample, the alkylation products are the most abundant although the alkylation is undoubtedly a secondary reaction. The two samples S(860)L0 and S(860)Lll.5, poor in strongly acidic hydroxyls are hardly active and give similar results shown in Figure 5A. With the mild leached samples (L3.75, L5, L8.75), the counting of strongly acidic 3600 hydroxyls give similar values as can be seen in Figure 3. In the same way, these catalysts lead to similar activities as shown in Figures 5B and 5C. With these samples, direct hydrogenation can also be observed, the conversions and selectivities are almost the same. In conclusion, the catalytic activity of this series of leached HY zeolites follows the number of acidic hydroxyls, the progressive mild leachings dissolve the aluminic amorphous phase effectively but lead to solids whose acidities are not very different. REFERENCES
1. P.O. Fritz and J.H. Lunsford, J Catal., 118 (1989) 85. 2. G. Garralon, A. Corma and V. Fornes, Zeolites, 9 (1989) 84. 3. A. Chambellan, T. Chevreau, S. Khabtou, M. Marzin and J.C. Lavalley, Zeolites, 12 (1992) 306. 4. M.A. Makarova and J. Dwyer, J. Phys. Chem., 97 (1993) 6337. 5. F. Lonyi and J.H. Lunsford, J. Catal., 136 (1992) 566. 6. C.V. Mac Daniel and P.K. Maher, Molecular Sieves, Society of Chemical Industry, London, 1968, p. 186. 7. J. Scherzer and J.L. Bass, J. Catal., 28 (1973) 101. 8. S. Khabtou, T. Chevreau and J.C. Lavalley, Micropor. Materials 3 (1994) 133. 9. O. Cairon, S. Khabtou, E. Balanzat, A. Janin, M. Marzin, A. Chambellan, J.C. Lavalley and T. Chevreau, Stud. Surf. Sci. and Catal., 84 (1994) 997.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
507
Adsorption of Nitrogen and Methane on Natural Clinoptilolite L. Predescu, F.H. Tezel*, and P. Stelmack Department of Chemical Engineering, University of Ottawa, 161 Louis Pasteur, Ottawa, Ontario K 1N 6N5, Canada The volumetric method was used to study the adsorption of pure N 2 and CH4 gases on natural clinoptilolite from Bigadic, Turkey. The Langmuir equation and the Flory-Huggins form of the Vacancy Solution Theory were used to fit the pure gas adsorption isotherms obtained for N 2 and CH4, and the binary adsorption isotherms as well as the binary adsorption phase diagrams of these gases were predicted by the Extended Langmuir Model, the Flory-Huggins form of the Vacancy Solution Theory, and the Ideal Adsorbed Solution Theory. The influence of the structural, physical and chemical characteristics of these adsorbates on clinoptilolite is discussed to explain the behaviour predicted by each model.
1. INTRODUCTION The objective of this work was to provide additional pure gas adsorption data that would improve our understanding of the phenomena which occur during physical adsorption on clinoptilolite (which is abundantly available in nature), and to permit an insight on the potential of this molecular sieve in the separation ofNz-CH 4 gas mixtures. For this purpose, pure adsorption isotherms were determined experimentally for N 2 and CH4 gases for this natural clinoptilolite at room temperature and at 40~ These isotherms were fitted with the Langmuir equation, as well as the Flory-Huggins form of the Vacancy Solution Theory (FH-VST) [1, 2]. Binary isotherms for N2-CH4 mixtures as well as XY diagrams for adsorption were then predicted using the Extended Langmuir Model (ELM) [3], the Ideal Adsorbed Solution Theory (IAST) [4], and the FH-VST for mixtures [ 1, 2]. Conditions for these predictions were 101.3 kPa total pressure at room temperature and at 40.0~ Frankiewicz and Donnelly [5] have studied the diffusivities of these gases in clinoptilolite and have shown that the kinetic separation factor values depend on the composition of the sample which results from the AI content (the number and type of cations neutralizing the anionic framework). These separation factors increase with temperature because of faster N 2 diffusion. Ackley and Yang [6] have used cation exchange in clinoptilolite to produce selectivity values higher than 1 for the difficult adsorptive separation of the N2-CH4 mixture.
* to whom correspondence should be addressed
508 The adsorption isotherms o f N 2 and CH 4 they determined at 300 and 323 K on various ionic forms of clinoptilolite confirmed that both the molecular sieving effect and the energetic heterogeneity exhibited by the Ca2§ ions in the zeolite were responsible for higher capacities for N 2 compared to CH4. 2. EXPERIMENTAL The pure gas adsorption isotherms of N 2 and CH4 gases on a natural clinoptilolite sample from Bigadic, Turkey (Si/AI = 4.25) rich in divalent cations, predominantly Ca 2§ [7], were determined at room temperature and 40.0~ using the volumetric method. The particle mesh size of the zeolite was 25-42 mesh. 3. RESULTS AND DISCUSSION
3.1. Pure Gas Adsorption Isotherms Figures 1 and 2 present the experimental pure-gas adsorption isotherms of the two adsorbates on the Turkish clinoptilolite rich in divalent cations at room temperature and 40~ along with the Langrnuir and FH-VST fits. The pure-component isotherms of N 2 and CH4 cross at a low pressure (4 kPa at room temperature and 8 kPa at 40.0~ as seen in Figure 3. Ackley and Yang [6] have reported similar observations for natural clinoptilolite samples from California.
1
1
_~0.8
,.~ 0.8
22.6 C
m
22.7 C
0.6
9, ' "
"~ 0.4
"t~ 0.4
0.2
r,.,) 0.2 I
20
I
.--
0.6
I
I
40 60 80 Pressure, kPa
I
100
Figure 1. Nitrogen adsorption on Turkish clinoptilolite Points are experimental data Langmuir fit FH-VST fit
120
0
I
I
I
I
I
20
40
60
80
100
120
Pressure, kPa
Figure 2. Methane adsorption on Turkish clinoptilolite Points are experimental data Langmuir fit FH-VST fit
509
(a)
,.~ 0.8
CH4 ,
0.6
,111.
.....a- ........ 9
,.~ 0.8 CH4
9. . . . . . . . . . I .
......
...-"""
0.4 "6 r r~ 0.2
..-
.. 9
/ .m-"
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"~ 0.4
N2
,, ,
!
...-
.1~..--~
0.6 ~'---"~
. ....... ~ ......... .....
~
[]
.....
,..
N2
$-
0.2
..o
r I
0 0
20
1
I
I
40 60 80 Pressure, kPa
1
100 120
0
I
0
20
I
I
1
40 60 80 Pressure, kPa
I
100
120
Figure 3. Comparison between CH 4 and N 2 adsorption on Turkish clinoptilolite at: (a) room temperature; (b) 40.0~ (experimental points and FH-VST fits)
FH-VST reflects the capacity of clinoptilolite more realistically for the adsorbates in the pressure range that is studied, and the presence of the vacancy-adsorbate interaction parameter appears to be responsible for the accuracy associated with each fit. The excellent FH-VST fits represent the pure-component adsorption system much better than the Langrnuir fits. This model forecasts the formation of an azeotrope in the binary mixture adsorption of these two adsorbates on the natural clinoptilolite studied, at both room temperature and at 40.0~ It appears that at low coverages the adsorbent retains N 2 in higher amounts because of the higher energetic heterogeneity of the adsorbent and the quadrupole moment of N 2 molecules. This mutually advantageous relationship leads to a more rapid increase of N 2 loading of the sieve in the low pressure range (below 8 kPa). As loading increases, the close-range repulsions between the adsorbed N~ molecules become more and more intense and these intermolecular interactions lead to a flattening of the isotherm. As far as CH 4 is concerned, this non-polar molecule uses its higher polarizability with respect to that of N 2 and the polarizing strength of the divalent ions (Ca 2§ Mg 2§ present in the adsorbent. At pressures above 8 kPa, the capacity of clinoptilolite is higher for CH 4 than for N 2. The Langrnuir equation tends to underestimate the capacity at the extremes of the pressure range, and overestimates the capacity in the middle of the investigated pressure range (see Figures 1 and 2). The fitting inaccuracies introduced by the use of the Langmuir equation for the systems involving clinoptilolite as adsorbent may be attributed to the fact that the assumptions upon which the Langmuir model is based (monolayer adsorption and energetically homogeneous surfaces), are not entirely valid. Lateral interactions between neighboring molecules strongly adsorbed on sites characterized by high adsorption potentials cannot be ignored, and the adsorbent in this system is not homogeneous from the point of view of the energy of adsorption characterizing the sites.
510 3.2. Binary Mixture Adsorption Isotherm Predictions All predictions have been determined for a total pressure of 101.3 kPa, at room temperature and at 40.0~ In the case of binary adsorption data, the reliability of the fsinformation provided by the application of each of the three theories chosen is a result of the accuracy resulting from the fitting of the pure gas adsorption isotherms. Predicted binary isotherms are given in Figure 4, with corresponding adsorption equilibrium phase diagrams in Figure 5. For N2-CH4 mixtures at 101.3 kPa total pressure, FH-VST predicts azeotropes at approximately Y(CH4) = 0.66 at room temperature and at Y(CH4) = 0.55 at 40.0~ (Figure 5). The decrease of the FI-I-VST predicted azeotrope mole fraction with increasing temperature is due to higher kinetic energy of the adsorbate molecules at higher temperatures, and lower adsorption capacity of the adsorbent. In the absence of close-range repulsive interactions at lower coverages, N 2 molecules are preferentially adsorbed at higher temperatures compared to CH 4 molecules, and the azeotrope composition shit, s towards higher Y(N2), or lower Y(CH4) values.
11 [ (a) 0.8 ~ 0.6
1
1
(b) Total
~
.. . . ~ . . ~ ~
i/,,/
or 0.4 r,.)
1
0.8
0.8
0.6
~0.6
0.4
0.4
0.8 Total .___.......... ...........~-:zz-~:......."~ 0.6 ......~.-:S~---'..,/.. 0.4 N2 " - ~ ' ~ ~
0.2
0.2 0
0 0
0.2
0.6 Y(CH4)
0.4
0.8
1
CH4
r~ 0.2
0.2
0
0
0
0.2
0.4 0.6 Y(CH4)
0.8
1
Figure 4. Predicted binaryisotherms for CH4-N2 mixture adsorption on Turkish clinoptilolite at 101.3 kPa total pressure: (a) room temperature; (b) 40.0~ ELM FH-VST IAST
ELM produces constant separation factor values below unity (0.714 at 40.0~ and 0.743 at room temperature), while IAST and FH-VST provide composition-dependent selectivity values (1.7 - 2.0 for IAST, and 2.4 - 0.4 for FH-VST at 40.0~ 1.8 - 2.1 for IAST, and 2.0 - 0.5 for FH-VST at room temperature). These values are in agreement with the selectivity ratios reported by Ackley and Yang [6], which show that selectivity reversal occurs for N 2 and CH 4 gases with natural clinoptilolite at low pressures. ELM yields values for the equilibrium separation factors lower lhan unity, in contrast with the other two models. Because of the poorer fits yielded by this model for the pure-gas adsorption, the predictions obtained by ELM are questionable.
511
/ .,I
.Io,2 .oJ
//~/'/ //
a)
0.8
~_0.6
/ .,,,;;?""
/ ~""///..-".-" ~./ ...""i,.o/
0.8
~" 0.6
.- ....-
//,..,::-'., ....
r~
/..'IIIS: .......
~----"0.4
X 0.4
X-Y
0.2
0.2
/ ojO/~
0
~"
0
0.2
I
I
I
0.4
0.6
0.8
Y(CH4)
1
0
I
I
I
I
0.2
0.4
0.6
0.8
1
Y(CH4)
Figure 5. Predicted XY diagrams for CH4-N2 mixture adsorption on Turkish clinoptilolite at 101.3 kPa total pressure: (a) room temperature; (b) 40.0~ ELM FH-VST .... IAST IAST fails to predict the formation of the azeotrope. This model may be very sensitive to the difference in the spreading pressures of the two components, which is quite high for this particular system given the difference in the adsorption capacities of the molecular sieve for the two gases. The basic assumption in IAST being the ideal behaviour of the adsorbed phase (which is more likely to occur only in the case of the adsorption of nonpolar gases on energetically homogeneous adsorbents) makes it difficult to accept, its predictions for the adsorption systems investigated in this study. Since FH-VST is the only model which accounts for the non-ideality of the adsorbed phase and is the only one which predicts the azeotrope formation and selectivity reversal for the separation of the N2-CH4 mixtures, its estimations are likely to be more accurate. Supporting this argument is the selectivity reversal of natural clinoptilolite with CH 4 and N 2 gases at low pressure (about 10 kPa) for 27~ and 50~ reported by Ackley and Yang [6]. Collection of experimental binary adsorption data would be necessary for a final say on the predictive ability of each model. 4. CONCLUSIONS The pure-gas adsorption isotherms ofN 2 and CH4 on Turkish clinoptilolite reveal that this natural zeolite has higher capacities for CH4. The increase in temperature does not appear to reduce the adsorption capacity drastically. The Langmuir fits of the pure adsorption isotherms for the clinoptilolite sample underestimated the capacity in the lower and upper portion of the investigated pressure range and overestimated the capacity in the intermediate pressure range. FH-VST proves to be an excellent fitting tool for the single-component adsorption systems investigated in this work, since the interactions in the adsorbed phase are accounted for. The FH-VST fits of the
512 N: and CH4 isotherms intersect each other at both temperatures, thereby forecasting an azeotrope. As far as predictions of binary mixture behaviour at the two temperatures and 101.3 kPa total pressure are concerned, ELM predicts lower capacities for the mixture than either the IAST or FH-VST at both temperatures. IAST and FH-VST selectivity values are composition dependent, unlike ELM which predicts constant equilibrium separation factors lower than unity. ACKNOWLEDGMENTS The natural clinoptilolite sample was kindly donated by Dr. Ayse Erdem-Senatalar from Istanbul Technical University, Turkey. The financial support received from the Natural Sciences and Engineering Research Council (NSERC) of Canada and from the Department of Chemical Engineeering at the University of Ottawa is gratefully acknowledged. REFERENCES
1. Cochran, T. W., R. L. Kabel, and R. P. Darmer, AIChE Journal, 31(2), 268-277 (1985a). 2. Cochran, T. W., R. L. Kabel, and R. P. Danner, AIChE Journal, 31(12), 2075-2082 (1985b). 3. Markham, E. D. and A. F. Benton, dr. Am. Chem. Soc., 53, 497 (1931). 4. Myers, A. L. and J. M. Prausnitz, AIChEJ., 11, 121 (1965). 5. Frankiewicz, T. C. and R. G. Donnelly, Industrial Gas Separations, Whyte, T. E., Jr., et al. (eds.), American Chemical Society, Washington, D.C., 1983. 6. Ackley, M. W. and R. T. Yang, Ind. Eng. Chem. Res., 30(12), 2523-2530 (1991). 7. Sirkecioglu, A., Y. Altay, and A. Erdem-Senatalar, in print, Sep. Sci. & Technol. (1995).
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
513
D e a l u m i n a t i o n and acidity m e a s u r e m e n t of H E M T zeolites modified by steaming and leaching O. Cairon, S. Sellem*, C. Potvin*, J.M. Manoli* and T. Chevreau Catalyse et Spectrochimie - URA CNRS 414 - ISMRA-UNIVERSITE - 14050 CAEN Cedex (France) *Reactivit~ de Surface - URA CNRS 1106, Casier 178 - Universite P. et M. Curie75252 PARIS Cedex 05 (France) HEMT zeolites steamed over a wide temperature range ( 7 2 0 - 1010 K) unleached or leached with HCI(1N) were characterized by spectroscopic techniques and chemical analyses. Strong Br6nsted acidity was quantified by pyridine adsorption at 470 K. Dealumination and acidity results of two series of cubic and hexagonal steamed faujasites are matched. 1.
INTRODUCTION
Acidic properties of zeolites, in particular the strength and number of acid sites, govern their catalytic activity. With steamed faujasites there is a wide variety of possible sites (framework and extraframework Br6nsted and Lewis sites). Quantitative results obtained by FTIR spectroscopy regarding the acidity of steamed HY zeolites leached to greater or lesser extent by acid solutions have already been published [1, 2]. The aim of this paper is to compare the solid properties of HEMT and HY zeolites steamed at various temperatures : dealumination level, crystallinity and numbers of acidic Br6nsted sites determined by pyridine adsorption at 470 K. 2. E X P E R I M E N T A L
SECTION
The starting zeolites were i) NH4EMT solid obtained according to the procedure described by Delprato [3], followed by Na+-NH4 + exchanges, ii) LZY62 solid (Union Carbide). Their molar formulae are (NH4)15Na5(AIO2)20(SiO2)76 and (NH4)43Na10 (AIO2)53 (SIO2)139, respectively. The steaming procedure and the rotating reactor used are fully described in [4], the steam pressure was 5.104 Pa. A double steaming was carried out, the first at 810 K and the second at higher temperature (T), except for samples S(720) on which a double (HY) or a single (HEMT) steaming was applied at 720 K. In all cases, the first steaming was followed by a double exchange Na + e-> NH4 +. Taking into account their hexagonal and cubic cell, the steamed solids stemming from HEMT and HY zeolites are designated as hS(T) and cS(T). The leaching strength is given by the volume of HCI(1 N) per gram of dehydrated zeolite (5 cm 3 g-l) i.e. the number of H+ millimoles per gram. The two series of leached solids are denoted hS(T)L5 and cS(T)L5. Several trials were realized for
514
each temperature. Chemical analysis of eluents (exchanges and leaching) was carried out. The samples were characterized by various spectroscopies : i) 29Si solid-state NMR to determine the Si/AIf ratio. 29Si MAS NMR was recorded at 79.5 MHz with a Bruker MSL 400. The acquisition parameters were : ~/4 pulse length of 2 l~s, 10 s recycle delay, rotor spinning rate of 5 kHz, 2000 to 20000 scans cumulated. The spectra were deconvoluted with the program WlNFIT using Gaussian line shapes. ii) XRD to determine the unit cell parameters and the crystallinity of the various rehydrated zeolites. The degree of X-ray crystallinity was estimated from the intensity of all reflections in the range 28 = 14.5~ ~ compared with those of NH4EMT zeolite in the same range, iii) FTIR to follow the effects of the treatments on the structural bands (500-1300 cm -1) and to study the acidity (v(OH) range). The IR spectra were recorded from wafers obtained either by dilution of the zeolite in KBr (1/200) or in the form of self-supported discs (10 mg), activated at 720 K, under reduced pressure (10 -3 Pa).
