Studies in Surface Science and Catalysis 105 PROGRESS IN ZEOLITE AND MICROPOROUS MATERIALS
Studies in Surface Science and Catalysis 105 PROGRESS IN ZEOLITE AND MICROPOROUS MATERIALS
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Studies in Surface Science and Catalysis A d v i s o r y E d i t o r s : B. D e l m o n a n d J.T. Yates Vol. 105
PROGRESS IN ZEOLITE AND MICROPOROUS MATERIALS
PART A Proceedings ofthe 1lth International Zeolite Conference, Seoul, Korea, August 12-17, 1996 Editors HakzeChon Son-Ki Ihm Korea Advanced Institute of Science and Technology, Taejon, Korea
Young Sun Uh
Korea Institute of Technology, Seoul, Korea
1997 ELSEVIER Amsterdam -- Lausanne -- New York-- Oxford -- Shannon -- Tokyo
ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 P.O. Box 211, 1000 AE Amsterdam, The Netherlands
ISBN 0-444'82344-1 © 1997 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 Part A xxxi
Preface Organizing Committee of the 11th IZC
XXXII1
International Advisory Board to the 1lth IZC
xxxvi
Financial Supports
xxxix
I. Synthesis Directed Synthesis of Organic/Inorganic Composite Structures G.D. Stucky, Q. Huo, A. Firouzi, B.F. Chmelka, S. Schacht, I.G. VoigtMartin and F. Schiith Incorporation and Stability of Trivalent Cations in Mesoporous Silicas Prepared Using Primary Amines as Surfactant S. Gontier and A. Tuel
29
Synthesis ofLamellar Aluminophosphates via the Supramolecular Templating Mechanism A. Sayari
37
Synthesis and Hydrothermal Stability of a Disordered Mesoporous Molecular Sieve R. Ryoo, J.M. Kim, C.H. Shin and J. Y. Lee
45
Preparation of Silica-Pillared Molecular Sieves from Layered Silicates S.-Y. Jeong, O.-E Kwon, J.-K. Sub, H. Jin and J.-M. Lee
53
A New Synthetic Route and Catalytic Characteristics of Pillared Rectorite Molecular Sieves J. Guan, Z Yu, Z Chen, L. Tang and X. Wang
61
Textural Control ofMCM-41 Aluminosilicates F. Di Renzo, N. Coustel, M. Mendiboure, H. Cambon and F. Fajula
69
New Routes for Synthesizing Mesoporous Material Y. Sun, W. Lin, £ Chen, Y. Yue and W. Pang
77
Synthesis and Characterization ofFeSiMCM-41 and LaSiMCM-41 N.-Y. He, S.-L. Bao and Q.-H. Xu
85
Synthesis of Titanium-Containing Mesoporous Molecular Sieves with a Cubic Structure K.A. Koyano and T. Tatsumi
93
Short Range Order of MCM-41 and Mesostructured Aluminiumphosphate C. Pophal, R. Schnell and H. Fuess
101
Syntheses ofMesoporous Aluminosilicates from Layered Silicates Containing Aluminum S. lnagaki, Y. Yamada and Y. Fukushima
109
Structure Descriptors for Organic Templates Employed in Zeolite Synthesis R.E. Boyett, A.P. Stevens, M.G. Ford and P.A. Cox
117
Quantitative Aspects in the Crystallization of Zeolites H. Lechert, T. Lindner and P. Staelin
125
A Computational 'Expert System' Approach to Design Synthesis Routes for Zeolite Catalysts T. Selvam, D.N. lyer, R.C. Deka, A. Chatterjee and R. Vetrivel
133
A New Method for Enhancing Zeolite Crystallization by Using Oxyacids/Salts of Group VA and VIIA Elements as Promoters A. Bhaumik, A.A. Belhekar and R. Kumar
141
The Influence of Mixed Organic Additives on the Zeolites A and X Crystal Growth V.P. Petranovskii, Y. Kiyozumi, N. Kikuchi, H. Hayamisu, Y. Sugi and F. Mizukami
149
Studies of the Crystallization of ZSM-5 under High Gravitational Force Field W.J. Kim, D.T. Hayhurst, S.A. Lee, 3/1.C. Lee, C. IV..Lira and J.C. Yoo
157
vii Structure Directing Role ofNa ÷ and TMA + Cations in 18-Crown-6 Ether Mediated Crystallization of EMT, MAZ and SOD Aluminosilicate Zeolites E.J.P. Feijen, B. Matthijs, P.J. Grobet, J.A. Martens and P.A. Jacobs
165
Synthesis of High-Silica FAU-,EMT-,RHO- and KFI-Type Zeolites in the Presence of 18-Crown-6 Ether T. Chatelain, J. Patarin, E. Brendl~, F. Doughier, J.L. Guth and P. Schulz
173
Synthesis of Zeolites in a Microwave Heating Environment J.P. Zhao, C. Cundy and J. Dwyer
181
Synthesis of Octahedral Molecular Sieves C.-L. O'Young and S.L. Suib
189
Syntheses and Crystal Structures of Two "Organozeolites" K. Maeda, J. Akimoto, Y. Kiyozumi and F. Mizukami
197
ERS-8: A New Class of Microporous Aluminosilicates G. Perego, R. Millini, C. Perego, A. CaratL G. Pazzuconi and G. Bellussi
205
Synthesis and Characterization of Levyne Type Zeolite Obtained from Gels with Different SiO2/Al203Ratios C.V. Tuoto, J.B. Nagy and A. Nastro
213
Synthesis ofETS-10 Molecular Sieve from Systems Containing TAABr Salts P. De Luca and A. Nastro
221
Synthetic Clinoptilolite and Distribution of Aluminum Atoms in the Framework of liEU Type Zeolites M. Kato, S. Satokawa and K. ltabashi
229
Preparation of Ultramarine Analogs from Zeolites S. Kowalak, M. Str6zyk, M. Pawlowska, M. Miluska and J. Kania
237
Exploration of Non Conventional Routes to Synthesize MFI Type Titanoand (Boro-Titano)- Zeolites M. Shibata, J. G~rard and Z Gabelica
245
Synthesis, Characterization and Catalytic Properties of Vanadium Containing VPI-5 K. Chaudhari, T.Kr. Das, A.J. Chandwadkar, J.G. Chandwadkar and S. Sivasanker
253
viii Crystallization of Titanium Silicalite-1 from Gels Containing Hexanediamine and Tetrapropylarrm,onium Bromide A. Tuel
261
Synthesis and Characterisation of Gallium and Germanium Containing Sodalites G.M. Johnson and M. T. Weller
269
Synthesis and Characterization of Chromo, Ferro, Mangano and Vanadio Silicates with MTW Structure M.L.S. Corrda, M. Wallau and U. Schuchardt
277
Improved Synthesis of (Ga)- and (Ga, Al)- Faujasites Z. Gabelica, K Norberg and T. Ito
285
A Study on the Crystallization of Binderless Zeolite X X. Luo, X.. He and J. Shen
293
Synthesis of Large Crystals of Molecular Sieves-A Review S. Qiu, W. Pang and R. Xu
301
Synthesis and Characterization of ZSM-5 in Fluoride Medium: The Role of NH4+ and K ÷ Cations E. Nigro, R. Mostowicz, F. Crea, F. Testa, R. Aiello and J.B. Nagy
309
Three-Dimensional Real-Time Observation of Growth and Dissolution of Silicalite Crystal A. lwasaki, L Kudo and T. Sano
317
Synthesis of Mordenite and ZSM-11 Zeolites from Very Dense Systems" Formation of Self-Bonded Pellets P. De Luca, F. Crea, R. Aiello, A. Fonseca and J.B. Nagy
325
Studies on Crystallization of ZSM-12 Type Zeolite A. V. Toktarev and K. G. lone
333
Synthesis of Nanocrystalline Zeolite Beta in the Absence of Alkali Metal Cations M.A. Camblor, A. Corma, A. Mifsud, J. P~rez-Pariente and S. Valencia
341
Synthesis of Zeolite Beta with Low Template Content M. W.N. C. Carvalho and D. Cardoso
349
Synthesis and Characterization of Sn- Containing ZSM-48 Type Molecular Sieves Using Different Templates N.K. Mal, V. Ramaswamy, B. Rakshe and A. V. Ramaswamy
357
Inorganic Cations in AIPO4 Synthesis E. Halvorsen, A. Karlsson, T. Haug, D. Akporiaye and K.P. Lillerud
365
New Insights into the Study oflndiumphosphate Molecular Sieves L.L. Koh, Y. Xu, H. Du and W. Pang
373
Synthesis and Characterization of Novel Open-Framework Cobalt Phosphates from Aqueous-Alcoholic Systems J. Yu, Q. Gao, J. Chen and R. Xu
381
A Family of Unusual Lamellar Aluminophosphates Synthesized from NonAqueous Systems Q. Gao, J. Chen, S. Li and R. Xu
389
Synthesis of Various Indium Phosphates in the Presence of Amine Templates H. Du, J. Chen and W. Pang
397
Steric-Electronic Model of Templating Effect Z. Liu and R. Xu
405
II. Characterization The Synthesis and Characterization ofUTD-I: The First Large Pore Zeolite Based on a 14 Membered Ring System K.J. Balkus Jr., M. Biscotto and A. G. Gabrielov
415
The Nature of the Acid Sites in Mesoporous MCM-41 Molecular Sieves A. Liepold, K. Roos, W. Reschetilowski, R. Schmidt, M. StOcker, A. Philippou, M. W. Anderson, A.P. Esculcas and J. Rocha
423
Solid Mesoporous Base Catalysts Comprising ofMCM-41 Supported Intraporous Cesium Oxide K.R. Kloetstra and H. van Bekkum
431
Non-Framework Aluminium in Highly Dealuminated Y Zeolites Generated by Steaming or Substitution W. Lutz, E. LOftier, M. Fechtelkord, E. Schreier and R. Bertram
439
Spectroscopic Studies of a Magnesium Substituted Microporous Aluminophosphate DAF- 1 S.J. Thomson and R.F. Howe
447
MASNMR Chemical Shit, s and Structure in Frameworks M.T. Weller, S.E. Dann, G.M. Johnson and P.J. Mead
455
A New Method for the NMR-Spectroscopic Measurement of the Deprotonation Energy of Surface Hydroxyl Groups in Zeolites E. Brunner, J. Ka'rger, M. Koch, H. Pfeifer, H. SachsenrOder and B. Staudte
463
~70 NMR Studies of Siliceous Faujasite L.M. Bull andA.K. Cheetham
471
Deuteron Magnetic Resonance Studies of Ammonia in AgNaY-Zeolites M. Hartmann and B. Boddenberg
479
Spectroscopic Studies of 170 and 180 Labelled ZSM-5 Zeolites F. Bauer, H. Ernst, E. Geidel, Ch. Peuker and W. Pilz
487
Anisotropic Motion of Water in Zeolites EMT, L and ZSM-5 as Studied by D- and H-NMR Line Splitting A. Wingen, W.D. Basler and H. Lechert
495
EXAFS and NMR Studies of the Incorporation of Zn(II) and Co(II) Cations into Tetrahedral Framework Sites of AIPO4 Molecular Sieves N.N. Tusar, A. Tuel, 1. Arcon, A. Kodre and K Kaucic
501
Si, AI Solid Solution in Sodalite: Synthesis, 298i NMR and X-Ray Structure M. Sato, E. Kojima, H. Uehara and M. Miyake
509
Substitution of Silicon and Metal Ions in Small Pore Aluminophosphate Molecular Sieves with Chabazite Structure: Synthesis and MASNMR Study D.K. Chakrabarty, S. Ashtekar, A.M. Prakash and S. K K Chilukuri
517
Inclusion of Sodium Chloride in Zeolite NaY Studied by 23NaNMR Spectroscopy U. Tracht, A. Seidel and B. Boddenberg
525
Spectroscopic Investigation of the State of Aluminium in MCM-41 Aluminosilicates S. Viale, E. Garrone, F. Di Renzo, B. Chiche and F. Fajula
533
Boiling-Point Elevation of Water Confined in Mesoporous MCM-41 Materials Probed by 1H NMR E.W. Hansen, R. Schmidt and M. StOcker
543
In Situ Studies of Catalytic Reactions in Zeolites by Means of PFG and MAS NMR Techniques J. Karger and D. Freude
551
Vibrational Study of Benzene Adsorbed in NaY Zeolite by Neutron Spectroscopy H. Jobic and A.N. Fitch
559
Infrared Holeburning Spectroscopy in Acid Zeolites M. Bonn, M.J.P. Brugmans, H.J. Bakker, A.W. Kleyn and R.A. van Santen
567
Exploring the Sites of Adsorbed Pyrrolidine Derivatives in Y Zeolites by Joined Infrared Spectroscopic and Computer Simulation Studies E. Geidel, K. Krause, J. Kindler and H. FOrster
575
Preparation and Characterisation of Ru-Exchanged NaY Zeolite: An Infrared Study of CO Adsorption at Low Temperatures S. Wrabetz, U. Guntow, R. SchlOgl and H. G. Karge
583
New Insight into the Mechanism of Zeolite Catalyzed Nucleophilic Amination Via In Situ Infrared Spectroscopy C. Griindling, V.A. Veefkind, G. Eder-Mirth and J.A. Lercher
591
Coke Formation in Zeolites Studied by a New Technique: Ultraviolet Resonance Raman Spectroscopy C. Li and P. C. Stair
599
Preparation of Titanium-Containing Large Pore Molecular Sieve from H-AIBeta Zeolite X. Guo, X. Wang, G. Wang and G. Li
607
Syntheses and Raman Spectroscopic Study of Bis- and Tris-(1,10Phenanthroline) Manganese(II) Complexes Encapsulated in Faujasite-Y B. Zhan and X. Li
615
Chemometric Analysis of Diffuse Reflectance Spectra of CoA Zeolites: Spectroscopic Fingerprinting of Co2+-Sites A.A. Verberckmoes, B.M. Weckhuysen and R.A. Schoonheydt
623
xii Raman Characterization of the Selenium Species Formed inside the Confined Spaces of Zeolites V. V. Poborchii
631
Determination of Basic Site Location and Strength in Alkali Exchanged Zeolites D. Murphy, P. Massiani, R. Franck and D. Barthomeuf
639
A Spectroscopic Study of the Initial Stage in the Crystallization of TPASilicalite-1 from Clear Solutions B.J. Schoeman
647
Characterization and Catalytic Properties of the Galliumphosphate Molecular Sieve Cloverite R. Fricke, M. Richter, H.-L. Zubowa and E. Schreier
655
Preparation of Titanosilicate with Mordenite Structure by Atom-planting Method and Its Catalytic Properties for Hydroxylation of Aromatics P. Wu, T. Komatsu and T. Yashima
663
Characterization of Zeolite Basicity Using Iodine as a Molecular Probe S.Y. ChoL Y. S. Park and K.-B. Yoon
671
Ship-in-Bottle Synthesis of Pt and Ru Carbonyl Clusters in NaY Zeolite Micropore and Ordered Mesoporous Channels ofFSM-16; XAFS/FTIR/TPD Characterization and Their Catalytic Behaviors M. Ichikawa, T. Yamamoto, W. Pan and T. Shido
679
Characterization and Reactivity ofNi,Mo-Supported MCM-41 Catalysts for Hydrodesulfurization J. Cui, E-H. Yue, E Sun, W. -Y. Dong and Z Gao
687
Probing the Hydrophobic Properites of MCM-41-Type Materials by the Hydrophobicity Index R. Glaser, R. Roesky, T. Boger, G. Eigenberger, S. Ernst and J. Weitkamp
695
Characterisation of Acid- Base- and Redox- Type Sites in ZSM-5 Zeolites by Sorption Rate "Spectroscopy" Gy. Onyesty6k, J. Valyon and L. V. C. Rees
703
A Picosecond Spectroscopic Study on the Proton Transfers of 6Hydroxyquinoline in Zeolite Cages H. Yu, J. Park, N.W. Song and D.-J. Jang
711
xiii Ethylene Dimerization in Nickel Containing SAPO Materials Studied by Electron Spin Resonance and Gas Chromatography: - Influence of the Channel Size M. Hartmann and L. Kevan
717
The Electronegativity Equalization Method(EEM) as a Promising Tool for the Analysis of Zeolite Catalyzed Reactions G.O.A. Janssens, H. Toufar, B.G. Baekelandt, W.J. Mortier and R.A. Schoonheydt
725
Synthesis and Characterization of Iron Modified L-Type Zeolite Y.S. Ko, W.S. Ahn, J.H. Chae and S.HMoon
733
Synthesis, Characterization and Catalytic Properties of VS-2 H. Du, G. Liu, Z. Da and E. Min
741
Preparation and Characterization of Manganese Bipyridine Complexes in Zeolites with Different Pore Architectures S. Ernst and B. Jean
747
Metal Substituted ATS Aluminophosphate Molecular Sieves D. Akolekar and R.F. Howe
755
The Modified Hydrophobicity Index as a Novel Method for Characterizing the Surface Properties of Titanium Silicalites J. Weitkamp, S. Ernst, E. Roland and G. F. Thiele
763
The Thermal Stability of GaUophosphate Cloverite W. Schmidt, F. Schiith and S. Kallus
771
Electron Spin Resonance Studies of 02" Adsorbed on Aluminophosphate Molecular Sieves S.B. Hong, S.J. Kim, Y.-S. Choi and Y.S. Uh
779
The Affinity Order of Organics on Hydrophobic Zeolite Silicalite-1 Studied by Thermal Analysis Y. Long, H. Jiang and H. Zeng
787
Chemistry of CoAPO-11 and VAPO-5" ESR Studies of Molecular Oxygen Adducts C. Naccache, M. Vishnetskaya and K.J. Chao
795
Cupric Ion Species in Cu(II)-Exchanged Gallosilicate K-L and Comparison with Aluminosilicate K-L J.-S. Yu, S.B. Hong and L. Kevan
801
xiv
Part B 111. Catalysis - First Part Structure-Reactivity-SelectivityRelationships in Reaction of Organics over Zeolite Catalysts P.B. Venuto
811
Conversion of Propan-2-01 on Zeolite LaNaY Investigated by in situ M A S
853
Structure and Catalytic Activity of Co-Based Bimetallic Systems in NaY Zeolite: Low Temperature Methane Activation L. Guczi, Z. Koppany, K. V. Sarma, L. Borkd and I. Kiricsi
86 1
The Aromatization of Methane over MokIZSM-5 Zeolites without Using Oxidants M. Xie, X Yang, W. Chen, L. Tao, X Wang, G. Xu, L. Wang, Y.-D. Xu,S.Liu andX-X Guo
869
Properties of PtSn/KL Catalysts for n-Hexane Aromatization J.H. Chae and S.H. Moon
877
A New Process of Light Naphtha Aromatization Using a Zeolite-Based Catalyst with Long-Time Stability S.Fukase, N Igarashi, K. Aimoto, H. Inoue and H. On0
885
Preparation of Zeolites Incorporating Molybdenum Sulfide Clusters with Unusual Carbon Number Distribution in c 0 - H ~ Reactions M. Taniguchi, Y. Ishii, T. Murata, M. Hidai and T. Tatsumi
893
CO Hydrogenation over Pd/HZSM-5 Catalysts: Temperature-Programmed Desorption, 13CO/C180Isotope Analysis, and in-situ Infrared Spectroscopy S.-K. Ihm, J-K. Jeon and D.-K. Lee
901
Reaction Mechanisms of Heptane Isomerization and Cracking on BifunctionalPt/H-Beta Zeolites E. Blomsma. LA. Martens and P.A. Jacobs
909
NMR Spectroscopy under Continuous-Flow Conditions M. Hunger, T.Horvath andJ Weitkump
XV
Pt/Zeolite Catalysts for Hydrocracking: A Comparative Study on FAU and EMT K Zholobenko, A. Garforth, F. Bachelin and J. Dwyer
917
Acidity in Working Zeolites: Use of a Stabilised Carbenium in a New Route for the Synthesis of Secondary Amides on ZSM-5 F. Thibault-Starzyk, M.M. Bettahar, J. Saussey and J.-C. Lavalley
925
Temperature Effects on Deactivation Rate and on Nature of Coke Formed from Propene over Mordenites C.A. Henriques, J.C. Afonso, P. Magnoux, M. Guisnet and J.L.F. Monteiro
933
Catalytic Degradation of High Density Polyethylene by HZSM-5 Zeolite KJ. Fernandes Jr., A.S. Araftjo and G.J.T. Fernandes
941
The Use of Cyclohexanol Dehydration, Isobutane Cracking and 2,6-DIPN Synthesis over Dealuminated Mordenite to Probe Acidity A.W. O'Donovan and C.T. O'Connor
949
Characterization and Reactivity Study of Rhenium-Impregnated Zeolite Y Catalysed Metathesis of Olefins H. Hamdan and Z. Ramli
957
Propene Oligomerization over Dealuminated Mordenite 1. Gigstad and S. Kolboe
965
The Induction Period in Ethylbenzene Disproportionation over Large Pore Zeolites U. Weifl, M. Weihe, 3/1. Hunger, H.G. Karge and J. Weitkamp
973
Effect of Ga on the Hydrogen Transfer Activity of Zeolites with the Offretite Structure P.-S.E. Dai, C.M. Tsang, R.H. Petty, M.. Somervell, B. Williamson andM.L. Occelli
981
TPR and XPS Studies of Iron-Exchanged Y Zeolites and Their Activity during Dibenzothiophene Hydrodesulfurization M.. Nagai, O. Uchino, J. Okubo and S. Omi
989
Hydroconversion of Aromatics on Metal-Zeolite Catalysts L.P. Poslovina, V.G. Stepanov, L. V. Malysheva, E.A. Paukshtis, L.A. Vostrikova and K. G. lone
997
xvi Low Temperature Hydrocracking of Paraffinic Hydrocarbons over Hybrid Catalysts 1. Nakamura, K. Sunada and K. Fufimoto
1005
A Comparative Study of Titanium-Containing Aluminophosphate Molecular Sieves TAPO-5, TAPO-11 and TAPO-36 M.H. Zahedi-Niaki, P.N. Joshi and S. Kaliaguine
1013
MCM-41-Type Molecular Sieves as Carriers for Metal-Phthalocyanine Complexes S. Ernst, R. Glaser andM. Selle
1021
Application of CoAPO-5 Molecular Sieves as Heterogeneous Catalysts in Liquid Phase Oxidation of Alkenes with Dioxygen H.F. ~ J . van Breukelen, M.E. Gerritsen, KM. Ummels, J.S. Broens and J.H. C. van Hooff
1029
Selective Oxidation of Aromatic Hydrocarbons over Copper Complexes Encapsulated in Molecular Sieves R. Raja and P. Ratnasamy
1037
Cation Effects in the Oxidation of Adsorbed Cyclohexane in Y Zeolite: An in situ IR Study D.L. Vanoppen, D.E. De Vos and P.A. Jacobs
1045
Influence of Extra-Framework Alumina in H-[AI]ZSM-5 Zeolite on the Direct Hydroxylation of Benzene to Phenol J.L. Motz, H. Heinichen and W.F. HOlderich
1053
The Stability of Chromium in Chromium Molecular Sieves under the Conditions of Liquid Phase Oxidations with tert-Butyl Hydroperoxide H.E.B. Lempers and R.A. Sheldon
1061
Oxidation of Olefins and Alkanes with Various Peroxides, Catalyzed by Triamine Containing Manganese Faujasites D.E. De Vos, J.L. Meinershagen and T. Bein
1069
Characterization and Reversible Reduction/Oxidation of Zeolite-Hosted Tiand V-Oxide Dispersions G. Grubert, M. Wark, W. Griinert, M. Koch and G. Schulz-Ekloff
1077
Zirconium Containing Mesoporous Silicas: New Catalysts for Oxidation Reactions in the Liquid Phase S. Gontier and A. Tuel
1085
xvii Synthesis of Aluminium Free Titanium Silicate with the BEA Structure Using a New and Selective Template and Its Use as a Catalyst in Epoxidations J.C. van der Waal, P. Lin, M.S. Rigutto and H. van Bekkum
1093
Incorporation of Vanadium into ZSM-5, Mordenite and Y Type Zeolite and Their Catalytic Properties G.-J. Kim, J.-H. Kim and H. Shoji
1101
Catalytic Tuning of the Olefin Epoxidation with Hydrogen Peroxide and Faujasite Y Occluded Mn Bipyridine Complexes P.-P. Knops-Gerrits, H. Toufar and P.A. dacobs
1109
The Selective Oxidation of Cyclohexane Using Iron(Ill) Incorporated Zeolite Y and Iron(III) Supported Zeolite Y in a Heterogeneous Biomimetic Oxidation System C.-H. Park, S.-S. Nam, S.B. Kim, S.-B. Kim, K.-W. Jun and K.-I4(. Lee
1117
Solid Catalysts for the Hydroxyalkylation of Aromatics with Epoxides. Intermolecular Hydroxyalkylation versus Intramolecular Hydroxyalkylation ~A. Elings, R.S. Downing and R.A. Sheldon
1125
Heterogeneously-Catalyzed Hydroalkoxylation of Limonene and alphaPinene in the Presence of Beta Zeolite K. Hensen, C. Mahaim and I~.F. HOlderich
1133
Photochemistry of Alkyl Ketones Included within the Zeolite Cavities: The Effect of Ion-exchanged Alkali Metal Cations and Types of Zeolites H. Yamashita, N. Sato, M. Anpo, T. Nakafima, M. Hada and H. Nakatsufi
1141
Acylation of Phenol with Acetic Acid. Effect of Density and Strength of Acid Sites on the Properties ofMFI Metallosilicates F. Jayat, M. Guisnet, M. Goldwasser and G. Giannetto
1149
Structural Design for Y-type Zeolite on Large Molecule Conversion. Alkylation of Phenol with Long Chain Olefin. X-W.. Li, M. Han, X.-Y. Liu, Z.-F. Pei and L. She
1157
Zeolite-Catalysed Rearrangement of lsophorone Oxide J.A. Elings, H.E.B. Lempers and R.A. Sheldon
1165
Vapour-Phase Beckmann Rearrangement Using B-MFI Zeolites J. ROseler, G. Heitmann and I'EF. HOlderich
1173
xviii Catalytic Properties of Fluorinated CeY S. Kowalak, M. Laniecki, M. Pawlowska, K.£ Balkus Jr. and A. Khanmamedova
1181
Evaluation of Catalyst Deactivation on Beckmann Rearrangement over Indiosilicate Modified with Noble Metals M.N.A. Nasution, T. Takahashi and T. Kai
1189
Selective Fries Rearrangement of Phenyl Acetate into Hydroxy Acetophenones Catalyzed by High-Silica Zeolite NCL-1 M. Sasidharan and R. Kumar
1197
Selective Hydrogenation of Cinnamaldehyde to Cinnamyl Alcohol on LZeolite Supported Catalysts G. LL T. Li, Y.-D. Xu, S. Wong and X.-X Guo
1203
Zeolite Catalysts for the Friedel-Craffs Alkylation of Methyl Benzoate, a Strongly Deactivated Aromatic Substrate B. Janssens, P. Carry, R. Claessens, G. Baron and P.A. Jacobs
1211
Methanol Amination over Small-Pore Zeolite Catalysts K. Segawa and M. C. Ilao
1219
Synthesis of Aniline from Phenol and Ammonia over Zeolite Beta N. Katada, S. lo'ima, H. lgi and M. Niwa
1227
Cyclodimerization ofBicyclo[2.2.1 ]Hepta-2,5-Diene in the Presence of Rhodium-Containing Zeolite Catalysts N.F. Goldshleger, B.L Azbel, Ya.I. Isakov, E.S. Shpiro and Kh.M. Minachev
1235
Vinyl Chloride Synthesis on Zeolite Catalysts: The Role of Strong Lewis Acid-Base Pair Sites E.B. Uvarova, L.M. Kustov, I.L Lishchiner, O.V. Malova and V.B. Kazansky
1243
IV. C a t a l y s i s - S e c o n d P a r t
Influence of Zeolite Pore Structure on Catalytic Reactivity A. van de Runstraat, P.£ Stobbelaar, J. van Grondelle, B.G. Anderson, L.£ van IJzendoorn and R.A. van Santen
1253
xix Regio Selectivity in the Hydrogenation of Geraniol over Platinum Containing Zeolites D. Tas, R.F. Parton, K. Vercruysse and P.A. Jacobs
1261
Zeolite Catalyzed Regioselective Synthesis of lndoles P.J. Kunkeler, M.S. Rigutto, R.S. Downing, H.J.A. de Vries and H. van Bekkum
1269
Shape-Selective Zeolite Catalysed Synthesis of Monoglycerides by Esterification of Fatty Acids with Glycerol E. Heykants, W.H. Verrelst, R.F. Parton and P.A. Jacobs
1277
Selective Key-Lock Catalysis in Dimethylbranching of Alkanes on TON Type Zeolites W. Souverijns, J.A. Martens, L. Uytterhoeven, G.F. Froment and P.A. Jacobs
1285
Kinetics Study ofEthylbenzene Disproportionation with Medium and Large Pore Zeolites N. Arsenova, W.O. Haag and H. G. Karge
1293
Application of a Kinetic Model for Investigation of Aromatization Reactions of Light Paraffins and Olefins over HZSM-5 D.B. Lukyanov
1301
Conversion of n-Pentane to Benzene Toluene and para-Xylene over Pore Size Controlled Ga203 Incorporated ZSM-5 Zeolite KS. Bhat, J. Das and A.B. Halgeri
1309
The Influence of Reagents on Shape-Selective Alkylation of Biphenyls over H-Mordenite M. Matsumoto, X. Tu, T. Matsuzaki, T. Hanaoka, E Kubota, E Sugi, J.-H. Kim, K. Nakajima, A. Igarashi and K. Kunimori
1317
Selective Benzene Isopropylation over Fe-Containing Zeolite Beta A.V. Smirnov, F. Di Renzo, O.E. Lebedeva, D. Brunel, B. Chiche, A. Tavolaro, B.V. Romanovsky, G. Giordano, F. Fajula and 1.1. Ivanova
1325
Para-Selective Gas Phase 02 Oxidations of Alkylaromatics over CVD Fe/Mo/Borosilicate Molecular Sieve J.S. Yoo, P.S. Lin and S.D. Elfline
1333
A Model Study on Zeolite Composites for Improved Catalyst Selectivity N. van der Pull, LB. Janto-Saputro, H. van Bekkum and J.C. Jansen
1341
XX
Toluene Disproportionation over ZSM-5 Catalysts Covered with Silicalite Shell C.S. Lee, T.-J. Park and IE.Y. Lee
1349
Ethylation of Ethylbenzene to Produce para-Diethylbenzene X. Wang, G. Wang, H. Guo and X. Wang
1357
Mechanisms of the Skeletal Isomerization of n-Butene over a HFER Zeolite. Influence of Coke Deposits. M. Guisnet, P. Andy, N.S. Gnep, C. Travers and E. Benazzi
1365
Reaction of n-Butene over H-ZSM-5 Zeolite. Influence of the Acid Strength on the Isobutene Selectivity P. M&iaudeau, T. Vu. Anh, H. Le Ngoc and C. Naccache
1373
Skeleton Hydroisomerization ofHexene-1 in the Presence of Synthesis Gas on Zn-Cr/HZSM-5 Catalyst V.M. Mysov, V.G. Stepanov and K. G. lone
1381
Skeletal Isomerization of 1-Butene over Zeolite Catalysts: A Computational Study R. Millini and S. Rossini
1389
Studies of Crystallization of SAPO-11 Molecular Sieve and Applications for Catalytic Skeletal Isomerization of Linear Butylenes H. Tian and C. Li
1397
Pt-Cu Bimetallics in H-Y Zeolite: Microcalorimetric Study of the Effect of Copper on the Catalyst Acidity A. Auroux, Y. Ben Taarit, M. Lokolo, P. Meriaudeau and C. Naccache
1405
La-EMT, a Promising Catalyst for Isobutane/2-Butene Alkylation H. Mostad, M. StOcker, A. Karlsson, H. Junggreen and B. Hustad
1413
Skeletal Isomerization of n-Butenes on Modified ZSM-35 Catalysts B.S. Kwak, J.H. Jeong and S.H. Park
1423
Selectivity to the Skeletal Isomerization of 1-Butene over Ferrierite(FER) and ZSM-5 (MFI) Zeolites G. Seo, H.S. Jeong, J.M. Lee and B.J. Ahn
1431
xxi
V. Environment High Potential of Novel Zeolitic Materials as Catalysts for Solving Energy and Environmental Problems T. Inui
1441
Modification and Stabilization of Cu-ZSM-5 by Introduction of a Second Cation A. V. Kucherov, C.P. Hubbard, T.N. Kucherova and M. Shelef
1469
Reactivity of Adsorbates in the Decomposition of Nitric Oxide over CuZSM-5 Catalysts S.S. C. Chuang and B. Lopez
1477
Quantum Chemical Investigation of Reactants in Selective Reduction of NOx on Ion Exchanged ZSM-5 M. Yamadaya, H. Himei, T. Kanougi, E Oumi, M. Kubo, A. Stir#ng, R. Vetrivel, E. Broclawik and A. Miyamoto
1485
The Role of Water for NO Reduction by Hydrocarbons over Copper IonExchanged Mordenite Type Zeolite Catalysts M.H. Kim, I.-S. Nam and E G. Kim
1493
Catalytic Reduction of Nitrogen Monoxide by Methane over Pd-Loaded ZSM-5 Zeolites. Roles of Acidity and Pd Dispersion M. Misono, Y. Nishizaka, M. Kawamoto and H. Kato
1501
Zeolites in the Environmental Protection - Decomposition of Chlorofluorocarbons over Zeolite Catalysts Z. K6nya, L Hannus and 1. Kiricsi
1509
Preparation of Cu-Na-ZSM-5 Catalysts by Thermal Spreading Techniques W. Griinert, T. Liese and C. Schobel
1517
Preparation, Characterization and Catalytic Activity Towards Lean NOx Reduction of Over-exchanged Cu-ZSM-5 Catalysts G. Moretti, G. Minelli, P. Porta, P. Ciambelli, P. Corbo, M.. Gambino, F. Migliardini and S. Iacoponi
1525
Factors Controlling Catalytic Activity of H-Form Zeolites for the Selective Reduction of NO with CI-L A. Satsuma, M. Iwase, A. Shichi, T. Hattori and Y. Murakami
1533
xxii CO Oxidation and NO Reduction by CO on Differently Prepared CuO/Mordenites K.-H. Lee and B.-H. Ha
1541
Steam Deactivation of Transition Metal MFI Zeolite Catalysts for NOx Reduction P. Budi, E. Curry-Hyde and R.F. Howe
1549
ln-situ IR Studies of Surface Species during the Selective Catalytic Reduction(SCR) of NO by Propene over Cu-ZSM-5 Zeolites D.H. Kim, I.C. Hwang and S.I. Woo
1557
Reduction of NO by CO Using a Zeolite Catalyst Obtained from Fly Ash E. L6pez-Sa#nas, P. Salas, L Schifter, M. Mor6n, S. Castillo and E. Mogica
1565
ln-situ FT-IR and Catalytic Studies of the Selective Reduction of Nitric Oxide by Carbon Monoxide over Au/NaY Catalysts: Effect of Adding Hydrogen to the Reaction Gas Mixture T.M. Salama, R. Ohnishi and M. Ichikawa
1571
Role of Oxygen in the SCR of NOx with Propane over Co/ZSM-5: Reaction and TPD Study A. Yu. Stakheev, C. W. Lee, S.J. Park and P.J. Chong
1579
Sharp Contrast in Thermal Stability between MFI-Type Metallosilicates and Metal-Ion-Exchanged ZSM-5 and Their Catalytic Performances for NO Removal S. Iwamoto, S. Kon, S. Yoshida and T. Inui
1587
Formation of Active Sites for Reduction of NO2 with Methane by Solid State Exchange of ln203 into H-Zeolites M. Ogura, N. Aratani and E. Kikuchi
1593
Purification of NOx on Pt-ZSM-5 and Mg-Cu-ZSM-5 Catalysts under LeanBurn Engine Emission Conditions S. Choung, B. Shin and J. Bae
1601
The In-situ Characterization of Titanium Oxides Prepared in the Zeolite Cavities and Framework and Their Photocatalytic Reactivities for the Direct Decomposition of NO into N2 at 275K Y. IchihashL H. Yamashita and M. Anpo
1609
xxiii Photocatalytic Decomposition of Trichloroethylene over Aluminosilicate Zeolites with Isomorphous Incorporation of Titanium I.H. Cho, J.H. Kwak, R. Ryoo, W.S. Ahn, K.Y. Jung and S.B. Park
1617
Catalytic Decomposition of Organic Sulfur Compounds - Effect of Zeolite Acidity M. Ziolek, P. Decyk, J. Czyzniewska and H.G. Karge
1625
Use of Natural Clinoptilolite for the Optimization of Mineral Clay Liners for Waste Deposits M. W. Upmeier and K.A. Czurda
1633
Removal of Highly Concentrated Ammonium Ions by Natural Mordenite S. Noda
1641
Palladium Ion-Exchanged SAPO-5 for a Low Temperature Combustion of CI-h Y. Takita, T. lshihara, H. Nishiguchi and H. Sumi
1647
Kinetics of CH4 Complete Oxidation on CuH-ZSM-5 Catalyst A. V. Kucherov, N. V. Nekrasov, A.A. Slinkin, E.A. Katsman and S.L. Kiperman
1655
Ion-Exchange Behavior of Zeolite NaA and Maximum Aluminum Zeolite NaP E.v.R. Borgstedt, H.S. Sherry and J.P. Slobogin
1659
Zeolite MAP: the New Detergent Zeolite C.J. Adams, A. Araya, S.W. Carr, A.P. Chapple, K.R. Franklin, P. Graham, A.R. Minihan, T.J. Osinga and J.A. Stuart
1667
Part C VI. Adsorption and Diffusion Zeolites as Adsorbents and Catalysts. The Interactive System Encaged Molecule/Zeolite Framework D. Barthomeuf
1677
Methanol Adsorption and Activation by Zeolitic Protons S.R. Blaszkowski and R.A. van Santen
1707
xxiv Carbon Dioxide Adsorption Kinetics in the Presence of Light Paraffins on NaA and CaA Zeolites A. Khodakov and L. V.C. Rees
1715
Specific Adsorption from Aqueous Phase on Apolar Zeolites C. Buttersack, 1. Fornefett, J. Mahrholz and K. Buchholz
1723
Adsorption Studies on Ordered Mesoporous Materials (MCM-41) J. Jdinchen, M. Busio, M. Hintze, H. Stach and J.H.C. van Hooff
1731
Rapid-Scanning FT-IR Study on the Adsorptions of Methanol and Water on H-ZSM-5 Zeolite F. Wakabayashi, M. Kashitani, T. Fujino, J.N. Kondo, K. Domen and C. Hirose
1739
Onthe Sorption of Ethylbenzene in ZSM-5 R. Schumacher, P. Lorenz and H. G. Karge
1747
Ethylene Adsorption on HNaZSM-5: Kinetic Study S.N. Vereshchagin, N.P. Kirik, N.N. Shishkina and A. G. Anshits
1755
Adsorption of Acetylacetone on Layer Silicate Containing Various Interlayer Cations JR. Sohn and S.I. Lee
1763
Sorption of Water Vapor on HZSM-5 Type Zeolites T. Sano, T. Kasuno, K. Takeda, S. Arazaki and Y. Kawakami
1771
~H-NMR Relaxation Times of Water and Benzene Adsorbed in Zeolite Beta S. Sardar, W.D. Basler and H. Lechert
1779
Adsorption of Sulfur Dioxide on Y-Type Zeolites Y. Teraoka, Y. Motoi, H. Yamasaki, A. Yasutake, J. Izumi and S. Kagawa
1787
Vanadium Derivatives of MFI Type Molecular Sieves Investigated by Sorption and Catalytic Tests J. Kornatowski, M. Sychev, M. Rozwadowski and W. Lutz
1795
Desiccant Selection Criteria for Enthalpy Exchange Systems M.P.F. Delmas, W.D. Holeman, C.N. Blystad, W.A. Belding and J.H.D. Tantet
1803
XXV
Atomistic Mechanism of the Adsorption of CFCs in Zeolite as Investigated by Monte Carlo Simulation K. Mizukami, H. Takaba, Y. Oumi, M. Katagiri, M. Kubo, A. Stirling, E. Broclawik, A. Miyamoto, S. KobayashL S. Kushiyama and K. Mizuno
1811
The Crystal Structures of Dehydrated Fully Cd2+-Exchanged Zeolite X and of Its Carbon Monoxide Sorption Complex S.B. Jang, J.H. Kwon, S.H. Song, Y. Kim and K. Serf
1819
Structural Property of Methane(CD4) and Hydrogen(D2) Sorbed Phases on MCM-41 (0=25A) J.P. Coulomb, C. Martin, Y. Grillet, P.L. Llewellyn and J. Andr~
1827
Composition, Location, Modes of Formation and of Removal of Coke Deposited on a 5A Adsorbent P. Magnoux, M. Misk, G. Joly, S. Jullian and M. Guisnet
1835
Self-Diffusion and Diffusive Transport in Zeolite Crystals J. Kdirger and D.M. Ruthven
1843
Simulation of Hydrocarbon Diffusion in Zeolites E.J. Maginn, R.Q. Snurr, A.T. Bell and D.N. Theodorou
1851
Methane Diffusion in Zeolites of Structure Type LTA in Dependence on Physical and Chemical Parameters- An MD Study S. Fritzsche, 3/1. Gaub, R. Haberlandt, G. Hofmann, J. Kgirger and M. Wolfsberg
1859
Study of the Molecular Diffusion in the Internal Porosity of ZSM-5 and HMOR Zeolites L.C. de M~norval, J.G. Kim and F. Figueras
1867
Single File Counterdiffusion in Pores of Infinite and Finite Length J.M..D. MacElroy and S.-H. Suh
1875
Hydrogen Separation by Two-Bed PSA Process J. Yang, J.-H. Lee, C.-H. Lee and H. Lee
1883
Pressure Swing Adsorption of Organic Solvent Vapors on Mesoporous Silica Molecular Sieves S. Namba, N. Sugiyama, M. Yamai, I. Shimamura, S. Aoki and J. lzumi
1891
xxvi
VII. Modifications Post-Synthesis Modification of Microporous Materials by Solid-State Reactions H. G. Karge
1901
A New Layered (Alumino) Silicate and Its Transformation into a FER-Type Material by Calcination L. Schreyeck, P. Caullet, J.C. Mougenel, J.L. Guth and B. Marler
1949
From the Keggin Complex Containing Solution to Pillared Layer Clays - A Comprehensive NMR Study J.B. Nagy, J.-C. Bertrand, L Pdlink6 and L Kiricsi
1957
Inactivation of Acid Sites on External Surface of Zeolites with Methoxytripropylsilane J.-H. Kim, M. Okajima and M. Niwa
1965
Generation of Acid Sites by Incorporation of Cobalt in the AFR Structure J.P. Lourengo, M.F. Ribeiro, F.R. Ribeiro, J. Rocha, Z Gabelica, B. Onida and E. Garrone
1973
Acidic Properties of Galliosilicate Molecular Sieves with the Offretite Structure M.L. Occelli, H. Eckert, C. Hudalla, A. Auroux, P. Ritz and P.S. Iyer
1981
The Thermal Expansion of the Zeolites MFI, AFI, DOH, DDR and MTN in Their Calcined and as Synthesized Forms S.H. Park, R.-W. Grofle Kunstleve, H. Graetsch and H. Gies
1989
Activity Enhancement of H-Zeolites by Ag Ion-Exchange and Sulfiding with Hydrogen Sulfide M. Sugioka and L. Andalaluna
1995
Effect of Pre-dealumination by (NI-h)zSiF6 on the Properties of ZSM-5 with Steam Aging Treatment L. Huang, Q. Li, Z. Xue, G. Ding, G. Niu, Z. Li and Z. Shi
2003
Effect of the Dealumination Procedure on Surface Properties and Catalytic Performance of UHP-Y Zeolite Z. Chang, C. Ruan and G. Tong
2011
xxvii Formation of Alkali Nanoparticles in NaY Zeolite Cages and in A1PO4-5 Molecular Sieves: NMR Studies L. C. de M~norval and F. Rachdi
2019
Iridium in Pentasil: Redox Behavior and Reactivity T. V. Voskoboinikov and E.S. Shpiro
2027
Para-Chlorination Activity of Solid-State Ion-Exchanged Zeolite KL Catalysts J. ~ Yoo, D.S. Kim, J.-S. Chang and S.-E. Park
2035
Solid-State Ion Exchange of Fe(III) into Y Zeolite under Deep-Bed Conditions P. Hudec, A. Smieskovd, Z. Zidek, ~ Jorik, 3/1. Miglierini and J.B. Nagy
2043
Preparation, Characterization, and Catalysis of lntrazeolite Iron Oxide Clusters Y. Okamoto, H. Kikuta, Y. Ohto, S. Nasu and O. Terasaki
2051
Zeolite Pore Size Engineering by Chemical Liquid Deposition Y.-H. Yue, Y. Tang and Z. Gao
2059
Incorporation of Molybdenum into Mesoporous MCM-41 S.D. Djajanti and R.F. Howe
2067
Chemical Vapor Deposition on Basic Zeolites Y. Chun, Q.-H. Xu, A.-Z Yan and X. Ye
2075
Metal-Oxide Interactions and Catalytic Behaviors of La203- and V2OsPromoted Rh/NaY Catalysts K. KunimorL K. Yuzaki, T. Yarimizu, fill.. Seino and S. Ito
2083
VIII. Novel Materials Modification of a Pyroelectric Detector by Controlled Electrocrystallization of Thin Zeolite Layers G.J. Klap, M. Wiibbenhorst, J. van Turnhout, J.C. Jansen and H. van Bekkum
2093
Fabrication, Luminescence and Photoacoustic Spectroscopic Studies of the Semiconductor Nanoclusters in Zeolites G. Tel'biz, 1. Blonskij, S. Shevel and E Voznyi
2101
xxviii Zeolites as Sensitive Materials for Organic Vapour Detection: An Exploratory Study C. CantalinL M. Pelino, M. Pansini and C. Colella
2109
Adsolubilization Equilibrium of Rhodamine B by Zeolite/Surfactant Complexes K. Hayakawa, A. DobashL Y. Miyamoto and L Satake
2115
Investigations into the Engineering of Inorganic/Organic Solids: Hydrothermal Synthesis and Structure Characterization of One- and TwoDimensional Molybdenum Oxide Polymers Y. Xu, L.L. Koh, L.H. An, D.G. Roshan and L.H. Gan
2123
Synthesis and Structure of Cadmium Chalcogenide Beryllogermanate Sodalites S.E. Dann andM.T. Weller
2131
Optical and Magnetic Properties of Na-K Alloy Clusters Incorporated into LTA T. Kodaira, E Nozue, O. Terasaki and H. Takeo
2139
Growth of Oriented Molecular Sieves on Organic Layers S. Feng and T. Bein
2147
Factors Affecting the Porosity of ZSM-5 Layer on the Surface of Stainless Steel Z. Shah, E. Min and H. Yang
2155
Synthesis of Films of Oriented Silicalite-1 Crystals Using Microwave Heating J.H. Koegler, A. Ararat, H. van Bekkum and J. C Jansen
2163
News from AiPO4-5: Microwave Synthesis, Application as Medium to Organize Molecules for Spectroscopy and Nonlinear Optics, Material for One-Dimensional Membranes J. Caro, F. Marlow, K. Hoffmann, C Striebel, J. Kornatowski, 1. Girnus, M. Noack and P. KOlsch
2171
Silylation of Silicalite Membrane and Its Pervaporation Performance T. Sano, K. Yamada, S. Ejiri, M. Hasegawa, Y. Kawakami and H. Yanagishita
2179
Vertically-Aligned MeAPO4-5 Crystals Grown on Anodic Alumina Membrane K.£ Chao, CN. Wu, H.C Shih, T.G. Tsai and Y.H. Chiou
2187
xxix Synthesis ofFER Membrane on an Alumina Support and Its Separation Properties N. Nishiyama, K. Ueyama and M. Matsukata
2195
Synthesis of Ultra Thin Films of Molecular Sieves by the Seed Film Method J. Hedlund, B.J. Schoeman and J. Sterte
2203
Synthesis of SAPO-34/Ceramic Composite Membranes L. Zhang, M. Jia and E. Min
2211
Synthesis of ZSM-5 Zeolite Membrane on the Inner Surface of a Ceramic Tube H.-S. Oh, M.-H. Kim and H.-K. Rhee
2217
Synthesis of Oriented Zeolite Film on Mercury Surface Y. Kiyozumi, F. Mizukami, K. Maeda, T. Kodzasa, M. Toba and S. Niwa
2225
Growth of Oriented Zeolite Crystal Membranes M. Cheng, L. Lin, W. Yang, Y. Yang, Y.-D. Xu and X. Li
2233
Preparation of Fibrous Titanium Silicalite-l(FTS-1) from Nano TS-1 Crystals K.T. Jung, J.H. Lee, J.H. Hyun, D.S. Kim, J. G. Kim and Y.G. Shul
2241
IX. Theory Structure of the Linear Na32÷ Cluster in Zeolite X W. Shibata and K. Serf
2251
Supralattices: Another Dimension in Materials Science- Theoretical Investigation A.A. Demkov and O.F. Sankey
2259
Experimental and Theoretical Studies of Siliceous Zeolites A.K. Cheetham, L.M. Bull and N.J. Henson
2267
Charge-Transfer Molecular Dynamics ofProtonated Faujasite L.J. Alvarez, P.B. Giral, C.Z. Wilson andJ.E. Sdnchez-S6nchez
2275
Quantum-Chemical Study of Hydride Transfer in Catalytic Transformations of Paraffins on Zeolites KB. Kazansky, M. K Frash and R.A. van Santen
2283
XXX
Computer Modelling of the Structure and Synthesis of Microporous and Mesoporous Materials D. l~. Lewis, R.G. Bell, P.A. Wright, C.R.A. Catlow and J.M.. Thomas
2291
Ab Initio Computer Modelling of Zeolite Frameworks I. Modelling of Basic Clusters M. Sato and H. Uehara
2299
Intrinsic and Enhanced Broensted Acidity in Zeolites J. Dwyer, K Zholobenko, A. Khodakov, S. Bates and M.A. Makarova
2307
Molecular Sieve Effect of Chemically Modified Na-A Type Zeolite and Its Molecular Dynamic Simulation J. lzumi, A. Yasutake, N. Tomonaga, N. Oka, H. Ota, N. Akutsu, S. Umeda andlVl. Tafima
2315
Computational Study of Structural and Thermal Properties of the Microporous Titanosilicate ETS- 10 M.E. Grillo, J. Lujano and J. Carrazza
2323
Complete Redox Exchange of Indium for TI÷ in Zeolite A. Synthesis and Crystal Structure of Fully Indium-Exchanged Zeolite A N.H. Heo, H.C. Choi and K. Serf
2331
Location and Orientation of Pyrrole and Acetaldehyde Molecules inside Siliceous Faujasite as Predicted by Electronic Structure Calculations A. Chatterjee, R. Vetrivel, M. Kubo and A. Miyamoto
2339
Commensurate Freezing of Hydrocarbons in Silicalite IJ(.jM. van Well, J.P. Wolthuizen, B. Stair, J.H.C. van Hooff and R.A. van Santen
2347
Phase Transition Types Observed During the Sorption of van der Waals Gases on Model Zeolites: Silicalite I and AIPO4-5 J.P. Coulomb, C. Martin, P.L. Llewellyn and Y. Grillet
2355
Author Index
2363
Subject Index
2375
xxxi
Preface The 11th International Zeolite Conference was held successfully in Seoul, Korea, from August 12 to 17, 1996. The main conference was preceded by the Pre-Conferenee Summer School on Zeolite held in Taeduck Science Town and followed by the PostConference Symposium on Catalysis related to Environmental Application of Zeolite in Kyoungju, Korea. We would like to thank members of the IZA Council, liaison to the council, Dr. Ono, and members of the International Advisory Board for their active support for the conference. The number of registrants totaling about 500 was a little bit less than the organizing committee hoped for. Considering the shortened interval between the successive International Zeolite Conference from three to two years, this number nevertheless reflected quite a favorable response to this conference. During the 6 day meeting of the main conference 5 plenary lectures and 113 papers were presented orally in 40 sessions and 165 full papers as well as 127 recent research reports were presented as posters. Ever increasing interest and continuous developments in the zeolite science and technology are reflected in the number of submitted contributions in various areas of research and application. We would like to thank all those who have submitted the papers. It is regrettable, however, we could not accommodate many contributions of high quality due to the limited time and space available for presentation. We also had to make certain consideration on the balance among the different disciplines. Each manuscript for the proceedings was reviewed by two reviewers and in case of a conflicting reviewing results a third referee was asked to recommend as to the acceptance of the paper for publication in the proceedings. The organizing committee is most grateful to the experts who spared their precious time to review abstracts as well as the full manuscript for the proceedings. Our special appreciation goes to Dr. David Olson who headed the third referee group. The Proceeding of the 1l th Zeolite Conference is published in three volumes containing 5 plenary lectures and 274 full papers. Part A contains Synthesis and Characterization (99 papers), Part B Catalysis, Environment (102 papers), and Part C Adsorption and Diffusion, Modifications, Novel Materials and Theory (78 papers).
xxxii Although 161 papers out of 279 are presented as posters no difference is made whatsoever between the oral and poster papers in publishing the full manuscript in the proceedings. Zeolite catalysis has been and continues to be an area of major interest. Growing interest in the synthesis, and the characterization of zeolite and microporous materials is reflected in the large number of contributions. Other area of growing interest is novel materials. Adsorption, theory and modeling remain as attractive areas. Our ambitious original plan to publish the proceedings before the conference had to be modified and three volume proceedings will be published by the end of this year. Generous donations were received from a number of organizations whose names are given in the sponsor's list. The organizing committee is most grateful to them for their support. Finally we would like to express our sincere appreciation to the authors for their fine papers and all the reviewers who made careful reviewing of the submitted abstracts. We are also grateful to the members of the scientific committee who have spent so much time and effort to select the papers, especially to Prof. Gon Seo, Prof. Yang Kim, Prof. Jong Rack Sohn, Dr. Sang-Eon Park, and Prof. Ryong Ryoo. We would like to thank also Mr. Sang lck Lee and Dae-Chul Kim who helped in the preparations of the proceedings.
Seoul, August, 1996
Hakze Chon Son-Ki Ihm Young Sun Uh
xxxiii
Organizing Committee of the 1 lth IZC Chairman Hakze Chon
Korea Advanced Institute of Science & Technology, Taejon, Korea
Vice-Chairmen Hanju Lee Baik-Hyon Ha Wha Young Lee
Yonsei University, Seoul, Korea Hanyang University, Seoul, Korea Seoul National University, Seoul, Korea
Secretary Young Sun Uh
Korea Institute of Science & Technology, Seoul, Korea
Finance Sub-Committee Chairman Hanju Lee
Yonsei University, Seoul, Korea
Members Hyun-Ku Rhee Ki Woong Sung Bong H. Chang
Seoul National University, Seoul, Korea. DaeLim Industrial Co., Ltd., Seoul, Korea AeKyung-PQ Advanced Materials Co. Ltd., Seoul, Korea
Program Sub-Committee Chairman Sang Heup Moon Members Wha Seung Ahn Suk-Jin Choung Sung Hwan Han Suk In Hong Kyung Lim Kim Ho-ln Lee
Seoul National University, Seoul, Korea
Inha University, Inchon, Korea Kyunghee University, Suwon, Korea Korea Institute of Science & Technology, Seoul, Korea Korea University, Seoul, Korea Yonsei University, Seoul, Korea Seoul National University, Seoul, Korea
xxxiv Hyun Ryul Park Tae-Jin Park Kee Jun Yoon
Chungang University, Seoul, Korea Korea Institute of Science & Technology, Seoul, Korea Sungkyunkwan University, Suwon, Korea
Scientific Sub-Committee Chairman Son-Ki lhm
Korea Advanced Institute of Science & Technology, Taejon, Korea
Vice-Chairman Gon Seo Members Byung Joon Ahn Hee Kwon Chae Paul J. Chong Jong Shik Chung Kee Sung Ha Chong Soo Han Nam Ho Heo Suk Bong Hong Seon-Yong Jeong Wha Joong Kim Jae Chang Kim Man Hoe Kim Yang Kim Chul Wee Lee Dong-Keun Lee Jae Sung Lee Jung-Min Lee Tae Jin Lee Hee Moon Sang Sung Nahm In-Sik Nam Kyung Tae No
Chonnam National University, Kwangju, Korea
Chonbuk National University, Chonju, Korea Hankuk University of Foreign Studies, Yong-ln Korea Korea Research Institute of Chemical Technology, Taejon, Korea Pohang University of Science and Technology, Pohang, Korea Pusan National University of Technology, Pusan, Korea Chonnam National University Kyungpook National University Korea Institute of Science & Technology, Seoul, Korea Korea Research Institute of Chemical Technology, Taejon, Korea Konkuk University, Seoul, Korea Kyungpook National University, Taeku, Korea Air Force Academy, Chongju, Korea Pusan National University, Pusan, Korea Korea Research Institute of Chemical Technology, Taejon, Korea Kyeongsang National University, Taeku, Korea Pohang University of Science and Technology, Pohang, Korea Korea Research Institute of Chemical Technology, Taejon, Korea Yeungnam University, Taeku, Korea Chonnam National University, Kwanju, Korea Korea Research Institute of Chemical Technology, Taejon, Korea Pohang University of Science and Technology, Pohang, Korea Soongsil University, Seoul, Korea
XXXV
Korea Advanced Institute of Science & Technology. Taejon, Korea Seung Bin Park Korea Research Institute of Chemical Technology, Taejon, Korea Sang-Eon Park Korea Advanced Institute of Science & Technology, Taejon, Korea Ryong Ryoo Yonsei University, Seoul, Korea Yong Gun Shul Kyungpook National University, Taeku, Korea Jong Rack Sohn Korea Institute of Science & Technology, Seoul, Korea Dong-Jin Suh Hongik University, Seoul, Korea Sung-Sup Suh Korea Advanced Institute of Science & Technology, Taejon, Korea Seung lhl Woo Ajou University, Suwon, Korea Jae Eui Yie Kyung Byung Yoon Sogang University, Taejon, Korea Jong-Sung Yu Hannam University, Taejon, Korea
Pre-Conference Summer School on Zeolites Chairman
Seung lhl Woo
Korea Advanced Institute of Science & Technology, Taejon, Korea
Co-Chairman
Sang-Eon Park
Korea Research Institute of Chemical Technology, Taejon, Korea
Members
Byung Joon Ahn Oh Bong Yang Sang Sun Nahm
Chonbuk University, Chonju, Korea Chonbuk University, Chonju, Korea Korea Research Institute of Chemical Technology, Taejon, Korea
Post-Conference Symposium on Catalysis Chairman
Young Gul Kim
Pohang University of Science and Technology, Pohang, Korea
Member
In-Sik Nam Kyung Hee Lee Jong Shik Chung Jae Sung Lee
Pohang University of Science and Technology, Pohang, Korea Pohang University of Science and Technology, Pohang, Korea Pohang University of Science and Technology, Pohang, Korea Pohang University of Science and Technology, Pohang, Korea
xxxvi
International Advisory Board A. Alberti J.R. Anderson J.N. Armor R. von Ballmoos
T. Bein A.T. Bell H. van Bekkum G. Bellussi H.K. Beyer D.M. Bibby M. Btilow K.-J. Chao A. Corma E.G. Derouane J. Dwyer F. Fajula
F. Fetting E.M. Flanigen P. Gallezot D. Goldfarb L. Guczi X. Guo A.B. Halgeri W. H01derich J. van Hooff R.F. Howe T. Inui K. lone P.A. Jacobs K.-J. Jens
S. Kaliaguine
University of Ferrara, Italy Monash University, Victoria, Australia Air Products & Chemicals Inc., USA Engelhard Corp., Ohio, USA Purdue University, Ind, USA University of California, Berkeley, USA Delft University of Technology, The Netherlands Eniricerche, Milano, Italy Hungarian Academy t?fSciences, Hungary The New Zealand Inst. For Ind. Res. & Dev., New Zealand The BOC Group Technical Center, USA National Tsing Hua University, Hsinchu, Taiwan Valencia University of Technology, Spain University of Namur, Belgium University of Manchester, UK ENSCM, Montpellier, France Darmstadt University of Tech., Germany UOP, Tarrytown, USA Institut de Recherches sur la Catalyse, France Weizmann Institute of Science, Israel Hungarian Academy of Sciences, Hungary Chinese Academy of Sciences, China Indian Petrochemicals Corp., India RWTH Aachen, University of Tech., Germany Eindhoven University of Tech., The Netherlands University of New South Wales, Australia Kyoto University Kyoto, Japan Boreskov Inst. of Catalysis, Novosibirsk, Russia Katholieke Universiteit, Leuven, Belgium Statoil Petrochemicals and Plastics, Norway Universit~ Laval, Canada
xxxvii
J. Kiirger
Hungarian Academy of Sciences, Hungary Fritz Haber Institute, Max Planck Society, Germany University of Leipzig, Germany
V. Kaub,i~,
University of Ljubljana, Slovenia
H. Kessler
University of Haute Alsace, France Ryukoku University, Japan Woodbury, NJ, USA Helsinki University of Technology, Finland University of Twente, The Netherlands University of Kiel, Germany Worcester Polytech. Inst., USA Katholieke Universiteit Leuven, Belgium ETH-Zentrum, Ziirich, Switzerland ETH-Zentrum, Zi~rich, Switzerland Res. Inst. of Petroleum Processing, China The Nishi-Tokyo University, Japan CNRS, Villeurbanne, France Biosym Technologies Inc., USA University of Cape Town, South Africa Mobil R&D Corp., USA Tokyo Inst. of Tech., Japan University of Leipzig, Germany National Chemical Lab., Pune, India University of Edinburgh, UK University of Maine, USA Northwestern University, Ill., USA Eindhoven University of Tech., The Netherlands Gunma University, Japan Sophia University, Japan Russian Academy of Sciences, Russia University of California, Santa Barbara, USA University of Connecticut, Conn., USA University of Clark Atlanta, Ga., USA University of Tokyo, Japan
D. Kall6 H.G. Karge
M. Koizumi G.T. Kokotailo A.O.I. Krause J.A. Lercher F. Liebau
E. Ma J. Martens L.B. McCusker W.M. Meier E. Min S. Namba C. Naccache J.M. Newsam C.T. O'Connor D.H. Olson Y. Ono H. Pfeifer P. Ratnasamy L.V.C. Rees D.M. Ruthven W.M.H. Sachtler R.A. van Santen
M. Sato K. Segawa E.S. Shpiro G.D. Stucky
S.L. Suib R. Szostak T. Tatsumi
xxxviii J.M. Thomas R.P. Townsend J.W. Ward J. Weitkamp T.E. Whyte, Jr T. Yashima R. Xu K.I. Zamaraev M. Zi61ek
The Royal Institution of Great Britain, UK Unilever Research, Bebington, UK UOP, Brea, USA University of Stuttgart, Germany The PQ Corporation, Conshohocken, USA Tokyo Inst. of Tech., Tokyo, Japan Jilin University, Changchun, China Boreskov Inst. of Catalysis, Novosibirsk, Russia Adam Mickiewicz University, Poznan, Poland
Liaison to the 11 th IZC Y. Ono
Tokyo Institute of Technology, Tokyo, Japan
Members of the IZA Council Roland von Ballmoos
President
Jens Weitkamp
Vice President
Koos Jansen
Secretary
J. Michael Bennett
Treasurer
T. Bein, G. Bellusi, K.-J. Chao, H. Chon, T. Inui, H. Karge, L.B. McCusker, W.J. Mortier, D.E.W. Vaughan, T. Yashima, S.I. Zones
xxxix
Financial Support The Organizing Committee gratefully acknowledges support by the following institutions and companies: (As of August 30, 1996) Korea Science and Engineering Foundation, Seoul, Korea Korea Research Foundation, Taejeon, Korea LG-Caltex Oil Corporation, Seoul, Korea YukongLimited, Seoul, Korea SsangYong Oil Refining Co., Ltd., Seoul, Korea DaeLim Industrial Co., Ltd., Seoul, Korea AeKyung-PQ Advanced Material Co., Ltd., Seoul, Korea Samsung Fine Chemicals Co., Ltd., Seoul, Korea Korea General Chemical Corporation, Seoul, Korea Zeobuilder Co., Ltd., Seoul, Korea Isu Chemical Co., Ltd., Seoul, Korea Cosmo Industrial Co., Ltd., Cheong-ju, Korea
This Page Intentionally Left Blank
I.
Synthesis
This Page Intentionally Left Blank
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 1997 Elsevier Science B.V.
DIRECTED SYNTHESIS OF ORGANIC/INORGANIC COMPOSITE STRUCTURES
Galen D. Stucky, Qisheng Huo, Ali Firouzi, and Brad F. Chmelka Departments of Chemistry, Materials, and Chemical Engineering University of California, Santa Barbara, CA 93106, U.S.A.
Stefan Schacht, I. G. Voigt-Martin, and Ferdi Sehiith Institut ftir Anorganische Chemie, Frankfurt University, Frankfurt, Germany
1. INTRODUCTION The impact of the discovery of the synthesis of periodic mesoporous material using amphiphilic surfactants 1,2,3,4,5,6 is readily apparent from the demographics of papers that have appeared in this area, as illustrated in Figure 1 for MCM-41 publications. In addition to providing for the first time access to high surface area, monodispersed mesopores (lnm to > 10nm), the research has provided, in a more generic sense, a new synthesis paradigm on how to bring together spatially distinct, nanostructured organic and inorganic arrays into two- and threedimensional periodic, composite structures. Designed materials synthesis and properties based on the molecular level interplay of the kinetics and energetics of organic and inorganic domain and interface
200-,,,,
assembly are of vital interest to many areas including biomineralization, conducting and optical display polymer composites, chemical sensors, fine chemical and bio-catalysis, and the creation of composite phases with useful
mechanical
and
.o 150 .~ "~ ~ 100 "~, 50
....................
J
i
................
~ .................
~ .................
~ .................
................
~ .................
~.................
~ .....................................................
thermal
properties for insulation and packaging applications. It is clear that this discovery is a major entry not only in the "breakthrough" library of zeolite and molecular sieve syntheses, but also
o
....
1991
*
....
....
i
~
................
~................
. . . . . . . . . . .
,
1 9 9 2 1993 1 9 9 4 1995 1 9 9 6 1997 Year
Figure 1 MCM-41 publications as found by STN of the American Chemical Society. The value for 1996 is an extrapolation based on the first five months.
in advancing the field of materials synthesis in general. In this paper, I will focus on some observations concerning this composite synthesis paradigm. The presentation is admittedly only a very selected sampling, with some brief excursions into implications related to biomineralization and porous materials design at longer (10 nm to 1000nm) length scales. Much progress has been made 7,8,9,10,11,12,13,1't,15,16 in what has been appropriately described as the "lofty goal of molecular sieve synthesis by design ''17. For these microporous (<15/~) phases, organic-inorganic (OI) interactions dominate during synthesis while what is generally the kinetically slower inorganic condensation (II) progresses so that each structure directing agent (SDA) is encapsulated within a cage and/or pore opening with only secondary van der Waals organic-organic (OO) interactions found in the final product (Figure 2, OI > OO, II > OO).
Charge matching of the available charge/unit area of the organic with that of the I Inorganic Molecular Species ~
HO
+ n[~
~
~}
~
~ O ~ Organic ' ~ - ~ Molecules/Arrays X"
Interface .S~ssembly
N ~N
Ol > OO
-
O > Ol II "Biomimetics" ' ~ . . ~
/
%~-~o
.. ~ SEQUENr~L
FLMGROWTH
OO,II << OI
/
II > OO
~ ~'. ~~'
Organic/l~t~organic Array A.,sembly ~,~,~ ~ q~
Inorganic Polymerization On OrganicArrays MESOPOROtSTmN FILMS
/
/
EMULSION"TEMPLArING~' Silica Acid Synthesis
MOLECULAR SIEVES
~
~.MESOPOROI.S MATERIALS
//~v
Silica Base Synthesis
Inorganic Polymerization Figure 2 Schematic illustrating organic-organic (OO), organic-inorganic (OI) interface, and inorganic-inorganic (II) control of composite materials synthesis. The ordering processes kinetically and/or thermodynamically determined and also must include solvation and cosolvent effects
encapsulating inorganic is required. Water molecules of solvation play a key role in this assembly process, both in terms of solubilizing the SDA and as an entropic thermodynamic driving force provided upon release of the water molecules as inorganic-organic assembly takes place. Taking into account these various factors, rigid, bulky and relatively short (<10A for the longest axis) molecules with moderate hydrophobicity are the best SDAs for high-silica molecular sieve synthesis 17. If solvation and cosolvent effects are for the moment implicitly included in the OO, OI, and II assembly processes, an interesting variation in materials synthesis takes place if OO, II are both much less kinetically and/or thermodynamically favorable than OI (OO, II << OI in Figure 2). This permits the very clever sequential growth process developed by Mallouk 18,19, in which monolayers of organic, then inorganic, etc., composition can be deposited to give lamellar phases that can be structurally designed over more than 100 repeats. This spatial control of composite ordering makes it possible to alter, for example, every nth repeating unit or to create compositionally laddered Ionic
arrays. The two situations of particular interest in this discussion arise when OI > OO>Ilor
O O > O I , II.
Again
should
it
emphasized
that
be these
Covalent
I / o-
I
o-
~
Silicate Anion Oligomer
ordering processes can be Bilayer (Base Synthesis) kinetically thermodynamic determined.
and/or ally
,o,o. 0
0 \ O" NSi/ Si" O/ ~ O / \ O \ H
Direct Framework Surfactant Inclusion Figure 3 Strong ionic 1-6 and covalent 20-22 interface bonding in silica mesostructure assembly.
The first situation with OI > OO; OO,II << OI (Figure 3) suggests that one can expect the original organic array, if present, to be disrupted upon interaction with the inorganic species; and then to reorganize to a new configuration determined by kinetic and thermodynamic assembly parameters associated with the newly formed molecular or oligomeric organic-inorganic unit. The formation constant for OI is large in this case with strong OI bonding due to interactions such as multidentate binding and chelation, or the forces between highly charged inorganic/organic species. The same result is obtained by creating the organic-inorganic molecular or oligomeric unit using covalent bonding20,21, 22 (Figure 3) between the desired organic and inorganic moieties. This
situation has been recently experimentally confirmed by in situ studies of the base (pH > 9) synthesis of mesoporous silicate phases using colloidal silica and cetyltrimethylammonium cations as the surfactant amphiphile23,24; and by the use of covalently bound organosilicon or transition metal 25 organometallic precursors in mesoporous phase synthesis. The second situation (OO > OI, II Figure 2) means that an organized organic array controls the assembly and also defines the ultimate configuration of the composite phase. In fact, this is the basis for what has been described as a central tenet of biomineralization 26,27,28,29,30,31that states that nucleation, growth, and the final morphology of biominerals are determined by the existence of a preorganized assembly of organic molecules. "Biomimetic" approaches and modelling of biomineralization have relied on this paradigm for experimental design and have accordingly focused on the use of known stable organic arrays or stabilization through, for example, covalent attachment to a substrate or crosslinking of the organic groups. Some examples include (i) hollow submicro-diameter silica cylinders obtained by depositing silica onto phospholipid tubules 32, (ii) bulk inorganic iron oxide deposited on charged bio-lipid substrates 33, and (iii) ceramic thin film processing by deposition of bulk inorganic phases on surfaces functionalized with ionic organic surfactants 34. A well known biological example of a strongly bound organic array whose surface is used as a template for inorganic oxide aggregation is apoferritin. Since composite assembly is determined by the
Hydrogen Bonding
relative strengths of the thermodynamic driving forces and the relative rates of the kinetic processes, stabilized organic arrays do not have be used if the OI binding interaction is weak. An example is the tri-layer (S§
35 hydrogen
bonding shown in Figure 4 that is obtained by combining cationic surfactants with cationic silica species at acidic pH values below the aqueous silica isoelectric point (SIp)36,37. In those cases where one wishes to retain or only slightly
N
N
R/+/~R
R/I~+~
CI" I
CI-
CI- CIi
I
I
modify the organic morphology that exists in the absence of
silica S+ X - I + trilayer (Acid Synthesis)
the silica, the (S§ but not the (S§ 3 base synthesis structure directing approach has been demonstrated to be successful. Examples include the synthesis of mesoporous
Figure 4 Structure direction by hydrogen bonding36,37 in mesostructure synthesis.
thin films on inorganic substrates 38 and at the air-water interface 39, mesostructure templating using
preformed organic liquid crystals 4~ and in the creation of micron scale shaped mesoporous structures by using amphiphilic surfactants embedded in the oil phase of oil-water microemulsions or oil water interfaces (see below).
2. SURFACTANT CONSIDERATIONS It is important to emphasize that the two situations described above (OI > OO > II, and OO > OI, II) are two different synthetic strategies, that not unexpectedly lead to composite and porous materials that have distinctly different properties. These are further described later in this paper. The common denominator for both routes is the dominating role that the surfactants have in determining the overall structural symmetry of the final product, a role also reflected in the observation that the same relatively small group of space group symmetries are obtained for conventional amphiphilic, hydrophilic, and polymer-based surfactant systems even though the underlying compositions, molecular structures, chemical and physical properties differ substantially 4i,42. Among the structures41,43 that can be observed in surfactant or lipid containing lyotropic systems, hexagonal (2d, p6m) and lamellar phases are the two most common mesophases. Six lyotropic cubic phases, in which Pm3n and Ia3d are two typical mesophases, may be found in many surfactant and lipid systems (for a recent review, see reference 42). Several intermediate phases can be formed44,45. Similar systems follow the same succession of phases, but not all of the phases are always present. However, the formations of additional new lyotropic mesophases are also possible, as indicated by the fact that new cubic phases, which consist of micelles of type I (oil-in-water), appear to be present in some surfactant systems46,47,48, 49. However, at this time no clear conclusions about the exact nature of the new phases have been obtained since the quality of the diffraction data is not adequate to determine their structures. Inorganic mesophases with good long-range ordering quality and excellent stability are helpful in characterizing new liquid crystal-like structures. In the conventional charged surfactant-water mixture at a given composition and temperature, from a molecular point of view the micellar shape or packing of the surfactant is determined by a balance between three general types of free energy contributions. One is associated with the tendency of the alkyl chains to minimize their water contact and maximize their inter-organic interactions. The second involves the coulombic and dipolar interactions among the charged headgroups and their associated anions. This contribution determines the mean area-perhead-group, a 0, that is available to each surfactant head group in an aggregate. In most classical discussions of liquid crystal aggregates, the counter-ion of the surfactant is implicitly included in
a o. The third type of free energy contribution includes solvation energies that arise from the presence of water, alcohol, or organic molecules in the hydrophilic, intermediary hydrophobichydrophilic
"palisade",
and
hydrophobic regions. Because
the
synthesis
intermediates and final composite structure are a consequence of the organization of spatially distinct organic and inorganic nanophase regions, the structure can also be readily defined in terms of surfaces generated by the above collective interactions (Figure 5). The mathematical perspective of the structure becomes one based on a differential calculus description of the spatial continuum described by the
Figure 5 Cartoon illustrating hypothetical cross section of mean surface continuums defined by collective interactions of organic surfactants and inorganic species. Curvature is defined by incompatible local packing requirements and structure frustration. The resulting modulated structures may exist in "lamellar", "hexagonal", etc., structured phases.
surfaces rather than the usual matrix algebra that is commonly used to describe a structure consisting of a collection of atoms located at discrete points in spaceSO,51. In this description, the inorganic and organic arrays of these mesostructures meet at an interface surface. The interface curvature is energetically defined so as to optimize charge repulsion and van der Waals interactions, resulting in a minimal surface structure for MCM-48 52,53,54. Phase transitions are associated with changes in the curvature of interface and may be understood phenomenologically as a competition between the elastic energy of bending the interfaces and energies resulting from the constraints of interfacial and charge separation 55. The different entropic and interaction energies in the nanoscale organic, inorganic and interface regions result in structure frustration with incompatible local packing constraints that forbid an optimal geometry where the energy is everywhere minimized. The inorganic/organic structures therefore readily undergo structural changes or transformations 56 to relieve this stress through rotational displacements of the surfaces (disclination defect). This structural modification is experimentally driven by entropic changes in the organic array (temperature), silica polymerization which modifies local density and interface charge properties23,24, 52 or through coordinated solvent (water) and organic cosolvent displacement21,23,24,73.
Because the mesostructured composite phases are frustrated structures, it is not surprising that one finds phases that for example would be classified as lamellar from the standpoint of the relatively crude average structure given by low-angle X-ray diffraction, but in fact on closer inspection by electron microscopy are found to have in-plane modulation, and a periodic rippled lamellar structure 24. BET (Brunaer-Emmett-Teller) measurements of this calcined "lamellar" phase show a high surface area and monodispersed pores. Similarly, there is considerable evidence to expect 3,4,5,56,57 that the honeycomb p6m (MCM-41) structure defined by X-rays very likely includes structures with modulated pores such as those indicated in Figure 5. The implication is that it should be possible to kinetically tune not only pore dimensions3,4,5, 21,37,58 for structures such as MCM-41, but also wall and pore morphologies. The characterization of these modulated structures with periodic necking of the honeycomb pores in a structure like MCM41 requires careful TEM and organic guest absorption/desorption isotherm studies. It is particularly important to note that the symmetries determined by the Bragg peak positions of X-ray or even electron diffraction are best viewed as "average" symmetries and a careful analysis of the diffraction intensities, freeze fracture or cryomicroscopy, solid state NMR, and adsorption or desorption data are required in order to characterize the details of the cage, wall and pore structures. Because of the limited amount of scattering data available for materials with high void densities and large unit cells, diffraction modelling must be done on the continuum surface basis and not at the discrete atom level used for zeolite structure determination. The continuum surface model has been particularly useful in understanding structures and phase transformations of materials in which surfactants play a dominating role in determining the overall structural symmetry, and there is every reason to believe that it applies equally well to inorganic mesostructured composites. However, the chemist begins with precursors defined on a molecular basis so a key question is how to relate these surfaces to surfactant molecular structure. Fortunately several investigators have shown that it is possible to mathematically relate molecular size, charge, and shape to the more global surface curvature, bending energies and morphology5~
The classical and contemporary molecular description of surfactant organization
in amphiphilic liquid crystal arrays has been described in terms of the local effective surfactant packing parameter 59,60, g __ V/a J, where V is the total volume of the surfactant chains plus any co-solvent organic molecules between the chains, a o is the effective head group area at the micelle surface defined above, and 1 is the kinetic surfactant tail length or the curvature elastic-energy55. The interface surface bending energy can be written in terms of g, the actual surfactant packing
10 parameter adopted by the aggregating chains in the phase 5~
The counterion in this classical
model is not explicitly included. It is not immediately clear that this relatively simple molecular model can be used as a first approximation to explain and predict product structure and phase transitions for the inorganic mesostructures.
Our preliminary goal was a very pragmatic one, to determine whether the
molecular packing parameter model used in liquid crystal chemistry is useful in designing inorganic/organic composite mesostructures. In classical micelle chemistry, as the g value is increased above critical values, mesophase transitions occur. The expected mesophase sequence as a function of the packing parameter is : 43,50 Packing Parameter, g Mesophase Example These
1/3
1/2
1/2- 2/3
1
Cubic (Pm3n)
Hexagonal (p6m)
Cubic (Ia3d)
Lamellar
transitions
reflect
a g =
decrease in surface curvature from cubic (Pm3n)
through
lamellar.
For
V/a o i
i,,'j MCM-4 (Pm3n) ,!(P63/mmci!j (p6m) li ;:~
surfactants to associate in a spherical structure, the surface area occupied by the surfactants polar head group should be large.
~i(la3d) i~ i~ sBA-5 ~ SBA-4
!i (R3c)i-!
_~~
If the head groups are
base ~'cltalyzed ~ ' ~
permitted to pack tightly, on the other
I I
I
EinSteinbehavior| Cluster-like I i~ I
hand, the aggregation number will
r..-
Im3m? ti Vj SBA-1 ~SBA-7 i~ SBA-3 (Pm3n) ~(P63/mmc)~,.,~ (pSm~
increase, and rod or lamellar packing will be favored. The values of g (between 1/2 and 2/3) for cubic (Ia3d) phase
~,~
depend upon the volume fraction of surfactant chains 50.
I
For the systematic investigation of
the
formation
~
~ ~ i~
. ;Lamellarl L,i I
~
~,~
acid ~.* -------~ catalyzed i'i
~Li Su "~ be vi r
of mesoporous
materials, we selected a series of surfactants (Figures 7, 8), with and without organic additives, which favor a
Figure 6 Mesophase structures obtained by systematically varying V, ao and 1 of packing parameter.
11
Figure 7. Examples of surfactants used in this research III
IIII
I IIIII
Name
I
Structure Example
ALKYLAMMONIUM CnNR1R2R3 = CnH2n+ 1NR1R2R3 n = 10, 12-16, 19, 20, 22 R = H, Car~2m+ 1
cn3 cH3
m = 1, 2, 3
GEMINIAMMONIUM Cm-s-m CmH2m+IN(CH 3)2(CI-I2)sN(CH3)2CmH2m+I m - 12, 14, 16, 18, 20, 22 s - 2 -12
~
N+
| ~CH3 CH3
DIVALENTS URFACTANT
Cn-s-I CnH2n+1N(CH3)2(CH2)sN(CH3)3 n= 12, 13, 16, 18,20,22 s=2, 3, 6
~H3+
CH3 l+
. N . ~ , , ~ N ~ CH
HYDROXY- FUNCTIONALAMMONIUM CnH2n+IN(CH 3)m[(CH2)POH]3_m n = 16 m = O , 1,2,3 p = 0 , 1,2,3
[§ N~OH
~H3
CH3
BENZALKONIUM
CnH2n+1N(CH3)2(CH2)mC6H5 n = 14, 16, 18,20
m = 1,2,3
BI-CHAIN AMMONIUM CnH2n+ 1N(CH3)2CmH2m+I n = 12, 16, 18 m
cH3 = 2-6,
12, 16, 18
~
\cH3
cH3
~cH3
CnH2n+1[N(CH3)2]m(CI-I2)pSi(OCH3)3
l+
~i__OCH3
n = 14, 18
~H3
ORGANOSILANE
m=0, 1
p=0, 3
cn3
ZWlTrI~RIQN
CnH2n+IN+(CH3)2RXn = 12 - 20 RX-= sulfonate, carbonate,
OCH3
etc
~H~ (Crb.)3-SO3-
12 range of g values when used as structure directing agents to synthesize silica-based mesophases in different reaction conditions. An examination of a large number of surfactants synthesized in our laboratory, some of which are shown in Figure 8, coupled with a study of the effects of cosolvents, has confirmed that to a first and relatively good approximation the molecular packing parameter model can be used in a predictive way to generate structures analogous to those found in conventional liquid crystal chemistry (Figure 6) 36,37,21. For example, if one wishes to create a silica structure in which there is considerable wall curvature and possible
cage
structure,
R1
the
effective head group area can be
R2
R3
Product
R1
H CH 3 CH 3 SBA-3 (p6m) CH 3 CH 3 CH 3 SBA-3 (p6m) CH 3 CH 3 CiH 5 SBA-3 (p6m) I CH 3 CiH 5 CiH 5 SBA- 1 (Pm3n) R3 CiH 5 CiH 5 CiH 5 SBA-1 (Pm3n) Figure 8 Headgroup (ao) definition of mesostructure phase at pH values below the isoelectric point
modified by simple head group I_1. substitution (Figure 8) or with the C16H33 - - N ~ R 2 Cn_s_l surfactants (Figure 7). Cn_s_l surfactants
have
charge
and
density
high large
headgroups, and therefore favor globular micellar aggregates 47. Thus, in an acid media p6m hexagonal phase with symmetry Pm3n are obtained with methyldiethyl or triethyl substitution of the head group (Figure 8), but p6m structures result if dimethyl substitution is used with hydrogen, methyl or ethyl in the third position21,36, 37. Larger surfactants are required in basic media62, and have given unit cells as large as 180/~ on edge, with cell volumes of over 5.8 million/~3. BET data and preliminary modelling of the X-ray diffraction data of the silicate phase 63 suggest a cage structure for Pm3n, similar to those proposed for a conventional liquid crystal Pm3n phase by Sadoc 64 and for the much smaller
Figure 9 Proposed Pm3n structurc for mesosilicate (SBA- 1)
(13.4/~) high temperature unit cell of melanophlogite 65 (Figure 9). A second more subtle and variable way to fine tune the surfactant molecular shape and charge is by using multicharged oligomeric units. This has been done in our laboratory with bi(gemini), tri, tetra and polymeric cationic systems with considerable success. In this discussion,
13 we will briefly review the gemini and divalent surfactants (Figure 7). "Gemini surfactant", Cm.s_m, is a name assigned to a family of synthetic amphiphiles possessing, in sequence, a long hydrophobic chain, an ionic group, a spacer, a second ionic group, and another hydrophobic tail 66,67,68,69. The divalent quaternary ammonium surfactant, Cn_s_ l, may be considered as a end member of gemini surfactant or a highly charged large headgroup surfactant. These surfactants are particularly interesting from a fundamental point of view: their structure can be considerably modified by acting independently on the length and nature of either side chain and the spacer group. The relative positions and distances of headgroups of conventional mono(quaternary ammonium) surfactants are determined primarily by electrostatic interactions and also by the packing requirements of disordered alkyl chains. Formally, the Cm_s_m surfactants
may
be
considered
as
dimers
(double-headed)
of
the
two-chain
CmHEm+I(CHE)s/EN+(CH3)2 surfactants. For bis(dimethylalkyl-ammonium) surfactants, two quaternary ammonium head group species CmHEm+IN+(CH3)2 are chemically linked through an adjustable polymethylene spacer (CsHEs). The presence of the spacer makes it possible to fine tune the distance between the head groups and thereby control the effective head group size, a o , as a function of charge (for detail see reference 70). By this means we can change V/aol of a surfactant by adjusting its spacer length 69. We used the surfactants in this family as structure directing agents in order to synthesize a variety of silica-based mesophase products. Their structure directing behavior is similar to that in surfactant-water binary system 68,69,70,71 and give the structures expected for charge density matching. Small s (2 or 3) surfactants favor MCM-50, medium s (5 or 6) surfactants favor MCM41. C16.12_16 gives MCM-48 at both room temperature and high temperature (100~
while C12 -
12-12gives MCM-41 at room temperature. The latter observation illustrates the ability to fine tune with the individual tail lengths. Note that aqueous solutions of C 12-12-12 remain micellar over the entire range of composition and do not form lyotropic liquid crystal phases 68. This circumstance demonstrates the importance of the inorganic species in the cooperative structure directing mechanism for the concentration region in which the syntheses are carded out. If the tail of one of the two surfactant head groups is eliminated to give a Cn_s_ 1 molecular shape, the effective head group area relative to total hydrophobic tail volume (V) and length (1) is effectively doubled, greatly decreasing g, the packing parameter. This puts us back in the cage
14 side of the structural phases. While a phase with P63/mmc symmetry had not been previously reported for conventional liquid crystal phases72, we found that both liquid crystal surfactant and silicate phases with symmetry P63/mmc73,21 can be obtained with varying unit cell and cage sizes by using different chain lengths (n) of the form Cn_s_1 over a wide synthesis range (from cell parameter c = 77/~ for C12_3_1 to c = 108/~ for C20_3_1). SBA-2 (below SIP) and SBA-7 (base synthesis) with P63/mmc 3-d hexagonal symmetry are readily synthesized using divalent quaternary ammonium surfactants, Cn_s_1 (e.g., C 12-3-1, C 14-3-1, C16-2-1, C 16-3-1, C 16-6-1, C 18-3-1, C 18-6-1, C20-3-1) in both basic and acidic media. X-ray, AFM and TEM experimental results show that SBA-2 has 3-d hexagonal symmetry, space group P63/mmc (No. 194), and is derived from a hexagonal close packing of globular surfactant/silicate arrays 73. The crystal growth is plate-like and excellent for making thin films and membranes at either an air-water interface 74 or an oil-water interface75 with the sixfold axis normal to the sheet direction. As expected for this geometry, the unit cell parameter c/a ratio is about 1.62. After calcination, the large cage structured mesoporous silica framework remains. The structure directing agent in SBA-2 can be removed by calcination at high temperature (500-600~
This material is thermally stable up to 8000C. The
calcined SBA-2 has a N 2 BET surface area of 500-800 m2g 1. The N 2 adsorption-desorption isotherms is type IV with a H2 hysteresis for even small pore SBA-2 (< 25 A). Thus by systematically varying the surfactant molecular structure as prescribed by the simple packing parameter model, the structural phase space associated with conventional surfactants can be extended to silicate mesoporous structures (Figure 6). Undoubtedly other symmetries and fine details of the nature of the modulated versions of these structures will be forthcoming in the near future.
2.1
M I X E D SURFACTANTS21, 73
The effect of mixing unlike surfactants can be thought of as a simple average of two surfactant packing parameters. For example, a mixture of C16_12_16 and C16_3_1 is used in silicate mesophase synthesis. The products vary from MCM-48 to SBA-2 through MCM-41 as the fraction of C16_3_ 1 increases in the mixture. The mixture of C n-3-1 and CmTMA+ can result in the formation of good quality MCM-41. The MCM-41 easily gives five or more well defined XRD peaks. It is worth noting that high-
15 quality MCM-41 still can be obtained when n = 22, while single surfactant CmTMA § (m > 20) favors the lamellar phases and does not give MCM-41 at 100*C. The high-quality MCM-41 obtained using a mixture of surfactants is thermally stable and the calcined sample still has at least 5 XRD peaks. As for MCM-41 obtained from CnTMA § surfactant, small pore calcined materials are more hydrothermally stable than large pore ones. For example, MCM-41 calcined (at 500~
from
a C12TMA § synthesis system gives a good XRD pattern after 3 hrs heating in water at 100~ while a large pore (-- 55/~) material loses its structure under the same condition. High temperature calcination can increases the hydrothermal stability of these materials. The calcined (at 800~ large pore (~ 55 A) sample shows a clear MCM-41 XRD pattern (5 or more peaks) after 2 hrs heating in water at 100~ When a swelling agent (e.g., TMB) is introduced into this synthesis system, the product MCM-41 has a large unit-cell (dl00 > 60/~) and shows good XRD patterns (4 or more narrow peaks). Our synthesis results indicate the CnTMA § is a good, but not ideal structure directing agent for the formation of MCM-41, even though CnTMA § (e.g., C14TMABr ) itself can give a high quality lyotropic liquid crystal hexagonal phase with five or more sharp reflections 76. CnTMA + has only one charge per hydrophobic chain. More charges (from Cn.3_l, two charges per chain) in the surfactant headgroups apparent are more favorable for the formation of high quality MCM-41. We have found in general for all structural phases that we have investigated that it is possible to fine-tune the synthesis and the quality of the phase even further by using a mixed surfactant approach 73.
2.2 COSOLVENTS
Org an ic c o- s o 1v e n t s are particularly effective in controlling phase
ao
(
palisade ~ \ regi~ FI~-'~'
and interface geometry during the synthesis of both mesoporous inorganic solids 2,3,21,73 and lyotropic mesophases
headgroup
V/aol < 1/3
~ ~
P63/mmc (SBA-2) hydrophobic core
Figure 10 Solvation regions accessible to of surfactant-solvent binary systems77,78. molecules with different dielectric and polar The control comes from being able to properties. The example of g < 1/3 leads to hexagonal close packed geometries. "solvate" the interface head group,
16 palisade and hydrophobic regions associated with the organic surfactant arrays (Figure 10). When a hydrophobic, apolar solvent such as trimethylbenzene is added, it seeks the most hydrophobic region (Figure 11) which is at the tail end of the surfactant array and swells the micelle size. Both V and 1 are affected and the net result can be either a phase change and/or an increase in the effective pore or cage diameter. Thus, when trimethylbenzene (TMB) is added as a swelling agent relatively large pore size changes are observed. This approach has been used in large pore MCM-41 synthesis 2,3,37 and frequently, but not always, works for other mesostructure
phases. C16.3.1 gives SBA-2 (P63/mmc) with a = 62/~, c = 100/~ when TMB/TEOS = 1.1; without TMB, a cell with a = 54/~, c = 87/~ is obtained. In both of these examples, the result is as if a longer chain surfactant (increased 1) has been used to increase the pore size. However, phase changes in some instances are also induced. C16TMA+ favors MCM-41 over a wide range of reactant compositions. At moderately high pH values, if TMB is added, the MCM-41 is replaced by a lamellar phase suggesting that the increased surfactant tail volume is more important. If this lamellar phase sample is heated before silica condensation, it reverts back to the hexagonal trimethylbenzene
configuration 24.
1 increases
A suitable polar additive is able to enter the hydrophobic-hydrophilic palisade region (first
V/aol < 1/3 P63/mmc large cell (SBA-2) Figure 11 Addition of hydrophobic, apolar molecule to surfactant array.
few carbon atoms) of the micelle, with a relative increase in the volume of the hydrophobic core to form surfactant molecule aggregates with a lower curvature surfaces, e.g.,from sphere to rod. Thus when t-amyl alcohol, a polar additive, is added into the synthesis mixture at basic pH, the SBA2 product is replaced by MCM-41 (Figure 12). In the acid synthesis one can make SBA-1 (Pm3n) using C16H33N+(C2Hs) 3 as template without t-amyl alcohol, but SBA-3 (p6m) if t-amyl alcohol is used 21. In our experience the effect of addition of polar solvents is quite predictable and one can very effectively use this to generated desired phases.
~ ~ - ~
t-amyl alcohol increases
i73
J ~
~
Hexagonal (MCM-41)
Figure 12 Phase change from P63/mmc to MCM-41 induced by addition of polar alcohol as solvent
17
2.3 HYDROXY-FUNCTIONALIZED SURFACTANTS The hydroxyl group in the functional surfactant, CnH2n+IN+(CH3)2(CH2)mOH, decreases the hydrophobicity of the headgroup and the headgroup charge is more shielded by water of solvation79 (or silicate or other anions in solution), thus decreasing the affective cationic headgroup area a o. It therefore plays an important role in the entropic and enthalpic contributions of water organization to structure direction. The hydroxyl headgroup surfactants favor formation of mesophases with low surface curvatures such as p6m or lamellar due to the smaller effective a o 37. The product is a lamellar phase when C16H33N+(CH3)2(CH2)2OH is used, while a similar structured surfactant with a smaller headgroup, C16H33N+(CH3)2C2H5, gave only a relatively high surface curvature mesophase, MCM-41. A hydroxyl group in the hydrocarbon chain of the surfactant, e.g., g-substituted, C14H29CH(OH)CH2N+(CH3)3, also has a small effective headgroup surface area. A highly ordered lamellar silicate (to sixth order in Bragg reflections) is obtained by using C14H29CH(OH)CH2N+(CH3)3. In addition, high-angle diffraction peaks are observed that are characteristic of the hydrocarbon chain packing within organized surfactant layer structuresS~
82. The use of zwitterionic surfactants 36 and other functionalized head groups is a
promising area of investigation. 3. INORGANIC CONDENSATION The processes that drive the co-assembly of organic and inorganic units into a bicontinuous composite with spatially distinct organic and inorganic regions of nanostructure are strongly correlated, which
a priori
makes the separation of the various contributing factors difficult to
resolve. Certainly, one would expect that the process of mesophase organization would be strongly coupled to the time-dependent polymerization kinetics of silicate species at the inorganic-organic interface 24. In order to separate the effects of silica polymerization from the thermodynamics of mesophase self-assembly, we have used low temperatures and careful pH control (within 0.1 pH units) to control silica polymerization relative to the overall mesophase assembly. This approach has been used to show that in the absence of inorganic polymerization, these mesophases have liquid crystalline properties, similar to those of conventional aqueous lyotropic liquid crystal systems 24. In order to maintain these liquid crystal-like properties and optimize long range composite ordering during polymerization of the inorganic species, the inorganic and organic domains must
18 be able to reorganize on the same kinetic time scale into mutually compatible configurations. Khushalani et a183 have recently shown that at high temperatures (to 150 ~
and high pH values
(base synthesis mother liquor), one can very nicely get restructuring of the silica phase with a systematic increase in pore size. Under these conditions there is considerable organic thermal disorder, but the kinetic molecular volumes are still effective in generating monodispersed pores. Our approach 21 to optimizing long range ordering and structure has been to maximize the ordering influence of the organic surfactants during the initial polymerization of the silica walls, and then to reduce OI (the organic-inorganic interface interaction) so that the organic and inorganic self assembly are less strongly coupled I
(Figure 13). We chose to do this by 1) using low temperatures to minimize
0.5 to 2
organic disorder and short reaction times
I
hours Iwater 1 powder / o~ | ~product
to kinetically create only partially condensed silica frameworks that can structurally
follow
the
organic
organization and minimize interphase frustration 37, 2) "annealing" the air-dried product at room temperature to further
filter,
as-made sample
air .dry
25~
optimize long range order, and then 3) carrying out the silica polymerization in deionized water (pH -7). The latter greatly reduces the silica charge relative to what it would have in the mother liquor so that
add to H20 at pH - 7
............................. I~ ~7 days, 1 0 0 o c
I =o,., I I -"'" ' a ~ , , , , ~
final product
the organic-inorganic (OI) interactions are correspondingly reduced. This and the
Figure 13 Low temperature, low pH, synthesis of large pore mesostructured phases.
reduced solubility at that pH makes it possible to retain the templating introduced at low temperatures by the more organized organic to the partially polymerized silica. For example, when silicon alkoxides are used as starting reagents at a surfactant-to-silicon ratio o f - 0.1, pH -12, at room temperature or lower, polymerization of the silica begins and a precipitate rapidly forms 37. Silica polymerization of this partly condensed phase is interrupted by using short reaction times (0.5 to 2 hours depending on pH) and then "ripening" the filtered, air dried, solid product at room temperature for 6-10 hours. The solubility of amorphous silica
19
minimizes in water at approximately a pH of 7 to 8, and is more than an order of magnitude less than that at the normal mother liquor pH used in MCM-41 synthesis 84. The X-ray diffraction patterns for a large unit cell (a= 77A calcined) for these MCM-41 phases show seven to eight peaks (Figure 14) and retain their structure on calcination with about a 3% cell shrinkage. N 2 BET measurements reveal that this material has a BJH pore size of 60/~,
Surfactants
5.7 105
22-3-1 + C18TMA+ 100C a = 79.6/~
a pore volume of 1.6 cma/g and a surface O
area of 1086 mE/g. An important feature is that the apparent wall thickness based on
FWHM 0.2 2 0
cps
the absorption isotherm and X-ray data is 17 A,, which is substantially greater than
0
l_2 v,-I
that obtained from conventional MCM-41 synthesis (--8-10/~). This is not surprising in view of the reduced charge associated
01
....
I,,,
2
,k~l~,,~u
3
l-,.,
4
. ! ,_=.~;-~_:--t..
5
6
,
7
8
with the silica phase at the lower pH. Three well-distinguished regions of the adsorption
isotherm
are
noticed:
1.2 106 calcined at 500 C a = 77.0/~
monolayer-multilayer adsorption, capillary condensation, and multilayer adsorption on the outer surface. In contrast to N 2
cps
adsorption results 85,86 of MCM-41 with pore size less than 40/~, a clear type H1 hysteresis
loop
in the
adsorption-
desorption isotherm is observed and the capillary condensation occurs at high relative pressure, consistent with the large pore 87.
0 , ~ .... - : ! . - - . I . . . . 1 2 3 4 5 6 7 8 Figure 14 a) X-ray diffraction pattern after treatment ol room temperature prepared sample at 100 C with wate] at pH 7 and b) after calcination
In all cases that we have examined the treatment improves the structural ordering, however in some cases there is no significant expansion of the unit cell. In the above MCM-41 example, using the indicated mixed surfactants the as made cell has a cell dimension of 54.5/~. Treatment at 2 weeks at 100 ~ in distilled water, pH = 7, gave a cell of 78.2/~, and two additional weeks of treatment gave a cell of 79.6/L However, no significant expansion of the unit cell is observed for MCM-41 and MCM-48 containing a single CnTMA+ molecule, although seven to eight peak
20 diffraction patterns are obtained for the MCM-41 and about twenty peak patterns are generated with MCM-48. The kinetic matching of inorganic and organic ordering during assembly and silica polymerization is critical to the morphological, structural and property design of mesophase materials. 4. INTERFACE CONSIDERATIONS One of the early possible models suggested for mesostructured materials synthesis using surfactants was that of coating preassembled organic arrays with the inorganic phase and then assembling these coated organic arrays into a 3-d periodic structure 88,89. There are features of this model that make it attractive, it gives a direct explanation for the analogous symmetries of the silicate structures to those of liquid crystal chemistry, and it is consistent with what at that time was the paradigm for biomimetic synthesis: first create an organized organic array, and then condense an inorganic phase on the preorganized organic surface 28. At this time, there is convincing evidence 90,91,92 that while stable organized organic arrays are an important part of inorganic nucleation and phase formation in biomineralization, total control by such an array is an extreme condition that is never completely realized. Complete structural phase changes can in fact be induced by soluble proteins 91. Generally, mutually induced structural modifications of the organic and inorganic phases are required to create the higher-order complex microstructures. A more general biomimetic approach must take into account the dynamic balance of solvation, soluble proteins and other soluble organics, organic array assembly, inorganic polymerization and the corresponding interface chemistries. Nevertheless, using pre-organized organics to control morphology and nucleation is a potentially powerful approach to composite materials synthesis, particularly in terms of macroscale shaping, as a template that can be created with the required acid-base or molecular structure characteristics, and as a liquid support phase for bulk processing or synthesis. The concept of using an organized organic array as a template is a statement that the most important free energy and/or kinetic contribution to biphase composite formation is the organization of the organic array. Inorganic deposition and subsequent polymerization do not significantly perturb that array morphology. Several possible ways to approach that goal are 1) to strengthen the organic intraarray coupling by cross-linking; 2) to stabilize the organic array by interfacing it to an inorganic substrate and 3) to decrease the organic-inorganic (OI) interface interactions relative to the organicorganic (O-O) interactions (Figure 2). As far as 3) is concerned, using hydrogen bonding at the interface (Figure 4) rather than
21 ionic or covalent interface interactions (Figure 3) is obviously a step in the right direction. A neutral synthesis route using uncharged (dodecyl amine) or nonionic surfactants (polyethylene oxide) has been explored by Pinnavaia and co-workers93,94 in the near neutral pH range where silica charges on oligomeric silica species (pK a ~ 6 -7) are greatly reduced from that expected at high pH. This is an especially intriguing study since the question of how biosilicates such as diatoms are so exquisitely assembled in nature in this pH region remains unanswered. With neutral surfactants, the primary forces that drive the self assembly of the composite are hydrogen bonding, van der Waals and dipole interactions. Monodispersed porous structures have been obtained in these investigations; however long-range order of the pores is lacking. In this pH regime above the SIP (silica isoelectric point), the silica species are still slightly negatively charged with a relatively high condensation rate. Item 2) above can be achieved by covalent linking to the inorganic substrate 95, so that the patterning of the organic array and the orientation of the organic molecules are defined by the connecting sites to the substrate. In this case the organic array is strongly bound so that its integrity is preserved upon addition of other inorganic phases. Alternatively, cationic organic amphiphiles can be bound to the substrate either by ionic (e.g. mica) or charge
image
and
van
der
Waals
bonding
(graphite) 96,1~ In this case one generates organized, periodic arrays of hemicylinders of CTAB (cetyltrimethylammonium bromide) on graphite and cylinders on mica, in both cases with the long axis of the CTAB organic array parallel to the substrate
Figure 15 Hemicylinders of CTAB formed on graphite. Measurements were made using continuous flow AFM cell with surfactant concentration near CMC 1 concentrations 91.
surface 96 (Figure 15). The advantage of this approach over the covalent attachment of surfactants is clear in that the organic phase is given more freedom and is able to organize in a variety of known liquid crystal geometries. Can these organized organic arrays be used as templates for silica thin film mesostructure nucleation? The answer to this question is yes 97,98, but in the published examples to date only when the mesostructure is formed at pH values below the SIP 36,37 (Figure 4) with the weak OI hydrogen bonding and indirect structure direction. Like the surfactant, silica films prepared on mica have their pore directions oriented parallel to the film surface. Syntheses carried out at high pH values2, 3 are less likely to be successful because of the strong OI interface interactions that can
22 be expected to disrupt the organic array assembly. The divalent surfactants used to create the P63/mmc structures form more stable organic arrays, and are much easier to convert into mesostructured films, even at basic pH values 74. The same situation applies to oriented thin silica films prepared at the air-water interface. Beautiful flexible sheets of periodic mesostructured films with p6m or subgroup symmetry (SBA-3) and the channels parallel to the film plane are formed with silica chemistry carried out at pH values below the SIP 99, but similar thin films of the MCM-41 phase have not so far been successfully made at basic pH values. Similarly, Attard and co-workers attempts to use liquid crystal structural phases formed in concentrated solutions of surfactants as templates required low pH values below SIP 100. In these examples the mechanism does not necessarily involve the simple coating of the organic array, since the hydrolysis of the silicon alkoxide used as a precursor may change or initially disrupt the organic phase structure. The important point however, is that with a weakly interacting inorganic-organic interface, the thermodynamic and kinetic factors that gave the original organic assembly its structural properties are still dominant and can control the composite organization. It should be emphasized that the acid(below SIP) and base synthesized silica mesophases have little in common other than sometimes the same space group symmetry. They do not have the same composition since mesophase samples synthesized below the silica isoelectric point require a counter anion, generally a halide anion, for each surfactant molecule that is present. Terminal Si-Ogroups are protonated so that the bulk compositions of M41S and acid prepared materials (APM) made with the same surfactants are completely different in hydrogen and halide ion content. The ion-pair surfactants of the APM materials are readily removed by washing with distilled water/ethanol at --70 ~
since the wall charge is neutral or slightly positive. Removal of
surfactant from M41S samples requires ion exchange by refluxing with acidic ethanol because of the negatively charged terminal oxygen atoms. Absorption and desorption properties similarly differ substantially. The ultimate periodic symmetry is determined in both cases by the nanophase surfactant packing requirements, so that similar space group and lattice symmetries may be observed by x-ray diffraction transmission electron micrographs. However the Bragg peaks of the two phases for a given surfactant have clearly different diffraction intensities, indicating different pore and wall structure. As pointed out by Brinker and Scherer 101, silica hydrolysis below SIP results in Huggins or chain like polymerization while M41S silica polymerization conditions lead to Einstein or cluster
23 like configurations with extensive cross-linking so that different silica wall structures are expected for the acid and base synthesized structures. The difference in wall and pore structure is evident in BET absorption isotherm measurements that show that APM exhibit a very different sorption behavior from M41S samples with a step in the isotherm at appreciably lower P/P0 values (p, pressure) than samples synthesized from alkaline media with a similar lattice spacing. These data and diffraction results show that the acid silica walls are effectively thicker than those of the corresponding M41S phases prepared at pH values above 10. Nevertheless, BET surface areas calculated for such samples can be a factor of two higher, indicating the presence of micropores or highly ruffled pore surfaces. Even taking into account the limitations of BET analysis for such materials, the difference in the values obtained indicates a major difference in the pore and wall structure of APM and MCM-41 type materials.
5. MACROSCALE STRUCTURES WITH PERIODIC MESOPORES75,102 Organic-inorganic hybrids are ubiquitous in nature as biominerals and inherently offer many opportunities for the creation of new materials with unusual features. As diphasic structures, they can be made shaped and multifunctional. The key to the integration of the organic and inorganic components is to use low temperature chemistry to control the kinetics of assembly of the organic and inorganic components, and an integral part of that process lies at the interface between the spatially distinct organic and inorganic regions. The extent to which the organic and inorganic domains have properties and structure that are characteristic of the corresponding bulk phases depends on the strength of the interface inorganic-organic (IO) interaction. The significance of this in hybrid organic-inorganic materials design has been recognized by Clement 103, who divides hybrid organic-inorganic materials into two distinct classes. In Class I, only weak bonds (e.g., hydrogen, van der Waals) give cohesion to the whole structure. In Class II the organic and inorganic components are linked together by strong chemical bonds (e.g., covalent or ionocovalent bonds). This differentiation is fundamental to the property design of organic-inorganic hybrid materials for a wide range of applications. The differences in the properties of the mesostmctured silica phases that are generated by base synthesis, an example of Class II (Figure 3) organic/hybrid materials, from the Class I silica mesostructures made at pH values below the aqueous silica isoelectric point (Figure 4) support this view. As another example of how weak OI interactions can be applied to materials synthesis, we have used the combination of long-range oil-in-water emulsion and oil-water interface physics with shorter-range molecular assembly of silica and surfactants at the emulsion interface to create
24 ordered composite mesostructured phases that are also macroscopically structured and shaped75,102. Microemulsions and emulsions occupy a special place in the hierarchy of structures, in that their formation involves long-range forces with an energy of assembly, including shape fluctuations and interaggregate interactions, about the same as the thermal energy kT 104. Hydrodynamic, long-range forces can therefore be used to define emulsion morphology and the configuration of the emulsion oil-water interface. With self-assembly energies approaching kT, emulsions are often close to the limit of stability. In order to stabilize the emulsion phase, current commercial practice is to use surfactants. Thus, oil-in-water emulsions are stabilized by short range (10 -7 m), and relatively weak van-der Waals interactions between the hydrophobic tails of amphiphilic surfactants and the emulsion organic phase. This combination provides an ideal starting place for periodic mesostructured inorganic phase synthesis. If an oil-in-water interface is used as an inorganic growth medium with the growth direction into the aqueous phase, morphological control of the resulting inorganic/organic composite assembly can be achieved at micron and longer length scales. The morphology of the preorganized organic liquid phase is preserved during the periodic mesopore synthesis by working at acid pH below the SIP, making use of the weak organic-inorganic interactions shown in Figure 4 (S§
+ structure directing during synthesis).
Mesoporous silica fibers, diatom-like hollow
spheres, and thin films are some of the morphologies that have been synthesized using conventional emulsion hydrodynamics, p6m, P63/mmc and Pm3n wall structures can be made. Surface areas of the calcined products are greater than 1000 m2/gm with narrow pore distributions. The results are of interest with regard to packaging, perhaps in slow-release applications. Hydrodynamics and emulsion technology are highly advanced commercially and the connection of this to inorganic/organic hybrid materials synthesis presents new process possibilities for hierarchically structured composite phases. 6. S U M M A R Y
As indicated in the Introduction, the discovery of periodic mesoporous structures is a major advance in composite organic-inorganic materials synthesis. In the short presentation given here, much has been omitted concerning our growing knowledge in this very rapidly expanding field, but hopefully it is clear that the organic and inorganic phases can be synergistically integrated in a pre-designed fashion. It also seems possible to carry out this co-assembly of organic and inorganic species using fluid mechanics to define macroscale shapes. Of particular interest are the
25 increasingly sophisticated studies of the dynamic processes and organic-inorganic interfaces that are present in biomineralization. These investigations seem to be converging with those being made on synthetic hybrid organic-inorganic materials, and the result could be an even more rapid evolution of technological applications. The confluence of these two trains of thought is certain to enhance an already exciting era in materials synthesis. REFERENCES
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26 24. Firouzi, A.; Stucky, G.D.; Chmelka, B.F. Synthesis of Microporous Materials, M.L. Occelli and H. Kessler, Eds., Marcel Dekker (in press). 25. Antonelli, D.M.; Ying, J.Y. Angew. Chem. Int. Ed. Engl. 1995, 34, 2014. 26. Weiner, S. Crit. Rev. Biochem., 1986, 20, 365. 27. Weiner, S.; Traub, W. Phil. Trans. Royal Society of London, 1984, B304, 428. 28. Mann, S.; Archibald, D.D.; Didymus, J.M.; Douglas, T.; Heywood, B.R.; Meldrum, F.C.; Reeves, N.J. Science, 1993, 261, 1286. 29. Mann, S. Nature, 1993, 365, 499. 30. Archibald, D.D.; Mann, S. Nature, 1993, 364, 430. 31. Heywood, B.R.; Mann, S. Adv. Materials, 1994, 6, 9. 32. Baral, S.; Schoen, P. Chem. Mater, 1993, 5, 145. 33. Archibald, D. D.; Mann, S. Nature, 1993, 364, 430. 34. Bunker, B.C.; Rieke, P.C.; Tarasevich, B.J.; Campbell, A.A.; Fryxell, G.E.; Graff, G.L; Song, L.; Liu, J.; Virden, J.W.; McVay, G.L. Science, 1994, 264, 48. 35. This notation refers to the solution species used in the synthesis, S = surfactant or surfactant precursor, X = the acid anion which is usually C1-or Br- in syntheses carried out below the aqueous silica isoelectric point (SIP) (pH ~2), and I§ or I- is used to designate the charge of the solution inorganic species that are present. Neutral silica species are also present, with a concentration that varies with pH; however in our experience the condensation rate and the quality of the mesostructure that is formed varies inversely with the neutral silica concentration in the pH region immediately below the SIP. 36. Huo, Q.; Margolese, D.I.; Ciesla, U.; Feng, P.; Gier, T.E.; Sieger, P.; Leon, R.; Petroff, P.M.; Schtith, F; Stucky, G.D. Nature, 1994, 368, 317. 37. Huo, Q.; Margolese, D.I.; Ciesla, U.; Demuth, D.G; Feng, P.; Gier, T.E.; Sieger, P.; Chmelka, B.F; Schtith, F; Stucky, G.D. Chem. Materials, 1994, 6, 1176. 38. Yange, H.; Kupermann, A.; Coombs, N.; Mamiche-Afara, S.; Ozin, G. Nature, 1996, 379, 703. 39. Yang, H.; Coombs, N.; Sokolov, I.; Ozin, G.A. Nature, 1996, 381,589. 40. Attard, G.S.; Glyde, J.C.; Goltner, C.G. Nature,1995, 378, 366. 41. Tiddy, G.J.T. Physics Reports, 1980, 57, 1 42. Luzzati, V.; Vargas, R.; Mariani, P.; Gulik, A.; Delacroix, H. J. Mol. Biol., 1993, 229, 540. 43. Henriksson, U.; Blackmore, E.S.; Tiddy, G.J.T.; Soderman, O. J. Phys. Chem., 1992, 96, 3894. 44. Husson, F.; Mustacchi, H.; Luzzati, V. Acta Crystallogr., 1960, 13, 668. 45. Hagslatt, H; Soderman, O; Jonsson, B. Liquid Crystals, 1994, 17, 157. 46. Jahns, E.; Finkelmann, H. Colloid Polymer Sci., 1987, 265, 304. 47. Hagslatt, H.; Soderman, O.; Jonsson, B. Langmuir, 1994, 10, 2177. 48. Kratzat, K.; Finkemann, H. Colloid & Polymer Science, 1994, 272, 400. 49. Gulik, A.; Delacroix, H.; Kirschner, G.; Luzzati, V. J. Phys. H, 1995, 5, 445. 50. Hyde, S.T. Pure and Applied Chemistry, 1992, 64, 1617. 51. Hyde, S.T.J. Phys. Chem., 1989, 93, 1458. 52. Monnier, A.; Schtith, F; Huo, Q.; Kumar, D; Margolese, DT; Maxwell, R.S.; Stucky, G.D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B.F. Science, 1993, 261, 1299. 53. Stucky, G.D.; Monnier, A.; Schtith, F.; Huo, Q.; Margolese, DT; Kumar, D.; Krishnamurty, M.; Petroff, P.; Firouzi,A.; Janicke, M.; Chmelka, B.F. Mol. Cryst. Liq.Cryst., 1994, 240, 187. 54. Alfredsson, V.; Anderson, M. W. Chem. Mater, 1996, 8, 1141.
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28 Speck, J.S.; Stucky, G.D. Chem. Mater., 1996, 8, 679. 92. Falini, G. et al., Science, 1996, 271, 67. 93. Tanev, P.T.; Pinnavaia, T.J. Science, 1995, 267, 865. 94. Bagshaw, S.A.; Prouzet, E.; Pinnavia, T.J. Science, 1995, 269, 1242. 95. Bunker, B.C.; Rieke, P.C.; Tarasevich, B.J.; Campbell, A.A.; Fryxell, G.E.; Graff, G.L.; Song, L.; Liu, J.; Virden, J.W.; McVay, G.L. Science, 1994, 48. 96. Manne, S.; Cleveland, J.P.; Gaub, H.E.; Stucky, G.D.; Hansma, P.K. Langmuir, 1994, 10, 4409. 97. Yang, H.; Kuperman, A.; Coombs, N.; Mamiche-Afara, S.; Ozin, G.A. Nature, 1996, 379, 703. 98. Aksay, I.A., et al., Materials Research Society Meeting; Boston, Massachusetts, November 1994; European Science Foundation Symposium on Biomineralization; Granada, Spain, September 1995; International Symposium on Synergistic Synthesis of Inorganic Materials, Tagungsst~itte Schloss Ringberg der Max-Planck-Gesellschaft, March 1996. 99. Yang, H.; Coombs, N.; Sokolov, I .; Ozin, G.A. Nature, 1996, 381,589. 100. Attard, G.S.; Glyde, J.C.; G61tner, C.G. Nature, 1995, 366. 101. Brinker, C.J.; Scherer, G.W.J. Non-cryst. Solids, 1985, 70, 301. 102. Schacht, S., Diploma Thesis, Mainz, 1995. 103. Sanchez, C.; Ribot, F. New. J. Chem., 1994, 18, 1007. 104. Israelachvili, J. Colloids and Surfaces A: Physicochemical and Engineering Aspects 1994, 91, 1. 105. Manne, Srinivas; Gaub, Hermann E. Science,1995, 270, 1480.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
29
Incorporation and Stability of Trivalent Cations in Mesoporou_s Silicas Prepared using Primary Amines as Surfactant S. Gontier and A. Tuel Institut de Recherches sur la Catalyse. C.N.R.S. 2, av. A. Einstein 69626 Villeurbanne Cedex France
Abstract A series of trivalent metal (A13+, Ga 3+, Fe 3+, B 3+) containing mesoporous silicas (Me-MS) have been synthesized using hexadecylamine as organic templating surfactant. All materials possess mesopores of about 37/~ diameter and surface areas above 900 m2/g. A solvent extraction has been used to remove the template from the solids. As compared to a conventional thermal treatment, this procedure preserves the mesopore structure and the coordination of the cations. Extracted samples are thermally stable and can be calcined in air at high temperature without observing changes in the cation coordination.
1. I N T R O D U C T I O N The family of silica-alumina based mesoporous molecular sieves M41S has received considerable interest over the last years because of their praticularly attractive characteristics like very high surface areas and regular mesopores whose diameter can be varied between 20 and 100/~ [1]. MCM-41, the well-known hexagonal member of this family is usually prepared with cetyltrimethylammonium (CTMA) cations and possesses mesopores in the 35-40/~ range [2]. As for zeolites, several trivalent cations can be incorporated in MCM-41, whose composition can be varied within a quite large domain of Si/Me ratios. Recently, Tanev et al. [3] have reported the synthesis of Hexagonal Mesoporous Silicas (HMS) by a neutral templating route using primary alkylamines in C 8 to C18 as surfactant. These materials are very similar to pure silica MCM-41, but differ by the arrangement of the mesopores. The neutral templating route offers several advantages with respect to the conventional preparation using ammonium cations. In particular, the synthesis is performed at room temperature and the template can be removed by ethanol extraction. The removal of organics by solvent extraction is not only interesting from an environmental point of view but it also preserves the mesoporosity of the samples, which is not always the case upon thermal treatment at high temperature
[3].
30 In the present paper, we report the synthesis of various trivalent cations (A13 +, Ga 3+, Fe 3+ and B3+) containing mesoporous silicas using hexadecylamine as surfactant. Template-free materials were obtained either by calcination in air at 650~ or by a solvent extraction. The influence of both procedures on the preservation of the mesoporosity and on the nature of the trivalent metal coordination is discussed.
2. E X P E R I M E N T A L In a typical synthesis, a solution containing 1 tool of tetraeth)~l' orthosilicate (TEOS), 6.5 mol of ethanol and 1 tool of isopropyl alcohol is mixed to a second solution containing 0.3 tool of hexadecylamine in 36 mol H20. Depending on its nature, the trivalent metal precursor is introduced either in the first solution (aluminium isopropoxide, tributyl borate) or with the amine (aluminium nitrate, gallium nitrate, iron nitrate or boric acid). The gel is vigorously mixed at room temperature for about 30 min and aged under static conditions for 12 h. The solids are then recovered, washed abundantly with distilled water and air-dried. For the solvent extraction of the organics, 1 g of dried solid is dispersed in 100 ml ethanol containing 1 g NaC1 and the suspension is refluxed for 1 h. The template-free samples are then dried at 80~ for 12 h. Samples are characterized using X-ray diffraction (CGR Theta 60 diffractometer using Cu Ka radiation), N 2 adsorption/desorption (Catasorb apparatus) and Solid State NMR (Bruker MSL 300). EPR spectra are obtained on a Varian E9 spectrometer. Chemical analysis are performed by atomic absorption after solubilization of the samples in HF-HC1 solutions.
3. RESULTS AND DISCUSSION The chemical composition of the various samples is given in Table 1. For all samples (except B-containing materials) the amount of metal in the solid corresponds approximately to that introduced in the synthesis gel. As a general trend, the yield in solid decreases with the metal content. The case of boron is interesting as it is the single example where the boron content is lower in the solid than in the gel, particularly for samples prepared with low Si/B ratios. Moreover, it is possible to prepare samples with low Si/B ratios in relatively high yields. Comparison of samples 1 and 2 or 14 and 15 shows that the nature of the precursor does not greatly influences the metal content in the final product. The maximum metal incorporation-is approx, the same for A1, Ga and Fe and corresponds to Si/Me ratios of about 10 in the solid phase. When syntheses are performed with lower ratios in the gel, the yield in solid is usually very low and spectroscopic characterization shows that as-synthesized samples contain non-framework octahedrally coordinated species.
31 Table 1 Chemical composition of the different samples Si/Me No
Sample
Gel
Product
Yield (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
A1-MS(100) A1-MS(100) a A1-MS(50) A1-MS(20) A1-MS(15) Ga-MS(100) Ga-MS(50) Ga-MS(20) Ga-MS(15) Fe-MS(100) Fe-MS(50) Fe-MS(20) Fe-MS(15) B-MS(100)_ B-MS(100) ~ B-MS(50) B-MS(15) B-MS(3) B-MS(l)
100 100 50 20 15 100 50 20 15 100 50 20 15 100 100 50 15 3 1
88 97 38 21 8 86 48 14 13 85 55 18 15 150 166 95 27 17 7
86 89 85 63 50 92 83 56 30 89 81 63 48 86 88 82 97 96 81
aThe sample was prepared with aluminium ethoxide, bThe sample was prepared using boric acid. As-synthesized samples are characterized by a single broad X-ray line around 2.2 ~ (20), as already reported for HMS [4]. After calcination in air at 650 C, all samples exhibit high surface areas but Horvath-Kawazoe pore size distributions show that the mesopore system partially collapses upon thermal treatment, particularly for samples containing high metal contents (Table 2). This was also confirmed by X-ray diffraction. Solid state NMR characterization of as-synthesized samples shows that A13+, Ga 3 + and B 3 + are tetrahedrally coordinated, at least for samples with Si/Me > 10. For low A1 contents, spectra of both as-synthesized and calcined samples show a single line at-about 52 ppm, as already reported for MCM-41 [5] (Fig. 1). For high A1 contents, the spectra of calcined samples show an additional peak around 0 ppm attributed to octahedrally coordinated A1 species resulting from a partial removal of framework aluminium. This removal probably occurs during the collapse of the mesopore structure.
32 Table 2 Characteristics of various samples Si/Me No
Sample
Gel
Product
S(m2/g)
~p(/~)a
1 4 5 7 11 14 17 18
AI-MS(100) AI-MS(20) AI-MS(15) Ga-MS(50) Fe-MS(50) B-MS(100) B-MS(15) B-MS(3)
100 20 15 50 50 100 15 3
88 21 8 48 55 150 27 17
1215 (1166) 983 (1079) 1252 929 700 (935) (1056) 1237 (935)
30 (36) 32 (37) 22 28 22 (37) (36) 30 (38)
,
a~p is the pore diameter obtained from N 2 isotherms. Valuesbetween parentheses have begn obtained on solvent extracted samples.
52
52
I
A (
CALCINATION
I
~ 650~
m
"
o-'
o'
, 6o
5 0'
o' - 5 0
ppm/Al(H20]8 s§ Fig. 1 27A1 NMR spectra of samples 1 (a) and 4 (b) before (left) and after (right) calcination in air
The 11B NMR spectra of B-MS materials show a single line at -1.5 ppm, characterisitc of B3+ in tetrahedral sites [6]. Upon calcination, the spectrum is modified and a second resonance, assigned to BO 3 units, is observed (Fig. 2). The 71Ga NMR spectra of Ga-MS materials show a broad line at 167 ppm, already observed on Ga-containing zeolites and assigned to tetrahedrally coordinated Ga species [7]. The EPR spectra of FeMS solids are very similar to those obtained on Fe-containing zeolites and show a sharp peak at -~+ge-- 4.3 usually attributed to F cations in a tetrahedral coordination [8].
33
-1.5
I
= 0.24
6
I
_
-
-
-
~
.
.
.
.
(b)
(a)
40'
'
2 "0
'
0'
. - 2. 0 . . - 4 0
ppm/Et2OBF3
Fig. 2. 11B NMR spectra of sample 18 as-synthesized (a) and calcined (b).
'60
-8
-100 -1 0 ppm/TMS
-140
Fig. 3. 29Si NMR spectra of sample 3. As-synthesized (a), extracted (b) and calcined in air (c)
29Si MAS NMR spectra of as-synthesized samples show 3 distinct lines at -90, -98 and -110 ppm, attributed to Q2, Q3 and Q4 species, respectively. After calcination, the spectrum is less well resolved and a deconvolution shows that approx. 50 % of the silanol groups have been removed with respect to as-synthesized products (Fig. 3). For pure silica materials, Tanev et al. [3] have reported that washing the mesoporous silica with boiling ethanol resulted in the total removal of the amine from the mesopores. That could be possible because of the weak interactions (hydrogen bonding) between the neutral organic micelle system and the inorganic framework. However, when trivalent cations are tetrahedrally coordinated in a silica matrix, they create a defect charge that has to be compensated by cations. When no cations are present in the gel, the charge is balanced by templating molecules, usually tetraalkyla .mmonium cations. We have first followed that recipe, i.e. the dried samples were refluxed in ethanol for about 1 h. The procedure removes nearly all the organics, as evidenced by the strong decrease of specific absorption bands in i.r. spectra. However, absorptions characteristic of the template are still present in the i.r. spectrum after 3 ethanol extractions. The 13C
34 NMR spectrum of the extracted sample is quite similar to that of as-synthesized samples but shows additional bands, particularly one at 41 ppm/TMS, attributed to carbon atoms directly bonded to primary ammonium cations [9]. These cations, that are not removed by ethanol extraction, are more likely in the proximity of Me 3+ cations and serve to maintain the electric neutrality of the samples. Therefore, the incorporation of trivalent cations in mesoporous silicas necessitates the protonation of a small amount of the primary amine. We thus tried to add inorganic cations to the extraction solvent to exchange primary ammonium cations. The procedure was similar to that previously described except that NaC1 was preliminary dissolved in ethanol (lg/100 ml). I.r. and NMR spectroscopies show that all organics are then removed, even in samples containing high metal contents. However, new absorptions are observed in the i.r. spectrum around 2900 cm -1, characteristic of Si-O-C2H 5 species formed after partial esterification of the samples, which was confirmed by a significant decrease of the OH band at 3745 cm "1. Chemical analysis of the samples show that the extraction does not modify the composition of the samples. After removal of the template, all samples exhibit high surface areas and pore sizes of about 37,1. (Table 2). 52
i
(~
1
I
50
I
0
I
-50
ppm / AI (H20)O*
Fig. 4. 27A1 NMR spectra of sample 4. As-synthesized (a), calcined in air (b), extracted (c) and extracted and calcined in air at 500~ (d).
35 Fig. 4 compares the 27A1 NMR spectra of sample 4 as-synthesized, calcined at 650~ in air and submitted to the solvent extraction. As clearly demonstrated in the figure, the solvent extraction preserves the coordination of AI3+ , as n o octahedrally coordinated species are detected. The spectrum is very similar to that of the assynthesized sample. Moreover, the spectrum is unchanged when the solvent extracted sample is submitted to a calcination in air at 500 ~ which confirms that-the formation of octahedrally A1 species more likely occurs during the decomposition of the organics at high temperature. Similarly, the liB NMR spectra of solvent extracted B-MS samples are very different from those of the corresponding calcined samples (Fig. 5).
-1.5
I
'
40
I
20
I
0
I
!
-20
-40
.
ppm/Et2OBF 3
Fig. 5. llB NMR s~ectra of sample 18. As-synthesized (a), extracted (b) and extracted and calcined at 500 C in air (c).
Only one line is observed, as for as-synthesized samples, and characterizes B(OSi)4 units. As for aluminium containing materials, a subsequent calcination does change the NMR spectrum. EPR spectroscopy shows that the solvent extraction also preserves the coordination of Fe 3 + cations in mesoporous silicas. EPR spectra are identical to those of as-synthesized samples and not modified by a subsequent calcination in air.
36 The 29Si NMR spectra of solvent extracted samples are strictly similar to those of as-synthesized samples (Fig. 3). The fraction of silanol groups are the same as for starting materials, thus confirming once more the preservation of the framework. However, a small amount of the species observed around -100 ppm are probably Si-OC2H 5 moities due to the partial esterification of silanol groups during the extraction.
4. CONCLUSION We have shown that trivalent metal containing mesoporous silicas could be prepared using a primary amine as surfactant molecule. The physical properties of the materials, in particular their surface area, did not significantly change with the amount of metal incorporated. Following this recipe, mesoporous silicas containing tetrahedrally coordinated cations could be synthesized with Si/Me ratios as low as 10 without observing the presence of octahedral species. The totallity of the organics could be removed from the mesopores using a solvent extraction. The procedure preserved not only the mesoporosity of the materials but also the coordination of the trivalent cations. Extracted samples were thermally stable and could be calcined in air at 500~ without modification of the cation coordination. This calcination had the advantage to remove ethoxy groups bonded to the silica framework and formed upon the extraction process.
5. R E F E R E N C E S
C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli ande J.S. Beck, Nature, 359 (1992) 710. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. P.T. Tanev and T.J. Pinnavaia, Science, 267 (1995) 865. P.T. Tanev, M. Chibwe and T.J. Pinnavaia, Nature, 368 (1994) 321. A. Corma, V. Fornes, M.T. Navarro and J. Perez-Parient6, J. Catal., 148 (1994) 569. A. Sayari, I. Moudrakovski, C. Danumah, C.I. Ratcliffe, J.A. Ripmeester and K.F. Preston, J. Phys. Chem, 99 (1995) 16373. Y.X. Zhi, A. Tuel, Y. Ben Taarit and C. Naccache, Zeolites, 12 (1992) 138. D.H. Lin, G. Coudurier and J.C. Vedrine, in P.A. Jacobs and R.A. Van Santen (Eds), Zeolites: Facts, Figures and Future, Elsevier Science Publishers, B.V. Amsterdam, 1989, p. 1431. G. Boxhoonn, R.A. Van Santen, W.A. Van Erp, G.R. Hays, N.C.M. Alma, R. Huis and A.D.H. Clague, Proc. 6th Int. Zeolite Conf. (Reno, 1983), Butterworth, Guilford, 1985, p. 694.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
37
Synthesis of lamellar aluminophosphates via the supramolecular templating mechanism Abdelhamid SAYARI Department of Chemical Engineering and CERPIC, Universit~ Laval, Ste-Foy, Qc, Canada G1K 7P4. The liquid-crystal templating approach for the synthesis of mesostructured materials was extended to aluminophosphates. Long chain primary and tertiary amines were used as templates. The molar gel composition was varied in a systematic manner over a wide range. Samples were thoroughly characterized using XRD, TEM, TGA, and 3~p, 27AI, 15N and lsC solid state NMR. Several lamellar phases with doo~distances in the 2 to 4 nm range were obtained. However, no three dimensional structures were detected. The gel composition was found to have a strong effect on the connectivity of aluminum and phosphorus in the final "crystalline" phase, as well as on their doo~ distances. TEM showed that some samples exhibit extended areas with unique structural features. They consisted of coaxial cylinders of alternating inorganic aluminophosphate material and organic surfactant bilayers, all wrapped around a central rodlike micelle. Such coaxial cylinders had an overall diameter of ca. 150 nm. They were aggregated into a hexagonal-like structure.
1. INTRODUCTION The crystalline mesoporous materials designated as M41S [1] have been for the last few years the subject of increasing attention. These materials are prepared hydrothermally via a supramolecular templating technique in the presence of surfactants. Synthetic methods using anionic, cationic, gemini or neutral surfactants, under either very basic or strongly acidic conditions [2-4] were developed. Thermally stable structures, particularly the so-called MCM-41 hexagonal structure have promising applications as catalysts and as advanced materials. Potential catalytic applications of such materials were reviewed recently [5]. Early investigations focussed on silicate and aluminosilicate materials [1]. Further work dealt with the incorporation of other metal cations such as Ti [6], V [7] and B [8] into MCM-41 silicates. In addition, Huo et al. [2] first reported on open-structure networks of a number of metal oxides like W, Sb, Zn, Pb, Mg, AI, Mn, Fe, Co, Ni and Zn oxides. Most of these oxides exhibited lamellar structures, except for W (hexagonal and lamellar), Sb (hexagonal and cubic) and Pb (hexagonal and lamellar) oxides. None of these oxide mesophases including the hexagonal phases was stable upon calcination. More recently, we were able to synthesize lamellar and hexagonal ZrO2
38 [9] and to stabilize the hexagonal phase [10]. Stable hexagonally packed mesoporous titania was also synthesized [11]. Abe et al. [12] prepared hexagonal vanadiumphosphorus oxides, but no information regarding their thermal stability was provided. Aluminophosphates (AIPO4) are crystalline microporous materials prepared hydrothermally, mostly in the presence of amine templates [13]. Several AIPO4s were also prepared using linear alkylene diamines [14] or cyclic diamines [15]. Recently, we extended the so-called liquid crystal templating mechanism to the synthesis of lamellar AIPO4s with d-spacings in the nanometer range [16,17]. Lamellar AIPO4s prepared in the presence of surfactants were also the subject of two other reports [18,19]. In this paper we present an overview of our findings with particular emphasis on samples prepared with the following gel composition P2Os : 0-2 AI203 : C~2H2sNH2 : 60 H20. 2. E X P E R I M E N T A L
Several series of AIPO 4 materials were prepared hydrothermally using the gel composition: x P205 : Y AI208 : z R-NR' 2 : w H20, where x = 1.0 (or 0 for P free samples), y = 0 to 2.0, z = 0.125 to 2.0 and w = 60 to 300. The template R-NR' 2 was a primary (R' = H) or a tertiary (R'= CHs) amine with a long alkyl chain (R = CnH2n§ with n = 8 to 16). However most samples were prepared in the presence of dodecylamine. These samples will be referred to as AIPO4-x:y:z:w. The following is a typical synthesis procedure of a AIPO4 sample with a molar gel composition: P2Os : AI203 : C~2H25NH2 : 60 H20. A suspension of 2.42 g of alumina (72 % pseudoboehmite alumina, Catapal B from Vista) in 5 g of water was mixed with 4 g of phosphoric acid (Fisher Scientific, 85 %) diluted with 13 g of water and stirred for about 1 h. Finally 3.2 g of dodecylamine surfactant was added to this mixture and stirred for one additional hour. The gel was then heated under autogenous pressure at 100 ~ for 24 h in a Teflon lined autoclave with no stirring. X-ray diffraction measurements were carried out on a D5000 Siemens diffractometer (CuKec radiation, ~, = 0.15418 nm). Transmission electron micrographs were obtained as reported elsewhere using a Philips CM20 instrument operated at 200 kV [17,20]. Thermogravimetric measurements were performed on a Mettler TG50 thermobalance in a flow of air. The temperature was raised at a rate of 10 ~ up to 600 ~ s~p and 27AI MAS NMR spectra were obtained on a Bruker AMX-300 (magnetic field 7.05 T, Larmor frequencies 78.18 and 121.47 MHz, respectively) and a Bruker AMX600 (magnetic field 14.1 T, Larmor frequencies 156.36 and 242.95 MHz) spectrometers. Typical MAS speeds of rotation were 10-14 kHz, and the delay times were set at 60 s for 3~p and 0.3 s for 27A1. A conventional one-pulse sequence in combination with high power proton decoupling (40 kHz) was used for both nuclei. Very short radiofrequency pulses were employed for 27AI (0.6 l~S) in order to obtain spectra for quantitative measurements [21]. A 5 mm High Speed Probe and a 5 mm Ultrasonic Speed Probe, both from DOTY Scientific were used on the AMX-300 and AMX-600, respectively. 85% H3PO4 and a 1 M solution of aluminum nitrate were used as external references. All values for the 27AI chemical shifts reported here were corrected for the second order quadrupolar interactions [22].
39 ~3C and ~SN CP MAS spectra were collected on a Bruker AMX-300 spectrometer (Larmor frequencies 75.5 and 30.1 MHz, respectively). The speed of rotation was within 3-3.5 kHz, and the CP contact time was 2 and 5 ms for ~3C and ~SN, respectively. Signals from tetramethylsilane (TMS) and the NO3" group of solid NH4NO3 were used as external references. 3. RESULTS AND DISCUSSION
Tanev et al. [6a] were the first to use long chain primary amines as supramolecular templates for the synthesis of pure and Ti-modified hexagonal mesoporous silicates. The same technique was extended to V-modified silicates [7] and to aluminophosphates [16,17]. Recently, Oliver et al. [19] prepared lamellar AIPO4s using decylamine in a non-aqueous tetraethylene glycol solvent. In the present study both primary and tertiary amines were used. ~SN and ~3C CP MAS NMR showed that amines occluded in the as-synthesized materials were actually protonated. The ~SN signal shifted from -346.0 ppm for pure dodecylamine to -340.0 + 0.4 ppm for the occluded molecule. Likewise, the ~3C chemical shift was 43.1 and 40.2 + 0.4 ppm for pure and occluded dodecylamine, respectively. The magnitude of these shifts corresponds to protonation. The effects of AI2OJP205, C~2H2s-NH2/P2Osand H20/P205 ratios as well as the effect of the alkylamine chain length were investigated systematically. All our AIPO4 materials had lamellar structures, and consequently collapsed upon high temperature calcination. The lamellar nature of these phases was inferred from the presence of only ( 001 ) XRD peaks, and also from direct TEM observations. Figure 1 shows a series of XRD patterns for AIPO4-1:y:1:60, with y = 0 to 1.8. Samples with low AI content (y = 0 to 0.4) exhibited a lamellar phase with doo~ = ca. 22.5 A. As seen below, 8~p NMR data of these samples are consistent with the occurrence of dodecylammonium dihydrogen phosphate. Recently, Oliver et al. [20] used a similar procedure to synthesize decylammonium dihydrogen phosphate. Gels with A I / P ratios higher than 0.6 gave a lamellar AIPO4 phase with doo~= 32.5 + 0.5 ,~ (Table 1). Figure 2, a representative micrograph of AIPO4-1:1:1:60, shows alternating dark and light fringes, indicative of the occurrence of layers viewed edge-on. The electron diffraction pattern is also consistent with the presence of a layered structure with a primary repeat distance of 31 A, in good agreement with XRD measurements. In addition, as shown in Figure 3, some samples exhibited extended areas with unique structural features. These areas consist of disks with an overall diameter of ca. 150 nm aggregated into a hexagonal-like army. A close-up (Figure 4) shows that each disk is comprised of alternating dark and bright concentric rings. The primary distance between dark rings was 31 A, indicating that this new mesophase is related to the layered structure shown in Figure 2. The central tubule of the disk had a diameter of ca. 36 A, consistent with the presence of a surfactant rodlike micelle. The concentric growth of rings to form self-organized, large disks with comparable diameters was interpreted as follows [17]. Because of their small head, long chain alkylammonium surfactants tend to self-organize into planar bilayers [23]. Consequently, in the presence of inorganic species, the formation of lamellar structures is strongly favored.
40
y=1.8 1.6
1.2 1.0
5
: ".';-:,.':~Y.. ~:.'Y/',,d,~ c ?;,.-x./lx.,r, fl.fJ'~ 9 . ..... ;11 ~
",<.x.,, :,.
0.8 k0.6 k.
jk o.o '
1.5
I
'
I
,
I
3.5 5.5 7.5 2 Theta (degrees)
,
I
9.5
Figure 1: XRD pattems of AIPO4-1:y:1:60 samples. Values of y are shown on the right-hand side.
Figure 2: TEM image and its corresponding selected area electron diffraction (Ref. 17).
Table 1 Properties of AIPO4-1:y:1:60 samples. l
AI/P (y)
doo1 (A)
31p ppm
27AI (Td) ppm
0 0.2
0.8 1.0 1.2 1.4 1.6 1.8 2.0 oo
32.5 32.5 33.2 32.7 32.2 32.2 32.7 am c
0.6; 2.3 s 1.0 s; -19.3 s 1.0 s; -19.0 s 0.65 s; 2.26 s -18.6 s; -19.1 s -23.8 s; -25.8 s -13.0 b -13.0 b -13.0 b -13.0 b -13.0 b -3.8 s; -13.0 b -3.0 s; -13.0 b .
-
0.6
22.3 22.6 22.6 29.7
0.4
27AI (Oh) ppm
-
-
47.4 sm 44.5 sm 47.7 m,b 46.8 m,b 46.5 m,b 47.5 sm,b 46.8 sm,b 46.8 sm,b 46.3 sm,b .
-9.8
sh
-
-9.9 sh
-
-10.2
m,sh 10.4 s,sh 10.5 s,sh 10.4s,sh 10.7s,sh 10.4s,sh 10.0s,sh 10.3 s,sh
10.4
27AI (Oh) ppm
s
-8.5 s,b -8.7 s,b -8.5 m,b -8.0 sm,b -6.8 sm,b -6.6 sm,b -6.5sm,b
(a) bulk composition; (b) framework composition; (c) amorphous. b: broad; m, medium; s: strong; sh: sharp; sm: small.
P" AI"
-
P" A!b
-
1:0.54
-
-
-
1:0.92 1:0.80 1:1.05 1:0.79 1:1.26 1:0.84 1:1.47 1:0.90 1:1.69 1:0.95 1:1.84 1:0.89 1:2.11 1:1.07
41
Figure 4: Close-up image showing altemating dark and bright concentric rings.
Fig re 3: TEM image showing disks of coP, :entdc rings packed into a he" tgonal-like array (Ref. 17).
y
1.0
20
(x = 0)
0
-20
y =2.0
D
-40 ~,.-,~
Jl\
2.0 1.8 1.6 D
1.2
(~k.~
~.o
/
_ o~
J~JL
0.6
m
r--
20(:
__>.~
9 ,~
_
0.4
0.4
0.2
0.2
0.0
'
~o
'
~
"
-1;~o"
Chemical Shift, ppm
Fig re 5.27AI MAS-NMR spectra of AIF ~)4-1:y:1:60. Values of y are shown on he left-hand side.
"2()0
100
'
510
Chemical Shift, ppm
Figure 6. 3~p MAS-NMR spectra of AIPO4-1 :y:1:60. Values of y are shown on the left-hand side.
42 However, in the present system, the occurrence of concentric growth suggests that the system has a tendency to form some rodlike micelles, but not enough to selfaggregate, for example into a hexagonal structure. These rodlike micelles play the role of nuclei for further concentric growth of alternating rings of inorganic AIPO4 materials (dark rings) and cylindrical vesicles of surfactant (bright rings). This unique morphology is to be regarded as an example of the occurrence of new surfactantinorganic mesophases which have no lyotropic surfactant liquid crystal counterparts. Using gemini surfactants, Huo et al. [24] also discovered a surfactant-silicate mesophase with three dimensional hexagonal symmetry which has no analog among known surfactant liquid crystal structures. Likewise, Oliver et al. [19] while studying the synthesis of lamellar aluminophosphates in the presence of decylamine in a nonaqueous tetraethylene glycol solvent, found that parts of their samples exhibit remarkable morphologies and surface patterns akin to the naturally occurring silicious skeletons of diatoms and radiolaria. Figures 5 and 6 show the 27AI and 31p NMR spectra of AIPO4-1:y:1:60 samples. Detailed data are given in Table 1. Figure 5 shows that most samples exhibit three different 27AI NMR signals. Based on literature data [25], the signal at ca. 47 ppm was assigned to tetrahedral aluminum (species A) bonded to four P atoms via oxygen bridges. In agreement with Rocha et al. [27] who found that 27AI in AI(OP)4(OH2)2 resonates between -9.5 and -12 ppm, the peak at -6 to -10 ppm was attributed to framework octahedral aluminum (species B) coordinated with water and PO4 groups. Samples with low AI content (y = 0.2 and y = 0.4) exhibited only one sharp 27AI NMR peak at ca. -10 ppm corresponding to the hydrated six-coordinated AI in AIPO4 framework. Upon vacuum treatment of the samples at room temperature, the peak of species A decreased in favor of species B. This indicates that (i) both species are related to each other, and (ii) species B is coordinated to at least two water molecules. At higher AI loading (y > 0.8) a third signal with a chemical shift of ca. 10.3 ppm developed. As seen, the amorphous P free sample exhibits only one 27AI NMR signal at 10.3 ppm. It is therefore inferred that the 10.3 ppm peak observed for AI rich samples corresponds to extraframework alumina. Figure 6 shows the 31p MAS-NMR spectra of the same AIPO4-1:y:1-60 series of samples. The aluminum free sample exhibited two 3~p NMR signals at 2.3 (40 %) and 0.6 (60 %) ppm. The anisotropy (AS = -75 + 5) and the asymmetry parameter (11 = 0.3 __. 0.1) were very similar for both species. These parameters, common for acid ammonium phosphates, were assigned to two non equivalent PO2(OH)2 anions belonging to dodecylammonium dihydrogen phosphate. Samples with AI to P ratios in the range 0.8 to 1.6 exhibited a broad 3~p NMR peak centered at -13 ppm, thus excluding the presence of P sites with P in their second coordination shells. This peak was attributed to tetrahedral P bonded to (4 - X) aluminum tetrahedra and X hydroxyl groups (where X = 1 or 2). The chemical shifts of 3~p in microporous AIPO4s generally fall in the range of -19 to -30 ppm [27]. The downfield shift observed for our samples may due to several factors, particularly for the hydrophillic nature of the materials [28]. The origin of the peak broadening is most likely attributable to the occurrence of a distribution of P sites with similar but not identical environments. This conclusion stems from the fact that at higher field (14.1 T) the resolution of the peak hardly improved. The first derivative of the 31p NMR signal
43 indicates the presence of at least five subgroups of P sites (Figure 6, inset). In addition to the -13 ppm 31p signal, samples with the highest levels of AI displayed a low intensity (< 4%), sharp peak at ca. -3.4 ppm attributed to an impurity phase. For samples with very low AI contents (y = 0.2 and y = 0.4) there was a sharp peak at -19 ppm in addition to the Sip peak close to 0 ppm observed in the AI-free sample. This -19 ppm 31p peak together with the -10 ppm 27A! peak observed for the same sample may correspond to variscite: AIPO4-2H20 with 5(~IP) = -19.5 ppm and ~(27AI) = -11 ppm [29]. If this assignment is correct, the variscite phase must be highly dispersed not to be observed by XRD. As seen in Table 1 (column 7), the overall AI/P ratios of the samples are comparable to those of the corresponding gels. The framework P to AI ratios shown in the last column represent ratios of the sum of AI species A and B to P calculated based on chemical analysis, quantitative AI NMR data and assuming complete retention of phosphorus. It is seen that AI/P is usually below one. As inferred from NMR data, this indicates the occurrence of some P-O +NH3-C12H2slinkages. Additional data on the effect of other synthesis parameters will be published elsewhere [30].
4. CONCLUSIONS A variety of lamellar aluminophosphates with doo~distances in the range of 2-4 nm were synthesized via the supramolecular templating mechanism using long chain primary and tertiary alkylamines as templates. The effects of other synthesis parameters were also studied. The occurrence of lamellar phases was inferred from XRD data and by direct TEM observations. 81p and 27AIdata were consistent with the presence of aluminophosphate. Even though the synthesis variables had strong effect on the quality of the products formed and on the connectivities of AI and P, they did not favor the formation of three dimensional AIPO4 structures. In addition to the main phase with planar lamellae, some samples exhibited extended areas consisting of coaxial cylinders of altemating inorganic aluminophosphate material and organic surfactant bilayers, all wrapped around a central rodlike micelle. Such coaxial cylinders which were aggregated into a hexagonal-like structure had an overall diameter of ca. 150 nm.
Acknowledgments Partial funding by the Natural Sciences and Engineering Research Council (NSERC) of Canada is acknowledged. I wish to thank I.L. Moudrakovski, J.S. Reddy, V.R. Karra, C.I. Ratcliffe, J.A. Ripmeester, K.F. Preston, A. Chenite and Y. Le Page for significant contributions to this work.
REFERENCES (a) C.T. Kresge, M.E. Leonowicz, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710.
44
.
=
4. 5. 6.
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.
.
10. 11. 12. 13. 14.
15. 16. 17. 18.
19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.
Q. Huo, D.I. Ciesla, D.G. Demuth, P. Feng, T.E. Gier, P. Sieger, A. Firouzi, B. Chmelka, F. Sch0th and G.D. Stucky, Chem. Mater., 6 (1994) 1176. P.T. Tanev and T.J. Pinnavaia, Science, 267 (1995) 865. S.A. Bagshaw, E. Prouzet and T.J. Pinnavaia, Science, 269 (1995) 1242. A. Sayari, Chem. Mater. (1995), submitted for publication. (a) P.T. Tanev, M. Chibwe and T.J. Pinnavaia, Nature, 368 (1994) 321. (b) A. Corma, M.T. Navarro and J. Perez-Pariente, J. Chem. Soc., Chem. Commun., (1994) 147. (c) A. Sayari, V.R. Karra and J.S. Reddy, Mat. Res. Soc. Symp. Proc., 371 (1995) 87. (a) K.M. Reddy, I.L. Moudrakovski and A. Sayari, J. Chem. Soc., Chem. Commun., (1994) 1059. (b) J.S. Reddy and A. Sayari, J. Chem. Soc., Chem. Commun., (1995) 2231. (d) J.S. Reddy and A. Sayari, Appl. Catal., (1995), submitted for publication. (a) A. Sayari, C. Danumah and I.L. Moudrakovski, Chem. Mater., 7 (1995) 813. (b) A. Sayari, I.L. Moudrakovski, C. Danumah, J.A. Ripmeester, C. Ratcliffe, C. and K.F. Preston, J. Phys. Chem., 99 (1995) 16373. J.S. Reddy and A. Sayari, Catal. Lett., (1996), in press. J.S. Reddy, P. Liu and A. Sayari, 1996 Spring Meeting of the Materials Research Society, San Diego, (1996). D.M. Antonelli and J.Y. Ying, Angew. Chem. Int. Ed. Engl., 34 (1995) 2014. T. Abe, A. Taguchi and Iwamoto, Chem. Mater., 7 (1995) 1429. (a) S.T. Wilson, B. Lok, C.A. Messina, T.R. Connan and E.M. Flanigen, J. Am. Chem. Soc., 104 (1982) 1146. (b) S.T. Wilson, Stud. Surf. Sci. Catal., 58 (1991) 137. (a) R.H. Jones, A.M. Chippindale, S. Natarajan and J.M. Thomas, J. Chem. Soc., Chem. Commun., (1994) 565, and references therein. (b) B. Kraushaar-Czametzki, W.H.J. Stork and R.J. Dogterom, Inorg. Chem., 32 (1993) 5029. P.A. Barrett and R.H. Jones, J. Chem. Soc., Chem. Commun., (1995) 1979. A. Sayari, V.R. Karra, J.S. Reddy and I.L. Moudrakovski, J. Chem. Soc., Chem. Commun., (1996), in press. A. Chenite, Y. Le Page, V.R. Karra and A. Sayari, J. Chem. Soc., Chem. Commun., (1996), in press. C.A. Fyfe, W. Achwieger, G. Fu and G.T. Kokotailo, Prepr., A.C.S. Div. Petrol. Chem., 40 (1995) 266. S. Oliver, A. Kuperman, N. Coombs, A. Lough and G. Ozin, Nature, 378 (1995) 47. A. Chenite, Y. Le Page, Y. and A. Sayari, Chem. Mater., 7 (1995) 1015. P.P. Mann, J. Klinowski, A. Trokiner, H. Zanni and P. Papon, Chem. Phys. Left., 151 (1988) 143. G. Engelhardt and D. Michel, High-Resolution Solid-State NMR of Silicates and Zeolites, Wiley, Chichester, 1987. J.N. Israelachvili, Intermolecular and Surface Forces, Academic Press, NY, 1991. Q. Huo, R. Leon, P.M. Petroff and G.D. Stucky, Science, 268 (1995) 1324. D. Muller, E. Jahn, G. Ladwig, G. and V. Haubenreisser, Chem. Phys. Lett., 109 (1984) 332. J. Rocha, W. Kolodziejski, H. He and J. Klinowski, J. Am. Chem. Soc., 114 (1992) 4884. I.L. Moudrakovski, V.P. Schmachlova, N.S. Katsarenko and V.M. Mastikhin, J. Phys. Chem. Solids, 47 (1987) 335. L.S. de Saldarriaga, C. Saldarriaga and M.E. Davis, J. Am. Chem. Soc., 109 (1987) 2686. C.S. Blackwell and R.L. Patton, J. Phys. Chem., 92 (1988) 3965. A. Sayari et al., Chem. Mater. (1996), submitted for publication.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
45
Synthesis and Hydrothermal Stability of a Disordered Mesoporous Molecular Sieve Ryong R y o o a, J. M. Kim', C. H. Shinb and J. Y. Lee c
"Departmem of Chemistry, KAIST, Taejon, 305-701, Korea; bCatalysis Research Division, KRICT, Taejon, 305-606, Korea; CDepartment of Materials Science and Engineering, KAIST
A noncrystalline mesoporous molecular sieve has been synthesized by hydrothermal polymerization of silicate and aluminate anions surrounding the molecular organization of hexadecyltrimethylammonium chloride in the presence of various organic polyacids. The noncrystalline molecular sieve is very similar to the well-known mesoporous molecular sieve MCM-41 in the aspect of the high specific surface area and uniform pore sizes, but the channel arrangement interconnected in a three dimensional disordered way distinguishes it most conspicuously from the MCM-41 which exhibits hexagonal arrangement of straight channels. The disordered structure has remarkably high hydrothermal stability and thermal stability, compared with MCM-41 and MCM-48.
1. Introduction In recent years, there have been dramatic advances in the concept of molecular sieves exhibiting uniform pore sizes. The structures of early molecular sieves were typified by crystalline microporous materials such as zeolites and AIPO4 in which the arrangement of channels (or pores) and the arrangement of framework atoms are ordered over crystallographically long range. Later, the discovery of crystalline mesoporous molecular sieves such as MCM-41 and MCM-4813 has opened a new class of molecular sieves in which only the channel arrangement is crystallographically ordered while the atomic arrangement is disordered similar to amorphous silica. Since then, a few silicate materials exhibiting disordered arrangement of mesopores with very high specific surface areas of approximately 1000 m2g1 began to attract our attention. One such material was synthesized by Chela et al. 4 using hydrothermal reaction of Na-kanemite and hexadecyltrimethylammonium (HTA) chloride. The material gave a N2 adsorption isotherm similar to that for MCM-41, which indicated that the pore size distribution was similar to that for the MCM-41. The structure of the material was illustrated by the authors with straight mesoporous channels which were entangled randomly. Bagshaw et al. s obtained another disordered mesoporous material designated MSU=I using tetraethylorthosilicate and nonionic surfactants. Guo 6 also reported a similar material. The pore size distribution curves for these two mesoporous silicate materials showed peak widths greater than 0.9 nm, which was about three times broader at the
46 half height than that for MCM-41. Although the pore structures of the materials were still very heterogeneous compared with the pore sizes for the crystalline MCM-41 and MCM-48, their discovery showed a possibility of finding noncrystalline molecular sieves with uniform pore sizes. Very recently, efforts to obtain such noncrystalline molecular sieves with truly uniform pore sizes have succeeded by the present authors using hydrothermal polymerization of silicate and aluminate anions surrounding the molecular organization of hexadecyltfimethylammonium chloride in the presence of various organic polyacids. 7 The disordered molecular sieve thus obtained is very similar to the MCM-41 in the aspect of the high specific surface area and uniform pore sizes, but the channel arrangement interconnected in a three dimensional disordered way distinguishes it most conspicuously from the MCM-41 which exhibits a hexagonal arrangement of straight channels. Besides, the disordered structure has higher thermal stability and hydrothermal stability than MCM-41 structure. Here, we describe details of the synthesis method and the hydrothermal stability of the disordered mesoporous molecular sieve exhibiting uniform pore sizes.
2. Experimental A fully disordered surfactant-silicate mesostructure has been obtained using sodium silicate, alkyltrimethylammonium (ATA) halide (CnHz~+IN(CH3)3X, n = 12 - 18, and X = Cl or Br) and sodium salt of organic polyacid. Typical procedures to obtain the disordered mesostructure were as follows: a clear solution of sodium silicate with a Na/Si ratio of 0.5 was first prepared by combining 46.9 g of 1.00 M aqueous NaOH solution with 14.3 g of a colloidal silica, Ludox HS40 (39.5 wt% SiO2, 0.4 wt% Na20 and 60.1 wt% H20, Du Pont) and heating the resulting gel mixture with stirring for 2 h at 353 K. The sodium silicate solution was dropwise added to a polypropylene bottle containing a mixture of 0.29 g of 28 wt% aqueous NH3 solution, 23.8 g of ethylenediaminetetraacetic acid tetrasodium salt (Na4EDTA), 20.0 g of 25 wt% HTACI solution and 28.0 g of doubly distilled water, with vigorous magnetical stirring at room temperature. The resulting gel mixture in the bottle had a molar composition of 4 SiO2 91 HTACI : 4 Na4EDTA : 1 Na20 : 0.15 (NH4)20 : 350 H20. After stirring for 1 h more, the gel mixture was heated to 370 K for 2 d. The resulting mixture was cooled to room temperature. Subsequently, pH of the mixture was adjusted to 10.2 by dropwise addition of 30 wt% acetic acid with vigorous stirring. The reaction mixture after the pH adjustment was heated again to 370 K for 2 d. This procedure for pH adjustment to 10.2 and subsequent heating for 2 d was repeated twice more. The precipitated product was filtered, washed with doubly distilled water and dried in an oven at 370 K. The product was calcined in air under static conditions using a muffle furnace. The calcination temperature was increased from room temperature to 823 K over 10 h and maintained at 823 K for 4 h. The calcined product is designated KIT-1. The product yield was more than 90%, based on the silica recovery. Aluminum-containing KIT-1 (AIKIT-l) samples with Si/Al as high as 5 have been obtained by adding 5 wt% aqueous solution of sodium aluminate (Strem, 99.9% on metal basis) during the formation of the above disordered surfactant-silicate mesostructure. The aluminate solution was added to the reaction mixture at room temperature following the second heating
47 step to 370 K, dropwise with vigorous mixing. The second pH adjustment o f the reaction mixture with acetic acid was carried out after the resulting surfactant-aluminosilicate gel mixture was heated for 2 d. The remainder of the synthesis procedure was the same as the preparation of the above pure silica K/T-1. Hydrothermal stability of the samples was investigated by measuring the intensity decrease in powder X-ray diffraction (XRD) pattern during heating in doubly distilled water. The sample to water ratio was fixed as 1 g.L q. Samples at~er the heating in water were filtered, washed in doubly distilled water, and immediately placed in oven to dry for 2 h at 400 K. XRD patterns were obtained from the samples with a Cu K~ X-ray source using a Rigaku D/MAXIII (3 kW) instrument.
3. Results and Discussion
Characterization of a disordered molecular sieve. Figure 1 shows XRD patterns for the disordered HTA-silicate mesostructure obtained using Na4EDTA The surfactant is easily
5
3
m
m
|
m
20
I
I
2
4
I
6 Two theta
!
8
10
Figure 1. Powder X-ray diffraction patterns for a fully disordered molecular sieve, KIT-l, synthesized using HTAC1 as a template. Inset, the relationship between dl0o spacing and number of carbon atoms in the surfactant chain: (0) as-synthesized and (r-l) calcined.
48 removed from the mesostructure by calcination in air under static conditions at any temperatures between 823 - 1173 K. Strict conditions using gas flows are not required for the calcination. Both the as-synthesized product and the calcined material exhibit same XRD patterns with three broad peaks indexed to (100), (200) and (300) diffraction with dl00 = 4.0 4.2 nm. The XRD intensity increases approximately 3 times upon calcination due to the removal of the surfactant. The dl00 spacing decreases very slightly (___0.1 nm) upon the calcination. The three-line XRD pattern is similar to the XRD pattern for a layered material. But, the KIT-1 is not a layered material since the removal of the surfactant by the hightemperature calcination does not lead to the structure collapse. The pore size distribution for the calcined KIT-1 material has been obtained from N2 adsorption-desorption isotherms at liquid N2 temperature following the Horvath-Kawazoe analysis, s The pore-size distribution curve in Figure 2 shows a mesopore with the pore diameter 3.4 nm at maximum of the distribution. The specific surface area for the KIT-1 has been obtained to be 1000 megq by the BET method, which is similar to MCM-41. It is remarkable that the mesopore structure of the KIT-1 (~ 0.3 nm peak width) is as uniform in pore size as the crystalline MCM-41 (~ 0.3 nm peak width) la on the basis of the width at half maximum for the pore size distribution curve. Thus, the individual pore widths are truly uniform inside the disordered structure of the KIT- 1.
-
~, 700 ~
600-
500 400 300 200 lO0 0 0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
I
I
I
I
I
I
I
I
I
I
1
2
3
4
5
6
7
8
9
10
Effective pore size (nm) Figure 2. Pore size distribution curve obtained by the Horvath-Kawazoe analysis for a disordered molecular sieve, KIT-I, alter calcination. Inset, the corresponding N2 adsorption-desorption isotherms.
49 As shown by a transmission electron micrograph (TEM) in Figure 3, the disordered surfactant-silicate mesostructure is similar to a bicontinuous structure arranged in a threedimensional disordered way. The calcined product KIT-1 also gives essentially the same transmission electron micrograph as the as-synthesized form. No long range structural order has been found from the electron diffraction pattern. Compared with the hexagonal structure of MCM-41, it is believed that the organic polyacids anions causes fluctuation in the surfactant micellar arrangement, giving rise to the formation of a stable isotropic labyrinth or disordered sponge phase that is similar to the so called L3 phases known in surfactant solutions. 9 However, the arrangement of the mesoporous channels in the KIT-1 structure is distinguished from the L3 phase by the intercormection of the mesoporous channels by numerous branches.
30nm
Figure 3. Typical transmission electron micrograph: (a) MCM-41 and Co) fully disordered molecular sieve, KIT-1. We have performed the following two experiments in order to investigate if the structure of the KIT-1 consists of one-dimensional mesoporous channels entangled in a disordered way, or the structure has very short channels interconnected by numerous branches. In one of the experiments, aluminum has been incorporated within the KIT-1 framework over the range of Si/AI = 5 - oo using sodium aluminate during synthesis, and the experimental conditions for the removal of surfactant by calcination have been compared with those for AIMCM-41. Our results show that the synthesis of AIMCM-41 leads to progressive decreases in the XRD intensity and increases in the line width, due to losses in the structural order as the AI content ihcreases beyond 15 Si/A1.~~ Moreover, calcination of the as-synthesized products with high AI content leads to the formation of coke due to the surfactant decomposition, which causes
50 channel blockage. The coke formation can be prevented by washing a large fraction of the surfactant using an ethanol-HCl mixture prior to the calcination. However, significant dealumination occurs during the washing. The resulting A1MCM-41 gives low ion exchange capacity, and furthermore much of the structural order is lost during the cation exchange due to weak hydrothermal stability of the MCM-41 samples, a~ Compared with the AIMCM-41, calcination of the AIKIT-1 can be performed using air under static conditions without washing with the ethanol-HCl mixture, which is indicative of facile diffusion of gases through numerous branches in the three dimensional channel structure during calcination. In the second experiment, Pt clusters about 3 - 4 nm in diameter have been supported inside the mesoporous channels of MCM-41 and KIT-1 following an impregnation technique using HzPtCI6. Catalytic activity of the Pt-supporting samples for hydrogenolysis of ethane with 1-12 has been measured using a batch recirculation reactor. The Pt clusters are large enough to cause multiple pore blockage in the MCM-41 channels, and thus the surface atoms on the Pt clusters located inside the one dimensional channel of the MCM-41 are not accessible for the catalytic hydrogenation, n On the other hand, the Pt clusters supported inside the three dimensional channel structure of the KIT-1 are fully accessible for ethane hydrogenolysis with H2. Thus, the catalytic activity for the Pt/KIT-1 is proportional to the Pt loading. It is clear that the presence of the three-line XRD pattern for the KIT-1 comes from a short range structural order with very uniform pore sizes. Similar products can be obtained if the Na4EDTA is substituted by sodium salts of adipic acid, other polycarboxylic acids and polysulfonic acids. The products obtained with different polyacids give similar XRD patterns with three broad diffraction bands. The line widths and line shapes can be somewhat dependent on the different polyacids and also on the salt concentration. This is probably due to differences in the density of the branches interconnecting the channels and the distance between adjacent branches, which determine the local structure of the KIT-1. It is believed that the channel disorder in the KIT-1 structure can be controlled by the nature and concentration of the chemical agents used to induce the fluctuation in the micelle arrangement. Likewise to the MCM-41,1'2 it is also possible to tailor the channel widths by use of surfactants with suitable sizes. The dl00 spacings for the calcined materials are plotted against the number of carbon atoms in the ATA surfactant in Figure 1.
Hydrothermal stability of mesoporous molecular sieves. MCM-411a and other mesoporous molecular sievess'6,a3 found recently have opened new possibilities as a support for adsorption and catalysis, and also as a template for architecting nanosize materials. The mesoporous material, MCM-41 has excellent thermal stability up to 1170 K or higher in air and 02. The stability is not affected considerably by the presence of' water vapor up to 2.3 kPa in the 02 flow. 14'as Furthermore, there is a report that the MCM-41 constructed with silica framework can be stable even in a 100%-steam flow under atmospheric pressure at 820 K. t~ However, contrary to the good stability at high temperatures, the MCM-41 is reported to lose the structure easily during storage in humid air and aqueous solutions at relatively low temperatures around 370 K. 11 The loss of the structure involves silicate hydrolysis as shown by Kim and Ryoo using XRD and magic angle spinning 29Si ~ spectroscopy, at The loss of the structure makes it difficult to obtain high levels of ion exchange with MCM-41. as In
51 addition, although MCM-41 has been reported to be useful for many catalytic applications, l~qs it is expected that the loss of the structure can lead to a rapid decrease in the catalytic activities with time under experimental conditions containing water or saturated with water vapor. Thus, the poor hydrothermal stability of the MCM-41 in water can be a critical problem limiting the applications. Therefore, improvement of hydrothermal stability is a target for ultimate successful uses of the MCM-41 type materials. We have obtained pure silica MCM-41 and MCM-48 samples, following hydrothermal synthesis procedures reported in the literature, 1-3 and compared hydrothermal stability of the
KIT-1
MCM-48
MCM-41
C
~
C
b ~~_______
b
!
2
4
6
8
Two theta
2
4
6
8
Two theta
i
i
i
i
2
4
6
8
10
Two theta
Figure 4. Powder X-ray diffraction patterns for two crystalline mesoporous molecular sieves, MCM-41 and MCM-48, and a fully disordered molecular sieve, KIT-l, exhibiting the transmission electron micrograph in Figure 3: (a) calcined samples, (b) heated in water at 343 K for 12 h, (c) heated in boiling water for 12 h, and (d) heated in boiling water for 48 h. samples in boiling water with that of KIT-1. Figure 4 displays XRD patterns for the mesoporous materials against heating temperatures in water. The XRD patterns for the calcined MCM-41 samples before the heating in water show four dif~action lines characteristic of the hexagonal structure of the MCM-41. The XRD pattern for the MCM-48 sample agrees with the cubic 1,3d structure known in the literature. 13 No distinct changes in the structures are indicated by the XRD lines during 12 h in distilled water at room temperature. As the water temperature increases to 343 K, the intensity of the XRD patterns decreases conspicuously. Moreover, the decrease in the XRD intensity depends considerably on details of the synthesis procedures. 11 The stability difference is consistent with previous conclusion lsa9 that the stability of MCM-41 can be enhanced by repeating pH adjustment to around 11 during hydrothermal synthesis, due to an equilibrium shift of the synthesis reaction
52 condensation. 16~~ However, all the structures disappeared completely during the heating at 373 K. Compared with the weak hydrothermal stability of the MCM-41 and MCM-48 samples as shown in Figure 4, it is remarkable that the structure of the KIT-1 is stable in boiling water for 48 h. We have confirmed that the structure of the KIT-1 does not change under 100%-steam flow for 2 h at 1020 K. The disordered structure is also stable during heating with air flow containing 2.3 kPa water vapor for 2 h at 1220 K. In summary, the disordered mesoporous material, KIT-l, found in the present work belongs to a new class of molecular sieve in which the channels are interconnected in a three dimensional, fully disordered way. The structure of the material corresponds to a disordered bicontinuous phases known in surfactant solutions, in contrast to the hexagonal structure of MCM-41. Since the disordered structure, compared with the ordered MCM-41, has remarkable advantages due to the three dimensional diffusion and high hydrothermal stability, our findings of the new class of molecular sieves are expected to provide new opportunities for rational design of heterogeneous catalysts, adsorbents and other related materials requiring the three dimensional pore structure. References
1. C.T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. J.S. Beck, J. C. Vartuli, W. J. Roth, M. El Leonowicz, C. T. Kresge, K. D. Schmitt, C. T.-W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, J. B. Higgins and J. L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 2. A. Monnier, F. Schi~th, Q. Huo, D. Kumar, D. Margolese, R. S. Maxwell, G. D. Stucky, M. Krishnamurty, P. Petroff, A. Firouzi, M. Janicke and B. F. Chmelka, Science, 261 (1993) 1299. 3. Q. Huo, R. Leon, P. M. Petroffand G. D. Stucky, Science, 268 (1995) 1324. 4. C.-Y. Chen, S.-Q. Xiao and M. E. Davis, Microporous Materials, 4 (1995) 1. 5. S.A. Bagshaw, E. Prouzet and T. J. Pinnavaia, Science, 269 (1995) 1242. 6 C.J. Guo, Stud. Surf. Sci. Catal., 97 (1995) 165. 7. R. Ryoo, J. M. Kim, C. H. Ko and C. H. Shin, J. Phys. Chem., in press. 8. R. Ryoo et al., in preparation. 9. G. Horvath and K. J. Kawazoe, J. Chem. Eng. Japan, 16 (1983) 470. 10. R. Ryoo, S. J. Cho, C. Pak and J. Y. Lee, Catal. Lett., 20 (1993) 107. R. Ryoo, S. J. Cho, C. Pak, J.-G. Kim, S.-K. Ihm and J. Y. Lee, J. Am. Chem. Soc., 114 (1992) 76. 11. D. Roux, C. Coulon and M. E. Cates, J. Phys. Chem., 96 (1992) 4174. 12. Z. Luan, H. He, W. Zhou, C.-F. Cheng and J. Klinowski, J. Chem. Soc. Faraday Trans., 91 (1995) 2955 and references on AIMCM-41 therein. 13 J. M. Kim and R. Ryoo, Bull. Korean Chem. Soc., 17 (1996) 66. 14. R. Ryoo, C. H. Ko, J. M. Kim and R. Howe, Catal. Lett., 37 (1996) 29. 15. C.-Y. Chen, H.-X. Li and M. E. Davis, Microporous Materials, 2 (1993) 17. 16. J. M. Kim, J. H. Kwak, S. Jun and R. Ryoo, J. Phys. Chem., 99 (1995) 16742. 17. E. Armengol, M. L. Carlo, A. Corma, H. Garcia and M. T. Navarro, J. Chem. Soc., Chem. Commun. (1995) 519. 18. C.-G. Wu and T. Bein, Science, 264 (1994) 1757. 19. K. R. Kloetstra and H. van Bekkum, J. Chem. Soc., Chem. Commun. (1995) 1005. 20. R. Ryoo and J. M. Kim, J. Chem. Sot., Chem. Commun. (1995) 711.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
53
Preparation of silica-pillared molecular sieves from layered silicates Soon-Yong Jeong a, Oh-Yun Kwon b, Jeong-Kwon Suh a, Hangkyo Jin a, and Jung-Min Lee a aChemical Engineering Division, Korea Research Institute of Chemical Technology, Yusung, P. O. Box 107, Taejon, Korea 305-606 bDepartment of Chemical Engineering, Yosu National Fisheries University, Yosu, Korea 550-749 Magadiite and kenyaite were hydrothermally synthesized in Teflon-sealed stainless steel autoclave. The intercalation of TEOS(tetraethylorthosilicate) into the interlayers of layered silicates was carried out by amine preintercalation, and the effects of acid and base catalysts during gelation of TEOS into interlayers were investigated. It was found that the samples silica-pillared by acid- and basecatalyzed reactions show well-ordered basal spacing and super-gallery heights. Also, they exhibit relatively narrow pore size distributions in the range of 18-40 ,A, and show high surface areas in the range of 533-845 m2/g, depending on the catalyst types. These results indicate that the variations in the conditions of gelation contribute to improvement in physical properties of silica-pillared molecular sieves. 1. INTRODUCTION Layered silicates have attracted widespread interest over the past 20 years due to their catalytic, adsorptive and ion-exchange properties [1]. Recently, several researchers have investigated the pillared reactions of layered silicates such as magadiite, and kenyaite [2-5]. Magadiite and kenyaite were primarily found by Eugster [6] in the lake beds of the lake Magadi in Kenya. Afterward, other occurrences of these have been continuously reported in various regions [7], and these materials were mostly found from sodium carbonate-rich alkaline lake waters. They also have been successfully synthesized under hydrothermal conditions [8,9]. Their basic structures are composed of duplicated SiO 2 tetrahedral sheets, and are similar to clay minerals except to be free of aluminum [10]. Silica-pillaring of magadiite was reported by a few researchers. Landis et al. [4] found that the pillaring of magadiite could be facilitated by using a preswelling step in which the interlayers are exposed to organoammonium ion or amine. The calcined sample obtained from TEOS [Si(OC2H5)4] pillaring exhibited a high surface area, 530 m2/g. Sprung et al. [5] reported that the pillared derivatives of magadiite
.54 can be obtained from the calcination of hydrolyzed phenyltrichlorosilane-magadiites. Daily and Pinnavaia [3] synthesized supergallery derivatives on the basis of Hmagadiite by gelation of TEOS with EtOH suspension. After calcining to remove organic compounds, pillared magadiite with surface areas of about 520-680 m2/g, depending on the amount of gelled TEOS, was formed. However, the pillaring of kenyaite was rarely reported. Recently, Landis et al. [4] prepared pillared derivatives on the basis of H-kenyaite by gelation of TEOS with EtOH suspension. The calcined sample exhibited a high surface area, 600 m2/g. In the sol-gel process, solvent such as EtOH is added to prevent the liquid-liquid separation during the initial stage of the hydrolysis reaction and to control the concentration of silicate species and water that influence the gelation kinetics [11]. However, the reaction of TEOS gelation by alcohol suspension occurs very slowly, and because TEOS is alcohol-soluble, it can be released outside the layered phases during gelation. Aelion et al. [12] observed that the rate of hydrolysis of TEOS was influenced by the strength and concentration of the acid and base catalysts. The fast gelation using the catalyst such as acid or base can minimize the release of TEOS from the layered phases during gelation. Generally, TEOS gelation by acidor base- catalyzed hydrolysis could diversify the interfacial properties of products and result in such products as bulk gel, film, fiber, powder, and catalyst support. In the present work, we report the effects of acid or base catalysts on the hydrolysis and condensation polymerization of intercalated TEOS in H-type layered silicates.
2. EXPERIMENTAL 2.1 Syntheses of Na-magadiite and Na-kenyaite Synthetic Na-magadiite and Na-kenyaite were prepared by the reaction of NaOH/Na2CO3-SiO2 system under hydrothermal conditions using methods analogous to those described by Fletcher and Bibby [9]. Materials used were silica gel (Wakogel Q-63) and analytical reagent grades of NaOH and Na2CO3. Namagadiite was synthesized in a stainless steel autoclave without stirring at 150~ for 72 hrs under autogenous pressure, using mole ratios of SiO2 " NaOH NaCO3 9 H20 9 = 15 19 29 300. 9 Na-kenyaite was synthesized at 150-160 ~ for 70-80 hrs under autogenous pressure, using mole ratios of SiO2 NaOH 9 Na2CO3 9 H20 9 = 9 19 29 9 600. The products were filtered, and washed with deionized water in order to remove excess NaOH or Na2CO3, and dried at 40~ 2.2 Preparation of silica-intercalated layered silicates H-magadiite and H-kenyaite was prepared by titration of Na-kenyaite with 0.1 N HCI using the method of Beneke and Lagaly [8]. A suspension of 40g of Nakenyaite per 500ml of deionized water was stirred for 1 hr. The suspension was then titrated with 0.1 N HCI to a final pH of 2.0, and then maintained at the same value for one week in a refrigerator. H-magadiite and H-kenyaite was recovered by filtering, and washing with deionized water until CI-free and then dried in air at 40 ~
55 Octylamine/octylammonium-magadiite gel was reacted for 24 hrs at room temperature by adding 5.0 g of excess octylamine to 1.0 g of air-dried H-magadiite. An organic pillar precursor 20g of TEOS, was added to octylammonium-magadiite gel and then stirred for 24 hrs at room temperature. TEOS was then absorbed into the organophilic interlayer region. The TEOS-intercalated magadiite was separated by centrifugation from the mother liquid. Also, octylamine/octylammonium-kenyaite gel was formed by allowing air-dried H-kenyaite (0.86g, 0.57 mmol) to react at room temperature with excess octylamine (2g, 15 mmol) for 48 hrs. During octylamine addition, H-kenyaite absorbs the liquid amine, immediately forming a gelatinous mixture that will not flow. Silica-intercalated derivatives of kenyaite were prepared by the reaction of excess TEOS (15g, 72 mmol) with a gel composed of octylammonium-kenyaite solvated by excess octylamine for 24 hrs at room temperature. The TEOS-intercalated product was separated by centrifugation from the mother liquid. Gelation of the intercalated TEOS without catalyst was carried out by drying EtOH (10ml) suspension of TEOS-intercalated products at 40~ in air. EtOH was mixed with 3 N NH4OH and 0.1 N HCI in order to examine the effects of base and acid catalysts during the gelation. The compositions of acid and base catalysts are shown in Table 1. Gelation was conducted with stirring for 20 min. after addition of 10 ml of each catalyst to the TEOS-intercalated magadiite and kenyaite at room temperature. The stoichiometry and methodology of gelation of TEOS are wellknown, and the physical characterization of gelled silicate has been studied by several researchers [11, 13]. The gelled samples were filtered from the mother liquid, and dried in air, and then calcined at 538~ for 4 hrs in air to remove water, intercalated organoammonium ion, and organic byproducts from TEOS hydrolysis. Basal spacings of samples were determined from the 00~ X-ray powder diffraction using a Rigaku diffractometer equipped with CuK(z radiation. Nitrogen adsorption/desorption isotherms were determined by Micromeritics ASAP 2000 at 77K. All samples were outgassed at 300~ under a vacuum for 4 hrs. Surface area was determined by the BET equation. Micropore volume was obtained from t-plot methods [14], and the pore size distributions of silica-pillared products were determined by the BJH equation [15]. Table 1 Compositions of catalysts (wt%) EtOH
H20
HCl
NH3
No catalyst
95.0
5.0
Base-catalyst
16.5
79.2
-
4.3
Acid-catalyst
64.8
35.2
0.01
-
56
3. RESULTS AND DISCUSSION 3.1 Syntheses of magadiite and kenyaite The basic hydrolysis of silica gel at 150~ according to the method of Fletcher and Bibby [9] produced well-crystallized Na-magadiite and Na-kenyaite. The X-ray powder diffraction patterns of the air-dried products, shown in Fig. l(a, c) exhibited several 00~ reflections corresponding to a basal spacing of 15.6 A for Na-magadiite and 20 A for Na-kenyaite. The peak positions for this synthetic product agree closely with values reported previously [8]. The slow titration of Na-magadiite and Na-kenyaite with 0.1 N HCI resulted in the exchange of sodium ions for protons in the layered structure. The X-ray of powder diffraction patterns of H-magadiite and H-kenyaite exhibited 00~ reflections corresponding to a basal spacing of 12.6 A for H-magadiite and 18.0 A for H-kenyaite (Fig. 1 (b, d)), in agreement with earlier work [9]. The decrease in basal spacing indicated a loss of interlayer H20 upon replacement of Na* by H*.
. =,..,
==
C
=.=.=
. =,,=,
,=..=
n,' d I
I
I
I
I
I
i
I
I
I
I
I
i
I
I
I
I'
I
I
I
I
I
5 1015202530354045505560
5 1015202530354045505560
20
20
Figure 1. X-ray diffraction patterns of (a) Na-kenyaite (b) H-kenyaite (c) Namagadiite and (d) H-magadiite.
3.2 Preparation of silica-intercalated layered silicates 3.2.1 Silica-pillared magadiite The X-ray diffraction patterns and basal spacings of the calcined silica-pillared magadiites are shown in Fig. 2 (a, b, c) and Table 2. The silica-pillared magadiites gelled by base- and acid-catalyzed reactions indicate a large increase in basal spacing of 39.2 and 33.3 A, compared with the basal spacing of EtOH gelled product (17A). Table 2 shows the physical properties of the porous silica-pillared magadiites. The surface area of the sample gelled by EtOH suspension was 587
57 m2/g, coinciding with the result of Daily and Pinnavaia [3]. The sample produced by base or acid catalyst has a higher surface area and larger total pore volume than that gelled by no catalyst. These results can be explained by the point that the hydrolytic polycondensation of intercalated TEOS by acid and base catalyst could form silica clusters of more highly branched and stiff network structure. Pillared silica clusters expand the space between layered phases and affect the development of micropocity and the increase of surface area. These effects are most evident when intercalated TEOS is not released outside the layered phase during gelation. Figure 3(a) shows that the pore size distributions of silica-pillared magadiites. The sample treated with the base-catalyzed gelation has more microposity (diameter < 20 A ) and shows a sudden increase in mesopore volume near 36 A with a narrow pore size distribution. These results indicate that abrupt gelation by base catalyst is closely related to the formation of a more uniform pore. On the other hand, the sample gelled by the suspension of EtOH has a broad pore distribution. In case of gelation by acid catalyst, microporosity decreases and mesoporosity increases between 40 and 100 A compared with gelation with no catalyst.
IN,
t~ t-'
E
i
1 e
n,,
I
2
'
1'2
Degree 2e
I
18 20
4 6 8
I()121416
1'820
Degree 20
Figure 2. X-ray diffraction patterns of the calcined silica-pillared layered silicates: (a) magadiite gelled by EtOH suspension (b) base-catalyzed magadiite (c) acidcatalyzed magadiite (d) acid-catalyzed kenyaite (e) base-catalyzed kenyaite (f) kenyaite gelled by EtOH suspension.
.58 Table 2 Physical properties of silica-pillared magadiites Basal spacinga(A) 17.0
Gallery heightb(A) 5.8
Surface area (m2/g) 587
Total pore volume c (cc/g) 0.60
Base catalyst
39.2
28.0
845
0.73
Acid catalyst
33.3
22.1
648
0.62
Catalyst No catalyst
a : Sample calcined at 538~ b : Gallery height = Basal spacing - 11.2 A (thickness of H-magadiite) [16]. c : Total pore volume obtained from Gurvisch rule [17] of nitrogen adsorption isotherm at 77K.
3.2.2 Silica-pillared kenyaite
The gelation of intercalated-TEOS by catalyst produces siloxane-intercalated derivatives with well-ordered basal spacings as well as the expansion of gallery height. The X-ray diffraction patterns and basal spacings of the products gelled by EtOH suspension, base-catalyzed reaction, and acid-catalyzed reaction are shown in Fig. 2(d, e, f), and Table 3, and these products exhibit reflections corresponding to basal spacings of 29.5, 39.9,, and 39.5 A, respectively. The silica-pillared products gelled by base- and acid-catalyzed reactions exhibits a large increase in the gallery height of 22.2 and 21.8 A, compared with the gallery height of the product gelled by EtOH suspension (11.8 A). A distinctive increase of the gallery height is related to the size and the structure of pillared silica, which could be associated with the amount of intercalated TEOS, the gelation condition (catalyst type, solvent composition, pH etc.), and the rate of gelation. Gelation by base and acid catalysts could minimize the release of TEOS outside the layered phase during the gelation of the intercalated TEOS, because gelation time is markedly reduced. The hydrolytic polycondensation of intercalated TEOS by acid or base catalyst could form silica clusters of highly branched and stiff network structure. These conditions could derive the effective gelation of intercalated TEOS in the interlayer, and contribute to develop the size and structure of pillared silica clusters which bring about the large expansion of the gallery height of pillared kenyaite. The adsorption/desorption isotherms of nitrogen were obtained at 77 K. Several pore characteristics calculated from them are listed in Table 3. The specific surface areas were calculated by BET equation from the adsorption isotherm below P/Po-0.1. The specific surface area of H-kenyaite shows the low specific surface area of 84 m2/g. The calcined silica-pillared products have high surface areas between 533-606 m2/g, depending on the catalyst types. The total pore volume of
59 Table 3 Physical properties of silica-pillared kenyaites Basal spacing a (A)
Galler~ height u (A)
Surface area (m2/g)
Total Micropore surface area c (m2/g)
Total pore volume (cc/g)
H-ke nyaite
18.0
0.3
84
56
0.10
No catayst
29.5
11.8
606
509
0.49
Acid catalyst
39.5
21.8
584
490
0.46
Base catalyst
39.9
22.2
533
427
0.52
Items
a : Sample calcined at 538~ b : Gallery height = Basal spacing - 17.7 A (thickness of H-kenyaite) [8]. c : Total micropore surface area obtained from t-plot of the nitrogen adsorption isotherm at 77K. base-catalyzed sample is the largest among three pillared samples, indicating that the average pore size of base-catalyzed sample is the largest. Fig. 3(b) shows the pore size distributions of silica-pillared kenyaites. The basecatalyzed sample shows a sudden increase in mesopore volume near 22A with a narrow pore size distribution relatively. The acid-catalyzed sample shows a narrow pore size distribution with a sharp peak near 18A. The abrupt gelation of acid and base catalysts is closely related to the formation of a more uniform pore. On the other hand, the sample gelled by the suspension of EtOH has a broader pore distribution and more microporosity (diameter < 20A). It is interesting that acid- and base-catalyzed products ae very similar in physical properties to surfactanttemplated mesoporous silica. 4. CONCLUSION The acid- or base-catalyzed reaction of hydrolysis and condensation polymerization of TEOS into a layered silicate gallery could affect the physical properties of silicapillared magadiite and kenyaite. The samples that were silica-pillared by acid- and base-catalyzed reactions shows a large increase in basal spacing. Also, they exhibit relatively narrow pore size distributions in the range of 18-40A and show high surface areas in the range of 533-845 m2/g, depending on types of the catalyst and layered silicate. These results indicate that the variations in the conditions of gelation contribute to improvement in physical properties of silica-pillared molecular sieves.
60
[ No Catalyst ~ -- - Base Catalyst - - - Acid Catalyst
o~ ~2
a
......... Acid - - - - EtOH Base
o'~' 1.0 "~u m0.8 ~ 0.6
N o 0.4 ~ 0.2 o i
20
i
i
i
I
40 60 80100
i
200
Pore Diameter, [.~,]
(a) Silica-pillared magadiite
0.0
i
i
10
i
i
i
i
i i I
I
I
i
1
100
Pore Diameter(~) (b) Silica-pillared kenyaite
Figure 3. Pore size distributions of silica-pillared products.
REFERENCES 1) K. H. Berke, W. Schwieger, and M. Porsch, Chem. Tech., 39 (1987) 459. 2) O.Y. Kwon, S. Y. Jeong, J. K. Suh, H. Jin, and J. M. Lee, J. Colloid and Interface Science, 177, JCIS PN 3928 (1995). 3) J. S. Daily, and T. J. Pinnavaia, Chem. Mater., 4 (1992) 855. 4) M. E. Landis, A. B. Aufdembrink, P. Chu, I. D. Johnson, G. W. Kirker, and M. K. Rubin, J. Am. Chem. Soc., 113 (1991) 3189. 5) R. Sprung, M. E. Davis, J. S. Kauffman, and C. Dybowski, Ind. Eng. Chem. Res., 29(1990) 213. 6) H. P. Eugster, Science, 157 (1967) 1177. 7) J. McAtee, R. House, and H. P. Eugster, Amer. Mineral., 53 (1968) 1026. 8) K. Beneke and G. Lagaly, Amer. Mineral., 68 (1983) 818. 9) R.A. Fletcher, and D. M. Bibby, Clays and Clay Minerals, 35 (1987) 318. 10) A. Brandt, W. Schweiger, and K. H. Bergk, Rev. Chem. Miner., 24 (1987) 564. 11) C. J. Brinker, and G. W. Scherer, "Sol-Gel Science The Physics and Chemistry of Sol-Gel Processing." Academic Press, London, (1990), P 97-160. 12) R. Aelion, A. Loebel, and F. Eirich, J. Am. Chem. Soc., 72 (1950) 5705. 13) M. Nogami, and Y. Moriya, J. Non-Crysralline Solids, 37 (1980) 191. 14) J. H. Boer, B. G. Linsen, T. V. D. Plas, and G. J. Zondervan, J. Catalysis, 4 (1965) 649. 15) P. B. Barrett, L. G. Joyner, and P. P. Halenda, J. Am. Chem. Soc., 73 (1951) 373. 16) G. Lagaly, and K. Beneke, American Mineralogist, 50 (1975) 650. 17) L. Gurvitsch, J. Phys. Chem. Soc. Russ., 47 (1915) 805.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
61
A New Synthetic Route and Catalytic Characteristics of Pillared Rectorite Molecular Sieves
Jingjie Guan Zhiqing Yu Zhenyu Chen Liwen Tang and XieqingWang Research Institute of Petroleum Processing, China Petro-Chemical Corporation
This paper presents a new synthetic route and catalytic characteristics of pillared rectorite molecular sieves (PR-MS).
The PR-MS was prepared with aluminum pillaring
agent improved by poly (vinyl alcohol) according to route of mixing, drying to take shape, washing, and calcination.
It has wide pore structure with basal spacings of 27-30 ,~.
Under same conversion their activity for cracking heavy oil is 3-4 wt % higher and total yields of gasoline and light cycle oil are 6-7 wt % higher than that of commercial cracking catalysts containing USY zeolites. The catalysts developed from the PR-MS and Y-zeolites have high catalytic activity and low bottom yield. The PR-MS has also good light olefin selectivity. The catalysts containing the PR-MS and ZSM-5 type molecular sieves are a class of cracking catalysts for maximizing olefin production.
1. INTRODUCTION It is an important way for developing new energy resources to convert more heavy oil into liquid petroleum gas, gasoline and light cycle oil products. Although Y-type zeolites are commercially widely used as cracking catalysts their pore sizes are within the limits of less then 9,~. The pore sizes are not effective for cracking reactants with large molecule sizes. There is a need for new molecular sieves that are able to crack heavy oil and are easy to be commercialized in catalyst manufactory industry. Al-pillared rectorites developed by RIPP in 1985 are a class of new pillared clay molecular sieves [1]. The molecular sieves have excellent hydrothermal stability, solid acid sites and greater versatility than that of the faujusite [2]. These characteristics are advantageous for cracking heavy oil feedstock. However the prior pillared clay molecular sieves were prepared according to route of pillaring reaction of thin clay slurry and A1pillaring agent, washing, filtering, drying and calcination [3]. In general the clay particles are less than 2 mi'cra that are difficult to be filtered in commercial scale. In 1991 to tackle
62 the filtering problem a synthetic method without filtering operation of thin clay slurry was provided. However catalytic activity of the catalysts prepared by the method is not as good as Y-zeolite catalysts. For example, under same evaluated conditions by riser pilot unit the conversions of the two catalysts are 63 wt % and 67 wt % respectively [4] Up to now the pillared rictorite catalysts have not been used in FCC process. The object of this paper is to present a new synthetic route and catalytic characteristics of PR-MS with high catalytic activity. On the basis of direct research possibility of developing the PR-MS into commercial molecule sieves and microspheric cracking catalysts is studied. 2. EXPERIMENTAL 2.1 Synthetic routes of samples
Raw clays used for preparing samples are naturally rectorites with regularly interstratified mineral structure. Their quality specifications are conformable to properties in Table 1-2. Table 1 Physicochemical properties ofthe rectorites D001 by X-ray method /~
Cation exchange capacity meq/100g ....
Phase transformation temperature oC
23-24
40-60
1050
Table 2 Chemical composition of the rectorites Item Contents
_
Na20
CaO
Fe203
A!203
SiO2
wt %
1.2-2.0
3.5-6.0
< 1.5
39-43
43-51
Pillaring-bonding agent used for preparation of samples is Al-sol solution improved by poly (vinyl alcohol) (abbreviated PVA). Its 27NMR spectra are shown in Fig. 1. The PR-MS and catalysts containing PR-MS and other zeolite components were prepared by synthetic route A in Fig. 2 according to operation conditions reported in the literature [5]. The prior pillared interlayer rectorites (PIR) were prepared by conventional method [6] in accordance with synthetic route B in Fig. 2.
63
~.~ lOO
. . .~! ,~, ,~,~.~,
........ 50
0
-50
ppm
Figure 1 27NMR spectra of AI- pillaring-bonding agent
=---H ....
Rectorites Pillared-bonding agent Other composition ,.
H
Mixingand drying to take shape
Wa,shingFiltering of microspheric
m
Drying ..Samples Calcination
A: Synthetic route of the PR-MS and related catalysts
Rectorites-~
H Filteringof H Washing, .... FilteringU.... Drying
Pillaring
Pillaring agent..I I reaction
clay slurry
of clay slurry
..Samples
] i Calcination
B: Synthetic Route of the prior pillared rectorites Figure 2 Principle scheme of sample preparation It can be seen that in the prior PIR sample preparation special pillaring reaction is involved and the filtertion of clay slurry is difficult. However in the PR-MS preparation the special pillaring reaction operation required in prior art has been omitted because pillaring reaction can be completed in presence of PVA during mixing process prior spray drying. Also the pillaring agent improved by PVA is not only good pillaring agent used in thick clay slurry but also good bonding agent. The reaction mixture of the pillaring agent and clay is directly dried to take shape without filtering operation for fine clay slurry. It is easy to be produced in commercial scale. Besides this, it can contain other bonding agents required for preparing microspheric samples. Therefore, the new synthetic route can be used for preparation of microspheric cracking catalysts containing the PR-MS and other molecular sieves as well as the PR-MS
2.2. Physicochemical analysis 27NMR spectra were obtained by using Bruker Am-300 with operation conditions of SW-25000HZ SI=4K DE-300 LB=30 AQ-0085 RD--0.25 PW=3.
64 X-ray diffraction measurement was obtained by using Geigerflex D-9C X-ray diffractometor at a scan rate of 2~ 0/min and with monchromatic Cuka radiation. Surface areas and pore volumes were measured by using BET method from nitrogen adsorption isotherms. 2.3. Catalyst testing
Microactivity test was used for evaluating catabr characteristics of samples. The samples were deactivated at 800~ for 4 hours with 100% steam before evaluation. Catalytic activity for cracking light gas oil (235-337~ operation conditions of reaction temperature of 500 ~ (WHSV) of 16hr-1 and catalyst to oil ratio (c/o) of 3.2. Catalytic activity for cracking heavy oil (330-520~ conditions of reaction temperature of 520~
was obtained according to weight hourly space velocity
was evaluated by
operation
WHSV of 16hr-: and C/O of 3.0.
3. RESULTS AND DISCUSSION 3.1. Evaluation of new synthetic route
The chemical composition, physico-chemical properties and X-ray diffraction pattern of the PR-MS samples are shox~ Table 3-4 and Fig. 3. Table 3 Chemical composition of the PR-MS Contents Samples . . . .
wt %
Na20
CaO
Fe203
A1203
SiO2
The PR-MS
1.5
3.9
0.9
49.8
44.6
The prior P,IR
1.5
3.3
1.3
48.5
42.5
Table 4 Surface areas and pore volumes of the PR-MS Samples
Surface areas m2/g Fresh Steamed at
pore volumes ml/g Fresh Steamed at
800~ for 4hrs
800~ for 4hrs
The PR-MS
145
127
0.12
0.10
The prior PIR
144
101
0.13
0.10
65
29A
29A
=o cD
27/~
.>.
..>
3
I~
l
I
2 Theta
2 Theta
A: Sample prepared by present work
B: Sample prepared by prior method
Figure 3. X-Ray diffraction patterns of pillared rectorites 1" fresh sample. 2" sample steamed at 800~ for 4 hrs The results from Table 3-4 and Fig.3 indicate that the PR-MS prepared by new synthetic route in present work has physicochemical properties similar to prior PIR. The PR-MS has basal spacing of 29/~ for fresh samples and basal spacing of 27,~, for samples steamed at 800~ for 4 hrs that is the same as prior PIR, Especially, the height of d 001 peak for the PR-MS is higher than that of prior PIR indicating that a class of good pillared clays can be obtained by the simplified preparation method in present work. The new synthetic route of pillared clays and related catalysts has a bright future for commercial production. Catalytic activity of the PR-MS as compared with prior PIR is listed in Table 5. Table 5 Catalytic activity of samples for cracking light gas oil The PR-MS at present work
The prior PIR
Samples
Commercial USY catalyst
Microactivi~ %
75
71
72
66 The data in Table 5 indicate that PR-MS has microactivity of 75 wt % versus microactivity of 71 wt % by prior PIR and microactivity of 72 wt % by commercial USY catalysts.
It
means that catalytic activity of the PR-MS prepared by the new synthetic route is higher than that of prior PIR and commercial USY catalysts. It fully proves that the new synthetic route provided in present work is successful for preparing
pillared clays and related
catalysts.
3.2. Catalytic properties of the PR'MS and catalysts containing the PR-MS 3.2.1 Reaction characteristic for cracking heavy oil The activity and selectivity for crackling heavy oil with the PR-MS and catalysts containing the PR-MS and Y zeolites as compared with commercial USY catalysts are shown in Table 6. Table 6 Activit), and selectivity of th e PR-MS for cracking heavy oil Conversion Samples
Yield ,,.
wt
%
LCO
Bottom
wt %
Gas
Coke
Gasoline
USY catalysts The PR-MS
77.2
21.3
2.1
53.8
12.9
9.9
.at present work
77.4
16.1
4.7
56.6
16.4
6.2
The catalyst containing PR-MS and Y zeolites
81.8
22.3
3.6
55.9
13.8
4.4
Commercial
The data from Table 6 indicate that in cracking reaction with commercial USY catalysts a 9.9 wt% of bottom are remained but in the same operation conditions with the PR-MS and their catalysts only 6.2 wt % and 4.4 wt % of bottom are remained respectively. Obviously the PR-MS and its catalysts have catalytic activity for cracking heavy oil much better than that of the commercial USY catalysts. Also under almost same conversion level (77.2-77.4 wt %) the PR-MS yields gasoline of 56.6 wt % and light cycle oil of 16.4 wt % versus gasoline of 53.8 wt % and light cycle oil of 12.9 wt % by commercial USY catalysts indicating that the PR-MS has good gasoline and light cycle oil selectivity. The results are corresponding to wide pore structure of the PR-MS which is favored for cracking reactants with large size molecules.
67 3.2.2 Reaction characteristic for maximizing olefin production. The olefin selectivity of the PR-MS catalysts as compared with commercial REHY catalysts are listed in Table 7. Table 7 Light olefin selectivity of the PR-MS and for cracking heavy oil Samples Evaluation conditions
Conversion
wt %
Catalyst-1 REHY+Matrix
REHY+PR-MS
520~ C/O 3 WHSV 16-1
520~ C/O 3 WHSV 16-1
71
.Catalyst-3 ZSM-5+PR-MS 520~ C/O 4.5 WHSV 16-1 J
73
71
20.2 2.8 47.9 15.3 13.8
17.5 4.1 51.2 16.8 10.4
38.1 2.4 30.5 16.9 12.1
C~
0.8
0.6
1.7
C4
4.1
4.4
14.5
C5 C2 "C5
5.4
6.5
15.0
3.4 13.7
5.5 17.0
8.5 39.8
1.2
1.7
6.1
2.0
3.1
5.8
3.2
4.8
11.9
Product yield wt % Gas Coke Gasoline Light cycle oil Slurry Olefin yield wt%
iC] iC~
yield wt %
iC~-iC5
The data from Table 7 indicate that the PR-MS has not only high capability for converting heavy oil but also good light olefin selectivity. Under almost same conversion level (71-73 wt %) the catalyst-2 containing the PR-MS and the REHY zeolites has C:-C5 yield of 17 m
wt % and iC4-iC5 yield of 4.8 wt % but catalyst-1 containing only the REHY zeolites as active composition has C, -C5 yield of 13.7 wt % and iC4-iC~ yield of 3.1 wt %. It means that the light olefin selectivity of the PR-MS components is better than that of the Y-type
68 zeolites. Catalyst-3 developed from the PR-MS and the ZSM-5 type zeolites have the best light olefin selectivity in the three samples. It has C2 "C5 yield of 39.8 wt % and iC~-iC~ yield of 11.9 wt %. Obviously, the catalysts containing the PR-MS and ZSM-5 zeolites are a class of cracking catalysts for maximizing olefin production. The results above mentioned have clearly demonstrated that the PR-MS is both active components like molecular sieves and high activity matrix composition of catalysts. It can be used for preparation of cracking catalysts to convert more heavy oil .into light olefins gasoline and light cycle oil products.
4. CONCLUSION 1 The pillared rectorite molecular sieves (PR-MS) and related catalysts with high catalytic activity can be obtained by special Al-pillaring agent according to synthetic route of mixing, drying to take shape, washing and calcination. The preparation procedures are easy to be put into effect in commercial scale. 2 The PR-MS and microspheric cracking catalysts developed from the PR-MS and Yzeolites have high catalytic activity for cracking heavy oil and good selectivity of gasoline and light cycle oil. It is much better than that of commercial USY microspheric catalysts. 3 The PR-MS has better selectivity of light olefins than that of Y-zeolites. The catalysts containing the PR-MS and ZSM-5 type zeolites are a class of new cracking catalyst for maximizing light olefin production. REFERENCES !. 2. 3. 4. 5. 6.
Jingjie Guan; Enze Min and et al, Proceeding 9th International Congress on Catalysis Vol. 1, p104-111. Calgary, Canada ,(1991 ). Jingjie Guan and Thomas J. Pinnavaia, Abstract Proceeding International Symposium on Soft Chemistry Routes to New Materials in Nantes,France (1993). N. Lahav, U. Shani and J. Shabtai, Clays and Clay Minerals, Vol. 26, No. 2, p107-115 (1978). Jingiie Guan and et al , Proceeding of the International conference on Petroleum Refining and Petrochemical Processing, Beijing, P.R.China Vol. 3, p1255 (1991). C.N. 92114024 X (1995) U.S.Patent 4,757,040 (1988)
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
69
Textural Control of M C M - 4 1 Aluminosilicates Francesco Di Renzo, Nicole Coustel, Miren Mendiboure, H616ne Cambon and Franqois Fajula Laboratoire de Mat6riaux Catalytique et Catalyse en Chimie Organique, URA 418 du CNRS, Ecole Nationale Sup6rieure de Chimie de Montpellier, 8 rue de l'Ecole Normale, 34053 Montpellier, France, fax (33)67144349
The thickness of the walls between mesopores in MCM-41 sieves can be controlled by modifying the solubility of silica and aluminosilicate species, notably by changing the alkalinity of the synthesis system. This effect strongly influences the thermal stability of aluminosilicate MCM-41 in activation conditions.
1. INTRODUCTION The disclosure of the properties of micelle-templated mesoporous silicas [1-3] has represented a twofold breakthrough. From the point of view of the materials scientist, it has marked the starting point of a blossoming research on the selforganization of inorganics and surfactant molecules [4-6]. From the point of view of the catalysis and sorption technologist, it has fullfilled a long expectation for solids with custom-tailored uniform pores of size larger than the micropores of zeolites [7]. The pore size is easily controlled by choosing a surfactant able to form micelles of the required diameter. For instance, the diameter of the hexagonally-packed cylindrical pores of MCM-41 silica can be adjusted at any value in the 18-37 A range by incorporating alkyltrimethylammonium ions with hydrocarbon chains ranging from 8 to 16 carbons [2]. Less attention has been paid to another textural parameter of the MCM-41 honeycomb: the thickness of the silicate walls which separate the micelles of the as-synthesized material and the mesopores of the activated sieve [8]. Molecular
70 dynamics simulations have shown this parameter to control the stability of the MCM-41 framework [9]. This communication deals with the correlations between the synthesis and activation parameters and the texture of the final mesoporous sieve, and the informations thereby provided about the mechanism of self-assembly of cationic surfactant and silicates.
2. E X P E R I M E N T A L
MCM-41 aluminosilicate samples were prepared from synthesis systems of composition xNa20 2.8CTMA20 A1203 55SIO2 1800H20, where CTMA stands for cetyltrimethylammonium. The samples were numbered from 1 to 6 in the order of increasing
alkalinity,
expressed
as
[(Na++CTMA+-AI(OH)4-Br-2SO42")/SiO2].
Alkalinity values of the synthesis mixtures are reported in Table 1. Reagents were mixed with stirring at 70~
in the order:
deionized water,
NaOH
(SDS),
AI2(SO4)318H20 (Rh6ne-Poulenc Prolabo), CTMABr (Aldrich), precipitated silica (Rh6ne-Poulenc Zeosil 175MP, 175
m2/g, grain
wt%). The gel was heated up to 120~ autoclave, kept 15 minutes at 120~
size 2-201.t, Na 0.07 wt%, AI 0.17
in 1 hour in a 130 ml stirred stainless steel
and rapidly cooled down.
Table 1 Alkalinity of the synthesis medium and properties of mesoporous aluminosilicates.
Sample OH-/SiO2
a~ A
a550 A
AW<373K AW>373K p/po
Vmp
SBET
Woo
ml/g
mE/g
Woo
AI (Si+AI)
1
0.02
no MCM-41 formed
2
0.14
57
52
0.08
0.39
0.38
0.49
630
0.031
3
0.29
54
48
0.12
0.56
0.36
0.66
860
0.042
4
0.40
51
47
0.05
0.59
0.33
0.73
960
0.050
5
0.52
50
44
0.10
0.74
0.32
0.82
1090
0.069
6
0.65
49
44
0.12
0.77
0.28
0.55
940
0.083
71 The products,
washed with water and ethanol and dried at 80~
were
characterized by powder X-ray diffraction (XRD, CGR Th6ta 60 diffractometer, 0.25 mm slits, monochromated Cu Kt~ radiation), scanning electron microscopy (SEM) and electron probe microanalysis (EDX) (Cambridge Stereoscan 260 apparatus), thermal gravimetry (TG, Setaram B85 instrument, air flow, 15 mg samples, heating rate 5 K/rain), and N2 sorption at 77K (Micromeritics ASAP 2000C, 250~ 850~
outgassed samples) after calcination at temperatures in the 550-
range (air flow, heating rate 1 K/rain, 8 hours isotherm).
3. RESIILTS In Figure 1 a typical XRD diagram is reported, showing a peak slightly above 1 ~
with a width at half height of 0.4 ~
and a broad band centered around 2 ~
The two features correspond, respectively, to the 100 line and to the convolution of the 110 and 200 lines of a MCM-41 structure with low long-range order [10, 11]. The parameter a of the hexagonal lattice of all samples as synthesized and after calcination at 550~
are reported in Table 1. A shrinking of the hexagonal
cell with calcination is observed, as a decrease of the lattice parameter for the samples prepared at higher alkalinity. Intensities of the XRD lines significantly increase with calcination, probably due to the better contrast which results from
Figure 1. X-ray diffraction diagram of sample 3 after calcination at 550~
,,~1,,,i,,,i,~,1,~,1,,,i,,,i,,,i,,,i,,~1,,,
1
2
Theta Degrees
72
the extraction of the organic phase. The TG patterns are in good agreement with literature reports [3] which allow to attribute the weight loss below 100~ above 100~
to water desorption and the weight loss
essentially to the degradation and extraction of the organic phase.
Weight losses below and above 100~
expressed as fractions of the final mass are
reported in Table 1 for all MCM-41 samples. The weight loss corresponding to the surfactant decomposition is larger for samples prepared at higher alkalinity. In Figure 2 the isotherms of N2 sorption at 77 K are reported for a typical sample after calcination at 550 and 850~
(Figures 2a and 2b, respectively).
Sorption curves are typical type IV isotherms [12] without any hysteresis below p/po 0.9. The sorption step in the p/po range 0.3-0.4 is sharp for the samples calcined at 550~
(Figure 2a), indicating a narrow pore size distribution. The step
of the isotherm is less sharp and shifted to a lower pressure for the samples
700
Figure 2. Nitrogen sorption
600 n CO
400
o
300
O W
m
nt (D c0 Q
.2 0
/
500
c~
%, 0
-f
200 100
f
isotherms at 77K for sample
J
3 calcined at 550~
a) and 850~ (curve b). (+) adsorption, (*) desorption.
0
600 500 400 300
-//
f
200 100
0
~
0.0
~
I
T
0.2 RELATIVE
(curve
t
0.4
1
T ~ T
r
0.6
PRESSURE
0.8 ,
(P/Po)
I
73 calcined at higher temperature (Figure 2b), indicating a broader size distribution of mesopores with a smaller average diameter. The isotherm of sample 6 already features a less sharp sorption step after calcination at 550~ The partial pressure of the sorption step is reported in Table 1 for all samples calcined at 550~
as the mesopore volume evaluated at the top of the step and the
surface area from the BET equation for P/P~
The partial pressure of the
sorption step slowly decreases as the alkalinity of the synthesis system increases. Mesopore volume and surface area increase at increasing alkalinity up to a maximum at a OH/SiO2 0.5, and decrease for further increases of alkalinity. The c parameters of the BET equation are 102+5 for samples calcined at 550~ and 62+10 for samples calcined at 850~
A significant sorption takes place
beyond the filling of mesopores, in the partial pressure range 0.4-0.9, suggesting a large outer-surface area for the MCM-41 grains. The t-plot of the data of Figure 2a is reported in Figure 3. It indicates that no micropores are present (absence of downward deviation) [13] and confirms that capillary condensation takes place in mesopores with narrow size distribution. The gentle slope of the t-plot beyond the capillary condensation step corresponds to sorption on the outer surface of the mesoporous grains. SEM of all samples shows large aggregates of less than 100 nm grains. The aluminium mole fraction measured by EDX is reported in Table 1 for all samples. The aluminium content increases at increasing alkalinity of the synthesis medium.
EL i-O3
000 00
400 121 \ o o
0 W
m [~ o LO n CE
-
Figure 3. t-plot for the N2 sorption isotherm at 77K of sample 3 calcined at 550~
200
1 lZllZl
_J 0
IZl
-
!
IZI
1
u... t-HARKINS
i
i
i
1
4 ,~, J U R A
b ~.
(A
74 4. DISCUSSION The alkalinity of the
synthesis
system affects
several properties
of the
surfactant-silicate assembly. For any alkalinity value high enough to satisfactorily dissolve the source of silica, materials corresponding to the definition of MCM-41 sieves [14] are formed. The width of the XRD peaks does not correspond to a wide pore size distribution, suggesting that the lumping of the 110 and 200 peaks in one broad signal corresponds to a lack of long-range order, in agreement with the observed size of the MCM-41 grains, no larger than a score of lattice patterns. The small grain size can be accounted for by heterogeneous nucleation of MCM41 on colloidal silica [ 15]. In Figure 4 the values of the lattice parameter a and the CTMA content of the as-synthesized MCM-41 samples are reported as a function of the alkalinity of the synthesis medium. As alkalinity increases, the lattice parameter a, corresponding to the distance between micelle axes, decreases from 57 to 49 /~, while the CTMA/SiO2 mass ratio increases from 0.39 to 0.77. A simultaneous increase in the organic fraction and decrease of the distance between micelle axes corresponds to a lower volume fraction occupied by silica, hence to thinner walls between micelles. A simple geometric model, assuming hexagonal pore section [16], CTMA density equal to the density of hexadecane (0.77) and amorphous silica density 2.2 [17], allows to calculate the average wall thickness t. The calculated t values, reported in Figure 4, regularly decrease at increasing alkalinity and account for the observed decrease of the lattice parameter a. The micelle diameter of the as-
e.....
Fig. 4. Lattice parameter a ( 9 ), 1.0
50
.<
U
f
0.5
10. . . . OH'/tSi02
015
'
wall thickness t ( A ) , O b5 < I-"
o
and organic
mass fraction ( O ) of MCM-41 as a function of the alkalinity of the synthesis medium.
75 synthesized samples can be considered as independent of the alkalinity. The solubility of silica rapidly increases with alkalinity [18], whereas most aluminate reacts with excess silica in the whole synthesis field to form less-soluble aluminosilicate. As a consequence, the higher the alkalinity the higher the amount of soluble silica left in the solution at the end of the synthesis, and the higher the aluminiun content of the solid product. This result implies that the silicate coating of each micelle tends to be at the equilibrium with the surrounding solution throughout the precipitation of MCM-41, and strongly supports a mechanism of formation by self-assembly of inorganic-coated micelles [4]. The solubility of silica is the main factor which influences the equilibrium between silicate coating and solution, and the wall thickness depends on the ratio between the amount of outof-equilibrium
silicate
and the
area
of the
micellar
surface
available
for
condensation. The wall thickness significantly influences the thermal stability, as indicated in Figure 5, where the results of N2 sorption on MCM-41 calcined at 550~
are
reported as a function of the organic fraction in the parent material. The mesopore volume is proportional to the organic volume fraction and the pore diameter is nearly constant for all samples with walls thicker than 8 A. For all these samples, it can be assumed that calcination at 550~
has not altered the basic geometry of
MCM-41. In the case of sample 6, with average wall thickness lower than 8 A_, the pore volume and diameter are much lower than expected, and the pore size
.___e_._.=__.
Fig. 5. Mesoporous volume (O) and average pore diameter ( & ) 30
MCM-41
calcined at 550~
as a
function
of
mass
the
organic
fraction of the as-synthesized solid.
0.5
20 "4
10
E 01/ 0
of
!
CTMA/SiO 2
0.5
76 distribution is wider than for the other samples, indicating that the material has already begun to sinter in the activation conditions at 550~ REFERENCES
1. 2.
T. Yanagisawa, T. Shimizu, K. Kuroda & C. Kato, Bull. Chem. Soc. Jpn 63 (1990) 988. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli & J.S. Beck, Nature 359 (1992) 710.
3.
J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins & J.L. Sehlenker, J. Am. Chem. Soc. 114 (1992) 10834.
4.
C.Y. Chen, S. Burkett, H.K. Li & M.E. Davis, Microporous Materials 2 (1993) 27.
5.
Q. Huo, D.I. Margolese, U. Ciesla, P. Feng, T.E. Gier, P. Sieger, R. Leon, P.M. Petroff, F. Seh~th & G.D. Stucky, Nature 368 (1994) 317.
6.
7. 8.
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.F. Chmelka, Science 267 (1995) 1138. D . E . W . Vaughan, Studies in Surface Science and Catalysis 49 (1989) 95. N. Coustel, F. Di Renzo & F. Fajula, J. Chem. Soc. Chem. Commun. 1994, 967.
9. 10.
B.P. Feuston & J.B. Higgins, J. Phys. Chem. 98 (1994) 4459. P.T. Tanev & T.J. Pinnavaia, Science 267 (1995) 865.
11. 12.
C.Y. Chen, S.Q. Xiao & M.E. Davis, MicroporousMaterials 4 (1995) 1. S. Brunauer, L.S. Deming,W.S. Deming & E. Teller, ,LAmer. Chem. Soc. 62 (1940) 1723.
13.
J.H. de Boer, B.G. Linsen & Th.J. Osinga, J. Catal. 4 (1965) 643.
14.
J.S. Beck, C.T.W. Chu, I.D. Johnson, C.T. Kresge, M.E. Leonowicz, W.J. Roth & J.C. Vartuli, WO pat. 91/11390 (1991).
15.
J. Liu, A.Y. Kim, J.W. Virden & B.C. Bunker, Langmuir 11 (1995) 689.
16.
V. Alfredsson, M. Keung, A. Monnier, G.D. Stucky, K.K. Unger & F. Schtith, J. Chem. Soc. Chem. Commun. 1994, 921.
17. R.K. Iler, The chemistry of silica, Wiley, New York 1979, 22. 18. Ref. 17, 126.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
77
N e w routes for synthesizing mesoporous material Yan Sun a, Wenyong Lin a, Jiesheng Chen a, Yong Yue b, and Wenqin Panga* aKey Laboratory of Inorganic Hydrothermal Synthesis, Department of Chemistry, Jilin University, Changchun, 130023, P.R. China bWuhan Institute of Physics, Academia Sinica, Wuhan, 430071, P.R. China New routes including direct thermal treatment, room-temperature crystallization and microwave heating were developed for the formation of mesoporous material MCM-41, and the properties of the materials from these new routes were compared with MCM-41 hydrothermally synthesized. The pore sizes of the MCM-41 materials are normally distributed with an effective pore diameter that falls into the range of 20-40 A. An unusual product which possesses a bimodal mesopores distribution was obtained from wet-gel thermal treatment with no presence of any auxiliary organic molecules. The well-defined smaller pores are around 30 A in diameter, and the large pores are within the range 80-200A MAS NMR spectroscopy reveals that after calcination to remove the organic template in the AIcontaining MCM-41, a small part of the tetrahedrally-coordinated framework aluminum atoms become octahedrally-coordinated and a considerable amount of Si-OH species are generated.
1. INTRODUCTION In 1992 novel mesoporous molecular sieves designated as MCM-41 were reported by Mobil scientists[I-4].. These materials possess hexagonally arranged unidimensional pores, the size of which can be tuned within 20-100 A by varying the preparation conditions. Unlike other mesoporous materials such as intercalated clays, the pore size of an MCM-41 molecular sieve is considerably uniform, and its uniformity is comparable with that of microporous crystalline materials. The MCM-41 materials were normally obtained by hydrothermal preparation and their structures were found to be constructed mainly from amorphous inorganic silica walls around surfactant molecules. By adding guest molecules such as mesitylene into the synthesis system, MCM-41 samples with larger pore sizes can be obtained. However, the pore size of MCM-41 materials is affected by other factors as well.
78 Several mechanisms for the hydrothemml formation of MCM-41 have been proposed in the literature [5-8]. To further the understanding of the formation of MCM-41 molecular sieves, we investigated the factors that afti~ct the hydrothermal synthesis of these materials, and new synthetic routes other than the hydrothermal one have been developed. It is the purpose of this paper to describe the new routes to the formation of MCM-41 materials and its properties in comparison with the MCM-41 synthesized hydrothermally. 2. EXPERIMENTAL
2.1 Materials and Samples preparation 1. Hydrothermal synthesis of MCM- 41.
Sodium hydroxide, distilled water and a
surfactant C,H2,+I(CHa)3Nx(X=CI, Br; n=12, 16) were mixed with or without mesitylene (MES). Then the sodium aluminate solution was dropped in to the mixture with stirring. When the mixture became homogeneous, Aerosil (99% SiO2) or tetraethyl orthosilicate (TEOS) was added. The final reaction gel Was loaded in a PTFE-lined stainless steel autoclave followed by heating at 110-160 ~
for 1-7 days. The product was recovered by
filtration, washed with water and dried at ambient temperature. The as-synthesized samples were calcined at 550~ for 5 hours to obtain the template-free MCM- 41 materials. 2. Room-temperature crystallization of MCM-41. This synthetic route involved tetraethyl orthosilicate exclusively as the silicon source. H20, a surfactant and TEOS were mixed and stirred until homogeneous. To this mixture, either NaOH or hydrochloric acid was added followed by stirring for 30 minutes. The solid product was filtered, washed with water and dried at ambient temperature. 3. Thermal formation of MCM-41. There are two sub-routes for the thermal formation of MCM-41. The wet-gel route is as follows: NaOH, H20, a surfactant and tetraethyl orthosilicate were mixed with stirring for 2 hours. The mixture was transferred int6 a crucible followed by heating at 550~ in an oven for about 1 hour. Another sub-route is the precursor route: an amorphous solid precursor obtained by drying a hydrothermally treated gel was heated at various temperatures in an oven and the products were subjected to XRD Characterization. The gel used for the hydrothermal treatment was prepared in a way similar to that for the hydrothermal synthesis of MCM-41. 4. Microwave synthesis of MCM-41. Homogeneous reaction mixture containing NaOH, H20, a surfactant, the silicon source with or without NaAIO2 was sealed in a cylindrical PTFE container followed by heating in a 650W microwave oven for 1-30 rain. The solid product was recovered by filtration and dried at ambient temperature.
79
2.2 Characterization techniques X-ray powder diffraction patterns were obtained on a Scintag XDS-2000 diffractometer with Cu KGt radiation. The 20 scanning speed was 0.5~ The scanning electron micrographs were taken on a Hitachi X-650B scanning electron microscope. An ASAP 2400 automatic adsorption instrument was used to perform the N2-adsorption and the pore size distribution measurements, and a Bruker MSL-400 NMR spectrometer was used to record the 27A1 and 29Si NMR spectra. Cross-polarization technique was applied to obtain the 29Si NMR signals. 3. RESULTS AND DISCUSSION With Aerosil as the silicon source, the XRD patterns of products depend on the H20/SiO/ ratio in the reaction mixture. If H20/SIO2>80, a lamellar phase or a MCM-41 sample (designated MCM-4 l-A) with a well-defined XRD pattern showing the (100), (110), (200), and (210) reflection is. readily formed. However, when the H20/SiO2 falls into the range between 10-70, a mesoporous material (denoted MCM-41-B) which exhibits a strong broad XRD reflection at a 20 angle lower than 2 ~ and a weak one at around 3.5 ~ was obtained (Fig. l b). The typical gel composition for synthesis is shown in Table 1. By varying the synthetic
500~ C 400"C 250~ MCM-41-A 175~ without treatment MCM-41-B
2.0
4.0
6.0 20, o
Fig. 1 XRD patterns of MCM-41-AI and B2.
8.0
2.0
4.0
6.0
8.0
10.0
20, ~ Fig. 2 XRD patterns of typical solid precursor treated at different temperature for l h.
80 Table 1. Gel compositions and synthetic conditions for MCM-41 materials Sample
Gel composition a
Reaction
pH
temp(~
Reaction time (Hs)
MCM-4 I-A 1
0.06AlzO3:SiO2:0. 5( 16Br):0.18Na20:107H20
140
11
72
MCM-4 i -A2
0.05A1203:SIO2:0.25(16CI):0.23Na20:107H20
140
!1
72
MCM-4 ! -B 1 MCM-4 ! -B2
0.04AlzO3:SiO2:0.25(l 2CI):0.30Na20:30H~O
150
12
72
0.04A1203:SIO2:0.47(16CI):0.65Na20:23H~O
150
11
72
MCM-41-Cl MCM-4 ! -C2
SIO2:0.09(16C!):0.13(TMA)20: ! 8H20 SIO2:0.09(16CI):0.13(TMA)20:0.4MES: 18H20
150 ! 50
13 13
1 I
MCM-41-D1
TEOS:0.1 (16Br):(0.23-0.47)NazO: 118H20
-25
>10
0.5-10
MCM-41-D2
TEOS:0. i 3(16Br): 12HCI:94H20
-25
<1
0.5-5
MCM-41 -E MCM-41 -F 1
TEOS:0.63(I 6CI):0.23Na20:65H:O
....