3. RESULTS AND DISCUSSION 3.1. Physicochemical characteristics of steamed faujasites The physicochemical characteristics of steamed hexagonal faujasites are given in Table 1. After steaming, the crystallinity decreases, whereas the steaming temperature increases because the framework dealumination increase generates more and more amorphous phase. The limiting acid leaching leads to a higher crystallinity as observed for steamed HY zeolites [5], except for the solids steamed at 720 K, less dealuminated and therefore less stable toward acid attack. As generally observed, the cell parameters decrease as well as the AI content of the network and undergo a slight contraction after acid leaching because of partial extraction of extralattice species. Linear correlations between the parameters (a and c) and the number of framework AI atoms have been tested and disregarded due to the values of their correlation coefficients (< 0.95). The results obtained by NMR show that the mild acid leaching does not affect the number of framework AI atoms per u.c., N(AIf), except for samples 5 and 5'. In this case the determination of Si/AIf ratio is difficult because the intensity corresponding to Si(1AI) is very low. The structural bands reported here, named Vl, v2, v3, are those often used in the literature to determine the numbers of framework AI atoms for dealuminated HY zeolites, [6-9]. As established in [9], the Vl wavenumber is sensitive to the silicaalumina amorphous phase, explaining the differences observed with the sample pairs 2-2' and 3-3', consequently, this band will no longer be considered. For each solid couple, the v2 and v3 wavenumbers are unaffected by mild acid leaching, therefore this fact confirms the framework conservation. The linear correlation established between these wavenumbers and N(AIf)are given in Figure 1. It is worth noting that silica and silica-alumina do not present any band in the v3 region, giving a better correlation and a specific application of this band assigned to the double-6 ring vibration [6] and characteristic of these cubic and hexagonal faujasites studied in this paper.
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Since, the linear correlations are established using all the experimental results stemming from NMR and IR spectroscopies, they lead to a good evaluation of N(AIf) for every sample. In Table 1, the number N(Aif) is the average value of N(AIf) obtained for each solid, from the two equations given in Figure1. In the case of highly dealuminated zeolites (samples 5, 5', 6, 6'), this number N(AIf) allows the determination of Si/Alf ratio with a better accuracy. Table 1 and Figure 2 contain the ratios calculated by this method. Curve 2a shows the correlation between Si/AIf ratio and the steaming temperature with the steamed HY zeolites studied in reference [4, 5]. The present data obtained for hexagonal faujasites (hS(T) series) are plotted (curve 2b). The two curves are nearly superimposable. At a given steaming temperature, the framework dealuminations for cubic and hexagonal faujasites are almost equal, therefore, the dealumination level of HEMT steamed at various temperatures can readily be deduced from curve 2a.
3.2. Br6nsted acidity studies After steaming, the number of zeolitic hydroxyls is less than that of framework AI atoms because the aluminic cations formed counterbalance partly the negative lattice charges. FTIR spectroscopy allows quantitative study of Br6nsted acid sites. The v(OH) massif of HY zeolites steamed at 860 and 1010 K studied here has already been described [2, 5, 10]. Spectra of HY zeolites steamed at 720 K and HEMT steamed at 720, 860 and 1010 K are shown in Figure 3. In addition to the (HF)OH and (LF)OH groups and silanols, the IR spectra presents a band at c.a. 3600 cm -1 and sometimes a shoulder at 3525 cm -1, as already reported for steamed HY zeolites [1, 5, 11]. Regarding the HEMT zeolites steamed at 860 and 1010 K (curves 4, 4', 5, 5') and the corresponding steamed HY [2, 10], acid leaching increases the v(OH) massif and slightly increases the SiOH band. As expected from their low crystallinity after leaching, the zeolites steamed at low temperature are less stable towards acid attack, therefore, the intensity of their v(OH) massif decreases after leaching, whereas the SiOH band increases (Fig. 3, curves a,a' and 2,2'). The v(OH) massif of steamed zeolites is complex and results from the superposition of bands due to framework and extraframework OH groups. Nevertheless, the acidic framework hydroxyls can be counted through the number of pyridinium species formed by pyridine adsorption at 470 K [1]. Figure 4A shows the difference spectrum (before adsorption minus after py adsorption) i.e. the v(OH) bands of zeolitic acidic hydroxyls perturbed by pyridine adsorption at 470 K, after a brief evacuation. It appears that the series of steamed HEMT (except sample 4') hardly displays the (3600)OH band due to strongly acidic hydroxyls. This band shown on the direct spectra (Fig. 3) is mainly due to the nonacidic extraframework hydroxyls [1, 2]. The same observation can be noted concerning the HY solids steamed at low temperature (curves 4A, a, a') but this result is quite different from that observed for HY zeolites steamed at 860 K. In this latter case, the (3600)OH bands is well-defined and its intensity is maximum for the leaching L5, [2]. Knowing the molar extinction coefficient (EpyH+ = 1.8 l~mo1-1 cm, [1]), the number of pyH + species formed can be determined from the intensity of the characteristic band at 1543 cm -1 shown in Figure 4B. The results are given in Table 2 for the two series of hexagonal and cubic steamed faujasites. The data are expressed in l~mol
517
Table 2- BrSnsted acidity of various HEMT and HY zeolites
Zeolites 0 u C
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283 102
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59 59
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Figure 3. v(OH) bands ; a, 2, 4, 5 samples 9 cS(720) and hS(720, 860 or 1010) ; a', 2', 4', 5' 9the corresponding leached solids.
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3800 3700 3600 3500 3400 1700 1600 1500 1400 Figure 4. A v(OH) 9 bands perturbed by pyridine adsorption, after evacuation at 470 K (acidic OH groups). B bands 9 at 1543 and 1454 cm -1, characteristic of pyH + and Lpy species formed.
518
of pyH + formed per gram of hydrated zeolite, i.e., in ~mol of perturbed zeolitic hydroxyls per gram. At present, the numbers of zeolitic hydroxyls cannot be directly determined for the HEMT series owing to the fact that their molar extinction coefficients are unknown. The numbers of pyH + species formed with HEMT(3.8) and HY(2.9) highly exchanged (> 90 and 99 % respectively) show the strongest acidity of HEMT as already reported [12, 13]. With HEMT zeolites, npyH+ is higher (852 ~mol.g -1) and the pyH + species are more stable : only 42 % are desorbed at 620 K, on evacuation. Moreover, IR spectra show partial disappearance of (LF)OH groups which contribute to the pyH+ formation, whereas the (LF)OH hydroxyls of the sample HY(2.9) are unperturbed after evacuation at 470 K [1]. For steamed zeolites, as mentioned above, the acid leaching increases npyH+, i.e., the number of zeolitic acidic hydroxyls, except for sample cS(720)L5, more sensitive to the acid attack. At medium steaming temperature (860 K), the higher amount of pyH + formed (283 ~mol.g -1) with the cubic zeolite leached studied in [2] could be explained by the appearance of strongly acidic hydroxyls ((3600+3525)OH). Nevertheless, the global acidity strength of hexagonal and cubic faujasites seems to be equal: c.a. 55 % of pyH + species are desorbed at 620 K. In conclusion, HEMT and HY zeolites steamed under the same conditions lead to the same framework dealumination but the steamed HEMT zeolites unleached or leached never show a large amount of strongly acidic hydroxyls ((3600 + 3525)OH). This specific property is probably due to their hexagonal structure. The authors wish to thank M. Marzin and J. Maquet for their collaboration. REFERENCES
1. S. Khabtou, T. Chevreau and J.C. Lavalley, Micropor. Materials, 3 (1994) 133. 2. L. Mariey, S. Khabtou, M. Marzin, J.C. Lavalley, A. Chambellan and T. Chevreau, to be published. Stud. Surf. Sc. and Catal., Internat. Zeolite Symp. Quebec, 1995 3. F. Delprato, L. Delmotte, J.L. Guth and L. Huve, Zeolites, 10 (1990) 546. 4. A. Chambellan, M. Marzin, O. Cairon and T. Chevreau, React. Kin. and Catai. Letters, accepted for publication (1995). 5. A. Chambellan, T. Chevreau, S. Khabtou, M. Marzin and J.C. Lavalley, Zeolites, 12 (1992) 306. 6. E.M. Flaningen, H. Khatami and H.A. Szymanski, Molecular Sieves I, Adv. Chem. Ser., 101 (1971)201. 7. P. Pichat, R. Beaumont and D. Barthomeuf, Compt Rend. C., 272 (1971) 612. 8. J.R. Sohn, S.J. De Canio, J.H. Lunsford and D.J. O'Donnell, Zeolites, 6 (1986) 225. 9. O. Cairon, S. Khabtou, E. Balanzat, A. Janin, M. Marzin, A. Chambellan, J.C. Lavalley and T. Chevreau, Stud. Surf. Sci. and Catal., Vol. 84, Elsevier Science B.V. (1994) p. 997. 10. T. Chevreau, A. Chambellan, J.C. Lavalley, E. Catherine, M. Marzin, A. Janin, J.F. H~midy and S. Khabtou, Zeolites, 10 (1990) 226. 11. F. Lonyi and J.H. Lunsford, J Catal., 136 (1992) 566. 12. Bao Lian Su, J.M. Manoli, C. Potvin and D. Barthomeuf, J. Chem. Soc. Faraday Trans., 89 (1993) 857. 13. Bao Lian Su and D. Barthomeuf, Zeolites, 13 (1993) 626.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
The synthesis of UTD-1, Ti-UTD-1 and Ti-UTD-8 using Cp*2CoOH as a structure directing agent K. J. Balkus, Jr. *l, A. G. Gabrielov 1 and S. I. Z o n e s 2 ~Department of Chemistry, University of Texas at Dallas, Richardson, TX 75083-0688, United States 2Chevron Research and Technology Co., Richmond, CA 94802-0627, United Sates SUMMARY As part of our effort to advance the application of metal complexes as structure directing agents for molecular sieve synthesis, we discovered that a novel, high silica zeolite (UTD-1) could be prepared using bis(pentamethylcylcopentadienyl) cobalt(HI) hydroxide, CP*2CoOH. UTD-1 has been characterized as a large pore molecular sieve having one dimensional channels running in parallel. The incorporation of titanium into the synthesis gel results in the formation of the titanosilicate, Ti-UTD-1 as well as the novel phase Ti-UTD-8. The preliminary characterization of these materials is described. 1. INTRODUCTION We have been exploring the structure directing properties of various metallocenes for the synthesis of molecular sieves. Cobalticinium ion, Cp2Co§ has proven to be a very versatile template, especially for clathrate type structures. So far molecular sieves have been prepared with the AST [1-3], AFI [1], LEV [4], NON [5-7] and DOH [6] topologies, all of which had previously required an organo-cation to form. These results encouraged us to evaluate various other metallocenes as structure directing agents. In particular, bis(pentamethylcylopentadienyl)cobalt(III) hydroxide, Cp*2CoOR has been employed in the preparation of several new molecular sieves [3]. A novel high silica zeolite referred to as UTD-1 has been prepared using Cp*2CoOH as the template. Preliminary characterization of UTD-1 seems to indicate that UTD-1 is a large pore molecular sieve that has one dimensional channels running in parallel [3,8,9]. Our ability to add aluminum to UTD-1 suggests that other heteroatoms might be incorporated into the framework. The addition of titanium into the UTD-1 synthesis gel results in the isostructural titanosilicate Ti-UTD-1. A slight modification of the synthesis conditions also results in a novel phase referred to as Ti-UTD-8. The Titanosilicates prepared using Cp*2CoOH as a template have been characterized by XRD, FT-IR and UV-Vis spectroscopy as well as scanning electron microscopy (SEM). Preliminary experiments suggest that UTD-1 and Ti-UTD-1 have potential as catalysts for the peroxide based oxidation of alkanes.
519
520 2. EXPERIMENTAL The synthesis of UTD-1 has previously been described [3,8,9]. All reagents were used as received unless otherwise specified. Electronic spectra of zeolites were obtained from samples prepared as nujol mulls between quartz plates using a Hitachi U-2000 UVVis spectrophotometer. Mid-IR spectra were obtained from KBr pellets using a Mattson 2025 FT-IR spectrophotometer. X-ray powder diffraction patterns were recorded on a Scintag XDS 2000 diffractometer using CaF2 as an internal standard. Scanning electron micrographs were obtained using a Philips XL30 SEM equipped with Philips PV8500 EDAX spectrometer. Elemental analyses were performed by Galbraith laboratories, Knoxville, TN.
2.1 Ti-UTD-I Synthesis The titanosilicate Ti-lYrD-1 was prepared by first combining 3.37 grams of a 19.8% by weight aqueous solution of Cp*2CoOH with 9 mL of deionized water and 0.027 grams of NaOH. The preparation of Cp*2CoOH has previously been described [3,9].The template solution was mixed with 0.81 grams of fumed silica and stirred at room temperature for one hour. Then 0.065 grams of a titanium ethoxide (20% Ti) solution (Aldrich) were added dropwise to the silicate solution with stirring. The gel was stirred at 80~ for 2 hours during which time the gel became more viscous and transparent in appearance. At this point three additional milliliters of deionized water were added. The resulting titanosilicate gel had a molar ratio of SiO2 : TiO2 : Cp*2CoOH : Na20 : 1-120 = 1 : 0.02 : 0.14 : 0.025 : 50. The gel was transferred to a 23 mL Teflon-lined autoclave and then heated under static conditions at 175~ for 6 days. The crystallization mixture was cooled to room temperature, then the yellow Ti-UTD-1 was isolated by suction filtration, washed with deionized water and dried at 90~ overnight.
2.2 Ti-UTD-8 Synthesis The titanosilicate Ti-UTD-8 was prepared by combining 9.82 grams of a 13.6% Cp*2CoOH aqueous solution [3,9] with 11 mL of deionized water, 0.108 grams of NaOH and 1.62 grams of fumed silica. The silicate solution was stirred for one hour at room temperature. Then 0.216 grams of a titanium ethoxide solution (20% Ti) were added dropwise to the mixture with stinS~g. The solution was covered and stirred for one hour at room temperature followed by additional stirring at 50~ for three hours. The resulting opaque gel had a molar ratio ofSiO2 : TiO2" Cp*2CoOH : Na20 : H20 = 1 : 0.033 : 0.14 : 0.05 : 40. The gel was transferred to a Teflon fined autoclave and heated under static conditions at 175~ for 6 days. The crystallization mixture was cooled to room temperature and the liquid was decanted from the yellow solid on the bottom of the reactor. The Ti-UTD-8 was washed with copious amounts of deionized water and dried at 90~ overnight.
521 3. RESULTS AND DISCUSSION The Cp2CoOH complex clearly has a tendency to form cage type structures and appears to parallel the behavior of quinuclidine. A comparison of different types of zeolite structures with the C/N ratio of the corresponding organic templates that make these phases, reveals several interesting trends [10]. For example, as the C/N ratio for the structure directing agent increases the number of zeolite phases that can be made decreases. Additionally, at Si/Al ratios > 25, organic templates with C/N ratios less than 10 have a propensity to form clathrate structures, while for C/N >10 large pore, one dimensional channel systems are favored. The C/Co ratio for Cp2Co § ion is 10 and this complex forms several different phases that could be classified as clathrates. Even though we have limited data, it is tempting to predict that larger metal complexes may form high silica zeolites with large pores and non-intersecting channel systems. By adding methyl groups to the Cp ring periphery to form the Cp*2CoOH complex, we effectively increase the size from---5.2 to 7.2 A and the C/Co ratio now becomes 20. We have prepared a high silica molecular sieve using Cp*2CoOH as the template and interestingly, this novel zeolite referred to as UTD-1 appears to have a large pore, one dimensional channel system. Additionally, the UTD-1 phase is the only high silica molecular sieve we have crystallized with the Cp*2CoOH template. This is in contrast to the smaller Cp2CoOH that facilitates the formation of several high silica phases. Furthermore, UTD-1 actually crystallizes in a fairly narrow window from the perspective of both time and template concentration [9]. As synthesized UTD-1 can be indexed in an orthorhombic (Ibam) crystal system with unit cell parameters a=18.824(5), b=23.519(7) and c=8.401(2). Further structure refinement is in progress, however, high resolution electron microscopy has revealed channels running parallel in one direction [9]. The micropore volumes (cc/gm) determined from N2, Ar and cyclohexane adsorption (0.113, 0.117 and 0.108 respectively) are also consistent with a channel type zeolite. Additionally, the conversion of methanol to hydrocarbons with a narrow product distribution composed largely of higher molecular weight aromatics further suggests a one dimensional channel type structure [ 11]. The size and structure of the channels is uncertain, however, unhindered adsorption of cyclohexane (kinetic diameter 6A) and triethylamine (7.8A) suggests channel openings composed of at least 12 membered rings with adsorption behavior of argon consistent with 14MR apertures [9]. The as synthesized UTD-1 is bright yellow in color indicating the presence of the Cp*2Co+ template which is confirmed spectroscopically [9]. UTD-1 is thermally stable to calcination at >500~ in air, however, the metal complex decomposes at -~3700C to form a pale green-gray material. The nature of the intrazeolite cobalt after calcination is under further investigation. The speculation that UTD-1 could be a new zeolite with greater than a 12 membered ring channel structure has generated an effort to evaluate the catalytic potential of this molecular sieve. An important issue is framework modification which includes the incorporation of aluminum as well as reactive metal centers such as titanium and vanadium. The incorporation of titanium into the UTD-1 precursor gel dramatically modifies the synthesis such that after 2 days heating, which is normally the optimum crystallization time, only amorphous materials result. If on the other hand the same gel is heated for 6 days, then a crystalline product, that appears isostructural with UTD-1, is
522
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Figure 1. XRD pattern for Ti-UTD-1 produced. In contrast, the silicate or aluminosilicate gel forms the dense phase cristobalite after 2 days heating [9]. The XRD pattern for Ti-UTD-1 is shown in Figure 1 and is nearly the same as the all silica UTD-1. Likewise, crystal morphologies of Ti-UTD-1 and UTD-1 are quite similar as shown in Figure 2. The Ti-UTD-1 crystals are composed of bundles of needles which on average are considerably smaller than those observed for UTD-1 [9]. The SEM images also confirm the lack of impurity phases in this Ti-UTD-1 sample. ::::::.::::.:::
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Figure 2. Scanning Electron Micrograph of Ti-UTD-1 Elemental analysis of Ti-UTD-1 indicates 1% Ti by weight (sifri molar ratio of 67.5) which corresponds to -74 % incorporation of the titanium that was in the precursor gel. XRF analysis of the Ti-UTD-1 crystals indicates a fairly homogeneous distribution of Ti at a level consistent with the elemental analysis. Also the amount of cobalt incorporated (2.5%) is quite similar to the UTD-1 synthesis [9]. The intrazeolite location of titanium is difficult to establish, however, Ig spectroscopy can provide partial evidence of framework substitution. Figure 3 shows the FT-IR spectra of UTD-1 and Ti-UTD-1 which again illustrates the structural similarity between the two phases. The most notable difference is
523
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Figure 3. Mid-FT-IR spectra of A) UTD-1 and B) Ti-UTD-1 recorded as KBr pellets. a band at 960 cm ~. This band is generally observed for titanosilicate molecular sieves and increases in intensity upon calcination [ 11-17]. Corma et al [ 11 ] assigned this band to SiO- defects which transform to Si-OH upon calcination. Others have attributed this band to Ti=O or Si-O-(Ti) group vibrations or to a stretching mode of a [SiO4] unit bonded to a Ti 4+ ion (O3SiOTi) [13]. All this suggests that titanium is incorporated into the UTD-1 structure. One must be careful in a titanosilicate because it is relatively easy to precipitate TiO2 during the gel preparation. We discovered that on those occasions when the TiUTD-1 precursor gels were not transparent we observed an impurity phase. We were able to optimize the synthesis of this material and the XRD pattern is shown in Figure 4.
s. . . .
~'o. . . . t5. . . . do. . . . ~ . . . . ~o. . . . ~ . . . . ~o 20 Figure 4. XRD pattern for Ti-UTD-8.
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Figure 5. Scanning Electron Micrograph of Ti-UTD-8. We are unaware of any known silicate phases with the same XRD pattern as this sample we refer to as Ti-UTD-8. This material appears to be a single phase as evidenced by the SEM shown in Figure 5. XRF measurements also indicate a relatively high concentration of titanium (Si/Ti = 4.5).In contrast to the UTD-1 structure, the Ti-UTD-8 structure collapses at -350~ which coincides with metal complex decomposition. Therefore, it will be difficult to measure meaningful adsorption data Figure 6. shows the UV-Vis spectra of as synthesized Ti-UTD-1 03) and Ti-UTD8 (A). The broad band at 306 nm is associated with the Cp*2CoOH complex which is redshifted in Ti-UTD-8. The band at --.211 nm as well as a shoulder at 292 nm for Ti-UTD-8
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]
'%
'..,i -,,,_., i
200
i
250
I
300
~A -,.
I
I
i
350 400 450 WAVELENTH, nm
i
500
i
550
600
Figure 6. UV-Vis spectra of A) Ti-UTD-8 and B) Ti-UTD-1 recorded as nujol mulls.
525 may indicate the presence of framework titanium [18]. A band centered at 290 nm has previously been observed for tetrahedral titanium [19]. The bands associated with extrafi'amework titanium species usually appear at 250-280 nm [18]. Further characterization of Ti-UTD-8 is in progress. We have shown that Cp*2CoOH is an effective template for the synthesis of the novel large pore zeolite UTD-1 as well as the titanium substituted analog Ti-UTD-1. Preliminary experiments using these molecular sieves as catalysts for the oxidation of alkanes using peroxides are promising. Additionally, we are optimistic that other metal ions can be incorporated into the UTD-1 structure. Titanium clearly modifies the gel chemistry and we anticipate the addition of other metal ions may also result in novel phases such as Ti-UTD-8. ACKNOWLEDGMENTS We thank the National Science Foundation and the Robert A. Welch Foundation for financial support of this work. REFERENCES .
2. 3. .
5. 6. .
8.
9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
K.J. Balkus, Jr., A.G.Gabrielov and S.Shepelev, Micropor. Mater., 3 (1995) 665. K.J. Balkus, Jr., and A.G. Gabrielov, Porous Mater., 1 (1995) 199. K.J. Balkus, Jr., A.G. Gabrielov, and N. Sandier,. Mater. Res. Soc. Symp. Proc., 368 (1995). E.W. Valyocsik, U. S. Patent No. 4,556,549 (1985). K.J.Balkus, Jr. and S.Shepelev Micropor. Mater., 1 (1993) 383. P. Behrens and G. van der Goor, Poster at the 8th International Symposium on Molecular Recognition and Inclusion, Ottawa, (1994). E.W. Valyocsik, U. S. Patent No. 4,568,654 (1986). K.J. Balkus, Jr., A.G. Gabrielov and S.I. Zones, Petrol. Preprints, 40 (1995) 296. K.J. Balkus, Jr., A.G. Gabrielov, S.I. Zones and I.Y. Chan., ACS Symp.Ser.,In Press. S.I. Zones, Y. Nakagawa and J.W. Rosenthal Zeoraito, 11 (1994) 81. L.T. Yuen, S.I. Zones, T.V. Harris, E.J. Gallegos and A. Auroux, Micropor. Mater., 2 (1994) 105. M.A Camblor, A. Corma and J. Peres-Pariente, J. Chem. Sot., Chem. Comm., (1993) 557. A. Corma, M.T. Navarro and J. Peres-Pariente, J. Chem. Sot., Chem. Comm., (1994) 147. A. Thangaraj, Kumar, R., Mirajar, S.P. and Ratnasamy, P., J.Catal., 130 (1991) 1. A. Tuel, Y. Ben Taarit and C. Naccache, Zeolites, 13 (1993) 454. T. Ito, H. Kanai, T. Nakai and S. Imamura, React.Kinet. Catal. Lett., 52 (1994) 421. P. Ratnasamy, and R. Kumar, Catal.Lett., 22 (1993) 227. R. Kumar, A. Raj, S. Baran Kumar and P. Ratnasamy, Stud.Surf. Sci.Catal., 84 (1994) 109. J. Klaas, K. Kulawik, G. Schultz-Ekloff and N.I. Jaeger, Stud. Surf. Sci. Catal., 84 (1994) 2261.
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Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
ZEOLITE CRYSTALLIZATION
ON MULLITE
527
FIBERS
V. Valtchev ~, S. Mintova a, B. Schoeman b, L. Spasov~ and L. Konstantinov ~ a Central Laboratory of Mineralogy and Crystallography, Bulgarian Academy of Sciences, 92 Rakovski St., 1000 Sofia, Bulgaria b Chemical Technology, Lulea University of Technology, 971 87, Lulea, Sweden Institute of Kinetics and Catalysis, Bulgarian Academy of Sciences, G. Bonchev St., 1113 Sofia, Bulgaria
In situ crystallization of zeolite A, Y and silicalite-1 on crystalline and amorphous mullite fibers is studied. It was found that the fiber degree of coverage is determined by the chemical composition of the initial gel and by the physicochemical features of fibers. The role of the fiber pretreatment is also discussed.
INTRODUCTION Microporous materials are of interest as sorbents and gas separators due to their large internal surface and well-defined pore system [1]. In industry zeolites have been usually used in the form either of granules with adhesive additives or as pallets and blocks of bulk particles. Neither the use of zeolites as bulk material nor as a part of a composite has been fully utilized so far. This is due to the obvious disadvantage that in the bulk usage a large part of the zeolite consists of material which is not available in the interior, but in other instances, when the zeolite is embedded within the matrix material, the effective surface~substantially decreases. A combined usage of the whole zeolite surface can be achieved through deposition of the zeolite on a supporting substrate. During the past few years a large number of articles and patents have been published on the preparation of composite materials containing continuous zeolite films [2-7]. Suitable substrates for this purpose are fibrous materials due to their flexibility and the ability to use them for forming matrices of various shape and size [8,9]. Therefore, the synthesis of zeolite-containing fibers of high thermal and acid stability is of a considerable technological importance.
528 In the present communication we discuss the in-situ synthesis of zeolites on mullite fibers. In addition, the effect of the chemical pre-treatment of the fibers on their reaction ability is studied.
EXPERIMENTAL Two types of muUite fibers were used by us with A1203 to SiO2ratios equal to 80/20 and 72/28. The mullite fibers were prepared by the sol-gel method with a centrifuge equipped with ejection nozzles.The spinning composition was prepared from basic aluminium chloride (A1/C1 atomic ratio of 1.85), silicasol, and a rheology modifier (polyvinyl alcohol of molecular weight 70 000). Further, the amorphous precursor obtained (the so called Green fibre) was calcined at a given temperature to form an oxide fibre. The 80/20 muUite was amorphous, while the 72/28 muUite crystalline. These two types of fiber were chemically treated before synthesis in 10% H3PO4for 30 mill under ultrasound action. The hydrothermal synthesis of zeolite A, Y and silicalite-1 was performed on fibers either treated or treated. The chemical composition of the gels used for the synthesis were: zeolite A: 4,1 Na20: A1203: 2,4SIO2:155H20 zeolite Y: 8Na20: A1203: 20SIO2:400H20 silicalite-l: 25Na20: 10TPA: 26SIO2:1528H20 After preparing the initial gels the muUite fibers were directly immersed into the autoclaves. After the hydrothermal synthesis the fibers were filtered, washed and dried at 110~ The zeolite coated muUite fibers were investigated by thermogravimetric analysis (Stanton Redcroft 781). The type of synthesized zeolites was specified by X-ray powder diffraction using a DRON-3M diffractometer with Cu K~ radiation, while the distribution of the zeolite particles by scanning electron microscopy (Philips 515).
RESULTS and DISCUSSION
The main results on the synthesis of zeolite A, Y and silicalite-1 on muUite fibers are systemized in Table 1. SEM measurements revealed that the crystallization of the three types of zeolite on untreated fibers had proceeded in different way. The most effective zeolite formation was observed for zeolite A on both type of fibers. Although the fiber coverage in both cases was not dense,
529 Table 1. Zeolites A, Y and silicalite-1 grown on different mullite fibers MuUite fibers Degree of coating "A12(~3/SiO2 80/20* 80/20** 72/28* 72/28** * untreated fibers
zeolite A medium hi$1a medium medium ** treated fibers
zeolite Y low medium none low
silicalite-1 none low none low
one can see that the content of zeolite A is higher on amorphous than on crystalline fibers (see Figure la). Zeolites Y and silicalite-1 crystallize much worse and only isolated crystals were observed on the fibers. It is worth noting that among all zeolites stidied the amount of siliealite-1 grown on the fibers is the smallest. The results obtained point that the composition of the initial gel plays an important role for the zeolite growth on mullite fibers. The highest is the reaction capacity of the gel yielding zeolite A, which is of the highest content of hydroxyl groups. The zeolite crystallization on the mullite fibre surface was confirmed by X-ray phase analysis and thermogravimetric analysis. X-ray diffraction method indicated the presence of crystalline 72/28 mullite and zeolite A. In the case of zeolite Y and siliealite-l,the X-ray phase analysis revealed a very low content of zeolite phase as only the most intense 1• were detected. The thermal analysis indicated the formation of a composite of zeolite A and mullite. Figure 2 shows DTA curves of 72/28 mullite fibers (curve a), zeolite A (curve b) and the composite (curve c). The DTA curve of mullite fibers shows a low-temperature endothermal effect caused by the surface desorption of water at 77~ The DTA curve of zeolite A is typical of zeolites of stable three-dimensional structures, which indicates a continuous water desorption in the temperature range 100-200~ The DTA curve of muUite fibers covered by zeolite A indicates relatively high-temperature exothermal effects at 265,350 and 420~ which are not characteristic both of zeolite A and mullite. The presence of these effects points to the formation of a combined zeolite-mullite structure which decomposes after heating and speaks in favour of a strong bond between the zeolite and mullite. On the other hand, we performed a series of experiments with samples deposited on pre-treated fibers. It was found that after the pre-treatment the fiber coverage has generally improved, the effect being more pronounced on amorphous fibers. For example, as Figure l b shows, zeolite A forms a homogeneous dense coating on treated amorphous fibers, while does not on untreated ones. In addition, one can see that the size of crystals decreases from
530
.....
:
~:
.......
Figure 1. SEM photographs of coatings of zeolite A on" untreated (a) and treated (b) 72/28 mullite fibers. M=I 0 ~tm
a
b
I
I
I
100 300
l
!
500~
Figure 2. DTA curves of: 72/28 mullite fibers (a), zeolite A (b) and zeolite A crystallized on mullite fibers (c).
531
()
0
~
~ 0
~o
I~
0
,~0 o
1/,
~
2/,
34.
~.6
2 Thefo Figure 3. X-ray diffraction pattern of zeolite A(o) on 72/28 treated mullite fibers (v)
P
...........
~
~ ~.~
Figure 4. SEM photographs of zeolite Y (a) and silicalite-1 mullite fibers. M=100 ~tm
(b) on treated 72/28
3 ~tm for untreated to 1 ~tm for treated fibers. The high-quality powder diffraction pattern of zeolite A on 72/28 mullite indicates the high content of the zeolite on treated fibers (Figure 3). Figure 4 shows that the content of zeolite Y and silicalite-1 also increases after treatment of the mullite fiber. These two
532 zeolites do not cover densely the fibers, but the crystals are homogeneously distributed on the surface of pre-treated fibers. In contrast, on untreated fibers the zeolite crystals are distributed inhomogeneously. The preliminary treatment of fibers leads to an increase of their surface and to intensification of their reaction ability. These effects are more pronounced in the case of amorphous fibers, on which the combined action of ultrasound and acid treatment leads to a disorientation of the structural dements and to the formation of active centres serving as sites for zeolite formation. The great number of seeds in the latter case leads to a reduction in the size of the zeolite crystals. The effect of the pre-treatment is weaker for crystalline mullite fibers, where the reaction ability depends on the crystalline-to-amorphous ratio. In this case the treatment by acid influences only the amorphous areas of the fibers which results in the observed weaker effect of the fiber chemical treatment on the zeolite deposition.
CONCLUSION The crystallization of zeolite A, Y and silicalite-1 on amorphous and crystalline muUite fibers is studied. The composition of the initial gel is the main parameter controlling the zeolite crystallization on the surface. Another important parameter is the fiber physicochemical state. The reaction ability of amorphous muUite fibers has been found higher than that of crystalline ones. The fiber ability capacity can be improved by chemical treatment which increases the amount of seeds for zeolite formation on the surface. REFERENCES
1. D. Breck, Zeolite Molecular Sieves, John Wiley & Sons, New York, (1974) 2. H. SumS, US Patent 4 699 892 (1987) 3. E. Geus, A. Mulder, D. Vischjager, J. Schoonman and H. van Bekkum, Key Engineering Materials, 61 &62 (1991) 57 4. T. Sano, Y.Kiyozumi, M.Kawamura, F.Mizukami, H.Takaya, T. Mouri, W. Inaoca, Y. Toida, M. Watanabe and K. Toyoda, Zeolites, 11 (1991) 842 5. W. Haag and J. Tsikoyianis, US Patent 5 019 263 (1991) 6. G. Brattom and T. Naylar, Eur. Pat. 481 660 (1991) 7. J. Dong, T. Dou, X. Zhao and L. Gao, J. Chem Soc., Chem. Commun., (1992) 1056 8. E. Albers and G. Edwards, 3 730 910 (1973) 9. L. Dimitrov, L. Spasov, P.Dimitrov and L.Petrov, J. Mater. Sci. Lea., 1994, 13,905
Zeolites: A Refined Tool for Designing CatalyticSites L. Bonneviot and S. Kaliaguine(editors) 9 1995 Elsevier Science B.V. All rights reserved.
533
Synthesis and Characterization of V - B e t a Zeolite Shu-Hua Chien and Jang-Cheng Ho Institute of Chemistry, Academia Sinica, Taipei 11529, and Department of Chemistry, National Taiwan University, Taipei 107, Taiwan, ROC
SUMMARY H-form and vanadium-containing Beta-zeolite (V-Beta) were synthesized by the hydrothermal method. XRD and IR spectra confirm that the structure of V-Beta is isomorphous with Beta zeolite. EPR spectra indicate VO 2§ ions formed in both V-Beta and solid mixture of V20 5 and H-Beta zeolite and most likely located at cationic sites. The 29Si MAS-NMR spectra indicate that Si-O-A1 angle of V-Beta is larger than that of H-Beta. Infrared spectra of pyridine adsorption show both H-Beta and V-Beta possess highly Lewis acid property but more acid sites in the V-Beta. INTRODUCTION The localization of vanadium ions in synthesized zeolites has gained particular interest owing to their bifunctional and notable catalytic properties. In this paper, we prepared V-Beta zeolite by the direct hydrothermal method and characterized well the structure of the synthesized material. For comparison, the solid mixture of vanadium oxide and H-beta zeolite was also investigated. EXPERIMENTAL The H- and V- Beta zeolites were hydrothermally synthesized using aluminum nitrate, amorphous Aerosil silica and tetraethylammonium hydroxide (TEA-OH) as starting materials. In the preparation of V-Beta, the V205 was used and preoxidized by H20 2. The reactant mixtures were placed in a teflon-lined autoclave and heated at 140~ for 20 days. The resulting sample was centrifuged and washed, then dried at 120~ Calcination was carried out in 0 2 stream at 550~ for 15 hours. The mixture of V20 5 and H-Beta with 0.3 mmol V per 1 g H-Beta was ground mechanically in a vibration mill. The synthesized zeolites were well characterized by ICP-AES, TGA, XRD, SEM-EDS, FT-IR, UV-VIS, 29Si and 27A1 MAS-NMR and EPR spectroscopy.