11
TEOS:0.2(I 6C!):0.03Na20:71HzO TEOS:0.4(I 6Ci):0.05Na20:9 i HzO
550 550
11 11
MCM-41 -F2
0.2 72 72
"~16Br=C16H33(CH3)3NBr, 16CI=CI6H33(CH3)3NCl, 12Ci=CI2H25(CH3)3NCI conditions, the main peak for MCM-41-B shifts between 1.3 ~ and 2.0 ~ The chain length of the surfactant used for the preparation has no distinct effect on the d-spacing of the (100) reflection. A similar phenomenon was observed previously by Tanev et al.[9]. On the other hand, the addition of mesitylene into the reaction mixture invariably increases the d-spacing of the main peak. After calcination, the main peak for all MCM-41 materials shifts toward higher 20 angle and the maximum shift observed reaches 7 A ind-spacing. It is very interesting that some amorphous solid precursors, which were produced by hydrothermal treatment of gels with HzO/SiOz<80 at 100-140~ can be transformed into crystalline MCM-41 (designated MCM-41-C) after direct calcination at elevated temperatures. If the H20/SiOz ratio is larger than 80 in the initial reaction gel, the recovered amorphous precursors remain non-crystalline after the thermal treatment. Fig. 2 shows the effect of the thermal treatment temperature on the transformation of typical amorphous solid to MCM-41-C. It is seen that at 175~ the XRD-amorphous phase starts to gain some c.rystallinity and at 250~ the crystallinity increases greatly. At 500~ the product reaches its maximum. The formation of the crystalline MCM-4 I-C from an amorphous precursor is probably due to the rearrangement of the disorderly-placed inorganic walls in the precursor. With an appropriate gel composition, MCM-41 can be prepared at room temperature. The reaction mixture for room temperature preparation is either alkali (pH>10) or acidic (pH<_l), and TEOS is preferably used as the silicon source. To obtain highly crystalline MCM-41, the reaction mixture must be kept under stirring for at least 30 minutes, and longer crystallization times have no further effect on the Crystailinity of the product. The incubation probably helps the hydrolysis and the polymerization of the SiO44- species in the reaction mixture. The XRD pattern of the MCM-41 material (designated MCM-41-D) prepared at room temperature is similar to that for MCM-4 l-A, suggesting that as long as the ratio of H20/SiO2 falls into a
81 certain range, the variation of crystallization temperature has no influence on the formation behavior of MCM-4 I. Nowadays, microwave heating technique has been widely used for inorganic and organic synthesis. The microwave synthesis of zeolite-A, X, analcime and other crystalline microporous materials also appeared in the literature[10]. We investigated the microwave synthesis of mesoporous materials and found that it is an effective route to obtain highly crystalline MCM-41 (denoted as MCM-41- E). The typical preparation of MCM-41 takes only 15 minutes of microwave heating time using a 650W microwave oven whereas a similar reaction gel takes 1 day to form crystalline MCM-41 under hydrothermal preparative conditions. All the reaction gels used for microwave treatment are amorphous on the basis of XRD patterns, excluding the possibility that crystalline MCM-41 is formed before the microwave treatment. For the wet-gel thermal formation of MCM-41, the amorphous gel should be prepared from TEOS, and the room-temperature aging time (with stirring) also plays a key role in the formation of crystalline MCM-41. When the incubating time is less than 30 min, only amorphous phase is obtained after the thermal treatment, and after 2 hours of incubation, reasonably crystalline MCM-41 (denoted MCM-41-F) forms upon thermal treatment. The XRD spectra show that the gel at room temperature is highly amorphous since there is no diffraction peak on its XRD pattern whereas the sample calcined at 550 ~ exhibits some crystallinity as suggested by the diffraction peak at around 1.7 ~ corresponding to a d-spacing
MCM-41-A
/
600 MCM-41-A
MCM-41-B
400
200
MCM-41-F
MCM-41-F !
10 20 PORE DIAMETER, nm
Fig. 3 The pore distribution curves of MCM-41-AI, D1 and FI
0
0.2 0.4 0.6 0.8 RELA'FIVE PRESSURE, p/p, Fig. 4 The N2-adsorption isotherms of MCM-41 -A 1, B2 and F 1
1.0
82 of 50 A. The pore size distribution based on the desorption branch reveals that there are two mesopores sizes in the products (Fig. 3): one with an average pore diameter of about 30 A, the other
within the range 80-200 A. The majority of mesopores are within the range
between 80-200 A and the maximum of the distribution curve is at around 110 A. It is believed that these large mesopores are similar to those present in amorphous mesoporous silica gels, being formed by the packing of small silica particles. The total pore volume for narrow pores (30 A) is only 0.12 cm 3gl corresponding to a total surface area of 150 m/g -l, whereas that for the large pores (110 A) is 1.35 cm3g-1 with a total surface area of 430 m 2 g~. From the N2-adsorption isotherms for MCM-41 materials (Figure 4), we can see that the pore regularity of MCM-41-FI is less than that of MCM-4 l-A1 and B2 [11]. The Na20/SiO2 ratio in the gel affects to a great extent the appearance of the XRD patterns of the precursor and the calcined samples. For instance, with Na20/SiO2>0.25 leads to a gel precursor with XRD pattern showing clearly a peak between 1~ and 2 ~ Whereas with Na20/SiO2<0.12, no diffraction peak appears at all. Figure 5 shows the SEM graphs of three typical MCM-41 samples selected from Table 1. The particle size of the sample prepared at room temperature (MCM-41-D1) is about 20~tm whereas those for MCM-41-A1, B2, C1, and E1 are between 0.1 and 0.3 lam. It seems that low temperatures favor the formation of relatively large particles.
MCM-41-B2 samples
possess lower BET surface areas, larger pore volumes and pore sizes compared with MCM41-A1 and D1 samples. The pore size of MCM-41-D1 is exceptionally small (about 20 A). Both MCM-41-AI and D1 were prepared from gels with higher H20/SIO2 ratios. Therefore, it is believed that the H20 content in the reaction mixture plays an important role in determining the pore features of the product. MCM-41-D 1 and E 1 have a pore distribution
Fig. 5 The SEM micrographs of MCM-41-A I, B2 and D l
83
as-synthesized
/
calcined calcined
I
200
J
l
I
100
,
I
0 8, ppm
I
9
-100
, .
100
0
-100
-200
-300
8, ppm
Fig.6 27A1MAS NMR spectra of MCM-41-A1 Fig.7 29Si MAS NMR spectra of MCM-41-A1 similar to that for MCM-41-A1. The 27A1 MAS NMR spectra of the as-synthesized and calcined Al-containing MCM-41-A1 sample is shown in Fig. 6. The signal at -51 ppm is attributed to tetrahedrally coordinated AI, and it is seen that only tetrahedral AI is present in the as-synthesized material. Upon calcination, a small amount of octahedrally-coordinated AI was formed which corresponds to the weak signal at about 0 ppm. It is quite likely that during calcination, a part of the tetrahedrally-coordinated framework AI atoms move outside to the extraframework positions where they become 6-coordinated. The 29Si MAS NMR spectra (Fig. 7) indicates that Q3(Si(O)3(OH)) and Q4(Si(O)4) species exist in both assynthesized and calcined MCM-41-A1 sample. After calcination, however, a signal (-91 ppm) corresponding to Q2(Si(O)2(OH)2) species appears clearly, suggesting that the removal of the template molecules increases the amount of the Si-OH groups in the structure. 4. CONCLUSIONS Under hydrothermal preparative conditions, reaction gels with a H20/SIO2>80 give rise to MCM-41 materials possessing high surface areas and small pore sizes. Whereas that with 1-110/SIO2<80 the products possess lower surface areas and larger pore sizes. New routes including direct thermal treatment, room-temperature crystallization and microwave heating were developed for synthesizing the mesoporous MCM-41. With appropriate gel
84 compositions, MCM-41 materials crystallize at room temperature or under microwave treatment. Direct thermal treatment of certain wet gels at an elevated temperature (550~ also leads to cr3stalline MCM-41 materials. The products possess a bimodal mesopores distribution: one is about 30 A and the large one within 80-200 fi, Some solid samples recovered from hydrothermally treated gels are XRD-amorphous, but as used as precursors for thermal treatment they transform to MCM-41 molecular sieves. In contrast to hydrothermal synthesis, these new routes exhibited obvious features in the following: very short time, very simple, and selective formation of the pore size. MAS NMR spectroscopy reveals that after calcination a small part of the tetrahedraily-coordinated framework aluminum atoms becomes octahedrally-coordinated and a considerable amount of Si-OH species is generated. ACKNOWLEDGMENT We are grateful to the National Natural Science Foundation of China, the National Doctoral Science Foundation of China, and the KeyLaboratory of Inorganic Hydrothermal Synthesis of Jilin University for financial support. REFERENCES
[1]C.T. Kresge, M.E. Leonowicz, W.J. Roth, and J.S. Beck, Nature, 359 (1992), 710 [2]J.S. Beck, J.C. Vartuli, W.J. Roth, et al., J. Am. Chem. Sot., 114 (1992), 10834 [3]J.S. Beck, C.T. Chu, et al., US Patent No. 5 108 725 [4]C.Y. Chen, H. Li, M.E. Davis, Microporous Materials, 2 (1993), 17 [5]Q, Huo, D.I. Margolese, U. Ciesla, et al., Nature, 368 (1994), 317 [6]A. Monnier, F. Schuth, Q. Huo, et al., Science, 261 (1993), 1299 [7]V. Aifredsson, M. Keung, A. Monnier, G. Stucky, J.C.S. Chem. Commun., 921 (1994) [8]A. Steel, S.W. Carr and M.W. Anderson, J.C.S., Chem. Commun., 1571 (1994) [9]P.T. Tanev, M. Chibwe and T.J. Pinnavaia, Nature, 368 (1994), 321 [10]P. Chu, F.G. Dwyer, J.C. Varfull, Eur. Patent Appl., 358 (1990), 827 [1 I]R. Schmidt, M. Stocker, E. Hansen, Microporous Mater., 3 (1995), 443
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
85
Synthesis and characterization of FeSiMCM-41 and LaSiMCM-41 Nong-Yue He, Shu-Lin Bao and Qin-Hua Xu* Department of Chemistry, Nanjing University, Nanjing 210093, P.R.China Mesoporous materials SiMCM-41, FeSiMCM-41, LaSiMCM-41, A1SiMCM-41, and La~SiMCM-41 were synthesized. The mesoporous structure of these materials was characterized by XRD, FT-IR, nitrogen and benzene adsorption methods. EPR and Mossbauer spectra suggested the incorporation of Fe(III) in the channel wall of -synthesized FeSiMCM41 sample. The introduction of Fe(IIl) and La([II) inproved the thermal and hydrothennal stabilities of SiMCM-41. I. INTRODUCTION The synthesis of mesoporous materials with the regular array of uniform, One-dimensional mesopore in the range of - 1.5 to greater than 10 nm designated MCM-41 by the Mobil researchers is widely interesting[ I-9]. One of interested objects is to improve its hydrothermal stability[ 10-12]. It was well known that ferric and rare earth elements play an important role in modifying zeolite molecular sieves. However, though the lamellar or unidentified iron oxide mesostructured materials have been reported [5,7] and a Fe-containing MCM-41 mesoporous molecular sieve with SiO2/Fe203 molar ratio 150 using fumed silica as silica source has been reported too[13], the hexagonal FeSiMCM-41 has not yet been studied in detail In the present study, we detailly discussed the synthesis and characterization of FeSiMCM-41 and LaSiMCM-41, and investigated their thermal and hydrothertmal stabilities. 2. EXPERIMENTAL The FeSiMCM-41 mesoporous materials with different SiO2/Fe203 ratio were prepared according to the following synthesis procedures: 11 g distilled water, 0.4 g sulphuric acid(95%), and optionally, different ammounts ofFe(NO3)3 - 9H20, varying from 0 to 0.71 g, were mixed and stirred for 10 rain. Then 8.6 g sodium silicate (20.3%SIO2, 6.7%Na20) were added. After stirring the formed mixture for 10 rain, the appropriate ammotmt template solution(25% CTABr) was added. The formed gel was stirred for 30 rain before adding 6.7 g distilled water. Then the gel was transfered into a stopped teflon-lined steel bottle and heated without stirring at 373K for 7 or 14 days. After cooling to room temperature, the resulting solid product was recovered by filtration, extensively washed with distilled water, and dried in air at ambient temperature. The colour of the as-synthesized FeSiMCM-41 from gel mixture with SiO2/Fe203 ratio from 67 to 133 was white, but the color of as-synthesized FeSiMCM-41 sample from the gel mixture with SiO2/Fe203 ratio 33 was pale yellow. The preparation of LaSiMCM-41 mesoporous materials was similar to that mentioned above except for substituting lanthanum
86 nitrate for ferric nitrate. For comparison, AISiMCM-41 samples were also synthesized by means of adding a given ammount of sodium aluminate into the g d mixture instead of fenic nitrate. The dried products was calcined at 813 K in flowing nitrogen followed by air for 4h to remove the template. The calcined samples were characterized by low-angle X-ray powder diffraction (Rigaku,D/max-rA) with Cu-K cx radiation, FT-IR(Nicolet, 5DX), EPR (Bruker EP 200-DSRC), the Mossbauer spectrum (recorded on a constant acceleration Mossbauer spectrometer with a source of 57Co in Pd matrix), the adsorption of nitrogen at 77K (Micromeritics, ASAP2400 instrument) and benzene adsorption at 298 K (on an conventional gravimetfic method). To compare the thermal stability, the above calcined samples were calcined once more in a muft]e stove in air at 1153 K for 2 h respectively, followed by XRD detection. The composition of ~mples were obtained on a Jarrell-Ash 1100 inductively coupled plasma quantometer. The hydrothermal stability of samples treated with 100% steam at 933 K for 2 h were investigated by benzene adsorption technique. 3. RESULTS AND DISCUSSION 3.1 XRD The powder X-ray di~action patterns of calcined samples are shown in Figure 1. q]~e observation of four peaks of samples which can be indexed on a hexagonal lattice is typical of MCM-41 materials[I]. Owing to the lack of good long-range hexagonal order[3], the (110) and (200) peaks of sample 8 overlapped each other. Table 1 The composition of gel mixtures and samples* Molar composition of.the gel Sample CTAB SiO2 A1203 Fe203 La203 Na20 H20 1 2 3 4 5 6 7 8 9
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
1 0.18 1 0.008 0.18 1 0.015 0.18 1 0.030 0.18 1 0.008 0.18 1 0.015 0.18 1 0.030 0.18 1 0.015 0.015 0.18 1 0.015 0.18
57 57 57 57 57 57 57 57 57
As-synthes~ed samples SiO2/m1203 SiO2/Fe203 SiO2/La203 pure siliceous 124 62 32
40 58
147 83 48 116
* Crystalized at 373 Kfor 7 days(samples 1 and 9) and 14 days (samples 2-8). The hexagonal FeSiMCM-41 and LaSiMCM-41 mesoporous molecular sieves were synthesized from the given gel mixtures as shown in Table l.The white color of as-synthesized
87 samples 2 and 3 exln'bites the absence of coloured oxides of iron and may give an indication of Fe(IlI ) incorporated into the framework position.
I
a a
b
b C
d e
2
4
6
8
2 | (degrees) Figure 1 XRD patterns of calcined samples a--sample 1, b--sample 3, c--sample 6, d--sample 8, e--sample 9
Figure 2 EPR spectra of as-synthesized samples FeSiMCM-41 samples. a--sample 2, b--sample 3, c--sample 4
The d-spacing and lattice parameters of samples are listed in Table 2, it is shown that the dl0o and the lattice parameter of calcined MeSiMCM-4 I(Me=AI, Fe and La) are greater than those of SiMCM-41, consistent with the Me-O bonds longer than Si-O bond and giving an evidence of Me(III) incorporation into framework position too. Table 2 Characterization of samples Before calcination Sample dloo a (nm) (nm) 1 3 6 8 9
4.27
4.93
4.29
4.95
* Lattice parameter a=dlo02v/3.
After calcination d~0o a (nm) (nm) 4.03 4.46 4.08 4.19 4.15
4.65 5.15 4.71 4.84 4.79
pore size (BJl-Lnm) 3.2 3.3 3.3 3.0 3.3
surface area (m2/g) 1340 947 932 944 1084
88 3.2 EPR spectra
The EPR spectra ofFeSiMCM-41 ~mples containing different SiO2/Fe203 ratio are shov~n in Figure 2. There are two signals in all the spectra, g--4.3 and g=2.0, assigned to tetrahedrally and octahedraUy coordinated Fe(Ill) respectively[14]. We found the relative intensity of sig~lal with g=4.3 to that of g=2.0 decreased with the increase of Fe(III) content in FeSiMCM-41 samples. Therefore, the amount of Fe( III ) incorporated into the framework position was limited. 3.3 Mossbauer Spectra
The Mossbauer spectra of sample 3 are shown in Figure 3a. The isomer shift (IS) of assynthesized sample is 0.28 mm/s, correspondent with tetrahedrally coordinated Fe( [II ) in framework[15]. Therefore, Fe(III) was mainly incorporated in fxamework positions of assynthesized FeSiMCM-41 sample. However, after calcination at 813 K for 4 h in air, The isomer ~ifl increases to 0.43 ram/s, which was ascribed to octahedrally coordinated Fe(I!!) (Figure 3b). It meant that the tetrahedrally coordinated Fe(III) had been mostly transformed to octahedrally coordinated Fe(Ill). This result is consistent with the EPR data of calcined samples(see Figure 7c). a
/b ~
.. '. :: ~" :.-. ~
a
--~........r C
Z
J
o
" " " ."
~'.'..X':
-)(..':'"."
"''"
fd
9
te
o
O
-.,.~'~
.,.
.... :, .. ~.?~.. ".. I
-5.0
-2.5
9:
.o
~?..
."
.~'...~.."':':. ~"
|
t
0.0
2.5
5.0
__t
1200
1000
800
600
400
VELOCITY (mm/s) WAVENUMBERS (cm-~) Figure 3 Mossbauer spectra of sample 3. (a)--as-synthesized (b)--after calcination at 813 K for 4 h in air
Figure 4 FT-IR spectra for calcined samples and fumed silica. a--sample 1, b--sample 3, c--sample 6, d--sample 9, e--fumed silica
3.4 F r - I R
FT-IR spectra of samples 1,3,6,and 9 are similar to the amorphous fumed silica (Figure 4), therefore, the channel wall of the as-synthesized mesostructured materials is amorphous[2l Compared with the spectrum of SiMCM-41, all the bands of MeSiMCM-41 (Me=Fe, La and
89 AI) shift to lower wavenumbers, indicating Fe(III), La(III) and Al(IlI) may be incorporated into the chaired wall, for Me-O bonds are longer than Si-O bond.
3.5 Adsorption experimentais Figure 5 shows the nitrogen adsorption-desorption isotherms for the samples at 77 K. All the isotherms exhibit a sharp inflection at a relative pressure about 0.36 characteristic of capillary condensation with uniform mesopores. The BJH pore size distribution indicates a narrow pore size distribution for all the samples at about 3.0-3.3 nm (see Table 2).
700
b
900
500
400
100
100
f I
I
0.2
0.4
I
0.6
I
rl
0.8
1.0
700
0
,,,
0.2
_1
i I
0.4
0.6
. I
0.8
1.0
900 C
d
400
500 f
100
100 4
!
0.2
0.4
I
0.6
i
0.8
900
1.0
0
0.2
i
0.4
e
r~
500
100 r
I
0.2
0.4
0.6
0.8
P/Po Figure 5 Adsorption isotherems of nitrogen of calcined samples. a--sample 1, b--sample 3, c--sample 6, d--sample 8, e--sample 9
1.0
,
i.
I
0.6
0.8
1.0
90 The BET surface area are also given in Table 2. Compared with the pure siliceous sample, the surface area of samples containing Fe, La and AI decreases slightly. 3.6 Thermal and hydrothermal stabilities It was noted that the (100) peak intensity of XRD for the template-removed SiMCM-4 I was very strong before calcination at 1153 K (Figure 6(a)), but it become weaker after the calcination with a decrease of d-spacing about 0.33 nm Conversely, the peak intensity of FeSiMCM-41(sample 3) was decreased slightly and the d-spacing only changed 0.05 mn after the same calcination (Fig.6(b)), while A1SiMCM-41 lost most of its structure after the calcinmation. Then compared samples calcined in air with that in N2 followed by air, the XRI) patterns ofFeSiMCM-41 and LaSiMCM-41 made no significant changes, while the (100) peak intensity of AISiMCM-41 decreased obviously, which suggested the incorporation of AI decreased the thermal stability of SiMCM-41. DTA curves for samples showed that FeSiMCM-41, LaSiMCM-41, SiMCM-41 and AISiMCM-41 collapsed starting at 1163, I 160, 1153 and 1149 K respectively. Thus, the order of thermal stability of samples obtained fi'o~n XRD and DTA data is shown as following: FeSiMCM-41 = LaSiMCM-41 > SiMCM-41 "; A1SiMCM-41.
e ~
.ill~,
"" i
l..,, IJ
__
i
2
4
6
2 | (degrees)
8
2
4
6
8
2 | (.degrees)
Figure 6 The XRD patterns for calcined (a) sample 1 and (b) sample 3, solid lines befbre calcination, dashed lines after calcination at 1153 K in air for 2 h. It was reported that the framework of A1SiMCM-41 collapsed at 963 K in stea~n treatmemt[3]. In our study, AISiMCM-41 (sample 9) steamed at 933 K lost its XRD pattten~ almost completely, which indicated its poor hydrothermal stability. However, FeSiMCM41(sample 3) and LaSiMCM-41(sample 6) still remained a stronger (100) peak and a high equih'bdum adsorption capacity for benzene under the same treatment conditions. Compared XRD values of Ill0 and c/c0 listed in Table 3, the sequence of the hydrothermal stability of samples is, FeSiMCM-41 = LaSiMCM-41 > SiMCM-41 > LaSiMCM-41 > AISiMCM-4 I.
91 From the above results, we suggest that the introdoction of Fe or La into the SiMCM-4 I increase both the thermal and hydrothermal stab'dities, while AI decrease them. Table 3 Hydrotherm__al stability of samples Before steam treatment
Sample
d,oo(nm)
1 3 6 8 9
4.03 4.46 4.08 4.19 4.15
After steam treatment
co(g/g)
dl0o(llm)
0.680 0.605 0.548 0.465 0.655
3.72 4.12 3.72
c(g/g) 0.381 0.408 0.348 0.234 0.278
1/I0(%) 54 81 83 very weak peak very weak peak
c/c0(%) 56 67 64 50 42
I0, I : The intensities of(100) peaks before and after steam treatment respectively. co, c : The adsorption capacity for benzene at P/P0=0.5 before and after steam treatment respectively. Figure 7 shows the EPR spectra ofFeSiMCM-41 at different calcination time. It was found that the color of FeSiMCM-41 samples changed from white to brown by increasing the calcination time, accompanied by decreasing the signal intensity at g=4.3 and increasing that at g=2.0 (Figure 7 b-d), suggesting that the tetrahedrally coordinated Fe(III) in as-synthesized sample FeSiMCM-41 progressively converted to octahedrally coordinated Fe(Ill) as hmreashlg the calcination time. The Mossbauer spectra also shows the same change of the coordination
8 a
b
d
Figure 7 EPR spectra of calcined sample 3. a--as-synthesized, b--1 h, c--4 h, do-8 h calcined at 813 K in air
92 enviroment of Fe(III) before and after the calcination (Figure 3). Thus, we suggest that the octahedrally coordinated Fe( Ill ) species in FeSiMCM-41 may increase the thermal and hydrothermal stabilities of the channel wall ofmesoporous material 4. CONCLUSION The hexagonal mesoporous FeSiMCM-41 and LaSiMCM-41 have been synthesized using sodium silicate as silica source. Fe( III ) and La( III ) species may impove the thermal and hydrothermal stabilities of SiMCM-41 but AI(III) destabilizes it. It seems that octahedrally coordinated Fe(Ill) species led to increase of thermal and hydrothermal stabilities of the channel wall ofmesoporous FeSiMCM-41. REFERENCES
1. C.T.Kresge, M.E. Leonowicz, W.J.Roth, J.C.Vartuli and J.S.Beck, Nature, 359 (1902) 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.Olson, E.W.Sheppard, S.B. McCullen, J.B.Higgins and J.L.Schlenker, J.Am. Chem Soc., 114(1992) 10834. 3. A.Monnier, F.Schuth, Q. Huo, D. Kumar, D. Margolese, R.S. Maxwell, G.D. Stucky, M. Kishnamurthy, P. Petroff, A.Firouzi, M. Janicke and B. F. Chmelka, Science, 261 (1993) 1299. 4. C.Y. Chen, S.L. Burkett, H. X. Li and M. E. Davis, Microporous Materials, 2 ( 19q3 ) 27. 5. Q. Huo, D.I. Margolese, U. Ciesia, P. Feng, T.E. Gler, P. Sieger, IL Leon, P. M. Petroft~ F. Schuth, and G. D. Stucky, Na~-~, 368 (1994) 317. 6. P. T. Tanev, M. Chibwe and T. J. Pinnavala, Nature,368 (1994) 331. 7. P. L. Llewellyn, U. Ciesla, H. Decher, R. Stadler, F. Schuth and K.K. Linger, Stud. Surt~ Sci. Catal., 84 (1994) 2013. 8. A. Corma, M. T. Navarro, J. Perez-Pariente and F. Sanchez, Stud. Sure Sci. Catal., 84 (1994) 69. 9. O. Franke, J. Rathousky, G. Schulz-Ekloff~ J. Starek and A. Zukal, Stud. Surf Sci. Catal.. 84 (1994) 77. 10. S.B.McCullen, J.C. Vartttli and W. Chester, Method for Stabili~ng Synthetic Mesoporous Crystalline Material, U.S Patent No. 5 516 829(1992). 11. N. Coustel, F. Di Renzo and F. Fajula, J. Chem. Sot., Chem. Commun., (1994) 967. 12. R. Ryoo andJ. M. Kim, J. Chem. Sot., Chem. Commtm., (1995) 711. 13. Z.Y. Yuan, S.Q.Liu, T. I-L Chen, J.Z. Wang and H.X. Li, J.Chem.Soc., Chem. Commum.. (1995) 973. 14. 1LSzostak, V.Nair and T.L.Thomas, J.Chem.Soc.Faraday Trans.I.(1987) 487. 15. P. Ratnasamy and 1L Kumar, Catal. Today, 9 (1991) 329.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
93
Synthesis of titanium-containing mesoporous molecular sieves with a cubic structure K. A. Koyano and T. Tatsumi Engineering Research Institute, Faculty of Engineering, The University of Tokyo, Yayoi, Tokyo 113, Japan 1. INTRODUCTION Recently a new family of mesoporous molecular sieves named M41S was discovered by the researchers at Mobil [1, 2]. The M41S family is classified into several members: MCM-41 (hexagonal), MCM-48 (cubic) and other species. These materials have uniform pore, and the pore size are able to be tailored in the range 16/~ to 100/~ through the choice of surfactant, auxiliary chemicals and reaction conditions. Because of the favorable uniformity and size of the pore, synthesis and utilization of mesoporous materials have been investigated by numbers of researchers. Some circumstantial reports about the synthesis have been made [3-5]. Adsorption property and catalytic activity of silica-based and Al-containing MCM-41 have also been reported [6, 7]. Ti-, V-, B-, Fe- and Mn-substituted MCM-41 and Ti-substituted hexagonal mesoporous silica (Ti-HMS) have also been synthesized [8-15]. These Ti- and V-substituted mesoporous molecular sieves pioneered the potential to oxidize bulky molecules which cannot enter into the micropores of zeolites such as TS-1, TS-2 and Ti-beta. MCM-48, characterized by a three-dimensional channel system, may have several advantages over MCM-41 with a one-dimensional channel system when applied to catalytic reactions: for instance, the three-dimensional pore system should be more resistant to blockage by extraneous materials than the one-dimensional pore system. Here we report the synthesis of Ti-MCM-48 and its use as a catalyst for epoxidation of alkenes and unsaturated alcohols. The effects of gel composition and the gel preparation method on the structure of mesoporous materials are also reported. 2. EXPERIMENTAL
Mesoporous materials were synthesized under hydrothermal conditions at 373 K in a static Teflon bottle for 10 days. The procedures of gel preparation were as follows. For the preparation of pure silica mesoporous materials, an aqueous solution of cetyltrimethylammonium chloride / hydroxide (CTMACI/OH, CI/OH - 70/30) was added dropwise to tetraethyl orthosilicate (TEOS) under vigorous stirring at 278 K. After stirred for 1 h, the mixture was heated at 358 K for 4 h to remove the ethanol produced in the hydrolysis of TEOS. For the synthesis of Ti-containing mesoporous materials, two types of hydrolysis method were employed. Ti-MCM-48(1) was prepared by a one-stage hydrolysis method: TEOS and tetrabutyl orthotitanate (TBOT) were hydrolyzed simultaneously after being mixed for 30 minutes at 298 K. Ti-MCM-48(2) was prepared by a two-stage method : To a 44% solution
94 of TEOS in propan-2-ol, CTMAOH in methanol and water (water / TEOS (molar) = 2) were added to partly hydrolyze TEOS at 278 K. After 1 h, a propan-2-ol solution of TBOT was added to this resultant mixture very slowly under vigorous stirring. The.mixture was then stirred for 1 h, when the aqueous solution of CTMAC1 was finally added. When water-glass (SiO 2 = 28 - 30%, Na20 = 9 - 10%) was used as the Si source, sulfuric acid was added to the mixture in order to adjust the pH to 11.6 and CTMAC1 was used as the template. The molar compositions of the gels subjected to hydrothermal synthesis (373 K, 10 days) were as follows : SiO 2. xTiO 2 9yNa20 9CTMA 9zH20, where 0 _< x _<0.02, y = 0 (TEOS) or 0.29 0.35 (water-glass), and 46.5 _
3. RESULTS AND DISCUSSION 3.1. Time course of hydrothermal synthesis Before we attempt to introduce Ti into the framework of MCM-48, we investigated the optimal conditions for the synthesis of pure silica MCM-48. Since it was found that the incorporation of titanium into zeolite frameworks was negatively affected by the presence of alkali cations, TEOS was used as the Si source. In the preparation of MCM-48, we conducted the hydrothermal synthesis in a Teflon
(211)
(420) .,..~ r~
(c) (B)
(A) 2
4
6
20 / degree
8
10
Figure 1. Change of XRD pattern with time in hydrothermal synthesis of MCM-48. (A) 2.5 days, (B) 6 days, (C) 10 days.
95 bottle at 373 K after keeping the mother liquids at 358 K for 4 hours to remove alcohols. The composition of reaction mixture was TEOS / CTMA / H20 = 1 / 1 / 46.5. As shown in Figure 1, mesoporous structure was formed after 2.5-day hydrothermal treatment. The typical pattern of the cubic structure with one strong peak and 7 weak ones in the 20 range of 2-7 ~ distinctly appeared after 10 days. This finding indicates that 10-day hydrothermal treatment is required to synthesize MCM-48 with the cubic structure, using TEOS as the Si source under our conditions. It is to be noted for the synthesis of MCM-41 under the same hydrothermal conditions except for the composition of reaction mixture ( TEOS / dodecyltrimethylammonium (DTMA) / H20 = 0.75 / 1 / 84 ), the formation of hexagonal structure also required 10 days while mesoporous structure was formed before the hydrothermal treatment. 3.2. Effect of surfactant concentration
Vartuli et al. have reported that the surfactant/silica molar ratio (Surf/Si) is a critical variable in the formation of M41S materials; cubic mesoporous material MCM-48 was formed when the Surf/Si ratio was adjusted to 1-1.5 [5]. We attempted the synthesis of mesoporous materials by varying the concentration of the surfactant with the Surf/Si ratio kept constant at 1.0. At 20.0 wt% of CTMAC1/OH (70/30) (H20/CTMA = 69.8), MCM-41 was exclusively obtained. The structure was transformed from hexagonal into cubic with increasing concentration of the surfactant (Table 1), in good agreement with the effect of concentration on the formation of a liquid-crystal phase for the CTMA system [16]; the cubic phase is favored over the hexagonal phase at higher concentrations for the chloride containing system. Obviously, the structure of the products was affected not only by Surf/Si but also by the surfactant concentration. Using TBOT as the Ti source, Ti-MCM-48-(1) and -(2) molecular sieves were successfully obtained from the gels with the following molar compositions: SiO 2 9 0.01TiO 2 90.7CTMACI 90.3CTMAOH 946.5I-I20. The increase in the Ti content to Si/Ti = 50 (two stage hydrolysis method) resulted in the formation of an ill-defined mesoporous material, which showed only one strong intensity peak at ca. 20 = 2 o. Table 1 Effect of concentration of surfactant and Si source on the structure of products Si source a
Si/Ti b
TEOS TEOS TEOS TEOS
0 0 100 50
H20/CTMA 69.8 c 46.5 c 46.5 c 46.Y
structure hexagonal cubic cubic ill-defined mesoporous
aTEOS: tetraethyl orthosilicate, Si/CTMA (cetyltrimethylammonium) = 1.0. bTi source : tetrabutyl orthotitanate, c CTMAC1/OH = 70/30. Hydrothermal synthesis conditions: 373 K; 10 days; under static condition. 3.3. Effect of Si source
Using colloidal silica as a Si source instead of TEOS, MCM-48 was obtained; however, the crystallinity was slightly lower than the product formed from TEOS. (Figure 2) When water-glass was used as the Si source, MCM-41 was obtained from the gels with the same Surf/Si and HzO/Surf ratios from which MCM-48 was obtained. There is an important
96
,i..o
(c) O
/
/
(A) !
0
2
4 6 20 ! degree
8
10
Figure 2. XRD patterns of as-synthesized mesoporous materials prepared from various Si sources. (A) TEOS, (B) colloidal silica, (C) sodium silicate.
difference between the Si sources; water-glass giving MCM-41 contains a large amount of Na, in contrast to TEOS and colloidal silica which give MCM-48. When a small amount of Na § (Na/Si molar ratio = 0.15) was added to TEOS together with the surfactant, the ill-defined mesostructure was formed. Even when Na § was added to the gel prepared from TEOS immediately before the hydrothermal treatment, we obtained poorly crystalline material. It has been shown that the addition of Na § dramatically changes the phase diagram of aqueous CTMA [ 17]. Thus it is conceivable that the presence of Na seriously affected the structure of the products. The details will be reported elsewhere. 3.4. Effect of pH
The pH is one of the important factors in the synthesis of all the kind of zeolites. We investigated the effect of pH by varying the ratio of anion of surfactant (CTMAC1/OH 70/30, 80/20, 90/10). As shown in Figure 3, the cubic structure was formed at the CTMAC1/OH = 70/30. With decreasing pH, the product structure was transformed from cubic into ill-defined
d .,..~ r~
(c)
O
(B) ~ ,-"-7~--4
,,
,(A)
6
8
20 / degree
Figure 3. XRD pattems of as-synthesized mesoporous materials prepared by variety of CTMACI/OH ratios. (A) 90/10, (B) 80/20, (C) 70/30.
97 mesostructure and finally amorphous. No solid products were obtained at the pH higher than 12. These results lead to the conclusion that the appropriate pH for the synthesis of MCM-48 is the range of 11 - 12.
3.5. Ti-eontaining MCM-4$ Titanium-containing mesoporous material with the cubic structure, Ti-MCM-48, has been synthesized by the addition of tetrabutyl orthotitanate as a Ti source. As shown in Figure 4, Ti-MCM-48(1) exhibited a very strong peak at d - 35.6 A and medium or weak peaks a t d = 31.0, 23.4, 21.8, 19.5, 18.6, 17.9 and 17.2/~. These eight peaks were indexed on a cubic unit cell with a - 87.2/~. The nitrogen physisorption isotherm was characteristic of mesoporous materials with uniform pore size. The average pore diameter determined by the Dolimore-Heal method was 25/~ and the BET surface area was ca. 1000 m2g1 . These data totally agree with those of pure silica MCM-48. We studied the pore structure of Ti-MCM-48 by high resolution transmission electron microscopy. Figure 5 exhibits a monoclinic type repeat pattern containing a 109.5 ~ angle, on the cubic [110] phase. Besides, typical regular two-dimensional patterns, such as [111] and [100], were observed, although most of the observed sheets show [111] or [110] patterns. These observations were in good agreement with those made by Mobil researchers for pure-silica MCM-48 [5]. From the XRD and TEM patterns, it has been revealed that the symmetry of Ti-MCM-48 was consistent with an I a 3 d space group, as was proposed by Schmidt et al. [18]. No differences in the TEM patterns were found between pure silica MCM-48 and Ti-MCM-48. As shown in Figure 6, solid-state 29Si MAS NMR spectrum of Ti-MCM-48 closely resemble that of pure silica MCM-48. These spectra are able to be separated into three broad peaks from Q4, Q3 and Q2 silicones by an appropriate choice of CP conditions. Though the peaks were rather broad, Q3 content (27%) roughly agreed with the N/Si molar ratio (= 0.30) observed for the as-synthesized pure silica materials [5]. This agreement in absolute numbers, .
.
.
.
.
.
.:
..
.. . . . . . .
...
..
:,::
,..
:...
:
.
.
.
.
.
r~
.~
(B___2____) (A) I
0
2
I
I
4
6
I
8
10
-'~-- bU n m ~
.t ~ .~:, ~. - %~ . - %m ~.m-. ~' m. . . - 9~
20 / degree
Figure 4. XRD patterns of MCM-41. (A) pure silica MCM-48, (B) Ti-MCM-48(1)
Figure 5. Transmission electron micrographs of Ti-MCM-48(1) showing the [110] projection.
98 Q3 (27%) and the N/Si ratio (30%) may indicate that Q3 is associated with the surfactant. The UV-VIS spectra for the Ti-MCM-48 samples are shown in Figure 7. The band at 220 nm has been assigned to isolated framework titanium in tetrahedral coordination [19]. TiMCM-48(1) showed a broad shoulder at ca. 270 nm attributed to extraframework titanium [20]. The anatase band, which occurs at 312 nm, seems to be superimposed on this band. In contrast, Ti-MCM-48(2) proved to be essentially free of extraframework titanium. These results indicate that the two-stage hydrolysis method is favorable for the isomorphous substitution of Ti for Si. This is interpreted in terms of the prerequisite condition to the efficient incorporation of Ti in the zeolite framework observed for TS-I: the rate of hydrolysis of Ti alkoxide should match that of Si alkoxide [21 ].
3
(B) (A) ! . . . . . . .
. . . . .
. . . .
. . . .
'-14o
ppm from TMS Figure 6. 29Si MAS NMR spectra of MCM-48. (A) pure silica MCM-48, (B) Ti-MCM-48(1).
200
!
400 600 Wavelength / nm
800
Figure 7. UV-VIS spectra of Ti-MCM-48. (A) Ti-MCM-48(2), (B) Ti-MCM-48(1).
3.6. Catalytic activity The oxidation of cyclododecene (c&/trans = 75/25) was performed using Ti-MCM-48, Ti-MCM-41, TS-1 and amorphous TiO2-SiO 2 with H202 or TBHP as the oxidant at 323 K. Ti-MCM-41 was synthesized at Surf/Si = 0.6, H20/CTMA = 75, and Si/Ti = 80 by the two stage method. As shown in Table 2, cyclododecene was epoxidized to cyclododecene oxide with either TBHP or H202 on Ti-MCM-48 and Ti-MCM-41. Preference for the epoxidation of trans-isomer was obtained for both oxidants. No other products were detected. The high activity of Ti-MCM-48 materials compared to Ti-MCM-41 may be due to the three-dimensional pore structure of Ti-MCM-48; Ti-MCM-48(2) was slightly more active than Ti-MCM-48(1). Amorphous TiO2-SiO 2 showed a very low activity in the epoxidation using H202. Compared with Ti-MCM-48(1), TiO2-SiO 2 showed a much more substantial difference in activity with H20 2 and TBHP, probably owing to severe inhibition of the catalyst by water [22]. No products were obtained with TS-1 probably as a consequence of the inability of cyclododecene to diffuse into the pores of TS-1 (5.6 x 5.3/~). Ti-MCM-48 was also applicable for the epoxidation of other alkenes. In the oxidation of pent-2-en-l-ol (10 mmol) using Ti-MCM-48(2) (100 mg) with H20 2 (60 mmol), oxidation
99 products were obtained in 17% yield (turnover number: 95); selectivities for epoxidation of the double bond and oxidation of the alcoholic group to aldehyde were 46 and 54%, respectively. The high reactivity of pent-2-en-l-ol compared to cyclododecene may be ascribed to the enhancement effect of the OH group [23]. Table 2 Epoxidation of cyclododecene on titanium-containing materials
Catalyst Ti-MCM-41(2) Ti-MCM-48(1) Ti-MCM-48(2) TS-1 TiO2-SiO 2
Si/Ti 71 80 83 79 85
Turnover Number (mol/mol-Ti) Oxidant H20 2 TBHP 0.73 (54/46)" 1.50 (59/41) ~ 1.61 (58/42)" 0 0.057 (-)"
--7.1(34/66)" --m 2.6 (30/70)"
"cis/trans ratio of products. Reaction conditions: 50 mg catalyst; 20 mmol cyclododecene (cis/trans = 75/25); 24 mmol oxidant; 323 K; 2 h. 4. CONCLUSION Titanium-containing mesoporous material with a cubic structure, Ti-MCM-48, has been successfully synthesized by means of using tetraethyl orthosilicate as a Si source under appropriate reaction conditions. Ti-MCM-48 has been found to show activity for epoxidation of bulky alkenes with H202. as well as tert-butyl hydroperoxide as an oxidant. The high catalytic activity of Ti-MCM-48 compared to Ti-MCM-41 may be related to its threedimensional channel system. 5. A C K N O W L E D G E M E N T S
The authors are grateful to Prof. S. Namba (The Nishi-Tokyo University) for measuring high resolution transmission electron micrographs, and also wish to thank Dr. S. Nakata (Chiyoda Corporation) for measuring 298i MAS NMR spectra. REFERENCES
1.
C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359
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. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. J.S. Beck, J.C. Vartuli, G.J. Kennedy, C.T. Kresge, W.J. Roth and S.E. Schramm,Chem. Mater., 6 (1994) 1816. J.C. Vartuli, C.T. Kresge, M.E. Leonowicz, A.S. Chu, S.B. McCullen, I.D. Johnson and E.W. Sheppard, Chem. Mater., 6 (1994) 2070.
(1992) 710.
3. 4.
100
10. 11. 12. 13. 14. 15. 16. 17. 18 19.
20. 21. 22. 23.
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 and E.W. Sheppard, Chem. Mater., 6 (1994) 2317. P.J. Branton, P.G. Hall and K.S.W. Sing, J. Chem. Soc., Chem. Commun., (1993) 1257. A. Corma, V. Fornes, H. Garcia, M.A. Miranda and M.J. Sabater, J. Am. Chem. Soc., 116 (1994) 569. A. Corma, M.T. Navarro and J.P. Pariente, J. Chem. Soc., Chem. Commun., (1994) 147. K.M. Reddy, I. Moudrakovski and A. Sayari, J. Chem. Soc., Chem. Commun., (1994) 1059. P.T. Tanev, M. Chibwe and T.I. Pinnavaia, Nature, 368 (1994) 321. T. Blasco, M.T. Navarro and J.P. Pariente, J. Catal., 156 (1995) 65. S. Gontier and A. Tuel, Zeolites, 15 (1995) 601. A. Sayari, C. Danumah and I.L. Moudrakovski, Chem. Mater.,7 (1995) 813. Z.Y. Yuan, S.Q. Liu, T.H. Chen, J.Z. Wang and H.X. Li, J. Chem. Soc., Chem. Commun., (1995) 973. D. Zhao, and D. Goldfarb, J. Chem. Soc., Chem. Commun., (1995) 875. P.K. Vinson, J.R. Bellare, H.T. Davis, W.G. Miller and L.E. Scriven, J. Colloid Interface Sci., 142 (1991) 74. C.Y. Chen, S.L. Burkett, H.S. Li and M.E. Davis, Microporous Mater., 2 (1993) 27. R. Schmidt, M. Stocker, D. Akporiaye, E.H. Torstad and A. Olsen, Microporous Mater., 5(1995) 1. A. Zecchina, G. Spoto, S. Bordiga, A. Ferrero, G. Petrini, G. Leofanti and M. Padovan, Zeolite Chemistry and Catalysis, ed. P. A. Jacobs, N.I. Jaeger, L. Kubelkova and B. Wichterlova, Elsevier, Amsterdam, 1990, p. 251. Y.L. Kim, R.L. Riley, M.J. Huq, S. Salim, A.E. Le and T.E. Mallouk, Mat. Res. Soc. Symp. Proc. 233 (1991) 145. A. Thangaraj, R. Kumar, S.P. Mirajkar and P. Ratnasamy, .1. Catal., 130 (1991) 1. R.A. Sheldon, J. Mol. Cat., 7 (1980) 107. R. Kumar, G.C.G. Pais, B. Pandey and P. Kumar, J. Chem. Soc., Chem. Commun., (1995) 1315.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 1997 Elsevier Science B.V.
101
S H O R T R A N G E ORDER OF MCM-41 A N D M E S O S T R U C T U R E D ALUMINIUMPHOSPHATE
C. Pophal, R. Schnell, H. Fuess FB Materialwissenschaft, Fachgebiet Strukturforschung, TH Darmstadt, Petersenstr. 20, D-64287 Darmstadt, Germany
The X-ray amorphous walls of MCM-41 possess a well defined short, but also a middle range order. The addition of Ti or AI to the synthesis gel of MCM-41 does not significiantly affect the short and middle range order of the walls. Mesolamellar aluminiumphosphates can be prepared by the liquid crystal templating (LCT) method. The aluminium atoms are within a tetrahedral coordination of oxygen.
Keywords Mesoporous, MCM-41, aluminiumphosphate, WAXS.
I. Introduction Since their discovery by the Mobil Oil Corporation 111, mesoporous inorganic materials, especially MCM-41, are extensively studied. Several mesostructured inorganic compounds, e. g. transition-metal oxides have been prepared and characterized 12-31. In the course of our investigations we have synthesized mesostructured aluminiumphosphate using the LCT mechanism. The sample has been analysed by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and thermal analysis (TGA). To elucidate the structural organisation of the inorganic walls, we have investigated calcined purely silica MCM-41, AI-MCM-41, TiMCM-41 and as-prepared mesostructured aluminiumphosphate by wide angle X-ray scattering (WAXS). This method provides a radial distribution function (RDF) from the corrected experimental X-ray data without any a priori assumptions of the atomic arrangement. Interatomic distances and angles are directly derived from the RDF. Additional 27A1- and 3 Ip_ MAS NMR measurements were performed to check and complete the results obtained by WAXS on aluminiumphosphates.
102
2. Synthesis 2.1. MCM-41 Samples containing AI were prepared as described by Chen et al. [4]. The Si/AI molar ratios in the reaction mixtures were fixed to oo, 30, 28 and 17, respectively. Ti-MCM-41 with a Si/Ti ratio of 52 was synthesized following the procedure reported by Corma et al. [5]. All MCM-41 molecular sieves were prepared using hexadecyltrimethylammoniumchloride (25%, Fluka) as template. The as-prepared samples were washed with water and calcined for 12 hours at 540~ to remove the organic surfactant.
2.2. Aluminiumphosphate An aqueous solution of pseudoboehmite (aluminiumoxide-hydrate containing 74% A1203, Condea) and ortho-phosphoric acid (85%, Merck) was combined with an aqueous solution of hexadecyltrimethylammoniumhydroxide (Fluka). The mixture was stirred at room temperature and placed in a static autoclave at a temperature of 150~
for two days. The product was
recovered by filtration on a Buchner funnel, washed with water, and dried in air at ambient temperature. The pH-value was adjusted by addition of the template. Best results were obtained in the pH range between 8 and 11 161.
3. Analysis The XRD data at room temperature were collected on a powder diffractometer described by Hsu et al. 171 using CUKal radiation. Data were taken in the angular range of 1~ to 11 ~ (20). The transmission electron micrographs were performed on a Philips CM-20 transmission electron microscope operated at 200kV. To increase the contrast the picture of MCM-41 was taken at strong underfocus. Thermogravimetric analyses were carded out on a TGA 92-18 in the temperature range from 293K up to 1673K. The data for the WAXS investigations were taken from 3 ~ to 140~ (20) using MOKal radiation. For the data collection a scintillation counter was used with an integration time of 400s per data point in steps of 0.05 ~ The influence of the special sample environment (quartz-glass capillary, diameter 0.5mm) has been determined by measuring the background scattering caused by the empty capillary. The observed intensities were corrected, both for absorption and polarization according to Paalman and Pings [81, using the LASIP software package from Lecante 19]. The corrected intensities were normalized by the method of Kroegh-Moe [10]. Finally, the RDF has been obtained by Fourier transformation of the corrected and normalized intensifies. The MAS NMR spectra were recorded at room temperature on a Bruker MSL-400 spectrometer.
103
4. Results and discussion 4.1. MCM-41
The obtained XRD patterns from the as-prepared and calcined samples are in excellent agreement with the data reported by Beck et al. 111. Direct TEM imaging unambigously reveals the regular array of uniform channels, which are the main characteristic of MCM-41. A representative TEM image is shown in Figure 1. The d~00-spacing for all as-synthesized samples is about 40A. The high temperature treatment of the samples during the calcination reduces the mean pore diameter by about 10%, as described by Janicke et al. [11]. As a paradigm case, the radial distribution function of as-synthesized, purely siliceous MCM-41 obtained by WAXS is shown in Figure 2 and will be discussed. The RDF reveals spurious maxima corresponding to distances smaller than 1A due to series termination effects. The first concentration of electron density is assigned to the average Si-O distance and appears at 1.60A. Distances of two oxygen atoms within a SiO4-tetrahedron are observed at 2.60A. The average electron density of two silicon atoms of comer-sharing SiO4-tetrahedra appears at 3.05A. This implies, under the assumption of regular tetrahedra, an average Si-O-Si angle of 145 ~ Based on crystal chemical considerations, the distances in the RDF up to 3A can easily be assigned. Most remarkable are the rather sharp peaks in the RDF. This clearly reflects a high degree of short and middle range order within the inorganic walls.
~ '~'~' ~ ~ " .~ ,~ ~'~, .~ ~9
~_~
j
~..
9' ~ , i i ~ i
~
i~. "....~";.~,~;. '.i~ .~,,% ".,-, "~' " " ' ~.,~:.-.,~.r ....., ~ ..... ~.~'~' ".,~
~"
~
~"
"
looA Figure 1. Transmission electron micrograph of calcined purely siliceous MCM-41.
104
11~ ,,-..,.,,
=
5000
a
-5000
' ' '
2
0
4
6
a [,N
8
10
Figure 2. RDF of calcined purely siliceous MCM-41. Interatomic distances caused by the surfactant do not contribute significantly to the RDF, due to its rather low scattering strength and concentration. The RDF of the samples with incorporated A! and Ti do not reveal significant differences compared to purely siliceous MCM-41.
4.2. Aluminiumphosphate The XRD pattern recorded, reveals three reflections in the angular range 1~ to 11~ (20), which correspond to a lamellar phase with do01 of 31A (Figure 3). The periodic lamellar arrangement of the inorganic layers is also confirmed by direct TEM imaging. The TGA analysis indicate a ratio of suffactant to Al of about 3.5.
'
I
'
'
400
'
001
I
'
'
'
I
'
'
'
I
'
'
"
I
'
300 .n t/) r-
Q) t---
n
200 100 I L l ,
, 2
002 ~ .
L_ _ ~ l
.
4
.
.
.
6 20
_ 003 L....r-.
8
'
.--!--
'--
10
Figure 3. XRD pattern of as-prepared lameUar aluminiumphosphate with a mean d-spacing of 31A.
105
. .
,~,
.,
...
~.,,..
,.,
9
..
~
. ?
..
... . . . .
...'
,.
Figure 4. Transmission electron micrograph of as-prepared lameUar aluminiumphosphate. The radial distribution function obtained by WAXS is displayed in Figure 5. Model calculations, based on the data of berlinite, were performed in order to assign the observed intensities in the experimental RDF to interatomic distances.
110 4
"E
5000
*"
0
.IQ L_
'
9
9
II
|
II
g
II
|
g
,
,
,
9
9
9
U.
a
n-
-5000
-1 10 4
9
,
9
I
2
,
9
9
I
4
R
[A]
I
,
,
6
Figure 5. RDF of as-prepared lamellar aluminiumphosphate.
.
I
8
,
'
I
10
106 The RDF reveals rather sharp electron concentrations. This indicates a narrow distribution of interatomic distances and a high degree of short and middle range order. The electron density at 1.60A is assigned to the average distance of aluminium and phosphorous to oxygen atoms within a tetrahedral arrangement. Because of restrictions caused by the experimental resolution, it is not possible to distinguish between AI-O and P-O distances. AI is tetrahedrally coordinated, since no electron density around 1.80-2.00A is observed. The maximum at about 2.55A is attributed to the average distances of two oxygen atoms within PO4- and AIO4-tetrahedra. This implies, under the assumption of regular tetrahedra, an average AI-O-P angle of 145~
l .
.
.
.
!
80
1
. . . .
60
:
l I
9 J
40
J
J
,
J
PPM
20
J
]
J
I
0
J
J
....
1
-20 '-
,
,
J_
-40
:
-
Figure 6. 27AI-MAS NMR spectrum of as-prepared lamellar aluminiumphosphate.
1
_
I
0
.L
I
-5
.
I
-I0
l,
I
-15
I
-20
1
PPM
-25
t
-30
I
-35
.l
I
-40
!
-45
,
Figure 7.3 IP_MA S NMR spectrum of as-prepared lamellar aluminiumphosphate.
I
i
107 Within this structural model all distances up to 5A as derived from the experimental RDF can be explained. The observed maxima at distances above 5A cannot be uniquely assigned to interatomic distances, because of the increasing number of contributions to the electron density. The 27Al-MAS NMR spectrum (Figure 6) consists of one broad signal with a chemical shift of 40ppm, which corresponds to the 27A1 chemical shifts for solid A1PO4 compounds in which Al is covalently bound to four P atoms via oxygen bridges [12]. This implies the absence of octahedrally coordinated Al. The peak is asymmetric, due to AIO4-tetrahedra which are linked to less than four PO4-tetrahedra. The broadening of the signal implies a certain distribution of angles within the tetrahedral coordination around Al. Therefore, the calculated AI-O-P angle derived from the RDF can only be seen as an average angle. The lack of strong sidebands and the sharp maxima in the RDF indicate a high degree of short range order. The 31P_MAS NMR spectrum (Figure 7) shows three strong peaks at -17ppm, -21ppm and -29ppm. In the range of -25 to -30ppm most of the signals of phosphorous atoms with four covalent oxygen bridges to aluminium are observed in the various crystalline forms of aluminiumphosphate 1111. The signals at -17ppm and -21ppm are caused by phosphorous atoms with mixed coordination P(OH)x(OAI)4_x.
5. Conclusion Our WAXS investigations reveal a high degree of short and middle range order of the inorganic walls of silica-MCM-41, AI-MCM-41, Ti-MCM-41 and lamellar aluminiumphosphate. The addition of AI or Ti (ratio see 2.1.) to the synthesis gel does not significantly affect the structural organisation of the walls of MCM-41. With regard to the coordination of AI within the aluminiumphosphate layers, solid-state NMR studies support the result obtained by WAXS. Modified Rietveld refinements, using the program ARITVE from Le Bail [13] are in progress to obtain an improved model for the structural organisation of the inorganic walls of MCM-41. This method is a variation of the reversed Monte Carlo technique (RMC). It has been successfully applied to the least squares refinement of glassy SiO2. In-situ experiments are in progress and expected to provide the
experimental
background for an improved
"understanding" of the structural alterations during the formation of MCM-41 and other mesostructured inorganic materials.
Acknowledgment We thank Dr. Engelhardt and M. Feuerstein at the Universit~t Stuttgart for the performance of the 27A1- and 31p_MAS NMR measurements, Dr. Stamm at the Max Planck Institut Mainz for providing of XRD equipment and Dr. Franke and Prof. Schulz-Eckloff at the Universit~t Bremen for the support on aluminiumphosphate.
108 References
C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck. Nature, 359 (1992) 710. .
U. Ciesla, D. Dehmut, R. Leon, P. Petroff, G. Stucky, K. Unger, F. Schiith. J. Chem. Soc., Chem. Commun., (1994), 1387.
.
Q. Huo, D. I. Margolese, U. Ciesla, P. Feng, T. E. Gier, P. Sieger, R. Leon, P. M. Petroff, F. SchUth, G. D. Stucky. Nature, 368 (1994) 317.
.
5.
C.-Y. Chen, H.-X. Li, M. E. Davis. Microporous Mater., 2 (1993) 17. A. Corma, M. T. Navarro, J. P6rez-Pafiente, F. Sanchez. Studies in Surface Science and Catalysis, Vol. 84 (1994) 69.
.
7.
R. Schnell. Diplomarbeit, TH Darmstadt (1994). T. C. Hsu, B. Hiiser, T. Pakula, H. W. Spiess, M. Stamm. Makromol. Chem., 191 (1990) 1597.
~
H. H. Paalman, C. J. J. Pings. Appl. Phys., 33 (1962) 2635.
9.
P. Lecante. Th~se at the Centre National de la Recherche Scientifique Toulouse, (1990).
10.
J. Kroegh-Moe, Acta Cryst., 9 (1956) 951.
11.
M. Janicke, D. Kumar, G. D. Stucky, B. F. Chmelka. Studies in Surface Science and Catalysis, Vol. 84 (1994) 243.
12.
M. A. Harmer, A. J. Vega. Solid State Nuclear Magnetic Resonance 5 (1995) 35.
13.
A. Le Bail. Journal of Non-Crystalline Solids 183 (1995) 39.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
109
Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
Syntheses of mesoporous aluminosilicates from layered silicates containing aluminum. S. Inagaki, Y. Yamada and Y. Fukushima Toyota Central R&D Labs., Inc., 41-1, Nagakute, Aichi, 480-11, JAPAN Addition of Al(NO3)3~
or NaAIO 2 as an AI source formed crystalline layered sodium
silicates containing AI. Aluminosilicate mesoporous molecular sieves were synthesized by heating
the
layered
sodium
silicates
with
various
amounts
of
AI
in
hexadecyltrimethylammonium chloride solution. The mesoporous aluminosilicates with hexagonal regularity and uniform pore-size distribution had only tetrahedral Al in the framework and large amount of acidity. Although the addition of Al(NO3)3~
formed not only the
layered sodium silicates but also cristobalite (SiO2) crystal, the addition of NaA10 2 formed no crystobalite. It suggested isomorphous substitution of Al for Si in the SiO 2 framework of the layered sodium silicates and the mesoporous materials. 1. INTRODUCTION Mesoporous molecular sieves such as MCM-411) and FSM-162) have attracted much attention, because of their applicability to catalysts and adsorbents for large size molecules. FSM-16 is synthesized from a layered sodium silicate by using surfactants 3). A folded sheet mechanism has been proposed for the formation of FSM-16 3'4).The topochemical synthetic method of FSM16 is expected for a new strategy for synthesizing various inorganic materials. Several attempts 5-9)have been made to incorporate AI in siliceous framework of the mesoporous materials and apply the aluminosilicate mesoporous materials to various catalytic reactions such as a synthesis of porphyrin 1~ Although aluminosilicate MCM-41 materials containing only tetrahedral Al in the framework have been synthesized, their structural regularity were lower than those of silicious MCM-41 materials 6'7). Only one or two peaks at a low angle region in X-ray diffraction patterns were observed for the aluminosilicate MCM-41 materials with a relatively low Si/Al ratio under 50. The structural regularity decreased with increasing Al contents and their diffraction peaks due to the hexagonal regularity disappeared for the samples with the Si/
110 AI ratios of 26) or 57). We also reported that an impregnation of FSM-16 with aluminum chloride aqueous solution produced a mesoporous aluminosilicate 5), although, the mesoporous materials contained AI in not only a tetrahedral site but also an octahedral site. The amount of acid was about an half of that on ZSM-5 with the same AI203 contents. Here we have tried to synthesize aluminosilicate mesoporous molecular sieves with high regularity and only tetrahedral AI from a layered sodium silicates containing AI. 2. EXPERIMENTAL
2.1. Preparation of Layered Sodium Silicates Containing Aluminum. Aqueous solutions of Al(NO3)309H20 or NaAIO 2 were added in sodium silicate aqueous solution (SiO2/Na20=2), and the mixtures were stirred at 50* C for 3h. The solutions were dried at 100 *C for 12h and vacuum-dried at 70 *C for 12h to yield sodium aluminosilicate glasses. The glasses were calcined at 700 *C for 6h to crystallize to layered sodium silicates containing AI. The Si/AI molar ratio in the initial mixtures ranged from 2.5 to 100 as listed in Table 1.
2.2. Conversion to Mesoporous Aluminosilicates. The layered sodium silicates were dispersed in water and stirred at room temperature for 3h. The filtered samples (50 g) were dispersed in 0.1 mol dm3 hexadecyltrimethylammonium chloride aqueous solution (1000 cm 3) and heated at 70 *C for 3h. 2N HC1 aqueous solution was added to the dispersion to adjust the pH value to 8.5, and they were maintained at 70 ~ for further 3 h. The filtered samples were dried at 60 ~ and calcined at 550 ~ for 6 h to remove organic fractions. Consequently, we got Al-containing FSM-16 samples with the bulk Si/AI molar ratio between 7.2 and 188 as listed in Table 1. The Si/AI ratios in the Al-containing FSM-16 materials were determined by inductively coupled plasma-atomic emission spectrometry. The higher Si/A1 ratios of the final products than the initial solutions were due to a dissolution of A1 during the dispersion of the layered sodium silicates in water.
2.3. Characterizations. X-ray powder diffraction pattems were obtained
by
using
a
Rigaku
RAD-B
Table 1 AI sources and Si/A1 molar ratios. Si/A1 ratio A1 sources
Initial solution 100 AI(NO3)3"9H20 50 20 5 4 NaAIO2
50 5 2.5
Final products 188 67 38 9.9 7.7 132 12 7.2
111 diffractometer with Cu-Kct radiation 9 Solid state MAS NMR spectra were measured on a Bruker MSL-300WB spectrometer 9 295i MAS NMR spectra were recorded at a frequency of 59.620 MHz spinning 4 kHz using pulses at 90-s intervals and 360 scans 9 27A1MAS NMR spectra were recorded at a frequency of 78.205 MHz spinning 4 kHz using pulses at 2-s intervals and 1000 scans. Ammonia-TPD spectra were measured with a conventional TPD apparatus, in which the desorbed materials were detected by a thermal conductivity detector. About 0.1 g of the samples were vacuum-dried at 500* C for lh. The dried samples were then exposed to 13.3 kPa of an ammonia gas at 100 ~C for 45 min, followed by evacuating at 100 ~C for lh. The TPD spectra were measured from 100 ~ to 1000 *C at a heating rate of 10 ~C/min. in a helium flow as carrier gas controlled at 4.0x10 3 of W/F.
3. RESULTS AND DISCUSSION Figs. 1 and 2 show X-ray diffraction pattems of Al-containing FSM-16 prepared by using Al(NO3)3 9
and NaAIO z as AI sources 9 The FSM-16 samples with Si/A1 ratios of 188, 132,
9cristobalite
/ 8 r
1..8i~ ' /] -t / :
~
~,,~,~,,.~_~__'
10 20 20 (CuKa)
30
67 / .."
/
-
/ /
38 ~
....9..,.9.....
....7..,2... 10
40
Figure 1. X-ray diffraction patters of aluminosilicate FSM-16 materials various Si/A1 ratios prepared by using AI(NO3)a.9H20 as AI source.
112
,
0
5
10
.
.
10
1
3
2
"
20
' ~
30
40
20 (CuKet) Figure 2. X-ray diffraction patters of aluminosilicate FSM-16 materials various Si/AI ratios prepared by using NaAIO 2 as A1 source.
67, 38 and 12 showed three Or four obvious peaks assignable to hexagonal symmetry at an angle smaller than 10 ~ The diffraction patterns indicate that their structural regularities were higher than those of MCM-41 materials with the same Si/AI ratio reported previously6-9). For the samples with the Si/A1 ratio of 9.9, though the peak intensity was reduced, two or three peaks were still observed in a low angle region, which suggested preservation of hexagonal regularity. The unit cell dimensions of the samples prepared by using AI(NO3)3-9H20 decreased from 4.3 to 4.0 nm with decreasing the Si/A1 ratio from 188 to 9.9. Those of the samples with the Si/A1 ratios of 132 and 12 prepared from NaAIO 2as an AI source were 4.4 nm. While, the products with the low Si/AI ratios of 7.7 and 7.2 showed only one broad peak in a low angle region. On the othe hand, the product (Si/AI=7.7) prepared from AI(NO3)3-9H20 showed several peaks due to the other crystal in a high angle region. Those peaks were assigned to sodium aluminum silicate hydroxide hydrate crystal. FSM-16 materials were not formed in such a high AI content, which was attributable to using non-layered sodium silicates as starting materials formed from the sodium aluminosilicate glasses mentioned later. Diffraction peaks due to cristobalite crystal were also observed in a higher angle region for all the samples prepared by using AI(NO3)3"9H20, and their intensity was increased with increasing the added amount of AI(NO3)3.9H20. The samples prepared by using NaA102 had almost no formation of cristobalite as shown in Figure 2. These results suggested that the introduction of A1 ions into
113 the sodium silicates without any supply of Na ions expelled Si from the framework of the layered sodium silicate and FSM-16.
The AI introduction incorporating with Na formed sheet
silicates including A1 and FSM-16 without ejecting Si to form crystobalite crystals, which is represented by the following chemical equations, (1) and (2). Na2Si205 + xAI(NO3)3 + (3x/2)H20 ---> Na2Si2.xAl H30s +xSiO 2 +3xNO 2 +302
(1)
(1-x)Na2Si20 s + (2x)Na~O 2 + xH20 -> Na2Si2zAl2xH2xOs
(2)
4-AI
03 04 9
i
/AI= 12
/A,J= 38 ,
,
200 100
0 -100 -200 ppm Si / AI m
-
m
-
u
-
-80
|
-
u
-
-100 ppm
u
-120
Figure 3. ZrAl and 29Si MAS NMR spectra of as-synthesized FSM-16 materials. The FSM-16 samples with Si/AI ratios of 12 and 38 were prepared by using NaAIO 2 and AI2(NO3)a'9H20, respectively. At
o' /Al = 12 I
~
i
/
A
l
= 38
200 100 0 -100-200 ppm Si / Al 9
.
,
-80
.
,
-
,
-
-100 ppm
,
.
9
-120
Figure 4. 27A1and 29Si MAS NMR spectra of calcined FSM-16 materials. The FSM-16 samples with Si/AI ratios of 12 and 38 were prepared by using NaAIO 2 and AI2(NO3)3~ O, respectively.
114
6- Na2Si20 5
Cristobalite---
~
~
~ . . . .
_
=
1
8
8
=
.
9.9 . . . .
10
;0 20(CuKet)
Figure 5. X-ray diffraction patterns of calcined products at 700 ~C of aluminosilicate glasses with various Si/AI ratios prepared by using A12(NOa)a~
These results strongly suggest that AI is incorporated in the silicate framework of the layered sodium silicates and the FSM-16 materials. BET surface areas were 800-1300 mZ/g for the Al-containing FSM-16 samples and 30-300 m2/g for the sodium aluminum silicate hydroxide hydrate crystals. NMR spectra of the as-synthesized FSM-16 samples and the calcined samples are shown in Figures 3 and 4. 27A1MAS NMR spectra of the Al-containing FSM-16 samples with the Si/A1 ratios of 38 and 12, prepared by using Al(NOa)3*9H20 and NaAIO2 as A1 sources respectively, showed almost only one signal due to tetrahedral AI before and after calcination. Broadening of the signal during calcination suggesting somewhat dealumination from the framework was obObvious broadening was not observed served for the sample prepared by using AI(NO 3)3~ for the sample prepared by using NaA102, which suggested its higher thermal stability than the sample prepared by using Al(NOa)a~ The incorporation of A1 in the silicious framework was also confirmed by 29SiMAS NMR spectra. Lower shifts at 3-7 ppm of signals assigned to Q3 and Q4environmental SiO4were observed for both as-synthesized and calcined samples after introduction of AI as shown in Figures 3 and 4. Such a peak shift was also observed for zeolites with Si (OSi)3(OA1). Figure 5 shows X-ray diffraction patterns of the calcined sodium aluminosilicate glasses prepared by using AI(NO3)3~
The diffraction patterns due to a layered sodium silicates, 6-
Na2Si205 were observed for the calcined products with Si/AI ratio over 9.9, while those were
115
|
[-
l~ "6~
|
IF
l
ii
(This work)
//
== 0.5~[ /
'
FSM-16
)llrSi/Al=9.9
0""""
FSM-16
I
(Impregnation)
o ........
..O ......
silica-alumina
<
0.0 0
10
20
30
AI20 3 contents (wt%) Figure 6. Relationship between amount of acid and AI203 contents of various aluminosilicate materials determined by NHa-TPD. hardly observed for the samples with the low Si/AI ratio of 7.7. Cristobalite was contained in the layered sodium silicates, and its content increased with added amount of AI(NOa)a.9H20 as shown in Figure 5. Although 6-Na2Si205 crystal also formed in the calcined products prepared by using NaAIO 2with the Si/Al ratio higher than 12, cristobalite did not formed in all samples. These results support the expelling of Si from the framework of the layered sodium silicates mentioned above. The d-spacings of the diffraction peaks of those layered silicates containing Al were similar to those of •-Na2Si205. Acid amounts of the aluminosilicate FSM-16 materials were larger than those prepared by impregnating aluminum chloride solution and usual amorphous aluminosilicate with the same Si/Al ratio reported previously~3as shown in Figure 6. The acid amounts were 0.7 times of those of ZSM-5. It indicates highly level incorporation of Al in the framework of FSM-16 prepared from the layered silicate containing Al. NHa-TPD profiles of the aluminosilicate FSM-16 materials resembled in those observed in amorphous aluminosilicates, which suggested broad distribution of acid-strength. The successive formation of mesoporous molecular sieves with high regularity and high alumina would be attributable to using a layered silicate containing Al as starting material, although further studies are necessary to clarify the mechanism.
116 4. CONCLUSION The aluminosilicate mesoporous molecular sieves with high alumina and highly regular structure were prepared from layered sodium silicates containing A1. The expelling of Si from the framework of the layered silicates, the large amount of acidity and the NMR results indicated the high level incorporation of AI in the framework of the FSM-16 materials. REFERENCES
1) C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck,, Nature, 359 (1992) 710. 2) S. Inagaki, Y. Fukushima, K. Kuroda, J. Chem. Soc., Chem. Commun., (1993) 680. 3) S.Inagaki, A. Koiwai, N. Suzuki, Y. Fukushima, K. Kuroda, Bull. Chem. Soc. Jpn, in press. 4) S. Inagaki, Y. Fukushima, K. Kuroda, In Zeolite and Related Microporous Materials: State of the Art 1994, Eds. J. Weitkamp, H. G. Karge, H. Pfeifer, W. Holderich, Elsevier, (1994) 125. 5) S. Inagaki, Y. Fukushima, A. Okada, T. Kurauchi, K. Kuroda, C. Kato, Proceedings from the Ninth International Zeolite Conference, Montreal 1992, I, 305. 6) R. B. Borade, A. Clearfield, Catal. Lett., 31 (1995) 267. 7) Z. Luan, C-F. Cheng, W. Zhou, J. Klinowski, J. Phys. Chem., 99 (1995) 1018. 8) A. Corma, V. Fornes, M. T. Navarro, J. Perez-Pariente, J. Catal., 148 (1994) 569. 9) K. R. Kloetstra, H. W. Zandbergen, H. van Bekkum, Catal. Lett., 33 (1995) 157. 10) T. Shinoda, Y. lzumi, M. Onaka, J. Chem Soc., Chem. Commun., (1995) 1801.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved 9
117
STRUCTURE DESCRIPTORS FOR ORGANIC TEMPLATES EMPLOYED IN Z E O L I T E S Y N T H E S I S R E Boyett,* a,b A P Stevens, a,b M G Ford b and P A Cox a,b Division of Chemistry, University of Portsmouth, St. Michaels Building, White Swan Road, Portsmouth, Hants, PO 1 2DT, UK. b Centre for Molecular Design, University of Portsmouth, Halpem House, 1/2 Hampshire Terrace, Portsmouth, Hants, PO 1 2QF, UK. a
ABSTRACT
A structural analysis has been carried out upon 130 organic templating agents employed in the synthesis of zeolites from 18 different framework types. The orthogonal principal axes of inertia of these molecules provide quantitative structure descriptors which can be plotted to produce three-dimensional 'shape-space' diagrams. Groups of templates which produce different zeolite framework architectures plot in discrete areas of these graphs. If the set of templates for a single framework type occupy more than one region of this shape-space, it can indicate that several modes of incorporation are found for the template molecules within the relevant zeolite porespace. Large, complex templates can often be demonstrated to comprise multiple structural units, or sub-shapes, the latter having similar dimensions to the smallest agents in any one set of templates, i.e. those that fulfil the minimum void-filling requirements of particular pores. Regions of overlap may occur between whole families of templates in the principal axes of inertia shape-space. The templates which plot within an overlap region may direct the formation of any of the different zeolite frameworks whose template shape-space envelopes interpenetrate at that point. The actual outcome of syntheses that deploy templates from these regions thereby depend more critically than usual upon factors besides just template shape, such as the composition of the reaction media and the reaction conditions. The quantitative description of template size and shape afforded by the use of principal axes of inertia highlights the importance of template shape in determining the zeolite product formed.
1. INTRODUCTION
Zeolite catalysts are being synthesised to an increasing extent via the use of gels that contain specific organic templating or structure directing agents. The exact r61e which these agents play in zeolite formation has not been established, but previous research has shown the size and the shape of any particular template molecule to be an important factor in defining the lattice structure of the eventual zeolite product, t1'2'3'4]as, indeed, is the template orientation. [51 9
118 Although a number of molecular modelling techniques have been applied in zeolite research, [6'7~none have yet provided a tool facilitating the quantitative description of template structure. In order to address this need we have investigated techniques that have a proven record in the disciplines of Quantitative Structure-Activity Relationships (QSAR), I8~ and drug design, applying the lock and key analog)Jg~ for the conformational interaction between drug and receptor site in a novel way, to rationalise the relationship between the structures of template molecules which direct the formation of the same products. Several different properties are suitable for the task of structure representation, t1~ the present study has employed the molecular principal axes of inertia. The results discussed below identify such descriptors as potential aids to our understanding of template-framework relationships, and we discuss their potential for application in the rational design of novel materials.
2. E X P E R I M E N T A L
Structure analysis was carried out upon 130 organic templating agents which have been employed in 229 syntheses documented in research literature and in patent applications. [1il The zeolites formed using these 130 agents exhibit 18 different framework structures; all examples investigated here were of a siliceous or aluminosilicate composition, and our conclusions are therefore restricted to zeolite-template systems of this type. Molecular mechanics calculations were used to defme a low potential energy conformation for each molecule, utilising the potentials and parameters in the CVFF force field, implemented within the program Discover. c12! The orthogonal principal axes of inertia, RN, were calculated using the Tsar Quantitative Structure-Activity Relationships (QSAR) software, t13~ These quantities are customarily described in ,~mgstroms, and define an ellipsoid (see Figure 1) which is scaled in inverse proportion to the molecular moments of inertia for a given template. RY
cr---
~
i
~
- - ~ '
RX
Figure 1. Inertial ellipsoid for a typical template molecule. The dimensions of the ellipsoid, RN, are inversely proportional to the molecular moments of inertia, IN. Two assumptions have been made in order to apply the principal axes of inertia in this way. Firstly, the derived molecular conformations are taken to represent those of templates located inside pore systems, without the explicit consideration of either thermal motion or of any conformational change of the template within the host arising from lattice-template interaction. Secondly, the masses of the C, N, and O atoms defining the 'backbones' of the templates are considered to be similar enough to permit a direct comparison of moments, and therefore RN values, between all molecules in the survey. These assumptions do not affect the employment of principal axes of inertia as a tool to describe the structure of templates at a first
119 approximation, indeed, the axes retain inherent advantages over alternative shape descriptors, being convenient to calculate, and requiring no pre-alignment, or definition of reference atoms.
3. RESULTS AND DISCUSSION The use of a template during zeolite synthesis can impart a very strong influence on the pore architecture developed within an eventual product. The extent to which the influence is controlled by the shape of the template is illustrated in the three-dimensional plots below, which depict the principal axes of inertia for several systems. The templates making different products are found to occupy discrete RN 'shape-space.' It should be noted that a little distinction is lost owing to the reduction of the 3-D plot onto a plane projection, and that only a limited groups of structures may be included in any one plot without significant loss of clarity. The data shown in Figure 2 represent six different framework types in which the templates are enclosed in clearly defined shape-space, this degree of enclosure may be rationalised quite simply. For example, the templates that form the cage-like cavities in NON-type zeolites are small molecules whose structures essentially occupy a spherical or toroid volume, agents such as adamantanes and unsaturated ring compounds, respectively. These molecules share similar RN values, where all three components are of approximately the same magnitude. The other framework types depicted in the figure are also shape specific in their templating requirements. Docking studies t4] indicate that only molecules of very precise dimensions may locate within the pockets along the pores of zeolite EU-1 (EUO). Similarly, the templates found to direct the crystallisation of zeolites ZSM-18 (MEI) and ZSM-57 (MFS) are seemingly very few, and occupy small, specific volumes of shape-space on the graphs. This is arguably because only very few compounds have the correct size and shape to direct the formation of these particular pore architectures. 4 3.6 3.2 2.8
i
11a
I !
I
2.4
m
ml
m
m
I
ml
m
1.2
0
2 RY
4
6 ~
lo lOO
80
60
40
20 RX
Figure 2. Principle axes of inertia (]k) for templates that produce zeolites assigned to the framework categories" NON ~ , MEL IlL EUO O, LEV &, MEI 1r and MFS ~.
120
All but one of the agents represented in Figure 2 and which form the MEL-type structure are homologous linear diamines which exhibit closely related RY and RZ values but have large and variable RX components, such that they plot as a parallel band in the figure. These templates are encapsulated along straight pore channels in ZSM-11 and its analogues. In contrast, tetrabutylammonium, whose RX and RY values are identical but whose RY and RZ values are comparatively larger than the diamines, is located centrally at pore intersection sites. The principal axes of inertia can clearly distinguish between these different modes of template accomodation within one pore system, as well as between framework varieties, evidence for such modes arising from the division of the shape-space for a given template set into subregions, one being found for each mode.
2.4 2.2 2
1.8 1.6 1.4 1.:
I
100
80
ou
--
RX
Figure 3. Principle axes of inertia (/~) of templates making up zeolites with MTT frameworks. Templates are coded thus: ,ik basic unit, occupying a single lobe; Ill double unit, occupying two lobes; and 9 multiple lobe-occupying units. Table 1. Maximum and minimum values for principal axes of inertia of templates which form zeolites with a MTT type framework.
Lobes occupied
Value
R X / / ~ RY / A R Z / A
Surface Molecular Area/]k z V o l u m e / A 3
Single
Max Min Only Max Min
11.9 2.7 25.8 97.0 39.0
108 82 167 378 290
Double Multiple
2.70 1.47 1.50 2.24 2.21
1.56 1.31 1.46 2.23 2.21
63 44 97 269 203
Unidimensional zeolite pore systems may exhibit several modes: there are at least three ways in which templates can be incorporated within the pores of zeolites which have the MTT-
121 type pore system, (e.g. ZSM-23). In addition, this framework type can also be used to introduce the concept of template shape and 'sub-shape.' The five smallest templates represented in Figure 3 and Table 1 can effectively be considered as void-filling traits ('mode 1') whose shape and molecular volume, c. 50 A, define the most basic requirements for the unidimensional MTT pore channels, with their regularly spaced, undulating lobes. Figure 4 illustrates the relationship of two small templates to these pore lobes, the organic agents pyrrolidine and 2-aminoethanol having been docked into the channels manually using computer graphics. The poremap in this and other similar figures has been defined by showing the space accessible to a template after van der Waals surfaces have been generated around the zeolite's framework. The other templates in the MTT set occupy more than one pore lobe, either by spanning two adjacent lobes (dipropylamine, 'mode 2') or by bridging between more distant cavities (as do the diammonium ions, 'mode 3'). The basic units such as pyrrolidine may therefore be envisaged as effective sub-shapes of the larger compounds. The four diammonium compounds typify a feature frequently seen amongst sets of templates in our investigation, in that they comprise a co/nmon chemical form with repeated structural units, and their total volume is approximately a multiple of that found for the basic void-filling templates, (Table 1). Only heptamethonium is shown within the pore in the figure, although presumably the other agents, octa-, undeca- and dodecamethonium, are accomodated in a closely-related fashion, with their quarternary nitrogen centres housed in pore lobes. It is not clear from Figure 4 whether nona- and decamethonium are unable to produce MTT systems in this way, or whether they have .been used successfully to synthesise such structures but were simply not identified in our primary search of the literature. If the lobe spacing does restrict the alkyl chains linking the nitrogens in diammoniums to certain permitted lengths, it appears from the figure and table that another pair of templating agents may be available with RX values of approximately 140 A, and molecular volumes of around 310 A 3. I
....
I"
"..~" ~
I .~...
'I
.~ i
....~I ....... 1 '
9
!! ~IB"
....:~" ",. ...I"
I ...-.:..
.....-~" \
i~/
< ..................
........." ~.
~ I
...,.I
I
!i
..
.~I ....... .....I ....
.~i . . . . ......I ....
~.i
!
!
!
"q!ll"
"l!tl"
........ I
"~IV
".~" ",,. ~.I
I ...~..
9
~ .,.~..
!
....:.:" ",,.
....... I
i ...~,..
.,
9
'.1....
~q!P',
..,..i
!
....!
9
.....i
"q
{.::::~::;;~:::;~:::.:i..::==========================:~::::~..:::.i..::~:::::~::::~:::..~.:::~i:;;~;::;~:::.:i.:::~;::::~:;::~:::.:~
Figure 4. Poremap showing the lobed one-dimensional channel system found in zeolites with the MTT structure. Manually docked molecules illustrate possible sites available within the lobes, from top left: pyrrolidine; 2-aminoethanol; dipropylamine; and heptamethonium.
122 Plotting the principal axes of inertia for a particular template can give some indication as to which zeolite framework structures may be synthesised using that agent. The dibenzyldimethylammonium ion (DBDMA) forms at least three types of zeolite lattice (see Figure 5), the actual outcome of a preparation being dependant not only on the template shape, but also on the chemical composition of the reaction media, and the conditions under which the synthesis is performed. The DBDMA ion can therefore be considered to be a member of three distinct sets of templates. The principal axes of inertia for these templates are plotted in Figure 6; DBDMA occupies a point which lies at the intersection of the three regions of 'shape-space' relating to the structure types BEA, EUO, and MTW. The ion is accomodated in different ways in each lattice, as illustrated in Figure 5.
a
b
C
Figure 5. Three different modes of accomodation for dibenzyldimethylammonium in zeolite framework types a) BEA, b) EUO, and c) MTW. The overlap region between sets of template structures becomes very pronounced towards the origin of the RX plots. In examples such as the NON, SGT, MTN, DDR, and DOH frameworks, factors other than RX values may also need to be considered in order to differentiate between what are actually very closely related sets of templates. The small template molecules that plot near the origin can, like pyrrolidine, produce a variety of structures, depending on a range of other variables in any given synthesis. Other shape descriptors from QSAR could yet be suitable for template analysis, and although none might have the convenience characteristic of the principal axes of inertia, they
123
3
x
2.8 2.6
.,,
=// =1://,
"1= l/l/
2
1.4
RY
3 4
70 60 50 40 30 20 10 0 RX 5
80
Figure 6. Principal axes of inertia (/~) for templates forming BEA it, EUO m, and MTW (part of set) ~ framework types. DBDMA O, falls at the intersection of the three types. may provide additional structural information. Comparative Molecular Field Analysis [14'151 (CoMFA) may be used to investigate the steric and electrostatic fields which define templates. However, CoMFA requires that target molecules be aligned identically before any comparison ac be made, and therefore analyses are restricted only to very closely related sets of templating agents. It may however provide a tool which can identify the template charge characteristics necessary to produce zeolites with a certain Si / A1 ratio, or crystal morphology. It has been suggested that CoMFA be applied directly to describe the quantitative structure-activity relationships of zeolites with known catalytic properties, t141
4. CONCLUSIONS The principal axes of inertia provide a basic quantitative description of the shape of organic templating agents, and if taken into account with molecular volumes, can increase the current understanding of the influence of template structure on the structure of zeolite pore systems. These descriptors also offer an indication as to the differing manners by which various templates are incorporated into the channel system of any one zeolite, t16] The use of structure descriptors is complementary to Monte Carlo-Simulated Annealing [4'171 and, perhaps, D e N o v o methods, and could establish which shape criteria are necessary to allow rational computer assisted design of novel catalytic zeolite systems to be realised. New and improved templating agents could be sought by screening the RN values of candidate molecules, and comparing them against the known values from previously successful templates. Novel templates may also be pursued using the concept of shape and sub-shape, whereby, for instance, cheaper and smaller agents may be identified as effective sub-units of larger molecules, and vice versa. Plotting a new template upon a principal axes of inertia 'shape-space diagram' may predict which zeolite structures it might produce in addition to any
124 (more desirable) target type, and then the optimum reaction conditions necessary for its deployment may begin to be established. The authors are currently seeking to supplement the described preliminary survey by accounting for the importance of template shape in respect to other variables in zeolite synthesis, notably the electrostatic profiles of organic templates, gel chemistry and reaction conditions. A detailed chemometric or quantitative structure-property relationships (QSPR) analysis of the available data may yield information which will permit specific preparative methods to be identified for the deployment of novel templating agents.
REFERENCES
10
11
12 13 14 15 16 17
H. Gies and B. Marler, Zeolites, 12, (1992), 42. B.M. Lok, T.R. Cannan, and C.A. Messina, Zeolites, 3, (1983), 282. L.B. McCusker, Materials Science Forum, 133-136, (1993), 423. A.P. Stevens, A.M. Gorman, C.M. Freeman, and P.A. Cox, submitted to J. Chem. Soc., Faraday Trans., (November 1995). A.P. Stevens and P.A. Cox, J. Chem. Soc., Chem. Commun., (1995), 343. See, for example, "Modelling of Structure and Reactivity in Zeolites," (ed. C.R.A. Catlow), Academic Press, London, (1992). C.M. Freeman, D.W. Lewis, T.V. Harris, A.K. Cheetham, N.J. Henson, P.A. Cox, A.M. Gorman, S.M. Levine, J.M. Newsam, E. Hemandez, and C.R.A. Catlow, in "ComputerAided Molecular Design: Agrochemicals, Materials and Pharmaceuticals," (eds. C.H. Reynolds, M.K. Holloway, and H.K. Cox), ACS, Washington DC, (1995), 326. D. Livingstone, "Data Analysis for Chemists," Oxford University Press, (1995). P.M. Dean, "Molecular Foundations of Drug-Receptor Interaction," Cambridge University Press, (1987), 254. P.C. Jurs, S.L. Dixon, and L.M. Egolf, in "Chemometric Methods' in Molecular Design, Methods' and Principles in Medicinal Chemistry Volume 2," (ed. H. van de Waterbeemd), VCH, (1995), 15. A list of calculated values for the principal axes of inertia relating to the templates included in this study are available on request from the authors, together with full details of the literature source for each. Discover 3.1 program, Insight H User Guide, Version 2.3.6, Biosym Technologies Inc., San Diego, USA, (1993). Tsar version 2.02, Oxford Molecular Ltd., UK, (1993). R.D. Cramer IU, D.E. Patterson, and J.D. Bunce, J. Am. Chem. Soc., 110, (1988), 5959. K.H. Kim, in "Molecular Similarity in Drug Design," (ed. P.M. Dean), Blackie Academic and Professional, Glasgow, (1995), 291. See also R.E. Boyett, A.P. Stevens, M.G. Ford, and P.A. Cox, to be submitted to Zeolites, 1996. P.A. Cox, A.P. Stevens, L. Banting, and A.M. Gorman, in "Zeolites and Related Microporous Materials: State of the Art 1994," Studies in Surface Science and Catalysis, Vol 84, (eds. J. Weitkamp, H.G. Karge, H. Pfeifer, and W. H/51derich), Elsevier, Science B.V., (1994), 2115.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 1997 Elsevier Science B.V.
125
QUANTITATIVE ASPECTS IN'I'HE CRYSTALLIZATION OF ZEOUTES H. Lechert, T. Lindner and P. Staelin Institute of Phgsical Chemistrg, Universitg of Hamburg, Bundesstr. 45, D-20146 Hamburg Kegwords: Zeolite Crgstallization, Si/AI-ratio, crgstallization rate 1. S U M M A R Y
The Si/AI-ratio in zeolite products can be described in a wide range of batch compopositions bg a simple relation with the SiO~- and the OH-- content in the solution phase. With some simplifications this Si/A~ratio can be related to the Si/AI-ratio n and the excess alkalinitg rn in the batch. Further is shown that the rate constant of linear growth k depends on the SiO..-, the [AI(OH). ]-, and the OH-- conctentration in the solution bg a relation similar ~o the equation o4f LANGMUIR-HINSHELWOOD, showing that the formation of the surface structures tgpical for the growing zeolite is the rate determining step of the crgstallization. 2. INTRODUCTION The cr'gstallization of zeolites is still more an "art" in which experience and sometimes tricks lead to the desired results rather than well established theoretical models, connecting the wanted properties of the product and the starting parameters of the batch composition. Usuallg the solution concentrations of possible building units of different zeolite tgpes are related to these starting parameters (see e.g. CAULLET and GUTH [1] ). For the description of the batch composition the Si/AI-ratio n, the excess alkalinitg m, and the water content p are usuallg used in NaAIO 2 n ( SiO 2 m NaOH) p H 20 (1) The silicate and aluminosilicate species in the solution, which are regarded to serve as building units, are preferably studied by NMR -spectroscopg [2,3]. A second approach is the study of the crystallization kinetics. Here two ideas are discussed. One is related to the AVRAMI-theory, trying to describe the whole process of crystallization by a single function [4-6]. Often, the processes of nucleation and growth are separated. The growth can be discussed in terms of the common crystallization kinetics ( see e.g. [7,8]). The kinetics of nucleation can be described from the knowledge of the growth parameters and the particle size distribution [4]. In [9] this procedure has been used for an estimation of the size of the nuclei in nucleation gels. In this paper the Si/Al-ratio and the rate constant of linear growth of crystallizing
126
zeolites are related with the concentrations of different species in the solution phase and with the parameters n and m in Eq. 1. 3. EXPERIMENTAL The experiments have been carried out as described in our former papers [7,8]. The gels were prepared from Na- aluminate, NaOH and water glass ( d = 1.37 ). For the aluminate solution AI(OH)_ was dissolved in NaOH so that 1000 g solution contained 2.5 mol AI(OH)_ and 5 mol NaOH. The silica source was water glass with 3 273.5 g (4.6 mol) SiO 2 and 83.3 (1.3 tool) NaOH in 1000 g solution. The total concentration of the solid was chosen at 50g ( AIO - + n SiO )/1000 g H O for all ex2 periments. The general composition of the batches can 15e described ~y Eq.1. The solutions were mixed at room temperature in PE-bottles. After 1 h NaX nuclei or nucleation gel were added. Before heating to 90~ the mixture was shaken for another hour. The NaX nuclei were obtained from batches with the composition NaAIO_ 2 (SiO~ 2.4 NaOH ) 400 H~O [7,8 ], the nucleation gel from batches with NaAIO 2 7.5 ( STO2 2 NaOH ) 150 H~3::) [9,10]. The nucleation gel was aged for 5 days at room temperature before use. "l-Se products were filtered, washed, and dried. The characterization was done by XRD comparing the intensity of the (555) - reflex with that of a well crystallized zeolite. The Si/AI ratio was determined by RFA or by EDX. The solutions of the earlier experiments have been analyzed with classical chemical methods, the other by ICP-AES. Some of the samples have been studied by 29 Si and by 27AI- MAS-NMR spectroscopy. For the kinetic studies, 10 ml samples were taken from the reaction vessel which were characterized in the described way. In later experiments the crystallization was carried out in 10 ml teflon vessels which were taken from the oven after appropriate times. 4. RESULTS AND DISCUSSION From a large set of data relating the Si/AI-ratio in the products with the silicate,the aluminate and the OH- content in the solution can be seen that (Si/AI)prod = 1 + b*lSiO 2 I~o~/I OH-Isol
(2)
with b = 1.92 for faujasite and some related zeolites [10]. The concentrations in the solution can be estimated from the batch parameters n, m and p with some accuracy. The aluminate concentration in the solution is about two orders of magnitude lower than the concentration of the silicate. Therefore it can be concluded that almost all of the aluminate and a corresponding amount of Na vanishes in the gel phase.
127
For IOH-I ~oI can be obtained (3)
]OH-Isol = n * m * 5 5 . 5 / p
Assuming that after sufficient aging the Si/AI-ratio in the gel phase is nearly that of the crystallizing zeolite (Si/AI) , ~ (Si/AI) , . (4) gem proaucl the silicate concentration in the solution can be estimated by
ISiO2 Isol = [(si/AI
)batch - (Si/AI)
,]. 55.5/p =
[ n ~'(Si/AI )prod ] •55.5/p
(5)
An experimental check of Eq. 4 and also data from the literature [11 ] show, that for typical zeolite batches this assumption is fulfilled with good accuracy. With Eq.2 can then be concluded, that (Si/AI)
prod
=
n * (b + m ) /
(b + n , m )
(6)
where for the faujasites and for related zeolites b = 1.92 ~ 2 can be used.. This argumentation and especially Eq.4 is possibly no more correct for low m and high n in the batch, because of the presence of colloidal SiO 2 in the gel as well as in the solution. In these cases higher values of b may be expected. Preliminary experiments with zeolite Beta and some pentasils confirmed this. The importance of Eq.6 is that it relates directly the Si/AI ratio of the product to the parameters m and n in the batch. The reliability of Eq.6 is shown in Fig. 1 for data from [10 ] and from this work in which the solution phase has been carefully analyzed. It can be seen that Eq.6 is fulfilled with b = 2 with a surprisingly high accuracy. A further test with data from the early literature summarized in the book of BRECK [12] is demonstrated in Fig.2. At the present stage of argumentation Eq.6 has to be regarded as a more or less empirical relation. As long as the assumptions made in the derivation of Eq.6 from Eq.2 are true, the explanation of Eq.6 is connected to the explanation of Eq.2. In a preceding paper we have discussed a model for the dependency of the Si/AIratio of a crystallizing zeolite on the OH- concentration in the solution[13]. This model starts from the assumption that the S i / A I - ratio is determined at the surface of the growing zeolite. The surface consists of the groups - AI-OH ]- , - Si-OH .and Si-O-. Between the silicate groups an equilibrium can be assumed: -=Si-OH + OH- = -Si-O+ H20. (7)
128
5.0 u
I,
4.0
-i-.....L
t
0
o_
+
!
ii
V
i
! iv
i i
I I
!i
i
2.0
3.0
,i "........................
2.0
i
1.0 1.0
n9
;
(2 + m)*n/(2
5.0
n=3
a
n=4
II
n--5
o
n-6-9 9
i
4.0
1.4,1.5
9 n = 2-2.5
3.0
E
tJ)
i
v
n>10
Mordenit
+n'm)
Fig. 1 Si/AI- ratios of faujasites with batch compositions with different n and m compared with values predicted by Eq.6
+ u :3 "13 O Q..
Mordenite 9 Analcime 9 A, HS,Zh, Cancrinite
E
0
Om
o
<
0
o /
r
a
P,R,S,X,Y
-
ZSM3,Mordenite with Li
i
0
!
0
!
|
|
|I
|
|
2
(2 + m)*n/(2
|
w
4
6 +n'm)
Fig.2 Si/AI ratios in different zeolites from batch compositions
with different n and m, and different cations taken from BRECK [12] compared with the values predicted by Eq.6
129
If it is further assumed that the aluminate from the solution reacts preferablg with the = Si-O- groups via a nucleophilic substitution reaction and the silicate with the = Si-OH via a condensation reaction, for the Si/AI- ratio an equation of the kind (Si/AI)prod
=
1 + a /IOH-I
(8)
can be derived. This relation describes the Si/AI-ratio of the product onlg if the Si09-concentration in the solution is held constant. It can be seen that the model usec~ for the derivation mag be correct with some further modifications taking into account the influence of the silicate which is found experimentallg in Eq.2. Regarding that the formation of a zeolite is a preferablg kinetical phenomenon, in which the Si/AI-ratio mag be given bg some kind of steadg state of the attachment of silicate and aluminate to the surface, in the following an extended number of kinetical experiments from this and our earlier papers shall be analgzed [7,8]. In these experiments the rate constants of linear growth k have been studied. The k have been related to the concentrations of [AI ( OH )4 ] - ' S i 0 2 ' and OH- in the solution phase. In Fig.3 the rate constant of linear growth k is plotted against the ratio of IOH -I , /ISiO.. I~ I" IOH-I_ I is an estimate of the concentration of the free alkali sqi .. ~ q~ in the sotutlon, given t)y the di~erence of the total alkali content and the silicate content in the solution, because from [1 ] can be seen that in the range of alkalinities in discussion here, the silicate in the solution phase is present preferablg as NaOSi(OH) 3 . As can be seen in Fig.3, k is a linear function of I OH -Isol/ISiO.2 ] , almost independent of n if n > 4. For low ] OH -I , / ISiO_! correspono~ng~h high Si/AIratios in the products the function o ~ ' o n thisz , ~ / b e c o m e s uniform for" all n. For lower values of n, k decreases distinctlg. In this region k is proportional to I OH - Isol * ISi02 ! sol with some accuracg. Both relations can be brought into a common equation bg k
=
or"
k
=
C 9( I OH -
- I SiO2 I=o )'1 sio2]~o t ( 1 + I SiO 21 sol ) 2
C *9 I AI(OH)4 - I~ol * ISiO 2 I sol )2 ( 1 + I Si02 I~ot
(g)
(10)
130
c-
E I
O tO r"
o
"-U
o
~-
c~i
4.0
+
Si/AI = 1.4
!
, ~ d
3.0 J 2.0
i
-
1.0
aJ
........................i.....................
--'i .......
i-"
121(
',
0.0 -2.0
2.0
i
1
I
6.0
10.0
14.0
(I NaOH l-IS~O= I)/I s~o= I
9
1.5
9
2.0
a
3.0
,,
4.0
o
5.0 9 6.0-9.9
9
>
10.0
Fig. 3 Rate constant of linear growth k in dependence on the ratio
IOH-Iso~/ISi021so~ c-
4,0
E I
o ,,I,--.
tO t/} tO u
o L_
, !
3,o
I
Si/AI = 1.4
i
i i
L -T--'
i io@O
o 9
2,0 ~- .... : ........... ,.-.-~s-; .............. 1,0
t
i
I~
~
i
1
i
0,0
0,2
0,4
!
=
i i
2.0
v
3.0
+
4.0
o
5.0 9 6.0-9.9
o,o -0,2
1.5
0,6
9
>
10.0
(I NaOH I-ISiO2 I)*1 s~o= I/(] + I SiO2 I) = Fig. 4 Rate constant of linear growth k in dependence on the relation given bg Eq.g
131
because it is [AI(OH) 4 ]-sol "" l OH-J,ol over a wide range of concentrati0ns. The data corresponding with Eq.9 are demonstrated in Fig.4. It can be seen that the data are described by this equation with good accuracy. Eq.9 is together with Eq. 10 similar to a LANGMUIR-HINSHELWOOD -equation for the formation of a structure of the kind (11) - Si(OH)2 - 0 - AI (OH)2 This means that the concentration dependence of the crystallization rate can be understood by a model in which the formation of a surface structure as e.g. a fourmembered ring is a rate determining event [14]. For a decision whether the ring closure or the attachment of the silicate or aluminate may be rate determining a more detailed discussion with the more extended relations of HOUGHEN and WATSON [15] has been carried out For the attachment of aluminate as rate determining step can be written
C'.( I AI(OH)4- I~o~ - SS/(ISiO21.ot*K) )
k =
(1+
CAt
AI(OH)4 -I ~ot * cslSiO
2
Isol }2
(12)
SS is the synonym for the surface structure and K the equilibrium constant of the formation of the ring structure from neigbouring - Si(OH) and -AI(OI-i) - 3 3 groups at the surface. K can be regarded to be large taking into account the low solubility of the aluminosilicate. Eq.12 becomes different from Eq.10. It can be shown that a uniform description of the experimental data is not possible by Eq. 12. Therefore, it can be seen that the dependence of k on the concentrations in the solution phase can be described by the formation of the rings given by (11) - or possibly structures formed in a consecutive reaction - as rate determining step. From Eq.10 follows that k is for the whole range of faujasites proportional to the concentation of the aluminate in the solution. The crystallization rate should, therefore, be increased with the concentration of the aluminate.. In two separate papers [16,17] the effect of the addition of fluoride and a series of complexing agents for the aluminate was thoroughly studied. It could be seen that the crystallization rate can be appreciably increased, especially for the crystallization of Y-zeolites supporting the foregoing arguments.
5. A
C
~
~
The authors thank for the support of this work by the "Deutsche Forschungsgemeinschaft.
132
6. R E F E ~ E S 1. P. Caullet and J. L. Guth, ACS Symp Series, 398 (1989) 83. 2. G. Harvey, L. S. Dent Glasser, ACS Symposium Series, 398 (1989) 49. 3.A.T. Bell, ACS Symp. Series, 398(1989)66. 4. S.P. Zhdanov and N.N. Samulevich Proc. 5th Intern. Conf. on Zeolites L. V. C. Rees Ed., Heyden, London 1980, p. 75. 5.M. Avrami, J.Chem.Phys., 9 (1941) 177. 6. R.W.Thompson, Zeolites, 12 (1992) 680. T. H. Kacirek and H. Lechert, J. Phys. Chem. 79 (1975) 1589. 8. H.Kacirek and H. Lecher-t, J.ChemPhys. 80(1976) 1291. 9 H.Lechert, P. Staelin, M. Wrobel and U. Schimmel, Studies in Surface Science and Catalysis 84A (1994) 147. 10. H.Lechert, P. Staelin, and Ch Kuntz, Zeolites 16 (1996) 149. 11. S.P. Zhclanov, S.S. Khvoshchev, and N.N. Feoktistova, "Synthetic Zeolites", Gordon and Breach Science Publishers, New York, Philadelphia, London, Paris, Montreux,Tokyo, Melbourne, 1990. 12. D. W. Breck, "Zeolite Molecular Sieves" John Wiley, New York, 1974 13. H. Lechert, H. Kacirek, and H. Weyda, in "Molecular Sieves" M.L. Occelli and H. Robson Eds., Van Nostrand Reinhold, New York, 1992, p. 494. 14. H. Lechert, Zeolites (in press). 15. O. A. Houghen and K. M. Watson "Chemical Process Principles" Part III Wiley & Sons, New York, 1948. 16. T. Lindner and H. Lechert, Zeolites, 14 (1994) 582. 17. T. Lindner and H. Lechert, Zeolites, 16(1996) 196.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
133
A computational 'Expert System' 'approach to design synthesis routes for zeolite catalysts T. Selvam, D.N. Iyert , R.C. Deka, A. Chatterjee* and R. Vetrivel* Catalysis Division, National Chemical Laboratory, Pune-411 008, INDIA. Fax: +91-212-334761 /330233.
We have developed a knowledge based database system on zeolites for IBM-PCs operating on MS-DOS. The system contains a database of physico-chemical and structural properties of all the molecular sieves. The data could be retrieved from the database by choosing answers to questions from a menu driven sot~ware and hence no background knowledge of computers is assumed. Further, the wide experience in the synthesis of molecular sieves, particularly zeolites, ZSM-5, ZSM-11, AIPO4-5 and AIPO4-11 available in the literature are collected and conserved. An 'expert system' approach is developed to derive a set of conditions to achieve one's goal in the synthesis of zeolites. The most suitable conditions of synthesis are decided based on the published data in the literature or based on the correlations derived from the reported data. This system includes the graphical display of the powder diffraction pattern for various molecular sieve structures, as well as their mixtures. By pattern matching procedure, it is possible to identify and analyze the impurity phases and yield in the synthesis batches.
1. INTRODUCTION Molecular sieve materials which comprise zeolites, ALPOs, etc., are the single class of catalysts having maximum commercial applications. The application of zeolite catalysts has penetrated into the areas of environmental catalysis and fine chemical synthesis, in addition to petroleum and petrochemical industries. Nearly 100 molecular sieves with established structures are reported [1] and more than 200 hypothetical molecular sieve structures are predicted [2]. In proportion to their scientific and technological importance, knowledge and information on existing as well as new zeolite structures are accumulating over the years. The design of a new catalyst involves logical application of all the available information. It is becoming a difficult task to manage the available information on molecular sieves by human experts. , Present address : Dr. Reddy's Research Foundation Bollaram Road, Hyderabad 500 138, India * Present address : Tohoku National Industrial Research Institute, AIST - MITI, 4-2-1, Nigatake, Miyagino-ku, Sendai 983, Japan * Author for correspondence.
134 Earlier attempts in the direction of systematically storing and effectively utilizing the information on catalysts include: i) a data base on C~ catalytic chemistry by Ito et al [3] ii) computer aided design of catalysts for oxidation reaction using expert systems approach [4,5] and iii) expert system approach to design a catalyst [6]. Computerization of information on zeolite catalysts have also been reported and different databases concentrate on specific properties [7-10]. They contain extremely useful information as shown in Table 1. In this study, we have attempted a systematic information technology approach and an Expert System (ES) approach to computationally design the synthesis routes for zeolites. Table 1 Existing; databases containing; information on zeolitic system. Database Hardware Information Content Advantages Platform Zeofile Macintosh Physico-chemical Simulation of powder property, diffraction pattern for a crystallographic known crystal structure is information, possible. Any textual and structure and graphical information topology of all could be exported to Word materials in Ref. 1. processor files.
Reference 7, 10
Zeopak
VAX system
Powder diffraction patterns of synthetic and natural zeolites,
Identification and quantification of zeolite phases. Characterization is possible by pattern matching.
8
Zeobase
IBM-PC AT 286 and above
Crystalstructure of Entries on as many as 1300 various modifications materials are available. Xof zeolites, ray patterns and molecular models of zeolitic materials can be viewed on screen, plotted on printer or plotter.
9
2. METHOD OF APPROACH
Our approach is to develop an user-friendly software system which can aid the design of molecular sieve catalysts. The task of 'molecular sieve design' can be resolved into many smaller tasks and the smaller tasks could be completed in a step-wise manner. The smaller tasks in catalyst design are listed out as follows:
135 i) propose the list of required physico-chemical properties in a zeolite for a specific end-use, ii) screen out zeolites by searching for a known material possessing all required properties, iii) if one is available with all desired properties the search is completed successfully, which is most unlikely. Alternatively if few molecular sieve materials are available whose properties are reasonably closer but not exactly as needed, find out the ways of modifications to improve or bring it to the level of satisfaction. iv) list out the side-effects or unfavorable properties that may get introduced due to modifications and find ways of eliminating them and v) design a viable and efficient synthesis route for the zeolite proposed in step (iii), taking into considerations of the points listed in step (iv). When the above tasks are completed, the number of experiments needed to design a molecular sieve catalyst could be restricted to a limited number and many trial and error process could be minimized. As recently pointed out, systematic correlations are derived between the critical factors that influence the zeolite synthesis and the kinetics, the quality as well as the yield of the zeolites formed [11]. The availability of all information as a single source, that too in a convenient electronic media is an asset, in our own experience. The retrieval of information on any specific known molecular sieve or on all molecular sieves with specific property is straightforward with our well structured database. An inference engine using logical computer language such as PROLOG is developed to make decision from the data provided in the form of mathematical relations. Thus an efficient research tool which reduces the burden involved in the above mentioned steps in the molecular sieves catalyst design is developed. Additionally, this shall be an effective educational tool for any beginner in the-field of molecular sieves. A 'question-answer' interactive session between system and user is useful as a tutorial for novice in the field of molecular sieves.
3. APPLICATIONS The structure of the system is designed to perform three salient functions as shown in Fig. 1. The first feature provides access to a large database of physico-chemical properties and crystallographic information of all reported zeolites [1]. The second feature provides interactive access to the expert system for the synthesis of zeolites. At the end of the session, the most logical route for the synthesis of a desired zeolite structure is provided. The third feature is a graphic tool box application to simulate X-ray powder diffraction patterns for zeolite phases with different amounts and nature of purity. 3.1.
Source of information
The salient information which are incorporated in the database for different zeolites are given in Table 2. Molecular sieves can be sorted out based on specific value or a range of values of a property. For example, one or all molecular sieves having a framework density of 14.0 T/1000/I,3 or in the range 10.0 to 15.0 T/1000A 3 could be sorted out. Additionally, a combination search for molecular sieves could be performed. This allows one to choose all materials which have a desired value or range of values for multiple properties. For example it is possible to select all molecular sieves having a framework density values in the range 10.0 to 20.0; bidimensional channel architecture; small pore system and orthorhombic symmetry could
136
OPENING
MENU
| EXPERT SYSTEM
DATA B A S E
ZEOLITE- b
KNOWLEDGE F,
WISE
BASE
'ROPERTIES R-C--I'PRO~~T O y(
GRAPHICS TOOL
WISE
QUESTION -
'
INFERENCE ENGINE
PDP FOR PURE PHASES
/~A ETERM INESX ZEOLITE WITH DESIRED PROPERTIES
M INE$ 7 DAETER LOGICAL ROUTE FOR SYNTHESIS
I" ~
II~IPHASES WITH III I I M P U R I T Y
INTERACTIVE SESSION
ANSWER SESSION
/
h
INTERACTIVE SIMULATION ~ THETERMIN ES" E P HASE COMPOSITION
Figure 1. Outlook of the sot~ware
Table 2 Salient information on each molecular sieve stored in the data base General 3 letter code Expansion of the code - full name P h y s i c o - c h e m i c a l properties Structural properties Framework density (Fwd) Secondary building unit Dimensionality of the channel (Dim) Crystal symmetry (Xst) Pore size Crystal space group (Xsg) Pore dimension Unit cell dimensions Isotypic framework structure Atomic co-ordinates of unit cell Miscellaneous
Catalytic properties Dimension of reactant molecules Dimension of template molecules Template, synthesis conditions and reference
137 Table 3 A typical sample screen is shown for a combination search performed. INPUT SELECTION Data Entr~ Menu
I SearchMenu ] Reports Exit IF i
[,Zeolite
I
Starting Material Exit Zeolite
Property [ Combination Properties
I
Exit Framework Density Between : 10.0 and 20.0 Dimensionality 92
Pore Size Crystal Structure
: SP : ORT
OUTPUT PRODUCED Zeolite
Fwd
Dim
Xst
APC APD ATT MON
18.00
2
19.80
2
16.70 18.10
2 2
ORTHORIIOM ORTHORHOM ORTHORHOM ORTHORHOM
Total
Xsg Pbca
Pca21 P212121 Fdd2
4 zeolites
be sorted out as shown in Table 3. A boolean search with 'and', 'or' and'not' options can be performed on zeolite names, zeolite structure codes, framework density, keywords, space group, crystal system, unit cell volume, catalytic performance and authors of the work. The output always appears on the screen and there is option to store the information to the hard disk of the computer as a file for future reference or to output as a hard copy in the printer for the usage on a WYGlWYS ( what you get is what you see) basis. There is always a hot-key (F l) help facility i.e., whenever F 1 key is pressed, a help information regarding the menu items on the screen are displayed.
138 A list of reactions catalyzed by a certain zeolite is also included. The dimensions of many hydrocarbon reactant molecules are part of the data base. Hence it is possible to get a first hand information above the suitability of a molecular sieve to adsorb reactant molecules. Sorption capacity of molecular sieves from experimental reports for various molecules are also added.
3.2. Designing a synthesis route by ES approach Nearly 75 different conditions of synthesis for ZSM-5 [12] and 25 different conditions of synthesis for each of ZSM-11, AIPO4-5 and AIPO4-11 were collected from the literature. These different "conditions" include the variations in the templating organic molecule, temperature, time, static or stirring, composition and source of hydrogel as well as pH. Templates were arranged in the order of descending efficiency. Inverse linear correlations were observed between temperature and time of crystallization as well as between efficiency of the template and time of crystallization. The dependence of time of crystallization on the above mentioned conditions were plotted and fitted into suitable mathematical relations. The system initially starts by enquiring the user for the final "output" of preferred phase, yield, crystallinity and morphology. The database is searched to see if these requirements could be met by match-making with one of the existing reports in the literature. In the absence of a synthesis route in the reported literature, the system chooses the conditions from the mathematical relations already established. Here, it is assumed that the data published in the literature are correct and they could be extrapolated or interpolated to valid regions, where there are gaps in the literature. The conditions are varied within a predefined range, so that finally all conditions fit into the mathematical relations between the "conditions" and "output", within an error count of 10%. The system registers an error message, in case the required output cannot be achieved within 10% error of variation in conditions. Thus the system chooses optimal conditions for the synthesis to satisfy the requirement of the user. The system solves problems which are often poorly defined or understood. At each decision making point, the system starts by providing an introductory note about the conditions being varied as well as typical case studies which are available in the literature.
3.3. Analysis of phase purity in synthesized samples Simulated powder diffraction patterns (PDP) of all the molecular sieve materials are presented in the form of a graphic display or in the form of a table of 20 and relative intensity. It is possible to simulate PDP of mixture of more than one zeolite. When PDP of more than one structure is displayed, the pattern for each structure is displayed in different color. This facility is useful to analyze the phase purity of any synthesized sample. For example, it is well known that ZSM-11 phase occurs as impurity while synthesizing ZSM-5 phase and vice-versa. In Fig.2a and 2b, the PDP of ZSM-5 and ZSM-11 are shown, respectively. Fig.2c shows the PDP for a 50:50 mixture of ZSM-5:ZSM-11, which is a typical example. If the PDP of synthesized sample matches with Fig.2c, the phase concentration could be inferred. Similarly PDP can be simulated for different proportions of the individual component, till it matches with the experimentally observed pattern. As an another typical example, the PDP of AIPO45, AIPO4-11 and the simulated PDP for 75:25 mixture of AIPO4-5:AIPO4-11 are shown in Figs. 3a, 3b, and 3c, respectively. If a sample obtained from AIPO4-5 synthesis batch showed a pattern as in Fig.3c instead of Fig.3a, then 25% yield of A1PO4-11 could be inferred. Thus using this facility, the impurity phases and the selectivity in the synthesis could be identified.
28
28
28
Figure 2. The PDP generated by the system for ZSM-5 (a), ZSM-11 @) and 5 0 5 0 ZSM-5:ZSM-11 100. F
- 100
100.
(a 1
-
80
-
-
80 60
60
-
1
w40-
40
40
-
320-
20
20
-
Ero E60
I
(c 1
W
a
t
0
1
5
I
10
L
IS 28
a
20
1L
25
30
0
5
10
IS 28
20
2530
0
I
1
5
10
Figure 3. The PDP generated by the system for AlP04-5 (a), M 0 4 - 1 I (b) and 75:25 ~ 0 4 - 5 : M 0 4 - 1 1
I
I l A
IS 2 0 2 5 28
30
140 4. SUMMARY Molecular sieves literature today is vast and versatile. Although one may remember that an information is available in the literature, it becomes a hard or sometimes impossible task to search and retrieve that information from the books and journals. However, one should recognize that the 'information science' has benefited largely by the advent of powerful computers and efficient software packages. Desktop personal computers which have attained the status of'essential equipment' of any researcher are good enough to handle vast amount of information efficiently. We felt that one massive attempt to collect all the information, classify and store them systematically as well as to create a 'user-friendly system' to retrieve will help the whole molecular sieve research community. The database is further upgraded as an ES to design a logical route for the synthesis of zeolite. The routes are decided from either well documented synthesis procedures or from correlations derived from the reported data. Thus the feasibility of ES approach has been tested and shown as powerful tool for computer aided design of catalysts.
ACKNOWLEDGMENTS The authors (TS, RCD & AC) thank UGC and CSIR for financial support in the form of Research Fellowships. We thank Dr. S. Krishnan and Dr. P. Ratnasamy for initiating the building up of the database reported here. We acknowledge the programming expertness provided by S. Mani, S. Hajamis, S. Ganorkar and V. Dhayagude. Many people in Catalysis Division, National Chemical Laboratory actually used the system in its primitive stage and suggested improvements which made it user-friendly. The work was partly funded by UNDP.
REFERENCES
1. W.M. Meier and D.H. Olson, in: 'Atlas of zeolite structure types', Butterworth Heinmann, London 1992. 2. J.V. Smith, Chem. Rev., 88 (1988) 149. 3. T. Ito, H. Hamada, Y. Kintaichi and M. Sasaki, Catal. Today 10 (1991) 223. 4. S. Kito, T. Hattori and Y: Murakami, Appl. Catal., 48 (1989) 107. 5. T. Hattori, H. Niwa, A. Satsuma, S. Kito and Y. Murakami, Appl. Catal., 50 (1989) L 11 6. T. Hattori and S. Kito, Catal. Today 10 (1991) 213. 7. J.M. Newsam and M.M.J. Treaey, 9th Int. Zeolite Conf., Extended Abstracts and Program, No.RP 193 (1992). 8. D.K. Smith, S.Q. Hoyle and G.G. Johnson, Jr., 9th Int. Zeolite Conf., Extended Abstracts and Program, No.RP 192 (1992) 9. W.H. Baur and R.X. Fischer, 9th Int. Zeolite Conf., Extended Abstracts and Program, No.RP 216 (1992). 10. J.M. Newsam and M.M.J. Treaty, Zeolites 13 (1993) 183. 11. E.J.P. Feijen, J.A. Martens and P.A. Jacobs, Stud. Surface Sr Catal., 84A, 1 (1994). 12. A. Chatterjee and R. Vetfivd, J. Chem. Sor Faraday Trans., 91 (1995) 4313.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
141
A new method for enhancing zeolite crystallization by using oxyacids / salts of group VA and VIIA elements as promoters Asim Bhaumik, A.A.Belhekar and Rajiv Kumar Catalysis Division, National Chemical Laboratory, Pune - 411 008, INDIA. ~ A novel and general method for highly efficient, fast synthesis (with 4-6 fold increase) of zeolite and related materials has been developed using a new concept of addition of some promoters (oxy anions like C104, PO43, AsO43, C103,etc in the form of salt or acid). The materials
produced using promoters exhibit similar, if not better, yield, crystallinity, morphology and catalytic properties compared to those obtained by conventional method. 1. INTRODUCTION Microporous, crystalline alumino- or metallo-silicate molecular sieves / zeolites [1-4] exhibit unique catalytic, adsorption and ion-exchange (viz. as detergent builders) properties. Zeolites are largely used as catalyst in various chemical, petroleum refining, petrochemical, bulk chemical and fine chemical processes [4-10]. Although, the nature of surface and catalytic properties of zeolites and related materials is relatively better understood, their synthesis continues to be intriguing and challenging. Zeolites are generally synthesised hydrothermally from an alkaline aluminosilicate reaction mixture which may or may not contain organic guest molecule (template), at autogenous pressure for few days to few weeks time. Inui [ 11 ] has pointed out various drawbacks of long crystallization time (few days or more) extensive labor coupled with delays and expense, low reproducibility and formation of larger crystals with inhomogeneous particle size distribution mainly because of secondary crystallization. However, the optimisation of recipes for each structure is done on case to case basis. Hence, ever increasing efforts are going on to reduce the synthesis time of these materials and thus to save energy and time. Now, we report, for the first time, a general method using some promoters (like C104, PO43, AsO43, C103", BrO 3" etc. in the form of acids or their Na/K salts) for highly efficient, fast
1 AB thanks CSIR, New Delhi for granting a senior research fellowship.
142 synthesis (with 4-6 fold or more increase) of zeolite and related materials with similar, if not better, yield, crystallinity and catalytic properties compared to those obtained by conventional method. A variety of zeolite structures representing small (NU-1 & FER), medium (ZSM-5 & ZSM-48) and large (Beta & ZSM-12) pore zeolites as well as aluminous / low silica (Si/A1 = 1-5, e.g. zeolite Y) and silicious / high silica (Si/A1 > 5: such as ZSM-5, ZSM-12, Beta etc.) molecular sieves have been chosen as representatives to demonstrate the generality and wide applicability of our present method.
2. EXPERIMENTAL All synthesis experiments were carded out in 200 ml capacity stainless steel autoclaves with teflon coatings under static conditions. In a typical synthesis, silica source was stirred with required amount of template and alkali dissolved in water for 1 h. Then the aluminium source (NaA102 for most of the cases) taken in water was added into it. Finally required amount of promoter (added in the form of oxyacid or their sodium / potassium salt of their corresponding oxyanions) taken in the rest amount of water was added slowly to the stirring gel. Stirring was continued for another 1 h and then the resulting gel was autoclaved. Na § / SiO2 molar ratio was kept constant for a particular zeolite by adding appropriate amount of NaC1. Gel compositions in terms of moles of oxides are given in Table 1" Table 1" Starting molar gel compositions of various zeolites Zeolite
Molar gel composition
NU-1
40
S i O 2 " A1203 " 5.0
FER
60
S i O 2 9A1203 "
ZSM-5
40
S i O 2 " A1203 " 5.0
ZSM-48
90
S i O 2 " A1203 " 9 . 0 N a 2 0
pH
Na20 " 10 (TMA)20 91000
H20"
18 N a 2 0 " 7.5 Pyrrolidine 92400 Na20" 5 (TPA)20 91200 9 15
DIQ-6 92700
12.2 _+0.2
H 2 0 " 6P
12.0 _+0.2
4P
10.8 _+0.2
H20"
n20"
4P
9P
11.6 _+0.2
ZSM-12
120
Beta
30SIO2 9A1203 "Na20 " 0.5K20 97.5(TEA)20 9600H20 93P
12.5 _+0.2
Y
9SiO 2 9A1203:4 Na20" 270 H20 9P
13.4 _+0.2
SiO 2 " AI203 " l0 Na20"
12 DIQ-6 " 3600
H20 "
12P
10.8 _+0.2
Where P = promoter chosen from HC104, NaCIO 4, H3PO 4, NaH2PO 4, Na2HPO4, Na3PO4, Na2HAsO4; TEA = tetra ethyl ammonium, TPA = tetra propyl ammonium, TMA= tetramethyl ammonium, DIQ-6 = hexamethylene bis (benzyl dimethyl ammonium), P =
C 1 0 4 , P O 4 3 , AsO43-
143 or none. The crystallisation temp (~
was = 170 (NU- 1, ZSM-48), 160 (FER, MFI, MTW) and
140 (BEA). In case of zeolite Y the standard seeding procedure reported by Ratnasamy et al.[ 12] was followed. 3. RESULTS AND DISCUSSION In Table 2,effect of various promoters used in the synthesis of different zeolite structures is shown. The use of small amount of promoters signigicantly enhances the overall crystallisation process in the order : C10 4- > PO43 > AsO43- >> none, independent of structures. Some stabilising effect of certain neutral sodium salts of some anions (e.g. NaC1, NaNO 3, Na2SO 4, NaC103 etc.) during hydrothermal transformation of kaolinite into sodalite and cancrinite has been observed [ 13] where the inclusion of these salts in the sodalite / cancrinite structure was found to be the main cause of stabilising a particular phase. In the present case of direct synthesis no such inclusion of promoters (anions or their salts) in crystalline molecular sieves was obtained. Table 2: Effect of various promoters on zeolite synthesis Zeolite
Cryst.
Si/AI
Crystallization time, h
Temp.~
(solid) a
CIO 4"
PO43"
msO43"
None
NU- 1
170
17-19
28.0
30.0
36.0
120.0
FER
160
10-15
18.0
26.0
26.0
60.0
ZSM-5
160
15-17
6.0
8.0
12.5
36.0
ZSM-48
170
30-38
40.0
44.0
40.0
108.0
ZSM- 12
160
45-50
36.0
40.0
42.0
132.0
Beta
140
12-14
30.0
32.0
36.0
156.0
Y
100
2.4-2.8
4.0
4.0
5.0
11.0
a: Obtained by chemical analysis of various zeolites using different promoters In Fig. 1, the crystallinity of zeolite Y (curves x and y) and ZSM-5 (curves a-d) is plotted against their corresponding crystallization time. The synthesis time was considerably reduced by the addition of the promoters in both the cases. However, in the case of zeolite Y, not only the crystallization rate but also the stability of fully crystalline material in the mother liquor was found to be more in the presence of a promoter (PO43-). While, following the conventional / standard method (without using promoter) zeolite P, a common impurity in Y synthesis [12], was detected immediately after 30 - 60 min. of the complete formation of Y (curve y, Fig. 1), in our method (curve x, Fig. 1) zeolite P was not observed even after 2 hours of the complete crystallization of zeolite Y. It is again clear from Fig. 1 that under otherwise same synthesis
144 conditions, the presence of promoters enhances the nucleation and crystallisation in the order: ClO 4" > PO4 3" > AsO4 3" >> none. The polarisability of the central element of the promoter (i.e.
charge / radius, Z/r value) also decreases in the same order. Further, from the nature of crystallisation curves (a-c vis-a-vis d in Fig 1), it can be inferred that the presence of promoters enhances the process of both nucleation and crystallisation.
lOOt
.~ 80-
w~'='e=w~n~=~J
,'
!
X
a
,,~r[
b
I iJY
I
I
>"
I
I
_~60-
I
I
i
I
Z
"J
I
I
"J 4 0 "
t
I
j=
I
l
U')
-/,'
-
l
I
,
'
I
~ 2O-
I !I
, '
/
4
I'
12
16
"
CRYSTALLIZATION
I
I
5
TIME , h
Fig. 1" Crystallization kinetics of zeolite Y (curve x with Na3PO4 and y with no promoter, arrow indicates the appearance of zeolite P) and ZSM-5 (curves a,b,c & d represent the use of HC104, H3PO4, Na2HAsO4 and none, respectively as promoter). For gel composition see Table 1. After having acertained the utility of various promoters, in zeolite crystallisation, it was thought worthwhile to study the effect of concentration of a particular promoter on a particular zeolite structure. NaH2PO 4 and ZSM-5 were chosen as representative. It was found that with
145 increasing the amount of promoter, the crystallisation time decreases upto Si / promoter molar ratio = 5. By still increasing the amount of promoter no further enhancement in crystallisation was observed (Table 3). Table 3: Effect of phosphate promoter concentrations on crystallisation of ZSM-5 T = 160 ~
gel composition is given in Table 1.
Si / Promoter molar ratio
Cryst. time, h a
oo
20
10
5
2.5
40
12.5
8.0
7.0
6.5
a: Time taken for obtaining fully cryStalline material, calculated from crystallisation kinetic curves
Hence, above mentioned data clearly demonstrate that a considerable increase in the nucleation and crystallisation processes of various zeolites can be achieved by adding a small amount of certain oxyacids (or their salts) of group V and VII A elements. After achieving the goal of faster synthesis of zeolites, next obvious question is to varify the quality of these materials. All samples were thoroughly characterised through XRD, SEM, EDX and weight chemical analysis. It is pertinent to mention that in all the cases comparable or better crystallinity and yield along with better uniformity of particle size distribution was obtained with the use of promoters over that prepared in the absence of promoter. The scanning electron micrographs of zeolite Y (Fig.2, a and b) and ZSM-5 (Fig.2, c and d) taken as representative, exhibited that the crystallite size of the samples synthesised in the presence of promoter (e.g. PO43, a and c) vis-a-vis its absence (b and d) is smaller along with better uniformity of particle size distribution. The crystallite size of zeolite Y was 0.6 to 0.8 Ixm and 0.8 - 1.5 l.tm in the presence and absence of Na2HPO4 respectively. Corresponding values for ZSM-5 were 0.8 - 1.0 ~tm and 1.0 - 2.0 ktm, respectively in the presence and absence of promoter. Same observations are obtained for other topologies also. Smaller and uniform crystallites size distribution indicates the absence of secondary crystallisation. When both nucleation and crystallisation processes are quite fast, the secondary crystallisation is expected to be eliminated or suppressed. The n-hexane cracking reaction was carried out over H form of ZSM-5 samples represented by Fig.2C and Fig.2d. The value of moles of n-hexane converted
146 per mole of A1 per h was 18.0 and 17.2 respectively for H-ZSM-5 samples synthesised in the presence and absence of promoter (T = 623 K) clearly suggesting that the material synthesised with promoters is quite comparable, if not better, in quality.
"'7
r
Fig.2: SEM photograph of zeolite Y ( a: with and b: without promoter, PO43-) and ZSM-5 (c: with and d: without promoter, PO43-) One of the most fundamental basis of the hydrothermal synthesis of zeolites is the mineralising property of water, which is greatly assisted by free OH concentration in the solution/gel. Apart from this basic requirement of mineralisability, other factors like, Si/A1 ratio, pH, aging at low temperature, crystallisation temperature and time etc. influence the type and quality of the crystalline material in rather specific ways. For example, in the crystallization of aluminous zeolites (Si/A1 = 1-5) the synthesis becomes faster with decreasing Si/A1 ratio while for high silica/silicious molecular sieves (Si/A1 > 5) the reverse is true [13-15]. Further, the range of
147 synthesis temperature for low and high silica zeolites is 80-120~ and 120-200~
respectively.
Similarly, organic bases (templates) play particularly significant role only in the synthesis of high silica zeolites [ 13-15]. However in our present method of using oxyacids / oxysalts of Gr VA and VIIA as promoters the enhancement in nucleation and crystallization is observed for all aluminosilicates zeolites independent of silica alumina ratio, pore size of the structures and temperatures of the synthesis. Although, the role of the promoters at mechanistic level is not clear, a direct correlation is found between Z / r of the central cation of the promoter with the decrease in crystallisation time. 4.
CONCLUSIONS The addition of small amount of some oxyacids (or their Na / K salts) of group V and VII
A elements (like C104, PO43", AsO43", C10 3- etc.) significantly promotes the nucleation and crystallisation of zeolites. Further, this method is applicable to all types of low, medium and large pore zeolites. Nearly 4-6 fold reduction in crystallision time could be achieved in all the cases. The quality (crystallinity, morphology, catalytic activity) of the samples obtained using promoters was comparable, if not better, than those synthesised by standard recipies without using any promoter. Promoters with more polarisability are more effective in enhancing the crystallisation. REFERENCES 1.
D.W.Breck,"Zeolite Molecular Sieves", Weley, New York, 1974.
2.
R.M.Barrer, "Hydrothermal Chemistry of Zeolites", Academic Press, NewYork, 1982.
3.
R.Szostak, "Molecular Sieves : Principle of Synthesis and Identification" Van Nostrand, Reinhold, New York, 1989. H.G.Karge, and J.Weitkamp, (Eds.), "Zeolites as Catalysis, Sorbents and Detergent Builders : Applications and Innovations," Elsevier, Amsterdam, 1989.
4. 5.
W.Holderich, M.Messe, and F.Naumann, Angew.Chem.Int.Ed.Engl. 27 (1988) 226.
6.
P.B.Venuto, Microporous Materials 2 (1994) 297.
7.
P.Kumar, R.Kumar and B.Pandey, Synlett. (1995) 289.
8.
A.Bhaumik and R.Kumar, J.Chem.Soc.Chem.Commun. (1995) 349.
9.
P.G.Schultz, Angew.Chem.Int.Ed.Engl. 28 (1989) 1283.
10.
M.E.Davis, Acc.Chem.Res. 26 (1993) 111.
11.
T.Inui, in "Zeolite Synthesis" (M.L.Occelli & H.E.Robson, Eds.) ACS Symp. Ser. 398, Am.Chem.Soc., Washington, D.C., 1989, ch.33, pp. 479-492.
12.
P.Ratnasamy, A.N.Kotasthane, V.P.Shiralkar, A.Thangaraj and S.Ganapathy, in "Zeolite Synthesis" (M.L.Occelli & H.E.Robson, Eds.) ACS Symp. Ser. 398, Am.Chem.Soc., Washington, D.C., 1989, ch.28, pp. 405-419.
148 13.
R.M.Barrer, in "Zeolites : Synthesis, Structure, Technology and Application," (B.Drzaj, S.Hocevar and S.Pejovnik, Eds.), Elsevier, Amsterdam, 1985, pp 1-26.
14.
P.A.Jacobs and J.A.Martens, Stud.Surf.Sci.Catal. 33 (1987) 58.
15.
J.S.Reddy, R.Kumar and S.M.Sciscery, J.Catal. 145 (1994) 73.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
149
The influence of mixed organic additives on the zeolites A and X crystal growth V. Petranovskii', Y. Kiy0zumib, N. Kikuchib, H. Hayamisub, Y. Sugi~, F. Mizukamib "Institute of Physics, UNAM, Ensenada, B.C. 22800 Mexico" bNational Institute for Materials and Chemical Research, AIST, Tsulmba, Ibarald 305, Japan
~
University, Cfifu,~01-11 Japan
The state of Al in the initial solutions for gel preparation is found to be important for the results of zeolite synthesis. Polynuclear complex of AI with chemical shift 8 = 57 ppm in the 27AI NMR spectra influences significantly the size and the shape of NaA and NaX crystals. The concentration of this complex depends on the composition of organic additives. 1. INTRODUCTION Zeolites have wide applications in diverse areas. Nevertheless, the mechanism of their growth is not completely clear [1]. A model that includes "secondary building units" (SBU) packing was suggested [2]. The problem of this model is to explain formation of these SBU from initial compounds. Another question is connected with the role of organic molecules in the zeolite synthesis. Organic compounds are frequently used as templates for zeolite growth [3, 4]. Usually it is supposed that the organic molecules direct crystal structures as space-filling agents. Joining these two models, a "can-and-cement" model of nucleation in zeolite synthesis was developed [5]. The formation of inorganic-organic composite structures was proposed as the explanation of the mechanism directing the structure [6, 7]. Chamell [8] found that the addition of triethanolamine (TEA) in the reaction mixture resulted in the growth of large single crystals of zeolites A and X. TEA molecules are not included in their crystals during the growth process. Hence TEA can not be the real template. It plays the role of a complex forming ligand. Only the polynuclear complex of tetrahedral surrounded Al with the chemical shift 8 = 62.6 ppm in the r~Al NMR spectra was detected in the initial solution [9, 10]. Such complexes (for example, similar to alumoxanes [11 ] or to well-known silicate ions [SisO-x]s" [12]) can be the prototypes of SBU. The aging of the initial solutions increases the size of NaX single crystals from 0.15 mm [8] to 0.25 mm [13]. The small concentration (less than 1%) of a new complex of AI with 8 = 57 ppm appears in the process of aging [10]. The same complex was found to be formed in the Al containing solutions in the presence of diethanolamine (DEA) or diisopropanolamine (DIPA), but other AI complexes were different [14].
On leavingfrom A.F. IoffePhysicalTechnical Institute, RAS, Saint Petersburg, 194021,Russia
150 Transformation of the shape of NaA crystals was observed in the case of their growth in DEA- or DIPA-containing gels. The reason for the shape change can be the variation of the structure of different aluminosilicate ions (potential SBU) in the presence of unlike ligands [ 14]. On the base of the data described above it is possible to expect that the simultaneous addition of di- and trialkylamines to the solution will lead to the generation of two complexes (with 8 -- 62.6 ppm and 6 = 57 ppm) at the same time. A variation of the ratio of organic additives can change the relative concentration of different complexes of aluminum in the solution and thus controls the results of synthesis. The aim of this work is to investigate the influence of the composition of organic additives on the process of crystal growth ofzeolites A and X. 2. EXPERIMENTAL SECTION
2.1. Preparation of solutions Sodium aluminate and sodium metasilicate ermeahydrate were used. The solutions with the compositions 0.42 mol NaAIO2 + 0.95 mol TEA + 55.51 mol H20 for NaX synthesis, 0.84 mol NaAIO2 + 0.95 mol TEA + 55.51 mol H20 for NaA synthesis and 0.44 mol Na2SiO3 + 0.95 mol TEA + 55.51 mol H20 for both cases were prepared following the procedure described by Charnell [8]. TEA was replaced in these solutions by the equivalent amount of DEA, DIPA or triisopropanolamine (TIPA). The concentration of dialkylamine (DEA or DIPA) in mixture was chosen to save total number of - - R - - O H groups. For that reason their molar concentration was half as much again as the concentration of TEA or TIPA. Mixtures of additives ( T I P A - DIPA, TIPA - DEA, T E A - DIPA, T E A - DEA) that contain 0%, 50%, 80%, 90%, 98% and 100% of NHR2 were selected to perform the zeolite synthesis. Analytical grade reagents (produced by Wako Pure Chemical Industries Ltd., Japan) were used. 2.2. 27A! NMR spectra Measurements of 27A1 NMR spectra for clear aluminate solutions were carried out on a JEOL GSH-200 spectrometer, operating at 51.90 Mt-k, using a 10 mm probe tube. The observations of 2~AI chemical shiRs were quoted relative to AI(I-I20)63+ (8 = 0 ppm). The references were placed in capillaries; the last ones were coaxially inserted in the NMR tubes. The measurements were done with a 90 ~ pulse of length 28.0 I~s, 256 scans. Spectra were recorded at 293 K. 2.3. Zeolites synthesis and samples examination The gels were prepared using TEA-, TIPA-, DEA- and DIPA-containing solutions, indicated above. The same silicate solutions in combination with different aluminate solutions were used for synthesis of NaA and NaX zeolites. Equal amounts of silicate and aluminate solutions were mixed at room temperature. The crystallization process was held at 75 ~ at static conditions. The size and shape of zeolite single crystals were detected with an optical microscope Olympus B061 and a scanning electron microscope Hitachi S-800.
151 3. RESULTS AND DISCUSSION
3.1. Influence of additive nature on the aluminum complexes in the solutions Only the monomer four-coordinated ions AI(OH)4" exist in the clear aluminate solution in the lack of any organic additives (Fig. 1, a). The chemical shift of AI in this compound is equal to 79.8 ppm. Addition of tri-substituted amines (TEA or TIPA) to the solution results in almost complete disappearance of the monomer ions. Simultaneously the polynuclear complex with 8 = 62.6 ppm appears (Table 1, Fig. 1, e, f). In the presence of DEA or DIPA the ion AI(OI-I)4" is kept in the solutions side by side with a new complex with 8 - 57 ppm (Fig. 1, c, d). The concentration of the last complex is small (Table 1). This is the same complex that was found for aged solutions [10]. Table 1 Relative intensity of different peaks in 27A1 NMR spectra for initial solutions with changed organic additives. Intensity of peaks (%) with 8:
Compounds
79.8 ppm
62.6 ppm
57 ppm
< 1
> 99
< 1"
< 1
> 99
-
95
-
5
90
-
10
100
-
-
2-Diethylaminoethanol (C2Hs)2NCH2CH2OH Tds(Hydroxymethyl)aminomethan
100
-
-
100
-
-
Triethylamine
100
-
-
2,2',2" =Nitrilotriethylamine
100
-
-
N(CH2CH2NH2)3 3,3',3"'-Nitrilotdpropiorfic N(CH2CH2COOI--I)3
100
-
-
100
-
-
Triethanolamine (TEA) N(CH2CH2OH)3 Triisopropanolamine (TIPA) Diethanolamine (DEA)
[CH3CH(OH)CH2]3N
HN(CH2CH2OH)2 Diisopropanolamine (DIPA) [CH3CH(OH)CH2]2NH Monoethanolamine (MEA)
H2NCH2CH2OH
(HOCH2)3CNH2 (C2Hs)3N
Nitrilotdacetic acid ~
acid ~r
N(CH2COOH)3 Glycine ~ 100 H2NCH2COOH Nitrilotds~ethylenephosphonic Acid) ~ 100 N(CH2POaH2)a *For aged solution The acids were neutrahzed by NaOH, and corresponding sodium salts were used '~
~
9
~
-
152 An aluminum ion interacts with TEA through its alcohol termination. The complex of AI with 5 = 62.6 ppm contains aluminum as ~AI--O---AI-- bridges only [9]. Moreover it is interesting that the number of identical alkyl groups connected with a nitrogen atom influences so significantly the structure of AI complexes in the solutions (see Fig. 1, b, c, e). Specifically, monodentate ligand MEA can not form polynuclear complexes of AI. Bidentate ligands, such as DEA or DIPA, form complex with 5 = 57 ppm as well as monomer AI(OH)g. Tridentate ligands (TEA or TIPA) convert all AI in the solution into the polynuclear complex with 5 = 62.6 ppm.
23 __....~ ~
11
C
j |
9'0"
23
a
'7'0'''50
!
: - _
s
ppm
9
9'0' ' '70
I
a
i
l
1
.
'
5'0 ppm
.
90'' 70
1
23 9b"
'7b"
b "5o' ppm"
_ilL_2& 9'0''''70''
3 i
I
'
i
I
'
50 ppm
3
d
'50''ppm"
,,,
90'' ' '70'' '5'0''ppm'
Figure 1.2~AI NMR spectra of the solutions for NaA synthesis: a - without organic additives; with addition of: b - MEA; c - DEA; d - DIPA; e - TEA; f - TIPA. Concentrations of additives are as described in "Experimental". Not only the number of the alkyl groups is important, but also their nature. Consideration of the data, summarized in Table 1, results in the conclusion that only in the case of two or three - - C - - C - - O H groups bonded to a nitrogen atom, does the aluminum form the polynuclear complexes. For example, in contrast to TEA, 2-diethylaminoethanol and triethylamine do not form any complexes. The same is correct for tris(hydroxymethyl)aminomethan that contains three alcohol groups, but they are not so flexibly connected with a nitrogen atom as those ones in TEA. Most likely, the structure of the complexes of AI with tri-substituted amines is similar to atranes [15] or pro-atranes [16]. However, substitution of oxygen by nitrogen (in the case of 2,2',2"-nitrilotriethylamine) leads to the disappearance of any form of the aluminum polynuclear complexes.
153 Results of interaction depend also on the nature of groups that substitute for the hydrogen atoms in the alkyl radical. Methyl-substituted derivatives (DIPA and TIPA) produce the same complexes (Fig. 1, c, d and e, f respectively). There are only the ions of monomer AI(OH)4" in the clear solutions with additives, which content carboxyl groups (see Table 1). Aluminate solutions with low Na20/AI203 ratio are usually unstable. An alumina precipitate appears after keeping them for several days at room temperature. The TEA containing solution was stable under the same conditions for at least several years. The structure of aluminum - TEA complex is still not clear. TEA influences significantly the chemistry of aluminum in the solutions. Thus, during the preparation of alumina from gels with different additives, only a gel synthesized with TEA remains amorphous up to 650 ~ [17]. It was supposed that TEA plays the role of a nucleation suppressant during the zeolite synthesis [ 18]. Probably all these phenomena are connected with the properties of the complex with 8 = 62.6 ppm. TIPA-, DIPA- and DEA-containing aluminate solutions were stable also. In contrast, the precipitates appeared after several days of storage for all other solutions (see Table 1). 3.2. Influence of additive mixtures on aluminum complexes in the solutions The simultaneous addition of di- and trialkylamines to the solution decreases the monomer ion concentration and increases the concentration of both complexes (with 8 = 62.6 ppm and 8 - 57 ppm). Synergism of their action is observed. Half amount of TEA in the presence of DEA or DIPA connect all AI in the complex with 8 - 62.6 ppm (see Fig. 2, a and Fig. 3, a).
'"|
90
9
it''
.'
k'
70
"-"
" 5'0
l~pm
Figure 2. 27A1NMR spectra of solutions for NaA synthesis with mixed TEA - DEA additive: a - 50% and b - 90% of DEA.
:i
90
-
u
10
9
|
.
|
50
,
~
9
___~
ppm
Figure 3.27A1 ~ spectra of solutions for NaA synthesis with mixed TEA - DIPA additive: a- 50% and b - 90% of DIPA.
3.3. Influence of additive mixtures on the zeolite A and X growth Variations of crystal size during zeolite growth in bath with mixed additives were investigated. Results are Summarized in Table 2 and in Fig. 4. Different dependencies were found for growth of NaA and NaX zeolites.
154 Table 2. Size of zeolite crystals grown in the presence of mixture of additives. Zeolite type
~ e
Crystal size, tim, for NHR2 concentration, % 50 80 90 98
composition
0
NaA
TEA + DEA TEA + DIPA TIPA + DEA TIPA + DIPA
24 24 16 16
28 26 21 21
37 29 27 23
32 20 29 24
47 38 23 15
39 33 22 19
NaX
TEA + DEA TEA + DIPA TIPA + DEA TIPA + DIPA
26 26 22 22
45 35 39 33
27 20 31 17
28 21 31 19
41 30 44 28
68 59 68 59
100
70
b 40 i
10
I
g
40 10 0
40
80
0
40
80
0
th 40
80
O
t 40
80
Figure 4. Dependencies of crystal size (ttm) on additive composition (% of NHR2 in mixture NR3 - NHR2) for NaA (a - d) and NaX (e - h) zeolites: a, e - TEA - DEA; b, f - TEA - DIPA; c, g - TIPA- DEA; d, h - TIPA- DIPA. For NaA zeolite the concentration dependencies of the crystal size go through a maximum when NHR2 concentration changes. Positions of these maxima depend on the nature of NR3. For TEA and TIPA they correspond approximately 98% and 80% of NHR2 in the mixture, respectively (Table 2, Fig. 4, a - d). For NaX zeolite these dependencies also have wellpronounced maxima for the 1:1 ratio of additives. Nevertheless the biggest crystals were grown in the case of pure N t ~ 2 additives (Table 2, Fig. 4, e - 11). In the all cases TEA and DEA show better results than more bulky TIPA and DIPA, both unmixed and in mixtures. Common features of these dependencies confirm that, for the results of the synthesis, the chemical state of framework components in the solutions is more important than the origin of the individual organic compounds. The chemical shift 8 = 57 ppm is known for solid state MAS ZTAlNMR spectra of the solid phase of gels and zeolites A and X [19]. Thus the action of dialkylamine changes the state of AI in the solutions to that state found in the zeolite crystal
155 lattices. The simultaneous action of TEA leads to the slow release of AI due to properties of TEA - AI complex with 8 = 62.6 ppm. The variation of the results of synthesis for NaA and NaX zeolites may be due to their growth with the participation of different SBU. At the same time, the shape of NaA crystals varies essentially. The contribution of the planes { 110} increases with increase of NHR2 concentration for all systems examined. This dependence is illustrated for NaA crystals grown in the presence of TEA - DEA mixture (Fig. 5). In contrast, the shape of NaX crystals does not change in all cases.
Figure 5. The NaA crystals grown in the presence of TEA- DEA mixture with content of DEA: a-0%; b - 50%; c - 80%; d- 90%; e- 98%; f-100%. Evidently, the synthesis results are strongly influenced by the local chemical composition of the aluminosilicate gel network. They are defined by the proportion and the mutual position of AI and Si in - - O - - S i O---At O - - chains. When a gel is formed, its structure and composition are controlled by the structure and the composition of the initial solutions. Hence the results of zeolite synthesis depend strongly on the structure of AI complexes in the initial solutions. R is well known that Lowenshtein's rule is not violated for zeolite frameworks [2]. Nevertheless, the clear solutions that contain polynuclear complexes of aluminum give the best results for the particle size of NaA and NaX zeolites [9, 10, 13, 14].
156 4. CONCLUSIONS It is shown that amines used as organic additives for NaA and NaX zeolite growths play the role of complex-forming ligands. The structure and the number of alkyl groups in an amine molecule determine the kind of the formed aluminum complex. The changes of the concentration ratio of three different forms of AI complexes influence the zeolite A and X crystal growth. These concentrations depend on the ratio of di- and trialkylamines in the mixture of organic additives. The most important complex for zeolite growth is characterized by the chemical shift 8 = 57 ppm. This complex appears in the presence of DEA or DIPA. Synergism of (NHR2 + NR3) mixture action is observed. The concentration of NHR2 influences the shape of NaA crystals for all systems examined. In contrast, the shape of NaX crystals does not change. ACKNOWLEDGMENTS The authors thank Dr. N. Bogdanchikova for fruitful discussion, and Dr. A. Slavin for careful reading of manuscript. This work was supported by AIST, MITI, Japan. REFERENCES 1. M.E. Davis and R.F. Lobo, Chem. Mater., 4 (1992) 756. 2. D.W. Breck, Zeolite Molecular Sieves, A Wiley Interscience Publ.: New York, 1974. 3. P.A. Jacobs and J.A. Martens, Synthesis of High-Silica Ahminosilicate Zeolites (Stud. Surf. Sci. Cat., Vol. 33), Elsevier: Amsterdam, 1987. 4. S.I. Zones and R.A. Van Nordstrand, in Novel Materials in Heterogeneous Catalysis (Eds. R.T.K. Baker and L.L Murrell) ACS Symp. Set., 437 Am. Chem. Soc.: Washington, DC, 1990, Chap. 2, p. 14. 5. G.O. Brunner, Zeolites, 12 (1992), 428. 6. S.L. Burkett and M.E. Davis, Chem. Mater., 7 (1995) 920. 7. S.L. Burkett and M.E. Davis, Chem. Mater., 7 (1995) 1453. 8. J.F. Charnell, J. Cryst. Growth, 7 (197 l) 29 I. 9. A. Efimov, V. Petranovskii, M. Fedotov, M. Khrepoun and L. Myund, J. Structural Chem., 34 (1993) 548. 10. V.P. Petranovskii, in Proceedings of 9th Zeolite Research meeting, Chem. Soc. of Japan: Tottori, 1993, p. 6. 11. A.W. Apblett, A.C. Warren and A.R. Barron, Chem. Mater., 4 (1992) 167. 12. M. Wiebcke, M. Grube, H. Koller, G. Engelgardt and J. Felshe, Microporous Mater., 2 (1993) 55. 13. V.N. Bogomolov and V.P. Petranovskii, Zeolites, 6 (1986) 418. 14. V.P. Petranovskii, Y. Sugi. Unpublished results. 15. M.G. Voronkov and V.P. Baryshok, J Organomet. Chem., 239 (1982) 199. 16. J.G. Verkade, Ace. Chem. Res., (1993) 483. 17. H. Tayaa, A. Mosset and J. Galy, Europ. J. Solid State Inorg. Chem., 29 (1992) 27. 18. E.N. Coker, P.S. Hees, C.H. Sotak at al., Microporous Mater., 3 (1995) 623. 19. L.V.C. Rees and S. Chandraseckhar, Zeolites, 13 (1993) 528.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
Studies of the Crystallization Gravitational Force Field
157
of ZSM-5
Under High
W. J., Kim1, D. T., Hayhurst2, S. A., Lee1, M. C., Lee1, C. W., LimI and J. C., Yoo1 1Department of Industrial Chemistry, Kon Kuk University, Seoul, Korea 2 D e p ~ e n t of Chemical Engineering, University of South Alabama, Mobile,AL, USA
The 23 factorial method was applied to the crystallization of large ZSM-5 under high gravity to optimize the synthesis condition. The optimum composition was determined and the growth rate, size distribution, and morphology were studied. The activation energies for various systems under high gravity were calculated and found to be consistently lower for elevated gravity synthesis. In addition, the effect of mixing order on the crystallization of ZSM-5 under high gravity was investigated. 1. INTRODUCTION The synthesis of large zeolite crystals has received much attention in both the open and patent literature. Among various zeolites, much attentions have focus~ on the pentasil zeolite, in particular ZSM-5/silicalite. The largest ZSM-5/silicalite are reported to range up to 420 Vm in length[i,2]. Most reports, however, have focused on the optimization of the synthesis mixture. Since the effect of high gravity on the crystallization of silicalite was relx)rted by Hayhurst et al.[3], several investigators[4-8] have reported on high gravity synthesis. According to these reports, high gravity has affects on crystal size, yield and morphology. Although most reports focus on silicalite, there are no report on the preparation of ZSM-5 under high gravity. This is perhaps due to the significant differences in synthesis chemistry between ZSM-5 and silicalite due to aluminum content in ZSM-5. In this research program, the composition of the reaction mixture for synthesis of large ZSM-5 crystal was optimized using 23 factorial method. Experiments were performed at 1 and 30 g. The effects of gravity on the percent crystallization, size distribution and morphology were evaluated.
2. EXPERIMENTAL 2.1. Synthesis The reactants used in this study were a colloidal silica,Ludox AS-40(Du Pont),
158 reagent-grade tetrapropylammonium bromide(Tokyo Chemicals), aluminum nitrate nanohydrate, aluminum hydroxide, 50 wt.% sodium hydroxide solufion(Junsei Chemical Co.) and deionized water. The reaction mixture had the oxide formula, aNa~-bA1203-100SiOz-5TPABr-cI-hO-dGravity where a, b, c, and d were varied by 23 factorial method. The synthesis was designed to perform in four different ways. In scheme I, the reactant solutions were prepared in two beakers. A half of water required , AI(NO3)a.9H20 and 50 wt.% NaOH solution were mixed in beaker I and the remaining water, Ludox AS-40 and TPABr were mixed in beaker If, respectively. After mixing these two solutions separately, they were mixed together and sitrred for enough time to give rise to homogeneous solution. Upon completion of mixing, the reactions were carried out at 180~ under i and 30 g for 24 hours. Scheme II followed exactly the same p r i e s as that of scheme I except that the concentration of Na20 was fixed at 8 moles while the concentration of A1203 was varied from 0.5 moles to 1 mole. In scheme IE based on the 23 factorial experimental results in schemes I and IL 4Na~-100SiO2-3.8TPABr-0.3A12033500H20-xGravity(x = 1 and 30 g) was used as an optimum composition to produce large crystals. The reactions were carried out at 170~ 180~ and 190~ up to 7 days under 1 and 30 g. In scheme IV, the effect of mixing order on crystal growth under high gravity was studied using the same batch composition as an optimized scheme 11I. Unlike schemes I, 1I, and Ill where total NaOH required was added into the alumina solution(be~r I), the amount of NaOH required was divided into two portions by 50%, 70%, and 100%, respectively. It was then added into the alumina solution(beaker I) and the silica solution(beaker II) finally mixing them together. Upon completion of each reaction, linear growth rates were obtained at three different temperatures and the activation energies were calculated. 2.2 Characterization Powder X-ray diffraction analysis(Rigaku Model D/Max-II~) was performed to identify crystallinity and phase. In order to measure crystal size and to investigate morphology, image analyzer(KanImager) and SEM(Shimazu Alpha 25A) were used.
3. RESULTS AND DISCUSSIONS 3.1. Scheme I Following the e ~ m e n t a l procedures described in previous section, the reactions were carried out at 180~ for 24 hours under the same conditions. The compositional ranges for three main factors, namdy, Na~3, H20, and gravity, were varied by 23 factorial method. Thus, each factor was varied from 4 moles, 2 ~ moles, and 1 g, to 8 moles, 3500 moles, and 30 g, res~:tively. Table 1 shows the effect of each combination on crystal size and the combination(bc) has the most significant positive effect while the combination(a) has the most significant negative effect. The negative effect means the decrease in crystal size while the positive effect enhances the crystal size. Aspect ratio(length divided by width) of product crystals was not significantly influenced by gravity.
159 Table 1 Compositional combinations for the effects of different factors on crystal size Crystal size(pm) Effect Significance Combination Factors A B C I II Mean a b ab c ac bc abc
8 4 8 4 8 4 8
2800 3500 3500 2800 2800 3500 3500
1 1 1 30 30 30 30
40.8 50.7 41.1 47.3 24.9 73.6 42.4
43.8 52.3 40.6 44.1 24.1 74.5 39.8
42.3 51.5 40.8 45.7 24.5 74.1 41.1
-20.5 8.9 - 1.3 - 2.2 - 6.5 13.6 - 4.6
+ + -
+ " singinficant at 99 % SEM of ZSM-5 crystals obtained from combinations (a) and (bc) are shown in Figure 1.
a
,~~
Ilc
,..~2
Figure. 1. SEM of ZSM-5 obtained from combinations (a) and (bc) Conversely, the crystallinity decreased slightly with high levels of I-~O and gravity. In order to identify the phase of solid product and crystallinity, x-ray powder diffraction analysis was performed and the crystallinity for each combination was calculated by the area between 22.5~ and 25~ in 29. Figure 2 shows XRD pattern of product obtained from combination (b) and no other phase except ZSM-5 was found. The crystallinity for each combination was summarized in table 2. Increasing the concentrations of I-IzO and gravity resulted in a slight decrease in crystallinity due to the majority of nucleation and crystallization occuring at the interface between the top-liquid phase and the segregated solid gel. As a conclusion, the gravity does not have significant effects on crystallinity while it enhances the crystal size significantly. Furthermore, it is interesting that the effect of gravity on the size distribution strongly depends on the concentration of I-hO as shown in Figure 3. At the low concentration of I-I20, the gravity resulted in positive effects on size distribution(narrow size distribution) while the gravity showed a negative effect on size distribution at the high concentration of I-I20.
160
GO 13.. 0
5.00 10.00
20.00
20
30.00
40.00
5000
Figure 2. XRD pattern of product obtained from combination (b) Table 2 Compositional combinations for the effects of different factors on crystallinity Effect Significance Combination Factors Crystallinity A B C I II Mean a b ab c ac bc abc
8 4 8 4 8 4 8
2800 3500 3500 2800 2800 3500 3500
1 1 1 30 30 30 30
96.9 98.9 98.2 96.5 98.4 96.4 72.8
96.9 96.9 97.7 100.0 97.7 96.4 82.8
96.9 97.9 98.0 98.3 98.1 96.4 82.8
-
4.6 5.0 4.6 4.7 4.8 6.1 4.6
+ -
+ " significant at 99 % 3.2. Scheme II In scheme II, AlzOa, I-hO and gravity were the parameters that were varied from 0.5 moles, 2800 moles and 1 g to 1.0 mole, 3500 moles and 30 g, respectively. In this case, the concentration of Na20 was fixed at 8 moles and the reaction was carried out for 1 day. As shown in table 3, an interesting result is that the crystal size was increased with aluminum content under high gravity while vice versa under 1 g. The effect of gravity on the crystal size seemed to become significant as aluminum content increased; that is, regardless of I-hO content, the crystal size decreased by 6 to 38% upon increasing aluminum content at 1 g while it was significantly enhanced at 30 g by 17% to 47%. This is attributed to the greater consumption of aluminum for crystal growth rather than for nucleation due to the liquid-solid segregation resulting from applying gravity. In case of crystallinity, combination(bc) in which I-hO content and gravity were high shows a negative effect on crystallinity indicating that gravity is not important as shown in table 4.
161 (a)
25
I
w
~,20
I
i
.:""~-->30G
o 10
....m.,':
7=0
(b)
/
/~\-I
"'" . ".. / /
35 _ ,
,
30
~'25 "6 20 '- 15 E 10
0
--1
=
Z
1 .i i t 40 50 60 Crystal Size(um)
5
0
! --
~
1G<--! .
_,'
i.
i .__2-.~ 1 20 30 40 Crystal Size(um)
50
(d)
(c) m
,4--= r
4O 35
~, 30 0 25 o 20 .Q r 115 -
\-->IG
0 5 0
_
\
_
E z
3O 1 25 0 20 -o 15 -Q 10 E = 5 j~ z 0 30
I 1).._~,
m
I
I - - - I .
_
I..
3OG<-:.
-
L
50
.,/ /
NI--
9
';
.
: -
60 70 80 Crystal Size(um)
9
I
I
-
l-->lG I
--
'~
I H
,,-
t
-
40 50 60 Crystal Size(urn)
Figure 3. Comparisons of the size distributions for the samples obtained from xNa20-0.5AI~O3-100SiO2-5TPABr-yH20 under 1 and 30g, respectively" (a) x=4, y=2800, (b) x=8, y=2800, (c) x=4, y=3500, and (d) x=8, y=3500. Table 3 Compositional combinations for the effects of different factors on crystal size Effect Significance Crystal size(pm) Combination Factors A B C I II Mean a b ab c ac bc abc +
1.0 0.5 1.0 0.5 1.0 0.5 1.0
2800 1 3500 1 3500 1 2800 30 2800 30 3500 30 3500 30
28.1 41.1 38.8 24.9 34.7 42.4
47.1
24.6 40.6 37.5 24.5 38.0 39.8 48.9
26.4 40.9 38.2 24.7 36.4 41.1 48.0
- 0.03 9.6 2.1 0.6 9.3
m
+ +
4.4
-
4.5
-
9significant at 97.5%
3.3. Scheme I n Based on the results obtained from schemes I and IL the optimum composition for large crystal was determined. It was for NaOH and alumina as low as possible and for I-hO and gravity as high as possible. The molar composition was
162 Table 4 Compositional combinations for the effects of different factors on crystallinity Combination Factors Crystallinity Effect Significance A B C I II Mean a b ab c ac bc abc
1.0 0.5 1.0 0.5 1.0 0.5 1.0
2800 1 3500 1 3500 1 2800 30 2800 30 3500 30 3500 30
84.0 98.2 100.0 98.4 89.9 72.8 90.4
87.3 97.7 81.5 97.7 94.8 82.8 79.4
85.7 97.9 90.8 98.1 92.4 77.8 84.9
- 4.3 - 5.4 4.2 -4.5 5.0 -8.5 2.2
+ " significant at 97.5 % 4Na20-100SiO2-3.8TPABr-0.3AI2(h-~H20. The reactions were carried out at three different temperatures up to 7 days. Figures 4 (a) and (b) show the aveyage crystal size with reaction time at 1 and 30 g.
(a) 1 G
v
(b) 3 0 G
E
E -!
v
w U oO
w
oo
~< .J
.J
Q: O
rr (3
0
1
2
3
4
5
REACTION TIME(HRS)
6
7
0
1
2
3
4
5
6
7
REACTION TIME(HRS)
Figure 4. Average crystal size vs. reaction times for the crystal obtained at various temperatures under (a) 1 g and (b) 30 g. The average crystal size with time at 190~ was significantly fluctuated in case of 1 g while the average crystal sizes with time were increased in proportion to temperature for 30 g. It also can be realized that the size distribution of crystals obtained at individual time interval for 30g was much narrower than that for 1 g. In addition, the maximum crystal sizes at each individual time for 170~ 180~ and 190~ were shown in Figures 5 (a) and (b) to investigate the reaction kinetics for crystal growth under 1 and 30 g. As shown in these Figures, the crystal sizes of ZSM-5 for three different temperatures show linear growth rotes up to 24 hours. The activation energies were calculated from the crystallization curve slope where a linear growth rate is observed. The activation energies of 63.19 kJ/mol and 56.54
163 (b) 30 G
(a) 1 G 160
19o"c
9
o-180='c
E
160
~ 120-
O''''C
.._.1
~ 80
~80.
D 40
~ 40.
>-. nO
Z~:lgO~ O:180~ O:170~
X
< ~
0
0
4
8
12
16 20 24
REACTION TIME(HRS)
0
4
8
12 16 20 24
REACTION TIME(HRS)
Figure 5. Maximum crystal size vs. reaction time for the crystal obtained at various t e ~ a t u r e s under (a) 1 g and (b) 30 g. kJ/mol were obtained for 1 g and 30 g, respectively. Lower activation energy for 30 g might be attributed to the fast crystal growth due to tl~ limited crystallization mainly at liquid-solid interphase. The value, 63.19 kJ/mol for 1 g is fairly consistent with 64.5 kJ/mol reported by Feokfistova et all9]. 3.4. S c h e m e IV
In scheme IV, the effect of mixing procedure in preparing reactant solution on crystal growth rate was studied using the same molar composition as that of scheme HI.. Unlike schemes I, ]l, and Ill where total NaOH solution required added into alumina s o l u f i o n ( ~ e r I), the reactant solutions were prepared in three different ways ; those are, 50%, 70% and 100% of 50 wt.% NaOH solution required were added into the silica solution(beaker II), respectively. The reactions were carried out at 170~ 180~ and 190~ under 1 and 30 g up to 24 hours. The logarithmic plots of the linear rate of ZSM-5 crystallization with respect to reciprocal temperature for 1 and 30 g were shown in Figures 6 (a), (b) and (c). In case of adding a half of 50 wt.% NaOH solution into silica containing solution, the activation energies for 1 and 30 g are 68.17kJ/mol and 64.85 kJ/mol(Fig.6(a)). However, as the addition of 50 wt.% NaOH solution into silica solution increased to 70% of total NaOH required(Fig.6(b)), the difference in the activation energies for 1 and 30 g became larger than that of the previous case. The activation energies for 1 and 30 g are 61.52 kJ/mol and 51.55 kJ/mol, respectively. This might be attributed to the more dissolution of highly stable silica particles upon addition of NaOH solution. On the other hand, the activation energies for the case of adding total NaOH solution required into the silica solufion(Fig.6(c)) increased to 66.51 kJ/mol and 59.85 kJ/mol for both 1 and 30 g compared to the second case. This seems to be caused by the formation of much more silicate species which could require more activation energies. It is interesting, however, to note that the activation energy under high gravity is smaller than under normal gravity regardless of mixing procedures.
164 (a)
(b)
(c) 1.85
1.85
1,85
,,r
~= 1.35
_=1.35
-%.,o .
lIT X IO00(K)
%
2~
lIT X IO00(K)
2ao
0 ~
I-
2.10
:
.~
.
220 230 1/T x IO00(K)
Figure 6. Logarithmic plots of the linear rate of ZSM-5 crystallization vs. reciprocal temperature for 1G and 30G.
4. CONCLUSIONS Several conclusions were obtained through this work. The gravity does not have significant effects on % crystallization while it enhances the crystal size significantly. In addition, the results suggest that the effect of gravity on the size distribution strongly delxmds on the concentration of I-hO. At low concentration of H ~ , the gravity resulted in narrow size distribution while it caused broad size distribution at the high concentration of I-hO. Unlike normal gravity, aluminium content shows positive effects on crystal size under high gravity. Finally, regardless of mixing procedures, a high gravity gives rise to lower activation energy than a normal gravity.
REFERENCES
1. D.T. Hayhurst and J.C. Lee, in New Developments in Zeolite Science and Technology(Fxts. y. murakami, A. Iijima and J.W. Ward) Kodansha, Tokyo and Elsevier, Amsterdam(1986),239. 2. J. Komatowsld, J. Zeolites 8(1988), 77. 3. D.T. Hayhurst, P. J. Melling, W. J. Kim and W. bibby, Zeolite Synthesis, ACS Symp. Set. No. 398(Eds. M..L. Occelli and H.E. Robson)(1989), 233. 4. W.J. Kim, Ph.D. Dissertation, Cleveland State University(1989). 5. W.J. Kim and J. Lee, J. Korean Ind. & Eng. Chem. Vol. 2, No. 2(1991), 97. 6. H. Zhang, S. Ostrach and Y. Kamotani, Trans~rt Phenomena in Materials Processing and Manufacturing, HTD-Vol. 196(1992). 7. H. Zhang, S. Ostrach and Y. Kamotani, 31st Aerospace Sciences Meeting and Exihibit, 1(1993), 11. 8. H. Zhang, S. Ostrach and Y. Kamotani, Processings of 10th International Conference on Crystal Growth(1993). 9. N.N. Feoktistova and S.P. Zhdanov, Zeolites Vol. 9(1989), 5.
H. Chon, S.-K. Ibm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
165
Structure directing role o f N a + and T M A + cations in 18-crown-6 ether mediated crystallization o f EMT, M A Z and SOD aluminosilicate zeolites E.J.P. Feijen, B. Matthijs, P.J. Grobet, J.A. Martens and P.A. Jacobs Centrum voor Oppervlaktechemie en Katalyse, KU Leuven Kardinaal Mercierlaan 92, B-3001 Heverlee (Leuven), Belgium
Summary
Zeolite crystallizations are performed in the system 10 SiO2; 1 A1203; 0.97 18-crown-6 ether; x Na20; y TMA20; 135 H20. EMT, MAZ and SOD type phases are obtained depending on the relative concentrations of sodium and TMA in the gel. The nature of the phases obtained is rationalized by the extended-structure approach with specific structure directing roles for the different cations in the formation of extended structures as well as in their mutual condensation.
1. INTRODUCTION Although much progress has already been made in the identification of precursors and template effects, the crystallization mechanisms responsible for the formation of zeolites are not yet fully understood [1]. According to the extended-structure approach of zeolite crystallization, zeolites grow by condensation of extended-structures (ES) whereby primary cations template the formation of ES units, and secondary cations organize the condensation of ES units [2]. Recently, this approach has been validated in the crystallization of FAU and EMT phases and their intergrowths in the presence of crown-ethers [3]. A mechanism of condensation of faujasite sheets decorated with the crown-ether molecules rationalizes and quantitatively predicts the exact nature of the faujasite polytype with respect to intergrowth pattern and phase composition. In this work, a hydrogel leading typically to the crystallization of the EMT phase was modified by adding tetramethylammonium cations (TMA) and varying the concentration of sodium. From these systems, EMT, MAZ and SOD zeolite phases were obtained. The nature of the zeolite phases obtained can be rationalized based on the known primary and secondary structure directing roles of sodium, TMA and sodium-crown-ether complexes. 2. EXPERIMENTAL Hydrogels were prepared with the following standard molar composition: 10 SiO2; 1 A1203; 0.97 18-crown-6 ether; x Na20; y TMA20; 135 H20. The Na and TMA contents of the gel are summarized in table 1. The 18-crown-6 ether (1,4,7,10,13,16-hexaoxacyclooctadecane, Janssen Chimica) was added to a colloidal silica source (Ludox HS-40). Gibsite (Fluka) was dissolved in an aqueous solution of NaOH and TMAOH under heating at 353K. The latter solution and the silica sol were combined and stirred for 15 minutes. The gels were transferred
166 into teflon lined autoclaves and aged at room temperature for 3 days. Crystallization was interrupted after 9 days of heating at 373K. The solids were recovered by centrifugation at 39 kG and washed with deionized water until the pH of the wash water was lower than 9. X-ray diffraction patterns were recorded on an automated Siemens D5000 diffractometer. Infrared spectra were taken on a Nicolet 730 FTIR spectrometer using the KBr pellet technique. Scanning electron micrographs on gold-coated samples were obtained using a Jeol superprobe 733 instrument. Thermogravimetric analyses and differential thermoanalysis profiles were recorded on a Setaram TGA 92 thermobalance in oxygen/helium (20/80 vol/vol) atmosphere. 13C MAS NMR with proton decoupling was performed on a Bruker 400 MSL spectrometer at 100.6 Mhz., with a pulse length of 4gs, a repetition time of 10s and a spinning rate of 4kHz. Quantitative 27A1 MAS NMR spectra were recorded on the same spectrometer at 104.2 Mhz, with a pulse length of 0.61 gs, a repitition time of 0.1 s and a spinning rate of 14 kHz using zeolite samples with the same topology and known A1 content as references.
3. RESULTS AND DISCUSSION 3.1. Gel composition and crystallization products The crystalline phases present in the products, as determined by XRD, are summarized in Table 1. In a first series of experiments (series A: sample 1 to 5), the influence of the addition of TMA cations to the gel was studied. The experiment in absence of TMA (sample 1), is a typical 18-crown-6 ether mediated crystallization of the hexagonal faujasite phase (EMT). Gradual addition of TMA cations yields the co-crystallization of more MAZ phase next to traces of SOD. XRD studies of series A showed, however, that the crystallization of MAZ which is most abundant in sample 2, (Figure 1) is suppressed at increasing TMA levels, the SOD phase becoming more abundant (sample 5, Figure 2). The weakly intense diffraction lines at a 2e value of 6 ~ in sample 2 indicate the presence of traces of EMT (Figure 1). The SEM picture in Figure 5 shows that the MAZ phase in sample 2 consists of spherulitic crystals, with a diameter of 1 to 2 gin. The spherulitic morphology is typical of MAZ type zeolite crystals grown under conditions of high supersaturation [4].
Table 1 Na20 (x) and TMA20 (y) concentrations in the standard hydrogel and XRD-visible crystalline phases I
Sample
x
y
Phases in product
Sample x
y
Phases in product
2.04
0.48
MAZ>EMT, SOD
8
1.92
0.48
MAZ>EMT
9
2.30
0.24
MAZ>EMT, SOD
2.21
0.24
MAZ > EMT, SOD
11
2.16
0.24
MAZ> EMT
MAZ>EMT, SOD Ij 12
1.68
0.72
SOD
!
1
2.40
0.00
EMT
i7 I
2
2.40
0.24
MAZ>EMT, SOD
3
2.40
0.48
MAZ>EMT, SOD I
4
2.40
0.72
MAZ>EMT, SOD . 10 I
5
2.40
0.96
SOD>MAZ I
6
2.28
0.48
167
I
5
l
I
l0
l
I
,
I
I
.
t
15 20 25 30 35 40 45 50 55 2O
Figure 1: XRD pattem of sample 2.
1'o1'5 ' ~o' ~5' ~o' ~5' ~o' ~5' go 55 2O Figure 2: XRD pattern of sample 5.
1 5
10 15 20 25 30 35 40 45 50 55 20 Figure 3: XRD pattem of sample 10.
' lb' l'5' J.o ~
~o ~5 do ,is go' 55 2O
Figure 4: XRD pattern of sample 12.
In order to further examine the influence of sodium and TMA cations, additional series of experiments were set up with variable amounts of sodium at a constant TMA content (series B: sample 6 to 8; series C: sample 9 to 11). For series B, all samples contain a considerable amount of MAZ and SOD, and possibly traces of EMT. In series C, three phases (EMT, MAZ and SOD) were unambiguously identified as illustrated with the XRD pattern of sample 10 (Figure 3). The SEM picture of sample 11 in Figure 6 clearly shows physical mixtures of hexagonal crystals of the EMT phase and spherulitic crystals assigned to the MAZ phase. Abundant TMA addition results eventually in the crystallization of the dense SOD phase only (sample 12, Figure 4). The Si/A1 ratio of the SOD phase of sample 12 is 6.3 according to quantitative 27A1 MAS NMR. For these experiments it is concluded that from Na and 18-crown-6 ether containing hydrogels, the crystallization of EMT is readily suppressed by addition of TMA. Instead, the MAZ phase is preferred at intermediate TMA contents in the hydrogel, while from TMA-rich hydrogels, a pure SOD phase crystallizes under the conditions investigated.
168
Figure 5: SEM picture of sample 2.
Figure 6: SEM picture of sample 11 (after 6 days of heating).
3.2. Quantification of phases in zeolite mixtures
During zeolite crystal growth, TMA cations are often occluded in cavities such as gmelinite and sodalite cages. Such occlusion phenomena of TMA have been observed previously for the crystallization of mazzite and sodalite type zeolites [5,6]. Thermoanalysis profiles confirm the presence of TMA cations in the present samples (Figures 7 and 8). The oxidative decomposition of TMA in gmelinite and sodalite cages is observed at 820 and 870 K, respectively. From the TG profile in Figure 8, the TMA content of sample 12 containing only SOD could be estimated. It was found that approximately each sodalite cage contains 1 TMA. The presence of these cations in gmelinite and sodalite cages was confirmed by Hdecoupled 13C MAS NMR. Indeed, for sample 2, the spectrum in Figure 9 shows resonance lines at 58.8 and 57.9 ppm, indicating the presence of sodalite cage and gmelinite cage occluded TMA cations, respectively [7]. Furthermore, a weak resonance is observed at 70 ppm, originating from Na-18-crown-6 ether complexes in the EMT structure [3,8]. These findings are in good agreement with the presence of three phases, viz. MAZ, SOD and EMT according to the XRD analyses (Table 1 and Figure 1). The amount of MAZ and SOD derived from the amount of TMA decomposed at 820 and 870 K, respectively, is plotted in Figure 11 against the Na20/(Na20 + TMA20) ratio in the synthesis gel. The amount of EMT in the samples (Figure 11) was estimated based on the intensity of the Double 6 Ring (D6R) vibration in the IR spectra, relative to that of the TObending vibration (450 cm 1) [3]. For sample 11, the IR spectrum is displayed in Figure 10.
169 The D6R vibration (585 cm l ) is clearly present as a shoulder at lower frequency on the Single 6 Ring vibration band (621 cm 1) of the MAZ structure.
3.3. Crystallization mechanism It is evident from Figure 11 that there is a relationship between the fraction of the Na/(Na+TMA) cations in the synthesis mixture and the formation of a particular structure type. Indeed, only intermediate fractions yield MAZ type zeolites, while from Na-rich and TMA-rich gels preferentially the EMT and SOD type zeolites are crystallized, respectively.
75
0
0
TG
150 50
-5
100~
~ ~ -lO
-10
-15 t t a Flow
0
'
~
'
460
'
660
'
-20
'
200
'
Terrp (~
Ternp (~
Figure 8:TG-DT analysis profile of sample 12.
Figure 7" TG-DT analysis profile of sample 11.
0.6
0.5
i~ 0.3
32' ~0' gS' 6'6 ' 6'4 ' g2 ' 6'0 ' 5'8 ' 5P6 (ppm)
O.
1000
~ Wa~nt~
Figure 9" 13C MAS NMR profile of sample 2
1~
~
~
5(~0 ' 4t~
(crab
Figure 10: IR spectrum of sample 11.
170
9
9 9
80 o~
~,,,,,,
9
40
20
.
",,,,.
/ .
mA
0.75
0.-80
- 0.85
0.-9ff
0.95
1
Na20 / Na20 + TM~O Figure 11" Influence of the Na and TMA content of the synthesis mixture on the crystallization of EMT, MAZ and SOD.
Following the extended-structure approach [2,3], the Na and TMA cations are classified as primary cations, responsible for the formation of extended structures (ES) during crystallization. For the formation of EMT and MAZ phases, these ES units are faujasite and mazzite sheets, respectively (Figure 12). The secondary cations that control ES condensation are Na-crown-ether complexes [3] and Na ions (Figure 12), respectively. Indeed, only Na and TMA cations were found in the MAZ materials, the latter cations being exclusively present in the gmelinite cages being part of the ES units. The crystallization pathways drawn in Figure 12, with each pathway controlled by specific primary and secondary cations, now fully explain the formation of the different zeolite types (Figure 11). Indeed, in absence of TMA, only pathway A is available, and EMT formation dominates. In presence of Na and TMA, pathway B becomes possible as mazzite sheets can be formed. Indeed, Na acts as primary as well as secondary cation for MAZ, and is assisted by TMA and its templating effect for gmelinite cages. If TMA cations become more abundant compared to Na, the structure directing potential of TMA for sodalite cages leads to SOD phases built from condensed sodalite cages. For reasons of charge compensation in the lattice, these TMA containing sodalite cages cannot be organized into an 18-crown-6 ether containing EMT lattice [3]. Pathway B, yielding MAZ, also becomes unfavourable under these conditions, as the concentration of sodium, i.e. a primary as well as secondary cation, is too low.
171
SOD
mazzite sheet
fauj asite sheet
+
i.
TMA Na+ _. TMA+
Na+1
( ~
Na+
[ [Na-18-crown-6]
g/
MAZ
EMT
Figure 12: Crystallization routes for EMT, MAZ and SOD.
4. CONCLUSIONS The crystallization of EMT, MAZ and SOD type aluminosilicate zeolites is controlled by the relative amounts of Na and TMA in the synthesis hydrogel. Their effect on the nature of the crystalline products is explained by their specific structure directing contribution in the formation of extended structures and the further organization of these units into a crystalline zeolite lattice.
ACKNOWLEDGEMENTS
This work is sponsored by the Belgian Ministry of Science Policy in the frame of an IUAP-PAI program and by the Flemish N.F.W.O. EJPF acknowledges KU Leuven for a postdoctoral fellowship, JAM and PJG the Flemish NFWO for a research position.
REFERENCES
1 E.J.P. Feijen, J.A. Martens and P.A. Jacobs, Zeolites and Related Microporous Materials: State of the Art 1994 (part A), Studies in Surface Science and Catalysis, Vol. 84., J.
172
2 3 4
5 6 7 8
Weitkamp, H.G. Karge, H. Pfeifer and W. H/51derich (Editors), Elsevier Science B.V., Amsterdam, (1994) p.3. D.E.W. Vaughan, Catalysis and Adsorption by Zeolites, G. Ohlmann, H. Pfeifer and R. Fricke (Editors) Elsevier Science B.V., Amsterdam (1991) p.275. E.J.P. Feijen, K. De Vadder, M.H. Bosschaerts, J.L. Lievens, J.A. Martens, P.J. Grobet and P.A. Jacobs, J. Am. Chem. Soc., 116 (1994) 2950. F. Di Renzo, F. Fajula, F. Figueras, S. Nicolas and T. Des Courieres, Zeolites" Facts, Figures, Future, Studies in Surface Science and Catalysis, Vol. 49 A., P.A. Jacobs and R.A. van Santen (Editors), Elsevier Science B.V., Amsterdam, (1989) p. 119. R.M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press, London (1982) p. 166. C. Baerlocher and W.M. Meier, Helv. Chim. Acta., 52 (1969) 1853. S. Hayashi, K. Suzuki, S. Shin, K. Hayamuzi and O. Yamamoto, Chem. Phys. Let. (1985) 368. F. Delprato, L. Delmotte, J.L. Guth and L. Huve, Zeolites 10 (1990) 546.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 i997 Elsevier Science B.V. All rights reserved.
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Synthesis of high-silica FAU-, EMT-, RHO- and KFI-type zeolites in the presence of 18-crown-6 ether. T. Chatelaina, J. Patarina, E. Brendl~a, F. Dougniera, J.L. Gutha and P. Schulzb a Laboratoire de Mat~riaux MinOraux URA-CNRS 428 ENSCMu - UHA 3 rue Alfred Werner, 68093 Mulhouse Cedex, France b Centre de Recherche Elf Antar-France de Solaize, Chemin du Canal, BP22, 69360 Saint Symphorien d'Ozon, France FAU-, EMT-, RHO- and KFI-type zeolites were synthesized by heating an aqueous alkaline aluminosilicate gel containing 18-crown-6 ether as an organic template with either sodium, or sodium and cesium, or potassium and strontium cations. The products were characterized by elemental analysis, scanning electron microscopy , powder X-ray diffraction , thermal analysis, solid-state nuclear magnetic resonance spectroscopy, and n-hexane adsorption. The 18-crown-6 ether is incorporated in each structure. Its presence turns out to be necessary to crystallize EMT-type materials and, in the other cases, it allows reproducible preparations of well crystallized materials with high Si/AI ratios (3.7-4.6). The retrieval of the 18-crown-6 ether from the EMT-type zeolite is possible with solvothermal treatments. Keywords
Synthesis, high-silica zeolites, 18-crown-6 ether
1. INTRODUCTION
Recently, in our laboratory the use of crown-ethers in an aluminosilicate gel containing sodium cations led to the crystallization of high-silica FAU- and EMT- type zeolites [1]. Later, other studies have been published on these materials [2-4]. Knowing the industrial interest for these high-silica zeolites [5], an extensive optimization work has been performed in order to decrease their synthesis cost. Moreover, as other zeolites have pores which are potentially able to accomodate crown-ether-cation complexes, a synthesis study was undertaken with different alkaline cations associated with 18-crown-6 ether. The aim was to increase the Si/AI ratio of zeolites already known and to take advantage of the structure-directing effect of crown-ether for developing more specific synthesis procedures for these zeolites. 2. EXPERIMENTAL 2.1. Reactants
The reactants were 18-C-6 ether (1,4,7,10,13,16-hexaoxacyclo-octadecane, > 98%, Lancaster), sodium hydroxide (purum, > 98%, Fluka), potassium hydroxide (normapur, > 86%, Prolabo), cesium hydroxide (50wt% CsOH, 50wt% H20, Aldrich) and strontium nitrate (normapur, >99%, Prolabo). The silicon and aluminum sources
174
were colloidal silica (40wt% SiO2 in water, CecasoI,Ceca or Ludox AS40, Du Pont de Nemours ) and sodium aluminate (56wt% AI203, 37wt% Na20, Carlo Erba) or aluminum hydroxide (purum, 65 wt% AI203, Fluka).
2.2. Synthesis procedure for the FAU-, EMT- and RHO-type materials
The samples were obtained by hydrothermal synthesis at 110 ~ optimization, the molar composition of the starting mixtures was as follows:
After
10 SiO2 : 1 AI203 : x Na20 :y Cs20:0.4-0.5 (18-crown-6) : 100 H20 where x = 2.6 and y = 0 for FAU-type zeolite; x = 2.1 and y = 0 for EMT-type zeolite; x = 1.1-1.8 and y = 0.3 for RHO-type zeolite. In all three cases, the starting mixture was prepared according to method II described elsewhere [6,7]. The resulting gel was aged at room temperature for 24 hours in a closed polypropylene bottle under continuous stirring. The crystallization was carried out under static conditions in PTFE-lined stainless-steel autoclaves during 2 to 15 days. The solids obtained were filtered, washed with distilled water until the pH of the filtrate was neutral and then dried at 80 ~
2.3. Synthesis procedure for the KFI-type materials
The samples were obtained by hydrothermal synthesis at 150~ composition of the starting mixture was:
The molar
10 SiO2 : 1 AI203 : 1.8-2.3 K20:0.10 SrO :0.5-1.0 (18-crown-6) : 160-220 H20 The synthesis procedure was similar to that developed by Verduijn [8]. A mixture of KOH and AI(OH)3 with a portion of water was boiled, under continuous stirring, until a clear solution A was formed and then cooled to room temperature. Sr(NO3)2 and 18-crown-6 were successively dissolved in another portion of water and colloidal silica was slowly poured in the thoroughly stirred solution before adding solution A. Before heating under static conditions in PTFE-lined stainless-steel autoclaves during 4 to 5 days, the resulting gel was stirred for about 30 minutes. After crystallization, the products were treated as previously described. A classical KFItype zeolite sample, i.e., without organic species, was also prepared according to the procedure described by Verduijn [8].
2.4. Chemical analysis
Si, AI, Na, K, Cs and Sr analysis was performed by atomic absorption spectroscopy. The amount of water and organic species of the as-synthesized materials was determined by thermogravimetry. Carbon analysis was performed by coulometric determination after calcination of the samples at 1050 ~ under air.
2.5. Powder X-ray diffraction
The powder patterns were obtained on a Philips PW 1800 diffractometer equipped with a variable divergence slit (CuKo0. For the FAU-type materials the relative XRD intensity was determined according to the ASTM D3906-85a procedure [9]. A similar procedure was set up for the EMT samples (18 peaks in the 2e range 1530~ and for the RHO-[7] and KFI-type materials.
2.6. Thermal analysis
Prior to analysis, the solids were equilibrated over a saturated aqueous solution
175
of NH4CI (p/po=0.85). Thermogravimetry (TG) was performed on a Mettler 1 thermoanalyzer by heating in air at 4~ -1. Differential thermal analysis (DTA) was carried out in air on a BDL-Setaram M2 apparatus between 20 and 750 ~ at a heating rate of 10 ~
2.7. Adsorption measurements The sorption studies were carried out on protonated samples, obtained according to a procedure previously described for FAU and EMT zeolites [10]. The adsorption capacity measurements were performed by using a computerized thermogravimetric equipment TG 92 from Setaram. About 100 mg of the calcined solid was activated at 450~ under flowing dry N2 (heating rate : 5~ After 1 hour at 450 ~ the sample was cooled to room temperature during 1 hour. Then the solid was subjected to a flowing mixture of dry nitrogen and n-hexane, during 10 hours. The relative pressure P/PO of n-hexane was 0.5. 2.8. 13 C,27AI, 29Si solid-state MAS and CP MAS NMR spectroscopy The spectra were recorded on a Bruker MSL 300 spectrometer. The recording conditions of the CP MAS and MAS spectra are given in ref.7. 2.9. Determination of the occluded organic species by liquid 1H NMR A known amount of the as-synthesized zeolites (~ 80 mg) was dissolved into 1.5 cm3 of a 40 wt% aqueous HF solution. Thereafter 200 mg of a lwt % dioxane-D20 solution was added as internal standard to the dissolved zeolites. After centrifugation, ,,, 0.5 cm 3 of the liquid was transferred with an equivalent volume of pure D20 in a PTFE tube. The latter was then placed in a classical glass tube for the NMR analysis. The spectra were recorded on a Bruker AC spectrometer. The recording conditions were: frequency =250.13 MHz; recycle time =8 s; pulse width =2 ms; pulse angle = 30 ~ 2.10. Retrieval of 18-crown-6 ether from EMT zeolites The retrieval of the 18-crown-6 ether was performed by a solvothermal treatment of a suspension of the as-synthesized zeolitic samples in water or in an alcohol with or without a salt at a temperature ranging from 180~ to 200~ 3.RESULTS AND DISCUSSION 3.1.Synthesis, crystal morphology, chemical composition and thermal analysis In the absence of Cs + cations and for x= 2.1, a pure EMT zeolite could be synthesized with a lower amount of the expensive 18C6 ether. Indeed, the previously used stoichiometry was reduced from 0.7 [1-4] to 0.4 (Table 1, sample D). Under these new synthesis conditions, no 18C6 remains in the mother liquor. A scale-up study has shown that with this new gel composition, batches producing several kilograms of very pure EMT-type zeolite were easy to reproduce [11].For x = 2.6, the soda content is too high for the 18C6 ether to direct the crystallization towards the EMT structure-type, the FAU phase was obtained (see Table 1, sample A ). For intermediate values ( 2.1<x< 2.6), intergrowths or overgrowths (FAU/EMT) of the two structural types were produced (Table 1, sample C). For instance, according to Treacy et al.[12], an (x value close to 0.5 was found for x= 2.4. In the absence of 18C6 ether, the materials were amorphous(see sample B) and no EMTtype zeolite could be obtained.
176
Table 1 Typical synthesis conditions of FAU-, EMT-, or RHO-type zeolites (starting molar gel composition : 10 SiO2 : 1 AI203 : x Na20: y Cs20 : z18-crown-6 : 100 H20). Heating time Sample x y z at 110~ XRD results (estimated crystallinity) a (days) A 2.6 0.0 0.4 8 FAU (100%) B 2.6 0.0 0.0 8 Amorphous C 2.4 0.0 0.4 8 FAU/EMT (or,~ 0.5) D 2.1 0.0 0.4 8 EMT (100%) E 1.8 0.3 0.5 8 RHO (100%) F 1.8 0.3 0.5 2 RHO (90%) G 1.8 0.3 0.0 2 RHO (45%) + CHA b + ANA b + amb H 1.1 0.3 0.5 15 RHO (90%) a : The reference samples are A for FAU materials and E for RHO materials. b : CHA = chabazite, ANA = analcime, am : amorphous material. The partial substitution of cesium for sodium in the gel composition (1.1< x< 1.8) gave rise to the crystallization of a pure high-silica RHO -type zeolite [7]. Here also the use of crown-ether seems to be necessary for obtaining a pure phase with good reproducibility. As a matter of fact, without the crown-ether molecule in the starting mixture, only a poorly crystalline RHO material was obtained with some chabazite and pollucite [( Na, Cs) analcime] as by-products (compare samples F and G). As it has been observed above for the RHO-type solids, the introduction of 18-C6 in the starting mixture of a KFI-type zeolite led also to a pure material (compare samples I and M, Table 2). Whereas, in the case of RHO zeolite, the soda content can be decreased to x=1.1, the K20 amount (k) has to be close to 2.3- 2.5 otherwise no crystallisation occurs after 5 days (see Table 2, sample L). Table 2 Typical synthesis conditions of KFI-type zeolites. Starting molar gel composition : 10 SiO2 : 1 AI203 : k K20:0.10 SrO :t 18-crown-6 : w H20 Heating time Sample k t w at 150~ XRD results (estimated crystallinity) a (days) Ib 2.3 0 160 5 KFI (90%) + (n.i.)c J 2.3 1 220 5 KFI (100%) K 2.3 0.5 160 5 KFI (90%) L 1.8 1 160 5 Amorphous M 2.3 1 160 5 KFI (100%) a : The reference sample is sample M b : Sample prepared according to the procedure described in ref. 8 c : (n.i.) Traces of a unidentified impurity The FAU-type crystals display a typical octahedral morphology (Figure l a), whereas the EMT-type samples consist of fairly hexagonal platelets (Figure lb). In the latter case, a decrease of the 18-crown-6 ether content (0.4 instead of 0.7) [1-4] and of the water content (100 H20 instead of 140 H20) [1-4] led to a significant
177
decrease of the crystal size (1-21~m instead of 3-4pm) which should enhance the activity of this zeolite in catalytic craking reactions. The RHO-type samples prepared in the presence of crown-ether display a sphere-like shape with an average size of ll~m (Figure lc). Whereas, aggregates of cubic crystals with a size close to 3-41~m were obtained for the KFI-type materials (Figure 1d).
(b)
(c)
(d)
Figure 1. Scanning electron micrographs of high-silica zeolites: (a) FAU sample A; (b) EMT sample D; (c) RHO sample E; (d) KFI sample M The chemical composition of some samples is reported in Table 3. The framework Si/AI molar ratios determined by 29Si MAS NMR spectroscopy are in good agreement with the values obtained from chemical analysis. This confirms that there is no extra-framework aluminum species in the as-made samples as checked by 27AI NMR spectroscopy and that no unreacted gel is present. Zeolites, whose synthesis is possible without crown-ether, i.e., FAU-, RHO- and KFI-type zeolites, show higher Si/AI ratios when the synthesis is performed in the presence of 18C6 ether. There are probably three reasons for this: -for a given Si/AI ratio in the gel, crystallization is generally possible with a lower alkalinity which does not favor the AI incorporation. -the incorporation degree of the bulky 18C6-cation complex is lower and consequently the substitution of AI for Si. -the 18C6-cation complex has better stabilization interactions than hydrated cations when the hydrophobicity of the framework increases (higher Si/AI). It can be seen from the analytical results in table 3, that there is a significant amount of organic matter incorporated in the zeolite. A strong signal close to 70ppm
178
on all 13C MAS n.m.r, spectra gives evidence of the presence of the crown-ether ( between 1 and 8 molecules per unit cell). The Rietveld refinement of the crystalstructure of an EMT-type sample showed that each large cage, i.e., both the hypocage and the hypercage contains one 18C6-cation complex [13]. It can be assumed that in the RHO- and KFI-type materials the crown-ether complexes are occluded in the large LTA-type cavities. But according to the chemical analysis only 50 to 70% of these cavities are occupied. In the case of the FAU-type zeolite the occupation factor of the supercage is close to one. The d.t.a.results show that the thermal decomposition of the crown-ether occurs at much higher temperature for the RHO-and KFI-type materials ( between 300 and 380~ than for FAU- and EMT-type materials ( between 160 and 280~ This difference can be related to the size of the apertures which are circumscribed by 8 membered rings in the former and by 12 membered rings in the latter. Table 3 Chemical composition (wt%) of some zeolite samples in their as-synthesized form (a) wt % Sample Struct. SiO2 AI203 Na20 K20 Cs20 SrO H20 Organic species type TG a CA b NMR c A FAU 46.8 14.3 8.6 / / / 17.0 12.4 12.1 n.d. D EMT 52.7 11.8 7.7 / / / 16.5 12.3 12.2 n.d. E RHO 54.4 11.9 5.2 / 10.6 / 12.5 6.0 5.9 5.9 H RHO 57.9 10.6 4.5 / 9.1 0 9.3 9.1 9.0 n.d. I KFI 51.5 12.6 / 12.1 / 1.2 n.d. / / ./ M KFI 57.1 12.8 / 10.3 / 1.2 16.4 3.1 3.1 3.0. (b) Si/AI molar ratios and 18-crown-6 per unit cell Sample Si/AI molar ratio 18-crown-6 CAb NMR d per unit cell A 2.8 3.0 ~8.5 a determined by thermogravimetry b determined by chemical analysis D 3.8 3.7 ~4.0 E 3.9 3.9 ~1.0 c determined by liquid 1H NMR H 4.6 4.6 ~1.4 d determined by 29Si MAS NMR I 3.5 3.6 / n.d. 9not determined M 3.8 4.0 ~1.0 .
.
.
.
3.4. A d s o r p t i o n
measurements An original result is observed for the RHO-type material. Indeed, the n-hexane sorption capacity (Table 4) is high and the corresponding porous volume is equal to 0.26 cm31iq.g-1.
Table 4 Adsorption of n-hexane Sample Structure type D E M
EMT RHO KFI
Adsorption capacity (wt%) 19.5 17.1 13.7
Porous volume (cm31iq.9-1) 0.29 0.26 0.21
179
This value, which is very close to the theoritical one ( 0.33 cm31iq.g -1114].) is larger than that observed for a classical RHO material (prepared in the absence of 18-crown-6), where only 50% of the theoritical volume is accessible [14]. This higher sorption capacity can be related to the non-distorded high-silica RHO framework [7]. 3.5. R e t r i e v a l of 1 8 - c r o w n - 6 e t h e r f r o m E M T z e o l i t e s
Given the high cost of the crown ether molecule, it is particularly relevant to find a procedure leading to the recovery of the intact macrocycle, and possibly allowing further recycling in synthesis. A few studies dealing with the removal of the organic templating agents from molecular sieves without calcining the samples are reported in literature. In 1986 Gelsthorpe and Theocharis [15] succeeded in extracting triethylamine from AIPO4-5 and SAPO-5 by treating these materials with a solution of methanol and hydrochloric acid at 243~ This method was further used and extented to other porous aluminophosphates by Malla and Komarneni ([16]. The EMT-type zeolite samples were treated in autoclaves by using various solutions as shown in Table 5. Table 5 Extraction media and yields (%) (extraction conditions" liquid/solid ratio 5-50, salt concentration 1-3 mol.I -t, pH 3- 7) Extraction medium Extraction yields (%) Solvent alone H20, C1 -C4 alcohols 15- 40 Solutions of salts of small cations Solutions of salts of large cations
e.g., K+, NH4 +. -protonated amines: e.g.,Et2NH, Et3N, Pr3N -quaternary ammonium salts: e.g., Me4N+ , Et4N+
30- 60 60- 95
According to the nature of the extraction medium, three extraction levels were reached: - with a solvent alone (H20 or alcohols from C1 to C4 or H20-alcohols mixture), the extraction yields did not exceed 40%. - with a solution containing small cations like alkaline or ammonium cations, the yields increase to 60%. - with a solution containing large cations like protonated amines or quarternary ammonium cations, almost all the crown-ether could be extracted. This is in agreement with the conditions which have to be fulfilled by extraction medium: it has to be a good solvent for the crown-ether or its cationic complex and it has to replace in the zeolite,with similar interaction, the extracted ether or its cationic complex.Thus water or alcohols are good solvents for the crown ether, but as guests in the silica-rich zeolite their interaction with the sodium cations is weaker than for the crown-ether. The presence of cations in the solvent helps probably the extraction because the crown-ether is stabilized in solution in the form of complexes. Moreover there can be a simultaneous cation exchange. When the cations present in the zeolite after exchange are similar to the sodium cations, this exchange does not improve the extraction. Only when the new cations exhibit stabilizing interactions
180
and volumes comparable to those of the crown-ether cation complexes, the exchange has a beneficial effect on the extraction. This is the case of bulky and relatively hydrophobic alkylammonium cations which seem to be the most suitable to mimic the 18C6-Na + complex. Among those the protonated triethylamine led to the highest extraction yield (95%). 4. CONCLUSION
FAU-, EMT-, RHO- and KFI-type zeolites were prepared by using the 18-crown-6 ether in an aluminosilicate gel composition. The addition of such an organic species to the starting mixture brought about some significant improvements. The first of them is purity since in a completely inorganic medium usually unwanted by-products appeared. Moreover, until now, the EMT structure-type cannot be obtained, even as traces, in the absence of 18-crown-6. The second outcome was a better crystallinity of the desired products. The use of the crown-ether molecule also resulted in an increased framework Si/AI molar ratio. This result can be related to the fact that the crown-ether is always incorporated and acts as a specific stabilizing template 5. ACKNOWLEDGEMENTS
The authors would like to thank Dr. H. Kessler for fruitful discussions and S. Einhorn for taking the photographs. Financial support of this work was provided by the European Union (Brite Euram program). REFERENCES
1. 2. 3. 4.
F. Delprato, L. Delmotte, J.L. Guth and L. Huve, Zeolites, 10 (1990) 546. S.L. Burkett and M.E. Davis, Microporous Materials, 1 (1993) 265. C. N. Wu and K. J. Chao, J. Chem. Soc. Farad. Trans., 91 1 (1995) 167. E.J.P. Feijen, K. de Vadder, M.H. Bosschaerts, J.L. Lievens, J.A. Martens, P.J. Grobet and P.A. Jacobs, J. Am. Chem. Soc., 116 (1994) 2950. 5. T. Des Couri~res, J.L. Guth, J. Patarin and C. Zivkov, U.S. Pat.5 273 945 (1993). 6. T. Chatelain, J. Patarin, M. Soulard, J.L. Guth and P. Schulz, Zeolites, 15 (1995)90. 7. T. Chatelain, J. Patarin, E. Fousson, M. Soulard, J.L. Guth and P. Schulz, Microporous Materials, 4 (1995) 231. 8. J.P. Verduijn, U.S. Pat. 4 944 249 (1990). 9. American Society of Testing Materials, ASTM Designation D 3906-85a (1985). 10. F. Dougnier, J. Patarin, J.L. Guth and D. Anglerot, Zeolites, 12 (1992) 160. 11. D. Anglerot, F. Fitoussi, P. Schulz, T. Chatelain, F. Dougnier,J. Patarin and J.L. Guth, ACS meeting, Anaheim 1995, accepted. 12. M.M.J. Treacy, J.M. Newsam and M. W. Deem, Proc. Roy. Soc. Ser. A, 433 (1991 ) 499. 13. C. Baerlocher, L.B. McCusker and R. Chiapetta, Microporous Materials, 2 (1994), 269. 14. W.H. Flank, ACS Symp. Ser., 40 (1977) p.43. 15. M.R. Gelsthorpe and C.R. Theocharis, J. Chem. Soc., Chem. Commun., (1986) 781. 16. P.B. Malla and S. Komarneni, Zeolites, 15 (1995) 324.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
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Synthesis of zeolites in a microwave heating environment Jing Ping Zhao, Colin Cundy and John Dwyer Centre for Microporous Materials, Department of Chemistry, UMIST, P.O. Box 88, Manchester M60 1QD, UK
ZSM-5, zeolite /3 and hexagonal Y (EMT), are successfully synthesised using microwave heating. NH 4 form [Ti]ZSM-5 and [A1]ZSM-5 are also obtained under microwave heating conditions in the presence of fluoride anions. NaY can be very rapidly crystallised in the microwave environment. High temperature (150 ~ C) can be used in NaY synthesis without inducing crystalline impurity, demonstrating the tendency of the microwave procedure to provide and maintain phase purity under favourable conditions. By comparison with conventional heating, microwave methods can significantly shorten the overall nucleation and crystal growth periods for most of the zeolites investigated, but the effect is greater in some cases than in others. In a microwave environment, a higher temperature can often be used in zeolite synthesis to gain extra benefits.
1. INTRODUCTION Zeolites are usually synthesised under hydrothermal conditions in a period from a few hours to a few days depending on the nature of the zeolite, mixture composition and the synthesis temperature. Frequently, low temperatures must be used to reduce the formation of undesired phases. However, working at a low temperature necessitates longer synthesis times which may prove too costly or impracticable for industrial application. Therefore, reducing the crystallisation time may be one of the most important targets for commercial synthesis. Recent research shows that microwave techniques present new possibilities for achieving such a target [1,2]. This paper examines the syntheses of various zeolites under microwave heating conditions and discusses the possible contribution of microwave energy to the formation of zeolites. [AI]ZSM-5, zeolite/3, EMT (Hexagonal Y) and NaY have been prepared using hydroxide ion as mineraliser. NH4 form [Ti]ZSM-5 and [A1]ZSM-5 are also synthesised from mixtures containing fluoride anion. By comparison with a conventional heating system, microwave heating can significantly reduce overall crystallisation time. It is also observed in the synthesis of Na-Y that microwave heating can significantly accelerate the formation of the FAU structure, and with a much reduced occurrence of P-type (GIS) impurity. Because of this "preference" or "selectivity", a higher temperature can be applied in NaY synthesis, making the process faster and potentially more cost effective.
182 2. EXPERIMENTAL The raw materials used were sodium aluminate (AlzO3=32.75 %, NazO=28.5%, BDH), NH4F (BDH), NaF (BDH), NaOH (BDH), 18-Crown-6 (BDH), Tetrapropylammonium bromide [TPABr] (Aldrich Chemicals), Silica sol (Ludox AS 40, SiO2=40%, Aldrich Chemicals), Fumed silica (BDH), Tetraethyl-ammonium hydroxide (20% solution, Aldrich) and TIC13 (BDH). The microwave equipment used in this work was a CEM MDS-2100 sample preparation system using a fibre optic temperature controller and a pressure controller and an adjustable power output (maximum 950W at 2450 MHz). Teflon PFA autoclaves (100 ml, unstirred), were used. The samples synthesised were characterised using 295i MAS N M R , XRD, SEM and EDAX.
3. RESULTS AND DISCUSSION 3.1 [Ti]ZSM-5 and [A1]ZSM-5 [A1]ZSM-5 was synthesised from reaction mixtures of composition xNa20-yA120360SiO2-zTPABr-nHaO, where x=4.5-7, y=0-2, z=4-8 and n=800-1600, prepared according to the following procedure. First, a silica-containing mixture (mixture A) was made by mixing silica sol (Ludox) and template (TPABr) with 1/3 of the total water. Secondly, mixture B was prepared by mixing NaA102 and NaOH with a further 1/3 of the calculated water. Both mixtures were stirred for 30 minutes. Finally, mixture B was slowly added to mixture A with vigorous stirring and the reaction gel was continuously stirred for 2 hours. The final composition was then transferred into autoclaves and heated by microwave. The temperature was controlled by a programme which in essence contains two stages which are D E temperature elevation and C temperature maintenance. The first stage usually takes 1-2 minutes and the A B second stage depends on the reaction temperature and mixture composition. 5 15 25 35 5 Twotheta Using a reaction mixture with a typical composition 5.0NazO-0.2AlzO3-60SiOz- Figure 1. XRD patterns of [A1]ZSM-5 synthesised from 4.0TPABr-900H20, a an alkaline system under microwave heating. very crystalline ZSM-5 A) 1.65 h B) 2.0 h C) 2.35 h D) 2.65 h E) 3.0 h
183 can be obtained at 170~ without seeding in 2-3 hours (Figure 1). The XRD patterns of the [A1]ZSM-5 show that the nucleation takes less than 1.65 hours and the crystal growth needs only about 1 hour. This demonstrates that [A1]ZSM-5 can be synthesised very rapidly from a conventional reaction mixture using microwave heating. The particle size of the [A1]ZSM-5 is 3-4 25 35 45 5 15 ~tm and no TwoTheta crystalline impurity phases are observed. Figure 2. XRD patterns of [A1]ZSM-5 synthesised from a In order to fluoride system under microwave heating. study the effects of 1) 7.0 hours; 2) 8.0 hours 3) 9.0 hours microwave heating on a different synthesis system, the method based on fluoride anion was chosen. The reaction mixtures were prepared using reported procedures [3,4]. Two typical reaction mixtures were used to study the effects of microwave energy on the formation of [Ti]ZSM5 and [A1]ZSM-5 in the presence of fluoride anions. Their compositions were 1.0NH4F0.03TiCI3-1.0SiO2-0.5TPABr-30H20 and 2.0NH4F-0.14A1C13-1.0SiO2-0.5TPABr-30H20. Both used silica sol (Ludox AS 40) as silica source. The reaction mixtures are usually aged overnight, but such a step is not essential. On microwave I ~ l heating, the MFI structure can be detected by XRD from the reaction mixtures after about 5 15 25 35 Two "Fheta 45 6-7 hours at 170~ C. The crystallisation will then take a Figure 3. XRD patterns of [Ti]ZSM-5 synthesised from a fluoride further 1-2 hours to system under microwave heating, complete. The XRD 1) 7.0 hours 2) 8.0 hours 3) 9.0 hours !
184 patterns of NHa-[AI]ZSM-5 and NH4-[Ti]ZSM-5 are given in Figure 2 and Figure 3 which show that they are both very good crystalline products. As can be seen the nucleation takes about 7 hours and the crystal growth needs only about 1.5 hours which is similar to that in an alkaline system. This suggests that microwave energy has no unique influence on the crystal growth in either the fluoride or in the alkaline synthesis system. It is also appearent that nucleation is the major factor controlling the overall crystaUisation time of the MFI structure in the fluoride system. Under conventional heating, the crystallisation takes about 20 hours at 175~ C using comparable reaction compositions [4], demonstrating an effective acceleration of 2-3 times using the microwave method.
3.2 Syntheses of zeolite/3 and hexagonal NaY (EMT) Zeolites 13and EMT can also be synthesised in a microwave heating environment. For zeolite/3, the reaction mixture was prepared by adding a solution containing NaA102 and NaOH to a mixture of fumed silica and TEAOH. A seeding slurry was then B optionally added. The final mixture was aged at room temperature overnight. The seeding slurry has a similar composition to the main reaction mixture but has been pre-heated 5 1~5 25 35 in a conventional oven Two Theta to nucleate. In the seeding slurry, zeolite XRD patterns of zeolite/3 synthesised at 140~ /3 can be detected using Figure 4. under microwave heating. XRD analysis. A A) 10 hours B) 14 hours typical reaction mixture has the composition 2.5Na20-1.0A1203-40SiO2-6.0(TEA)20-560H20. Usually, the amount of seeding slurry is between 4 % and 8 % based on SiO2 + A1203 in the mixture. Figure 4 gives the XRD pattern of the product after 14 hours microwave heating at 140~ C. The wellcrystallised zeolite has a Si/A1 ratio of 14 as measured by 29 Si NMR spectroscopy. Hexagonal Y (EMT) is one of most difficult aluminous zeolites to synthesise, normally taking 4-12 days at 110~ C by conventional heating [5]. However, it can be obtained in one day using microwave energy by working at a higher temperature. The composition of synthesis mixture was 2.4Na20-0.5NaF-(0.85-1.0)A1203-10SiO2-(0.5-0.8)[18-Crown-6]140H20, prepared as described previously [5]. At a lower temperature (110 ~ C), EMT can be detected after microwave-induced crystallisation for about 1.8 days and the crystal !
!
185 growth takes a further 2 days, which is nearly as long as in a conventional hydrothermal synthesis at that temperature. It is clear that the crystallisation of the EMT under these conditions is very difficult. When a higher temperature ( 115~ C) is used, a very crystalline EMT can be obtained in about 2.5 days (Figure 5). The c fast increase of crystallisation with a very small rise in temperature may suggest that the formation of EMT is very sensitive to temperature in a microwave environment. With further increase of ! 0 1~0 20 Two Theta 30 40 the temperature (130150~ C), EMT can be obtained between 820 hours depending Figure 5. XRD pattern of EMT synthesised without seeding on the reaction under microwave heating. t e m p e r a t u r e . A) 3.0 days at 110~ B)l.8 days at 115~ C)2.5 days at 115~ However, small amounts of amorphous or other impurity phases are generally unavoidable. If a seeding technique similar to that used in standard zeolite Y synthesis is used, the purity of the product can be improved, particularly at a higher temperature (150~ C).
3.3 NaY synthesis Highly crystalline NaY was synthesised in a very short time using microwave heating. The reaction mixtures were prepared by adding a solution of NaA102 and NaOH to a diluted silica sol. The compositions employed were xNazO-1.0A1203-ySiO2-240H20 , where x=4-7 and y = 10-20. When y = 10 and x=6.2, a very crystalline NaY can been obtained at 110~ in less than 1.5 hours under microwave heating without seeding. Using the same reaction mixture, the synthesis takes 9-10 hours at the same temperature under conventional heating conditions in order to achieve a good crystalline product. The synthesis time is about 6 times shorter using microwave heating. Zeolite P is an impurity phase frequently obtained in conventional NaY preparations, usually rendering NaY production impossible at higher temperature. Therefore, most industrial processes use a lower temperature (usually ca. 100~ At this temperature, the crystallisation usually takes from 10 to 30 hours. In order to study the formation of zeolites at different temperatures in a microwave environment, the NaY synthesis system was chosen. It is found that, using microwaves,
186 there is far less conversion to P-type zeolite even at higher temperatures (150 ~ C). At such a high temperature, formation of P-type zeolite is usually unavoidable by conventional heating. For example, heating the same reaction mixture at 150~ C by conventional means, produced a P-type zeolite resembling Gobbinsite [6] without any NaY. At 150~ C, the FAU structure can be detected by XRD after about 5 minutes microwave heating and the crystal growth takes a further 10-15 minutes (Figure 6). The crystalline product had an average crystal size of about 0.5-1.0/~m without any detectable crystalline impurity. Continuing the microwave heating for a total of 120 minutes yielded no evidence of any other crystalline phase. This suggests that microwave energy has made an effective and apparently selective contribution to the formation of the FAU structure D which is also stable in the microwave C reaction environment. Again, it appears that the use of B microwave /" energy may have a particular , , i 310 i advantage in 0 lb ' 2'0 Two Theta 40 encouraging the .
.
.
.
.
.
_
_
,
:
.
.
.
.
.
_
-
7
nucleation of a Figure 6. XRD patterns of NaY synthesised at 150 ~ C without s in g 1e p h a s e seeding under microwave heating. under favourable A) 5 min. B) 10 min. C) 15 min. D) 20 min. conditions rather than exerting any unique influence upon crystal growth. The effect of microwave heating on the synthesis of high-silica NaY has also been studied. When Na20=5.8 and SiO2 = 15, a very crystalline NaY can be obtained in less than 1.5 hours, with seeding, at 150~ C. A typical product had SiO2/AlzO 3 = 4.7 as calculated from the unit cell parameter [7]. This demonstrates that a fairly siliceous NaY can also be obtained very rapidly at a higher temperature without crystalline impurity. Unfortunately, the true comparison of microwave heating with conventional heating at higher temperatures cannot be made because pure NaY cannot be obtained from oven-heated autoclaves under these conditions.
4. CONCLUSIONS The syntheses of [AI]ZSM-5 and [Ti]ZSM-5 can be accelerated in a microwave heating environment without seeding using either traditional alkaline media or in the presence of fluoride ions. With microwave heating, [A1]ZSM-5 can be obtained from a normal OH-
187 based composition in 2.5 hours. In most cases, crystaMsation can be speeded up by 2-3 times by using microwave heating as compared with conventional heating. Very crystalline zeolite/3 can be crystallised in 14 hours from the standard NaOH/TEAOH system using microwaves. Highly crystalline EMT can similarly be obtained at 115~ in 2.5 days without seeding. The formation of EMT is very sensitive to the synthesis temperature. The most notable contribution so far observed for microwave energy input is found in NaY synthesis. By using microwave heating, under given conditions, a reduction in overall synthesis time of up to 6 times can be achieved compared to conventional methods. NaY can be crystallised in times ranging from 10 minutes to 1.5 hours depending on the composition of the reaction mixture. In NaY synthesis, it is found that for approximately every 15 degree increase in reaction temperature, the crystallisation time is reduced by a half. Microwave heating can use this to advantage by limiting the formation of impurity phases (particularly zeolite P) and providing, in effect, selectivity towards NaY. A much higher temperature can therefore be applied in microwave-mediated NaY synthesis than in conventional synthesis procedures. Consequently, there is considerable scope for the improvement of existing procedures in terms of time and cost and perhaps also for the development of new continuous processes.
ACKNOWLEDGEMENTS We thank the EPSRC for their financial support (GR/K06877) of this work and R J Plaisted for helpful discussions.
REFERENCES 1. 2. 3 4. 5. 6. 7.
A. Arafat, J. C. Jansen, A. R. Ebaid and H. van Bekkum, Zeolites 13 (1993), 162-5. P. Chu, F. G. Dwyer and V. J. Clarke, Eur. Pat. 358 827 (1990). J. Zhao, J Dwyer and D Rawlence, Proceedings of the 9th International Zeolite Conference. Montreal (1992), 155-61. J. Zhao and J. Dwyer, paper in preparation. K. Karim, J. Zhao, D. Rawlence and J. Dwyer, Microporous Materials, 3 (1994) 695-698. R. von Ballmoos and J. B. Higgins, Collection of Simulated XRD Powder Patterns for Zeolites, Zeolites, 10(5), (1990). D . W . Breck, Zeolite Molecular Sieves, Wiley, New York, p49, 1974.
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H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
SYNTHESIS OF OCTAHEDRAL
MOLECULAR
189
SIEVES
Chi-Lin O'Young' and Steven L. Suibb 'Texaco Inc. P.O. Box 509, Beacon, NY 12508 USA ~Dept. of Chemistry~ U. of Connecticut, Storrs, CT 06269-3060 USA A large number of families of manganese oxide octahedral molecular sieves (OMS) and their precursors, octahedral layered materials (OL), have been synthesized and characterized by various methods. The materials include OMS-1, OMS-2, and OL-1; they all use MnO6 octahedra as the basic structural unit to form (3X3) tunnels, (2X2) tunnels, and layered structures, respectively. Different metal cations can incorporate into the OMS and OL structures through framework, tunnel, and interlayer substitutions. These materials can be synthesized by the reflux, hydrothermal, precipitation, calcination, ion-exchange, isomorphous substitution, and sol-gd methods. The materials have been characterized in detailed by a variety of techniques to study the structure, composition, stability, and morphology. This paper will discuss and review the different methods used to synthesize OMS and OL materials and the resultant changes in physical and chemical properties. 1. INTRODUCTION Synthesis of novel zeolites and molecular sieves such as intersecting 10- and 12-ring pore zeolites [1] and mesoporous materials [2] have been a major focus of several research groups in the past few years. Unique structural materials that result in porous and open l~amework systems such as vanadium phosphates with 18.4 A tunnels [3] and porous TiO2 materials [4] are continually being discovered. The synthesis, characterization, and applications of todorokite [5] and cryptomelane (or hollandite) materials [6-8] have been reported. These OMS materials use MnO~ octahedra as the basic structural unit to form mono-directional tunnel structures. The synthetic todorokite (OMS-1) and cryptomelane (OMS-2) have (3x3) and (2x2) tunnels with pore sizes of 6.9 x 6.9 A and 4.6 x 4.6 A, respectively (Figure 1). The precursors of OMS, bimessites or octahedral layered materials (OL-1), consist of layers of edge and comer linked MnO6 octahedra with water molecules and metal cations in the interlayer voids and have an interlayer distance of 7 to 10 A. Certain metal cations can incorporate into the OMS and OL structures through framework, tunnel, and interlayer substitutions [7,9]. The framework substitutions of OMS and OL are represented as [M]-OMS and [M]-OL, while the tunnel and interlayer substitutions are represented as M-OMS and M-OL, respectively. These OMS and OL materials can be synthesized by the reflux, hydrothermal, precipitation, calcination, ion-exchange, isomorphous substitution, and sol-gel methods and result in materials that
190 have different physical and chemical properties. We will discuss and review these various methods used to prepare OMS and OL materials and resultant changes in physical and chemical properties.
OMS-1 Todorokite
OL-1 Birnessite
. . . . .
H20
G
~
9
G
~
|
.,
OMS-2 Hollandite
.....o
M n+
9
9
r//.
.
Mn2~ Co2§ , Ni2§ Cu 2., Zn2§
(sx3)
(2xe)
Cd 2+, Mg2+ Figure 1. Structure of OL-1, OMS-1, and OMS-2 T.~...b.Le..~..~.~.~.m.~!.~...?.f..~s...~..~..~L~.~..a.t.e.~..gr..e.p...~.r.e...~....~r.~y.~..d?...".t..~rth.~.~.
Structure 1. K-OMS-2
Reactants KMn04, MnSO,
2. K-OMS-2 3. K-OMS-2 4. K-OMS-2 5. M-OMS-2 6. [M]-OMS-2
12. Mg-OMS- 1
KMn04, MnSO4 K-OL-1 from #8 KMnO4,maleic acid K-OMS-2 form # 1, 2 # 1, 2 & dopants added to initial solution MnCI2, NaOH, Mg0VlnO4)2 MnSO4, KOH, 02 KMnO4, sugar Na-OL- 1 from #7 #7 & dopants added to initial solution Mg-OL- 1 from # 10
13. M-OMS- 1
M-OL-I from #I0
14. [M]-OMS- I
[M]-OL- I from # I I
7. Na-OL-1 8. K-OL-1 9. K-OL-1 10. M-OL- 1 1 !. [M]-OL-1
............................................................................................................
.............................................
Method/Conditions Hydrothermal, autoclave 100 ~ 17 h, low pH's Reflux 100 ~ 17 h, low pH's Calcination, 600 *C, 17 h Sol-gel, 450 *C, 4 h Ion-exchange Isomorphous substitution, reflux or hydrothermal Precipitation, 25 ~ aging, high pH's
Precipitation, 25 ~ high pH's Sol-gel, 450 ~ 2 h Ion-exchange, 25 ~ Isomorphous substitution, 25 ~ aging, high pH's Hydrothermal, autoclave 175 *C, 20 h Hydrothermal, autoclave 175 ~ 20 h Ion-exchange, hydrothermal, ~.t..o..~ay~..!.7...5....*...C..,...~O.~
References 6
7 7 8 7 7 5 7 10 5, 9
5 9
.........................................................................
191 2. EXPERIMENTAL SECTION Table 1 shows fourteen families of OMS and OL materials have been prepared by the reflux, hydrothermal, precipitation, calcination, ion-exchange, isomorphous substitution, and sol-gel methods.
2.1. Synthesis of OMS-2 with (2X2) Tunnel Structure K-OMS-2 have been synthesized by the hydrothermal [6], reflux [7], calcination [7], and sol-gel [8] methods (Methods 1 - 4). A typical hydrothermal or reflux synthesis was as follows: 100 mL of 0.4 M KMnO4 was added to a mixture of 30 mL of 1.74 M MnSO4 and 3 mL of concentrated HNO3. The final mixture was autoclaved or refluxed at 100 *C for 17 h; the product was filtered, washed, and dried. A standard calcination preparation was as follows: 200 mL of 3.12 M KOH was added to 200 mL of 0.89 M MnSO4. Oxygen was bubbled (12 L/min) through the solution for 4 h. The black product was filtered, washed, and calcined at 600 *C for 17 h. A typical sol-gel preparation was as follows: 3 m mole of maleic acid was added to 100 mL of 0.1 M KMnO4 and the mixture was stirred for 30 min. A dark brown sol was formed at room temperature which started to gel in 30 rnin. The resultant gel was filtered, washed, and calcined at 450 *C for 4 h. The tunnel substitution of K-OMS-2 was prepared by the ion-exchange method (Method 5) [7]. The framework substitution of K-OMS-2 was prepared by adding metal dopants (a total concentration of 0.01 M) into the initial solution, followed by reflux or autoclave treatment as described before (Method 6) [7]. 2.2. Synthesis of OL-I The precursor of OMS-1, Na-OL-I, was prepared by reacting MnCl2 with NaOH to form pyrochroite, Mn(OH)2, followed by adding a solution of Mg(MnO4)2 to the suspension to form NaOL-1 (Method 7) [5]. KMnO, or NaMnO, also could be used to prepare the same quality of precursor by adding an equivalent amount of Mg 2+ into the mixture. A typical preparation was as follows: 50 mL orS.0 MNaOH solution was added to a mixture of 40 mL of 0.5 M MnCl2 and 0.1 M MgCl2 in a plastic bottle with stirring. 40 mL of 0.2 M KMnO4 or NaMnO4 was then added to the suspension. The mixture was aged at room temperature for 7 days. The precipitate was filtered and washed thoroughly with water. XRD of the wet precipitate showed peaks at 10.1, 5.0, and 3.33 A. The interlayer peak shined from 10.1 to 7.1 Jk after drying. MgCl2 also could be mixed with the KMnO, or NaMnO4 solution, and the results were the same. The precursor of OMS-2, K-OL-l, was prepared by reacting MnSO4 and KOH with oxygen bubbling through the solution as described previously (Method 8) [7]. The sol-gel K-OL-1 was prepared by reacting KMnO4 and simple sugars (glucose or sucrose) to form a brown gel, followed by drying and calcining at high temperatures (Method 9) [10]. A standard preparation was as follows: 50 mL of 0.38 M KMnO4 was added to 20 mL of 1.4 M glucose to form a brown gel, and the gel was then washed, dried, and calcined at 450 *C for 2 h. The interlayer substitution of Na-OL-1 was prepared by the ion-exchange method at room temperature (Method 10) [9]. For the l~amework substitution, [M]-OL-1 was prepared by adding metal dopants (6 mL of 0.1 M metal salts) into the initial solution as described previously (Method
II).