534 RESULTS AND DISCUSSION
We have successfully synthesized H-Beta and V-Beta zeolites by direct hydrothermal method with Si/A1 atomic ratio of about 25/1. ICP-AES measurement gives a V content of 0.047 wt% in V-Beta, but no vanadium signal was detected by EDS. TGA profiles show the organic materials were removed below 600~ in both as-synthesized zeolites. The XRD patterns of both zeolites are highly crystalline and match very closely with the Beta zeolite structure. The SEM micrographs exhibit almost the same morphologies in cubic shape. The average crystal size of H-Beta is about 0.5 larn and the V-Beta is slightly larger. FT-IR spectra exhibit the characteristic peaks of Beta zeolite but the peaks at 1098 and 1230 cm 1 of H-Beta shift to 1103 and 1237 cm -1 and the peak at 953 cm 1 almost disappeared in V-Beta. UV-VIS spectrum of H-Beta gives a weak peak at 222 nm. In V-Beta, a very broad absorption band appeared from 200 nm up to 370 nm with overlapping peaks likely appearing at 222, 248, 287 and 330 nm. Both FT-IR and UV-VIS spectra reveal that the incorporation of vanadium indeed affects the structure of H-Beta. EPR spectra of V-Beta at 77 K showed intense signals at g//= 1.936 and ga_ = 1.987 with A// = 201 G and At_ = 84 G, with distinct eight-lines hyperfine splittings due to 51V (I = 7/2). After the sample being evacuated at 298 K, the spectrum retained almost the same g-values and hyperfine constants, but a super hyperf'lne structure with A = 4 G appeared. The mixture of V205 and H-Beta also gave the eight-lines hyperfine splitting spectra at g / / = 1.931 and g.t. = 1.985 with A// = 199 G and AJ_ = 81 G, but no super hyperfine structure was observed. The results are comparable to those results as VO 2+ adsorbed on ZSM-5 and X, Y zeolite [ 1,2]. Therefore, we believe that the VO 2+ species were formed in V-Beta zeolite and most likely located at cationic sites. The 29Si MAS-NMR spectrum of H-Beta showed a strong peak centered at -110.2 with a weak shoulder at-104.1 ppm. The spectrum of V-Beta also showed a strong but much narrower peak centered at -110.7 ppm, besides, a distinct small sharp peak appearing at -114.4 ppm. The Si/A1 ratio calculated from 29Si NMR using Loewenstein rule is 24.5 for H-Beta and 235 for V-Beta. Probably, these results are due to the increase in SiO-A1 angle [3] when VO 2+ occupy the cation sites. In the case of the 27A1 NMR spectrum, the resonance peak corresponding to tetrahedral A1 is composed of two overlapping peaks at 56.5-ppm and 54.2-ppm. For H-Beta, the peak intensity of 56.5-ppm is slightly larger. On the other hand, the intensity of 56.5-ppm peak of V-Beta decreased and became to about half of the 54.2-ppm peak. Both H-Beta and V-Beta exhibited a signal at 0.1 ppm corresponding to octahedral A1, but a broader peak with a visible shoulder appeared at higher ppm for V-Beta. Moreover, the infrared spectra of pyridine adsorbed on both H-Beta and V-Beta zeolites indicate that both zeolites possess highly Lewis acidic and very weak BrCnsted acidic properties with more acid sites in the V-Beta. We believe that the results are important and helpful in the related catalytic processes. REFERENCES
1. M. Petras and B. Wichterlova, J. Phys. Chem., 96 (1992) 1805. 2. G. Martini, M.F. Ottaviani and G.L. SeravaUi, J. Phys. Chem., 79 (1975) 1716. 3. J.M. Thomas, J. Klinowski, S. Ramdas, B.K. Hunter and D.T.B. Tennakoon, Chem. Phys. Lett., 102 (1983) 158.
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
535
T i t a n i u m boralites with M F I structure characterized using X R D , IR, U V - V i s X A N E S and M A S - N M R techniques D. Trong Ona, b, M. P. Kapoorb, S. Kaliaguine b, L. Bonneviot a and Z. Gabelica c Department of aChemistry and bChemical Engineering and CERPIC, Laval University, SteFoy, G1K 7P4, Canada, CDepartment of Chemistry, Laboratory of Catalysis, Facult6s Universitaires de Namur, B-5000 Namur, Belgium Two series of boralites and titanium boralites of MFI structure have been synthesized. The structural state of titanium and boron in the silicalite framework has been investigated using XRD, IR, UV-Vis, XANES and MAS-NMR spectroscopic techniques. The quantitative determination of framework titanium and tetrahedral framework boron was made by XANES at the Ti K-edge and liB MAS-NMR. It was shown that both titanium and boron can be incorporated in the framework. In spite of the formation of extra-framework titanium in presence of boron, even at low Ti/Si ratio, the maximum of Ti incorporated seems not affected by boron and optimized by an excess of H202 during the gelification. In calcined samples, the concentration of tetrahedral framework boron is smaller for higher framework titanium content. 1. INTRODUCTION The discovery of crystalline titanium silicalites with MFI or MEL structures, TS-1 and TS-2 respectively, has extended the use of molecular sieves to the catalysts of oxidation reactions [ 1,2]. The simultaneous incorporation of a trivalent metal ions (e.g., B 3+, A13+, Ga 3+, or Fe 3+) along with Ti 4+ in MFI, MEL [3-5], and beta [6] structures has been reported. It may be anticipated that such solids are active both in oxidation reactions like titanium silicalites and in acid-catalysed reactions like aluminosilicates [3,4]. Recently, it has been demontrated that the epoxydation reaction occuring on framework titanium is followed by an hydrolysis of the epoxide into ot-diols in the presence of a mild acidity [7]. Such a bifunctional catalyst is obtained with a zeolite containing titanium and aluminium. Since in a silicalite containing boron, designated as boralite, framework boron is known to introduce milder acidic bridging hydroxyl groups, B-(OH)-Si, than aluminium, this element has a better potential for such reaction. Nevertheless, both boron and titanium have to be incorporated in the framework to yield the proper bifunctionnality. 2. EXPERIMENTAL Two series of boralites (BS-1) and titanium boralites (TBS-1) catalysts were prepared from a gel containing TEOS, TEOT, H3BO3, H202, H20 and the appropriate template by a method previously described [8]. The gel composition was: 0.40TPAOH- xTiO2-0.1 B203-SiO235H20-yH202 where x =0.00-0.03 and y = 0.04 - 0.4). The gel was charged in an autoclave and maintained in hydrothermal conditions for five days at 488 K. The solids were calcined under continuous flow of dry ammonia, or oxygen gas at 450~ and 550~ respectively. Sodium exchange was carried out by stirring 0.5 g of molecular sieve in 30 ml of 1M aqueous sodium bromide during 24 h at room temperature. For deboronation of calcined
536 samples, an aqueous hydrochloric acid (pH= 1) was used at room temperature. The elemental analysis was performed by flame atomic absorption spectroscopy for titanium and sodium, and induced coupled plasma atomic emission spectroscopy for boron analysis. The X-ray diffraction patterns of the samples were recorded on a Rigaku D-MAX II VC X-ray diffractometer using nickel filtered Cu K0~ radiation. The unit cell volumes were calculated from XRD using silicon as an internal standard. The Ti K-edge X-ray absorption spectra were collected at the radiation synchrotron facility of the LURE (France). The white radiation Was monochromatized by a Si (111) two-crystal monochromator. The normalized XANES spectra were analyzed using a classical edge normalization procedure and energy calibration to the first peak of the K-edge of a titanium foil [9]. Dehydrated samples were obtained by evacuation at 573 K in vacuum and transferred under dry argon into a vacuum tight cell for measurements. Diffuse reflectance UV-Vis spectroscopy was performed using a Perkin-Elmer Lambda 5 spectrophotometer using MgO as a standard. IR spectra were recorded with a Bomen 102 FTIR spectrometer on self supporting pellets of samples mixed with KBr. 11B_MAS NMR spectra were obtained on a Bruker CXP-300 spectrometer by a "one-cycle" type measurement: resonance frequency: 128.288 MHz, r.f. field: 9.4 G (10 ~ pulses); repetition time: 0.2s, sweep width: 62.5 kHz, with 4K data points. Typically 2000 free induction decays were accumulated per sample. MAS was 10 kHz, using a 4 mm zirconia rotor. The chemical shifts were determined from BF3.OEt2, used as an external reference.
3. RESULTS AND DISCUSSION XRD: The X-ray pattern of the silicalite revealed a Pnma-orthorhombic symmetry from which the indexation was performed. The 2.9% boron leads to a decrease of the unit cell volume from 5335 to 5280 s consistently with previous reports (Table 1) [10]. On the other hand, the [3.4]TS-1 exhibits a unit cell expansion of 35 ~3 in agreement with literature data [ 11 ]. This observation is logical on account of the differences in the M-O distances (1.79, 1.61 and 1.47 .~ for M=Ti 4+, Si 4+ and B 3+ respectively) and thereby supports the incorporation of Ti 4+ and B 3+ in the framework lattice. A mixte effect is observed for the titanium , 1 ' ' ' ' 1 .... I .... I .... I .... I .... I ' " ' ' ~ ] boralites for which unit cells are 5400 larger than pure boralite, 9 a 9 [2.9]BS and smaller than pure eq silicalite (Table 1). Taking the 5360 b , boralite as reference, it is clear that the unit cell volume progressively expends when the o 5320 ;> titanium loading increases at constant boron concentration. A 5280 linear fit of the expansion can be I ~ J l ~ l ~ , , , I , , , , I , ~ , , I .... I , . , , ! .... proposed as in the case of TS-1 0 1 2 3 (Fig. 1). The slopes (dV/dx) T~Si in molar% associated to the unit cell expansion is smaller for TBS Figure 1. Evolution of the unit cell volume with the titanium than TS (15 instead of 20 A3, content for, a) TS (A this work, 9 ref. 11), b) TBS versus respectively)[ 11 ]. Further framework titanium obtained from XANES spectra and c) investigation is indeed necessary TBS versus total content of titanium. as this expansion must be correlated to the effective level of Ti incorporation.
537
Table 1 Chemical composition of crystalline solids and XANES, MAS-NMR and XRD characteristics for titaniumboralites and boralites.
zeolite
XANES
NMR
tetrahedral Ti b
tetrahedral B c
hydrated
dehydrated
T/(O+T) (%)
T/(O+T) Ti/Si (%) (%)
a [ 1.5-3.0]TBS-A [1.5-2.9]TBS-C e [ 1.4-2.7]TBS-A [ 1.4-2.7]TBS-C f [2.3-2.7]TBS-A [2.2-2.7]TBS-C [3.2-2.8]TBS-A [3.4-2.7]TBS-C [3.3-3.8]TBS-A [3.3-3.6]TBS-C [3.4]TS-C [3.0]BS-C [2.9]BS-C silicalite-C [1.5]TS-C
20 . 25 25 30 . 60
35 .
.
.
. 100
B/u.c.
2.8 1.6
. 1.5
XRD
Ti/Si B/Si (%) (%)
unit cell d volume (A3)
1.5 1.5 1.4 1.4 2.3 2.2 3.2 3.4 3.3 3.3 3.4 -
3.0 2.9 2.7 2.7 2.7 2.7 2.8 2.7 3.8 3.6 3.0 2.9
5305
1.5
-
line (-3.7 ppm)
0.5 . 0.8 0.9 1.5 1.7 -
55 40 45 50 -
chemical analysis
2.3 0.7 2.1 0.4 2.4 0.3 2.8 2.1 . -
.
5325 5325 5330 5370 5280 5335 5375
a) 1.5 and 3.0% for Ti/Si and B/Si atomic ratio respectively in as-synthesized or calcined TiBoralite are referred to as [ 1.5-3.0]TBS-A or C respectively, b) percentage of tetrahedral (T) and octahedral (O) titanium calculated from XANES results, c) content of tetrahedral boron calculated from NMR results, d) calculated from XRD, e) and f) H202/B=0.3 and 1.0 in the gel respectively.
Framework IR: The IR spectra o f the boralite and titanium boralite samples with different treatments are given in Figure 2. In all cases, the IR spectra w e r e typical of pentasil zeolites. T h e well defined IR bands at 800 and 455 c m -1 and the saturated region 1000-1300 c m -1 are characteristic of SiO4 tetrahedra, while the vibrational band at 555 cm-1 confirms the presence of five m e m b e r tings of the pentasil structures. The IR band at 920 cm-1 can be assigned to the presence o f the tetracoordinated f r a m e w o r k boron and it is o b s e r v e d in all b o r o n pentasils. H o w e v e r , for the calcined sample (Fig.2A), a strong band is also appearing at 1380 c m -1 which can be assigned to tricoordinated f r a m e w o r k boron [ 12]. The transformation o f the absorption band at 920 c m -1 to 1380 c m -1 upon calcination is logically assigned to the change o f b o r o n coordination from tetrahedral to trigonal. On the other hand, after the N H 3 treatments (Fig. 2Ab), f o l l o w e d by N a +- e x c h a n g e (Fig. 2A-c) and calcination (Fig. l d), the IR spectra do not s h o w the characteristic BO3 absorption at 1380 c m -1. It is c o n c l u d e d that tetrahedral f r a m e w o r k b o r o n is stable w h e n N H 4 + or N a + ions are c o u n t e r b a l a n c i n g the f r a m e w o r k negative charge, and unstable when the counterion is the proton (Fig. 2A-e)[ 12]. The IR spectra o f the titanium boralites are similar to those of boralite samples (Fig. 2B). A
538 new band appears at 960 cm-1 and corresponds to the fingerprint of titanium incorporation in boron free silicalites [ 13]. The presence of IR bands at -920, 960 and 1380 cm-1 in all samples strongly suggests the simultaneous incorporation of Ti and B in the silicalite framework.
4-
NMR: Assuming that all b o r o n atoms are N M R A 1380 visible. The M A S - N M R method can be used for a 1800 1500 1200 900 600 300 1100 900 700 quantitative determination of -1 -1 cm cm the amount of boron actually substituted for Si in zeolite Figure 2: IR spectra of (A) the boralite, [4.0]BS and (B) titanium frameworks, whereas boralite, [3.4-4.0]TBS samples: a) as-synthesized, b) after NH3 chemical analysis can only treatment at 450~ for 4h, c) sample (b) exchanged with 1M NaBr provide a bulk boron solution at ambient temperature, d) sample (c) calcined at 550~ in air concentration. The NMR line for 6h, e) sample (a) calcined at 550~ in air for 6h. located at 5 - - 3 . 7 ppm is unambiguously assigned to BO4- tetrahedra in a crystalline boralite structure. This line is very sensitive to the local structure and the electronic environment [ 14,15]. A quantitative determination of tetrahedral boron content in the samples can thus be estimated by measuring the intensity of NMR line at -3.7 ppm. The 11B NMR spectrum of as-synthesized [3.0]BS sample exhibits a single narrow peak at --4.0 ppm (Fig. 3A), consistent with most of the boron ions incorporated as tetrahedral BO4units in the framework. Two very broad lines of much weaker intensity around -2 ppm correspond to framework linked trigonal boron [15]. Upon calcination, the intensity of tetrahedral boron line is decreased and is slightly shifted to -3.7 ppm. The trigonal framework lines are relatively increased in addition a hump assigned to an extra-framework boron arises at - 6 ppm. The observed small shift for the tetrahedral boron can be explained by a change in the chemical environment and a sharp decrease of the dipolar interaction between boron and other nuclei upon calcination. The 11B NMR spectra of the as-synthesized and calcined TBS samples containing both Ti and B are essentially similar to those of the corresponding boralite samples (Fig. 3B). The intensity of the tetrahedral boron line at - -3.7 ppm for the calcined TBS sample is found to be decreased while the intensity of the framework and extra-framework trigonal lines are clearly enhanced as compared to calcined BS sample of same boron concentration. The NMR quantification of boron by 11B NMR yields less boron than the amount measured by chemical analysis for several of the samples, especially for those calcined at 550~ (Table 1). It seems that the amount of framework and extra-framework trigonal boron at - -2 and - 6 ppm are not totally detected by 11B NMR, because the NMR lines are highly asymmetric species and may be broadened beyond the detection limit. Therefore, the presence of observed NMR lines accounts only qualitatively for the existence of framework and extra-framework trigonal boron. UV-Vis spectroscopy. The pure boralite exhibits no UV band in the 200-400 nm range where lie the features of TBS and TS samples which indeed for the two materials are very
539 0.5 -3.6
:j
%
J 8 6 4 2 0 -2 -4 -6 -8 PPM
0 -3.6
500
I
I
I
I
I
I
450
400
350
300
250
200
nlrl ~ +6
-I.
9
i
.
i
.
i
.
i
,
i
,
i
,
i
b
Figure 4. UV-Visible diffuse reflectance spectra of titanium boralites with the same Ti and B loadings of 3.0 and 1.5 atom % respectively and varying H202/B ratio in the gel: a) [1.5-3.0]TBS (H202/B-0.3), b) [1.4-2.7]TBS (H202/B=l.0) and c) [1.5]TS.
I
similar. Between 270 to 400 nm, the absorption exhibits a broad peak assigned to TiO2 anatase, while below 270 nm, an intense peak is assigned to isolated Figure 3. l I B MAS-NMR titanium species incorporated in the framework [16]. spectra of (A) boralite, [3.0]BS The UV-Vis spectra of TBS samples synthesized in the and (B) titanium boralite, [3.4presence of boron show a framework titanium band at 210 nm along with the extra-framework band of varying 2.7]TBS: a) as-synthesized b) intensifies. The series of titanium boralites prepared calcined samples. with varying H202/B ratio in the gel (Fig. 4). This is apparently consistent with a diminution of extraframework Ti concentration when an excess of hydrogen peroxide is added. The UV band of extra-framework titanium is blue shifted (at 325 and 340 nm for samples (a) and (b) respectively in Fig. 4) in comparison with the position expected for bulk TiO2 (at 350 nm). This blue shift clearly indicates a change in the nature of extra- framework species. In addition, the titanium loading chosen for these samples is low enough to avoid the formation of extraframework Ti species in the boron free TS sample as indicated by the absence of I.W bands in the 270-400 nm range for [1.5iTS (Fig. 4). Conversely, the systematic presence of a UV band in this region for TBS clearly shows that extra-framework Ti species are systematically generated in presence of boron. The hydrogen peroxide concentration seems to be an important parameter influencing the extra-framework species nature and concentration. Thus it is concluded that framework incorporated Yi increases in concentration as the hydrogen peroxide concentration increases. By contrast, the inconsistent intensity variation of the peak in the 270400 nm range versus titanium loading indicates that no quantification of this species should be attempted by UV spectroscopies. 8 6 4 2 0 -2 -4 -6 -8 PPM
XANES: The Ti K-edges of dehydrated and hydrated TBS-1 were compared to those of bulky TiO2 anatase and dehydrated [1.5iTS-1. The pre-edges of TBS samples exhibit intermediate features compared to the two reference materials (Fig. 5). This suggests the presence of a mixture of sites in the TBS samples. Taking into account the absence of anatase
540 or any form of extra-framework Ti in the [1.5]TS, the Ti K-edge of this sample 2 was taken as a reference for 100% o incorporation of titanium in the ~. 1.6 framework. O .~ 1.2 Accordingly, the Ti K-edge of anatase was adopted as reference for octahedral bl 0.8 titanium. Simulations of the titanium Kedges using a linear combination of the 0.4 two reference edges led to excellent fits of O z the dehydrated TBS- l(Fig. 5, doted o lines). A quantitative determination of tetrahedral and octahedral titanium -0.4 distribution in TBS-1 samples is 4960 4980 5000 5020 5040 proposed on this basis [9] (Table 1). Energy/eV For dehydrated boralites prepared with various titanium loadings (1.5, 2.2 and Figure 5. Ti-K-edge XANES spectra of: a) anatase, b) 3.4 %, see Table 1), one observes that [I.5-3.0]TBS, c) [1.4-2.7]TBS and d) [1.5]TS; the higher the titanium loading the higher Linear combinationfit (---) of XANES spectra using as the incorporation in tetrahedral site (35, reference spectra those of anatase for octahedral 40, 45 %). In the series where the hydrogen peroxide concentration was coordination (a) and [1.5ITS for tetrahedral varied at a constant Ti loading, from 3 5% coordination (d). up to 55% of tetrahedral titanium was found for samples made from gels 3.5 containing a H202/B higher than 1 (Table 3 1). According to the XANES results, the maximum titanium incorporated in ~ 2.5 iiiiiiiiiiiiiiiiiii iiiiiii presence of boron is about 1.5%. For hydrated samples, the fit quality was not ~ 2 as good as for dehydrated samples and ~ 1.5 ........i...........i..................!....................!.....................!.................... the confidence on site occupancy was not 1 as reliable. This is attributed to a change :: :: .i :: of symmetry more likely due to the 0.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . addition of a water molecule in the 0 coordination sphere of Ti [ 17,18]. -0.5 0 0.5 1 1.5 The quantitative data on tetrahedral Ti/Si (%) framework or total boron and framework or total titanium contents in titanium Figure 6: Total boron content obtained by chemical boralites obtained by using XANES, analysis (curve a), and tetrahedral boron content NMR, and chemical analysis (Table 1) are gathered in Fig. 6 to facilitate the obtained from liB MAS-NMR intensities for asdiscussion for the series of assynthesized (curve b) and calcined (curve c) boralite and synthesized (Fig. 6b) and calcined 9 titanium boralite samples samples (FI~. 6c). The slope d(B/Si)/d(Ti/Si) is slightly lower tbr curve (b) than for curve (a) and much lower for curve (c). This strongly suggests that during calcination more boron is converted into trigonal and extra-framework species in samples with higher framework titanium content. The tetrahedral framework boron is decreased from 2.1% for the boralite sample to -~0.4% for the titanium boralite sample as the framework titanium increased from 0% to 1.6%. Finally, it is quite clear that boron hinders the incorporation of titanium into framework during crystallization whereas titanium favors the extraction of boron out from the framework during calcination. ,
...................
N
/
i
,
,
,
i
...............
,
,
,
i
i
,
,
. . . . . .
j
i
....
a
...........
541 4. CONCLUSION The combined XRD, IR, UV-Vis, XANES and MAS-NMR investigations on the structural state of titanium and boron in the silicalite of MFI structure led to the following conclusive points: i) both titanium and boron can be simultaneously incorporated in the MFI-framework, ii) the presence of boron leads to some extra-framework titanium formation even at low Ti/Si ratio but does not affect significantly the limit of titanium incorporation, iii) high hydrogen peroxide concentrations were necessary to optimise the incorporation of Ti in the framework, iv) less tetrahedral framework boron was left behind for high framework titanium contents during calcination. REFERENCES
.
3. 4.
,
.
.
8.
.
10. 11. 12. 13. 14. 15. 16. 17. 18.
M. Taramasso, G. Perego and B. Notari, UK Pat., GB 2 071 071 B (1983), assigned to Snamprogetti, Italy; G. Bellussi, European Patent, A1 0 272 496 (1987). D. R. C. Huybrechts, L. De Bruycker and P. A. Jacobs, Nature, 345 (1990), 240. K. K. Lam Shang Leen, US Patent 4, 623, 526 and US Patent 4, 519, 998 (1985). A. Thangaraj, R. Kumar, and P. Ratnasamy, Appl. Catal., 57, (1991) L 1; A. Thangaraj, S. Sivasanker, and P. Ratnasamy, Zeolites 12 (1992) 135; J. S. Reddy, R. Kumar, and S. M. Csicsery, J. Catal. 145 (1994) 73. L. Forni, and M. Pelozzi, J. Mater. Chem., 1101 (1990), L.Forni and M. Pelozzi, A. Guisti, G. Fornasari, and R. Millini, J. Catal. 122 (1990) 44. M. S. Rigutto, R. de Ruiter, J. P. M. Niederer, and H. Van Bekkum, Stud. Surf. Sci. Catal. 84 (1994) 2245. G. J. Hutchings, D. F. Lee, J. Chem. Soc., Chem. Com., (1994) 1095. D. Trong On, S. Kaliaguine and L. Bonneviot, J. Catal., (1995) submitted.; D. Trong On, M. P. Kapoor, L. Bonneviot, S. Kaliaguine and Z. Gabelica, J. Chem. Soc., Faraday Trans. (1995) submitted. C. Cartier, C. Lortie, D. Trong On, H. Dexpert, L. Bonneviot, Physica, B, 208 & 209 (1995) 653. B. L. Meyers, S. R. Ely, N. A. Kutz, J. A. Kaduk and E. Van Den Bossche, J. Catal., 91 (1985) 352. G. Perego, G. Bellussi, C. Corno, M. Taramasso, F. Buonomo, A. Esposito, Stud. Surf. Sci. Catal. 28 (1986) 129. R. de Ruiter, A. P. M. Kentgens, J. Grootendorst, J. C. Jansen and H. Van Bekkum, Zeolites, 13 (1993) 128. M. R. Boccuti, K. M. Rao, A. Zecchina, G. Leofanti and G. Petrini, Stud. Surf. Sci. Catal. 48 (1989) 133. Z. Gabelica, J. B. Nagy, and G. Debras, Stud. Surf. Sci. Catal., 19 (1984) 113. K. F. M. G. J. Scholle and W. S. Veeman, Zeolites, 5 (1985) 118. M. S. Rigutto, R. de Ruiter, J. P. M. Niederer, and H. Van Bekkum, Stud. Surf. Sci. Catal. 84 (1994) 2245. A. Lopez, H. Kessler, J. L. Guth, M. H. Tuilier and J. M. Topa, Proc. of 6th Int. Conf. on X-ray Absorption Fine Structure, York (1990) p. 549. L. Bonneviot, D. Trong On and A. Lopez, J. Chem. Soc., Chem. Comm., (1993) 685.
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Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
543
Acidity and Structural State of Boron in Mesoporous Boron Silicate MCM-41 D. Trong On, P.N. Joshi, G. Lemay and S. Kaliaguine D6partement de G6nie Chimique et CERPIC, Universit6 Laval, Qu6bec, Canada
ABSTRACT Boron containing MCM-41 molecular sieve has been hydrothermally synthesized from the system of S i t 2 - B20 3 - CTAC1 - Na20 - TMAOH - NH4OH - H20 (CTAC = cetyltrimetylammonium chloride and TMAOH - tetramethylammonium hydroxide). The products were characterised by XRD, FTIR, BET and liB MAS-NMR and compared with their A1 analogue. The framework boron incorporation was confirmed by the decrease in "d" spacings and the appearance of 1380 and 940 cm 1 bands. The simultaneous presence of tetra- and tri-coordinated boron was observed in the calcined form whereas the as-synthesised form exhibited only tetra-coordinated boron. On the basis of pyridine adsorption and almost identical trivalent cation content, the trend in both acid strength and density of acid sites was found as: pure silica MCM-41 < B-MCM-41 < A1-MCM41 << B-ZSM-5. INTRODUCTION Zeolites are widely used as catalysts in acid-catalyzed reactions for the production of petrochemicals and fine chemicals. The activity of zeolites is attributed to their acidic character wherein Br6nsted and/or Lewis sites are involved [ 1]. However, catalysis by zeolites has been severely limited in the past by the available dimensions of their micropores and therefore, the maximum molecular size of substrate or reaction products that can diffuse through the internal voids [2]. Among the different types of molecular sieves, a large-pore and a medium pore zeolites are the ones used in larger quantifies. Very recently, the discovery by researchers at Mobil Oil Corporation of a new family of mesoporous materials designated as M4IS has opened a new perspective in the hydrothermal synthesis of porous materials [3,4]. MCM-41 is one of the members of this family and has a hexagonal arrangement of unidimensional pores, varying in size from around 15 ,~ to more than 100 ,A,. There is no doubt that the presence of these very large pores combined with acidic properties opens new possibilities for processing and/or producing large molecules. The substitution of Si by B during the synthesis of MCM-41 with mild Br(~nsted acid sites is expected to modify the acidic properties of these materials and thereby their catalytic properties [5]. In this report, we describe the synthesis and characterization of mesoporous boron molecular sieve, B-MCM-41. EXPERIMENTAL
The hydrothermal synthesis of B-MCM-41 was carried out using gels with the following molar composition:
544
SiO2: 0.05B203:0.27CTACI: 0.13Na20: 0.26TMAOH: 0.27 NH4OH: 60H20. A typical synthesis procedure was as follows: (1) 10 g of a 25% solution of tetramethylammonium hydroxide (TMAOH) was combined with 7.6 g of sodium silicate dispersed in 20 g of water with stirring, and 34.2 g of 25% cetyltrimethylammonium chloride (CTAC1) mixed with 3.20 g of NH4OH (- 29%) and 12 g of ludox (33.6% SiO 2) were added and stirred for 2 h. (2) 0.95 g of NaEB407 was dissolved in 40 g of water and then slowly added to the gel. (3) The pH was adjusted with dilute sulfuric acid to 11.5 and the homogeneous mixture was then transferred into a Teflon-lined autoclave and heated to 423 K for 48 h. (4) The solid product was filtered, washed with distilled water, dried in air at 353 K and finally calcined in air at 813 K for 6 h (heated from room temperature to 813 K with a heating rate of 1 K/min). The A1-MCM-41 was synthesized using the same procedure but the boron source was replaced by aluminum sulfate, and pure-silica of MCM41 was synthesized with no boron and aluminum sources. The borosilicalite (B-ZSM-5) was prepared by the method described in [6a]. The amorphous borosilicate was obtained by same hydrothermal synthesis conditions but with no template in the initial gel mixture. The elemental analysis was performed by atomic absorption spectroscopy for aluminum and sodium. ICPAES (induced coupled plasma atomic emission spectroscopy) was used for boron estimation. The powder X-ray diffraction patterns of the samples were recorded on a Rigaku D-MAX II VC X-ray diffractometer using nickel filtered Cu Kot (~, = 1.506 ,~) radiation. Surface area measurements were carried out on an Omnisorp-100 apparatus following the BET l~rocedure. IR spectra were recorded on a Digilab FTS-60 spectrometer, using wafers of 10 mg cm -~ treasted in a vacuum cell at 773 K for 6 h. The samples were cooled down to room temperature under vacuum and then immediately exposed to pyridine vapours for 15 min. Desorption of pyridine was carried out by evacuation for 4 h at 323, 373 and 423 K. IR spectra for in situ thermal desorption were obtained by treatment of samples at 573 and 773 K in vacuum for 4 h each time. liB and 27A1 MAS-NMR measurements were carried out by 90 ~ pulse excitation in a Bruker ASX 300 spectrometer at 96.25 MHz and 78.17 MHz frequencies respectively. 11B MAS-NMR spectra were obtained with 90 ~ pulse duration 2.5 las, repetition time 2s, and spinning rate of 3.5 kHz and chemical shifts were determined relative to BF30(ET) 2. 27A1MAS-NMR spectra were obtained with 4 kHz spinning to sufficiently remove contribution from side bands, using 90 ~ pulse duration 1.1 tas and repetitive time of 1 s. A solution of AI(NO3) 3 with pH - 1 was used as an external reference for chemical shift. RESULTS AND DISCUSSION
Material: Table 1 shows the chemical composition of gels and solids, the d-spacing and the BET surface area of B-MCM-41, AI-MCM-41 and their pure silica analogue. BET surface areas were 950, 900 and 650 m2/g for pure-silica, B-MCM-41 and AI-MCM-41 respectively. Table 1 Chemical composition (atomic ratio) of gels and calcined samples and their texture properties Sample Pure-silica MCM-41 B-MCM-41 A1-MCM-41 B-ZSM-5
B/Si
Al/Si
...... 0.1 --0.1
--0.03 ......
Na/Si
B/Si
A1/Si
Na/Si
XRD d l o o o d-spacing/A
0.13 0.13 0.13
...... 0.024 --0.029
0.05 57 --0.12 55 0.045 0.50 - 60 . . . . . . . . .