192 2.3. Synthesis of OMS-I with (3X3) Tunnel Structure Mg-OMS-1 was prepared by the rearrangement of Mg-OL-1 under hydrothermal conditions (Method 12) [5]. The tunnel substitution of OMS-1 was prepared by the ion-exchange of Na-OL-1 with metal ions (Method 10), and consequently convened to M-OMS-1 by the hydrothermal treatment (Method 13) [9]. The fiamework substitution of OMS-1 was prepared by adding metal dopants into the initial solution to form [M]-OL-1 (Method 11), followed by the ion-exchange with Mg2§ then converted to [M]-OMS-1 by the hydrothermal treatment (Method 14). 2.4. Characterization Techniques Various characterization techniques have been applied to study the structure, composition, morphology, and thermal stability of these materials. They included X-ray powder diffraction (XRD), microanalysis, transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermal analysis, temperature programmed desorption (TPD) and reduction (TPK), cyclic voltammetry (CV), and others. Details of the procedures have been disclosed in the literature [512].
3. RESULTS 3.1 Synthesis The hydrothermal and reflux preparations of K-OMS-2 involved reactions between MnO4" and Mn2§ solutions. The size and concentration of counter cation, pH, and temperature were identified as important preparation parameters. The optimal conditions are: pH's < 2, 80 to 140 ~ and [KMnO4]/([KMnO4] + [Mn2"]) > 0.4. The template effects of counter cations were clearly observed in both methods [6,7]. The calcination method involved the heat treatment of the layered precursor, K-OL-1, at 6OO~ The sol-gel method involved reactions between KMnO4 and pure maleic acid. The critical preparation parameters included the concentration of permanganate, nature of the counter cation and the organic acid reducing agent, temperature of gelation and calcination. A mixture of OMS-2 and OL-I was formed if the concentration of permanganate was too high (0.35M). KMnO4 and CsMnO4 yielded most crystalline and thermally stable (800 oC) sol-gel OMS-2. NaMnO4, LiMnO4, Ca(MnO4)2, and Mg0VInO4)2leaded to less stable and impure products. The optimal conditions are: a concentration of KMnO4 around 0.1 M, pure and flesh maleic acid, reaction temperature around room temperature, and calcination temperature around 400 ~ [8]. Several critical parameters for the synthesis of Mg-OMS-1 and the precursor, Na-OL-1, were identified; they included the pH, ratio of MnO4" to Mn2§ Mg2+concentration, time and temperature of aging, time and temperature of autoclaving. The optimal conditions for the synthesis of OMS-1 are: a MnO4"/Mn2§ ratio of 0.3 to 0.4, pH of 13.8, aging at 25 to 35 ~ for 1 to 7 days, and autoclave treatment at 160 to 180 oC for 4 to 7 days. The effects of pH, MnO4"/Mn2+ ratio, and autoclave conditions have been discussed in the literature [5]. The crystaUinity and thermal stability of OMS-1 strongly depended on the crystallinity of the precursor. Table 2 shows the effects of aging temperature (5 to 35 *C) and time (1 to 7 days) on the crystallinity of Na-OL-1. The integrated intensity of the d(001) peak at 10.1 A was measured and used as an index of the crystaUinity. The results show that the crystallinity of Na-OL-1 is quite sensitive to the aging time and temperature. For example, aging at 35 *C for 1 day has about the same crystallinity as aging at 5
193 oC for 7 days. Aging at higher temperature (80 *C) significantly increases the crystallinity; however, the resultant Na-OL-1 loses the ion-exchange capacity and cannot form Mg-OL-1 and OMS-1. Table 2. Effects of aging time and temperature on crystallinity of Na-OL-I Aging Temperature Aging Time ~ Days 5 20 5941 9050 1 7345 11100 2 11982 14700 4 13069 15170 7 Values are the integrated intensity of the d(001) peak at 10.1 K
35 13571 16254 22100 24486
The Mg2§ concentration is another important preparation parameter. The most crystalline and thermally stable OMS-1 materials are prepared by Mg(MnO4)z. Using NaMnO4 or KMnO4 yields less stable and impure products. However, the same quality of OMS-1 can be formed when an equivalent amount ofMg 2+is added into the initial solution. For the sol-gel synthesis of K-OL- 1, the counter cation of permanganate plays an important role in reactions of MnOi with sugars. Different cations influence both gel formation and the nature of the final product. Only KMnO, yields sol-gel and pure products; NaMnO4 and Mg(MnO,)2 do not produce sol-gel K-OL- 1 products [ 10]. 3.2 Ion Exchange Properties K-OMS-2 prepared by the reflux and hydrothermal methods exhibit some ion-exchange properties. At room temperature, about 70% of tunnel K§ can be exchanged with Rb§ Li§ and Na + cannot exchange K § easily [7]. At 80 *C, K-OMS-2 can be exchanged with NH4+, Co2+, Cu2+, and Ni2+to form tunnel substituted [M]-OMS-2. The percentage of exchange capacity shows the order: NI-I4+ (36%) > Co2§ (18%) > Ni 2+(5.8%) ~ Cuz+(4.4%). Na-OL-1 prepared by Method 7 has very good ion-exchange capacity when it is wet. However, dehydration of Na-OL-1 decreases the interlayer distance from 10.1 to 7.1 ~ and loses the ionexchange capacity. Mg 2§ Ni2+, Co2+, Cu2+, and Zn2+ can be exchanged easily at room temperature with wet Na-OL-1 to from M-OL-1 and consequently converted to M-OMS-1 under the hydrothermal conditions [9]. Direct ion-exchange of Mg-OMS,1 to form tunnel substituted MOMS-I has not succeeded. Sol-gel K-OL-1 doesn't have any ion-exchange capacity at room temperature. 3.3. Framework Substitution The isomorphous substitutions of cations intothe OMS and O L l~amework structures involved addition of metal dopants into the solution precursors prior to any precipitation or reaction. A variety of cations, such as Cr~§ Fe3+, Co2§ Ni~§ Cu2+, and Zn2+ have been incorporated into the OMS-2 framework. At 0.01 M of dopant concentration and the standard conditions, the degree of substitution was between 1.5% to 1.7%. Divalent cations with ionic diameters similar to Mn2§ such as Mg 2§ Co2§ Niz§ Cuz§ and Zn2§ have been incorporated into the framework of OMS-1 by Method 14. Results of compositional
194 analysis showed that all doped cations (about 2%) were totally retained in the corresponding OMS1 materials. 3.4 Characterization K-OMS-2 prepared by the four different methods all have the same XRD patterns; however, they have quite different properties of crystaUinity, thermal stability, and morphology (Table 3). The reflux, autoclave, and sol-gel materials have higher average manganese oxidation states than the calcination material. XRD of the calcination and sol-gel materials show sharper peaks than those of the reflux and autoclave materials. The reflux and autoclave materials are thermally stable up to 600 ~ whereas the calcination and sol-gel materials are stable up to 800 *C. The morphology of the reflux and autoclave materials are needle-like; however, the morphology of the calcination and solgel materials are clump and irregular. Temperature programmed desorption (TPD) data for 02 evolved show that the reflux and autoclave materials have much higher oxygen loss than the sol-gel material. Table 3. Comparison of OMS-2 prepared by Reflux, Autoclave, Calcination, and Sol-Gel Methods Parameter Reflux Autoclave Calcination Sol-Gel broad broad sharp sharp XRD 3.80 3.96 3.68 3.80 Ave. Mn Oxidation state 600 ~ 600 ~ 900 ~ 800 ~ Thermal Stability needles needles clumps irregular SEM 9.41 of16 O 9.41 of16 O N.A. 0.48 of16 O Oxygen Desorption (TPD)
Tunnel cations have profound effects on the thermal stability and morphology ofM-OMS-1 [9]. Results of TGA and XRD show that Mg-OMS-1 is thermally stable up to 600 ~ Co-OMS-1 and Ni-OMS-1 are stable to 500 ~ and Cu-OMS-1 and Zn-OMS-1 are stable to 300 *C. SEM results show that the crystal morphologies of M-OMS-1 can be plates, needles, or fibrous shapes, depending on the nature of the cations. TPD results show that the tunnel cations do not markedly affect the evolution of TPD peaks, but remarkably influence TPR in both H2/Ar and CO/He with respect to both emerging temperature and population. Cu-OMS-1 appears to process more oxygen species that are especially reactive with 1-12and CO at low temperatures [ 11]. Results of XRD and TGA show that framework substituted [Zn]-, [Co]-, [Ni]-, and [Cu]OMS-1 are all thermally stable up to 400 ~ which are different from the tunnel substituted OMS1. The strongest evidence for fi-amework substitution comes from cyclic voltammetry measurements [12,13]. Results of CV studies show Cu2§ migration out of the OMS-1 tunnels for Cu-OMS-1 with most Cu 2§ in the tunnel, while no such Cu2§ migration is found for [Cu]-OMS-1, indicating the existence of Cu2§ in the framework. Results of electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) also indirectly support the framework substitution [ 13]. 4. DISCUSSION OMS-1 and OMS-2 have very similar tunnel structures; however, they are synthesized by quite different methods and conditions. Scheme 1 shows two different preparation routes to OMS. Both routes involve redox reactions between MnO; and Mn2+ at different pH's. Low pH's are used for
195 Route 1 tO form OMS-2. XRD of the flesh precipitate shows an amorphous phase. OMS-2 is formed by selecting suitable templates and temperatures. K§ is the major template for the synthesis of OMS-2. Attempts to synthesize OMS-1 by using larger templates are not s u ~ . High pH's are used for Route 2 to form OL-1 as precursors to OMS. XRD of OL-1 prepared by this route show either 10 or 7 A layered phases. However, the critical preparation parameters and conditions for the precursors of OMS-1 and OMS-2 are different. OMS-1 prepared by NaMn04, KMn04, or oxygen [14] are less thermally stable and impure. The effects of Mg2+suggest that a small amount of Mg2§ is in the ~ o r k of layered precursor and OMS-1, which stabilizes the OMS-1 structure. Mg2§ is the major template for the synthesis of OMS-1. K+ and high temperature are essential for the formation of OMS-2 by this route (Method 3). The rearrangement of layered silicates to mesoporous ahminosilicates also has been reported [2]. Scheme 1. Two Different Preparation Routes to OMS
Route 1:
Route 2:
Mn 2+ + MnO4
Mn 2+ +
MnO4 or Oxygen
Low pH's -"
Amorphous Phase
Hi pH's
Temperature ~ OMS-2 Template
Temperature ,~- OL-I
P
Template
OMS-I and
OMS-2
The synthesis" of OMS with (2X5) tunnel structure has been reported in the literature [16]. It was prepared by reacting pyrolusite, (1X 1) tunnel structure, with RbOH solution in a gold capsule under hydrothermal conditions at 350 ~ and 200 Mpa. Another known OMS structure with (2X3) tunnel structure, psilomelane, has not been synthesized. It is still not clear why so different procedures and conditions are needed to synthesize these OMS. Sol-gel methods for the preparation of OMS and OL materials provide several advantages, such as easy incorporation of dopants and template agents directly into the sol. Preparation of thin films via spin coating techniques that might be used in electrochemical or sensor application are also possible. The sol-gel synthesis involves reactions between MnO4" and organic reducing agents. KOL-1 prepared by the sol-gel method is very stable compared to the OL-1 materials prepared by the precipitation, hydrothermal treatment at 160 ~ for 2 days or calcination at 800 ~ for 2 h has essential no effect on its XRD pattern. 5. SUMMARY We have reviewed here the various routes that can be used tO prepare an extensive family of OMS and their precursors, OL. The syntl~c methods include the reflux, hydrothermaL precipitation, calcination, ion-exchange, isomorphous substitution, and sol-gel techniques. The materials compri~ OMS-1, OMS-2, and OL-1. The different synthetic methods have been compared, the important preparation parameters and reaction conditions have been identified. Various characterization techniques have been used to study the structure, comlx~sition,
196 morphology, and ~ stability of these materials. Both tunnel ( i o n - e x ~ e ) and framework substituted (dopants in solution precursors) materials have been reported. We have shown quite different physical and chemical properties of these matenals and how they are related to the resultant strucaae and to particular synthetic methods used to prepare such materials. REFERENCES
1. Lobo, R.F., Pan, M; Chan, I., Li, H.X.; Medmd, R.C., Zones, S.I.; Crozier, P.K, Davis, M.E. Science, 1993, 262, 1543-1546. 2. (a) Kresge, C.T.; Leonowicz, ME.; Roth, W.J., Vartuli, J.C., Beck, J.S. Nature, 1992, 359, 710712. (b) Inagaki, S.; Fukushima, Y.; Kuroda, IC J. Chem. Soc. Chem. Comm. 1993, 680. 3. Soghomonian, V.; Chen, Q., Haushalter, R.C.; Zubieta, J. Angew. Chem. Int. Ed Engl., 1993, 32, 610~11. 4. Tanev, P.T., Chibwe, M.; Pinnavaia, T.J. Nature, 1994, 368, 321-323. 5. Shen, Y.-F.; Zerger, I~P.; [kGuznum, R.N.; Suib, S.L.; McCurdy, L.; Potter, D.I.; O'Young, C.-L. Science, 1993, 260, 511-515. 6. O'Young, C.-L. in Expanded Clays and Other Microporous Solids; Occeili, M.L., Robson, H., Eds. Vol. II 333-340, Van Nostrand Reinhold, NY 1992. 7. DeGuzman, R.N., Shen, Y.-F., Neth, E.J., Suib, S.L., O'Ytmng, C.-L., ~ , S., Newsam, J. M., Chem. Mater., 1994, 6_,815-821. 8. Dumt, N., Suib, S.L., O~roung, C.-L. J. Chem. Soc. Chem. Comm. 1995, 1367-1368. 9. Shen, Y.-F., Suib, S.L., O~Young,C.-L., J. Amer. Chent Soc., 1994, 116, 11020-11029. 10. Ching, S.; Landfigan, J.K, Jorgensen, M.L., Duan, N., Suib, S.L.; O'Young, C.-L. Chem. Mater. 1995, 7, 1604-1606. 11. Yin. Y.-G., Xu, W.-Q., DeGuzma~ R.N., Shen, Y.-F., Suib, S.L., O'Young, C.-L., in Zeolites and Related Microporous Materials: State of the Art 1994, J. Weitkamp et al. Eds. pp. 1671-1676, Elsevier, 1994. 12. DeCnmnan, R.N., Shen, Y.-F., Shaw, B.I~, Suib, S.L.; O'Yom~ C.-L. Chem. Mater., 1993, 5, 1395-1403. 13. Shen, Y.-F.; Suib, S.L.; O'Young, C.-L. submitted for publication. 14. Golden, D.C., Chen, C.C., Dixon, J.B. Science, 1986, 231,717-719. 15. Tamada, O., Ymnan~to, N. Minerlt~c~ J., 1986, ~ 130-140.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
197
S y n t h e s e s a n d c r y s t a l s t r u c t u r e s of t w o " o r g a n o z e o l i t e s " K. Maeda, J. Akimoto, Y. Kiyozumi, and F. Mizukami National Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305, Japan
Synthesis conditions of two isomeric aluminum methylphosphonates were examined. A1MepO-~t crystallized from a well-dispersed mixture of pseudo-boehmite, methylphosphonic acid and water on hydrothermal reaction. Neutral or acidic organic additives also favored formation of A1MepO-[~ with crystal size enlargement in most case. Static mixing followed by aging or use ofglycolic solvent instead of water caused to form AlMepO-a. Single crystal X-ray structural analysis of the two products revealed that both compounds have unidimensional channels lined with methyl groups. Also two new phases, designated A1MepO-~ and -5, were found. 1. I N T R O D U C T I O N Much attention has been paid to metal organophosphonate with a lamellar s t r u c t u r e as i n t e r c a l a t i o n hosts or d e s i g n e d l a y e r e d m a t e r i a l s [1]. Organophosphonic acid (RPO3H2) have an organic group (R) covalently bonded with the phosphorus center and the structure of metal organophosphonate generally has similarities with that of related phosphate [2,3]. However, structures other t h a n layered type were rare so far. Aluminum [4] and copper [5] methylphosphonates, and zinc aminoethylphosphonate [6] reported recently possess three-dimensional neutral frameworks and unprecedented organically lined tunnel structures. Such three-dimensional frameworks will provide possibilities for designed channel structures. The copper and zinc compounds have so small space surrounded by organic moieties that even nitrogen molecules perhaps cannot intrude. On the other hand, both of the isomeric aluminum methylphosphonates, A1MepO-a [7] and A1MepO-[~ [4,8], reported by us have as _large channels as can adsorb 2,2-dimethylpropane with a kinetic diameter of 6.2/~. They can be called the first "organozeolites" with molecular sieving properties. Their frameworks are different from existing aluminum phosphate molecular sieves [9] because P/A1 ratio should be 1.5 to build neutral framework and each phosphorus center should be connected to only three aluminum centers via oxygen atoms. We report here the synthesis condition and comparison of these two isomeric structures.
198 2. EXPERIMENTAL 2.1. S y n t h e s i s a n d c h a r a c t e r i z a t i o n Aluminum source was pseudo-boehmite powder (PURAL SCF, Condea Chemie, 74.4 wt.% A1203, 25.6 wt.% water). Methylphosphonic acid was obtained from Aldrich. Other organic reagents were obtained from Tokyo Kasei and used without further purification. Fundamental synthesis procedures were as follows: ten mmol of pseudo-boehmite powder and 15 mmol of methylphosphonic acid were dispersed in 400 mmol of water. The mixture (1.0A1 : 1.5P : 40H20) was stirred at ambient temperature for 1 h. The suspension was hydrothermally treated using an autoclave with a teflon sleeve at 160 ~ for 48 h in an thermostated oven under an autogenous pressure. The solid product was filtered, washed with water, and air-dried. In some runs, 5 mmol of an organic additive listed in Table 1 were added to the starting mixture or 10 g of organic solvents listed in Table 2 were used instead of water. Static mixing, namely gentle pouring of water onto boehmite covered with methylphosphonic acid followed by static aging of the mixture, was also examined instead of stirring in the mixing procedure. X-ray powder diffraction analyses (XRD) were performed with a MAC Science MXP18 diffractometer. Scanning electron microscopy (SEM) images were taken on a Hitachi S-800 microscope. 2.2. X-ray c r y s t a l l o g r a p h y Large crystals suitable for single crystal X-ray diffraction study were obtained from a different composition of mixture (1.0A1 : 1.0P : 40H20, reacted at 220 ~ for 48 h) for A1MepO-a and by addition of dioxane (1.0A1 : 1.5P : 40H20 : 0.5dioxane) for A1MepO-f3. X-ray diffraction data were taken on Rigaku AFC-4 diffractometer for A1MepO-a and Rigaku AFC-7 for A1MepO-f3. Crystallographic data for A1MepO-a: space group, trigonal P31c; a=13.9949(13), c=8.5311(16), Z=6; for A1MepO-~: space group, trigonal R3c; a=24.650(2), c=25.299(5), Z=18. Refinements were based on F2; the final Rw(F2) were 0.1081 (for 1795 reflections, 104 parameters) for A1MepO-a and 0.2115 (for 2514 reflections, 312 parameters) for A1MepO-f~.
3. R E S U L T S AND DISCUSSION
3.1. Synthesis conditions Figure 1 shows XRD of samples prepared with no additive (la) and aqueous ammonia (lb). With no additive the product gives XRD pattern with an intense reflection at d=12.30 corresponding to A1MepO-~. A1MepO-~ was obtained also at 130 ~ whereas a different compound, designated A1MepO--8, giving the strongest reflection at d=9.59 became the major product above 200 ~ Addition of aqueous ammonia caused to form another product, designated AlMepO-~, of non-porous layered structure with the composition AI(OH)(O3PCH3)'H20 [10]. Products obtained with various high-boiling organic additives were listed in
199 Table 1. Most of carboxylic acids, alcohols and ethers, namely acidic and neutral additives, gave A1MepO-[3 as the main product. Glycolic acid and oxalic acid did not give A1MepO-[3 probably owing to their strong chelation of aluminum. Basic additives like q u a t e r n a r y ) 9 9 b ammonium hydroxide, aqueous ammonia always gave A1MepO-~. Quaternary ammonium halide had no effect on product. These results revealed that quaternary ammonium 5 ' io ' a'o ' 4'0 hydroxide does not work as 20 structure-directing agent like in A1PO4 system [11] but works simply as base. Figure 1 XRD of A1MepO-[3 and -~ prepared with Hydrophobic interaction be- no additive (a), and aqueous ammonia (b), respectween methyl groups must tively. assist formation of the chan- 9 : A1MepO-[3, A : A1MepO-~ nel structure of A1MepO-[3. When the starting materials were mixed statically and left standing before the hydrothermal reaction, products changed depending on standing time. When the starting mixture was hydrothermally treated immediately after the water addition, the products contained mainly A1MepO-[5 (Figure 2a). On standing of the starting mixtures for 24 or 48 h A1MepO-a was mainly formed with small amount of A1MepO-[3 ,-~, -5 (Figure 2b and 2c). The maximum A1MepO-a content of the
I
9
9
.[
9
qp
.
9
9
9
Table 1 A1MepO products obtained with various organic additives Additive
Product
no additive [3>>~ AcOH [3>>~ AcOH (1.0) ~>~ PhCOOH (5 CH3(CH2)loCOOH ~ HOCH2COOH ~,Uk (COOH)2 (0.25) Am HOOC(CH2)4COOH* ~
Additive
Product
1-BuOH [3 2-BuOH [3>>~ t-BuOH [3>~ Ethylene glycol [3>~ 1,4-HO(CH2)4OH [3>~ Dioxane [3 Dioxane (1.5) [3 18-crown-6 [3>Uk
Additive
Product
NH3 NH3 (1.5)
[N(CH3)4]O H [N(C2Hs)4]OH [N(C3H7)4]OH [N(C4H9)4]O H [N(C4H9)4]Br
Additive/A1 = 0.5 unless ratio is given in parenthesis, *Additive/A1 = 0.25 Uk: Unknown phases, Am: Amorphous,
%>>Uk
[3
200 Table 2 Products from nonaqueous solvents. Solvent
Product
Ethanol a,~ Ethylene glycol a>>~ Diethylene glycol a,~ Tetraethylene glycol 1,2-Propanediol a>~ 1,3-Propanediol a,~ 1,4-Butanediol Glycerol a>>~ Dioxane Am>~ Water Am: Amorphous
o 9
J
9 li
1
9? 9
9
..
9
,"
t--
Lt
c-
purest sample so far obtained • 9 9 is c a . 9 1 % according to 27A1 9 9A 9 A 9 9 9 A~A 9 9 J MAS-NMR. Pure and large crystals of A1MepO-a were obtained from starting mixture ,, . /~ 1 .. , a) of the composition 1.0AI : 1.0P : 40H20 reacted at 220 ~ for 1'o ' go ' 3b ' 4'o S 48 h (Figure 2d) as described 2(1 above. Careful s e p a r a t i o n from boehmite-derived mass, Figure 2 XRD of products obtained with however, was necessary and different aging time in static mixing. a) 0 h, b) 24 h, c) 48h, d) single crystals the yield was very low. Nonaqueous organic solvents prepared from composition 1.0A1 : 1.0P : 40H20 have been used to produce at 220 ~ novel materials and large crys9 : A1MepO--a, 9 : A1MepO-~, • : AIMepO--5, tals in zeolite-related material A : A1MepO-~ synthesis [12,13]. The effect was claimed to be owing to reducing amount of water in the reaction media. In our system, alcohol was effective as solvent to obtain microporous products; non-hydric dioxane gave small amount of crystalline product. Many alcohols listed on Table 2 tended to give product containing A1MepO-a in spite of mixing by stirring, although static mixing is necessary for formation of A1MepO--a in the case of water solvent. Especially, ethylene glycol and glycerol was most suitable to obtain A1MepO-a. With no additive the dimension of A1MepO-~ needle crystals ranges c a . 30-300 ~tm in length and less than 10 ~tm in width (Figure 3a). Sometimes small amount of AlMepO-~ was observed as plate crystals together with A1MepO-~. A1MepO-~ synthesized with aqueous ammonia was aggregates of plate crystals (Figure 3c). Instead of no templating ability of the acidic and neutral additives, most of them stimulate growth to give larger crystals; among them dioxane was most effective to
J
201
a)
V. I l l l l l l
b)
Him
V. ! ! [1111
el)
Figure 3 SEM Image of A1MepO products a) A1MepO-[3 with no additive, b) A1MepO-[3 with dioxane, c) A1MepO-~ with aqueous ammonia, d) A1MepO-a by static mixing.
202 give approximately ten times larger crystals than no additive (Figure 3b). Elemental analysis revealed that a considerable amount of dioxane remains in the crystals even after thorough washing with water, although a trace amount remain in products from other additives. Good fit of dioxane molecules into channels probably encourages crystal growth along the channels, i.e. in the direction of the needle. As can be seen in some SEM images the needle crystals were often observed as radial aggregates typically in spherical or conical shape or as needles twinned on the side. This indicates that nuclear growth starts from small boehmite particles dispersed in water followed by collapse of the aggregates into separate needle crystals. Crystals of AlMepO--a synthesized by static mixing were also long needles similar to A1Mel~O-~ of 50-1000 ~m in length (Figure 3d). Optimum synthesis conditions of A1MepO-a suggest that less dispersed boehmite in reaction media favors formation of this phase. Anyhow, further study is necessary to obtain full understanding of the reaction mechanism. 3.2. C r y s t a l s t r u c t u r e s Both structures have the same framework composition A12(CH3PO3)3 and contain both tetrahedral (/kiTh) and octahedral aluminum (A1oh) in the ratio of 3 : 1 as also confirmed by 27A1 MAS-NMR. The frameworks are composed of vertex-shared [A1ThO4], [A1ohO6] and [CH3PO3] polyhedra. The arrangement of Aloh is similar between the two frameworks. Each Aloh is surrounded by eight nearest A1Oh; two are located almost along the c-axis and the other are along the channel walls. The connectivity among A1Ohis different between the isomers as shown in Figure 4. In AlMepO-a [7] three-fold rotation axes parallel to the c-axis run through the positions Where every A1Oh (All in Figure 4a) locates. Therefore, there are only two independent phosphorus sites (P1 and P2). The connectivity along the c-axis is
I a
I ~ b
Figure 4 A1-P linkage of A1MepO-a and-[5. a) left, viewed along [110], b) right, viewed along [120] white circle: A1oh, black circle: methyl group
a
C
203 9
9
~l
eQ.
.~
.\
Io
\X'"
T ,
9 ;
9
Figure 5 Stereoplots of AlMepO-a (top) and-13 (bottom) viewed along [001]. whitecircle: A1oh, black circle: methyl group based on the stacking of 6343 polyhedra sharing vertex A1Oh(Figure 4a). Along the channel wall two A1Ohare connected by single linkage. In A1MepO-~ [8], three-fold screw axes parallel to the c-axis run beside the positions where every Aloh (All in Figure 4b) locates. Each A1oh is connected with two AlOh almost along the c-axis by linearly fused triple four-rings in which the two extreme rings are on the same side of the central four-ring (Figure 4b). Each AlOh is connected with further two Aloh among the remaining six neighbors by similar triple four-rings in which the two extreme rings are on the opposite sides of the central ring. Figure 5 showed stereo drawing of the frameworks along the c-axes. Unidimensional straight channels are running along the c-axes. The oxide frameworks surrounding the main channels contain triangular 18-ring as large as VPI-5 [14], a representative large pore aluminophosphate. So far reported phosphonates [5,6] and phosphites [15,16] with channel structures lined with organic groups or hydrogen atoms connected to phosphorus centers contained 12-rings at largest. The channels of both compounds are lined with methyl groups. In A1MepO-a and -~, large channels composed of 18rings allow to leave still large space where the inner wall are covered with methyl groups.~ A cross section of a channel of A1MepO-a looks triangle, a side of which is c a . 7.0 A. In A1MepO-~ a channel section is of similar size as A1MepO-a and looks
204 rather rounded because the channel wall are more rugged and twisted along the triangular channel than A1MepO-a.
CONCLUSIONS Optimization of synthesis conditions including organic additive, reaction solvent, mixing and aging conditions enables selective synthesis of two structural isomers of aluminum methylphosphonate "organozeolites". Organic additives do not work as template like A1PO4 synthesis but affect crystal size. Channel structures lined with organic groups are confirmed by single crystal X-ray structural analysis.
REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
T.E. Mallouk and H. J. Lee, Chem. Edu., 67 (1990) 829. Y.P. Zhang and A. Clearfield, Inorg. Chem., 31 (1992) 2821. G. Cao, H.-G. Hong and T. E. Mallouk, Acc. Chem. Res., 25 (1992) 422. K. Maeda, Y. Kiyozumi, F. Mizukami, Angew. Chem. Int. Ed. Engl., 33 (1994) 2335. J. Le Bideau, C. Payen, P. Palvadeau, B. Bujoli, Inorg. Chem. 33 (1994) 4885. S. Drumel, P. Janvier, D. Deniaud, B. Bujoli, J. Chem. Soc., Chem. Commun., (1995) 1051. K. Maeda, J. Akimoto, Y. Kiyozumi, F. Mizukami, Angew. Chem. Int. Ed. Engl., 34 (1995) 2335. K. Maeda, J. Akimoto, Y. Kiyozumi, F. Mizukami, J. Chem. Soc., Chem. Commun., (1995) 1033. J.A. Martens and P. A. Jacobs, in Advanced Zeolite Science and Applications, eds. J. C. Jansen, M. StScker, H. G. Karge and J. Weitkamp, Elsevier, Amsterdam, 1994, p. 653-685. K. Maeda, to be published. E.M. Flanigen, B. M. Lok, R. L. Patton and S. T. Wilson, inNew Developments in Zeolite Science and Technology, eds. Y. Murakami, A. Iijima and J. W. Ward, Elsevier, Amsterdam, 1986, p. 103. Q. Huo, R. Xu, S. Li, Z. Ma, J. M. Thomas, R. H. Jones and A. M. Chippindale, J. Chem. Soc., Chem. Commun., (1992)875. A.F. Kuperman, S. Nadimi, S. Oliver, J. Garces, M. M. Olken and G. A. Ozin, Nature, 365 (1993) 239. M.E. Davis, C. Saldarriaga, C. Montes, J. Garces and C. Crowder, Nature, 331 (1988) 698. R.E. Morris, M. P. Attfield and A. K. Cheetham, Acta. Crystallogr. Sect. C, 50 (1994) 981. M. Sghyar, J. Durand, L. Cot and M. Rafig, Acta. Crystallogr. Sect. C, 47 (1991) 2515.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
205
ERS-8: a N e w Class of Microporous A l u m i n o s i l i c a t e s
Giovanni Perego, Roberto Millini, Carlo Perego, Angela Carati, Giannino Pazzuconi and Giuseppe Bellussi Eniricerche S.p.A., Via F. Maritano 26, 1-20097 San Donato Milanese (Italy)
The synthesis of micro-mesoporous aluminosilicates via gelation of a reaction mixture containing Si(OC2H5)4, AI(OC3H7)3, C2HsOH, H~O and alkali-free NR4-OH (R = C3H7, C41-I9, CsH m C8H~3) is described. X-ray amorphous mesoporous aluminosilicates (MSA) or microporous alominosilicates, characterized by a broad peak in the low angle region of XRD pattern (ERS-8), are obtained, depending on the NR4-OI-I/SiO2molar ratio and on the number of C atoms in the R groups. A structural model is proposed concerning the arrangement of the NR4§ cations within the gel, which accounts for the experimental data.
1. INTRODUCTION M41S constitutes a well known class of ordered mesoporous materials which is formed in hydrothermal conditions, starting from an aqueous solution containing silica and alumina sources together with CiH2i§ N§ cations (i > 7; usually 12 or 16) [ 1,2]. The same procedure can be used for preparing mesoporous materials containing oxides of V [3], Ti [4], Mn [5], W [6], Sb [6]. A mechanism of formation of M41S has been proposed, based on the templating effect of a liquid crystal structure formed in the reaction mixture by tetraalkylammonium cations, due to their surfactant properties [1,2]. For the MCM-41 derivative, Stucky et al. [7] suggested the initial formation of a lamellar structure of surfactant molecules interposed between sheets of silica oligomers, which transforms into the hexagonal mesophase as polymerization of silica proceeds. After the discovery of M41S-type compounds many attempts have been made to synthesize new ordered porous materials. We claimed the possibility to synthesize mesoporous X-ray amorphous aluminosilicates (MSA) with a narrow pore size distribution [8,9]. The procedure initially adopted was based on the gelation, eventually followed by a hydrothermal treatment at 180~ of an alkali-free mixture of silica and alumina sources in the presence of (C3H7)4N-OH. Successively, an easier and more reproducible procedure was adopted, based on
205 refluxing a hydroalcoholic solution of the silica and alumina sources, in the presence of (C3H7)4N-OH as gelating agent [10]. Though it is known that the textural properties of amorphous aluminosilicates can be controlled by gel formation in acidic [11] or slightly basic media [12], the synthesis of MSA is performed under strongly basic conditions in order to stabilize tetrahedral A1 [13]. This gives MSA interesting properties in acid-catalyzed reactions [8-10]. Starting from the formation mechanism postulated for MSA, we investigated the synthesis parameters which in principle could modify the arrangement of NR4§ cations in the gel, arriving at the discovery of a new class of materials (named ERS-8). The paper deals with the synthesis and characterization of ERS-8's compared to MSA's. 2. EXPERIMENTAL The materials investigated were prepared using Si(OC2H5)4 (Dynasil-A, Nobel), AI(i-OC3H7) 3 (Fluka) or AI(8-OC4H9) 3 (Fluka), aqueous alkali-free NR4-OH (R - C2H5, C3H7, C4H9, C5Hll, C6H13), ethanol and distilled water. The molar ratios EtOH/SiO 2 = 8, H20/SiO 2 = 8, SiO2/A1203 - 50 were kept constant for all the runs, varying the molar ratio NR4+/SiO2 in the range 0.03 to 0.40. In a typical preparation, AI(/-OCsH7) 3 is dissolved in aqueous NR4-OH at 60~ to which an ethanolic solution of Si(OC2H5)4 is added under vigorous stirring. When using (C6H13)4N-OH (40% wt in water), this is diluted with EtOH and then with the required amount of water before being mixed with a solution of Si(OC2Hs) 4 and AI(s-OC4H9) 3. In both procedures the initial clear solution becomes a viscous gel in a few minutes. After about 15 hours aging at room temperature, the gel is dried at lOO~ and calcined at 550~ during 8 hours in air flow. The solid products were characterized by: - X-ray powder diffraction (XRD) on a computer controlled Philips diffractometer using CuKa radiation (~ = 1.54178 A). In the angular range 1< 2(} < 10 ~ the data were collected stepwise with 1/6~ receiving slit set. The position and breadth of the Bragg peak were accurately determined by means of the FIT routine contained in the software package DIFFRAC (from Siemens), assuming a Split-Pearson VII function for the peak profile. - Thermogravimetric analysis (TGA), with a Mettler TG50 thermobalance controlled by a Mettler TC 3000 microprocessor, running in the range 25 - 900~ heating rate 10~ and 300 ml/min air flow. - Nitrogen physisorption on a computer controlled Fisons Sorptomatic 1990 system. The calcined samples were degassed under high vacuum (~ 10.5 torr) at 300~ for 4 hours. The data were analyzed with the Horvath-Kawazoe method [14]. 3. RESULTS AND
DISCUSSION
As previously reported [8-10], X-ray amorphous aluminosilicates (MSA), characterized by a narrow pore size distribution, can be obtained by using
207 (C3H7)4N-OH as a gelling agent. The same type of materials can be obtained by using other NR4-OH compounds (R = C4H9, C5Hll, Cell13), provided the synthesis is carried out with low NR4-OH/SiO 2 molar ratio [8,10,15]. When performing gelation with higher NR4-OH/SiO 2 molar ratios, we obtained materials different from MSA, characterized by the presence of a peak in the low angle region of the XRD pattern and by a very narrow pore size distribution, with pore radius constantly lower than 20 A [15]. Table 1 Characteristics of the synthesized materials. Sample a NR4-OH/SiO 2 Vol. fraction (%)b
Phase type
d (A) ~
dried d
calcined e
C3(1)
molar ratio 0.403
NR4§ 75
SiO 2 25
ERS-8
19
56
C3(2)
0.253
65
35
ERS-8
20
54
C3(3)
0.197
60
40
ERS-8
18
51
C3(4)
0.153
53
47
ERS-8
19
47
C3(5)
0.113
45
55
MSA
C3(6)
0.105
42
58
MSA
C3(7)
0.093
40
60
MSA
-
C6(1)
0.256
78
22
ERS-8
25
4O
C6(2)
0.108
60
40
ERS-8
24
32
C6(3)
0.082
53
47
ERS-8
26
31
C6(4)
0.068
48
52
ERS-8
25
32
C6(5)
0.062
45
55
MSA
C6(6)
0.047
41
59
MSA
C6(7) C6(8)
0.038 0.034
35 33
65 67
MSA MSA
C6(9)
0.032
30
70
MSA
C6(10) 0.030 28 72 MSA (a) The labels C3(X) and C6(X) stand for materials prepared with R = C 3 H 7 and C6H13, respectively. (b) Referred to dry gels; assumed mass densities of 0.9 and 2.0 g/cm 3 for NR4+ and SiO2, respectively. (c) Bragg distance in XRD pattern. (d) At 100~ in air. (e) At 550~ in air during 8 hours. Starting from these results, a systematic varying both the NR4-OH/SiO 2 ratio and the the other synthesis parameters constant (see volume fractions of SiO 2 and organic matter
investigation was carried out by chain length of R groups, keeping Experimental). By referring to the in the dried materials (derived by
208 assuming for both components reasonable values of mass density, see Table 1), it appears very clearly that a threshold exists around 50% volume fraction of NR4§ Below this threshold, MSA is formed while above this ERS-8 is formed, independent on NR4§ used in the synthesis (Figure 1) In a previous paper, a detailed characterization was given for MSA prepared with (C3H7)4N-OH [9]. To better describe the properties of ERS-8's compared to those of MSA's, attention will be focused on the materials prepared with R = C3H 7 and Cell13 (Table 1) considering that the same conclusions are valid for materials obtained with R = C4H9 and CsHlr The only exception is R = C2H5 which gives MSA over the whole range examined. A peak is observed in the XRD pattern of as-synthesized, 100~ and calcined ERS-8's; however, the position of the peak varies significantly as a function of the thermal history of the sample (Figure 2). i
9
i ERS-8
MSA
D
!i5
I,m i
3
A
l'o 2~ 3'0 4~ 50 6'0 7o ~'o ~, ioo
0
NR4 Vol.fraction(%)
Figure 1. Relationship between the NR4+ volume fraction in wholly dried precursor and the type of material obtained ~ I S A , @ERS-8).
4
2
6
8
2-Th eta [o]
10
Figure 2. Low angle region of XRD patterns for as-synthesized (A), dried at 100~ (B) and calcined at 550~ (C) ERS-8 (sample C6(2)), compared to that of MSA (D) (sample C6(7)). 40
N
"
i /
\
:x
/
i
"|
""
l '~-
34
~:ir-dried
.~
32
-o
263028 24
-
-
-
i
-
-
i
.
.
.
.
f
.
-
-
i
"
-
Figure 3. TG ( - - ) and DTG (.... ) curves of dried C6(2) sample.
as-synthesized gel
38
36
22
gel
~""B'"r"~
1()0
-
2()0
3()0
Temp. (~
400
5(}0
600
Figure 4. Variation of d as a function of calcination temperature, for ERS-8 sample C6(2).
209 The FWHM (3 - 4 ~ 20) of the peak is quite large and certainly indicative of very low structural order; however, it is significantly lower than the FWHM (8 10 ~ 20) of the peak (observed in both MSA's and ERS-8's) at ca. 23 ~ 20, due to the Si(A1)/Si(A1) pair correlation. Whether the low angle peak has to be considered a Bragg peak may be matter of discussion. In any case, it seemed reasonable to consider the related Bragg distance, d. The trace of the TGA curve, shown in Figure 3 (the same features were observed in all the dried gels), is characterized by several steps of weight loss, which are interpreted as in the following. Below 150~ residual H~O and C2HsOH are lost, while the organic matter is eliminated in two well defined steps. The first one (at ca. 270~ corresponds to the elimination of NR4-OH molecules simply "embedded" in the aluminosilicate matrix; in the second one (at ca. 350~ the elimination of NR4+ acting as counterions to the tetrahedrally coordinated A1 ions occurs. As a matter of fact, both in MSA's and in ERS-8's, the moles of NR4§ lost at higher temperature nearly correspond to the total moles of AI present in the material. The slight weight loss observed at T > 450~ probably corresponds to the combustion of some coke formed during the TG analysis and/or to the elimination of H20 deriving from condensation of surface silanol groups. Portions of C6(2) dried gel were treated in the T G furnace at the temperatures indicated in Figure 3. The X R D peak is observed for all the samples and the related Bragg distance, d, varies as a function of the treatment tempoerature (Figure 4). The value of d decreases from ca. 38 A (wet gel) to ca. 24 A in the totally dried sample (150~ and increases up to ca. 32 ik after calcination at 550~ It is worthy to note that, for dried samples, the value of d is practically independent of the NR4-OI-I/SiO 2 molar ratio, but depends on the number of C atoms of the R groups (Table 1).
9 o
9
o
9
MSA
9
o 0"6
~
0.4
L/.~':":"...... ~RS~
~
.eo.eO-eo''
9
9
9
ERS-g
,,P o
04
o g
o o
o
9
MSA
mO~ 0 0 ~0.2 o~d~
. . . . . . . .
i i0
.
.
.
Pore radius
. (~)
.
. 100
.
.
. . . . . . .
. 1000
o~
| 10
,j 100 Pore radius
. . . . . . . . 1000
(,~)
Figure 5. Cumulative pore volume of MSA Figure 6. Cumulative pore volume of MSA (sample C3(5)) and ERS-8 (sample C3(3)) (sample C6(7)) and ERS-8 (sample C6(2)) synthesized with (C3H7)4N-OH. synthesized with (CsH13)4N-OH. The pore size distribution is narrow in both materials. However, while in MSA there is a predominance of mesopores, in ERS-8 only micropores (r in the range 3
210
10/k) are present, independent on NR4 § used (Figures 5 and 6). Both MSA's and ERS-8 are characterized by specific surface area in the range 700 - 1000 m~/g. Pore volume of M S A is ranging from 0.5 to 0.7 cmS/g (C3 series) and from 0.4 to 0.5 cm3/g (C6 series);for ERS-8, values of 0.3 - 0.4 cm3/g (C3 series) and 0.4 - 0.5 cmS/g (C6 series) are measured. The following model is given, which reasonably accounts for the experimental data. In the gelation step, the formation of clusters of NR4 § cations is postulated. The picture shown in Figure 7 represents a possible structure for these clusters, where the alkyl chains in liquid-like conformation are arranged parallel to one another; the O H anions solvated by ethanol and water molecules are placed on the border of the alkyl chains assembly. In MSA-forming mixtures, the growth of these clusters is limited because of the excess of silica (volume fraction exceeding that of NR4 § see Table 1) which tends to form a three-dimensional aluminosilicate structure all around them (Figure 8a). As a consequence, the ethanol- and water-containing NR4 § domains become the pore precursors of M S A (Figure 8b) and their dimensions control the pore size of the material. In an ERS-8-forming mixture, due to the excess of NR4 § component with respect to silica(see Table 1), the NR4 § clusters are expected to grow much more in the directions perpendicular to that of the elongation of the alkyl chains. Practically, the formation of monolayers of NR4 § solvated by ethanol and water molecules, is admitted, on the surface of which aluminosilicate sheets grow. According to this hypothesis, the ERS-8 precursor has to be considered essentially as a layered-type material. The layers so formed should be embedded in ethanol and water (which constitute about 90% of the total volume of the gel) with occasional, if any, layers stacking. Therefore the d value of 38 A would represent the average distance between the aluminosilicate sheets grown on the two surfaces of each NR4 + layer (Figure 8c). It is interesting to note that the formation of a layered-type intermediate has been proposed also for M C M - 4 1 [7]. After drying, the structure should collapse with a resulting packing of the layers roughly parallel to one another. The decrease of d value for the dried gels is well accounted for by the removal of most of ethanol and water molecules from the interlayer region (Figure 8d). Consistently with the different length of the alkyl chains of NR4 + cations, the d values observed for C3 samples (18 - 20/k) are lower than those observed for C6 samples (24 - 26/k, see Table 1). However, according the poor order of the structure, only occasional correlation exists among the layers along the direction parallel to the stacking direction (no correlation at all in the other directions). As the organic matter is removed during calcination, the two aluminosilicate sheets grown on each NR4 § layer will condense forming double sheets; these constitute the building blocks which form a three-dimensional structure (through inter-sheet condensation of silanol groups), maintaining the plane of the sheets roughly parallel to each other. Due to the likely difference in the dimensions (mainly length and width) of the sheets, a regular assembly cannot occur and pores are inevitably formed. In Figure 8e a simplified, very schematic picture of the 3D structure proposed for ERS-8 is shown. According to this model, pore -
211
dimensions in the direction perpendicular to the plane of the sheet (arrows in Figure 8e) is controlled by the double-sheet thickness, which is expected to be much more regular than length and width (for this reason, a broad distribution of pore dimensions is expected along the directions parallel to the sheet plane). Since the average thickness of a single sheet is estimated to be around 5 A (by considering the d value together with the volume fractions of
- Nl~luster
MSA
ERS4~
3s~ a
I dryingand ! calcination
~ dr~.g
d
Figure 7.
Proposed structure for a
solvated (C2Hs-OH + H20)(C6H13)4 N+
/calcination
Figure 8. Schematic model proposed for pores formation in MSA and ERS-8
clusters (number of molecules arbitrary). NR4§ and S i O 2 ) , w e should expect pore dimensions of ca. 10/k (one double sheet), ca. 20/~ (two double sheets), etc. The curves of Figures 5 and 6 really agree with the presence of pores of ca. 10 and ca. 20/~ in ERS-8's. Inter-sheet distances can be identified within the structure (arrows in Figure 8e), not correlated to each other, which account for the broad Bragg peak observed in calcined ERS-8's. These distances correspond to multiples of the average thickness of the double sheet. The d values observed for calcined ERS-8 (Table 1), agree with an average of 3-4 double sheets (C6 series) and 4-5 double sheets (C3 series). Because the pore size distribution is practically identical for the two materials, the larger spacing observed for C3 series may be accounted for by a more dense
212 packing of the sheets. The lower value of total pore volume observed for ERS-8 in C3 series, relative to that in C6 series, agrees with the above hypothesis. 4. CONCLUSIONS Gelling a solution containing silica and alumina sources with an alkali-free tetraalkylammonium hydroxide (NR4-OH, R = C3H 7, C4H 9, CsH n, C8H13) leads to the formation of mesoporous (MSA) or microporous (ERS-8) aluminosilicates. In particular, the formation of ERS-8 occurs when the volume fraction of NR4 § exceeds that of silica-altunina, independent on the length of R alkyl chain. ERS-8's represent a new class of aluminosilicates characterized by a narrow pore size distribution in the range of micropores. Due to their acidic properties, these materials are suitable for applications in acid-catalyzed reactions. REFERENCES 8
Q
~
*
5. 6. 0
0
1
10. 11. 12. 13. 14. 15.
C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature 359 (1992) 710. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. I~M. Reddy, I. Moudrakovski and A. Sayari, J. Chem. Soc., Chem. Commun., 1059 (1994) P.T. Tanev, M. Chibwe and T.J. Pinnavaia, T.J., Nature 368 (1994) 321. D. Zhao and D. Goldfarb, J. Chem. Soc., Chem. Commun., 875 (1995) Q. Huo, D.I. Margolese, U. Ciesla, P. Feng, T.E. Gier, P. Sieger, R., Leon, P.M. Petroff, F. Schuth and G.D. Stucky, Nature, 368 (1994) 317. A. Monnier, F. Schuth, Q. Huo, D. Kumar, D. Margolese, R.S. Maxwell, G.D. Stucky, M. Krishnamurty, P. Petroff, A. Firouzi, M. Janicke and B.F. Chmelka, Science 261 (1993) 1299. G. Bellussi, M.G. Clerici, A. Carati and F. Cavani, US Patent No. 5 049 536 (1991). G. Bellussi, C. Perego, A. Carati, S. Peratello, E. Previde Massara and G. Perego, Stud. Surf. Sci. Catal. 84 (1994) 85. C. Perego, S. Peratello and R. Millini Eur.Patent No. 659,478 A1 (22.12.1993) T. Pecoraro, U.S. Patent No. 4,988,659 (1991). M.R. Manton and J.C. Davidtz, J. Catal. 60 (1979) 156. J. Livage, Stud. Surf. Sci. Catal. 85 (1994) 1. G. Horvath and I~ Kawazoe, J. Chem. Eng. Jpn. 16 (1983) 470. G. Pazzuconi, G. Bassi, C. Perego, G. Bellussi, R. Millini and G. Perego, It. Patent No. 94A001399 (1994).
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
213
S y n t h e s i s a n d c h a r a c t e r i z a t i o n of l e v y n e t y p e zeolite o b t a i n e d f r o m gels w i t h different SIO2/A1203 ratios C. V. Tuoto a, J. B. Nagyb and A. Nastro a aDepartment of Chemical Engineering and Materials, University of Calabria, Arcavacata di Rende, 87030 Rende (CS), Italy bUnit~ de R.M.N., University of Namur, Rue de Bruxelles 61, B-5000 Namur, Belgium. Levyne type zeolites have been successfully synthesized from gels 4.5Na206MeQI-xAI203-30SiO2-500H20 with 0.6_<x_<3, at 150~176 The 29Si spectra show unambiguously the presence of two different crystallographic sites in the structure. The combined 13C-NMR and TA data allowed one to interpret the nature of two different types ofMeQ + ions in the levyne channels. 1. INTRODUCTION The structure of the levynite, a natural zeolite discovered in 1825 [1] can be described by a sequence of single six member rings [2]. The unit cell has a trigonal symmetry and contains 54 tetrahedral sites [36 T1 sites and 18 T2 sites]. The chemical composition of the levynite is Ca3(Al18Si36Olo8)50H20 and the diameter of the pores is 4.8 x 3.6A [2]. Its use as a molecular sieve and as a catalyst for the transformation of methanol to low molecular weight olefins has been recently demonstrated [3]. Although the levynite was one of the first discovered zeolite, its first synthesis was only reported in 1969, the zeolite ZK-20 was obtained from an aluminosilicate gel containing DABCO (1-Methyl 1-Azonia 4-azabyciclo [2,2,2] Octane) [4]. Later the synthesis of similar structures were reported such as Nu-3 [5], LZ-132 [6] and LZ-133 [7], b a s e d on t h e use of m e t h y l q u i n u c l i d i n e (1Methylazabicyclo[2,2,2]octane). The structure of LZ-133, not yet resolved, was claimed as being analogous to the levynite structure, although the X-ray pattern is quite different from that of levynite [7]. Zeolite ZSM-45 also of levynite structure was obtained in presence of 2-hydroxyalkyltrialkylammonium chloride [8]. It has to be noted that synthetic levyne-type zeolites all have a quite high Si/A1 ratio, because they were synthesized in presence of organic molecules. Quite recently, different hetero-atoms have also been introduced in the levyne structure [9,10]. Finally, levyne was also successfully obtained in fluorine containing media [11]. As the synthesis of levyne type zeolites is essentially described in the patent literature, it is interesting to explore the field of crystallization, the influence of the SiO2/Al203 ratio in particular [12,13]. In the present work are reported the results on the synthesis and on the characterization of levyne-type zeolite obtained from sodium aluminosilicate gels
214 containing methylquinuclidine iodide (MeQI) as organic compound and a variable SiO2/Al203 ratio. 2. E X P E R I M E N T A L
As the amount of sodium and methylquinuclidine iodide (MeQI) were previously optimised in the initial gels [12], in this study the relative amount of A1 was varied following=.. 4.5Na20-6MeQI-xA1203-30SiO2-500H20 with 0.6 < x< 3 -
-
n
The hydrogels were prepared by mixing the reagents in the following order: 30% aqueous solution of NaOH (pellets RPE, Carlo Erba), MeQI synthesized from quinuclidine (Aldrich) and iodomethane (Aldrich) following refs. 6 and 7, AI(OH)3 (dry gel, Pfaltz and Bauer), distilled water and SiO2 (fumed silica, Serva). The so obtained homogeneous gel mixtures were heated in sealed Teflon autoclaves (8 cm 3) at 150~ 170~ and 190~ in static conditions. A ~ r programmed periods of time, the autoclaves were quenched in cold water and the solid product filtered, washed with cold water and dried at 100~ for 12 hours. The gels, after 48 h of ageing at room temperature, were dried at 100~ for 24 h. The nature of the solid and the degree of crystallinity were determined by x-ray powder diffraction using a Philips PW 1730/10 diffractometer equipped with a PW 1050/70 vertical goniometer (CuKal). The reference sample for quantitative measurement were obtained from the final crystalline products treated with ultrasounds in order to separate the well crystallized material from the remaining amorphous gel. Thermal analyses (DSC, TG and DTG) of dried gels and of the crystalline products were made on a Netzsch STA 409 thermal analyzer scanning from 30~ to 800~ at a rate of 10~ under a 15ml/min air flow. The size and shape of the crystallites were measured by SEM on a JEOL JSMT 330A microscope. The chemical analysis was made by atomic absorption. The NMR spectra were recorded on either a Bruker MSL 400 or a Bruker CXP 200 Spectrometer. For 29Si (39.7 MHz) a 6.0~ts (0= ~/2) pulse was used with a repetition time of 6.0 s. For 13C (50.3 MHz), 5.0~ts (0=~/2) pulses, a single contact sequence with 5.0 ms contact time and a recycle time of 4.0s were used. For 27A1 (104.3 MHz), a 1.09s (0= ~/12) was used with a repetition time of 0.2s. 3. RESULTS AND DISCUSSION The crystallization field obtained at various temperatures and various SiO2/Al203 ratios is illustrated in Figure 1. Pure levynite crystals can be obtained at low temperature (150~ for SiO2/Al203 ratios up to 40, while at high temperature (190~ gels of only low SiO2/Al203 ratios can lead to pure levyne.
215
9
O
190 "
I
'
I
k
'
I
'
I
'
I
#"
~LEV+LZ133
O
d
~LEV+~
+Q
170
-
LEV
150 9
I
9
10
I
9
I
,
I
30
i
I
,
50
8iO2/A1203 ratio Fig. 1 Crystallization field of various crystalline phases obtained after 10 days of synthesis from 4.5Na20-6MeQI-xAI203-30SiO2-500H20. The absence of amorphous phase in the crystallization field underlines the importance of MeQ + in the formation of either LEV or LZ-133. Finally, at high SiO2/Al203 ratios, the more stable quartz phase also starts to form. Figure 2 shows the crystallization curves for systems with different x values at 150~ The induction rate (1/tind in h-l) and crystallization rate (R in % day "1) are determined from Figure 2. If log 1]tind and logR are plotted as a function of log x a linear correlation is obtained with a negative slope of-l.4. It seems thus that the amount of A1203 in the gel has a great negative influence on both the induction and crystallization rate.
100 80 60
D x=3.0 9 x=2.0 o x=l.5 9 x=l.2 A x=l.0
-r-0
~
40
~
20 0
A
0
5
10
x=0.8
15
Time, days Figure 2. Crystallization curves oflevyne as a function of x at T=150~ from gels 4.5Na20-6MeQI-xA1203-30SiO2-500H20.
216 Table 1. Physicochemical characterization of final crystalline levyne samples obtained at 150~ from the gels 4.5Na20-6MeQI-xA1203-30SiO2-500H20. Si/A1 gel 5
Si/Al H20/u.c 6.8
5.7
NaJu.c 2.5
Al/u.c MeQ+tot/u.c 7.2
4.8
MeQ+/u.c MeQ+/u.c DSC=480~ DSC=580~ ....
4.8
7.5
8.7
2.7
1.2
5.8
4.9
....
4.9
10.0
10.7
4.6
0.8
4.8
6.6
2.0
4.6
12.5 15.0
14.1 15.0
3.7 2.5
0.7 0.9
3.7 3.5
6.7 7.1
2.9 3.5
3.8 3.6
17.5
20.5
3.8
0.9
2.6
7.1
3.7
3.4
e
X=0.8
X O
X=l.O
x=1.2 x=1.5 e n
x=2.0
d O
X=3.0 0
200
400
600
800
TEMPERATURE, ~ Figure 3. DSC curves of crystalline levyne phases obtained at 150~ from gels 4.5Na20-6MeQI-xAI2 O3-30SIO2-500H20.
217 Table I shows the physicochemical characterization data of the final crystalline levyne samples. It is shown that the aluminium incorporation in the structure is quite effective, the Si/A1 ratios in the initial gels and in the final samples are quite close. The amount of Na + ions is quite low and decreases with decreasing A1/u.c. Oppositely, the total amount of MeQ + increases with decreasing Al/u.c. It is interesting to note that the thermal decomposition of MeQ + ions shows a low temperature peak at ca. 480~ and a high temperature peak at ca. 580~ (Figure 3). It can be interpreted, as it was previously shown for TPA + in ZSM-5 [15] or MeQ + in levyne of high Si]A1 ratio [16] , that the 480~ peak is due to the decomposition of MeQ + neutralizing SiO- defect groups, while the 580~ peak is due to the decomposition of MeQ + neutralizing (SiOA1-) negative charges. For the first two samples, with the highest A1 contents, only the high temperature peak is detected and the A1/u.c. is equal to (MeQ + M)/u.c. (Table 1). As the Al/u.c. is decreasing, the low temperature peak appears and its amount increases with decreasing Al/u.c. In these cases the sum of the amount of high temperature MeQ and the amount of M/u.c. is always higher than the Al/u.c. (Table 1), showing the presence of defect groups in the structure. The presence of these defect groups is confirmed by the high resolution solid state 29Si-NMR spectra of levyne samples. Figure 4 and Table 2 show the 29SiNMR results. The NMR line at-115 ppm can be tentatively assigned as stemming from Si(OA1)configurations ofT2 sites. The-108.5 ppm is then assigned to Si(1A1) configuration on T2 and to Si(0A1) of sites T1. The -103.2 and -97.3 lines are s u p p o s e d to s t e m , r e s p e c t i v e l y , from Si(2A1)T2+Si(1A1)T1 and Si(3A1)T2+Si(2A1)T1. In addition, a rather large contribution of the defect groups SiOM and SiOM2 (M=Na and/or H) can be found in lines -103.2 and -97.3 ppm respectively. Indeed, if the spectra of the precursor samples are compared with those of the samples heated at 700~ in air, a notable decrease in these two peaks are observed. The spectral decomposition of both the precursor and heated sample could allow one to compute the Si/A1 ratio and the amount of SiOM defect groups. Work is in progress to quantify the 29Si-NMR spectra. The 27Al-NMR spectra show the presence of tetrahedral A1 in structure. The thermal treatment in air at 700~ decomposes some of the tetrahedral AI and an extra-framework octahedral A1 appears in the spectra. The size and shape of the levyne crystallites of as made samples are illustrated in Figure 5 and 6. It can be seen that the size of the crystal increases with increasing A1 content in the gel. As the nucleation and the crystallization rates are decreasing with increasing A1 concentration, less nuclei are formed in the beginning of the reaction with high A1 contents, leading to the formation of larger crystals. Methylquinuclidinium ions (MQ +) are incorporated intact in the zeolite channels. The NMR lines at 58.6 and 24.9 ppm stem from CH2 groups at position 2 and 3, respectively, the 54.6 ppm line is assigned to the methyl group and the 18.8 ppm line to carbon 4 (CH group). If the spectra are taken in quantitative conditions, the relative intensities confirm these contributions. The chemical shifts of the occluded MeQ + ions are slightly influenced with respect to the solid state MeQI spectra. Indeed the chemical shifts in the latter case are
218 Table 2. 29Si-NMR results of crystalline precursor levyne samples syntesized at 150~ from gels 4.5Na20-6MeQI-xA1203-30SiO2-500H20. x
d(ppm)
Io(%)
d(ppm)
Io(%)
d(ppm)
Io(%)
d(ppm)
Io(%)
3.0 2.0 1.5 1.2 1.0 0.86
-96.7 -98.5 -98.7 -98.7 -98.7 -98.2
11.3 10.2 16.6 13.1 19.8 18.9
-103.0 -103.2 -103.3 -103.6 -103.7 -103.6
42.0 31.7 27.8 30.7 24.5 26.6
-108.8 -108.7 -108.6 -108.6 -108.6 -108.5
34.1 41.1 41.1 38.9 37.5 37.6
-115.3 -115.3 -115.3 -115.3 -115.3 -115.2
12.6 17.0 14.5 17.3 18.2 16.6
/
-90
-100
-110
-120
ppm Figure 4. MAS 29Si-NMR spectra of precursor levyne (a) and of calcined (b) sample up to 700~ in air (x=10). C2:56.7 ppm; C3:25.9 ppm; C1:19.9 ppm and CH3:54.7 ppm. The chemical shift of the methyl group ofMeQ + ions remains constant (54.4 ppm); a low field shift is observed for C-2 carbon (58.6 ppm) and a high field shift is observed for both C-3 and C-4 carbons (24.9 and 18.8 ppm, respectively). In order to identify the nature of both the low and high temperature peaks in DSC curves both 13C-NMR and mass spectrometry measurement were carried out on samples partially calcined up to 550~ It is shown unambiguously, that after the partial calcination in N2, part of the MeQ + ions is still intact in the
219
structure and the chemical shifts did not change. The spectra also show the presence.of aromatic compounds, stemming from the decomposition of the MeQ + ions characterized by the first DSC peak. Some pyridine could also be formed and a large amount of benzene can be present among the aromatic products. As a conclusion, one can say that the high temperature DSC peak contain both the MeQ + ions linked to the presence of framework negative charge, i.e. (SiOA1)-, and the decomposition products stemming from the MeQ + ions counterion to SiO" defects groups.
:.
.,,
L Figure 5 SEM micrographs oflevyne crystals obtained with SiO2/Al203 ratio 25. ..
.
,.
9
,
7,
b Figure 6 SEM micrographs oflevyne crystals obtained with SIO2/A1203 ratio 10.
220 4. C O N C L U S I O N S Highly crystalline levyne type zeolite samples are obtained from aluminosilicate gels at 150~176 in presence ofmethylquinuclidinium iodide (MeQI). The MeQ + ions are incorporated intact in the zeolitic channels and neutralize either the (SiOA1)- framework negative charges and/or the SiO" defect groups. The presence of two crystallographically different sites is confirmed by the 29Si-NMR spectra. REFERENCES
1. G. Gottardi, E. Galli, Natural Zeolites, (eds) Springler-Verlag, Berlin 1985 192. 2. W. M. Meier, D. H. Olson, Atlas of Zeolites Structure Types, ButterworthHeinemann (1992) 114. 3. Z. Tvaruzkova, M. Tupa', P. Jiru', A. Nastro, G. Giordano, F. Trifiro', Catalysis Letters, 2 (1989) 369. 4. G. T. Kerr, US Patent No 3 459 676 (1969). 5. G. D. Shorte, T.V. Wittham, Eur. Patent No 0 040 016 (1981). 6. T. R. Cannan, M. T. Brent, E. M. Flanigen, Eur. Patent Appl. No 0 091 048 A1 (1983). 7. M. T. Brent, M. T. Lok, T. R. Cannan, E. M. Flanigen, Eur. Patent Appl. No 0 091 049 A1 (1983). 8. E. J. Rosinski, M. K. Rubin, Eur. Patent Appl. No 0 107 370 (1983). 9. G. Bellussi, A. Carati, M. Millini, Ital. Patent Appl. 2 289 9A (1988). 10. A. Grunewald-Luke, H. Gies, Microporous Materials 3 (1994) 159. 11. P. Caullet, J. Patarin, A. C. Faust, in Proc. III Convegno Nazionale Scienza e Tecnologia delle Zeoliti, R. Aiello (ed), Cetraro, Italy 28-29 Sett. 1995. p.55, and P. Caullet,J. L. Guth, A. C. Faust, J. L. Loly, F. Raatz, FR. Patent Appl. N~ 9 17 163 (1989). 12. C.V. Tuoto, F. Testa, A. Nastro, Materials Engineering, 5 (1994) 175. 13. C. V. Tuoto, F. Testa, R. Aiello, A. Nastro, Atti 2 ~ Convegno Nazionale sui Materiali per Ingegneria, P. G. Orsini (ed), Trento, Italy 19-21 Sett. 2 p 173. 14. C. V. Tuoto, A. Regina, J. B.Nagy, A. Nastro, in Proc. III Convegno Nazionale Scienza e Tecnologia delle Zeoliti, R. Aiello (ed), Cetraro, Italy 28-29 Sett. 1995. p113. 15. J. Hage A1 Asswad, N. Dewaele, J. B.Nagy, R. A. Hubert, Z. Gabelica, E. G. Derouane, F. Crea, R. Aiello, A. Nastro, Zeolites, 8 (1988), 221. 16. Y. L. Casci, T. V. Whittam, Stud. Surf. Scie. Catal., 24 (1985) 39.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved. _
_
_
221
Synthesis of ETS-10 molecular sieve from systems containing TAABr salts P. De Luca and A. 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 The synthesis conditions of the ETS-10 molecular sieve from the system a N a 2 0 - b K F - c T i O 2 - d T A A B r - l . 2 8 a H C l - l . 4 9 S i O 2 - 3 9 . 5 H 2 0 where TAABr corresponds to TMABr, TEABr, TPABr and TBABr and.0.9
222 TAABr contents and of SiO2/TiO2 ratio in the reaction mixtures. Moreover the results of thermal analysis on purified ETS-10 are reported. 2. EXPERIMENTAL The system studied was: aNa20-bKF-cTiO2-dTAABr-1.28aHCl-1.49SiO239.5H20 where TAABr corresponds to TMABr, TEABr, TPABr and TBABr and.0.9
223
'''
'I
"
'
I.
'
I
'
I
:
I
I
1.0 C~
'
I
'
1.0
o
"
#.
r~
o.4
0.4
. r"' i
ETS-4
~.~
r
0.2
0.2 I,
I
1.0
I
1
9
t
,
l
1.5 2.0 2.5 N a 2 0 , Moles
v P~''~
t
i
1.0
3.0
Figure 1. Crystallization field of the crystalline phases obtained from the system aNa20-0.6t~-cTiO2-1.28aHC]1.49SIO2-39.5H20, 0.9
9
+
0.6-
0.6
I
U'9
o
o.s
Ti407
0.8
I
oi
t
f
I
1.5, .2.0 2.5 Na20, Moles
:
!
3.0
Figure 2. Crystallization field of the crystalline phases obtained from the system aNa20-bKF,0.25TiO2-1.28aHCl1.49Si02-39.5H20, 0.9
ETS-10 is obtained in a ve~- small area, because the synthesis conditions are highly influenced by Ti02 content in the reaction batch (Figure 1). The variation of KF content in the reaction batch does not modify the area of exSstence of ETS-10 (Figure 2), and it does not produce a variation of the kinetic parameters, of the }deld of the batch and of the crystal size and morpholo~, (cubes of about 1.5 microns). The addition of any TAABr (0.2 moles) to the system aNa20-0.6KF-cTiO21 . 2 8 a H C l - l . 4 9 S i O 2 - 3 9 . 5 H 2 0 where 1.0
T,&A moles nucleation time, hours rate, %/h m~ximum yield, %
0.2 14 5.28 95
0.4 6
0.2 9
5.27 4.17 95 ~
0.4 6
0.2 6
3.46 5.22 92 94
0.4 6
0.2 9
0.4 14
2.81 93
4 . i 9 1.35 ~ 65
..... 24 1.85 89
224
.....
1.0 IID
9
0.2TMABr I
I
'
I
"
I
9
Ti407
0.8
0.8 o
0.6 ~9 0.4 0.2
"
/
i
"
li
"
i
I
.
I
.
I
"
"
Ti407 /
0.6
0.2 i
.
I
,
1.5
I
,
2.0
I
~
2.5
I
.
r "
9
I
,
1.0
3.0
I
9
1.5 2.0
2.5 3.0
Na20, Moles
Na20, Moles
0.8
|!
0.4
1.0
Q)
"
ETS-10 9
1.0
!
1.0 "0"2TEABrI
"
1.0
Ti407 / s
Ti407
0.8 o
O
N o.6
~ o.6 9 N o.4
~ 0.2
u iETS_ O \ 9
I
1.0
9
I
.
.
--'----
U
0.2 I
.
|
1.5 2.0 2.5 Na20, Moles
i
I
3.0
1.0
1.5 2.0 2.5 Na20, Moles
3.0
Figure 3. Crystallization fields of the crystalline phases obtained from the system aNa20-0.6KF-cTiO2-0.2TAABr-l.28aHCl-l.49SiO2-39.5H20, 1.0
225
1.0
1.0
0.8
0.8 ETS-10
ETS-10 0.6
0.6
0.4
~" 0.4
0.2
DTS+ETS-10 |
ETS-10+DTS DTS
DTS a
0.2
I
9
.
I
9
0.6
0.2
I
0.2
1.0
i
I"
1.0 _TPABr I
'
"
'
"
'
"
'
~
ETS-10
I i'
I
"
l
"
I
,zTs-, o.+DTs. 0.6
TAABr, Moles
"
I
ETS-10
0.6
U
0.2
I
TBABr
0.8
0.4 0.2 U
9
1.0
0.8 0.6
1.0
TAABr, Moles
TAABr, Moles 9
0.6
U
~0.4
TS-10+DTS
0.2 , I
1.0
~
0.2
I
9
I
0.6
9
I
1.0
TAABr, Moles
Figure 4. Crystallization fields of the crystalline phases obtained from the system 1.26Na20-bKF-dTAABr-0.25TiO2-1.63HCl-l.49SiO2-39.5H20, 0
226 Table 2. Chemical analysis of purified crystals of ETS-10. Water and TAA content, detected by thermal analysis, Na, K, Si and Ti per unit cell detected by EDAX. The samples are identified by the KF and the TAABr content in the reaction batch. H20/u.c. 1" 2**
SAMPLE 0.6KF
H20 %wt
TAA %wt
O.OTAABr O.2TMABr O.2TEABr 0.2TPABr O.2TBABr O.4TMABr O.4TEABr O.4TPABr O.4TBABr
14.50 10,31 6,07 5,67 4,82 8,71 4,98 3,80 6,05
.... 4.32 7.71 6.51 4.88 5.25 8.48 6.61 4.07
27,46 21,30 12,40 11,35 9,38 17,86 9,91 7,70 11,87
99,59 6,85 6,78 6,19
4.24 7.57 6.57 4,07
19,57 14,10 13,77 12,12
TAA/u.c. 1" 2**
Si/Ti Na/u.c. K/u.c.
3,43 2,66 1,54 1,40 1,17 2,23 1,29 0,94 1,48
...... 2,16 0,27 2,17 0 , 3 9 1,29 0,15 0,75 0,08 2,61 0 , 3 2 2,33 0,30 1,25 0,15 0,59 0,07
5.05 5.11 5.15 4.99 4.55 5.30 5.04 4.98 4.73
1.22 1.25 1.31 1.28 1.38 1.12 1.31 1.24 1.14
0.80 0.91 0.76 0.71 0.80 0.93 0.81 0.81 0.89
2,44 1,76 1,72 1,50
2,10 2,15 1,28 0,59
5.31 4.91 5.03 4.94
1.32 1.12 1.37 1.42
0.73 0.65 0.85 0.55
***
***
***
0.4KF O.2TMABr O.2TEABr O.2TPABr O.2TBABr
0,26 0,26 0,16 0,07
*) Results calculated on the basis of the formula Si4oTi8Olo416- (basic building block for the disordered structure) (6). **) Results calculated on the basis of the formula Si5TiO132- (stechiometry of the framework) (6). ***) Results Mean of 10 experiments on the basis of the formula Si5TiO132(stechiometry of the framework) (6). reaction gel in agreement with result of Figure 4. The results are reported in Table 1. The templating action of the tetralkylammonium salts is also confirmed by the chemical analysis data of purified ETS-10 crystals, reported in Table 2. In particular the TG data show that the water adsorbed into the pore of ETS-10 precursors, synthesized from a system without organic salt, is partially substituted by the TAA added. The TA~u.c. and the H20/u.c. detected are not constant, their amounts are a function of the molecular weight and that of the molecular size of the organic cation. The presence of TAABr in the batch composition does not substantially modify the Ti content in the crystals of ETS-10. Only higher values of Si/~ ratio are observed in the crystals obtained from systems containing TMABr. This anomalous Si/Ti ratio in the ETS-10 structure, probably, derives from the
227 combined effect of the presence of some crystalline defects, of the ability of TMA+ to produce open structures and of its hydrated size similar to hydrated Na +. More investigations are in progress to clarify this problem. The cations content per unit cell is in good agreement with the negative charges of the structure. Any inorganic anion is detected by EDAX therefore the positive charge of TAA+ cation, probably, is balanced by OH'. The amounts of Na + and K + detected are enough to balance the negative charges of the framewok. This behaviour suggests that the TAA+ cations do not replace the inorganic cations in the ETS-10 structure. These results confirm that in general the organic ions play a predominant role of pore filling. The course of DTA patterns of the different TAAETS-10 suggest the presence of one type of organic cation incorporated inside the ETS-10 channels
E X 0
-10
l
-10
AT
S-IO E N D 0
10
0
1 1 O0
! 200
1 300
1 400
! 500
! 600
700
TEMPERATURE, ~ Figure 5. DTA thermograms of the different TAAETS-10 obtained from the system:l.25Na20-0.6KF-0.4TAABr-0.25Ti02-1.6HCl-l.49Si02.39.5H20.
228 (Figure 5). In fact the thermal decomposition of TAA is connected with the single endothermal effect, its temperature is different for any type of organics. The molecular weight of TAA incorporated in ETS-10 channels plays a role on the desorbtion kinetic, in fact the resolution of the endothermic effect is more evident when the molecular weight of the TAA increases. Variation of the KF content does modify the interaction between the organic cation and the framework of ETS-10. The thermograms obtained from TAAETS-10 containing less t h a n 0.4 moles show a not well-defined effect. In agreement with the results of Table 2, the temperature of water desorption is observed at different temperatures as a function of the type of TAA incorporated inside the ETS-10 channels. More investigations are in progress to define the mechanism of ETS-10 activation. 4. CONCLUSIONS The results above reported show that the addition of a TAA+ cation to the batch composition modifies the area of existence of ETS-10 molecular sieve, and extend the range of pH where ETS-10 exists (10.0<_pH_
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
229
Synthetic clinoptilolite and distribution of aluminum atoms in the framework of l I E U type zeolites M.Kato, ~ S. Satokawa, b'~ and K. Itabashi b "Research Center for Chemometrics, Toyohashi University of Technology, Toyohashi 441, JAPAN bNanyo Research Laboratory, Tosoh Corporation, Shinnanyo 746, JAPAN
Pure (K,Na)-clinoptilolite was crystallized in good reproducibility without seed crystals. Ordered A1 distributions in frameworks of synthehized and natural clinoptilolite and natural heulandite, which are isostructural framework, were also analyzed. It was found that A1 atoms in clinoptilolite were condensed in T1 and T2 sites with space group P2/c. On the other hand, A1 atoms in heulandite were distributed in T1, T2, T3, and "I'4 sites with space group P2~, which violates against the 2 A1/5 ring avoidance rule. 1.
INTRODUCTION
There has been long discussion on the distinction between clinoptilolite and heulandite because of the difficulty of synthesis of these pure zeolite crystals in laboratory [1]. In this work, we attempted to synthesize pure (K,Na)-clinoptilolite without seed crystals. It is well known that distribution of A1 atoms in a zeolitic framework takes an important role in a catalytic activity. However, A1 distribution has been ambiguous because the atomic scattering factor of A1 atoms in XRD measurement is nearly equal to that of Si atoms. Therefore, in this work, we examined the ordered distribution of A1 atoms in natural and synthetic clinoptilolite, and natural heulandite on the basis of analysis of 298i MAS NMR spectra and their framework structures with the aid of Loewenstein's rule [2] and 2Al/5-ring avoidance rule [3], and attempted to clarify the difference between clinoptilolite and heulandite. 2. EXPERIMENTAL Materials (K,Na)-clinoptilolite was hydrothermaUy synthesized without seed crystals by homogeneous mixing of reactant mixture of (K,Na)-aluminosilicate gel during heating at 150~ for six days. Natural clinoptilolite(Na3.nKz.4Cao.7A16.6Si29.407z: Arizona, USA) and heulandite (Nal.sK0.1Ca2.9A18.6Si27.4072: Maharashtra, India) were used. Apparatus 29Si MAS NMR spectra were measured at 79.4586MHz and spinning frequency of 6 kHz with Varian Unity-400 plus spectrometer. Thermal stability was measured by TG/DTA with Rigaku Thermoflex 8100. Chemical composition of samples was determined by ICP and EDX elemental analysis. c Present address : Fundamental Technology Research Laboratory, Tokyo Gas Co. Ltd.,
Minato-ku, Tokyo 105, Japan
230 3. RESULTS AND DISCUSSIONS
3.1 Synthetic clinoptilolite Pure (K,Na)-clinoptilolite with composition of Na2.1K4.sA16.9Si29.1072 nn20 was crystallized from the reactant mixture of Si/Al=5.5, (K+Na)/Si=0.3 and K/(K+Na)--0.5. All of the diffraction peaks of the products could be assigned to those of HEU structure [4]. Pure clinoptilolite was produced within very restricted reactant composition with good reproducibility [5], and phillipsite, mordenite, or ferrierite was crystallized with clinoptilolite around the composition. The crystals of synthesized clinoptilolite have morphology like thin plate as shown by SEM photograph in Figure 1. DTA curve of natural heulandite showed two endothermic peaks at about 160~ and 300~ corresponding to dehydration [5]. In the case of synthetic and natural clinoptilolite, continuous dehydration curves in TG/DTA up to 600~ were obtained, and there was no structural change in XRD patterns after heat treatment at 600~ This phenomena agree with Figure 1. Scanning electron micrograph of synthesized the discussion of Mumpton clinoptilolite crystal. [6] that clinoptilolite structure is maintained after heating overnight at 450~ but that heulandite is decomposed in the same treatment. 3.2 NMR spectra The 29Si NMR spectrum of synthetic clinoptilolite was illustrated in Figure 2. By a peak deconvolution, four peaks(1 to 4) were obtained as shown in the figure, and the populations of Si(OA1)n(OSi)4_, (abbreviated as Si(nA1) hereafter) in the four peaks are calculated. The difference of the chemical shifts of these peaks were about 5 ppm from each other, which is almost equal to that observed in the difference of A1 number n in Si(nA1) [7]. However, if they are assigned to Si(3A1), Si(2A1), Si(1A1), and Si(0A1) from low magnetic field, the A1 content of the framework is much greater than the observed values by chemical -90 -100 -110 -120 -130 analysis. ~/ppm The chemical shift of Si atoms in the framework were reassigned on the basis of Figure 2. 29SiMAS NMR spectrum of empirical equation derived by Engelhardt synthetic clinoptilolite. [7] using the atomic coordinates of
231 tetrahedron sites (abbreviated T sites hereafter) and oxygen atoms [8]. The calculated chemical shifts were widely spread from -95 ppm to -113 ppm as illustrated in Figure 3. The chemical shift of tetrahedron sites, TI(nA1), T3((n+I)A1), and T4((n+I)A1), were almost equal value, and , , 'r:zo^l) 1 those of Tz((n+l)A1) and f rs(,^,) v,(o.,,) T4(IAI) ~ 3 _ / T4(2AI) Ts(nA1) sites were located in 2 I"~ 'r1(l^l) I a':3(2^t) both sides of them. When ~ . 'r2(2,u) } they can be divided into four 1 Ts(~^,) . . . . r~(o.,,) 4 TI(2AI)~ r - T4(0AI) groups as shown in Figure 3 which were labeled as peaks 1 to 4 from low magnetic field, it is found that the calculated chemical shifts -95 -100 -105 -UO -115 8/pi,,,, are in good agreement with the values of the observed peaks shown in Figure 2. Figure 3. Calculated chemical shifts for clinoptilolite, HEU. TS(0AI)
T2(0AI )
9
l
,
i
3.3 Ordered distribution of A! atoms in HEU type framework 3.3.1 Candidates of ordered distribution of A! atoms The framework of clinoptilolite, HEU, which was assigned to space group C2/m by Alberti [8]. However the difference between A1 and S i atoms was ignored in his assignment. So it is predicted that the assigned symmetry is higher than that of the real one. In order to distinguish A1 and Si atoms, the discussion was started from the most primitive P1 symmetry in Table 1 Configurations of eight A1 atoms in the HEU type framework, and the p 9pulation of Si(nA1). Expected intensity of peaks in Fig. 2 [Si(nA1)] Space group No. n=3 2 1 0 1 2 3 4 Configuration of A1 atoms I
TI,I* T1,2 T1,3 T1,4 T1,5 T1,6 T1,7 T],s
C2/m
0
12
8
8
4
8
8
8
II
T1,1 TI,z T1,7 Tl,s T2,3 "1"2,4Tz,5 Tz,6
P2/c
0
10 12
6
2
12
10
4
III
P2/c
0
10 12
6
6
4
14
4
P21
0
10
12
6
2
14
6
6
P21/m
0
8
16
4
4
12
8
4
VI
TI,1 T1,2 T1,7 TI,s T3,3 T3,4 T3,5 T3,6 TI,1 T1,4 T1,6 T1,7 T2,2 T2,5 T3,2 T3,5 TI,1 T1,4 Tl,6 T1,7 T3,2 T3,3 T3,5 T3,8 TI,1 T1,6 "1"2,2T2,5 T3,2 T3,5 %,4 T4,7
P21
0
10 12
6
2
14
8
4
VII
T2,1 T2,2 T2,5 T2,6 T3,1 T3,2 T3,5 T3,6
C2
0
12
8
8
4
12
8
4
VIII T2,1T2,2 T2,7 T2,s "1"3,3T3,4 T3,5 T3,6
P2/c
0
8
16
4
4
8
16
0
T2,1T2,3 .T2,6 Tz,s T3,1 T3,3 T3,6 T3,8
P2a/C
0
12
8
8
4
12
8
4
T2,1 "1"2,6%,1 T3,2 T3,3 T3,s T3,6 T3,s
P21
0
10
12
6
6
6
14
2
IV V
IX X XI
T3,1 T3,2 T3,3T3,4 T3,5 T3,6 T3,7 T3,s C2/m 0 8 16 4 8 0 20 0 *In Ti,/, i corresponds to the site in Ref.[8] andj the number of the equivalent position of Ref. [10].
232 of atomic coordinates by the same manner described in previous papers [3,9]. Using the connectivity table, candidates of ordered distribution of A1 atoms in the HEU type framework were searched under the restriction of Loewenstein's rule and 2A1/5-ring avoidance rule proposed in the previous paper [3]. Some of the candidates with high symmetry were listed in Table 1 out of hundreds of them, along with the calculated population of Si(nA1) and the expected intensity of the peaks 1 to 4 in the 29Si MAS NMR spectra. It is found from the table that the maximum Figure 4. Projection to bc-plane of ordered number of A1 atoms in the framework is eight per unit cell. The A1 content per unit distribution of A1 atoms in clinoptilolite with cell observed in most of natural space group P2/c. @ 9A1. clinoptilolite is in good agreement with this value. In order to contain more than eight A1 atoms per unit cell in the framework, the A1 distribution must violate against the 2 Al/5-ring avoidance rule. Also, it is shown that Si(3A1) cannot exist in the framework except for only one configuration with space group P1. 3.3.2
Clinoptilolite
The ordered distribution of A1 atoms in the framework must satisfy the observed 298i MAS NMR spectra and A1 atoms should be condensed in "I'2 site [8,10]. In the candidates listed in Table 1, the configuration with P2/c (P12/al in non-standard form) symmetry (No. II) was selected. In the ordered distribution, A1 atoms are located in TI.~ T1,2 TI,7 Tl,s T2.3 T2.4 T2.5 and T2.6sites shown in Figure 4. Real crystals of clinoptilolite contain less A1 atoms than eight for the ideal distribution. Therefore, substitutions of A1 atoms by Si atoms in the framework must be considered. The substitutions are summarized in Table 2 along with the population change of [Si(nA1)] in five To sites and A1 content. The peak intensities were calculated from the ideal populations and their changes by these substitutions. The results were listed in Table 3. It is found that the calculated intensities agree well with the observed intensities. This means that a misplacement of A1 atoms in the framework is almost negligible, because only the substitutions of A1 atoms by Si atoms were considered in this calculation. A1 atoms are ideally located in T1 and "I'2sites. In these sites, A1 atoms in T1 sites are easily substituted by S i atoms in real crystals, and occupation of A1 atoms in the site decreases from 50 % to 37%. On the other hand, A1 atoms in "I'2sites are hardly substituted, and the occupation is ca 45%. The value of occupation in T2 site is in good agreement with the value estimated from XRD data by Alberti [8]. It is reported that cations in clinoptilolite are located near T2 site [8,10], Which also agree with the model of ordered distribution of A1 atoms of this work In XRD measurements, space group C2/m [8,10] has been assigned to the framework of clinoptilolite. The space groups P2/c proposed in this study have lower symmetry than C2/m,
233 Table 2 of NMR spectrum by substitution of A1 atom Si in clinoptilolite. Peak no. 1 2 3 n in Si(nA1) 2 2 2 2 1 2 1 mill 0 l ! l l l ~ Site T1 Ts T2 T3 T1 T4 Ts Ideal population of [Si(nA1)]/u.c. 0 2 4 4 4 0 0 Population changes by -1 substitution in T2 site(Si) -1 +1-1 +1-1 -1 -2 -4 substitution in T1,7 and Tl,s sites(Sa) +2 +4 +2 +1 +2 +2 +2 -2 -2 -2 substitution in T1,1 and T1,7 sites(Sin) +, - 9indicate the population changes relative to the ideal
4 [AI] 0 0 /u.c. T 2 T 3 T4 004 8 0
+1 +1 +1 -1 -2
,.
+2
-2
Table 3 Observed and calculated NMR spectra of clinoptilolite. peak No. Synthesized clinoptilolite 0.49x Sn+ 0.11 x SI observed Acalc-obs Natural clinoptilolite 0.26 x SI + 0.53 x Sn+0.05 x Sm observed Aca|c-obs
1
[Si(nA1)]/u.c. 2 3
4
[A1] /u.c.
1.5
8.8
14.4
4.3
6.9
1.6 -0.1
8.8 _+0.0
14.2 +0.2
4.5 -0.2
6.91 -0.01
1.4 1.2 +0.2
8.2 8.0 +0.2
15.0 15.2 -0.2
4.9 5.1 -0.2
6.5s 6.58 :tO
but one of the subgroups of C2/m. The configuration of A1 atoms presented here has always the topologically same structure, e.g., the configuration of (T1,3, T1,4, T1,5, T1,6, T2,1, T2,2, T2,6 and T2,8 ) is topologically the same as the configuration of (TI,1, T1,2, T1,7, T1,8, T2,3, T2,4, T2,5 and T2,6). In real crystal, both of these configurations should be realized with equivalent probability. Therefore the space group determined by XRD can be higher than the space group assigned by the consideration of the ordered distribution of A1 atoms. In this case, eight sites of T1 and T2 which are divided into two groups in space group P2/c become equivalent, and apparent space group in XRD measurement becomes C2/m. 3.3.3 Heulandite In the above configuration for clinoptilolite, there are two sites which can be occupied by additional A1 atoms. However the calculated intensities of the peaks of NMR spectrum for heulandite did not agree with the observed one, where chemical shifts calculated using the atomic coordinates reported by Alberti [11] were almost equal to those of clinoptilolite. So another configuration for heulandite must be selected from the candidates listed in Table 1, since typical A1 content per unit cell of heulandite in natural samples [ 1] is reported as nine and has not exceeded ten, which is very close to the value of clinoptilolite. If the Loewenstein's rule is only one restriction of distribution of A1 atoms, more A1 atoms should be contained in the framework. The candidate of ordered distribution of A1 atoms should satisfy that expected intensity of peak 1 should be less than four because the observed population was 3.6 and it should
234 increase when A1 atoms inserted. Consequently the ordered configuration illustrated in Figure 5 was selected as a candidate, whose symmetry is P21. The configuration was derived from configuration VI in Table 1 which is inserted two Al atoms into T2,~ and "1"2,6sites. Ten A1 atoms are contained in the ideal distribution, and they are located at T~,~, T1.6, T2,1, "1"2,2,"1"2,5, "I'2,6, T3,2, T3,5, "1"4,4,and T4,7 sites. In the configuration, no more Al atoms can be inserted by the restriction of Loewenstein's rule, that is, the number of Al atoms is limited to 10 per unit cell in heulandite [ 1]. In order to compare with a real crystal, the substitutions of A1 atoms by Si atoms were considered as clinoptilolite. The population changes of Si(nA!)s are T: summarized in Table 4. The calculated peak intensities were in good agreement with .2 the observed values as listed in Table 5. T ~,7 Although extra A1 atoms were added to T2 L sites, the substitution is occurred at T1 and . T3 sites. Consequently, in heulandite, A1 atoms are condensed in "/'2 sites ! (occupation is 50%), and small amount of A1 atoms distributed in T1 (16%), T3(16%), and "1"4(25%) site. Alberti [ 11] concluded from the bond distances between T site and oxygen site that occupancy of A1 atoms in "I"2 Figure 5. Projection to be-plane of site atoms was about 40%, and 25% in T1, "1"3 and T5 sites. On the other hand, configuration of Alatomsinheulandite. Markle and Slaughter [12] concluded that A1 @ 9Al atoms. atoms were mainly co_ndensed in T2 (50%) and "1"4sites (50%), and small amount of Al atoms is in T1 site (13%) in notation of space group C2/m ( they refined the structure in space group Cm). Their conclusions agree .....with the present results from the view point of the condensation of A1 atoms in T2 site. It is reported that cations in heulandite are located near T2 site [11], which also agree with this work. The A1 content of heulandite is greater than eight per unit cell predicted from the 2 A1/5ring avoidance rule. It means that heulandite violates against the rule. Moreover, the Table 4 Chan of NMR s~ substitution of A1 atom b, Si in heulandite. Peakno. 1 2 3 4 [A1] n in Si(nA1) 3 3 2 3 2 2 2 1 2 1 1 1 0 1 0 0 0 0/u.c. Site T2 T3 T1 T4 T5 'I'2 T3 T1 '1"4 T5 T~ T2 T3 T1 "I'4 T5 T2 T3 "I'4 Ideal population of [Si(nAl)]/u.c. 2 2 2 0 0 2 0 4 2 4 ~DmHwmmm~ Population changes by substitution inT1 site(Sin) -1 -1 +1 +1 -1 ma,mmmmmmm substitution in T3 site(S~) 0 -1 -1 -1 mmmmmmmmmlm +, - 9indicate the population changes relative to the ideal
235 ordered configuration of A1 atoms differs Table 5 from that of clinoptilolite. These Observed and calculated NMR spectra of natural difference seems to be the reason that heulandite thermal stability of heulandite is lower than [Si(nAl)]/u.c. [AI] that of clinoptilolite. Peak No. 1 2 3 4 /u.c. The occurrence of heulandite shows 3.9 11.3 10.8 1.4 8.6 0.7 x (Sin + San) that 2 Al/5-ring avoidance rule may be observed 3.6 11.8 11.1 1.0 8.6 weaker restriction of A1 distribution than +0.3 -0.5 -0.3 +0.4 __+0.0 Aeale-obs the Loewenstein's rule. In the case of heulandite, driving force of the violence against the rule seems to be a charge compensation of Ca cation by A1 atoms in the framework.. 4. CONCLUSIONS Clinoptilolite can be synthesized from very narrow region of reactant mixture with good reproducibility without seed crystals. In the framework of clinoptilolite, eight A1 atoms are theoretically ordered in T1 and I'2 sites with space group P2/c. Heulandite has the same framework as clinoptilolite, and 10 A1 atoms are located in T~, T2, T3, and '1"4sites. In both zeolites, A1 atoms mainly occupy "I"2site. It is considered that the thermal instability of heulandite against clinoptilolite is attributed to the violation against 2 A1/5-ring avoidance rule. Acknowledgment
The authors thank Prof. T. Takaishi for his helpful suggestions. References
1. G. Gottardi and E. Galli, Natural Zeolites, Springer-Verlag, Berlin 1985. 2. W. Loewenstein, Am. Mineral., 39 (1954) 92. 3. T. Takaishi, M. Kato and K. Itabashi, Zeolites, 15 (1995) 21. 4. W.M.Meier and D.H.Olson, Atlas of Zeolite Structure Type, Butterworth-Heinemann, Stoneham, 1992. 5. S. Satokawa and K. Itabashi, to be submitted. 6. F.A.Mumpton, Am. Mineral., 57 (1972) 1463. 7. G. Engelhardt, Proceedings of 1989 Meeting of British Zeolite Assoc., (1989) 151. 8. A. Alberti, Tschrmaks Min. Petr. Mitt., 22 (1975) 25. 9. T. Takaishi and M. Kato, Zeolites, 15 (1995) 689. 10. K. Koyama and Y. Takeuchi, Z. Kristallogr., 145 (1977) 216. 11. A. Alberti, Tschrmaks Min. Petr. Mitt., 18 (1972) 129. 12. A.B.Markle and M. Slaughter, Amer. Mineral., 53 (1968) 1120.
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H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 1997 Elsevier Science B.V.
237
Preparation of ultramarine analogs from zeolites S. Kowalak, M. Str6~,yk, M. Pawtowska, M. Milu~ka and J. Kania Adam Mickiewicz University, Faculty of Chemistry, ul. Grunwaldzka 6, 60-780 Poznafi, Poland
Novel methods for ultramarine analogs preparation involving generation of sulfur radicals inside the synthetic zeolites are presented. Sulfur compounds were introduced into the zeolites porous structures either by impregnation or hydrothermal treatment or even during crystallization of zeolites in their presence. The samples were then calcined for short time in order to generate the radicals. Oppositely to conventional syntheses of ultramarine the emission of SO2 was found negligible. INTRODUCTION The technology of the artificial ultramarine production has been developed as early as in 1820's independently by J.B. Guimet in France and C. G. Gmelin in Germany [ 1]. The largescale production started in Germany already in 1830. The technology consisted in calcination of mixtures of kaolin, sodium carbonate, elemental sulfur and reducting agents for long time (at least one week) at high temperature (800~ or higher). The inexpensive substrates and the simple technology made the resulting ultramarine a very cheap and common product. The production raised very fast and almost one hundred factories producing ultramarine were operating in Europe in the beginning of twentieth century. The technology did not change considerably until now. The main drawback of this method is the presence of large amount of sulfur dioxide in waste gases. About one half of the total sulfur in the substrate mixture is transformed into SO2 upon calcination. The contemporary regulations on environment protection can not accept the pollution resulting from this production. Neutralization of sulfur oxides is complicated and expensive. Although ultramarine is still a product of high market demand, most of the factories in Europe closed, because of the pollution problem. Both, natural (Lazurite, Lapis lazuli) and synthetic ultramarine show the structure of sodalite. The anion-radicals $3-, responsible for the intense blue color, are sitting inside the sodalite cages (6.4A in diameter) which are very suitable for their accommodation [2]. Encapsulation of sulfur radicals inside the 13-cages provides their stability and subsequently makes the ultramarine blue an extremely durable dye. The sodalite cages are part of the structure of several molecular sieves (LTA, FAU) as secondary building units. It seemed reasonable that
238 the 13-cages of most common commercial zeolites such as A, X, Y, could be also adequate to encapsulate the sulfur radicals. The first attempts to use zeolites for the preparation of ultramarine have been already reported in the beginning of the 1950's [3-5]. The conventional synthesis of ultramarine consists in a high temperature crystallization of sodalite structure with a simultaneous generation of the sulfur radicals inside its cavities. It is likely that the introduction or generation of the radicals inside the porous structure of zeolites could be carried out at much milder conditions. The gentle condition should prevent a high emission of sulfur oxides. We have undertaken several approaches to prepare ultramarine analogs from zeolites. The anion-radicals S3- identical to those found in ultramarine have been obtained by dissolving certain alkali polysulfides in electron pair donor solvents such as DMSO, DMF, NH3 [6-8 ]. We tried to introduce the radicals into zeolites from DMSO or liquid NH 3 solutions of Na2S 3. The results have not been very promising so far [9]. The blue color of impregnated zeolites vanished upon evacuation of the solvent and the ESR spectra did not indicate the presence of S3- radicals in the resulting samples. Generation of radicals in zeolites from precursors introduced into zeolites has appeared as a more suitable approach. The precursors such as sodium sulfides or polysulfides have been incorporated by impregnation of zeolites [ 10,11 ]. The samples were then calcined for several hours either at high temperatures (above 500oc) in the presence of a reducing agent (coal tar) or at a relatively low temperature (350oc) without any reducing agent. In another series of experiments the impregnation was replaced by a hydrothermal treatment of zeolites with aqueous precursor solutions in an autoclave at elevated temperatures (100 -200oc). The obtained samples usually showed a light blue color which turned deep blue after a short calcination [ 12]. Another approach comprised a crystallization of zeolites (A,X, hydroxysodalite) in the presence of sulfur compounds in order to incorporate them into the intracrystalline cavities already on nucleation of the zeolite structure. This method of encapsulation has been proved successful for various metal complex compounds [ 13]. The crystallization products were then heated to generate the sulfur radicals. The crystallization mixture was mostly obtained from sodium silicate and aluminate. In some experiments kaolin was used as starting material for crystallization [ 14]. Kaolin was also employed in other experiments similar to traditional ultramarine synthesis. The main difference was the use of sodium sulfide or polysulfide in the substrate mixture instead of elemental sulfur. The formal charge of the anion-radical S3- is -1/3. Thus, the use of negatively charged sulfur compounds should be advantageous to reach the formal charge value of-l/3 in the oxidative condition of thermal procedure. This can also prevent the total oxidation of the sulfur compounds to sulfur dioxide which always occurs in the conventional method [ 15]. Besides the experiments of commercial interest we have also studied thermochromic effects with ammonium polysulfide [ 16], the influence of cation zeolite modification on generation of sulfur radicals [ 17] and AIPO4-20 ultramarine analogs [ 18 ]. The following work summarizes some results of our previous study [9,10, 11, 14,15, 16] and presents the new data.
239 2. EXPERIMENTAL The commercial zeolites NaA, NaX, NaY supplied by SODA, Ma,twy, Poland, Na mordenites (Leuna Werke) and Na ZSM-5 ("Ultrazet" from Institute of Industrial Chemistry, Warsaw) were used as principal parent materials. We also used hydroxysodalite, AIPO4-5 [19] and AIPO4-20 [20 ] prepared in our laboratory according to published recipes. Kaolin (A1203-32,5%, SIO2-53,7% ) was supplied by Otdrzychrw mine, Poland and Na2S. 91-120 by POCH, Poland. Elemental sulfur was provided by Siarkopol, Poland. The impregnation of molecular sieves (NaA, NaX, NaY, Na-hydroxysodalite, Na-mordenite, Na-ZSM, AIPO4-5, A1PO4-20) has been carried out using an incipient wetness technique. Polysulfide Na2S 3 was mostly used in the preparations presented in this study. It has been obtained by merging Na2S with adequate amounts of elemental sulfur. The quantity of sulfur introduced as polysulfide constituted 15 wt.% of the whole composite material. The impregnated samples were dried at 100~ Then the reducing agent, coal tar, was admitted (1/5 of zeolite weight) and then they were calcined in covered ceramic crucibles at temperatures 500-850~ for two hours. The samples were heated for additional two hours in open crucible at 600~ In other experiments impregnated samples were heated without any reducing agent at 350~ for two hours. Hydrothermal treatment of zeolites was carried out in Teflon lined autoclave at temperatures ranging from 100 to 200~ for 22 hours. Zeolites were immersed in concentrated aqueous solution (-~30%) of Na2S 3. The number of sulfur (as Na2S3) was about 15 wt.% of the zeolite. The samples were then filtered, washed and dried at 100~ Finally they were calcined at 800 ~ C for 15 minutes without any reducing agent. Crystallization of zeolites (A,X, sodalite) was conducted under conventional condition [19]. The composition of the initial mixtures was however completed by various amounts ofNa2S 3 (Table 3). The initial aluminosilicate gel was formed from sodium aluminate and silicate solutions. Some preparations were conducted with kaolin and NaOH as crystallization substrates. In other experiments elemental sulfur replaced sodium polysulfide. The samples after synthesis were washed with water and dried and then calcined with a reducing agent similarly as for impregnated zeolites. The conventional thermal method of ultramarine synthesis has been modified by using sodium sulfide or polysulfide instead of elemental sulfur. The substrates were ground together with coal tar and calcined in covered crucibles at 850~ for 3 hours and then additionally heated in full contact of air at 600~ The color of the samples under study was estimated visually. The electronic spectra were recorded on Shimadzu UV-160 spectrometer, using glycerine suspension and they were compared to those of commercial ultramarine (produced by Reckitt's Colours or Polifarb). Crystallinity was measured by XRD (TUR M-62). ESR spectra have been recorded at room temperature (Radiopan EPR Spectrometer, 9.4 GHz, magnetic modulation 100 KHz), the g value was estimated by calibration of magnetic induction and microwave freqency. Thermogravimeric analyses were conducted in static air atmosphere (Derivatograph MOM OD- 102).
240 RESULTS AND DISCUSSION
Introduction of anion-radicals S 3- into zeolites NaA and NaX from DMSO and liquid ammonia solutions did not result in ultramarine analogs [9]. It is very likely that solvated radicals approach only the large cavities. Evacuation of the solvent causes their transformation or recombination before reaching the 13 cages. Although the ESR indicated the presence of some radical species but these were certainly not the S3" radicals. Table 1 Results of impregnation of zeolites with Na2S 3 followed by calcination at various temperature Parent zeolite Calcination Color Final structure temperature (XRD) NaA 850~ deep blue SOD 650~ blue SOD+LTA 500~ light blue LTA+SOD 350~ * blue LTA 850~ intense blue SOD NaX 650~ light blue SOD+FAU 500~ very light blue FAU+SOD 350~ * white FAU 850~ grey blue SOD NaY 650~ grey blue SOD+FAU 500~ very light blue FAU+SOD 350~ * light blue FAU 850~ dark blue SOD Hydroxysodalite 650~ blue SOD 500~ light blue SOD 350~ * light blue SOD 850~ grey almost Na-ZSM-5 amorphous 850 ~ grey amorphous Na-mordenite 850~ light blue SOD AIPO4-20 * calcined at 350 without a reducing agent Table 1 shows that most of the parent molecular sieves impregnated with sodium polysulfide (Na2S3) show a blue color similar to that of ultramarine, after short calcination. It is significant that the blue color is attained only for the molecular sieves containing sodalite cages (A, X, Y, sodalite and AIPO4-20 ). It seems the 13 cages are very important for generation and stabilization of the radicals. Although the color of modified AIPO-20 is less intense than that of zeolites it is remarkable that the same geometry of the host matrix provokes encag
241 ing the radicals, regardless their composition. Zeolites involving a channel pore system (mordenite and ZSM-5) do not turn blue upon modification. Although sodalite cages are very suitable to accommodate radicals, there are also reports [21,22] that other cavities (e.g. of zeolite P) can be filled with sulfur radicals and form ultramarine analogs. Table 1 shows that high temperature treatments of impregnated zeolites results in structural transformations. 612 g
2.0286
1
/
' 3
9 605 605 2.0292
200
600
1000 nm
Fig. 1 Electronic spectra of: 1. Ultramarine Reckitt's, 2. Zeolite 13X hydrothermally treated with Na2S 3 and calcined at 800~ 3. Zeolite 4A after the same modification, 4. Zeolite 13X after hydrothermal procedure, 5. Zeolite 4A after hydrothermal procedure, 6. Zeolite Y impregnated with Na2S 3 and calcined at 350~
320
~
330
S
340 B[mT]
Fig.2 ESR spectra of." 1.Ultramarine Reckitt' s, 2. Zeolite 4A, impregnated with Na2S 3 and calcined at 850~ 3. Zeolite 4A after hydrothermal process, 4. Zeolite 4A hydrothermally treated with NazS 3 and calcined at 800~ 5. Zeolite 4A impregnated with Na2S 3 and calcined at 350~
The sodalite structure is detected noticeable in modified zeolites A and faujasites already after calcination at 500~ The contribution of SOD structure increases with temperature and it becomes the only crystalline phase aider calcination at 850oc. Neither the ordinary calcination of the parent zeolites of these types nor those impregnated with NaOH leads their transformation into the SOD structure. It therefore seems that the sulfur compounds are
242 responsible for directing the recrystallization, which means they act as templates. They probably play similar a role in the conventional thermal crystallization of ultramarine. It is conceivable that the sodalite cages filled with sulfur derivatives do not undergo the decomposition before the recrystallization but they are rather rearranged upon calcination towards sodalite. Zeolites ZSM-5 and mordenite (that do not involve cages ) undergo an amorphization upon thermal treatment. Recrystallization of zeolites A and X in the presence of sodium polysulfides is already noticeable up on hydrothermal modification in autoclave above 180~ (Table 2). Recrystallization can result from increased sodium amount or from templating effect of sulfides. The samples treated at 195oc show a light blue color. Table 2. Properties of zeolites hydrothermally treated with Na2S3, followed by calcination at 800~ Zeolite Autoclave Structure before Color before Color a~er temperature calcination calcination calcination A 100~ LTA white white 150~ LTA light green white 195~ SOD white deep blue X 100~ FAU white white 150~ FAU white white 195~ SOD light blue deep blue Table 3. Results of zeolite crystallization in the presence of Na2S3. Zeolite Molar ratio Structure Si: AI:Na: S :H20 A, 85~ 4h 1:1:2:0:20 LTA 1:1:3.2:1.8:20 LTA+SOD 1:1:4.4:3.6:20 SOD+LTA X, 90~ 14h 1.5:1:3.5:0:145 FAU 1.5:1:4.5:1.5:145 FAU+SOD 1.5:1:6.7:4.4:145 SOD+FAU Sodalite, 95~ 25h 1.5:1:2.8:0:34 SOD 1.5:1:3.1:0.5:34 SOD 1.5:1:3.7:1.4:34 SOD Sodalite (kaolin), 1.4:1:1.9:2.8:20 CAN 195~ 23h 1.4:1:3.8:5.7:20 CAN "1.4:1:2.7:5.9:25 CAN * elemental sulfur
Color atter calcination white light blue light blue white light blue blue white lisht blue blue blue blue blue
The presence of S 3- radicals was confirmed by ESR (Fig. 2). Short (15 min.) heating leads to a very deep blue color (Fig.l). Calcination of polysulfide impregnated zeolites (A, X, Y) at
243 350oc without reducing agents results in light blue products of the retained zeolite structure. Their ESR signal anisotropy can result from the extra cage $3" radicals. Crystallization of zeolites in the presence of various amounts ofNa2S 3 followed by calcination gives rise to a blue product of relatively low color intensity (Table 3). Small amounts of polysulfide introduced do not affect the crystallization and the parent structures (LTA or FAU) are maintained, but most of sulfur compounds is washed out atter synthesis. Calcination of such samples results in colorless products. Increase in polysulfide content causes the formation of sodalite. The latter becomes the predominant structure for high they Na2S concentration. Such a direction of crystallization can result from increased Na2S 3 contents reflecting a templating effect of the polysulfide. Kaolin resulted mostly in cancrinite atter a crystallization and then turned to SOD upon calcination.
L ....
n A
I I
TG
I I
0,5%..
DTA 9
100
500
900~
100
,
,
] |
I 500
900 ~
Fig. 3 Thermogravimetric curves of conventional mixture (kaolin, S, Na2CO3),-A and zeolite 4A impregnated with NazS3.-B. Modification of the traditional ultramarine synthesis by using sodium sulfide of polysulfide results in product of similar quality as produced with elemental sulfur. Emission of SO 2, however, is several times lower. Thermogravimetric curve of zeolites impregnated with polysulfide (Fig. 3) shows that the weight loss caused by evolving SO2 is almost negligible. CONCLUSION Ultramarine analogs can be produced from zeolites under much milder conditions than those used in conventional technologies. It allows to significantly reduce the SO2 evolution. Impregnation of zeolites (particularly these involving sodalite cages in their structure) with sulfur radical precursors followed by short calcination results in products comparable to synthetic ultramarine. High temperature calcination causes transformation of zeolites A, X and .Y into sodalite. Mild calcination results in light blue products of unchanged structure. The above procedure was also successful for generation of sulfur radicals in AIPO4-20.
244 Hydrothermal treatment of zeolites A and X with Na2S in autoclave at elevated temperatures gives rise to their recrystallization to sodalite. A certain amount of radicals is generated during this procedure. Further calcination yields intense blue products. Crystallization of zeolites in the presence of polysulfides or elemental sulfur followed by further calcination results in ultramarine analogs of relatively low color intensity. The sulfur compounds affects the crystallization and facilitates the formation of sodalite or of cancrinite. Conventional synthesis of ultramarine can be modified by replacing elemental sulfur by sodium sulfide or polysulfide. This modification provides substantial reduction of SO2 emission.
Acknowledgement We acknowledge a support from the Polish Committee for Science Research (grant 7 $203 051 05). REFERENCES 1. F. Seel, Studies in Inorganic Chemistry, 5 (1994), 69. 2. G.D. Stucky, V.I. Srdanov, W.T.A. Harrison, T.E. Gier, N.L. Keder, K.L. Moran, K. Haug, and H.I. Metiu, T. Bein (eds.), ACS Symposium Series 499, (1992), 294. 3. A.E. Gessler, S. Kumins and C.A. Kumins, U.S. Patent 2535 057, (1950). 4. C.A. Kumins, U.S. Patent 2535 057, (1951). 5. C.A. Kumins and A.E. Gessler, Ind. Eng. Chem., 43,(1953),3. 6. F. Seel, G. Simon, J. Schuh, M. Wagner, B. Wolf, L. Ruppert and A.B. Wi~ckowski, Z. Anorg. Allg. Chem.., 536, (1986), 177. 7. V. Pinon, E. Levillain and J.P. Lelieur, J. Mang. Reson., 96, (1992), 31. 8. P. Dubois, J.P. Lelieur and G. Lepoutre, Inorg. Chem., 27, (1988), 73. 9. S. Kowalak, M. Pawtowska, M. Milu~ka, M. Str62yk, J. Kania, W. Przystajko, Colloids and Surfaces A,101, (1995), 179. 10. S. Kowalak, M. Milugka, A.B. Wi~ckowski and J. Goslar, Mol. Phys. Rep., 5, (1994), 223. 11. S. Kowalak, Polish Patent pending, P-302065,(1994). 12. S. Kowalak, M. Str62yk, J. Kania, to be published. 13. K.J. Balkus, S. Kowalak, U.S. Patent 5,167,942, (1992). 14. J. Kania, M. Str62yk, M. Pawtowska, S. Kowalak, in Characterization and Properties of Zeolitic Materials, M. Rozwadowski (eds), N. Copernicus Univ. Press, Torufi, (1994), 43. 15. B. Gliniany, S. Kowalak, ibid., 49. 16. S. Kowalak, M. Str62yk, J. Chem. Soc. Faraday Trans. in press. 17. S. Kowalak, M. Str62yk, M. Wr6bel, in preparation. 18. S. Kowalak, M. Pawtowska, M. Str62yk, to be published. 19. D. W. Breck, Zeolite Molecular Sieves, Wiley, New York, (1974). 20. M. Szostak, Molecular Sieves, Van Nostrand Reinhold Catalysis Series, New York (1989). 21. W.D. Galstian, A.K. Nadjarian, S.S. Karakhanian, E.B. Oganiesjan, F.S. Szakhazarian, S.A.Grigorjan, S.U.Patent 1638147 A1, (1984). 22. S. Kowalak, J. Matycha, to be published.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105. 9 1997 Elsevier Science B.V. All rights reserved.
245
E x p l o r a t i o n of n o n c o n v e n t i o n a l routes to synthesize M F I type titano- and (boro-titano)- zeolites M. Shibata*, J. G6rard and Z. Gabelica Facult6s Universitaires de Namur, I)6partement de Chimie 61, Rue de Bruxelles, B-5000 Namur, Belgium A wide series of non conventional, simple, rapid and inexpensive synthesis routes to prepare well crystallized (Ti)- and (Ti,B)-MFI zeolites from short chain alkylamine media have been explored. A new procedure involving an in situ seeding phenomenon was successfully used to considerably accelerate the crystallization. Ti is readily incorporated into the MFI framework, either alone or along with boron, as confirmed by XRD, IR, UV-vis and NMR techniques. Both ions show a homogeneous distribution throughout the zeolite crystallites grown in the presence of methylamine. B-Ti concentration gradients could appear when methylamine is admixed with fluoride ions used as co-mineralizing species. 1. I N T R O D U C T I O N MFI type titanosilicates have been shown to be excellent catalysts in various reactions involving olefin epoxidation and hydroxylation with hydrogen peroxide [1]. On the other hand, most of the known (M)-MFI zeolites (where M stands for a trivalent ion such as A13+, Ga 3+, B 3+... thus generating Br6nsted type acidic sites) are currently used in acid catalyzed reactions. It was recently anticipated [2,3] that mixed (Ti,M)-MFI materials would behave as bifunctional catalysts for both types of reactions. In that respect, the presence of milder acidic centers like the bringing hydroxyls induced by B 3+ ions within the titanosilicate framework, is expected to synergize the overall oxidation properties of such catalysts [3,4]. An efficient incorporation of Ti in the MFI framework requires quite severe preparation conditions such as, for example, the absence of any trace of alkali cations. Consequently, the various routes leading to (Ti)-MFI or (Ti,B)-MFI necessarily involve the use of elaborated and cosily reactants such as for instance various Ti-alkylates and alkali free TPAOH. The aim of this work was to explore a series of more readily accessible, less time consuming and inexpensive alternative routes, yielding both (Ti)- and (Ti,B)- MFI zeolites. The preparation procedures described here are based on the use of alkali free mineralizing agents such as methylamine. This compound was shown to efficiently mobilize through complexation a wide series of metallic ions [5]. Moreover, it does not compete with TPA for the templating role in crystallizing MFI zeolites. Finally, its basic properties represent the advantage, not only to solubilize silica, but also to use of alkali free tetrapropylammonium bromide (TPABr) instead of using the costly tetrapropylammonium hydroxide (TPAOH) [6].
* Present address: Kao Corporation, Tokyo Research Laboratories 1-3, Bunka 2-chome, Sumidaku, Tokyo 131, Japan
246 We also wanted to test a recently elaborated method that consists in adding ingredients into an already preheated gel precursor to silicalite, in which a very low amount of silicalite crystals were already generated from preliminary heating. This procedure was designated as the in situ seeding method because the silicalite crystals were shown to act as true fresh seeds dramatically accelerating the crystallization of various MFI metallosilicates [7].
2. EXPERIMENTAL 2.1 Synthesis of titanosilicates and boro-titanosilicates The short chain amine used as the mineralizing agent was methylamine (aq. 40% from Fluka). Alkali-free TPABr (Janssen Chimica) was used as a template for the MFI structure. Titanium tetrachloride (Merck) and boric acid (UCB Chemicals) were the main Ti and B reagents respectively. Although SiC14 can be advantageously co-hydrolyzed with TiC14 so as to yield homogeneous Si-Ti hydrogels [8], fumed silica (Aerosil 200 from Degussa) was preferentially used in order to accelerate the crystallization process. Some other potential complexing agents towards titanium, namely hydrogen fluoride, hydrogen chloride and oxalic acid, were optionally added to the initial gels. Two procedures for preparing the reaction gel were selected.
i) Conventional synthesis Titanium tetrachloride was carefully added to an aqueous solution of TPABr and boric acid which was cooled to 5~ beforehand. A complexing agent was optionally added to the solution. After adding SiO 2 in small portions, the resulting gelatinous suspension was admixed with methylamine. Optionally, ground silicalite crystallites were added as seeds to the reaction gel.
ii) In situ seeding synthesis Titanium-free borosilicate or silicate gel were prepared similarly to the above described method and heated at 185~ for 6h in autoclaves. The so-obtained intermediate phase (gel and about 3-5 wt. % of borosilicate or silicalite crystals generated "in situ" and considered as "seed" ) was cooled to ambient temperature and then mixed with TiCI4, prior to a second heating step. The reaction gels prepared by the both methods were heated at 185~ for several days in Teflon coated stainless steel autoclaves, in static conditions. The final solids were filtered, washed thoroughly with cold water and then dried at 100~ for 12h. More detailed synthesis conditions and some properties of the various crystalline phases are summarized in Table 1. For products still containing residual amorphous phase after heating (thus that were not 100% crystalline), the unreacted gel could be removed by achieving phase separation with the help of an ultrasonic generator (ultrasonic washing), before measuring some characteristic properties of the pure crystals, such as their composition by EDX analysis or lattice constant determination. Calcination was performed under nitrogen (heating rate: 10~ from 20 to 550~ the samples were further maintained at 550~ for 6h in flowing air prior to cooling.
2.2 Characterization All the products were checked for their nature and purity by SEM (Philips XL 20 electron microscope). Spot EDX quantitative analysis of Si and Ti on selective areas (core and edge) across individual crystallites was performed using an EDAX P.V. 9800 Phillips analyzer coupled with SEM. X-ray powder diffraction patterns were recorded on a Philips P.W. 1349 / 30 diffractometer (Cu-K0~ radiation) using a-alumina as internal standard. Crystallinities were also determined by probing the internal pore volume by n-hexane adsorption at 90~ within a
247 Stanton Redcroft ST-780 thermobalance (simultaneous TG-DTA-DTG), as described previously [9]. FT-IR spectra were obtained using a Bio-Rad FTS-60A spectrometer with KBr pellets containing 1 wt. % sample. UV-vis spectra were recorded by a Shimadzu UV-3100PC spectrometer. ~IB-MAS NMR spectra were recorded on a Bruker MSL-400 spectrometer. The detailed experimental conditions for NMR were described elsewhere [ 10]. 3. RESULTS AND DISCUSSION 3.1 Crystallization of titanosilicates While 100% crystalline silicalite was formed after 2 days heating in the methylamine medium, no crystalline phase was detected after heating the same gel for 14 days in the presence of TiC14 (TS-I-a). This shows that, as in the case of A13+ ions and Ga 3+ ions [6], the presence of Ti 4+ impedes the growth of MFI framework. The absence of any XRD peaks corresponding to TiO 2 (anatase ...) indicates that the amorphous phase is still composed of stable Ti-Si oligomers. Indeed methylamine and TIC14 admixtures heated under similar conditions without silica, yield pure anatase. The effect of some other potential Ti-complexing agents on the crystallization of titanosilicates was examined. Adding either HC1 or oxalic acid did not improve the crystallization kinetics, the reaction mixtures being still amorphous after 10 days heating (TS-II-a and TS-VI-a). In contrast, the presence of hydrogen fluoride markedly increased the kinetics of crystallization. A highly crystalline (84%) MFI product was obtained after 6 days heating (TSIII). Indeed, fluoride ions are excellent complexing (solubilizing) agents towards silica [ 11 ]. As they also form soluble complexes with titanium ions, it is expected that their adding would increase the solubility of both titanium and silicon species, even at basic pH values achieved in the methylamine medium, and hence improve the crystallization kinetics. The most efficient way to dramatically accelerate the crystallization of titanosilicates was the use of the in situ seeding method (TS-I-b). Compared with classical seeding (dry silicalite added to the final gel prior to heating, TS-I-c), the in situ seeding method proved to be far more efficient: 81% crystalline titanosilicate was obtained after 6 days heating, versus a 50% crystalline phase recovered after 10 days heating in the former case. Homogeneous crystallites were also as rapidly obtained when TIC14 was diluted with either HC1 or oxalic acid solution, before it is added to the preheated silicate gel (TS-II-b and TS-VI-b). 3.2 Crystallization of boro-titanosilicates In the case of titanosilicates synthesized by the conventional method and without using F-ions, the reaction mixture was still amorphous after 14 days heating (TS-I-a). However, upon simultaneous adding of boron and titanium (BTS-I involving 4 B and 4 Ti / 96 T atoms; T=Si+B+Ti), some MFI crystals were detected (34% crystallinity after 14 days heating). As the amount of initial titanium decreased from 4 mole / 96 T to 0 while the boron concentration stayed constant (4 B / 96 T), the crystallization accelerated and a shorter time was required to obtain highly crystalline products (compare BTS-I, BTS-III-a, BTS-IV and BS, Table 1). These findings indicate that the presence of titanium in the initial gel inhibits the crystallization; irrespectively to boron, that accelerates the crystallization. Adding of hydrogen fluoride led to the rapid crystallization of boro-titanosilicate (BTSII) just as in the case of titanosilicate synthesis. The in situ seeding method was also tested in the case of boro-titanosilicates. The preheated borosilicate gel without titanium contained a small amount of borosilicate crystallites (5% crystallinity estimated from n-C6 sorption data). As in the case of titanosilicates, the in situ seeding procedure accelerated the crystallization rate of boro-titanosilicates (compare samples BTS-III-a and BTS-III-b, Table 1).
Table 1 Boro-titanosilicatessynthesized through the methylamine route Samples (code) Silicalite TS-I-a TS-I-b TS-I-c TS-11-a TS-11-b TS-111 TS-IV TS-V TS-VI-a TS-VI-b BS BTS-I BTS-I1 BTS-111-a BTS-111-b BTS-IV
Procedure
Complexing ~el.comu.') agent (z mol) B Ti
conventional
-
conventional in situ seeding conventional (seeds) conventional in situ seeding conventional conventional conventional conventional in situ seeding conventional conventional conventional conventional in situ seeding conventional
-
-
8.0 HCl 8.0 HCl 100 HF 100 HF 100 I-IF 20 ox. acid 20 ox. acid -
50 HF
-
-
-
'
Cry st. time Crystallinity
~roducts'
(days)
(%)
B')
2
100
-
Unit cell
T~(EDx)~)T~(xRD)~)vo~u~~ (A3)
-
-
5339
Amorph. 81 50 Amorph. 57 84 100 88 Amorph. 75 100 34 84 75 96 100
Gel composition: (96-x-y) Si02 - x H3B03 - y T i c 4 - 25 TPABr - 300 MeNH2 - z complexing agent - 3500 H20 1) mole 1 96T atoms (T=Si+B+Ti) 2) by 1 1 NMR ~ 3) by EDX surface analysis 4) calculated from the unit cell volume expansion and taking into account the contraction due to the presence of framework boron
249 SEM images indicated that uniformly and similarly shaped crystallites were obtained, from both the conventional and the in situ seeding method. 3.3 Characterization of titanosilicates IR spectroscopy proved to be an easy tool to characterize titanosilicates. The unique band characteristic of titanosilicates and occurring near 960 cm-1 has been attributed to an asymmetric stretching mode of SiO4 bonded to Ti4+ ions in tetrahedral zeolite sites [ 12]. The titanosilicate TS-III synthesized by the conventional method in the presence of HF showed a distinct IR peak at 960 cm ~ which is not detected in the spectrum of silicalite (Fig. 1). All titanosilicates synthesized by the in situ seeding method also clearly exhibit this band. The unit cell volume variation calculated from the shift of specific XRD lines also gives important information on the possible heteroatom incorporation in the zeolite framework. When a Ti 4+ ion, larger than the substituted S i4§ incorporates the MFI framework, the longer Ti-O bond results in an expansion of the unit cell volume of the titanosilicate with respect to that of silicalite. The unit cell parameters calculated from XRD data for various titanosilicate phases are presented in Table 1. The unit cell volume of silicalite synthesized from the methylamine media is identical to the one characterizing silicalite grown from current media using TPAOH. The unit cell volume of titanosilicates synthesized from gels involving initial Ti / 96 T ratios of 4 and 6 through conventional procedures in the presence of HF (samples TS-III and TS-V) showed a marked increase (about 5370 ,~3) with respect to that of silicalite (5339 A3), while the lattice of the sample TS-IV, involving 2 Ti / 96 T initial ratios is logically slightly less expanded (5362 ~3). The unit cell volume of samples obtained by the in situ seeding method (TS-I-b and TS-VI-b) also showed a similar increase (respectively 5366 and 5361 ,~3) with respect to that of silicalite. These results strongly support the IR data suggesting that titanium ions were indeed incorporated in the framework. A relationship between the unit cell volume of a titanosilicate and the quantity of lattice Ti has been proposed [12]. Using this correlation, we could evaluate the amount of lattice titanium in the different titanosilicate phases obtained by using the methylamine route. The incorporation extends to about 1.7 Ti / 96 T for initial ratios of 4 Ti / 96 T, thus smaller than the amount measured in crystals grown from conventional media involving Ti and Si alkoxides and TPAOH (about 2.5 Ti / 96 T, as reported in the original patent [13]). Sample TS-III (titanosilicate synthesized in the presence of HF by the conventional method) showed a neat UV band at about 230 nm, corresponding to isolated framework titanium ions involving a tetrahedral coordination [ 14]. This constitutes a further evidence of the existence of framework titanium ions. The sample synthesized from gels involving initial 6 Ti / 96 T ratios (TS-V) exhibits, along with this UV band, an additional very weak shoulder at 320 nm which was not detected in samples TS-III and TS-IV (respectively involving initial ratios of 4 Ti / 96 T and 2 Ti / 96 T). This broad band has been attributed to octahedrally coordinated Ti in anatase [14]. These findings indicate that a too large amount of titanium (more than 4 mole / 96 T ) in the initial gel leads to the formation of anatase as side phase, in quantities too small to be detected by XRD. 3.4 Characterization of boro-titanosilicates l lB-NMR has been shown to be the key technique to determine the quantity and the coordination of boron in various borosilicate [10,15] and boro-titanosilicate [4] zeolites. All MFI-borosilicates [16] and boro-titanosilicates (BS and BTS samples, Table 1) synthesized in the presence of methylamine showed only one narrow peak at -3.6 ppm which was assigned to framework tetrahedral BO 4 units. No peaks occurring at +6 ppm (trigonal boron) and at-2 ppm (tetrahedrally coordinated boron in amorphous phase) were detected, confirming the efficiency of the methylamine route in incorporating boron in the T sites of the MFI structure, sole or in the presence of titanium.
250 The amount of boron in as-synthesized I crystallites was calculated from llB-NMR line intensities using the appropriate calibration curves [ 10,15] and the results are reported in Table 1. Trong On et al. [3] reported that boro-titanosilicates synthesized in the presence of TPAOH and H202 contained almost the same amount of boron as the corresponding Ti-free borosilicates: they coneluded that the quantity of titanium in the initial gel did not affect the amount of framework boron under l I I ' I I i their synthesis conditions. In contrast, the boron 1400 1200 1000 800 600 400 content in our boro-titanosilicate phases varied with Wave number (cm- 1) the initial quantity of titanium. The amount of Fig. 1 IR spectra of zeolites synthesized 2.72 mole B per unit cell from methylamine media decreased to 1.86 mole (a) TS-III: titanosilicate (conventional synthesis) when titanium ions (1 Ti / (b) TS-I-b: titanosilicate (in situ seeding) 96 T) were added in the (c) BTS-II: boro-titanosilicate initial gel (sample BTS-IV) (d) BS: borosilicate and a further increase of the titanium content up to 4 mole/96 T led to a further slight decrease of the boron amount (sample BS, to be compared to BTS-IV, BTS-III-a and BTS-I). These findings support the assumption that methylamine forms complexes with both Ti and B species while it also solubilizes the various Si species by bringing the pH to basic values. The stability of these complexes could be mutually affected by assuming that the presence of titanium ions in the system prevents to some extent the formation of the adequate boronmethylamine complexes, and vice-versa, at least at certain stages of the crystallization process (see below). The IR spectra of several metallosilicates are shown in Figure 1. All samples show the IR bands at 455, 800 and 1100 crn- that typically characterize the vibration of the SiO 4 tetrahedra in the MFI framework, as well as the band at 555 cm-1 which corresponds to the vibration of five membered rings in the pentasil structure. In addition to these bands, the borosilicate shows a shoulder peak at 920 cm -I and very week band at 1380 cm ~. The IR band at 920 cm 1 has been assigned to tetrahedrally coordinated framework boron, while the one at 1380 cm -~was found to correspond to trigonal boron species [ 17]. As described above, the titanosilicate samples as well as the BTS-II sample showed a quite well resolved band at 960 cm 1 which is generated by the presence of the lattice titanium. The combination of 11B-NMR and IR data confirms the simultaneous presence of Ti and B in the framework of boro-titanosilicate crystals. The framework titanium content in the bulk of the boro-titanosilicates could be approximately estimated from the unit cell volumes calculated from XRD data, taking into Ca)
/'x
251 account the lattice contraction caused by the presence of boron; the concentration of which was evaluated by NMR (Table 1). The maximum framework Ti concentration as evaluated by XRD does not exceed 0.7 mole / 96 T. All the values are actually slightly lower than those found in the corresponding titanosilicates: 1.3 to 1.6 mole / 96 T, suggesting that the presence of boron also hinders the incorporation of titanium into MFI frameworks. Spot EDX measurements of the Ti concentrations in individual crystallites also indicate that boro-titanosilicates contained less titanium than the corresponding titanosilicates. On the other hand, the higher EDX values with respect to the global bulk Ti concentrations evaluated by XRD could indicate that the surface of the boro-titanosilicate samples is slightly enriched in Ti with respect to their core. Indeed, in the case of large crystals, the EDX technique would probe elemental concentrations more readily in their outer shell region than in their core [6, 18]. These findings suggest that the hindering of Ti incorporation by the presence of boron is apparent and essentially occurs in the beginning of the crystal growth process, when boron is more readily incorporated in the framework than Ti, possibly in relation to the different complexation power of both metals with methylamine or HF. By contrast, similar EDX and XRD values suggest that the corresponding titanosilicates involve a less pronounced Ti gradient throughout the crystallites when they are crystallized in the presence of both methylamine and HF mineralizing species (samples TS-III, -IV and-V). Moreover, titanosilicates involve a quasi homogeneous Ti framework distribution when methylamine is the only mobilizing species used (sample TS I-b), as already observed for A1- and Ga-MFI [6]. The gradient concentration of Ti in large crystals grown in the presence of various mineralizing-complexing agents is now being investigated by combining XPS data with other depth profiling evaluation techniques [19]. Finally, while all the titano- and boro-titanosilicates samples showed the typical UV band at about 230 nm corresponding to isolated framework titanium ions in tetrahedral coordination, all the boro-titanosilicate samples exhibited a very weak additional shoulder which was attributed to anatase impurities. This could confirm that the addition of boron in the initial gel first leads to the easy formation of a borosilicate (Ti poor) framework (core of the final crystal) and that more Ti is preferentially incorporated during a later stage of the growth process when the gel is somewhat depleted in boron, resulting in a Ti enriched outer rim. This situation would result in an overall restricted Ti incorporation in the zeolite so that any excess of Ti would incorporate in the other Ti species currently stable under mild hydrothermal conditions, such as anatase. 4. C O N C L U S I O N Pure MFI type titanosilicates and boro-titanosilicates were synthesized rapidly and in good yield by using the new procedure involving alkali free alkylamine (optionally admixed with HF) as mobilizing agent, and TPABr. Silica and titanium tetrachloride were used as Si and Ti sources respectively. One of the advantages of this new procedure compared to conventional methods leading to fitanosilicates, is that highly crystalline titanosilicates can be obtained without the use of costly TPAOH or Si and Ti alkoxides. The presence of titanium in the methylamine bearing gel slows down the crystallization of Ti-MFI. Its crystallization could be dramatically increased by a further addition of F ions. Compared with titanosilicates, boro-titanosilicates crystallize fairly rapidly in methylamine media. The in situ seeding method (addition of titanium reactants into a preheated silicate or borosilicate gel) markedly accelerated the crystallization of both fitanosilicates and borotitanosilicates, the obtained crystals still exhibiting a uniform shape and size. IR data, the lattice volume expansion (XRD) and UV-vis spectroscopy confirmed the incorporation of titanium into the MFI framework of titanosilicates or boro-titanosilicates grown by using either the conventional or the in situ seeding methods, llB-NMR confirmed the presence of framework boron along with titanium in the framework of boro-titanosilicates. The
252 amount of boron in boro-titanosilicates is lower than in the corresponding borosilicates. The relative hindering of Ti incorporation by the presence of boron and vice-versa was interpreted by considering the preferential incorporation of boron in the beginning of the growth process allowing an easier further Ti incorporation at the end of the crystallization. These somewhat sequential incorporation may be related to the different strength of the corresponding metallic complexes with HF and/or methylamine. This can also explain the formation of traces of extra framework Ti species (anatase) during the last stages of the growth. REFERENCES
10 11 12 13 14 15 16 17 18 19
M.G. Clerici and. P.J. Ingulina, J. Catal. 140 (1993), 71 J.S. Reddy, R. Kumar and S.M. Csicsery, J. Catal., 145 (1994), 73 D. Trong On, S. Kaliaguine and L. Bonneviot, J. Catal.,157 (1995), 235 D. Trong On, M.P. Kapoor, S. Kaliaguine, L. Bonneviot and Z. Gabelica, Stud. Surf. Sci. Catal., 97, (1995), 535 Z. Gabelica, R.J. Monque and G.P. Giannetto, US Patent N ~ 5 354 719 (1994) Z. Gabelica, G. Giannetto, F. Dos Santos, R, Monque and R. Galiasso, in "Proc. 9th Intern. Zeolite Conf., Montreal, 1992, R. Von Ballmoos et al., Eds,Butterworth- Heinemann, New York, 1993, Vol. I, pp 231 Z. Gabelica, M. Wiame, J. G6rard and M. Shibata, submitted for publication M. Shibata, J. G6rard, T. Inui and Z. Gabelica, submitted for publication Z. Gabelica, J. B.Nagy, E.G. Derouane and J.P. Gilson, Clay Minerals, 19 (1984), 803 Z. Gabelica, J. B.Nagy and G. Debras, Stud. Surf. Sci. Catal., 19, (1984), 113 H. Kessler, J.M. Chezeau, J.L. Guth, H. Strub and G. Coudurier, Zeolites, 7, (1987), 360 M. R. Boccuti, K. M. Rao, A. Zecchina, G. Leofanti, and G. Petrini, Stud. Surf. Sci. Catal., 48, (1989), 133 M. Taramasso, G. Perego and B. Notari, US Patent N ~ 4 410 501 (1983). A. Zecchina, G. Spoto, S. Bordiga, A. Ferrero, G. Petrini, G. Leofanti, and M. Padovan, Stud. Surf. Sci. Catal., 69, (1991), 251 Z. Gabelica, J. B.Nagy, P. Bodart and G. Debras, Chem. Letters, 1984, 1059 Z. Gabelica and M. Shibata, submitted for publication R. de Ruiter, J.P.M. Kentgens, J. Grootendorst, J.C. Jansen, and H. Van Bekkum, Zeolites, 13, 128 (1993). Z. Gabelica, N. Blom and E.G. Derouane, Appl. Catal., 5, (1983), 227 M. Shibata, Ph.D. Thesis, in preparation
H. Chon, S.-K. lhm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
253
Synthesis, characterization and c a t a l y t i c p r o p e r t i e s of vanadium containing VPI-5 Karuna Chaudhari, Tapan Kr. Das, A.J. Chandwadkar, J.G. Chandwadkar" and S. Sivasanker* National Chemical Laboratory, Pune 411 008, India 'Chemistry Department, University of Poona, Pune 411 007, India A novel vanadium containing molecular sieve with VPI-5 structure has been synthesized XRD, UV-vis, ESR, NMR and sorption studies suggest that the vanadium ions are in the framework positions. V-VPI-5 is found to be active in the catalytic oxidation of large organic molecules such as 2-methyl naphthalene in the presence of both organic peroxides and 1-1202. 1. INTRODUCTION A number of vanadium containing molecular sieves possessing selective oxidation properties have been reported so far. However, the application of these molecular sieves as catalysts is limited as all the materials, except the mesoporous vanadosilicate, V-MCM-41, catalyse the oxidation of only small molecules. VPI-5 is a large pore aluminophosphate molecular sieve with pores made up of 18-membered tings and having a diameter of-- 1.2 nm [1]. We now report the synthesis, characterization and catalytic properties of vanadium containing VPI-5. The physico-chemical characterization of the samples reveals the presence of V ions in the lattice. Besides the material is found to catalyse the selective oxidation of the large molecule, 2-methyl naphthalene in the presence of both organic peroxides and 1-1202. 2. EXPERIMENTAL The synthesis of V-VPI-5 was carried out hydrothermally using psuedoboehmite (CatapalB, 73% A1203, Vista Chemicals), vanadyl sulphate trihydrate (Aldrich, 99.99%), phosphoric acid (S.D. fine chemicals, 85% solution) and tetrabutyl ammonium hydroxide. The composition of the gel in terms of moles of oxides was Al203 " P205 "x VO2" TBAOH 950 H20 ; x < 0.04. In a typical s3nlthesis, pseudoboehmite (4.2 g, Catapal-B) was mixed with water (8.0 g). A diluted solution of orthophosphoric acid (85%; 6.93 g) in water (3 g) was added slowly with stirring to the alumina slurry. Vanadyl sulphate trihydrate (0.521 g) dissolved in water (2.0 g) was then added to the above AIPO4 gel and aged at 298K for 24 hours. Then, tetrabutyl ammonium hydroxide (19.46 g; Aldrich, 40 wt.% solution) was added to the aged gel and stirred for one hour. The gel was transferred to a stainless steel autoclave and allowed to crystallize at 418K for 24 hours. The crystalline material thus obtained was washed with hot water to remove occluded templates. The sample was dried at room temperature. The sample was treated with NH4OAc to remove surface vanadium present [2]. It was then treated two times with dilute H202 (1"10, wt.) to convert the V4§ to V s+ , washed with water and dehydrated slowly at 623K in high vacuum overnight. For comparison purposes, a sample
254 of VPI-5 was synthesized following the procedure reported earlier [3]. The as-synthesized, dried and oxidized samples were examined for phase purity by XRD, morphology by SEM and the state of vanadium by UV-vis, NMR and ESR spectroscopies. The V4+ and V 5+ contents in the as-synthesized, NI-I~OAc washed and H202 treated samples were estimated by chemical analysis: atomic absorption spectroscopy, inductively coupled plasma analysis and permanganometric titrations [4] (Table 1). Adsorption of water, n-hexane and tri-isopropyl benzene was carried out to ascertain void volumes. The catalytic reactions were carried out in a batch (100 ml) reactor in organic solvents or in aqueous medium at 353K. 3. RESULTS AND DISCUSSION
3.1 Chemical composition The four V containing VPI-5 samples were prepared with different V (as V4§ inputs in the gel (Table 1). The percentage incorporation of vanadium in the crystalline material increased with decrease in V-content in the gel. On extraction with NH4OAc (1 M solution), a small quantity of extralattice V (mostly V4+) was removed from all the samples. In the case of the sample with the least vanadium content (V-VPI-5(1)), only 3% of the vanadium ions was extracted, while a much larger and nearly constant amount of --11-13% vanadium was extracted from the other (V-rich) samples. After H202 treatment only V 5§ was present in the samples. Table 1 Chemical composition of V-VPI-5 samples. Composition (atom ratio); A I : P : V( %V4+, %V5+)'
Sample Synthesis gel
As-synthesized
NH4OAc treated
H202 treated
V-VPI-5(1)
1:1:0.0033
1:0.98:0.0032(92,8)
1:0.98:0.0031(83,17)
1:0.98:0.0031
V-VPI-5(2)
1:1:0.0100
1:0.97:0.0084(81,19)
1:0.97:0.0075(70,30)
1:0.97:0.0075
V-VPI-5(3)
1:1:0.0200
1:0.96:0.0140(76,24)
1:0.96:0.0123(66,34)
1:0.96:0.0123
V-VPI-5(4)
1:1:0.0400
1:0.98:0.0160(71,29)
1:0.98:0.0140(62,38)
1:0.98:0.0140
'The numbers in brackets refer to the % of V4+ and V 5§ in the samples.
3.2 X-ray diffraction The XRD patterns of the VPI-5 and the V-VPI-5 samples are presented in Figure 1. They are all similar and do not reveal the presence of impurity phases. Since V4+ and V 5+ are larger than AI3+ and pS+ (ionic radii : V4+(CN = V), 0.053; Vs+(IV), 0.036; AIa+(IV), 0.039 and Ps§ 0.017 nm), their incorporation in the lattice of the VPI-5 is expected to increase its unit cell size [5,6]. We do observe a systematic increase in the d-values of all the lines in the XKD patterns, on incorporation of vanadium in VPI-5. For example, in the case of the typical line at 20 -- 5.38 ~ the d-value increases from 1.6412 nm (VPI-5, pattern A in Figure 1) to 1.6536, 1.666, 1.6695 and 1.6723 nm (V-VPI-5, patterns B, C, D and E respectively in Figure
255 1). The above results suggest that V ions are probably incorporated in the framework in the samples. It is likely that the vanadium ions are replacing both AI3+ and pS+. The V4§ ions probably replace both AI3+ and pS+, the V 5+ ions replacing preferentially the pS+ ions. However, in view of the much larger radii of the V 4§ and V s§ ions (compared to the AI3+ and P~+ ions), the isomorphous replacement is like to lead to defect sites in the framework as has already been reported by earlier workers in the case ofvanadosilicates [7]. The crystallinity of all these samples remained the same even on vacuum dehydration. 3.3 SEM The SEM pictures do not reveal noticeable differences in morphology between V free and V containing samples. The crystals are seen as long needles (10-60 ~tm) arranged in a definite manner to produce agglomerates of 300 to 500 l.tm diameter, the entire structure suggesting growth from the center to the outer surface of the agglomerates (Figure 2).
I0
20
30
40
20---=-
Figure 1. XRD patterns of as-synthesized samples. A:VPI-5, B:V-VPI-5(1), C:VVPI-5(2), D:V-VPI-5(3), and E:V-VPI5(4).
Figure 2. SEM picture of as-synthesized V-VPI-5(3).
256 3.4 UV-vis
The UV-vis reflectance spectra of the as-synthesized and H202 treated samples revealed a band around 320 nm in both the samples. This band has been suggested to be due to low energy charge transfer (LCT) between oxygen and V 5+ ions present in tetrahedral environments [2,8]. The intensity of the band did not decrease on extraction of the samples with NH4OAc suggesting that the V s+ species are strongly held in the samples and are probably present in the framework positions. The intensity of the 320 nm band increases on treatment of the sample with H202 due to an increase in V 5+ions in the samples. 3.5 ESR
All the as-synthesized vanadium samples exhibit sharp 8-line ESR spectra at room temperature. A representative spectrum of V-VPI-5(3) (Figure 3) reveals spectral parameters of g l l - 1.907, gL = 1.966, All - 204.28G and A• = 75G suggesting that V4+ ions are present in square pyramidal or distorted octahedral symmetries [9]. Sharma et aL [10] have shown that the magnitude of the quantity B (= Agj}/AgL) can be used to decide whether the species is in a distorted octahedral or a square pyramidal symmetry, values closer to 1 signifying nearly octahedral symmetry. The value of 2.6 obtained in our case suggests that the species are probably in a distorted octahedral symmetry. The intensity of the spectral lines decreases slightly on extraction with NH4OAc (Figure 3, spectrum B). The ESR parameters of the assynthesized sample (A) and the NH4OAc-extracted sample both after gentle drying at 300K and after calcination at 623K (sample B) were nearly identical.
C
2000
Figure 3. ESR spectra of V-VPI-5(4). A:as-synthesized, B:NH4OAc treated and C:H202 treated and dehydrated.
0
- 2 0 0 0 -~)(X) PPld
Figure 4. 51V NMPx spectra of oxidized and dehydrated samples. A:V-VPI-5(1), B:V-VPI-5(2) and V-VPI-5(3).
257 As the ESR spectra are attributable to distorted Oh species, it appears likely that the vanadium ions are attached to the framework at defect sites with partial framework connectivities, the additional ligands necessary for Oh co-ordination probably being tightly held water molecules. The samples lost ESR activity on oxidation with H202 and dehydration at 623K in vacuum (Figure 3, spectrum C). 3.6 NMR StV spectra of the oxidized and dehydrated V-VPI-5(1), V-VPI-5(2) and V-VPI-5(3) samples were recorded under static conditions using a Bruker MSL300 spectrometer (Figure 4). All these samples exhibit a single broad signal centred at 8 = -600 + 50 ppm (Figure 4). The near isotropic nature of the signals and their positions around -600 ppm suggest that these are probably due to Q0 species present in nearly regular tetrahedral positions and associated with O-ions [11,12].
It is likely that the V4§ions present in the octahedral symmetry (I; shown below) in the assynthesized sample transform into tetrahedral V s§ species (II) through oxidation and loss of H20 ligands during H202 treatment and evacuation. The absence of a signal at 8 = - 3 l0 ppm suggests that V2Os-like phases are not present in the samples.
H
H
~AI~o 0 O~V4~'OH HO~AI~ ,, OXIDN.--- ~ V HO--At -H20 / 7 0 / ' ! ~ O x AI 70 O~ 9jAI H/O\H ~ AI AI -qAk
x? /
4. SORPTION STUDIES Sorption measurements of water, n-hexane and tri-isopropylbenzene by VPI-5 and VVPI-5 samples were obtained at 298K ( P/po = 0.5) using a McBain-Baker type gravimetric unit having a calibrated silica spring. The sorption values obtained for VPI-5 are 30.9%, 17.3% and 9.4% for water, n-hexane and tri-isopropylbenzene, respectively. On incorporation of V (e.g. sample V-VPI-5(3)), these values were 31.53%, 17.8% and 9.8%, respectively. The similarity in values suggests that the incorporation of vanadium did not lead to blockage of the pores. In fact, the small increase in sorption by the V-VPI-5 sample probably suggests an increase in void volume clue to expansion of the lattice by the V-ions.
258 5. CATALYTIC ACTIVITIES The results of the hydroxylation of phenol with H202 and the oxidation of 2-methyl naphthalene with H202 and tertiary butylhydroperoxide (TBHP) over V-VPI-5 samples are presented in tables 2 and 3, respectively. Table 2 Activity of the samples in the hydroxylation of phenol' Sample
Conv. (mol.%)
H202 Select.(mol%)
Product distribution (wt.%) BQb
CATb
HQ b
V-VPI- 5( 1)
12.81
38.5
29.70
68.7
1.60
V-VPI-5(2)
14.88
44.7
24.60
67.3
8.10
V-VPI-5(3)
16.35
49.1
23.50
68.2
8.30
V-VPI-5(4)
18.10
54.3
15.28
70.6
14.12
V-VPI-5(4) NH4OAc treated
17.90
52.6
16.30
69.8
13.90
V-impregnated VPI-5 (2% V205)
1.00
3.9
50.00
25.0
25.00
'Reaction conditions : Catalyst = 100 mg, Solvent(water) = 10 g, Phenol/H202(mole) = 3, Temp. = 353K, Reaction time = 24 h, Substrate = lg. bBQ, benzoquinone; CAT, catechol; HQ, hydroquinone. The catalytic activity tests were carried out using H202 treated samples with and without NH4OAc treatment. Both the samples exhibited almost the same catalytic activities (Table 2). As some amount of vanadium was extracted by NH4OAc, it appears that only non-extractable vanadium (V in the framework) has been catalytically active. In general, in the case of both phenol hydroxylation and oxidation of 2-methyl naphthalene, the activities and peroxide selectivities increase with increase in V-content of the samples. This is similar to earlier reports over titano and vanadosilicates [ 13,14]. The hydroxylation of phenol produces more benzoquinone than generally reported over titanosilicates [13]. Such an overoxidation has already been reported in the case of the vanadosilicate, VS-2 [5]. Again, the oxidation of 2-methyl naphthalene produces more side chain oxidation products than ring hydroxylation products. Such a selectivity for side chain oxidation products has also been reported in the case of VS-2 during the oxidation of toluene [5]. The V-impregnated VPI-5 samples are found to possess very little activity confirming that the activity of V-VPI-5 samples arises from V-ions incorporated in the framework.
259 Table 3 Oxidation of 2-methyl naphthalene over V-VPI-5" Sample
Conv. (mol.%)
Product distribution (wt.%)
TBHP select. (mol.%)
2-naphthaldehyde
2-naphthalene methanol
Others b
V-VPI-5(1)
5.7
18.80
80.7
12.3
7.0
V-VPI-5(2)
6.8
21.63
75.0
16.1
8.9
V-VPI-5(3)
8.1
25.40
72.8
16.0
11.2
V-VPI-5(4)
9.7
30.00
70.1
18.5
11.4
V-VPI-5(4) c
8.6
26.60
71.2
17.5
11.3
V2Os(2%) impregnated VPI-5 "Reaction conditions" Catalyst = 100 mg, Solvent(acetonitrile) = 10 g, Substrate/TBHP(mole) = 2, Temp. = 353K, Reaction time = 24 h, Substrate = lg. bMostly ring hydroxylation products. cHzOz used as oxidant; reaction conditions same as in the case of TBHP runs except, substrate~zOz (mole) = 2. In general, medium pore vanado and titanosilicates such as VS-1 or TS-1 can not oxidize molecules such as 2-methyl naphthalene due to pore size restrictions. Besides,. they can not utilize TBHP as an oxidant due to its large size and spatial restrictions inside the pores. VVPI-5 is able to not only oxidize the large molecule, 2-methyl naphthalene, it is also able to use TBHP as the oxidant. As the impregnated VPI-5 samples do not possess any activity, the activity of the V-VPI-5 samples is due to the V-ions in the framework and inside the pore system. Thus, V-VPI-5 appears to be a good large-pore selective oxidation catalyst. 6. CONCLUSIONS Highly crystalline V-containing VPI-5 samples have been synthesized. These samples contain both V 4§ and V s§ mainly in framework positions. The small amount of non-framework ions (mostly V 4+) is easily removed by washing with an NH4OAc solution. While direct evidence for V-incorporation in the framework is obtained from XRD data, the ESR, UV-vis and NMR studies also suggest such incorporation. The V-VPI-5 samples are catalytically active in the hydroxylation of phenol and the oxidation of 2-methyl naphthalene.
260 ACKNOWLEDGMENTS We thank Dr. D. Chakrabarty for ICP analysis. TKD thanks CSIR for a research fellowship. SS thanks IFCPAR, New Delhi for partial funding. REFERENCES
.
9. 10. 11. 12. 13. 14.
M. Davis, C. Saldarriga, C. Montes, J. Garc~s aM C. Crowder, Nature, 331 (1988) 698. G. Centi, S. Perathoner, F. Trifiro, A.. Aboukasis, C.F. Aissi and M. Guelton., J. Phys. Chem., 96 (1992) 2617. M.E. Davis and D. Young, Stud. Surf. Sci. Catal., 60 (1991) 53. I.M. Kolthoff, Treatise on Analytical Chemistry, Interscience, New York, Part II, 8 (1963) 226,260. P.R.H.P. Rao, A.V. Ramaswamy and P. Ratnasamy., J. Catal., 137 (1992) 225. A. Tuel and Y. Ben Taarit, Appl. Catal., A 102 (1993) 201. T. Sen, V. Ramaswamy, S. Ganapathy, P.R. Rajamohanan and S. Sivasanker, J. Phys. Chem. (in press). A. Corma, J.M. Lopez Nieto and N. Parades, Appl. Catal., 104 (1993) 161. A. Davidson and M. Che, J. Phys. Chem., 96 (1992) 9909. V.K. Sharma, A. Wokaun and A. Baikar, J. Phys. Chem., 90 (1996) 2715. O.B. Lapina, V.M. Mastikhin, A..A.. Shubin, V.N. Krasilnikov and K.I. Zamaraev, Progress in NMR Spectroscopy, 24 (1992) 457. I. Moudrakovski, A. Sayari, C.I. Ratchffe, J.A. Ripmeester, K.F. Preston, J. Phys. Chem., 98 (1994) 10895. A. Thangaraj, R. Kumar and P. Ratnasamy, J. Catal., 131 (1991) 294. P.R.H. Rao and A.V. Ramaswamy, Appl. Catal. A, General, 93 (1993) 123.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
261
Crystallization of Titanium Silicalite-1 from Gels Containing Hexanediamine and Tetrapropylammonium Bromide A. Tuel Institut de Recherches sur la Catalyse. C.N.R.S. 2, av. A. Einstein 69626 Villeurbanne Cedex FRANCE
Abstract
Titanium substituted silicalite-1 (TS-1) samples have been synthesized using hexanediamine (C6DN) and tetrapropylammonium bromide (TPABr). Following this method, titanium could be incorporated up to a level of about 2 Ti/unit cell in the framework of silicalite-1. Crystals had a typical habit, very different from those obtained with standard syntheses, which led to exceptionnally high para/ortho ratios in the hydroxylation of phenol with hydrogen peroxide.
1. I N T R O D U C T I O N Usually, the titanium containing silicalite-1 (TS-1) is synthesized using tetrapropylammonium hydroxide (TPAOH) solutions, which provides both the templating molecule and the alkalinity necessary for the crystallization of the zeolite [1]. However, the synthesis requires severe conditions, particularly regarding the presence of alkali cations in the precursor gel [2]. Therefore, TS-1 is a quite expensive catalyst whose synthesis requires the preparation of alkali-free TPAOH solutions. Thus, it would be of great interest to synthesize TS-1 with TPABr as templating molecule as the latter is less expensive than TPAOH and usually free from alkali cations, in contrast to most of the commercial TPAOH solutions. Several authors have shown that pure silicalite-1 could be obtained from gels containing TPABr and an organic (or inorganic) base like piperazine or NH4OH [3,4]. However, crystals were usually very large and syntheses were complicated by the high volatility of NH4OH solutions. Because of the large crystals thus obtained, it was not conceivable to obtain Ti-containing silicalite-1 active in oxidation reactions in the liquid phase following these methods.
262 In the present paper, we report on the synthesis of TS-1 using TPABr as templating agent and hexanediamine (C6DN) as only base. The system was particularly interesting as C6DN has also been reported to direct zeolite structures in the absence of quaternary ammonium cations [5]. We have explored the possibilities of synthesizing TS1 by varying synthesis parameters such as the Ti content, the nature of the silica source or the crystallization conditions. Catalytic performance of such materials in the hydroxylation of phenol is reported and compared with that of a conventional TS-1 sample prepared with TPAOH.
2. E X P E R I M E N T A L TS-1 was synthesized in the following manner: 1.75 g of C6DN was first dissolved in 40 ml of distilled water before the addition of 3 g of silica A 200 (Degussa). Then 0.17 ml of tetrabutyl orthotitanate in 10 ml dry isopropyl alcohol was added and the resulting mixture stirred for about 30 min. Finally, 1.33 g TPABr in 25 ml of distilled water was added and the gel heated at 80~ for 3 h to remove isopropyl alcohol. The gel, whose composition was the following" SiO 2 - 0.01TiO 2 - 0.3C6DN - 0.1TPABr - 50H20 was transferred into a stainless steel teflon-lined autoclave and crystallized at 180~ for 5 days under stirring (250 rpm). The solid was then recovered, washed with distilled water and dried at 120~ in an oven. Occluded organics were removed by calcination of the sample in air at 530~ for 10 h. For comparison a TS-1 sample was prepared with TPAOH using a patent recipe [1] and contained 0.92 wt % Ti. Samples were characterized using conventional methods. X-ray powder patterns were recorded on a Philips PW 1710 diffractometer using the CuKa radiation. U.v-vis spectra were collected on a Perkin Elmer Lambda 9 spectrometer. I.r. spectra were obtained on a Perkin Elmer 580 spectrometer using KBr wafers (1 wt % zeolite). 13C Solid state n.m.r, spectra were recorded on a Bruker MSL 300 spectrometer using a 1H13C CP/MAS sequence with 2 ms contact time and 10 s delay. SEM pictures were collected on a Jeol microscope. Chemical analyses were obtained by atomic absorption after solubilization of the samples in HF-HCI solutions. The hydroxylation of phenol was carried out batchwise following a method previously described [6].
263 3. RESULTS AND DISCUSSION The X-ray powder pattern of as-synthesized sample (corresponding to the gel composition given in the experimental section) shows all the reflections characteristic of the MFI structure but reveals important differences in peak intensities, particularly those corresponding to (0k0) reflections as compared to a conventional TS-1. The systematic increase in (0k0) reflection intensities probably indicates the presence of preferential planes in the crystals. Indeed, crystals are in the form of elongated prisms of about 7 x 2.5 x 0.5 /~m (Fig: 1), a very unusual habit for TS-1 whose particles are generally very small and round-shaped.
(c)
IL--
Fig. 1. SEM pictures of sample 1 (Si/Ti = 76) (a), sample 3 (Si/Ti - 31) (b) and corresponding axes (c) The well developed (ac) planes are thus responsible for the intense (0k0) reflections in the X-ray pattern. As expected, 13C NMR spectrum of the as-synthesized sample shows that only TPA + cations are occluded in the zeolite pores. I.r. spectrum of calcined sample shows an absorption band at 960 cm "1, absent from the spectrum of the pure silica material, and characteristic of metal-substituted zeolites. The optical density of this line, 1960/1800, increases with the Ti content in the samples (Fig. 2). Moreover, the i.r. absorption band at 550 cm "1, due to the presence of double rings in the structure, is
264 clearly splitted, giving a band at 540 cm -1 more intense than that at 560 cm "1. As for Xray reflection intensities, this was attributed to the typical morphology of the crystals obtained by the present synthesis route.
u.c. (A3)
1960/1800 2.0
5380
o~~ ~
~~
5360
1.5
D
5340 _ o
o.6
o.b2
Ti/(Si+Ti)
o.6a
0
0.04
Fig. 2. Evoultion of the u.c. volume and of the optical density of the i.r. band at 960 cm -1 with the Ti content. 3.1. I n f l u e n c e o f t h e t i t a n i u m c o n t e n t
We have synthesized a series of samples by varying the amount of titanium in the synthesis gel. The chemical composition as well as the characteristics of the various samples are listed in Table 1. Table 1 Characteristics of the samples prepared with various Ti contents Si/Ti Sample
Gel
Zeolite
u.c. (~3)
1
100
76
5367 _+ 2
2 3 4
50 33 25
43 31 24
5384 _ 2 5387 _+ 2 5390 _+ 2
265 The unit cell volume increases linearly with the titanium content in the samples up to about 2 wt % Ti. Then, a deviation is observed for higher values, suggesting that samples do not contain only tetrahedrally coordinated titanium species (Fig. 2). Indeed, the presence of extraframework titanium in the samples prepared with low Si/Ti ratios was confirmed by u.v-vis spectroscopy. In contrast to samples 1 and 2 for which the spectrum was composed of a single absorption band at 210 nm, sample 3 shows an additional shoulder at 280 nm, resulting from partially condensed hexacoordinated titanium species [7]. However, we never observed a dinstinct band at 330 cm -1 characteristic of TiO 2. SEM pictures of the various samples show a change in the crystal habit and size with Ti incorporation (Fig. 1). Starting from about 0.1-0.2 for sample 1, the a/c ratio increases to 0.65 for sample 3. In the same time, the thickness (along the b-axis) increases with the Ti content from 0.3-0.4 ~tm for silicalite-1 to 1.5/~m for sample 3. The change in crystal habit has an influence on X-ray diffraction intensities and the relative intensities of the i.r. bands at 540 and 560 cm -1. For sample 3, both X-ray powder pattern and i.r. spectrum are very similar to those of a conventional TS-1 synthesized with TPAOH. 3.2. Effect of the diamine concentration Samples have been prepared with Si/Ti = 100 in the gel and varying the
C6DN/SiO 2 ratio from 0.075 to 1.2. As a general trend, this ratio has not a great influence on the composition of the TS-1 obtained (Table 2). However, as for the titanium content, increasing the amount of C6DN in the precursor gel changes the crystal morphology, from elongated to cubic-like particles. As an example, crystals of sample 9 prepared with C6DN/SiO 2 = 1.2 are very similar to those of sample 3 obtained with C6DN/SiO 2 = 0.3 but Si/Ti = 33 (see Fig. 1). Table 2 Characteristics of the samples synthesized with various amounts of hexanediamine Si/Ti Sample 5 6 7 8 9
C6DN/SiO 2 0.075 0.15 0.3 0.6 1.2
'
Gel
Zeolite
100 100 100 100 100
77 86 76 89 78
u.c. (/~3) 5349 5362 5367 5363 5365
_+ 2 +_ 2 _ 2 _+ 2 +_ 2
266 3.3. Influence of the TPABr concentration
Assuming 4 TPA + cations per unit cell of TS-1, the minimum amount required to fully crystallize TS-1 corresponds to TPA/SiO 2 = 0.042 in the gel. For lower values, there could be a competition between TPA + and hexanediamine to direct the MFI and ZSM-48 structures, respectively. However, it has been shown that addition of TPABr in a pure silica gel containing C6DN strongly favoured the formation of silicalite-1 with respect to ZSM-48, even for TPA/SiO 2 ratios as low as 0.025 [5]. We have performed a series of syntheses for which the TPA/SiO 2 ratio was varied from 0 to 0.1. For TPA/SiO 2 ratios > 0.045, highly crystalline TS-1 is obtained. For lower ratios, TS-1 is obtained together with amorphous material but Ti-ZSM-48 is never observed, suggesting that this zeolite can probably not be obtained under the present synthesis conditions. The amount of TS-1 in the batch, estimated from the intensity of the (501) reflection in the X-ray pattern, increases linearly with the TPA/SiO 2 ratio (Fig. 3).
100
I TS-1 Yield (%)
oj,,,.~,..~~
o
8O 6O 4O 2O O0
TPA/SiO2 !
0.025
I
0.050
!
0.075
0.1
,...._
Fig. 3. Percentage of TS-1 in the batch as a function of the TPA/SiO 2 ratio 3.4. Influence of the silica source
In conventional syntheses, the nature of the silica source greatly influences the morphology of TS-1 crystals and the Ti incorporation. In the present case, syntheses performed with tetraethyl orthosilicate or colloidal silica Ludox AS-40 led to TS-1 crystals very similar to those obtained with A 200, indicating that the nature of the silica
267 source was not as critical as for syntheses performed with TPAOH [8]. Moreover, the Ti content was approximately the same in all samples.
3.5. Catalysis Among all the reactions catalyzed by TS-1, the hydroxylation of phenol is of particular interest as it permits to obtain para and ortho dihydroxybenzenes with an excess in the para isomer. However, Van der Pol et al. [9] have reported that the crystal size strongly influences the performance of the catalyst and that good activities can only be obtained with TS-1 crystals smaller than 0.5 um. Using the Weisz theory., the authors could predict the catalytic efficiency, assuming that the average length of the diffusion path was half the dimension of the crystals along the b-axis. Following the present synthesis route, we have shown that it was possible to prepare TS-1 crystals with well developed (ac) planes and a dimension along the b-axis of 0.3-0.4 ,urn (sample 1), thus leading to an average length of the diffusion path of 0.15-0.2 um. Therefore, these TS-1 samples are expected to have a catalytic efficiency similar to that of conventional TS-1 crystals of 0.3-0.4 um diameter. Results of the hydroxylation of phenol over sample 1 are reported in Table 3 and compared with those obtained over the standard TS-1. Results show that sample 1 is active in this reaction and that the p/o ratio at the end of the reaction is higher than that obtained over the standard TS-1. This can be attributed to the lower external surface of sample 1 as compared to the standard TS-1 (about half that of TS-1, as estimated from SEM pictures). Indeed, we have reported that catechol, the ortho isomer, was preferentially formed on the surface sites of the crystals whereas hydroquinone, the para isomer, was essentially formed inside the channels [10]. Table 3 Hydroxylation of phenol over sample 1 and a conventional TS-1 sample. Sample
t (min)
Cph (%)
SH202 (%)
CAT (%)
HYD (%)
HYD/CAT
1
60 120
15 19
65 90
4 5
10 13
2.5 2.6
TS-1
60
20
95
6.5
12.5
1.9
Cph is the phenol conversion and SH20 2 the selectivity in dihydroxybenzenes. CAT: catechol, HYD: hydroquinone
268 4. CONCLUSION We have shown that it is possible to synthesize highly crystalline TS-1 from gels containing hexanediamine and TPABr as organic molecules. The present synthesis route leads to a maximum Ti incorporation of about 2 Ti/u.c. Crystals have an unusual habitfor TS-1, very different from that obtained in conventional syntheses performed with TPAOH. The size and morphology of the crystals strongly depend on the synthesis parameters, particularly the Ti and C6DN concentrations. Highly crystalline TS-1 was obtained for TPA/SiO 2 ratios > 0.045; for lower ratios, amorphous material was also observed. Samples obtained following the present synthesis route and having a relatively low Ti content have shown to be active in the hydroxylation of phenol with hydrogen peroxide.The para/ortho ratio at the end of the reaction was higher than that obtained over a conventional TS-1 sample, which was attributed to the difference in external surface areas between the two samples.
5. REFERENCES
10
M. Taramasso, G. perego and B. Notari, US Pat 4 410 501 (1983). C.B. Khouw and M.E. Davis, J. Catal., 151 (1995) 77. K.R. Franklin and B.M. Lowe, Zeolites, 8 (1988) 501. M. Ghamani and L.B. Sand, Zeolites, 3 (1983) 155. K.R. Franklin and B.M. Lowe, Zeolites, 8 (1988) 495. A. Tuel and Y. Ben Taarit, Appl. Catal. A: General, 102 (1993) 69. T. Blasco, M.A. Camblor, A. Corma and J. Perez-Parient6, J. Am. Chem. Soc., 115 (1993) 11806. S. Gontier and A. Tuel, Zeolites (in press). A.J.H.P. Van der Pol, A.j. Verduyn and J.H.C. Van Hooff, Appl. Catal., 92 (1992) 113. A. Tuel, S. Moussa-Khouzami, Y. Ben Taarit and C. Naccache, J. Mol. catal., 68 (1991) 45.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
269
Synthesis and Characterisation o f Gallium and G e r m a n i u m Containing Sodalites Geoffrey M. Johnson and Mark T. Weller Department of Chemistry, University of Southampton, Highfield, Southampton, SO17 1BJ U.K.
A series of gallosilicate, alurninogermanate and gallogermanate halide sodalites have been synthesised and subsequently characterised using powder diffraction, infra-red and magic angle spinning nuclear magnetic resonance spectroscopy. Correlations of structural parameters with the spectra demonstrate trends similar to those found with the aluminosilicate analogues. 1. INTRODUCTION Aluminosilicate sodalites may be represented by the general formula M8[A1SiO4]6.X2 where M = Na, Li, Ag,..., and X = C1, Br, C104,... The sodalite framework consists of alternating SiO 4 and AIO 4 corner sharing tetrahedra forming four and six rings which make up the basic B-cage unit common to many zeolites. A semi-condensed structure is formed through linkage of the B-cages via six rings, with M4X clusters located at the centre of each cage as shown in Figure 1. Replacement of the framework aluminium and silicon by the larger gallium and germanium affects such properties as cell parameter and T-OT bond angles, where T represents the tetrahedral framework species. The introduction of larger framework atoms may also allow the enclathration of larger anions which cannot normally be incorporated into aluminosilicate sodalites. In this paper, a series of halide sodalites, Na8[ABO4]6.X 2, where A = A1 and Ga, B - Si and Ge and X = C1, Br and I, will be discussed and their structures and spectroscopic properties Figure 1: Sodalite cage containing M4X cluster compared with their aluminosilicate analogues. 2. EXPERIMENTAL All framework substituted sodalites were synthesised by hydrothermal reaction of basic solutions of A1203, Ga203, SiO 2, GeO 2 and an excess of the anion to be entrapped. Variation of pH, temperature and reaction stoichiometries has allowed the incorporation of a wide range of anions including CI', Br, I", CIO4", (SO4) 2" and (MOO4)2". MASNMR spectra were recorded on a Varian 300 MHz at Durham or a Bruker AM300
270 spectrometer at Southampton with typical spinning rates between 2.5 and 4.5 kHz. Reference samples used were TMS for 29Si, a saturated solution of Al(acac) in C6H 6 for 27A1 and 3 M Ga(NO3) 3 in D20 for 71Ga. IR spectra were recorded in the range 4000 to 500 cm 1 with a resolution of 2 cm l on a Perkin Elmer FT-IR 1710 spectrometer equipped with a Perkin 3600 data station. Powder neutron diffraction data were recorded on POLARIS at Rutherford Appleton Laboratories and D2B at The Institut Laue Langevin, Grenoble. 3. R E S U L T S 3.1 Structure Refinement All the products crystallised in the space group P43n with the cubic sodalite structure, and profile refinement of PND data achieved using the General Structure Analysis System (GSAS) l with the aluminosilicate analogues as starting models. Silicon/germanium was placed on the 6(c) ( 88189 site, galliurn/aluminium on the 6(d) ( 88189 site, sodium the 8(e) ( x , x , x ) site 2O w i t h x~0.19, and the 15 framework oxygen, O(1), on the 24(i) (x,y,z) site with x~0.14, y~0.15, -~ 5 z~0.45. The anion was placed at the cage r~ 0 centre on the 2(a) (0,0,0) site. Final refinement parameters ~ ~ ~b dz 1~ f6 1'8 and atomic positions Time Of Flight / milliseconds for aluminogermanate Figure 2: Profile fit to powder neutron data for aluminogermanate sodalite iodide sodalite are given as an example in Table 1 with the profile fit shown in Figure 2.
"•
i , i m u
Table 1 Final refinement parameters, atomic positions and thermal parameters estimated errors in parentheses for aluminogermanate iodide sodalite at 298 K Site
x
Na
8(e)
0.1935(1)
I
2(a)
0
AI
6(d)
Ge
6(c)
O
24(i)
z
Biso
0.1935(1)
2.156(2)
0
0
3.347(5)
0.25
0
0.5
0.535(7)
0.25
0.5
0
0.663(8)
0.43837(6)
1.214(5)
0.14547(9)
y
(A 2) with
0.1935(1)
0.1475(1)
271 Table 2 shows selected derived bond distances and angles from data collected at 298 K for aluminosilicate, gallosilicate, aluminogermanate and gallogermanate halide sodalites. The aluminogermanate results are in concordance with those reported by Fleet 2, and the structural parameters of gallosilicate chloride sodalite show good agreement with those of Newsam 3. Table 2 Selected derived bond distances (A) and angles (~ for aluminosilicate, gallosilicate, aluminogermanate and gallogermanate halide sodalites with estimated errors in parentheses AISi-Cl
GaSi-Cl
AISi-Br
GaSi-Br
AISi-I
GaSi-I
a
8.8812(3)
8.9603(6)
8.9304(3)
9.0000(3)
9.0318(4)
9.0742(3)
Si-O
1.623(6)
1.6341(4)
1.670(7)
1.6367(3)
1.665(8)
1.630(1)
A1/Ga-O
1.739(6)
1.7983(5)
1.683(7)
1.793(1)
1.687(8)
1.802(1)
Al/Ga-O-Si
138.10(1)
133.58(2)
140.40(1)
136.11(9)
144.50(2)
138.37(8)
Na-X
2.734(8)
2.6601(6)
2.872(8)
2.803(4)
3.151(9)
3.045(3)
Na-O
2.352(6)
2.3211(7)
2.363(7)
2.334(2)
2.374(8)
2.317(2)
4.53/2.33
3.18/1.88
8.15/3.91
4.40/2.76
5.54/2.60
3.71/2.10
AIGe-CI
GaGe-CI
AIGe-Br
GaGe-Br
9.0325(4)
9.1159(6)
9.09147(9)
9.1725(6)
9.1735(1)
9.26691(5)
Rwp/Rex
a
AIGe-I
GaGe-I
Ge-O
1.7282(9)
1.758(2)
1.7287(9)
1.750(2)
1.7277(9)
1.7825(1)
AI/Ga-O
1.753(1)
1.812(3)
1.752(1)
1.822(2)
1.752(1)
1.7875(1)
Al/Ga-O-Ge
133.06(3)
129.04(6)
134.86(3)
130.46(5)
137.52(4)
133.20(1)
Na-X
2.704(2)
2.676(3)
2.851(2)
2.808(3)
3.075(2)
3.0286(1)
Na-O
2.3331(9)
2.307(2)
2.3290(9)
2.299(1)
2.3276(8)
2.2998(1)
2.62/ 1.60
4.61/2.76
2.72/ 1.93
4.44/2.55
2.67/2.52
4.97/3.45
Rwp/Rex
3.2 MASNMR Spectroscopy 27A1, 29Si and 71Ga MAS NMR spectroscopy have been used to confirm the framework ordering indicated by profile refinement in the space group P43n by the observation of a single resonance corresponding to a single framework A(OB)4 site, where A =A1 and Ga and B = Si and Ge. The 29Si chemical shifts for gallosilicates are displaced typically 2.5 ppm upfield from their aluminosilicate analogues at a particular T-O-T angle. This fits in with the data reported for silicon surrounded by four silicons 4, and silicon
272
surrounded by four aluminiums5; as the silicon neighbour is changed from silicon to gallium to aluminium the resonance frequency is shifted downfield due to the increasing electron withdrawal from the shared oxygen deshielding the silicon nucleus. Although no correlations have been plotted here the general trends relating chemical shift with chemical shift and bond angle can be seen. In previously reported work on gallosilicate sodalites 6, a shift in resonance position of 10 ppm from Si(OA1)4 to Si(OGa)4 was observed. However, this was not just a consequence of Ga substitution, but due additionally to the associated change in T-O-T angle, a factor which affects chemical shift irrespective of whether or not framework substitution has taken place. A summary of the results obtained for halide sodalites of different framework composition is given in Table 3. Table 3 Chemical shift values (ppm) for a series of halide sodalites Chemical Shift Framework
Anion
29Si
27A1
71Ga
AlSi
Cl
-85.3
63.8
-
Br
-86.7
62.0
-
I
-89.1
60.0
-
GaSi
AIGe
GaGe
Cl
-78.8
-
-153.0
Br
-80.2
-
- 155.6
I
-82.3
-
-157.1
CI
-
69.7
-
Br
-
68.2
-
I
-
66.1
-
CI
-
-
- 137.1
Br
-
-
-140.5
I
-
-
-145.8
Less research has been carried out on the 27A1 nucleus due to its quadrupolar nature, thus complicating spectral interpretation. Trends relating chemical shift with structural parameters are again evident from the data, with chemical shift becoming more negative with increasing T-O-T angle due to the greater s contribution to the A1-O bond. The effect of germanium substitution is also clear: for the less electropositive germanium, 5 is higher than for the silicon species for a similar T-O-T angle. Examples of 71Ga M A S N M R spectra in the literature are scarce, and those reported show very broad bands indeed. For the sodalites discussed here, the Ga environment is close to the ideal tetrahedral angle of 109.48 ~ and hence quadrupolar interactions are reduced; this gives rise to sharp bands with typical halfwidths of 20 ppm. Correlations are analagous to 29Si and 27A1, with the chemical shift decreasing as the Ga-O-T angle increases.
273 3.3 Infra-red Spectroscopy The main IR framework absorptions of sodalites can be divided into four distinct categories: the asymmetric stretch, Vas(T-O-T ), located in the range 1200-900 cm "1, the symmetric stretch, Vs(T-O-T ), in the range 850-550 cml; the 6(O-T-O) bend in the region 550-400 cm "1 and the 6(T-O-T) bend < 300 cm l . The infra-red spectra of the framework substituted sodalites have been recorded: the 8 bands have not been studied since they are found at values beyond the lower limit of the instrument used here. Certain trends in the framework absorption positions are evident: for the asymmetric stretching band, Vas, the number of observable bands changes with framework composition. For the aluminosilicates the three modes are generally separated by 20 cm "1 and are observed as a single broad peak since they cannot be resolved. This is also the case for the gallogermanates, but with the gallosilicates and aluminogermanates two distinct bands can be seen. Henderson and Taylor 7 reported the IR absorptions of several framework substituted sodalites including all of the bromide versions, and a comparison of their data with that obtained in the current study is shown in Table 4. Table 4 IR absorption bands for framework substituted bromide sodalites. The results of Henderson and Taylor are given in parentheses; dashed line indicates no band reported IR Absorption bands Sample
Vas(T-O-T)
Vs(T-O-T)
Na8[GaSiO4]6.Br2
943(945), 927(-)
641(620), 552(550), 454(451)
Na8[AIGeOa]6.Br2
861(860), 839(-)
634(633), 602(607)
Na8[GaGeOa]6.Br2
801(785)
518(530)
The results from the halide sodalites studied here indicate that for a particular framework, as the halide size, and hence T-O-T angle, is increased Vs(T-O-T ) is shifted to lower energy, whereas Vas(T-O-T ) is shifted to higher energy. The substitution of Ge for Si for a particular anion shifts Vas(T-O-T ) to lower energy by approximately 85 cm 1, and Ga for A1 by approximately 60 cm 1. It is this band which shows the most significant change on framework substitution. Aluminosilicates have higher wavenumbers for all modes and show three modes for Vs(T-O-T); for aluminogermanates only two modes have been resolved, gallosilicates display two to three modes, and gallogermanates only one mode.
4. DISCUSSION 4.1. Structural Studies Table 2 above shows cell parameters and selected bond lengths and angles, indicating that the gallosilicates are slightly bigger than aluminosilicates, aluminogermanates larger still and gallogermanates the most voluminous. This is in agreement with the results of Nenoff et al. 8 for the Na3(ABO4)3.4H20 system who reported that, compared with the parent aluminosilicate, the approximate increase in cage size was 11.2 % for gallogermanate, 7.3 % for the aluminogermanate and 3.5 % for the gallosilicate framework. For a particular framework the increase in cell parameter associated with the entrapment of larger halide anions
274 is accompanied by an increase in T-O-T framework bond angle rather than any significant tetrahedral tilting. It is this particular structural property which principally accounts for framework expansion and collapse: as the cell expands the T-O-T angle becomes larger and assumes a greater degree of linearity. This variation in T-O-T angle is also evident when the structures of sodalites with different frameworks containing the same anion are examined: in order to attain reasonable bond distances for a particular entrapped anion, the larger frameworks undergo greater relative cell collapse which is principally accommodated by a decrease in T-O-T angle. The biggest T-O-T angle in this data set is that for aluminosilicate iodide sodalite, since the A1-O-Si bonds require this increased planarity in order to accommodate the large iodide ion; large tetrahedral anions such as permanganate 9 can also be incorporated within the aluminosilicate cage, the cell parameter and T-O-T angle for which are 9.10811(5) A and 149.16 ~ respectively. It is at these sorts of values that the limit of framework expansion is being reached; similarly large T-O-T angles for gallogermanates would allow the possibility of enclathrating much larger anions than aluminosilicates if appropriate synthetic conditions could be achieved. Such a study is currently being undertaken as part of this work. A theoretical modelling study of sodalitic species has been reported by Beagley et al. Io, in which isomorphous framework substitution has been investigated. The maximum cell parameter achievable for aluminosilicate, gallosilicate and aluminogermanate sodalites is reported as being 9.356, 9.590 and 9.696 A respectively, which is in turn governed by a maximum T-O-T angle of 160.5 ~ Although such large cell parameters may not be experimentally feasible, this demonstrates the potential of framework substituted sodalites for entrapping large anions. The difference between theoretical and experimental observations is epitomised by aluminosilicate sodalites: an estimation of the most stable framework shows an unstrained T-O-T angle of 144 ~ with a cell parameter of 9.01 A. However, experiment seems to indicate that an A1-O-Si angle of 138.1 ~ and cell parameter of 8.88 A corresponding to aluminosilicate chloride sodalite is the favoured structure. 4.2. MASNMR Spectroscopy Magic angle spinning nuclear magnetic resonance (MASNMR) spectroscopy has been used principally to confirm framework ordering. However, the positions of the resonances also provide information related to structural parameters such as T-O-T angle, demonstrated for a wide range of aluminosilicate sodalites by Weller and Wong 5. Structural parameters correlated with chemical shift include cell parameter, T-O-T angle, average T-T distance, degree of s hybridisation 11 and effective electronegativity 4 of the framework oxygens. All of these factors will be affected by substitution of framework cations and resonance positions have been shown to shift in a certain manner. The vast majority of these correlations apply to 29Si, a nucleus ideal for study by MASNMR spectroscopy due to the fact that it possesses no quadrupolar moment and hence gives a spectrum which is straightforward to interpret; this is particularly important for systems such as zeolites for which there is more than one silicon environment. Analogous relationships have also been obtained for a7A1 and 71Ga, and will be discussed in detail elsewhere 12. These may be used to provide structural information on amorphous and more complex non-sodalite systems for which Rietveld refinement is problematic. Such systems would include novel zeolites and those for which isomorphous framework substitution had been undertaken.
4.3. Infra-red Spectroscopy In a similar manner to MASNMR spectroscopy, the positions of the IR absorption bands change with framework composition. This is caused not only by a change in the
275 framework species themselves but also by the associated change in T-O-T angle. This relation between band position and T-O-T angle has been reported by Creighton et aL 13, 14 who, in their study of aluminosilicate sodalites, correlated IR band positions with cell parameter and framework bond angles, and concluded that the force constants in the framework are uneffected by framework geometry. The vibrational mode positions were subsequently shown to be predominantly related to the T-O-T bond angle or to O-T-O tetrahedral distortion angles. The discrepancies between the data obtained in this work and that of Henderson and Taylor7, as shown in table 4, can be principally assigned to differences in sodalite composition. It is clear from their obtained lattice parameters and preparative method using borosilicate glassware that the sodalites obtained were not of ideal stoichiometry Na8[ABO4]6.X 2 where A, B and X are as defined previously. 5. CONCLUSIONS The substitution of gallium and germanium for aluminium and silicon in the sodalite framework has been successfully achieved to yield a range of halide sodalites which have been subsequently characterised by powder neutron diffraction, MASNMR and IR spectroscopy. The effect of a particular framework substitution, along with that of halide size, on the spectral band positions has been rationalised. ACKNOWLEDGEMENTS We thank the EPSRC for a studentship for GMJ, and for use of the POLARIS and D2B neutron diffraction and VXR300 NMR facilities; Johnson Matthey for their financial support, and P.J. Mead of Southampton University for helpful discussions. REFERENCES
1) A.C. Larson and R.B. Von Dreele, GSAS General Structure Analysis System MS-H805, Los Alamos, NM 87545, 1990. 2) M.E. Fleet, Acta Cryst., C45, 843, 1989. 3) J.M. Newsam and J.D. Jorgensen, Zeolites, 7, 569, 1987. 4) R. Radeglia and G. Engelhardt, Chem. Phys. Lett., 114, 1, 28, 1985. 5) M.T. Weller and G. Wong, J. Chem. Soc. Chem. Commum., 1103, 1988. 6) D.E.W. Vaughan, M.T. Melchior and A.J. Jacobson, lntrazeolite Chem., 14, 231, 1983. 7) C.M.B. Henderson and D. Taylor, Spectrochim. Acta, 35A, 929, 1979. 8) T.M. Nenoff, W.T.A. Harrison, T.E Gier, N.L. Keder, C.M. Zaremba, V.I. Srdanov, J.M. Nicol and G.D. Stucky, Inorg. Chem., 33, 2472, 1994. 9) M.E. Brenchley and M.T. Weller, Zeolites, 14, 682, 1994. 10) B. Beagley and J.O. Titiloye, Structural Chemistry, 3, 429, 1992. 11) B.L. Sheriff and H.D. Grundy, Nature, 332, 819, 1988. 12) M.T. Weller, S.E. Dann, G.M. Johnson and P.J. Mead, "Chemical Shifts and Structure in Frameworks", this conference. 13) J.A. Creighton, H.W. Deckman and J.M. Newsam, J. Phys. Chem., 95, 2099, 1991. 14) J.A. Creighton, H.W. Deckman and J.M. Newsam, J. Phys. Chem., 98, 448, 1994.
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H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
277
SYNTHESIS AND CHARACTERIZATION OF CHROMO, FERRO, MANGANO AND VANADIO SILICATES WITH MTW STRUCTURE
Maria Luiza S. Corr~a 1, Martin Wallau2 and Ulf Schuchardt2 llnstituto de Quimica, Campus Universithrio de Ondina, Universidade Federal da Bahia, 40170-290 Salvador-BA, Brasil; 2Instituto de Quimica, Universidade Estadual de Campinas, Caixa Postal 6154, 13083-970 Campinas-SP, Brasil The preparation of metallosilicates with MTW structure with pores consisting of 12membered rings and pore openings of 5.5x5.9 A in the presence of the redox metals Cr, Fe, Mn and V is described. The incorporation of Cr(III), Fe(III) and V(IV) in the lattice was confirmed by physico-chemical characterization. In the Mn-silicate, the major part of the Mn cations was present as extraframework manganese oxide. After the calcination, oxidation to Cr(V) and Cr(VI), V(V) and Mn(VII) occurred while for the Fe-silicate the formation of extraframework iron oxide was observed. The metallosilicates were tested as catalysts for cyclohexane oxidation with aqueous H202 or tert-butylhydroperoxide. Cr, V and Mn MTW showed good activities, which increased in the absence of water, indicating that the metallosilicates are hydrophilic. 1. INTRODUCTION Since thediscovery and successful application of the titanosilicate with MFI structure (TS-1), large efforts have been done to incorporate other redox metals in the structure of microporous molecular sieves and to study their application as catalysts in liquid phase oxidation [1 ]. The advantages of these so called "redox molecular sieves" over conventional metal-supported catalysts are [2]: (1) incorporation of the redox metals as isolated sites in the framework, which prevents the deactivation of the catalyst via leaching and oligomerization of the active species; (2) pore size, hydrophobicity and acidity of the framework are amenable to "fine tuning", thus leading to specific activity and selectivity, and possibly shape selectivity; (3) the redox molecular sieve can extract the substrate and the oxidant out of the bulk solvent like a second solvent; (4) if the molecular sieve can accomodate the substrate with little or no room for solvent molecules, the reaction may take place more easily since no solvation energy has to be overcome and the reaction can be compared to a reaction under gas phase conditions. To allow the oxidation of bulkier molecules, which is of great interest for the selective preparation of fine chemicals, special attention has been given to the preparation of redox molecular sieves with pores greater than those of the MFI structure. The MTW structure contains a one-dimensional system of 12-membered ring channels with a pore diameter of 5.5x5.9 A [3]; it was suggested that molecular sieves with MTW structure might be interesting
278 catalysts for the transformation of molecules slightly too large to enter the pores of the MFI structure [4]. The preparation of V [5] and Fe [6] containing MTW silicates has already been described. Here we will give a general procedure for the preparation of silicates with MTW structure in the presence of Cr, Fe, Mn and V cations. In order to confirme the phase purity and the incorporation of the metals, the catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), elemental analysis, thermogravimetry (TG), UV/vis-, IR-, 29Si solid state NMR-, and ESR-spectroscopy. Oxidation of cyclohexane was studied to determine the catalytic activity and selectivity of these new catalysts. 2. EXPERIMENTAL SECTION 2.1. Synthesis The synthesis of the metallosilicates was carried out with tetraethylammonium hydroxide (TEAOH) as template in 130 ml stainless steel autoclaves lined with PTFE, under static conditions, in an oven at 413 K for 12 days. To avoid the precipitation of the metal hydroxides in the basic gels (pH= 12), oxalic acid was added [7]. The final gels had the following composition: 1 SiO2 : x M : 0.04 Na20 : 0.02 I(20 : 0.33 TEAOH : 0.051 oxalic acid : 25 H20 (x= 0.034 for M = Cr, Fe and x = 0.017 for M = Mn, V). They were prepared by adding 7.22 g (0.120 mol) SiO2 (Aerosil 200; Degussa) to a solution of 0.19 g (0.005 mol) NaOH, 0.12 g (0.002 mol) KOH and 16.4 mL (0.039 mol) aqueous TEAOH (35wt%; Aldrich) in 33 mL water. After stirring for 24 h at room temperature, a solution of 8.33 g (0.17 mol) Fe2(804)3"5 H20, 13.61 g (0.034 mol) Cr(NO3)3.9 H20, 4.17 g (0.017 mol) Mn(CH3COO)2-4 H20 and 3.69 g (0.017 mol) VOSO4"3H20 (all Aldrich), respectively, and 6.43 g (0.051 mol) oxalic acid dihydrate in 10 mL water was added. The resulting gel was transferred to an autoclave and introduced into the preheated oven. After the reaction the solid products were separated by centrifugation, dried at 373 K in an oven and subsequently characterized. Prior to the catalytic tests the occluded template was burned off by calcination in air for 2h at 423 K, 2h at 623 K and 4h at 823 K. 2.2. Characterization A Shimadzu XD-3A diffractometer with Cu Kc~ radiation was used for the XRD analysis between 219= 5~ 35 ~. The morphology and the size of the crystals were determined with a Jeol JSM-T 300 electron microscope. The metal content was determined with a Spectrace TX-5000 X-ray fluorescence spectrometer. Mechanical mixtures of SiO2 and the metal oxides were used for the calibration curve. A Perkin Elmer 2401 elemental analyzer was used for the C/H/N analysis. The thermogravimetric measurements were carried out in a flow of synthetic air, on a DuPont 951 thermobalance, between 293 and 1073 K, with a heating rate of 10 K/min. The IR spectra were recorded between 400 and 4000 cm 1, on a Perkin Elmer 1600 M-80 spectrometer. For the UV/vis spectroscopy between 200 and 800 nm, a Perkin Elmer Lambda-9 serie 1645 spectrometer was used. The 29Si CP MAS NMR spectra were obtained with a Bruker AC 300/P spectrometer using the following conditions: frequency 59.628 MHz, rotation frequency 4000 Hz, pulse length 11.3 ~ts, contact time 3 ms, with 3 s between each scan. The X-band ESR were recorded at room temperature at the Centro Polit6cnico by Prof. Mangrich at the Federal University of Paranfi, Brazil. IR-, UV/vis-, and
279 ESR-spectra were recorded for the as-made and the calcined metallosilicates. Only the asmade samples were characterized by 29Si NMR spectroscopy. 2.3. C a t a l y t i c R e a c t i o n
The catalytic tests were carried out in a 30 ml PTFE-lined stainless steel autoclave which was thermostated in an oil bath at 353 K, under magnetic stirring, for 24 h. Cyclohexane (2 mL) was oxidized with 100 mg catalyst and 2 mL H202 (30%) in 15 mL acetone. With tert-butylhydroperoxide (TBHP) as oxidant the reactions were carried out in 19 mL cyclohexane without solvent. The products were analyzed with a HP 5890 gas chromatograph using a packed 3 m Carbowax 20 M (15%) column connected to a FID. The temperature was raised from 313 to 443 K with a rate of 10 K/min. Cyclooctane was added as an internal standard for the quantification of the products. 3. RESULTS AND DISCUSSION 3.1. E l e m e n t a l A n a l y s i s , X R D , S E M a n d T G
The X-ray diffractograms show that, besides the MTW structure, no other crystalline phase was present. However, the Mn- and V-silicates still contain amorphous material. SEM pictures show that they crystallize as individual crystals with average lengths of ca. 7 pm, 3 ~tm, 30 lam and 6 ~tm for the Cr-, Fe-, Mn- and V-silicate, respectively. The elemental analysis shows that the Fe content in the crystals is slightly higher than that of the gel [(Si/Fe)gel= 29; (Si/Fe)crystal= 24], while the Cr-, Mn- and V-silicates contain less metal than the gels [(Si/M)gel Cr= 29, Mn = 59, V = 59; (SifM)crystal Cr = 83, Mn= 87, V= 232]. During calcination, the Cr, Fe and Mn-silicates maintain their metal content, given in table 1, while the metal content of the V-silicate decreases from 0.335 wt% to 0.224 wt%. The content of organic material, determined by C/H/N analysis, is in relative good agreement with the weight loss above 373 K observed by TG, also given in Table 1. The C/H/N analysis show no decomposition of the occluded template during the synthesis. Table 1" Results of elemental and TG analysis of the as-made MTW metallosilcates metal Cr
Fe Mn V
metal content/wt% 1.445 3.313 0.974 0.335
organic content/wt% 10.00 10.35 8.45 5.29
T<373K
weight loss/wt% 373K
T>673K
-
7.5
3.3
-
5.4
6.5
-
10.8
-
12.0
7.8
-
For the Cr- and Fe-silicates the TG shows a weight loss in two steps. The step below 673 K can be attributed to the desorption of occluded TEAOH molecules, while the step above 673 K can be assigned to the desorption of TEA + cations which are compensating the negative charge of the framework, expected for the isomorphous substitution of Si(IV) by Cr(III) and Fe(III), respectively. Assuming that the weight loss above 673 K corresponds to the amount of charge compensating TEA + cations, the observed TEA+/M ratios of 1.3 and 1.1 for the Cr- and Fe-silicate, respectively, are an indication for the presence of these metals in the lattice. The
280 fact that the TEA§ ratios are higher than the theorectical value of 1 can be explained by the dehydroxylation of SiOH groups in defect sites of the lattice which occurs also at temperatures above 673 K and increases the weight loss. The incorporation of V(IV) would not lead to a negatively charged framework; only weight losses below 373 K, due to the desorption of adsorbed water, and between 373 K and 673 K, due to the desorption of uncharged TEAOH are observed. The incorporation of Mn(II) in the lattice would lead to two negative charges for each incorporated Mn cation. The absence of charge compensating TEA + cations in the Mnsilicate might indicate that the Mn is not incorporated. However, the TEA § cations are probably too bulky to compensate the two negative charges. Thus, they are compensated by Na or K cations also present in the solid.
3.2. IR spectroscopy While the IR spectra of the as-made and the calcined Fe-silicate are similar to that of metal flee MTW silicate, prepared by the same method, the spectra of the as-made and the calcined Cr-silicate show strong additional shoulders at 1030, 800 and 540cm l and a weak additional band at 640 cm l , which can be attributed to Cr in framework positions [8]. The spectra of the as-made and the calcined V-silicate show an additional shoulder around 960 c m l which is attributed to vanadium cations in the lattice [ 1]. For the as-made Mn-silicate also an additional shoulder at 960 cm l is observed. However, this shoulder vanished in the calcined material. An attribution to silanol groups, which are expected to resonate in the same frequency region [1 ], is, therefore, more likely. 3.3. UV/vis spectroscopy The green color of the Cr-silicate indicates the presence of Cr(III) in an octahedral coordination, which is confirmed by bands at 440 and 625 nm, which are assigned to octahedral Cr(III) [9]. Although tetrahedral coordination is expected for metals incorporated in the lattice, the observed octahedral coordinaton might be explained by the coordination of two additional water molecules to the framework Cr(III), as it was suggested for Cr(III) incorporated in microporous aluminophosphates [10]. After the calcination, the Cr-silicate keeps its green color. However, the occurence of broad bands around 450, 350 and 260 nm indicates the oxidation to Cr(VI) [9]. The white colored Fe-silicate shows, besides the intense charge-transfer bands at 220 and 260 nm, very weak bands at 370 and 410 nm due to forbidden transitions between states of different multiplicity and which are attributed to tetrahedrally coordinated Fe(III) [11], thus indicating the incorporation in the framework during the synthesis. After the calcination, the Fe-silicate shows a light brown color and broadening of the charge transfer bands, which indicate the formation of iron oxide particles in the pores [11]. The Mn-silicate crystals has a pink color, which indicates the presence of octahedral Mn(II). In the UV/vis spectrum, only broad charge-transfer bands at 210 and 250 nm and a very weak band at 330 nm due to a forbidden transition are observed, which does not allow a decision between tetrahedral and octahedral coordination of the Mn(II) cations. The violet color observed after the calcination indicates oxidation to Mn(VII); in the UV/vis spectrum only broad charge-transfer bands can be observed. Bands around 210, 250 and 350 nm indicate the presence of vanadyl cations coordinated to the lattice in the as-made Vsilicate. The spectrum of the calcined V-silicate demontrates the presence of V(V) cations in tetrahedral coordination [12].
281
3.4.
29Si NMR spectroscopy The 29Si MAS NMR spectrum of the
Cr-silicate shows one band at -112 ppm, which can be attributed to Si(OSi)4 units, and a smaller band at -101 ppm, which can be assigned to the presence of (SiO)3SiOH and/or (SiO)3SiOCr units in the framework. However, this peak is only slightly increased in the 29Si CP MAS NMR spectrum, confirming the incorporation of the Cr(III) cations in the the lattice [13]. The 298i MAS NMR spectrum of the Fe-silicate shows a broad peak at -110 ppm with a shoulder at -99 ppm. This shoulder can be taken as an indication for the presence of Fe(III) cations in framework position [14]. However, the presence of (SiO)3SiOH groups, which will also give a resonance in this range cannot be excluded because in the CP mode the much faster relaxation of the protons in the presence of iron diminishes all 29Si signals for the Fe-silicate under the employed conditions. For the Mnsilicate, also a broad peak at -111 ppm with a shoulder at -99 ppm is observed. The broadening of the shoulder in the 29Si CP MAS NMR spectrum indicates the presence of (SiO)3SiOH groups in the framework, as it was already suggested by the IR spectrum. Thus, the incorporation of Mn(II) cations in the framework is unlikely. For the V-silicate two separated signals at -109 ppm and -100 ppm are observed in the 29Si MAS NMR spectrum. The latter is strongly increased in the 29Si CP MAS NMR spectrum and can be clearly attributed to the presence of (SiO)3SiOH groups in the framework. The results reported here for the V-silicate are in good agreement with those obtained by Moudrakovski et al. [5] for V-silicate with MTW structure. They found that the vanadium is present as vanadyl cations coordinated to hydroxyl groups in defect sites of the framework, which is in agreement with the results of the UV/vis spectroscopy.
3.5. ESR spectroscopy The ESR spectrum for the uncalcined Cr-silicate shows a broad signal at g= 1.98, indicating the presence of isolated Cr(III) cations in octahedral coordination. After calcination, the ESR spectrtma shows the presence of isolated Cr(V) cations. The signals at g= 1.99 and 1.91 and at g= 1.98 and 1.96 can be attributed to two Cr(V) components [13] differing in the coordination number: the first (g= 1.99; 1.91) to Cr(V) in a distorted tetrahedron and the second (g= 1.98; 1.96) to Cr(V) in a distorted five coordinated environment. Kucherov et al. confirmed for a chromosilicate with MFI structure that these Cr(V) species are incorporated in the framework [13]. The ESR spectra of the as-made and the calcined Fe-silicate show two signals at g= 4.3 and g= 2.3, usually assigned to framework iron and iron in interstitial oxides, respectively. However, Goldfarb et al. [15] and Ratnasamy and Kumar [6] questioned these assignments. It was shown that also iron in framework position can give a signal at g= 2.3 and that the absence of a signal at g= 4.3 does not exclude the framework incorporation [15]. Therefore, the ESR spectra of Fe-silicates do not provide evidence for Fe in framework or extraframework sites unless they are combined with other chemical methods. Due to the results obtained by TG, UV/vis and 29Si NMR spectroscopy it can be concluded that after synthesis the iron is incorporated in framework sites, while the calcination leads to the formation of extraframework iron oxide located in the pore system. The formation of extraframework iron is confirmed by the increase of the ESR signal at g= 2.3 in the calcined sample. For the calcined Mn-silicate no ESR signal was obtained which can be explained by the oxidation of Mn(II) to ESR silent Mn(VII). The ESR spectrum of the as-made sample shows an intense broad signal at g= 2.01 which can be attributed to aggregated Mn(II) cations,
282 probably in the form of extaframework manganese oxides. This signal is superimposed by a weak second signal, which shows a hyperfine structure typical for isolated and immobilized Mn(II) cations in framework positions [16]. For the calcined V-silicate also no ESR signal was obtained, which indicates oxidation to V(V). The ESR spectrtma of the as-made sample is in agreement with that obtained by Moudrakovsky et al. [5] for V-silicate with MTW structure and indicates the presence of isolated vanadyl cations.
3.6. Cyclohexane Oxidation The results obtained for the oxidation of cyclohexane catalyzed by the metallosilicates with MTW structure are given in Table 2. These results were already corrected to the blank experiments. The turnover number (TN) was calculated as mmol of oxidized products / mmol of metal atom in the catalyst. The selectivity is given by the cyclohexanone (one)/ cyclohexanol (ol) ratio. Table 2: Results of the cyclohexane oxidation catalyzed by redox silicates with MTW structure TN one/mmol ol/mmol oxidant solvent catalyst 53.4 1.02 0.48 acetone Cr-MTW H202 100.3 2.55 0.26 TBHP Cr-MTW cyclohexane 2.2 0.07 0.06 acetone Fe-MTW H202 3.7 0.16 0.06 TBHP Fe-MTW cyclohexane 4.4 0.07 0.01 acetone Mn-MTW H202 226.3 2.47 1.83 TBHP Mn-MTW cyclohexane 88.5 0.29 0.18 acetone V-MTW H202 394.1 1.4 0.72 TBHP cyclohexane V-MTW solvent/cyclohexane/peroxide = 15/2/2 (v/v/v), 100 mg catalyst, 24 h, 353 K
selectivity 2.1 9.8 1.2 2.7 7.0 1.3 1.6 1.9
It can be seen that the activity of the catalysts is markedly increased in the absence of water. This can be explained by the fact that the incorporation of highly charged cations such as Cr(V), Cr(VI), V(V) and Mn(VII), which are present in the calcined samples, leads to a polar and hydrophilic framework, which adsorbs water easily and hampers the diffusion of the hydrocarbons in the channels of the catalysts, thus decreasing their activity. The effect of water on the activity is less pronounced for the Fe-silicate. Here the formation of iron oxide particles in the pores leads to pore blockage, so that the oxidation mainly occurs at the surface of the crystals, which results in a low activity. It is further shown that the Mn-silicate in the water free system is the most active catalyst, indicating that water adsorbed at the active sites hinders the oxidation. However, it was shown that only a small part of the Mn cations are incorporated after the synthesis and no evidence for Mn in framework positions is given for the calcined sample. The decrease of the Mn content of the catalyst after the oxidation further indicates the leaching of the metal from the solid, so that homogeneous catalysis is more likely than heterogeneous catalysis. The selectivity towards cyclohexanone increases, with exception of the Mn-silicate, also in the water free system. Here the Cr-silicate shows a high selectivity (9.8) towards the ketone.
283 4. CONCLUSIONS It is shown that silicates with MTW structure can be prepared in the presence of Cr(III), Fe(III), Mn(II) and V(IV) cations. The physico-chemical characterization demonstrates that Cr(III) is incorporated in the silicate lattice after the synthesis and that it is oxidized to Cr(V) and Cr(VI) after calcination. To the best of our knowlegde this is the first report of Cr incorporation in the structure of MTW silicates. Fe(III) incorporation in the lattice during the synthesis is observed. After calcination the loss of iron from the framework and the formation of iron oxide particles inside the pores is observed. After the synthesis of the Mn-silicate no evidence for the incorporation of Mn(II) cations can be found by TG, IR, UV/vis and 29Si NMR spectroscopy. ESR spectroscopy indicates that the major part of the Mn(II) cations forms extraframework manganese oxide and that only a small part of the Mn(II) is present as isolated and immobilized cations, probably in framework positions. After calcination, the manganese is oxidized to Mn(VII) cations and no evidence for their incorporation in the framework could be found. In the V-silicate, isolated vanadyl cations coordinated to the framework via SiOH or SiO groups are present, which are oxidized to V(V) during calcination. The catalytic tests reveal that the synthesized metallosilicates are active catalysts for cyclohexane oxidation. However, the activity of the Fe-silicate is very low, probably due to the pore blockage by iron oxide particles. The activity of the catalysts increases in the absence of water. This indicates that these catalysts with highly oxidized cations in framework positions are, in contrast to the unpolar and hydrophobic TS-1 with Ti(IV) in framework position, polar and hydrophilic, so that a water free system should be employed. However, for the Mn-silicate strong metal leaching is observed, which results in catalysis in the homogeneous phase. The highest selectivity towards cyclohexanone was observed for Crsilicate in the water free system. ACKNOWLEDGEMENTS Acknowledgements are given to Prof. Mangrich, Universidade Federal de Paranfi, Curitiba, for recording the ESR spectra and valuable discussions, and to CNPq and FAPESP for financial support. REFERENCES
[ 1] G. Bellussi, M.S. Rigutto; Stud. Surf Sci. Catal., 85 (1994) 117 [2] R.A. Sheldon; Top. Curt. Chem., 164 (1993) 21 [3] W.M. Meier, D.H. Olson; Zeolites, 12 (1992) 449 [4] S. Ernst, J. Weitkamp; Catal. Today, 19 (1994) 27 [5] I.L. Moudrakovsky, A. Sayari, C.I. Ratcliffe, J.A. Ripmeester, K.F. Preston; J. Phys. Chem., 98 (1994) 10895 [6] P. Ratnasamy, R. Kumar; Catal. Today, 9 (1991) 329 [7] R. Kumar, A. Raj, S.B. Kumar, P. Ratnasamy, Stud. Surf Sci. Catal., 84 (1994) 109
284 [8] J.S.T. Mambrin, H.O.Pastore, C.U. Davanzo, E.J.S. Vichi, O. Nakumura, H. Vargas; Chem. Mater., 5 (1993) 166 [9] B.M. Weckhuysen, R.A. Schoonheydt; Stud. Surf Sci. Catal., 84 (1994) 965 [ 10] J.D. Chen, PhD thesis, Delft 1995 [ 11] D.H. Lin, G. Coudurier, J.C. Vedrine; Stud. Surf Sci. Catal., 49 (1989) 1431 [12] K. Tran, M.A. Harming-Lee, A. Biswas, A.E. Stiegman, G.W. Scott; J. Am. Chem. Soc., 117 (1995) 2618 [13] A.V. Kueherov, A.A. Slinkin, G.K. Beyer, G. Borbely; Zeolites, 15 (1995) 431 [14] W.J. Ball, J. Dwyer, A.A. Garforth, W.J. Smith; Stud. Surf Sci. Catal., 28 (1986) 137 [ 15] D. Goldfarb, M. Bemardo, K.G. Strohmeier, D.E.W. Vaughan, H. Thomann J. Am. Chem. Soc., 116 (I 994) 6344 [16] G. Brouet, X. Chen, C.W. Lee, L. Kevan; J. Am Chem. Soc., 114 (1992) 3720
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 10S 1997 Elsevier Science B.V.
285
Improved synthesis of (Ga)- and (Ga, A1)-faujasites Z. Gabelica(a)(*), V. Norberg(a) and T. Ito(b) (a) University of Namur, Chemistry Department, 61, Rue de Bruxelles, B-5000 Namur, Belgium (b) Tamai Sangyo Co. Ltd., Zenibako 3-chome, 524-11, Otaru 047-02, Hokkaido, Japan A direct and efficient method to prepare in one step mixed (Ga,A1)-faujasites as well as the pure (Ga)- and (A1)- end members is proposed. Slow depolymerization of silica governs the formation of adequate gallosilicate precursors that probably undergo a solid state restructuration yielding crystalline Ga-faujasites. The true incorporation of gallium in the FAU framework tetrahedral sites was ascertained by measuring the unit cell expansion and confirmed by 29Si-NMR. Both trivalent ions are incorporated with a 100% efficiency and their total amount is fairly constant (about 72 per unit cell). 129Xe-NMR suggests that all samples are true X zeolites despite the average Si / M3+ molar ratios close to, or larger than 1.6, and shows that their pore size is independent on the Ga content. Rutherford Back Scattering profiles reveal, along with 29Si-NMR, that Ga is homogeneously distributed throughout the lattice and that it remains stable and well dispersed when the Ga faujasites are calcined at 500~ 1. INTRODUCTION Since the pioneering work of the group of the Linde Division of Union Carbide in the early 1950's, a host of recipes to prepare (Al)-faujasite (X and Y), one of the most important catalysts for the petrochemical industry, has been proposed and extensively evaluated in the literature [ 1]. Surprisingly, little is known about possibilities to insert gallium in the FAU lattice, although such materials would potentially also exhibit extremely interesting catalytic properties [2, 3]. Four main early works proposed detailed recipes for hydrothermal synthesis of gallofaujasite materials. Selbin and Mason [4] were the first to describe a recipe allowing to prepare partially and totally Ga-substituted 13 X faujasites. These materials could be synthesized in quite particular hydrothermal conditions from gels involving GaO(OH) or GaHC14. The final products were obtained in pure form only under very specific heating conditions (temperature not higher than 70~ long crystallization times...), otherwise they were contaminated by other crystalline or amorphous Ga bearing phases. These syntheses could be further reproduced and slightly modified [5]. G. Ktihl [6] has described the preparation of gallosilicate faujasite materials in the presence of phosphate complexing ions which contributed to increase the Si ! Ga molar ratio of the final product to about 2.55, but also prevented the formation of Ga sodalite after long heating times. Vaughan et al. [7, 8] and Newsam et al. [9-10] have described a series of preparation methods leading to Ga rich X and Y (Ga, A1) faujasites, from Na-Ga-Si bearing gels seeded
286 by aluminosilicate faujasite crystallites. These studies emphasized more particularly the structural and absorption properties of these materials [8-10]. More recently, Occelli [2] patented a quite elaborated method yielding gallofaujasite zeolites upon heating a mixture of two different hydrogels containing Ga, Na, and Si reactants of selected molar composition. His recipe does not allow to prepare mixed (Ga,Al) faujasites. Finally, gallo-aluminofaujasite materials could also be prepared through post synthesis modification of the A1 analogs. Dwyer et al. [3] inserted Ga ions in the lattice of a pre dealuminated faujasite by using GaF3 in gaseous phase as reactant. On the other hand, Chu et al. [11] happened to prepare high silica Ga-Y zeolites by refluxing a pre steamed and acid exchanged USY material with a basic solution containing a Ga salt dissolved in NaOH. All these works either provide reliable but very specific methods of preparation of gallo or gallo-aluminofaujasites, or are devoted to the study of very specific properties of these materials. However none of them neither presents any systematic and complete study of the synthesis variables, nor any thorough physicochemical characterisation of the amount, structural state, framework distribution or stability of the Ga 3+ ions in the faujasite lattice. Addressing and answering these questions was the aim of the present investigation. 2. EXPERIMENTAL
2.1 Synthesis of galio- and gallo-aluminofanjasites In a first attempt to evaluate the importance of most of the synthesis parameters allowing to prepare in an efficient way pure (Ga)- or (Ga,A1)-faujasites, were basically followed all the procedures already described in the literature [2, 4, 6, 8], that essentially deal with gels prepared from various Si, Al and Ga sources admixed and dissolved (or dispersed) in NaOH solution in variable proportions. Such a screening resulted in an understanding of the role of the principal synthesis variables that govern the nucleation and growth of faujasite under mild hydrothermal conditions and that favor the easy incorporation of large cationic species such as Ga 3+ ions in its framework. These results, described in more detail elsewhere [ 12], allowed us to propose new optimized conditions leading to a simple and straightforward preparation of not only the (A1)- and (Ga)- pure faujasites but also of the whole series of the intermediate mixed (Ga,Al)-analogs. The principle of the new procedure consists in using Ludox HS-40 silica, quite diluted gels involving variable amounts of Ga (or A1) nitrates and appropriate amounts of Na20 so as to prevent any preliminary precipitation of Ga(OH)3 or GaO(OH). The final gel composition in terms of molar ratios of the ingredients was: Ga203 (or A1203). 10 SiO2. 5.6 Na20. 200 H20 The closest gel composition proposed in the early literature is the one detailed in the patent by Occelli [2], except that in our case, nitrates were used as the trivalent source in place of oxides and that all the ingredients were admixed in one single step instead of being first divided into two (pre heated) different gels prior to their final mixing. The gels were agitated at ambient temperature for 2 h without having been aged overnight and then heated in Teflon plugged containers, at 100~ for various periods of time. They were filtered, washed thoroughly with cold water and dried at 90~ for 12 h.
2.2 Characterization All the as synthesized products were checked for their morphology, particle size and purity by SEM (Philips XL-20 electron microscope). The crystallinity and unit cell parameters of the various phases were evaluated by X-ray powder diffraction patterns recorded on a Philips P.W. 1349/30 diffractometer coupled to a Philips PW-1730 X-ray generator, using Cu-Ko~ radiation, a quartz monochromator, a Ni filter and alpha alumina as internal reference. Elemental analyses were performed by combining Atomic Adsorption
287 (total A1 and Ga contents) and spot EDX quantitative determination of Si, AI and Ga on individual crystallites by using an EDAX P.V. 9800 Phillips analyzer coupled with SEM. Performances and limitations if this latter technique have been discussed elsewhere [13, 14]. Crystallinities of the final phases were also determined by probing the internal pore volume of the zeolite by n-hexane absorption at 90~ within a Stanton Redcroft ST-780 thermobalance (simultaneous TG-DTA-DTG), as described previously [15]. Si framework distribution and the Si / trivalent molar ratios were evaluated by 29Si-NMR using a Bruker CXP-200 spectrometer and specifically defined experimental conditions [16]. 27A1- and 71Ga_MA S_ NMR spectra were recorded on a Bruker MSL-400 spectrometer. Detailed experimental setup, conditions for quantitative measurements of framework A1 and Ga and their limitations have been described and discussed in a preceding paper [17]. Probing the exact geometry of the internal pore volume was performed using 129Xe-NMR of adsorbed xenon, as described earlier [18, 19]. Ga concentration gradients through zeolite crystallites could be evaluated by Rutherford Back Scattering (RBS) using a He 4+ beam of 0.5 to 3 MeV energy generated by an electrostatic Van de Graaf accelerator manufactured by High Voltage Engineering (HVE Europa). The beam was focused on a 3 mm area of a 20 mg sample pellet (representing approximately 20,000 zeolite particles), each sample being exposed for 10-15 min to the incident beam. The total charge exposure of the sample was 7.10 -3 C (10 nA current). Information was collected at a detector angle of 10~ backwards. Results were expressed in at. % with reference to Si. A more critical evaluation of the efficiency of the RBS technique for a quantitative analysis (depth profiling) of various heavy elements in zeolite crystals is being presented in a subsequent paper [20]. 3. RESULTS AND DISCUSSION
3.1. Optimized synthesis parameters One of the pre-requisite conditions to achieve a simple and rapid synthesis of mixed (Ga,A1)-faujasites over the whole range of A1 / Ga ratios being the flexible use of similar sources of trivalent elements, we have explored the possibility to use A1 and Ga nitrates and their admixtures. When A1 nitrate is progressively replaced by Ga nitrate in classical recipes leading to faujasite, such as those proposed by Breck [21] and modified by Bodart et al [22], 100 % crystalline faujasite is obtained when gels involving A1 / (A1 + Ga) ratios varying from 1 (A1faujasite) to about 0.5 (Ga,Al-faujasite with 50 % of Ga with respect to A1), are heated at 100~ for 12 h. A higher Ga concentration causes a dramatic decrease in crystallinity, while zeolite P is formed for gels involving more than 70 % of Ga. Replacing Na aluminate or gallate by the corresponding nitrates in the recipes proposed by Selbin and Mason [4] or by Ktihl [6] did not yield any crystalline faujasite, at least when all the other synthesis variables proposed by these authors were maintained. Mixed (Ga, Al)-faujasites were anyhow easily obtained when their (quite elaborated) operatory conditions were carefully followed, but the Ga-A1 compositional range was narrow, probably drastically conditioned by the other synthesis variables. The recipe such as exactly proposed by Occelli in his patent [8] yielded amorphous phases when Ga nitrate was used or when both Ga and AI were admixed as oxides, or when only A1203 was used. However, when the Na20 concentration was slightly increased so as to avoid a preliminary precipitation of Ga or A1 oxy-hydroxy species that further need more time to re dissolve, and also when the total amount of water was increased, pure faujasite phases were obtained in the presence of A1 and Ga nitrates, admixed in the whole range of molar ratios. Moreover, in such conditions, pure faujasite readily crystallized from a gel of final composition corresponding to the one achieved when the two hydrogels of different intermediate composition were admixed, as suggested in the patent. Avoiding this time consuming step constituted another improvement of our above described new synthesis route.
288
3.2. Crystallization kinetics of (Ga)-, (AI)- and (Ga,Al)-faujasites Following our recipe, 100% crystalline pure (AI)- or mixed (Ga,A1)-faujasites with variable A1/Ga ratios were obtained after 1 day heating at 100~ They were completely transformed into sodalite for longer synthesis times. More unexpectedly, the pure Ga analog took 5 days to crystallise but then remained remarkably stable under hydrothermal conditions after prolonged times; no traces of sodalite were observed after 10-12 days heating. Sodalite was also the first and the only phase to crystallize after heating for 1 to 2 days gels involving the above optimized composition, in which Ga nitrate was replaced by Ga203. Faujasite never crystallized under these conditions. To explain such a dramatic influence of the nature of the Ga reactants on the selective formation of sodalite or of faujasite, we could tentatively invoke kinetic effects. Soluble Ga(OH)4" mononuclear species are generated by a slow and progressive dissolution of Ga203 in NaOH [23]. They will therefore react with silica species that are available at that moment in the gel mixture. The depolymerization of the quite polymeric Ludox HS 40 silica being also quite slow even at high pH [24], the lower oligomeric or nearly monomeric silica species are also slowly generated, possibly at a similar rate as the Ga(OH)4species with which the silica will have to react. The resulting gallosilicate precursor would therefore involve a low Si/Ga molar ratio and thus generate upon crystallization a Ga rich crystalline phase such as sodalite (in which the Si/Ga ratio was systematically of about 1). By contrast, Ga nitrate would rapidly hydrolyze m GaO(OH) that will immediately re dissolve in an excess of NaOH and generate rapidly a large number of Ga(OH)4"mononuclear species. The silica species present in the gel after such a short time will be rather polymeric because the depolymerization of the Ludox would not be completely achieved. It is therefore reasonable to assume that the resulting gallosilicate species will be richer in Si than in the preceding case and that such precursors will yield a crystalline phase richer in Si than sodalite, such as is faujasite (Si / Ga ratio of about 1.6). The rather long time required for faujasite to start to nucleate is therefore possibly related to the slow depolymerization of Ludox silica into smaller monomers ready to interact with the Ga(OH)4-entities and generate gallosilicate species with sufficiently low Si / Ga molar ratio so as to correspond to the final composition of faujasite. The fact that this latter does not re dissolve at the expense of sodalite after longer heating times could either be due to the relative stability of the gallosilicate precursor that remains in the synthesis medium, or for the stability of the Ga-faujasite framework itself, with respect to the Al-analog. Ga-faujasite crystals were also obtained from gels involving Ga nitrate and soluble Na silicate (waterglass) of which the degree of polymerization is similar to that of Ludox silica [24, 25]. Both silica sources are therefore also expected to depolymerize in basic medium at a similar rate and provide quasi simultaneously adequate precursors to faujasite. Indeed, about 28 % crystalline gaUofaujasite was formed after 5 days heating a gel in which Ludox was replaced by waterglass. This qualitatively suggests that waterglass could even be slightly more polymerized than the Ludox silica, although other variables could also play a role in the crystallization rams. In contrast, Aerosil type silicas are quite polymeric and their dissolution in concentrated NaOH first yields highly polymeric silicate [24] or aluminosilicate [25] species. They are not expected to depolymerize as rapidly as the two "soluble" silicas. In fact, the use of Aerosil did not even favor the crystallization of the Si rich zeolite (A1)-ZSM-20, that currently crystallizes in the presence of less polymeric silicas [26]. A new A1 faujasite polytype crystalline phase, that was even richer in Si than ZSM-20, was obtained in this case, but after more than 10 days heating and at temperatures higher than 100~ probably when the (alumino)silicate species stemming from Aemsil were not completely depolymerized. It is therefore not surprising that no trace of any crystalline gallosilicate phase was observed after 5 days heating a gel involving the above defined optimized composition but in which Ludox was replaced by Aemsil.
289 Finally, we also observed that both A1 nitrate and A1203 could be used along with Ludox silica to prepare the pure Al-faujasite at 100~ in 1 to 2 days. This is not at all in contradiction with the above discussion, if we consider that AI(OH)4- monomers are always very rapidly generated in basic pH madia upon hydrolysis of any A1 precursor [24], at least more rapidly than Ga(OH)4-. In such a case the alumino or gallo-aluminosilacate precursors to faujasite will be more readily generated than the A1 richer species precursors to sodalite, just as in the case of the system Ga nitrate-Ludox. As soon as A1 is present alone or admixed with Ga in the gel phase, faujasite starts to crystallize after about 1 day heating, in contrast to the pure Ga system that yields Ga faujasite after at least 5 days (Fig. 1).
100 80
._z, .c: tll ,.~ 60 (/)
o
.m !._
r
"5
~0
40 20
,~o ~.po
0
9
I
100
9
I
200
9
300
Cristallization time (h) Figure 1. Crystallization curves for A1 -faujasite (Q) and for Ga-faujasite (O) at 100~ in static conditions. This seems to indicate that it is the hydrolysis of the trivalent hydroxy-oxy species that governs the kinetics of the metallosilicate precursor formation and that A1 governs the rate of formation of the mixed AI-Ga silicate precursors. Such precursors can also further readily incorporate Ga(OH)4" species when they are present, without undergoing a major restructuration or (de)polymerization and then rapidly start to nucleate. Only in the total absence of AJ, the hydrolysis of the gallium precursor, possibly along with the silica depolymerization, is the rate limiting step for the gaUosilicate precursors to form and to crystallize.