Surface area (m2/g)
950 900 650 440
545
The X-ray diffraction pattern of B-MCM41 (Fig. 1) matches well with that of the boronfree silica MCM-41 and also the patterns reported by Kresge et al. [3]. One major peak at 20 - 1.6~ along with three peaks at 20 - 2.8, 3.2 and 3.6 ~ were observed. Beck et al. [2] indexed these peaks for a hexagonal unit cell the parameter of which was calculated using ao = 2dloo/~3. It was also observed that the dl0 o spacing increases with the chain length of the organic template. A variation in the d-spacing should account for the differences in M-O bond distances (1.47, 1.61 and 1.76 A for M = B 3+, Si 4§ and A13§ respectively). Accordingly, the decrease in the dspacing of B-MCM-41 compared to its pure silica analogue (Table 1) suggests the presence of boron in the silicate framework. This observation was reported for all microporous boron silicalites [6b,7] as a consequence of shorter M-O bond distance. The d-spacing increases for the AIMCM-41 indicating that A1 is incorporated into the framework. This is also confirmed by the 27A1 0 2 4 6 8 10 MAS-NMR spectrum of this sample which showed 2O only 4-coordinated aluminum [8]. MAS-NMR spectroscopy: 11B MAS-NMR Fig.l: Powder X-ray diffraction pattern spectra of the as-synthesised and calcined form of of calcined MCM-41 samples B-MCM-41 are shown in Figure 2. The asa) pure silica M C M - 4 1 b) B-MCM-41 synthesised product exhibited only one signal at 2.5 ppm which is significantly different from amorphous borosilicate (- 1.9 ppm), BO 4 in borate solutions [9] and in solid state [10,11 ]. Since the range of framework boron shift is believed to reflect the range of average T-O-T angles, the signal at -2.5 ppm may be assigned to tetrahedral BO 4 units in the MCM-41 phase. The reversible conversion of tetrahedral boron into trigonal boron has been reported [ 12] following of dehydration and rehydration. However, the conformation and/or charge distribution around boron atoms was found to be influenced by the removal.of the templace upon calcination. The liB MAS-NMR spectrum of calcined B-MCM-41 sample shows the simultaneous presence of both tetrahedral and trigonal boron. The observed small shift at lower field (from -2.5 to -2.3 ppm) for the tetrahedral boron signal may be attributed to the change in chemical environment and to decrease dipolar interactions with other nuclei. It is important to investigate the co-ordination state of aluminum in aluminosilicate molecular sieves as Br6nsted acidity originates from the quantity, location and the co-ordination state of aluminum. The 27A1MAS-NMR spectra of calcined AI-MCM-41 sample showed (Fig. 3) a single resonance at - 51 ppm exhibiting exclusive incorporation of 4-coordinated aluminum into the framework [8]. Infrared Spectroscopy and Acidity Measurements: The IR spectra in the 300-1800 cm -1 region contain a series of bands which are characteristic of the SiO 4 tetrahedron unit and its modification by introduction of boron (Fig. 4). The location of boron is confirmed from the presence of IR bands at 1380 and 940 cm 1 which are characteristic of tri- and tetra coordinated boron in the silicalite framework, respectively and these bands are observed in all borosilicates [7,13]. Trigonal
546
60
40
20
0
-20
-40
-50
-60
0
50
ppm
150
Fig. 3:27A1MAS-NMR spectrum of calcined A1-MCM-41 sample
Fig. 2:11B MAS-NMR of B-MCM-41 in (a) as-synthesized and (b) calcined form. boron was produced upon dehydration and could be reversibly converted back to tetrahedral boron by rehydration [12-15], However tetrahedral framework boron (IR band at - 940 cm -1) is stable when sodium is counterbalancing, but unstable in the hydrogen form [13]. Therefore, stability of these sites is thought to be caused by formation of Na § -B(OSi) 4units, which are coordinatively saturated and stable to hydrolysis [13,14]. The calcined B-MCM-41 form exhibited a week band at 1380 cm 1, suggesting a presence of tricoordinated boron, which can be due to low Na/B ratio in the solid product. These bands at 1380 and 940 cm 1 are absent in the spectrum of the pure silica analogue (Fig. 4). Figure 5 shows the IR spectra of samples of pure silica MCM-41, BMCM-41 and AI-MCM-41 dehydrated in vacuum at 573 and 773 K, in the hydroxyl range (4000 - 3000 cm-1). In the figure 3a, two bands are clearly
100
ppm
I
I 84
a
r/3
1380 455 1800
1500
1200
900
Wave vector in cm
600
300
-1
Fig. 4: IR spectra of calcined MCM 41 samples a) Pure silica MCM-41 b) B-MCM-41
547
visible: a broad one centered at - 3400 cm 1 and a sharp one a t - 3740 cm -1. Both bands can be assigned to silanol groups, the first one being shifted to lower wavenumber and enlarged due to strong interaction with residual water located in the channels and second one is attributed to noninteracting external silanol groups. In fact, after treatment at 773 K (Fig. 5B), only a narrow and very intense band at 3740 cm 1 was observed in the IR A 3740 spectra, due to free external silanol groups, indicating reduced silanol ~3400 interactions [16]. The intensity of this band is not significantly different for all three samples. Figure 6 shows the IR spectra of pyridine adsorbed on pure silica MCM-41, B-MCM-41, AI-MCM-41 and Figure 7 the ones of B-ZSM-5 all pretreated under the same conditions in the IR cell. The B-ZSM-5 containing 2.9 atom % B was studied for comparison with MCM-41 analogues. A IR broad o band at 1380 cm -1 which is characteristic ,.Q B of framework trigonal boron was observed < on the boron samples. It is seen that pyridine adsorbed on pure silica MCM-41 sample exhibit IR Bands at 1441, 1446 and 1600 cm -1. The bands at 1441 and 1600cm -1 which are characteristic of physisorbed pyridine [17], decreased sharply upon evacuation at elevated temperatures. The band at 1446 cm 1 has been assigned to hydrogen-bonded pyridine (hydroxyl groups which do not protonate pyridine). The B-MCM-41, 3900 3600 3300 AI-MCM-41 and B-ZSM-5 samples show W a v e vector in cm- 1 additional bands which are absent in pure silica MCM-41 (Fig. 6 and 7). Two weak Fig. 5: IR spectra in the hydroxyl region of MCM-41 samples bands at 1545 and 1455 cm -1 which are calcined at (A) 573 K and 03) 773 K characteristic of Br(insted-acid and Lewis a) pure silica MCM-41 b) B-MCM-41 sites were observed in the IR spectrum of c) A1-MCM-41 AI-MCM-41, and another band due to contributions of both Br0nsted-acid and Lewis sites at 1492 cm 1". The B-MCM-41 sample exhibited a very weak band at 1545 cm -1 and a new band at 1462 cm q, the latter has been assigned to strong Lewis sites generated by the polarisation effect of hydroxyl nests and/or an electrophilic B atoms (Fig. 5 and 7) [18]. The IR spectra of B-ZSM-5 sample showed the bands at 1446, 1462, 1490, 1545 and 1600 cm -1, even after evacuation at 573 K, while these bands had disappeared in B-MCM-41 and A1-MCM-41 samples, though they were evacuated at lower temperature, ca. 423 K (Fig. 6 and 7). Finally, weak and mild acid sites (Br6nsted and Lewis) were detected in the B-MCM-41 and AI-MCM-41 samples. However, the B-MCM-41 IR spectrum exhibits a new band at 1462 cm -~ indicating a higher amount of Lewis sites
L_
I I
I
c
I
548
as compared to the A1-MCM-41. The acid strength of B-MCM-41 sample was found almost identical with AI-MCM-41 sample. The B-ZSM-5 sample has a stronger acidity and a higher amount of acid sites than B-MCM-41 and AI-MCM-41. The order of the acid strength is: pure silica MCM-41 < B-MCM-41 < AI-MCM-41 << B-ZSM-5.
A
1446 !
1600
1446
'"2 I
ab
L
1600
B
1441
1490 1 4 6 ~ jIA [ ~
e
J ~.~___.___jkj I
I
I
1600
1500
1400
W a v e v e c t o r in c m - 1 Fig. 6: IR spectra of pyridine after desorption at (A) 323 K and (B) 373 K a) pure silica MCM-41 b) B-MCM-41 c) A1-MCM-41
: ,
1600
1550
1500
1450
,
1400
W a v e v e c t o r in c m - 1 Fig. 7: IR spectra of pyridine on B-ZSM-5 sample after desorption at different temperatures a) room temperature b) 323 K c) 4 2 3 K d) 4 7 3 K e) 5 7 3 K
CONCLUSION Framework incorporation of boron in the MCM-41 structure has been achieved by the direct hydrothermal synthesis. The decreased "d" spacings and the presence of 1380 and 940 cm -1 IR bands confirm the framework occupancy of boron in the MCM-41 structure, lZB MAS-NMR results showed the partial conversion of tetra-coordinated boron into tri-coordinated boron during the process of template removal. The calcined form showed the simultaneous presence of tetra- and tricoordinated boron. B-MCM-41 molecular sieve possesses a weak acidic strength as compared to B-ZSM-5, almost identical to the one of the AI analogue of MCM-41. The authors wish to acknowledge Dr. L. Le Noc for providing skillful assistance and fruitful discussions in connection with MAS-NMR spectra and Prof. L. Bonneviot for stimulating discussions and comments.
549
REFERENCES
1. 2. 3. 4.
J.A. Rabo, Zeolite Chemistry and Catalysis, Am. Chem. Soc., ACS Monograph (1979) 171. M.E. Davis, ACC. Chem. Res., 26 (1992) 111. 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. Bartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, T.U. Chu, D.H. Olsen, E.W. Sheppard, S.B. McCullen, J.B. I-Iiggins and J.L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 5. M.R. Klotz, US Patents 4 268 420; 4 269 813; 4 285 919 (1981). 6a. D. Trong On, S. Kaliaguine and L. Bonneviot, J. Catal. (1995) accepted. 6b. G. Perego, G. Bellussi, C. Corno, M. Taramasso, F. Buonomo, A. Esposito, Stud. Surf. Sci. Catal. 28 (1986) 129. G. Coudurier, A. Auroux, J.C. V6drine, R.D. Farlee, L. Abrams and R.D. Shannon, J. Catal. 108 (1987) 1. Z. Luan, C.F. Cheng, W. Zhou and J. Klinowski, J. Phys. Chem., 99 (1995) 1018. 9. C.G. Salentine, Inorg. Chem., 22 (1983) 3920. 10. S. Schramm and E. Oldfield, J. Chem. Soc. Chem. Commun. (1982) 980. 11. Z. Gabelica, J.B. Nagy, P. Bodard and G. Debras, Chem. Lett. (1984) 1059. 12. K.F.M.G.J. School and W.S. Veeman, Zeolites, 5 (1985) 118. 13. R. de Ruiter, A.P.M. Kentgens, J. Grootendorst, J.C. Jansen and H. van Bekkum, Zeolites, 13 (1993) 128. 14. G.H. Kuehl, US Patent 4 661 467 (1987). 15. H. Kessler, J.M. Chezeau, J.L. Guth, H. Strub and G. Coudurier, Zeolites, 7 (1992) 360. 16. A. Corma, V. Fom6s, M.T. Navarro and J. P6rez-Pariente, J. Catal., 148 (1994) 569. 17. J. Dwyer and P.J. O'Malley, Stud. Surf. Sci. Catal., 35 (1988) 1. 18. A.K. Ghosh and G. Curthoys, J. Chem. Soc., Faraday Trans. 1, 79 (1983) 805. .
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Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviotand S. Kaliaguine(editors) 9 1995 Elsevier Science B.V. All rights reserved.
551
THE ROLE OF Na AND K ON THE SYNTHESIS OF LEVYNE-TYPE ZEOLITES. Claudia Vilma Tuoto, Janos B.Nagy* and Alfonso Nastro D e p a r t m e n t of Chemical Engineering and Materials, University of Calabria, Arcavacata di Rende, 87030 Rende (CS), Italy. *) Laboratoire de R.M.N., University Notre Dame de la Paix, Rue de Bruxelles 61, 8000 Namur, Belgium. The synthesis of levyne-type zeolite from aluminosilicate gels containing methylquinuclidine as template and sodium and potassium as inorganic cations, has been investigated by varying the K/Na ratio in the batch, at two different temperatures (150~ and 170~ The synthesis conditions in order to crystallise the levyne-type zeolite free of parasite phases have been identified for potassium containing systems. The variation of the kinetic parameters and the i n t e r a c t i o n of the i n o r g a n i c cations (Na, K) and of the organic methylquinuclidine ion (MeQ +) with the levyne framework have been defined. 1. INTRODUCTION The levyne was one of the first zeolitic s t r u c t u r e s discovered and synthesised utilising organic compounds (1). Recently, the isomorphous substitution of A1 by B, Ga and Fe has been reported (2). In 1983, two patents reported the synthesis of LZ-132 (3) with a levyne structure and LZ-133 (4), both obtained from the same batch composition but at different temperature. The metylquinuclidine (MeQ +) as organic templating cation was used together with sodium. Only in the patent on the synthesis of ZSM-45, another zeolite isostructural with levyne, some examples containing both Na and K are reported (5). In the earlier studies, the role of methylquinuclidine (MeQ) on the levynetype zeolite synthesis was investigated (6-8). In particular, the area of existence and the crystallisation kinetics of levyne as a function of the reaction temperature, the N a 2 0 and MeQI content (6, 8) and the SIO2/A1203 ratio (7) was reported. The interaction of the inorganic and organic methylquinuclidine ion MeQ + with the levyne framework and with its precursor aluminosilicate amorphous gel was also reported (9). In the present work the results on the synthesis of levyne-type zeolite from aluminosilicate systems containing the MeQI as organic compound and a variable amount of sodium and potassium are discussed. 2. EXPERIMENTAL The batch composition studied was xNa20-yK20-6MeQI-A1203-30SiO25 0 0 H 2 0 where x+y=6. The reaction gels were prepared by mixing 30% NaOH aqueous solution (pellets RPE, Carlo Erba), MeQI, AI(OH)3 (dry gel, Pfaltz and
552 Bauer), distilled water and Si02 (fumed silica, Serva). MeQI was prepared mixing quinuclidine (1-azabyciclo-2-2-2.octane, Aldrich) and iodomethane according to the procedure reported elsewhere (3,4). The reaction mixtures were heated at 150~ and 170~ under autogeneous pressure and in static conditions for programmed times, using modified autoclaves type Morey. The characterisation was carried out using X-ray powder diffraction, chemical analysis, SEM, TPD and thermal analyses. Thermal analysis characterisation (DSC, TG and DTG) was carried out from room t e m p e r a t u r e to 900~ using a Netzsch STA409 simultaneous thermal analyser. Adequate sensitivity was obtained with a sample weight of about 10mg. Measures were performed in air flow (15ml/min) on a heating rate of 10~ p e r minute. The volatiles evolved during the thermogravimetric analysis were identified by TPD analysis carried out with a Carlo Erba QTMD (quadrupole thermalprogrammed mass detector). The identification was made from 1 to 150 a.m.u. (atomic mass units). The ion species signals, six selected ion species for each analysis, were followed as a function of temperature. 3. RESULTS A_N]:)DISCUSSION The system reported containing only sodium (K/(Na+K)=0) produces the cocrystallisation of levyne and LZ-!33 zeolites at 150~ and at 170~ as shown previously (6) and reported in Figures 1 and 2, respectively. The induction time of reaction is equal to one day at 150~ and to few hours at 170~ After seven days of reaction at 150~ the levyne yields is equal to about 80%, while after 6 days at 170~ the 3,Jeld is only 68% since LZ-133 zeolite crystallisation occurs.
100
80
o-,,t
>.I-z
60 m
_..! ..._!
< I.0o n" o
40
20
6Na
A
LZ-133
O
4K2.Na
Z~
3K3Na
r]
2K4Na
9 eK 0
50
1 O0
150
200
250
300
TIME, HOURS
Figure 1. Cristallization kinetics of levyne-type zeolite from system xNa20yK20-6MeQI-A1203-30SiO2-500H20 where x+y=6 at 150~
553 100
>..-
0
80
I--.-
_z
6o
.d
<
co >.. rr"
Al
40
6Na LZ-133 2K4Na 3K3Na
2O
4K2Na 6K I--" T . C ' a
0
g
~
,
1
5O
1
9 .
1
15O
100
a
2~0
20O
TIME, HOURS
Figure 2. Cristallization kinetics of levyne-type zeolite from system x N a 2 0 yK20-6MeQI-A]203-30SiO2-500H20 where x+y=6 at 170~
g58
oO
o
~)0 158
100
n n
50
m
al
15ooc
A, 0T
0
,
I
93
,
,
,
40
I
60
,
I
80
170~ a
!
100
K/(Na+K) % Figure 3. Induction time as a function of K/(Na+K) % ratio in the reaction batch at the t e m p e r a t u r e s of ( I ) 150~ and ( k ) 170~ The addition of potassium to the reaction batch prevents the formation of LZ133 zeolite as parasite phase at both temperatures. The increase of potassium
554
amount in the batch, at both temperatures of reaction, produces an increase of the induction time as shown in the Figures 1 and 2. The course of the nucleation time variation as function of the amount of sodium and potassium in the batch composition is reported in Figure 3. The increase of the ratio K/(Na+K) in the reaction mixture, produces a linear variation of the nucleation time. The system containing just potassium (K/(Na+K)=I) does not follow this course because the absence of sodium modifies completely the rate of the induction period, The results shown in F i b r e s 1-3 suggest that the levyne-type /zeolite can be obtained from both systems containing just sodium (K/(Na+K)=0) or potassium (K/(Na+K)=I). The kinetic parameters are different because the sodium is more important in forming the single six rings secondary building units and the potassium favours their assembling but does not increase the rate of crystal growth. In fact, the slope of the kinetic curves is practically constant with a value of 5.4+__0.2 %/hour and a larger size of the levyne crystals is observed increasing the potassium content in the batch composition. In the system containing only sodium at 170~ the LZ-133 zeolite is not just a parasite phase but it is a zeolite that co-crystallises with levyne (6-8). The
E X 0
E N D 0
00 ....
9I
. . . .
100
|
. . . .
200
i.
300
,,,
It.l.,
400
,,
i . . . .
500
I
,
600
,
,
,
I
,
700
, , ,
I
800
TEMPERATURE, ~ Figure 4. DSC thermograms of levyne-type zeolite obtained from batches with different K/(Na+K) ratios.