290
3.3 Physicochemical properties of (Ga)-, (AI)- and (Ga,Al)-faujasites Table 1 lists some properties of a typical series of (Ga,Al)-faujasites of variable Ga and A1 contents and of the pure Ga and Al end members crystallized from hydrogels involving appropriate amounts of Ga and/or A1 nitrates and Ludox silica and heated at 100~ for 24 h. (A1- and Ga, Al-phases) and for 5 days (Ga-phase). Table 1 Physicochemical characteristics of some (Ga)-, (A1)- and (Ga,A1)-faujasites Sample (code) VN VN VN VN VN VN
23 35 36 36 38 28A
% Ga (a) % crvstallinitv Ca) in gel in zeol. n-hex. XRI) 0 25 50 75 95 100
0 22 49 74 (94) 100
100 97 98 102 71 100
97 91 89 110 82 100
unit cell composition (e) AI Ga (A1 + Ga) 76 0 50 14 36 36 17 49 (4) (67) 0 72
76 64 72 66 (71) 72
Si / (A1 + Ga) (mol. ratio) 1.53 2.00 1.67 1.91 (1.71) 1.67
(a): percentage of Ga with respect to the total (Ga + A1) amount introduced in the gel or measured in the final as synthesized zeolite, as mentioned in (c) (b): The % of crystallinity of each sample was calculated from the normalized intensities of the 8 most intense XRD peaks, and compared to sample VN 28A arbitrary considered as the most crystalline (100 %) phase, and from n-hexane sorption capacities achieved as described in ref. [26] and compared to the same arbitrary selected standard sample. (c): for unit cell formula Nax+yAlxGaySi192.(x+y)O384; A1 content as determined by quantitative 27Al_NMR [ 17] and Ga content as evaluated by atomic absorption. Data from Table 1 show that in all cases each trivalent ion is incorporated from the gel phase to the zeolite framework with a nearly 100 % efficiency. Such an efficiency was never observed for (Ga,A1)-faujasite samples synthesized by using classical recipes [21, 22]. All samples except VN 38 proved to be highly crystalline phases. The low crystallinity of sample VN 38 could result from the high Ga/A1 ratio in the gel, that possibly would have needed more than 24 h to crystallize completely. The rather high amount of Ga probably slightly slows down the crystallization rate. The fact that the VN 38 intermediate phase (gel + crystalline faujasite) also has the same composition as the initial gel, suggests that the Ga-faujasite is predominantly governed by a solid phase type restructuration mechanism, although both liquid phase transportation and gel restructuration mechanisms were shown to govern the crystallization of Al-faujasites [22, 27]. This is also in line with the assumption that the main precursor to crystalline Gafaujasite is a rather insoluble gallosilicate oiigomeric amorphous species, as suggested above. The further analysis of the remaining amorphous phase separated from the crystals of sample VN 38 by ultrasonic cleaning, confirms that it contains about the same Ga concentration as the pure crystalline phase. Moreover, the mother liquor separated from phase VN 38 only contained 2 % of the total gallium introduced in the gel, showing that very little Ga remains solubilized in the liquid phase, irrespectively to A1 [22]. These trends were also duly confirmed by analyzing the gallium content in all the intermediate phases of different crystallinities isolated after heating the pure gallosilicate gel for various periods of time (phases from the kinetic curve in Fig. 1). All of them involve Si / Ga ratios between 1.56 and 1.68, indicating that the Ga is probably never transferred from the gel to the liquid phase during crystallization,
291 The very homogeneous Ga distribution throughout the Ga-faujasite crystallites as detected by RBS (see below) also favors a solid phase rcstructuration mechanism at the expense of a liquid Ga transportation, which would more readily result in compositional gradients through the crystallites. The homogeneous distribution of Ga from the core to the outer rim of the crystallites was also indirectly but elegantly confirmed by 29Si-NMR. The Si / Ga ratios measured by this technique were always constant (1.5 to 1.6) in all the pure Ga faujasite phases isolated after various heating times, thus at different crystallization stages (the intermediate phases were separated from the contaminating amorphous phase by ultrasonic decantation prior to taking the NMR spectra). 129Xe-NMR data (chemical shift values plotted versus Xe partial pressures) definitely suggested that all the samples belong to the X family of faujasites, despite the quite high value of Si / (Ga+A1) ratios in some samples (Table 1). Similar chemical shift values for all samples also showed that the pore size was not dependent on the framework Ga content. Finally the plot of the chemical shift values versus the number of Xe atoms per gram of dry zeolite gave an accurate measurement of the pore volume that is also related to the phase crystallinity, conf'Lrrning and completing the n-hexane sorption data. A more complete 129XeNMR characterization of a wide series of gaUo and gaUo-alumino faujasites is reported in a subsequent paper [ 12]. SEM micrographs showed that all samples involve a typical octahedral morphology and a quite uniform size of 0.1-0.2 nm, irrespectively to their Ga-A1 content. The Ga rich members however exhibit more readily polycrystalline clusters (aggregated octahedra) of a mean diameter of about 3 nm.. The regular unit cell volume, as directly determined from XRD line positions, regularly increases from 14,400/~3 (M-end member) to 17,400 A3 (Ga-end member) as a function of a progressive A1 replacement by Ga. Along with 29Si-NMR data, that clearly show the presence of Si (nGa) configurations (not shown here but discussed in ref. 12) and that allow the evaluation of the amount of framework Ga in the pure Ga-faujasite [8, 9], the unit cell expansion definitely demonstrates a true positioning of Ga 3+ ions in the T positions of the faujasite lattice. This suggests that the total amount of Ga as determined by atomic absorption or EDX is only framework gallium. Actually, this should in principle be possible to confirm by using 71Ga-NMR that proved very efficient in determining quantitatively the amount of tetrahedraUy coordinated framework Ga 3+ ions in zeolites, provided specific conditions are respected [ 14, 17]. In the case of Ga-faujasites however, no correlation between the intensities of the 71Ga-NMR lines located near 150 ppm and the total amount of Ga as measured by atomic absorption or EDX, could be derived. The possible reason, irrespectively to (Ga)ZSM-5 for which such a correlation is straightforward [ 17], is the presence of too numerous Ga and/or Na neighbors around the experienced Ga nucleus, that exert in various ways strong perturbing quadrupolar interactions, thereby affecting the intensities of the NMR lines. Nevertheless, the 71Ga-NMR line intensities were quite similar for each as synthesized and calcined sample, suggesting that the amount of framework Ga in each phase does not change upon calcination. This is also confirmed by the same amount of framework Ga 3+ concentration evaluated by 29Si-NMR in the as synthesized (72 Ga/u.c.) and calcined (68 Ga/u.c.) samples. RBS is being considered as a key technique for quantitative analysis of composition and depth profiles of solid samples near the surface region. For heavier elements like Ga, concentrations could be probed up to a depth of 1 micron [28]. Potentialities and limitations of this technique in probing gallium concentrations in zeolites are discussed elsewhere[20]. RBS profiles showed that the Ga 3+ ions were homogeneously distributed throughout the crystallites from the surface to the core and that this profile stayed unchanged when the Ga-faujasites were calcined [20]. This result at least shows that Ga does not migrate towards the external surface upon calcination so it is probable that it is even not extracted from the
292 lattice, as suggested by 71Ga-NMR, 29Si-NMR and the very same Ga amount in the as synthesized and calcined samples evaluated from RBS profiles (68 Ga/u.c, mean bulk analysis). This def'mitely demonstrates the remarkable (hydro)thermal stability of gaUo- and gallo-aluminofaujasites, leaving much hope for their potential use as stable catalytic materials. Moreover, the absence of any extra framework Ga-bearing species confirms that the initial concentration and distribution of Brtinsted acid sites is always maintained [29] but also that no bifunctional catalysts could be generated by calcination. REFERENCES
[1] [2] [3]
[4] [5]
[6] [7] [8] [9] [101 [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
see for example D.W. Breck, Zeolite Molecular Sieves: Structure, Chemistry and Use, Wiley, New York, 1974 M.L. Occelli, US Patent N ~ 4 803 060 (1989) J. Dwyer, J. Dewing, K. Karim, S. Holies, A.F. Ojo, A.A. Graforth and D.J. Rawlence, Stud. Surf. Sci. Catal., 69 (1991), 1 J. Selbin and R.B. Mason, J. Inorg. Nucl. Chem., 20 (1961), 222 Z.G. Zulfugarov, A.S. Suleimanov and C.R. Samedov, Stud. Surf. Sci. Catal., 18 (1984), 167 G.H. Kiihl, J. Inorg. Nucl. Chem., 33 (1971), 3261 D.E.W. Vaughan, G.C. Edwards and M.C. Barrett, US Patent N ~ 4 340 573 (1982) D.E.W. Vaughan, M.T. Melchior and A.J. Jacobson, ACS Syrup. Series 21.8 (1984), 231 J.M. Newsam, A.J. Jacobson and D.E.W. Vaughan, J. Phys. Chem., 90 (1986), 6858 J.M. Newsam, and D.E.W. Vaughan, Stud. Surf. Sci. Catal., 28 (1986), 457 C.T. Chu, R.D. Partridge and S.E. Schramm, US Patent N ~ 5 057 203 (1991) V. Norberg; T. Ito and Z. Gabelica, in preparation J. B.Nagy, P. Bodart, H. Collette, J. E1 Hage-Al Asswad, Z. Gabelica, R. Aiello, A. Nastro and C. Pellegrino, Zeolites, 8 (1988), 209 Z. Gabelica, G. Giannetto, F. Dos Santos, R. Monque and R. Galiasso, in "Proc.9th Intern. Zeolite Conf., Montreal, 1992", Eds: R. Von Ballioos et al., ButterworthHeinemann, New York, 1993, Vol. I, pp 231 Z. Gabelica, J. B.Nagy, E.G. Derouane and J.P. Gilson, Clay Minerals, 19 (1984), 803 N. Dewaele, L. Maistriau, J. B.Nagy, Z. Gabelica and E.G. Derouane, Appl. Catal., 37 (1988), 273 Z. Gabelica, C. Mayenez, R. Monque, R. Galliasso and G. Giannetto, in "Synthesis of Microporous Materials": Vol. I: "Molecular Sieves", Eds: M.L. Occelli and H. Robson, Van Nostrand Reinhold, New York, 1992, pp 190 (chap. 15) C. Pellegrino, T. Ito, J. B.Nagy, Z. Gabelica, and E.G. Derouane Appl. Catal., 61 (1990), L1 T. Ito, N. Dumont, J. B.Nagy, Z. Gabelica, and E.G. Derouane, Stud. Surf. Sci. Catal., 60 (1991), 11 G. Demortier, J.L. Ruvalcaba-Sil, V. Norberg and Z. Gabelica, in preparation D.W. Breck, US Patent N ~ 3 130 007 (1964) P. Bodart, J. B.Nagy, Z. Gabelica and E.G. Derouane, J. Chim. Phys., 83 (1986), 777 C.F. Baes, Jr. and R.E. Messmer, The Hydrolysis of Cations, Wiley, New York, 1976 R.K. Iler The Chemistry of Silica, Wiley, New York, 1979 E.G. Derouane, S. Detremmerie, Z. Gabelica and N. Blom, Appl. Catal., 1 (1981), 201 Z. Gabelica, N. Dewaele, L. Maistriau, J. B.Nagy and E.G. Derouane in: "zeolite Synthesis" Eds: M.L. Occelli and H. Robson, American Chemical Society, Washington, DC, 1989; ACS Syrup. Series 398 (1989), 518 N. Dewaele, P. Bodart, Z. Gabelica and J. B.Nagy, Acta Chim. Hung., ! 19 (1985), 233 S.B. Abdul Hamid, E.G. Derouane, G. Demortier, J. Riga and M.A. Yarmo, Appl. Catal. A, 108 (1994), 85 B. Onida, E. Garrone, V. Norberg and Z. Gabelica, in preparation
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
293
A Study on the Crystallization of Binderless Zeolite X" Luo, Xiaoming" He, Xihua ~ Shen, Jijingb Depamnent of Chemistry, Nanjing University, Nanjing, 210008 P. R. CHINA Preformed Binderless Zeolite X was synthesized by using silica gel spheres as starting material. The products were essentially in the same size and shape as the raw silica gel. The crystallinity of the zeolite was higher than 90 ~ . Since the speciality of the silica material, the hydrothermal system is different from ordinary S i O ~ - AlzO3- NazO--H20 system. The experiment results of chemical analysis, XRD, SEM, TEM ,ZTA1NMR and the results of determination of particle size distribution gave an indication that in the system investigated the transformation of most amorphous alumina-silica gel to crystal zeolite X proceeded in the solid phase which was coexistent with an alkaline solution, but not through dissolution-reprecipitafion processes. 1. INTRODUCq'ION Although a lot of new molecular sieve materials have been synthesized in the last three decades, the earliest synthesized zeolite A and faujasite type zeolites are the varieties which have been used most extensively. Since the complexity of the reactions in the synthesis system and the difficulties to clarify the status of the Si- and A l - ~ g species and their change, different viewpoints to the mechanism of nucleation and crystallization still exist[1-ts]. Two mechanisms of zeolite crystMliTation have been proposed. Many investigations have revealed evidences later. One of the mechanisms is based on that the transformation of the amorphous gel to zeolite crystal occured in the solid phase of gel through rearrangement and surface diffusion [z-6]. The other mechanism is based on that crystallization processes occured in the equilibrium state through a continuous dissolving of the amorphous gel phase and reprecipitating to form and grow into the zeolite crystal [7-1z3. The kinetic researches by H. Lechert and H. Kacirek were based on the hypothesis which is the same as the second mechanismC~3-~s].
Until now there were few reports related to the crystallization mechanism of binderless zeolites. The aim of the present work was to obtain information about the erystalli,ation in the system for preparation of preformed binderless zeolite X, and add understandings to the
9,
9 This work has been sponsored by China Petro-Chemical Corporation. T o w h o m the correspondence shallbe forwarded to. a. Present address. Department of Chemistry, Wuhan University, Wuhan, 430072 P. R. CHINA b. Present address: Department of Chemistry, Peking University, 45--1128, Beijing, 100871, P. R. C H I N A
294 mechanism of zeolite formation. We paid more attention to the crystallization process than to the nucleation process. More and more attention has been paid to the preparation of binderless zeolites E19-z33, since they possess better effects than that of zeolites involving inert binders. In preparation of preformed binder--free zeolites the starting solid materials, for example, silica gel, aluminosilicate gel, some kinds of clay or several kinds of extrudates from the mixtures of zeolite powder with binders, are all preformed. Preformed binderless zeolites were synthesized essentially in the same size and shape as the starting materials. There is substantially no amorphous phase or just a small amount in the preformed binderless zeolites. In ordinary hydrothermal synthesis most of the silica source materials are soluble in the alkaline solution at the reaction conditions. Many researchers investigated the zeolite formation processes started from different silica sources Exs-ls3. Different silica sources strongly affect the chemical state of the gel and may affect the mechanism of crystallization. In this work silica gel spheres were used to prepare preformed binderless zeolite X. The conditions of synthesis were different from that in ordinary synthesis. 2. EXPERIMENTAL
Silica gel spheres (20--40 mesh) were dipped into sodium-aluminate solution and aged at about 318K for 16 hours. Then raised the temperature to 358--368K. The crystallization started and continued for 6--72 hours. Several synthesis experiments with same formulation proceeded at the same conditions, but stopped at different time. Solid and liquid phases were separated by centrifugation. The concentrations of SiOz,Al~O3 and NazO in the solutions were determined by chemical analysis. Solid phases were tested by XRD, sometimes by S E M , T E M and solid state ~A1 NMR. XRD patterns were obtained with D/MAX RA diffractometer. SEM photographs were obtained using JSM 6300 microscope. TEM photographs were obtained using JEM CX microscope. Solid state Z~A1NMR measurements of stationary samples were performed with a Bruker-MSL-300 spectrometer using a pulse of 0.6/as ( < ~ / 1 0 ) . Before every pe~ormance one gram of powdered sample was impregnated in 2ml 38% (vol.) solution of acetylacetone (acac) in ethanol at room temperature for 24 hours according to the references Ez4-~s3. As a reference for chemical shift measurements an aqueous solution of A1C13 was used. 3. RKSULTS AND DISC3U/SSION 3. 1. The products showed the typical X-ray diffraction pattern of crystalline zeolite X. The crystallinity of the products was higher than 90 ~ . The shape and the size of the zeolite products were essentially the same as that of original silica gel spheres. The Si/A1 ratio (measured by XRD) were about 1.2. The strength of the spherical products was 0. 2--0. 4 Kg/particle. The benzene adsorption capacities were more than 220 mg/g. SEM photographs showed that the length of the crystal zeolite X on the surface of the spherical products was in the range of 0. 5--1.5ttm, which is always bigger than that inside the spheres of the products. SEM photographs showed no crystals appeared on the solid gel samples at the end of aging stage. But TEM and electron diffraction patterns gave an evidence of existing crystal
295 nuclei on the same samples, which was X-ray amorphous. The crystallinity of the products, the length of the octahedral crystals on the spherical products and the particle size distribution were Considerably affected by the properties of the starting silica gel. 3.2. Partite Size Distribution Usually the silicagel spheres in the range of 20--40 mesh were used as starting material. For comparison of the particle size distribution at two stages of the synthesis, we used two kinds of silicagel (gel 1 and gel 2) as starting silicasource. Silica gels in a narrower size range of 30--40 mesh were used in this experiment. The experiments proceeded at the same conditions, and stopped at the end of aging stage or crystallizationstage respectively. The solid phase was filtered, washed and then dried at 383K for 3 - 4 hr. Dried solid samples were sieved and separated to four particle size ranges, then weighted. The results are indicated as percentages in table 1.
Table 1 Particle Size Distribution at the end of aging
20--30 mesh 30-- 40
40--60 >60
at the end of crystallization
use gel 1
use gel 2
use gel 1
use gel 2
0. 7% 90. 6 %
0. 7 ~ 96.5 ~ 1.8~ I. 0 ~
1.1~ 80. 3 ~
4.1~ 88, 7 1.0~
6.5~ 2. 2~
9.5~ 9. 1~
6.2~
The results showed that more than 900~ samples kept original particle size at the end of aging stage and that more than 80 ~ product kept original particle size at the end of crystallization when the gel 1 usedas silica source (Most of our experiment results in this work related to the ease using gel 1). That is one of the reason to be eonsidered that most of the amorphous gel had not passed a dissolution-repreeipitation in their crystallization processes. 3.3. Chemical analysis results The chemical analysis in one of the series of experiments gave the concentrations of AlzO3, Na20 and SiOz in the solution at different stages of synthesis. The contents of AlzO3,NazO and SiOz are indicated as the percentages of the amount of every ingredient related respectively to that in the starting synthesis systems. All the results are shown in Table 2 and Fig 1. The chemical analysis results showed that the content of AlzO3 in the solution reduced quickly at the beginning of the aging stage. Before the crystallizationstarted at an elevated temperature, almost 9 8 ~ AlzOa had attached to the solid gel phase, which was X-ray amorphous at that time. Throughout 8 hours crystallization period the content of A1203 in the solution did not increase and did not fluctuate, just reduced a little. It indicated that no detectable aluminosilicate gel dissolved into the solution. At the end of 8 hours crys~lh'zation, the crystallinity of the product was higher than 90 ~ .
296 Table 2 The contents of AI,O3,NazO,SiO2 in the solution at different stages of synthesis Aging stage Time(hr. ) A1203~
Crystallization stage
NazO~
SiOz~
Time(hr. ) A12Os~
NazO~
SiOz~
1
27.4
74. 7
2.5
O. 5
I. 9
68. 4
22. 2
2 4 7 ll 16
15. 6 8. 3 2. 6 2. 3 2.2
68. 4 66.9 65.4 64. 8 61.7
I. 7 18. 1 21.9 22. 0 22. 7
1 2 4 6 8
I. 7 1.3 O. 9 O. 8 O. 8
67.7 68.2 69. 3 69.8 68. 5
22.5 23.0 23. 6 24. 0 25.7
o o
/.
/ l AI,O3~
r\ .0
so,, ;
:
"
,q
SiO,~
9_ 03
-20.0
"
8O. 0
Na,O~
10.
O"--O-
60.0
---O
.
.
.
.
,-- - - 0
.
.
.
.
.
.
0--
I0.0
- - - - .-- - - I J
A1,03% 4
8
12
16
2
4
6
8
Aging time, hour---~ Crystallization t i m e , hour m - ~ Figure 1. The contents of AlzO3,NazO,SiOz as functions of time during synthesis The fact can be served as another reason to be considered that the transformation of the amorphous gel to zeolite crystal p r o ~ e d through rearrangement, depolymerization and condensation in the solid phase (including on its surface). It can hardly be proved that the solid gel spheres, which obviously can be seen from the beginning to the end of crys~llizadon, had changed thoroughly in an equilibrium state from amorphous phase to 90 ~ crystallized phase through dissolving and reprecipitation just in a period of time which was not long. If the dissolution-reprecipitation happened in a very thin surface diffusion layer, which was an interface between the network of the gel and the liquid phase, tO detect the changes is really a difficult problem now. According to the chemical armJysis results, the content of SiOz in the solutions increased rapidly at the initial stage of aging. In the crystallization stage the content of the SiOz in the solution increased not very much. About 220~ SiOz dissolved into the solution at the end of aging stage. About 25 0~ SiO, existed in the solution when crystallization proceeded to 8 hours. Since the SiO,/AI,O3 ratio in the mixture of starting materials was much higher than that in the product zeolite X, so part of SiOz, which did not attend the arrangement of
297 Si--O--AI framwork, dissolved in the alkaline solution. The dissolution of SiO2 did not results in most of the spheres fallen-apart in the synthesis conditions of this work. Most of the spheres essentially retained the origial particle size. It meant that some kinds of silica-alumina framwork appeared and maintained from the early stage to the end of synthesis process. The cherrdcal analysis results indicated that about 70 ~ of NazO remained in the solution when the crystallization had proceeded for 8 hours. 3.4. The results of solid state Z~Al NMR experiments Figure 2 ,represented the solid state ~,AI NMR experiment results in this work, which gave the information about the local environment of AI nuclei in the solid samples at different stages of the synthesis.
(d)
(a) It
IIIIIII
I t l l l l l l l t l l l l l t t l l l
II
100 0 100 0 ppm ppm Figure 2. real NMR spectra of solid samples (a)after 2hr. aging (c)after lhr. crystallization (b) after 16hr. aging (d) after 8hr. crystallization We tried to use acetylacetone (acac)impregnation method ca-263 to obtain the signals of aluminium, which resided in the low syrmnetry environments, together with the signals of aluminium in the fourfold oxygen coordinated environments. According to the reference :2s3 38 ~ (vol.) acac solution in ethanol does not affect the tetrahedrally coordinated aluminium including the aluminium existed in the same coordination state in an amorphous silica matrix. (In this aspect further investigation is in progress now) The aluminium in the low symmetry environments, like extra-framwork aluminium in the dealuminated zeolites, was converted with acac into AI (acac) 3 complexes. From the investigation of solid state ~AI NMR it was known that the signals at about 60 ppm chemical shift are attributed to fourfold oxygen coordinated AI indicating the building units AI (OSi)4. The AI (acac)3 complexes exhibit a chemical shift of about 0 ppm as a narrow peak in the ~ AI NMR spectra. The relative area of the peaks at about 60 ppm
298 proportional to the quantities of the fourfold coordinated A1 in the samples Ez~3. Fig. 2 showed that the quantity of fourfold coordinated AI increased obviously along with the synthesis process. Fig. 2 also showed that at the initial aging stage (2hr.) fourfold oxygen coordinated aluminium already appeared in the gel. Engelhardt et alE,s:! had also detected this signal at 59 ppm in the precursor gel. At the end of aging stage (after 16hr. aged) in our experiments, a large number of fourfold coordingated A1 existed. Accxrrding to the results it was considered that the coordinated environment of aluminium at the end of aging somewhat approached to the environment in the ordered assemblages of tetrahedra in the zeolite crystal framwork. Maybe the fourfold oxygen coordinated A1 served as structure directors. So the crystallization proceeded rather fast when the temperature raised. The results of ~ AI NMR experiments were consistent with the assumption that the transformation of the amorphous gel to crystal zeolite proceeded in-situ by rearrangement of the atoms and building units in the solid phase. 3.5. The effect of the alkaline solution on the crystallization
For understanding the effect of the solution around the gel on the crystallization, another four experiments had been carried out. Every one started with the same formulation and aged in the same conditions. At the end of aging stage, the solution were decanted as far as possible, and the solid alumina-silica gel was left in the wet state. The f'rrst solid sample was washed to approach neutral. Then the sample was separated to two parts. One part of the sample was put in 383K for 48 hours. Another part was put in the room temperature for 50 days. XRD patterns showed that all these two parts of the sample were X-ray amorphous. Let the second sample (which was in the wet state and coexisted with a small amount of original solution) to crystallize at 363K for 24 hours with no additional solution added in. XRD pattern showed that zeolite X with crystaUinity of 85 ~ formed. In the third sample a NaOH aqueous solution was added to instead of the original solution which had been decanted. The concentration and volume of the NaOH solution was the same as the original one. Let the sample to crystallize at 363K for 24 hours. XRD result indicated the zeolite X pattern with crystallinity of 80 o/~. The fourth sample was washed with water one time. The solution was der.anted again. Just a small m o u n t of solution with lower pH value remained in the sample. Let the fourth sample to crystallize at 363K for 24 hours. XRD pattern showed the zeolite X with crystallinity of 52 ~ . The impurity of zeolite A and P was much more than that in the second and third samples. It meant that the solution with lower pH value was disadvantageous for crystallization. The above listed results showed that without the alkaline solution coexisted with the solid phase, the amorphous gel can hardly be transformed to crystal zeolite, although there was some crystal nuclei on the gel at the end of aging stage. According to the facts it was considered that when an alkaline solution coexisted with the amorphous gel, the atoms and the structural units in the network of the gel and on its surface had a larger amplitudes of thermal vibration and a lower activation energies for diffusion than that in the network of the gel without coexistance of alkaline solution. According to the reference Ez93it was also considered that in the crystallization process OH- ions in the solution were necessary for condensa-
299 tion proceeded in the gel, and that some HzO produced in condensation. The coexistance of the alkaline solution with solid gel was favourable to remove this kind of H,O and unnecessary SiOz and NazO in the gel. The alkaline solution promoted the transformation of the gel to zeolite framwork. 3.6. On the SEM photographs it was shown that the crystal size of zeolite X on the surface of the preformed spheres was always bigger than that inside the spheres. This is one of the fact to be noted. It can be expected that the atoms and structural units on the interface diffused and rearranged more easily than that inside the spheres. Another fact is that in the crystallization process, small part ( < 10 ~ ) of the spheres became smaller than that in the beginning of the crystallization stage. According to that two facts it was considered that in the synthesis system investigated a small amount of the zeolite crystals formed and grew probably through dissolution of the gel and reprecipitation on the interface. 4. CONCLUSION 1. Preformed binderless zeolite X with crystallinity higher than 90 ~ were synthesized by using spherical SiO2 gel as starting material. 2. It was considered that most of the amorphous alumina-silica gel in the synthesis system investigated transformed to crystal zeolite X in the solid phase. 3. In the solid phase, fourfold oxygen coordinated aluminium AI (OSi)4 increased along with the synthesis process, The existence of fourfold oxygen coordinated A1 in the amorphous gel before crystallization proceeded indicated a suitable environment in the solid phase for in-situ transformation to ordered framwork. 4. The crystaUizadon process can hardly be able toproceed without coexistence of alkaline solution with the gel. 5. Small amount of the zeolite X crystal in the system investigated probably formed and grew through dissolution of the gel and repreeipitation on the interface.
Admowledgement The authors wish to express their thanks to Li shanan, Ma Jian and their colleagues (Nanjing Jinling Petro-Chemieal Corporation Refinery), Hu Cheng (Laboratory of Solid State Microstruetures, Nanjing University) for their assistance during the experiments and their helpful discussions. REFERENCES 1. R.M. Barter, in "Zeolite Synthesis" (Eds. M . L . Oeeelli and H. E. Robson), ACS Syrup. Ser. 398, Am. Chem. Soc., Washington, D.C. (1989) P. 11. 2. B.D. McNicol, G.T. Pot-t, K. R. Loos and N. Mulder, Chem. Sex. , 121(1973)152. 3. E. M. Flanigen, Adv. Chem. Sex. 121(1973)119. 4. H. Khatami, E. M. Flanigen, unpublished work. 5. B. Fahlke, P. Starke, V. Seefeld, W. Wicker and K. P. Wendlandt, Zeolites 7 (1987) 209. 6. R. Aiello, R. M. Barrer and I. S. Kerr, Adv. Chem.Ser. , 101(1971)44.
300 7. S. P. Zhdanov, Adv. Chem. Ser. 101(1971)20. 8. S. P. Zhdanov, and N. N. Samulevieh, "Proe. Fifth Int. Conf. on Zeolites" (Ed. L. V. C. Rees) Heyden, London (1980) P. 75. 9. Ali Culfaz and L.B. Sand, Adv. Chem. Sex. 121(1973)140. 10. C. L. Angell and W.H. Flank, in "Molecular Sieves I " (Ed. J.R. Katzer) ACS Syrnp. Sex. 40, Am. Chem. Soe., Washington, D.C. (1977) P. 194. 11. Peter M. Budd, Graham J. Myatt Colin Price, and Smart W. CarT, Zeolites 14(1994) 198. 12. S. Ueda, N. Kageyama and M. Koizumi, Proc. 6th Int. Zeolite Conf. in 1983 (Ed. David Olson and Attilio Bisio), Butterworth, UK (1984) P. 905. 13. H. Lechert and H. Kacirek, Zeolites 11(1991)720. 14. H. Lechext and H. Kacirek, Zeolites 13(1993)192. 15. H. Kacirek and H. Lechert, J. Phys. Chem. 80 (1976)1291. 16. Ryozi I-Iino, Ryohei Matuura and Kenzi Toki, Bull. Chem. Soc. ]pn. 56(1983)3715. I7. Kenneth E. Hamilton, Eric N. Coker, Albext Sacco, Jr. Anthony, G. Dixon and Robert W. Thompson, Zeolites 13 (1993) 645. 18. W. Meise and F. E. Schwochow, Adv. Chem. Sex. 121 (1973) 169. 19. E. Michalko, US Patent No. 3,428,574(1969). 20. K. D. Vesely, US Patent No. 3,492,089(1970). 21. Carl V. McDaniel, Philip K. Maher and Joseph M. Pilato, US Patent No. 3,472,617 (1969). 22. Johannes P. Verduijn, WO Patent No. 92/12928. 23. T. Kawamoto, Y. Taga, I. Tosawa, T. Nishimura and W. Inaoka, Jpn Kokai Tokkyo Koho 92,198,011. 24. P. J. Grobet, H. Geerts, J. A. Martens and P. A. Jacobs, J. Chem. Soc. , Chem. Commun, (1987)1688. 25. P. J. Grobet, H. Geerts, M. Tielen, J. A. Martens and P. A. Jacobs, Studies in Surface Science and Catalysis 46(1989)721. 26. V. Bosacek, D. Freude, T. Frohlich, H. Pfeifer and H. Schmiedel, J. Colloid and Interface Science 85(1982) 502. 27. D. Freude, T. Frohlieh and H. Pfeifer, Zeolites 3(1983)171. 28. G. Engelhardt, B. Fahlke, M. Magi and E. Lippmaa, Zeolites 3(1983)292. 29. Fred Roozeboom, Harry E. Robson and Shirley S. Chan, Zeolites 3(1983)321.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
301
S Y N T H E S I S OF L A R G E C R Y S T A L S OF M O L E C U L A R S I E V E S - A R E V I E W Shilun Qiu, Wenqin Pang and Ruren Xu Key Laboratory of Inorganic Hydrothermal Synthesis, Department of Chemistry Jilin University, Changchun 130023, P. R. China More than 50 kinds of single crystals of molecular sieves (50-1000 um) including aluminosilicates, aluminophosphates, gallophosphates and indiumphosphates were crystallized from the systems of organic solutions, weak acid, aqueous solutions and fluoride ions, respectively. 1. I N T R O D U C T I O N There is considerable interest in synthesis of large crystals of molecular sieves since they are useful in many research studies, development of advanced zeolite materials and industrial applications[ 1]. However, molecular sieves are normally metastable phases, large and perfect crystals are often difficult to obtain. Here we report the synthesis of more than 50 kinds of large crystals (some of them are single crystals) of molecular sieves including zeolite A, Mordenite, Beta, ZSM-5, -39,-48, cancrinite, aluminophosphate AIPO4-5,-11,-17, JDF-20, gallophosphate -LTA, Cloverite, GaPO4-C3, - C 4 , - C 7 , indiophosphate as well as aluminoarsenate, some of them are novel phases. They were prepared via, mainly, the following four novel routes: in organic solutions, in a weak acidic medium, in aqueous solutions, in the presence of fluoride ions. 2. E X P E R I M E N T A L The large crystals of the molecular sieves are generally crystallized by hydrothermal synthesis, except for the synthesis from nonaqueous solutions, in which alcohols and / or amines are employed as the solvent. The reaction mixture is added to a Teflon-lined autoclave (10-1000 ml in volume) and heated at 100-200 ~ for 1-30 days. The large crystals were recovered by filtering, washing, sonicating (if necessary) and drying. They have the sizes of 50-1000 ~tm. 3. R E S U L T S A N D D I S C U S S I O N
3.1. Synthesis in organic solutions In 1985, Bibby and Dale[2] first reported the synthesis of pure-silica sodalite using organic solvents. Following years saw the synthesis of zeolites and metal-substituted alurninophosphate microporous materials from this system in Jilin University. To extend the study, various non-aqueous solvents are successfully applied for the preparation of Silica-
302 Sodalite as well as other kind of zeolites and phosphate microporous materials (e.g., ethanol amine and ethylenediamine-glycerol media) by Xu and coworkers[3]. A significant phenomenon is found in this system that single crystals of the molecular sieves are often obtained. Further investigation indicated that the crystallization rate is rather slow than in hydrothermal synthesis owing to the lower dielectric constant of alcohols and/or amines than those of water. Fig. l a shows crystallization curves for AIPO4-21 by hydrothermal and solvothermal synthesis, respectively. It can been seen that crystallization rate in solvothermal system is much more slower than in hydrothermal system, although the nucleation rate in solvothermal system is faster. The lower polymerization and slow crystallization rate of the reactants allows the growth of large crystals of the molecular sieves. On the contrary, in hydrothermal synthesis the fast crystallization, due to the quick exhaustion of the nutrient for crystallization, impedes the crystal growth, as a result, small crystals are often obtained in this system. Different solvent plays a different affect in crystallization period which depends mainly upon the molecular configuration and polarity of the solvent molecules. It is found that large crystals of zeolites and microporous materials prefer to grow from the alcoholic medium like ethyleneglycol, butyl alcohol, hexanol and others. The crystals resulted from the gel by using small amines are likely to grow to large dimension. The following single crystals are prepared from the alcoholic systems, Si-zeolite (such as Si-sodalite from ethylene glycol, EG)[4], Si-Sodalite (Fig. 2a)[5], Si-ZSM-48 (EG)[6], AIPO4-n like AIPO4-CI (chain structure, BuOH)[7], A1PO4-CA (EG)[8], AIPO4-CC (EG)[9], A1PO4-F (HexOH) [9], AIPO4-H (BuOH) [9], AIPOa-B (EG) [9] and GaPO4-C3 (EG or HEXOH)[9]. By reason of slow crystallization in the medium, a favorable circumstance is provided for the growth of the single crystals. Some single crystals for novel microporous materials are prepared, such as gallophophate-LTA[ 10] (Fig. 2b), ZnPO4, CoPO4-1, -2, -3[ 11 ], as well as JDF-20 [9, 12, 13], which contains 20-ring channel. The preparation of JDF-20 single crystals is carded out in the 1.0 A1203 : 1.8 P205 : 4.7 Et3N : 18 TEG (Triethylene glycol) system at 180 ~ for 5 days. In addition, single crystal of cancrinite is crystallized from butane-l, 3-diol solvent[14]. Adsorption measurement shows that it possesses type I adsorption isotherms. Its adsorption capacity to water, hexane and cyclohexane are 13.7, 9.5 and 9.2%, respectively. These values are different from those of the natural cancrinite, and the cancrinite synthesized from an aqueous medium. In fact, the cancrinite obtained from the system does not exhibit adsorptive properties since the channels are blocked by intercalated salts or due to stacking faults. (%)1oo (%)100
,
/ X
.
// 0
0
5
10
15
20(x1000 s)
i
o
i
2
'
~X
Z/
~
L..
J ....
L.__L
_
4
6
8
l 0 (d)
Time
a
b
Fig. 1 Crystallization curves for AIPO4-21 at 200 ~ (a), (I) in water, (II) in HOCH2CH2OCH3, (HI) in O(CH2CH2OH)2; and for Mordenite at 145 ~ (b), (A) Silica sol as a silica source, (x) Silica sol and aerosil as "double silica sources".
303
b
V
d
e
f
Fig.2. SEM images of single crystals of Si-Sodalite (a), GaPO4-LTA (b), GaPO4-C4 (c), A1AsO4-1(d), InPO4-C11 (e) and InPO4-C12 (f).
304 3.2. Crystallized from acidic medium A novel microporous family containing the third and fifth element, M0]~X(V)O4, has been discovered by R. Xu[15]. Single crystals of the microporous MOI~X(V)O4 prefer to grow from the acidic medium, pH values of the starting mixture being 3-6. Among over 30 species of the M([II)X(V)O4 family, more than six have been grown single crystals from the acidic medium containing templates. These include GaPO4-C3[15],-C4 (Fig. 2c) [16],C7[ 17], AIAsO4-1 (Fig. 2d)[ 18], -2[ 19] and GaAsO4-2[20]. Morerecently, Y. Xu, et al.[21 ] have synthesized a novel microporous indium phosphate, InPO4-1 single crystal, from acidic medium. The structure determination indicated that the In and P atoms have octahedral and tetrahedral coordination respectively bridged through oxygen atoms. Followed by Du et al. [22], they successfully synthesized 4 kinds of large crystals of microporous InPO4 from acidic medium. They are InPO4-7, -9 (trigonal, Fig. 2e), -12 (monoclinic, Fig. 2f). According to our experience for the synthesis of single crystals of microporous M(I]I)X(V)O4, the stronger metal the M(I]I) is, the lower pH value is needed. In contrary the stronger nonmetal the X(V) is, the higher pH value is needed. 3.3 Crystallized from aqueous solutions The molecular sieves are normally crystallized from a reaction mixture gel. In this way, the concentration and solubility in the gel is very difficult to control at crystallization temperature. Along with the increasing of the crystallization rate, the nutrient for crystallization is rapidly exhausted, so it is very difficult to grow up large crystal. Crystallization from homogeneous solution is a good way to obtain single crystal since the supersaturation of the reactants can be easily controlled. By using this method, single crystals of zeolite A [23] was crystallized by adding more sodium hydroxide in the mixture to get homogeneous solution. Similarly, HF was added to the mixture of AIPO4 and diethylamine to create an aqueous solution, single crystal A1PO4-5124], FAPO-5125] wereobtained. Growth of single crystals of zeolites mordenite(Fig. 3 (a)), Beta (Fig. 4 (a)) and ZSM-5 has been successfully accomplished by using "two silica sources" technique. Pang and coworkers [26] have studied the preparation of large single crystals of zeolite mordenite from the aqueous solutions in the absence of templates. During the preparation stage, sodium silicate solution and aerosil are jointly used as silicon sources. This results in mordenite single crystals of large dimension. In the usual case, the crystallization of zeolite mordenite is completed in a short period which will result in small particles. With the use of the mixed silicon sources, the crystallization rate is reduced which is believed to be due to the different reactivity of sodium silicate solution and aerosil. Using the "two silica sources" technique, Pang and coworkers [27] also prepare large single crystals of zeolite Beta and ZSM-5, each using tetraethylarnmonium cation and n-butylamine as templates. The synthetic conditions and products are shown in Table 1 a and b. 3.4 Synthesis in the presence of fluoride ions. Guth et al. [26] developed a novel route to preparing single crystal of ZSM-5 in a nonalkaline medium in the presence of flouride and tetrapropylammonium bromite (TPABr) was employed as a template. To extend the investigation, Qiu et al. [27] studied the crystallization mechanism and reported for the first time the growth of perfect single crystals of boron and titanium substituted ZSM-5 (Borozeosilite and Titanozeosilite) (Fig. 4b and 4c). It is believed that silicon, boron and titanimn complex fluorides are formed during the beginning stage of
305
Table la Crystallization from aqueous solutions Reaction comp. (mol) SiO2 Na20 Me Cod A1203
H20
Temp (~
Time (d)
Reaction mixture
Crystal size/pro
Product
550 550 550 550 550 550
150 150 150 150 150 145
15 15 15 5 5 25
c c c c c g
185x125 27x30 85x50 8x3 2xl 110x55
MOR MOR MOR MOR MOR MOR
e
A B C D E F
1 1 1 1 1 1
60a§
b
60"+ 15b 60%15b 75a§ b 0a§
b
60a+50b
15 15 15 15 15 15
4NaC1 4NaAc 4KC1 4NaC1 4NaCI 4NaC1
a: silica from aerosil; b:silica from sodium silicate; Me: amount of salt used; c:clear solution; g: gel. Table 1 b Crystallization from aqueous solutions Gel composition (mol ratio)
Temp
No.
A1203
SiO2
Re20
Na20
H20
NaC1
A B C D
1 1 1 1
79a§ b 70a+63b 35a§ b
16TEA 27TEA 71NBA 76NBA
36 36 23 23
1466 1466 1466 1466
12 12 10 10
43a+40b
Time
(~
(d)
140 140 155 155
12 12 15 15
Crystal size/~n 28 20 63 52
Product Beta Beta MFI MFI
a: silica from aerosil; b: silica from sodium silicate; TEA: tetraethylammonium hydroxide; NBA" n-butylamine. crystallization and then slowly hydrolyzed to other less fluorinated silicates, borates and titanates, respectively, which supply, slowly and continuously, the nutrients for the growth of the single crystals [27]. Besides, Zhao and coworkers [28] successfully prepared large crystals of ZSM-5 using seven different organic templates in fluoride system. They are triethylamine, tetraethylammonium, diethylamine, ethylenedimine, choline, piperazine and 1,4-diazabicycle(2,2,2) octane, respectively. Single crystals of ZSM-12 and ZSM-39 (Fig. 4d)[29] were also crystallized from this medium. Qiu and coworkers [30] reported for the first time the growth of single crystals of aluminophosphate molecular sieves by adding fluoride to the starting mixture. A big and perfect single crystal (1000 um in length) of A1PO4-5 (Fig. 3b and 3c) was obtained. Using the similar technique, Si-, Li-, B-AIPO4-5 (Fig. 4e) and AIPO4-1 l(Fig. 40 single crystals [31 ] can be crystallized from fluoride system. Based on experimental observations, the controlling parameters for the growth of single crystals are summarized [24]. They are (1) low temperature aging of the hydrogel, (2) low temperature crystallization, (3) high water content, (4) presence of fluoride ions in proper amount and (5) pre-treatment of vessels to remove the trace amount of impurities from the surface before crystallization. By controlling these parameters, the nucleation and crystal growth processes can be controlled for the preparation of single crystals of certain dimensions.
306
b
c
Fig. 3 SEM images of single crystals of mordenite (a), and A1PO4-5 (b), (c). 4. CONCLUSIONS Novel routes to synthesis of large crystals of molecular sieves have been developed, which are crystallization in the media of (a) organic solution, (b) weak acid, (c) aqueous solution and (d) fluoride ions. ACKNOWLEDGEMENT This work was supported by National Natural Science Foundation of China and Changchun Center of Applied Chemistry.
307
a
b
c
d
e
f
Fig. 4 SEM images of single crystals of Beta (a), B-ZSM-5 (b), Ti-ZSM-5 (c), ZSM-39 (d), SAPO-5 (e) and AIPO4-11 (f).
308 REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 10. 20. 21. 22. 23. 24. 25. 26. 27. 28. 20. 30. 31.
L.B. Sand, in Proc. of the 5th Intemational Zeolite Conference on Zeolites, 1980, P. 1. D.M. Bibby and M.P. Dale, Nature, 317 (1985), 157. Q.S. Huo, S.H. Feng and R. R. Xu, Acta Chimica Sinica, 48 (1990), 639. S.H. Feng, J.N. Xu, R.R. Xu, G.D. Yang, C. G. Chen and G.P. Li, Chem. J. Chinese Univ., 4 (1988), 9. S.H. Feng, Ph. D Thesis, Jilin University, PRC, 1986. Q.S. Huo, S.H. Feng and R.R. Xu, Zeolites: Facts, Figures, Future, eds, P.A. Jacobs and R.A. van Santen, Elsevier, Amsterdam, 1989, P291. R.H. Jones, J.M. Thomas, R. Xu, Q. Huo, Y. Xu, A.K. Cheetham and D. Bieber, J. Chem. Sot., Chem. Commun., 1170 (1990). R.H. Jones, J.M. Thomas, R. Xu, Q. Huo, .A.K. Cheetham and A.V. Powell, J. Chem. Soc., Chem. Commun., 1266 (1991). Q.S. Huo, PH.D. Thesis, Jilin University, PRC, 1992. J. Yu, J. Chen and R. Xu, Microporous Mater., 5 (1995), 333. J. Yu, J. Chen and R. Xu, Poster Presentation in this Conference. Q.S. Huo and R.R. Xu, in Proc. 9 th International Zeolite Conference (Montreal, Canada), Butterworth-Heinemann, 1992, P. 279. R.H. Jones, J.M. Thomas, R.R. Xu, J.Chen, Q.S. Huo, S.G. Li and Z.G. Ma, J. Solid State Chem., 102 (!993), 204. S.G. Li, C.H. Liu and R.R. Xu, J.Chem. Soc., Chem. Commun., 1645 (1993). J. Chen, Ph.D Thesis, Jilin University, PRC, 1989; R. Xu, J. Chen, S. Feng, Chemistry of Microporous Crystals (Kodansha-Elsevier), 1990, 63. S.H. Feng, R.R. Xu, G. Yang and H. Sun, Chem. J. Chinese Univ. (English edition), 4 (1988) 1. T. Wang, G.Tang, S. Feng, C.Shang and R. Xu, J. Chem. Sot., Chem. Commun., 948 (1989). G.Yang, L.Li, J.Chen and R.Xu, J. Chem. Soc., Chen. Commun., 810 (1989). L.Li, L.Wu, J. Chen and R. Xu, Acta Crystallor., Sect. C, 286 (1991). J.Chen, L.Li, G.Yang and R.Xu. J. Chem. Soc., Chem. Commun., 1217 (1989). Y.Xu, L.L.Koh, L.H.An, S.Qiu and Y.Yue, in Proc. 10th International Zeolite Conference (Garmisch-Partenkirchen), Elsevier, 1994, P. 2253. H.Du and W.Pang, to be published. W.Pang, S. Ueda, and M. Koizumi, in Proc. 7th International Zeolite Confenrence. , Tokyo, Elsevier, 1986, P. 177. S.Qiu, Ph. D Thesis, Jilin University, PRC, 1988. W.Pang, S.Qiu, Q. Kan, Z. Wu, S. Peng, in Proc. 8th International Zeolite Conference, Amsterdam, Elsevier, 1989, P. 281. J.L.Guth, H.Kessler and R.Weg, in Proc. 7th IntemationalZeolite Conference, Tokyo, Elsevier, 1986, P137. S.Qiu, W.Pang and R.Xu, Chem. J. Chinese Univ.(English Edition), 5(1989) 8. D.Zhao, S.Qiu and W.Pang, Zeolites, 13.(1993) 478. D.Zhao, S.Qiu and W.Pang, J. Chem. Soc., Chem. Commun., 1313(1990). S.Qiu, W. Pang, H. Kessler and J.L. Guth, Zeolites, 9 (1989) 440. S.Qiu, W. Tian, W. Pang, T. Sun, D. Jiang, Zeolites, 1-1(1991) 371.
H. Chon, S.-K. lhm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis,Vol. 105 © 1997ElsevierScience B.V. All rights reserved.
309
S y n t h e s i s a n d c h a r a c t e r i z a t i o n of Z S M - 5 in f l u o r i d e m e d i u m : t h e role of N H 4 ÷ a n d K ÷ c a t i o n s
E. Nigro a, R. Mostowicz b, F. Crea a, F. Testa a, R. Aiello a and J. B.Nagy c aDipartimento di Ingegneria Chimica e dei Materiali, Universit~ della Calabria, 87030 Rende (CS), Italy bIndustrial Chemistry Research Institute, 01-793 Warsav, Poland cUnit~ de R.M.N., Facult6s Universitaires Notre Dame de la Paix, B-5000 Namur, Belgium
The role of NH4 + and K ÷ cations in the synthesis of ZSM-5 using fluoride anions as mineralizing agents was investigated as a function of the amount of aluminium and of the cations (9 < MF < 24 and (0.16 < SiO2/AI(OH)3 _ 1). The potassium containing system is the most effective in the incorporation of aluminium into the zeolite framework while NH4 is the least effective one. The crystallization curves and morphology of the crystals are also strongly influenced by the nature of fluoride salt added in the reaction mixtures.
1. I N T R O D U C T I O N The first synthesis of zeolites with MFI structure obtained in fluoride medium was reported in 1986 [1]. Guth and his co-workers patented different synthesis methods for the preparation of both purely siliceous (silicalite-1) microporous materials and of the MFI-type zeolites, with Si partly substituted by T m elements (T=B, A1, Fe, Ga) [2], by utilizing N I ~ F and HF. The effectiveness of other fluorides (NaF, KF and CsF) in the synthesis of silicalite-1 [3,4], silicalite-2 [5] and zeolite Beta [6] opened new routes for the preparation of zeolites in non-alkaline media. MFI type zeolites with boron [7] and iron [8] incorporated into the framework were successively synthesized using different fluoride salts. These studies clearly demonstrated that the presence of the different alkali cations has a marked influence on the amount of heteroatom included in the structure, rate of crystallization, morphology and crystal size. In this paper we report the synthesis and characterization of [A1]ZSM-5 type zeolite obtained in the presence of various fluoride salts.
310 2. EXPERIMENTAL P A R T The initial batch composition was: 10Si02-xMF-yAl(OH)3-1.25TPABr-330H20 with x=9, 15, 24 and y=0.16, 0.5, 1, with M=NH4 and K. The reactants were mixed in the following order: tetrapropylammonium bromide (TPABr, Fluka), distilled water, MF (M=NH4 and K, Carlo Erba), AI(OH)3 (Pfaltz and Bauer) and fumed SiO2 (Sigma). The syntheses were carried out in Morey-type PTFE-lined 20 cm 3 autoclaves at 170_+2°C for a prefixed time, without stirring, and under autogeneous pressure. At the end of the reaction the products were filtered, washed with distilled water and dried overnight at 105 °C. The nature of the solid phase and the degree of crystallinity were determined by using powder X ray diffraction. For the calculation of the crystallinity, the intensity of the main peak at d= 3.85/~ (or 23.10 ° 20) were compared with the intensity of a reference sample, for each system, purified through ultrasound treatment. The amount of aluminium and alkali cations in the crystals were determined by atomic absorption analysis. The amount of TPA + cations and water trapped into the crystals was obtained using TG analysis. DSC curves were used to evaluate the path of decomposition of the organic molecule. TG and DSC analyses were carried out in an N2 atmosphere with a heating rate of 10°C/min. Morphology and crystal size were determined by SEM. The MAS-NMR spectra of the samples were recorded on either a Bruker MSL 400 or a CXP 200 spectrometer. For 29Si (39.7 MHz) a 6.0 ~s (0=~/2) pulse was used with a repetition time of 6.0 s. For 27A1 (104.3 MHz) a 1.0 ~ts (0=x/12) pulse was used with a repetition time of 0.1 s. The number of accumulations varied between 8.000 and 15.000 to obtain a good signal-to-noise ratio.
3. RESULTS AND DISCUSSION Preliminary experiments showed that in systems where x<6 and y>l ZSM-5 crystallization does not occur. MFI-type zeolite was the only phase obtained when 9y~x<24 and 0.16~y~l. Table 1 reports the physico-chemical characterization of NH4- and K-ZSM-5 zeolite. Although the A1 incorporation in the ZSM-5 crystals regularly increases with AI(OH)3 in the batch, the Si/A1 in the crystals is always lower than in the gel. Moreover, the variation of A1 incorporation as a function of type and concentration of fluoride is markedly different for NH4+ and K ÷ cations. With NH4F A1 incorporation is more favoured by low fluoride concentrations in the batch for all the AI(OH)3 concentrations investigated, while, in the presence of KF, A1 incorporation in the crystals increases with the amount of fluoride in the batch. In correspondance of the highest AI(OH)3 amount in the gel a decrease of A1 incorporation is however observed for the highest fluoride content. In case of KF, the optimum fluoride concentration lies between 15 and 24 moles of fluoride. The ZSM-5 crystallization kinetics in the presence of the various fluorides concentrations containing 1.0 AI(OH)3 moles, are reported in Figure 1 and Table 2 summarizes the induction times (tind) and crystallization rates (R). Both log 1/tind and logR show quasi similar variations as a function of either the initial AI(OH)3 content or the total NH4 ÷ or K ÷ ion amount. For the NH4-gels, both the induction
311 Table 1 Physico-chemical characterization of ZSM-5 samples obtained from the system: 10Si O2-xAI(OH)3-yMF- 1.25TPABr-330H20 ~(OH)3 inifi~ A1/u.c.aM/u.c. a
TP~u.c. ~420°C
~470°C
H20/u.c. c
9NH4F
0.16 0.5 1.0
1.3 2.9 3.9
-
2.5 2.2 2.0
3.8 3.8 3.7
1.8 2.3 2.6d
2.0 1.5 1.1
0 0 0
15NH4F
0.16 0.5 1.0
0.4 1.6 2.8
-
3.7 3.0 2.3
3.8 3.8 3.8
1.9 2.3 2.3
1.9 1.5 1.5
0 0 0
24NH4F
0.16 0.5 1.0
0.6 0.7 1.2
-
3.0 2.8 2.5
3.8 3.8 3.8
2.0 1.9 1.8
1.8 1.9 2.0
0 0 0
9KF
0.16 0.5 1.0
0.2 2.9 4.9
0.2 1.0 5.0
3.7 2.2 1.9
3.8 3.8 1.9
1.7 1.5
2.1 2.3 1.9
0 0.4 11.4
MF
F/u.c. b
total c
-
0.16 0.3 0.8 4.3 3.8 2.6 1.2 0 2.2 15KF 0.5 3.7 1.4 2.0 3.6 1.1 2.5 12.0 1.0 6.5 3.0 1.5 2.0 2.0 0.16 0.15 1.7 4.8 3.8 2.6 1.2 0.4 24KF 0.5 3.2 2.6 1.9 2.5 2.5 7.9 1.0 6.1 3.1 1.2 3.0 3.0 8.1 a D a t a from atomic absorption analyses (NH4 not determined); the a m o u n t of f r a m e w o r k t e t r a h e d r a l A1 was computed from 27Al-NMR results; b d e t e r m i n e d using ion-selective electrode; cdata from thermogravimetric analyses; d m a x i m u m at ca 450°C. rate and the crystallization rate are decreasing with increasing AI(OH)3 content, except for 24NH4F, where the induction rate r e m a i n s constant. Both induction rate and crystallization rate are increasing with increasing NH4F content, and a p l a t e a u seems to be reached a t 15NH4F. For the K-gels, a decrease in both induction rate and crystallization rate as a function of AI(OH)3 content can only be noticed at low initial KF content (9KF). For 15 and 24 KF in t h e gels, both induction r a t e s and crystallization r a t e s are almost d e p e n d e n t on the AI(OH)3 content. The variation of the induction rates and the crystallization rates as a function of K ÷ is similar to t h a t observed with NH4F. The total yields and the yield with respect to A1 are reported in Table 3. NH4 + is more efficient for the total yield t h a n K ÷ for quasi all MF and AI(OH)3 contents. The total yield decreases for both cations as a function of increasing MF content. NH4 ÷ is more efficient to introduce A1 in the solid sample at low MF content (9MF), while K ÷ is favoured for higher MF contents (15 and 24 MF). These results confirm the previously reported results on the ZSM-5 synthesis in the presence of NH4F and in p a r t i c u l a r t h a t the increase of a m m o n i u m fluoride
312 Table 2 I n d u c t i o n time (h) and crystallization r a t e (% h -1) of M-ZSM-5 s a m p l e s crystallized from the system: 10SiO2_xAl(OH)3-yMF-1.25TPABr-330H20
MF
AI(OH)3
tind
R(%h -I)
0.16 0.5 1.0 0.16 0.5 1.0 0.16 0.5 1.0
9.2 8.5 36 3.1 3.2 9.1 3.1 3.2 3.6
7.1 2.1 2.7 21.7 17.2 10.4 35.7 17.2 10.4
0.11 0.12 0.03 0.32 0.31 0.11 0.32 0.31 0.28
2.14 2.08 1.48 2.50 2.49 2.14 2.50 2.49 2.45
0.85 0.32 0.43 1.34 1.24 1.02 1.55 1.24 1.02
0.16 0.5 1.0
2.9 10 72
5.3 1.9 1.5
0.34 0.10 0.014
2.53 2.00 1.15
0.72 0.28 0.18
0.16 0.5 1.0 0.16 0.5 1.0
2.9 3.1 5.8 2.9 3.1 5.8
23.8 32.3 26.3 13.5 10.0 14.5
0.34 0.32 0.17 0.34 0.32 0.17
2.53 2.50 2.23 2.53 2.50 2.23
1.38 1.51 1.42 1.13 1.0 1.15
Initial
9NH4F
15Ntt4F
24Ntt4F
9KF
15KF
24KF
(h)
I/tind(h"I) 3+logl/tind log R
100-
NH4F
80 >, _=
KF
60
.....
40
m
20 0
I 0
20 Tlme (hours)
30
0
100 200 Tlme(hours~
300
Figure 1. Crystallization kinetics of ZSM-5 from gels of molar composition: 10SiO2-xMF-1.0Al(OH)3-1.25TPABr-330H20 with x=9 (D), 15 ( o ) and 24 (/1 ). and M=NH4 and I~
313 Table 3 Total reaction yield in (%) and yield with respect to A1 content obtained from the s~,stems:10 SiO2-xAl(OH)3-~'MF-1.25TPABr-330H20 NH4 K AI(OH)3 Total a Aluminium Total a Aluminium initial Yield (%) Yield (%) Yield (%) Yield (%) MF 0.16 44.8 81.0 39.0 33.8 0.5 47.6 60.5 40.0 65.6 1.0 47.6 42.0 39.0 56.3 0.16 39.8 23.1 30.4 52.0 0.5 40.8 13.4 28.7 72.5 15 1.0 41.0 31.6 37.1 86.3 0.16 29.5 35.4 27.0 48.9 24 0.5 30.1 13.5 29.5 72.3 1.0 33.5 13.0 30.0 80.4 aTotal yield is determined with respect to the weight of SiO2 + A1203 in the batch
content in the reaction mixture leads to a decrease of the A1/Si ratio in the zeolite framework [9]. This was explained by considering that, once ensured a fluoride concentration suitable for the AI(OH)3 in the starting mixture, a further increase of fluoride concentration, corresponding to higher pH, stabilizes the fluoride complexes of the heteroatom in the liquid phase so disfavouring the isomorphous substitution of silicon into the framework. In all compositions investigated (see Table 1) the Si/A1 ratio in the product is higher than in the initial reaction mixture, meaning that only part of A1 in the gel was introduced into the zeolite framework. A previous study has clearly shown the effect of the F- concentration on the Si/A1 ratio in the crystals [9]. Fluoride ions act as solubilizing agents initially and then assess the polycondensation and crystallization processes. The stability of fluoride complexes of the heteroatom in the liquid phase determines the degree of substitution of silicon in the products. It is already evident that, if the mixtures prepared with different fluoride salts can give ZSM-5 samples with different amounts of tetrahedral aluminium, inorganic cations control the mechanism of crystallization. Both type and concentration of fluoride strongly influence morphology and size of ZSM-5 crystals, as reported in Table 4 and Figure 2. While the crystals obtained from NH4-containing systems are, in fact, always in the form of long and narrow prisms, in the presence of K fluoride ZSM-5 crystals show a prismatic morphology only for low AI(OH)3 content (0.16 moles) and a spherulitic morphology in correspondance when the AI(OH)3 content is larger (0.5 and 1 mole). The 27A1-NMR spectra of all the samples show clearly that the NH4-ZSM-5 zeolites only contain framework tetrahedral A1 atoms characterized by a chemical shift of 52 ppm vs AI(H20)63+. The K-ZSM-5 zeolite samples contain, in addition, some octahedral extraframework A1, the presence of which is shown by the NMR line at ca 0 ppm (Table 5) [10]. It is interesting to compare the total formal positive charges given by (TPA+M)/u.c. with those of the negative charges (Al+F)/u.c. (Table 5). In most of the cases for the K-ZSM-5 samples, the two charges are equal, showing that the [SiOA1]- framework negative charges and the
314 Table 4 Crystal size (in ~m) of the crystals obtained from the system: 10SiO2.xAI(OH)3-yMF- 1.25TPABr-330H20 yMFa xAI(OH)3 a NH4F KF 0.16 50x15x10 58x27x26 0.5 45x16x10 ~56 b 1.0 48x27x10 ~45 b 0.16 51x15x10 37x37x16 15 0.5 36x10x10 ~36 b 1.0 68x24x16 ~24 b 0.16 61x17x10 60x50x50 24 0.5 60x20x15 ~26 b 1.0 82x22x18 ~11 b a In the batch; b spherulitic crystals. F" ions are neutralized by both TPA ÷ and K ÷ cations. In two cases the (A]+F)/u.c. values are higher than the (TPA+M)u.c. values, suggesting that protons could also neutralize the framework negative charges. Although no information is available at present on the nature of fluoride species, whether they are present as F- ions or framework -Y--Si-F species, the presence of protons as countercations seems very probable, because the A1/u.c. alone is higher than the (TPA+K)/u.c. values for these two cases. When the A1/u.c. increases in the framework of the final K-ZSM-5 samples (Table 1) , the TPA/u.c. decreases, while the K/u.c. increases together with H20/u.c. The same phenomenon was observed previously for syntheses of ZSM-5 in alkaline media [ 11 ].
Figure 2. Scanning Electron Micrographs of ZSM-5 crystals obtained from gels of molar composition: 10SiO2.15MF-1.0Al(OH)3-1.25TPABr-330H20 with M=NH4 and K.
315 Table 5 27A1 and 29Si-NMR data and concentration variations in K-ZSM-5 crystals MFa AI(OH)3a 5(ppm) b I(%)c (TPA+K)/u.c.d (Al+F)/u.c.d SiOM/u.c. e octahedral A1 0.16 -2 12 4.0 3.9 14.3 9KF 0.5 0 2 4.8 5.1 12.0 1.0 0 2 6.9 6.8 5.0 0.16 -1.2 17 4.6 4.6 20.0 15KF 0.5 0.4 4 5.0 5.7 2.2 1.0 -0.8 6 5.0 8.0 2.0 0.16 -1.0 80 5.5 5.0 18.6 24KF 0.5 -1.0 30 5.1 5.1 2.3 1.0 -0.9 4 6.1 7.3 2.0 aIn the batch; bmeasured by 27A1-NMR; c Relative intensity; dsee Table 1 for individual values; ecomputed from 29Si-NMR spectra: 1.105ppm=ISiOM+Isi(1A1) and ISi(1A1)= 100/0.25(Si/A1) All the 29Si-NMR spectra are characteristic of ZSM-5 in an orthorhombic form. The SiOM/u.c. defect groups, where M=H in most of the cases, are in a much smaller amount t h a n in samples synthesized in alkaline media [10, 12]. In addition, their value decreases with increasing Al/u.c.
CONCLUSIONS The synthesis of M-ZSM-5 zeolites in presence of either NH4 + or K + cations shows similar induction and crystallization rates in fluoride containing media. The A] incorporation into the framework is favoured by low fluoride concentrations in the presence of NH4F, while the reverse is observed with KF. In correspondance of the highest A](OH)3 amounts in the gel, the optimum KF concentrations lies between 15 and 24 moles of fluoride. K + cations are, in any case, more efficient in introducing aluminium into the framework. Size and shape of the crystals are also strongly influenced by the nature of the cations.
ACKNOWI~DGMENT The authors thank Mr G. Daelen for his skillful help in taking the NMR spectra and the Italian National Research Council (CNR) - Progetto Strategico "Tecnologie Chimiche Innovative" for its finantial support. Janos.B.Nagy is indebted to the Belgium Programme on Inter University Poles of Attraction initiated by the Belgium State Prime Ministre's Office of Science Policy Programming.
316 REFERENCES
1. J.L. Guth, H. Kessler and R. Wey, Stud. Surf. Sci. Catal. 28 (1986) 121. 2. H. Kessler, J. Patarin and C. Schott-Darie, Stud. Surf. Sci. Catal, 85 (1994) 75 and references therein. 3. F. Crea, R. Mostowicz, F. Testa, R. Aiello and J. B.Nagy, in Proc. "9th International Zeolite Conference", R. von Ballmoos et al. (eds) Part. I, Butterworth-Heinemann, Stoneham, 1993, p. 197. 4. R. Mostowicz, F. Crea, and J. B.Nagy, Zeolites, 13 (1993) 678. 5. R. Mostowicz, A. Nastro, F. Crea and J. B.Nagy, Zeolites, 11 (1991) 732. 6. R. Mostowicz, F. Testa, F. Crea, A. Nastro, R. Aiello, A. Fonseca and J. B.Nagy, Stud. Surf. Sci. Catal., 84A (1994) 171. 7. F. Testa, R. Chiappetta, F. Crea, R. kiello, A. Fonseca and J. B.Nagy, Stud. Surf. Sci. Catal., 94 (1995) 349. 8. F. Testa, F. Crea, R. Aiello, and S. Teti, Proc. "2 ° Convegno Nazionale sui Materiali per rIngegneria', Trento (Italy), (1994), 165. 9. J. Dwyer, J. Zhao and D. Rawlence, in "Proc. 9th International Zeolite Conference", Part I, R. von Ballmoos et al. (eds), Butterworth-Heinemann, Stoneham (1993), p. 155. 10. Q. Chen, J.L Guth and J. Fraissard, in Proc. "10th International Congress on Catalysis", Budapest, Hungary, 19-24 July, 1992, Akad~miai Kiad6, Budapest, 1993, p. 2577. 11. J. B.Nagy, P. Bodart, H. Collette, C. Fernandez, Z. Gabelica, A. Nastro and R. Aiello, J. Chem. Soc., Faraday Trans, 1, 85 (1989) 2749. 12, G. Debras, A. Gourgue, J. B.Nagy and G. De Clippeleir, Zeolites, 5 (1985) 377.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All fights reserved.
317
Three-dimensional real-time observation of growth and dissolution of silicalite crystal A. Iwasaki a, I. Kudo a and T. Sano b a Electrotechnical Laboratory, Tsukuba, Ibaraki 305, Japan
b Japan Advanced Institute of Science and Technology, Tatsunokuchi, Ishikawa 923-12, Japan A three-dimensional real-time observation technique was applied in the study of the growth and dissolution processes of silicalite crystals. The behavior of each crystal face was investigated by varying the growth and dissolution conditions. The growth of the (010) face of the crystals started from the center, and the dissolution, from the edge. The evolution of hillock on the (010) face was revealed. 1. INTRODUCTION Since the supersaturation level during the hydrothermal synthesis of zeolite crystal is not well clarified, the growth mechanism has not yet been perfectly understood. Such knowledge on the growth process would enable the synthesis of larger crystals with better quality. As noted by many authors, ~-2) the growth morphology of zeolite varies with the composition of the synthesis mixture, i.e., aluminum and alkali concentrations, as well as the synthesis temperature. This means that the growth process for each crystal face is strongly dependent on the synthesis conditions. Dissolution of ZSM-5 zeolite crystals in alkali bases (NaOH and Na2CO3) has been studied to remove silica species preferentially, and the aluminum concentration at the rim was reported. 3-5) As-synthesized crystals were relatively stable in alkali solution compared with calcinated ones because the template molecules in the pores prevent the entrance of hydroxide ions. Fabre refluxed silicalite crystals in 2M NaOH solution and showed that the external surface of the crystal was dissolved. 4) Chemical etching is usually used to investigate defects in many crystals, and makes the characterization of crystals possible. Real-time observation by optical microscopy was applied in the measurement of the growth rate of the relatively large crystals (larger than 10 Ixm) to gain knowledge on the growth process of zeolite crystals, such as the anisotropic features of the growth process. 6-1~) Very recently, we developed an in situ interferometric observation apparatus, which enables precise investigation of the growth behavior throughout the crystal depth. ~2) We could measure the crystal growth rate of the three faces of silicalite crystals simultaneously under hydrothermal synthesis conditions. It was found that the growth rate and the apparent activation energy for each crystal face depend markedly on the synthesis conditions, such as the temperature and the composition of the starting mixture. In this paper, the growth morphology of silicalite crystal during hydrothermal synthesis is presented in detail.
318 A three-dimensional image is helpful in understanding the growth mechanism. The dissolution behavior in a NaOH solution, for which the real-time observation is much more effective, is also addressed. 2. EXPERIMENTAL
Colloidal silica (Cataloid SI-30 from Shokubai Kasei Co.; 30.4 wt% SiO2, 0.38 wt% Na20, 69.22 wt% water; particle size about 10-14 nm) was added with stirring to a mixture of tetrapropylammonium bromide (TPABr) and sodium hydroxide in solution, giving a clear aqueous solution. The mixture had a negligible amount of aluminum. A small amount of the synthesis mixture (about 0,5-1.0 ml) was poured into a Teflon sleeve. To protect the quartz glass window from dissolution in a highpH solution, PFA [poly(tetrafluoroethylene-perfluoroalkylvinyl ether)] film with a thickness of 50 I~m was attached to the surface of the quartz glass. The synthesis of silicalite crystals was conducted at 135, 150, 165 and 180°C. Crystals settling on the quartz window were observed with a microscope (Mitsutoyo VMU-3H) and measurements were carried out continuously over several days. For the dissolution experiment, silicalite crystals of about 65 x 20 x 6 p,m3 were prepared from the synthesis mixture of 0.1TPABr-0.05Na20-SiO2-200H20 (0.3 ml). Crystals grown on the PFA film were taken out from the cell, and were returned to the cell filled with a 1.4M NaOH solution (1.5 ml). Special care was taken not to break crystals during the procedure. Within 2 hours, the temperature of the experimental cell was raised and kept at the desired temperature. The focus of the microscope was adjusted to the (010) face of a crystal which was free from twin and isolated from other crystals. Real-time observation using an interferometric microscope was another key point Depth (d) of our experiment (Fig. 1). Light beams reflected by the upper and lower surfaces of the (010) face interfered with each other, ++:+:+:++:+++++++;+++++++++++++++++++++++++?;+++++++++++++++++++++:++++":: +;+ry ++:+++% stal :+: producing interference fringes corresponding to the crystal thickness. As a light source, a light-emitting diode with a wavelength of 660nm was used, which produced clear o++°,,v+ Lon+ fringes without speckles and was suitable for observation of small crystals (the order of observed fringe (m) was less than 20 because ~l Light of low coherence of the light beam). Difference in the thickness relative to the neighboring fringe was 220 nm (assuming refractive index n=1.49). 12) On the basis of the displacement rate of fringes, the growth rate in the depth ~ Interference direction was calculated. The measurement i accuracy of the growth rate in the depth L H H 1 direction was limited by that of the wavelength of light. The interferogram due to the internal interference was stably observed for more Figure 1. Experimental setup for than several days because the two faces were interlerometric observation. almost parallel and reflected the light in the same direction.
~2nd=m~,
319
3. RESULTS AND DISCUSSION 3.1 Effect of composition of synthesis mixture on the growth behavior The effects of the composition of the synthesis mixture on the growth rate and the morphology were investigated using the following synthesis mixtures. ~~) (1) 0.1TPABr-0.05Na20- SiO2- xH20 (x=100-1000) (2) yTPABr-0.05Na20- SIO2-300H20 (y=0.012-0.1) (3) 0.1TPABr- zNa20- SIO2-300H20 (z=0.025-0.2) (4) 0.1TPABr-0.05Na20-wSiO2-300H20 (w=-0.5-2) The growth rate and the crystal morphology were affected markedly by the composition of the synthesis mixture. By systematic study, the following relationships were obtained at 165°C. (length growth rate ) o~(TPABr)O.39 (Na20)o.52 (SiO2)-o.2o(H20)-o.75 (width growth rate ) =: (TPABr)0.68 (Na20)1.05(SIO2)-0.23(H20)-1.12 (length-to-width ratio) o~(TPABr)-O.29 (Na20)-0.53(SiO2)0.03(H20)0.37 Taking into account the fact that the growth condition of silicalite crystals is mainly characterized by the supersaturation of the primary building unit for the crystallization, we can evaluate the supersaturation level by the form of crystals. Elongated crystals were formed in a solution diluted with water, where the population of chemical species was small and the supersaturation of the primary building units was low. From the morphology of the crystal prepared with low concentrations of Na20 or TPABr, it was suggested that a decrease in the concentration of these species caused a reduction in the number of the primary building units. The concentration of colloidal silica negligibly affected the growth behavior, which indicates that only dissolved silica species contribute to the crystal growth. Addition of both tetramethanolamine (TMA) to form another primary building unit and ethanol for dilution also caused the growth of elongated crystals because of the reduction in the supersaturation. 7) The above result indicates that a good relationship exists between the concentration of the primary building units and the crystal morphology. It should be noted that the composition of the synthesis mixture affected both the growth and the nucleation process, and large crystals were obtained with an appropriate composition. 13) At low concentration, we further observed that the degree of supersaturation was too low to promote the growth in the width direction at higher temperature, and thus the apparent activation energy obtained by Arrhenius plot of the growth rate in the width direction was very small.~) The "apparent" effect shows that the supersaturation level is not the same at each temperature. Since the effect of the H20:SiO2 ratio was significant, we further investigated the growth process using a synthesis composition of 0.1 TPABr-0.05 Na20-SiO2-xH20 (x=80-1000). Figure 2 shows the influence of the crystallization temperature and the H20:SiO2 ratio (x) on the growth form of the silicalite crystal. At higher crystallization temperature, elongated crystals with a high length-to-width ratio were observed. Elongated silicalite crystals were obtained with a high H20:SiO2 ratio, while cubic crystals were obtained with a low H20:SiO2 ratio. The contrast of black fringes (visibility) corresponded to the thickness of crystals.
320
•1TPABr-0.05Na20-SiO 2-xH20 = 100,300) 180 ?o o v
L_
)01 [ ~ ~ . ~ ' ~ Q.
E I--
150
Observation
100
300
401~m
H20:SiO2 Ratio Figure 2. Growth forms of silicalite crystal obtained by in-situ observation. Figure 3 shows the relationship between the width-to-length and depth-to-length ratios of crystals prepared from our system (0.1TPABr-0.05Na20-SiO2-xH20) under several conditions (temperature and H20:SiO2 ratio) at the stationary growth stage. It was found that the width-to-length and depth-to-length ratios were correlated, L e., the crystal with a high width-to-length ratio had a form with a high depth-to-length ratio. The relationship was valid in a system with a higher Na20:SiO2 ratio. Further study using systems with other compositions, such as a Na+-free solution, is necessary to clarify the growth mechanism. 0
0.5
•
I
"
I
"
I
"
I
"
H20:SiO2
o.===
IT" 0.4
H20:SiO2
t.-.
.=80 . ~ ~ O ..... i ~.::-.:~:~:~:~..
= 00
~0.3 t,-" _J
6 0.2
,
o:sio
¢..
Q.0.1 a 0.0
0.0
............
=300 ....% i ! i i ! ~ ~,,~'I 0.1 0.05 ............ ,:~:~::~_ _ _ _~ET.=" i O 15n°c'~i":#:~"~~~~~:~:~":"!";:~:~"~!~i~:.... ~:~:~:~ Na20:SiO2 m N a 2 0 . S i OL2 r'l 18" ^,-o,-,0o ~
dl..J I
,
I
0.2
,
I
0.4
,
I
0.6
,
Width-to-Length Ratio
i
0.8
i
1.0
Figure 3. Relationship between width-to-length and depth-to-length ratio of silicalite crystals.
321
Figure 4. Optical microscope images of growing silicalite crystals: (a) 26.5 and (b) 31 hours of growth (165°C).
Figure 5. Optical microscope images of dissolving silicalite crystals: (a) 2.7 and (b) 3.3 hours of dissolution (172°C).
3.2 Crystal form during the growth and the dissolution Figures 4 (a) and (b) show optical microscope images of silicalite crystals growing from the synthesis mixture with a composition of 0.1TPABr-0o05Na20SIO2-200H20 at 165°C. The black fringes show the contour lines in the depth dimension. From the movement of fringes, it was found that the growth of the (010) face started from the center of the (010) face and expanded outwards. During the stationary growth stage, the growth rates in length, width and depth directions were 1.0, 0.4 and 0.21~m/h, respectively. It was puzzling that the fringe pattern was concentric and isotropic, while the crystal showed anisotropy. The lateral displacement rate of fringes in the (001) direction was high at the center but low at the periphery, suggesting that the reduction of crystal growth due to some impurity took place at the rim. Next, the dissolution behavior of silicalite crystals in a NaOH solution was studied (Figures 5(a) and (b)). The crystal dissolved while retaining its flat faces, which showed that the etching process in surface resulted from the interaction with NaOH solution. Since black fringes moved from the periphery of the crystal to the center, the crystals dissolved from the edge. 14) The visibility of black fringes became better during dissolution, indicating that the crystals became thinner. Since the dissolution rate in the width direction was almost the same as that in the length direction, the crystal gradually became elongated. Etch pits, which are usually seen in the etching process of quartz crystals, were not observed in this observation.
Figure 6. Optical microscope images of growing silicalite crystals: (a) 53.6, (b)72 .8 and (c) 80.1 hours of growth (165°C).
Figure 7. Optical microscope images of dissolving silicalite crystals: (a) 2.1 and (b) 2.2 hours of dissolution (172°C).
322
Figures 6 (a)-(c) show the images of silicalite crystal growing from a 0.1TPABr0.05Na20-SiO2-300H20 solution at 165°C. The concentric fringes indicate the generation of a single hillock on the (010) face. The growth rate of this hillock was much higher than that of the mother crystal and a large protruded area grew on the face. Figures 7 (a) and (b) show the dissolution process of silicalite crystal with a hillock. The hillock dissolved while retaining its form. On the (010) faces, waviness of fringes was also observed at the upper part of the figures. Since the dissolution rate was influenced by the presence of impurity in the crystal, the waviness was attributable to impurity zoning during the growth. In the presence of aluminum species, aluminum zoning with some concentration waviness toward the rim of the crystals has been reported.IS, 16) Further study on the impurity effect is needed. t a j 3
.
,
3
E 2
.
1 . 26.5h
A
,
0
~
.
A
h
~
i
, A
e-
o
0
0. ] TPABr-0.05Na20-SiO2-200H20 ,
I
-40
(b) 3
i
I
-20
|
0 •
0.1TPABr-0.05Na20-SiO2-300H20
E ~2 e--
80.1h
040 (C) 3 E ~2
40
I
"~ •
a
I
20
,
53.6h ,
'
"
-20
1.4M NaOH
Hillock Evolution
I
I
|
'
•
0
i
20
40
I
e-
•- ~ 1 -r
0 -40
3
. -20
3
~ 0 Length (l~m)
20
40
Figure 8. Configuration of the (010) faces (cross section): (a) during growth, (b) during growth with hillock and (c) during dissolution. Figures 8 (a), (b) and (c) show the cross sections of silicalite crystals at the center on a plane parallel to the (100) face, which were constructed from the data in Figs. 4 (a)-(b), Figs. 6 (a)-(c) and Figs. 5 (a)-(b), respectively. The growing crystal became convex in form, as shown in Fig. 8 (a). In the case of a diluted solution
323 (H20/SIO2=300), an elongated crystal grew, where the number of interferometric fringes generated from two opposite (010) faces was small, indicating that the faces were almost parallel (Fig. 8(b)). When there was a hillock, many interferometric fringes arose. The growth rate of the hillock should be calculated assuming that the hillock occurred at one face of the crystal. Tangential growth at the hillock was much faster than that of the mother crystal. Configuration of the crystal face became more convex during dissolution than that during growth, as shown in Fig. 8(c). The inclination of the (010) face becomes steeper at the edge, where inclination was kept almost constant. Since the movement of interferometric fringes was fast at the edge and slowed down at the center, the inclination of the face at the center was gentle. Figure 9 shows the three-dimensional view of the crystal constructed from interferometric images in Fig. 4 (b) and Fig. 5(a), and indicates that the center of the crystal is thicker than the periphery in the (001) direction. The form of the (010) face changed due to the dissolution. Although the resolution in the length and width dimensions was worse as compared with the electron microscope, good resolution of up to 55 nm (corresponding to phase of ~/4 of light) was obtained in the depth direction in our system. This analysis has been recently applied to the examination of optical elements and will be promising for the study of transparent crystals.
(a)
(001 ....
(ol
. "
~,,,:.~ii~i~i,~ii!!i~~, i~.i;
.............................
)
~ i ~ , .....
Figure 9. Three-dimensional images of the crystals: corresponding to (a) Fig. 4 (b) and (b) Fig. 5 (a). Table 1 Apparent activation energies for the growth and dissolution of silicalite.
Apparent activation energies (kd/tool) ,
(001)
(100)
(010)
tangential*
0.1 TPABr-0.05 Na20-SiO2-300 H20
52
28
44
69
0.1 TPABr-0.05 Na20-SiO2-200 H20
6O
36
31
85 94 97 86 Dissolution in 1.4M NaOH solution * on the growth and the dissolution of the (010) face in the (001) direction
Table 1 shows the apparent activation energies for the growth ~o) and the dissolution process, ~7) which were obtained by measuring the growth and dissolution rates at several temperatures. During the growth, colloidal silica dissolved and crystals grew. As the synthesis mixture for the growth is more viscous than that of the dissolution, the activation energies for diffusion of chemical species
324 seems higher. Therefore, the silicalite crystal is more stable than noncrystalline colloidal silica in the synthesis mixture. 4. CONCLUSIONS The internal interferometric technique, which could measure a crystal depth with a resolution of up to 55 nm, was applied to the three-dimensional observation of the growth and dissolution behaviors of silicalite crystals. From the systematic study of the growth and dissolution rates of each crystal face of silicalite, it was found that the growth of the (010) face of the crystal started from its center and the dissolution, from its edge. The growth form of silicalite and its relation with the supersaturation level were speculated. The evolution of a hillock on the (010) face was visualized as concentric fringes started from one point. The technique was also effective for characterizing the crystal quality in terms of surface flatness by obtaining the light reflected by two opposite (010) faces, which enables us to evaluate the internal optical properties of the crystals. ACKNOWLEDGMENTS
We thank Dr. K. Onuma for useful discussions on the growth process of zeolite crystals, and Dr. M. Tanimoto and Dr. K. Koyama for technical advice concerning the interferometric measurement. REFERENCES
1. S.P. Zhdanov and N.N. Samulevich, in Proceedings of the 5th International Zeolite Conference (Ed. L.V.C. Rees) Heyden, London, 1980, p.75. 2. J.C. Jansen, Stud. Surf. Sci. Catal., 58 (1991) 77. 3. R. von Ballmoos, Ph.D. Dissertation, Zurich 1981. 4. R.A. Fabre, Ph.D. Dissertation, Leiden 1989. 5. R.M. Dessau, E.W. Valyocsik, and N.H. Goeke, Zeolites, 12 (1992) 776. 6. B.M. Lowe, Stud. Surf. Sci. Catal., 37 (1988) 1. 7. C.S. Cundy, B.M. Lowe, and D.M. Sinclair, Faraday Discuss., 95 (199) 235. 8. T. Sano, Y. Kiyozumi, F. Mizukami, A. Iwasaki, M. Ito and M. Watanabe, Microporus Mater. 1 (1993) 353. 9. T. Sano, S. Sugawara, Y. Kawakami, A. Iwasaki, M. Hirata, I. Kudo, M. Ito and M. Watanabe, Stud. Surf. Sci. Catal., 84 (1991) 187. 10. A. Iwasaki, I. Kudo, M. Hirata, T. Sano, S. Sugawara, M. Ito and M. Watanabe, Zeolites, 15 (1995) 308. 11. S. Sugawara, K. Takeda, T. Sano, Y. Kawakami and A. Iwasaki, Nippon Kagaku Kaishi, (1995) 621. 12. A. Iwasaki, I. Kudo, M. Hirata and T. Sano, Zeolites, 16 (1996) 35. 13. D.T. Hayhurst and J.C. Lee, in Proceedings of the 7th International Zeolite Conference (Ed. Y. Murakami, A. lijima, L.W. Ward) Tokyo, 1986, p.113. 14. J.P. Hirth and G.M. Pound, J. Chem. Phys., 26 (1057) 1216. 15. R. von Ballmoos and W.M. Meier, Nature, 289 (1981) 782. 16. R. von Ballmoos, R. Gubser and W.M. Meier, in Proceedings of the 6th International Zeolite Conference (Ed. Olsen and A.Bisio) Reno, 1984, p.803. 17. A. Iwasaki, I. Kudo and T. Sano, in preparation.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
325
Synthesis of Mordenite and Z S M - I I zeolites from very dense systems: formation of self-bonded pellets. P. De Luca a,F. Crea a,R. Aielloa,A~ Fonsecab and J. B.Nagy b aDipartimento di Ingegneria Chimica e dei Materiali, Universit~ della Calabria, 87030 Rende (CS), Italy
bUnitd de R. M. N., Facultds Universitaires Notre Dame de la Paix, B-5000 Namur, Belgium Zeolite synthesis was performed from very dense systems of initial gel composition: aM20-bNa20-bAI2Oa-I50SiO2-2bTAABr-490H20 with M= Li, Na, K; TAA = tetramethyl, tetraethyl and tetrabutyl-ammonium; 0.9
1. INTRODUCWION Self-bonded ZSM-5 pellets were recently prepared using Li salts in the reacting gels [1-3]. Self-bonded zeolitie pellets have also been prepared from natural products of volcanic origin [4-6]. The advantage of the direct synthesis of pelleted zeolites is to avoid the necessary formation of agglomerates from the fine zeolitie crystals using an inorganic binder, such as a clay [7]. In this paper, direct syntheses of pelleted zeolites are attempted from aluminosilieate gels obtained with Na ÷, Na ÷ - Li÷ and Na÷-K÷ cations [8]. The role of tetraalkylammonium ions (tetramethyl, tetraethyl and tetrabutyl) is examined in order to yield different types of pelleted zeolites.
2. E X P E R I M E N T A L P A R T
The following materials were used for the preparation of the gels: sodium aluminate (Carlo Erba), NaOH (pellets, Baker), KOH (pellets, Baker), LiOH (monohydrate, Baker), t e t r a m e t h y l (TMA), t e t r a e t h y l (TEA) and
326 tetrabutylammonium (TBA) bromides (all Fluka products), precipitated silica (BDH) and distilled water. The initial gels were prepared by adding an aqueous solution of MOH to solid NaA]O2 followed by addition of tetraalkyammonium (TAA) bromide. The sooted mixtures were homogenized in the mortar. Finally, the precipitated silica was added and the homogenization was continued until a uniform gel was obtained. The final compositions of the gels were: aM20-bNa20-bA120s-150SiO2-2bTAABr-490H20 with M=Li, Na or K; 0.9~__<8.82 and 1.66~_<15. A series of gels were also prepared without TAA bromides. The amorphous gels were then prepared in the required form of pellets using pressures of 0.1-10kg em -2. These pelleted gels were put into a special autoclave where the water phase was separated from the pelleted gels (see Figure 1 ofref. 3), and heated up to 170+- 2°C under autogeneous pressure. The volume of the teflon autoclaves was 80 em S and 5 cm S of water was introduced at the bottom of the autoclaves. The syntheses were interrupted after various times, and the corresponding soformed intermediate phases removed, filtered, washed with cold water and dried overnight at 100°C. The pure crystnlline phases were obtained by ultrasound treatment from the final samples, separating them from the remaining amorphous phase. Their chemical composition was determined by atomic absorption (A~A.- Perkin Elmer 380). The products were identified and their erystaUinity was determined using powder X-ray diffraction patterns, recorded on a Philips 1730/10 diffractometer using Cu Ka radiation and 0.5 ° 20 per minute. Crystal size and morphology were investigated by optical microscopy and scanning electron microscopy (SEM) using a JEOL T 330 electron microscope. Simultaneous t.g.- d.t.g.- d.t.a, analyses of initial gels, intermediates and end products were perfomed by a Netzseh thermal analyzer 414/429. Weight losses and thermal effects due to the release of decomposition products under dry nitrogen atmosphere (flow rate 10 em S min. -1) were evaluated at a rate of 10°C rain. -1 from 30 to 700 ° C. The M R spectra were recorded on either a Bruker MSL 400 or a CXP 200 spectrometer. For 29Si(39.7MHz) a 5.0 ~s(0=~/2) pulse was used with a repetition time of 4.0 s. For 1sC(50.3 MHz), 5.0 ~s(O=~/2) pulses, a single contact sequence with 5.0 ms contact time and a recycle time of 4.0 s were used. For 27AI(104.3MHz), a 1.0 ~s(0=rJ12) was used with a repetition time of 0.2 s.
3. RESULTS AND DISCUSSION Self-bonded pellets have been obtained from two dense systems. The first one is without any organic cation and the inorganic cation is Na + only. Mordenite containing pellets were formed from the initial gel systems: 0.75-1Na20-0.325-0.75A1203-150SiO2-490H20. Self-bonded ZSM-11 pellets were obtained from initial gels of composition: 8.SLi20-0.325-0.75Na20-O.325-0.75A1203-150SIO2-065-1.5 TBABr-490H20. The samples obtained with N a + alone, show the formation of self-bonded mordenite pellets (Figure 1). If the alealinity is still increased up to 3 and 4 moles of NaOH, self-bonded pellets are still obtained, the mechanical resistance of which is
327
90
a3
100
7O
A
b
80
v
o
,.60
E5o o
g e~
N
~40
o ~
if)
30
m •._
u 20 10
ANA-I-PC
c;.5
1.'o
NaOH, molal
,|
o
2.0
Figure 1. Crystallization fields obtained from Na-gels: aNa20bAl203-150SIO2-490H20 with 2.26
•
0
I
I
"
I
2
"
I
"
I
•
3 4 Tlme (days)
I
5
Figure 2. Crystallization curves obtained at 170°C for Na-MOR from gels" a N a20-7.5A1203-150SIO2490H20 with a=8.5 (a) and a=9 (b).
however lower than that of the pellets obtained with 1.5 and 2 moles of NaOH. In order to better understand the origin of differences in mechanical properties, the curves of crystallization of mordenite obtained with 2 and 3 moles of NaOH have been studied (Figure 2). The rate of crystallization is higher with 3 moles of NaOH at the beginning of crystallization (Figure 2b). The higher nucleation and crystallization rates lead to smaller crystals, as more nuclei are formed which grow
Figure 3. SEM micrographs of Na-MOR self bonded pellets obtained from gels: aNa20- 7.5 A1203 -150 SIO2-490 H20 with a= 8.5 (a) and a=9 (b) after 3 days of synthesis at 170°C.
328
90!
100 f%
m o m O
v
7O
U+MEL
80
MEL
ESO & O
IB
~ 4o
¢~30 ~
,IWEL.,..~J
O
C
20 ¸ I,
0.5
i
ANA
I
1.0 1.5 LiOH, molal
I
2.0
Figure 4. Crystallization fields obtained after 3 days of synthesis at 170°C from gels: aLi20-bNa20bA12Os-150Si02-2bTBABr-490H20 with 0.9<15. For symbols see Figure 1.
0
0
1
2
Time
3
(days)
4
5
Figure 5. Crystallization curves of ZSM-11 obtained at 170°C from gels: 4.4M20-bNa20-bA1203-150Si022bTBABr-490H20 with M=Na and b=2.5 (a); M=K and b=2.5 (b~, M=Li and b=7.5 (c).
then faster. Indeed, the SEM micrographs show without ambiguity, that the crystal sizes are greater for 2 moles of NaOH (ca 8-15 tam) than for 3 moles of NaOH (ca 1-3 ~m) (Figure 3). It seems then that the greater crystallites are better bound to each other with the remaining amorphous phase than the smaller crystals. In presence of t e t r a m e t h y l a m m o n i u m ions sodalite is formed more frequently. With tetraethytammonium ions both mordenite and ZSM-5 zeolites are formed in a large crystallization field. Finally, with tetrabutylammonium ions, ZSM-11 is the more interesting zeolite formed. In the presence of IA+, at SIO2/A1203 ratios of 10-20 and an amount of Li close to 8.8 moles, high mechanical strength self-bonded pellets were also obtained (Figure 4). The area of pellets formation is indicated as "MELANA pellets" because the crystallization field corresponds to a reaction time of 3 days but ZSM-11 pellets without analcime can be obtained at shorter reaction times (Figure 5c). It is true, that the available concentrations for pelleted MEL are more restricted than for the formation of MFI [3]. The crystallization curves are illustrated in Figure 5. The SIO2/A1203 ratio was equal to 60 for the Na ÷ and Na+-K+ systems, while it was equal to 20 for the Li + system. It can be seen, that MEL structure is formed faster in presence of Na + than with K + ions (Figures 5a and 5b). The formation of MEL in pelleted form is much slower, and the MEL structure is slowly transformed into the more stable analcime structure (Figure 5c). The thermal analysis of the samples show t h a t TBA + ions are essentially decomposed at ca 480°C and hence it can be supposed that these ions neutralize the (AIOSi)" species. The low temperature peak at ca 405°C is quasi absent [8].
329
Table 1 Chemical composition of M E L zeolitesobtained from initialgels of composition 8.SLi20-7.5Na20-7.5~2Oa-150SiO2-15TBABr-490H20 (sample Li-MEL) and 4.4M20-2.5Na20 2.5A1208-150 SIO2-5 TBABr-490 H20 (samples Na- and K-
ME, L) ..........................................................
Samples
Al/u.c.a~ K/.u.c:a,,Na/u..c..a..,,.Li/u.c.a,.TBA/u.cb
(M+TBA:AI)/u.c.,C.
Li- (MEL)
6.2
---
2.4
0.6
1.6
Na-(MEL)
2.4
---
2.9
---
2.1
1.6
K- (MEL)
2.1
1.1
1.5
---
1.7
2.2
afrom atomic absorpti'on' measm'ements;' bfrOm' therm'al analysis data: csio: " defect groups The chemical analysis also shows that TBA]u.c. is systematically lower than A1/u.c. (Table 1), hence part of alkali cations also neutralizes the (A1OSi)- species, while the remaining amount neutralizes the SiO- defect groups. The SEM photographs show clearly the difference between the pelleted MEL zeolite and the individual large crystals obtained in presence ofK +ions (Figure 6). The small crystals of ca 5 pm are well embedded in the remaining amorphous phase (Figure 6), while the large crystals of ca 10 pm are not easily bonded together. It is interesting to note, that the bigger crystallites form better pellets in the case ofmordenite, while the opposite is true for the ZSM-11 zeolites. This apparent contradiction can be explained by the different binders which play an essential role in the compactness of the pellets. For mordenite pellets, the binder contain sodium
Figure 6. SEM micrographs of ZSM-11 zeolites obtained after 3 days of synthesis at 170°C from : 4.4M20-bNa20-hA12Os-150SiO2-2bTBABr-490H20 with M=Li and b=7.5(a); M=K and b=2.5 (b).
330 Table 2 27A1-NMR data of selected samples obtained from gels aM20-bNa20-bAl20a,150SiO~2bTAABr,490H20 :
..
_
TAA ,
, .
.
.
.
.
.
.
.
,
a
,
. . . . . . . . . . . . .
j
b
,,
,
,,
:
Zeolite
.
_
.
.
....
.
.
.
_
_
:
_ _ . .
O(ppm)/,AH !Hz,)~
._
_
L
AI m ~ g
104 ....
/
8.8Na20
7.5
MOR pellets
55.3/600
22.54
TEA
8.8Na20
7.5
MOR
54.1/600
13.17
TBA
8.8Li20
7.5
ZSM-11 pellets
55.6/1000
9.87
6.6/1000
2.75
TMA
8.8Na20
15
SOD
50.6/600
23.12
TMA
6.75Na20
15
SOD
50.6/6OO
23.34
TMA
8.8Na20
7.5
SOD+Amo
53.6/600
14.07
,,
,,
,
,
,
,
,.J
,,i
.
.
.
.
.
J
,
,,
Table 3 29Si-NMR data of selected samples obtained from gels aM20-bNa20-bA120a,,1 5 0 S i O ~ - 2 b T A A B r - 4 9 0 H ~ O
TAA .............. a
,
..................................
b ...... Ze,°li~
...........
/
8.8Na20
7.5
MOR pellets
TEA
8.8Na20
7.5
MOR
TBA
8.8Li20
7.5 ZSM-11 pellets
° ( p v m ! ,,I-%) (,
Broad band between-97 and -112 ppm -98 (18)
-105 (37)
-412.5 (45)
-97 (10)
-103 (12)
-113 (78)
TMA 8.8Na20
15
SOD
-104.7 (11.8) -110.5 (63.7) -116.3 (24.5)
TMA 6.75Na20
15
SOD
-104.7 (11.8) -110.6 (63.7) -116.3 (11.5)
TMA 8.8Na20
7.5
SOD+Amo
-104 -108 -110 -114 (unresolved spectrum)
Table 4 13C NMR data of selected samples obtained from gels aM20-bNa20-bA120a150SiO2-2bTAABr-490H20 TAA
.
~- a
............................................... b ....... Zeolite ...... 0(ppm)/AH(H.z) ' . ~ p p m ) / A H
!Hz),
TEA
8.8Na20
7.5
MOR
53.3/350
7.3/100
TBA
8.8Li20
7.5
ZSM-11 pellets
62.1/330
20.5/180
24.2/180
14.2/180
TMA
8.8Na20
15
SOD
58.3/10
TMA
6.75Na20
15
SOD
58.3/10
TMA
8.8Na20
7.5
SOD+Amo
57.5/10
,
331 salts for the small and large crystallites. The total compactness depends on the adhesion of the crystallites to the amorphous binder and the adhesion properties of the binder containing more sodium are bett~r despite the fact that the crystallites are bigger. For ZSM-11 zeolites, the nature of the binder is also different from one case to the other. Only the Li-containing binder is able to possess good adhesive properties, as it was also observed for ZSM-5 42, 37, while the K-containing salts are not able to form pellets. Some selected well crystallized and pelleted samples were also characterized by 27A1-,29Si- and 18C-NMR. The data are reported in Tables 2-4. Interestingly, the aluminium is essentially in a framework tetrahedral form in MOR pellets not containing any organics, in the crystalline MOR and in SOD (Table 2). Only some 22% octahedral AI is detected in pelleted ZSM-11. However, a great part of tetrahedral A1 is certainly in the binder in MOR pellets, as the crystalline MOR has a Si/A1 = 5.5 (see below) corresponding to 13.17 10"4 mg Al per g sample, while the MOR pellets contain a much higher amount of A1 (22.54 10-4 mg per g sample). The 29Si-NMR spectra show the characteristic feature of MOR, ZSM-11 and SOD (Table 3). The spectrum of MOR pellets shows a broad band between -97 and -112 ppm. The crystalline MOR and SOD have Si/AI ratios of 5.5 and 4.6, respectively, as computed from the relative intensities of the 29Si-NMR lines [9]. TEA is occluded intact in MOR, TBA in ZSM-11 and TMA in SOD (Table 4).
4. CONCIAISION Self-bonded pellets can be obtained from two dense systems. The first one is without any organic cation and the inorganic cation is Na + only. Mordenite containing pellets are formed from the initial gel systems: 0.75-1Na20-0.325-0.75 A1203-150SiO2-490H20. Self-bonded ZSM-11 pellets were ob~ined from initial gels of composition: 8.8Li20-0.325-0.75NasO-O.325-0.75A1203-150SiO2-0.65-1.5TBABr-490H20. Without any organic cation, mordenite is the main interesting zeolitic product obtained from these dense systems. In presence of tetramethylammonium ions sodalite is formed more frequently. With tetraethylammonium ions both mordenite and ZSM-5 zeolites are formed in a large crystallization field. Finally, with tetrabutylammonium ions, ZSM-11 is the more interesting zeolite formed.
ACKNOWLEDGMENT This work was partly carried out with financial support of Italian Research Council (CNR).
REFERENCES 1. J.P. Gilson, US Pat. 4 537 866 (1985)
332 2. 3. 4. 5. 6. 7. 8. 9.
R. Aiello, J. B.Nagy, F. Crea and/~ Nastro, Brevetto Ital. 47891A89 (1989). F. Crea, R. Aiello, A. Nastro and J. B.Nagy, Zeolites, 11 (1991) 521. R. AieUo,/~ Nastro, F. Crea and C. ColeUa, Zeolites, 2 (1982) 290. R. Aiello, C. Colella,/~ Nastro and R. Sersale, in Proc. "Int. Zeolite Conf.", Reno, (1983) (D. Olson and/~ Bisio, eds), Buttherworths, Guildford, UK, (1984)957. R. Aiello and C. Colella, Brevetto Ital. 47735A84 (1984). D.W. Breck, Zeolite Molecular Sieve, Structure, Chemistry and Use, John Wiley, New York (1974) 742. R. Aiello, F. Crea, & Nastro and P. De Luca, Brevetto Ital. 9410190 (1995). G. Engelhardt and D. Michel, High-Resolution Solid-State NMR of Silicates and Zeolites, Wiley, Chichester (1987).
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
333
Studies on Crystallization of ZSM-12 Type Zeolite. A. V. Toktarev and K. G. Ione Boreskov Institute of Catalysis, Ak. Lavrentiev Av., 5, Novosibirsk, 630090, Russia.
SUMMARY Formation of ZSM-12 zeolite in aluminosilicate system in presence of N(Et)4Br as an organic template was studied. The influence of nature of silica source, concentration of chemical ingredients in reaction mixture, the nature of the element other than aluminum, parameters of hydrothermal synthesis on phase composition of the product was examined. Solid phase transformation mechanism is proposed for ZSM-12 formation in the studied system. Pore-filling function of the template is shown. Peculiarities of ZSM-12 crystallization are related with structural features of this zeolite. 1. INTRODUCTION ZSM-12 (structure type code recommended by the IZA Stucture Commission MTW) is a high-silica zeolite which one-dimensional pore architecture is formed by linear and parallel channels, composed by puckered 12-membered rings. The non-circular apertures of the channels have minimum and maximum values of 5.7A and 6.1A respectively [1]. By this value ZSM-12 has an intermediate position between middle- and wide-pore zeolites and can be an effective shape-selective catalyst for conversion of relatively bulk organic molecules [2, 3]. Catalytic properties of zeolites are strongly influenced by synthesis conditions in which zeolite was prepared. Zeolites with MTW type structure can be synthesized with a wide range of organics: tetraethylammonium (TEA) salts, triethylmethylammonium (MTEA) salts, N,N'-dimethyl-2methylimidazolinium (DMMI) salts, polymer of 1,4-diaminobutane and DABCO and others [4]. For the cases of MTEA [5] and DMMI [6] it was shown that organocations act as porefilling agents but not as true-template ones. It is believed that the other organics also play a similar role in MTW type zeolite syntheses because in their presence the zeolite from the reaction mixtures with more limited range of SiO2/A1:O3 ratio can be synthesized. X. Shouhe et al. [7] showed that ZSM-12 forms in presence of MTEA cation from the reaction mixture with SiO2/A1:O3 ratio as low as 45 against of 120 in case of TEA cation. But, TEA salts are cheaper and more available, and, moreover, ZSM-12 zeolite with SiO:/AI203 ratio of 100 and higher has reasonable interest from catalytic point of view. Thus, detailed and systematic information on ZSM-12 zeolite crystallization in the presence of TEA cation would be very desired. In this paper the results on the investigation of the influence of chemical nature of different silica sources, isomorphously substituting elements, composition of reaction mixture, reaction conditions on crystallization of zeolite with MTW
334 type structure in presence of tetraethylammonium bromide (TEABr) as an organic template are discussed.
2. E X P E R I M E N T A L Reagents used were silicagel (wide-pore, granulated, technical product, sp. surface area 260 m2/g; A1203 - 0.08 wt.%, SiO 2 - 93.00 wt.%, Fe203 - 0.08 wt.%, Na20 - 0.03 wt.%, 1-120 - 6.80 wt.%. Material before use was ground on a disc mill up to a particle size of 30 microns in average), silica sol (colloidal silica in water, technical product (Russia); SiO2 29.54 wt.%, Na20 - 0.40 wt.%, SiO2/A1203 > 3440 (molar). Mean particle size approx. 110 A), silicic acid (reagent grade, Reakhim (Russia), 21.78 wt.% 1-120, sp. surface area 1000 m2/g), fumed silica A-175 (99.8 wt.% SiO2, technical product (Russia), sp. surface area 175 m2/g), sodium metasilicate (Na2SiOy9H20, C.P., Reakhim (Russia)), aluminum nitrate nonahydrate (reagent grade, Reakhim (Russia)), tetraethylammonium bromide (purified, Reakhim (Russia)), sodium hydroxide (C.P., Reakhim (Russia), 98 wt.% NaOH), compounds of isomorphously substituting elements were respective metal nitrate salts, with the exception of boric acid in case of system with boron (all reagent grade). In a typical reaction, appropriate amounts of NaOH, TEABr and water were mixed to obtain alkaline solution. Water solution of element nitrate (El = Be, A1, Ga, Zn) was added to the above solution. To the obtained solution silica reagent was finally added. In cases when E1 = Fe, Cr, Co corresponding nitrate salt water solution was initially mixed with silica reagent. Then obtained mixture was added to the alkaline solution. Hydrothermal syntheses were carried out in 100 ml SS autoclaves lined with teflon in static conditions in temperature interval of 420 - 450 K with different time of crystallization at chosen temperature. XRD analyses of the crystallization products were carried out on HZG-4 diffractometer (GDR) with CuK~ radiation. Content of crystalline phase in the products was calculated as a ratio of heights of the most intensive XRD peak for analyzed sample and of corresponding peak for standard sample.
3. RESULTS 3.1 The influence of chemical composition of the reaction gel From the literature data, especially patent ones (see Ref. [4] and references therein), it is known that ZSM-12 can be synthesized from the reaction mixtures with molar ratios of components falling to the intervals: SIO2/A1203 = 100 - 500, OH/SiO2 = 0.1 - 0.3, H20/SiO2 = 10 - 60 and TEA/SiO2 = 0.1 - 0.5. To find the optimal composition of initial reaction mixture toward crystallization of pure ZSM-12 and, at the same time, to decrease the number of experiments the method of fractional factorial experiment designs for factors at two levels was applied. Having four independent parameters for aluminosilicate reaction composition, 241 fractional design was chosen. Basic levels for optimized reaction mixture' parameters and variation intervals for them (in parentheses) were chosen as follows" SIO2/A1203 = 150 (50), OH/SiO2 = 0.15 (0.04), H20/SiO2 = 18 (4) and TEA/SiO2 = 0.20 (0.05). Temperature was maintained at 430 K and reaction time was fixed at two days for all experiments. Data on these experiments are presented in Tab. 1.
335 Table 1. Factorial designs experiments for ZSM-12 zeolite synthesis optimization. Level of Level of Level of factor Level of ZSM-12 factor factor OH/Si and its factor H20/Si content, % Si/A12 and TEA/Si and value and its value its value its value -1 100 -1 0.15 -1 0.11 -1 14 21 * +1 200 +1 0.25 -1 0.11 -1 14 60 -1 100 +1 0.25 -1 0.11 +1 22 0 -1 100 -1 0.15 +1 0.19 +1 22 37" +1 200 -1 0.15 +1 0.19 -1 14 84 +1 200 -1 0.15 -1 0.11 +1 22 5 -1 100 +1 0.25 +1 0.19 -1 14 62 * +1 200 +1 0.25 +1 0.19 +1 22 90 * Product contains a-cristobalite and trace amount of ZSM-5 Analysis of data showed that OH]SiO2, H20/SiO2 and SIO2]A1203 are the main factors determining the preferential formation of ZSM-12 phase, and TEA/SiO2 is second-order of importance factor in respect of its structure-determining role (in borders of studied compositions). Products obtained from the gels with SIO2/A1203 - 100 along with the goal phase contained impurities of tx-crystobalite and ZSM-5. The increase in OH/SiO2 ratio and the decrease in H20/SiO2 ratio lead, in general, to more crystalline ZSM-12 products. Obtained results allowed to apply directly second step of optimization. Optimal composition of the reaction mixture towards crystallization of pure ZSM-12 zeolite was determined by factorial experiments design to be: SIO2/A1203 = 200, OH/SiO2 = 0.2, H20/SiO2 = 15 and TEA/SiO2 = 0.23. The next experiments on crystallization of ZSM-12 were carried out by one-factor variation of chemical composition of the reaction mixture with the determined optimal components ratios. It is known [8] that the nature of silica source not only influence on the kinetic of zeolite crystallization, but in some cases determines a type of zeolitic phase will be formed too. To examine the influence of the nature of silica source on ZSM-12 formation we used sodium metasilicate, silica sol, fumed silica, silicic acid and silicagel. The reaction mixture was crystallized at 430 K for two days. In all cases, except of sodium metasilicate and silicagel, well crystalline ZSM-5 type zeolite Was formed. In case of Na2SiO3 analcime was formed as a crystalline product. In case of silicagel crystallization product was characterized by coexistence of comparable amounts of ZSM-12 and ct-cristobalite phases and ZSM-5 phase in trace. Thus, with the increase of degree of silica condensation one can consequently obtain analcime, MFI or MTW type zeolite as a product. The most suitable reagent for ZSM12 zeolite formation was found to be a wide pore silicagel. In the next experiments part of silicagel was substituted for silica sol. Results on product phase composition after 64 hours of heating at 420 K are: SiO2 (silica sol)/SiO2 (total) ratio 1/20 1/10 1/5 1/2.5 ZSM-12 phase content 100 92 18 18
336 It is seen that silica from silica sol is not involved into the formation of ZSM-12 zeolite but converts to ZSM-5 phase, which is the second crystalline phase as it was revealed by XRD analysis. ZSM-12 zeolite forms from silicagel only. Variation of SIO2/A1203 molar ratio in the reaction mixture with other parameters being fixed at optimal values was carried out at three levels: 2000 (no aluminum was added), 150 and 100. XRD analysis showed that ZSM-12 is the only phase in the first two cases and major phase with small impurity of ZSM-5 zeolite in the second case. These results confirm the thesis that ZSM-12 is a true high-silica zeolite. The decrease of template concentration leads to proportional decrease ZSM-12 zeolite phase in the solid product. Thus, the decrease of TEA Br content up to 75 % of optimal one led to decrease of content of ZSM-12 phase in the product up to 86 %. The use of a half of optimal content of TEA Br gave 23 % of ZSM-12 in the product only. At a very low content of TEABr, as well as in the absence of it, a layered sodium silicate similar by its XRD pattern to magadiite is formed. The last fact is in agreement with the observations of S. Zones [6]. The influence of water content in the reaction mixture was studied for reaction mixtures with H20/SiO2 molar ratios of 15, 10 and 5. Experiments showed that phase purity of products towards ZSM- 12 is equal and high for the first two cases (100% of ZSM- 12 after 38 hours of crystallization). Further decrease of water content up to H20/SiO2 = 5 resulted in lengthening of crystallization time (56 % of ZSM-12 after 38 hours), but the phase purity remained high. Nature of isomorphously substituting element (B 3÷, Be 2÷, A13+, Ga 3÷, Fe 3÷, Cr 3÷, Zn 2÷, Co 2÷) influences on kinetic of zeolite crystallization (at a constant Men+/si molar ratio), as well as on the possible level of substitution of silicon atom for particular element. Thus, addition of Co or Cr cations into the reaction mixture (Si/EI = 50) practically doesn't influence on the rate of ZSM-12 formation. Chemical analysis of calcined and NHaexchanged samples didn't showed the presence of these elements in the samples. So, it can be concluded that there is no isomorphous substitution of Si atoms for Co or Cr in ZSM-12 in the studied system.. Reaction mixture with Zn didn't produce any crystalline product even after 5 days of heating. Maximal content (expressed in terms of molar ratios of oxides Me2On/SiO2) of introduced element in the product, obtained in studied reaction system and representing pure phase with MTW type structure, may be achieved as high as 66 in case of B, 100 - for Be, 150 - for AI, 200 - for Ga, 300 - for Fe. The rate of crystallization and the level of substitution are higher for elements on the left wing of the above row and v i c e v e r s a . These observations are in accordance with the changing of ionic radii of elements. 3.2 The influence of the reaction conditions on ZSM-12 crystallization. ZSM-12 zeolite readily crystallizes from the optimal reaction mixtures in temperature interval of 450 - 420 K. Kinetic curves of the zeolite crystallization at the two extreme temperatures of this interval are shown on Fig.1. It is seen that crystallization curves have Sshape typical for zeolite crystallization processes, which are believed to be autocatalytic [8]. It is possible to estimate apparent activation energy of ZSM-12 formation in our crystallization system. For this purpose kinetic data were approximated by the Avraami Erofeev equation and maximal rates of crystallization were calculated. The apparent activation energy of crystallization in our system was estimated to be equal to 4.8 kcal/mole. This value is surprisingly much lower than that obtained in the system with MTEA template [7] and corresponds by its magnitude to mass-transfer processes like diffusion.
337 Experiments at 420 K with seeding (Tab. 2) of the reaction mixture with MTW (up to 10 % of total SiO2) or MFI (up to 0.3 % of SiO2) type zeolite crystals showed small, if any, influence of the seeds on crystallization rate and on phase purity of the product. Increase of amount of MFI seed crystals up to 1% of SiO2 had retardation effect on ZSM-12 formation owing to concurrent process of ZSM-5 formation. It is very interesting that, at the same time, both types of seed crystals have definite accelerating and structure-directing influence ( more pronounced in case of MFI seeding, of course) on formation of ZSM-5 zeolite in the reaction mixture with SIO2/A1203 = 100 and with aliphatic alcohol as organic agent (Reaction temperature - 443 K), i.e. in the system which is sensitive to proper seeding. Table 2. Content of ZSM-12 and ZSM-5 in the crystallization products in seeding experiments. Time, h Content of the seed, % of total SiO2 ZSM- 12 synthesis 0 10 (MTW) 0.1 (MFI) 0.3 (MFI) 1 (MFI) Content of 47/64 26/93 26/97 -/93 86+8 MFI 75+25MFI the goal ZSM-5 synthesis zeolite 0 3 (MTW) 3 (MFI) phase, % 24/49/65 - / 67/91 + magadiite + quartz 100/100/60/100/1,0
I
I
,8
'N
,5
v
r~ ,a ,3
~
/f
0,0 0
i 20
/ZX
419K[
[A
,a i 40
i 60
i 80
100
Crystallization Time, h ,,
Figure 1. Kinetic curves of crystallization.
3.3 Characterization
, ,
, , ,
,
|
,
,
,
,
, , ,
Figure 2. Powder X-ray diffraction pattern of as-synthesized ZSM- 12.
of ZSM-12.
Crystallization product obtained from the reaction mixture with optimal molar ratios of components after two days of heating was determined to be a highly crystalline ZSM-12 phase substantially free of any impurities, which XRD powder pattern and TEM micrograph are shown on Fig.2 and Fig.3. It can be observed from the photograph that the crystals are thin plates with the length ca. 0.02 microns.
338
5O
5O
ZSM-12 .
~,.
ZSM-5
lO0 50 o l~pha Figure 3. TEM micrograph of as-synthesized Figure 4. 27A1 MAS NMR spectra for asZSM- 12. Bar is equal to 0.1 micron, synthesized ZSM-12 and ZSM-5. Asterisk denotes spin-side band. To confirm the inserting of aluminum into the zeolite framework 27A1MAS NMR and thermal analysis measurements Of as-synthesized ZSM-12 were done. The results were compared with those of ZSM-5 zeolite prepared from the reaction mixture of the same composition and substantially the same reagents but with silicagel ground inplanetary mill (see Discussion part) taken as a silica source. From 27A1 MAS NMR spectra for the ZSM-12 (Fig.4) it is seen that the most part of aluminum atoms sample is in tetrahedral occupancy by oxygen (chemical shift 50 ppm), i.e. they are incorporated into zeolite framework. Only about 10 % of aluminum are non-framework (chemical shill 6.7 ppm). Thermal analysis data (Fig.5) showed that the as-synthesized sample of ZSM-12 contained ca. 1.5 wt.% of water and 10.5 wt.% of organic. Exothermic effect on DTA curve has a maximum at 370 °C with a broad shoulder from the high-temperature side. This observation differs with that for ZSM-5 case, where maximal exo-effect is observed at 455 °C and a shoulder from the low-temperature side. This difference is consistent with wider channel openings for ZSM-12 than for ZSM-5. From the other hand, thermal data for ZSM12 are in general agreement with those, given in Ref. 7, for ZSM-12 synthesized with MTEA template. Our DTA data are intermediate between thermal effects for two samples: with SIO2/A1203 =1010 and with SIO2/A1203 = 99 (Fig.2 in Ref.7). On DTA curve of our ZSM-12 sample endo-effect at 845 °C is observed. This effect may point out on some amorphization processes in ZSM-12 framework. Also, it may be related with non-framework fraction of aluminum, but it is difficult to explain endo-thermal character of the effect in this case. Specific surface area determined by argon adsorptionat 77 K of the sample calcined at 550 °C in air for 4 hours was found to be 343 m2/g. Thus, it may be concluded that good quality ZSM-12 zeolite can be synthesized from the reaction mixture, having determined optimal composition.
339
TG
1 Tridymite-like
DTG
unit
845
L/
zsM-l
ZSM-uSni i like
DTA
365
Figure 5. Thermal analyses of as-synthesized Figure 6. Fragment of MTW type structure ZSM-12 and ZSM-5 zeolites. Temperatures are given in centigrade scale. 4. DISCUSSION It is the most striking feature that ZSM-12 zeolite crystallization, being autocatalytic process (owing to the S-shape of kinetic curves, Fig.l), is insensitive to seeding. It means that in the reaction system there is no mass transfer of reactive species from amorphous gel onto seed crystals surface, i.e. liquid phase mechanism of ZSM-12 zeolite crystallization is improbable. So, we have another altemative - solid phase transformation mechanism [8], according to which zeolite nuclei are formed by rearrangement of the amorphous gel around template molecules. To our opinion, this mechanism is consistent with the observation of proportional dependence of ZSM-12 content in the crystallization product with the content of TEA Br in the mixture. It may be proposed the low value of the apparent activation energy of ZSM-12 formation process (4.8 kcal/mole) points on diffusion of TEA cations (along with Na + cations) inside of silicagel particles as a rate-determining step of the zeolitization process. When internal space of silicagel particles is saturated enough by TEA template species, rearrangement of silicagel leads to the formation of ZSM-12 phase. Easy rearrangement (it is not a limiting step !) assumes that the silicagel short-range structure is similar to that of formed crystalline phase. Indeed, MTW type structure has the same building units as in cristobalite and tridymite, from one hand, and in MFI type structure, from the other hand (Fig. 6) [9]. Such description of the MTW type structure may easily explain coexistence of cristobalite phase and ZSM-5 along with ZSM-12. The fail of formation of ZSM-12 with silica sources other than silicagel is explained in the frame of proposed mechanism by absence of cristobalite- or tridymite-like units in their short-range structures. This view is supported by the fact that silicagel activated in planetary mill produce ZSM-5 zeolite only. It is known that activation in planetary mill results in quick and deep amorphization of the material because this kind of milling brings in much energy in comparison with other types of grinding [10]. Another evidence of existence of MFI-like units in MTW structure can be seen in accelerating and structure-directing effect of ZSM-12 seed crystals on crystallization of ZSM-5 zeolite system with aliphatic alcohol.
340 It is necessary to outline that this conclusion about the mechanism of ZSM-12 zeolite crystallization is based on the experimental data obtained with reagents and in reaction conditions used in this investigation.
5. CONCLUSION It is shown that formation of ZSM-12 zeolite in presence of tetraethylammonium bromide is sensitive to the nature of silica source. Pure ZSM-12 forms when silica gel is used as a silica source. Optimal chemical composition of the reaction mixture was determined. The limits on possible variation of chemical composition of the reaction mixture without loss of product purity towards ZSM-12 zeolite phase are determined. Observed results on ZSM-12 zeolite formation in alumino(metal)silicate system in presence of tetraethylammonium bromide as a template are in compliance with the MTW framework structure, which can be built from tridymite- and MFI-like structural units. Solid phase transformation mechanism of ZSM-12 zeolite crystallization in studied reaction system has been proposed and discussed on the basis of experimental data.
REFERENCES 1. R.B. LaPierre, A. C. Rohrman, Jr., J. L. Schlenker, J. D. Wood, M. K. Rubin and W. J. Rohrbaugh, Zeolites, 5 (1985) 346. 2. A.S. Loktev, K. G. lone, G. P. Snytnikova, A. V. Toktarev, F. D. Klebanova and A. G. Karshenbaum, in Zeolite Catalysis for the Solution of Environmental Problems. Chemistry, Ecology, Health, K. G. lone, (ed.), Nova Science Publishers, Inc., 1995, P. 109. 3. A.S. Loktev and P. S. Chekriy, in Zeolites and Related Microporous Materials: State of the Art 1994, J. Weitkamp et al., (eds.), Stud. Surf. Sci. Catal., 84 (1994) 1845. 4. P.A. Jacobs and J. A. Martens, Synthesis of High-Silica Aluminosilicate Zeolites, Stud. Surf. Sci. Catal., 33 (1987) 297. 5. S. Ernst, P. A. Jacobs, J. A. Martens and J. Weitkamp, Zeolites, 7 (1987) 458. 6. S.I. Zones, Zeolites, 9 (1989) 458. 7. X. Shou-he and Li Hexuan, in New Developments in Zeolite Science and Technology, Preprints of Poster Papers, The 7th International Zeolite Conference, Japan Association of Zeolite, Tokyo, 1986, P. 25. 8. R.M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press, London, 1982. 9. J.V. Smith and J. J. Pluth, in Ninth International Zeolite Conference, Extended Abstracts and Program, J. B. Higgins et al. (eds.), 1992, RP 207. 10. G. Heinicke, Tribochemistry, Akademie-Verlag, Berlin, 1984.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
341
Synthesis o f nanocrystalline zeolite Beta in the absence o f alkali metal cations a,*
a
M.A. Camblor , A. Corma, A. Mifsud a, J. P6rez-Pariente b and S. Valencia a alnstituto de Tecnologia Quimica (UPV-CSIC), Universidad Polit6cnica de Valencia, Avda. Los Naranjos, s/n, 46071 Valencia, Spain blnstituto de Cat~lisis y Petroleoquimica (CSIC), Campus Universidad Aut6noma, Cantoblanco, 28049 Madrid, Spain. Contrarily to previous reports, pure aluminosilicate zeolite Beta can be synthesized in the complete absence of alkali cations without the use of seeds. Thus, the presence of alkali cations is not necessary for the nucleation and crystal growth to occur, although they greatly affect the crystallization kinetics. By this synthesis method stable suspensions of colloidal zeolite Beta particles in the nanoscale size (100 to 10 nm) were prepared. It is also possible to synthesize 100% crystalline zeolite Beta with up to 7.6 framework A1 atoms per unit cell (Si/AI=7.4). 1. INTRODUCTION Zeolites are usually synthesized in the presence of alkali cations, which strongly influence the crystallization kinetics and the nature of the phase which crystallizes [ 1 ]. While the role of alkali cations is not fully established at present, it is clear that there are cases, like the synthesis of new Ti-substituted zeolites, where they have a detrimental effect, whereas in other cases the presence of a specific alkali cation is claimed to be necessary for the crystallization of a given zeolitic structure. While Ti-substituted zeolite Beta has been prepared in the absence of alkali cations [2] aluminosilicate zeolite Beta was reported not to crystallize in their absence when the synthesis was carded out in basic media [3]. In fluoride media without alkali cations, pure aluminosilicate zeolite Beta was obtained only when seeds of zeolite Beta were used [4]. Here we present for the first time the unseeded synthesis of aluminosilicate zeolite Beta in basic media in the absence of alkali cations, and speculate on the role of these cations during the crystallization process. This new crystallization method yields stable colloidal suspensions of highly crystalline zeolite Beta which can find a wide variety of applications [ 5]. 2.EXPERIMENTAL Aluminosilicate gels were prepared avoiding the presence of alkali cations. The appropriate amount of metal AI (Merck, 99.9%) was first dissolved in alkali free tetraethylammonium hydroxide (35 wt % aqueous solution, Aldrich, Na<2ppm, K<0.5ppm). This solution was then added to a mixture made by dispersing amorphous silica (Aerosil 200, Degussa) in a solution of tetraethylammonium hydroxide and the mixture was homogenized by
342 stirring. A fluid gel was obtained in all the cases, its viscosity being much lower than that obtained in the presence of alkali cations. Gels with different SIO2/A1203 ratios were prepared and the molar ratios (OH-AI)/SiO2 and H20/SiO2 were kept constant at 0.52 and 15, respectively. The mixtures had then the following oxide molar composition: AI203 : xSiO2 : (0.26x+l)TEA20 : 15xH20 and x was varied between 400 and 14. The crystallizations were carried out in PTFE lined stainless steel 60ml autoclaves at 413 K under rotation (60 rpm). After quenching at different crystallization times the solids were separated by centrifugation (16,000 rpm, 90 min), washed with distilled water until pH<9 and dried at 100°C. These synthesis conditions are very similar to those previously described for the synthesis of zeolite Beta in the presence of alkali cations [ 6 ]. Crystallinity and phase purity of the solids were determined by powder X-ray diffraction (XRD) using a Philips PW1710 (Cu K~ radiation) equipped with an automatic slit (constant area mode) and a graphite monochromator. After background subtraction, the area under the peak in the 19.33-23.87 ° 20 range was taken as a measure of the crystallinity, by comparison to a highly crystalline standard sample. Infrared spectroscopy (IR) in the region of framework vibrations was also used to ascertain the high crystallinity of the final samples, using the KBr pellet technique and a Nicolet 710 FTIR spectrometer. Thermogravimetric (TGA) and differential thermal (DTA) analyses were performed simultaneously using a Netzsch STA 409 EP thermal analyzer with about 0.0200 g of sample, a heating rate of 10 K/min and an air flow of 6 1/h.Chemical composition was determined by atomic absorption (Varian SpectrAA-10 Plus, A1) and elemental chemical analysis (C, H, N, Fisons EAll08CHN-S). Si was determined by difference. Transmission electron microscopy (TEM) was performed on a Philips 400 mycroscope. N2 Adsorption experiments were performed on a Micromeritics ASAP 2000. 3. RESULTS AND DISCUSSION Pure and highly crystalline zeolite Beta was obtained in the 200-8 range of initial Si/A1 molar ratios in the complete absence of alkali cations (impurity levels in the synthesis mixtures:
343 The powder XRD pattems of the final crystalline solids obtained at different initial Si/A1 ratios are plotted in Fig. 1. For Si/Al=200 in the gel the final product shows the typical pattern of a highly crystalline zeolite Beta, with sharp and broad reflections in agreement with the structure proposed for this zeolite [7]. However, as the Si/Al ratio in the gel decreases there is a significant broadening and a decrease in height for all the peaks. This is a consequence of the decrease in size of the crystallites (see below) and not an indication of a decrease in crystallinity, as the area under the peaks remains in all cases the same. Furthermore, the high crystallinity of these samples can be ascertained from their framework IR spectra (see Fig. 2) which show in the 500-650cm "l region the sharp features typical of zeolite Beta. It is worth to note that the sample with the poorest XRD pattern (the one obtained from Si/Al=8 in the gel) shows the best resolved IR spectnma. Additionally, this sample shows a clear band at around 950-900cm 1, that we assign to terminal Si-O groups in the external surface of the crystallites. These groups are present in that sample in a very high concentration due to its small crystal size (see below). There is little if any contribution to this band from internal Si-O defect groups, which from charge balance considerations can be considered to be practically absent in the as made highalumina alkali-free sample (see below).
2.4
16
2.0
14 12
12.5
~. 10
25
._~8 ~
6
:::J
1.6 1.2
:~0.8
4
0.4
2 0
10
20 30 2o (degrees)
40
Figure 1.- XRD pattems of zeolite Beta synthesized from alkali-free initial reaction mixtures with Si/A1 ratios as indicated.
0.0
i
1600 ' 12'00 800 Wavelenght (cm-1)
400
Figure 2. IR spectra of zeolite Beta obtained from alkali-free initial reaction mixtures with Si/AI=200 (bottom) and 8 (top).
Figure 3 shows the effect of decreasing the Si/A1 ratio of the initial reaction mixture on the crystallization kinetics. For zeolite Beta it was previously shown that in the presence of alkali cations there is a great variation in the yield of solids during the crystallization process
344 [ 3 ], [ 6 ] and that this greatly depends on the initial Si/AI ratios. This is also observed when the crystallization is carded out in the absence of alkali cations, and thus a good description of the crystallization kinetics requires to consider the variation with time of the amount of zeolite produced (calculated as "yield of solids" x "crystallinity"/100, Fig. 3), rather than the change in crystallinity of the recovered solids. As shown in Fig. 3, the crystallization time needed to obtain zeolite Beta in the absence of alkali cations strongly increases when the Si/A1 ratio of the synthesis mixture decreases. Actually, there is a steady and strong increase in the "induction peIiod" (time before any crystallinity is observed) as the Si/A1 ratio decreases (Fig. 3). This effect was not observed in the presence of alkali cations, where the increase in induction period was only apparent for Si/AI ratios below 25 [6]. On the other hand, the effect of the Si/A1 ratio on the maximum rate of production of zeolite Beta, as deduced from the maximum slope of the crystallization curves (Fig. 3), seems to be less marked.
16
~12 N
8
o
2 ~--
0
.--
~r,
.
,
.
,
.
,
.
,
5 10 15 20 25 Crystallization time (days)
.
30
Figure 3. Crystallization curves for zeolite Beta in the absence of alkali-metal cations: Si/AI= 200 (!1), 50(O), 25 (A), 12.5 (V), 10 (~) and 8 (~) in the initial reaction mixture. It is also worth to mention that once the maximum yield of zeolite Beta is produced, the system is highly stable for a very long time at the crystallization temperature. Thus, for a Si/A1 ratio of 8 in the initial reaction mixture, the yield of zeolite, its crystallinity and its chemical composition as well as the physical properties of the colloidal suspension obtained are not affected by additional heating at the crystallization temperature for more than one month. There is a noteworthy increase in the yield of crystalline solids (up to 17% for Si/AI=8) as the A1 content of the gel increases (Fig. 4). In parallel to this effect, there is also an increase in
345 the efficiency of Si incorporation from the gel to the zeolite. Thus, for Si/AI ratios of 8 in the gel about 90% of the Si initially present in the gel is finally incorporated to the zeolite, while less than 25% is incorporated for Si/AI=200. The incorporation of A1 is always 85 to 100%. As a consequence, the Si/AI ratio of the zeolite is much smaller than that of the gel for high Si/A1 ratios, while it approaches that value for low Si/AI ratios (Table 1). An interesting result is that it is now for the first time possible 25 _-< to synthesize 100% crystalline zeolite Beta with Si/AI ratios as low 20~ as 7.4 (7.6A1 atoms per unit cell). Considering that in these conditions only TEA + cations can counterbalance the negative charges introduced in the framework by AI, 10~ and that zeolite Beta can accommodate no more than 8 TEA [__. -----------__~ cations per unit cell, a Si/A1 molar ratio of 7 appears to be the lower limit for this zeolite in the absence of alkali cations. The amount of TEA + charge-balancing framework A1 can be calculated by thermal analysis from the weight losses
0
~
11~ 1~ ~/AI ingel
~0
Figure 4.- Yield of crystalline solids (11) and efficiency in the incorporation of Si (0).
associated to the two exothermic processes occurring at higher temperatures [8] (Table 1). For the higher Si/A1 ratio materials an excess in total TEA relative to A1 is a clear indication of the presence of defect Si-O defect groups, whose concentration decreases as the Si/A1 ratio decreases. In the material synthesized from gels with Si/AI=8 all the TEA is used to chargebalance framework aluminium. Thus, in its as synthesized form, such a zeolite have essentially no Si-O defect groups. This could be the explanation for the lack of reactiveness of gels with Si/AI<8, where the excess of A1 can not be balanced by TEA. Table 1 Chemical composition of zeolites synthesized in the absence of alkali cations Si/A1 r~tio C/N Composition per unit cell in the gel in zeolite molar ratio AI TEA +a TEA b 200 49.0 8.4 1.28 2.3 7.2 50 20.5 8.4 2.98 3.0 6.1 25 14.0 8.4 4.27 4.2 6.6 12.5 10.7 8.5 5.47 5.5 6.4 10 9.5 8.2 6.10 6.2 6.2 8 7.4 8.0 7.62 7.3 7.3 a.- TEA cations balancing framework aluminium, calculated from TGA (see text). b and c._ total TEA calculated from TGA and chemical analysis, respectively.
TEA c 6.3 6.3 6.4 6.3 6.2 6.9
346 It was previously reported [ 8 ] [ 6 ] that in the presence of alkali cations the gel becomes less reactive as its AI content increases and that for Si/AI<15 it is difficult to transform into a crystalline product the high-alumina amorphous solid that precipitates in the initial stages of heating. As a consequence, workers trying to increase the A1 content of zeolite Beta in the presence of alkali cations had little success, getting low crystallinity solids (-75% in ref. [9] ) and thus quite an uncertainty on the actual Si/A1 ratio of the zeolite produced. As it has been demonstrated here, the synthesis in the absence of alkali cations appears to be a better route to increase the concentration of framework AI, and consequently it opens a new possibility to increase the number of potential acid sites in pure zeolite Beta [ 5]. The crystal size of the crystalline zeolite samples has been estimated from the TEM micrographs (Fig. 5) and the values are listed in Table 2. While for the sample synthesized at Si/AI=200 the crystal size has been accurately determined (~ 100 nm), the others have much smaller crystal sizes and the crystaUites tend to aggregate, making difficult to get a reliable crystal size distribution. Thus, the crystal sizes listed in Table 2 are just estimated representative values. There is anyway a clear tendency for the crystal size to decrease as the A1 content in the gel increases, this trend being more marked the lower the Si/A1 ratio. This is supported by the change in width (FWHM) of the most intense peak of the XRD patterns (Fig.6). We have not tried to derive absolute crystal sizes from the FWHM values because the peak width also depends on the microstrains and because zeolite Beta is an intergrowth of at least two polymorphs. In an attempt to further verify the values obtained by TEM we have performed 0.9 N2 adsorption experiments on the as-made samples with Si/AI=7.4 and 49. In order to . . 0.8 m~. t/J do this, the as-made samples were outgassed ® ® 0.7 at 150 and 100°C, respectively, under I1) dynamic vacuum (2x10 "6 atm). Optimum E 0.6 outgassing temperatures were determined -r- 0 . 5 from TGA experiments. In those conditions the template was not removed, as ,, 0.4 demonstrated by the null micropore ~ ~ adsorption capacity (Table 2). Thus, N2 is 0.3 only adsorbed on the external surface of the 0.2 crystallites. The sample with Si/AI=7.4 0 50 10 0 15 0 200 S i/AI in g e l exhibits a very high external surface area and remarkable mesoporous adsorption properties, due to the interstitial void space Figure 6. Width of the 22.4 ° 219 XRD peak between the crystallites. This is an of zeolites samples synthesized from gel indication of the very small crystallite size. with different Si/A1 ratios. We have derived an apparent crystal size from the calculated external surface area assuming an spherical morphology of the crystallites. The results are shown in Table 2 and are in good agreement with the crystal size estimated from the TEM micrographs. It is remarkable the extremely small crystal size (~ 10 nm) of the sample synthesized from a gel with Si/Al=8. Actually, up to the best of our knowledge the crystal sizes of the zeolites obtained from Si/AI < 50 are clearly the smallest ones ever reported for 100 %
---..._.
347
I"')
t---4!
Figure 5. TEM pictures of zeolites obtained from gels with Si/AI=200 (a), 12.5 (b), 10 (c) and 8 (d). Scale bar is =__40 nm. crystalline zeolites. Jacobs and coworkers reported the preparation of materials with ZSM-5 crystals smaller than 8 nm embedded in an amorphous matrix and thus not fully crystalline [ 10 ]. Schoeman and coworkers prepared materials with around 80-100nm crystal size (silicalite [ 11 ], TS-1 [ 12], A and Y [ 13] ) going in the case of sodalite down to 37nm [ 14]. Finally, these results, when compared to our previous reports on the synthesis of zeolite Beta [ 3 ], [ 6 ] suggest that alkali cations determine the crystallization kinetics in two ways. One refers to their influence on the "gel chemistry" (solubility of aluminosilicate species, viscosity of the gel...). The other one, already proposed in reference [3], refers to the likely relationship between Si-O defect groups and the crystallization kinetics. In [3] we showed that, for a given total alkali cation concentration, the optimium Na/(Na+K) ratio leading to the shortest crystallization time produced the zeolite with the highest SiO defect concentration. Clearly,
348 while alkali cations are not needed for zeolite Beta to nucleate and grow they have a benefitial effect on these processes, maybe by allowing a high SiO defect concentration and thus the formation of "less perfect" crystals requiering a lower activation energy. Further work is in progress to check this hypothesis. Table 2 Crystal size and N2 adsorption properties of as made zeolite Beta samples (see text) Si/A1 in gel TEM crystal BET surface Micropore Mesopore Crystal size a (nm) size (nm) area (m2/g) Volume (cc/g) Volume (cc/g) 200 115 30.7 0 0.036 102 12.5 ~20 10 10-20 8 5-15 288.5 0 0.43 11 a.. Crystal size calculated from the extemal surface area assuming a spherical morphology. 4. CONCLUSIONS The synthesis of zeolite Beta in the absence of alkali cations yields stable colloidal suspensions of nanocrystalline material, the average crystal size steadily decreasing from around 100 nm to around 10 nm as the Si/A1 ratio of the parent gel decreases from 200 to 8. For a Si/A1 initial ratio of 8 the zeolite contains essentially no defects and has the lower Si/A1 ratio (7.4) ever reported for a 100% crystalline zeolite Beta. While it is shown that alkali cations are not needed for zeolite Beta to nucleate and grow their large influence on the crystallization kinetics may be related to its ability to promote a high concentration of defect Si-O groups, as well as to their influence on the gel chemistry. REFERENCES
1. Barrer, R.M., Hydrothermal Chemistry of Zeolites, Academic Press, London, 1982. 2. Camblor, M.A., Corma, A., Martinez, A., P&ez-Pariente, J., d. Chem. Soc., Chem. Commun., 589, 1992. 3. Camblor, M.A., P&ez-Pariente, J., Zeolites, 11,202, 1991. 4. Caullet, P., Hazm, J., Guth, J.L., Joly, J.F., Lynch, J., Raatz, F., Zeolites, 12, 240, 1992. 5. Valencia, S., Camblor, M.A., Corma, A., P&ez-Pariente, J., Spanish Pat., P9501552, 1995. 6. Camblor, M.A., Mifsud, A., P&ez-Pariente, J., Zeolites, 11, 1991,792. 7. Newsam, J.M., Treacy, M.M.J., KoetsierW.T., de Gruyter, C.B., Proc. R. Soc. Lond. A, 420, 375, 1988. 8. P6rez-Pariente, J., Martens, J.A., Jacobs, P.A., App. Catal., 31, 35, 1987. 9. Vaudry, F., di Renzo, F., Fajula, F., Schulz, P., Stud. Surf Sci. Catal., 84, 163, 1994. 10. Jacobs, P.A., Derouane, E.G., Weitkamp, J.,d. Chem. Soc., Chem. Commun., 591, 1981 11. Persson, A:E., Schoeman, B.J., Sterte, J., Otterstedt, J.E., Zeolites, 14, 557, 1994. 12. Zhang, G., Sterte, J., Schoeman, B., d. Chem. Soc., Chem. Commun., 2259, 1995. 13. Schoeman, B.J., Sterte, J., Otterstedt, J.E., Zeolites, 14, 110, 1994. 14. Schoeman, B.J., Sterte, J., Otterstedt, J.E., d. Chem. Soc., Chem. Commun., 994, 1993.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors)
Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
349
Synthesis of zeolite Beta with low template content. Maria Wilma N. C. Carvalho a and Dilson Cardoso b Chemical Engineering Department a Federal University of Paraiba, Campina Grande - PB - Brazil. b Federal University of S~o Carlos, P.O. Box 676, 13565-905 - S~o Carlos - SP -Brazil.
The influence of template content and crystallization temperature during the synthesis of Beta zeolite has been studied. Crystallization performed at 130°C showed that reducing the template ratio (TEA)20/AI203 from 12.5 to 7.5, there is a considerable increase in the time of crystallization to accomplish 100% crystalline Beta zeolite. For the lower template ratio the required time for its crystallization was reduced by increasing the crystallization temperature to 150°C. No formation of Beta zeolite occurs for 8 days at a higher temperature (170°C), but formation of ZSM-5 was detected. Using the lower template ratio of 7.5 the crystal size of zeolite Beta increases considerably, and its Si/AI ratio approaches that of the gel.
I. I N T R O D U C T I O N Zeolite Beta is a crystalline aluminosilicate of large pores synthesized from a gel with alkali metal and tetraethylammonium cations. It was first synthesized in 1967 by Wadlinger et al [1], and it was the first zeolite synthesized using an organic template. Its structure has been reported to be an intergrowth of two or three polymorphs having a tridimensional system of interconnected 12-membered ring channels [2,3]. Zeolite Beta is potentially an important catalyst, because, like other zeolites with high silica content, it can possess high thermal and acid treatment stability, high strength acid sites and hydrophobicity [4]. However, zeolite Beta is generally formed as very small crystals, less than 1 ttm, which diminish its stability. On the other hand, it is known that the crystal size of a zeolite catalyst can influence the reaction activity and selectivity. The crystallite size of medium pore zeolites, for example, has a pronounced influence in the selectivity of formation of the para-dialkylated aromatic compounds [5]. Zeolite Beta has been used as a cracking catalyst f o r higher production of olefins, and an increase in the formation of these products was observed, as well as in the catalyst stability, by increasing the crystal size [6].
350 Another difficulty encountered in the use of Beta zeolite is that it is relatively expensive due to the high consumption of template [8] normally used in its synthesis (z=12.5 equation 1). The high template content during the synthesis is also the reason for the formation of small size crystals [7]. These reasons thus justify the study of the synthesis of zeolite Beta with lower template content, observing the influence in their crystallization and crystal size.
2. EXPERIMENTAL The synthesis of zeolite Beta was carried out based on the procedures used by Camblor [8] but without stirring of autoclaves in the course of the crystallization. The following reagents have been used: amorphous Silica aerogel, (Aerosil 200, Degussa), sodium aluminate (Riedel-de-Ha~n), tetraethylammonium hydroxide (20% aqueous TEAOH, Sigma) and sodium chloride (Smith). The composition of reaction mixture used in the synthesis of zeolite Beta was: 3Na20, z(TEA)20, A1203, 50SIO2, 1000H20, 6HC1
(1)
Where: z=12.5, 10.0; 7.5; 5.0. The reacting gel was prepared from amounts of reagents needed to obtain a total weight of about 30g in the following way: NaCI and sodium aluminate were dissolved in the amount of water needed to prepare the reaction mixture; to this solution the template (TEAOH) and finally the silica were added while stirring. The reaction mixture was distributed in Teflon vessels which were placed inside of stainless-steel autoclaves. The autoclaves were heated at the desired temperature (T¢=130°C, 150°C or 170°C) and the crystallization was carried out under autogenous pressure. The crystallization time (t~) was varied from 1/2 to 8 days and afterwards the autoclaves were cooled under running water. Next, the solids were centrifuged, washed with ethanol and water until the solution had a pH of about 8 and dried at 120°C. The characterization of the solids was done using the following techniques' X-Ray Powder Diffraction (Rigaku-Rotaflex Diffractometer, model RU-200B, using CuKt~ radiation); Scanning Electron Microscopy (ZEISS, Model DSM 960, operating with focal distance of 9 mm), used to determine the crystal size and morphology, and Atomic Absorption to determine the global chemical composition. The degree of crystallinity of zeolite Beta sample (Beta %) was determined by comparing the area of the diffraction peak at 20 = 22.4 ° (under CuKt~ radiation), to that of a reference sample, which had the most intense peak area and was synthesized with a gel having z=7.5, (equation 1) and 8 days of crystallization at 150 °C (Sample E, Table 1).
351 To determine the morphology and distribution of crystal size of zeolite Beta an image analyzer software was used to count and measure the crystal dimensions from micrographs. This software incorporates a series of facilities to processing and analyzing images, including mathematical morphology and granulometry by Hough transform.
3. R E S U L T S AND D I S C U S S I O N 3.1 C R Y S T A L L I Z A T I O N C U R V E S Zeolite Beta was, in general, the only crystalline phase obtained under the synthesis conditions previously described. The X-ray diffractograms, for the zeolite Beta samples, indicate that they are formed by an intergrowth of tetragonal and monoclinic polymorphs, similar to those reported by Newsam et al [21. As shown in Figure 1, there is no appreciable change in the crystallization curve of zeolite Beta when the template ratio, z = (TEA)20/AI203, is reduced from 12.5 to 10.0 and the crystallization is carried out at 130°C. When the template ratio is decreased to z=7.5, the Crystallization time necessary to obtain high-pure Beta zeolite increases from 2 to 8 days. Finally, further decrease of the template ratio to z=5.0 prevents the formation of any crystalline material in 8 days at this temperature. Figure 2 presents the influence of template content and time of crystallization at 130°C on the synthesis yield of zeolite Beta, measured in grams of zeolite Beta / 100g of gel. These yields are similar to those found by Camblor and Perez Pariente [7] and the results show that the time needed to reach the maximum yield decreases with an increase of template content. Accordingly, 7 days of crystallization are necessary to obtain the maximum yield at this temperature, if we use z=7.5. Otherwise, 2 days are necessary at z=10.0, and only 1 day is needed to obtain the maximum yield of the Beta zeolite when the template ratio is further increased to z=12.5. However, when figures 1 and 3 are compared, it can be concluded that crystallinity and solids yield are playing in opposite ways : although the crystallinity of the zeolite is higher with increase of template content, there is a reduction of solids yield under this condition. The lower solid formation is certainly the result of the higher final pH of reaction mixture after the crystallization (Figure 4), which increases the depolymerization and therefore the solubility of silica in aqueous medium. This conclusion is supported by the reduction of the 8iO2/A1203 ratio of Beta zeolite, when the template content is increased, as indicated in Table 1. Table 1 shows also that, under our synthesis conditions, the template quantity present in the reaction medium has no influence in the zeolite sodium/aluminum ratio. Furthermore, about all aluminum atoms are counterbalanced by sodium, differing from the results of Bourgeat-Lami et al [9], who postulate that more than 50% are associated with TEA + cations.
352 100
10 [3
80
n
f
0
•
....I
u
~ 6o
8
o
6
<
4
O
40 2O
"' (9
FLU
o/I
m
oj /
2 0
~_______ ~
v
(]' 2 "4 " 6 " 8 '1'0'1'2'14 Time (days)
TEA20/AI203
Figure 2: Template ratio and yield of Beta zeolite at 130°C, (V) 1 day, (O) 2 days, (m) 7 days of crystallization.
Figure 1' Crystallinity versus crystallization time at 1300C; (1"!) z=12.5, (m) z=lO.O,
(o)
z=7.5,
(v)
z=5.0.
15
O
12,5
i
o~ 12 0
o
9
"o .i ._=
12,0 T tn
6
0
o~
11,5
3
04
'
15 '
8
' 1'0 ' 1'2 ' 14
TEA20/AI203
Figure 3: Yield of solids versus template ratio (z) at 130°C; (0) 5 days; ( l ) 7 days of crystallization.
4 ' (5 ' 8
' 1'0 ' 1'2 ' 14
TEA2OIAI203
Figure 4" pH after 5 days of crystallization as a function of template ratio.
353 Table 1 Global Composition of Beta zeolite synthesized under different conditions. Crystallization conditions
Solid Properties
Sample
tc (days)
Tc (°C)
z
Na20/AI203
SIO2/A1203
Beta %
APS* (~tm)
A
7
130
12.5
0.99
18.7
90
0.3
B
7
130
10.0
0.95
26.1
70
0.5
C
4
150
7.5
-
-
82
2.0
D
7
130
7.5
-
-
70
1.4
E
8
150
7.5
0.98
34.1
100
5.3
*Average particle size
It was possible to synthesize Beta zeolite using a low template content (z=7.5) when 80 the crystallization temperature was elevated to 150°C (Figure 60 5). However, no more Beta < I.40 zeolite is formed when the LU ED crystallization temperature is 20 further increased to 170°C. At this higher temperature the S --~ --. -----. -solids yield decreased and a 0 small amount of ZSM-5 was Time (days) formed. Similar results, although Figure 5" Crystallinity versus crystalusing higher template content, lization time employing z = 7.5, were obtained by Perez-Pariente ( 0 ) 130°C, ( a ) 150°C, (11) 170°C. et al [10], who studied the influence of temperature in the synthesis of zeolite Beta, observing that the formation of ZSM-5 and erystobalite is in agreement with a general observation that at higher temperatures the formation of denser phases is favored. According to these authors, the formation of ZSM-5 instead of Beta zeolite at higher temperatures indicate that the TEA + cation is not a specific template for the nucleation of 100
354 zeolite Beta, but works essentially like a pore filler agent in the process of crystal growing of this material. 3.2. C R Y S T A L SIZE Figure 6 presents the micrographs of zeolite crystals synthesized with z-7.5 and 12.5, under the conditions specified in Table 1.
•
i I¸:i
i :>.. ~ i!i~~ :~ :F~!'.!.I....;.~...
(A)
....~.,~i:.i;i!i;i~,.i
(c)
,,.
~:~....
(E) Figure 6: Micrographs of Beta samples A, C, D and E (see Table 1).
As shown by the micrographs, in all the cases the particles of zeolite Beta present a rounded shape morphology with a tendency to form a tetragonal bipyramid as the template content in the reaction mixture is lowered (sample E). It's also possible to observe that keeping z-7.5 and a crystallization time of
355 about one week, increasing the temperature from T=130°C to T=150°C increases the crystals size and their uniformity (Figure 6, samples D and E). The crystal size distribution is presented in figure 7 and reveals that the standard deviation decreases from 80 % to 60 % when the crystallization temperature is elevated from 130°C to 150°C. It can also be observed that there is a pronounced increase in crystal size with longer crystallization times (Figure 6, samples C and E) which can be ascribed to heterogeneous nucleation of the amorphous material (smaller particles in Figure 6C), serving as nutrient to the rounded shaped crystals already formed.
=
.
,
.
,
.
,
60
~
.
,
.
,
.
,
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.
,
.
,
.
,
.
,
.
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.
,
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.
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,
.
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.
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.
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.
,
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.
160 40
40.
m
0
20.
00 - 1
2
3
4
5
6
7
0
1
Crystal size (~tm)
2
3
4
5
6
Crystal size (~tm)
7
0
1
2
3
4
5
6
7
Crystal size ( ~tm )
Figure 7" Distribution of the crystal sizes of samples A, D and E.
// /
/
/
/
/
A
/
/
//
E=1.
/
iI iii I/l/IlIA
v
fit. <
,...._o..)/_............ .o......... ~a .................. /o ..........
? -~L- 4
0
2
,
.
4
?
.J
6
.
,
8
Time (days) Figure 8: Average particle size and crystallization time. (El) z = 12.5 and 130°C, (O) z = 7.5 and 130°C, (A) z = 7.5 and 150°C.
Figure 8 shows, in squared dots, the evolution of the crystal size obtained under usual synthesis conditions [7,8,10] and that the average particle size increases as template content is decreased from 12.5 to 7.5, especially if the temperature is increased from 130°C to 150°C. The last behavior can be ascribed to a conjunction of two factors: (1) a decrease of the number of zeolite nuclei when the template content is lowered and (2) a reduction of system supersaturation, then the elevation of temperature favors the solubility of aluminate and silicate ions.
356 ACKNOWLEDGEMENTS Our acknowledgments to FAPESP and CAPES for financial support, and to Luciano F. Costa of Cybernetic Vision Center, Institute of Physics of S~o Carlos-USP for software analysis of image of zeolite crystals.
REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
R. L. Wadlinger, G. T. Kerr and E. J. Rosinski, US Pat. 33088069, (1967). J. M. Newsam, M. M. J. Treaty, W. T. Koetsier and C. B. de Gruyter, Proc. R. Soc. Lond. A., 420, (1988), 375. J. B. Higgins R. B. LaPierre, J. L. Schlenker, A. C. Rohrman, J. D. Wood, G. T. Kerr and W. J. Rohrbaugh, Zeolites, 8, (1988), 446. J. Scherzer, ACS Syrup. 248, (1984), 157. W . W . Keading. J. Catal, 67, (1981), 159. L. Bonetto, M. A. Camblor, A. Corma e J. Perez-Pariente, Anales del 13 ° Simposio Ibero Americano de Catalisis, (1992), 759. M . A . Camblor and J. Perez-Pariente, Zeolites, 11, (1991), 202. M. A. Camblor, A. Mifsup and J. Perez-Pariente, Zeolites, 11, (1991), 792. E. Bourgeat-Lami, F. Di Renzo, F. Fajula, P. H. Mutin and T. Courieres, J. Phys. Chem., 96, (1992), 3807. J. Perez-Pariente, J. A. Martens, P. A. Jacobs, Zeolites, 8, (1988), 46.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
357
Synthesis and characterization of Sn- containing ZSM-48 type molecular sieves using different templates N. K. Mal, Veda Ramaswamy,. B. Rakshe, and A.V. Ramaswamy° National Chemical Laboratory, Pune - 411 008, India M-free, tin-containing medium pore ZSM-48 type molecular sieves (Si/Sn > 50) have been synthesized using two different organic templates, viz., 1,8 diamino octane and pyrrolidine. The Sn-ZSM-48 samples were characterized by XRD, SEM, FT-IR, UV-Vis and adsorption techniques. An increase in the unit cell volume, an absorption around 970 cm1 in the IR region and 210 nm in the UV region point to the formation of Sn-O-Si linkages. The possible location of Sn4+ ions which may be gratted to the ZSM-48 structure is indicated. The Snsamples are active in the hydroxylation of phenol, toluene and cyclohexane with aqueous H202. The higher activity of the samples derived from 1,8 diamino octane is probably due to the smaller particle size and the absence of SnO2 type of species. 1. INTRODUCTION Tin-containing molecular sieve zeolites are interesting materials, as they could be useful in their application as adsorbents [1], as ionic conductors [2] and as catalysts in many chemical reactions [3,4]. Substitution of tin for aluminium or silicon in molecular sieves has been attempted both by post synthesis procedures [3-6] and by hydrothermal crystallization [1,710]. The post synthesis procedures which involve the treatment of crystalline zeolites with SnCI4 or SnF4 lead to some loss of crystallinity of the resulting materials. The hydrothermal synthesis of M-free Sn- or Zr- containing MFI silicalites and their use in the hydroxylation of phenol and phenol-ethers are described in a recent patent [7]. We have, earlier, reported the synthesis of Sn-incorporated, medium pore silicalites (MFI and MEL) [9,10] and a new large pore Sn-containing analogue of ZSM-12 (MTW topology) [11] using organic templates. In these reports, we have shown that some of the Sn4÷ ions may be in the framework positions and that well-dispersed Sn4+ ions are capable of catalyzing hydroxylation of phenol, toluene and other organic substrates with aqueous H202. ZSM-48 is a unidimensional, medium-pore, high-silica zeolite synthesized by various methods in presence of a number of organic templates [12]. Pure silica ZSM-48 can be synthesized in the absence of alkali cations [ 13]. Among the metallo-silicate molecular sieves, the possibility of incorporation of Ti in ZSM-48 framework was announced by Davis et al [14] and subsequently by Kaliaguine and co-workers [15,16]. The synthesis and characterization of vanadium-ZSM-48 have been attempted by Tuel and Ben Taarit [ 17]. This report describes, for the first time, the hydrothermal synthesis of Al-free, Sn-ZSM-48 using Detailed two different organic additives, viz., 1,8 diamino octane and pyrrolidine. characterization of the sample.,; having different Si/Sn ratios has been attempted in order to see the differences in the location and environment of Sn species in ZSM-48 structure.
"For correspondence: Fax: 91-212-334761, e-mail: [email protected]
358 2. EXPERIMENTAL
2.1. Synthesis The synthesis of Sn-ZSM-48 samples was carried out using fumed silica (Sigma, 99.8%) and SnCIa.5H20 (Loba Chemic, 98%) as the source of Si and Sn, respectively. In a typical synthesis using 1,8 diamino octane (1,8 DAO) (Aldrich, 98%), 6.0 g of fumed silica was treated with 63 g of distilled water under vigorous stirring for 30 min. 0.33 g of SnCI4 in 20 g of water was then added and stirred for 2 h. Finally, 7.65 g of 1,8 diamino octane in 25 g of water was added and stirred for another 3 h. The precursor gel having a molar composition of 1.0 SiO2 : 0.52 1,8 DAO : 0.0125 SnO2 : 60 H20 was transferred to an autoclave and crystallized at 453 K under rotation of the autoclave for 8 days. This sample is referred to as Sn-ZSM-48 (A). In the synthesis that used pyrrolidine, 6 g of fumed silica was hydrolyzed by 1.75 g of NaOH in 50 g of water for 45 min. 4 g of pyrrolidine was added during a period of 30 min under stirring. And then, 0.33 g of SnCI4 and 1.75 g of H2SO4 in 31 g of water were added and stirred for 105 min. The precursor gel with a molar composition of 1.0 SiO2:0.53 pyrrolidine : 0.0125 SnO2 : 0.096 OH : 0.16 SO4" : 45 1-120, where [OH ] = [Na ÷ ] -21504"" ], was transferred to an autocalve and crystallized at 453 K under stirring for 1 day. This sample is referred to as Sn-ZSM-48(B). Samples having three different Sn concentrations (Si/Sn = 50, 80 and 150) were thus synthesized by the two routes. The resultant crystalline material was filtered, washed with deionised water, dried at 383 K and calcined at 823 K for 12 h. All samples of Sn-ZSM-48 (A) and (B) were treated with 1 M ammonium acetate solution and further calcined at 823 K for 8 h. For comparison, a Si-ZSM-48 and a Sn-impregnated SiZSM-48 samples were also prepared. The latter was prepared by impregnating Si-ZSM-48 with SnCI4 solution followed by calcination at 823 K for 12 h. 2.2. Characterization The chemical analyses of the samples were done using both ICP (John Yvun-38 VHR instrument) and XRF (Rigaku, model 3070) techniques. The crystalline samples were characterized by XRD (Rigaku, D-Max III VC model, using Ni-filtered Cu Kt~ radiation and a graphite monochromator), SEM (Jeol, model 5200), FT-IR (Nicolet, model 60 SXB) and DRUV-Vis (Perkin Elmer, model 2101 PC) spectral techniques. The sorption measurements of water, n-hexane and cyclohexane as adsorbents after equilibration for 3 h, were carried out gravimetrically in an electrobalance (Cahn, model 2000G) at 298 K and at a fixed p/po ratio of 0.5. Surface areas (BET) of the samples were measured from N2 adsorption isotherms at liquid nitrogen i:emperature in an Omnisorb (Coulter, 100 CX) instrument. The samples were calcined in vacuum at 773 K for 6 h prior to adsorption experiments. 2.3. Catalytic reactions The hydroxylation of phenol and toluene was carried out in a glass reactor and the oxidation of cyclohexane, in a Paar autoclave at 353 K using Sn-ZSM-48 (A) and (B) samples (Si/Sn = 80) as catalyst and H202 as the oxidant. A substrate to catalyst (wt.) ratio of 5 and a substrate to H202 (mole) ratio of 3 were employed. In a typical run, 1 g of phenol, 200 mg of the catalyst and 5 g of water were placed in the reactor and heated to 353 K in an oil bath under stirring. 0.46 g of H202 (26 % aqueous solution) was added in one lot. Aliquots of reaction mixture were withdrawn at every 2 h intervals for analysis in a capillary GC (HP 5880) fitted with a 50 m long silicon-gum column. Acetonitrile was used as solvent in the oxidation of toluene and cyclohexane.
359 3. RESULTS AND DISCUSSION 3.1 Synthesis, Crystallization and Structure The chemical analysis of the crystalline Sn-ZSM-48 samples synthesized using the two templates shows that the uptake of Sn from the gel is fairly complete, as seen from similar Si/Sn ratios of the gel and the product samples (Table 1). The XRD profiles of all the samples are similar to that of pure Si-ZSM-48, with no impurity peaks. The peaks of Si-ZSM-48 are somewhat sharper than those of Sn-containing samples (Fig. 1). The interplanar d spacings were corrected with respect to silicon. The orthorhombic unit cell parameters were refined using two different least squares fitting programs. The replacement of Si by the larger Sn
%
-
-2410
i -2402
__. b I
,
•
35
A
|
25 15 2 e (degrees)
5
0
0.01
002
Sn I ( S i + Sn)
Figure 1. XRD profiles of calcined Sn-ZSM-48(A) (curve a), Sn-ZSM-48(B) (curve b) (Si/Sn = 80) and Si-ZSM-48 (curve c) and unit cell expansion with tin content. ions gratted in the framework brought about an increase in the unit cell parameters and hence the unit cell volume. Table 1 shows that this volume is larger for the samples of series (A) than observed for samples of (B) series for a given Sn content and that it is possible to synthesize Sn-ZSM-48 with different Sn contents. All the samples of Sn-ZSM-48 examined by SEM were composed of random agglomerates of small needle shaped crystals. For lower Sn content (Fig. 2a and c) the rice shaped agglomerates were typically 1.5 - 2.0 to 3.0 - 3.5 ~tm long with a maximum diameter of about 0.5 and 0.2 ~tm for Sn-ZSM-48 (A) and Sn-ZSM-48(B), respectively. For higher Sn content (Fig.2 b and d) dumbbell and spherical agglomerates of fine needles were observed in SnZSM-48(A) and Sn-ZSM-48(B) samples respectively. Such unusual morphology was also observed for Ti-ZSM-48 samples prepared using diamino hexane and diamino octane as templates [16]. The silica analogue of ZSM-48 prepared by similar method also showed fine
360 needles of 1.0 to 4.0 l.tm long with about 0.2 lam in diameter. The smaller crystals of SnZSM-48 samples in general, are responsible for the higher surface area compared to Si-ZSM48 sample (Table 1).
3.2. Texture and Sorption properties The sorption capacities of the Sn-ZSM-48 samples for H20, n-hexane and cyclohexane are given in Table 1. The sorption data for water show that the Sn-samples are more hydrophilic than the Si-analogue. Comparable sorption capacity for n-hexane and cyclohexane between Sn-containing samples and Sn-free ZSM-48 sample indicates that the pores of Sn-ZSM-48 (A) are free from any occluded SnO2 type species and that the Sn-ZSM-48(B) samples may contain some occluded SnO2 type species.
Figure 2. Scanning electron micrographs of calcined Sn-ZSM-48(A) with Si/Sn = 150 (a); 50 (b) and Sn-ZSM-48(B) with Si/Sn =150 (c); 50 (d).
361 Table 1 Sample composition and physico -chemical characteristics Si/Sn Vuc, Sample (Molar Sorption Capacity, wt% ° ratio) A3
Sn-ZSM-48(A)
in gel 50
in prod 55
2407
1-120 10.4
Cyclo hexane 37
Uv-Vis Abs.max nm
Surface area m 2 g'~
nhexane 6.6
205
285
6.4
205
280
205
270
Sn-ZSM-48(A)
80
86
2408
10.2
36
Sn-ZSM-48(A)
150
153
2399
10.1
32
6.0
Sn-ZSM-48(B)
50
47
2400
10.8
28
4.8
205,280
305
Sn-ZSM-48(B)
80
76
2399
10.7
28
5.0
205,280
296
Sn-ZSM-48(B)
150
133
2398
10.4
27
4.9
205,280
285
-
-
2396
4.5
32
5.7
-
210
Si-ZSM-48
" Gravimetric adsorption at p/po = 0.5 and at 298 K; (A) Samples synthesized using 1,8 diamino octane as template; (B) Samples synthesized using pyrrolidine as template.
3.3. Spectral characterization The framework IR spectra of the Sn-ZSM-48 samples are shown in Fig. 3. In both (A) and (B) Sn-samples, a band at 970 cm ~ which is absent in Si-ZSM-48 sample, is attributed to SiO-Sn linkages [9,10]. In titanium silicalites, a band at 960 cm ~ was attributed to the vibrations of a silicon tetrahedron perturbed by interaction with a neighbouring Ti atom
I
7
%
,
t
i
....
F b
Q
t:~tx)
,A L
tooo
~
i
413o
Figure.3. Framework FT-IR spectra of Sn-ZSM-48(A) (curve a), Sn-ZSM48(B) (curve b) and Si-ZSM-48(curve c)
~
t.£1~'rl.t ( am I
Figure 4. DR-UV-Vis spectra of Sn-ZSM48(A) (curve a), Sn-ZSM-48(B) (curve b) and Sn- impregnated ZSM-48 (curve c)
362 linked to the framework [ 18]. According to a recent asssignment by Corma et al [ 19] for Ti-silicalites, the IR band at 960 cm ~ is due to Si-O- defect groups generated upon Ti incorporation in the framework. It is possible that Sn incorporation leads to similar defect silanol groups within ZSM-48 structure. The diffuse reflectance spectra (UV-Vis) of Sn- samples prepared by the two routes are compared with that of Sn-impregnated ZSM-48 sample in Fig. 4. The Sn-ZSM-48(A) sample shows an absorption with a maximum at around 210 nm (curve a) which may be attributed to the presence of Sn species mainly in Td configuration. In the Sn-sample(B), there is an additional band at 280 nm (curve b), which is similar to the one observed in Sn-impregnated ZSM-48 sample (curve c). This arises from octahedral SnO2 type species [11], which is probably formed due to the presence of sodium during the synthesis of sample (B) using pyrrolidine.
3.4. Catalytic activity The results on the hydroxylation of phenol, oxidation of toluene and cyclohexane over Sn-ZSM-48 samples are summarized in Table 2. For comparison, blank runs were performed over pure SiO2 and Sn-impregnated ZSM-48 samples. Both of them show negligible activity in these reactions and hence it is seen that well-dispersed Sn species are responsible for the catalytic activity reported. In the hydroxylation of phenol, the TON of phenol are 80.8 and 57.6 with a H202 efficiency of 43.6 and 35.1 mol%, respectively, for Sn-ZSM-48(A) and (B) samples. A significant difference in the product distribution between these two runs is also noticed. The catechol (CAT) to hydroquinone (HQ) ratios are 1.3 and 2.2 for Sn-ZSM-48(A) and Sn-ZSM-48(B) samples, respectively. It may be noted that a CAT to HQ ratio of 0.9 to 1.1 has been reported for the titanium silicalites Table 2 Catalytic activity over Sn-ZSM-48 molecular sieves a H202 Product Distribution d, (wt%) Substrate
TON b Sel. c (mol %)
CAT/ ocresol
HQ/ pcresol
Phenol ¢
80.8
Phenol f
57.6
43.6
55.0
43.5
35.1
67.2
31.0
PBQ/ mcresol 1.5
benzyl alcohol/ Cyclohexanol -
benzaldehyde/ Cyclohexanone -
1.8
-
-
Others
-
Toluene ¢
16.8
29.3
3.2
6.9
2.1
13.5
71.3
3.0
Toluene f
13.6
22.4
3.0
6.2
2.3
15.3
69.4
3.8
Cyclohexane ~
12.6
18.9
-
-
-
39.8
58.7
1.5
Cyclohexane f
7.9
12.7
-
-
-
45.3
52.9
1.8
a Reaction conditions: catalyst = 200 mg and 100 mg (phenol); Substrate = 1 g; solvent (acetonitrile) = 10 g and 5 g HzO (phenol); substrate/H202 (mole) = 3; Temp. = 353 K; Reaction time = 24 h; b Number of mol of substrate converted per mol of Sn; c H202 consumed (mol%) in the formation of products excluding heaviers (others); d Break up (wt%) of products: CAT-- catechol; HQ = hydroquinone; PBQ = parabenzoquinone; ~ catalyst used: Sn-ZSM-48(A) and f catalyst used: Sn-ZSM-48(B).
363
(TS-1 and TS-2) [20]. These results indicate that in the case of samples synthesized using 1,8 DAO (Sn-ZSM-48(A) samples), the Sn4÷ ions are well-dispersed and are probably located within the channels of the ZSM-48 structure. The Sn-ZSM-48(B) samples synthesized using pyrrolidine show relatively a lower activity probably due to the bigger particle size and a higher CAT to HQ ratio, indicating that the hydroxylation occurs at the surface as well, where a part of Sn species may be located. In the oxidation of toluene, the products from the oxyfunctionalization of the methyl substitutent (benzyl alcohol and benzaldehyde) predominate over the products of aromatic ring hydroxylation (the cresols) (Table 2). In the product distribution, about 88% is accounted for the side chain oxidation on both the Sn-samples. In this respect, the SnZSM-48 samples are similar to the vanadium silicate molecular sieves (VS-1 and VS-2) [21]. The Sn-ZSM-48(A) sample is somewhat more active than Sn-sample(B), even though the product selectivities are similar. The oxidation of cyclohexane gives mainly two products, viz., cyclohexanol (the primary oxidation product) and cyclohexanone. In this reaction also, a higher TON and a better 1-1202 selectivity have been observed with Sn-ZSM-48(A) sample. The cyclohexanol to cyclohexanone ratio in the product distribution are 1.5 and 1.2 for Sn-ZSM-48(A) and Sn-ZSM-48(B) samples, respectively, the former being more efficient in the secondary oxidation. Summarizing the oxidation of the three substrates, it may be noted that the lower TON observed for toluene and cyclohexane is probably due to a lower rate of diffusion of the reactant in the unidimensional pore system of ZSM-48. However, we have observed that these Sn-molecular sieves can be reused aider calcination at 823 K without significant loss in the initial activity. 3.5 Topology of Sn4+ locations in ZSM-48 An orthorhombic lattice with pseudo-I or pseudo-C centering was proposed for the ZSM-48 lattice [12]. The structure of the lattice (space group = Imma; a = 8.40 A, b = 14.24/~ and c = 20.14 )k ) and the locations of 4 crystallographically distinct T sites are shown to the right. The structure contains 1-dimensional channel characterized by the 10 membered ring. The average T-O and T-O-T for the 4 T sites are in the range of 1.60 to 1.61 /~ and 141.01 to 155.74 °, respectively. Among the 4 T sites, T1 site has the average T-O value of 1.61 A and the T-O-T angle of 141.01°. Earlier quantum chemical calculations have shown that T sites with maximum T-O distance and minimum T-O-T angle are the preferred location for larger cations [22]. Thus, typically the location of larger Sn4÷ (ionic radius = 0.55 A ) compared to Si4÷ (ionic radius = 0.26 A) will be T1 site. T1 site is at the junction of 5-5 ring and at the intersection of 10-m ring and 6-ring sheets. There will be two such T1 / sites in a 10-m ring. Therefore, the framework Sn4÷ ions are accessible to the reactant adsorbing and diffusing in the 1dimensional 10-membered channel.
364 4. CONCLUSIONS Sn-ZSM-48 samples have been synthesized hydrothermally using two different templates.The samples synthesized in 1,8 diamino octane show relatively a larger increase in the unit cell volume (XRD), smaller particle size (SEM), the presence of Si-O-Sn linkages (IR) and absence of SnO2 type (octahedral) species (UV-Vis). These are also catalytically more active in the oxidation of phenol, toluene and cyclohexane with H202 as oxidant than the Sn-samples synthesized using pyrrolidine. The possible location of framework Sn species in the ZSM-48 structure is indicated.
Acknowledgement NKM is grateful to CSIR, New Delhi for a research fellowship. An assistantship by DST, New Delhi to BR is gratefully acknowledged. The authors thank Dr. R. Vetrivel for computer modelling of the ZSM-48 structure and useful discussions. REFERENCES 1. E.W. Corcoran. Jr., and D.E.W. Vaughan, US Pat. 5, 192,519 (1993). 2'. I.G.K. Anderson, E.K. Anderson, N. Knudsen and E. Skou, Solid State Ionics, 46 (1991) 89. 3. G.W. Skeels and E.M. Flanigen, Stud. Surf. Sci. Catal., 49A (1989) 331. 4. D.E.W. Vaughan and S.B. Rice, US Pat. 4,933,161 (1990). 5. G.W. Skeels and E.M. Flanigen, Eur. Pat. 321,177 (1989). 6. W. Pang, B. Zhao and S. Qin, Shiyou Xuebao, Shiyou Jiagong, 5 (1989) 93; CA, 112:237640c (1990). 7. M. Costantini, J.L. Guth, A. Lopez and J.M. Pope, Eur. Pat. 466,543 (1992). 8. F.G. Dwyer and E.E. Jenkins, US Pat. 3,941,871 (1976). 9. N.K. Mal, V. Ramaswamy, S. Ganapathy and A.V. Ramaswamy, J. Chem. Soc., Chem. Commun., (1994) 1993. 10. N.K. Mal, V. Ramaswamy, S. Ganapathy and A.V. Ramaswamy, Appl. Catal. A: General, 125 (1995) 233. 11. N.K. Mal, A. Bhaumik, R. Kumar and A.V. Ramaswamy, Catal. Lett., 33 (1995) 387. 12. J.L. Schtenker, W.J. Rohrbaugh, P. Chu, E.W. Valyocsik and G.T. Kokotailo, Zeolites, 5 (1985) 355. 13. A. Araya and B. Lowe, J. Catal., 85 (1984) 135. 14. D.P. Servano, Hong-Xin Li and M.E. Davis, J. Chem. Soc., Chem. Commun., (1992) 745. 15. K.M. Reddy, S. Kaliaguine and A. Sayari, Catal. Lett., 23 (1994) 169. 16. K.M. Reddy, S. Kaliaguine, A. Sayari, V. Ramaswamy, V.S. Reddy and L. Bonneviot, Catal. Lett., 23 (1994) 175. 17. A. Tuel and Y. Ben Taarit, Appl. Catal. A: General, 102 (1993) 201. 18. M.R. Boccuti, K.M. Rao, A. Zecchina, G. Leofanti and G. Petrini, Stud. Surf. Sci. Catal., 48 (1989) 133. 19. M.A. Camblor, A. Corma and J. Perez-Pariente, J. Chem. Soc., Chem. Commun., (1993) 557. 20. P.R. Haft Prasad Rao and A.V. Ramaswamy, Appl. Catal. A: General, 93 (1993) 123. 21. A.V. Ramaswamy and S. Sivasanker, Catal. Lett., 22 (1993) 239. 22. A. Chatterjee and R. Vetrivel, Microporous. Mat. 3 (1994) 211.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 © 1997Elsevier Science B.V. All fights reserved.
365
Inorganic cations in AIPO4 synthesis Erling Halvorsena, Ame Karlssonb, Thomas Haug a, Duncan Akporiayeb and Karl Petter Lillerud a. aUniversity of Oslo, Department of Chemistry, P.O.Box. 1033 Blindem. N-0315 Oslo, Norway. bSINTEF, P.O.Box 124 Blindem, N-0314 Oslo, Norway. The structure directing effect of Fluorine and alkali metal cations in the TEAOH • x.HF: y.MF : A120 3 : P205 : 40-H20 ( 0<x<2.0; 0
1. INTRODUCTION The first microporous A1PO4s, were made in an inorganic system [1] but it was not until the researchers at Union Carbide made and patented the A1PO4s in the early -80's [2], that they were identified as zeolite like microporous materials. The main difference in the synthesis reaction mixtures of d'Yvoir[1 ] and Union Carbide, was the use of organic templates. The use of organic templates resulted in both new structures, but above all, gave pure phases that made structure solutions possible. Probably as a result of this success nearly all A1PO4 synthesis investigations have so far focused on using different organic templates. One of the few exceptions are the fluorine modified synthesis developed by Kessler et al. [3]. The success of the fluoride route in molecular sieve synthesis is demonstrated by the large number of structures obtained using this method [4]. We reported the concentration dependent structure directing effect of fluoride, in a previous paper [5].
366 Motivated by the demand for low cost synthesis procedures of catalysts for industrial applications, we have investigated the structure directing effect of alkali metal cations in combination with fluoride anions in AIPO4 synthesis. The objective is to investigate the possibility of substituting, partially or fully, organic templates by inorganic ions and to explore the potential for new structures.
2' EXPERIMENTAL The synthesis gels, were made as follows: Pseudobohemite alumina (VISTA), was mixed with water and phosphoric acid (85 wt. %, KEBO), to this mixture was added a solution (designated solution II) consisting of hydrofluoric acid (40 wt. % in water), TEAOH (tetraethylammonium-hydroxide, 40 wt. %, Aldrich), and KF (Fluka) or altematively NaF (Merck). Different mixing orders, were also investigated, however the same crystalline products were obtained. The gel compositions in terms of oxide molar ratios, were as follows: 1 A1203 : 1 P205 : x HF : y MF : 1 TEAOH : 401-120 ( 0<x<2.0; 0
3. RESULTS Experimental parameters and the compositions of the crystalline products, are given in Table 1. In this TEA system three topologies, AFI, UiO-6 (new) and CHA (UiO-4), are identified. Diffractionpatterns of these pure phases are given in Figure 1. An unidentified layer type compound, that gives rise to a broad peak in the diffraction patterns at around 5 degrees twotheta is also often formed. This phase disappears when heated above 100 °C. Most syntheses result in rather complex mixtures of two or more phases. The amount of each phase is estimated using integrated peak intensities, from the x-ray diffractograms obtained. From these data, a phase diagram, Figure 2, showing the crystallisation field and quantitative distribution of each phase was drawn. A simplified phase diagram which shows representative xrd patterns for different gel composition ranges, in the HF - NaF system is given in Figure 3. Alkali metal concentrations higher than 0.6 have been omitted in the phase diagrams, since no interesting results were obtained in this region.
Table 1 . Selected syntheses from the KF- HF system. Gel composition Product composition KF ALPO-5 Ui0-4 Ui0-6 Sample-ID HF 003-001 0.00 0.50
0
100
0
Analysis results Chryst. T/F T/K 15
14
8
368 3.1. AFI As expected, pure samples of A1PO4-5 were obtained for low concentrations of alkali cation and fluoride in the reaction mixture. When either the amount of alkali cation or fluoride was increased, a significant decrease of A1PO4-5 was observed. To our surprise, pure A1PO4-5 was also obtained at HF = 0.3 and KF = 0.3. By closer inspection of the xray pattern of AIPO4-5, displayed in Figure 1, it is seen that two of the peaks in the triplet at -~20 degrees two theta, are quite broad. Onishi et al. [6] discussed the hexagonalorthorhombic phase transition of an A1PO4-5, which was also made in a fluoride containing medium. Further increase in alkali cation and fluoride concentration suppresses any AFI formation. AFI crystallised from potassium rich gels incorporates both F and K +, with data from elemental analysis indicating typically one ion pair per unit cell (24 T-atoms). Fluorine forms bridges between two Al-atoms in the fourtings in the AFI structure. This bridge shortens the distance between these Al-atoms and gives rise to the orthorhombic distortion. We observe a whole series of AFI materials with a variable amount of ions and distortion in symmetry, depending on the location of F. High loading of F sometimes gives a low degree of distortion due to a symmetrical arrangement [7]. Structure refinement of distorted AFI materials, is in progress.
5
10
15
20
25
UiO-5 orthorhombic AFI
UiO-6
i
UiO-4
triclinic CHA
3.2. UiO-6
UiO-6 crystallises as a pure phase in a narrow alkali cation - fluoride range around HF, KF 0.5, 0.2 and HF, NaF = 0.4, 0.1. Without alkali cations UiO-6 is formed as a minor phase in combination with AFI. UiO-6 is a deg. twoTheta unidimensional 12-ring structure like AFI but with slightly more narrow channels, free Figure 1. Diffraction pattems of the pure aperture 6.2A (AFI = 7.3A) [8]. phases: AFI, orthorhombic-AFI, UiO-6 (new) and CHA (UiO-4) in as-synthesised forms.
369 UiO-6 always incorporates fluoride in the lattice, typically 1 - 2 atom per unit cell (32 T-atoms). 3.3. UiO-4 (CHA) The crystallisation field of UiO-4 (CHA) is quite broad, and is formed from gels with both high alkali metal and high fluorine concentrations. This structure seems to be extremely flexible to both fluorine and alkali cation concentrations in the gels and it is crystallised with different amounts of ions incorporated in the structure. Fluorine incorporation spans from F/T=1/6 to F/T=1/24 (T = A1 + P). The potassium concentration is typically half the fluorine concentration. Only minor differences in the diffraction patterns are observed. UiO-4 is a triclinic version of the CHA topology with a unit cell up to 4-times larger than the ordinary Chabazite cell. The size of the cell depends on the fluoride concentration in the structure. The most detailed structure characterisation has been done on a material synthesised using HF = 1.5, KF = 0 gel ratios and with product composition F/Tatom ratio of 1/24. The unit cell of this material is a=18.54 A, b= 18.63A, c= 9.22A, tx= 98.86 °, 13= 92.33 °, T= 94.80 °. The cell expansion is caused by a
systematic incorporation of fluoride in
thelattice.
0.6
t; ~.
0.4
~ :~
.
.
.
.
.
.
.
.
.
.
.
0.2
L..
"~ l~ ~ "~
0.0 ~i'~i~'~'~'~'i'O'~'"i 0.4
~
........................................................................................ ¢0 0.2
Ii ~
0.0
0.4
o.2
o.0
0.0
0.4
0.8
1.2
HF/Ai203 (Molar gel ratio)
The x-ray diffractogram of UiO-4, has little resemblance with other known Figure2. Crystallinity of thephases. zeolite structm'e types, but upon heating, the material undergoes a phase transition, and the well-known CHA-type XRD-pattern appears.
4. DISCUSSION So far nearly all systematic studies on factors regulating the formation of AIPO4 structures have focused on the geometric constraints of the organic base (normally an amine) added to the crystallisation system. In this study three different topologies have been made with the
370
NaF 0.2
0.1
,
0
5
10
15 20[ De8. tth
25
30
,, 5 ,0 Deg.,5tth 20 25 3o
.~
5 j
0
5
0 Del.Stth 0 20
25
30
25
30
// 0.01
| 0
0.0 I
0.0
I
I
I
0.2
I
I
5 I
0.4
10 I
15 20 Deg.tth I
25 I
0.6
[10
15
~tl
30 I
I
I
0.8
I
I
I
1.o
HF
Figure 3. A simplified phase diagram with representative XRD-pattems for different ranges of gel composition. same template, based on systematic variations of the F" and the K ÷ concentrations. A systematic occurrence of these three phases is observed when the gel concentration of fluorine and alkali cation are changed. Consequently there seems to be three interacting structuredirecting parameters: the organic base, the F anion and the inorganic cation. The different effects might be rationalised from similarities and differences in the structure of the phases.
AIPO4-S(AFI )
UiO-6
UiO-4 (CHA)
Figure 4. Comparison of the large channals/cages in the three competing phases.
371
AFI
UiO-6
CHA
Figure 5. Comparison of the secondary building unit in the three structures. 4.1 Effect of template All the three structures AFI, UiO-6 and CHA have channels/voids with dimensions that fit very well to the TEA molecule. AFI and UiO-6 are both uni-dimensional 12-ring structures and the CHA structure can be described as an interrupted 12-ring channel structure, the channel is sectioned by the double 6rings. The dimensions of the largest voids in these structures are compared in Figure 4. The 12-ring channel is largest in the AFI structure with 7.3 A as the free aperture in the channel. UiO-6 has a narrow 12 ring channel with only a 6.3 A free opening. Based on molecular mechanics calculations, TEA fits this channel but there is no free space left. The CHA structure is intermediate in geometrical restrictions, the diameter of the barrel shaped cages is 7.0 A and the space between these blocks is 7.5A. This means that TEA fits well but leaves approximately 20% free space. A pure A1PO4-structure with altemating A1 and P as T-atoms is electrically neutral. When organic cations such as TEA are incorporated into these structures, some anionic species have to be present in the extra framework space as well. These systems are crystallised from nearly neutral or slightly acidic conditions. The anion that occurs in highest concentration in the gel in the pH range of the crystallisation is H2PO4".
4.2 Effect of fluorine All the three structures might be described as a combination of six-tings fused together face to face. Four-rings are generated from the way the six-tings connect into a three-dimensional lattice. There are several ways to connect two six-tings, three of them are represented in these three structures, Figure 5 illustrates these different secondary building units. In CHA the two 6-rings are aligned with bonds between all T-atoms, forming the well-known double-six-ring, generating six four-ring windows per unit. The opposite extreme is found in the AFI-structure, where two six-rings connect at every second T-atom, forming a unit with no four-rings. The secondary building unit in the new structure UiO-6 is of the intermediate type with two four-rings and two six-tings.
372 All the three structures incorporate variable amounts of fluorine into the lattice. The geometry of the four-ring and its surroundings, seems to be the crucial factor regulating whether the fluorine bridges are formed or not. UiO-4 (CHA) is the topology that forms from gels with high fluoride concentration and combined high fluoride, alkali cation concentration. This phase always has a high loading of fluorine located as bridges between aluminium T-atoms across the four-rings that connect the D6R units in the structures. 4.3 Effect of inorganic cation The structure-stabilising effect of the cations is probably derived from the electrostatic interaction between the alkali metal and the fluoride ion. Without the alkali metal cation the bulky TEA + ion is the only available cation to interact with the fluoride ion. In the AFI and CHA structures the four-rings that might incorporate a fluorine bridge are located next to the large channel with sufficient space to accommodate the TEA+ ion. In UiO-6 on the other hand, the fluorine bridge found in the planar four-ring located in the "star"-like cage is separated far away from the 12-ring channels. Incorporation of smaller ions is therefore particularly beneficial in this structure. 5. CONCLUSION There has been synthesised a new topology where the stabilisation by cation - fluorine interaction is essential for the formation of this new phase. Alkali cations are located in the intra crystalline space in fluorine containing A1POas in the same way as known for zeolites. ACKNOWLEDGEMENTS The authors would like to acknowledge the financial assistance of the Research Council of Norway. We also thank Norsk Hydro and Statoil for their cooperation in the use of the Biosym Industries Software. REFERENCES 1. F. d'Yvoire. Bull.Soc.Chim.de France. 1961. 2. S.T. Wilson, B.M. Lok and E.M. Flannigen U.S. Patent 4 310 440, 1982. 3. Zeolite synthesis in the presence of fluoride ions. A comparison with conventional synthesis methods. J.L. Guth, H. Kessler, J.M. Higel, J.M. Lamblin, J. Patarin, A. Seive, J.M. Chezeau and R. Wey. American Chemical Society symposium series 398. 4. Further results in the synthesis of microporous alumino- and gallophosphates in the presence of fluoride. C. Schott-Darie, H. Kessler and E. Benazzi. Stud.Surf.Sci.Catal 83 (1994) 3-10. 5. E. Halvorsen, A.Karlsson, D. Akporiaye and K.P. Lillerud. Presented at the 209th ACS national meeting. 1995. In press 6. N. Ohnishi, S. Qiu, O. Terasaki, T. Kajitani, K. Hiraga. Microporous materials, 2 (1993). 7. S. Qiu, W. Pang, H. Kessler and J-L. Guth, Zeolites, 9, 440, 1989 8. In preparation.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
373
NEW INSIGHTS INTO THE STUDY OF INDIUMPHOSPHATE MOLECULAR SIEVES L.L. Koha, Y. Xu b, H.B. Du e and W.Q. pangC aDepartment of Chemistry, National University of Singapore, Singapore 119260 bDivision of Chemistry, School of S¢ience/NIE, Nanyang Technological University, Singapore 259756 eDepartment of Chemistry, Jilin University, Changchun 130023, P.R. China A new microporous indiumphosphate InPO4-2, In2P2Os(OH)(H20)(NH4) • H20 (the second member of the series) is obtained by the hydrothermal synthesis (473K, 11 days, autogenous pressure) from a mixture of 1.0In203 : 1.0P205 : 1.2HF : 2.0Piperidine : 160H20. It is monoclinic, space group P21/n with a=10.043(2)A, b=9.136(2)A, c=10.325(2)A, 13=102.60(3)°, V=924.6(5) A 3 and Z=2. The three-dimensional framework is built up from two basic building units, tetramer where two InOs(OH) and InO4(OH)(H20) octahedra are joint by sharing both comers and edges, and PO 4 tetrahedra which serve as bridges linking InO6 octahedra. An unusual linkage between two InO 6 octahedra via shared-edge and (~t4-OH) are found in InPO4-2 where the oxygen is shared by three InO 6 octahedra. The structure contains 8-ring zig-zag channels along b-axis. NH4+ and H20 are situated within the main channels. The crystallization and structural features of InPO4-2 exhibit certain similarity to that of InPO4-1 and contrast strongly to those of M(ni)X(v)O4-type molecular sieves. It suggests the unusual hydrothermal chemistry of the microporous indiumphosphate. 1. INTRODUCTION Recent studies on M(m)X(v)O4-type molecular sieves has demonstrated their practical applications as catalysts and hosts for catalytic reactions as well as their significant role in fundamental chemistry [1-2]. It has been shown that M(lli)X(v)O4's (M=AI, Ga, In. X=P, As) have some common features, however, the differences among them appear to be more drastic. These can be shown in three aspects: (i) coordination of M, (ii) structural topology and (iii) the role and characteristic of templates [1]. In general, the structural versatility and thermal stability 0f M(m)X(v)O4's decrease in the order of AIPO 4- n > GaPO 4- n > InPO4-n [3] as the r ~ r x ratio and the tendency of M to have higher coordination number increase. The structuredirecting units used in the synthesis seem to play less important role from AIPO4-n , GaPO4-n to InPO4-n. In order to gain more insights into M(lll)X(v)O4-type molecular sieves, we have investigated the hydrothermal synthesis of indiumphosphate by focusing on the role of templates and its correlation with structural topology of products. Synthesis and structure of InPO4-1 has been reported [3]. Here, we report the synthesis and structure of a new
374 indiumphosphate molecular sieve, InPO4-2. The similarity and difference between InPO4-1 and InPO4-2 are also discussed. 2.
EXPERIMENTAL SECTION
2.1. Synthesis and characterization InPO4-2 was prepared by hydrothermal synthesis at 473K and autogenous pressures from weak acidic to neutral medium (pH=3.0-7.0). The reactants were indium oxide, phosphoric acid (85%, H3PO4), hydrofluoric acid, piperidine and distilled water. A typical preparation was conducted as follows: 1.04g In203 was mixed with 15 mL H20, to which 1.0 mL HF was added. The mixture was stirred until a homogeneous solution was formed. Next, 0.7 mL piperidine was added to the mixture dropwise. The reaction mixture was stirred vigorously for about 1 hour before it was transferred to a PTFE-lined stainless steel autoclave. The crystallization was conducted in the static conditions at 200°C and autogenous pressures. The microcrystallites were filtered off, washed with distilled water and air-dried at RT. The crystals suitable for single crystal experiment were obtained after 10 days at the designated conditions. The details of the synthesis conditions are summarized in Table 1. Powder X-ray diffraction patterns (XRD) were collected using a Rigaku D/MAX-IIIA diffractometer with Cu Kcz radiation (20mA, 30kV. 5.0°<20<80.0°). Thermal gravimetric analysis (TG) was performed in air on a Perkin-Elmer TG-7 thermal analyzer. Differential thermal analysis (DT) was done using a Perkin Elmer DTA 1700. Fourier-transform Infrared (FT-IR) spectra were recorded using a Nicolet 5DX FT IR instnunent (KBr pellet, 4000 to 450cm'l). Selected IR bands are summarized in Table 2. 2.2. Structure determination using the single crystal X-ray diffraction method The crystal structure of the as-synthesized InPO4-2 was determined by means of the single crystal X-ray diffraction method. The crystallographic data were collected on a Siemens R3MN2000 diffractometer with Mo Ktz radiation by co scan (3.5°<20<50.0°). Lorentz and polarization correction were made using the applied program [4]. The absorption correction was made by psi-scan [5]. The crystal structure was solved by direct method using the XS program of SHELXTL-PLUS and refined by full-matrix least-squares analysis on F with XLS program of SHELSX-76 [4]. The non-hydrogen atoms were refined anisotropically. The hydrogen atoms were located from difference maps with fixed isotropic thermal parameters. The final values of the refinement are R=2.10% and wR=3.08%. Selected bond lengths and angles are shown in Table 3. 3.
RESULTS AND DISCUSSION
3.1. Synthesis and characterization of InPO4-2 The typical composition of the reaction mixtures (in molar ratio) and crystallization conditions are shown in Table 1. InPO4-2 prefers high-temperature crystallization in weak acidic to neutral medium. Low-temperature conditions and strong alkaline medium tend to
375 lengthen crystallization process and induce the formation of amorphous phases. This is similar to that encountered during the crystallization of InPO4-1 [3]. The preparation of InPO4-2 has also been tried out by using other organic amines such as eyclohexaneamine and triethylamine, however, no crystalline phase is produced except for the dense phase. This forms drastic contrast to the synthesis of InPO4-1 where amines with different configurations and properties are able to induce and organize the same structure. The synthesis studies also indicate the " t e m p l a t e - s p e c i f i c " feature of InPO4-2 and the critical role that piperidine plays during crystallization. Another striking point of the synthesis of InPO4-2 is the crucial role of fluoride ion. Without F', no crystalline InPO4-2 can be obtained. This is different from the synthesis of InPO4-1, where the presence of F is not required. °
Table 1.
1
2 3 4 5 6 7 8 9 •
Synthesis conditions and the products
Compositions of reaction mixture Temp Time In203 P205 PD HF H20 (°C) (days) 1 1.8 4.0 2.0 160 180 25 1 1.8 2.0 0.4 160 180 25 1 1 2.0 0.4 160 180 25 1 1 2.0 1.2 160 180 35 1 1 4.0 2.0 160 180 25 1 1.5 4.0 2.0 160 200 11 1 1 4.0 2.0 160 200 11 1 1 2.0 2.0 160 200 11 1 1 2.0 1.2 160 200 11 a m : Amorphous; u n : Unknown crystalline phases
Product* am am
InPO4-2 + InPO4-2 +
un un
am
InPO4-2 + InP04-2 + InP04-2 InP04-2
un un
The infrared spectrmn of InPO4-2 shows a complicated band structure in the stretching and bending regions of In-O and P-O bonds as indicated by the following data (Table 2): The stretching vibrations of In-O and P-O appear between 950 and 1150 cm "l and those of In-O-In and In-O-P bending vibrations are seen between 540 and 630 cm "l. In AIPO4-n, the IR absorption associated with P-O stretching vibrations appear between 1250 and 1100 cm "l [6] and bending vibrations near 500 cm "l which are significantly different from those in InPO4-2. It indicates that PO4 units in InPO4-2 are situated in the drastic different environments. The complication in the band structure suggests the presence of various types of In-O and P-O bonds in the structure of InPO4-2. Table 2. InPO4-1 InPO4-2
IR bands for InPO 4-1 and InPO4-2 Stretching (cm q) 1031-1007 (broad), 1110 971, 1003, 1078-1103 (shoulder) 1153
Bending (cm "j) 576, 580, 613-636 (shoulder) 543,578-596 (shoulder), 621
376 The TG/I)T analysis of InPO4-2 shows the occurrence of three major thermal changes as the change of total weight. The first thermal change is endothermic occurring between 400523K with 6.4% weight loss. This is due to the loss of surface and included water molecules (in theory: H20/total = 3.7%). The second endothermic change appears between 660-850K with a weight loss of 3.6%. It is assigned to the removal of NH4÷ cations (in theory: NH4÷/total=3.7%). The temperature required to remove NH4÷ suggests that there exists fairly strong H-bonds between NH4÷ and the structure of InPO4-2. The removal of H20 and NH4÷ causes no deformation of the host structure. The last thermal change between 900-970K is accompanied by an irreversible phase transformation and a weight loss of---2.5%. It may be attributed to the partial release of structural H20 and OH" groups associated with In atoms. 3.2. Description of the structural features
InPO4-2 has a three-dimensional open framework structure with 1-D 8-ring channels running along b direction as shown in Figure 1. There are two types of building units, namely PO 4 tetrahedra and In4Ol2 tetramers (Figure 2) which share comers to form the 3-D framework as shown in Figure 3. An asymmetric unit is composed of {InO2-(OH)InO2(OH2)}(PO2)z(H20)(NH4). The structure of InPO4-2 is built up by the linkage of two asymmetric units through shared edges. This results in "puckering" of the 8-ring channel which is circumscribed by 3-member openings composed of two InO6 octahedra bridged by P O 4 tetrahedron.
%
O,~ ~.
<
I% 9
-
Figure 1. Stick-ball representation of the InPO4-2 structure along b-axis. H20 and NH4÷ are indicated by open and full circles respectively.
The direct connection between InO 6 octahedra via shared edges is found in InPO4-2 as shown in Figure 2. This is a distinct feature of InPO4-2 which has never been seen in I n P O 4-1 and other M(III)X(v)O 4 molecular sieves! The In40~2 tetramer consists of a pair of two
377 crystallographic independent In atoms, Inl and In2, which are related by a center of symmetry. The two symmetry-related Inl octahedra (Inl, Inla) are joined by sharing the edge containing 04 and O4a as shown in Figure 2.
0(11
O(9b)
0(I0c1
~O(3b)
0(8c1~~(~016c } ~ O(7b}
0(10)
o(g)
Figure 2. Perspective view of In40!2 tetramer in ballstick representation.
O(lb)
The two In2 octahedra are each connected to In l octahedra by sharing a vertex through 04 and O4a. O4 is thus coordinated to three In atoms with the average In-O distance of 2.189/1,. It is also bonded to one H as suggested by X-ray diffraction results, having 4-coordination. The InlO4(OH)2 octahedron consists of four 2- and two 4-coordinated oxygen ligands and the In204(OH2)(OH) octahedron consists of four 2-, one 3- and one 4-coordinated oxygen ligands. The Inl octahedron is distorted whereas, surprisingly, that of In2 retains the reasonable geometry of octahedron as shown in Table 3. As compared to the average In-O distances of 2- and 3-coordinated In (In-O=2.059(3) A and 2.089(3) ,A, respectively), the In-O in InPO4-2 is much longer (2.189/1,) due to the 4-coordination of 04. In2 consists of a terminal H20 molecule (07) where the oxygen 07 has 3-coordination. It is noteworthy that the In2-O7 bond length (2.089(3) A) is not much different from those of 2-coordinated O ligands (2.059(3) A).
I Figure 3. Structural motif of InP04-2 in polyhedral representation.
378 As shown in Table 3, the P O 4 units retain reasonable conformation of tetrahedron. Both P1 and P2 consist of a pair of short and long P-O bonds with average lengths of 1.467(3) and 1.522(3) A for P1, and 1.458(3) and 1.536(2) A for P2. This contrasts to that found in AIPO-n and other MPO-n where P-O lengths are generally longer and vary in a small range. 0(20)
..
:::
0 ( 8 0 ) ~ 0(20~:'
_// cO
~ ( 1 0 )
(JlS~"~ nil)/// .... 0(3) ~._
"~
0(90) C ~ 0(30) 0 ....
O( .... n(2)
Figure 3. Structural motif of InPO4-2 in stick-ball representation
P(lb)
-('~
#;z~ 0 -
1 P(ic)
#
The pairing of P-O bonds is attributed to the anisotropic environment of Inl and In2. If the two O ligands forming the short P-O bonds are shared with two In l octahedra, then the remaining two O ligands forming the long P-O bonds are shared with two In2 octahedra, and vice-visa. This suggests that the bonding effect of one InO6 octahedron to the PO4 unit significantly affects its interaction with other InO6 octahedra. Table 3. In-O
Selected bond lengths (A) and angles (o) 2-coordination
""fni ....................... h'i":6"i'i"~['iS~"iS"~"i"
2.101(3) 2.086(2) 2.083(3) 1.956(2) In2-O6, 08, 09, O10" 2.064(2) 2.087(2) 2.041(2) 2.053(3)
In2 ....iS-~n-iSia~/ai~
P-O P1 P2 ....i5-i~-6
P1 P2
In2-O7: 2.089(3)
..................
2.251(2) 2.135(2) In2-O4: 2.155(2)
....... i'Tig:~~i'~ ..... i'3"~3"i~i'~ ................................... ........................................................................................
172.5(1) 172.1(1) 171.1(1)
""/i~:'6:i ............... h'i'-'6:i:i~'i .,,
4-coor hi-6~"6:iai
.............................................................................................................................................................................................
"ini ....................... iY/~~i'5
In2
3-coor .............................................................................................
........... "i"i'~';~'i'5" .............................................. h'i':'i57Fihi'ai
In2-O4-Inl a: 119.6(1) short 1.452(2) 1.482(3) 1.450(2) 1.465(3)
....... b'g?g~i'5 ..............................
Inl-O4-H4a: 97.4(1) long 1.517(3) 1.526(2) 1.523(2) 1.549(2)
...................................................................................................................................................................................................................
O6-1~1-O5, O6-P1-O2a, O6-P1-O8a: 113.4(1) 109.0(1) 108.0(1) O10-P2-Ola, O10-P2-O3a, O10-P2-O9a: 113.4(1) 105.6(1) 106.8(2)
379 Each unit cell consists of two H20 and NH4+.(supported by chemical analysis and TG/DT analyses) locating separately along the 8-ring channels [shortest O20-N1 a = 3.561(3) A. a(o.5+X, 1.5-)', -0.5+Z)], as shown in Figure 1. They form H-bonds of various strengths with the anions of the three-dimensional framework. The shortest contact between the included H20 and the framework O atoms is O20-O7 b= 2.656(3) A [b(x, y, z)] and that between NH4÷ and the framework O atoms is N1-O1 c= 2.817(3) A [C(-x, l-y, l-z)].
3.3. Comparative study of the microporous indiumphophates 3.3.1. Structure features InPO4-2 is the number two indiumphosphate rendered with micropores. The largest channel is formed by stacking of 8-member tings in which InO6 and PO 4 appear in the In-P alternation similar to that in InPO4-1 [3]. The main channels are staggered along b direction and separated by 3-member rings which are built up from two InO6 octahedra and one PO 4 tetrahedron. The distinct feature of InPO4-2 is the existence of In-O-In bonds which results in its unusual building units of In4012 tetramer (a pair of In206 dimers related by a center of symmetry). This forms a striking contrast to the situation in InPO4-1 where In-P are arranged in a strict alternate order. Two crystallographic independent In atoms (Inl and In2) are different from each other by their oxygen ligands (Figure 2). Both Inl and In2 are composed of four 2-coordinated oxygen attaching to four P atoms. The other two oxygen ligands are 4-coordinated (04 and O4A) for Inl, and 3- and 4-coordinated (07 and 04) for In2. The average In-O distance varies from 2.059, 2.089 to 2.180 ,& depending on if the oxygen has 2-, 3- or 4-coordination. The In-O bond lengths in InPO4-2 appear to be shorter than their coordination-equivalencies in InPO4-1 (from 2.119 to 2.187 A) [3]. The PO4 tetrahedra consist of common 2-coordinated oxygen ligands where the four P-O bond lengths fall into two ranges of average distances, 1.463 and 1.529 ,~, respectively. This is different from that in InPO4-1 where the P-O bond lengths appear to be uniform and longer with an average P-O = 1.538 A. In comparison to those in AIPO4-n (P-O=---1.52 A) and GaPO4-n (P-O=~1.526 A), the shorter one is smaller than that in A1PO4-n whereas the longer one is greater than that in GaPO4-n. The irregular shift of P-O bond lengths may be due to the special coordination environment of In. 3.3.2, Thermal-stability The crystal structure of InPO4-2 remains intact until 900 K. It is thermally more stable than InPO4-1 which deforms at around 600K. The enhanced thermal stability is probably attributed to the combination of unique coordination and geometry of In atoms, and the location of NH4+. In In4Ol2 tetramer, InO 6 octahedra are bridged by sharing vertices and edges in which the geometric tension around In is minimized. Although negative charges are generated for the (In4O12H6)6" tetramer which causes electrostatic repulsion, they are
380 neutralized by the immediate neighbor of PO4 tetrahedra and extra-framework NH4+ cations. In InPO4-2 , NH4+ is readily removable, and Br6nsted acidic sites are thus created with strengths yet to be investigated. This appears to be another striking difference between InPO 41 and InPO4-2. No amine is found in the framework structure of InPO4-2 as supported by the small residue and adequate fit of X-ray diffraction analysis. The presence of the extra-framework H20 and NH4+ may have certain role to play in the organization of crystal structure which is yet to be understood. This is similar to what has been observed in InPO4-1, however, significantly different from other M(in)X(v)O 4 molecular sieves. ACKNOWLEDGMENTS: This work was supported by the National University of Singapore and Natural Science Foundation of China.
REFERENCES (1)
(2) (3)
(4) (5) (6)
T. Inui, S. Namba and T. Tatsumi (eds.), Chemistry of Microporous Crystals, Kodansha-Elsevier, 1990 63. M. Estermann, L.B. McCuster, C. Baerlocher, A. Merrouche and H. Kessler, Nature 352 (1991) 320. Y. Xu, L.L. Koh, L. An, R. Xu and S.Qiu, J. Solid State Chem. 1 ! 7 (1995) 373. Simens Analytical X-ray Instruments, Inc., Madison, WI 1990. A.C.T. North, D.C. Phillips and F.S. Mathews Acta Cryst. A24 (1968) 351. N. Tapp, N. Milestone, M. Bowden and R. Meinhold, Zeolites, 10(1990) 105.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
Synthesis a n d c h a r a c t e r i z a t i o n of novel p h o s p h a t e s from aqueous-alcoholic s y s t e m s
381
open-framework
cobalt
Jihong Yu, Qiuming Gao, Jiesheng Chen and Ruren Xu* Key Laboratory of Inorganic Hydrothermal Synthesis, Department of Chemistry, Jilin University, Changchun 130023, China Three amine-occluding open-framework cobalt phosphates, CoPO4-n (n=1-3) have been synthesized from aqueous-alcoholic systems of Co(Ac)2-H3PO4-R-EGH20, where R stands for an amine and EG for ethylene glycol. It is found that diamines can be exclusively used as the structure-directing templates during the crystallization of open-framework cobalt phosphates. The structures of CoPO4-n were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), inductively coupled plasma (ICP) analysis, infrared spectroscopy (IR) and UV-Vis diffuse reflectance spectroscopy (DRS). The as-synthesized compounds are all blue in color and exhibit UV-Vis spectra characteristic of tetrahedfallycoordinated Co atoms. Thermal analysis shows that the templates occluded in CoPO4-n decompose endothermally at 400-450 °C, and after the template decomposition, the materials become X-ray amorphous. 1. I N T R O D U C T I O N
Following the discovery of microporous aluminophosphates (A1PO4-n, where n signifies a distinct framework structure)[1], many silicon and/or metalsubstituted analogues of aluminophosphate molecular sieves (SAPOs, MeAPOs, and MeAPSOs)[2-7], as well as open-framework compounds such as M(III)X(V)O4s (M=A1, Ga, In; X=P,. As)[8-10], beryllium and zinc phosphates[ll,12] are also synthesized. Furthermore, the synthesis of 3dtransition metal (e.g. V and Mo) phosphates with an open-framework structure was also successful[13-16]. Recently, a cobalt phosphate designated DAF-2 (CoPO4-0.5C2H10N2), the first open-framework constructed from transition metal phosphate in which the transition metal is exclusively in a tetrahedral environment and is present in a metal/P ratio of unity, was synthesized hydrothermally using ethylenediamine as a structure-directing template[17]. Non-aqueous synthesis has proved to be an effective route for microporous materials and new open-framework compounds [18-20]. In order to extend the non-aqueous technique to the synthesis of transition meta!lophosphates, we
382 focused on synthesis from the system Co(AC)2-H3PO4-R-EG-H20 , and three new open-framework cobalt phosphates with occluded templates were successfully obtained. In this paper we report the preparation and characterization of these compounds.
2. EXPERIMENTAL CoPO4-n were synthesized from an aqueous-alcoholic system in which ethylene glycol (EG) was used as the main solvent. Co(Ac)2.4H20 and phosphoric acid (85 wt% H3PO 4) were used as the cobalt and phosphorus sources, respectively. A typical synthetic procedure involved the following steps: Co(Ac)2.4H20 was slurred in the solvent, and then an amine was added to the slurry. Phosphoric acid was added dropwise to the mixture thus formed. The final reaction mixture was stirred thoroughly for 0.5 h, charged into a Teflon-lined autoclave and heated at 180-200 °C for 3-12 days. The product was filtered and washed with water and dried at ambient temperature. X-ray powder diffraction patterns of the samples were recorded on a Rigaku 3DX diffractometer with Cu-Ka radiation. Scanning electron micrographs (SEM) were taken with a Hitachi X-650B electron microscope. Inductively coupled plasma (ICP) analysis was carried out on a Jarrzall-ASH 800 Mark-II ICP instrument, and infrared spectra (IR) were recorded on a Nicolet 5DX FTIR instrument with KBr pellet. UV-Vis diffuse reflectance spectra were recorded on a Rigaku 1_W-365 spectrometer: Differential thermal analysis and thermogravimetric analysis (DTA-TG) were performed on a Perkin-Elmer DTA 1700 and a TGA 7 thermal analysers, respectively. 3. R E S U L T S AND D I S C U S S I O N
3.1 S y n t h e s i s and c o m p o s i t i o n s Table 1. The batch compositions of the initial hydro-gels in molar ratio and crystallization conditions for cobalt phosphates Initial hydro-gels in molar ratio (a)
Crystallization conditions Time(days) Temp.(°C) Product
Co(Ac)~'4H20:3.6H3PO4:4.3 en:44EG 3-4 Co(Ac)~'4H20:3.6H3PO4:3.7HDA:44EG 10-12 Co(Ac)2"4H20:5.4H3PO4:5.5DPA=44EG 5 Co(Ac)2"4H20:9.5H3PO4:8.2DPA:44EG 5 Co(Ac)~'4H20:3.6H3PO4:4.3iPrNH2:44EG 3
180 180 180 180 200
DAF-2 COPO4-1 COPO4-2 COPO4-3
C07(PO4)2(HP04)4
(a) For clarity, the water brought from the phosphorus source is not included.
383 The batch compositions of the initial hydro-gels in molar ratio and crystallization conditions for the cobalt phosphate compounds are summarized in Table 1. Previously, DAF-2 was prepared hydrothermally and the existence of a dense phase of cobalt phosphate (C%(HPO4)2(OH) 2[21] in the product was very difficult to avoid. However, when EG is used as the solvent, pure DAF-2 single crystals were easily obtained. It is found that three new cobalt phosphates (designated CoPO(1, COPO4-2 and COPO4-3, respectively) are readily synthesized in the presence of diamines, such as 1,6-hexanediamine (1,6-HDA) and 1,2diaminopropane (1,2-DPA). Taking into account the fact that DAF-2 can only be crystallized in the presence of ethylenediamine (en), it seems that diamines may be crucial for the formation of open-framework cobalt phosphates. When isopropylamine (iPrNH 2) is used as the template, a known cobalt phosphate CoT(PO4)2(HPO4)4122] crystallizes. A number of mono-amines were also tried in our synthetic system, but no open-framework cobalt phosphates were obtained. The mechanism is yet to be understood. On the other hand, it is found that in aqueous system, the compound C%(HPO4)2(OH) 2 forms readily in the presence of amines other than ethylenediamine. The successful preparation of CoPO4-1, CoPO,-2 and COPO4-3 further reveals that non-aqueous synthesis is an effective route for a variety of materials w.ith an open-framework structure. Cobalt phosphates can be synthesized at temperatures between 180-200 °C. An attempt to synthesize open-framework cobalt phosphates below 150 °C led to the presence of a large amount of unreacted gel in the product. On the other hand, at a synthesis temperature above 200 °C, the product was highly impure. ICP analysis shows that the Co/P molar ratio in the three CoPO4-n is 1.0, suggesting that their structures are based on a network of strictly alternating Co and P atoms, like that of the open-framework cobalt phosphate DAF-2. Elemental analysis indicates that CoPO,-I contains 15.13% C, 3.63% H and 5.51% N with C:H:N=l:2.88:0.31. In fact, the C:H:N ratio of 1,6-hexanediamine, Table 2 Elemental and ICP analysis results and empirical formulae for openframework cobalt phosphates C% COPO4-1 COPO4-2 COPO4-3
15.13 9.39 4.84
H% 3.63 3.35 2.15
N% 5.51 6.92 3.76
Co/P 1.0 1.0 1.0
empirical formula CoPO4"0.45C6H14N2"0.3H20 CoPO4"0.54C3H,oN2"0.8H20 CoPO 4.0.25 C3H,oN2-0.75H20
which is believed to act as a template in the preparation is 1:2.66:0.33. The disagreement between the two ratios is attributed partly to the presence of water molecules in CoPO,- 1. The empirical composition calculated is
384 CoPO4-0.45C~H14N~-0.3H~O. The elemental analysis, ICP analysis results and empirical formulae for the three open-framework CoPO4-n are listed in Table 2. 3.2 S t r u c t u r a l c h a r a c t e r i z a t i o n
The as-synthesized open-framework cobalt phosphates are all deep blue crystals. Their scanning electron micrographs indicate that the crystals are homogenous, and there are no any other phases co-existing the samples.
J
4
I
10
I
20
I
30
,
~
~
40
4
1
b
i
i
20
30
_
40
2e (degrees)
29 (degrees)
Figure l(a) XRD pattern of DAF-2
Figure l(b) XRD pattern of CoPO,- 1
I
4
10
20
30
40
20 (degrees)
Figure l(c) XRD pattern of COPO4-2
4
10
20
30
40
2e (degrees)
Figure l(d) XRD pattern of CoPO,-3
385 The X-ray powder diffraction patterns of CoPO4-n are shown in Figure 1. It is found that the products have good crystallinity. Comparison of the XRD patterns of CoPO4-n with those of known cobalt phosphates suggests that the assynthesized CoPO4-n have unique structures. Figure 2 shows the IR spectra of CoPO4-1, CoPO4-2 and COPO4-3 as well as DAF-2 in the frequency range 2000-400 cm 1. The four crystals all exhibit absorptions mainly within 1300-920 cm 1, 820-600cm 1 and 600-400 cm ~. According to the framework vibration models of A1PO4-n[23,25] , it is believed that the absorption between 1300-920 cm ~ are attributable to asymmetric stretch vibration of PO 4, the bands at 820-600 cm ~ are associated with symmetric stretch vibrations of PO 4, while the ring and bending vibrations are responsible for the absorptions at 600-400 cm ~.
o o e-¢U
o O c
¢U
E
E
c
r-
I--
F-•
•
!
I
2000 1560 1120 840 620 Wavenumbers (cm"1)
-I
i
400
2000 1560
1120
_
840
•
620
400
Wavenumbers (cm -1)
Figure 2(b) IR spectrum of COP04-1
Figure 2(a) IR spectrum of DAF-2
0 c
o o c
E
E t~
c
c
t
2000 1560.
.
1120
j
840
J
620
J
400
Wavenumbers (cm "1)
Figure 2(c) IR spectrum of COPO4-2
2000 1560
1120 8 4 0 620 400 Wavenumbers (cm "1)
Figure 2(c) IR spectrum of COPO4-3
C o II with 3d 7 electron configuration has a characteristic d-d transition absorption[26]. The blue color of the as-synthesized open-framework CoPO4-n as
386 well as DAF-2 is usually considered as proof of the presence of tetrahedrally coordinated CoII centers. The UV-Vis diffuse reflectance spectra for CoPO4-1, CoPO4-2 , CoPO4-3 and DAF-2 (Figure 3 (c)-(f)) exhibit three absorptions with maxima at 540-550, 580-586 and 610-626 nm, respectively, typical of tetrahedral CoIIO4, in contrast to the spectrum for the cobalt phosphate Co3(HPO4)2(OH)2 in a
1
,o
_= ¢.-
4
250
450
!
650
850
Wavelength (nm)
Figure 3. l.W-Vis diffuse reflectance spectra of (a) Co3(HPO4)2(OH)2, (b) CoT(PO4)2(HPO4)4, (c) CoPO4-1, (d) COPO4-2, (e) COPO4-3 and (f) DAF-2. pink color characteristic of octahedrally coordinated Co~ (Figure 3 (a)), and the spectrum for cobalt phosphate CoT(PO4)~(HPO4)4 with 5- and 6- coordinated Co H (Figure 3(b)).
3.3 Thermal analyses Differential thermal and thermogravimetric analyses for CoPO4-n show that the template 1,6-HDA and 1,2-DPA occluded in CoPO4-1, CoPO4-2 and COPO4-3, decomposes endothermically at 400-450 °C, corresponding to a marked weight loss. For CoPO4-1, TG analysis illustrates that there are three major weightlosses mainly occurring at 100, 359 and 634 °C with weight losses of 4%, 11.8% and 7.7%, respectively (Figure 4). The first effect can be associated with the removal of water molecules present in the channel, the second is attributed to the decomposition of occluded 1,6-HDA, and the third due to the complete collapse of the crystal structure. The exothermic effect at 600-700 °C is believed to arise
387 from a phase transition. A decomposition temperature as high as 400 °C for an amine which normally has a boiling point lower than 200 °C implies strong interaction between the template and the framework of CoPO4-n. XRD studies show that after the template decomposes, CoPO4-n become X-ray amorphous, thus, as expected, the thermal stability of CoPO4-n is inferior to that of most members of the A1PO4-n family. At 700 °C, CoPO4-n undergoes recrystallization. When held for prolonged periods at the recrystallization
loo.
TG
.
.
.
.
.
rv
92.5 DTA
~
O *0 C W
'i
. = .
~77.5 70
30
190
350
5=10 670 Temperature (oc)
830
Figure 4 DTA-TG curve of CoPO4-1 temperature, CoPO4-n convert via an amorphous phase into the known compound C%P207 [27] on the basis of XRD. 4.
CONCLUSION
In summary, three new open-framework cobalt phosphates , designated CoPO4-n, have been synthesized from alcoholic system. It is found diamines have evident structure-directing effects during the crystallization of CoPO4-n. Composition analysis and structural characterization indicate that the structures of CoPO4-n are based on a network of strictly alternating Co and P atoms, like that for open-framework cobalt phosphate DAF-2. The as-synthesized CoPO4-n are blue in color and exhibit UV-Vis spectra characteristic of tetrahedrallycoordinated Co atoms. The thermal stability of CoPO4-n is inferior to that of most members of the A1PO4-n family. Acknowledgment This work is supported by the National Natural Science Foundation of China.
388 REFERENCES
1. S.T. Wilson, B. M. Lok and E. M. Flanigen, US Patent No. 4310440 1982. 2. E.M. Flanigen, B. M. Lok, R. L. Patton and S. T. Wilson, Pure Appl. Chem., 58 (1986) 1351: 3. S.T. Wilson and E. M. Flanigen, Am.Chem.Soc.Symp.Ser., 398 (1989) 329. 4. J.M. Thomas, Y. Xu, C. R. A. Catlow, J. W. Couves, Chem. Mater., 3 (1991) 668. 5. C. Montes, M. E. Davis, B. Murray, M. Narayana, J. Phys. Chem., 94 (1990) 6425. 6. N.J. Tapp, N. B. Mileston, L. J. Write, J. Chem. Soc. Chem. Commun., (1985) 189. 7. J. Chen, G. Sankar, J. M. Thomas, R. Xu, G. N. Greaves and D. Waller, Chem. Mater., 4 (1992) 1373. 8. J.B. Parise, J. Chem. Soc.,Chem. Commun., 22 (1985) 606. 9. R, Xu, J. Chen and S. Feng, Stud. Surf. Sci. Catal., 60 (1991) 63. 10. Y. Xu, L. Koh, L. An, S. Qiu and Y. Yue, Stud. Surf. Sci. Catal., 84 (1994) 2253. 11. T.E. Gier and G. D. Stucky, Nature, 349 (1991) 508. 12. T. Song, M. B. Hu~house, J. Chen, J. Xu,. M. A. Malik, R. H. Jones, R. Xu and J. M. Thomas, Adv. Mater., 6 (1994) 679. 13. H.E. King, L. A. Mundi, K. G. Strohmaier and R. C. Haushalter, J.Solid State Chem., 92 (1991) 154. 14. R.C. Haushalter, K. G. Strohmaier and F. W. Lai, Science, 246 (1989) 1289. 15. V. Soghomonian, Q. Chen, R. C. Haushalter, J. Zubieta, Chem. Mater., 5 (1993) 1595 16. Y. Zhang, A. Clearfield, R. C. Haushalter, Chem. Mater., 7 (1995) 1221. 17. J. Chen, R. H. Jones, S. Natarjan, M. B. Hursthouse and J. M. Thomas, Angew. Chem. Int. Ed. Engl., 33 (1994) 639. 18. Q. Huo, R. Xu, S. Li, Z. Ma, J. M. Thomas, R. H. Jones and A. M. Chippindale, J. Chem. Soc., Chem. Commun., (1992) 875. 19. Q. Huo and R. Xu, J. Chem. Soc., Chem. Commun., (1992) 1391. 20. J. Yu, J. Chen and R. Xu, Microporous Mater., 5 (1995) 333 21. T.L. Pizarro, G. Viuenuve and P. Hagenmuller, J. Solid State Chem., 92 (1991) 273. 22. P. Lightfoot and A. K. Cheetham, Acta Cryst., 44 (1988) 1331. 23. Y. Xu, X. Jiang, X. Meng and R. Xu, Acta Petrol Sin., 3 (1987) 101. 24. R.A. van Nordstrand, D. S. Santilli, S. Z. Zones, ACS Symp.Ser., 368 (1988) 236. 25. M.E. Davis, P. E. Hathaway, C. Montes and J. M. Gares, Stud. Surf. Sci. Catal., 49 (1989) 199. 26. A.B.P. Lever, Inorganic Electronic Spectroscopy, Elsevier, Amsterdam, (1984) 479. 27. N. Krishamachari and C. Calvo, Acta Cryst., 1328 (1972) 2883.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
389
A family of unusual lameUar aluminophosphates synthesized from non-aqueous systems Qiuming Gao, Jiesheng Chen, Shougui Li, and Ruren Xu* Key Laboratory of Inorganic Hydrothermal Synthesis, Department of Chemistry, J'flin University, Changchun 130023, China A family of unusual lamellar aluminophosphates designated APO-Ln (L stands for lamellar and n=6, 8, 10, 12, the number of carbon atoms for the occluded template molecules) were synthesized from mixtures of phosphoric acid, aluminum triisopropoxide, ethylene glycol, an unbranched alcohol and a long-chain primary amine. The compounds were characterized by Xray powder diffraction, scanning electron microscopy, magic angle spinning nuclear magnetic resonance spectroscopy, differential thermal analysis and thermogravimetric analysis. APO-Ln are composed of P04, Al04 and A10~ structural units, and the ratio of the octahedral Al to the tetrahedral Al in the as-synthesized compounds is about 1:1. The APO-Ln exhibit a large weight loss when treated at elevated temperatures, and upon dehydration they adsorb a considerable amount of water. The water adsorption isotherms are linear, being different from the five conventional types of physical adsorption isotherms. 1. INTRODUCTION During the past two decades, considerable effort has been devoted to metal phosphates and phosphonates with low-dimensionalities (1-D chain and 2-D lamellar structures) as they are potentially applicable in the areas of catalysis, ion exchange, molecular recognition, optics and electronics[I-4]. A number of lamellar metal phosphates and phosphonates have been discovered[5-13], and those falling into the category of mesophases have an interlamellar distance of 20 to 100 A. Among them are the molecule-intercalated inorganic phosphates and metal-phosphonate multilayer films. The former possess inorganic layers with organic molecules sandwiched between the adjacent layers whereas in the latter the organic groups are chemically bonded to the phosphorus atoms. Organic groups can also be bonded to the O atoms of the phosphate anions which construct the inorganic layers with metal cations. Whereas layered phosphates and phosphonates of divalent and tetravalent metals with a large interlamenar distance have been frequently encountered, lamellar mesophases oftrivalent-metal phosphates or phosphonates are rare. The interlamellar distance of the layered aluminophosphates and gallophosphates reported previously[10-15] is invariably less than 15 A, and although their structures and compositions can be varied to a great extent, few of the lameUar ah~minophosphates and gallophosphates exhibit si~ificant water adsorption capacities since the layers either are hydrophobic or lack void space to accommodate guest molecules. In this paper, we describe the synthesis and characterization of a family of unusual lamellar
390 aluminophosphate mesophases with occluded long-chain primary amines. Upon dehydration, the compounds exhibit a considerable water adsorption capacity. 2. EXPERIMENTAL APO-Ln were synthesized from a predominantly hon-aqueous system in which the mixture of ethylene glycol (EG) and an unbranched alcohol (n-CmH2m+lOH,m=4, 5, 6; 7, 8) was used as the medium and an unbranched primary amine (n-C~-I2.+INH2, n=6, 8, 10, 12) as the template. Thus, ahtminum triisopropoxide ((iPrO)3A1), phosphoric acid (85% H3PO4) and the unbranched primary amine were successively added into the mixture of an unbranched alcohol and ethylene glycol. Alter sthaSng for a few hours, a gel with an empirical molar composition of (iPrO)3AI: 1.8H3PO4:3.4n'C~-I2.+lNH2:3.4n-CmH2m+lOH:13.8EG:1.7H20 (n=6, 8, 10, 12; m=4, 5, 6, 7, 8) was formed. The gel was sealed in a Teflon-lined stainless autoclave and heated under autogenous pressure at 180 °C for 8 days. The crystalline product was filtered, washed with water and dried at ambient temperature. The products were characterized by element analysis, X-ray powder diffraction, scanning electron microscopy, the differential thermal analysis thermogravimetric analysis, adsorption and 27A1, 13C and 31p magic angle spinning nuclear magnetic resonance spectroscopy. 3. RESULTS AND DISCUSSION 3.1 Formation and Compositions of APO-Ln.
The use of a mixed solvent is very important for the synthesis of APO-Ln. If ethylene glycol and the unbranched alcohols are used independently as the solvent, APO-Ln is not obtainable. On the other hand, it seems that the polarity of the co-solvents (unbranched alcohols) also plays an important role for the formation of APO-Ln as suggested by the fact that the compounds crystallize readily in the presence of both ethylene glycol and a larger unbranched alcohol such as n-butanol, n-pentanol, n-hexanol, n-heptanol or n-octanol as the medium, whereas only amorphous phase exists in the product when a shorter unbranched alcohol (methanol, ethanol or n-propanol) is used with ethylene glycol. Crystallization temperature is also an important factor for the synthesis of APO-Ln. A temperature of 200 °C or above results in a large amount of A1PO4-tridymite in the product without appearance of APO-Ln; whereas when the temperature is below 160 °C, only amorphous material exists in the product. The results of chemical, element and TG analyses for APO-Ln are listed in Table 1, and the calculated compositions and normalized formulae based on these results in Table 2. Table 1. Composition analysis data for APO-Ln Compound
APO-L6 APO-L8 APO-L10 APO-L12
Element analysis (wt%)
Chemical analysis (wt%)
C
N
H
AI
P
TGA (wt%) amine I-I20
21.46 22.66 24.00 28.24
4.32 3.30 2.80 4.32
6.07 6.22 6.51 6.07
16.02 16.91 16.14 15.82
9.20 9.71 9.26 9.08
29.1 30.0 31.1 36.1
23.2 15.6 17.0 13.6
391 Table 2. The empirical compositions and normalized formulae for APO-Ln Compound APO-L6 APO-L8 APO-L10 APO-L12
Empirical composition AI4P2011"2.03 C 6I-I13NI-I2"7.07H20 AlsP4022"3.07 CsH171~2"11.02H20 Al12P6Oa3"4.02C 1oH2l]~]n2"15.04H20 All 2P6033"3.9 8C 12H25NH2"15.04H20
Normalized formula AI4P201l'2C6H,3NH2-7H20 AlsP4022"3CsH17NH2"11H20 Al12P6033"4Cloll21NH2" 15H20 A112_P6Oa3"4C12H25NH2"15H20
3.2 X-Ray Powder Diffraction and Scanning Electron Microscopy.
The X-ray powder diffraction pattern (Figure 1) of each member of APO-Ln shows *•Q only three peaks. As for the lamellar silicate mesophase[9, 16, 17], ~ e peaks are attributable to the (001), (002) and (003) reflections of the lamellar struetttres. The materials synthesized using n-hexylamine, noctylamine, n-decylamine, or n-dodecyhmine, respectively, as the template, have different d• (b) spacings (22.50, 26,53, 30.68, and 3.4.70 A), further revealing that the structure of APO~ _ j e,i ,-. ~~ (a) Ln (n=6, 8, 10, 12) is lamellar. The interlamellar distance of APO-Ln is shorter than that (ca 40 A) of the surfactant-containing 2 6 I0 14 lamellar silicate[16, 17], owing to the length 20 of the unbranched fatty chain of the template in APO-Ln being shorter than that of the Figure I. X-ray powder diffraction surfactant in the lamellar silicate. However, it patterns for APO-Ln; (a) n=6, (b) n=8, must be emphasized that the interlamellar (c) n=10, and (d) n=12. distance of APO-Ln is much greater than that (ca 10 A) of the lamellar aluminophosphates[10, 14] reported previously, where the templates used are amines with short chains. The scanning electron micrographs show that APO-Ln crystals appear as thin plates with diameters of 10-20 ~tm, characteristic of layered materials. The shape of the plate is irregular, different t~om that of the known lamellar aluminophosphates which normally exhibit welldefined three dimensional morphologies. Le3
t'--
0
~
,
1
i
|
l
•
3.3 MAS NMR Spectra. In the 31p MAS NMR spectrum of the reaction gel (Figure 2), only one peak at -8.2 ppm is observed, suggesting the presence of phosphorus atoms in a reasonably uniform tetrahedral coordination environment[18-20]. The 31p MAS NM~ spectra of the APO-Ln samples each also exhibit one peak but the maximum of the peak shiiis slightly to -7.8 ppnl The similarity between the spectra indicates that the local environment of the P atoms remains more or less the same when the gel is transformed to APO-Ln.The 27A1MAS NMR spectrum of the gel shown in Figure 3 exhibits one signal at 4.3 ppm, characteristic of aluminum atoms in an
392 octahedral coordination of O atoms[18-20]. The ligands for the AI atoms are probably water molecules. The 27A1MAS NMP~ spectru.1]l of the as-synthesized APO-Ln sample shows a peak centered at 45.4 ppm, attributable to AI in a tetrahedral symmetry, and another one at -8.1 ppm assigned to A1 in an octahedral symmetry. It is interesting to note that the areas of the two peaks are more or less equal, suggesting that the molar ratio of the octahedral AI to the tetrahedral AI is c a 1:1. The 31p and 27A1MAS ~ spectra of the four APO-Ln materials are quite similar to one another, revealing that the inorganic parts of these materials are akin. 13C MAS NMR spectra of APO-Ln samples show that the templates are intact in the compounds[21].
(a!
L
.
1
I
I
I
J
50
0
-50
- 100
200
ppm Figure 2. 311)MAS M R spectra of (a) the reaction gel and (b) APO-Ln. The spectra o f the four members of APO-Ln (n=6, 8, 10, 12) are the same. The asterisks represent sidebands. 3.4 Thermal
I
J
100
!
I
0
I
J
I
_
- 100
ppm Figure 3.27A1MAS NMR spectra of (a) the reactiongel and (b) APO-Ln. The spectra of the four members of APO-Ln (n=6, 8, 10, 12) are the same. The asterisks represent sidebands.
Properties.
Thermogravimetdc analysis (Figure 4) indicates that there are weight losses of 23.2, 15.6, 17.0, 13.6 wt% l~om c a 100 °C to 180 oC and of 29.1, 30.0, 31.1, 36.1 wt% t~om c a 180 °C to 430 oC for APO-L6, APO-L8, APO-L10, APO-L12, respectively. The low-temperature weight
393 losses are associated with the removal of water and the high-temperature ones with the decomposition of the templates. Correspondingly, two endothermic peaks at about 120 °C and 270 °C are observed on the DTA curves for all the four compounds (Figure 5). The temperature for the desorption of water is marginally higher than that of the boiling point of water, indicating that the interaction between the water molecules and the ahuninophosphate layers is not very strong. The decomposition temperature of the templates is high enough for the material to stand the removal of the water molecules without structure destruction. We can see that the temperature of the decomposition of templates for the four APO-Ln are similar, revealing that the interactions between the inorganic layers and different templates have more or less the same strength. The total weight losses are over 50%, much larger than those (less than 40%) for the known lamellar aluminophosphates containing a template and for as. synthesized aluminophosphate molecular sieves, such as AIPO4-41122] with a one dimensional medium-pore framework, AIPO4-17123] with a cage-containing framework and /DF-20124, 25], an aluminophosphate possessing 20-membered ring channels. XRD patterns of samples treated at various temperatures show that APO-Ln convert to amorphous phases at 350 °C and to A1PO4-tridymite over 400 °C. 3.5 Isothermal Adsorption of Water and the Nature of Adsorbed Water. The water-adsorption isotherms at 293 K for APO-Ln are shown in Figure 6. Prior to the adsorption measurement, the samples were dehydrated at 373 K and 10.3 Tom One sees that (d) -
"~
100
-
=
(a)
~82.5
(b) -
(c)_ G5
~
~1
(g)
42.5
(0 (e)(d)-
30
[
110
270
430
590
750
Temperature (oC) Figure 4. The TGA curves for (a) A1PO4-41 (b) A1PO4-17, (c) JDF-20, and APO-Ln: (d) n=6, (e) n=8, (f) n = 10, (g) n =12.
110 270
430
590
750
Temperature (oC) Figure 5. The DTA curves for APO-Ln: (a) n=6, (b) n=8, (c) n=10, and (d) n=12.
the isotherms appear as a straight line within the experimental relative pressure values, being different from the five types of conventional adsorption isotherms for physical adsorptions. The XRD and 31p MAS NMR spectra of the hydrated samples undergo no change after dehydration, indicating that the layers of the compounds are not affected by the adsorption-
394 desorption, and the templates are not affected because the d-spacing values remain unchanged, in agreement with the results of TGA-DTA. The 27A1MAS NMR spectrum (Figure 7) of the APO-L6 sample dehydrated at 373 K and 10 -3 Tort shows a strong signal at 38.9 ppm and a very weak one at -15.0 ppm The former is attributable to tetrahedral AI whereas the latter to octahedral A1. This implies that almost aH the A1 atoms in dehydrated APO-L6 are in a tetrahedral symmetry. Obviously, the sixcoordinated AI atoms in the as-synthesized APO-L6 are transformed into four-coordinated ones after dehydration. The presence of six-coordinated AI atoms in the as-synthesized APOL6 must be due to the adsorbed water molecules as extra ligands coordinating to the AI atoms. This is in sharp contrast with the lack of coordinating water molecules in the previously reported layered aluminophosphates with smaller interlamellar distances[10-14], which contain only four-coordinated A1 atoms. The other three members 0fthe APO-Ln behave similarly.
0.25 OD ¢)
o
ol
0.20
'0
"0
0.15
"0 ca
*-
0.10
0
<E
0.05 0.00
0.0
0.2
0.4
0.6
0.8
P/Po
Figure 6. The water adsorption isotherms at 293 K for dehydrated APO-Ln: O n=6, & n=8,0 n=10, • n=12.
1.0
I
!
I
I
100
50
0
-50
ppm Figure 7. The 27A1 MAS NMR spectrum of APO-L6 after dehydration at 373 K and 10-3 Torr for 2 hrs.
Assuming that each six-coordinated AI in the as-synthesized APO-L6 is bound to two extra water ligands, the fact that the octahedral/tetrahedral AI ratio is about 1:1 on the basis of the NMR signal suggests that four of the seven water molecules in the composition are coordinated to two A1 atoms, and the other three are accommodated somewhere between the layers. Similarly, the water molecules in other APO-Ln can be classified as coordinating ones and non-coordinating ones. Therefore, it is more appropriate for the formulae of APO-Ln to be written as in Table 3, where the 1-120 in the brackets represent water molecules coordinating to AI atoms whereas the 1-120 outside the brackets stand for water molecules between layers.
395 Table 3. The rationalized formulae for APO-Ln Compound
Formula
APO-L6 APO-L8
[ALIP2011"2C6HlsNH2"4H20]"3H20 [A18P4022"3CsH17]~-H2"8H20]'3H20 JAIl2P6033"4CloB21NH2"12H20]"3H20 [All2P6033"4C12H25NH2"12H20]]"3H20
~a,O-L10 APO-L12
ACKNOWLEDGEMENTS We are grateful to the National Natural Science Foundation of China and the PhD Studentship Foundation of the State Education Commission of China for financial support.
REFERENCES
1. T. Kanazawa, Inorganic Phosphate Materials, Elsevier, Tokyo, 1989. 2. 1LH. Jones, J.M. Thomas, 1L Xu, Q. Huo, Y. Xu, A.I~ Cheetham, D. Bieber, J. Chem. Soc., Chem Commun., (1990) 1170. 3. G. Cao, H. Hong, T.E. Mallouk, Acc. Chem. Res., 25 (1992) 420. 4. G.A. Ozin, Adv. Mater., 4 (1992)612. 5. A. Cleartield, Chem. Rev., 88 (1988) 125. 6. J. Chen, S. Natarajan, P.A. Wright, 1LH. Jones, J.M. Thomas, C.1LA. Catlow, J. Solid State Chem., 103 (1993) 519. 7. G. Cao, V.M. Lynch, L.N. Yacullo, Chem. Mater., 5 (1993) 1000. 8. M.I. Khan, Y.S. Lee, C.J.O, connor, 1LC. Haushalter, J. Zubieta, J. Ant Chem. Soc., 116 (1994) 4525. 9. Q. Huo, D.I. Margolese, U. Ciesla, D.G. Demuth, P. Feng, T.E. Crier, P. Sieger, A.Firouzi, B.F. Chmelka, F. Schuth, G.D. Stucky, Chem_ Mater., 6 (1994) 1176. 10.1LH. Jones, J.M. Thomas, 1L Xu, Q. Huo, A.K. Cheetham, A.V. Powell, J. Chem. Sot., Chem. Commtm., (1991) 1266. 11. A.M. Chippindale, A.V. Powell, L.M. Bull, l~H. Jones, A.I~ Cheetham, J.M. Thomas, 1L Xu, J. Solid State Chem., 96 (1992) 199. 12. J.M. Thomas, 1LH. Jones, 1~ Xu, J. Chen, A.M. Chippindale, S. Natarajan, A.I~ Cheetham, J. Chem. Sot., Chem. Commtm., (1992)929. 13. A.M. Chippindale, S. Natarajan, J.M. Thomas, 1LH. Jones, J. Solid State Chem., 111 (1994) 18. 14. D. Riou, Th. Loiseau, G. Ferey, J. Solid State Chem., 102 (1993) 4. 15.1LH. Jones, J.M. Thomas, Q. Huo, R. Xu, M.B. Hursthouse, J. Chen, J. Chem. Soc., Chem. Commtm., (1991) 1520. 16. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, I~D. Schmitt, C.T-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Ant Chem. Soc., 114 (1992) 10834.
396 17. Q. Huo, D.J. Margolese, U. Ciesla, P. Fcng, T.E. Gier, P.Sieger, 1L Leon, P.M. Petrofl~ F. Schuth, G.D. Stucky, Nature, 368 (1994) 317. 18. L. Maistrian, Z. Gabelica, E.G. Derouane, E.T.C. Vogt, J. van Oene, Zeolites, 11 (1991) 583. 19. I~ Nakashiro, Y. Ono, S. Nakata, Y. Moyimura, Zeolites, 13 (1993) 561. 20. N.J. Tapp, N.B. Milestone, D.M. Bibby, Zeolites, 8 (1988) 183. 21. Q. Shen, I3C ~ Spectroscopy; Peking University Press, Beijing, 1988. 22. Q. Gao, J. Chen, S. Li, 1L Xu, in preparation. 23. Q. Gao, S. Li, 1L Xu, J. Chem. Sot., Chem_ Commtm., (1994) 1465. 24. Q. Huo, 1L Xu, S. Li, Z. Ma, J.M. Thomas, 1LH. Jones, A.M. Chippindale, J. Chem_ Sot., Chem_ Commtm., (1992) 875. 25.1LH. Jones, J.M. Thomas, J. Chen, 1L Xu, Q. Huo, S. Li, Z. Ma, A.M. Chippindale, J. Solid State Chem_, 102 (1993) 204.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
397
Synthesis o f various indium phosphates in the presence o f amine templates Hongbin Du, Jiesheng Chen and Wenqin Pang* Department of Chemistry, Jilin University, Changchun 130023, China Novel indium phosphates were synthesized in the presence of amines. Differing from other MmXvo4 type open-framework materials, indium phosphates invariably have an octahedraltetrahedral framework. By varying the amines used as a structure-directing agent, the solvents and the compositions of reaction mixtures, a number of indium phosphates (InPOa-Cn, n=l 12) with new structures crystallized, and the products were characterized by means of XRD, TGA, adsorption and single crystal structure analysis. 1. INTRODUCTION A large number of MKIxVo4 (M=A1, Ga and X=P, As) type open-framework materials, represented by A1POas, GaPOas, A1AsOas and GaAsOas, have been synthesized by using a wide range of amines as templates. On calcination at elevated temperatures the majority of the materials become microporous and are potentially applicable as new catalysts, adsorbents and molecular sieves. Structurally, of most MmXVO4 compounds the primary building units are exclusively XO4 and MO4, with only a few containing XO4, MO5 and/or MO6 units. Recently, a novel family of open-framework materials composed of TOa-tetrahedra and TO6-octahedra has been discovered[ 1]. And like zeolites, these materials exhibit adsoption capacities and are promising shape-selective catalysts due to their unusual compositions. Most of open-framework MmXvo4 compounds are synthesized hydrothermally by using an amine or quaternary ammonium as template and water as solvent. However, Recently, many open framework materials can also be crystallized from non-aqueous, or predominantly nonaqueous medium[2,3], especially in the presence ofF- ions[4]. In contrast to open-framework aluminophosphates and gallophosphates which have been extensively investigated, only a few open-framework indium phosphates appeared in the literature [5-7], and their structures are invariably composed of octahedral InO6 and tetrahedral PO4 units. In this paper we present the synthesis and characterization of a number of novel indium phosphates (InPOa-Cn) synthesized from alcoholic solvent and/or in the presence of F- ions using an amine as the structure-directing template. 2. EXPERIMENTAL The reaction mixtures for InPO4-Cn were prepared from indium hydroxide (67% a s I n 2 0 3 ) , phosphoric acid (85%), hydrogen fluoride (42%), a template and the solvent. The templates used were ethylamine, ethylenediamine, pyrrolidine, piperidine, and the solvents were water, ethylene glycol, 1,2-propanediol , 1,4-butanediol, n-butanol. In a typical synthesis, indium
398 hydroxide was dispersed in distilled water or alcohol, to which aqueous hydrogen fluoride and phosphoric acid were successively added with stirring, to form a clear solution. Finally, an amine was added dropwise under vigorous stirring. The gel thus formed was stirred until being homogenous. The crystallization of the final reaction mixture was carried out in a PTFE-lined stainless steel autoclave at 473' K or 453 K for several days. The products were filtered, washed with distilled water and dried at room temperature. Powder X-ray diffraction (XRD) data were obtained using a Rigaku D/MAX-IIIA diffractometer with Cu Ka radiation. Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer TGA-7 thermal analyzer under a flow of gaseous N2 at a heating rate of 20 K min 1. A Hitachi X-650B scanning electron microscope was used to take the micrographs. Adsorption measurements were carried out isothermally in a Cahn-2000 electron recording balance. The samples were activated at 473 K and 10.3 Torr for about 3 hrs prior to the measurements. 3. RESULTS AND DISCUSSION The presence of F- ions favored the formation of InPO4-Cn, and without F- ions in the reaction mixtures, only dense indium phosphates were found to crystallize. The batch compositions of the reaction mixtures for InPOa-Cn were: xR:In203:yP2Os:zHF:(20-100)S, where R represented an amine and S an alcoholic solvent. The typical mixture compositions and crystallization conditions are listed in Table 1.
Table 1. Typical reaction mixture compositions and crystallization conditions for InPOa-Cn R x ethylamine ethylamine ethylenediamine ethylenediamine ethylenediamine ethylenediamine ethylenediamine ethylenediamine ethylenediamine ethylenediamine ethylenediamine ethylenediamine ethylenediamine ethylenediamine
1,6-hexenediamine pyrrolidine piperidine
2.5 2.0 4.0 4.0 4.0 2.0 2.0 2.0 2.5 1.0 1.0 1.0 1.0 1.0 2.0 1.0 1.0
Mixture composition y z S 1.8 1.0 1.2 1.2 1.2 1.8 1.8 1.2 1.5 1.0 1.0 1.0 1.0 1.0 1.2 1.0 1.0
1.0 ethylene glycol 0.1 H20 3.0 ethylene glycol 5.5 1,2-propanediol 5.5 1,4-butanediol 2.0 H20 0 ethanol 5.5 n-butanol 7.5 ethylene glycol 6.0 1-120 0.1 H20 0.4 1-120 1.0 H20 2.0 H20 0 n-butanol 2.0 H20 1.2 H20
Temp. (°C)
Product
180 180 180 or 200 180 or 200 180 or 200 180 180 180 180 180 180 180 180 180 180 200 200
InPO4-C 1 InPO4-C2 InPO4-C3 InPO4-C3 InPO4-C3 InPO4-C3 InPO4-C4 InPO4-C5 InPO4-C6 InPO4-C6 InPO4-C7 InPO4-C8 InPO4-C9 IRPO4-C10 InPO4-C 11 InPO4-C12 InPO4-C12
399 3.1. Ethylamine as template With ethylamine as the template, InPO4-C1 crystallizes from ethylene glycol solvent. To obtain pure phase of InPO4-C 1, the reaction conditions have to be controlled carefully, and the favorable pH range is 4.5-5.5. A pH value out of this range results in the formation of dense phases or layered indium phosphates with poor crystallinities. Use of other solvents than ethylene glycol has failed to produce InPO4-C1. The powder diffraction pattem (Figure 1) reveals that InPO4-C1 has a new structure and the scanning electron micrograph (Figure 2) shows that the compound is a pure phase. With water instead of ethylene glycol as the solvent, another indium phosphate (InPO4-C2) forms at a lower HF content. The TGA curve of InPO4-C 1 exhibits two weight losses. The first occurs from about 353 K to 773 K with an amount of 16.2 %, which maybe due to the evolution of H20 and the template. The second occurs from 873 to 1043 K and is attributed to the condensation of hydroxyl groups. After calcination at above 723 K, InPO4-C1 becomes X-ray amorphous, indicating that the compound is not as stable as most open-framework A1POas and GaPO4s. When InPO4-C1 is activated at 423 K and 10-3 Torr for 3 hours, it exhibits wateradsorption capacity. The adsorption isotherm is of type II (Figure 3a) and the amount of adsorbed water at room temperature and P/P0=0.8 is 8.0%.
3.2. Ethylenediamine as template When ethylenediamine is used as the template, a series of indium phosphates can be obtained by varying the solvent and the composition of the reaction mixture. From an alcoholic medium, the product relies on the HF content, the alcohol and the order of the addition of the reactants. InPO4-C3 with an XRD pattern shown in Figure 4a and a morphology shown in Figure 5a is easily formed from diol (e.g..ethylene glycol, 1,2propanediol, 1,4-butanediol) systems. On the other hand, InPO4-C5 (Figure 4c and Figure 5c)
1
!
10
l
I
2O 20
I
I
3O
Figure 1. XRD pattern of InPO4-C1.
I
I
4O
Figure 2. SEM of InPO4-C1.
400
10.0 8.0 6.0 ©
4.0
c
2.0 ~
0
i
I
0.2
I
I
0.4 P/Po
t
I
0.6
|
t
0.8
Figure 3. Water adsorption isotherms for (a) InP04-C1, (b) InP04-C5, (c) InP04-C3 and (d) InP04-C4. is obtained using n-butanol as the solvent. 1,2-Propanediol can also be used for the preparation of InPO4-C5. In case of high HF content in the gel, needle-like InPO4-C6 crystallizes from the diol systems whereas with a minor amount of HF or no HF, InPO4-C4 (Figure 4b and Figure 5b) is formed from an ethanol system. Thermogravimetric analysis of lnPO4-C3 sample shows weight losses of 21.2% in the 573823 K range and about 6% at 1073 K. Again the sample heated at about 573 K is amorphous. InPO4-C3 adsorbs a small amount of water (Figure 3c) after evacuation at 473 K. In case of InPO4-C5, the weight losses below 573 K are about 3% due to removal of water and the weight loss from 623 K to 873 K is attributed to the evolution of the occluded templates. After evacuation at 573, InPO4-C5 adsorbs about 10% water at P/P0=0.6 and room temperature, and the adsorption isotherm is of type I (Figure 3b). The sample of InPO4-C4 after dehydration at 523 K under vacuum has essentially no water-adsorption capacity (Figure 3d). The decomposition of templates occurring from 573 K to 873 K with a weight loss of 20% also leads to the collapse of the structure of InPO4-C4. With water as the solvent, the formation of indium phosphates was markedly influenced by the pH values, especially the HF content in the gel. It is found that several indium phosphates obtained from alcoholic solvent, such as InPO4-C3, InPO4-C6' can also be synthesized from aqueous system with the same template by varying the HF content. The resulting crystal size of the samples from aqueous system is larger than that of the corresponding samples from alcoholic solvent. Large single crystals of InPO4-C7 with prismatic shape (Figure 5e) are formed at a low HF content. Slight deviation in HF content leads to the formation of a mixture oflnPO4-C7 and InPO4-C8, an indium phosphate as shown in Figure 5f. Therates of nucleation and crystal growth for InPOa-Cn increase with the HF content. However, high HF contents result in highly twinned or aggregated indium phosphates. On the other hand, possibly due to conformation variation at different pH, ethylenediamine directs the formation of a number of indium phosphates with different framework structures on the basis of XRD patterns of the samples (Figure 4).
401 a
d
i
b
l
10
1
I
20 20
i
I
30
I
I
40
•
I
10
I
I
I
20
I
30
I
I
40
2O
Figure 4. XRD pattems of (a) InPO4-C3, (b) InPO4-C4, (c) InPO4-C5, (d) InPO4-C6, (e) InPO4-C7 and (0 InPO4-C9. A few indium phosphates have been structurally characterized by means of single crystal X-ray structure analysis. It is found that all of the structurally characterized InPO4-Cn have an octahedral-tetrahedral framework, namely, they are composed of InO6 and PO4 primary building traits. The polyhedral views of InPO4-C7, -C8, and -C9 are shown in Figure 6. The microporous indium phosphate InPO4-C7 has a 10-membered-ring channel system. It is of interest that ethylenediamine molecules traverse the 10-membered rings and each are
402 coordinated to two In atoms (Figure 6a). The In/P ratio in InPO4-C7 is 5/4, in contrast to the A1/P ratio of unity or less than unity for open-framework aluminophosphates. InPO4-C8 and InPO4-C9 are two layered compounds. The [NH3CH2CH2NH3]2+ cations (not shown in Figure 6b) are situated between two adjacent layers of InPO4-C8, and the [NH2CH2CH2NH3]+ cations pendantly coordinated to In atoms at one end in InPO4-C9 (Figure 6c). The refinements of the structures are still underway.
a
b
c
d
e
f
Figure 5. SEM of (a) InPO4-C3, (b) InPO4-C4, (c) InPO4-C5, (d) InPO4-C6, (e) InPO4-C7 and (f) InPO4-C8.
403
e
¢
o
o
o
o
o
o
oo
a
b
c
Figure 6. Polyhedral view of indium phosphates (a) InPO4-C7, (b) InPO4-C8 and (c) InPO4-C9 along the b axis.
3.3. Piperidine and pyrrolidine as templates
In the presence of piperidine or pyrrolidine, another novel 3-dimensional indium phosphate InPO4-C12 is obtained. Single crystals of this compound form after crystallization of the reaction mixture at 473 K for about two weeks. InPO4-C12 crystallizes in the monoclinic space group P21/n with a=10.325(2) A, b=9.136(2) A., c=10.325(2) A and 13=102.63°(3). The structure of the compound consists of 4-membered ring and 8-membered ring channels connected by InO6 octahedra and PO4 tetrahedra, in which NH4+ and H30 + were occluded. Upon calcination at 823 K for 2 hours, the material converts into an amorphous phase. Although F- ions play an important role in the crystallization of open-framework indium phosphates, novel indium phosphates can also be synthesized in the absence of F- ions, especially from alcoholic system. Among them is InPO4-C 11, which crystallizes in n-butanol at 353 K after two months by using 1,6-hexenediamine as the structure-directing template. 4. CONCLUSIONS In In203-P2Os-template-solvent systems, a variety of template-occluding indium phosphates (InPOa-Cn) can crystallize under hydrothermal or solvothermal conditions. Template, pH value and solvent are all crucial in determining the structures of the compounds. A few indium phosphates exhibit water-adsorption capacities after dehydration under vacuum. Structural analysis indicates that InPOa-Cn crystals are invariably composed of octahedral InO6 and tetrahedral PO4 units. The successful synthesis of open-framework indium phosphates leads to new insights into the MmXVO4 family.
404 ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China. The authors are grateful to Dr. I. D. Williams (HKUST), Dr. Y. Xu and Prof. L. L. Koh (National University of Singapore, Singapore) for structural analysis. REFERENCES R.C. Haushalter, L.A. Mundi, Chem.Mater., 1992, 4, 31. H. Du, S. Qiu, W. Pang, Chem.Res.Chin. Univ., in press Q. Huo, Ph.D thesis, Jilin University, 1992. J.L. Guth, H. Kessler, P. Caullet, J. Hazm, A. Merrouche, J. Patarin, Proceedings of the 9th International Zeolite Conference, Montreal 1992, Eds. P.von Ballmoos et al., 1993, p.215. 5. S.S. Dhingra, R.C. Haushalter, J. Solid State Chem., 1994, 112, 96. 6. S.S. Dhingra, R.C. Haushalter, J. Chem.Soc., Chem.Commun., 1993, 1665. 7. Y. Xu, L.L. Koh, L.H. An, S.L. Qiu, Y. Yue, Stud.SurfSci.Catal., Vol.84, J. Weitkamp, H.G. Karge, H. Pfeifer, W. H61derich (Eds.), Elsevier Science B.V., 1994, p. 2253.
1. 2. 3. 4.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
405
S t e r i c - E l e c t r o n i c Model of T e m p l a t i n g Effect Zaiqun Liu and Ruren Xu* KeyLaboratory of Inorganic Hydrothermal Synthesis, Department of Chemistry, Jilin University, Changchun 130023, P.R.China The function of organic amines during synthesis of A1PO4-21, A1PO4-11 and A1PO4-5 is studied by means of linear free-energy-relationship (LFER) analysis, and it is found that electronic effect of amines on crystallization-transition-state converts from strong nucleophilicity to electrophilicity with an increase of major channel from 8-ring to 12-ring. Furthemore, we observed steric effect of amines retards the crystallization rate of A1PO4-21 and accelerates the crystallization rate of A1PO4-5. Therefore, steric-electronic model of ~mplating effect in c~ystallization-transition-state is proposed, based on these experimental results. 1. INTRODUCTION It is well known that one type of template can be used to crystallize various aluminophosphate molecular sieves (A1PO4-n) whereas the same type of A1PO4-n can be crystallized by using different templates. From literature, we observed that researchers have paid more attention to the relationship between framework of A1PO4-n as host and amines as guest, but they neglected the dynamical factor[I,2,3] such as steric-electronic effect of amines on crystallization-transition-state. One of the important methods of researching template effect of amines on the crystallization of aluminophosphate molecular sieves is to detect the kinetic parameters of the reaction between initial gel of aluminophosphate and amines, because we couldnot see the intermediate stlxlcmres along the path[4,5] in the crystallization. Recently, physical organic chemists have used many tools to gather evidences of reaction mechanism, and one of the most useful ideas is the concept that a given structural feature will affect reactions as described by a linear free-energy-relationship (Taft equation) in the following: ln(k/ko)=pZa*+sZEs
(1)
where Z(~* and ZEs are numbers characteristic of inductive and steric properties of substiuent of amines, and k stands for the crystallization rate, ko stands for the rate of model reaction, p and s represent the electronic and steric properties of the reaction, respectively. The formation of A1PO4-n with a special designed structure could be reasonably regarded as a chemical reaction process, which hydroxyl groups attaching toA1 and P atoms dehydrate to bind characteristic bond AI-O-P catalyzed by organic amine. The reactivity of synthesis of A1PO4-n is
405 affected by factors such as the electronic-steric effect of functional groups of organic amines. Therefore, the method of linear free-energy-relationship analysis may be also useful to solve the problem of templating effect of amines. A1PO4-n (n=21,11,5) w i t h similar substructural units are chosen to investigate stericelectronic effect of amines on synthesis of A1PO4-n (n=21,11,5)[6]. 2. EXPERIMENTAL 2.1 S a m p l e preparation A typical synthetic procedure was in accordance with a certain batch composition of starting mixture of (1.0~2.0) Amine/1.0A12Os/1.0P2Os/54H20. Pseudoboehmite is firstly mixed with part of amount of water, then H3PO4 diluted by the other amount of water was dropped into above mixture. The aluminophosphate gel was formed after stirring 2 hours, followed by adding various organic amines to regulate the pH value range from 5.5 to 6.5. Finally, crystallization of the reaction mixture was carried out in a stainless steel autoclave at 202°C (475K) under autogenous pressure. The samples w e r e occasionally extracted, filtered, washed with distilled water and dried at ambient temperature. 2.2 S a m p l e c h a r a c t e r i z a t i o n The crystalline products were identified by X-ray powder diffractometer (Rigaku D/MAX with CuK~ radiation) and the crystallinities were calculated from the peak area range from 4 ° to 40 °. The crystallization rate (k) was calculated by the ratio of crystallinity with reaction time. The chemical environments of A1 and P atoms in the crystal structure of ALP04-21 synthesized by using (CH3)2NH, C2HsNH2 and n-CsH7NH2 as templates respectively were identified by 27A1- and 3:P-MASNMR. 3. RESULTS AND DISCUSSIONS 3.1 The linear free energy r e l a t i o n s h i p analysis of AIPO4-21 The crystallization rates (refered k) of AIPO4-2117,8] synthesized by using Me2NH, EtNH2 a n d n-PrNH2 as template agents are determined respectively. The methyl (-CH3) is regarded conventionally as the basic substituent defined as either inductive constant (a*) or steric constant (E0 as 0.0. The rate of AlPO4-21 by using Me2NH as template agent is chosen to be a model reaction whose crystallization rate is refered as ko. The other crystallization rate (k) campare with ko to establish the function of relative crystallization rate (refered as ln(k/ko)). The relationship between crystallization rate and substituent constants are shown in Table 1. With the increase of Z~* and ZE~ from propylamine to dimethylamine, the crystallization rate (k) is accelerated distinctly. This phenomeno~ indicates electronic and steric effect of organic amines stabilizes the transition state of this crystallization process. The negative charge of crystallization-transition-state is dispersed to more atoms than that in initial state. The reaction with this transition state is called nucleophilic mechanism. Moreover, ]n(k/ko) is regressed
407 Table 1 Relationship between crystallization rate of A1PO4-n (n=21,11,5) and substituent constants of organic amines at 475K AlPO4-n Template Inductive Steric Crystallization Structure constant(Z(~*) constant rate (xl03s 1) (EEl) A1PO4-21 (CH3)2NH 0.0 0.0 97.22 C2H5 NH2 -0.10 -0.07 37.04 n-C3HTNH2 -0.115 -0.36 25.83 ALP04-11 (n- C3HT)2NH -0.23 -0.72 9.042 (n- C4Hg)2NH -0.26 -0.78 9.417 (i-C3HT)d~qH -0.38 -0.94 10.33 ALP04-5 (C2Hs)sN -0.30 -0.21 16.22 (n-C3H7)3N -0.345 - 1.08 27.30 (HO CH2CH2)3N 0.595 - 1.08 11.95 linearly by the substituent constants (Z~* and EEl) to set up quantitative relationship in accordance with equation (1) as follows. ln(k/ko)=9.11Z(~*+ 0.77ZEs
(2)
The equation (2) characterizes quantitatively the influence of steric-electronic effect of amine as "attacking reagent" on the initial aluminophosphate gel as substrate. In this equation, the magnitude of p is so high as 9.11, which indicate the transition state of crystallization is stabilized most efficiently by electronattracting group attaching to N atom of amines. Also, since the value of p is about two times of typical organic nucleophilic reaction, it can be deduced that a complex is formed by coordination of amines to chemical species with positive charge in the initial aluminophosphate gel. On the other hand, the coefficient of ZEs is positive, so the coordination is limited by the increase of space of substituent of amines. In 1985, J.M.Bernett et al[8] reported the crystal structure of A1PO4-21 which contains 4A13PaO12OH'1.33N2CTH2x in a monoclinic cell. The framework contains three type of tetrahedral P, two trigonalbipyramidal A1 and one tetrahedral A1. All these types of A1 atoms and P atoms could be determined by MAS NMR method, so it could be compared with the difference in chemical environment of A1PO4-21 synthesized by using various amines as template agents. The crystallization mechanism of ALP04-21 is nucleophilicity and steric hindrance of amine in the transition state, which the electronic effect of amine improve the interaction between the framework of A1PO4-21 and amine, the space of amine decreases this kind of interaction. This proposal could be demonstrated by the 27A1-MAS NMR and 31p-MAS NMR, as shown in Figure 1. Because the most strong nucleophilicity during synthesis of AlPO~-21 by using Me2NH as template cause the most week shielding effect to
408 A1v, which the chemical shift (5=38.244ppm) locate more low field than that of the other samples synthesized by EtNH~. and n-PrNH~ as templates. 3.2 The l i n e a r free energy r e l a t i o n s h i p of A1PO4-11 and A1PO4-5 The c17¢stallization rates(k)of A1PO4-1119] are also determined respectively while (n-Pr),.2qH, (i-Pr)2NH and (n-Bu)2NH are used as template agents. The results similar to A1PO4-21 are also presented in Table 1. The linear free-energyrelationship equation of A1PO4-11 is presented as equation (3). ln(k/ko)=0.40Zt~*-0.88ZEs-2.91
.
.
.
.
.
- -
- _
(3)
.
43.960
-16.759---1 /
~
[
/t
hr ' ' - - - 2 8 " 9 0 0 ]
a 1 44.511---1 ,
. O. 8 4 4
i
-18.512"--1
I a2
~
-
-23.414
A b2
42.133----n~ 41.699 -19.323-
----.-_~ c j /~
. . . . 23.056 ~-29.654
a3 °
..
8O
4O
5 (ppm)
0
0
.-30
5(ppm)
Figure 1 MAS NMR spectra of (a) 27A1 and (b) sip on A1PO4-21 synthesized by using (1) MezNH,(2) EtNH2, (~) n-PrNH2 as template agents.
409 Table 2. Relationship between crystallization rate of AIPO4-n (n=21,11,5) and substiment constants of amine at 448K and AH~;AS~ of A1PO4-n (n=21,11,5) A1PO4-n Template Za* ZE, k (xl0-Ss -1) AH~ ASs Structure (kJ.mo1-1) (J.mol-l.K-1) APO-21 (CHs)2NH 0.0 0.0 18.45 105.1 -47.0 C2HsNH2 -0.10 -0.07 10.15 81.0 -105.7 n-CsH7NH2 -0.115 -0.36 8.921 65.8 -104.6 APO-11 (n-CsH7)2NH -0.23 -0.72 2.233 87.8 -103.1 (n-C4Hg).oNH -0.26 -0.78 2.040 96.4 -84.6 (i-C3H7)2NH -0.38 -0.94 2.524 88.5 - 100.5 APO-5 (C2Hs)sN -0.30 ,0.21 9.808 29.1 -221.7 (n-C3H7)3N -0.345 -1.08 14.49 37.7 -199.4 (HOCH2CH2)3N 0.595 -1.08 1.893 116.9 -39.5 The magnitude of p decrease from 9.11 of A1PO4-21 to 0.40 of AIPO4-11. Along with the increase of electron-donating effect of subsituent attaching to N atom, the nucleophilicity of amine decrease rapidly, which result in the decrease of nucleophilicity. In contrast, since the coefficient of ZEs is -0.88, a negative value, the steric factor of amine play an important role to support the formation of channel of A1PO4-11. As a result, it may be deduced that the steric-electronic effect of A1PO4-5, a large size channel with the similar structural subunit to A1PO4-21 and A1PO4-11 would get steric acceleration and electrophilicity, that is, the value of the coefficients of Z~* and ZE, are negative. Then, the crystallization rates of A1PO4-5 synthesized by using Et3N, (n-Pr)sN and (HOCH2CH2)sN as template agents are determined respectively and the results are also presented in Table 1. The linear free-energy-relationship equation of A1PO4-5 is presented as
(4).
ln(k/ko)=-0.88Za*-0.55ZEs-2.17
(4)
The inference of A1PO4-5 is demonstrated correct by the results of experimental equation (4). So, the electronic effect of A1PO4-n (n=21,11,5) could be concluded that the nucleophilicity of amines get to electrophilicity with the increase of size of channel; the steric effect of amine retard the crystallization of A1PO4-21 but accelerates the crystallization of AlPO4-11 and A1PO4-5. 3.3 The linear free e n e r g y r e l a t i o n s h i p of AIPO4-n (n=21,11,5) at 175°C (448K) All the experimental results as mentioned above are repeated at 175°C (448K) to obtain the correspond linear free-energy-relationship equation to discuss the influence of steric-electronic factor on the crystallization process at relative low temperature. Table 2 presents the crystallization rates of A1PO4-n (n=21,11,5) at 448K and active-enthalpy, active-entropy of A1PO4-n (n=21,11,5). The free-energy is related with reaction temperature. To investigate the influence of steric-
410 electronic effect of organic amines on crystallization transition state, the freeenergy is divided into active-enthalpy and active-entropy as follows: k~=kT/h.eAS*~.eA~T
(5)
where T stands for reaction temperature, l~ represents the crystallization rate, k is Boltzmann constant, and AH*,AS* represent active-enthalpy and active-entropy respectively. The crystallization of AlPO4-21 by using Me2NH as template agent at 448K, whose the crystallization rate is refered as ko*, is also chosen to be model reaction. The linear free-energy -relationship equations of A1PO4-21 (equation (6)), AIPO4-11 (equation (7)), AIPO4-5 (equation (8). In (k/ko*)=5.88 Za*+ 0.14ZEs ln(k/ko*)=- 11.4Z(~*+7.18YEs+0.45 ln(k/ko*)=-2.17Z(~*-0.34ZEs- 1.35
(6) (7) (8)
Comparison equation (2,3,4) with equation (6,7'8), the corresponding linear free-energy-relationship equations indicates that all the nucleophilicity decrease. For instance, the magnitude of coefficient of Za* of A1PO4-21 changed from 9.11 to 5. 88, which indicate that they keep nucleophilicity property. The value of coefficient of Z(~* decrease from -0.88 to -2.17 for AIPO4-5 suggesting that the crystallization mechanism of A1PO4-5 keep electrophilicity property. In contrast, the crystallization mechanism of AlPO4-11 converts from week nucleophilicity (p=0.40) to strong electrophilicity (p=-11.4). Relatively, with the decrease of crystallization temperature, in the process of synthesis of A1PO4-21 the steric hindrance of amine is decreased, and in the process of crystallizing A1PO4-5 the steric acceleration of amine decrease but still keep the steric facilitation effect; in the process of synthesis of A1PO4-11 the steric acceleration of amines convert to retardation. So, the conclusion could be obtained that steric effect of amines only determine the crystallization rate according to the inductive effect of amines control the structural style of AlPO4-n (n=21,11,5). 3.4 R e l a t i o n s h i p of linear active enthalpy-entropy equations of A1PO4-n. The quantitative relationship between active enthalpy and substituent constants is established as linear active-enthalpy-relationship equations and linear active-entropy-relationship equations as presented as in Table 3. The inductive effect of amines lead to the decrease of active-enthMpy for A]PO4-n (n=21,11,5) and the steric effect results in the increase of the activeenthalpy of AIPO4-11 and AIPO4-5 and decrease of A1PO4-21. On the other hand, the inductive effect of amines lead to the increase of the absolute value of active entropy of all these three kinds of A1PO4-n, and steric effect of amines results in the decrease of that of A1PO4-11 and A1PO4-5, in contrast, increase that of A1PO4-21. Considering influence of inductive effect of substituent on either active-entropy or active-enthalpy, the amines took part in the whole crystallization process by means of electron-attracting or electron-donating effect of substituent on the structure of gel with positive or negative
411 Table 3 The linear active-enthalpy-relationship equations A1PO4-n The linear active enthalpy relationship A1PO4-21 ln(AH~/H%)=2.18Z~*+0.60EEs ALP04-11 ln(AH~/AH%)=8.37Za*-5174YEs-2.39 ALP04-5 ln(AH~/AH%)=l.21Za*-O.36ZEs-l.O0 ALP04-21 ln(AS'/AS%)=- 7.69Z~*-0.592Es ln(AS~/AS%)=-17.5Za*+12.1ZE~+5.44 ALP04-11 ALP04-5 ln(AS~/AS%)=- 1.72Za*+0.21ZE~+ 1.08
(8) (9) (10) (11) (12) (13)
charge. The steric effect of substituent of amine showed different properties to different A1PO4-n. 4. CONCLUSION We concluded that steric effect of amines determined the crystallization rates and the electronic effects of amines directed remarkbaly the style of A1PO4-n. Nucleophilicity is related to small channel of A1PO4-n such as ALP04-21, and electrophilicity had the relationship with large channel of A1PO4-n such as ALP04-5. Week nuc!eophilicity is related to middle channel of A1PO4-n such as ALP04-11, electrophilicity of amines can also direct this kind of aluminophosphate molecular sieve at relative low crystallization temperature. Moreover, all factors that can change charge distribution of initial gel strongly influence channel type of A1PO4-n. The steric factor of template agent play an important role in the crystallization rate. And this inference is demonstrated by the result that Et3N, a electrophile in synthesizing ALP04-5 at 475K, shows nucleophlicity in transition state for synthesizing ZnAPO-34 with a small size channel, and (i-Pr)2NH a weak nucleophile of crystallizing ALP04-11 at the same temperature as above showed strong electrophilicity for synthesizing MgAPO-5. The results were explained by addition of Zn 2÷, Mg2+ which extensively changed charge distribution of initial aluminophosphate gel. ACKNOWLEDGEMENT This work was supported by the National Natural Fundation of China and Key Laboratory of Inorganic Hydrothermal Synthesis in Jilin University of China. REFERENCES [1] M. E. Davis and R. F. Labo; Chem.Mater.4(1992)756 [2] S. Minton, V. Vaitchev and I. Kanev; Zeolites 13(1993)102 [3] E. Narita, J. Crystal Growth,78(1986) 1 [4] X. Ren, S. Komarneni, D. M. Roy, Zeolites,ll(1991)142 [5] J. Shorter, Correlation Analysis in Organic Chemistry,Oxford, 1973 [6] B. Parlitz, U. Lohse, E. Schreier, Microporous Materials,2(1994)223 [7] J.M.Bennett, W.J.Dytrych,J.W.Richardson Jr, J.V.Smith, Zeolites,6(1986)349 [8] J. M. Bennett, J. M. Cohen, G. Artioli, J. J. Pluth, Inorg.Chem.,24(1985)188 [9] J. W. Richardson Jr, J. J. Pluth, J. V. Smith, Acta Cryst. B44(1988)367
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II. Characterization
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H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 1997 Elsevier Science B.V.
415
THE SYNTHESIS AND C H A R A C T E R I Z T I O N OF UTD-I: THE FIRST LARGE PORE Z E O L I T E BASED ON A 14 M E M B E R E D RING SYSTEM Kenneth J. Balkus, Jr*, Mark Biscotto and Alexei G. Gabrielov Department of Chemistry, University of Texas at Dallas, Richardson, TX 75083-0688 United States
SUMMARY The high silica zeolite UTD-1 was prepared via a hydrothermal synthesis using bis(pentamethylcyclopentadienyl) cobalt(m) hydroxide (2) and bis(tetramethylcyclopentadienyl) cobalt(Ill) hydroxide (3) as a templates. The structure of UTD-1 consists of a one dimensional channel system where the pores are def'med by elliptical 14 membered rings based on T atoms. Preliminary, adsorption and reactivity studies reflect the large pore (7.5 X 10 A) nature of this material. 1. INTRODUCTION We have developed a new strategy for the preparation of zeolite ship-in-a-bottle complexes [1] which involves the encapsulation of metal complexes during zeolite synthesis. This work encouraged us to explore the relatively unknown structure directing p r i e s of metal complexes. In particular, we have shown that bis(cyclopentadienyl)cobalt (HI) ion, Cp2Co÷ (1) is a template for several clathrate structures as well as the one dimensional channel type molecular sieves AII~4-5 [2] and more recently UTD-12 which appears to be isostructural with ZSM-48 [3]. All of these molecular sieves are known but had previously required an organo-cation to form. These results suggested that certain metal complexes may function as structure directing agents towards potentially new materials. We anticipated the addition of methyl groups to the cyclopentadienyl tings would encourage the crystallization of larger pore materials. Therefore, we examined the templating properties of bis(pentamethylcyclopentadienyl)cobalt(III) hydroxide, Cp*2CoOH (2) and discovered a novel high silica zeolite which we refer to as UTD-1 [4-8]. More recently we have shown that bis(tetramethylcyclopentadienyl) cobalt(Ill) hydroxide, Cp'2CoOH (3) is also a template for UTD-1. As anticipated a structure determination [9] revealed UTD-1 to be a large pore zeolite having a one dimensional channel system. An exciting aspect of the UTD- 1 structure is the fact that the channels are defined by 14 T atoms. Therefore, this represents the first zeolite having greater than a 12 membered ring pore structure. The pores are elliptical in shape having dimensions of 10 x 7.5 A which corresponds to the largest pore structure known for a zeolite. Additionally, the UTD-1 structure appears to be stable to at least 1000°C in air. As a result this novel large pore material may have many interesting applications in catalysis. In this paper, we will describe the synthesis and characterization of UTD-1 using metal complexes 2 and 3 as templates as well as a preliminary account of the catalytic activity.
416 2. EXPERIMENTAL Metal complex 1 can be purchased from Aldrich or prepared as previously described [10]. Template 2 can be purchased from Strem or prepared according to the literature procedure [11]. The tetramethylcyclopentadiene ligand was purchased from Aldrich and complexed with Co(Ill) in the same manner as for the Cp* ligand to produce complex 3. The preparation of the hydroxide forms of these templates was accomplished by ion exchange as previously described [8]. The synthesis of the high silica form of UTD-1 using Cp*2CoOH has been described [5,8]. The synthesis of UTD-1 using Cp'2CoOH was conducted as follows. NaOH, an 18% aqueous Cp'2CoOH solution and fumed silica were combined to form a gel with the following molar ratio: SiO2 : Cp'2CoOH : NaOH :H20 = 1 : 0.125 : 0.1 : 60. The gel was aged with stirring for 1 hour and then transferred to a 23 mL Teflon-lined pressure reactor (Parr). The reactor was heated at 175°C under static conditions for 2 days. The resulting yellow crystalline product was isolated and then washed with deionized water and dried at 90°C for 2 hours. 3. RESULTS AND DISCUSSION Both the Cp*2Co÷ (2) a n d C p ' 2 C o ÷ (3) complexes in Figure 1 shown below are templates for the synthesis of UTD-1. So far no other low density silica phases have been prepared using these complexes. Interestingly, CpECO÷ (1) forms the 12 ring channel type zeolite UTD-12 (ZSM-48) which is structurally related to UTD-1. Increasing the size of the cobalt(III) complexes by adding four or five methyl groups to the Cp ligand results in UTD-1. The Addition of one methyl group to the Cp ligand results in a template for only dodecasil-lH so far [12]. The structure directing properties of cobalticinium complexes having two or three methyl substituents remains to be studied.
1 Figure 1. Metal complex templates
2 Cp2Co +
3 (1), Cp*2Co+ (2) and
Cp'2Co +
(3)
417
This new UTD-1 phase forms using 2 and 3 over a fairly narrow range of template concentration and crystallization times [8]. The all silica form of UTD-1 forms as aggregates of plank shaped crystals. Figure 2 shows a scanning electron micrograph of UTD-1 prepared using Cp'2Co ÷as a template. The crystals are larger than those prepared using Cp*2Co ÷ [51.
Figure 2. Scanning electron micrograph of the as synthesized UTD-1 prepared using Cp'2CoOH as the template. The structure of the all silica version was determined by a combination of model building and distance least squares refinement using synchrotron powder diffraction data [9J. The structure of UTD- 1 has an orthorhombic unit cell with lattice parameters of a = 18.981 A, b = 8.415 ,/~ and c = 23.040/~. The simulated XRD pattern of the pure structure using the space group Imma differed slightly from the experimental XRD pattern. However, the introduction of ~ 20% faulting into the model provides a reasonable match for the XRD pattern. A polymorph having Bmmm symmetry was used to simulate the XRD pattern. The Bmmm polymorph does not alter the channel size or shape but rather the nature of the walls. The channel walls of the Imma structure are composed of six membered rings, while the Bmma polymorph has six membered rings as well as eight and four membered tings. Figure 3 shows a projection of the UTD-1 framework viewed along the [010] direction which reveals the elliptical channels (7.5 x 10,/~) that are def'med by 14 silicon atoms. The basic building units, six membered tings surrounded by 4 fwe membered rings, are also found in other zeolites particularly high silica zeolites such as ZSM-48. The projection of ZSM-48 (UTD-12) can be generated from UTD-1 if the 4 four membered tings are removed. It is interesting that the
418 smaller metal complex Cp2Co + forms the ZSM-48 structure and the larger C p * 2 C o + or C p'2Co ÷ form UTD- 1.
Figure 3. The structure of UTD-1 viewed along the [010] direction.
It does not a ~ that the metal complex templates can be removed by solvent extraction or ion exchange. However, the metal complexes can be decomposed by heating the UTD-1 at 500°C which does not affect the zeolite structure. In fact the UTD-1 structure appears to be stable to at least 1000°C in air. The TEaM shown in Figure 4 shows the small dusters of cobalt oxide that result from thermal decomposition of the intrazeolite Cp*2Co+ template. The clusters are a few nanometers in size and impart a blue/gray cast to the calcined UTD-1. The nature of the cobalt oxide in this case is uncertain, however, the thermal decomposition of Cp2Co÷ inside A1PO4-5 has been reported to result in cobalt(III) ions bound to six oxygens [13]. In A1PO4-5 the resulting cobalt oxide is not entirely consistent with Co304 and may involve some coordination of lattice oxygens. In UTD-1 the cobalt oxide clusters that are visible by TEM a ~ to have formed on the outer surface of the crystals with some of the larger clusters being located at the ends of the channels. This would suggest that cobalt species migrate out of the channels during thermal decomposition of the template. There is still some cobalt species in the channels or at least partially blocking the channels because the pore volume increases when the cobalt is completely removed. For example, the pore volume obtained from cyclohexane adsorption are 0.079 and 0.111 ml/g for the calcined and cobalt free UTD-1 zeolites respectively. There is no evidence of cobalt incorImmtion in the UTD-1 lattice and all the detectable cobalt can be removed by washing the calcined crystals with HCI.
419
Figure 4. Transmission electron micrograph of UTD-1 after calcination at 500°C. The large pores of UTD- 1 provides an opportunity to study reactions of substrates too large for other zeolites. The pore volume calculated from the adsorption of 1,3,5triisopropylbenzene is approximately the same as cyclohexane (0.111 ml/g) which bears testament to the size of these pores. The unhindered diffusion of such large molecules suggest that catalytic transformations that are difficult or not possible with 12 MR zeolites may be realized with IYrD-1. In order, to affect such reactivity the silica framework must be modified to include other T atoms. We have incorporated Al, B, Ti, V and Zn into the structure of UTD1 during synthesis. The lYrD-1 zeolite synthesized with very high Si/AI ratios has been reported [8]. Alternatively, the boron from B-UTD-1 can be removed by acid treatment and rel;~aced by aluminum where Si/Al ratios of 50 are more accessible [14]. We have previously shown that Bronsted acidity can be generated in UTD-1 is evidenced by the conversion of
420 methanol to hydrocarbons at 400°C. The detection of large aromatics such as hexamethylbenzene (kinetic diameter 7.1A) in the product stream is consistent with the characterization of UTD-1 as a large pore zeolite. Similarly, an evaluation of the constraint index using n-hexane cracking indicates a rather low value of -0.2 which is also consistent with the large pore structure. Further details of these catalytic cracking experiments will be r ~ elsewhere. Other elements such as titanium may provide a different type of reactivity and preliminary results for the oxidation of alkanes and using peroxides are quite promising [15]. More recently we have shown that large substrates such as 2,6-ditertbutylphenol can be oxidized to the corresponding quinone using Ti-UTD-1 with conversions and selectivity comparable to Ti substituted mesoporous silica [ 16]. The conversion of a substrate of this size is not possible over a TS-1 catalyst. Similarly, t-butylhydrogen peroxide which is too large for TS-1 can be effectively utilized as the oxidant for Ti-UTD-1 catalyzed reactions. Ti-UTD- 1 has also been shown to be an effective epoxidation catalyst using hydrogen peroxide as the oxidant, clearly a great deal of work remains in the characterization of UTD-1 and the framework modified analogs. However, these preliminary studies suggest that some novel reactivity may be observed. 4. CONCLUSIONS There are relatively few zeolites that crystallize an all silica form. UTD- 1 and UTD-12 (ZSM-48) are now two among others that include ZSM-11, ZSM-12, ZSM-22, ZSM-5, SSZ31, SSZ-24 and ferrierite. It is anticipated that UTD-1 will behave in a similar fashion some of these other high silica zeolites. For example, The framework of UTD-1 is easily modified with elements such as Ti, V, Zn and B. The incorporation of aluminum by direct synthesis is more difficult and Si/A1 ratios below 10 seem improbable. However, post synthesis modification with aluminum has proven to be a convenient method for generating Bronsted acidity. It is clear even from preliminary data that UTI)-I has potential as a catalyst in petroleum refining. This novel zeolite was the product of using metal complexes as templates which certainly warrants the continued development of this strategy towards new microporous materials. ACKNOWLEDGMENTS We thank the Chevron, the National Science Foundation (CHE-9157014) and the R.A. Welch Foundation for financial support. The contributions of S. Zones, I. Chan (TF~) and C. Y Chen (adsorption data) are greatly appreciated. REFERENCES 0
.
3. .
5. 11
0
SIP
K.J. Balk-us, Jr. and A.G. Gabrielov, in Inclusion Chemistry with Zeolites, Nanoscale Materials by Design, N. Herron and D. Corbin (Eds), Kluwer, (1995) 159. K.J. Balk-us, Jr., A.G. Gabrielov and S. Shepelev, Micropor. Mater, 3 (1995) 489. K.J. Balkus, Jr. and M. Biscotto, manuscript in Preparation. K.J. Balkus, Jr. and A.G. Gabdelov, U. S. Patent No. 5,489,424 (1996). K.J. Balkus, Jr. and A.G. Gabrielov, U. S. Patent No. 5,489,424 (1996). K.J. Balkus, Jr., A.G. Gabrielov and N. Sander, Mater. Res. Soc. Symp. Proc., 368 (1995) 369 K.J. Balkus,Jr., A.G. Gabrielov and S.I. Zones, Stud. Surf. Sci. Catal. 97 (1995) 519. K.J. Balkus, Jr., A.G. Gabrielov, S.I. Zones and I.Y. Chan Petr. Preprints 40 (1995) 296. K.J. Balkus, Jr., A.G. Gabrielov, S.I. Zones and I.Y. Chan in Synthesis of Porous Materials, M. Occelli and H. Kessler, (Eds), Marcel Dekker, NY (1996) 77.
421
11
10. 11. 12. 13. 14. 15. 16.
C.C. Freyhardt, M. Tsapatsis, R.F. Lobo, M.E. Davis and K.J. Balkus,Jr., Nature, 381 (1996) 295. K.J. Balkus, Jr. and S. Shepelev, Micropor. Mater., 1 (1993) 383. U. Kolle and F. Khouzami, Chem. Ber., 114 (1981) 2929. G. van de Goor, B. Lindlar, J. Felsche and P. Behrens, J. Chem. Soc., Chem. Commun., (1995) 2559. M. Endregard, D.G. Nicholson, M. Stocker and G. M. Lamble, J. Mater. Chem. 5 (1995) 785. K.J. Balkus, Jr. and A. Ramasaran, manuscript in preparation. K.J. Balkus, Jr. A. Khanmamedova, A.G. Gabrielov and S.I. Zones, Stud. Surf. Sci. Catal., 101 (1996) 1341. P.T. Tanev, M. Chibwe, T.J. Pinnavaia, Nature, 368 (1994) 321.
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H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 1997 Elsevier Science B.V.
423
The nature of the acid sites in mesoporous MCM-41 molecular sieves A. Liepold', K. Roos', W. Reschetilowski', R. Schmidt b, M. Strcker b, A. Philippou °, M.W. Anderson °, A.P. Esculcas d and J. Rocha d 'Karl Winnacker-Institut der DECHEMA e.V., Theodor-Heuss-Allee 25, D-60486 Frank~rt am Main, Germany b SINTEF Oslo, P.O. Box 124 Blindern, N-0314 Oslo, Norway °Department of Chemistry, UMIST, PO Box 88, Manchester M60 1QD, UK dDepartment of Chemistry, University of Aveiro, 3800 Aveiro, Portugal The objective of this presentation is to give a characterization of aluminosilicate MCM-41, in particular with respect to acidity and the nature of the acid sites. 295i MAS NMR spectra of MCM-41 closely resemble those of amorphous silica, suggesting that the pore walls are amorphous. This fact could also imply that the types of acid sites in these materials are different from those generally found in zeolitic materials. The obtained results from 1H MAS NMR and combined FTIR and temperature-programmed ammonia desorption (TPAD) investigations indicate that the washed, template-free MCM-41 and its protonic form behave as a solid-state acid with a broad acid strength distribution. With regard to the presence of bridging hydroxyl groups in the investigated MCM-41 materials, the findings of these two methods are not consistent. Therefore, it is assumed that the broad absorption between 3650 3400 cm 1 in the FT1R spectra is connected with hydrogen-bonded vicinal silanol pairs. The results of the catalytic testing suggest that the weak BrOnsted acid sites interact with neighbouring coordinatively unsaturated aluminium species and that synergistically stronger acid sites are formed.
1. INTRODUCTION The invention of a new family of mesoporous materials, designated M41S, by scientists of Mobil Oil Corporation [ 1] has dramatically expanded the range of pore size and really seems to make the mesoporous regime accessible to applications typical of zeolites and related materials. One of the members of this family, MCM-41, shows a hexagonal arrangement of uniformly sized mesopores whose pore dimensions may be engineered in the range of about 1.5 nm to greater than 10 nm in diameter, depending on the template and synthesis conditions. By inserting aluminium or other trivalent cations in the framework of the silicious MCM-41, this material should potentially become catalytically active. Aluminosilicate MCM-41 with a pore size of 2.5 nm seems to be a suitable active component for "deeper" cracking of high
424 boiling hydrocarbons [2]. Hitherto, only few reports deal with the acidity of these materials describing its strength as medium [3] or as mildly acidic [4]. However acid sites of medium strength could not explain the high catalytic activity of aluminosilicate MCM-41 materials in hydrocarbon cracking, because this reaction requires strong acid sites. Therefore, the objective of this presentation is to give a characterization of aluminosilicate MCM-41, in particular with respect to acidity and the nature of the acid sites, in order to explain the raised catalytic activity of this material in hydrocarbon cracking.
2. EXPERIMENTAL
SYNTHESIS AND CATALYTIC TESTING The synthesis procedure of aluminosilicate MCM-41 materials (Si/A1 = 17.3) has been described by Schmidt et al. [5]. Prior to using this material in physico-chemical investigations and catalytic testing, it is necessary to remove the organic templating agents to empty the pores. To avoid severe damage to the structure, the template removal procedure by calcination is supplemented by an intermediate washing step, described in detail earlier [2]. This sample is further denoted as-prepared. The obtained template-free material is used for the protonic ion exchange procedure performed with 0.1 M NH4NO s solution at 80°C (with liquid-to-solid ratio of 5 ml/g) to form NH4-MCM-41. The so-called protonic form of MCM-41 (H-MCM-41) was obtained by deammoniating the NHa-MCM-41 sample at 500°C in flowing nitrogen. MCM-41 was catalytically tested with n-hexadecane as the model feed and different cat./oil ratios at three temperatures in the range of 432°C to 532°C by cyclic MAT (microactivity lest) according to an improved ASTM standard D3907-92. This means that after each MAT test the inserted sample was regenerated in flowing synthetic air (flow rate 150 ml/min) before the next run.
METHODS The as-synthesized, modified and catalytically tested MCM-41 materials were characterised by X-ray powder diffraction and N 2 adsorption/desorption at 77 K. Solid state 298i MAS NMR spectra were recorded on a Bruker MSL 400P spectrometer at 79.49 MHz. The 29Si MAS NMR spectra were recorded with 50 ° pulses and 5 - 5.5 kHz spinning rates. The recycle delay was 120 s. The 1H -29Si CP/MAS NMR spectra were recorded with 6.0 ~s 1H 90 ° pulses and a contact time of 5 ms. Chemical shifts are given in ppm from TMS. The 1H MAS NMR spectra were recorded on a Bruker MSL 400 (9.4 T) operating at 400.13 MHz. A rotor synchronised Hahn echo method was used with spinning rates of 5 - 9 kHz and
425 spectra were obtained by accumulating 200 to 400 transients with 5 s recycle delay. The dehydrated samples were spun in 4 mm zirconia rotors. The temperature-programmed ammonia desorption (TPAD) technique was used to study the acidic properties of template-free MCM-41 materials and an amorphous aluminosilicate. The combined FTIR spectroscopy and TPAD apparatus and measurement procedure is described elsewhere [6].
3. RESULTS AND DISCUSSION 29Si ]~AxS
By applying 29Si MAS NMR the short-range ordering changes involved during the calcination and modification procedures of aluminosilicate MCM-41 are monitored. 1H- 295i CP/MAS NMR spectroscopy has
-loo
been used to monitor the defect sites selectively. The broad 295i MAS NMR spectra of cyclic MAT MCM-41 materials, cf. Figure 1, are identical to those from amorphous silica, indicating k~.~MCM-41 that the local arrangement of the Si-O-Si bonds in the pore walls is irregular and that a wide range of bond angles are present [7,8]. The I H - 298i CP/MAS NMR spectra, see inset in Figure 1, clearly show that, as in amorphous silica, part of the silicon atoms exist as silanol groups [8,9]. The spectra / \ contain three resonances at - 110, - 100 and / -91 ppm attributed to Si(4Si), Si(OSi)3(OH ) and Si(OSi)~(OH)2 environments rrt•i•liiilitll•llliiIllliIllljl•li•iIiillliI•li•jI=li=•lil1ljilwillllii•ll•lIill -90 -110 -130 respectively. Considering the Si/A1 ratio, it is -70 ppm from TMS possible that the resonances at -100 and -91 ppm contain contributions from Si(3Si, IA1) and Si(2Si,2A1) sites respectively. Figure 1. 29SiMAS NMR spectra of A close inspection of the spectra reveals only MCM-41 materials. As an example the minor differences in the intensities of the inset depicts the IH -29Si CP/MAS NMR observed resonances. spectrum of calcined MCM-41.
426 TEMPERATURE-PROGRAMMED AMMONIA DESORPTION AND 1H MAS NMR The broad acid strength distribution of MCM-41 and H-MCM-41, determined by TPAD, which resembles that of amorphous aluminosilicates has been reported previously [10]. The number of stronger acid sites of the MCM-41 materials differs from that of an amorphous aluminosilicate which is indicated by the ammonia desorption traces above 400°C. On account of the ammonia desorption traces, it is not possible to distinguish between Br6nsted- and Lewis-type acid sites. In accordance with Corma et al. [3], the FTIR spectra of MCM-41, cf. Figure 2, and its protonic form in the hydroxyl range show a very intense band at 3740 cm"], correlating with free Si-OH vibrations found on silicas [11] and appearing superimposed on a broader absorption between 3750 - 3400 cm 1. On adding ammonia, intense bands appear at 3352 cm 1, 3285 cm1, 3201 cm 1 in the region of N-H bond stretching vibrations. The absorption bands at 3352 cm"1 and 3285 cm ~ could be assigned to ammonia bonded to coordinatively unsaturated species and the asymmetric stretching vibration of the ammonium ion respectively. We tentatively assigned them, in accordance with Roev et al. [12], to stretching vibrations of ammonia adsorbed on surface aluminium sites since the two bands do not completely recover upon heating to 550°C. Uytterhoeven and colleagues [13] identified the band at 3200 cm~ as originating from an ammonium ion, which was also responsible for that at 1680 cm ~.
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Figure 2. FTIR spectra of MCM-41 ( ..... ) before ammonia adsorption; (. . . . . . . . . . . .) ammonia adsorption at 100°C; (- . . . . . . . . ) TPAD at 300°C; (. . . . . . ) TPAD at 550°C.
427 In accordance with Uytterhoeven et aL [ 13], the bands at 1660 cm4, 1625 cm 4 and 1450 cm4 in the region of N-H bending vibrations were assigned to the interaction between ammonia and weak Brensted type acid sites, to ammonia coordinatively bonded to Lewis acid sites and to ammonium ions formed by the interaction between ammonia and strong Br6nsted acid sites. Surprisingly, the broad absorption between 3650 - 3400 cm4, generally characteristic of bridging hydroxyl groups in zeolites, is not affected. Since no change could be observed after saturating MCM-41 with ammonia on the former bands it can be concluded that the broad band between 3650 - 3400 cm4 is connected with hydrogen-bonded vicinal silanol pairs. Therefore, the bands at 1450 and 1660 cm4 could not be ascribed to bridging hydroxyl groups. On the basis of theoretical calculations Sauer et aL [14] pointed out that the deprotonation energy of these vicinal silanol pairs is markedly lower than that of terminal silanol groups. An additionally performed protonic ion exchange of aluminosilicate MCM-41 seemed to cause only a slight increase in acidity with regard to the region of hydroxyl vibrations. When heating to 300°C the bands in the region of the N-H deformational vibrations decrease, but the intense band at 3740 cm4 slightly recovers, see spectra in Figure 2. After heating to 550°C, the band at 1625 cm4 has not completely disappeared, whereas the other bands in the region of N-H bending vibrations have. Simultaneously, the incomplete recovery of the band at 3740 cm4, representing terminal silanol groups at 550°C, is observed. By applying ~H MAS NMR techniques, the actual acid strength could be determined by differentiating the various types of hydroxyl groups. The present spectra, cf. Figure 3, indicate that the investigated materials are completely dry, because of the non-appearance of a broad
MAT tested
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Figure 3. ~H MAS NMR of deammoniated and cyclic MAT tested H-MCM-41.
428 signal centred at 4.7 ppm, which is characteristic of intracrystalline water. The spectra of the protonic form ofMCM-41 before and atter cyclic MAT testing both display a prominent signal around 1.7 ppm which is generally assigned to terminal silanol groups [15]. The shoulder between 3.5 and 2.0 ppm in the spectra of the H-MCM-41 samples could be related to non-lattice A1-OH groups materials. This is supported by 27A1 MAS NMR measurements, indicating the presence of octahedrally coordinated aluminium in the case of aluminosilicate MCM-41. It is generally accepted [ 16] that these species may be responsible for the Lewis acidity of zeolites. No signals related to bridging hydroxyl groups, so-called Brrnsted signals, could be observed in the spectra. When comparing the results of the FTIR and ~H MAS NMR spectra a concept of Basila et al. [ 17] was applied to explain this inconsistency. In the case of amorphous aluminosilicates, by means of a surface model Basila ruled out the possibility of Brrnsted acidity being caused by the interaction of silanol groups with ammonia or another base molecule, which in turn was adsorbed on a Lewis acid site. On the basis of the result discussed above, certain similarities of MCM-41 to amorphous aluminosilicates could be ascertained. Since 298i MAS NMR spectra exhibit local disorder in the pore walls, a semi-quantitative model for the surface and surface chemistry of a silica-alumina surface described by Peri [ 18] is considered suitable to be applied to MCM-41 materials [10]. With regard to Peri's model, a possible explanation of the higher acidity of MCM-41 compared to amorphous aluminosilicates is the closer distance in the local geometry of Lewis acid site and vicinal silanol groups [ 19]. CATALYTIC TESTING The catalytic performance of MCM-41 and its protonic ion-exchanged form has been presented previously [2]. Protonic ion exchange increases the conversion of the MCM-41 catalyst from 47% before the exchange to about 60 % in the first run, cf. Figure 4. After several runs, the conversion decreased to 40%, as obtained with the sample that had not been ion exchanged, but only calcined. A possible explanation is that protonic ion exchange creates a greater number of so called 73 sites, described by Peri for a silica-alumina surface, which could be removed by the formation of ~ and 13 sites under the influence of hydrocarbons in the catalytic tests. According to Peri, the formation of a Y3 site requires tricoordinated aluminium and, therefore, these sites ought to be highly acidic. The results of the catalytic cracking measurements imply that the investigated mesoporous materials ought to possess a sufficient number of catalytically active acid sites. According to the ~H MAS NMR results these centres could not be bridging OH groups. On account of this fact, two possible mechanisms could be discussed for the catalytic activity of aluminosilicate MCM-41 in hydrocarbon cracking. Considering the conversion and selectivity in cracking
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RUN Figure 4. Catalytic activity ofH-MCM-41 and MCM-41 at constant cat./oil = 3 and 482°C. n-hexadecane, Lewis acid sites could be alone responsible for the cracking activity, as proposed for amorphous aluminosilicate catalysts [18]. Regarding the acidity measurements, the catalytic activity suggests an electron-attracting effect of a Lewis acid centre on a neighbouring Si-OH group, which is regarded as responsible for the enhanced proton donor ability [ 19].
4. CONCLUSIONS The results obtained from physico-chemical characterization methods and catalytic testing in a microactivity reactor indicate that MCM-41 and its protonic form behave like a solid-state acid. 2 9 S i MAS NMR spectra suggest similarities between MCM-41 materials and amorphous aluminosilicates. Furthermore, ammonia desorption curves indicate a broad acid strength distribution of MCM-41 and H-MCM-41, again resembling amorphous aluminosilicates. The addition of ammonia results in the appearance of certain N-H bending vibration bands at 1450 cm], 1625 cm ] and 1660 cm] in FTIR spectra, suggesting the existence of BrOnsted, Lewis sites and weak BrOnsted-type acid sites. 1H MAS NMR spectra do not correspond to this observation: there is no clear evidence of the presence of bridging hydroxyl groups in the investigated MCM-41 materials. It is, therefore, assumed that the ammonium ions identified in the FTIR spectra were formed by the interaction of an ammonia molecule with a silanol group acting as a BrOnsted acid site which is adjacent to a conventional Lewis acid site. Proceeding from these considerations about the nature of acid sites in mesoporous aluminosilicate
430 MCM-41, it seems possible to apply Peri's structure model of amorphous aluminosilicate catalyst to the pore walls of MCM-41 materials. Apart from their broad acid strength distribution, which closely resembles that of amorphous aluminosilicates, the investigated MCM-41 materials possess a greater number of stronger acid sites than amorphous aluminosilicates. The results of the catalytic testing imply that the weak BrOnsted acid sites interact with neighbouring coordinatively unsaturated aluminium species and that consequently synergistically stronger acid sites are formed. It is assumed that this synergism combined with the pore geometry of MCM-41 molecular sieves promotes the high catalytic activity and particular selectivity in cracking hydrocarbons.
5. ACKNOWLEDGEMENTS
The authors gratefully acknowledge financial support from the Commission of the EU within the JOULE II Programme.
REFERENCES
1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. 2. K. Roos, A. Liepold, W. Reschetilowski, R. Schmidt, A. Karlsson and M. St0cker, Stud. Surf. ScL Catal., 94 (1995) 389. 3. A. Corma, V. Fornes, M.T. Navarro and J. Prrez-Pariente, J. Catal., 148 (1994) 569. 4. K.R. Kloetstra and H. van Bekkum, Jr. Chem. Res. Synop., (1995) 26. 5. R. Schmidt, D. Akporiaye, M. St0cker and O.E. Ellestad, Stud. Surf. Sci. Catal., 84 (1994) 61. 6. A. Liepold, K. Roos, W. Reschetilowski, A.P. Esculcas, J. Rocha, A. Philippou and M.W. Anderson, Jr. Phys. Chem., in prep.. 7. C.-Y. Chen, H.-X. Li and M.E. Davis, Microporous Mater, 2 (1993) 17. 8. Luan, Z., Cheng, C.-F., Zhou, W. and Klinowski, J., J. Phys. Chem., 99 (1995) 1018. 9. W. Kolodziejski, A. Corma, M.T. Navarro and J. P&ez Pariente, Solid State Nucl. Magn. Reson., 2 (1993) 253. 10. A. Liepold, K. Roos and W. Reschetilowski, Chem. Eng. Sci., in press. 11 M.R. Basila, T.R. Kantner and K.H. Rhee, Jr. Chem. Phys., 35 (1961) I 151. 12. L.M. Roev, V.N. Filimonov and A.N. Terenin, Optika Spektrosk., 4 (1958) 328. 13 J.B. Uytterhoeven, L.G. Christner and W.K. Hall, J. Phys. Chem., 69 (1965) 2117. 14 J. Sauer and W. Schirmer, Stud. Surf. Sci. Catal., 37 (1987) 323. 15 D. Freude, Stud. Surf. Sci. Catal., 52 (1989) 169. 16 J. Dwyer, Stud. Surf. Sci. Catal., 37 (1987) 333. 17 M.R. Basila, T.R. Kantner and K.H. Rhee, J.Phys. Chem., 68 (1964) 3197. 18 J.B. Peri, Jr. Catal., 41 (1976) 227. 19. G.A. Olah, Angew. Chem., 107 (1995) 1519.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 1997 Elsevier Science B.V.
Solid Mesoporous Base Catalysts comprising of intraporous Cesium Oxide
431
MCM-41
supported
K. R. Kloetstra and H. van Bekkum Delft University of Technology, Laboratory of Organic Chemistry and Catalysis, Julianalaan 136, 2628 BL Delft, The Netherlands
Abstract Mesoporous MCM-41 containing intraporous cesium oxide particles have been prepared by impregnation of aqueous cesium acetate and subsequent calcination, and characterized by r33Cs MAS NMR, CO 2 temperature programmed desorption and nitrogen physisorption. The amount of framework aluminum of the MCM-41 support appeared to be very important regarding the strength of the basic sites. The Michael addition of diethyl malonate to neopentyl glycol diacrylate catalyzed by MCM-41 supported cesium oxide showed a high regioselectivity, in contrast to bulk cesium oxide. keywords" MCM-41; cesium oxide; C O 2 TPD; Michael addition; regioselectivity 1. INTRODUCTION Base catalysis by heterogeneous systems, in particular zeolites, is a rather underexposed area. The use of alkali-exchanged zeolites as solid catalysts is well-known. One of these systems is used commercially in the side-chain alkylation process of toluene with methanol. 1 However the basicity of these zeolites is rather limited. The introduction of cesium oxide particles in zeolite X and Y improves their basic properties to a certain extent. 2'3'4'5 The general method to prepare these materials is by impregnating the zeolites with cesium acetate followed by calcination at high temperature. Catalysis by cesium oxide occluded in zeolites is limited to relatively small molecules. The mesoporous molecular sieve MCM-416 can be of assistance in this field. Recently we reported on the basic and acid properties of alkali-exchanged MCM-41 in several organic reactions. 7 Preliminary results on MCM-41 containing cesium oxide particles in base catalyzed Michael additions looked very promising for further investigations. 7 Here we report on more detailed work on the cesium oxide-MCM-41 systems. The cesium oxide-MCM-41 materials were subjected to several characterization techniques to investigate the accessibility and the strength of the active basic sites and the catalytic activity in fine organic chemical synthesis. 2. EXPERIMENTAL
2.1. Catalyst preparation The starting MCM-41 was synthesized by mixing 16.8 g TMA-SiO 2 solution (TMA/SiO 2 = 0.5; 10 wt% SiO2; tetramethylammonium hydroxide (TMAOH) purchased
432
from Aldrich and the silica source was Cab-osil M5 from Fluka) with 6.60 g of sodium silicate (Aldrich; 27 wt% SiO2). Subsequently 31 g of water and 4.56 g of silica were added to the mixture. Under vigorous stirring 15.5 g of cetyltrimethylammonium bromide (ACROS) in 104 g of water was poured in the mixture. Finally the gel was enriched with an appropriate amount of sodium aluminate (Riedel de Hgen; 54 % A1203 and 4 1 % Na20) to obtain the desired Si/A1 ratio. The gel was stirred for 2 hours at room temperature and subsequently held in an oven at 100 °C statically overnight. The resulting solid was filtered and thoroughly washed, dried at 90 °C under vacuum, and calcined at 540 °C under air for 10 h. The cesium added MCM-41 materials, denoted CsMCM-41A, were prepared by impregnating MCM-41 materials in a concentrated solution of cesium acetate (ACROS). 6 Different loadings were achieved by applying different molarities. The mixtures were shaken overnight at room temperature, centrifuged and subsequently the upper solution was decanted. The solids were dried at 100 °C under vacuum. The sodium and cesium ion-exchanged MCM-41 samples (denoted NaMCM-41E and CsMCM-41E, respectively) were prepared according to reference (7) by using aqueous NaC1 or CsC1, respectively. Reference cesium oxide was purchased from Aldrich.
2.2. Characterization 2.2.1. X-ray powder diffraction Some of the samples were characterized by powder X-ray diffraction on a Philips PW 1840 diffractometer using monochromated CuKa radiation. Patterns were recorded from 1° to 40 ° (20) with a resolution of 0.02 ° and a count time of 1 s at each point. 2.2.2.133Cs NMR spectroscopy Solid-state 133CsMAS NMR spectra were recorded at room temperature on a Varian VXR-400s spectrometer, equipped with a Doty Scientific 5 mm Solids MAS Probe. A resonance frequency of 52.5 MHz, a recycle delay of 1.0 s, acquisition time of 0.1 s and short 2 #s pulses, a spectral width of 100 KHz and a spin rate of 4-7 KHz were applied. The lines were referenced to a 1M solution of CsC1 in water (0 ppm). 2.2.3. CO 2 temperature programmed desorption Temperature programmed desorption (TPD) of carbon dioxide was performed on a Micromeritics 2900 TPD/TPR instrument. First the materials were calcined at 600 °C for 5 h under a helium flow. During cooling down to 110 °C the activated materials were saturated with dry carbon dioxide. Physisorbed carbon dioxide was removed by purching under a helium flow at 110 °C till a stable baseline was monitored (about half an hour). The TPD was performed under a helium flow (10 mL/min) by heating from 110 °C to 600 °C with a heating rate of 10 °C/min. 2.2.4. Nitrogen physisorption Multipoint BET surface areas, pore volumes and pore size distributions of the materials were calculated from N 2 adsorption/desorption isotherms at -196 °C using a Quantochrome Autosorb 6 apparatus. The samples were outgassed for 16 h under vacuum at 350 °C prior to use.
433
2.3. Catalytic testing The substrates used were dried on NaA sieves. A Cs/MCM-41 sample was calcined in situ at 500 °C under vacuum for 6 h. At t=O the activated sample (6 wt% with respect
to the total amount of reactants) was added to a stirred mixture of 10 mmol of diethyl malonate (1.61 g; Aldrich), 5 mmol neopentyl glycol diacrylate (1.06 g; Aldrich) and 15 ml of toluene (ACROS) at room temperature in a nitrogen atmosphere. The course of the reaction was monitored by GC (CP sil 5CB column). 3. RESULTS AND DISCUSSION X-ray powder diffraction was carried out on the original MCM-41 support and the uncalcined cesium acetate impregnated MCM-41 materials. Moisture sensitivity of the cesium oxide-MCM-41 materials leads to wet samples which makes it impossible to measure with a conventional X-ray powder diffractometer. The original MCM-41 showed clearly the dlo 0 spacing at 40 ,~ and the high order reflections. The dlo o spacings of the impregnated CsMCM-41 materials are at the same position, but the high order diffraction peaks decreased going to higher Cs loadings of CsMCM-41A. Solid state 133Cs MAS NMR was performed to investigate the nature of Cs in the CsMCM-41 materials. Fig. 1 shows the 133Cs NMR spectra of Cs ion-exchanged and Cs added MCM-41 samples. Hydrated Cs ion-exchanged MCM-41 (Fig. la) shows a resonance at -14.4 ppm with a very small line width indicating very mobile Cs species. 8'9
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Fig. 1. 133Cs MAS NMR spectra. (a). Hydrated CsMCM-41E. (b) Dehydrated CsMCM41E (asterisks are spinning side bands). (c) Calcined CsMCM-41A (12.4 wt% Cs). (d) Calcined CsMCM-41A (21.9 wt% Cs).
434 Upon dehydration the resonance shifts to -88 ppm (Fig. l b) and line broadening appears as a consequence of a different geometry. The calcined CsMCM-41A samples show a 133Cs resonance shift from 21 ppm via 48 ppm (Fig. lc,d) to 89 ppm (not shown) by increasin~ the Cs content from 12.4 wt% to 32.5 wt% Cs indicating occluded Cs20 particles. ~ A physical mixture of 40 wt% Cs20/SiO 2 shows a resonance at 109 ppm. The resonances at -18.5 ppm and -4.2 ppm are tentatively assigned to partially hydrated cesium species formed by reaction of cesium oxide with moisture. Fully hydrated CsMCM-41A materials by contacting them in air show a resonances around 13 to 18 ppm. When a hydrated CsMCM-41A (21.9 wt% Cs) is calcined again then the 133Cs resonance does not appear at its original position before hydration, but shifts about 13 ppm units downfield to 61 ppm. This indicates a reorganization of the Cs atoms forming probably (larger) cesium oxide clusters. The basic strength and the accessibility of the cesium oxide particles were measured by CO 2 temperature programmed desorption (TPD). 4'5 TPD plots of some Cs containing MCM-41 samples are shown in Fig. 2. By increasing the Cs loading the amount of desorbed CO 2 increases and the maximum peak temperature (Tmax) shifts to higher temperatures compared to Cs ion-exchanged MCM-41. This indicates that stronger basic sites are generated by impregnation with cesium acetate followed by heat treatment. CsMCM-41A with higher Si/A1 ratios showed in general a lower Tmax, indicating weaker basic sites. A remarkable result was found for Na ion-exchanged MCM-41 (NaMCM41E). 7 NaMCM-41E gives a strong and relatively sharp peak in the CO 2 TPD plot. The desorbed amount of CO 2 corresponds to 1 CO 2 per 2 Na atoms. This suggests formation of sodium carbonate species and indicates very loosely bounded Na ions. This correlation
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Tempereture (~<~)
Fig. 2. CO 2 TPD plots of alkali containing MCM-41 with Si/A1 = 12.5.a) NaMCM-41E; b) CsMCM-41E; c) CsMCM-41A (19.2 wt% Cs); d) CsMCM-41A (27.5 wt% Cs).
435
is not found for CsMCM-41E. Fig. 3 shows the correlation between the number of Cs atoms per desorbed CO 2 and the Cs loading. If MCM-41 contains a monolayer of Cs20, then a desorption of 1 CO 2 per 2 Cs atoms is expected. However a desorption of 1 to 2.2 is found for the lower Cs loadings of around 10 wt% (corrected for the Cs ion-exchange capacity) indicating an reasonable homogeneous dispersion of the Cs. Less CO 2 is desorbed at high Cs loadings and is probably a consequence of formation of cesium oxide clusters.
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Fig. 3. Amount of CO 2 (open markers) and BET area (closed markers) as a function of the cesium loading of MCM-41. Triangles denote CsMCM-41 with Si/AI = 21 and circles denote CsMCM-41 with Si/A1 = 25. Table 1 illustrates the nitrogen physisorption data of some calcined Cs/MCM-41 materials. The BET areas and pore volumes decrease with increasing Cs content. At a certain amount of Cs the BET area (also the pore volume) decreases dramatically. The border case at which this happens depends on the Si/A1 ratio of the MCM-41 support. In contrast to CsMCM-41A with 24.2 wt% Cs and a Si/A1 ratio of 25, CsMCM-41A with 27.5 wt% Cs and a Si/A1 ratio of 13 shows no collapse of the mesoporous framework. Duplicate measurements were carded out on some CsMCM-41A samples, which implies evacuation for a second time for 16 h under vacuum at 350 °C. It appeared that all the BET areas decrease significantly (about 70 %). This indicates that the occluded cesium oxide particles reacts with the MCM-41 framework probably forming cesium silicate species. A drastic decrease in the surface areas of Cs added zeolite X and ZSM-5 after calcination has also been reported by other workers. 3 It was suggested that the lower the Si/A1 ratio, the more stable is the zeolite framework. The collapse of the framework is rather a result of breaking of the Si-O-Si bonds than of the Si-O-A1 bonds. 3 More or less the same picture is observed of the Cs added MCM-41 materials. An increase in framework stability is observed for the MCM-41 materials with lower Si/A1 ratios.
436 Table 1. Nitrogen physisorption data of Cs containing MCM-41 samples.
Sample
Cs content (wt%)a
BET surface (mE/g)
Vcub (ml/g)
MCM-41 c CsMCM-41E CsMCM-41A CsMCM-41A CsMCM-41A CsMCM-41A CsMCM-41A
_ 3.7 12.4 21.9 24.2 32.5 45.4
1069 957 510 176 42 70 24
0.819 0.471 0.248 0.094 0.002 0.034 0.004
MCM-41d CsMCM-41A CsMCM-41A CsMCM-41A CsMCM-41A
_ 5.7 9.4 15.3 23.6
1072 803 656 448 214
0.855 0.405 0.322 0.190 0.084
1087 534
0.892 0.304
1002 423 206
0.785 0.188 0.089
MCM-41 e C sMCM-41A MCM-41 e CsMCM-41A CsMCM-41A
_
15.6 _
19.2 27.5
a) Measured by flame AES. b) Cumulative pore volume, c) Si/A1 = 25. d) Si/A1 = 21. e) Si/A1 = 13.
Fig. 3 illustrates the correlation between the BET areas and the cesium atomloading. The BET area decreases linearly with the Cs content which is thus rather a result of the expression of area per weight than a collapse of the framework. However, a horizontal line is observed for loadings higher than 22 wt% Cs. The activity of some of the materials was tested earlier in the base catalyzed Michael addition of diethyl malonate to chalcone, a substrate with an activated double bond. 7 To study the influence of the mesoporous framework of the MCM-41 support on the chemoselective addition on neopentyl glycol diacrylate, possessing two isolated activated double bonds, was used as the probe. The reaction of neopentyl glycol diacrylate with diethyl malonate can lead to a mono-adduct 1 and/or a bis-adduct 2 (eq. 1). Table 1 illustrates the performance of the used catalysts. It can be observed that the Cs ionadded MCM-41 materials are very active in the this reaction. The activity of CsMCM41A increases with higher Cs atom loading and lower Si/A1 ratios. These observations agree with CO 2 TPD data (Tmax). The influence of the MCM-41 support compared to bulk cesium oxide is illustrated by the selectivity ratio of both products. Selectivities for
437 1 of up to 98 % are achieved at 20 °C. Bulk adduct 2.
0 II
CH
Cs20 shows
0 II
I s
c.~cH--c--o-- CH,,--CcH--CH;--O--C--CH =CH~ .
a higher preference for the bis-
/C--'02Et
2 .,,C.co, et
$ 0
OH
0
II I a II c.,-- CH=--c--o-CH,--,~-- CH,--O--C--CH----CH,
H - - C--C02Et I C02Et
c. .-c-co,
co,Et
CH
a
1
(1) 0 II
OH
0 II
I s
o -o-
E,
c. ,
2
.-c-co,
co, et
t
Table 2. Michael addition of diethyl malonate to neopentyl glycol diacrylate over CsMCM-41A samples, a
Catalyst
Cs content (wt%)
Tb (°C)
tc (min)
conversiond (% m/m)
Selectivitye 1:2 (%:%)
CsMCM-41A f
23.6
110
CsMCM-41A g CsMCM-41A g
19.2 27.4
110 20
15 60 60 30 60 10
41 68 36 38 55 100
63:37 69:31 85:15 98:02 58:42 0:100
Cs2 Oh
20
(a) Reaction formulation and conditions see experimental part 2.3. (b) Reaction temperature. (c) Reaction time. (d) Conversion of neopentyl glycol diacrylate at t. (e) Selectivity ratio of the mono-adduct 1 and bis-adduct 2; the overall selectivity to both products is around 85 % - 90 %. (f) Si/A1 = 22. (g) Si/A1 = 13. (h) 8 wt% bulk cesium oxide based on total amount of substrates.
438 3. CONCLUSIONS Cesium oxide loaded MCM-41 materials seem to be promising catalysts for base catalyzed fine chemical reactions. The presence of cesium oxide particles is responsible for the strong basicity of these materials. A disadvantage is the poor regenerability of the material leading to a drastic decrease of the surface area and pore volume. The influence of the mesoporous framework of the MCM-41 support is illustrated by the high regioselectivity in the Michael type addition of diethyl malonate to neopentyl glycol diacrylate. ACKNOWLEDGEMENT We thank J.C. Groen for doing the physisorption measurements, A. Sinnema for the NMR data and NIOK institute, the Dutch School of Catalysis, for financial support. REFERENCES 1. 2. 3. 4. 5. 6.
7. 8. 9.
T. Yashima, K. Sato, T. Hayasaka and W. Hara, J. Catal. 26 (1972) 303. P.E. Hathaway and M.E. Davis, J. Catal. 116 (1989) 263. F.Y.N.Kanuka, H. Tsuji, H. Kita and H. Hattori, Stud. Surf. Sci. Catal. 90 (1994) 349. M. Lasp6ras, H. Cambo, D. Brunel, I. Rodriguez and P. Geneste, Microporous Mater. 1 (1993) 343. J.C. Kim, H-X. Li and M.E. Davis, Microporous Mater. 2 (1994) 413. 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. K.R. Kloetstra and H. van Bekkum, J. Chem. Soc., Chem. Commun. (1995) 1005. P-J. Chu, B.C. Gerstein, J. Nunan and K. Klier, J. Phys. Chem. 91 (1987) 3588. T. Tokuhiro, M. Mattingly, L.E. Iton and M.K. Ahn, J. Phys. Chem. 93 (1989) 5584.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
439
N O N - F R A M E W O R K ALUMINIUM IN HIGHLY DEALUMINATED Y ZEOLITES GENERATED BY STEAMING OR SUBSTITUTION W. Lutz 1, E. L6ttler 2., M. Fechtelkord 3, E. Schreier 4, and R. Bertram 1 1 Institut ftir Angewandte Chemie Berlin-Adlershof, Rudower Chaussee 5, 12484 Berlin, Germany Umwelttechnik - Forschung - GmbH, Rudower Chaussee 5, 12484 Berlin, Germany 3 Institut ftir Mineralogie der Universit~t Hannover, Welfengarten 1, 30167 Hannover, Germany 4 Institut lqir Chemie der Humboldt-Universit~t Berlin, Hessische Strage 1-2, 10115 Berlin, Germany 2 Analytik -
Non-framework aluminium formed in steamed DAY-T or substituted and aluminated DAYSalum zeolites constitutes Al-rich aluminosilicates with 4-fold coordinated aluminium. Its negative charge is compensated by highly condensed Al/O (DAY-T) and Na cations (DAY-S). 1. INTRODUCTION In general the term "non-framework aluminium" characterizes aluminium species which are formed from framework AI during the dealumination of the zeolite. Such species occur in DAY-T zeolites dealuminated in a thermochemical process ( T ) b y steaming of NH4Y [1]. Their nature is repeatedly discussed with respect to the Lewis and Bronsted acidity of dealuminated faujasites. Recently, the influence of non-framework A1 on the stability of DAY-T in water and alkaline solutions has been described [2,3]. The stabilizing effect was explained in terms of blocking the terminal OH groups and the energy rich Si-O-Si bonds at the external surface of crystals and the surface of mesopores. By means of molybdate measurements [4], we detected mono- and dimeric silicate units additionally to the polymeric units of the dealuminated framework in DAY-T zeolites [5]. This observation suggested the occurrence of a newly formed N-rich aluminosilicate phase with four-fold coordinated aluminium. Such negatively charged aluminosilicate layers may protect high-silica zeolites and non-zeolite products against the attack of water and, especially, of hydroxyl ions [4]. Supplementary to Stockenhuber and Lercher [6] who describe an alumina-silica phase, we discuss the non-framework A1 of DAY-T zeolites as an amorphous aluminium aluminosilicate present address: Institut ftir Brennstoffchemie und physikalisch-chemische Verfahrenstechnik der RWTH Aachen, Worringerweg 1, 52074 Aachen, Germany
440 containing highly condensed aluminium cations with octahedrally coordinated A1 as well as alumosilicate anions with tetrahedrally coordinated A1. In DAY-Salum zeolites prepared by substitution of the framework aluminium by silicon in SiC14 vapour [7] and subsequent alumination by sodium aluminate solution [8], non-framework aluminium occurs as a sodium aluminosilicate [2]. In this contribution we describe in detail the nature of aluminosilicates as so-called nonframework A1 in DAY-T and DAY-Salum zeolites by use of chemical techniques in addition to spectroscopic methods. 2. EXPERIMENTAL The DAY-T samples were prepared by steaming NH4Y zeolite of a framework Si/AI ratio = 2.4 at 773 K, 873 K, and 973 K for 5 h at the steam pressure of 1 atm in shallow bed. The treatment produced Si/AI ratios of the frameworks of 3.5, 6.9, and 11.7, respectively. The non-framework A1 extracted DAY-Tex (acid) and DAY-Tex (alkal+acid)
iiiiiiiiDiiiiiii E, 1 iii!
samples were obtained by leaching 1 g DAY-T in 100 ml HC1 at pH = 3 or by combined leaching in 0.25 M KOH for 1 h followed by the acid treatment under the above mentioned conditions. The DAY-S samples (Degussa AG, Hanau) were prepared at 623 K by SiC14
vapour in a technical process similar to that described by Beyer et al. [7]. The substitution of framework aluminium by silicon produced Si/A1 ratios far above 100.
t
The DAY-Salum zeolites were obtained by stirring DAY-S in as-dissolved 0.025 M solutions of sodium aluminate for 1 h at room temperature using a liquid/solid mass ratio of the slurry of 100. No insertion of aluminium into framework position was detectable.
The samples were characterized by I ~ 27A1 and 29Si MASNMR spectroscopy as well as by chemical analysis of the solids and liquids with respect to the A1203 and SiO2 contents. The MASNMR spectra of the hydrated zeolites were measured by means of a Bruker ASX 400 (B 0 = 9.34 T). The DRIFT spectroscopy was used for recording the spectra in the region of OH stretching vibrations. The samples were dehydrated at 673 K and measured at 473 K with a resolution of + 2 cm-1.
441
3. RESULTS AND DISCUSSION 3.1. DAY-T zeolites
Dealumination by steaming For DAY-T dealuminated at 873 K, the 29Si MASNMR spectra (Fig. 1) characterize the successful dealumination by the significant increase of the Si(0A1) peak at -106 ppm and the simultaneous decrease of all other peaks.
c) DAY-Tex(acid)
a) NaY i ~ l
b)
d) DAY-Tcx( •
i
-
i i |
-8o
"'2~io " - ~
"-t~io -a6 ppnl
"-too
"-t:~o
-8o
-t6o
-.t~io
"-t6o "
DAY-Tcx (
.
.-> Figure 1 29Si MASNMR spectra of chemically treated DAY-T (873 K) zeolite
-ao
-too
"-t2o
b) D
21
too
6o"
-
2o "-ao
too" do"
too 60
20 -2o
tOO
Figure 2 27A1MASNMR spectra of chemically treated DAY-T (873 K) zeolite
60
20
-20
/
2o " - i o
ppm
" -t~o
~,/
~
"-)
..... - t 6 o
DAY'Tex( a c i d + a l k a i )
DAY'Tex (acid)
a
-e6
DAY-T
AY' Tex 0dkal+~id)
S too
60 "
=o "-:/o
1oo" 6 o
zo'-2o
442 In the 27A1MASNMR spectra (Fig. 2) in addition to the line at 60 ppm of the tetrahedrally coordinated framework Al, broad lines centred at about 30 ppm and 0 ppm are observed. These additional lines are caused by tetrahedrally (or pentacoordinated) and octahedrally coordinated non-framework aluminium, respectively [9]. The latter probably occurs as highly condensed A1 species acting as counter-cations for the negatively charged aluminosilicate bulk. A cid leaching DA Y- T
The cationic and anionic non-framework AI was extracted up to 90% in 80 h by careful addition of HCI to the stirred batch. From 1 g DAY-T (873 K) 258 mg AI20 3 were removed. Thereby, the aluminosilicate bulk was hydrolysed and the highly condensed AI cations were decomposed step by step. The finally formed [Al(H20)6] 3+ and weakly condensed AI cations being stable at a pH < 3.5 [ 10] were mainly isolated with the filtrate. Since the acidity of the batch does not fall below pH = 3, the 29Si MASNMR spectrum of DAY-Tex (acid) (Fig. 1) is nearly unchanged with respect to the Si and AI content of the framework. Due to the condensation of silica released during the decomposition of the aluminosilicate bulk, the spectrum is worse dissolved. The 27A1 MASNMR spectrum of DAYTex (acid) (Fig. 2) shows a decreased content of tetrahedraUy coordinated non-framework AI due to the hydrolysis of the aluminium aluminosilicate. The presence of aluminium cations compensating the negative charge of the zeolite framework is demonstrated by the peak at 0 ppm, typical for octahedrally coordinated AI. Alkaline and acid leached DA Y-T
In order to separate the highly condensed AI cations from the aluminosilicate bulk, we treated the DAY-T zeolite (873 K) with alkaline and acid solution subsequently. Alkaline step: The 27A1MASNMR spectrum ofDAY-Tex (alkal) (Fig. 2) shows a lack of octahedral A1 while the tetrahedral A1 in the aluminosilicate bulk being relative stable in the KOH solution [2,3,11 ] is decreased to a smaller extent. However, the filtrate of the DAY-Tex (alkal) batch contained only low amounts AI203 (< 30 mg) being not adequate to the dissolved amount of the highly condensed AI cations of DAY-T (873 K) (~ 135 mg Al203 = 40% Al203 of the non-framework AI). Furthermore, since the solution contained also SiO2 and the 29Si MASNMR spectrum (Fig. 1) showed a decrease of the Si(0Al) peak, the formation and partial dissolution of a new potassium aluminosilicate generated from the dissolved highly condensed Al cations and silicon removed from the framework has been assumed. In order to understand this phenomenon in more detail, we compared the batch process with a stepwise treatment. We leached 1 g DAY-T (873 K) in 100 ml 0.25 M KOH for 1 h under
443 the same conditions as given above but in ten steps. The sample was stirred in 10 ml flesh KOH for 6 min. in each case. The filtrates were individually analysed. Table 1 gives the AI20 3 and SiO 2 concentrations of the batch and the sum of the stepwise leaching. In addition to the DAY-T (873 K), the data for zeolites dealuminated at 773 K and 973 K are presented, too. Table 1 Concentrations of AI203 and SiO2 in the filtrates from batch and stepwise leached DAY-T mg
Alkaline leaching of DAY-T obtained at different temperature
in 100
in batch
ml KOH
stepwise
773K
873K
973K
773K
873K
19
27
13
67
125
30
23
34
139
22
38
113
1.95
2.14
18.3
0.56
0.61
6.5
Al203 SiO2 SiO2/Al2Oa
I
973K
The filtrates of the batchwise treated samples (773 K and 873 K) contained actually SiO 2 and AI20 3 in a ratio of about 2 what is typical for Al-rich aluminosilicate gels [12]. According to SiO2/AI20 3 ratios of 0.56 and 0.61, the filtrates of the stepwise treatment contained an excess of A1203. This hints at a higher dissolution rate of the highly condensed AI cations compared with that of the framework silicon (Fig. 3). With the permanent removal of the aluminate the driving force for a stronger removal of silicon from the zeolite framework, consisting in the formation of the hydrothermally stable potassium aluminosilicate gel, does not exist. With increasing dealumination and, thus, with rising alkaline solubility of the strongly disturbed framework [2,3], the dissolution rates are inversely related. From DAY-T (973 K, Si/AI = 11.7), a great amount of silicon is removed. The formed silicate binds nearly the whole amount of the dissolved former highly condensed AI cations as potassium aluminosilicate. The values in Table 1 demonstrate that this effect is still more pronounced for the batch process. 30 nag/ 100ml DAY-T (773 K) 25
30 mg/ 100ml 25.
20
20-
• sio 2
-A1203
30 nag/ 100ml 30 DAY-T (973 K) 25
DAY-T (873 K)
20
1510. . . . .
10 !
P •
O
•
•
•
• •
•
-,~" |
•
•
2 4 6 8 10 steps of alkalinetreatment
0
0
•
•
•
•
•
•
•
•
•
•
. 2 4 6 8 10 steps of alkalinetreatment
O 0
2 4 6 8 10 steps of alkalinetreatment
Figure 3 Concentrations ofAl203 and SiO2 with respect to the steps of alkaline leaching of DAY-T
444 According to our observations, the discussed ion exchange of the "cationic aluminium oxide" species by a treatment with aqueous solution of ammonium hydroxide, reported by Stockenhuber [6], seems to be a result from the dissolution of the highly condensed AI cations and their replacement by ammonium ions. Furthermore, we believe that the observed SiO2removal from the zeolite framework is the actual explanation for the "reinsertion of nonframework A1 into the framework of steamed zeolites by alkaline treatment" discussed e.g. by Zhang [ 13]. Acid step: By the subsequent acid leaching of DAY-Tex (alkal) (873 K) also the AI of the aluminosilicate bulk is dissolved. The amount of AI20 3 removed by the combined leaching of DAY-T (265 mg/g ) corresponds to the mass (258 rag/g) set free on the exclusive acid route. The 29Si MASNMR spectrum of the DAY-Tex (alkal+acid) sample (Fig. 1) is changed due to the precipitation of additional silica resulting from the decomposition of the aluminosilicate bulk. It can not be excluded that a further dealumination of the strongly disturbed zeolite framework took place. In analogy to the acid route, the 27A1 MASNMR spectrum of DAYTex (alkal+acid) shows a decreased content of tetrahedrally coordinated AI in the aluminosilicate bulk and the octahedrally coordinated A1 acting as counter-cations.
1R spectroscopic investigations According to a suggestion of Falabella Sousa-
3638
3550
_
8
Aguiar [14] who reported on condensed and non-condensed aluminium species as components of the non-
37,4'2
framework AI (in analogy to our highly condensed A1 cations and the aluminosilicate bulk), we tried to distinguish the octahedral and tetrahedral non-framework AI species by measurement of the OH stretching vibration bands. The parent HY zeolite shows three bands which can
I
I
......
=x(acid)
be attributed to terminal SiOH groups (3742 cm-1) and bridged SiOHAI groups (3638 and 3550 cm-1) (Fig. 4). After the dealumination at 873 K, the bands of the bridged OH groups disappear and a broad one appears at 3618 cm-1. An explanation could be the
4000
t
i
wavenum ber/cm
I 320O
I
-
I
!
Fig. 4 IR spectra of DAY-T zeolites
shitt of the band at 3638 cm-1 to lower wavenumbers due to an interaction of these OH groups with nonframework AI [6]. The additional band of DAY-T at
445 3688 cm-1 can also be assigned to OH groups of non-framework AI species. After decomposition of the aluminosilicate bulk by the acid leaching the bridged OH groups appear again and in addition to the band at 3688 cm"1 a further one at 3670 cm-1 is visible characterizing the interaction of the remaining aluminium cations with the framework of DAYTex (acid). After the alkaline leaching of DAY-T the bands of OH groups almost disappear because of their deprotonation and dissolving the highly condensed AI cations. The strong band at 3694 cm -1 can be assigned to OH at potassium cations. By the subsequent acid leaching, the aluminosilicate bulk is decomposed and lower condensed cations (e.g. [AI(H20)6] 3+) are formed. Therefore, the spectra of both DAY-Tex (acid) and DAY-Tex (alkal+acid) are very similar.
3.2. DAY-Salum zeolites As opposed to the steaming of NH4Y, where the dealumination of the framework is limited to Si/AI ratios of about 30 [3 ], the substitution of aluminium by silicon in NaY by SiC14 generates Si/AI ratios far above 100. The aim of the subsequent alumination of DAY-S crystals is the generation of non-framework AI as surface layer in order to stabilize such water and alkaline sensitive microporous adsorbents [2,8]. Sodium aluminosilicate as non-framework A1 is formed by silicate removed from the crystal and aluminate offered by the sodium aluminate solution in a consecutive reaction. Since thereby no detectable amount of aluminium is inserted into the framework as shown by the 29Si MASNMR spectra ofDAY-S (a) and DAY-Salum (b) (Fig. 5), DAY-Salum keeps its hydrophobicity. By means of molybdate measurements, monoand dimeric silicate units were found for DAY-Salum [3]. This hints at a high content of aluminium inside the formed non-framework AI phase. The 27A1 MASNMR spectra (Fig. 6) show tetrahedral framework A1 ofDAY-S (a) and another type of tetrahedral AI as shoulder at
(a)
446 the signal of the framework AI phase in DAY-Salum (b). The octahedral aluminium in (b) is attached to alumina precipitated additionally to the formed aluminosilicate due to the decrease of the alkalinity of the sodium aluminate solution during the alumination process. 4. CONCLUSIONS Under the hydrothermal conditions of steaming NH4Y zeolite, a partial phase transformation takes place. Aluminium removed from the framework forms with silicon an X-ray amorphous aluminium aluminosilicate onto the external crystal surface and surface of mesopores. Due to a deficit of the original sodium ions, the highly condensed AI cations compensate the negative charge of the framework and the aluminosilicate bulk. The cationic and anionic "non-flamework AI species" of DAY-T are detectable by 27A1MASNMR spectroscopy, IR spectroscopic measurements within the range of the OH vibration bands, and by chemical methods, e.g. molybdate measurements and chemical analysis after the alkaline and acid leaching of steamed zeolites. Aluminosilicates may be generated as non-framework AI species also by the alumination of highly dealuminated DAY-S zeolites obtained by aluminium substitution. The treatment of DAY-S with aqueous solution of sodium aluminate yields surface modified zeolites without realumination of the framework. ACKNOWLEDGEMENT The authors are grateful to the Degussa AG, Hanau for providing the DAY-S zeolites. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11 12. 13.
C.V. Mc Daniel et al., Molecular Sieves, Soc. Chem:Ind., London, 1968, p. 168 W. Lutz, B. Zibrowius and E. LOffler, Stud. Surf. Sci. & Catal. 84, Elsevier, 1994, p 1005 W. Lutz, E. LOftier and B. Zibrowius, Stud. Surf. Sci. & Catal. 97, Elsevier, 1995, p. 327 R.K. Iler, The Chemistry of Silica, Wiley, New York, 1979 W. Lutz, Cryst. Res. Technol., 25 (1990) 921 M. Stockenhuber and J.A. Lercher, Microporous Materials 3 (1995) 457 H. Beyer et al., J. Chem. Soc., Faraday Trans. I, 81 (1985) 2889 W. Lutz, E. L6ffler and B. Zibrowius, ZEOLITES 13 (1993) 685 H. Pfeifer and H. Ernst, Annual Reports on NMR Spectroscopy, 28 (1994) 91 J.W. Aldtt et al., J. Chem. Soc., Dalton Trans., (1972) 604 S.P. Zdanov and E.N. Egorova, Chimija Ceolitov, Izd. Nauka, Leningrad, 1968 S.P. Zdanov et al., Sinteticeskie Ceolity, Izd. Chimija, Moskva, 1981 Z. Zhang, X. Liu, Y. Xu and R. Xu, ZEOLITES 11 (1991) 232 14. E. Falabella Sousa-Aguiar, personal communication
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
447
Spectroscopic Studies of a Magnesium Substituted Microporous Aluminophosphate DAF-1. Stuart. J. Thomson and Russell. F. Howe Department of Physical Chemistry, University of New South Wales, Sydney, Australia, 2052 A magnesium containing aluminophosphate DAF-1, was characterised using 129Xe NMR, XPS, and MAS NMR of the nuclei 27Al, 31p, and 13C. Single crystals of DAF-1 (up to 100~tm in diameter) were characterised using SEM and WDS, whilst Raman and IR microspectroscopies were used to study the organic material within single crystals ofDAF-1. I. INTRODUCTION The substitution of divalent elements into microporous aluminophosphates has attracted interest because of the resulting ion exchange and catalytic properties of the socalled MeAPO materials [1,2]. Thomas et al. recently described a new magnesium substituted ALPO named DAF-1 with an unusual pore structure [3]. The framework structure of DAF-1 contains two parallel channel systems with approximately circular apertures of ca. 6.1 and 7.5A respectively. The smaller diameter pores are approximately cylindrical, while the larger diameter pores contain supercages 16.3A in diameter. The two parallel pore systems are interconnected laterally by 8-ring pores with a minimum free diameter of 3.9A. The smaller pores are also interconnected laterally with each other (but not with the larger pores) by oval 10-ring apertures. This pore structure is unique in ALPO or MeAPO materials synthesized to date. The novelty of the pore structure and the dearth of characterization studies on DAF-1 in the literature, prompted us to undertake a detailed spectroscopic investigation of the material. We were also intrigued by the possibility of conducting single crystal experiments with DAF-1, since large non-twinned single crystals can be grown and studied by infrared and Raman microspectroscopy. The unidirectional pore structure of DAF-1 should allow polarization studies of the orientation of molecules adsorbed in the pores in such single crystal experiments. This paper presents results of the initial experiments undertaken; further work will be reported subsequently. 2. EXPERIMENTAL Synthesis of DAF-1 was conducted using slightly modified conditions to those previously reported [3], with the aim of producing large single crystals. Typically, a gel prepared for hydrothermal synthesis had the composition ; 0.9 A1203 : P205 : 0.2 MgO : 0.2 HOAc : R : 20 < 1-120< 40, where R represents the template, decamethonium hydroxide (DMOH). The reagents used were phosphoric acid (Aldrich, 85 wt %), boehmite "CATAPAL B" (Vista), magnesium acetate (Aldrich) and DMOH. The template was prepared by adding decamethonium bromide (Aldrich) to an excess of Ag20 in distilled water. The resulting mixture was filtered and rotary evaporated to obtain a concentrated solution.
448 Mixing of the synthesis gel involved adding phosphoric acid dropwise, with constant stirring, to a concentrated slurry of boehmite, magnesium acetate, and water. The prepared template was then added, and the mixture heated in a Teflon lined autoclave at temperatures of 170-190°C for 48 hours. Once completed, the product was triply decanted and filtered to obtain only the larger portion of DAF-1 crystals. Calcination was performed by flowing high purity oxygen, at a temperature of 600°C, over DAF-1 for 30 hours. As the calcined material is unstable in the presence of water, methylcyclohexane was adsorbed following calcination to stabilise the structure. X-ray powder diffraction patterns were obtained with a Siemens 122 diffractometer using Cu K~ radiation. Surface area (SA) measurements were conducted using a Quantasorb surface area unit. Nitrogen was used as the probe molecule and surface areas were calculated using the Langmuir isotherm model. The crystal morphology and atomic composition of DAF-I single crystals, were studied by Scanning Electron Microscopy and Wavelength Dispersive Spectrometry (WDS) respectively. SEM results were obtained using a Cambridge $360 instrument and WDS measurements were obtained using an Cameca SX50 spectrometer. Samples characterised by WDS were embedded in an epoxy matrix and mechanically polished until smooth crystal cross sections were obtained. Analysis was performed by measuring atomic compositions at a number of pre-defined positions on a crystal cross section. X-ray Photoelectron Spectroscopy measurements of pressed pellets and single crystals of DAF-1 used respectively a Perkin Elmer 5500 (Mg Ko0 and a Fisons EscaLab (monochromatized AI Ko0 instrument equipped for micro spot analysis (50 micron spot size). Solid state MAS NMR experiments were recorded on a Bruker MSL 300 spectrometer. The 27A1and 31p nuclei were studied at 78.188 and 121.44 Mhz respectively, using a 4mm probe and magic angle spin rate of 10kHz, whilst 13C used a 7mm probe and a magic angle spin rate of 3kHz. The 27A1, 31p, and ~3C chemical shifts were referenced to kaolin (8=-2.5 ppm), ammonium dihydrogen phosphate (8= l ppm) and adamantane (8 =38.7 ppm). A Bruker 300ACP spectrometer was used to study the 129Xe NMR of Xe adsorbed on calcined DAF-1. All 129Xe measurements were performed under static conditions and all spectra were referenced to pure Xe gas. Measurements were conducted by evacuating the sample overnight at 150°C to remove the methylcyclohexane and any adsorbed water. Once cooled, samples were exposed to various Xe pressures between 90 and 650 torr and the NMR spectrum recorded at each pressure. Xenon adsorption isotherms were measured in separate experiments using a vacuum microbalance. Infrared measurements were conducted on both calcined and uncalcined crystals of DAF-1 using a Spectra-Tech microscope platform attached to a Bomem MB Series spectrometer. Samples were placed in an in situ cell which enabled heating and evacuation to remove adsorbed water. Polarisation studies were undertaken using a ZnSe wire grid, as in earlier studies[4,5,8]. Raman spectra were obtained using a Renishaw microscope equipped with an Argon ion laser (~,=514nm) source. Raman spectra were measured in air, and polarisation measurements utilized a polarising filter and half wave plate.
449 3. RESULTS AND DISCUSSION The best synthesis conditions, found to enhance both crystal size and morphology, were obtained when the synthesis gel contained as little water as possible and was heated at 170°C for a period of 48 hours. Figure 1 shows an SEM photograph of a batch produced using this method. The larger crystals form as hexagonal cylinders with good morphology, diameters up to 100 microns, and few exhibit evidence of twinning or intergrowths. Calcination of DAF-1 batches containing large crystals, involved slowly heating the material at 1-2 C mln to mlmmlze blockage of the channel structure due to coking of the template. Samples were heated at 1-2 C min- to 150 C and held at this temperature for 3 hours to remove adsorbed water. The temperature was then ramped at 1-2°C min -l until a temperature of 600°C was reached. This temperature was held for 30 hours and the material cooled at 2°C min -~ to minimise crystal degradation through thermal shock. Powder XRD measurements of the calcined material stabilised with methylcyclohexane, showed the material was still crystalline DAF-I. Nitrogen adsorption measurements, using a Langmuir isotherm model, yielded a surface area of 814 m 2 g-l. The total internal surface area estimated from the crystal structure was --1200m 2 g-I. The ratio of observed to calculated surface areas is similar to that reported by Tapp [6] for ALPO-5 and ALPO-11, and gives confidence that template removal was complete without pore blockage. •
o
•
-1
•
•
•
o
1
o
The homogeneity of DAF-I single crystals was confirmed by the use of WDS. Figure 2 shows WDS scans for AI, P, Mg, and O across a cleaved single crystal of calcined DAF-I. Identical results were obtained with the as-synthesized material (i.e. there is no change in composition or migration of material during calcination). The average atomic composition of the crystal calculated from the WDS data was 0.53P, 0.37A!, and 0.10Mg. The starting gel composition corresponds to an atomic composition of 0.5P, 0.45A1 and 0.05Mg. However the unit cell composition reported for DAF-I by Thomas et al. is MgI4.5AI51.5P660264.7R.40H20 [3], which corresponds reasonably well with the WDS data. Initial XPS measurements were carried out on pressed wafers of the DAF-1 crystals. These showed that the average surface composition measured by XPS agreed moderately well with the WDS results (e.g. surface composition of 0.48P, 0.43A1 and 0.09Mg), suggesting that the outer surface composition of the crystals is not very different from that of the interior. (We note that XPS samples the outer 3-5nm of the crystals, whereas the spatial resolution in the WDS experiments was 3 microns).Attempts to confirm this conclusion by depth profiling through argon ion etching of the surface did not succeed; it was found in all cases that argon ion etching caused selective sputtering of phosphorus from the surface and could not therefore give a true indication of the sub surface composition. The XPS binding energies, referenced to Ols at 532.4 as described by Borade and Clearfield [7], were identical for as synthesized and calcined DAF-I" AI 2p = 74.9eV, P 2p = 134.2eV and Mg 2p = 50.5 eV.
450
Figure 1 "SEM ofDAF-1 single crystals.
80
r
60 40 20
v
r
0
0
-
Ai
=
P
--v---
0
v
="
I
10
--
n
I -~
20
,
±
I
30
I
40
Distance p m Figure 2 • WDS plot obtain from scanning a single crystal cross section ofDAF-1.
451 XPS spectra were also measured from single crystals of DAF-1 using the small area analysis capability of the Fisons Escalab system. Such measurements should, in principle, give a more accurate surface composition than those on a polycrystalline sample without interference from amorphous impurities or other phases (and have been previously applied to determine the surface composition of the single crystals of ZSM-5 [8]). Signal to noise in the initial data from single crystals of DAF-1 has been insufficient to allow quantification of the Mg, but the Al and P surface concentrations agree with those determined from polycrystalline samples. Solid state 27A1 and 31p MAS NMR of the synthesised material showed the aluminium and phosphorous to be tetrahedrally coordinated. The 27A1 spectrum exhibited a single peak at /5 =36.8 ppm, with no contribution from octahedral aluminium. Figure 3 shows the 31p MAS NMR spectrum of as synthesized DAF-1 (proton decoupled). This contains two major peaks at -30.8 and-24.5 ppm, plus small additional shoulders at lower field. These fall in the chemical shift range expected for tetrahedral phosphorus in ALPO structures. Very similar 31p spectra are observed for other magnesium substituted MAPO materials e.g. MAPO-11, -31 and -24.4 pprn, MAPO36, -28 and -22.9 ppm, and MAPO-39, ,I 31.6 and -24.5 ppm. [9]. One O0 10 29 20 68 30 86 -41.15 interpretation of the two peaks is that they are due to P atoms having Figure 3 : MAS NMR spectrum of 31p showing the 4 component peaks fitted to the respectively, 4 and 3 aluminium next nearest neighbours (i.e. P(OAI)4 and [spectrum and used to derive the experimental P(OAI)3(OMg ) ). This interpretation, [values described in the text. which is analagous to the assignment of the multiple 29Si NMR signals for aluminosilicate zeolites, was first applied to MeAPO materials by Barrie and Klinowski [ 10], and more recently to a range of different MeAPO s by Akolekar and Howe [9]. An alternative possibility proposed by Ono et al.[11] is that the lower field peak is due to P which is. hydroxylated or interacting with adsorbed water. We have taken the first interpretation and used it to estimate the lattice composition of DAF-1 from the NMR spectra using the method previously described [12]. For this purpose, the observed spectrum was fired with 4 overlapping Gaussian components as shown, assigned to P(OAI)4 , P(OAI)3(OMg), P(OAI)2(OMg)2 ' and P(OAI)(OMg)3. From the integrated intensities of the 4 components the composition of the lattice was calculated to be P:AI = 1.3, P:Mg = 4.2 and Mg : (Mg+P+AI) = 0.12, in good agreement with the unit cell composition and the WDS results, considering the uncertainties in deconvolution. We have
452 also taken the composition of the sample deduced by WDS, and calculated the relative intensities of the 4 31p NMR peaks assuming Mg is randomly distributed through the DAF-1 lattice (the intensity of the P(OMg)4 peak is zero at this Mg loading). An acceptable agreement with the observed relative intensities was achieved, implying that there is no zoning of magnesium in the DAF-1 lattice. The consistency between the NMR data and the WDS analyses of single crystals strongly supports the first interpretation of the 31p spectra.
Using 13CMAS NMP~ the spectrum of as-synthesised DAF-1 was compared to that of solid decamethonium bromide. The only major difference between the 13C spectra of the decamethonium template in the as synthesized DAF-1 and the solid salt was in the 20-3 5ppm region characteristic of the methylene groups in the decamethonium chain. In the salt, this region contains an overlapping triplet of three signals at 31, 29 and 23 ppm. For the template in DAF-1, these are shit~ed to higher field (24, 23, and 20 ppm), presumably because the intermolecular interactions between methylene chains in the solid are no longer present for isolated template ions in the pores of DAF-1. The ~3C signal from methyl groups attached to the positively charged nitrogen head groups in decamethonium shitied ca. l ppm to lower field in the DAF-1 pores, perhaps due to interaction with the negatively charged MAPO lattice. To further probe the channel system of DAF-1, 129Xe NMR was .... used. Surprisingly, the Xe spectra at all coverages of Xe showed only a single ....... ~-~......... | [ narrow peak (Figure 4). The chemical ~~_-i~~_-~._~_i ~ ~ ~ ~~'_~~_i _J[ shitt of this peak is plotted versus the . . . . . . . . . . . . amount of adsorbed xenon in Figure 5; _-'_ ~ / ~ i ~ - ~ - i - i "~i-~~1 the curve is linear, and extrapolates to an intercept at zero coverage of I 76ppm. The absence of any curvature in this plot indicates (in accord with the results described above) that there are no divalent Mg 2+ cations within the 110 100 90 80 70 60 50 (ppm) pore system [13]. The Xe chemical shitt at zero coverage will be Figure 4 " 129Xe NMR spectra at various determined by electrostatic field effects pressures; 600, 530, 450, 300, 200, and 101 and by interaction with the pore walls. Torr shown from top to bottom respectively. In the absence of electrostatic field effects, the chemical shift will increase with decreasing pore size as pore wall interactions increase.The chemical shitt at zero coverage determined for DAF-1 is much higher than that expected for xenon adsorbed in a relatively large pore molecular sieve containing no divalent cations. For example, the value for ALPO-5 (12 ring pores) is given by Chen [14] as 56ppm. Xenon adsorbed in the large pore system of DAF-1 (containing supercages linked by 12 ring pores) would be expected to show a smaller xenon chemical shitt than in ALPO-5, while xenon adsorbed in the smaller pore system of DAF-1 (uniform 12 tings pores)may resemble more closely that in ALPO-5. Furthermore, since the crystal size is too large to allow intercrystalline diffusion on the NMR time scale, two distinct Xe signals from the two different pore systems may be anticipated.
453 The observation of a single line only at a higher chemical shift than expected for xenon in either pore system, together with nitrogen and xenon adsorption measurements indicating that the entire pore system in the calcined DAF-1 is accessible to xenon, means we suggest that xenon is able to diffuse through the oval 8-ring aperture connecting the two pore systems. The minimum free diameter of this aperture is quoted by Thomas et al. to be 3.9 A [3], smaller than the free kinetic diameter of xenon. This may not however be a sufficient barrier to inhibit diffusion over the short distance involved. The unusually high chemical shit~ of Xe in DAF-1 can be explained in this way; the observed chemical shit~ of xenon is averaged over the different environments sampled by xenon as it diffuses through the entire pore system, but the interaction of xenon with the 8-ring aperture dominates the chemical shift. We have measured spectra at lower temperatures to see if the diffusion can be inhibited, but only a single line could be observed at temperatures down to 173K (the chemical shift increased by 25ppm between 298K and 173K).
95.
~
90-
~¢/j
85-
~
80-
×e atoms per unit cell
Figure 5 : Plot of chemical shift versus number of Xe atoms per unit cell.
Raman and IR microspectroscopy has also been carried out on the DAF-1 crystals as synthesized. The objective was to determine if the decamethonium template adopted any preferred orientation within the pore system. Polarized infrared and raman spectra were measured of the template in single crystals oriented with respect to the polarization axes, but no differences were observed as the polarization was varied. We interpret this to mean that the decamethonium cations do not adopt a linear configuration along the pore axes but are randomly oriented in a folded configuration. The process of template decomposition during calcination could be monitored by IR microspectroscopy, but the onset of intense laser induced fluorescence in calcined crystals prevented their characterization by Raman spectroscopy.
454 4. CONCLUSIONS Large single crystals of the MAPO DAF-I have been grown and characterized. Substitution of magnesium for aluminium in the structure has been confirmed by microanalysis on single crystals and by MAS NMR on polycrystalline samples. The unusual pore structure of this material has produced interesting behaviour of adsorbed xenon; further study of host: guest interactions in DAF-1 is certainly warranted. 5. ACKNOWLEDGMENTS The authors would like to acknowledge the assistance provided by Prof. Ryong Ryoo, Korean Advanced Institute of Science and Technology, in obtaining the 129XeNMR data and Dr. Jim Hook, UNSW NMR Facility, for assistance with the MASNMR. Thanks must also go to Ms. Narelle Brack, UNSW Surface Science Department, for help in obtaining XPS data, and to Professor Jack Lunsford, Texas A and M University, for allowing use of the Perkin Elmer ESCA instrument in his laboratory. Financial support from the Australian Research Council is also acknowledged. 6. REFERENCES
1 Wilson, S. T., Lok, B. M., and Flanigen, E. M., Eur. Pat.O043562 1981. 2 Natarajan, S., Wright, P. A., and Thomas, J. M., J. Chem. Soc. Chem. Commun., p633635, 1993. 3 Wright, P. A., Jones, R. H., Natarajan, S., Bell, R. G., Chen, J., Hursthouse, M., B, Thomas, J. M., J. Chem. Soc. Chem. Commun., p 1861-1863, 1993. 4 Schuth, F., Demuth D., Zibrowius, B., Kornatowski, J., and Finger G., J. Am. Chem. Soc, p 1090, 1994. 5 Thomson S., Howe R., First Australian Vibrational Conference Procedings; 1994. 6 Tapp, N. J., PhD thesis 1988. 7 Borade, R. B. and Clearfield, A.; Applied Catal. A, 80(1), p59, 1992 8 Howe, R., in Surface Science, Principles and Applications (Howe, R., Lamb, R. N., and Wandelt, K., eds) Springer Verlag 1993, p242. 9 Akolekar, D. and Howe, R., J. Chem. Soc. Faraday. Trans., submitted for publication. 10 Barrie, P. J., and Klinowski, J., J. Phys. Chem., 93, p5972, 1989. 11 Nakashiro, K., Ono, Y., Nakata, S. and Morimura, Y., Zeolites, 13, p561, 1993. 12 Barrie, P. J., and Klinowski, J., J. Phys. Chem., 93, p5972, 1993 13 Barrie, P. J., and Klinowski, J., Progress in NMR spectroscopy, 24, p91, 1992 14 Chen, Q. J., Spinguel-Huet, M. A., and Fraissard, J., Chem. Phys. Lett., 159, p 117, 1989
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
455
M A S N M R C h e m i c a l Shifts and Structure in F r a m e w o r k s
M.T. Weller, S.E. Dann, G.M. Johnson and P.J. Mead Department of Chemistry, University of Southampton, Highfield, Southampton, Hants, SO17 IBJ. United Kingdom
The study of a wide range of members of the sodalite family Ms[TT'O4]6.X2 where T, T' = B, Be, AI, Si, Ga, Ge using 29Si, 'TAI, 7~Ga and ~IB MASNMR and Rietveld refinement of powder diffraction data has provided definitive correlations between structural features and chemical shift parameter. The effects of changing T-O-T' bond angle and the electrostatic effect of T' on the NMR spectrum of T are resolved.
I. INTRODUCTION Most framework materials are aluminosilicates and aluminophosphates with structures formed from linked SiO4, PO4 and AIO4 tetrahedra. However, frameworks may contain other tetrahedral oxoanions specifically BO4, GaO4, G¢O4, ZnO4, BeO4, CoO4 and AsO4. Interest in these non-aluminosilicate framework materials is growing rapidly with potential applications resulting from the control of channel size, the nature of catalytic sites within the pores, the modification of intercage interactions as the size of the TO4 unit changes, the control of acidity and, for transition metal ions, the potential of generating redox sites within the framework. MASNMR has been a central technique in the charaeterisation of framework materials. Numerous studies have been published and examples of the application can be taken from the determination of silicon and aluminium distributions in zeolites [1], the assignment of aluminium sites in aluminophosphates [2] and work on gallosilicates [3]. More recently the study of sodalites using specifically MASNMR has been highlighted by Newsam [4] and Engelhardt [5]. The sodalites are a well known class of framework of the general formula Ms[TO2112.X2, where M represents a mono or divalent cation, T a tetrahedron forming species such as Si or AI and X is a monovalent or divalent anion. They consist solely of 13cages, formed from ( Be, B, AI, Si, P, Ga, Zn, Ge or As )04 tetrahedra and the anions incorporated into in Ms[SiAIO4]6.X n (n=l,2) have included CI, Br, I, SCN, NO 2, NO3, 804, 8203, PO4, VO4, MnO4, MOO4, CrO4, WO4, CIO4, CIO3, S, C204, CO3, OH. Hence for each framework type a range of materials exists with different cations and anions encapsulated in the B cages. With different sized cations and anions the framework geometry changes markedly in order to co-ordinate well to these ions. The sodalite family therefore offers an ideal model system for studying the effects of framework geometry on the NMR spectra of the various framework ions. We have synthesised more than one hundred materials of the sodalite family with
456 frameworks containing silicon, aluminium, gallium, germanium, boron and beryllium, e.g. Ms[SiAIO4]6.X2, Ms[SiGaO4]6.X 2 Ms[GeAIO4]6.X2 Ms[GeGaO4]6.X 2, Ms[SiBeO4]6.X2, Ms[GeBeO4]6.X2 and Ms[AIO2],2.X2, M,[BO2],2.X2 with a variety of different anions and cations trapped within the framework. The changes in the framework geometry, in terms of T-O-T bond angles and TO4 tetrahedron deformations which occur in order to co-ordinate effectively to the different sized entrapped species, have been refined by us in order to delineate these structural parameters accurately. MASNMR, specifically Figure 1. A sodalite cage viewed through a six-ring and for the nuclei 29Si, 27A1, liB and showing the T-O-T angle, 3t. 71Ga, has also been used to study these sodalite derivatives with respect to the chemical shift. In this paper the chemical shift has been correlated to the structural information in order to understand the separate roles which framework geometry, bond lengths and electrostatic effects have on the chemical shift. This data from a model system may then be used to define the distributions of species such as gallium doped into more complex zeolitic structures.
2. EXPERIMENTAL A wide range of sodalites containing the species Si, AI, Ga, Ge and Be, with the general formulae as detailed above, have been synthesised. Full details of the preparations have been or will be published elsewhere [6,7]. The structures of these materials were studied using refinement of either powder x-ray or neutron diffraction data. Powder x-ray diffraction data were collected on a Siemens D5000 diffractometer typically over a period of 15 hours with a step size of 0.020 over the angular range 15-120o. Powder neutron diffraction data were collected on the POLARIS or LAD diffractometers on the time-of-flight source at ISIS, Rutherford-Appleton Laboratory or on D2B at the ILL, Grenoble, typically over a period of 4 hours. Data were refined using the GSAS package [8]. Full details of these refinements and the structural results have been or will be published elsewhere. Information available from these structural techniques which influences the NMR spectrum are the bond angles around the tetrahedral species, i.e. the T-O-T and O-T-O angles and the T-O distances; each sodalite contains a single crystallographically distinct type of T or T' atom. Note that materials containing only one type of tetrahedral atom e.g. Zns[BO2]~:.S: crystallise in the space group 1-43 m whilst those with two tetrahedral atom types and a perfectly ordered arrangement adopt the P-43n space group.
457
-60 -70 -80 ~
-90
~
-100
~
-110 -120
~
SiSi
,
120
130
140 T-O-T / o
150
160
FigUl~ 2. 298i chemical shift as a function of Si-O-T angle. The nature of T is shown in the terminology TSi. MASNMR data were collected from each of the materials as appropriate for the nuclei 298i, 27A1, 7~Ga, ~B. Spectra were recorded on a Varian 300 MHz at the EPSRC Solid State NMR facility at the University of Durham or a Bruker AM300 spectrometer at Southampton. Reference compounds were TMS, 3M Al(acac) in benzene, B F 3 in Et20 and 3M Ga(NO3) 3. Typical spinning rates were 4.5 kHz for all nuclei.
3. RESULTS. 3.1 298i The correlations between structure and 298i chemical shift in aluminosilicates have been studied extensively by a number of authors. One of the most general relationships has been determined by Klinowski and Ramdas [9] which applies to a full range of aluminosilicates containing a variety of Si(-OAl)4_n(O-Si), environments. This expression of the form 8/ p.pan. = 14.3.03 - 20.34.~ drr / A where
458
¢1
drr -- [ 3.37n + 3.24(4-n) ] sin" 2 drr is the separation of the tetrahedral species n is the number of aluminium units surrounding the silicon and 0 is the T-O-T bond angle. This origin of this variation of the chemical shift with T-O-T angle has been discussed by a number of authors. A semi-empirical quantum chemical consideration put forward by Radeglia and Engelhardt [10] considers the changing effective electronegativity of the oxygen due to the level of s-p hybridisation varying as the T-O-T angle changes. This produces an expression of the type 8 =80+an+beos~
m
eos~-I
where 8 is the observed chemical shift, n the number of neighbouring AIO 4 tetrahedra, a, b and 80 are constants and o~ the Si-O-T bond angle. This expression can be used to fit data from a range of aluminosilicates with differing n values. The value of a represents the effect of replacing one silicon by one aluminium in the second co-ordination shell. The origin of this contribution has been discussed by a number of authors. Ramdas and Klinowski [9] assigned it to a change in the paramagnetic contribution to the chemical shift but it is unclear whether in aluminosilicates such a contribution is significant. More likely is that the change in charge on the neighbouring T cation affects the chemical shift through its bonding with the shared oxygen. A more highly charged electropositive species as the neighbouring T ion will deshield the silicon nucleus through its electron withdrawal from the shared oxygen. The results obtained in this work are presented in Figure 2 where the 29Si chemical shift is plotted as a function of Si-O-T angle for the different framework types; data from the Ms[SiAIO4]6.X 2, Ms[SiGaO4]6.X2 and M8[SiBeO4]6.X2 systems were obtained in this work, data for the Mn[SiO2]12.X m system were taken from Englehardt [5]. For all these systems a more negative chemical shift is obtained with increasing T-O-T bond angle. This reflects the higher s orbital contribution to the Si-O bond as the effective hybridisation of the oxygen moves between the limits of sp 2 (900) and sp (1800). In general the rate of change is similar though a slightly more rapid variation is found with gallium and aluminium. For a particular bond angle the chemical shift is in the order Si
459 and the location of this gallium in this structure and in others, is therefore, of considerable interest but has not yet been determined. By combining the 29Si(-OA1)4 and 29Si(-OGa)4 data shown in figure 2 and allowing for the increased size of gallium it should be possible to locate the preferred site of gallium occupancy in such structures.
3.2 27AI MASNMR spectra of aluminium are complicated by the quadrupolar nature of the aluminium nucleus, 1=5/2. Asymmetry in the local charge distribution around the aluminium nucleus results in quadrupolar broadened lines. For the aluminosilicates and aluminogermanates studied in this work electric field gradients at the aluminium sites were small and with magic angle spinning at moderate speeds no evidence of peak broadening was observed. In previous work on pure aluminate sodalites [12] the effects of quadrupolar coupling were accounted for by theoretical fitting of the broadened spectra. The difference between the aluminosilicate and aluminogermanate sodalites on one hand and the pure aluminate sodalites on the other is the level of distortion of the AIO4 tetrahedron and possibly the charge distribution around it (in pure aluminate sodalites the non framework cations are divalent rather than monovalent as in the AISi/AIGe systems). In the pure aluminate frameworks the O-AI-O bond angles around
85 80 75 .
~
A1A1
~
¢~ 70 ;2 A1Ge ~ 65 A1Si
x: 6 0
55 50
! 120
i
p 130
I
I
140 T - O - T angle / °
, 150
, 160
Figure 3. 27A1 chemical shifts as a function of AI-O-T bond angle, the nature of T is given by the terminology AIT.
460 the aluminium can be as large as 118o and this produces strong field gradients at the aluminium nucleus. In contrast in the aluminosilicates and aluminogermanates the O-T-O bond angle never exceeds 113o and the charge distribution around aluminium is close to cubic in symmetry. Due to these complexities the number of 27A1 spectra reported in the literature from framework materials is therefore much lower than those of 298i and such detailed analysis as undertaken for silicon has not been carried out. The effect of the neighbouring T atoms in AI(OT)4 tetrahedra has been described; typical chemical shift ranges reported were 85-70ppm for AI(-OAI)4,65-50 ppm for AI(-OSi)4 and 45-35ppm for AI(-OP)4, however within these ranges no full analysis has been carried out for the effect of changing AI-O-T angle. Studies of framework materials containing linked AIO4 tetrahedra have yielded information on the chemical shift as a function AI-OAI bond angles and also the relationship between the geometry of the distorted AIO4 tetrahedron and the quadrupolar coupling constant [12,13]. With the range of materials studied in this work it is now possible to construct correlations between the AI-O-T bond angle and chemical shift for a variety of different T. Figure 3 summarises the data for compounds containing AI-O-AI, AI-O-Si and AI-O-Ge links with each aluminium surrounded by four of the other atom type. As with silicon as the T-O-T bond angle increases the s contribution to the AI-O bonding increases producing a more negative chemical shift with increasing effective oxygen electronegativity. The chemical shift ranges for the differing neighbouring T atoms also follows the trends seen for 298i. For the most electropositive element, Si, the chemical shift range lies lower than for Ge and for aluminium the chemical shift range lies noticeably more positive.
-120 -130 ¢~ -140
• p,,,(
-150
~ -160 -170 120
125
130 135 T-O-T / o
140
145
Figure 4. 71Ga chemical shifts as a function of Ga-O-T bond angle in gallosilicate and gallogermanate sodalites.
461 3.3 7~ Ga
Few 7~Ga spectra of framework materials have been reported in the literature and results reported indicate very broad spectra. The high quadrupolar moment of this nucleus will lead to spectra broadened by quadrupolar coupling unless the environment is close to cubic in symmetry. In a number of the sodalites studied in this work the gallium environment is close to a perfect tetrahedron, the O-Ga-O bond angles being close to 109.40 leading to reasonably narrow spectra ( typical halfwidth 15ppm) Figure 4 plots the 71Ga chemical shift as a function of Ga-O-T bond angle for gallosilcate and gallogermanate sodalites. The expected variation with more negative, deshielded gallium, with increasing Ga-O-T bond angle is seen. For a particular set of bond angles around gallium and aluminium it is now possible to compare the chemical shifts characteristic of these environments. Hence when gallium is doped into a structure where a number of geometrically different aluminium sites occur MASNMR data could be used to investigate the gallium dopant distribution. 3.4 liB
Data were available from only three borate sodalites from the series Zns[BO2]12X 2 and these are presented in Figure 5. Despite ~B being a quadrupolar nucleus the quadrupolar moment is small and all spectra were observed as a single sharp peak. The expected trend with decreasing chemical shift with increasing B-O-B angle is seen, though the range of chemical shift is small. 4. DISCUSSION For each of the tetrahedral species, T, studied using MASNMR the chemical shift decreases as the T-O-T' bond angle increases. This can be related the changes in the effective electronegativity of the oxygen resulting from the level of sp hybridisation. The rate of change of the chemical shift with bond angle is greater if T' is more electropositive. For a particular bond angle the more electropositive T' produces the most negative chemical shift so for 29Si the order of chemical shifts is Be>Ga>AI>Si and for ZTAI AI>Ge>Si. In considering the use of MASNMR in determining
0.3 0.2
~0.1 ""
0
N -0.1 -0.2
-0.3 -0.4
k
127
129 131 133 T-O-T angle / o
Figure 5 I~B Chemical shifts as a function of B-O-B angle in borate sodalites
462 the distributing of dopant ions in a framework the effects of changing bond angles and electrostatic influences may now be deconvoluted. These results show that, for example, replacing Si(OAI)4 by Si(OGa)4 in a particular sodalite Ms[SiTO4]6.X2, with a certain M and X, changes the 298i chemical shift by ~6.5ppm. Of this the bond angle change (typical 60 for the larger gallium replacing aluminium) alters the chemical shift by 3 ppm and the electrostatic contribution to the change is 2 - 4 ppm depending on the bond angle range. At present this analysis takes no account of variations in the T-O distance. It is noteworthy that the Si-O distance in sodalites varies between 1.65 and 1.59A and this too should be correlated with chemical shift. This analysis requires very accurate determinations of Si-O distances and this work is currently in progress.
5. ACKNOWLEDGEMENTS We thank the EPSRC for grants in the support of this work including the provision of MASNMR facilities. We also thank Johnson-Matthey for a CASE studentship for GMJ and Dr M.E.Brenchley for the synthesis of the borate sodalite samples.
6. REFERENCES [1 ] J.Klinowski. Progress in NMR Spectroscopy 16 (1984) 237. G.Engelhardt [2] G.Engelhardt and W.Veeman. J.Chem See. Chem Comm. (1993) 622. [3] S.Hayashi, K.Suzuki,S.Shin,K.Hayamizu and O.Yamamoto. 58 (1985) 52. [4] J.Newsam. J.Phys Chem 91 (1987) 1259. [5] G.Engelhardt, P.Sieger and J.Felsche. Analytica Chimica Acta 283 (1993) 967. [6] P.J.Mead and M.T.Weller Zeolites 15 (1995) 561 [7] M.E.Brenchley and M.T.Weller. Chemistry of Materials 5 (1993) 970 [8] A.C.Larson and R.B.Von Dreele. GSAS Generalised Structure Analysis System MS-H805 Los Alamos NM 1990. [9] S.Ramdas and J.Klinowski. Nature(London) 308 (1984) 521 [10] R.Radeglia and G.Engelhardt. Chem. Phys. Lett. 114 (1985) 28 [11]P.Meaudeau, G.Sapaly, G.Wicker and C.Naccache, Catal. Lett 27 (1994) 143. [12] M.T.Weller, M.E.Brenchley, D.C.Apperley and N.A.Davies. Solid State Nuclear Magnetic Resonance 3 (1994) 103. [13] G.Engelhardt, H.Koller, P.Sieger, W.Depmeier and A.Samoson. Solid State Magnetic Resonance 1 (1992) 127.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
463
A new m e t h o d for the N M R - s p e c t r o s c o p i c m e a s u r e m e n t of the deprotonation e n e r g y of surface h y d r o x y l groups in zeolites E. Brunner, J. K~ger, M. Koch, H. Pfeifer, H. SachsenrOder and B. Staudte Universit~t Leipzig, Fakult~it fSr Physik und Geowissenschaften, Abteilung Grenzfl~ichenphysik, Linn6str~e 5, D-04103 Leipzig, Germany A new 1H MAS NMR-spectroscopic method for the determination of the deprotonation energy AEDpof surface hydroxyl groups in zeolites is described. This method is based on the measurement of the induced ~H NMR chemical shift A 8 caused by the interaction between the surface hydroxyl groups and weakly basic probe molecules such as C2C14 or CO. The new ~H MAS NMR-spectroscopic method is compared with the formerly established methods based on the measurement of the induced wavenumber shift Av of the O-H stretching vibration caused by the adsorption of weakly basic probe molecules [1] or on the measurement of the ~H NMR chemical shift 8H in activated samples [2,3].
1. INTRODUCTION Surface hydroxyl groups especially the bridging hydroxyl groups in zeolites can act as catalytically active Bronsted acid sites. The catalytic activity of H-zeolites with respect to Bronsted acid catalyzed reactions such as the cracking of n-paraffines [4] or the disproportionation of ethylbenzene [5] is determined by (i) the concentration, (ii) the accessibility and (iii) the strength of acidity of the bridging hydroxyl groups. 1H MAS NMR spectroscopy allows the measurement of the concentration of the different types of surface hydroxyl groups since the intensity of the corresponding 1H NMR signals is directly proportional to the concentration of resonating nuclei. The accessibility of the different types of surface hydroxyl groups can be studied on samples loaded with suitable probe molecules. The present contribution is devoted to the determination of the strength of acidity of surface hydroxyl groups by 1H MAS NMR spectroscopy. 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 AGDp of the deprotonation reaction TO-H~
TO-
+ H
+
.
(1)
It could be shown [6] that ~GDp is the sum of the deprotonation energy AEDp(heterolytic dissociation energy) and a constant contribution for surface hydroxyl groups in zeolites. Therefore, AEDp is a convenient measure for the strength of gas phase acidity. However, the spectroscopic measurement of the deprotonation energy AEDpis still a subject of discussion.
464 On the basis of experimental results it was suggested [2] to use the 1H NMR chemical shift 8H of surface hydroxyl groups in evacuated samples as a measure for the strength of acidity. In complete agreement with this suggestion recent ab initio quantum chemical calculations [7] revealed a linear correlation between the deprotonation energy AEDp and the chemical shift ~H for surface hydroxyl groups which are responsible for the Bronsted acidity of catalysts. These hydroxyl groups are bound to atoms (B, AI, Si or P) whose first coordination sphere consists of oxygen atoms only. The slope of AEDp amounts to - (84 ___ 12) kJ mol -~ ppm -~. Therefore, the difference AE = AEop - t~EDpsi°H between the deprotonation energy of the considered surface hydroxyl groups and non-acidic SiOH groups can be calculated according to AE
kJmo1-1
--- 84
6H -
$iOH ~H
(2)
ppm
where I~HSiOH denotes the 1H NMR chemical shift of SiOH groups which amounts to (2.0 + 0.1) ppm. Since it was possible [8] to calculate AEDpsi°H with a relatively high accuracy (AEDsi°H = (1400 __ 25) kJ mol-1), eq. (2) can be used to determine the absolute value of the deprotonation energy of surface hydroxyl groups from their 1H NMR chemical shift [3]. Since chemical shift differences can be measured with an experimental error of + 0.1 ppm it is possible by this method to determine differences in the deprotonation energy of surface hydroxyl groups with an accuracy of _ 8 kJ mol -~. It has however to be mentioned that the application of this method is restricted to free surface hydroxyl groups, i.e., surface hydroxyl groups which are not influenced by a hydrogen bond or an additional electrostatic interaction with the zeolite framework which may be a critical restriction, e.g., if surface hydroxyl groups located in pores of different diameter are compared. The mobility of the protons in bridging hydroxyl groups was recently studied by ~H MAS NMR spectroscopy at elevated temperatures [9,10]. Two remarkable phenomena could be observed for temperatures where the mean residence time xc of the protons on a certain framework oxygen atom approaches (2~tVr) -1. Here, v~ denotes the sample spinning rate. In agreement with the predictions of extended calculations [11] one observes (i) a characteristic broadening of the central line and (ii) a continuous decrease of the relative intensity of the spinning sidebands, i.e., an increase of the relative intensity of the central line. Sarv et al. [10] have observed that xc for protons of bridging hydroxyl groups in H-ZSM-5, H-mordenite and H-Y follows the sequence ~c(H-ZSM-5) < ~c(H-mordenite) < ~c(H-Y) at a given temperature, i.e., the proton mobility seems to be correlated with the strength of acidity. On the other hand, Baba et al. [9] have found a continuous decrease of xc for HZSM-5 with increasing A1 concentration. That means, the proton mobility increases with decreasing average distance between the framework A1 atoms although the strength of acidity of the bridging hydroxyl groups in the silicon-rich H-ZSM-5 is known to be approximately constant. In ref. [10] it is also discussed that the proton mobility may be correlated with the average proton affinity difference between the four oxygen atoms surrounding a framework AI atom. Further investigations are necessary in order to elucidate the interdependence between the proton mobility and the strength of acidity. An IR spectroscopic method for the determination of differences in the deprotonation energy of TO-H groups was developed by Paukshtis and Yurchenko [ 1] which is based on the
465 measurement of the induced wavenumber shift Av = V O H . . . M - V OH , where V O H . . . M denotes the wavenumber of the stretching vibration of these surface hydroxyl groups which form a hydrogen bond with the adsorbed probe molecules M. Provided that lay[ ~ 400 cm -~ (weak hydrogen bonding) the difference AE = A E D p - A E D p S i O n (see above) can be calculated according to the formula AE kJmo1-1
=-
1
l°g
A
[Av[
(3)
]A v sionl
with A = 0.00226 [1]. The deprotonation energy of different types of surface hydroxyl groups in zeolites was determined successfully by this method using CO as the probe molecule M [12,13]. 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. For bridging hydroxyl groups in H-ZSM-5 zeolites Makarova et al. [14] have found a linear correlation between the induced wavenumber shift and the line width of the stretching vibration band. Denoting the full width at half maximum of this band by a one can write [14]
a= ao ( 1 . O.Olo[Avl-1
(4)
where ao denotes the full width at half maximum for the unperturbed bridging hydroxyl groups. An induced wavenumber shift of ca. 300 cm -1 which is caused by the adsorption of CO on bridging hydroxyl groups therefore leads to an increase of the line width by a factor of four. This results in a relatively large experimental error for Av which limits the accuracy of the measurement especially of small differences in the deprotonation energy.
2. EXPERIMENTAL The investigated zeolite H-ZSM-5 was kindly provided by Degussa. The total Si/AI ratio of 14 was determined by chemical analysis which was performed in the laboratory of Dr. H.G. Karge, Fritz Haber Institute of the Max Planck Society, Berlin. It was also proved by chemical analysis that the proton exchange degree is higher than 98%. The framework Si/AI ratio of 17 was determined by 27A1 and 29Si MAS NMR according to the methods described in ref. [15]. The hydrothermal treatment was performed in an apparatus mainly consisting of a horizontally arranged quartz glass tube surrounded by a furnace. About 1.5 gramme of the zeolite was placed in the tube (bed depth: 1 mm) and heated up to 813 K (heating rate: 10 K/h) under a pressure of ca. 10 Pa. Steaming was conducted over a period of 2.5 h with 13 kPa water vapour pressure using nitrogen as carrier gas (flow rate: 40 l/h). Zeolite Na-Y was provided by Chemie AG Bitterfeld-Wolfen and ion exchanged in the Department of Chemistry of the University of Leipzig to an ammonium ion exchange degree of 30 %. The framework Si/AI ratio of 2.5 for this 0.3 NH4Na-Y zeolite was determined by
466 295i MAS NMR spectroscopy. 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 mm) 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 C2C14. After loading the samples were sealed. MAS NMR spectroscopic investigations have been carried out on a Bruker MSL 500 spectrometer at low temperatures (down to 130 K) with a sample spinning rate of 5 kHz. Zeolite 0.3 NHaNa-Y is denoted as 0.3 HNa-Y after the activation since the ammonia is then removed. All NMR chemical shifts are given relative to tetramethylsilane (TMS).
3. RESULTS AND DISCUSSION It is known (see, e.g., ref. [16]) that the interaction between surface hydroxyl groups and hydrogen bond forming probe molecules M causes an induced 1H NMR chemical shift A8 = bia M - 8H, where 8HM denotes the chemical shift of the surface hydroxyl groups influenced by the probe molecules. It could be shown [17] that bH and VOH are linearly correlated at least in limited ranges. The slope of 8H amounts to - 0.0147 ppm/cm -~ for surface hydroxyl groups in zeolites and to - 0.0092 ppm/cm -1 for hydrogen bonded protons in various solids. It can therefore be supposed that the correlation between A8 and a v is given by
/x~
B Imvl
ppm
em -1
~5)
with B-values between 0.0092 and 0.0147. Provided that this is true it should be possible to make use of A8 and A SsioH instead of Av and AvsioH in eq. (3). The induced 1H NMR chemical shift A b caused by the adsorption of hydrogen bond forming probe molecules on surface hydroxyl groups can strongly be influenced by rapid thermal motions and/or exchange processes of the probe molecules (see, e.g., ref. [18]). Therefore, in most cases it is necessary to carry out the corresponding 1H MAS NMR measurements at low temperatures. Fig. 1A shows the 1H MAS NMR spectrum of zeolite 0.3 HNa-Y. The spectrum exhibits an intense signal at 3.9 ppm (line (b)) caused by bridging hydroxyl groups in the large cavities. Two further, relatively weak signals occur at 4.9 ppm (line (c)) and 2.0 ppm (line (a)) which are due to bridging hydroxyl groups in the small cavities and SiOH groups, respectively. The total concentration of bridging hydroxyl groups amounts to ca. 17 OH per unit cell (u.c.). More than 90 % of the bridging hydroxyl groups are placed in the large cavities. Figs. 1B and 1C exhibit the 1H MAS NMR spectra of zeolite 0.3 HNa-Y loaded with 8 molecules C2C14 per unit cell measured at 293 K and 130 K, respectively. At a temperature of 293 K the signal due to bridging hydroxyl groups in the large cavities is broadened and completely shifted from 3.9 ppm to ca. 4.5 ppm despite the fact that the coverage is considerably lower than the concentration of bridging hydroxyl
467 groups in the large cavities. Assuming that the C2C14molecules exchange rapidly between bridging hydroxyl groups in the large cavities one expects only one signal at a mean position given by 6 = 6H +pMA6
(6)
where PM denotes the probability that a bridging hydroxyl group in the large cavities is occupied by a probe molecule (C2C14).PM is given by the ratio NM/Nbwhere NM denotes the concentration of probe molecules (C2C14)adsorbed on bridging hydroxyl groups in the large cavities and Nb is the concentration of bridging hydroxyl groups in the large cavities. Since ArM is less than or equal to the coverage it follows PM ~ 0.5. Using ti = 4.5 ppm and 8H = 3.9 ppm eq. (6) yields A8 ~ 1.2 ppm. For the limiting case of slow exchange, i.e., for sufficiently low temperatures one therefore expects a line at 8H...M ~ 5.1 ppm besides the signal at 3.9 ppm due to unperturbed bridging hydroxyl groups in the large cavities. In fact, the spectrum measured at temperatures below 150 K exhibits two well resolved signals at ~iH = 3.9 ppm and 8H...M = 5.5 ppm (see Fig. 1C). The existence of these two signals indicates that the limiting case of slow exchange is reached for temperatures T ~ 150 K. Therefore, the measurement of A8 should be carried out at temperatures below 150 K in order to suppress the influence of the exchange processes upon the spectra.
Figure 1. 1H MAS NMR spectra of zeolite 0.3 HNa-Y. Unloaded sample measured at 293 K (A) and sample loaded with 8 molecules C2C14 per unit cell measured at 293 K (B) and at 130 K (C). The spectra shown in Figs. 1B and 1C are enlarged by a factor of 2.
_ _ _ _ ~ L-.___A
B
C 6a / ppm
468 Fig. 2 shows the low-temperature ~H MAS NMR spectra of H-ZSM-5 loaded with different amounts of C2C14. The spectrum of the unloaded sample (see Fig. 2A) exhibits the wellknown signals at 2.0 and 4.2 ppm which are caused by SiOH and free bridging hydroxyl groups. Furthermore, a broad signal at ca. 7 ppm occurs which could be assigned [19-21] to bridging hydroxyl groups influenced by an additional electrostatic interaction with the zeolite framework (bridging hydroxyl groups of type 2 [19,20]). It should be mentioned that these species give rise to a broad IR band at ca. 3250 cm -~ [22]. A quantitative analysis yields the following concentrations of the different types of surface hydroxyl groups" (0.9 + 0.2) SiOH per u.c., (3.4 ___0.4) free SiOHA1 per u.c. and (1.8 ___0.4) SiOHA1 of type 2 per u.c. A part of the signal due to free bridging hydroxyl groups is shifted from 8H = 4.2 ppm tO 8HM = 6.1 ppm after loading with 2 C2CI4/u.c. (see Fig. 2B). It could furthermore be shown that C2C14molecules are adsorbed on bridging hydroxyl groups of type 2 for coverages higher than the concentration of free bridging hydroxyl groups. The corresponding complexes give rise to a "shoulder" at ca. 6.2 - 6.4 ppm nearby the above described signal at 6.1 ppm in the 1H MAS NMR spectrum (see Fig. 2C). That means that the bridging hydroxyl groups of type 2 form similar complexes with C2C14as the free bridging hydroxyl groups. The same behaviour could be found by IR and 1H MAS NMR spectroscopy for the interaction between bridging hydroxyl groups of type 2 and other probe molecules [19,20,22]. Furthermore, it is remarkable that the SiOH groups are completely shifted from 2.0 ppm to 2.8 ppm.
Figure 2. ~H MAS NMR spectra of H-ZSM-5 measured at 130 K. Unloaded sample (A), sample loaded with 2 molecules C2C14per unit cell (B) and with 12 molecules C2C14per unit cell (C). The spectra shown in Figs. 2B and 2C are enlarged by a factor of 2.
~ A
C i
i
i
i
,
i
+,
i
,
i
i
i
l
1
0
8
8H ] ppm
Tab. 1 summarizes values for the induced 1H NMR chemical shift A8 and the induced wavenumber shift Av of various free surface hydroxyl groups in zeolites. It could be found that A 6 and Av are linearly correlated as it was supposed above (see eq. (5)). The slope B amounts to 0.01 ppm/cm -1. Therefore, AE can be calculated according to eq. (3) by using A b instead of Av. It can be seen from Tab. 1 that the values calculated for AE by using A8 and Av are in reasonable
469
agreement. The experimental error of this method will be discussed and compared with the method based on the measurement of 8H (see eq. (2)) for the following example. For bridging hydroxyl groups in the large cavities of zeolite 0.3 HNa-Y values of A~i = (1.6 + 0.1) ppm and Av = -(155 _ 15) cm-'were found. Using the induced wavenumber shifts Av and AVsioH eq. (3) yields AE = - (127 _ 35) kJ mol -~. Replacing Av and AvsioH in eq. (3) by the induced 'H NMR chemical shifts A8 and A SSiOH, respectively, it follows AE = - (146 _ 30) kJ mo1-1. Both these values are in reasonable agreement with AE = - (160 + 8) kJ mol -~ which follows from (SH - 8Hsi°H) = (1.9 _ 0.1) ppm by the use of eq. (2).
In summary, it has to be stated that the experimental error of AE calculated from ~, according to eq. (2) is still considerably smaller than the experimental error of AE determined from A ~ (new method) or A v (method of Paukshtis and Yurchenko) by the use of eq. (3) if C2Cl 4 is chosen as the probe molecule M. A reduction of the experimental error, i.e., an enhancement of the sensitivity of the latter methods requires probe molecules M causing higher shifts. The measurement of A ~ (i.e., the new method) should then be preferred instead of the measurement of Av (method of Paukshtis and Yurchenko) since the broadening of the IR bands due to the hydrogen bond formation leads to larger relative experimental errors for t~v than for AS. A promising candidate for such investigations is CO [23]. It could however be shown for CO molecules adsorbed on zeolites that the suppression of their rapid thermal motions and/or exchange processes requires measurement temperatures below 60 - 80 K [24] under magic angle spinning conditions. These temperatures can only be achieved by using a helium cooled MAS NMR device which is now available in our laboratory. Table 1 Values for the induced wavenumber shift A v and the induced ~H NMR chemical shift A~i of free surface hydroxyl groups in zeolites using C2C14 as the probe molecule M. The difference ZXE = AEDp- AEDpsi°H between the deprotonation energy of the corresponding surface hydroxyl groups and SiOH groups in SiO: was determined from Av and from t~8 (instead of ~v) by using eq. (3). Values of ASsioH = (0.75 + 0.07) ppm and AvsioH = - (80 _ 7) cm -~ could be determined for SiOH groups in SiO2. H-ZSM-5 HT denotes a hydrothermally treated H-ZSM-5.
sample
OH group
0.3 HNa-Y
SiOHA1 (HF)
H-ZSM-5
H-ZSM5 HT
•v/cm-'
A ~/ppm
- 155
1.6
SiOH
- 80
0.8
SiOHA1 (free)
- 175
1.9
SiOH
-
75
AE/kJmo1-1 (from A v)
AE/kJmol-' (from A b)
-
127
-
146
-
150
-
177
0.8
A1OH
- 105
1.1
- 52
- 74
SiOHA1 (free)
- 185
1.9
- 161
- 177
470 ACKNOWLEDGEMENT Financial support by "Deutsche Forschungsgemeinschaft" (SFB 294 "Molek~ile in Wechselwirkung mit Grenzfl~ichen") is highly appreciated.
REFERENCES .
2. 3. 4. 5. 6. 7. .
9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
E.A. Paukshtis and E.N. Yurchenko, Usp. Khim., 52 (1983) 426. H. Pfeifer, NMR Basic Principles and Progress, Vol.31, Springer, Berlin 1994, p. 31. E. Brunner and H. Pfeifer, Z. Phys. Chemie, 192 (1995) 77. W.O. Haag, Stud. Surf. Sci. Catal., 84 (1994) 1375. H.G. Karge, J. Ladebeck, Z. Sarbak and K. Hatada, Zeolites, 2 (1982) 94. J. Sauer, J. Mol. Catal., 54 (1989) 312. 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. T. Baba, Y. Inoue, H. Shoji, T. Uematsu and Y. Ono, Microporous Materials, 3 (1995) 647. P. Sarv, T. Tuherm, E. Lippmaa, K. Keskinen and A. Root, J. Phys. Chem., 99 (1995) 13763. D. Fenzke, B.C. Gerstein and H. Pfeifer, J. Magn. Reson., 98 (1992) 469. L. Kubelkov~., 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. M.A. Makarova, A.F. Ojo, K. Karim, M. Hunger and J. Dwyer, J. Phys. Chem., 98 (1994) 3619. G. Engelhardt and D. Michel, High-Resolution Solid-State NMR of Silicates and Zeolites, Wiley, Chichester 1987. J.L. White, L.W. Beck and J.F. Haw, J. Am. Chem. Soc., 114 (1992) 6182. E. Brunner, H.G. Karge and H. Pfeifer, Z. Phys. Chemie, 176 (1992) 173. M. Koch, E. Brunner, D. Fenzke, H. Pfeifer and B. Staudte, Stud. Surf. Sci. Catal., 84 (1994) 709. E. Brunner, K. Beck, M. Koch, H. Pfeifer, B. Staudte and D. Zscherpel, Stud. Surf. Sci. Catal., 84 (1994) 357. E. Brunner, K. Beck, M. Koch, L. Heeribout and H.G. Karge, Microporous Materials, 3 (1995) 395. L.W. Beck, J.L. White and J.F. Haw, J. Am. Chem. Soc., 116 (1994) 9657. V.L. Zholobenko, L.M. Kustov, V.Yu. Borovkov and V.B. Kazansky, Zeolites, 8 (1988) 175. E. Brunner, Stud. Surf. Sci. Catal., 97 (1995) 11. M. Koch, E. Brunner, H. Pfeifer and D. Zscherpel, Chem. Phys. Lett., 228 (1994) 501.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
471
170 N M R STUDIES OF S I L I C E O U S F A U J A S I T E
L.M. Bull and A.K. Cheetham Materials Research Laboratory, University of California, Santa Barbara, CA 93106, U.S.A. ABSTRACT By using a combination of magic angle spinning (MAS) and double rotation (DOR) NMR techniques, and also various enrichment methods, different oxygen sites in the asymmetric unit of a zeolite (siliceous faujasite) have been observed for the first time. 1. I N T R O D U C T I O N 170 nuclear magnetic resonance (NMR) is potentially a very powerful technique for studying zeolites because the oxygen atoms in the framework are intimately involved in adsorption and catalytic processes. 170 has a nuclear spin of I=5/2 and so the observed NMR lineshape of the central transition and its relative peak position are determined by the quadrupole coupling constant, Cq, the asymmetry parameter of the electric field gradient tensor, 11, and the chemical shift tensor, all of which are dependent upon the coordination of the oxygen and the type of bonding in which it is participating. However, there are a number of reasons why relatively few 170 NMR studies have been performed on zeolites: 170 is only 0.037% in natural abundance and it is expensive to isotopically enrich, its relative NMR receptivity is poor, and the Cq parameters are believed to be large. In addition, many zeolite structures have large numbers of symmetry inequivalent oxygen sites, and these sites may be locally disordered due to the distribution of aluminum in the framework and the presence of charge compensating cations. The potential of 170 NMR to study silicatesland zeolites 2 has been demonstrated by Oldfield and coworkers using magic angle spinning (MAS) and variable angle spinning techniques. Chemically distinct sites such as Si-O-Si and Si-O-A1 were distinguished in zeolites A and Y, and from empirical correlations and theoretical considerations based on the Townes-Dailey model, 30ldfield and Schramm4 were able to predict the Size of Cq from the Si-O-Si or Si-O-A1 bond angle. More recently, high resolution 170 NMR spectra of condensed silicatesS,6,7,8 have been obtained by the implementation of techniques such as double rotation (DOR), 9 dynamic angle spinning (DAS) 1° and satellite transition spectroscopy (SATRAS), ~ methods that average the second-order effects of the quadrupole interaction. Narrow spectral lines for distinct oxygen sites in the asymmetric unit have been
472 obtained, and Grandinetti et al., 7 again using a Townes-Dailey model for predicting the effective field gradient at the oxygen from the bridging oxygen bond angle, were able to completely assign the spectrum of coesite to the 5 distinct oxygen sites in the asymmetric unit. No correlation with the bond angle at the oxygen was found for the 170 NMR chemical shift tensor, unlike that for 29Si.12 However, quantum mechanical cluster calculations are becoming more accurate for predicting 170 chemical shifts in solids. 13 Using a variety of NMR techniques, and also various enrichment methods, we have studied a prototypic zeolite system, siliceous faujasite (Sil-Y), which has four oxygen sites in the asymmetric unit and no aluminum. Figure 1 shows the structure of this zeolite determined from neutron diffraction, 14highlighting the four distinct oxygen sites. The insert tabulates the bond angles and bond lengths for each oxygen. Site Si-O-Si angle Si-O length (degrees) (/~) O(1)
138.4(2)
1.607(2)
0(2)
149.3(2)
1.597(2)
0(3)
145.8(2)
1.604(2)
0(4)
141.4(2)
1.614(3)
Figure 1. The structure of siliceous zeolite Y (SiI-Y) showing the four oxygen sites in the asymmetric unit determined from neutron diffraction.
473
2. EXPERIMENTAL Sil-Y, prepared according to Hriljac et al., 14 was 170 enriched with both 1702(g ) and H2170. The first method involved evacuating the SiI-Y for 1 hour at room temperature in a quartz tube before adding 1 atmosphere of 1702(g). The tube was then sealed and placed in a furnace and heated to 750oc. The sample was left at this temperature for 24 to 120 hours. The percentage enrichment for the sample was estimated to be --11% from the 170 MAS NMR spectrum of a non-enriched sample. The second method of enrichment was to treat the Sil-Y hydrothermally in H2170 at 95°C for 1 to 3 days. The crystallinity of the ~esulting 170 enriched samples was examined by X-ray diffraction and 29Si MAS NMR (Figure 2). 170 NMR experiments were performed at magnetic field strengths of 11.7 T and 9.4 T using Chemagnetics CMX spectrometers. DOR data were collected using a Chemagnetics probe. Recycle times of at least 10 s were found to be necessary in order to accurately quantify the relative intensities of the 170 signals. Rotor synchronization in the DOR experiments was used to remove the odd spinning sidebands. ~5 X-ray powder diffraction (XRD) data were acquired on a Scintag PAD X using Cu-Ka radiation and a liquid nitrogen cooled germanium solid-state detector.
(a) (b) CPS
1166.41036.890"/.2-
648.0" 518.4-
i , , , , i , , , , i , , ,", i , , ,
-100
ppm
-150
259"i129
oZ
Figure 2. 1702(g) enriched SiI-Y examined by: (a) 29Si MAS NMR (spinning speed = 6kHz, recycle delay 180 s, reference to TMS at 0ppm), and (b) X-ray diffraction.
474
3. R E S U L T S A N D D I S C U S S I O N Figures 2(a) and 2(b) show, respectively, tile 29Si MAS NMR spectrum aud X-ray diffi'action pattern of SiI-Y after being 170 enriched by the gaseous method described previously. Tile si~lgle narrow resonance observed in the 29Si MAS NMR spectrum confirms the high crystallinity and low almninum content of the sample after enrichment. The X-ray diffraction pattern consists of two components, one crystalline, consistent with SiI-Y, and the otller amorphous. The amorphous phase is thought to arise from the sample degrading under tile extreme temperature and pressure conditions used during the gaseous enrichment procedure. This phase is predicted to give a broad 170 NMR spectrum that may not be narrowed under MAS or DOR conditions because of the large distribution in local enviromnenls around the oxygens. It will not therefore be considered further in the analysis of the NMR data. Figure 3 colnpares the 170 MAS and DOR NMR spectra of SiI-Y enriched by 1702(g ), collected in an 11.7 T magnetic field. The MAS NMR spectrmi.l'(Figure 3(a)) shows a broad, fairly featureless pattern that arises from the second order quadrupolar broadening that is not averaged completely by rapid splinting around 54.7 o, the magic angle. The DOR techuique partially averages the second order quadrupolar broadening resulting in the significantly narrower sl~eCtl'Um shown in Figure 3(b). Three resonances and many spinning sidebands are observed, in ratios of approximately I:1:2, in accordance with 4 sites of equal occupancy observed fiom neutron diffraction. 14
b)
*
....
1200
I I00
I 0
I -100
I -200
-
'I'
200
i
100
'"
l
0 ppm
i
-t00
ppm
Figure 3. 170 NMR spectra of SiI-Y enriched with 1702(g) collected at l l.7T using (a) magic angle spinning (rotor speed = 6.5 kHz), (b) double rotation, rotor synchronized with all outer rotor speed of 710 Hz (isotropic lines are indicated by *). Both spectra are refelenced to 1120 at 0 ppm.
475 The observed shift from a quadrupolar nucleus is a combination of the chemical and the quadrupolar shifts. ~6 In order to separate the contributions of the two shifts to the observed shift, experiments need to be performed at two different magnetic field strengths as the quadrupolar shift is field dependent. In order to extract the chemical shifts for each site in the Sil-Y we recorded the DOR spectrum at 11.7 T and 9.4 T. Figure 4 compares the two spectra. It can be seen that the spectrum measured at 11.7 T is shifted to higher frequency than that measured at 9.4 T, in accordance with the quadrupolar shift being inversely dependent on the external magnetic field. The chemical shifts calculated from the spectra collected at the two fields are shown in Table 1, together with the Cq and rl parameters extracted from simulating the anisotropic dimension of a dynamic angle spinning experiment (data not shown here). ~7Again, only three resonances are observed at the lower field strength, with no significant broadening of any of the peaks. We conclude that two oxygen sites in SilY have vel3' similar chemical and quadrupolar shifts. This is not very surprising because all the sites have very similar Cq and 11 parameters, consistent with the small dispersion in the bridging oxygen bond angles. Clearly, the 170 NMR chemical shift is unpredictable, with factors, as yet unknown, contributing to its value. The chemical shifts derived from this work are within the range of those previously observed for silicates. Measurements at 14.09 T are cungntly in progress to obtain more precise values for the chemical shifts in Sil-Y.
(a)
50
(b)
0
ppm
51t
0
ppm
Figulg 4. 170 DOR NMR spectra collected at (a) 11.7T and (b) 9.4 T. Both sets of data are collected with rotor synchronization and are referenced to H20 at 0ppm.
476 Table 1. The observed shifts at 11.7 T and 9.4 T, referenced to H20 at 0ppm, and the extracted chemical shifts, quadrupolar coupling constants, Cq, and asymmetry parameters, rl, for each resonance, t7 Observed shift (ppm)
Chemical
@ 11.7 T
@9.4 T
shift (ppm)
Cq (MHz)
Peak 1
0.7
-18.1
33.5
5.1_+0.1
0.23_+0.1
Peak 2
10.9
-4.7
37.1
4.7-&-0.2
0.105:0.1
Peak 3
15.3
-2.1
44.1
4.9-L-9.2
0.03+0.1
We also investigated the possible effect of different oxygen sites having different rates of 170 exchange by heating Sil-Y at 750oc in 1 atmosphere of 1702(g) for 5 days. The 170 DOR spectrum showed no significant deviations from the spectrum shown in Figure 2, taken from the a sample heated for only 1 day at 750oc. This result confirms that an exchange equilibrium between the 1702(g) and the framework oxygen is reached fairly rapidly at 750°C. H2170 enrichment was also investigated as it is known that O2(g) may not access the small cages in the FAU structure, giving rise to the possibility of site selective enrichment based on steric arguments. Sil-Y enriched by suspending the zeolite in isotopicaUy enriched 170-water and heating to 95°C gave a similar 170 DOR NMR spectrum to that obtained from the gas enriched sample, with no additional resonances or measurable broadening of the peaks. Hydrothermal treatment of the Sil-Y at higher temperatures Was also attempted, but even though the rate and percentage of 170 enrichment increased, severe structural degradation of the sample occurred, to the point where virtually no reflections could be seen in the X-ray diffraction pattern. We have also investigated the effect of adsorbates on the 170 DOR NMR spectrum of Sil-Y to see if different oxygen sites would interact with the molecules in different ways, and thus increase dispersion of the peaks in the spectrum. Water and hexane were introduced separately into the sample, but no differences in the spectra were observed. This can be attributed to the weak interaction of the siliceous host with adsorbates, as previously noted from studies on adsorbed benzene. TM
477
4. C O N C L U S I O N S We believe that all the evidence presented above is consistent with the conclusion that the 170 NMR spectrum of Sil-Y has three resolvable peaks, with each site having similar quadrupolar coupling constants and asymmetry parameters. Two of the oxygen sites in the asymmetric unit have the same chemical shift. Assignment of the spectra using correlations of Cq or rl with Si-O-Si bond angle does not seem to be possible for this sample where the quadrupolar parameters are, within the error of the simulations, identical. This result is consistent with the small dispersion in Si-O-Si bond angles determined with high precision from neutron diffraction. ACKNOWLEDGEMENTS This work was supported by the MRL Program of the National Science Foundation under award No. DMR-9123048.
REFERENCES a N. Janes and E. Oldfield, J. Am. Chem. Soc., 108 (1986) 5743. 2H. Kyung, C. Timken, G.L. Turner, J.-P. Gilson, L.B. Welsh and E. Oldfield, J. Am. Chem. Soc., 108 (1986) 7231. 3C.H. Townes and B.P. Dailey, J. Chem. Phys., 17 (1949) 782. 4 S. Schramm and E. Oldfield, J. Am. Chem. Soc., 106 (1984) 2502. s K.T. MueUer, Y. Wu, B.F. Chmelka, J. Stebbins and A. Pines, J. Am. Chem. Soc., 113 (1991) 32. 6 K.T. Mueller, J.H. Baltisberger, E.W. Wooten and A. Pines, J. Phys. Chem., 96 (1992) 7001. r p.j. Grandinetti, J.H. Baltisberger, I. Farnan, J.F. Stebbins, U. Werner and A. Pines, J. Phys. Chem., 99 (1995) 12341. 8 C. J~iger, R. Dupree, S.C. Kohn and M.G. Mortuza, J. Non-Cryst. Solids, 155 (1993) 95. 9 A. Samoson, E. Lippmaa and A. Pines, Mol. Phys., 65 (1988) 1013. lo K.T. MueUer, B.Q. Sun, G.C. Chingas, J.W. Zwanziger, T. Terao and A. Pines, J. Magn. Reson., 85 (1990) 470. u C. J~iger, J. Magn. Reson., 99 (1992) 353. 12 R.E. Morris, S.J. Weigel, N.J. Henson, L.M. Bull, M.T. Janicke, B.F. Chmelka and A.K. Cheetham, J. Am. Chem. Soc., 116 (1994) 11849. 13 j. Sauer and B. Bussemer, private communication. 14 J.A. Hriljac, M.M. Eddy, A.K. Cheetham, J.A. Donohue and G.J. Ray, J. Solid State Chem., 106 (1993) 66. as A. Samoson and E. Lippmaa, J. Magn. Reson., 84 (1989) 410. 16 B.F. Chmelka and J.W. Zwanziger, NMR: Basic Principles and Progress, B. Bltimich (ed.) Springer-Verlag, Berlin, 33 (1994) 80. 17 L.M. Bull, J. Shore, S. Gann, Y. Lee, R. Dupree, A. Pines and A.K. Cheetham, in
preparation. 18 L.M. Bull, N.J. Henson, A.K. Cheetham, J.M. Newsam, S.J. Heyes., J. Phys. Chem., 97 (1993) 11776.
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H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 1997 Elsevier Science B.V.
479
Deuteron Magnetic Resonance Studies of Ammonia in A g N a Y Zeolites
M. Hartmann* and B. Boddenberg
Lehrstuhl ~ r Physikalische Chemic II, Universitat Dortmund, D-44221 Dortmund, Germany
The adsorption of ammonia is investigated in silver exchanged AgNaY zeolites. Silverincorporation into NaY increases the ammonia adsorption probably due to the formation of silver-diammine-complexes. 2H-NMR spectra show fast isotropic reorientations of the ammonia at room temperature, which transform into a rigid lattice behavior with decreasing temperature. Comparison with Ag(ND3)2+-complexes ion-exchanged into NaY show the same dynamic behavior giving additional evidence for a silverdiammine-complex formation upon adsorption of ammonia.
1. INTRODUCTION Transition metal ions are catalytically active in a variety of chemical reactions [1]. Incorporation of transition metal ions into zeolite cavities or channels may result in catalysts with unique properties. 2H nuclear magnetic resonance (NMR) spectroscopy can be used to investigate the dynamics of adsorbed deuterated molecules as well as their interaction with different adsorption sites. This technique has been successfully used for studies of adsorbed benzene and propene in zeolites [2]. Ammonia may be assumed to interact specifically with the silver sites in zeolites since Ag(ND3)2+-complexes are known in solution, rendering this molecule a well-suited candidate for this study on the locations and the properties of silver ions in Y zeolites.
2. EXPERIMENTAL SECTION Starting from NaY (Union Carbide LZY-52, Si/AI =2.4) silver exchanged zeolites Ag(x)NaY with x = 14, 28, 50 and 100 % were prepared with aqueous solutions of different * Present address" Institute of Chemical Technology I, Universit~it Stuttgart, D-70550
Germany.
Stuttgart,
480 AgNO 3 concentration. The silver contents of the samples were determined by. electron microprobe analysis (EMPA). The samples were dehydrated under vacuum (p < 10-5 hPa) for 18 h at 420 °C and subsequently oxidized for 6 h at the same temperature. The ammonia adsorption isotherms were measured volumetrically at 298 K. Atter completion of the first adsorption isotherm, the ammonia was pumped off and the sample was evacuated at 298 K for 18 h (p < 10-5 hPa). Subsequently a second isotherm was measured. For the 2H-NMR experiments the samples were pretreated as described above and ammonia-d3 (MSD, Montreal, Canada) was adsorbed up to a pressure of 100 hPa. Then the sample was evacuated overnight, sealed under cooling in liquid nitrogen and stored in the dark. The 2H-NMR spectra were recorded at a resonance frequency c00/2rt = 52.72 MHz using a Bruker CXP 100 spectrometer. The spectrometer as well as the measuring procedures applied have been described elsewhere [3]. For comparison an Ag(ND3)2Y zeolite was prepared by exchanging the complex ion Ag(ND3)2 + into the NaY zeolite. Under dry nitrogen ammonia-d3 was introduced into a solution of AgNO 3 in D20. Under stirring the addition of ammonia was performed until the solution became transparent. Now the AgfND3)2 + complex has been formed in solution. Adding a calibrated amount of NaY and additional stirring in the dark formed an Ag(ND3)2Y zeolite with an exchange degree of 55 %. The zeolite was then separated from the solution and the sample was subsequently carefully dehydrated, sealed and stored in the dark.
3. RESULTS Figure 1 shows the adsorption isotherms at 298 K of ammonia in the zeolites NaY, Ag(14)NaY, Ag(28)NaY, Ag(50)NaY and AgY (Ag(100)NaY). These isotherms were obtained for the zeolites activated as described above. The isotherms obtained after 18 h ambient temperature evacuation are only displayed for the zeolites AgY, Ag(28)NaY and Ag(14)NAY. Generally, for each zeolite the pairs of isotherms steeply increase at low pressure and run almost parallel to each other yielding difference amounts Nirr that are collected in Table 1. In comparison to NaY, Nirr is enlarged considerably up to a factor of about two in the case of the most highly silver exchanged zeolite AgY.
Table 1 Irreversible adsorbed amounts of ammonia at 298 K. sample NaY
Nirr/(NH3/u. c. ) 30
Ag(14)NaY
30
Ag(28)NaY Ag(50)NaY AgV
30 35 63
481
[
N/(NH3/u. c.)
120
.
.
.
.
.
.
.
.
.
.
.
~
x
'
"
,oo
X=50 ~
+ ,X
80
60
0
2. adsorption
4O
+
20
NaY
- 0 - Ag(14)NAY 0 0
10
20
30
I 40
I
50
-
-I
60
I
70
80
1
90
100
p/hPa
Figure 1. Ammonia adsorption isotherms at 298 K in silver exchanged Y zeolites Figure 2 shows the 2H-NMR spectra of ammonia in NaY zeolite at selected temperatures between 293 and 80 K. With decreasing temperature the spectra develop from singlets of increasing width into solid state powder patterns of the Pake-Type with Av = 51 kHz prominent edge splitting. T he spectral shape transition range extends from about 200 K to 100 K. The 2H-NMR spectra of (a) AgY loaded with ammonia (Nirr = 63/u.c.) and (b) Ag(ND3)2Y at selected temperatures between 290 and 80 K are compared in Figure 3. In both cases, the spectra develop in the same fashion with decreasing temperature from sing,lets of increasing widths into temperature-independent solid state powder pattern. From the edge sI~litting of Av = 53 kHz the deuteron quadrupolar coupling constant is readily calculated as e"~qQ/h = 71 kHz. The appearance of this rigid pattern indicates that in comparison with the characteristic NMR time XNMR all motions of the N-D bonding except the C3-axis rotation proceed very slowly. The breakdown of the solid state Pake pattern into temperature dependent Lorentzian-type singlets occurs in both samples in the temperature interval between 167 and 235 K.
482
250 K
293K
1~
1~
~
0
~0
-1~
-1~
150
1IX)
50
150
143K
....
-100
-150
i .... 100
i . . . . . . . . . . . . . . . . . . . 50 0 -50 -100
-1,50
+ ....
..+,5o
125 K
•,+o . . . .
100K
.,,;o . . . .
,; . . . .
- ; o
+...,~
-~
-1~
+
80 K
(P-IPo)/kHz
4 (P-Po)/kHz 1~
-50
167 K
200 K
1~
0
~
0
-50
-1~
-1~
1~
1~
~
0
Figure 2.2H-NMR spectra of ammonia adsorbed in NaY zeolite (Nirr = 30 NH3/u.c. )
-1~
483
(a)
I---25 k Hz
(b)
I----! 25; k Hz
L
.
.
.
.
.
.
.
•
~
_
~
:
. . . .
J _
,
_
290K
290K
222K
222K
167K
162K
.
.
.
___/ 80K
80K
Figure 3.2H-NMR spectra of ammonia in (a) AgY (63 ND3/u.c.) and (b) Ag(ND3)2Y
484 4. DISCUSSION The adsorption of ammonia in NaY zeolites can be increased by ion-exchange of Ag(I) ions. With increasing silver content the overall adsorption increases (Figure 1). The amount of strong adsorbed ammonia molecules Nirr also increases with the degree of exchange. In NaY 30 NH 3 molecules can not be removed from the sample due to their adsorption on strong adsorption sites. Due to absence of strong Lewis or Broensted acid sites only the sodium cations in the supercages are able to adsorb ammonia strongly [4,5]. The sodium cations are located on site SII and adsorb ammonia with an adsorption enthalpy of about 44 kJ/mol, which is close to typical chemisorption enthalpies [6,7,8]. With increasing degree of silver exchange the amount of strongly adsorbed ammonia molecules also increases, showing that silver cations are able to adsorb more than one ammonia molecule. It is well known that Ag + form silverdiammine-complexes Ag(ND3)2 + in solution and solids [9]. It was shown previously that Cu 2+ and Ni 2+ are also able to form amine complexes in solution [10] and zeolites [11,12]. Therefore, it is very likely that Ag + also forms silverdiamminecomplexes in zeolites [11], but no experimental evidence could be presented so far. The increase in ammonia adsorption in Ag(x)NaY zeolites can very well be explained by the formation of Ag(NH3)2 + complexes in the supercages. At a low level of silver-exchange, the increase is very small, showing that most of the silver-cations are no__!tlocated in the supercage. This is in excellent agreement with X-ray and xenon adsorption data, which show a preference of Ag+ for the SI position in the double six ring [ 13]. It can be concluded from our data that not all silver ions are accessible for ammonia and only some silver ions migrate into accessible sites. Therefore the irreversible adsorbed amount of ammonia Ni~ does not increase linearly with the degree of silver exchange. In AgY 63 ammonia molecules were found to be strongly adsorbed corresponding to about 32 Ag + cations on the SII positions in the supercages. X-ray crystallographic data assign between 25 and 30 silver cations to sites in the supercage [14]. This was also confirmed by xenon adsorption and xenon NMR data [ 15]. The formation of silverdiammine-complexes in zeolites can be confirmed by our NMR data. Investigation of the ion-exchange with silverdiammine-complexes indicate that these complexes can only replace all sodium cations in the supercage, but are not able to enter the 13cage [16]. Therefore, the situation in AgfND3)2Y should be comparable to the AgY after adsorption of 64 ammonia molecules. In fact, we observed that the spectra looked almost identical at all temperatures. In NaY the spectral shape develops very slowly in a temperature interval of about 100 K. This transformation interval shortens with increasing degree of silver exchange down to 68 K for AgY and Ag(ND3)2Y. In all samples a low temperature powder pattern is observed, which is characteristic for an axially symmetric electric field gradient (EFG) tensor being operative. The appearance of this rigid pattern indicates that in comparison with the characteristic NMR time XNMR all motions of the N-D bond except the C3-axis rotation proceeds very slowly [17]. The effective deuterium quadrupole coupling constant (DQCC) is 69 kHz for NaY and 71 kHz for AgY. Single crystal 2H-NMR measurements of the [Ag(ND3)2]Ag(NO2)2-complex show a DQCC of 71.6 + 0.5 MHz at room temperature [9]. Therefore, it is most likely that at least at low temperatures AgfND3)2+-complexes are also present in zeolites.
485 With increasing temperature some molecular motion runs the spectrum shapes from the slow into the fast oriental exchange limits measured on the time scale XNMR. At present it is still unclear whether this motion involves only the ammonia molecules or the Ag(ND3)2 + complex.
5. CONCLUSIONS The exchange of Ag + ions into NaY zeolites leads to an increase in ammonia adsorption most likely due to the formation of Ag(NH3)2+-complexes in the supercage of zeolite Y. The 2H-NMR-results show that the dynamic properties of the silverdiammin-complexes formed by ion-exchange or adsorption are almost identical. They show a fast isotropic reorientation at room temperature and a rigid lattice behavior at low temperature.
ACKNOWLEDGMENTS
Support of this research is gratefully acknowledged by "Fonds der Chemischen Industrie".
REFERENCES
[ 1] I. E. Maxwell, Adv. Catal. 31 (1982) 1. [ 2] B. Boddenberg and R. Burmeister, Zeolites 8 (1988) 488. [ 3] B. Boddenberg and R. Burmeister, Zeolites 8 (1988) 480. [ 4] V. Kanazirev and N. Borisova, Zeolites 2 (1982) 23. [ 5] V. P. Shiralkar and S. B. Kulkami, J. Colloid and Interface Sci. 1.08 (1985) 1. [ 6] R. M. Barrer and R. M. Gibbons, Trans. Farad. Soc. 59 (1963) 2569. [ 7] K. Morishige, S. Kittaka and S. Ihara, J. Chem. Soc. Faraday Trans. 1 81 (1985) 2525. [ 8] V. B. Shiralkar and S. B. Kulkarni, J. Colloid. Interface. Sci. 109 (1986) 115. [ 9] H. M. Maurer and A. Weiss, J. Chem. Phys. 69 (1978) 4046. [ 10] A. F. HoUemann and N. Wiberg, Lehrbuch der Anorg. Chemic, Walter de Gryter: Berlin, 1985. [11] B. Coughlan and J. J. McEntee, Proc. A. Ir. Acad. 76B (1975) 473. [12] A. Gedeon, J. L. Bonardet and J. Fraissard, J. Chem. Soc. Faraday Trans. 86 (1990) 413. [ 13] L. R. Gellens, W. J. Mortier and J. B. Uytterhoeven, Zeolites 1 (1981) 11. [ 14] L. R. Gellens, W. J. Mortier and J. B. Uytterhoeven, Zeolites 1 (1981) 85. [ 15] R. Grol3e, J. Watermann, A. Gedeon, J. Fraissard and B. Boddenberg, Zeolites 12 (1992) 909. [ 16] P. Fletscher and R. P. Townsend, J. Chromat. 201 (1980) 93. [ 17] S. W. Rabideau and P. Waldstein, J. Chem. Phys. 46 (1966) 4600.
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H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
487
Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
Spectroscopic studies o f
170 and 180 labelled
Z S M - 5 zeolites
F. Bauer a, H. Ernst a, E. Geidel b, Ch. Peuker c and W. Pilz ° University of Leipzig, Permoserstr. 15, 04303 Leipzig, Germany b University of Hamburg, Bundesstr. 45, 20146 Hamburg, Germany Humboldt University Berlin, Rudower Chaussee 6, Geb. 19.5, 12489 Berlin, Germany Using mild hydrothermal conditions zeolite ZSM-5 was labelled with 170 and 180 and studied by multi-nuclear MAS NMR, Raman and IR spectroscopy. The 170 NMR experiments gave a quadrupole coupling constant of about 5.5 MHz. In the case of 180 substitution frequency shifts of Raman and IR bands up to 50 crn"1, in good agreement with model calculations, were found. The IR spectrum of coke deposits on ~80-labelled HZSM-5 showed no isotope shitS. 1. INTRODUCTION In addition to the well established characterisation of zeolites by 1H, 27A1 and
29Si MAS
NMR the labelling of zeolite ZSM-5 with the isotope 170 allows NMR studies with an framework element which is otherwise not accessible for NMR investigations. Shiits of Raman and IR bands due to 180 isotope substitution may be used to characterise the zeolite framework and facilitate the assignment of observed bands to the normal modes. Furthermore, detailed information on ZSM-5 framework may be obtained by the comparison of the experimental 180 band shitts with frequency shifts calculated for various framework cluster models. BaUmoos [ 1] published fundamental investigations of 180 isotope exchange and dealumination of zeolite ZSM-5 using D2~80. Exchange of 180-labelled carbon dioxide and zeolite A was studied by Takaishi et al. [2]. The introduction of metal cations lowered the temperature required for isotope exchange between gas phase oxygen and framework oxygen of zeolite ZSM-5 [3]. But, only few infrared studies have been published of isotope substituted zeolites [4,5]. Significant shifts of the IR bands up to 51 cm~ were found in the vibrational spectra of the two SiO2 phases, tridymite [6] and quartz [7]. In this work, labelled zeolites were also used to study the formation of carbonaceous materials deposited on acid zeolites during hydrocarbon reactions. The intensity of the socalled 'coke' IR band around 1600 cm1, which is usually attributed to carbon-carbon stretching in hydrogen deficient ring structures, proved to be a suitable measure for the coke content [8]. Bands around 1360
-
1390 cm~ and 1440 - 1490 cm~ may be assigned to CH bending modes
488 of paraffinic species. However, Eischens [9] observed after acetylene exposure to A1203 or Pt/AI203 at 523 K that the bands at 1580 and 1460 cm1 grow at the same rate, as would be expected for a pair of bands from a single species. Both of the bands showed a shift of about 20 cm 1 when oxygen in the alumina was replaced with 1SO [10]. This shift has been taken as proof that the origin of these bands are vibrations with significant oxygen displacements, such as for carboxylate. The existence of carboxylate or acetate-like complexes was also assumed on Pt/AI203 [11] and on dealuminated HY [12]. Therefore, studies with oxygen-labelled zeolites may be also helpful for the elucidation of the surface-bounding of coke residues on zeolites. 2. EXPERIMENTAL SECTION A commercial NaZSM-5 zeolite (Chemiewerke Bad K6stritz GmbH, Germany), with a Si/AI ratio of 19 (measured by 29Si MAS M R
and X-ray fluorescence analysis), was ion
exchanged three times with aqueous solution of NH4NO3 (0.1 N)and heated in air for 12 hours at temperatures up to 823 K in order to yield the H-form. Both NaZSM-5 and HZSM-5 samples were treated with vaporized H2170 (24 % 170, Isotec, USA) and H21sO (98 % 180, Chemotrade, Germany) at 723 K for one hour. Changes in the Si/AI ratio resulting from hydrothermal treatment were checked by
29Si MAS NMR. For coking studies, n-hexene was fed at
693 K on HZSM-5 yielding 4.1 wt.% coke (H/C ratio of 1.8). IR measurements were performed on a IFS 66 spectrometer (Bruker). Samples were pressed to pellets with KBr or polyethylene to obtain mid-IR transmission spectra. The DRIFT measurements were carried out on the same spectrometer, equipped with a Nz cooled MCT detector, using a Praying Mantis DRIFT attachment (Harrick) connected with a heated vacuum cell (Harrick). The samples were measured at temperatures up to 873 K under a vacuum better than 10-5 mbar. Raman spectra were taken using a DILOR XY instrument with a microscope. The 514.5 nm line of an Ar + laser (Carl Zeiss, Jena) with a power of 50 mW was used. Only NaZSM-5 samples could be measured because of the strong background of all protonated zeolites. The IR and Raman bands were measured with a resolution of 2 cm1. The calculation of normal modes was carried out by a procedure using the method of normal coordinate analysis and is described elsewhere [13]. 1H, 170, 27A1 and Z9Si MAS NMR spectra of hydrated samples were obtained at 7.0 and 11.7 T on a Bruker MSL 500 spectrometer. For the quantitative detection of Bronsted, extraframe-work and silanol species by ~H MAS NMR activated and sealed samples were used. The estimation of the Si/AI ratio and the determination of the framework and extra-framework species as well as the 170 NMR experiments were done with hydrated samples.
489 3. RESULTS AND DISCUSSION Especially the interaction of water with zeolites may affects catalytic and structural properties. As demonstrated by von Ballmoos [1] in ~SO exchange studies with zeolite ZSM-5 and water at temperatures as low as 368 K the framework of zeolite ZSM-5 is considerably less inert than usually assumed. Under steaming conditions at 873 K nearly complete exchange was found within one hour which shows the reactivity of the Si-O-Si and the Si-O-AI bridging oxygen in a temperature range typical for hydrocarbon processing. The temporary cleavage of T-O-T bonds may bring about both a widening of the zeolite windows and the generation of reactive hydroxyl groups. Takaishi and Endoh [2, 14] showed with C~sO2 that the T-O-T linkage in zeolite A was not so easily broken as in X- and Y, type zeolites. AIPO-5 was far less reactive than the aluminosilicates. All zeolite samples, irrespective of their composition, contained few, extremely reactive site for oxygen exchange. Such sites may be ascribed to amorphous (colloidal) impurities [2] and/or defects within the silicate framework [ 15]. For the study of defects in zeolites, combined use of 1SO exchange with infrared and MAS NMR spectroscopy is essential. 3.1. MAS NMR spectroscopy
During the hydrothermal oxygen isotope exchange of HZSM-5 dealumination of the framework occurred which is indicated in the ~H, 27A1, 29Si MAS NMR, and IR spectra. The 27Al MAS NMR spectrum of the hydrated sample gives hints to non-framework aluminium
A
Fig. 1: 170 MAS NMR spectra of 170-labelled NH4ZSM-5 (A) and HZSM-5 (B). (.) Spinning side bands
. . . . . . . . . . . . . . . !00
. .....
. . . . . . . . . . . . . . . . . . . . . . . . . 60
20
0
-20
-60
, .......
.,., -100
8 (ppm) species due to peaks at 33 ppm and 0 ppm. The ~H N~_S ~
spectrum of the activated
sample shows a line at 1.8 ppm and a shoulder at 2.5 ppm due to silanol and non-framework Al, respectively [ 16]. The Si/AI ratio of the HZSM-5 sample increased by water treatment at
490 800 K from 19 to 65, whereas no dealumination of NaZSM-5 was observed during the hydrothermal oxygen exchange. The results of the 170 NMR experiments (Fig. 1) give, in agreement with Timken et al. [ 17] and Yang et al. [18], a quadrupole coupling constant of about 5.5 MHz and an electric field gradient tensor asymmetry parameter of about zero. The 170 MAS NMR spectra of the zeolites NaZSM-5 and NH4ZSM-5 are identical. 3.2. Infrared and Raman spectroscopy IR and Raman frequency shifts caused by oxygen isotope substitution were detected in the spectra of NaZSM-5 and HZSM-5 for framework and hydroxyl vibrations as well as for the combination tones [ 19]. The largest shifts of about 50 cm"~ were found for the most intense IR bands at about 1200 cm~ (Table 1), which are attributed to asymmetric T-O-T stretching Table 1" Experimental 180 shifts oflR and R a m a n framework bands of NaZSM-5. Sample
Wavenumber [cm"1]
NaZSM-5
1222 1091 824
797
795
548
453
381
380
360
292
180-NaZSM.5
1166 1064 817
789
790
537
444
364
370
350
285
8
5
11
9
17
10
10
7
isotope shift
56
27
7
modes, and for the strongest Raman band at 381 cm"~ (Fig. 2), which is assigned to a symmetric bending vibration whereby the oxygen atoms move along the bisecting line of the T-O-T angle. This finding reflects the significant participation of oxygen atoms in these vibrational
Fig. 2: Raman spectra of NaZSM-5 and 180-labelled NaZSM-5
o
L)
.Os l
.
,
. . . ' -. ~ ~ v ¢ ' ,
-NaZSM-5 |
1000
500
W a v e n u m b e r (cm -1)
491 modes. Only slight shifts were observed between 825 and 785
cm"l (Fig.
3). This can be
understood by assuming a nearly symmetric T-O-T stretching mode with small displacement of oxygen atoms.
=~
e:i
i1
.e-
I
,," ,,,i,,
180-ZSM.5
~\ I~,. ~,O'ZSM'5
0 ¢..)
ms._,
.40 r,,n '~
Fig. 3" IR spectra of hydrated HZSM-5 samples
,,'.,,K \",
.2-
_--'~/'
"-'7,
1300
i.i" ,
\ ',.
,
.--'-~_
,
1 2 0 0 1100 1000
900
800
_4A~'_~" 700
600
600
'i~l
1
400
Wavenumber (cm -1) In addition, since dealuminated HZSM-5 samples show a slight shift to higher wavenumbers [20], this shill by dealumination have to be added to the observed isotope shift compared with the untreated sample. The frequency of acid hydroxyl groups (Fig. 4) were shifted from 3596 to 3587 cm"1 while no 180 shift was found in terminal SiOH groups (3735 cm'l). Indicated by the appearance of the band at 3650 cm"~ non-framework AI species were formed during the hydrothermal
,l Fig. 4 OH bands in the DRIFT spectra.
O O
£,
HZSM-5
.1
3600
37"50
3T'O0
3650
36'00
3550
W a v e n u m b e r (cm -1) treatment. Similarly to the terminal SiOH groups, no 180 substitution was detected for the OH groups bound to these aluminium species. These findings may be explained by an easy exchange of both hydroxyls with ambient water even at room temperature. This view is
492 supported by the results, due to von Ballmoos [1] and Takaishi et al. [2], that amorphous impurities and modified silanol groups are extremely reactive sites for the exchange of oxygen isotopes. 3.3. Calculations
Calculations of vibrational frequencies by normal mode analyses for framework cluster models with different oxygen isotopes were carried out. Lorentzian lineshapes with a halfwidth of 50 cm 1 and without intensity weighting are assumed in the density of states for each calculated normal mode. The calculated density of states for the pentasil unit Si13034 with all 160, 170, and 180 atoms are shown in Fig. 5. Calculated frequencies above 1250 cm 1 and near
//~O-ZS.M-5 sO.ZSM.5
15-
=
• w,,,l r,¢2
= o
Fig. 5" Calculated density of vibrational states of the pentasil unit with 160, 170, and 180 atoms.
105-
o
c,.)
O-
14100
12100
lOlO0
800
600
W a v e n u m b e r (cm -1) 900 cm 1, which arise from model artifacts due to terminal oxygen atoms [13], have been omitted. Frequency shifts 180 vs. 160 up to 50 cm"1 were obtained for the asymmetric Si-O-Si stretching modes near 1100 cm1. This seems to be caused by the high degree of oxygen displacement in these vibrational forms in the high frequency region. The lowest calculated shift was found in the region near 800 cm1, which was also shown experimentally. Displacements of oxygen atoms are found to be low during the calculated normal modes in these region. The comparison of Fig. 3 and Fig. 5 confirms that calculated frequency shit, s for the pentasil unit are in good agreement with the experimental ones in nearly all regions of the framework spectrum. Therefore, model calculations with ~So substitution may be used to test the suitability of the force field under study, but the problem of lack of experimental data to adjust force constants for zeolite lattices can not be overcome in this way.
493
3.4. Coking studies For the elucidation of the bounding of coke residues on the surface of zeolites, MAS NMR and IR studies with oxygen isotope substituted zeolites at low coke content may be very helpful. The IR spectra of carbonaceous deposits formed during n-hexene conversion at 693 K on HZSM-5 and on the ~sO-labelled sample are shown in Fig. 6, and they are typical for coke
Fig. 6: IR spectra of coke formed during n-hexene conversion on HZSM-5 at 693 K.
HZSM-5 .16
d.) .1
o
r./3
<
.05
0
!
i
!
!
!
!
1650
1600
1550
1500
1450
1400
|
1350 "
!
1300
Wavenumber (cm-1) on zeolites [21]. No frequency shift of coke bands between the H z160 and H z180 hydrothermal treated samples is observed. Neither the broad bands at 1595 cm"1 (with shoulders at 1618 and 1573 cm ~) nor the bands around 1465 cm~ give any hint to an oxygen-containing species. This finding supports the result ofDatka et al. [22] that only on spent alumina catalysts carboxylatelike species are responsible for the intense bands at 1580 and 1460 c m "l and that the observation of carboxylate is not attributable to differences in experimental procedures. Rather, it is due to differences in the surface properties of hydroxyl groups on alumina and zeolites. Nevertheless, the absence of anyisotope shill in the spectrum of coke on ~gO-labelled HZSM-5 confirms the results of IK 13C MAS M R
and UV-vis studies [21] that, if at all, only
positively charged carbonaceous residues seems to be bound to the zeolite surface. 4. CONCLUSIONS Vibrational and MAS NMR spectroscopy are useful techniques for oxygen isotope exchange studies with zeolites. The post-synthesis labelling of zeolite ZSM-5 under mild hydrothermal conditions indicates a quick microscopic rupture and re-formation of T-O-T bonds in the macroscopic stable zeolite. Moreover, oxygen-labelled samples enable a more detailed spectroscopic characterization of surface-bound intermediates during the catalytic conversion of hydrocarbons on zeolites. Solid-state 170 NMR has been shown to be an advantageous method in the characterization of zeolites which promises further interesting results. In the case of lSo substitution of zeolite
494 ZSM-5 the shifts of the IR band at about 1100 cmq and of the Raman band at about 380 cmq may be helpful to estimate the degree of exchange. Hydroxyl groups associated to nonframework AI species and terminal silanol groups are assumed to be extremely reactive sites for the exchange of oxygen isotopes. No frequency shift was found in the spectrum of carbonaceous deposits on ~so-labelled HZSM-5 which confirms the absence of carboxylate species on coked HZSM-5.
Acknowledgements The authors express their gratitude to Dr. W. Geyer (FTIR) and Prof. D. Freude (MAS NMR) for stimulating discussions and to D. Jung and I. Saul for technical support.
REFERENCES
[1]
[2] [3] [4]
[5] [6]
[7] [8] [9] [ 10] [ 11] [12] [ 13] [ 14] [15] [ 16] [ 17] [18] [19] [20] [21] [22]
R. von Ballmoos: The nO-exchange method in zeolite chemistry; Otto Salle Verlag, Frankfiart, 1981. T. Takaishi and A. Endoh, J. Chem. Sot., Faraday Trans. 1, 83 (1987) 411 Y.F. Chang, G.A. Somorjai and H. Heinemam~ J. Catal., 154 (1995) 24 L.M. Parker, D.M. Bibby and G.R. Bums, Zeolites, 13 (1993) 107 Ch. Peuker, J. Mol. Struct., 349 (1995) 317 A.M. Hofmeister, T.P. Rose, T.C. Hoering and I. Kushiro, J.Phys.Chem., 96 (1992) 10213. R.K. Sato and P.F. McMillan, J.Phys.Chem., 91 (1987) 3494. H.G. Karge and E.P. Boldingh, Catalysis Today 3(1988) 53. R.P. Eischens, Stud. Surf. Sci. Catal. 49 (1988) 51. J. Najbar and R.P. Eischens, in Proceedings of the 9th International Congress on Catalysis, Paper 184 Ab, Calgary, 1988. B.B. Zarkov, W.L. Medschinskii, L.F. Butocnikova, O.M. Oranskaja and W.B. Marischev, Chim. Technol. Topliv. Masel (1988) 18 J. Novakova, L. Kubelkova, V. Bosacek and K.Mach, Zeolites, 11 (1991) 135 E. Geidel, F. Bauer, H. BOhlig and M. Kudra, Acta Chim.Hung., 132 (1995) 349 A. Endoh, K. Mizoe, K. Tsutsumi and T. Takaishi, J. Chem. Sot., Faraday Trans. 1, 85 (1989) 1327 R. von Ballmoos and W.M. Meier, J. Phys. Chem., 86 (1982) 2698 H. Pfeifer and H. Ernst; Annual Reports on NMR spectroscopy, 28 (1994) 91 H.C. Timken, G.L. Turner, J.P. Gilson, L.B. Welsh and E. Oldfield, S. Yang, K.D. Park and E. Oldfield, J.Am.Chem.Soc., 111 (1989) 7278 F. Bauer, E. Geidel, Ch. Peuker and W. Pilz, Zeolites (in press) E. Loeffler, Ch. Peuker and H.G. Jerschkewitz, CatalysisToday, 3_(1988)415 H.G. Karge, Stud. Surf. Sci. Catal., 58 (1991) 531 J. Datka, Z. Sarbak and R.P. Eischens, J. Catal., 145 (1994) 544
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
495
Anisotropic Motion of Water in Zeolites EMT, L and ZSM-5 as studied by D-and H-NMR Line Splitting A. Wingen, W. Basler and H. Lechert Institute of Physical Chemistry, University of Hamburg, Bundesstr. 45, D-20146 Hamburg, Germany 1. ABSTRACT Water sorbed in zeolites EMT, L, Y and ZSM-5 has been investigated by D- and H-NMR in the temperature range of 200K to 390K. A doublet splitting, indicating anisotropic motion, has been found in EMT, L and ZSM-5. D20 in zeolite Y, however, gave one single line, characteristic for isotropic motion. 2. INTRODUCTION Extended knowledge about the motion of sorbed guest molecules inside the host framework of a zeolite is of fundamental interest, as it reflects the interaction between the molecules and the inner surface of the zeolite and their geometries. Specific interactions (like H-bonds) may induce more or less anisotropic reorientation. Doublet splittings in NMR spectra indicate anisotropic motion. For proton resonance, water is a system of two nuclei with spin 1/2, coupled by direct magnetic dipole-dipole-interactions. This causes a splitting of the resonance line, according to KH = KH, static * (3 c O s 2 ® - 1 )
(1)
with KH, static = 46 kHz. ® is the angle between the proton-proton vector and the applied magnetic field B0. If diffusion of water is isotropic, this interaction will be averaged to zero by the molecular diffusion. For deuteron resonance the splitting is given by the electric interactions of the quadrupole moment of the deuteron with the electric field gradient. Since the electric field gradient is nearly rotational symmetric, equation 1 is valid even in this case. KH, static and KH must be substituted by KD, static = 176 kHz and KD, ® is given by the direction of the OD-connection vector and the field. Splitting in H-NMR spectra may be masked by intermolecular interactions and exchange processes. Splittings in D-NMR spectra, however, are nearly insensitive against
496
exchange effects of the hydrogen nuclei, since smaller magnetic interactions may be neglected. Therefore D-NMR spectra are better suited for studying anisotropic motion than H-NMR spectra. The order parameter S in table 1 and 2 is given by KD/KD, static and KH/KH, static respectively, multiplied by 100%. This paper presents D-NMR spectra of D20 in zeolites EMT, L, Y and ZSM-5 for different coverages in the temperature range of 200K to 390K. The order parameters are compared to those achieved by H-NMR experiments. 3. EXPERIMENTAL
Zeolite Na-Y (SK 40, union carbide, Si/AI = 2.11) and zeolite (0.97 K, 0.03 Na)-L (SK 45, union carbide, Si/AI = 3.07) have been used as synthesized. Na-EMT and Na-ZSM-5 were prepared and calcinated by standard methods in our laboratory. After dehydration by heating to 673K at 104 Torr for about 12 hours, adsorption of degassed H20 respectively D20 was performed. Free induction decays were taken using a Bruker BKR 322 pulse spectrometer at 60MHz. The D-NMR spectra have been measured by a Varian DP 60 wide-line spectrometer at 11MHz. The residual splitting constant KD and the line width Av(FWHH) were obtained by comparing the derivative spectra with calculated derivative powder patterns. Calculation of powder patterns was performed by folding a Pake doublet [1] with a Lorentzian line shape numerically. The residual splitting KH was obtained from the free induction decay of pulse NMR. It is related to the times tl and t2 of the first and second zerocrossing by [2] KH * tl = 2.16(rad/s) * s = 0.344Hz * s KH * t2 = 5.63(rad/s) * s = 0.896Hz * s
and
(2)
4. RESULTS AND DISCUSSION
Doublet splittings were found in EMT, L and ZSM-5. Both splitting constants KD and KH and the order parameter S are listed in table 1 and 2 respectively. For the line width Av(FWHH) in table 1 a Lorentzian line was presumed. For zeolite EMT, containing 174 mg D20/g dry sample, doublet splitting could be detected between 235K and 339K. At 204K splitting was masked by the increasing line width. The corresponding derivative D-NMR spectra are shown in figure 1. Coverage with 76 mg/g led to splitting at 298K. Only the fully hydrated sample (368 mg D20/g EMT) did not show any splitting at all (Av(FWHH) = 0. lkHz at 298K). For comparison D20 in zeolite Y showed a single line between 200K and 390K, no matter what coverage has been chosen. The line width was Av(FWHH) = 0. lkHz for a sample with 154 mg D20/g at 298K.
497
For zeolite L, containing 78 mg D20/g, doublet splitting could be detected between 235K and 333K. A fully hydrated sample (151 mg D20/g L) gave a splitting only at 244K, but not at 298K and above. At 204K splitting couldn't be observed furthermore. ZSM-5 with 27 mg/g, did not show any splitting at 298K and 339K. Both intermediate coverage (59 mg D20/g) and full coverage (76 mg D20/g) led to splitting between 204K and 321K.
204K
235K
244K
298K
309K
327K
339K 354K
Fig. 1" Derivative D-NMR spectra of D20 in zeolite EMT (174 mg/g), sweep 20G.
498 Tab. 1 Doublet splitting KD, line width Av(FWHH) and order parameter S of D-NMR spectra of D20 in zeolites EMT, L and ZSM-5. zeolite
coverage [mg/g]
T[K]
Ko[kHz]
Av(FWHH)[kHz]
S[%]
EMT
76
298
5.8
2.1
3.3
174
235 244 269 298 309 327 339
2.8 3.6 4.1 3.9 3.6 3.8 2.4
1.2 1.0 1.2 0.5 0.7 1.1 1.3
1.6 2.0 2.3 2.2 2.0 2.1 1.4
368
ZSM-5
no splitting between 235K and 370K
78
235 244 278 298 333
151
244 7.0 2.6 no splitting between 298K and 389K
27
15.3 17.3 19.6 17.9 17.6
3.7 3.5 3.1 5.2 3.3
8.7 9.8 11.1 10.2 10.0 4.0
no splitting at 298K and 339K
59
204 235 261 298 321
7.7 5.3 3.9 3.6 3.3
4.2 2.2 1.7 2.0 1.7
4.4 3.0 2.2 2.1 1.9
139
204 235 261 298 321
7.6 5.6 3.7 2.5 3.9
3.4 2.2 1.8 1.4 2.2
4.3 3.2 2.1 1.4 2.2
The difference between cubic faujasites like Y on the one hand, hexagonal faujasites EMT, ZSM-5 and L on the other hand can be explained by the different channel geometry. In cubic faujasites the large cages are arranged in a diamond
499
like structure. Therefore, no space direction is preferred by the diffusing water molecules and the term (3cos2®-1) of equation 1 is averaged to zero by the molecular diffusion. In EMT, L and ZSM-5 there is a 1-dimensional system consisting of straight channels. Here the term (3cos2®-1) is not averaged to zero by the channel geometry. The splitting reflects the anisotropic motion of the molecules relative to the inner surface. As can be seen in table 1 and 2, the residual splitting for water in zeolite L is greater than those for water in EMT and ZSM-5. Probably this can be explained by the fact that in zeolite L there is no averaging of (3cos2®-1) by the linear channel geometry. In contrast, EMT and ZSM-5 have a branched channel system, allowing some averaging of (3cos2®-1).
Tab. 2 Doublet splitting KH and order parameter S of H-NMR spectra of H20 in zeolites EMT, L and ZSM-5. zeolite
coverage [mg/g]
T[K]
KH[kHz]
s[%]
EMT
50
250 270 300 320 340
8.9 3.5 3.0 2.1 1.0
19.3 7.6 6.4 4.5 2.2
146
220 229 250 269 289 298 310
30.3 3.0 2.8 2.6 2.4 2.3 2.7
65.9 6.4 6.1 5.7 5.1 4.9 5.8
L
73
210 230 250 270 298
7.3 7.0 6.9 6.4 5.6
15.9 15.3 15.0 13.9 12.3
ZSM-5
59
230 250
26.1 2.7
56.8 5.9
500 In comparison with to the results of D-NMR measurements, a fully hydrated sample of EMT did not show any splitting of H-NMR spectra in the observed temperature range. For zeolite EMT, containing 50 mg H20/g, splitting occurred up to 340K. Coverage with 146 mg H20/g led to splittings reaching temperatures of about 310K. For zeolite L, containing 73 mg H20/g, doublet splitting could be detected up to room temperature. Equal to L for ZSM-5 only intermediate coverage (59 mg D20/g)led to splitting up to 250K. If samples with a lower content of iron impurities were examinated, splittings would be found even for low and high coverages of zeolites L and ZSM-5. Nevertheless, the results listed in table 2 do not change remarkably. Obviously the residual splitting of both D- and H-NMR-spectra is rather small. This excludes that the reorientation is a rapid rotation around a single fixed axis. As only one splitting is observed for the two deuterons of the water molecul, the properbility distribution of the orientation must be symmetrical to the two-fold-axis of the water molecule. The molecular reorientation is almost isotropic. The residual splitting is approximately independent of temperature. At higher temperatures line broadening occurs in case of EMT at intermediate coverage. This broadening may be due to an increased rate of exchange between two types of reorienting water molecules. According to this, we found a maximum in the transverse relaxation time T2 as a function of temperature [3], examining a sample of EMT covered with 62 mg/g H20 and 108 mg/g D20 by H-NMR. This indicates, according to Pfeiffer [4], exchange with small amounts of protons with no or slow motion, e.g. OH-groups. This exchange dominates the H-NMR spectra at temperatures about 270K, but the D-NMR spectra at sligthly higher temperatures, as can be seen in table 1. This difference may be caused by isotopic effects. REFERENCES
[1] [2] [3] [4]
Pake, G.E., J. Chem. Phys., 16 (1948) 327-336 Woessner, D.E., Snowden, B.S., Jr., 50/4 (1969) 1516-1523 Diplomarbeit Annette Wingen, University of Hamburg Pfeifer, H., Nuclear Magnetic Resonance and Relaxation of Molecules Adsorbed on Solids, NMR Basic Principles and Progress,. Springer Verlag, Berlin-Heidelberg-New York, Z (1972) 55-153
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
501
EXAFS and N M R studies of the incorporation of Zn(II) and Co(II) cations into tetrahedral framework sites of AIPO 4 molecular sieves N. Novak Tu~ar a, A. Tuel b, I. Ar6,onc, A. Kodre e and V. Kau6,i6,a,d aNational Institute of Chemistry, Hajdrihova 19, 61000 Ljubljana, Slovenia blnstitut de Recherches sur la Catalyse, CNRS, Avenue Albert Einstein 2, 69626 Villeurbanne Cedex, France CJo~.ef Stefan Institute, Jamova 39, 61000 Ljubljana, Slovenia dFaculty of Chemistry and Chemical Technology, University of Ljubljana, A~ker6.eva 6, 61000 Ljubljana, Slovenia
CoAPO-11 and CoZnAPO-11 materials were synthesised hydrothermally in the presence of diisopropylamine (DIP) as a template, and ZnAPO-34 and CoZnAPO-34 in the presence of tetraethylammonium hydroxide (TEAOH) as a template. EXAFS and 31p and 27A1 MAS NMR studies on as-synthesised and calcined samples indicate the incorporation of cobalt and zinc cations into tetrahedral framework sites of AIPO4-11 and A1PO4-34.
1. INTRODUCTION Following the discovery of aluminophosphate (A1PO4) molecular sieves I in 1982 much attention has been focused on the substitution of framework phosphorus by silicon (SAPO) and framework aluminium by di- and trivalent metal cations (MeAPO). Although the synthesis and characterisation of MeAPO molecular sieves have been described by several groups 2, very little attention has been paid to the study of their framework structure details. The present paper shows the results of Co and Zn K-edge EXAFS and 31p and 27A1 MAS NMR studies on four MeAPO materials, CoAPO-11, CoZnAPO-11, ZnAPO-34 and CoZnAPO-34, studied in their as-synthesised and in calcined forms. The possibility of successful incorporation of cobalt and/or zinc into tetrahedral framework sites of A1PO4-11 and A1PO4-34 is described.
F. Villain and S. Benazeth from LURE provided expert advice on the beamline operation. The work was performed with a financial support by Ministry of Science and Technology of Slovenia.
502 2. EXPERIMENTAL SECTION
2.1. Synthesis The syntheses of MeAPO-11 and MeAPO-34 have been performed using the reaction gels of the molar compositions: 1) 0.8A1203 2) 0.8A1203 3) 0.8A1203 4) 0.8A1203
: : : :
1.0P205 1.0P205 1.0P205 1.0P205
: : : :
1.0DIP : 50H20 : 0.30COO 1.0DIP : 50H20 : 0.14COO : 0.17ZnO 1.0TEAOH : 75H20 : 0.33ZnO 1.0TEAOH : 50H20 : 0.15COO : 0.15ZnO
following the procedure of crystallisation in stainless-steel teflon-lined autoclaves, described by Wilson et al. 3 Blue crystalline powders of CoAPO-11 and CoZnAPO-11 were obtained at 150°C for 5 days, white crystalline powder of ZnAPO-34 at 150°C for 3 days and blue crystalline powder of CoZnAPO-34 at 100°C for 7 days. Templates were removed under vacuum at 500°C.
2.2. Instrumentation Powder diffraction data were taken on a Philips PW 1710 X-ray powder diffractometer using the CuKot radiation in 0.02 ° steps from 5 to 135°20 with 1 s per step. Morphology of the samples was studied with a Jeol JSM-T220 scanning electron microscope. Elemental analysis was carded out using an EDS (Energy dispersion analysis by X-ray) analytical system TRACOR EDX, attached to the scanning electron microscope JXA-840A. TEAOH and H20 were determined thermogravimetrically on TA 2000 thermal analyser (TA Instruments, Inc.). EXAFS spectra at Co and Zn K-edges were measured at XAS 3 station at DCI storage ring (running at 1.5 GeV and 223 mA) in LURE, Orsay, with an Si(311) double-crystal monochromator with 0.5 eV energy resolution at 8 keV. Powdered samples were prepared on multiple layers of adhesive tape. The absorption thickness of ~td ~ 2 above K-edge of investigated elements was chosen. Reference spectra on empty tapes were taken under identical conditions. NMR spectra were recorded on a Bruker MSL 300 spectrometer. 31p spectra were taken using a HP/DEC sequence with 2 ms (30 °) pulse lenght and 20 s delay. For 27A1 MAS spectra a one-pulse sequence was used with 1 ms (20 °) pulse length and 1 s delay. T 1 measurements were performed on static samples using an inversion-recovery sequence. Chemical shifts for 31p and 27A1 were referred to H3PO 4 and Al(H20)63+, respectively.
3. RESULTS AND DISCUSSION X-ray powder diffraction (XRD) patterns show the as-synthesised products as pure, wellcrystallised and containing only one phase. SEM photographs reveal prismatic crystals and small spherical particles for MeAPO-11 (Figure 1). For MeAPO-34 SEM photographs reveal crystals of cubic morphology (Figure 1).
503
....i~
El ~ - ~ :
~ ....
,,~
.
ZnAPO-34
CoAPO-11
.
.
.
.
,
.
CoZnAPO-34
CoZnAPO-11
Figure 1. Scanning electron micrographs of MeAPO-34 and MeAPO-11.
The XRD diffractograms of calcined samples MeAPO-11 (exposure to the ambient atmosphere at room temperature) show significantly different powder patterns (Figure 2). The same structure changes were reported 4 for AIPO4-11. Results of Rietveld refinement 5 of MeAPO- I 1 show that, if Co 2÷ and Zn 2+ replace aluminium in AIPO4-11 molecular sieve, one of the three of crystallographically non-equivalent A1 sites is probably preferred for substitution.
504
t.t-
as-synthesised CoAPO- 11
5
10
15
20
2O Figure 2. X-ray powder diffraction pattern of as-synthesised CoAPO-11 (lower) and calcined CoAPO- 11 (upper). The XRD patterns of calcined samples MeAPO-34 (exposure to the ambient atmosphere at room temperature) show only a small decrease in crystaUinity with respect to assynthesised samples with no indication of crystal structure changes. On the basis of elemental analyses of the crystals (EDS) the general formula (MexAlyPz)O2 was calculated (the amount of water and template is calculated from TG and DSC analyses): CoAPO- 11 CoZnAPO- 11 ZnAPO-34 CoZnAPO-34
(Co0.08A10.42P0.5)O2 : 0.03DIP : 0.08H20 (Co0.02Zn0.04A10.44P0.5)O2 : 0.03DIP : 0.04H20 (Zn0.1A10.4P0.5) : 0.07TEAOH : 0.5H20 (Zn0.04Co0.05A10.41P0.5) : 0.04TEAOH : 0.4H20
From x + y = z = 0.5 in the general formula indicates the isomorphous substitution of aluminium by metal is indicated 6. Elemental analyses (EDS) on the spherical particles present in MeAPO-11 products show that they contain large amounts of cobalt as compared to aluminium and phosphorous; they are probably amorphous as no extra-phases were detected in XRD patterns. 27A1 MAS NMR spectra of all as-synthesised and calcined samples show a strong symmetrical peak corresponding to a tetrahedral AI(4P) environment. This confirms that, if Co and/or Zn are constitutive parts of the aluminophosphate framework, they replace exclusively A13+. 31p MAS NMR spectra of as-synthesised and calcined ZnAPO-34 show 4 lines at about -30, -24, -20 and -14ppm/H3PO 4 (Figure 3). The lines can be attributed 7 to various configurations of P atoms in the A1PO4-34 structure (P(4AI), P(3A1, 1Zn) and
505 P(2A1, 2Zn) units). The calculation of the fraction of zinc in the sample from deconvolution of the 31p NMR spectrum was in very good agreement with elemental analysis, thus confirming the incorporation. 31p MAS NMR spectra of CoAPO- 11, CoZnAPO- 11 and CoZnAPO-34 show significant sidebands and line broadening which arise from strong interaction of phosphorus nucleus with paramagnetic cobalt. The incorporation of cobalt into AIPO4-11 framework significantly decreases T 1 and T 2 relaxation times (Table 1). The same trend has been observedS,9 for Co-containing A1PO4-5 and A1PO4-34 and suggested as convincing evidence for cobalt incorporation into the lattice. Table 1. T 1 and T 2 relaxation times for A1PO4-11, CoAPO-11 and CoZnAPO-11. Sample
T 2 (s) x 10-4 2 0.72 1.1
A1PO,-11 CoAPO- 11 CoZnAPO- 11
. . . . . . .