555 presence of potassium at 170~ does not favour the growth of the LZ-133 zeolite because the assembling of the single six rings secondary building units is favoured by the presence of potassium. This result confirms the hypothesis that the LZ-133 zeolite is not a zeolite with the same structure as levyne, as previously proposed (6-8). The behaviour of the sodium and the potassium ions, the first-ones are able to produce the nuclei and the second-ones are able to increase the crystal growth, has been also observed in the synthesis of other high silica zeolites (10). The variation of the course of the DSC thermograms reported in Figure 4 shows that the interaction of the methylquinuclidine ion with the framework of levyne-type zeolite is influenced by the synthesis conditions. In fact, the thermal decomposition of the MeQ is observed at different temperatures for levyne obtained from sodium (K/(Na+K)=0) or potassium (K/(Na+K)=I) system (Table 1). In agreement with the results obtained with other organo-zeolites (11), the peak a is attributed to organic compound partially occluded in the zeolitic channels. The peak b is attributed to the MeQ + balanced by an OHanion and the peak c is due to the MeQ + linked with the zeolitic negative charge. The levyne obtained from the systems containing both sodium and potasium shows the same behaviour probably because the ratio Na+/K + in the framework is not a function of the amount of potassium in the initial batch. In this case we suppose that during the crystallisation, an ion exchange balances the ratio Na+/K +. This hypothesis is suggested by the analysis reported elsewhere (5) where independently of the amount of potassium added to the initial mixture of reaction, a higher amount of potassium is detected in the crystals of levyne-type zeolite. More analytical tests are in progress to verify this hypothesis. In any cases the mass spectrometer analysis of the volatile phases due to decomposition of MeQ, has not shown the presence of iodine. This suggests that the MeQ + neutralises some of the negative charges of the levyne framework. Table. 1 Data computed from the DSC and TG characterisation of levyne-type zeolite obtained from systems at different K/(Na+K) content. TEMPERATURE, ~ I! MeQ+u.c.* weight losses, % II batch peak b peak c total peakb peak c total peakb p e a k c 6Na 458.2 589.5 23.0 11.3 8.7 6.7 3.8 2.9 4Na2K 463.1 590.0 21.8 8.1 11.5 6.4 2.7 3.8 3Na3K 456.2 585.3 22.0 8.3 11.4 6.5 2.7 3.8 2Na4K 460.0 586.0 22.6 8.9 10.3 6.4 2.9 3.4 6K 439.8 584.3 22.6 8.7 11.3 6.6 2.9 3.8 ,,,
,,
* Computed on the basis of a framework of 54 T atoms. CONCLUSIONS The above reported results show that: - The levyne-type zeolite can be obtained from systems containing potassium but with a longer induction time; - The presence of the potassium in the batch composition gives the possibility to synthesize the levyne-type zeolite free of parasite phases at a temperature higher than 150~
556 - The systems containing sodium and potassium produce a levyne-type zeolite with a yield of 80%; - The MeQ + is counter-ion of the zeolite framework ACKNOWI,EDGEMENT Work supported by Italian Research Council, CNR, "Progetto Strategico Tecnologie Chimiche Innovative" RE~'~CI~ 1. G.T. Kerr, US Patent 3 459 676, (1969). 2. R. Millini, A. Carati and G. Bellussi, Zeolites, 12, 265, 1992, and references therein. 3. T. R. Cannan, M. T. L. Brent and E. M. Flanigen, Eur. Patent Appl. 0091048 A1, 1983. 4. M. T. L. Brent, T. R. Cannan and E. M. Flanigen, Eur. Patent Appl. 0091049 A1, 1983. 5. E. J. Rosinsky and M.K. Rubin, Eur Patent Appl. 0107370, (1983) 6. C.V. Tuoto, F. Testa, R. Aiello and A. Nastro, Materials Engineering, 2, (1994), 175, and references therein. 7. C.V. Tuoto, F. Testa, R. Aiello and A. Nastro, Atti 2 ~ Conf. AIMAT, (Ass. It. Ing. dei Materiali) P. Giordano Orsini Ed., vol.1, Universit~ di Trento, (1994), 173. 8. C.V. Tuoto, F. Testa and A. Nastro, Proc. 10th Int. Zeolite Conf., Garmisch Partenkishen, July 1994, Recent research report, in press. 9. C.V. Tuoto, A. Regina, J. B.Nagy and A. Nastro, Proc. III It. Nat. Conf. "Scienza e Tecnolog~a delle Zeoliti", Cetraro, Sept. 1995, in press. 10. R. Aiello, F. Crea, A. Nastro and C. Pellegrino, Zeolites, 7, 549, 1987. 11. F. Crea, J. B.Nagy, A. Nastro, G. Giordano and R. Aiello, Thermochimica Acta, 135, (1988), 353.
557
AUTHOR INDEX
An, L.-D. Auroux, A. Azuma, N. B alkus Jr., K.J. Bandyopadhyay, R. Bates, S.P. Baur, W.H. Bell, R.G. Benazzi, E. Ben Ta~rit, Y. Boddenberg, B. Bonneviot, L. Bordiga, S. Brunel, D. Brunner, E. Btilow, M. Buzzoni, R. Cairon, O. Cambon, H. Cardoso, D. Carrazza, J. Cartier dit Moulin, C. Cauvel, A. Catlow, C.R.A. (~ejka, J. Chambellan, A. Channon, Y.M. Chevreau, T. Chien, S.-H. Ciambelli, P. Clacens, J.-M. Clark, L. Corbo, P. Corma, A. Curtiss, L.A. Davis, M.E. De Luca, P. Descorme, C.
F-l-3 Th-2-4 F-l-1 W-l-I, P-34 F-l-2 M-2-6 Th- 1-4 PL-2 F-2-3 P-20 W-l-3 M- 1-3, P-42 KL-3 Tu-2-5, Th-2-5 M-l-1 Th-l-5 KL-3 P-31 Th-2-5 P-6 P-26 M-l-3 Tu-2-5 Tu- 1-3, PL-2 F-2-4 P-28 Tu-l-3 P-28, P-31 P-41 Th-2-2 Tu-2-1 F-l-4 Th-2-2 M-l-4 Tu-l-1 KL-1 P-15 Th-2-1
Diaz, A.C. DiRenzo, F. Djajanti, S. Downing, R. Dwyer, J. Eder, F. Eder-Mirth, G. Eic, M. Eissa, M. Ellestad, O.H. Fajula, F. Feng, Y. Fonseca, A. Fricke, R. Fyfe, C.A. Gabelica, Z. Gabrielov, A.G. Gale, J.D. Gambino, M. Garforth, A. G61in, P. Geneste, P. Geobaldo, F. Ghorbel, A. Giordano, G. Goldfarb, D. Gontier, S. Goursot, A. Grondey, H. Guisnet, M. Gunnewegh, E.A. Guo, C.J. Hartmann, M. Ho, J.-C. Howe, R.F. Huang, M. Huang, Y.
PL-1 Tu-2-5 W-l-2 P-16 M-2-6, F-1-4, P-7 P-27 M-2-5 Th-l-3 W-l-1 Tu-2-2 Tu- 1-2, Tu-2-5, KL-2 PL-1 M-2-3 P-21 PL-1 Tu-2-1, P-42 P-34 PL-2 Th-2-2 F- 1-4 Th-2-1, P-19 Th-2-5 KL-3 P-20 M-2-3 Tu-2-6 Tu-2-3 Tu-l-2 PL-1 F-2-3 P-16 Tu-2-4 F-l-1 P-41 W-l-2 Th-2-4 PL-1
558 Inui, T. Iton, L.E. Ivanova, I.
KL-4 Tu-l-1 M-l-4
Jackson, R.A. Jaeger, N.I. Jansen, J.C. Jorda, E. Joshi, P.N.
Tu-l-3 W-l-3 F-2-1 P-20 P-43
P-11 Kai, T. Th-2-4, P-42, P-43 Kaliaguine, S. P-42 Kapoor, M.P. M-2-3 Katovic, A. M-2-4, P-2 Kazansky, V.B. F-l-1 Kevan, L. P-28 Khabtou, S. W-l-1 Khanmamedova, A. P-25 Kinrade, S.D. P-25 Knight, C.T.G. PL-1 Kokotailo, G.T. P-14 Komatsu, T. Tu-l-4 Koningsberger, D.C. P-36 Konstantinov, L. Th-l-4 Komatowski, J. KL-5 Kumar, R. M-2-4, P-2 Kustov, L.M. P-15 Kuznicki, S. Lasp6ras, M. Lavalley, J.C. L6cuyer, C. Lemay, G. Le Noc, L. Lercher, J.A. Lessard, S. Lewis, A.R. Lewis, D.W. Liao, B. Loeffler, E. Lortie, C. Lujano, J. Lutz, W. Makarova, M.A.
Th-2-5 P-28 Th-2-1 P-43 M-l-3 M-2-5, P-27 M-l-3 PL-1 PL-2 Th-l-3 M-2-4, Th-2-6 M-l-3 P-26 Th-l-4, Th-2-6 M-2-6
Manoli, J.M. Mariey, L. Marzin, M. Micke, A. Migliardini, F. Miller, J.T. Minelli, G. Mintova, S. Monteiro, J.L.F. Moretti, G. Moudrakovski, I.L. Mueller, K.T. MUller, G. Murta Vane, M.L. Naccache, C. Nagy, J.B. Nakagawa, Y. Nastro, A. Nasution, M.N.A. Ng, F.T.T. Nkosi, B. Noronha, Z.M.
P-31 P-28 P-28 Th-l-5 Th-2-2 Tu-l-4 Th-2-2 P-36 P-19 Th-2-2 Th-l-2 PL-1 M-2-5 P-6 P-20 M-2-3, P-45 M-2-1, M-2-2 P-15, P-45 P-11 F-2-2 F-2-2 P-19
Occelli, M.L. Owens, S.L.
Th-l-3 Tu-l-3
Papai, I. Porta, P. Potvin, C. Predescu, L. Primet, M.
Tu-l-2 Th-2-2 P-31 P-29 Th-2-1
Rao, B.S. Ratcliffe, C.I. Ratnasamy, P. Rees, L.V. Rempel, G.L. Ribeiro, F.R. Ribeiro, M.F. Ricchiardi, G. Richter-Mendau, J. Ripmeester, J. Rodenburg, E.C. Rodriguez, I.
F-l-2 Th-l-2 KL-5 Th-l-1 F-2-2 F-2-3 F-2-3 KL-3 P-21 Th-l-2 F-2-1 Th-2-5
559 Roland, U. Romero, Y. Rozwadowski, M. Ruthven, D.M. Saint-Just, J. Salzer, R. Scarano, D. Schmidt, R. Schoeman, B. Schreier, E. Schwenn, H.-J. Sellem, S. Shaikh, R.A. Shen, D. Silva, J.M. Singh, P.S. Smekalina, E.V. Sobrinho, E.V. Sobry, R. Solomykina, S. Sooknoi, T. Sousa-Aguiar, E.F. Spasov, L. Spoto, G. Steinike, U. Stelmack, P. Stockenhuber, M. StOcker, M. Strobl, H. Su, B.L. Stimmchen, L. Syvitski, R.T. Takahashi, T. Tezel, F.H.
P-18 P-26 Th-l-4 Th- 1-3, PL-4
Trong On, D. Tuel, A. Tuoto, C.V. Tvarfi~&ov~i, Z.
M-l-3, P-42, P-43 Tu-2-3, P-20 P-45 F-2-4
Th-2-1 P-18 KL-3 Tu-2-2 P-36 P-21 W-l-3 P-31 F-l-2 Th-l-1 F-2-3 F-l-2 P-2 P-6 Tu-2-1 M-l-3 P-7 P-6 P-36 KL-3 P-21 P-29 P-27 Tu-2-2 PL-1 Th-2-3 P-18 P-25
Uddin, Md. A. Uvarova, E.B.
P-14 P-2
P-11 P-29
Valtchev, V. van Bekkum, H. van den Bossche, G. van der Puil, N. Vasilyev, V. Wang, H.-L. Wang, K.-J. Wark, M. Wamken, M. Wichterlov~i, B. Wong-Moon, K.C.
P-36 F-2-1. P-16 Tu-2-1 F-2-1 Tu-I-2 F-l-3 P-17 W-l-3 W-l-3 F-2-4 PL-1
Xiao, T.-C.
F-l-3
Yang, S.-M. Yashima, T.
P-17 P-14
Zecchina, A. Zhao, D. Zholobenko, V.L. Zibrowius, B. Zilkov~i, N. Zones, S.I. Zubowa, H.-L. Zygmunt, S.A. v
KL-3 Tu-2-6 M-2-4, F- 1-4 Th-2-6 F-2-4 M-2-1, P-34 P-21 Tu-I-1
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561
S U B J E C T
Th-2-4, P-36
A - zeolite Alcohols - sorption of
M-2-5
Alkane oxidation
W - l - l , KL-5
ALPO4-5
Th-l-2, F-l-4, P-17
ALPO4-8
Th-l-2
ALPO4-11
Th-l-2, F-l-2, F-1-4, P-17
Anisole acylation of
P-16
I N D E X
Brr
acid sites
M-l-l, M-2-4, M-2-5, M-2-6, Tu-l-1, F- 1-4, P-31, PL-2, KL-2, KL-3, KL-4
Butene-1 - isomerisation
F-l-4
Butenes - oligomerisation
F-2-2
Butylbenzene acylation
P-16
CH4 - oxidation by NO x
P-2
As -
KL-5
zeolites
Basic zeolites
Th-2-4, Th-2-5, P-7
Beckmann rearrangement
C2C14 - adsorption of
M-l-1
Chloroaniline oxidation of
Tu-2-3
P-11 Clays
Benzene T-h- 1-4, Th-2-3 - adsorption of P-28 - hydroconversion of BETA
M-2-2, P-16, P-17
BF3 - adsorption of B
M-2-6
-
pillared and expanded
Tu- 1-3, P-29
Clinoptilolite Cloverite
Th-l-3
M-2-5, W- 1-2, P-21
Clusters of cesium oxide - models
Th-2-5 Tu-l-1
oron silicalite - MCM-.' 1
M-2-1 P-42 P-43
Borosilicate sieves
M-2-1
-
CO oxidation by NO x - adsorption of
COAPO-5
P-2 M-l-l, P-18, P-29 F- 1-4
562 COAPO- 11
F- 1-2
Co - ZSM-5
Th-2-2, KL-4
of binary mixtures - of n-paraffins in clays
3,7-Diazabicyclononane templating studies
CP*2CoOH as template Cracking
P-6
Crystallization kinetics and mechanism -
M
-
2
-
3
Cyclohexane adsorption of
Tu-2-4
Cyclohexene oxidation of
Tu-2-3
-
Dimethylacetal elimination of methanol
P-17
DSC
P-45
-
,
KL-1
DTA/DTG
EMT
P-21, P-36, KL-4
M-2-6, Th-2-3, P-31
Encapsulated complexes
-
E
Cyclohexanone oxime
P
R
,
E
S
R
P-11
Cu
NaA - ZSM-5
M-2-2
-
P-34
-
-
Th-1-1 Th-l-3
-
KL-4 Th-2-2, KL-4
W-l-1
Tu-2-6, F-I-I, F-l-2 P-20, P-41
Erionite acid sites in
M-2-4
-
ESEM
F-l-1
DAF-1
PL-2
ETS-10
PL-2
Dehydrogenation of n-heptane
F-2-1
EU-1
PL-2
Density Functional Theory (DFT) Tu-l-1, Tu- 1-2
EXAFS A1 K-edge Mo K-edge - W Liii-edge Ti K-edge
Tu-l-4 W-l-2 W-l-2 M-1-3, P-42
-
-
Design of synthesis - strategies for
KL-1
Diffusion coefficient of n-butane, n-hexane and butenes PL-2 - coefficient of water in MCM-41 Tu-2-2 coefficient of water in MCM-48 Tu-2-2 measurements of- coefficients PL-4 -
-
FAU
M-2-1, KL-2
Fe -
ZSM-5
-
-
zeolites
F-2-4, P- 11, P- 14, PL-2, KL-4, KL-5 KL-5
-
Frequency response (FR)
Th- 1-1, PL-4
563
Friedel - Crafts
P- 16
Isomerisation - of C 8 aromatic cuts
F-2-3
Ga - ZSM-5 Gravimetry
Hartree - Fock theory H-D exchange
P-11, KL-4
Knoevenagel condensation
Th-2-5
L - zeolite
Th-2-3
P-27
M-2-6, Tu-1-1 P-18, P-20
P-45, PL-2
Levyne type zeolites Lewis acids
M-l-l,
M-2-5,
M-2-6,
Th-2-4, KL-2 Heat of adsorption Host-guest interaction HREM
Th-2-4, P-27 PL-2, KL-3
Light alkanes - sorption of
P-27
Tu-2-2
Hydrogenation - competitive - of 1-heptene and 3-3 dimethyl-1butene F-2-1
MAPO- 11
F- 1-2
Mazzite
KL-2
MCM-41
Tu-2-1, Tu-2-2, Tu-2-5, W- 1-2, P-26
MCM-48
Tu-2-2, Tu-2-6, P-26
In -
ZSM-5
P-11
IR
-
-
-
-
-
diffuse reflectance M-2-4, P-2 framework P-42 of alkanes P-27 of ammonia M-2-5, KL-3 of benzene Th-2-3 of CD3CN KL-2 of CO P-19 of cyclohexene KL-2 of methanol F-1-3 of methylcyclopentene KL-2 of NO Th-2-1, P-14 of OH (and OD) M-2-6, P- 18, P-28, P-31, KL-3 of pyridine M-2-5, F-l-4, P-27, P-31, P-41, P-43, KL-2, KL-3 of pyrrole Th-2-4
MCM-L
Tu-2-1, P-26
MeAPO
F- 1-2, F- 1-4
Mesoporous materials Tu-2-1, Tu-2-2, Tu-2-3, Tu-2-4, Tu-2-5, Tu-2-6, P-26, P-43 Methane adsorption of P-29 Methanol to olefins (MTO)
F-l-3
MgAPO- 11
F- 1-4
Microcalorimetry
Th-2-4
Milling
Th-2-4
564
Mn - MCM-41 - MCM-48 - MCM-L
Tu-2-6 Tu-2-6 Tu-2-6
MOCVD
W-l-2
Mo(CO) 6
W-l-2
Modelling - of adsorption properties '
Mr
N
Tu-l-2, Tu-l-3
Plesset perturbation (MP2) M-2-6, Tu- 1-1
Mordenite acid sites in M-2-4 dealumination of F-2-3, P-19 - H-form P-19, P-27, KL-3 - Na Th-l-2, P-17
2
- adsorption of
Tu-l-2, Tu-2-4, Tu-2-5, Th- 1-4, Th-2-6, P-29
Ni - SAPO-5 and SAPO-11
F-l-1
NO x - adsorption complexes reduction with ammonia reduction with methane - reduction with propane - decomposition
P-2 P-14 Th-2-1 Th-2-2 KL-4
-
Nano-particles of ZnO, CdO and SnO 2 W-l-3
-
Mullite fibers
Nu-3
PL-2
Nucleation rate
M-2-1
P-36 02
- adsorption of NMR - 27A1 - liB -
13C
-
2 D
- 1H - 71Ga 31p
_
M-1-3, Tu-2-1, Th-2-1, P-6, P-19, P-41, PL-1, KL-2 P-42, P-43 M - l - l , M-l-4, Tu-2-5, F-l-3 PL-1 M - l - l , M-l-3, Tu-2-2 Tu-2-1 PL-1
- pulsed field gradient NMR PL-4 - 29Si M-1-3, M-2-3, Tu-2-1, Tu-2-2, Th-2-1, P-6, P-19, P-25, P-31, P-41, PL-1 ll7Sn, 119Sn W-l-3, P-25 - two dimensional Th-l-2, P-25 PL-1 - 129Xe Th-l-2, F-1-3
Tu-l-2 KL-5
Oxidation reactions with H202
Pd - exchanged zeolites Perfluorotributylamine
Th-2-1, F-1-3 Th-l-3
2-Phenylethanol hydrogenolysis -
P-7
Quasi-elastic neutron scattering (QENS) PL-4
-
Ruthenium perfluorophtalocyanines W-1-1
565 SAPO-5
F-l-l, F-l-3, P-17
Ti boralite cloverite MCM-41 MCM-48 - NCL-1 - 13 - UTD-1, UTD-8 ZSM-48
P-42 P-21 Tu-2-3 Tu-2-3 KL-5 Tu-2-3, KL-5 P-34 KL-5
-
SAPO-11
F-l-l, F-l-2, F-l-4, P-17
-
-
-
SAPO-34
P-17
SAXS
Tu-2-1
-
Tu-2-4, F- 1-2, F-2-1, P-21, P-34, P-36, P-41
SEM
Toluene acylation P-16 - alkylation M-l-4, F-2-4, P-7 disproportionation F-2-4 -
Silanes- alkoxy grafting
Tu-2-5
-
Silicalite- 1
M-l-3, Th-l-1, Th-l-4, F-2-1, P-36, P-42, PL-4
Silicate speciation
P-25
-
TPD F- 1-3, F- 1-4, P- 11 Tu-2-1 - of carbon dioxide Th-2-5 of methanol M-2-5
-
of ammonia
-
Sn -
zeolites
KL-5
Sorption kinetics
Th- 1-5
Spillover
F-2-3
Triisopropylbenzene - cracking of
P-6
P- 18
SSZ-24
M-2-2, Th- 1-2
SSZ-31
M-2-2
SSZ-35
M-2-2
SSZ-37
M-2-2
Styrene hydrogenation of -
TEM
Transalkylation
TS- 1
M- 1-3, Tu-2-3, P-20, P-42, KL-5
TS-2
KL-5
UV irradiation of TS-1
P-20
UV-visible - diffuse reflectance P-7
Tu-2-5, W- 1-3, Th-2-2, F- 1-2, P-34, P-42
Tu-2-4, Tu-2-6, Th-2-6, F-2-1 BETA MCM-41 - NCL-1 - VAPO- 11 -
Tetralin - adsorption of
-
Tu-2-4
P-41, KL-5 Tu-2-3 KL-5 F-l-2
566 - ZSM-12
KL-5
VPI-5
Th- 1-2, PL- 1
VS- 1
Tu-2-3, KL-5
VS-2
- CsNa - dealuminated -
H
-
L a
-
N a
KL-5 -
Water - adsorption of
Tu-l-1, Tu- 1-3, Tu-2-4, Th-l-4, P-18
W(CO)6
W-l-2
-
Th-2-4, Th-2-5 Th-2-6, P-6, P-28, P-31, Tu-l-4 Tu-l-4 Tu-l-4, W-l-2, Th-l-2, Th-2-3, P-17, P-18, PL-4
NiNa - alkaline-earth PtNa
F-2-2 P-18
Zero length column (ZLC)
Th-l-3, Th- 1-5, PL-4
M-2-2, Tu-l-1, F-2-1, F-2-4, P-17, P-27, PL- 1, PL-2, KL- 1, KL-2, KL-3 M-2-4 acid sites in Th-l-4, F-2-3 - dealuminated P-11, KL-1 high silica
ZSM-5
X acid sites in - alkali exchanged -
- CsNa
M-2-4 W- 1-1, Th-l-2, Th-2-3, Th-2-4, PL-4 Th-2-4, Th-2-5, P-7
-
-
XPS Xylenes disproportionation
Th-2-2, Th-2-4
Y - acid sites in - alkali exchanged Ca -
ZSM-11
M-l-4, PL-1, PL-2
ZSM-12
Th-l-2, PL-1
ZSM-18
PL-2
ZSM-48
M-2-3
F-2-3
M-2-4 Th-2-4 Tu-l-4
567
STUDIES IN SURFACE SCIENCE AND CATALYSIS Advisory Editors: B. Delmon, Universite Catholique de Louvain, Louvain-la-Neuve, Belgium J.T. Yates, University of Pittsburgh, Pittsburgh, PA, U.S.A.
Volume
1
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Volume 13 Volume 14
Preparation of Catalysts I.Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 14-17,1975 edited by B. Delmon, P.A.Jacobs and G. Poncelet The Control of the Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, with Special Emphasis on the Control of the Chemical Processes in Relation to Practical Applications by V.V. Boldyrev, M. Bulens and B. Delmon Preparation of Catalysts I1. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Second International Symposium, Louvain-la-Neuve, September 4-7, 1978 edited by B. Delmon, P. Grange, P.Jacobs and G. Poncelet Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings ofthe 32nd International Meeting ofthe Societe de Chirnie Physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon Catalysis by Zeolites. Proceedings of an International Symposium, Ecully (Lyon), September 9-11, 1980 edited by B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud Catalyst Deactivation. Proceedings of an International Symposium, Antwerp, October 13-15,1980 edited by B. Delmon and G.F. Froment New Horizons in Catalysis. Proceedings of the 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.l. Yermakov, B.N. Kuznetsov and V.A. Zakharov Physics of Solid Surfaces. Proceedings of a Symposium, Bechyhe, September 29-October 3,1980 edited by M. Lazni~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 andMetaI-Additive Effects in Catalysis. Proceedings of an International Symposium, Ecully (Lyon), September 14-16, 1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau, P. Gallezot, G.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. Benard Vibrations at Surfaces. Proceedings of the Third International Conference, Asilomar, CA, September 1-4, 1982 edited by C.R. Brundle and H. Morawitz
568 Volume 15 Volume 16
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Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.I. Golodets Preparation of Catalysts III. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Third International Symposium, Louvain-la-Neuve, September 6-9, 1982 edited by G. Poncelet, P.Grange and P.A. Jacobs Spillover of Adsorbed Species. Proceedings of an International Symposium, Lyon-Villeurbanne, September 12-16, 1983 edited by G.M. Pajonk, S.J. Teichner and J.E. Germain Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July 9-13, 1984 edited by RA. Jacobs, N.I. Jaeger, R Jin3, V.B. Kazansky and G. Schulz-Ekloff Catalysis on the Energy Scene. Proceedings of the 9th Canadian Symposium on Catalysis, Quebec, P.Q., September 30-October 3, 1984 edited by S. Kaliaguine and A. Mahay Catalysis by Acids and Bases. Proceedings of an International Symposium, Villeurbanne (Lyon), September 25-27, 1984 edited by B. Imelik, C. Naccache, G. Coudurier, Y. Ben Taarit and J.C. Vedrine Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Uxbridge, June 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 and Application. Proceedings of an International Symposium, Portoro~-Portorose, September 3-8, 1984 edited by B. Dr~aj, S. Ho~evar and S. Pejovnik Catalytic Polymerization of Olefins. Proceedings of the International Symposium on Future Aspects of Olefin Polymerization, Tokyo, July 4-6, 1985 edited by T. Keii and K. Soga Vibrations at Surfaces 1985. Proceedings of the Fourth International Conference, Bowness-on-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 and Technology. Proceedings of the 7th International Zeolite Conference, Tokyo, August 17-22, 1986 edited by Y. Murakami, A. lijima and J.W. Ward Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H. Kn6zinger Catalysis and Automotive Pollution Control. Proceedings of the First International Symposium, Brussels, September 8-11, 1986 edited by A. Crucq and A. Frennet Preparation of Catalysts IV. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Fourth International Symposium, Louvain-la-Neuve, September 1-4, 1986 edited by B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet Thin Metal Films and Gas Chemisorption edited by P. Wissmann Synthesis of High-silica Aluminosilicate Zeolites edited by RA. Jacobs and J.A. Martens Catalyst Deactivation 1987. Proceedings of the 4th International Symposium, Antwerp, September 29-October 1, 1987 edited by B. Delmon and G.F. Froment
569 Volume 35 Volume 36
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Keynotes in Energy-Related Catalysis edited by S. Kaliaguine Methane Conversion. Proceedings of a Symposium on the Production of Fuels and Chemicals from Natural Gas, Auckland, April 27-30, 1987 edited by D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-17, 1987 edited by P.J. Grobet, W.J. Mortier, E.F.Vansant and G. Schulz-Ekloff Catalysis 1987. Proceedings 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 IUPAC Symposium (COPS I), Bad Soden a. Ts., April 26-29,1987 edited by K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral Physics of Solid Surfaces 1987. Proceedings of the Fourth Symposium on Surface Physics, Bechyne Castle, September 7-11, 1987 edited by J. Koukal Heterogeneous Catalysis and Fine Chemicals. Proceedings of an International Symposium, Poitiers, March 15-17, 1988 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, C. Montassier and G. Perot Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by Z. Paal Catalytic Processes under Unsteady-State Conditions by Yu. Sh. Matros Successful Design of Catalysts. Future Requirements and Development. Proceedings ofthe Worldwide 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 byH.H. Kung Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an International Symposium, W~rzburg, September 4-8,1988 edited by H.G. Karge and J. Weitkamp Photochemistry on Solid Surfaces edited by M. Anpo and T. Matsuura Structure and Reactivity of Surfaces. Proceedings of a European Conference, Trieste, September 13-16, 1988 edited by C. Morterra, 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 P.A. Jacobs and R.A. van Santen Hydrotreating Catalysts. Preparation, Characterization and Performance. Proceedings ofthe Annual International AIChE Meeting, Washington, DC, November 27-December 2, 1988 edited by M;L. Occelli and R.G. Anthony New Solid Acids and Bases. Their Catalytic Properties by K. Tanabe, M. Misono, Y. Ono and H. Hattori Recent Advances in Zeolite Science. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-19, 1989 edited by J. Klinowsky and P.J. Barrie Catalyst in Petroleum Refining 1989. Proceedings of the First International Conference on Catalysts in Petroleum Refining, Kuwait, March 5-8, 1989 edited by D.L. Trimm, S. Akashah, M. Absi-Halabi and A. Bishara
570
Future Opportunities in Catalytic and Separation Technology edited by M. Misono, Y. Moro-oka and S. Kimura New Developments in Selective Oxidation. Proceedings of an International Volume 55 Symposium, Rimini, Italy, September 18-22, 1989 edited by G. Centi and F.Trifiro Olefin Polymerization Catalysts. Proceedings of the International Symposium Volume 56 on Recent Developments in Olefin Polymerization Catalysts, Tokyo, October 23-25, 1989 edited by T. Keii and K. Soga Volume 57A Spectroscopic Analysis of Heterogeneous Catalysts. Part A: Methods of Surface Analysis edited by J.L.G. Fierro Volume 57B Spectroscopic Analysis of Heterogeneous Catalysts. Part B: Chemisorption of Probe Molecules edited by JoL.G. Fierro Introduction to Zeolite Science and Practice Volume 58 edited by H. van Bekkum, E.M. Flanigen and J.C. Jansen Heterogeneous Catalysis and Fine Chemicals II. Proceedings of the 2nd Volume 59 International Symposium, Poitiers, October 2-6, 1990 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. Perot, R. Maurel and C. Montassier Chemistry of Microporous Crystals. Proceedings of the International Symposium Volume 60 on Chemistry of Microporous Crystals, Tokyo, June 26-29, 1990 edited by T. Inui, S. Namba and T. Tatsumi Natural Gas Conversion. Proceedings of the Symposium on Natural Gas Volume 61 Conversion, Oslo, August 12-17, 1990 edited by A. Holmen, K.-J. Jens and S. Kolboe Characterization of Porous Solids II. Proceedings of the IUPAC Symposium Volume 62 (COPS II), Alicante, May 6-9, 1990 edited by F. Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing and K.K. Unger Preparation of Catalysts V. Scientific Bases for the Preparation of Heterogeneous Volume 63 Catalysts. Proceedings of the Fifth International Symposium, Louvain-la-Neuve, September 3-6, 1990 edited by G. Poncelet, P.A. Jacobs, R Grange and B. Delmon New Trends in CO Activation Volume 64 edited by L. Guczi Catalysis and Adsorption by Zeolites. Proceedings of ZEOCAT 90, Leipzig, Volume 65 August 20-23, 1990 edited by G. (~hlmann, H. Pfeifer and R. Fricke Dioxygen Activation and Homogeneous Catalytic Oxidation. Proceedings of the Volume 66 Fourth International Symposium on Dioxygen Activation and Homogeneous Catalytic Oxidation, Balatonf~red, September 10-14, 1990 edited by L.I. Simandi Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis. Volume 67 Proceedings of the ACS Symposium on Structure-Activity Relationships in Heterogeneous Catalysis, Boston, MA, April 22-27, 1990 edited by R.K. Grasselli and A.W. Sleight Catalyst Deactivation 1991. Proceedings of the Fifth International Symposium, Volume 68 Evanston, IL, June 24-26, 1991 edited by C.H. Bartholomew and J.B. Butt Zeolite Chemistry and Catalysis. Proceedings of an International Symposium, Volume 69 Prague, Czechoslovakia, September 8-13, 1991 edited by RA. Jacobs, N.I. Jaeger, L. Kubelkova and B. Wichterlova Volume 54
571 Volume 70
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Poisoning and Promotion in Catalysis based on Surface Science Concepts and Experiments by M. Kiskinova Catalysis and Automotive 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 of the 3rd European Workshop Meeting on New Developments in Selective Oxidation by Heterogeneous Catalysis, Louvain-la-Neuve, Belgium, April 8-10, 1991 edited by P. Ruiz and B. Delmon Progress in Catalysis. Proceedings of the 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 Current Applications edited by S.D. Kevan New Frontiers in Catalysis, Parts A-C. Proceedings of the 10th International Congress on Catalysis, Budapest, Hungary, 19-24 July, 1992 edited by L. Guczi, F. Solymosi and P.Tetenyi Fluid Catalytic Cracking: Science and Technology edited by J.S. Magee and M.M. Mitchell, Jr. New Aspects of Spillover Effect in Catalysis. For Development of Highly Active Catalysts. Proceedings ofthe Third International Conference on Spillover, Kyoto, Japan, August 17-20, 1993 edited by T. Inui, K. Fujimoto, T. Uchijima and M. Masai Heterogeneous Catalysis and Fine Chemicals III. Proceedings ofthe 3rd International Symposium, Poitiers, April 5-8, 1993 edited by M. Guisnet, J. Barbier, J. Barrault, C. Bouchoule, D. Duprez, G. Perot and C. Montassier Catalysis: An Integrated Approach to Homogeneous, Heterogeneous and Industrial Catalysis edited by J.A. Moulijn, P.W.N.M. van Leeuwen and R.A. van Santen Fundamentals of Adsorption. Proceedings of the Fourth International Conference on Fundamentals of Adsorption, Kyoto, Japan, May 17-22, 1992 edited by M. Suzuki Natural Gas Conversion II. Proceedings ofthe Third Natural Gas Conversion Symposium, Sydney, July 4-9, 1993 edited by H.E. Curry-Hyde and R.F. Howe New Developments in Selective Oxidation II. Proceedings of the Second World Congress and Fourth European Workshop Meeting, Benalm~dena, Spain, September 20-24, 1993 edited by V. Cortes Corberan and S. Vic Bellon Zeolites and Microporous Crystals. Proceedings of the International Symposium on Zeolites and Microporous Crystals, Nagoya, Japan, August 22-25, 1993 edited by T. Hattori and T. 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 and W. H61derich Advanced Zeolite Science and Applications edited by J.C. Jansen, M. St6cker, H.G. Karge and J.Weitkamp
572 Volume86 Volume 87
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Oscillating Heterogeneous Catalytic Systems by M.M. Slin'ko and N.I. Jaeger Characterization of Porous Solids III. Proceedings of the IUPAC Symposium (COPS III), Marseille, France, May 9-12, 1993 edited by J.Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger Catalyst Deactivation 1994. Proceedings of the 6th International Symposium, Ostend, Belgium, October 3-5, 1994 edited by B. Delmon and G.F. Froment Catalyst Design for Tailor-made Polyolefins. Proceedings of the International Symposium on Catalyst Design for Tailor-made Polyolefins, Kanazawa, Japan, March 10-12, 1994 edited by K. Soga and M. Terano Acid-Base Catalysis II. Proceedings of the International Symposium on Acid-Base Catalysis II, Sapporo, Japan, December 2-4, 1993 edited by H. Hattori, M. Misono and Y. Ono Preparation of Catalysts VI. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Sixth International Symposium, Louvain-La-Neuve, September 5-8, 1994 edited by G. Poncelet, J. Martens, B. Delmon, P.A. Jacobs and P.Grange Science and Technology in Catalysis 1994. Proceedings of the Second Tokyo Conference on Advanced Catalytic Science and Technology, Tokyo, August 21-26, 1994 edited by Y. Izumi, H. Arai and M. Iwamoto Characterization and Chemical Modification of the Silica Surface by E.F.Vansant, R Van Der Voort and K.C. Vrancken Catalysis by Microporous Materials. Proceedings of 7EOCAT'95, Szornbathely, Hungary, July 9-13, 1995 edited by H.K. Beyer, H.G.Karge, I. Kiricsi and J.B. Nagy Catalysis by Metals and Alloys by V. Ponec and G.C. Bond Catalysis and Automotive Pollution III. Proceedings of the Third International Symposium (CAPoC 3), Brussels, Belgium, April 20-22, 1994 edited by A. Frennet and J.-M. Bastin Zeolites: A Refined Tool for Designing Catalytic Sites. Proceedings of the International Zeolite Symposium, Quebec, Canada, October 15-20, 1995 edited by L. Bonneviot and S. Kaliaguine