1 . . . . . . . 2fl
. . !
. . . . . . . . . m
I,-
....
-2fl
T1 (S) 6.5 0.19 1.37
...!
.... -4fl
~ .....
! . . . . . . . . . -6fl
PPH
Figure 3. 31p MAS NMR spectrum of as-synthesised ZnAPO-34.
I -8fl
...
506 The local structure around zinc and cobalt in as-synthesised and calcined samples has been studied with EXAFS. Two examples of (FT) magnitude of the k 3 weighted EXAFS spectra is shown in Figure 4a. The FT calculations were performed over the range from 4/~-1 to 12/~-1 using Hanning window function. All the spectra exhibit a single peak, which can be ascribed to oxygen atoms in the first coordination shell around each metal atom. Raw EXAFS spectra were Fourier filtered in the range of 1.1/~ - 2.0/~ and fitted with a single shell EXAFS model, constructed in the FEFF code.10,11 In this way parameters of the first coordination shell i.e. number of oxygen atoms, their distance from the metal atom and Debye-WaUer factor were obtained. Typical examples of the Fourier filtered spectra for zinc are shown in Figs. 4b and 4c. Parameters for all samples are given in Table 2. Table 2. Parameters of the first coordination shell of oxygen atoms around metals in different assynthesised and calcined MeAPO-11 and MeAPO-34 samples: N - number of oxygens; R metal-oxygen distance (uncertainty less than _+0.006 /~); a 2 - Debye-Waller factor (uncertainity of the last digit is given in parentheses). sample
el. "N
CoAPO- 11 Co CoZnAPO- 11 Co CoZnAPO-34 Co CoZnAPO- 11 Zn CoZnAPO-34 Zn ZnAPO-34* Zn *data from ref. 12.
3.0(9) 3.6(9) 3.6(4) 4.0(4) 3.9(4) 4.0(2)
as-synthesised R (/~) c~2(A2) 2.01 2.02 1.93 1.93 1.93 1.94
0.004(2) 0.007(2) 0.004(1) 0.004(1) 0.0027(7) 0.0050(7)
N 2.4(7) 2.0(7) 3.4(9) 4.3(8) 3.9(5) 3.1(2)
calcined R (A) o2(A 2) 2.00 1.98 2.04 1.94 1.93 1.96
0.006(2) 0.002(1) 0.009(2) 0.006(1) 0.003(1) 0.0054(9)
All the fits are characterised by a very strong correlation coefficient of +0.9 between the parameters N and a 2. Therefore, noninteger values for N as obtained from the fit are listed in the table. For most samples the intervals of probable values are centered close to integers. (The large uncertainties are a consequence of correlations and can be reduced by fixing either of the correlated parameters.) The number of oxygen atoms around zinc for all assynthesised samples and around cobalt for as-synthesised CoZnAPO-34 show tetrahedral coordination for zinc and cobalt, proving directly incorporation of the metals. The number of oxygen atoms around cobalt found for as-synthesised MeAPO-11 samples is unexpected and does not agree with elemental analysis and NMR data which strongly suggest the incorporation of cobalt into A1PO4-11 aluminophosphate lattice. Such a low coordination in these samples could not be explained, but it may be related to the presence of amorphous phase as evidenced in SEM pictures. Table 2 shows a decrease in the number of neighbour oxygen atoms of the order of unity in some samples as a result of calcination. Actually, the decrease can be surmised for all samples except Zn in CoZnAPO-34, if the high correlation between N and a 2 is taken into account:
507
20
/
--,,../ r',
.,.-. _ . \ .....,..,.-"
i
:~
~
'
-
R(A)
b 2
-2 -4
~t
6
8
1'0
1'2
1'4
16
k (A")
c 2
-2 -4
~
6
~
k(A-I)
f0
1'2 1 ' 4
16
Figure 4. Zn EXAFS on CoZnAPO-11 samples: a) Fourier transforms (as-synthesised sample - solidiine, calcined sample - dotted line); b) First shell Fourier filtered EXAFS spectra using the range of 1.0A-2.0A, of as-synthesised sample (experiment - solid line, fit- dotted line); c) First shell Fourier filtered EXAFS spectra using the range of 1.0A-2.0A, of calcined sample (experiment - solid line, fit - dotted line).
508 the decrease in N can be compensated by an increase in a2. In reality, it is the EXAFS amplitude that diminishes upon calcination (Fig. 4a), as a result of loss of oxygen atoms from the metal neighborhood. The decrease of the amplitude can be interpreted by the model either as a smaller coordination number or as a larger static disorder. In another instance, such a decrease has been explained by a removal of one oxygen atom around metal sites upon calcination. 12 The presence of such local defects in the aluminophosphate lattice could be important from the catalytic point of view. Some of these materials have been found to posess interesting properties in catalytic reactions. 2
4. CONCLUSIONS Cobalt and zinc containing aluminophosphate molecular sieves with chabazite and AEL structures have been synthesised and characterized by means of EXAFS and solid state NMR spectroscopy. Experimental data show that zinc and cobalt atoms isomorphously substitute aluminium in both structures, up to a level of 20% in MeAPO-34 and up to about 10% in MeAPO- 11.
REFERENCES 1 S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan and E.M. Flanigen, J. Am. Chem. Soc., 104 (1982) 1146. 2 R.A. Sheldon and J. Dakka, Catalysis Today, 19 (1994) 215. 3 S.T. Wilson, S. Oak and E.M. Flanigen, US Pat. 4,567,029 (1986). 4 P.J. Barrie, M.E. Smith, J. Klinowski, Chem. Phys. Letters, 180 (1991) 6. 5 A. Meden, N. Novak Tugar, V. Kau~i6., Materials Science Forum, (1995), in press. 6 E.M. Flanigen, R.L. Patton, S.T. Wilson, Stud. Surf. Sci. and Catal., 37 (1988) 13. 7 F. Deng, Y. Yue, T. Xiao, Y. Du, C. Ye, L. An and H. Wang, J. Phys. Chem., 99 (1995) 6029. 8 S.H. Chen, S.P. Sheu, K.J. Chao, J. Chem. Soc.,Chem. Commun., (1992) 1054. 9 M.P.J. Peeters, L.J.M. van de Ven, J.W. Haan, J.H.C. van Hooff, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 72 (1993) 87. 10 E.A. Stern, M. Newville, B. Ravel, Y. Yacoby, D. Haskel, Physica B, 208&209 (1995) 117. 11 J.J. Rehr, R.C. Albers and S.I. Zabinsky, Phys. Rev. Lett. 69 (1992) 3397. 12 N. Novak Tu~,ar, V. Kau6i6., S. Geremia, G. Vlaic, Zeolites, 15 (1995) 708.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
509
Si,AI S O L I D S O L U T I O N IN S O D A L I T E : S Y N T H E S I S , 29Si N M R A N D X-RAY STRUCTURE Mitsuo Sato *, Eiji Kojima, Hirofumi Uehara and Michihiro Miyake Department of Chemistry, Gunma University, Kiryu, Gunma 376, Japan A formation of complete solid solution series of Si,AI in the sodalite framework is tried in a non aqueous solution. The formation of two types of solid solutions, type A and type B, are successfully confirmed, which are separated at two discontinuous points of 1.4 and 2.0 in the Si/AI ratio. The characteristic X-ray powder diffraction patterns, crystal structure, thermoanalysis and 29 Si MAS NMR data are shown.
1. INTRODUCTION A complete solid solution series of Si,A1 in zeolite frameworks has not been reported. Comparatively extensive ease is the faujasite series including Zeolite X and Y. Most of natural socialite samples are nearly or equal to 1.0 in the Si/A1 ratio. However, Baerlocher et al. [1] reported the synthesis of sodalite of Si/AI = 5.0 using TMA as a template, while Bibby et al. [2] showed the preparation of pure silica sodalite in ethylene glycol solvent. These results suggest a possibility of formation of extensive solid solution in the sodalite series. In this paper, a systematic preparation ~of these sodalite series in a non aqueous solution, and their characterization by X-ray diffraction, thermoanalysis and 29Si NMR spectroscopy are reported. 2. EXPERIMENTAL Both solid NaOH pellets and metallic AI powders were added in the ethylene glycol(EG) solvent, and heated at 200 °C for 2 days in a Teflon coated reaction tube. After confirming the complete solution of A1 metals, a homogeneous gel was prepared by adding fumed silica, and heated at 200 °C for one week in the reaetiontube. In this experiment the mole ratio of EG:SiO2:NaOH was fixed to be 50:5:1. The resultant crystalline materials were washed, air dried and characterized by X-ray~•powder diffraction, differential thermal, electron microprobe and 29Si MAS NMR analyses. X-ray powder data were collected on a Rigaku
510
RAD-C diffictometer using graphite monochrometer CuKa radiation. Lattice parameters were refined using the data corrected with an internal standard Si sample. X-ray Rietveld analysis was performed using the program DBWS9006 [3], in which the data were recorded in consecutive steps of 0.02" in 20 with 10s per step between 10" and 80" in 20. Differential thermal curves were recorded on a Rigaku Thermoflex 8112-BH. 29Si NMR measurements were carried out on a JNM-EX270 spectrometer using magic angle spinning and cross polarization techniques, i.e., a resonance frequency of 54.74 MHz, chemical shifts referenced to tetramethylsilane, and the recycle delay of 8.0. Deconvolution of the complex peak into the components Si(4Al), Si(3AI), Si(2Al), Si(lAl), Si(0Al) was performed using a program LISA(Version5.83)[4]. Chemical analyses of the products were performed using a JXA -733 electron microprobe analyzer. 3. RESULTS and DBCUSSION 3.1. Lattice constants and A1 contents After c o d m i n g the validity of cubic symmetry on these samples, the lattice parameters were refined by least squares method. Figure 1 shows the relationship between the lattice
Si/AI mole ratio
0
1
2
3
4
5
Al atoms in an unit cell
Figure 1. Lattice parameters vs. Al atoms per unit cell.
6
511
constants and AI contents in the framework. From the figure, continuous solid solutions of Si,AI seem to be formed in the framework, but some discontinuous points can be noticed at the Si/AI = 1.4, 2.0 and 5.0. The samples ranging from Si/AI ratio oo to 2.0 are conveniently denoted as type A, and 1.4 to 1.0 as type B. The samples ranging from Si/AI ratio 2.0 to 1.4 are mixtures of both types. Typical X-ray powder diffraction patterns of them are shown in Figure 2. It is obvious that the type A and silica sodalite are characterized with the appearance of strong X-ray reflections of 200 and 220, while the type B remarkable reduction of them. 3.2.
Thermoanalysb
All the samples include the template of ethylene glycol in the sodalite cage. Byheating the sample in air, the desorption of template occurs at different temperatures. Figure 3 shows their differential thermal and thermogravimetric curves. Al-free silica sodalite has its strong exothermic peak at around 470 °C, while AI containing sodalite has two exothermic peaks at around 400 and 6000(3 in the type A, and around 410 and 510°(3 in the type B.
(a) (b)
(a) ¢/)
o
.... I L
-~-~
(c)
.E °~ (9
~
-12.6wt%
(d)
(b)
-lo.7 ~t%
(e)
~ ~
~
-15.2 wt%
~
-12.0 wt%
~
-11.5 wt% 471
c
A
(c) (a)
(d) (d) I ¸ ,
20
,ll,
40
I ...... 20(" )
60
o
Figure 2. X-ray diffraction patterns synthesized in ethylene glycol solvent system. (a) Silica socialite (b) Type A (c) Type B (d) Natural socialite
I
I
I
I
200
400
600
8oo
looo
Temperature('C)
Figure 3. DTA-TG curves of sodalite. (a) Silica sodalite (b) Type A (Si/AI=14. 5) (c) Type A (3.5) (d) Mixture (1.8)
(e) Type B (1.2)
512 Combining the thermogravimetric data with the estimated density, the number of ethylene glycol in a socialite cage can be determined. The contents of EG per one sodalite cage decrease parabolically from 1.0 in the silica socialite to 0.5 in the type B socialite(Si/AI=I.0). 3.3.
29Si NMR spectra 29Si NMR spectra are examined on some selected samples of their Si/AI ratio = co, 9.68, 5.09, 2.87, 1.81, 1.29, and 1.08 respectively, which are shown in Figure 4. Deconvoluting the patterns into individual eomP0nents of Si(4AI), Si(3AI), Si(2AI), Si(1AI), Si(0A1), estimating their intensities and plotting them against A1 contents in the framework, it earl be certainly realized that there are three inflection points at Si/AI = 4.0, 2.0 and 1.5., which are shown in Figure 5. These points are consistent with those predicted on the SCCL (Substituted Concentric Cluster) theory by Sato[5]. This means that the AI distributions in the framework are obeyed by Dempsey rule [6], i.e., minimum number of AI-AI pairs in the 2nd neighbor.
3.4 X-ray structure Crystal structures of silica sodalite, type A and type B were solved by means of X-
(b) (c)
(d)
(g) -50
-1 oo
-150
Chemical shift ( 6 ppm from TMS )
Figure 4. 29Si MAS NMR spectra of sodalites. (a) SOD(0) (b) SOD(l) (c) SOD(2) (d) SOD(3) (e) SOD(4) (f) SOD(5) (g) SOD(6) Numbers in parentheses show Al atoms per unit cell.
ray powder Rietveld analysis. Chemical compositions of them were Si120242.0EG for silica sodalite, Na2.6 (Si9.9A12.1024)(OH)o.5 I.TEG for type A and NAT.6(Si6.7A15.3024)(OH)2.3 1.0EG for type B. Space groups adopted were Im3m for silica sodalite, 143m for type A, and P43n for type B. The space groups for silica socialite and type B were from Richardson et al. [7] and Hassan et al.[8] respectively. The space group of I43m for type A was taken from that by Hassan et al. for the disordered structure. Initial Rietveld refinements were applied on the
513
Si/AI mole ratio 3.99
100 o~ co (1)
\
80
¢/)
60
r-
40
• n
2O
u
I
2.00
1.5
I
I
)1
/'
Ios'c ~') • s,c,,,) I
"~i(z~l)
I
/
.,
r..=.
1 N N
i
I
0
Figure 5.
i
l
0.1
i
I
l
0.2 0.3 AI/(Si+AI)
I
i
0.4
0.5
29Si MAS NMR peak intensity of components vs. Si/AI ratio in the framework.
Si/AI ratio 11.0
160 A
='-" 155
! o
! o o o
! o o o
¢.n
o
,....,
I--
150
3.0 !
2.0
14
! I o o o o o o o o o o o
uy} t-
5.0
i )
"
-<..
I
On 145
,'t--.....
F-
! !
140
i
0
Figure 6
1
2 3 4 5 AI a t o m s in an unit cell
6
T-O-T angles vs. AI atoms l~r unit cell
514 framework structures and extra cations. Judging from the validity of
the R factor
convergence, the difference Fourier synthesis was applied to find the configuration of ethylenglycol involved in the sodalite cage. And then, Rietveld refinement was processed. The final R ~ factors were 22.0% for silica sodalite, 15.7% for type A and 13.5 % for type ]3. The final atomic parameters, bond distances and bond angles are shown in Table 1. There are no differences of fundamental frameworks between them, but the bond distances of T-O and the bond angles T-O-T are remarkably changed with increasing A1 contents in the framework. Figure 6 shows the changes of T-O-T angles between
neighboring TO4
tetrahedron against the AI contents in the framework. It is noteworthy that the
behavior of
characteristics 200 and 220 reflections is closely related to the changes of T-O-T angles. There are three different conformations of ethylene glycol, i.e., cis, trans and gauche. Their conformations in the sodalite cage are found to be vans and gauche for silica sodalite, and cis and gauche for both type A and B. The cis and gauche formation may be due to the presence of Na ions in the sodalite cage. The difference of DTA curves between the silica sodalite and the other types seems to due to that of the conformations formed. Refinement of the EG groups, which appear to be disordered, is still in progress.
Table I. X-ray crystallographicdata of silica,type A and type B sodalite.
Silica sodalite "atomic parameters Space group Im3m Atom Si
O1 C 02
Site 12d 24h 16f 12e
Chemical formula Si12024 • 2.0EG x
0 0 0.5745(2) 0.70(1)
y
114 0.6484(2) 0.5745(2) 1/2
Cell parameter(A) 8.8366(3) z
112 0.6484(2) 0.5745(2) 1/2
B
1.8(1) 2.8(2) 3(1) 2(1)
S!.lica sodalite • bond distances and angles Distances(A) Si-O1 1.589(4) 1.4(1) C-O2 C-C 2.28(2)
Ang!es( ° ) O1-Si-O1 111.2(2) Si-O1-Si
C-C-O2
158.8(3)
85(4)
N
0.13 0.25 0.074(1) 0.010(1)
515 Sodalite type A" atomic parameters Space group 143m Atom Si O1 Na C 02
Site 12d 24g 8c 12e 8c
Chemical formula Naze(Sig.~lzlO=4)OHo5 • 1.7EG x
0 0.1455(2) 0.162(1) 112 0.418(3)
y
114 0.1455(2) 0.162(1) 112 0.418(3)
z
112 0.4600(5) 0.162(1) 0.611(5) 0.418(3)
Cell parameter(A,) 8.8306(4) B
1.9(1) 2.8(1) 6.3(5) 7.5(8) 5.0(9)
N
0.25 0.5 0.05 0.07 0.07
Sodalite type A • bond distances and angles Distances(A) Si-O1 1.621(2) C-O2 1.05(1) C-C 1.93(2) 1.39(3)
(gauche)
(cis)
Angles(') O1-Si-O1 108.9(2) Si-O1-Si 148.8(2) C-C-O2 75(2) 108(2)
(gauche) (cis)
Sodalite type B • atomic parameters Space group P43n Atom Si AI O1 Na 02 C 03
Site 9d 6c 24i 8e 8e 12f 8e
Chemical formula NaT.6(Si6.~ls.3024)OHz3 • 1.0EG x 1/4 1/4 0.1397(3) 0.189(1) 0.292(2) 112 0.399(1)
y 0 1/2 0.1549(3) 0.189(1) 0.292(2) 112 0.399(1)
z 1/2 0 0.4510(3) 0.189(1) 0.292(2) 0.393(1) 0.399(1)
Cell parameter(A) 8.9916(2) B 3.66(1) 2.1(1) 3.9(1) 3.5(1) 5.9(1) 8.4(5) 4.0(6)
N 0.25 0.25 1.0 0.1 0.18 0.1 0.25
Sodalite type B • bond distances and angles Distances(,A,) Si-O1 1.582(3) AI-O1 1.766(3) O3-Na 1.61(2) C-O2 1.29(1) C-C 1.93(2) 1.36(1)
Angles( ° ) O1-Si-O1 107.0(1), 114.6(1) O1-AI-O1 108.4(1), 111.7(1) Si-O1-AI 143.4(2) (gauche)
(cis)
C-C-O2
75(2) 108(2)
(gauche) (cis)
516 4. CONCLUSION The present study confirmed that there are three discontinuous points in the Si,AI solid solution of the sodalite. The points of 1.4 and 2.0 are exactly the same as those in the faujasite series presented by Dempsey et al. [6]. In addition, the three points confirmed here are fairy consistent with the predicted points by Sato[5], which means the A1 atom distributions in the framework to be obeyed by Dempsey rule. The sodalite solid solution can be classified into two types, i.e., type A and type B on the basis of characteristic X-ray diffraction patterns as well as their unit cell parameters. The appearance and disappearance of characteristic X-ray reflections of 200 and 220 are closely related to the change of T-O-T angles between neighboring TO4 tetrahedrons. The difference of thermoanalysis data between the silica sodalite and the other types data is due to that of ethylene glycol conformations involved in the sodalite cage.
REFERENCES 1. 2. 3. 4. 5. 6. 7.
Ch. Baerlocher and W.M. Meier, Helv.Chim.Acta,52 (1969) 1853. D.M.Bibby and M.P.Dale, Nature,317 (1985) 157. A.Sakthivel and R.A.Young, Programs DBWS-9006, Georgia Inst.Tech.,(1990). H.Kurosu, Program LISA,Tokyo Institute of Technology(1990) M.Sato, Chem.Lett.,1195(1985). E.Dempsey, G.H.Kuhl and D.H.Olson, J.Phys.Chem.,73 (1969) 387. J.W.Richardson, J.J.Pluth, J.V.Smith, W.J.Dytrych and D.M.Bibby, J.Phys.Chem., 92 (1988) 243. 8. I.Hassan and H.D.Grundy, Acta.Cryst.,B40 (1984) 6.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
517
S u b s t i t u t i o n of silicon and metal ions in small pore aluminophosphate molecular sieves with chabazite s t r u c t u r e • synthesis and MASNMR study D.K. Chakrabarty, Sunil Ashtekar, A.M. Prakash and S.V.V. Chilukuri Solid State Laboratory, Department of Chemistry, Indian Institute of Technology, Bombay 400076, India
Syntheses of MeAPO-34, MeAPO-44, MeSAPO-34 and MeSAPO-44 where Me = Mn or Mg with varying amounts of silicon and metal ions have been achieved. 31p, 27A1 and 29Si MASNMR spectra of these compounds have been studied. The 31p MASNMR of MnAPO-44 suggest the ordering of the Mn2+ ions give a P(Mn,3A1) environment. MASNMR of MgAPO-44 and MgAPO-34 showed a random distribution of Mg 2+ ions. The P/A1 ratios calculated from MASNMR are in good agreement with the chemical analyses. The substitution of silicon in the MeSAPOs are strongly influenced by the amount of Me present. Samples with small amount of Me essentially have Si(4A1) environments, but increase in the amount of the metal ions leads to the formation of silica-rich regions in the structure. keywords" aluminophosphate, chabazite, manganese, magnesium, MASNMR
1. I N T R O D U C T I O N Although the aluminophosphate analogs - A1PO4-34 and A1PO4-44 of the zeolite chabazite are not known, these structures can be stabilised by partial substitution of the T atoms by silicon and divalent metal atoms such as Co 2+, Mn 2+ or Mg 2+ etc. Substitution of Mn 2+ in the AIPO4 or SAPO molecular sieve framework has been attempted by several groups. Thus, Levi et al.[1] found that, at low concentration, Mn 2+ entered T sites in MnAPO-5, but when the manganese concentration in the gel was higher, the majority of the Mn 2+ remained outside the framework. Similar conclusions were arrived at in the case of MnSAPO-44 by Olender et al.[2] and for MnSAPO-11 by Lee et al,[3]. Reports on the substitution of Mg2+ are limited. Flanigen et al.[4] were the first to report the substitution of Mg2+ in the aluminophosphate molecular sieve framework. Evidence on the incorporation of Mg 2+ at the T site of aluminophosphate framework was presented by Barrie and Klinowski [5] for MgAPO-20. Goepper et al.[6] studied magne-
518 sium incorporation in the structure types -5, -11 and -34. Very recently, Deng et al.[7] have reported the preparation and MASNMR of MgAPO:34. 2. E X P E R I M E N T A L Morpholine and cyclohexylamine have been used as templates for the syntheses of the structures -34 and -44 respectively. Details of synthesis procedures have been described elsewhere [8,9]. MASNMR spectra were recorded on a Varian VXR-300S spectrometer with a Doty scientific CP-MAS probe. The frequencies were 78.15, 121.41 and 59.59 MHz for 27A1, 3ap and 29Si respectively. Data were acquired at a MAS speed of 4.5 KHz. Aluminium nitrate in water, 85% phosphoric acid and tetramethylsilane were employed as references. Chemical analysis of the samples was carried out after calcination at 500°C. The samples were dissolved in aqua regia for analysing aluminium and phosphorus. The undissolved portion was fused with lithium metaborate and subsequently dissolved in dilute nitric acid. Analysis was done on an atomic emission spectrometer with ICP source (Labtam Plasma Lab 8440). 3. R E S U L T S A N D D I S C U S S I O N 3.1 Synthesis MnAPO-44 could be crystallized in pure form only at 180°C. At lower temperature, the pure phase could not be obtained even after prolonged heating for 7 days. X-ray powder diffraction patterns (XRD) of MnSAPO-34 and MnSAPO-44 were very similar to those of SAPO-34 and SAPO-44 respectively [10]. MnAPO-34 and MnAPO-44 showed diffraction fines with d values similar to those of the respective SAPOs. The intensity of these lines in the case of MnAPO-44 were very similar to those of CoAPO-44, showing that the (101) peak had intensity far greater than the others [11]. This would mean that in MnAPO-44, manganese atoms were occupying the same sites as cobalt did in CoAPO-44. The intensity of the peaks for MnAPO-34 was similar to that of SAPO-34 and MnSAPO34 rather than that of CoAPO-34. This appears to be due to low substitution of Mn 2+ at the T sites, as will be discussed later. The compositions of the as-synthesized samples based on chemical analysis are given in Table 1. The MgAPOs could be crystallized in pure form at 180°C. It was noticed that the MgAPO samples with these structures do not form below an optimum concentration of magnesium. Two compositions of MgAPO-34 and -44 each were prepared by taking 0.4 and 0.6 moles of magnesium in the initial gel. MgSAPO-34 and -44 samples were prepared with three different compositions each with a specified magnesium content of the initial gel (0.1, 0.2 or 0.4 moles). The highly crystalline nature of the samples was evident from the X-ray diffraction patterns and SEM. An increase in magnesium content in the initial gel above 0.4 mole led to the formation of MgSAPO-20 as a second phase along with MgSAPO-34 crystals. The compositions of the calcined samples based on chemical analysis are given in Table 2. In all the MeSAPOs, the ratio (Si+P)/(AI+Me) is greater than or equal to unity. In
519 Table 1: Chemical Composition and Acidity of the Samples from TemperatureProgrammed Desorption of Ammonia (P+Si)/(AI+Mn) sample MnAPO-34 MnAPO-20 MnSAPO-34/1 MnSAPO-34/2 MnAPO-44 MnSAPO-44/1 MnSAPO-44/2
Mn 0.13 0.10 0.007 0.028 0.14 0.0075 0.028
Al P 0.42 0.45 0.40 0.50 0.49 0.37 0.46 0.38 0.36 0.50 0.48 0.38 0.45 0.40
Si 0.14 0.13 0.13 0.12
0.82 1.0 1.02 1.04 1.0 1.05 1.09
acidity (mmol/g) moderate + strong 0.66 1.94 1.75 1.39 1.86 1.53
principle silicon can substitute for aluminium (mechanism 1) or for phosphorus (mechanism 2) or two silicon atoms can replace an (A1 + Si) pair (mechanism 3). However, the known SAPO compositions could be understood if one assumed that substitution occurred according to mechanism 2 along with the formation of silica rich regions in the structure. It has been shown from 29Si MASNMR results that in SAPO-34 and SAPO-44 structures, most of the silicon atoms go to the phosphorus sites (mechanism 2), although a small portion of the silicon atoms form silica islands that prevent the formation of Si-O-P linkages [10]. In the case of metal atoms, it has been shown that the Me 2+ ions almost exclusively go to the aluminium sites in the framework [12]. Based on this assumption, the ratio (Si+P)/(Me+A1) in the MeSAPOs should be unity, if silica islands were not formed. The fact that almost all the samples of MeSAPO-34 and MeSAPO-44 have this ratio greater than 1 is an indication that at least a part of the silicon has formed silica-rich regions. Such silica patches are more prominent in the samples with higher amounts of manganese or magnesium. This will be discussed further along with the MASNMR results. The samples without silicon, MnAPO-20 and MnAPO-44 have (Mn + A1) : P ratio equal to one, while this value is greater than 1 for MnAPO-34 suggesting that the latter has extra-framework manganese. Since pure A1PO4-34 and A1PO4-44 could not be synthesized so far, the formation of the-34 and -44 structures with manganese in the absence of silicon would suggest that manganese is indeed occupying some of the T sites in order to stabilize the structure. Crystallization behaviour of MnAPO-34 from a gel containing morpholine is interesting. No crystallization of MnAPO-34 was noticed at temperatures below 200°C. At 200°C, MnAPO-34 crystallized within 3 hours. Use of tetramethylammonium hydroxide has been reported to take seven days or more to crystallize this structure. Thus, morpholine can be used for rapid synthesis of MnAPO-34. Heating the gel beyond 6 hours resulted in the transformation of MnAPO-34 to MnAPO-20. The latter is isostructural with the very small pore zeolite-sodalite. After 12 hours of heating, the product obtained was pure MnAPO-20. Interestingly, MnAPO-20 was also unstable in the mother gel and a third phase began to crystallize after 24 hours of heating. After heating for 30 hours, the product had only this third phase. XRD of this new phase did not match with any
520 of the known AIPO4-type structures. Table 2: Chemical Composition and Acidity of the Samples from TemperatureProgrammed Desorption of Armnonia (P+Si)/(AI+Mg) sample MgAPO-34/1 MgAPO-34/2 MgSAPO-34/1 MgSAPO-34/2 MgSAPO-34/3 MgAPO-44/1 MgAPO-44/2 MgSAPO-44/1 MgSAPO-44/2 MgSAPO-44/3
Mg AI 0.13 0.39 0.16 0.35 0.03 0.47 0.04 0.45 0.10 0.38 0.10 0.40 0.16 0.35 0.02 0.47 0.04 0.45 0.09 0.40
P 0.48 0.49 0.37 0.40 0.43 0.50 0.49 0.38 0.40 0.43
Si 0.13 0.11 0.09 0.13 0.11 0.08
0.92 0.96 1.00 1.04 1.08 1.00
0.96 1.04
1.01 1.04"
structural acidity (mmol/g) 0.27 0.30 0.52 0.44 0.28 0.39 0.37 0.53 0.43 0.36
.
In the MgAPO samples, on the other hand, P/(AI+Mg) ratio was less than one (except for MgAPO-44/1, when had the ideal composition). This would require the presence of a small amount of extraframework Mg/Al in the sample. In order to find out the amount of extraframework Mg 2+, the calcined samples were exchanged with 0.5 molar ammonium acetate at 60°C for 6 hours. According to the chenfical compositions (Table 2), one would expect the MgAPO-34/1 to have larger amount of exchangeable Mg2+. Experiments, however, showed that this had only 9.3% exchangeable magnesium as compared to 27.7% in case of MgAPO-44/1, which has the ideal composition. It is clear that during the exchange experiments, some magnesium comes out of the framework. This is further established by the partial loss of crystallinity of the samples after the exchange experiments. 3.2 M A S N M R 27A1 and alp MAS NMR spectra of MnAPO-34 and MnAPO-44 are shown in Fig.1. Similar spectra were shown by the MnSAPOs also. All the spectra showed spinning side bands the intensity of which increased with the Mn z+ content. MnAPO-34 and MnAPO-44 showed 2rAl signals at 39.5 and 35.9, ppm respectively that can be assigned to tetrahedral aluminium [13]. Chemical shifts observed in MnAPO samples showed greater deshielding as compared to the corresponding SAPO samples [10]. The 2rAl spectrum of MnAPO-44 showed broad bands and intense side bands because of the presence of a large amount of manganese in this sample. There is an additonal peak at 5.5 ppm which may be due to extra coordination of A1. alp spectra of MnAPO-34 showed a central band at -26.5 ppm, which can be easily identified with tetrahedral phosphorus. In this sample, most of the manganese was outside the framework and the spectrum is similar to those of the MnSAPO-34 samples, except that the side bands were more intense. However, in MnAPO-44, which appeared to have a large amount of manganese in the framework, the central peak was shifted to -4.1 ppm.
521
zTAw o t
(b)
Figure 1: 27AI and 31p MASNMR spectra of (a) MnAPO-34 and (b) MnAPO-44. This unambiguous but unusually large shift we assign to P(Mn, 3A1) environment. If there is to be a complete ordering of this type throughout the structure, the composition should have a Mn:A1 ratio 1:3. Within the error of chemical analysis, this was indeed so. I !
tffl
t~lo
34/3 34/2
4/1
- ~ 4"0 -~0-~6 -rio P ~ Figure 2. 29Si MASNMR spectra of the MnSAPO samples.
,.:o 6'o Figure 3.29Si MASNMR spectra of the MgSAPO samples.
Figures 2 and 3 show the 29Si spectra of the MeSAPO samples. There was an intense peak at -90.8 ppm and weak resonances at -94, -99 and -109 ppm for MnSAPO-34/1. These peaks suggested multiple silicon environments. The peak at -90.4 is due to Si(4A1) environment, and the other peaks are due to Si(nA1, 4-n Si) environments formed because of the presence of silica-rich regions in the structure [5,10]. The spectra of MnSAPO-44 samples were similar to those of MnSAPO-34. It can be seen that there was a significant decrease in the intensity of the Si(4AI) peak with the increase in the manganese content.
522 This means that the increase in the amount of manganese gave rise to more silica-rich regions as seen from the 29Si spectra. In the MgSAPO samples, when the amount of Mg 2+ was small, the peak due to Si(4A1) at -92 ppm was predominant. As the amount of magnesium was increased, the multiple silicon environments became clearly visible. They arise due to the formation of silica-rich regions. The interesting part of the 29Si MASNMR of the MgSAPO samples was that the silica-rich region became more prevalent with higher amount of magnesium in the sample, although the silicon content was actually less. Mn 2+ (r = 0.66 /~) and Mg (r = 0.58/~,) are bigger than A13+ (r = 0.43/~). Since Me2+ replaces A13+, it distorts the tetrahedron pushing the oxygen towards the P atoms. Hence it becomes difficult for Si4+ (r = 0.33/~) that are larger than ps+ (0.25/~) to occupy the T sites next to Me2+. Thus, isolated substitution of P by Si becomes less prevalent. As the amount of metal ions increase, silicon prefers to form more silica-rich regions in which silicon atoms occupy both aluminium and phosphorus sites, thus minimising the distortion. It is also evident that silica-rich regions are more abundant in the MeSAPO34 samples than in the MeSAPO-44 samples; that is also reflected in their difference in ~ciditv. ~. ?
--
•
.6o ~o-2o ~ i
•
•
I~"
"
~lp
io 6 '2"o~
~
Figure 4. 2r'.~l and sip MASNMR Spectra of (a) MgAPO-44/1 and (b) MgAPO-44/2. Figure 4 presents the 27A1 and alp MASNMR spectra of the MgAPO-44 samples with varying amounts of magnesium. 27A1 NMR showed a sharp resonance at 37 ppm characteristic of tetrahedral aluminium in the framework [12]. The sample MgAPO-44/2 that had a nearly ideal composition showed 31p NMR peaks at -28.8;-23.0 and -16.8 ppm. Based on the MASNMR of SAPO-44 that has all the phosphorus atoms in P(4AI) environment and showed a single NMR peak at -29.2 ppm [10], we assign the -28.8 ppm peak to P(4A1). Assuming that Mg-O-Mg linkages are absent (extended Lowenstein's rule), the other possible phosphorus environments are P(3AI,IMg), P(2AI,2Mg), P(1AI,3Mg) and P(0A1,4Mg). Considering the relative amounts of magnesium and aluminium in the sample, the last coordination is highly unlikely. Barrie and Klinowski [5] identified the NMR bands P(4Al) (-34.9 ppm), P(3AI,1Mg) (-28.0 ppm), P(2A1,2Mg) (-21.1 ppm) and a very weak P(1AI,3Mg) (-14.0 ppm) signal for a MgAPO-20 sample having the same composition as MgAPO-44/2 reported here. It is clear that the phosphorus atoms
523 in MgAPO-20 are more shielded than those in MgAPO-44. But then it is known that the MASNMR peak positions for the same coordination of the atoms depend on the structure type. In fact, the only 31p peak in A1PO4-20 is due to P(4AI) that appeared at -36.0 ppm, whereas the same phosphorus P(4AI) environment gave a peak at -29.2 ppm in SAPO-44. Therefore, the peaks at -23.0 ppm and -16.8 ppm in MgAPO-44 are assigned to P(3AI, IMg) and P(2A1,2Mg) coordinations respectively. Barrie and Klinowski [5] calculated the P/AI ratio in MgAPO-20 using the equation: 4 En=0 ]P(nAl) (1) P/AI
=
4
~-,n=O0.25nlp(nAl)
which assumes that Mg2+ ions substitute AI3+ only and all the aluminium should be in an AI(4P) coordination and all the Mg 2+ should be in a Mg(4P) coordination. Using this equation, we calculated the P/A1 ratio in MgAPO-44/2 to be 1.42, which is close to the chemical composition of the sample (P/AI = 1.40). 2¢A1 and 31p MASNMR spectra of the MgAPO-34 samples are shown in Fig.5. 27A1 MASNMR spectra of all the samples of MgAPO-34 and MgSAPO-34 showed sharp resonances at about 38 ppm, characteristic of tetrahedral aluminium in the framework [12]. 31p MASNMR spectra of MgAPO-34 samples showed peaks at -8.6, -13.1, -16.2, -20.8, -23.9 and -27.3, whereas Goepper et al.[6] observed only three resonances at -21.4, -25.6 and -28.3 ppm. By comparison with 31p NMR spectra of SAPO-34 [10], we assign the peak at -27.7 ppm in MgAPO-34 samples to P(4A1) environment. The peaks at -23.9 ppm, -16.2 ppm and -8.6 ppm may be assigned to P(3AI,IMg), P(2A1,2Mg) and P(1A1,3Mg) coordinations respectively by comparing with the observed resonances in MgAPO-44 as both structures -34 and -44 are the structural analogs of the zeolite chabazite. This leaves us with the two peaks at -20.8 ppm and -13.1 ppm. These resonances may be arising from the splitting of the P(3AI, IMg)and P(2A1,2Mg) peaks respectively, due to the presence of structurally nonequivalent sites. Similar assignments for the 31p MASNMR peaks have been recently made by Deng et al.[7]. cq
~d
o
27,, I'~
~"'
Figure 5. 27A1 and 3~p MASNMR spectre of (a) MgAPO-34/] and (b) MgAPO-34/2. The existence of two crystaUographically nonequivalent T sites was shown by Ito et al.[15] in SAPO-34. These sites, however, were not distinguished in the 31p MASNMR spectra
524 of SAPO-34 [10]. Using the above assignments , we calculated the P/Al ratio to to be 1.43 for MgAPO-34/2 which is close to the value (1.40) obtained from chemical composition. We have shown here that it is possible to calculate the P/A1 ratio from 31p MASNMR spectra in the MgAPO-44 and MgAPO-34 by assuming a random distribution of magnesium at the aluminium sites. ACKNOWLEDGEMENT This work has been funded by a research grant from the CSIR, New Delhi. SA and AMP are grateful to the CSIR for the award of research fellowships. The authors thank RSIC, IIT Bombay, for making available the various characterization facilities.
REFERENCES 1. 2. 3. 4.
Z. Levi, A.M. Raitsmiring and D. Goldfarb, J. Phys. Chem., 95 (1991) 3830. Z. Olender, D.Goldfarb and J. Batista, J. Am. Chem. Soc., 115 (1993) 1106. C.W. Lee, X. Chen, G. Brouet and L. Kevan, J. Phys. Chem, 96 (1992) 3830. E.M. Flanigen in Zeolite Synthesis, eds. M.L. OceUi and H.E. Robson, Am. Chem. Soc., Washington DC, 1989, p.329. 5. P.J. Barrie and J.K. Klinowski, J. Phys. Chem., 93 (1989) 5972. 6. M. Goepper, F. Guth, L. Delmotte, J.L. Guth and H. Kessler, in Zeolites : Facts, Figures, Future, eds. P.A. Jacobs and R.A. van Santen. Elsevier, Amsterdam, 1989, p.857. 7. F. Deng, Y. Yue, T. Xiao, Y. Du, C. Ye, L. Au and H. Hong, J. Phys. Chem., 99 (1995) 6029. 8. S. Ashtekar, S.V.V. Chilukuri, A.M. Prakash and D.K. Chakrabarty, J. Phys. Chem. (in press). 9. S. Ashtekar, A.M. Prakash, S.V.V. Chilukuri and D.K. Chakrabarty, J. Chem. Soc., Faraday Trans., (in press). 10. S. Ashtekar, C.V.V. Satyanaryana and D.K. Chakrabarty, J. Phys. Chem., 98 (1994) 4878. 11. S. Ashtekar, A.M. Prakash, S.V.V. Chilikuri, C.S. Harendranath and D.K. Chakrabatty, J. Phys. Chem., 99 (1995) 6937. 12. C.S. Blackwell and R.L. Patton, J. Phys. Chem., 88 (1984) 6135. 13. D. Hasha, L.S. Saldarriga, C. Saldarriga, P.E. Hathaway, D.F. Cox and M.E. Davis, J. Am. Chem. Soc., 110 (1988) 2127. 14. R.J.B. Jakeman, A.K. Cheetham, N.J. Clayden and N.J. Dobson, J. Am. Chem.
Soc.,
105 (1985) 6249. 15. M. Ito, Y. Shimoyama, Y. Saito, Y. Tsutita and M. Otake, Acta Cryst., C41 (1985) 1698.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
525
Inclusion of sodium chloride in zeolite N a Y studied by 23Na N M R spectroscopy U. Tracht*, A. Seidel and B. Boddenberg Lehrstutd fiir Physikalische Chemie 11, Universifiit Dortmund, Otto-Hahn-Str. 6, D-44227 Dortmund, Germany
A mechanical mixture of polycrystalline NaC1 and zeolite NaY in the weight-ratio of 0.145 was heated at 823 K under vacuum to produce a salt dispersion in the voids of the zeolitic matrix. The obtained sample was subjected to various further pretreatment steps, and was studied with solid-state 23Na NMR spectroscopy, XRD, and low-temperature nitrogen adsorption. It is demonstrated that the inclusion of NaC1 in the sodalite ([3)- and supercages leads to characteristic well-resolved M R lines under magic-angle spinning (MAS) conditions. Washing the sample with water removes the salt from the supercages but leaves the salt in the 13-cages.
1. Introduction
Due to the regular arrangement of channels and cavities, zeolites provide interesting host matrices for dispersions of solid materials on the nanometer scale. Various procedures for loading zeolites with such materials, especially salts, have been employed, e. g. impregnation with concentrated salt solutions and subsequent evaporation of the solvent [1], and monolayer dispersion whereby the zeolite and the crystalline solid are mechanically mixed and treated at elevated temperatures [2]. In the present contribution the latter method is used to produce sodium chloride dispersions in zeolite NaY. The loaded zeolite is studied with the aid of solidstate 23Na NMR spectroscopy which has proven a useful tool for the detection of sodium cations in zeolites NaX and NaY [3-6] and of sodium salt inclusions in sodalites [7-8].
2. Experimental
Hydrated zeolite NaY (Linde LZ-Y 52; Union Carbide; Si/Al=2.4) and polycrystalline NaC1 (Fluka) were carefully mixed mechanically in such proportion to yield a nominal loading of 5.3 NaC1 per 1/8 unit cell (uc) of the zeolite. The mixture was heated under high vacuum in a quartz tube up to 823 K and maintained at this temperature for 24 h. After cooling to ambient temperature, part of the material ('as-prepared') was transferred into a ZrO2 MAS NMR rotor (4 mm o. d.) in dry argon atmosphere. Another part was held at the normal *Present address: Max-Planck-Institut fiir Polymerforschung, Mainz, Germany
526 atmosphere for several days ('rehydrated') and then transferred into the rotor for NMR measurements. Part of the rehydrated material was repeatedly washed with bidistilled water at ambient temperature and subsequently dried at 353 K. NMR samples of this material after saturation with water (~ydrated') as well as after treatment at 673 K under high vacuum ('dehydrated') were prepared. Static and magic-angle spinning (MAS) 23Na NMR spectra of the differently pretreated samples were obtained with a high-power NMR spectrometer (MSL 400, Bruker, Karlsruhe) operating at the resonance frequency coo /2 ~r= 105.84 MHz. Excitation with ~r/8-pulses was always employed. The MAS experiments were performed at the rotor spinning frequency 10 kHz. 1.0 mol dm"3 aqueous NaC1 solution was used as the external reference. In order to evidence the salt inclusion, XRD powder patterns of the mechanical mixture of hydrated zeolite NaY and NaC1 before heating, and of the inclusion compound in the rehydrated state were recorded with monochromatic Cu-Ka radiation (A=0.154178 nm). Nitrogen adsorption isotherms measured at 77 K in an all-steel adsorption aparatus were used to determine the free supercage volume of dehydrated zeolite NaY as well as of the NaY/NaC1 inclusion compound in the as-prepared and dehydrated states.
3. Results
Figure 1 shows the XRD powder patterns of the mechanical mixture of hydrated zeolite NaY with NaC1 (a) and of the material heated at 823 K in the rehydrated state (b). Obviously,
CI
I
15
25
35
20
45
55
15
25
35
45
55
20
Figure 1. XRD powder patterns of a mechanical mixture of NaY/NaC1 (a) and of the rehydrated NaY/NaC1 compound (b). Asterisks denote the reflexes of crystalline NaC1.
the relative intensities of the NaC1 reflexes are drastically reduced by the heating procedure. The ratio of the intensities of the strongest observed reflexes of NaC1 (20= 31. 7 °) and the zeolite matrix (20= 23.7 °) decreases from 10.0 for the reference mixture to 1.3 for the
527 NaY/NaC1 compound. These findings indicate that about 90 % of the crystalline NaC1 present in the mechanical mixture has been dispersed upon heating. The nitrogen adsorption isotherms (77 K) of dehydrated zeolite NaY as well as of the inclusion compound in the as-prepared and dehydrated states each steeply increase at low equilibrium pressures and reach saturation at p / Po ~ 0.05. The obtained saturation capacities are collected inTable 1.
Table 1 Nitrot~en saturation capacities (77 K) NaY dehydrated nso~/ (N 2 / uc)
132+_3
NaY / NaC1 as-prepared dehydrated 82+_3
129+_3
In comparison to dehydrated NaY, the nitrogen saturation capacity of the as-prepared sample NaY/NaC1 is reduced by about 40 % indicating the blocking of the supercage pore space by NaC1 deposits. After washing and subsequent dehydration the saturation capacity of NaY is practically restored giving evidence that NaC1 is almost completely removed from the supercages by this treatment. Figure 2 shows the 23Na MAS NMR spectra of the NaC1 inclusion compound in the asprepared state (a) and after rehydration at the air (b). The former sample exhibits two overlapping lines centered at -14 and +2 ppm as well as a hardly resolved line at +7 ppnt The rehydrated sample shows overlapping lines at -2 and +2 ppm as well as two very sharp overlapping lines at about +7 ppm The lines in the +7 ppm region can be attributed to nonoccluded NaC1 since pure microcrystalline NaC1 is known to resonate at +7.3 ppm relative to the standard used here. Figure 3 shows the :3Na MAS NMR spectra of the inclusion compound after washing and saturation with water (a), and after subsequent dehydration (b). For comparison, the figure contains the :3Na MAS NMR spectra of the hydrated (c) and dehydrated (d) parent zeolite NaY. The compound samples both in the hydrated and dehydrated state exhibit a line at +2 ppm but otherwise show different spectral behaviour. In the hydrated state a fitrther line is observed at -2 ppm, whereas in the dehydrated form a double-humped spectrum portion with maxima at about -22 and-48 ppm appears. A similar double hump pattern with maxima at -23 and -54 ppm is observed for dehydrated NaY with a further Gaussian line centered at -4 ppm Hydrated zeolite NaY exhibits a rather narrow line at -1 ppm and a broad component at about -5 ppm It is remarkable that each of the samples containing NaC1 shows the line at +2 ppm which in all cases has a linewidth in the range 0.6-0.8 kHz. It was observed [9] that under static conditions this line is considerably broader by a factor of about 4. Actually, this line does not appear in the spectra of NaY, neither in the hydrated nor in the dehydrated state.
528
I
I
I
I
I
I
I
I
I
20
0
I
C 40 I
-20
I
-40
I
I
-100
-150
i! ..... /:
b
":. •, .....................................
40
_.. .......................
I
I
I
I
20
0
-20
-40
-60
8 / ppm
Figure 2. 23Na MAS NMR spectra of NaY/NaC1 samples as-prepared (a) and rehydrated (b).
100
50
t
t
0
-50
~ -200
5 / ppm Figure 3.23Na MAS NMR spectra of washed NaY/NaC1 samples in the hydrated (a) and dehydrated (b) state, as well as of hydrated (e) and dehydrated (d) zeolite NaY.
4. D i s c u s s i o n
We begin the discussion with a comparison of the 23Na ~ spectra of the washed and subsequently dehydrated sample NaY/NaC1 (Figure 3, b) and the dehydrated zeolite NaY (Figure 3, d). It has been shown [3-5] that in the MAS spectrum of NaY the Gaussian line at -4 ppm is due to sodium cations at the crystallographic position SI in the hexagonal prisms. The double-hump high-field portion which represents a second-order quadmpole pattern originates from Na + cations residing at the hexagonal window sites SH in the supercages and sr in the ~-cages [3-4]. From these assignmems we can immediately conclude that in the NaY/NaCbsample the Na + on sites SI are completely removed or, at least, strongly disturbed (missing of the Gaussian line at -4 ppm), and that the Na + ions at hexagonal window sites - at least part of them- experience similar environments as in NaY (almost no change of the quadmpole pattern). The latter aspect becomes more evident by inspection of Figure 4 where the experimental NMR spectra are decomposed and compared with the corresponding
529 simulations. The quadrupole patterns of the samples are characterized by only slightly different values of the quadmpole coupling constant ( Q C C ) and of the isotropic chemical ~qhiR( g ~ ) whereas the asymmetry parameter of the electric field gradient tensor (r/) remains unchanged. The most remarkable feature of the spectrum of dehydrated NaY/NaC1 is the intense Gaussian line at +2 ppm which has no counterpart in the spectrum of NaY. I
50
I
. J~
I
100
I~
0
-50
-100
-150
-200
~5/ppm
-t . . . . . 50
I
..\
I"
...... 100
I
.............
Jl
\---q 0
..... -50
q"- . . . . -100
-I . . . . . . . . -150 -200
8/ppm
Figure 4. Comparison of experimental (upper spectra) and simulated (lower spectra) 23Na MAS NMR spectra. The decomposition into a Gaussian line (GL) and a quadrupole pattern (QP) is shown by dotted lines: (a) dehydrated NaCI/NaY (GL: g~ = +2 ppm; QP: g~ = - 1 ppm, Q C C = 4 . 0 MHz, I?=0.3); (b) dehydrated NaY [3] (GL: g~ = - 4 ppm; QP: 8~o = +3 ppm, QCC = 4.3 MHz, rl = O.3).
The physicochemical picture of the dehydrated compound sample NaY/NaC1 that emerges from the just discussed spectral features becomes clear when we take into account the nitrogen adsorption data as well as observations from the literature. From the data in Table 1 it is evident that the washing procedure restores the supercage pore volume by removing the incorporated salt deposits therein. Actually, Rabo has shown that sodium chloride introduced into NaY by the impregnation technique can be completely washed out of the supercages whereas the salt is irreversibly held in the ~-cages at the maximum level of 1 NaC1 per cage [ 1]. Assuming that in the presently studied sample NaY/NaC1 the situation is comparable, we have a straightforward explanation for the observed spectral features of the dehydrated sample: the line at +2 ppm and the high-field quadrupole pattern originate from Na + cations in the 13cage under the influence of a halide anion, and from ions in the supercages, respectively. Let us now consider the 23Na NMR spectra of the washed NaY/NaC1 sample in the hydrated state (Figure 3, a) and of hydrated NaY (Figure 3, c). In hydrated NaY the narrow
530 line at -1 ppm and the broad component at -5 ppm are due to hydrated Na + cations in the super- and 13-cages, respectively [3]. The observation that in the NaY/NaC1 inclusion compound the broad component at -5 ppm is missing and instead the line at +2 ppm appears, whereas the narrow line at -1 ppm of NaY is still observed with only a small upfield shitt, is strong support for the aforementioned assignment of the signal at +2 ppm to Na + ions in the 13cages which appears here with identical position and linewidth. At this stage it seems to be in order to have a closer look at the +2 ppm line. First, we conclude from the previously mentioned observation of considerable line-broadening under static conditions that the Na + cations giving rise to this line, experience strong anisotropic coupling with their surroundings and are immobile on the NMR time-scale. Interestingly, a line of similar features with, however, somewhat different isotropic chemical shift (c~i~o ~ +6 ppm) is observed for NaCl-sodalite where (Na4C1)3+ complexes with one CI" anion residing in the center of the sodalite cage have been proposed [8]. We suggest a similar complex in the sodalite(f3)-cage of the presently studied faujasite structure where the Na + ions occupy the four tetrahedrally arranged s r positions. From di~action studies [10,11] it is known that the overall Na + content at SI and s r positions in dehydrated NaY corresponds to ca. 3 Na + cations per 1/8 unit cell. So, stoichiometrically, the aforementioned complexes can readily be formed ff a NaCl-tmit is added to each ~3-cage, and if the small amount ofNa + ions residing on sites SI in dehydrated NaY is completely displaced to the s r position under the influence of the negative charge of the 13-cage-C1- anion in the inclusion compound. An interesting question concerns the degree of occupation of the f3-cages by NaC1. Since the sodalite-cages without salt are known to be accessible to water molecules, a broad component at -5 ppm should be expected in the NMR spectra of hydrated and rehydrated NaY/NaC1 if some cages of this type remain unoccupied by NaC1. From the circumstance that this line is completely missing in the respective NMR spectra, i. e. the line is below the limit of detection, we conclude that the ~-cages are occupied by NaC1 to a high degree. This result is in agreement with Rabo's findings obtained from chemical analysis [ 1]. The preceding discussions of the washed samples have led us to two important conclusions. First, the inspection of the 23Na MAS NMR spectra allows us to differentiate unequivocally between Na + cations residing in the super- and f3-cages. Secondly, the signal of Na + in the f3-cages is uninfluenced by the physical state of the supercages. On the basis of these conclusions, the interpretation of the spectra of the as-prepared and rehydrated samples is a matter of straightforward analysis. Since the line at +2 ppm occurs in both non-washed samples with features unchanged in comparison to the washed materials, the lines at -14 ppm and -2 ppm observed for the as-prepared and rehydrated sample, respectively, must be due to sodium species in the supercages. We assign the line at -14 ppm to [Nax(NaC1)y]x+ clusters, in the formation of which both occluded NaC1 and SII-Na + cations of the zeolite matrix are involved. This is concluded from the missing of the quadrupole pattern typical for Na + on SII as is found in the dehydrated washed sample, and from the drastically reduced nitrogen adsorption capacity. In the rehydrated sample it is the line at -2 ppm which is characteristic of the state of the Na + cations in the supercages. Since the width of this component increases only slightly from 0.3 to 0.6 kHz if the sample is not rotated, the signal must come from rather mobile, most probably, hydrated sodium cations. Hence, the state of the supercages in the respective material is best described to be a highly concentrated aqeous cationic salt solution.
531 5. Conclusions
In the present contribution it has been shown that 23Na NMR spectroscopy can favourably be used to study sodium salt inclusions in the void system of faujasite type zeolites. The sodium chloride that is irreversibly occluded in the small ~-cages, can readily be detected, and can unequivocally be distinguished from salt as well as cationic salt solutions in the supercages.
Acknowledgement
Financial support of this work by 'Fends der Chemischen Industrie' is gratefiflly acknowledged.
References
.
°
8. 9. 10. 11.
J. A. gabo, in Zeolite Chemistry and Catalysis (J. A. Rabo, Ed.) ACS Monograph 171, Am. Chem. See., Washington, D.C., 1976, p. 332. Y. C. Xie and Y. Q. Tang, Adv. Catal., 37 (1990) 1. A. Seidel and B. Boddenberg, Z. Natufforsch, 50a (1995) 199. M. Hunger, G. Engelhardt, I~ Keller, and J. Weitkamp, Solid State Nuclear Magnetic Resonance, 2 (1993) 111. G. Engelhardt, M. Hunger, H. Keller, and J. Weitkamp, in Zeolites and Related Microporous Materials: State of the Art 1994 (Eds.: J. Weitkamp, H. G. Karge, H. Pfeifer, and W. H61derich), Studies in Surface Science and Catalysis Vol. 84, Elsevier, Am~erdam, 1994, p. 421. M. Feuerstein, M. Hunger, and G. Engelhardt, Solid State Nuclear Magnetic Resonance, in press. N. Ch. Nielsen, H. Bildsoe, H. J. Jakobsen, and P. Norby, Zeolites, 11 (1991) 622. G. Engelhardt, P. Sieger, and J. Felsche, Analytica Chimiea Acta, 283 (1993) 967. B. Boddenberg, A. Seidel, and U. Tracht, unpublished results. W. J. Mortier, Compilation of Extra Framework Sites in Zeolites, Butterworth, Guildford, 1982, and references cited therein. A. N. Fitch, H. Jobic, and A. Renouprez, J. Phys. Chem., 90 (1986) 1311.
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H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 1997 Elsevier Science B.V.
533
Spectroscopic investigation of the state of aluminium in M C M - 4 1 aluminosilicates Stefania Viale a, Edoardo Garrone a, Francesco Di Renzo b, Bich Chiche b, Francois Fajula b a
Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universith di Torino, via P. Giuria 7, I-10125 Torino, Italy
b Laboratoire de Materiaux Catalytiques et Catalyse en Chimie Organique, CNRS UMR 5618, ENSCM, 8 rue de l'Ecole Normale, F-34053 Montpellier, France
The synthesis procedure brings about a low threshold value for the surface concentration of A1 species and renders MCM-41 a model system for the study of silicaalumina phases. On severely outgassed samples, two types of Lewis sites are present, differing in their protrusion from the surface, as well as Bronsted sites engaged in lateral H-bonding, evidenced by the interaction with NH3 and CO. The conversion of Lewis sites into Bronsted ones by water adsorption shows that the Bronsted species are Si(OH)A1, typical of zeolites, and Si-AI(OH)-Si, found in dealuminated zeolites. The flexibility of the amorphous framework accounts for the differences with zeolitic aluminosilicates.
1. INTRODUCTION MCM-41 are a family of recently synthesized amorphous siliceous materials with strictly controlled mesoporosity [ 1], coming from a honeycomb distribution of cylindrical pores of nearly equal size, a consequence of the peculiar preparation method, which involves rod-shaped surfactant micelles as templates. The same systems may be prepared either as purely siliceous or with a variable AI content: in the latter case, such systems represent a new generation of classical catalytic systems, i.e. silica-aluminas. The availability of aluminosilicates with mesopores of uniform diameter has raised the hope of extending the catalytic applications of zeolites to the conversion of bulky molecules, unable to enter the 7 A-wide micropores of wide-pore zeolites [2]. The insertion of aluminium at the tetracoordinated site of a silicate network gives rise to a structural anion, responsible for cation-exchange properties and protonic acidity in zeolites. The question thus arises: is any zeolite-type Bronsted acidity present in MCM41? ! The characterization of framework aluminium and related acidity in MCM-41 has received indeed some attention in the recent literature. Reports agree on the tetrahedral coordination of aluminium in the as-synthesized MCM-41 [3-5], but such evidence is lost after the depletion of the organic template. Upon calcination, the aluminium environment becomes distorted enough to severely blur the 27A1 MAS-NMR signal [3, 4]. It is therefore difficult to completely assess the coordinative situation of AI, also if NMR evidences of
534 the presence of tetrahedral AI have been given [5, 6]. NH3-TPD experiments indicate that the acid strength of activated MCM-41 is slightly lower than that of a conventional silicaalumina of the same composition [3, 7]. Pyridine and acetonitrile adsorption experiments have shown that aluminosilicate MCM-41s feature a significant Lewis acidity, which strongly depends on the activation conditions [7, 8]. The purpose of the present work is the spectroscopic characterization of the surface aluminium sites of MCM-41. The spectroscopic techniques used have been 29Si and 27A1 MAS-NMR and FT-IR, this latter mainly as spectroscopy of adsorbed species. Probe molecules used were CO (both at room and low temperature), NH3, pyridine and CO2. The samples have been studied both after severe dehydration, so to magnify the role of Lewis centers, and after dosing small amounts of water, which converts Lewis centers into Bronsted ones [9]. These results have already been presented at the l lth International Congress on Catalysis, Baltimore, June 30 - July 5, 1996.
2. EXPERIMENTAL Reagents used in the synthesis of the aluminosilicate samples were cetyltrimethylammonium bromide (Aldrich), Aerosil 200V (Degussa) or Zeosil 175MP (Rh6ne-Poulenc) silica, AI2(SO4)318H20 (Aldrich) and NaOH (Prolabo). The reagents have been mixed at 70°C under stirring in a stainless steel vessel, then sealed and heated at 120°C. The solid formed has been separated by filtration, washed first with deionized water to pH 9, then with ethanol and dried at 80°C. Five samples have been studied with an Al content, expressed as percentage of AI over tetrahedral sites, of 0.0 (purely silicical), 0.4, 1.2, 3.4 and 7.1, respectively. These are indicated in the following with numbers from 0 to 4, in the order of increasing Al content. Powder X-ray diffraction (CGR Th&a-60 diffractometer with Cu Kct monochromated radiation and 0.25-0.40-0.40-0.25 mm slots) featured a main peak centered between 42 and 45 A for all as-synthesized samples. Pore diameters, evaluated from the ratio between the volume of N2 sorbed at 77 K and the BET surface area, measured between 33 and 37 A for the samples activated at 800°C under vacuum. The alkali content of the samples was always negligible with respect to the AI content. IR spectra were taken on a FT-IR Perkin Elmer 1760-X, both at room temperature and at a nominal temperature of 77 K. Samples were outgassed under vacuum up to 800°C before IR studies, in order to decompose the template. NMR measurements have been carried out at RT, after treatment in air at 550°C, on a Bruker AM-400 instrument.
3. RESULTS N M R spectra. 27Al MAS-NMR spectra of the aluminium-containing samples feature one
sharp peak at 53 ppm (tetrahedral AI). The intensity of this resonance strongly decreases with the activation, probably due to increased site anisotropy, and a much less intense signal at 0 ppm (octahedral AI) appears.
535
298i MAS-NMR spectra are less modified by the activation. They always feature two broad bands at about -110 and -100 ppm, corresponding to the Q(4Si) resonance and to the overlap of the Q(3Si, A1) and Q(3Si, OH) bands, respectively. The center of the Q(4Si) band shifts from -110 to -107 ppm for increasing AI content. In the case of zeolites, a down-field shift of the Q(4Si) signal corresponds to the incorporation of A1 in the silicate framework [ 10]. IR spectra of the hydroxyl region. Figure 1 compares the spectra of the samples 0, 2, and 4, atter outgassing at 800°C in the range 3800-3400 cm 1. The presence of a substantial amount of A1 does not alter much the band profile. The silanol peak at 3747 cm -1 appears slightly broader in sample 4 than in the all-silica sample 0. To check whether the acidity of the silanol species is affected by the presence of AI, ammonia was adsorbed at room temperature at different pressures on all samples. Besides other phenomena, described below, interaction takes place reversibly with the silanol species, the strength of which is a measure of the acidity of the silanol. By measuring the changes in intensity of the 3747 cm 1 peak as a function of ammonia pressure, the adsorption isotherm is obtained, as far as the interaction SiOH/NH3 is concerned. The sorption equilibrium constants, determined by the Langmuir method, are very close, indicating that all samples present the same silanol acidity. As to the hydroxyl species other than SiOH, there is little evidence in the IR spectra. Two extremely weak bands at 3660 and 3610 cm 1 are observed rather randomly, as are two very broad bands (hardly distinguishable from the background) at 3460 and 3220 cm
-1
.
Figure 1. IR spectra of the samples 0 (spectrum a), 2 (spectrum b) and 4 (spectrum c) in the O-H stretching region.
_/34i0 0 3610
. . . . . . .
b
j
~,......,,.._.,.,...._____ '3doo'
' '34'oo' ' Wovenumbers(cm-1)
'3000
Pyridine and ammonia adsorption. The IR spectra (not reported) indicate the presence of Lewis sites, on which pyridine is irreversibly adsorbed at room temperature with the 8a
536 mode at 1625 cm4. Species with 8a mode at 1596 c m -1 also appear, related to the interaction with silanol groups. The most interesting feature is however the absence of any absorption related to pyridinium species. Ammonia is irreversibly adsorbed in molecular form on Lewis sites: note that, in contrast, modest amounts of ammonium species are formed (vide infra, Figure 5). Adsorption of carbon dioxide. By contact of CO2, signals only develop in the 2400-2250 cm1 region, as reported in Figure 2, which refers to sample 2. No absorption related to carbonate (or similar) species are present: the carbon dioxide molecule acts in the present case as a probe for Lewis acidic centers. A complex spectrum is observed for the adsorption of CO2 alone, with components at 2400, 2371, 2358 and 2345 cm1 (upper part of the Figure 2). NH3 presorption yields a drastically simplified spectrum (lower part), coinciding with the one observed with sample 0. This observation allows to infer that the 2345 cm 1 component is due to CO2 adsorbed on the silica surface. The bands at 2371 and 2358 cm ~ are due to two families of acid sites, of different strength (not distinguished by either pyridine or ammonia), as indicated by both the different frequency of the asymmetric stretch of CO2 adsorbed on them, and the different pressure-dependence. These are referred to in the following as site S (strong) and W (weak), respectively. The band at 2400 cm ~ (showing the same pressure dependence as the 2371 cm~ one) is most probably related not to sites even stronger than S sites (for which irreversible CO2 adsorption should be expected), but to a combination mode of carbon dioxide adsorbed on S sites. 2371
7' •
/
5
a
|
'24-'10'
'
'23'70' ' '23'30' ' '22'90' Wavenumbers (cm--1)
|
2250
Figure 2. IR spectra of CO2 adsorbed on sample 2. Upper section (a) bare sample; lower section (b)" after NH3 adsorption at room temperature and successive evacuation.
Adsorption of CO. Figure 3 shows that, at room temperature, two CO species are
present, with stretching modes at 2229 and 2173 c m -1, readily assigned to sites S and W, respectively. The contribution of the gas-phase signals is also evident, with the P branch
537 Figure 3. IR spectra of CO adsorbed on sample 2 at room temperature.
229
background i
'
22'20
'
' ' 21'4-0 ' Wovenumbers (cm--
i
t
2060 1)
overlapping the signal of site W. The figure refers to sample 2: similar spectra are observed, however, with the other samples, only differing as to the intensity of the two peaks. Figure 4 illustrates the adsorption on the same sample 2, carried out at the nominal temperature of 77 K. The CO stretching mode region shows the same 2229 and 2173 c m "l bands, the former of which is no longer pressure-dependent, as barely is the 2173 cm 1 component. The two bands at 2154 and 2139 c m "1 a r e respectively due to CO interacting with silanol species and adsorbed in a liquid like phase. From the spectra in Figure 4 and
o
g 3675
'
'
'36'oo'
'
'3;oo'
W~enumbers (cm- 1)
'
'
3200
' 22~20
!
!
!
2i'40
!
!
Wcwenumbers(ern-1)
!
20'60
Figure 4. IR spectra of CO adsorbed on sample 2 at a nominal temperature of 77 K. Right: CO stretching region; left: OH stretching region.
538 similar, the intensities at full coverage of the 2229 and 2173 cm-1 bands are evaluated: these are utilized below to estimate the populations of S and W sites. In the O-H stretching region, besides the expected band at 3675 cm ~, related to the interaction of CO with silanol species, two new absorptions arise at about 3485 and 3420 cm -~, the former showing a more marked pressure dependence than the latter. Presorption of small doses of water. The lower spectra in Figure 5 show, in both the OH stretching region and the NH3 deformation region, the effect of ammonia adsorption on sample 4, briefly described above. The weak band at 1465 cm -~ is clear evidence of the occurrence of some amount of ammonium species. The band at 1620 cm 1 is due to ammonia molecules irreversibly adsorbed, as is the absorption around 3400 cm "1.
a
3688
1465
3610
¢.1
"-
8
o
r-~
¢,¢1
H~+ NH3 ]
•1
3600
i
|
31%)0
3400
Wavenumber= (cm--
I
)
3200
1600
14@0
Wavenumber'~ ( c m - - 1 )
1300
Figure 5. Effect on Bronsted acidity of water presorption on sample 4. Left: IR spectra in the OH stretching region; right: IR spectra in the NH3 deformation region. Lower spectra: bare sample as such (spectrum a) and after NH3 adsorption (spectrum b). Upper spectra: sample after chemisorption of a small amount of water (sample a) and successive adsorption of NH3 (spectrum b).
Adsorption of a small dose of water on the same sample converts the lower (a) spectrum into the upper (a) spectrum, characterized by a definitely asymmetric, sizeable band at 3688 cm -l and a tiny increase of absorption at 3610 cm -1, where the zeolitic-type Bronsted hydroxyls absorb. The same happens with the other samples, though the intensity of the tiny 3610 band is rather erratic. The band at 3688 cm l is likely to be due to molecular water, also responsible for the 1620 cm 1 band in the upper spectrum (a) (note that both NH3 and H20 do absorb at the same frequency!). There is evidence, however, that the tailing at lower frequency of the 3688 cm"1 is due to another component (at about 3660 cm-~), of acidic nature. Indeed, contact with ammonia, besides displacing molecular water, gives rise to substantial amounts of ammonium species (upper spectrum (b): bands
539 at 1465 cm-1 and at 3280 cml). The low-temperature interaction with CO (figures not reported) shows an erosion of the low-frequency side of the 3688 cm 1 band, with the appearance of a new band shifted some 200 cm ~ to lower frequency.
4. DISCUSSION
29Si MAS-NMR indicates that the presence of A1 modifies the average O-T-O angle. This implies that most A1 is connected to the silicate network in both the as-synthesized and calcined samples. Thermal treatments do not bring about any extensive segregation of alumina-like phases, as shown both by CO2 adsorption (no carbonate formation) and 27A1 MAS-NMR (only traces of octahedral A1 detected). The activation deeply modifies the geometry of the AI site. With the calcined samples, 27A1MAS-NMR does not detect most AI, indicating that AI is in less symmetrical environments than the tetrahedral sites of the as-synthesized samples. Lewis sites. No CO adsorption has ever been observed at room temperature on zeolite-type Bronsted sites. That allows to confidently attribute both S and W absorptions to Lewis sites of different strength. The rather high stretching frequency of CO adsorbed on site S (2229 cm~) suggests a highly uncoordinated state, e.g. trigonal as in transition aluminas [11 ]. A model for site S, in agreement with NMR and IR data, is depicted in Figure 6a. The weaker interaction of both CO and CO2 with sites W suggests, as a reasonable hypothesis, that this site correspond to a similar AI(OSi)3 species with a different degree of protrusion from the surface (Figure 6b).
•
Si//~.
...... /\ a
i .....
b
0 ''ill
< s,
"', c
Figure 6. Models for: (a) strong Lewis site S, (b) weak Lewis site W, (c) Bronsted site with OH stretching mode at 3660 cm~.
Br~nsted sites. The O-H stretching mode region of all samples basically shows the 3747 cm1 band typical of amorphous silica. The presence of AI species does not alter the acidity of the SiO-H stretching, nothwithstanding an observable band broadening. Internal Si(OH)AI bridges, which must be present according to NMR evidence, escape IR detection, just as is the case with classical silica-aluminas [9]. No direct observation of the O-H stretching modes corresponding to Bronsted surface sites is possible on the outgassed dry samples. However, the indirect, clear evidence of the existence of these sites comes from the interaction of NH3 and CO. Bronsted sites may be created by the dissociative adsorption of water molecules yielding two bands at 3610 cm"1 and 3660 cm"1. Whereas the former correspond to the well known Bronsted site in zeolites, the structure of the latter is unknown. However,
540 both the location of this band and its acidity, as measured by the shift imparted by CO, coincide with what found for a hydroxyl species formed on zeolites in the course of dealumination [12], involving an Al-linked hydroxyl species, described in Figure 6c and designated as Si-AI(OH)-Si hereafter. For the conversion of Lewis sites into Brensted ones, the mechanism represented in Figure 7 may thus be proposed, which envisages the possible formation of both bands at 3610 and 3660 cm -1.
Figure 7. Scheme for conversion of Lewis sites S the two Bronsted sites with stretching mode at 3660 3610 cm "1.
H
....... Si\o/ .,AI.. l[,,p / ...., , S --i ~
%
i k,.ll'
~ -
~\. -_ S i \ _ / I
(J
........ Q.. /
S,--o-.~sL.1 \ I""',i
the into OH and
',l!
I
I ......
0~i I o---si.,.~,,, \
As to the spectra of the bare samples, very broad bands at 3440 and 3210 cm 1 can be observed in some aluminum-rich samples. These seem ascribable to acidic hydroxyls Hbonded in a lateral interaction with some other functionality on the silica surface, e.g. a siloxane or silanol oxygen. Such hydroxyls are acidic enough to transfer their proton to ammonia, though not to pyridine. It seems possible to advance the hypothesis that the species responsible for the observed behavior correspond to the Si(OH)A1 and to the SiAI(OH)-Si species featuring, in their free form, absorptions at 3610 and 3660 cm 1. The shift imparted by the unknown surface functionality (siloxane oxygen?) to the two species (400 and 210 cm "1, respectively) scale reasonably well with the acidity of the two species as measured in zeolitic systems by the free interaction with CO [ 12]. With CO, two bands are observed again, at 3485 and 3420 cm "1, which should correspond to the H-bonded complexes of the two "hidden" hydroxyl species. Note that, with free zeolitic species, different shifts are measured. In the case of MCM-41, however, any interaction of the already H-bonded hydroxyls is subjected to constraints, absent from continuous-network zeolite systems: e.g., proton transfer takes place to ammonia and not to pyridine, probably because the endothermic step involved in the disrupture of the lateral H-bond is compensated in the former case, and not in the latter. The H-bonded Brensted sites, detected by CO adsorption, probably contribute to the CO absorption at 2173 cm -~, considered so far only as due to a Lewis site. Ouantifvina a d d sites. From the integrated intensities of adsorbed CO, by assuming a reasonable dependence of the extinction coefficient on the frequency [ 13 ], it is possible to determine the surface concentration of S and W sites, reported in Figure 8. The relative populations of sites S and W do not vary much: S and W sites thus appear as two equally probable configurations of the same surface entity. The overall population of measured
541 acid sites is in good agreement with the global AI content for the samples 1 and 2. This result suggests that the vast majority of the acid sites is measured by CO adsorption at 77 K. In the Al-richer samples 3 and 4, no more Al equipartition between surface and bulk is observed. Even if allowance is made that some A1 species may be involved in further "hidden" Bronsted sites, and thus escape detection as Lewis sites, the comparison between surface and bulk composition show that there is a dramatic loss of A1 species at the surface. A threshold value is met, and no more than 1 A1 over 60 tetrahedra is measured at the surface. Such a fact, in good agreement with data on the thermal degradation of the charge-compensating cetyltrimethylammonium [8], is probably related to the peculiar synthesis mechanism of MCM-41 rather than to energetic considerations. Multiplycharged anions, e.g. SiO2(OH)22 [14], are probably preferentially adsorbed on the templating micelles [ 15], whereas the single-charged aluminate anions preferentially react with the silica layer already coating the micelles, and are no more accessible after the aggregation of the silicate-coated micelles [ 16, 17].
0.02
~ o.o16 U
~
A
0.012 A O
0.008
©
r~
0.004
0
0.62
0.04
0.()6 0.68 All (Si+Ai)
Figure 8. Surface population of S sites ( • ), W sites ( O ), and overall measured acid sites ( & ) as a function of the global A1 population.
4. CONCLUSIONS The surface of MCM-41 materials deeply differs from the zeolite surface. In the case of silicic materials, the inner surface of MCM-41 interrupts a framework of amorphous silica and is essentially silanol-lined. The inner surface of a silicic zeolite is limited by a completely connected network of tetrahedra and is lined by siloxane bridges. The nature of the aluminium sites are also different for the two kinds of materials. In zeolitic networks, aluminium isomorphously substitutes silicon in isolated tetrahedral sites and gives rise to Si(OH)AI Bronsted acidic sites, with O-H stretching mode in the range 3630-3610 c m "1. Other sites, Bronsted and Lewis, are only formed by partial extraction of aluminium from the framework. In the case of activated MCM-41 aluminosilicates, Lewis
542 sites are mostly present in two types. Two Bronsted sites are also possible, Si(OH)AI and Si-AI(OH)-Si. The O-H stretching modes of the surface hydroxyl sites are blurred by the systematical H-bonding with some surface functionality. Such an easy lateral interaction is more readily understood on the basis of the flexible amorphous framework of silica than of a quite rigid zeolite network. The Lewis sites of MCM-41 are easily converted into Bronsted sites by partial hydrolysis. In both as-synthesized and activated MCM-41, aluminium occupies isolated framework sites. The high surface area, the low surface concentration of AI induced by the synthesis method and the presence of well-defined Lewis and Bronsted sites, among which interconversion is possible, make aluminosilicate MCM-41 a model system for amorphous silica-aluminas. The presence of acidic sites with strong lateral interactions and, probably, different levels of protrusion from the surface, suggests that the flexibility of the silicate framework can play an especially important role in defining the acidic properties of highsilica silica-aluminas.
REFERENCES
.
3. 4.
.
10. 11. 12. 13. 14. 15.
16. 17.
J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins & J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. P. Behrens, Adv. Mater. 5 (1993) 127. Cong-Yan Chen, Hong-Xin Li & M.E. Davis, Microporous Materials 2 (1993) 17. R. ~Schmidt, D. Akporiaye, M. Strcker & O.H. Ellestad, J. Chem. Soc. (7hem. Commun. 1994, 1493. Zhaohua Luan, Heyong He, Wuzong Zhou, Chi-Feng Cheng & J. Klinowski, jr. Chem. Soc. Faraday Trans. 91 (1995) 2955. Zhaohua Luan, Chi-Feng Cheng, Wuzong Zhou & J. Klinowski, J. Phys. Chem. 99 (1995) 1018. A. Corma, V. Fornrs, M.T. Navarro & J. Prrez-Pariente, J. Catal. 148 (1994) 569. M. Busio, J. J~inchen & J.H.C. van Hooff, Microporous Materials 5 (1995) 211. V.B. Kazansky, Kinet. Katal. 23 (1982) 1334. C. Fyfe, H. Grondey, Y. Feng, H. Cries & G.T. Kokotailo, NAT() ASI Series C352 (1992) 225. A. Zecchina, S. Coluccia & C. Morterra, Appl. Spectrosc. Rev. 21 (1985) 259. E. Garrone, R. Chiappetta, G. Spoto, P. Ugliengo, A. Zecchina & F. Fajula, Proc. 9th IZC, Butterworth, Boston 1993, 2-267 V. Bolis, B. Fubini, E. Garrone, C. Morterra & P. Ugliengo, J. (?hem. Soc. Faraday Trans. 88 (1992) 391. J.L. Guth and P. Caullet, J. Chim. Phys. 83 (1986) 155. 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.F. Chmelka, Science 267 (1995) 1138. Cong-Yan Chen, S.L. Burkett, Hong-Xin Li & M.E. Davis, Microporous Materials 2 (1993) 27. Guoyi Fu, C.A. Fyfe, W. Schwieger & G.T. Kokotailo, Angew. Chem. Int. Ed. Engl. 34 (1995) 1499.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
Boiling-point elevation of water materials probed by 1H NMR
543
confined
in
mesoporous
MCM-41
Eddy W. Hansen, Ralf Schmidt and Michael St5cker SINTEF Applied Chemistry, P.O. Box 124 Blindern, N-0314 Oslo, Norway Dedicated to Professor Dr. Klaus K. Unger on the occasion of his 60th birthday. Characterization of water saturated mesoporous MCM-41 materials with narrow pore-size distributions by 1H NMR revealed two temperature transitions above 373 K. The first transition temperature (373 -391 K) was assigned to the boiling point of "free" water within the pores, while the second transition temperature (408 - 413 K) was associated with desorption of less mobile "surface" water. The boiling point, T b, of the "free" water increased with decreasing pore diameter, D (A), according to: T b =a0+a,.i D-l+a2 D-2with a 0 = (373+ 1) K, a 1 = ( 7 0 + 5 9 ) KA. and a 2 = (5.7+ 1.2). 10L~K.,A,~. 1. INTRODUCTION Understanding of pore structure and how it affects physical properties is an important challenge in many aspects of science and technology [1]. Such understanding may lead to development of new materials with improved performance and broaden the range of their applicability. Porous media typically contain interconnected three-dimensional network of channels of non-uniform size and shape. The distribution of pore sizes is therefore an important characteristic of such materials. The techniques most commonly used to characterise pore structure include adsorption or desorption methods and mercury intrusion porosimetry [2]. NMR spectroscopy is another technique used to characterise pore geometry including spin-spin relaxation time (T2), spin-lattice relaxation time (T 1), and diffusion measurements [3-7]. Reliability as well as analysis of relaxation curves to give pore size distribution is a continuous and active area of research. It is well known that physical properties of a liquid confined within small pores can be radically different from those of bulk materials. For instance, the freezing point of a confined liquid is depressed [8]. The relation between the freezing point depression (ATf) and pore radius (r) was originally developed by Gibbs and Thompson (Lord Kelvin) [9] and takes the form: ATf = -2yMT0/rpAH f where y, M, p, T O and AHf are the surface tension, the molecular weight, the density, the freezing point and the molar heat of fusion of the bulk fluid, respectively, and will be referred to as the "Kelvin" equation.
.544 The freezing point depression phenomenon has recently been studied by NMR [1016] to provide a new method of determination of pore size. In this work, a mathematical model will be applied, enabling the boiling point of a liquid (containing protons) confined in a porous material to be determined from 1H NMR signal intensity vs temperature measurements (IT-curve) with the highest possible precision. Such measurements facilitate an empirical correlation between the boiling point and the pore size of the host material to be established. Three sizes of mesoporous MCM-41 materials, with pore diameters of 20, 24 and 40 A, respectively, will be used to demonstrate the method.
2. EXPERIMENTAL The MCM-41 materials were prepared according to synthesis procedures similar to those reported by Beck et al. [17]. The three mesoporous powder materials, denoted by the letters A, B, and C, were saturated with water under vacuum and loaded into 5 mm NMR tubes. The diameter (D) of the materials were 20 A (A), 24 A (B) and 40 A (C), respectively. The structure of the MCM-41 samples was maintained during the temperature treatment (checked by XRD). A Varian Gemini spectrometer, operating at 300 MHz proton resonance frequency was used. A bandwidth of 50 kHz and an acquisition time of 0.030 s were applied with a repetition time of 15 s between pulses. A longer acquisition time was not necessary due to the rather broad spectral lines with half widths of more than 300 Hz. The long interpulse timing of 15 s was imposed by the long spin-lattice relaxation time of the silanol protons of approximately 2 - 4 s [16, 18]. All measurements were performed with a 900 if-pulse of 8 its, on resonance. Each spectrum was composed of 4 transients. Less than 50 mg of material was used which filled the NMR tube to a height of less than 2.5 mm. The temperature of the powder sample was determined with an accuracy of + 1 K. The intensity or area (I) of the resonance peak was determined by numerical integration and corrected for temperature (T) according to an empirical equation, which was determined using a glycerol sample. This temperature correction deviated somewhat from the expected Curie law (I = l/T) [16, 19, 20] and was attributed to the probe design. The same glycerol sample was used to calibrate the temperature. The temperature was increased with a rate of 1.5 K/minute, if not otherwise stated in the text. The NMR spectra were sampled periodically with time, corresponding to a temperature interval of 1.5 K between each NMR cycle of 4 transients. A cotton wool was inserted into the NMR tube, above the detection coil, to prevent condensed liquid water to re-enter the coil area.
2.1. Methodology and underlying theory The 1H NMR signal intensity (I) vs the absolute temperature (T) of water confined in a porous material, denoted as IT-curve, can be expressed by equation 1, where the parameters z~H and AB represent the motional activation enthalpy of the water molecules and the width of the log-normal distribution of correlation times imposed
545
on the water molecules, respectively. T c represents the transition temperature.
,o E
.,,
_,]))
I(T) - --~ . l + erf ( R . AB " (-~ -
(1)
The symbol "eft" is the accepted short hand notation of the error function, defined by equation 2, where u is an integration variable y
(2)
erf(y) = - ~ ! exp(-u 2)du If more than one transition temperature exists, equation 1 can be generalised;
I(T)-
,:, I,(T) =~.,--~,:1
1 + e~. (R. ~
(-~-~,
(3)
))
where 10i is the intensity contribution of water from phase "i". More details concerning the derivation of equations 1 and 3 are shown in the appendix. 3. RESULTS AND DISCUSSION
Figure 1 shows the 1H NMR signal intensities vs temperature of water confined in samples A and C and demonstrates the decrease in intensity with increasing time due to phase transition of the pore confined water which evaporates and leaves the pore. Since the heating rate is constant (1.5 K/min) in the two experiments, the phase transition of sample A shows up at a higher temperature compared to sample C.
I I l
l
J
!
1
s
c
|
0
|
i
5
,
,
,
,
|
10
,
,
,
,
I
15
.
.
.
.
J
'
"
20
25
TIM E ( m i n u r e s )
Figure 1.1H NMR signal intensities vs time of samples A and C. Heating rate 1.5 K/minute.
The experimental data presented in Figure 1 are in qualitative agreement with
546
equation 4: A ~ = 2yMT° .]-pA/-I~, r
(4)
which predicts a higher boiling point temperature of water confined in smaller pores (smaller pore radius r) when all other parameters in equation 4 are constant. Equation 4 represents an analogous version of the modified "Kelvin" equation presented earlier in this work, which has been shown to be valid for the freezing of pore confined water [ 10,14-16,18]. The temperature-corrected intensity (area) of the NMR peaks in Figure 1 are plotted against the inverse absolute temperature (ITcurve) shown in Figure 2. The dotted line represents the IT-curve of bulk water, while the solid lines represent non-linear least squares model-fits of equation 3 to the experimental IT-data. The observed decrease in signal intensity at approximately 373 K (Table 1) is found to be independent of pore radius (Table 1) and is caused by evaporation of residual bulk water located between the crystallites of the powder materials. This transition temperature is equivalent to the boiling point of bulk water (~373 K; dotted line) and serves as a 0seful temperature calibration point. The second temperature transition observed in the IT-curve (Figure 2, Table 1) represents the main transition, i.e., the temperature at which most of the mobile water evaporates and is provisionally interpreted as the boiling point of "free" water confined in capillary pores of radius r, as predicted by equation 4. Table 1 The average (Tci) and the standard deviation (RABi/AHi) of the transition temperature of samples A(D=20A), B(D=24A) and C(D=40A). Heating rate 1.5 K/min. SAMPLE
Tci (K)
,~Bi/AH i (K-1)
Corr. coeff.
A(i=l) A(i=2) A(i=3)
373.5 + 0.9 391.3 + 0.3 416.3 + 5.9
1.4 + 1.5 5.3 + 0.7 15.6 + 8.4
0.9988
B(i=2) B(i=3)
385.7 + 0.3 412.1 + 1.3
7.9 + 0.4 11.4 + 6.1
0.9984
C(i=l) C(i=2)
371.5 378.8 + 0.1
0.3 1.7 + 0.2
0.9973
The two samples containing the smaller pores, samples A and B, reveal a third transition temperature at approximately T = 414 K which is tentatively assumed to represent desorption of the more strongly bounded surface water. Another point of
547
interest is the width or standard deviation (R~B/AH) of the second temperature transition, as can be inferred from the observed IT-curves (Figure 2, Table 1). This width is significantly smaller for sample C, which contains the larger pores, compared to samples A and B. From equation 1 this might be rationalised according to a decrease in the enthalpy of motion (AH) of the water molecules or an increasing spread (AB) of molecular correlation times with decreasing pore radius. The latter seems intuitively reasonable. However, the profile of the IT-curve might also depend on typical external parameters, as for instance the heating rate. A large heating rate might cause a broadening of the IT-curve due to the time needed for heat transfer, evaporation and mass transport between sample and surrounding. However, reducing the heating rate from 1.5 K/min to 0.2 K/min had no significant effect on the shape of the IT-curve, i.e., neither the transition temperature nor the width of the temperature transition changed.
.
z
uJ i--
8O
Z
. <
60
Z
®
4O
o')
d
_.,,
2O Z 7-
'-
-
0
-
o
-
2.26
-
-
-
m
-
-
m
m
m
m
-
I
-
m
2.36
-
-
-
-
m
I
I
2.46 2.56 1000/T (K- 1)
/
D I
2.66
Figure 2.1H-NMR IT-curves of samples A, B, C and D (bulk water). Heating rate was 1.5 K/min. Figure 3 shows the observed boiling point temperature (Tb) of the second temperature transition vs inverse pore diameter (D 1 ) of the samples investigated, The boiling point of bulk water corresponds to water within a pore of infinite size (D -1 = 0). The curve is not straight but has a positive curvature towards smaller pore diameters. Referring to equation 4, this might originate from a temperature dependence of one or all of the three parameters, density (p), surface tension ('if), or molar heat of evaporation (AHb). The change in density of liquid water within the actual temperature range (373-390 K) is probably small. The surface tension is
548
known to increase with temperature, however, the increase is probably not more than 10 % within the actual temperature range studied [21]. Thus, we are left with the temperature dependence of the heat of evaporation (AHb), which might be an indirect function of pore radius. We have not been able to find any published correlation between AH b and pore radius, however, Jackson et al. [8] have shown experimentally that the heat of fusion (AHf) of a number of organic liquids confined in pores decreases with decreasing pore radius below approximately 100 /!i,. The existence of a similar relation between pore size and heat of evaporation of pore confined water, might thus explain the shape of the curve displayed in Figure 3. 395
ILl
¢v
A 390
iJJ 13_ 385
UJ
Z
0
v 380
375
Z
_J
o
370
I 0
i 0.01
i 0.02
i 0.03
i 0.04
0.05
]/D (,k-1 ) Figure 3. Boiling point temperature of the second temperature transition vs inverse pore diameter (D-l) of samples A, B and C. Hansen et al. [16] and co-workers [14,15] have recently presented experimental NMR results on the freezing of water confined in mesoporous materials. For instance, they found [16] that sample A froze at a temperature of 193 K, corresponding to a freezing point depression ATf = 80 K. Combining equation 4 and the equivalent expression relating the freezing point depression to pore radius, the boiling point elevation, AT b, can be expressed by equation 5; =
T+
(5)
where Tob and Tof are the boiling point and the freezing point of bulk water, respectively. The density and surface tension are assumed to be temperature independent. The enthalpies of fusion and of evaporation of bulk water are AHf = 1.44 kcal/mol and AH b = 9.82 kcal/mol, respectively. Inserting these values into equation 5 predicts a boiling point elevation of water confined in sample C of approximately 20 K, which is in good agreement with the observed value of 18.2 + 0.4 K. It is clear from the results presented in this work that the boiling point elevation
549 is less sensitive to pore size than the corresponding freezing point depression of water within capillary pores [14-16]. However, one disadvantage when performing cooling experiments below the freezing point of pore confined water is the potential destruction of pore structure upon phase transformation from liquid water to solid ice. It should be emphasised, however, that the sensitivity regarding the boiling point elevation vs pore size depends on the molar heat of evaporation, equation 4. Replacing water with another liquid, having a smaller molar heat of evaporation, might thus improve the sensitivity.
4. APPENDIX Equations 1 and 3 can be derived using the following arguments. Since the motion of fluid molecules are restricted by the pore walls, a distribution of correlation times is expected. A number of available distribution functions exist, among which the lognormal distribution of correlation times seems to be the most commonly accepted model for describing the molecular motion in solids and liquids, and takes the form:
1
P(x)dx-
zxBJ-ff
exp(-
) d Z with
Z = In(--~. ) ¢:~ 1; - 1;* e x p ( Z ) I;
(A1)
where x* represents the average correlation time, defined by the median of the distribution function, and AB characterises the width of the distribution function. Moreover, we will assume that a critical correlation time % exists, above which the fluid molecules are transformed into gas phase molecules and escape from the sample and out of the NMR receiver coil area. Identifying this critical correlation time (%) with a corresponding critical temperature, T c and assuming x* and xc to be related to temperature by an Arrhenius type function, we obtain:
1;* - 1:0 e x p ( - ~ )
and
1;c - 1;o exp(
)
(A2)
where AH represents the activation enthalpy of the restricted motional freedom of the water molecules, z0 is a constant. The NMR signal intensity of the liquid water molecules within the pores at a temperature T can thus be calculated;
2
1 exp(_~B2)dZ_ ~l I(T)- ~~P('r.)d'r,-"(~'~'~ S AB.f-~ 0
0
~-~-~
j'exp(-u=)du
-,,,,
(A3)
5. CONCLUSION The pore radius of mesoporous MCM-41 materials are shown to be obtainable from
550
1H NMR IT-data above the normal boiling point of bulk water. An improved estimate of the boiling point is obtained by a model fit to the observed IT-data. The boiling point, T b, of the free water increased with decreasing pore diameter. However, the sensitivity is significantly less than what can be obtained from similar NMR experiments at sub-zero temperature ("freeze" - NMR). However, use of another liquid having a lower molar heat of evaporation, might improve the sensitivity of the present NMR technique. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
F.A.L. Dullien, Porous Media, Academic Press, New York, 1979, Chap.1. P.L.Pratt, Materials and Structure, 21 (1988) 106. W.P.Halperin, F.D'Orazio, S.Bhattacharija and J.C.Tarczon, Molecular Dynamics in Restricted Geometries, edited by K.Klafter and J.M.Drake, John Wiley and Sons, New York, 1989. G.C.Borgia, A.Brancolini, R.J.S.Brown, P.Fantazzini and G.Ragazzini, J.Magn.Res. Imaging, 12 (1994) 191. R.L.Kleinberg, W.E.Kenyon and P.P.Mitra, J.Magn.Res., Ser.A, 108, (1994) 206. L.L.Latour, R.L.Kleinberg, P.P.Mitra and C.H.Sotak, J.Magn.Res., Ser.A, 112 (1994) 83. P.T.Callaghan, A.Coy, D.MacGowan and K.J.Packer, J.Mol.Liq., 54 (1992) 239. C.J.Jackson and G.B.McKenna, J.Chem.Phys. 93 (1990) 9002. B.J.Mason, The Physics of Clouds, Clarendon Press, Oxford, Second Edition (1971)2. K. Overloop and L.Van Gerven, J.Magn.Res.,Ser. A, 101 (1993) 179. J.H.Strange and M.Rahman, Phys.Rev.Lett., 71 (1993) 3589. S.M.Alnaimi, J.H.Strange and E.G.Smith, Magn.Res.lmaging, 98 (1994) 1926. W.L.Earl, Y-W. Kim, E.G.Smith, IUPAC Symp. Marseille, France, (1993) 21. D.Akporiaye, E.W.Hansen, R.Schmidt and M.StScker, J.Phys.Chem., 98 (1994) 1926. R.Schmidt, E.W.Hansen, M.StScker, D.Akporiaye and O.H.Ellestad, J.Am.Chem.Soc., 117 (1995) 4049. E.W.Hansen, R.Schmidt, M.St5cker and D.Akporiaye,J.Phys.Chem., 99 (1995) 4148. 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. K.Overloop and L. Van Gerven, J.Magn.Res., Ser.A, 101 (1993) 147. A. Abragam, The Principles of Nuclear Magnetism, Clarendon Press, Oxford, U.K. (1961)2. D.R. Kinney, I.-S.Chuang, G.E.Maciel, J.Am.Chem.Soc., 115 (1993) 6786. B.R.Puri, L.R.Sharma, M.L.Lakhanpal, J.Phys.Chem., 58 (1954) 289.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
551
In situ studies of catalytic reactions in zeolites
by means of PFG and M A S N M R techniques J. K~.rger and D. Freude Universitat Leipzig, Fakultat ~ r Physik und Geowissenschaften, Abteilung Grenzflachenphysik, D-04103 Leipzig, LinnrstraBe 5, Germany
NMR spectroscopy is shown to be a most efficient tool for the #1 situ study of catalytic reactions in zeolites. MAS NMR permits an analysis of the evolution of the intrinsic concentrations of the species taking part in the chemical reaction under study, while PFG NMR provides direct information about the molecular mobilities and their time dependence. A special design for sample heating allows the in situ observation of chemical reactions up to 800 K. We present examples for the application of these techniques to elucidate the elementary processes during standard reactions of acid and basic catalysis.
1. INTRODUCTION Being able to provide quantitative information about the concentration and mobility of the different constituents, NMR spectroscopy has become a most efficient tool for the study of chemical reactions in zeolitic adsorbate-adsorbent systems [1-3]. It is remarkable that new developments in high resolution NMR such as the application of ~3C NMR [4] or of the multiple pulse sequences of solid-state NMR in combination with magic angle spinning (MAS) of the sample to adsorbate-adsorbent systems [5] has soon been followed by the application of these methods to the study of chemical reactions [6-7]. First selective self-diffusion measurements in multicomponent adsorbate-adsorbent systems by the pulsed field gradient (PFG) NMR method have been carried out by preparing the NMR samples in such a way that only one species contains the nucleus (~H [8] or ~gF [9]) to be observed. Only recently, high resolution ~H NMR has been applied to studying the diffusivity of different compounds in adsorbate-adsorbent systems selectively [10-12]. Thus, NMR spectroscopy is potentially able to provide all relevant information to characterize the intrinsic processes during zeolite catalysis on a microscopic scale. On applying these possibilities to a given process of zeolite catalysis, however, one is otten in conflict with the limitations of the technique. Very often, the reaction temperatures are far above the range generally accessible by NMR. In these cases, special arrangements to enhance the range of measurement are inevitable. A fundamental problem one has to deal with on studying chemical reactions is the finite resolution of the measurement. This limitation is particularly stringent in PFG NMR where line narrowing by MAS would be in conflict with the mechanical stability required during high performance self-diffiasion measurement [13].
552 A direct measurement of the translational mobility of the compounds participating in the reaction will therefore be possible only in exceptional cases. The examples given in this contribution are to show how such complications in the application of NMR spectroscopy to the in situ study of catalytic reactions may be overcome. A new technique making use of a laser beam makes it possible to switch from the temperature of the beating gas (usually room temperature), at which the reaction is too slow to be measured, to temperatures up to 800 K, at which the reaction takes place within 60 s. Among MAS NMR studies the catalytic conversion of methanol to hydrocarbons (the MTG process) using zeolite H-ZSM-5 has attracted a great deal of attention, cf. [14-15]. Results of laser supported high-temperature MAS NMR experiments were first reported in Ref. [16], confirmed in Ref. [ 17] and explained in detail in Ref. [ 18]. Heterogeneous catalysis using solid acids such as zeolites is initiated by transferring protons from Bronsted acid sites, cf. [19], to adsorbed reactant molecules. The dynamics of such transfer can be studied by NMR, when the zeolite is loaded with fully deuterated probe molecules. At a given temperature, the time evolution of the 1H line intensity of the probe molecules depends on the deprotonation energy of the acid sites in the zeolite. Hence, the in situ determination of H-D exchange times between deuterated molecules and different hydrogen forms of zeolites at different temperatures provides information about the catalytic activity of the zeolite. We discuss H-D exchange times of deuterated cyclohexane in zeolites. 2. EXPERIMENTAL The self-diffusion measurements have been carded out by means of the home-built PFG NMR spectrometer FEGRIS 400 [20]. The spectrometer can supply pulsed field gradients up to 24 T/m and operates at a proton resonance frequency of 400 MHz. Intracrystalline selfdiffusivities were measured with IH,~ac and 19F PFG NMR. The measurements have been carried out with closed sample tubes containing the adsorbate-adsorbent system under study. Prior to adsorption, the zeolite samples were activated at 400°C at a pressure of less than 0.01 Pa for 24 h. The activation temperature was attained at a ramp rate of 10 K/h. Chemical reactions were initiated by subjecting the sample to a corresponding temperature programme. ,,Stop and go" experiments have been performed as follows. The sample is heated by maximum power of a laser beam within a short time up to the desired temperature _< 800 K and kept there for some ten seconds. The location of a sealed glass ampoule in a boron nitride container in the MAS rotor decreases the temperature gradient in the sample and avoids laserinduced reactions. This is the go period where the reaction takes place and the IH MAS NMR line is measured in intervals of about one second. In the stop period the laser is switched off and the sample goes back to room temperature. If the reaction is frozen at room temperature, the stop period can be used for the measurement of a ~3C MAS NMR spectrum, which characterizes the reaction state after the previous go period. A few thousand scans can be accumulated for the 13C MAS NMR spectrum. By repeating the stop and go periods several times either the time dependent 1H spectra of fast reversible reactions can be accumulated or a complete non-reversible reaction can be measured by both ~H and ~3C MAS NMR. MAS samples were pretreated in glass tubes of 5 or 3 mm outer diameter and then loaded under vacuum at room temperature with cyclohexane or methanol, 99% enriched in 13C or 99.5% enriched in 2H. The samples were kept frozen until the start of the NMR experiment.
553 3. RESULTS AND DISCUSSION 3.1 PFG NMR diffusion studies with C3 hydrocarbons in X type zeolites as typical reactant and product molecules in zeolite catalysis The conversion of isopropanol is a well established test reaction to discriminate between acid and basic zeolites [22-23]: Isopropanol is dehydrated to propene on acid catalysts, while it is dehydrogenated to acetone on basic catalysts. Figure 1 shows the results of IH PFG NMR self-diffusion measurements of the relevant reactant and product molecules under the conditions of single-component adsorption on Cs(60%)NaX [24]. The diffusivities of acetone and isopropanol are found to be quite close to each other, while the diffusivity of propene is seen to be more than one order of magnitude larger. If the acid and base catalysed reactions occured as parallel reactions, the faster diffusion of propene could therefore favour the macroscopically observable production of this species. i0 -s
Figure 1. Intracrystalline self-diffusion coefficients of acetone, isopropanol and propen e in CsNaX-60 at a sorbate concentration of three molecules per cavity
I0 -o
7
v
• propene
0
•
•
• acetone
isopropanol
II
m
E
I0 -I0
I=
I0 -11
10
-12
t
2
3
4
I
5
I
,
IO00/T (K- t)
The simultaneous observation of two or of even more components in their mixture is a necessary and sufficient condition for the in situ diffusion measurement during a catalytic reaction. So far, this type of measurement could only be performed for the conversion of cyclopropane to propene in NaX [25]. The ]H NMR spectra of these two species turned out to be sufficiently different to allow the separate measurement of their diffusivities. For the species considered in Figure 1 such a discrimination was iml~ossible. Owing to the larger chemical shifts, ]3C NMR offers much better prospects for a discrimination between different compounds than ]H NMR. However, this advantage may be dramatically corrupted by the lower sensitivity of ]3C NMR. Hence, only recently, first ]3C PFG NMR studies of zeolitic adsorbate-adsorbent systems have become possible [26]. We are presently trying to take advantage of these novel possibilities for the in situ observation of the conversion of isopropanol in X type zeolites. First ]3C PFG NMR measurements of the diffusivity of isopropanol (1.4.10 -1° m2s-]) and of propene (2.10 -9 m2s-]) in zeolite CsNaX at 473 K are in reasonably good agreement with the ]H PFG NMR data given in Figure 1.
554 3.2. Transport inhibition due to coke deposition during catalytic reactions Catalytic conversion is very often accompanied by coke depositions, which may dramatically reduce the efficiency of the catalytic process. Besides a mere blockage and/or poisoning of the active sites, the applicability of the catalyst under study may also be terminated by the deterioration of its transport properties. The in situ observation of the transport properties of a catalyst during chemical reaction is therefore of immediate practical relevance. Measurements of this type may be easily carried out by the PFG NMR method employing an inert probe molecule. Figure 2 shows the result of an in situ 19F PFG NMR measurement of the self-diffusivity of CF4 in a sample of zeolite H-ZSM-5 which has initially been loaded with 12 ethene molecules per unit cell [27]. For the considered temperature of 343 K, ethene in H-ZSM-5 is well-known to be converted preferentially into alkanes [28-29]. The measurements show, that this conversion is accompanied by a dramatic decrease in the mobility of CF4 indicating a marked deterioration in the transport properties of the zeolite crystallites. During the first three hours, the mobility of the probe molecule drops by a factor of about 6. Then the mobility of the probe molecules remains essentially constant. This indicates that after this time the supply of reactant molecules is more or less consumed, and that consecutive reactions- if present at all - do not lead to a significant further enhancement of transport inhibition.
Figure 2. Self diffusivity of CF4 in H-ZSM-5 at a concentration of 4 molecules per unit cell during ethene conversion at 343 K
10"~ f
I
o
10-~0
•
0
1
2
3
4
5
6
•
7
$
9
10
time of reaction at 343 K (hr) 3.3. Stop and go in the in situ-MAS NMR spectroscopy In order to combine the information from the time resolved 1H spectra with those from ~3C MAS NMR, we performed a "stop and go" experiment (about 9 min go periods at 600 K and 1 hour stop periods at room temperature) for the methanol conversion. The Figure 3 consists of the t3C MAS NMR spectra of the methanol loaded (8 molecules per unit cell) zeolite H-ZSM-5 in a 3 mm glass ampoule measured at room temperature before the first go period (A), atter the first go period 03) and after the second go period (C). The change between the spectra A and B is due to the decomposition of methanol (51 ppm) and the formation of dimethyl ether (60 ppm) and methoxy groups (59 ppm). No formation of alkanes can be observed in spectrum B. This can be explained by an induction period of the reaction [17].
555 Figure 3. ~SC MAS NMR spectra of the methanol loaded zeolite H-ZSM-5
A
1
.
.
.
.
.
I
60
'
'
'
40
"
'
'
'
20
'
'
,
.
.
.
.
0 8 / ppm
The inset in the Figure 3 shows the 160-200 ppm region of spectrum B, enhanced by a factor of 10. A weak line at 184 ppm is attributed to carbon monoxide formed during the first go period (induction period). A small amount of methane gives a signal at ca. -7 ppm. Figure 4 presents a contour plot of the ~H spectra, which were acquired during the second go period of the methanol conversion. The first free-induction decay (FID) was measured immediately aider switching on the laser beam, the next FID after 4 seconds and so on. A total of 128 onedimensional spectra has been acquired with one scan for each spectrum during this go period. In order to obtain a two-dimensional representation of both the chemical shift of the ~H nuclei in the reacting species and the reaction rate, the last spectrum representing the final state is subtracted from all spectra, which were measured as a function of the reaction time. Then a second Fourier-transformation is performed with respect to the reaction time tl. The F2dimension of the obtained 2D spectrum gives the usual Fourier transform with respect to the acquisition time t2 (chemical shift). The F 1-dimension is characterized by positive or negative lines (the negative area is shaded) for species which were decomposed or formed in the reaction, respectively. The line width of the corresponding lines is twice the reaction rate, if the reaction is simple exponential. Thus, quickly or slowly running first-order reactions give broad or narrow Lorentzian lines in the Fl-dimension, respectively. The following species and tH resonance positions can be assigned: CH3 groups of methanol and dimethyl ether give rise to lines in the range of 3-5 ppm, CH3 groups of isobutane and propane were observed at 1.1 ppm, methane at 0.3 ppm. The scalar ~H- ~3C coupling gives rise to doublets for the observed isobutane, propane and methane lines. The experiment presented in Figure 4 lies between two stop states characterized by the ~SC spectra 03) and (C) in Figure 3. The latter spectra show that only alter the second go period mainly the compounds isobutane (24 ppm), propane (16 ppm) and methane (-8 ppm) can be observed. The two-dimensional representation of the ~H spectra in Figure 4 shows positive lines of decomposed dimethyl ether/methanol/methoxy compounds and a superposition of negative lines due to formed alkanes. The two minima centred at 1.1 ppm are due to the dominant isobutane/propane formation. From the linewidths in the F 1 dimension a reaction rate of 0.12 min-~ at 600 K can be determined. Figure 4 confirms that after the induction period C3/C4
556 alkanes are formed prior to methane and are not decomposed later on. The negative areas at 4.3 and 4.5 ppm are found in a region, where intermediate methoxy groups should be expected. Further studies are necessary, in order to make the methoxy groups visible during the reaction. In conclusion, the "stop and go" experiment is an appropriate tool for the investigation of different periods of a reaction, especially if an induction period is taken into consideration.
dr/ ppm
3
+16
2
-16 -1
1
-20
0 '
I
'
0.016
'
'
I
'
0.008
'
'
I
0
'
'
'
I
'
0.008
'
'
i
'
0.016
Figure 4. Contour plot of the series of ~HMAS NMR spectra acquired during the second go-period of the methanol conversion. Single FID's were acquired in steps of 4 s. After subtracting the final spectrum from all spectra a second Fourier transformation with respect to the reaction time was performed. Species which were decomposed or formed during the reaction are characterized by positive or negative (shaded areas) signs, respectively. Intensity values of the contour lines and peak intensities (italic) are given in the figure.
k/Hz
3.4. H-D exchange reactions IH spectra of a dealuminated zeolite H-Y loaded with 2.5 C6D~2molecules per supercage were measured in dependence on the temperature [ 18]. The IH NMR spectrum of gaseous cyclohexane consists of one line at 1.44 ppm according to reference values measured in solution of CC14. The shift of the cyclohexane line from 1.44 ppm (gas phase) to our experimentally obtained value of 1.7 ppm (adsorbed phase) can be explained by adsorption interaction. A chemical shift difference of 0.22 ppm has also been observed between methane molecules in the intra- and inter-crystalline space of zeolite H-ZSM-5 [ 19]. After switching the temperature to 600 K the H-D exchange takes place within less than two minutes and a new line appears at 1.0 ± 0.1 ppm, growing rapidly until after 20 min an equilibrium state seems to be approached, whereas the line at 1.7 ppm due to IH nuclei in cyclohexane disappears after this time [18]. In order to assign the 1.0 ppm line correctly we performed additional ~H and natural abundance ~3C experiments of samples of zeolite HY with deuterated cyclohexane. The 500 K 13C spectrum shows a strong 27.6 ppm line, which is due to cyclohexane. Three additional lines in the 13C spectrum at 35.8, 26.3 and 20.4 ppm and five additional lines in the 1H spectrum between 1.2 and 2.2 ppm can be identified as methyl cyclopentane which is an isomer of cyclohexane. The proton transfer between the bridging hydroxyl groups and the molecules can be described by the equation CM (t)= CM(t=oo)[1-exp(-t/tox)], where CM denotes the proton concentration in the molecules as obtained from their IH intensities and tex is the exchange
557 time. The values of rex were measured as a function of temperature using the intensity of the ~H signal of the molecules. Figure 5 shows the thus obtained exchange times tcx in an Arrhenius representation. For temperature control both laser heating and gas flow heating was applied. As to be expected the exchange times turned out to depend on the reaction temperature rather than on the mode how this temperature was maintained. The Arrhenius representation exhibits a marked deviation from linearity in the considered temperature range, resulting in a temperature dependence of the apparent activation energy E. The limiting values in the considered temperature intervals are E = 29 kJ mol-t at about 600 K and E = 112 kJ mol-I at about 400 K. Extrapolating the exchange times with this latter activation energy to room temperature yields a value of some years. This is in agreement with the fact that no H-D exchange can be observed at room temperature. The maximum value of 112 kJ mol-~ can be interpreted as the activation energy for the composition of a protonated intermediate complex for the H-D exchange. Since the activation energy for this process should not depend on the temperature, the decrease of the apparent activation energy with increasing temperatures should be considered as a hint that in this temperature region the composition of the transient complex is no more the rate limiting step in the proton transfer reaction. Thus it turns out that even the first step of acid catalysis is far more complicated than it might appear at a first glance. Further considerations of the processes contributing to the proton transfer reaction are therefore inevitable. Combined PFG NMR and MAS NMR studies of this topic are in progress. t ex / rain Figure 5. Arrhenius plot of the H-D exchange times
1000
+ gas flow heating +
× laser heating 100
4-
10
1
0.0016
×
.
.
.
×
.
,, T'I /K-1
0.0018 0.0020 0.0022 0.0024 0.0026
ACKNOWLEDGEMENT We have reported most recent results of the application of different NMR techniques to the in situ study of zeolite catalysis, which have been obtained in our group. We are deeply obliged to Prof. Dr. Dr. Harry Pfeifer, who has chaired this group over many years and who has laid the foundation for these results. We gratefully acknowledge the contributions of our coworkers Dr. H. Ernst, DP. T. Mildner, DI. D. Prager, Dr. H. B. Schwarz, Dr. R. Q. Snurr (now at the Department of Chemical Engineering, Northwestern University, Evanston, IL, USA) and DP I. Wolf as well as the help of Dr. W.-D. Hoffmann. The Deutsche Forschungsgemeinschaf~ and the Bundesministerium ftir Bildung und Forschung have supported this work.
558 REFERENCES
1. H. Pfeifer and H. Ernst, Ann. Rep. NMR Spectr. 28 (1994) 91. 2. J.F. Haw, in NMR Techniques in Catalysis, A. T. Bell, A. Pines, Edts., Marcel Dekker, New York, 1994, pp. 139-194. J.B. Nagy, M. Guelton and E. G. Derouane, J. Catal. 55 (1978) 43. 4. D. Michel, Surface Sci. 42 (1974) 453. 5. G. Engelhardt and D. Michel, High Resolution Solid State NMR of Silicates and Zeolites, Wiley, New York, 1987. D. Michel, W. Meiler and H. Pfeifer, J. Mol. Catal. 1 (1975/76) 85. 7. M. Anderson and J. Klinowski, Nature 339 (1989) 200. 8. P. Lorenz, M. BiJlow and J. Kiirger, Izv. Akad. Nauk SSSR, Set. Khim. (1980) 1741. 9. J. K~irger, H. Pfeifer, S. Rudtsch, W. Heink and U. Grof3, J. Fluorine Chem. 39 (1988) 349. 10. U. Hong, J. K~irger and H. Pfeifer, J. Am. Chem. Soc. 113 (1991) 4812. 11. K.P. Datema, J.A. Bolt-Westerhoff, G.J. Nesbitt, P.K. Maarsen, W. Ylstra, P.N. Tutunjian, H. Vinegar and J. Karger, in Magnetic Resonance Microscopy, B. Bltimich, W. Kuhn, Edts., VCH, Weinheim, 1992, pp. 395-416. 12. S.S. Nivarthi and A.V. McCormick, J. Phys. Chem. 99 (1995) 4661. 13. N.-K. BAr, J. Kiirger, C. Krause, W. Schmitz and G. Seiffert, J. Magn. Reson. A 113 (1995) 278. 14. M.W. Anderson and J. Klinowski, J. Am. Chem. Soc. 112 (1990) 10. 15. E.J. Munson, A.A. Kheir, N.D. Lazo and J.F. Haw, J. Phys. Chem., 96 (1992) 7740. 16. H. Ernst, D. Freude and T. Mildner, Chem. Phys. Letters 229 (1994) 291. 17. D.B. Ferguson and J.F. Haw, Anal. Chem. 67 (1995) 3342. 18. H. Ernst, D. Freude, T. Mildner and I. Wolf, Solid State Nuclear Magnetic Resonance, in press 19. D. Freude, Chem. Phys. Letters 235 (1995) 69. 20. J. Karger, N.-K. Bar, W. Heink, H. Pfeifer and G. Seiffert, Z. Naturforsch. A50 (1950) 186. 21. J. Kiirger and D.M. Ruthven, Diffusion in Zeolites and Other Microporous Solids, Wiley, New York, 1992. 22. P.E. Hathaway and M.E. Davis, J. Catal. 116 (1989) 263. 23. T. Yashima, H. Suzuki and N. Hara, J. Catal. 33 (1974) 486. 24. H.B. Schwarz, H. Ernst, J. Kiirger, T. ROser, R.Q. Snuff and J. Weitkamp, Appl. Catal. A 130 (1995) 227. 25. U. Hong, J. K~irger, B. Hunger, N.N. Feoktistova and S.P. Zhdanov, J. Catal. 137 (1992) 243. 26. F. Stallmach, J. Karger and H. Pfeifer, J. Magn. Reson. A 102 (1993) 270. 27. R.Q. Snuff, A. Hagen, H. Ernst, H.B. Schwarz, S. Ernst, J. Weitkamp and J. K~irger, J. Catal., submitted. 28. E.G. Derouane, J.-P. Gilson and J. B. Nagy, Zeolites 2 (1982) 42. 29. E.J. Munson, A.A. Kheir, N.D. Lazo and J.F. Haw, J. Phys. Chem. 96 (1992) 7740. .
.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
559
Vibrational study of benzene adsorbed in NaY zeolite by neutron spectroscopy H.
JOBIC a
and A. N. FITCH b
Institut de Recherches sur la Catalyse-CNRS, 2 Ave. A. Einstein, 69626 Villeurbanne Cedex, France
a
b European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex, France
Neutron spectroscopy is the only vibrational technique where the intensities can be calculated with reasonable accuracy. This method is applied here to benzene adsorbed in NaY zeolite. The force field of the adsorbed molecule has been refined directly to the observed neutron spectrum so that all the internal vibrational frequencies of benzene can be determined. The external modes of benzene relative to the cations were measured in the frequency range 10-100 cm -1. This allows the sodium-benzene bond strength to be estimated.
1. I N T R O D U C T I O N The vibrational modes of benzene adsorbed in different zeolites have been measured by several spectroscopic techniques: infrared, Raman and UV diffuse reflectance. Inelastic neutron scattering (INS) has also been used to study the vibrational spectrum of benzene adsorbed in NaY [1]. In contrast to infrared, the INS bands due to the bare NaY zeolite are negligible compared to the scattering from benzene because the hydrogen atom has the largest scattering cross section of all atoms. Therefore all the vibrational modes of the adsorbed molecule can be measured. For a loading of three molecules per supercage, on average, the INS results indicated the presence of two different molecules, in agreement with neutron diffraction work where two adsorption sites were found [2]. A lower loading was used in this work, one molecule per supercage on average, since it is known from neutron diffraction and infrared spectroscopy that at this loading the predominant species is the one bonded to the SII sodium ions (at higher loading, a second adsorption site was located in the 12-ring windows [2]). INS is analogous to infrared and Raman spectroscopies in that transitions between vibrational levels can be measured. However, there are several features which make INS complementary to the optical methods, e.g. the lack of selection
560 rules and the possibility to compute the intensities [3]. Low-frequency vibrations can also be observed more easily with INS than with infrared spectroscopy, and in this work the spectral range has been extended down to 16 cm -~ compared with 200 cm -~ in our previous study [1], so that the external modes of benzene relative to the cations can be measured.
2. E X P E R I M E N T A L INS spectra were obtained with the spectrometer IN1BeF at the Institut LaueLangevin in Grenoble, France, and with the Spectrometer TFXA, at the Rutherford Appleton Laboratory, U.K. The spectrum of bulk benzene was recorded only on IN1BeF, between 200 and 2000 cm -1, using Cu (220) and (331) monochromator planes and a beryllium filter situated between the sample and the detector [3]. This setting gives a moderate energy resolution, the FWHM of the peaks varies from 30 -1 cm to 42 cm -1. A better resolution can be obtained on this instrument, using an additional graphite filter, but at the expense of the signal intensity [4]. INS spectra of benzene adsorbed in NaY were obtained on both spectrometers using different samples. The NaY was heated at 720 K under flowing oxygen and outgassed at a final pressure of 10 -3 Pa, at this temperature. The activated zeolite (~ 10 g for TFXA and 20 g for IN1BeF) was transferred into cylindrical aluminium containers in a glove box. Spectra of the bare zeolite were first recorded at 20 K. After these measurements, the zeolite was warmed to room temperature and benzene was adsorbed onto the zeolite, the loading was the same for the 2 experiments" 1 benzene molecule per supercage, on average. The samples were left overnight at 370 K to equilibrate. Spectra were recorded from the samples cooled to 20 K. Such a low temperature is required to decrease dynamical disorder of benzene molecules and thus to sharpen the vibrational features [3]. On the IN1BeF spectrometer, the Cu (220) monochromator plane was used to cover the energy range 200-2000 cm -1. Lower frequencies were measured on TFXA.
3. RESULTS AND DISCUSSION 3.1. Bulk Benzene The experimental INS spectrum of polycrystalline benzene, recorded at 5 K on IN1BeF, is shown in Figure 1 as a dotted line. The resolution is slightly better above 1000 cm -~, compared with our previous study [1 ]. The C-H stretching region, around 3000 cm -~, was not investigated because only one broad band is obtained in that range. This is due to the energy resolution which worsens at large energy transfers and also to the Debye-Waller factor, exp(-Q2 ), where Q is the neutron momentum transfer and < u2> the mean-square atomic amplitude. For a molecular system, the mean-square amplitude can be split into contributions from the
561
20000 •
A
15000 o cj
10000
o
5000
Z
0
.4
Ii 300
Iil I, Iil, 600
900
II, It I, 1200
1500
1800
E (cm -1) Figure 1. Comparison of experimental and calculated inelastic neutron scattering spectra of bulk benzene.
low frequency external modes < u 2 > ~xt and the high frequency internal modes < u 2> i,t. The Debye-Waller factor decreases the intensifies from the fundamentals as Q2 increases, (on both spectrometers used in this work, Q2 is proportional to the energy transfer). The intensity which is taken from the fundamentals is redistributed into side bands (or phonon wings) as a function of Q. The frequencies of the fundamentals is indicated as sticks in Figure 1. Side bands due to multiphonon scattering can be clearly observed in between fundamentals. The force constants of benzene in the solid phase were refined directly to the observed INS profile, starting from the force field of La Lau and Snyder [5]. The refinement includes the intensifies from fundamentals, overtones and combinations, the contributions from all atoms being added up. The final force field is close to the original. Since the C-C stretch and the C-C bond interaction force constants hardly varied, they were fixed to their initial values, but other force constants have been modified. Apart from the force constants, the only other parameters are a scale factor and the mean-square amplitude for a hydrogen atom due to the external modes ,xt • The mean-square amplitude due to the internal modes < u 2 > i,t is calculated from the fitted frequencies and has a value of 0.012 ]k2. The final value of
562
30000 O~
25000 20000
o r,.)
15000
o
10000
q)
5000
Z
e¶
,
-00
~,e
•
.%
I,, I ii,, I I Iiil, i1,, I! Il I, 300
600
900
1200
1500
1800
E (cm -1) Figure 2. Comparison of experimental and calculated INS spectra of benzene adsorbed in NaY (1 molecule per supercage, on average).
< U 2 > ext is 0.011 A 2. The calculated profile, shown as a continuous line in Figure 1, is in good agreement with the experimental spectrum. Not all the frequencies are resolved, but the method to treat overlapping modes is similar to the Rietveld method in powder diffraction [6]. The entire INS profile is refined instead of the individual normal modes. The difference between the observed and calculated INS spectra is minimised using a least-squares procedure. Some frequency shifts are observed with respect to gas phase frequencies because of intermolecular forces. For example, the out-of-plane C-H bending mode Vll is shifted up to 694 cm 4, compared with 673 cm 4. On the other hand, the modes which have a strong C-C stretch character in their potential energy distribution (PED) are not shifted. The intensities of the vibrational peaks are related to atomic displacements, so the modes involving the largest displacement vectors for the hydrogen atoms have the highest intensities because of the large cross section of this atom. Therefore, the out-of-plane C-H bending mode v~0 at 855 cm 4 has a much larger intensity than the in-plane mode v6 at 605 cm 4, which is mainly C-C stretch and C-C-C deformation.
563 3.2 Benzene adsorbed in NaY The experimental INS spectrum of benzene adsorbed in NaY (1 molecule per supercage on average) is shown in Figure 2. The data were recorded at 20 K on IN1BeF. The contribution from the bare zeolite, which is almost fiat on the same intensity scale, has been subtracted. Therefore all the vibrational modes of adsorbed benzene can be measured in that range, whereas there are strong absorption bands due to the framework in infrared below 1300 cm -t. The force constants of adsorbed benzene were refined to the observed INS profile, starting from the force field obtained for the molecule in the solid phase. The adsorption geometry, shown in Figure 3, was taken from the diffraction work: the planar benzene molecule is located in the supercage, with its center on the cube diagonal, at a distance of 2.7 A from the sodium cation in site II [2]. The sodium cation was considered to be bonded only to three 0(2) oxygen atoms, at a distance of 2.35 A from each oxygen
Na
Figure 3. Adsorption geometry of benzene in the supercage of NaY [2]. The calculated profile is in good agreement with the experimental data so that the frequencies of all fundamentals are known, whereas they have to be derived from overtones and combination bands in infrared. The out-of-plane C-H bending mode Vl~ is measured at 700 cm -1, implying an upward shift of 27 cm q when compared to the gas phase. Such shifts to higher frequencies of v~ have also been found in the formation of metal 7t-complexes, e.g. Vl~ shifts to 785 cm -~ in (l'16-c6n6)Cr(CO)3 [7]. In previous infrared measurements performed at different temperatures [8] and in the previous INS study [1], a splitting of v~ was observed at high loadings, with
564 components at 700 and 725 cm 4. The band at 725 cm 4 was assigned to benzene bonded to the cation and the low-frequency one to benzene adsorbed in the 12-ring windows. The correct assignment is the reverse since only the component at 700 -1 cm is observed in this work. Indeed, it has been since realised that the previous infrared measurements were incorrectly interpreted [9]. The other out-of-plane C-H bending modes are also found to be shifted up in energy: v~0, v17, and v5 are found at 865, 977, and 1009 cm 4, respectively. This explains quite nicely the combination modes observed in infrared at 1844 cm -1 (v~0 + V17) and at 1985 cm -1 (vs + v~7) [10]. The INS features of adsorbed benzene (Figure 2) are broader and less structured compared with solid benzene (Figure 1). This is due to a slight increase in the intrinsic width of the modes, due to small differences of interaction energy, and also to the Debye-Waller factor, which has decreased. Therefore the intensities of the fundamentals is further decreased, and is transferred to multiphonon features, hence broader bands. The smaller Debye-Waller factor for adsorbed benzene is due to a larger value of the mean-square amplitude due to the external modes < u2> ext • The fitted value of oxt is 0.017 ]ka, instead of 0.011 ~2 in the solid. This arises because the molecules are more loosely bound in the zeolite so that the external modes are situated at lower frequency and their contribution to ex, is increased. This can be checked from the position of the external modes of the adsorbed molecule. In the zeolite, these modes become hindered translations and
4.0
3.5 3.0
, it
2.5 ~
2.0
°
~
1.5
2
i.o
-
'--~"v~..,~
0.0
I
0
I
50
i
I
100
~
./-..
,. , , ~ . . ~ . . . .
I
150
200
250
E (cm -1) Figure 4. Low-frequency modes measured by INS for benzene adsorbed in NaY (1 molecule per supercage, on average).
565
rotations. They have never been observed with optical spectroscopy but they can be measured by INS. The low-frequency spectrum of benzene adsorbed in NaY, at the same loading, is shown in Figure 4. The data were recorded on TFXA at 20 K. The most intense bands are found in the range 10-100 cm -~, whereas the density of states of the external modes of benzene has peaks extending up to 130 cm -~. This difference affects the position of the side bands in our INS spectra. In Figure 1, the side band at 494 cm -~ is well separated from the v~6 peak at 407 cm -1, while in Figure 2 the side band becomes a shoulder at 465 cm -~. A simulation was performed by taking into account Na-C stretching force constants and several valence angles introduced by the bonding between the benzene moiety and the NaO3 fragment. The calculated spectrum is shown in Figure 5. The agreement with the experimental data, Figure 4, is reasonable considering that the external modes probably do not comply with the hypothesis of harmonic motions. Furthermore, our force field between benzene and sodium is only schematic of the rttype interaction. The range of frequencies for the external modes is however well reproduced, the calculated modes corresponding to coupled translations and rotations. In our previous paper [1], frequencies below 200 cm -~ could not be recorded so we had assigned two small bands above 200 cm -~ to the stretching modes of benzene relative to the cations. The new experiments reported here allow us to correct this assignment, the modes having Na-C stretch character in their PED are
4.0
3.5 °
D
3.0 2.5 2.0 1.5 1.0 0.5 0.0
I
I
50
I
m
I00
E
i
I
150
,
I
200
l
250
(cm -1)
Figure 5. Calculated low-frequency spectrum of benzene adsorbed in NaY.
566
situated between 30 and 80 cm -1. The calculated Na-C force constant, 0.06 mdyn/,/k, indicates a small benzene-sodium bond strength, in agreement with the small perturbation of the internal modes. In the organometallic compound (rl 6C6H6)Cr(CO)3 , the benzene-chromium bond strength is much larger, 4.5 mdyn/A [7].
4. CONCLUSION Inelastic neutron scattering has been used to investigate the vibrational modes of benzene adsorbed in NaY zeolite, for a loading of one molecule per supercage. The frequency range 16-2000 cm 1 was covered with two spectrometers, allowing measurement of both the internal and the external modes of the molecule. Since it is known from neutron diffraction and from infrared experiments that one type of benzene predominates for low loadings, a normal coordinate analysis of the adsorbed molecule has been performed. The force constants related to the internal modes of benzene were refined directly to the observed profile, which makes INS a very powerful vibrational technique. The modes of benzene relative to the cations were measured in the range 10-100 cm -~, yielding a relatively small benzene-sodium bond strength.
ACKNOWLEDGMENTS We thank Dr. H. J. Lauter (Institut Laue-Langevin, Grenoble, France), and Dr. J. Tomkinson (ISIS, Rutherford Appleton Laboratory, U.K.), for their help with running the spectrometers. We also thank these two institutions for allocation of beam time. REFERENCES 1. H. Jobic, A. Renouprez, A. N. Fitch and H. J. Lauter, J. Chem. Soc., Faraday Trans. 1, 83 (1987) 3199. 2. A . N . Fitch, H. Jobic and A. Renouprez, J. Phys. Chem., 90 (1986) 1311. 3. H. Jobic in "Catalyst Characterization, Physical Techniques for Solid Materials", B. tt~t~k ~td J. C. V6drirte, Eds., Plenum, New York (1994) p. 347. 4. H. Jobic and H. J. Lauter, J. Chem. Phys., 88 (1988) 5450. 5. C. La Lau and R. G. Snyder, Spectrochim. Acta, 27A (1971) 2073. 6. H . M . Rietveld, J. Appl. Crystallogr., 2 (1969) 65. 7. H. Jobic, J. Tomkinson and A. Renouprez, Mol. Phys., 39 (1980) 989. 8. M. Primet, E. Garbowski, M. Mathieu and B. Imelik, J. Chem. Soc., Faraday Trans. 1, 76 (1980) 1942. 9. M. Primet (private communication). 10. A. de Mallmann and D. Barthomeuf, Proc. 7th Int. Zeolite Conf., Eds. Y. Murakami, A. Iijima and J. W. Ward, Kodansha-Elsevier, Tokyo-Amsterdam (1986) p. 609.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
567
Infrared Holeburning Spectroscopy in Acid Zeolites Mischa Bonn ~ b, Marco J.P. Brugmans a, Huib J. Bakker a, Aart W. Kleyn a, Rutger A. van Santen b ~FOM-Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands bSchuit Institute of Catalysis Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
With time-resolved (picosecond) infrared holeburning spectroscopy, we directly demonstrate the inhomogeneity of acid sites in zeolites. Remarkably, the degree of relative inhomogeneity decreases upon adsorption of weakly interacting molecules. 1. I N T R O D U C T I O N Infrared spectroscopy is one of the foremost spectroscopic techniques used in modern catalytic studies.[1] It has led to a better characterization and understanding of catalytic processes on a molecular scale; the vibrational frequencies of the catalyst and the reactant are a rich source of information on reaction centers and intermediates. Additionally, the width of an infrared absorption line contains information on the nature of the interaction of a molecule or oscillator with its surroundings.[2] This will be the subject of this Paper. There are essentially two contributions to the linewidth: homogeneous and inhomogeneous contributions. Inhomogeneous broadening occurs when the oscillators in the sample have slightly different center frequencies, due to the different environments they experience. This type of broadening is of a static nature. This is in contrast with broadening arising from homogeneous contributions, which is due to the dynamics of the vibration: Due to dynamic interactions (e.g. energy exchange, dephasing or collisional broadening processes) with its surroundings the absorption line of a single oscillator will be broadened. Hence an inhomogeneous absorption line will be composed of a sum of homogeneous (Lorentzian) lines with slightly different center frequencies Non-linear spectroscopy allows for the separation of homogeneous from inhomogeneous contributions, and thus enables the measurement of the homogeneous linewidth, even in case of a strongly inhomogeneously broadened absorption line. Hence, non-linear spectroscopy enables us to unravel the IR absorption lines and obtain information inaccessible with conventional linear spectroscopy. For the zeolite O-H absorption lines under investigation here, we will use this technique to prove directly the inhomogeneity of active sites in acid zeolites. Upon adsorption of simple molecules, the relative inhomogeneity decreases. This can be explained by considering the nature of the hydrogen bond potentials.
568 2. E X P E R I M E N T A L
The principle of the technique described in this paper is to excite a significant fraction (,-~ 15%) of the O-H groups to the v=l first excited state by means of an intense short infrared laser pulse (pump pulse) which is tuned to the absorption peak. As a result of the pump pulse, the original absorption band will decrease in amplitude (the exited oscillators are saturated; they can no longer absorb the light). Also, excited state absorption will occur, from v = l to v=2, at somewhat lower frequencies due to the anharmonicity of the vibration. The consequent transmission changes are monitored by a much weaker probe pulse. This probe pulse is independently tunable in wavelength and can be variably delayed with respect to the pump pulse. This allows for two types of experiments. The first is a delay-scan experiment. In this experiment, pump and probe are both tuned to the peak of the absorption band, and the time evolution of the pump-induced transmission changes are monitored. This will give the lifetime of the oscillators in v = l (T1); the pump-induced transmission will decay back to equilibrium with a time-constant that is the lifetime of the first excited state. This population lifetime T1 is known to be a good measure for the degree of hydrogen-bonding of the zeolite hydroxyl groups. The second is a wavelength-scan experiment, in which the delay between pump and probe is fixed (at small delays) and the wavelength of the probe is scanned. Thus the spectral effect of the pump pulse is monitored. This so-called spectral holeburning technique will reveal possible inhomogeneous broadening and render the homogeneous linewidth, associated with the intrinsic dynamics of the vibration.
Nd:YAG ~
PROBE
2500-4000 cm-1, 25 ps I r'=6-30 cm-1 1°° g J
~ ~
PUMP ~
variable delay
sample~ ~detector
Fig. 1. Outline of the experimental setup for non-linear IR spectroscopy.
The picosecond infrared pulses are obtained by downconversion of 1064 nm, 35 ps pulses from a Nd-YAG laser by means of parametric generation and amplification of infrared in LiNbOa crystals. A schematic picture of the set-up is shown in Fig. 1 (for details, see Ref.
[a]).
569 3. R E S U L T S A N D D I S C U S S I O N We have investigated the deuterated BrCnsted sites of zeolites Y and Mordenite, since the linewidth of our laser is smaller at O-D than at O-H stretching frequency allowing for a better spectral resolution.
1.5
O-D"'Olatt
0.8
,.,%
1.2 ~-
8 0.6
n~ k ; ~ k ,
0.6
o.o...,
0.4 #'E"
~ ~
-- 0.2
"~ 0.3 2500
"
2~0
2&O 2&O 2;00 wavenumber (crn-1)
2750
Fig. 2. Absorption spectra for deuterated n0.27Y (solid), NaH0.07Y (dashed) and the latter with adsorbed nitrogen (dotted line).
HF-OD
A o
HF-OD ' N2 LF_OD ... Olatt
~1=171 (4) ps
a°
0.0 0.0
"
-1;0
;
T1=44(1 ) 1;0 200 300 1:delay(ps)
%
4;0
-
-] 500
Fig. 3. Results of (one-colour) pump-probe delay-scan experiments for the three O-D groups of Fig. 2: Transmission changes (To is the transmission without pump pulse) versus delay between pump and probe.
In Fig. 2 three absorption spectra of Y zeolite, recorded with a Perkin-Elmer 881 double beam IR-spectrometer, are shown in the O-D stretching region. By exposing the 5 mg, 1.3 cm 2 pressed zeolite discs to 1 bar of D2-gas (Messer Griesheim, 99.7%) at 693 K for 1 hour, 70 % H ~ D exchange occurs. For fully proton-exchanged deuterated H0.27Y zeolite (Si/AL=2.7), we see two absorption peaks, one at 2680 cm -1, associated with high-frequency (HF) O-D groups situated in the supercages, and one at 2620 cm -1 originating from the low-frequency (LF) sites in the smaller cages.[4] The LF O-D peak is redshifted compared to the HF O-D peak, because the deuterons from the LF O-D groups are hydrogen-bonded to zeolite lattice oxygen atoms.J5] For the same zeolite with a lower degree of exchange, NaH0.07Y, mainly HF O-D groups in the large cages are present. Upon addition of 50 mbar of nitrogen (Messer Griesheim, 5.0) at 100 K, we see that the nitrogen adsorbs to these O-D sites.J6] The weak hydrogen bond between the O-D group and the adsorbate weakens the original O-D bond, accounting for the redshift of the 2682 cm -1 peak to 2622 cm-1.[6] Note the similarity between the LF O-D peak of H.27Y and that of the HF O-D of NaH.07Y with adsorbed nitrogen, further referred to as the O-D.. "Olatt and O-D...N2 groups, respectively. In Fig. 3 the results of the delay scans are shown: upon adsorption of the nitrogen, the vibrational population lifetime decreases from 171 to 48 ps. We have shown recently [7]
570 that this enhancement of the vibrational relaxation rate is not due to desorption of the nitrogen (breaking of the weak hydrogen bond would be a very efficient way for the O-D group to lose a considerable amount (~ 1000 cm-116]) of its excess vibrational energy). The enhancement is attributed to an increased coupling of the O-D group to the lattice caused by the hydrogen bond. Note that, as with the absorption spectra, there is a striking similarity between the population lifetimes of the (LF-OD) O-D...Otatt and (HF-OD) O-D.. "N2 groups, demonstrating that the two O-D groups are hydrogen-bonded with an equal strength.
1.2 1.0 5
0.8
[]
•
'
'
'
without pump with pump
o~
...
..Q L_ 0 . 4 0 O3
0.2
O0 2450
Ep [] [] [] []
| latl
|
[]
without pump
•
with pump
| O-D:N2
Qo
D
oanPoooO
0.6 r.,-
•. 0
uu=-o...-
,,,
B B:. ~ 25~00'
• 2550'
8
000
II
°o
•
oO0°O0°
26'50
24so'
2 oo'
W°
[]
o•
o° otAmae e mwaSB~ooo°°°°°°°~--
puIm p ". 26~00'
[]
•
T
~$g
pump
2 oo'
wavenumber (cm -1)
Fig. 4. Regular and transient absorption spectra (recorded with the infrared laser probe pulses) with and without the pump pulse tuned to the peak of the absorption band. For the O-D-. "Olatt groups, the width of the hole is almost completely determined by the 6 cm -1 width of the laser pulses, revealing a strong inhomogeneity. For the O-D-. "N2 groups, the absorption line is clearly more homogeneous. For both experiments T=100K.
The results of the wavelength scan (2 colour pump-probe) experiments are depicted in Fig. 4, where the absorption lines with and without the pump pulse are presented. Recording a transient spectrum at a delay between pump and probe of 30 ps, it is found that the pump pulse burns a hole in the absorption spectrum for the O-D.. "Olatt groups, whereas the whole absorption band goes down in amplitude for the O-D...N2 groups. Similarly, the excited state absorption line, ( v = l ) ~ ( v = 2 ) , is also much narrower for the O-D.. "Olatt groups than for the OD.-.N2 groups. The fact that a hole is burnt in the O-D...Olatt absorption spectrum implies that this line is inhomogeneously broadened [8]; there is a distribution of vibrational frequencies for the O-D groups, and the laser pulse can only excite those O-D groups resonant with the frequencies within the laser pump pulse. This means there is an inhomogeneous distribution of catalytic sites in acid zeolites[3], which resolves an old issue in catalysis.J9]-[13] A possible cause for the inhomogeneous broadening is the inhomogeneous distribution of silicon and aluminum atoms throughout the lattice. It is known that even the second coordination shell of
571 AI/Si atoms is 'felt' by a Si or A1 atom.[14] Since we have observed that within the LF O-D (O-D"-Olatt) absorption band the lifetime decreases strongly with decreasing frequency[15], the inhomogeneous distribution of A1 atoms evidently gives rise to a static distribution of hydrogen bond strengths: More strongly hydrogen-bonded O-D groups will absorb at lower frequencies, and exhibit shorter lifetimes. From the width of the hole (determined by the laser bandwidths and the homogeneous linewidth) we extract[3] homogeneous linewidths of 0.5 cm -1 f o r the O-D.. "Otatt groups and 13 cm -1 for the O-D...N2 groups. For the HF O-D groups, the same homogeneous linewidth of 0.5 c m - 1 w a s observed, confirming that this absorption line is also strongly inhomogeneously broadened. For the O-D.. "Ot~tt and O-D--.N2 groups, this means that, although the absorption lines are indistinguishable, the homogeneous linewidths hidden under the Overall absorption lines differ enormously. Contrary to the O-D groups in vacuum, homogeneous broadening mechanisms play a significant role for the O-D groups with nitrogen adsorbed.
I
A
"...IN ' - - NI '
'
"
•
•
i
B
y
,
oct i" \~2600cm -1 V
-0
OD-
lk~ ,
roN2
I
VOD=0 ,
I
,
I
ro...O,at t
Fig. 5. Schematic hydrogen bond potential for the two differently hydrogen-bonded systems: the lower curve for the O-X stretch vibration in the ground state, upper curve first excited state. For a hydrogen bond to an adsorbate, strongly anharmonic (A) and for a hydrogen bond to fixed lattice oxygen atoms, very harmonic (B).
The large difference in the homogeneous linewidths can be understood by considering the hydrogen bond potentials for the two systems schematically depicted in Fig. 5. Shown in this figure is the variation of the energy of the system with the hydrogen-bond coordinate. For the LF-OD groups this coordinate is the OD" "Olatt distance, for the HF-OD
572 groups with adsorbed nitrogen, it is the OD'" "N2 distance. In the presence of nitrogen, the potential is strongly anharmonic. Furthermore, it is well known that the potentials for the ground and excited state of the O-D stretch vibration are different[16]; the hydrogen bond is stronger for v = l than for v=0, and hence the potential energy minimum is situated at smaller (O...N2 distance) for v=l.[16] This means that a variation of the O..-N2 distance will result in a change (modulation) of the v=0--.v=l transition frequency. In other words, the exact transition frequency for the O-D group depends on the O.. "N2 distance. This will result in a broadening of the homogeneous line, since for an oscillator a wide range of transition frequencies is available due to the nitrogen. In contrast, for the OD.. "Otatt the potential energy curves are dictated by lattice parameters; the OD'" "Olatt distance is not determined by the electrostatic interaction between the deuterium and the Ol~tt, but by the geometry of the zeolite lattice. To move the Ot~tt away from the O-D group requires a local deformation of the zeolite lattice. Hence the potential is very narrow and harmonic, and very similar for the ground and excited state of the O-D vibration. The fact that the ground and excited state potentials are the same implies that the transition frequency does not depend on the O.. "Ol~tt distance; the presence of the hydrogen bond will not lead to a broadening of the homogeneous absorption line. Hence in this case, the homogeneous line is very narrow. The overall inhomogeneous absorption line, in contrast, is determined by a static distribution of hydrogen-bond strengths, as demonstrated above. Summarizing, for the LF O-D groups without adsorbates, there is a strong static inhomogeneity. In contrast, the HF O-D groups interacting with nitrogen exhibit large homogeneous linewidths, of the same order as the inhomogeneous absorption line. The results for nitrogen, methane and xenon adsorption on acidic Mordenite (H0.13Mor Si/AI=6.7) are shown in Fig. 6. For nitrogen adsorption, a larger overall inhomogeneous linewidth is observed, but the homogeneous linewidth is found to be 13 cm -1, exactly what was found for Y-zeolite. For methane, which produces the same shift of the O-D absorption band, it is clear that the pump-induced hole in the absorption spectrum is much narrower. A much smaller homogeneous linewidth of 8 c m - 1 is extracted from the data. It is interesting that two adsorbates producing the same perturbation of the O-D groups, give rise to such different homogeneous lines. For adsorption of xenon, which is more weakly adsorbed than both methane and nitrogen, a homogeneous linewidth of 28 c m - 1 is observed! In this case, the absorption band is almost totally homogeneously broadened. The explanation for the strong variation of the homogeneous linewidth with different adsorbates is as follows: As the adsorbate is (thermally) moving to and from the O-D group, the frequency of the O-D stretching vibration will change, since this frequency is very sensitive to the OD-adsorbate distance (Fig. 5a). This means that the transition frequency is modulated by the adsorbate, and changes in time due to the movement of the adsorbate. It is known from nuclear magnetic resonance spectroscopy (NMR) that the width of an absorption line depends on the rate with which the frequency of the associated transition is modulated: A more rapid modulation of the transition frequency will result in a narrowing of the absorption line.J17] This phenomenon is known as 'motional narrowing'. [17]
573
0.8
/...a--.a.~
O-DN 2
0.6
0.6
0.4 0.4
v II) ¢O C
l pump
0.2 '
"0.5 O u)
<
2ds0
'
2s'80
'
0.2
/
2640 2550 wavenumbers (cm1)
28'10
o..e..o
0.3
/,...j
/-'"
O " " O°
0.1
"~. '
z /:/.'. "w"•
T
t~....~
pump |
25'80
'
26'10
i
2640
O-DXe
0.4
0.2
".l:l
)...a.%'
O_D.I.CH 4
258o
[]
"',,.\
I
-x
\\}::1
p u mp '
28'1o
'
"ooQ~x 214o
'
28'7o
2700
Fig. 6. Regular and transient absorption spectra for D-Mordenite with nitrogen, methane and xenon adsorbed. For Xe the transient spectrum exhibits two components; one broad feature due to the OD...Xe groups, and a narrow feature due to the O-D groups in the small cages (with a very narrow homogeneous linewidth) to which no Xe is adsorbed[19]. Temperatures are 100 K for N2 and CH4, and 170 K for Xe. As stated above, the transition frequency for the O-D groups with adsorbates depends on the varying position of the adsorbate. The position of the adsorbate will change in time, due to the oscillations of the hydrogen bond mode, the OD'' .X stretching mode (typical frequency ,-,100 cm-1). Hence the time-scale associated with the modulation of the O-D stretch transition frequency is determined by the frequency of this hydrogen-bond mode. The frequency of this mode, in turn, is determined by the strength of the hydrogen bond as well as the reduced mass of the vibration (determined predominantly by the mass of the adsorbate). For methane, which has half the mass of nitrogen, the frequency of the OD.-.X stretching (hydrogen bond) mode will be , - , ~ higher compared to nitrogen. As a result, the O-D transition frequency will undergo a more rapid modulation, resulting in a homogeneous line that is narrower than that for nitrogen adsorption. The reverse argument holds for xenon: Due to its high mass, the movement of the xenon atom from and to the O-D group will be very slow (low O D . . . X stretching frequency). Consequently, in this case less motional narrowing of the O-D absorption line will occur, accounting for the large homogeneous linewidth. Extensive calculations presented elsewhere demonstrate the quantitative correctness of this picture.J18]
574 4. C O N C L U S I O N S By means of infrared transient holeburning spectroscopy, we have directly demonstrated the proton inhomogeneity in acid zeolites. This technique is unique in the sense that it enables resolution of the proton inhomogeneity with an energy difference on the order of 1 cm -1. The probable cause for the proton inhomogeneity is the difference in local A1 concentrations. The difference between the proton energies is, however, quite small, since a weakly interacting adsorbate strongly decreases the effect of the inhomogeneity. We show that in the presence of adsorbates, a mechanism known in NMR as motional narrowing plays a significant role in determining the homogeneous linewidth. REFERENCES
H. Niemantsverdriet, Spectroscopy in Catalysis, (VCH publishers, New York, NY,
1993). . 3.
o
9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
D.W. Oxtoby, Adv. Chem. Phys. 40, 1 (1979). M.J.P. Brugmans, H.J. Bakker, A. Lagendijk, J. Chem. Phys. accepted for publication. M. Czjzek, H. Jobic, A.N. Fitch, and T. Vogt, J. Phys. Chem. 96, 1535 (1992). M.J.P. Brugmans, A.W. Kleyn, A. Lagendijk, W.P.J.H. Jacobs and R.A. van Santen, Chem. Phys. Lett. 217, 117 (1994). F. Wakabayashi, J. Kondo, W. Akihide, K. Domen, and C. Hirose J. Phys. Chem. 97, 10761 (1993). M. Bonn, M.J.P. Brugmans, A.W. Kleyn, and R.A. van Santen, Chem. Phys. Lett. 233, 309 (1995). A.E. Siegman, Lasers, (University Science Books, Mill Valley, 1986). J. Datka, M. Boczar, P. Rymarowycz, J. Catal. 114, 368 (1988). V.B. Kazansky, Acc. Chem. Res. 24, 379 (1988). K.P. SchrSder, J. Sauer, M. Leslie, C.R.A. Catlow, Zeolites 12, 20 (1991). K.P. SchrSder, J. Sauer, M. Leslie, C.R.A. Catlow, J.M. Thomas, Chem. Phys. Left. 188, 320 (1992). G.J. Kramer and R.A. van Santen, J. Am. Chem. Soc. 115, 2887 (1993). J. Dwyer in: Innovation in Zeolite Material Science, eds. P.J. Grobet et al. (Elsevier, Amsterdam, 1988) p. 333. M. Bonn, M.J.P. Brugmans, A.W. Kleyn, and R.A. van Santen, A. Lagendijk Stud. Surf. Sc. Cat. 84, 493 (1994). C. Sandorfy in: Hydrogen bonds, P. Schuster (Springer-Verlag, Berlin, 1984), p. 41. C.P. Slichter, Principles of Magnetic Resonance, (3~d edition, Springer-Verlag, Berlin, 1989). M. Bonn, M.J.P. Brugmans, A.W. Kleyn, R.A. van Santen and H.J. Bakker, Phys. Rev. Lett. submitted for publication. V.L. Zhoblenko, M.A. Makarova and J. Dwyer, J. Phys. Chem. 97, 5962 (1993).
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
575
Exploring the sites of adsorbed pyrrolidine derivatives in Y zeolites by joined infrared spectroscopic and computer simulation studies
E. Geidel, K. Krause, J. Kindler and H. F6rster Institute of Physical Chemistry, University of Hamburg, BundesstraBe 45, D-20146 Hamburg, Germany
The nature and strength of interaction of pyrrolidine (PY), N-methylpyrrolidine (NMP) and their deuterated derivatives with Y zeolites were investigated. IR measurements show a protonation of both probe molecules in the case of HY. In alkali metal-exchanged zeolite Y the probes are strongly bound via the nitrogen lone pair to the cations in the sequence NaY>KY>RbY>CsY. Computer simulation techniques were applied to verify the results obtained by vibrational spectroscopy.
1. INTRODUCTION Vibrational spectroscopic techniques are widely applied for the characterization of zeolites and of processes occurring on their internal surfaces. However, a straightforward interpretation of the spectral data is mostly difficult. This is especially the case when polyatomics are used as probe molecules for the study of surface acidity or basicity. The aim of this study is to determine the nature and strength of the interaction of pyrrolidine derivatives with Y zeolites by IR spectroscopy and to use computer modelling techniques to facilitate the interpretation of the experimentally measured spectra. In order to understand the spectral alterations during adsorption reliable assignments of the vibrational spectra of the free probes are necessary. Therefore besides PY and NMP additionally the deuterated derivatives PY-dl (pyrrolidine-dl CaHsND), NMP-d3 (N-methyl-d3-pyrrolidine CaHsNCD3) and NMP-d8 (N-methylpyrrolidine-d8 CaD8NCH3) were investigated. We should like to demonstrate that these molecules are suitable probes to get information about hostguest interactions in zeolites.
2. EXPERIMENTAL SECTION Zeolites HY, KY, RbY and CsY with Si/AI=2.6 were prepared from the sodium form by conventional ion exchange in aqueous solution at 343-353 K. Three successive exchanges were performed with a duration of 24 h for each exchange. After washing and drying, the samples were pressed into self-supporting wafers and mounted in a vacuum cell equipped with calcium fluoride windows. The samples were activated at 623 K in vacuo. PY, PY-dl,
576 NMP, NMP-d3 and NMP-d8 were each adsorbed at room temperature at a pressure of about 500 Pa for 1 min followed by evacuation. Mid-IR spectra were recorded in the range 40001200 cm l using a Digilab FTS-20E spectrometer with a resolution of 2 cm -I accumulating 512 scans and ratioed against the background of the activated zeolites. In order to record far-IR spectra, self-supporting wafers were mounted in a cell with polyethylene windows. After dehydration at 623 K in a stream of dry nitrogen and cooling to room temperature, the probe molecules were added stepwise until saturation, followed by heating up to 353 K in order to remove physisorbed molecules. Far-IR spectra were recorded in the range 400 - 20 cm l with a resolution of 4 cm l using a Digilab FTS 15E spectrometer, equipped with a mercury vapour source and a liquid-helium-cooled silicon bolometer (Infrared Laboratories) as high sensitive detector. IR and Raman spectra of the pure ~robe molecules were obtained on a Perkin-Elmer System 2000R with a resolution of 1 cm ° coadding 1024 scans.
3. C O M P U T A T I O N A L DETAILS To clarify vibrational assignments of the free probe molecules normal coordinate analyses (NCA) were performed by the Wilson GF matrix method [ 1] using the Jones program package [2]. The geometric structures of PY and NMP were taken from electron diffraction studies of Pfafferott et al. [3]. The vibrational analyses in terms of internal coordinates were carried out for all molecules using the general valence force field determined in a scaled quantum mechanical ab initio calculation [4]. To get information about the low energy sorption sites of the probe molecules in the host structures, Monte Carlo (MC) and molecular dynamics (MD) calculations were performed using the Solids_Docking and the Discover modules of the Biosym software [5]. For the host structures simulation boxes of one crystallographic unit cell of faujasite with Si/A1=2.76 of the formula M51Si141A1510384 were created. Following the sequence of preferred sites for protonation found by Schr6der et al. [6], for M=H + the protons are equally distributed at 03 and O 1 positions. In addition, the protons are assumed to lie in the Si-O-A1 plane. For M = Na + the cations were randomly distributed over possible sites I, I', II, II' and III followed by a MD equilibration run. For all calculations the CFF91 force field [7] with the extensions proposed by Hill and Sauer [8] was applied. As there are a series of potential minima for the guest molecules [3] MC procedures were started with high temperature MD calculations of the probes in vacuo (10 ps duration, collection of 50 conformers, T=1500 K) to arrive at different conformations. A maximum of 10000 random trial insertions of these conformers into the host lattice were attempted using periodic boundary conditions. The intermolecular interaction energies were calculated and compared to a chosen threshold value of 1500 kcal/mol in order to avoid extreme contact between host and guest. Provided the calculated energy exceeds the limit, the configuration is neglected and the docking procedure is started again, otherwise the host-guest-combination is saved for further investigations. In the last step the created docked structures are allowed to relax performing a molecular mechanics computation for a maximum of 100000 iterations until the maximum derivative was less than 0.01 kcal/A. In the MD calculations periodic boundary conditions were applied with a short range cutoff of 12.5 A. Centering one guest molecule in the supercage and treating the host lattice to be rigid, an equilibration run of 20 ps and a data run of 100 ps at 300 K with an integrating time step of 1 fs were performed. Data were collected every 10th step.
577 4. RESULTS AND DISCUSSION 4.1. Probe molecules In the case of PY it is evident to use the NH(D) stretching vibrations as an indicator for host-guest interactions. The corresponding absorptions in the IR spectra of the free PY molecules are found at 3358 (vapour) and 3265 cm l (liquid) for PY and at 2496 (vapour) and 2427 cm 1 (liquid) for PY-dl. In the gas phase these bands are difficult to obtain because of their very low intensity, whereas their intensity increases remarkably in the liquid. The potential energy distribution calculated by NCA reveals that in these normal modes more than 98% of the potential energy is located in the NH(D) stretching coordinate. Thus the NH(D) mode is - as expected - a typical candidate for a characteristic group vibration which makes it especially suitable for describing intermolecular interactions. In the vibrational spectra of NMP derivatives one of the most intense bands was observed near 2780 c m -1. This band was first identified by Bohlmann in some cyclic imines [9] and is explained by a trans effect of the nitrogen lone pair which lowers the wavenumber of the stretching mode of the CH bond in trans position [10]. In a refined attempt with high resolution a splitting of this so-called Bohlmann band into three components at 2754, 2771and 2780 cm l was observed for NMP. In Fig. 1 the Raman spectra of NMP, NMP-d3 and NMPd8 are shown in the CH(D) stretching range. The IR spectra of gaseous NMP are more complicated in this region because of the rotational fine structure of the vibrational bands. Raman scattering
i'
il !i
i/l' ira
:!,! i:i
INMP-d8 I
!i
i'
. iNMP_d3 I I I .
3000
2800
!t
;,
0',
,
2;00
2400
~°
2500 2000 wavenumber (cm "1)
Figure 1. Raman spectra ofNMP, NMP-d3 and NMP-d8 in the CH(D) stretching range. As can be seen analogues in the CD stretching range are observed at 2036 cm "l for NMP-d3 and at 2035 and 2046 cm" 1 for NMP-d8. Normal mode analyses prove that the C-H(D) bonds
578 in trans position to the lone pair of the a-methylene groups in the ring as well as the methyl group are responsible for these bands. IR spectra of NMP in different solvents show an increasing upscale shift of the Bohlmann bands up to 2813 cm -1 (CD3OD) with growing polarity of the solvent• This indicates the sensitivity of these normal modes to electronic changes on the nitrogen atom of NMP and gives a detectable spectral response induced by intermolecular interaction.
4.2. Host-guest interactions The IR spectra in the CH stretching range of NMP adsorbed in several Y zeolites are shown in Fig. 2. It can be seen that in the case of HY no Bohlmann band is detectable.
..........................-.........
1
"N~"~ / A ' ~ [ \ /
i
,.,..,
,!
.,~,~"
/
I
I
3000
I
,,'
.....
I
t
i<......,<."/ . I
/ /"salt-like"-bands, r.............................;@.....
,,,, ,
i
3100
!"
]
....
j
~,,.jJ I
2900
I
I
2800
I
I
2700
I
I
I
I
2600 2500 wavenumber [cm- 1]
Figure 2. IR spectra of NMP adsorbed in NaY (1), KY (2), CsY (3), HY (4) and gas-phase NMP (5) in the CH stretching range. Instead some broad bands appear in the range 2750-2500 cm -1. The same pattern was found in the IR spectrum of NMP hydrochloride and explained as modes of the salt. At the same time the high frequency (HF) and low frequency (LF) bands of the bridging OH groups of the zeolite near 3650 and 3550 cm 1 disappear. These bands are usually assigned to O(1)-H and O(3)-H groups. Thus a strong protonation of NMP in HY must be concluded leading to a saltlike NMP in which the trans lone pair effect is lost. During desorption of the probe molecules with increasing temperature the HF and LF OH bands reappear at unshifled positions. The significant increase in intensity of the HF OH band at higher desorption temperatures indicates a strong protonation of NMP by O(1)-H groups. This is in agreement with the results obtained by TPD experiments of NMP in HY [11]. The same behaviour was obtained in all spectral regions during adsorption and desorption experiments of PY in HY.
579 In contrast the IR spectra of NMP sorbed in alkali metal zeolites show an upscale shift of the Bohlmann band decreasing in the sequence of growing size of the cation (Tab. 1). As the IR spectra of NMP in solution reveal the same effect in solvents of growing polarity, it can be concluded that an interaction withthe cations, acting as Lewis acid sites, takes place. Even if the effect for CsY is minor the slaifts of the Bohlmann bands follow the polarizing power of the cations as well as the HSAB principle and indicate the strength of interaction. This is additionally supported by a DRIFT study of NMP in a HY which was activated at 873 K thus yielding an increased amount of silanol groups. In this case a Bohlmann band at 2798 cm "l was obtained monitoring the interaction of NMP with the less acidic silanol groups [ 12]. Upon adsorption of PY in alkali metal Y zeolites a downscale shift of the NH stretching frequency was detected in the sequence NaY>KY>RbY>CsY. The same effect was found for the ND stretching band of PY-dl. The results are summarized in Tab. 1. Table 1 Experimental wavenumbers of characteristic IR bands of the probe molecules in the gas phase and adsorbed in alkali metal Y zeolites (the dominant band of NMP is marked bold) Pyrrole I PY NMP I v(CH) [cm "1] I v(NH) [cm "l] I v(ND) [cm "11 v(NH) I v(ND) IR
[cmll
2496 2438 2442 2451 2464
3358 3282 3292 3297 3311
ivapour
NaY KY RbY CsY arb. units
NaY+PY
i
x,,.. I
300
I
I
200 100 wavenumber (cm"1) Figure 3. Far-IR spectra of NaY and KY and of PY adsorbed in NaY and KY.
2754, 2771, 2780 2768, 2800, 2812 2762, 2787, 2806 - ,2783, 2805 - ,2780, 2804
3532 3397 3327 3327 3322
2621 2538 2490 2490 2485
The fact that the Na + cation with its high polarizing power yields the largest shift indicates a direct interaction of the lone pair electron of the strong basic PY with the cations. For comparison the NH(D) frequencies measured for pyrrole under the same experimental conditions are also given in Tab. 1. In this case the sequence of the observed shifts is inversed. Thus an interaction of the NH group of pyrrole with the framework oxygens of the zeolite under formation of hydrogen bonds is supposed. This is in agreement with the results obtained by Barthomeuf [13] from which pyrrole was concluded to be a wellsuited probe to investigate sites of different basicity. Also in the far-IR spectra changes were detected upon adsorption of the probe molecules. As an example far-IR spectra of PY adsorbed in NaY and KY in comparison with the spectra of the pure zeolites are shown in Fig. 3. On the one hand no additional band for the interaction of probe molecules with the cations could be
580 observed. Only a small additional shoulder at 65 c m -1 o c c u r s in some adsorbate spectra. Because of an absorption at the same position in the far-IR spectra of the free probe molecules - assigned to the ring puckering m o t i o n - this shoulder should not be interpreted as an indication for host-guest interaction. On the other hand the comparison between the pure zeolite spectra and adsorbate spectra shows broadenings and shifts of nearly all bands. Usually the absorptions below 250 cm in the far-IR spectra of alkah metal zeohtes are assigned to vibrations of cations at specific sites [ 14]. However, no correlations between cation sites and vibrational frequencies were found in theoretical investigations where the far-IR bands were interpreted as simultaneous motions of cations in all sites coupled with framework vibrations [15, 16t. The spectra in Fi.~. 3 support the latter assignment. Especially the shoulder at 159 cm- (NAY) and 104 c m ( K Y ) - assigned to cations at SI position following the "site concept"- are remarkably changed during adsorption. This is in contradiction to the fact that cations at site I are not accessible by the PY molecule. The diameter of PY is about 4 A while the free diameter of the 6-ring window is only 2.8 A. Hence the concept of Ozin [14] assigning each band in the far-IR spectra of zeolites to the motion of a cation on a distinct site seems to be an oversimplification. The results of MC and MD calculations can be summarized as follows: In all MC calculations sites of minimal interaction energy are located near the 12-ring window of the zeolites. The minimum energy found was -73,1 kcal/mol. For NaY a strong orientation of the nitrogen atom to the cation at site II was observed for PY as well as for NMP. The conformations with the lowest interaction energies were determined at distances between sodium at site II and the nitrogen atoms of the guest molecules of about 3.2 A for NMP and of about 2.9 A for PY. The orientation of NMP in NaY at the low energy sorption site is illustrated in Fig. 4. •
-1
.
.
.
.
O D
\
"
Figure 4. Low energy sorption site of NMP in a NaY lattice section• The sodium ions are represented by large gray spheres, the distance between the nitrogen at the nearest cation is indicated by a dotted line.
581 MD simulations confirm the strong interaction between the probe molecules and the sodium cations at site II. It was found that the probe molecules preferentially slide along the cage surface. This is illustrated in Fig. 5 for NMP in NaY. At the chosen temperature of 300 K the sorbate molecules are confined to regions close to the adsorption site calculated by MC.
j1-~
f\
•
•
Figure 5. Single molecule trajectory of NMP in NaY. Our results demonstrate that pyrrolidine derivatives are well-suited probes for the investigation of host-guest interactions especially due to their sensitive Bohlmann band. For both PY and NMP adsorbed in HY and in alkali metal Y zeolites two different types of interaction can be discriminated. The good agreement between experiment and theory in the case of NaY encourages the application of computer simulation techniques to study the sorptive behaviour of the guest molecules. 5. ACKNOWLEDGEMENT The financial support by the Deutsche Forschungsgemeinschaft (Ge 783/1-1) and by the Graduiertenf6rderung der Universit~it Hamburg is gratefully acknowledged. We thank Dr. F. Bauer (University of Leipzig) for preparing the deuterated NMP derivatives.
582 REFERENCES
1. 2. 3.
.
.
6. 7.
8. 9. 10. 11. 12. 13.
14. 15. 16.
E.B. Wilson Jr., J.C. Decius and P.C. Cross, Molecular Vibrations, McGraw-Hill Book Company, Inc., New York (1955). R.N. Jones, Computer Programs for Infrared Spectrophotometry- Normal Coordinate Analysis - N.R.C.C. Bulletin No. 15, Canada (1976). G. Pfafferott, H. Oberhammer, J.E. Boggs and W. Caminati, J. Am. Chem. Soc., 107 (1985) 2305, G. Pfafferott, H. Oberhammer and J.E. Boggs, J. Am. Chem. Soc., 107 (1985) 2309. F. Billes and E. Geidel, Proc. 10th Intern. Conf. on Fourier Transf. Spectrosc., Budapest (1995) A9.3. Catalysis Version 2.3.6, Biosym Technologies, San Diego (1993). K.-P. Schr6der, J. Sauer, M. Leslie, C.R.A. Catlow and J.M. Thomas, Chem. Phys. Letters, 188 (1992) 320. J.R. Maple, U. Dinur, A.T. Hagler, Proc. Nat. Acad. Sci. USA, 85 (1988) 5350, J.R. Maple, T.S. Thatcher, U. Dinur, A.T. Hagler, Chemical Design Automation News, 5(9) (1990) 5. J.-R. Hill and J. Sauer, J. Phys. Chem., 99 (1995) 9536. F. Bohlmann, Chem. Ber., 91 (1958) 2157. D.C. McKean, Chem. Soc. Rev., 7 (1978) 399. B. Hunger and M. v.Szombathely, Stud. Surf. Sci. Catal., 84 (1994) 669. M. Ackermann, J. Nimz, M. Kudra and E. Geidel, Proc. 5th German Workshop on Zeolites, Leipzig (1993) PC9. D. Barthomeuf, Spectroscopic Investigations of Zeolite Properties, in: E.G. Derouane et al. (eds.), Zeolite Microporous Solids: Sythesis, Structure, and Reactivity, Kluwer Acad. Publ., Netherlands (1992). M.D. Baker, G.A. Ozin and J. Godber, Catal. Rev.-Sci. Eng., 27 (1989) 591, J. Godber, M.D. Baker and G.A. Ozin, J. Phys. Chem., 93 (1989) 1409. K.S. Smirnov, M. Le Maire, C. Br6mard and D. Bougeard, Chem. Phys., 179 (1994) 445. K. Krause, E. Geidel, J. Kindler, H. F6rster and H. B6hlig, J. Chem. Soc., Chem. Comm., (1995) 2481-82.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
583
Preparation and Characterisation of R u - E x c h a n g e d N a Y Zeolite: A n Infrared Study of C O A d s o r p t i o n at L o w T e m p e r a t u r e s S. Wrabetz, U. Guntow, R. Schl6gl and H. G. Karge Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, D-14195 Berlin, Germany
RuNaY zeolite was prepared by ion exchange of NaY with an aqueous solution of [Ru(NH3)6]C13. The resulting complex was decomposed and reduced. The IR spectra of CO adsorbed at 110 K on the highly dispersed Ru ° clusters inside the supercages (1-1.5 nm) and on the sintered clusters at the external surface (5.5, 10-15 and 30 nm) display a broad band at ~ 2040 cm-x and a low frequency shoulder at ~2000 cm~. The former corresponds to linearly bonded CO species and the latter to CO adsorbed on the Ru atoms with low coordination numbers. The autoreduced sample produced an additional shoulder at 2098 cm1 which is assigned to the CO species weakly o-bonded to the residual Ru x+ ions in the zeolite or to CO adsorbed on single Ru atoms perturbed by surrounding oxygen atoms. The band width was found to be sensitive to reduction conditions and hence particle sizes. 1. INTRODUCTION Ru-containing NaY zeolite is an active and temporally stable catalyst for the synthesis of ammonia and for CO hydrogenation [1-3]. Samples of [Ru(NH3)6]NaY were prepared by ion exchange of NaY with an aqueous solution of [Ru(NH3)6]C13. In our efforts to study the structure-activity relationship of Ru clusters in catalysis we apply low temperature adsorption of CO as a molecular probe monitored by IR and complement the data by techniques such as TP1L TEM and XRD. The effect of Ru particle size on the spectrum of CO adsorbed at RT on alumina-supported Ru has recently been discussed by Dana Betta [4]. For particles 90 A in diameter only a single band at 2028 cm -1 was observed, while for particles of 60 A in diameter and smaller ones, three bands in the vicinity of 2140, 2080 and 2040 cm1 were noted. The exact positions of the three bands changed with decreasing particle size. The low-frequency band was assigned to CO adsorbed on low-index planes of Ru, while the two higher-frequency bands were associated with CO adsorbed on low-coordinated edge and comer metal atoms. These band assi~ments have been questioned, however, by Brown and Gonzalez [5]. Based upon their own studies performed with silica-supported Ru, they noted that CO adsorption on a reduced sample produced a strong band at 2030 cm1 and weak bands at 2150 and 2080 cm1, whereas CO adsorbed on an oxidized sample produced a strong band at 2080 cm1 and bands of medium intensity at 2135 and 2030 cm1. The low-frequency band was assigned to CO adsorbed as Ru-CO. The high- and medium-frequency bands were assigned to CO adsorbed on a surface oxide and CO adsorbed on a Ru atom perturbed by a surrounding oxygen atom, respectively. Miessner [6], in a study of CO adsorption on RuNaY at temperatures at or above room temperature, observed multiple CO adsorption on Ru metal. Gelin and Yates, Jr. [7] have shown that during adsorption below 300 K, a linear CO species may extensively form and
584 that this adsorption process is accompanied by a stoichiometric conversion of preexisting bridged CO species to linear CO species. In this work, the influence of preparation and activation conditions on size, location and micromorphology of the Ru clusters is investigated. The aim of this paper is to selectively influence the linearly bonded CO on Ru metal and then to compare our IR spectra obtained from differently reduced RuNaY with Ru/SiO: and single crystals from the literatur [8, 9]. 2. EXPERIMENTAL NaY was obtained from DEGUSSA, KM-390. [Ru(NH3)6]NaY samples were prepared by ion exchange of NaY with an aqueous solution of [Ru(NH3)6]C13 (HERAEUS) at 298 K (charge 1) and 333 K (charge 2) [3] for lh. Alter the exchange reaction, the sodium and ruthenium contents were determined by AAS and UV-VIS. Some characteristics of the samples are shown in Table 1. Table 1 Characteristics of samples used Sample Degree of Exchange AAS [%] UV-VIS [%1 theoretical [%] NaY charge 1 49 50 51 charge 2 47 50 51 * refined cell parameter of Y-zeolites (Fd3m)by XRD
colour
a* [A]
white pink violet
24.651 (__ 0.003) 24.634(_+0.002) 24.672 (_+ 0.002)
The samples used for the IR measurements were pressed into self-supporting wafers (57mg/cm2) and transferred to a low-temperature IR cell connected to a contamination-free highvacuum apparatus. The low-temperature IR cell was described by Karge et al. [ 10]. The samples were reduced with the same heating rate at 5 K/rain as shown in chart 1. The IR spectra were recorded using a Perkin Elmer 580B spectrometer. The spectral resolution was 2.3 c m "1. CO ( purity 99.997 %) and 02 ( purity 99.998 %) were purchased from Messer Griesheim, Germany and used without further purification. Hydrogen (99.999 %, Linde, Germany) was purified using an Oxisorb and a Hydrosorb trap from Messer Griesheim~ Chart 1 Summary of reduction conditions Autoreduction Ru(NHa)6NaY Dehydrated, deammoniated (723 K or 823 K, 12h) A utoreduced and CO-reduced Dehydrated, deammoniated (673 K, 12 h) CO adsorption (298 K, 5-10 mbar, 1 h) Desorption of CO (823 K, 12 h)
A utoreduced and H2_-reduced Dehydrated, deammoniated (673 K, 12 h) Flow (50 cm3/min) of H2 (673 K, 3 h) Desorption of H20 (673 K, 12 h) A utoreduced and oxidized Dehydrated, deammoniated (673 K, 12 h) Flow (50 cm3/min) of HE (673 K, 3 h) O2 adsorption (673 K or 773 K, 1000 mbar, 3 h) Evacuation of 02 (673 K, 12 h) Re-reduction with HE Desorption of H20 (673 K, 12 h)
585 The TPR (IMR-MS 100) and high-temperature in-situ IR experiments provided insight into the autoreduction process. The high-temperature in-situ IR spectra were recorded using a Perkin Elmer 2000 FTIR spectrometer (4 cm"1 resolution, average of 20 scans). The transmission electron micrographs were obtained on a Philips EM 400 T microscope ( beam energy: 100 kV). TEM images enabled us to estimate the Ru particle size and to locate the Ru particles. The crystallinity of the samples was characterized using XRD carded out in the transmission mode on a Stoe Stadi P using monochromatized Cu radation. The mean diameter of the sintered Ru particles was estimated by XRD from line broadening. The catalytic activity was checked by NH3 synthesis [3]. 3.
RESULTS AND DISCUSSION
3.1 Charactedsation of two differently prepared R.u(NHa)6 NaY zeolites.
Figure 1 shows the TPR spectra of references [(NHa)sRum-O-RuW(NHa)4-O-Rum(NHa)~]CI6 and [Ru(NH3)6]C13 and of NaY exchanged with Ru amine complexes. The N H 3 profiles of charges 1 and 2 are very broad. Contrary to RuNaY, the TPR profiles of the reference compounds exhibit narrower peaks with maxima at 563 and 543 K. The maxima of the charges 1 and 2 occurred at 593 and 613 K, respectively. The differing maxima indicate Ru complexes with various binding energies. The profile of both charges exhibits a plateau in the 433 - 463 K range. The N H 3 evolution of charges 1 and 2 was detected up to about 720 and 790 K. Figures 2 and 3 show the infrared (IR) spectra in the OH, NH, NO and H20 region of charges 1 and 2 during the autoreduction process. The spectra of Ru(NI-I3)6NaY degassed at 723K showed bands at 3550 cm] and 3641 cm1 (Brensted-acid sites) and a small terminal SiOH band at 3742 cml( insets from figures 2 and 3 ). For charges 1 and 2, three intense bands are visible: (a) -1643 cm"1 (8(asyn~) of coordinated NH3, 8(synl) mode of zeolitic H20), (b) -1451 cm1 (carbonates from the NaY, 8(NH) of the NH4+ ions) and (c) -1327 cm1 105 ~
(~ /~
/
104 "
,,:. /'" aLaJ ,
;'
::i
,'# ," I I I
Im/e = 17
',1 °'1 ',l ® .....~"1"'.
3740
i ,.i
/A,l_o.
y,l
~, 1.2
:1,
~, ,I Iii
,,
3641
1.6 1.4
' ....."~,~,4)
: .......... r
102
"644
"
/-,~
1
".
"~.. '"-
14sl I//F~I
< 0.6
!
1327
~
.
0.2
¢°f 0.2
101 ,',', ,',,,i 400
i ( ,3
~ 0.8
0.4 !''
I
I
I
I
500 600 700 Temperature [K]
Figure 1. NI-Ia-TPRprofiles of the autoreduction of (a) Ru red, (b) [Ru(NH3)6]CI3, (c) charge 1 and (d) charge 2.
2000
1800
1600
Wavenumber
1400
[cm"1]
Figure 2. In-situ IR spectra of charge 1 deammoniated at temperatures from 298 to 973 K in steps of 50 K (top to bottom) in high vacuum.
0
2000
1800 1600 1400 Wavenumber [cm "1]
Figure 3. In-situ IR spectra of charge 2 deammoniated at temperatures from 298 to 973 K in high vacuum.
586 (~(sy~) mode of coordinated NH3 in the [Ru(NH3)6] 3+ complex) [ 11, 12]. The intensity of the band at -1451 cm1 decreased sharply upon heating in the 423 - 473 K range indicating that it is mainly due to NH species. The change in the IR spectrum of the N-H vibrations is typical of the reduction of Ru m. For both charges, one band was visible at -1869 cm1 which decreased as a new band appeared at -1930 cm1. These infared bands indicated two forms of Ru-nitrosyl complexes. For charge 1 changes in this part of the IR spectrum started above 523 K and for charge 2 above 573 K. Heating above these temperatures attenuated the nitrosyl bands while a new band appeared at about 2030 cm"1 due to a very stable [Ru(Ozeol)3(Nn3)y(NO)]complex formed at site II [ 11]. This complex was completely decomposed at 773 K for charge 1 and at 823 K for charge 2. The results from the high temperature IR investigations are in excellent agreement with the results from our TPR experiments. The results indicate that the intercalation complex ion [Ru(NH3)6]C13 exhibited a rich redox and ligand exchange chemistry. Both techniques show that charge 1 is obviously easier to reduce. Ion exchange at 298 K and 333 K leads to a significant change in the cell parameter compared to NaY (Table 1). The TEM images of charges 1 and 2 after autoreduction revealed the presence of Ru particles in the supercages ranging from 1 - 1.5 nm particle size. Furthermore, charge 1 contains some 3 - 5 nm large Ru particles. The Ru particles seem to agglomerate on the external surface. Therefore, it appears that for cell parameters greater than for NaY, the Ru complexes are located inside the zeolite. For smaller cell parameters the Ru complexes are most probably outside the crystallites. Catalytic testing by NHa-synthesis of both charges showed only insignificant differences between the two, so that in the following we present only the results obtained from charge 2. 3.2 Dimension and location of the metal particles (charge 2) The mean diameter of the reduced Ru particles (Chart 1) in charge 2 were evaluated by XRD, TEM and CO chemisorption methods (Table 2). The CO chemisorption method is described in section 3.3.3. The typical TEM micrographs of autoreduced, HE reduced, CO reduced and oxidized samples are shown in figure 4. Autoreduction of charge 2 produced particles ranging from 0.7- 1.5 nm. Reduction under an H2 flow produced finely dispersed Ru particles (1 - 1.5 nm). CO reduction resulted in Ru species aggregated to apparent TEM sizes of 2 - 4 nm which is larger than one supercage. After oxidation, 5.5 - 15 nm and several --35 nm large Ru particles were found. Smaller particles migrated to the exterior surface of the zeolite, where they aggregated. Detailed inspection of the TEM image of sintered particles provided evidence that elongated fibres, circular particles and hexagonal platelets were favoured. Table 2 Ru particle dimensions obtained b~¢three different techniques Teetmique Mean diameter dp(Ru) [nm] autoreduction HE reduction XRD TEM CO chemisorption CO/Ru = 1 CO/Ru = 0.75
CO reduction
oxidation
2-4
36(loo), 23(002), 32(lOl) 5.5- 35
0.7-1.5
1- 1.5
-
1.2 + 0.3
1.6 ± 0.3
10.8 + 0.3
-
1.6 ± 0.3
2.1 ± 0.3
14.4 ± 0.3
587
Figure 4. TEM micrographs of (A) autoreduced, (B) H2-reduced, (C) CO-reduced and (D) sintered (oxidized) samples. cell parameters [ Figure 5 displays XRD patterns of difcell parameters a (fa) [A] I samples samples a (fg) [A] 1.5"104 NaY (Fd3m) 24,65-1(-~,003) 1073 K, UHV 24.717 (0.002)] ferently reduced Ru(NH3)6NaHY and NaY Ru[NH3)sNaY 24,672 (0,002) 673 K, H2 24,667(0,001)1 samples. Their comparison shows that the 723 K, UHV 24,651 (0,075) 823 K, CO 24,615 (0,004)1 823 K, UHV 24,656 (0,027) 673 K, 02 24,636(0,006)l background intensity increases with ther973 K, UHV 24,731 (0.001) 773 K, 02 24,622(0,OO6)I 1.2.104 Rull01) mal treatment. We note a loss in crystallinity of a fraction of the zeolite material • Ru(O02)][ caused by local damage of the zeolite ma- "g A~ Ru(lO0) I,I trix [ 13]. Ion-exchanged charges exhibited ._&,. 9000 a characteristic change in the intensity distribution of the reflections. The inten- -= 6000 sity ratio of the reflections 2® = 10° and 2 0 = 11 ° was completely reversed como , A AI,'i ~ ' - ~ - ¢ ~ - ~ - ~ pared to NaY. The series of patterns 3000 shows that at the end of the sequence of autoreduction at lower temperatures, autoreduction at higher temperatures, H2 0 10 20 30 40 50 2 Theta reduction, CO reduction and oxidation, this intensity ratio approximated the value of NaY. It appears that this ratio is pre- Figure 5. XRD transmission patters of zeolite NaY, dominatly influenced by Ru complex ions Ru(NH3)6NaYand differently reduced RuNaHY samples. and Ru particle sizes. All reduced samples except the CO reduced and sintered sample show cell parameters greater than observed with NaY. Comparison of X-ray diffraction and TEM results suggests a location of Ru particles (< 2 nm) inside the zeolite. Larger particles are located on the outer zeolite surface. The di~action pattern of the sintered sample additionally exhibits Ru metal reflections. For Ru(100), the average diameter was consistently larger than for Ru(002) and Ru(101) faces (Table 2).
3.3 IR spectra of adsorbed CO 3.3.1 CO adsorption experiments at 298 - 75 K Figure 6 shows the spectra obtained after adsorption of CO (0.13 mbar) on autoreduced samples of charge 2 in the 298 - 75 K range. Spectra at 298 K showed bands due to Ru(CO)3 (2154, 2098 and 2086 cml), Ru(CO)2 (2090 and 2054 cm1) and linear (2017 cm1) forms of
588
CO adsorbed on ruthenium The intensity of the Pxu(CO)3 bands decreased as the adsorption temperature is lowered from 298 K to 185 K. The most dominant band in the 185, 175 and 135 K spectra showed a maximum around 2060 cm ~ with a shoulder at 2093 cm 1 due to Ru(CO)2. Further cooling diminishes the intensity of these two bands. Bridged CO (~1855 cm -1) was formed during cooling down to 175 K. Furthermore, a single band due to linearly bonded CO (Ru-CO) at 2017 cm 1 with a high frequency shoulder due to CO adsorbed on Ru x+ ions [8] around 2093 cm ~ and a shoulder at ~2000 cm ~ was observed above 110 K. On cooling, these bands undergo a continuous shift to higher frequencies. Their intensities decrease sharply below 110 K. Bands observed in this study between 2200 and 2100 cm -1 following low-temperature CO adsorption are assigned to CO molecules adsorbed on true Lewis acid sites (2174 cm 1) [14] and physisorbed CO to form sioI-rs+.-.co (~2160 cm 1) [15]. The former species are known to be formed during dehydration [ 14]. 3.3.2 CO adsorption on differently reduced samples at 110 K In figure 7, IR spectra of differently reduced samples are compared; the spectra were normalized and corrected for dispersion. The inset presents the IR spectra of adsorbed CO at increasing coverage on an autoreduced sample (823 K). The spectrum of CO on this autoreduced sample (823 K) consists of a broad band (Av~-- 94 cm 1) with a component at about
J
.2174 2160
l
l 75 K
2086
2035
~
O
..Q ° ,<
'-" 90 K
5 3 t'~
0 0/j
\
0.8
"~ 0.6~
2
I
T=110K
,
1855
@
2°98~/
~~
2021
~ ® /
110K
2019 2064 2054
___.._.~093
2045
017
2000 2020
oc -
2050
- 2200 1 8' 2000 ' '
'00
/@12000
135 K 175 K
185 K 2154
<
1865
0.2 -
I
o L-_,/__ 2200
298 K 0 2000
1800
Wavenumber [cm1] Figure 6. Ill spectra of CO adsorption (0.13mbar) on autoreduced samples of charge 2 in the 298 - 75 K range.
i 2400
i
i
2200
2000
1800
W a v e n u m b e r [cm 1] Figure 7. IR spectra of CO chemisorbed on differently reduced RuNaHY (charge 2) samples, i. e. autoreducued at (a) 723 K, (b) 823 K, (c) reduced with CO, 823 K, (d) reduced with H2, 673 K and (e) sintered in 02, 673 K.
589 2050 cm 1 due to linearly bonded CO, a shoulder at 2098 c m "1 due to CO adsorbed on the tinreduced Ru complexes or on single Ru atoms perturbed by surrounding oxygen atoms, and a shoulder around 2000 cm "1 which can be attributed to CO bonded to preferred adsorption sites, e. g. low coordinated Ru atoms located on steps and comers [8] and defects. Bond et al. [16] showed that the presence of a low-frequency component at 1990 cm 1 is a fingerprint of isolated Ru-CO species formed on very small and less perfect Ru crystals supported on TiO2. When the coverage increased, the central peak progressively grew while the low frequency band remained as a small shoulder. Under equal conditions, the IR spectra of CO adsorbed on RuNaY (charge 1)were similar. The spectra of CO adsorbed on CO-reduced and H2-reduced samples exhibited an infrared band at around 2021 cm 1 and at around 2045 cm 1 (Av~ = 86 cm 1 and 81 cml), respectively. These bands were asymmetric with a tail on the low frequency side. CO adsorption on the sintered sample at 673 K displays a single narrow band with Av~= 62 cm 1. This half width, which is close to the value o f - 1 5 cm 1 found on Ru(001) faces [9], suggests that well-crystallized particles with low index faces predominate. On the well-crystallized Ru platelets of average (100) orientation generated by high temperature oxidation at 773 K, no CO adsorption was detectable at 110 I~ Increased band widths indicate a rough surface o f th e Ru particles. Hence, the main band at about 2040 cm 1 is characteristic of CO molecules linearly bonded to edges, steps and comers of low index planes of Ru. For larger Ru particles located on the external surface we find, in combination with XRD and TPD results, reduced roughness and an average (100) orientation. The spectral features obtained in this work are similar to those obtained from Ru(001)/SiO2 [8] and compare well to observations of Ru single crystals, e. g. Ru(001), Ru(100) [9, 17]. 3.3.3 Isotherms at 110 K and Ru particle size estimation As shown in figure 8, the chemisorption isotherms of CO on reduced samples were determined (except for the autoreduced sample) from their IR spectra according to Lambert-Beer's law, in combination with pressure measurements in a high-vacuum tight system of known volume. The isotherm points correspond to the increasing band intensity around 2050 cm 1 as shown in Figure 7. It seems that beyond an equih'brium pressure o f ~ l x l 0 2 mbar (curves a, b) and 3 x 1 0 "3 mbar (curve c) the 110 K isotherms exhibit a plateau which indicates formation of a monolayer of CO on the Ru surface. The metallic dispersion of Ru, D, defined as the ratio of the total number of surface atoms (Ns) to the total number of the metal atoms present (NT), can be estimated from the total amount of chemisorbed CO on the Ru surface, assuming a stoichiometric CO chemisorption of one monolayer coverage. Based on meas0.012 IT = 1 lO K[ a: H2 reduction urements of hydrogen bonded to the Ru ~ 0.ol d(Ru)co 1.2nm; surface of a 5 wt% RuNaY sample (H/Rus E ° = 0.008 = 1) and CO chemisorbed on a Pd surface at ~ b: COreduction a ratio R = CO/Pds = 0.75 [18], we as- E 0.006 dIRu)co=1.6nm; sumed the same stoichiometry for CO on ~ 0.004
our
Ru-containing
samples.
Assuming oo 0.002
spherical particles, the average diameter of ~" the Ru particles, d, was calculated from d=6
mR" nC0adsorption X R c o / R u X N A x
A xp
,
oxidation/re-reduction
d(Ru)co= 10nm; •
o
0
0.1
0.2
~ . . a _ _0.4 _ _ _ _ ~ OJ 0.3
p(CO) [mbaO
Figure 8. CO adsorption isotherms and Ru particles size estimation of RuNaHY.
590 where m is the mass of Ru in the sample, NA is the Avogadro-constant, A is the average Ru area [4] and p is the density of Ru. These particle sizes are in good agreement with the particle sizes evaluated by TEM and XRD (Table 2). 4. C O N C L U S I O N S Four types of Ru particles with different sizes can be discriminated in the differently activated samples. For Ru particles smaller than or comparable to the diameter of a supercage we generally observe a larger cell parameter than for NaY. For larger Ru particles the cell parameter is greater. We conclude that all particles smaller than 1.3 nm are located inside the Yzeolite, up to around 2 nm they are also accommodated in the interior of the zeolite crystallites, particles of 2- 4 nm size are most probable outside, and particles larger than 4 nm are definitely located on the external surface of the zeolite crystallites. CO adsorption at 110 K shows that the widths of the band around 2040 cm 1 are sensitive to the reduction conditions and hence to particle sizes. In the sequence of low-temperture autoreduction, high-temperature autoreduction, CO-reduction, H2-reduction and oxidation, the band width decreased. A lack of the high frequency shoulder at about 2098 cm 1 indicates a completely reduced sample and the absence of single Ru atoms or very small Ru particles. Further, the generally broader band widths compared to that of Ru(001) indicate a rough surface of Ru particles. For larger particles located at the external surface it follows, also from XRD results, a reduced roughness and and an average (100) orientation. In general the features of the spectra obtained in this work are similar to those obtained from Ru(001)/SiO2 [8] and compare well to observations on Ru single crystals [9]. CO adsorption isotherms obtained at low temperature are suitable to determine the average size of the Ru particles. Above room temperature, CO adsorbs not only on metallic Ru but also on the non-reduced ruthenium species and on Ru atoms perturbed by adjacent oxygen atoms to form Ru carbonyls which are responsible for the migration of ruthenium to the external surface in agreement with TEM and XRD. For this reason determining the catalytically active surface area by CO adsorption is reliable only for temperatures below 298 I~ REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [ 12] [13] [14] [15] [16] [17] [18]
J.J. Verdonck, R.A. Schoonheydt and P.A. Jacobs, J. Phys. Chem. 85, 2393 (1981). J. Wellenbiischer, U. Sauedand, W. Mahdi, G. Ertl and R. Schl6gl, Surf. Interface Anal. 18, 650 (1992). U. Guntow, F. Rosowski, M. Muhler, G. Ertl and R. Schl6gl, Proc. Int. Conf. Scientific Basis of Catalyst Prep. Louvain-la Neuve, 1994 ed. P. A. Jacobs, Elsevier. R.A. Dalla Betta, J. Phys. Chem. 79, 2519 (1975). F.M. Brown, and R. P. Gonzalez, Phys. Chem. 80, 1731 (1976). H. Landmesser and H. Miessner, J. Phys. Chem. 95, 10544 (1991). P. Gelin, and J. T. Yates, Jr., Surf. Sci. 136, L 1-L8 (1984). E. Guglielminotti, G. Spoto and A. Zecchina, Surf. Science 161, 202 (1985). H. Pfniir, D. Menzel, F.M. Hoffanann, A. Ortega and A.M. Bradshaw, Surf. Science 93, 431 (1980). H.G. Karge, W. Krauss and S. Wrabetz, Review of Scientific Instruments, submitted. J.R. Pearce, B.L. GustafsonandJ. H. Lunsford,Inorg. Chem. 20,2957 (1981). J.J. Verdonck, R.A. Schoonheydt and P.A. Jacobs, J. Phys. Chem. 87, 683 (1983). J.J. Verdonck, P. Jacobs, M. Genet and G. J. Poncelet, J. Chem. Soc. Faraday Trans. 1 76, 403 (1980). J. Howard and J. M. Nicol, Zeolites 8, 142 (1988). T.P. Beebe, P. Gelin and J.T. Yates, Jr, Surf. Sci. 148, 526 (1984). E. Guglielminotti and J.C. Bond, J. Chem. Soc. Faraday Trans. 86(6), 979 (1990). G. Lauth, T. Solomun, W. Hirschwald and K. Christmann, Surf. Sci. 210, 201 (1989). A. Palazov, C.C. Chang, andR.J. Kokes, J. Catal. 36, 338 (1975)
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
591
N e w Insight Into The Mechanism of Zeolite Catalyzed Nucleophilic Amination Via In Situ Infrared Spectroscopy
Christian GrOndling, Victor A. Veefkind, Gabriele Eder-Mirth, and Johannes A. Lercher University of Twente, Department of Chemical Technology, P.O. Box 217, 7500 AE Enschede, The Netherlands
SUMMARY Amination of alcohols over Bronsted acidic molecular sieves such as H-mordenites proceeds via a bimolecular precursor consisting of an ammonium ion and an alcohol molecule. The rate of
transformation of that complex into a sorbed alkyl ammonium ion is fast. Desorption of the formed amines, however, is difficult under reaction conditions (633K) and only proceeds in the presence of excess ammonia. Two mechanisms are proposed for the release of the alkylamines in the gasphase, i.e., adsorption assisted desorption and alkyl scavenging by ammonia. The vast majority of the alkylamines are formed by scavenging alkyl groups of chemisorbed alkyl ammonium ions by ammonia. A linear relation of the initial rate of monomethylamine formation and the total surface concentration of methyl groups (present as mono- up to tetramethylammonium ions) inside the pores of the mordenites was found. INTRODUCTION Solid acid catalyzed direct amination using ammonia emerges as an industrially very important route to produce alkyl and alkyldiamines [ 1,2].While it is straightforward to produce amines with the help of solid acid catalysts, selectivity to a particular product is more difficult to achieve. A well-known example concerns the shape selective formation ofmethylamines, such as mono- (MMA) and dimethylamine (DMA) over mordenite based catalysts. Two strategies are reported in literature to achieve high selectivities towards these products: (i) modification of the outer surface of Br~nsted acidic mordenites by silylating agents to narrow the pore openings [3,4] and (ii) the use of alkali exchanged mordenites [5,6]. Previous sorption experiments led us to propose that the mechanism of methanol amination is dramatically different over Bronsted acidic and alkali exchanged zeolites [7]. A detailed study of the reaction mechanism over Bronsted acidic mordenites has been reported recently [8]. The present contribution adresses open questions with respect to the mechanism of shape selective and non shape selective catalyzed amination over mordenite catalysts. Experimentally, this is realized by the parallel use of in situ i.r. spectroscopy and kinetic measurements.
592 EXPERIMENTAL Brensted acidic mordenites with a SiOJA1203 ratio of 20 (HMOR20), 15 (HMOR15), and 10 (HMOR10) were obtained from the Japanese Catalysis Society [9]. HMOR10-E was obtained by treating a sodium exchanged MOR10 with EDTA solution, followed by calcination and ion exchange with NH4NO3. HMOR20-M was prepared by adding tetraethoxysilane to a suspension of the activated HMOR20 in n-hexane, followed by intensive stirring at room temperature, removal of the solvent and subsequent calcination in air. The BET surface area and the micropore volume of all catalysts were measured on a Micromeritics Accelerated Surface Area and Porosimeter (ASAP 2400). The concentration of extra framework aluminum (EFAL) was determined by 27A1MAS NMR. The concentration of strong Bronsted acid sites was calculated from the amount of irreversibly adsorbed ammonia at 3 73 K. These physico-chemical properties of the samples are summarized in Table 1.
Table 1: Physico-chemical properties of the investigated Bronsted acidic mordenites Catalyst
SpecificArea
Si/A1
(m2/g)
Micropor.Vol. (cm3/g)
EFAL (%)
Bronsted acid sites (mol/g)
HMOR20-M
353
0.15
10
10
1.1.10"3
HMOR20
390
0.21
10
n.d.
1.3.10.3
HMOR15
350
0.15
7.5
11
1.7.10.3
HMOR10-E
280
0.10
5
n.d.
2.0.103
HMOR10 n.d. = not detected
130
0.05
5
8
2.1.10.3
All i.r. spectra were recorded in situ using a Bruker IFS88 FTIR spectrometer in the transmission absorption mode with a resolution of 4 crn"~. The i.r. spectra were baseline corrected in the range between 3800 and 1300 cm"a. For graphical representation, the differences of the i.r. spectra of the sample in contact with the adsorbate (reactant) and of the activated zeolite are used. For the experiments under reaction conditions, an i.r. cell was used which approximates a continuously stirred tank reactor with a reactor volume of 1.5 cm3. The effluent gas stream was stored in 16 loops of a multi port valve and subsequently analyzed by means of gas chromatography. This experimental setup allows simultaneous analysis of the products inside the zeolite pores and in the gas phase (a more detailed description is given in ref. [10]). Typically, the catalyst was pressed into a self-supported wafer and activated at 823 K in flowing helium for 1 hour and then cooled to reaction temperature (633 K). The reaction was carried out at 5.103 Pa ammonia and 5.103 Pa methanol balanced with helium to 105 Pa. In order to obtain quantitative information on the concentration of the surface species under reactive conditions, a multi component fit (in the spectral range between 1700 and 13 50 cm"t) was applied to the i.r. spectra recorded during the reaction. The reference spectra of the reactants and products were obtained upon sorption of the individual components under non reactive conditions (at 633 K and 1 Pa). All bands observed under these conditions are indicative for molecules bound to Br~nsted acid sites (1:1 stoichiometry).
593
R E S U L T S AND DISCUSSION
Formation of Methylamines At 633 K and low methanol conversion, ammonia was present in large excess over the other N-containing compounds (> 95%) and MMA was the favored reaction product (> 85% amine selectivity) over all catalysts. Quantitaitve analysis of the i.r. spectra of the working catalysts, however, revealed a quite different distribution for the adsorbed phase (see Table 2).
Table 2: Concentration of the species sorbed on mordenites and the rate of MMA formation under differential reaction conditions (p(MeOH) = 5.103 Pa, p(NH~) = 5.10 JPa, T = 633 K) Catalyst
Conv.
Conc. chemisorbed species (10 -4mol/g)
Rate MMA
TOF MMA
(%)
NH3
MMA
DMA
TMA
TET
(mol/g.s)
(molec./s.H+)
HMOR20-M
6
1.1
3.8
0.8
2.0
3.1
5.2"10.6
4.7"10.3
HMOR20
5
2.5
6.4
2.7
0.2
1.2
2.4"10.6
1.9"10.3
HMOR15
4
1.7
7.1
0.2
3.1
4.5
1.1"10.5
6.7"10.3
HMOR10-E
8
4.4
6.4
2
2.8
4.4
1.3"10.5
6.4"10.3
HMOR10
9
4.1
10
2.3
1.6
2.8
6.8"10.6
3.2"10.3
This clearly indicates that the relative surface concentrations of reactants and products in the mordenite pores is not determined by an adsorption-desorption equilibrium of the substances in the reactor.
I 0.2
0.0 1.0 2.0
3.0 4.o
o O
TI~
!
T
3500
3000
2500
1650
1500
1350
Wavenumber (era"l)
Figure 1: I.r. spectra of HMOR20 under reactive conditions (5.103 Pa methanol and 5.103 Pa ammonia at 633 K) with increasing time on stream [13]
594 First, the formation of surface species under non reactive conditions was probed by coadsorption of ammonia and methanol at ambient temperature. A coadsorption complex formed by an ammonium ion (ammonia protonated at the Bronsted acid sites of the catalyst) and a methanol molecule was found to exist. The interaction in such a complex, however, was rather weak. Since all nitrogen orbitals are engaged in chemical bonds, the alkylation of the ammonium ion is speculated to involve the protonation of the alcohol by the ammonium ion, followed by a rapid release of water and the formation of a C-N-bond (see Fig.2 [13]). To probe the role of intermediates, a series of transient experiments was performed and will in the following be described in detail for HMOR20. In a typical experiment (see Fig. 1), the activated mordenite was saturated with ammonia at 633K. All hydroxyl groups of the zeolite were interacting with ammonia by forming quantitatively ammonium ions. When methanol was added to the reactant gas stream, protonated methylamines were rapidly formed and, with time on stream, replaced the ammonium ions. The amines increased in concentration sequentially from MMA, via DMA and TMA up to TET. Sorbed methanol was not observed. This suggests that not the formation of sorbed amines, but rather their displacement from the acid sites is rate determining. To test this hypothesis, a methanol containing stream of He was passed over the ammonium form of H-mordenite at 633 K. Upon contact, all methylamines were formed rapidly in the zeolite pores, again in sequential order. At steady state (which was quickly reached), the methylammonium ions MMA, DMA, TMA and TET were present in approximately equal surface concentrations. However, none of the amines formed under such conditions (T=633K) was able to desorb from the zeolite pores and dimethylether was the only product observed in the gas phase. This is in line with the higher base strength of the substituted amines compared to ammonia [ 11,12]. Temperature programmed desorption experiments ofsorbed methylamines suggest that even at higher temperatures these amines rather decompose than desorb from the strong Bronsted acid sites. However, when ammonia was passed over HMOR20 loaded with alkylammonium ions at 633K, the amines were able to desorb from the active sites. It should be noted that their rate H
H
/ H ..........0 ~ NI j "
CH3
CH3
Nil bond cleavage N
H
0\
/
O® \
/ AI / \
Si / \
0
/ ~\ HH
O0
/k\
Proton transfer
H j.
O\ /
O0
/ Si
0
0\
/
O® \
/ A1 / \
Si / \
\
0
0
%1 /~\
o\
/
0
Al / \
O0
/
O0
/
0
si \ o
/
H H
o® \ /
Si / \
O0
.
O\
~H,O
CH,
H
H H
o\ /
O0
/
Si
o
\
0
Figure 2: Proposed reaction mechanismfor the formation of sorbed methylammonium ions
595 ofdesorption was much lower than their rate of formation (as measured in the above described transient experiment). The individual rates of displacement of the amines by ammonia do not correspond to their gas phase basicities of the amines, i.e., TMA and TET disappear completely before the surface concentrations of MMA and DMA (which attain an almost constant level at moderate time on stream) decrease [13]. However, in the gas phase mainly MMA and DMA were observed as reaction product. Whereas the amine selectivity to DMA was initially high, MMA was the favored product as the concentration ofmethylammonium ions inside the zeolite pores decreased. The formation of TMA in the gas phase was low under these reaction conditions. It is interesting to note that the initial rate of MMA formation in the gas phase in such an experiment was approximately equal to the rate of MMA formation under normal methylamine synthesis conditions (when both reactants, ammonia and methanol, are present).This gives a strong indication that the removal of the chemisorbed methylammonium ions from the active sites is the rate determining step for the overall amine synthesis reaction. (a) Adsorption assisted desorption
n
CH3
H'cx
..I H
o 0
\
Si f \
f
o® \
J AI 7 \
O0
o\ f
Si
O0
J \
o
o\
0
0
f
J\
Si
iiII
Oo \
J
oN
AI
7 \ f O0 O0
J
Si
\
o 0
(b) Methyl scavenging mechanism H H
H
(i) methylscavenging
tt/
aN ~Ix
H~C
o 0
\
Si J \
7
cHCH~
o®
00
\
f
IP
Al
f \
f
O0
/ix
(ii) proton transfer
o\
Si
f \
o
n, Ce
n 3 c cHjCH3
o® 0% / o AI Si f\ f \ f \ o oo oo o / % /
Si
0
Figure 3: Proposed reaction mechanism for the removal of methylamines from Bronsted acid zeolites via adsorption assisted desorption (a) and methyl scavenging (b) Two mechanisms might be proposed to explain these observations: (i) ammonia helps the amines to desorb (adsorption assisteddesorption) (see Fig. 3a) or (ii) ammonia scavenges methyl groups from surface bound amines by leaving a lower substituted amine behind (methyl scavenging mechanism) (Fig. 3b).Whereas both pathways seem feasible to explain the decrease of surface bound MMA and DMA, for the highly substituted amines (TMA, TET), the scavenging mechanism seems to prevail. The high importance of the methyl scavenging mechanism is underlined by the linear correlation of the total number of methyl groups present inside the zeolite pores (in form of the methylammonium ions sorbed on the Bronsted acid sites) and the rate of MMA formation under differential reaction conditions over the investigated mordenites (Fig. 4).
596 Table 3: Amine selectivities over investigated mordenite catalysts (p(MeOH) = 5.10 ~ Pa, p ( N H ) = 5.10 ~ Pa, T = 633 K)
Catalyst
W H S V(11"l)
2.8
HMOR20-M
Conv.(%)
35
Amine selectivity (mol%) Total
MMA
DMA
TMA
96
67
30
3
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2.3
HMOR20
35
76
55
20
25
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4.0
34
87
67
18
14
6.5
33
79
71
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HMORIO
34
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i',',!',i',i', Selectivity in methylamine synthesis In order to relate the physico-chemical properties of the mordenites to the selectivity in methylamine synthesis, the amine selectivities are compared at two methanol conversions (i.e., at 35% and 90%). The data are summarized in Table 3. Note that the catalysts, which showed the highest rates for the formation of MMA at low conversions (i.e., HMOR10-E and HMOR15), also exhibited the highest activity at intermediate and high conversion (compare space velocities required for the same conversion over the different samples). This indicates that measuring the rate of MMA formation under differential conditions is a valid tool to compare catalytic activity in methylamine synthesis. Upon variation of the conversion, similar trends were observed for all catalysts. With increasing conversion, the selectivity towards MMA decreased, whereas the selectivity towards TMA markedly increased. For HMOR20-M, however, the formation of TMA in the gas phase (but not in the zeolite pores) was almost completely suppressed. In accordance with the proposed reaction mechanism, this increase in the selectivity to the higher substituted amines is not attributed to the sequential formation of chemisorbed amines (which did not change markedly in their relative concentration as the conversion increased) but to an increasing contribution of methylamines as scavenging agents. Despite these general similarities, the selectivity towards the various methylamines strongly depended on the catalyst composition and pore volume. To give an example, a high selectivity towards TMA (55%) was observed at 90% conversion over HMOR20 (which is close to the thermodynamic equilibrium distribution) whereas over HMOR10 the selectivity towards TMA was only 24% at the same conversion. In general, the selectivity towards MMA increased and the selectivity towards TMA decreased in the order HMOR20 < HMOR15 < HMOR10-E < HMOR10. This trend can be correlated to the increase in concentration of Bronsted acid sites and the decrease in the micropore volume. If one considers that all acid sites are covered under reaction conditions, both effects lead to a restriction in void space inside the zeolite pores. Along the same line, Abrams
597
15 HMOR10-E X o
HMOR15
'9
10O ow,,¢
10
~o .¢ _
o
HMOR20 ~,
G,)
t~
1
!
!
2
3
Fig.4. Concentration of methyl groups ofmethylammonium ions (10 -4 mol/g)
et al. [ 14] showed over a series of RHO type zeolites that the yield in TMA decreased in parallel with the gravimetrically determined amount of TMA inside the zeolite pores. The decrease in the micropore volume apparently causes spatial constraints to form T M A via methyl scavenging by DMA. Alternatively, the constraints could also lead to a restriction of the diffusion of the bulkier methylamines which would favor transalkylation to chemisorbed methylammonium ions. The most prominent example for a selective catalyst due to severe diffusional constraints to the bulkier products is HMOR20-M. Although a high concentration of higher methylated ammonium ions (i.e., TMA and TET; compare Table 2) was found inside the pores of this catalyst, hardly any TMA molecules (amine selectivity < 5 %) could leave the zeolite pores, even at high methanol conversion (90 %). Poisoning of non-selective sites on the outer surface by the modification procedure can be excluded, because the rate of amine formation was higher over the HMOR20-M as compared to the parent sample (see Table 2 and Table 3). Similar results were obtained by Segawa et al. [3] who used a mordenite modified via chemical vapor deposition with SIC14 for methylamine synthesis. From their catalytic data and ditfusivity measurements (the diffusivity of the amines over the modified catalyst decreased in the order MMA > DMA > TMA), they concluded that the observed improvement in the selectivity towards the lower substituted products, MMA and DMA, could only be attributed to a narrowing of the pore openings of the mordenite channels.
CONCLUSIONS All Bronsted acidic mordenites investigated are highly active in methylamine synthesis. The principal mechanism for the formation of amines was the same for all samples studied. Methylammonium ions (including tetramethylammonium ions) are rapidly formed on the Bronsted
598 acidic sites of the zeolites via a bimolecular complex of a chemisorbed (methyl)ammonium ion and hydrogen bonded methanol. However, the release of the amines into the gas phase is the rate determining step. At low methanol conversion, this removal is proposed to occur via (i) ammonia adsorption assisted desorption or (ii) scavenging of a methyl group of chemisorbed amines by gas phase ammonia. The activity in methylamine synthesis (expressed as the initial rate of formation ofmonomethylamine) can be directly correlated to the total concentration of surface methyl groups present in the zeolite pores. This suggests that the methyl scavenging mechanism is the more important route. At high conversions, formed methylamines take over the role of ammonia as scavenging agent. Consequently, the high initial selectivity towards monomethylamine decreases in favor of the secondary (dimethylamine) and tertiary (trimethylamine) products with increasing methanol conversion for most catalysts. Despite this general trend, the amine selectivity differs drastically over the various mordenite samples at high methanol conversion. As the highest selectivity towards the lower substituted amines was observed for the catalyst with the smallest micropore volume (and as it decreased with increasing micropore volume of the different mordenites used), limitations in the rate of methyl scavenging by methylamines and/or transport limitations for the bulkier trimethylamine are concluded to cause the increase in selectivity with increasing aluminum concentration. The very high selectivity towards the lower substituted products obtained over the silylated mordenite sample is concluded to be a direct consequence of the narrowing of the pore openings by silylation. Although all methylamines can be formed inside the pores of thiscatalyst, TMA is retained in the channel system, and undergoes rapid transmethylation to form the lower methylated amines, which are able to diffuse out of the zeolite pores. ACKNOWLEDGEMENTS The authors are indepted to the Christian Doppler Society and NIOK for partial support of this work. REFERENCES
1. M.G. Turcotte and T.A. Johnson, in J.I. Kroschwitz (Ed.), Kirk-Othmer Encyclopedia of Chemical Technology, 4th edition, John Wiley & Sons, New York, 1992, Vol.2, p.369. 2. A.B. van Gysel and W. Musin, in B.Elvers, S. Hawkins, and G. Schultz (Eds.), UUmann's Encyclopedia of Industrial Chemistry, 5th edition, VCH, Weinheim, 1990, VoI.A16, p.535. 3. K. Segawa and H. Tachibana, J.Catal., 131, 482 (1991). 4. T. Kiyoura and K. Terada, Eur.Patent Appl. 593.086, 1994. 5. Y. Ashina, T. Fujita, M. Fukatsu, K. Niwa and J. Yagi, Stud. Surf. Sci. Catal., 28, 779 (1986). 6. F. Weigert, J. Catal., 103, 20 (1987). 7. A. Kogelbauer, Ch. GrOndling, and J.A. Lercher, Stud. Surf. Sci. Catal., 84, 1475 (1994). 8. A. Kogelbauer, Ch. GrOndling, and J.A. Lercher, J. Phys. Chem., accepted for publication. 9. M. Sawa, M. Niwa, and Y. Murakami, Zeolites, 10, 532 (1990). 10. G. Mirth, F. Eder, and J.A. Lercher, J.Appl. Spectrosc., 48, 194 (1994). 11. N. Cardona-Martinez and J.A. Dumesic, J. Catal., 128, 23 (1991) 12. D.J. Parrillo, R.J. Gorte, and W.E. Fameth, J. Am. Chem. Soc., 115, 12441 (1993). 13. Ch. GrOndling, G. Eder-Mirth, and J.A. Lercher, Res. Chem. Intermed, accepted 1995. 14. L. Abrams, D.R. Corbin, and M. Keane, Jr., J. Catal., 126, 610 (1990).
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
599
Coke formation in zeolites studied by a new technique: ultraviolet resonance Raman spectroscopy Can Li ° and Peter C. Stair b
"State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, China bCenter for Catalysis and Surface Science, Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA
An ultraviolet (UV) Raman spectrometer was recently set up with the goal of improving Raman spectroscopy for catalysis and surface science studies. Using UV Raman spectroscopy, coke formation and oxidation in ZSM-5 and USY have been investigated. The coke species were generated by the reaction of propylene in the zeolites at temperatures from 297 to 773 K. The gaman bands of coke species can be clearly resolved since the fluorescence interference is successfully avoided in UV Raman spectra. There are three groups of strong Raman bands observed at 1360-1400, 1580-1640, and 2900-3100 cm"t. Various carbonaceous species like olefinic, polyolefinic, aromatic, polyaromatic and pregraphite species can be discriminated based on the positions and relative intensities of these Raman bands. At lower temperatures, olefinic and aromatic species are dominant for both zeolites, and these species desorb or partly convert into polyaromatic and pregraphite species at high temperatures. The coke species formed at high temperatures are quite different for the two zeolites: polyolefinic and aromatic species are predominant in ZSM-5, and polyaromatic and pregraphite species are the major species in USY. I. INTRODUCTION A common problem with zeolite catalysts used in hydrocarbon conversion is deactivation by coke deposition [ 1]. A detailed study of the chemical nature of coke species in zeolites, as they are forming, can contribute importantly to the understanding of coke formation mechanism that is absolutely necessary to improving the catalytic performance of petrochemical processes. There have been extensive studies of coke formation, but the nature and mechanism of coke formation are still not clear[2]. In principle, vibrational spectroscopy can offer the opportunity to study the coke formation mechanism. Raman spectroscopy has recently received considerable attention in the field of catalysis because it offers a number of potential advantages over other methods of vibrational spectroscopy in the characterization of real catalysts [3,4]. For example, Raman spectroscopy can potentially obtain information about both surface adsorbed species and the structure ofc~slysts under working conditions. But Raman spectroscopy has gencndly not lived
600 up to its potential because the Raman scattering cross section is inherently small for many catalysts, and the Raman signal is often overwhelmed by strong fluorescence from the catalyst surface. The mrface fluorescence problem becomes particularly serious when the catalyst is contaminated with carbonaceous species. Consequently normal Raman spectroscopy using visible laser as the excitation source can not be applied in many situations involving catalytic hydrocarbon conversions. A few normal Raman spectroscopic studies have been conducted to examine the coke on catalysts[5-7], however, for most coked catalysts it is difficult to measure the Raman spectrum because of strong fluorescence interference. We have recently performed ultraviolet resonance Raman spectroscopy using a continuous wave ultraviolet laser as the excitation source [8,9] with the purpose of avoiding fluorescence and enhancing the Raman intensity. This technique has been applied to catalyst characterization and it was found that by using an ultraviolet wavelength below 260 nm not only is the Raman scattering enhanced, but the fluorescence background is avoided. In this paper, UV Raman specua are presented for coke species formed in ZSM-5 and USY zeolites which are difficult or impossible to measure by normal Raman spectroscopy. The results indicate that LN Raman spectroscopy can identify different coke species formed in zeolites and is capable of characterizing coke formation under working conditions.
2. EXPERIMENTAL The ultraviolet laser beam for exciting UV Raman spectra was generated by frequency doubling the 514.5 nm output of a 12-watt Ar+ ion laser to 257.2 nm using a temperature-tuned KDP crystal. The power of the 257.2 nm line can be as large as 30 mW, but in our studies the power delivered to the sample was kept below 5 mW to avoid heating. The Raman scattering from the sample surface was collected by an AIMgFz coated ellipsoidal reflector using a back-scattering geometry, and then focused into a 0.32 m single grating spectrograph through a notch filter. A 3600 groove/mm holographic grating was used to disperse the Raman scattered light onto a pyroelectrically cooled imaging multichannel photomultiplier tube (IPMT) with a spectral coverage of 2100 cm"~. A quartz reaction cell was specially fabricated for the UV Raman spectroscopic studies. A detailed description of this cell is given in a previous paper[10]. The cell consists of an outer part and an inner part which are connected and sealed by an o-ring firing. The outer part is surrounded by a furnace for pretreatment and reaction, and terminates at one end in a spherical bubble, and its center is located at the focus point of the collection mirror. The inner part has a sample holder which is movable between the furnace and the measurement position without exposure to air. The sample can be treated over a wide temperature range, 293-1200 K, under various atmospheres. ZSM-5, USY, and coked ZSM-5+SiO, catalysts were provided by Amoco Oil Co. The powder samples were pressed into wafers for Raman measurements. He (99.99%) was used as carrier gas. 02 (99.5%) and C3Hs (> 99%) were used for pretreatment and coke formation reaction, respectively. The coke generation was carried out in a C3I-I~(25%)+He(75%) flow with a flow rate of 150 ml/min. The oxidation of coke was performed in 02 flow with a flow rate of 100 ml/min.
601 3. RESULTS AND DISCUSSION 3.1. U V and normal R a m a n spectra of ¢oked samples
Figure 1 shows spectra recorded by normal Raman and UV Raman spectroscopy for ZSM-5 + SiO2 coked at 773 K with naphtha. No distinct Raman bands are detectable in the normal Raman spectrum even at a laser power of 100 mW. The Raman bands are buried by an intense background due mainly to fluorescence from the surface, indicating that the normal Raman signals are typically obscured by strong fluorescence. By contrast, Raman spectra of surface coke species can be clearly and easily detected using UV Raman spectroscopy In the 600-2200 cm"t region there are two groups of Raman bands observed near 1375 and 1610 cmt which are characteristic bands of coke species in olefmic and aromatic forms. It is obvious that UV Raman spectroscopy effectively avoids the fluorescence background. We have compared normal and UV Raman for a number of catalysts. In every case the UV Raman spectra are clear and strong while only strong fluorescence is observed by normal Raman spectroscopy. 3.2. C o k e formation in Z S M - 5 and U S Y at elevated temperatures
Coke formation in two typical zeolites, ZSM-5 and USY, were compared using UV Raman spectroscopy[ 10]. UV Raman spectra were recorded for ZSM-5 and USY treated in C3I-I6+ He
A: Normal Raman B: UV Raman
ca
©
tt3 03
'
I
1000
i
I
1500
'
I
2000
Raman shift/cm-1 Figure 1. Normal Raman and UV Raman spectra of ZSM-5+SiO2 catalysts ¢oked with naphtha at 773 K. (Normal: 514.5 rim, 100 mW; UV: 257.2 rim, 5 roW).
602 flow at 300 K and 773 K and the time for each temperature is about 1 h. For the sake of brevity, the observed Raman bands and their a s s i s t are summarized in Table 1. At room temperature, the adsorbed species for the two zeolites are olefms and the spectra are almost identical, meaning that the adsorbed species formed in the two zeolites at room temperature are similar. However, as the reaction temperature increased, the spectra for the two samples become quite different. At 573 K, the species in ZSM-5 still keep the identity of olefin but the species in USY tend to convening into polyolefm and aromatic species because their bands at 1390 and 1635 cm"1 shift down to 1380 and 1610 cm4, respectively. In addition, the bands near 3000 cma hardly change for ZSM-5 but almost vanish for USY. This clearly shows that the coke formation reactions in the two zeolites are different in nature. Obviously, for USY some of the adsorbed species desorb, and some of them convert into highly dehydrogenated carbonaceous species since the bands of CH stretching vibrations are attenuated dramatically. The striking shift of the band in the 1600 cm"~ region is also indicative of chemical changes in the adsorbed hydrocarbon species. These changes suggest that the olefinic species convert into polyolefm and aromatic, and this conversion appears to be easier in USY than in ZSM-5. When the temperature was increased to 773 K, the bands near the 3000 cm~ region of C-H stretch vibrations disappear but the spectrum in lower frequency region still shows the feature of olefinic species as indicated by the bands at 1375 and 1620 cm~. This means that part of the olefmic species in ZSM-5 transform into polyolefinic species through polymerization and/or dehydrogenation. The band at 1620 cm"~ slowly shifts to lower frequency if the reaction is prolonged, eventually shifts to 1610 cm~ which is in the characteristic region of aromatic species. Therefore, it appears that the coke formation in ZSM-5 is initiated from adsorbed olefinic species that p r ~ through polyolefms and terminate with aromatic species. By contrast, for USY the Raman bands at 1380 and 1610 cm1 shift respectively down to 1365 and 1595 cm~ , indicating the formation ofpolyaromatic and pregraphite species because the observed Raman bands are very close to the characteristic frequencies of polyaromatic and pregraphite species [11,12].
Table 1 UV Raman bands of coke species derived from the reaction of CsI-I~ in ZSM-5 and USY at different temperatures in C~-I~+ He flow for 1 h
Zeolite
ZSM-5 USY ZSM-5 USY ZSM-5 USY
Temperature (K)
297 297 573 573 773 773
(w): weak band.
Raman shift (cm"l)
1390, 1390, 1390, 1380, 1375, 1365,
1560, 1635, 2970, 2990 1635, 2980, 3010 1560, 1630, 2970, 2990 161O, 2960(w) 1620, 2970(w) 1375, 1 5 9 5
Assignment
olefin olefin olefin polyolefin+aromatic polyolefin polyaromatic+pregraphite
603 3.3. Coke formation in ZSM-5 and USY with time The evolution of coke species not only depends on the reaction temperature, but also varies with reaction time[2]. Figure 2 shows the UV Raman spectra recorded for different reaction times of propene with ZSM-5 at 773 K. At the beginning the spectrum resembles that at room temperature, and the observed bands at 1395 and 1630 cm"t are primarily due to adsorbed olefm species. After a reaction for 3 11, the two bands shift down to 1390 and 1620 cm"t respectively. These bands continue shifting down to 1375 and 1610 cm"~for another 3 h as seen in Figure 2D. This slow change proves that the adsorbed olefm species gradually convert into polyolefin species and finally into aromatic species because the band at 1610 cm"t is close to the band of aromatic species[13]. The 1610 cm"t band no longer shifts even after a further prolonged reaction at this temperature. Meanwhile it is interesting to note that the Raman band intensities decline slightly rather than develop with reaction time. This may be interpreted as evidence for no further coke accumulation after a certain amount of coke has formed since the channels in ZSM-5 are not large enough to host bigger coke particles. Figure 3 presents the UV Raman spectra for USY reacted with propene at 773 K. The coke band at 1610 cm"~grows considerably in the first hour and keeps on developing, indicating that
A: Background B: 5 min C" 1800 min D" 3600 min
143 CO t43
o ,qp,,, ¢D
Q
qff-
¢D
11!
A: Background B: 60 min C" 1800 min
CO
C C B 0
800 1600 Raman shift/cm-1
2400
Figure 2. UV Raman spectra of coke species formed in ZSM-5 with propene at 773 K for different reaction time.
1600 2400 3200 Raman shift/cm-1 Figure 3. UV Raman spectra of coke species formed in USY with propene at 773 K for. different reaction time.
604
the coke species build up in the USY with reaction time. Another noticeable change is that with a prolonged reaction at 773 K, the band at 1610 cm"t shifts to 1585 cm"xwhich is very close to the characteristic band of graphite at 1575 cm"~[12] and the band at 1375 crn"~ shifts to 1360 cm~ which is due to edge defects of the graphite [14], These remits strongly suggest that pregraphite species are formed in USY at 773 K, and the coke particle becomes bigger with longer time. Apparently, the coke formation in USY is quite different from ZSM-5 where mainly polyolefin and aromatic species are dominant at this temperature. 3.4. Oxidation of coke species in ZSM-S and USY The chemical nature of coke species could be distinguished through oxidation of the coke because the different coke species may show different reactivity towards oxygen. UV Raman spectroscopy was used to follow the coke species lett in the coked ZSM-5 and USY after different stages of oxidation treatment in 02 flow. The coke species were formed at 773 K in a C3I-I6+ He flow for more than 3 h and then the sample was purged with He alone for 30 min. Figure 4 exhibits the Raman spectra recorded for coked ZSM-5 oxidized at various temperatures. The spectrum is scarcely altered when the coked ZSM-5 is exposed to 02 at room temperature. When the sample was treated at 573 K, a dramatic decrease of band intensities at 1375 and 1610 cm"t
A: Coked ZSM-5 B: 02,573 K,1 h C: 02,773 K,5 h D: 02,873 K,1 h E: 02,873 K,3 h
IZ3 OO t43
o
T-
A: Coked USY B: 02,773 K,1 h C" 02,873 K,1 h
(D IZ) 03
A E)
B
I"
c l
800
16oo
2400
Raman shift/cm-1 Figure 4. UV Raman spectra of coked ZSM-5 treated in 02 flow.
1600
2400
3200
Raman shift/cm-1 Figure 5. UV Raman spectraof cokmi USY treated in Oz flow.
605 is clearly observed from Figure 4A to 4B. This can be attributed to a removal of coke species. Further removal of coke is more difficult as can be seen by the persistence of the coke band through spectra 4C and 4D. When the sample was treated in O5 flow at 873 K for 3 h, the coke species are finally removed except for a tiny/band at 1610 cm"~still remained no matter how long the oxidation was continued. The two st~es of coke oxidation probably manifest two kinds of coke species, polyolefm and aromatic formed in ZSM-5. The former is reasonably easier to oxidize than the latter. The slow oxidation of aromatic species might be explained in terms of slow diffusion of oxygen in the channels of ZSM-5. Figure 5 shows the Raman spectra of coked USY treated in O~ flow at 773 K and 873 K. No evident change in the spectrum was observed when the oxidation treatment was carried out at temperatures lower than 773 K. This implies that the coke species in USY are chemically inert towards oxygen, consistent with the assignment that the coke species in USY are mostly in the form ofpregraphite. A remarkable attenuation ofthe bands at 1365 and 1585 cm4 occurred at 773 K(Figure 5B), and the band intensity was reduced further with increasing temperature to 873 K(Figure 5 C). Two residue bands at 1605 and 1615 cm"~ survived from the oxygen treatment at 873 K. By comparing with the coke in ZSM-5, the coke species in USY seems easier to remove. This may be due to the bigger pores of USY which allows a faster diffusion of 05. A very important phenomenon in Figure 5 is that the band position at 1585 cm~ shifts up to above 1600 cm"~when the majority of the coke species was removed. It can be assumed that the pregraphite is removed by gradual oxidation at the edge of the graphite sheets. As a consequence, the particle becomes smaller and smaller which produces the up-shifting of the coke band from 1585 to 1610 cm-~. , 3.4. Mechanism of coke formation
The coke formation in the ZSM-5 and USY are different especially at high temperatures. The fact that the Raman spectra of adsorbed propene at room temperature are similar for the two zeolites suggests that the species are mainly adsorbed propene or/and polypropene. At elevated temperatures, the adsorbed olefin is dehydrogenated and polymerized, resulting in the polyolefin and aromatic species. Because of the limitations set by the pore size, the polyolefin and aromatic species can not grow fluter in ZSM-5. For USY, with increasing temperature, the polyolefin and aromatic species gradually aggregate into pregraphite species which mainly accumulate in the cages of the zeolite. It is also assumed that the difference in coke formation in the ZSM-5 and USY is not only due to the pore structures but also due in part to the acidity of the two zeolites. A study of the relationship between coke formation and acidity is under way.
4. SUMMARY UV Raman spectroscopy has been demonstrated to be a powerful tool for characterizing coke formation in zeolite catalysts. The sensitivity of Raman spectroscopy is improved significantly owing mainly to avoiding fluorescence interference. Coke formation in zeolites is initiated with adsorbed olefinic species and terminated with polyolefin and aromatic species in ZSM-5, but proceeds to pregraphite in USY. This difference is attributed to the pore structure and acidity of the two zeolites.
606 ACKNOWLEDGMENT We gratefully acknowledge Frank Modica and Jeffrey l~_tller for providing the zeolites and coked industrial catalysts. The Raman spectrum of figure 1A was measured by Maritoni Litorja. Acknowledgement is made to the Donors of the Petroleum Research Fund, administered by the American Chemic~ Society for partial support of this research. This project was also supported by the Center for Catalysis and Surface Science of Northwestern University.
REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
M. Guisnet and D. Magnoux, Appl. Catal., 54(1989)1. H. G. Karge, Studies in Surface Science and Catalysis, 58(1991)531. G. J. Hutchings, A. Desmartin-Chomel, R. Oiler and J.-C. Volta, Nature, 368(1994)41. J. Miciukiewicz, T. Mang and H. Knozinger, Appl. Catal. A: General, 122(1995)151. P.D. Green, C. A. Johnson and K. M. Thomas, Fuel, 62(1983) 1013. C. A. Johnson and K. M. Thomas, Fuel, 63(1984) 1073. D. Espinet, H. Depert, E. Freund and G. Martino, Appl. Catal., 16(1985)343. C. Li and P. C. Stair, Catal. Lett., 36(1996) 119. C. Li and P. C. Stair, Pro¢. of Inter. Congr. Catal., 1996, Baltimore, USA. C. Li and P. C. Stair, Catal. Today, in press. P. Kwizera, M. S. Dresselhaus and G. Dresselhaus, Carbon, 21 (1983) 121. M. Nakamizo, Carbon, 29(1991)257. D, Lin-View, N. B. Colthup, W. G. Fateley and J. G. Grasselli, The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules, Academic Press, Inc., Boston, 1991. 14. P. Lespade, R. AI-Jishi and M. S. Dresselhaus, Carbon, 20(1982)427.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
607
Preparation of Titanium-containing large pore molecular sieve from HAl-Beta zeolite Guo Xinwen, Wang Xiangsheng, Wang Guiru and Li Guangyan Institute of Industrial Catalysts, Daliam University of Technology, Dalian 116012, P.R. China ABSTRACT Ti-AI-Beta zeolite has been synthesized by gas-solid isomorphous substitution with H-AI-Beta zeolite as precursor. X-ray diffraction pattern showed that its structure was similar to that of zeolite H-AI-Beta. The only difference between their IR spectra was an extra adsorption band appearing at 960cm-~. The study of IR and UV-Vis spectra showed that Ti was incorporated into the framework of Beta zeolite and Ti was tetrahedrally coordinated as part of the framework. After the precursor was treated with acid, the incorporation of titanium became easy. With increasing acid concentration, the content of Ti into the framework increased. In addition, increasing substitution time, the content of Ti into the framework also increased. Key words Gas-solid isomorphous substitution, H-AI-p zeolite, Ti-AI-~zeolite. 1. INTRODUCTION In recent years isomorphous substitution of silicon or aluminium by titanium in the lattice of molecular sieves has attracted a considerable attention, especially the syntheses of TS-1 ~J and TS-2 [=3zeolites, which possess interesting catalytic properties. However, in the field of fine
608
chemicals, it is sometimes required to oxidize large molecules that can not penetrate in the narrow pores of the MFI structure. Ti-Beta zeolite has less steric constraints than TS-1 for oxidation of cycloalkanes because of its large pore. In 1992, Camblor Es~reported the synthesis of I-Ti,Al']-Beta in the hydrothermal system, then, Rigutto E4] reported the secondary synthesis of Ti-~ zeolite from boron-Beta. However, little information on the secondary systhesis of Ti-p zeolite from H-AI-Beta zeolite was available in the literature. In this paper. We present that Ti has been introduced by secondary synthesis into the structure H-AI-Beta zeolite.
2. Experimental 2. 1 Materials and Methods Thestarting material AI-Beta (SiOz/AI203 -- 25) synthesized with organic template (TEAOH) Es3was calcined for 6 hours at 540°C. The Hforms of AI-Beta were prepared as follows= 10g of the freshly calcined powder was treated with 200ml of 0. 4N aqueous solution of ammonium nitrate (4h at 80°C), and then deammoniated by heating at 540°C for 4h (sample I ). Another portion of the freshly calcined powder was treated with 2N (sample I ) or 5N HCI solution (sample I ) at 80°C for 4hr. "Concentration" of zeolite in the suspension was 50g/dm s of HCI solution. The secondary synsthesis method was performed by reacting H-AIBeta (predried at 450"C) at 500°C in a flow of dry N2 saturated with TiCI4 vapour.
2. 2 Characterization X-ray powder diffraction patterns were recorded on a XR-3A diffractometer using the CuKo radiation. UV-Vis spectra were collected on a UV-240 spectrometer, and Framework IR spectra were obtained on a IR-435 spectrometer using the KBr method (1 w t ~ zeolite).
609
3. RESULTS AND DISCUSSIONS 3. 1 Characterization of TI-AI-Beta zeolite
Powder X-ray diffraction patterns in Fig. 1. Show that the samples kept pure BEA structure after their transformation into H-form (spectrum a) and subsequent treatment of the H-form with TiCI4/Nz(spectrum b). After reacting H-AI-Beta with TiCI~, the crystallinity of the sample does not decrease and there is no variation inthe spacing of the (600) plane. Framework IR spectra indicates that the zeolites are highly crystalline (See Fig. 2). Moreover, the presence of an IR band at ~60cm-~ strongly suggests the incorporation of Ti atoms as a framework element. Eel The chemical composition is obtained by AAS after dissolution of the sampies (Table 1). a
f .
5
.
.
,, .
|.
.
10
L
15
.
.
L~
.=
20
25
211
,,.,
!.5oo
•
..
iooo
500
Wavenumber (ore- 1) Fig. 1 XRD patterns of sample I (a) and the corresponding Ti-Al-j3(b) zeolites
Fig. 2 IR patterns of sample I (a) and the corresponding Ti-Al-j3(b) zeolites
Table 1. Chemical composition of sample 1 and the corresponding Ti-AI-p sample Sample
Composition (mol ~ ) SiO~.
AI=O~
Ti02
H-AI-~
99.05
0. 88
0. 07
Ti-AI-~
96.80
1.00
2.20
610
The equivalence between remained SiO2 and incorported TiO2(see Table1) indicates that Ti is incorporated into the framework by mainly isomorphous exchange with framework Si, From (Fig3),
UV-Vis
spectra
it can be observed
that curve a shows a weak peak in the 210-230nm range because of a small amount of T i in the sample, and curve b shows a intense band in the 210-230nm range and a shoul-
~ 8
der at -~, 270nm. The band at ,~-225nm in the UV-Vis pectrum of calcined hydrated TS-1, has
a
b
been assigned to the ligand to metal charge transfer (CT) involving isolated Ti atoms in
190
290
390
._,
Wavelength(nm)
490
octahedral coordination rTl. De- Fig. 3 UM-vis spectra of sample ! (a) and the hydration of this sample shifts correspondingTi-AI-19(b)zeolites the band to ca. 205nm, characteristic for a CT transition involving tetra coordinated Ti (IV) in the FTiO,I or [TiOsOHl structure cSJ. Taking into account these assignments, we could conclude that in these Ti-AI-p samples, most of titanium exists i~ the form of isolated tetra coordinated Ti species.
3. 2 The effect of various treatment condition.
611
a
1500. . . .
,,
~
.=
~_
1000
j~...
t__=
500
Wavenumber (cm - * )
Fig. 4 IR framework spectra of Ti,AI-beta zeolites which precursors treated under different conditions • a. no treatment b. c. 2NHCI d. SNHCI
NH,NO=
190
290
390
490
590
Wavelength (nm) Fig. 5 UV-Vis spectra of Ti-AI-Beta zeolites which precursors treated under different condition: a. no treatment b. NH4NOs c. 2N HCI d. 5N HCI
Fig. 4 shows that the intensity of 960cm -~ increases with increasing acid concentration, and the increase of the intensity of 960cm -~ indicates that the content of titanium which is incorporated into the framework increases. From Fig. 4, we can s e e , when using as synthesed powder as precursor, there is a small peak at about 960cm -~ because of a large amount of AI 3+ and Na + existing in it. When precursor was treated with NH4NO3 solution, the intensity of 960cm -~ increases slightly. Although Na + is removed through ion exchange, there is a large amount of AI 3+ in the precursor. When the precursor was treated with 2N HCI, the
612
intensity of 960cm -~ obviously increases. Inereasing acid concentration, the intensity of 960cm -~ continues to increase. The reason is that acid treatment produces hydroxyl nests because of dealumination, which leads to a more efficient incorporation of titanium into the frameworkCg- ~01. Fig. 5 shows that the intensity of the band at
d
220nm increases with increasing treatment degree, this indicates that the content of Ti into the framework increases. It is in total agreement with the IR result. We can also see, when using as-synthesed powder as precursor (curve small
a),
there is a
amount
of
TiOz
J..;.
1500
1000
..
500
(anatase) in the product. Wavenurnber (cm -~) This also indicates that Fig. 6 IR spectra of Ti-AI-p prepared with AI s+ and Na + in the predifferent substitution time (sample ! cursor are detrimental to as precursor) a)0 b) 6h c)18h d)26h the incorporation of titanium and make titanium exist in the form of extra framework titanium. When the precursor is treated with NH,NOs or HCI, the amount of extra framework Ti decreases. The framework spectra of Ti-AI-Beta zeolites show that the content of Ti into the framework increases with increasing substitution time (Fig. 6). 4, CONCLUSION Titanium-containing large-pore molecular sieves with the BEA struc-
613
ture can be prepared by reacting H-AI-Beta with titanium chloride at 500°C. Dealumination ~ d s to a more efficient incorporation of titanium into the beta framework. AOKNOWLEDGE~NTS The authors express their sincere thanks to professor Z. H. Zou (Department of applied chemistry, Dalian University of Technology) for his help in registering the UV-Vi= diffuse reflectance spectra.
REFERENCES El] M. Taramasso, G. Perego and B.Notari, US Pat. ,4410501(1983). [-2-] J. S. Reddy and R. J. Kumar, J. Catal., 130(1991)440. E3] M. A. Camblor, A. Corma,J. Perez-Pariente,zeolites, 13(1993)82. E4-1 M. S. Rigutto, et al., Studies in Surface Science and Catalysis, 84 (1994)2245. C5-] R. L. Wadlinger, G.T. Kerr and E. J. Rosinski, US. Pat. 3308069 (1967) [6"] M. R. Boccuti, et al., Stud. Surf. Sci. Catal. ,48(1989)133. E7] A. Zecchina, G. Spoto, S. Bordiga, et al., Studies in Surface Science and Catalysis,69(1991)251. E8~ F. Geobaldo, S. Bordiga, A. Zecchina, et al., Catal. Lett., 16 (1992)109. E9-] R. M. Barrer and M. B. Makki, Canadian Journal of Chemistry, 42 (1964)1481. ElO-] Guo Xinwen, Ph. D Thesis, Dalian University of Technology,China, 1994.
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H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
615
S y n t h e s e s a n d R a m a n s p e c t r o s c o p i c s t u d y of bis- a n d t r i s - ( 1 , 1 0 - p h e n a n t h r o l i n e ) m a n g a n e s e ( I I ) c o m p l e x e s e n c a p s u l a t e d in f a u j a s i t e - Y B.-Z. Zhan and
X.-Y. Li
Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong ABSTRACT We report the syntheses of three different types of occluded Mn(II)-l,10phenanthroline complexes in faujasite-Y. We present a detailed characterization of the encapsulated complexes by Raman spectroscopy and by other analytical techniques. We show Raman evidences for a strong interaction between oxygen atoms of the supercages and metal ion of the occluded complexes. We illustrate that the relative intensity ratio between Raman marker peak of metal complexes and that of zeolite framework can be use to estimate the occlusion concentration quantitatively. Finally, the preliminary results of the title guest-host composites as selective allylic hydroxylation catalysts are presented. 1. INTRODUCTION The lasting extensive interests in zeolite encapsulated transition metal complexes lie in their established applications in catalysis, gas adsorption and separation, and in their potentially useful electrochemical and photochemical properties. The Y-type faujasite is particularly attractive because their well organized "supercages" have an internal diameter of 13/~ and are sufficiently large to host redox-active metal complexes of such multidentate ligands as phthalocyanine[1], schiff base[2] and polypyridine[3]. The main difficulties in the study of molecular sieve confined chemistry, however, are the characterization of guest molecules in the host-guest composites, the assessment of the effect of cage environment on the guest, as well as the in situ monitoring of the behaviors of guest molecules in a chemical process[4]. In this paper, we report (a) the synthesis of three different t y p e s of occluded Mn(II)-l,10-phenanthroline (Phen) complexes in faujasite-Y, and (b) a detailed characterization of the encapsulated complexes by R a m a n spectroscopy. We demonstrate that Raman-scattering is a very powerful technique for characterizing and monitoring the guest molecules occluded in the zeolite cages. We show the Raman evidences for a strong interaction between the supercages and the occluded complexes. We illustrate that the relative intensity ratio between Raman marker peak of metal complexes and that of zeolite framework can be use to estimate the occlusion concentration quantitatively. Guest molecules having visible or near ultraviolet absorption
616 can be s t u d i e d by resonance R a m a n s c a t t e r i n g , while those d i s p l a y i n g interference of intramolecular fluorescence can be studied by using the n e a r i n f r a r e d excited F o u r i e r t r a n s f o r m R a m a n technique[5]. F i n a l l y , t h e p r e l i m i n a r y r e s u l t s of the title g u e s t - h o s t composites as h y d r o x y l a t i o n catalysts are presented. 2. E X P E R I M E N T A L 2.1. Synthesis M n ( P h e n ) 2 2 + / Y , M n ( P h e n ) 2 ( O z ) 2 ~ , a n d Mn(Phen)32+/Y : The sodium form of faujasite-Y (NAY) with Si/AI = 2.6 was obtained from H u n a n Petro Co., China. All chemicals were purchased from Aldrich and used as received. The synthesis of the title complexes in NaY was carried out by the following steps. First, Na + cations in NaY were exchanged by Mn 2+, at a loading of I Mn 2+ pel; two s u p e r c a g e s , in a vigorously s t i r r e d aqueous solution c o n t a i n i n g the stoichiometric a m o u n t s of NaY and Mn(OAc)2 at 85°C for 3 hours u n d e r N2 atmosphere. The pH value of the solution was retained at ~ 6 during cation exchange by t i t r a t i n g with 2M HC1 solution. Second, the suspension after cation exchange was filtrated and w a s h e d by redistilled w a t e r a n d t h e n dehydrated at 150°C for 3 hours under vacuum. Third, MnY thus obtained was thoroughly mixed with methanol solution of P h e n in a specified ligand-tometal ratio at room t e m p e r a t u r e . The methanol was t h e n removed at 80°C using an oil bath. Each template synthesis(the diffusion of ligands into the supercages) was conducted in a sealed v a c u u m glass tube for 24 hours. A specific type of occluded complex was obtained by controlling the ratio of r e a c t a n t s (ligand-to-metal) and the reaction temperature, as listed in Table 1. Upon the completion of reaction, the powder s a m p l e was extracted by 10% NaC1 aqueous solution, acetonitrile and dichloromethane respectively in order to remove the unreacted ligand and surface adsorbed complexes. Table 1 The specifications of the synthesized samples Sample Label
Loada
Reactant Ratio b
Reaction Product Temp(°C) Color
Product Ratioc
Product Assignmentd
Mnl00211
4
2.1:1
100
gray
2.16
Mn(Phen)22+/Y
Mn200211
4
2.1:1
200
pale pink
2.05
Mn(Phen)2(Oz)2~
Mn200351
4
3.5:1
200
pale pink
2.83
Mn(Phen)32+[Y
a. number of Mn2+ per unit cell; b. the mole ratio of Phen-to-Mn2+ in reactants; c. the mole ratio of Phen-to-Mn 2+ in products from elemental analysis(for C,N) and atomic absorption analysis(for Mn2+); d. see the text for discussion. C/s-Mn(Phen)2Cl2 and Mn(Phen)3(CIO4)2 were synthesized according to the l i t e r a t u r e methods[6,7] and were further confirmed by elemental analysis. Calcd. for cis-Mn(Phen)2C12:C,59.2; H,3.29; N,11.5%. Found: C,58.3; H,3.15;
617 N,11.2%. Calcd. for Mn(Phen)3(C104)2: C,54.4; H,3.02; N,10.6%. Found: C,55.0; H,2.95; N,11.2%. R a m a n spectrum of Mn(Phen)2C12 confirms t h a t it is cis isomer[6b] . 2.2. C h a r a c t e r i z a t i o n Atomic absorption analysis(A.A.A.) of Mn 2+ content was conducted with a Model PU9100X(Philips) AA spectrometer equipped with a Mn lamp. Elemental analysis(E.A.) was carried out for C, N elements for each sample. FT-Raman spectra were collected on a Bruker IFS 100 spectrometer equipped with a CW Nd:YAG laser(1064 nm excitation) and a Ge detector cooled at liquid N2 temperature. All Raman spectra were collected with 180 ° scattering geometry and ~ 4 cm -1 spectral resolution. Typically, a laser power of 200mW was used to i r r a d i a t e onto a loosely packed powder sample held in a aluminum holder. Usually, 2000 scans need to be averaged in order to reach a reproducible signal-to-noise ratio. FT-IR spectra were collected using a Bruker IFS66 spectrometer with the sample being thoroughly mixed in a KBr pellet (sample/KBr ratio = 1:100 by weight ). 3. RESULTS AND DISCUSSION 3.1. R a m a n s c a t t e r i n g v e r s u s IR abs or pt i on: N e a r i n f r a r e d excited F o u r i e r R a m a n mmttering is a m u c h s u p e r i o r a n d a v e r y sensitive p r o b e of
ill
NaY ....
,
~
,
Mn100211
Mn200351
-J-
1700
i
J
I
I
1250 lOOO 750
Wavenumber
(A)
cm-1
I
400 1700
I000
Wavenumber (B)
'1
500
cm-1
I00
Fibre-1 : FT-IR spectra(A) and FT-Raman spectra(B) of NaY and its two occlusion oom~ Mn(Phms)22+/Y and Mn(Phen)32+/Y. See text for the experiments] conditions and ~scussion8.
618 For the purpose of comparison, we displayed in Figure 1 the FT-IR (A) and FTRaman(B) spectra of NaY and its two occlusion composites Mn100211 and Mn200351, respectively. As can be clearly seen from the IR spectra(A), NaY itself absorbs strongly in almost full mid-IR region, leaving only a narrow window between 1250-1550cm "1 for the analysis of the encapsulated molecules. Of particular disappointing is the finger-printing region below 800 cm "1 where strong absorption of faujasite-Y obscured any hope to extract useful information about the occluded molecules. FT-IR spectra of MLx/Y composites, in spite of its very limited information, do indicate the successful occlusion of Mn(Phen)x 2+ complexes within the faujasite-Y supercages as evidenced by the peaks at 851, 1432, 1473, 1523, 1545 cm -1, respectively. It is almost impossible, for example, to structurally distinguish the occluded complexes in the samples Mnl00211 and Mn200351 from their IR spectra. R a m a n spectra, as illustrated in Figure-lB, provide a much superior probe than FT-IR for the occluded complexes. First of all, NaY itself, like most of the SiA10 zeolites, is a poor light scatterer. Its Raman spectra is very simple and quite well-defined. Therefore the complication from the internal vibrations of zeolite framework in the Raman spectra of guest-host composite can be easily identified and removed. Secondly, almost whole mid-IR and far-IR regions can be used to study the occluded molecules. This is of particular significance because the subtle differences between the occluded complexes prepared under different conditions can thus be studied using both the functional group and the finger-printing regions. Thirdly, a well-defined zeolitic internal Raman peak at -500 cm "1 ( mainly the skeleton's T-O-T bending character[8] ) provides an ideal internal standard to study such quantitative information as the occlusion concentration and hydration level of the zeolite, etc. 3.2. Mn(Phen)22+/Y v e r s u s Mn(Phen)32+/Y: R a m a n m a r k e r s for d i f f e r e n t ligand-to-metal ratio. E.A. and A.A.A. of the two samples prepared under very different conditions, Mn100211 and Mn200251, show that Phen-to-Mn ratios are -2:1 and -3:1, corresponding to the occluded complexes of Mn(Phen)22+ and Mn(Phen)32+, respectively. At least four set of Raman bands are identified that clearly mark the differences of the ligand number in the occluded complexes. For the occluded Mn(Phen)22+, these band are located at 1600 (shoulder), 1302, 722, 276 cm -1, respectively, while for the occluded Mn(Phen)32+, they are found at 1592 (sharp),1314, 727, 286 cm -1 respectively. The sensitivity of both Phen internal modes at high frequency and the Mn-L mode at low frequency to the change of coordination number is expected in t h a t they reflect different strengths of complex-cage interaction due to different shapes and sizes, as well as different strengths of Mn-Phen interaction[9-11]. To further confirm the observations made above, we have synthesized homogeneous cis-Mn(Phen)2C12 and Mn(Phen)3(C104)2. The Raman spectra of the homogeneous complexes were acquired and compared with those of occluded complexes in the finger-printing region (Figure-2). Indeed, the Raman spectra show remarkable similarity for the same type of complex no
619 m a t t e r it is in the occluded or homogenous forms. While for complexes with different n u m b e r of ligands, the differences in t h e i r R a m a n spectra are clearly visible. For bis-complex, the characteristic peaks were observed at ~ 419(with shoulder) and 177/152 (doublet) cm -1, respectively, while for tris-complex, the features are at 423/411(doublet) and 162 cm -1, respectively.
c/s-Mn(Phen)2Cl2
tJ
3.3 Mn(Phen)22+fY v e r s u s M n ( P h en)2(Oz)2fY: R a m a n evidences for the d i r e c t i n v o l v e m e n t of the s u p e r c a g e o x y g e n a t o m s in m e t a l c o o r d i n a t i o n sphere. A striking observation was made on a p a r t i c u l a r p r e p a r a t i o n of the I I I I I sample Mn200211 which was made 55O 4OO 300 2O0 I00 with a starting Phen-to-Mn ratio o f - 2 , but with the synthesis being carried Wavenumber c m ' l Figure-2 : Raman spectra of the fingerout at 200°C. E.A. and A.A.A. results printing region for homogeneous and indicate that the occluded complexes occluded Mn(II)-Phen complexes. indeed has a Phen-to-Mn ratio of ~ 2, in consistence with the expected occlusion of Mn(Phen)22+ complex. Yet, its Raman spectrum bears remarkable similarity to that of Mn(Phen)32+ftr, with m a r k e r bands at 1592(sharp), 1315, 727, 286cm -1, respectively, indicating that the occluded Mn 2+ is in a six-coordinated state ( F i g u r e - 3 ) . Several new Raman bands were also observed at 1401, 673, 567cm -1, respectively.
Mn(Phen)2(Oz)2/Y~
1500
i
I
z
i
l'
l
l
i
1250 750 700 650 600 550 500 450
~I
390
Wavenumber cm'l
Figure-3 • Comparison of Raman spectra of Mn(Phen)22+/Y, Mn(Phen)2(Oz)2/Y, and Mn(Phen)32+/Y. See text for the experimental conditions.
620
We are therefore compelled to conclude that, at high temperature of synthesis, the oxygen atoms of supercages break away from the zeolite framework, and start to strongly interact with the occluded Mn 2+ ion or Mn(Phen)22+ complex. We denote this sample by Mn(Phen)2(Oz)2~ where Oz is the oxygen atom from the zeolitic supercage. This idea, together with the occlusion composites synthesized under other conditions, can be expressed in the Scheme-1. The new band observed at 673 cm -1 is presumably due to the Si(Al)-O- stretching of the broken Si-O-Al skeleton. The other two new bands at 1401 and 567 cm -1 are attributable to the splitting of the nearby Phen band due to strong steric distortion of the complex.
s Si
W,
o.AI"
..Si
,,
MnNaY ""'1~
A
..~
y
'o
c
M'n .....O-si'
•
Scheme-1 • Three types of faujasite-Y occluded Mn2+-Phen complexes. 3.4. T h e g u e s t v e r s u s t h e h o s t : R s m a n i n t e n s i t y r a t i o a s a q u a n t i t a t i v e estimation of occlusion concentration.
For a given type of occluded complex, the intensity ratio between a well-defined characteristic Raman band of the guest molecule and that of the host matrix should correlate linearly to occlusion concentration (the average number of complexes per gram of sample). This is indeed what we observed by using four different levels of loading in Mn(Phen)3 2+ /Y. In Figure-4, the intensity ratio of 1050 cm -1 ( guest mode ) over 500 cm -1 ( host mode ) peaks was plotted against the concentration of the guest. An excellent linear relationship was obtained.
I
~
0.4 0.3-
'0.2
0.1
8 o.o
I 0.0
0.5
I 1.0
Iloso / I soo PiJmm-4 : 110601 ~ ss a m s r k ~ for the E m l t ~ m c e n t r a t i ~ in J&m(l~n)$2+/Y.
621 3.5. Catalysis The oxidation of cyclohexene was used as a reference reaction to study the catalytic properties of the three types of occluded Mn-Phen complexes. They show different catalytic behaviors, in agreement with the composition/ structure studies reported in the previous sections. Two competitive pathways of oxidation were observed with one mainly leading to epoxide and the other to allylic hydroxylated products. We have achieved the selection of one pathway over the other by using different oxidants. The optimization of the catalytic conditions is currently in progress. 4. CONCLUSIONS We have demonstrated that the type and the structure of the occluded molecules depend not only on the ligand-to-metal ratio used as reactants, but also on the t e m p e r a t u r e applied during the synthesis. We have shown that Raman spectroscopy is a very sensitive probe for the structure of the occluded molecule, as well as for the interaction between the guest molecule and the supercage. We i l l u s t r a t e d t h a t R a m a n spectroscopy can be used to quantitatively estimate the occlusion concentration, and therefore be utilized to optimize the synthesis of catalyst with desired concentration of catalytic site. Optical fiber guided and time-resolved Raman spectroscopy will enable us to study and monitor the reaction intermediates formed during catalysis. Finally, the title occluded complexes were shown to be good catalysts for the selective allylic hydroxylation of alkenes. ACKNOWI,~EMENTS
We acknowledge the Research Grant Council, Hong Kong and the Hong Kong University of Science and Technology for the financial support (to XYL). R~'ElCENCES 1. (a) V. Yu. Zakharov and B. V. Romanovsky, Vestn. Mosk. Univ., Ser. 2: Khim., 18 (1977) 142 [Eng. Trans. in Sov. Mosc. Univ. Bull., 32 (1977) 16]; (b) B. V. Romanovsky, R. E. Mardaleishvili, V. Yu. Zakharov, and O. M. Zakharova, Vestn. Mosk. Univ., Ser. 2: Khim., 18 (1977) 232; (c) V. Yu. Zakharov and B. V. Romanovsky, Vestn. Mosk. Univ., Ser. 2 KhJm., 18 (1977) 348; (d) G. Meyer, D. Wohrle,D. Mohl and G. Schultz-Ekloff, Zeolites 4 (1984) 30; (e) T. Kimura, A. Fukuoka, and M.Ichikawa, Shokubai, 31 (1988) 357; (f) R. F. Patton, L.Utytterhoeven, and P. A.Jacobs, Stud. Surf. Sci. Catal. 59 (1991) 395; (g) E.Paez-Mozo, N.Gabriunas, F. Lucaccioni,D.D. Acosta, P. Patrono, A. L. Ginestra,R.Ruiz,and B.Delmon, J. Phys. Chem., 97 (1993) 12819; (h) R. F. Parton, I. F. J. Vankelecom, M.J.A. Casselman, C. P. Bezoukhanova, J. B. Utytterhoeven, and P. A. Jacobs, Nature, 370 (1994) 541; (i) K. J. Balkus, Jr., A. G. Gabrielov, S. L. Bell, F. Bedioui, L. Rouk, and J. Devynck, Inorg. Chem., 33 (1994) 67. 2. (a) D. Chatterjee, H.C. Bajaj, A. Das, and K. Bhatt, J. Mole. Catal., 92 (1994) L235; (b) D.E. Devos, F. Thibault-Starzyk, and P. A. Jacobs, Angew.
622
3.
4. 5.
6. 7. 8. 9. 10. 11.
Chem. Int. Ed. Engl., 33 (1994) 431; (c) F. Bedioui, L. Roue, E. Briot, J. Devynck, S. L. Bell, and K. J. Balkus, J. Electroanal. Chem., 373 (1994) 19. (a) K. Maruszewski, D.P. Strommen, and J.R. Kincaid, J. Am. Chem. Soc., 115 (1993) 8345; (b) K. Maruszewski and J.R. Kincaid, Inorg. Chem., 34 (1995) 2002; (c) P.P. Knopes-Gerrits, D.D. Vos, F. Thibault-Starzyk, and P.A. Jacobs, Nature, 369 (1994) 543. S.L. Suib, Chem. Rev., 93 (1993) 803. (a) D. E. De Vos, D. L. Vanoppen, X.-Y. Li, S. Libbrecht, Y. Bruynseraede, P.P.Knopes-Gerrits, and P.A. Jacobs, Angew. Chem.: Chem. Eur. J., 1(2) (1995) 144; (b) P.P.Knopes-Gerrits, E. Feijen, X.-Y. Li, and P.A. Jacobs, Angew. Chem.: Chem. Eur. J., in press(1996). (a) B. P. Sullivan, D. J. Salmon, and T.J. Meyer, Inorg. Chem., 17 (1978) 3334; (b) R.E. Morcom and C. F. Bell, J. Inorg. Nucl. Chem., 35 (1973) 1865. A.A. Schilt and R.C. Taylor, J. Inorg. Chem., 9 (1959) 211. (a) P. K. Dutta, K.M. Rao, and J.Y. Park, J. Phys. Chem., 95 (1991) 6654; (b) C. Bremard and M. Le Maire, J. Phys. Chem., 97 (1993) 9695; (c) A. J. M. de Man and R. A. van Santen, Zeolites, 12 (1992) 269. (a) N. Abasbegovic, N.Vukotic and L. Colombo, J. Chem. Phys., 41 (1964) 2575; (b) E.R. Lippincott and E. J. O'rielly, Jr., ibid., 23 (1955) 238; (c) A. A. Schilt and R.C. Taylor, J. Inorg. Nucl. Chem., 9 (1959) 211. K. Krishnan and R.A. Plane, Spectrochim. Acta, 25A (1969) 831. K. Nakamoto, B. Hutchinson, and J. Takemoto, J. Am. Chem. Soc., 92 (1970) 3332.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors)
Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
623
Chemometric Analysis of Diffuse Reflectance Spectra of CoA Zeolites: Spectroscopic Fingerprinting of Co2+-Sites An A. Verberckmoes*, Bert M. Weckhuysen and Robert A. Schoonheydt
Centrum voor Oppervlaktechemie en Katalyse, Departement lnterfasechemie, K.U.Leuven, Kardinaal Mercierlaan 92, B-3001 Heverlee, Belgium 1. ABSTRACT A new method for spectroscopic fingerprinting is proposed for CO 2+ in zeolite A. The method is based on the use of different mathematical (GRAMS) and chemometrical techniques (PCA and SIMPLISMA) which were applied on series of diffuse reflectance spectra of CoA zeolite as a function of the Co-content and taken after dehydration at 400°C. Two Co 2÷ species could be determined, which were assigned to trigonal and tetrahedral coordination at the hexagonal windows. Their relative concentrations as a function of the Coloading were determined.
2. INTRODUCTION The coordination of TMI on surfaces is characterized by low symmetry and incomplete coordination. Different sites can be occupied simultaneously, which can lead to overlapping spectra. This is also the case for zeolites. However, the sites are crystallographically known and therefore well-defined. Here, we present an analysis method of such overlapping spectra 2+ with the most simple example Co A, where according to XRD only one site is present: the oxygen six-ring. Diffuse reflectance spectroscopy is ideally suited for probing the coordination environment via d-d transitions measured in the VIS region. The DRS spectra of CoA zeolites as a function of Co-content are unraveled with chemometrical techniques, like Principal Component Analysis (PCA) 1'2 and SIMPLe-to-use Interactive Self-Modeling 35 Analysis (SIMPLISMA) -. Chemometric analysis is not yet generally used in spectroscopic 67 investigations of catalysts. ' Here, the results on the number of components and the obtained spectroscopic signatures of the different coordination sites of Co 2+ will be discussed.
3. EXPERIMENTAL SECTION
3.1. Sample preparation and spectroscopy Zeolite A (HMV 7) was exchanged with COC12.6H20 to obtain cobalt zeolites with variable cobalt content. 6 The Co 2+ contents of the samples as determined by atomic absorption spectroscopy (AAS) after acid dissolution of the solids were 0.25, 0.5, 0.86, 1.32, 1.85, 2.18, 2.48 and 2.72 Co2+/UC. The samples were dried, granulated (0.25-0.40 mm) and calcined at
624 400°C in a DRS flow cell during 24h in an oxygen stream. Diffuse reflectance spectra were taken on a Varian Cary 5 UV-VIS-NIR spectrophotometer at room temperature in the 2002500 nm region. The spectra were recorded against a BaSO 4 standard (KODAK). Before applying mathematical/chemometrical techniques, the spectrum of the NaA support after dehydration at 400°C was subtracted from each of the CoA spectra and a baseline correction was performed.
3.2. Mathematical and chemometrical techniques (i) The spectra were decomposed in Gaussian bands with a Grams 386 software package of Galactic. Industries. Corp.. The band positions were estimated by eye and were kept almost constant when decomposing the series of spectra. Also the bandwidths were kept almost constant. (ii) P C A 1'2 is a factor analysis method and the model is:
A=T.B+EA A (mxn) is the spectral data matrix of m samples taken at n digitized wavelengths. B is a hxn matrix with h the number of PCA basis vectors, also called the loading vectors or loading spectra. T is a mxh matrix with in the columns the intensities ('scores') of the h loading vectors for the m samples. The columns in T are orthogonal. E A is the matrix of the spectral residuals. PCA describes the spectral information via principal components. This means that each component maximally describes the spectral variation in A. The PCA analysis is performed by using the Chemometric toolbox of MATLAB. (iii) SIMPLISMA (SIMPLe-to use Interactive Self-Modeling Analysis) 35 is a method to resolve the spectral data matrix A (mxn) in pure component spectra. The method is based on the principle of the pure variable. This is a variable, in this case a given wavelength, at which the intensity comes from one component only. The model is:
-jr= C.S T is the transpose of the spectral data matrix. S is a (kxm) matrix with the unknown pure spectra in the mixture and C a (nxk) matrix with in the columns the fractional contributions of the pure spectra, k is the number of pure spectra. When using in C the observed intensities m
of the pure variables, S can be resolved by the method of least-squares. The SIMPLISMA software has been developed by Willem Windig of KODAK and runs under MATLAB.
4. RESULTS In order to gather information on the speciation of cobalt, different techniques have been applied on the DRS spectra of Co2+A and the visible region has been selected because of the resolution. A flowshart of the applied methods is shown in figure 1. With the Grams decomposition method the spectra were systematically resolved in Gaussian bands. In addition to the mathematical fitting of spectra, chemometrical techniques were introduced. First PCA was used to obtain the number of components. This number is necessary in the
625 SIMPLISMA analysis. The latter method results in pure component spectra and intensities of the pure components in the individual spectra, which allows spectroscopic fingerprinting.
0.5 0 -0.5
500
550
600
650
700
750
800
WAVELENGTH (nm)
Chemom
Mathemati
DECOMPOSITION
PCA
_ component number prediction
,b
SIMPLISMA
_ pure component spectra 56O
60e
~de
_ intensity profiles
WAVELENGTH(nm)
Figure 1. Flowshart for the method followed to obtain information on the speciation of C o 2+ in zeolites. The upper figure contains the DRS spectra of CoA with increasing Co-content. The figure left below is the decomposed spectrum of Co2.48A. Figure 2 gives the overall spectrum of Co].85A after dehydration at 400°C. In the visible region a band is present around 390 nm. A triplet can be observed with maxima around 538, 580 and 637 nm with a shoulder at 733 nm. In the near infrared a broad overlapping region of cobalt exists with at 1385 nm and 2200 nm respectively the overtone and combination bands of hydroxyls.
626
tm-
Nanometers
Figure 2. DRS spectrum of Con 85A after dehydration at 400°C. 4.1. S p e c t r a l d e c o m p o s i t i o n
The visible region of the CoA spectra was decomposed in Gaussian bands at 505, 538, 580, 637, 692 and 733 nm. The band positions were kept constant with a small variation of + 3 nm. Figure 3 shows the intensity courses of the band areas of the band decomposition of CoA dehydrated at 400°C. The band evolutions are shown for the separate bands at 505, 692 and 733 nm. A summation of the band areas has been taken for the triplet bands around 538, 580 and 637 nm. There is a global increase of the band areas with increasing Co2+-content, except for the band at 733 nm. The upper curve in figure 3 is the overall intensity of the visible region (sum of the band areas of the six bands). The intensity course is not strictly linear, which is an indication that more than one component contributes. 480
6O / / % , A / A
48
384 288
/g"
,/,"
/7s
~
,, [
A i.
/ +. "6% 24
96
12
2
-+-
505nm
- A-
538 nm+580 nm+637 nm
~
-o-
692 nm
~
,,.,,
733
-A-
sum of 6 bands
36
192
1
~
nm
3
# Co/UC Figure 3. Band areas of the separate bands at 505, 692 and 733 nm; of the sum of the bands at 538, 580 and 637 nm; and of the sum of the six bands, all as a function of Co content for CoA dehydrated at 400°C. 4.2. P r i n c i p a l C o m p o n e n t A n a l y s i s
PCA is a chemometrical tool for the determination of the number of principal components. It is an explorative technique and can be used as a predictive step before u
SIMPLISMA analysis. Because normalization of the spectral data in A is a necessary step in the SIMPLISMA procedure, the same data pretreatment must be performed for a well
627 matched PCA analysis. The normalization formula of SIMPLISMA for a set of spectra (j = 1...m) taken in a wavelength region with n wavelengths (i = 1...n) and equal intervals, is: _
X~
Xu
_
IIx,ll
~ +~t~)
with ~i and ~ti respectively the standard deviation and mean at wavelength i of the m spectra. This normalization procedure can be simulated by variance scaling (VARSCALE) the data, which is an optional function of the chemometrics toolbox of MATLAB. Determining how many of the principal components to keep is a crucial step in factor-based techniques like PCA. The indicator function PCAREV calculates the Reduced Eigenvalues (REV) according to the method of Malinowski. 8 It looks at the eigenvalues associated with each eigenvector and is proportional to the amount of variance in the data. Tabel 2 gives the reduced eigenvalues for the CoA data. From the REV% values in table 2, it is derived that approximately 97% of the variance can be explained with two eigenvectors or PCA components. If more than two factors are kept, one is in danger of overfitting the data and adding noise. Table 2 Reduced Eigenvalues (REV) of the VARSCALED spectral CoA data RANK REV REV% 1 0.1206 94.88 2 0.0025 1.9669 3 0.0012 0.944 4 0.0012 0.944 5 0.0007 0.0055 6 0.0005 0.0039 7 0.0003 0.0024 8 0.0001 0.0008
4.3. SIMPLISMA Taking into account the PCA prediction of two components for CoA, SIMPLISMA can highlight the pure spectra and their intensity profile. -3 A. -3 B. xlO xl0 lO ~, 011 5
o 700
600
500
WAVELENGTH (nm)
700
600
500
WAVELENGTH (nm)
Figure 4. Pure component spectra of component 1 (4A) and component 2 (4B).
628 Figures 4A-B show the pure spectra. The first component has two main absorption bands at 666 and 616 nm, accompanied by a band at 512 nm. The second component has three bands at 635,582 and 546 nm and a small band at 738 nm. Figure 5 gives the intensity contributions of the pure spectra to the individual CoA spectra. Both components increase with increasing C o 2+ c o n t e n t .
O
200
comp. 2,..-" ~Z L)
/..'*
100
~.l-
/ /
/~
comp 1
Z
0 0
r"
'
0
1
2
3
# Co/UC Figure 5. Intensity contributions of component 1 and 2 to the individual DRS spectra of CoA. Table 3 gives the P3ure variables and the corresponding weight, purity and purity-corrected standard deviation. The values in the third row of table 3 are almost zero, which is an indication that only noise is left after two components were selected. Table 3 Relative total intensities of the weight, purity and purity-corrected standard deviation. Pure variable Weight Purity Stdev selected -
100
100
100
688 552
1.5371 0.0102
1.2587 0'0068
1.2512 0.0030
4.4. Comparison of the different techniques
The best fit decomposition of the CoA spectra gives six Gaussian bands. The band positions and widths are given in the first and second column of table 4. The third and fourth column of table 4 give respectively the absorption maxima of the pure spectra of component 1 and 2, resolved with SIMPLISMA. There is a good agreement between the band positions at 538, 580, 637 and 733 nm decomposed with Grams and the absorption maxima of component 2 of SIMPLISMA. Correspondence also exist between the intensity contributions of the sum of the three bands in figure 3 and component 2 in figure 5.
629 Table 4 Band positions and widths of the Gaussian bands in which the CoA spectra are decomposed and positions of the absorption maxima of the pure spectra obtained with SIMPLISMA. Gaussian bands Gaussian bands component 1 component 2 (SIMPLISMA) (SIMPLISMA) L/nm width L/nm L/nm L/nm 512 505 + 3 40 + 4 546 538 + 3 38 + 3 582 580 + 3 46 + 6 616 637 + 3 70 635 666 692 + 3 551+ 6 733 + 3 20 738
5. DISCUSSION
The three spectral analysis techniques (decomposition in Gaussian bands, PCA and SIMPLISMA) point together to the presence of two Co 2+ species in CoA dehydrated at 400°C. One of the components (component 2, figure 4B) closely matches the experimental spectra both in band position and in intensity course. For proper use of SIMPLISMA the following must be considered: (1) the components must be pure, (2) the different components may not be correlated and (3) the law of Lambert-Beer must be valid, which means that spectra in the non-linear absorption regime can't be used for the analysis. Klier proposed a single nearly trigonal symmetry of Co 2+ in CoA, calcined at 350°C, where Co 2+ is coordinated to three framework oxygens almost in the plane of the six-ring. 9 Heilbron and Vickerman suggested the existence of a pseudo-tetrahedral Co(Ox)3 O2 or Co(Ox)3OH- species (Ox=lattice oxygen) after dehydration at 400°C and the development of trigonal CoO3 at higher temperatures, l° From our chemometrical methods a co-existence of two coordinations after dehydration at 400°C is most probable. The bonding of cations with non-lattice oxygens is common for polyvalent cations such as C 2+, La 3+ and Ce 3+ in X- and Y-type zeolites. 1113 These two components are also present in Co2+-exchanged faujasite-type X- and Y-zeolites, as was found in a recent study. 6 The question is how to assign the coordination types to real sites in zeolite A. There are three six-ring sites in zeolite A: in the cubo-octahedron, in the plane of the six-ring and in the supercage. 14'15 When located in the plane of the six-ring, the coordination is trigonal. In the two other cases the coordination is pseudo-tetrahedral if a fourth extra-lattice ligand is present. We suggest that the pure spectrum of the first component with two absorptions in the 610-680 region and one at 525 nm (figure 4A) corresponds to pseudo-tetrahedral symmetry and that the pure spectrum of the second component with three bands at 635,582 and 546 nm and with a small band at 730 nm (figure 4B) corresponds to trigonal symmetry. Pseudotetrahedral cobalt is thus assumed to make up part of the coordination when CoA is fully dehydrated, but the component which matches best the experimental spectrum is trigonal. For the exact interpretation of the pure spectra calculations of theoretical spectra of the coordination of Co 2+ at the six-rings are in progress.
630 6. CONCLUSIONS A combination of band decomposition, PCA and SIMPLISMA applied on DRS spectra, has proved to be useful for the determination of Co 2+ coordinations in zeolite A. After dehydration at 400°C two components have been identified, which were assigned to trigonal and pseudo-tetrahedral symmetry of cobalt at the six-ring sites of zeolite A. Both components are common to two of the three earlier defined components in faujasite-type X-and Y-zeolites. Future work will be directed towards an extension of the chemometric techniques which can aid in the spectroscopic investigation of zeolites and of heterogeneous catalysts in general. A.A.V. acknowledges a grant of the I.W.T. (Belgium) and B.M.W. a grant as research assistant of the National Fund for Scientific Research of Belgium (N.F.W.O.). This work was financially supported by the Geconcerteerde Onderzoeksactie (GOA) of the Flemish Government and by the Fonds voor Kollectief Fundamenteel Onderzoek (FKFO) under grant no. 2.0050.93.
REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
S. Wold, Chemometrics and Intelligent Laboratory Systems, 2 (1987) 37. E.V. Thomas, D.M. Haaland, Anal. Chem., 62 (1990) 1091. F. Cuesta Shnchez, D.L. Massart, Analytica Chimica Acta, 298 (1994) 331. W. Windig, C.E. Heckler, F.A. Agblevor, R.J. Evans, Chemom. Intell. Lab. Syst., 14 (1992) 195. W. Windig, S. Markel, J. of Molecular Structure, 292 (1993) 161. A.A. Verberckmoes, B.M. Weckhuysen, J.A. Pelgrims, R.A. Schoonheydt, J. Phys. Chem., 99 (1995) 15222. B.M. Weckhuysen, A.A. Verberckmoes, A.R. De Baets, R.A. Schoonheydt, submitted to J. Catal.. E.R. Malinowski, J. ofChemometrics, 1 (1987) 33. K. Klier, R. Kellerman, P.J. Hutta, J. Chem. Phys., 61 (1974) 4224. M.A. Heilbron, J.C. Vickerman, J. Catal., 33(1974) 434. M.L. Costenoble, W.J. Mortier, J.B. Uytterhoeven, J. Chem. Soc., Faraday Trans. 1, 74 (1978) 466. P.P. Lai, L.V.C. Rees, J. Chem. Soc., Faraday Trans. 1, 72 (1976) 1809. P.P. Lai, L.V.C. Rees, J. Chem. Soc., Faraday Trans. 1, 72 (1976) 1827. R.A. Schoonheydt, Catal. Rev., Sci. Eng., 35 (1993) 129. W.J. Mortier, Compilation of Extra-Framework Sites in Zeolites, Butterworths, Guildford, 1982.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
631
R a m a n characterization of the selenium species formed inside the confined spaces of zeolites V.V.Poborchii Ioffe Physico-Technical Institute, St.Petersburg 194021, Russia Institute for Materials Research, Tohoku University, Sendai 980-77, Japan Five types of zeolites containing adsorbed Se species have been studied by Raman scattering. A strong influence of the confinement geometry of the zeolite pores on the structure of the Se species have been observed. New cyclic molecules 5e12 have been found to be stabilized in the large cavities of the zeolite A. Six-membered cyclic Se molecules have been observed in the chabazite and mordenite. In the mordenite, these molecules have been found to coexist with the helical Se chains. Amorphous-like array of the irregular Se chains has been found in the zeolite X. Linear chain of the interacting Se22 anions has been found in the cancrinite channels, chain properties being influenced significantly by the one-dimensional incommensurability between the chain and the cancrinite matrix. 1. INTRODUCTION Zeolites possessing regular system of cavities or channels with diameters -1 nm are very attractive for preparation of arrays of microclusters in the cavities or 1-dimensional chains in the channels. Selenium can be easily injected into the zeolites by adsorption, and some kinds of species can be stabilized in zeolites. Se species confined in the zeolite pores have been studied during almost 20 years. However, this subject is rather complicated, and many questions about the structure and properties of the zeolite-confined selenium are under discussion till now. Bonding of atoms in neutral Se species is organized by orbitals hybridized from s and p atomic orbitals. Each Se atom has two nearest neighbors distanced at 0.23-0.24 nm, the bond angle being 101-106 °. Most stable species of bulk selenium are helical chains in trigonal selenium and Ses molecules in monoclinic selenium [1] . The dihedral angle in selenium structures is quite flexible and its value and sign can be varied in different species. It is helpful to consider chain-like fragments (no change of the sign of the dihedral angle) and ring-like fragments (with change of the sign of the dihedral angle) (fig. 1). In the confined spaces of zeolites, we can expect stabilization of a variety of combinations of these fragments, because the sign of the dihedral angle can easily be changed according to the topology of the zeolite framework. The main purpose of the present work is to determine the structures of Se species confined in the pores of a variety of zeolites, namely: chabazite (Ch), mordenite (M), A, X, and cancrinite (C) using Raman spectroscopy. Why so many different zeolites have been examined? The first reason for this is a possibility to demonstrate the influence of the confinement geometry and of the interaction with the zeolite framework on the structure of the stabilized Se species. The second reason isa possibility to identify some species confined in different zeolites by means of comparison of their Raman spectra (RS).
632 In this work, single crystals and powder samples of zeolites containing adsorbed selenium (A-Se, X-Se, Ch-Se, M-Se, C-Se) have been studied. Structural models for zeolite-confined Se are proposed. This work is a systematic study, which includes critical analysis of some published data (Ch-Se [2] ; A-Se [3-5]; X-Se [3-5]; M-Se [4,6-8]; C-Se [4,7,9,10]). This is a first attempt to combine Raman data of many different zeolites in one paper to deduce logical scheme of material design (selenium is only suitable example) inside zeolite pores with different framework topology, the spectra in this work being presented with some new details compared to the published ones, new interpretations being given. 2. EXPERIMENTAL Synthetic zeolites A (Na12A1125i12048), X (Na92A192Si1000384),and hydrocancrinite and natural chabazite (Caa.6Na0.4A13.6Sis.5021.6) and mordenite (Ca2Na4Si40A18096) have been used. The sizes of crystals were from 30- 50 ~tm (zeolite A) to 2-3 mm (cancrinite along c-axis). Chalcogens have been adsorbed into the zeolites at the temperatures 450 - 550 ° C during several days after the hydration of zeolites. It is not so easy to make Raman measurements of microcrystals such as mordenite using only macrooptical Raman devices. In this work, RS have been studied using the microoptical equipment as well as the traditional macrooptical technique. Usage of a microoptical Raman device consisting of a microscope optically connected with a double or triple monochromator, allowed to find easily microcrystals, to choose high quality crystals or high quality part of a crystal, and using microobjective, to collect effectively the light scattered from the few microns area excited by laser microprobe. Triple Dilor-Z, and double DFS-24 monochromators have been used. 647.1 nm line of the Kr-laser and 514.5 nm line of the Ar-laser have been used for the excitation of RS. The laser light probe power was 1-20 mW and the size was 10-30 l.tm. It is well known that RS of zeolite matrices, which are excited by the visible light, are much weaker than the spectra of the adsorbed chalcogens. The bands of the zeolite vibrations have been weaker than the noise level in RS of all the samples examined. (NasSi6A16024(OH)2)
3. RESULTS AND DISCUSSION. 3.1 IRREGULAR Se CHAINS IN THE ZEOLITE X. The spectrum of X-Se displays a broad band with the maximum at 258 cm -1 in the bondstretching mode region and very weak features in the bond-bending mode region. The spectrum of X-Se is similar to that of amorphous Se (a-Se) [ 11 ] (fig.2). This means that the Se species, which are stabilized in the zeolite X, are similar to the Se species in a-Se, namely,. irregular chains consisting of combinations of the ring-like and chain-like fragments and some portion of the ring molecules. This is not surprising, because the large cavities of the zeolite X (diameter-- 1.3 nm) are associated through the wide windows with diameters -~0.7 nm. Within this confinement geometry, Se atoms can easily construct quite long chains, which penetrate from one large cavity to another one through the windows. Thus, adsorbed species can be associated into the 3-dimensional continuum similar to the bulk amorphous solid. The feature at-~330 cm "1 can be assigned to the charged selenium molecules, probably to Se2, which can be located not only in large cavities, but also in sodalite cages. RS of low-loaded X-Se displays the 330 cm -1 band intensity comparable with the intensity of the 258 cm 1 band.
633
Fig.1. Selenium chain-like (a) and ring-like (b) fragments.
o~-Se8 __/
a-Se
/
L /
t
/I
t
/ \
/
25 28
I !
/
X-Se
!
330 "t""
/
J
1
I
25 I I
, I
A-Se
55
'
88 L
:
175fl
f
i
_._J ,
3OO
200
i
100
RAMAN SHIFT, cm-1 Fig.2.Structural fragments and Raman spectra of o¢-monoclinic selenium (o~-Se8) [11], amorphous selenium (a-Se) [11], X-Se and A-Se (k0=514.5nm).
634 3.2 STABILIZATION OF THE Sel2 RINGS IN THE LARGE CAVITIES OF THE ZEOLITE A. In contrast with the zeolite X, large cavities of the zeolite A (diameter - 1.14 nm) are connected through the narrow windows with the diameter- 0.42 nm. This is a good condition for the stabilization of separate clusters in the cavities. RS of A-Se (fig.2) displays specific bands, which are not similar to the spectra of known Se species. (Some versions of RS of A-Se are also presented in ref. [3-5]. The relative intensities of the bands seem to be more reliable in the spectrum presented in fig.2) The spectrum doesn't depend on the concentration of selenium. It is not so difficult to show that the Se species responsible for the spectrum differ from the Ses molecules, which are usually considered as species stabilized in the zeolite A (see review [12]). The Ses molecules should display a strong band associated with the symmetric bond-bending mode. This band is clearly seen at 112 cm 1 in the spectrum of the a-monoclinic Se [ 11 ] (fig.2), consisting of the Ses molecules. However, this band is absent in the spectrum of A-Se. On the other hand, one can find strong bands in the spectrum of A-Se at lower frequencies (fig.2). RS of A-Se can be explained, if we consider ring clusters larger than Ses. Let us consider the molecule Sel2 (fig.2, A-Se) with the structure similar to that of the cyclo-dodecasulfur S12 [13]. This molecule consists of alternating ring-like and chain-like fragments and possesses D3a symmetry. Six Se atoms in this molecule occupy positions in the middle plane (black circles in fig.2, A-Se), and six others occupy three positions in the plane higher (white circles) and three positions in the plane lower (gray circles) than the middle plane. The structure of the Sel2 molecule is compatible with the structure of the large cavity of the zeolite A. In fact, six Se atoms from the middle plane can occupy the positions near 4membered rings of (si,ml)O4 tetrahedra (fig.2, A-Se), the molecule being oriented by the threefold axis along the threefold axis of the zeolite. Probably, the orientations of the rings in the neighboring cavities are not correlated. We can estimate roughly the expected frequency of the symmetric bond-bending mode of the molecule Se12 v(Se12) using data for the frequencies of symmetric bond-bending modes of v(Ses)=112 cm "1 [1], v(512)=128 cm1 [14], v(Ss)=218 cm 1 [14]. v(Se12)-v(S12)xv(Ses)/v(Ss) = 128xl 12/218 -~ 65 cm ~. This value is quite close to the frequency 55 cm ~ of a strong band in RS of A-Se, and so we can attribute this band to the symmetric bond-bending mode of 5e12. A strong band at 28 cm "1 can be attributed to the libration of the Se~2 ring in the cavity. More careful calculations of the frequencies and Raman intensities of the Se~2 molecule vibrations [5] show reasonable agreement with RS of A-Se, calculated frequency of the symmetric bond-bending mode being equal to the experimental one 55 cm ~. The assignment of A-Se Raman bands to the vibrations of Se~2 molecule is supported by the A-Se loading density data. The loading density-10.5 Se atoms per cavity is determined in the work [5]. This value corresponds well to the expected value 12 atoms per cavity. 3.3 Se6RING MOLECULES IN THE CHABAZITE CAVITIES. One of the most important problems in the studying of zeolite-confined Se species is to find simple system among zeolites with selenium, which can be used as a base for characterization of more complex systems. The chabazite is a good candidate for the preparation of some kind of simple Se clusters in its cavities. The chabazite cavity sizes
635 (0.67nm x 0.67nm xl.0nm) (fig.3) are too small for Ses ring molecules (Ses diameter is -0.75nm). We can expect stabilization of smaller ring molecule Se6 (fig.3). The symmetry of the Se6 molecule D3d and its size (diameter -- 0.65nm) are compatible with the symmetry and size of the chabazite cavity. According to RS of rhombohedral selenium consisting of Se6 molecules [15], there are four Raman-active internal modes of the Se6 molecule with the frequencies 102 cm l (E e - bond-bending), 129 cm l (Ale- bond-bending), 221 cm 1 (E e - bondstretching), 247 cm ~ (Ale- bond-stretching). RS of Ch-Se (fig.3) displays bands correlating with the Se6 modes. Obviously, the band at 104 cm"1 should be attributed to the E e bondbending mode, the band at 135 cm "l to the Ale bond-bending mode, the band at--220 cm ~ to the E e stretching mode. It should be noted that the band at --220 cm -~ is clearly seen in contrast with the spectrum presented in ref. 2. It is due to the change of the excitation wavelength from 647.1 nm to 514.5 nm and to more sensitive detection. The band at 274 cm "l should be attributed to the Ale bond-stretching mode, but the spectral position of this band differs significantly from the position of the corresponding band in RS of the rhombohedral selenium 247 cm "~. The reason for this is a strong interaction of the Se6 molecules in the rhombohedral selenium, which influences the internal bond strength. If we extrapolate data on the dependence of the frequency of the Ale bond-stretching mode of the rhombohedral selenium on the intermolecular interaction [ 16], we can expect that the frequency of the Ale stretching mode of a separate Se6 molecule should be higher than that of the rhombohedral selenium, the frequency 274 cm ~ being quite reasonable. The band at 135 cm l displays a shoulder at 145 cm "l. It can be attributed to the combination of the Ale bending mode with the libration of the molecule in the cavity or to the forbidden in RS A2u bond-bending mode which become active due to distortion of the molecule. 3.4 HELICAL Se CHAINS AND Se6 RINGS INSIDE MORDENITE CHANNELS. Mordenite channels (elliptic cross section 0.67nm x 0.7nm) formed by 12-membered tings of (Si, AI)O4 tetrahedra are attractive for the preparation of 1-dimensional structures. One can expect that Se atoms form single chains inside channels. In many works, arguments for the stabilization of single Se chains inside mordenite channels have been found. However, the structure of the chains is unclear. Moreover, in all the previous studies (see review [ 12] ) all the observed phenomena had been considered with the assumption that the mordenite-confined selenium forms only the chains. In this section, another point of view is proposed and experimental evidence is given for stabilization of two types of species in the mordenite channels, namely helical Se chains and Se6 ring molecules. If we consider polarized RS of M-Se (fig.3), we can distinguish two types of the bands. We attribute the bands active only for cc-polarization (256 cm l and the low frequency broad band) to the first type and other bands (104, 135, -0220, 274 cm "1) to the second type. The first type bands should be attributed to the Se chain, high Raman activity of the bands in the ccpolarization being associated with the resonant enhancement due to the absorption of the Se chain for the light polarized parallel to the chain. Obviously, the 256 cm l band should be attributed to the symmetric bond-stretching mode, and the low-frequency broad band centered at-- 40 cm "1 to the acoustic-like mode active due to the finite chain length (according to our calculations this band corresponds to a set of 10-20-atomic Se chains). The structure of the chains is probably close to that of trigonal one. In the works [4,7] another chain structure has been proposed to explain anisotropy of RS in the a-b-plane, but as it is shown below and in ref.
636
I
'l
104
o
~hain.
If,~~I eh~ a~
7-
M-Se
tb t
=18.13
~
300 b=20.29
--~
~/
200
c=7.50
100
RAMAN SHIFT, cm'l
Fig.3. Structural fragments and Raman spectra of Ch-Se (2,0=514.5nm) and M-Se (~0=647.1nm); "aa", "bb", and "cc" indicate polarizations of the incident and scattered light beams in respect to the mordenite axes./~ l
220K C-Se
100K y
b q a=12.67 c=5.165
, 300
, 200
_ 1()0
RAMAN SHIFT, cm-1 Fig.4. Structural fragments and Raman spectra of C-Se (~0=514.5nm) for the polarizations of incident and scattered light beams parallel to the c-axis of the cancrinite. Linear dimerized Se chain and periodic (period is equal to c/2) potential of the cancrinite in the center of channel are schematically shown.
637 [8], the a-b-anisotropy of RS is associated with another kind of Se species in the mordenite channels. It is clear that the second type bands coincide with the bands of Ch-Se. Obviously, the mordenite channel contains the same molecules as the chabazite cavity contains, namely Se6. The polarization dependence of RS of M-Se corresponds to the Se6 molecule oriented by the threefold axis along the b-axis of the mordenite crystal. In fact, the bands at 104 cm 1 and 135 cm ~, which can be assigned to the Eg and A~g bond-bending modes, should be less active, when the incident and scattered light beams are polarized parallel to the threefold axis of the molecule, Eg mode being forbidden in this geometry. Alg symmetric bond-bending mode is not forbidden, but it is much more active for the polarizations of the incident and scattered light beams parallel each other and perpendicular to the threefold axis of the molecule (our calculations show that the Raman activity of the Axg bond-bending mode is negligible for this polarization). To summarize two types of Se species, namely helical chains and Se6 ring molecules are stabilized in the mordenite channels, the rings being oriented by the threefold axis along the baxis of the mordenite. It is interesting to note that the intensity ratio of the chain Raman bands to the Se6 bands is almost the same for. different M-Se samples prepared under different conditions. Probably, it means that some kind of regular arrangement of chains and Se6 rings occurs.
3.5 LINEAR CHAIN OF INTERACTING Se22" ANIONS IN THE CANCRINITE CHANNELS AND INCOMMENSURABILITY BETWEEN THE CHAIN AND THE CANCRINITE MATRIX. Cancrinite channels as well as mordenite ones are formed by the 12-membered rings of (Si,A1)O4 tetrahedra. However, free space of the cancrinite channel is smaller than that of mordenite, because there are 2 Na ÷and 2 OH per unit cell in the cancrinite channel [9]. RS of C-Se (fig.4) at the temperatures 50-400 K displays dominant band at ---246 cm 1 (this value is determined at 300 K) which practically doesn't change when the temperature changes, but the spectrum at lower frequencies changes significantly. (RS of C-Se for smaller temperature interval are also presented in ref. [4,7,9].) According to the x-rays diffraction data [9,10] Se atoms occupy positions in the center of the channel (fig.4) and display wide distribution along the c-axis of the cancrinite. All these data can be explained, if we suppose existence of the interacting selenium dimers in the channels, linear dimerized chain of Se atoms being formed. In fact, the 246 cm 1 band can be attributed to the internal dimer mode. The wide distribution and the temperature dependence of RS can be explained, if we suppose incommensurability between the chain and the cancrinite matrix along the c-axis. A misfit between the lattice parameter of the chain and that of the cancrinite depends on the temperature, and so the arrangement of dimers in the chain depends on the temperature also. Our examination of the C-Se x-rays photoelectron spectra (ESCA) shows that Se22 anions are stabilized in the cancrinite. The frequency 246 cm "1 is quite reasonable for the vibration of Se22". During adsorption, probably, 2OH" are substituted by one Se22 in the channel. In this case, interaction between Se22 and closely connected with the cancrinite framework 2Na ÷ should be quite strong. This interaction determines coupling between the chain and the incommensurate cancrinite matrix. Such a system can be basically described as a
638 1-dimensional incommensurate system. Molecular dynamics simulation of the temperature dependence of the structure and RS of linear dimerized chain under the action of incommensurate periodic potential [ 10,17] show qualitative agreement with the experimental data. 4. CONCLUSION A variety of structures from Se atoms have been experimentally designed in free spaces of different zeolites and corresponding RS have been studied. The structural models for zeoliteconfined Se species are proposed. Most important conditions, which influence the structure of the Se species, are the topology of the zeolite framework and the interaction with the host zeolite lattice, incommensurability being important. Se species, which are unstable in other conditions, have been found to be stable inside zeolite pores, giving rise to new types of the zeolite-based solids. Acknowledgments. The author is grateful to V.N.Bogomolov for supplying mordenite and cancrinite, to V.P.Petranovskii, S.G.Romanov, and Y.A.Barnakov for the sample preparation, to A.V.Shchukarev for the ESCA of C-Se and to the International Science Foundation (grant R4P300) for the partial support of the work. REFERENCES 1. R.M.Martin, G.Lucovsky, K.Helliwell, Phys.Rev.B 13 (1976) 1383. 2. Yu.A.Barnakov, V.V.Poborchii, A.V.Shchukarev, Phys. Solid State 37 (1995) 847. 3. V.N.Bogomolov, V.V.Poborchii, S.V.Kholodkevich, JETP Lett. 42 (1985) 517. 4. V.V.Poborchii, Proc. of the 1-st Japanese-Russian Meeting "Material Design Using Zeolite Space", Kiryu, Japan, 1991, p. 1. 5. V.V.Poborchii, M.S.Ivanova, V.P.Petranovskii, Yu.A.Bamakov, A.Kasuya, Y.Nishina, Materials Science & Engeneering A, in press. 6. V.N.Bogomolov, V.V.Poborchii, S.G.Romanov, S.I.Shagin, J.Phys. C: Solid State Phys. 18 (1985) L313. 7. V.V.Poborchii, J.Phys.Chem.Sol. 55 (1994) 737. 8. V.V.Poborchii, Chem.Phys.Lett., in press. 9. V.N.Bogomolov, A.N.Efimov, M.S.Ivanova, V.V.Poborchii, S.G.Romanov, Yu.I.Smolin, Yu.F.Shepelev, Sov.Phys.Solid State 34 (1992) 916. 10.Yu.A.Barnakov, A.A, Voronina, A.N.Efimov, V.V.Poborchii, M.Sato, Inorganic Materials, 31 (1995) 748. 11. A.Mooradian, G.B.Wright, "The Physics of Selenium and Tellurium" (Pergamon, London, 1969), p.269. 12. G.D.Stucky, J.E.MacDougall, Science 247 (1990) 669. 13. A.Kutoglu, E.Hellner, Angew. Chem. 78 (1966) 1021. 14. R. Steudel, Spectrochimica Acta, 31A (1975) 1065. 15. K.Nagata, K.Ishibashi, Y.Miyamoto, Jap.J.Appl.Phys., 20 (1981) 463. 16. K.Nagata, K.Ishibashi, Y.Miyamoto, Jap.J.Appl.Phys., 22 (1983) 1129. 17. V.V.Poborchii, A.N.Efimov, to be published.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
639
D e t e r m i n a t i o n of basic site location and strength in alkali e x c h a n g e d zeolites. D.Murphy, P.Massiani*, R.Franck and D.Barthomeuf Laboratoire de Rdactivitd de Surface, URA 1106 CNRS, Universitd Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France ABSTRACT IR studies using adsorbed pyrrole were used to characterise the basicity of alkali exchanged X, Y and EMT. Specific IR bands due to pyrrole adsorbed on the basic framework oxygens adjacent to alkali cations at sites I', I and II were identified. The relative strengths and various locations of the basic sites were determined for the first time. The inherent heterogeneity of basic site strengths is shown to vary significantly from site to site for a given cation. Within the series NaEMT, NaY and NaX the relative strength of the basic sites increases for the framework oxygens associated with Na + cations from sites I' to I to II. 1. I N T R O D U C T I O N The basicity of alkali exchanged zeolites has received growing attention of late because of the intrinsic catalytic properties of these materials [1,2]. Recent studies on basic zeolites have focussed on the characterisations of the basic sites themselves in order to obtain more accurate information on the strength and density of these sites [3-6]. Although various techniques have been used to study basicity in zeolites, on an experimental basis no single technique is currently able to locate exactly which oxygens are basic in the framework [2]. Adsorption of IR spectroscopic probes is now widely used to characterise basicity on oxides and zeolites. Pyrrole (C4H4NH) has in particular been fruitfully used for more than a decade as an acid probe ofbasicity in zeolites [3-6, 8-11]. The H-donor pyrrole molecule interacts with the basic framework 0 2- sites (Lewis bases) and the vNH stretching frequency of the NH .... O hydrogen bridge is then used as a measure of the overall basic strength in the zeolite [2-4]. In all these previous IR studies of adsorbed pyrrole, the Vmax of the broad vNH stretching band was taken as the measure of the overall basicity in the zeolite. The idea t h a t pyrrole may be used to probe not only the overall basicity in a
640 zeolite but also the localised basicity was first suggested in the results of Scokart and Rouxhet [4]. This idea was also developed by Kaliaguine based on FTIR, XPS and microcalorimetric studies of the adsorbed pyrrole in different alkali exchanged zeolites [5,7,9]. It was concluded that the basic sites in alkali exchanged faujasites are the framework oxygens adjacent to the alkali cations, and the basicity is determined mainly by the local environment [5,9]. A recent publication by our group using pyrrole confirmed that the basicity in alkali EMT (a hexagonal faujasite) is indeed determined by the local environment and that the basic framework oxygens are those adjacent to the alkali cations at specific extraframework sites (i.e., sites I, r and II) [11]. In other words the pyrrole was employed to probe not only the local basicity but furthermore it permitted the identification of basic sites with different strengths and their specific locations in the zeolite framework. The aim of the present contribution is to extend our previous work on alkali EMT to the detailed characterisation of basicity in faujasites. It will be shown that by deconvolution of the IR spectra of adsorbed pyrrole and comparison with known cation populations (assuming no cation migration), both the locations and relative strengths of the individual basic sites may be determined. 2. E X P E R I M E N T A L
NaX and NaY zeolites were supplied by Union Carbide and NaEMT by Elf Co. Alkali exchanged Y zeolites were prepared from NaY by liquid exchange with a cationic chloride solution as described elsewhere [12]. Unit cell compositions were determined by atomic absorption spectroscopy (Table 1). The zeolites were pressed into self supported wafers for IR analysis. Dehydration of the zeolites was performed as follows: slowly heated (rate=lK min" 1) under a flow of dry air to 773K, followed by evacuation (pressure=10 "3 Pa) for 15 hours at the same temperature. Pyrrole (supplied by Aldrich) was stored over molecular sieves and distilled under vacuum before use. Admission of a known amount (0.7 moYsupercage (s.c.) for Y and 0.62 mol/s.c, for X zeolite) of the probe onto each sample was performed as described previously [11]. The FTIR spectra were recorded on a Bruker IFS 66V spectrometer with a spectral resolution of 2 cm "1. Deconvolution of the spectra were performed using a standard Bruker Opus/IR soi~ware program.
641 TABLE 1. Chemical composition of the zeolitesamples % EXCHANGED CATION
SAMPLE
FORMULA
LiY
Li37Na18(A102)55(SiO2) 137
NaY
Na56(A102)56(SiO2) 136
100% Na +
KY
K54Na2(A102)56(SiO2) 136
96%
K+
RbY
Rb45Na9(A102)54(SiO2) 138
83%
Rb +
CsY
Cs45Na9(A102)54(SiO2)138
83%
Cs +
NaEMT
Na20(A102)20(Si02)76
100% Na +
NaX
Nas6(A102)86(Si02)106
100% Na +
67%
Li +
3. R E S U L T S Figure 1(a-e) shows the IR spectra in the 3600-2750 cm -1 region of pyrrole adsorbed on dehydrated Li, Na, K, Rb and CsY respectively. The broad band located between 3450 and 3200 cm -1 is generally assigned to the vNH stretching frequency of the NH .... O hydrogen bridge of chemisorbed pyrrole interacting with a basic site [3,5,10]. The complete IR spectrum of pyrrole adsorbed on basic zeolites is quite complex and has been reported elsewhere [3-6,10,11]. The complex series of narrow bands in the low wavenumber region have been recently discussed in detail [10]. Figure 2 shows the analogous spectrum of pyrrole adsorbed on dehydrated NaX. It can be clearly seen from these figures that the vNH band profile changes dramatically from the LiY to CsY series and to NaX, evidencing an inherent complexity in the strength and heterogeneity of the basic sites. In order to better understand this complexity and to identify the various intrinsic components of the broad vNH band, a band simulation program was used to deconvolute the experimental spectra in Figures 1 and 2. The upper trace in each figure represents the experimental spectrum while the lower trace represents the computer fitted spectrum. The accuracy of the fit was determined from the RMS error which was less than 0.0005 in each case. The individual vNH component bands are plotted in Figures 1 and 2 (the narrow combination bands are not shown). For all samples the position (a) and relative integrated intensities (b) of each component band are listed in Table 2 together with the available percentage cation distributions at specific extraframework sites as reported in the literature. The analogous IR spectra and deconvoluted bands for NaEMT have been presented elsewhere [11], so that only the resulting data are listed in Table 2 for completeness.
642
,
1
i .....
a
tq
IM
d
u~d
d
3500
:~50
3000
IJavenumber ca -I
2750
3500 I
1
3250 " 3000 gavenumber cm"l
I
2250
C
d
.e.,i
..ac~
d
. . . . . .
1
"
_
3500
3250
3 )oo
Javenumber ~'
,1
d~
.~,~
275o !
.,,.4
:S
d
t5
\
_
d 3500
3250 3000 14aventeber ol -I
2750
3500
3250 3000 l~avenumber cm-1
2750
Figure 1. Experimental and fitted IR spectra of pyrrole adsorbed on dehydrated (a) LiY, (b) NaY, (c) KY, (d) RbY and (e) CsY showing the vNH component bands.
643 T A B L E 2. (a) Wavenumbers of deconvoluted IR bands (cm -1) assigned to basic sites adjacent to alkali cations in inner or supercages sites and (b) relative i n t e g r a t e d i n t e n s i t y of the deconvoluted band together with the (reported
percentage alkali cation distributions per unit cell from the literature). SAMPLE
I n n e r sites
S u p e r c a g e sites
Ref.
I! I H 3432 3415 3293 (b) 7,9 % (7.9%) (13) 48.0% (45.0%) 23, !% (23.2%) NaX? .... (a) 3380 3319 3269 (b) 30,1% (31.3%) 5,2% (3.2%) 35,4% (33.4%) (14) LiY (a) 3445 3403* 3359* 3325 3228 (b) 45.6% (57.9%) 24.3% 10.4% 12.9% (12.8%)6.8% (15) NaY (a) 3405 3356 3273 (b) 29.2% (28.3%) 9.3% (5.7%) 62.5% (66.0%) (16) KY (a) 3383 3303 3246 36.1% (36.0%) 12.3% (10. 7%) (b) (17) 51.6% (53.3%) RbY (a) 3403* 3354* 3283 3180 5,1% 43.5% 40.8% (b) 10,5% CsY (a) 3399* 3359* 3280 3173 2,0% 47.3% 37.4% (b) 13,3% * Component bands associated with unexchanged Na + cations, t R e m a i n i n g cations located at other specified framework sites.
N a E M T t (a)
4. D I S C U S S I O N 4.1 C h a r a c t e r i s a t i o n of basic sites in alkali Y zeolites The bathochromic shift in the vNH stretching vibration of adsorbed pyrrole is used to monitor the oxygen framework basicity in zeolites [3-5]. Component bands with different vNH frequencies should then suggest the presence of various basic sites with different relative strengths. In alkali exchanged EMT, a heterogeneous distribution of basic sites was observed and related to the localised nature of the basicity [11]. Since the charge on the framework oxygen will depend not only on the SiOA1 angle and T-O distance, but also on the M+-O distance (where M + is the alkali cation), the basicity of the oxygens adjacent to these exchanged cations will vary from site to site as the M +O distance varies. In EMT, each individual component band of the deconvoluted vNH band was assigned to the basic framework oxygens adjacent to an alkali cations at sites r, I and II. This assignment was proposed based on the similarities between the relative integrated intensities of the different component bands and the known distribution and population of cations per unit cell [11]. A similar interpretation of the component bands in Figure 1 can be made for the alkali Y zeolites. The percentage distribution of Na + cations per unit cell of dehydrated NaY are 28.3, 5.7 and 66.0% Na + at sites I', I and II respectively, as
644
1
_,_1
. . . .
I
_
vNH/cm-I 3450 r
|
~ S i t e
32503300~ 0,32
I
[ _
0,37
0,42
Negative Oxygen Charge
0
dJ
3500
~ 3250 ~
°'~
2750
@NaEMT
~NaY
@NaX
F i g u r e & Relationship of oxygen F i g u r e 2. Experimental and fitted IR dmrge and vNH frequency for pyrrole ads. at basic sites in EMT, Y and X. spech-a of pyn~le on dehydrated NaX. determined by 23Na NMR [16] in the absence of adsorbate (Table 2). The relative integrated intensities of the three IR component bands at 3405, 3356 and 3273 cm-1 were 29.2, 9.3 and 62.5% respectively. Based on the similarities with the above percentage Na + distribution, the three IR bands can be assigned to the basic framework oxygens adjacent to Na÷ cations a t sites I', I and II. In addition it is well known that the cations of sites I and F are connect~ to oxygens 0(3) while the II cations are linked to 0(2) oxygens. Since adjacent I' and I sites are not occupied simultaneously, this menn~ that three different types of"potential basic" oxygens e ~ in NaY, in agreement with the above pyrrole results. Confirmation of the above assignments for basic sites in NaY can be obtained from the LiY, RbY and CsY deeonvoluted spectra. In these three samples unexdmnged Ha + cations (Table 1) are present which should be in inner cavities (I and/or I') since the supercage II cations are more easily exchanged. Therefore evidence of basic sites associated with inner cavity Na + cations should be apparent in these IR spectra. Component bands are indeed visible at 3403 and 3359 cm-1 in LiY, RbY and CsY which were also visible in the NaY spectnnn and assigned to the basic sites associated with the Na + cations at sites I' and I. Moreover chemical analysis reveals that 17% of the Na + cations remain unexchanged in RbY and CsY while 33% remain unexchanged in LiY. In agreement with these values the relative intensities of the Na + related bands (3403 and 3359 cm-1) in RbY, CsY and LiY were 15.6, 15.3 and 34.7% respectively. Based on the comparison between the percentage integrated intensities of t h e remaining deconvoluted bands in LiY and the reported % Li+ cation populations, the various component bands in LiY can be identified as the basic
645 sites adjacent to the Li + cations at sites r and II (Table 2). A minor band visible at 3465 cm -1 in LiY (and also observed in NaY) can be assigned to pyrrole adsorbed on a Lewis acid site as discussed previously [11]. The two r e m a i n i n g bands in RbY and CsY may be assigned to framework oxygens associated with Rb + and Cs + cations in the supercages. It was recently proposed t h a t some Cs + cations may occupy inner cavities (I and I') [18] and the consequences of this on our present findings is currently under investigation. In KY the a g r e e m e n t between component band intensities and K + cation distribution is quite good (Table 2), so t h a t the deconvoulted bands in Figure lc may be identified as the framework oxygens associated with K + at sites r, I and II.
4.2. D e p e n d e n c e of basic strength on alkali cations at specific sites. It is well known t h a t the overall basicity of the framework oxygens increase with an increase in the electropositivity of the countercation. This dependence also occurs at localised sites as evidenced from the present results. In Y zeolite all the exchanged alkali cations occupy site II. (In RbY and CsY the majority of the Rb + and Cs + cations also occupy supercage sites II). The vNH frequency for the site II related component bands (where two "supercage" bands are observed a weighed average of the two was taken) shifts to lower cm -1 from LiY (3299 cm -1) to NaY (3273 cm -1) to KY (3246 cm -1) to RbY (3233 cm -1) to CsY (3230 cm-1). In other words for the same basic site adjacent to the alkali cation at site II, the relative basic strength increases as the alkali cation is exchanged from Li + to Cs +. 4.3. Comparison of localised basic sites in NaEMT, NaY and NaX. The location of the basic sites in N a E M T [11] and NaY (section 4.1) have been identified. The same comparative procedure when applied to NaX containing adsorbed pyrrole, again assuming no cation migration, also enables the identification of the basic site locations in this zeolite (Figure 2 and Table 2). N a E M T is a hexagonal faujasite and the "types" of cations sites in this zeolite are the same as those in the NaY and NaX faujasite structures (i.e., the r, I and II sites are structurally equivalent in all three zeolites). The basic sites adjacent to the Na + cations at these specific sites I', I and II have been identified in all three zeolites. The values of negative oxygen charge calculated from the Sanderson electronegativity equalization principle [19] for NaEMT, NaY and NaX are plotted in Figure 3 as a function of the vNH frequency of pyrrole adsorbed on basic oxygens adjacent to site r, I and II Na + cations. The basicity in alkali exchanged zeolites is well known to depend on the alkali cation present and the Si/A1 ratio. For a given cation (Na +) as the Si/A1 ratio is increased from X to Y to EMT the
646 respective negative charge on the oxygens increases. This graph clearly illustrates the further dependence of zeolite basicity on specific local environments and the distribution of basic site strengths depending on the site of the adjacent cation. The trends of increasing basicity for the basic sites adjacent to the Na + cations from sites r to I to II are similar in all three zeolites (X, Y and EMT). 5. CONCLUSION Pyrrole was used to characterise the basicity of alkali exchanged zeolites. Using a curve fitting program, the broad vNH band of pyrrole adsorbed on framework basic sites of alkali Y and NaX zeolites was deconvoluted into several component bands. Based on the comparison between cation site populations and integrated intensities of these deconvoluted bands, the locations of the different basic sites can be identified and their relative strengths determined. The basicity in these alkali zeolites depends therefore not only on the Si/A1 ratio or the nature of the alkali cation but also on the localisation of the basic sites themselves.
Acknowledgements. Financial assistance from the EU under the HCM network (contract no. CHRX-CT94-0477) is gratefully acknowledged.
REFERENCES 1. Hathaway, I.E. and Davies, M.E., J.Catal., 116 (1989) 263. 2. Barthomeuf, D., Catal.Rev., (1996) submitted. 3. Barthomeuf, D., J.Phys.Chem. 88 (1984) 42. 4. Scokart, P.O. and Rouxhet, P.G., Bull.Soc.Chim.Belg., 90 (1981) 983. 5. Huang, M. and Kaliaguine, S., J.Chem.Soc., Faraday Trans., 88 (1992) 751. 6. Xie, J., Huang, M. and Kaliaguine, S., Catal.Lett., 29 (1994) 281. 7. Huang, M., Adnot, A. and Kaliaguine, S., J.Catal., 137 (1992) 322. 8. Akolekar, D.B., Huang, M. and Kaliaguine, S., Zeolites, 14 (1994) 519. 9. Huang, M., Kaliaguine, S., Muscas, M., Auroux, A., J.Catal., 157 (1995) 266. 10. Binet, C., Jadi, A., Lamotte, J. and Lavalley, J.C., J.Chem.Soc., Faraday Trans., (1995) in press. 11. Murphy, D., Massiani, P., Franck, R. and Barthomeuf, D., J.Phys.Chem., (1996) submitted. 12. Prasad Rao, P.R.H., Massiani, P. and Barthomeuf, D., Stud.Surf.Sci.Catal., 84 (1994) 1449 13. Lievens, J.L., Verduijn, J.P., Bons, A-J., Mortier, W.J., Zeolites, 12 (1992) 698. 14. Olson, D.H., Zeolites, 15 (1995) 439. 15. Franklin, K.R., Townsend, R.P., Whelan, S.J. and Adams, C.J., in Proceedings of 7th International Zeolite Conference, Eds., Y.Murakami, A.Iijima and J.W.Ward, (1986) 289. 16. Engelhardt, G., Hunger, M., Koller, H. and Weitkamp, J., Stud.Surf.Sci.Catal., 84 (1994) 421. 17. Mortier,W.J., Bosmans,H.J., Uytterhoeven, J.B., J.Phys.Chem., 76 (1972) 650 18. Koller, H., Burger, B., Schneider, A.M., Engelhardt, G., Weitkamp, J., Micro. Mater., 5 (1995) 219. 19. Sanderson, R.T., Chemical bonds and bond energies, Academic Press, NY, 1976
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
A spectroscopic s t u d y of t h e i n i t i a l s t a g e in crystallization of TPA-silicalite-1 from clear solutions
647
the
B r i a n J. S c h o e m a n
Department of Chemical Technology, Lule~i University of Technology, S-971 87 Lule~i, Sweden Discrete sub-colloidal (2-4 nm) particles have been identified in TPA-silicalite-1 precursor solutions and isolated from aqueous solution by extraction. The powdered extract sample was shown to possess microporosity, entrapped TPA + cations and a short range order by Raman and FT-IR spectroscopy and electron diffraction. An in-situ light scattering study shows that the sub-colloidal particles are subject to an Ostwald ripening mechanism and the evolution of a second particle population is detected on the sub-colloidal size range.
1. INTRODUCTION Numerous studies dealing with the crystallization of zeolite from apparently clear homogeneous solutions have dealt primarily with the events occurring in solution during the intermediate and latter stages of crystallization. Information on the initial stage in the crystallization event is normally extracted by extrapolating growth kinetics data to zero time according to the method of Zhdanov and Samulevich [1]. One of the difficulties associated with the direct analysis of the initial stage in zeolite crystallization lies in the fact that the crystal sizes are in the sub-colloidal size range, 1-20 nm, as well as that the crystals co-exist with macroscopic amorphous phases. It is generally accepted that the amorphous phase undergoes dissolution thus supplying the solution phase with species that form the first zeolitic structures that grow to the desired crystalline phase. The amorphous silica phase in the crystallization of TPAsilicalite-1 is typically one of several silica sources, for example silicic acid or a colloidal silica sol and the templating agent may be tetrapropylammonium (TPA) hydroxide, TPAOH/TPABr or a mixture of TPABr/NaOH [2,3]. Alternatively, the silica source may be derived from the hydrolysis of tetraethoxy silane with TPAOH [4] thus creating a reaction mixture free from macroscopic amorphous material. Such solutions are idealy suitable for light scattering studies and furthermore, the hydrolysis of silanes is a well investigated topic [5].
648 The purpose of this report is to describe the nature of the TPA-silicalite-1 precursor solution prepared via the hydrolysis of tetraethoxy silane with TPAOH and the events taking place during the initial stage of crystallization in the subcolloidal size range.
2. EXPERIMENTAL 2.1. Materials and preparation of the precursor sol A solution with the molar composition 9TPAOH 25SIO2 480H20 100Ethanol was made up by hydrolyzing a dilute solution of tetrapropylammonium hydroxide, TPAOH (Sigma, 1M in water, 143 ppm Na, 4200 ppm K, < 10 ppm A1) and tetraethoxy silane, TEOS (Merck, > 98%) for at least 24h at room temperature. The hydrolyzed TPA-silicate solution was pre-filtered through a Gelman Sciences Supor Acrodisc membrane filter, 0.2 ~m pore size, whereafter the solution was passed through a Millipore ultrafiltration membrane (Ultrafree-PF) with a nominal molecular weight limit of 100 000. 2.2. Extraction of sub-colloido! precursor particles Sub-colloidal silica particles were extracted [6,7] from the precursor sol prior to hydrothermal treatment. The pH of the silicate solution was reduced to ca. 2 by adding a strong cationic ion-exchange resin (Dowex HCRS-E, duPont) in the hydrogen form whereafter the resin was separated from the acidic sol and a hydrogen-bonding agent, t-butyl alcohol (Riedel-de-Haen, p.a.) was added with stirring. The organic phase was salted out by adding NaC1 (Merck, p.a.) and the organic phase containing the polymeric silica was separated and freeze dried to a powder for further analysis. 2.3. Spectroscopic analyses Diffuse reflectance FTIR (DRIFT) analyses on undiluted freeze dried samples of extract-powder and Ludox® SM (duPont) were performed with a Perkin-Elmer FT-IR 1760X spectrometer. Raman spectr a of uncalcined samples of the extract powder and a reference sample of well crystallized TPA-silicalite-1 synthesized according to the method given in reference 4 were obtained using a Perkin-Elmer PE1700X NIR FT-Raman spectrometer and N2 adsorption data on outgassed (100°C) samples was collected with a Micromeritics ASAP 2010 instrument. Electron diffraction patterns from the extract powder were collected using a JEOL 2000EX transmission electron microscope (TEM) in the diffraction mode.
649
2.4. ln-situ synthesis Quasi-electric light scattering spectroscopy (QELSS) was used to monitor the crystallization of TPA-silicalite-1 with a Brookhaven Instruments BI-200SM goniometer couPled to a Lexel Ar laser operating at 514.5 nm and a laser output effect of 500 roW. F u r t h e r details concerning these analyses are reported elsewhere [8]. The hydrolyzed synthesis solution, ca. 17 ml, was hydrothermally treated i n - s i t u at 70°C. The alignment of the optics was confirmed once the temperature of crystallization was reached.
3. RESULTS AND DISCUSSION The fact that the TPA-silicate solution passes freely through a Millipore ultrafiltration membrane with a nominal molecular weight limit of 100 000 would appear sufficient evidence to term the solution as being a "homogeneous" clear solution. Analysis of the undiluted (viscosity 6.4 cP at 22°C) solution prior to hydrothermal treatment with QELSS shows however that sub-colloidal particles are present in solution as an essentially monodisperse population with an average particle size of 2-3 nm. The particle size as estimated by the reaction of monomeric silica with molybdic acid [7] yields a size of 2.8 nm, in good agreement with the QELSS results. The particle size and narrow distribution has also been confirmed with Cryo-TEM (ca. 2 nm) [8]. A Raman spectroscopic analysis of the aqueous sol was undertaken to identify the presence of structurally entrapped TPA-cations [9] which could support the notion that the sub-colloidal particles possess a structure resembling that of the MFI phase. As shown in Figure la, only the prominent bands of the free TPAOH in the solution phase are visible. In order to detect structurally entrapped TPA+, the free TPA as well as TPA associated with the particle surface should be removed. For this reason, the polymeric silica was e x t r a c t e d from the solution phase - a process entailing removal of free and surface TPA by deionization and separation of the particles from smaller silicate species by hydrogen bonding of tbutyl alcohol with the particle surface. The resulting particle is lyophobic in a saturated saline solution thereby resulting in a phase separation - the organic phase containing the polymeric silica. Raman spectra, Figure 1 of the reference material, TPA-silicalite-1, crystal size 60 nm, shows the charactersitic peaks due to entrapped TPA cations [9]. Note, essentially no other forms of TPA are present in this sample. This spectrum may be compared to that of the freeze dried powder
650 containing the polymeric silica, Figure 1. Since the free and surface associated TPA is essentially absent in this sample, these peaks may be assigned as being due to entrapped TPA in the silicate structure. Since TPA cations are too large to enter the channel structure after the formation of the channels, they must be incorporated during the formation of the sub-colloidal particles, i.e. during the polymerization of silica species released as a result of the base (TPA) catalyzed hydrolysis of TEOS.
°,
(ii)
I
1415
"
?
/\~
I
1300
l\~
I
I
"
I
1200 1100 1000 FREQUENCY SHIFT (¢m-l)
~
!
I
I
I
900
800
740
Figure 1. R a m a n spectra of i) the aqueous synthesis solution showing the absorption bands primarily due to free TPAOH, ii) the structurally entrapped TPA present in the extract powder sample and iii) the structurally entrapped TPA present in XRD crystalline silicalite crystals following deionization. The peaks marked by * are the peaks of interest. DRIFT analysis of the extract powder yields the result that the absorption band at ca. 560 cm -1 assigned to highly distorted double six rings present in the MFI structure [10] is present as shown in Figure 2. The presence of both the 560 cm -1 and the absorption band at ca. 450 cm -1 can be indicative of the presence of the MFI phase [11] although the absorption band at ca. 1220 cm -1 normally present in the DRIFT spectra of well crystallized TPA-silicalite-1 ( all be it a very weak band ), is not evident in the DRIFT spectra of the extract powder. Calcining the powder at 480°C for a time as short as 4 minutes results in the
651 disappearance of the absorption band at 560 cm -1 yielding a DRIFT spectra more similar to truly amorphous silica than that of the MFI phase as seen in Figure 2.
I 1200
800 (cm"l)
6O0
Figure 2. DRIFT spectra of a freeze dried powder (i) containing extracted subcolloidal silicate particles, (ii) of the extract material following calcining at 480°C, 4 minutes and (iii) of truly amorphous silica particles, Ludox~ SM. TEM micrographs of the extract powder show large aggregates of siliceous (as shown by EDX analysis) material as well as a few areas with apparently discrete particles with sizes less than 5 nm. The reason that aggregates are present is due firstly to the extraction and freeze drying process as well as to the method of the TEM sample preparation. A light field image of such a discrete particle is shown in Figure 3a and the diffraction pattern due to this particle is shown in Figure 3b. These diffraction spots correspond to the d-spacings 1.45, 2.11, 2.65, 2.79, 3.853,91, 5.16, 5.43 and 5.61A which are similar to certain peaks in the XRD pattern for crystalline TPA-silicalite-1 in the 2-theta range 16-66 °. This result may seem surprising since it has been stated that precursor particles believed to be nuclei, presumably the size of a few unit cells, contain too few repeat units to yield electron diffraction patterns [12]. No details concerning the minimum particle size detectable by electron diffraction were given. The same particle, analyzed with EDX-analysis, was shown to be a silica particle thus indicating that the
652 siliceous particle may possess an ordered structure similar to that of the MFI phase. N2 adsorption data shows that the freeze dried powder contains microporous material with a pore diameter in the range 4-8 A comparable to t h a t of TPAsilicalite-1 (pore diameter 5.5 A). The BET specific surface area of the powder is 212 m2/g. According to Scholle et.al. [13], microporosity may be detected by N2 adsorption even though TPA is present since the structures are solids with many defects. It is also possible t h a t the observed microporosity arises from the intraparticle cavities that are formed upon sample drying. Caution in the interpretation of these results is therefore necessary.
a)
b)
ii;ii
......................................... ". . . . . . . . . . . . . .
Figure 3. a) A light field image of aggregated siliceous particles imaged with TEM and b) the diffraction p a t t e r n of this particle showing diffraction spots corresponding to short interplanar distances. A critical appraisal of the results presented above indicate t h a t it is most probable that these sub-colloidal particles are highly defected structures and the DRIFT, N2 adsorption data and Raman results (and possibly the electron diffraction data) can not solely support the view that the X-ray amorphous particles possesses a form of short range order similar to that of the MFI phase. In order to reach a final conclusion in this respect, further detailed investigations are necessary. An in-situ QELSS study of the hydrothermally treated sol was performed at 70°C, a relatively low temperature which was chosen so that the slow kinetics would allow for the accumulation of reliable data. The intensity of the scattered light due to the particles in solution as a function of crystallization time is shown in Figure 4a for the initial stage of the crystallization. The appearance of the
653 curve in the interval 0-40 minutes is particularly interesting - the intensity of the scattered light decreases initially and after 40 minutes, it increases almost exponentially. This observation may seem to be unexpected since the temperature increase during the s~mple heating period, ca. 25 minutes, will result in the increase in the Brownian motion of the particles and thus an increase in the intensity of the scattered light should be observed. The temperature increase will however result in a higher solubility of the siliceous particles and a redistribution of silica will take place via an Ostwald ripening mechanism, i.e. smaller silica particles will depolymerize and soluble silica will be deposited onto the larger particles [14]. The number of particles will thus decrease and the net result is a reduction in the intensity of the scattered light. The results of the particle size analysis are shown in Figure 4b. "~
a)
300
-
~ m Z
b) 250 -
200 -
ffl
0o
0
0
20
0
NI
[7
Sm_~]!size-fraction
O
Large size-fraction
Cx:)
b~
Oo
15
1°I
c9
0
o@
150 -
0
25
I
I
I
2
4
6
CRYSTALLIZATION TIME (h)
o
o
5
lo
CRYSTALLIZATION
~ T I M E (h)
Figure 4. a) The scattered light intensity as a function of crystallization time and b) the increase in the average particle size with crystallization time. The average particle size increases initially from 2-3 nm, at room temperature, to 3.5 nm at 70°C. The particle size continues to increase to ca. 6 nm during the first 12 hours of hydrothermal treatment during which period, the particle size distribution (PSD) is monomodal. After ca. 12 hours, a second particle population appears, the PSD changes to a bimodal PSD and the average particle size of the small size-fraction reverts to the original size of 3.5 nm. A reasonable interpretation of these results is that the monomodal PSD's initially observed actually represent the average of two separate particle populations that are not resolved by the light scattering technique. Once the technique is able to resolve the two populations, the PSD of the small size-fraction reverts to its original state as stated above.
654 4. CONCLUSIONS Spectroscopic studies of the polymeric silica in a hydrolyzed silicalite-1 precursor solution indicate that the sub-colloidal particles may possess a short range order but the defect structures require further characterization before their role in the crystallization of silicalite may be determined. The use of a high effect laser scattering system allows one to monitor the events taking place on the subcolloidal size range thus enabling the study of the initial stage in the crystallization of silicalite. 5. ACKNOWLEDGEMENTS
The financial assistance by the Swedish Research Council for Engineering Sciences (TFR) is gratefully acknowledged. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
S.P. Zhdanov and N. N. Samulevich, Proc. 5th Int. Conf. on Zeolites, (Ed. Rees, L. V. C.), Heyden, London, (1980) 75. J . J . Keijsper and M. F. M. Post, "Zeolite Synthesis" - ACS Symposium Series 398, (Eds. Occelli, M. L. and Robson, H. E.), Washington, DC, (1989) 28. C.S. Cundy, B. M. Lowe and D. M. Sinclair, J. Crystal Growth, 100 (1990) 189. A.E. Persson, B. J. Schoeman, J. Sterte and J-E. Otterstedt, Zeolites, 14 (1994) 557. See for example E. P. Plueddemann, Silane Coupling Agents, Plenum Press, 2nd ed., New York, 1991. R.K. Iler, "Soluble Silicates"- ACS Symposium Series 194, (Ed. Falcone Jr., J. S.), Washington, DC, (1982) 95. B.J. Schoeman, To be submitted for publication. B.J. Schoeman and O. Regev, Submitted to Zeolites for publication. P.K. Dutta and M. Puri, J. Phys. Chem., 91 (1987) 4329. P.A. Jacobs, E. G. Derouane and J. Weitkamp, J. Chem. Soc., Chem. Commun., (1981) 591. G. Coudurier, C. Naccache and J. C. Vedrine, J. Chem. Soc., Chem. Commun., (1982) 1413. J. Dougherty, L. E. Iton and J. W. White, Zeolites, 15 (1995) 640. K . F . M . G . J . SchoUe, W. S. Weeman, P. Frenken and G. P. M. van der Velden, Applied Catalysis, 17 (1985) 233. R.K. Iler, The Chemistry of Silica, Wiley, New York, (1979).
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
655
Characterization and catalytic properties of the galliumphosphate molecular sieve cloverite R. Fricke~, M. Richter a, H.-L. Zubowa ~ and E. Schreierb alnstitute for Applied Chemistry and bHumboldt-University Berlin, Rudower Chaussee 5, D-12484 Berlin, Germany
Modification of the galliumphosphate cloverite by various heteroelements has been carried out. In the case of Ti-cloverite at least partial incorporation of titanium into the galliumphosphate lattice could be shown. Catalytic properties of both the pure and the Ti- and Si-modified cloverite catalysts show remarkable activity and selectivity in the etherification of isobutene by methanol and ethanol producing MTBE and ETBE, respectively. Modification with titanium influences the Catalytic properties in a negative way whereas Si-modification shows nearly no effect. The nature of acid centers is discussed.
INTRODUCTION During the last years attempts have been made to synthesize molecular sieves with super wide pores. Following the aluminophosphate structure VPI-5 having 18-membered pore openings (18MR) the synthesis of the 20MR type JDF-20 with the same composition and the gaUiumphosphate cloverite (20MR) were the most recent results (not counting the mesoporous M41S system). Cloverite is the only molecular sieve with a three-dimensionally arranged super wide pore system having pores of 13.2 A and large cavities of about 30 A of diameter. In addition, small pores (< 4 A) not intersecting the large pores are present [ 1]. The characterization is mainly concentrated on the template containing samples because the structure easily collapses after detemplation under the influence of moisture. This might be the reason why only limited information on the catalytic properties of cloverite are available. Recently it has been shown for the first time that cloverite, following appropriate in situ decomposition of the template, catalyzes the gas phase etherification of isobutene to methyl tert-butyl ether (MTBE) under atmospheric flow conditions in the temperature range from 363 to 383 K with good performance [2]. The examination of the formation of ethyl tert-butyl ether (ETBE) is a continuation of the former work, despite the fact, that at present the replacement of methanol by ethanol is of minor commercial relevance but could become an alternative as the efficiency of the exploitation of biomass will be improved. As dehydration of ethanol to diethyl ether (and water) is a more facile process than that of methanol we are concerned with the extent of this side reaction and with the influence of the higher gas phase concentration of water on the stability of the cloverite molecular sieve. The present contribution deals also with physico-chemical and catalytic properties of cloverite samples containing Si and Ti as heteroatoms.
656 EXPERIMENTAL Molecular sieves
The synthesis of cloverite was carried out according to [3] but in a microwave oven at 443 K for 1 h. The gel composition was as follows: 1 Ga20 3 : 1 P205 : z HF : 6.0 Q : x EIO : y H20 (Q" quinuclidine; EIO: heteroelement; x = 0-0.3; y = 70-600; z = 1 1-4.1). Catalytic measurements were performed in a flow reactor with a catalyst volume of 1.9 cm 3 containing 90 wt.% cloverite and 10 wt.% SiO 2 binder at a flow rate of 0.9 cm3s -1 and a molar alcohol to iso-butene ratio of 1. The weight hourly space velocity (alcohol and isobutene) was 5.9 h -1. Detemplation was performed in situ under a flow of either nitrogen or air. This procedure is described in detail in Ref. [2]. The fluoride content in the spent cloverite samples was determined after removal of organic residues by calcination of the samples at 773 K in air for 2 h. Alter dissolving the material in diluted H2SO 4 and neutralization with NaOH the F- content was measured with a F- ionselective electrode (Mettler-Toledo). Characterization
Infrared spectra were taken with a Specord M 85 (Carl Zeiss, Jena) FTIR spectrometer. Self-supporting wafers of the samples placed in an IR cell were used for measurements. UV-vis measurements were carried out with a Perkin-Elmer Lambda 19 spectrometer. BaSO 4 was used as standard. The reaction
H3C~ H3c/C~---CH2 + ROH ~
H3C~ /OR H3c/C~cH3
(1)
The etherification reaction of isobutene and relevant side reactions are shown in R = -CH3; -CH2CH3 Figure 1. Equation (1) describes the main reaction leading to the corresponding tertiary 2 ROH ~-~ R20 + H20 (2) ethers MTBE or ETBE. The dehydration of the alcohols to symmetric dimethyl ether H3C~ H3C,,x _jOH (3) H3c/C~CH2 + H20 ~ H3c~C~cH3 (DME) or diethyl ether (DEE) is one undesired side reaction consuming the alcohol and producing water (eq. (2)). Isobutene can be converted to tert-butanol by reaction with H3C~~CH,g---C~CH 2 H3C~ ,7 water (eq. (3)) and/or can oligomerize to CH3 (4) predominantly the two isomers 2,4,4 2 m H3 ~ H3 trimethyl pent-l-ene and 2, 4, 4 trimethyl pentH3c/CmCH2"~ H3C----~mCH--~-~CmCH3 2-ene (eq. 4). As will be shown in a forthcoming paper, CH3 the trimethylpentenes are also etherified by H~ H3C~ ~OR the alcohol according to eq. (5). In the data C~---CH2+ ROH ~ RC,,,,C~cH3__ (5) given in the tables, this reaction is not considered. However, because it additionally consumes ethanol, this side reaction has to be R 1= -CHg--H3 taken into account when comparing the x3 experimental overall ethanol consumption with calculated thermodynamic values which are based exclusively on the reaction Figure 1. Etherification of isobutene with methanol and ethanol. For explanation see text. according to eq. (1). i..x
657
Activity and selectivity Because the reaction proceeds with volume reduction, conversion values were calculated on the basis of the gas phase composition at the reactor outlet taking into account the reaction scheme given in Figure 1. Accordingly, the alcohol concentration at the reactor inlet is given by the sum of unreacted alcohol found at the reactor outlet plus the percentage of tertiary ethers formed and the percentage of symmetric ethers, the latter multiplied by 2 due to stoichiometry. The dehydration of the alcohols leads to 2-3 vol.% DME and ETBE, respectively, at the highest reaction temperature (403 K) but does practically not proceed to a substantial extent in the low temperature range (353-393 K). The selectivity to the tertiary ethers is referred to the conversion of isobutene. Relating the selectivity of these ethers to the conversion of the alcohols would yield values of 100% due to the low percentage of side reactions consuming additional alcohol. Thus, the selectivity to the tertiary ethers is expressed as SE =
100 CE/(CE + 2CDIB) (%)
where cE is the molar ether concentration and CDiB is the molar concentration of diisobutenes (2, 4, 4 trimethyl pent-l-ene and 2, 4, 4 trimethyl pent-2-ene). Higher oligomers when present are considered correspondingly to their carbon number.
RESULTS AND DISCUSSION
Modification by heteroatoms In a former investigation a series of heteroatoms (Ti, Si, AI, Ni, Co, Fe, Mg) has been introduced into the synthesis gel in order to modify properties of cloverite [3]. When applying microwave heating the products have been found to be of enhanced crystallinity. The influence of the heteroatoms on the XRD pattern (compared to pure cloverite) was small. Ti cloverite (Ti-Clo) showed the highest crystallinity and adsorption capacity of all modified samples. For the sake of comparison a highly crystalline Si-Clo sample is additionally involved into this study. In the case of Ti-Clo the possible incorporation of titanium into the cloverite lattice has been proven by ESR measurements of the reduced samples as well as by UV-vis measurements. After reduction in hydrogen at 773 K the Ti-Clo sample shows an axial ESR spectrum of Ti 3+ ions which is temperature dependent ( ~ = 1.93, g tl = 1.88). In particular, . the temperature dependence is usually taken as evidence for (distorted) tetrahedral coordio nation of the Ti 3+ ions. This 200 400 6(30 BOO would mean that it is possible to WovelenQth (nr~) reduce Ti4+ incorporated into Figure 2. UV-vis spectrum of Ti cloverite. the cloverite lattice to the three-
A v-N
. . . . .
|
__
=
,
,
,
658 valent state. On the other hand, it still seems difficult to distinguish between titanium located on lattice or extra-lattice positions on the basis of these ESR results alone. However, further evidence for tetrahedral coordination of titanium is obtained by UV-VIs measurements (Figure 2). Well-resolved bands at 214 and 247 nm were shown in the spectra as well as an additional shoulder at about 289 nm. Following an interpretation of UV-vis spectra of Corma et al. [4] for Ti-containing MCM-41 the band at 214 nm should be caused by titanium in low coordination (probably tetrahedral). It should be mentioned at this occasion that the position of this band slightly depends on the conditions of the synthesis, i.e. on the HF concentration used for gel preparation. The band at 247 nm and the shoulder show that "Ti clusters" are also present, i.e. Ti in higher coordination and/or aggregation. Summarizing the ESR and UV-vis results it may be concluded that at least part of titanium is located on lattice positions. At present it is, however, not known which consequences the different location of titanium might have concerning catalytic properties because, up to now, this question has not been investigated in detail. Catalytic results 1. Formation o f MTBE and ETBE over pure cloverite Without any side reaction, conversion data for the alcohol component and isobutene should be the same for the molar reactant ratio of one. This is indeed observed in good approximation for both alcohols at reaction temperatures up to 363-368 K due to the marginal extent of side reactions occuring below 373 K. Therefore, the selectivity of the tertiary ethers is high (95.8 % MTBE and 98.5 % ETBE at 363 K). At higher reaction temperatures the isobutene conversion grows considerably due to the onset of oligomerization reactions. Consequently, the selectivity of the ether formation deteriorates, because it is referred to the isobutene conversion. The use of ethanol for the etherification instead of methanol is thermodynamically less favourable since the possible maximum conversion is generally lower than that for methanol (values are given in parentheses). Practically, at a reaction temperature of 373-383 K, the ethanol conversion reaches its thermodynamic equilibrium value which is as low as 24 % at 373 K and 18 % at 383 K. In case of MTBE formation the methanol equilibrium conversion is 64.4 % at 373 K and 54.1% at 383 K. Characteristically, the conversion of ethanol on cloverite was found higher than allowed by thermodynamics if the temperature exceeded 383 K. This is attributed to the additional consumption of ethanol by the formation of DEE (one mole of DEE formed consumes two moles of ethanol) and by the etherification of diisobutene isomers. The extent of these two side reactions corresponds to 4.5 % of the ethanol consumption at 403 K (Table lb, last row), so that the actual conversion of ethanol to ETBE is reduced to 7.5 %. This agrees excellently with the thermodynamic prediction and underlines that under the applied reaction conditions the thermodynamic equilibrium of the ETBE formation is reached. 2. Formation of ETBE over pure and modified cloverite Adequate data are given for Ti-containing and Si-containing modifications of the cloverite material. The performance of catalysts cannot be appropriately compared at the point of thermodynamic equilibrium. Considering the conversion at 353 and 363 K, it is striking that the Ti-cloverite is significantly less active than the other two samples, whilst the non-modified and the Si-modified cloverite are not largely different in their activity.
659 Table la Catalytic data for the MTBE reaction over non-modified cloverite after in situ oxidative detemplation T/K
Conversion/%
Selectivity/%
MeOH
Isobutene
MTBE
IB dimers
363
35.8 (37.1)
36.0
95.8
4.2
368-
38.0 (69.0)
38.6
91.9
8.1
373
36.7 (64.4)
40.7
83.4
16.6
383
30.5 (54.1)
44.0
57.6
42.4
Methanol conversions in parentheses are the calculated equilibrium values. Table lb Catalytic data for the ETBE reaction over non-modified cloverite after in sire oxidative detemplation T/K
Conversion/%
Selectivity/%
EtOH
Isobutene
ETBE
I]3 dimers
353
8.2 (54.0)
7.9
99.5
0.5
363
15.6 (34.0)
15.1
98.5
15
373
19.7 (24.0)
19.6
97.3
2.7
383
18.6 (18.0)
19.9
91.5
8.5
393
13.9 (12.0)
16.6
80.5
19.5
403
11.0(8.0)
13.4
72.1
27.9
Ethanol conversions in parentheses are calculated equilibrium values. Table 2 Catalytic data for the ETBE reaction over Ti-modified cloverite after in situ oxidative detemplation T/K
.
Conversion/%
Selectivity/%
EtOH
Isobutene
ETBE
IB dimers
353
1.7
1.6
98.9
1.1
363
3.0
2.9
97.0
3.0
373
4.7
4.8
93.5
6.5
383
6.9
7.6
87.3
12.7
393
8.9
8.7
88.5
11.5
403
9.5
8.7
88.2
11.8
660 Table 3 Catalytic data for the ETBE reaction over Si-modified cloverite after in sire oxidative detemplation T/K
Conversion/%
353
Selectivity/%
EtOH
Isobutene
ETBE
IB dimers
10.9
10.9
99.3
0.7
363
19.5
19.7
98.4
1.6
373
21.0
21.5
96.3
3.7
383
19.3
21.2
86.6
13.4
A c i d centers
It is well acknowledged that the etherification reaction of iso-butene by alcohols requires strong acid sites. The industrial production of MTBE is performed (in liquid phase) over
Q
0.2 "
~
"~-"~~
d
1612
I
0.2
1448 1448
ls4o t,jb .~~,...~./
1800
.
~
1600
1400
Wavenumber/cm- 1
1800
1600
.~j~a '
i 400
Wavenumber/cm- 1
Figure 3. IR spectra of the pure clovefite (left) and Ti-cloverite (fight) after (a) vacuum treatment at 753 K, (b) adsorption/desorption of pyridine (4 kPa) at room temperature, (c) after annealing at 423 K (18 h) and (d) 673 K (1 h).
661 sulphonated ion exchange resin catalysts with high acidity. The question arises, therefore, of what nature the acid centers on the ¢loverite catalysts are. In the literature there are some suggestions concerning the acidity of pure ¢loverite [5,6]. Already the very first IR measurements of the OH region have shown that ¢loverite exhibits two bands at 3670 and 3700 ¢m-1 [7]. Following the structure of ¢loverite which shows that it contains two structtiral OH groups these bands have been attributed to P-OH and Ga-OH groups, respectively. In a recent paper Mtiller et al. [6] have identified these groups to be of moderate (P-OH) and high (Ga-OH) acid strength. In particular the Ga-OH groups are considered to be responsible for a concerted BrOnsted and Lewis type interaction with adsorbed polar molecules, for instance methanol. However, no catalytic data are given in their paper. In the present study these high (HF) and low frequency (LF) hydroxyl bands are also observed, usually the I-IF band with much lower extinction than the LF band. Surprisingly, no difference in the IR spectra were obtained for the pure or Ti-modified cloverite catalyst although Ti-eloverite is much less active than pure doverite in the formation of ETBE (Tab. lb, 2). Comparative adsorption of pyridine on samples that have been evacuated for 1 h at 753 K, oxidized in 0 2 and again evacuated shows about the same concentration of Lewis (1448 and 1612 cm-1) as well as Br6nsted (1540 ¢m-1) centers on both pure and Ti-eloverite catalysts. Increasing the desorption temperature to 423 K shows, however, that pyridine desorbs faster from the pure doverite samples suggesting that Ti-cloverite should possess stronger Lewis and Br/3nsted acid sites. This conclusion does, however, not fit the expectations connected with the lower catalytic activity of the Ti-cloverite catalyst. There are several patents which enclose the post-synthesis modification of zeolites by hydrogen fluoride, fluorsulphonic acid, fluorphosphoric acid and other highly acidic media [8]. Following a completely different idea, it seems, therefore, reasonable to assume that residual fluoride ions within the cloverite solid aPter in situ detemplation might be responsible for the good catalytic performance of the cloverite catalysts in the etherification reactions studied. In a first attempt to confirm this approach a pure cloverite and a Ti-cloverite sample have been analyzed with respect to their F- concentration after reaction. The result shows that both cloverite catalysts indeed contained fluoride but the pure cloverite sample had a higher fluorine concentration than the Ti-containing one. This might point to a certain contribution of the fluoride ions to the catalytic activity which would be in coincidence with the acidification of industrial catalysts by fluorine compounds. However, at the present stage of investigation, it cannot evidently be shown how fluoride ions come into action when located within the cloverite lattice. According to the proposed structure aider synthesis, F- ions should be located in the four-ring subunits where they act as counter-ion to the quinuclidine cation Q+ [1]. After detemplation the residual fluoride ions could modify the acidity of adjacent hydroxyl groups. However, no direct evidence for this idea is available at present; further studies concerning this question are under investigation. Nevertheless, it is obvious that the question whether fluorine contributes to the overall acidity, i.e. also to the catalytic activity, in a dominating or insignificant way is crucial. Taking into account that the amorphous galliumphosphate sample that has been synthesized in completely the same way as cloverite (i.e. also with the same concentration of HF) exhibited a distinctly lower catalytic activity for the formation of MTBE than a crystalline cloverite catalyst [2] it has to be concluded that the cloverite structure is an essential property with respect to the catalytic appearance.
662 Acknowledgement. The authors kindly acknowledge analytical and technical assistance of Mrs. E. Lieske, Mr. R. Eckelt, and Mr. U. Marx. R. F. and M. R. are indebted to the qTonds der Chemischen Industrie' (VCI) for financial support. REFERENCES 1. M. Estermann, L.B. McCusker, C. Baerlocher, A. Merrouche, H. Kessler, Nature, 352 (1991) 320. 2. M. Richter, H.-L. Zubowa, R. Eckelt and R. Fricke, Microporous Materials, in press. 3. H.-L. Zubowa, E. Schreier, K. Jancke, U. Steinicke and R. Fricke, Collect. Czech. Chem. Commun., 60 (1995) 403. 4. A. Corma, M.T. Navarro and J. P6rez Pariente, J. Chem. Soc., Chem. Commun., 1994, 147. 5. A. Janin, J.C. Lavalley, E. Benazzi, C. Schott-Darie and H. Kessler, Proc. ZEOCAT '95, Szombathely (Hungary), July 9-13, 1995, H.K. Beyer, H.G. Karge, I. Kiricsi and J. B. Nagy (editors), Elsevier Sci. Publ. Amsterdam, 1995, 124. 6. G. MOiler, G. Eder-Mirth, H. Kessler and J.A. Lercher, J. Phys. Chem., 99 (1995) 12327. 7. T.L. Barr, J. Klinowsky, H. He, K. Alberti, G. MOiler and J.A. Lercher, Nature, 365 (1993) 429. 8. US Patent No. 5,364,981 (1994).
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
663
Preparation of Titanosilicate with Mordenite Structure by Atomplanting Method and Its Catalytic Properties for Hydroxylation of Aromatics Peng Wu, Takayuki Komatsu and Tatsuaki Yashima Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan Titanium mordenites (Ti-M) with different titanium and aluminum content have been prepared by the solid-gas reaction at elevated temperatures (atom-planting method) between highly dealuminated mordenites and TiCI4 vapor. Ti-M samples were characterized with MAS NMR, IR and UV spectroscopies, which indicates that Ti atoms have been incorporated tetrahedrally into the mordenite framework through the reaction of TiCI4 with hydroxyl nests composed of SiOH groups. The catalytic properties of Ti-M for the liquid phase hydroxylation of various aromatic substrates having different molecular sizes with 1-1202were studied by comparing with those of MFI-type titanosilicate (TS-1). Ti-M showed comparable specific activity to that of TS-1 for the hydroxylation of smaller substrates which can diffuse easily into the pores of both catalysts. However, Ti-M was more active than TS-1 in the hydroxylation of bulkier aromatics, because Ti-M has larger window size of pore than TS-1. 1. INTRODUCTION MFl-type titanosilicate, TS-1, has opened new possibilities for using zeolites as oxidation catalysts under liquid-phase conditions. TS-1 has been shown to be a remarkable catalyst for the selective oxidation of a large family of organic substrates using H202 as an oxidant under mild conditions, i.e., hydroxylation of alkylbenzenes and phenol [1], epoxidation of olefins [2], oxidation of paraffins to the corresponding alcohols and ketones [3], oxidation of alcohols [4] and ammoximation of ketones [5]. These successes on TS-1 have induced subsequent researches on the synthesis of other Ti-containing zeolites, e.g., TS-2 (MEL) [6] and Ti-ZSM-48 [7]. These titanosilicates show unique catalytic activity due to isolated and tetrahedrally coordinated Ti atoms in the framework. However, they are restricted to the oxidation of relatively small substrates because of their medium-pore structures. It is not ambitious to say that the synthesis of large-pore titanosilicates is one of the main research subject in the field of developing oxidation catalysts. These motives have led to the synthesis of titanosilicates with 12-ring channels such as Ti-Beta [8], TAPSO-5 [9] and Ti-ZSM-12 [10]. More recently, a mesoporous Ti-MCM-41 material has been synthesized and found to have an advantage over TS-1 and Ti-Beta in the
664 oxidation of large organic molecules [11]. The hydrothermal synthesis of Ticontaining zeolite with mordenite structure, however, has been reported seldom. Atom-planting method, i.e., a treatment of highly siliceous zeolites with metal chloride vapor at elevated temperatures, has been proved to be a useful way for preparing metallosilicates with MFI a n d M O R structures [12-14]. In this study, we have performed the incorporation of Ti atoms into the mordenite framework by the atom-planting method to prepare Ti-containing mordenite and compared its catalytic properties for the hydroxylation of various aromatics with those of TS-1. 2. E X P E R I M E N T A L H-Mordenites, M ( l l ) (framework Si/AI atomic ratio of 11) and M(8.2) were used as starting material for the dealumination to obtain various dealuminated mordenites with Si/AI ratios of 41-325. The dealumination was carried out by the calcination in air at 973 K followed by HNO3 reflux, as described in detail elsewhere [14]. The atom-planting procedure with TiCI4 vapor was similar to the alumination treatment [14]. After dehydration at 773 K for 4 h, 2 g of dealuminated mordenite was treated with TiCI4 vapor (1.7 kPa) in a flowing helium at 673 K for a prescribed process time (5 min-4 h). The sample was then purged with pure helium at 673 K for 1 h. After cooling it to the room temperature, the TiCl4-treated sample was washed with deionized water and dried at 383 K for 24 h to obtain Ti-containing mordenite, Ti-M(n), where n was the Si/AI ratio of the parent dealuminated mordenite. The reference catalyst, TS-1 (Si/Ti=104)was synthesized hydrothermally according to the patent [1]. 29Si MAS NMR (Bruker MSL-400), IR (Shimadzu FTIR-8100) and diffuse reflectance UV-visible (Shimadzu MPS-2000) spectroscopies were used for the characterization of Ti introduced into mordenite structure. Hydroxylation of aromatics with H202 was performed in a 50 ml flask with a magnetic stirrer at 363 K. In a typical run, 50 mg of catalyst, 2 ml of water, 20 ml of aromatic substrate and 1 ml of H202 (30 wt%) were mixed in the flask. The reaction was then carried out under vigorous agitation for 2 h. The reaction mixture was analyzed with gas chromatography using p-ethylphenol or 2,5-xylenol as an external standard. 3. RESULTS AND DISCUSSION 3.1 Preparation and characterization of Ti-M The incorporation of Ti by the atom-planting treatment with TiCI4 vapor at elevated temperature was first investigated to know where Ti atoms were located. The effect of the process time of TiCI4 treatment was investigated using M(71) as a parent. As shown in Fig. 1, the amount of Ti introduced (bulk Ti) increased rapidly with increasing the process time to 1 h. The longer process time than 1 h did not make the amount of Ti increase further. The amount of bulk AI changed very slightly during the TiCI4 treatment. As a result, Ti-M with the amount of Ti larger than that of
665
|
"6 0.6 Bulk Ti • P,,,i
O'2 .~ 0.4
ofo
< •[--. ; 0.2
--------U 3
o <
--O
-10311
Bulk AI 1-'1----Si released
0
!
0
1031~
1
2 3 4 Process time/h
. . . . .
5
Figure 1. Effect of process time on the atomplanting of M(71). Atom-planting: temp., 673 K; TiCI4 vapor pressure, 1.7 kPa.
Figure 2. 29Si MAS NMR spectra without CP (a, c) and with CP (b, d) for M(151) (a, b) and Ti-M(151) (c, d).
AI was prepared. More importantly, the amount of Si released from the zeolite framework was negligible compared with that of Ti introduced. These phenomena were very similar to those observed for the alumination process of dealuminated mordenites with AICI3 vapor [14]. 1H-29Si cross-polarization (CP) MAS NMR technique was adopted to clarify where Ti atoms have been incorporated. The 29Si MAS NMR spectra of M(151) and Ti-M(151) measured without and with CP are shown in Fig. 2. Non-CP spectrum of M(151) showed two peaks at-112 and -114 ppm due to Si(0AI) together with a wellresolved peak at-103 ppm (a). The-103 ppm peak was greatly enhanced using the CP technique (b). Therefore, it is reasonably assigned to the internal SiOH groups in SiOH(OSi)3 units developed during the dealumination treatments [14]. After atomplanting treatment, the peak a t - 1 0 3 ppm was hardly observed in the non-CP spectrum of Ti-M(151) (c). Only spectrum with very poor signal-to-noise ratio was obtained even after using the CP technique (d). These results are consistent with Fig. 1 to indicate that Ti atoms were incorporated mainly through the reaction between TiCI4 molecules and the internal SiOI-I groups at hydroxyl nests but not through an isomorphous substitution of Ti for the framework AI or Si. Figure 3 shows IR spectrum of M(71) and difference spectra of Ti-M(71) prepared by treating M(71) with TiCI4 vapor at 673 K for a different period of process time. The spectra of Ti-M(71) samples always exhibited an absorption band at 963 cm "1 not observed for M(71). Such a band at ca. 960 cm -1 attributed to the Si-O-Ti bonds has been reported in the IR spectra of TS-1, TS-2 and Ti-Beta, and is taken as a characteristic evidence for the presence of Ti atoms in the framework sites [1-8]. The intensity of this band increased with increasing the process time from 5 min to 2 h. The relationship between the absorbance of the 963 cm-1 band and the amount of bulk Ti is shown in Fig. 4 for Ti-M prepared at 673 K from various parent dealuminated mordenites. Independent of the Si/AI ratios of mordenites, the absor-
666 12
./
.MfTl) t
~ 10
t 963 cm"1 30 m i n ~ /
t
0
,~ ,,
o <
,
I|
1% ,= ,,,,
[
II
6
o o
4
~o
2
<
0
XP/ t1 ~'A /11
• • 4, X
~1
!
1200 I000 SO0 Wavenumber/cm1
Figure 3. IR spectra of M(71) and Ti-M(71) samples prepared by the TiCI4 treatment at 673 K for a different period of process time.
0
Ti-M(71) Ti-M(123) Ti-M(169) Ti-M(195) Ti-M(245)
I
I
I
I
'
0.1
0.2
0.3
0.4
0.5
0.6
Bulk Ti/mmol g-X Figure 4. The absorbance of 963 cm-1 band vs Ti amount for various Ti-M samples.
bance linearly increased with the amount of bulk Ti. These results indicate that Ti atoms have been incorporated into the mordenite framework sites. Figure 5 shows diffuse reflectance spectra in UV-visible region of various samples. Ti-M(71) 330 exhibited a band at ca. 220 nm which was also ~. Ti02 observed for TS-1 but not for M(71). A similar L 240 ~ M ( 7 1 ) impregnated band has been previously reported for TS-1 [1] ~"\~ ~ withTi(SO4)2 and Ti-Beta [8], and assigned to isolated Ti ~\\ /~Ti'M(71) atoms in tetrahedral coordination. Note that in the spectrum of Ti-M(71), there were no peaks around 330 nm corresponding to anatase TiO2 nor at ca. 240 nm due to six-fold coordinated M(71) \ amorphous Ti species in the zeolite impregnated f ! I I I with Ti(SO4)2. In agreement with the data given 200 400 600 by MAS NMR and IR spectra, these results Wave length/nm further support that most of Ti atoms introduced into the mordenite crystals by the TiCI4 treatment occupy the framework positions in a tetraFigure 5. Diffuse reflectance spectra hedral coordination. in UV-visible region.
3.2 Catalytic properties of Ti-M for hydroxylation of aromatics Influence of Al content on catalytic activity of Ti-M in the hydroxylation of toluene. Ti-M catalysts with various A1 and Ti contents were tested for the hydroxylation of toluene at 363 K with H202. The products were mainly cresol isomers (> 99 %) together with trace amounts of benzaldehyde and benzyl alcohol
667 resulted from the side-chain oxidation. The activity of the incorporated Ti was "6 found to be dependent greatly on the oOI.o-O-°--o-o composition of the parent dealuminated / mordenites. The specific activity o / (turnover number per Ti atom) for the 20 "6 o hydroxylation of benzene ring to yield corresponding cresols increased with Z ' I I 0 0 J ~ Si/AI ratio from 11 to 200, then it only [--, 100 200 300 changed slightly with further dealuminaSi/AI ratio tion (Fig. 6). This indicates that a low AI content in Ti-M is favorable for the high Figure 6. The specific activity of toluene specific activity. Similar behavior has hydroxylation as a function of Si/AI ratio been observed on hydrothermally synin Ti-M. Reaction conditions: eat., 50 mg; thesized [AI, Ti]-Beta in the olefin oxidatemp., 363 K; substrate, 20 ml; H202 (30 tion [8]. As all of Ti-M catalysts with wt%), 1 ml; H20, 2 ml; time, 2 h. various AI contents exhibited only the 220 nm band in their UV spectra and showed the characteristic band at 963 cm-1 in their IR spectra, the lower activity observed over Ti-M with Si/AI ratio below 200 cannot be due to the presence of nonframework Ti species. The dealumination is suggested to increase the hydrophobicity of the zeolites, which may result in higher catalytic activity, since the reaction was performed under aqueous condition. Furthermore, the electronic density around Ti sites is reported to be altered by the AI atoms nearby [8]. Therefore, the lower AI content is expected to result in M-free Ti sites in Ti-M, which generates the higher activity of Ti atoms. Hydroxylation of various aromatics. The catalytic properties of Ti-M sample were studied by comparing with those of TS-1 for the hydroxylation of various aromatics with different molecular sizes. Ti-M(245) was used to lower the influence of AI on the activity assuggested by Fig. 6. The Ti content in Ti-M(245) was almost the same as that in TS-1 (Si/Ti=104). For both TS-1 and Ti-M, products generated by three reaction paths were observed, that is, the direct hydroxylation of benzene ring to cresols (phenol for benzene), the substitution of hydroxyl group for side-chain alkyl groups to yield phenol and the oxidation of side-chain alkyl groups to corresponding alcohols, aldehydes and ketones. The amounts of alcohols, aldehydes and ketones through the third path were generally small (< 1%) and comparable to those obtained without catalysts. Thus, the oxidation of side-chain alkyls is a noncatalyzed reaction and is not considered for the activity comparison between TS-1 and Ti-M. Figure 7 compares the catalytic activity and selectivity of TS-1 (a) and Ti-M (b) for the hydroxylation of aromatic substrates with single alkyl group. It can be seen that TS-1 and Ti-M showed comparable turnover number in the case of the hydroxylation of the smallest substrate, benzene. The activity of TS-1 for toluene decreased to less than half of that for benzene, and decreased further when bulkier substrates were
668
~
Benzene ring hydroxylation Side-chain substitution
6O
16
>-5_0
).,, 12
"6 4O 3o
20 o
1o
r-I a
b
Benzene
a
b
a
gxl b
T E Substrate
0 b a
a
C
b t-B,
Figure 7. Hydroxylation of various aromatics over TS-1 (a) and Ti-M(b). Cat.: TS-I(104), (Ti: 0.151 mmol g-l); Ti-M (245), (Ti: 0.150 mmol g-l). Reaction conditions: see Figure 6. T=toluene; E= ethylbenzene; C=cumene; t-B=t- butylbenzene.
a b p-Xy
a b a b o-Xy m-Xy Substrate
Figure 8. Hydroxylation of xylene isomers over TS-1 (a) and Ti-M (b). reaction conditions: see Figure 6.
used. TS-1 was completely inactive for t-butylbenzene. On the other hand, Ti-M showed a little higher activity for toluene than that for benzene. Although the activity of Ti-M decreased gradually for larger substrates, Ti-M was still active in the case of t-butylbenzene hydroxylation. There could be two factors dominating the reactions of these alkyl aromatics on zeolite catalysts. Electron-donating alkyl groups attaching to the benzene ring would increase the electrophilicity of substrates, and subsequently promote the ring hydroxylation in the order: -C(CH3)3 >-CH(CH3)2 >-CH2CH3>-CH3. The bulkier alkyl groups, however, are expected to retard the reaction rate due to the diffusion limitation and/or to steric hindrance for transition states. The reactions of the bulkier substrates on the medium-pore TS-1 catalyst might be dominated by the second factor. Therefore, the activity of TS-1 decreased to zero in the hydroxylation of t-butylbenzene. In the case of large-pore Ti-M, the first factor might dominate slightly in the hydroxylation of toluene to make its activity a little higher than that for benzene hydroxylation. For the substrates larger than toluene, the second factor~ the diffusion limitation, becomes to play a leading role in the reaction, resulting in the decrease in activity. Nevertheless, Ti-M always showed higher activity for ring hydroxylation of alkylbenzenes than TS-1, indicating that Ti-M is a potential catalyst especially for the hydroxylation or oxidation of bulky molecules. As shown in Fig. 7, Ti-M showed surprisingly high activity yielding phenol from cumene. Phenol was produced accompanied with a similar amount of 2-propanol and acetone, just like the commercial process of cumene
669
oxidation for phenol formation. Figure 8 compares the catalytic activities of Ti-M with those of TS-1 in the hydroxylation of xylene isomers. The main products were xylenols (>99 %) over both catalysts. Ti-M showed comparable specific activity to that of TS-1 for pxylene, while it showed much higher activity than TS-1 for bulkier o- and m-xylene. The lower activity for p-xylene than for o- and m-xylene over Ti-M must be due to the lowest reactivity of p-xylene itself among the three isomers. Therefore, these three isomers may have comparable diffusion rates within the 12-ring channels of mordenite. On the other hand, TS-1 was almost inactive for o- and m-xylene, indicating o- and m-xylene have much lower diffusion rates than p-xylene within the 10-ring channels of TS- 1. Competitive hydroxylation of toluene with other aromatics. In order to clarify the diffusion of aromatics inside the zeolite channels, the competitive hydroxylation of toluene with ethylbenzene, cumene and t-butylbenzene was carried out on both Ti-M and TS-1. Figure 9 shows the results obtained over Ti-M. When the competitive reaction of toluene with ethylbenzene was performed, the specific activity for toluene decreased to lower than half of the activity observed for the independent hydroxylation of toluene and was almost the same as that for ethylbenzene. When the competitive molecules were changed to cumene and tbutylbenzene, the activity for toluene hydroxylation decreased further to be similar to those for cumene and t-butylbenzene, respectively. It is indicated that all of the substrates used here are able to diffuse into the mordenite channels. When two kinds of molecules are present simultaneously within the channels of zeolite, the diffusion
60
20 Ti-M
"-' 50
-r
TS-I 15
o
"~30 "6 20
~1o o
Z 5 O 0 T
T
E T C Substrate
L.~
r'-I
T
t-B
Figure 9. Competitive hydroxylation of toluene with other aromatics over Ti-M. Reaction conditions: toluene, 10 ml; competitive substrate, 10 ml; others, see Figures 6 and 7. Abbreviations are the same as those used in Figure 7.
0 T
T
[--I ,---. E T C T Substrate
t-B
Figure 10. Competitive hydroxylation of toluene with other aromatics over TS-1. Reaction conditions: toluene, 10 ml; competitive substrate, 10 ml; others, see Figures 6 and 7.
670 rate of the smaller molecule is controlled by the larger molecules having lower diffusion rate [15]. Consequently, the activity for toluene was lowered by the presence of larger molecules. In the case of TS-1, the hydroxylation of toluene was retarded by presence of ethylbenzene and cumene but was little affected by tbutylbenzene (Fig. 10). The molecules of ethylbenzene and cumene with very low diffusion rate would hinder the diffusion of toluene in the channels of TS-1, while tbutylbenzene hardly diffusing into the channels would not hinder the diffusion of toluene significantly. 4. CONCLUSION Ti-containing mordenites, Ti-M, with tetrahedrally coordinated Ti atoms in the framework sites can be prepared by the atom-plating method using dealuminated mordenites and TiCI4 vapor. Ti-M with a low AI content is an active catalyst for the hydroxylation of aromatic substrates with 1-1202. For the reactions without serious steric restrictions, Ti-M shows a specific activity comparable to that of TS-1. On the other hand, the large-pore Ti-M is more effective for the reactions of bulkier aromatics than the medium-pore TS-1. REFERENCES
1. T. Taramasso, G. Perego and B. Notari, US Patent No. 4 410 50(1983). 2. C. Neff, A. Esposito, B. Anfossi and F. Buonomo, Eur. Patent, Appl. 100 119(1984). 3. T. Tatsumi, M. Nakamura, S. Negishi and H. Tominaga, J. Chem. Soc. Chem. Commun., (1990)476. 4. G. Bellusi and M. S. Rigutto, Stu. Surf. Sci. Catal., 85(1994)177. 5. A. Thangaraj, S. Sivasanker and P. Ratnasamy, J. Catal., 131(1991)394 6. J.S. Reddy, R. Kumar and P. Ratnasamy, Appl. Catal., 58(1990)L1. 7. D.P. Serrano, H.-X. Li and M. E. Davis, J. Chem. Soc. Chem. Commun., (1992) 745. 8. A. Corma, M. A. Camblor, P. Esteve, A. Martfnez and J. P6rez-Pariente, J. Catal., 145(1994)151. 9. A. Tuel, Zeolites, 15(1995)228. 10. A. Tuel, Zeolites, 15(1995)236. 11. T. Blasco, A. Corma, M. T. Navarro and J. P6rez-Pariente, J. Catal., 156(1995)65. 12. K. Y amagishi, S. Namba and T, Y ashima, Stu. Surf. Sci. Catal., 49(1989)459. 13. P. Wu, T. Nakano, T. Komatsu and T. Yashima, Stu. Surf. Sci. Catal., 90(1994)295. 14. P. Wu, T. Komatsu and T. Yashima, J. Phys. Chem., 99(1995) 10923. 15. S. Namba, K. Sato, K. Fujita, J. H., Kim and T. Yashima, in" Proceedings of the 7th International Zeolite Conference", Y. Murakami, A. Iijima and J. W. Ward (eds.), Kodansha, Tokyo, (1986)661.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
671
C H A R A C T E R I Z A T I O N O F Z E O L I T E B A S I C I T Y U S I N G IODINE AS A MOLECULAR PROBE S. Y. Choi, Y. S. Park and K. B. Yoon* Department of Chemistry, Sogang University, Seoul 121-742, Korea The visible absorption band of iodine adsorbed on zeolite blue shifted with the increase in the electropositivity of the counter cation and the aluminum content in the framework. Since the visible band of iodine in solution has been known to blue shift with the increase in the basicity (donor strength) of the solvent, the blue shift over zeolite was attributed to the increase in the basicity of the zeolite framework, from the consideration of zeolite as a solid solvent. The framework structure, moisture content and the degree of NH3 loss from NH4+-exchanged zeolites also greatly affected the absorption band of iodine. The results established the charge-transfer interaction between the adsorbed iodine and the zeolite oxide surface which allow iodine to be used as a novel molecular probe for the systematic evaluation of the zeolite basicity. 1. I N T R O D U C T I O N
Iodine has long been known as a prototypical solvatochromic compound whose color, hence the visible absorption band changes dramatically depending on the (electron) donor strength of the solvent[i,2]. For instance, it is violet in carbon tetrachloride as in the vapor, red in benzene, various shades of brown in alcohols and ethers, and pale yellow in water. The visible absorption of free iodine arises from the transition of an electron from n* to o*. However, it has been proposed that the relative energy of the latter is significantly perturbed in electron-rich solvents because of the donor-acceptor interaction between iodine and solvents[3] Thus the stronger the donor strength of the solvent, the higher the energy level of o" orbital shifts up resulting in the hypsochromic shift of the visible absorption band. Accordingly, the visible absorption bands of iodine can be related to the donor strengths of various solvents. In this abstract, we present the dramatic shifts of the visible bands of iodine adsorbed on various zeolites, with the change in the Si/A1 ratio, the electronic nature of the cation, the dehydration temperature, and the framework structure, and the interpretation of the shifts in terms of the change in the donor strengths of zeolites, from the consideration of zeolites as electrolytic solvents[4]. 2. E X P E R I M E N T A L
LTA(1.0), FAU(2.6), LTL(3.3), MOR(21), MOR(34), ZSM-5(900) were purchased from Union Carbide. FAU(1.2) was purchased from Strem. LTA(x) (x -- 1.4, 2.1, and 2.3), FAU(x) (x = 1.0 and 3.4), ZSM-5(x) (x = 50 and 150) were obtained from S. B. Hong of the Korea Institute of Science and
672 Technology. MAZ(4.2) was obtained from D. R. Corbin of the DuPont Company. ZSM-5(x), x -- 14 and 28, were kind gifts from ALSI-PENTA Zeolithe GmbH. All the organic templates included within zeolites during the preparations were removed by heating at 500 °C under flowing oxygen. The ion exchange was carried out by refluxing the zeolites in an aqueous solution of concentrated (0.5-1 M) salts at least 5 times to ensure the complete or maximum exchange. H*-exchanged zeolites were prepared from the corresponding NH4-forms~ The ion-exchanged samples were rigorously dried at 300 °C in uacuo (<10 -~ Tort) for 15 h, unless stated otherwise. The dried zeolites were stored in an argon-filled glovebox and 200 mg of each sample was transferred into a flat cylindrical cell equipped with a greaseless stopcock. The cell was removed from the glovebox and connected to an arm of an inverted glass U-tube which has an access to a vacuum line through a greaseless stopcock. Independently, a glass tube containing solid iodine was connected to the other arm of the inverted U-tube. After brief evacuation of both arms in a sequential manner, the zeolite sample was allowed to adsorb iodine vapor at room temperature. The white zeolite powder turned intense yellow to purple within 10-15 min depending on the types of zeolites and cations. The amounts of iodine adsorbed onto zeolite samples were measured by weighing the sample cell on an electronic balance (Mettler 240). The diffuse reflectance UV-Vis spectra of the colored samples were recorded on a Shimadzu UV-3101PC equipped with an integrating sphere. 3. R E S U L T S AND DISCUSSION The various colorless zeolite powders immediately turned light yellow, brown or pink upon exposure to iodine vapor. Accordingly, the diffuse reflectance UV-VIS spectra of the colored samples revealed distinct absorption maxima (kmax) in the visible region. The colors intensified upon increasing the adsorbed amounts with the exception of K+-LTA zeolites which adsorbed only a limited amount of iodine (less than 10 mg per gram of zeolite), due to its smaller pore opening (3 A) than the kinetic diameter of iodine (4 A). Interestingly, the increase in the adsorbed amount generaUy led to a significant red shift of the visible band in consistent with the result of Barrer and Wasilewski[5a]. This phenomenon was attributed to the long-range electron withdrawing effect of the adsorbed iodine molecules through the zeolite framework, since the portion of electron density to be donated from a zeolite crystal to each iodine would decrease with the increase in the number of the adsorbed molecule, by considering each zeolite microcrystal as a giant polymeric inorganic molecule possessing multiple basic sites. In consistent with this interpretation, Barter and Wasilewski observed a sharp decrease in the isosteric heat of adsorption with the increase in the adsorbed amount at the initial stage of adsorption [5b]. In order to compare the visible bands of iodine on various zeolites, the adsorbed amount was fixed to --30 mg per g of zeolite which gave rise to the absorbance of --0.5 (arbitrary) in the diffuse reflectance spectra[6]. Figure 1 shows the visible absorption bands of iodine adsorbed on a series of alkali-metal-exchanged synthetic faujasite designated as M+-FAU(n), where M" and n represent alkali metal cation and the Si/A1 ratio, respectively[7]. Most of all, all the absorptions occurred at significantly shorter wavelengths than the absorption band of iodine in CC14 (which closely resembles that of free iodine in the vapor phase). Assuming zeolites as solid solvents[4], the blue shifts were proposed to arise from the remarkable donor strengths of the negatively charged zeolite frameworks. Interestingly, the direct comparison of
Table I. The Visible Absorption Band of Iodne Adsorbed on Various Zeolitesa. Effect of Cation and Si/Al Ratio. LTA
FAU
ZSM-5
Si/Al
Si/Al
Si/Al
Cation 1 1.4 2.1 2.3 1 -
1.2 2.6 3.4
14
-
28
50
LTL
150 900
--
434
448
475
485
420 421
478
486 483 501 507 511
Li'
481
483
502
502
443
501
507
Amax in nm. 'weak absorption. 'Dehydrated at 500C.
MOR
Si/A1 Si/Al
Si/Al
--
Na'
a
MAZ
--
-
3.3
4.2
21 34
477
486
474 490
493
493
482 498
-
674
IFAU(I.2) K+
Na + Li +
1
CCl 4
" " .q~ °.,~ ° " ' " °~,l o
",,,
.~.o%
oO'°. . . . . . °%. o°." °Oo.
.~.:
L, , : , . '
J
"....
- "% oO°"
""
".,
I
,,
~,
-i
"%,
°°%
%°°°
....... a.... ..-,.-'."';'"'?"
-
AU(2.s)
. v
#m
K+ j'~
-
Na+Li +
J"
.j~,"
. _
.."
-''.~..____..,~-~ ........ -
....-...'•
.................
' ........ "":'":"
%
%.
~ ~
~. "...'~..,.. "..-.~
oO
•
K+
J-----~
L
CCI.~
~,." , . ' ~ <..'~..~..... ,,. ..- ,,, ",,... .~ ~ s" ." "~ ""'. ~j,1
. . . .
'
. . . . .
,
,
,
,
• ....
I
,
,
i''¢
~
N a + Li + CCl~
~ -- ~ ~.~--'-~......
n
•%,
400
_
'~..~
*%. °% %°°° ' ~q" q,m
Wavelength(nm) 500
600
Figure 1. Diffuse reflectance UV-Vis spectra of iodine adsorbed on various alkali metal exchanged faujasites with Si/A1 ratios of 1.2 (top), 2.6 (middle), and 3.4 (bottom), respectively. For comparison, the absorption band of iodine in CC14 is represented in the dotted line ( .... ). the absorption maximum (~max) of iodine on K+-FAU(1.0) listed in Table I (387 nm) with the values obtained from various solvents suggested that the anionic framework of the particular zeolite (W-exchanged zeolite-X with Si/A1 of 1) has a remarkable donor strength that far exceeds that of alkyl or aromatic amines (Xm~x = 410-420 rim). However, the validity of the literal comparison remains to be checked since the strong electrostatic fields of zeolites also give rise to the blue shifts of iodine band. Most interestingly, the result clearly demonstrated that the donor strength
675
A 3.4
I ....
B
I ....
) ....
3.4
o FAU(1.2) FAU(2.6) @ FAU(3.4)
3.2
"
)
"0
3.2
3.0
e..). ~ @. " ' "•~.....,,
(D
",,.
....
~
,.Q • r..,I rn 2 . 6
O r-FAU Na+-FAU @ Li+-FAU Na÷-LTA
O
.
\% "~."<:..
\\
"Oo~
~.~
~.
",,.~k.~ -
.r-,l
-
2.6
"~,, Q - . ~
:a..
-
\
"t', @ \ x
*o % ~. "%. o.. % ' % @ "o .,, -o o.
" .
2.4 K I
•
0.40
,
,
,
I
.
2.8
O
"".'b....~ ~o
r.-,4
I ....
7 =
\
I ....
(~ .
3.0
,.a 2 . 8
''l
,
0.45
,
,
,
Na I
,
Li
0.50
Ip (MJ/mol)
,
2.4 ,
,
.,,
,
I
1
,
,
,
,
I
,
,
,
,
i
2
3
Si/A1
ratio
,
"',
,
,
4
Figure 2. The negative linear correlation of the visible absorption energy of iodine adsorbed on various alkali metal exchanged zeolites (as indicated) with (A) the ionization potential (Ip) of the alkali metal element and (B) with the Si/A1 ratio. of the framework increases with the increase in the electropositivity of the counter cation in consistence with other experimental[8-14] and theoretical results[15]. Thus as illustrated in Figure 1 for a series of FAU(n) zeolites, the visible band of iodine experienced a pronounced blue shift upon changing the cation from Li+ to Na+ and to K+. The plot of the absorption energy against the ionization potential (Ip) of the alkali metal element shows a well correlated negative linearity as shown in Figure 2A. Since the electrostatic field strength is expected to decrease with the increase in the size of the cation, the result could not be attributed to the increase in the electrostatic field strengths. As a corollary, it was concluded that iodine does not adsorb on the cation, but forms a charge-transfer complex with the negatively charged framework oxygen in consistence with the X-ray crystallographic result of Serf and Shoemaker[16] and the result of Barrer and Wasilewski[Sb] obtained from the calculation of the energy of iodine adsorption. Interestingly, H-exchanged zeolites generally gave iodine bands at longer wavelengths than the alkali-metal-exchanged zeolites, indicating the more acidic nature of the frameworks (see Table I). The degree of hypsochromic
676 shift induced by the counter cation gradually diminished with the decrease in the aluminum content, hence with the decrease in the number of the cation. This further emphasized the importance of the cation in governing the donor strength of the zeolite framework (see Figure 1 and also compare LTL with MAZ or MOR in Table I). In consistence with this, the variation of the divalent cation (M2÷ - alkaline earth metal ion) showed essentially no effect on the absorption maxima of iodine over the channel-type zeolites _with relatively high Si/A1 ratios, in contrast to the sensltwe shift over M -FAU(2.6) with a lower Si/A1 ratio. The somewhat n-regular value obtmned from Mg -FAU(2.6) dehydrated at 300 C was ascribed to the incomplete removal of hydrated water from Mg 1on as discussed later. This result was attributed to the substantial decrease in the number of the cation (only half the number of the corresponding monovalent cation) within the aluminum-pool zeolites. Nevertheless, it was noted that most of the iodine bands over MZ*-exchanged zeolites shifted to longer wavelengths than those of M*-exchanged zeolites, despite the diminished number of cations. This coincide with the greater extent of electron withdrawing from the framework by the more electronegative divalent cations. Therefore, the result further supported the donor-acceptor interaction between the framework and the counter cation in zeolites. The close examination of the absorption maxima in Figure 1 also revealed that iodine band progressively red shifts with the increase in the S i/A1 ratio (compare top to bottom). Since the electrostatic field strength in the vicinity of the cation increases with the increase in the Si/A1 ratio, this result cannot be attributed to the decrease in the field strength. Instead, it was attributed to the decrease in the donor strength of the framework as a result of the decrease in the negatively charged aluminum sites, in consistent with other reports[8-15]. In support of this, the plot of the absorption bands vs Si/A1 ratio shown in Figure 2B revealed an excellent negative correlation. The appearance of the iodine absorption band at 520 nm on the virtually aluminum-flee zeolite (Si/A1 = 900) i.e., silicalite, further supported this argument (see Table I). The same trend was observed from other zeolites in Table I. This result further supported the conclusion that iodine interacts with the framework oxygen. The careful examination of the data in Table I and Figure 2B also revealed that the visible band of iodine over Na*-LTA(n) invariably appears at lower energy region than the corresponding value over Na~-FAU(n). This fact allowed us to suggest that the framework of FAU, in general, has higher donor strength than that of LTA. The iodine adsorption could also be utilized to delineate the effect of moisture on the framework donor strength. For example, as listed in Table II, the iodine band over Na~-FAU samples, progressively blue shifted upon increasing the dehydration temperature (To). Interestingly, however, the values remained nearly constant above certain temperatures, i.e., above --200 °C for Na*-FAU(2.6) and --250 °C for Na+-FAU(1.2), respectively. We believe the initial blue shifts arise from the removal of the surface-lining water which otherwise would prevent a direct interaction between iodine and the negatively charged frameworks. The near constant values of absorption maxima at higher Td indicated that the framework donor strengths are not further affected by subsequent dehydration at higher temperatures. The more pronounced degree of spectral shift over FAU(1.2) than over FAU(2.6) was attributed to the higher donor strength of FAU(1.2) than FAU(2.6) in the dry state. Furthermore, the higher Td (> 250 °C) observed over FAU(1.2) than over FAU(2.6) to reach a constant wavelength also reflected the higher •
•
•
•
o
•
Z+
•
•
•
•
•
2+
~-~-
•
677 Table U. The Visible Absorption Band of Iodine Adsorbed on Na +and NH4*FAU. a Effect of Dehydration Temperature.
Temp b
Na + - FAU
NH4 + - FAU
Si/Al
SVAI
1.2
2.6
1.2
2.6
25
-- 440¢
491
-- 443 ¢
-- 472 ¢
50
-~ 440 ¢
491
-- 432 ~
---471 ¢
1O0
437
486
-~ 432 ~
462
150
431
482
447
466
200
424
478
469
479
225
422
470
488
250
420
470
490
300
421
472
492
485
491
487
492
478
350 400
421
479
450 500 a ~ max
492 420
480
487
492
in nm. bin °C. ~Shoulder band.
hydrophilicity of FAU(1.2) than FAU(2.6), due to the greater numbers of cations and anionic sites in the framework. By contrast, however, the iodine band on NH4*-FAU(2.6) progressively red shifted at Td between 150 and 300 °C followed by an initial blue shift due to water loss until Td reached --100 °C. This contrasting behavior was ascribed to the generation of highly electronegative H ÷ from NH4÷ by the gradual loss of NH3 at Td between 150 and 300 °C. This result further revealed that deamminafion of NH4*-FAU(2.6) starts above ---100 °C. A similar trend was observed from NH4+-FAU(1.2). However on this zeolite, a sudden, discontinuous red shift of iodine band was observed at Td > 300 °C. An independent X - r a y powder diffraction analysis revealed that serious breakdown of the framework occured at Td > 300 °C. By the s~n+e analogy, the unusually blue shifted iodine band observed from M g " - e x c h a n g e d FAU(2.6) which was d ~ y d r a t e d at 300 °C (see Table I) was attributed to the strong hydration of Mg ~÷ owing to its extremely high positive charge density, since hydration would decrease the electronegativity of Mg z+ in the same way NH3 did to H + in the above experiment. In conclusion, the results established that iodine can serve as a convenient and highly efficient molecular probe for the quantitative evaluation of zeolite
678 basicity (donor strength) which varies depending on the Si/A1 ratio, the nature of cation, the structure, and the dehydration temperature. ACKNOWLEDGMENT
We thank the Aimed Basic Research Program of the Korea Science and Engineering Foundation (KOSEF), the Ministry of Education (MOE), Korea, and the Center for Molecular Catalysis (CMC) of Seoul National University for financial support. We also thank S. B. Hong and D. R. Corbin for providing some of the zeolite samples and the referees for the valuable comments. REFERENCES
1. J. H. Hildebrand, Science, 150 (1965) 3695. 2. E. M. Voigt, J. Phys. Chem., 72 (1968) 3300. 3. R. S. Drago, Physical Methods for Chemists, Saunders College Publishing: Ft. Worth, 1992, 2nd ed. p. 134. 4. J. A. Rabo, C. L. Angell, P. H. Kasai and V. Schomaker, Discuss. Faraday Soc., 41 (1966) 328. 5. (a) R. M. Barrer and S. Wasilewski, Trans. Faraday Soc. 57 (1961) 1153. (b) R. M. Barrer and S. Wasilewski, ibid, 1140. 6. Although the controlled amounts were very small compared to the maximum adsorbed amounts ( ) 500 mg per g of zeolite)[~], we believe most df them are incorporated within the pores, since the external surface area corresponds to only - - 1 % of the total surface area, and a much lesser amount was adsorbed on K*-LTA whose pore opening is smaller than the kinetic diameter of iodine. 7. The terms used for the zeolites used in this report were mostly taken from their structure codes since some zeolites have different names depending on the Si/A1 ratio, despite the identical structures. 8. Y. Okamoto, M. Ogawa, A. Maezawa and Imanake, T., J. Catal., 112 (1988) 427. 9. (a) M. Huang, A. Adnot and S. Kaliaguine, J. Am. Chem. Soc., 114 (1992) 10005. (b) M. Huang, A. Adnot and S. Kaliaguine, J. Catal. 137 (1992), 322. 10. (a) T. L. Barr and M. A. Lishika, J. Am. Chem. Soc., 108 (1986) 3178. (b) T. L. Barr, Zeolites, 10 (1990) 760. 11. V. K. Kaushik, S. G. T. Bhat and D. R. Corbin, Zeolites, 13 (1993) 671. 12. J. Stoch, J. Lercher and S. Ceckiewicz, Zeolites 12 (1992) 81. 13. (a) D. Barthomeuf, J. Phys. Chem. 88 (1984) 42. (b) See also D. Barthomeuf, in Acidity and Basicity of Solids, Fraissard, J.; Petrakis, L., Eds. NATO ASI Series C 444, Kluwer Academic, 1994, p. 181. 14. M. Huang and S. Kaliaguine, J. Chem. Soc., Faraday Trans. 88 (1992) 751. 15. W. J. Mortier and R. A. Schoonheydt, Prog. Solid St. Chem. 16 (1985) 1. 16. K. Serf and D. P. Shoemaker, Acta Cryst. 22 (1967) 162.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
679
Ship-in-Bottle Synthesis of Pt and Ru Carbonyl Clusters in NaY Zeolite Micropore and Ordered Mesoporous Channels of FSM-16; XAFS/FTIR/TPD Characterization and Their Catalytic Behaviors Masaru Ichikawa, Takashi Yamamoto, Wei Pan#, and Takafumi Shido Catalysis Research Center, Hokkaido University, Sapporo 060, Japan
Abstract Ru3 (CO)I 2, H4Ru4(CO)I 2, [Ptg(CO)18] 2- and [Pt12(CO)24] 2- were synthesized in NaY cages by "ship-in-bottle" technique. Thermostable robust platinum clusters [Ptl5(CO )30] 2- combined with R4 N+ (R=Me,Et,Bu and Hex) and MV2+ Were encapsulated in the ordered hexagonal mesoporous channels of FSM-16(27.5A diameter) which were characterized by FTIR, EXAFS and HRTEM methods. They remain their flexibility of duster frameworks and exhibited remarkedly higher catalytic activities for the water-gas-shift reaction, compared with Pt9-Ptl2 clusters restricted in NaY micropores(12A). Highly dispersed Pt aggregates (ca 15A size) in FSM-16 channels were uniformally prepared from the Ptl5 carbonyl clusters by the evacuation at 323-473K to controlled removal of CO. The coordinatively unsaturated Pt aggregates in FSM-16 exhibited catalytic activities in the hydrogenation of ethene and butadiene selectively towards butenes. 1. INTRODUCTION Zeolites are aluminosilicate crystallines consisting of microporous cages of molecular dimensions(8-12 A) which are interconnected with smaller windows and channels(6-8A). Such micropores can supply "templating" circumstances for the selective synthesis of some bulky metal carbonyl clusters which fit the interior cages as ultimate "nanometer reaction vessels". This is coined with "ship-in-bottle" synthesis and may open new opportunities for the rational design of tailor-made catalysts[l] of discrete metal/alloy dusters having uniform sizes and metal compositions and with sufficient stability against a metal sintefing and leaching under the prevailing reaction conditions. Previously, uni- and bimetal carbonyl clusters such as Rhf_xIrx(CO)16(0-6) [2], [Pt3(CO)6]n2-(n=3,4)[3], [Ru6(CO)1812-[4], [Fe2Rh4(CO)1512-[5] and [HRuCo3(CO)12] [6] were synthesized in NaY and NaX zeolite cages, which were characterized by EXAFS/ XANES, FTIR,129Xe NMR, HRTEM and Raman spectroscopies. They are useful for preparing discrete metal/alloy clusters(less than 10A size) which catalyze the alkane hydrogenolysis[2], CO hydrogenation towards C 1-C5 alcohols[2,6] and olefin hydroformylation reaction[5]. There are current interests in mesoporous materials such as MCM-41[7] and FSM-16[8] having their honeycomb structures with ordered enormous channels of 20100 A diameters, which are larger than microporous cavities of conventional zeolites such as NaY, ALPO4-5 and ZSM-5. They are potential hosts for inclusion of bulky organic and inorganic species for new application to design of tailored metal catalysts[l] acx~ssible for larger substrates. In view of the relative interests in their mobility/flexibility and reactivity of #Permanent address: DeI~mment of Chemical Engineering, Chinghua University, Beijing, China.
680 the intrazeollitic clusters, we have extended to proceed a "Ship-in-Bottle" synthesis of Ru and robust Pt carbonyl clusters in NaY micropores and the ordered mesoporous channels of FSM -16. Their structmal flexibility and thermostabilities of metal clusters in micro and mesoporous constraint have been discussed in conjunction with their catalytic behaviors in 13CO exchange reaction, water-gas-shift reaction(WGSR) and hydrogenation of ethene and butadiene in terms of reactivities and selectvities.
2. EXPERIMENTALS AND PROCEDURES 2.l.Catalyst Preparation Ru3(CO)12/NaY (2-4 wt% Ru) was prepared by vapor deposition method. The mixture of Ru3(CO)12 and NaY was heated at 354K for 4 days, where sublimation of Ru3(CO)12 proceeded onto NaY. Ru3(CO)12/NaY was converted by the reaction with with H2 at 343K into H4Ru4(CO)12/NaY(2082m, 2064s, 2032m, 2021m and 2001w cm1). 2.0-5.0 wt% pt2+/NaYwere prepared by cation-exchange of NaY(LZY-52 from TOSO Chem. Co.; Si/AI=5.6) with Pt(NH3)4CI 2 at 363K in aqueous solution. [Pt12(CO)24] 2/NaY (v(CO)=2080vs and 1841s cm-1; 290, 445 and 640 nm) and [Pt9(CO)18] 2/NaY(2056vs and 1798s cm-1; 435 and 710 nm) were synthesized from pt2+/NaY and Pt(NH3)4/NaY(4 wt% Pt) by the reductive carbonylation with CO/H20 from 288-373K, respectively[3]. The host FSM-16 was synthesized using a polysilicate (Kanemite; NaHSi2053H20) and trimethyl-hexadecylamine chloride as a surfactant template according to published proceAures[8]. After calcination at 823K, the resulting material (surface area,950 m 2/g) presents ordered hexa-gonal channels(27.SA) with silanol groups(3745 cm1) characteristic of the X-ray powder patterns in low angle region(20=2.26, 3.44, 4.50 and 5.90) and the TEM observation. The NaY/FSM-16 encaged Ru and Pt carbonyl clusters were subjected to the controlled evacua-tion at 323-423K and 10-4 torr to remove CO, followed by the reduction in H2 flow(1 bar, 40 ml/min) at 293-673 K. 2.2 Reactivity Study The reactions were carried out using a dosed circulating microreactor in which 150-350 mg of the catalyst was charged prior to the catalytic probing reactions such as WGSR and olefin hydrogenation. Product analysis was performed by a periodical sampling of the effluent gas, which was quantitatively analyzed by TCD and FID gas chromatography using silica/Porapak Q and VZ-10 columns and on line with Gc-mass (Parkin-Elmer ; 50 m capillary columun) for isotope-labbeled products. 2.3. Infrared and UV-vis experiments The catalysts were pressed into self-supporting wafers(15 mm i.d., 12-20 mg/cm2) which were placed into an infrared cell equipped with CaF2 windows. Infrared and diffuse reflectance UV-vis spectra were measured at various coniditions using a bouble-beam Fourier transform IR with a resolution of 2 crn"1 and Hitachi-330 spectrophotometer. For the isotope-labelling experiments 13CO (90% enriched) was purchased from MSD Isotope Co. 2.4. EXAFS Spectroscopy The samples(2-5 wt%Ru and Pt) were charged under N2 in an in-situ EXAFS cell with KAPTON film windows(500 mm) to prevent exposure to air. EXAFS(Extended X-Ray Absorption Fine Structures) measurements were carried out similarly as reported[2-6] at SOR beam line 10B with a Si(311) channel-cut monochrometor in the Photon Factory in the National Laboratory for High Energy Physics(KEK-PF) using synchrotron radiation with an electron energy of 2.5 GeV at current of 250-360 mA. The EXAFS spectra were measured at the Ru K edge(22120 ev) and Pt L3(11549 ev).
681 3. RESULTS AND DISCUSSION 3.1. Ship-in-bottle Synthesis of Ru3 and Ru4 Carbonyl Clusters in NaY About 1.0 gr of NaY (evacuated at 673K to remove water) was mixed with Ru3(CO)12 crystal (Strem Chemicals Co.99.% purified from n-hexane solution) under nitrogen atmosphere, which was heated at 354K for 4 days, where sublimation of Ru3(CO)12 proceeded onto NaY. The XRD(x-ray diffraction, Cu-K) patterns suggested that the crystal-line phase of Ru3(CO)I 2 at 20 =12.60, 16.44, and 30.42) mixed with NaY zeolite phase was broaden and diappeared after the thermal heattreatment at 354K, leaving only those of NaY crystals. The Ru contents of the aging samples were determined by using an inductively coupled plasma-atomic emission(ICP) spectrometer to evaluate the residual Ru laoding which was almost invariant before and after the heattreatment of the mechanical mixture. This suggests that a gaseous Ru3(CO)12 is diffused and highly dispersed into the NaY supercages by sponteneous monolayer dispersion, similar to [Au(I)CI]/NaY[9]. The IR spectra gave intense bands at 2068s and 2028s crn-1, which quite resemble those of Ru3(CO)12 (2060s, 2026s and 2002m tan -1) in hexane solution. The resulting orange-colored material was very reactive with H2 (100 torr) using a closed circulating system by removal of a trace of water with a liq.N2 trap at 323-363K led to the formation of hydridocarbonyl cluster in NaY(2082m, 2064s, 2032m, 2021m and 2001w crn-1), which resemble those of H4Ru4 (CO)12 in cyclohexane solution (2081m, 2067s, 2031m and 2024m cm-1). To obtain more insight into the structure of Ru carbonyl clusters formed in NaY zeolites, EXAFS spectra of Ru K-edge were measued for the samples of Ru3(CO)1.2/NaY (I) and H4Ru4(CO)12/NAY (II) under N2 atmosphere at 300K. The Fourier transform(FT) parameters for the sample of (I) CN(Ru-Ru)=2.0,R= 2.86A) and (II) (CN(Ru-Ru)=3.2,R= 2.80A) are very similar to those of Ru3(CO)12 and H4Ru4(CO)12 diluted in boron nitride. 3.2. Ship-in-bottle synthesis of Ptl$(CO)302" Channel Host and FTIR/EXAFS Characterization.
Encapsulated
in
FSM-16
A sample contaning 5.0 mass% Pt was prepared by impregnation of FSM-16 with an aqueous solution of H2PtCI6. This sample was exposed to CO(200 torr) in a closed circulating system by ramping the temperature from 300 to 323K, resulting in the IR bands at 2188, 2149 and 2119cm -1, as presented in Fig.l(a). From the analogy of the previously reported Pt carbonyl species[10], the IR bands at 2188, 2149 and 2119 cm "1 can be ascribed to cis-Pt(CO)2C12(2188 and 2149 cm -1) and Pt(CO)C13(2119 em-1), respectively. The Pt carbonyls were converted by subsequent admission of H20 vapor(15 torr) onto the CO atmosphere to make an olive-green product(sample A) exhibiting a steady-state spectrum (Fig. l(b)) of carbonyl bands(2081s and 1880m cm -1) and UV-vis reflectance (~,max;452 and 855 nm). The final specman closely resembles those of [E~N]2[PtlS(CO)30] in MeOH (2056s(terminal) and 1872m(bridged) cm -1, Z.max; 408 and 697 nm) and crystal[ll], but with shifting of both carbonyl bands to higher wavenumber(Av=29-10 cm -1) and the visible absorption peaks to higher wave length, respectively. These shifts are consistent with the formation of contact ion pairs[8] between the Pt carbonyl anions and proton or Lewis acid sites on the wall of FSM-16 channels. Attempts to extract the platium carbonyl species from sample A with THF(tetrahydrofuran) and MeOH were tmsucgesful, but with [(Ph3P)2N]C1 in THF proceeded, which gave specifically an appreciable amount of [(Ph3P)2N]2[Ptl5 (CO)30] (2056s and 1872m cm -1; kmax; 405 and 702 nm in T H e . This is consistent with an uniform entrappment of [Ptl5(CO)30] 2- in mesoporous channels of FSM-16, which was prepared by a reductive carbonylation of H2PtCI6/FSM-16, analogous to its synthesis in solution[ll]. It was found that [Ptl5(CO)30] 2- in FSM-16(sample A) is relatively unstable,
682 and evacuation of. the sample A at 10-4 torr and 300-323K led to the irreversible transformation owing to partial removal of CO to give the brownish product (2063s and 1820w cm'l), which resembles those of higher nuclearity Pt carbonyl dusters reported as [Pt55(CO)x] n- .and [Pt38(CO)4412-(2060-2043s and 1832-1820w cm"1) in THF solution [12]. As presented in Fig.2, the electron micrograph of the evacuated sample A showed that platinum aggregates having ca.20 A diameter were uniformly distributed along the ordered
0.2
0
0
c~ /
|
\ct
/
i
2200 2000 18'00 1600 14'00 Wavenumber / cni 1
Fig. 1. In-situ FI'IR spectral changes in reductive carbonylation of H2PtCI6 /FSM-16 with CO and CO+H20 at 323K
Fig.2. TEM of [Pt15(CO)3012-/ FSM- 16 after evacuation(343K)
mesopomus channels of FSM-16 crystals with a negligible formation of the external particles. This was in good agreement of the observed EXAFS parameters (C.N.=8.6;R(Pt-Pt) =2.74A) for the sample A after evacuation at 323K. On the other hand, the thermostable [Pt15(CO)30] 2- in FSM-16 was successfully prepared by using the FSM-16 which was coimpregnated with H2PtCI6 and quartemary alkyl ammonium salts (R4NX;R=Methyl, Ethyl, Butyl and Hexyl; X = CI, Br and OH) and methyl viologen chloride([MV]2Cl) ([CH3N N CH3]2CI') from each aqueous solution. The reductive carbonylation of each coimpregnated sample resulted in an olive-green product (UV-vis reflect-ance AAA and 859 nm), showing the intense CO bands which appeared at 2075-2079s and 1875-1884m cm'I(R4NCI; R=Methyl, Ethyl, Butyl and Hexyl), relatively shifted to higher frequencies by using the larger alkyl ammonium cations. It is worthy to note that those organic cations stabilize the robust Ptl5 carbonyl cluster dianion and increase their thermostabilities in the FSM-16 channels by varying the used quaternary ammonium cations(R4N + and MV2+); None
683 Table 1.Structural Parameters of Pt Carbonyl Clusters Encapsulated in NaY and FSM-16 Compound / shell
C.N.
R / ,/k
AEd eV
Ate2 / A
R factor (%)
[NEt4]2~t_ CO)30]aIPt,4 Pt-Pt Pt-C Pt-C Pt-O
2.1 1.5 1.5 1.0 2.3
2.68 3.07 2. I0 1.89 2.99
6.5 -8.5 0.4 -5.9 -4.6
-1.1 x 5.8 x 2.0 x 1.3 x 4.5 x
10-3 10-3 10-3 10-3 10-3
7.5
[Pt12(CO)2412-/NaY Pt-Pt Pt-Pt Pt-C Pt-C Pt-O
2.2 1.4 1.6 0.9 2.2
2.67 2.99 2.14 1.99 2.99
6.5 -8.5 0.4 -5.8 -4.6
-1.6 7.4 2.0 0.9 1.3
x x x x x
10-3 10-3 10-3 10-3 10-3
12.2
[Pt 15(CO)3o]2-/Et4NCI/FSM-i 6 Pt-Pt Pt-Pt Pt-C Pt-C Pt-O
2.3 1.7 2.2 1.3 2.3
2.68 3.08 2.08 1.90 3.01
3.7 -6.4 -7.6 -10.0 -2.3
-0.7 7.0 5.8 2.6 3.0
x x x x x
10-3 10-3 10-3 10-3 10-3
9.6
,
.
.
.
.
.
.
R/A:interatomic distance CN:coordination number AEo:inner potential correction Ao2:the difference Debye-Waller factor from that of reference compounds
,-,,
of interest to find that for [Ptl5(CO)30] 2Fig.3. FF of z(k)k3 of [Pt9(CO)1812-/ encapsulated in FSM-16(sample B) the avaragNaY and [Pt15(CO)3012-/NEt4/FSM-16 ed interdistance of Pt-Pt between adjoining triplatinum planes (R2= 3.08A; CN2=-1.6) are slighly enlonged, compared with that of [NEt4]2 [Ptl5(CO)30]/BN(D) (R2= 3.07 A;CN2=-I.5) and X-ray analysisof [PPN]2[Ptl5(CO )30] in crystal [Et4N]2[Pt3(CO)6]5 (R2=3.05 A" Nc2 =1.5)[11]. By contrast, the intertriplatinum distances of Ptl2 duster frameworks in NaY micropores (C)(CN2=1.5, R2= [Pt3(CO)6142" 2.99A) are significantly shorter(AR= 0.07A) than that of [PPN]2[Ptl2(CO)30]/BN[ll] (R2=3.06A; CN2=1.5) within the experimentalerrors (AR= [Pt3(C0)615 2" 0.02A). The NMR study by Heatonet. al.[13] on the structure of [Pt15(CO)30]2- in solution 3 4 5 6 0 1 2 showed that the Pt3 triangle was rotated wA fluxionally fluxionally in the Pt3 plane, which cause the wide distribution of the Pt-Pt distances of interPt3 triangle planes. Hence, the slight enlongment of Pt-Pt distance of inter Pt3 planes for sample B may be caused by the fluxional rotation of Pt3 triangle of the Pt15 cartx)nyl clusters (8X12.3A van del Waals diameter) in the enormous channels of FSM-16(27.SA). By contrast, the Pt12 carbonyl clusters(8X10 A rod) encapsulated in NaY micropores may be frozen owing to the intrazeolitic constraint(10-12A effective diameter). •
3.3.
,
,
WGSR on Pt Carbonyl Clusters in NaY and Mesoporous FSM-16.
The WGS reaction(CO+H20-->CO2+H2) was performed at reduced pressure(Pco=50 -200 Torr,PH20=15 Torr) by using a closed circulating Pyrex glass reactor charged with the powdered samples of [Pt15(CO)30][R4N+]/FSM-16(R=Me, Et, Bu), [Ptl5(CO)30] [MV2+]/FSM-16, [Pt12(CO)2412-/NaY and [Pt9(CO)18]2-/NaY at 300-373K. As shown in Table 2, [Pt15(CO)30] 2- clusters with organic cations in FSM-16 exhibited remarkably high activities in the WGS reaction to form an equimolar mixture of CO2 and H2, compared with the Ptl2-Pt9 cluster in NaY and conventional Pt/AI203 catalyst(4 mass %Pt). It was worthy to note that the Ptl5 carbonyl anions coupled with methyl viologen
684 cation as a redox charge-carrier in FSM-16 showed the highest activities for the reaction. The rates of WGS reaction at 300K based on TOF(turn-over-frequency) on [Ptl5(CO)30] 2in FSM-16 are higher(20-100 times) than those on [Pt12(CO)24] 2- and [Pt9 (CO)1812-(4 mass%Pt) entraped in NaY cavities. Moreover, it was demonstrated that the carbonyls of [Ptl5(CO)30] 2- in FSM-16 underwent facile isotopic exchange of 13CO at 300K. On exposure of [Ptl5(CO)30] [Et4N+]/FSM-16 to 13CO(100 torr) for 2 h, the bands of linear CO at 2078 cm -1 was completely replaced by new band at 2013 cm -1. As shownin Table 2, the exchange rates in TOF varied upon the sorts of organic cations in FSM-16, whereas the carbonyl exchange of Pt9 and Ptl2 dusters in NaY micropore proceeded very slowly. From these evidences, it is suggested that the robust carbonyl duster anions e.g., [Ptl5(CO)30] 2(8X13A rod) are acz,omodated and stabilized with organic cations entrapped in the ordered mesoporous channels of FSM-16(27.5A diameter). They exhibit higher activities in the WGSR and 13CO exchange reactions probably owing to their flexible cluster fameworks and sufficient diffusibility of reactant gases, compared with the Pt9 and F'tl2 carbonyl clusters which are restrected in NaY micropore constrain(12A diameter). 3.4. Trasformation of PtI$(CO)302" in FSM-16 by evacuation to Pt aggregates and catalytic behaviors in hydrogenation of ethene and butadiene. It was demonstrated by the in-situ FTIR and EXAFS observzation that a thermostable [ P t l 5 ( C O ) 3 0 ] 2 - ~ / F S M - 1 6 (2075s and 1875m cm -1) was gradually transformed by evauation at 10-4 torr by lamping temperatures from 300-343K, resulting in a partially decarbonylated Ptl5 characteristic of CO IR bands (20(~s and 1880w cm -1) . EXAFS data show the following; the resulting Pt cluster species kept the prismatic triangular Pt frame- work and linear CO ligands unchanged(CNl=2.0;R1 = 2.68A; CN2= 1.5;Rl=3.10A) but their bridging carbonyls are almost completely disappeared by the thermal evacuation upto 450K. Table 2. Catalytic performances in 13CO Exchange and Water-Gas-shift Reaction on [Ptl5(CO)30]2"/NR~FSM-16(R=Me, Et, Bu, Hex and MV), [Pt9(CO)18]2"/NaY, [Ptl2 (CO)2412-/NaY and the conventional Pt/y-A1203(4 wt%Pt) catalyst. Pt carbonyl clusters/ FSM- 16 or NaY
13CO exchange reaction k/min(300K)a
[Pt 15(CO)3012- [NEt4]+/FSM- 16 [Pt 15(CO)30]2-[NBu4]+/FSM- 16 [Ptl 5(CO)3012- [MV]2+/FS M - 16 [Pt12(CO)2412-/NaY [Pt9(CO)18]2-/NaY Pt/y-A1203 c
123 89
WGSR k'/min(300K)b 12 4.8 22
7 9
0.42 0.75
-
0.02
a) 13CO (100 torr);TOF(mmol/Pt atom/min) b) CO(200 torr)+H20(15torr);TOF(CO2)(mmol/Pt atom/min)X10 -2 c) The catalyst was prepared by H2-reduction at 673K for 2 h after H2PtCI6 impregnated on ~,-A1203(4 mass%Pt).
685 A further evacuation of the Ptl5 clusters in FSM-16 at 363K led to the substantial transformation to spherical aggregates of ca 15A diameter coordinated with CO ligands(2065s and 1820w cm -1) which were characterized by EXAFS parameters of CN(Pt-Pt) =7.6; R= 2.74A). The exceeding thermal evacuation of the sample at 393-473K resulted in naked Pt aggregates of 15A size owing to the complete removal of CO. According to the analysis of the EXAFS and PTIR data, the transformations of Pt carbonyl dusters were proposed in the ordered mesoporous channel hosts FSM-16 as dipicted in Fig.4, owing to the controlled removal of CO by evacuationat at 323-473K. As catalytic probing reactions, hydrogenaFig. 4 Proposed transformation of tion of ethene and butadiene has been conducted [Ptl5(CO)30] 2" in FSM-16 by thermal evacuation by IR and EXAFS. at 300K on the [Ptl5(CO)30]2-/NEt4/FSM-16 after the controlled removal of the channel host FSM-16 FSM-16 was inactive for the catalytic hydrogenation of ethene and butadiene when an H2PtCI6 / Et4NC1/ FSM-16 mixture of olefin at H2(P(C2H4)= P(H2)=P (C4H6)=P(H2)=13.3 kPa) was exposed at 300K. The hydrogenation of ethene was initiated but negligibly for butadiene on the dusters in FSM-16 after partial removal CO by evacuation at various temperatures. As shown in Fig. 5, it was interesting to find that the original [Ptl5(CO)30] 2- encaged in of CO by evacuation evac. at 3 2 3 K / evac. at 343K at less than 343K, where the Ptl5 cluster framet i work was kept remained. By contrast, ~i~ ~Pt50"60 butadiene was firstly hydrogenated the resulting P- ~ particle size sample of the Pt carbonyl aggregates of 15A =15A sizeafter the evacuation at 364K, which - v at 473K proceeded selectively to give butenes(1-butene at 363K C.N= 7.6 e :CO C . N = 7.7 (78%)+2-butenes(20%)) with Fig. 5. TOF of hydrogenation of ethene and butadiene on [Ptl5(CO)30]2-/NEt4/FSM-16 after evacuation at various temperatures
70.0
Fig.6. Product selectivities in butadiene on [Ptl5(CO)30]2"/NEt4/FSM-16 after evacuated at various temperatures
80
r
45"01 20.0 [-" 1.00 o so
0.00
. . . . 300 350 400 450 500 Evacuation temperature / K
~
c-C4' 450 500 Evacuation Temperature / K a negligible formation of butane. The in-situ IR spectna suggested that the Ptl5 carbonyl clusters after the thermal evacuation at 364K kept CO cordinated(2065s and 1820w cm -1)
0
360
350
4~
686 during the butadiene hydrogenation selectively towards 1-butene. By contrast, the naked Pt aggregates obtained by complete removal of CO at 4(D-475K exhibited non selective hydrogenation of butadiene towards a mixture of butenes and butane under the atmosphere of butadiene and hydrogen(l: Iv/v) at 300K, as indiated in Fig.6. The butadiene hydrogenation proceeded on the decarbonylated Pt clusters in FSM-16 channels with the TOF and selectivities for butane formation same as those on the conventional Pt/SiO2 catalyst. 4. C o n c l u s i o n
1. Ru 3 (CO)I 2, H4Ru4(CO)12, [Pt9(CO)18] 2- and [Pt12(CO)24] 2- were synthesized in NaY cages by "ship-in-bottle" technique. 2. Thermostable robust [Pt15(CO )30] 2- combined with organic cations were encapsulated in the ordered mesoporous channels of FSM-16(27.5A) by "ship-in-bottle technique, which were characterized by F'FIR, EXAFS and HRTEM methods. 3. They remain their flexibility of duster frameworks in the mesoporous channels of FSM16(27A diameter) and exhibited remarkedly higher catalytic activities for the water-gas-shift reaction, compared with Pt9-Ptl2 dusters which are more restricted in NaY micropores (12A). 4. Ptl5 carbonyl clusters in FSM-16 channels was uniformly transformed to highly dispersed Pt aggregates(ca 15A size) owing to the controlled CO removal by the evacuation at 323-473K. The resulting Pt aggregates in FSM-16 exhibited specific catalytic activities for the competitive hydrogenat-ion of ethene and butadiene owing to their coordinatively unsaturation. 5. R e f e r e n c e s
[1] M. Ichikawa, Adv. Catal., 38 (1992) p. 283-400. [2] a)M. Ichikawa, L.-F. Rao, N. Kosugi,Faraday Discuss.,(1989)87, 232;b)M.Ichikawa, L.-F.Rao, T.Kimura, and A. Fukuoka, J. Mol. Catal.,(1990) 62, 15. [3] G.-J. Li, T. Fujimoto, A. Fukuoka, and M. Ichikawa, J. C.S., Chem. Commun.,(1991) 1331;R.-J.Wang,T.Fujimoto,T.Shido,and M. Ichikawa, ibid,(1992), 962; Catal. Lett., (1992),12,171. [4] A.M. Liu, T. Shido, and M. Ichikawa, J.C.S., Chem. Commun.,(1995)507. [5] A.Fukuoka, L.-F.Rao, N.Kosugi, H.Kuroda and M.Ichikawa, Appl. Catal., 50, 295 (1989). [6] M. Ichikawa, A.M. Liu, G. Shen, T. Shido, Topics. Catal., 2, 141 (1995). [7] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kreage, K.D.Smitt, T.-W.Chu, D.H. Olson, E.W.Sheppard,S.B.McCulleni,J.B.Higgins and J.L. Schlenker, J. Am. Chem. Soc.,(1992), 114, 10834. [8] T. Yanagisawa, T. Shimizu, K. Kuroda and C. Kato, Bull. Chem. Soc.,Japan, (1990), 62, 763, 1535; S. Inagaki, Y. Fukushima and K. Kuroda,J.C.S., Chem. Commun., (1993), 680. [9] S.Qiu,R. Ohnishi, and M. lchikawa, J.C.S.,Chem. Comm.,(1992)1423. [10] R.J. Irving and E.A. Magnusson, J. Chem. Soc.,(1956),1860;(1958), 2283. [11] J.C. Calabrese, L.F. Dahl, P. Chini, G. Longoni and S. Martinengo, J.Am. Chem. Soc., (1974), 96, 2614. [12] T. Fujimoto, A. Fukuoka, S. Iijima, and M. Ichikawa,J. Phys. Chem.,(1993), 97, 279; J.D. Roth, G.J. Lewis, L.K. Safford, X. Jiang, L.F.Dahland M.J. Weaver, J. Am. Chem. Soc.,(1992), 114, 6159. [13] C.Brown, B.T.Heaton, A.D.C. Towel, and P. Chini, J. Organomet. Chem., (1979), 181,233.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
687
Characterization and reactivity o f N i , M o - s u p p o r t e d M C M - 4 1 catalysts for hydrodesulfurization J. Cui Y.-H. Yue Y. Sun W.-Y. Dong Z. Gao* (Department of Chemistry, Fudan University, Shanghai 200433, P. R. China)
The adsorption isotherms of MCM-41 molecular sieve and catalysts exhibit two regions in their reversible parts and also two hysteresis loops. The first hysteresis at relative pressure below 0.4 is assigned to the capillary condensation in the mesoporous channels, and it is sensitive to the filling or blockage in the channels. The adsorption isotherms together with XRD results can be used to characterize the dispersion of nickel and molybdenum oxides on MCM-41. MCM-41 supported Ni and Mo catalysts are in general more active than similar NaY supported catalysts, but they are more strongly dependent on the dispersion of metal oxides owing to the unidimensional channel system of MCM-41.
1. INTRODUCTION A new family of mesoporous molecular sieves designated as MCM-41 has been discovered recently [ 1]. MCM-41 possesses large surface area, a uniform system of mesoporous channels and high thermal stability. These characteristics are of particular advantage for this type of material to be used as catalyst supports for processing larger molecules, such as the heavy feedstocks in oil refinery. In this work Ni and Mo supported MCM-41 catalysts were prepared. The dispersion of the metal oxides on MCM-41 was studied by XRD and adsorption method. The reactivities of the Ni,Mo-supported catalysts for thiophene hydrodesulfurization were tested and compared with those of supported Y zeolite catalysts having the same Ni and Mo contents.
2. EXPERIMENTAL The MCM-41 sample was synthesized hydrothermally, following the procedures described in Ref. [2]. In a typical preparation, 60.3 g of sodium metasilicate was mixed with 114 mL H20 and 3.66 g 98% H2SO4, and then 150 ml of 25% CI6H33(CH3)3NBr solution and 60 mL H20 were added under vigorous agitation. This gel mixture was transferred to a stainless steel
688 autoclave and kept-in an oven at 373 K for 11 d. The product was filtered, washed, dried in air, calcined at 813 K for 1 h in flowing N2 and then calcined at 813 K for 6 h in a flow of air to remove the organics. The NiO(x)/MCM and NiO(x)/NaY catalysts were prepared by impregnation of MCM-41 and NaY zeolite with Ni(NO3)2 solutions of appropriate concentration, followed by drying at 383 k and calcining at 923 K for 6 h. MoO3 was introduced into the catalysts through solid state dispersion method [3] in order to avoid the loss of crystallinity of the molecular sieves in the impregnation method. A mixture of MoO3 and the molecular sieve was ground and mixed thoroughly, and then calcined at 733 K for 6 h to prepare MoO3(y)/MCM or MoO3(y)/NaY catalysts, x and y in the formulas are the weight percentages of the oxides in the samples. X-ray powder diffraction (XRD) was carried out on a Rigaku D/MAX-IIA equipment using the Cu Ka radiation at 40 kV and 20 mA. Specific surface area, pore volume and the most probable pore diameter were calculated from the adsorption-desorption isotherms of N2 at 77 K on a ASAP-2000 apparatus. The thiophene hydrodesulfurization activity of the catalysts was tested in a fix-bed flow microreactor at ambient pressure and 673 K. The catalyst load was 100 mg, and thiophene was fed from a saturator with H2 as a carrier gas (30 mL min i) to give a H2" thiophene molar ratio of 27. The reaction products were analyzed by means of a gas chromatograph equipped with a flame ionization detector.
3. RESULTS AND DISCUSSION 3.1. Unusual adsorption isotherm of MCM-41 The adsorption isotherms of MCM-41 samples are very unusual, as shown in Figure 1. They exhibit two regions in their reversible parts. The adsorption increases gradually with pressure in the first region, whereas its rise in the second region is much steeper. There is a hysteresis loop in each region. The hysteresis in the second region is normal, which is often assigned to the capillary condensation in mesopores of the aggregates of crystals. The appearance of the hysteresis in the first region is unexpected, because in the literature [4] it has been mentioned many times that there is only one hysteresis and its lower limit is located at a relative pressure above 0.4. The shape of this new hysteresis loop is similar to the H 1 type hysteresis in the new classification recommended by IUPAC, corresponding to capillary condensation in cylindrical pores with uniform size and array. The position of the hysteresis shows that the diameter of these cylindrical pores is approximately 3 nm, which is almost the same as the value (2.96 nm) of the most probable pore diameter of the MCM-41 sample calculated from the desorption isotherm by BJH method and close to the value (~ 4 nm) of the layer spacing of MCM-41 determined by XRD. Therefore, this new hysteresis loop can be assigned to the capillary condensation in the uniform channels of MCM-41. The traditional concept of capillary condensation without hysteresis loop at a relative pressure below 0.4 in the literature [4,5] is incorrect in this case. The contradiction comes probably from the lack of
689
!
0.0
0.2
o.4 RELATIVEPRESSURE (P/Po)
(a) ~cx~.a ~.1
M.c~.a 1 sar~l~
~M..41.3 (a) S~cM.,41.1
1.0
690 a typical mesoporous material abundant of pores with uniform size like MCM-41 in the previous works. The properties of some selected MCM-41 samples with different crystallinity are listed in Table 1, and their adsorption isotherms are illustrated in Figure 1. As the relative crystallinity of the sample is decreased, not only the specific surface area and pore volume are reduced but the first hysteresis on the adsorption isotherm becomes smaller as well. The reason is that the pore channels are narrowed or blocked by extra-framework impurities. To prove the validity of this explanation, 10 wt% of SiO2 was deposited on MCM-41 via chemical liquid deposition with SIC14 [6]. As expected, the first hysteresis of the deposited sample was reduced considerably. The above results tell that the first hysteresis on the adsorption isotherm is rather sensitive to any sort of filling or blockage in the channels of MCM-41, so it can be quite useful to probe the dispersion of active components on supported MCM-41 catalysts. Table 1 Properties of MCM-41 samples Sample
i
i
Pore volume (cm3/g)
Surface area (m2/g) ill
MCM-41-1 MCM-41-2 MCM-41-3 SiMCM-41 - 1*
i
i
i
Relative crystallinity (%)
2.96 2.96 2.96 2.96
100 73 49 95
i
0.97 0.94 0.84 0.86
1127 1054 1031 1048
Most probable diameter (nm)
i
* MCM-41-1 deposited with 10 wt% of SiO2
3.2. Dispersion of metal oxides XRD results show that MoO3 is easily dispersed on MCM-41. Increasing the content of MoO3 to 20 wt%, the characteristic peaks of MoO3 are still not observed in the XRD patterns of the catalysts, showing that MoO3 has dispersed as a monolayer or submonolayer on the surface of MCM-41 [3]. NiO is not so easy to disperse as MOO3. When the content of NiO is below 6 wt%, the characteristic peaks of NiO are not observed. The peaks of NiO appear when the NiO content exceeds 6 wt%, demonstrating that under this condition larger NiO crystallites detectable by XRD have formed. The dissimilarity in the dispersion behavior of the metal oxides is related with their differences in melting point and the affinity between the metal oxides and the support [3]. The specific surface area, pore volume and relative crystallinity of the catalysts are listed in Table 2, and their adsorption isotherms are illustrated in Figure 2. For MoO3/MCM catalysts with MoO3 content < 20 wt% the first hysteresis can be visualized clearly in the figure, but it becomes smaller as the MoO3 content is increased, implying that MoO3 is dispersed on the
REJ..,ATIVE PRESSURE (P/Po)
F ' i 2 -tion isotherms of supposed MCM-41 cablysb (a) M~O~(lo.O)hlcM @) MoO,(l5.O= (c) M o 4 ( 2 0 . 0 ~ M (d) N0(4.0)MCM (e) NQ6.0yMCM (f) xO(lO.O)/MCM
692 surface of MCM-41 quite homogeneously and the channels are gradually narrowed. Nevertheless, a further increase in MoO3 content will cause the disappearance of the first hysteresis and a significant reduction in surface area. Under the circumstances, the mesoporous channels in the catalysts are seriously narrowed or partly blocked. The first hysteresis on the isotherms and the surface area of all the NiO/MCM catalysts are smaller in comparison with those of the MoO3/MCM catalysts, although the loadings of NiO are much lower. This suggests that unlike the MoO3/MCM catalysts, the NiO/MCM catalysts become seriously narrowed or partly blocked at very low NiO loading owing to the low dispersive ability of NiO. It is also surprising to find that the first hysteresis and the surface area of NiO(6.0)/MCM are slightly larger than those of all the other NiO/MCM catalysts. In the author's previous study on the dispersion of NiO on NaY zeolite by XRD and EXAFS [7], it has been found that as the loading of NiO on the zeolite is increased it tends to migrate out from the zeolite cage to form larger oxide crystallites on the external surface of the zeolite. This may also happen in the NiO/MCM systems, so that the release of a part of the blockage in the pore channels of the NiO(6.0)/MCM catalyst due to oxide migration is probably responsible for the increase in the hysteresis and the surface area. As more NiO is added, the NiO crystallites on the external surface will grow and block the pore-opening of MCM-41 again, so the first hysteresis and the surface area ofNiO(10.0)/MCM are reduced. Table 2 Properties of supported MCM-41 catal~,sts . . . . . . . . Sample
Surface area (mZ/g)
MCM-41-1 NiO(4.0)/MCM NiO(6.0)/MCM NiO(10.0)/MCM MoO3(l 0.0)/MCM MOO3(15.0)/MCM MoOa(20.0)/MCM |l
1127 834 872 781 1018 883 670
Pore volume (cm3/g)
Relative crystallinity
0.97 0.60 0.65 0.56 0.87 0.70 0.52
100 67 68 71 88 85 83
(%)
i
3.3. Thiophene Hydrodesulfurization The catalytic activity and selectivity of the metal oxide supported catalysts were measured at 120 min on stream and compared with those of the same type of supported NaY zeolite catalysts in Table 3. The hydrodesulfurization activity of the MoOa/MCM catalysts increases with the MoO3 content in the catalysts, and obviously the MoOa/MCM catalysts are more active than the MoOa/NaY catalysts. The activity of the NiO/MCM catalysts increases with the NiO content below 6 wt% and decreases as more NiO is added into the catalyst. The
693 NiO/MCM catalysts are less active than the NiO/NaY catalysts. Correlating the reactivity of the catalysts with the results on the dispersion of the metal oxides shows that in comparison to supported NaY zeolite catalysts the catalytic activity of the supported MCM catalysts is more strongly dependent on the dispersion of the active components. Obviously, this is because the blocking of the pores by the small oxide crystallites will do more harm to the unidimensional pore system of MCM-41 than to the three-dimensional pores of NaY zeolite. Hence, in order to improve the performance of supported MCM-41 catalysts it is necessary to pay more attention to the dispersion of the active components and if possible any sort of blockage of the pore channels should be avoided. Table 3 Hydrod,esulfurizat!0n activ!t,¢ 9 f th e catalysts Catalyst
Conversion (%)
MOO3(10.0)/MCM MOO3(15.0)/MCM MoO3(20.0)/MCM NiO(2.0)/MCM NiO(4.0)/MCM NiO(6.0)/MCM NiO(10.0)/MCM MOO3(10.0)NiO(6.0)/MCM MOO3(10.0)/NAY NiO(5.7)/NaY NiO(8.6)/NaY MOO3(10.0)NiO(8.6)/NaY
10.3 16.1 17.1 0.81 2.06 2.76 1.89 7.95 6.29 2.93 3.87 4.90
ii
,,
i
ii
....................... Product (%)* Ca C4+
i
i
i
i
i
i
82 83 80 100 100 100 100 100 90 100 100 100 i
ii
Activity (mmol g-I h-l) i
i
i
i
ii
18 17 20 10 i
i
i
i
0.98 1.54 1.63 0.08 0.20 0.26 0.18 0.76 0.60 0.28 0.37 0.47 i
i
i
i i
i
* C4 includes a small amount of C2 and C3; and C4÷ includes aromatics The hydrodesulfurization activities of MoO3(10.0)NiO(6.0)/MCM and MoO3(10.0)NiO(8.6)/NaY catalysts are higher than NiO(6.0)/MCM and NiO(8.6)/NaY catalysts, but lower than the respective MoO3(10.0)/MCM and MoO3(10.0)/NaY catalysts. A synergy between Ni and Mo surface active species in the reaction is not so evident for these catalyst systems, because the promotion of the reaction and the blockage of the pore channels occur simultaneously when adding the two metal oxides together.
694 REFERENCE
1. C.T. Kresage, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710 2. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834 3. Y.C. Xie and Y.Q. Tang, Advances in Catalysis (D.D. Eley, H. Pines and P.B. Weisz, eds.), Vol. 37, p. 1, Academic Press Inc., San Diego, 1990 4. S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982 5. O. Franke, G. Schulz-Ekloff, J. Rathousky, J. Starek and A. Zukal, J. Chem. Soc., Chem. Commun., (1993) 724 6. Y.H. Yue, Y. Tang and Z. Gao, Ind. Eng. Chem. Res., in press 7. Z. Gao and J. Cui, Acta Physico-Chimica Sinica, 10 (1994) 992
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
695
Probing the hydrophobic properties of M C M - 4 1 - t y p e materials by the hydrophobicity index R. Gl~isera, R. Roeskya, T. Bogerb, G. Eigenbergerb, S. Ernst a and J. Weitkampa a Institute of Chemical Technology I, University of Stuttgart, D-70550 Stuttgart, Germany
b Institute of Chemical Process Engineering, University of Stuttgart, P.O. Box 106037, D-70049 Stuttgart, Germany The surface properties of MCM-41-type materials with different nsi/nAl-ratios and at temperatures from 30 °C to 120 °C were characterized by the Hydrophobicity Index (HI). As expected, HI increases with decreasing almninum content of the molecular sieve. It is demonstrated that MCM-41-type materials are truly hydrophobic and possess high adsorption capacities for organic compounds. A model is developed which describes the binary adsorption equilibrium in terms of a superposition of sorption and capillary condensation. 1. INTRODUCTION Members of the MCM-41-family of materials have attracted considerable interest in many fields of research since their discovery by scientists at Mobil Oil Corp. [ 1]. The pore system of MCM-41-type materials consists of parallel tubes with a hexagonal packing. The diameter of these tubes can be taylored in the range between 1.5 nm and 10 nm with a very narrow pore size distribution. MCM-41 has therefore been suggested as a model adsorbent [2]. Due to its high surface area and pore volume it possesses a high adsorption capacity. In recent adsorption studies, low water loadings [3,4] and high loadings of several organic compounds [ 1,2,4,5] have been reported. In these investigations, a type IV isotherm was found which indicates an adsorption process based on sorption and condensation. Here we report our results of systematic studies on the influence of the nsi/nAl-ratio and the adsorption temperature on the hydrophobic properties of MCM-41-type materials. As a quantitative measure for hydrophobicity/hydrophilicity, the Hydrophobicity Index (HI) as defined earlier [6,7] was determined from breakthrough curves using toluene and water as adsorptives. So far, only single component adsorption has been studied on MCM-41. It was therefore one of the aims of the present study to provide some new data on multicomponent adsorption. In addition, a mathematical model is presented which allows to describe the experimentally observed binary adsorption equilibria from single component data.
696 2. E X P E R I M E N T A L SECTION Samples of MCM-41 with four different nsi/nm-ratios were synthesized according to a modified procedure given in the literature [1,8]. For the synthesis, the quaternary ammonium surfactant CH3(CH2)13N(CH3)3Br (ClaTMABr) was used as templating agent. Waterglass and N a ~ O 2 were used as sources for silicon and aluminum, respectively. The molar oxide ratio of the resulting gels was y A120 3 • 10 SiO 2 • (3.0 + 1.25y) Na20 • 2.7 (C14TMA)20 • 1.3 H2SO 4 • 480 H20 with 0 < y < 0.25. The gels were heated for 12 h at 150 °C to induce the formation of MCM-41. The assynthesized products were washed with ethanol and water and dried at ambient conditions. They were calcined at 540 °C for 6 h in a nitrogen atmosphere to remove the organic template. XRD powder patterns were collected on a Siemens D5000 instrument using CuKot radiation. The bulk chemical composition was determined by AES-ICP (Perkin Elmer, Plasma 400) and atomic absorption spectrometry (Varian, SpectrAA-300). 27A1 M.AS NMR spectroscopic measurements were conducted on a Bruker MSL 400 instrmnent. A solution of aluminum nitrate in water was used as external reference. Nitrogen adsorption isotherms at 77 K were determined in a Micromeritics ASAP 2010 equipment. The samples were outgassed at 523 K for 12 h before each experiment. For the determination of the Hydrophobicity Index, a mixture of w a t e r (Pwater = 2.34 kPa) and toluene vapors (,Ptoluene = 2.92 kPa) was passed through a fixed bed adsorber with a bed height of h = 15 - 20 mm. The adsorbent was used as a powder (unpressed) or was pressed at p = 40 N/mm 2, crushed and sieved to a particle fraction of d = 0.2 - 0.3 mm. Nitrogen was used as carrier gas with flow rates around u = 7.0.10 -3 m/s. The gas stream leaving the adsorber was analyzed every 3 minutes using a capillary gaschromatograph equipped with a thermal conductivity detector. The fmal loadings (Xi) were calculated directly from the obtained breakthrough curves. The Hydrophobicity Index was then calculated as HI = Xtoluen e / Xwate r [6].
3. RESULTS AND DISCUSSION 3.1 MCM-41-type materials The results of the physico-chemical characterization of the calcined materials are summarized in Table 1. As can be seen from the nNa/nAl data, the charge induced by the aluminum atoms in the samples is not completely balanced by sodimn cations. This suggests that the molecular sieves are in a mixed Na~-form. The XRD patterns of the calcined samples were similar to those reported in the literature [1] and there were no major differences in the shape of the patterns for the different nsi/nAl-rafios. It was ascertained by 27A1 MAS NMR spectroscopy that the almninum atoms of the samples were exclusively in tetrahedral coordination (signal at around 54 ppm). Only for the sample with nsi/nAl = 20 a minor signal at 0 ppm (octahedral coordination of aluminum) was observed (intensity ratio of the signals at 0 ppm and 54 ppm: 8/100).
697
Table 1. Results of the physico-chemical characterization of the calcined MCM-41 powders sample
chemical analysis
d-value of nNa/nA1 [100] reflex/ nm 0.29 3.74 0.17 3.45 0.09 3.40 0.04 3.28
No. nsi/nA1 1 2 3 4
XRD
20 64 90 266
N2-adsorption pore diameter / nm 2.49 2.52 2.58 2.48
BET surface / m2/g 815 851 894 845
total pore volume / cm3/g 0.72 0.74 0.78 0.73
3.2 Breakthrough curves The breakthrough curves for two typical experiments at Tads = 50 °C and 80 °C are shown in Figure 1. It is obvious that water breaks through first and is even displaced by toluene. Water displacement ceases when toluene starts to break through. In the breakthrough curve for toluene at 50 °C (Figure 1A) a step is observed at 130 min to 170 min time on stream. This is typical for adsorbents with type IV isotherms (as is the case for the adsorption of toluene on MCM-41 [8]), when equilibrium is achieved throughout the adsorbent bed [9]. The first increase of the toluene concentration in the effluent occurs after 110 min time on stream and is attributed to the saturation of the adsorption on the pore walls. After 170 min the second from, this time due to capillary condensation, arrives at the adsorber exit. Similar breakthrough curves are observed at different temperatures, provided that adsorption conditions allow capillary condensation to occur. At 80 °C the partial pressure of the feed is too low for capillary condensation to occur, and hence no step is observed (Figure 1B). Using MCM-41 pellets obtained by pressing and sieving, the step in the breakthrough curve cannot be observed, because the two fronts are wiped out due to increased mass transfer resistances. '
'
'
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I
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1
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1
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1
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1
1.2 e-
:-- 0.8
Q.
•Q.
0.4 0.0 0
50
100
150
200
TIME
ON
0 STREAM
50
100
150
200
/ min
Figure 1. Breakthrough curves for the competitive adsorption of toluene/water vapors on MCM-41 (nsi/nAl = 266, powder), mads = 0.90 g; A: Tads = 50 °C; B: Tads = 80 °C.
698
3.3 Hydrophobicity Index and equilibrium loadings The Hydrophobicity Indices 40 for MCM-41-type materials ,~ O nsi/nAI = 266 Ihaving three different nsi/nAr \ [] nsi/nAi = 90 1 ratios are depicted in 30 dependence of the adsorption temperature in Figure 2. As already observed for zeolites I 20 [6], HI increases with decreasing aluminum content of the 10 molecular sieve. At variance to zeolitic adsorbents [7], however, for constant nsi/nArratio the Hydrophobicity Index passes 40 80 120 through a maximum at around TEMPERATURE / °C 50 °C. An explanation can be Figure 2. Hydrophobicity Index of MCM-41 deduced from Figure 3, where with different aluminum contents in the equilibrium loadings dependence of the adsorption temperature. determined from the breakthrough curves for toluene (Xtoluene) and for water (Xwater) are shown. From 30 °C to 50 °C, the toluene loadings increase slightly. The water loadings, which are generally one order of magnitude smaller, decrease monotonously with increasing nsi/nArratio. For toluene, there is no clear influence of the nsi/nArratio on the loading of the adsorbent. Rather, the pore volume (see Table 1) seems to play the decisive role. At temperatures above 50 °C the toluene uptakes decrease drastically. Hence, HI decreases. •
I
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'
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•
•
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1
.
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z
UJ
I
.
.
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0.8
0.08
0.6
0.06
O []
nsi/nAI = 266 nsi/nAi = 90 nsi/nAi = 20
n,, 0.04
0.4
LU
V
_1
o IX 0.2
0.0
I
=
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,
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.
,
60
.
,
80
0.02
0.00
TEMPERATURE
'
/ °C
'
40
'
•
60
.
80
Figure 3. Equilibrium loadings of toluene Xtoluene and of water Xwate r for MCM-41 adsorbents with different nsi/nArratios in dependence of the adsorption temperature.
699
3.4 Model for the prediction of the binary adsorption equilibrium It has been shown recently, that a model based on the potential theory of adsorption [ 10] can be used to describe the adsorption equilibrium of various pure components on MCM-41-type materials [8]. The basic assumption of the potential theory is that the adsorbed volume v is correlated to the adsorption potential Ai,which is defined as A i - RTlnlPi'°l, ~,P0J
(1)
with the gas constant R, the adsorption temperature T, and Pi and Pi,0 as the actual partial pressure and the partial pressure at saturation, respectively. A separation of the adsorption process into two regions is made" region I, describing sorption in the intraparticle pores, and region II, describing adsorption in the voids between the particles. The total uptake X i is the sum of the contributions of the two regions. Furthermore, capillary condensation is assumed to take place in region I at a discrete value Ai, c of the adsorption potential. This results in the following two equations, describing the total uptake X i for the region A i > Ai, c (eq. 2) and A i < Ai,c (eq. 3). Xi = V[im "Pi,s -exp -
Ai - Ai,iim
I Ei
"Pi,s .exp -
Ai - Al,~im.~ -.-~ -
+ VII "Pi,s .exp -
Ai -_ Ai,lim ~ gilI j
Ei
(2)
)
IX
Xi = V~im " IDi,s
(3)
In these equations, viim denotes the total intraparticle pore volume (see Table 1) and the subscripts s, c indicate the sorbed and the condensed phase, respectively. Ai.l~ n I is the potential at which the pore volume is filled due to physical adsorption, v0D is the interparticle volume filled at A i,lim a and a value of 0.27 cm3/g is used for MCM-4118]. I Negative values of Ai,~ and A ai.~m~mean that a pressure higher than Po is required to fill the limiting pore volume by physical adsorption. E I and E u are the characteristic energies in the two regions and are related to the interactions between adsorbent and adsorbed molecules. The density of the sorbate ios is assumed to be equal to its liquid I m and density. The parameters E I , EII , Ai,li A ui,l~ were determined from single component data [8] and are listed in Table 2. For the adsorption potential Ai,c, at Table 2. Model parameters E and Ali m for single component adsorption (in kJ/mol) toluene water nsi/nm
EI
A~im
En
AlIIm
EI
A~im
EII
AlliI
20 64 90 266
4.14 3.72 3.64 3.29
3.00 3.10 3.52 3.60
2.15 2.15 2.15 2.15
0.00 0.00 0.00 0.00
3.60 3.20 3.15 2.90
-3.94 -4.72 -4.65 -6.43
3.60 3.20 3.15 2.90
-17.30 -20.80 -16.40 -19.80
700
which capillary condensation occurs, temperature independent values of A c = 5.0 kJ/mol for toluene and ofA c - 1.6, 1.46, 1.42 and 1.16 kJ/mol for water (samples with nsi/nA1 = 20, 64, 90 and 266) were also obtained from single component data [8]. For the calculation of the binary adsorption equilibrium it has to be taken into account that two different phases can exist inside the pores at the same time. One phase is physically adsorbed on the pore walls and another one is condensed in the void space of the pores. Different mixing rules have to be applied to these two phases. For the condensed phase it is assumed that it can only occupy the volume not filled by the sorbed phase. Thus the maximum volume Vo,~n filled by the condensate amounts to Vc,lim -- V]im --(Xtoluene,s I I / Ptoluene,s + Xwater,s / Pwater,s) •
(4)
Furthermore, it is assumed that both components behave like pure components, which only share the same volume but do not interact with each other. The loading of the component i (i" toluene or water) in the condensate (X~.~) is equal to f" V~,l~ "Pi,s. The factor f can adopt values of 0, 1 or 1/2 depending on whether component i does not condense, only component i condenses or both components condense, respectively. The composition of the adsorbed phase is calculated separately in both regions according to the ideal adsorbed solution theory (IAST)[ 11]. This theory requires that at equilibrium both components possess the same spreading pressure @. If q~ is expressed as a function of the adsorption potential [ 12], one obtains for region I (M i is the molar weight of component i, and the superscript 0 denotes the pure component) oo
IV ~dAi I _ Pi,s Mi
(I)i
- V~im • Pi,s'E[ .exp (
Mi
A iIO - Ai,li I I m .1
Ei
for AIo > Aic, '
(5)
for A~°
(6)
AI°
--V~imMi'Pi'~.IA~ - A ~ ° + E l .
exp(- A~'°c -A~'liml]E~
The mole fraction of the component i in the adsorbate xi,i can then be calculated by x! -
"~
Pi .exp
Pi,o
(A o)
with A]° - A}° ((I)I)
(7)
~, RT J
in analogy to Raoult's law. AI° can be understood as the pure component adsorption potential corresponding to the solution spreading pressure • and the temperature T. The loadings of the components in the sorbed mixture X~,,are then given by x! • M i x.I = ',' l,s Mtoluene. Xtohene,s + Mwater. Xwater,s IO XI0 ~oluerie,s water,s
with x~,° - Xl,°(Al°) .
(8)
The equations for the sorption in the interparticle voids can be derived in a completely analogous way.
701
In Figure 4, the experimental and the predicted loadings for the binary mixture are compared for adsorbent No. 1. In Figure 5, calculated and experimental values of HI are shown in dependence of the nsi/nAl-ratio. The agreement of calculated and experimental values is very satisfactory. According to the model, the loadings of both components increase with increasing pore volume (eq. 2,3,8) which supports the influence of the pore volume on the adsorption capacity. But since HI is calculated as the ratio of two loadings, it is independent on the pore volume. HI depends on the nsi/nAl-ratio only, because the pore diameter, and hence A c, were kept constant. This dependence is mainly a result of the strong influence of the adsorption of the pure component water. A calculated maximum is obtained for HI at a temperature of ca. 58 °C (Figures 2 and 5). This can be rationalized by a superposition of sorption and condensation. Up to 30 °C, both water and toluene exist in the condensed phase (calculated loadings in Figure 4). There is no clear selectivity for either of the components. Above 30 °C, however, only toluene is existent in the condensed phase, whereas water is only physically adsorbed on the pore walls. At temperatures above 60 °C no condensate is formed at all, since mtoluene > mtoluene,c. The toluene loading, as well as HI, decrease abruptly. The binary adsorption equilibrium is now completely determined by the competitive physical adsorption on the internal surface, as it is the case for zeolites [7]. Since the characteristic energy of toluene exceeds the one of water, the toluene loading decreases more slowly with increasing temperature. Thus HI is expected to rise again upon increasing the temperature. This is contradictory to the experimental results (Figure 2). However, one should bear in mind that the loadings determined experimentally for temperatures above 80 °C are very low (< 0.01 kg/kg). Hence, the error in calculating HI as the ratio of two loadings becomes considerable. Therefore, the information which can be obtained from HI at high '
O}
I
'
I
•
I
'
I
"
I
- - calc. toluene I [] exp. toluene I. . . . calc. water
0.6 - r " ~
l , =, • &,V O
50
calculated experimental maximum HI
40 n,' W
0.4
I
T =60 *C
30
X LU
Z LU
20
T = 30 °C
0.2
C) .J
10
o i--X
u-l~. 0.0
,
I
40
,
I
" " r -(~
60
80
-= . . . .
,
O_.
100 120
TEMPERATURE / ° C
Figure 4. Experimental and calculated mixture loadings for MCM-41 (nsi/nA1 = 20, powder).
10
,
,
• ||||1
1O0
,
,
,
,
,,|,
1000
nsi/nAi
Figure 5. Hydrophobicity Index in dependence of the nsi/nAl-ratio (maximum HI: Tcal¢" = 58 °C, Texp. = 50 °C).
702 temperatures is very limited. The HI values obtained at temperatures between 60 °C to 80 °C, however, seem to be suitable for characterization and comparison of the surface hydrophobicity of MCM-41-type adsorbents with that of zeolites. In this temperature range, the loadings are reliable, and there is only a minor influence of condensation, if any. 4. CONCLUSIONS The competitive adsorption of a binary mixture (toluene/water) on MCM-41-type molecular sieves was studied for the first time, using a flow-type apparatus with fixed bed adsorber. The Hydrophobicity Index of MCM-41 increases with decreasing aluminum content of the adsorbent and is in the same order of magnitude as those of hydrophobic high-silica zeolites. The experimental data from multicomponent adsorption can be reasonably well described by a model which is based on single component data. Due to their high adsorption capacity, MCM-41-type materials are interesting adsorbents for the selective removal of organic compounds from moist gas streams. ACKNOWLEDGEMENTS
Generous fmancial support of this work by Volkswagen-Stiftung, Hannover, Germany, under grant No. 1/67574, is gratefully acknowledged. R.G., S.E. and J.W. moreover acknowledge financial support by Deutsche Forschungsgemeinschaft, Fonds der Chemischen Industrie and Max-Buchner-Forschungsstiftung. The authors thank Dr. M. Hunger for performing the MAS NMR measurements. REFERENCES
1. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Sot., 114 (1992) 10834-10843. 2. P.J. Branton, P.G. Hall and K.S.W. Sing, Adsorption, 1 (1995) 77-82. 3. P.L. Llewellyn, F. Schiith, Y. Grillet, F. Rouquerol, J. Rouquerol and K.K. Unger, Langmuir, 11 (1995) 574-577. 4. C.Y. Chen, H.-X. Li and M.E. Davis, Microporous Materials, 2 (1993) 17-26. 5. J. Rathousl~, A. Zukal, O. Franke and G. Schulz-Ekloff, J. Chem. Soc., Faraday Trans., 91 (1995) 937-940. 6. C.H. Berke, A. Kiss, P. Kleinschmit and J. Weitkamp, Chem.-Ing.-Tech., 63 (1991) 623-624. 7. J. Weitkamp, P. Kleinschmit, A. Kiss and C.H. Berke, in: Proc. 9th Intern. Zeolite Conf., R. von Ballmoos, J.B. Higgins and M.M.J. Treacy (eds.), Vol. II, Butterworth-Heinemann, Stoneham, Massachusetts, 1993, pp. 79-87. 8. T. Boger, R. Roesky, R. Gl~iser, S. Ernst, G. Eigenberger and J. Weitkamp, to be submitted to Microporous Materials. 9. D. Basmadjian, Ind. Eng. Chem., Proc. Des. Dev., 19 (1980) 129-137. 10. M. Polanyi, Trans. Faraday Sot., 28 (1932) 316-333. 11. A. Myers and J.M. Prausnitz, AIChE J., 11 (1965) 121-127. 12. U. Eiden, PhDthesis, University of Karlsruhe (1989).
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 1997 Elsevier Science B.V.
703
Characterisation of acid-base- and redox-type sites in ZSM-5 zeolites by sorption rate "spectroscopy" Gy. Onyestyfika, J. Valyona and L. V. C. Rees b aCentral Research Institute for Chemistry of Hungarian Academy of Sciences, H-1525 Budapest, P.O.Box 17, Hungary bDepallment of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, Scotland, UK The fiequency-response (FR) technique was used to distingtfishsimtdtaneoussorptionprocesses occuning at different rates on various sites ofzeolites. The H-, Na-, and Cu-forms of ZSM-5 zeolite were studiexl. The rate "spectra" of NH3,1-t20, CH4, 02, CO, CO2 and NO were determined in the 0.01-100 Hz frequency whldow and 195-673 K temperature range. A peak was observed at the highest experimental resonance fiequealcies (about 10 Hz) for each sorption system and was ascribed to the fastest sorption interaction of the gases. Due to the weak interaction and/or the small number of sorption sites, no useful spectra could be obtained with H20 between 473-673 K. The sorption of methane could be examined at 195 K only. Conclusions were &'awn for file acid-base properties of the sample~ The complex FR speclaa fotmd tbr sorption of 02, CO, and NO on Cu-ZSM-5 above 373 K were interpreted as redox-type chemisoq~tion processes. Adspecies detected by frequency-response technique surely play a role in any dynamic system of practical interest. Combination of this technique with other methods is proposed to obtain, finally, a more complete chaa'actefisation of the zeolitic sites.
1. INTRODUCTION Analogous to a spectroscopic method various, simultaneous rate-processes of a heterogeneous catalytic system, e. g., diffusion ill micro- and macropores, adsorption and desorption involving different sites, and complex multistep reactions, can be investigated and distinguished by the frequency response technique (FRT) [ 1]. The frequency response (FR) method has been successfully applied for the examination of mass transfer kinetics in zeolites and has become one of the most powerful experimental methods for studying intra- and intercrystalline ditfiasional resistances [2, 3]. In spite of all the advantages of the technique, only a few papers have been published on the application of the FRT for studying chemisorption. Naphthali and Polinski [4, 5] demonstrated the usefulness of the method; then the technique was perfected and applied to study different supported catalyst systems [ 1, 6-11]. The growing importance of zeolites in adsorption and catalytic processes induced extensive researdl on these materials using a wide variety of techniques. In catalysis, one of the most important properties of the zeolites is the acidity. Amlnonia is a generally used probe molecule for the charac-
704 terization of acidic sites. In recent studies various H- and Me-zeolites were investigated by FR and FTIR spectroscopy [ 12, 13]. It was shown that adsorption of ammonia can be conveniently studied by the FRT. Comparing FR results of ammonia sorption with results obtained by conventional methods, such as IR, TPD, and calorimetric meastu'ements, the high-temperature FR peaks could be assigned to solption on Br6nsted acid sites in zeolites [ 14]. The application of the FRT for charactefzing basic or redox centers has not been attempted yet. Such examination requhes the use of a probe specific for these sites and conditions where the ratecontrolling step is the adsorption/desorption of the probe molecules on the sites. Such sorption studies can give infonnation about the catalytically active centers. In principle, the FRT can be also applied to study a rate process involving heterogeneous catalytic reaction. However, an indispensable precondition of such a study is the acctunulation of data which will allow us to distinguish between the spectral COlnponents of diffusion, sorption, and reaction. In the present work, the potenrials of the FR method are storeyed in the hwestigation of sorption sites. The site-specific adsorption of various probe molecules was studied on acidic, basic, and redox sites in zeolites.
2. EXPERIMENTAL The FR teclmique consists of measuring the pressure variation resulting from a periodically perturbed volume. By determining the dependence of phase difference and adsorption amplitude on the frequency, it is possible to study adso~9tion/desorption phenomena occuring on different surface sites. The principle of the FR teclufique has been described previously [15, 16]. The experimental FR data, the "FR spectra" of a system, are described by the in-phase (fiin, real) and out-of- phase (~iout, imaginary) characteristic functions [ 1, 6]: (PB/PZ)cos~z_B-l=Z (KilK.j) (K_i2/(K_j2+~02))=8~,,,real (PB/Pz)sin@z_ B=Z(~i/rc_i) (K.jg0/(K_i2+O)2))=~out,imaginary
(1) (2)
where Kj/K_j-(~)Aj/~)P)eR'I'/V e is con'elated to tile gradient of the adsorption isotherm stemming fiom Aj, tile amount of adspecies on the j sorption centre; K_j is the time constant of the ad/desoq)tion process for adsorbate on the site j; PB and PZ are the pressure responses to the +1% vohlme pel~urbations ill the absence (B) and the presence (Z) of a sorbent; ~Z-B is the difference between phase lags, and co is the angular velocity of the wave generator. The inphase fimction (~Sin) tends to Kj/K_j in the lower frequency region. The maxima of the phase difference and the out-of-phase function (~5out) appear at the perturbation frequencies of resonance, dependent on the type and strength of adsorption sites, the temperature and the sorbate pressure. The associated dynamic parameters of the FR spectra (Kj/~j and r,..j which determine local maxima on a curve of the out-of phase component) could be determined by fitting the characteristic fimctions generated by an appropriate theoretical model. The adsorption and desorption rate constants can be determined fi'om the pressure dependence of K:.j values.
705 The frequency window used in this study was 0.01-10 Hz About 50-80 mg of zeolite was placed into a sorption chamber and outgassed at 673 K for 1 hour before the sorption experiments were carried out. The sorbate was admitted to pretreated samples and allowed to come to equih'briumat 1.0 Torr in a temperature range of 195-673 K. Measurements were carried out in the presence and absence of sorbent zeolite samples to obtain the difference of the respective FR parameters. The sorbates used were NH3, CI-I4, 02 and NO obtained from ARGO International; CO from Matheson; CO2 from Linde; and distilled water. The ZSM-5 powder was a gitt from the Zentralinstitut ftir Physikalische Chemie, Berlin, (}DR (Si/AI=I5, crystal size: ~3pro). It was ion exchanged with a IN aqueous solution of NH4Ct The fonnula of the NH4-ZSM-5 catalyst is Na0.03(Nnn)5.77Sig0.2Als.8O192.This sample was transformed into Na,NH4-form (66% Na ÷) by ion exchange using 0.2N NaNO3 solution at room temperature and, similarly, into Cu,NH4-form (72% Cu2+) using 0.01N Cu-acetate solution. The same procedure was followed for preparing Cu-Y,FAU (95% of Cu2+) by ion-exchange from Na-Y obtained from Wolfen, GDIL Deeply ion-exchanged Cu-ZSM-5 catalyst was made by long-term (35 days) ion exchange with 0.01N Cu-acetate solution. A non-acidic, monocationic Na-ZSM-5 was obtained using the solid-state ion exchange method [17], where the powdered mixture of Na,NH4-ZSM-5 and NaCI was calcined in vacuum at 823 K for 12 hours.
3. RESULTS AND DISCUSSION The appearance of a maximum at about 10 Hz was a common feature of all the different sorption systems studied. Spectra showed characteristic changes with temperature. The changes were different for the various zeolites and gases. The temperature dependence of the spectra suggests that none of the peaks can originate from a diffusion process. Since, under the conditions applied, sorption was the rate-determining process for all the systems studied the FR data points were fitted using a sorption model The assignment of the 10-Hz peak to some kind of solid/gas adsorption interaction is uncertain at present. Conceivably this peak reflects the fastest sorption process in the systems with a dynamic time constant close to 0.1 s. The peaks characteristic for slower processes appear at lower frequencies. The lower experimental limit is 0.01 I-Ix Measurements below this frequency could be of interest when very slow processes must be detected. The study of slow chemisorption processes may require the extension of the range of measurement in this direction. In connection with the systems concerned in this work, it seems probable that part of the infolltlation was lost due to this limitationwhen chemisorptionwas examined on redox siteg It was easy to record excellent rate-spectra for adsorption of ammonia on the ZSM-5 samples between 373-673 K (Fig. 1). The main response peaks appear at frequencies close to 10 Hz on all of the ZSM-5 forms. Smaller-amplitude responses at lower-frequencies were also observed for all of the samples, although at different temperatures. Fig. 1 demonstrates that the sorption properties of the ZSM-5 samples in various cationic forms are significantly different for ammonia. The ten~etamle dependence of total FK intensifies (EK/r~) for forms of ditFetmt cationic compositio~ are shown in F'~ 2. A miahnan was observed around 573 K for H-ZSM-5. In a previous study [14], we have shown that the 10-Hz peak txx~omesamller if the temperaaae is inaeasett At tet~etaaaes above 573 K, a newpeak appears at about 2-3 Hz
706
1.ot
(A) n
~o0_n~on_
0.5- H-ZSM-5 .SUMS5': 0.0
~
,,-" ~ , , \
\ "'-._
Z1
o...c~~......,~
o ....
/
a
o
..... °
.r\
(B) @ 1.0 C
n O00n~O~_
1~0.5i
................................
Na-ZS
° n,
o.o
................................1 7 !
400
......................
10 0 o0
o.ol
.
5 o.1
~ 1
lo
F r e q u e n c y I Hz
loo
Fig. 1. The in-phase (U) and out-of-phase (0) characteristic functions for ammonia sorption on H-(A), Na-(B) and Cu-
500
600
Temperature I K
700
Fig. 2. The influence of temperature on the total intensity of ammonia responses for H-ZSM-5 (o), Na-ZSM-5 (Ak), Na,H-ZSM-5 (A), Cu-ZSM-5 (ll) and Cu, H-ZSM-5 (0).
The appearance of the two response peaks and the minimum of the Z~:/rq (Fig 2) can be explained by the temperatrue-dependent conmqaaJon of two sorption processes to the development of the rate spectra_ Most probably only the ZSM-5 (C) at 573 K. The FR rate spectra hy&'ogeu-bonding process of anmaonia on NH4 + ions is were recorded with 50rag zeolite under meaa,'ed at tempeaattues below 573 K. In addition to this 1Torr NH~ pressure. process the ~'ect inteaaction of the anmaonia with the Br'Oltsted acid sites is also obseaved ill the spectla at highea"telr~eaattu'es [14]. The total intensity passes tluougll a nfilinnun and beghis to incTeaseat tempemtmes when the solption ofanmao~ on the Br'onsted acid sites in a protonated species, i. e., ill the fonn ofNH4+ion, becomes the plevailing soIption process The sodium cations are weak Lewis acid sites. The chemisorption of ammonia is weaker on these sites than on protons. In the absence of protons the intensity of FR spectra rapidly decreases with increasing temperature in the whole temperature range examined. Above 673 K, no interaction could be detected. It is well known that Cu-salts can fonn ammhle complexes hi aqueous solutions and that some of these complexes, e.g., the Cu(H)-diamlnhle-cldolJde, are thenmlly stable up to 540 K. In Cu-ZSM-5 the Cuz+ can fonn complexes with a maximum number of fore" molecules of ammonia [18, 19]. The FR data in Fig. 2 suggest that the bindhlg of NH~ in the presence of 1-Torr gas-phase ammonia is stronger to the Cu2+- than to the Na +- or NI-t4+ ions of the ZSM-5 zeolite at 573 K. The total intensity ofthe NH3 signals of Cu-ZSM-5 as a function oftempeaattu'e shows a nmximtun at about 573 K (Fig 2). At about this tempemaue, three peals can be fotmd in the FR speclaa. Strongly bound ammonia could not be removed even by evacuation at 573 K as was indicated by the appearance ofthe attnnonia bands in the IR speanan ofthe san~le.
707
The temperature dependence of Y.•j/r._j for the mixed ionic forms, see, e. g., the Na,H-ZSM-5and the Cu,H-ZSM-5 curves in Fig. 2, can be interpreted as a combination of the curves obtained for the pure H- and Me-forms. Since the resolution of the FRT is such that in 0.01-10 Hz frequency window only 4 to 5 peaks can be resolved, the use of pure cationic forms is recommended to allow the assi~ment of distinct peaks to sorption processes on given cationic sites. Water was used as a basic probe and the ~H broad-line NMR technique was applied to characterize Br0nsted acid sites in solids. This way the oxy-protonated species formed from the water adsorbed could be identified and quantified [20]. A previous study suggested that Fit spectra of water sorption could also be informative in studying the acidity of cation exchanged Yzeolites [13]. However, our attempt to characterize the ZSM-5 zeolite has failed due to the weak response obtained between 473-673 K. The methane sorption rate-spectra are well resolved at 195 K, but, on raising the temperature to 273 K the response signals became too weak to be useful A single peak appeared in the out-ofphase curve at 9 Hz and 5 Hz for the H- and the Na-ZSM-5 forms, respectively. The data obtained 1 .o[ (A) are preliminary, but, promising. It seems possible, although further work is needed, to establish correo.5 lations between the FR parameters of methane adl sorption and the acid strength of the sorption sites. The carbon dioxide FR spectra recorded on o.o different ZSM-5 zeolites at 273 K are shown in 1.0 Fig. 3. A strong, low-frequency peak was found • (B) to develop at any temperatures between 273 and e...................... 373 K. Similar peak was observed earlier with ~o.s Na-ZSN-5 Na-X,FAU [3], Ca-A [211 and Cs-X,FAU [22]. • " The peak was most intense with the Cs-Y,FAIJ ne which is well known to be the most basic of the o.o samples compared. It is suggested that the low1 .o frequency peak can be diagnostic for the basic sites of the samples. More detailed investigation Cu,H-ZSM-5 is needed before any explanation can be offered 0.5 / for the differences of the spectra shown in Fig. 3. 0.0 Three peaks were found in the FR spectra of 02, 0.01 0.1 1 10 100 NO and CO on Cu,H-ZSM-5 between 373 K and Frequency I Hz 673 K indicating the formation of copper adsorption Fig. 3. Response curves for carbon dioxide on complexes, while at lower temperatures only one H-ZSM-5 (A), Na-ZSM-5 (B) and Cu,H- peak was detectable. Results suggest that at low temZSM-5 (C) at 273 K. Rate spectra were re- peratures only weak, physical adsorption of the gases corded with 75mg zeolite under 1 Tort CO2 is involved and sorbates become activated for cherub pressure, sorption reactions at elevated ten~eratures.
° i
708
1
.
o
~
0.8
(A)
(A)
0.6] 0
u~UL~----'
'
0.4
0.2
o.oJ
o.2Q.o.1
(B) 473K
......
0.0
o . o ~
(C)
0.4
er
o Q. ¢n
0.2
195K
o
[] . . []~
. ~ ~
.........
(B)
o.1
¢YO0
(C)
o.2-
0.1
0.2 0.0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.01 0.1 1
10
F r e q u e n c y / Hz
Fig. 4. Cad)on monoxide solption responses of Cu,H-ZSM-5 at 195 K (A), 473 K(B) and 673 K(C). Rate spectra were recorded with 75 mg of zeolite under I Torr CO pressure.
100
0 . 0
0.01
--:':':':;:,--
0.1
•
. . . . . . .
,
1
. . . . . . .
,-,
10
. . . . . . . .
Frequency I Hz
100
Fig. 5. Frequencyresponses of NO SOlbed in--75mg of CklM-ZSM-5 under 1Torr at 195 K (A), 373 K (B) and 673 K (C).
Figure 4 shows tile development of the rate spectra of CO sorption as the temperature was increased. At 195 K the CO coverage of the surface is high. The spectrum is simple with a single peak at high fiequency. The soq)tion process is fast substantiating that physical adsorption is the dominating form of the solid-gas interaction. On increasing the temperature, the coverage decreases and the FR spectlvm becomes characteristic of parallel site-specific chemisorption processes. The three peaks resolved can be associated with adsorption on various copper species ill the Cu,H-ZSM-5 catalyst. Above 473 K, the copper ions are reduced to Cu+ state by CO [23]. Ill order to avoid the complication that the appearance of the product CO2 in the system could cause, samples were prereduced with CO at the temperature of the FR measurement before the sorption rate-spectra for CO were recorded. The differences between the 1ow-(_<473 K) and high-temperature (673 K) FR spectra may reflect the effect of reduction.
709 Figure 5 shows that NO is a sensitive probe molecule for Cu-zeolites at 373 IC At higher temperatures (above about 623 K) NO decomposition proceeds on the Cu,H-ZSM-5 catalyst [24]. Due to the reaction, the response spectrum takes up a complex character at 673 K, because the batch method used is not suitable for studying systems wherein catalytic reaction proceeds. Desol9tion and readsorption of oxygen occurs at elevated temperatures with the si/~1 multaneous change of the oxidation state of 3 (A) copper in the Cu-exchanged zeolites [24, 25]. This process was followed by the FR "O ~ : ~ " 1 method. For Cu,H-ZSM-5 peaks could be _~ detected from the sorption of 02 at 673 K at low frequencies as can be seen in Fig. 6. No ¢J e. 0 / ........-.~::.~:~-~:-~,......----:. . . . . . . . . similar FR spectrum could be obtained with a. t (B) I Cu-Y. Under identical conditions no re3 sponses for oxygen sorption could be de2 tected on H- and Na-ZSM-5. These observations correlate well with the results of lo catalytic tests [24] and oxygen TPD measurements [25]. The exceptionally high active.el o.1 1 lO lOO ity of Cu,H-ZSM-5 in the decomposition of NO has been related to the presence of meF r e q u e n c y I Hz bile oxygen in the catalyst. These remits Fig. 6. Phase lag versus frequency curves for tend to confirm that Cu-ZSM-5 carries meoxygen solption in-75mg of Cu,H-ZSM-5 (A) bile oxygen, probably in the form of Cuand Cu-Y,FAU (B) at 673 K under 1Torr oxygen bound extra-lattice oxygen playing an important role in the NO decomposition reaction.
CONCLUSIONS The d3mamics of tile ad/desorption processes can be characterized by the FR method. These processes are of primary catalytic simlificance. A new, direct, macroscopic FR technique is proposed for measurhlg and interpreting the ad/desorption kinetics of various small probe molecules and for characterizing catalytically active sites in zeolite catalysts. Combination of the FR lnethod with other methods is necessary to reach a more complete characterization of the acid-base and redox centers. The application of rate "spectroscopy" has been shown also to offer a new approach to the study of redox catalytic reaction mechalfism.
710 ACKNOWLEDGMENTS The valuable advices of Dr. Dongmin Shell are gratefully acknowledged. Thanks is due to the National Science Foundation of Hungaly (grant number TO10761) for financially supporting this research. One of the authors (Gy. O.) would like to acknowledge the scholarship of the Hungarian Academy of Sciences and the Royal Society making tiffs study possible.
REFERENCES 1. Y. Yasuda, Heterog. Chem. Rev. 1 (1994) 103. 2. L.V.C. Rees and D. Shen, Proc. Charact. Porous solids, France, Marseilles, 1993.Elsevier, Amsterdam, Stud. Surf. Sci. Catal. 1994, No. 87 p563. 3. Gy. Onyesty~ik, D. Shen and L. V. C. Rees, J. Chem. Soc. Faraday Trans. 91 (1995) 1399. 4. L. M. Naphtali and L. MI Polinski, J. Phys. Chem. 67 (1963) 369. 5. L. M. Polinski and L. M. Naphtali, Advances in Catalysis 19 (1969) 241. 6. Y. Yasuda, J. Phys. Chem. 80 (1976) 1867. 7. Y. Yasuda, J. Phys. Chem. 80 (1976) 1870. 8. G. Marcelin and J. E. Lester, React. Kinet. Catal. Lett. 28 (1985) 281. 9. G. Marcel in, J. E. Lester and S. F. Mitchell, J. Catal. 102 (1986) 250. 10. J. G. Goodwin, J. E. Lester, G. Marcelin and S. F. Mitchell, ACS Symposium Series 288 (1985) p67. 11. Y. Li, D. Willcox and R. D. Gonzalez, AIChE Journal 35 (1989) 423. 12. Gy. Onyesty~ik, D. Shen and L. V. C. Rees, ill Catalysis by Microporous Materials, (eds. H. K. Beyer, H. G. Karge, I. Kiricsi and J. B. Nagy), Elsevier, Amsterdam, 1995, p 116. 13. Gy. Onyesty~ik, D. Shell and L. V. C. Rees, in Proc. of Int. Symp. on Zeolites in Chhla, 1995, 1-32. 14. Gy. Onyesty~ik, D. Shen and L. V. C. Rees, J. Chem. Soc. Faraday Trans. 92, (1996), 307. 15. N.G. van Begin and L. V. C. Rees, in Zeolites: Facts, Figures, Future, (eds. P. A. Jacobs and R. A. van Santen), Elsevier, Amsterdam, 1986, p915. 16. L. V. C. Rees and D. Shen, Gas Separation & Purification 7 (1993) 83. 17. H. K. Beyer, H. G. Karge and G. Borb61y, Zeolites 8 (1988) 79. 18. C. E. Sass and L. Kevan, J. Phys. Chem. 92 (1988) 5192. 19. M. W. Anderson and L. Kevan, J. Phys. Chem. 91, (1987) 4174. 20. C. Doremieux-Morin, Acidity and basicity of solids (Eds. J. Fraissard and L. Petrakis) Kluver Academic Publishers (1994) p279. 21. Gy. Onyestyak, D. Shen and L. V. C. Rees, Microporous Materials 5 (1996) 279. 22. Gy. Onyestyak and L. V. C. Rees, unpublished results 23. J. Valyon, and W. K. Hall, J. Phys. Chem. 97, (1993) 1204. 24. Y. Li and W. K. Hall, J. Catal. 129 (1991) 202. 25. J. Valyon and W. K. Hall, J. Catal. 143 (1993) 520.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
A
711
picosecond spectroscopic s t u d y
on the p r o t o n t r a n s f e r s of 6 - h y d r o x y q u i n o l i n e in zeolite c a g e s
Hyunung Yu,-Jiho Park, Nam Woong Song T and Du-Jeon Jang -t-'* Department of Chemistry, Seoul National University, Seoul 15i.-742, Korea TKorea Research Institute of Standards and Science, Taejon 305-606
Excited and ground state proton transfer reactions as well as other photophysical behaviors such as triplet state dynamics are studied for 6-hydroxyquinoline adsorbed in zeolite cages by measuring picosecond fluorescence kinetic profiles, microsecond transient reflectance kinetic profiles, emission spectra, and diffuse reflectance spectra. Protropic equilibrium species of 6-hydroxyquinoline undergo photon-initiated proton transfer cycles in catalytically important zeolite systems. In dehydrated zeolite surface, proton transfer reactions are much slower than those in aqueous solutions but also deprotonation is completed faster than protonation in the first excited singlet state as well as in the ground state. Protonation reaction in the excited state occurs only when the nitrogen atom of the molecule is located near a proton of zeolite surface otherwise the molecule relaxes downward. 1. INTRODUCTION Proton transfer reaction is an essential step in many important processes, including both chemical and biological processes such as enzymatic reactions and signal transductions. It is one of the simplest chemical reactions but it has provided us with a vast amount of information on equilibria, kinetics, isotope effects, free energy relationships, etc.. A protropic species with a functional group which has a large pK difference between ground and excited states may undergo protonation or deprotonation in excited states. 1 The produced excited protropic species relaxes to the ground state and then goes through reverse reprotonation, completing a four well proton transfer cycle. 2 On-the occasion that one molecule in amphoteric solvent environment has two groups with opposite pK tendencies in excited states, a photon may initiate protonation and deprotonation to yield zwitterion. Studies on proton transfer cycles of this type with a minimum of six potential wells are interesting since these may serve as experimental models for proton relays and proton pumps. 3 A variety of molecular systems have been extensively studied~-8 for photon-initiated proton transfer cycles associated with excited and ground
712 electronic states. However, proton transfer cycles involving excited states have been rarely reported in zeolites which show excellent catalytic effects. Investigations on the photochemistry and photophysics of molecules adsorbed on solid surfaces have been vigorously carried out, 9'1° since zeolites have well-defined structures, supercage effects and various unusual cation interactions so that the investigations may lead us to understand heterogeneity, surface and supramolecule in general. Zeolite acidity(basicity) increases(decreases) as Si/A1 ratio increases, as metal cations are exchanged with protons or as metal cations are exchanged with those of a higher charge density. 11 Time-resolved and static spectroscopic techniques are employed to understand proton transfer reactions of 6-hydroxyquinoline (6HQ) as well as their protropic equilibria and photophysical properties in zeolite microenvironments. Interactions of acidic a n d basic sites in zeolites with the N and OH groups of 6HQ may exhibit interesting phenomena such as protropic equilibria, cage effects, etc., since the participations of solvent molecules are reported 12 to be important in the excited state and ground state proton transfers of hydroxyquinolines. 6HQ in amphotefic solvent environment exists as one of four protropic equilibrium species, normal (N), protonated cationic (C), deprotonated anionic (A) and zwitterionic (Z) species. 13 Our results indicate that protonafion reaction in the excited state occurs only when the nitrogen atom of the molecule is located near a proton otherwise the species relaxes downward.
2. EXPERIMENTAL SECTION
The synthesized X and Y zeolites were washed by hot water and dried in a vacuum oven. A glass tube c0nt/fining Na÷-exchanged X ( N a X ) o r Y (NAY) zeolite was jointed with a vacuum line and evacuated at 670 K for 2 h. 6HQ, purchased from the Aldrich, was purified by vacuum-sublimation and transferred to a quartz tube after mixing with zeolite powder. Then the tube was evacuated, sealed and maintained at 570 K for 12 h to make 6HQ v~ipofize and diffuse into zeolite cages. 14 Diffuse reflectance spectrometer was assembled with a W/2H2 lamp, a 0.25-m monochromator (Kratos GM 252) and a Hamamatsu R374 photomulfiplier tube. Reflectance was detected at a fight-angle to the beam, which was collimated to the sample mounted at the tangent of an internally MgO coated integrating sphere. For the measurements of emission spectra, samples were excited by a wavelength-selected 350-W Xe lamp (Schoeffel, LPS 255HR) using a 0.25-m monochromator (Kratos, GM 252). Luminescence was collected from the front surface of sample excitation and focused to a 0.25-rn- monochromator (Kratos, GM 252) which was attached with a Hamamatsu R376 photomultiplier tube. Emission spectra reported here were not corrected for the wavelength-dependent sensitivity variation. Picosecond time-resolved fluorescence kinetic profiles were measured using 10 a previously described time-correlated single photon counting system of a
713 70-ps-fwhm response time equipped with a Coherent Antares YAG-pumped, hybrid mode-locked and cavity-dumped dye laser. Samples were excited only by frequency-doubled R6G dye laser pulses of 290 nm because of limited laser availability. Microsecond transient reflectance kinetic profiles were measured by monitoring reflectance changes of the above Xe lamp beam incident on a sample, which was excited by 0.6-ns N2 laser (Laser Photonics, LN1000) triggered by a digital pulse/delay generator (SRS, DG 535). The probe beam was detected with a photomultiplier tube (Hamamatsu R928), digitized with an oscilloscope (Tektronics, TDS 350) and accumulated with a computer.
3. R E S U L T S AND DISCUSSION
Figure 1 shows that protropic equilibria of 6HQ in zeolites are very different from those in aqueous solution la and that the spectra in NaX and NaY are significantly different each other. The absorption contribution of Z species is enhanced while that of A species is remarkably reduced in NaY. The major fraction of the molecules interact with strong acid and base sites of zeolites at an adsorbed position. Observed zeolite pHs of near neutrality are established by a near balance in numbers of many strong acid and base sites rather than by small numbers or weakness of acid and base sites. The fluorescence spectra in Figure 2 indicate that excited and ground state proton transfer cycles of protropic species can be triggered by photons in organized zeolite media as well as in solutions. ~5 The emission peaks of the spectra in NaX and NaY approximately correspond to the fluorescence peaks of A* and Z* respectively. The increased basicity in NaX may explain the dominant A* fluorescence but the increased absorption contribution of Z in
¢j Z < _ o (D <
3OO WAVELENGTH (nm) Figure 1. Diffuse reflectance spectra of 6HQ in NaX and NaY.
714
gJ °p..~
a~
fir] Z Z ,....___
360
. . . . .
-.J
.-"
430
5oo
WAVELENGTH
57o
64o
(nm)
Figure 2. Emission spectra of 6HQ in NaX and NaY, excited at 320 nm.
NaY is not enough to explain the dominant Z* fluorescence. The main reason of the spectral difference in two different zeolites is kinetic difference in the excited state proton transfer rather than the static population differences of protropic equilibrium species A and Z in the ground states at the moment of excitation. We can see the evidence of kinetic difference from the fluorescence kinetic profiles in Figure 3. The kinetic profile at 370 nm brings in the decay time constants of 40 (49%), 150 (13%) and 780 (38%) ps while that at 430 nm does those of 1000 (49%) and 7600 (51%) ps. The rise time of 40 ps and the decay time of 7700 ps are deconvoluted from the kinetic profile at 490 nm. We suggest that the excited normal species in dehydrated zeolite surface undergoes deprotonation in 40 ps but that protonation takes 1000 ps. Excited protonation process is possible only when the nitrogen atom of 6HQ has a proton nearby in zeolite surface otherwise A* relaxes to the lower triplet or ground state. This is because a long range excited protonation, which is slower than the relaxation of A*, is impossible to occur. Since a minor fraction of 6HQ molecules have a proton near to the N atom, the major fraction of A* relaxes downward without excited state protonation process. A larger fraction of A* species undergo protonation, forming Z* as the final photoproduct in the first excited singlet state, in NaY which has a higher acidity than NaX. Triplet state dynamics and reverse ground state proton transfer reactions are studied by measuring microsecond transient reflectance kinetic profiles as shown in Figure 4. The protonation time of 70 ms is also larger than the deprotonation time of 15 ms for the ground state Z species in dehydrated zeolite. .... In dehydrated zeolitic cages, both protonation and deprotonation processes are not only much slower than those in aqueous solutions but also protonation is even slower than deprotonation in contrast to the p r o c e s s e s in aqueous
715
() ~5 p~ E-~ t..-4 (/3
-
2; E-~
zW--4
i000
500
~,~,~.~,,
m
~~ .
r .~r~,"~2'L."~,d~-, i ~,
°e-4
,d
• ,,~,.
TIME
,
(p~)
1500
.
2000
(b)
'
l'
,"~".:ii~,.,,~,,....-~.
49q ,rim
rd
p~ E-~ [/3
z E-~ 2; P----4 0
2o'oo
4o'oo
TIME
(ps)
6o'oo
aooo
Figure 3. Fluorescence kinetic profiles of 6HQ in NaX. Excitation wavelength is 290 nm and collection wavelengths are indicated in the respective profiles.
solutions. 15 In addition to these dynamic differences, the spectra (energies), transitions (oscillator strengths) and equilibria (distributions) in zeolite cages are greatly different from those in aqueous solutions. These unusual observations are quite interesting and informative for understanding the catalytic roles of zeolites. During our presentation we will discuss in detail the mechanisms of these excited and ground state proton transfer cycles but also stress the importance of our observations for understanding both zeolitic catalysis and proton transfer reactions. A c k n o w l e d g e m e n t . We thank Dr. Dongho Kim at the KRISS for allowing us to use the picosecond emission spectrometer. This work was financially supported by the Hallym Academy of Sciences, Hallym University and the Korea Ministry of Education.
716
L I
z< -
i
o
2obo
40'00 TIM ,
6ooo
80'00
loooo
Figure 4. Transient reflectance kinetic profiles, excited at 337 nm, of 6HQ in NaX at 440 and 520 nm.
REFERENCES
-t-Also a member of the Center for Molecular Science, Taejon 305-701, Korea. *To whom correspondence should be addressed. 1. A. Weller, Progr. React. Kinet. 1 (1961) 188. 2. M. Kasha, J. Chem. Soc., Faraday Trans. 2 82 (1986) 2379. 3. A. B. Kotlyar, N. Borovok, S. Kiryati, E. Nachliel and M. Gutman, Biochemistry 33 (1994) 873. 4. Y. Chen, F. Gai and J. W. Petrich, J. Am. Chem. Soc. 115 (1993) 10158. 5. A. Douhal and R. Sastre, Chem. Phys. Lett. 219 (1994) 91. 6. L. M. Tolbert, L. C. Harvey and R. C. Lum, J. Phys. Chem. 97 (1993) 13335. 7. S.-I. Lee and D.-J. Jang, J. Phys. Chem. 99 (1995) 7537. 8. T.-G. Kim, S.-I. Lee, D.-J. Jang and Y. Kim, J. Phys. Chem. 99 (1995) 12698. 9. M. Huang and S. Kaliaguine, J. Chem. Soc. Faraday Trans. 88 (1992) 751. 10. J. Park, W.-K. Kang, R. Ryoo, K.-H. Jung and D.-J. Jang, J. Photochem. Photobiol. A: Chem. 80 (1994) 333. 11. D. Barthomeuf, J. Phys. Chem. 88 (1984) 42. 12. K. Tokumura and M. J. Itoh, J. Phys. Chem. 88 (1984) 3921. 13. S. F. Mason, J. Philip and B. E. Smith, J. Chem. Soc. A (1968) 3051. 14. B. F. Chmelka, J. G. Pearson, S. B. Liu~ R. Ryoo, L. C. de Menorval and A. Pines, J. Phys. Chem. 95 (1990) 303. 15. E. :Bardez, A. Chatelain, B. Larrey and B. Valeur, J. Phys. Chem. 98 (1994) 2357.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
717
Ethylene Dimerization in Nickel Containing SAPO Materials Studied by Electron Spin Resonance and Gas Chromatography: - Influence of the Channel Size
Martin Hartmann and Larry Kevan Department of Chemistry, University of Houston, Houston, TX 77204-5641, USA
Dehydrated nickel-containing silicoaluminophosphates SAPO-n (n---5,8,11) are catalytically active for ethylene dimerization. The catalytic activity of the SAPOs for this reaction is shown to be due to nickel (I) species in ion-exchange or framework positions. The ethylene dimerization activity and the selectivity for the formation of nbutenes is dependent on the location (ion-exchange vs framework site) of the nickel (I) species and on the channel size of the SAPO-n material. Electron spin resonance studies support the gas chromatography expedments showing that in SAPO-34 no reaction occurs, whereas in SAPO 5, 8 or 11 materials dimerization products are found. 1. INTRODUCTION
Silicoaluminophosphates (SAPOs) are microporous inorganic oxides, which are comparable to the well known aluminosilicate zeolites. The incorporation of transition metal ions into these materials increases the range of possible catalytic applications. It has been shown, that Ni(I) ions can be active sites for ethylene dimerization in zeolites [1] and on silica [2]. Traditionally Ni(ll) is incorporated into zeolites and SAPOs by liquid or solid state ion-exchange. If the transition metal ion can be incorporated substitutionally into the molecular sieve lattice, it is surmised that the stability and the catalytic properties of such a framework ion might be advantageous compared to an ion-exchange site. By comparison of catalytic and electron spin resonance (ESR) results, different catalytic activity of Ni(I) in ion-exchanged versus framework positions is indeed demonstrated and also an influence of the channel size is found. We have demonstrated this by studies of Ni(I) in SAPO-34 (3.8 A pore openings), SAPO-11 (6.3 A), SAPO-5 (7.3 A) and SAPO-8 (7.9 A).
718
2. EXPERIMENTAL SECTION
Sample Preparation. The starting materials H-SAPO-n (n : 5, 8, 11, 34) were synthesized as described in the patent literature with some modification developed in our laboratory [3,4]. NiH-SAPO-n was prepared by solid state ion-exchange using 0.01 g of NiCI2 and I g of the SAPO material. NiAPSO-n (n : 5, 8, 11, 34) were synthesized according to the p r ~ u r e s developed in our laboratory [5,6]. The nickel content of these samples was determined by electron probe microanalysis with a JEOL JXA 8600 spectrometer. The nickel content in all samples was about 0.1 atom %. Sample Treatmentand Measurements. Prior to the catalytic reaction the samples were dehydrated at 723 K in vacuum (p < 10.4 hPa) for 18 h. The catalytic reaction was performed inside a glass reactor under 400 hPa ethylene at 353 K. The gas phase over the sample was analyzed periodically by withdrawing an aliquot into a gas chromatograph. ESR spectra were recorded at 77 K on a Bruker ESP 300 X-band spectrometer. The magnetic field was calibrated with a Varian E-500 gaussmeter. The microwave frequency was measured by a Hewlett Packard HP 5342A frequency counter. Electron spin echo modulation (ESEM) was obtained at 4.2 K with a Bruker ESP 380 pulsed spectrometer using a three pulse x/2 - ~ - r,J2 - T - :/2 sequence. The deuterium modulation was analyzed by a spherical approximation for powder samples in terms of N nuclei to the nearest integer at distance R to typically 0.01 nm
[7].
3. RESULTS 3.1 Catalytic activity of Ni containing SAPO materials The parent catalysts H-SAPO-n were found to be essentially inactive for ethylene dimerization. In the SAPO-34 system no reaction occurs at 80 °C showing that the entrance to the cage is not large enough for the butenes. The other systems with the larger channels are active for ethylene dimerization. Analysis of the gas phase indicates that ethylene is dimedzed to isomers of nbutene and some side products are also formed (Figure 1). The overall turnover after 24 h at 80 °C is higher in SAPO-11 and SAPO-5 than in SAPO-8. Most likely the stabilization of Ni(I) is less effective in SAPO-8. In NiH-SAPO-5 and NiH-SAPO-11 the selectivity for the formation of n-butenes is quite low (Table 1). The side-products formed are isobutene, propane, propylene and butane. Incorporation of Ni(I)into the framework of SAPO-5 and SAPO-11 leads to catalysts with improved selectivity for the formation of n-butenes (Figure 2). While the total tum0ver decreases to some extent in NiAPSO-11, the total conversion increases in NiAPSO-5 compared to NiHSAPO-5. It has been reported that ethylene is initially dimerized to 1-butene, which on acidic materials is subsequently isomerized to cis-2-butene and trans-2-butene [8]. The cis/trans-2-butene ratio in NiH-SAPO-5 and NiAPSO-5 as well as in the type 8 system is close to the thermal equilibrium value (Table 1). However, in NiH-SAPO11 and NiAPSO-11 the distribution of n-butenes is quite far away from the thermal
719
equilibrium value indicating, that the reduced channel size (10-ring versus 12-ring or larger) has significant influence on the product distribution.
D sidet:xodJ:ls
.
I
I 1 ~
®5
4~
"040s _ O.
o~3 ~2 NiH-SAPO-34NiH-SAPO-11 NiH-SAi::~5
NiH-SAPO-8
Figure 1. Distribution between n-butenes and side products in the gas-phase after 24 h. Reaction conditions: T = 353 K, p = 400 hPa ethylene, 0.1 g catalyst.
Table 1 Ethylene Dimerization on Nickel Containing SAPO Materials after 24 h at 353 K. catalyst " NiH-SAPO-34/ NiAPSO-34 NiH-SAPO-11 NiAPSO-11 NiH-SAPO-5 NiAPSO-5 NiH-SAPO-8 NiAPSO-8
wt % conversion
wt % seleclN~
cis / trans ratio
0.10 4.74 3.05 6.12 6.99 2.20 1.55
76 93 21 74 83 99
49 6 0.45 0.45 0.46 0.45
720
[] s i < ~
|
a.
i,,.
.;;.~~,.-11 L .
.
.
.
.
.
.
.
.
.
.
.
.
II~11 .
.
~
II ::
F N I ~ :
-
::
-
-
=
",
Figure 2. ~ - b e t w e e n n-butenes and aide pnxltcm in the ~ ~er a 24 h reaction period at 353 K in different nickel oont~nlng SAPO-5 and ~ 1 1 materials. 3.2 Spectroscopic Studies All samples do not show any ESR signal at 77 K prior to any treatment. Thus, the Ni species exist in the form of Ni(ll). Figure 3a shows the ESR spectrum of NiHSAPO-11 dehydrated at 673 K. Dehydration at this temperature leads to formation of Ni(I) species A, which has been assigned to an isolated Ni(I) [9]. Ni(I) species A is also detected in all other samples. Adsorption of ethylene at room temperature on all type 5 and type 8 samples leads to the spectrum in Figure 3b. Species A is partially converted into species E (gl - 2.689, g2 = 2.489 and g3 - 1.959). Due to the limited diffusion of ethylene in the smaller channel type 34 and type 11 materials, heating of the samples to 353 K for 30 rain is required to obtain species E. Prolonged heating of NiH-SAPO-34 and NiAPSO-34 does not change the ESR spectrum. This supports the catalytic studies, showing that at 353 K no dimerization occurs in this system. ESEM analysis at g = 1.92 of the Ni(I)-C2D 4 complexes in NiH-SAPO-n shows the interaction of four deuteriums with a Ni-D distance of 0.35 + 0.02 nm indicating a typical ~ - bond interaction between Ni(I) and one ethylene molecule. But in NiAPSO-5 and NiAPSO11 the ESEM data show a different coordination of C2D4 towards Ni(I) consistent with end-on ~ - bonding geometry [10]. After ethylene adsorption and subsequent heating to 353 K several new species can be seen in the other materials. In NiH-
721
SAPO-11 and NiAPSO-11 a new species B1 (gl = 2.720, g2 = 2.255 and g3 = 1.995) appears (Figure 4a). The g values of this species do not change after a longer reaction time. Species B1 can also be obtained by adsorption of n-butenes in this system. In NiH-SAPO-5 and NiAPSO-5 the formation of two new species B2 (gl = 2.353, g2 = 2.179 and g3 = 2.061 ) and B3 (gu = 2.052 and gj. = 2.007) (Figure 4b) is observed. In the type 8 sample Ni(I) species B3 and B4 (giso = 2.07) are obtained. These species B2, B3 and B4 are also obtained after n-butene adsorption. The analysis of the deutedum modulation of species B1 to B4 shows the interaction of 8 deuteriums with one nickel(I) center indicating butene formation.
dehydrated 673 K gl~'=2.489
gA = 2.108
gE= 2.689 I gE= 1.959
Figure 3. ESR spectra at 77 K of NiH-SAPO-11 after (a) dehydration at 673 K and (b) subsequent adsorption of C2D4 and warming to 353 K for 30 rain.
722
$APO-11
B1
g3 = 1.996 framework defect SAPO-5 B2
g2 = 2.174 g3"= = 2.067
-.4
2
^"
Yiso :o .U/ glT" = 2.
200 G F-- : , ~
B3 g.L =2"
Figure 4. ESR spectra at 77 K of nickel-containing SAPO-n liter 24 h at 353 K. 4. DISCUSSION
Ni(I) ions formed by reduction of Ni(ll) can be stabilized in zeolites and SAPO materials with different channel sizes and structure. It has been shown that cations with a d9 configuration like Pal(I) and Ni(I) cataliyze olefin dimerization [1,11]. Independent of channel size and location, isolated nickel species A can be stabilized in all these materials. However, stabilization of Ni(I) in NiH-SAPO-8 is less efficient
723
due to the lower amount of silicon present in the lattice [12] and the large size of the channels. Species A was found to decrease significantly upon the adsorption of ethylene at RT or 353 K accompanied by the formation of species E with a rhombic g-tensor indicating lower symmetry of this Ni(I) species. Uke in zeolites, species E can be ascribed to a Ni(I)-C2D4 complex. After heating the samples to 353 K species E slowly disappears and Ni(I)-C2D 8 species B1 to B4 appear. In the SAPO material with the smallest channel size (SAPO-11) an orthorhombic spectrum with the largest g-factor anisotropy is observed. In SAPO-5 due to the increasing channel size the complex has a greater symmetry resulting in a lower g-factor anisotropy (species B2). Species B3, which is also found in SAPO-8, has an axially symmetric g-tensor, which indicates even higher symmetry in the large pore SAPO-8 material. These more symmetric Ni(I)- C4D8 complexes are also found in nickel-containing AIMCM41, a meSoporous material with channels of 3.5 nm diameter [13]. In NiCa-X and NiCa-Y axial or isotropic Ni-butene complexes are also seen [11]. Therefore it can be concluded that the channel size of the SAPO-material has a s i g n ~ n t influence on the local geometry of the Ni(I)-C4D8 complex. The formation of a Ni(I)-butene complex is suPlx)rted by ESEM analysis, which shows the interaction of eight deuteriums near the Ni(I) center [14]. It has been reported that ethylene is initially dimerized to 1-butene, which is subsequently isomedzed to an equilibrium composition of n-butenes with predominant trans-2-butene over various transition metal cation exchanged zeolites. It has been shown before that H-SAPO-11 and H-SAPO-5 are highly active for 1butene isomerization at reaction temperatures of 300 °C and more [15]. The main products besides the n-butenes are isobutene, propane, propylene and butane. High temperatures and strong acid sites usually favor the formation of isobutenes over 2butenes [15]. Our results show that the selectivity for the formation of n-butenes is higher in NiH-SAPO-8 than in NiH-SAPO-5 or NiH-SAPO-11. Since the acidity of these materials is comparable and not high, the difference in selectivity must result from the effect of the pore size. SAPO-8 has very large 0.87 nm by 0.79 nm pores, while SAPO-5 is a large pore molecular sieve with pore openings around 0.73 nm. SAPO11 is a medium pore molecular sieve with elliptical pore openings around 0.63 nm by 0.39 nm, while SAPO-34 has a chabazite-like cage strtc~re. The entrance to the SAPO-34 cavities is via eight-ring pores with 0.38 nm diameter. Therefore it is quite logical that in SAPO-34 no reaction products can be detected. In X-zeolites and AIMCM-41, which have pore openings of 0.75 nm and 3.5 nm, respectively, the selectivity for the formation of n-butenes is close to 100 %. X- and Y- zeolites are more acidic than these SAPO materials and should be less selective for n-butene formation if only the acidity determines the selectivity. In medium pore materials the diffusion of the reaction products out of the channels might be slower, so that subsequent reactions are more able to occur. The increase in selectivity in NiAPSO-n materials containing nickel in framework sites compared to the ion exchanged samples, NiH-SAPO-n, is most probably due to the location of the active nickel (I) center. Being in a framework site Ni(I) is less accessible to the large butenes and therefore oligomerization to hexenes and subsequent cracking is suppressed.
724 5. CONCLUSIONS Ethylene dimerization occurs in nickel-containing SAPO materials with different pore sizes. In the small pore SAPO-34 system no dimerization occurs, while in NiHSAPO-n (n = 5, 8, 11) and NiAPSO-n (n = 5, 8, 11) reaction products can be detected. The decrease of the channel size from 14-ring SAPO-8 to 12-ring SAPO-5 and 10-ring SAPO-11 causes a higher amount of side products (isobutene, propane and butane) to be formed, probably because of slower product diffusion out of the channels. Incorporation of nickel into the framework of SAPO-5 and SAPO-11 increases significantly the selectivity for the formation of n-butenes which shows that Ni(I) in this location has distinct catalytic advantages. ESR and ESEM experiments support the catalytic measurements and show that paramagnetic Ni(I) species are observed prior to ethylene dimerization. After adsorption of ethylene a Ni(I)-(C2D4) n complex is observed, which is converted into a Ni(I)-C4D8 complex with different symmetry dependent of the channel size after reaction. No Ni(I) product complex is observed in the SAPO 34 in agreement with its lack of catalysis. ACKNOWLEDGMENTS This research was supported by the National Science Foundation and the Robert A. Welch foundation. REFERENCES
1. 2. 3. 4. 5. 6. 7.
A. K. Ghosh and L Kevan, J. Phys. Chem., 94 (1990) 3117. L. Bonneviot, D. Oliver and M. Che, J. Mol. Catal., 21 (1983) 415. N. Azuma, M. Hartmann and L. Kevan, J. Phys. Chem., 99 (1995) 6670. C.W. Lee, X. Chen and L. Kevan, J. Phys. Chem., 95 (1991) 8626. 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) 10988. L. Kevan, In Time Domain Electron Spin Resonance; L. Kevan, R. N. Schwartz (Eds.); Wiley: New York, 1979, Chapter 8. 8. T. Yoshima, Y. Ushida, M. Ebisawa and N. Hara, J. Catal. 36 (1975) 320. 9. J. Michalik, H. Lee and L. Kevan, J. Phys. Chem. 88 (1984) 5236. 10. M. Hartmann, N. Azuma and L. Kevan, In Zeolites: A Refined Tool for Designing Catalylic Sites; L. Bonneviot, S. Kaliguine (Eds.); Studies in Surface Science and Catalysis Vol. 97; Elsevier: Amsterdam, 1995, p. 335. 11. A. K. Ghosh and I_ Kevan, J. Phys. Chem., 92 (1988) 4439. 12. M. Hartmann and L. Kevan, J. Phys. Chem., 100 (1996) 0000. 13. M. Hartmann, A. P6ppl and L. Kevan, Proceedings of the 11. Int. Conf. on Catalysis, Baltimore 1996, submitted. 14. M. Hartmann and L. Kevan, J. Chem. Soc. Faraday Trans., 92 (1996) 0000. 15. S. M. Yang, D. H. Guo, J. S. Lin and G. T. Wang, In Zeolites and Related Microporous Materials: State of the Art 1994; J. Weitkamp, H. G. Karge, H. Pfeiffer and W. H61dedch (Eds.); Studies in Surface Science and Catalysis Vol. 84; Elsevier: Amsterdam, 1994, pp 1677-1684.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
725
The Electronegativity Equalization Method (EEM) as a promising tool for the analysis of zeolite catalyzed reactions Geert O. A. Janssens,* Helge Toufar, Bart G. Baekelandt, Wilfried J. Mortier and Robert A. Schoonheydt Centrum voor Oppervlaktechemie en Katalyse, Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92, 3001 Heverlee, Belgium
The H-exchange of methane in FAU and MFI is analyzed with the Electronegativity Equalization Method (EEM). The polarization channels and their hardnesses are presented as promising criteria for estimating the reactivity of acid zeolite catalyzed reactions. The reaction mode hardness is shown to depend on structural parameters (topology) as well as chemical composition (Si/A1 ratio and the A1 distribution), the latter being dominant. The results theoretically confirm the Next Nearest Neighbors (NNN) principle, which stresses the importance of the number of A1 atoms in the second coordination sphere as a key reactivity parameter. 1. INTRODUCTION Zeolites are widely used as solid catalysts in petrochemical industry due to their acid character. There is a general agreement that the acidity is brought about by protons that are connected to the framework oxygens. Carbenium/carbonium chemistry known in liquid superacids can be successfully applied to rationalize reaction mechanisms for hydrocarbon rearrangements [1]. This accounts for the extensive body of literature that exists on the theoretical determination of acidity from the proton affinity [2-4]. Recent developments in theoretical modelling propose a "bifunctional" active site, consisting of the Bronsted acid (bridging hydroxyl) and a neigboring framework oxygen which acts as a Lewis base [5-11]. The dependence of the acidity of zeolites on the structure type and the chemical composition, and its relation to catalytic activity, continues to be a major research topic. The application of ab initio methods to solid state problems is hampered by the infinite dimensions of the solids [ 12]. Therefore, small isolated parts of the lattice, usually referred to as clusters, are being used to study the acidic properties of zeolites. Up to now, quantum chemical nor density functional methodologies allow rigorous reaction path calculations for systems incorporating the next neighbor coordination sphere. A comparison between different zeolite structure types is therefore not straightforward and only possible in an indirect way. Moreover, compositional effects such as e.g. the Si/A1 ratio and the influence of the A1 ordering are difficult to deal with. Larger systems can be dealt with by the Electronegativity Equalization Method (EEM) which can give information on the effects of chemical composition (Si/AI ratio) and structure type on the activated complex, and on reactivity criteria along the reaction path which are not readily accessible by traditional computational methods. In this paper the potential of the
726 model is demonstrated by analyzing the H-exchange of methane for faujasite (FAU) and ZSM5 (MFI). 2. M E T H O D O L O G Y
The EEM Charge Sensitivity Analysis (CSA) is a semi-empirical density functional method, which allows calculations on molecules interacting with infinite crystals, whatever the structure type and chemical composition, when the atomic coordinates and the calibration coefficients of all atoms in the structure are known. For a thourough description of EEM [ 13], and the related CSA [ 14] we refer to the original literature. Recent advances in the EEM-CSA have significantly increased the conceptual framework for describing various stages of a chemical reaction in terms of charge redistribution processes. Particularly, the independent normal modes of charge transfer (CT) and/or polarization (P) are powerful concepts, as they quantify the tendency of a reacting molecule towards charge displacements facilitating this reaction. More interestingly, they describe charge displacements upon interaction with another molecule or a catalyst. A major conceptual breakthrough for the validity of the charge sensitivity approach towards the analysis of chemical reactions came recently with the derivation of the so-called mapping relations [15], which explicitly relate charge rearrangements to changes in atomic positions along the reaction coordinate. The positive correlation between the "force" needed for a nuclear vibration (force constant) and the "hardness" of the associated charge redistribution (principal hardness) was clearly demonstrated and provides us with a valid starting point for the study of molecular interactions occurring at zeolite surfaces. When a molecule is perturbed an electron redistribution will occur, which can be computed with the EEM formalism starting from the hardness matrix 11, originally defined by Nalewajski et al. [ 16]
[ 2n~ ri _ [rl~f~ ] _ [k/RBa
k/Roll3 ..- k/Roan 2~.~.
:i'. k/R-Bn
Lk/Rno~ k/Rn~
.--
(1)
21qn
This matrix contains the atom-in-molecule (AIM) hardnesses, rl~c~ = rl*ct, which depend on the atom type, as well as the inter atomic distances, R~I3, which specify the geometry of the system. Notice that k is a conversion factor (14.4) to obtain the energy in units of eV, when the distance is expressed in A. For the calculation of molecule-crystal interactions, the geometric information necessary for the construction of the hardness matrix is extracted from a Bertaut-Ewald Madelung type summation which is implemented within the EEM formalism. The Polarization (P) and Charge Transfer (CT) channels, also referred to as the electronic Populational Normal Modes (PNM's), are the eigenvectors U which diagonalize the hardness matrix. uT~u
= h
with
UU T =
1
and
h = {h~136o~13}
(2)
727 A new orthogonal set of charge displacement coordinates accompany the rotation of the coordinate system of the AIM electron populations, N (or charges, q), to the principal axis system, Q
aQ- dqV
(3)
Polarization modes deal with a purely internal redistribution of electron density so that EUia = 0. Charge Transfer modes describe a net in- or outflow of electrons to or from the molecular system, zUia ~ 0. Associated with each channel is a principal hardness, h (eigenvalue), which denotes the difficulty of its charge redistribution. To visualize the normal modes, the following notation is adopted: the modes are represented by diagrams showing the relative changes in the electron populations; white circles correspond to an electron outflow (Uo~7 < 0) and white circles with a black cross to an inflow (U~7 > 0), while the radius of the circle is proportional the amount of electron density displacement I Matrix algebra states that the sum of the diagonal elements of a matrix before and after diagonalization is identical. In the specific case of the hardness matrix this means that: n
n
~2'rl~- Lhi (z=l i=1
(4)
The molecular geometry will directly influence the principal hardnesses. Some will shift to lower values, some remain constant, while others increase in value, if the composition is constant. Identification of the polarization channel(s) which favor a reaction pathway as a first step and then systematically manipulating the geometrical configuration of the catalysts so as to lower the principal hardness(es) of the reaction channel(s) provides us with a sensitive design tool for the formulation of structure/composition activity/selectivity relationships. The normal representation of the energy expression: dE
- j~'n°rdQT
+ ~-l ~ h i=l
i
dQ~
(5)
is obtained upon orthogonal transformation of the matrix equivalent of the EEM energy expression: dE - z*dq T + dqrldq T
(6)
It is a particularly useful reactivity criterion, since the most important PNM's, which describe a charge reorganization process along the reaction path, can be selected from an energetical viewpoint. In eq 5 Z n°r is a row vector consisting of the normal electronegativities, %nor = c~
aQi
(7)
728 which measure the force behind the inflow or outflow of electrons via the independent electron population channels Q. In our recent work the electronic normal mode analysis of the H-exchange reaction of methane on a zeolite cluster has been presented, using the transition state proposed by Kramer et al. [6]. It was shown that the PNM's can be divided into three groups. When the charge redistribution mainly involves the atoms of methane, we define them as molecular modes. Cluster modes are concentrated on the cluster. Interaction modes involve charge rearrangements on methane and on the cluster. On the basis of the energy criterion (Eq. 5) the softest interaction mode was selected as the polarization channel responsible for driving the reaction, with a contribution of more than 80 % to the energy at the transition state. Figure 1 shows the softest interaction mode, referred to as the reaction mode, and the associated nuclear displacements.
Figure 1. The reaction polarization channel and the reaction coordinate The proton of the bridging hydroxyl group decreases its electron population, or increases its net positive charge and thus its acidity. At the same time, the neighboring bridging oxygen decreases its electron density, hereby stimulating an increase in the electron population of the proton connected to the carbon, consequently, weakening the C-H bond. Both electron displacements favor a proton transfer from the cluster to the methane and simultaneously from the methane to the cluster. The difficulty of the charge rearrangement of the reaction mode is reflected in the value of the reaction mode hardness. We can now safely use the reaction mode hardness as a reactivity parameter, where a more reactive situation is connected with a lowering of the hardness. 3. RESULTS 3.1 The impact of structural effects on the reaction mode hardness As the geometry directly impacts on the value of the reaction mode hardness, the importance of structural differences between zeolite crystals can be envisaged. The typical test cases are faujasite (FAU) and ZSM-5 (MFI). Faujasite is a highly symmetric zeolite with one unique tetraheder site surrounded by four different oxygen types. The unit cell comprises 192 T-atoms. MFI is a much more complex structure consisting of 12 topologically different Tatoms and 26 O-sites. When the size of the unit cell is doubled, it also contains 192 T-atoms. Both zeolite structures FAU and MFI are converted into idealized crystals with prescribed distances (r(Si-O) =1.61~ r(A1-O) = 1.73 A) and bond angles (O-T-O = 109.47 °, T-O-T = 145 ° with T = Si or A1), using the DLS program [17]. To avoid compositional influences only
729 one Si atom is substituted with an A1 atom in the P1 unit cell. Calculations thus refer to a Si/A1 ratio of 191 for both FAU and MFI . . . . . The transition state geometry of the CH5 moiety has been taken from the literature [6]. The methane carbon atom and the two exchanging H-atoms are placed in the O-AI-O plane, with H-O distances identical to the reported ones. The sterically accessible pathways for MFI and FAU have been determined by graphical analysis, using the Hyperchem software [ 18]. The obtained reaction mode hardnesses are given in Table 1. For MFI the atom numbers refer to the structure determined by Olson et al.[19] It is also indicated whether the methane molecule is situated in the Straight channel (S), the Zigzag channel (Z), or at the intersection. For FAU O 1 and 04 belong to the 12-ring. /
Table 1 Structural influence on the reaction mode hardness position CH56+ MFI
FAU
A1 T2 T3 T4 T5 T6 T8 T9 T10 T11 T12 T12 T
O O1 02 04 05 05 07 O18 O15 O11 Oll 020 O1
O 02 020 O 17 O21 O19 08 025 026 022 020 / 024 04 /
/
Z-channel x x x
S-channel x x x x x
....
x x x x x
h (V/e) 12.5808 12.5500 12.5698 12.5282 12.5935 12.5644 12.5521 12.5596 12.5882 12.6085 12.5897 12.5554
/
The reaction mode hardness varies from 12.53 V/e to 12.61 V/e due to topological differences only. Three T-sites of MFI (T3, T5 and T9) are more reactive (lower hardness) compared to the T-site of FAU. They are situated, respectively, at the intersection of the two channels (T3), in the straight channel (T5) and in the zigzag channel (T9). There is no relation between the reaction mode hardnesses and the channel type (Z or S). Both the most reactive (T5_O5-O21) and the least reactive (T12_Oll-O20) methane are situated in the straight channel. 3.2 The impact of AI distribution on the reaction mode hardness To study the influence of the AI distribution on the reaction mode hardness, we restrict ourselves to the most reactive site on the basis of the topology, i.e. T5 of MFI. An AI atom is inserted at each of the 12 Next Nearest Neighbors (NNN) positions ofT5 by substitution of Si while'keeping the geometry fixed. The accompanying O-H bond is placed in the AI-O-Si plane, bisecting the AI-O-Si angle. The length is set at 0.96 A. The protons are positioned on the oxygens, bridging the Nearest Neighbors (NN) to the NNN's. Figure 2 shows the labels of
730 the NNN T-sites relative to T5. The reaction mode hardnesses are shown in Figure 3 and Table 2.
y
r.
12.59.
T3
O16 T4
~ 12.58. T2
T2
12.57. 06_T2 • ,.~
-
•
12.56.
01
T2
O18- T9
"~ 12.55. T3
019 T3
015 TIO
•
.~ 12.54. O22T7
12.53. 12.52
Figure 2. Transition state of H-exchange of methane at T5 of MFI.
: ..............
VVV
m
Ol1_T12 O10_T10 O17 37 m
0
,
2 4 6 8 10 12 i
,
i
•
i
,
i
•
i
•
i
Figure 3. Impact of AI distribution on the reaction mode hardness.
Table 2 Influence of A1 distribution on the reaction mode hardness Si_NN H-pos'i'tion ................ AI_NNN T6 06 T2 O18 T9 O19 T3 Tll Oll T12 022 T7 O10 T10 T1 O1 T2 O16 T4 O15 T10 T4 O16 T1 O 17 T7 03 T3 . . . . . . . . . . . . . . . . . . . . .
O16 T1 O3_T2
m
VVV
, . . . . . . . . . . . . . . .
h (V/e) 12.5676 12.5610 12.5704 12.5295 12.5303 12.5282 12.5618 12.5875 12.5554 12.5283 12.5282 12.5283 ,,
,,
,,
The data clearly show that A1 can harden the reaction, but not in all of the NNN positions. This depends whether the A1 is bonded to a Si_NN tetrahedron, which contains one of the two oxygens that take part in the H-exchange. If A1 is connected to S~6 and Sil, the reaction mode hardness increases. This variation is in the same order of magnitude as the reactivity differences due to the topology. For all other AI positions the influence is negligible.
731 3.3 The impact of AI content on the reaction mode hardness
The compositional influence on the reaction mode hardness is probed by a systematic increase of A1 at the NNN positions of T5, while keeping Si at the other T-sites in the unit cell. Taking into account the rule of Loewenstein (no A1-O-A1), a maximum of 8 A1 atoms can be inserted. Figure 4 shows the dependence of the reaction mode hardness on the number of A1 atoms that have been added. The labels correspond to those mentioned above and indicate the T-site which has been substituted, keeping the A1 distribution of the previous A1 content fixed.
13.0-
O6T2
~ 12.9.
O15_T10~
12.8.
O1%T1
Compositional influence
12.712.6.
/ O18_T9 O1T:201~_T12 •
12.5- ~
Structural influence
6 ~ ~ ~ 8
# AI at NNN-positions
Figure 4. Impact of the A1 content on the reaction mode hardness The results clearly show that the A1 content directly influences the hardness of the reaction mode. The higher the A1 content, the harder it becomes, and therefore the more difficult the H-exchange reaction will be. The plateaus that appear in the relation can be explained by the details of A1 distribution, which are superposed on the results of the AI content. When the newly added A1 atoms are connected to Si4 (O16_T1) or Sil 1 (O1 l_T12 and O 17_T7), we obtain no significant increase in the reaction mode hardness. The composition affects more drastically the reactivity, compared to the structural differences, as is evidenced by the large variation in the reaction mode hardness. 4. DISCUSSION In this paper the H-exchange between CH4 and a bridging hydroxyl in the faujasite and the ZSM-5 structure has been studied. In order to include the long range electrostatic effects, we combined the ab initio optimized transition state geometry proposed by Kramer et al. [6] with the EEM-CSA approach. In this way topologically different systems can be compared, excluding any cluster size effects. The hardness of the reaction polarization channel provides
732
us with a sensitive tool to probe the influence of both structural and compositional properties of the zeolite catalysts. Using idealized structures, which contain one A1 per unit cell (Si/A1 = 191), the different zeolite topologies-can be compared directly. It is interesting to see that the structural properties o f MFI tend to make it more active than FAU, especially if one assumes that the most reactive site determines the overall activity. From experimental studies, it is known that the number of A1 atoms at the N N N positions of the reaction center has a tremendous impact on the reactivity. According to the N N N principle, the maximal intrinsic acidity is obtained when no A1 is present in the second coordination sphere of the acid proton. The here proposed findings theoretically verify the importance of the NNN's. The higher the number of A1 atoms at the N N N positions, the harder the reaction will be. However, an increase of A1 at the NNN sites, not connected to the bridging oxygen atoms that take part in the reaction, will not result in a lower reactivity. In conclusion, both structural effects and the chemical composition (Aluminum distribution and Si/A1 ratio) determine the hardness and therefore difficulty of the H-exchange reaction, compositional effects being dominant. G.O.A.J. thanks the Flemish Institute for the Support of Scientific-Technologic Research in Industry (I.W.T.). The authors acknowledge financial support from the Belgian State Secretariat for Scientific Research in the form of a Concerted Research Action (G.O.A) REFERENCES
1. P.A. Jacobs and J.A. Martens in Introduction to Zeolite Science and Practic (Eds. H. van Bekkum, E.M. Flanigen and J.C. Jansen), Stud. Surf. Sci. Catal. No 58, Elsevier, Amsterdam, 1991, p. 445. 2. W.J. Mortier, J. Sauer, J.A. Lercher, H. Noller, J. Phys. Chem. 88 (1984) 905. 3. G.J. Kramer, R.A. van Santen, J. Am. Chem. Soc. 115 (1993) 2887. 4. A. Redondo, P. Jeffrey Hay, J.Phys.Chem 93 (1993) 11754. 5. V.B. Kazansky in Advanced Zeolite Science and Applications (Eds. J.C. Jansen, M. St0cker, H.G. Karge and J. Weitkamp), Stud. Surf. Sci. Catal. No 85, Elsevier, Amsterdam, 1991, p. 251. 6. G.J. Kramer, R.A. van Santen, C.A. Emeis, A.K. Nowak, Nature 363 (1993) 529. 7. E.M. Evleth, E. Kassab, L.R. Sierra, J.Phys.Chem 98 (1994) 1421. 8. J.A. Lercher, R.A. van Santen, H. Vinek, Catal. Lett. 27 (1994) 91. 9. I.N. Senchenya, V.B. Kazansky, Kinet. Catal. 35 (1994) 61. t0. S.R. Blaszkowski, A.P.J. Jansen, M.A.C. Nascimento, R.A. van Santen, J.Phys.Chem 98 (1994) 12938. 11. G.J. Kramer, R.A. van Santen, J. Am. Chem. Soc. 117 (1995) 1766. 12. J. Sauer, Chem. Rev. 89 (1989) 199. 13. a) W. J. Mortier, S. K. Ghosh, S. Shankar, J. Am. Chem. Soc. 108 (1986) 4315.;b)B.G. Baekelandt, W.J. Mortier, J.L. Lievens, R.A. Schoonheydt, J. Am. Chem. Soc. 113 (1991) 6730.;c) G.O.A. Janssens, B.G. Baekelandt, H. Toufar, W.J. Mortier, R.A. Schoonheydt, J. Phys. Chem. 99 (1995) 3251.;d) H. Toufar, B.G. Baekelandt, G.O.A. Janssens, W.J. Mortier, R.A. Schoonheydt, J. Phys. Chem. 99 (1995) 13876. 14. R.F. Nalewajski, Int. J. Quantum Chem. 56 (1995) 453. 15. a) G.O.A. Janssens, B.G. Baekelandt, H. Toufar, W.J. Mortier, R.A. Schoonheydt, Int. J. Quantum Chem. 56 (1995) 317.;b) B.G. Baekelandt, G.O.A. Janssens, H. Toufar, W.J. Mortier, R.A. Schoonheydt and R.F. Nalewajski, J. Phys. Chem. 99 (1995) 9784. 16. R.F. Nalewajski, J. Korchowiec, Z. Zhou, Int. J. Quant. Chem.: Quant. Chem. Symp. 22 (1988) 349. 17. C. Baerlocher, A. Hepp, W. M. Meier, DLS-76: A Program for the Simulation of Crystal Structures, ETH: Zurich, Switzerland (1978). 18. HyperChem, Release 3 for Windows, Molecular Modeling System; Hypercube, Inc., and Autodesk, Inc.: Waterloo, Ontario (1993). 19. D.H. Olson, G.T. Kokotailo, S.L. Lawton, W.M. Meier, J. Phys. Chem. 85 (1981) 2238.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
S y n t h e s i s and Characterization of I r o n m o d i f i e d L - t y p e
733
Zeolite
Y. S. Koa, W. S. a h n a, J. H. Chae b, and S. H. Moonb aDepartment of Chemical Engineering, Inha University, Inchon 402-751, Korea bDepartment of Chemical Engineering, Seoul National University, Seoul 151-742, Korea
Iron modified L-type zeolite was synthesized hydrothermally, partially substituting iron atoms for the framework aluminum. XRD, SEM, IR, EPR, XAS, TG/DTA were performed confim~g the isomorphous substitution of iron into the zeolite structure. Catalytic studies reveal that benzene selectivity in n-hexane aromatization reaction is enhanced for the Fe-subsfituted L.
I. INTRODUCTION Isomorphous substitution of iron into the framework of zeolites would induce changes in both acidity distribution and pore size resulting in a modification of the catalyst selectivity. Trace amounts of non-framework iron in the zeolites has also been shown to contribute to the overall catalytic activity[l]. The replacement of A13. by Fe 3. in the framework of zeolite ZSM-5 has been reported by various workers[2-4]. McNicol and Pott[5] have shown the existence of Fe 3. impurity in the faujasite and mordenite framework using EPR, phosphorescence and chemical studies. Szostak and Thomas[6] were the first to deliberately synthesize a sodalite with significant quantities of Fe 3÷ in the framework. Over the years, a wide range of iron analogues such as MEL[7], MOR[8], MTT[9], MTW[10], NU-I[ll], and TON[12] were reported and techniques such as ESR, EXAFS, UV-visible diffuse reflectance, and Mossbauer spectroscopy were employed to demonstrate the tetrahedral coordination of iron. Though the methods of synthesis of iron-substituted L-type zeolites have been recently investigated[13,14], further work on the synthesis of iron substituted L-type zeolites with extensive characterization would be beneficial to the understanding of the system. In this work, we present the details of
734 synthesis and report the results of a wide range of characterization techniques including XRD, SEM, IR, EPR, XAS, and TG/DTA. Catalyst selectivity for n-hexane aromatization is also briefly discussed.
2. E X P E R I M E N T A L 2.1. S y n t h e s i s The reagents used in preparing the substrate were Ludox HS-40 (Dupont Co., 40% SiO2), potassium hydroxide (Tedia Co., 85%), aluminum hydroxide (Junsei Co., 51.1% AbO3), nanohydrate ferric nitrate (Shinyo Co., 98%) and distilled water. The synthesis of A1- and Fe-modified L-type zeolites was carried out following the method of Verduijn[15], partially Fe-substituted L-type zeolite samples were synthesized from substrates having the following compositions : 2.35 I~O - x Fe2Os - A12Os - 10 SiO~ - 160 H~.O, where x = 0, 0.015, 0.03 and 0.06. The reaction mixture was transferred to a 100ml teflon-lined stainless steel autoclave and maintained in an air oven at 170°C under unstirred conditions. Autoclaves were removed at different time intervals from the oven and were quenched immediately in cold water for identification. The solid products were separated by suction-filtration. Excess alkali was washed with distilled water, and :the products were dried in an air oven at 120°C for 12h. 2.2. Characterization X-ray diffraction patterns of the different crystalline samples were determined by X-ray diffractometer (Phillips, PW-1700) using Ni-filtered monochromatic CuKa radiation. Unit cell parameters were obtained by a least-squares fit. The crystallite size and morphology of the crystalline phase were examined using a scanning electron microscope (Hitachi, X-650) after coating with a Au-Pd evaporated film. Framework i.r. spectra of samples were recorded in air at room temperature on a Perkin Elmer 221 spectrometer with wafers of zeolites mixed with dry KBr. Electron spin resonance spectra were measured with a Bruker E-2000 spectrometer in the temperature range of 100K--400IL The XAS spectra were measured above the Fe K-edge at beamline 1 0 B , Photon Factory of National Laboratory for High Energy Physics in Tsukuba, Japan. Thermogravimetric analysis was made on a Dupont 2000 thermal analyzer in the temperature range 298--873K at a heating rate of 10K min -1. Chemical analysis was performed using ICP (Jobin Yuon-JY-38 VHR),
735
2.3. Catalysis For n-hexane aromatization, catalysts containing 2wt% Pt were used. The n-hexane aromatization was performed under atmospheric pressure at 773K in a conventional fixed bed microreactor system with 0.2g catalyst. The reaction products were analyzed by gas chromatograph (Shimadzu, GC-14A) equipped with flame ionization detector using Porapack-Q and SE-30 columns.
3. RESULTS AND DISCUSSION The chemical compositions of A1- and Fe-modified L-type zeolites are given in Table 1. For each sample, an increase in the iron contents cause the A1 contents of the crystalline material to decrease, which suggests that Fe a÷ and A13÷ could perform a similar role in the synthesis of L-Wpe zeolite and compete for its incorporation into the framework. The X-ray diffraction patterns of the crystals are shown in Figure 1. Virtually identical diffractograms of L-type zeolite were obtained irrespective of the iron contents of the sample. Figure 2 shows the influence of iron contents on the unit cell volume, V. The unit cell volume of the Fe-modified L-type zeolite increased linearly with increases in iron contents. These results strongly support that the F e 3÷ ions are incorporated in the silicate framework.
tt Q
~9
i
10
i
I
,
20
I
30
i
i
40
2 Theta Figure 1. XRD patterns of (a) A1- and (b) Fe-modified L-type zeolite.
736 Table 1 Chemical composition of A1- and Fe-modified L-type zeolites Samples (X) 0.000 0.015 0.030
SiO2 60.1 61.5 60.8
0.060
60.7
Chemical composition (wt%) A1203 Fe203 17.1 0.000 16.3 0.308 16.1 0.645 15.7
1.231
I~O 15.9 15.4 15.2 15.3
Figure 3 shows the SEM photograph of the as-synthesized Fe-modified L-type zeolite. Morphology appeared sensitive to the Fe2Oa/Al203 ratio of the synthesis mixture : A distinctive characteristic of iron addition was the formation of cylindrical shaped crystals accompanied by a decrease in crystal size as the iron contents increased. On the other hand, clam shaped crystals of a domed basal plane was synthesized in the absence of iron. IR spectra of normal A1- and partially Fe-substituted L-type zeolite with different iron contents are shown in Figure 4. Partially Fe-substituted L-type
2212 I
~ 2208[
:>
"~=~ 2204 t
2200 t / ° 21961~
, !
i
I
0.000 0.015 0.030 0.045 0.060 Fe203 / AI203 molar ratio Figure 2. Unit cell volume of the Fe-modified L-type zeolites.
Figure 3. S EM photograph of Fe-modified L-type zeolite.
737 zeolites showed a pattern similar to the normal L-type zeolite. As the amount of iron increases, IR absorption bands near 1027 and 1099 cm -1 shift progressively towards the lower frequency region compared with the absorption band of iron free L-type zeolite. The shift to lower frequency in the spectra is due to the presence of heavier Fe a+ existing in tetrahedral sites. In the partially Fe-substituted L-type zeolite, new Si-O-Fe bond vibrations could be found at near 668cm -1, which is absent in the i.r. spectrum of normal L-type zeolite, and its intensity increased with increasing iron contents.
(d)
1200
11 O0
1000
900
800
700
600
Wavenumbev (cm "1) Figure 4. IR spectra of A1- and Fe-modified L-type zeolites. (a) x = 0, ( b ) x = 0.015, ( c ) x = 0.03, ( d ) x = 0.06
Figure 5 shows EPR spectra of as-synthesized partially Fe-subsitiuted L-type zeolites. The EPR spectra of the samples reveal two main signals at g=2.0 and 4.4, and the sharp signal at g=4.4 assigned to tetrahedral Fe(m)[16], increased in intensity with increasing iron contents. According to the X-ray absorption spectroscopy(XAS), the Fe-O distance and Fe-O coordination number for Fe-modified L-type zeolite(Feg:)a/A12Oa = 0.06) were 1.85A and 4.2, respectively. The XAS results showed conclusively that the majority of the iron was substituted into the L-type zeolite framework.
738
g=4.4
(a)
I
,
1000
,
I
2000
,
3000
I
,
4000
5000
B (Gauss) Figure 5. EPR spectra of Fe-modified L-type zeolites at 100K. (a) x = 0.015, ( b ) x = 0.03, ( c ) x = 0.06
Figure 6 shows thermogravimetfic analysis patterns of the as-synthesized A1- and partially Fe-substituted L-type zeolite. The weight loss is due to dehydration of physically sorbed or occluded water, and TGA shows that Fe-modified samples are more hydrophobic than normal L-type zeolite.
100 98
o
94
} 92
(a)
L
(b) 8
,
0
i
100
•
!
•
200
I
300
•
I
400
,
I
500
•
600
Tcmlpel'at~e (°C) Figure 6. TGA patterns of (a) A1- and (b) Fe-modified L-type zeolite.
739 Pt supported on zeolite L has been commercially applied as a catalyst for n-hexane aromatization, and in Table 2, a comparison was made on benzene selectivity for the L-type zeolites with or without Fe 3+ in the framework. Significant enhancement in selectivity to benzene was obtained for the partially iron-substituted L-type zeolites. However, large reduction in conversion was also observed. This may be a consequence of Fe 3. released from the zeolite structure blocking the 1D pores at high reaction temperature. Further investigation is in progress. Table 2 Products distribution in n-hexane aromatization Samples (X) C1 = C5
Selectivity C6 isomer
MCP
C6I-I6
Conversion
0.000
3.66
20.97
1.63
73.74
90.47
0.015
2.22
9.19
1.10
87.49
35.70
0.030
4.31
4.77
1.05
89.87
34.27
0.060
3.36
1.92
2.05
92.67
32.02
Reaction conditions : temp. 773K, atmospheric pressure, Hz/HC=6, TOS 2h
4. CONCLUSIONS Fe-modified L-type zeolite was synthesized hydrothermally, partially substituting Fe 3÷ for the framework aluminum of L-type zeolite from substrates having the following compositions : 2.35 K20 - x Fe~O3 - A1203 10 SiO2 - 160 H20, where x - 0, 0.015, 0.03 and 0.06. The unit cell volume of the Fe-modified L-type zeolite was found to increase linearly following the increases in the iron contents of the substrate. The crystal morphology also changes depending on the iron contents of the sample. The mid range i.r. spectra shows a band shift to lower frequencies as iron incorporation into the lattice increases, and new S i - O - F e bond vibration was located at near 668cm -1. The EPR spectra shows a signal at g =4.4, 'which can be assigned to Fe 3. isomorphously subsitituted in the tetrahedral positions. For n-hexane aromatization reaction, enhanced benzene selectivity could be achieved by introducing Fe 3. into the zeolite L.
740
Acknowledgements We would like to thank Dr. Nomura of photon factory, Tsukuba, Japan for helping us using the synchrotron radiation source at beamline 10B and Pohang Accelerator Laboratory in Korea for providing the travel expanses. Professor R. Ryoo's group in KAIST for their assistance in analyzing EXAFS data is also acknowledged. YSK and WSA thank Inha university for providing the research fund in 1994.
REFERENCES 1. R. Szostak, Molecular Sieves, Principles of Synthesis and Identification, Van Nostrand Reinhold, New York, 1989. 2. R. Szostak and T. L. Thomas, J. Catal., 100 (1986) 555. 3. A. Meagher, V. Nair and R. Szostak, Zeolites, 8 (1988) 3. 4. R. B. Borade, Zeolites, 7 (1987) 398. 5. B. D. McNicol and G. T. Port, J. Catal., 25 (1972) 223. 6. R. Szostak and T. L. Thomas, J. Chem. Soc., Chem. Commun., (1986) 113. 7. J. S. Reddy, K. R. Reddy, R. Kumar and P. Ratnasamy, Zeolites, 11 (1991) 553. 8. A. J. Chandwadkar, R. N. Bhat and P. Ratnasamy, Zeolites, 11 (1991) 42. 9. R. Kumar and P. Ratnasamy, J. Catal., 121 (1990) 89. 10. Zhao Yanan and Li Xi Hexuan, Shiyou Xuebao, Shiyou Jiagong, 6 (1990) 33. 11. G. Bellusi, R. MiUini, A. Carati, G. Madinelli and A. Gervasini, Zeolites, 10 (1990) 642. 12. R. B. Borade, A. Adnot and S. Kaliagine, Zeolites, 11 (1991) 710. 13. P. N. Joshi, S. V. Awate and V. P. Shiralkar, J. Phys. Chem., 97 (1993) 9749. 14. V. A. Duke, K. Latham and C. D. Williams, Zeolites, 15 (1995) 213. 15. J. P. Verduijn, Exxon Chemicals, Int. Pat. WO 92/13799 (1992). 16. B. Witcherlova, Zeolites, 1 (1981) 181.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
741
Synthesis, characterization and catalytic properties of VS-2 Hongwei Du, Guanghua Liu, Zhijian Da and Enze Min Research Institute of Petroleum Processing P.O.Box. 914-28 100083, Beijing, ElL China Microporous vanadium silicalite with MEL structure (VS-2) has been synthesized by a new method. XRD, IR, and ESR examinations show that vanadium ions are atomically and immobilely dispersed in V-Si molecular sieves, and exist in the form of V +4 and V +5 in assynthesized and calcined san~les, respectively. It is found that the incorporation of vanadium makes the pore structure of VS-2 more uniform and regular. A tentative mechanigic explanation is given for the increase of BET area upon vanadium insertion. The results of phenol oxidation with aqueous H:O: show that VS-2 molecular sieves have good catalytic activity and selectivity. 1. INTRODUCTION Vanadium-silicalite-2 (VS-2) is a new class of oxidation catalyst with good activity and selectivity in the oxidation of a variety of organic substances by diluted H202 [1-3]. However, since the electronic structure of vanadium element is quite different from that of silicon element and the ratio of electronic charge to radius of vanadium ion is higher , the insertion of vanadium into zeolite t~amework is very difficult. Up to now, the effective method reported in literature [4-5] for synthesizing VS-2 is the hydrothermal crystallization using organic materials. In the method, organic tetraethyl orthosilicate was used as silicon source, large amount of expensive organic base, tetrabutyl ammonium hydroxide (TBAOH), was used as base sourse and template , and the synthesis procedure is rather critical and complicated. All these make VS-2 a high cost product. In the present paper, a cheap and simple method for preparing vanadium-silicalites-2 will be introduced, in which inorganic SiO2 pellets are used as silicon source, and V205 is used as vanadium source. Expensive TBAOH is effectively utilized by greatly reducing the amount of water in the system The synthesized VS-2 molecular sieves are characterized by various physicochemical methods and tested by probe reaction of phenol oxidation with H202. 2. EXPERIMENTAL Vanadhm~silicalite-2 was synthesized as follows: a certain quantity of V205 was dissolved in a solution of TBAOH (40% aqueous solution) and the mixture was stirred for 0.5-1h. Dried micro spheric silica with a definite surface area and pore size was added to the above mixture and stirred uniformly. The typical molar composition of the gel was as follows: SiO2: x VO2: 0.14TBAOH: 4H20 (x = 0.005--43.03)
742 The resultant mixture was tranferred into a Telflon, lined autoclave and crystallized at 413K for 5 days. After crystallization, the product was filtered, washed with deionized water and dried at 383K for 8h. The organic template was removed by calcining the product at 813K for 5h. The samples of VS-2 molecular sieves were characterized by XRD (D/max-IliA, D/ max-TA), IR spectroscopy (Perkin-Elmer-580, FT-IR, Bruker-IFS-113V ), ESR measurement (E200-D), ICP analysis (Plasma Spectrovac) and adsorption techniques (ASAP-2400, Micromeritics ). Phenol oxidation by H202 ( 30% ) was carried out in a batch glass reactor and the products were analyzed by gas chromatography (Varian 3400) using OV-01 capillary column (30mx0.25mm). 3. R E S U L T S AND D I S C U S S I O N 3.1. The incorporation of vanadium ions into zeolitic framework
The X-ray powder diffraction patterns are presented in Fig.1. It can be seen that the XRD pattern of VS-2(Si/V=78) is similar to that of silicalite-2, showing the typical MEL structure. The absence of peaks at 20 = 9.06 ° and 24.06 ° indicates that there is no MFI type impurity in the product. The unit cell parameters given in Table 1 show that the unit cell volume of vanadium-silicalite-2 is larger than that of silicalite-2, and increases with the vanadium content to a certain extent. This can be explained by the fact that larger vanadium ions are incorporated into zeolitic framework of MEL structure.
b
--
10
20
30
40
20 Fig. 1. XRD pattems of silicalite-2 (a) and VS-2 (Si/V=78) (b). Table 1 Unit cell parameters of vanadium-silicalite-2. Sample Si/V molar ratio Unit cell Gel Product a Silicalite-2 oo oo 2.002 VS-2 200 378 2.003 137 224 2.002 78 127 2.004 33 82 2.004
=
b 1400
1000
600
400
Wavenumber(cm.t) Fig. 2. IR spectra of silicalite-2 (a) and VS2 (Si/V=78) (b).
parameter/nm b c 2.002 1.336 2.003 1.337 2.003 1.338 2.004 1.339 2.004 1.338
Volume/nm 3 5.354 5.364 5.369 5.378 5.376
743 The insertion of vanadium ions in the framework is further confirmed by IR experiments. The IR spectra of vanadium-silicalite-2 and silicalite-2 are shown in Fig. 2. The main peaks of VS-2 shift slightly to lower wavenumber, which is due to that the length of V-O bond is longer than that of Si-O bond and also the mass of vanadium atom is larger than that of silicon atom. In addition, a new absorption band at around 970 cm-1 appears in the spectrum of vanadium-silicalite-2, which is absent in the spectrum of vanadium-free silicalite-2. This can be accounted for by the influence of the incorporation of vanadium ions on the asymmetric stretching vibration of Si-O8+ "V ~ [6]. The above experimental results prove that the larger vanadium ions are incorporated into the framework position. 3.2. The chemical state of vanadium ions
The ESR spectra of both as-synthesized and calcined VS-2 samples have been recorded at room temperature. As shown in Fig. 3a, the spectrum of as-synthesized VS-2(Si/V=78) is composed of 8-splitting lines arising from the b hyperfine interactions of the d electrons of the V +4 ion with the I=7/2 spin of the 5Iv nucleus. i This kind of ESR spectrum is characteristics of atomically dispersed and immobile V ÷4 ions [7]. After calcination of the as-synthesized sample in air at 813K for 5h, no ESR signals are observed (Fig. 3b), which indicating that V ÷4 ions ( d 1 ) are completely oxidized to V +5 ions Fig. 3. ESR spectra of VS-2 (Si/V=78) (dO). After reducing the calcined sample in H2 at as-synthesized (a), calcined (b) and 753K for 4h, the typical ESR spectrtun of reduced (c). atomically dispersed V ÷4 ions with multisplitting lines appears again, showing that the reversibility of the transformation between V +s and V +4 and that V +5 ions are still linked to the silicalite structure in calcined sample. The above results indicate that vanadium ions in V S-2 zeolites exist as atomically and immobily dispersed V +4 and V +5 state in as-synthesized and calcined samples, respectively. The transformation of V +4 ¢:> V +5 is completely reversible, showing the good redox properties of vanadium-silicalite-2.
1
3.3. Pore distribution and surface area
The pore distribution profiles of silicalite-2 and VS-2 measured by N 2 adsorption and calculated on the basis of BJH methods are shown in Fig.4 and Fig.5, respectively. It can be seen that both the meso pore with 4nm diameter in average and the large pore about 55nm diameter in average exist in silicalite-2, whereas only the meso pore with 3.2nm diameter in average exists in vanadium-silicalite-2. Moreover, the pore volume of vanadium-silicalite-2 (0.025ml/g) is less than that of silicalite-2 (0.044ml/g). It seems that the introduction of vanadium plays a role in reducing the volume of medium pore as well as inhibiting the formation of larger pore, which leads to a more regular and uniform pore structure of vanadium-silicalite-2. This may be the main reason causing the increase of BET area after the insertion of vanadium (Table 2). When too much vanadium is added to the synthesis system,
744 the BET area of vanadium-silicalite-2 decreases. This is probably due to the formation of extra-framework vanadium cluster which might block the pore of VS-2.
•
-
0.40
..
-I--~ :'
.
A
''i
4nm_..:. . . . A n m
~:::"
~ 0.30
.~ 0.20 ;~ 0
o t~
.
'
./
,..-:....
"-,-:-:.i-z i
•
~
'.
."
:ii!iiifil if:ii!!! ..
l
'.
.. , t . . :
• J-:
":
I • ::'"
0.10 t--~=~-~.---v--t, 55nm ---~'"i ]._.,
0.00
:"
'
T ...... ~ : . 2 _ 5 I. ---. -'. -.,...., - . . . . . . . . .
1
~
"
10
0.28
!ii
~ :'
• A
"~ 0.20 E o
area ofvanadium-silicalite-2. BET surface area/mZ.g4 co 267 200 489 504 517
,
~ !
"i
. . . . . . . . . . . . . .
. . . . . . i ............................ I ...............
.
:
.
! " t ..... ..........
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"
.a .........
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• ._:_
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-
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Pore diameter(nm)
Table 2 BET surface Sample Si/V VS-2
..
.......
;> 0.10
: .....
i
0
-I ........ ..
Fig. 4. Pore distribution curve of silicalite-2.
• '
|
1. I . . . . . . . . . . . . . . . .
:"' . . . . .
,,...
-
100
................. ......
'ell
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. ,,
.~. . ,
_--
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.--z---r-
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.
j
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10 100 Pore diameter(rim)
Fig. 5. Pore distribution curve of VS-2 (SifV=78).
137 531
78 538
33 511
3.4. Adsorption properties The data of adsorption measurements are listed in Table 3. It can be seen that the sorption capacity over VS-2 decreases with the adsorbates in the following order: n-hexane> cyclohexane > water. The RAL values of VS-2 with different Si/V molar ratios are in the range of 1.81 to 1.90, exhibiting the obvious hydrophobicity. The hydrophobical property of vanadium-silicalite-2 makes it possible to use low concentration aqueous HzO2 (30%, safe and cheap) as oxidant and water as solvent in catalytio reactions, which makes the reaction process both convenient and cost-effective. Comparing with the case of silicalite-2 (Table 3), the adsorption capacity of H202 over vanadium-silicalite-2 is enhanced evidently, and it increases with the increase of vanadium content, which can be accounted for by the cl~emical absorption of H202 on vanadium-silicalite-2.
Table 3 Adsorption properties ofvanadium-silicalite-2 Sample Adsorption capacity/wt.% n-Hexane cyclo-Hexane Water Silicalite-2 12.9 7.89 6.68 VS-2.(200) 13.1 8.12 6.91 VS-2(137) 13.2 8.23 7.11 VS-2(78) 13.1 8.09 7.38 VS-2(33) 13.4 8.16 7.27
H2Oz 6.4 8.5 9.2 11.1 11.3
RAL= n-Hexane(wt. %) / Water(wt.%) . 1.93 1.90 1.87 1.81 1.85
745 3.5. Surface acidity NH3-TPD profiles of different samples are shown in Fig.6. The absence of the significant high temperature peak on VS-2 ( curve b) reveals the lack of strong acid sites. Fig. 7 illustrates the IR. spectrum of pyridine adsorption on VS-2 (Si/V=78) after evacuation at 302K. Three absorption bands at 1548cm-1, 1447cm-1 and 1492cm-1, which can be assigned to Bronsted acid sites, Lewis acid sites, both Bronsted acid sites and Lewis acid sites, respectively, are identified in the spectrum. It is calculated that the ratio of the number of Bronsted acid sites to that of Lewis acid sites on VS-2 (Si/V=78) is approximately 0.62.
I
i
|
l
i
I
!
373 573 773 Temperature(K)
Fig. 6. NH3-TPD profiles of silicalite-2 (a) VS-2(Si/V=78) and ZSM-5(Si/Al=65)
1700
t
1600 1500 1400 Wavenumber(cm"~)
Fig. 7. FTIR spectrum of VS-2(Si/V=78) after pyridine adsorption. "
3.6. Oxidation of phenol with H20 2 The results of phenol oxidation by diluted H202 over VS-2 with different Si/V molar ratio in water solvent are given in Table 4. The samples of VS-2 were treated with 0.5M ammonium acetate solution at room temperature and then calcined at 773K for 6h before use in the catalytic reaction. Reaction conditions were as follows: catalyst/phenol-4.8wt.%; phenol/H202 =3.0mol/mol; HEO/phenol=5:l(wt.); temperature(K)=353; reaction time=8h. It can be seen that vanadium-silicalite-2 samples are catalytically active in hydroxylation of phenol to catechol and hydroquinone. Phenol conversion over VS-2 (Si/V=78) could reach a level of 18.6% with a selectivity of 99.1%. The main products are catechol (58%) and hydroquinone (41%). Para-benzoquinone is obtained in an amount less than 1%.
Table 4 Catalytic properties of phenol hydroxylation over vanadium-silicalite-2 Sample Phenol Phenol H202 Products distribution / mol% conversion/% selectivity/% efficiency/% CAT HQ PBQ VS-2(33) 18.2 99.3 54.5 58.8 40.3 0.89 VS-2(78) 18.6 99.1 55.3 58.2 41.0 0.83
746 4. CONCLUSIONS Vanadium-silicalite-2 has been synthesized by a low cost method. XRD and IR experiments confirm that vanadium ions are incorporated into the zeolitic l~amework. ESR spectra indicate that vanadium ions are atomically and immobily dispersed respectively in as-synthesized and calcined samples. The transformation of V+4c:~V+5 is found to be completely reversible, showing the good redox properties of VS-2. BET, NH3-TPD, FTIR and sorption measurements reveal that vanadium-silicalite-2 possesses regular and uniform pore structure, surface hydrophobicity and medium acid strength with a ratio of Br&asted to Lewis acid sites of 0.62. The synthesized VS-2 molecular sieve exhibits good catalytic activity and selectivity in phenol oxidation by aqueous H202 solution. REFERENCES
1. P. 1~ Hari Prasad Rao, A. V. Ramaswamy and P. Ratnasamy, J. CataL,141(1993)604. 2. P. P,. Hari Prasad Rao and A. V. Ramaswamy, AppL Cata[ A: General, 93(1993)123. 3. P. P,.Hafi Prasad Rao, K.Ramesh, A. V. Ramaswamy and P. Ratnasamy, Stud. Sure Sci. CataL, 78(1993) 385. 4. Hari Prasad Rao, A.A. Belhekar, A.V. Ramaswamy et al., J. CataL, 141(1993)595. 5. P. lk Haft Prasad Rao, Rajiv Kumar, A. V. Ramaswamy and P. Ratnasamy, Zeolites, 13(1993)663. 6. R. M. Boccuti, M. K. Rao, et aL, Stud. Sure Sci~ CataL, 48(1989)133. 7. ~ Takahashi, M. Shiotani~ H. Kobayashi and J. Sohma, J. CataL 14(1969)134.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All fights reserved.
747
Preparation and characterization of manganese bipyridine complexes in zeolites with different pore architectures S. Ernst and B. Jean Institute of Chemical Technology I, University of StuRgart, D-70550 Stuttgart, Germany
[Mn(bpy)2]2+-complexes have been synthesized and immobilized in the supercages of zeolite Y and, for the first time, in the large intracrystalline cavities of zeolite MCM-22. No complex formation could be observed with zeolite EU-1 as host material, most probably due to steric constraints. The synthesized host/guest compounds are characterized by physico-chemical methods and their catalytic properties are tested in the liquid phase oxidation of cyclohexene using aqueous hydrogen peroxide.
I. INTRODUCTION Zeolites and related mieroporous materials are currently under intensive study as inorganic host materials for the immobilization of catalytically active guests, i. e., transition metal complexes [1-3]. In many eases, zeolite inclusion is applied to avoid the formation of polynuclear aggregates and to strongly reduce the self-oxidation tendency of the complexes. Typical organic ligands used for the synthesis of zeolite encapsulated transition metal complexes are phthalocyanines, the Sehiff base salen and 2,2"-bipyridine (bpy) [1-3]. With very few exceptions (e. g., metal phthaloeyanines in AIPO4-5 [4,5], VPI-5 [6] or in zeolite EMC-2 [7]), only faujasite-type zeolites have so far been explored as host materials. There are, however, several additional framework topologies currently available having large intracrystalline voids which render them promising candidates for the immobilization of transition metal complexes. Among these framework topologies are those of zeolite MCM-22 and zeolite EU-1. Zeolite MCM-22 contains two independent pore systems: One consists of large supereages (ca. 0.71 x 1.82 rim) which are accessible and interconnected via 10-membered ring windows. The second channel system is made up from two-dimensional, sinusoidal channels with 10-membered ring openings [8]. The pore system of zeolite EU-1 essentially consists of unidimensional, non-interconnected 10-membered ring channels with very large side pockets of ca. 0.65 nm in diameter and 0.8 nm deep [9]. Hence, these two zeolitic materials could potentially act as hosts for transition metal complexes of suitable size. Here we report on our attempts to synthesize manganese bipyridine complexes in the intracrystalline voids of zeolites MCM-22 and EU-1. For
748 comparative purposes, zeolite Y is also included in the present study. Manganese-bisbipyridine complexes encapsulated in the supercages of faujasite-type zeolites were recently reported to catalyze selective oxidation reactions of alkenes in the liquid phase [10], in particular oxidation of cyclohexene to adipic acid using tertiarybutylhydroperoxide as the oxidant [11 ].
2. EXPERIMENTAL SECTION Zeolites MCM-22 (nsi/nAl = 21, [12]) and EU-1 (nsi/nAl = 20, [13]) were synthesized according to published procedures. The as-synthesized materials were calcined at 540 °C in air in order to remove the organic templates and then extensively ion exchanged at room temperature with a 0.1 n aqueous solution of NaC1. The sample of zeolite NaY used in this study was a commercial product supplied by Union Carbide Corp., Tarrytown, N. Y., USA, and had a nsi/nAl-ratio of 2.6. Predetermined amounts of Mn 2+ were introduced into the three zeolites with different pore architectures by ion exchange with Mn(CH3COO)2 in aqueous suspension at room temperature. For complex formation, the Mn 2+ exchanged and dried zeolites were mixed in a glove box with 2,2"-bipyridine ligand with a ratio of nbpy/nMn2+ - 2.5. The mixture was then transferred to a glass ampoule, sealed and kept in an oven at 90 °C for a period from two to three days. The obtained products were soxhlet-extracted with dichloromethane to remove uncomplexed bipyridine ligand. After hydrothermal synthesis and after each further modification step it was ascertained by X-ray powder diffraction (Siemens D 5000) that the crystallinity of the samples remained virtually unchanged. The prepared materials were further characterized by chemical analysis using atomic emission spectroscopy with inductively coupled plasma (AES/ICP), UV/VIS-spectroscopy in the diffuse reflectance mode, solid-state IR-spectroscopy using the KBr pellet technique and simultaneous thermogravimetry/differential thermal analysis (TGA/DTA). The catalytic properties of the prepared host/guest compounds were tested in the liquid phase oxidation of cyclohexene using hydrogen peroxide (35 wt.-%) as the oxidant.
3. RESULTS AND DISCUSSION After the complex formation and soxhlet extraction steps, the obtained materials exhibit a slight pink color in the cases of zeolite Y and zeolite MCM-22, whereas zeolite EU-1 remained white. This is a first indication that complex formation occurred in the former zeolites but not in the latter. Most probably, the space in the side pockets of zeolite EU-1 is not large enough to allow for the bipyridine complexes to form. The positions of the absorption maxima in the UV/~S-spectra of the Na +- and Mn2+exchanged zeolites and after treatment with bipyridine and soxhlet extraction are summarized in Table 1. Replacing part of the initially present Na+-cations by Mn 2+cations results in a shift of the absorption maximum from 225 nm to 235 nm for zeolite Y and from 250 nm to 260 nm for zeolites MCM-22 and EU-1. For the samples of
749 zeolites Y/l, Y/2 (two samples with different Mn2+-contents were prepared from zeolite Y) and MCM-22 which had been treated with bipyridine, new absorption bands appear which can be attributed to the formation of [Mn(bpy)2]2+-complexes. In particular the absorption maxima at 495 nm and 530 nm (519 nm for [Mn(bpy)2] 2+MCM-22) can be attributed to the typical metal-to-ligand charge transfer. The absorption maximum around 360 nm is most probably due to ligand-to-metal charge transfer. No new absorption bands can be observed for the bipyridine-treated sample of zeolite MnEU-1. As already suspected from the unchanged color of this zeolite, obviously no complex formation had occurred. At present it is tentatively assumed that this is due to the steric constraints in the intracrystaUine voids of zeolite EU-1. The UV/VIS-spectra of the prepared host/guest compounds are depicted in Figure 1. As expected, the absorption bands for the faujasite-type zeolite with a higher concentration of Mn2+-cations are more intense. This indicates a higher density of [Mn(bpy)2]2+-complexes in the former sample. The weak shoulder around ca. 360 nm (ligand-to-metal charge transfer) almost disappears for the samples with lower complex content. UV/WIS-spectroscopy of the solutions recovered after soxhlet extraction reveals that during the extraction step only uncomplexed 2,2"-bipyridine is removed from the zeolite. The Mn2+-containing complexes are obviously efficiently retained in the zeolites, i. e., they are encapsulated.
Table 1. Positions of the absorption maxima in the solid-state UV/VIS-spectra of the Na +- and Mn2+-exchanged zeolites and after the treatment with 2,2"-bipyridine and soxhlet-extraction. Sample
Positions of the absorption maxima, nm
NaY NaMCM-22 NaEU-1
225 250 250
MnY MnMCM-22 MnEU-1
235 260 260
300 300
247 247 245 260
295 295 295 300
[Mn(bpy)2]2+-y/1 [Mn(bpy)2]2+-y/2 [Mn(bpy)212+-MCM-22 [Mn/bpy)E]E+-EU- 1
356 356 361
495 495 495
530 530 519
750
Fourier transform infrared spectra of the encapsulated complexes reveal only minor frequency shifts as compared to the homogeneous [Mn(bpy)2]2+-complex (the latter was prepared as described in [14]). Knops-Gerrits et al. [10] reported that in faujasitetype zeolites the cis-bipyridine complex is preferentially formed over the transconfiguration, which is indicated by the occurrence of two absorption bands at ca. 757 cm-1 and ca. 772 cm-1. A single band around 772 cm-1 would be indicative for the trans-configuration. The host/guest compounds prepared in the present study possess a dublett with absorption bands at ca. 757cm-1 and 772 cm-1. Hence, cis-[Mn(bpy)2] 2+ seems to be the preferentially formed complex in the cavities of both, zeolite Y and zeolite MCM-22. It is known from the free homogeneous complex that oxygencontaining ligands may favor the formation of the cis-species. In a zeolite, this role could be played by oxygen ions of the zeolite lattice. This would also result in an enhanced retention of the complex in the zeolite.
356 Z
•
0
530
i
Ifl_ n,"
0
cO nn
<
495 530
~
a
495519
I
C .
200
.
.
.
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.
.
.
.
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400
.
.
.
.
I
,
,
,
500
i
I
600
,
,
,
,
700
WAVELENGTH, nm
Figure 1. UV/VIS-spectra of [Mn(bpy)2]2+-complexes encapsulated in zeolite Y (a: [Mn(bpy)2]2+-y/1; b: [Mn(bpy)2]2+-y/2; the complex concentration in sample a is higher than that in sample b) and in zeolite MCM-22 (c: [Mn(bpy)212+-MCM-22).
751 The thermal stability of the encapsulated [Mn(bpy)2]2+-complexes was characterized by TGA/DTA. A typical result obtained with sample [Mn(bpy)2]2+-y/1 is depicted in Figure 2. In the temperature range from ca. 35 °C to ca. 150 °C a weight loss is observed which is accompanied by an endothermic effect. This weight loss can be attributed to the desorption of water from the intracrystalline voids of the zeolite. Upon fitaher heating, an additional weight loss is observed starting at ca. 400 °C. In the DTA-trace an exothermic maximum appears at ca. 450 °C with a shoulder on the high temperature side at ca. 550 °C. The observed exothermic weight loss is most probably due to the decomposition~urning of the encapsulated complexes. At present, however, it is not dear why this happens in two steps. Perhaps, the complexes decompose by first splitting-off one of the bipyridine ligands and, at a slightly higher temperature, the second ligand is removed from the coordination sphere of the metal cation. The thermal stability of the homogeneous complex was also investigated. A similar two-step decomposition of the complex was also observed in this case, however, at considerably lower temperatures, viz. in the range from ca. 300 °C to 400 °C. Obviously, the immobilization of the complexes leads to a considerably improved thermal stability. From a combination of the results of chemical analysis and TGA/DTA it can be seen that all samples (except of course zeolite EU-1) contain slightly more of the bipyridine ligand than one would have expected on the basis of a 2 • 1 stoichiometry in the bis-bipyridine complex (i. e., nbpy/nMn2+ typically amounts to 2.2 to 2.3). At present, two possible explanations cati be offered for this experimental result: In view '
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200
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300
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400
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500
600
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700
,
800
TEMPERATURE, °C Figure 2. Characterization of sample [Mn(bpy)2]2+-y/1 by simultaneous TGA/DTA. Temperature program from 35 °C to 800 C, heating rate: 5 K/rain; purge gas: air; Vai r = 30 cm3/rain. O
.
752
of the excess of bipyridine ligand applied in the synthesis of the complex (% P y/nMn2÷ _ = 2.5) the possibility exists that not all of the remaining uncomplexed ligands were dissolved from the zeolite during the soxhlet extraction step. Alternatively, one could envisage that, under the conditions applied in the present study, also a small amount of tris-coordinated complexes are formed which, due to their low concentration, have not been detected by U V / ~ S - and IR-spectroscopy. From the data obtained from chemical and TGA analysis the loadings of the zeolites with complex can be calculated. They amount to five and three [Mn(bpy)E]E+-complexes per unit cell of the Y-type zeolites (sample Y/1 and Y/2, respectively) and to approximately one complex in every third unit cell of zeolite MCM-22. The catalytic properties of the immobilized [Mn(bpy)2]2+-complexes were explored in the liquid phase oxidation of cydohexene using an aqueous solution of hydrogen peroxide as oxidant. Pertinent results obtained with samples [Mn(bpy)2] 2+Y/1 and [Mn(bpy)E]2+-MCM-22 are depicted in Figures 3 and 4, respectively. Both host/guest compounds are active catalysts for the oxidation of cydohexene. Main reaction products are 1,2-cydohexanediol, 2-cydohexenol, 2-cydohexenone and cydohexene-oxide. The latter is most probably the primary product from the oxidation of cyclohexene on the immobilized [Mn(bpy)E]E+-complexes [11]. As a whole, the catalytic results obtained in the present work are comparable to those observed earlier by Knops-Gerrits et al. [10,11] with [Mn(bpy)E]E+-complexes immobilized in zeolites 50
10
[Mn(bpy)212*-y/1 l
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,
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,
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I
40
,
0
50
h
Figure 3. Catalytic oxidation of cyclohexene with aqueous hydrogen peroxide over [Mn(bpy)2]2+-Y/1. Conditions: meat. = 0.2 g; T = 60 °C; nCHx= = 40 mmol, nH202 = 40 mmol; solvent: acetone, Vacetone = 20 cm 3.
753
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Figure 4. Oxidation of cyclohexene on [Mn(bpy)212+-MCM-22
(conditions see Fig. 3).
X and Y. However, no further reaction to 1,2-cyclohexanone and to adipic acid could be observed in the present case. The reason is obviously that the oxidant was added only in a 1 : 1 stoichiometry rather than in a large excess as in the work of KnopsGerrits et al. [11 ]. A comparison of the catalytic results obtained with [Mn(bpy)2] 2+ immobilized in zeolites Y and MCM-22 reveals that the product distribution is very similar for both zeolites. Hence, the nature of the zeolitic host material does not seem to play the decisive role in this respect. Rather, the catalytic behavior of the host/guest compounds seems to be governed by the immobilized guest complex. The lower catalytic activity of the complexes immobilized in zeolite MCM-22 can be attributed to the fact that the complex concentration in this zeolite is much lower than in the zeolite Y sample and that diffusional restrictions may occur for reactant and product molecules due to the relatively narrow 10-membered ring windows in zeolite MCM-22.
4. CONCLUSIONS It has been shown that the synthesis strategies for the preparation of manganese-bisbipyridine complexes in zeolites of the faujasite structure can also be applied to zeolites with other framework topologies. This way [Mn(bpy)2]2+-complexes were prepared and immobilized for the first time in the large intracrystalline cavities of zeolite MCM-22. As in the case of faujasite-type host materials, the complexes seem to be preferentially formed in the cis-configuration. No complexes are formed in zeolite EU-1, most probably due to steric constraints. The prepared host/guest compounds are active catalysts for the selective liquid phase oxidation of cyclohexene with hydrogenperoxide.
754 ACKNOWLEDGEMENTS
The authors gratefully acknowledge financial support by Deutsche Forschungsgemeinschaft and Fonds der Chemischen Industrie. Moreover, financial support from the Ecole Europeenne des Hautes Etudes des Industries Chimiques de Strasbourg to cover the stay of B. Jean at the University of Stuttgart is gratefully acknowledged.
REFERENCES
1. R. Parton, D. De Vos and P. A~ Jacobs, in: Zeolite Microporous Solids: Synthesis, Structure and Reactivity, E. G. Derouane, F. Lemos, C. Naccache and F. Ramoa Ribeiro (eds.), Kluwer Academic Publishers, Dordrecht, 1992, pp. 555-578. 2. J. Weitkamp, in: Proceedings from the Ninth International Zeolite Conference, 1L von Ballmoos, J. B. Higgins und M. M. J. Treaty (eds.), Part I, ButterworthHeinemann, Stoneham, 1993, pp. 13-46. 3. D. E. De Vos, F. Thibault-Starzyk, P. P. Knops-Gerrits, R. F. Parton and P. A. Jacobs, Macromolecular Symposia, 80 (1994) 157-184. 4. S. Kowalak and K. J. Balkus, Jr., Collect. Czech. Chem. Commun., 57 (1992) 774780. 5. D. W6hrle, A. K. Sobbi, O. Franke and G. Schulz-Ekloff, Zeolites, 15 (1995) 540550. 6. R. F. Patton, L. Uytterhoeven and P. A. Jacobs, in: Heterogeneous Catalysis and Fine Chemicals II, M. Guisnet, J. Barrault, C. Bouchole, D. Duprez, G. P6rot, R. Maurel and C. Montassier (eds.), Studies in Surface Science and Catalysis, Vol. 59, Elsevier, Amsterdam, 1991, pp. 395-403. 7. S. Ernst, Y. Traa and U. Deeg, in: Zeolites and Related Microporous Materials: State of the Art 1994, J. Weitkamp, H. G. Karge, H. Pfeifer and W. H61derich (eds.), Studies in Surface Science and Catalysis, Vol. 84, Part A, Elsevier, Amsterdam, 1994, pp. 925-932. 8. M. E. Leonowicz, J. A. Lawton, S. L. Lawton and M. K. Rubin, Science, 267 (1994) 1910-1913. 9. N. A. Briscoe, D. W. Johnson, M. D. Shannon, G. T. Kokotailo and L. B. McCusker, Zeolites, 5 (1985) 74-76. 10. P. P. Knops-Gerrits, D. De Vos, F. Thibault-Starzyk and P. A. Jacobs, Nature, 369 (1994) 543-546. 11. P. P. Knops-Gerrits, F. Thibault-Starzyk and P. A. Jacobs, in: Zeolites and Related Microporous Materials: State of the Art 1994, J. Weitkamp, H. G. Karge, H. Pfeifer and W. H61derich (eds.), Studies in Surface Science and Catalysis, Vol. 84, Part B, Elsevier, Amsterdam, 1994, pp. 1411-1418. 12. S. Unverricht, M. Hunger, S. Ernst, H. G. Karge and J. Weitkamp, in: Zeolites and Related Microporous Materials: State of the Art 1994, J. Weitkamp, H. G. Karge, H. Pfeifer and W. H61derich (eds.), Studies in Surface Science and Catalysis, Vol. 84, Part A, Elsevier, Amsterdam, 1994, pp. 37-44. 13. G. W. Dodwell, R. P. Denkewicz and L. B. Sand, Zeolites, 5 (1985) 153-157. 14. C. C. Addison and M. Kilner, J. Chem. Soc. A, (1966) 1249-1254.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
755
Metal Substituted ATS Aluminophosphate Molecular Sieves
Deepak Akolekar a and Russell F Howe b * aDepartment of Applied Chemistry, RMIT, Melbourne, VIC 3001, Australia bDepartment of Physical Chemistry, UNSW, Sydney, NSW 2052, Australia MeAPO-36 molecular sieves containing V,Mn,Co or Zn have been synthesized and characterized by XRD,SEM,EXAFS,EPR,NMR and N2 sorption capacity measurements. The V and Mn materials are compared with the corresponding A1PO-5 materials. In all cases pure crystalline phases are obtained. The spectroscopic evidence for incorporation of the transition element into the A1PO lattice is discussed. 1.INTRODUCTION Transition element substituted aluminophosphate molecular sieves have attracted increasing attention because of their novel chemical and catalytic properties [1]. Most interest has focussed on A1PO-5, the AFI structure, and metals such as Co[2,3], Fe[4], Cr[5,6], V[7], Zn[8], Sn[9] and Mn[10] have been incorporated into the A1PO-5 synthesis. Incorporation of the transition metal into the A1PO synthesis does not however guarantee isomorphous substitution of a dior tri-valent ion for A13+ or a tetravalent ion for p5+ in the A1PO framework. Only in the case of CoAPO-5 is the spectroscopic evidence for isomorphous substitution completely convincing [2,3]. In other cases spectroscopic studies indicate that the transition metal ion may be anchored to the A1PO-5 framework but not substituted into it [4-9]. From a catalytic viewpoint, of course, such A1PO anchored transition metal ions may in fact be more reactive than truly substituted metals. This paper is concerned with the less widely studied A1PO-36 which has the ATS structure, consisting of a unidirectional elliptical 12-ring channel system with staggered annular side pockets[ll]. The ellipticity of the channel makes its minimum free dimension slightly smaller than that of A1PO-5, but the side pockets mean that the total pore volume is slightly larger than that of A1PO-5. A1PO-36 has not so far been prepared in pure aluminophosphate form, but is readily synthesized in the presence of Mg 2+ using a tripropylamine template (MAPO-36, [11]). Mg 2+ substitutes isomorphously for A13+ in MAPO-36, generating an anionic framework which can be proton exchanged to generate Bronsted acidity[12-14]. An analagous ZnAPO-36 material containing Zn 2+ was recently described by Akolekar[15]. We report here synthesis and characterization
756 of A1PO-36 incorporating together V4+ and Mg 2+, (VMAPO-36), Co 2+ (CoAPO-36), Mn 2+ (MnAPO-36) and further characterization of ZnAPO-36. For comparison purposes, we have also prepared and characterized the corresponding MeAPO-5 materials. Our principal objective has been to determine the extent to which isomorphous substitution can be proven or disproven by means of different physical and spectroscopic measurements. The acidity and catalytic properties of the same MeAPO-36 samples have been described previously [16] 2.EXPERIMENTAL.
MeAPO-36 materials were prepared by hydrothermal crystallization of gels having the following general composition: 2.0 nPr3N, 0.17 MeO ,0.92 A1203 ,1.0 P205, 40 H20, where MeO represents the transition metal component. The binary system VMAPO-36 was prepared from a gel with composition 1.8 nPr3N, 0.085 MgO, 0.043 V205,0.92 A1203,1.0 P205., 40 H20. Gel compositions used for MeAPO-5 were 1.5 nPr3N ,0.17VO, 0.92A1203, 1.0 P 2 0 5 , 4 0 H20 and 1.5 nPr3N, 0.085MnO , 0.96 A1203 , 1.0 P205 , 40 H20. Pseudo-boehmite or aluminium isopropoxide were used as alumina sources, orthophosphoric acid, and metal acetate salts. Gels were transferred to a teflon lined stainless steel autoclave and heated under static conditions for typically 96h at 373K and 24h at 423K.(MeAPO36) or 423K 24h (MeAPO-5). Solid products were washed then air dried at 373K. The organic template was removed by calcination in flowing nitrogen at 763K for 16h (MeAPO-36) or 773K for 12 h (MeAPO-5). MeAPOs were characterized by X-ray powder diffraction (Philips PW1730, Cu radiation), elemental analysis (AAS), scanning electron microscopy (JEOL LSM840A) and nitrogen sorption capacity (Quantasorb). 27A1 and 31p MAS NMR spectra were recorded on a Bruker MSL300 instrument at 78.188 and 121.44 MHz respectively, using a spinning rate of 10 kHz. EPR spectra were recorded at Xband on a Bruker ESP300 instrument. XAS data were collected in transmission mode on the Australian National Beam Line Facility at the Photon Factory, Tsukuba, Japan. 3.RESULTS 3.1 Bulk Characterization Figure 1 shows XRD patterns for selected MeAPO-36 and MeAPO-5 samples as synthesized. All were well crystalline materials, with no evidence of any other phases present. CoAPO-36 was deep blue as synthesized, and turned yellow-green on calcination. The V containing samples were light grey as synthesized, and turned yellow after calcination and exposure to air. The Mn containing samples were pale yellow as synthesized, but turned pink after calcination, while the ZnAPO-36 material remained white. Elemental analyses, and nitrogen sorption capacities are summarized in Table 1. The elemental analyses are consistent with the phase purity deduced from XRD. For CoAPO and MnAPO materials the analyses indicate substitution of the transition element for A13+. For the VAPO materials, the P:A1 ratios are also greater than unity. The pore volumes of the MeAPO-36 materials are similar to those reported for MAPO-36 (N2 capacity of
757 5.56 mmol g-l[ll]). Unsubstituted A1PO-5 has a nitrogen capacity of 4.86 mmol g1 [17]; the values for VAPO-5 and MnAPO-5 are similar to this.
e •
5
~s
2s
35
!
s
~5
25
35
45
Figure 1. XRD patterns of a) MnAPO-36; (b) CoAPO-36; (c) ZnAPO-36; (d) VAPO-5 and (e) MnAPO-5. Table 1 Characterization of MeAPO materials. Sample CoAPO-36 ZnAPO-36 VMAPO-36 MnAPO-36 MnAPO-5 VAPO-5
Chemical composition (molar) (0.04Co 0.46A10.50P)O2 (0.04Zn 0.46A10.50P)O2 (0.02V 0.02Mg 0.46A10.50P)O2 (0.04Mn 0.46A1 0.50P)O2 (0.023Mn 0.048A10.5P)O2 (0.041V 0.46A10.5P)O2
N2 capacity mmol g-1 5.45 5.43 5.3 5.4 4.89 4.92
3.2 NMR Spectra 27A1 and 31p NMR spectra of MeAPO-36 and MeAPO-5 m a t e r i a l s (as synthesized) are presented in Figure 2. All gave a 27Al signal at 38-40ppm; only in the case of VAPO-5 was a second 27A1 signal detected at 5ppm. No 31p NMR signal could be detected from the MnAPO-5 and MnAPO-36 materials. VAPO-5 and VMAPO-36 gave broad 31p signals at around -30ppm with no obvious s t r u c t u r e . The CoAPO-36 and ZnAPO-36 gave more complex 31p spectra containing two major components. As described in detail elsewhere [18,19], the
758 31p spectra of MeAPO materials can be deconvoluted into signals arising from P atoms having differing numbers of A1 next nearest neighbours, and the relative intensities of the component signals used to estimate the composition of the A1PO framework. Such deconvolution carried out here gave acceptable agreement between the framework composition deduced from the NMR spectra and the chemical analyses (e.g. CoAPO-36, (P/A1)nmr= 1.092, (P/A1)bulk = 1.087, ZnAPO36, (P/A1)nmr = 1.084, (P/A1)bulk = 1.087). The 31p spectra were unchanged when measured under proton cross polarization conditions. The 31p spectra measured after calcination of the MeAPO-36 materials were considerably broadened, and deconvolution was not attempted.
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Figure 2 27A1 NMR spectra of (a) MnAPO-5,(b) VAPO-5, (c) CoAPO-36, (d) ZnAPO-36. • 31p NMR spectra of (e) VAPO-5,(f) VMAPO-36.(g) CoAPO-36,(h) ZnAPO-36.
3.3 EPR Spectra EPR spectra were recorded at room temperature and 77K of the VAPO-5, VMAPO-36, MnAPO-5 and MnAPO-36 materials as synthesized. Figure 3 shows spectra of the vanadium A1POs measured at room temperature: identical signals were obtained at 77K. Also shown in Figure 3 is a spectrum simulated with the following parameters: gxx= 1.974, gyy= 1.974, gzz= 1.935, Axx= 70.3 gauss, Ayy= 70.3 gauss, Azz=196.0.gauss. Figure 4 shows the corresponding EPR spectra of the MnAPO-36 and MnAPO-5 materials (as synthesized). Both gave a broad symmetrical line centred on g= 1.99. In the case of MnAPO-5 this could be clearly resolved in the second derivative trace into 6 55Mn hyperfine lines with a splitting of 89 gauss. The hyperfine splitting is less clearly resolved for MnAPO-36, but can be estimated to be approximately the same as that for MnAPO-5.
759
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,
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Figure 3. EPR spectra (298K, 9.7GHz). (a), VMAPO-36, (b), VAPO-5, (c) simulated powder spectrum, using the parameters listed in the text.
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4000
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Figure 4. EPR spectra (298K, 9.7GHz) of as synthesized (a) MnAPO-5, (b) MnAPO36. Inset are second derivative traces.
760
3A X-ray Absorption Spectra Figure 5 shows XANES spectra of Co and Mn containing ALPOs. These all show a significant pre-edge peak (at 6540eV for Mn and 7710eV for Co). A preedge peak was not seen in the XANES spectra of ZnAPO-36 (not shown). The other effect noted in the XANES spectra is a sharpening and intensity increase in the so-called white line peak just above the absorption edge on calcination. This effect w a s seen in Co, Mn and Zn samples. Vanadium K-edge XAS data were not available . Preliminary analysis of the corresponding EXAFS data indicates a single shell of 4 oxygen nearest neighbours around Co and Mn, whereas the Zn EXAFS of ZnAPO-36 shows evidence of a second coordination shell. Full analysis of the EXAFS will be reported later.
,
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Figure 5. XANES spectra of (a) MnAPO-5, as synthesized, (b) MnAPO-36 calcined, (c) CoAPO-36, as synthesized, (d) CoAPO-36, calcined.
4.DISCUSSION The XRD and chemical analysis data indicate that the MeAPO-36 and MeAPO-5 materials synthesized are all pure crystalline phases, and that furthermore the transition metal components appear to be substituting for aluminium in the ALPO framework. VAPO-5 has been previously studied in detail by Schoonheydt et al. [7], who found that at low vanadium loadings (V/(V+AI+P) up to 0.02) chemical analysis was consistent with V substitution for A1, whereas at higher loadings excess vanadium was present. In our case, (V+A1)/P=I.0 at a vanadium content of 0.04, i.e. the synthesis reported here using tripropylamine as the template gives a higher level of vanadium incorporation than that used in [7]. The EPR spectrum of the as synthesized VAPO-5 is closely similar to that reported by Schoonheydt et al. (Figure 3(b)), who fitted their spectra with 2 overlapping orthorhombic signals, differing only in the gzz value. The parameters used for the simulation in Figure 3(c) are not identical to those of either signal in
761 reference 7, but our attempts to simulate the observed spectrum with the parameters used by Schoonheydt et al. gave significantly inferior agreement.A weak shoulder on the lowest field peak in Figure 3(b) does suggest a second signal is present. We did not observe the broad signal at g = 2 assigned in [7] to an extraframework oxidic phase, and the intensity of the observed signal varied with temperature in the normal manner, suggesting it is due to magnetically isolated V4+. The V 4+ EPR signal obtained from VMAPO-36 (Figure 3(a)) is almost indistinguishable from that of VAPO-5. Although the parameters of these two signals are not identical to those reported by Schoonheydt et al., they are sufficiently similar to assign the signals to the same kind of anchored VO 2+ species with distorted octahedral symmetry. This assignment is discussed in detail in reference [7]. The important point is that the signals are not due to tetrahedrally coordinated V 4+. In VMAPO-36, as in VAPO-5, the vanadium is not isomorphously substituted for A13+ (or p5+), (i.e. tetrahedrally coordinated to 4 lattice oxygens) but is atomically dispersed as anchored VO 2+ replacing A13+ in distorted octahedral symmetry. The observation of octahedral A13+ in the 27A1 NMR spectrum of as-synthesized VAPO-5 further supports the suggestion of local distortion of structure. CoAPO-36 as synthesized shows the same blue colour characteristic of tetrahedrally coordinated Co 2+ seen in CoAPO-5 [2,3]. Furthermore, the XANES of the as-synthesized CoAPO-36 shows the same pre-edge peak, with the same relative intensity, as that reported for CoAPO-5 (and tetrahedral Co 2+ in cobalt aluminate) [20]. The pre-edge peak is due to ls to 3d transitions which are allowed only in non centrosymmetric systems. After calcination, this peak is still present (although reduced in relative intensity). The colour change of CoAPO-5 from blue to yellow-green on calcination (also seen for CoAPO-36) was originally interpreted in terms of partial oxidation of Co 2+ to Co3+[2], but EPR experiments of Kevan et al.[3] have shown conclusively that calcination causes a distortion of the tetrahedral symmetry of Co 2+ rather than oxidation. Such a distortion would explain at least qualitatively the change in shape of the white line peak on calcination (also evident in the data of Zhang and Harris [20]). The XAS results, together with the chemical analyses and 31p NMR evidence for framework incorporation, strongly suggest that Co 2+ is substituting isomorphously for A13+ in the CoAPO-36 framework. Low temperature EPR experiments to definitively confirm this conclusion are in progress. The XAS data for MnAPO-5 and MnAPO-36 show the same kind of pre-edge feature seen with the Co substituted materials, suggesting that Mn 2+ is also tetrahedrally coordinated. The EPR evidence for tetrahedral coordination is suggestive rather than definitive. The 55Mn hyperfine splitting of 89 gauss falls between the values typical of octahedral and tetrahedral Mn 2+ [21], and may indicate distorted tetrahedral symmetry. In a previous EPR study of MnA1PO-5, Goldfarb [10] reported a hyperfine splitting of 95 gauss, which was attributed to an octahedral extraframework Mn 2+ species in the A1PO pores. In that study, some incorporation of Mn 2+ into the framework also occurred, as evidenced by a broad underlying EPR signal and the appearance of paramagnetic shift anisotropy in the 31p and 27A1 NMR spectra. In our case, the dipolar
762 paramagnetic coupling between Mn 2+ in the framework and 31p broadens the NMR signal beyond detection, suggesting a greater degree of Mn incorporation. The corresponding 27A1 spectra measured at 10 kHz spinning speed did not show the complex sideband patterns reported by Goldfarb (at 2.5-4 kHz). The XAS, EPR and NMR data taken together suggest that in both MnAPO-5 and MnAPO-36 materials synthesized here the Mn 2+ is incorporated into the A1PO framework, probably isomorphously replacing A13+. We have no direct spectroscopic signature of tetrahedral Zn 2+ to confirm that this ion isomorphously substitutes for A13+, since there is no ls to 3d transition possible in the XANES spectrum. The 31p NMR spectrum of ZnAPO-36 is similar to that of MAPO-36[19], suggesting that the framework composition is similar, and the changes in white line profile on calcining the ZnAPO-36 were very similar to those seen with CoAPO-36 and MnAPO-36. Isomorphous substitution of Zn2+ for A13+ is thus likely, but not proven. ACKNOWLEDGEMENTS We thank the Australian Research Council and the Access to Major Research Facilities Program for financial support. REFERENCES 1. E.M.Flanigen,B.M.Lok,R.L.Patton and S.T.Wilson in "New Developments in Zeolite Science and Technology" (Y.Murukami,A.IIjima and J.W.Ward,eds), Kodansha, Tokyo,1986, pp 103-112. 2. L.E.Iton,I.Choi,J.A.Desjardins and V.A.Maroni, Zeolites 9 (1989) 535. 3. V.Kurshev,L.Kevan,D.J.Parillo,C.Pereira,G.T.Kokotailo and R.J.Gorte, J.Phys.Chem.98 (1994) 10160. 4. G.Catana,J.Pelgrims and R.A.Schoonheydt, Zeolites 15 (1995) 475. 5. J.D.Chen,M.J.Haanepen,J.H.C. van Hoof and R.A.Sheldon, "Zeolites and Related Microporous Materials:State of the Art 1994" ( J.Weitkamp, H.G.Karge, H.Pfeiffer and W.Holderich,eds),Elsevier,Amsterdam,1994, 973 6. B.Weckhuysen and R.A.Schoonheydt, Zeolites 14 (1994) 360. 7. B.M.Weckhuysen, I.P.Vannijvel and R.A.Schoonheydt, Zeolites 15 (1995) 482. 8. R.Roque-Malherbe, R.Lopez-Cordero, J.A.Gonzales-Morales, J.OnateMartinez and M.Carreras-Gracial, Zeolites 13 (1993) 481. 9. K.Vinje and K.P.Lillerud, reference 5 p227. 10. D.Goldfarb, Zeolites 9 (1989) 509. ll.J.V.Smith,J.J.Pluth and K.J.Andries, Zeolites 13 (1993) 166. 12. S.T.Wilson and E.M.Flanigen, ACS Symp.Ser. 398 (1989) 329. 13. D.B.Akolekar, J.Catal. 143 (1993) 227. 14. D.B.Akolekar, Zeolites 14 (1994) 53. 15. D.B.Akolekar, Appl.Catal. 112 (1994) 125. 16. V.R.Chowdary,D.B.Akolekar,A.Singh and S.D.Sansare, J.Catal. 111 (1988) 23. 17. D.B.Akolekar, J.Chem.Soc.Faraday Trans. 90 (1994) 1041 18. P.J.Barrie and J.Klinowski, J.Phys.Chem. 93 (1989)5972. 19. D,B,Akolekar and R.F.Howe, J.Chem.Soc. Faraday Trans., submitted. 20. G.Zhang and T.V.Harris, Physica B 208,209 (1995) 697. 21. H.Levanon and Z.Luz, J.Chem.Phys. 49 (1968) 2031..
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
The m o d i f i e d h y d r o p h o b i c i t y index as a novel m e t h o d characterizing the surface properties o f titanium silicalites
763
for
J. Weitkamp a, S. Ernst a, E. Roland b and G. F. Thieleb alnstitute of Chemical Technology I, University of Stuttgart, D-70550 Stuttgart, Germany bDegussa AG, ZN Wolfgang, Rodenbacher Chaussee 4, D-63403 Hanau, Germany
Competitive adsorption measurements using n-octane and water on TS-1 samples prepared acxording to two different synthesis methods reveal that the Modified Hydrophobicity Index (HI*) decreases linearly with the titanium content of the molecular sieve for nsi/nTi >40. For TS-1 samples with a lower nsi/nTi-ratio , i.e., a higher Ti content, HI* strongly depends on the synthesis procedure and may deviate considerably from linearity. These differences are tentatively interpreted in terms of a possible formation of additional non-framework titanium species which might represent adsorption sites with more hydrophilic properties. The Modified Hydrophobicity Index of the different TS-1 samples can be correlated with their activity in the liquid phase hydroxylation of phenol with aqueous hydrogen peroxide. In particular, catalysts with high titanium content are the more active, the lower their Modified Hydrophobicity Index.
1. INTRODUCTION
The first synthesis of a microporous titanium silicate with the framework topology of zeolite ZSM-5 (titanium silicalite-1, TS-1) was reported in 1983 [1,2]. It has been demonstrated that this material is a useful catalyst for the selective oxidation of various organic substrates by H20 2 [2,3]. However, the quality of the synthesized materials appeared to be a crucial point for obtaining good activities and selectivities. Therefore, several methods have been developed to probe the quality of TS-l-type materials. Among them are catalytic testing (e.g,, in the hydroxylation of phenol [4,5]) and the use of various physico-chemical techniques [6]. Recently, it was reported that titanium silicalites adsorb more water than their titanium-free counterpart, viz. silicalite-1, which was attributed to the presence of polar Si-O-Ti bridges in the former [7]. In the present study, the Modified Hydrophobicity Index HI* as introduced recently for more conventional alumosilicate molecular sieves [8] was extended to the characterization of titanium silicalites. HI* is defined as the ratio of the final loadings of the molecular sieve with n-octane and water,
764 detennined from the breakthrough curves under specified gas phase conditions [8]. Furthermore, an attempt is made to correlate HI* with the titanium content of various titanium silicalite samples which were synthesized according to different methods. In addition, the materials used in this study have been characterized by UVNISspectroscopy, and they were tested for their catalytic properties in the hydroxylation of phenol with H20 2.
2. EXPERIMENTAL SECTION Samples of TS-1 with different Ti contents were synthesized according to two different procedures designated as method A and method B. Basically, method A was adapted ]?rom ref. [1] using tetraethylorthosilicate (TEOS) and tetraethylorthotitanate (TEOT) as the silica and the titania source, respectively. Tetrapropylammoniumhydroxide (TPAOH) was used as templating agent. Method B which is based on the work of Thangaraj et al. [9] makes use of tetrabutylorthotitanate (TBOT) which hydrolyzes at a lower rate than tetraethylorthotitanate. After hydrothermal synthesis, the as-synthesized materials were calcined to remove the organic template.This occurred first in flowing air for 12 hours at 450 °C and then for an additional 12 hours at 550 °C in a purge of oxygen. The bulk titanium content of the samples was determined after dissolution in a melt of LiBO 2 by atomic emission spectroscopy using an inductively coupled plasma. The microporous materials were further characterized by UV/VIS-spectroscopy in the diffuse reflectance mode using a Perkin Elmer Lambda 16 instrument with integration sphere. For the determination of the Modified Hydrophobicity Index, a mixture of water (Pwater = 2.3 kPa) and n-octane vapors (Pn-octane= 1.4 kPa) was passed through a fixed bed adsorber in a flow-type apparatus. Hydrogen was used as carrier gas with a flow rate of 12 cma/min. Typically, 1 g of adsorbent was used. The gas stream leaving the adsorber was analyzed periodically using a capillary gas chromatograph equipped with a thermal conductivity detector. The final loadings (Li) were calculated directly from the measured breakthrough curves. The Modified Hydrophobicity Index was then calculated as HI* = Ln_oetane/Lwater [8]. The catalytic properties of selected titanium silicalite samples were tested in the liquid phase hydroxylation of phenol using aqueous hydrogen peroxide (35 wt.-%) as oxidant. These experiments were conducted in a continuously stirred tank reactor at a temperature of 100 °C after the catalysts had been activated at 500 °C. Periodically, samples were withdrawn l~om the reaction mixture using a syringe and analyzed by capillary gas chromatography. Yields of hydroxylation products are expressed as moles of product formed per moles of hydrogen peroxide addec[
3. RESULTS AND DISCUSSION Typical breakthrough curves for n-octane and water obtained with a standard sample of TS-1 are depicted in Figure 1. It can be seen that water begins to break through soon after the start of the experiment and, after a time on stream of c~ 160 minutes, is replaced
765
in part by n-octane flom its adsorption sites. This is the typical behavior of a mildly hydrophobic adsorbent [8]. HI* values obtained for TS-1 samples synthesized with different titamum contents after methods A and B are shown in Figure 2. For samples synthesized according to method A and having nsi/nTi-ratios above 40, HI* decreases linearly with increasing titanium content. Samples with a higher titanium content and flee of amorphous TiO 2 (as derived from UWVIS-spectroscopy in the diffuse reflectance mode, cf. Figure 3), i.e., sample A35, have lower values for HI* which do not fit the straight line in Figure 2. On the other hand, sample A30 which does contain some amorphous (i.e., non-framework) TiO2, as indicated by a broad absorption band between 300 and 370 nm in the UV/VIS-spectrum (cf. Figure 3), possesses a considerably higher HI* value. HI* values for TS-1 samples synthefized according to method B are generally lower than those of group A. The deviation observed for the titanium-rich samples of group A and the generally lower hydrophobicities of the group B samples are tentatively explained by the following model: Both sample A35 and sample B49 exhibit a considerably broadened absorption band as compared to sample A48, which is vimmlly free fxom extra-framework titanium species. The enhanced absorption in the region between about 220 nm and 300 nm can be due to either the presence of extremely small TiO2 particles which leads to a hypsochromic shift of the TiO2 absorption band usually appearing at ca. 320 nm to 340 nm due to the quantum size effect, or to the existence of atomically dispersed titanium in octahedral coordination (bathochromic shift of the Ti-O charge transfer band due to a larger number of oxygen ligands). The presence of defect sites associated with edge sharing species as suggested by Trong On et al. [10] can most probably be ruled out because their formation was recently shown to be very unlikely [11]. 1,2
•
•
1,0
water
n-octane
mads. =1.0 g
o 0,8
T
....."
Q.
= 3 0 °C
"7._ 0,6 0,4
0,2 0,0
0
50
100 150 200 TIME ON STREAM, min
250
300
Figure 1. Typical breakthrough curve for n-octane and water observed with a standard sample of TS-1. HI* of this sample mounts to 8.3.
766 11
I
T X uJ a z
10
--
9
'
I
'
I
• synthesized after method A - - ~ - - synthesized after method B
=..-
A30
0 I n O n," Q >T
A41
8B49 7
d
A35
O
6
I
,
I
40
20
,
I
60 nsi/nTi
80
100
- RATIO
Figure 2. Influence of the titanium content and the mode of preparation of titanium silicalites on their Modified Hydrophobicity Indices.
I
'
I
"
I
'
.....
1,0
z 0 F-
.....
A48 A35 B49 A30
0
co 0,5 m <
%%
. --.. ~
o
--: 0,0
I
200
.
I
--~..
,
,
300 WAVELENGTH,
.-..
I
400
500
nm
Figure 3. Diffuse reflectance UV/VIS-spectra of selected samples of titaalum silicalites with different titanium contents and prepared according to different methods.
767
In general, the following conclusions can be drawn l~om the Modified Hydrophobicity Indices shown in Figure 2: For titanium silicalite samples with low t i t ~ u m content (nsi/nTi > 80), both synthesis methods probably yield products with a comparable density of lattice defects because they produce materials with similar Modified Hydrophobicity Indices. Upon successive incorporation of titanium atoms into the silicate framework, the materials become more and more hydrophilic which is indicated by a decreasing HI*. To account for this decrease in HI* it is assumed that water molecules can coordinate to framework Ti-atoms. Hence, samples with nsi/nTi > 40 show a decrease in HI* with increasing titanium content. From the l.W/VIS-spectmm of sample A30 (cf. Figure 3) it can be deduced that beside titanium in tetrahedral coordination in the silicate framework a separate crystalline titanium-containmg phase is present. It is assumed that the latter phase adsorbs practically no water. Hence, HI* of sample A30 is between those of samples A40 and A48, which presumably contain a comparable titanium content in the zeolite lattice. Sample A35 and those samples prepared acx~rding to method B for which the presence of a second titanium species is suggested by their UVNIS-spectra, are significantly more hydrophilic than those samples synthesized after method A which possess only tetrahedrally coordinated titanium in the ffamenwork At present, the following structural models can be envisaged for these hydrophilic titanium species in sample A35 and the samples synthesized according to method B: (i) titanium dioxide with a partide diameter in the nanometer range and an unusually large number of surface Ti-OH groups, (ii) an amorphous hydrophilic titanium silicate with titanium in octahedral coordination and, (iii) the incorporation of titanium species into defect sites of the silicate framework. If this occurs in octahedral coordination (titanium incorporation as "framework satellite"), hydrophilic hydroxyl nests are created which are analogous to those formed upon silicon removal from the t~amework Based on the experimental data presently available, no dear decision can be made as to which of the structural models discussed above comes closest to reality. The catalytic behavior of selected titanium silicalite samples was tested in the liquid phase hydroxylation of phenol with aqueous hydrogen peroxide. Typical results obtained with sample A35 as catalyst are depicted in Figure 4. After co. 70 minutes the peroxide initially added to the reaction mixture is completely converte~ The yields of the hydroxylation products, i.e., hydroquinone and catechol reach almost 40 % each. Hence, a total yield of hydroxylation products of nearly 80 % is obtained at the end of the experiment. These results are in principal agreement with published data obtained under comparable reaction conditions with a Euro-TS-1 sample [4]. The sample used in the present study (A35) was prepared in a similar manner as Euro-TS-1 and its nsi/nTi-ratio was similar as well. The catalytic behavior of selected additional samples (A30, A48, B49) prepared in this study is compared in Figure 5 with that of sample A35. As a measure for the quality of the _cat___al_ysts, the total yield of hydroxylation products (viz. the sum of the yields of hydroquinone and catechol) was choserL Titanium silicalite A30 contains amorphous
768
o-e,
>_a ._1 LU .=.
100 80
>-
•
YHydroquinone
•
Yc=t,~ol
~
m T='t
/ ~
n,
O x
/
60
Z O
= 0.5 g = 100oc
nH2o/nphenol =
1/4
solvent -
acetoneI H20 J
_
40
00
n, LU >
20
Z
O (3
,
I
20
I
I
,
I
,
40 60 REACTION TIME, min
I
=
80
Figure 4. Hydroxylation of phenol on titanium silicalite sample A35 at 100 °C.
80 •
A 35
• •
A48 B 49
•
o"9, 60
~-~--
................e~
tO ",~
m=,t
=
0.5 g
T
=
100 °C
nH202/ nR,.,~
solvent
= •
114 acetonelH=O
I
~" 20
0
20
40
REACTION T I M E ,
60
80
min
Figure 5. Yield of hydroxylation products over TS-1 smnples prepared ac~rding to different synthesis procedures and with different nsi/nTi-ratios.
769 titanium dioxide; samples A48 and B49 possess similar titanium contents but most probably do not contain amorphous titanium dioxide (cf. Figure 3). It can be seen from Figure 5 that the rate of hydroxylation is somewhat lower for catalyst A48 than for sample A35. This is in agreement with the lower titamum content of the former sample. At the relatively high reaction temperature applied in the present study, hydrogen peroxide is always consumed to a certain extent by thermal decomposition. Therefore, the maximum yield of hydroxylation products increases with increasing ratio of the rate constants of hydroxylation and decomposition. Hence the maximum yield which can be achieved with sample A48 is somewhat smaller than with sample A35. Catalyst B49 contains approximately the same overall amount of titanium as sample A48. However, hydroxylation yields are dearly lower (cf. Figure 5). The relatively broad absorption in the UV/VIS-spectmm of B49 suggested the presence of an additional titanium species. We anticipate that this species, regardless of its exact nature, possesses a somewhat lower catalytic activity than tetrahedrally coordinated titanium atoms in framework positions. Titanium silicalite A30 possesses the highest titanium content of all catalysts prepared in the course of this study. However, it exhibits by far the lowest hydroxylation activity due to the presence of large amounts of amorphous titanium dioxide. From the results presented here it can be deduced that synthesis method A allows to incorporate more titanium into framework positions than synthesis method B. In addition, for a comparable overall content of titanium, the former possess a considerably higher catalytic activity than the latter.
4. CONCLUSIONS It has been shown that samples of titanium silicalite-1 having different nsi/nTi-ratios and prepared according to two different synthesis procedures can differ significantly in their hydrophobic/hydrophilic surface properties as revealed by the Modified Hydrophobieity Index HI*. For nsi/nTi-ratios above ca. 40, HI* decreases linearly with increasing titanium content which has been attributed to the increased formation of polar Si-O-Ti bridges in the zeolite framework For TS-1 samples with higher titanium content, HI* strongly depends on the method of preparation and is considerably influenced by the formation of additional titanium-containing species in extra-framework positions. From the results of the catalytic characterization in the hydroxylation of phenol it can be concluded that, in particular, TS-1 samples with a high titanium content (i.e., nsi/nTi below 40) are the more active, the lower their Modified Hydrophobicity Index. Hence, the determination of HI* seems to be a useful method for the characterization of titanium silicates which furnishes valuable structural information especially in combination with other suitably selected characterization techniques.
770 ACKNOWLEDGEMENTS J. W. and S. E. gratefully acknowledge financial support by Deutsche Forschungsgemeinschaft, Fonds der Chemischen Industrie and Max-Buchner-Forschungsstithmg,
REFERENCES
1. M Taramasso, G. Perego and B. Notari, US Patent 4,410,501 (1983). 2. G. Perego, G. Bellussi, C. Como, ~ Taramasso, F. Buonomo and/L Esposito, in: New Devdopments in Zeolite Science and Technology, Y. Murakami, A fijima and J. W. Ward (eds.), Kodansha, Tokyo, and Elsevier, Amsterdam, 1986, pp. 129-136. 3. B. Notari, in: Innovation in Zeolite Materials Science, P. J. Grobet, W. J. Mortier, E. F. Vansant and G. Schulz-Ekloff (eds.), Studies in Surface Science and Catalysis, Vol. 37, Elsevier, Amsterdam, 1988, pp. 413-425. 4. B. Kraushaar-Czametzky and J. H. C. van Hooff, Catal. Lea., 2 (1989) 43-47. 5. J. A~ Martens, P. L. Buskens, P. A. Jacobs, A~ van der Pol, J. H. C. van Hooff, C. Ferrini, H. W. Kouwenhoven, P. J. Kooyman and H. van Bekkum, Appl. Catal. A, 99 (1993) 71-84. 6. A~ Zecchina, G. Spoto, S. Bordiga, A~ Ferrero, G. Pelrini, G. Leofanti and M Padovan, in: Zeolite Chemistry and Catalysis, P./L Jacobs, N. I. Jaeger, L. Kubelkovfi and B. Wichterlovfi (eds.), Studies in Surface Science and Catalysis, Vol. 69, Elsevier, Amsterdam, 1991, pp. 251-258. 7. S. P. Mirajkar, A, Thangaraj and V. P. Shiralkar, J. Phys. Chem., 96 (1992) 30733079. 8. J. Weitkamp, P. Kleinschmit, A~ Kiss and C. I-L Berke, in: Proceedings from the Ninth International Zeolite Conference, 1L von Ballmoos, J. B. Higgins and ~ M J. Treacy (eds.), Part II, Butterworth-Heinemmm, Boston, 1992, pp. 79-87. 9. /k Thangaraj, M J. Eapen, S. Sivasanker and P. Ratnasamy, Zeolites, 12 (1992) 943950. 10. D. Trong On, A. Bittar. A~ Sayari, S. Kaliaguine and L. Bormeviot, Catal. Lett., 16 (1992) 85-95. 11. G. Bellussi and ~ S. Rigutto, in: Advanced Zeolite Science and Applications, J. C. Jansen, M. St6cker, H. G. Karge and J. Weitkamp (eds.), Studies in Surface Science and Catalysis, Vol. 85, Elsevier, Amsterdam, 1994, pp. 177-213.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
771
T h e T h e r m a l S t a b i l i t y of t h e G a l l o p h o s p h a t e C l o v e r i t e W. Schmidt, F. Schiith, S. Kallus Institut fiir Anorganische Chemie und Analytische Chemie, Johann Wolfgang Goethe-Universitiit, Marie Curie-Stral3e 11, D-60439 Frankfurt, Germany
The thermal stability of cloverite, [GasPsO31(OH)2](QF)2(H20)n, was studied by in situ x-ray diffraction (XRD), in situ infrared spectroscopy (IR), thermogravimetric analysis combined with differential thermal analysis (TG/DTA) and nitrogen adsorption measurements. During the heating process the framework remained stable to at least 500°C in air as well as in vacuum. Cooling down to room temperature in ambient air led to a structure break down, while under vacuum the cloverite structure remained stable during the cooling process. When the calcined cloverite is kept at temperatures above 100°C its framework remains intact while storage under aliphatics produces a material similar to cloverite.
1. I N T R O D U C T I O N The gallophosphate cloverite has gained much attention due to its unique pore system consisting of three different types of cages including an extremely large supercage with approximately 3 nm in diameter [1]. All cages are connected and accessible via windows consisting of at least 8 T-atoms. Thus, unique adsorption properties should be expected from this material, which consists mainly of large and open cavities. During the synthesis water molecules and organic template molecules are enclosed inside the pore system of cloverite, which have to be removed prior to any application of that material as molecular sieve. Attempts to activate cloverite by calcination in ambient air failed and only amorphous material remained after the samples had been taken out of the furnace [2]. The instability of the calcined cloverite makes it useless for any application. The aim of our investigations was, and still is, to get insight into the behavior of cloverite during thermal treatment and, perhaps, find a possibility to preserve the crystalline structure of the material, even after the calcination process, in
772 order to make the material amenable to adsorption studies and for the use of cloverite as host material for organic and inorganic guest molecules. Several research teams work on very similar topics [3,4], which shows the interest t h a t activated cloverite gains.
2. E X P E R I M E N T A L All cloverite samples were synthesized in teflon lined autoclaves, which were placed in an oven at 150°C for 24 hours. HF (Merck) and N a F (Aldrich) were used as fluoride sources. Both of them can be used to synthesize cloverite, the products only show differences in the morphology of the crystals and slightly differences in their weight loss during the calcination. The molar compositions of the reaction gels were Ga203 : P205 : 6 Q : 0.75 HF : 71.3 H20 and Ga203 : P205 : 6 Q : 0.75 NaF : 71.3 H20 (Q = quinuclidine, Fluka), respectively. The amount of water in the reaction gel was fixed by the water content of the employed gallium sulphate solution (8 wt.% Ga, VAW AG), the phosphoric acid (85 wt.%, Merck), and the hydrofluoric acid (40 wt.%, Merck). For the preparation of the reaction mixture the phosphoric acid was poured into the gallium sulphate solution. Then the quinuclidine was added slowly which results in a highly viscous gel. The gel became less viscous in the case of HF and remained as gel in the case of N a F after t h e addition of the fluoride source. The reaction mixture was stirred during the whole preparation. The homogeneous reaction mixture was poured into the teflon lined autoclaves. For in situ XRD measurements in ambient air and under vacuum CUK~ radiation and a Paar HTK 10 heating chamber were employed. The powder samples were prepared on a platinum sample holder serving as a resistance heater. IR experiments were carried out in a heatable vacuum cell [5] using a SpectraTec Research Plan Microscope attached to a Nicolet 5SBX optical bench. DRIFT measurements were performed on the same spectrometer using a heatable DRIFT cell (Harrick). Samples were heated with a rate of 5°C/min for in situ XRD m e a s u r e m e n t s and data sets were recorded every 50°C up to 500°C. One m e a s u r e m e n t took about 20 minutes. In order to treat the samples in the IR cell in a similar way they were heated with the same heating rate using the same t e m p e r a t u r e steps. The samples were kept at this temperatures for 20 minutes before they were measured. TG/DTA experiments were performed on Linseis L81 and Setaram TG-DTA 92-16 thermobalances in an air flow of 10 lPa with a heating rate of 10°C/rain. Adsoi'ption isotherms were recorded on a Micromeritics ASAP 2010 adsorption instrument using nitrogen as adsorbent at 77 K. The samples were activated at 350°C under vacuum.
773 3. R E S U L T S AND D I S C U S S I O N According to the synthesis routes mentioned above cloverite was obtained in good quality without amorphous impurities. The crystals obtained by using NaF as fluoride source were much larger than those from the reaction mixtures containing HF. They were occasionally elongated up to 200 ~m in one direction as shown in figure lb). Besides those elongated rods one finds typical cubeoctahedrally shaped cloverite crystals [6] with 40 ~m in sizes in the NaF system. Figure 1. SEM images of a) cloverite crystals obtained using HF as fluoride source scale b a r - 20 ~m
b) a cloverite crystal elongated in one direction obtained using NaF as fluoride source scale b a r - 100 ~m
The crystals obtained from the HF containing reaction mixtures have the same cube-octahedraUy shape but only sizes of 10 ~m or are even smaller (figure la)).
774 The different sizes of the crystals from both synthesis systems can be explained by two effects. The pH of a mixture containing NaF is higher than that of those containing HF. The dissolution of the reaction gel is slowed down and during the nucleation period only a small number of crystal nuclei is formed, which then grow much larger since the gel is consumed by only a few crystals. The NaF containing reaction mixture has a gel like consistency and the crystals remain longer in the growth area of the reactor. The HF mixture is a clear solution and once the growing crystals reach a distinct size their sedimentation starts. At the bottom of the reaction vessel the growth of the crystals is restricted, since the number of growing crystals is much higher and, thus, the concentration of molecules with the ability to condense on the crystal surfaces is much lower than in the upper parts of the vessel. TG/DTA measurements in flowing air show that the calcination behavior of crystals from both systems is similar up to a temperature of 900°C. Using a heating rate of 10°C/rain water is desorbed in an endothermic process up to 200°C. The desorption of the quinuclidine, followed by its combustion, takes place in four distinct exothermic steps in the temperature range between 300-700°C (maxima of exothermic peaks: 340°C, 430°C, 490°C, 540°C). In general the samples contain 6-10 wt.% water and approximately 13-14 wt.% quinuclidine. At temperatures above 900°C the samples crystallized in the NaF system show an additional weight loss of about 2-3 wt.%. It might be due to the combustion of an impurity of only little crystallinity which is not detectable by XRD. Since this additional weight loss did not appear with samples synthesized in the HF system, we used only material synthesized with HF for the investigations described below.
~lL~l~.~~' ~ Ib
~
211ela
~o
~
Figure 2. In situ XRD pattern of a) Cloverite, recorded during heating up to 500°C and cooling down to 20°C under vacuum
~~~i.
~~.~,~,
2111ela
b) Cloverite, recorded during heating up to 500°C and cooling down to 20°C under vacuum and exposure to ambient air
XRD patterns of cloverite, recorded during the heating and cooling down period, are shown in figure 2. Diffraction patterns were measured in steps of
775
50°C. For clearness not all patterns which were recorded are presented and only the range up to 30 ° 2 theta is shown. When cloverite is exposed to a vacuum of 10 .4 mbar at room temperature its XRD pattern has a much lower underground t h a n an as synthesized sample. Figure 2a) reveals t h a t once the vacuum is stable the diffraction patterns remain basically unchanged when the sample is heated up to 500°C. The intensities of some peaks change slightly, but the structure of cloverite remains stable and the crystallinity of the sample does not decrease. Cooling down to room temperature under vacuum does not change the XRD pattern. Heating in ambient air also does not affect the crystallinity and the structure of cloverite. The crystal structure can withstand t e m p e r a t u r e s up to 500°C without getting damaged and the XRD pattern is basically the same as under vacuum at t h a t temperature. Differences occur during the cooling in air as shown in figure 3. While no structural changes are detectable down to 100°C a rapid amorphization of the crystalline cloverite is observed within a few minutes when the temperature is below 100°C. This amorphization is not observed when the sample which was heated under vacuum is exposed to ambient air. Figure 2b) presents the diffraction patterns of that sample after its exposure to air.
1oo
_r
I
,,iw,~ku iuiiiw,11~~]~A" " - ~
,, ] ~ , A ~
IF vkJ v ~' ~ k.~L,~.'"'-.,,~,~,,,J ~ 200L . . . . . ~ .... o
"
~)
~
~0
"%. ~ - ~ ~k ~
~
~
"~'
\/.~ _ v\ k
/
20°C 100"6
~'~',",,,,,,,~~13oo'c,m , o3n:L
, ~"~'"~1 ~s
'B~m'aoom i 'lX~mb.oon£ nm.n~o0ra
2theta
Figure 3. In situ XRD p a t t e r n of Cloverite, recorded during heating in ambient air up to 500 °C and cooling down to 50°C in air
3600
3400
3200
3000
Wavenumbers (crn "1)
2800
2600
Figure 4. In situ IR spectra of cloverite, recorded at different t e m p e r a t u r e s in air
IR experiments can help to explain the differences in the t h e r m a l stability of cloverite under vacuum and in air. IR spectra were recorded in steps of 50°C up to 500°C. In figure 4 parts of the spectra at four different t e m p e r a t u r e s are shown. At 20°C a broad band in the range form 2500-3700 cm -1, caused by water in the material, is superimposed with three additional bands. The one at about 3175 cm -1 can be assigned to a quinuclidiunium NH vibration, proving the protonated state of the template. The bands at 2886 cm "1 and 2948 cm "1 are due to symmetric and asymmetric CH streching vibrations of the template. At a temperature of 100°C the broad water band disappeared, indicating t h a t no more
776 water is present inside the cloverite channels at this temperature. The template molecules can withstand a temperature of 300°C. At higher temperatures the desorption and decomposition of the template takes place and at 450°C no more NH and CH streching vibrations were observed. A part of the template desorbs physically at temperatures above 3000C. When we used a heatable DRIFT cell in static air quinuclidine condensed at the colder KBr windows as a blank measurement showed. The above mentioned experiment had to be performed either using an air stream or under vacuum where the desorbed molecules were carried away from the sample. Therefore, we employed a heatable cell for an IR microscope in which, due to its dimensions and to the gas flow or vacuum, respectively, no condensation of the quinuclidine occured. Measurements under vacuum showed that the template does not desorb totally from the sample. It remains ~nside the cloverite structure even at temperatures of 500°C. The template inside the pores of cloverite seems to stabilize the structure when the sample is cooled down and then exposed to ambient air. Calcination in air leads to a total combustion of the template and the structure collapses at temperatures below 100°C, probably by interaction with water molecules from the humid air. Bedard et al. [4] assume the formation of defect sites during the calcination process caused by the loss of the template. Once these defect sites are formed water molecules might interact with them and initiate the disintegration of the crystal structure. A large number of defect sites should affect the XRD pattern of the material, which was not observed. The half widths and the intensities of the reflection peaks did not change essentially during the heating periods. The long range order of the crystal structure still exists. 1074 1046 11
Figure 5. IR spectra of cloverite ~
"-J
a
1070
o c ,.o o L-
1154 ~
1200
982
1000
800
Wavenumbers
600
a) as synthesized, b) calcined 20 h in ambient air at 500°C, c) heated" in a DRIFT cell to 500°C in dry air, d) same as c) then cooled down in dry air to 300°C, e) same as c) then cooled down in dry air to room temperature in dry air * Heating rate = 5°C/rain
(ore"1)
Alterations were observed in the short range order. The IR spectra differ significantly between 500-1300 cm -1 during the heating period in air as shown in figure 5 . Due to vibrations of the crystal framework as synthesized cloverite exhibits six distinct bands in that region at about 594, 633, 665, 1046, 1074, and
777
1198 cm -1. At 500°C the bands at 594, 633, and 1046 cm -1 disappear, while the intensity of the band at 665 cm -1 increases. Three broad new bands at 774, 982, and 1154 cm -1 were observed. The bands at 982 and 1154 cm -1 appear as shoulders of the one at about 1070 cm -1 and become more pronounced when the sample is cooled down below 350°C. After calcination in ambient air these three bands become one broad one and cannot be resolved any longer. Additionally, a broad band occurs at about 640 cm -1 as shown in figure 5b). The band at 774 cm -1 also disappears. The two broad single bands after the calcination were also observed by Bradley et al. [3]. They concluded t h a t the long range order as well as the short range order were lost after the exposure of calcined cloverite to ambient air. The IR spectra recorded at higher t e m p e r a t u r e s during the calcination show t h a t the short range order changes, but is still intact. We have to keep in mind t h a t the XRD patterns recorded at those t e m p e r a t u r e s have not changed dramatically. One can conclude t h a t the long range order inside the crystal is not affected by the thermal treatment, the crystal structure is still intact, but the nearest neighbour interaction between the T-atoms changes during the heating period. In order to stabilize the structure of calcined cloverite there seem to be two possibilities. Either keeping the material at elevated t e m p e r a t u r e s to avoid water adsorption from the ambient air or a blocking of the pore system with hydrophobic adsorbents. By keeping the material at t e m p e r a t u r e s above 100°C after its calcination in air it was possible to achieve a type 1 isotherm, typical of microporous materials, in a nitrogen adsorption experiment. The calcination of the sample for the adsorption measurement was performed at 450°C in ambient air. In the glass sample holders no higher temperatures could be achieved. The sample contained about 4.5 wt.% coke which burnt off at 530°C in an exothermic process in an TG/DTA experiment. The coke seems to block the pore system, since the pore volume, detected by the nitrogen adsorption, is less t h a n expected. Further experiments are in progress to obtain adsorption isotherms of pure cloverite. Nevertheless, the samples obtained in the above experiments were microporous without any detectable mesopores. Figure 6. XRD p a t t e r n of cloverite a) as synthesized b) after 16 h at 550°C in air at 20°C c) calcined at 550°C in air and exposed to ambient air at room t e m p e r a t u r e d) calcined and stored in decane e) calcined and stored in hexane f) calcined and stored in hexane for one day () .... ' " ' i O ' ....
.... 2 0 " " 2 theta
.... 3 0 ' " '
'
''
'1
40
778 When calcined cloverite, still at temperatures above 100"C, is poured into hexane or decane a material is obtained which exhibits XRD patterns similar to those of cloverite as shown in figure 6. The intensities of the peaks are different but the positions of the peaks are similar to those from cloverite. The material was dried and stored in ambient air for several month without changing its XRD pattern. The aliphatics seem to be adsorbed inside the pore system, since weight losses of about 13-14 wt.% were observed in the range of 100-3000C by TG/DTA experiments. For a sample, stored under decane over night and dried in ambient air, a weight loss of 13.6 wt.% was detected which corresponds to a pore volume of 0.19 cma/g which agrees with pore volumes found by Merouche et al. by adsorption measurements on cloverite using aliphatic and aromatic hydrocarbons as adsorbates at 25°C [2]. Nitrogen adsorption experiments at 77 K proved that the material stored under aliphatics is still microporous, even when the micropore volume is only half of that of a sample calcined in air at 450°C. Mesopores could not be found in any of the samples. Thus, the aliphatics must be adsorbed in the micropores.
4. C O N C L U S I O N S Our experiments showed that the cloverite structure can be kept intact in ambient air at elevated temperatures. Once the material is calcined one has to keep it above 100°C for further experiments. The long range order of the structure is not affected by the thermal treatment while changes of the short range order are observed during the calcination process. Storage in aliphatics leads to a material which seems to be very similar to cloverite. This material is microporous and the micropore volume is still remarkable. Further investigations on the real structure of that material must show whether a structure conversion took place.
REFERENCES 1 2 3 4 5 6
M. Estermann et al., Nature 352 (1991) 320 A. Merrouche et al., Zeolites 12 (1992) 226 S.M. Bradley et al., Solid State Nucl. Magn. Reson. 2 (1993) 37 R.L. Bedard et al., J. Am. Chem. Soc. 115 (1993) 2300 F. Schfith et al., J. Am. Chem. Soc. 116 (1994) 1090 J. Patarin et al., Proc. 9th Int. Zeolite Conf., Montreal 1992, Eds. R. von Ballmoos et al., (1993) by Butterworth-Heinemann, 263
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
779
E l e c t r o n spin r e s o n a n c e studies of 02- a d s o r b e d on a l u m i n o p h o s p h a t e m o l e c u l a r sieves Suk Bong Hong ~, Sun Jin KiIn a, Young-Sang Choib and Young Sun U h a aKorea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea bDepartment of Chemistry, Korea University, Seoul 136-701, Korea Electron spin resonance (ESR) spectra of the superoxide ion, 02-, generated in dehydroxylated aluminophosphate (A1PO4) molecular sieves and related materials are presented. Among the molecular sieves studied here, the extra-large-pore VPI-5 shows the largest spin concentration of O2-, which corresponds to 2.9 x 10~8 ions per gram. Analysis of the g values of 02- in AIPO4 molecular sieves reveals that the crystal field splittings, A, of the adsorbed O2- are in the range of 1.32-1.60 eV and suggests that the crystal field interaction is dependent on the pore size of the molecular sieves.
1. INTRODUCTION Aluminophosphate (AIPO4) molecular sieves were first synthesized by Wilson et al. in the early 1980s [1,2]. Although these microporous solids exhibit remarkable diversity in the framework structure, the use of AIPO4 molecular sieves as catalysts and separation media has been severely limited. This may be due to the lack of Bronsted acidity, which originated from the neutrality of their framework. The A1 and P atoms in A1PO4 molecular sieves occupy tetrahedral framework positions in strict alteration, with AI/P=I for a perfect framework. However, actual AIPO4 materials cannot be crystallographicaUy perfect because of framework defects generated during the synthesis and/or post-synthesis treatment steps. It has been recognized that many important properties of molecular sieves can be influenced by the presence of these defect sites [3]. Therefore, it is of fundamental interest to accurately identify the nature of these defect sites. There are a number of investigations on the defect sites in zeolites. However, no attention has been directed toward the characterization of structural defects in AIPO4 molecular sieves. Here we present the results obtained from the ESR spectra of the superoxide ion, 02-, adsorbed on the defect sites of a wide variety of AIPO4 molecular sieves and related materials. The crystal field interaction between the adsorbed 02- ion and the A1PO4 framework is also discussed.
780 2. E X P E R I M E N T A L The molecular sieves AIPO4-5, A1PO4-11, AIPO4-17, A1PO4-18 and A1PO4-20 were synthesized according to the procedures described in the Union Carbide patent [2]. VPI-5 was prepared by the procedures given by Davis and Young [4]. A1PO4-8 was synthesized by heating hydrated VPI-5 at 473 K for 24 h. A1PO4 tridymite was prepared by the procedures given by Cheung et al. [5]. Amorphous A1PO4 (AI/P=I) was obtained by heating a layered A1PO4 material in air at 873 K for 24 h. CoAPO-5, SAPO-5 and SAPO-34 molecular sieves were synthesized by the procedures described in the literature [6,7]. As-synthesized molecular sieves were calcined in 02 at 823 K for 16-24 h to remove the structurally incorporated organic structure-directing agents. The calcined samples were refluxed in water for 4 h and then dried at room temperature. Structural information on A1PO4 molecular sieves and related materials prepared in this study is given in Table 1. All the molecular sieves were phase-pure and show good crystallinities, as seen by powder X-ray diffraction (XRD) using a Rigaku D / M a x - H A diffractometer (Cu Ir~ radiation). This can be fftrther supported by the nitrogen BET surface area measurements, which were performed on a Micromeritics ASAP 2000 analyzer (see Table 1). Chemical analysis for CoAPO and SAPO materials was performed by a Jarrell-Ash Polyscan 61E inductively coupled plasma (ICP) spectrometer. The ESR spectra at 77 K and room temperature were obtained on a Bruker ER-200D Spectrometer operating at X-band (~ 9.45 GHz) with 100-kHz field modulation. Prior to ESR measurements, approximately 30 mg of the samples were placed into a quartz tube of 3-mm inner diameter, slowly heated in a vacuum of 10-5 Torr to 773 K, and then kept at this temperature for 2 h. After cooling to room temperature, the dehydroxylated samples were exposed to O~. at a pressure of about 10 Torr for 5 min. Finally, the gas-phase O~. was removed by evacuation to better than 10-4 Torr. The spin concentration of 02- adsorbed on molecular sieves was determined by comparing the intensities of the doubly integrated superoxide spectra with those obtained from various weighted amounts of DPPH. The estimated error in spin concentration is _+50%. The spectra at 4 K were measured on a Bruker ESP-300 Spectrometer in combination with an Oxford ESR 900 cryostat.
3. R E S U L T S AND DISCUSSION Figure 1 shows the ESR spectra at 77 K developed after adsorption of 02 on A1PO4 molecular sieves with different structures dehydroxylated under a vacuum' of 10-5 Torr at 773 K for 2 h. Very recently, we have found that the structural defect present in hydrated AIPO4 molecular sieves can change into the paramagnetic centers when the molecular sieves are dehydroxylated at high temperatures. Distinct differences in the position and number of the
781 Table 1 Physical data of AIPO4 molecular sieves and related materials used in this study Sample
Structure Ring Anhydrous unit Molecular wt. Surface area type size cell composition per unit cell (m2.g -1)
tridymite AIPO4 AIPO4-20 A1PO4-17 A1PO4-18 SAPO-34( I ) SAPO-34(II) AIPO4-11 AIPO4-5 SAPO-5 CoAPO-5( I ) CoAPO-5(II) AIPO4-8 VPI-5 amorphous A1PO4
SOD ERI AEI CHA CHA AEL AFI AFI AFI AFI AET VFI .
6 6 8 8 8 8 i0 12 12 12 12 14 18 .
AlzP208 A16P6Ou AllsP18072 AluP24096 A]I8.oPI6..6Sil.4072 AIIs.oPI5.8Si2.20v2 AI2oP2oOso AII2P12048 AIn.3PIo.sSil.90~ AIn.gP12.oCoo.IO~ Aln.oP12.oCoLoO~ Ala6Pa6Om AllsP18072 . .
244 732 2195 2927 2191 2189 2439 1463 1461 1467 1495 4390 2195
458 568 604 639 185 337 302 331 329 62 409 14
observed ESR signals from dehydroxylated AIPO4 molecular sieves reveal that the nature of the paramagnetic defects formed is significantly different in the structure type of the molecular sieves. We are investigating this fln-ther by using variable-temperature ESR studies, and the results will be given elsewhere (Hong et al., in preparation). The introduction of 02 into dehydroxylated AIPO4 molecular sieves resulted in an immediate loss of the ESR signals from paramagnefic defect centers. As seen in Figure 1, however, three new ESR signals are observed. Many papers on the physicochemical properties of the charged dioxygen species in various metal-ion-exchanged zeolites have been published, and comprehensive ESR studies of the adsorbed dioxygen are available [8,9]. A comparison of the ESR spectra in Figure 1 with the data report~ in the literature reveals that the paramagnetic oxygen species formed in dehydroxylated A1PO4 molecular sieves is 02- [9]. All the ESR spectra in Figure 1 were not significantly broadened by the presence of 10 Torr of 02, indicating that the spin-spin interaction between the chemisorbed 02- ion and physically adsorbed O~. is negligible. However, they were destroyed when the molecular sieves were exposed to 10 Torr of water vapor. The dehydroxylation of hydrated A1PO4 materials at the condition stated earlier and the subsequent adsorption of 02 at room temperature give rise to the complete regeneration of the 02- ESR spectra. Therefore, it is most
782 likely that the formation of 02- in A1PO4 molecular sieves is reversible. On the other hand, the absence of any hyperfine structure in the ESR spectra of Figure 1 suggests that no hyperfine interaction between the unpaired electron in 02- and the A1 atoms (•=5/2) in the A1PO4 framework exists. This may be result of the localization of the unpaired electron onto the adsorbed 02 molecule only. To more accurately ensure this speculation, we have performed ESR measurements on the 02 adsorbed on A1PO4 molecular sieves at 4 K. As expected, the 02- spectra obtained at this temperature are the same as those in Figure 1 and no hyperfine structures in the spectra are observed.
20G (i)
g=z = 2.01 ~ .
~
9
9
2.0198
(g) .
.
x 100
w
-.~=f
.
_
.
x
(f) (e) _
.
_
(d) .u.z.__
.
2.0231 ,,.,~ .
.
.
.
.
.
.
.
--.~
~-====-~--
2.0236
x
40 __
-
x ~
--
2
_
_...,.~ k
x.
,
~
j~,~.._
.._ . . =
L
L , = _
,,__
1 _
__
~ L
V
Figure i. ESR spectra at 77 K of O~- adsorbed on (a) tridymite A1PO4, (b) AIPO4-20, (c)A1PO4-17, (d)AlPO4-18, (e)AlPO4-11, (f)ALP04-5, (g) AIPO4-8, (h) VPI-5 and (i) amorphous A1PO4 dehydroxylated under a vacuum of 10-5 Ton" at 773 K for 2 h.
783 Table 2 g Values, spin concentrations and crystal splittings, A, at 77 K of 02- species adsorbed on AIPO4 molecular sieves and related materials Spin g Value concentration A" (eV) Sample gzz
tridymite AIPO4
A1PO4-20
AIPO4-17 A1PO4-18 SAPO-34( I ) AIPO4-11 AIPO4-5 SAPO-5 CoAPO-5( I ) CoAPO-5( 11) AIPO4-8 VPI-5
amorphous AIPO4
2.0209 2.0201 2.0236 2.0231 2.0216 2.0214 2.0215 2.0216 2.0215 2.0211 2.0202 2.0198 2.0199
g~
2.0103 2.0104 2.0106 2.0106 2.0103 2.0103 2.0102 2.0104 2.0102 2.0104 2.0105 2.0109 2.0101
g~
2.0033 2.0041 2.0035 2.0033 2.0036 2.0033 2.0039 2.0039 2.0038 2.0037 2.0041 2.0045 2.0034
(1016.g-1) 1.3 2.4 2.4 0.8 5.4 15.1 163 65 85 0.9 17.5 293 0.6
1.51 1.57 1.32 1.35 1.45 1.47 1.46 1.45 1.46 1.49 1.57 1.60 1.59
"Calculated using the simplified equation g ~ = g~ + 2k/A [12]. k has been taken equal to 0.014 eV so that comparison with earlier results can be made
[9]. Another interesting observation obtained from the ESR spectra in Figure 1 is that the relative intensity of the 02- signals differs significantly according to the structure type of the molecular sieve where the 02- ions are adsorbed. The spin concentrations of the 02- formed in A1PO4 molecular sieves and related materials studied in this work are listed in Table 2. In general, the larger pore size the molecular sieve has, the higher concentration of the adsorbed 02- ions it shows. For example, the concentration of the 02- ions formed on the small-pore AIPO4-20 is much small as compared to that of the extra-large-pore VPI-5. This result can be due to differences in the paramagnetic defects generated in dehydroxylated A1PO4 molecular sieves. However, a linear relationship between the concentration of 02- ions and the pore size of the molecular sieves was not observed, indicating that the formation of 02- in AIPO4 molecular sieves is more complicated than our expectation. Figure 1 and Table 2 also show that the 02- ion can be formed on amorphous and tridymite A1PO4 phases, although its concentrations on these nonmicroporous A1PO4 materials are very small as compared to those on the AIPO4 molecular sieves. Therefore, it appears that microstructure is not necessary to produce 02- ions in A1PO4 materials, but it plays an important
784 role in achieving high concentrations of 02- ions. The substitution of heteroatoms such as Si or Co into the AIPO4 framework gives rise to a significant decrease in the intensity of the 02- ESR spectra. Figure 2 shows the ESR spectra at 77 K of the 02- adsorbed on SAPO-34 with low and high Si contents. The 02- ESR spectrum from SAPO-34( I ) with Si/A1 = 0.08 exhibits three peaks at g = 2.0216, 2.0103 and 2.0036 while no noticeable 02- signals are observed for SAPO-34(II) with Si/A1 = 0.12. The same trend was also observed from C o A I ~ - 5 samples with different Co contents (see Table 2). Therefore, it is clear that the concentration of the 02- ions formed in AlPO4-based molecular sieves decreases as the heteroatom content in their framework increases. This can be directly related to the decrease in concentration of paramagnetic defect centers in these molecular sieves. The g values of the O~.- adsorbed on A1PO4 and related materials are listed in Table 2. These data reveal that all the 02- ESR spectra obtained here exhibit only one g ~ value, which is in contrast to 02- formed on T-irradiated alkaline-earth-exchanged zeolites where several different gz~ values can be observed [10]. Therefore, it appears that each molecular sieve contains only one type of 02- sites. This can be further support~ by the fact that the average g values of the 02- ions on A1PO4 molecular sieves at 77 K are quite similar to those obtained from the 02- ions on the corresponding materials at room temperature. The g values listed in Table 2 also show that the g,, values are different in the structure type of AIPO4 molecular sieves, while g ~ and g= remain almost unchanged. Figure 3 illustrates the electronic energy diagram of 02-. The crystal splitting, A, of the / / ~ level of 02- can be calculated from the g values. The theoretical expressions for calculation of the
20 G (b)
x 150
gzz= 2.0216 ~
(a) ,
,
J _
x 40 .
.
.
.
Figure 2. ESR spectra at 77 K of 02- adsorbed on (a) S A P O - 3 4 ( I ) and (b) SAPO-34(II). The signal intensity is referenced relative to the ESR specmm~ of 02- on VPI-5 in Figure lh.
785
IA
rq rg rg
E
z
Figure 3. The simplified energy level diagram for 02- in the ground state. A and E indicate the crystal field splitting and the energy difference of the lowest and highest occupied energy levels, respectively.
A and E energy splittings were derived by Kanzig and Cohen [11] and simplified by Kasai [12]. For the case E > A >> k, the expressions were simplified to g~, ~ g~ + (2k/A)
(1)
S~ ~ g~ + (2k/E) - (k2/A2) - ( k 2 ~ )
(2)
s=
(3)
= g,-
(X2/a~) + (X~/E A)
where k is the spin-orbit coupling constant = 0.014 eV and ge is the g value of the free electron. The calculated A values of the 02- in AlPO4 molecular sieves and related materials studied in this work are in the range of 1.32-1.60, as seen in Table 2. These A values are larger than any reported values from 02- ions on alkaline- or alkaline-earth-exchanged zeolites, and are comparable to those of 02- species on TiO2 or Ti3+-exchanged Y zeolites [9]. This suggests that 02- is more strongly held in AIPO4 molecular sieves than in zeolites. It is interesting to note that the A values of the 02- ions on AIPO4-8 and VPI-5 materials are quite similar to those of the 02- ions on the exterior surfaces of tridymite and amorphous AIPO4 phases, and AIPO4-20. This can be attributed to the large interior surface areas of these extra-large-pore materials. Finally, the A values listed in Table 2 clearly demonstrate that the crystal field interaction gets stronger when the pore size of an AIPO4
786 molecular sieve is larger. This suggests that the nature of 02- sites present in each A1PO4 molecular sieve is not the same. 4. CONCLUSIONS
The results presented here show that the 02- ion can be formed by dehydroxylation of AIPO4 molecular sieves under a vacuum of 10-5 Torr at 773 K for 2 h and then adding 02. This is in contrast to most of the ESR studies of O2- on zeolites in that the formation of 02- ions in AlPO4 molecular sieves is possible only by dehydroxylation, without 7 or UV irradiation of the molecular sieves. This property enables us to suggest that AIPO4 molecular sieves may find possible applications in catalysis and molecular separation technology. ACKNOWLEDGMENT
This work was support~ by the Technology under contract No. 2N13723.
Korea Institute
of Science
and
REFERENCES
1. S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan and E.M. Flanigen, J. Am. Chem. Soc., 104 (1982) 1146. 2. S.T. Wilson, B.M. Lok and E.M. Flanigen, US Patent No. 4 310 440 (1982). 3. G. Engelhardt and D. Michel, High-Resolution Solid-State NMR of Silicates and Zeolites, Wiley, New York, 1987. 4. M.E. Davis and D. Young, Stud. Surf. Sci. Catal., 60 (1991) 53. 5. T.T.P. Cheung, ICW. Willcox, M.P. McDaniel and M.M. Johnson, J. Catal., 102 (1986) 10. 6. B.M. Lok, B.K. Marcus, L.D. Vail, E.M. Flanigen and S.T. Wilson, European Patent No. 159 624 (1985). 7. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan and E.M. Flanigen, US Patent No. 4 440 871 (1984). 8. J.H. Lunsford, Adv. Catal., 22 (1972) 265. 9. M. Che and A.J. Tench, Adv. Catal., 32 (1983) 1. 10. K.M. Wang and J.H. Lunsford, J. Phys. Chem., 74 (1970) 1512. 11. W. Kanzig and M.H. Cohen, Phys. Rev. Lett., 3 (1959) 509. 12. P.H. Kasai, J. Chem. Phys., 43 (1965) 3322.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
787
Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
THE
AFFINITY
ZEOLITE
ORDER
SILICALITE-1
OF
ORGANICS
STUDIED
BY
ON
HYDROPHOBIC
THERMAL
ANALYSIS
Yingcai Long *, Huiwen Jiang, and Hong Zeng Department of Chemistry, Fudan University, Shanghai 200433, People's Republic of China
Abstract The AT values determined by TG/DTG are used to compare the affinity order of 27 organics adsorbed in the hydrophobic zeolite silicalite-1, which possesses a perfect Si-O-Si micropore surface without cations and silanol defect. Silicalite-1 has a strong affmity for some organic compounds with high polarity. The physical meaning of the AT values, and the nature of two type supermolecular host/guest interactions are discussed. Keywords" aff'mity order of organics; thermal analysis; silicalite-1 1. I N T R O D U C T I O N Silicalite-1 is a hydrophobic zeolite and very useful as a separation medium, such as an adsorbent or a membranes. Its hydrophobic/organophilic character is based on the interaction between the guest molecules adsorbed and the zeolite framework. A hydrophobicity (h) [1J has been def'med as the ratio of the weight loss at 150 °C to the weight loss at 400 °C for a dealuminated zeolite Na-Y. A hydrophobicity index(HI) [2'31 has been introduced. It was defined as XtoluenefXwater where X is the loading, i.e., the mass of adsorbed compound per mass of dry adsorbent (such as ZSM-5, silicalite-1, and zeolite Y). The hydrophobicity index can be used to compare the hydrophobic properties and the aff'mities for hydrocarbons. Actually, the sorption and desorption behavior of an organic compound with different functional groups is strongly influenced by small amounts of cations and defects t4-61. There is a perfect Si-O-Si micropore surface in the hydrophobic zeolite silicalite-1. The temperatures for complete desorption of most compounds adsorbed within silicalite-1 are usually lower than 300 °C in a narrow region without catalytic effects. A new concept AT, the aff'mity of the perfect micropore surface in silicalite-1 to an organic compound adsorbed was recently developed. AT = T a- Tb was defmed, where T d is the temperature of the weight loss peak in the DTG curve and Tb the boiling point of the compound [61. In this paper we will use the AT values to compare the atTmity order of 27 organics, and reveal the nature of the host/guest interaction. * Author for correspondence. This work was supported by the Science Fund of the Chinese Academy of Science.
788 2.
EXPERIMENTAL
2.1. Preparation of zeolite samples The zeolite samples used in this study were hydrothermally synthesized in an amine-Na20SiO2-H20 system by using water glass as a silicon source [7]. The as-synthesized products were washed, filtered, dried by IR lamp, and then calcinated in an oven to remove the organic template at about 600 °C for 2 hr. After treated with a 0.5N HCI solution at 95 °C for 4 hr, it was washed, filtered, dried and calcinated at 550°C for 2 hr to get H-ZSM-5. The H-ZSM-5 sample was calcinated at 800 °C in a quartz tube for 100 hr using an air flow, which was saturated at room temperature by water steam, to get the silicalite-1. The flow rate of water steam was controlled by a micro pump at 105 _+5 ml/h.
2.2. Adsorption and Determination of The AT Value Reagents used as adsorbates were chromatographic grade or analytically pure. Adsorption of vapors of a guest compound was carried out in a fixed bed of the zeolite sample at room temperature for more than 24 hr in order to fully saturate the sample. When the guest compound was a solid at room temperature, the adsorption was done at higher temperature to get vapor from the melt. TG/DTG/DTA measurements were carried out by using a PTC10A thermal analyzer with an air flow of 70 ml/min at a rate of 5 °C/min from room temperature to 700 °C. About 10 mg of a zeolite sample were used in a test. The sensitivities of TG and DTA used were 0.0ling and +25gV respectively. The adsorption data were calculated from TG, and Td data from the temperature of the weight loss peak in the DTG curves. Tb is the boiling point of the guest compound at standard condition and can be found in any handbook of physical chemistry. If there are two weight loss peaks appearing in a DTG curve, the higher one is taken as Td to calculate the AT value.
3.
RESULTS AND DISCUSSION
3.1. Structural Characterization of the Silicalite-1 Sample SEM showed that the as-synthesized MFI zeolites were in a prismatic form of single crystals with a size of 6 x 15 ~tm. The XRD patterns indicate that the samples were pure MFI phases with a high crystallinity. The silicalite-1 Sample has monoclinic symmetry according to the diffraction peaks doublets at 24.4 °, 29.2 °, and 48.6°(20). 29SiMAS NMR spectra show that the Si-OH peak at -103 ppm and the Si(1A1) peak at -106 ppm, which appear in the patterns of H-ZSM-5 sample, disappear in the pattern of silicalite-1. The small amount of framework aluminum in the H-ZSM-5 sample came from the silica source as an impurity in the water glass. The high resolution 29Si resonance with 20 peaks indicates a symmetry transition from orthorhombic to monoclinic, accompanied by a perfection of the framework upon steam treatment N. In the process of preparing the sample of silicalite-1, the dealumination and desilanation effect of steam treatment can also be detected by the FT-IR spectra. A [TO4] external asymmetrical stretching vibration at 1226 cm 1 and an internal
789 asymmetrical stretching vibration at 1095 cm 1 move to higher frequencies. At the same time the vibrations of silanol groups at about 3730 and 3450 cm ~ disappear in the spectra of silicalite- 1. All these facts indicate that the sample of silicalite- 1 used in this study possesses a perfect crystalline structure, whose framework is constructed by Si-O-Si without Si(1A1) The and Si-OH defects. This is the typical character of silicalite-1 [ 8 - 1 2 ] . hydrophobic/organophilic properties greatly change with the content of framework defects. Silicalite-1 has high hydrophobicity, its adsorption of water vapor at room temperature is less than lwt%.
3.2. The Thermal Desorption Behavior Typical TG/DTG/DTA spectra of paraffins, aromatics, alkyl-alcohols, multivalue hydroxyl alcohols, alkyl-amines, and organic acids (see Fig. 1) indicate that the temperatures for complete desorption of these compounds are lower than 300 °C, and that the desorption occurs in a narrow region without catalytic effects, so far no exothermic effect in the DTA curves is found. There is no significant thermal effect on desorption for most hydrocarbons. An endothermic effect can be found from the DTA curves of silicalite-1 for p-xylene, alkylalcohols, multivalue hydroxyl alcohols, alkyl-amines, and formic acid respectively. The hydrophobicity of the siliceous zeolite is based on the absence of exchangeable cations and the substitution of aluminum atoms in the framework by silicon atoms. HOwever the influence of the structural defects on the hydrophobicity is not negligible. The hydrophobic/organophilic property is a characteristic of the siliceous zeolite and is a generalization of the difference between organics and water upon adsorption and desorption. It is a reflection of an interaction between the micropore surface of the zeolite and certain organic molecules. In order to reveal the nature of the interaction, it is necessary to exclude an influence of cations and structural defects of Si-OH. The behavior of thermal desorption can be used to characterize the interaction in silicalite-1 since there is no the influence [13] . That is why we used the sample of silicalite-1 to determine the AT values, and investigate the host/guest interaction.
3.3. The Physical Meaning of the AT Values Considering that there are no strong electrostatic sorption centers on the micropore surface of the perfect silicalite-1 framework, the adsorption of guest molecules can approximately be treated as a physical capillary condensation. If a liquid is immersionally wetting(i.e., philic) a capillary surface, the capillary ascent phenomenon will cause the boiling (i.e., violent desorption) temperature of the liquid condensed within the capillary to be higher than its boiling point in the free state at normal atmospheric pressure. Obviously, if a liquid is nonimmersionally wetting (i.e., phobic) a capillary surface, the capillary depression phenomenon will cause the boiling temperature of the liquid within the capillary to be lower than its boiling point at normal conditions. It is reasonable to believe that the AT value is positive if the micropore surface of silicalite-1 is playing a role of the philicity with the guest molecule. The AT value is negative if the micropore surface is phobic with the adsorbate. No unit for the AT values is required while comparing the philicity or the phobicity of the interaction.
790
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l
l
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i
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72 !
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....
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138
141 I
200
I
400 °C
I
200
I
400 °C
Figure 1. TG/DTG/DTA spectra of some organics on zeolite silicalite-1, a) pentane, b) ethylbenzene, c) p-xylene, d) 1-pentanol, e) glycerol, f) ethylamine, g) acetic acid, h) ethylacetate; . . . . . . TG ....... DTA, ~ DTG, $ endothermic effect
791
3.4. The AT Values of Organics The AT values and the adsorption data of organics are listed in Table 1. The AT values are 60~90 for the saturated hydrocarbons, which indicate the organophilic property of silicalite-1. The AT values are 6~13 for benzene, toluene, ethyl-benzene and p-xylene, but -91 for naphthalene. The facts indicate that silicalite-1 has weaker aff'mity with the unsaturated hydrocarbons than that with the saturated hydrocarbons. The AT value from -4 for ethanol increases to 13 for 1-propanol. The AT values decreases by about 55 as increasing one hydroxyl group on an alkyl-alcohol with the same bulk structure, such as from 1-propanol to 1.2-propanediol, from 1.3-propanediol to glycerol, and from ethanol to ethylene glycol as well. The AT values decreases by about 40 from benzene to phenol. The negative AT values for alkyl-alcohols with multivalue hydroxyl groups indicate that silicalite-1 is usually phobic to the organics with hydroxyl groups. The AT values are 160, 127 and 116 respectively for methylamine, ethylamine and acetaldehyde, which are much higher than that for other alkylamines and organics with oxygen atom. It is possible that there is a double molecule associate existing in the channel intersections of the zeolite. 3.5. The Sub-Hydrogen Bond Between Hydrocarbons and Framework 0 2. It is a simple way to investigate the degree of the interaction between a zeolite(i.e., the host) and an adsorbate(i.e., the guest). During temperature programming, the temperature for mass desorption of a guest molecule from micropore(i.e., Td- the temperature of the weight loss peak in the DTG curve) is related to the interaction between the surface of silicalite-1 and the molecules. The framework of silicalite-1 is constructed by [SiO4] tetrahedra linked by sharing 02. . Since the radius of oxygen atom is much larger than that of silicon atom, the whole surface of the framework is actually covered with 02-. In the framework of silicalite1 the distance between two neighboring oxygen atoms is 0.149 nm, which is very close to 0.154 nm, the distance between two neighboring carbon atoms in saturated hydrocarbons. The bond angle of C-C-C or H-C-H in saturated hydrocarbons is 109°23 ", which is equal to the bond angle of O-Si-O in the framework of silicalite-1. The C-H groups of saturated hydrocarbons can freely rotate along the C-C axis. It provides a greater opportunity to play a role of the interaction between the mass hydrogen atoms of saturated hydrocarbons and 02. in the framework with a sub-hydrogen bond. It leads Td to be much higher than Tb for most saturated hydrocarbons adsorbed as a monomolecular layer within the micropore of the zeolite. The influence of the number of carbon atom on the interaction is not obvious. The weak sub-hydrogen bond increases the AT values, but can not induce a visible thermal effect of desorption.
792 Table-I Adsorption and the AT Values on Silicalite-I guest molecule
M
molecule size(nm)
Au /u.c.
Tb
(b.p.) °C
Td weightl o s s
AT (Td-Tb)
103 79, 127 63, 185
67 58 87
peak temp.°C
pentane n-hexane heptane
72.15 86.18 100.21
.90 1.03 1.16
8.3 8.5 6.5
36.1 69 98.4
benzene toluene ethyl-benzene p-xylene naphthalene
78.12 92.15 106.17 106.17 128.19
.58 .91 1.03 1.32 1.16
7.8 8.0 5.5 7.8 3.1
80.1 110.6 136.2 138.3 218
90 124 149 94, 144 127
10 13 13 6 -91
ethanol 1-propanol 1-pentanol phenol
46.07 60.11 88.15 94.11
.69 .82 1.07 .84
15.0 12.4 9.2 6.9
78.5 97.4 137.3 181.7
75 110 141 78, 144
-4 13 4 -38
ethylene glycol 1,2-propanediol 1,3-propanediol glycerol
62.07 76.11 76.11 92.11
.85 .82 .99 .99
15.6 9.9 10.9 4.0
198.9 189 213.5 290
104, 144 134 166 193
-55 -55 -48 -97
31.06 45.07 59.11 87.11 101.19
.50 .63 .75 1.10 1.13
19.8 15.6 10.8 9.1 7.7
-6.3 16.6 47.8 104.4 130
154 72, 144 94 121 107, 194
160 127 46 15 64
formic acid acetic acid
46.03 60.05
.36 .69
28.0 17.5
100.7 117.9
86 128
-15 10
acetaldehyde ethyl-ether acetone ethylacetate
44.05 74.12 58.08 88.12
.44 .85 .62 1.18
17.6 8.6 12.5 8.8
20.8 34.5 56.2 77.1
137 87 95 138
116 53 39 61
methylamine ethylamine n-propylamine pentylamine n-hexylamine
The number of hydrogen atoms in an aromatic molecule is less than that of in a saturated hydrocarbon with the same number of carbon atoms. An aromatic ring is in a plate form. The hydrogen atoms on the ring can not rotate along C-C axis. The opportunity of playing the role of the interaction between the framework 0 2- and the molecules adsorbed with subhydrogen bond for aromatics is much less than that for saturated hydrocarbons. It leads to a lower AT value for aromatics. The AT values decrease to negative because of a stronger polarity of the hydroxyl groups in molecules for multivalue hydroxyl alcohols, which are
793 repulsed by the framework O 2". organics with hydroxyl groups.
This is the reason that silicalite-1 is usually phobic to
3.6. The Effect of Associate on the AT Value Restricted by micropore size, the molecules of p-xylene can form an associate with hydrogen bond in the channel intersections of MFI zeolite [~31. A hydrogen atom of the methyl group in the molecule of p-xylene combines with an aromatic ring in a neighbor molecule of pxylene to form the associate with hydrogen bond. The situation is similar to that in the pure crystal of p-xylene, and may cause the visible thermal effect on the desorption in the DTA curve [~41 . The plate molecules of naphthalene are easier to form a tight crystal structure in the free state. The boiling point of naphthalene is 218 °C, much higher than that of nnonane (C9H20, b.p. = 150.7 °C), which has a similar molecular weight. The dimension of the naphthalene molecule is 0.58 x 1.16 nm. The adsorption of naphthalene is about three molecules per unit cell. It is impossible for the molecules to form an associate with each other within the micropores of silicalite-1. The situation leads Td to decrease to 127 °C, and the AT value decreases to -91. The endothermic effects on desorption for alcohols or phenol possibly come from a deassociation of the associate, which is formed in a combination with a neighboring molecules with hydrogen bond within the channel intersections.
3.8. Abnormal AT Values Amine groups are also polar, and their polarities are weaker than that of hydroxyl groups. So the AT values of alkyl-amines are usually positive. The molecular size of methylamine and ethylamine are 0.50 and 0.63 nm respectively. It is easy to form a double molecule associate by combining -NH2 groups with an adjacent molecule with stronger hydrogen bond in the channel intersections. The double molecule associate may lead the AT value to be abnormally high for methylamine and ethylamine, and cause the visible endothermic effect on the desorption. The molecules of pentylamine can not form double molecule associate in the channel intersections because of their larger molecular size. The hydrogen bond, which exists between two neighboring molecules in the free liquid state, diminishes since the molecules of pentylamine are isolated by the zeolite channels. The diminution of the hydrogen bond makes the Td close to the boiling point of n-hexane, which has a similar molecular weight with pentylamine. The endothermic effects of desorption for alkyl-amines are a sign of forming the associate in the channel intersections with hydrogen bond. The AT value is abnormally high for acetaldehyde. It is also possibly caused by a double molecule associate forming with hydrogen bond at the channel intersections. The tendency of the AT values is abnormal for heptane (1 = 1.16 nm), 1-pentanol (1 = 1.07 nm) and pentylamine (1 = 1.13 nm) in comparison with n-hexane, 1-propanol and npropylamine respectively. There are four channel intersections per unit cell of silicalite-1. The space of the intersection can be seen as a cross of four channels with length of 1.05 nm and cross section of 0.54 x 0.56 nm. If a molecule has a length < 1.05 nm, the molecule is adsorbed in the intersections with higher probability. Otherwise, a molecule with the length > 1.05 nm is not easy to be adsorbed at the intersections. In this situation, the interaction
794 between the alkyl groups and the framework O 2", and the interaction between the polar functional groups of the molecules will change more. It leads the Av values to become abnormal. CONCLUSION TG/DTG/DTA can be used for quick analysis of affmity on zeolites. The AT values compare the order of host/guest interaction and are useful for separation practice. The difference in the A7 values and the thermal effects of desorption are brought by different interactions between the micropore surface and the functional groups of organics. It is also influenced by the different situations of the associates located at the channel intersections. These facts are a reflect of different type supermolecular interaction for the host/guest system.
REFERENCES
1. M.W. Anderson, and J. Klinowski, J. C. S., Faraday Trans., 1, 1986, 82, 1449. 2. C.H. Berke, A. Kiss, P. Kleinschmit and J. Weitkamp, J. Chem. -Ing. -Tech., 1991, 63, 623. 3. J. Weitkamp, P. Kleinschmit, A. Kiss and C. H. Berke, Proceedings from the 9th IZC, Montreal 1992, Ed. R. Von Ballmoos et al, 1993 by Butterworth-Heinemann, VII, P79. 4. N.B. Milestone, and D. M. Bibby, J. Chem. Tech. Biotechnol., 1983, 34A, 73. 5. N.B. Milestone, and D. M. Bibby, J. Chem. Tech. Biotechnol., 1981, 31,732. 6. Long Y-C., Jiang H., and Zeng H., J. Fudan Univ., 1994, 33(1), 101. 7. Long Y-C., Sun Y-J., Wu T-L., Wang L-P., Qian M., and Fei L., CN apply No. 92 1 13807.5. 8. Sun Y-J., Huang Y-F., Wu T-L., Wang L-P., Fei L., Yang H., and Long Y-C., ACTA CHIMICA SINICA, 1994, 52, 573. 9. E.M. Flanigen, R. L. Patton, USP, 4073 565, (1978). 10. C.A. Fyfe, J. H. O'Brien, and H. Strobl, Nature, 1987, 326(19), 281. 11. D.H. Olson, W.O. Haag, and R.M. Lago, J. Catal., 1980, 61,390. 12. E.L. Wu, S.L. Lawton, D.H. Olson, A.C. Rohrman, Jr., and G.T. Kokotaillo, J. Phys. Chem., 1979, 83(21), 2777. 13. Zeng H., Jiang H., Long Y-C., Sun Y-J., Wu T-L., Wang L-P., ACTA PHYSICO CHIMICA SINICA, 1995, 11(3), 242. 14. Y-C. Long, H. Zeng, Y-J. Sun, T-L. Wu, L-P. Wang, "Two Types of P- Xylene / Silicalite-1 Associate and Their Formation Studied by XRD, TG/DTG/DTA, 29Si and 13C MAS NMR", to be published.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
795
Studies in Surface Science and Catalysis, Vol. 105 1997 Elsevier Science B.V.
CHEMISTRY OF CoAPO-11 AND MOLECULAR OXYGEN ADDUCTS.
VAPO-5
• ESR
STUDIES
OF
C. NACCACHE 1, M. VISHNETSKAYA 2 and Kuei-Jung CHAO 3 • hstitut de gecherches sur la Catalyse, CNKS, Villeurbanne, France. 2. Moscou University, Chemical Department, Moscou, Russia. 3. National Tsing Hua University, Department of Chemistry, Hsinchu, Taiwan. 1. ABSTRACT CoAPO-11 and VAPO-5 were synthesized in the presence of diisopropylamine. XRD indicates high crystalline materials with the isotopic structure of respectively ALPO-11 and ALPO-5. CoAPO-11 UV-spectra and the existence of acidic OH revealed by infrared, indicate isomorphous substitution of divalent cobalt for aluminium Removal of the template by 02treatment at 773 K produced a diamagnetic oxo-type cobalt Co = O. This species activates molecular oxygen only after H2-treatment at high temperature. The esr spectra of the oxygen adduct formed upon 02 adsorption on H2-reduced CoAPO-11 correspond to the paramagnetic species Co(2+)-O-O'. The amount of this peroxo cobalt adduct increases with the severity of the H2-treatment. One concludes that Co = O is converted into an active Co(2+) species by H,treatment. Co(2+)- O-O" is very reactive toward H2, hydrocarbons, and is regenerated by cyclic 02 adsorption. At high temperature about 573-773 K Co(2+)-O-O" is converted into Co = O oxo species non active to activate molecular oxygen. VAPO-5 was studied by esr. The esr spectra revealed the presence of V 4* both in the sample as synthesized and in the 773 K O2-treated sample to remove the template. Tetravalent V is rather stable in O2 at high temperature which indicates that V 4+ ion substitutional framework position is resistant towards oxidation. Like CoAPO-11, oxygen adducts V- O-O" are formed, as revealed by esr, only aiter H2-treatment at high temperature. The esr results are interpreted similarly to those on CoAPO- 11. 2. INTRODUCTION There has been much interest in the synthesis of isomorphously substituted aluminophosphates (1). It has been established that aluminophosphates containing cobalt possess redox properties which contribute to the catalytic properties for oxidation (2). In addition the isomorphous substitution of Co(2+) into the framework of ALPO generates acidic properties associated with P-OH sites (3). Although the Bronsted acidity linked with P-OH is generally weak, CoAPO exhibited interesting catalytic properties in reactions catalyzed by acid such as isomefisation ofbutene (4) and isomefisation of alkanes (5). The aim of this work was to investigate the redox properties of CoAPO-11 and VAPO-5 in relation to their interaction with oxygen and/or hydrogen. Esr technique was employed to investigate these interaction
796 3. EXPERIMENTAL SECTION Preparation of CoAPO- 11 and VAPO-5. The CoAPO- 11 was synthesized using a gel having the following molecular composition : H3PO4 (85%) = 1, A1203 = 0.475, COSO4, 71-120 = 0.05, diisopropylamine = 0.75 1-120 = 40 The crystallization was carried out in a teflon bottle in an autoclave at 463 K for 2 days. Highly crystalline CoAPO-11 was obtained following the above procedure as indicated by XRD analysis (4). The crystals of CoAPO-11 with the template were thoroughly washed, filtered and dried in air at 373 K. The CoAPO-11 showed a deep blue color. The organic template was removed by calcination in air at 773 K. The sample turned green and is designated by Ox-CoAPO-11. Ox-CoAPO-11 samples which have been reduced by H2 in the temperature range 473-773 K are designated by Redu-CoAPO- 11. The gel composition for the synthesis of VAPO-5 was the following in molar ratio • P r 3 N " A1203 : 1.9 H3PO4 : 0.05 V205 : 401-120. The crystallization was carried in an autoclave at 423 K during two days. The n-tripropylamine template was removed by calcination of VAPO-5 in air at 773 K (Ox-VAPO-5). The sample turned yellow. Upon 1-12reduction at 773 K (ReduVAPO-5) the VAPO-5 returned to almost its initial green color. XRD spectra of the assynthesized, oxidized, reduced VAPO-5 s a b l e s are in agreement with those of A1PO-5 published in the literature (4). Esr spectra were recorded with an X-band Varian E9 spectrometer operating with 100 KHZ frequency modulation. g-values were determined using a dual cavity and DPPH (g = 2.0036) as reference. All esr spectra were recorded at 77 K unless mentioned in the text. Prior esr measurement, samples were outgassed directly in the esr quartz tube. Oxygen adsorption was performed on samples having experienced an activation procedure which will be specified in the text. 4. RESULTS AND DISCUSSION As indicated in the experimental section the as-synthetized blue CoAPO-11 showed well characterized x-ray line pattern of the isostructural A1PO- 11. The x-ray line pattern was almost unchanged upon removal of the organic template by calcination in air. CoAPO-11 sample was formed of spherical particles with 10 Bm size. The infrared spectrum of the ox- CoAPO-11 in the OH stretching vibration showed band at 3640 cm~ attributed to P-OH. The acid character of these P-OH groups was demonstrated by their reaction with a base like pyridine. The IR band at 3640 cm"1 disappeared on adsorption of pyridine. Simultaneously bands at 1550 and 1490 cmt characteristic ofpyridinum ions and at 1490 and 1450 cmt characteristic of pyridine coordinated with Lewis centres appeared. According to these in~ared results the oxidized CoAPO-11 sample shows some Bronsted and Lewis acidity. Similarly the H2-reduced CoAPO- 11 sample exhibited almost the same infrared features as the ox- CoAPO- 11 that is IR band at 3640 cm ~ which disappeared upon pyridine adsorption with the subsequent formation of IR bands at 1540, 1490 and 1450 cmt indicating the Bronsted and the Lewis acid character of Redu-CoAPO- 11. The aluminophosphate molecular sieves have neutral framework. Isomorphous substitution of divalent cations Co 2+ for trivalent A13+ in A1PO generates negative framework charge, neutralized by H +. CoAPO-11 will show Bronsted acidity for divalent Co ~-+. By contrast substitution of trivalent cations for A1 3+ will leave neutral the framework. CoAPO- 11 where cobalt is at + 3 oxidation state should not exhibit any no proton acidity. The infrared results presented above have indicated that calcined ox-CoAPO-11 and
797 reduced Red- CoAPO-11 exhibited almost the same proton acidity. Therefore one must necessary conclude that the charge of the substituted cobalt in these samples is always + 2 since no Bronsted acid site would form for Co 3+ substitution. It is tempted to conclude in agreement with recent work based on esr and uv studies of CoAPO-5 that no oxidation of Co 2+ into Co 3+has occurred upon calcination ofCoAPO- 11 (5). The esr spectrum at 77 K of the as synthesized blue sample CoAPO-11 consists of a very broad asymmetrical signal which spreads within more than 2000 gauss. The approximative gvalues are around g~ = 4.5 g2 = 2. The signal was previously ascribed to Co 2+ (5,6). The esr signal of ox- CoAPO-11 calcined at 773 K has a considerable lower intensity and apparently is much more broad than the previous one. These changes in esr spectra were interpreted either in terms of cation oxidation Co 2+ to Co a+ (6) or in terms of lattice distortion of the molecular sieve, therefore the local tetrahedral field symmetry at Co 2+ ions was changed (5). The calcined ox- CoAPO-11 sample apparently did not activate molecular oxygen. Indeed no paramagnetic oxygen species visible to esr appeared upon 02 adsorption at 77 and 293 K on ox- CoAPO-11 outgassed in the temperature range 293-473 K. However upon H2 treatment in the temperature range 473-773 K the Red- CoAPO- 11 became active towards Ozadsorption. Indeed as Oz (1 torr) was allowed to react with Red-CoAPO-11, previously outgassed at 473 K, a strong esr signal with anisotropic g-values g~ = 2.0210 g2 = 2.0093 g3 2.0024 was observed (figure 1). The species responsible for this 3-g values esr signal were unambiguously identified to the cobalt superoxide (or peroxide) ions. It was observed that the superoxide esr signal intensity (hence the amount of paramagnetic Co-O2 adduct) increased as the temperature of the initial H2-treatment was higher, the Hz-treatment at 773 K producing the highest quantity of superoxide ions. The thermal stability of these superoxide ions was found to depend on the sample temperature. At about 423 K under outgassing conditions the esr signal of the paramagnetic oxygen adducts disappeared within few minutes. However re-adsorption of molecular oxygen on this sample regenerated immediately the superoxide species. Within these experimental conditions (Red- CoAPO- 11 never contacted with O~ at high temperature) the Red-CoAPO- 11 sample can perform several cycles of oxygen activation. By contrast as the temperature of the Co-O2 adduct was raised up to 773 K, the esr signal of the superoxide ions disappeared, and a subsequent adsorption of molecular oxygen did not generate, as previously, the superoxide esr signal. This 02 treated at 773 K, was rendered again active to oxygen adsorption only after it was H2-treated in the temperature range 473-773 K. The superoxide species were found reactive towards hydrocarbons, particularly olefins and cycloolefins, the esr signal of the paramagnetic Co-O2 adducts disappearing upon contacting the sample with for example cyclohexene. From the known structure of AIPO-11 it is logical to assume that if the Co 2÷ ions, in cobalt substituted AIPO, are part of the aluminophosphate lattice, they will be in a tetrahedral environment, replacing partially some lattice A13÷ ions. The resulting tetrahedral crystal field experienced by Co 2÷ would give to this ion the high spin 3/2 electron configuration which for powdered samples produce very broad esr signal as observed with CoAPO-11. The negative excess charge, resulting from the substitution of A13÷ for Co 2+ is in the as-synthesized sample compensated with the protonated di-isopropylamine. The removal of the template by calcination would leave protons only if the overall charge of the A1 substituted cation species is at the 2+ oxidation state. As stated above the increase of the oxidation state of Cobalt from 2+ to 3+ would render the CoAPO framework neutral. The presence of proton acid sites on the ox- CoAPO-11 suggests that upon calcination Co(2+) forms with oxygen a cobak oxo species such as Co = 0 where the two p orbitals beating one impair electron form with d orbitals of Co the Co = 0 bonds. The total charge of the oxoadduct would be (Co = 0) 2+ while the cobalt ion =
798 would be virtually at 4+ oxidation state. Such cobalt-oxo adduct apparently could not further activate molecular oxygen. Co = 0 oxo complexes were stable towards decomposition up to 773 K, However upon H2-treatment at high temperature (473-773 K) the oxygen ad-atom is removed following the reaction • (Co = 0) 2+ + H2 ~ Co z+ + He0. The above discussion explains how and why Oe-treated and H2-treated CoAPO-11 exhibited the same proton acidity. The He-treated CoAPO-11 adsorbed oxygen in the form of a paramagnetic superoxo or peroxo like cobalt species. On the He-reduced sample tetrahedral Co(2+) would form bonds with 02 molecule either via electron transfer between Co(2+) and 02 (ionic bonding) forming Co (3+) - O2" species or via coupling of one unpaired d electron of Co (2+) and one unpaired electron of molecular oxygen forming the covalent peroxo cobalt adduct Co-O-O'. The relatively high thermal stability of the cobalt peroxo adduct, and the relatively high ionization potential for Co (2+) ~ Co (3+) render the covalent structure Co-O-O" more favorable. One can predict that the oxidation mechanism over CoAPO catalysts will be of radical type. The Co-O-O" adducts react with hydrogen donor molecules, H2, RH in a catalytic cycle following 2He Co (2+) + 02 --~ Co-O-O" ) Co (2+) + 2H20 At high temperature, 773 K, the peroxocobalt adducts are converted into the cobalt oxospecies Co-O-O" -~ Co=O + ½ 02 (T = 773 K) which is inactive toward the activation of molecular oxygen. The solid recovered its activity toward 02 after the oxo species were removed by H2 (or hydrocarbons) at 773 I~
12.02
I DPPH
..___
IDPPH I~ 2.009
..2.0093 H lOLgau,ss
1
i
I 2.003
I 2.0024
Figure 1. esr spectrum of Co-O-O"in CoAPO-11
Figure 3. esr spectrum of V-O-O"in VAH3-5
799
[DPPH 100 gauss
Figure 2. esr spectrum at 77 K of V(+4) in VAPO-5 Esr study of VAPO-5. The esr spectrum of the as-synthesized VAPO-5 is shown in figure 2. The esr spectra recorded at 77 K and at 293 K extfibited identical esr features (g-values and hyperfme splittings). By decreasing the temperature the esr line intensities increased following the 1/T Curie Law. The magnetic parameters of this esr signal are" gx = 1.94 g:= 1.99, A//= 185 gauss A± = 72 gauss. These parameters are typical for V (4+) ions in distorted axial symmetry. V (4+) has a 3d ~ configuration. This ion in a tetrahedral symmetry such as experienced for T site in ALPO-5 framework will present a very short spin-lattice relaxation time, and since the line width is related to the inverse of the relaxation time, the esr spectrum of V (4+) in a non distorted tetrahedral crystal field will be broadened beyong detection at 77 and 293 K. Indeed V (4+) in VCI4 compound was not observed by esr (7). Similarly (nor)4 V has no detectable esr signal at temperatures down to 143 K (8). Below this temperature the esr signal of V (4+) with" gl = 1.984 g2 = 2.036 AI = 40 g and A¢/= 120 gauss becomes discernible. This was attributed to V (4+) species in a tetrahedral crystal field with a tetragonal distortion. The detection of an esr signal at 293 K attributed to V (4+), and the relatively large hyperfme splitting (A//= 185 gauss, A± = 72 gauss) therefore indicate that if vanadium ions present in VAPO-5 are in substitutional position, then the symmetry around V (4+) is considerably distorted. It is likely that the symmetry around V (4+) observed by esr in VAPO-5 is provided by three identical VO bonds and one short V-O bond. Suggesting that V (4+) replaces P (5+) in AIPO-5 then the local structure will be (02)3 V-O(H)...AI(O2)3. The results have also shown that the V(4+) esr signal observed on the as synthesized VAPO5 remained almost unchanged alter calcination. It is clear that V (4+) in VAPO-5 was stable
800 toward oxidation into V (5+) which is diamagnetic. It appeared that the crystal energy of the lattice was able to stabilize V (4+) ion. The calcined VAPO-5 does not form paramagnetic oxygen species by 02 adsorption on the outgassed sample. However the sample submitted to H2-treatment at 773 K, adsorbed O2 to form the vanadium peroxospecies V-O-O" whose esr spectrum is shown in figure 3. The esr gvalues are g~ = 2.021 g2 = 2.009, g3 = 2.002. The mechanism for the formation of this peroxospecies is apparently similar to that proposed for CoAPO sample' (02")3 V-O(H)
773 K (OZ')3V 02 (02")3 V-O-O" H2 > ) At high temperature (773 K) the (02")3 V-O-O" species is again converted into : (O2")3V-O(H) 5. REFERENCES E.M. Hanigen, B.M. Lok, ILL. Patton and S.T. Wilson, in Y. Muzakami, A. figima and J.W.Ward (Editors), Studies in surface Science and Catalysis, New Developments in Zeolite Science and Technology, Kodansha Elsevier (1986) p. 103. B. Kraushaar-Czametzld, W.G.M. Hoogervorst and W.H.J. stork in J. Weitkamp, H.G. Karge, H. Pfeifer and W. HSldefich eds Zeolites and Related microporous Materials • State of the Art 1994 Studies in Surface Science and Catalysis Vo184, (1994) p. 1869. J. Jiinchen, M.J. Haanepen, M.P.J. Peeters, J.H.M.C. van Wolput, J.P. Wolthuizen, J.H.C. van Hooffin J. Weitkamp, H.G. Karge, H. Pfeifer and W. H61derich eds, Zeolites and related microporous Materials • State of the Art 1994. Studies in Surface Science and Catalysis vo184, (1984)p. 373. J.M. Bennett, J.P. Cohen, E.M. Hanigen, J.J. Pluth, J.V. Smith, ACS Symposium Series, 218 (1983) 109. V. Kurshev, L. Kevan, D.J. Parillo, C. Pereira, G.T. Kokotailo and ILJ. Gorte, J. Phys. Chem, 98 (1994) 10160. 6.
L.E. Iton, I. Choi, J.A. Desjardins, V.A. Maroni, Zeolites, 9 (1989) 535.
7.
J.C.W. Chien and C.IL Boss, J. Phys. Chem. 83 (1961) 3767. B.K. Bower, M. Findlay, J.C.W. Chien, Inorg. Chem., 13 (1974) 759.
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.
Cupric Ion Species in Cu(II)-Exchanged and Comparison with Aluminosilicate K-L
801
Gallosilicate
K-L
Jong-Sung Yu. a Suk Bong Hongb and Larry KevanC aDepartment of Chemistry, Han Nam University,Taejon, Chungnam, 300-791,Korea bKorea Institute of Science and Technology, P.O.Box131,Cheongryang,SeouI,Korea CDepartment of Chemistry, University of Houston, Houston, TX, 77204-5641,USA
The locations and adsorbate interactions of Cu(II) in Cu(II)-exchanged gallosilicate with the zeolite L channel structure were investigated by electron spin resonance(ESR) and electron spin echo modulation (ESEM) spectroscopies, and compared with those in Cu(II)-exchanged aluminosilicate K-L zeolite previously studied. Similar results to those in aluminosilicate CuK-L zeolite are observed in its gallium analog, suggesting that gallium substitution has little effect on the interaction of Cu(II) with adsorbates in the L framework structure. 1.
INTRODUCTION
Isomorphous substitution of elements other than Si and AI into T-site framework positions in zeolite has been a long standing interest in zeolite chemistry as this can provide a route to modify the framework characteristics and geometries. Gallosilicate is one such important material where zeolitic aluminum atoms can be replaced by gallium atoms[I-4]. The gallium analogs of zeolites usually possess physical and chemical properties different from their aluminum analogs[1,2]. Transition metal ions on zeolite surfaces are increasingly being exploited as controllable catalytic active sites[5]. This requires knowledge of both the location and adsorbate interactions of the transition metal ion. Various experimental techniques have been used in the elucidation of these properties. In particular, electron spin resonance (ESR) has been widely used in delineating information concerning the number of different species, oxidation state and coordination environment of a paramagnetic metal center such as Cu(II)[6-8]. A type of pulsed ESR, known as electron spin echo modulation (ESEM) spectroscopy can provide additional quantitative information concerning the number of surrounding adsorbate nuclei and their interaction distance[9]. In earlier work, the location and adsorbate interactions of Cu(II)in Cu(II)exchanged K-L a!uminosilicate zeolite was investigated by electron spin resonance techniques[10,11]. Zeolite L is a channel type zeolite with a main channel diameter of about 0.75 nm[12]. The replacement of AI by Ga in the framework
802
may change the type of cation coordination and its reaction properties. In the present work, the interaction of paramagnetic Cu(II) with various adsorbates in Cu(II)-exchanged K-L gallosilicate is studied by electron spin resonance(ESR) and electron spin echo modulation (ESEM) spectroscopies to deduce the locations and coordination geometries of Cu(rl)with adsorbates in this material. These results are compared to those in Cu(II)-exchanged K-L aluminosilicate zeolite. 2. EXPERIMENTAL SECTION Synthetic K-L zeolite was obtained from Union Carbide Corp. A gallium analog of L zeolite was synthesized according to the procedure described by Newsam and Vaughan[1,2]. The structure of the materials was determined by X-ray diffraction. These K-L materials were then exchanged at room temperature for 12 h by dropwise addition of a 10 mM solution of cupric nitrate (Alfa Products) according to the procedure described earlier[13]. Copper exchange was 0,1-0.4 % by weight of the K-L sample, assuming complete exchange. Dehydration of the sample was carried out with the sample in a Suprasil quartz ESR tube (2 mm i.d. by 3 mm o.d.)reactor by degassing at room temperature following evacuation and oxidation with oxygen at at 400 oC according to the procedure described in the earlier work[10,11] In particular, gallosilicate is known to have a lower thermal stability than the corresponding aluminosilicate[3]. Thus no evacuation was usually made at temperatures higher than 210oc in this work. This heat treated sample with oxygen is termed as a dehydrated sample. After dehydration, adsorbates as gases were admitted at room temperature to the sample tubes and left to equilibrate. ESR spectra were measured at 77 K on a modified Varian E-4 spectrometer interfaced to a Tracor Northern TN-1710 signal averager. ESEM spectra were recorded at 4.5 K with a Bruker ESP 380 pulsed ESR spectrometer. Three-pulse echoes were measured by using a 90°- x-90 °- T90 ° pulse sequence with the echo measured as a function of T to analyze the deuterium modulation from deuterated adsorbates. B o t h the theory and methods used for simulation of the data are described in detail elsewhere[9]. 3. RESULTS AND DISCUSSION Figure 1 shows the ESR spectra of fresh hydrated CuK-L aluminosilicate zeolite at room temperature and 77 K. At room temperature the ESR spectrum shows an almost isotropic signal at giso = 2.17 as shown in Figure la. The fresh hydrated samples of gallium anologs of CuK-L also give a similar ESR spectrum mainly consisting of a broad isotropic line at ambient temperature. Such a broad ESR signal at room temperature is indicative of a mobile species which is rotationally unrestricted on the ESR timescale. Analysis of the three-pulse ESEM spectrum of zeolite K-L which has been rehydrated with D20 and which exhibited an identical ESR spectrum to the fresh zeolite indicates a water solvation number of six around Cu(II), i.e. [Cu(H20)6] 2+ located in the main channel for both gallium and aluminum analogs of CuK-L. At 77 K this [Cu(H20)6] 2+ complex becomes immobilized and gives rise to an asymmetric spectrum with two species, denoted as species A and B, typical of Cu(II) axial powder spectra as shown in Figure lb.
803
Fresh Hydrated CuK-L
giso=2.166
RT ESR
~
b
I
X8
'
200 G
I
'
I
'
gilA=2.412
'
I
!
77K ESR I
I
gllS=1.943
i
Figure 1. ESR spectrum of fresh, hydrated CuK-L zeolite recorded (a) at room temperature and (b) at 77 K. Interestingly, species B, which is a minor species, shows ESR parameters with reversed g values of gll = 1.94, All = 97 x 10 -4 cm -1 and g_L= 2. 16. From a consideration of the symmetry expected in L zeolite and the analysis of the deuterium modulation obtained for species B, the Cu(II) species is suggested to be a diaquo complex [ C u ( O z ) 3 ( H 2 0 ) 2 ] with trigonal bipyramidal geometry. Interestingly, the minor Cu(II) diaquo species seen in aluminosilicate is not observed in the gallosilicate. Similar changes in ESR profile were observed during evacuation in the gallosilicate and aluminosilicate analogs of CuK-L. When the samples are evacuated at room temperature, the isotropic components of the ESR signal recorded at ambient temperature decrease and the ESR signal is no longer broadened at room temperature. This is indicative of the copper losing some water ligands and becoming immobilized by coordination to several lattice oxygens. The three-pulse ESEM results indicate that Cu(II) is now coordinated to only three water molecules. Upon further evacuation at increasing temperature Cu(II) ion moves from the main channel towards recessed sites. Complete dehydration produces one major Cu(II)species
804 assigned to the center of a hexagonal prism in a six-ring channel based on a lack of broadening of its ESR lines by oxygen for both gallium and aluminum analogs. Interestingly, water in Cu(II)-exchanged K-L gallosilicate is removed more easily than in the corresponding Cu(II)-exchanged aluminosilicate. This may be verified by the temperatures at which no further change of the ESR signal is observed during evacuation which may indicate the point of complete dehydration. This is around 2 0 0 o c for CuK-L gallosilicate and around 350 oC for CuK-L aluminosilicate. Adsorption of molecules such as water, alcohols, ethylene, benzene, ammonia, pyridine and dimethyl sulfoxide causes changes in the ESR spectrum of the Cu(II) indicating migration from a recessed site toward cation positions in the main channels where adsorbate coordination can occur. Table I summarizes the ESR parameters of Cu(II) in CuK-L gallosilicate compared with those of CuK-L aluminosilicate zeolite after various sample treatments. Table [. ESR parameters at 77 K of Cu(II) in CuK-L gallosilicate aluminosilicate zeolites observed after various sample treatments Gallosilicate
Treatment
glla
fresh/RT ESRc
2.17d
fresh
2.400
Aiib
and CuK-L
Aluminosilicate
g_La
glla
Allb
g_La
2.166d 134
2.08
2.412
137
2.08
97 159
2.157 2.05 2.08
dehydrated
2.338
153
2.07
1.943 2.334
+CH3OH
2.381
132
2.09
2.389 2.394
134 134
+CH3CH2OH
2.381
128
2.09
2.382
134
2.08
+ CH3CH2CH3OH
2.380
131
2.08
2.384
133
2.08
+C2H4 + NH3
2.344 2.254
148 175
0.07 2.05
2.343 2.255
159 177
2.07 2.05
+ pyridine
2.255
187
2.06
2.260
181
2.06
+ benzene + (CH3)2SO
2.350 2.394
140 129
2.07 2.09
2.341 2.392
161 117
2.05 2.06
aEstimated uncertainty is +_ 0.008. bThe unit of All is 10 -4 cm -1 and the estimated uncertainty is + 6 x l 0 -4 cm -1. CESR measured at room temperature, dgiso value. Figure 2a shows the ESR spectrum at 77 K observed after adsorption of ethanol on dehydrated CuK-L gallosilicate samples. Adsorption of methanol produces a new ESR spectrum with its g value shifted from 2.338 to 2.381 and a decrease in the All coupling. Similar ESR spectra are observed upon
805
adsorption of ethanol and propanol. The corresponding three-pulse ESEM spectrum with adsorbed CH3CH2OD is shown in Figure 2b. The simulation indicates interaction with two deuterium nuclei, .i.e. two ethanol molecules, with a Cu(U)-D distance of 0.28 nm. Cu(II) also forms complexes with two molecules of methanol and propanol based on ESEM analyses. Cu(II) in CuK-L alumin0silicate forms similar complexes with two molecules of methanol, ethanol and propanol, respectively. Cu(II) also forms new complexes with one molecule of ethylene and benzene based on ESR and ESEM data for both gallium and aluminum analogs. Figure 3 shows the ESR spectra after adsorption of NH3 and pyridine onto dehydrated CuK-L. CuK-L gallosilicate with adsorbed 15NH3 shows five hyperfine lines centered at gll = 2.054 and split by 18 x 10-4 cm -1 which are shown in the expanded second-derivative spectrum in Figure 3a. Since 15N has a nuclear spin of 1/2, the five lines indicate four ammonias coordinated to the Cu(II). A similar ESR spectrum with five hyperfine lines was observed in dehydrated CuK-L aluminosilicate upon 15NH3 adsorption. In this case, the Cu(lI) species is suggested to be located in the center of 12-ring main channel coordinating to four ammonias in a square-planar geometry. However, ESR spectra with different nitrogen hyperfine interactions in dehydrated CuK-L gallosilicate and CuK-L aluminosilicate measured after
b. CH=CH=OH I ~ 1.0 z
c6 0.8 nv
X8
~
V !
,
-a'°e ,
gll --2.381
0.6
oo z 0.4 u.I I.z O 0.2 ..r. O u.I
,
CuK-L gallosilicate
==,=.
a. C H 3 C H 2 O D
0
1
2
3
4
5
T, lU= Figure 2. (a) Experimental(---)and simulated ( - - - ) three-pulse ESEM spectrum recorded at 4 K. The best simulation indicates N = 2, R = 0.28 __.0.01 nm and Also = 0.28 MHz. (b) The corrresponding ESR spectrum at 77 K of dehydrated CuK-L gallosilicate with adsorbed CH3CH2OD.
806
a. ~ S N H 3 / G a - C ~
~..... f g=2.052 . b. P y r i d i n e / G a - ~ , ,
,
, , .... ~,
,
f ~
c. P y r i d i n e / A I - - C u ~
o,,
if
("''""r" V g=2.055
Figure 3. ESR spectra at 77 K of (a) dehydrated CuK-L gallosilicate with 70 Torr of 15NH3 added at room temperature, (b) dehydrated CuK-L gallosilicate equilibrated with pyridine containing 14 N and (c) dehydrated CuK-L aluminosilicate equilibrated with pyridine (the g±region is expanded as the second derivative to more clearly show the hyperfine structure).
807
pyridine adsorption are shown in Figures 3b and 3c. A new cupric ion species due to complex formation with pyridine is observed with at least nine hyperfine lines centered at g_L =2.055 and split by 15 x 10 -4cm -1 and seven lines centered at g_L = 2.055 and split by 16 x 10-4 cm-1 as shown in the expanded second-derivative spectra for CuK-L gallosilicate and CuK-L aluminosilicate, respectively. Since 14 N in pyridine has a nuclear spin of 1, the nine and seven lines indicate four and three pyridine molecules directly coordinated to the Cu(H), respectively. Thus, the Cu(II) species in K-L gallosilicate is suggested to be located in the center of a 12-ring main channel coordinating to four pyridines in a square-planar geometry, while Cu(II) species in CuK-L aluminosilicate is located near an eight-ring window of a main channel coordinating to four framework oxygens and three pyridine molecules. Dimethyl sulfoxide also produces new Cu(II) ESR signals. The analysis of three-pulse ESEM data for CuK-L gallosilicate with adsorbed deuterated dimethyl sulfoxide(D3CSOCD3) shows no deuterium modulation as shown in Figure 4. Since no modulation is usually observed at a distance beyond about 0.5 nm, the new Cu(II) species may be due to Cu(II) indirectly interacting with dimethyl sulfoxide at a longer distance. Interestingly, we observed strong deuterium modulation for aluminosilcate CuK-L zeolite with adsorbed deuterated dimethyl sulfoxide. The simulation indicated that the Cu(II) in aluminoslicate CuK-L zeolite directly interacts with one molecule of dimethyl sulfoxide with a Cu(II)-D distance of 0.37 nm. 1.0
CuK-L + SO(0D3)2
z q.8
E" 1.o z
~d 0.8
n-
CuK-L gallosilicate
" 0.6
0.4
Z I
CuK-L aluminosilicate
0 0.2
'-1tO uJ
0
1
2
3
4
5
T, 14s Figure 4. Experimental ( ~ ) and simulated (- - -) three-pulse ESEM spectra recorded at 4.5 K of dehydrated CuK-L gallosilicate and CuK-L aluminosilicate with adsorbed D3CSOCD3. The simulation parameters for CuK-L aluminosilicate are N = 6, R = 0.37 + 0.01 nm and Aiso = 0.02 MHz.
808
4.
CONCLUSIONS Rather similar results to those for CuK-L aluminosilicate were observed in CuK-L gallosilicate but there are also significant differences. The main Cu(II) species in hydrated CuK-L aluminosilicate and gallosilicate is an octahedrally coordinated hexaaquo species [Cu(H20)6] 2+ which resides in the main channel with rotational freedom at room temperature. The minor Cu(II) species with reversed g values assigned to a diaquo complex in hydrated CuK-L aluminosilicate is not observed in the gallosilicate. Upon partial dehydration at room temperature, the fully hydrated cupric ion loses some of its coordinated water and becomes anchored to the zeolite lattice by partial coordination to zeoliticoxygens. The coordinated water is removed more easily in the gallosilicate than in the aluminosilicate. When completely dehydrated, the cupric ions are located in cation sites recessed from the main channels. Absorption of adsorbate molecules such as water alcohols, ammonia, pyridine, dmethyl sulfoxide, benzene and ethylene causes migration of Cu(II) into the main channel and coordination with the molecule. Cu(II) forms complexes with two molecules of alcohols and one molecule of ethylene and benzene based on ESEM data for both the gallium and aluminum analogs. Cu(II) also forms complexes with four molecules of ammonia in the center of twelve-membered ring main channels based on resolved nitrogen superhyperfine. However, Cu(II) coodinates with four molecules of pyridine in K-L gallosilicate and only three molecules of pyridine in K-L aluminosilicate based on resolved nitrogen superhyperfine. Cu(II) also interacts directly with dimethyl sulfoxide in KL aluminosilicate, but only indirectly at a longer distance with dimethyl sulfoxide in K-L gallosilicate based on ESEM data. 5.
ACKNOWLEDGMENTS
This research was supported by the Korea Science and Engineering Foundation (951-0303-046-2), the U.S. National Science Foundation and the Robert A. Welch Foundation. REFERENCES °
2. 3. .
5. 6. 7. 8. 9. 10. 11. 12. 13.
J. M. Newsam and D.E.W. Vaughan, Stud. Surf. Sci. Catal. 28 (1986) 457. J. M Newsam, Mat. Res. Bull. 21 (1986)661. R. Sz0stak, Molecular Sieves, Principles of Synthesis and Identification; Van Nostrand Reinhold: New York, 1989; p.212. J. M. Thomas and X.-S. Liu, J. Phys. Chem. 90 (1986) 4843. I.E. Maxwell, A d v . Catal. 31 (1982) 1. M. Narayana, S. Contarini and L. Kevan, J. Catal. 94 (1985)370. R. G. Herman and D.R. Flentge, J. Phys. Chem. 82 (1978) 720. J.S. Yu and L. Kevan, J. Phys. Chem. 95 (1991)3262. L. Kevan, In Time Domain Electron Spin Resonance; L. Kevan and R. N. Schwartz, Eds. ; Wiley- Interscience: New York, 1979; Chapter 8. J. S. Yu, J.M. Comets and L. Kevan, J. Phys. Chem. 97 (1993) 11047. J. S. Yu and L. Kevan, J. Phys. Chem. 98 (1994) 12436. A. Sayari, J. R. Morton and K. F. Preston, J. Phys. Chem. 93 (1989) 2093. J.S. Yu and L. Kevan, J. Phys. Chem. 94 (1990) 5995.
